JP2002241894A - High hardness prehardened steel for cold working having excellent machinability, die for cold working using the steel and working method for the steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high hardness prehardened steel which has excellent machinability, a die for cold working using the steel, and a working method for the steel. SOLUTION: The prehardened steel for cold working has a composition containing, by mass, 0.3 to <0.5% C, 0.7 to 2.0% Si and 0.08 to 0.25% S, and is refined to a hardness of 50HRC. Preferably, the prehardened steel concretely has a composition containing 0.3 to <0.5% C, 0.7 to 2.0% Si, 0.1 to 2.0% Mn, 0.08 to 0.25% S, 0.5 to 15.0% Cr, one or two kinds selected from W and Mo by <=3.5% in terms of (Mo+1/2 W), <=4.0% V and <=0.15% N, and the balance Fe with inevitable impurities. Then, the die for cold working is obtained subjecting the prehardened steel to cutting, e.g. at a cutting speed of >=50 m/min. Further, in the working of the steel having a hardness of >=50HRC at a cutting speed of >=50 m/min, the steel contains 0.3 to <0.5% C, 0.7 to 2.0% Si and 0.08 to 0.25% S.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold working mold used for a die for punching, bending, drawing and trimming of a steel sheet used for automobiles, household appliances, printed circuit boards, agricultural equipment and the like. The present invention relates to a prehardened steel having hardness, a mold using the same, and a method of processing steel that contributes to achieving these means.

[0002]

2. Description of the Related Art Processing such as punching is used for manufacturing parts such as automobiles and home appliances. The mold material used for the mold, especially the mold material for cold working, contains a large amount of carbide for imparting abrasion resistance, and further has excellent quenching properties and a large Cr content for securing toughness. For example, a high C-high Cr steel such as SKD11 which is an alloy tool steel specified in JIS G4404 is used.
When abrasion resistance is not particularly required, a low alloy steel such as SKS3 is also used after tempering at 300 ° C. or less after oil quenching.

[0003]

However, in recent years, in order to overcome price competition, automobile manufacturers and the like have been implementing cost reductions in all fields, and it has become necessary to reduce the number of mold manufacturing steps in relation to dies. On the other hand, as a general social trend, there is a shift to high-mix low-volume production, and from that point, it is increasingly important how quickly molds can be made.

[0004] At present, a general mold is manufactured by roughing a shape of a material in an annealed state mainly by cutting, quenching and tempering, and obtaining a hardness necessary for processing of a product such as punching. Enhanced. When quenching and tempering are performed, size changes due to heat treatment,
Since deformation occurs, finishing such as cutting and grinding is performed, and the manufacture of the mold is completed. Therefore, in order to improve the manufacturing efficiency of the mold, it is desirable that the machinability in the annealed state is good, and that the heat treatment does not change in size or deform.
A cold tool steel such as No. 71 has been proposed.

[0005] However, in the current situation where the demands for further man-hours and cost reduction for molds are increasing, cutting from the quenching and tempering state can omit the heat treatment and finishing after cutting, which are current general steps. There is a growing need for so-called pre-hardened steel, which can improve mold manufacturing efficiency.

[0006] At present, mold making using pre-hardened steel is performed with some plastic molds and hot forging dies, but the hardness is about 40 HRC, and 50 HRC required for punching by cold working or the like. It does not reach the above hardness. This is because the higher the hardness, the higher the resistance and impact to the tool during the cutting process, which exceeds the strength of the tool, causing early fracture and extending the life of the tool.

On the other hand, in the field of cutting, the cutting efficiency has been improved by high-speed machining. However, in the case of a high-hardness material exceeding 50 HRC, when the high-speed machining is performed, the cutting temperature is excessively increased in addition to the above-mentioned impact on the tool. As a result, the softening of the tool or the welding of the workpiece to the tool progresses, and the tool life also reaches an early stage. JIS SKD11 of existing cold working tool steel
Is the cutting process in the high hardness state after quenching and tempering,
For the above reasons, the efficiency and tool life requirements cannot be met in many cases from the viewpoint of shortening the man-hours for mold production.

Further, JIS SKS3 has better machinability in the quenched and tempered state than SKD11, but does not always satisfy the requirement for reduction of the mold manufacturing efficiency. The problem of distortion arises in electric discharge machining, which is often used in mold making together with machining.

As described above, it is practically difficult to cut a pre-hardened state with a hardness of 50 HRC or more for tool steel conventionally applied to molds and the like. Therefore, the present invention provides a 50 HR
Tool steel with improved hardness after quenching and tempering so that high hardness of not less than C can be obtained and processing with pre-hardened, a mold using the same, and a steel that contributes to the achievement of these means. A processing method is provided.

[0010]

Accordingly, the present inventors have studied the components which can obtain a high hardness of 50 HRC or more and have good machinability at the high hardness. And have led to the present invention.

That is, in mass%, C: 0.3% or more and less than 0.5%, Si: 0.7 to 2.0%, S: 0.08
Cold working steel containing ~ 0.25%, 50 HRC
As described above, it is a high hardness prehardened steel for cold working which has been tempered to a hardness of 55 HRC or more.

As a specific component composition, desirably, in mass%, C: 0.3% or more and less than 0.5%, Si: 0.7 to 0.7%
2.0%, Mn: 0.1-2.0%, S: 0.08-
0.25%, Cr: 0.5 to 15.0%, W or Mo
(Mo + ／ W) 3.5% or less, V: 4.0% or less, N: 0.15% or less, balance Fe
And a hardened prehardened steel for cold working comprising inevitable impurities.

More desirably, in mass%, C: 0.3
-0.45%, Si: 0.8-2.0%, Mn: 0.1
2.0%, S: 0.08-0.25%, Cr: 4.0
~ 6.0%, one or two of W or Mo (Mo
+ 1 / 2W), 2.0% or less, V: 1.0% or less, N:
It is a high-hardness prehardened steel for cold working comprising 0.15% or less, with the balance being Fe and unavoidable impurities.

Further, in addition to the above composition, Nb, Ta,
0.4% or less of one or more of Ti in total, N
i: 4.0% or less, Cu: 2.0% or less, Co: 5.0
% Or less, Zr: 0.2% or less, Se: 0.15% or less,
High hardness prehardened steel for cold working with Ca: 100 ppm or less or Al: 1.5% or less.

Further, in the tempered high-hardness pre-hardened steel for cold working, the high-hardness pre-hardened steel for cold working is used after being cut from the tempered state. High-hardness prehardened steel for cold working with a speed of 50 m / min or more. And, it is a mold for cold working formed by cutting the high hardness pre-hardened steel for cold working of the present invention.

The achievement of the prehardened steel and the cold working mold according to the present invention is achieved by cutting a steel having a high hardness.
It depends largely on where the optimum conditions for extending the life of the cutting tool have been established. That is, the steel processing method of the present invention is a steel processing method in which a cutting speed of 50 m / min or more is applied to steel tempered to a hardness of 50 HRC or more, and the composition of the steel is mass%. C: 0.3% to less than 0.5%, Si: 0.7 to 2.0%, S: 0.0
A steel working method characterized in that the steel contains 8 to 0.25%.

[0017]

DETAILED DESCRIPTION OF THE INVENTION The features of the present invention are 50 HRC or more,
Furthermore, the components, especially C, Si and S content, are optimized so that a hardness of 55 HRC or more can be obtained and the machinability in a high hardness state after quenching and tempering is good. . Hereinafter, the reasons for limiting the components in the present invention will be described.

C is an important element in the present invention. C
Is necessary for improving the hardenability and maintaining the hardness after the heat treatment. C combines with Cr, Mo, W, and V to form a carbide, and improves wear resistance and tempering softening resistance. Further, an MX type compound (TiC, VC, etc.) having a sufficient film thickness in a surface treatment performed for imparting wear resistance.
Is important in generating

However, in the case of high hardness after quenching and tempering, it is important to reduce the amount of C in order to improve machinability. When the hardness becomes higher than 50 HRC, the temperature of the workpiece increases during cutting, the tool is softened, and the tool life is extended. In addition, the welding of the workpiece to the tool increases, and when the welded material peels, the tool also peels off at the same time, leading to chipping and chipping of the tool, leading to a long tool life.

To improve machinability, the addition of S or T
Although a method based on morphological control of sulfide by addition of e or the like is used, it is important to reduce the amount of C in order to suppress a decrease in machinability due to a rise in cutting temperature at high hardness. The present inventors have found. Since the excessive increase in cutting temperature during cutting is suppressed by reducing the amount of C, machinability is improved. However, if it is too small, a high hardness of 50 HRC or more cannot be obtained. Therefore, it is important for the present invention to appropriately adjust the amount of C so as to reduce the amount of C within a range in which the required hardness can be obtained.
%, Desirably 0.3 to 0.45%.

Si is an important element for improving machinability. The oxide lowers the melting point during cutting due to the addition of Si, and an effective oxide film is formed to prevent welding between the tool and the workpiece.
This prevents direct contact between the tool and the workpiece and reduces tool wear, ie, improves machinability. In order to obtain this effect, it is necessary to add at least 0.7% or more. Further, Si is also contained for the purpose of improving the Si deoxidizing agent and castability. However, excessive content also causes intense segregation of the components of the matrix and lowers the toughness. Therefore, the content of Si is preferably 0.7 to 2.0%. Was set to 0.8 to 2.0%. More preferably 0.8 to
1.7%.

S is an element important for sulfide generation which enhances machinability. However, if added too much, the toughness and weldability are reduced, so the content was made 0.08 to 0.25%. Preferably it is 0.1% to 0.25%, especially 0.13 to 0.25%.

Next, a description will be given of specific component compositions which are preferable for achieving the high hardness prehardened steel for cold working of the present invention.

Mn is contained for improving hardenability.
If it is less than 1%, it is insufficient to obtain a stable quench hardness. It is also necessary for the production of MnS, a sulfide that improves machinability. On the other hand, if the content is too large, the segregation of the components in the matrix becomes intense as in the case of Si. Preferably it is 0.6 to 1.5%.

Cr combines with C to form carbides, thereby improving wear resistance and increasing hardenability.
This has the effect of preventing a kind of burning cracking phenomenon that occurs during cooling after surface treatment into a complex shape by VD treatment, salt bath method, or the like. However, if it is too large, it causes a decrease in toughness and machinability due to an increase in Cr carbide. Further, the temperature range of the solid-liquid coexistence becomes large, and the risk of casting defects increases, which substantially causes difficulty in manufacturability. Therefore, the amount of Cr added is 0.
It was set to 5 to 15.0%. In achieving the present invention, it is more preferably 4.0 to 6.0%.

Mo and W improve hardenability. Also C
To form hard carbides and improve wear resistance. In many cases, the effects on the characteristics of Mo and W are similar, and the degree of the effect is such that Mo is twice as large as W in terms of mass ratio. Therefore, the content contributing to the effect is (Mo + 1 /
2W). In the present invention, Mo, W
One or two of Mo, Mo,
May be replaced with a double W content, or a part of Mo may be replaced with a corresponding W content. Mo is preferably used from the viewpoint of economy. Excessive addition increases the amount of crystallization of Mo and W-based carbides and deteriorates machinability and toughness. Therefore, the content is set to 3.5% or less, preferably 2.0% or less. 0.2-1.1% is preferred.

V is an element that increases the softening resistance required for tool steel. However, when V is excessively contained, it crystallizes a huge V-based carbide at the time of solidification, which causes a reduction in machinability. Therefore, in the present invention, the content is set to 4.0% or less, preferably 1.0% or less. 0.1-0.5% is preferable.

N forms a solid solution in a matrix or carbide to refine crystal grains and increase toughness. It also increases the quenching and tempering hardness.
In the present invention, it does not matter whether it is added or contained. However, in order to obtain the above effects, a large amount of addition is not required, and the content is limited to 0.15% or less due to the limitation of the solid solubility limit. In the case where the above effects are obtained, 0.02% or more is sufficient.

In addition, in the case of the high hardness prehardened steel for cold working of the present invention, the following elements can be contained.

Nb, Ta, and Ti all suppress the growth of crystal grains during quenching and holding, refine the crystal grains, and increase the toughness. However, excessive addition generates coarse carbides and lowers the toughness. Therefore, the total content of Nb, Ta, and Ti is set to 0.4% or less. In the case where the above effects are obtained, the content is preferably 0.008% or more.

Ni is an element whose toughness is improved by increasing the quenchability and impact transition temperature. However, in the present alloy system, deterioration of weldability can be particularly prevented, and the surface treatment region which can be practically operated is expanded. Works. However, excessive addition deteriorates machinability, so that the content is set to 4.0% or less. When the above effects are obtained, the content is preferably 0.01% or more.

Cu is an element that improves quenching and tempering hardness. Even if the amount of C is reduced to improve machinability, hardness can be secured by adding Cu. Excessive content impairs the hot workability during the production of the material, so it is set to 2.0% or less. When the above-mentioned effect is obtained, it is preferably at least 0.5%.

Co is an element that suppresses agglomeration of finely precipitated carbides during tempering, and provides high hardness. Therefore, even if the amount of C is reduced to improve machinability, hardness can be secured by adding Co. Excessive content increases the production cost, so is made 5.0% or less. When the above effects are obtained, it is preferably 1.0% or more.

Zr becomes ZrO ₂ or ZrN which has an effective inoculating action on sulfides, so that the sulfides are uniformly distributed and the machinability is improved. ZrO ₂ or Z
Since the amount of rN increases and the machinability is impaired, the content is set to 0.2% or less. When the above effects are obtained, the content is preferably 0.005% or more.

Se is an element that suppresses elongation of sulfide by forging and improves machinability. However, an excessive content causes a decrease in mechanical properties, so the content was made 0.15% or less. When the above effects are obtained, it is preferably at least 0.03%.

Ca is a free-cutting element that does not cause a decrease in mechanical properties. The free-cutting mechanism lowers a small amount of oxides dispersed in the steel to lower the melting point, which melts out by cutting heat and forms a protective film on the cutting edge. Further, it has an effect of suppressing the sulfide from being stretched in the forging and stretching direction and suppressing a decrease in toughness in the forging and stretching perpendicular direction. However, since the vapor pressure is high, it easily escapes from the molten steel, and up to about 100 ppm is the current technical level of addition. In order to obtain the above effect, the content is preferably 10 ppm or more.

In addition, the tool steel of the present invention has Pb, Te, Bi, I
There is no problem if one or more of n, Be, and Ce are contained in a total amount of 0.2% or less.

In addition, rare earth elements can be contained in a total amount of 0.2% or less for the purpose of improving machinability in the tool steel of the present invention. The total amount of inevitable impurities is preferably 0.5% or less. Further, if further wear resistance is required,
Can be added to increase the nitriding hardness by 1.5% or less. In this case, the content is preferably 0.05% or more and 0.5% or less in order to prevent chipping or the like due to an excessive increase in nitriding hardness.

In the present invention, the precipitation hardening of NiAl can be used so that the hardness after quenching and tempering can be obtained at 50 HRC or more, and even at 55 HRC or more even if the C content is low. For this purpose, Ni is 4.0% or less, and Al is 1.0% or less.
It is effective to add 5% or less and 1.5% or less of Cu.

Next, the tempering temperature and hardness will be described. The die for cold working used for punching, bending, etc. is 50HRC or more from the viewpoint of abrasion resistance, furthermore 54HRC or more,
Desirably, hardness of 55 HRC or more is required. Therefore, the hardness in the present invention is 50 HRC or more, preferably 55 HR.
C or higher.

For machining these dies, electric discharge machining is widely used in addition to cutting. In this case, if the tempering temperature is low, quenching strain in the material remains, and the strain is released after electric discharge machining, which may cause deformation or cracking. Furthermore, since surface treatments such as nitriding and PVD performed for imparting wear resistance are generally performed at least at 400 ° C. or higher, the tempering temperature of the material in performing this is required to be higher than the surface treatment temperature. Tempering is 500 ℃
It is desirable to perform the above.

Next, the cutting speed will be described. Even a high hardness material of 50 HRC or more can be cut to some extent by performing low-speed cutting using a coated carbide end mill so as to suppress an excessive increase in the cutting temperature. However, in this case, the number of machining steps is increased, and as a result, it is impossible to improve the mold manufacturing efficiency. Therefore, in the present invention, the excessive increase in the cutting temperature is suppressed mainly by reducing the amount of C,
The feature is that the machinability is improved.

That is, with the high hardness pre-hardened steel of the present invention, the cutting temperature does not increase so much even if the cutting speed is increased. An improvement in mold processing efficiency can be achieved. In this case, in consideration of the mold processing efficiency, it is desirable that the cutting speed is 50 m / min or more in order to exert the effect of the present invention. The machining at the cutting speed according to the present invention is particularly performed by end milling, face milling, turning, and the like.

As described above, the high hardness pre-hardened steel of the present invention enables efficient die cutting even at a high hardness of 50 HRC or more, and even 55 HRC or more. Can be shortened. Furthermore, the steel of the present invention has high hardness and high machinability,
It can also be used for auxiliary tools such as resin-type, rubber-type, plastic-type, hot-working-type spacers and holders that require wear resistance including glass.

[0045]

EXAMPLES Next, examples of the present invention will be described in detail, but the present invention is not limited to these examples.

Example 1 A predetermined alloy was melted in a high-frequency furnace to produce a steel ingot having the chemical composition shown in Table 1. Comparative steel 18 is a component equivalent to JIS SKD11. These steel ingots were forged at a forging ratio of 5 to finish them into steel and then annealed. Next, these annealed materials are heated at 1030 ° C. in an atmospheric furnace.
, And then air quenched and tempered at 500 to 600 ° C. Hardened and tempered material is 40 × 50 × 200m
m and finished as a sample for cutting test. Table 1 also shows the hardness after quenching and tempering. The comparative steel 19 is C
Since the amount is too low, a hardness of 50 HRC or more has not been obtained.

[0047]

[Table 1]

A cutting test using a square end mill was performed using these samples, except for Comparative Steel 19, which did not achieve a hardness of 50 HRC or more. The test conditions are shown in Table 2.
For the machinability, the cutting length until the flank wear of the tool reached 0.1 mm was evaluated as the tool life. Table 3 shows the machinability evaluation results.

[0049]

[Table 2]

[0050]

[Table 3]

In processing at a relatively low cutting speed of 30 m / min, some comparative steels can be cut to some extent.
In the high-speed machining at a cutting speed of 150 m / min, the tool life of the steel of the present invention, that is, the machinability is good, while the comparative steel 1
8 has a high C content and a low Si content, the comparative steel 20 has a high C content, the comparative steel 21 has a low Si content, the comparative steel 22 has a low S content, and the comparative steel 23 has a high C content. Machinability is poor. The comparative steel 24 has a low hardness but a high C content, and the comparative steel 25 has a high C content and a low S content, so that the machinability is inferior.

(Example 2) Using pre-hardened materials of the present invention steel 2 and comparative steels 18 and 24 in Table 1, the cutting temperature at the time of end mill cutting was measured. The dimensions of the prehardened material are 40 × 50 × 200 mm plates. Table 4 shows the hardness of the pre-hardened materials of Invention Steel 2, Comparative Steels 18 and 24,
Table 5 shows the amount of carbide and the amount of solute carbon, and Table 5 shows the cutting conditions and the method of measuring the cutting temperature. In addition, when compared with steel of the same composition, if the hardness is low, the cutting temperature is naturally low. Therefore, in this evaluation, the hardness of the steel 2 of the present invention is made higher than that of the comparative steels 18 and 24, thereby making the effect of the working method of the present invention more remarkable.

[0053]

[Table 4]

[0054]

[Table 5]

FIG. 1 shows the cutting temperature measurement results. When the cutting speed was 30 m / min, there was no significant difference in cutting temperature with any steel type, but Comparative Steel 24 was the lowest in the order of hardness, followed by Comparative Steel 18 and Steel 2 of the present invention. Next, the cutting temperature of all steel types increased with the increase of the cutting speed. However, the difference in the increasing tendency of the cutting temperature began when the cutting speed exceeded about 50 m / min. This depends on the concentration of carbon dissolved in the material base,
In the case of a cutting speed of 150 m / min, the cutting temperature of the steel 2 of the present invention having the lowest amount of solute carbon of about 0.35% is the lowest, and then the comparative steel 18 having a cutting temperature of about 0.60%, and about 0.05%.
The cutting temperature of the comparative steel 24, which is 80%, is the highest, and each temperature difference reaches about 100 ° C.

High-hardness steel is generally cut with a tool coated with TiAlN. A factor that affects the life of the coated tool is the oxidation start temperature of the coating film. For this purpose, it is effective to cut steel at a cutting temperature lower than this temperature. The oxidation start temperature of the TiAlN coating film applied to the tool is generally around 800 ° C. In the case of the steel 2 of the present invention, the cutting temperature of 800 ° C. or less is maintained even when the cutting speed reaches 200 m / min. It has excellent machinability.

Further, as a factor largely affecting the machinability, there is a carbide distribution in the raw material. In the case of the comparative steel 18, in addition to the above, a large number of hard carbides are precipitated, so that the abrasive wear is remarkably caused, which also has an influence. It has poor machinability.

(Example 3) Using a pre-hardened material of the steel 2 of the present invention and the comparative steel 18 shown in Table 1, a mold production test was performed assuming a printed circuit board type. The dimensions of the pre-hardened material are 200 × 200 × 10 mm plates,
The drilling of m is performed for 100 holes.

As for the comparative steel 18, a mold was also manufactured before the quenching and tempering, that is, from a material in an annealed state. In other words, it is a current general mold manufacturing process, in which holes are formed in an annealed state, and then heat treatment is performed, followed by finishing and finishing. The size of the annealed material is 0.1% of the thickness of the pre-hardened material.
It is increased by 5 mm. The heat treatment conditions for the annealed material were quenching at 1030 ° C., vacuum pressure cooling, and tempering at 540 ° C.

Table 6 shows the hole cutting conditions (finish processing conditions for the comparative steel 18 manufactured from the annealed state) in the above-described die manufacturing, and Table 7 shows the die manufacturing man-hours from the drilling. .

[0061]

[Table 6]

[0062]

[Table 7]

In the drilling of the comparative steel 18 from the pre-hardened material, the drill was broken at 30 holes, and the die could not be machined. Also, in the production of a mold from an annealed material, which is a current general mold production process, when the comparative steel 18 is used as a material, heat treatment is performed after drilling, and a dimension change due to the heat treatment occurs. Production man-hours increase.

On the other hand, in the case of producing a mold from the pre-hardened material of the steel 2 of the present invention, it is possible to make a hole after the tempering, and it is not necessary to perform a heat treatment or a finishing process thereafter. Can be reduced. Even if the cutting efficiency of the comparative steel 18 from the annealed material is increased by using a carbide drill for drilling, it does not reach the man-hours required for processing the steel 2 of the present invention from the pre-hardened material.

[0065]

As described above, according to the steel of the present invention, the hardness of 50 HRC or more required for the die for cold working such as punching, and the hardness of 55 HRC or more can be obtained, and quenching and tempering. Even in the so-called pre-hardened state, since the machinability with high hardness is good, it is possible to expect an improvement in mold manufacturing efficiency and a reduction in cost. The industrial value according to the invention is high.

[Brief description of the drawings]

FIG. 1 is a diagram showing an increase in cutting temperature with an increase in cutting speed, and is a diagram for explaining an example of an effect of the present invention.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Yoshihiro Kada 2107-2 Yasugi-cho, Yasugi-shi, Shimane Pref.

Claims (15)

[Claims] 1. In mass%, C: 0.3% to less than 0.5%, Si: 0.7 to 2.0%, S: 0.08 to 0.25
% Cold-working hardened steel with excellent machinability, characterized in that it is tempered to a hardness of at least 50 HRC. 2. The high-hardness prehardened steel for cold working having excellent machinability according to claim 1, wherein the hardened steel has a hardness of 55 HRC or more. 3. A high-hardness pre-hardened steel for cold working, which is used after being cut from the tempered state,
The high-hardness prehardened steel for cold working having excellent machinability according to claim 1 or 2, wherein the cutting speed is 50 m / min or more. 4. In mass%, C: 0.3% or more and less than 0.5%, Si: 0.7-2.0%, Mn: 0.1-2.0.
%, S: 0.08 to 0.25%, Cr: 0.5 to 15.
0%, one or two of W or Mo (Mo + 1 /
2W) 3.5% or less, V: 4.0% or less, N: 0.1
The high-hardness prehardened steel for cold working excellent in machinability according to any one of claims 1 to 3, wherein the steel comprises 5% or less, the balance being Fe and inevitable impurities. 5% by mass, C: 0.3% to 0.45%,
Si: 0.8 to 2.0%, Mn: 0.1 to 2.0%,
S: 0.08 to 0.25%, Cr: 4.0 to 6.0%,
One or two of W or Mo (Mo + 1 / 2W)
The machinability according to any one of claims 1 to 3, wherein 2.0% or less, V: 1.0% or less, N: 0.15% or less, and the balance is Fe and unavoidable impurities. High-hardness pre-hardened steel for cold working with excellent performance. 6. The machinability according to claim 1, wherein at least one of Nb, Ta, and Ti is at most 0.4% by mass. High-hardness pre-hardened steel for cold working with excellent performance. 7. The high-hardness prehardened steel for cold working excellent in machinability according to claim 1, wherein Ni is not more than 4.0% in mass%. 8. The high-hardness prehardened steel for cold working excellent in machinability according to claim 1, wherein Cu is not more than 2.0% by mass%. 9. The high-hardness prehardened steel for cold working excellent in machinability according to claim 1, wherein the content of Co is not more than 5.0% by mass. 10. The high-hardness prehardened steel for cold working excellent in machinability according to any one of claims 1 to 9, wherein Zr: 0.2% or less by mass%. 11. The high-hardness prehardened steel for cold working excellent in machinability according to claim 1, wherein the mass% is Se: 0.15% or less. 12. The high-hardness prehardened steel for cold working excellent in machinability according to claim 1, wherein the mass ratio is Ca: 100 ppm or less. 13. The high-hardness prehardened steel for cold working excellent in machinability according to claim 1, wherein Al is not more than 1.5% by mass%. 14. A cold working mold obtained by cutting the high-hardness pre-hardened steel for cold working according to any one of claims 1 to 13. 15. A method of processing steel, wherein a cutting speed of 50 m / min or more is applied to steel tempered to a hardness of 50 HRC or more, wherein the composition of the steel is C: 0.3 in mass%.
% To less than 0.5%, Si: 0.7 to 2.0%, S:
A method for processing steel, comprising steel containing 0.08 to 0.25%.