Precipitation Mechainsm Of Second Phase Particles And Control Of Surface Cracks In Continuous Casting Slab Of Microalloyed Steel

Microalloyed steel has been utilized in defferent areas since it was introduced to china, however, there has mass of defects on microalloyed steel which has limited the development. Surface cracks of microalloyed steel is affected seriously by precipitates. Much more studies has been done on the reheating and rolling. But few studies on the precipitation mechanism of as cast micro-alloyed steel. Therefore, the surface cracks of micralloyed steel (PQ700, P550CL) during continuous casting were studied. The hot ductility of microalloyed steel, the solute distribution, the relationship between phase transformation and precipitates on surface crack has been studied by Scanning electron microscopy (SEM), energy dispersve spectroscopy (EDS), metallographic microscope, transmission electron microscopy(TEM), thermal expansion, confocal laser scanning microscope (CLSM) and in situ analyzer, respectively. The hot ductility and precipitates of microalloyed steel were studied by Gleeble3500and TEM. Meanwhile, the effect of elements on the brittle zone were also analyzed by B-F model. The results show that plastic trough becomes deeper with the cooling rate decreasing. While, with the cooling rate decreasing and strain rate increasing, the plastic trough move to the right side of R.A-σ curve. Moreover, the hot ductility of the steel decreases with the quantity of precipitate increasing and the size of precipitates decreasing. According to the calculation results, the critical stress of PQ700steel and P550CL steel are58.3MPa,53.58MPa at1248K, respectively. Therefore, the temperature should be controlled above1248K during continuous casting. The size of precipitation can be expressed as which based on the methd of dimensional analysis. The molar fraction, atom ratio and precipitation regularity in solute-rich regions and solute-depleted regions of microalloyed steel was calculated by thermodynamic model and kinetic model. The calculation results were verified by the experiment using TEM and EDS. In solute-rich regions and solute-depleted regions of PQ700steel, the effect of Ti(CxN1-x) on theoretical tensile strength is48MPa,47MPa, respectively. The effect of Nb(CxN1-x) on the theoretical tensile strength is96MPa and104MPa. In additions, in solute-rich regions and solute-depleted regions of P550CL steel, the effect of Ti(CxN1-x) on theoretical tensile strength is52.5MPa,46.7MPa, the effect of Nb(CxN1-x) on the theoretical tensile strength is82.13MPa and80.03MPa and the effect of V (CxN1-x) on the theoretical tensile strength increased obviously to171MPa and163MPa, respectively.The precipitates andγ/α transformation were affected by temperature fluctuation in the secondary zone. Therefore, the hot ductility of microalloyed steel was studied with thermal cycling method by Gleeble3500. The results indicated that brittle zone would be inlarged with temperature fluctuation increasing. In addition, A’r3, A’r1, decreases with cooling rate increasing. While, the phase transformation temperature changed unobviusly with the heating rate increasing. Moreover, molar fraction of Nb increases to1.6times at grain boundary. Furthermore, precipitation tremperautre improves25K with100ppm Nb content increasing.Industrial tests showed that the cooling water flow rate decreased from10.3m/s to9.3m/s while in and out of the water temperature increased1-2K and mold copper plate about10K after the optimization of mold cooling system. In addition, the nozzles of secondary zone at conrer are closed on1650mm slab, while, reducing flow rate of nozzle by half on1650mm slab. Therefore, the coner temperature increases to1213K to avoid the low temperature brittle zone. The results show that the surface peel defects rate of hot-rolled coil decreased from27.69%to11.48%.