Development of Ultra High-strength TRIP-aided Forging Steels by Ausforming-austempering Process

为了开发贝氏体铁素体基超高强度TRIP辅助锻造钢（TBF钢），研究了一种新的热锻-热处理工艺对TBF钢冲击韧性、疲劳强度和氢脆性能的影响。主要研究结果如下。(1)0.2%C-1.5%Si-1.5%Mn钢在800-900℃热锻后400℃等温淬火时，获得了高韧性的小尺寸试样。复合添加0.5%Al和0.05%Nb显著改善了SCM 440钢的强韧性平衡，其强韧性比常规调质钢有较大提高。这与Al对残余奥氏体的增碳作用和Nb对组织的细化作用有关。(2)TBF基体钢退火后400℃等温淬火的疲劳强度最高。与常规马氏体钢相比，该钢具有较高的缺口疲劳强度和较低的缺口敏感性。复合添加Al、Nb、Mo使合金的疲劳性能进一步提高。(3)TBF钢具有比马氏体钢更高的延迟断裂强度。添加0.5%Al可进一步改善TBF钢的氢脆性能，但与添加Mn不同。Al稳定了残余奥氏体，从而捕获了大部分氢。另一方面，Nb有助于细化原奥氏体晶粒。

In order to develop ultra high-strength TRIP-aided forging steels with bainitic ferrite matrix (TBF steels), the effects of a new hot forging-heat-treatment process on (1) impact toughness, (2) fatigue strength and (3) hydrogen embrittlement performance of the steels were investigated. The main results are summarized as follows.(1) High toughness of a base steel (0.2%C-1.5%Si-1.5%Mn) was achieved when hot forging at 800-900℃ and subsequent austempering at 400℃ were conducted with small size specimens. Complex additions of 0.5%Al and 0.05%Nb considerably enhanced the strength-toughness balance, which was higher than that of conventional SCM440 steel quenched and tempered. This was associated with TRIP effect of retained austenite carbon-enriched by Al and uniform microstructure refined by Nb.(2) The TBF base steels exhibited the highest fatigue strength when austempered at 400℃ after annealing. Also, the steel were characterized by higher notched fatigue strength and lower notch sensitivity than the conventional martensitic steel. Complex addition of Al, Nb and Mo resulted in further good fatigue performance.(3) The TBF steels possessed higher delayed fracture strength than martensitic steel. Addition of 0.5%Al improved further the hydrogen embrittlement performance of the TBF steels, differing from addition of Mn. Al stabilized the retained austenite and consequently trapped most of hydrogen. On the other hand, Nb contributed to size down prior austenite grain.