Systematic Research Of Prediction For Microstructure Evolution And Property Of High Strength Wire Rods

The microstructure matelloghraphy of hot-rolled wire rods of high carbon steel has great influence on the shape properties of the metalwork. The interaction between the temperature and deformation with physical metallurgy of stock result in the mcrostructure evolution, such as multipass groove rolling increasing the complication of deformation, storage energy of deformation materials influencing by the thermal mechanical history. So hot rolling processing is a key procedure to the final mechanical properties of wire rods. In order to control the mechanical properties of wire rods, the thermerl mechanical condition, rolling processes and cooling parameters must be optimized. The method of prodiction of the microstructure and property by building mathematical models is feasible.The achievements have provided a scientific basis for computer aid control product's property, designing new steel grades and new rolling technology.It is difficult to predict the microstructure of high carbon steel wire rods due to high strain rate and high deformation heat and irregular section. Nowadays the microstructure prediction of high carbon steel is about indenpented physical metallurgy processes such as reheating, recrystallization and phase transformation, absence the validation of industial trials. So it's not full reason to explain the unstable properties of high carbon steel wire rods. A integrated mathematical models of microstructure evolution from reheating to coiling should be developed by systematic researching considering chemical composition, which have great influence on the characterilitics different of rolling and phase changes existing more definte microalloy elements in demestic high carbon steel. The models of recrystallization evolution and phase transformation have developed by laboratory simulated experiment. As well as the analysis of finish production's texture,cementite morphy,decarbonizes, carbon separate et al metalloghraphy。Base the mathematical models developed, a software which validated by industrial practice is programmed to simulated the production and to provide the optimal processs parameters of hot rolling of high carbon steel.The fellow are main conclusions of the research including.(1) The cooling system located the entry and delivery of the finishing mill train preserves to a large extent the refinement of the austenitic grain structure produced by hot deformation, hereinafter called Rapid Cooling System (RCS).The finite difference method was employed to calculate the stock's temperature profile at the RCS according the difference heat transfer mechanism on the difference condition of surroundings such as air, liquid water and vapor-liquid mixture and vapor film. The computing results indicated surface temperature of temperature profile is well agree with the measured, computing time is short and meet the requirement of online control. The effects of the cooling parameters on the temperature profile were examinated, and the conclusion indicated that the recovery stage of wire rods cooling in the RCS should be taken into account, the cooling efficiency depended the water pressure of the nozzle, the delivery temperature of furnace decreased as soon as possible due to the more deformation heat, the spray cooling sequence have influence on the uniform of the temperature profile implied that the fore nozzle of water cooling box should be considered priority.(2) The activation energy of hot deformation, recrystallization critical strain, kinetics of dynamic recrystallization was measured by single-pass compression test on thermomechanical simulator, the model of recrystallization critical strain on the condition of elevated temperature and high strain rate were systematically investigated. A high precision criterias of static recrystallization softening mechanics was proposed for high speed rolling of high carbon steel.(3) The CCT curves of high carbon steel 82A were obtained by way of thermal dilation and metallography. The transformation behavior of steel 82A such as start transform temperature during continuous cooling was investigated by means of THERMECMASTOR-Z thermomechanical simulator. The microphotoghraph of the sample after CCT experitment was taken, the result indicated that the fine pearlite (volume 96% are sorbite) and fine interlamellar spacing of pearlite (interlamellar spacing is 0.15～0.21μm) were obtained when continuous cooling speed was less than 10℃/s. Futermore the experiment of the austente isothermal transformation of high carbon steel 82B was conducted on the thermomechanical simulator to gain the curve of pearlite volume transformed to transform time, and the effect of the isothermal temperature on the transformation volume are analyzed comparatively. Transformation kinetics model of austenite decomposition of high carbon steel was developed on the base of the equation Avarim modified by the grain size.(4) The relationships of strength and interlamellar spacing of pearlite have been developed by regression analysis utilizing production datum.(5) A set of integrated mathematical models for simulating hot rolling and controlled cooling of wire rods have been developed through extensive laboratory research work and software coded by Visual Basic Language. The reliability of the simulating software has been validated directly by in-situ sample and temperagture measured and testified indirectly by mechanical properties. The predicted results are agree well with measured of the industrial tests.The result implied that the models including rolling deformation submodels and algorithm adopted can provide the guidance to industry production.(6) The chemical composition, morphology of cementite, decarburized depth and inclusion of wire rods have been exiaminated and analized to find the cause of wire drawing failure. The main cause of brittle fracture in drawing and lower specific elongation of finished wire are local martensite existence and carbon segregation leading net cementite, It was shown that index of carbon central segregation in bloom should be less than 1.12 and index of total central segregation should be less than 1.20 to avoid martensite.In summary, the industrial application of software implied that it can reveal the essentiality of microstructure evolution of wire rods rolling of high carbon steel, and it is effective tool to develop new steel grades and opitimize process parameters, and the software developed may provided theoretical basis for online microstructure prediction in the future. The study is helpful to better understand of microstructure evolution and industrial practice, and is valuable to parameter optimal for guaranteeing stable operation of this system.