Development Of High Strength Low Alloy Steel And Research Of Related Formability

The development of automobile industry not only brings convenient transportation to human beings,but also causes serious environmental pollution.Energy and safety issues are getting more and more attention,which is increasingly demanding indicators such as lightweight,safety,energy saving and environmental protection.High-strength steel is the material of choice for lightweight vehicles,safety and energy saving,and is the main development trend of automotive steel in the future.Taking Nb-Ti and Nb-V-Ti low-alloy high-strength steels as research objects,the cold-rolled high strength low alloy steels with good comprehensive properties above 500MPa level have been developed through process optimization and control.The main work content and research results are as follows:(1)The experimental steel after cold rolling were annealed by a thermal simulation test machine,and the microstructural evolution of the experimental steel at different annealing temperatures was analyzed.The isothermal ferrite recrystallization kinetics models of the two experimental steels were established by using JMAK model,which provided a theoretical basis for the subsequent annealing process.The recrystallization activation energies of Nb-Ti steel and NbV-Ti steel are 179.54 kJ/mol and 244.88 kJ/mol.Under the same annealing conditions,the recrystallization rate of Nb-V-Ti steel is lower.(2)Aiming at the production process of cold rolled low alloy steel,simulated experiments of batch annealing and continuous annealing were carried out for two kinds of experimental steels.The effect of annealing temperature on the structure and mechanical properties of batch annealing was studied.Improve the performance difference between inside and outside of coil due to different heating process,and improve the stability of coil.In the process of continuous annealing,aiming at the experimental steel with slower recrystallization rate,a process system of high temperature annealing and short time heat preservation was proposed to obtain fine and uniform structure and improve comprehensive properties.After batch annealing and continuous annealing,the strength of the experimental steel can reach more than 550MPa.(3)The precipitates of annealed experimental steel were observed and the precipitation behavior during annealing was studied.The size and number of precipitated particles were counted.The contribution of particle size to the strength of Nb-Ti steel and Nb-V-Ti steel was calculated by Ashby-Orowan modified model.The precipitation strengthening effect accounted for about 30%of the total strength.(4)In the aspect of formability,the influence of initial structure of hot rolling and alloying elements under batch annealing on the performances of hole expansion was studied,so that excellent strength-hole expansion matching was obtained,and the hole expansion rate could reach more than 95%.It provided theoretical support for improving the performances of experimental steel.Aiming at the surface quality problems of annealed plate caused by Ludes band phenomenon,the causes were analyzed,and the effects of continuous annealing process and Mn content on yield platform were studied.The yield platform effect is improved,and the surface quality and processing performance of the material are improved.