| Due to its high tensile strength,low yield strength,and good strength-toughness matching,dual-phase steel has been widely used in automobiles.As people's requirements for body corrosion resistance increase year by year,automotive steels usually need to be galvanized.However,the Si element in steel is susceptible to oxidation during hot-dip galvanizing,and the oxides formed on the surface of the steel sheet can cause problems such as decreased plating adhesion and missing plating.Therefore,the Si content needs to be strictly controlled.However,reducing the Si content causes a reduction in strength.Adding microalloying elements such as Nb,V,and Ti to steel can effectively improve the strength and can compensate for the decrease in strength caused by reducing Si.Therefore,the research objects of this article are two kinds of dual-phase steels,one is silicon-free dual-phase steel(A steel),and the other is added with a small amount of microalloying elements(B steel).The effects of process parameters on microstructure evolution and properties during annealing were studied.The main research contents and results are as follows:(1)Through thermal simulation experiments,combined with nano-hardness and micro-hardness,the recrystallization rules of two experimental steels during continuous annealing were studied.The results show that with the increase of the heating rate,the recrystallization of ferrite is suppressed,so that the recrystallization of ferrite is delayed to the intercritical region,which overlaps with the austenite transformation.The state of recrystallization when ferrite enters the intercritical region will seriously affect the nucleation and growth of austenite in the later stage.Comparing A steel and B steel,it was found that the recrystallization process of B steel was severely delayed.Under the same annealing conditions,its recrystallization fraction is always lower than that of A steel.Isothermal experiments were performed for a longer period of time above Ac1,and it was found that the addition of microalloying elements obviously played a role in refining the grains and making the structure more uniform.(2)The phase transformation experiments combined with JMAK model analysis were used to study the dynamics of austenite phase transformation under different heating methods.The results show that when the ferrite is partially recrystallized,the austenite transformation rate is accelerated.This is because the deformed energy storage retained in the unrecrystallized ferrite increases the driving force for austenite nucleation,thereby reducing the nucleation activation energy and increasing the nucleation rate.And more defects in the unrecrystallized ferrite also provide a path for the diffusion of alloying elements,thereby accelerating the growth of austenite.The addition of microalloying elements caused a decrease in the austenite fraction by hindering carbon diffusion.(3)Through continuous annealing experiments combined with performance testing,the effects of annealing processes on microstructure and properties were studied.The experimental results show that the strength of A steel is 682 MPa,which can reach the level of DP590.The strength of B steel is 927 MPa,which can reach the level of DP780 and still has a large margin.Bending experiments showed that no crack was observed in steel A,and the bending angle of steel B was 140°.Although the bending angle of the B steel becomes smaller,the strength and the formability match better. |
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