Study On Controlled Cooling Process And Tempering Process Of Engineering Machinery Steel Q690

In recent years, in the lead with a trend towards lighter, bigger engineering machinery, people have growing requirement for properties of high strength steels. Requirements, not only high strength, but also high toughness and low costs, have been proposed for engineering machinery steel Q690. The whole research work was supported by the project "Ultra fast cooling technology development—product and process research of high-performance plate". The influence of controlled cooling process based on ultra fast cooling technique and tempering process in engineering machinery steel Q690had been studied. Chief original work is as follows:(1) Thermal expansion method was employed to determine the CCT curves and transformation behavior with continuous cooling for the tested steel. Results showed that with the cooling rate increased, the microstructure of the test steel was bainiteâ†'granular bainite+lath bainiteâ†'lath bainite under deformed and undeformed conditions. The bainite phase transition temperature range decreased, and the deformation promoted bainite transformation.(2)The results of controlled cooling research showed that lath bainite was easier to be obtained on low finish cooling temperature and high cooling rate, which can provide high mechanical properties to the test steel. Bainite/martensite dual-phase structure was obtained after online quenching using the ultra fast cooling system. The strength of the steel was200MPa higher than that of conventional controlled cooling process, and the toughness was fine.(3) Results of tempering process research showed that when the tempering temperature was higher than550℃, good comprehensive mechanical properties of test steel could be obtained. The optimum tempering temperature was550℃～600℃. The size, shape and distribution of M/A islands were influential to impact toughness. Big M/A islands can not effectively prevent propagation of the cracks. By contrast, fine M/A islands can effectively prevent propagation of the cracks. (4)The results of Q550upgrading Q690showed that when the finish rolling thickness were12mm and15mm, the mechanical properties of the test steel can meet the criteria of Q690under ultra fast cooling conditions. In UFC+tempering conditions, the mechanical properties of the test steel can meet the criteria of Q690as the thickness of the test steel were12mm,15mm and20mm,(5) Industry trials were conducted in Anshan iron and steel plant field. The results showed that the strength of the test steel after hot rolling can meet t the engineering machinery Q690standards, but the elongation and impact energy were low. The mechanical properties of the steel can meet the criteria of Q690through a series of tempering tests. The optimal tempering temperature was600℃.