A hot rolled a high strength steel sheet with a high ratio of yield strength and excellent formability usually bar or point and its production

摘要

To provide a high-yield-ratio hot-rolled high-strength steel sheet which is excellent in formability and spot weldability and contains at least 5 % of retained austenite and a process for producing the same. A high-yield-ratio hot-rolled high-strength steel sheet which contains as the main components either 0.05 to less than 0.16 wt % or 0.16 to less than 0.30 wt % of carbon, 0.5 to 3.0 wt % of silicon, 0.5 to 3.0 wt % of manganese, more than 1.5 to 6.0 wt % of silicon and manganese in total, 0.02 wt % or less of phosphorous, 0.01 wt % or less of sulfur, 0.005 to 0.10 wt % of aluminum, and iron, and has a microstructure constituted of three phases of ferrite, bainite and retained austenite, a ratio of the space factor (VF) of ferrite to the grain diameter (dF) thereof of 20 or above (or 7 or above when the carbon content is 0.16 to less than 0.30 wt %), a space factor of the retained austenite with a grain size of 2 νm or less of 5 % or above, a yield ratio (YR) of 60 % or above, a strength-ductility balance (tensile strength times total elongation) of 2,000 kgf/mm2.% or above, a hole expansion ratio (d/d¿0?) of 1.4 or above (or 1.1 or above when the carbon content is 0.16 to less than 0.30 wt %), and a uniform elongation of 15 % or above (or 10 % or above when the carbon content is 0.16 to less than 0.30 wt %).