| 摘要 |
PURPOSE:To convert the structure of a low-carbon steel having a specific structure to a mixed structure of ferrite and martensite, bainite, and in some case, residual austenite by heating said steel at a high temp. to a ferrite-austenite region then cooling the same. CONSTITUTION:A steel material contg. <0.3% C, <2.0% Si and 0.5-2.5% Mn is subjected to controlled rolling at <=30% cumulative draft at <=950 deg.C in which the rolling ends at Ar3+ or -50 deg.C to convert the structure thereof to a structure consisting principally of martensite or bainite or a mixed structure of ferrite the phase formed in low-temp. transformation of martensite or bainite in the case of forming said steel material to a plate material. The plate material is in succession heated at a heating rate of >=1,000 deg.C/min to a ferrite-austenite region and is then cooled to convert the structure to a mixed structure of ferrite and martensite, bainite and in some case residual austenite, by which the steel plate having stable yield strength without change in strength is obtd. |
