【作者】 张莉芹；

【导师】 袁泽喜；

【作者基本信息】 武汉科技大学 ， 材料学， 2009， 博士

【摘要】 本文采用碱土金属元素Ca、Mg与Nb、V、Ti等元素,按单个、两个和三个氧化物形成元素系列复合微合金化,设计了抗拉强度490MPa级非调质大线能量焊接低合金高强度钢。系统研究了试验钢母材及30kJ/cm、60kJ/cm、100kJ/cm不同线能量焊接热模拟热影响区(HAZ)力学性能、夹杂物形态分布及微观组织特征等。结果表明:试验钢母材力学性能达到设计要求,对于提高材料的强韧性而言,以Nb配合Ca、Mg、Ca-Mg和Ti-Ca-Mg复合微合金化要优于以V、Nb-V配合Ca、Mg、Ca-Mg和Ti-Ca-Mg复合微合金化。试验钢均可承受100kJ/cm大线能量焊接,表明选用的氧化物形成元素中除传统的Ti之外,碱土金属元素Ca、Mg可改善大线能量焊接HAZ强韧性。采用Ti-Ca-Mg三种氧化物形成元素微合金化比单独采用Ti、Ca或Mg单个氧化物形成元素以及Ti-Ca、Ti-Mg、Ca-Mg两个氧化物形成元素微合金化对提高100kJ/cm大线能量HAZ强韧性更为有利。试验钢母材组织为内部存在亚结构的铁素体+少量珠光体组织,随焊接线能量增大,HAZ组织按贝氏体(或贝氏体+少量IAF)→少量贝氏体+IAF→晶界铁素体+IAF组织变化。试验钢母材与不同线能量焊接HAZ冲击断口扫描电镜及透射电镜复合析出物成分能谱分析结果与相应的微合金化系列一致。Ca系、Mg系、Ca-Mg系和Ti-Ca-Mg系试验钢与Ti系试验钢相比洁净度指数明显降低,其中Ti-Ca-Mg系试验钢洁净度指数最小,洁净度最高,且Ti-Ca-Mg系试验钢夹杂物的平均粒径最小。Ca、Mg微合金化对夹杂球化变性作用较显著,Ti-Ca-Mg微合金化对夹杂物球化效果最好。碱土金属元素Ca、Mg以复合氧化物的形式出现,且大多数复合析出物都富含Mn和S。IAF以含Ca、Mg的尺寸为500nm~3μm的非金属氧化物夹杂为核心形核生成。不同微合金化系列有效的IAF形核核心数量不同,导致HAZ组织中IAF体积分数和HAZ强韧性相应发生变化。IAF形核核心较多的微合金化系列,其IAF体积分数较高,且IAF相对比较细小,IAF数量的增多可抑制晶界铁素体的长大和粗化。HAZ组织中IAF相对数量及晶界铁素体的形态是影响HAZ强韧性的决定性因素。基于Ca、Mg对钢质的净化作用和夹杂物变性特性,一方面可提高母材强韧性,另一方面,可使HAZ获得小尺度表面含MnS的析出物,促进IAF形核,提高HAZ强韧性。而Ti与碱土金属元素Ca、Mg或Ca-Mg的复合微合金化,可促进MnS在碱土金属Ca、Mg及Ca-Mg钢复合析出物表面的析出,又增强了析出物对IAF的形核能力,进一步提高了HAZ强韧性。HAZ不同析出物的IAF形核能力按Ca(O,S)-MnS或Mg(O,S)-MnS、氧化钛-MnS或CaMg(O,S)-MnS、TiCa(O,S)-MnS或TiMg(O,S)-MnS、TiCaMg(O,S)-MnS的顺序增大。碱土金属Ca、Mg微合金化钢HAZ中IAF形核机制包括两个方面:一方面,在HAZ析出物表面均不同程度存在MnS,该MnS可通过适当提高复合析出物与铁素体之间的晶格匹配度和适当降低复合析出物与奥氏体的平均热膨胀系数差异,抑制高温下先共析铁素体的生成,提高IAF形核能力和体积分数;另一方面,HAZ中的一定量的尺寸为500nm~3μm的复合析出物,可提高其周围的应变能,增大IAF相变驱动力。不同微合金化系列占主导地位的IAF形核机制不尽相同。试验优化出新钢种的化学成分,并结合某钢铁企业设备及工艺流程状况,提出了新钢种合理的内控冶炼化学成分范围及控制轧制生产工艺,结合相关技术标准制定了工业化生产工艺规程及交货条件,并申报国家发明专利一项。所有工作为新钢种的商品化打下基础。更多还原

【Abstract】 High heat input welding non-quenched and tempered high strength low-alloyed (HSLA) steels with tensile strength of 490MPa grade were designed by composite-micro-alloying with single, double and tri-oxide-forming elements of alkaline earth metal elements of calcium(Ca), magnesium(Mg) and niobium(Nb), vanadium(V), titanium(Ti) in this paper. Mechanical properties, morphological distribution of inclusions and microstructure characteristics of the as-rolled steel plates and the heat-affected zone (HAZ) of 30kJ/cm, 60kJ/cm, 100kJ/cm heat input welding thermal simulation were studied systematically. It was found that:The mechanical properties of the as-rolled steel plates met the target requirements. For improving the strength and toughness of the materials, Nb composite-micro-alloying with Ca, Mg, Ca-Mg and Ti-Ca-Mg was superior to that of V and Nb-V. All of the experiment steels could bear 100kJ/cm high heat input welding, indicating that in addition to the traditional elements of Ti, the selected oxide-forming elements of the alkaline earth metal elements Ca, Mg could improve the strength and toughness of HAZ. The tri-oxide-forming elements of Ti-Ca-Mg micro-alloying was more favorable for improving the strength and toughness of 100kJ/cm HAZ than single oxide-forming element of Ti, Ca or Mg micro-alloying as well as double oxide-forming elements of Ti-Ca, Ti-Mg, Ca-Mg micro-alloying.The microstructures of the as-rolled steel plates were ferrite with sub-structures and a small amount of pearlite, and the microstructures of HAZ changed from bainite (or bainite and a small amount of intragranular acicular ferrite--IAF) to bainite and IAF, and further changed to a small amount of grain boundary ferrite and IAF as the welding heat input increased.The compositions of the composite precipitates analysised with scanning electron microscopy(SEM) and transmission electron microscopy(TEM) both in the impact fracture of the as-rolled steel plates and in different heat input welding HAZ were consistent with the corresponding micro-alloying steel series. The cleanliness index of Ca, Mg, Ca-Mg and Ti-Ca-Mg steels were significantly lower compared to that of Ti steel, Ti-Ca-Mg steel had the smallest of cleanliness index and the highest degree of cleanliness, and the average particle size of inclusions in Ti-Ca-Mg steel was the smallest. Ca, Mg micro-alloying had significant spheroidizing capabilities of the inclusions, and Ti-Ca-Mg micro-alloying had the best spheroidizing effect of the inclusions. Alkaline earth metal elements Ca, Mg appeared in the form of composite oxides, and the majority of composite precipitates were rich in manganese(Mn) and sulphur(S). The nuclei of IAF were the non-metallic oxides inclusions containing Ca, Mg with the size of about 500nm to 3μm. The number of effective IAF nuclei were inconsistent for different microalloying series, resulting the corresponding changes of the volume fraction of IAF in HAZ and strength and toughness of HAZ. The larger the number of IAF nuclei in HAZ of the micro-alloying series, the higher the volume fraction and the relatively finer of IAF became in HAZ, the increase of IAF inhibited the growing up and coarsening of grain boundary ferrite. The relative amount of IAF and the morphology of grain boundary ferrite in HAZ were the decisive factors affecting the strength and toughness of HAZ.Based on their features of steel purification of and inclusion modification, Ca and Mg micro-alloying improved strength and toughness of the as-rolled steel plates, and formed small-size precipitates containing MnS on its surface which promoting the nucleation of IAF in HAZ and then improved the toughness of HAZ. The composite micro-alloying of Ti with alkaline earth metal elements Ca, Mg or Ca-Mg could promote the MnS precipitating on the surface of precipitates containing Ca, Mg or Ca-Mg, enhanced IAF nucleation ability of precipitates, and further improved the strength and toughness of HAZ. The IAF nucleation capability of precipitates in HAZ increased in the order of Ca(O,S)-MnS or Mg(O,S)-MnS, Ti-MnS or CaMg(O,S)-MnS, TiCa(O,S)-MnS or TiMg(O,S)-MnS, TiCaMg(O,S)-MnS.The IAF nucleation mechanism in HAZ of alkaline earth metal Ca, Mg micro-alloyed steels including two aspects: on the one hand, as almost all of the surface of precipitates in HAZ containing different amounts of MnS, the MnS could appropriatly increase the lattice matching between precipitates and the ferrite, and also appropriatly reduce average coefficient of thermal expansion differences between precipitates and austenite, thus inhibited the high temperature formation of proeutectoid ferrite, and improved the IAF nucleation capability and its volume fraction; on the other hand, the certain amount precipitates with the size of 500nm to 3μm in HAZ could increase the strain energy around the precipitates, and increased the driving force of IAF transformation. The dominant nucleation mechanism of IAF in HAZ of different micro-alloyed series was not the same as each other.The chemical compositions of a new steel were optimized, and the internal controlled smelting chemical compositions ranges and controlled rolling production processes of the new steel were suggested in combination with the equipment and technological process of a steel enterprises. The industrial production processes and delivery terms of the new steel were also established combining with the technical standards related to the new steel, and a national invention patent for the new steel were declared. All of the above works laid the foundation for the commercialization of the new steel.更多还原