Steel sheet with strain induced transformation type composite structure and method of manufacturing same

摘要

There is provided a steel sheet with strain induced transformation type composite structure having a predetermined chemical composition in which a microstructure at the ¼ thickness of the steel sheet includes, by area ratio, 50% to 85% of a polygonal ferrite, 3% to 10% of a residual austenite, 5% to 47% of bainite and 1% or less of a fresh martensite and a tempered martensite in total and satisfies the equation of 0.01Ex.C/fsd≤0.015, the microstructure includes 1×10¹⁶ pieces/cm³ or more of precipitates containing TiC, an average grain diameter of the residual austenite 1.0 μm to 5.0 μm at an equivalent circle diameter, an average of closest distances of the austenite is 3.0 μm to 10.0 μm, and an average diameter of the precipitates is 3 nm or less.