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

建议的焊接参数应用于某一钢种和个别高强度钢的实际使用参数之间有很大的差异。这归因于以下事实，即目前已经利用具有微合金添加的不同合金化设计概念来产生弥散型微合金化钢。由于这种差异，即使具有相似水平的机械性能的钢也表现出不同的可焊性。然而，在焊接组织演变过程中的微合金元素的作用和机械性能的知识是不够先进的允许可靠的预测焊接微合金钢的性能。对微合金元素的添加、焊缝冶金和机械性能之间关系的基本理解应在本研究项目范围内进行。对于各种设计的微合金钢，关键是要建立一个机制，管理热影响区软化性能的理解。这一重大问题的原因是，具有严重局部软化区域的焊缝的强度和塑性低于母材。当考虑结构完整性时，机械性能的这种降低是有问题的。利用数字图像相关技术，可实现在拉伸试验中确定软化区对断裂行为的影响。通过热影响区的物理模拟，以了解不同的微合金添加剂在影响软化行为中的作用。定量分析了相变产物的成分、复合碳化物的析出及粗化效果。通过热力学和动力学模型对碳化物粗化过程进行了模拟。实验和模拟结果将进行评估，以揭示软化机制。研究结果可为微合金钢的设计提供指导，以确保焊接件具有良好的焊接性和完整性。

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.