| In recent years,with the rapid development of automotive industry in our country,the demand for automotive steel is increasing remarkably and the demand for car crash safety performance is also improved continuously.Also because the steel industry is confronted with the environmental pollution problem and the shortage of raw material and energy,the energy conservation and emission reduction of automobile has become a key research subject for the automobile industry development.For the heavy-duty automobile accounted for thirty percent of the automobile output of China,lightweight technology is the fastest and most efficient measure of automobile energy conservation and emission reduction currently.So it is necessary to make study on the process of automotive steel to develop high strength thin steel plate with high quality and low production cost to replace the original low strength thick steel plate to achieve the lightweight of body.This paper took Nb-Ti-Mo microalloying ultra-high strength strip as research object and made basic theoretical research on high temperature deformation behavior and continuous cooling transformation behavior of austenite.Hot rolling experiments have also been done.The main work involved are as follows:(1)By using the thermal simulation experiment technology,the study of austenitic high temperature deformation behavior of experimental steel was conducted by single pass compression experiments.The effect of deformation temperature,strain and strain rate on austenitic dynamic recrystallization behavior and deformation resistance was analyzed.Results showed that low strain rate and high deformation temperature were advantageous for austenitic dynamic recrystalization.Activation energy of dynamic recrystalization,dynamic recrystalization models and mathematical models for deformation resistance were obtained.(2)The austenitic continuous cooling transformation behavior was studied by continuous cooling phase transformation experiments.The static and dynamic CCT curves were drew and the roles of deformation,cooling rate in continuous cooling phase transformation were investigated.Deformation promoted ferritic transformation and refined ferritic grain size.Deformation indirectly affected perlitic transformation and bainitic transformation through promoting ferritic transformation.(3)Hot rolling experiments were carried out for tested steel at laboratory and the TMCP process for tested steel was studied.The effects of coiling temperature and cooling rate on its microstructure and mechanical properties were investigated.The results showed that with the decrease of coiling temperature and the increase of cooling rate,the strength and hardness of tested steel were improved,the ductility of tested steel was declined.(4)The metal foil sample was observed by using transmission electron microscope,the fine microstructure and precipitation of tested steel were analyzed.The strengthening mechanism of tested steel was determined and the strength contribution amount to tested steel for each mechanism was studied.Microstructure of tested steel was dominated by bainitic ferrite.There was a large number of diffusion nearly circular precipitation(Nb,Ti,Mo)C in bainitic ferrite matrix,its size was between 5-20nm and the nucleation mechanism was mainly dislocation nucleation.The strengthening mechanism of tested steel consisted of solid solution strengthening,dislocation strengthening,precipitation strengthening,fine-grain strengthening.The contribution amount of each mechanism was 118MPa,75MPa,218MPa,299MPa respectively.(5)Impact toughness of tested steel was investigated by low-temperature impact experiment.The result showed that with the decrease of experimental temperature,the impact absorbing energy was declined.The fracture mechanism of tested steel was changed from ductile fracture to ductile fracture and cleavage fracture. |
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