GCr15SiMn轴承钢中大尺寸TiN生成与控制

 以西宁特钢EAF→LF→VD→IC工艺路线生产GCr15SiMn轴承钢为研究背景,采用矿物解离分析仪、夹杂物自动分析系统、化学分析及X射线荧光光谱仪等检测手段与热力学计算相结合的方法,研究了铸锭中大尺寸TiN夹杂物的析出机理,分析了冶炼阶段钢液中钛含量增加的原因,并提出了相应的改进工艺。通过对轴承钢中残余钛、氮含量的热力学计算,发现大尺寸TiN主要是在凝固过程中析出和长大,降低冶炼过程中钛含量是控制TiN生成的主要途径。对原工艺冶炼过程钛含量变化进行分析,钢液中钛增量主要发生在EAF出钢→LF结束阶段,其中52%钛增量来自于炉渣。为了避免炉渣中钛进入钢液,借助于七元炉渣CaO-SiO₂-MgO-FeO-Al₂O₃-MnO-TiO₂与钢液之间平衡的钛分配比(LTi)模型,计算了铝含量和炉渣中CaO含量对LTi 的影响,预测了最佳的平衡炉渣成分。结果表明,适当降低铝含量和炉渣中CaO含量可以提高渣-钢之间的钛分配比,降低钢液的钛含量;当铝质量分数为0.015%～0.025%、CaO质量分数为50%～55%时,其他炉渣组元的最佳成分(质量分数)为18%～24% Al₂O₃和12%～17% SiO₂。此外,定量描述了不同LTi条件下EAF下渣量与LF终点钛含量的关系。最后,采用“低炉渣碱度、低铝含量以及严格控制电炉下渣量”的改进措施进行了优化试验,优化后冶炼终点钛质量分数可控制在小于0.002 0%,铸锭TiN平均数密度可由原工艺的2.09个/mm2降低至0.73个/mm2,且TiN尺寸几乎都小于10 μm。

Formation and control of large-size TiN in GCr15SiMn bearing steel

Based on the production of GCr15SiMn bearing steel by EAF→LF→VD→IC process route in Xining Special Steel Plant, the characteristics and quantity of large-size TiN inclusions in ingot are studied by means of mineral dissociation analyzer, automatic inclusion analysis system, chemical analysis, X-ray fluorescence spectrometer and thermodynamic calculation, and the reasons for the increase of Ti content in steel during smelting stage are analyzed, the improved process is put forward. By the thermodynamic calculation of residual Ti and N content in bearing steel, it was found that TiN were mainly precipitated and grown up during solidification, and reducing Ti content in refining process was the main approach to control the formation of TiN. By the analysis of the change of Ti content during the original process, the results show that the titanium increase mainly occurs in the process from EAF tapping to the end of LF, in which 52% of total titanium increase comes from slag. In order to prevent titanium in slag from entering molten steel, the effects of Al content and CaO content in slag on LTi are discussed and the best equilibrium slag composition is predicted with the help of the previously established thermodynamic model of LTi between equilibrium slag CaO-SiO₂-MgO-FeO-Al₂O₃-SiO₂-TiO₂ and molten steel. The results show that the LTi between slag and steel can be increased and the titanium content in molten steel can be reduced by appropriately reducing the Al content and CaO content; When the Al content is 0.015%-0.025% and the mass perceng of CaO is 50%-55%, the optimum composition range of other slag components (mass percent)is 18%-24% Al₂O₃ and 12%-17% SiO₂. Besides, the relationship between the amount of slag from EAF and the titanium content at the end of LF under different LTiconditions is further described quantitatively. Finally, the amount of roughing-slag of EAF, Al content and slag composition in LF refining process were adjusted, and the improved process tests were carried out. Result showed that Ti content at the final point of smelting could be controlled below 0.002 0%, and the average TiN density of ingot could be reduced from 2.09/mm2 to 0.73/mm2, and the size of TiN is almost no more than 10 μm.