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The influence of V-N microalloying on the high-temperature mechanical behavior of high strength weathering steel is discussed through thermomechanical simulation experiment. The difference of tensile strength caused by variation of [%V][%N] appears after proeutectoid phase change, and the higher level of [%V] [%N] is, the stronger the tensile strength tends to be. The ductility trough apparently becomes deeper and wider with the increase of [%V] [%N]. When the level of [%V][%N] reaches to 1.7 Â 10 −3 , high strength weathering steel shows almost similar reduction of area to 0.03% Nb-containing steel in the temperature range of 800-900℃, however, the ductility trough at the lowtemperature stage is wider than that of Nb-containing steel. Moreover, the net crack defect of bloom is optimized through the stable control of N content in low range under the precondition of high strength weathering steel with sufficient strength.

The strain-induced precipitation kinetic model of vanadium carbonitride [V(C, N)] precipitates with the cubic structure in vanadium-nitrogen (V-N) microalloyed high-strength weathering steels was constructed and validated. The V(C, N) precipitates possess an fcc structure with the lattice sites and octahedral interstitial sites (O-sites) being occupied with V atoms, and C and N atoms, respectively. The model is based on the cubic structure for V(C, N) precipitates instead of the spherical one, since recent experimental results clearly demonstrate that the strain-induced precipitation of V(C, N) in austenite is of the cubic structure. The nitrogen content effect on precipitation-temperature-time (PTT) curve pattwerns was also studied. With the nitrogen content, the PTT curve nose temperature increased, and the curves shifted left. When the nitrogen content in steels was dropped, PTT curves of V(C, N) precipitates in austenite acquired an S-shape, since free energy conversion is nonlinear. Additionally, thermodynamic software was employed to calculate the phase equilibrium of V(C, N) precipitates in the new high-strength weathering steels. The N site fraction of N during the V(C, N) precipitation was much larger for steels with high N contents, compared to those with low ones.

YQ450NQR1 high-strength weathering steel is prone to cracking. The cracking patterns and morphology were detected by metallographic and scanning electron microscopies. The decarburized layer around the cracks is indicative of their origination in the continuous casting bloom. Then, the controlling factors were established by analyzing the carbon content of molten steel, residual elements in the alloy, purity and performance of the mold powder, and secondary cooling water flow of the mold. Results show that the process optimization reduced the cracking reject ratio from 3.5 to 0.78%.

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