Understanding and Controlling Transformations in the HAZ of 3G AHSS

政府间气候变化专门委员会报告说，到2055年，温室气体排放必须减少到净零，以将全球变暖限制在比工业化前水平高1.5°C的范围内。汽车制造商计划通过在其车辆中使用极限拉伸强度（UTS）大于1 GPa的超高强度钢（UHSS）来减少车辆的温室气体排放。UHSS允许将更薄的钢材用于零件，减少零件重量，同时保持零件强度。然而，当UHSS进行电阻点焊（RSW）时，其复杂的成分和显微组织会导致焊缝附近的硬度峰值、软化和液态金属脆化（LME）裂纹。 这些现象还没有被完全理解，但是，众所周知，热历史会影响焊缝硬度和LME。拟议的CRD项目将更好地了解在UHSS的RSW过程中发生的转变和LME的形成。 这些知识将用于修改RSW工艺，以最大限度地提高接头性能并最大限度地减少LME开裂。 为了促进这一点，具有980 MPa和1180 MPa UTS的可焊性钢。将对具有双相（DP）微观结构的在用钢和最先进的淬火和分配（QP）钢进行比较，从而了解基础材料强度和微观结构如何影响焊接和LME期间的转变。该项目将通过观察到的结构和机械性能的变化来表征热影响区（HAZ）的转变。 还将表征这些钢的LME敏感性。将绘制LME敏感性与焊接参数的关系图，并对裂纹形成进行现场观察。 了解热历史如何影响HAZ转变和LME将用于优化焊接工艺，以最大限度地提高焊后质量，为预测焊后性能和开裂严重程度的模型提供信息。 这些结果将使加拿大受益，因为新的UHSS钢将越来越多地用于汽车，从而为加拿大钢铁制造商制造这些高价值钢创造需求，这些高价值钢将用于加拿大汽车生产。

The Intergovernmental Panel on Climate Change reported that greenhouse gas (GHG) emissions must decrease to net-zero by 2055 to limit global warming to 1.5°C above pre-industrial levels. Automakers plan to decrease the GHG emissions of their vehicles by using ultra high strength steels (UHSS), with ultimate tensile strength (UTS) greater than 1 GPa, in their vehicles. UHSS allows thinner steels to be used for parts, reducing part weight, while retaining part strength. However, when UHSS are resistance spot welded (RSW), their complex composition and microstructure can lead to hardness peaks, softening, and liquid metal embrittlement (LME) cracks near the weld. These phenomena are not fully understood, but, it is known that thermal history will affect both weld hardness and LME.The proposed CRD project will better understand transformations and LME formation occurring during RSW of UHSS. This knowledge will be used to modify the RSW process to maximize joint performance and minimize LME cracking. To facilitate this, the weldability steels with UTS of 980 MPa and 1180 MPa. In-use steels with dual-phase (DP) microstructure and state-of-the-art Quench & Partition (QP) steels will be compared, enabling an understanding of how base material strength and microstructure affect transformations during welding and LME. The project will characterize transformations in the heat affected zone (HAZ) through changes in observed structures and mechanical properties. LME sensitivity in these steels will also be characterized. LME sensitivity will be mapped as with respect to welding parameters and in-situ observations of crack formation will be made. Understanding how thermal history affects HAZ transformations and LME will be used to optimize the welding process to maximize post-weld quality, informing a model to predict post-weld properties and cracking severity. These results will benefit Canada by increasing adoption of new UHSS steels into vehicles, creating a demand for Canadian steelmakers to make these high value steels, that will be used in Canadian automotive production.