Effect of Heat Treatment on Properties of Rare Earth Microalloyed 1200 MPa High Strength Steel

Combined with the advanced controlled rolling and cooling process, which is represented by the 2250 mm hot continuous rolling production line, in China, an on-line heat treatment process (DQ&P&T) is developed, which uses dense laminar flow cooling to conduct sectional quenching, isothermal distribution and low-temperature coil after tempering followed by rolling, and the rare-earth-microalloyed ultra-high strength steel (R-m >= 1200 MPa) was prepared through the traditional off-line quenching and tempering heat treatment process (RQ&T) for comparison. The microstructure transformation morphology and mechanical properties of the test steels under two heat treatments were systematically analyzed by OM, SEM equipped with electron backscatter diffractometer, tensile testing machine, impact testing machine, Vickers hardness tester, and wet sand/rubber wheel wear testing machine. The results show that the microstructure of the test steels at room temperature is composed of lath martensite, bainite ferrite and a small amount of residual austenite under the two heat treatment processes. Compared with that prepared by RQ&T process, the lath martensite content of the steel prepared by DQ&P&T process decreases and the bainite ferrite content increases. The bainitic ferrite formed by isothermal transformation penetrates and divides the deformed undercooled austenite, which promotes the grain refinement and the increase in the proportion of large angle grain boundaries, thereby improving the strength, toughness and wear resistance of the material. However, the uniformity of hardness in the thickness direction of the product prepared by DQ&P&T process is more inferior than treated by RQ&T process. Compared with the off-line heat treatment, the on-line heat treatment has higher efficiency and lower manufacturing cost in the production of high strength steel, and the product performance meets the standard requirements, which proves that the on-line heat treatment process is feasible.