Study On Strengthening And Mechanism Of Stress-tempering And Warm Deformation Of Cold Rolled Martensitic Steel

As an important structural material,steel has a wide range of applications in industry.The properties of steel largely depend on the internal structure.The introduction of appropriate stresses and strains during processing can significantly change the microstructure and properties of the material.Combining external stress and temperature deformation with traditional heat treatment techniques to control the precipitation of carbides and the grain size of the matrix,and to increase the strength and toughness of the material simultaneously,and to explore the strengthening and toughening mechanism of the material,is of far-reaching significance.In this paper,cold-rolled low-carbon martensitic steel was selected as the research object,followed by multi-temperature gradient stressless conventional tempering,external stress tempering and temperature deformation processing,using metallography(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and other analytical techniques,combined with mechanical properties such as hardness and tensile tests,systematically studied the effect of stress and temperature deformation on the microstructure and properties of deformed martensite tempering,and explored its mechanism of action.The cold-rolled martensite own good mechanical properties after the conventional tempering at 500°C-1h,the tensile strength and elongation are 1076 MPa and 11.2%,respectively.The microstructure is mixture of nano-sized ferrite and dispersed carbides.Combined with mechanical properties and TEM analysis,the conventional temper grain refinement mechanism is martensitic transformation refinement,plastic deformation refinement and continuous recrystallization refinement.The effect of applied stress on tempering structure and properties was studied.The presence of critical stress causes significant changes in the microstructure and mechanical properties of the stress tempered specimens.Tempering at 200°C~500°C and 120 MPa or more stress conditions promotes carbide precipitation and slat width,promote the precipitation of carbides and the widening of martensite laths,which will reduce the solid solubility and dislocation density,lead to stress-tempering-induced softening,and increase the softening effect with increasing stress.The tempering hardness of 200MPa-300°C is 347 HV,which is much lower than that of 486 HV without stress tempering at 300°C.The hardness difference between the two is 139 HV.When tempered at 120MPa-550°C,the degree of recrystallization increased,so that the recrystallized grain size increased slightly,leading to stress tempering induced softening.The tempering tensile strength at 120MPa-500°C is 929 MPa,which is lower than the 1076 MPa without stress tempering at 500°C.The tempering under the stress condition of 550°C and 80 MPa or more promotes the increase of the degree of recrystallization,inhibits the growth of recrystallized grains,improves the stability of the recrystallized structure,and induces stress-induced hardening.The tempering tensile strengths of 80 MPa and 120 MPa at 550°C are 726 MPa and 757 MPa,respectively,which are higher than 550 MPa without stress tempering at 550°C.The mechanism of stress grain refinement was preliminarily established by TEM analysis: the stress distribution inside the structure under the uniaxial compressive stress was not uniform,and the stress concentration part caused a slight plastic deformation,so that the nucleation site increased and the nucleation rate increased.Increased number of carbides enhance pinning of grain boundaries.The deformed martensite was deformed by 50% at 400°C and 500°C and annealed at 550°C for 1 hour,which hardness were 225 HV and 203 HV,respectively,which was higher than 165 HV that was conventionally tempered at 550°C.The distribution of carbides in the recrystallized structure was more obvious.The order is conducive to improve the plasticity of the material;after 50% temperature deformation at 550°C,600°C,and 650°C,respectively,the hardness of the recrystallized structure is 205 HV,174HV,160 HV,respectively,which is higher than the 165 HV,163HV,137 HV of the conventional tempering microstructure at the same temperature.The mechanical properties and TEM analysis show that the effects of warm deformation and applied stress on microstructure and mechanical properties have a similar effect: the hardness of the specimen after 400 °C and 500 °C deformation decreases,and the main strengthening mechanism is solid solution strengthening and dislocation strengthening;The hardness of the samples after the temperature change from 500°C to 650°C increases,and the main strengthening mechanisms are fine-grained strengthening and second-phase particle strengthening.