Aiming at the problem of micro-cracks about 0.1-0.3 mm in length inside the low-carbon and low-alloy structural steel bars produced by a steel plant,the micro-cracks inside the steel bars were analyzed and studied through low power structure inspection,optical microscope and scanning electron microscope observation,and process improvement measures were proposed.The results show that after hot rolling,multiple micro-cracks with a length of about 0.1-0.3 mm,which were not connected to each other and no directionality,appear inside the steel bars.There were no metallurgical defects such as abnormal inclusions around the micro-cracks.The micro cracks propagated along the pearlite strip with a banded structure,and a mixed structure of bainite and martensite was distributed around the pearlite strip.Compositional segregation resulted in the formation of high-grade banded structures in steel bars,and mixed structures of bainite and martensite existed around the banded structures,which were more likely to produce micro cracks when large plastic deforma-tion occurred.During continuous casting,the superheat was controlled at 20-30 ℃,the electromagnetic stirring parameters were set at 200 A and 2.0 Hz,the specific water content in the secondary cooling zone was reduced to 0.41 L/kg,and the casting speed was con-trolled at 1.2 m/min,which could reduce the banded structure from the source of the continuous casting billet.The composition segrega-tion could be improved by appropriately prolonging the holding time of billet at high temperature of(1 180±10)℃ to 250-305 min.The slow cooling of the hot rolled steel bars into the pit could avoid the generation of bainite and martensite structures.After the process improvement,no internal crack defects were found in the non-destructive testing of hot rolled low-carbon low alloy steel bars,and the improvement effect was significant.
low-carbon and low-alloyhot rolled steel barsinternal micro-crackscomposition segregationband structureoptimi-zation of continuous casting process
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