Microstructure And Mechanical Properties Of Medium Carbon Silicon-rich Steels Processed By Ausformed Isothermol Transformation

In this dissertation,the effect of supercooled austenite deformation with different strains on isothermal bainite transformation behavior,microstructure and mechanical properties of two kinds of medium carbon silicon-rich steels with different carbon contents of 0.28 wt.% and 0.38 wt.% was studied.Compression deformation and subsequent isothermal transformation treatment were carried out on a Gleeble-3800 thermomechanical simulator.Microstructure characteristics of the samples were analyzed by optical microscopy,scanning electron microscopy,transmission electron microscopy and X-ray diffraction.The mechanical properties,such as hardness,tensile and impact properties were measured and analyzed.The strain of 30% reduces the Ms point of the two steels by 75? and 10?,respectively.But the strain of 10% only reduces the Ms point of 0.28 C steel,and has no effect on the Ms point of 0.38 C steel.The decrease of Ms point allows the isothermal bainite transformation can be carried out at a lower temperature to produce finer bainite ferrite laths.Austenite deformation can shorten the incubation period,accelerated the early bainite transformation,while inhibited the late bainite transformation.Therefore,in 0.38 C steel the holding time for isothermal transformation at 330? after compression deformation with 10% strain at 500? increased from 60 min to 90 min,and the holding time for isothermal transformation at 360? was the same as that of undeformed.The transformation time for other processes can be reduced.Bainitic ferrite lath and retained austenite were obtained by isothermal transformation of deformed austenite,while no carbides were found.The austenite deformation causes a certain directionality in the spatial distribution of bainitic ferrite and reduces the number of variants of bainitic ferrite.The supercooled austenite deformation increases the retained austenite content and reduces the amount and size of the block retained austenite and the thickness of the bainitic ferrite lath.The hardness and strength of the samples were enhanced to some extent by the supercooled austenite deformation,and the impact energy of the 0.28 C steel sample was increased to 86 J by the austenite deformation.For 0.38 Csteel,the impact energy of the sample without austenite deformation was higher than that with austenite deformation due to the effect of variation selection.However,the impact energy of the 30% strained deformation sample was higher than that of the 10% strained one.The impact energy of test steels is much greater than that of high-C nano bainite steel.