Effects of Nb on Elevated-Temperature Properties of Fire-Resistant Steel

Abstract

Objective: Two kinds of fire-resistant steel with different Nb content (Nb-free and 0.03 wt.%) were prepared for studying the effects of Nb addition on the elevated-temperature strength of fire-resistant steel. Methods: Two stages of heat treatment were carried out on the steels to obtain different microstructures. Typical microstructures, dislocation, and precipitates morphology of steels were observed by SEM and TEM. The dislocation density was calculated by the X-ray data from the microstructures. High temperature and room temperature mechanical properties of steels were determined by tensile testing. Results: The results showed that the YS of N2-HR steel (addition of 0.03 wt.% Nb) at RT and 600 °C was higher than N1-HR steel (Nb-free) by about 81 and 30 MPa, respectively. This indicates that Nb is an alloying element as effective as Mo in increasing the elevated-temperature strength of fire-resistant steel. The dominant strengthening mechanisms of Nb addition on elevated-temperature yield strength are precipitation strengthening and bainite strengthening. Conclusions: Theoretical analysis shows that there are two precipitation strengthening stages in fire-resistant steel: (1) increasing dislocation density during hot rolling, and (2) blocking dislocation movement and recovery in tensile testing. The results also show that the effect of fine grain strengthening is not obvious at high temperature, but is obvious at room temperature.

1. Introduction

In recent years, due to the requirements of fire safety regulations, the number of multi-story buildings built with fire-resistant steel has increased greatly. Different from conventional structural steel, the yield point of fire-resistant steel at 600 °C (offset 0.2%) is guaranteed to be two-thirds of the yield point at room temperature [1,2,3]. It is generally considered that Mo is the most effective element to improve the high-temperature strength of steel [3,4]. If the Mo content is reduced, the high temperature strength of the steel will be significantly reduced. However, as a precious metal element, an increase in Mo content will inevitably increase the production cost of fire-resistant steel, which, in some degree, limits its wide application.

Previous studies [5,6] have showed that Nb can improve the high temperature strength of fire-resistant steel as effectively as Mo, and the Mo content and production cost of steel is reduced by microalloying Nb. However, the strengthening mechanisms of Nb addition on the high temperature strength were not discussed by the previous studies. In addition, some studies [7,8,9] point out there are three strengthening mechanisms of Nb added to steel: (1) grain refinement strengthening, (2) precipitation strengthening, and (3) bainite strengthening, which is due to the addition of Nb promoting the transformation of bainite. Unfortunately, these studies on the effect of Nb were carried out at room temperature. Furthermore, comparatively limited scientific attention has been focused on the high-temperature strengthening mechanisms of Nb addition.

Microstructure and properties of hot rolled or heat treated low-Mo fire-resistant steel with two kinds of Nb addition (no Nb and 0.3 wt.%) were studied in this investigation. The effect of grain refinement strengthening, precipitation strengthening and bainite strengthening by Nb addition on high temperature strength properties are discussed. The dominant high temperature strengthening mechanism of Nb addition in fire-resistant steel was revealed by theoretical analysis.

2. Materials and Experimental Procedure

The chemical compositions of low-Mo fire-resistant steels are given in

Table 1. Chemical composition of steels (wt.%).

Steels	C	Si	Mn	Mo	Cr	Nb	Fe
N1	0.12	0.35	1.20	0.27	0.35	/	Bal.
N2	0.11	0.36	1.22	0.26	0.32	0.03	Bal.

. These steels were reheated to 1180 ± 5 °C for 3 h, and then were hot-rolled into 30 mm thick plates, and finally air cooled to room temperature.

Two stages of heat treatment were carried out on the steels. First, the steels were held at 1160 °C for 60 min to form austenite and dissolve any Nb-containing alloy carbides/carbonitrides present in the steels, followed by water-quenching to room temperature. Second, the steels were reaustenitized at 930 °C for 30 min, austempered for 30 min at 750 °C to form ferrite–pearlite microstructures, and finally air cooled to room temperature.

X-ray experiments were carried out on a SHIMADZU XRD-6000 diffractometer at a scan rate (2θ, where θ is the Bragg angle) of minimum 2° lower than the range 2θ = 35–90°, with unfiltered Cu K_α radiation. The system was operated at 40 kV and 40 mA.

The tensile specimens, in accordance with standard ASTM E8-16 [10], were processed from hot rolled plate along the longitudinal direction. Tensile tests were performed using a SHIMADZU-100kNA tensile tester at room temperature (ASTM E8-16) and at elevated-temperature up to 600 °C (E21-17 [11]) with strain rates of 10⁻³ s⁻¹. Before the high temperature test, the samples were placed at the test temperature for 15 minutes. Test temperatures were: room temperature, 100 °C, 200 °C, 300 °C, 400 °C, 500 °C and 600 °C. Each tensile data point was at least the average of two samples.

After polishing and etching with 4% Nital, the optical microstructure (OM) was examined longitudinally in the thickness direction. The average grain size of ferrite and the volume fraction of bainite were determined by linear intercept method (ASTM E112-13 [12]) and grid method (E562-19 [13]), respectively. The etched samples were further examined by JSM-7600F scanning electron microscope (SEM) at a voltage of 15 kV. Thin foil samples for transmission electron microscope (TEM) observation were prepared by spray polishing in 5% perchloric acid solution. They were observed by using a conventional TEM (JSM-2100) operating at a voltage of 200 kV.

3. Results

3.1. Microstructure

Figure 1 shows ferrite–pearlite (P) microstructures in heat-treated (HT) steels. A typical ferrite–pearlite–bainite microstructure of hot-rolled (HR) steels is demonstrated in Figure 2. Pearlite, the black area (as shown by the arrow in Figure 1 and Figure 2), lies in a ferrite (F) matrix (white), and bainite (B) is the region where black alternates with white (as shown by the arrow in Figure 2). The microstructure difference between pearlite and bainite under SEM is shown in Figure 3. Figure 4 depicts a ferrite–pearlite–bainite microstructure of N2-HR steel after elevated-temperature hardness tests. It was found that the microstructure of N2-HR steel was almost the same before and after the high temperature hardness test, which indicated that the bainite microstructure had high stability even at high temperatures.

Figure 5 shows the change trend of pearlite volume fraction, bainite volume fraction and ferrite grain size of steels. A limited quantity of bainite (about 5 vol.%) was observed in N1-HR steel (Nb-free), and the volume fraction of bainite was 11.1% in N2-HR steel (Nb-added). As is known to all, the γ/α transformation of austenite of low-carbon steel is greatly affected by the addition of Nb. The grain growth and recrystallization of austenite was significantly suppressed by the precipitation of Nb (C, N) and the solute Nb prior to the γ/α transformation during hot rolling [14,15]. Nb made a significant contribution to promoting bainite transformation. In contrast, all steels had a similar volume fraction of pearlite, which was about 14%. Therefore, the influence of pearlite on steel strength was ignored in this study. It was also found that the average ferrite grain sizes of all steels were about 14 μm, except N1-HR steel. The average ferrite grain size of N1-HR steel was about 18 μm. The finer ferrite grain size was observed in N2-HR and heat-treated steels due to the Nb addition and recrystallization in heat treatment, respectively.

3.2. Precipitation and Dislocation

Typical microstructures, dislocation, and precipitates morphology of steels observed by TEM are shown in Figure 6. The typical pearlite morphology is illustrated in Figure 6a. In addition, as shown in Figure 6b, precipitation and low dislocation density were almost not observed in the ferrite of N1-HR steel. Figure 6c shows a typical bainite morphology, in which discontinuous carbides are distributed between ferrite sheets with high dislocation density. Bhadeshia also observed similar bainite morphology under TEM [16]. Meanwhile, the steel N2-HR showed higher density dislocation and fine precipitates in the ferrite (Figure 6d). It was observed that the density dislocation in heat-treated steels was lower than in hot-rolled steels (Figure 6e,f). Fine precipitates were also observed in N2-HT steel.

In order to identify the composition of precipitate in N2 steel, Figure 7 shows the element distribution map which was drawn by electron probe micro-analysis (EPMA). The results showed that only Nb was enriched in precipitates (as arrow-marked in Figure 7), which indicated the precipitates in N2 steel were Nb (C, N). Uemori et al. [17] also stated that those fine precipitates, which are found in steels with similar chemical compositions, are NbC or Nb (C, N).

3.3. Dislocation Density

The dislocation density was calculated by the X-ray data from the microstructures. The dislocation density, ρ, is calculated by Equation (1) [18]. The parameter b is the Burgers vector of dislocations in α-Fe. The slope of a plot of β_hkl cos{θ_hkl} versus 4sin{θ_hkl} is equal to a measure of the non-uniform strain ε [19]. The parameter β is the measured peak broadening.

ρ = 6πε²/b²

(1)

The dislocation density calculated by Equation (1) is illustrated in Figure 8. It was found that the variation trend of calculated dislocation density conformed to the variation trend of dislocation density under TEM. The dislocation density of hot-rolled steels was higher than that of heat-treated steels. Meanwhile, the dislocation densities of N2 steels (HR and HT) were higher than those of N1 steels (HR and HT). The dislocation density of N2-HR steel was the highest of all steels. It was also found that the dislocation density of N2-HT steel was approximately equal to that of N1-HR steel.

3.4. Mechanical Properties

Figure 9 shows the yield strength (YS), ultimate tensile strength (UTS) and elongation (El.) of steel at different temperatures. The YS of all steels dropped when the temperature rose (Figure 9a). The YS of hot-rolled steels showed a marginal drop, while the YS of the heat-treated steels showed a rapid drop. It was observed that the hot-rolled (HR) steels retained two-thirds of their room-temperature yield strength at 600 °C, while the heat-treated (HT) steels were below the required level. It was also found that the YS of N2 steels (HR and HT) was higher than of N1 steels (HR and HT) from RT to 600 °C.

The UTS of steels dropped nonlinearly with rising temperature (Figure 9b). Meanwhile, the elongation of steels curved in an opposite manner to the UTS with rising temperature, as shown in Figure 9c. The result showed that the highest UTS and the lowest El. appeared at 300 °C. It was also found that the YS of the steels had a peak value at 300 °C. These phenomena were due to the dynamic strain ageing (DSA) at 300 °C [20].

4. Discussion

4.1. Bainite Strengthening

The YS of N2-HR steel at RT and 600 °C was higher than that of N1-HR steel, by about 81 and 30 MPa, respectively. This indicates that Nb plays an important role in improving the strength of fire-resistant steel, whether at room temperature or at 600 °C. The YS of hot-rolled steels had minor variation as the temperature rose. In contrast, the YS of heat-treated steels dropped rapidly as the temperature rose. This was due to the different microstructures in the steels. The hot-rolled steels showed a ferrite–pearlite–bainite microstructure while the heat-treated steels showed a ferrite–pearlite microstructure. This indicated that the ferrite–pearlite–bainite microstructure, especially bainite, plays an important contribution to the high temperature strength of steel. Thus, bainite strengthening has an important influence on the high-temperature strength of fire-resistant steel. As mentioned before, Nb plays a significant contribution to promoting the transformation of bainite. Thus, bainite strengthening is one of the important elevated-temperature strengthening mechanisms of Nb addition.

The TEM image (Figure 6c) depicted that bainite had a combination of high-density dislocation in the ferrite lath and many cementite precipitates between the ferrite laths; these dislocations and substructures play a significant role in improving the high-temperature strength [3]. It was also found that good high temperature stability can ensure the high strength of bainite, even at high temperature. Thus, the high temperature strength of bainite was higher than that of ferrite. In addition, with the increase in bainite volume fraction, a bainite/ferrite composite structure was formed, in which the ferrite was surrounded by bainite (Figure 2b). These combined microstructure interfaces strengthen the high temperature strength of the ferrite matrix. Therefore, when the volume fraction of bainite increased, the high-temperature strength of the steel increased significantly.

4.2. Precipitation Strengthening and Dislocation

Due to the similar ferrite–pearlite microstructures and the lower density dislocation in the heat-treated steels, the disparity of YS between heat-treated steels was considered as precipitation strengthening by Nb addition. The disparity of YS in heat-treated steels increased when the temperature rose. This indicates that the effect of precipitation enhancement is important for enhancing high temperature YS. Thus, precipitation strengthening is another important high temperature strengthening mechanism of Nb addition. It was found that the disparity of YS between N2-HR and N2-HT steels at 600 °C (64.1 MPa) was much higher than that between heat-treated steels (16.2 MPa) and between hot-rolled steels (30.1 MPa). It was observed that the influence of bainite strengthening on high temperature strength of N2-HR steel was below 30 MPa. This represents the existence of other elevated-temperature strengthening mechanisms between N2-HR and N2-HT steels. It was also found that the TEM images and X-ray data calculation illustrated higher density dislocation in N2-HR steel, compared with N2-HT steel. This indicates that higher density dislocation as well as bainite strengthening is helpful to ensure the high-temperature strength of fire-resistant steel.

Thus, there are two stages of precipitation strengthening for fire-resistant steel. First, some precipitates were observed to be distributed on the dislocations of N2-HR steel (Figure 6d). This showed that the presence of precipitates during hot rolling hindered the recovery of dislocations, and due to the pinning effect of Nb (C, N) precipitates, the dislocation density of N2-HR steel was higher than that of N1-HR steel. Second, during the tensile test, especially in the high temperature tensile test, the precipitate had the effect of hindering the climbing movement and the recovery of dislocations. Thus, the elevated-temperature strength of steels showed a remarkable improvement due to the precipitation strengthening of Nb. Moreover, the YS of N2-HR steel observed a rapid drop at 600 °C due to the lowering of dislocation density on soaking above 500 °C. The dislocation was recovered quickly with rising temperature, especially above recrystallization temperature (about 450 °C). Thus, the effect of dislocation strengthening on enhancing high-temperature YS showed a rapid drop above 500 °C.

4.3. Grain Refinement Strengthening

It is well known that finer grains make a remarkable contribution to enhancing yield strength at room temperature. The YS of N1-HT steel was higher than that of N1-HR steel when temperature was below 300 °C due to the finer grains in N1-HT steel. However, the YS of N1-HT steel rapidly dropped with rising temperature, and was lower than that of N1-HR steel when temperature was above 300 °C. This indicates that grain refinement strengthening at high temperature is weaker than at room temperature, and has minor effect on the high-temperature strength of fire-resistant steel.

As we all know, high mechanical properties are considered to be due to a reasonable balance between grain boundary strength and strength within grains [21]. Grain refinement is beneficial to the improvement of room temperature strength of steel because the strength of the grain boundary at room temperature is higher than that in the grain. Thus, the YS of N1-HT steel is higher than that of N1-HR steel at room temperature. However, with the increase in temperature, the strengthening effect of the grain boundary becomes weaker. At high temperatures, the strength of the grain boundary is equal to or even lower than that in the grain. Therefore, The YS of N1-HT steel is approximately equal to that of N1-HR steel at 300 °C, and the YS of N1-HT steel is lower than that of N1-HR steel at 600 °C. Grain refinement has little effect on improving the high temperature strength of steel.

5. Conclusions

The YS of N2-HR steel (addition of 0.03 wt.% Nb) at RT and 600 °C were higher than N1-HR steel (Nb-free) by about 81 and 30 MPa, respectively. This indicates that Nb plays an important role in improving the strength of fire-resistant steel, whether at room temperature or at 600 °C. Nb is an alloying element as effective as Mo in increasing the elevated-temperature strength of fire-resistant steel. Precipitation strengthening and bainite strengthening are the dominant strengthening mechanisms of Nb addition in enhancing elevated-temperature strength. Theoretical analysis shows that there are two precipitation strengthening stages in fire-resistant steel: (1) increasing dislocation density during hot rolling, and (2) blocking dislocation movement and recovery in tensile testing. The results also show that the effect of fine grain strengthening is not obvious at high temperature, but is obvious at room temperature.