Drawn heat treated steel wire for high strength spring use and pre-drawn steel wire for high strength spring use

Abstract

Drawn heat treated steel wire for high strength spring use is provided containing, by mass %, C: 0.67% to less than 0.9%, Si: 2.0 to 3.5%, Mn: 0.5 to 1.2%, Cr: 1.3 to 2.5%, N: 0.003 to 0.007%, and Al: 0.0005% to 0.003%, having Si and Cr satisfying the following formula: 0.3%≦Si−Cr≦1.2%, and having a balance of iron and unavoidable impurities, having as impurities, P: 0.025% or less and S: 0.025% or less, furthermore having a circle equivalent diameter of undissolved spherical carbides of less than 0.2 μm, further having, as a metal structure, at least residual austenite in a volume rate of over 6% to 15%, having a prior austenite grain size number of #10 or more, and having a circle equivalent diameter of undissolved spherical carbides of less than 0.2 μm.

1 . Pre-drawn steel wire for high strength spring use characterized by containing, by mass %,
 C: 0.67% to less than 0.9%,   Si: 2.0 to 3.5%,   Mn: 0.5 to 1.2%,   Cr: 1.3 to 2.5%,   N: 0.003 to 0.007%, and   Al: 0.0005% to 0.003%,   having Si and Cr satisfying the following formula:
   0.3%≦Si−Cr≦1.2%,
 
   having a balance of iron and unavoidable impurities,   having P and S as impurities comprising   P: 0.025% or less and   S: 0.025% or less, and,   furthermore,   having a circle equivalent diameter of undissolved spherical carbides of less than 0.2 μm.   
     
     
         2 . Pre-drawn steel wire for high strength spring use as set forth in  claim 1  characterized by, further, containing, by mass %, one or more of
 V: 0.03 to 0.10%, 
 Nb: 0.015% or less 
 Mo: 0.05 to 0.30%, 
 W: 0.05 to 0.30% 
 Mg: 0.002% or less, 
 Ca: 0.002% or less, and 
 Zr: 0.003% or less, 
 when containing V 
 satisfying 1.4%≦Cr+V≦2.6% and 0.70%≦Mn+V≦1.3%, and, 
 when containing Mo and W, 
 satisfying 0.05%≦Mo+W≦0.5%. 
 
     
     
         3 . Drawn heat treated steel wire for high strength spring use characterized by containing, by mass %,
 C: 0.67% to less than 0.9%,   Si: 2.0 to 3.5%,   Mn: 0.5 to 1.2%,   Cr: 1.3 to 2.5%,   N: 0.003 to 0.007%, and   Al: 0.0005% to 0.003%,   having Si and Cr satisfying the following formula:
   0.3%≦Si−Cr≦1.2%, and
 
   having a balance of iron and unavoidable impurities,   having P and S as impurities comprising   P: 0.025% or less and   S: 0.025% or less,   furthermore,   having a metal structure comprised of at least residual austenite in a volume rate of over 6% to 15%,   having prior austenite grain size number of #10 or more, and   having a circle equivalent diameter of undissolved spherical carbides of less than 0.2 μm.   
     
     
         4 . Drawn heat treated steel wire for high strength spring use as set forth in  claim 3  characterized by, further, containing, by mass %, one or more of
 V: 0.03 to 0.10%, 
 Nb: 0.015% or less 
 Mo: 0.05 to 0.30%, 
 W: 0.05 to 0.30% 
 Mg: 0.002% or less, 
 Ca: 0.002% or less, and 
 Zr: 0.003% or less, 
 when containing V 
 satisfying 1.4%≦Cr+V≦2.6% and 0.70%≦Mn+V≦1.3%, and, 
 when containing Mo and W, 
 satisfying 0.05%≦Mo+W≦0.5%. 
 
     
     
         5 . Drawn heat treated steel wire for high strength spring use as set forth in  claim 3  characterized in that said drawn heat treated steel wire for high strength spring use has a tensile strength of 2100 to 2400 MPa. 
     
     
         6 . Drawn heat treated steel wire for high strength spring use as set forth in  claim 5  characterized in that said drawn heat treated steel wire for high strength spring use has a yield strength of 1600 to 1980 MPa. 
     
     
         7 . Drawn heat treated steel wire for high strength spring use as set forth in  claim 3  characterized in that said drawn heat treated steel wire for high strength spring use has a surface Vicker's hardness of HV750 or more and an internal Vicker's hardness of HV570 or more aftersoft nitriding of keeping at 500° C. for 1 hour. 
     
     
         8 . A method of production of pre-drawn steel wire for high strength spring use characterized by taking a bloom containing, by mass %,
 C: 0.67% to less than 0.9%,   Si: 2.0 to 3.5%,   Mn: 0.5 to 1.2%,   Cr: 1.3 to 2.5%,   N: 0.003 to 0.007%, and   Al: 0.0005% to 0.003%,   having Si and Cr satisfying the following formula:   0.3%≦Si−Cr≦1.2%,   having a balance of iron and unavoidable impurities,   having P and S as impurities comprising   P: 0.025% or less and   S: 0.025% or less,   heating the bloom to 1250° C. or more, then hot rolling the bloom to produce a billet and heating the billet to 1200° C. or more, then hot rolling to produce pre-drawn steel wire.   
     
     
         9 . A method of production of pre-drawn steel wire for high strength spring use as set forth in  claim 8  characterized by the bloom further, containing, by mass %, one or more of
 V: 0.03 to 0.10%, 
 Nb: 0.015% or less 
 Mo: 0.05 to 0.30%, 
 W: 0.05 to 0.30% 
 Mg: 0.002% or less, 
 Ca: 0.002% or less, and 
 Zr: 0.003% or less, 
 when containing V 
 satisfying 1.4%≦Cr+V≦2.6% and 0.70%≦Mn+V≦1.3%, and, 
 when containing Mo and W, 
 satisfying 0.05%≦Mo+W≦0.5%. 
 
     
     
         10 . A method of production of pre-drawn steel wire for high strength spring use characterized by further heating pre-drawn steel wire as set forth in  claim 8  to 900° C. or more, then patenting it at 600° C. or less. 
     
     
         11 . A method of production of heat treated steel wire for high strength spring use characterized by
 drawing said pre-drawn steel wire which was produced by the method of production of pre-drawn steel wire as set forth in  claim 8 ,   heating it at a heating rate of 10° C./sec or more up to an A 3  point,   holding it at a temperature of the A 3  point or more for 1 minute to 5 minutes,   then cooling it at a cooling rate of 50° C./sec or more down to 100° C. or less.   
     
     
         12 . A method of production of heat treated steel wire for high strength spring use characterized by
 drawing said pre-drawn steel wire which was produced by the method of production of pre-drawn steel wire as set forth in  claim 10 ,   heating it at a heating rate of 10° C./sec or more up to an A 3  point,   holding it at a temperature of the A 3  point or more for 1 minute to 5 minutes,   then cooling it at a cooling rate of 50° C./sec or more down to 100° C. or less.   
     
     
         13 . A method of production of heat treated steel wire for high strength spring use as set forth in  claim 11  characterized by further holding and tempering it at 400 to 500° C. for 15 minutes or less. 
     
     
         14 . A method of production of heat treated steel wire for high strength spring use as set forth in  claim 12  characterized by further holding and tempering it at 400 to 500° C. for 15 minutes or less. 
     
     
         15 . Drawn heat treated steel wire for high strength spring use as set forth in  claim 4  characterized in that said drawn heat treated steel wire for high strength spring use has a tensile strength of 2100 to 2400 MPa. 
     
     
         16 . Drawn heat treated steel wire for high strength spring use as set forth in  claim 4  characterized in that said drawn heat treated steel wire for high strength spring use has a surface Vicker's hardness of HV750 or more and an internal Vicker's hardness of HV570 or more aftersoft nitriding of keeping at 500° C. for 1 hour. 
     
     
         17 . A method of production of pre-drawn steel wire for high strength spring use characterized by further heating pre-drawn steel wire as set forth in  claims 9  to  900 ° C. or more, then patenting it at 600° C. or less. 
     
     
         18 . A method of production of heat treated steel wire for high strength spring use characterized by
 drawing said pre-drawn steel wire which was produced by the method of production of pre-drawn steel wire as set forth in  claim 9 ,   heating it at a heating rate of 10° C./sec or more up to an A 3  point,   holding it at a temperature of the A 3  point or more for 1 minute to 5 minutes,   then cooling it at a cooling rate of 50° C./sec or more down to 100° C. or less.