700MPa级低碳微合金钢的连续冷却相变组织及强韧性

Continuous Cooling Transformation Microstructure and Strength-Toughness of 700 MPa Low Carbon Microalloyed Steel

利用Gleeble-3500型热/力模拟试验机测定了700 MPa级低碳微合金钢的连续冷却相变（CCT）曲线,分析了冷却速率对该钢连续冷却相变及显微组织的影响,研究了该钢的强韧性。结果表明：该钢CCT曲线呈现扁平状,可在较大冷速范围内获得低碳贝氏体组织;冷却速率对试验钢各相的形态、数量、分布和显微硬度均有影响;随着冷却速率的提高,显微组织中依次出现多边形铁素体（PF）、针状铁素体（AF）、粒状贝氏体（GB）和板条贝氏体（LB）,且各相的显微硬度也依次增加;当冷速在10~30℃·s-1范围时,显微组织主要为板条贝氏体组织,M/A组元弥散分布于晶界上,且晶粒随着冷却速率的增加而逐渐细化;利用冷却制度控制中温转变组织类型能优化其综合力学性能。

Gleeble-3500 thermo-simulation machine was employed to determine the continuous cooling transformation（CCT） curves of 700 MPa low carbon microalloyed steel. The effects of cooling rate on phase transition behavior and microstructure were investigated. The results indicate that the CCT curves were in flat shape and the low carbon bainite structure was obtained in a large cooling rate range. The cooling rates had great effects on the form, proportion, distribution and micro-hardness of phases. The microstructure in experimental steel were changed from polygonal ferrite, acicular ferrite, granular bainite to lathe bainite with micro-hardness increasing when cooling rate became faster. When the cooling rate was between 10 ℃ ·s-1 and 30 ℃ ·s-1, the microstructure of the experimental steel was mainly compound of lathe bainite, and the M/A constituents dispersed on the grain boundaries. Futhermore, the size of lathe bainite became smaller with the cooling rate increasing. Therefore controlling the intermediate temperature transformation products using a proper cooling process could improve comprehensive mechanical property of experimental steel on the basis of analyzing of its phase transition characteristics.