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

[0037] S can be added as an element increasing the free cutting property. Depending on the contents of the carbide-forming elements such as Cr, Mo and V, addition to as much as 0.04% or more is preferable in view of obtaining the effect of increasing the free cutting property. Excessive addition of the element considerably decreases the toughness, or mechanical properties including surface roughness after discharge processing and cutting, so that the upper limit is preferably adjusted to 0.20%. For applications making a great account of machinability, the element is added considering the balance with the mechanical properties. On the other hand, for applications in which a greater account is placed on the mechanical properties rather than on the machinability, the amount of addition of S is set to 0.02% or less, and more preferably to 0.01% or less, considering the balance with the cost for manufacturing. The mechanical properties can be satisfied by adjusting the amount of the addition to 0.01% or more and 0.02% or less for the practical operation. This makes it possible to obtain the S content described in the above.

[0038] Also Se, Te, Ca, Pb and Bi can be added for the purpose of increasing machinability. Se and Te can be used as substitutive elements of S in Mn sulfide. Ca improves the machinability by forming a protective film on the surface of a tool during cutting, by forming an oxide or by dissolving itself into Mn sulfide. Pb and Bi segregate in the grain boundary, to thereby lower the grain boundary strength and to improve the machinability. The elements are necessarily added to as much as the lower limits or more in view of obtaining these effects. On the other hand, excessive addition results in degraded mechanical properties, so that the upper limits should be met.

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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