Influence of Normalization on the Properties and Nb、V Precipitations of Grade 420 MPa Steel Plate for Construction

The microstructure and mechanical properties and Nb,V precipitations of Q420 GJ steel plate for construction was investigated after hot rolling and normalizing.Quantitative analysis of Nb,V precipi-tates had been performed by electrolytic method for hot rolled and normalized steel plates,the precipitates were observed under transmission electron microscope.Tensile tests indicated that strengths of the steel plate after normalizing remarkable decreased.TEM observation showed there was no visible difference in vanadium precipitation through thickness direction of the plate after hot rolling,and about 30% of total added vanadium precipated.Normalizing process could not effectively improve vanadium precipitation in low nitrogen steels.Niobium precipitation increased from the surface to the centre of the as-hot rolled plate,the amount of precipitated niobium after air cooling is larger than that after controlled cooling.The amount of precipitated niobium after normalizing was somewhat higher than that of as-hot rolling.