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通过OM,TEM和EBSD等分析手段,并结合热膨胀测试法,研究了Cu对690 MPa级HSLA钢焊丝熔敷金属组织转变及组织细化的作用,并对细化机理进行了探讨.研究结果表明,熔敷金属的组织主要由板条状贝氏体(LB)、粒状贝氏体(GB)和残余奥氏体(AR)组成,Cu含量从0.24%增加到0.53%时,马氏体/奥氏体(M/A)组元数量由0.62%减少到0.31%,并且形状也由小块状、条状向颗粒状转变;残余奥氏体数量增多;同时,组织的亚结构得到明显细化,贝氏体板条块尺寸和板条平均宽度分别从2.18和0.39μm减少到1.99和0.36μm,可阻碍裂纹扩展的大角度晶界的比例也由68.5%增加到71.0%.通过对晶粒细化原因进行分析发现,Cu能降低奥氏体转变温度,增加奥氏体的稳定性,使相变时铁素体自由能(G_α)和奥氏体自由能(G_γ)差值增大,减小临界晶胚尺寸;同时,相变温度的降低也降低了C的扩散速率,使已形核的晶胚长大速率减慢,最终细化了亚晶粒结构.
The effects of Cu on the microstructure and transformation of 690 MPa HSLA steel wire were investigated by means of OM, TEM and EBSD, and the thermal expansion test method was also used to study the mechanism of refinement. The results show that The microstructure of the deposited metal consists mainly of lath-shaped bainite (LB), granular bainite (GB) and retained austenite (AR). When the Cu content increases from 0.24% to 0.53%, the martensite / The number of austenite (M / A) group decreased from 0.62% to 0.31%, and the shape changed from small to strip to granular. The amount of retained austenite increased. At the same time, the microstructure of the structure was obviously thinned The size of the bainite slab and the average width of the slats decreased from 2.18 and 0.39μm to 1.99 and 0.36μm, respectively, and the proportion of large-angle grain boundaries that could hinder the crack growth from 68.5% to 71.0% The reason of grain refinement shows that Cu can reduce the austenite transformation temperature and increase the stability of austenite so that the difference of ferrite free energy (G_α) and austenite free energy (G_γ) increases during the phase transformation , Reducing the size of the critical embryos; the same time, reducing the phase transition temperature also reduces the diffusion rate of C, so that nucleated embryo growth rate slowed down, the most Refined sub-grain structure.