Full length articleInteraction of carbon partitioning, carbide precipitation and bainite formation during the Q&P process in a low C steel
Introduction
The “quenching and partitioning” (Q&P) process is known as a promising method for developing steels with good combinations of strength and ductility [1]. The Q&P process involves rapid quenching of an austenitic microstructure to a temperature lower than the martensite-start temperature (Ms) to form a controlled fraction of initial martensite (M1). Then, the treatment is followed by isothermal holding either at or above the initial quenching temperature to stabilize austenite via carbon partitioning from supersaturated martensite to austenite. The Q&P process is ended by quenching the microstructure to room temperature during which some secondary martensite (M2) may form if the carbon enrichment is not sufficient to stabilize all austenite [2]. Secondary martensite contains a high concentration of carbon and is detrimental for ductility [3]. Formation of M2 can be controlled by knowledge of the carbon partitioning process as well as its interactions with other possible reactions such as martensite-austenite interface migration, carbide precipitation in martensite and decomposition of austenite to bainite.
The thermodynamics of the carbon partitioning process can be well described on the basis of “constrained carbon equilibrium” (CCE) [4], [5]. In this approach, the partitioning process across an immobile austenite-martensite interface is ended when martensite (i.e. ferrite) is in metastable equilibrium with austenite. Santofimia et al. [6] adapted the model to simulate the interaction between the carbon partitioning and the possible migration of martensite-austenite interfaces. These approaches are limited to well-controlled systems in which carbide precipitation in martensite and decomposition of austenite to bainite are totally suppressed. However, these reactions are often unavoidable, even in low-carbon steels containing a relatively high concentration of Mn and Si [7], [8], [9].
Precipitation of ɛ-carbide prior to the partitioning step or of cementite during the isothermal holding affects the carbon partitioning process. Precipitation of cementite reduces the amount of carbon in solid solution in martensite and therefore decreases the degree of carbon enrichment that can be reached in austenite [10], [11]. Generally, a high concentration of Si is added to the steel composition to control cementite formation [12]. However, Si improves the coherency at the carbide-martensite interface during the nucleation stage of the ɛ-carbides [13] and consequently increases the stability of the ɛ-carbides [2], [13], [14]. Consequently, suppressing precipitation of ɛ-carbide is really challenging and practical designing of Q&P treatments requires knowledge of the interaction between ɛ-carbide precipitation and carbon partitioning process.
The carbon partitioning process may also overlap with the decomposition of austenite to bainite. Formation of carbide-free bainite associates with carbon enrichment of austenite and could stabilize austenite. Therefore, bainite formation has an important influence on the final microstructure. Developing a model that indicates the interaction between bainite formation and carbon partitioning process assists in better controlling the microstructure.
In this paper, the microstructural evolution during the Q&P process of a 0.3C-1.6Si-3.5Mn (wt.%) steel with non-homogenous chemical composition is analysed. The influence of the carbide precipitation as well as the formation of carbide-free bainite on the microstructure is discussed based on experimental observations and microstructural modelling.
Section snippets
Experimental procedures
Cylindrical specimens with length of 10 mm and diameter of 3.5 mm were machined parallel to the hot-rolling direction of 0.3C–1.6Si–3.5Mn (wt.%) steel sheets. A scheme of the applied heat treatments is shown in Fig. 1. The specimens were austenitized at 900 °C for 180 s, quenched to 180 °C, 200 °C, 220 °C, 240 °C and 260 °C, isothermally treated at 400 °C for 5 s, 10 s, 50 s, 100 s and 200 s and finally quenched to room temperature in a Bähr DIL 805 A/D dilatometer. In this paper, the code QTxxx
Analysis of compositional gradients in the steel
Fig. 2a shows an optical micrograph of the specimen QT260-5. The microstructure consists of internally etched features, M1, and blocky white features, M2, which are distributed forming bands. In this specimen, as in the other Q&P specimens, microstructural bands are parallel to the rolling direction. This shows non-homogeneous distribution of the alloying elements in the transverse direction. Concentrations of Mn and Si along the transverse direction, the black arrow in Fig. 2a, were determined
Effect of the non-homogeneity of the chemical composition
According to Fig. 2a and b, higher fractions of M1 are formed in low-alloying regions than high-alloying regions. This can be discussed on the basis of the relation between chemical composition and Ms [21], which is given in Eq. (1). Fig. 11 shows the influence of Mn and carbon segregation on Ms under two extreme conditions; (a) Mn concentration changes in the range of 2.90–4.20 wt.%, as it was measured by EPMA (Fig. 2b), and carbon concentration is constant (0.30 wt.%). (b) Carbon
Conclusions
The interaction between carbon partitioning, carbide precipitation and carbide free bainite formation is studied during the application of the Q&P process to a 0.3C-1.6Si-3.5Mn (wt.%) steel with non-homogenous chemical composition. The main conclusions are:
- 1.
Precipitation of ɛ-carbides during the first quenching reduces the amount of carbon in solid solution in martensite. Therefore the amount of carbon partitioning to the austenite is lower than according to simulations of carbon partitioning.
Acknowledgements
This research was carried out under the project number M41.10.11437 in the framework of the Research Program of the Materials innovation institute M2i (www.m2i.nl). The support of Tata Steel RD&T to this project is acknowledged.
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