重轨钢中MnS夹杂物形貌分析与控制研究

发布时间：2018-03-10 07:17

本文选题：MnS夹杂物　切入点：重轨钢　出处：《北京科技大学》2017年博士论文　论文类型：学位论文

【摘要】：高速重轨钢要求高洁净度、高强度、高韧性、细珠光体。非金属夹杂物对重轨钢的性能有着重要的影响,目前国内某厂家生产的重轨钢中A类夹杂物(主要是MnS)评级较高,达到甚至超过3.0级。本文通过对U75V重轨钢的连铸坯和对应的钢轨进行研究,得到其凝固组织及MnS夹杂物在铸坯和钢轨内的形貌特征和分布规律,结合热力学和动力学计算分析MnS夹杂物的形成机理并提出控制措施,再根据实验室保温实验给出最优的控制参数,为进一步提升我国重轨钢的质量提供理论依据及生产指导。通过观察重轨钢连铸坯和钢轨的凝固组织及MnS夹杂物的二维和三维形貌特征,发现在铸坯内MnS夹杂物的分布规律为：从边缘到中心,纯MnS的形貌由球状和纺锤状,逐渐变为长条状、花瓣状、多面体和不规则状,最后变成沿晶界分布和沿树枝晶疏松处分布的片状,尺寸逐渐增大：而MnS在氧化物表面析出生成的氧硫化物,在边缘处MnS完全包裹氧化物,随着铸坯厚度增加,硫化物层越来越薄,在铸坯厚度1/4区域的氧化物表面大部分是点状的MnS夹杂物。在钢轨内,MnS夹杂物普遍沿轧制方向呈长条状,沿重轨横截面方向,夹杂物普遍呈圆状、椭圆状和褶皱状,但在轨头和轨底处的MnS夹杂物更加细长，，在轨腰处比较扁平。根据无水有机溶液电解法得到MnS完整的三维形貌,结合前人的研究成果,对其形貌分类进行了修改和补充：第一类椭球状、第二类长棒状、第三类板片状、第四类不规则形状或多面体、第五类树枝状和第六类在氧化物表面析出的点状。通过热力学计算，发现MnS夹杂物在1619 K、即靠近固相线才开始析出。利用偏析模型计算出MnS在固相分数为0.94时形成。且发现S的富集程度远远大于Mn,固相分数为0.9时残余液相中的Mn和S的质量分数分别达到1.427%和0.12%,分别是其初始质量分数的1.64和9.23倍。通过调整钢中Mn和S含量及冷却速率,可以控制MnS的析出。动力学计算结果表明,MnS粒子在重轨钢凝固过程以均质形核和晶界形核为主导：MnS的有效形核温度为1634 K，把硫含量降低到50 ppm以下,能够推迟MnS析出、降低MnS的形核率、临界直径和尺寸长大：提高冷却速率也能够降低MnS临界析出半径,减小其尺寸。通过研究重轨钢连铸坯和钢轨在加热保温过程中MnS夹杂物的形态演变,发现在1200℃下,MnS形态演变缓慢；在1300℃下,保温时间超过30 min有助于MnS粒子分裂：而在1400℃情况下,MnS发生分解和固溶;当冷却再析出后,MnS在晶界处析出量最多,且二维观察到单个MnS尺寸较小,但聚集一起会形成超大尺寸的MnS夹杂物。对高硫的铸坯试样,在1300℃保温30 min左右时MnS的形态演变由Mn或S元素的扩散控制向固溶反应速率控制转变。而对于钢轨内的长条状MnS,发现在1300℃下,形态演变限制性环节的转变节点处于保温时间为30～60 min。通过研究“重熔→冷却→保温”条件下MnS的析出行为,发现降低冷却速率,MnS的形貌从近球状和纺锤状逐渐向棒状和长条状演变,且1300℃时保温30～60 min是限制性环节的转变节点。对比高硫([S]=130 ppm)、低硫([S]=81 ppm)的试样,发现在1200℃时,随着保温时间增加,MnS的数量密度在高硫试样中降低,在低硫试样中增加；而在1400℃时,随着保温时间增加,MnS的数量密度在高硫试样中增加,在低硫试样中降低。这说明高温有助于控制低硫样中的MnS。因此,将钢中的硫含量降低到81 ppm以下,能够使加热温度和保温时间对控制MnS夹杂物更为合理有效。针对该厂的实际情况建议将钢中的硫含量降到40 ppm以下。此外建议增加轨头对应的铸坯宽面右侧的柱状晶区比例,达到降低MnS评级的目的。
[Abstract]:High speed rail steel Gao Jie clarity, high strength, high toughness, fine pearlite. Have an important effect on performance of non-metallic inclusions on steel, a production of heavy rail steel inclusion in the current domestic manufacturers (mainly MnS) reached higher ratings, even more than 3. The research is conducted through the continuous casting billet of U75V steel heavy rail steel and the corresponding, the solidification microstructure and MnS inclusions in the cast slab and morphology characteristics and distribution in the rail, combined with thermodynamic and kinetic analysis of the formation mechanism of MnS inclusions and puts forward the control measures according to the control parameters of the optimal experimental laboratory of thermal insulation, to further enhance the quality of heavy rail steel in China to provide a theoretical basis and production guidance. Through the two-dimensional microstructure observation of heavy rail steel bloom and rail and MnS inclusions and 3D morphology, found in the slab of MnS inclusion Distribution: from the edge to the center, pure MnS morphology from spherical and spindle shaped, gradually changed into a long strip, petal shaped polyhedron and irregular shape, finally become distributed along the grain boundaries and along the dendrite loose distribution of sheet size increases: MnS in oxygen from the surface of sulfide oxide formation. At the edge of MnS completely wrapped with oxide, the slab thickness increases, the sulfide layer is thinner, most of the oxides on the surface of the slab thickness 1/4 area is MnS. In the rail in punctate inclusions, MnS inclusions generally along the rolling direction is a strip along the rail cross-section direction, inclusions generally cylindrical, oval shaped and fold, but MnS inclusions in the rail head and rail at the bottom of the more slender waist, in relatively flat. According to the electrolysis of anhydrous organic solution to obtain three-dimensional morphology of MnS complete, combined with previous research results, the morphology of points The class were modified and added: the first kind of ellipsoidal, second long rod, third sheet, fourth kinds of irregular shape or polyhedron, fifth types of dendrimers and sixth types of precipitates at the surface oxide. Point by thermodynamic calculation, MnS inclusions at 1619 K, which is close to the solidus began by precipitation. The segregation model to calculate the formation of MnS in the solid fraction is 0.94. And the enrichment degree of S is greater than Mn, the solid fraction is 0.9 when the residual liquid Mn and S respectively reached 1.427% and 0.12%, respectively is the initial mass fraction of 1.64 and 9.23 times. By adjusting Mn and S content in steel and the cooling rate, can precipitate MnS. The kinetic calculation results show that the MnS particles in the steel solidification process with homogeneous nucleation and grain boundary nucleation dominated: effective nucleation temperature of MnS is 1634 K, the sulfur content was reduced to 50 to ppm Next, can delay the MnS precipitation, MnS reduced the nucleation rate, the critical diameter and size: increase the cooling rate can also reduce the critical radius of MnS precipitation, reducing its size. Through the research of heavy rail steel billet and steel rail in the heating process of the morphological evolution of MnS inclusions, it is 1200 DEG C, MnS slow evolution; at 1300 DEG C, the holding time is more than 30 min help MnS particles: and in the 1400 C case, MnS decomposition and solution; when cooling precipitation, MnS precipitates at grain boundaries and the amount is maximum, two-dimensional observed single MnS size is small, but will gather MnS large size inclusions the formation of the slab. Specimens of high sulfur, at 1300 DEG C for about 30 min MnS shape evolution by the diffusion of Mn or S element to control reaction rate control. But for the rail in the long strip MnS, found at 1300 DEG C, the morphological evolution Change of node link in the holding time is 30~60 min. by precipitation of remelting - cooling - insulation under the condition of MnS, found that the cooling rate decreases, the morphology of MnS from nearly spherical and spindle shaped and elongated gradually to 1300 DEG C and evolution, holding 30~60 min is to change the node restriction link comparison of high sulfur ([S]=130 ppm), sulfur ([S]=81 ppm) samples, found at 1200 DEG C, with the increase of the holding time, the number density of MnS in high sulfur samples decrease, increase in sulfur in the sample; and at 1400 DEG C, with the holding time increasing, the number density of MnS in high sulfur in the sample increase, decrease in sulfur in the sample. This shows that high temperature helps to control sulfur in MnS. so the sulfur content in steel is reduced to below 81 ppm, the heating temperature and holding time on the control of MnS is more reasonable and effective. According to inclusion The actual situation of the company suggested that the sulphur content in steel is reduced to less than 40 ppm. In addition the corresponding suggestion to increase the right head slab width of the columnar zone proportion, to reduce the MnS rating.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG142.1

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