Study On Microstructure And Properties Of Cold Rolled High Strength Low Alloy Steel

In the recent years,energy saving and environmental protection is the main direction of the development of autornotive industry,and automotive lightweight is one of the most effective measures to reduce pollution and save energy.High strength low alloy steel is mainly used in automobile structural parts,strengthening parts and so on.It is widely used in automobile industry.On the basis of the conventional production process of HC420LA,this article aim to reduce cost by using Ti instead of Nb and Ti.In addition,the yield strength is improved by adding V.The main contents and results are as follows:(1)The recrystallization law of ferrite of the experimental steels containing different microalloying elements during annealing was studied.Based on the JMAK model,values of n of the experimental steels at different annealing temperatures were calculated.The values of n were divided into two phases,indicating that the recrystallization process was mainly divided into two stages:rapid recrystallization nucleation and growth stage,slow recrystallization completion stage.At the same annealing temperature,the values of n of Ti steel was bigger than that of Nb-V-Ti steel.It meaned that the time required to complete recrystallization process was shorter.Calculation results show that the recrystallization activation energy and deformation storage energy of Ti steel were lower.Because of the pinning effect of a large number of precipitated particles in Nb-V-Ti steel,the recrystallization process was delayed.(2)The evolution of recrystallization textures during annealing was studied.The results showed that the proportion of low angle grain boundaries would decrease with the increase of holding time.The evolution law of recrystallization textures of the experimental steels was basically the same.In the early stage of recrystallization,<111>//ND oriented grains would preferentially nucleate because of higher storage energy,and the nucleation mode was dominated by oriented nucleation,the nucleation site was mainly located at the grain boundaries of ?/? deformed bands.The presence of<001>//ND oriented deformation bands was the reason for the slow end stage of recrystallization.Most of the recrystallization textures were y fibre orientations.(3)The precipitation law during annealing and the contributions of fine grain strengthening and precipitation strengthening to strength were studied.The precipitation particles of Nb-V-Ti steel were more,and the distribution was uniform The results showed that fine grain strengthening was the main strengthening mechanism for two kinds of experimental steels.And the values of fine grain strengthening and precipitation strengthening of Nb-V-Ti steel were bigger.(4)The effects of annealing processes on the microstructure and properties were studied.Under the new technological condition,bell furnace annealing temperature of Ti steel could be reduced to 650?,and strength level could reach to 450MPa.Ti steel could have good tissue uniformity.Under new technological condition,strength level of Nb-V-Ti steel could reach to 550MPa at 700? after annealing,and Nb-V-Ti steel had good tissue uniformity.In the continuous annealing process,the annealing temperature had more influence on the structures and properties.Nb-V-Ti steel had small grain sizes and uniform microstructures after annealing at high temperature and short time,and the strength level could reach to 550MPa.