Structure and Properties of High-Strength Low-Alloy Cold-Resistant Steel after Reheat and Direct Quenching with Temperin
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L SCIENCES. METALLURGY
Structure and Properties of High-Strength Low-Alloy ColdResistant Steel after Reheat and Direct Quenching with Tempering O. V. Sycha, *, E. I. Khlusovaa, E. A. Yashinaa, E. V. Svyatyshevaa, and E. A. Vasilievaa aNational
Research Center Kurchatov Institute—Central Research Institute of Structural Materials Prometey, St. Petersburg, 191015 Russia *e-mail: [email protected] Received November 21, 2019; revised November 26, 2019; accepted November 28, 2019
Abstract—This paper is focused on the comparative investigation of the structure of rolled plates from lowalloy cold-resistant steel 08KhN2MDB with a guaranteed yield strength of 750 MPa after traditional reheat quenching and quenching from rolling heat (direct quenching) with subsequent high-temperature tempering. The investigation has been carried out by means of optical metallography and scanning and transmission electron microscopy. The parameters of bainite-martensite structure which influence the strength level in the initial (quenched) state have been estimated. In addition, the tempering impact on the structure and properties of the rolled plates after reheat and direct quenching has been assessed. Keywords: high-strength steel, reheat quenching, direct quenching, tempering, structure, bainite, martensite, parent austenite grain, grain average misorientation (GAM), EBSD, TEM DOI: 10.1134/S2075113320060258
INTRODUCTION Recently, the interest in high-strength low-alloy steels which can significantly reduce the metal consumption of critical welded structures has increased [1–3]. However, better strength should be accompanied by high plasticity, impact strength, and crack resistance at low test temperatures. This goal is achieved by improvement of the metallurgical quality of steels and the use of special process methods to form the structure with a given ratio, morphology, and dispersion of structural components throughout the sheet metal. The common high-strength steels with a yield strength of 500 MPa or more and with a Cr–Ni–Cu– Mo alloying composition are characterized by high contents of expensive alloying elements and a long production cycle including hot rolling, separate reheat quenching (Q), and high-temperature tempering [4, 5]. The current experience in the development of high-strength cold-resistant steels testifies to the effectiveness of reducing the alloying level without degradation of the mechanical properties and performance characteristics [6, 7] by direct quenching (DRQ) with high-temperature tempering. This technology involving controlled thermal deformation and direct cooling provides a higher density of crystal structure imperfections in austenite, inherited by the final structure during the subsequent γ → α transformation, which ensures a high strength of rolled sheets [8–13].
To date, voluminous experimental data have been accumulated on the effect of reheat quenching and tempering modes on the structure and mechanical properties of high-strength steels [4–6], which is not the case of direct quenching followed by te
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