| Hot stamping steel,as one of the highest strength automotive sheets,is mainly used in key safety components such as front/rear bumpers and B-pillars.At present,the most widely used hot stamping steel is 22MnB5.The tensile strength can reach 1500?1600MPa,and the total elongation is 5?6%after hot stamping.In the application process,there are some problems,such as low plasticity,hydrogen induced delayed cracking and demand for higher strength.In this study,through the optimization of material composition and the combination of various strengthen-toughening mechanisms,a kind of ultra-strong hot stamping steel with excellent energy absorption properties was developed,which can achieve high strength and sufficient plasticity,improve hydrogen induced delayed cracking,and have tensile strength more than 2000 MPa and total elongation between 6%and 9%.The main work is as follows:(1)By composition optimization on the basis of traditional 22MnB5,developed 2000 MPa grade hot stamping steel with high strength and toughness.Sufficient solid-dissolved carbon content was obtained by designing reasonable carbon content range.The increase of Si content can inhibit carbide precipitation and ensure a certain proportion of retained austenite in the final microstructure.The addition of Nb can refine the grain size.When the cooling rate is higher than the critical cooling rate of 2 ?/s,the vickers hardness of the microstructure is greater than 640 HV.Compared with the critical cooling rate of 22MnB5 at 27 ?/s,the critical cooling rate of 38MnBNb decreases significantly.Non-martensitic transformation of the experimental steel is significantly inhibited,and low Ms and Mf are beneficial to obtain a certain amount of retained austenite in the final microstructure.(2)Based on the precipitation law of the particles in the experimental steel calculated by Thermal-Calc,the hot rolling temperature was set in the range of 870?1150 ?.The amount of precipitates in the experimental steel is obviously higher than those in the 1500 MPa grade hot stamping steel.Nb can refine the microstructure after hot rolling and improve the strength and elongation at the same time.The maximum total elongation of 22.88%can be obtained by batch annealing at 700 ? for 5 h.In the continuous annealing process,the minimum ultimate tensile strength 918 MPa and the maximum total elongation 15.64%are obtained after annealing at 740 ?.The batch annealing process has lower strength and higher elongation.Microstructure of the quenched experimental steel is composed of martensite(fresh martensite and tempered martensite),3?5%retained austenite,and carbide precipitation.With increasing austenite holding temperature and austenite holding time,the grain size increases.Compared with the austenitizing holding time,the austenitizing temperature has greater influence on the mechanical properties of the experimental steel.With the decrease of quenching temperature,the total elongation of the experimental steel decreases significantly.The decrease is related to the excessive internal stress in the whole martensite microstructure.(3)Quenching-tempering process can significantly improve the yield strength,reduce the internal stress and enhance the toughness of the experimental steel.In quenching and partitioning process,the volume fraction of retained austenite and the carbon content of retained austenite in experimental steel increase significantly with the increase of partitioning time,which greatly improves the plasticity of experimental steel.Compared with commercial 35MnB5 anti-collision beam,tempered martensite and retained austenite in 2000 MPa grade hot stamping steel can improve the plasticity of experimental steel,and the addition of microalloying elements such as Nb and Ti can refine the grain size.(4)The Nb microalloyed 2000 MPa grade hot stamping steel 38MnBNb exhibits the lowest strength loss and elongation loss in the slow strain rate tensile test.The addition of Nb and Ti can significantly reduce the hydrogen diffusion coefficient in the steel.The hydrogen diffusion coefficient of 38MnBNb is at a low level in current automotive sheets.The addition of Nb element can increase the proportion of small angle grain boundaries and the number of reversible hydrogen traps in the materials.Fractures before and after hydrogen charging are composed of the core fracture zone and the instantaneous fracture zone at the edge.After hydrogen charging,the area of the core fracture zone increases.Large cracks appeared in the core fracture zone of the commercial 35MnB5 after hydrogen charging,but no crack were found in 38MnB and 38MnBNb after hydrogen charging.The microcracks after hydrogen charging mainly expand along the rolling direction,which is related to the elongation of low hardness inclusions after rolling,and the width of microcracks in 38MnBNb is less than that in 35MnB5 and 38MnB.The cracks of 35MnB5 and 38MnB mainly propagate along grain boundaries,while those of 38MnBNb mainly propagate within the grain.The low proportion of ?3 grain boundaries in 38MnBNb structure also contributes to the improvement of hydrogen induced cracking.The hydrogen induced cracking susceptibility of 2000 MPa hot stamping steel was reduced by means of grain refinement,second phase precipitation,retained austenite and martensite morphology control.(5)On the whole,with the increase of strain rate,the yield strength,ultimate tensile strength,uniform elongation,post uniform elongation,strain hardening rate and dislocation density at fracture surface of the experimental steel increase.At high strain rates stage(100?103 s-1),the adiabatic heating near the fracture surface can soften the matrix and increase the post uniform elongation.With the increase of strain rate,the macroscopic fracture changed from "V",shape to 45°direction.Compared with the commercial automotive sheets DP590,DP780 and 30MnB5,at the necking point,the energy absorption capacity of 2000 MPa grade hot stamping steel in this study is superior to those of three kinds of contrast steels at various strain rates.When the strain rate is 103 s-1,the energy absorbed by the experimental steel reaches 21721 MPa·%,which shows a good energy absorption. |
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