WO2009088027A1 - Cold-work die steel and dies for cold pressing - Google Patents

Abstract

The invention provides a cold-work die steel useful as material for dies for cold pressing which is excellent in basic characteristics such as hardness, toughness and dimensional change by heat treatment and which is also unproblematic with respect to roughness on machined surface and machine life; and dies made of the steel for cold pressing. A cold-work die steel which contains by mass C: 0.5 to 0.7%, Cr: 5.0 to 7.0%, Si: 0.5 to 2.0%, Mn: 0.1 to 2.0%, Al: 0.001 to 0.010%, Cu: 0.25 to 1.00%, Ni: 0.25 to 1.00%, Mo+0.5xW: 0.5 to 3.0%, V: 0.5% or below; P: 0.05% or below, S: 0.1% or below, and O: 0.005% or below and satisfies the requirements: [C]x[Cr]<4, FP=[Si]/5+[Cr]/5+ 2x[Mo]+[W]+2x[V]+10x[Al]<5.0, and AP=[Mn]+3x([Cu]+[Ni])<2.5; and dies for cold pressing which are made by using the steel.

Description

Cold mold steel and cold press mold

INDUSTRIAL APPLICABILITY The present invention is a cold mold useful as a material for a cold press mold used when press forming (punching, bending, drawing, trimming, etc.) of steel sheets for automobiles, steel sheets for household appliances, etc. Steel and its cold press mold.

Cold stamping dies used for press forming of steel plates for automobiles, steel plates for household appliances, etc. are required to have an improved service life as the strength of the steel plates increases. Especially for automobile steel sheets, environmental issues are taken into consideration, and high-tensile steel sheets with a tensile strength of 590 MPa or more are increasingly used in order to improve the fuel efficiency of automobiles. Expected.

When press-forming the high-tensile steel plate, the surface film of the cold-press die that has been surface-treated is damaged at an early stage, causing a phenomenon of seizure during press-forming called mold galling or galling. The occurrence of problems such as extremely shortening the die life of the hot press die is increasing.

The cold press mold is manufactured by applying a hard coating to the surface of the cold mold steel as a base material. Cold mold steel as a base material is generally manufactured through steps of annealing, cutting and quenching and tempering in this order.

Conventionally, high-C tool steels such as JIS SKH51 with high wear resistance and high-C high Cr alloy tool steels such as JIS SKD11 have been widely used as cold mold steels. In these tool steels, the hardness is improved by precipitation hardening of Cr-based carbides, Mo, W, and V-based carbides. Furthermore, low alloy high-speed tool steel called matrix high speed steel, which has improved both toughness and wear resistance by reducing the alloy elements such as C, Mo, W, and V contained in JIS SKH51, Used in mold steel. Moreover, the technique of patent document 1 and the technique of patent document 2 are proposed as what aimed at the further improvement of the characteristic of these steel for cold molds.

In Patent Document 1, an appropriate amount of Ni or Al is added for the purpose of obtaining excellent dimension suppressing properties, high hardness properties, and galling resistance without impairing necessary properties such as machinability and wear resistance. A cold die steel is disclosed in which a proper amount of Cu is added in accordance with the addition, and the content of C and Cr is further adjusted to finely disperse the carbide distribution in the structure.

Further, in Patent Document 2, even if the quenching temperature is lower than that of the conventional matrix high speed, characteristics such as hardness and toughness after heat treatment can be obtained at the same level as the conventional matrix high speed. Disclosed is an alloy tool steel having a structure in which 2 to 5 vol% of M ₂₃ C ₆ type carbide is generated in a tempered state and having a quenched and tempered structure in which at least one of MC type carbide and M ₆ C type carbide is dispersed and precipitated. Has been.

JP 2006-169624 A JP 2004-169177 A

The cold press mold is manufactured by applying a hard coating to the surface of the cold mold steel as a base material. As this hard film treatment, there are a TD process for forming a film made of VC by thermal diffusion, a CVD process for forming a film made mainly of TiC, a PVD process for forming a film made mainly of TiN, and the like. These hard coating treatments are appropriately employed depending on the circumstances of the mold user and the press manufacturer. Therefore, it is required to develop cold mold steel that can cope with any hard coating treatment. Needless to say, the cold press mold is required to have basic characteristics such as hardness, toughness, and heat treatment size change.

Furthermore, the cold press mold has a problem that stagnation occurs during cutting. When peeling occurs, the finished surface roughness becomes large, so that the lapping work after the heat treatment becomes difficult, and further the life of the mold is reduced. In addition, the cutting tool life is shortened and the manufacturing cost is increased. In order to solve these problems, it is necessary to suppress the precipitation of Al-based inclusions (Al ₂ O ₃ , AlN) that is the cause of the problem. On the other hand, when the content is reduced, there is a risk of adversely affecting basic properties such as hardness reduction, toughness reduction, and heat treatment dimensional change. Therefore, it is desired to develop a cold press die that ensures these basic characteristics and has no problems in terms of cutting finish surface roughness and cutting tool life.

The present invention has been made as a solution to these conventional problems, and has the required basic properties such as hardness, toughness, heat treatment size change, and can also handle various hard coating treatments. An object of the present invention is to provide a steel for cold mold that is useful as a material for a cold press mold that has no problems in terms of roughness of the finished surface and cutting tool life, and to provide a cold press mold. Is.

The gist of the present invention is shown below.
[1] C: 0.5 to 0.7% by mass;
Cr: 5.0 to 7.0% by mass;
Si: 0.5 to 2.0% by mass;
Mn: 0.1 to 2.0% by mass;
Al: 0.001 to 0.010 mass%;
Cu: 0.25 to 1.00% by mass;
Ni: 0.25 to 1.00% by mass;
N: 0.003 to 0.025 mass%;
P: larger than 0 and 0.05% by mass or less;
S: greater than 0 and 0.1% by weight or less;
O: greater than 0 and 0.005 mass% or less; and containing at least one of Mo and W;
The balance contains iron and inevitable impurities,
And 0.5 ≦ [Mo] + 0.5 × [W] ≦ 3.0; and [C] × [Cr] ≦ 4,
Further, the FP (parameter made of ferrite-forming elements) satisfies the requirement of [Si] / 5 + [Cr] / 5 + 2 × [Mo] + [W] + 2 × [V] + 10 × [Al] ≦ 5.0. ,
A cold mold steel characterized in that AP (a parameter composed of an austenite-forming element) satisfies the requirement of [Mn] + 3 × ([Cu] + [Ni]) ≦ 2.5.
However, in the above formula, [] indicates the content (% by mass) of each element.
[2] The steel for cold mold as set forth in [1], further comprising V: greater than 0 and 0.5% by mass or less.
[3] The description according to [1] or [2], further comprising at least one element selected from the group consisting of Ti, Zr, Hf, Ta, and Nb in a total of more than 0 and 0.5% by mass or less Cold mold steel.
[4] The steel for cold mold as set forth in any one of [1] to [3], further containing Co of greater than 0 and 10% by mass or less.
[5] A cold press die produced by processing the steel for cold die according to any one of [1] to [4] and performing a surface treatment.

By using the steel for cold molds of the present invention as a material for cold stamping molds, it has the required basic properties such as hardness, toughness, heat treatment size change, and also supports various hard coating treatments. Furthermore, it is possible to obtain a cold press die that is free from problems in terms of the finished surface roughness and cutting tool life. In addition, a cold press die obtained by using the cold die steel can be suitably used particularly for forming a high-tensile steel plate having a tensile strength of 590 MPa or more.

This shows the mechanism of damage to the TiN film by Cr carbide, (a) is a longitudinal sectional view showing the original cold press mold, (b) is a crack in the TiN film of the cold press mold The longitudinal cross-sectional view which shows the state which carried out, (c) is a longitudinal cross-sectional view which shows the state which peeling generate | occur | produced in the TiN membrane | film | coat from the crack. It is explanatory drawing which shows the Charpy impact test piece used by the measurement of the Charpy impact value of an Example. It is explanatory drawing which shows the heat processing conditions at the time of heat-processing to the test body used by the measurement of the Example maximum heat processing size change amount.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Cold mold steel 2 ... TiN film 3 ... Cr type carbide 4 ... Crack

Hereinafter, the present invention will be described in more detail based on embodiments.

The present inventor first eagerly searched for the cause of galling caused by damage to the TiN film formed by PVD treatment in a conventional cold press mold using JIS SKD11 or matrix high speed.

As a result of the search, the cause of galling in the TiN film is the coarse Cr-based carbide produced in the cold mold steel used as the base material, and galling is generated starting from the Cr-based carbide. I found out. The mechanism of damage of the TiN film by the Cr-based carbide is as shown in FIG.

First, as shown in FIG. 1 (a), a cold press mold having a TiN film 2 formed on its surface is prepared by applying a hard film treatment to the surface of the cold mold steel 1 as a base material. To do. When this cold mold steel 1 is made of JIS SKD11 or matrix high speed steel, coarse Cr carbide 3 is deposited on the surface of the cold mold steel 1 as a base material. . When press molding is performed using this cold press die, as shown in FIG. 1B, when the molding slides in the direction of the arrow, a crack 4 is generated in the TiN film 2. The part where the crack 4 is generated is a part where the Cr-based carbide 3 is deposited on the base material below the TiN film 2. Furthermore, when the molding is slid, as shown in FIG. 1 (c), the crack 4 is the starting point, and the TiN film 2 is peeled off, which causes galling.

As described above, the cause of galling in the TiN film is Cr-based carbide. The present inventor has found that by suppressing the formation of this Cr-based carbide, it is possible to prevent the TiN film from peeling off and to suppress the occurrence of problems such as extremely shortening the mold life.

In order to suppress the formation of coarse Cr-based carbide 3 precipitated on the surface of the cold mold steel as a base material and to prolong the life of the TiN film formed by PVD treatment, the inclusion of C in the steel The amount and the Cr content may be reduced. However, if the C content is excessively reduced, it becomes difficult to form a VC film by TD treatment or a TiC film by CVD treatment on the surface of the steel for cold mold. Therefore, in the present invention, the content of C is defined as 0.5 to 0.7 mass%, the content of Cr is 5.0 to 7.0 mass%, and the product of these contents is specified. Thus, the coarse Cr carbide 3 is not deposited on the surface of the cold mold steel, and on the other hand, it is possible to form a VC film or a TiC film having a sufficient thickness as required.

In the present invention, parameters made of ferrite-generating elements such as Si, Cr, Mo, W, V, and Al and parameters made of austenite-generating elements such as Mn, Cu, and Ni are also defined.

When the total content of ferrite-forming elements such as Si, Cr, Mo, W, V, and Al is too large, the balance between hardness and toughness of the cold mold steel is lost, and the accuracy of the finished surface of the machining work is also deteriorated. Therefore, in the present invention, the parameter (FP) defined by the ferrite-forming element is formulated into a mathematical formula, and the total content of the ferrite-forming elements is defined so as to satisfy the formula, whereby the hardness of the steel for cold molds is increased. In addition to improving the balance of toughness, the finished surface accuracy of the machined work was also improved.

Also, if the total content of austenite-generating elements such as Mn, Cu, and Ni is too large, the amount of retained austenite increases, resulting in an increase in variation in heat treatment size change and a shortened tool life during cutting. Therefore, in the present invention, the parameter (AP) defined by the austenite-generating element is formulated, and the total content of the austenite-generating element is defined so as to satisfy the formula, thereby reducing the retained austenite in the steel. In addition to reducing the variation in heat treatment dimension, the tool life during cutting was extended.

Hereinafter, the reason for limiting the range of the content of the chemical component in the cold mold steel of the present invention will be described in detail for each element. In addition, all% described in this specification shows the mass%.

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

Cr: 5.0 to 7.0%
Cr is an element useful for ensuring a predetermined hardness. Specifically, if the Cr content is too small, the hardenability is insufficient and a portion of bainite is generated, so that the hardness is lowered and the wear resistance cannot be ensured. Furthermore, Cr is an element useful for ensuring the corrosion resistance of the mold. Therefore, the lower limit of the Cr content is set to 5.0%. Further, the lower limit is preferably 5.5%. However, if the content is excessive, a large amount of coarse Cr-based carbide is generated, and the TiN film formed by the PVD treatment is easily peeled off. On the other hand, if the Cr content is excessive, the durability of the hard coating is reduced by shrinkage after heat treatment. Furthermore, if the Cr content is excessive, the toughness is also adversely affected. Therefore, the upper limit of the Cr content is set to 7.0%. Moreover, it is preferable that the upper limit is 6.5%.

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

Mn: 0.1 to 2.0%
Mn is an element useful for ensuring hardenability. However, if the content is excessive, retained austenite increases, so that the desired hardness cannot be obtained unless tempering is performed at a high temperature, and the toughness also decreases. Taking these into consideration, the Mn content is determined to be in the range of 0.1 to 2.0%. The lower limit of the Mn content is preferably 0.15%, and the upper limit is preferably 1.0%, more preferably 0.5%, and still more preferably 0.35%.

Al: 0.001 to 0.010%
Al is an element useful as a deoxidizer. However, if the content is less than 0.001%, the effect cannot be sufficiently obtained. Therefore, the lower limit of the Al content is set to 0.001%. The lower limit is preferably 0.002%. On the other hand, Al-based inclusions such as Al ₂ O ₃ and coarse AlN cause peeling during cutting and reduce the accuracy of the finished surface of the cut. Therefore, the upper limit of the Al content is set to 0.010%. The upper limit is preferably 0.008%.

Cu: 0.25 to 1.00%
Cu is an element necessary for improving the hardness by precipitation strengthening of ε-Cu, and contributes to the suppression of HAZ softening. However, if the content is excessive, toughness is reduced and forging cracks are likely to occur. Therefore, the upper limit of the Cu content is set to 1.00%. The upper limit is preferably 0.80%. Further, the lower limit of the Cu content is 0.25%. The lower limit is preferably 0.30%.

Ni: 0.25 to 1.00%
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 content is excessive, the retained austenite increases, and unless it is tempered at a high temperature, a predetermined hardness cannot be secured, and it expands after the heat treatment. Further, if the Ni content is excessive, toughness is also lowered. Taking these into account, the Ni content is determined to be in the range of 0.25 to 1.00%. The lower limit of the Ni content is preferably 0.30%, and the upper limit is preferably 0.80%.

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 was 0.003%. The lower limit is preferably 0.004%. Further, the upper limit of the N content was 0.025%. The upper limit is preferably 0.017%.

Mo + 0.5W: 0.5-3.0%
Mo and W are elements that contribute to precipitation strengthening by forming M ₃ C type carbides and M ₆ C type carbides as well as forming Ni ₃ Mo intermetallic compounds. However, if these contents are excessive, the above carbides and the like are excessively generated, resulting in a decrease in toughness, and a change in size after heat treatment becomes large. Therefore, the total content of Mo and W when applied to the formula of Mo + 0.5 × W is determined in the range of 0.5 to 3.0%. The content of Mo alone is also preferably in the range of 0.5 to 3.0%. The content of W alone is preferably 2.0% or less (including 0%). That is, Mo is an essential element and W is a selective element. However, the lower limit of the content of W alone is more preferably 0.02%. Further, the lower limit of the content of Mo alone is more preferably 0.7%, and the upper limit is more preferably 2.5%. More preferably, the lower limit of the content of W alone is 0.05%, and the upper limit is 1.5%.

P: greater than 0 and not more than 0.05% P is an element that is unavoidably present in the molten raw material and is an element that inhibits toughness. Therefore, the upper limit of the P content is set to 0.05%. The upper limit is preferably 0.02%. In addition, although content of P is so preferable that it is small, since it is inevitably contained, the lower limit becomes substantially 0.005%.

S: More than 0 and 0.1% or less S is an element useful for ensuring machinability. From the viewpoint of securing machinability, it is recommended that S is a content of preferably 0.002% or more, more preferably 0.004% or more. However, if the content is excessive, weld cracks occur. Therefore, the upper limit of the S content is set to 0.1%. The upper limit of the S content is preferably 0.07%, more preferably 0.05%, and still more preferably 0.025%.

O: More than 0 and 0.005% or less O is an element contained in molten steel, and is unavoidably contained in steel. When the content of O is high, it reacts with Si, Al, etc. to form oxide inclusions. Therefore, the upper limit of the O content is set to 0.005%. The upper limit is preferably 0.003%, more preferably 0.002%. The lower the O content, the better. However, since it is inevitably contained, the lower limit is substantially about 0.0005%.

Furthermore, the present invention makes it essential to satisfy each mathematical formula described above. In addition, [] shown to each numerical formula shows content (mass%) of each element.

[C] × [Cr] ≦ 4
The above formula is a formula set for the purpose of suppressing the formation of coarse Cr-based carbides. When the product of the content of C and the content of Cr exceeds 4, the durability of the hard coating is reduced and the size after heat treatment is increased. In addition, from the viewpoint of suppressing the formation of coarse Cr-based carbides and suppressing the change in size after heat treatment, the product of the C content and the Cr content is preferably as small as possible, but the above-described addition of C and Cr In consideration of effectively exerting the action, the lower limit of this product is preferably about 0.8.

FP = [Si] / 5 + [Cr] / 5 + 2 × [Mo] + [W] + 2 × [V] + 10 × [Al] ≦ 5.0
The above mathematical formula is a mathematical formula defining and parameterizing the total content of ferrite-forming elements such as Si, Cr, Mo, W, V, and Al. If this parameter (FP) is larger than 5.0, the balance between hardness and toughness of the steel for cold mold is lost, and the accuracy of the finished surface of the machining work is also deteriorated. This parameter (FP) is more preferably 4.8 or less. The FP value 2.11 determined from the lower limit value of elements essential to the cold mold steel according to the present invention, such as Si and Cr, is the substantial lower limit value of this parameter (FP).

AP = [Mn] + 3 × ([Cu] + [Ni]) ≦ 2.5
The above formula is a formula that defines and defines the total content of austenite-generating elements such as Mn, Cu, and Ni. When this parameter (AP) is larger than 2.5, the retained austenite increases, the variation in the heat treatment sizing amount increases, and the tool life during cutting decreases. This parameter (AP) is more preferably 2.3 or less. AP value 1.6 determined from the lower limit values of Mn, Cu, and Ni is substantially the lower limit value of this parameter (AP).

The requirements regarding the basic components in the steel for cold mold according to the present invention are as described above. The balance contains iron and inevitable impurities. Examples of the impurity include Sn and Pb. In the present invention, the following selective components may be further contained for the purpose of improving other characteristics.

V: 0 to 0.5%
V is an element effective for suppressing HAZ softening, in addition to forming carbides such as VC and contributing to improvement in hardness. 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 on the surface of the base material by nitriding such as gas nitriding, salt nitriding, plasma nitriding, etc. . In order to effectively exhibit such an action, it is preferable to add approximately 0.05% or more of the V content. However, when the content is excessive, the amount of dissolved C decreases, the hardness of the martensite structure which is the parent phase is decreased, and the toughness is decreased. Therefore, the upper limit of the V content is set to 0.5%. The upper limit of the V content is preferably 0.4%, more preferably 0.3%.

At least one element selected from the group consisting of Ti, Zr, Hf, Ta, and Nb: 0.5% or less in total These elements are all nitride-forming elements, and nitrides of these elements and AlN It is an element that contributes to fine dispersion and, as a result, prevents coarsening of crystal grains and contributes to improvement of toughness. In order to effectively exhibit the above-described actions, in general, 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 contain 02% or more. However, when these total contents are excessive, the amount of solid solution C falls and the hardness of a martensite falls. Therefore, the total content of these elements is set to 0.5% or less. The total content of these elements is preferably 0.4% or less, more preferably 0.3% or less. In addition, these elements may be contained alone or in combination of two or more.

Co: 10% or less Co is an element that increases the Ms point and is effective in reducing retained austenite, and can thereby improve the hardness. In order to effectively exhibit this action, the Co content is preferably approximately 1% or more. However, if the content is excessive, the cost is increased, so the upper limit is made 10%. The upper limit of the Co content is preferably 5.5%. Here, the Ms point is one of the transformation temperatures (the temperature at which the phase change occurs, and when the transformation occurs over the temperature range, the temperature at which the transformation starts or ends). It means the temperature that begins to transform into martensite.

A cold press mold is manufactured using cold mold steel that satisfies the requirements described above. An example of the manufacturing method of this cold press mold will be described. For example, after the cold mold steel of the present invention is melted, it is hot forged and then annealed (for example, held at about 700 ° C. for 7 hours). Then, after cooling in the furnace to about 400 ° C. at an average cooling rate of about 17 ° C./hr and then allowing to cool, it is softened and then roughly processed into a predetermined shape by cutting or the like, and then 950 to 1150 A cold press mold is manufactured by quenching at a temperature of 0 ° C. and further tempering at 400 to 530 ° C. to give a desired hardness.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limited by these examples.
In this example, a total of 26 types of steel compositions described in Table 1 (No. 1 is JIS SKD11 conventionally used as a steel for cold molds) and 150 kg in a vacuum induction melting furnace are used. After melting the ingot, it was heated to 900 to 1150 ° C. to forge a plate of 40 mmT × 75 mmW × about 2000 mmL, and then gradually 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 gradually cooled at an average cooling rate of about 50 ° C./hr (annealing). The following various tests were conducted using the annealed material obtained as described above.

(1) Measurement of maximum hardness A test piece of 20 mm T × 20 mm W × 15 mm L size is cut out from the above-mentioned annealed material to make a test piece for hardness measurement, and heat treatment, specifically, a quenching treatment ( Heating at 1030 ° C. for 120 minutes), air cooling, tempering treatment (kept at 450 to 520 ° C. for 180 minutes) and cooling were performed in this order. The hardness when the tempering temperature was changed within the range of 450 to 520 ° C. was measured with a Vickers hardness meter (standard AVK manufactured by AKASHI, load 5 kg), and the maximum hardness was examined. In this test, those having a maximum hardness of 650 HV or more obtained by measurement were regarded as acceptable. The test results are shown in Table 2.

(2) Measurement of Charpy impact value (measurement of toughness)
Heat treatment, specifically quenching (heating at 1030 ° C. for 120 minutes), air cooling, tempering (holding at 450 to 520 ° C. for 180 minutes), and air cooling or cooling are performed in this order on the above annealed materials. Treated. Next, a test piece having an R notch portion of 10 mmR as shown in FIG. 2 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 for each steel type, and the average value of these specimens was taken as the Charpy impact value. In this test, a Charpy impact value obtained by measurement of 20 J or more was regarded as acceptable. The test results are shown in Table 2.

(3) Investigation of cutting finish surface roughness The above-mentioned annealed material was used as a specimen, and finishing was performed with a ball end mill, and the cutting finish surface roughness was examined. The test conditions are as follows.
Machine: MORI (BT40, 5.5kw)
Tool: Mitsubishi SRFH30S32M φ30
Chip: Mitsubishi SRFT30 VP10MF φ30
Protrusion length: 118mm
Cutting direction: Down cut Cutting speed: 250 mm / min
Feeding speed: 0.31mm / rev
Cutting depth: Ad0.3mm, Rd0.7mm
Cutting oil: None (Air blow)
Processing distance: 257.1m

The cutting finish surface roughness Ra was an average value of values obtained by investigating five 10 mm length ranges of the specimen. In this test, the cutting finish surface roughness Ra obtained by the test was 0.40 mm or less. The test results are shown in Table 2.

(4) Judgment of cutting tool life The above-mentioned annealed material was used as a test specimen, rough machining was performed with a high-feed cutter, and the cutting tool life was investigated. The test conditions are as follows.
Machine: OKK (BT50, 7.5kw)
Tool: Mitsubishi AJX148R503SA42S φ50
Chip: JOMW140520ZDSR-FT VP15TF
Cutting speed: 10 m / min
Feed amount: 1.0mm / rev
Cutting depth: Ad1mm, Rd35mm
Protrusion length: 80mm
Cutting oil: None (Air blow)
Life judgment: Tool wear, chipping

When the cutting tool life is “1” when roughing is performed using a specimen (No. 1) made of JIS SKD11, the cutting tool life is roughened using each specimen. The life of the cutting tool when performed was determined by how many times the life of the cutting tool when roughing was performed using a test body (No. 1) made of SKD11. The determination value of 4.0 or higher was regarded as acceptable. The test results are shown in Table 2.

(5) Measurement of maximum heat treatment change amount From the above-mentioned annealed material, 6 blocks of 40 mm T × 75 mm W × 100 mm L are cut out for each anneal material to make a test piece for measuring the maximum heat treatment change amount. The heat treatment was performed under the conditions shown in FIG. The maximum heat treatment change amount was determined from the dimensional change amount before and after the heat treatment of six specimens. The amount of dimensional change in three directions (x direction, y direction, z direction) orthogonal to each specimen is obtained, and the maximum numerical value among the obtained three directions × 6 absolute values is the maximum heat treatment dimensional change amount. did. In this test, this maximum heat treatment dimension change amount was 0.08 or less. The test results are shown in Table 2.

As described in Table 1 and Table 2, the contents of each chemical component, the product of the content of C and the content of Cr, the parameter composed of a ferrite-forming element, and the parameter composed of an austenite-forming element are all requirements of the present invention. No. which is an example of the invention satisfying 7 to 9, 11, and 14 to 20, all of the maximum hardness, Charpy impact value, cutting finish surface roughness, cutting tool life, and maximum heat treatment change amount were within the range of acceptance criteria. On the other hand, No. 1 which is a comparative example which does not satisfy even one requirement of the present invention. For 1 to 6, 10, 12 to 13, and 21 to 26, at least one acceptance criterion is removed, and there is some defect.

No. The comparative examples 1 to 6, 10, 12 to 13, and 21 to 26 have some problems by removing one or more of the requirements of the present invention described above. A characteristic example for each requirement described in the above was used as a comparative example. Hereinafter, a comparative example corresponding to each requirement defined in the present invention will be described.

A comparative example with too much C and Cr content is No. 1 and No. 2, on the contrary, the comparative example in which the content of C and the content of Cr are too small is No. 3 and no. 4. Both the comparative examples with too much and too little of these contents deviate from the acceptance criteria in all or any of Charpy impact value (toughness), cutting finish surface roughness, cutting tool life, maximum heat treatment size change amount. It was.

No comparative example with too much Si content. 21, on the contrary, there are too few comparative examples. 1. In particular, No. whose content is excessive. In No. 21, the toughness was greatly lowered and the Charpy impact value deviated from the acceptance criterion. Moreover, although it was within the range of the acceptance criterion, the size change after the heat treatment became relatively large.

Comparative example with too much Mn content is No. 22. In this comparative example, the toughness was greatly lowered and the Charpy impact value deviated from the acceptance criterion. Furthermore, the cutting tool life and the maximum heat treatment size change are also outside the acceptance criteria.

A comparative example with too much Al content is No. 10, on the contrary, there are too few comparative examples. 6. No. of comparative example having too much Al content. In No. 10, peeling occurred when finishing with a ball end mill, and the accuracy of the finished surface was deteriorated. In addition, the comparative example No. In 6, the Charpy impact value deviated from the acceptance criterion.

No comparative example with too much Cu content. 23, on the contrary, there are too few comparative examples. 5. No. whose content is excessive. In No. 23, the toughness decreased and the Charpy impact value deviated from the acceptance criterion. Furthermore, the cutting tool life and the maximum heat treatment size change are also outside the acceptance criteria. On the other hand, the comparative example No. 5 also deviated from the acceptance criteria in terms of Charpy impact value and maximum heat treatment change.

Comparative example with too much Ni content is No. 23, on the contrary, there are too few comparative examples. 1. No. whose content is excessive. In No. 23, the acceptance criterion was exceeded by the Charpy impact value and the maximum heat treatment change amount. Also, the cutting tool life is out of the acceptance criteria.

The comparative example where the numerical value calculated from Mo + 0.5W is too small is No. 24, and the numerical value is within the range of the present invention, but the case corresponding to the maximum of 3.0% which is the boundary value is No. 24. 25. No. In 24, the maximum hardness and Charpy impact value deviated from the acceptance criteria. No. 25, the Charpy impact value is lowered, although there is an influence that other requirements are removed.

Comparative example with too much V content is No. 26. No. of this comparative example. In No. 26, since the content of V was excessive, the toughness was lowered, and the acceptance criterion was deviated by the Charpy impact value. Moreover, the acceptance criteria were also out of the finished surface roughness.

A comparative example in which the product of the content of C and the content of Cr is too large is No. 1 and No. 2. No. 1 and No. In No. 2, due to this influence, the life of the cutting tool was remarkably shortened and the size after heat treatment was increased.

A comparative example with too much N content is No. 27. As a result, the toughness decreased and the Charpy impact value deviated from the acceptance criterion.

A comparative example with too large a parameter consisting of ferrite-forming elements is 1 to 4 and no. 25. Due to this influence, in these comparative examples, the balance of toughness is lost, or the accuracy of the finished surface of the cutting work is deteriorated. In particular, no. In No. 25, the toughness was greatly reduced, and the Charpy impact value deviated from the acceptance criterion.

A comparative example with too large a parameter composed of austenite-forming elements is 2-5, no. 12, 13, 22, and 23. Due to this influence, in these comparative examples, the amount of retained austenite increases, the amount of heat treatment change increases, and the tool life during cutting is shortened. In particular, no. 12 and no. No. 13 is out of the acceptance criteria only by the cutting tool life and the maximum heat treatment change amount.

Although the present invention has been described in detail and 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 based on a Japanese patent application filed on Jan. 10, 2008 (Japanese Patent Application No. 2008-003524), the contents of which are incorporated herein by reference.

By using the steel for cold molds of the present invention as a material for cold stamping molds, it has the required basic properties such as hardness, toughness, heat treatment size change, and also supports various hard coating treatments. Furthermore, it is possible to obtain a cold press die that is free from problems in terms of the finished surface roughness and cutting tool life. In addition, a cold press die obtained by using the cold die steel can be suitably used particularly for forming a high-tensile steel plate having a tensile strength of 590 MPa or more.

Claims (5)

1. C: 0.5 to 0.7% by mass;
   Cr: 5.0 to 7.0% by mass;
   Si: 0.5 to 2.0% by mass;
   Mn: 0.1 to 2.0% by mass;
   Al: 0.001 to 0.010 mass%;
   Cu: 0.25 to 1.00% by mass;
   Ni: 0.25 to 1.00% by mass;
   N: 0.003 to 0.025 mass%;
   P: greater than 0 and 0.05% by weight or less
   S: greater than 0 and 0.1% by weight or less;
   O: greater than 0 and 0.005 mass% or less; and containing at least one of Mo and W;
   The balance contains iron and inevitable impurities,
   And 0.5 ≦ [Mo] + 0.5 × [W] ≦ 3.0; and [C] × [Cr] ≦ 4,
   Further, the FP (parameter made of ferrite-forming elements) satisfies the requirement of [Si] / 5 + [Cr] / 5 + 2 × [Mo] + [W] + 2 × [V] + 10 × [Al] ≦ 5.0. ,
   A cold mold steel characterized in that AP (a parameter composed of an austenite-forming element) satisfies the requirement of [Mn] + 3 × ([Cu] + [Ni]) ≦ 2.5.
   However, in the above formula, [] indicates the content (% by mass) of each element.
2. Furthermore, the steel for cold molds of Claim 1 containing 0.5 mass% or less larger than V: 0.
3. The cold mold 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 of more than 0 and 0.5% by mass or less. steel.
4. Furthermore, the steel for cold molds in any one of the Claims 1 thru | or 3 which contain more than 0 and 10 mass% or less of Co.
5. A cold press die manufactured by processing the steel for cold die according to any one of claims 1 to 4 and performing a surface treatment.

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