The effect of molybdenum on precipitation behaviour 2 in austenitic strip-cast steels containing niobium 3

: Two low-C steels microalloyed with Nb were fabricated by simulated strip casting, one with 11 Mo and the other without Mo. Both alloys were coiled at 900 °C to investigate the effect of Mo on the 12 precipitation behaviour in austenite in low-C strip-cast Nb steels. The mechanical properties results 13 show that during the coiling at 900 °C the hardness of both alloys increases and reaches a peak after 14 3000 s and then decreased after 10,000 s. Additionally, the hardness of the Mo-containing alloy is higher 15 than that of the Mo-free alloy in all coiling conditions. Thermo-Calc predictions suggest that MC-type 16 carbides exist in equilibrium at 900 °C, which are confirmed by transmission electron microscopy (TEM). 17 TEM examination shows that precipitates are formed after 1000 s of coiling in both alloys and the size 18 of the particles is refined by the addition of Mo. Energy dispersive spectroscopy (EDS) and electron 19 energy loss spectroscopy (EELS) reveal that the carbides are enriched in Nb and N. The presence of Mo 20 is also observed in the particles in the Nb-Mo steel during coiling. The concentration of Mo in the 21 precipitates decreases with increasing particle size and coiling time. The precipitates in the Nb-Mo steel 22 provide significant strengthening increments of up to 140 MPa, much higher than that in the Nb steel, 23 ~ 96 MPa. A thermodynamic rationale is given, which explains that the enrichment of Mo in the 24 precipitates reduces the interfacial energy between precipitates and matrix. This is likely to lower the 25 energy barrier for their nucleation and also reduce the coarsening rate, thus leading to finer precipitates 26 during coiling at 900 °C.


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For the high-strength low-alloy (HSLA) steels containing niobium (Nb)   austenite by lowering interfacial energy [11], and thus refines the precipitates [11,12]. However, despite 41 the extensive investigations on the effect of Mo on the formation of Nb-rich precipitates in austenite, route with extremely rapid solidification and high cooling rates [1][2][3], which reduces cost and saves 1 energy-consumption up to 90%. Dorin et al. [13] studied the formation of Nb(C, N) in strip-cast steels 2 and found that chemically complex Nb-rich precipitates containing C, N, Si and S were formed in 3 austenite during coiling at a high temperature (850 °C), which provides a significant strength increment 4 of up to ~ 150 MPa. This early work also emphasized significant differences in the Nb(C, N) precipitates 5 formed in strip-cast materials as compared to the ones formed in conventionally processed steel. One 6 of the main differences is that strip casting produces as-cast samples where the Nb is supersaturated 7 and the precipitates can then be formed during a controlled coiling treatment [14]. Consequently, 8 precipitation occurs in non-deformed austenite. Thus, the mechanism behind the precipitation can be 9 different to the strain-induced precipitation in previous literature. To the best of our knowledge, the 10 effect of Mo on the precipitation of Nb(C, N) in the austenite phase field of strip-cast steels has rarely 11 been studied.

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In the present paper, two Nb-containing low-carbon steels were prepared via strip casting with and 13 without Mo. The as-cast samples were then coiled at 900 °C for different durations and hardness testing 14 was used to monitor the hardening response during coiling. Transmission electron microscopy (TEM) 15 was performed to observe the precipitation for different coiling times. Finally, the thermodynamic 16 effects of Mo on Nb-rich precipitates and corresponding precipitation strengthening are also presented.

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The compositions of the alloys studied in this work are listed in Table 1. The steels were cast using a 19 lab-scale strip casting simulator, known as a dip tester [15]. The steels were melted in a 75 kW induction 20 furnace using high purity starting elements. The details of the casting process can be found in [16,17].

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After casting, the samples were immediately transferred into a preheated muffle furnace within 2 s, 22 and within this time the samples did not cool below the coiling temperature of 900 °C. The detailed 23 thermal profile and schedule diagram of coiling treatment can be found in [16]. Coiling times varied 24 from 100 to 10,000 s, and all samples were air-cooled to room temperature at the end of the coiling 25 treatment.

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Hardness tests were carried out at room temperature to monitor the hardness evolution of both steels 5 coiled at 900 °C for various times. Average values were generated from at least 7 measurements for 6 each condition.

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Samples examined by optical microscopy were prepared by sectioning parallel to the casting direction.

8
The specimens were ground using increasingly finer grades of silicon carbide paper, followed by

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Afterwards, a more rapid hardening occurs in both alloys by reaching the peak hardness after 3000 s at

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Through calculation, the equilibrium ratio of Mo/(Nb+Mo) at 900 °C is 0.00177, which indicates that the 14 precipitates observed in this work were thermodynamically unstable and did not have an equilibrium 15 composition, as shown in Fig. 8.

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It is well-known that microalloyed carbonitrides formed in the austenite matrix typically keep a cube-

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Consequently, the coarsening rate of precipitates in the Nb-Mo steel coiled at 900 °C is smaller than 21 that in the Nb steel, leading to a smaller particle size (Fig. 5). Additionally, as mentioned above, the 22 diffusion of Mo between precipitates and matrix also slows the growth and coarsening of particles.

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which suggests that a small amount of very fine precipitates is formed, likely due to the lower interfacial 31 energy from Mo ( Fig. 9) that reduces the energy barrier to nucleation. However, no Nb-rich 32 carbonitrides are seen in the Nb-Mo steel coiled at 900 °C for 100 s using the carbon replica sample 33 preparation method (Fig. 4). This is probably due to the low extraction capability of carbon replica for 34 the particles with diameters < 10 nm.
In recent work by Dorin et al. [13], the shearing to by-passing of the Nb-carbonitrides formed in the 1 austenite was found to occur for a radius of ~ 6 nm. In the present work, the precipitates formed at 2 900 °C are always larger and we hence assume that they will be by-passed by moving dislocations. As  Table 2.

1
In the present study, the effect of Mo on the precipitation behaviour in austenite in low-carbon strip-2 cast steels containing Nb has been investigated. The key conclusions are as follows:

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(1) The hardness values of both as-cast steels were similar. During coiling at 900 °C, the hardness 4 of the two steels increased and reached the peak at 3000 s, then decreased after 10000 s coiling.

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Generally, the Nb-Mo steel had higher hardness than the Nb steel at all coiling times.

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(2) The microstructures of both steels coiled at 900 ºC were bainite, only some grain boundary 7 allotriomorph (GBA) were formed in the Nb steels coiled for 10,000 s at 900 °C.

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(3) Nb-carbonitrides precipitated in both steels after coiling for 1000 s at 900 °C, and the size of the 9 particles in the Nb-Mo steel was finer than that in the Nb steel. In the Mo-containing steel, Mo 10 also participated in the precipitation, and the concentration of Mo in Nb-rich carbonitrides 11 decreased with increasing particle size and coiling time.

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(4) The enrichment of Mo in the Nb-rich carbonitrides reduces the interfacial energy between 13 precipitates and the matrix, which lowers the nucleation energy barrier and the coarsening rate