Abstract: A method for producing a low alloy hot rolled steel strip or sheet comprising hot rolling a steel slab at a finishing temperature not higher than the Ar.sub.3 transformation temperature plus 60.degree. C. and cooling and coiling the hot rolled strip at a temperature not higher than 500.degree. C. The steel slab contains not more than 0.20% carbon, 0.50 to 2.50% manganese and 0.05 to 1.0% chromium and optionally contains not more than 1.0% silicon, the balance being iron and unavoidable impurities. The resultant strip or sheet has a high tensile strength of not less than 40 kg/mm.sup.2, a low yeild ratio of not more than 70% and an excellent total elongation of not less than 25%.

Abstract: An apparatus for continuously producing a high strength dual-phase steel strip or sheet having a chemical composition which produces a dual-phase structure composed of ferrite grains and a low temperature transformed phase when rapidly cooled from the (.alpha.+.gamma.) temperature range or from the .gamma. temperature range. The apparatus has a continuous heat treatment furnace in which the strip moves vertically and which is composed of a heating zone, a soaking zone and a cooling zone, and conventional equipment associated therewith. The cooling zone has an upper hearth roll group, a lower hearth roll group and cooling equipment for cooling the strip at a rate in the range between 1.degree. and 300.degree. C./sec. for retaining the austenite in the steel of the strip.

Abstract: Process for producing a high strength, low yield ratio and high ductility dual-phase structure steel sheet having a structure composed mainly of a ferrite phase and a rapidly cooled transformation phase and having excellent formability with a tensile strength of 40 kg/mm.sup.2 or higher. The process comprises cooling the continuously annealed steel sheet under the following conditions:(1) 1.degree. C./second.ltoreq.R.sub.1 .ltoreq.30.degree. C./secondwherein R.sub.1 represents an average cooling rate from the continuous annealing temperature down to an intermediate temperature T.degree. C. in the cooling process.(2) 4.degree. C./second.ltoreq.R.sub.2 .ltoreq.100.degree. C./secondwherein R.sub.2 represents an average cooling rate from T.degree. C. to a temperature not higher than 200.degree. C.,(3) R.sub.1 <R.sub.2 and(4) 420.degree. C..ltoreq.T.ltoreq.700.degree. C.

Abstract: A method for producing a steel sheet of dual-phase structure composed of ferrite phase and transformed phase (or phases) produced by rapid cooling, which comprises isotropically dispersing a carbon concentrated phase by a pretreatment of the steel sheet and continuously annealing the steel sheet thus pretreated in the alpha-gamma range, followed by cooling at a rate not larger than about 10000.degree. C/min., as defined herein.