Effect of microstructure on the impact toughness transition temperature of direct-quenched steels

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING

卷:712

Article

Pallaspuro, Sakari^1,3  Kaijalainen, Antti¹  Mehtonen, Saara²  Komi, Jukka¹  Zhang, Zhiliang³  Porter, David¹ 

[1] Univ Oulu, Mat & Prod Engn, Ctr Adv Steels Res, POB 4200, Oulu 90014, Finland

[2] SSAB, POB 93, Raahe 92101, Finland

[3] Norwegian Univ Sci & Technol, Dept Struct Engn, Richard Birkelands Vei 1 A, N-7491 Trondheim, Norway

关键词: Charpy impact test;    Brittle-ductile transition;    Stress intensity factor;    Characterisation;    Grain size;    Microstructure;   

DOI  :  10.1016/j.msea.2017.12.037

来源: Elsevier

【 摘 要 】

A sufficient level of toughness at low temperatures is paramount for the use of structural steels intended for arctic applications. Therefore, it is important for the steel industry to identify the factors that control brittle fracture toughness. In this study, the quantitative effect of microstructure on the impact toughness transition temperature has been investigated with 18 different thermomechanically rolled and direct-quenched low-carbon ultra-high-strength steels with varying martensite and bainite contents. The steels were produced by altering their chemical composition, the finish rolling temperature and the total reduction of the prior austenite grains in the non-recrystallisation temperature regime, i.e. austenite pancaking, and characterised in terms of micro structural constituents, grain size distributions and texture as well as by using Charpy-V impact and tensile testing. It is shown for the first time that the impact toughness transition temperatures T-28J and T-50 closely follow a dynamic reference toughness, defined by yield strength and the size of the coarsest grains in the effective grain size distribution at 80th percentile. Decreasing the area fraction of {100} cleavage planes oriented within 15 degrees of the macroscopic fracture plane by increasing austenite pancaking is also shown to improve T-28J. The best toughness is achieved with the lowest finish rolling temperatures that are nevertheless high enough to avoid the subsequent formation of granular bainite, which weakens both the toughness and strength. The results show that it is perfectly possible to produce untempered ultra-high-strength martensitic and martensitic-bainitic structural steels with adequate low-temperature toughness when the grain size is properly controlled.

【 授权许可】

Free