Study on the sensitivity of the internal cracks induced by mechanical reduction of different steel grades

Mechanical reduction can mitigate most internal defects. However, internal cracks are more frequently observed in high-carbon steels than in low-carbon steels following the mechanical reduction process. This study presents theoretical calculations of the mechanical properties of low-carbon steel Q235 and high-carbon steels U71Mn and GCr15 to evaluate the mechanism of internal cracking in high-carbon steels. The calculations indicate that U71Mn and GCr15 exhibit lower critical strains and broader mushy zones compared to Q235, thereby increasing the crack sensitivity of high-carbon steels. The mechanism of internal cracking in high-carbon steels is further examined through microstructural analysis. The results reveal that internal cracks in U71Mn and GCr15 typically form in the columnar crystal zone. Elevated concentrations of carbon, phosphorus and sulphur are often found near the final solidification zone, which weakens the grain boundaries and promotes crack propagation along them.