Patent ID: 9580779
Date: 2017-02-28
CPC Classifications: B21B,B32B,C21D,C22C,C23C

Claim:
1. A method for manufacturing a high strength steel sheet having excellent formability, the method comprising: hot-rolling a steel slab having a chemical composition containing, by mass %, C: 0.03% or more and 0.35% or less, Si: 0.5% or more and 3.0% or less, Mn: 3.8% or more and 10.0% or less, P: 0.1% or less, S: 0.01% or less, N: 0.008% or less and the balance comprising Fe and inevitable impurities; coiling the hot-rolled steel sheet at a temperature range of the Ar cooling the coiled steel sheet down to 200° C. or lower; heating and holding the cooled steel sheet at a temperature range of the Ac pickling the heated steel sheet; cold-rolling the pickled steel sheet under the condition that the rolling reduction is 20% or more; and heating and holding the cold-rolled steel sheet at a temperature range of the Ac wherein the steel sheet includes a microstructure including: in terms of area fraction, 30.0% or more and 80.0% or less of ferrite, in which the ratio of the amount of Mn (mass %) in the ferrite divided by the amount of Mn (mass %) in the steel sheet is 0.20 or more and 0.80 or less; in terms of volume fraction, 10.0% or more of retained austenite, in which the amount of Mn in the retained austenite is 6.0% or more and 11.0% or less, and in which the average grain size of the retained austenite is 0.1 μm or more and 2.0 μm or less; and, in terms of area fraction, 3.0% or less of bainite, and an aspect ratio of the retained austenite is 2.0 or less.