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Evaluating Strengthening and Impact Toughness Mechanisms for Ferritic and Bainitic Microstructures in Nb, Nb-Mo and Ti-Mo Microalloyed Steels
CEIT and TECNUN, University of Navarra, 20018 San Sebastian, Basque Country, Spain
Author to whom correspondence should be addressed.
Received: 10 February 2017 / Accepted: 17 February 2017 / Published: 22 February 2017
Low carbon microalloyed steels show interesting commercial possibilities by combining different “micro”-alloying elements when high strength and low temperature toughness properties are required. Depending on the elements chosen for the chemistry design, the mechanisms controlling the strengths and toughness may differ. In this paper, a detailed characterization of the microstructural features of three different microalloyed steels, Nb, Nb-Mo and Ti-Mo, is described using mainly the electron backscattered diffraction technique (EBSD) as well as transmission electron microscopy (TEM). The contribution of different strengthening mechanisms to yield strength and impact toughness is evaluated, and its relative weight is computed for different coiling temperatures. Grain refinement is shown to be the most effective mechanism for controlling both mechanical properties. As yield strength increases, the relative contribution of precipitation strengthening increases, and this factor is especially important in the Ti-Mo microalloyed steel where different combinations of interphase and random precipitation are detected depending on the coiling temperature. In addition to average grain size values, microstructural heterogeneity is considered in order to propose a new equation for predicting ductile–brittle transition temperature (DBTT). This equation considers the wide range of microstructures analyzed as well as the increase in the transition temperature related to precipitation strengthening.
microalloyed steels; niobium; molybdenum; titanium; mechanical properties; yield strength; impact toughness; modeling; microstructure; EBSD
2. Materials and Methods
In the present study, three low carbon Nb, NbMo and TiMo microalloyed steels are selected. Their chemical composition is listed in Table 1
Plane strain compression tests were performed at different simulated coiling temperatures (Tcoiling
, °C) following the thermomechanical schedule represented in Figure 1
. The plane compression specimens were reheated at 1200 °C for 5 min, followed by a multipass deformation sequence. The first two deformations (ε = 0.4) at 1100 and 1000 °C were designed in order to ensure fine recrystallized austenite. Then, the specimens were deformed at 900 °C, below the non-recrystallized temperature, to obtain a deformed austenite prior to transformation. After the last deformation, the samples were cooled down at 10 °C/s to three different coiling temperatures (700, 600 and 500 °C), where the specimens were maintained for 90 min to simulate the coiling process. Finally, the samples were cooled down slowly (1 °C/s) to room temperature.
The strain distributes heterogeneously through the section of the plane strain compression specimen, as a result of friction and specimen/tool geometry. Therefore, the specimens used for the microstructural and mechanical (tensile and Charpy specimens) characterization were obtained from the central part of the plane strain compression specimens in order to minimize strain gradients. The microstructures were characterized after etching in 2% Nital via different characterization techniques: optical microscopy (OM, LEICA DMI5000 M, Leica Microsystems, Wetzlar, Germany) and field-emission gun scanning electron microscopy (FEGSEM, JEOL JSM-7000F, JEOL Ltd., Tokyo, Japan). In order to quantify the crystallographic features, electron backscattered diffraction (EBSD) scans were performed for all samples. For that purpose, the samples were polished to 1 µm, followed by a polish with colloidal silica. Orientation imaging microscopy was carried out on the Philips XL 30CP SEM with W-filament, using TSL (TexSEM Laboratories, Salt Lake City, UT, USA) equipment. Different scan step sizes were used depending on the resolution needed, varying from 0.1 µm for high resolution scans to 0.4 µm for unit size measurements. The total scanned area was about 200 × 200 µm2. The study of the precipitation was performed using a transmission electron microscope (TEM, JEOL 2100, JEOL Ltd., Tokyo, Japan) with a voltage of 200 kV and a LaB6 thermionic filament. Carbon extraction replicas and electropolished thin foils were used for this purpose.
Cylindrical tensile specimens with a gauge length of 17 mm and a diameter of 4 mm were machined from the plane strain compression samples. The tensile tests were performed at room temperature and with a strain rate of 10−3
on an Instron testing machine (Instron, Grove City, PA, USA) under strain control. The 0.2% proof stress and the ultimate tensile strength were determined as the mean value of two tests for each condition. Additionally, Charpy sub-size specimens (~4 × 10 × 55 mm3
) were machined and Charpy tests were performed (within an interval between −120 °C and 20 °C) in a Tinius Olsen Model Impact 104 pendulum impact tester with maximum capacity of 410 J. Specimens with a thickness of 4 mm are within the range of applicability of the proportionality rule [6
are the impact energy for specimens that are 10 mm or B
mm thick, respectively. The impact transition curves that are determined consider the modified hyperbolic tangent fitting algorithm proposed by Wallin [7
]. Based on these curves, the temperature at which the sample shows a 50% ductile–brittle appearance transition temperature (DBTT) was calculated.