Ti-Nb微合金钢中第二相粒子对CGHAZ组织及韧性的影响

EFFECT OF SECOND PHASE PARTICLES ON MICROSTRUCTURE AND TOUGHNESS OF CGHAZ IN Ti-Nb MICROALLOYED STEEL

利用碳萃取复型技术研究了含Ti-Nb微合金钢模拟粗晶区(CGHAZ)中的第二相粒子,并利用OM(光镜)、TEM(透射电镜)及系列冲击试验对含Ti-Nb微合金钢CGHAZ的组织及韧性进行了研究。研究结果表明,含Ti-Nb微合金钢中含有大量尺寸细小的TixNb1-x(CyN-y)粒子,粒子中Nb的相对质量分数(WNb/(WNb+WTi))在25%～82%之间,尺寸较大的粒子含有较多Ti,形状接近方形;尺寸较小的粒子含有较多的Nb,形状接近球形。焊接热循环后,粒子的数量显著减少、平均尺寸增大,残留的粒子变为方形。在焊接热循环过程中这些粒子能有效地阻止奥氏体晶粒长大、抑制粗大贝氏体的形成、促进针状铁素体析出及M—A组元的分解,从而显著改善低合金高强钢CGHAZ的韧性,在500～800℃的冷却时间t越长,这种改善作用越明显。

Carbon extraction replica analysis is employed to measure the quantities and size distributions of particles in Ti-Nb microalloyed steel and its simulated CGHAZ (coarse grain heat affected zone) to reveal the response behavior of second phase particle to the welding thermal cycle. The micro-structure and impact toughness of CGHAZs in Ti-Nb microalloyed steel and a kind of high strength low alloyed steel with similar chemical content and manufacture process are investigated with OM, TEM and series charpy impact test. Experiment results and analysis show that the particles in the steel are irregular in shape, and exhibited a slight size-dependence in that the relative content of Nb decrease with the increase of particle size. The Nb contents of particles in parent metal fell in a wide range from 25%-82%. After weld thermal cycle, the shape of remaining particles in simulated CGHAZ changes from irregular shape to cubicle one, the particle number per unit area decrease, and the mean particle size increases. During the weld thermal cycling, the particles prohibit the original austenite grain from coarsening, prevent the formation of coarse upper bainite, accelerate the formation of acicular ferrite, and promote the decomposition of M-A constituent into ferrite and cemen-tite, thus improve the toughness of CGHAZ in high strength low alloyed steel remarkably. With time t (cooling time form 800 ℃ to 500 ℃) increasing from 16 s to 120 s, the toughness of CGHAZ in Ti microalloyed steel increases because that secondary microstructure of CGHAZ change from upper bainite to acicular ferrite and the austenite grain size coarsening is not serious.