| In order to achieve the goal of light weight,high strength steel and super high strength steel are used to make a large number of automobile components.With the increase of the strength of the steel plate,the elastic resilience of the cold formed components increases and the size accuracy decreases.More and more automobile components are formed by hot stamping.After the steel plate is heated and formed,the martensite is quenched and formed in the mould to reach the super high strength.After the steel plate is heated and formed,the martensite can be quenched and formed in the mould to reach the super high strength level,but the safety still needs to be improved because of the low plasticity.Recent studies have found that the strength of the steel can be effectively improved through the diffusion of retained austenite into martensite through the Q&P process.Combined with the characteristics of hot stamping technology,the double partition treatment of medium manganese steel(dual-phase annealing+rapid heating and quenching+dynamic partitioning)can significantly refine the microstructure and increase the content of retained austenite,and it has great potential to enhance the cooperation of strength and plasticity.In this paper,24Mn3Si medium manganese steel is used as the research object,the effect of process parameters on microstructure and mechanical properties and the mechanism of strengthening and toughening are studied in the aspects of dual-phase annealing and rapid austenitizing and quenching.The conclusion was obtained as follows:(1)The microstructure and properties of hot rolled plates of steel 24Mn3Si are greatly affected by the annealing temperature.The lower annealing temperature,the higher proportion of ferrite will be,thus content of the retained austenite at room temperature is lower,and the carbide can not be completely dissolved,the performance of the steel can be adversely affected.If the annealing temperature is too high,the volume fraction of austenite will increase,however,the concentration of alloy elements in austenite will decrease,thus the stability of the supercooled austenite will decrease,some of austenite will converte to secondary martensite when it is cooled after annealing.For the hot rolled plate of 24Mn3Si medium manganese steel,the optimum annealing temperature is 680℃,the microstructure consists of about 20%lamellar austenite and it uniformly distributed on the ferrite matrix.The yield strength of the experimental steel is 560 MPa,the tensile strength is 1050 MPa,the elongation is 31.6%,and the impact absorbing energy is 31 J.(2)When 24Mn3Si steel is fast heated to austenitizing temperature and quenches after dual-phase annealing treatment,microstructure and properties are affected by austenitizing temperature greatly.When the austenitizing temperature is too low,the austenitizing is insufficient.There may be ferrite and twin martensite except the lath martensite,it is not beneficial to obtain excellent comprehensive mechanical properties.When the austenitizing temperature is too high,alloying elements in austenite uniformly distributed,the effect of dynamic distribution in quenching process will be bad.24Mn3Si steel air quenches from 920℃ after dual-phase annealed,it obtained the mixed structure of martensite and 3.5%austenite.The yield strength of the experimental steel is 959MPa,the tensile strength is 1716 MPa,the elongation is 13.9%,and the impact absorbing energy is 28 J,there is a 16%improvement in plasticity than hot rolled sample.(3)More crystal defects and cold deformed storage energy are introduced in the tissue of cold rolled 24Mn3Si medium manganese steel after annealing,increasing isotropy of the tissue can also be discovered.It will result in the morphology of austenite during the process of reannealing in two phase region that changes from laminar to equiaxial,size and spacing of austenite grain also increase.Fast heated of cold rolled sheet to austenitizing temperature after dual-phase annealing,substitutional alloying elements can not uniformly distributed.It result in that the Ms temperature rises and the Mf temperature decreases in the cooling and quenching,expanding the temperature range of dynamic distribution.The volume fraction of retained austenite in the quenched structure is further increased,and the strength and plasticity are obviously improved.(4)Cold rolled sheet of 24Mn3Si medium manganese steel air quenches from 920℃ for 5 min after annealed at 680℃ for 5 h,the yield strength of the steel is 948 MPa,the tensile strength is 1776 MPa,and the elongation is 11.7%.The plasticity has been raised by about 60%. |
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