| 摘要 |
PURPOSE: To produce a steel material with high strength and high coefficient of thermal expansion by heat-treating a high carbon steel having a specific composition containing Al, Si, etc., under specific temp. conditions. CONSTITUTION: A steel stock, which has a composition consisting of by weight, 0.40-2.0% C, 0.50-3.0% Si, <2.50% Mn, <0.50% Cr, <1.5% Al, and the balance Fe and satisfying (Al+Si)=0.6 to 4.5% or further containing one or >=2 kinds among specific amounts of B, Ni, Mo, Cu, and N and further, as machinability improving elements, one or >=2 kinds among specific small amounts of S, Te, Pb, Ca, and Bi, is heated to a temp. not lower than the Ac, point, cooled down to a temp. between the Ms point and 550 deg.C at a cooling rate higher than the critical cooling rate for pearlite formation, isothermally held at that temp., and cooled down to room temp. By this method, the steel material, having a mixed structure consisting of 30-70vol.% of austenite with a thermal expansion coefficient higher than that of ferrite and the balance bainite or bainite and martensite and also having >343Mpa yield strength and >13×10<-6> / deg.C thermal expansion coefficient, can be obtained.
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