Abstract: The present invention relates to A high strength steel sheet having a structure which is mainly composed of MD structure (Micro Duplex structure) comprising a ferrite matrix, and as a secondary phase, martensite or martensite and retained austenite, finely dispersed in said matrix, wherein the proportion that the MD structure occupies in the whole structure is 90% or more, wherein the proportion that the secondary phase present in the whole structure occupies in the whole structure is from 10 to 60%, wherein the secondary phase in the MD structure is present in ferrite grains and at grain boundary, in which the proportion of the secondary phase present in the ferrite grains is 50% or more, and wherein the average grain size of the secondary phase in the whole structure is 3 ?m or less. The secondary phase is constituted of martensite, or martensite and retained austenite.

Abstract: A high tensile strength, hot or cold rolled steel sheet having improved ductility and hole expandability consists essentially, on a weight basis, of: C: 0.05-0.3%, Si: 2.5% or less, Mn: 0.05-4%, Al: greater than 0.10% and not greater than 2.0% wherein 0.5.ltoreq.Si(%)+Al(%).ltoreq.3.0, optionally one or more of Cu, Ni, Cr, Ca, Zr, rare earth metals (REM), Nb, Ti, and V, and a balance of Fe and inevitable impurites with N being limited to 0.01% or less. The steel sheet has a structure comprising at least 5% by volume of retained austenite in ferrite or in ferrite and bainite. A hot rolled steel sheet is produced by hot rolling with a finish rolling end temperature in the range of 780.degree.-840.degree. C., cooling to a coiling temperature in the range of 300.degree.-450.degree. C. either by rapid cooling to the coiling temperature at a rate of 10.degree.-50.degree. C./sec or by initial rapid cooling to a temperature range of 600.degree.-700.degree. C.

Abstract: A hot-rolled high tensile titanium steel plate and production thereof are disclosed. The steel plate has improved toughness and cold formability and is made of a killed steel which consists essentially of:______________________________________ C: 0.05-0.20 wt %, Si: not more than 1.2 wt %, Mn: 0.5-2.0 wt %, Ti: 0.04-0.20 wt %, P: not more than 0.025 wt %, S: not more than 0.015 wt %, sol. Al: 0.005-0.15 wt %, O: not more than 0.0080 wt %, N: not more than 0.0080 wt %, B: 0-0.0030 wt %, Cr: 0-1.0 wt %, Ca: 0-0.010 wt %, ______________________________________the balance being Fe and incidental impurities, the Ti content comprising not less than 0.02 wt % of incoherently precipitated Ti and not more than 0.015 wt % of coherently precipitated Ti, and said killed steel containing 20 to 90% by volume of a bainitic structure and not less than 10% by volume of a ferritic structure.