| 摘要 |
PURPOSE:To make the stable production of a titled steel sheet possible with less amounts of hardening alloy elements, low equipment costs and low rates of cooling by preliminarily annealing the steel sheet at specific temps. below an AC1 transformation point prior to two phase area heating, in producing a steel of mixed structure. CONSTITUTION:A steel sheet obtained by rolling high tensile steel contg. by weight % 0.05- 0.20 C, <=1.0 Si and 0.5-2.0 Mn is first preliminarily annealed at temps. above 550 deg.C and below the AC1 point. Thereby, the chances for progression of distribution and reaction of Mn, Cr, Mo are given and the diffusion and migration of these elements from an alpha phase into FeC take place, resulting in an increase in said elements in carbide. Next, if this steel is subjected to alpha+gamma 2 phase area heating of the temps. between the AC1 and AC3 points, the parts around the carbide turn to a gamma phase and therefore the thickened gamma phase of the above-described elements is created by the preliminary annealing. Hence, the hardenability in the subsequent cooling process is improved, the formation of mixed structure is mode easy, and mechanical properties are improved. With the preliminary annealing temp. above the AC1 point, no object is achieved and below the lower limit, a long time is required. |
