| With the great advantages on safety,lightweight and manufacturing cost,high strength steels occupy the largest proportion of lightweight materials in the automotive industry.The use of high strength materials can effectively reduce body weight and energy consumption.At present,1500MPa grade steel can only be prepared by hot stamping.In the application of automobile body,the proportion of hot stamping steel is also increasing year by year.However,how to further enhance the strength and plasticity of the new generation of hot stamping steel is an urgent problem.In this thesis,by combining the alloy design of medium manganese steel with Quenching&Partitioning(Q&P)process,synthetical understanding he whole process of automobile steel material preparation,hot working and follow-up treatment,1800 MPa hot stamping steel with high plasticity is developed.Meanwhile,this thesis systematically studied the evolution of microstructure and properties of materials under the specific process path.The main work is as follows:(1)A medium manganese material with the addition of V was designed for hot stamping process.The original austenite grain was refined by austenitizing at low temperature and VC precipitation during hot forming,in order to obtain nano size retained austenite during the cooling process.Based on the design of nano-scale retained austenite in the material,the baking and coating process of 170? for 20 min in automobile manufacturing process was proposed,to realize the distribution of C from martensite to austenite and replace the traditional tempering process.Ultimately,a‘bake-partitioning'medium manganese steel designed for press hardening,with the strength of 1800 MPa and the elongation of 16%,was obtained.(2)The influence mechanism of Si and V on the microstructure evolution of‘bakepartitioning'hot stamping steel has been studied.The mechanism of VC precipitation on microstructure and properties of the medium manganese hot-formed steel was proposed.VC precipitation can improve the stability of retained austenite during tempering through grain refinement,reduce the thickness of austenite strip,and shorten the distance for C partitioning.Thus,the strength and plasticity of the material was enhanced.Solid solution strengthening of Si in hot-formed steel can improve the strength of the material,but its effect is mainly reflected in the inhibition of carbide precipitation and stabilization of austenite during the tempering process at 300-40?.(3)The effect of martensitic transformation on the reverse transformation of austenite during the baking-partitioning process was studied.The control mechanism of yield strength of baking-partitioning hot-formed steel was proposed.It is found that,with the increase of cooling rate,martensitic transformation happens in front of Ms temperature,with the accelerated martensitic transformation rate.In this situation,dislocation martensite is the main type of martensite and the retained austenite content in the initial microstructure is relatively low.In the contrast,more twin-martensite is formed in the low cooling rate situation,and more retained austenite is obtained after the baking process.During the baking-partitioning process,part of twin martensite transformed to austenite,leading to the different fraction of retained austenite and the difference in yield strength.(4)The mechanism of dislocation strengthening of retained austenite and TRIP effect of retained austenite in medium manganese hot-formed steel was studied.The strengthening provided by the transformation of retained austenite into martensite during deformation is greater than the weakening caused by the decrease of carbon content in the original martensite matrix.Thus,‘bake-partitioning' hot stamping steel has high ductility and higher strength than full martensite steel.Due to the addition of a large amount of Mn,the retained austenite in the medium manganese press hardening steel can be stabilized with lower carbon content,which makes the average carbon content in martensite higher than that of the conventional Q&P steel.At the same time,the strength of martensite matrix did not decrease significantly during the baking process of 170? for 20 min.Therefore,the medium manganese hot-formed steel designed in this paper has higher strength and plasticity under the process of more retained austenite content.(5)A medium manganese steel containing Al was designed and an‘intercritical annealing+Q&P'treatment was used in the steel,which is used for soft zone material to assist hot stamping steel.By means of intercritical annealing,the Ms point of the material was reduced to about 100?,so that 20-80? was the reasonable quenching temperature(QT)range,and the heat treatment was carried out by means of‘water quenching+partitioning'.Thus,1200 MPa grade strength,18%elongation,and high controllability of microstructure and properties are obtained.Meanwhile,by comprehensive utilization of fine grain strengthening,phase transformation strengthening and ferrite strengthening brought by carbon capture during tempering,the goal of‘one material,different processes,gradient properties'was achieved.Specifically,different initial micro structure was obtained by austenitizing at different temperature for the first heat treatment after cold rolling.The medium manganese steel with different strength grades in the range of 9001200 MPa was prepared by the same critical zone annealing+Q&P process. |
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