Apparatus for continuously producing a high strength dual-phase steel

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. The hearth rolls of each hearth roll group are arranged to guide the strip coming from the soaking zone up and down through the cooling zone from the inlet end to the outlet end, alternately passing around the upper hearth rolls and the lower hearth rolls. The hearth roll at the position where the strip is subjected to straining and begins to work harden as a result of the bending, and the hearth rolls subsequent to such position, have diameters such that the strain induced phase transformation of the retained austenite and consequently the work hardening of the strip when it is turned around the hearth rolls is avoided.