HOT ROLLED STEEL SHEET EXCELLENT IN IMPACT RESISTANCE AND STRENGTH-AND-ELONGATION BALANCE AND HAVING HIGH STRENGTH AND HIGH WORKABILITY, AND ITS PRODUCTION

摘要

PROBLEM TO BE SOLVED: To improve formability, impact resistance, amounts of work hardening and baking hardening, and a balance between strength and elongation by providing a specific composition containing C, Si, Mn, and Cr, also providing a structure where primary phase is composed of pro-eutectoid ferrite and secondary phase is composed of martensite, acicular ferrite, and retained austenite, and regulating the crystalline grain size of pro-eutectoid ferrite to a specific value or below. ;SOLUTION: This steel sheet has a composition consisting of, by mass, 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, and the crystalline grain size of pro-eutectoid ferrite is regulated to ≤5 μm. A slab having the above composition is roughed, and finish rolling is finished at 780-980°C while regulating the draft at the final pass to ≥15%, followed by cooling down to 620-780°C. Subsequently, the resultant plate is cooled down to 350-500°C by isothermal holding or slow cooling treatment, coiled, and then cooled down to ≤300°C at (10 to 100)°C/h cooling rate. By this method, the steel plate can be produced.;COPYRIGHT: (C)1999,JPO