【作者】 齐江华；

【导师】 薛正良；

【作者基本信息】 武汉科技大学 ， 钢铁冶金， 2009， 博士

【摘要】 随着管线钢的强度和韧性的不断提高,高级别管线钢的焊接性能已经成为管线钢质量好坏的重要判据。自新日铁研究人员提出氧化物冶金概念以来,有关钛的脱氧产物就受到了冶金和材料学术界的重点关注和广泛研究。对夹杂物诱发的IGF能使HAZ保持良好的韧性和较高的强度,从而提高钢材的可焊性已形成共识。但目前氧化物冶金的机理并未得到统一,学术观点各异,对钛氧化物的物相组成(主要是TiO,TiO2,Ti2O3和Ti3O5)也存在较大争议。由于这一技术由新日铁等少数厂家垄断,有关氧化物冶金技术的机理及其工艺开发进展比较缓慢。为尽快推动这一技术实现工业应用,就必须对脱氧工艺和脱氧产物进行全面认识,从而在基本掌握IGF形成规律基础上进行精确的冶炼过程控制。本文在调研大量文献的基础上,通过针对应用氧化物冶金的高级别管线钢脱氧工艺控制和脱氧产物析出的热力学与动力学计算,在实验室条件下开展了高级别管线钢的冶炼工作,对高级别管线钢的脱氧工艺和氧位控制;真空下耐火材料分解向钢液增氧的规律;有利于IGF形成的Ti2O3析出的热力学条件和动力学因素;钛脱氧产物的大小、成分,形貌、内部结构以及物相组成;钛脱氧产物对凝固组织的影响;氧化物冶金机理以及微米级夹杂物的研究方法等方面进行了深入细致的研究和探讨。通过钛氧化物析出的热力学计算,在一定条件下钛的氧化物中Ti2O3总能比其它钛氧化物优先析出,控制氧含量小于0.002%更有利于钛的氧化物以Ti2O3颗粒的形式析出;通过对高级别管线钢钛铝竞争脱氧的理论分析,确定了实验室采用无铝脱氧的工艺路线,即采用真空碳脱氧再用钛终脱氧的脱氧工艺;通过计算在炼钢温度和固液两相区内Ti2O3和TiN的竞争析出表明,在不同钛含量情况下,只要控制钢液中初始氮含量小于0.003%,Ti2O3总比TiN优先析出;通过建立的钛脱氧产物析出的动力学模型研究表明,要生成细小的Ti2O3夹杂,必须首先控制钢液凝固过程中有较高的冷却速率的基础上,应适当控制好钢液中的氧含量。通过对真空碳脱氧熔炼钢液时,MgO炉衬与CaO炉衬热分解向钢液供氧的理论研究表明,当钢液中实际氧含量高于与炉衬分解反应平衡时钢液氧含量时,炉衬不向钢液供氧,反之,则向钢液供氧,相比之下,CaO炉衬的稳定性更强;在实验室开展了真空碳脱氧的实验研究,在CaO坩埚内,在满足高级别管线钢C含量要求情况下(0.04%～0.06%),能够将钢液中的氧含量降到0.001%～0.002%左右;通过在金相试样和小样电解萃取的凝固析出的钛脱氧产物外部形貌观察,凝固析出的钛脱氧产物主要是Ti-O-Mn-S系形成的尺寸为1～2μm球形复合夹杂物;通过夹杂物颗粒分布统计分析发现,钢中夹杂物的数目是随着氧含量的降低而减少,但平均尺寸却随着氧含量的降低是先减小后增大;通过对凝固析出的超细夹杂物的内部形貌、结构和物相组成的SEM+EDS+EBSD研究表明,控制氧含量在50ppm以下,夹杂物的物相组成都是有利于针状铁素体形核的Ti2O3和凝固过程中以Ti2O3为形核核心的MnS,并通过透射电镜下选区电子衍射证实了EBSD的分析结果,总结归纳出了研究微米级夹杂物内部结构和物相组成的新思路;通过对微观组织的观察发现,钛脱氧钢的组织为针状铁素体,且随着钛含量的增加,组织细化越明显;诱发针状铁素体的夹杂物都是Ti2O3和MnS以及个别含Al2O3的复合夹杂物,同时发现细小的MnS也能诱发针状铁素体的形成。诱导针状铁素体形核的夹杂物尺寸在1～3μm左右,形成针状铁素体的宽度在1～3μm,长度约在5～15μm之间。综合以上分析表明,钛含量控制在0.015%～0.02%,氧含量控制在0.002%左右时,最有利于析出大量的诱发针状铁素体形核的Ti2O3的复合夹杂物;通过空冷组织和淬火组织的对比,发现钛脱氧钢形成针状铁素体除了与夹杂物本身的性质有关外,还与冷却速度也有非常重要的关系。在金相试样上和RTO表征法制样的扫描电镜元素分布分析表明,Ti、O元素主要分布在夹杂物的中心,S元素主要分布在夹杂物的外围,而从Mn元素的分布来看,在夹杂物中心和外围都含有Mn;通过RTO表征法制样的夹杂物透射电镜分析,在夹杂物最外围有单独的S元素存在,即夹杂物本身存在Mn贫乏区;通过对夹杂物拉曼光谱分析表明,钛脱氧钢样夹杂物除了Ti2O3和MnS外,还存在有MnTiO3;在对钛脱氧产物深度解析的基础上,发展并完善了Ti2O3诱发IGF的贫锰区机制。更多还原

【Abstract】 With the improving of strength and toughness of pipeline steel, welding performance of high grade pipeline steel has become the criterion of the quality of pipeline steel. Since the conception of oxide metallurgy has been advanced by Nippon Steel, Ti-deoxidized products draw the attention and are studied in the metallurgy and material academic community. It has been accepted that IGF induced by the inclusions can maintain HAZ in a good condition of strength and toughness to improve the weldability of steel. But at present there are still controversies in the mechanism of oxide metallurgy and the phase-component of titanium oxides (mainly including TiO, TiO2, Ti2O3 and Ti3O5). Due to the technical monopoly from Nippon Steel and some other enterprises, the mechanism and technology development about oxides metallurgy are improved slowly. To promote this technology to apply in the industry as soon as possible, it is necessary to recognize the deoxidization process and products comprehensively, through which the smelting process can be controlled precisely in the base of mastering the forming law of IGF.Referring to a great deal of the literatures, this research has first calculated the thermodynamics and dynamics in deoxidization process controlling and precipitating products from high grade pipeline steel and then carried out this high grade pipeline steel smelting in laboratory. It has been made a conscientious study of the following: the deoxidization technology and oxygen potential controlling in high grade pipeline steel, the law of increasing oxygen in molten steel from decomposed refractory material in vacuum, the thermodynamical and dynamical factors of precipitating Ti2O3 which is favorable in forming IGF, the size, composition, shape and appearance, inner structure and phase composition of Ti-deoxidized products, the influence to coagulation structure from the Ti-deoxidized products, the mechanism of oxide metallurgy, research method of micron-sized inclusions, etc.By calculating the thermodynamics of precipitating Ti-oxidized products, it is known that Ti2O3 will always precipitate before other Ti-oxidized products and it is favorable to precipitate Ti2O3 in the form of particles with the oxygen content below 0.002%. And it has been determined to use Al-free deoxidization technology (with carbon-deoxidization first and Ti-deoxidization finally in vacuum) through analyzing the theory of competition in deoxidization between Ti and Al. Studying the precipitating competition between Ti2O3 and TiN in liquid-solid phase region at smelting temperature, it indicates that Ti2O3 is always precipitated preferentially than TiN in different Ti content level with the initial nitrogen content below 0.003%. By modeling the dynamics of precipitating Ti-deoxidization products, it also shows that getting tiny inclusions shall be in a high cooling rate first and let the oxygen content in a proper level.The theory of oxygen contamination from MgO and CaO crucible material to molten steel in smelting with carbon-deoxidization in vacuum indicates that if the practical oxygen content in molten steel is higher than the equilibrium oxygen content when the crucible material decomposing reaction reaches to equilibrium, crucible material will not supply oxygen to molten steel; and if the content is lower, it will supply to molten steel. Compared with these two crucible materials, the stability of CaO material is stronger. The carbon-deoxidization research has been carried out in vacuum in laboratory. The oxygen content in molten steel can reduce to 0.001%~0.002%, which can also meet the requirement of carbon content in high grade pipeline steel (0.04%~0.06%). By observing the outer shape and appearance of the Ti-deoxidization precipitating products extracted from metallographic and electrolyzation specimen, the main Ti-deoxidization products are 1~2μm spherical compound inclusions coming from Ti-O-Mn-S system. Following the reducing oxygen content the amount of the inclusions in steel reduces. But the average size first decreases then increases, which are showed by the statistic of the inclusion particle distribution. The observing the inner appearance, structure, phase composition of the superfine inclusions by SEM, EDS and EBSD have indicated that with the oxygen content controlled below 50ppm the inclusions are all composed by Ti2O3 which is favorable for nucleating IGF and also by MnS of which the nucleation core is Ti2O3 in cooling. The selected area electron diffraction under TEM has proved the analysis results from EBSD. The following are the new ideas of micron-sized inner structure and phase composition of inclusions. Ti-deoxidization steel is composed of IGF of which the microstructure refinement will be smaller with the increasing Ti content. The inclusions inducing IGF are Ti2O3, MnS and a few of compound inclusions containingAl2O3. Also tiny-sized MnS can promote to form IGF. The size of the inclusions inducing IGF is 1~3μm and the width and length of the IGF are 1~3μm and 5~15μm. From the above it is showed that the favorable condition of producing great deals of compound inclusions with Ti2O3 as the nucleation core which can induce to nucleate IGF are the Ti content in 0.015%~0.02% and oxygen content at 0.002%. Compared with the structure of air cooling and quenching, forming IGF in Ti-oxidization is related not only to the inclusion’s natural property but also to the cooling rate. By analyzing the element contribution by STM from metallographic specimen and RTO it has been indicated that Ti and O exist mainly in the middle of the inclusions while S is distributed over the outer. Referring to the distribution of Mn, Mn exists both inner and outer. S exist the most part of the outer layer, which is to say the inclusions contain Mn-depleted zone (MDZ) in itself. Besides Ti2O3 and MnS by analyzing Raman Spectra The inclusions in Ti-deoxidization steel still contain MnTiO3. This research has been developed and completed the MDZ mechanism of Ti2O3 nucleating IGF by studying Ti-deoxidization products in depth.更多还原