Influences of Thermomechanical Treatment and Nb Micro-alloying on the Hardenability of Ultra-High Strength Steels
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IN the recent decades, thermomechanical processing with subsequent direct quenching (TM-DQ) has generated an attractive way to produce ultra-high-strength steels.[1–6] Their high demand arises from an improved payload-to-weight ratio for mobile crane applications. The requirements of the user and manufacturer were met through both economic and efficient production routes. The highest strength and proficient toughness as well as good weldability are realized with these steel grades.[7] This is accomplished by the systematic combination of an alloying and a rolling schedule in which the recrystallization processes are tuned to achieve a fine-grained austenitic microstructure prior to martensitic transformation. The influence of the condition of the prior austenite grain (PAG) on the properties of the emerging steel product has been investigated in depth.[8,9] The dimension of the emerging substructure
RAPHAEL ESTERL and RONALD SCHNITZER are with the Department of Materials Science, Montanuniversita¨t Leoben, Franz Josef-Str. 18, 8700 Leoben, Austria. Contact e-mail: raphael.esterl@ unileoben.ac.at MARKUS SONNLEITNER is with the voestalpine Stahl GmbH, voestalpine-Straße 1, 4020 Linz, Austria. Manuscript submitted December 14, 2018. Article published online April 29, 2019 3238—VOLUME 50A, JULY 2019
such as packets and blocks[10] is proportional to the size of the PAGs.[11] A decrease in the PAG size results in smaller martensitic constituents.[12] Consequently, the Hall–Petch correlation can also be extended on martensitic microstructures. A decrease of the size of the prior c-grain promotes a strength improvement through the smaller dimensions of martensite. The number of barriers for the movement of dislocations increases, which is followed by a rise in strength. Furthermore, rolling in the nonrecrystallization c-regime (TNR) promotes an elevated number of nucleation sites for the following c to a transformation enhancing the grain refining.[4,8,13] It has been well established that decreasing the finish rolling temperature improves both strength and toughness.[6,14–16] For a further improvement in the strength of these steels, a martensitic transformation is performed by instant quenching.[4,8,17] Therefore, alloying elements such as Mn, Cr, Mo, Si, and B come to application, which, in dissolved condition, decelerate the diffusion-controlled c to a transformation and enable full martensitic strength.[18] During cooling, Mo retards the pro-eutectoid ferrite and perlite transformation[19] and prevents the Nb-precipitation to Nb(CN).[20] Si, Mn, and Cr decelerate the perlite and bainite transformations through different mechanisms. The role of Si is based on its inhibition to form carbides due to its low solubility in cementite.[18] The influence of Cr on the hardenability is less pronounced than the effect of Mn, as Cr requires higher additions to decrease the critical METALLURGICAL AND MATERIALS TRANSACTIONS A
cooling rate,[21] yet, like Mo, it counteracts the softening during tempering.[18,22] The strongest impact on th
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