Heat treatment for manufacturing spring steel excellent in high-temperature relaxation resistance

A method for manufacturing a spring steel, excellent in quenchability and high-temperature relaxation resistance, which comprises the steps of:

• providing a steel consisting essentially of, each by 0.4-0.8% C, 0.5-2.5% Si, 0.3-2.0% Mn, 0.1-1.5% Cr, 0.1-0.5% Mo, and the balance being Fe except inevitable impurities, wherein:
• -7.ltoreq.4.times.Si(%)-10Cr(%).ltoreq.5;
• which method comprises:
• hot rolling said steel;
• annealing said hot rolled steel sheet;
• subjecting said annealed steel sheet to at least one cycle of cold rolling at a reduction of 10-80% and annealing at a temperature of about 550.degree.-730.degree. C. for a time sufficient to precipitate spheroidal carbides of about 2 .mu.m or less in average particle size;
• heating said annealed steel sheet at a temperature above its Ac.sub.3 transition point for a time sufficient to substantially dissolve said spheroidal carbides in an austenite matrix;
• rapidly cooling said heated steel sheet at a lower critical cooling speed to an extent sufficient to completely transform the steel matrix in said heated steel sheet to a martensite state oversaturated with carbon;
• tempering said cooled steel sheet at a temperature of about 450.degree.-600.degree. C. for a time sufficient to precipitate fine Mo.sub.2 C particles in the martensite matrix;
• and then
• cooling the tempered steel sheet to a room temperature.

A method for manufacturing a spring steel, excellent in quenchability and high-temperature relaxation resistance, which comprises the steps of:

• providing a steel consisting essentially of, each by weight, 0.4-0.8% C, 0.5-2.5% Si, 0.3-2.0% Mn, 0.1-1.5% Cr, 0.1-0.5% Mo, and the balance being Fe except inevitable impurities;
• hot rolling said steel;
• annealing said hot rolled steel sheet;
• subjecting said annealed steel sheet to at least one cycle of cold rolling at a reduction of 10-80% and annealing at a temperature below its Ac.sub.1, transition point, to precipitate fine spheroidal carbides;
• heating said annealed steel sheet at a temperature above its Ac.sub.3 transition point for a time sufficient to substantially dissolve said spheroidal carbides in an austenite matrix;
• rapidly cooling said heated steel sheet at a lower critical cooling speed under conditions sufficient to completely transform the steel matrix in said steel sheet to a martensite state oversaturated with carbon;
• tempering said cooled steel sheet at a temperature condition to adjust temper hardness to about HV 400-550;
• and then
• cooling said tempered steel sheet to a room temperature.