| The development and application of the third-generation automobile steel provides the necessary prerequisites for the lightweight technology of automobiles,especially for the improvement of the cruising range of new energy vehicles.However,while reducing the weight of the car,the safety of the car body must be ensured.Therefore,in the development of the third-generation automotive steel,how to further improve its plasticity under the premise of obtaining ultra-high strength has always been a research scholar at home and abroad.One of the research hotspots concerned by the automotive industry.This paper takes the new low-cost automobile steel 20Mn2 Cr as the research object,and analyzes the recrystallization annealing,austenitizing temperature,tempering temperature,isothermal temperature and other methods using SEM,EDS,EBSD,TEM,hardness testing,and room temperature stretching.The influence of heat treatment parameters such as cooling rate on the microstructure characteristics and microhardness of 20Mn2 Cr cold-rolled steel,and on this basis,a better heat treatment process for this steel is proposed;for typical heat treatment processes,the ultra-high strength conditions of 20Mn2 Cr steel are specifically analyzed The underlying plasticity promotion mechanism provides a scientific basis for the development of new high-strength plastic-product automotive steel and its heat treatment process.Research indicates:1)The microhardness of 20Mn2 Cr cold-rolled sheet is 274.3HV.With the increase of the austenite temperature(800～1000℃),the microhardness of 20Mn2 Cr steel gradually increases,among which the microhardness reaches 293.3HV when austenitizing at 900℃.When the austenitizing temperature is further increased,Although the microhardness increases slightly,the coarsening of austenite grains and the coarsening of martensite laths are not conducive to the improvement of plastic toughness.After the austenitization treatment,take out air cooling to slightly higher than the bainite transformation temperature,and then put the sample into the furnace for low temperature tempering,which is beneficial to the formation of bainite structure and can achieve the purpose of improving plasticity.The recrystallization annealing before the austenitizing treatment can refine the original austenite grains and martensite laths,and the water quenching after the recrystallization annealing can prevent the growth of recrystallized grains.During the tempering process,the martensite structure is decomposed and a large number of second phase particles with a size of about50 to 300 nm are precipitated.As the tempering temperature increases,the microhardness of the steel plate first increases and then decreases,reaching a peak of 380.0 HV at 400°C.Quick cooling after tempering can prevent the second type of temper brittleness,but it will increase stress and reduce plastic toughness.Based on the above research,two typical heat treatment processes T1 for 20Mn2 Cr steel are determined: 550℃/1h air cooling +900℃/2min water quenching to room temperature + 400℃/10 min water quenching and T2:550℃/1h water quenching +850℃/2min air cooling 11s+300℃/10 min air cooling.2)The microstructure obtained under T1 heat treatment conditions is mainly tempered martensite,the martensite structure is significantly enhanced,the microhardness reaches380.0HV,the tensile strength is 1300 MPa,the total elongation at break is 10.03%,and the strong plastic product is 13.04 GPa?%.The fractured specimen shows a mixed fracture morphology of cleavage and dimples.The structure obtained under T2 heat treatment is a multiphase structure of bainite + ferrite + a small amount of martensite,with a microhardness of 296.5HV,a tensile strength of 938 MPa,a total elongation at break of 19.21%,and a strong plastic product of 18.02 GPa ?%,the tensile fracture is a typical ductile fracture.The fine structures of T1 and T2 were compared and analyzed.The results showed that the microstructure of T1 treated was mainly lamellar,while that of T2 treated was "soft" homogeneous massive grain;the average grain size of T2(1.076μm)is larger than that of T1(1.507μm)is 40.06% smaller,which is beneficial to hinder crack propagation;and the original size of T2(5.27μm)is 34.9% smaller than T1(7.11μm),and the martensite and bainite laths produced by phase transformation are also fine,which is conducive to improving ductility and toughness;the retained austenite content of T2 is about 3.14%,which is significantly higher than 0.0791% of T1.It can avoid local stress concentration and reduce plasticity during deformation.Therefore,the plasticity of T2 is better,almost double that of T1.3)A large number of second phase particles are precipitated in T1 and T2(volume fraction> 10%),through EDS and TEM composition analysis,it can be seen that the granular second phase part is the second phase of Cr,and the rod-shaped second phase is cementite.The theoretical contribution of precipitation strengthening of the second phase particles to the T1 and T2 yield strength by the bypass mechanism is 290.64 MPa and 138.16 MPa,accounting for 25.7% and 19.6% of the actual total yield strength.The strengthening effect of the second phase is significant.However,the second phase particles are also prone to cause micro-cracks in steel,and rod-shaped particles are prone to fracture in the direction of smaller size when subjected to external force.Precipitating at grain boundaries,it is easy to become a crack propagation path during stretching,reducing plasticity. |
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