以复合添加Nb, V和Ti的低碳微合金钢为研究对象, 采用热模拟试验机模拟高温轧制+超快速冷却+缓冷工艺, 采用OM, HRTEM和显微硬度计等对超快冷至不同温度实验钢的组织转变和析出规律进行研究. 结果表明, 随着超快冷终冷温度的升高, 显微组织由贝氏体向珠光体和铁素体转变, 碳化物形核位置从贝氏体转变为铁素体, 铁素体中的析出物密度大于贝氏体中的, 且在620 ℃达到最大. 超快冷至不同温度时析出物的尺寸均小于10 nm, 纵横比均接近于1, 即析出物形态更接近于球形, 且随终冷温度的降低, 析出物尺寸逐渐减小. 利用Orowan机制计算了析出强化增量, 得出在620 ℃析出强化对屈服强度的贡献最大, 可达到25.6%.

High strength low-alloy (HSLA) steel has been widely used in buildings, bridges, ships and automobiles because of the remarkable high strength and forming property. Conventional HSLA steels are strengthened by a combination of grain refinement, solid-solution strengthening and precipitation hardening, and the contribution of precipitation hardening is considered to be minor, since many of the alloying elements are added to HSLA steels in the past basically for the strengthening of grain refinement. However, in recent research, yield strengths up to 780 MPa have been achieved in Ti and Mo bearing HSLA sheet steels by producing microstructures that consist of a ferritic matrix with nanometer-sized carbides, and the precipitation strengthening has been estimated to be approximately 300 MPa. Nowadays, thermo mechanical controll process (TMCP) is widely used to process HSLA steels, the final temperature of ultra-fast cooling (UFC) plays a decisive role for microstructure evolution and precipitation behavior, and finally determines the mechanical properties of the steels. In this work, the effects of final temperature after UFC on microstructural evolution, precipitation behavior and micro-hardness of Nb-V-Ti bearing low alloy steel were studied by using the thermal mechanical simulator, OM, HRTEM and micro-hardness instrument. The results showed that the microstructure and nucleation sites of micro-alloy carbides changed with final temperature after UFC. The microstructure changed from bainite to pearlite and ferrite and the nucleation sites changed from bainite to ferrite with final cooling temperature increasing. The number density of the precipitates in ferrite matrix was greater than that in bainite. Furthermore, the number density of the nanometer sized carbides got the maximum values at 620 ℃. The aspect ratios of the precipitates were close to 1, which meat that the precipitation morphology close to spherical. The sizes of the carbides were all less than 10 nm and became smaller with the decrease of final cooling temperature. Through the calculation by Orowan mechanism, the contributions of the precipitation strengthening to yield strength could reach 25.6% at the final cooling temperature of 620 ℃.