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

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,
• holding the coiled 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 for 5 hours or more and 12 hours or less;
• 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.

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;
• optionally further holding the coiled 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 for 5 hours or more and 12 hours or less;
• 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;
• 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;
• cooling the heated steel sheet down to a temperature of 200° C. or lower;
• and
• heating and holding the cooled 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 10 seconds or more,
• 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.

A high strength steel sheet having excellent formability, the steel sheet 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;
• and
• a microstructure including,
• in terms of area fraction, 30.0% or more and 80.0% or less of ferrite,
• wherein 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, and
• in terms of volume fraction, 10.0% or more of retained austenite,
• wherein the amount of Mn in the retained austenite is 6.0 mass % or more and 11.0 mass % or less,
• 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,
• wherein the aspect ratio of retained austenite is 2.0 or less.