| 摘要 |
PROBLEM TO BE SOLVED: To impart superior formability, impact resistance, machining and baking characteristic to a steel plate by making it contain a specific quantity of C, Si, Mn and Cr, making a mixed structure consisting of pro-eutectoid ferrite for the principal phase, martensite, acicular ferrite and residual austenite for the second phase, and also specifying the surface roughness. SOLUTION: The composition of the steel plate is constituted, in mass %, of 0.05-0.40% C, 1.0-3.0% Si, 0.6-3.0% Mn, 0.2-2.0% Cr, and the balance essentially Fe. In addition, with the structure consisting of pro-eutectoid ferrite for the principal phase and of martensite, acicular ferrite and residual austenite for the second phase, the roughness on the surface of the steel plate is made 3μm or below in Ra. It is desirable that the ratio of the second phase is made 3-40% in the structure of the steel and that the ratio of each phase in the second phase is 10-80% martensite, 8-30% residual austenite, and 5-60% acicular ferrite. As necessary, the structure is designed to contain for example 0.01-0.2% P and 0.01-0.3% Al.
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