Cooling process of SG255 steel

Cooling of SG255 steel adopts two-stage cooling process. The degree of supercooling can be increased through a period of cooling. When SG255 steel is cooled to the critical temperature region that is conducive to ferrite transformation, the cooling should be terminated and the air cooling should be continued for a short time. This can increase the driving force of phase transformation and effectively promote the transformation of ferrite. A large amount of precipitation increases the content of ferrite in SG255 steel, so as to obtain relatively uniform equiaxed ferrite; after the phase transformation, the microstructure is ferrite grains of uniform size, which not only ensures the strength of SG255 steel but also ensures the Low yield-to-strength ratio requirements.

The crimping temperature has an important influence on the microstructure and grain size after phase transformation. If the crimping temperature is high, the grains of the structure after the phase transformation are coarse, the strength is low. The crimping temperature is low, the grains are small after the phase transformation. Reasonable control of the crimping temperature is also the key to obtain good performance of SG255 steel. The increase of ferrite precipitation will inevitably enrich the carbon in ferrite to untransformed austenite.

Therefore, the two-stage cooling process can effectively inhibit the continued growth of ferrite, and can also promote the pearlite phase transformation process of austenite with high carbon concentration, so as to obtain high-quality SG255 steel.