The Role of Retained Austenite on the Mechanical Properties of a Low Carbon 3Mn-1.5Ni Steel
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07/s11661-017-4362-9 The Minerals, Metals & Materials Society and ASM International 2017
I. JUN CHEN, WEI-NA ZHANG, ZHEN-YU LIU, and GUO-DONG WANG are with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, People’s Republic of China. Contact e-mails: [email protected], [email protected], and [email protected] Manuscript submitted April 26, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
INTRODUCTION
IN searching for modes to improve cryogenic temperature Charpy impact toughness, besides grain refinement and nickel alloying design, more studies focus on retained austenite.[1–8] The retained austenite can be introduced by additions of C, Mn, or Ni in conjunction
with quenching and partitioning or austenite reverted transformation processes.[2,9] However, the C deteriorates weldability and Charpy impact toughness at cryogenic temperatures while the cost of using Ni is high.[10] Hence, the cost-effective Mn attracts materials scientists’ attention and Fe-Mn-C martensite steels containing 3 to 10 wt pct Mn (i.e., medium Mn steels) have been developed.[2,8,11–16] Although it has been reported that martensite Fe-Mn alloys tend to show strong embrittlement at cryogenic temperatures due to intergranular fracture caused by segregation of Mn at grain boundaries,[8,16,17] fortunately, when medium Mn steels are intercritically treated, the formation of retained austenite at grain boundaries can greatly reduce grain boundary segregation of Mn.[16] Therefore, the cryogenic temperature Charpy impact toughness can be improved. Recently, several reports showed that the cryogenic temperature Charpy impact toughness of medium Mn steels can be improved significantly after intercritical heat treatment.[15,16,18–20] Hu et al.[19] compared hot-rolled and annealed medium Mn steel with and without retained austenite, showing that the cryogenic temperature Charpy impact toughness can be enhanced significantly by introducing retained austenite. The formation of retained austenite at grain boundaries can greatly weaken the effect of Mn on grain boundary cohesion.[16] Besides the grain boundary segregation of Mn reduced by retained austenite, the cryogenic Charpy impact toughness is also related to retained austenite characteristics. Han et al.[15] compared the cryogenic temperature Charpy impact toughness of hot/cold-rolled and annealed Fe-7.22Mn-0.093C-0.49Si (in wt pct) steels with lath-shaped and globular-shaped grain morphologies, showing that the steel with globular-shaped grain morphology possesses better cryogenic temperature Charpy impact toughness owing to prior austenite grain boundary break-up mechanism. Our works[18,20] indicated that the cryogenic temperature Charpy impact toughness significantly depends on stability of retained austenite in medium Mn steels with different characteristics of retained austenite. Although previous works provided insights into the effect of retained austenite on cryogenic temperature Charpy impact toughness, the dislocation density in a¢-marten
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