| Since the development and uprising of micro-alloying technology for steels, the application of micro-alloyed steels has been widely spread in every field such as traffic, construction as well as machinery manufacturing. The precipitation of secondary phase particles in micro-alloyed steel is the main deciding factor for steel quality. Over the past decades, researchers from domestic and abroad have paid deep attention on the reheating and rolling process of micro-alloyed steels and fruitful application results have been obtained. However, the quality of micro-alloyed rolled products is determined by the quality of the corresponding continuously-cast strands to quite a great distant. Unless the cooling process of continuously-cast strands is controlled properly, the formation of solidification defects such as cracks and segregation would easily be transferred and kept to the final rolled products, causing the occurrence of edge cracks and banded microstructures. Hence, it would be necessary to deeply analyze the interrelationships between the solidification characteristics in continuous casting process and microstructure properties of micro-alloyed steel, clarify the formation mechanism of steel solidification defects and finally lay a solid foundation for cooling optimization of micro-alloyed steel continuous casting process. For this reason, as-cast vanadium micro-alloyed steel YQ450NQR1 bloom is selected as target of this paper to research and analyze the evolution of solidification characteristics and microstructure properties in continuous casting process.Regarding the high temperature properties of YQ450NQR1 steel bloom, DSC and materials calculation method are utilized to determine the liquidus and solidus temperature of YQ450NQR1 steel, which is 1511? and 1466 ?, respectively. Considering the variation of alloying element content in steel, thermal simulation method is used to test the reduction of area curves and stress-elongation curves of YQ450NQR1 steel bloom. Results have shown that with the [%V][%N] content in steel increases from 1.5×10-3 to 2.02x10-3, the austenite dynamic temperature rises from 950 ? to 1000? and the hot ductility through widens from 700??917? to 700??970?, correspondingly. With the heat cycle being stable, the high temperature end of hot ductility through is mainly affected by the onset precipitation temperature of secondary phase particles. The depth and width of hot ductility through is determined by volume fraction of secondary phase and formation temperature of grain ferrite, respectively.Regarding the problem of abrupt cooling rate fluctuation and high hot cracking susceptibility near mold region for YQ450NQR1 steel continuous casting process, micro-segregation model for main solute elements is established based on actual production parameters and steel compositions. The model is validated with data from previous literatures combining original position analysis of element distribution along the cross section direction of YQ450NQR1 steel bloom. On this basis, the influence cooling rate on element micro-segregation are analyzed and the quantitative relationships between cooling rate and ZST, ZDT, LIT are obtained with non-linear fitting method. Moreover, index for internal crack susceptibility (IICS) is defined and the internal crack susceptibility prediction model is established accordingly.Regarding the tight links between surface horizontal cracks and non-equilibrium solid phase transformation, Gleeble thermal simulator, thermal dilatometer, confocal laser scanning microscope as well as scanning electron microscope are applied to study the influence of ??? phase transformation and secondary phase precipitation on the evolution of hot ductility for YQ450NQR1 steel bloom in continuous casting process. Effect of cooling rate variation on ??? phase transformation is researched and the quantitative relationships between residual ? ferrite amount, cooling rate, a ferrite precipitation time consumption and onset a ferrite precipitation temperature with multiple non-linear regression method. Research have shown that effect of V(C, N) precipitation on a-ferrite phase transformation is mainly revealed in two aspects:(i) Precipitated V(C, N) particles act as inoculant particles to promote proeutectoid ferrite nucleation. (ii) Local carbon concentration along the y-austenite grain boundaries is decreased with the precipitation of V(C, N), which in turn promotes a-ferrite precipitation.To study the influence of cooling process parameters for YQ450NQR1 steel on solidification structure and element segregation behavior, Cellular Automaton-Finite Element (CA-FE) method is used to established heat transfer and solidification structure coupling model and the effect of superheat and secondary cooling intensity on equiaxed grain ratio and grain size. On the basis of solidification structure simulation/control and evolution of non-equilibrium solid phase transformation, a new cooling strategy for YQ450NQR1 steel continuous casting process is obtained to optimize solidification structure and improve surface microstructure strength of the steel bloom, namely "low pouring temperature (with molten steel superheat of 23 ?)+intensive cooling after temperature revival ahead of straightening position (with secondary cooling water amount of 2.6 L/min in III zone and 165.6 L/min in ? zone) " thus providing theoretical basis for actual continuous casting process optimization.Considering the fact that YQ450NQR1 steel bloom would undergo complex hot deformation and violent flow stress variation in hot rolling process, thermal simulation experiment is carried out to obtain the flow stress curves under different deformation amount (30%,60%), strain rate(1/s,10/s,30/s) and deformation temperatures. Besides, constitutive equation for high temperature deformation of YQ450NQR1 steel based on the hyperbolic sine form model is established. A correlation coefficient of 0.988 and an average absolute relative error of 4.18%between the experimental and the calculated flow stress have been obtained. These indicate that the constitutive equations could be used to accurately predict the flow behavior of YQ450NQR1 steel during high temperature deformation process. |
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