Research And Development Of Heat Treatment Of Q690Grade High Strength Steel Plates For Engineering Mechanism

In recent years, Q690grade high strength steels were widely applied in the engineering mechanical equipment. However, there exist some technical problems for the domestic enterprises, such as single specification, low qualification rate, high added quantity of alloy elements and so on. Some advanced foreign steel enterprises (SSAB, DillingerHiitte, JFE) have fulfilled the constant production and good performance. But the prices of steel plates are very expensive.Based on status quo, as the background of the heat-treatment process development of high strength steel plates for construction machinery, according to the key technology and important problem during development and industrial application for Q690grade structural high strength steels, the heat-treatment process was systematically studied in this paper. And the quenched-tempered high strength steels were developed successfully relying on the modern roller hearth heat treatment production line. The technological innovations and product features are as follows:The cost of composition is decreased by the thought of multiple element and small amount for composite strengthening. In view of the plate thickness≤16mm,17-34mm,≥35mm, the steelmaking and continuous casting processes are simplified by only adjusting the alloying elements of Cr, Mo, Ni. The product thickness has wider scope of5-70mm. The higher product qualified rate can be guaranteed by the optimal processes of smelting, rolling and quenched-tempered heat treatment. The main works involved as follows:(1) The chemical composition of Q690quenched-tempered steel was designed by the concept of low alloying. By the principle of adding complex alloy elements with small amounts respectively, beneficial effect of the chemical composition and their mutual promotion results were fully developed in order to achieve high performance and low costs.(2) High temperature deformation of low alloy high strength steel was investigated. The dynamic recrystallization and static recrystallization were studied by the single/double pass reduction deformation experiment on thermo-mechamical simulator. The results show that the dynamic recrystallization happened under the conditions of low strain rate and high deformation temperature, so only dynamic recovery occurred for the usual rolling speed and temperature. The model of the deformation resistance of tested steel was also established. Static recrystallization could complete entirely within10s deforming at and above1100℃. And static recrystallization slowed down or was inhibited below950℃.(3) To develop the Q690high-strength quenched and tempered steel plates for construction machinery, the effects of quenching and tempering treating regime on the microstructures and mechanical properties were investigated. The results show that the optimum quenching temperature was930℃, and the holding time was extended with the increasing of plate thickness. During the high-temperature tempering, the elongation and the impact energy exhibited an increasing tendency with the increasing of tempering temperature and holding time. All indexes come up to the National Standard GB/T16270-2009under the optimum process parameters.(4) The effects of intercritical heat-treatment process on microstructure and mechanical properties were investigated. The intercritical heat-treatment was an effective means to improve toughness and plasticity of steel plate, which put forward some requirements for prior microstructure. The results show that the hot rolling microstructure of experimental steel contained massive ferrite, incompletely transforming cluster pearlite and granular bainite. The impact toughness of test steel with prior microstructure containing plenty of massive ferrite could be deteriorated while the intercritical quenching temperature was780℃. The impact energy at-40℃was only59J. However, by the same heat treatment in γ+α region, the final microstructure of test steel with prior lath martensite structure consisted of much finer martensite and acicular ferrite existed among martensite phase. The Charpy impact energy at-40℃reached253J.(5) According to the requirements of microstructural controlling during quenching process for plates, the equations of heat conduction of quenched plates were established. And the computational methods of cooling ratio and quench-hardened case during quenching process for plates with different thickness were also studied. Through the analysis of the effects of quenching parameters, such as flowing amount, roll gap value and roll speed, on the profile controlling, the controlling technology of high flatness was developed. The establishment of automatic system during quenching process was the important condition for the industrial production of Q690, which laid the foundation for industrial trial manufacture of Q690steels.(6) Based on laboratory research results, the Q690high strength steel plates with5～60mm thickness specification had been developed successfully by the recommended composition system and heat-treatment process. The product qualification reached99.57%. The industrial products with excellent properties, flatness and weldability can meet the requirements of construction machinery, mining machinery and port machinery.