成分与冷却速率对高硅不锈钢凝固模式的影响

 高硅奥氏体不锈钢因其较高的Si元素含量所表现出的优异耐蚀性能而成为制酸行业普遍应用的一种特殊钢种。然而,高含量Si元素的加入会引发铸造缺陷和成分偏析加剧以及钢中析出相增多,热加工过程中易产生热裂纹等问题。高硅奥氏体不锈钢凝固过程中δ铁素体的含量、形态和分布与合金化学成分和热加工历史紧密相关,其室温组织取决于析出相的析出顺序和随后的固态相变,因此,奥氏体不锈钢的凝固模式势必会影响合金的热塑性。为此通过调整高硅奥氏体不锈钢中Si元素与Cr元素的含量,采用金相显微镜(OM)、X射线衍射仪(XRD)、扫描电镜能谱分析(SEM/EDS)、电子探针(EPMA)、JMatPro软件计算等方法,探究了合金成分变化与冷却速率对高硅奥氏体不锈钢凝固模式的影响,并对经典铬镍当量算法进行了评估。结果表明,Schneider铬镍当量算法相较于Rajasekhar铬镍当量算法对大多数合金的凝固模式预测较为准确;随着合金中Si元素与Cr元素含量的提高,合金凝固模式由AF模式转变为FA模式,合金凝固过程中经历更多的“δ→γ”固态相变,其中质量分数为6.0%Si成分的合金对应的δ铁素体增幅减缓;随着质量分数为5.0%的Si铸锭冷却速率的提高,合金凝固模式由AF模式转变为A模式;Hammar and Svensson凝固路线判据可以准确预测高硅奥氏体不锈钢的初始析出相。研究为合理制定高硅奥氏体不锈钢的合金成分与成形工艺提供理论依据。

Effect of elements and cooling rate on solidification mode of high-silicon stainless steel

High-silicon austenitic stainless steel as a special steel is commonly used in the acid industry due to its excellent corrosion resistance for high silicon content. However, the addition of high silicon content will cause problems such as aggravation of casting defects, component segregation and an increase of precipitation phases in the steel, resulting in hot cracking during hot working. The content, morphology and distribution of δ ferrite in the solidification process of high-silicon austenitic stainless steel are closely related to the chemical composition and hot processing history of the alloy, while the solidification structure depends on the precipitation order of the precipitates and subsequent solid phase transformation. As a result, the solidification mode of stainless steel will inevitably affect the thermoplasticity of alloy. This work adopted metallographic microscope (OM), X-ray diffractometer (XRD), scanning electron microscope/energy spectrum analysis (SEM/EDS), electron probe (EPMA), JMatPro calculation and other methods to study the influence of alloy composition and cooling rate on the solidification mode of high-silicon austenitic stainless steel by adjusting the contents of Si and Cr elements, and the classic chromium-nickel equivalent algorithm was evaluated. The results show that the Schneider chromium-nickel equivalent algorithm is more accurate in predicting the solidification mode of most alloys than the Rajasekhar chromium-nickel equivalent algorithm. The solidification mode of alloy changes from the AF mode to the FA mode with the contents of Si and Cr in the alloy increasing, the alloy undergoes more “δ→γ” solid phase transition during solidification, and the increase of δ ferrite with a mass fraction of 6%Si alloy slows down. The solidification mode of alloy changes from the AF mode to the A mode with the cooling rate increasing of 5%Si ingot. The Hammar and Svensson solidification route criterion can accurately predict the initial precipitation phase of high-silicon austenitic stainless steel. This study provides a theoretical basis for rationally formulating the alloy composition and forming process of high-silicon austenitic stainless steel.