| Microalloyed non-quenched and tempered steel has excellent strength,toughness and corrosion resistance,and its production has the advantages of high efficiency,energy saving and so on,so it has been widely used.However,due to the particularity of its chemical composition,the second phase precipitated in steel is the key factor restricting the quality of the strand during solidification,which puts forward stricter requirements for secondary cooling of continuous casting.The surface and subcutaneous cracks of continuous casting bloom can be caused by the mismatch between the water flowrate of secondary cooling and the solidification characteristics of steel,as well as the non-uniformity of secondary cooling.At present,the studies about the secondary phase precipitation and γ→a transformation of microalloy non-quenched and tempered steel are inadequate.There is no customized secondary cooling control method to guide the actual production,and the research on“longitudinal-horizontal" solidification cooling of continuous casting bloom is not systematic and comprehensive.In this paper,based on the solidification characteristics of steel,the solidification evolution rule of micro-alloy non-quenched and tempered steel during continuous casting production was clarified,and the secondary cooling control strategys conformed to the solidification characteristics of steel were put forward,which provided theoretical basis and guidance for the continuous casting production of micro-alloy nonquenched and tempered steel.High temperature properties of steel involve the high temperature mechanical properties and the high temperature physical properties.Firstly,the effects of different cooling rates and composite cooling mode on the hot ductility of SG02 steel were studied by thermal simulator.The results show that increasing the cooling rate deteriorates the overall hot ductility of steel,the third brittleness temperature zone expands from below 794℃ at 0.5℃/s to below 903℃ at 3.0℃/s.However,when the temperature is at the stage of secondary phase precipitation and preeutectoid ferrite transformation,increasing the cooling rate can improve the hot ductility of steel,and the composite cooling mode of "first intensive and then mild"has the best hot ductility in single-phase austenite region.The effect of cooling rate on the thermal expansion property of steel was studied by using quenching deformation dilatometer.The results showed that the thermal expansion rate of austenite decomposition increases’ and linear expansion coefficient and the ferrite transition temperature decreases with the increase of cooling rate between 0.1～1.0℃/s.The amount of ferrite transformation decreases with the increase of cooling rate,and the volume fraction of ferrite increases in an "S" shape with the decrease of temperature.For the study of secondary phase precipitation and solid phase transformation of SG02 bloom,the effect of cooling rate on the precipitation and γ→α phase transformation was studied by high-temperature confocal laser scanning microscope,field emission scanning electron microscopy and transmission electron microscopy.The results show that secondary phase precipitation is accompanied by the appearance of "relief",which is mainly caused by the volume expansion of the"depleted zone" around the secondary phase.The "fast-growing zone" of "relief"is 960～910℃,and the increased amount accounts for about 78%of the total amount.Finally,the quantitative relationship between the number of "relief" and temperature was established,namely N=3462.6+90207.9/(1+e0.084T-78.786).The effect of cooling rate on the average size and volume fraction of the secondary phase particles was significant.The results showed that the average size and volume fraction of the secondary phase particles decrease with the increase of cooling rate,and the secondary phase particles transition from a chain-like distribution at the grain boundaries to a uniform distribution in the matrix.Based on the above research,the pinning force factor was defined to characterize the ability of the secondary phase pinning grain boundaries,and the prediction model of pinning force factor was established by nonlinear regression fitting,namely σ=0.430.1/(1+2.76 Vc3.48).In view of the fact that small cracks on the surface and subsurface of SG02 steel bloom are frequent and difficult to be effectively controlled,the crack morphology was observed by scanning electron microscopy,and the formation mechanism of cracks was clarified.In addition,a heat transfer mathematical model was established to reveal the distribution rule of secondary phase on the bloom surface.The results show that secondary phase particles precipitate slowly in the continuous casting process,the precipitated position is located within 80 mm from bloom surface center and more than 3.6 m from the meniscus,and the "fast-growing zone" is mainly distributed from the secondary cooling zone Ⅳ to the air cooling zone.On the basis of clarifying the reasonable cooling rate and temperature range(especially the "fast-growing zone")for controlling the secondary phase precipitation,a new control strategy and water distribution scheme of"intensitive cooling in the front zones+mild cooling in the back zones" was proposed,which are more consistent with the solidification characteristics of steel.This strategy takes into account the high-temperature mechanical properties of SG02 steel,aiming at strengthening the surface microstructure and reducing surface and subcutaneous cracks of continuous casting bloom.The effectiveness of the technical scheme was confirmed through plant trials.Aiming at the problem of "longitudinal-horizontal" cooling non-uniformity of the steel bloom caused by the unreasonable nozzle configuration and nozzle clogging during the actual production process,the spray water distribution in the secondary cooling zone was tested by using the independently-developed continuous casting spray test platform.Moreover,a heat transfer mathematical model considering the actual water distribution in the secondary cooling zone was established to clarify the influence of the spray water covering mode on the"longitudinal-horizontal" cooling non-uniformity of continuous casting bloom.In order to reduce the surface transverse temperature difference and control the longitudinal reheating of the bloom,it was proposed to change the four-segment cooling mode to the three-segmen cooling mode,which is in accordance with the continuous casting production characteristics of microalloyed non-quenched and tempered steel.The results show that the"longitudinal-horizontal" cooling nonuniformity of steel bloom is improved,and the corner temperature of the bloom at the original secondary cooling zone Ⅲ and Ⅳ increases from 840～739℃ to 906～865℃,avoiding the groove zone of the hot ductility curve(850～650℃).In view of the problems that the crack defects of continuous casting bloom were difficult to monitor online,and the quality of continuous casting bloom and rolled products was poor and unstable,factors affecting internal cracks were analyzed and summarized,and collected a large number of historical production data of continuous casting.Data mining methods such as boxplot graph,heat map and principal component analysis(PCA)were used to clean data and conduct dimension reduction.On this basis,an internal crack prediction model based on PCA and depth neural network(DNN)was established,and the trial and error method was used to optimize the model network parameters.The results show that when the network structure is 18-(15-8-3)-1,the activation function is logsig,and the iteration number is 2500,the performance of DNN model is the best.K-folding cross verification shows that the hit ratio of DNN model is 91.8%,which is better than BP neural network,Extreme Learning Machine(ELM),Regularization Extreme Learning Machine(RELM),Decision Tree(DT)and other algorithms,and can be used for online monitoring of bloom quality. |
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