Characteristics of 30MnB5 boron steel at elevated temperatures Hande Güler n , Rukiye Ertan, Reşat Özcan Uludag University, Faculty of Engineering and Architecture, Department of Mechanical Engineering, 16059 Gorukle, Bursa, Turkey article info Article history: Received 20 March 2013 Received in revised form 25 April 2013 Accepted 30 April 2013 Available online 9 May 2013 Keywords: Flow properties 30MnB5 Hot ductility Dynamic recrystallization Fractography abstract The hot ductility behavior of boron steel at high strain rates is of great importance because of its relationship with problematic brittleness observed during hot-forming processes. The proper hot-forming temperature must be established to manufacture high-quality products of complex geometries. In this investigation, the mechanical properties of 30MnB5 boron steel has been examined by hot tensile tests performed at temperatures ranging from 400 to 900 1C, with 100 1C increments, and at a strain rate of 0.083 s -1 . The deformation and fracture mechanisms under hot tensile testing were considered in relation to the testing data and structural observations performed by SEM on longitudinal specimen sections. The results indicate the minimum ductility of the tested boron steel observed as a function of deformation temperature. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Currently in the automotive industry, designing reduced weight vehicles with high safety is the major requirement to meet the needs for a very wide range of products. The critical objectives for a vehicle designer are cost saving and the reduction of gaseous emissions. For this purpose, the cross sectional areas of the vehicle parts are decreased and thinner parts from high-strength steel are used. The most popular among these steels are dual phase (DP) steels, transformation-induced plasticity (TRIP) steels, complex phase (CP) steels, boron steels (BS), martensitic steels (MART) and the more recently developed Twinning Induced Plasticity (TWIP) steels. Boron– manganese hot-rolled alloyed steel, also called Boron steels or MnB steels, are very popular because of their excellent mechanical proper- ties. The most commonly used ultra-high strength boron steels are 22MnB5, 27MnCrB5, 30MnB5 and 37MnB4. For a long time, research- ers have studied the effect of boron in steels [1, 2], particularly to increase the hardenability. Several studies clearly noted the beneficial influence of boron on hot ductility [3–6]. On the other hand, the hot- working parameters for optimum forming of 30MnB5 have not been well studied, even though metal-forming operation is an essential step in shaping materials into useful products that require not only desired shape, changing in limited dimensional accuracy, but also specific microstructures, mechanical and physical properties. The forming of the ultra-high strength steels has become a large problem because of their high strength. The metal-forming techniques consist of set of heating and shaping processes using rolling, forging and extrusion operations, through which relatively simple materials are converted into complicated shapes using a single step without fracture at elevated temperatures (namely, hot working). Therefore, the forming processes are conducted at elevated temperatures to achieve the desired shape of the steel at minimum loads. Hot working (or hot forming) involves deformation at temperatures above the recrystallization temperature [7] and results in plastic deformation of the coarse metal grains to fine grains with an increase in strength, ductility and resistance to impact and an elimination of porosity [8]. Prior to the hot-working process, the material must be subjected to testing to determine the optimum parameters for the process. The proper processing temperature and deformation rate must be estab- lished to produce high-quality products. The two characteristics that determine the forming properties of a material are its resistance to plastic flow (strength) and its ductility [9]. The percent reduction of area (RA) is a measure of the ductility and is therefore the primary result obtained from the hot tension test. This ductility measurement is used to determine the maximum allowable deformation ability of the material to withstand crack propagation. The RA measurement has the disadvantage that it is difficult to measure the final cross- sectional area of a fracture with very ductile materials [10]. In this study, a 30MnB5 boron-steel sheet was subjected to hot tensile tests to obtain a better understanding of the ductility and plasticity occurring during hot-deformation processing and char- acterize the fracture surfaces to determine the fracture mechan- isms associated with the hot-ductility process. 2. Experimental procedures The chemical composition and the mechanical properties of the hot-rolled 30MnB5 steel (wt%) with 2.5 mm sheet thickness are Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.04.116 n Corresponding author. Tel.: +90 224 2941944; fax: +90 224 2941903. E-mail address: handeguler@uludag.edu.tr (H. Güler). Materials Science & Engineering A 578 (2013) 417–421