钛微合金化IF钢第二相粒子应变诱导析出行为

Strain induced precipitation of second phase in titanium microalloyed IF steel

在Gleeble-3500热模拟试验机上,利用应力松弛试验研究了钛微合金化IF钢奥氏体区第二相粒子析出行为。试验结果表明：因第二相粒子析出钉轧位错与晶界,应力松弛曲线呈现出3个阶段的特征。试验钢的析出-时间-温度曲线呈现出典型的＂C＂曲线形状,最快析出鼻子点温度约为900℃,在此温度下,第二相粒子析出开始时间与结束时间分别为10与160 s;随着等温弛豫时间的延长,第二相粒子析出数量逐渐增加,当弛豫时间超过析出结束时间后,析出物数量将不再增加而尺寸增加。试验用Ti-IF钢主要观察到Ti N、Ti4C2S2、Ti C 3种析出物,Ti4C2S2与Ti C沉淀物的形状和尺寸相差不多,无法从形状和大小上来区分,易于沿奥氏体晶界或亚晶界析出,奥氏体亚晶的尺寸大约分布在0.1~0.4μm。900℃应变诱导析出物数量较800℃多,该结果与试验钢PPT曲线析出鼻子点温度900℃是相符的。对于Ti-IF钢,其热轧工艺过程应有利于C,N化物的析出和聚集长大,最终形成粗大、稀疏的第二相粒子,因此宜采用＂三低一高＂快速大压下的热轧工艺制度,以提高Ti-IF钢的深冲性能。

The second phase precipitation behavior of the Ti-microalloyed interstitial free steel was studied by using stress relaxation method on Gleeble 3500 thermal simulation testing machine. The results showed that the strain induced curves showed three stages, which was caused by precipitation. The precipitation - time - temperature curve of the Ti-IF steel showed a typical ＂C ＂shape, the fastest precipitation temperature was about 900 ~C which was called nose point. And the start and finish time of the second phase precipitation were 10 seconds and 160 seconds, respectively at 900 ℃. With the increase of the isothermal relaxation time, the number of precipitates increased significantly. When the relaxation time exceeded the finish time of precipitation, the amounts of precipitates increased hardly. And the precipitates will grow dur- ing the precipitation process. Three kinds of precipitates were observed in Ti-IF steel, which were TiN, Ti4C2S2, and TiC. Because of almost the same shape and size of Ti4C2S2 and TiC precipitates, they can＇t be distinguished from each other. And they were easy to precipitate along the austenite grain boundaries or sub-grain boundaries, sub-grain size of Austenite distributed 0.1-0.4μm. Strain-induced precipitation amount at 900 ℃ was more than that of 800 ℃, which is consistent with the results of PPT curve. For Ti-IF steel, in order to obtain coarse and sparse second phase particles, the hot-rolled process of ＂three low and one high＂ rapid and large depress should be adopted to improve the drawability.