An as hot rolled approach to production of molybdenum and chromium microalloyed dual phase steels.

Abstract

Because of improved strength-ductility combination over HSLA steels, dual phase steels have recently become of commercial importance to both the sheet users and producers. These steels possess good properties by virtue of their microstructure which consists, typically, of about 15-20% martensite uniformly distributed in a soft matrix of ferrite. Although, the desired microstructural features of a dual phase steel can be obtained by various process routes, the most economical method is the production of this steel in as hot rolled condition. The sucessful production of dual phase steels in the hot strip mill, however, requires a careful control of process parameters particularly the finishing temperature, the cooling of the sheet on the runout table, the coiling temperature and the subsequent cooling of the coils.As a development effort some commercial heats of dual phase steel in C-Mn-Si-Cr-Mo chemistry have been produced in as hot rolled condition at Rourkela Steel Plant. The effect of coiling temperature and cooling rate on the final structure and properties of the steel has been discussed in detail. All the coils coiled at lower temperatures of about 470°C or less showed dual phase structure with uniform properties where as coiling at about 500°C or above did not yield the desired microstructure and properties. Possible reasons have been given to explain the effect of coiling temperature on microstructure. Acceleration of the cooling rate of the coil, after coiling, has been found to improve the tensile strength without significantly affecting the ductility of the material.