| In recent years, the technology of hot charge continuous casting slabs has beenwidely used in the steel industry. However, continuously cast low alloyed steel slabstend to form more surface cracks through hot charge process than cold charge process.In order to analyse the formation of hot cracks during hot charge process, themicrostructure evolution and precipitate behaviors of Nb containing steel were studied.To prevent the hot cracking problem, quched-onlined tenique was also studied inlaboratory.The results of the CCT static curve of Nb containing steel show that: thetransformation microstructure was made up of ferrite and pearlite, when the cooling ratewas below1℃/s. And the transformation microstructure was consist of ferrite, bainiteand pearlite, during a cooling rate of2℃/s~10℃/s. When a cooling rate of15℃/s,pearlite was not observed, instead of ferrite and bainite. If the cooling rate was fasterthan100℃/s, the transformation microstructure was entirely lath martensite.The effect of different hot charge process on microstructure evolution formicro-alloyed steel was studied. It shows that: the microstructure of water-quenchedspecimen was lath martensite, as the hot charge temperature higher than850℃. Themicrostructure morphologies of specimens charged at750°C and700°C respectivelywas lath martensite, banite, polygonal ferrite and the film-like ferrite along the prioraustenite grain boundaries. As the specimen was charged at700°C and reheated to800°C~1150°C, it can be obviously observed that cracks both distributed on thefilm-like ferrite matrix of water-quenched specimens and furnace-cooling specimens, byusing optical microscopy and SEM observation. For further understanding the formationmechanism of hot cracks, the volume fraction of phases was determined by using IPPstatistical software, and the results show that the film-like ferrite was3.6％of all and9.5％of the austenite. As the hot temperature below650℃, the microstructure wasferrite and pealite, and the difference of prior austenite grain size of specimen reheatedfrom R.T~650℃to1150℃was not change obviously due to the result of phasetransition refinement.The theory calculation of microalloying carbonitride precipitates show that: theprecipitation sequence in matrix start with Ti-containing precipitates, followed byNb-containing precipitates, V-containing precipitates was formed at last. And the nucleation of precipitation was firstly at grain boundary, followed by dislocations, andfinally homogeneous nucleation. The results of TEM show that: a square shape of Ticontaining precipitates firstly appeared from the austenite grain boundary. After theoccurrence of phase transformation of austenite to ferrite, a shape of irregular or laminarprecipitate was observed in the ferrite matrix. For instance, laminar precipitates at700℃was found in ferrite along the austenite boundary, the distribution was like chain, andEDS analysis results for (Ti,Nb)C. By comparing the precipitates distribution ofspecimen under different hot charge process, it is found that the proportion of smallprecipitates was the highest as the specimen was reheated from700℃to800℃, andthe chain-like precipitates trend to gather into a cluster, then grow up to be a largeparticle. Therefore, by compared with the former two, the destruction of precipitates onthe ductility of grain boundary was the most dangerous.In order to prevent the generation of micro cracks during hot charge process, thequench-online of slab was studied in laboratory. Research shows that: the ferrite grainsize was smaller, because of the effect of grain refinement after the quenched-reheatedprocess. But the quench-online parameters including the start and final coolingtemperature, water volume and the cooling rate, still need further investigated. |
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