| With the increase of car ownership,environmental pollution and energy consumption are increasingly serious,and the lightweight technology of automobiles can effectively alleviate such problems.The application of ultra-high-strength hot stamping forming steel can reduce the car on the premise of ensuring safety.The purpose of weight.At present,the hot forming steel on the market is mainly composed of B steel.Although B element can greatly improve the hardenability of steel,its content is difficult to control.Low B content leads to insufficient hardenability,and high B content causes composition segregation.Therefore,it is of great significance to design a new type of hot forming steel.A high-strength automotive steel with a composition of 0.2C-1.77Mn-1.27Cr-0.2Si(20Mn2Cr)was designed for this project.The microstructure and mechanical properties of the hot rolled and cold rolled steel 20Mn2 Cr with different heat treatment processes were studied.In order to provide a theoretical basis for the regulation of the structure and properties of super high strength hot forming steel 20Mn2 Cr,the main results are as follows:(1)In this paper,the effect of Mn and Cr on the hardenability of experimental steel was analyzed by using JMatPro.The new hot stamping automotive steel 20Mn2 Cr designed and developed has better hardenability than 22MnB5 steel.(2)Using Gleeble 3500,20Mn2 C hot rolled steel plate was simulated by thermal simulation.The experimental steel was completely austenitized and quenched at two different cooling rates.As the cooling rate increased,the original austenite grain size became smaller and the microhardness increased;Austenitizing the experimental steel and austempering at 250℃,the effect of holding time on the morphology of the matrix is not obvious,but the microhardness of the experimental steel increases with the holding time.Compared with quenching in the single-phase zone,the steel used in the quenching of the two-phase zone has Higher microhardness values;hardness values in the isothermal quenching of the two-phase zone are lower than hardness values in the direct quenching.(3)The 20Mn2 Cr steel cold rolled sheet was heat treated according to the actual continuous annealing process.The 20Mn2 Cr steel after simulated continuous annealing treatment had good mechanical properties.The tensile strength was up to 1000 MPa,the elongation rate reached 20.4%,and the strong plastic product reached 20.4.GPa·% has reached or even surpassed the mechanical properties of Q&P 980 steel produced by Baosteel.The experimental steel was directly quenched after austenitizing,and the properties obtained after quenching in the critical zone of 800℃ for 5 min were better.The tensile strength reached 1680 MPa,the elongation was 10.3%,and the strong plastic product was 17.3 GPa·%.The tensile strength of the directly quenched and completely martensitic structure is 1610 MPa,the elongation is 10.7%,and the tensile strength reaches 17.2 GPa.%.Compared to 22MnB5,the new hot-formed steel has an increase in strength and almost the same elongation.Doubled,good mechanical properties.(4)In order to explore the strengthening and toughening mechanism of experimental steel,the microstructure of 20Mn2 Cr was characterized by TEM.The experiment shows that after austenitizing at 850℃ for 2 min,air cooling was taken for 12 s and the heat treatment process was carried out at 300℃ for 10 min to cool to room temperature.Martensite matrix and Cr23C6 and Cr7C3 carbides dispersedly distributed on the matrix showed obvious dislocation packing around the carbides after tensile deformation,indicating that the strengthening and toughening of the experimental steel was due to solid solution strengthening and dislocations.Strengthened,second-phase particle enhancement results from a combination of effects. |
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