硫化物改质对20MnCr5齿轮钢夹杂物和切削性能的影响

 齿轮钢广泛应用于汽车、机械等传动系统,易切削化是提高齿轮加工效率、降低制造成本的主要途径之一。提高钢中硫元素含量是改善齿轮钢切削性能的有效方式,然而过多的硫元素在轧制时会形成条带状MnS,增大钢的各向异性,因此需要对硫化物进行改制处理。分别采用Ca处理和Mg处理两种方式对20MnCr5齿轮钢进行硫化物改质,通过对力学性能、组织形态、夹杂物分布以及切削性能等表征,对比分析不同改制方式对材料的影响。结果表明,Ca处理和Mg处理后试验钢的强度和塑性保持一致,而Mg处理试样的晶粒尺寸较小,因而韧性较高。Ca处理试验钢中MnS夹杂物数量较多,长条状夹杂物比例相对较高;而采用Mg处理可以有效降低钢中夹杂物的数量,夹杂物平均尺寸有所增大,同时小长径比夹杂物数量增多,但是复合氧化物型夹杂物的数量也有所增加。在240~280 m/min条件下进行了干切削试验,结果表明两种试验钢均出现前刀面磨损、后刀面磨损和边界磨损,其中Ca处理试样的刀具磨损较为严重,同时在较高切削速度下出现了积屑瘤和崩角现象,而Mg处理试样则具有更长的刀具使用寿命,对切削速度的敏感性也较低。分析可知,Mg处理后钢中存在较多的大尺寸球状硫化物夹杂物,提高了应力集中效应,更有利于改善切削性能,因而改制效果更好。

Influence of sulfide modification on inclusions and cutting properties of 20MnCr5 gear steel

Gear steel is widely used in automobile, machinery and other drive systems. Free cutting is one of the main ways to improve gear processing efficiency and reduce manufacturing cost. Increasing sulfur content in steel is an effective way to improve the cutting performance of gear steel. However, excessive sulfur will form strip MnS during rolling, which will increase the anisotropy of steel. Therefore, it is necessary to modify the sulfide. 20MnCr5 gear steel was employed and modified with sulfide by Ca treatment and Mg treatment respectively. Through the characterization of mechanical properties, microstructure, inclusion distribution and cutting performance, the influence of different modification methods on the material was compared and analyzed. The results show that the strength and plasticity of steels treated by Ca and Mg are consistent, while the grain size of the sample treated by Mg is smaller, leading to a higher toughness. The amount of MnS inclusions in Ca-treated steel is larger, and the proportion of long strip inclusions is relatively higher by comparing with Mg-treated one. However, Mg treatment can effectively reduce the number of inclusions in steel, increase the average size of inclusions and the number of small aspect ratio inclusions at the same time, however, the number of composite oxide inclusions also increases. The dry cutting tests were carried out at speed range of 240-280 m/min. The results show that both steels appear the front tool surface wear, the flank wear and the boundary wear. The tool wear of the Ca-treated sample is more serious, and the built-up edge and edge breakage phenomenon occur at the higher cutting speed. On the contrary, the sample treated with Mg has a longer tool life and a lower sensitivity to cutting speed. Based on the results obtained, there are more large-size spherical sulfide inclusions in Mg-treated steel, which improves the stress concentration effect and is more conducive to improving the free cutting performance, so the sulfide modification effect is better.