Cold working die steel

Abstract

This invention is aimed at providing a cold working die steel successfully reduced in the hardness, and increased in readiness in the cold workability (forging, pressing and so forth) and machinability (milling, drilling, endmilling processing, grinding and lathe turning). A cold working die steel aimed at solving the above-described subject consists essentially of, in % by mass, 0.6%≦C≦1.60%, 0.10%≦Si≦1.20%, 0.10%≦Mn≦0.60%, 5.5%≦Cr≦13.0%, 0.80%≦Mo+0.5W≦2.10%, 0.10%≦V≦0.40%, 0.0002%≦O≦0.0080%, 0.001%≦A1≦0.10%, and the balance of Fe and inevitable impurities; has transformation point Ar3 in the range from 750° C. to 850° C., with both ends inclusive; has a mean circle-equivalent diameter of a carbide, which belongs to a circle-equivalent diameter range from 0.1 μm to 3 μm observed in a section of a structure obtained after spherodizing a sample that was heated at a temperature of (Ar3+50° C.) or above and 1,050° C. or below, of 0.25 μm to 0.8 μm, with both ends inclusive; has a Brinell hardness attained after the spherodizing of HB179 to HB235, with both ends inclusive; has a steel cleanliness of (dB+dC)60×400 in the group C inclusion and the group B inclusion specified by JIS G0555 of 0.05% or less; and has a K value defined as Cr(%)−6.8×C(%) of 0.1 to 3.5, both ends inclusive.

Description

FIELD OF THE INVENTION [0001] This invention relates to a cold working die steel used for cold die and structural components processed by forging or pressing in cold working; mechanical components demanding wear resistance; punch and die for cold forging; mold die for high tensile steel sheets; bending dies; cold forging dies; swaging dies; thread rolling dies; punch components; slitter knives; punch dies for lead frames; gauges; deep-drawing punches; bender punches; shear blades; benders for stainless steel; drawing dies; tools for plastic working such as heading; punches for gears; cam components; press punch dies; progressive punch dies; seal plates for soil conveyers; screw components; rotary plates for concrete spraying machines; IC molding dies; precision press mold demanding high dimensional accuracy; and dies used for the above-described applications after being surface-treated by CVD, PVD or TD. BACKGROUND ART [0002] SKD11, SKD12 and so forth, which are representative steel...

AI Technical Summary

Benefits of technology

[0010] On the contrary, a smaller mean circle-equivalent diameter results in a larger hardness by virtue of the denseness of the secondary carbide having relatively small sizes. More specifically, adjustment of the mean circle-equivalent diameter of the secondary carbide in the above-described range makes it possible to obtain a Brinell hardness of as low as HB179 to HB235, with both ends inclusive (conventional steels showed a Brinell hardness of as large as HB241 to HB255 or around), and to distinctively improve the efficiency in machining and cold working. (2) Control of the Size of the Secondary Carbide by Annealing Conditions

[0040] These elements can be added for the purpose of obtaining an effect of increasing the toughness through fineness of the carbide grains and fineness of the crystal grains. Mg and REM have an effect of increasing the toughness and machinability, through formation of oxides, which contribute to the reduction of the O content and consequently in coarse oxide grains. Addition to as much as the lower limit or above is preferable in view of obtaining these effects. On the other hand, excessive addition results in decreasing in the toughness and weldability, so that the amount of addition is preferably set to the upper limit or less. A single species or two or more species of rare earth metals may be used as REM.

Problems solved by technology

Moreover, no description is made on the hardness after quench-and-temper, despite a known problem in that annealing carried out at a temperature higher than hardening temperature results in lowering in the hardness after quench and temper than usual.

With cold tool steel, however, it is hard to achieve a low level of hardness by annealing intrinsically because of the composition thereof.

Steel having large amounts of coarse carbide are used for applications in particular demanding wear resistance such as dies, tools and mechanical components, in view of the wear resistance, but making too much account of wear resistance inevitably results in degradation of the material characteristics, such as a lowering of the overall toughness.

On the other hand, a decrease in the coarse carbide or an addition of a large amount of element enhancing free-cutting properties, such as S, aiming at improving the machinability, lowers the wear resistance required for tool steel.

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