Ultra-high strength steel (UHSS) has been widely applied in a variety of industries to satisfy lightweight requirements. For now, it is still a challenge to realize both high strength and excellent toughness in welded joints of UHSS, especially in the coarse grain heat affected zone (CGHAZ). In this research, an innovative step cooling process was proposed to control microstructures and mechanical properties in the thermal simulated CGHAZ of UHSS via thermal simulation test. The results show that the proposed step cooling process can realize the collaborative improvement of strength and toughness. With the cooling rate of 150 ^°C/s, the yield strength, the ultimate strength and the toughness are comparable to those of base metal (BM), even the total elongation exceeds that of BM. The combination of high strength and toughness of CGHAZ is primarily attributed to the synergetic effect from martensitic transformation, microstructure refinement and transformation-induced plasticity effect induced by the retained austenite.

超高强度钢 (UHSS) 已广泛应用于各个行业以满足轻量化要求。目前，UHSS焊接接头特别是粗晶热影响区（CGHAZ）的焊接接头要同时实现高强度和优异的韧性仍然是一个挑战。在这项研究中，提出了一种创新的阶梯冷却工艺，通过热模拟试验来控制 UHSS 热模拟 CGHAZ 中的微观结构和机械性能。结果表明，所提出的阶梯冷却工艺可以实现强度和韧性的协同提高。以150 ^°的冷却速度 C/s，屈服强度、极限强度和韧性与母材（BM）相当，即使总伸长率超过BM。CGHAZ高强韧性的结合主要归因于残余奥氏体引起的马氏体相变、组织细化和相变诱导塑性效应的协同作用。