Effect Of Rolling Process On The Microstructure And Mechanical Properties Of Ti-Mo Alloyed Low-carbon Martensitic Steel

Martensitic steel with high strength at the same time there are still goodformability. However, with the strength increasing, the plasticity and toughness ofmartensitic decreases, which restricts the application scope. By adding micro-alloyelements, the nanoscale MC precipitates dispersed in martensitic steel. Themicro-alloying elements can form precipitates as carbides or nitrides or nitrides inaustenite, which can lower the grain growth rate by pinning the boundaries. It is wellknown that the most common micro-alloying elements are Nb、V and Ti. Someresearcher have contributed to the study of effect of Nb and V elements on therecrystallization and grain refinement, However, in comparison with Nb and Velements, the research on Ti-microalloyed was relatively few. The Ti-microalloyedsteels experienced were investigated. The microstructure and precipitates wereexamined by scanning electron microscopy (SEM), transmission electronmicroscopy (TEM), X-rays diffraction (XRD) and phase analysis method.The hot deformation characteristics of low carbon Ti-Mo MicroalloyedMartensitic Steel have been investigated by means of Gleeble-3800thermalmechanical simulator. The results show that the dynamic recrystallization wasdifficult to occur under high deformation temperature, low strain rate and largedeformation. The deformation activation energy of the experiment steel is deducedto462.8KJ/mol，the temperature of austenitic recrystallization is1000℃. Theisothermal kinetics curve (PPT) of carbide precipitation in austenite is a typical “C”type, precipitate the fastest “nose” temperature at about920℃,the incubation time is10s.To compare the effect of the two rolling ways of non-recrystallization zone rolling and recrystallization zone rolling on the microstructure and the mechanicalproperties of the martensitic steel it was found that the microstructure was flattedand the percentage of the MC precipitation increased from0.116%to0.128%withinthe non-recrystallization zone rolling. After reheating process, the grain wasimpeded to grow up due to the pining effect of the precipitate phase and the grainwas refined from8.6μm to4.7μm, and the refinement of the microstructure wasbenefit for the improvement of the comprehensive performance of the martensiticsteel. Compared with the recrystallization zone rolling, the tensile strength wasalmost the same but the impact toughness increased by10.5%, as high as42J underthe non-recrystallization zone rolling. It showed that the comprehensive mechanicalproperties of the steel were improved under low temperature rolling.Increasing the deformation strain on non-recrystallization zone rolling from80%to90%, the number of small size precipitated phase was increased. The grain wasrefined due to the pinning effect of the smaller size precipitated phase and theaverage austenite grain size was refined from7.8μm to5.4μm. The tensile strengthand yield strength of steel increased from1610MPa and1146MPa to1690MPa and1334MPa respectively. At the same time, the low-temperature impact toughness wasimproved by16J, as high as58J. Compared with traditional martensitic steel withthe same strength level, the impact toughness was obviously improved. It is shownthat the increasing of rolling deformation strain on non-recrystallization zone canimprove the comprehensive mechanical properties of the steel.Increase the amount of deformation on non-recrystallization zone, thedeformation was increased from80%to90%, the number of precipitated phaseincreases, and the size was smaller, smaller size of the precipitated phase of pinninggrain boundary effect makes the grain refining and average austenite grain size from7.8μm reduce to5.4μm, the tensile strength and yield strength of steel respectivelyincreases from1610MPa and1146MPa to1690MPa and1334MPa, at the sametime, the low-temperature impact absorbing energy also improves the16J, reach upto58J. Compared with traditional martensitic steel with same strength level, theimpact toughness obviously improves. It is shown that increase of rollingdeformation on non-recrystallization zone can improve the comprehensive mechanical properties of the steel.With the increase of heating temperature, the austenite grain of the test steel wascoarsened gradually and grain coarsening temperature was1050℃. The grain sizewas refined after re-austenitizing for the different initial grain size. Comparing themechanical properties of samples with the different reheating temperature, thetensile strength and yield strength was1657MPa and1343MPa for the samplesreheated at880℃, and the toughness was better than conventional martensitic steel.So the optimum heat treatment process was the880℃reheating for5min with oilquenching...