复合N/B对低碳Mo-V-Ti钢焊接热影响区IGF组织及强韧性的调控作用

Low-carbon V-N-Ti steel is a kind of promising constructison steel of new generation potential for high heat input welding due to its tougher heat affected zone (HAZ) as a result of enhanced nucleation of fine-grained intergranular ferrite (IGF) on (V,Ti)(C,N) complex precipitates by an increasing nitrogen content. However, the microstructure refinment of HAZ is still insufficient and accordingly the strength/toughness combination is inadequate owing to the formation of coarse-grain intragranular polygonal ferrite (IGPF), which prohibits its further development and applications. In the present project, a new low-carbon Mo-V-N-Ti-B steel will be obtained for high efficiency welding with a HAZ of upgraded strength/toughness combination by simultaneous additon of nitrogen and boron in reasonable control. The effects of N/B on the revolusions of solution-precipitation of relative particles and boron segreation at pior auteninte grain boundaries during welding thermal cycles will be investigated for futher estimating their effects on thermodynamics/kenitics of IGF transformation. Based on the results, the endeavors of precisely controlling the homogeneous fine-grained IGAF will be carried out, and the emphases should be paid to understanding thoroughly how and why the additions of N/B affect the morphology of IGF. The subsequent investigations will cover the effect of IGF morphology on the strength/toughness combination and fracture behaviors in HAZ of the experimental steels imposed by high heat inputs. The efforts will be also performed to understand the role of IGAF in controlling the strength/toughness combination and the mechanism of improved resistance to brittle fracture by IGAF in comparison to IGPF. The project aims to create a new low-carbon Mo-V-N-Ti-B steel more suitable for high heat input welding by providing an improved HAZ with a stronger and tougher IGAF microstructure. The results will not only enrich the theory of solid-state phase transformation for heterogeneous nucleation control but also lay a foundation for developing a new construction steel with superior resistance to high efficiency welding and pushing its extensive applications.

低碳V-N-Ti钢利用增N促进沉淀粒子在高热输入焊接热影响区（HAZ）诱发晶内铁素体（IGF）形核而细化组织、提高韧性，是一种极具发展潜力的高效焊接用钢。但该钢的焊接HAZ因晶内块状铁素体（IGPF）不够细化而强韧性仍有不足，已成为制约其发展和应用的瓶颈。本项目拟采用复合添加N/B，制备HAZ具有高强韧性晶内针状铁素体（IGAF）组织的高热输入焊接用低碳Mo-V-N-Ti-B钢。研究复合N/B对热影响区沉淀粒子固溶-析出和B晶界偏聚行为、以及它们对IGF相变热/动力学的影响规律，准确调控均匀细化的IGAF，揭示复合N/B对IGF的调控作用及机理；研究IGF形态对HAZ强韧性及断裂行为的影响规律，阐明IGAF提高强韧性的作用机理。上述研究通过复合N/B调控高热输入焊接HAZ形成IGAF组织而提高强韧性，对发展异质形核固态相变控制理论和新型高效焊接用结构钢材料、促进其广泛应用具有重要意义。

本项目研究了复合N/B对低碳Mo-V-Ti钢热影响区IGF相变热力学/动力学的影响规律，揭示复合N/B对IGF的调控作用及机理；研究IGF形态对HAZ强韧性及断裂行为的影响规律，取得如下主要结果：. 低碳Mo-V-N-Ti-B钢在小热输入（≤50 kJ/cm）条件下，粗晶热影响区组织主要为板条贝氏体和粒状贝氏体；热输入增加到75 kJ/cm，焊接热影响区组织主要为IGF，热输入进一步增加时（≥100 kJ/cm），热影响区组织主要为块状铁素体和退化珠光体，并且随着热输入的增加，晶粒发生粗化。. 随着B含量的增加，B在晶界偏聚含量增加，试验钢Ar3降低。热输入75 kJ/cm条件下，B含量由2ppm增加到13ppm时，可以抑制晶界铁素体的形成，降低相变点，促进针状铁素体的转变；B含量进一步增加到19ppm时，钢的淬透性明显增加，过冷奥氏体在更低的温度发生转变，形成粒状贝氏体。. 随着N含量的增加，试验钢粗晶热影响区中微米级亚微米级和纳米级析出增多，纳米级析出钉扎原奥晶界，原奥晶粒发生细化，并且增N促进了基体中B的析出形成BN，降低了基体中固溶B的含量，提高了 CGHAZ 的相变点。另一方面，增N促进了富V粒子在未溶解的富Ti粒子上形核，促进了晶内多边形铁素体和针状铁素体的形成。. 低碳Mov-N-Ti-B钢的屈服强度以细晶强化为主，位错强化和固溶强化次之。随着主要组织由IGPF转变为IGAF和GB，晶粒细化强化对YS的贡献显著提高。组织形态由IGPF变为IGAF，硬相由P转变为M/A，应变硬化效果减弱，使抗拉强度和屈强比升高、应变硬化n值和均匀延伸率降低；由于IGAF的晶粒尺寸较细，位错积累率比IGPF试样大，导致n1的值较低。. 当组织由IGPF转变为IGAF时，第二相组织由粗大的珠光体转变为细小的M/A组元，从而使得裂纹萌生功升高，同时组织发生明显细化，大角度晶界含量增多，阻碍裂纹扩展能力增强，韧脆转变温度降低，冲击韧性得到改善。在-20 °C条件下，组织由IGPF转变为IGAF时，其断裂行为由脆断转变为混合断裂。