U.S. patent application number 12/598328 was filed with the patent office on 2010-06-03 for cold-work die steel and die.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel,Ltd.). Invention is credited to Shogo Murakami, Shigenobu Namba, Tsuyoshi Tonomura, Kenji Yamamoto.
Application Number | 20100135844 12/598328 |
Document ID | / |
Family ID | 40638544 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100135844 |
Kind Code |
A1 |
Murakami; Shogo ; et
al. |
June 3, 2010 |
COLD-WORK DIE STEEL AND DIE
Abstract
The present invention relates to a cold-work die steel,
comprising by mass %: 0.5 to 0.7% of C; 0.5 to 2.0% of Si; 0.1 to
2.0% of Mn; 5 to 7% of Cr; 0.01 to 1.0% of Al; 0.003 to 0.025% of
N; 0.25 to 1% of Cu; 0.25 to 1% of Ni; 0.5 to 3% of Mo; 2% or less
(including 0%) of W; and 0.1% or less (excluding 0%) of S, with a
remainder being iron and an unavoidable impurity; wherein the
following requirements (1) to (3) are satisfied: (1)
[Cr].times.[C].ltoreq.4; (2) [Al]/[N]: 1 to 30; and (3)
[Mo]+0.5.times.[W]: 0.5 to 3.00%, wherein the bracket means a
content (%) of an element written therein.
Inventors: |
Murakami; Shogo; (Hyogo,
JP) ; Namba; Shigenobu; (Hyogo, JP) ;
Yamamoto; Kenji; (Hyogo, JP) ; Tonomura;
Tsuyoshi; (Toyama, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel,Ltd.)
Kobe-shi
JP
Nippon Koshuha Steel Co., Ltd.
Chiyoda-ku
JP
|
Family ID: |
40638544 |
Appl. No.: |
12/598328 |
Filed: |
September 18, 2008 |
PCT Filed: |
September 18, 2008 |
PCT NO: |
PCT/JP2008/066870 |
371 Date: |
October 30, 2009 |
Current U.S.
Class: |
420/87 ; 420/91;
72/462 |
Current CPC
Class: |
C22C 38/002 20130101;
C22C 38/00 20130101; C22C 38/44 20130101; C21D 6/002 20130101; C22C
38/46 20130101; C21D 9/00 20130101; C22C 38/04 20130101; C22C 38/02
20130101; C22C 38/06 20130101; C22C 38/42 20130101 |
Class at
Publication: |
420/87 ; 420/91;
72/462 |
International
Class: |
C22C 38/42 20060101
C22C038/42; C22C 38/60 20060101 C22C038/60; B21D 37/00 20060101
B21D037/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 13, 2007 |
JP |
2007-294326 |
Claims
1. A cold-work die steel, comprising by mass %: 0.5 to 0.7% of C;
0.5 to 2.0% of Si; 0.1 to 2.0% of Mn; 5 to 7% of Cr; 0.01 to 1.0%
of Al; 0.003 to 0.025% of N; 0.25 to 1% of Cu; 0.25 to 1% of Ni;
0.5 to 3% of Mo and 2% or less (including 0%) of W; and 0.1% or
less (excluding 0%) of S, with a remainder being iron and an
unavoidable impurity; wherein the following requirements (1) to (3)
are satisfied: [Cr].times.[C].ltoreq.4; (1) [Al]/[N]: 1 to 30; and
(2) [Mo]+0.5.times.[W]: 0.5 to 3.00%, (3) wherein the bracket means
a content (%) of an element written therein.
2. The cold-work die steel according to claim 1, further comprising
at least one of the following (a) to (c): (a) V in a content of
0.5% or less (excluding 0%); (b) at least one element selected from
the group consisting of Ti, Zr, Hf, Ta and Nb in a total content of
0.5% or less (excluding 0%); and (c) Co in a content of 10% or less
(excluding 0%).
3-4. (canceled)
5. A die obtained by using the cold-work die steel according to
claim 1.
6. A die obtained by using the cold-work die steel according to
claim 2.
Description
TECHNICAL FIELD
[0001] The present invention relates to a cold-work die steel and a
die, and more specifically to a die steel useful as a material of
dies used in carrying out cold/worm press forming (stamping,
bending, drawing, trimming, etc.) of steel plates for cars, steel
sheets for home electric appliances and so on.
BACKGROUND ART
[0002] With increases in strength of steel plates and sheets, dies
used in carrying out forming of steel plates for cars, steel sheets
for home electric appliances and the like are required to undergo
further improvement in their life. As to the steel plates for car
in particular, in consideration of environmental issues and with
the intention of enhancing fuel economy of cars, demand for
high-tensile steel plates having tensile strengths of about 590 MPa
or more has grown sharply. With the increase in demand for such
high-tensile steel plates, there has arisen a problem of damaging
the surface coating films of dies at an early stage and thereby
causing "galling" (a seizing-up phenomenon occurring under press
forming) to result in extreme loss of die life.
[0003] A die is generally made by giving hard coating treatment to
the surface of a base material of the die (die steel). In
manufacturing the die steel as a base material, processes of heat
treatment or annealing, cut working and quenching-tempering
treatment are generally carried out in order of mention. In the
present specification, there may be cases where the quenching
treatment and the tempering treatment in particular are referred to
as solution treatment and aging treatment, respectively.
[0004] As the die steel (cold-work die steel), not only high-C,
high-Cr alloy tool steel, which is represented by JIS SKD11, but
also high-speed tool steel having further improved abrasion
resistance, which is represented by JIS SKH51, has generally been
used so far. Improvements in hardness of these tool steels are
mainly made by precipitation hardening of Cr carbide or Mo, W and V
carbides. In addition, low-alloy high-speed tool steels (usually
referred to as matrix high speed steels) which are improved in both
toughness and abrasion resistance by reducing the contents of alloy
elements in JIS SKH51, such as C, Mo, W and V, are currently in use
as die steels.
[0005] A variety of methods aiming further improvements in
properties of die steels have been proposed (e.g. Patent Documents
1 and 2).
[0006] Patent Document 1 discloses the cold-work die steel to which
proper amounts of Ni and Al are added and further Cu is added in an
amount appropriate to the amounts of Ni and Al added for the
purposes of reducing the quantities of dimensional changes (changes
in dimension) by quenching-tempering treatment, particularly
changes in dimension by expansion under tempering, and increasing
the hardness. In this document, it is also described that galling
resistance can be improved by making adjustments to contents of C
and Cr and finely dispersing the distribution of carbide in the
texture.
[0007] With the intention of attaining properties (hardness and
toughness) on the same levels as those of conventional matrix high
speed steels even when quenching is performed at temperatures lower
than those adopted for the conventional matrix high speed steels,
Patent Document 2 discloses the alloy tool steel that has a
microstructure in which Cr-based M.sub.23C.sub.6-type carbide is
formed in an amount of 2 to 5 vol % under tempered conditions
(conditions before heat treatment), and besides, that has a
quenched-tempered microstructure including either V-based MC-type
carbide or Mo- and W-based M.sub.6C-type carbide precipitated in a
dispersed state.
[0008] Patent Document 1: JP-A-2006-169624
[0009] Patent Document 2: JP-A-2004-169177
DISCLOSURE OF THE INVENTION
[0010] As described above, a die is generally made by giving hard
coating treatment to the surface of a die steel. Examples of
general hard coating treatment currently in use include TD
treatment by which a VC coating film is formed through thermal
diffusion, CVD treatment by which TiC is mainly formed, and PVD
treatment by which TiN is mainly formed. Herein, the term "TD
treatment" refers to the treatment that C in a steel material is
allowed to react with V by immersing the steel material in a bath
of fused salt including V and the VC coating film having a
thickness of about 5 to about 15 .mu.m is diffused and permeated on
the surface of a base material under high temperature conditions of
about 900 to about 1,030.degree. C. These hard coating treatments
are adopted as appropriate according to the circumstances of die
users and press makers. Therefore, it is required to develop die
steels having satisfactory adaptability to any of the hard coating
treatments (or capable of forming long-life hard coating films). In
addition, as a matter of course, die steels are also required to
ensure excellent basic properties (including hardness and
toughness).
[0011] The invention has been made in view of these circumstances,
and objects thereof are to provide a cold-work die steel which
exhibits excellent basic properties (including hardness and
toughness), and besides, which is adaptable satisfactorily to a
variety of hard coating treatments, and to provide a die.
[0012] Namely, the present invention provides a cold-work die
steel, comprising by mass %:
[0013] 0.5 to 0.7% of C;
[0014] 0.5 to 2.0% of Si;
[0015] 0.1 to 2.0% of Mn;
[0016] 5 to 7% of Cr;
[0017] 0.01 to 1.0% of Al;
[0018] 0.003 to 0.025% of N;
[0019] 0.25 to 1% of Cu;
[0020] 0.25 to 1% of Ni;
[0021] 0.5 to 3% of Mo and 2% or less (including 0%) of W; and
[0022] 0.1% or less (excluding 0%) of S,
[0023] with a remainder being iron and an unavoidable impurity;
and
[0024] wherein the following requirements (1) to (3) are
satisfied:
[Cr].times.[C].ltoreq.4; (1)
[Al]/[N]: 1 to 30; and (2)
[Mo]+0.5.times.[W]: 0.5 to 3.00%, (3)
[0025] wherein the bracket means a content (%) of an element
written therein.
[0026] In addition, the cold-work die steel preferably comprises at
least one of the following (a) to (c):
[0027] (a) V in a content of 0.5% or less (excluding 0%);
[0028] (b) at least one element selected from the group consisting
of Ti, Zr, Hf, Ta and Nb in a total content of 0.5% or less
(excluding 0%); and
[0029] (c) Co in a content of 10% or less (excluding 0%).
[0030] The die of the invention is obtained by using any of the
cold-work die steels specified above.
[0031] Because in the cold-work die steels of the invention, as
specified above, the alloy components and balances between the
specified elements are appropriately adjusted, they can have high
hardness and toughness, and besides, long-life hard coating films
can be formed on the surface thereof even by a variety of hard
coating treatments. Dies obtained by using the cold-work die steels
of the invention are particularly suitable as dies for forming
high-tensile steel plates having tensile strength of about 590 MPa
or more.
BRIEF DESCRIPTION OF DRAWINGS
[0032] FIG. 1(a) is an optical photomicrograph showing a state in
which galling occurs at the die surface having a TiN coating film
formed by PVD treatment in the case of using JIS SKD11 as a die
steel, FIG. 1(b) is an optical photomicrograph of the base material
for a die which is provided with no TiN coating, and FIGS. 1(c) and
1(d) are partially enlarged optical photomicrographs shown in FIG.
1(a).
[0033] FIG. 2 is a schematic diagram showing a shape of the Charpy
impact test piece used in Examples.
BEST MODE FOR CARRYING OUT THE INVENTION
[0034] The present inventors have made extensive studies in order
to provide a cold-work die steel that can satisfactorily exhibit
its basic properties, such as hardness and toughness, and that has
sufficient adaptability to a variety of hard coating treatments. As
a result, it has been found that prevention of exfoliation of a TiN
coating film formed thereon and improvements in hardness and
toughness can be achieved by not only controlling the contents of
various alloy elements to within individually specified ranges but
also adjusting specified elements to have respectively appropriate
balances as shown in the above items (1) to (3). As the result of
such a finding, it has further been found that, even when hard
coating treatments of various types, including TD treatment, CVD
treatment and PVD treatment, are carried out, long-life hard
coating films can be formed on the surface, thereby achieving the
invention.
[0035] Details about the achievement of the invention are described
below.
[0036] First of all, the present inventors have researched causes
of impairment of a TiN coating film formed by PVD treatment given
to a die using conventional JIS SKD11 or matrix high speed steel
and the galling brought about thereby.
[0037] FIG. 1(a) is an optical photomicrograph showing a state in
which galling occurs at the surface of a die made by using JIS
SKD11 as a die steel and forming a TiN coating film thereon by PVD
treatment. In addition, an optical photomicrograph of the base
material for a die which is provided with no TiN coating is shown
in FIG. 1(b). The areas looking whitish in FIG. 1(b) indicate the
presence of Cr carbide. FIGS. 1(c) and 1(d) are partially enlarged
optical photomicrographs shown in FIG. 1(a). As is apparent from
FIGS. 1(c) and 1(d), it is understandable that hard, coarse Cr
carbide (carbide mainly containing Cr and Fe and having a size of
about 1 .mu.m to about 50 .mu.m) precipitates out on the surface of
the areas in which TiN coating film is exfoliated, and cracks are
produced from the carbide spots.
[0038] From these observation results, the present inventors find
that, since the galling occurrence in the TiN coating film begins
at spots where coarse Cr carbide is present, minimization of the
carbide formation allows prevention of exfoliation of the TiN
coating film and improvement in die life.
[0039] For inhibiting the formation of coarse Cr carbide and
thereby increasing lifetime of a TiN coating film formed by PVD
treatment, it is appropriate to reduce both contents of C and Cr in
a steel. However, a too large reduction in content of C makes it
difficult to form a VC coating film or TiC coating film with a
sufficient thickness by TD treatment or CVD treatment on the
surface of a die steel (base material). Therefore, one of the
features of the invention is attainment of the sufficiently thick
VC coating film and TiC coating film without precipitating coarse
Cr carbide by appropriate control of contents of C and Cr in a die
steel and the product of these contents (the foregoing requirement
(1)).
[0040] In the die steel of the invention, formation of the coarse
Cr carbide is inhibited in order to increase the lifetime of a TiN
coating film formed by PVD treatment. However, unless the Cr
carbide is formed, grain growth during quenching cannot be
prevented, and excellent toughness cannot be achieved after
quenching. Therefore, another feature of the invention is formation
of fine AlN by precise control of content of Al, content of N and
balance between them (the requirement (2)), thereby attaining
excellent toughness after quenching. In the present specification,
the expression "excellent toughness" means that the Charpy impact
value determined by the method described in the following Examples
section is 20 J or more. Additionally, the expression "fine AlN"
means AlN having an average grain size of about 5 .mu.m or
less.
[0041] In addition, since the die steel of the invention contains
fine AlN, the die steel of the invention thought to have
improvements in adhesion to nitride coating films (e.g. CrN and
TiN) formed by PVD treatment.
[0042] In the die steel of the invention, as mentioned above, both
contents of C and Cr are reduced to low values compared with those
in JIS SKD11 as a conventional steel for the purpose of inhibiting
formation of coarse Cr carbide. In the invention, therefore,
hardness reduction by reduction of both contents of C and Cr is
supplemented with positive addition of alloy components
(particularly Al, Cu, Ni, Mo and W). More specifically, in the die
steel of the invention, high hardness is achieved particularly
through the use of hardening by precipitation of an Al--Ni
intermetallic compound under the control according to the
requirement (2) and secondary hardening by formation of carbide
from C and Mo or W under the control according to the requirement
(3). Additionally, the expression "high hardness" in the present
specification means that the maximum hardness determined by the
method described in the following Examples section is 650 HV or
more.
[0043] Chemical components in the steel of the invention are
described below in detail on an element basis. Additionally, all
percentages in the present specification are by mass unless
otherwise noted. And all percentages and so on defined by mass are
identical with those defined by weight, respectively.
[0044] C: 0.5 to 0.7%
[0045] C is an element that ensures hardness and abrasion
resistance and contributes to inhibition of HAZ softening. In
addition, when a coating film of carbide, such as VC and TiC formed
by a TD method or by a CVD method, is formed on the surface of a
base material for dies, a low content of C therein causes a problem
that the coating film formed has a small thickness, and so on.
Considering these circumstances, the lower limit of the content of
C is set to 0.5% for the purpose of achieving the above effect
effectively. And the content of C is preferably 0.55% or more.
However, an excessive content of C causes production of coarse Cr
carbide and makes it easy for a TiN coating film formed by PVD
treatment to exfoliate. In addition, an excessive content of C
causes an increase in residual austenite content, as a result, the
desired hardness cannot be attained unless aging treatment is
performed at a high temperature, and besides, a great dimensional
change occurs through expansion after the aging treatment.
Moreover, an excessive content of C affects adversely the
toughness. Therefore, the upper limit of the content of C is set to
0.7%. And the content of C is preferably 0.65% or less.
[0046] Si: 0.5 to 2.0%
[0047] Si is useful as a deoxidizing element at the time of
steelmaking, and it is an element that contributes to a hardness
improvement and ensures machinability. In addition, Si is useful
for inhibiting the softening of martensite in a matrix by tempering
and inhibiting HAZ softening. For the purpose of fulfilling such
functions effectively, the lower limit of the content of Si is set
to 0.5%. However, an excessive content of Si brings about a
reduction in toughness. In addition, increases in segregation and
dimensional change after heat treatment are caused. Therefore, the
upper limit of the content of Si is set to 2.0%. The content of Si
is preferably 1.0% or more, more preferably 1.2% or more, and
preferably 1.85% or less.
[0048] Mn: 0.1 to 2.0%
[0049] Mn is an element useful for ensuring hardenability during
quenching. However, an excessive content thereof brings about an
increase in residual austenite content, as a result, the desired
hardness cannot be attained unless aging treatment is performed at
a high temperature, and besides, the toughness is lowered.
Considering these circumstances, the content range of Mn is defined
as the above. The content of Mn is preferably 0.15% or more, and
preferably 1% or less, more preferably 0.5% or less, further more
preferably 0.35% or less.
[0050] Cr: 5 to 7%
[0051] Cr is an element useful for ensuring the proper hardness.
Specifically, a too low content of Cr brings about insufficient
hardenability during quenching and leads to partial production of
bentonite, as a result, the hardness is lowered, and the abrasion
resistance cannot be attained. Moreover, Cr is an element useful
for ensuring corrosion resistance of dies. Therefore, the lower
limit of the content of Cr is set to 5%. And the content of Cr is
preferably 5.5% or more. However, an excessive content of Cr causes
an increased production of coarse Cr carbide and makes it easier
for a TiN coating film formed by PVD treatment to exfoliate. In
addition, an excessive content thereof causes a reduction in
durability of the hard coating film through shrinkage after heat
treatment. Moreover, an excessive content of Cr affects adversely
the toughness. Therefore, the upper limit of the content of Cr is
set to 7%. And the content of Cr is preferably 6.5% or less.
[0052] Al: 0.01 to 1.0%
[0053] Al is an element useful as a deoxidizer, and besides, it is
an element that contributes to not only hardness improvement by
precipitation hardening of an Al--Ni intermetallic compound, such
as Ni.sub.3Al, but also inhibition of HAZ softening. Moreover, Al
is an important element for attainment of excellent toughness by
formation of AlN precipitates in conjunction with N and prevention
of grain growth during quenching. Considering these circumstances,
the lower limit of content of Al is set to 0.01%. The content of Al
is preferably 0.02% or more, more preferably 0.03% or more.
[0054] In the field of tool steels, for the purpose of improving
the quality of inclusions, the content of Al is generally
minimized. In the invention, however, Al is positively added for
the purpose of increasing the hardness of a die steel, preferably
for the purposes of inhibiting HAZ softening and preventing grain
growth. The positive addition of Al in the invention makes one of
significant differences as compared with the related arts.
[0055] On the other hand, an excessive content of Al brings about a
reduction in toughness, and besides, it causes great segregation
which leads to an increase in dimensional change after heat
treatment. Therefore, the upper limit of the content of Al is set
to 1.0%. And the content of Al is preferably 0.8% or less.
[0056] N: 0.003 to 0.025%
[0057] N is an important element for attainment of excellent
toughness by formation of AlN precipitates in conjunction with Al
and prevention of grain growth during quenching. For the purpose of
attaining excellent toughness, the lower limit of the content of N
is set to 0.003%. However, an excessive content thereof brings
about a reduction in toughness. Therefore, the upper limit of the
content of N is set to 0.025%. And it is preferable that the
content of N is 0.004% or more and 0.020% or less.
[0058] Cu: 0.25 to 1%
[0059] Cu is an element necessary to aim at hardness improvement by
precipitation hardening of .epsilon.-Cu, and contributes also to
inhibition of HAZ softening. However, an excessive content thereof
causes a reduction in toughness, and it tends to produce forging
cracks. Therefore, the upper limit of the content of Cu is set to
1%. And it is preferable that the content of Cu is 0.30% or more
and 0.8% or less.
[0060] Ni: 0.25 to 1%
[0061] Ni is an element necessary to aim at hardness improvement by
precipitation hardening of an Al--Ni intermetallic compound, such
as Ni.sub.3Al, and contributes also to inhibition of HAZ softening.
In addition, the use of Ni in combination with Cu allows control of
hot embrittlement by Cu addition in an excessive amount, and
thereby the forging cracks can also be prevented. However, an
excessive content thereof causes an increase in residual austenite
content, as a result, the proper hardness cannot be attained unless
aging is performed at a high temperature, and besides, expansion
occurs after heat treatment. In addition, an excessive content of
Ni causes a reduction in toughness. Considering these
circumstances, the content of Ni is specified to fall within the
range specified above. And it is preferable that the content of Ni
is 0.30% or more and 0.8% or less.
[0062] Mo: 0.5 to 3%, and W: 2% or Less (Including 0%)
[0063] Mo and W are elements that contribute to precipitation
hardening because each of Mo and W forms M.sub.6C-type carbide, and
besides, a Ni.sub.3Mo intermetallic compound is formed. However,
excessive contents of these elements result in overproduction of
those carbides and so on, which leads to not only a reduction in
toughness but also an increase in dimensional change after heat
treatment. Therefore, the content of Mo and the content of W are
specified so as to fall in the above-specified ranges,
respectively. In the invention, Mo is an essential element, while W
is an optional element. However, they may be used in combination.
The suitable lower limit of the content of W is 0.02%. And it is
preferable that the content of Mo is 0.7% or more and 2.5% or less,
and that the content of W is 0.05% or more and 1.5% or less.
[0064] S: 0.1% or Less (Excluding 0%)
[0065] S is an element useful for ensuring machinability. From the
viewpoint of ensuring machinability, it is recommended that the
content of S be 0.002% or more, preferably 0.004% or more. However,
an excessive content thereof results in occurrence of welding
cracks. Therefore, the upper limit of the content of S is set to
0.1%. The content of S is preferably 0.07% or less, more preferably
0.05% or less, further more preferably 0.025% or less.
[0066] Further, it is necessary for the die steel of the invention
to fulfill the following requirements (1) to (3) (wherein the
bracket means the content (%) of each element written therein).
[0067] (1) [Cr].times.[C].ltoreq.4
[0068] The requirement (1) is set for the purpose of inhibiting the
production of coarse Cr carbide. When the product of [Cr].times.[C]
is more than 4, coarse Cr carbide is formed to result in easy
exfoliation of TiN coating films. In addition, when this product is
more than 4, there occurs not only degradation in durability of
hard coating films but also increase in dimensional change after
heat treatment. Thus, [Cr].times.[C] is preferably 3.8 or less,
more preferably 3.7 or less. From the viewpoints of reducing the
formation of coarse Cr carbide, inhibiting the dimensional change
after heat treatment and the like, the smaller the lower limit of
this product is, the better it is. However, further considering
significant achievement of the effects from the addition of Cr and
C, it is preferably basically 0.8.
[0069] (2) [Al]/[N]: 1 to 30
[0070] The requirement (2) is set for the purpose of forming fine
AlN and ensuring toughness after quenching. When [Al]/[N] is too
small or too large, it becomes difficult to form fine AlN
precipitates, so excellent toughness cannot be achieved. Therefore,
it is important that the balance between them is precisely
controlled. And [Al]/[N] is preferably 2 or more and 20 or
less.
[0071] (3) [Mo]+0.5.times.[W]: 0.5 to 3.00%
[0072] Mo and W, as mentioned above, are elements that contribute
to precipitation hardening, and the requirement (3) is selected as
a parameter for mainly ensuring hardness improvement by
precipitation hardening of these elements. In addition, the control
of this parameter allows satisfactory inhibition of HAZ softening.
In order to achieve these effects effectively, the lower limit of
the requirement (3) is set to 0.5%. However, excessive contents of
Mo and W result in addition of excess carbides, which leads to not
only a reduction in toughness but also an increase in dimensional
change after heat treatment. Therefore, the upper limit of the
requirement (3) is set to 3.00%. The lower limit of the requirement
(3) is preferably 1.0%, more preferably 1.2%, and the upper limit
of the requirement (3) is preferably 2.8%. In the requirement (3),
(0.5), the coefficient of [W], is defined by considering that the
molecular weight of Mo is about 1/2 in comparison with that of
W.
[0073] The basic components in the steel of the invention are as
mentioned above, with the remainder being iron and unavoidable
impurities. The unavoidable impurities are elements unavoidably
mixed e.g. in the process of manufacturing, with examples including
P and O. In general, the content of P is preferably 0.05% or less,
more preferably 0.03% or less, and the content of O is preferably
0.005% or less, more preferably 0.003% or less, further more
preferably 0.002% or less. In addition, for the purpose of
improving other properties, the following optional components may
further be included.
[0074] V: 0.5% or Less (Excluding 0%)
[0075] V contributes to an improvement in hardness by forming
carbide such as VC, and besides, it is an element effective in
inhibiting HAZ softening. In addition, when a diffusion hardening
layer is formed by giving nitriding treatment, such as gas
nitriding, salt bath nitriding or plasma nitriding, to the surface
of a base material, it is an effective element for improvement in
surface hardness and increase in hardening layer depth. For
achieving these effects effectively, it is appropriate that V be
basically added in an amount of 0.05% or more. However, V added in
an excessive amount lessens the amount of C dissolved in solid and
causes a reduction in hardness of the martensite texture as a
matrix, and besides, it reduces the toughness. Therefore, when V is
added, the upper limit of the content thereof is set to 0.5%. The
content of V is preferably 0.4% or less, more preferably 0.3% or
less.
[0076] At Least One Element Selected from the Group Consisting of
Ti, Zr, Hf, Ta and Nb: 0.5% or Less in Total (Excluding 0%)
[0077] All of these elements are nitride-forming elements, and they
contribute to a finely dispersed state of their nitrides and AlN,
accordingly they are elements allowing prevention of grain growth
and contribution to improvement of toughness. For achievement of
such effects effectively, it is basically appropriate that 0.01% or
more of Ti, 0.02% or more of Zr, 0.04% or more of Hf, 0.04% or more
of Ta and 0.02% or more of Nb be contained. However, when the
contents of these elements become too high, the amount of C
dissolved in solid is lessened to result in a hardness reduction of
martensite. Therefore, it is preferable that the total content of
these elements is set to 0.5%. The total content of these elements
is preferably 0.4% or less, more preferably 0.3% or less.
Additionally, these elements may be contained alone or in
combination with two or more thereof.
[0078] Co: 10% or Less (Excluding 0%)
[0079] Co is an element effective in heightening an Ms point and
reducing residual austenite, and thereby enhancing the hardness.
For achievement of such an effect effectively, it is basically
appropriate that the content of Co be 1% or more. However, an
excessive content thereof brings about rises in cost and so on.
Therefore, it is appropriate that the upper limit of content of Co
be set to 10%. The content of Co is preferably 5.5% or less.
[0080] The invention further relates to dies obtained by using the
die steels described above. In the invention, though there is no
particular restriction as to the manufacturing method of the dies,
a manufacturing method which can be adopted is e.g. as follows:
After producing the above steel by melting, the steel is subjected
to hot forging, and then softened by undergoing heat treatment or
annealing (e.g. by being kept at about 700.degree. C. for 7 hours,
and then subjected to furnace cooling to about 400.degree. C. at an
average cooling rate of about 17.degree. C./hour, and further to
standing to cool). Thereafter, the resultant is crude-worked into
intended forms by e.g. a cutting work, and then hardened so as to
acquire an intended hardness by undergoing solution treatment at
temperatures ranging from about 950.degree. C. to about
1,150.degree. C., and subsequently undergoing aging treatment at
temperatures ranging from about 400.degree. C. to about 530.degree.
C.
EXAMPLES
[0081] Now, the invention will be illustrated in more detail by
reference to the following examples, but the invention should not
be construed as being restricted by these examples. In carrying out
the invention, it is possible as a matter of course to make changes
and modifications as appropriate so long as they conform to the
foregoing and following imports, and any of modes undergoing such
changes and modifications are included in the technical scope of
the invention.
[0082] A variety of steel species listed in Table 1 were used and,
from each of these steel species, 150 kg of ingot was produced by
melting in a vacuum induction melting furnace. Then, each ingot was
heated to a temperature in a range of about 900.degree. C. to about
1,150.degree. C., and forged into two plates each having a size of
40 mmT.times.75 mmW.times.about 2,000 mmL. Thereafter, each plate
obtained was slowly cooled at an average cooling rate of about
60.degree. C./hour. After cooling to a temperature of 100.degree.
C. or less, the resultant was re-heated up to a temperature of
about 850.degree. C., and subsequently cooled slowly at an average
cooling rate of about 50.degree. C./hour (heat treatment or
annealing).
[0083] The following tests (1) to (3) were carried out on each of
the materials thus heat-treated or annealed.
[0084] (1) Hardness Test (Determination of Maximum Hardness)
[0085] Test species basically having a size of 20 mmT.times.20
mmW.times.15 mmL were cut from each of the heat-treated or annealed
materials, and used as the test specimens for hardness measurement.
Each test specimen was subjected to the following heat
treatment.
[0086] Solution treatment (quenching treatment): heating at
temperatures ranging from about 1,020.degree. C. to about
1,030.degree. C. for 120 minutes.fwdarw.air cooling.fwdarw.aging
treatment (tempering treatment): keeping for about 3 hours at a
temperature in a range of about 400.degree. C. to about 560.degree.
C..fwdarw.standing to cool
[0087] While changing the tempering temperature within the range of
about 400.degree. C. to about 560.degree. C. range as described
above, hardness measurements were made with a Vickers hardness
tester (manufactured by .LAMBDA.K.LAMBDA.SHI Co., Ltd., .LAMBDA.VK
standard, load of 5 kg), and the maximum hardness (HV) was
determined. In these examples, test specimens showing maximum
hardness of 650 HV or more in the measurements were regarded as
acceptable. The test results are shown in Table 2.
[0088] (2) Toughness Test (Measurement of Charpy Impact Value)
[0089] Each of the heat-treated or annealed materials was subjected
to the following heat treatment.
[0090] Solution treatment (quenching treatment): heating at
temperatures ranging from about 1,020.degree. C. to about
1,030.degree. C. for 120 minutes.fwdarw.air cooling.fwdarw.aging
treatment (tempering treatment): keeping for about 3 hours at
temperatures ranging from about 400.degree. C. to about 560.degree.
C..fwdarw.air cooling or standing to cool
[0091] Then, test species each having a V-notch section of 10-mmR
as shown in FIG. 2 were cut therefrom, and used as test specimens
for toughness measurement (Charpy Impact test specimens). Charpy
impact testing was carried out on these specimens, from which
absorption energy at room temperature (Charpy impact value) was
determined. Three test species were taken for each Charpy impact
testing, and the average thereof was taken as Charpy impact value.
When the Charpy impact value obtained in this testing was 20 J or
more, the test specimen exhibiting such a Charpy impact value was
regarded as excellent in toughness. The results obtained are shown
in Table 2.
[0092] (3) Property Evaluation of Hard Coating Film
[0093] (3-1) Formation of Hard Coating Film
[0094] Test pieces basically having a size of 4 mmt.times..phi.50
mm were cut from each of the heat-treated or annealed materials,
subjected to the same heat treatment as in the toughness test, and
used as test specimens for property evaluation of hard coating
films. On separate surfaces of these test specimens, a VC coating
film, a TiC coating film and a TiN coating film were formed by TD
treatment, CVD treatment and PVD treatment, respectively, under
general conditions.
[0095] (3-2) Thickness Measurement of Hard Coating Film
[0096] Photographs of each of the hard coating films formed in the
foregoing manner (VC, TiC and TiN coating films) were taken under a
magnification of 2,000 by use of a scanning electron microscope
(SEM), and thickness measurements at 5 points randomly chosen from
them were carried out. The average of the values measured at the 5
points was taken as the thickness (.mu.m) of each hard coating
film. In these examples, test specimens allowing formation of both
a VC coating film and a TiC coating film having a thickness of 7.0
.mu.m or more were regarded as acceptable. The results obtained are
shown in Table 2.
[0097] (3-3) Measurement of Exfoliation Limit Load of Hard Coating
Film
[0098] An exfoliation limit load was measured on each of the hard
coating films (VC, TiC and TiN) by pin-on-disk testing.
Specifically, a diamond indenter having a tip of 200-.mu.m radius
(R) was indented into and made to travel through each hard coating
film under conditions that the load increasing rate was 100 N/min
and the indenter travel rate was 10 mm/min, and the load (N)
applied to the point where the hard coating film was exfoliated
primarily was evaluated as the exfoliation limit load. In these
examples, test specimens ensuring an exfoliation limit load of 20 N
or more on any of the hard coating films were regarded as
acceptable. The results obtained are shown in Table 2.
TABLE-US-00001 TABLE 1 [Mo] + [Cr] .times. [Al]/ 0.5 .times. No. C
Si Mn Cr Al N Cu Ni Mo W S P O V Others [C] [N] [W] 1 1.49 0.35
0.42 12.10 0.050 0.0130 0.05 0.08 1.04 0.35 0.005 0.018 0.0015 0.25
-- 18.03 3.85 1.22 2 1.01 1.06 0.60 8.38 0.330 0.0068 0.40 0.44
0.91 0.39 0.007 0.019 0.0007 0.09 Nb: 0.1 8.46 48.53 1.11 3 0.25
1.32 0.28 4.95 1.091 0.0148 3.01 2.95 1.20 0.02 0.004 0.018 0.0013
0.20 -- 1.24 73.72 1.21 4 0.40 1.35 0.25 4.45 1.030 0.0140 3.00
2.98 1.21 0.02 0.004 0.019 0.0013 0.20 -- 1.78 73.57 1.22 5 0.60
1.00 0.40 5.87 0.009 0.0170 0.04 0.67 0.93 0.02 0.004 0.020 0.0015
0.32 -- 3.52 0.53 0.94 6 0.58 1.01 0.42 5.95 0.017 0.0154 0.30 0.29
0.95 0.02 0.004 0.018 0.0014 0.28 -- 3.45 1.10 0.96 7 0.58 1.01
0.42 5.95 0.050 0.0165 0.30 0.29 0.95 0.02 0.004 0.018 0.0014 0.28
-- 3.45 3.03 0.96 8 0.59 1.02 0.43 5.95 0.100 0.0165 0.30 0.30 0.96
0.02 0.004 0.019 0.0015 0.29 -- 3.51 6.06 0.97 9 0.58 1.01 0.42
5.97 0.220 0.0164 0.29 0.29 0.95 0.02 0.005 0.018 0.0014 0.28 --
3.46 13.41 0.96 10 0.60 1.02 0.42 5.97 0.310 0.0165 0.30 0.30 0.95
0.02 0.004 0.018 0.0015 0.28 -- 3.58 18.79 0.96 11 0.58 1.01 0.43
5.95 0.300 0.0195 0.30 0.29 0.95 0.02 0.004 0.019 0.0014 0.28 --
3.45 15.38 0.96 12 0.59 1.02 0.44 5.96 0.550 0.0196 0.28 0.30 0.96
0.02 0.005 0.018 0.0014 0.29 -- 3.52 28.06 0.97 13 0.58 1.02 0.43
5.97 1.050 0.0194 0.30 0.29 0.95 0.02 0.005 0.018 0.0014 0.28 --
3.46 54.12 0.96 14 0.58 1.75 0.42 5.95 0.100 0.0165 0.29 0.30 0.95
0.02 0.004 0.018 0.0013 0.28 -- 3.45 6.06 0.96 15 0.58 1.02 1.10
5.96 0.110 0.0161 0.30 0.30 0.95 0.02 0.004 0.019 0.0014 0.29 --
3.46 6.83 0.96 16 0.60 1.02 0.42 5.95 0.100 0.0165 0.73 0.75 0.95
0.02 0.004 0.018 0.0014 0.28 -- 3.57 6.06 0.96 17 0.58 1.02 0.42
5.95 0.100 0.0165 0.30 0.30 1.70 0.02 0.004 0.019 0.0014 -- -- 3.45
6.06 1.71 18 0.60 1.01 0.43 5.96 0.110 0.0162 0.29 0.30 0.95 0.02
0.080 0.018 0.0014 0.28 -- 3.58 6.79 0.96 19 0.59 1.02 0.42 5.95
0.110 0.0165 0.30 0.30 0.96 0.02 0.004 0.018 0.0013 0.28 Ti: 0.04
3.51 6.67 0.97 20 0.58 1.02 0.43 5.95 0.100 0.0165 0.29 0.29 0.95
0.02 0.004 0.018 0.0014 0.29 Nb: 0.1 3.45 6.06 0.96 21 0.60 1.01
0.42 5.96 0.110 0.0162 0.30 0.30 0.96 0.02 0.004 0.019 0.0015 0.28
Zr: 0.1 3.58 6.79 0.97 22 0.58 1.01 0.44 5.95 0.100 0.0163 0.29
0.30 0.95 0.02 0.005 0.019 0.0014 0.29 Hf: 0.1, 3.45 6.13 0.96 Ta:
0.1 23 0.59 1.02 0.43 5.97 0.100 0.0165 0.30 0.29 0.95 0.02 0.004
0.018 0.0013 0.28 Co: 5.2 3.52 6.06 0.96 24 0.58 2.21 0.42 5.96
0.110 0.0165 0.29 0.30 0.96 0.02 0.004 0.018 0.0014 0.28 -- 3.46
6.67 0.97 25 0.60 1.02 2.10 5.97 0.100 0.0165 0.30 0.30 0.96 0.02
0.005 0.018 0.0013 0.29 -- 3.58 6.06 0.97 26 0.58 1.02 0.42 5.96
0.100 0.0164 1.50 1.48 0.95 0.02 0.004 0.018 0.0014 0.28 -- 3.46
6.10 0.96 27 0.59 1.02 0.43 5.95 0.110 0.0165 0.30 0.29 0.20 0.19
0.004 0.019 0.0014 0.29 -- 3.51 6.67 0.30 28 0.60 1.01 0.42 5.95
0.100 0.0162 0.30 0.30 2.98 0.05 0.005 0.019 0.0013 0.28 -- 3.57
6.17 3.01 29 0.59 1.02 0.43 5.96 0.100 0.0164 0.30 0.29 0.96 0.02
0.004 0.018 0.0015 0.60 -- 3.52 6.10 0.97 30 0.60 1.01 0.42 5.95
0.110 0.0165 0.29 0.30 0.95 0.02 0.005 0.018 0.0014 0.28 Ti: 0.25,
3.57 6.67 0.96 Nb: 0.3 31 0.58 1.02 0.43 5.97 0.100 0.0260 0.30
0.29 0.96 0.02 0.005 0.019 0.0013 0.29 -- 3.46 3.85 0.97 Unit: mass
%, Remainder: iron and unavoidable impurities
TABLE-US-00002 TABLE 2 Charpy Maximum Impact Thickness of Hard
Exfoliation Limit Load of Hardness Value Coating Film (.mu.m) Hard
Coating Film (N) No. HV J VC-TD TiC-CVD TiN-PVD VC-TD TiC-CVD
TiN-PVD 1 690 10 7.8 6.7 5.1 27 22 17 2 720 13 7.5 7.9 5.0 23 23 18
3 685 22 4.3 4.3 5.1 12 10 33 4 710 17 6.5 6.2 4.9 18 15 32 5 700
15 7.5 7.3 5.0 24 25 31 6 710 30 7.4 7.5 5.0 27 26 32 7 715 35 7.3
7.5 5.0 25 26 30 8 724 35 7.5 7.6 5.0 26 24 29 9 728 35 7.4 7.7 5.1
25 27 30 10 729 35 7.3 7.8 5.0 27 25 33 11 734 31 7.5 7.5 4.9 24 23
31 12 750 25 7.5 7.5 5.0 25 24 29 13 745 18 7.4 7.4 4.9 28 28 30 14
726 21 7.3 7.7 5.0 23 24 32 15 719 22 7.5 7.6 5.0 26 23 30 16 721
23 7.9 7.5 5.1 25 26 30 17 730 22 7.4 7.5 5.0 20 21 31 18 722 15
7.8 7.5 5.0 28 29 30 19 709 40 7.3 7.3 5.1 21 22 33 20 711 41 7.5
7.6 5.0 22 21 30 21 708 39 7.5 7.4 4.9 20 22 31 22 710 28 7.6 7.8
5.0 20 22 32 23 724 35 7.5 7.3 4.9 24 24 30 24 729 17 7.3 7.7 5.1
25 26 33 25 730 16 7.9 7.5 5.0 26 25 28 26 728 17 7.5 7.6 5.1 24 23
31 27 648 20 7.4 7.5 5.0 29 30 30 28 741 17 7.3 7.6 5.1 21 22 32 29
718 15 7.6 7.8 5.0 28 29 30 30 600 20 7.7 7.9 5.1 24 25 31 31 717
16 7.5 7.5 5.1 24 24 30
[0099] As is apparent from Tables 1 and 2, each of the steels Nos.
6 to 12 and 14 to 23, which fulfills all the requirements of the
invention, is excellent in all of the (maximum) hardness, toughness
(Charpy impact value), thickness of the VC or TiC coating film and
exfoliation limit load of the hard coating film (the VC coating
film, TiC coating film or TiN coating film). By contrast, the
steels Nos. 1 to 5, 13 and 24 to 31, each of which does not fulfill
at least one of the requirements of the invention, have the
following problems.
[0100] The steels Nos. 1 and 2 are insufficient in exfoliation
limit load of the TiN coating film because all of their contents of
C, contents of Cr and [Cr].times.[C] values are too high, and
because they contain coarse Cr carbide. In addition, there are
reductions in their toughness because they are too high in contents
of C and Cr.
[0101] The steels Nos. 3 and 4 are insufficient in thicknesses of
the VC and TiC coating films because of their low contents of C,
and as a result, the exfoliation limit load of those coating films
are reduced.
[0102] The steel No. 5 is insufficient in toughness because of its
low content of Al and small value of [Al]/[N].
[0103] The steel No. 13 is insufficient in toughness because of its
high content of Al and large value of [Al]/[N].
[0104] All the steel Nos. 24, 25 and 26 are insufficient in
toughness because it is too high in content of Si in the steel No.
24, it is too high in content of Mn in the steel No. 25 and it is
too high in contents of Cu and Ni in the steel No. 26.
[0105] The steel No. 27 is insufficient in hardness because of its
low content of Mo and small value of [Mo]+0.5.times.[W].
[0106] The steel No. 28 is insufficient in toughness because of its
large value of [Mo]+0.5.times.[W].
[0107] The steel No. 29 is insufficient in toughness because it
contains V as an optional element in the excessive amount.
[0108] The steel No. 30 is insufficient in hardness as a result of
the content of C dissolved in solid being reduced by addition of Ti
and Nb as optional elements in the total amount in excess of
0.5%.
[0109] The steel No. 31 is insufficient in toughness because of its
too high content of N.
[0110] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope
thereof.
[0111] This application is based on Japanese Patent Application No.
2007-294326 filed on Nov. 13, 2007, and their contents are
incorporated herein by reference. In addition, all the references
cited herein are incorporated as a whole.
INDUSTRIAL APPLICABILITY
[0112] Because in the cold-work die steels of the invention, as
specified above, the alloy components and balances between the
specified elements are appropriately adjusted, they can have high
hardness and toughness, and besides, long-life hard coating films
can be formed on the surface thereof even by a variety of hard
coating treatments. Dies obtained by using the cold-work die steels
of the invention are particularly suitable as dies for forming
high-tensile steel plates having tensile strength of about 590 MPa
or more.
* * * * *