Hardening Mechanism in Low-Carbon Low-Alloy Steels with a Simultaneous Increase in Ductility and Fracture Toughness
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ening Mechanism in Low-Carbon Low-Alloy Steels with a Simultaneous Increase in Ductility and Fracture Toughness P. V. Kuznetsov1,2*, V. E. Panin1,2,3, and N. K. Galchenko1 1
Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences, Tomsk, 634055 Russia 2 National Research Tomsk Polytechnic University, Tomsk, 634050 Russia 3 National Research Tomsk State University, Tomsk, 634050 Russia * e-mail: [email protected] Received July 29, 2019, revised September 25, 2019, accepted September 30, 2019
Abstract—The paper analyzes the nucleation and growth of bainite in low-carbon low-alloy 09Mn2Si steel doped with titanium carbonitride nanoparticles in impact toughness testing. The analysis shows that such particles segregate at low-angle boundaries, retarding the formation of high-angle ones, and when impacted into the steel, they curve the lattice and generate a new bainite phase at the curvature interstices. The mechanism of bainite nucleation and growth is sympathetic, obeys the angular momentum conservation law, and provides the formation of multilayered packets of bainite plates capable for unlimited thinning to sub-subsubunits during deformation. Such bainite plates can respond to their stress-strain state by one or another rotation, showing a high relaxation capacity and providing a high impact toughness of the steel at low temperatures. Keywords: low-carbon low-alloy steel, high impact toughness, lattice curvature, bainite, rotation DOI: 10.1134/S1029959920040098
1. INTRODUCTION By now, various technologies have been proposed to mitigate the loss of ductility in hardened structural materials. In particular, low-carbon low-alloy steels can be treated so that they can gain both strength and ductility at a time [1–7] and can display such an effect up to a temperature of −70°C [3]. One of the concepts explaining the simultaneous increase in strength and ductility is based on the notion of lattice curvature [4, 8–11]. According to this concept, the electronic subsystem of a deformed material generates nanoscale mesoscopic structural states at its interstices, allowing the material to gain an additional degree of freedom and to develop nonequilibrium martensite structures with a high relaxation capacity. If the bond force in such nonequilibrium structures grows, their transformations can serve to increase the material ductility and strength. Low-alloy pearlite steels are hardened via Fe3C cementite plates formed in their pearlite system, and at temperatures below T = −30°C, the impact toughness of this type of steel, e.g., 10Mn2VNbAl steel, de-
creases steeply (Fig. 1, curve 1). However, with a lattice curvature of a few degrees per micrometer created in the material by helical rolling at T = 850C, its impact toughness can remain high up to T = −70°C (Fig. 1, curve 2) [4]. After such treatment, the pearlite content in the steel drops but its primary bainite persists as decisive in the mechanical behavior of the steel. Here, using scanning tunneling microscopy, we analyze the morph
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