易切削非调质汽车用钢采用410 mm × 530 mm大断面连铸坯轧制成规格Φ130～220 mm的钢材,表面探伤合格率较低,钢材表面出现裂纹缺陷,通过对钢轧全流程系统分析发现为连铸坯表面角部横裂纹和星状裂纹所导致,其与结晶器铜管倒锥度及R角不合理、钢中硫化物形态及偏析聚集、保护渣理化指标适配性差、二次冷却系统工艺有待优化等方面原因有关.通过改进结晶器铜管倒锥度及R角、微钙化处理工艺改善钢中硫化物形态、调整结晶器保护渣的理化性能指标、优化二次冷却系统,酸洗试样表面质量合格率提高了近30％,铸坯表面磁粉探伤合格率提高至99.2％,连铸坯所对应轧材的表面探伤最高炉次达98.6％、最低炉次达92.4％,铸坯纵剖酸洗试样V型偏析改善明显,且碳、硫的中心偏析指数分别为1.04、0.97,符合±0.05的高标准均质化要求.
FORMATION MECHANISM AND PROCESS OPTIMIZATION OF FREE-CUTTING NON-TEMPERED AUTOMOTIVE STEEL SURFACE CRACKS
Free-cutting non-tempered automobile steel is rolled into with a specification of Φ130～220 mm by continuous casting slab with a cross section of 410 mm x530 mm,then the qualified rate of surface flaw detection is low,and cracks appear on the steel surface.Through the analysis of the whole steel-rolling process system,it is found that the cracks are caused by transverse cracks and star-shaped cracks on the surface of continuous casting slab.The surface defects of the slab are connected with the unreasonable reverse taper and R angle of the mold copper tube,sulfide forms and segregation in steel,poor adaptability of the physical and chemical indexes of the mold powder,and the process of the secondary cooling system needs to be optimized.By improving the reverse taper and R angle of copper tube in mold,improving the sulfide form in steel by micro-calcification treatment process,adjusting the physical and chemical performance index of mold flux and optimizing the secondary-cooling-system,it shows the qualified rate of surface samples increased by nearly 30％after pickling,the qualified rate of magnetic particle flaw detection on slab surface increased to 99.2％,the highest number of heats and the lowest number of heats for surface flaw detection of rolled products corresponding to continuous casting slab reached 98.6％and 92.4％,the V-shaped segregation of longitudinal section pickling samples of slab was obviously improved,and the central segregation indexes of carbon and sulfur were 1.04 and 0.97 respectively,which were inline with the requirements of±0.05.
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