Study On Hot Rolling Process Of 700MPa Ti Micro-alloyed High Strength Steel

In recent years,the rapid growth of economy and society led to the fast development of automobile and equipment manufacturing industry,The application of micro-alloyed high strength steel used in automobile,engineering machinery and other fields is more and more extensive,and it is widely produced by TMCP to develop in the direction of higher strength,higher toughness,lower cost and quantitative reduction.Due to the yield of Ti was difficult to control before,and the performance of the Ti micro-alloyed steel is not stable,therefore,The application of Ti in high strength hot rolled plate and strip products is comparatively few.However,with the development of vacuum smelting technology,the yield of Ti gets increased greatly.At the same time,the production of the steel with tensile strength above 700MPa grade by using the Ti micro-alloying technology is cheaper than that by using composite micro-alloying,meanwhile remarkable enhancement effect can be achieved.In this paper,precipitation behavior of strain-induced precipitation TiC,the isothermal y-a phase transformation and precipitation behavior of TiC in phase transition,the effect of TMCP parameters on the microstructure evolution and hardness was investigated through laboratory thermal simulation experiment and industrial trial production.The best hot rolling process to guide the industrial production was obtained.Major researches and results in this paper are as follows:(1)The strain-induced precipitation kinetics of TiC and its effect on the recovery and recrystallization of austenite in the hot rolling process was investigated through double passes deformation method,The results showed that,The strain-induced precipitation phase was single TiC with FCC structure,which precipitated on dislocation line or dislocation substructure.TiC phase obeyed the parabolic growth law in the growth stage and grown slower in the coarsening stage.Meanwhile,it’s found that the size of deformed austenite sub-grain was much smaller than that of initial austenite grain.(2)The isothermal phase transformation kinetics of Ti micro-alloyed steel at different temperatures,and the precipitation behavior of TiC in isothermal γ-α phase transformation and coiling process were studied through laboratory thermal simulation test.The results indicated that isothermal γ-α phase transformation phenomenon occurred in Ti micro-alloyed steel during coiling process,the interphase precipitation and supersaturation precipitation in ferrite of TiC occurred separately in isothermal phase transformation and coiling process.Its precipitation strengthening effect can significantly increase the strength of steel.(3)By TMCP experiment in thermal simulation test system,the influence of TMCP parameters(heating temperature,finishing rolling temperature,cooling rate,coiling temperature,etc)on the microstructure evolution and hardness of Ti micro-alloyed high strength steel was studied,and optimal industrial trial production plan was put forward based on the thermal simulation test results.The results showed that,the decrease of heating temperature can suppress the coarsening of austenite,thus the solid solution content of Ti should be guaranteed with 1230～1260℃ of heating temperature in the industrial production.The reduction of finishing rolling temperature will increase the rate of nucleation of ferrite and restrain the precipitation and growth of the-precipitates during the deformation process,so the finishing rolling temperature should be kept at 870～900 ℃.Increasing cooling rate can refine ferrite grains and get quasi-polygonal ferrite or needle-like ferrite,so the cooling rate should be kept at 15～30℃/s.Rising coiling temperature will increase steel strength test,in order to ensure the precipitation strengthening effect of Ti(C,N)in the ferrite acting fully,the coiling temperature should be selected at 620 ± 20℃.(4)Industrial production was carried out,and the microstructure and mechanical properties of experimental steel were measured systematically.the results indicated that,the microstructure of 700MPa Ti micro-alloyed hot rolled high strength steel was ferrite + little pearlite,its yield strength and tensile strength were greater than 650MPa and 700MPa respectively.The optimum parameters for the hot rolling process are:heating temperature of 1230～1260℃,finishing rolling temperature of 885 ±20℃,cooling rate of 15～30℃/s,and coiling temperature of 620 ± 20 ℃.