COLD-WORK DIE STEEL AND DIES FOR COLD PRESSING

Abstract

The present invention provides a cold-work die steel useful as a material of dies for cold pressing, which has basic properties such as hardness, toughness and dimensional change by heat treatment, and besides, which causes no problem in terms of machined surface roughness and cutting tool life, and also its dies for cold pressing. The invention relates to a cold-work die steel comprising: 0.5 to 0.7 mass % of C; 5.0 to 7.0 mass % of Cr; 0.5 to 2.0 mass % of Si; 0.1 to 2.0 mass % of Mn; 0.001 to 0.010 mass % of Al; 0.25 to 1.00 mass % of Cu; 0.25 to 1.00 mass % of Ni; 0.5 to 3.0 mass % of Mo+0.5×W; 0.5 mass % or less of V; 0.05 mass % or less of P; 0.1 mass % or less of S; 0.005 mass % or less of O, wherein the following requirements are satisfied: [C]×[Cr]≦4; FP=[Si]/5+[Cr]/5+2×[Mo]+[W]+2×[V]+10×[Al]≦5.0; and AP=[Mn]+3×([Cu]+[Ni])≦2.5, and also relates to a die for cold pressing which is manufactured by using the cold-work die steel.

Description

TECHNICAL FIELD

The present invention relates to a cold-work die steel useful as a material for dies for cold pressing, which are used in carrying out press forming (stamping, bending, drawing, trimming, and the like) of steel plates for cars, steel sheets for home electric appliances and so on under cold working or the like, and further relates to its dies for cold pressing.

BACKGROUND ART

With increases in strength of steel plates and sheets, dies for cold pressing which are employed for press 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 use in particular, for enhancement of fuel economy of cars with consideration given to environmental issues, high-tensile steel plates having tensile strengths of 590 MPa or more have come to be adopted in growing numbers, and it is conceivable that its demand will increase more and more from now on.

In carrying out the press forming of high-tensile steel plates, there is an increase in the frequency of occurrence of a problem that the surface coating film of a surface-treated die for cold pressing suffers damage at an early stage and thereby a seizing-up phenomenon referred to as compact attach or galling is caused during the press forming to result in extreme loss of life to the die for cold pressing.

A die for cold pressing is manufactured by giving hard coating treatment to the surface of a cold-work die steel as a base material. The cold-work die steel as a base material is generally manufactured by undergoing processes of heat treatment or annealing, cut working and quenching-tempering treatment in order of mention.

As the cold-work die steel, not only high-C, high-Cr alloy tool steel, such as JIS SKD11, but also high-speed tool steel having further improved abrasion resistance, such as JIS SKH51, has generally been used in the past. Improvement in hardness of these tool steels are made by giving them precipitation hardening of Cr carbide or Mo, W or V carbide. In addition, low-alloy high-speed tool steels referred to as matrix high speed steels which are improved in both toughness and abrasion resistance by reducing contents of alloy elements contained in JIS SKH51, such as C, Mo, W and V, are used as cold-work die steels. Further, there are proposals of the arts disclosed in Patent Document 1 and Patent Document 2 with the intention of further improving properties of those cold-work die steels.

With the intension of attaining excellent properties of inhibiting dimensional change and securing high hardness and galling resistance without impairment of the required properties such as machinability and abrasion resistance, Patent Document 1 discloses a cold-work die steel which allows carbide to have finely-dispersed distribution in its texture by adding Ni and Al in proper amounts, and further by adjusting C and Cr contents in concert with Cu addition in an amount appropriate to the amounts of Ni and Al added.

On the other hand, with the intention of attaining properties, such as hardness after heat treatment 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 2 to 5 vol % of M₂₃C₆-type carbide is formed under tempered conditions, and besides, that has a microstructure including at least either MC-type carbide or M₆C-type carbide precipitated in a dispersed state after quenching-tempering.

Patent Document 1: JP-A-2006-169624

Patent Document 2: JP-A-2004-169177

DISCLOSURE OF THE INVENTION

Problems that the Invention is to Solve

A die for cold pressing is manufactured by giving hard coating treatment to the surface of a cold-work die steel as a base material. Examples of the hard coating treatment include TD treatment by which a coating film comprising VC is formed through thermal diffusion, CVD treatment by which a coating film mainly comprising TiC is formed, and PVD treatment by which a coating film mainly comprising TiN is formed. These hard coating treatments are adopted as appropriate according to the circumstances of die users and press makers. Therefore, it is required to develop cold-work die steels adaptable to any of hard coating treatments. In addition, as a matter of course, dies for cold pressing are also required to ensure basic properties including hardness, toughness and dimensional change by heat treatment.

Dies for cold pressing have an additional problem of suffering from plucking during cut working. When the plucking occurs, roughness of the machined surface becomes great, wrapping operation after heat treatment becomes difficult, and besides, reduction in die life is caused. In addition, the cutting tool life is shortened and the production cost is increased. For solving these problems, it is required to inhibit precipitation of Al inclusions (Al₂O₃, AlN) as a cause of the problem occurrence. However, there is a fear that reduction in the content of Al as an element causing precipitation of Al inclusions affects rather adversely the basic properties such as reduction of hardness, reduction of toughness and increase in dimensional change by heat treatment. Under these circumstances, it has been awaited to develop dies for cold pressing, which can ensure those basic properties, and besides, which have no problems from the viewpoints of roughness of machined surface, cutting tool life and so on.

The invention has been made in order to solve these conventional problems, and subjects thereof are to provide a cold-work die steel useful as a material of dies for cold pressing, which has not only basic properties required, such as hardness, toughness and dimensional change by heat treatment, but also adaptability to various types of hard coating treatment, and besides, which causes no problems in terms of machined surface roughness and cutting tool life, and to provide its dies for cold pressing.

Means for Solving the Problems

The gist of the invention is described below.

[1] A cold-work die steel comprising:

0.5 to 0.7 mass % of C;

5.0 to 7.0 mass % of Cr;

0.5 to 2.0 mass % of Si;

0.1 to 2.0 mass % of Mn;

0.001 to 0.010 mass % of Al;

0.25 to 1.00 mass % of Cu;

0.25 to 1.00 mass % of Ni;

0.003 to 0.025 mass % of N;

more than 0 to 0.05 mass % of P;

more than 0 to 0.1 mass % of S;

more than 0 to 0.005 mass % of O; and

at least one of Mo and W,

with a remainder comprising iron and an unavoidable impurity;

wherein the following requirements are satisfied:

0.5≦[Mo]+0.5×[W]≦3.0;

[C]×[Cr]≦4;

FP=[Si]/5+[Cr]/5+2×[Mo]+[W]+2×[V]+10×[Al]≦5.0; and

AP=[Mn]+3×([Cu]+[Ni])≦2.5;

wherein FP is a parameter associated with the ferrite-forming elements, AP is a parameter associated with the austenite-forming elements, and the bracket means a content (mass %) of an element written therein.

[2] The cold-work die steel according to [1], further comprising more than 0 to 0.5 mass % of V.
[3] The cold-work die steel according to [1] or [2], further comprising at least one element selected from the group consisting of Ti, Zr, Hf, Ta and Nb in a total content of more than 0 to 0.5 mass %.
[4] The cold-work die steel according to any of [1] to [3], further comprising more than 0 to 10 mass % of Co.
[5] A die for cold pressing, which is manufactured by working the cold-work die steel according to any of [1] to [4] and giving surface treatment thereto.

ADVANTAGES OF THE INVENTION

The use of the cold-work die steel of the invention as a material for dies for cold pressing allows provision of dies for cold pressing, which each have not only basic properties required, such as hardness, toughness and dimensional change by heat treatment, but also adaptability to various types of hard coating treatments, and besides, which cause no problems in terms of roughness of machined surface and cutting tool life. Moreover, the dies for cold pressing made by use of the cold-work die steel are particularly suitable for use in forming high tensile steel plates having tensile strength of 590 MPa or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a mechanism for damaging a TiN coating film by Cr carbide. Therein, (a) is a vertical cross-section view showing an original die for cold pressing, (b) is a vertical cross-section view showing a state in which cracks are formed in the TiN coating film of the die for cold pressing, and (c) is a vertical cross-section view showing a state in which exfoliation of the TiN coating film occurs from the cracks as starting points.

FIG. 2 is an explanatory diagram representing a Charpy impact test piece used for determining Charpy impact values in Examples.

FIG. 3 is an explanatory diagram demonstrating heat treatment conditions adopted in giving heat treatment to test samples used for determining a maximum rate of dimensional changes by heat treatment in Examples.

DESCRIPTION OF REFERENCE NUMERALS

• • 1: Cold-work die steel
  • 2: TiN coating film
  • 3: Cr carbide
  • 4: Crack

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is described below in more detail on the basis of its illustrative embodiments.

In the first place, the inventors have researched on causes of damage to a TiN coating film formed by PVD treatment and occurrence of galling in dies for cold pressing which were made by using as a material conventional JIS SKD11 and matrix high speed steels.

As a result of the research, it has been found that a cause of occurrence of galling in the TiN coating film lay in coarse Cr carbide formed in the cold-work die steels used as a base material, and the Cr carbide formed the starting point of occurrence of galling. The mechanism of damage of the TiN coating film by the Cr carbide is as shown in FIG. 1.

As shown in FIG. 1(a), to begin with, the surface of a cold-work die steel 1 to be used as a base material is subjected to hard coating treatment, thereby preparing a die for cold pressing in which a TiN coating film 2 is formed on its surface. When this cold-work die steel 1 is formed using as a material JIS SKD11 or a matrix high speed steel, coarse Cr carbide 3 precipitates out at the surface of the cold-work die steel 1 as the base material. In carrying out press forming by use of this die for cold pressing, as shown in FIG. 1(b), cracks 4 are formed in the TiN coating 2 by sliding the material in the direction of the arrow. The sites at which cracks 4 are formed lie in areas where Cr carbide 3 precipitates out in the base material beneath the TiN coating film 2. When the material is further slid, as shown in FIG. 1(c), the cracks 4 act as starting points and cause exfoliation in the TiN coating film 2, resulting in occurrence of galling.

As mentioned above, the cause of galling occurrence in the TiN coating film lies in Cr carbide. The inventors have found that exfoliation of the TiN coating film can be prevented by inhibiting formation of the Cr carbide, and thereby a problem of extreme reduction in die life can be inhibited from arising.

In order to inhibit formation of coarse Cr carbide 3 precipitating out at the surface of a cold-work die steel to be used as a base material, thereby increasing the lifetime of a TiN coating film formed by PVD treatment, it is appropriate to reduce both C content and Cr content in the steel. However, an excessive reduction in C content makes it difficult to form a VC coating film by TD treatment and a TiC coating film by CVD treatment on the surface of the cold-work die steel. Therefore, the invention makes it possible to prevent coarse Cr carbide 3 from precipitating out at the surface of a cold-work die steel, and moreover to form a VC coating film and TiC coating film having necessary and sufficient thicknesses on the surface of a cold-work die steel by not only controlling the C content to be from 0.5 to 0.7 mass % and the Cr content to be from 5.0 to 7.0 mass % but also specifying the product of these contents.

In addition, the invention specifies both a parameter associated with ferrite-forming elements, such as Si, Cr, Mo, W, V and Al, and a parameter associated with austenite-forming elements, such as Mn, Cu and Ni.

When the total content of ferrite-forming elements, such as Si, Cr, Mo, W, V and Al, is too high, not only a balance between hardness and toughness of a cold-work die steel is lost, but also accuracy of worked-machined surface becomes worse. Therefore, the invention performs mathematization of the parameter (FP) associated with the ferrite-forming elements, and further adjusts the total content of ferrite-forming elements to satisfy the mathematic expression, and thereby the invention ensures not only a good balance between hardness and toughness in the cold-work die steel but also improvement in accuracy of worked-machined surface.

On the other than, when the total content of austenite-forming elements, such as Mn, Cu and Ni, is too high, there occurs an increase in residual austenite content to result in not only a wide variation in rate of dimensional change by heat treatment but also a reduction in tool life under cutting. Therefore, the invention performs mathematization of the parameter (AP) associated with the austenite-forming elements, and further adjusts the total content of the austenite-forming elements to satisfy the mathematic expression, and thereby not only the amount of residual austenite in the steel is reduced to result in a narrow variation in rate of dimensional change by heat treatment but also the tool life under cutting is increased.

Reasons for specifying the content ranges of chemical components in the cold-work die steel of the invention are described in detail on an element basis. Additionally, all percentages in the present specification mean percent by mass.

C: 0.5 to 0.7%

C is an element that ensures hardness and abrasion resistance and contributes to inhibition of HAZ softening. Additionally, when a carbide coating film, such as a VC coating film by TD treatment or a TiC coating film by CVD treatment, is formed on the surface of a base material for the die, 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 effectively fulfilling the above functions. And the lower limit thereof is preferably 0.55%. 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 tempering treatment is performed at a high temperature, and besides, a great dimensional change occurs through expansion or the like after the tempering 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 upper limit thereof is preferably 0.65%.

Cr: 5.0 to 7.0%

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 secured. Moreover, Cr is an element useful also for ensuring corrosion resistance of dies. Therefore, the lower limit of the content of Cr is set to 5.0%. And the lower limit thereof is preferably 5.5%. However, an excessive content thereof 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 of Cr 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 7.0%. And the upper limit thereof is preferably 6.5%.

Si: 0.5 to 2.0%

Si is useful as a deoxidizing element at the time of steelmaking, and 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%. Additionally, the content thereof is preferably 1.0% or more, more preferably 1.2% or more. However, an excessive content thereof 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%. And the content thereof is preferably 1.85% or less.

Mn: 0.1 to 2.0%

Mn is an element useful for securing 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 tempering treatment is performed at a high temperature, and besides, the toughness is lowered. Considering these circumstances, the content of Mn is so specified that the range thereof is between 0.1% and 2.0%. The lower limit of the content of Mn is preferably 0.15%, and the upper limit is preferably 1.0%, more preferably 0.5%, further more preferably 0.35%.

Al: 0.001 to 0.010%

Al is an element useful as a deoxidizer. When the content thereof is less than 0.001%, the effect cannot be fully achieved. Therefore, the lower limit of the content of Al is set to 0.001%. And the lower limit thereof is preferably 0.002%. On the other hand, Al inclusions, such as Al₂O₃ and coarse AlN, become a cause of the plucking during cutting, and aggravate accuracy of worked-machined surface. Accordingly, the upper limit of the content of Al is set to 0.010%. And the upper limit thereof is preferably 0.008%.

Cu: 0.25 to 1.00%

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 tends to cause forging cracks. Therefore, the upper limit of the content of Cu is set to 1.00%. And the upper limit thereof is preferably 0.80%. Further, the lower limit of the content of Cu is set to 0.25%. And the lower limit thereof is preferably 0.30%.

Ni: 0.25 to 1.00%

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 tempering treatment 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 so specified that the range thereof is between 0.25% and 1.00%. The lower limit of the content of Ni is preferably 0.30%, and the upper limit thereof is preferably 0.80%.

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%. And the lower limit thereof is preferably 0.004%. In addition, the upper limit of the content of N is 0.025%. And the upper limit thereof is preferably 0.017%.

Mo+0.5W: 0.5 to 3.0%

Mo and W are elements that contribute to precipitation hardening because each of Mo and W forms M₃C-type carbide or M₆C-type carbide, and besides, forms Ni₃Mo intermetallic compound or the like. However, excessive contents of these 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 sum of content of Mo and content of W, to which the expression Mo+0.5W is applied, is specified so as to fall in a range of 0.5 to 3.0%. Additionally, the content of Mo in itself is preferably in a range of 0.5 to 3.0%. On the other hand, the content of W in itself is preferably 2.0% or less (including 0%). In other words, Mo is an essential element, while W is an optional element. However, the lower limit of the content of W in itself is preferably 0.02%. Moreover, it is more preferred that the lower limit of the content of Mo in itself be a lower limit of 0.7% and an upper limit thereof be 2.5%. On the other hand, it is more preferred that the lower limit of the content of W in itself be 0.05% and the upper limit thereof be 1.5%.

P: more than 0 to 0.05%

P is an element that is unavoidably present in dissolved raw materials, and that impairs toughness. Therefore, the upper limit of the content of P is set to 0.05%. And the upper limit thereof is preferably 0.02%. Although the lower the content of P is, the better it is, the inclusion of P is unavoidable, so the practical lower limit of the content thereof is about 0.005%.

S: more than 0 to 0.1%

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 upper limit of the content of S is preferably 0.07%, more preferably 0.05%, further more preferably 0.025%.

O: more than 0 to 0.005%

O is an element included in molten steel, and it is unavoidably present in steel. When the content of O is high, it reacts with Si, Al and the like to form oxide inclusions. Therefore, the O content is so specified that its upper limit is 0.005%. The upper limit thereof is preferably 0.003%, more preferably 0.002%. Additionally, the lower the content of O is, the better it is, but inclusion of O is unavoidable, so the practical lower limit thereof is about 0.0005%.

Furthermore, it is an essential condition for the invention to satisfy all of the mathematical expressions described above. Additionally, the bracket in each mathematical expression represents the content (mass %) of an element written therein.

[C]×[Cr]≦4

The above mathematical expression is a mathematical expression defined for the purpose of inhibiting the production of coarse Cr carbide. When the product of content of C and content of Cr is more than 4, there occurs not only degradation in durability of hard coating films but also increase in dimensional change after heat treatment. From the viewpoints of inhibiting the formation of coarse Cr carbide and inhibiting the dimensional change after heat treatment, it is preferred that the product of content of C and content of Cr be minimized. However, further considering significant achievement of the effects from the addition of C and Cr, the lower limit of the product is preferably basically 0.8.

FP=[Si]/5+[Cr]/5+2×[Mo]+[W]+2×[V]+10×[Al]≦5.0

The above mathematical expression is a mathematical expression defined by parameterizing the sum relating of the contents of ferrite-forming elements such as Si, Cr, Mo, W, V and Al. When this parameter (FP) is more than 5.0, not only the balance between hardness and toughness of the cold-work die steel is lost, but also the accuracy of worked-machined surface is aggravated. This parameter (FP) is preferably 4.8 or less. And the FP value of 2.11 determined from the lower limit values concerning the elements essentially included in the cold-work die steel according to the invention, such as Si and Cr, is a substantial lower limit value of this parameter (FP).

AP=[Mn]+3×([Cu]+[Ni])≦2.5

The above mathematical expression is a mathematical expression defined by parameterizing the sum of the contents of austenite-forming elements, such as Mn, Cu and Ni. When this parameter (AP) is more than 2.5, the residual austenite is increased in content, and thereby not only the rate of dimensional change by heat treatment varies widely but also the tool life under cutting is shortened. This parameter (AP) is preferably 2.3 or less. And the AP value of 1.6 determined from the lower limit values concerning Mn, Cu and Ni is a substantial lower limit of this parameter (AP).

Requirements concerning the basic components in the cold-work die steel of the invention are as mentioned above. The remainder comprises iron and unavoidable impurities. As the impurities, e.g. Sn, Pb and so on are exemplified. In addition, for the purpose of improving other properties, the following optional components may be further included.

V: 0 to 0.5%

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 significant achievement of those effects, it is appropriate that the content of V be basically 0.05% or more. However, an excessive content of V 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, the upper limit of the content of V is set to 0.5%. And, the upper limit thereof is preferably 0.4%, more preferably 0.3%.

At least one element selected from the group consisting of Ti, Zr, Hf, Ta and Nb: 0.5% or less in total

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 improved toughness. For significant achievement of such effects, 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 total content of these elements becomes excessive, the amount of C dissolved in solid is lessened to result in a hardness reduction of martensite. Therefore, these elements are so specified that they have a total content of 0.5% or less. 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

Co is an element effective in heightening an Ms point and reducing residual austenite, and thereby enhancing the hardness. For significant achievement of such effects, 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, the upper limit of the content of Co is set to 10%. The upper limit of the Co content is preferably 5.5%. Herein, the term “Ms point” is one of transformation temperatures (a temperature at which a phase change occurs, or a temperature at which transformation begins or comes to an end when the transformation lasts over a temperature range), and refers to the temperature at which austenite begins undergoing transformation into martensite during cooling.

Dies for cold pressing are manufactured by using the cold-work die steel that satisfies the requirements described above. An example of a method of manufacturing such dies for cold pressing is explained below. For instance, after production by melting, the cold-work die steel of the invention is subjected to hot forging, and then softened by undergoing annealing (e.g. by being kept at about 700° C. for 7 hours, and then subjected to cooling to about 400° C. in furnace at an average cooling rate of about 17° C./hr, and further to standing to cool). Thereafter, the resultant is crude-processed into intended forms by e.g. a cutting work, and then hardened so as to acquire an intended hardness by undergoing quenching at temperatures ranging from 950° C. to 1,150° C. and further by tempering at temperatures ranging from 400° C. to 530° C. Thus, dies for cold pressing are manufactured.

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 limited to these examples.

In these examples, steel species having chemical compositions listed in Table 1 with 26 varieties in total (No. 1: JIS SKD11 conventionally used as a cold-work die steel) were used. From each of the steel species, 150 kg of ingot was produced by melting in a vacuum induction melting furnace, and then heated to a temperature of 900° C. to 1,150° C. and thereby forged into a plate having a size of 40 mmT×75 mmW×about 2,000 mL. Thereafter, each plate obtained was slowly cooled at an average cooling rate of about 60° C./hr. After cooling to a temperature of 100° C. or less, re-heating to a temperature of about 850° C. and subsequent slow cooling at an average cooling rate of about 50° C./hr (heat treatment or annealing) were carried out. Various tests were made on each of the heat-treated or annealed materials thus obtained.

(1) Determination of Maximum Hardness

A test piece having a size of 20 mmT×20 mmW×15 mmL was cut from each of the heat-treated or annealed materials, and used as the test specimen for hardness measurement. Each test specimen was subjected to heat treatment, and more specifically, it underwent such treatment that quenching (heating at 1,030° C. for 120 minutes), air cooling, tempering (keeping for 180 minutes in a temperature range of 450° C. to 520° C.) and standing to cool were carried out in order of mention. Hardness measurements thereof were made with a Vickers hardness tester (manufactured by ΛKΛSHI Co., Ltd., ΛVK standard, load of 5 kg) as the tempering temperature was shifted within the range of 450° C. to 520° C., and the maximum hardness thereof was determined. The test specimens showing maximum hardness of 650 HV or more in these measurements were regarded as acceptable. The test results are shown in Table 2.

(2) Measurement of Charpy Impact Value (Toughness Measurement)

Each of the heat-treated or annealed materials underwent heat treatment, specifically such treatment that quenching (heating at 1,030° C. for 120 minutes), air cooling, tempering (keeping for 180 minutes in a temperature range of 450° C. to 520° C.) and air cooling or standing to cool in order of mention. A test piece having an R-notch section of 10-mm R as shown in FIG. 2 was cut, and used as a test specimen for toughness measurement (Charpy Impact test specimen). Charpy impact testing was carried out on this specimen, and absorption energy at room temperature (Charpy impact value) was determined. From each steel species, three test species for Charpy impact testing were taken, and the average value thereof was taken as Charpy impact value. In this testing, when the Charpy impact value obtained by measurement was 20 J or more, the test specimens having such Charpy impact values were regarded as acceptable. These test results are shown in Table 2.

(3) Survey on Machined Surface Roughness

Each of the heat-treated or annealed materials was used as a test sample, subjected to finish working by means of a ball end mill, and examined for machined surface roughness. Testing conditions adopted were as follows.

Machine: MORI (BT40, 5.5 kw)

Tool: Mitsubishi SRFH30S32M φ30

Tip: Mitsubishi SRFT30 VP10MF φ30

Projection length: 118 mm

Cutting direction: Down cut

Cutting rate: 250 mm/min

Feed rate: 0.31 min/rev

Cut: Ad 0.3 mm, Rd 0.7 mm

Cutting oil: Nothing (air-blow)

Working distance: 257.1 m

The machined surface roughness Ra is defined as the average value of the values obtained by carrying out surveys at 5 points chosen from a length range of 10 mm in each test sample. In this testing, the test samples having machined surface roughness Ra of 0.40 mm or more were regarded as acceptable. Test results obtained are shown in Table 2.

(4) Determination of Cutting Tool Life Each of the heat-treated or annealed materials was used as a test sample, subjected to crude working with a high feed cutter, and examined on the cutting tool life. Testing conditions adopted were as follows.

Machine: OKK (BT50, 7.5 kw)

Tool: Mitsubishi AJX148R503SA42S φ50

Tip: JOMW140520ZDSR-FT VP15TF

Cutting rate: 10 m/min

Feed ratio: 1.0 mm/rev

Cut: Ad 1 mm, Rd 35 mm

Projection length: 80 mm

Cutting oil: Nothing (air-blow)

Life determination: Tool wear, chipping

The cutting tool life in the case of using each of the test samples and carrying out the crude working was determined by examining how many times it was longer than the cutting tool life in the case of using the test sample (No. 1) made from JIS SKD11 as a material and carrying out the crude working, with the latter cutting tool life being taken as “1”. When the values determined were 4.0 or more, the test samples concerned were regarded as acceptable. Test results obtained are shown in Table 2.

(5) Measurement on Maximum Rate of Dimensional Changes by Heat Treatment

Six blocks each having a size of 40 mmT×75 mmW×100 mL were cut from each of the heat-treated or annealed materials, used as test samples for measurement on maximum rate of dimensional changes by heat treatment, and subjected to heat treatment under the conditions as shown in FIG. 3. The maximum rate of dimensional changes by heat treatment was determined from the rates of changes caused in dimensions of 6 test samples between before and after heat treatment. More specifically, each of the test samples was measured on the rates of dimensional changes in orthogonal three directions (x, y and z directions), and the maximum numerical value among the absolute values of 3×6 dimensional change rates measured was defined as the maximum rate of dimensional changes by heat treatment. In this testing, every case where the maximum rate of dimensional changes by heat treatment was 0.08 or less was regarded as acceptable. Test results obtained are shown in Table 2.

	TABLE 1
	Chemical Component (mass %)
No.	Classification	C	Si	Mn	P	S	Al	Ni	Cu	Cr	Mo	W
1	Comparative Example	1.49	0.35	0.42	0.018	0.005	0.050	0.08	0.05	12.10	1.04	0.35
2	Comparative Example	1.01	1.06	0.60	0.019	0.007	0.330	0.44	0.40	8.38	0.91	0.39
3	Comparative Example	0.25	1.32	0.28	0.018	0.004	1.091	2.95	3.01	4.95	1.20	0.02
4	Comparative Example	0.40	1.35	0.25	0.019	0.004	1.030	2.98	3.00	4.45	1.21	0.02
5	Comparative Example	0.60	1.00	0.40	0.020	0.004	0.009	0.67	0.04	5.87	0.93	0.02
6	Comparative Example	0.58	0.95	0.42	0.018	0.004	0.0008	0.30	0.30	5.95	0.95	0.02
7	Example	0.58	0.98	0.42	0.019	0.005	0.002	0.29	0.30	5.97	0.95	0.02
8	Example	0.60	0.97	0.42	0.018	0.004	0.005	0.29	0.30	5.96	0.95	0.02
9	Example	0.59	0.98	0.43	0.019	0.004	0.009	0.30	0.30	5.97	0.96	0.02
10	Comparative Example	0.60	0.96	0.41	0.018	0.005	0.017	0.30	0.30	5.96	0.95	0.02
11	Example	0.58	1.70	0.42	0.018	0.004	0.003	0.30	0.29	5.95	0.95	0.02
12	Comparative Example	0.59	0.99	1.10	0.019	0.004	0.003	0.30	0.30	5.96	0.95	0.02
13	Comparative Example	0.60	0.98	0.42	0.018	0.004	0.003	0.75	0.73	5.96	0.95	0.02
14	Example	0.58	0.98	0.43	0.019	0.004	0.003	0.30	0.30	5.97	1.70	0.02
15	Example	0.59	0.97	0.41	0.018	0.080	0.003	0.30	0.30	5.96	0.95	0.02
16	Example	0.60	0.99	0.42	0.019	0.004	0.003	0.29	0.28	5.95	0.96	0.02
17	Example	0.58	0.97	0.41	0.018	0.004	0.003	0.29	0.29	5.95	0.95	0.02
18	Example	0.60	0.98	0.40	0.019	0.004	0.003	0.30	0.30	5.96	0.96	0.02
19	Example	0.58	0.97	0.42	0.019	0.005	0.003	0.29	0.29	5.95	0.95	0.02
20	Example	0.59	0.98	0.41	0.018	0.004	0.003	0.29	0.30	5.97	0.95	0.02
21	Comparative Example	0.58	2.11	0.42	0.019	0.004	0.003	0.30	0.29	5.96	0.96	0.02
22	Comparative Example	0.60	0.98	2.02	0.018	0.005	0.003	0.30	0.30	5.97	0.96	0.02
23	Comparative Example	0.58	0.97	0.41	0.019	0.004	0.003	1.48	1.49	5.96	0.95	0.02
24	Comparative Example	0.60	0.99	0.42	0.019	0.004	0.003	0.29	0.30	5.95	0.19	0.19
25	Comparative Example	0.58	0.98	0.43	0.019	0.005	0.003	0.30	0.30	5.95	2.97	0.05
26	Comparative Example	0.59	0.98	0.41	0.018	0.004	0.003	0.29	0.28	5.96	0.96	0.02
27	Comparative Example	0.58	0.96	0.42	0.019	0.005	0.003	0.29	0.28	5.97	0.96	0.02
	Chemical Component (mass %)	[Mo] +	[Cr] ×
No.	Classification	V	Ti	Nb	Zr	Hf	Ta	Co	N	O	[W]/2	[C]	FP	AP
1	Comparative Example	0.25	0	0	0	0	0	0	0.0130	0.0015	1.22	18.03	5.92	0.81
2	Comparative Example	0.09	0	0.1	0	0	0	0	0.0068	0.0007	1.11	8.46	7.58	3.12
3	Comparative Example	0.20	0	0	0	0	0	0	0.0148	0.0014	1.21	1.24	14.98	18.16
4	Comparative Example	0.20	0	0	0	0	0	0	0.0165	0.0014	1.22	1.78	14.30	18.19
5	Comparative Example	0.32	0	0	0	0	0	0	0.0170	0.0015	0.94	3.52	3.98	2.53
6	Comparative Example	0.28	0	0	0	0	0	0	0.0162	0.0013	0.96	3.45	3.87	2.22
7	Example	0.28	0	0	0	0	0	0	0.0161	0.0014	0.96	3.46	3.89	2.19
8	Example	0.29	0	0	0	0	0	0	0.0165	0.0013	0.96	3.58	3.94	2.19
9	Example	0.28	0	0	0	0	0	0	0.0165	0.0014	0.97	3.52	3.98	2.23
10	Comparative Example	0.29	0	0	0	0	0	0	0.0162	0.0014	0.96	3.58	4.05	2.21
11	Example	0.28	0	0	0	0	0	0	0.0162	0.0013	0.96	3.45	4.04	2.19
12	Comparative Example	0.29	0	0	0	0	0	0	0.0161	0.0014	0.96	3.52	3.92	2.90
13	Comparative Example	0.28	0	0	0	0	0	0	0.0165	0.0014	0.96	3.58	3.90	4.86
14	Example	0	0	0	0	0	0	0	0.0162	0.0014	1.71	3.46	4.84	2.23
15	Example	0.28	0	0	0	0	0	0	0.0165	0.0014	0.96	3.52	3.90	2.21
16	Example	0.28	0.04	0	0	0	0	0	0.0162	0.0013	0.97	3.57	3.92	2.13
17	Example	0.29	0	0.1	0	0	0	0	0.0162	0.0014	0.96	3.45	3.91	2.15
18	Example	0.28	0	0	0.1	0	0	0	0.0162	0.0015	0.97	3.58	3.92	2.20
19	Example	0.29	0	0	0	0.1	0.1	0	0.0163	0.0014	0.96	3.45	3.91	2.16
20	Example	0.28	0	0	0	0	0	5.2	0.0161	0.0013	0.96	3.52	3.90	2.18
21	Comparative Example	0.28	0	0	0	0	0	0	0.0162	0.0014	0.97	3.46	4.14	2.19
22	Comparative Example	0.28	0	0	0	0	0	0	0.0165	0.0013	0.97	3.58	3.92	3.82
23	Comparative Example	0.29	0	0	0	0	0	0	0.0164	0.0014	0.96	3.46	3.92	9.32
24	Comparative Example	0.28	0	0	0	0	0	0	0.0165	0.0014	0.29	3.57	2.55	2.19
25	Comparative Example	0.28	0	0	0	0	0	0	0.0162	0.0013	3.00	3.45	7.97	2.23
26	Comparative Example	0.60	0	0	0	0	0	0	0.0164	0.0015	0.97	3.52	4.56	2.12
27	Comparative Example	0.29	0	0	0	0	0	0	0.0254	0.0013	0.97	3.46	3.94	2.13
In the above table, compar. stands for Comparative Example, and ex. stands for Example.

TABLE 2
		Maximum	Charpy Impact	Machined Surface	Cutting Tool Life	Maximum Rate of Dimensional
		Hardness	Value	Roughness Ra	ratio	Changes by Heat Treatment
No.	Classification	HV	J	mm	(reference: No. 1)	%
1	Comparative Example	690	10	1.15	1	0.15
2	Comparative Example	720	13	0.85	1.5	0.11
3	Comparative Example	685	22	0.52	3.5	0.08
4	Comparative Example	710	17	0.66	3.8	0.10
5	Comparative Example	700	15	0.20	4.5	0.09
6	Comparative Example	720	19	0.19	4.4	0.07
7	Example	723	27	0.21	4.7	0.06
8	Example	722	26	0.23	4.6	0.06
9	Example	724	27	0.29	4.5	0.05
10	Comparative Example	726	30	0.41	4.6	0.07
11	Example	732	21	0.27	4.5	0.05
12	Comparative Example	717	22	0.28	3.9	0.10
13	Comparative Example	720	25	0.29	3.6	0.12
14	Example	728	21	0.38	4.3	0.07
15	Example	720	20	0.18	5.0	0.08
16	Example	708	38	0.33	4.5	0.06
17	Example	710	38	0.34	4.3	0.07
18	Example	705	36	0.37	4.3	0.06
19	Example	709	30	0.38	4.2	0.06
20	Example	719	34	0.29	4.7	0.07
21	Comparative Example	725	15	0.25	4.8	0.08
22	Comparative Example	727	14	0.28	3.5	0.09
23	Comparative Example	726	15	0.25	3.9	0.10
24	Comparative Example	645	18	0.23	4.7	0.05
25	Comparative Example	742	15	0.39	4.2	0.06
26	Comparative Example	716	14	0.45	4.1	0.06
27	Comparative Example	715	17	0.30	4.5	0.06
Acceptance Criteria	≧650	≧20	≦0.4	≧4	≦0.08

As listed in Table 1 and Table 2, each of Nos. 7 to 9, 11 and 14 to 20 as Examples of the invention, which satisfies all of the requirements of the inventions for the contents of individual chemical components, the product of content of C and content of Cr, the parameter associated with ferrite-forming elements and the parameter associated with austenite-forming elements, had all of its maximum hardness, Charpy impact value, machined surface roughness, cutting tool life and maximum rate of dimensional changes by heat treatment within the ranges of their respective acceptance criteria. By contrast, each of Nos. 1 to 6, 10, 12 to 13 and 21 to 26 as Comparative Examples for the invention, which does not satisfy at least one of the requirements of the invention, missed meeting at least one of the acceptance criteria, and had some problem.

Although it had some problem by missing out one or two or more of the requirements of the invention, each of Nos. 1 to 6, 10, 12 to 13 and 21 to 26 as Comparative Examples was chosen as Comparative Example to distinguish one of the requirements from another. In the following, each Comparative Example is explained in relation to some of the requirements specified in the invention.

No. 1 and No. 2 are Comparative Examples in which both of the content of C and the content of Cr are too high, and conversely, No. 3 and No. 4 are Comparative Examples in which both of the content of C and the content of Cr are too low. All the Comparative Examples in which those contents and too high and too low were outside all or some of the acceptance criteria for the Charpy impact value (toughness), the machined surface roughness, the cutting tool life and the maximum rate of dimensional changes by heat treatment.

No. 21 is Comparative Example in which the content of Si is too high, and conversely, No. 1 is Comparative Example in which the content of Si is too low. No. 21 in particular, in which the content of Si is too high, was greatly reduced in toughness, and the Charpy impact value thereof was outside the acceptance criterion. In addition, dimensional changes after heat treatment, though within the range of the acceptance criterion, were relatively large.

No. 22 is Comparative Example in which the content of Mn is too high. In this Comparative Example, the toughness was greatly reduced, and the Charpy impact value was outside the acceptance criterion. In addition, the cutting tool life and the maximum rate of dimensional changes by heat treatment were also outside their individual acceptance criteria.

No. 10 is Comparative Example in which the content of Al is too high, and conversely, No. 6 is Comparative Example in which the content of Al is too low. In No. 10 as the Comparative Example in which the content of Al is too high, plucking occurred when finish working was carried out with a ball end mill, and thereby the accuracy of worked-machined surface was aggravated. On the other hand, No. 6 as the Comparative Example in which the content of Al is too low, the Charpy impact value was outside the acceptance criterion.

No. 23 is Comparative Example in which the content of Cu is too high, and conversely, No. 5 is Comparative Example in which the content of Cu is too low. In No. 23 in which the content of Cu is too high, toughness was reduced, and the Charpy impact value thereof was outside the acceptance criterion. In addition, the cutting tool life and the maximum rate of dimensional changes by heat treatment were also outside their individual acceptance criteria. On the other hand, in No. 5 as the Comparative Example in which the content of Cu is too low, the Charpy impact value and the maximum rate of dimensional changes by heat treatment were also outside the acceptance criteria.

No. 23 is Comparative Example in which the content of Ni is too high, and conversely, No. 1 is Comparative Example in which the content of Ni is too low. In No. 23 in which the content of Ni is too high, its Charpy impact value and maximum rate of dimensional change by heat treatment were outside their individual acceptance criteria. In addition, the cutting tool life was also outside their individual acceptance criteria.

No. 24 is Comparative Example in which the numerical value determined from Mo+0.5W is too small, and No. 25 is the case in which, although it lies in the range specified in the invention, the numerical value falls on the upper limit of 3.0%. In No. 24, the maximum hardness and the Charpy impact value were outside their individual acceptance criteria. On the other hand, in No. 25, the Charpy impact value was reduced though it was also influenced by missing out other requirements.

No. 26 is Comparative Example in which the content of V is too high. In this Comparative Example of No. 26, since the content of V was too high, the toughness was reduced, and the Charpy impact value was outside the acceptance criterion. In addition, the machined surface roughness was outside the acceptance criterion.

No. 1 and No. 2 are Comparative Examples in which the product of the content of C and the content of Cr is too great. Under this influence, in No. 1 and No. 2, cutting tool life was seriously reduced and the dimensional changes after heat treatment became great.

No. 27 is Comparative Example in which the content of Ni is too high. As a result, the toughness was lowered, and the Charpy impact value was outside the acceptance criterion.

Nos. 1 to 4 and No. 25 are Comparative Examples in which the parameter associated with the ferrite-forming elements is too great. Under this influence, in Comparative Examples, toughness balance was lost and the accuracy of worked-machined surface was aggravated. In No. 25 in particular, in which only this requirement was outside, toughness was seriously reduced, and the Charpy impact value thereof was outside the acceptance criterion.

Nos. 2 to 5, 12, 13, 22 and 23 are Comparative Examples in which the parameter associated with the austenite-forming elements is too great. Under this influence, in Comparative Examples, the residual austenite was increased, and thereby not only the rate of dimensional change by heat treatment was increased but also the tool life under cutting was shortened. In No. 12 and No. 13 in particular, in which only this requirement was outside, the cutting tool life and the maximum rate of dimensional changes by heat treatment were outside the acceptance criteria.

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. 2008-003524 filed on Jan. 10, 2008, and their contents are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The use of the cold-work die steel of the invention as a material for dies for cold pressing allows provision of dies for cold pressing, which each have not only basic properties required, such as hardness, toughness and dimensional change by heat treatment, but also adaptability to various types of hard coating treatments, and besides, which cause no problems in terms of roughness of machined surface and cutting tool life. Moreover, the dies for cold pressing made by use of the cold-work die steel are particularly suitable for use in forming high tensile steel plates having tensile strength of 590 MPa or more.

Claims

1. A cold-work die steel comprising:

0.5 to 0.7 mass % of C;

5.0 to 7.0 mass % of Cr;

0.5 to 2.0 mass % of Si;

0.1 to 2.0 mass % of Mn;

0.001 to 0.010 mass % of Al;

0.25 to 1.00 mass % of Cu;

0.25 to 1.00 mass % of Ni;

0.003 to 0.025 mass % of N;

more than 0 to 0.05 mass % of P;

more than 0 to 0.1 mass % of S;

more than 0 to 0.005 mass % of O; and

at least one of Mo and W,

wherein the following requirements are satisfied: 0.5≦[Mo]+0.5×[W]≦3.0; [C]×[Cr]≦4; FP=[Si]/5+[Cr]/5+2×[Mo]+[W]+2×[V]+10×[Al]≦55.0; and AP=[Mn]+3×([Cu]+[Ni])≦2.5;

wherein FP is a parameter associated with the ferrite-forming elements, AP is a parameter associated with the austenite-forming elements, and the bracket represents a content (mass %) of an element written therein.

2. The cold-work die steel according to claim 1, further comprising more than 0 to 0.5 mass % of V.

3. The cold-work die steel according to claim 1, further comprising at least one element selected from the group consisting of Ti, Zr, Hf, Ta and Nb in a total content of more than 0 to 0.5 mass %.

4. The cold-work die steel according to claim 1, further comprising more than 0 to 10 mass % of Co.

5. A die for cold pressing, which is manufactured by working the cold-work die steel according to claim 1 and giving surface treatment thereto.

6. The cold-work die steel according to claim 2, further comprising at least one element selected from the group consisting of Ti, Zr, Hf, Ta and Nb in a total content of more than 0 to 0.5 mass %.

7. The cold-work die steel according to claim 2, further comprising more than 0 to 10 mass % of Co.

8. The cold-work die steel according to claim 3, further comprising more than 0 to 10 mass % of Co.

9. The cold-work die steel according to claim 6, further comprising more than 0 to 10 mass % of Co.

10. A process of making a die comprising working the cold-work die steel according to claim 1, and giving surface treatment thereto.