轻压下对GCr15轴承钢大方坯内部质量的影响

轴承钢棒材中心致密性和碳化物缺陷与大方坯铸态内部质量控制水平密切相关。以GCr15 轴承钢为研究对象,建立了大方坯连铸过程二维纵向凝固传热模型,结合现场测温试验验证了凝固模型的准确性。基于凝固末端轻压下补偿当地凝固收缩、控制中心缩孔的理论,通过对大方坯凝固进程的准确预测,揭示出其糊状区内合理的轻压下范围。其中,浇铸试验条件下对应铸坯中心固相率为0.30~0.75的合理压下区间为16.4~22.5 m。生产试验表明,轻压下对铸坯凝固组织转变与形貌影响不大,但可明显消除中心缩孔,中心疏松也可由1.5级以上稳定降至0.5~1.5级,满足轧制要求; 合理的轻压下位置和适度的轻压下量可明显改善轴承钢大方坯中心缩孔和中心疏松程度,提高轧材探伤合格率。同时也发现,压下位置与压下量分配不合理或不稳定可能诱发铸坯内裂纹,从而不利于轧材质量的稳定性和一致性。当前生产条件下,稳定拉速并在3~6号压下辊合理分配压下量可达到有效改善内部质量的目的。

Influence of soft reduction on internal qualityof GCr15 bearing steel bloom castings

Central soundness and carbide defects of bearing steel bars are closely related to the internal quality control of bloom castings. Taking GCr15 bearing steel as the research object, a two-dimensional longitudinal solidification and heat transfer model was established for the casting process of the bloom semi-products. The accuracy of solidification model is verified by field temperature measurement. Based on the theory of compensating the local solidification shrinkage and controlling the central shrinkage cavity under the soft reduction at the end of solidification, the reasonable soft reduction interval in the pasty region can be revealed through the accurate prediction of the solidification process. Among them, under the conditions of casting tests, the effective soft reduction interval corresponding to the solid phase ratio of the center of the casting is from 0.30 to 0.75, which refers to 16.4-22.5 m from the meniscus. The test results showed that the solidification structure and morphology of the bloom castings are little affected by soft reduction. However, after soft reduction, the central porosity index drops from greater than or equal to 1.5 to 0.5-1.5, which can meet the rolling requirements, and the central shrinkage cavity is basically disappeared, indicating that reasonable soft reduction position and moderate soft reduction amount can obviously improve the degree of central porosity and central shrinkage cavity of the bearing steel bloom castings, so as to improve the qualified rate of the following rolled material. However, the unreasonable or unstable distribution of the position and amount of press-down may lead to the occurrence of mid-way cracks, thus reducing the stability and consistency of the rolled material quality. For the present production, it is recommended to keep the constant casting speed and press down at No.3 to No.6 pressure roller to assure the internal quality of the steel.