Effects of combined silicon and molybdenum alloying on the size and evolution of microalloy precipitates in HSLA steels containing niobium and titanium
Introduction
Molybdenum is used in high-speed steels and tool steels to provide strength via carbide precipitation, often in conjunction with chromium or tungsten, in martensite [1], [2]. In the case of high-strength, low-alloy (HSLA) steels, molybdenum is generally added to control the austenite decomposition kinetics [3], [4], [5], [6], but molybdenum has also been reported to be present in relatively fine microalloy carbide or carbonitride precipitates that form in ferrite during or after transformation, and to a lesser extent in austenite [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Generally, precipitation of molybdenum carbide is not expected in austenite for typical compositions of HSLA steels because of the high solubility of molybdenum carbide in austenite [18]. A recent investigation by Enloe et al. [15] on hot-rolled molybdenum-bearing microalloyed steels supports the hypothesis that molybdenum is incorporated into niobium-rich carbonitride precipitates during cooling after conventional austenitic hot rolling. Using atom probe tomography and transmission electron microscopy, they observed elevated molybdenum concentrations near the surface of the precipitates compared to the core, which was richer in titanium than molybdenum. They attributed the concentration profiles to the precipitation sequence of the carbides and nitrides—titanium concentrations were greater at the core because the precipitates nucleated and grew in austenite and only became enriched in molybdenum during cooling and after transformation to ferrite. Furthermore, they also showed that the molybdenum concentration near the surface of the microalloy precipitates was reduced after simulated carburizing at 1373 K, indicating enrichment of austenite with molybdenum [15]. Similar assertions regarding molybdenum enrichment of precipitates were advanced by Houghton et al. [7] in an investigation of a series of molybdenum-containing steels microalloyed with niobium and titanium.
Several studies have considered alloying effects on microalloy carbonitride solubility, precipitation kinetics, and microalloy diffusivity in austenite [19], [20], [21], [22]. Molybdenum has been suggested to reduce the activity of niobium in austenite, similar to the effects of manganese [18], [23]. The decreased niobium activity can increase precipitate solubility and may also retard precipitation by decreasing niobium diffusivity. In contrast, silicon has been widely reported to increase the diffusivity of niobium in austenite [19], [20], [21]. Kurokawa et al. [20] showed that the diffusivity of niobium in austenite increased by 72% in a Fe–0.6Si (in mass percent) alloy compared to a pure Fe alloy. They also indicated that niobium diffusivity decreased with a 0.6% manganese addition to their base (pure Fe) alloy. Interestingly, when both silicon and manganese were added to the pure Fe alloy, niobium diffusivity increased by 87% in a Fe–0.6Si–1.5Mn alloy even though it decreased by 12% for the same manganese addition without any silicon, and this increase was greater than that measured for the Fe–0.6Si alloy. In other words, the combined silicon and manganese additions were reported to increase niobium diffusivity beyond the lone effects of silicon. It is unknown if a similar synergistic effect may also occur with combined silicon and molybdenum additions. Therefore, the objective of the current investigation was to examine the effects of combined silicon and molybdenum additions on microalloy carbide and nitride precipitation in austenite.
Section snippets
Materials
Table 1 lists the chemical compositions of the two steels investigated in this study. The steels are high-niobium (~ 0.09 mass%) HSLA grades that differ in silicon and molybdenum content. The two steel grades are designated based on their molybdenum content. The 0Mo steel contains no molybdenum addition and 0.14 mass% silicon, whereas the 15Mo steel contains 0.15 mass% molybdenum and 0.29 mass% silicon. Titanium is slightly hyperstoichiometric with respect to nitrogen in both steels in order to
Microstructure
Fig. 2 shows the microstructures of the 0Mo and 15Mo alloys in the fully-processed single-step (S) and double-step (D) deformation conditions for longitudinal cross-sections. The fully-processed material has a non-polygonal ferrite microstructure with martensite–austenite constituents. Deformation conditions (single- or double-step) may have affected ferrite packet dimensions but the ferritic microstructures were not quantified, as the focus of the current study was on precipitation. Molybdenum
Type I precipitates
Type I precipitates were defined as existing or forming in austenite during reheating at 1523 K for 900 s. These precipitates are cuboidal titanium-rich precipitates with an average size of ~ 105 nm.
Fig. 8 shows the effects of processing condition (RH-Q, NoStrain-Q, S-Q, D-Q, S, and D) on the size and size distribution of the Type I precipitates. There are no systematic effects of processing on the measured precipitate size and the average size is within the experimental uncertainty for all
Conclusions
Complex precipitate formation in austenite was examined in two HSLA steels microalloyed with titanium and high niobium (~ 0.09 mass%). The steels had the same nominal composition except for silicon and molybdenum. One steel contained 0.14 mass% silicon and no molybdenum (0Mo alloy) while the second steel contained 0.29 mass% silicon and 0.15 mass% molybdenum (15Mo alloy). The precipitation sequence in austenite was examined via an interrupted thermomechanical processing simulation. The state of
Acknowledgments
The authors would like to acknowledge the sponsors of the Advanced Steel Processing and Products Research Center, an industry/university cooperative research center at the Colorado School of Mines, for funding this study. They would also like to acknowledge Mr. Gail Smith at Evraz North America for supplying the steel used in this investigation.
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