钛镁镧复合处理钢中晶内针状铁素体与奥氏体晶界钉扎对HAZ的协同韧化机理研究

Large line energy welding is an urgent need of the current manufacturing industry for the development of welding technology. Developing steels that can be used in welding heat input >400 kJ/cm has great potential for improving construction efficiency and reducing manufacturing costs. Aiming at the problem of the decrease of toughness in heat affected zone (HAZ) during the welding process of heat input 400kJ/cm, this project intends to use complex treatment of C-Mn steel with titanium magnesium and lanthanum to obtain a large amount of active inclusions to induce the nucleation of intragranular acicular ferrite (AF) and stable pinning phases to pin austenite grain boundary (AGB). By controlling the active inclusions and pinning phases, making the formation of AF and the pinning of AGB organic synergy together to improve the low temperature impact toughness of HAZ under the large line energy welding. The research contents include: 1) Studying the generation rules of inclusions/second phase particles in the steel treated by titanium, magnesium and lanthanum, revealing the generation control mechanism of inclusions and second phase particles after the composite treatment; 2) Studying the nucleation mechanism of AF induced by active inclusions and the pinning rules of particle phases inhibit the coarsening of austenite grain; 3) Studying the synergetic toughening mechanism of AF formation and AGB pinning of the HAZ after the composite treatment, providing the basic theory for improving the low temperature impact toughness of HAZ under the condition of large line energy welding.

大线能量焊接是当前制造业对焊接工艺发展的迫切需求，开发大于400kJ/cm焊接用钢在提高施工效率和降低制造成本方面具有很好的潜力。针对400kJ/cm焊接过程中热影响区韧性降低的问题，本项目拟选用钛镁镧对C-Mn钢进行复合处理，使钢中同时产生大量诱导晶内针状铁素体形核的活性夹杂物与钉扎奥氏体晶界移动的稳定钉扎相，通过调控活性夹杂物与钉扎相使晶内针状铁素体形成与奥氏体晶界钉扎有机的协同在一起，改善大线能量焊接热影响区的低温冲击韧性。研究内容包括1）研究钛镁镧处理钢中夹杂物/第二相粒子的生成规律，揭示钛镁镧复合处理后夹杂物和第二相粒子的生成控制机制；2）研究阐明活性夹杂物诱导晶内针状铁素体形核的机理以及钉扎相抑制奥氏体晶粒粗化的作用规律；3）研究钛镁镧复合处理后晶内针状铁素体形成与奥氏体晶界钉扎的协同机制对焊接热影响区韧性的影响规律，为提高大线能量焊接下热影响区的低温冲击韧性提供基础理论依据。

针对大线能量焊接过程中热影响区（HAZ）低温冲击韧性（LTIT）降低的问题，本项目系统探究了脱氧剂-镧（La）复合处理钢中夹杂物的生成情况，明确了针状铁素体（AF）对性能的影响，探明了AF的生成条件，明晰了钛镁镧处理对HAZ的LTIT改善机制。项目研究结果对稀土工业化应用具有重要的参考意义。.研究脱氧元素与La复合作用对夹杂物的影响发现：0.021%La（质量分数，下同）能使钢中夹杂物由MnS+Mn-Si-Al-O变为La2O2S+MnS，尺寸细化、数量增加。0.028%Al-0.018%La钢夹杂物为LaAlO3、MnS和La2S3；0.001%Ca-0.028%La钢夹杂物为La-O-S-Ca和La-S-Ca，数量降低，尺寸略增大；0.001%~0.02%Ti-0.012%La处理钢Ti与La能形成复杂的复合夹杂物，数量降低，尺寸变化不大；0.001%~0.02%Ti-0.001%Mg-La钢La0.010%时，夹杂物为La-O-S-Ti-Mg+MnS，La0.068%时，夹杂物为La-O-P+La-P。La含量高的钢中夹杂物平均尺寸和数量均大于La低的钢。.研究脱氧元素与La复合处理对HAZ组织的影响发现：AF发射状铁素体能明显细化钢的组织。0.018%La能使HAZ中形成大量AF，形核核心主要是1~4μm的La2O2S-MnS。Al对形核核心具有强的改质作用，不利于AF形成，Al-La钢HAZ组织为贝氏体/魏氏体。0.001%Ca对La处理HAZ中AF影响不大，但Ca能增强La对组织的细化作用。0.001%~0.02%Ti-0.001%Mg-La钢La含量较低时HAZ中AF含量最高，形核核心为1~4μm的La-O-S-Ti-Mg+MnS，形核铁素体片数平均2.9片/核心。AF在含La夹杂物上形核主要是低错配度及锰贫乏所致。.对比研究不同处理方式HAZ性能发现：La使HAZ中形成大量AF，阻碍裂纹的扩展，显著提高HAZ的LTIT。Al会弱化La对HAZ性能的优化效果，适量Ca的作用与Al相反。Ti-Mg与适量La复合处理HAZ时LTIT明显提高。在超过400kJ/cm的热输入下，低La含量的Ti-Mg-La处理钢HAZ的LTIT优于其他处理方式，结果与项目预期一致。