Study On Microstructure Evolution And Mechanical Behavior Of Cold Rolled Low Carbon Martensitic Steel

Advanced high-strength steels have been widely used in the automotive industry,such as car weight loss,safety,energy saving,environmental protection and so on.Through the study of the microstructure evolution and mechanical properties of low carbon martensitic steel plates after cold rolling deformation,the method of preparing ultra-high strength steel plates which has important significance for the development of the automotive industry was obtained.In this paper,carbon steel-10#steel was used to obtain complete martensite structure through quenching,and then low-carbon martensite steel sheets were cold-rolled in order to prepare ultra-high strength steel.The effects of the amount of deformation on the mechanical properties and microstructures of the experimental steel were studied,at the same time,the changes in microstructure and properties of the experimental steel were analyzed after the low temperature aging treatment.The following research results have been obtained:After the low-carbon martensitic steel was cold-rolled and deformed,the spacing of deformed martensite laths gradually decreases as the deformation increases.When the cold-rolling reduction is small(<30%),the angle between the arrangement direction of the martensite laths and the rolling deformation direction is done distributed and there was no obvious orientation relationship.With the increase of the deformation amount,the original martensite structure was gradually arranged along the rolling direction,and it showed a distinct delamination characteristic.The initial low-carbon martensitic steel has a hardness of 440 HV,a tensile strength of 1302 MPa,an total elongation of 15%at break,and an impact toughness of 95 J/cm~2;as the cold rolling reduction increases,the hardness of the experimental steel increases,the total elongation at break decreases,and the impact toughness decreases.The steels were cold-rolled to 80%reduction,the hardness increases to 532 HV,the tensile strength increases to 2045 MPa,and the impact toughness decreases to 40 J/cm~2.The mechanical properties of cold-rolled low-carbon martensitic steels performed different mechanical properties at 0°,45°,and 90°(angles from the rolling direction),and the tensile properties at 0°are slightly higher than those at 45°and 90°,however,the differences between the tensile properties at 0°,45°,and 90°are not obvious.After the initial low-carbon martensitic steel and deformed martensitic steel were subjected to low-temperature aging treatment(120°C and 240°C),the strength of the experimental steel exhibited a trend of first increasing and then decreasing as the aging time increased.The intensity reaches a maximum at a certain time during aging,and the strength increases more obviously at the initial aging period,however,the strength does not decreases significantly as the aging time increased at the later aging period.The total elongation at break of the experimental steel gradually increases with the extension of the aging time,and the total elongation at break increases more obviously at the initial aging period,while at the later aging period,the total elongation at break basically increases no longer changes.