SURFACE DEFECTS ANALYSIS AND PROCESS OPTIMIZATION OF CASTING BILLET OF 20Cr STEEL
20Cr is a subperitectic low-carbon alloy steel.Transverse cracks were observed in the corners of the cast billet during pickling.The analysis of the transverse cracks in the cast billet was conducted using shot blasting,magnetic particle testing,and scanning electron microscopy.Based on the test results,the process plan was optimized around the mold area.The findings indicate that excessive flow rate of mold cooling water led to strong cooling with a water flow speed reaching 13.7 m/s.Uneven flow of liquid slag into air gaps was attributed to its low melting point and viscosity of protective powder.Additionally,deep vibration marks at the corner of casting billet were identified as the primary cause for transverse corner cracks in 20Cr casting billet.Theoretical calculations suggested reducing mould cooling water flow from 150 m3/h to 120 m3/h to slow down heat transfer rate and increasing slag melting point from 1 030℃ to 1 180℃ along with viscosity from 0.53 Pa·s to 0.95 Pa·s for more even distribution between billet and copper tube gap.Furthermore,by reducing amplitude from±5 mm to+3 mm and increasing vibration frequency from 155 cpm to 190 cpm,significant reduction in billet vibration depth resulted in absence of cracks in over 100 pickled billets.It consequently raises pass rate for shot blasting and magnetic particle detection from 75％to over 99％.
20Crtransverse crackmouldcooling waterprotective powdervibration mark
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