The Study Of Microstructure Control On Hotrolled Dual-phase Steel With High Product Of Strength And Elongation

This investigation systematically studied one low C contained steel with micro-alloying elements.It aims to allow full play to the mechanical properties of the hard and soft phase respectively,obtaining high product of strength and elongation,under the guideline of adjusting phase composition of the steels' microstructure.The investigative results were shown as follows.(1)The continuous transformation of the tested steels super-cooled austenite could be divided into three kinds i.e.F+P transformation,bainite transformation(B transformation)and martensite transformation(M transformation),in which B transformation has a broad cooling range.The tested steels obtained typically F+P microstructure at room temperature during the cooling range(0.05-1?/s).The super-cooled Austenite mainly occures B transformation within the cooling range(3-10?/s),and the obtained microstructure was typically granular bainite(GB)at cooling rate 5?/s.During the cooling range(3-10?/s),the GB morphology gradually evolved to lath bainite(LB)morphology with the increasing cooling rate.During the cooling range(10-20?/s),the obtained microstructure from the super-cooled Austenite was a mixture of GB,LB and LM.And the super cooled austenite fully transformed to LM when the cooling rate exceeds 30?/s.(2)The M and ferrite microstructure of the tested steels which subjected to IQ and IQ' process evolves with inter-critical temperature.a)IQ temperature not only affected the C,Mn containment of the transformed austenite,but also lead the M morphology which transformed from A.M mainly as the form of granular during AC1-800?,and partially converted to lath morphology during(800?-AC3).The M volume fraction increases with the inter-critical temperature.b)After the process of IQ',the microstructure of the typical banded structure during AC1-780?,and the ferrite grain was characteristic of polygon.The microstructure of the tested steels were typically dual-phase dispersive distribution above 800?,in which ferrite grain partially converted to lamellate and M began to appear lath morphology.(3)The tensile tests results of the tested steels subjected to IQ(760?780?800?810?820?)+ T(520?,1h)process were as follows: The strength(YS,TS)of the tempered(520?)samples surges with the IQ temperature,and the proportional elongation(A5)of the ones decline with the IQ temperature,which were the consequence of the increasing martensite volume fraction(MVF).However,the relation between YS/TS ratio of the tempered samples and IQ temperature expressed in Cartesian Coordinates is similar with “V”,and the lowest YS/TS ratio(0.835)was obtained at IQ temperature 800? because of the optimal dual-phase composition(hard phase volume fraction: some 50%).(4)The commercially produced plates(20mm)subjected to thermal simulating tests(TH=1320?,t8/5=10,20,30,60,120s)shows the results as follows.a)the CGHAZ didn't show the soften phenomenon compared with the parent metal;b)the different orientation of the M bound(B bound)or island structure dispersing all over the ferrite matrix of the GB,produced within t8/5?20s,have a low impaction on the impact toughness of CGHAZ due to the bound or prior A grain boundary restraint the extension of crack.When t8/5?30s,the impact toughness of CGHAZ dramatically reduce,causing from the coarsen island structure lessen the contact area with F matrix and made the crack extend easily.