... Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015. Edited by: Matthias Militzer, Gianluigi Botton, Long-Qing Chen, James Howe, ...
Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015 Edited by: Matthias Militzer, Gianluigi Botton, Long-Qing Chen, James Howe, Chadwick Sinclair, and Hatem Zurob
MICROSTRUCTURE AND HARDNESS EVOLUTION DURING SIMULATED COILING OF A DIRECT STRIP CAST LOW CARBON LOW NIOBIUM STEEL Thomas Dorin1, Peter Hodgson1, Nicole Stanford1 1
Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia Keywords: strip casting, low alloy steel, precipitation, age-hardening Abstract
This paper examines the impact of coiling temperature and duration on the phase transformation and precipitation behavior of a low carbon and low niobium direct strip cast steel. Coiling was performed at three carefully chosen temperatures: (1) in the ferrite (600 °C), (2) during the austenite decomposition (700 °C) and (3) in the austenite (850 °C). The coiling conditions were found to strongly affect the final microstructure and hardness response, thus highlighting the necessity to judiciously design the coiling treatment. Optical microscopy, and scanning and transmission electron microscopy were used to characterize the microstructural constituents (polygonal ferrite, bainite and pearlite) and the NbC precipitates. Vickers macrohardness measurements are utilized to quantify the mechanical properties. The differences in hardening kinetics for the three different temperatures are shown to come from a complex combination of strengthening contributions. 1. Introduction The use of rapid solidification techniques, such as direct strip casting (DSC), has spread significantly in the steel industry in the past 10 years [1, 2, 3]. The DSC process is extremly energy-efficient and consists of two main stages: (1) rapid solidification of liquid steel by contact with rotating Cu-rolls, and (2) a moderate temperature heat treatment of the steel which results from coiling of the sheet at the end of the process. Steps (1) and (2) are carried out continuously with no re-heating. Although the effect of coiling temperature on conventionally hot rolled sheet is well studied [4], very little is known about the effects of coiling on DSC steel sheet. After the sheet exits the casting rolls it cools rapidly, and it has been shown that this cooling is sufficiently high that the steel is supersaturated in elements such as Nb [5, 6]. Most laboratory scale experiments do not investigate the coiling phase, and there is little information in the literature pertaining to the effect of the coiling temperature on the microstructral development. In the case of high-strength low alloy steels, the coiling temperature will not only affect the microstructral development during the austenite decomposition, but may also influence the precipitate distribution and related bulk mechanical properties. It is thus necessary to design an experimental set-up that allows simulation of coiling on a laboratory scale in order to develop an understanding of the precipitation kinetics, the microstructural evolution and related strength increment in steels that are produced by DSC. In this paper, a common high strength low carbon, low niobium steel grade was chosen for study. The NbC precipitates act as one of the main strengthening contributors in these alloys, and their formation has been reported to occur both in the austenite and in the ferrite [7, 8]. In the present study, the continuous cooling diagram for this alloy, established in previous work [6], is used to select suitable coiling temperatures. An experimental set-up was designed to simulate DSC and coiling directly from the melt with no need for further re-heating. Optical
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microscopy, scanning electron microscopy and transmission electron microscopy were used to study the microstructure at different coiling stages. The Vickers hardness evolution during coiling was followed by performing macro-hardness measurements. The significant differences in hardening kinetics, as a function of coiling temperature, are explained in terms of the strengthening contributions from the microstructural features in these types of steels. 2. Materials and techniques The composition of the material investigated is reported in Table 1. Samples were sectioned and metallographically prepared using standard cutting and grinding methods. The final polishing step was at least 5 minutes using colloidal silica. For optical microscopy, samples were etched in a solution of 5% nitric acid in ethanol, but no etching was required for electron microscopy. Scanning electron microscopy (SEM) was carried out on a JSM 7800F equipped with a field emission electron source. Samples for transmission electron microscopy (TEM) were prepared by electropolishing in a solution of 5% perchloric acid in acetic acid. TEM observations were made on a Phillips CM20 at 160 kV. Table 1 – Composition of the steel tested in the present study in wt%. C 0.11
Nb 0.16
Mn 0.59
Si 0.16
S
