WO2008047806A1

WO2008047806A1 - Cold work die steel, die, and method for production of cold work die steel

Info

Publication number: WO2008047806A1
Application number: PCT/JP2007/070197
Authority: WO
Inventors: Shogo Murakami; Tsuyoshi Tonomura
Original assignee: Kabushiki Kaisha Kobe Seiko Sho; Nippon Koshuha Steel Co., Ltd.
Priority date: 2006-10-17
Filing date: 2007-10-16
Publication date: 2008-04-24
Also published as: TW200831682A; US20100074792A1

Abstract

Disclosed is a cold work die steel which comprises (by mass): C: 0.20-0.60%, Si: 0.5-2.00%, Mn: 0.1-2%, Cr: 3.00-9.00%, Al: 0.3-2.0%, Cu: 1.00-5%, Ni: 1.00-5%, Mo: 0.5-3% and/or W: 2% or less (including 0%), and S: 0.10% or less (excluding 0%) wherein these components satisfy the following requirements (1) to (3) [wherein each square bracket [ ] means a content (%) of each element]: (1) [Cr] x [C]≤3.00; (2) [Cu]/[Ni]: 0.5-2.2; and (3) [Mo]+0.5 x [W]: 0.5-3.0%, with the remainder being iron and unavoidable impurities. Also disclosed is a die produced by using the steel. Further disclosed is a method for producing the cold work die steel.

Description

Specification

COLD MOLD STEEL AND MOLD, AND METHOD FOR PRODUCING COLD MOLD STEEL

Technical field

TECHNICAL FIELD [0001] The present invention relates to cold mold steel and molds, and a method for producing cold mold steel, and more specifically, cold-warm press forming of steel sheets for automobiles and steel sheets for home appliances. The present invention relates to a mold steel useful as a mold material used for punching, bending, drawing, trimming, and the like, and a method for manufacturing the mold steel.

Background art

[0002] Molds used for forming automobile steel plates, home appliance steel plates, and the like are required to have an improved service life as the strength of the steel plates increases. In particular, for steel sheets for automobiles, the demand for high-tensile steel sheets with a tensile strength of about 590 MPa or more is increasing rapidly in order to improve automobile fuel efficiency in consideration of environmental problems. There is a problem that occurs when the tool life is drastically reduced due to damage to the die, resulting in an extremely short die life.

[0003] A mold is composed of a mold base material (mold steel) and a surface hardened layer (surface coating) applied to the surface thereof. The base metal mold steel is generally obtained by annealing → cutting → quenching / tempering (in this specification, quenching is called solution treatment and tempering is called aging). Manufactured.

[0004] As mold steel (cold die steel), high C high Cr alloy tool steel represented by JIS SKD11 and high speed represented by IS SKH51 with improved wear resistance. Tool steel has been widely used. In these tool steels, hardness is mainly improved by precipitation hardening of Cr-based carbides and Mo, W, and V-based carbides. In addition, for the purpose of improving both wear resistance and toughness, low alloy high speed tool steel (usually called matrix high speed steel) with reduced alloy content such as C, Mo, W and V of JIS SKH51 is also used. Has been.

[0005] With the aim of further improving the properties of cold mold steel, for example, Patent Documents 1 to 3 propose techniques for improving the components in steel.

[0006] Patent Document 1 has been proposed to further improve the hardness of the matrix high speed. In this case, a large amount of Nb and / or Ta is contained, and by suppressing the coarsening of crystal grains when subjected to high-temperature quenching, high-temperature quenching is possible and higher hardness (improved wear resistance) is achieved. The method of aiming at is described.

[0007] Patent Document 2 relates to a cold die steel that achieves a size change suppressing characteristic and a high hardness characteristic,

(A) The expansion change during tempering caused by the decomposition of retained austenite during quenching is offset by the effect of suppressing the change due to precipitation strengthening of Ni-Al intermetallic compounds. (Ii) Predetermined steel components It has been disclosed that the segregation index K calculated by FIG. 1 of Patent Document 2 shows that tempering is performed at a temperature at which the maximum hardness is obtained.

[0008] Patent Document 3 describes that the amount of dimensional change (size change) due to quenching and tempering treatment, in particular, the expansion change during tempering can be suppressed, and an appropriate amount of Ni or A1 for the purpose of increasing hardness. And cold die steel with the appropriate amount of Cu added is disclosed. Moreover, it is described that galling resistance is improved by adjusting the C and Cr contents and finely dispersing the carbide distribution in the structure.

[0009] On the other hand, in Patent Document 4, for the purpose of reducing the die manufacturing cost, cutting is performed from a quenching and tempering state, in which a quenching and tempering process is not performed after a conventional cutting process. The technique of “pre-hardened steel” to be performed (quenching and tempering → cutting) is disclosed. Specifically, as a steel that can exhibit good machinability even at high hardness and can be punched in the cold, especially pre-hardened steel with appropriately controlled contents of C, Si and S It is disclosed. However, at present, the life of molds using pre-hardened steel is short and has not been put into practical use.

[0010] The above-mentioned patent documents;! To 4 mainly suppress the size change (size change) after heat treatment (after aging treatment or tempering treatment) by controlling the components in steel. Patent Documents 5 to 7, which will be described later, mainly disclose techniques for suppressing deformation by controlling heat treatment conditions such as quenching and tempering! /.

[0011] Among these, Patent Document 5 discloses a method of suppressing dimensional changes after quenching and tempering by performing low temperature tempering at 150 to 450 ° C and high temperature tempering at 480 to 550 ° C at least once. It is disclosed. Patent Document 6 discloses a method of performing quenching → sub-zero treatment at 0 to −200 ° C. → low temperature tempering at 500 ° C. or lower. Specifically, a method is described in which the sub-zero treatment is performed at the above temperature, the dimensional change is controlled by adjusting the amount of retained austenite, and then the low-temperature tempering is performed to achieve the target dimension.

[0013] Patent Document 7 discloses a method of realizing a predetermined hardness by increasing the hardenability by adjusting the components in steel and controlling the cooling rate during quenching by pearlite nose and gas cooling. As a result, heat treatment distortion is reduced while ensuring the necessary hardness for the mold.

Patent Document 1: Japanese Patent Laid-Open No. 10-330894

Patent Document 2: JP 2006-152356 A

Patent Document 3: Japanese Unexamined Patent Publication No. 2006-169624

Patent Document 4: JP-A-2002-241894

Patent Document 5: JP-A-9-125204

Patent Document 6: Japanese Patent Laid-Open No. 200-172748

Patent Document 7: Japanese Patent Laid-Open No. 2002-167644

Disclosure of the invention

Problems to be solved by the invention

[0014] The properties required for the cold mold steel include not only the above-described increase in hardness and the ability to suppress change in size after heat treatment, but also excellent weld repairability.

[0015] Welding repair is performed mainly for the purpose of correcting and repairing damage to the mold (specifically, unevenness of the hardened surface layer) by welding and reusing the mold. For example, overlay welding by argon welding is widely used. As mentioned above, the tensile strength is about

Due to the drastic decrease in mold life due to an increase in demand for high-tensile steel over 580MPa, welding repairs to the mold are frequently performed to reduce costs.

[0016] However, when welding repair is performed on a mold having a hardened film, the hardness variation around the welded portion increases, and cracks and galling are likely to occur. In particular, the heat affected zone (Heat Affected Zone, HAZ) softening (HAZ softening) after welding is prominently seen, so a reduction in die life after welding repair is a problem. HAZ softening is applied to the bond part (welded metal and base metal The boundary, also called “weld melt line”. This is a phenomenon seen in a region slightly away from ()). In this region, transformation occurs from fine-grained austenite where the heating temperature is lower than that of the bond part, so the hardenability is reduced and the fraction of the soft ferrite phase increases. The further distance is tempered at high temperature, which is considered to reduce the hardness. Fig. 1 (a) is a diagram schematically showing how the base metals are welded together with weld metal, and Fig. 1 (b) schematically shows the hardness distribution in region A shown in Fig. 1 (a). Is shown. As shown in Fig. 1 (b), the HAZ hardness decreases and becomes softer as the distance from the bond portion increases. When the HAZ is softened, even if a surface hardening treatment is subsequently applied, the protective effect of the surface hardening layer is not fully exhibited, and the surface hardening layer is damaged early, and the life of the mold is reduced.

[0017] As described above, the welding repair may be performed after the surface hardening film is applied to the base material, or before the surface hardening film is applied to the base material. In particular, when press-molding high-tensile steel sheets with a tensile strength of about 590 MPa or more using a mold, it is difficult to press into the desired shape, so trial testing and welding repair (overlay welding) are required in advance. To obtain a desired shape. In the trial punching process, after welding repairs, press forming is performed without heat treatment, so wrinkles tend to occur in the HAZ softened part. Such wrinkles generated in the HAZ softened part also remain in the surface film formed by the subsequent hardening process, so this residual part is considered to be the starting point of film damage. Also, not only the HAZ softened part but also the hardened part is generated (see Fig. 1 and Fig. 7), and cracks and chipping occur in the hardened part, causing trouble immediately.

[0018] Accordingly, there is an urgent need to provide a mold steel with excellent weld repairability that can suppress HAZ softening during welding repair, and can easily perform overlay welding of corner portions. However, none of the above-mentioned patent documents considers weld repairability, and there is a concern that the mold life after weld repair will be reduced.

[0019] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel for cold molds that has high hardness, is excellent in restraining deformation after heat treatment, and has good weld repairability. And to provide molds.

[0020] Another object of the present invention is to provide a production method for efficiently obtaining a steel for a cold mold which has high hardness and is excellent in restraining to change in size after heat treatment. Means for solving the problem

That is, the present invention relates to the following;

1. By mass%

C: 0.20-0.60%,

Si: 0.5-2.00%,

Mn: 0.;! ~ 2%

Cr: 3.00—9.00%,

A1: 0.3—2.0%,

Cu: l.00—5%,

Ni: l.00—5%,

Mo: 0.5 to 3% and / or W: 2% or less (including 0%),

S: 0.10% or less (excluding 0%),

The following (1) to (3) {[] means the content (%) of each element. }

(1) [Cr] X [C] ≤3.00,

(2) [Cu] / [Ni]: 0.5 ~ 2 ・ 2,

(3) [Μο] + 0.5Χ [W]: 0.5—3.0%

Satisfy the requirements of

Remainder: Steel for cold mold, which is iron and inevitable impurities.

2. Further, for cold mold as described in 1 above, further containing V: 0.5% or less (excluding 0%)

3. Further, the cooling according to the above 1 or 2, containing at least one element selected from the group force consisting of Ti, Zr, Hf, Ta, and Nb in total of 0.5% or less (excluding 0%) Steel for molds.

4. Furthermore, the steel for cold mold as described in any one of 1 to 3 above, further comprising Co: 10% or less (not including 0%).

5. Martensitic transformation point (Ms point) expressed by the following formula:

Ms point

= 550-361 X [C] -39X [Mn] -35X [V] -20X [Cr] -17X [Ni] -10X [Cu] -5X ([Mo] + [W])

+ 15X [Co] + 30X [Al]

{In the formula, [] represents the content (%) of each element. }

The steel for cold molds as described in any one of 1 to 4 above, which has a temperature of 170 ° C or higher.

6. A mold obtained using the cold mold steel according to any one of 1 to 5 above.

7. Steel that satisfies the composition described in 1 above, and the following (4) {[] means the content (%) of each element. }

(4) [Cu] / [C]: 4.0 ~; 15

A process of preparing steel that satisfies the requirements of

A step of solution treatment and aging treatment under conditions satisfying the following formula (5);

TA-10≤T2≤TA + 10 (5)

Where

TA = 0.29XT1-2. 63X [Cu] / [C] + 225,

T1 is the solution temperature (° c),

T2 means aging temperature (° C),

A method for producing steel for cold molds including:

8. The manufacturing method according to 7 above, wherein the steel contains V: 0.5% or less (not including 0%).

9. The steel according to the above 7 or 7 containing a total of at least one element selected from the group force consisting of Ti, Zr, Hf, Ta, and Nb of 0.5% or less (excluding 0%) 8. The production method according to 8.

10. The manufacturing method according to any one of 7 to 9 above, wherein the steel contains Co: 10% or less (not including 0%).

11. Martensitic transformation point (Ms point) expressed by the following formula:

Ms point

= 550-361 X [C] -39X [Mn] -35X [V] -20X [Cr]

-17X [Ni] -10X [Cu] -5X ([Mo] + [W])

+ 15X [Co] + 30X [Al] 11. The production method according to any one of 7 to 10 above, which is 170 ° C. or higher.

12. A mold obtained by the production method according to any one of 7 to 11 above. The invention's effect

[0022] In the cold mold steel of the present invention, the alloy components are appropriately controlled as described above. Therefore, the hardness is high, the deformation resistance after heat treatment is excellent, and the weld repairability is also good. . Therefore, the mold obtained by using the above cold mold steel is particularly suitably used as a mold for forming a high-tensile steel sheet having a tensile strength of about 590 MPa or more, and has a long life, particularly after welding repair. Life is further increased.

[0023] Further, in the production method of the present invention, the components in the steel and the conditions of the solution treatment and the aging treatment are appropriately controlled! /. Excellent cold mold steel can be produced efficiently. Therefore, the mold obtained using the production method of the present invention is particularly suitably used as a mold for forming a high-tensile steel sheet having a tensile strength of about 590 MPa or more, and has a longer life, particularly after welding repair. Enhanced.

Brief Description of Drawings

[0024] [Fig. 1] Fig. 1 is a diagram schematically showing a state in which the base metals are welded to each other with a weld metal. Fig. 1 (a) is a cross-sectional view of a welded portion, and Fig. 1 (b ) Is a diagram schematically showing the hardness distribution of region A shown in FIG. 1 (a).

[Fig. 2] Fig. 2 (a) is an optical micrograph showing the condition of galling on the surface of a mold that uses JIS SKD11 as the mold steel and has a TiN coating on it. ) And Fig. 2 (c) are optical micrographs showing an enlarged part, and Fig. 2 (d) is an optical micrograph of the mold base material before the TiN film is applied.

FIG. 3 (a) is a schematic view showing the shape of a welding test piece used in the examples, and FIG. 3 (b) is an enlarged sectional view of a groove.

FIG. 4 is a schematic view schematically showing a state of a test piece subjected to buttering.

FIG. 5 is a schematic view showing the shape of a Charpy impact test piece used in the examples.

FIG. 6 is a graph showing the relationship between the [Cu] / [Ni] ratio and the HAZ softening width.

FIG. 7 is a graph showing a profile of hardness distribution.

[0025] [Fig. 8] Fig. 8 is a graph showing the relationship between the ratio of [Cu] / [C] and the change rate (average value, maximum value). [FIG. 9] FIG. 9 is a diagram schematically showing the influence of aging treatment on hardness and dimensional change (size change rate).

[FIG. 10] FIG. 10 is a diagram schematically showing the influence of aging treatment on the amount of change in size.

BEST MODE FOR CARRYING OUT THE INVENTION

[0026] Hereinafter, the present invention will be described in detail. Note that percentages in this specification are based on mass unless otherwise indicated. Also, all percentages defined by mass are identical to those defined by weight, respectively.

First, the cold mold steel according to the first aspect of the present invention will be described in detail.

[0027] Among the various properties required for cold mold steel, the present inventor has, among other things, a cold mold having improved properties of hardness, resistance to deformation after heat treatment, and weld repairability. In order to provide steel, we first investigated the cause of galling due to damage to the surface film of the mold using conventional JIS SKD11 and matrix high speed steel.

[0028] Fig. 2 (a) is an optical micrograph showing a state in which galling occurs on the surface of a mold having JIS SKD11 as a mold steel and having a TiN film formed thereon. b) and Fig. 2 (c) are optical micrographs of a part of it. For reference, Fig. 2 (d) also shows an optical micrograph of the mold base material before the TiN film is applied. In Fig. 2 (d), the white parts are Cr-based carbides. As is clear from Fig. 2 (b) and Fig. 2 (c), the area where the film was peeled off was hard and coarse Cr-based carbide (carbonization of about 1 to 50 m, mainly containing Cr and Fe. It can be seen that the product is precipitated on the surface and cracks are generated starting from the carbide.

[0029] From the above observation results, the present inventor has found that the starting point of galling is the coarse Cr-based carbide described above, and if the generation of the carbide is suppressed (not generated) as much as possible, peeling of the surface film is prevented. It was possible to improve the life of the mold.

[0030] Based on the above findings, the present inventors have further studied. As a result, in order to suppress the formation of coarse carbides and improve the characteristics described above, the amount of C should be controlled appropriately.

It was found that it is extremely important to actively add various alloy components and to properly control the alloy component design. Specifically, in order to obtain the desired properties, alloy components (particularly Al, Cu, Ni, Mo, etc.) that are not intended to increase hardness by controlling carbides as in the past. It is effective to increase the hardness by precipitation hardening of alloy elements by actively adding (w), mainly precipitation hardening by Al-Ni intermetallic compounds and formation of carbides of Mo, W and C. As a result of further experimentation, the present inventors have reached the configuration of the present invention.

In the present specification, “high hardness” means a material having a maximum hardness of 650 HV or higher when the maximum hardness is measured by the method described in the column of Examples described later.

[0032] In the first embodiment of the present invention, the "size after heat treatment (dimensional change rate)" is the average of the three directions of thickness, width, and length before and after aging treatment. Evaluation is based on both the value and the difference between the maximum and minimum values. For convenience of explanation, the former is referred to as “average size change rate” and the latter is referred to as “change in rate of change”. Thus, in the first aspect of the present invention, the change in size after heat treatment is evaluated using both the “average value of change rate” and the “difference in change rate”. This is different from the technique of Patent Document 2 in which only the former (the average value of the change rate) is measured. According to the experiment results of the present inventor, it is not enough to reduce the average value of the rate of change as in Patent Document 2 in order to sufficiently suppress the size change after heat treatment. It is indispensable to reduce the size change (variation) in all directions, and even if the average value of the rate of change is suppressed, the difference in size change rate may increase (and vice versa). (See examples below). In the first aspect of the present invention, “the size change after heat treatment is small! /” (Excellent in size change suppression) J is based on the method described in the column of Examples described later. When the dimensional change before and after heat treatment is measured, it means that the average value of the change rate is within ± 0.05% and the difference of change rate is 0.08% or less.

[0033] In the present specification, "/ weld repairability" is evaluated based on the HAZ softening width. “Excellent weld repairability” means that the HAZ softening width is measured within the range of 6.5 mm or less when measured by the method described in the column of Examples described later.

[0034] The components in the steel according to the first aspect of the present invention are as described in detail below, and the following formula is obtained only by controlling the contents of various alloy elements contributing to precipitation hardening within a predetermined range. As shown in (1) to (3), the balance with a predetermined element is also appropriately controlled, thereby improving the above characteristics. As shown in the examples described later, those which do not satisfy any of these requirements cannot secure desired characteristics. In particular, the present invention Therefore, it is indispensable to add all of Cu, Ni, and Al. For example, in steels having components that do not contain either one of them as described in Patent Document 1 and Patent Document 3, the desired effect is obtained. I can't get the result! /, I confirmed by experimenting! /, I'll (see the examples below).

[0035] Here, the components in the steel according to the first aspect of the present invention are classified into "weld repairability" (evaluated by the HAZ softening width) and "after heat treatment" which are the main improvement targets in the first aspect of the present invention. When organized in relation to the “restraint suppression” (evaluated by both the longitudinal change rate and the difference between the change rates), it is generally as follows.

[0036] First, in order to improve weld repairability (reduce the HAZ softening width), the upper limit of [Cr] X [C], Ms point (lower limit), C amount (lower limit), A1 amount ( Lower limit), Ni amount (lower limit), [Cu] / [Ni] (upper limit, lower limit), [Mo] + 0.5 X [W] (lower limit), V amount (upper limit) can be controlled appropriately is important. That is, as a design guideline for reducing the HAZ softening width, the amount of C to be reduced by hardening by martensite generation is set to about 0.2 to 0.60%, and the alloy composition (mainly Al , Cu, Ni, Mo, W) are used for precipitation hardening (eg, ε_Cu, Ni_Al intermetallic compounds, Ni-Mo intermetallic compounds). Since these precipitates are finely matched and precipitated in the matrix, the hardness is remarkably increased.

[0037] In particular, Cu, Ni, and Al are important as precipitation hardening elements, and are elements that greatly contribute to the suppression of HAZ softening. Experiments have confirmed that steels that do not substantially contain any of these elements do not have the desired HAZ softening suppression effect.

[0038] Furthermore, the ratio of [Cu] / [Ni] (ratio of [Cu] to [Ni]) is closely related to the suppression of HAZ softening, and the above ratio should be appropriately controlled. It was found that the softening of HAZ can be suppressed. FIG. 6 is a graph showing the influence of the ratio of [Cu] / [Ni] on the HAZ softening width when the HAZ softening width is measured by the method described in the examples described later. This draft plots the results of Nos. 7, 8, and 10 in Table 3 and Nos. 31-35 and 37 in Table 4 below. As shown in Fig. 6, the ratio of [Cu] / [Ni] has a close relationship with the HAZ softening width, and by controlling the above ratio within the range of 0.5 to 2.2. It can be seen that the HAZ softening width can be suppressed within the range specified in the present invention (6.5 mm or less).

[0039] On the other hand, in order to make the dimension after heat treatment as small as possible, the product of Cr and C content (upper limit of [Cr] X [C]), C amount (upper limit), Si amount ( Upper limit), Mn amount (upper limit), Ms point (lower limit), A It is important to appropriately control 1 amount (upper limit), Ni amount (upper limit), Cr amount (upper limit), and [Mo] + 0.5 X [W] (upper limit). Since the present invention is based on low C, the content of alloy components such as Cu, Ni, and A1 is appropriately controlled in addition to the fact that the amount of retained austenite is originally low due to the high Ms point. Therefore, in particular, expansion and contraction after aging treatment at about 400 to 550 ° C and after surface hardening treatment can be remarkably suppressed. This is about 400-500 if you don't need to use the above alloy components. Ε_Cu force is mainly about 450 to 530 in the low temperature range of C. Ni- (A1, Mo) -based intermetallic compounds are mainly produced in the intermediate temperature range of C, and Mo-V-based carbides are mainly produced in the high temperature range of about 500 to 550 ° C. The crystal structure of these precipitates ( Since the FCC structure is different from the matrix (BCC structure), the volume is shrunk, which is considered to contribute to the suppression of the size change after heat treatment. Further, in the present invention, since the composition design is such that coarse Cr-based carbides are not precipitated as much as possible, the crystal structure is isotropic in any direction, and even in the manufacture of a large complex mold. It is considered that the size change after heat treatment can be effectively suppressed.

[0040] Hereinafter, the components in steel in the first embodiment of the present invention will be described.

[0041] C: 0.20—0.60%

C is an element that ensures hardness and wear resistance and contributes to the suppression of the HAZ softening width. In addition, when a carbide film such as VC or TiC is formed on the surface of the mold base material by the CVD method, there is a problem that the film thickness becomes thin if the C concentration is low. Taking these into consideration, the lower limit of the C content was set to 0.20% in order to effectively exert the above-described effects. The amount of C is preferably 0.22% or more. However, if added in excess, the retained austenite increases and the desired hardness cannot be obtained unless high-temperature aging treatment is performed. 60%. The amount of C is preferably 0.50% or less, and is preferably 0.45% or less.

[0042] Si: 0.5 to 2.00%

Si is useful as a deoxidizing element during steelmaking, and is an element that contributes to improving hardness and securing machinability. In addition, Si suppresses the temper softening of martensite in the matrix and is useful for suppressing the HAZ softening width. In order to effectively exhibit such an effect, the lower limit of the Si amount is set to 0.5%. However, if added excessively, segregation increases and the size change after heat treatment increases. Therefore, the upper limit was made 2.00%. The lower limit of the Si amount is 1%, preferably S. 1.2% is more preferred, while the upper limit of the Si amount is preferably 1.85%.

[0043] Mn: 0.;! ~ 2%

Mn is an element useful for ensuring hardenability. When excessively added, the Ms point is significantly lowered and the retained austenite increases. Therefore, the desired hardness can be obtained without high-temperature aging treatment. Absent. Taking these into consideration, the Mn content was set within the above range. The lower limit of the Mn content is preferably 0.15%, while the upper limit of the Mn content is preferably 1%, more preferably 0.5%, and still more preferably 0.35%.

[0044] Cr: 3. 00—9.0%

Cr is an element useful for ensuring a predetermined hardness. If the Cr content is less than 3.00%, the hardenability is insufficient and a part of bainite is generated, so that the hardness decreases and the strength S cannot be ensured. The Cr content is preferably 3.5% or more, and preferably 4.0% or more. However, if added excessively, a large amount of coarse Cr-based carbides are formed, shrink after heat treatment, and the durability of the film decreases, so the upper limit was made 9.00%. The Cr content is preferably 7.0% or less, more preferably 6.5% or less, and even more preferably 6.0% or less.

[0045] Α1: 0. 3〜2 · 0%

Al is intended to improve hardness by precipitation strengthening of Al_Ni intermetallic compounds such as Ni A1.

Three

It is a necessary element for the metal and contributes to the suppression of the HAZ softening width. A1 is also useful as a deoxidizer. Taking these into account, the lower limit of A1 was set to 0.3%. However, if added in excess, segregation increases, dimensional change after heat treatment (especially the difference in change rate) increases, and toughness decreases, so the upper limit was made 2.0%. The amount of A1 is preferably 0.50% or more and 1.8% or less, and more preferably 0.7% or more and 1.6% or less.

[0046] Cu: l. 00 ~ 5%

Cu is an element necessary for improving the hardness by precipitation strengthening of ε-Cu and contributes to the suppression of the HAZ softening width. However, if excessively added, forging cracks are likely to occur, so the upper limit was made 5%. The amount of Cu is preferably 2.0% or more and 4.0% or less. [0047] Ni: l. 00—5%

Ni is intended to improve hardness by precipitation strengthening of Ni-based intermetallic compounds such as Ni Al.

Three

It is a necessary element for the metal and contributes to the suppression of the HAZ softening width. Ni can also be used in combination with Cu to suppress hot brittleness caused by excessive addition of Cu and to prevent cracking during forging. However, if added excessively, retained austenite increases, and if it is not aged at a high temperature, it will expand after heat treatment. The amount of Ni is preferably 1.5% or more and 4.0% or less.

[0048] Mo: 0.5-3% and / or W: 2% or less (including 0%)

Both Mo and W form MC type carbides and Ni-Mo intermetallic compounds.

6 3

It is an element that forms an object and contributes to precipitation strengthening. However, if Mo or W is added excessively, the above carbides and the like are generated excessively, leading to a reduction in toughness and increasing the size after heat treatment (especially the difference in size change rate). A range was set. In the present invention, Mo is an essential component and W is a selective element, but both may be used in combination. Mo is preferably 0.5% or more and 3% or less, and more preferably 0.7% or more and 2.5% or less. Further, W is preferably 2% or less, and more preferably 1.5% or less.

[0049] S: 0. 10% or less (excluding 0%)

S is an element useful for ensuring machinability. If it is added in excess of force, weld cracking occurs, so the upper limit was made 0.10%. The amount of S is preferably 0.07% or less, more preferably 0.05% or less, and still more preferably 0.025% or less.

[0050] Furthermore, in the first aspect of the present invention, it is necessary to satisfy the following requirements (1) to (3) {[] means the content (%) of each element. . }.

[0051] (1) [Cr] X [C] ≤3.0

The above (1) is set for the purpose of suppressing the formation of coarse Cr-based carbides. If the product of [C r] and [C] exceeds 3.00, the size change after heat treatment is large. As a result, the durability of the surface film decreases. The product of [Cr] and [C] is 1. 80 or less force S, preferably 1. 70 or less than force S. The lower limit is better from the viewpoint of suppressing the size change after the heat treatment, but considering the fact that the above effect is effectively exhibited by the addition of Cr and C, it is generally 0.8. It is preferable that [0052] (2) [Cu] / [Ni]: 0 · 5〜2 · 2

The above (2) is mainly set as a parameter for suppressing the HAZ softening width by utilizing precipitation strengthening of ε-Cu (see the examples described later). In order to effectively demonstrate such effects, the ratio of [Cu] to [Ni] was set to 0.5. However, if the ratio is increased, forging cracks occur, so the upper limit was set to 2.2. The ratio is preferably 0.7 or more and 1.5 or less, more preferably 0.85 or more and 1.2 or less.

[0053] (3) [Mo] + 0.5 X [W]: 0.5 to 3 · 0%

As described above, Mo and W constituting the above (3) are elements contributing to precipitation strengthening, and the above (3) is mainly used as a parameter for ensuring hardness improvement by precipitation strengthening. It is effective for suppressing the HAZ softening width. In (3) above, the [W] coefficient (0.5) was determined taking into account that the atomic weight of Mo is about half that of W. In order to exert these effects effectively, the lower limit of the above (3) is set to 0.5%. However, adding excessive amounts of Mo and W will add excessive amounts of the above carbides, leading to a decrease in toughness, and increase the size change (especially the difference in size change rate) after heat treatment. The upper limit of 3) was set to 3.0%. The lower limit of (3) is preferably 1 · 0%, while 1.2% is more preferred, while the upper limit is preferably 2 · 8%.

[0054] The components in steel in the first embodiment of the present invention are as described above, and the balance: iron and inevitable impurities. Examples of inevitable impurities include elements that are inevitably mixed in the manufacturing process, and examples thereof include P, N, and O. The amount of P is preferably about 0.05% or less, more preferably 0.03% or less. The amount of N is preferably 350 ppm or less, more preferably 200 ppm or less, and even more preferably 150 ppm or less. In general, the amount of O is preferably 50 ppm or less, more preferably 30 ppm or less, and even more preferably 20 ppm or less.

[0055] In the present invention, the following components may be added for the purpose of improving other characteristics.

[0056] V: 0.5% or less (excluding 0%)

V is an element that forms carbides such as VC and contributes to improved hardness, and is effective in suppressing the HAZ softening width. In addition, when a diffusion hardened layer is formed by nitriding such as gas nitriding, salt bath nitriding, or plasma nitriding on the surface of the base material, it is effective in improving the surface hardness and increasing the hardened layer depth. It is an effective element. In order to exert such an action effectively, it is preferable that the amount of V is approximately 0.05% or more. However, if added excessively, the amount of dissolved C will decrease and the hardness of the martensite structure which is the parent phase will be reduced, so the upper limit is preferably made 0.5%. The amount of V is more preferably 0.4% or less, and still more preferably 0.30% or less.

[0057] A total of at least one element selected from the group consisting of Ti, Zr, Hf, Ta, and Nb is 0.5% or less (excluding 0%). It is an element that contributes to toughness improvement by fine dispersion and crystal grain refinement of the nitride and A1N. In order to effectively exert such actions, generally, Ti is 0.01% or more, Zr is 0.02% or more, Hf is 0.04% or more, Ta is 0.04% or more, and Nb is 0.0. It is preferable to add 02% or more. However, if added in excess, the amount of dissolved C decreases and the hardness of the martensite decreases, so the total amount of the above elements is preferably 0.5%. The total amount of the above elements is preferably 0.4% or less, more preferably 0.30% or less. The above elements may be added alone or in combination of two or more.

[0058] Co: 10% or less (excluding 0%)

Co is an element that increases the Ms point and is effective in reducing retained austenite, and this improves the hardness. In order to effectively exhibit the above action, the amount of Co is preferably approximately 1% or more. However, if added excessively, the cost will increase, so the upper limit is preferably made 10%. The upper limit of Co content is preferably 5.5%.

[0059] Martensitic transformation point (Ms point) ≥170 ° C

Ms point

= 550-361 X [C] -39 X [Mn] -35 X [V] -20 X [Cr]

-17 X [Ni] -10 X [Cu] -5 X ([Mo] + [W])

+ 15 X [Co] + 30 X [Al]

{In the formula, [] represents the content (%) of each element. }

In the present invention, the Ms point mainly serves as an index for suppressing the change in size after hardness and heat treatment. When the Ms point is less than 170 ° C, the retained austenite increases, and it does not age at high temperatures. In addition to obtaining the desired hardness, it causes expansion after heat treatment. The higher the Ms point, the better. Generally, it is more preferable that the temperature is 230 ° C or higher, and it is more preferable that the temperature is 235 ° C or higher. The upper limit is not particularly limited from the viewpoint of the above action, but it is generally preferable that the upper limit is 350 ° C. in consideration of the action effect by addition of the above elements constituting the Ms point. More preferably, it is ° C.

[0060] The present invention also includes a mold obtained using the above steel for molds. The method for producing the mold is not particularly limited.For example, after the above steel is melted, hot forged and then annealed (for example, after holding at about 700 ° C for 7 hours, about 17 ° C / hr After the furnace is cooled to an average cooling rate of about 400 ° C and then left to cool, it is softened and then roughly processed into a predetermined shape by cutting, etc., and then about 950 to 1150 ° C There is a method of imparting a desired hardness by performing a solution treatment at a temperature → aging treatment at about 400 to 530 ° C.

[0061] Next, the method for producing the steel for cold mold according to the second aspect of the present invention will be described in detail.

[0062] Among the various properties required for cold mold steel, the present inventor, among other things, repairs hardness, resistance to deformation after heat treatment, weld repairability (such as damage to the mold by welding). In order to provide steel for cold molds with improved properties (such as mold life characteristics), investigations were made. As a result, it was found that the intended purpose can be achieved if the components in the steel are appropriately controlled (first aspect of the present invention).

[0063] Continuing with the first aspect of the present invention, the present inventor has made further improvements based on the components in steel disclosed in the previous application, in particular, in order to further improve the reduction in size after heat treatment. I have been studying it. As a result, if the steel according to the first aspect of the present invention is used and solution treatment and aging treatment are performed under appropriate conditions, the steel for cold molds, in which the dimensional change after the heat treatment is further suppressed, is efficient. I found that I can get it well.

[0064] That is, in the manufacturing method according to the second aspect of the present invention, in the first aspect of the present invention, in order to efficiently obtain the steel for cold mold in which the dimensional change after heat treatment is further suppressed. This is characterized by specifying suitable manufacturing conditions. In detail, the solution temperature and the aging temperature are characterized by the parameters (Cu / C mass ratio) that contribute most to the suppression of size change after heat treatment. According to the manufacturing method of the second aspect of the present invention, for example, a patent document As described in Table 5, there is no special heat treatment such as “one or more two-stage tempering treatments” or the sub-zero treatment described in Patent Document 6. By performing the treatment (aging treatment), it is possible to obtain a steel for cold molds in which the dimensional change after the heat treatment is further suppressed as compared with the conventional one, so that the productivity is extremely excellent!

[0065] First, how the cold mold steel manufacturing method according to the second aspect of the present invention is reached from the first aspect of the present invention will be described.

[0066] The present inventor first searched for the cause of galling due to damage to the surface film of the mold in the conventional mold using JIS SKD11 or matrix high speed. As a result, hard coarse Cr-based carbides (mainly containing Cr and Fe, approximately;! ~ 50 m carbide) are deposited on the surface in the area where the film is peeled off, and cracks start from the carbides. It was found that this occurred.

[0067] From the above results, the present inventor is able to prevent the peeling of the surface film if the generation of galling is the coarse Cr-based carbide described above and the generation of the carbide is suppressed (not generated) as much as possible. Thought that the life of the mold could be improved.

[0068] Based on the above findings, the present inventors have further studied. As a result, in order to suppress the formation of coarse carbides and improve the above-mentioned characteristics, various alloy components should be actively added after properly controlling the C content, and the alloy component design should be appropriately It was found that it is extremely important to control the system. Specifically, in order to obtain the desired characteristics, the alloy components (especially Al, Cu, Ni, Mo, W) that are not intended to increase the hardness by controlling carbide as in the past are actively added to the alloy. It is effective to increase the hardness by precipitation hardening of elements, and mainly use precipitation hardening by Al-Ni intermetallic compounds and secondary hardening by carbide formation of Mo, W and C. I found it.

[0069] The above is the reason why the first aspect of the present invention has been reached. After that, the present inventor performed a single solution heat treatment and aging treatment on the cold mold steel, which is more excellent in size change suppression after the heat treatment, as before without performing a special heat treatment. Further studies have been made in order to provide a highly productive manufacturing method that can be easily obtained by simply performing it. As a result, when performing solution treatment and aging treatment using the above steel, these temperatures (solution treatment temperature and aging temperature) are the most effective in reducing the size change after the heat treatment, as shown in the examples described later. Contributing " It was found that the intended purpose was achieved if it was well defined in relation to the “mass ratio of Cu and C”, and the manufacturing method according to the second aspect of the present invention was completed.

[0070] Specifically, the solution temperature (° C) is Tl, the aging temperature (° C) is T2, the mass ratio of Cu and C is [Cu] / [C],

0. 29 XT1-2. 63 X [Cu] / [C] + 225

T2 is expressed by the following formula (5) where TA

TA-10≤T2≤TA + 10 (5)

If the solution treatment and the aging treatment are performed within a range satisfying the above conditions (that is, TA soil 10 ° C), both the average size change rate and the maximum size change rate (details will be described later) after the heat treatment are It was found that a steel excellent in size change suppression that satisfies the range according to the second aspect can be obtained (see Table 7 in the Examples).

In the present specification, “solution treatment” is synonymous with quenching treatment, and “aging treatment” is synonymous with tempering treatment.

In this specification, “high hardness” means a material having a hardness of 650 HV or higher when the hardness is measured by the method described in the column of Examples described later.

[0073] In the second aspect of the present invention, the "size after heat treatment (dimensional change rate)" is the thickness (ΔX), width (Ay), and length (Δz) before aging treatment. When each of the three directions was measured, it was evaluated using both the average value [(Δχ + Ay + Δζ) / 3] and the maximum values (absolute values) of Δχ, Ay, Δζ above. Yes. For convenience of explanation, the former is referred to as “average value of change rate or average change rate”, and the latter is referred to as “maximum value of change rate or maximum change rate”. Thus, in the second aspect of the present invention, the change in size after heat treatment is evaluated using both the “average value of change rate” and the “maximum value of change rate”! In this respect, it is different from the technique of Patent Document 2 which measures only the former (average value of change rate). According to the experiment results of the present inventor, it is not sufficient to reduce the average value of the rate of change as in Patent Document 2 in order to sufficiently suppress the size change after the heat treatment, and the thickness, width, length It is indispensable to reduce the dimensional change (variation) in all directions, even if the average value of the dimensional change rate is suppressed, the difference in dimensional change rate may increase (and vice versa). (See Examples below). In addition, as in the second aspect of the present invention! /, “The change in size after heat treatment is small! /” (Excellent in suppressing change in size) is described later. Based on the method described in the example column, when measuring the dimensional change before and after the heat treatment, the average value of the change rate is within ± 0.03% and the maximum value of the change rate (Absolute value) means less than 0.05%.

It should be noted that the evaluation criteria (method and its level) in the above-described second aspect of the present invention are different from the above-described first aspect of the present invention in the following points.

[0075] First, in both the first aspect of the present invention and the second aspect of the present invention, the "average value of the change ratio" is adopted as the evaluation standard for the change in size after the heat treatment. In the first aspect of the present invention, it is ± 0.05%, while in the second aspect of the present invention, it is defined as ± 0.03% which is stricter than the first aspect of the present invention.

[0076] Further, in the first aspect of the present invention, the "difference in size change", that is, the difference (absolute value) between the maximum value and the minimum value among the above-mentioned Δχ, Ay, Δζ is adopted. In contrast, in the second aspect of the present invention, as described above, the “maximum change rate” is adopted. This is because, in order to provide a steel that is more excellent in restraining deformation than in the first aspect of the present invention, the portion (maximum value) where the size (variation) after heat treatment becomes the largest is as much as possible. Based on the recognition that “it is necessary to make it smaller”, the “maximum change rate” is adopted in addition to the “difference in change rate” described in the above description of the first aspect of the present invention. It depends on you. As shown in the examples described later, even if the “difference in size change” defined in the first aspect of the present invention is satisfied, the “maximum size change ratio” defined in the second aspect of the present invention is satisfied. There are some that do not satisfy the value! / (See the examples described later), but this is not the “steel that is excellent in restraining deformation after heat treatment” in the second aspect of the present invention. Absent.

[0077] The components in the steel according to the second aspect of the present invention are as described in detail below, and the following formula is obtained only by controlling the contents of various alloy elements contributing to precipitation hardening within a predetermined range. As shown in (1) to (4), the balance with a predetermined element is also appropriately controlled, thereby improving the above characteristics. As shown in the examples described later, those which do not satisfy any of these requirements cannot secure desired characteristics. In particular, in the second aspect of the present invention, it is indispensable to add all of Cu, Ni and A1, and for example, one of these is included as in Patent Document 1 and Patent Document 3 described above. In the case of steel with no component, it is confirmed by experiment that the desired effect cannot be obtained! [0078] In particular, in the second aspect of the present invention, in order to minimize the size change after the heat treatment, mainly the mass ratio of [Cu] and [C] constituting the above-described formula (5) is used. ,, Product of Cr and C content (upper limit of [Cr] X [C]), C amount (upper limit), Si amount (upper limit), Mn amount (upper limit), Ms point (lower limit), A1 amount (upper limit) ), Ni amount (upper limit), Cr amount (upper limit), and [Mo] + 0.5 X [W] (upper limit) are appropriately controlled. In the present invention, since it is based on low C, the content of alloy components such as Cu, Ni, and A1 is appropriately controlled in addition to the fact that the amount of retained austenite is originally low due to the high Ms point. Therefore, in particular, expansion and contraction after aging treatment at about 400 to 550 ° C. and after surface hardening treatment can be remarkably suppressed. This is about 400-500 if you don't need to use the above alloy components. Ε_Cu force is mainly about 450 to 530 in the low temperature range of C. Ni- (A1, Mo) -based intermetallic compounds are mainly produced in the intermediate temperature range of C, and Mo-V-based carbides are mainly produced in the high temperature range of about 500 to 550 ° C. The crystal structure of these precipitates ( Since the FCC structure) is different from the matrix (BCC structure), the volume shrinks, which is considered to contribute to the suppression of the size change after heat treatment. Further, in the present invention, since the component design is such that coarse Cr-based carbides are not precipitated as much as possible, the crystal structure is isotropic in any direction, and a large complex shape mold is manufactured. Therefore, it is considered that the size change after heat treatment can be effectively suppressed.

[0079] Further, in the second aspect of the present invention, in order to improve weld repairability (reduce the HAZ softening width), the upper limit of [Cr] X [C], the Ms point (lower limit), the C amount (Lower limit), A1 amount (lower limit), Ni amount (lower limit), [Cu] / [Ni] (upper limit, lower limit), [Mo] + 0.5 X [W] (lower limit), V amount (upper limit) Is properly controlled. In other words, as a design guideline for reducing the HAZ softening width, the amount of C to be removed by hardening by martensite generation should be as low as about 0.2 to 0.60%, and the alloy components (mainly Al , Cu, Ni, Mo, W) are used for precipitation hardening (for example, ε-Cu, Ni-Al intermetallic compounds, Ni-Mo intermetallic compounds). Since these precipitates are finely aligned in the matrix, the hardness increases significantly.

[0080] In particular, Cu, Ni, and A1 are important as precipitation hardening elements, and are elements that greatly contribute to the suppression of HAZ softening. Experiments have confirmed that steels that do not substantially contain any of these elements do not have the desired HAZ softening suppression effect.

[0081] Furthermore, the ratio of [Cu] / [Ni] (ratio of [Cu] to [Ni]) is closely related to the suppression of HAZ softening. It was found that HAZ softening can be suppressed by appropriately controlling the above ratio.

[0082] Hereinafter, the components in steel in the second embodiment of the present invention will be described.

[0083] C: 0.20—0.60%

[0084] Si: 0.5-2.00%

Si is useful as a deoxidizing element during steelmaking, and is an element that contributes to improving hardness and securing machinability. In addition, Si suppresses the temper softening of martensite in the matrix and is useful for suppressing the HAZ softening width. In order to effectively exhibit such an effect, the lower limit of the Si amount is set to 0.5%. However, if added in excess, segregation increases, the force that increases in size after heat treatment, and toughness also decreases, so the upper limit was made 2.00%. The lower limit of the Si amount is 1%, preferably S. 1.2% is more preferred, while the upper limit of the Si amount is preferably 1.85%.

[0085] Mn: 0.;! ~ 2%

[0086] Cr: 3. 00—9.0%

Cr is an element useful for ensuring a predetermined hardness. If the Cr content is less than 3.00% However, since hardenability is insufficient and a portion of bainite is generated, the hardness is reduced and the wear resistance cannot be ensured. The Cr content is preferably 3.5% or more, and preferably 4.0% or more. However, if added excessively, a large amount of coarse Cr-based carbides are formed, shrink after heat treatment, and the durability of the film decreases, so the upper limit was made 9.00%. The Cr content is preferably 7.0% or less, more preferably 6.5% or less, and even more preferably 6.0% or less.

[0087] Α1: 0. 3〜2 · 0%

Three

[0088] Cu: l. 00 ~ 5%

Cu is an element necessary for improving the hardness by precipitation strengthening of ε-Cu and contributes to the suppression of the HAZ softening width. However, if excessively added, forging cracks are likely to occur, so the upper limit was made 5%. The amount of Cu is preferably 2.0% or more and 4.0% or less.

[0089] Ni: l. 00—5%

Three

[0090] Mo: 0.5-3% and / or W: 2% or less (including 0%)

Both Mo and W form MC type carbides and Ni-Mo intermetallic compounds.

6 3

It is an element that forms an object and contributes to precipitation strengthening. However, if Mo or W is added excessively, the above carbides will be generated excessively, leading to a decrease in toughness and deformation after heat treatment ( In particular, the above range was set because the difference in size change ratio was large. In the present invention, Mo is an essential component and W is a selective element, but both may be used in combination. Mo is preferably 0.5% or more and 3% or less, and more preferably 0.7% or more and 2.5% or less. Further, W is preferably 2% or less, and more preferably 1.5% or less.

[0091] S: 0. 10% or less (excluding 0%)

[0092] Further, in the present invention, it is necessary to satisfy the following requirements (1) to (4): [[] means the content (%) of each element. }.

[0093] (1) [Cr] X [C] ≤3.0

The above (1) is set for the purpose of suppressing the formation of coarse Cr-based carbides. If the product of [C r] and [C] exceeds 3.00, the size change after heat treatment is large. As a result, the durability of the surface film decreases. The product of [Cr] and [C] is 1. 80 or less force S, preferably 1. 70 or less than force S. The lower limit is better from the viewpoint of suppressing the size change after the heat treatment, but considering the fact that the above effect is effectively exhibited by the addition of Cr and C, it is generally 0.8. It is preferable that

[0094] (2) [Cu] / [Ni]: 0 · 5〜2 · 2

[0095] (3) [Mo] + 0.5 X [W]: 0.5-3 · 0%

As described above, Mo and W constituting the above (3) are elements contributing to precipitation strengthening, and the above (3) is mainly used as a parameter for ensuring hardness improvement by precipitation strengthening. It is effective for suppressing the HAZ softening width. In (3) above, the [W] coefficient (0.5) was determined taking into account that the atomic weight of Mo is about half that of W. These actions Therefore, the lower limit of the above (4) is set to 0.5%. However, adding excessive amounts of Mo and W will add excessive amounts of the above carbides, leading to a decrease in toughness, and increase the size change (especially the difference in size change rate) after heat treatment. The upper limit of 3) was set to 3.0%. The lower limit of (3) is preferably 1 · 0%, while 1.2% is more preferred, while the upper limit is preferably 2 · 8%.

[0096] (4) [Cu] / [C]: 4 · 0〜; 15

The above (4) is mainly positioned as a parameter for shifting the hardness peak after heat treatment (after aging treatment) to a lower temperature side. Yes. In general, expansion deformation after aging treatment (tempering) is said to occur due to the release (decomposition) of retained austenite during solution treatment (quenching) (for example, see Fig. 9 below). As shown in Fig. 5, the mass ratio ([Cu] / [C] ratio) between Cu, which has the effect of shifting the hardness peak after aging to the low temperature side, and C, which has a close correlation with retained austenite, is appropriate. It was found that the size change after heat treatment can be remarkably suppressed by controlling to.

[0097] FIG. 1 shows the influence of the ratio of [Cu] / [C] on the rate of change when the rate of change (average value and maximum value) is measured by the method described in the examples described later. It is a graph. This graph plots the results for No. 44 (steel grade 8), 52 (steel grade, 56 (steel grade 0), 70 (steel grade), and 72 (steel grade K)) in Table 7 below. Contains almost the same amount of C, Si, Mn, Cr, Al, Cu, Ni, Mo, W. As shown in Fig. 8, the ratio of [Cu] / [C] By controlling the above ratio within the range of 4.0 to 15; the rate of change is within the range specified in the second aspect of the present invention (the rate of change). It can be seen that the average value of S is suppressed to 0.03% or less for soil and the maximum value of change rate is 0.05% or less.

[0098] When the ratio of [Cu] / [C] is less than 4.0, the aging temperature at which the hardness reaches a peak is considerably higher than the temperature at which the residual austenite begins to decompose. On the other hand, if the above ratio exceeds 15, shrinkage due to increase in aging temperature (cancellation with expansion after solution treatment) does not occur. Sex cannot be obtained. The ratio is preferably 5.0 or more and 13 or less, and more preferably 6.0 or more and 12 or less.

[0099] The components in steel in the second aspect of the present invention are as described above, and the balance: iron and impossibility It is an impurity. Examples of inevitable impurities include elements that are inevitably mixed in the manufacturing process, and examples thereof include P, N, and O. The amount of P is preferably about 0.05% or less, more preferably 0.03% or less. The amount of N is preferably 350 ppm or less, more preferably 200 ppm or less, and even more preferably 150 ppm or less. In general, the amount of O is preferably 50 ppm or less, more preferably 30 ppm or less, and even more preferably 20 ppm or less.

[0100] In the present invention, the following components may be further added for the purpose of improving other characteristics.

[0101] V: 0.5% or less (excluding 0%)

V is an element that forms carbides such as VC and contributes to improved hardness, and is effective in suppressing the HAZ softening width. In addition, it is an effective element for improving the surface hardness and increasing the depth of the hardened layer when a diffusion hardened layer is formed by nitriding such as gas nitriding, salt bath nitriding, or plasma nitriding on the surface of the base material. . In order to exert such an action effectively, it is preferable that the amount of V is approximately 0.05% or more. However, if added excessively, the amount of dissolved C will decrease and the hardness of the martensite structure which is the parent phase will be reduced, so the upper limit is preferably made 0.5%. The amount of V is more preferably 0.4% or less, and still more preferably 0.30% or less.

[0102] A total of at least one element selected from the group consisting of Ti, Zr, Hf, Ta, and Nb is 0.5% or less (excluding 0%)

These elements are all nitride-forming elements and contribute to the improvement of toughness by the fine dispersion of the nitride and A1N and the refinement of crystal grains. In order to effectively exert such effects, generally, Ti is 0.01% or more, Zr is 0.02% or more, Hf is 0.04% or more, Ta is 0.04% or more, and Nb is 0.02 It is preferable to add at least%. However, if added excessively, the amount of dissolved C decreases and the hardness of martensite decreases, so the total amount of the above elements is preferably 0.5%. The total amount of the above elements is 0.4% or less, and preferably S, more preferably 0.30% or less. Note that the above elements may be added alone or in combination of two or more.

[0103] Co: 10% or less (excluding 0%)

Co is an element that increases the Ms point and is effective in reducing retained austenite. , The hardness is improved. In order to effectively exhibit the above action, the amount of Co is preferably approximately 1% or more. However, if added excessively, the cost will increase, so the upper limit is preferably made 10%. The upper limit of Co content is preferably 5.5%.

[0104] Martensitic transformation point (Ms point) ≥170 ° C

Ms point

= 550-361 X [C] -39X [Mn] -35X [V] -20X [Cr]

-17X [Ni] -10X [Cu] -5X ([Mo] + [W])

+ 15X [Co] + 30X [Al]

{In the formula, [] represents the content (%) of each element. }

In the present invention, the Ms point is mainly used as an index for suppressing the change in hardness and the heat treatment after heat treatment. When the Ms point is less than 170 ° C, the retained austenite increases, and it is desirable that it does not age at high temperatures. Hardness cannot be obtained, and expansion after heat treatment is caused. The higher the Ms point, the better. Generally, it is more preferable that the temperature is 230 ° C or higher, and it is more preferable that the temperature is 235 ° C or higher. The upper limit is not particularly limited from the viewpoint of the above action, but it is generally preferable that the upper limit is 350 ° C. in consideration of the action effect by addition of the above elements constituting the Ms point. More preferably, it is ° C.

[0105] Next, a method for producing mold steel according to the second embodiment of the present invention will be described.

[0106] The production method of the present invention includes a step of preparing steel that satisfies the above-described requirements and a step of solution treatment and aging treatment under the conditions satisfying the following formula (5).

TA-10≤T2≤TA + 10 (5)

Where

TA = 0.29XT1-2. 63X [Cu] / [C] + 225,

T1 is the solution temperature (° c),

T2 means aging temperature (° C), respectively.

[0107] Specifically, after melting the steel that satisfies the above-mentioned requirements, hot forging, annealing (for example, holding at about 700 ° C for 7 hours, then about 17 ° C / hr. After the furnace is cooled to an average cooling rate of up to about 400 ° C, it is left to cool, and then softened and then roughed into a predetermined shape by cutting or the like. To perform aging and aging [0108] As described above, in the second aspect of the present invention, the force that is a component in steel with a small amount of retained austenite during solution treatment. Further, as shown in the above equation (5), the mass of Cu and C If the ratio ([Cu] / [C]) is controlled in relation to the solution temperature T1 and the aging temperature T2, the hardness after aging peaks before the retained austenite decomposes and expands after aging treatment. Therefore, it is possible to achieve both reduction in size after heat treatment and hardness. In general, in the manufacture of mold steel, solution treatment at a temperature of about 950 to 1150 ° C → aging treatment at a temperature of about 400 to 530 ° C gives the desired hardness. According to the inventor's experimental results, even when solution treatment and aging treatment are performed within the above range, there is a case in which the desired hardness cannot be obtained, and the deformation after the heat treatment cannot be sufficiently suppressed. Since it became clear (refer to the examples described later), it depends on the identification of the above formula (5).

[0109] When the above mechanism of the present invention is compared with the method of Patent Document 2 (equivalent to conventional high C high Cr steel) described above, FIG. 2 (corresponding to FIG. 1 of Patent Document 2) is shown in Patent Document 2. As shown in Fig. 1, when the retained austenite is decomposed to some extent, the tempering process is performed so that the change in size during tempering becomes zero, whereas in the present invention, the residual austenite is decomposed before or after the decomposition. They are different in that they are tempered at the temperature immediately after they begin! That is, the present invention uses an aging treatment at a lower temperature than a conventional high C high Cr steel (specifically, a low temperature of about 500 ° C. or lower). According to the present invention, a region in which a large amount of stable retained austenite is generated without performing an aging treatment in a region (A in FIG. 3) that undergoes severe deformation after heat treatment as in Patent Document 2. Since the aging treatment is performed in (B in Fig. 3), it is thought that a steel with a smaller variation in size compared to Patent Document 2 can be obtained. In addition, when aging treatment is performed at such a relatively low temperature, the stability of retained austenite is improved, and the temporal change of retained austenite is reduced. The effect of becoming smaller can also be obtained.

[0110] The aging temperature T2 is preferably TA ± 5 ° C represented by the above.

[0111] It should be noted that the solution temperature T1 can be a temperature lower than the range normally employed in the production of mold steel, and thereby heat treatment deformation can be reduced. Specifically, it is preferable that the temperature is approximately in the range of 900 to 1150 ° C. [0112] In the present invention, it is only necessary that the temperature of the solution treatment and the aging treatment be appropriately controlled as described above, and these times are not particularly limited, and are usually used for the production of mold steel. However, it is generally good to control the solution time (heating time) to about 1 to 5 hours and the aging time (holding time) to about 2 to 8 hours!

Example

[0113] Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples as well as the present invention, and is appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement by adding any of these, and they are all included in the technical scope of the present invention.

[0114] First, examples according to the first aspect of the present invention will be described below.

[0115] Using various steel grades listed in Tables 1 and 2, 150 kg of ingot was melted in a vacuum induction melting furnace, and then heated to about 900 to 1150 ^o C, 40mmT X 75mmW X It was forged into 2 plates of about 2000 mL and then slowly cooled at an average cooling rate of about 60 ° C./hr. After cooling to a temperature of 100 ° C. or lower, the sample was again heated to a temperature of about 850 ° C. and gradually cooled at an average cooling rate of about 50 ° C./hr (annealing).

[0116] Using the annealed materials obtained as described above, the following tests (1) to (4) were performed.

[0117] (1) Hardness test (maximum hardness measurement)

From the above-mentioned annealed material, a 20 mm TX 20 mm W X 15 mm L size test piece was cut out to obtain a hardness measurement test piece, which was subjected to the following heat treatment.

Solution treatment (quenching treatment): approx. 1020 ~; Heating at 1030 ° C for 120 minutes → Air cooling → Aging treatment (tempering treatment): Hold at approx. 400 ~ 560 ° C for approx. 3 hours → Allow to cool

[0118] As described above, the hardness when the tempering temperature was changed within the range of about 400 to 560 ° C was measured with a Vickers hardness tester (standard AVK manufactured by AKASHI, load 5 kg). Hardness (HV) was examined. In this example, a sample having a maximum hardness of 650 HV or higher was accepted (◯).

[0119] (2) Size change test (Measurement of average change rate and change rate)

From the above annealed material, a test piece of 40 mm TX 70 mm W X lOO mm L was generally cut out and used as a test piece for measuring deformation. This was subjected to the same solution treatment as the hardness test in (1) above, and then tempered at a temperature that reached the maximum hardness. Next, change The “average dimension value” and “difference in size change” were measured, and according to the following criteria, those having both evaluations of “0” were considered to have excellent size control after heat treatment (pass).

[0120] (2-1) Measurement of average value of change rate

Measure the thickness, width, and length of each of the above test pieces for dimension measurement (after annealing and before solution treatment) and the specimen after tempering. The difference in length and the difference in length were determined, and the average value (percentage) was defined as the “average value of change ratio”. In this example, “average size change ratio” within ± 0.05% was accepted (◯) and those exceeding ± 0.05% were rejected (X).

[0121] (2-2) Measurement of difference in change rate

Measure the thickness, width, and length of each of the above test pieces for dimension measurement (after annealing and before solution treatment) and the specimen after tempering. The difference in length and the difference in length were obtained. Among these, the difference (percentage) between the maximum value and the minimum value was defined as “difference in change rate”. Those with a difference in size change rate of 0.08% or less were accepted (◯), and those with a difference of 0.08% or more were rejected (X).

[0122] (3) Welding test (measurement of critical preheating temperature and HAZ softening width)

A test piece of 40 mm TX 45 mm W X 75 mm L was cut out from the above annealed material to make a test piece for welding. This was subjected to a solution treatment and a tempering treatment in the same manner as the size change test of (2).

[0123] Next, the tempered material thus obtained was processed to obtain the plate material of Fig. 3 (a). The plate material in FIG. 3 (a) has the groove shown in FIG. 3 (b). Next, the composition shown in Table 3 (the balance: iron and inevitable impurities, unit: mass ^_0/0) TIG wire (Nippon Yuteku Co. "TIG_Tectic 5H SS", 2. 4 mm) using, the plate Overlay welding was performed on the groove in the following manner. Welding conditions:

Current: 150A, Voltage: 11V, Welding speed: 9.5 ~; 14cm / mm

Interpass temperature: below preheating temperature

Heat input: 7 · 1 ~ 10.4 kj / cm

Preheating: None, Yes (100.C, 200.C, 300.C, 400.C)

[0124] In Table 2, No. 22 and No. 23 (both steels simulating conventional high C high Cr steel) As shown in Fig. 4, welding material was built up on the groove surface (battering) to prevent the influence on the base metal component during welding. For buttering, TIG wire for buttering welding with the following composition [“TGS_50”, 2.4 mm, manufactured by Kobe Steel, Ltd.] was used for single layer welding. The welding conditions are the same as above.

Composition of TIG wire for notching welding: 0.09% C-0. 93% Si-l. 95% Mn_0. 009% P-0. 01% S (remainder: iron and inevitable impurities, unit: mass%)

[0125] When the preheating conditions were changed as described above, cracks did not occur in both the weld metal (DEPO) and the HAZ part! /, And the minimum temperature (limit preheating temperature) was measured. The limit preheating temperature is low, which means that it is hard to crack. In this example, a sample with a critical preheating temperature of 200 ° C or lower was judged good (◯), and a sample with a temperature exceeding 200 ° C was judged defective (X).

[0126] Further, in order to investigate the hardness distribution of the cross section of the specimen subjected to overlay welding at the above-mentioned limit preheating temperature, the position was 30 mm away from the position of the weld melt line (bond) at the 1/4 part of the plate thickness. Hardness was measured continuously at lmm pitch. The distance from the center of the weld metal to the position where the hardness dropped below 600HV was defined as the “HAZ softening width”. For reference, the measurement area of the HAZ softening width is shown in Fig. 1 above. In this example, the HAZ softening width of 6.5 mm or less was evaluated as being excellent in weld repairability (◯), and the one exceeding 6.5 mm was evaluated as being poor in weld repairability (X).

[0127] (4) Toughness test

The annealed material was subjected to the following heat treatment.

Solution treatment (quenching treatment): approx. 1020 ~; heating at 1030 ° C for 120 minutes → air cooling → aging treatment (tempering treatment): holding at approx. 400 to 560 ° C for approx. 3 hours → air cooling or cooling

Next, as shown in FIG. 5, a test piece having a V notch of lOmmR was cut out to obtain a test piece for toughness measurement (Charby impact test piece). Using this specimen, a Charpy impact test was carried out, and the absorbed energy (Charby impact value) at room temperature was measured. Three Charpy impact test specimens were collected, and the average of these was taken as the Charpy impact value. In this example, a Charpy impact value of 15 J or more was evaluated as “excellent toughness”.

[0128] These results are shown in Tables 4-5.

[0129] [Table 1] 0197

7070197 2]

[0131] [Table 3]

[0132] [Table 4]

[0133] [Table 5] Change rate

Maximum hardness HAZ softening range Limit preheating temperature Difference in rate of change Charbi-Impact value

No. Average value

HV mm. C%% J

22 690 1 1.0 400 0.06 0.15 10

23 720 10.5 400 0.01 0.12 13

24 645 7.0 25 -0.02 0.04 35

25 715 5.2 300 0.06 0.10 14

26 725 4.5 100 0.05 0.09 12

27 722 5.1 100 0.06 0.08 15

28 688 5.3 300 0.01 0.04 19

29 638 8.4 100 0.04 0.05 33

30 700 4.6 100 -0.02 0.09 19

31 640 7.5 100 0.03 0.04 20

32 648 6.3 100 0.06 0.08 40

33 645 6.6 100 0.05 0.04 37

34 620 7.5 100 0.06 0.03 30

35 625 7.3 100 0.06 0.04 29

36 630 7.0 100 0.06 0.03 33

37 661 7.0 100 0.01 0.03 19

38 647 6.5 100 0.02 0.05 39

39 683 4.9 100 -0.06 0.09 21

40 629 6.9 100 0.05 0.05 35

41 723 5.0 100 0.03 0.10 13

42 640 7.0 100 0.02 0.05 35

43 630 7.7 100 0.02 0.04 33

From Table 4 and Table 5, it can be considered as follows.

[0135] Nos. In Table 4;! To 21 are examples using Nos. In Table 1 that satisfy all the requirements of the present invention;! To 21 and all have high hardness. In addition to excellent resistance to deformation and weld repair after heat treatment, it also has excellent toughness and a preheating temperature of 200 ° C or less.

[0136] On the other hand, Nos. 22 to 43 in Table 5 are examples using Nos. 22 to 43 in Table 2 that do not satisfy any of the requirements defined in the present invention, and have the following problems. is doing.

[0137] Nos. 22 and 23 in Table 5 are examples using Nos. 22 and 23 in Table 2 simulating a conventional high C high Cr steel, and the product of [Cr] and [C] Since the Ms point is large and the difference is high, the difference between the HAZ softening width and the size change rate increased. Since these steel types have higher hardness as the tempering temperature is lower, the tempering temperature when using the above steel types is set to 510 ° C, and various properties are obtained. It was measured.

[0138] No. 24 in Table 5 is an example using No. 24 in Table 2 with low C content.

An increase in the Z softening width was observed.

[0139] No. 25 in Table 5 is an example using No. 25 in Table 2 with a large C content and a large product of [Cr] and [C] and a low Ms point. Inferior in control!

[0140] No. 26 in Table 5 is an example using No. 26 in Table 2 with a large amount of Si. The average value of the change rate after heat treatment is good, but the difference in change rate is large. .

[0141] No. 27 in Table 5 is an example using No. 27 in Table 2 with a large amount of Mn and a low Ms point. The average value of the change rate after heat treatment is large! /.

[0142] No. 28 in Table 5 is an example using No. 28 in Table 2 with a large amount of S. The limit preheating temperature becomes high and there is a risk of weld cracking.

[0143] No. 29 in Table 5 is an example using No. 29 in Table 2 with a small amount of A1.

An increase in the Z softening width was observed.

[0144] No. 30 in Table 5 is an example using No. 30 in Table 2 with a large amount of A1, and the average value of the change rate after heat treatment is good, but the difference in change rate is large. .

[0145] No. 31 in Table 5 is an example of using No. 31 in Table 2 with a small amount of Ni and a large ratio of [Cu] / [Ni]. The hardness decreases and the HAZ softening width increases. It was observed.

[0146] No. 32 in Table 5 is an example using No. 32 in Table 2 with a large amount of Ni. The hardness decreased and the average value of the change rate after the heat treatment increased.

[0147] No. 33 in Table 5 is an example using No. 33 in Table 2 with a small amount of Cu and a small ratio of [Cu] / [Ni]. The hardness decreases and the HAZ softening width increases. It was observed.

[0148] No. 34 in Table 5 is an example of simulating a steel to which Cu content is not practically added, and the Cu content is 0.0.

Using No. 34 in Table 2 with an extremely small ratio of [Cu] / [Ni] of 5%, a decrease in hardness and an increase in the HAZ softening width were observed. Furthermore, the average value of the change rate after the heat treatment increased.

[0149] No. 35 in Table 5 is an example of simulating a steel that does not practically contain Ni. The amount of Ni is extremely low at 0.05%, and the ratio of [Cu] / [Ni] is low. The small No. 35 in Table 2 was used, so a decrease in hardness and an increase in the HAZ softening width were observed, as well as an increase in the average size change rate after heat treatment. Yes

[0150] No. 36 in Table 5 is an example of simulating a steel to which Al content is not practically added.

Since No. 36 in Table 2 was used, which was extremely low at%, a decrease in hardness and an increase in the HAZ softening width were observed, and the average value of the change rate after heat treatment increased.

[0151] No. 37 in Table 5 is an example using No. 37 in Table 2 in which the ratio of Cu and Ni satisfying the scope of the present invention is small. Softening width increased.

[0152] No. 38 in Table 5 is an example using No. 38 in Table 2 with a small amount of Cr, and the hardness decreased.

[0153] No. 39 in Table 5 is an example using No. 39 in Table 2 with a large amount of Cr, and is inferior in size reduction after heat treatment.

[0154] No. 40 in Table 5 is an example of using No. 40 in Table 2 where the total amount of [Mo] + 0.5 X [W] is small. Decrease in hardness and increase in HAZ softening width It was observed.

[0155] No. 41 in Table 5 is an example using No. 41 in Table 2 with a large total amount of [Mo] + 0.5 X [W]. Although it is good, the difference in the change rate is large.

[0156] No. 42 in Table 5 is an example using No. 42 in Table 2 with a large amount of Ti, and a decrease in hardness and an increase in the HAZ softening width were observed.

[0157] For reference, Fig. 7 shows the hardness distribution profile obtained by the method described above.

. In the figure, steel of the present invention (country) shows the results of No. ⁴ in Table 1, and SKD11 conventional steel (♦) shows the results of 22ο · 22 in Table 2. As shown in Fig. 7, it can be seen that if the steel of the present invention is used, the softening after welding is remarkably suppressed as compared with the conventional steel.

[0158] Next, examples according to the second aspect of the present invention are shown below.

[0159] Using the steel types 溶 to 150 listed in Table 6, about 900 to 150 after melting 150 kg ingot in a vacuum induction melting furnace. C was heated to heat, silver was produced on two 40 mm T x 75 mm W x about 2000 mm L plates, and then slowly cooled at an average cooling rate of about 60 ° C / hr. After cooling to a temperature of 100 ° C or lower, the mixture was again heated to a temperature of about 850 ° C and annealed at an average cooling rate of about 50 ° C / hr (annealing).

[0160] Using each of the annealed materials obtained as described above, the following tests (1) to (2) were performed.

[0161] (1) Hardness test (Measurement of hardness)

From the above-mentioned annealed materials, generally, a 20mmTX 20mmW X 15mmL size test piece It cut out to make a test piece for hardness measurement, and this was subjected to solution treatment → air cooling → aging treatment under the conditions shown in Table 2, and then allowed to cool. In each case, the solution treatment time was about 120 minutes, and the aging treatment time was about 3 hours.

[0162] The hardness after aging treatment was measured with a Vickers hardness tester (standard AVK manufactured by AKASHI, load 5 kg), and the hardness (HV) was examined. In this example, a sample having a hardness of 650 HV or higher was accepted (◯).

[0163] (2) Size change test (Measurement of average value of change rate and maximum value of change rate)

After roughly cutting out a 40mmTX 70mmW x lOOmmL test piece from the above annealed material to make a test piece for size change measurement, it was subjected to solution treatment → fan air cooling → aging treatment under the conditions shown in Table 2 and then released. Chilled. Next, measure the “average value of the change rate” and “the maximum value of the change rate” as described below. Excellent (passed).

[0164] (2-1) Measurement of average value of change rate (average change rate)

Measure the three dimensions of thickness, width, and length for the above-mentioned test pieces for measuring dimensions (after annealing and before solution treatment) and after aging. The difference and the difference in length were determined, and the average value (percentage) was defined as the “average value of change ratio”. In this example, “average size change ratio” within ± 0.03% was determined to be acceptable (◯) and those exceeding ± 0.03% were regarded as unacceptable (X).

[0165] (2-2) Measurement of maximum change rate (maximum change rate)

Measure the three dimensions of thickness, width, and length for the above-mentioned test pieces for measuring dimensions (after annealing and before solution treatment) and after aging. Differences and length differences were determined, and the absolute value (percentage) of these maximum values was defined as the “maximum value of the change rate”. When the maximum change rate was 0.05% or less, it was judged as acceptable (◯), and when it exceeded 0.05%, it was judged as unqualified (X).

[0166] (2-3) Measurement of difference in change rate

For reference, the “difference in size change” described in the description of the first embodiment of the present invention was also measured. Specifically, with respect to the above-mentioned test pieces for measurement of dimensions (after annealing and before solution treatment) and the test pieces after aging, the thickness, width and length were measured in three directions, respectively, before and after heat treatment. Thickness differences, width differences, and length differences were determined. Among these, the difference (percentage) between the maximum value and the minimum value was defined as “difference in size change”. A difference in size change ratio of 0.08% or less was accepted (◯), and a ratio exceeding 0.08% was rejected (X).

[0167] These results are shown in Table 7.

[0168] [Table 6]

L6l0L0 / L00ZdT / 13d 68 908 0/800 OAV . ^ ^^ ^ 秦 ^ τ) Sat r: 挲 [ο ΐο]

^{S + [0] / [η} Ο] X ε9 · Ζ- X 630 = * V丄※

L6lOLO / LOOZdT / 13d 908m 讀 00 OAV [0171] First, Nos. 44 to 47 in Table 7 show the characteristics when the solution temperature T1 and the aging temperature T2 are changed variously using the steel type A in Table 6 whose components in the steel satisfy the requirements of the present invention. It is what was investigated.

[0172] Among them, No. 44 and No. 45 are examples of the present invention in which the aging temperature T2 satisfies the range of the present invention (TA ± 10 ° C). Excellent in size control (not only the difference in change rate, but also the average change rate and the maximum change rate).

[0173] In contrast, No. 46 is a comparative example in which the aging temperature T2 exceeds the range of the present invention, and No. 47 is a comparative example in which the aging temperature T2 is lower than the range of the present invention. Although the difference in size change rate after heat treatment was low, the average size change rate and the maximum size change rate were reduced.

[0174] Nos. 48 to 51 in Table 7 show the characteristics when the solution temperature T1 and the aging temperature T2 are changed in various ways using the steel type B in Table 6 whose components in the steel satisfy the requirements of the present invention. It is what was investigated.

Of these, No. 48 and No. 49 are examples of the present invention in which the aging temperature T2 satisfies the range of the present invention (TA ± 10 ° C.). Excellent size control.

[0176] In contrast, No. 50 is a comparative example in which the aging temperature T2 exceeds the range of the present invention.

No. 51 is a comparative example in which the aging temperature T2 is lower than the range of the present invention. In all cases, the difference in the sizing rate after the heat treatment was good, but the average sizing rate and the maximum sizing rate were reduced. In addition, No. 51 also decreased in hardness.

[0177] Nos. 52 to 55 in Table 7 show the characteristics when the solution temperature T1 and the aging temperature T2 are changed in various ways using the steel type C in Table 6 whose components in the steel satisfy the requirements of the present invention. It is what was investigated.

[0178] Among them, No. 52 and No. 53 are examples of the present invention in which the aging temperature T2 satisfies the range of the present invention (TA ± 10 ° C), both of which are post-heat treatment changes with high hardness. Excellent size control.

[0179] In contrast, No. 54 is a comparative example in which the aging temperature T2 exceeds the range of the present invention.

No. 55 is a comparative example in which the aging temperature T2 is lower than the range of the present invention, and a decrease in hardness and an increase in the maximum size change rate after heat treatment were observed. No. 54 also increases the average size change rate after heat treatment. did.

[0180] Nos. 56 to 58 in Table 7 show the characteristics when the solution temperature T1 and the aging temperature T2 are changed variously using the steel type D in Table 6 whose components in the steel satisfy the requirements of the present invention. It is what was investigated.

[0181] Of these, No. 56 and No. 57 are examples of the present invention in which the aging temperature T2 satisfies the range of the present invention (TA ± 10 ° C). Excellent size control.

[0182] On the other hand, No. 58 was a comparative example in which the aging temperature T2 exceeded the range of the present invention, and the maximum size change rate after heat treatment increased.

[0183] Nos. 59 to 61 in Table 7 show the characteristics when the solution temperature T1 and the aging temperature T2 are changed variously using the steel type E in Table 6 whose components in the steel satisfy the requirements of the present invention. It is what was investigated.

[0184] Of these, No. 59 and No. 60 are examples of the present invention in which the aging temperature T2 satisfies the range of the present invention (TA ± 10 ° C). Excellent size control.

[0185] On the other hand, No. 61 is a comparative example in which the aging temperature T2 exceeds the range of the present invention, and although the difference in the size change ratio after heat treatment is good, the average size change ratio and the maximum size change The rate fell.

[0186] Nos. 62 to 64 in Table 7 show the characteristics when the solution temperature T1 and the aging temperature T2 are changed variously using the steel type F in Table 6 whose components in the steel satisfy the requirements of the present invention. It is what was investigated.

[0187] Of these, No. 62 and No. 63 are examples of the present invention in which the aging temperature T2 satisfies the range of the present invention (TA ± 10 ° C), both of which are post-heat treatment changes with high hardness. Excellent size control.

[0188] On the other hand, No. 64 is a comparative example in which the aging temperature T2 exceeds the range of the present invention, and the difference in size change after heat treatment is good, but the average size change rate and the maximum size change The rate fell.

[0189] Further, Nos. 65 to 67 in Table 7 show the characteristics when the solution temperature T1 and the aging temperature T2 are changed variously using the steel type G in Table 6 whose components in the steel satisfy the requirements of the present invention. It is what was investigated. [0190] Of these, No. 65 and No. 66 are examples of the present invention in which the aging temperature T2 satisfies the range of the present invention (TA ± 10 ° C). Excellent size control.

[0191] On the other hand, No. 67 is a comparative example in which the aging temperature T2 exceeds the range of the present invention, and the difference in the size change rate after heat treatment is good, but the average size change rate and the maximum size change rate are good. The rate fell.

[0192] The following Nos. Are comparative examples having various problems because the solution temperature and the aging temperature satisfy the requirements of the present invention but the components in the steel do not satisfy the scope of the present invention.

[0193] No. 68 and No. 69 are examples using steel types H and I in Table 6 simulating conventional high-C high-Cr steel, and the product of [Cr] and [C] Since steel grades with a large ratio of [Cu] and [C] and a low Ms point were used, all of the average sizing ratio, maximum sizing ratio, and sizing ratio after heat treatment increased. Since these steel types have high hardness when the tempering temperature is low, the tempering temperature when using the above steel types was set to 510 ° C, and various characteristics were measured.

[0194] No. 70 and No. 71 are examples using the steel bumps in Table 6 in which the ratio of [Cu] / [Ni] and the ratio of [Cu] / [C] are small, with a small amount of Cu. A decrease in hardness and an increase in the maximum size change rate were observed.

[0195] No. 72 and No. 73 are examples using the steel grade K in Table 6 where the ratio of [Cu]] and [C] is large.

In both cases, the maximum size change rate increased.

[0196] Note that this example does not show the change in size change over time, but if solution treatment → aging treatment is performed under conditions that satisfy the requirements of the present invention, high! /, Hardness and It is expected that the change over time of the change rate will be kept small while maintaining good change characteristics.

[0197] Although the invention has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is a Japanese patent application filed on October 17, 2006 (Japanese Patent Application 2006).

-283038), Japanese patent application filed on October 30, 2006 (Japanese Patent Application 2006-29

4528) and Japanese patent applications filed on February 27, 2007 (Japanese Patent Application No. 2007-04749)

Based on 0), which is incorporated in its entirety by Bow I.

Also, all references cited herein are incorporated as a whole.

Industrial applicability Since the steel for cold mold of the present invention has the alloy components appropriately controlled as described above, the hardness is high, and the deformability after heat treatment is excellent, and the weld repairability is also good. Therefore, the mold obtained by using the above cold mold steel is particularly suitably used as a mold for forming a high-tensile steel sheet having a tensile strength of about 590 MPa or more, and has a long life, particularly after welding repair. Life is further increased.

In the production method of the present invention, the steel components and the conditions of the solution treatment and the aging treatment are appropriately controlled! /. Steel for molds can be produced efficiently. Therefore, the mold obtained using the production method of the present invention is particularly suitably used as a mold for forming a high-tensile steel sheet having a tensile strength of about 590 MPa or more, and has a longer life, particularly after welding repair. Enhanced.

Claims

Claim C: 0.20—0.60%, Si: 0.5-2.00%, Mn: 0.;! ~ 2%, Cr: 3.00—9.00%, A1: 0.3—2.0%, Cu : l.00—5%, Ni: l.00—5%, Mo: 0.5 to 3% and / or W: 2% or less (including 0%), S: 0. 10% or less (0% The following (1) to (3) {[] means the content (%) of each element. }

(1) [Cr] X [C] ≤3.00,

(2) [Cu] / [Ni]: 0.5-5-2,

(3) [Μο] + 0.5.5Χ [W]: 0. 5—3.0%

Satisfy the requirements of

Remainder: Steel for cold mold, which is iron and inevitable impurities.

The cold mold steel according to claim 1, further comprising V: 0.5% or less (not including 0%). The cold according to claim 1 or 2, further comprising a total of 0.5% or less (excluding 0%) of at least one element selected from the group consisting of Ti, Zr, Hf, Ta, and Nb. For mold

[4] The steel for cold mold according to any one of claims 1 to 3, further comprising Co: 10% or less (not including 0%).

[5] Martensitic transformation point (Ms point) represented by the following formula:

Ms point

= 550-361 X [C] -39X [Mn] -35X [V] -20X [Cr]

-17X [Ni] -10X [Cu] -5X ([Mo] + [W])

+ 15X [Co] + 30X [Al] {In the formula, [] represents the content (%) of each element. }

The steel for cold mold according to any one of claims 1 to 4, which has a temperature of 170 ° C or higher.

[6] A mold obtained using the cold mold steel according to any one of claims 1 to 5.

[7] The steel satisfies the composition according to claim 1, and the following (4) {[] means the content (%) of each element. }

(4) [Cu] / [C]: 4.0 ~; 15

A process of preparing steel that satisfies the requirements of

TA-10≤T2≤TA + 10 (5)

Where

TA = 0.29XT1-2.63X [Cu] / [C] + 225,

T1 is the solution temperature (° c),

T2 means aging temperature (° C),

A method for producing steel for cold molds including:

8. The manufacturing method according to claim 7, wherein the steel contains V: 0.5% or less (not including 0%).

[9] The steel according to claim 7 or 8, wherein the steel contains a total of 0.5% or less (not including 0%) of at least one element selected from the group consisting of Ti, Zr, Hf, Ta, and Nb. The manufacturing method described.

[10] The manufacturing method according to any one of claims 7 to 9, wherein the steel contains Co: 10% or less (not including 0%).

[11] Martensitic transformation point (Ms point):

Ms point

= 550-361 X [C] -39X [Mn] -35X [V] -20X [Cr]

-17X [Ni] -10X [Cu] -5X ([Mo] + [W])

+ 15X [Co] + 30X [Al]

The production method according to any one of claims 7 to 10, wherein is 170 ° C or higher.

[12] A mold obtained by the production method according to any one of [7] to [11].