| With the intensification of energy and environmental issues,lightweight has gradually become the future development trend of the automotive field.As an important automotive material,advanced high-strength steel has undergone first,second,and third-generation technological changes.Due to the limitations of various technical conditions,researchers have gradually considered combining hot stamping processes with Q&P and other processes to solve the pain points of high-strength steel for difficulty forming and realize the control of the performance and cost of high-strength steel.The heat preservation in the dual-phase zone can carry out the comprehensive distribution of C and Mn elements,and the process of carbon distribution can further enhance the effect of C element distribution.Combining the above process with the hot stamping process can realize the control of the structure and performance while ensuring the formability of the high-strength steel,and obtain high-strength,highplasticity and more precise high-strength steel for automobiles.This paper mainly studies the effects of C and Mn elements on the microstructure evolution and mechanical properties of hot stamping steels.By controlling the temperature of the dual phase zone,the holding time,the temperature of carbon partitioning,and the time of carbon partitioning,the effect of the partitioning of C and Mn elements is controlled.Using OM,SEM,XRD,EBSD,tensile experiments and other methods to study the influence mechanism of high-strength steel structure evolution,retained austenite content,grain orientation,element distribution and mechanical properties,and obtained the following research results:(1)The microstructures of the test steel obtained at different dual-phase zone temperatures are massive ferrite,lath martensite and massive martensite.With the increase of the temperature in the dual phase zone,the content of lath martensite increases and the lath becomes slender,the content of massive martensite and the content of massive ferrite gradually decrease.EBSD analysis shows that the grain size in the dual-phase region is larger at 740°C,and the grain boundary is more blurred.While at 830°C,the grain boundary is clearer and the number increases significantly,and the grain size is also smaller;At 740 ?,there is no obvious enrichment phenomenon in the test steel neither C nor Mn.However,the bright part of the element distribution diagram at 830 ? is more obvious,and both Mn and C have obvious enrichment.With the temperature increasing,the content of retained austenite increases,the tensile strength first increases and then decreases,the maximum value is 1340MPa(800 ?),The elongation after fracture and the PSE decreased first and then increased,and the maximum elongation after fracture is 19.0%(740 ?),the PSE up to 19.9 GPa·%(830 ?).The overall hardness shows an upward trend,with the highest being 421.8 HV(830 ?).(2)When the test steel is held at 740 ? for 5 min to 20 min,the microstructure changes from most of the massive and scattered martensite to lath martensite,and the scattered martensite grains disappear completely at 20 min.With the extension of the holding time,the tensile strength gradually increases,the highest is 1110MP(20 min).The elongation after fracture gradually decreases,the highest is 19.0%(5 min).The PSE first decreases and then rises,the highest is 17.3G GPa·%(5 min).The hardness shows a gradual upward trend at740 ?,the highest is 380.6 HV(20 min).With the extension of the holding time at 830 ? in the dual phase zone,the grain growth is particularly obvious,the martensite is distributed in large areas,and microcracks appear,and the grain boundaries are gradually blurred.As the holding time increases,the retained austenite content gradually decreases.As the holding time increases,the overall trend of tensile strength will increase first and then decreases,the highest is 1390 MPa(15 min).The elongation after breaking gradually decreases,the highest is 15.2%(5 min).The PSE is a trend of decreasing-increasing in a small range-decreasing,the maximum is 19.9 GPa·%(5 min).The hardness presents a trend of increasing first and then decreasing,the highest is 452.6 HV(10 min).(3)The microstructure of the test steel was coarse massive martensite and massive ferrite at the carbon distribution temperature of 300 ?,while the bainite structure appeared at 360 ?and showed needle-like and feather-like structures.EBSD analysis shows that the grain orientation of carbon distribution at 300 ? is relatively single,and the grain orientation of360 ? is more diversified;the distribution of Mn element at 300 ? and 360 ? is similar with no significant difference,while the distribution of C element is significantly different.With the increase of carbon partition temperature,the content of retained austenite first increases and then decreases.The tensile strength and PSE of the test steel show a downward trend,the highest is 1340 MPa(270 ?),17.2 GPa·%(270 ?);the elongation after fracture is not much different except for 360 ?,the highest is 12.8%(270 ?).The hardness presents a trend of decreasing first and then increasing,and the highest hardness is 423.4 HV(360 ?).(4)The test steel is held in a mold at 300°C and carbon is partitioned for different times,the microstructures obtained are block,lath martensite and block ferrite.As the carbon distribution time increases,lath martensite decreases and massive martensite increases.The retained austenite content increases first and then decreases with the extension of the carbon partitioning time.With the extension of the holding time,the tensile strength of the test steel shows a trend of first decrease and then a slight increase,the highest is 1380 MPa(no pressure holding);the elongation after breaking increases first and then decreases with the extension of the holding time,the highest is 12.4%(1 min);The PSE decreases first and then rises,the highest is 17.0 GPa·%(no pressure holding).The hardness shows a trend of rising first and then falling,the highest is 456.8 HV(holding pressure 2 min). |
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