| The microstructures and the mechanical properties of medium and low-carbon steelswere systematically optimized by a series of heat-treatment processes, and the heattreatment process of low-temperature bainite in low-carbon steel was explored. Resultsshowed that the microstructure of low-temperature bainite in the low-carbon steel,containing fine plate of carbide-free bainitic ferrite and thin film of retained austenite,could be produced by continuous cooling transformation around the Ms temperature fromMs+10oC to Ms-20oC at cooling rate of0.5oC·min-1. A new model was proposed toevaluate the comprehensive mechanical properties of steel, which found that thelow-temperature bainite had the best comprehensive mechanical properties compared toany other microstructures for the low carbon steel. The higher dislocation density and finerbainitic ferrite plate in the low-temperature bainite resulted in the higher yield strength andthe higher toughness, but relatively lower ultimate tensile strength due to the lowerwork-hardening rate caused by the higher initial dislocation density. There were some veryfine particles in the bainitic ferrite of the steel after isothermal treatment at highertemperature. The ultimate tensile strength and the low-temperature impact toughness ofthe steel decreased with the volume fraction of the retained austenite increasing, while theelongation initially increased with an increase in the volume fraction of the retainedaustenite (<10%) and then remained constant. The retained austenite in thelow-temperature bainite has the best mechanical stability, and with the temperaturereducing, the stability improved. The low-temperature bainite had the best low cyclefatigue life. And similar results found in the43MnSiCrAlNiMo steel, and tempering canimprove the strength and the toughness of the material. |
