Ultrahigh Charpy impact toughness (~450J) achieved in high strength ferrite/martensite laminated steels

www.nature.com/scientificreports OPEN received: 28 October 2016 accepted: 19 December 2016 Published: 02 February 2017 Ultrahigh Charpy impact toughness (~450J) achieved in high strength ferrite/martensite laminated steels Wenquan Cao1, Mingda Zhang1, Chongxiang Huang2, Shuyang Xiao3, Han Dong1 & Yuqing Weng1 Strength and toughness are a couple of paradox as similar as strength-ductility trade-off in homogenous materials, body-centered-cubic steels in particular. Here we report a simple way to get ultrahigh toughness without sacrificing strength. By simple alloying design and hot rolling the 5Mn3Al steels in ferrite/austenite dual phase temperature region, we obtain a series of ferrite/martensite laminated steels that show up-to 400–450J Charpy V-notch impact energy combined with a tensile strength as high as 1.0–1.2 GPa at room temperature, which is nearly 3–5 times higher than that of conventional low alloy steels at similar strength level. This remarkably enhanced toughness is mainly attributed to the delamination between ferrite and martensite lamellae. The current finding gives us a promising way to produce high strength steel with ultrahigh impact toughness by simple alloying design and hot rolling in industry. Toughness is an indication of the capacity of a steel to absorb energy and depends on strength as well as ductility significantly. The critical impact toughness is strongly concerned as one of the screening parameters of properties in many steel applications to ensure safety1,2, such as pipeline steel, bridge steel, car steel, and aeronautic steel, et al. With today’s situation of severe environmental pollution and natural resource limitation, the application of high strength steels is increasingly requested to get the weight lightening and safety improvement. However, the impact toughness of widely used low alloy steels (≤8 weight % alloying elements) normally decreases with increasing tensile strength2–5. For instance, the room temperature (RT) impact toughness decreases from ~300– 350J at strength level of 0.5 GPa (pipeline steel), to 50–100J at 1.0 GPa (medium carbon steel) and further to only 10–40J at 1.5 GPa (high carbon steel)6–8. Generally, the toughness of these low alloy steels is controlled by their inherent resistance of microstructures (grain size, precipitates, etc.) to crack initiation and growth, namely intrinsic toughness mechanism1,5,7. High alloy steels exhibit much higher impact toughness than low alloy steels. For example, the impact energy of high alloyed precipitated hardening steels (austenite steel and nanobainite steel) could be about 200J at strength level of 1.5 GPa and about 50J at 2.0 GPa at RT9–11, due to the existence of austenite and addition of Ni and Co. These high alloy steels are much more expensive than low alloy steels, which restricts their application and drives the research and development of low alloy steels with high strength and high toughness. In order to get this target, the dependence of toughness on the strength of low alloy steels has to be altered, i.e., to find a way to evade the dependence of toughness on the intrinsic toughening mechanism in which the impact toughness is controlled by the ductility of constitutive materials. Remarkably enhanced impact toughness has been reported in composite materials with laminated structure12–19. Laminate composite consists of different constitutive materials that are alternately separated by discrete interfaces. By roll bonding, much better impact energy than those of constitutive steels can be obtained in laminate composites15,16. Delamination between layers, a typical extrinsic toughness mechanism1,2,12, was found to play an important role in deflecting crack, imposing new cracks and absorbing energy, which was responsible for high impact toughness and high fracture resistance of laminate composites12–19. However, the conventional fabrication procedure for laminate composites is very complex, which increases the cost significantly and restricts their manufacturing efficiency in large quantity in industry. 1 Special Steel department of Central Iron and Steel Research Institute (CISRI), Beijing 100081, China. 2School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China. 3School of Materials Science and Engineering, University of Science &Technology of Beijing, Beijing 100083, China. Correspondence and requests for materials should be addressed to W.C. (email: ) or C.H. (email: ) Scientific Reports | 7:41459 | DOI: 10.1038/srep41459 1 www.nature.com/scientificreports/ Figure 1. Representative tensile curves of hot rolled FeMnAlC DP-steels. (a) Engineering stress-strain curves, (b) true stress-strain curves. Dual phase steels (DP-steels) consisting of ferrite and martensite can be easily produced in steel industry, which however give impact toughness lower than 150J at strength level of 0.5–0.9 GPa at RT20,21. It could be expected that if laminated structure with ferrite and martensite is designed in the DP steels, their impact toughness could be improved significantly without sacrificing strength very much. Normally, laminate materials are fabricated by stacking and repeatedly rolling different steels and/or other materials22,23, which are very complicated and difficult to perform in industry. In addition, the stability of ferrite phase and austenite phase in conventional DP-steels is too low to form a laminated microstructure by hot rolling. Therefore, a redesign of alloying and hot rolling technique is essential to develop laminated structure in new designed DP-steels to fulfill commercial application and scientific research. In this research, a series of low carbon and low alloy steels alloyed with Mn and Al were designed and fabricated by conventional melting, casting and hot rolling. The ferrite/martensite laminated structure was produced by hot rolling the steel in ferrite and austenite dual phase temperature region and air cooling down to RT. A remarkably enhanced Charpy V-notch impact toughness was demonstrated in these laminate DP steels, 400–450J at strength level of 1.0–1.2 GPa at RT and 400J at −40 °C, when the notch was opened in the rolling plane with direction parallel to the transverse direction. Our results demonstrate a promising and easy pathway towards a new generation of super strong and tough steels for industrial fabrication and application in large quantity. Results Materials and mechanical properties. We have designed FeMnAlC ferrite/martensite DP-steels with C of 0.05–0.2%, Mn of 5% and Al of 3–4% (weight%, Supplementary Table S1), in which Mn is added to obtain air hardening capacity and Al is to widen the dual phase temperature region. Figure 1(a) shows the engineering stressstrain curves of the laminate steels hot rolled for 70–80% thickness reduction. These steels have tensile strength of 0.81–1.48 GPa and elongation-to-fracture of 12–20%, depending on C content (Supplementary Table S2). Normally, the (...truncated)