Effects of Cr and Mo on Mechanical Properties of Hot-Forged Medium Carbon TRIP-Aided Bainitic Ferrite Steels

metals Article Effects of Cr and Mo on Mechanical Properties of Hot-Forged Medium Carbon TRIP-Aided Bainitic Ferrite Steels Koh-ichi Sugimoto 1, * , Sho-hei Sato 2 , Junya Kobayashi 3 and Ashok Kumar Srivastava 4 1 2 3 4 * Department of Mechanical Systems Engineering, Graduate School of Science and Technology, Shinshu University, Nagano 380-8553, Japan Department of Production Engineering, Sato Press Co., Ltd., Toyota 473-0933, Japan; Department of Mechanical Engineering, Graduate School of Science and Engineering, Ibaraki University, Hitachi 316-8511, Japan; Department of Metallurgical Engineering, School of Engineering, OP Jindal University, Raigarh 496109, India; Correspondence: ; Tel.: +81-90-9667-4482 Received: 12 September 2019; Accepted: 27 September 2019; Published: 30 September 2019



 Abstract: In this study, the effects of Cr and Mo additions on mechanical properties of hot-forged medium carbon TRIP-aided bainitic ferrite (TBF) steel were investigated. If 0.5%Cr was added to the base steel with a chemical composition of 0.4%C, 1.5%Si, 1.5%Mn, 0.5%Al, and 0.05%Nb in mass%, the developed steel achieved the best combination of strength and total elongation. The best combination of strength and impact toughness was attained by multiple additions of 0.5%Cr and 0.2%Mo to the base steel. The excellent combination of strength and impact toughness substantially exceeded those of quenched and tempered JIS-SCM420 and 440 steels, although it was as high as those of 0.2%C TBF steels with 1.0%Cr and 0.2%Mo. The good impact toughness was mainly caused by uniform fine bainitic ferrite matrix structure and a large amount of metastable retained austenite. Keywords: hot-forging; microalloying; TRIP-aided bainitic ferrite steel; retained austenite; tensile property; impact toughness 1. Introduction In the past decades, first-, second-, and third-generation cold- and hot-rolled advanced high-strength steels (AHSSs) have been developed in the world [1,2]. Ferrite-martensite dual-phase steels, TRIP-aided steels with polygonal ferrite matrix structure, and complex steel are categorized as the first-generation AHSSs [1]. High-Mn twinning-induced plasticity (TWIP) steels are known as the typical second-generation AHSSs [2]. The typical third-generation AHSSs are transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) [3], bainitic ferrite/martensite (TBM) and martensite (TM) steels [4–7], quenching and partitioning (Q&P) steels [8–10], carbide-free bainitic (CFB) steels (or nano-structured bainitic steels) [11–13], and medium Mn (M-Mn) steels [14–16]. Cold rolled AHSSs of 980−1180 MPa grade with excellent cold formability have already been applied to automotive body in white and seat frame in order to reduce the weight and enhance the crush safety [17–19]. In addition, 1180 MPa hot-rolled AHSS has been successfully applied in truck cylinders for concrete mixer [20]. Low- and medium-carbon TBF, TBM, and TM steels [4–7] are produced by a similar heat treatment process to low- and medium-carbon Q&P [8–10], CFB steels [11–13], and martensite type M-Mn steels [15], except Q&P steel subjected to two step Q&P process and dual-phase type M-Mn steels [14,16]. The heat treatment consists of austenitizing and subsequent austemper or martempering. Metals 2019, 9, 1066; doi:10.3390/met9101066 www.mdpi.com/journal/metals Metals 2019, 9, 1066 2 of 13 Recently, an interesting project for weight reduction and size-down of automotive forging parts such as Powertrain components etc., “The Lightweight Forging Initiative”, was implemented in Germany [21,22]. In this project, V-bearing precipitation-hardening ferritic-pearlitic steels and bainitic steels without heat treatment after hot-forging were used on behalf of quenched and tempered martensitic steels for the weight reduction and size-down. For further weight reduction, TBF, TBM, and TM steels [23–25] are also very attractive as well as Q&P [26,27], CFB [26,28–33], and martensite type M-Mn steels [15], because their steels possess excellent mechanical properties such as tensile strength, impact toughness, fatigue strength, and delayed fracture strength. In order to develop a new hot-forged TBF steel, Sugimoto et al. [34,35] investigated the effects of hot forging in γ region and the subsequent austemper (FA) process on the microstructure and mechanical properties of TBF steels with chemical compositions of 0.4%C-1.5%Si-1.5%Mn and 0.4%C-1.5%Si-1.5%Mn-1.0%Al-0.05%Nb. They obtained the following interesting results: (1) (2) The FA process refined the microstructure and increased the volume fraction of retained austenite with a decrease in its carbon concentration. Good combination of yield strength and impact toughness was achieved when austemper was conducted at temperatures above Ms . If the TBF steels are applied to relatively large forging parts, high hardenability may be required to obtain the mixed microstructure of bainitic ferrite and metastable retained austenite. In general, hardenability of the steel is improved by the addition of alloying elements such as Cr, Mo, Ni, Mn, B, etc. However, there is no research investigating the effects of hardenability on microstructure and mechanical properties in the hot-forged medium-carbon TBF steels. In the present study, the effects of Cr and Mo additions on the microstructure and mechanical properties (such as tensile properties and impact toughness) of 0.4%C-1.5%Si-1.5%Mn-0.5%Al- 0.05%Nb TBF steels subjected to the FA process were experimentally investigated. The mechanical properties were related to the microstructural and retained austenite characteristics. In order to investigate the effects of carbon content, the mechanical properties were compared with those of hot-forged low-carbon TBF steels (0.2%C-1.5%Si-1.5%Mn-0.04%Al-0.05%Nb-(0–1.0)%Cr-(0–0.2)%Mo). In addition, the mechanical properties were compared with those of commercial JIS-SCM420 and 440 steels [36]. 2. Experimental Procedure Three 100 kg ingots of 100 mm in diameter were vacuum-melted and then hot-forged to 32 mm in diameter. Chemical compositions of the steel bars are shown in Table 1. Steel A is a base steel containing 0.40%C, 1.49%Si, 1.49%Mn, 0.49%Al, and 0.048%Nb. Al and Nb were added to the base steel to stabilize the retained austenite and refine the prior austenite grain size, respectively. Steel B was obtained by adding about 0.51% Cr to the Steel A. Steel C was produced by further adding of up to 0.20%Mo to the Steel B. For the Steels B and C, Cr and Mo were added to improve the hardenability. The martensite-start and -finish temperatures (MS and Mf ) of the steels were measured using a dilatometer (Thermecmaster-Z, Fuji Electronic Ind. Co., Osaka, Japan). The continuous cooling transformation (CCT) diagrams of Steels A–C are shown in Figure 1. To investigate the effects of carbon content, four kinds of low-carbon 1.5%Si-1.5%Mn steels with different Cr and Mo contents (Steels (...truncated)