Patent ID: 7909950
Filing Date: 2011-03-22
Classification: C21D,C22C

Abstract:
1. A method for manufacturing an ultra soft, high carbon hot-rolled steel sheet having a volume ratio of ferrite grains having a grain diameter of 10 μm or more which is 80% or more, comprising the steps of: performing rough rolling of a steel comprising on a mass percent basis: 0.2% to 0.7% of C, 0.01% to 1.0% of Si, 0.1% to 1.0% of Mn, 0.03% or less of P, 0.035% or less of S, 0.08% or less of Al, 0.01% or less of N, and the balance being Fe and incidental impurities, then performing a finish rolling at a reduction ratio of 20% or more and at a finish temperature of (Ar 3 −20)° C. or more in a final pass, then performing a first cooling within 2 seconds after the finish rolling to a cooling stop temperature of 600° C. or less at a cooling rate of more than 120° C./sec, then performing a second cooling so that the steel is held at 600° C. or less, then performing coiling at 580° C. or less, followed by pickling, and then performing a spheroidizing annealing at a temperature in the range of 680° C. to less than the Ac 1 transformation point by a box-annealing process, wherein in the texture of the ultra soft, high carbon hot-rolled steel sheet, an average ferrite grain diameter is 20 μm or more and an average carbide grain diameter is in the range of 0.10 to less than 2.0 μm.