Abstract: The effects of heating temperature and holding time on the austenite grain growth behavior of SCr420H gear steel samples with different AlN precipitation states were experimentally investigated. The equilibrium precipitation and dissolution law of AlN precipitates in the experimental steel was calculated by Thermo-Calc thermodynamic calculation software， and the precipitation behavior of AlN in the experimental steel was studied by field emission transmission electron microscopy. The results show that the hot-rolled and air-cooled samples are mixed grain structure after being carburized for 0.5 h at 950-1 050 ℃， and the average austenite grain size level is below grade 5. The austenite grains of the samples after 1 210 ℃ solution-water quenching treatment do not grow up after carburizing at 950-1 000 ℃ for 0.5-6 h or at 1 050 ℃ for 0.5-2 h， the austenite grain size level is grade 9.5-10. The abnormal grain growth phenomenon occurs after 2 hours carburizing at 1 050 ℃. The sample after 1 210 ℃ solution-water quenching is isothermally treated at 700 °C for 1 h and then water-cooling so that it can be carburized at 1 050 °C for 8 h without abnormal grain growth or mixed grain structure， the average austenite grain size is 14 µm， the grain size level is grade 9.4. The growth behavior of austenite grains during carburization process is mainly controlled by the solubility and the ripening rate of the secondary phase AlN in the sample. The size of AlN precipitates in the solution-treated sample after carburization at 1 050 ℃ for 0.5 h is less than 80 nm， with an average size of 27.8 nm. 14.3% AlN particles larger than 100 nm appear in the sample after carburization at 1 050 ℃ for 2.5 h， with an average size of 64.2 nm.

Key words: Gear Steel, Carburizing, Grain Growth, AlN

摘要： 实验研究了加热温度和保温时间对具有不同AlN析出状态的SCr420H齿轮钢的奥氏体晶粒长大行为的影响。用Thermo-Calc热力学计算软件计算了实验钢AlN析出相的平衡溶解规律，并用场发射透射电子显微镜研究了实验钢中AlN的析出行为。结果表明，热轧空冷态的试样在950~1 050 ℃渗碳0.5 h均出现混晶，奥氏体平均晶粒度级别低于5级。经过1 210 ℃固溶水淬处理的试样在950~1 000 ℃渗碳0.5~6 h和在1 050 ℃渗碳0.5~2 h奥氏体晶粒均不发生长大，晶粒度级别在9.5~10级；在1 050 ℃渗碳2 h以后开始出现异常晶粒长大。试样经1 210 ℃固溶水淬后在700 ℃等温1 h水冷处理，可使其在1 050 ℃渗碳8 h而不发生异常晶粒长大或混晶，奥氏体晶粒平均尺寸为14 µm，晶粒度级别为9.4级。试样在渗碳过程中的奥氏体晶粒长大行为主要受钢中第二相AlN的溶解度和熟化速率控制。固溶后在1 050 ℃渗碳0.5 h的试样中AlN析出相尺寸均小于80 nm，平均尺寸为27.8 nm，渗碳2.5 h的试样中出现了尺寸大于100 nm的AlN粒子，数量占比为14.3%，AlN粒子的平均尺寸为64.2 nm。

关键词: 齿轮钢, 渗碳, 晶粒长大, AlN