Development and application of online quenching technology for enhancing toughness of high strength steel

In recent years, the online quenching process widely used in the production of high-strength steel for medium and thick plates, which is a green and short process manufacturing process. However, compared with the offline quenching process, the problem of high strength and low impact toughness, which hinders the promotion and application of this process. It is found that the reason for this problem is the austenite grain of the rolled steel plate mainly presents a flat shape due to the improper control of the austenite structure during rolling. In the subsequent direct quenching process, this form of austenite is easy to form martensite lath through the original austenite grain, and the orientation is relatively consistent, which is not conducive to preventing the expansion of cracks. In this paper, based on the control of austenite structure, the rolled steel plate forms fine equiaxed austenite grains, and retains some dislocations during plastic deformation. In the subsequent quenching process, martensite lath with different orientations are formed. This structure is conducive to preventing crack propagation, thus improving the impact toughness of the steel effectively. The production of 550 MPa high strength steel can be achieved without adding expensive microalloying elements, the impact toughness at -20 ℃ has been significantly increased to over 200 J, reaching the equivalent level of offline quenching, while also increasing the production of rolling line. Compared with offline quenching, the microalloyed elements under this process can be more solidly dissolved in austenite, and then retained in martensite during quenching. The precipitation strengthening effect of microalloyed carbonitride is enhanced during tempering. Finally, the 690 MPa high strength steel produced by this process has the same performance and slightly higher strength as offline quenching, which expands a new idea for further promotion the process.