Investigation On Hot Deformation Feature And Dynamic Recrystallization Models Of300M Steel

As a widely used kind of low-alloy ultrahigh strength steel, the forming technologyof300M steel is forging, generally. Therefore, it is necessary to research the hotdeformation mechanism, especially the law of dynamic recrystallization evolution of300M steel, systemically. Based on the heat exposure experiment and the thermalsimulation compression experiment, the law of austenitic grain growth, the hotdeformation behavior and the law of dynamic recrystallization evolution of300M steelare investigated, and the mathematical models of them are constructed, respectively.Based on the heat exposure experiment, the law of austenitic grain growth and themodels of300M steel are researched. The results show that the austenitic grains of300M steel grow with the increase of temperature and the extension of time; theaustenitic grains of300M steel grow slowly in the temperature range of850-1050℃;and the austenitic grains of300M steel grow severely in the temperature range of1050-1180℃. Based on the measured austenitic grain size, the austenitic grain growthmodels of300M steel are constructed, which can be used to calculate the grain size ofthe material before the hot deformation.Based on the thermal simulation compression experiment, the hot deformationbehavior of300M steel is studied. The results show that there’re two kinds ofstress-strain curves which are the discontinuous dynamic recrystallization type of curveswith two peaks and the discontinuous dynamic recrystallization type with one peak indifferent hot deformation conditions. Then, the constitutive relation model of300Msteel was established, the deformation activation energy Q was calculated to be336.9788kJ/mol, and the values of Z parameter under different hot deformationconditions were calculated, according to Arrhenius hyperbolic sine equation. Thechange rule of the values of Z parameter shows that when the value of lnZ is more than33.37, the stress-strain curves of300M steel are the discontinuous dynamicrecrystallization type with two peaks, and when the value of lnZ is less than33.37, thestress-strain curves of300M steel are the discontinuous dynamic recrystallization typewith one peak.Based on the observation and analysis to the microstructure of300M steel, therelationship between dynamic recrystallization and hot deformation conditions isinvestigated. The results show that the grain size of300M steel after hot deformation decreases with the decrease of the grain size before hot deformation; the average grainsize after hot deformation increases and the uniformity of the deformation structureimproves with the decrease of strain rate and the increase of deformation temperature;there are two rounds of dynamic recrystallization during hot deformation if thestress-strain curve is the discontinuous dynamic recrystallization type with two peaks;there is only one round of dynamic recrystallization during hot deformation if thestress-strain curve is the discontinuous dynamic recrystallization type with one peak; ifthere are two rounds of dynamic recrystallization during hot deformation, the first roundof dynamic recrystallization finishes when the dynamic recrystallization volumefraction of the first round of dynamic recrystallization reaches80%.Based on the flowing deformation data and the measured dynamic recrystallizationdata of300M steel, the dynamic recrystallization peak strain, critical strain, volumefraction and average grain size models of two rounds of dynamic recrystallization areconstructed, respectively. The accuracy of the models is fine, therefore, the models canbe used to quantificationally describe the evolution of dynamic recrystallization of300M steel during hot deformation.