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.

1 . A cold working die steel consisting 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%≦Al≦0.10%, and the balance of Fe and inevitable impurities; 
 having transformation point Ar3 in the range of from 750° C. to 850° C., both ends inclusive;    having 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;    having a Brinell hardness attained after the spherodizing of HB179 to HB235, with both ends inclusive;    having 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    having a K value defined as Cr(mass %)−6.8×C(mass %) of 0.1 to 3.5, with both ends inclusive.    
   
   
       2 . The cold working die steel as claimed in  claim 1 , wherein the steel components are 0.60%≦C≦0.80%, 0.10%≦Si≦1.20%, 0.10%≦Mn≦0.60%, 5.5%≦Cr≦8.5%, 0.80%≦Mo+0.5W≦2.10%, 0.10%≦V≦0.40%, 0.0002%≦O≦0.0080% and 0.001%≦Al≦0.10%.  
   
   
       3 . The cold working die steel as claimed in  claim 1 , wherein the steel components are 0.90%≦C≦1.10%, 0.8%≦Si≦1.20%, 0.10%≦Mn≦0.60%, 7.0%≦Cr≦9.0%, 1.50%≦Mo+0.5W≦2.10%, 0.10%≦V≦0.40%, 0.0002%≦O≦0.0080% and 0.001%≦Al≦0.10%.  
   
   
       4 . The cold working die steel as claimed in  claim 1 , wherein the steel components are 1.40%≦C≦1.60%, 0.10%≦Si≦0.40%, 0.10%≦Mn≦0.60%, 11.0%≦Cr≦13.0%, 0.80%≦Mo+0.5W≦1.20%, 0.10%≦V≦0.40%, 0.0002%≦O≦0.0080% and 0.001%≦Al≦0.10%.  
   
   
       5 . The cold working die steel as claimed  claim 1 , further containing, as the steel component, either one of or both of 0.0030%≦N≦0.0500% and 0.001%≦P≦0.040%.  
   
   
       6 . The cold working die steel as claimed in  claim 1 , further containing any one of, or two or more of steel components selected from 0.01%≦Cu≦1.0%, 0.01%≦Ni≦1.0%, 0.2%≦Co≦1.0% and 0.0003%≦B≦0.010%.  
   
   
       7 . The cold working die steel as claimed  claim 1 , further containing any one of, or two or more of steel components selected from 0.001%≦S≦0.20%, 0.005%≦Se≦0.10%, 0.005%≦Te≦0.10%, 0.0002%≦Ca≦0.010%, 0.005%≦Pb≦0.10% and 0.005%≦Bi≦0.10%.  
   
   
       8 . The cold working die steel as claimed  claim 1 , further containing any one of, or two or more of steel components selected from 0.01%≦Nb≦0.12%, 0.005%≦Ta≦0.10%, 0.005%≦Ti≦0.10%, 0.005%≦Zr≦0.10%, 0.005%≦Mg≦0.10% and 0.005%≦REM≦0.10%.  
   
   
       9 . The cold working die steel as claimed in  claim 1 , wherein the spherodizing comprises keeping the steel heated, and cooling the steel to as low as 750° C. or lower at a cooling rate slower than 60° C./h.