JP2002003988A - Cold work tool steel with excellent machinability - Google Patents

Abstract

(57) [Summary] [Problem] While providing excellent machinability in the material state,
To provide a cold tool steel exhibiting excellent tool performance by quenching and tempering. SOLUTION: In mass%, C: 0.5 to 2.0%, S
i: 2.0% or less, Mn: 2.0% or less, Cr: 3-1
5%, Mo: 5% or less, W4% or less,
V3% or less, Nb: steel containing any one or more of 3% or less, and among the carbides constituting the steel, 5%
The average particle size of the fine carbide of less than μm is 1.0 to 2.0 μm
And In addition to the above chemical components, Ni of 3% or less, 5%
The following Co, and S, Te, Ca, Pb, Se, Z
Any one or more of r and Bi can be contained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a cold tool steel used for a press die, a bending die, a punch die, a drawing die, a die, a punch, a cutting blade, or the like.

[0002]

2. Description of the Related Art Cold dies such as press dies, bending dies, punch dies, draw dies, dies, punches, and plates require high hardness and wear resistance. ), Using tool steel such as special tool steel (SKS) and cold die steel (SKD), roughening by cutting in a state of being annealed and softened, then hardening and tempering to obtain the required hardness as a tool, The mold is finished with toughness and abrasion resistance. Finally, precision processing such as grinding and polishing is performed to improve dimensional accuracy and surface quality.

[0003] Conventionally, in cold tool steel,
Adjustment of the steel composition and metal structure has been performed with an emphasis on improving the mechanical properties after quenching and tempering, and the goal has been to finely and uniformly disperse carbides in the metal structure. . Originally, cold tool steel requires high hardness and abrasion resistance during use, so it has a high C content, and relatively contains elements that form strong carbides such as Cr, Mo, W, and V. Contains a lot. Therefore, it is difficult to perform cutting even in an annealed state, and it usually takes a lot of time to perform a rough machining step of a mold by cutting. However, in recent years, there has been a demand for more efficient cutting work due to the necessity for shortening the delivery time of die manufacturing, and it has been desired to improve the machinability of cold tool steel.

To cope with this, the machinability of cold tool steel has been improved by adding a free-cutting element, thereby improving the machinability to some extent. There have been problems such as deterioration in toughness after quenching and tempering, which affects performance.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a cold-working machine which has excellent machinability in the raw material state by adjusting the chemical composition and controlling the grain size of the carbide, and exhibits excellent tool performance by quenching and tempering. It aims to provide tool steel.

[0006]

Means for Solving the Problems To solve the above problems, the cold work tool steel of the present invention contains C: 0.5 to 0.5% by mass.
2.0%, Si: 2.0% or less, Mn: 2.0% or less,
Cr: 3 to 15%, Mo: 5% or less,
Any one of W4% or less, V3% or less, Nb: 3% or less
Containing at least two or more species, the balance being Fe and unavoidable impurities, of which 5 μm
The average particle diameter of the following fine carbide is 1.0 to 2.0 μm.

[0007] The cold tool steel of the present invention may contain not more than 3% of Ni in addition to the above chemical components in order to improve the hardenability and toughness of the steel. Further, in order to increase the high temperature tempering hardness of steel, Co can be contained at 5% or less.
Furthermore, in order to improve the machinability of steel, S: 0.03-
0.20%, Te: 0.005 to 0.05%, Ca:
0.0002-0.02%, Pb: 0.02-0.30
%, Se: 0.02 to 0.20%, Zr: 0.01 to
0.30%, Bi: 0.015 to 0.15%, any one or more of them can be contained.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a steel having the above-mentioned limited chemical composition, in which fine carbide grains capable of forming a solid solution with the steel contribute to the machinability of the steel, and the size of the carbide grains is large. It has been made based on a new finding that the machinability is reduced if the size is too large or too small. Hereinafter, the reason for limiting the content of the chemical component in the cold tool steel of the present invention will be described.

C: 0.4% to 2.0% C is a basic element of cold work tool steel, which increases the hardness of the steel and forms secondary carbides by tempering, thereby reducing the wear resistance of the steel. To improve the content. If the C content is less than 0.4%, quenching hardness required as a cold tool steel cannot be obtained, so the lower limit of the C content is set to 0.4%. Further, when C is excessively contained, coarse crystallized carbides are generated at the time of solidification of the steel, and a large amount of undissolved carbides remains at the time of quenching and heating, which causes a decrease in the toughness of the steel and a secondary carbide form. Therefore, the upper limit of the C content is set to 2.0% because the effect of improving the machinability due to the improvement of C cannot be secured.

Si: 2.0% or less Si is an element added to improve the hardenability of steel and to increase the temper hardness, but if contained excessively, the toughness is reduced. The upper limit is set to 2.0%.

Mn: 2.0% or less Mn improves the hardenability of steel. Further, it combines with S in the steel to form MnS, thereby preventing red hot embrittlement of the steel. However, an excessive content causes generation of retained austenite and lowers the toughness of the steel.
%.

Cr: 3 to 15% Cr is added to improve the hardenability of steel and increase the amount of secondary hardening by tempering. If the Cr content is less than 3%, the effect is small, so the lower limit of the Cr content is set to 3%. However, if the content exceeds 15%, the machinability of the steel decreases due to the increase in carbide having high hardness, so the upper limit of the Cr content is set to 15%.

Mo: 5% or less, W: 4% or less, V: 3%
Hereinafter, Nb: 3% or less Mo, W, V, and Nb all form hard carbides to improve the wear resistance of steel and are effective in increasing the high-temperature tempering hardness. Species or two or more species are contained. V and Nb also have the effect of preventing crystal grains from becoming coarse during quenching and heating. However, if these elements are contained excessively, they form coarse primary carbides and impair the toughness of the steel, so that Mo: 5%, W: 4%,
V: 3% and Nb: 3% are the upper limits of the contents, respectively.

Ni: 3% or less Ni is added to improve the hardenability of steel. When the Ni content exceeds 3%, retained austenite increases in the steel at the time of quenching, and it becomes difficult to secure necessary hardness.
Further, since the toughness of steel also decreases, the upper limit of the Ni content is set to 3%.

Co: 5% or less Co is added to improve the high-temperature tempering hardness of steel. However, if the content is excessive, the hardenability of the steel is reduced and the hardness and toughness are reduced, so the upper limit of the content is set to 5%.

S: 0.03-0.20%, Te: 0.0
05-0.05%, Ca: 0.0002-0.02%,
Pb: 0.02 to 0.30%, Se: 0.02 to 0.2
0%, Zr: 0.01 to 0.30%, Bi: 0.015
-0.15% S, Te, Ca, Pb, Se, Zr, and Bi are all added to improve the machinability of steel. S: 0.03% or more, Te: 0.0
05% or more, Ca: 0.0002% or more, Pb: 0.0
It is necessary to contain 2% or more, Se: 0.02% or more, Zr: 0.01% or more, and Bi: 0.015% or more. However, if contained excessively, the hot workability of the steel is impaired for S and Te, the toughness of the steel is reduced for Ca, Se and Bi, and the hot impact property of the steel is reduced for Pb. Since the content is reduced, the upper limit of the content is set to S: 0.20% and Te:
0.05%, Ca: 0.02%, Pb: 0.30%, S
e: 0.20%, Zr: 0.30%, Bi: 0.15%
And

[0017] The cold tool steel of the present invention is obtained by subjecting a steel having a limited chemical composition as described above to hot working such as hot forging and hot rolling and forming a shaped material having a required shape. After that, by applying an appropriate pre-heat treatment to the cast material, the grain size of the carbide in the steel is adjusted, and the machinability is improved.

According to the present invention, in the steel having the limited chemical composition, fine carbide grains having a size of 5 μm or less that can form a solid solution with the steel contribute to the machinability of the steel. It is based on the new finding that the machinability is reduced if the value is too large or too small. That is, the cold tool steel of the present invention is characterized in that, among the carbides constituting the steel, the average particle size of fine carbide of 5 μm or less is 0.8 to 2.0 μm. If the average particle size of the fine carbide of 5 μm or less exceeds 2.0 μm, the machinability of the steel decreases. Also, when the average particle size of the fine carbide of 5 μm or less is as small as less than 0.8 μm, the machinability of the steel also decreases.

Here, the average particle size of the fine carbide is an average particle size obtained by measuring the particle size distribution of a carbide extracted from steel by electrolytic extraction using a laser scattering type particle size measuring method. When two peaks appear in the frequency distribution of the carbide particle size obtained by the measurement, the two peaks are separated into single peaks by statistical processing, and among the separated peaks, the particle size is small. The average particle size obtained from the particle size distribution constituting the peak on the side is defined as the average particle size of the fine carbide.

By limiting the chemical composition as described above and adjusting the particle size of the fine carbides in the steel, it has excellent machinability in the raw material state and has excellent tool performance by quenching and tempering. Cold tool steel can be provided.

[0021]

EXAMPLE A steel containing the chemical components shown in Table 1 was melted by a high frequency induction furnace to produce a 150 kg steel ingot. The ingot was hot forged into a slab having a cross section of 40 mm × 60 mm. The slab was heated at 1050 ° C. × 1 h and then subjected to air-cooled heat treatment.
Next, after reheating to each pre-heat treatment temperature shown in Table 2 and holding for 2 hours, it was gradually cooled to 650 ° C. at a cooling rate of 15 ° C./h, and then allowed to cool to room temperature to obtain a test material. The following measurements and tests were performed on the test material.

[0022]

[Table 1]

Measurement of carbide particle size: 10 m in length from the test material
mx 10 mm wide x 10 mm high, a carbide test piece was collected, and a 0.5N hydrochloric acid aqueous solution was used as an electrolyte.
A was subjected to electrolytic extraction by the galvanostatic method, and the carbide obtained as a residue was centrifuged and dried in vacuum to obtain a carbide particle size measurement sample.

Using a 0.2% aqueous solution of sodium hexamethanoate as a dispersion medium, the carbide particle size measurement sample is dispersed in the dispersion medium by ultrasonic vibration, and the particle size distribution of settled particles is measured by a laser scattering particle size measurement method. did. When two peaks appeared in the frequency distribution, the two peaks were separated by statistical processing, and the average particle size obtained from the particle size distribution of the peak with the smaller particle size was defined as the fine carbide particle size. Cutting test:

A cutting test was performed on the test material under the following conditions. When the flank wear became 0.3 mm, the tool life was determined. The machinability of the test materials was compared.

Tool: carbide end mill (UTi20T), 1 flute Cutting width: 4.0 mm Cutting depth: 10. mm Feed: 0.035 mm / blade Cutting oil: None (dry type)

Hardness test: A test piece having a length of 40 mm, a width of 60 mm and a height of 10 mm was cut out from the test material, heated at each quenching temperature shown in Table 2 for 30 minutes, quenched by oil cooling, and quenched. After tempering for 1 hour at the tempering temperature shown in FIG. 2, the hardness of the surface portion was measured with a Rockwell C scale. Table 2 shows the test results.

[0028]

[Table 2]

As can be seen from Table 2, the tool life speeds of Comparative Examples 1 to 3 and Comparative Example 8 in which the chemical composition was within the range of the present invention but the fine carbide particle size was out of the range of the present invention. And the machinability is poor. In Comparative Examples 4 and 6, the fine carbide particle size is within the range of the present invention, but the chemical composition is out of the range of the present invention, and the tool life speed is low. In Comparative Examples 5 and 7, both the chemical composition and the fine carbide particle size are outside the limits of the present invention, and the tool life speed is low.

On the other hand, it can be seen that all the examples of the present invention show a high tool life speed and have excellent machinability. Further, as is apparent from Table 2, in the examples of the present invention, the hardness after quenching and tempering shows a sufficiently high value as a tool for cold working.

[0031]

As described above, by limiting the chemical composition and adjusting the particle size of the fine carbide of 5 μm or less in the steel, it is possible to obtain excellent machinability in the material state and to obtain quench hardening. A cold tool steel exhibiting excellent tool performance by reversion can be provided. Thus, it can be said that the economic effects of the present invention, such as shortening the production delivery time of the cold working tool and reducing the manufacturing cost, are extremely large.

Claims (4)

[Claims] 1. A mass% of C: 0.5 to 2.0%, Si: 2.0% or less, Mn: 2.0% or less, Cr: 3 to 15%, and Mo: 5% or less, W: 4% or less, V: 3% or less, Nb: 3% or less of any one or more of the following, the balance being Fe and unavoidable impurities; Among them, a cold work tool steel excellent in machinability, characterized in that the average particle size of fine carbide of 5 μm or less is 0.8 to 2.0 μm. 2. In addition to the above chemical components, Ni:
The cold work tool steel having excellent machinability according to claim 1, containing 3% or less. 3. In addition to the above chemical components, Co:
The cold work tool steel having excellent machinability according to any one of claims 1 and 2, wherein the cold work tool steel contains 5% or less. 4. In addition to the above chemical components, S: 0.03 to 0.20%, Te: 0.005 to 0.05%, Ca: 0.0002 to 0.02%, Pb: 0 0.02 to 0.30%, Se: 0.02 to 0.20%, Zr: 0.01 to 0.30%, Bi: 0.015 to 0.15%, or 2
The cold work tool steel excellent in machinability according to any one of claims 1 to 3, wherein the cold work tool steel contains at least one kind.