Note: Descriptions are shown in the official language in which they were submitted.
CA 02325453 2000-09-22
WO 99/50469 PCT/SE99/00346
COLD WORK STEEL
TECHNICAL FIELD
The invention relates to a new cold work steel, i.e. a steel intended for the
manufacture
of tools for cold working. Typical applications are blanking knives, punching
tools,
deep drawing moulds etc.
BACKGROUND OF THE INVENTION
The most important feature of a cold work steel is that it shall have a high
hardness. For
most applications also a good abrasion resistance and a toughness sufficient
for the
application are required. In order to satisfy these primary and a number of
other
requirements, a very great number of steel alloys have been developed. Most of
these
alloys, especially when toughness is more important than wear resistance, have
a
composition within the following alloying ranges: 0.8-1.2 C, 0.2-1.2 Si, 0.2-
0.5 Mn, 5-
12 Cr, 0.5-4 Mo, 0-3 W and 0.2-2 V. Further, small or moderately high contents
of Ni,
Nb, Cu and/or Al may be present. A steel bf the latter type, which contains
moderate
though significant contents of niobium and aluminium, is described in US-A-
5,160,553.
DISCLOSURE OF THE INVENTION
It is the purpose of the invention to provide a cold work steel with a
chemical
composition which is balanced such that the steel shall satisfy the following
requirements:
- it shall be easy to manufacture in a non-powder metallurgical way and have a
good
hot workability in order to get a high yield in production;
- it shall be able to be manufactured in dimensions ranging from the very
smallest
dimensions, i.e. 0 10 mm or less, up to 0 500 mm or corresponding sizes in
square
or flat sections;
- it shall not contain any large amount of coarse primary carbides;
- it shall have good heat treatment features, which among other things means
that it
shall be able to be hardened from a moderately high austenitising temperature;
- it shall have a good hardenability, i.e. a capacity to be through-hardened
also in case
of large dimensions;
- it shall have good dimensional stability on heat treatment as well as in
use, the latter
condition inter alia implying that it shall have small susceptibility to
ageing;
- it shall be able to secondary-harden in connection with tempering for the
achievement of a hardness of 60-64 HRC;
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WO 99/50469 2 PCT/SE99/00346
- it shall have good surface deposition features, which means that it shall be
able to be
nitrided, case hardened and surface coated through PVD and CVD;
- it shall have good sparking machinability;
- it shall have an adequate abrasive wear resistance;
- it shall have an adequate toughness;
- it shall have a high compressive strength; and
- it shall have good fatigue features, good cuttability and good grindability.
First a series of cold work steels known in the art were examined. The
chemical
compositions of these steels are given in Table 1.
CA 02325453 2000-09-22
WO 99/50469 PCT/SE99/00346
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WO 99/50469 4 PCT/SE99/00346
The steels of the table were examined or compared with reference to micro
structure,
including the type and character of inclusions, response to heat treatment,
hardenability,
hardness after austenitising and after tempering, dimensional stability, spark
machinability, toughness in terms of impact strength and bending strength,
abrasive
wear resistance, compressive yield point, grindability, and cuttability.
None of the examined steels had an in all respects desired combination of
features.
Then, during the continued development work, in view of the achieved results,
a revised
series of requirements was designed, wherein in the first place the influence
of hardness
and volume of carbides on toughness and wear resistance were considered. In
this
second phase of the development work it was examined more in detail how small
changes in the contents of C, N, Mn, V, and Mo have influence on a number of
such
critical features as toughness, response to heat treatment, hardenability,
secondary
hardening, resistance to tempering, and wear resistance. In this work, there
were made
seven 50 kg laboratory heats with a chemical composition in weight-% according
to
Table 2.
Table 2 Chemical cam osition in wei ht-%
Steel Q-heat C Si Mn P S Cr Mo V N Bal.
No. No.
12 9020 0.96 0.81 0.50 0.007 0.005 7.18 2.97 0.41 0.016 Fe
13 9021 0.98 0.95 0.47 0.008 0.005 7.05 2.90 0.41 0.016 -Fe
14 9024 0.92 0.93 0.53 0.009 0.005 7.06 2.53 0.40 0.074 Fe
15 9021 0.97 0.91 1.04 0.007 0.005 6.85 2.34 0.41 0.027 Fe
16 9023 1.03 1.06 1.20 0.008 0.005 6.97 1.99 0.66 0.047 Fe
17 9038 0.90 0.84 0.49 0.006 0.005 6.69 2.45 0.44 0.023 Fe
18 9039 0.85 0.86 0.47 0.007 0.004 7.28 2.46 0.43 0.022 Fe
All the heats were forged to the shape of bars, 60 x 60 mm. The material
examinations
showed that a steel that satisfies the raised requirements best in terms of
the different
features mentioned in the foregoing should have the following composition in
weight-
%: 0.82-0.97 C, 0.70-1.10 Si, 0.38-0.62 Mn, at least 7.6 but less than 8.0 Cr,
max 0.40
Ni, 2.30-2.70 Mo, max 0.25 W, 0.35-0.55 V, balance iron, impurities and
accessory
elements in normal amounts. Further, the steel normally contains max 0.15 N,
preferably max 0.03 N, max 0.30 Cu, and max 6 ppm H. The Al content must be
max
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WO 99/50469 5 PCT/SE99/00346
0.1 %, preferably max 0.045 %, but typically it amounts to 0.010-0.045 % as a
residual
element from the desoxidation treatment of the steel. Typically, the steel
should contain
0.92C,0.95Si,0.5Mn,7.8Cr,2.5Mo,0.45V.
As far as the micro structure of the steel is concerned, it consists, after
austenitising at
1000-1080 C, cooling to room temperature and tempering once or several times
at 180-
650 C, of tempered martensite, containing a total carbide volume of 3-6 vol-%,
preferably 3-5 vol-%, of which 0.25-0.45 vol-% consists of MC carbides and the
rest
essentially of M7C3 carbides. Suitably, the amount of primary carbides is
about 4 vol-%.
The steel of the invention can be manufactured in a conventional way through
production of a melt, which is cast to ingots, which can be hot worked to the
shape of
bars, plates, etc., of which there can be made tools or other articles, which
can be heat
treated for the achievement of a final product having the desired combination
of
features. The conventional production of ingots can be complemented by any
subsequent melt-metallurgical process step, such as e.g. Electro Slag Refining
(ESR) or,
as an alternative process, the building up of castings of solidifying drops of
the melt,
such as the OspreyTM process (trade-mark of Sandvik Osprey LLC).
Further characteristics and aspects of the steel of the invention will be
apparent from the
appending patent claims and from the following description of performed
experiments.
BRIEF DESCRIPTION OF DRAWING
In the following description of performed experiments, reference will be made
to the
drawing, which in a form of a diagram illustrates the punch wear versus the
number of
strokes in connection with punching ultrahigh strength steel plate.
DESCRIPTION OF PERFORMED EXPERIlVIENTS
The steel of the invention is intended to be used for the manufacture of tools
for cold
working. Cold work tools are used e.g. in the automotive industry for
blanking,
punching, pressing and bending thin steel plates. In this field, new,
ultrahigh strength
steels have been developed in recent years. One of these steels has been
developed by
SSAB Tunnplat AB and is known by its trade name Doco1T"" 1400 DP and contains,
besides iron and unavoidable impurities, in weight-% typically: 0.18 C, 0.50
Si, 1.80
Mn, 0.015 P, 0.002 S, 0.040 A, and 0.030 Nb. This steel is manufactured in
gauges
between 0.50 and 2.00 mm, in its delivery condition it has the mechanical
features
stated in Table 3.
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WO 99/50469 6 PCT/SE99/00346
Table 3- Mechanical Pro erties of Work Material
Steel Yield strength Yield strength after Tensile strength Elongation Min.
grade Rpo.2 or RzL 2 % deformation Rm A80 radius for
N/mm2 and bake hardening N/mm2 % 90 bend
170 C/20 min in transverse
min.-max. Rp2.0+BH direction
N/mm2
min. min.-max. min.
Docol(T"") 4.0 x
1400 DP 1200- 1450 1350 1400-1600 3 thickness
Details in side impact protect systems, bumper reinforcements, seat frames and
beams
and other structural parts in motor cars are typical applications of this
steel. The
performed investigations aimed at evaluating the feasibility of the steel for
tools for the
manufacture of products of the said kind and at comparing the features of the
steel with
other, commercially available steels for cold work tools.
The chemical compositions of the examined steels are listed in Table 4. Steel
No. 19 is a
steel of the invention. The steel was manufactured as a 35 tons production
heat in an
electric arc furnace. Of the steel there were cast ingots, which were forged
and rolled to
the shape of bars. The contents of nickel, niobium, titanium, and copper are
residuals
from used raw materials and are unintentional. Aluminium has been added for
the
desoxidation of the steel, and the stated content of aluminium is a residue
from that
process. Steel No. 20 is a steel according to the above mentioned US-A-
5,160,553,
which has been manufactured by another producer. The steel, which is
commercially
available, has been analysed by the applicant with reference to its chemical
composition. Steels Nos. 21, 22, and 23 are commercial steels, which are
manufactured
by the applicant. The contents of steels Nos. 21-23 stated in Table 4 are
nominal
contents. Steel No. 21 is a conventionally manufactured steel, while steels
Nos. 22 and
23 have been manufactured powder metallurgically. Besides the contents of the
different elements stated in the tables, these steels also contain impurities
in normal
amounts eminating from the raw materials which were used for the manufacture
of
steel.
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WO 99/50469 7 PCT/SE99/00346
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WO 99/50469 8 PCT/SE99/00346
Punches with the punch diameter 10 mm were manufactured of bars of steels Nos.
19-
23. The bar dimensions are listed in Table 5. All punches were taken out from
the centre
of the bars and cross-wise the bar direction, the longitudinal direction of
the punch
coinciding with the height direction of the bar. The working material
consisted of said
DocolTM 1400 DP with a thickness of 1.0 mm. The material was cold rolled and
heat
treated for the achievement of highest strength level and it therefore gave a
good
indication concerning abrasive wear resistance and ductility/toughness. The
punching
operatings were carried out in a 15 tons excenter press. The punching rate was
200
strokes/min; punching play 6 %; no lubrication. The measurement of the wear
was made
by means of a prism, the curve deviation being measured before and after the
punching
series. The difference was transformed to number of mZ, which represents the
wear.
Table 5 shows the testing parameters and the registered punch wear after
200,000
punching strokes. The table also shows the heat treatment of the tools. All
the tools had
been hardened from the stated austenitising temperature (TA as is shown in the
table)
and tempered twice after cooling, each time for two hours at the tempering
temperature
given in the table.
Table 5
Steel Punch wear Bar dimension Hardness Heat treatment
No. m2 (mm) (HRC)
19 13125 254x76.2 60 TA=1030 C/30 min+
550 /2x2h
36105 200x100 59.5 TA=1050 C/30 min+
550 /2x2h
21 18743 250x80 60,5 TA=1020 C/30 min+
550 /2x2h
22 9618 250x80 60 TA=1020 C/30 min+
525 /2x2h
23 7790 250x63 60.5 TA=1020 C/30 min+
525 /2x2h
In the drawing, the wear during the course of the complete punching test is
shown. The
results can be explained in the following way. The powder metallurgically
manufactured steels Nos. 22 and 23 have sufficient ductility to avoid
microchipping of
the punch edge, and the smallest abrasive wear stated for steel No. 23 is due
to the
higher vanadium content in that steel. Steel No. 19 of the invention, which
has a well
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balanced chemical composition of alloy elements, also has a balanced
combination of
features, where abrasive wear dominates over microchipping of the punch edge.
The
wear resistance was better than that of the substantially higher alloyed steel
No. 21 and
was comparable with that of the exclusive, powder metallurgically manufactured
steels
Nos. 22 and 23, which contained high vanadium contents. Especially, steel No:
20 had a
pronounced tendency to microchipping of the punch edge, which explains why
that
material is less good in this test.
