弱脱氧高碳钢中非氧化物夹杂的凝固析出及其在热加工过程中的固溶-再析出行为

Using Si/Mn killed processes for high-speed heavy rail steel and high carbon wire cord steel is to avoid the precipitation of nondeformable brittle inclusions in molten steel. During the solidification of molten steel, the large amount of big MnS particles precipitated from heavy rail steel and the inclusions of titanium carbonitride precipitated from the wire cord steel have a significant impact on the qualities of the products after hot-working. This research intends to add Ti or Ti-Mg into molten heavy rail steel to change the thermodynamic conditions of sulfide heterogeneous nucleation, so as to decrease sulfide inclusions particles and reduce the deformation ability of sulfide inclusions. In order to avoid precipitating titanium inclusions larger than 4μm during the solidification of SWRH92A, the factors influencing the nucleation of titanium inclusions and its precipitation and growth up need to be understand, the behavior of titanium inclusions decomposition and dissolution in the subsequent hot-working process need to be study. The research contents include: (1) The behavior of solutes segregation in solid-liquid phase region precipitated with coupled oxide, sulfide and titanium inclusions and its influence on the size of precipitated inclusions at the condition of Si/Mn killed high carbon steel. (2) The behavior of nucleation and growth up of sulfide inclusions during solidification of high speed heavy-rail steel after using Ti or Ti-Mg composite treatment. (3) The nucleation, precipitation and growth of titanium inclusions in 92A, and its dissolution and reprecipitation behavior in the subsequent hot-working process. This will provide theoretical guidance for product quality control and even product upgrade of high speed heavy-rail steel and extra high strength wire cord steel.

高速重轨钢和高碳帘线钢采用弱脱氧工艺冶炼是为了避免钢液中析出不变形夹杂物。钢液凝固过程中重轨钢中析出的数量众多的大型MnS颗粒和帘线钢中析出的碳氮化钛夹杂分别对这两个钢种轧后产品的质量产生重要影响。本课题拟采用在重轨钢中加Ti或Ti-Mg复合处理来改变硫化物夹杂的性质及其异相形核析出的热力学条件，达到细化硫化物夹杂和降低其变形能力的目的。对92A级帘线钢，为避免凝固析出＞4μm的钛夹杂，需要解明钛夹杂形核和析出长大的规律，探索钛夹杂在热加工过程中的分解及固溶行为。主要研究内容:(1)耦合氧化物、硫化物及钛夹杂析出的弱脱氧高碳钢固液两相区溶质偏析行为及其对析出夹杂尺寸的影响。(2)用Ti和Ti-Mg复合处理高速重轨钢，凝固过程中硫化物夹杂的形核和长大行为。(3)92A帘线钢中钛夹杂形核和析出长大行为，以及热加工过程中钛夹杂的固溶和再析出行为。为重轨钢和帘线钢质量控制乃至产品升级提供理论指导。

重轨钢U75V因MnS夹杂超声探伤超标，帘线钢SWRH92A因钛夹杂（TiN、 Ti(CxN1-x)）影响拉拔性能，这两类夹杂都是在钢液凝固过程中因溶质元素在枝晶间偏析不断富集，当夹杂物形成元素的实际浓度积大于平衡浓度积时，便为在凝固前沿不断析出长大。本项目针对U75V和SWRH92A两个典型钢种开展非氧化物夹杂析出与控制基础研究，为工业生产提供理论指导。项目系统分析了非氧化物夹杂的析出热力学、长大动力学以及钢材热加工过程中的固溶行为。采用FactSage 7.0、溶质元素微观偏析和夹杂物长大模型、Ti/Ti-Mg复合处理等对钢中的非氧化物夹杂问题展开研究。针对U75V重轨钢，研究发现：(1) MnS夹杂在钢液凝固达92.63%时开始析出；析出之后，Mn浓度缓慢上升，S浓度迅速下降。(2) 冷却越快，MnS析出越早，但尺寸越小。(3) Ti处理后钢中主要为TiOx-MnS复合夹杂，Ti含量越高，夹杂尺寸越小。Ti-Mg复合处理后钢中主要为MnS-MgO和MnS-TiN-MgO复合夹杂；Mg含量增加，夹杂尺寸减小。提高钢坯加热温度和延长保温时间有利于MnS的细化和球化。针对SWRH 92A帘线钢，研究发现，(1) 凝固开始后Ti(CxN1-x)先于TiN析出，析出之后，N浓度上升，Ti浓度下降。凝固过程中TiN的异相形核功小于均相形核功，异相形核半径大于均相形核半径。(2)凝固阶段溶解氧同样会发生偏析行为析出Al2O3，尺寸较小的Al2O3易被推至凝固前沿成为TiN的析出核心并形成Al2O3-TiN复合夹杂。TixOy在钢液凝固之前会优先形成，之后TiN将以TixOy为核心析出，MnS将以TiN为核心析出，最终形成TiN-TixOy-MnS复合夹杂。(3)TiN、Ti(CxN1-x)的固溶温度分别为1399 K和1416 K；热处理阶段Ti夹杂形貌会发生变化并进行固溶，大尺寸夹杂减少，小尺寸夹杂增多。钢样冷却至固溶温度以下时，固溶于钢中的Ti、N会以均质形核或异质形核方式再次析出；冷却速率越小，再析出Ti夹杂总数越大，大型夹杂占比增加。在1273~1423 K范围内Ti夹杂表面有低熔点氧化物生成；1523 K保温过程中Ti(CxN1-x)和低熔点氧化物均会溶解；Ti(CxN1-x)从棱角开始直至全部消融，形态从立方体逐渐转变成圆润类球形。

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