Elsevier

Materialia

Volume 8, December 2019, 100462
Materialia

Full Length Article
The effect of molybdenum on clustering and precipitation behaviour of strip-cast steels containing niobium

https://doi.org/10.1016/j.mtla.2019.100462Get rights and content

Abstract

Two high-strength low-alloy (HSLA) steels containing Nb-carbonitrides were produced, one contained Mo and the other was Mo-free. The alloys were produced by simulated direct strip casting, and were fully bainitic in the as-cast condition. Isothermal ageing treatments were carried out to precipitate harden the alloy, and the strength was measured using a shear punch test. The dislocation density was measured with X-ray diffraction (XRD), and was found to be larger in the alloy containing Mo in all ageing conditions. Atom probe tomography (APT) showed the presence of solute clusters in the as-cast condition, and the addition of Mo increased both size and volume fraction of these clusters. The solute clusters provided significant strengthening increments of up to 112 MPa, and cluster strengthening was larger in the Mo-containing alloy. Precipitation of Nb-carbonitrides was observed after longer ageing times, which were refined by the addition of Mo. This was attributed to the higher dislocation density that increased the number of nucleation sites. Precipitate chemistry was similar for both alloys, and contrary to some literature reports, minimal Mo was observed to segregate to the precipitates. A thermodynamic rationale is presented which describes the reasons that Mo segregates to the Nb-carbide in some alloys but not in others, despite the alloy chemistries being relatively similar.

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:

  • (1)

    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

References (56)

  • E.P. Gilbert et al.

    ‘Quokka’—the small-angle neutron scattering instrument at opal

    Phys. B: Condens. Matter

    (2006)
  • R.K.W. Marceau et al.

    Quantitative atom probe analysis of nanostructure containing clusters and precipitates with multiple length scales

    Ultramicroscopy

    (2011)
  • T. Dorin et al.

    Effect of coiling treatment on microstructural development and precipitate strengthening of a strip cast steel

    Acta Mater.

    (2016)
  • D.W. Saxey

    Correlated ion analysis and the interpretation of atom probe mass spectra

    Ultramicroscopy

    (2011)
  • S. Dhara et al.

    Atom probe tomography data analysis procedure for precipitate and cluster identification in a Ti–Mo steel

    Data Brief

    (2018)
  • E.V. Pereloma et al.

    Fine-scale microstructural investigations of warm rolled low-carbon steels with and without Cr, P, and B additions

    Acta Mater.

    (2006)
  • M.K. Miller et al.

    Precipitation in neutron-irradiated Fe–Cu and Fe–Cu–Mn model alloys: a comparison of APT and SANS data

    Mater. Sci. Eng.: A

    (2003)
  • ZhuK. et al.

    An approach to define the effective lath size controlling yield strength of bainite

    Mater. Sci. Eng.: A

    (2010)
  • A. Deschamps et al.

    Influence of predeformation and agEing of an Al–Zn–Mg alloy—II. Modeling of precipitation kinetics and yield stress

    Acta Mater.

    (1998)
  • A.G. Kostryzhev et al.

    Effect of niobium clustering and precipitation on strength of an NbTi-microalloyed ferritic steel

    Mater. Sci. Eng.: A

    (2014)
  • R. Uemori et al.

    AP-FIM study on the effect of Mo addition on microstructure in Ti–Nb steel

    Appl. Surf. Sci.

    (1994)
  • ChenC. et al.

    Precipitation hardening of high-strength low-alloy steels by nanometer-sized carbides

    Mater. Sci. Eng.: A

    (2009)
  • ChenC.Y. et al.

    Microstructure characterization of nanometer carbides heterogeneous precipitation in Ti–Nb and Ti–Nb–Mo steel

    Mater. Charact.

    (2014)
  • I.B. Timokhina et al.

    Precipitate characterisation of an advanced high-strength low-alloy (HSLA) steel using atom probe tomography

    Scr. Mater.

    (2007)
  • ParkD.B. et al.

    Strengthening mechanism of hot rolled Ti and Nb microalloyed HSLA steels containing Mo and W with various coiling temperature

    Mater. Sci. Eng.: A

    (2013)
  • N. Isasti et al.

    Microstructural and precipitation characterization in Nb–Mo microalloyed steels: estimation of the contributions to the strength

    Met. Mater. Int.

    (2014)
  • LeeW.B. et al.

    Carbide precipitation and high-temperature strength of hot-rolled high-strength, low-alloy steels containing Nb and Mo

    Metallur. Mater. Trans. A

    (2002)
  • YongQ. et al.

    The roles and appliactions of molybdenum element in low alloy steels

    International Seminar on Applications of Mo in Steels

    (2010)
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