Full Length ArticleThe effect of molybdenum on clustering and precipitation behaviour of strip-cast steels containing niobium
Introduction
It has been found that the addition of Mo into Nb or Ti microalloyed low-carbon steels can favour the formation of finer and more closely-spaced carbides, which significantly improves the strength whilst maintaining ductility [1], [2], [3], [4], [5], [6], [7]. Recent transmission electron microscopy (TEM) and atom probe tomography (APT) studies of interphase precipitation in Ti–Mo low-carbon steels [6], [7], [8], [9] reveal the co-existence of nano-clusters and precipitate particles, both of which are enriched in Ti, Mo and C. The Mo in TiC particles has been suggested to reduce the misfit between the carbides and ferrite matrix [10], leading to a lower interfacial energy that is beneficial for nucleation and inhibits precipitate coarsening [8], [10], therefore producing greater precipitation strengthening. Unlike the role of Mo in interphase precipitates in Ti–Mo steels, Mo in Nb-containing steels produced by conventional casting processes has been known to increase the solid solubility of Nb atoms in austenite [11] and available for precipitation during slow cooling [2], [11]. TEM and energy dispersive X-ray spectroscopy examinations of these Nb carbides in the Nb–Mo steels show them to be enriched in both Nb and Mo, suggesting that the Mo behaves synergistically with Nb during carbide precipitation [1], [2], [11], [12]. However, the underlying mechanism for Mo strengthening in Nb-containing low-carbon steels is not well described. Some research suggests that the addition of Mo refines Nb-carbide precipitates by providing more nucleation sites through a higher dislocation density [2], [11]. Other research suggests that Mo lowers the interfacial energy of the carbide with the matrix, and this is proposed to inhibit precipitate coarsening [12]. To date, a thorough microscopic examination of these complex steel alloys has not been carried out with sufficient rigour that a solid conclusion can be made regarding the exact mechanisms by which Mo improves strength.
Direct strip casting (DSC) is a near-net-shape casting technology which produces thin sheets directly from the liquid metal [13], [14], [15]. The cooling rate is sufficiently high (in the range of 100 °C/s to 1000 °C/s [16]) during strip casting that Nb-carbides have insufficient time to develop [14], [17], [18], and in this case precipitation is achieved during a post-solidification heat treatment, instead of during slow cooling as in conventional processing. Correlative TEM and APT work of Nb-containing strip-cast steels [19], [20] show that the peak hardness occurs rapidly and is caused by a fine dispersion of Nb-rich solute atom clusters that form at the early stage of ageing. The strength increment attributed to these clusters in a 0.084 wt% Nb strip-cast steel was ∼165 MPa after ageing at 700 °C for 4 min, which is apparently superior to the precipitation strengthening from Nb-rich carbonitrides, Nb(C, N). [19]. However, there is very little literature about how the addition of Mo affects these Nb-rich solute atom clusters and Nb-rich precipitates that form in strip-cast steels. The effect of Mo on clusters and carbide precipitation and subsequent strengthening in strip-cast steels may be different from that in steels produced by conventional casting processes. Furthermore, Mo is known to significantly affect the phase transformation behaviour in strip-cast steels [21], [22] and this may have an additional effect on the final mechanical properties [22]. Understanding how these higher cooling rates affect precipitation and properties of DSC steels is critical to ensure industrial uptake of these new “green” technologies to produce high-strength low-alloy (HSLA) steels. The present paper systematically describes the effect of Mo on the evolution of clustering, precipitation and mechanical properties of a HSLA steel containing Nb produced by direct strip casting. The microstructure is extensively studied with multiple microscopy techniques, and this information has been used to quantify the effect of Mo on the different microstructural contributions to strength.
Section snippets
Material
Two steel compositions were used in this study, one with Mo and the other without Mo, as given in Table 1. The direct strip casting experiments were carried out using a lab-scale direct strip casting simulator, known as a dip tester [23]. The steels were melted in a 75 kW induction furnace using high purity starting elements. Once the composition was confirmed using an inductively coupled plasma atomic emission spectroscopy instrument, two copper substrates were rapidly submersed into the
Mechanical properties
Fig. 1 shows the equivalent yield and ultimate tensile strengths of the two alloys, determined from the shear punch test measurements. It can be seen that the strengths of the two steels increased from that of the strip cast conditions after ageing for 100 and 500 s at 650 °C, and dropped after an extended ageing time of 10,000 s. The alloy containing Mo exhibited higher strength for all conditions.
Microstructural observations
Fig. 2 shows the electron backscatter diffraction (EBSD) examination for the microstructures of
Comparison of SANS and APT results
In general, the volume fractions of precipitates measured by APT (0.038% and 0.044% for the Nb and Nb–Mo alloys respectively) are consistent with those obtained by SANS (0.056% for both alloys). The comparison between SANS and APT with regard to size of precipitates is presented in Fig. 11. Both APT and SANS techniques showed the Nb–Mo alloy had smaller precipitates than the Nb alloy, however, overall APT shows larger precipitate diameters. One reason for this discrepancy could be that the APT
Conclusions
The current research has investigated the effect of Mo on the atomic-scale precipitation behaviour of strip-cast steels containing Nb. Two alloys were produced by rapid solidification to simulate the direct strip casting process, one alloy contained Mo and the other did not. These as-cast samples were then heat-treated to induce precipitation. The solute clusters and precipitates were examined by a range of microscopy techniques, and it was found that:
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The as-cast and aged microstructures of the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The work in this study was funded by ARC Discovery Grant (DP160101540). The authors would like to acknowledge the support of the Deakin Advanced Characterisation Facility. The authors also would like to acknowledge Australian Nuclear Science and Technology Organisation for allocating SANS beamtime on the Quokka beamline at the Bragg Institute (Proposal No: P4666). The authors are grateful to Mr David Grey and Dr Adam Taylor for assistance with casting the alloys, and the continued support of
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Achieving high strength and large elongation in a strip casting microalloyed steel by ageing treatment
2022, Materials Science and Engineering: ACitation Excerpt :Owing to the complexed thermo-mechanical processing process of conventional hot rolling process, highly dispersed nanoprecipitates could form at different stages, e.g., deformation induced precipitation in austenite during hot rolling, interphase precipitation at phase interface during phase transformation and precipitation in ferrite matrix during the subsequent cooling and coiling processes [18–21]. However, for strip casting process, owing to the high solidification rate during casting and the limited hot rolling reduction, i.e., single pass hot rolling, nanoprecipitates have insufficient time to form [22]. In addition, the coarse prior austenite grain size (∼200 μm) of strip casting steel greatly enhances the hardenability of strip casting HSLA steels, which facilitates the formation of low temperature phase transformation products, such as bainite and acicular ferrite (AF) [14,23].
Precipitation behavior of Ti–Nb–V–Mo quaternary microalloyed high strength fire-resistant steel and its influence on mechanical properties
2021, Materials Science and Engineering: AThe effect of molybdenum on interphase precipitation at 700 °C in a strip-cast low-carbon niobium steel
2020, Materials CharacterizationCitation Excerpt :It has been shown that there is no significant difference between the Nb and Nb–Mo steels [23], and the rapid cooling during direct strip casting retains most alloying elements in solid solution [21]. Since 0.33 wt% Mo only provides 3.63 MPa solid solution strengthening [22], the hardness of both steels in the as-cast condition is similar. The SEM observation shown in Fig. 2 demonstrates that polygonal ferrite is formed during coiling at 700 °C for the two alloys, with the remainder of the microstructure forming bainite.