US20100135844A1

US20100135844A1 - Cold-work die steel and die

Info

Publication number: US20100135844A1
Application number: US12/598,328
Authority: US
Inventors: Shogo Murakami; Shigenobu Namba; Kenji Yamamoto; Tsuyoshi Tonomura
Original assignee: Nippon Koshuha Steel Co Ltd; Kobe Steel Ltd
Current assignee: Nippon Koshuha Steel Co Ltd; Kobe Steel Ltd
Priority date: 2007-11-13
Filing date: 2008-09-18
Publication date: 2010-06-03
Also published as: TW200936785A; JP5143531B2; JP2009120886A; WO2009063690A1; CN101668874B; CN101668874A

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]×[C]≦4; (2) [Al]/[N]: 1 to 30; and (3) [Mo]+0.5×[W]: 0.5 to 3.00%, wherein the bracket means a content (%) of an element written therein.

Description

TECHNICAL FIELD

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

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.
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.
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.
A variety of methods aiming further improvements in properties of die steels have been proposed (e.g. Patent Documents 1 and 2).
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.
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 M₂₃C₆-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 M₆C-type carbide precipitated in a dispersed state.
Patent Document 1: JP-A-2006-169624
Patent Document 2: JP-A-2004-169177

DISCLOSURE OF THE INVENTION

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 μm is diffused and permeated on the surface of a base material under high temperature conditions of about 900 to about 1,030° 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).
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.
Namely, the present invention provides 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; and
wherein the following requirements (1) to (3) are satisfied:
[Cr]×[C]≦4; (1)
[Al]/[N]: 1 to 30; and (2)
[Mo]+0.5×[W]: 0.5 to 3.00%, (3)
wherein the bracket means a content (%) of an element written therein.
In addition, the cold-work die steel preferably comprises 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%).
The die of the invention is obtained by using any of the cold-work die steels specified above.
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

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).

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

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.
Details about the achievement of the invention are described below.
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.
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 μm to about 50 μm) precipitates out on the surface of the areas in which TiN coating film is exfoliated, and cracks are produced from the carbide spots.
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.
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)).
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 μm or less.
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.
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.
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.
C: 0.5 to 0.7%
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.
Si: 0.5 to 2.0%
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.
Mn: 0.1 to 2.0%
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.
Cr: 5 to 7%
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.
Al: 0.01 to 1.0%
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 Ni₃Al, 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.
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.
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.
N: 0.003 to 0.025%
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.
Cu: 0.25 to 1%
Cu is an element necessary to aim at hardness improvement by precipitation hardening of ε-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.
Ni: 0.25 to 1%
Ni is an element necessary to aim at hardness improvement by precipitation hardening of an Al—Ni intermetallic compound, such as Ni₃Al, 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.
Mo: 0.5 to 3%, and W: 2% or Less (Including 0%)
Mo and W are elements that contribute to precipitation hardening because each of Mo and W forms M₆C-type carbide, and besides, a Ni₃Mo 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.
S: 0.1% or Less (Excluding 0%)
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.
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).
(1) [Cr]×[C]≦4
The requirement (1) is set for the purpose of inhibiting the production of coarse Cr carbide. When the product of [Cr]×[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]×[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.
(2) [Al]/[N]: 1 to 30
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.
(3) [Mo]+0.5×[W]: 0.5 to 3.00%
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 ½ in comparison with that of W.
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.
V: 0.5% or Less (Excluding 0%)
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.
At Least One Element Selected from the Group Consisting of Ti, Zr, Hf, Ta and Nb: 0.5% or Less in Total (Excluding 0%)
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.
Co: 10% or Less (Excluding 0%)
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.
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° C. for 7 hours, and then subjected to furnace cooling to about 400° C. at an average cooling rate of about 17° 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° C. to about 1,150° C., and subsequently undergoing aging treatment at temperatures ranging from about 400° C. to about 530° C.

EXAMPLES

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.
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° C. to about 1,150° C., and forged into two plates each having a size of 40 mmT×75 mmW×about 2,000 mmL. Thereafter, each plate obtained was slowly cooled at an average cooling rate of about 60° C./hour. After cooling to a temperature of 100° C. or less, the resultant was re-heated up to a temperature of about 850° C., and subsequently cooled slowly at an average cooling rate of about 50° C./hour (heat treatment or annealing).
The following tests (1) to (3) were carried out on each of the materials thus heat-treated or annealed.
(1) Hardness Test (Determination of Maximum Hardness)
Test species basically having a size of 20 mmT×20 mmW×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.
Solution treatment (quenching treatment): heating at temperatures ranging from about 1,020° C. to about 1,030° C. for 120 minutes→air cooling→aging treatment (tempering treatment): keeping for about 3 hours at a temperature in a range of about 400° C. to about 560° C.→standing to cool
While changing the tempering temperature within the range of about 400° C. to about 560° C. range as described above, hardness measurements were made with a Vickers hardness tester (manufactured by ΛKΛSHI Co., Ltd., ΛVK 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.
(2) Toughness Test (Measurement of Charpy Impact Value)
Each of the heat-treated or annealed materials was subjected to the following heat treatment.
Solution treatment (quenching treatment): heating at temperatures ranging from about 1,020° C. to about 1,030° C. for 120 minutes→air cooling→aging treatment (tempering treatment): keeping for about 3 hours at temperatures ranging from about 400° C. to about 560° C.→air cooling or standing to cool
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.
(3) Property Evaluation of Hard Coating Film
(3-1) Formation of Hard Coating Film
Test pieces basically having a size of 4 mmt×φ50 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.
(3-2) Thickness Measurement of Hard Coating Film
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 (μm) 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 μm or more were regarded as acceptable. The results obtained are shown in Table 2.
(3-3) Measurement of Exfoliation Limit Load of Hard Coating Film
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-μm 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 1

																		[Mo] +
																[Cr] ×	[Al]/	0.5 ×
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 2

	Charpy
Maximum	Impact	Thickness of Hard	Exfoliation Limit Load of
Hardness	Value	Coating Film (μm)	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

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.
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]×[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.
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.
The steel No. 5 is insufficient in toughness because of its low content of Al and small value of [Al]/[N].
The steel No. 13 is insufficient in toughness because of its high content of Al and large value of [Al]/[N].
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.
The steel No. 27 is insufficient in hardness because of its low content of Mo and small value of [Mo]+0.5×[W].
The steel No. 28 is insufficient in toughness because of its large value of [Mo]+0.5×[W].
The steel No. 29 is insufficient in toughness because it contains V as an optional element in the excessive amount.
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%.
The steel No. 31 is insufficient in toughness because of its too high content of N.
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.
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

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.

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]×[C]≦4; (1)

[Al]/[N]: 1 to 30; and (2)

[Mo]+0.5×[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.