卷取温度对500 MPa级热冲压桥壳用钢组织与力学性能的影响

Effect of Coiling Temperature on Microstructure and Mechanical Properties of 500 MPa Grade Hot Stamping Axle Housing Steel

采用OM、SEM和TEM等研究了卷取温度对500 MPa级热冲压桥壳用钢的组织与性能的影响规律。结果表明,热冲压桥壳用钢在600和570℃卷取时的力学性能有显著差异,570℃卷取时的屈服强度和抗拉强度分别达到538和641 MPa,较600℃卷取时分别提高了165和117 MPa,而其20^-40℃的系列温度冲击功均低于600℃,尤其是-40℃及以下低温韧性明显恶化,这与热冲压桥壳用钢在不同卷取温度下的显微组织和析出物的差异有关。600℃卷取时的组织由铁素体和珠光体组成,铁素体晶粒平均尺寸为4.48μm,大角度晶界体积分数为68.1%,析出物平均尺寸为8.4 nm,其中10 nm以下的纳米级析出物体积分数约为70%;当卷取温度降低至570℃时,其组织主要由针状铁素体、粒状贝氏体、多边形铁素体/准多边形铁素体和珠光体组成,铁素体晶粒平均尺寸为4.39μm,大角度晶界体积分数约为54.5%,析出物的平均尺寸为6.4 nm,其中10 nm以下的纳米级析出物体积分数高达86%;这种差异主要是由于热冲压桥壳用钢的贝氏体转变温度高达580℃,以及570℃卷取时的形核率与形核速率均大于600℃所致。因此,考虑到热冲压桥壳对板形质量要求较高,以及热冲压工艺对热轧原料组织和析出物的影响,热冲压桥壳用钢的卷取温度为600℃更合适。

With the rapid development of the Chinese axle industry and the increasing demand for lightweight axle housings,the demand for a new type of hot stamping axle housing steel,which can ensure the yield strength meets the requirements after the hot stamping process,is becoming increasingly urgent.However,there are no published reports on the development of this new type of hot stamping axle housing steel.The aim of this study was to develop 500 MPa grade hot stamping axle housing steel.The effect of coiling temperature on the microstructure,precipitates,and mechanical properties of the 500 MPa hot stamping axle housing steel were studied by OM,SEM,and TEM.The results showed that the mechanical properties of the tested steel were significantly different when coiling at 600 and 570℃.When coiling at 570℃,the yield strength and tensile strength reached 538 MPa and 641 MPa,respectively,which were 165 MPa and 117 MPa higher than those at 600℃,whereas,the impact energy in the range of 20^-40℃was lower than that at 600℃,especially low-temperature toughness.These are related to the differences in the microstructure and precipitates in test steel when coiling at different temperatures,especially the difference in the proportion of the high angle grain boundary(HAGB).The microstructure of the tested steel was composed of ferrite and pearlite when coiling at 600℃,average grain size of ferrite was 4.48μm,proportion of HAGB was 68.1%,and average size of the precipitates was 8.4 nm,of which nanoscale precipitates with sizes below 10 nm accounted for approximately 70%.The microstructure was mainly composed of acicular ferrite,granular bainite,polygonal ferrite/quasi polygonal ferrite,and pearlite when the coiling temperature was reduced to 570℃,the average size of ferrite was 4.39μm,ratio of HAGB was approximately 54.5%,average size of the precipitates was6.4 nm,and nanoscale precipitates with a size below 10 nm accounted for 86%.The difference in the microstructure and the precipitates was mainly owing to the fact the bainite transformation temperature of the test steel was as high as 580℃,and nucleation rate and nucleation speed of the test steel was higher when coiling at 570℃than that at 600℃.Thus,considering the high requirements on the shape quality,effect of the hot stamping process on the microstructure,and precipitates of the tested steel,the coiling temperature of the test steel is more suitable at 600℃.