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:
• 3.5% 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;
• 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;
• 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.

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:
• 3.5% 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;
• 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;
• 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;
• 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;
• 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.

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:
• 3.5% 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;
• and
• a microstructure including,
• in terms of area fraction, 30.0% or more 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 0.80 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
• in which the average grain size of the retained austenite is 2.0 μm or less.