Influences of Quenching Temperature on the Microstructure Evolution and Strength−Toughness of a Novel Medium‐Carbon Ti−Mo‐Bearing Martensite Steel

Abstract

High-strength steels have attracted much attention in recent decades due to their positive roles in energy conservation, emission reduction, and cost reduction in the global setting. [1,2] Numerous methods have been adopted to improve the strength of steels. For example, microalloyed steels, individually or associatedly alloyed with Nb, V, and Ti, manifest better mechanical properties than other plain carbon steels. [3][4][5] The super-bainite steel proposed by Bhadeshia and coworkers [6,7] exhibits commendable properties due to the presence of nanoscale bainite structure and retained austenite. In addition, numerous works have been conducted on ultrafine-grain (UFG) steels consisting of microÀnanometer ferrite/austenite grains. [8][9][10][11][12][13][14] Especially, martensite steels are found to be promising due to their very high strength; however, the unsatisfactory toughness limits their commercial applications. [15,16] To improve the properties of martensite steels, secondary-hardening martensite steel, maraging steel, medium-carbon low-alloy martensite steel, and bainite/martensite duplex-phase high-strength steels have been successively proposed. [17][18][19][20][21][22][23][24] Kown et al. [17] prepared a new type of secondary-hardening martensite steel (0.24CÀ3.13WÀ3.07CrÀ14.18CoÀ9.83Ni) with a hardness of 55 HRC and room-temperature impact energy of <30 J. Mooney et al. [18] and Casalino et al. [19] both developed maraging steels with different Ti additions. The tensile strength was substantially improved to about 1200 MPa accomplished by a common failure elongation. Liu and coworkers [20] fabricated a novel ultrahigh-strength martensite steel with a strength of 1589À2446 MPa by quenching and tempering; however, the increased strength sacrificed the ductility of the steel. Multifarious schemes have been adopted to improve the toughness of martensite steels; unfortunately, the improvement of toughness is often accompanied by the sacrifice in strength.Very few studies reported the improved mechanical properties of medium-carbon martensite steel by simultaneous addition of Ti and Mo. In the present study, a novel TiÀMo-bearing martensite steel was developed by thermoÀmechanical control process (TMCP) and subsequent quenching and tempering (QÀT) process to optimize the strength and toughness of high-strength martensite steels. It is well known that the design of quenching temperature is crucial for the final service life of the low-alloy high-strength steels compared with isothermal holding time. A lower quenching temperature will result in an incomplete austenization for the martensite steel, whereas a higher quenching temperature leads to coarse austenite grains and deteriorates the combination property of the high-strength TiÀMo-bearing