Study On Recrystallization And Precipitation Behavior Of Cold Rolled High Strength Low Alloy Steel

High-strength low-alloy steel has been increasingly widely because of the advantages of low cost and high strength.However,there are still many problems in the production and application of cold-rolled high-strength low-alloy steel,such as instable mechanical properties,poor formability caused by heterogeneous microstructure quality and so on.These problems limit the application of high-strength low-alloy steel to a large extent.In this paper,the effect of the hot rolled microstructure on the recrystallization of ferrite was studied by means of thermal simulation machine,continuous annealing machine,XRD,EBSD,transmission electron microscope,tensile testing and hole expansion test.Texture evolution during the annealing process was investigated,and the contribution of precipitates on strengthening was evaluated.And the influence of initial microstructure and continuous annealing parameters on the strength-ductility was studied.The results and conclusions are as follows.(1)The differences in recrystallization behavior during the annealing of the tested steels is due to the differences of initial microstructure.2#experimental steel in the initial microstructure has higher dislocation density,cold rolling deformation of the microstructure after the deformation can be greater,which is conducive to the nucleation and growth of recrystallized grains,thus reducing the time required for complete recrystallization.Rapid heating will accelerate the recrystallization,heating rate of 80?/s make a large number of dislocations and tissue defects not eliminated in the recovery process,and the microstructures still exist large stored energy,thus the rate of ferrite recrystallization has been accelerated.At the same time,the recrystallization kinetics model at different annealing temperatures also been calculated based on the JMAK equation.The results showed that the ferrite recrystallization start earlier and the time to complete recrystallization become shorter at the higher temperatures.(2)The typical cold-rolling texture<001>orientation in the low-alloy steel is turned from the<001>to<111>direction due to the recrystallization behavior versus the annealing time.A large number of low angle grain boundaries were obtained in the early stage of annealing,the frequency of low angle grain boundaries was rapidly reduced as the annealing temperature and time increased.The frequency of low angle grain boundaries for the new process steel was significantly reduced faster than that of the traditional experimental steel,It is considered that the grain boundary angle between the cold-rolled deformed substrate and the new grain orientation is between 25°and 60°.(3)After cold rolling,the texture is mainly composed of a texture and y texture.During the initial stage of recrystallization,the experimental steel is composed of complete ?-fiber texture and y-fiber texture.The ?-orientation density gradually decreases and the ?-orientation density increases versus annealing time.The bainite structure changes to the ferrite and cementite during the annealing process,which may weaken the strength of the y texture.(4)The results showed that the contribution of precipitated particles to 1#steel and 2#steel is 123.93MPa and 150.37MPa respectively based on Ashby-Orowan correction model,and the strength increases with the increase of the number of precipitated particles and the decrease of the particle size.According to the analysis of the particles precipitated by TEM,the precipitated particle size of the experimental steel is more uniform under the new process,and the size is concentrated between 5-7 nm and the quantity is relatively large.(5)The influence of the initial microstructure on the mechanical properties and forming properties is related to the continuous annealing process parameters.When the initial microstructure is ferrite-pearlite,the ferrite is not sufficiently and the homogeneity of the microstructure is poor after annealed,which is more obvious under annealing of low temperature and the heating rate of 50?/s,which leads to the decline of its strength and reaming performance.When the initial microstructure is bainite,the distribution of ferrite is more uniform after annealing,and the microstructures with good plasticity can be obtained by heating at 5?/s for a certain time.