Materials Science and Engineering A 517 (2009) 212–218 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Effects of Cu and B addition on microstructure and mechanical properties of high-strength bainitic steels Sang Yong Shin a , Seung Youb Han a , Byoungchul Hwang b , Chang Gil Lee b , Sunghak Lee a,∗ a Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea b Ferrous Alloys Research Group, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea article info Article history: Received 23 November 2008 Received in revised form 18 March 2009 Accepted 23 March 2009 Keywords: Bainitic steel Tensile properties Charpy impact property Effective grain size abstract Effects of Cu and B addition on microstructure and mechanical properties of high-strength bainitic steels were investigated in this study. Six kinds of steels were fabricated by controlling the amount of Cu and B addition, and their microstructures and tensile and Charpy impact properties were investigated. Their effective grain sizes were also characterized by the electron back-scatter diffraction analysis. The tensile test results indicated that the B- or Cu-containing steels had the higher yield and tensile strengths than the B- or Cu-free steels because their volume fractions of acicular ferrite and martensite were quite high. The B- or Cu-free steels had the higher upper shelf energy than the B- or Cu-containing steels because of their lower volume fraction of martensite. In the steel containing 10ppm B without Cu, the best combination of high strengths, high upper shelf energy, and low energy transition temperature could be obtained by the decrease in effective grain size due to the presence of acicular ferrite having fine effective grain size. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Demands for new materials development, which can meet with the requirements of environmental-friendliness and more excel- lent mechanical properties, have been emphasized. To satisfy all requirements, active studies to develop structural steels and to enhance their functionality by materializing many properties from different microstructural combinations have been conducted [1–4]. Conventional alloying methods pose the problem of deteri- orating toughness of structural steels as strength is improved. Recently, however, efforts have been made to come up with a nice combination of high strength and toughness by promoting fine low-temperature transformation microstructures such as bainite at rapid cooling rates, together with low alloying methods [5–7]. For better formation of bainitic low-temperature transformation microstructures, it is necessary to stabilize austenite during cooling. It is known that Cu and B, as austenite stabilizing elements, well pro- mote the formation of bainite even though the cooling rate is slowed down [8–12]. By varying the alloying compositions, the kind and volume fraction of bainitic structure are changed, and mechanical properties consequently vary. Therefore, the systematical analy- sis of the correlation between microstructures and properties vs. alloying compositions is needed. ∗ Corresponding author. Tel.: +82 54 279 2140; fax: +82 54 279 2399. E-mail address: shlee@postech.ac.kr (S. Lee). In the present study, high-strength bainitic steel plates were fab- ricated by controlling the addition of copper and boron, and effects of their microstructure on tensile properties and absorbed energy and transition temperature obtained from the Charpy V-notch impact test were investigated. In particular, effective grain sizes of various microstructures were analyzed by electron back-scatter diffraction (EBSD) method to identify factors affecting fracture properties of high-strength bainitic steels. 2. Experimental Six kinds of high-strength bainitic steel plates having different compositions as listed in Table 1 were fabricated by a vacuum induc- tion melting method. The basic chemical composition the steels was Fe–0.07C–0.25Si–1.8Mn–0.3Ni–0.2Cr–0.2Mo–0.03Nb–0.06V– 0.015Ti (wt.%). 0.5–1.5wt.% Cu and/or 10ppm of B were added to fabricate five kinds of steels as shown in Table 1. For convenience, the steels containing 0, 0.5, and 1.5wt.% of copper are referred to as ‘0Cu’, ‘0.5Cu’, and ‘1.5Cu’, respectively, and the steels addition- ally containing a small amount of boron are referred to as ‘0Cu–B’, ‘0.5Cu–B’, and ‘1.5Cu–B’, respectively. An overall grain refinement effect was expected by rolling with a high rolling reduction ratio (over 60%) in the non-recrystallized region of austenite after austen- itization at 1150 ◦ C [13–16]. Rolling was finished at the temperature of austenite single phase region above Ar 3 . After the finish rolling, the steels were rapidly cooled from 800 ◦ C down to finish cooling temperatures of 350 ◦ C at a cooling rate of 20 ◦ Cs -1 . The final plate thickness was 15 mm. 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.03.052