Understanding and Controlling Transformations in the HAZ of 3G AHSS

政府间气候变化专门委员会报告称，到2055年，温室气体(GHG)排放量必须减少到净零，才能将全球变暖控制在比工业化前水平高1.5摄氏度的范围内。汽车制造商计划通过在汽车中使用极限抗拉强度(UTS)大于1 GPA的超高强度钢(UHSS)来减少汽车的温室气体排放。UHSS允许零件使用更薄的钢，减轻零件重量，同时保持零件强度。然而，当超高强度钢进行点焊时，其复杂的成分和组织会导致焊缝附近出现硬度峰值、软化和液态金属脆化(LME)裂纹。这些现象尚不完全清楚，但已知热历史将同时影响焊缝硬度和LME。建议的CRD项目将更好地了解超高强度钢在RSW过程中发生的相变和LME形成。这些知识将被用来修改RSW工艺，以最大限度地提高联合性能，并将LME裂解降至最低。为了促进这一点，焊接性钢的UTS为980兆帕和1180兆帕。将对具有双相(DP)组织的在用钢和最先进的淬火和隔断(QP)钢进行比较，以了解母材强度和组织如何影响焊接和LME过程中的转变。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)的变化。还将对这些钢材的LME敏感性进行表征。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，LME将用于优化焊接工艺，以最大限度地提高焊后质量，为预测焊后性能和裂纹严重程度提供模型。这些结果将使加拿大受益，因为车辆中越来越多地采用新的UHSS钢，这将创造对加拿大钢铁制造商的需求，使其生产这些将用于加拿大汽车生产的高价值钢。

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.