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

为了研制超高强度贝氏体铁素体基trip辅助锻造钢（TBF钢），研究了一种新型热锻热处理工艺对钢(1)冲击韧性、(2)疲劳强度和(3)氢脆性能的影响。主要研究结果总结如下：(1)对基体钢（0.2%C-1.5%Si-1.5%Mn）进行800 ~ 900℃热锻和400℃等温回火，获得了较高的韧性。复合添加0.5%Al和0.05%Nb显著提高了钢的强度-韧性平衡，优于常规调质后的SCM440钢。这与Al富集碳的残余奥氏体和Nb细化的均匀组织的TRIP效应有关。(2) TBF基材在退火后400℃等温回火时的疲劳强度最高。与传统马氏体钢相比，该钢具有较高的缺口疲劳强度和较低的缺口灵敏度。Al、Nb和Mo的复合添加进一步提高了材料的疲劳性能。(3) TBF钢比马氏体钢具有更高的延迟断裂强度。添加0.5%Al能进一步改善TBF钢的氢脆性能，但与添加Mn不同。Al稳定了残余的奥氏体，从而捕获了大部分氢。另一方面，Nb有助于减小先前奥氏体晶粒。

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.