奥氏体化温度对HSLA100高强度低合金钢组织及冲击韧性的影响

INFLUENCE OF AUSTENITIZING TEMPERATURE ON THE MICROSTRUCTURE AND IMPACT TOUGHNESS OF A HIGH STRENGTH LOW ALLOY HSLA100 STEEL

利用Gleeble-3500热模拟机研究了低冷速条件下奥氏体化温度对高强度低合金钢相变组织及-20℃冲击韧性的影响.研究发现,随着奥氏体化温度的升高,显微组织由粒状贝氏体逐渐变为板条贝氏体.奥氏体化温度为1000℃时冲击韧性最佳,显微组织中马氏体/奥氏体(M/A)岛细小弥散且大角晶界密度最大.低于1000℃奥氏体化时M/A岛粗化显著,大角晶界密度较低;而高于1000℃时,虽然M/A岛细小弥散,但是大角晶界密度有所下降.动力学分析表明,随着奥氏体化温度的升高,相变起始温度逐渐下降,转变速率不断加快,较低的相变起始温度及较快的转变速率有利于M/A岛细化.所有转变过程均可分为贝氏体转变及马氏体转变两个阶段,1000℃奥氏体化时贝氏体转变分数最大,转变最完全.晶体学分析进一步显示,当M/A岛得到细化时(奥氏体化温度1000℃及1300℃),除原奥氏体晶界外,更多大角晶界来源于发生协变相变时,晶体学集合内不同Bain组之间的界面.当奥氏体化温度过高时,在粗大的奥氏体晶粒内部,集合内的相变产物由单一Bain组主导,从而导致大角晶界密度的降低及冲击韧性的下降.

The effect of austenitizing temperature on the microstructures and -20℃ impact toughness of HSLA100 steel was investigated by Gleeble-3500 thermal simulator. Its microstructures were observed by SEM and EBSD, and the relevant transformation kinetics was also analyzed by means of dilatometer. The results show that the microstructure of HSLA100 steel changes gradually from granular to lath bainite with increasing austenitizing temperature. The highest impact toughness of samples was achieved at austenitizing temperature of 1000 ℃, in which martensite-austenite （M/A） islands are finer and dispersed and the density of high angle boundaries is maximum. M/A islands, however, become coarser and this density lowers below 1000 ℃, beyond 1000℃, these islands are refined, being accompanied by a dramatic decrease of this density of high angle boundaries. Kinetics analysis indicates that with increasing austenitizing temperature, the transformation start temperature decreases but the transformation rate increases. Both lower start temperature and faster rate would facilitate M/A islands refining. All the transformation occurring !n samples might be divided into two stages： bainite and martensite stages. The highest transformed fraction of bainite is achieved in the bainite stage at about 1000 ℃, resulting in the best impact toughness of HSLA100 steel. The crystallographic analysis of the well refined M/A islands at 1000 ℃ and 1300℃ shows that major high angle boundaries occur prior at the boundaries between different Bain groups belong to the same crystallographic group set to at austenite boundaries when covariance transformation occurring. When over-increasing austenitizing temperature, the covariance transformation products in coarser austenite grains are dominated by only one Bain group belong to the crystallographic group set, leading to the density of high angle boundaries and thus the impact toughness of HSLA100 steel decreasing.