JP5143531B2 - Cold mold steel and molds - Google Patents

Description

  The present invention relates to steel for cold molds and dies, and more specifically, used to cold-warm and hot-press steel plates for automobiles and home appliances (punching, bending, drawing, trimming, etc.). The present invention relates to a mold steel useful as a material for a mold to be manufactured.

  Metal molds used for forming automobile steel plates, home appliance steel plates, and the like are required to have an improved life as the strength of the steel plates increases. In particular, for steel sheets for automobiles, the demand for high-tensile steel sheets having a tensile strength of about 590 MPa or more is rapidly increasing in consideration of environmental issues and improving the fuel efficiency of automobiles. As a result, there is a problem in that “galling” (a phenomenon of burning during press molding) occurs and the life of the mold is extremely reduced.

  The mold is generally manufactured by applying a hard film treatment to the surface of a mold base material (steel for mold). The base metal mold steel is generally manufactured by annealing → cutting → quenching and tempering. In the present specification, in particular, the quenching process may be referred to as a solution treatment and the tempering process may be referred to as an aging process.

  As mold steels (cold die steels), high C high Cr alloy tool steel represented by JIS SKD11 and high speed tool steel represented by JIS SKH51 with further improved wear resistance. Etc. have been widely used. In these tool steels, the hardness is mainly improved by precipitation hardening of Cr-based carbides, Mo, W, and V-based carbides. Further, for the purpose of improving both wear resistance and toughness, a low alloy high speed tool steel (usually called matrix high speed) in which the amount of alloy elements such as C, Mo, W, V, etc. of JIS SKH51 is reduced. Has also been used.

  Various methods have been proposed with the aim of further improving the properties of mold steel (for example, Patent Documents 1 and 2).

  In Patent Document 1, an appropriate amount of Ni or Al is added for the purpose of suppressing dimensional change (size change) due to quenching and tempering treatment, particularly suppressing expansion change during tempering and increasing hardness, A cold die steel to which an appropriate amount of Cu is added is disclosed. Further, it is described that when the C and Cr contents are adjusted and the carbide distribution in the structure is finely dispersed, galling resistance is also improved.

In patent document 2, even if it hardens at the temperature lower than the conventional matrix high speed, in the tempered state (state before heat processing) in order to ensure the characteristic (hardness and toughness) comparable to the conventional one. has a composition M ₂₃ C ₆ type carbide mainly containing Cr to produce 2~5vol%, MC carbides mainly V after quenching and tempering, and Mo, any one of the M ₆ C type carbide was mainly W An alloy tool steel having a structure in which is dispersed and precipitated is disclosed.
JP 2006-169624 A JP 2004-169177 A

  As described above, the mold is generally manufactured by applying a hard film treatment to the surface of the mold steel. As this hard film treatment, at present, a TD process for forming a VC film by thermal diffusion, a CVD process for mainly forming TiC, and a PVD process for mainly forming TiN are generally used. Here, the TD treatment means that a steel material is immersed in a molten salt bath such as V to react C and V in the steel material, and a VC film of about 5 to 15 μm is formed in a high temperature environment of about 900 to 1030 ° C. This is a treatment for diffusing and penetrating the substrate surface. These hard coating treatments are appropriately employed depending on the circumstances of the mold user and the press manufacturer. Therefore, there is a demand for mold steel that can cope with any hard coating treatment (that is, a hard coating having a long life is formed). Moreover, of course, good basic properties (for example, hardness, toughness, etc.) are required for mold steel.

  The present invention has been made in view of the above circumstances, and its purpose is to provide excellent basic characteristics (hardness, toughness, etc.) and cold mold steel that can cope with various hard coating treatments, And to provide molds.

The cold mold steel of the present invention that has achieved the above-mentioned object,
C: 0.5 to 0.7% (meaning mass%, the same shall apply hereinafter),
Si: 0.5 to 2.0%,
Mn: 0.1 to 2.0%,
Cr: 5-7%,
Al: 0.01 to 1.0%,
N: 0.003-0.025%,
Cu: 0.25 to 1%
Ni: 0.25 to 1%,
Mo: 0.5 to 3% and W: 2% or less (including 0%), and S: 0.1% or less (not including 0%)
The balance is iron and inevitable impurities,
The following (1) to (3) {[] means the content (%) of each element. }
(1) [Cr] × [C] ≦ 4,
(2) [Al] / [N]: 1 to 30,
(3) [Mo] + 0.5 × [W]: 0.5 to 3.00%
A summary exists where the above requirements are satisfied.

  In a preferred embodiment, the cold mold steel further contains V: 0.5% or less (excluding 0%).

  In a preferred embodiment, the cold mold steel further includes at least one element selected from the group consisting of Ti, Zr, Hf, Ta and Nb in total of 0.5% or less (excluding 0%) )contains.

  In a preferred embodiment, the cold mold steel further contains Co: 10% or less (excluding 0%).

  The mold of the present invention is obtained using any one of the above cold mold steels.

  As described above, the steel for cold mold according to the present invention is appropriately controlled in the balance between the alloy components and the predetermined elements. Therefore, in addition to being excellent in hardness and toughness, in various hard coating treatments, A hard film having a long life is formed on the surface. A mold obtained by using the above steel for cold mold is particularly suitably used as a mold for forming a high-tensile steel sheet having a tensile strength of about 590 MPa or more.

  The inventors of the present invention have made extensive studies in order to provide steel for cold molds that can satisfactorily exhibit basic characteristics such as hardness and toughness and that can sufficiently cope with various hard coating treatments. As a result, not only the content of various alloy elements is controlled within a predetermined range, but also the balance between the predetermined elements is appropriately controlled as shown in the above (1) to (3), thereby preventing peeling of the TiN film. In addition, it is possible to improve the hardness and toughness, and as a result, it is possible to form a hard film having a long life on the surface even if a hard film process such as a TD process, a CVD process, or a PVD process is performed. The present invention has been completed.

  Hereinafter, the background to the present invention will be described.

  The present inventors first sought the cause of galling caused by damage to the TiN film formed by PVD treatment in a conventional mold using JIS SKD11 or matrix high speed.

  FIG. 1 (a) is an optical micrograph showing a state in which galling is generated on the surface of a mold in which JIS SKD11 is used as a mold steel and a TiN film is formed thereon by PVD treatment. FIG. 1B also shows an optical micrograph of the mold base material before the TiN film is applied. In FIG.1 (b), the part which looks white is Cr type carbide. 1 (c) and 1 (d) are optical micrographs showing an enlarged part of FIG. 1 (a). As is clear from FIGS. 1C and 1D, hard coarse Cr-based carbides (carbides mainly containing about 1 to 50 μm, mainly containing Cr and Fe) are present in the area where the TiN film is peeled off. It can be seen that cracks are generated on the surface, starting from the carbide.

  From the above observation results, the present inventors have found that the origin of galling of the TiN film is the coarse Cr-based carbide, and if the generation of the carbide is suppressed as much as possible, the TiN film can be prevented from peeling, It has been found that the mold life can be improved.

  In order to suppress the generation of coarse Cr-based carbides and improve the life of the TiN film by PVD treatment, the amount of C and the amount of Cr in the steel may be reduced. However, if the amount of C is reduced too much, it becomes difficult to form a VC or TiC film having a sufficient thickness on the surface of the mold steel (base material) by TD treatment or CVD treatment. Therefore, the present invention appropriately controls the amount of C, the amount of Cr, and the product thereof ((1) above) to prevent coarse Cr-based carbides from being precipitated and, on the other hand, sufficient thickness. One of the characteristics is to secure a VC film and a TiC film.

  In the steel for molds of the present invention, the production of coarse Cr-based carbides is suppressed in order to improve the life of the TiN film by PVD treatment. However, if Cr-based carbides are not generated, crystal grain coarsening during quenching cannot be prevented, and excellent toughness cannot be ensured after quenching. Therefore, the present invention is characterized in that fine AlN is formed by precisely controlling the amount of Al, the amount of N and the balance thereof (above (2)) to ensure excellent toughness after quenching. To do. In the present specification, “excellent toughness” means that the Charpy impact value is 20 J or more when measured by the method described in the column of Examples described later. The fine AlN means that having an average particle diameter of about 5 μm or less.

  Moreover, since the steel for molds of this invention contains fine AlN, it is thought that adhesiveness with the nitride-type membrane | film | coat (for example, CrN and TiN) by PVD process improves.

  As described above, in the mold steel of the present invention, in order to suppress the formation of coarse Cr-based carbides, the amount of C and the amount of Cr are reduced as compared with JIS SKD11, which is a conventional steel. Therefore, in the present invention, one of the characteristics is that the hardness reduction due to the reduction of the C content and the Cr content is compensated by positively adding alloy components (especially Al, Cu, Ni, Mo, W). . Specifically, the steel for molds of the present invention is particularly formed by precipitation hardening by an Al—Ni intermetallic compound by controlling the above (2), and carbide formation of Mo, W and C by controlling the above (3). High hardness is achieved by utilizing the secondary curing. In the present specification, “high hardness” means that the maximum hardness when measured by the method described in the column of Examples described later is 650 HV or more.

  Hereinafter, the components in the steel of the present invention will be described in detail one by one.

C: 0.5 to 0.7%
C is an element that ensures hardness and wear resistance and contributes to the suppression of HAZ softening. Further, when a carbide film such as VC or TiC is formed on the surface of the mold base material by the TD method or the CVD method, there is a problem that if the amount of C is low, the thickness of the film becomes thin. Considering these, the lower limit of the C amount is set to 0.5% in order to effectively exhibit the above-described action. The amount of C is preferably 0.55% or more. However, when the amount is excessive, coarse Cr-based carbides are generated, and the TiN film formed by the PVD process is easily peeled off. If the amount of C is excessive, retained austenite increases, and the desired hardness cannot be obtained unless high-temperature aging treatment is performed, and the size increases due to expansion after aging treatment. Further, if the amount of C is excessive, the toughness is adversely affected. Therefore, the upper limit of the C amount is set to 0.7%. The C content is preferably 0.65% or less.

Si: 0.5 to 2.0%
Si is useful as a deoxidizing element at the time of steelmaking, and is an element that contributes to improving hardness and securing machinability. Moreover, Si suppresses temper softening of the martensite of the matrix and is useful for suppressing HAZ softening. In order to effectively exhibit such an action, the lower limit of the Si amount is set to 0.5%. However, if the amount is excessive, the toughness decreases. Moreover, segregation increases and the size after heat treatment increases. Therefore, the upper limit of Si content was set to 2.0%. The amount of Si is preferably 1.0% or more, more preferably 1.2% or more, and preferably 1.85% or less.

Mn: 0.1 to 2.0%
Mn is an element useful for ensuring hardenability. However, if the amount is excessive, retained austenite increases, so that the desired hardness cannot be obtained unless high temperature aging treatment is performed, and the toughness is also lowered. Taking these into consideration, the amount of Mn was set within the above range. The amount of Mn is preferably 0.15% or more, preferably 1% or less, more preferably 0.5% or less, and still more preferably 0.35% or less.

Cr: 5-7%
Cr is an element useful for ensuring a predetermined hardness. Specifically, if the amount of Cr is too small, hardenability is insufficient and a part of bainite is generated, so that the hardness is lowered and the wear resistance cannot be ensured. Further, Cr is an element useful for ensuring the corrosion resistance of the mold. Therefore, the lower limit of the Cr amount is set to 5%. The amount of Cr is preferably 5.5% or more. However, if the amount is excessive, a large amount of coarse Cr-based carbide is generated, and the TiN film formed by the PVD process is easily peeled off. On the other hand, if the amount of Cr is excessive, the durability of the hard coating decreases due to shrinkage after heat treatment. Further, if the amount of Cr is excessive, the toughness is adversely affected. Therefore, the upper limit of Cr content is set to 7%. The amount of Cr is preferably 6.5% or less.

Al: 0.01 to 1.0%
In addition to being useful as a deoxidizer, Al is an element that contributes to the improvement of hardness by precipitation strengthening of Al—Ni-based intermetallic compounds such as Ni ₃ Al and the suppression of softening of HAZ. Furthermore, Al is an important element for forming an AlN precipitate together with N to prevent crystal grain coarsening during quenching and achieving excellent toughness. Considering these, the lower limit of Al was set to 0.01%. The amount of Al is preferably 0.02% or more, more preferably 0.03% or more.

  In the field of tool steel, the amount of Al is generally reduced as much as possible in order to improve the quality of inclusions. Nevertheless, Al is actively added in the present invention in order to improve the hardness of the mold steel, preferably to suppress the HAZ softening and to prevent the coarsening of the crystal grains. The positive addition of Al in the present invention is one of the major differences compared to the prior art.

  On the other hand, if the amount of Al is excessive, the toughness is reduced, segregation increases, and the size after heat treatment increases. Therefore, the upper limit of Al content was set to 1.0%. The amount of Al is preferably 0.8% or less.

N: 0.003 to 0.025%
N is an important element for forming an AlN precipitate together with Al to prevent crystal grain coarsening during quenching and achieving excellent toughness. In order to achieve excellent toughness, the lower limit of the N content is set to 0.003%. However, if the amount is excessive, the toughness is rather reduced. Therefore, the upper limit of the N amount is set to 0.025%. The N amount is preferably 0.004% or more and 0.020% or less.

Cu: 0.25 to 1%
Cu is an element necessary for improving hardness by precipitation strengthening of ε-Cu, and contributes to suppression of HAZ softening. However, if the amount is excessive, the toughness is lowered and forging cracks are likely to occur. Therefore, the upper limit of the amount of Cu is set to 1%. The amount of Cu is preferably 0.30% or more and 0.8% or less.

Ni: 0.25 to 1%
Ni is an element necessary for improving the hardness by precipitation strengthening of Al—Ni-based intermetallic compounds such as Ni ₃ Al, and contributes to the suppression of HAZ softening. Ni can also be used in combination with Cu to suppress hot brittleness due to excessive addition of Cu and to prevent cracking during forging. However, if the amount is excessive, the retained austenite increases, and unless it is aged at a high temperature, a predetermined hardness cannot be secured, and it expands after heat treatment. Further, if the amount of Ni is excessive, the toughness also decreases. Taking these into account, the amount of Ni was set to the above range. The amount of Ni is preferably 0.30% or more and 0.8% or less.

Mo: 0.5 to 3% and W: 2% or less (including 0%)
Mo and W are elements that contribute to precipitation strengthening by forming M ₆ C-type carbides and forming Ni ₃ Mo-based intermetallic compounds. However, if these amounts are excessive, the above carbides and the like are excessively generated, leading to a decrease in toughness, and the size change after heat treatment becomes large. Therefore, the above ranges were set as the Mo amount and the W amount. In the present invention, Mo is an essential component and W is a selective element, but both may be used in combination. A preferable lower limit of the amount of W is 0.02%. The amount of Mo is preferably 0.7% or more and 2.5% or less. The amount of W is more preferably 0.05% or more and 1.5% or less.

S: 0.1% or less (excluding 0%)
S is an element useful for ensuring machinability. From the viewpoint of ensuring machinability, it is recommended to contain S in an amount of preferably 0.002% or more, more preferably 0.004% or more. However, if the amount is excessive, weld cracks occur. Therefore, the upper limit of the amount of S is set to 0.1%. The amount of S is preferably 0.07% or less, more preferably 0.05% or less, and still more preferably 0.025% or less.

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

(1) [Cr] × [C] ≦ 4
The above (1) is set for the purpose of suppressing the formation of coarse Cr-based carbides. When [Cr] × [C] exceeds 4, coarse Cr-based carbides are generated and the TiN film is easily peeled off. On the other hand, when this product exceeds 4, not only the durability of the hard coating is lowered, but also the size after heat treatment is increased. [Cr] × [C] is preferably 3.8 or less, and more preferably 3.7 or less. The lower limit is better from the standpoint of suppressing the formation of coarse Cr-based carbides and further suppressing the change in size after heat treatment, but effectively exerting the above-described action by the addition of Cr and C. Taking this into consideration, it is generally preferable that the ratio is 0.8.

(2) [Al] / [N]: 1-30
The above (2) is set to form fine AlN and ensure toughness after quenching. If [Al] / [N] is too small or too large, it becomes difficult to obtain fine AlN precipitates, and excellent toughness cannot be ensured. Therefore, it is important to precisely control these balances. [Al] / [N] is preferably 2 or more and 20 or less.

(3) [Mo] + 0.5 × [W]: 0.5 to 3.00%
As described above, Mo and W are elements that contribute to precipitation strengthening, and the above (3) is mainly set as a parameter for ensuring hardness improvement by these precipitation strengthening. Moreover, by controlling this parameter, the HAZ softening can be satisfactorily suppressed. In order to effectively exhibit these actions, the lower limit of the above (3) is set to 0.5%. However, if the amount of Mo or the amount of W is excessive, carbides are added excessively, leading to a decrease in toughness, and the size after heat treatment becomes large. Therefore, the upper limit of (3) is set to 3.00%. The lower limit of the above (3) is preferably 1.0%, more preferably 1.2%, and the upper limit is preferably 2.8%. In the above (3), the coefficient (0.5) of [W] was determined considering that the atomic weight of Mo is about 1/2 of W.

  The basic components in the steel of the present invention are as described above, and the balance is iron and inevitable impurities. Examples of inevitable impurities include elements that are inevitably mixed in the manufacturing process, and specific examples include P and O. The amount of P is approximately 0.05% or less, more preferably 0.03% or less. The amount of O is generally preferably 0.005% or less, more preferably 0.003% or less, and still more preferably 0.002% or less. In the present invention, for the purpose of improving other characteristics, the following selective components may be contained.

V: 0.5% or less (excluding 0%)
V is an element effective for suppressing HAZ softening, in addition to forming carbides such as VC and contributing to improvement in hardness. Further, when the diffusion hardened layer is formed by performing nitriding treatment such as gas nitriding, salt bath nitriding, plasma nitriding on the surface of the base material, it is an element effective for improving the surface hardness and increasing the depth of the hardened layer. In order to exhibit such an action effectively, it is preferable that the amount of V is approximately 0.05% or more. However, if the amount is excessive, the amount of solute C is reduced, the hardness of the martensite structure that is the parent phase is reduced, and the toughness is reduced. Therefore, when V is contained, the upper limit of the amount is set to 0.5%. The amount of V is more preferably 0.4% or less, and still more preferably 0.3% or less.

At least one element selected from the group consisting of Ti, Zr, Hf, Ta and Nb is 0.5% or less in total (not including 0%)
All of these elements are nitride-forming elements and contribute to fine dispersion of the nitride and AlN. As a result, the elements prevent the coarsening of crystal grains and contribute to the improvement of toughness. 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 0.02%. It is preferable to make it contain in the quantity of% or more. However, if these amounts are excessive, the amount of dissolved C will decrease and the hardness of martensite will decrease. Therefore, the total amount of the above elements is preferably 0.5%. The total amount of the above elements is more preferably 0.4% or less, still more preferably 0.3% or less. Said element may be contained independently and may use 2 or more types together.

Co: 10% or less (excluding 0%)
Co is an element that increases the Ms point and is effective in reducing retained austenite, and thereby improves the hardness. In order to effectively exhibit the above action, the amount of Co is preferably approximately 1% or more. However, if the amount is excessive, the cost is increased, so the upper limit is preferably 10%. The amount of Co is more preferably 5.5% or less.

  This invention also includes the metal mold | die obtained using said steel for metal mold | dies. 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, held at about 700 ° C. for 7 hours, and then an average cooling of about 17 ° C./hr). After the furnace is cooled to about 400 ° C. at a speed, and then left to cool), it is softened and then roughly processed into a predetermined shape by cutting or the like, and then subjected to a solution treatment at a temperature of about 950 to 1150 ° C. A method of imparting desired hardness by performing an aging treatment at about 400 to 530 ° C can be mentioned.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Using various steel types listed in Table 1, 150 kg ingot was melted in a vacuum induction melting furnace, then heated to about 900-1150 ° C., forged into two 40 mm T × 75 mm W × about 2000 mm L plates, Slow cooling was performed 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 gradually cooled at an average cooling rate of about 50 ° C./hr (annealing). Using the annealed materials obtained as described above, the following tests (1) to (3) were performed.

(1) Hardness test (measurement of maximum hardness)
From the above-mentioned annealed material, a test piece having a size of 20 mmT × 20 mmW × 15 mmL was generally cut out to obtain a hardness measurement test piece, which was subjected to the following heat treatment.

Solution treatment (quenching treatment): heating at about 1020-1030 ° C. for 120 minutes → air cooling → aging treatment (tempering treatment): holding at about 400-560 ° C. for about 3 hours → cooling as described above, tempering temperature Was measured with a Vickers hardness meter (standard AVK manufactured by AKASHI, load 5 kg), and the maximum hardness (HV) was examined. In this example, a sample having a maximum hardness of 650 HV or higher was accepted. The results are shown in Table 2.

(2) Toughness test (measurement of Charpy impact value)
The annealed material was subjected to the following heat treatment.

Solution treatment (quenching treatment): heating at about 1020 to 1030 ° C. for 120 minutes → air cooling → aging treatment (tempering treatment): holding at about 400 to 560 ° C. for about 3 hours → air cooling or standing cooling Next, FIG. As shown, a test piece having a V notch portion of 10 mmR was cut out to obtain a test piece for toughness measurement (Charpy impact test piece). A Charpy impact test was performed using this test piece, and the absorbed energy (Charpy impact value) at room temperature was measured. Three Charpy impact test specimens were collected, and the average value of these was taken as the Charpy impact value. In this example, a Charpy impact value of 20 J or more was evaluated as “excellent toughness”. The results are shown in Table 2.

(3) Characteristic evaluation of hard coating (3-1) Formation of hard coating From the above-mentioned annealed material, a test piece of about 4 mmt × φ50 mm size is cut out and subjected to the same heat treatment as the toughness test to evaluate the characteristics of the hard coating. The test piece was used. The test piece was subjected to TD treatment, CVD treatment and PVD treatment under general conditions, and a VC coating, a TiC coating and a TiN coating were separately formed on the surface.

(3-2) Measurement of Hard Film Thickness A 2000 times photograph of each hard film (VC, TiC and TiN film) formed as described above was taken with a scanning electron microscope (SEM). The film thickness was measured at the points. The average value of the measured values at the five locations was taken as the film thickness (μm) of each hard coating. In this example, a film having a film thickness of 7.0 μm or more as a VC film and a TiC film was regarded as acceptable. The results are shown in Table 2.

(3-3) Measurement of peeling limit load of hard film The peeling limit load of each hard film (VC, TiC and TiN) was measured by a pin-on-disk test. Specifically, a diamond indenter with an R radius of 200 μm at the tip is pushed and moved into a hard film under the conditions of a load increase rate: 100 N / min and an indenter moving speed: 10 mm / min, and the load at the point where film peeling occurs first. (N) was determined as the peel limit load. In this example, each hard coating having a peeling limit load of 20 N or more was regarded as acceptable. The results are shown in Table 2.

  As is apparent from the results of Tables 1 and 2, the steel No. 1 satisfying the requirements of the present invention was obtained. 6-12 and 14-23 all have good (maximum) hardness, toughness (Charpy impact value), film thickness of VC or TiC film, and peeling limit load of hard film (VC film, TiC film or TiN film) It is. On the other hand, steel No. which does not satisfy any requirement of the present invention. 1 to 5, 13 and 24 to 31 have the following problems.

  Steel No. In Nos. 1 and 2, the amount of C, the amount of Cr, and [Cr] × [C] are excessive, and the peeling limit load of the TiN film is insufficient due to coarse Cr-based carbides. Moreover, since the amount of C and Cr is excessive, the toughness is also lowered.

  Steel No. Since 3 and 4 have a small amount of C, the film thicknesses of the VC and TiC films are insufficient, and as a result, the peeling limit load of these films is lowered.

  Steel No. No. 5 is insufficient in toughness because the amount of Al is small and the value of [Al] / [N] is small.

  Steel No. Since No. 13 has a large amount of Al and a large value of [Al] / [N], the toughness is insufficient.

  Steel No. No. 24 has an excessive amount of Si. No. 25 has an excessive amount of Mn. Since No. 26 has an excessive amount of Cu and Ni, both have insufficient toughness.

  Steel No. In No. 27, since the amount of Mo is small and the value of [Mo] + 0.5 × [W] is small, the hardness is insufficient.

  Steel No. Since No. 28 has a large value of [Mo] + 0.5 × [W], its toughness is insufficient.

  Steel No. No. 29 has insufficient toughness due to an excessive amount of V as a selective element.

  Steel No. In No. 30, since the total amount of Ti and Nb, which are selective elements, exceeds 0.5%, the amount of dissolved C decreases, and as a result, the hardness is insufficient.

  Steel No. No. 31 has insufficient toughness because the N amount is excessive.

FIG. 1 (a) is an optical micrograph showing a state in which galling occurs on the surface of a mold in which JIS SKD11 is used as a mold steel and a TiN film is formed thereon by PVD treatment. FIG. 1 (c) and FIG. 1 (d) are enlarged photomicrographs of part of FIG. 1 (a). FIG. 2 is a schematic view showing the shape of the Charpy impact test piece used in the examples.

Claims (5)

C: 0.5 to 0.7% (meaning mass%, the same applies hereinafter),
Si: 0.5 to 2.0%,
Mn: 0.1 to 2.0%,
Cr: 5-7%,
Al: 0.01 to 1.0%,
N: 0.003-0.025%,
Cu: 0.25 to 1%
Ni: 0.25 to 1%,
Mo: 0.5 to 3% and W: 2% or less (including 0%), and S: 0.1% or less (not including 0%)
The balance is iron and inevitable impurities,
The following (1) to (3) {[] means the content (%) of each element. }
(1) [Cr] × [C] ≦ 4,
(2) [Al] / [N]: 1 to 30,
(3) [Mo] + 0.5 × [W]: 0.5 to 3.00%
A steel for cold mold, which satisfies the requirements of   Furthermore, the steel for cold molds of Claim 1 containing V: 0.5% or less (excluding 0%).   The cold gold according to claim 1 or 2, further comprising at least one element selected from the group consisting of Ti, Zr, Hf, Ta and Nb in a total amount of 0.5% or less (excluding 0%). Mold steel.   Furthermore, the steel for cold molds in any one of Claims 1-3 containing Co: 10% or less (excluding 0%).   The metal mold | die obtained using the steel for cold molds in any one of Claims 1-4.