高强度帘线钢中Ti(CxN1-x)夹杂析出及演化行为研究

High-hardness birttle titanium incusions cause the fracture and stratification of tire cord steel in the drawing and stranding process, and the control of titanium inclusions is one of the major challenges in the smelting process of high strength tire cord steel. Generally, titanium inclusions precipitates during the solidification of tire cord steel and would dissolve or coarsen depending on the critical size in the reheating process. If the particle size and the distribution of titanium inclusions could be controlled effectively in the solidification process, the size of larger titanium inclusions would decrease and that of the small ones would dissolve in reheating process. The above idea has great significance to improve the quality of tire cord steel..Therefore, the project proposes to research the change rules of carbon, nitrogen and titanium concentration by establishing a coupled mathematical model of microsegregation and precipitation of MnS and TiN inclusions during solidification of tire cord steel. In addition, the project plans to study the effects of initial contens of carbon, nitrogen, titanium and the solidification conditions on the precipitation time, sizes, quantities and distribution characteristics of titanium inclusions, reveal the dissolving and coarsening behavior of titanium inclusions in the reheating conditions on the critical size of dissolving or coarsening of Ti(CxN1-x) inclusions. Through the investigation into the precipitation and the evolution mechanisms of titanium inclusions during the solidification and reheating process, this project can provide a theoretical basis for the control of titanium inclusions in the smelting and rolling process.

高硬度脆性钛夹杂会造成帘线钢在拉拔和捻股过程中断丝和分层，钛夹杂物的控制是冶炼高强度帘线钢急需解决的主要难题之一。通常，钛夹杂在帘线钢的凝固过程中析出，且在再加热阶段其溶解或粗化存在临界尺寸。若能在凝固过程中控制钛夹杂析出尺寸小于临界尺寸并使之弥散分布，在再加热阶段使钛夹杂尺寸变小甚至完全溶解，将对提高帘线钢产品质量具有重要意义。为此，本项目拟建立溶质元素微观偏析与MnS、TiN析出的耦合模型，查明帘线钢凝固前沿C、N和Ti元素浓度变化规律；在此基础上，研究Ti(CxN1-x)夹杂的成分、析出时机、尺寸、数量及分布特征与钢液初始C、N、Ti含量和凝固条件的关联性；最后，研究高强度帘线钢铸坯在再加热过程中钛夹杂分解固溶和粗化机制，明确加热条件对钛夹杂溶解或粗化临界尺寸的影响规律，从而揭示帘线钢凝固和铸坯再加热过程中钛夹杂的析出及其演化机理，为实现在冶炼及轧制过程中钛夹杂的控制提供理论依据。

帘线钢是用于生产汽车轮胎子午线的高品质钢种。脆性夹杂物严重影响着帘线钢的质量，是造成帘线钢后续加工过程中断丝、分层和疲劳性能恶化的主要因素。本项目针对上述问题，开展帘线钢凝固析出钛夹杂热力学和动力学、加热过程中钛夹杂分解和固溶规律等应用基础研究。结果表明：（1）钢液凝固过程中，由于溶质和杂质元素发生选分结晶，溶质和杂质元素不断在溶液中富集而产生浓度偏析。当82A钢水凝固比大于0.95时，Ti、N元素偏析比迅速增加，特别是凝固比超过0.97以后元素偏析十分强烈。凝固冷却速度对Ti和N的凝固偏析影响不大，但明显影响钛夹杂尺寸。（2）钢中N含量对凝固析出钛夹杂尺寸的影响远大于Ti含量对析出钛夹杂尺寸的影响。通过分析在同样的钢水Ti、N含量下82A中析出的钛夹杂尺寸明显大于72A中析出的钛夹杂的尺寸，提出在目前Ti含量控制水平下，应将帘线钢82A中的N含量控制在0.0035%以下。（3）82A中，当钢水凝固比达到0.3时，在凝固前沿就会析出Ti(C0.118N0.882)，而72A中析出的碳氮化钛为Ti(C0.095N0.905)。帘线钢强度级别越高，钢液凝固析出钛夹杂就越早，析出的钛夹杂（TiCxN1-x）中碳化钛的比例越高，钛夹杂尺寸也越大。因此，为了控制过共析帘线钢中钛夹杂的尺寸，必须降低钢水中的初始N含量。（4）通过对帘线钢中钛夹杂在加热过程中分解和固溶的规律进行研究，发现当帘线钢82A铸坯中初始浓度积w(Ti)*w(N) =0.00024%时，温度超过1074℃，TiN夹杂开始分解，分解出的Ti、N元素向周边的奥氏体中不断扩散；加热温度达到1087℃时，Ti(CxN1-x)夹杂将开始溶解；在1150℃~1250℃的加热升温阶段，钛夹杂会不断溶解，钛夹杂的平均尺寸会减小。在高温加热后的缓冷阶段，固溶状态的Ti和N会重新结合析出钛夹杂。（5）将轧钢加热炉温度提高到钛夹杂分解、固溶温度以上，促使铸坯中已经存在的钛夹杂分解、扩散，特别是尺寸大于4μm的钛夹杂通过分解、固溶，减小到4μm以下，对降低帘线钢盘条钛夹杂罚分值，提高钛夹杂罚分合格率具有重要意义。