527 ISIJ International, Vol. 61 (2021), No. 2, pp. 527–536 https://doi.org/10.2355/isijinternational.ISIJINT-2020-382 * Corresponding author: E-mail: [email protected] © 2021 The Iron and Steel Institute of Japan. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs license (https://creativecommons.org/licenses/by-nc-nd/4.0/). Deformation-induced Martensite Transformation Behavior during Tensile and Compressive Deformation in Low-alloy TRIP Steel Sheets Hiroyuki KAWATA, 1) * Takashi YASUTOMI, 1) Satoshi SHIRAKAMI, 2) Kohki NAKAMURA 1) and Eisaku SAKURADA 1) 1) Steel Research Laboratories, Nippon Steel Corporation, 20-1 Shintomi, Futtsu, Chiba, 293-8511 Japan. 2) East Nippon Works, Nippon Steel Corporation, 1 Kimitsu, Kimitsu, Chiba, 299-1141 Japan. (Received on June 26, 2020; accepted on August 31, 2020) Transformation-induced plasticity (TRIP) is a phenomenon that improves the deformability of high- strength steel. TRIP depends on deformation-induced martensite transformation behavior. To clarify the mechanism of the transformation in low-alloy TRIP steel, we evaluated the transformation behavior via in-plane tension and compression experiments. During tensile and compressive deformation, the volume fraction of austenite (V γ ) decreased as strain and stress increased. The rate at which V γ decreased during compressive deformation was slower than that during tensile deformation. However, after continuous deformation (i.e., tensile deformation under compression and vice versa), V γ depended on stress, not strain. The transformation behavior was controlled by the applied stress, regardless of strain path and stored strain. It is appropriate to apply a stress-dominant strain-induced transformation model to explain this macroscopic transformation behavior. KEY WORDS: TRIP steel; deformation-induced transformation; retained austenite. 1. Introduction High-strength steels that contain multiple phase structures are widely used. However, the applications for high-strength materials are limited owing to their poor formability. The transformation induced plasticity (TRIP) 1) is a phenomenon that counteracts this problem in steel. 2) Metastable austenite (γ ) in steel transforms to hard martensite with deformation, and this transformation increases the macroscopic work- hardening rate of steel, which dominates its uniform elon- gation (UEL). 3) Zackey et al. 4) produced high-performance steel containing martensite with metastable γ, which they called “TRIP steel.” Although the balance between strength and elongation in TRIP steel is excellent (e.g., 1.3 GPa max- imum tensile strength with 75% total elongation), using it is rarely economically feasible because it contains expensive alloying elements (Ni, Mo, and Cr) and the manufacturing process is difficult. Sakuma et al. 5,6) reported the fabrication of CSiMn simple composition steel sheets that contained a small volume fraction of metastable retained γ. This steel is called low-alloy TRIP steel, 7) and it has been primarily used to produce automotive steel sheets 8,9) because its per- formance and manufacturing cost are balanced. Recently, so-called “3rd generation steels” 10) are attracting attention as the next advanced high-strength steel. Representative 3rd generation steels, including quench and partition- ing steel, 11,12) medium manganese steel, 13,14) carbide-free bainitic steel, 15,16) and TRIP-aided annealed martensitic steel, 17) include metastable γ and utilize the TRIP effect to improve performance. 18) Therefore, it is important to under- stand the transformation behavior of metastable γ during deformation in such steels. Many studies on the effects of deformation on transfor- mation have been conducted. 2) In 1932, Scheil 19) found that the amount of deformation-induced martensite generated at temperatures higher than that of the initiation of martensite transformation, M s , decreases with increasing deformation temperature in high-Ni ferrous alloy. Therefore, no mar- tensite forms above a specific deformation temperature, M d . Scheil suggested that a critical resolved shear stress is required to initiate the transformation from γ to martensite. Olson and Cohen 20) classified the deformation-induced martensitic nucleation from γ into stress-assisted and strain- induced nucleation. A schematic diagram of their model is shown in Fig. 1(a) for FeNiC alloys studied by Bolling and Richman. 21–24) This diagram shows the critical stress Advances in TRIP Effect Research
Deformation-induced Martensite Transformation Behavior during Tensile and Compressive Deformation in Low-alloy TRIP Steel Sheets
Jun 29, 2023
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