Development Of Hot-rolled Microalloyed TRIP Steel Plate And Study On Its Microstructure And Property

With the progress and development of steel plate products and product quality, high additional value steel plates are needed strongly by domestic and international market. As high value-added key steel products, the development of hot-rolled TRIP steel plate with high strength, ductility and formability causes metallurgists’ attention. The basics in conventional controlled rolling include low temperature large reduction and addition of microalloying elements, mainly through grain refinement and precipitation strengthening to improve the strength of steel. However, the concept of low temperature rolling is restricted by equipment capacity and the application range of (ultra)fine-grained steel is limited to some extent. Ultra-fast cooling(UFC) and precise control of cooling route technologies emerging in recent years provide broad space for diversified transformation microstructure and material properties, and simultaneously create good condition for the development of hot-rolled microalloyed TRIP steel plate.Based on this, high temperature deformation behaviour, continuous cooling transformation behaviour, process simulation, finish rolling process, cooling routes on the run-out table with UFC as core, continuous cooling transformation kinetics of ferrite, strengthening mechanism as well as tensile deformation behaviour of five kinds of microalloyed steels were studied. The major innovative work of this paper is as follows:(1) Hot-rolled TRIP steel plates with14mm thickness of five kinds of chemical composition series, A, B, C, D and E, were developed by the adoption of appropriate microalloying design, reasonable controlled rolling process and various cooling routes with UFC as core, breaking through the limitation of conventional gauge. Tensile strength level of the steel plates was between690MPa and820MPa, total elongation varied from29.4%to37.5%and the product of tensile strength and ductility was between21000MPa-%and30750MPa’%.(2) The effect of controlled rolling temperature interval on the multiphase microstructure and mechanical properties of steels A, B and C was studied and characteristics and formation mechanism of various phases at different positions through thickness direction were discussed.1) With the decreasing of starting and final temperatures of finish rolling, ferrite grain got refined and tended to distribute along the rolling direction. In addition, ferrite content increased and the sizes of bainite packet and M-A island decreased. The number of dislocation on the ferrite matrix increased, strain induced-precipitation intensified and the average size of microalloying carbonitride particles precipitated within ferrite grains reduced.900℃～840℃was the optimum finish rolling temperature interval by comparing the mechanical properties of steels A, B and C under different controlled rolling temperature intervals.2) Due to strain inhomogeneity and thermal gradients, ferrite grain and bainite packet were smaller and smaller, and ferrite amount increased gradually from central region through a quarter of the thickness to surface layer of the specimen. Whereas M-A amount first increased, then decreased and reached peak value in a quarter of the total thickness. Under the condition of similar TMCP parameters, ferrite grain, bainite packet and M-A constituent sizes were smaller and ferrite amount was most for steel C at the same region through thickness direction, and microstructure difference in different regions of thickness direction was smaller than that of steels A and B, which was more favorable to reduce thickness effect.(3) The effect of chemical composition, relaxation stop temperature and finish cooling temperature on the multiphase microstructure and mechanical properties were studied by adopting "air-cooling relaxation&UFC" process and micro structural evolution of microalloyed steels A, D and E during the relaxation process after finish rolling was revealed.1) Ferrite grain size and its content increased with the increasing of relaxation time at the air-cooling stage. Dislocation density on the ferrite matrix continually declined and the cell substructure connected by dislocation wall disappeared. Precipitation amount of microalloying carbonitrides first increased, and then kept unchanged basically. Both the amount of retained austenite and carbon content of retained austenite first increased, and then reduced. Steels D, A and E demonstrated good comprehensive properties and the optimization of strength and ductility was achieved when relaxation stop temperature range was in720℃～700℃.2) When relaxation stop temperature was720℃, the mean ferrite grain size gradually increased from steel D, steel A to steel E, and the same tendency in ferrite quality and precipitation amount on the ferrite. However, the amount of retained austenite indictated the opposite trend. The product of strength and ductility of steel D was highest, and that of steel E was lowest while that of steel A was between them.3) There was optimum finish cooling temperature under the same controlled rolling process and relaxation stop temperature for three kinds of hot-rolled microalloyed TRIP steel plates A, D and E. When finish cooling temperature was too low, large amount of martensite appeared, which was unfavorable to the improvement of ductility and the enhancement of the product of strength and ductility. When finish cooling temperature exceeded the optimum value, the number of blocky retained austenite increased, even pearlite formed in the microstructure, leading to weakened TRIP effect.4) The relationship between formability index and stability parameter of retained austenite was as follows:Φ=0.577λ,+17.6(steel A),Φ=0.627λ,+17.9(steel D).(4) Three kinds of microalloyed TRIP steel plates A, D and E were studied by adopting "laminar cooling&air-cooling&UFC" and "UFC&air-cooling&UFC" cooling routes. And combined with "air-cooling&UFC" cooling process, the effect of the run-out table cooling routes on phase transformation process was investigated.440℃～460℃was the optimum finish cooling temperature range for steels A, D and E under "laminar cooling&air-cooling&UFC" and "UFC&air-cooling&UFC" cooling routes. After the air-cooling stage of controlled cooling, the steel plate was cooled rapidly to the bainite temperature range at cooling rate of more than50℃·s-1and retained austenite was obtained during the subsequent stack cooling process. By adopting "air-cooling&UFC","laminar cooling&air-cooling&UFC" and "UFC&air-cooling&UFC" cooling routes, respectively, ferrite grain in the microstructure got finer, ferrite content, the dislocation density and the amount of carbonitride precipitation on the ferrite matrix increased. Meanwhile, dislocation density within the bainitic ferrite laths increased gradually.(5) Transformation kinetics of ferrite at the air-cooling stage of different cooling routes was clarified and impingement mechanism was verified by hot simulation test.From the very early stage of the transformation, impingement between ferrite grains was nearly complete along the austenite-austenite grain boundaries and the latter became site saturated. Meanwhile, the number of grains per unit volume present in the microstructure decreased during transformation, accompanied by a significant increase in the mean grain size. When ferrite crystallites impinged, lattice rotations could be activated to reduce the energy associated with the boundaries and led to the final elimination of the lowest-misoriented crystallites through coalescence. Under the deformation and cooling rate conditions, the final ferrite grain size was not determined by the frequency of nucleation on austenite grain boundaries, but by normal grain growth after full impingement on the austenite grain boundary plane and coalescence between different ferrite grains with close orientation formed from the same crystallographic variant. At the later stage of the transformation, ferrite grains coarsened and the mean grain size shifted towards larger values.(6) Deformation behaviour and fracture characteristics of microalloyed TRIP steel under uniaxial tension were studied and the relationship between work-hardening rate, work-hardening exponent and true strain was analyzed.1) Strain-hardening behavior of microalloyed TRIP steel could be divided into three stages. Work-hardening rate of the first stage was relatively high, which was related to plastic deformation of ferrite. At early stage of plastic deformation, strain was concentrated in softer polygonal ferrite, causing it to flow around bainite. The interaction between polygonal ferrite and hard phase led to the formation of an additional plastic deformation field in the soft matrix. At the second stage, work-hardening rate decreased slowly, which resulted from the coordination deformation of bainite and the gradual transformation of retained austenite to martensite. At the third stage, the rate of strain hardening produced by dislocation interactions was inadequate to compensate for the increase in stress in the region of the neck. Meanwhile, plastic instability started and work-hardening fell rapidly.2) With the increase of strain, work-hardening exponent of microalloyed TRIP steel changed nonlinearly. When strain was0.1or so, work-hardening exponent reached the maximum value. After that, it decreased slowly. When neck took place it diminished rapidly.