HIGH-STRENGTH GALVANIZED STEEL SHEET HAVING EXCELLENT FORMABILITY AND CRASHWORTHINESS AND METHOD FOR MANUFACTURING THE SAME

摘要

A method of manufacturing a galvanized steel sheet having a composition containing, by mass %, C: 0.05% or more and 0.5% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.5% or more and 3.5% or less, P: 0.003% or more and 0.100% or less, S: 0.02% or less, Al: 0.010% or more and 0.5% or less, B: 0.0002% or more and 0.005% or less, Ti: 0.05% or less, a relationship of Ti>4N being satisfied, and the balance comprising Fe and inevitable impurities, and a microstructure containing 60% or more and 95% or less of tempered martensite in terms of area ratio and 5% or more and 20% or less of retained austenite in terms of area ratio

主权项

1. A method for manufacturing a high-strength galvanized steel sheet having excellent formability and crashworthiness, the method comprising:
hot-rolling a steel slab having a composition containing, by mass %, C: 0.05% or more and 0.5% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.5% or more and 3.5% or less, P: 0.003% or more and 0.100% or less, S: 0.02% or less, Al: 0.010% or more and 0.5% or less, B: 0.0002% or more and 0.005% or less, Ti: 0.05% or less, a relationship of Ti>4N being satisfied, and the balance comprising Fe and inevitable impurities at a finish rolling temperature of an A3 transformation point or higher; after the completion of finish rolling, subsequently cooling the resulting steel sheet to a coiling temperature at an average cooling rate of 30° C./s or more; coiling the steel sheet at the coiling temperature of 300° C. or higher and 550° C. or lower to form a hot-rolled steel sheet; then performing heat treatment on the hot-rolled steel sheet, the heat treatment including heating the hot-rolled steel sheet to an annealing temperature of (A3 transformation point−20° C.) or higher and (A3 transformation point+80° C.) or lower at an average heating rate of 5° C./s or more in a temperature range of 500° C. or higher and an A1 transformation point or lower, holding the steel sheet at the annealing temperature for 10 seconds or more, then cooling the steel sheet from 750° C. to a temperature range of 100° C. or higher and 350° C. or lower at an average cooling rate of 30° C./s or more, subsequently reheating the steel sheet to a temperature of 300° C. or higher and 600° C. or lower, and holding the steel sheet at the temperature for 10 seconds or more and 600 seconds or less; then galvanizing the steel sheet, and optionally performing an alloying treatment.