含Ca汽车控制臂用钢中复合硫化物控制机理

 汽车控制臂由于形状复杂,切削量大,部分汽车控制臂用钢在加入质量分数0.03%硫元素的基础上,又进一步添加了少量钙元素,希望将钢中常见的细长条状MnS转变为纺锤状(Ca, Mn)S以增加零件的切削性能。然而,硫质量分数为0.03%时,钙元素在钢液中的溶解度很低,冷却和凝固过程单一的纯(Ca, Mn)S生成量极少。因此,提出了利用钢液中生成的含CaO类的氧化物来诱导(Ca, Mn)S在其外围形核长大,形成大量双层结构复合硫化物的形貌控制机理。为了研究最佳双层结构复合硫化物形成机理,选取了3炉不同冶炼工艺的汽车控制臂用钢,利用带能谱分析的电子扫描显微镜观察了铸坯和轧材中典型复合硫化物形貌、成分特征,并手动测量了其尺寸,最后利用热力学软件FactSage计算了钢中夹杂物的生成行为。研究结果表明,当钢中不进行钙处理时,复合硫化物内部氧化物主要为Al₂O₃或低MgO比例的镁铝尖晶石,外围硫化物为纯MnS,轧制后成细长条状。当钢中进行钙处理后,可以得到两种不同类型的复合硫化物。一种内部氧化物中CaO组元含量较高,外围硫化物主要是高CaS比例的(Ca, Mn)S,基本不变形,成典型的D类或Ds类形貌;另一种核心氧化物中CaO组元低,外围硫化物主要是低CaS比例的(Ca, Mn)S,轧制后成纺锤状。控制钙处理后钢液氧化物中合适的CaO比例使得氧化物既具有高效的硫化物形核能力,又能促进合适CaS比例的(Ca, Mn)S在其外围生成,这是钢中得到大量纺锤状双层结构复合硫化物的关键。当钢中Ca/S比约为0.07时,外围硫化物中的钙元素质量分数为2%~5%最为理想。

Control mechanism of compound sulfides in steel for automotive control arms containing Ca

There is a large cutting workload during automotive control arm manufacturing processes because of its complex shape. In order to increase the cutting performance of parts, a small amount of Ca element is further added into steel with 0.03% sulfur, so that common slender strip MnS would be transformed into spindle (Ca, Mn)S. However, when the sulfur mass fraction is 0.03%, the solubility of Ca element in molten steel is very low, and the generation of pure (Ca, Mn)S during cooling and solidification processes is quite small. Therefore, controlling mechanism of sulfide morphology is proposed to induce (Ca, Mn)S to nucleate and grow on oxides containing CaO formed in molten steel, promoting generation of a large number of duplex compound sulfides. In order to obtain the formation mechanism of optimal duplex compound sulfides, 3 commercial steels with different smelting processes were selected. Morphologies and component characteristics of typical compound sulfide were investigated with scanning electron microscope and energy spectrum analysis (SEM-EDS), size characteristics were manually measured, and evolution behaviors of inclusions during cooling and solidification were calculated by thermodynamic software FactSage. The results show that inside oxides of compound sulfides are mainly Al₂O₃ or magnesium aluminum spinel with low MgO ratio, wrapped sulfides are pure MnS with long strip shape after rolling in the steel without Ca treatment. After Ca treatment, there are two different compound sulfides.One is typical D or Ds type inclusions, with high CaO ratio inside oxides and high CaS ratio wrapped (Ca, Mn)S, basically no deformation. The other is spindle-shaped, with low CaO ratio inside oxides and low CaS ratio wrapped (Ca, Mn)S. Controlling the proper proportion of CaO in liquid oxides of Ca-treated steel is the key to obtain a large amount of spindle-shaped sulfides in steel, owing to high sulfide nucleation ability of core oxides and proper CaS proportion in peripheral (Ca, Mn)S. When the Ca/S ratio in steel is about 0.07, the optimal Ca content in peripheral sulfide is about 2%-5%.