Effect of Nb Content on High-Temperature Strength and Precipitates of Nb-Containing Steel

Abstract

Reasonable control of the content of niobium (Nb) in a steel slab is of great significance to improve the strength of the slab and the regularity of precipitation of precipitates. In this study, the high-temperature strength and precipitates of the same steel with four different Nb contents (A: 0.006%, B: 0.031%, C: 0.050%, D: 0.065%) were tested and analyzed. The results show that when the Nb content is 0.031%, the tensile strength and yield strength of the steel reach the ideal state; the two-phase particles that precipitated in the Q355GJ steel after heat treatment are mainly square- and star-shaped (Ti, Nb) (C, N) composite precipitates, and their amount increases with the increase in Nb content. Most of the two-phase particles (84%) precipitated in the steel with the Nb content of 0.031% are smaller than 80 nm, and the continuous increase in the Nb content cannot increase the precipitation amount of the two-phase particles in the steel, but can increase the size of the two-phase particles.

1. Introduction

Q355GJ Nb-containing high-strength steel is a kind of low-alloy, high-strength steel for building structures, produced and consumed most in China [1,2,3,4,5]. It has the characteristics of high strength, suitable low-temperature impact toughness and efficient welding performance, and is widely used in large ships, medium- and low-voltage containers, high-voltage transmission towers, bridges and building steel structures [6,7,8,9,10,11,12,13].

At present, Nb, Ti, V and other microalloying elements are commonly added to high-strength steel containing Nb at home and abroad. The primary role of Nb, Ti, V and other microalloying elements is to produce a solute drag effect, that is, to prevent the recrystallization of austenite during rolling, and to provide a large temperature range for the combined process of rolling in the non-recrystallization zone and controlling the cooling temperature, so as to achieve the effect of efficiently refining ferrite grains. At the same time, strong carbide-forming elements (e.g., Nb, Ti, V) combine with C and N to precipitate stable fine particles which have a strong hindrance to the movement of austenite grain boundaries, called the phenomenon of pinning, which not only inhibits the recrystallization behavior of the matrix during rolling, but also effectively prevents the grain coarsening in the heat-affected zone during welding. When plastic deformation occurs under stress, the second-phase particles in the material also interact with dislocations in various ways, forming resistance to the movement of dislocations, thus producing a strengthening effect, which significantly increases the stress required for the plastic deformation of materials [14]. The above three microalloying effects can greatly affect the mechanical properties of steel, such as strength, toughness and formability. The literature [14,15,16,17,18,19,20,21] has reported the crystal structure, distribution morphology and strengthening effect of the second-phase particles precipitated from Nb, V, Ti, Mo, W, V-Ti, Nb-Ti, Nb-V-Ti and other microalloy systems under laboratory conditions, showing that this kind of carbonitride basically has a NaCl-type crystal structure, and its mismatch with ferrite is generally lower than that of austenite.

For high-strength building structure steel, the control and optimization relationship between controlled rolling and controlled cooling processes and specific structure and properties has been studied by scholars at home and abroad [22,23,24]; however, the influence of Nb on the microstructure, high-temperature mechanical properties and precipitation behavior of two-phase particles at different temperatures of Q355GJ steel with different Nb contents has rarely been reported. Therefore, in this paper, the high-temperature mechanical properties of four kinds of high-strength steels with different Nb contents were tested, and the thermodynamic calculation of the solid solution precipitation of two-phase particles in the solid phase was carried out. By analyzing the crystal structure, phase composition, particle size and distribution pattern of the precipitates of two-phase particles, the optimal Nb content range in high-strength steel is determined, which provides a basis for the smelting process of high-rise structural steel and improving product quality.

2. Experimental Scheme

2.1. Smelting Experiment

The smelting experiment was carried out in a vacuum induction furnace. The base material for smelting was IF (Interstitial-Free) steel continuous casting billet produced by a factory, and its main chemical composition is shown in

Table 1. The main chemical composition of IF steel slab.

C	Si	Mn	P	S	Al	Ti
≤0.003	≤0.03	0.1~0.2	≤0.006	≤0.007	0.02~0.05	0.04~0.08

. The alloy materials used in smelting experiments included ferrosilicon, low-carbon ferromanganese, ferrotitanium, ferroniobium, aluminum wire, etc. When the steel block was melted, the alloy material was added into the molten steel in a certain order, and it was cast into about 35 kg ingots.

During steel smelting of each furnace, it was required to ensure that the quality of the base material fed into the furnace was consistent, that the amount of alloy material was weighed according to the design requirements, and that the sequence of alloy material into the furnace and the situation of molten steel when added was as consistent as possible. After tapping, it was required to ensure that the cooling conditions of the four kinds of test steels during the cooling process are consistent, to reduce the impact of external factors on the ingot performance and improve the reliability of subsequent experimental results.

Table 2. Chemical composition of high-rise structural steel with different Nb contents (wt %).

	C	Si	Mn	P	S	Nb	Ti	Al	N
A	0.174	0.206	1.567	0.014	0.009	0.005	0.029	0.021	0.0060
B	0.173	0.202	1.555	0.014	0.008	0.030	0.026	0.013	0.0055
C	0.175	0.210	1.546	0.014	0.008	0.051	0.027	0.015	0.0056
D	0.173	0.208	1.557	0.014	0.008	0.064	0.026	0.013	0.0058

shows the chemical composition of four steels with different Nb contents.

2.2. High-Temperature Strength Performance Experiment

The high-temperature tensile test of Q355GJ steel was carried out in the Gleeble-3500 thermal simulation test machine (Dynamic Systems Inc., Poestenkill, NY, USA), with the test temperature range of 1573~1023 K and the test temperature interval of 323 K. According to the instrument requirements, cut a 14 mm × 14 mm × 135 mm long strip sample from Q355GJ steel smelted in the laboratory, then use the lathe to process the strip sample into a 120 mm × ø 10 mm round bar; an M10 thread was processed within 10 mm of both ends of the round bar, which was used to fix the sample during the experiment. Figure 1 shows the specific morphology of the round bar sample.

After the sample is held, vacuumize the sample chamber, and then introduce Ar gas as the shielding gas. Figure 2 shows the specific heating system of the high-temperature tensile test. Raise the sample to 1623 K at the heating rate of 283 K·s⁻¹ and keep it warm for 4 min. In order to simulate the thermal history of the slab in the secondary cooling zone of the continuous casting process, combined with the actual production, the cooling rate was selected as 373 K·min⁻¹ (6.22 K·s⁻¹) to reduce the sample to the predetermined test temperature. After holding for 2 min, tension the specimen at a strain rate of 10⁻³ s⁻¹ until the specimen breaks. After the specimen is pulled off, the fracture should be cooled by spraying water immediately to keep the original appearance of the fracture for later observation.

2.3. Precipitate Analysis

The steps of the carbon replica method are as follows: etch the metallographic polished sample in 4% (volume fraction) nitric acid ethanol solution, then evaporate a layer of 30~40 nm carbon film on the eroded surface under vacuum conditions, use a blade to cut a square with a side length of 2 mm, put the sample in 8% (volume fraction) nitric acid ethanol solution for extraction, wash the extracted carbon film repeatedly in ethanol, place it in deionized water for development and fish it out with a copper mesh. After drying, the transmission electron microscope sample is yielded.

First, use Thermo-Calc software to draw the equilibrium phase diagram of four kinds of Q355GJ high-strength steel with different Nb contents, as shown in Figure 3. It can be seen from the figure that at the beginning of solidification of four different Nb-containing steels, it first enters the δ ferrite region. After going through a small temperature range and reaching the solidus temperature, the liquid phase and δ ferrite are completely transformed into austenite, and the temperature range of austenite is relatively large. When the temperature drops to about 1100 K, α ferrite transformation begins.

According to the chemical composition of the experimental steel in Table 2, the liquidus and solidus of four kinds of steel with different Nb contents can be calculated by using the following Formulas (1) and (2). The results are shown in Table 3, which is basically consistent with the calculation results of Thermo-Calc software.

$${\mathrm{T}}_{\mathrm{L}}=1809-83\mathrm{w}(\mathrm{C})-7.8\mathrm{w}(\mathrm{Si})-5\mathrm{w}(\mathrm{Mn})-32\mathrm{w}(\mathrm{P})-31.5\mathrm{w}(\mathrm{S})-1.5\mathrm{w}(\mathrm{Cr})-2\mathrm{w}(\mathrm{Mo})-2\mathrm{w}(\mathrm{V})-3.6\mathrm{w}(\mathrm{Al})-18\mathrm{w}(\mathrm{Ti})$$

(1)

$${\mathrm{T}}_{\mathrm{s}}=1809-344\mathrm{w}(\mathrm{C})-12.3\mathrm{w}(\mathrm{Si})-6.8\mathrm{w}(\mathrm{Mn})-124.5\mathrm{w}(\mathrm{P})-183.5\mathrm{w}(\mathrm{S})-1.4\mathrm{w}(\mathrm{Cr})-4.1\mathrm{w}(\mathrm{Al})-4.3\mathrm{w}(\mathrm{Ni})$$

(2)

In the formula, w is the mass concentration of the element.

Table 3. Solid and liquidus temperature of samples.

	TS/K	TL/K
A	1731.69	1784.40
B	1732.46	1785.45
C	1731.84	1783.97
D	1732.50	1784.53

Table 3. Solid and liquidus temperature of samples.

	TS/K	TL/K
A	1731.69	1784.40
B	1732.46	1785.45
C	1731.84	1783.97
D	1732.50	1784.53

During the high-temperature tensile test, the maximum temperature of the sample was far lower than the solidus temperature. Therefore, only the solid solution of Nb (C, N) in the solid phase was considered in the calculation process.

3. Experimental Results and Analysis

3.1. Analysis of High-Temperature Strength Test Results

3.1.1. Tensile Strength Curve

Figure 4 shows the tensile strength curve of Q355GJ steel with different Nb contents, showing that the tensile strength of the four specimens decreases with increasing temperature in general. There is an obvious low strength area in the temperature range of 1073 K~1173 K for the tensile strength of sample A. At 1123 K, the minimum tensile strength is 68.15 MPa. At 1173 K~1073 K, there is a high strength point in sample B, and at 1173 K, the maximum tensile strength is 154.73 MPa. At 1173 K~1573 K, the tensile strength of sample A and sample B decreases with the increase in temperature, and at 1573 K, it reaches the minimum value, 25.7 MPa and 29.32 MPa, respectively. Although the Nb content of sample C is less than that of sample D, the high-temperature tensile strength of sample C is slightly higher than that of sample D as a whole. The tensile strength of the two curves decreases with the increase in temperature. The tensile strength of samples C and D reaches the maximum 229.12 MPa and 222.32 MPa at 1023 K, and the minimum 26.33 MPa and 25.33 MPa at 1573 K.

3.1.2. Yield Strength Curve

Yield strength is one of the important characteristics of high-temperature properties of materials, and the factors affecting yield strength are mainly related to lattice type, microstructure, precipitates, metal nature, etc. Due to the composition characteristics of samples A and B, Nb content has become a key factor. With the increase in Nb content, the microstructure of sample B is significantly more refined than that of sample A. The fine grain structure increases the number of grain boundaries in the unit area, and the dislocation movement process needs to overcome greater resistance to reduce the area of dislocation aggregation area in the grain, thereby improving the yield strength of steel.

Figure 5 shows the relationship between the yield strength of Q355GJ steel and temperature. The yield strength of the four samples decreased with the increase in temperature. The yield strength curve of sample A is below samples B–D as a whole, indicating that the three samples have adequate plastic deformation resistance at the test temperature. Sample A has an obvious low yield strength zone between 1173 K and 1073 K, and the minimum yield strength is 30.37 MPa at 1123 K. At 1023 K, the yield strength of the four samples reached the maximum values of 59.68 MPa, 110.25 MPa, 74.25 MPa and 156.28 MPa. The minimum yield strength values of the samples are 14.25 MPa, 20.36 MPa, 19.35 MPa and 19.64 MPa, respectively.

The strengthening effect of precipitates is an important internal factor affecting the yield strength of steel. With the increase in Nb content, it provides an element guarantee for the formation of more precipitates. Combined with the previous results, it can be found that the total number of precipitates in sample D is slightly less than in other samples, but the size of precipitates becomes larger, which increases the distance between adjacent particles. According to dislocation theory, dislocation movement must bypass some non-deformable particles such as Nb (C, N), which increases the line tension, slows down the dislocation movement speed and finally improves the yield strength of the sample.

With the addition of the Nb element, the microstructure of the sample becomes more uniform and fine, which improves the high-temperature tensile strength and yield strength of Q355GJ steel, and has strong resistance to deformation at high temperature, which is important to ensure the smooth progress of continuous casting.

The microstructure of the fracture is mainly martensite and bainite, with a small amount of ferrite. Martensite and bainite are transformed from austenite in the cooling process. In Figure 6, as the Nb content of sample B is higher than that of sample A, the grains are finer and the resistance to deformation is stronger [25,26,27]. it is found that there is reticular ferrite. Since the C content of Q355GJ steel is about 0.173%, proeutectoid ferrite is generated in the cooling process.

In Figure 6, proeutectoid ferrite is not found, and the main structures are martensite and bainite, which indicates that the sample does not undergo ferrite transformation at these three test temperatures, and it is in the single-phase austenite region. Combined with the high-temperature strength curve of Q355GJ steel, the sample has adequate strength. There are two main reasons for the better strength of austenite in the single-phase zone: (1) the austenite has a face-centered cubic structure, with many slip directions and slip systems, and the strength is better; (2) dynamic recrystallization of austenite occurs at high temperature, which increases the driving force of austenite grain boundary migration and reduces the difficulty of dislocation migration. As the migration speed of the grain boundary is relatively high, when the migration speed exceeds the sliding speed of the grain boundary, defects such as microcracks can be surrounded in the grain to prevent crack propagation, thus improving the high-temperature strength.

The carbon extraction replica samples were observed by transmission electron microscopy (JEM-F200) (JOEL, Akishima shi, Tokyo, Japan), and selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS) analyses were conducted for some representative particles to determine their crystal structure and the types and contents of constituent elements.

The distribution of precipitates is shown in Figure 7. It can be seen that the larger particles are mainly distributed in a linear arrangement, while the smaller particles are more dispersed, which is the same as the observation results of the strain-induced precipitation behavior of Nb-Ti-V microalloyed HSLA steel under laboratory conditions in the literature [15]. It shows that under the controlled rolling and cooling process, carbides and carbonitrides preferentially nucleate and grow up along the austenite grain boundary to form the second phase with a slightly larger particle size, and then the particle size precipitated in the grain is smaller. In order to make the quantitative calculation of precipitates more accurate, 15 bright-field images under the above two different fields of view were randomly selected, and the equivalent diameter and quantity of precipitates in all photos were counted with Image pro software. The results are shown in Figure 8.

We counted the number of two-phase particles in the microscopic morphology of two-phase particles. Figure 8 shows the size distribution of precipitates in the sample, and

Table 4. Size statistics of precipitates in the sample.

	A	B	C	D
Total number of particles/piece	346	585	477	509
Proportion of particle size >80 nm/%	24.57	16.07	19.92	20.83

shows the statistical data of large-sized precipitates in the sample. The results show that the total number of two-phase particles in samples A–D is 346, 585, 477 and 509, respectively. The number of two-phase particles precipitated in samples B–D is obviously greater than that of two-phase particles precipitated in sample A. Since the content of other elements in the sample is almost the same, it will hardly affect the precipitation of two-phase particles. Therefore, the change in Nb (C, N) precipitation can reflect the overall change in two-phase particles in the sample.

According to the size statistics of the precipitates, the size of the precipitates of the four samples is mostly concentrated in the range of 10 nm~80 nm, of which the large size (>80 nm) precipitates only account for 24.57%, 16.07%, 19.92% and 20.83% of the total precipitates, providing favorable conditions for the precipitates to exert their second-phase strengthening. In sample B, the proportion of precipitates smaller than 80 nm is larger, and the distribution in the steel matrix is more uniform and dispersed, which has a positive role in improving the overall performance of Q355GJ steel.

4. Analysis and Discussion

At present, many scholars mainly study the precipitation in steel through thermodynamic calculations, solution precipitation behavior and other means [28,29,30,31]. In the cooling process of slabs, the solid solution and precipitation of precipitates is a reversible process, because the change in temperature will change the direction of the reaction. Therefore, the thermodynamic calculation of precipitation is of great significance for analyzing the precipitation behavior of precipitates. The preferred research method for the observation of precipitation results is transmission electron microscopy, which can determine the precipitation location, particle size and overall morphology of two-phase particles, which provides an important theoretical basis for in-depth study of the high-temperature properties and crack generation mechanism of precipitates in steel.

4.1. Thermodynamic Analysis

The liquidus and solidus temperatures of four kinds of steels with different Nb contents were calculated according to Equations (1) and (2) above. During the high-temperature tensile test, the maximum temperature that the sample bore was far lower than the solidus temperature. Therefore, only the solid solution of (Nb, Ti) (C, N) in the solid phase was considered in the calculation process.

The commonly used solid solubility product formulas of NbC and NbN are as follows [32]:

$$\lg {\left([\mathrm{Nb}][\mathrm{C}]\right)}_{\mathrm{\gamma }}=2.206-6746/\mathrm{T}$$

(3)

$$\lg {\left([\mathrm{Nb}][\mathrm{N}]\right)}_{\mathrm{\gamma }}=2.394-9076/\mathrm{T}$$

(4)

$$\lg {\left(\left[\mathrm{Nb}\right]\left[\mathrm{C}\right]\right)}_{\mathsf{\alpha }}=5.137-11013/\mathrm{T}$$

(5)

$$\lg {\left(\left[\mathrm{Nb}\right]\left[\mathrm{N}\right]\right)}_{\mathsf{\alpha }}=4.383-10249/\mathrm{T}$$

(6)

where [Nb], [C] and [N] are the mass fractions of Nb, C and N in austenite and ferrite.

By further processing the calculated solid solubility of Nb compounds, the relationship between the precipitation amount of Nb-containing precipitates and temperature can be obtained, as shown in Figure 9. In the figure, the change curves of the four samples are the same as a whole, and the amount of Nb-containing precipitates decreases gradually with the increase in temperature. It can also be found that the amount of precipitates will increase with the increase in Nb content.

Although the content of Nb in samples A, B, C and D is different, at 1023 K, the precipitation amount of Nb compounds is 99.78%, 98.97%, 97.47% and 97.34%, respectively, almost all of which are precipitated. The increase in Nb content increases the initial precipitation temperature of Nb compounds (the initial precipitation temperatures of samples A, B, C and D are 1403 K, 1447 K, 1453 K and 1455 K, respectively). At the same temperature, the increase in Nb content promotes the precipitation of Nb compounds.

In a single Ti microalloyed steel, according to the binding ability of Ti and other elements, the sequence of compound precipitation is Ti₂O₃, TiN, Ti₄C₂S₂ and TiC. Generally, Ti₂O₃ and Ti₄C₂S₂ are separated into the steel slag, and only TiN and TiC exist in the matrix [33]. In a single Nb microalloyed steel, there are generally only NbN and NbC. Based on the analysis of EDS spectra and the mass ratio of Ti/Nb, it can be considered that with the decrease in temperature, the replacement behavior of Nb and C atoms occurs in TiN under the same lattice type, and finally, (Nb, Ti) (C, N) phase is formed.

The empirical formula is used to calculate the solid solution of Ti compounds in steel. The commonly used formulas for the solid solution product of Ti compounds are as follows [32]:

$$\lg {\left(\left[\mathrm{T}\mathrm{i}\right]\left[\mathrm{C}\right]\right)}_{\mathrm{\gamma }}=2.75-7000/\mathrm{T}$$

(7)

$$\lg {\left(\left[\mathrm{T}\mathrm{i}\right]\left[\mathrm{N}\right]\right)}_{\mathrm{\gamma }}=0.32-8000/\mathrm{T}$$

(8)

$$\lg {\left(\left[\mathrm{T}\mathrm{i}\right]\left[\mathrm{C}\right]\right)}_{\mathrm{\alpha }}=4.40-9575/\mathrm{T}$$

(9)

By further processing the calculated value of solid solubility of Ti compounds, the relationship between the precipitation amount of Ti-containing precipitates and temperature can be obtained, as shown in Figure 10. In the figure, the change curves of the four samples are the same as a whole, and the amount of Ti-containing precipitates decreases gradually with the increase in temperature.

In Figure 10, the change curve of the precipitation amount of Ti compounds in samples A, B, C and D with temperature is close, and the precipitation amount increases with the decrease in temperature. At 1573 K, the precipitation amount of Ti compounds in samples A, B, C and D is 73.06%, 74.35%, 75.57% and 74.86%, respectively. At 1073 K, the precipitation amount of Ti compounds is almost complete, being 99.85%, 99.88%, 99.92% and 99.75%, respectively.

According to the results of thermodynamic calculations, the precipitation amount of Ti precipitates in samples with different Nb contents is basically the same. At 1073 K, almost all of the Ti precipitates in Q355GJ steel samples with different Nb contents are precipitated. The precipitation of the second-phase particles in the samples can be judged by the precipitation of Nb precipitates.

4.2. Phase and Composition of Precipitates

Figure 11 shows the microscopic morphology of Nb-containing precipitates at 1373 K and 1473 K. According to the thermodynamic calculation results of two-phase particle precipitation, after heat treatment at 1373 K and 1473 K, the microalloyed elements combine with C and N in the steel to form carbonitride precipitation in the steel.

As shown in Figure 12, in sample A, the Nb content is only 0.006%, so the number of precipitated second-phase particles is small. At the same temperature, a large number of second-phase particles is precipitated in samples B, C and D, and the precipitation amount is significantly more than sample A, indicating that the increase in Nb content promotes the precipitation of Nb-containing second-phase particles, which is consistent with the precipitation thermodynamic calculation results in the previous section.

The precipitation rule of the second-phase particles in the sample after heat treatment at 1473 K is basically consistent with that after heat treatment at 1373 K. For the same sample, the higher the heat treatment temperature is, the greater the number and size of the second-phase particles are.

Since the initial precipitation temperature of Ti (C, N) is relatively high, more than 90% of Ti (C, N) is precipitated during the heat treatment at 1473 K and 1373 K. Therefore, the precipitation amount of the second-phase particles in the sample after heat treatment is mainly determined by the Nb (C, N) precipitated from the steel.

The increase in Nb content is bound to promote the formation of Nb (C, N) in the sample. During the cooling process, Nb (C, N) will continue to precipitate in the matrix. Because the Nb and Ti two-phase particles have a face-centered cubic structure and NaCl crystal form, they can be infinitely miscible with each other. With the increase in the precipitation amount of the two-phase particles, the size of the precipitated phase will grow. According to the thermodynamic calculation results of Nb (C, N) precipitation, the amount of Nb (C, N) precipitation will gradually increase with the decrease in heat treatment temperature; especially in the temperature region near the initial precipitation temperature, the increasing trend of the amount of Nb (C, N) precipitation is more obvious. After heat treatment at 1373 K for the sample with the same Nb content, the precipitation amount of Nb- and Ti-containing two-phase particles in the sample increases significantly.

5. Conclusions

The high-temperature tensile strength and yield strength of four kinds of test steels with different Nb contents were tested by the Gleeble-3500 thermal simulation machine. The morphology and composition of Nb-containing precipitates in the test steel were analyzed by a transmission electron microscope, and the following conclusions were obtained through thermodynamic calculation.

(1)

In the temperature range of 1373~1023 K, the increase in Nb content significantly improves the high-temperature tensile strength and yield strength of Q355GJ steel, which makes Q355GJ steel have strong resistance to deformation at high temperatures. When Nb content is 0.031%, the high-temperature strength performance of the steel is the best.

(2)

According to the thermodynamic calculation results, at 1023 K, almost all Nb compounds in samples A, B, C and D are precipitated. The increase in Nb content increases the initial precipitation temperature of Nb compounds (the initial precipitation temperature of Nb (C, N) in samples A, B, C and D is 1403 K, 1447 K, 1453 K and 1455 K, respectively).

(3)

After heat treatment at 1373 K and 1473 K, the precipitation amount of the two-phase particles in samples B, C and D is obviously more than that in sample A, and the precipitation amount of the two-phase particles is obviously increased as the heat treatment temperature increases. The second-phase particles are mainly square- and star-shaped (Ti, Nb) (C, N) composite precipitates. The precipitated two-phase particles in sample B have a smaller size and more uniform distribution; those smaller than 80 nm account for about 84% of the total number, which is more effective for improving the overall performance of Q355GJ steel.