Study on mechanism and control of hot rolling defects for high carbon martensitic stainless steel

Aiming at the defects such as edge cracking and voids readily occurring during hot rolling of high-carbon martensitic stainless steel 70Cr13Mo, the chemical compositions of molten steel from four batches of 70Cr13Mo and the hot-rolling production process were investigated. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and other testing methods were adopted to analyze the macrostructure of continuous-casting slabs, the microstructure of defect specimens, and carbide distribution. The results show that the aging of water-sealing equipment at the entry and exit of the roughing mill and cooling-water leakage lead to an increased cooling rate and sharp rise in strength at the strip edge. Meanwhile, massive premature precipitation of secondary carbides along grain boundaries significantly reduces the matrix plasticity, which serves as the primary cause for edge cracking of hot-rolled strips. In addition, a large number of (Ti,V)N and its composite inclusions formed in the microstructure tend to act as crack initiation sites, constituting another major factor for defect generation. By adopting rectification measures including strict control over the working condition of roughing-mill water-sealing equipment, optimization of raw-material proportioning, and stringent restriction of titanium-containing scrap steel charged into the furnace for smelting, the occurrence rate of hot-rolling defects has been successfully reduced to zero. This study effectively improves the finished-product yield of 70Cr13Mo steel, and provides theoretical basis and practical guidance for the prevention and control of hot-rolling defects of high-carbon martensitic stainless steel.