High Strength,High Toughness,Microstructure And Texture Of Low Carbon Microalloyed Steel

With the rapid development of economy,the demands for high strength,high toughness and weldable steels are obviously increasing,the traditional materials are already unable to meet the requirements of high performance steel and weight loss for equipment.Hence,it is necessary to research and develop high quality steel.High strength and high toughness steels with low carbon microalloying can be produced by controlled rolling and ultra-fast cooling technology.High strength steel with favorable low temperature toughness provides the guarantee for the equipment worked in severely cold environment.In this paper,the exploratory research related to properties of high strength and high toughness,microstructures and textures in low carbon high strength steel were carried out.In order to achieve the combination of high strength and good impact toughness,the study focused on evolutions of microstructure and texture,relationships among crystallographic texture,fracture behavior in impact specimen and toughness,relationships between crystallographic texture and delamination on fractured surfaces,as well as the effect of delamination,grain size and precipitation state on impact energy.The main work and research results are as follows:(1)The relationships among crystallographic texture,fracture behavior and impact toughness were investigated in detail.The three different hot-rolled processes were employed to obtain different crystallographic texture and impact toughness.The effect of texture orientation,effective grain size,fracture behavior of impact specimen and delamination in the ductile region on impact energy was systematically analyzed.The results show that the {001}<110>texture is unfavorable for toughness and makes steel brittle,while {112}<110>and{332}<113>textures contribute to better impact properties.The impact toughness of hot-rolled steel can be improved by controlling the development of austenite textures to weaken {001}<110>texture.The rolling practices also prove that the intensity of the {001}<110>component can be minimized by appropriate controlled rolling processes.The steel rolled above Tnr temperature will acquire strong<110>//RD orientation,weak<110>//RD orientation and insufficient quantities of high-angle boundaries,the cleavage fracture parallel to the fracture surfaces occurs more easily with a decrease in the test temperature.Therefore,the impact energy of steel rolled above Tnr temperature decreases rapidly with decreasing temperature,while the steel rolled below Tnr temperature can gain excellent impact energy at low temperature.The elongated grain structures in accompany with {001} cleavage planes along the grain boundaries are the primary reasons for delamination,and the {001} planes parallel to the rolling plane will lead to an increase in the probability of delamination.(2)Based on impact test results at different directions with respect to the RD(rolling direction),the anisotropic behavior of impact toughness in hot-rolled high strength steel was investigated.A series of experiments were carried out to establish the origin of anisotropy of impact toughness in the given high strength steel rolled in two stages,the effect of texture orientation and the occurrence of delamination on anisotropy of impact energy was discussed.The results show that the RD fiber centered at {112}<110>and {223}<110>is the main reason for anisotropy of impact energy,brittle {001}<110>texture can also produce anisotropy of impact energy,while the {332}<113>texture produces isotropy and provides favorable strength and toughness.The deep groove shape cleavage crack caused by delamination will decrease the impact energy of steel in the ductile region,while the convex-concave region due to delamination will produce dimples of ductile fracture and serve as a part of the fracture energy absorption process to improve impact energy in the ductile to brittle transition region.Delamination can also play an important role in the anisotropy of impact toughness.(3)The hot-rolled high strength steel with yield strength above 850MPa and favorable low temperature impact toughness was researched and developed.The results show that the hot-rolled high strength steel with 880MPa yield strength,1083MPa tensile strength,17.7%total elongation and impact energy of about 122J at-80? is successfully developed by rolling in two stages and ultra-fast cooling technology.The impact specimens keep the state of ductile fracture in the temperature range from 20? to-80?.The strong {112}<110>and{332}<113>textures,beneficial<110>//RD orientation as well as sufficient quantities of high-angle boundaries are the reasons for favorable low temperature impact toughness.(4)The properties of strength and toughness of high strength steel under different tempering processes were investigated,and the evolutions of microstructures and crystallographic texture under different tempering processes were analyzed.The results show that the increase in quantities of cementite and precipitation in high strength steel tempered below Acl temperature will lead to an increase in the nucleation site of potential crack and micro-void,and then impact toughness is reduced.The differences in texture components and quantities of high-angle boundaries in the tempered samples are not obvious,but the volume fraction of beneficial<110>//RD orientation will evidently decreased with the increase in tempered temperature or time,thus reducing impact toughness of tempered samples.The elongated grain structures are the primary reasons for delamination of tempered samples during impact process,the plenty of formation of cementite on the prior austenite grain boundaries increases crack initiation sites,cleavage cracks form more easily and quickly propagate on the prior austenite grain boundaries during impact process,then the quantities of delamination are increased,and impact energy of tempered sample is finally decreased.The high temperature softening in martensitic matrix partially compensates the toughness deterioration and gradually improves impact toughness when the tempered temperature is relatively high.(5)The proper intercritical temperature,austenitizing temperature and low tempering temperature were selected to study the effect of multi-stage heat treatment processes on properties of strength and toughness,microstructures and textures in high strength steel.The results show that the tempering process still retains the intensities of' as-rolled texture components.The intercritical heat treatment has a significant increase in the intensity of' {332}<113>texture,then the {332}<113>texture has the same intensity with the {112}<110>texture,and the overall a fiber textures possess higher intensity compared with tempering process as well.Hence,the intensities of {332}<113>texture and<110>//RD orientation can be enhanced by intercritical heat treatment.The austenitizing heat treatment inherits partially original texture intensity,but overall texture intensity is significantly weakened and the intensity of {001}<110>texture increases,the intensity of the most favorable {332}<113>component is greatly weakened and finally below intensity of {001}<110>texture.The average effective grain sizes of the austenitizing heat treatment,intercritical heat treatment and tempering process are 10.5?m,3.8?m and 4.6?m,respectively.The partial phase transformation at intercritical temperature by intercritical heat treatment will produce fine grains,while high austenitizing temperature by austenitizing heat treatment promotes grain growth,and many ferrites subsequently form during cooling process,then effective grain size is comparatively large.