Fast Salt Bath Heat Treatment for a Bainitic/Martensitic Low-Carbon Low-Alloyed Steel
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INTRODUCTION
BAINITE containing dual-phase (DP) steels have shown exceptional mechanical properties in addition to reduced cost. The microstructure of these steels exhibits great ductility, toughness and high strength.[1–4] These properties have been utilized in the automotive industry for decades. DP microstructure can be obtained by controlling the heat treatment. The more general ferrite-martensite DP steel is obtained when austenite is transformed to ferrite in the A1 to A3 intercritical temperature region, followed by transformation from the remaining austenite to martensite by rapid cooling.[5] Bainitic/martensitic steels have been studied by many researchers,[2,6,7] but low-carbon low-alloyed steels have not been investigated. The transformation from austenite to bainite is mainly focused on bainitic steels and other structural steels which contain more than 0.1 wt pct carbon and other alloying elements. In previous studies made on multiphase steels, accomplishing these microstructures required long heat treatment times,[8,9] as the general way to obtain bainitic/martensitic microstructure is to quench austenitized steel into the bainite transformation region for sufficient time followed by cooling to room temperature in air. In order to relieve the residual stresses and promote transformation of remaining austenite to bainite, tempering the specimens at the bainite forming temperature area for 1 to 2 hours is required.[2] It is of interest to see, whether shorter time could result in favorable mechanical properties. According to Pinto da Silva et al.,[10] austenite decomposes and rapidly transforms to bainite within JULIA URBANEC, Ph.D. Student, SEPPO KIVIVUORI, Professor Emeritus, and SEPPO LOUHENKILPI, Professor, are with the Department of Materials Science and Engineering, Aalto University, Vuorimiehentie 2, 02150 Espoo, Finland. ARI SAASTAMOINEN, Ph.D. Student is with the Department of Mechanical Engineering, University of Oulu, Erkki Koiso-Kanttilan katu 2S, 90570 Oulu, Finland. Contact e-mail: Julia.urbanec@aalto.fi Manuscript submitted March 2, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
the first minute of isothermal holding, thus supporting improved properties after short holding time. The goal of this research was to study and find a new heat treatment concept for low-carbon low-alloy steel (Table I) to produce good strength and total elongation with satisfactory toughness using salt bath holding. Conventionally, this steel is manufactured for hot forging products with direct quenching. This concept leaves the microstructure in the fully martensitic state, which has excellent strength but limited toughness. A new heat treatment concept is proposed in this paper with obtained material properties.
II.
MATERIALS AND EXPERIMENTAL PROCEDURE
The steel used in this research was obtained by a continuous casting process. Imaform is specifically tailored for the automotive industry and it is used mainly for forging processes, especially for hot forming. It has advantageous mechanical properties, fatigue
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