Abstract: A high yield ratio-type, hot rolled high strength steel sheet excellent in both formability and spot weldability, containing not less than 5% of retained austenite, and a process for producing the same are provided. The steel sheet contains 0.05 to less than 0.15% by weight or 0.15 to less than 0.30% by weight of C, 0.5 to 3.0% by weight of Si, 0.5 to 3.0% by weight of Mn, more than 1.5 to 6.0% by weight of Si and Mn in total, not more than 0.02% by weight of P, no more than 0.01% by weight of S, and 0.005 to 0.10% by weight of Al, the balance essentially being Fe, and is composed of three phases of ferrite, bainite and retained austenite as a microstructure, and having a ratio (V.sub.F /d.sub.F) of ferrite volume fraction (V.sub.F) to ferrite grain size (d.sub.F) of not less than 20 (not less than 7 in case of 0.15 to less than 0.30% by weight of C), a volume fraction of retained austenite having grain sizes of not more than 2 .mu.

Abstract: A hot rolled steel sheet with a high strength and a distinguished formability, and a process for producing the same are disclosed. The steel sheet comprises 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, and has a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. The steel sheet can be produced with a high productivity and without requiring special alloy elements.

Abstract: A hot rolled steel sheet with a high strength and a distinguished formability, and a process for producing the same are disclosed. The steel sheet comprises 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, and has a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being in a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. The steel sheet can be produced with a high productiviity and without requiring special alloy elements.