HIGH STRENGTH COLD ROLLED STEEL SHEET EXCELLENT IN PITTING CORROSION RESISTANCE AND CRUSHING CHARACTERISTIC, HIGH STRENGTH GALVANIZED STEEL SHEET, AND THEIR PRODUCTION

摘要

PROBLEM TO BE SOLVED: To produce a base material for high strength cold rolled steel sheet and galvanized steel sheet, excellent in pitting corrosion resistance and crushing characteristic, by hot-rolling a steel slab of specific composition under specific temp. conditions, cold-rolling the resultant plate into sheet, and then heating and cooling the sheet under respectively specified conditions. ;SOLUTION: A steel slab, having a composition containing 0.05-0.25% C and specific small amounts of Mn, P, Ti, Al, S, N, solid-solution Ti, and other elements, is hot- rolled at ≥800°C finishing temp., and the resultant steel plate is cooled down to ≤650°C at ≥5°C/sec average cooling rate and coiled. After the hot rolled steel plate is cold- rolled, the resultant steel sheet is heated in a continuous annealing furnace at a temp. not lower than the Ac₁ point for 10sec to 10min, cooled down to 850-650°C at ≥1°C/sec average cooling rated and successively cooled down to ≤450°C at ≥5°C/sec average cooling rate. By this method, the cold rolled steel sheet, consisting of ≤15vol.% ferrite and the balance, e.g. low temp. transformation structure of martensite, tempered martensite, and bainite and having 7500N/mm₂ tensile strength, is produced, or further, galvanizing, chromating treatment, and organic coating treatment are applied.;COPYRIGHT: (C)1997,JPO