Ausformed Martensite Transformation And Preparation Of Nanostructured Bainite In High-carbon Si-Al-rich Steel

In high–carbon high–silicon steel low–temperature isothermal transformation attemperatures slight above martensite start temperature （Ms） can form nanobainite structureconsisted of nanoscale laths bainite ferrite and retained austenite film, which have highstrength, ductility and toughness. But the weak joint performance restricted its applicationas structure steels. In this dissertation, a medium–carbon Si–Al–rich alloy steel was usedto study the ausforming behavior and subsequent transformation behaviors of martensiticand low temperature bainite on a thermomechanical simulator. X–Ray diffraction （XRD）,optical microscopy （OM） and transmission electron microscopy （TEM） were used toanalyze the effect of deformed austenite on Ms, behaviors of martensite and bainitetransformation, in order to explore a preparation method of nanobainte in medium orlow–carbon steel.Martensite laths and retained austenite were obtained in medium–carbon Si–Al-richalloy steel by quenched of deformed austenite. Ms、martensite laths thickness and latticetetragonality can be decreased by compressive deformation of austenite, and variantsselection reduced. With deformation temperature decreasing, Ms、martensite lathsthickness and lattice tetragonality decreased and the hardness increased. Compared toundeformed austenite, austenite deformed at900–300℃with strain rates of0.01s^-1andstrains in50%can depress Msfrom294℃down to268–200oC; martensite lathsthickness from409±15nm down to310±4–52±9nm; hardness increased from756±9HV1.0up to760±12–783±6HV1.0. With supercooled austenite deformation strainsincreased, Ms、martensite laths thickness and variants selection decreases, volume fractionof retained austenite and hardness increased. Austenite deformed at600℃and the strainsranging in10%–30%can depress Msdown to238–231oC; martensite lath thickness downto355±12–198±15nm and microhardness are increased to762±4–772±5HV1.0. Withaustenite deformation strain rates decreased, Ms、martensite laths thickness decreases andthe hardness enhanced. Austenite deformed at300℃with the strain rates of0.01–1s^-1and strains in50%can depress Msdown to200–228oC, martensite laths thickness down to50±4–115±10nm and the microhardness are increased to783±8–774±5HV1.0.Baintie ferrite and retained austenite can be obtained by austempering of deformedaustenite. The ausformed baintie ferrite is thinned by deformation of austenite, and thetransformation rates and the microhardness increased. As austenite deformationtemperature decreased, bainite laths thickness and variants selection reduces; bainitetransformation rates、 incompleteness and microhardness enhances. After austenitedeformed ranging in300–900℃with strains of1s^-1and250℃austempered, baintieferrite laths thickness are48±3–142±8nm and the hardness are627±20–511±6HV1.0.With austenite deformation strains increases, bainite laths thickness and variants selectionreduced; transformation rates、volume fraction of retained austenite and microhardnessenhanced. After austenite deformed at600℃with strains rates of1s^-1and strains rangingin10%–30%and250℃austempered, bainite laths thickness are115±9–76±5nm, andthe hardness are578±6–592±5HV1.0. As austenite deformation strain rates decreased,bainite laths thickness reduced; transformation rates and microhardness enhanced. Afteraustenite deformed at300℃with strain rates of0.01、1s^-1and235℃austempered,bainite laths thickness are40±7nm and45±5nm, and the hardness are655±16and645±10HV1.0.