Influence of Continuous Annealing Parameters on the Mechanical Properties of Cold Rolled Ultra High Strength Steels

Abstract

Two C-Si-Mn cold rolled ultra high strength steels are investigated to study the effect of cooling parameters on the mechanical properties. The fast cooling start temperature and fast cooling end temperature as well as overageing temperature have significant influence on the yield and tensile strength of the experimental steels. It is found that tensile strength as high as 980Mpa can be achieved when the cooling speed exceeds 50°C/s. The yield strength and tensile strength can also be increased by increasing the fast cooling start temperature and meanwhile maintain relatively high fast cooling speed. The strength is significantly reduced if the steel is overaged at temperature of 350°C or above. Microstructure study reveals obvious decomposition of martensite and precipitation of cementite at high overageing temperatures. Other than these, it is found that the high Si steel has better elongation and lower YP/TS ratio than low Si steel at identical tensile strengths.