Reasearch On Microstructure And Mechanical Properties Of Hot-roled And Cold-Rolled Rapid Annealing Low Silicon TRIP Steel Containing Phosphorus

The modern automotive industry develops to light-weight in order to reduce the fuel consumption and emissions under the premise of ensuring the safety performance as the concept of low-carbon, energy-saving and environment-friendly is advocating in the recent years. Considered costs, recovery and maintenance comprehensively, the preferred material of automobile light-weight is high strength steel, and because TRIP steel has its combination of high strength and high elongation, so as to provide an effective direction of solving the contradiction between strength and ductility, it has been one focus in the studies of high strength steel. However, there are still many problems during studying progress of improving the chemical constituents and further increasing strength. Based on this, in the present paper the design idea of chemical composition for low silicon containing phosphorus and vanadium micro-alloying was proposed, hot rolling process, cold rolling and continuous annealing process, continuous annealing process of hot-dip galvanizing TRIP steel were researched deeply and systematically. The main works are present as follows:(1) The continuous cooling transformation behavior for TRIP steel was investigated.The decrease in deformation temperature from900℃to800℃increased the starting transformation temperature of ferrite(Ars) and enlarged the region of ferrite formation of TRIP steel containing vanadium, namely two-phase region (y+a) was enlarged, and this is very beneficial to further improving mechanical properties by expanding windows of processes and using strain induced ferrite transformation to control the formation of finer ferrite grains. With an increase in cooling rate, the effect of deformation temperature on the starting transformation temperature of ferrite (Ar3) increased gradually. The vanadium nitrides and vanadium carbides both dispersed and precipitated within ferritic grains or at grain boundaries, and most of the precipated particle sizes were in the range from2to5nm even at the cooling rate0.5℃/s.(2) The effect mechanism of parameters for hot-rolling process on the microstructure and mechanical properties of low silicon TRIP steel containing phosphorus was investigated, to provide theoretical basis for making hot rolling process of vanadium micro-alloying low silicon TRIP steel containing phosphorus.Reducing appropriately the finishing temperature or final air-cooling temperature could improve the comprehensive mechanical properties of TRIP steel. When final air-cooling temperature was690℃and finishing temperature was above Ae3, bainitic ferrite was used as matrix microstructure; when the finishing temperature was reduced in the range of Ae3-Ar3, uniform and equiaxed ferrite grains could be obtained as matrix microstructure and the average ferrite grain size was～4.5μm. When finishing rolling temperature was820℃and final air-cooling temperature was in the range of630～700℃, the average ferrite grain size was about4μm; the bainite packets had chaotic orientations and the morphology of bainite was presented as larger lath after fuzzily combining with the final air-cooling temperature increasing; the retained austenite distributed in the ferrite grain boundaries or triradius was fine and dispersive, and their grain size was about0.4-1.9μm.(3) Based on the research of hot-rolling processes of low silicon TRIP steel containing phosphorus, the finishing temperature would be controlled in the range of Ae3-Ar3, the hot rolling processes suitable for production of high-strength vanadium micro-alloyed TRIP steel were successfully developed, and the effect mechanism on the microstructure and mechanical properties was clarified.The microstructure was consisted of polygonal ferrite, granular bainite and a small amount of retained austenite. The second morphology (such as fine M/A islands) in granular bainite effectively hindered the movement of dislocations by interacting with dislocations, and this is an important reason for increasing strength. After comprehensive analysis and comparison for elongation and energy-absorbing ability, the elongation rate was22.4%and the value of strength×elongation was20272MPa.%when finishing rolling temperature was nearby Ae3and final air-cooling temperature was620℃; the elongation rate was21.7%and the value of strength×elongation reached up to20181MPa.%when finishing rolling temperature was in the range of Ae3-Ar3and final air-cooling temperature was630℃. As the finish rolling temperature decreased from840℃to800℃, the average ferrite grain size reduced from5.2to4μm and most of the precipated particle sizes were in the range from2to5nm.(4) The effect of phosphorus and vanadium was analyzed and calculated by using Thermo-Calc software, to explore the effect of cold rolling reduction and annealing process, and the interactional relationship and control principle among annealing parameters, microstructure and properties were given.Addition of phosphorus and vanadium increased the starting transformation temperature of αâ†'γ, namely two-phase region (α+γ) was enlarged, this was very beneficial to expanding windows of processes, applying the rapid heating and ultra rapid heating in the annealing process of TRIP steel. The phase transformation driving force of αâ†'γ was decreased by addition of proper phosphorus and helpful to enrich carbon in austenite. The concentration of carbon in y had no influence when the addition of vanadium was less than0.1%, but meanwhile it was also not good for precipitation strengthening of vanadium; the concentration of carbon in y decreased slightly when the addition of vanadium was achieved0.2%.Rapid heating of the continuous annealing process can improve recrystallization temperature further by inhibitting the recovery, thus the recrystallization kinetics was greatly promoted and the ultra-fine microstructure of the TRIP steels was achieved. When the heating rate was80℃/s and intercritical annealing temperature was880℃, the tested steel got a superior combination of strength and plasticity; the ultra rapid heating rate of～300℃/s refined the grain size to≤3±0.5μm and decreased the width of lamellar for retained austenite to<50nm. The increase in cold rolling reduction increased the strength and plasticity simultaneously. Changing the matrix phase of polygonal ferrite into bainitic ferrite when the annealing temperature was increased to above the complete austenitizing temperature, the strength increased but plastic property decreased.(5) The continuous-annealing processes (changing the initial hot-rolled microstructures, different matrix microstructures, rapid heating and ultra rapid heating) of ultra-high strength TRIP steel suitable for continuous galvanizing were investigated.When the isothermal bainitic temperature was higher (460℃), the diffusibility of carbon was remarkably higher, so the carbon content in retained austenite was apparently enough to meet the need of deformation induced martensite transformation even though the holding time was shorter (-20s). Compared with initial hot-rolled microstructure of F+P, initial microstructure of F+B showed higher strength but lower elongation after cold rolling and continuous annealing, the value of strengthxelongation was still superior to that initial microstructure was F+P. Tensile strength (1045MPa) was higher for the matrix phase as BF than the matrix phase as PF (1030MPa), but the elongation rate and the value of strength×elongation were lower. When the matrix phase was polygonal ferrite, the elongation rate and the value of strength×elongation were up to24.5%and24720MPa.%respectively, besides relying on deformation induced martensite transformation of retained austenite, bainite structure and precipitation strengthening of vanadium, ferrite grain size refinement (2.5±0.5μm) for PF steel was also an important factor to increase strength. With increasing heating rate while the intercritical annealing temperature being increased appropriately during rapid continuous annealing, the combined mechanical properties were improved, when the heating rate was80℃/s and intercritical annealing temperature was880℃, the tested steel had excellent mechanical properties. When the heating rate was～300℃/s, the strength decreased with a prolongation in intercritical annealing time but the elongation rate showed an increasing tendency, the value of strength×elongation characterized on combined mechanical properties for strength and plasticity decreased from23976MPa.% for annealing time of10s to23625MPa.% for annealing time of180s. In this ultra-rapid heating experiment condition, the anisotropic index was well and higher than1.0, the fluctuation of yield ratio was not obvious.