Investigation of the softening mechanism during welding in the heat-affected zone of micro-alloyed high-strength steels

适用于某种钢等级的建议焊接参数与个别高强度钢的实际使用参数之间存在很大差异。这是由于目前不同的合金化设计理念被用于添加微合金来制造不同的微合金钢。由于这种差异，即使具有相似机械性能水平的钢也表现出不同的可焊性。然而，关于微合金元素在焊缝组织演变和力学性能中的作用的知识还不够先进，无法可靠地预测焊接微合金钢的性能。对添加微量合金元素、焊接冶金和力学性能之间关系的基本理解应在本研究项目的范围内完成。对于各种设计的微合金钢，建立控制热影响区软化性能的机制是至关重要的。造成这一重大问题的原因是，具有严重局部软化区的焊缝强度和延展性低于母材。当考虑结构完整性时，机械性能的降低是有问题的。在拉伸试验中采用数字图像相关技术来确定软化区对断裂行为的影响。利用膨胀仪对热影响区进行物理模拟，了解不同微合金添加量对软化行为的影响。定量分析了相变产物的组成、复合碳化物的析出及粗化效果。利用热力学和动力学模型对碳化物粗化过程进行了模拟。将对实验和模拟结果进行评估，以揭示软化机制。预期结果将为微合金钢的设计提供指导，旨在确保焊缝的优良可焊性和完整性。

There is a great divergence between the suggested welding parameters applied to a certain steel grade and the practical use of parameters for individual high strength steels. This is ascribed to the fact that currently different alloying design concepts with micro-alloy addition have been utilized to create diversely micro-alloyed steel. As a consequence of such a difference, the steels even having similar level of mechanical properties exhibit different weldability. However, the knowledge about the role of micro-alloy elements during weld microstructure evolution and for the mechanical properties is not advanced enough for allowing reliable predictions of the performance of welded micro-alloyed steel. This fundamental comprehension of the relationship between addition of micro-alloy elements, weld metallurgy and mechanical properties shall be worked out within the scope of the present research project. For various designed micro-alloy steels, it is crucial to build an understanding of the mechanism that governs the HAZ softening properties. The reason for this significant issue is that the strength and ductility of welds with severe local softening region are lower than that of the base metal. This reduction in mechanical properties is problematic when structural integrity is considered. To determine the influence of softening zone on the fracture behavior will be realized by using digital image correlation technique during tensile test. Physical simulation of HAZ by dilatometer will be performed to understand the role of different micro-alloy addition in affecting the softening behaviour. A quantitative analysis will be given to the phase transformation products composition, complex carbides precipitation and coarsening effect. The simulation of carbides coarsening by thermodynamic and kinetic modeling will be made. Both experimental and simulation results will be evaluated to reveal the softening mechanisms. The expected results will provide guidelines for micro-alloyed steels design with an aim to ensure excellent weldability and integrity properties of weldments.