Design And Development Of 1180 MPa Steel With High Elongation

Nowadays,the problem of environmental pollution is becoming increasingly tense,and people pay more and more attention to energy conservation and emission reduction.For the automobile industry,lightweight is of great significance for the sustainable development of the automobile industry.Under the premise of ensuring the safety performance of the automobile,reducing the weight of the whole automobile as much as possible,so as to improve the power performance of the automobile,reduce the fuel consumption and reduce the air pollution is the development direction of the automobile industry in the future.As the representative of the third generation high-strength steel,Q&P steel combines the advantages of martensitic steel,DP steel and TRIP steel.Compared with the first generation and the second generation of high-strength steel,Q&P steel has not only high strength,but also high elongation and good strength-ductility balance.But compared with the traditional DP steel and TRIP steel,Q&P steel has more heat treatment parameters to control,especially quenching temperature and composition temperature.It’s not easy to apply it in the efficient industrial production,especially in the continuous production line.Based on the conventional continuous annealing line,a kind of C-Mn-Si steel for dynamic austenite stabilization is designed.The designed alloy is fully austenitized,then quenched to Ms and cooled slowly during continuous annealing and over aging.The residual austenite can be enriched and stabilized by carbon distribution,which leads to higher strength and plasticity.In this paper,the effects of quenching temperature and partitioning time on the mechanical properties and microstructure of experimental steel under continuous annealing process are studied,and the effects and relations of microstructure and retained austenite on the properties are explored,so as to determine the best production process parameters.The main work and research results are as follows:(1)The transformation point and CCT curve of the experimental steel were measured by thermodynamic calculation and phase transformation apparatus experiment.The temperature of Ms and Mf were 344℃ and 202℃,respectively.The critical cooling rate was 15℃/s.The Complete martensite could be obtained under the cooling rate of more than 20℃/s.(2)Through single pass hot compression experiment,it is found that the peak stress of rheological curve of experimental steel will decrease with the increase of deformation temperature and increase with the increase of strain rate.The activation energy of hot deformation of experimental steel is calculated,and then the mathematical model of high temperature flow stress of experimental steel is established.According to the calculated rheological stress model,a reasonable hot rolling and controlled cooling process system is established,and the hot rolling structure of pearlite and ferrite is obtained,which is convenient for cold rolling.(3)The mechanical properties and microstructure evolution of the experimental steel at different quenching temperature and time were discussed by means of the deformation thermal expansion transformation instrument.With the increase of quenching temperature in the range of 340～400℃,the strength of experimental steel decreases and the elongation of experimental steel increases.When the quenching temperature is increased to 420℃,the strength and elongation of the steel decrease with the increase of quenching temperature.When the quenching temperature is the same,the strength of the experimental steel decreases and the elongation increases with the increase of the partitioning time.(4)According to the actual situation of the continuous annealing production line and the industrial production simulation,it can be known that the mechanical properties of the experimental steel are stable under the established continuous annealing process system,which has the feasibility of actual production.Moreover,when the continuous annealing process is austenitizing at 860℃,it will cool slowly to 310℃ within 18 minutes after quenching to 360℃,its tensile strength is about 1180mpa,and its elongation is more than 15%,which can meet the target mechanical property requirements.