微量Zr添加对Mn-Mo-Nb-Cu-B钢晶粒长大倾向性的影响

设计了一种低碳Mn-Mo-Nb-Cu-Zr-B钢,经热处理工艺,采用中等冷速冷却,可得到以板条贝氏体为主,含粒状贝氏体和针状铁素体的混合组织,轧态屈服强度大于850MPa,达到X120管线钢的强度要求.TEM观察表明,0.015%Zr(质量分数)添加到钢中形成大量含Zr的复杂的碳氮化物,它们的形状不规则,尺寸约为80~200nm;从形态看,它们在高温形成,并且由于其熔点高,再加热到1200℃时,这种析出物中的Ti、Nb会有部分溶解,使其尺寸有所减小,利于控制奥氏体晶粒长大;其他近椭球形的(Ti,Nb)(C,N)则在加热时逐渐溶解直至消失.由于这种含Zr析出物在钢的基体中均匀分布,加热到高温时,它们会明显阻碍晶界移动,从而使含Zr钢的奥氏体晶粒长大倾向性明显比不含Zr钢小.可见,添加微量Zr能够起到提高钢材焊接性能的作用.

Effect of micro-zirconium addition on the carbonitride precipitation and austenite grain coarsening in Mn-Mo-Cu-Nb-B structure steel

Low carbon Mn-Mo-Nb-Cu-Zr-B steel was designed, which was processed by thermo-mechanical controlling and cooled at medium cooling rate, and the microstructures including an abundant of lath bainite, a number of granular bainite and a bit of acicular ferrite were obtained. The yield strength of the as-rolled steel plate exceeds the 850MPa grade, which meets the need for the mechanical properties of X120 pipe line steel. As 0. 015% Zr (mass faction) was added in the steel, a number of complex Zr-bearing carbonitrides with irregular shape were observed by TEM, and their size ranged from 80 nm to 200 nm; considering anomalous shape, they formed at high temperature and had high melting points. The size of Zr-bearing precipitates would reduce when the reheat temperature exceeded 1200℃ because of some amounts of Ti and Nb in the precipitates dissolving, which is better for controlling the austenite grain coarsening. Whereas, quasi-elliptic Zr-free carbonitrides would dissolve even vanish when reheating. Zr-bearing precipitates distributing evenly can block the moving of grain boundaries effectively when reheating, which is the predominant reason that the tendency of austenite grain growth of Zr-bearing steel is lower than that of Zr-free steel. A conclusion should be obtained that 0.015% Zr (mass fraction) addition can improve the weld properties of low carbon Mn-Mo-Nb-Cu-Zr-B steel.