A FORGED ROLL MEETING THE REQUIREMENTS OF THE COLD ROLLING INDUSTRY AND A METHOD FOR PRODUCTION OF SUCH A ROLL

ROULEAU FORGE SATISFAISANT LES EXIGENCES DE L'INDUSTRIE DU LAMINAGE A FROID ET PROCEDE DE PRODUCTION D'UN TEL ROULEAU

English Abstract

This invention relates in general to the field of forged rolls and to production of forged rolls. More particularly the present invention relates to forged rolls for use in the cold rolling industry. The present invention relates to a forged roll for use in the cold rolling industry and a method for production of such a roll. Said forged roll, comprises a steel composition and a microstructure that comprises: - tempered martensite with a retained austenite rate less than (<) 5 % per volume; and - an open eutectic carbide network with eutectic carbides of less than (<) 5 % per volume; and wherein the roll exhibits: - a hardness between 780-840HV; and - internal compressive between -300 to -500 MPa in absolute values.

French Abstract

L'invention concerne en général le domaine des rouleaux forgés et la production de rouleaux forgés. Plus particulièrement, la présente invention concerne des rouleaux forgés destinés à être utilisés dans l'industrie du laminage à froid. La présente invention concerne un rouleau forgé destiné à être utilisé dans l'industrie du laminage à froid et un procédé de production d'un tel rouleau. Ledit rouleau forgé comprend une composition d'acier et une microstructure qui comprend : - de la martensite revenue présentant une teneur en austénite retenue inférieure à (<) 5 % par volume; et - un réseau de carbure eutectique ouvert présentant une teneur en carbures eutectiques inférieure à (<) 5 % par volume; le rouleau présentant: -une dureté comprise entre 780 et 840HV; et -une compression interne comprise entre -300 et -500 MPa en valeurs absolues.

Claims

38
Claims
1. A forged roll (1), comprising a steel composition comprising, in terms
of % per weight,
0.8 to less than (<) 1 % C,
0.2 to 0.5 % Mn,
0.2 to 2.0 Si,
7.0 to 13.0 % Cr,
0.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,
the remaining portion of the steel being substantially Fe and incidental
and/or
unavoidable impurities;
and wherein the microstructure of the roll (1) comprises:
- tempered martensite with a retained austenite rate less than (<) 5 % per
volume; and
- an open eutectic carbide network with eutectic carbides of less than (<)
5 % per volume;
and wherein the roll (1) exhibits:
- a hardness of between 780 to 840 HV; and
- internal compressive stresses of between -300 to -500 MPa.
2. The roll of claim 1, wherein the open eutectic carbide network delimits a
cell-like pattern of
eutectic cells.
3. The roll according to any one of claims 1 and 2, wherein the open eutectic
carbide network
comprises dendritic arms.
4. The roll of any one of claims 1-3, wherein the microstructure is present at
least in a working
layer of the roll.
5. The roll of any one of claims 1-4, wherein the steel composition
consists of, in terms of
% per weight;
0.8 to less than (<) 1 % C,
0.2 to 0.5 % Mn,
0.2 to 2.0 % Si,

39
7.0 to 13.0 % Cr,
0.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,
less than (<) 0.015 % P, and
less than (<) 0.015 % S, and
less than (<) 1.0 % Ni
less than (<) 30 ppm O2, and
less than (<) 100 ppm N2, and
less than (<) 3 ppm H2
less than (<) 2 % W, and
less than (<) 1 % Nb, and
less than (<) 1 % Ti, and
less than (<) 0.5 % Ta, and
less than (<) 0.5% Zr,
the remaining portion of the steel being substantially Fe and incidental
and/or
unavoidable impurities.
6. The roll of any one of claims 1-5, wherein the C content in the steel
composition is
between 0.8 - 0.99 % C in terms of % per weight of total roll weight.
7. The roll of any one of claims 1-6, wherein the C content in the steel
composition is
between 0.85 - 0.9 % C in terms of % per weight of total roll weight.
8. The roll of any one of claims 1-7, wherein the Mn content in the steel
composition is
between 0.4 - 0.5 % Mn in terms of % per weight of total roll weight.
9. The roll of any one of claims 1-8, wherein the Si content in the steel
composition is
between 0.2 - 1.5 % Si in terms of % per weight of total roll weight.
10. The roll of any one of claims 1-8, wherein the Si content in the steel
composition is
between 0.85 - 1.15 % Si in terms of % per weight of total roll weight.

40
11. The roll of any one of claims 1-10, wherein the Cr content in the steel
composition is
between 7.0 - 11 % Cr in terms of % per weight of total roll weight.
12. The roll of any one of claims 1-10, wherein the Cr content in the steel
composition is
between 7.3 - less than (<) 8.0 % Cr in terms of % per weight of total roll
weight.
13. The roll of any one of claims 1-12, wherein the Mo content in the steel
composition is
between 1.45 - 1.55 % Mo in terms of % per weight of total roll weight.
14. The roll of claim 5, wherein the Ni content in the steel composition is
less than (<) 0.3 Ni
in terms of % per weight of total roll weight.
15. The roll of any one of claims 1-14, wherein the V content in the steel
composition is
between 1.3 - 2.1 % V in terms of % per weight of total roll weight.
16. The roll of any one of claims 1-14, wherein the V content in the steel
composition is
between 1.3 - 1.6 % V in terms of % per weight of total roll weight.
17. A roll of any one of claims 1-16, wherein the steel composition consists,
in terms of % per
weight:
0.8 - 0.99 % C, and
0.4 - 0.5 % Mn, and
0.2 - 1.5 % Si, and
7.0 - 11 % Cr, and
0.6 - 1.6 % Mo, and
less than (<)1.0 Ni, and
1.0 - 2,1 % V, and
less than (<) 0.015 % P, and
less than (<) 0.015 % S, and
less than (<) 30 ppm O2, and

41
less than (<) 100 ppm N2, and
less than (<) 3 ppm H2, and
the remaining portion of the roll being substantially Fe and incidental and/
or unavoidable
impurities.
18. A roll of any one of claims 1-17, wherein the steel composition
consists, in terms of %
per weight:
0.85 - 0.9 % C, and
0.4 - 0.5 % Mn, and
0.85 - 1.15 % Si, and
7.3 - less than (<) 8.0 % Cr, and
1.45 - 1.55 % Mo, and
less than (<) 0.3 Ni, and
1.3 - 1.6 % V and less than (<) 0.015 % P> and
less than (<) 0.015 % S, and
less than (<) 30 ppm O2, and
less than (<) 100 ppm N2, and
less than (<) 3 ppm H2, and
the remaining portion of the roll being substantially Fe and incidental and/or
unavoidable
impurities.
19. The roll of any one of claims 1-18, wherein the roll is a working roll
in cold rolling.
20. The roll of any one of claims 1-19, further having a weight of more
than 400 kg.
21. The roll of any one of claims 1-20, further having a diameter in the
range of 215-
800mm.
22. A process for manufacturing a non-forged roll, the process comprising
the steps of:
a. Providing a steel composition comprising, in terms of % per weight,

42
0.8 to less than (<) i % C,
0.2 to 0.5 % Mn,
0.2 to 2.0 % Si,
7.0 to 13.0 % Cr,
0.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,
the remaining portion of the steel being substantially Fe and incidental
and/or
unavoidable impurities;
b. Manufacturing an ingot maintaining a solidification rate higher than
15°C/min in the
working layer of the ingot in the solidification interval;
c. Forging the ingot to a roll;
d. Hardening the roll by induction heating;
e. Tempering the roll at a temperature between 450-530°C to reach
hardness between 780
to 840HV;
thereby achieving a microstructure of the roll (1) that comprises:
- tempered martensite with a retained austenite rate less than (<) 5 % per
volume; and
- an open eutectic carbide network with eutectic carbides of less than (<) 5 %
per volume;
and wherein the roll (1) exhibits:
- a hardness of between 780 to 840HV; and
- internal compressive stresses of between -300 to-500 MPa.
23. A process according to claim 22 wherein the ingot is manufactured
maintaining a
solidification rate in the working layer as well as in the core in the range
of 15°C/min to 55 °C
/min.
24. A process according to claim 22 wherein the ingot is manufactured
maintaining a
solidification rate in the working layer as well as in the core in the range
of 17 °C /min -
50°C/min.
25. A process according to claim 22 wherein the ingot is manufactured
maintaining a
solidification rate in the working layer as well as in the core in the range
of 35 °C /min -

43
55 °C/min.
26. A process according to claim 22 wherein the ingot is manufactured
maintaining a
solidification rate in the working layer as well as in the core in the range
of 45 °C /min -
55 °C/min.
27. The process according to any one of claims 22-26, wherein the ingot is
manufactured
maintaining a solidification rate higher than 35 °C/min in the working
layer of the ingot in the
solidification interval.
28. The process of any one of claims 22-27 wherein the solidification
interval is between
1400-1200°C for said ingot.
29. The process of any one of claims 22-28, wherein the ingot is
manufactured maintaining a
pre-selected solidification rate in an electro-slag refining furnace (ESR)
technique process by
controlling the ampere current supply according to a predetermined function of
the solidification
rate.
30. The process of any one of claims 22-29, wherein the step of forging the
ingot to a roll
further comprises the steps of:
a. Heating the ingot to a temperature of between 800 - 1200 °C or
between 850 - 1100
°C;
b. Forging the ingot at a temperature above 800°C or above 850
°C;
c. Repeating steps a-b until the ingot has been formed to a roll that has
desired shape and
size.
31. The process of claim 30, wherein the step of heating the ingot to a
temperature comprises
heating the ingot to the temperature for a period of about 6 hours.
32. The process of any one of claims 22-31, further, after the forging
step, comprising a step
of preliminary heat treatment.

44
33. The process of claim 32, wherein the step of preliminary heat treatment
comprises
preliminary heating the ingot to a temperature of between 700 - 1100
°C.
34. The process of claim 32, wherein the step of preliminary heat treatment
comprises
preliminary heating the ingot to a temperature of between 800 - 900 °C.
35. The process of any one of claims 32-34, wherein the step of preliminary
heat treatment
comprises hydrogen diffusion treatment.
36. The process of any one of claims 22-35, wherein the step of tempering
the roll comprises
further the steps of:
a. Heating the roll to about 450 - 530 °C,
b. Air cooling the roll between the heating steps.
37. The process of claim 36, wherein the step of heating the roll to about
450 - 530 °C
comprises heating the roll to about 450 - 530 °C 3 times.
38. The process of any one of claims 22-37, further comprising machining
the roll to
texturing a white layer comprising eutectic carbides.
39. The process of claim 38 wherein said eutectic carbides in the white
layer are M7C3.
40. A method of producing a forged roll (1) comprising the steps of:
a. Providing a steel composition comprising, in terms of % per weight,
0.8 to less than (<) 1 % C,
0.2 to 0.5 % Mn,
0.2 to 2.0 % Si,
7.0 to 13.0 % Cr,
0.6 to 1.6 % Mo,

45
more than (>) 1.0 to 3.0 % V,
the remaining portion of the steel being substantially Fe and incidental
and/or
unavoidable impurities;
b. Manufacturing an ingot maintaining a solidification rate higher than
15°C/min in the
working layer of the ingot in the solidification interval;
c. Forging the ingot to a roll; d. Hardening the roll by induction heating;
e. Tempering the roll; thereby achieving a microstructure of the roll (1) that
comprises:
- tempered martensite with a retained austenite rate less than (<) 5 % per
volume; and
- an open eutectic carbide network with eutectic carbides of less than (<) 5 %
per volume;
and wherein the roll (1) exhibits:
- a hardness of between 780 to 840HV; and
- internal compressive stresses of between -300 to -500 MPa.
41.
An intermediate product ingot in the production of a roll according to any one
of claims
1-21, the ingot comprising the steel composition comprising, in terms of % per
weight,
0.8 to less than (<) 1 % C,
0.2 to 0.5 % Mn,
0.2 to 2.0 % Si,
7.0 to 13.0 % Cr,
0.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,
the remaining portion of the steel being substantially Fe and incidental and/
or
unavoidable impurities;
and wherein the microstructure of the final roll issued from the ingot
comprises:
- tempered martensite with a retained austenite rate less than (<) 5 % per
volume; and
- an open eutectic carbide network with eutectic carbides of less than (<) 5 %
per volume.

46
42. Use of a roll according to any one of claims 1-21 for cold rolling
material requiring a
high rolling load.
43. Use of a roll according to any one of claims 1-21 for cold rolling of
high strength
materials.
44. Use of a roll according to claim 43 for cold rolling of AHSS steel
grades.
45. Use of a roll according to any one of claims 1-21 for a selection of:
- cold rolling reduction mills for early and finishing stands, reversible and
non- reversible
stands for tinplate, sheet, silicon steel, stainless steel, aluminum and
copper; or
- cold rolling temper and/or skin pass mills; or
- mill configurations as 2-High, 4-High and 6-High stands with textured or non
textured
surface.
46. Use of a roll according to any one of claims 1-21 as a work roll.
47. The roll according to any one of claims 1-21 , wherein the roll is non-
coated.
48. The roll of any one of claims 1-21, wherein the roll is coated with a
selectable coating.
49. The roll of claim 48, wherein the coating comprises a chromium coating.
50. The process of any one of claims 22-40, wherein the roll is non-coated.
51. The process of any one of claims 22-40, wherein the roll is coated with
a selectable
coating.
52. The process of claim 51, wherein the coating comprises a chromium
coating.
53. The use of any one of claims 43-46 wherein the roll is non-coated.

47
54. The use of any one of claims 43-46 wherein the roll is coated with a
selectable coating.
55. The use of claim 54, wherein the coating comprises a chromium coating.

Description

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Title
A forged roll meeting the requirements of the cold rolling industry and a
method for
production of such a roll
Field of invention
This invention relates in general to the field of forged rolls and to
production of forged rolls.
More particularly the present invention relates to forged rolls meeting the
requirements of
and mainly being directed for use in the cold rolling industry.
Background
General background
The general trend for development in cold rolling both for the ferrous and the
non-ferrous
metal industries is to roll faster, thinner and wider. The current challenge
is to do this while
achieving perfect control of flatness, thickness and surface aspects
compatible with a high
productivity. Therefore, this trend calls for use of advanced rolling
technologies that control
key rolling parameters.
Some key parameters such as roughness retention and surface aspects can be
guaranteed
through chrome plating of work rolls. This practice is effective and
efficient, but is becoming
more and more questionable and in a near future unacceptable due to
environmental
restrictions.
Nowadays forged work rolls (2 to 6 %Cr) with surface chrome plating are
usually used in cold
rolling processes. Chrome plating of such rolls is applied to improve the wear
resistance in
terms of surface texture retention which, in turn, will ensure, for instance,
consistent and
higher gloss of car bodies after painting. Hard electrolytic deposit
techniques as chrome
plating were initially developed for temper/skin pass mill applications. In
these applications,
chrome plated work rolls exhibit 2 to 8 times longer lifetimes than uncoated
rolls, mainly
because of a better roughness retention. The implementation of this technique
was
progressively extended to the reduction mills.
There are also forged rolls made of high speed steel (HSS) which are made
intended for use
without coating but there is a need for a roll with low residual internal
stresses and there is
also a need for an industrial process for producing such a roll, which is
intended to be used
without coating in a mill while giving roughness retention which is at least
equivalent to that
of coated rolls.
Specific background
Rolls produced to be used within the cold rolling industry has to manage the
processing
conditions or the specific operating stresses during usage without getting
cracks or be prone

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to explode. Explosion of a roll can involve safety of operating persons and
collateral damage
in the mill. Therefore there is a need for a roll with low residual internal
stresses.
Prior art
Examples of prior art disclosing the development towards HSS rolls without
coatings for the
purpose of cold rolling:
C. Gaspard, C. Vergne, D. Batazzi, T. Nylen, P.H. Bolt, S. Mul, K.M. Reuver
"Implementation of in-service key parameters of HSS work roll grade dedicated
to
advanced cold rolling", 1ST Conference May 3-6, 2010, Pittsburgh, Pa, USA
C. Gaspard, S. Maine, D. Batazzi, P.Thonus: "Improvement For Advanced Cold
Rolling
Reduction Mills By Using Semi-HSS and HSS Rolls ", 7th International
Conference on Steel
Rolling (ISIJ), Makuhari, Chiba, Japan, 1998
P.H. Bolt, D. Batazzi, N.P. Belfiore, C. Gaspard, L. Goiset, M. Laugier, 0.
Lemaire, D.
Matthews, T. Nylen, K. Reuver, D. Stocchi, F. Stork, J. Tensen, M. Tornicelli,
R. Valle, E. van
den Elzen, C. Vergne, I.M. Williams: "Damage Resistance and Roughness
Retention of work
Rolls in cold Rolling Mills", 5th European Rolling Conference, 23-25 June
2009, London, UK
Other examples of prior art are shown in the patent publications: JPo9003603 ,
JP53077821,
JP57047849õ JP2002285284, JP2002285285, JP1o317102, JP1208437, EP0395477 and
JPo8158018 which describe work rolls for cold rolling to enhance wear and
spalling
resistance.
However, these pieces of prior art lack the disclosure of parameters and
properties necessary
to achieve and enable such an HSS roll that is operative during the conditions
in a cold
rolling mill.
Object of the Invention
General object
The general object of the invention is to provide a roll and an industrial
process for producing
such a roll that is operative during the conditions in a cold rolling mill,
preferably in a non-
coated form. A more specific object is to provide such a roll and process for
producing such a
roll while keeping tribological properties such as low friction coefficient,
high roughness
retention, no dust pollution by iron fines at least equivalent to prior art
coated rolls and
which exhibit improved mill performances in terms of higher crack resistance
and higher
safety in operation compared to known rolls.

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Partial problems
The invention further seeks to solve the partial problems of:
-Improving the roll surface which gives the roll higher performance.
-Avoiding roll spalling accidents
-Avoiding non-environmental rolling production processes
-Improving rolling distance or life span of a roll, allowing longer runs per
mill campaign.
Summary of the invention
The solution to the problem, partial problems and aspects listed above is a
roll according to
the invention with improved fire crack resistance and low crack propagation
which will
reduce the sensitivity to mill incidents while keeping higher wear resistance.
The present invention provides a forged roll for use in the cold rolling
industry and a method
for production of such a roll. The roll is preferably non-coated but may also
be coated.
A first aspect of the invention relates to a forged roll, comprising a steel
composition
comprising, in terms of % per weight,
o.8 to less than (<) % C,
0.2 to 0.5 % Mn,
0.2 to 2.0 % Si,
7.0 to 13.0 % Cr,
0.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,
the remaining portion of the steel being substantially Fe and possible
incidental and/or
possibly unavoidable impurities;
and wherein the microstructure of the roll comprises:
- tempered martensite with a retained austenite rate less than (<) 5 % per
volume; and
- an open eutectic carbide network with eutectic carbides of less than (<)
5 % per volume;
and wherein the roll exhibits:
- a hardness between 780 HV to 840HV; and
- internal compressive stresses between -300MPa to -500 MPa.
In other embodiments of the invention the roll of the invention comprises an
open eutectic
carbide network delimits a cell-like pattern of eutectic cells.
Further varieties of the roll comprising any of the following optional,
individual or
combinable aspects:

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A roll wherein the open eutectic carbide network of said roll comprises
dendritic arms.
A roll wherein the open eutectic carbide network of said roll is formed as
substantially
isolated portions of eutectic carbides network.
A roll wherein the microstructure of said roll is present at least in the
working layer of the
roll.
A roll with a steel composition consisting, in terms of % per weight;
o.8 to less than (<) % C,
0.2 to 0.5 % Mn,
0.2 to 2.0 % Si,
7.0 to 13.0 % Cr,
o.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,
less than (<) 0.015 % P, and
less than (<) 0.015 % 5, and
less than (<) i% Ni
less than (<) 30 ppm 02, and
less than (<) 100 ppm N2, and
less than (<) 3 ppm H2
less than (<) 2 % W, and
less than (<) i. % Nb, and
less than (<) i % Ti, and
less than (<) 0.5 % Ta, and
less than (<) o.5% Zr,
the remaining portion of the steel being substantially Fe and possible
incidental and/or
possibly unavoidable impurities;
The roll according to the invention, wherein the C content in the steel
composition is between
0.8 ¨ 0.99 % C in terms of % per weight of total roll weight.
The roll according to the invention, wherein the C content in the steel
composition is between
0.85 ¨ 0.9 % C in terms of % per weight of total roll weight.
The roll according to the invention, wherein the Mn content in the steel
composition is
between 0.4 ¨ 0.5 % Mn in terms of % per weight of total roll weight.

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The roll according to the invention, wherein the Si content in the steel
composition is
between 0.2.- 1.5 % Si in terms of % per weight of total roll weight.
The roll according to the invention, wherein the Si content in the steel
composition is
between 0.85 ¨ 1.15 % Si in terms of % per weight of total roll weight.
5 The roll according to the invention, wherein the Cr content in the steel
composition is
between 7.0 ¨ 11 % Cr in terms of % per weight of total roll weight.
The roll according to the invention, wherein the Cr content in the steel
composition is
between 7.3 ¨ less than (<) 8.o % Cr in terms of % per weight of total roll
weight.
The roll according to the invention, wherein the Mo content in the steel
composition is
between 1.45 ¨ 1.55 % Mo in terms of % per weight of total roll weight.
The roll according to the invention, wherein the Ni content in the steel
composition is less
than (<) 0.3 Ni in terms of % Per weight of total roll weight.
The roll according to the invention, wherein the V content in the steel
composition is between
1.3 ¨ 2.1 % V in terms of % per weight of total roll weight.
The roll according to the invention, wherein the V content in the steel
composition is between
1.3 ¨ 1.6 % V in terms of % per weight of total roll weight.
A roll according to the invention, wherein the steel composition consists, in
terms of % per
weight:
o.8 ¨ 0.99 % C, and
0.4 ¨ 0.5 % Mn, and
0.2 - 1.5 % Si, and
7.0-11%Cr,and
o.6 ¨ 1.6 % Mo, and
less than (<) to Ni, and
1.0 ¨ 2,1 % V, and
less than (<) 0.015 % P, and
less than (<) o.o15 % S, and
less than (<) 30 ppm 02, and
less than (<) 100 ppm N2, and
less than (<) 3 ppm H2, and
=

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the remaining portion of the roll being substantially Fe and possible
incidental and/or
possibly unavoidable impurities.
A roll according to the invention, wherein the steel composition consists, in
terms of % per
weight:
0.85 ¨ 0.9 % C, and
o.4 ¨ o.5 % Mn, and
0.85 ¨ 1.15 % Si, and
7.3 ¨ less than (<) 8.o % Cr, and
1.45 ¨ 1.55 % Mo, and
less than (<) 0.3 Ni, and
1.3 ¨1.6 % V and
less than (<) 0.015 % P, and
less than (<) 0.015 % S, and
less than (<) 30 ppm 02, and
less than (<) 100 ppm N2, and
less than (<) 3 ppm H2, and
the remaining portion of the roll being substantially Fe and possible
incidental and/or
possibly unavoidable impurities.
A roll according to the invention further being configured for use as a
working roll in cold
rolling.
A roll according to the invention further having a weight of more than 400 kg.
A roll according to the invention further having a diameter in the range of
215-800mm.
A further aspect of the invention provides a forged roll produced by a process
comprising the
steps of:
a. Providing a steel composition comprising, in terms of % per weight,
o.8 to less than (<) % C,
0.2 to 0.5 % Mn,
0.2 tO 2.0 % Si,
7.0 to 13.0 % Cr,
0.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,

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the remaining portion of the steel being substantially Fe and possible
incidental
and/or possibly unavoidable impurities; In other embodiments the composition
according to the invention is as any of the compositions or combinations of
compositions described above.
b. Manufacturing an ingot maintaining a solidification rate higher than 15
C/min
in the surface layer of the ingot, equivalent to the surface layer of the
roll, in the
solidification interval;
c. Forging the ingot to a roll;
d. Hardening the roll by induction heating;
e. Tempering the roll;
thereby achieving a microstructure of the roll that comprises:
- tempered martensite with a retained austenite rate less than (<) 5 % per
volume; and
- an open eutectic carbide network with eutectic carbides of less than (<)
5 % per volume;
and wherein the roll (1) exhibits:
- a hardness of between 780 HV to 840 HV; and
- internal compressive stresses of between -300MPa to -500 MPa.
Further varieties of the roll comprising any of the following optional,
individual or
combinable aspects regarding the chemical composition or microstructure of the
roll
mentioned above and further comprising the features of any of the comprising
any of the
following optional, individual or combinable aspects mentioned below.
A further aspect of the invention provides a process for manufacturing a non-
forged roll
according to the invention, the process comprising the steps of:
a. Providing a steel composition comprising, in terms of % per weight,
o.8 to less than (<) % C,
0.2 tO 0.5 % Mil,
0.2 to 2.0 % Si,
7.0 to 13.0 % Cr,
o.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,
the remaining portion of the steel being substantially Fe and possible
incidental
and/or possibly unavoidable impurities; In other embodiments the composition

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according to the invention is as any of the combinations of compositions
described above.
b. Manufacturing an ingot maintaining a solidification rate higher than 15
C/min
in the working layer of the ingot, equivalent to the working layer of the
roll, in
the solidification interval;
c. Forging the ingot to a roll;
d. Hardening the roll by induction heating;
e. Tempering the roll at a temperature between 450-530 C to reach hardness
between 780 HV to 840 HV;
thereby achieving a microstructure of the roll (1) that comprises:
- tempered martensite with a retained austenite rate less than (<) 5 % per
volume; and
- an open eutectic carbide network with eutectic carbides of less than (<)
5 % per volume;
and wherein the roll (1) exhibits:
- a hardness of between 780 HV to 840 HV; and
- internal compressive stresses between -300 to -500 MPa.
Further varieties of the roll comprising any of the following optional,
individual or
combinable aspects mentioned below.
A process according the invention wherein the ingot is manufactured
maintaining a
solidification rate in the working layer as well as in the core in the range
of 15 C/min to 55 C
/min, or alternatively 17 C /min - 50 C/min, or alternatively 35 C /min - 55
C/min, or
alternatively 45 C /min - 55 C/min.
A process according the invention, wherein the ingot is manufactured
maintaining a
solidification rate higher than 35 C/min in the working layer or surface of
the ingot in the
solidification interval.
A process according the invention wherein the solidification interval is
between 1400-1200 C
for said ingot.
A process according the invention, wherein the ingot is manufactured
maintaining a pre-
selected solidification rate in an electro-slag refining furnace (ESR)
technique process by

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controlling the ampere current supply according to a predetermined function of
the
solidification rate.
A process, wherein the step of forging the ingot to a roll comprises the steps
of:
a. Heating the ingot to a temperature of about 850 ¨ 1100 C or between
8 oo-i000 C preferably fora period of about 6 hours;
b. Forging the ingot at a temperature above about 800 C or above 850 C;
c. Repeating steps a-b until the ingot has been formed to a roll that has
desired shape and size.
A process further, after the forging step, comprising a step of preliminary
heat treatment,
applied on the roll blank, preferably to a temperature of about 700-1100 C or
between 800 ¨
goo C, which may include hydrogen diffusion treatment.
A process further comprising a step of superficial hardening by progressive
induction heating
, preferably at a temperature of about 900-1150 C.
A process wherein the step of tempering the roll comprises the steps of:
d. Heating the roll to about 450 ¨ 530 C or between 450 ¨ 520 C,
preferably 3 times,
e. Air cooling the roll between the heating steps.
A process further comprising machining the roll to texturing a white layer
comprising
eutectic carbides.
Further varieties of the process of the invention comprising any of the
following optional,
individual or combinable aspects regarding the chemical composition or
microstructure of
the roll mentioned above and further comprising the features of any of the
comprising any of
the following optional, individual or combinable aspects mentioned below.
A further aspect of the invention provides an intermediate product ingot in
the production of
a roll, the ingot comprising a steel composition comprising, in terms of % per
weight,
o.8 to less than (<) % C,
0.2 to 0.5 % Mn,
0.2 tO 2.0 % Si,
=
7.0 to 13.0 % Cr,
o.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,

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the remaining portion of the steel being substantially Fe and possible
incidental and/or
possibly unavoidable impurities;
and wherein the microstructure of the final roll issued from the ingot
comprises:
- tempered martensite with a retained austenite rate less than (<) 5 % per
volume; and
5 - an open eutectic carbide network with eutectic carbides of less than
(<) 5 % per volume.
Further varieties of the intermediate ingot of the invention comprising any of
the following
optional, individual or combinable aspects regarding the chemical composition
of the ingot
mentioned above and further comprising the features of any of the comprising
any of the
following optional, individual or combinable aspects mentioned below.
10 A further aspect of the invention provides the use of a forged roll
according to the invention
for cold rolling material requiring a high rolling load.
Other embodiments of the invention provide the use of a forged roll for cold
rolling of high
strength materials like AHSS steel grades.
The use of a forged roll according to the invention for a selection of:
- cold rolling reduction mills for early and finishing stands, reversible and
non-reversible
stands for tinplate, sheet, silicon steel, stainless steel, aluminum and
copper; or
- cold rolling temper and/or skin pass mills; or
- mill configurations as 2-High, 4-High and 6-High stands with textured or
non textured
surface.
The use of a forged roll according to the invention as a work roll.
The roll according to the invention is useful in many applications as a non-
coated roll.
However, in further aspects and embodiments of the invention, the roll may
also be provided
with a coating selected for any current of specific application. The coating
may for example
be a chromium coating. The roll may also be used in warm rolling applications.
Brief description of the figures
The invention will be further described by means of exemplifying embodiments
wherein:
Figure 1 shows a schematic picture of a roll according to the invention.
Figure 2 shows a schematic view of the roll production process according to
the invention.
Figure 3 shows a schematic picture of an ingot according to the invention.
Figure 4 shows a manufacturing process of an ingot according to the invention.

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Figure 5 A-B shows a cast microstructure of roll grade made using a production
process
according to the invention. The roll grade is shown in sectional view of the
working layers of
the roll grade.
Figure 6 A-B shows a cast microstructure of roll grade made using a production
process
according to the invention. The roll grade is shown in sectional view of the
working layers of
the roll grade.
Figure 7 shows cast microstructure of roll grade made using a production
process according
to the invention but with the deviation rendered when using too low
solidification rate. The
roll grade is shown in sectional view of the working layers of the roll grade.
Figure 8 shows a first set of examples of solidification rates for roll
production process
according to the invention.
Figure 9 shows a second set of examples of solidification rates for roll
production process
according to the invention.
Figure loA-B show a cast microstructure of an ingot made in laboratory
conditions when
using the production process according to the invention.
Figure ii A-B show a cast microstructure of an ingot made in laboratory
conditions when
using the production process according to the invention but with the deviation
rendered
when using too high Mo content.
Figure 12 shows a schematic view of forging according to the invention.
Figure 13 shows a schematic view of the steps of forming the ingot by forging
it to a roll
according to the invention.
Figure 14 shows a schematic view of progressive induction hardening with
different
frequencies of the roll according to the invention.
Figure 15 A-B shows a microstructure of the surface of a roll according to a
standard grade
after surface texturing (EDT texturing).
Figure 15 C-D shows a microstructure of the surface of a roll according to the
invention after
surface texturing (EDT texturing).
Figure 16 A-D shows detrimental defects on a roll generated during
manufacturing of rolls
with low chromium content and high molybdenum content.
Figure 17A shows an embodiment of a microstructure according to the invention
with an
open eutectic network.
Figure 17B shows an example of a microstructure with a closed eutectic network
wherein the
eutectic carbides 200 form a closed eutectic network with clearly separated
eutectic cells 212.
Figure 18 shows an example representing the microstructure of a roll surface
according to the
invention after Electro Discharge Texturing.
Figure 19 shows the roll microstructure of a depth of 4mm on the roll surface
after tempering
and induction hardening of the roll.

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Detailed description
Introduction
The invention relates generally to a forged roll 1 which preferably has a
weight of more than
400 kg, or, as in embodiments for common applications for example a weight of
more than
woo kg. The roll according to the invention is produced according to a forged
roll production
method which in its general steps is per se known but is specifically adapted
in accordance
with the inventive concept to be able to produce a roll according to the
invention.
The invention is mainly directed to rolls with a weight between 400 kg and 10
000 kg. The
roll according to the invention has a diameter 2 of typically more than 200 mm
and, for
example between 215-800 mm, and a length of the barrel 8 typically between 1-3
meters and
a maximum length of typically about 6 meters including the necks 10. The roll
1 has a
working layer 4 which corresponds to a part of the outer layer and is
typically ranging
between 20 mm and 120 mm in diameter, dependent on the application of the
specific roll
and/or dependent on the total roll diameter 2. Commonly, the outer 1/6 part 6
of the
diameter 2 of the roll is referred to as the working layer 4 of the roll 1,
see figure i.The outer
1/6 part 6 of the diameter 2 of the ingot 34 is also referred to as the
working layer 4 of the
ingot 34 in the text.
There are special problems and challenges involved in making large forged
rolls due to the
internal stresses involved when forming these large pieces of rolls. A roll
with a smaller
diameter would not need the same treatment because then the internal stresses
are lower and
those rolls are not as prone to for example exploding during hardening.
The roll production process 12 according to the invention is crucial for
manufacturing a roll 1
of this size according to the invention. The improved mechanical properties
such as low
residual internal stresses of the roll of the invention result from the roll
production process
12. To achieve the low level of residual internal stresses of the resulting
roll, the internal
stresses induced by thermal gradient and allotropic transformations have to be
minimized in
all stages of the production processes through casting, forging, heat
treatments and
machining. The microstructure of the roll 1 according to the invention
comprises tempered
martensite with a retained austenite rate lower than 5% in volume due to the
production
process of the roll and due to the chemical composition according to the
invention.
The roll production process according to the invention comprises a selection
of the following
basic steps schematically shown in the flow diagram of Fig 2:

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14. Providing a steel composition
16. Manufacturing an ingot 34
18. Forging said ingot 34 to a roll 1
20. Preliminary heat treatment of said roll 1
22. Rough machining said roll 1
24. Induction hardening said roll 1
26. Tempering heat treatment of said roll 1
28. Machining said roll 1
Intermediate products are obtained after the respective steps. Specific
control parameters as
well as a chemical composition of the roll are selected to produce a roll
according to the
invention.
Roll production process
The present invention relates to a forged roll (1) produced by a process
comprising the steps
of:
a. Providing a steel composition comprising, in terms of % per weight,
o.8 to less than (<) % C,
0.2 to 0.5 % Mn,
0.2 to 2.0 % Si,
7.0 to 13.0% Cr,
o.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,
the remaining portion of the steel being substantially Fe and possible
incidental
and/or possibly unavoidable impurities;
b. Manufacturing an ingot maintaining a solidification rate higher than 15
C/min
in the working layer of the ingot in the solidification interval;
c. Forging the ingot to a roll;
d. Hardening the roll by induction heating;
e. Tempering the roll;
thereby achieving a microstructure of the roll (1) that comprises:
- tempered martensite with a retained austenite rate less than (<) 5 % per
volume;
and

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- an open eutectic carbide network with eutectic carbides of less than (<) 5 %
per
volume;
and wherein the roll (1) exhibits:
- a hardness of more than 780 HV; and
- internal compressive stresses of less than -500 MPa in absolute values.
Wherein the provided chemical composition according to the invention used in
combination with the described process steps according to the invention gives
the
roll according to the invention the desired properties in the microstructure
of the
roll according to the invention.
A process of making a forged roll according to the invention comprises the
following steps:
Step 14: Providing of a steel composition.
In one embodiment of the invention the steel composition comprises an alloy
comprising or
consisting of the following constituents indicated in weight % as listed in
Table 1. In Table 1,
the impact of the constituents and the effect of the inventive roll that is
achieved by the
selected constituents and the specific intervals are explained.
Table 1
Chemical Alloy according to Impact (effect) of interval according to
the
composit embodiments of the invention
ion present invention - %
Elements weight.
0.8 ¨ 0.99 Carbon is the most important and
influential
alloying element in steel. In addition to carbon
however, any unalloyed steel will contain silicon,
manganese, phosphorus and sulphur, which occur
unintentionally during manufacture. The addition
of further alloying elements to achieve special
effects and intentional increase in the manganese
and silicon contents results in alloy steel. With
increasing C content, the strength and harden
ability of the steel increase, but its ductility,
forgeability, weldability and machinability (using
cutting machine tools) are reduced. In the.
invention the level of C is lower than 1% to avoid

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the formation of too large closed network of
eutectic carbides.
Mn 0.2 - 0.5 Manganese deoxidizes. It compounds with
sulphur to form Mn sulphide, thus reducing the
undesirable effect of the iron sulphide. This is of
particular importance in free-cutting steel; it
reduces the risk of red shortness. Mn very
pronouncedly reduces the critical cooling rate,
thus increasing hardenability. Yield point and
strength are increased by addition of Mn and, in
addition, Mn favourably affects forgeability and
weldability and pronouncedly increases hardness
penetration depth. In the invention Mn is kept
lower to o.5% to avoid excessive brittleness.
Si 0.2 -2.0 Silicon is contained in all steel in the same way
as
manganese, as iron ores incorporate a quantity of
it according to their composition. In steel
production itself, silicon is absorbed into the melt
from the refractory furnace linings. But only those
steels are called silicon steels which have Si
content of> 0.40%. Si is not a metal, but a
metalloid as are also, for example, phosphorus
and sulphur. Si deoxidizes. On account of
significant reduction of electrical conductivity,
coercive field intensity and low wattage loss, Si is
used in steels for electrical quality sheet.
Accordingly, in the invention, too high level of Si
influences the Eddy Current response during the
roll inspection leading to possible untrue reading
and must be kept under 1.5%
<0.015 Sulphur produces the most pronounced
segregation of all steel accompanying elements.
Iron sulphide, leads to red shortness or hot
shortness, as the low melting point sulphide
eutectics surround the grains in reticular fashion,
so that only slight cohesion of the latter occurs

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and during hot forming the grain boundaries tend
to break down. This is further increased by the
action of oxygen. As sulphur possesses a
considerable affinity for manganese, it is
combined in the form of Mn sulphide, as this is
the least dangerous of all existing inclusions,
being present distributed in point form in the
steel. Toughness in transverse direction is
reduced significantly by S. To be kept at the
lowest level.
<0.015 Phosphorus is usually regarded as a steel
parasite,
as P produces pronounced primary segregation on
solidification of the melt and the possibility of
secondary segregation in solid state due to the
pronounced restriction of the gamma phase. As a
result of the relatively low rate of diffusion, both
in the alpha- and in the gamma crystal,
segregation which has occurred can only be
corrected with difficulty. In accordance with the
invention, P is to be kept at the lowest level,
preferably <0.015 W%.
Cr 7.0 ¨ 13.0 Chromium renders steels oil and air-hardenable.
By reduction of the critical rate of cooling
necessary for martensite formation, it increases
hardenability, thus improving its susceptibility to
hardening and tempering. Notch toughness is
reduced however, but ductility suffers only very
slightly. The tensile strength of the steel increases
by 8o-loo N/mm2 per 1 % Cr. Cr is carbide
former. Its carbides increase the cutting ability
and wear resistance. High temperature strength
property is promoted by chromium. The element
restricts the gamma phase and thus extends the
ferrite range.
With a Cr content higher than 13%, extended
eutectic carbides tend to be formed.

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With a Cr content lower than 7%, the level of
hardness remains too low for cold rolling
application due to a deficit in secondary
hardening mechanisms.
Mo 0.6 ¨ 1.6 Molybdenum is usually alloyed together with
other elements. Reducing the critical cooling rate
improves hardenability. Mo significantly reduces
temper brittleness and promotes fine grain
formation. Increase in yield point and strength.
Pronounced carbide former; cutting properties
with high speed steel are improved thereby. Very
severe restriction of the gamma phase. Increased
high temperature strength. With increased Mo
content, forgeability is reduced. Accordingly, its
content is maintained under 1,6% to avoid the
detrimental formation of ferrite delta.
Ni < 1.0 Nickel in steel produces significant increase in
notch toughness, even in the low temperature
range, and is therefore alloyed for increasing
toughness in case-hardening, heat-treatable and
subzero toughness steels. Ni is not carbide former.
V >1 ¨ 3 Vanadium refines the primary grain and thus the
casting structure. Pronounced carbide former,
thus providing level of hardness compatible with
cold rolling process, increase in wear resistance,
high cutting ability and high temperature
strength. It is used therefore primarily as
additional alloying element in high speed, hot
forming and creep resistant steels. Significant
improvement in retention of temper, reduction of
overheating sensitivity. V restricts the gamma
phase and shifts the Curie point at elevated
temperatures.
With a V content lower than 1%, the level of

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hardness remains to low regarding cold rolling
process.
With a V content higher than 3%, the steel
grindability becomes prohibitive for cold rolling
process.
0.0 ¨ 2.0 Tungsten is a very pronounced carbide former (its
carbides are very hard) and restricts the gamma
phase. It improves toughness and prevents grain
growth. W increases high temperature strength
and retention of temper as well as wear resistance
at high temperatures (red heat) and thus cutting
ability. It is therefore alloyed primarily to high
speed and hot forming tool steels, as well as
creep-resistant steel types and to ultrahard steels.
Ti 0.0 ¨ 1.0 Titaniumn account of its very strong affinity for
oxygen, nitrogen, sulphur and carbon, Ti has a
pronounced deoxidizing, pronounced denitriding
and pronounced carbide forming action. Used
widely as carbide former. Also possesses grain
refining properties. Ti restricts the gamma phase
very pronouncedly. In high concentration, it leads
to precipitation processes and is added to
permanent magnet alloys on account of achieving
high coercive field intensity. Ti increases creep
rupture strength through formation of special
nitrides. Finally, Ti tends pronouncedly to
segregation and banding.
Nb 0.0 ¨ 0.5 Niobium (Nb) and Tantalum (Ta) occur almost
Ta 0.0 ¨ 0.5 exclusively together and are very difficult to
separate from one another, so that they are
usually used together. Very pronounced carbide
formers, thus alloyed particularly as stabilizers of
chemical resistant steels. Both elements are ferrite
formers and thus reduce the gamma phase. On
account of the increase in high temperature

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strength and creep rupture strength due to Nb.
Zr 0.0 ¨ 0.5 Zirconium is a carbide former;
metallurgical use
as alloying element for deoxidation, denitriding
and desulphurization, as it leaves minimal
deoxidation products behind. Additions of Zr to
fully deoxidized sulphur-bearing free-cutting
steels have a favourable effect on sulphide
formation and thus prevention of red shortness. It
increases the life of heating conductor materials
and produces restriction of the gamma phase.
and further optionally comprising, H2, N2, 02, Al, Cu, each in amounts lower
than 0.4
weight %; and wherein the remaining portion of the steel composition is
substantially Fe,
apart from incidental elements and possibly unavoidable impurities.
In an embodiment of the invention the steel composition comprises, in terms of
% per
weight,
o.8 to less than (<) % C,
0.2 to 0.5 % Mn,
0.2 to 2.0 % Si,
7.0 tO 13.0 % CT,
o.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,
wherein the remaining portion of the steel is substantially Fe, apart from
incidental elements
and possibly unavoidable impurities.
In different variants and embodiments of the invention the composition
comprises or
consists of a combination or a selection of the constituents (weight %)
according to the
following examples. In some instances, the before mentioned embodiment is
combined with,
substituted by or narrowed by the below variants of constituent amounts.
A roll with a steel composition consisting, in terms of % per weight;
o.8 to less than (<) % C,
0.2 tO 0.5 % Mri,
0.2 to 2.0 % Si,
7.0 to 13.0 % Cr,

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o.6 to 1.6 % Mo,
more than (>) 1.0 to 3.0 % V,
less than (<) 0.015 % P, and
less than (<) 0.015 % S, and
5 less than (<) i %Ni
less than (<) 30 ppm 02, and
less than (<) 100 ppm N2, and
less than (<) 3 ppm H2
less than (<) 2 % W, and
10 less than (<) i % Nb, and
less than (<) i % Ti, and
less than (<) 0.5 % Ta, and
less than (<) o.5% Zr,
the remaining portion of the steel being substantially Fe and possible
incidental and/or
15 possibly unavoidable impurities;
The roll according to the invention, wherein the C content in the steel
composition is between
o.8 ¨ 0.99 % C in terms of % per weight of total roll weight.
The roll according to the invention, wherein the C content in the steel
composition is between
20 0.85 ¨ 0.9 % C in terms of % per weight of total roll weight.
The roll according to the invention, wherein the Mn content in the steel
composition is
between 0.4 ¨ 0.5 % Mn in terms of % per weight of total roll weight.
The roll according to the invention, wherein the Si content in the steel
composition is
between 0.2 - 1.5 % Si in terms of % per weight of total roll weight.
The roll according to the invention, wherein the Si content in the steel
composition is
between o.85 ¨ 1.15 % Si in terms of % per weight of total roll weight.
The roll according to the invention, wherein the Cr content in the steel
composition is
between 7.0 ¨ ii % Cr in terms of % per weight of total roll weight.
The roll according to the invention, wherein the Cr content in the steel
composition is
between 7.3 ¨ less than (<) 8.o % Cr in terms of % per weight of total roll
weight.

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The roll according to the invention, wherein the Mo content in the steel
composition is
between 1.45 ¨ 1.55 % Mo in terms of % per weight of total roll weight.
The roll according to the invention, wherein the Ni content in the steel
composition is less
than (<) 0.3 Ni in terms of % per weight of total roll weight.
The roll according to the invention, wherein the V content in the steel
composition is between
1.3 ¨ 2.1 % V in terms of % per weight of total roll weight.
The roll according to the invention, wherein the V content in the steel
composition is between
1.3 ¨ 1.6 % V in terms of % per weight of total roll weight.
A roll according to the invention, wherein the steel composition consists, in
terms of % per
weight:
o.8 ¨ 0.99 % C, and
0.4 - 0.5 % Mn, and
0.2 - 1.5 % Si, and
7.0 ¨ ii % Cr, and
o.6 ¨ 1.6 % Mo, and
less than (<) to Ni, and
1.0 ¨ 2,1 % V, and
less than (<) 0.015 % P, and
less than (<) 0.015 % S, and
less than (<) 30 ppm 02, and
less than (<) loo ppm N2, and
less than (<) 3 ppm H2, and
the remaining portion of the roll being substantially Fe and possible
incidental and/or
possibly unavoidable impurities.
A roll according to the invention, wherein the steel composition consists, in
terms of % per
weight:
0.85 ¨ 0.9 % C, and
o.4 - 0.5 % Mn, and
0.85 - 1.15 % Si, and
7.3 ¨ less than (<) 8.o % Cr, and
1.45 ¨ 1.55 % Mo, and
less than (<) 0.3 Ni, and
1.3 ¨1.6 % V and

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less than (<) 0.015 % P, and
less than (<) 0.015 % S, and
less than (<) 30 ppm 02, and
less than (<) 100 ppm N2, and
less than (<) 3 ppm H2, and
the remaining portion-of the roll being substantially Fe and possible
incidental and/or
possibly unavoidable impurities.
Step 16: Manufacturing 16 of a cylindrical shaped ingot 34
In a typical application of the invention, an intermediate product, the ingot
34 produced
according to the method of the invention preferably has a diameter 32 of
between 450 and
1100 mm, length 30 up to 6 meters and weight between 400 to 30000kg, see
figure 3. The
method of making an ingot 34 according to the invention involves using a
technique which
enables fast cooling during the ingot 34 manufacturing. For example the ingot
34 can be
produced using different ingot forming techniques. Suitable manufacturing
techniques are
those which are capable of being controlled to achieve and maintain a specific
minimum
solidification rate.
According to embodiments of the invention the average solidification rate is
controlled to be
higher than 15 C/min in the surface and preferably also higher than io C/min
in the core
during the formation of the ingot. Preferably, this solidification rate is
maintained while
controlling cooling the ingot material in the solidification interval which
may for example be
between 1400 C to 1200 C. In other embodiments of the invention the average
solidification
rate is controlled to be higher than 35 C /min in the working layer in the
solidification
interval.
From a practical point of view it is generally difficult to achieve very high
solidification rates
when implementing the invention. Further embodiments of the invention comprise
the
average solidification rate in the working layer as well as in the core is
controlled to be in the
range of 15 C/min to 55 C /min, or alternatively 35 C /min - 55 C/min, or
alternatively 45
C /min - 55 C/min.
Techniques which are used in the invention to control the process with regard
to
solidification parameters in accordance with the invention are for example
different types of
electro-slag refining furnace (ESR), for example moving mold ESR melting or
ESR cladding
or spray forming techniques etc.

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An ingot made using a solidification rate and a chemical composition as
described in any of
the above embodiments according to the invention has the following
characteristics:
-Very fine dendrite macrostructure.
-Uniformity of chemistry,
-Lack of macro segregations and dark veining in the intermediate layers.
-No minor segregations.
Further, an ingot made using a process according to the invention has the
following
advantages on the rolled product:
-Elimination of "orange peel" effect (it consists of the appearance of the
dendrite patterns due
to the difference of wear of the interdendritic area).
- No pinhole problems.
- Very bright surface finish.
-Homogeneity of the texture obtained by texturing.
-Absence of marks related to the heterogeneity of the structure.
In one embodiment of the invention an electro-slag refining furnace (ESR) is
used for
manufacturing of the ingot 34 according to the invention, for a schematic view
see figure 4.
The electro-slag refining furnace (ESR) is capable of melting about 300-
1100kg/h, and
comprises an electrode clamp 36, a stinger 38, an electrode 40, a cooling
jacket outlet 42, a
cooling jacket inlet 50 for cooling water. In the ESR, the ingot is formed by
melting the
electrode 40 and thus different layers are formed in the ingot material such
as a slag pool
44, which is located near the electrode, and a molten metal pool 46.
The ESR also comprises a starting plate 52 which is water cooled 54, see
figure 4. The ESR
technique may require a starting ingot (electrode 40) obtained by a
conventional melting
process to be re-melted to form an ingot according to the invention. The re-
melting using
the ESR is carefully controlled in order to achieve the average solidification
rate according to
embodiments of the invention, for example an average solidification rate
higher than
15 C/min in the working layer and also in the core of the ingot during
formation of the ingot.
The electrode 40 is in the ESR process thus heated by an electric current, for
example a high
ampere current to re-melt the steel of the electrode to form an ingot
according to the
invention. The high ampere current of the electrode 40 is carefully controlled
to control the
speed of re-melting and this also affects the speed of cooling and thereby the
solidification
rate. The solidification rate depends on the ampere current fed to the
electrode according to a
predetermined function. Basically, the higher the ampere current, the higher
is the power

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supplied to re-melt the electrode 40 (see Ohm law). The higher the supplied
power, higher is
the slag temperature and the lower is the solidification rate.
By maintaining the correct re-melting rate and slag temperature, directional
solidification
can be achieved with a solidification rate according to the invention in the
core and in the
working layer while cooling the ingot in certain intervals. For example, in
one embodiment a
solidification rate which in average is higher than 15 C/min both in the core
and in the
working layer of the ingot while cooling the ingot in the solidification
interval from 1400 C to
1200 C.
According to the invention and as a consequence of the combination of the
steel composition
and the process of the inventive concept, the eutectic carbide content in the
ingot is held
below 5 volume %. This renders a good grindability of the resulting roll. The
grindability of
the roll is important since during usage of the final roll, grinding is an
important procedure to
achieve the adequate roughness of the roll regarding cold rolling process. It
is known that a
concentration of eutectic carbides higher than 5% gives unsatisfactory
grindability of such a
roll.
Moreover, another effect of the low eutectic carbide content is a low tendency
of the roll to
form dust during operation in the mill. In contrast, dust forming can be
generated in rolls
having high concentrations of carbides, which is negative for the rolled
products as well as
the working environment in the mill.
It is especially important to control the solidification rate when making
ingot from
compositions which comprises high levels of Cr (for example 7-13%). High
segregation which
is obtained if the solidification rate is too slow defects high chromium
ingots.
A solidification rate higher than 15 C/min during the solidification interval
when making the
ingot gives a low segregation rate resulting in an eutectic carbide content
lower than 5% in
volume.
The present invention will be understood more readily by reference to the
following
examples. However, these examples are intended to illustrate embodiment
variants of the
ingot forming step of the invention and are not to be construed to limit the
scope of the
invention.
Comparative examples

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Example 1 demonstrates the effect the method of the invention has on the
microstructure of
the roll 1 according to the invention. Example 2 is a comparative example. The
examples are
performed during production of roll prototypes in natural scale. The
experiments show the
important variation of the distribution of eutectic carbides and network shape
in the ingot
5 after casting depending on the used solidification rate, see examples 1
and 2 below and table
2. The distribution of eutectic carbides and network shape which is seen in
the ingot is
remained in the final roll after forging and tempering according to the
invention.
Example 1
10 This example shows the effect on the microstructure in the roll
according to the invention
when using a solidification rate higher than 15 C/min during formation of the
ingot 34
according to the invention.
Figure 5A-B show an example of a microstructure of INGOT 1 according to the
invention
which is made using a process with a solidification rate in average 50 C/min
(on gomm
15 depth of the ingot) while cooling the ingot from 1400 C to 1200 C. The
eutectic cells in the
example INGOT 1 according to the invention are small (940, 942), figure 5B
shows the
fragmented network with to an open eutectic network. See also figure 8 for the
different
solidification intervals in the different parts of the ingot during the
solidification showing the
temperature rate in the core 82, the mid-radius 84, gomm 86, 5omm 88, 3omm 90
and
20 surface 92. Figure 5B is a magnification of figure A. See also table 2.
Figure 6A-B show an example of a microstructure of INGOT 2 according to the
invention
which is made using a process with a solidification rate in average 18 C/min
(on gomm
depth of the ingot) while cooling the ingot from 1400 C to 1200 C. Figure 6
shows the
eutectic cells in the example INGOT 2 according to the invention, and these
are small, see for
25 examplecross sectional distance 1024. See also figure g for the
different solidification
intervals in the different parts of the ingot during the solidification 8o,
showing the
temperature rate in the core loo, the mid-radius 102, gomm 104, 5omm 106, 30mm
108 an
surface no. Figure 6B is a magnification of figure 6A. See also table 2.
Conclusion
The method according to the invention ensures absence of segregation in mid-
radius of the
ingot. Absence of segregation in mid-radius (or 5/6 of inner part of the
diameter of the
cylindrical roll) guarantees the integrity of the roll during the hardening
process. A
solidification rate higher than 15 C/min in the working layers thus generates
a finer
microstructure which, as explained above, is better in terms of grinding and
dust pollution, se
figures 5A-B and figure 6A-B.

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Example 2
This example shows the effect of using a solidification rate lower than 15
C/min during
formation of the TEST 1 ingot.
Figure 7A-C show an example of a microstructure of the TESTi ingot which is
made using a
process with a solidification rate of lower than 15 (in fact even lower than
10) C/min while
cooling the ingot in the solidification interval from 1400 C to 1200 C. The
cells 700 of the
comparative TEST 1 ingot in figure 7A-C are larger in size, see for example
cross section 708
which has a cross sectional length 708 is larger than the largest cross
section in for example
of the INGOT 1 in example 1 according to the invention. TEST 1 ingot also
shows shrinkage
porosities 704. The coarse conglomerate eutectic network 702 can also be seen
in figure 7A-
C. See also table 2. Figure 7B-C is a magnification of figure 7A.
Conclusion:
A solidification rate lower than 15 C/min within the solidification interval
gives a high
segregation of the carbides and a coarse carbide network 702 the mid-radius of
the TEST 1
ingot structure and also porosities 704, see figure 7A-C. A high segregation
of the carbides
and a coarse carbide network makes a white blank roll or a finished roll made
by an ingot
according to TEST 1 brittle and thus prone to explode during induction
hardening (a white
blank roll) or in the cold rolling mill (finished roll).
Example 2 also shows that a solidification rate lower than 15 C/min also makes
the size of
the eutectic cell structure larger and coarser compared to when an ingot is
made using
solidification rates higher than 15 C/min as according to the invention.
A solidification rate higher than 15 C/min during the solidification interval
when making the
ingot gives a low segregation rate resulting in an eutectic carbide content
lower than 5% in
volume.
Table 2
=Average C Mn Si Cr Mo Ni
V Effect on Effect on
solidification segregation micro-
rate* /eutectic
structure
carbide
formation
INGOT 1 50 C/rain o. 0.5 1.0 7.2 1.4 <1 1.8 Low See
figure
8 segregation 5A-B
rate+ control
on eutectic
carbides

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INGOT 2 18 C/min 0. 0.5 1.0 7.2 1.4 <1 1.8
Low See figure
8 segregation 6A-B
rate+ control
on eutectic
carbides
TEST 1 <15 C/min o. 0.5 1.0 7.2 1.4 <1 1.8
High See figure
8 segregation 7A-C
rate ¨No
control on
eutectic
carbides
Table 2 shows experimental data for test of ingots with different average
solidification rate
(*) while cooling the ingot from 1400 C to 1200 C on gomm depth of the ingot.
Comparative examples
Example 3 demonstrate for example the effect the method of the invention and
the chemical
composition of the ingot has on the microstructure of the ingot and thus also
on the roll of
the invention. Example 4 is a comparative example. Example 3 and 4 show
microstructure of
ingots produced by experimentation in the laboratory with controlled
solidification device
and controlled cooling speeds.
The shape of the eutectic carbide network in the ingot is affected depending
of the used
chemical composition, see also table 3..
Example a
This example shows an INGOT 1 microstructure produced according to the method
of the
invention by experimentation in the laboratory with controlled solidification
device and
controlled cooling speeds higher than 15 C/min in the solidification interval.
When a
chemical composition comprising Mo in 1,4 % is used according to the
invention, an open
eutectic carbide system 750 is achieved in the ingot structure, see figure loA-
B. See also table
3. This open eutectic carbide system 750 as is seen in the roll 1 according to
the invention is
characterized as a dendrite pattern and the eutectic carbide structures 752 is
not forming
closed eutectic carbides network(as in comparative example 4, TEST2) but
instead forms
dendrite arms in a network, see figure 10A-B which shows a picture of the
microstructure of
an ingot with 1.4% Mo is produced according to the process of the invention.
This open

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eutectic carbide system according to the invention makes the roll easier to
grind compared to
rolls made using higher amounts than 1.6% of Mo.
Example 4
A TEST2 ingot is made using a process of the invention and a composition where
the main
constituents are according to the above embodiments but with the difference
that the
chemical composition differs from the invention regarding the amount of Mo.
This TEST 2
ingot is produced according to the method of the invention by experimentation
in the
laboratory with controlled solidification device and controlled cooling speeds
higher than
15 C/min in the solidification interval. In TEST 2 the amount of Mo is 2.77%,
see also table
3. Using a chemical composition comprising Mo of 2.77% in the process of the
invention
producing an ingot makes the eutectic carbide system of the ingot shaped in a
cell of closed
eutectic carbides, see figure 11 A-B, and the eutectic carbides 852 forms
substantially isolated
portions 850, like islands or segregated cell structures in figure 11A-B
showing the
microstructure of TEST 2. The white areas in figure iiA-B represent a matrix;
mainly iron,
the black is secondary carbides.
The excessive addition of alloying elements in TEST2 leads to the formation of
a coarse
carbides network linked to segregation of carbides. See also table 3.
Table 3
Average C M Si Cr Mo Ni V Effect on
solidificati n micro-
on rate * structure
TEST2 18 C/min o.8 o.6 1.11 7.19 2.77 <1 0.44 Figure
nA-B.
Shows a
closed eutectic
carbide
network
INGOT 1 18 C/min o.8 0.5 1.0 7.2 1.4 <1 1.8 Figureio
A-B.
Shows an
open eutectic
carbide
network.

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29
Table 3 shows experimental data for test of ingots with different average
solidification rate (*)
while cooling the ingot from 14.00 C to 1200 C. The constituents other than Mo
are within
the intervals as described above.
Step 18: Forging said ingot '14 to a roll 1
In a typical application of the invention, the ingot 34 made according to the
previous step of
the invention is then forged. In one embodiment of the invention the ingot
34i5 hot press
forged using a a per se known process for simultaneously reducing the cross-
sectional area
and changing the shape by passing them between a hammer and an anvil forming
the ingot to
a roll 1 according to the invention. The ingot is heated in a dedicated
furnace, see figure 12 for
a schematic view of the forging step.
The forging step 18 according to the invention includes the following steps,
see figure 12;
-Pre-heating 56 of the ingot 34 for about 6h to a temperature of between 800-
1200 C or
between 850-1100 C. The pre-heating step 56 involves heating the ingot 34
from the surface
all the way into the core of the ingot. The temperature during forging is
adjusted within the
interval 8400-1200 C or between 850-1100 C since a higher temperature than
1200 C leads
to defects of the ingot structure due to burning of the roll. The reasons for
keeping the
temperature of the ingot at the indicated temperature interval is that a
temperature below
800 C leads to crack forming of the ingot. As the ingot 34 cools it becomes
stronger and less
ductile which may induce cracking if deformation continues.
After preheating (step 56) of the ingot 34, it is forged (step 6o) using a
forge ratio of 1.35-2Ø
The forging step 60 and the preheating step 56 are repeated, this forging
cycle commonly
being called a heat 58. A heat 58 is repeated as many times as needed to form
a roll according
to the invention, see figure 12.
In one embodiment the roll 1 according to the invention is forged using 3-6
heats 58 to forge
the ingot into a roll blank. A roll blank is a roll which has the shape of a
roll but still with a
barrel that lacks the final treatments to become a roll usable in the mill.
In another embodiment the ingot 34 is forged in several heats 58, see figure
13 for a
schematic view of forging a roll:
a) first, the ingot 34 adjusted in cross-sectional area in a few or 1-2 heats
58,
b) one neck of the roll is made in one heat,
c) the other neck of the roll is forged in the next heat.
Forging a steel composition according to the invention is more difficult to do
because of the

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high alloy content according to the invention than compared to forging example
standard
steel grades.
During forging, the diameter 32 of the ingot 34 is reduced by 30-50% while
forged into a roll
1 according to the invention. For example a roll 1 according to the invention
has preferably a
5 diameter 2 between 250-800 mm, see figure 1 and an ingot 34 according to
the invention has
preferably a diameter 32 between 40o-l000mm or between 450-1100 mm.
It is important that the ingot 34 has the desired eutectic carbide
microstructure formed
during the manufacturing process of the ingot 34 during the solidification
step 80. It is
10 shown that ingots 34 with the eutectic carbide microstructure according
to the invention with
amounts of eutectic carbides lower than 5 volume % are possible to forge using
hot pressure
forging techniques. Using an ingot with formed with another process, for
example with a
solidification rate lower than 15 C/min makes these large rolls to lead to
explosion during
induction hardening or in the mill.
Step 20: Preliminary heat treatment of said roll 1
In the manufacturing process of the invention the roll is treated with a
preliminary heat
treatment step. In one embodiment of the invention, the roll is heated to
between 700 C -
1100 C during the preliminary heat treatment 20 according to the invention in
a furnace and
then the roll is kept at that temperature for a certain time until
satisfactory hydrogen
diffusion has occurred. The preliminary heat treatment (normalizing and
spheroidal
annealing) is performed in order to improve machinability of the roll.
Step 22: Rough machining 22 of said roll
In the manufacturing process of the invention the roll is treated by a rough
machining step
22. Rough machining 22 of the formed roll 1 according to the invention means
removing the
outer layer of the forged roll. In one embodiment of the invention of the
outer layer is
removed during rough machining. The roll is called a black blank before it is
treated to rough
machining. By removing the oxidation layer on the surface of the roll the
black blank roll is
then transformed to a white blank.
Step 24: Induction hardening of said roll 1
In the manufacturing process of the invention the roll is treated by induction
hardening.
During induction hardening of the roll the hard surface of the roll is formed.
See figure 14 for
a schematic view of the induction hardening step.
In one embodiment of the invention the roll is slowly moved downwards while an
electric
current or voltage frequency between 50-1000Hz is applied on through the
inductor

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arrangement 70 during the induction hardening step. The roll 1 is cooled using
water cooling
72 after the heating step, see figure 14. The formed hard surface is also
called the working
layer 4 of the roll and is about 1/6 (see figure 1, number 6) of the total
diameter 2 of the roll 1.
The roll barrel surface is heated quickly when lowered through a series of
inductors
comprising electrical coils leading into a quench box. The fast heat
penetration of induction
heating and immediate quenching using water produces a defined layer of
uniform hardness
of the surface of the roll. Both the necks and core of the roll remain at low
temperature
throughout the process. During induction hardening a frequencies typically
between 50-
1000Hz are applied on the surface of the roll 1 and a frequency selected from
the lower parts
of that interval gives deeper working layer 4 of the roll 1. Other factors
that affect the depth of
the formed working layer are the gap between inductors 70 (if several
inductors are used).
Also the gap or distance between the inductor 70 and the roll 1 affects the
depth of the
formed working layer 4. The induction hardening step 24 according to the
invention could be
of single, double or more frequency/ies.
The roll according to the invention explodes using conventional hardening
techniques and
induction heating is the most suitable technique for hardening of the roll
according to the
invention. Cooling of the roll 1 during the induction hardening 24 is
performed by high flow
of cold water.
In one embodiment of the invention the induction hardening 34 is made by
double induction
hardening and the cooling of the roll 1 after the induction hardening 24 is
made by high flow
of water which has a temperature of 40 C and is transported at a flow of about
300 m3/h and
the roll is moved downwards at a speed of o.3mm to 1 mm/s.
In one embodiment the induction hardening step 24 takes between 0.5-2h.
Step 26: Tempering of said roll
In the manufacturing process of the invention the roll 1 is tempered. The
purpose of the
tempering step is to reduce the brittleness of the roll and to adjust the
level of hardness. The
tempering step 26 is a crucial step during the formation of the roll because
it decreases the
internal stresses. During the tempering step the roll achieves its final micro
structure by
diffusion and secondary precipitation of carbides. Air cooling is applied
between the
tempering heating steps. The rolls are tempered preferably 3 times at 450-530
C. The
tempering step makes the roll obtain the required hardness level higher than
780 HV or
between 780-840 HV. Precise control of time and temperature during the
tempering process
are critical to achieve a metal with well balanced microstructure for example
tempered
martensite so that the roll made according to the process of the invention
after tempering
comprises tempered martensite with a retained austenite rate lower than 5% in
volume.

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Step 28: Machining of said roll
In the manufacturing process of the invention the roll is preferably treated
by a machining
step 28 before used in the mill. For example at the mill an application
specific surface
treatment of the roll is performed by grinding and other surface treatments to
get the desired
roughness and the related friction on the surface of the roll. Examples of
surface treatments
of the roll are for example: Laser beam texturing (LBT), Electro beam
texturing (EBT) or
electro discharge texturing (EDT).
In one embodiment the roll is treated by grinding and electro discharge
texturing (EDT)
surface treatment. Figure 15 A-B show microstructure of the surface of a roll
comprising a
low chromium composition after Electro Discharge Texturing. Figure 15 C-D show
microstructure of the surface of the roll according to the invention after
Electro Discharge
Texturing. Underneath the white layer in
figure 15 D there are the re-austenitized layer
and a thinner softened zone, since this grade has a high tempering
temperature. It is also
noted that within the white layer in figure 15 D, the eutectic carbides 302
have not been
affected by the electric arc energy. For comparison, these sorts of carbides
are not present in
the roll described in figure 15 A-B. The roll according to the invention has
better properties
and performance than a standard grade roll (see figure 15 A-B) due to the
presence of the
hard eutectic carbides in the white layer.
Figure 18 shows a more schematic figure of figure 15D, representing the
microstructure of a
roll surface according to the invention wherein the newly formed eutectic
carbides 302,
formed due to the re-melting, are present within the white layer 304. Also
previously formed
eutectic carbides 300 are shown in figure 18. The roll surface in figure 18
illustrates how the
surface looks like after Electro Discharge Texturing according to the
invention. The scale 306
represents 5gm.
A roll i according to the invention made by the process described above
A typical roll according to the invention has a diameter of between 215 and
800 nun or
between 250-700mm, total length including the necks is up to 6 meters, wherein
the barrel
length is between 1-3 meters. The typical weight of the roll is between 400 to
l0000kg. The
microstructure of a roll according to an embodiment of the invention is
characterized in
comprising tempered martensite with a retained austenite rate lower than 5% in
volume, and
wherein the roll comprises an open eutectic carbide network of less than 5
volume % eutectic
carbides; and the roll (1) exhibits a hardness between 780 to 840 HV; and
internal
compressive stresses of between -300 to -500 MPa. These properties of the roll
are due to

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the roll production process of the invention and also due to the chemical
composition of the
roll according to the chemical composition of the invention.
The roll according to the invention is intended to be used in a cold strip
mill which requires
rolls that withstands high pressures. The roll according to the invention is
intended to be
used in the cold strip mill as a work roll and is suitable in any stand in the
rolling process and
is suitable in 2Hi to 6Hi mills and may have roughness on the surface from 0.3-
o.5pm which
is required in the finishing stands to a roughness of 1.5-2. 5um which is
required in the initial
stands.
The present invention will be understood more readily by reference to the
following
examples. However, these examples are intended to illustrate the roll
properties of the
invention and are not to be construed to limit the scope of the invention.
In table 4 different rolls are compared to the roll according to the
invention. All the rolls
comprise Mn in amounts between 0.2-0,5 in weight %.
Two examples of the invention
ROLLi according to the invention in table 4 is made using the process
according to the
invention, using a solidification rate of higher than 15 C/min in the working
layer during the
solidification interval and also using the induction heating using a frequency
of 50-250 HZ
and tempering 3 times at 450-530 C.
ROLL2 according to the invention in table 4 is made using the process
according to the
invention, using a solidification rate of 18 C/min in the working layer
during the
solidification interval and also using the induction heating using a frequency
of 50-250 HZ
and tempering 3 times; first at 490 C, then at 490 C and in the last
tempering at 480 C.
Figure 19 shows the microstructure of a roll after tempering and induction
hardening,
sampled on 4mm depth from the surface of ROLL 2. The microstructure 1034 with
the open
eutectic network and the eutectic carbides 1032 of the roll is also shown in
figure 19.
Table 4
Roll C Cr Mo V Hardness
Secondary- Remarks
(IW)
hardening
average
level peak
TEST4 0.6 5 1.1 0.25 700 No
To soft for work roll in cold

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rolling application
TEST5 0.8 10 1.1 0.25 730 Slightly To
soft for late stand-
Convenient in early stand
TEST6 0.7 5 2 0.5 750 Slightly Impossible to
produce.
Rejection due to formation of
ferrite delta at high
temperature during forging
heat. See figure 16 A-D shows
detrimental defects 502 on a
roll generated during
manufacturing of rolls with
low chromium content. The
detrimental defects 502 are
for example porosities and
shrinkage.
TEST7 0.9 8 2 2 820 Sharply Adapted for cold
rolling
(required for aluminium
rolling)
ROLLi 0.9 8 1.5 1.45 8o o Sharply
Adapted for cold rolling and
easier to grind compared to
TEST 7, for example.
ROLL2 0.87 7.8 1.5 1.5 8o o Sharply
Adapted for cold rolling and
easier to grind compared to
TEST 7, for example.
The Mn content for the rolls in table 4 are all within the range 0.4-0.5, Si
content for the rolls
in table 4 are all within the range 0.2-2,0, Ni is always below 1%.
Applications of the roll
Applications wherein the rolls are suitable are:
Aluminium industry:
single stands 4Hi no reversing mill
Steel industry:
4 Hi Single stand reversing
- 4 Hi Tandem 4 and 5 stands for sheet in continuous and discontinuous
process

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4 Hi Tandem 4 and 5 stands for tinplate
6 Hi Tandem mill for sheet
Roll usage
5 The forged roll according to the invention is suitable to be used for
example as a work roll or
intermediate roll in cold rolling mills or in for example;
-cold rolling reduction mills for early and finishing stands, reversible and
non-reversible
stands for tinplate, sheet, silicon steel, aluminum or copper.
- Cold rolling temper and/or skin pass mills;
10 - Mill configurations as 2-High, 4-High and 6-High stands with textured
or non-textured
surface.
- Cold rolling of AHSS steel grades.
15 Roll surface
Surface texture
One problem with known rolls is that the surface texture gets worn during
usage of the roll.
The surface texture is important because it ensures the friction coefficient
to avoid slippage
20 and/or derailment of the strip. Moreover it determines the strip surface
texture which gives
the superficial properties crucial for deep drawing and painting of rolled
strip. The rolls
according to the invention exhibit an increased ability to keep their surface
texture due to a
white layer of the roll and wherein the white layer comprises hard eutectic
carbides as M7C3.
In the working layer; the microstructure of the roll of the invention after
final heat treatment
25 consists of tempered martensite with a retained austenite rate lower
than 5% in volume and
carbides as MC and MX (M= metal, C= carbon) finely and homogeneously
distributed into
the matrix. This type of microstructure has shown to be important for keeping
the surface
texture of the roll.
Roughness transfer
30 The roughness transfer of the roll surface changes during usage of the
roll. The rolls
according to the invention exhibit an increased ability to keep roughness
transfer constant
during rolling which is important for the life time of the roll. This is due
to the special
claimed composition and also due to the production method used when making the
rolls.
Schedule-free rolling in the mill
35 A problem during usage of rolls is that dirt which build up on the roll
surface leaves a track
line on the strip. In the working layer, the roll according to the inventions
has a strong
surface due to that the microstructure of the roll of the invention comprises
tempered

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martensite with a retained austenite rate lower than 5% in volume and carbides
as MC and
M2C finely and homogeneously distributed into the matrix, where M indicates
metal and C
indicates carbon. This special microstructure increases the possibilities for
a schedule-free
rolling.
Spalling
Another problem with known rolls is that propagation of cracks inside the
rolls is governed
by the accumulative stresses, induced by the rolling operation and the field
of residual
internal stresses of the roll. A roll in service is submitted to a complex set
of stresses. The roll
according to the invention displays a low level of residual internal stresses
and thus a better
resistance to spalling and this makes the mill incident rate low.
The mechanical strength of the roll of the invention is better compared to a
roll with the same
alloy composition as the roll of the invention but made using another
production method.
The mechanical strength of the roll according to the invention is due to the
formed open
eutectic network in the working layer of the roll. This open eutectic network
is formed during
the cooling step in the roll-making process. A solidification rate higher than
15 C/min during
the cooling step when making the ingot is crucial for the formation of the
open network
which is present in the rolls according to the invention.
Also, the accumulation of various tempering treatments at high temperature
after hardening,
for example between 450-530 C during the production of the roll, induces an
important
relaxation of internal stresses of the roll. The internal stresses are
minimized by using
differential heating of the external layer. The hardness penetration depth of
the roll according
to the invention can be controlled between 20 and 120 mm on diameter measured
from the
roll surface and inwards. The internal compressive stresses of the roll of the
invention are
preferably between -300 to -500 MPa in absolute value or for example lower
than -400 MPa.
Roll microstructure
Figure 17A shows a schematic view of an exemplified roll microstructure
according to the
invention. In figure 17A is seen dendrite arms 210, comprising of eutectic
carbides forming
the eutectic cell structures 204 by forming an open carbide network. The open
eutectic
network comprising of dendrite arms 210 forming eutectic cells 204, which can
be seen in
figure 17A, is formed in the process due to the specific chemical composition
according to the
invention. The scale 208 represents loopm.

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In one embodiment of the invention, the microstructure of the roll of the
invention comprises
an open eutectic network which is only spread over one grain or two grains of
the cell
structures.
In comparison, figure 17B shows a closed eutectic network wherein the eutectic
carbides 200
form a closed eutectic network with clearly separated eutectic cells 212. This
type of network
is unwanted in the roll according to the invention due to brittleness of the
roll if it comprises
this type of microstructure. The scale 214 represents ioo
The invention has been explained by means of different embodiments within the
scope of the
accompanying claims.