High strength steel sheet having excellent formability and method for manufacturing the same

摘要

A method for manufacturing the high strength steel sheet having excellent formability includes 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: 3.5% 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 Ar1 transformation point to the Ar1 transformation point+(the Ar3 transformation point−the Ar1 transformation point)/2, cooling the coiled steel sheet down to 200° C. or lower, heating and holding the cooled steel sheet at a temperature range of the Ac1 transformation point−200° C. to the Ac1 transformation point for 30 minutes or more, 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 Ac1 transformation point to the Ac1 transformation point+(the Ac3 transformation point−the Ac1 transformation point)/2 for 30 seconds or more.

主权项

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 Ar1 transformation point to the Ar1 transformation point+ (the Ar3 transformation point− the Ar1 transformation point) / 2; cooling the coiled steel sheet down to 200° C. or lower; heating and holding the cooled steel sheet at a temperature range of the Ac1 transformation point−200° C. to the Ac1 transformation point for 30 minutes or more and 750 minutes or less; 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 Ac1 transformation point to the Ac1 transformation point+ (the Ac3 transformation point− the Ac1 transformation point) / 2 for 30 seconds or more and 360 minutes or less, 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.