Preparation,Microstructure Control And Mechanical Properties Of Low-Carbon Superfine Bainitic Steel

There is a continuing demand for advanced steels with both high strength and good ductility to ensure service safety by reducing the weight of steel components and to improve fuel efficiency by reducing the steel amount used on components.It has been discovered that a superfine bainitic microstructure composed of very fine bainitic ferrite laths(with thickness less than100nm)and thin carbon-enriched retained austenite films between the laths can be achieved in rich-silicon steels with high/medium carbon content.Such structure possesses an ultra-high strength and a noticeable uniform elongation,being a likely candidate to satisfy industrial demand for advanced steels.However,high/mediun carbon content leads to much prolonged bainitic transformation duration(dozens of hours or even several days),which restricts significantly the industrial application of such steel.Low-carbon bainitic transformation,by contrast,has a much faster process but leads to coarse bainitic ferrite lath.Accordingly,producing low-carbon bainitic steels with similarly ultrafine bainitic structure is much desirable for practical applications.In this dissertation,three low-carbon carbide-free steels with different carbon content(0.15C,0.18C and 0.28C,wt.%)were designed and manufactured;and pre-quenching,below-Ms austempering and ausforming processes were well-designed and conducted on the low-carbon steel,aiming to obtain the so-called superfine low-canbon bainitic steel.The effects of water-quenching pretreatment on austenization state and the subsequent kinetics of the isothermal bainitic transformation,bainitic microstructure and the mechanical properities of 0.18C steel(Fe-0.18C-1.68Mn-1.31Si)were investigated.The results show that when the as-untreated and water-quenched steel were austenized at950 ~oC for 20min,their prior-austenite grain sizes(PAGS)are 34.6?m to 7.6?m,respectively,indicating that the pre-quenching process can refine the PAGS remarkely.The decrease in PAGS for the pre-quenching sample helps to enhance the strength of austenite,which in turn reduces the martensite start temperature(Ms);meanwhile,the decrease in PAGS suppress the growth of bainitic ferrite lath,leading to reduced bainitic transformation rates.Besides,in the pre-quenching steel,the decrease in PAGS help to refine both the bainitic ferrite lath and the blocky martensite/austenite constituents,which enhance significantly both the yeild strength and impact toughness.The effects of above/below-Ms austempering on bainitic transformation behavior and the resultant microstructures and mechanical properties were investigated in two low-C steels(Fe-0.15C-1.41Si-1.88Mn andFe-0.28C-0.82Si-2.14Mn-1.62-1.21Al).It is found that bainitic transformation occurs at isothermal temperatures below Ms.The existing of prior martensite increase significantly the bainitic nucleation sites,which not only greatly accelerates the bainitic transformation rates,but also refines both the bainitic ferrite lath and the martensite/anstenite constituents,and the latter is giving rise to significantly increased impact toughness and improved tensile properties.The effects 50%ausforming at 600 ~oC/400 ~oC followed by above/below-Ms austempering on the phase transformation behavior,the resultant microstructure and mechanical properties of the 0.15C steel(Fe-0.15C-1.41Si-1.88Mn)were studied.Results indicate that an optimal combination of tensile strength and ductility(beyond 4.2 GPa%)and superior impact toughness(exceeding 150 J/cm2)were obtained by the process of ausforming+below-Ms austempering.The prior ausforming helps to recuce the Ms;and with decreasing the ausforming temperature from 600 ~oC to 400 ~oC,the Ms decreases from378 ~oC to 367 ~oC.This is because ausforming or decreasing ausforming tend to enhance the strength of the supercooled austenite,which in turn increases the shear resistance of austenite-to-martensite transformation,thereby lowering the Ms.Ausfoming also greatly accelerates the bainitic transformation rates because crystal defects induced by ausforming.can serve as sites for bainite nucleation and thus promote the bainite transformation.Since lowering the ausforming temperature can yield much more crystal defects in the parent-phase austenite,ausfoming at lower temperature of 400 ~oC accelerates more the bainitic transformation.Ausforming takes twofold competitive effects of accelerating nucleation and retarding growth on bainitic transformation,which helps to refine the bainitic ferrite for both above/below Ms samples but has radically different influences on the martensite/austenite(M/A)constituents between this two samples,which in turn toakes effect on the mechanical properities.For above-Ms samples,ausforming refined the bainite laths but resulted in coarsening MA blocks,leading to significantly increased strength but decreased ductility and impact toughness.While for the below-Ms samples,ausforming refined not only the bainite plates but the MA blocks and thus increased the final fraction of retained austenite,which gives rise to markedly enhanced strength with no deterioration of the ductility and impact toughness.