Determining role of heterogeneous microstructure in lowering yield ratio and enhancing impact toughness in high-strength low-alloy steel

International Journal of Minerals, Metallurgy and Materials Volume 28, Number 5, May 2021, Page 816 https://doi.org/10.1007/s12613-020-2235-5 Determining role of heterogeneous microstructure in lowering yield ratio and enhancing impact toughness in high-strength low-alloy steel Yi-shuang Yu1), Bin Hu1), Min-liang Gao1), Zhen-jia Xie1), Xue-quan Rong1), Gang Han1,2), Hui Guo1), and Cheng-jia Shang1,3) 1) Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China 2) Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China 3) State Key Laboratory of Metal Material for Marine Equipment and Application, Anshan 114021, China (Received: 22 October 2020; revised: 29 November 2020; accepted: 2 December 2020) Abstract: Here we present a novel approach of intercritical heat treatment for microstructure tailoring, in which intercritical annealing is introduced between conventional quenching and tempering. This induced a heterogeneous microstructure consisting of soft intercritical ferrite and hard tempered martensite, resulting in a low yield ratio (YR) and high impact toughness in a high-strength low-alloy steel. The initial yielding and subsequent work hardening behavior of the steel during tensile deformation were modified by the presence of soft intercritical ferrite after intercritical annealing, in comparison to the steel with full martensitic microstructure. The increase in YR was related to the reduction in hardness difference between the soft and hard phases due to the precipitation of nano-carbides and the recovery of dislocations during tempering. The excellent low-temperature toughness was ascribed not only to the decrease in probability of microcrack initiation for the reduction of hardness difference between two phases, but also to the increase in resistance of microcrack propagation caused by the high density of high angle grain boundaries. Keywords: heterogeneous microstructure; yield ratio; impact toughness; intercritical heat treatment; high-strength low-alloy steel 1. Introduction High-strength low-alloy (HSLA) steels with yield strength (YS) >690 MPa are in great demand in shipbuilding, construction, bridges, and offshore structures owing to their exceptional combination of strength and toughness, low yield ratio (YR) (ratio of YS to tensile strength), excellent weather resistance, and good weldability [1–3]. To achieve superior comprehensive properties, alloy design is very important. For example, low carbon content (<0.10wt%) is used for ensuring good weldability, Cr, Mo, and Ni for improving hardenability, microalloying elements (Nb, V, and Ti) for thermomechanical processing, and Cu for corrosion resistance and precipitation strengthening [4–5]. Traditionally, quenching and tempering (Q&T) processing is used for producing HSLA steels. Quenching is used to obtain the martensitic or bainitic microstructure for increasing the strength [6–7] while tempering improves ductility and toughness. However, the YR of HSLA steels by Q&T processing is usually higher than 0.90 [3]. How to decrease the YR became an increasCorresponding authors: Hui Guo E-mail: ; © University of Science and Technology Beijing 2021 ingly important issue for HSLA steels with a minimum YS of 690 MPa. Creating a heterogeneous microstructure in HSLA steels has proven to be a new method of addressing the above problem [8–11]. Heterogeneous microstructure has a significant difference in strength among different domains, generally containing a hard martensite phase embedded in the soft austenite and/or ferrite matrix [12–13], which can be fabricated by intercritical annealing [14–15]. Compared with conventional Q&T processing, intercritical annealing can markedly increase the ductility and toughness and lower the YR of low carbon low alloy steels. However, the excellent ductility and toughness comes at a loss of strength, which results from the presence of stable retained austenite induced by two-step intercritical heat treatment (intercritical annealing plus intercritical tempering) [16]. Hence, it is still a challenge to simultaneously achieve a superior balance of strength and toughness and a low YR in steels with low carbon and lean alloy. To ensure that HSLA steels possess high strength and toughness while sustaining a low YR, a new microstructure Cheng-jia Shang E-mail: Y.S. Yu et al., Determining role of heterogeneous microstructure in lowering yield ratio and enhancing ... design approach, introducing intercritical annealing between the conventional Q&T processing, is proposed. Through this novel method, a heterogeneous microstructure with intercritical ferrite and tempered martensite can be achieved [17]. The microstructure would be much different compared with that in steel with two-step intercritical heat treatment for the absence of retained austenite, which means that the YR and toughening mechanism may be different. However, the effect of a heterogeneous microstructure without retained austenite on YR and toughness in HSLA steel is poorly understood to date, and requires further investigation. This study aims to explore the significance of heterogeneous microstructure, namely intercritical ferrite and martensite, on the mechanical properties of HSLA steel. To clarify the YR variation and toughening mechanism of this steel, full martensitic microstructure was also adopted for comparison with heterogeneous microstructure. 817 mental steel was 812°C. Between 550 and 812°C, the volume fraction of austenite increases with temperature. At 762°C, 50vol% austenite (fcc) and 50vol% ferrite (bcc) can be obtained after holding for a long time. In this work, this temperature was selected as the temperature for IA. In addition, for comparison with heterogeneous microstructure, a full martensitic structure prepared by QT was also employed in the present study. Schematics of the two heat treatments are shown in Fig. 2. After austenitizing at 900°C for 60 min, the samples were quenched in water to obtain full martensitic structure (designated as Q), and then divided into two groups. One group was tempered at 450°C for 30 min (termed as QT), the other was intercritically annealed at 760°C for 30 min and then tempered at 450°C for 30 min (referred to as QIA and QIAT, respectively). 1.0 Volume fraction of phases 2. Experimental The chemical composition of the experimental steel is shown in Table 1. The steel was vacuum melted and cast into a 25 kg ingot. The ingot was forged and cut into blocks of ~80 mm thickness. The blocks were then soaked at 1200°C for 120 min and hot-rolled to ~12 mm thick strips, followed by air cooling to room temperature. Table 1. Chemical composition of experimental steel Temperature Temperature A3 (762, 0.5) 0.4 0.2 812°C 0 550 600 650 700 750 800 Temperature / °C 900°C for 60 min A3 760°C for 30 min A1 A1 450°C for 30 min T AC ( (...truncated)