Manufacturing method of plate

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PURPOSE : A method of manufacturing a plate is provided to reduce the strength difference between the surface and the center of the plate and to increase elongation. CONSTITUTION : A slab consists of C of 0.08-0.12wt%, Si of 0.25-0.35wt%, Mn of 1.40-1.60wt%, P of 0-0.02wt%, S of 0-0.005wt%, Al of 0.05-0.15wt%, Nb of 0.035-0.045wt%, V of 0.045-0.055wt%, Ti of 0.01-0.02wt%, Fe and other inevitable impurities. After the slab is heated at a temperature of 1100-1200°C, a hot rolling process is finished so that an accumulated reduction rate is within a range of 50-70% below a re-crystallizing stop temperature. After an uneven size of ferrite is uniformly distributed, it is cooled. The distribution period of time is within a range of 1-10min. The cooling is made at a speed of 5-20°C/sec to a temperature of 500-550°C, and then it is air-cooled to a room temperature.