Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HOT ROLLED AND STEEL SHEET AND A METHOD OF MANUFACTURING
THEREOF
The present invention relates to hot rolled steel sheets suitable for use as
steel sheet
for green goods such as parts or ancillary for agriculture machinery, mining
machinery
and engineering machinery.
Agricultural Machinery, mining machinery and engineering machinery, such as
plough wheels, dozer, shovel loader, excavator, wagon trernie and various
mining
machinery, grab bucket, stacker-reclaimer, crusher jaw, tractor shoe are
mandated to
have good wear resistance and steel used to manufacture these equipment depend
primarily on the hardness of the steel to achieve good wear resistance and
higher
hardness can provide good wear resistance. However, increase the hardness, is
detrimental for other properties, such as ductility and fatigue. In order to
obtain steels
having both very good wear-resistance and good suitability for use, therefore,
means
other than increasing the hardness have been sought.
Additionally, such aoriculture and rnining equipment loose efficacy rapidly
owing to
wear and tear, and waste of material that causes material as well as financial
loss.
Today modern industry demands high speed development and the demand running
speed of mechanical means is increasingly high, and market increases the
increasing
demand of wear resisting steel, therefore: Development of high wear resisting
steel is
zo mandated to reduce the loss that causes wearing and tearing.
Therefore, intense Research and development endeavors are put in to increase
the
hardness of the steel while keeping other properties same to improve the wear
resistance of the steel.
Earlier research and developments in the field of high strength and high
hardness steel
sheets have resulted in several methods for producing steel sheets, some of
which are
enumerated herein for conclusive appreciation of the present invention:
EP2695960 is an abrasion resistant steel plate or steel sheet suitable for use
in
construction machines, industrial machines, and the like and a method for
manufacturing the same. In particular, a steel plate or steel sheet has a
composition
containing 0.20% to 0.30% C, 0.05% to 1.0% Si, 0.40% to 1.20% Mn, P, S, 0.1%
or
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less Al, 0.01% or less N, and 0.0003% to 0.0030% B on a mass basis, the
composition
further containing one or more of Cr, Mo, and W, the composition further
containing
one or more of Nb, Ti, Cu, Ni, V, an REM, Ca, and Mg as required, the
remainder being
Fe and inevitable impurities. A semi-finished product having the above steel
composition is heated, hot rolling is performed, air cooling is performed,
reheating is
performed, and accelerated cooling is then performed or accelerated cooling is
performed immediately after hot rolling. However the steel of EP2695960 is not
able
to achieve the hardness of 550Hv or more.
The purpose of the present invention is to solve these problems by making
available
hot rolled steel sheets that simultaneously have:
- a hardness of greater than or equal to 580Hv and preferably above 600 Hv,
- a wear loss of steel of at most 0.085mm3/s in accordance to wear ASTM-G55
standard and preferably at most 0.080mm3/s..
In a preferred embodiment, the steel sheets according to the invention may
also
present a tensile strength 1800 MPa or more
Preferably, such steel can also have a good suitability for forming, in
particular for
rolling with good weldability and bendability.
Another object of the present invention is also to make available a method for
the
manufacturing of these sheets that is compatible with conventional industrial
zo applications while being robust towards manufacturing parameters shifts.
Other characteristics and advantages of the invention will become apparent
from the following detailed description of the invention.
Carbon is present in the steel of present invention is from 0.38% to 0.5%.
Carbon is
an element necessary for increasing hardness of the Steel of present invention
by
producing a low-temperature transformation phases such as Tempered Martensite,
carbon also impart the steel with strength by precipitate strengthening by
forming Iron
carbides, Vanadium Carbide or Niobium Carbides. But Carbon content less than
0.36% will not be able to impart the strength as well as hardness to the steel
of present
invention. On the other hand, at a Carbon content exceeding 0.5%, the steel
exhibits
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poor fatigue properties which limits its application for the agricultural
machinery parts.
A preferable content for the present invention may be kept from 0.39% to 0.48%
and
more preferably from 0.39% to 0.45%.
Manganese content of the steel of present invention is from 1 % to 2%. This
element
is gammagenous and also influence Bs and Ms temperatures therefore plays an
important role in controlling the Martensite formation. The purpose of adding
Manganese is essentially to impart hardenability to the steel. An amount of at
least 1%
by weight of Manganese has been found in order to provide the strength and
io hardenability to the steel sheet. But when Manganese content is more
than 2% it
produces adverse effects such as it retards transformation of Austenite during
the
cooling after hot rolling. In addition, the Manganese content of above 1.8% it
promotes
the central segregation hence reduces the formability and also deteriorates
the
weldability of the present steel. A preferable content for the present
invention may be
kept from 1.3% to 1.8%,
Silicon content of the steel of present invention is from 0.1% to 0.7%.
Silicon is solid
solution strengthener. In addition, a higher content of Silicon can retard the
precipitation of Cementite. However, disproportionate content of Silicon leads
to a
problem such as surface defects like tiger strips which adversely effects the
steel of
zo present invention. Therefore, the concentration is controlled within an
upper limit of
0.7%. A preferable content for the present invention may be kept from 0.2% to
0.6%
and more preferably from 0.2% to 0.5%.
Aluminum is an element that is present in the steel of the present invention
from 0.01%
10 0.1%. Aluminum is an alphagenous element and imparts ductility to steel of
present
invention. Aluminum in the steel has a tendency to bond with nitrogen to form
aluminum
nitride hence from point of view of the present invention the Aluminum content
must
be kept as low as possible and preferably from 0.02% to 0.06%.
Chromium of steel of present invention is from 0.3% to 1%. Chromium is an
essential
element that provide strength to the steel by solid solution strengthening and
a
minimum of 0.3% is required to impart the strength but when used above 1%
impairs
surface finish of steel. The preferred limit for the presence of Chromium is
from 0.3%
to 0.9 % and more preferably from 0.3% to 0.8%.
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Boron is an essential element for the steel of present invention and may be
present
from 0.002% to 0.05%. Boron forms boro-nitirides and impart additional
strength to
steel of present invention when added in an amount of at least 0.002%.
Phosphorus constituent of the steel of present invention is from 0.002% to
0.02%.
Phosphorus reduces the spot weldability and the hot ductility, particularly
due to its
tendency to segregate at the grain boundaries or co-segregate with manganese.
For
these reasons, its content is limited to 0.02% and preferably lower than
0.015%.
Sulfur is not an essential element but may be contained as an impurity in
steel and
from point of view of the present invention the Sulfur content is preferably
as low as
possible, but is 0.005% or less from the viewpoint of manufacturing cost.
Further if
higher Sulfur is present in steel it combines to form Sulfides especially with
Manganese
and reduces its beneficial impact on the steel of present invention, therefore
preferred
below 0.003%
Nitrogen is limited to 0.01% in order to avoid ageing of material, nitrogen
forms the
nitrides which impart strength to the steel of present invention by
precipitation
zo strengthening with Vanadium and Niobium but whenever the presence of
nitrogen is
more than 0.01% it can form high amount of Aluminum Nitrides which are
detrimental
for the present invention hence the preferable upper limit for nitrogen is
0.005%.
Molybdenum is an optional element that constitutes 0% to 0.5% of the Steel of
present
invention; Molybdenum increases the hardenabty of the steel of present
invention
and influences the transformation of austenite during cooling after hot
rolling. However,
the addition of Molybdenum excessively increases the cost of the addition of
alloy
elements, so that for economic reasons its content is limited to 0.5%.
Preferable limit
for molybdenum is from 0.01% to 0.3%.
Vanadium is an optional element that constitutes from 0 % to 0.5% of the steel
of
present invention. Vanadium is effective in enhancing the strength of steel by
forming
carbides, nitrides or carbo-nitrides and the upper limit is 0,5% due to the
economic
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reasons. These carbides, nitrides or carbo-nitrides are formed during the
cooling after
hot rolling. Preferable limit for Vanadium is from 0% to 0.3%.
Titanium is an optional element to the Steel of present invention from 0.001 %
to 0.1%.
It forms Titanium-nitrides appearing during solidification of the cast
product. The
amount of Titanium is so limited to 0.1% to avoid the formation of coarse
Titanium-
nitrides detrimental for formability. In case the Titanium content below
0.001% does
not impart any effect on the steel of present invention.
Niobium is an optional element for the present invention. Niobium content may
be
present in the steel of present invention from 0% to 0.05% and is added in the
steel of
present invention for forming carbides or carbo-nitrides to impart strength to
the steel
of present invention by precipitation strengthening.
Nickel may be added as an optional element in an amount of 0% to 1 A to
increase the
strength of the steel present invention and to improve its toughness. A
minimum of
0.01% is preferred to get such effects. However, when its content is above 1%,
Nickel
causes ductility deterioration.
Copper may be added as an optional element in an amount of 0% to 1 A to
increase
zo the strength of the of Steel of present invention and to improve its
corrosion resistance.
A minimum of 0.01% is preferred to get such effects. However, when its content
is
above 1 A, it can degrade the surface aspects.
Calcium can be added to the steel of present invention in an among from 0.001%
to
0.01%%. Calcium is added to steel of present invention as an optional element
especially during the inclusion treatment. Calcium contributes towards the
refining of
the Steel by binding the detrimental Sulfur content in globular form thereby
retarding
the harmful effect of Sulfur.
Other elements such as Mg, Sn , Pb or Sb can be added individually or in
combination
in the following proportions: Mg 0.0010%, Sn 0.1%, Pb 0.1% and Sb
Up to the maximum content levels indicated, these elements make it possible to
refine
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the grain during solidification. The remainder of the composition of the steel
consists
of iron and inevitable impurities resulting from processing.
The remainder of the composition of the Steel consists of iron and inevitable
impurities
resulting from processing.
The microstructure of the Steel sheet comprises:
Martensite constitutes at least 94% of the microstructure by area fraction and
preferably 95 to 99% in area fraction. The martensite of the present invention
can
comprise both fresh and tempered martensite. However, fresh martensite is an
optional microconstituent which is preferably limited in the steel at an
amount of from
0% to 4%, preferably from 0 to 2% and even better equal to 0%. Fresh
martensite may
form during cooling after tempering. Tempered martensite is formed from the
martensite which forms during the cooling after annealing and particularly
after below
Ms temperature and more particularly from Ms-I 0 C to 20 C.Such martensite is
then
tempered during the holding at a tempering temperature Temper from 100 C to
300 C.
The martensite of the present invention imparts ductility and strength to such
steel.
Preferably, the content of martensite is from 95% to 99% and more preferably
from
96% to 99%.
zo Residual Austenite is an optional constitutes for the steel of present
invention and may
be present from 0% to 5% by area fraction. When Residual Austenite is in
excess of 5
% it lowers the hardness of the steel of present invention below an acceptable
level.
In a preferred embodiment, residual austenite is from 0% to 4% and more
preferably
from 0% to 3%.
Carbides of alloying elements might be present in the steel of present
invention in a
cumulated amount from 0% to 5% by area fraction such as of Chromium, Niobium,
Vanadium and Iron. These carbides may increase the strength of the steel of
present
invention by precipitation strengthening, but whenever the presence of
carbides is 5%
or more, their precipitation consume partly the amount of Carbon required for
the
strengthening of tempered martensite.
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In addition to the above-mentioned microstructure, the microstructure of the
hot rolled
steel sheet is free from microstructural components, such as Pearlite, ferrite
and
Bainite but may be found in traces.
A steel sheet according to the invention can be produced by any suitable
method. A
preferred method consists in providing a semi-finished casting of steel with a
chemical
composition according to the invention. The casting can be done either into
ingots or
continuously in form of thin slabs or thin strips, i.e. with a thickness
ranging from
approximately 220mm for slabs up to several tens of millimeters for thin
strip.
For example, a slab having the above-described chemical composition is
manufactured by continuous casting wherein the slab optionally underwent the
direct
soft reduction during the continuous casting process to avoid central
segregation and
to ensure a ratio of local Carbon to nominal Carbon kept below 1.10. The slab
provided
by continuous casting process can be used directly at a high temperature after
the
continuous casting or may be first cooled to room temperature and then
reheated for
hot rolling.
The temperature of the slab, which is subjected to hot rolling, is preferably
at least
1100 C and must be below 1300 C. In case the temperature of the slab is lower
than
1100 C, excessive load is imposed on a rolling mill. Therefore, the
temperature of the
slab is preferably sufficiently high so that hot rolling can be completed in
the in 100%
zo austenitic range. Reheating at temperatures above 1275 C must be avoided
because
it causes productivity loss and is also industrially expensive. Therefore, the
preferred
reheating temperature is from 1200 C to 1275 C.
Hot rolling finishing temperature for the present invention is from 850 C to
975 C and
preferably from 860 C to 930 C.
The hot rolled strip obtained in this manner is then cooled wherein the
cooling starts
immediately after the finishing of hot rolling and in the cooling step hot
rolled strip is
cooled from finishing of hot rolling to a coiling temperature range fr0m550 C
to 750 C,
preferably at a cooling rate from1 C/s to 150 C/s. In a preferred embodiment,
the
cooling rate for the of cooling step is from1 C/s to 120 C/s.
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Thereafter the hot rolled strip from is coiled in the temperature range of 550
C to 750 C
and preferably fr0m570 C to 720 C and more preferably from 580 C to 700 C.
Then
cooling the coiled hot rolled strip to room temperature.
The coiled hot rolled strip may be optionally cut into steel pieces to
subjected to at
least one mechanical manufacturing operation. Mechanical operation may
comprise
tapering, cutting, forming, turning, honing or any other suitable mechanical
operation
or manufacturing procedure that is required to form the part or ancillary for
agriculture
machinery, mining machinery and engineering machinery.The preferred
temperature
for all the mechanical operations is from 20 C to Ac3 +300 C and more
preferable
temperature for all the mechanical operations is from 20 C and Ac3 +250 C.
After
the completion of the mechanical operations the part is cooled to room
temperature
to obtain an non-heat treated part or ancillary for agriculture machinery,
mining
machinery and engineering machinery. The obtained non-heat treated part or
ancillary
according to the present invention must be heat treated in the identical
manner as the
hot rolled strip to obtain final microstructure described herein below.
Thereafter, the hot rolled strip is being heat treated which will impart the
steel of present
invention with requisite mechanical properties and microstructure.
The hot rolled strip is then being heated, to an annealing temperature Tsoak
which is
from Ac3 to Ac3 + 100 C, preferably from Ac3 +10 C to Ac3 + 100 C, at a
heating rate
HR1 which is from 1 C/s to 100 C/s. In a preferred embodiment, the heating
rate HR1
is from 1 C/s to 50 C/s. Ac3 for the steel sheet is calculated by using the
following
formula:
Ac3 = 910 ¨ 203[C]"(1/2) ¨ 15.2[Ni] + 44.7[Si] + 104[V] + 31.5[Mo] + 13.1[W]
¨ 30[Mn] ¨ 11[Cr] ¨ 20[Cu] + 700[P] + 400 [Al] + 120[As] + 400[Ti]
wherein the elements contents are expressed in weight percentage of the cold
rolled
steel sheet.
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The hot rolled strip is held at Tsoak during 10 seconds to 1000 seconds to
ensure a
complete recrystallization and full transformation to austenite of the
strongly work
hardened initial structure.
Then the hot rolled strip is cooled from Tsoak at a cooling rate CR1 from 1
C/s and
150 C/s, to a temperature Ti which is in a range from Ms-75 C and 20 C. In a
preferred embodiment, the cooling rate CR1 for such step of cooling is from 15
C/s
and 120 C/s. The preferred Ti temperature for such step is from Ms-100 C and
20 C.
Ms for the steel sheet is calculated by using the following formula:
Ms = 545 ¨ 601.2 * (1 ¨ EXP(-0.868[C])) ¨ 34.4[Mn] ¨ 13.7[Si] ¨ 9.2[Cr] ¨
17.3[Ni]
- 15.4[MO] + 10.8[V] + 4.7 [CO] - 1.4[Al] - 16.3 [Cu] ¨ 361[Nb]
¨ 2.44[Ti] ¨ 3448[B]
Thereafter the hot rolled strip is reheated to a tempering temperature Ttemper
from
100 C to 300 C, preferably with a heating rate of at least 1 C/s and
preferably of at
least 2 C/s and more of at least 5 C/s during 10s and 10 hours. The preferred
temperature range for tempering is from 150 C to 250 C and the preferred
duration for
holding at Ttem per is from 200 s to 9hours.
Then, the hot rolled strip is cooled down to room temperature to obtain a the
hot rolled
steel sheet.
EXAMPLES
zo The following tests, examples, figurative exemplification and tables
which are
presented herein are non-restricting in nature and must be considered for
purposes of
illustration only, and will display the advantageous features of the present
invention.
Steel sheets made of steels with different compositions are gathered in Table
1, where
the steel sheets are produced according to process parameters as stipulated in
Table
2, respectively. Thereafter Table 3 gathers the microstructures of the steel
sheets
obtained during the trials and table 4 gathers the result of evaluations of
obtained
properties.
9
Table 1
0
Steels C Mn Si Al Cr
Cu Ni N Nb Mo V B Ti
11 0.4 1.36 0.25 0.03 0.319 0.007 0.0012 0.014 0.001 0.01 0.045
0.042 0.023 0.002 0.0347
1= according to the invention; R = reference; underlined values: not according
to the invention.
Table 2
Table 2 gathers the process parameters implemented on steels of Table 1.
Heating
Ms
HR Cooling Cooling Col.1i T HR1 .ng Reheating rate
TsoakingAnnealin
CR1 T1 rate to Ttemper Tempering Ac3 (
C) p
Steel Finish stop T
T ( C) T cc) ( C/s) cc) ( C) ( C/s) cc\ g
time ( C/s) ( C) tempering(' ( C) time (hr) ( C)
(s)
C/s)
u,
11 1170 880 9 600 600 15 840 900
20 25 9 170 3 776 418 u'
I = according to the invention; R = reference; underlined values: not
according to the invention. u,
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Table 3
Table 3 exemplifies the results of the tests conducted in accordance with the
standards
on different microscopes such as Scanning Electron Microscope for determining
the
microstructures of the steel.
The results are stipulated herein:
Martensite Carbides (%)
Trials (%) RA ( % )
11 97 3 <1
Table 4
Table 4 exemplifies the mechanical properties of the steel. In order to
determine the
tensile strength the tensile tests are conducted in accordance of JIS Z2241
standards
and the wear test is conducted in accordance of Wear ASTM-G55 standards.
The results of the various mechanical tests conducted in accordance to the
standards
are gathered
Table 4
Wear ASTM- Tensile
Trials Hardness (Hv) G55 (mm3/s) Strength(MPa)
11 615 0.075 1810
20
11
