Patent ID: 6554925
Filing Date: 2003-04-29
Classification: C21D,C22C

Abstract:
A method for manufacturing a cold-rolled steel sheet comprising the steps of:(a) providing a slab consisting essentially of 0.02% or less C, 0.5% or less Si, 2.5% or less Mn, 0.10% or less P, 0.05% or less S. 0.003% or less O, 0.003% or less N, 0.01to 0.40% of at least one element selected from the group consisting of Ti, Nb, V, and Zr, by weight, optionally 0.0001 to 0.005% by weight of B, and a balance being Fe; (b) rough-rolling the slab by a rough-rolling mill to form a sheet bar; (c) finish-rolling the sheet bar by a continuous hot finish-rolling mill to form a hot-rolled steel strip, the finish-rolling comprising finish-rolling the sheet bar so that a material temperature at a final stand of the finish-rolling mill becomes an Ar3 transformation point or more over the whole range of from a front end of the sheet bar to a rear end thereof; (d) cooling the hot-rolled steel strip on a runout table and coiling the cooled hot-rolled steel strip at a coiling temperature, the cooling on the runout table beginning with a time range of from more than 0.1 second and less than 1.0 second after completing the finish-rolling, the cooling on the runout table being conducted at an average cooling speed in a temperature range of from a hot-rolling finish temperature to 700Â° C. being 120Â° C./sec or more, an average cooling speed in a temperature range of from 700Â° C. to the coiling temperature being 50Â° C./sec or less, the coiling temperature of the hot-rolled steel strip being less than 700Â° C.; and (e) pickling and cold rolling the hot-rolled steel strip, and final annealing the cold-rolled steel strip.