摘要:
此项工作集中研究钒、氮、碳在控制奥氏体和铁素体内V(C,N)沉淀上的作用,以及在促成晶粒内铁素体的形成而使晶粒细化和通过中间相和不规则沉淀而形成沉淀强化上的作用。
在某一给定钒含量下,铁素体沉淀强化的程度取决于氨、碳的可利用量。已有结论表明,氮是一种非常可靠的合金元素。它可以增加钒微合金钢的屈服强度:每增加0.001%氮,可增加约6 MPa的强度,并且基本上与工艺条件无关。碳在沉淀强化上的作用较为复杂,目前的结论表明.随着碳含量的增加,钒傲合金钢的沉淀强化作用急剧增强,每增加0.01%碳,可增加约5.5 MPa的强度。其原因是.在相变期间,铁素体和过冷奥氏体之间亚稳定均势,极大地增加了碳在铁素体内的可溶性,因而有利于大量的V(C,N)微粒核化。碳的这种作用,在用于热轧钢筋和型材生产的中碳钢中,特别明显。
试验结果明显表明,钒不仅可以有敏地用于沉淀强化,而且也可以用于铁素体晶粒细化。钒有助于两种晶粒内核化铁素体的形成;生成多边(自发)铁素体和针状(侧板)铁素体。晶体内多边铁素体在氮化钒(VN)微粒上核化。在等温保持期或奥氏体范图内缓冷期内,钒氮微粒在奥氏体内生长。在低温约450℃时,在等温相变期间,针状铁素体微结构在钒微合金钢中形成。针状铁素体微结构在含高、中或非常低的氮含量的钒微合金钢中获得。这就说明钒本身可以促成针状铁素体的形成。
Abstract:
The present work has concentrated on the roles of vanadium, nitrogen and carbon in controlling the precipitation of V(C,N) in austenite and ferrite and their effects on:(i) grain refinement by promoting the formation of intragranular ferrite, and (ii) precipitalion strengthening by interphase and random precipitation.
The degree of precipitation strengthening of ferrite at a given vanadium content depends on the available quantities of carbon and nitrogen. It is concluded that nitrogen is a very reliable alloying element, increasing the yield strength of V-microalloyed steels by some 6 MPa for every 0.001%N, essentially independent of processing conditions. The role of tarbon in precipitation strengthening is complicated. The present results have shown that the precipitation strengthening of V-microalloyed steels increases significantly with the total C-content, ≈5.5MPa/0.01%C. The explanation is that the metastable equilibrium between ferrite and undercooled austenite greatly increases the solubility of carbon in ferrite, in the times available during transformation, thereby contributing to profuse nucleation of V(C,N) particles. This effect of carbon is particularly significant for medium carbon steels typically used for hot rolled bars and sections.
The experimental results strongly indicate that vanadium can be effectively used not only for precipitation strengthening but also for ferrite grain refinement. It was shown that vanadium contributes to the formation of two types of intragranulary nucleated ferrite; polygonal(idiomorph) ferrite and acicular (sideplate)ferrite. Intragranular polygonal ferrite nucleates on VN particles that grow in austenite during isothermal holding or slow cooling throughout the austenite range. Acicular ferrite microstructure forms in V- microalloyed steels during isothermal transformation at lower temperatures (~450 C). The acicular ferrite microstructure was obtained in V-microalloyed steels containing high, medium or very low nitrogen levels. This suggests that vanadium on its own can promote the formation of the acicular ferrite microstructure.
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