| The character of the microstructure and the morphology and distribution of second phase precipitation in micro-alloyed slab corner surface were investigated, and the causes of transverse corner cracks formation were analyzed, the slab was produced from the 3rd vertical-bend continuous caster in Baosteel. The results show that the fundamental causes of the cracks formation are the corner temperature is in the low ductility range(700-900°C) inevitably as the corner is subjected to two dimensional cooling when the slabs are bent, and due to the stress mismatch between the matrix and the fine precipitates during bending operations, the chain-like precipitated carbides and/or nitrides in the grain boundaries increased of cracking susceptibility of the slabs . Meanwhile, the film-like proeutectoid ferrite precipitated along the austenite grain boundaries during the austenite–ferrite transformation disrupted the continuity of the austenite matrix. In view of the fact that the strength of proeutectoid ferrite film is lower than that of the austenite grains, when the slabs were subjected to bending stress, the cracks were generated along the film-like proeutectoid ferrite.Based on the causes of transverse corner cracks formation, and combined with the parameters of caster and the produce process character, the microstructure evolution of the slab surface in micro-alloyed steel was investigated using the following instruments: optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrum (EDS), transmission electron microscopy (TEM), confocal laser scanning microscopy (CLSM), and remelting solidification cooling equipment. And the results show that the method of preventing transverse corner cracks is to improve the resist crack capability (or to decrease the cracking susceptibility) of slab surface microstructure.The precipitate dynamics of ferrite and the precipitate thermodynamics and dynamics of micro-alloyed elements carbonitride were analyzed based on the process path of slab surface microstructure control cooling (SMC), and the influence of cooling rate on microstructure and second phase precipitation of slab surface was investigated. The relation of cooling rate and precipitation as follows: the inverse exponential relationship between carbonitride precipitation volume fraction and cooling rate, as V f = A?exp( ?B?υc), and the inverse power function relationship between precipitation size and cooling rate, as D p = C?υc?n, that is the carbonitride precipitate behavior can be controlled by increasing the cooling rate, the precipitation volume fraction and size decrease and the proeutectoid ferrite was restrained with cooling rate increase. The influence of micro-alloyed elements'carbonitride on proeutectoid ferrite is very importantly, the proeutectoid ferrite nucleation is promoted as the carbonitride act as nucleating agents, and the carbonitride precipitation consume the carbon in steel during precipitating, which result in the concentration of carbon in local, and promote the proeutectoid ferrite to precipitate.Based on the above research results, the parameters of the slab surface microstructure control cooling(SMC) as follows: the best cooling rate range is 3-6°C·s-1, in which, the slab surface microstructure has no chain-like carbonitrides and the film-like proeutectoid ferrite precipitated along the austenite grain boundaries; and the best cooling temperature is correspondingγ→αstart transformation temperature; and the best reheating temperature is higher Ac3 of this micro-alloyed steel. And based on the parameter of the 3rd cater in Baosteel, the the slab surface microstructure control cooling(SMC) as follows: the best cooling rate is 5°C·s-1, and the best cooling temperature is 660°C, and the best reheating rate is 3°C·s-1, and the best reheating temperature is higher 870°C.The cooling conditions were simulated and optimized in the SMC process Solidification cooling through the remelting cooling experiments. The optimized SMC process scheme of the 3rd caster in Baosteel as follows: remove the bottom 5 rows nozzles and reserve the upside 5 rows nozzles in the narrow side, and change the nozzle 3/8KSH37128H for original nozzle 1/4KSH16113H, and change nozzle 1/4KSH16113H for original nozzle 1/4KSH0889H, then remove the bottom 4 rows nozzles near corner in the width side. After change nozzles, the nozzle install high of 3/8KSH37128H and 1/4KSH16113H is 65mm and 50mm in 300mm thick section, respectively, and which is all 50mm in 200mm thick section. Reserve the total water quantity of every zone, experiment for add or decrease 25% of total water quantity in 2nd zone, and optimize the best mill water quantity.Produce experiment in the pilot caster is conducted in order to verify the effect of SMC cooling on the microstructure. In the conventional continuous casting process, the microstructure and the distribution of precipitation from pilot cast slab consistent with these from mill, and the chain-like precipitated carbonitrides and the film-like proeutectoid ferrite precipitated along the austenite grain boundaries, meanwhile, the microstructure is Coarse. However, in the SMC cooling process, there is no chain-like precipitated carbonitrides and film-like proeutectoid ferrite precipitated along the austenite grain boundaries, the uniform fine precipitation is dispersed in martrix, and the microstructure is fine.Hot ductility of slab was compared with both cooling patterns in hot tensile test. Representative lower ductility curve is obtained under mild cooling in the temperature range 700-900°C. SMC cooling, however, gave a significant improvement on the ductility, thus empbrittlement trough almost disappeared. For specimen having low ductility, i.e. RA of 40%, crack lies within the grain boundary film-like proeutectoid of ferrite and the fracture mode is intergranular brittle. It is, however, transgranular ductile under SMC cooling, which possesses higher ductility.The result of lab and pilot caster experiment show that the hot ductility gave a significant improvement in the SMC cooling pattern, the empbrittlement trough in the conventional continuous casting process almost disappeared. The cracks susceptibility is decreased conspicuously, and the transverse corner cracks could be eliminated essentially by using this process.
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