不锈轴承钢真空制备过程洁净度及碳化物变化

 为了提高G102Cr18Mo高碳不锈轴承钢的洁净度、细化碳化物组织,采用真空感应熔炼、两次真空自耗重熔、大锻压比锻造的工艺路线,研究了真空处理及大锻压比锻造对化学成分、气体含量、夹杂物分布、二次枝晶间距及碳化物颗粒度的影响。研究结果表明,真空感应熔炼过程(VIM)中,随着铝含量的增加,碳的脱氧能力大幅降低,即使铝质量分数为0.003%也对碳的脱氧能力有明显的阻碍作用;真空自耗重熔过程(VAR)由于高的真空度、高的重熔温度等热力学条件以及反应动力学条件的改善,氧含量显著降低,第一次自耗重熔后氧质量分数从0.001 49%降低至0.000 57%,降低了61.7%,第二次自耗重熔后氧质量分数降低至0.000 50%。真空感应熔炼、真空自耗重熔过程,夹杂物的成分变化不大,主要以Al-Si夹杂为主,其次为Al₂O₃夹杂,再次为MnS夹杂、Mg-Al-Ca、Mg/Ca-Al夹杂。双真空冶炼后,钢中夹杂物主要为0～5 μm的细小夹杂物,未发现大于20 μm的夹杂,含有少量10～20 μm的夹杂,钢的洁净度大幅度提高。在真空自耗锭横断面上,从边部向芯部二次枝晶的形貌变化不大,二次枝晶间距逐渐增大,但是变化趋势缓慢,二次枝晶间距为85～95 μm,这主要得益于低的自耗重熔速度。对真空自耗锭进行大变形处理,最终锻造成40 mm的圆棒,碳化物颗粒的最大尺寸不大于20 μm,平均尺寸为15 μm,且没有碳化物聚集的现象。低的自耗重熔速度和大锻压比锻造是碳化物细化的关键。

Changes of cleanliness and carbide during vacuum preparation of stainless bearing steel

In order to improve the cleanliness and refine the carbide structure of G102Cr18Mo high carbon stainless bearing steel, the process route of vacuum induction melting(VIM), vacuum consumable arc remelting(VAR) and forging with large forging ratio was adopted to research the influence of vacuum treatment and large forging ratio on chemical composition, gas content, inclusions distribution, secondary dendrite spacing and carbide size. The results show that during VIM process, the deoxidation ability of carbon decreases greatly with the increase of aluminum content. Even 0.003% aluminum has an obvious obstacle function to deoxidation of carbon. The oxygen content is greatly reduced due to the improvement of thermodynamic conditions such as high vacuum degree, high remelting temperature and reaction kinetic conditions during VAR process. After the primary VAR, the mass percent of oxygen decreases to 0.000 57% from 0.001 49%, reduced 61.7%, and after the secondary VAR, the mass percent of oxygen decreases to 0.000 50%. During VIM and VAR, the composition of inclusions changes little, the main inclusions are Al-Si inclusions, followed by Al₂O₃ inclusions, MnS inclusions, Mg-Al-Ca and Mg/Ca-Al inclusions. The inclusions of diameter over 20 μm are completely removed, there are only a few inclusions with a diameter of 10-20 μm and the inclusions are mainly composed of fine inclusions with diameter less than 5 μm after double vacuum smelting. The cleanliness of G102Cr18Mo steel is greatly improved. It is found that the morphology of secondary dendrite changes little from edge to core on the cross section of VAR ingot. The secondary dendrite spacing increases slowly from the edge to the core, and varies from 85 to 95 μm. Such fine dendrite structure benefits from low remelting speed. The VAR ingot is forged repeatedly and finally forged into a round rod with a diameter of 40 mm. Through metallographic examination, it is found that the maximum size of carbide particles is no more than 20 μm and the average size is 15 μm. No aggregated carbides are found. Low VAR speed and forging with large forging ratio are the key to refinement of carbides.