28CrMnMoV钢过冷奥氏体连续冷却转变研究

Continuous cooling transformation of undercooling austenite about 28CrMnMoV steel

利用Gleeble-3500热模拟机测量28CrMnMoV钢以不同速度连续冷却时的膨胀曲线,结合差热分析法和金相-硬度法,确定临界点及相变温度点,绘制、分析、应用过冷奥氏体的连续冷却曲线（CCT图）,研究实验钢连续冷却时的奥氏体转变。研究结果表明：实验钢过冷奥氏体在高温区可能发生铁素体转变和珠光体转变,中温区可能发生贝氏体转变,低温区可能发生马氏体转变,当冷却速度为0.05-0.5℃/s时,转变产物为多边形铁素体、珠光体和少量贝氏体的混合组织,在1-5℃/s的冷却速度范围内,转变产物为贝氏体,当冷却速度大于5℃/s时,转变产物为马氏体。较低的含碳量提高了马氏体转变温度,合金元素Cr,Mo和V有抑制奥氏体扩散分解的作用,以低于1℃/s的速度冷却才会有先共析铁素体和珠光体,较高的锰含量对贝氏体转变起到了明显的促进作用,在0.05-10℃/s的较宽冷却速度范围内连续冷却都会发生贝氏体转变。根据测得的CCT图指导28CrMnMoV钢的分级控冷,会对减小钢管淬火应力产生很好的效果,为V150油套管的高韧性提供组织保障。

The dilatometric curves of 28 CrMnMoV steel during continuous cooling process at different cooling rates were measured on Gleeble-1500 thermal mechanical simulator. According to DSC analysis and microstructure observation combined with hardness test, the critical temperatures and phase transformation temperatures were determined from the dilatometric curves, which were used to determine the continuous cooling transformation（CCT）diagram of 28 CrMnMoV steel. The results show that ferrite（or pearlite） can be formed in supercooled austenite of tested steel at higher temperature intervals, and bainite transformation and martensite transformation can occur at intermedium temperature and low temperature zones, respectively. The massive morphology of ferrite and pearlite can be easily formed when the cooling rate is in the range of 0.05-0.5 ℃/s. The bainite appears when the cooling rate is in the temperature interval of 1-5 ℃/s, and the martensitic transformation occurs when the cooling rate is greater than 5 ℃/s.Lower carbon content increases the martensite transformation temperature. Because the alloy elements of Cr, Mo and V can inhibit the decomposition of austenite, pro-eutectoid ferrite and pearlite can be formed only in the condition of cooling rate below 1 ℃/s. However, higher manganese content can greatly accelerate the bainite transformation.Therefore, bainite can be easily formed during the continuous cooling process for a large range of cooling rate. It is an effective way of decreasing the quenching stress of steel tubes to use the obtained CCT diagram to guide the multiple controlled cooling process, which indicates that the CCT diagram can provide guarantee for excellent touthness of V150 tubing and casing.