**Comparative Effect of Mo and Cr on Microstructure and Mechanical Properties in NbV-Microalloyed Bainitic Steels**

#### **Andrii Kostryzhev 1,\*, Navjeet Singh 1, Liang Chen 2, Chris Killmore <sup>3</sup> and Elena Pereloma 2,4**


Received: 4 January 2018; Accepted: 13 February 2018; Published: 16 February 2018

**Abstract:** Steel product markets require the rolled stock with further increasing mechanical properties and simultaneously decreasing price. The steel cost can be reduced via decreasing the microalloying elements contents, although this decrease may undermine the mechanical properties. Multi-element microalloying with minor additions is the route to optimise steel composition and keep the properties high. However, this requires deep understanding of mutual effects of elements on each other's performance with respect to the development of microstructure and mechanical properties. This knowledge is insufficient at the moment. In the present work we investigate the microstructure and mechanical properties of bainitic steels microalloyed with Cr, Mo, Nb and V. Comparison of 0.2 wt. % Mo and Cr additions has shown a more pronounced effect of Mo on precipitation than on phase balance. Superior strength of the MoNbV-steel originated from the strong solid solution strengthening effect. Superior ductility of the CrNbV-steel corresponded to the more pronounced precipitation in this steel. Nature of these mechanisms is discussed.

**Keywords:** steel; thermomechanical processing; microstructure characterisation; mechanical properties; molybdenum

#### **1. Introduction**

In high strength low alloyed steels Mo is well known to provide phase balance strengthening, via facilitating the bainite transformation [1–5], and solid solution strengthening [6–9]. It can decrease the rate of dynamic recrystallization of austenite [10–12], which may lead to grain refinement. Sometimes Mo can contribute to precipitation strengthening through formation of Mo-rich carbides [13–16]. Although its main effect on precipitation is via the increase in solubility of Ti [17,18] and Nb [10] in austenite, resulting in decreased sizes and increased number densities of Ti- and Nb-rich particles [19–23], which are essential for the precipitation strengthening from Ti- and Nb-rich particles.

Similar to Mo, Cr facilitates the bainite transformation [24–26], may precipitate in complex Cr-rich carbides [27–29], and increases solubility of Ti [30,31] and Nb [30,32,33]. In particular, Cr was observed delaying Fe3C precipitation in low carbon steel [34]. However, the solid solution strengthening effect of Cr is ~6 times weaker than this of Mo [35], and, therefore, Cr is less affective in retarding recrystallization [36,37].

Amongst the published data, effects of Mo and Cr in multi-microalloyed steels, in particular containing V, are rarely reported. In 0.042C-0.3Mo-1.0Cr-0.08V steel coiled in the temperature range of 180–530 ◦C, Hutchinson et al. [38] observed bainitic microstructures with average ferrite grain size of 3 μm. Increased to 640–770 MPa proof stress was suggested to originate mainly from high dislocation density in bainite and, in particular, dislocation pinning by V(C,N) precipitates. In another work, Kong et al. [39] investigated mechanical properties of 0.064C-0.22Mo-0.21Cr-0.031Nb-0.031V steel thermomechanically processed in a temperature range of 1150–800 ◦C and cooled at the rate of 20–30 ◦C/s to 430–550 ◦C finish cooling temperature. The yield stress in the range of 530–710 MPa was attributed to the narrow width of bainitic ferrite lath (about 0.52 μm), although precipitation of TiNbV-rich particles was also observed. Abbasi and Rainforth [40] studied the microstructure and mechanical properties in MoNbV microalloyed ferritic steel. Simultaneous additions of 0.08 wt. % Mo and 0.04 wt. % Nb to 0.12C-0.16V ferritic steel resulted in precipitation of MoNbVC and decreased size of VC particles, which was explained by the improved temperature stability and reduced coarsening rate of multi-element precipitates. Increased steel hardness with Nb and Mo microalloying was related to finer ferrite grain size and higher number density of VC particles in the NbMoV-microalloyed steel. In this work we advance the knowledge of multi-microalloyed steels in the following aspects: (i) compare the effects of minor Mo and Cr additions on phase transformation and particle precipitation in low carbon NbV-microalloyed bainitic steels; (ii) analyse the microstructure-property relationship in the newly developed steels with 700–850 MPa of yield stress; and (iii) investigate the effect of high temperature strain (in the recrystallization temperature region) on room temperature microstructure and mechanical properties. The effect of high temperature (>1000 ◦C) strain is important to study because increased strain values may enhance recrystallization of austenite (refine grain size) and accelerate precipitation of MoNbV-rich particles [10,23,28,41] (reduce Mo solid solute concentrations). Consequently, the grain size, precipitate number density and solute atom concentrations will affect the ambient temperature mechanical properties.

#### **2. Materials and Methods**

Two steels containing 0.08C, 1.5Mn, 0.3Si, 0.2Ni, 0.03Al, 0.003S, 0.015P, 0.01N, 0.06Nb, 0.12V and either 0.3Cr-0.2Mo or 0.5Cr-0Mo (wt. %), denoted below as MoNbV-steel and CrNbV-steel respectively, were melted in a 60 kg induction furnace and cast as 75 × <sup>100</sup> × 150 mm<sup>3</sup> blocks by Hycast Metals Pty, Sydney, Australia. The blocks were homogenised at 1250 ◦C for 30 h, to equalise chemical composition, then forged in the temperature range of 1250–900 ◦C along the 100 mm side to 28 mm plate thickness, to assure 3.5 times reduction of the as-cast microstructure. The forged plates were cut into standard <sup>20</sup> × <sup>15</sup> × 10 mm<sup>3</sup> Gleeble samples. Thermomechanical processing in Gleeble (manufactured by Dynamic Systems Inc., Poestenkill, NY, USA) was conducted using two schedules:


The processing schedule parameters (deformation temperature range, total strain and strain per pass, strain rate, and cooling rate between passes) have been defined to model the industrial rolling process within reasonable limits of the Gleeble simulator.

Microstructure characterisation for the four studied conditions was carried out using optical, scanning (SEM) and transmission (TEM) electron microscopy. For optical and SEM microscopy, the Gleeble samples were cut parallel to the normal direction (ND)–rolling direction (RD) plane, where ND is the compression direction and RD represents the rolling direction in Gleeble simulation. For TEM and tensile properties testing the samples were cut parallel to the normal direction (ND)–transverse direction (RD) plane. Optical and SEM sample preparation included polishing with SiC papers and diamond suspensions followed by etching with 5% Nital. Foils for TEM were prepared by hand polishing with a number of SiC papers, pre-thinning on a dimple grinder, and ion milling on a Gatan PIPS machine (manufactured by Gatan, Pleasanton, CA, USA). Optical microscopy was conducted on a Leica DM6000M microscope (manufactured by Leica Microsystems, Wetzlar, Germany) equipped with Leica Application Suite (LAS) 4.0.0 image processing software (developed by Leica Microsystems). Scanning electron microscopy was carried out using a JEOL 7001F FEG scanning electron microscope (manufactured by JEOL, Tokyo, Japan) operating at 15 kV for imaging and 7 kV for energy dispersive X-ray spectroscopy (EDS) of precipitates. For the determination of size of bainitic ferrite (the shortest distance between the martensitic grains) more than 400 randomly located areas were manually measured in SEM images for each of four studied conditions. In the SEM visible size range precipitation was scarce. Thus, only a limited number of 50 particles was analysed for the determination of precipitate sizes, number density and area fraction values for each of four studied conditions. The EDS semi-quantitative point analysis was carried out for 20 particles for each studied condition using an AZtec 2.0 Oxford SEM EDS system (manufactured by Oxford Instruments, Abingdon, UK). Transmission electron microscopy was conducted on a JEOL JEM2010 TEM microscope (manufactured by JEOL, Tokyo, Japan). For the analysis of <15 nm particle parameters, 200–500 precipitates were imaged for each of four studied conditions. The precipitates type was analyzed using selected area diffraction. The foil thickness was measured to be ~80 nm; a convergent beam diffraction technique was applied for this measurement [42]. Imaging of dislocation structure was performed for the beam direction being close to [011] grain zone axis. Tensile testing for the four studied conditions was carried out on a Kammrath and Weiss GmbH tensile stage. Testing was performed using 3 mm wide, 1 mm thick and 7 mm gauge length flat specimens. The constant crosshead speed of 7 <sup>μ</sup>m·s−<sup>1</sup> was applied and resulted in 1 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> strain rate. Two specimens were tested per condition.

#### **3. Results**

#### *3.1. Grain Structure and Phase Balance*

Optical, SEM and TEM microscopy showed in both steels a microstructure of mixed granular bainite and bainitic ferrite (Figure 1). Blocky or elongated martensite was present as the second phase. In both steels, the martensite crystals comprise a number of sub-grains with low angle (~10◦) boundaries between them in accordance with possible intervariant misorientation of 10.53◦ [43] for Kurdjumov–Sachs relationship between the parent austenite (face centred cubic (fcc) crystal structure) and product martensite (body centred cubic (bcc) crystal structure, observe the rotation of diffraction patterns in neighbouring sub-grains in Figures 2 and 3). The diffraction analysis (points A, B and C in Figure 2b, and points A and B in Figure 3b) confirmed the bcc type of crystal structure of martensite. Retained austenite was not observed. The average size of bainitic ferrite (the shortest distance between bainitic ferrite-martensite boundaries across the bainitic ferrite area) was measured to be below 1 μm (Table 1). The variation in the average sizes and shape of size distributions (Figure 4b) of bainitic ferrite with steel composition and processing schedule was insignificant and could result from the measurement error. However, a noticeable variation in the size of martensite grains was observed (Figure 4c,d). Thus, for low strain processing, the maximum size of blocky grains was 16% smaller and the maximum length of elongated grains was 40% shorter in the CrNbV-steel. For high strain processing an opposite trend was observed: the average and maximum sizes of blocky grains were 64% and 3.7 times, respectively, larger in the CrNbV-steel; and average and maximum length of elongated

grains were 20% and 30%, respectively, larger in the CrNbV-steel. In addition for high strain processing, the total fraction of martensite was 1.5 times higher in the CrNbV-steel.

 **Figure 1.** Optical images of microstructures in (**a**,**b**) MoNbV-steel and (**c**,**d**) CrNbV-steel after (**a**,**c**) low and (**b**,**d**) high strain processing.

 **Figure 2.** TEM (**a**) bright field image of microstructure and (**b**) dark field image of martensite in MoNbV-steel after low strain processing; (**b**) is from the white frame in (**a**); A, B and C diffraction patterns were taken from the corresponding points in the dark field image.

**Figure 3.** TEM (**a**) bright field image of microstructure and (**b**) dark field image of martensite in CrNbV-steel after low strain processing; (**b**) is from the white frame in (**a**); A and B diffraction patterns were taken from the corresponding points in the dark field image.

**Figure 4.** (**a**) A representative SEM image of microstructure in CrNbV-steel and size frequency distributions of (**b**) bainitic ferrite; (**c**) blocky martensite and (**d**) elongated martensite for four studied conditions.


**Table 1.** Microstructural parameters and mechanical properties of the studied steels.
