Method for manufacturing high-strength cold-rolled steel strip excellent in press-formability

摘要

A method for manufacturing a high-strength cold-rolled steel strip excellent in press-formability, which comprises the steps of: preparing a slab of an aluminum-killed steel consisting essentially of, in weight percentage: -Carbon from 0.02 to 0.06%, -Manganese from 0.06 to 0.25%, -Phosphorus from 0.01 to 0.06%, -Soluble aluminum from 0.020 to 0.060%, -Nitrogen up to 0.005%, and - the balance iron and incidental impurities; hot-rolling said slab to prepare a hot-rolled steel strip; coiling said steel strip at a temperature within the range of from 650 DEG to 770 DEG C.; cold-rolling said hot-rolled steel strip thus coiled to prepare a cold-rolled steel strip; subjecting said cold-rolled steel strip to a continuous annealing treatment for a prescribed period of time at a temperature within the range of from 750 DEG to 880 DEG C.; cooling said cold-rolled steel strip continuously annealed at a cooling rate of at least: exp {-5.6 (Cwt. % gamma +Mn wt. %/6+Si wt. %/24)+7.8} DEG C./sec from a temperature region of from Ar1 to Ar1+60 DEG C. to convert the structure thereof into a dual-phase structure of ferrite and a low-temperature transformation phase; and then, subjecting said cold-rolled steel strip having said dual-phase structure to an over-ageing treatment for a prescribed period of time at a temperature within the range of from 260 DEG to 360 DEG C.