连铸高铝氮积齿轮钢第三脆性区形成机制与控制

钢中的Al、N含量对连铸及其后续加工热塑性和奥氏体晶粒度控制有重要影响,这也是高温渗碳钢与各种Al脱氧钢广泛关注的问题。使用Gleeble 3800热/力学模拟试验机测定了一种轨道交通用高铝氮积齿轮钢(SCM420H)的高温热塑性,并结合差示扫描量热仪(DSC)分析、AlN析出热力学模型以及Schwerdtfeger热塑性特征值计算模型揭示了其第三脆性区的形成机制与调控途径。结果表明,高铝氮积齿轮钢第三脆性区低谷温度范围为750~850 ℃,这是由应力诱导先共析铁素体膜的产生与AlN粒子的大量析出共同导致的。Schwerdtfeger热塑性特征值计算模型可以较准确地预测高铝氮积齿轮钢第三脆性区的上限温度与最小面缩率,但由其预测的热塑性曲线下限温度偏高,应进一步考虑先共析铁素体膜析出的影响,并依据Ar3温度对其进行修正。高Al高N齿轮钢第三脆性区的下限温度取决于其先共析铁素体开始析出温度,主要与钢种成分和铸坯冷却速率相关,连铸生产中可控性有限;但其上限温度则与铸坯应变速率、冷却速率以及钢中的Al、N含量和AlN析出行为均有关联,调控空间较大,应该是连铸生产中合理控制铸坯热塑性与表面裂纹倾向的正确途径。

Formation mechanism and control for the third brittle zone of continuous-cast gear steel with higher Al-N products

The content of Al and N in the steel has an important influence on the control of hot ductility and austenite grain size in continuous casting and subsequent processing, which is also a problem for high-temperature-carburized special steels and widely-used Al-killed steels. The high-temperature ductility of railway gear-steel SCM420H with higher Al-N products was measured by thermal-mechanical physical simulator, Gleeble 3800, and the formation mechanism and control of its third brittle zone was analyzed through the combination of Differential Scanning Calorimeter (DSC) analyzing, thermodynamic calculation model of AlN particles and the well-known Schwerdtfeger model. The results show that gear steels with higher Al-N products exist the third brittle trough between 750-850 ℃, which is caused by the formation of stress induced proeutectoid ferrite film and precipitation of adequate AlN particles. The Schwerdtfeger model is also suitable to predict the third brittle zone of gear steels with higher Al-N products, and the reliable temperature range and the minimum reduction of area can be obtained, but the A_r3temperature of proeutectoid-ferrite-film formation should also be considered to modify the model because the lower limit temperature of the predicted thermal ductility curve is too high, and the influence of proeutectoid ferrite precipitation should be further considered. The lower bound temperature of the third brittle zone for Al and N containing gear steel depends on the initial formation temperature of proeutectoid ferrite, which is related to the steel composition and the cooling rate of continuous casting slab, and its adjustability is very limited, while the upper bound temperature is closely related to the casting strand strain rates, cooling rates, Al and N contents and AlN precipitation behavior in steel, which shows a much bigger regulation range and should be adopted to control the thermal ductility of steel strands for a less surface crack tendency.