Research, Development, and Application of Cost-Effective Niobium-Vanadium Microalloyed HRB600E Reinforcing Bars
Release time:
2026-08-03
Abstract: To meet the evolving demands of the construction steel market, this study optimized the design of niobium-vanadium composite microalloying and the controlled rolling and cooling processes. By integrating research on the thermodynamic behavior of microalloying elements and phase transformation kinetics, the properties and microstructure of the prototype reinforcing bars were examined and analyzed. The results indicate that the prototype reinforcing bars exhibit excellent properties and microstructure.
0 Introduction
Over 60% of the world’s floor area is located in regions prone to earthquakes, and traditional 335 MPa-grade reinforcing bars struggle to meet the seismic requirements of high-rise buildings. Reinforcing bars with a yield strength of 500 MPa or higher are already widely used in Europe, the United States, and Japan [1–2]. China’s 12th Five-Year Plan explicitly states the need to “adapt to the trend of reduced steel consumption, upgrade standards for hot-rolled ribbed steel bars, and prioritize the development of high-strength ribbed steel bars and seismic-resistant steel bars with yields of 400 MPa and above” [3]. To address the issues of reliance on imports and high costs associated with high-strength steel bars, this study aims to develop cost-effective 600 MPa-grade seismic-resistant steel bars through chemical composition optimization and process innovation, thereby promoting carbon reduction in construction and the upgrading of the steel industry.
1 Composition Design and Strengthening Mechanisms
1.1 Product Requirements
Composition design is based on the required product performance and the influence of various chemical elements on the overall mechanical properties. The main mechanical properties of HRB600E are shown in Table 1.
Table 1 Key Mechanical Properties of HRB600E
1.2 Chemical Composition Design
Niobium and vanadium exhibit strong precipitation hardening effects. By adding niobium-vanadium alloy in specific proportions, it is possible to reduce continuous casting cracks in niobium-containing steel and address the issue of low elongation in reinforcing bars.
Based on product requirements, and incorporating the properties of the elements as well as strength and elongation calculation models, the design was conducted in accordance with the “Hot-Rolled Ribbed Steel Bars” standard. The composition design employs microalloying for strengthening and utilizes methods such as substituting niobium-phosphorus iron for niobium iron to reduce production costs per ton of steel. Conventional elements are controlled in accordance with standard requirements. The composition for HRB600E was designed, and the results are shown in Table 2.
Table 2 Chemical Composition of HRB600E
2 Study on the Thermodynamic Behavior of Microalloying Elements
The microalloying elements niobium and vanadium promote the precipitation of carbonitrides during cooling and rolling, thereby achieving precipitation hardening. Under the influence of interfacial energy and solute segregation, the second phase in steel typically precipitates preferentially at austenite grain boundaries. The extent of second-phase precipitation is closely related to its solubility in the matrix and its thermodynamic precipitation behavior. By calculating the thermodynamics of second-phase precipitation in experimental steels and studying the precipitation behavior of microalloying elements, appropriate production processes can be established to maximize the beneficial effects of microalloying elements in the experimental steels [4–5].
At temperatures above 1,100°C, the solubility of niobium in austenite is significantly influenced by nitrogen content. When the temperature is below 1,050°C, the mass fraction of dissolved niobium is generally low. At temperatures above 750°C, the niobium content in ferrite decreases as the nitrogen content in the steel increases. The lower the carbon mass fraction, the higher the corresponding solubility of niobium.
During the phase transformation from austenite to ferrite in high-strength reinforcing bars, the rapid precipitation of vanadium carbonitrides refines the ferrite grain size, thereby improving the strength and ductility of the bars. Thermodynamic calculations reveal the precipitation temperatures of the microalloying elements niobium and vanadium in HRB600E high-strength reinforcing bars, as shown in Table 3. A key aspect of the microstructure control process for high-strength reinforcing bars is managing the precipitation behavior of microalloying elements in the steel to achieve the desired microstructure.
Table 3 Precipitation Temperatures of Niobium and Vanadium in Steel
3 Study of Phase Transformation Kinetics
HRB600E steel was produced according to a specific chemical composition and machined into specimens measuring 8 mm in diameter by 80 mm in length. Thermal simulation experiments were conducted on a Gleeble 3800 thermal simulation machine.
3.1 Critical Points of the Test Steel
The test steel was heated at a rate of 0.05 °C/s and cooled at a rate of 100 °C/s. The critical heating and cooling transformation curves were plotted, yielding the following values for the test steel: Ac1 and Ac3 at 736 °C and 852 °C, respectively, and Ms and Mf at 424 °C and 291 °C, respectively.
3.2 Effect of Heating Rate on Ac1 and Ac3
As the heating rate increases, both the Ac1 and Ac3 values increase to varying degrees, as shown in Table 4. This is because the formation of austenite is accomplished through atomic diffusion, and this diffusion process requires a certain amount of time. When the heating rate increases, the diffusion time at a given temperature is shortened, thereby pushing the temperature at which the transformation originally begins to a higher temperature, and correspondingly raising the transformation completion temperature.
Table 4 Ac1 and Ac3 Values at Different Heating Rates
3.3 CCT Curves and Microstructure of the Experimental Steel Figure 1 shows the CCT curve of the experimental steel. When the cooling rate is below 1 °C/s, the microstructure consists of pearlite and ferrite, and the grain size decreases as the cooling rate increases. When the cooling rate exceeds 1 °C/s, bainite begins to form and increases with the cooling rate; the microstructure at cooling rates between 1 and 5 °C/s consists of bainite and ferrite. This occurs because, as the cooling rate increases, the diffusion rate of carbon in the steel slows down, and the diffusion distance decreases. Since the diffusion rate of carbon in ferrite is higher than that in austenite, carbon-rich zones in the austenite between ferrite bands precipitate discontinuous carbides, thereby forming bainite; When the cooling rate exceeds 5 °C/s, the microstructure consists of martensite and ferrite. The proportion of martensite increases with the cooling rate because the accelerated cooling further slows the diffusion of carbon, causing the phase transformation to shift toward shear transformation, which promotes the formation of martensite.
As the cooling rate increases, the hardness of the test steel gradually increases. When the cooling rate is between 0.3 and 4.0 °C/s, the ferrite content decreases, while the pearlite and bainite contents increase, resulting in higher hardness of the test steel; When the cooling rate exceeds 5 °C/s, the microstructural transformation products consist primarily of ferrite, bainite, and martensite. As the cooling rate increases, the rate of increase in bainite and martensite content slows, and the rise in hardness also slows [6].
4 Industrial Production Practices
4.1 Process Flow
Blast furnace molten iron → Converter → Argon blowing → LF → Continuous casting → Inspection → Finishing → Hot or cold delivery → Heating in a reheating furnace → Descaling → Rough rolling → Intermediate rolling → Finishing rolling → Over-length shearing → Cooling on a cooling bed → Sampling and inspection → Cut-to-length shearing → Bundling, weighing, tagging, and collection → Storage and shipment.
4.2 Smelting and Continuous Casting Processes
For HRB600E, niobium-phosphorus iron and re-precipitation alloys are used as substitutes. The target end points for converter smelting are shown in Table 5. Once the molten steel arrives at the refining station, argon gas is introduced and the flow rate is adjusted. Once the ladle is positioned at the refining station, add 200–500 kg of lime and 50–100 kg of fluorite. After 10 minutes of electroslag refining, measure the temperature and take samples. Add deoxidizers to produce white slag. Based on the analysis results of samples taken from the LF furnace, add the re-precipitation alloy simultaneously with the conventional alloying agents, and maintain argon blowing for ≥5 minutes. Once the target composition and temperature are reached, feed the calcium wire. The soft argon blowing time is ≥8 min. Maintain argon protection throughout the continuous casting process. Seal the large ladle’s long nozzle with argon; the long nozzle should be inserted ≥150 mm below the liquid surface. During normal pouring, the liquid level in the intermediate ladle should be ≥700 mm. Pour at a constant pulling speed. The cast billet is a 160 mm × 160 mm square billet. The chemical composition of the ingot is shown in Table 6.
Figure 1 CCT curve for the test steel
Table 5 Reference Values for Smelting End Points
Table 6 Chemical Composition (Melting Analysis)
4.3 Rolling and Cooling Processes
The microstructure of HRB600E after hot rolling consists of pearlite and ferrite. To prevent the formation of closed rings in the macrostructure, and to avoid abnormal microstructures such as martensite caused by uneven heating or inadequate cooling control, The billet must be heated uniformly, with strict control over heating duration and temperature to prevent overheating, overburning, or insufficient core temperature, which could lead to rolling cracks or roll breakage. The heating process is shown in Table 7; temperature-controlled rolling is employed to refine the grain structure and enhance the performance of the reinforcing bars.
Table 7 Temperature Control for the Heating Process Unit: °C
The cooling water supply must not be shut off during the production process. Rolled stock temperature control: Adjust the water pressure and flow rate to control the temperature of the rolled stock entering the finishing mill and the temperature upon entering the cooling bed. The controlled rolling and cooling process is shown in Table 8.
Table 8 Controlled Cooling Process Unit: °C
5 Product Performance
5.1 Results and Analysis of Mechanical Properties
The mechanical properties of HRB600E are shown in Table 9. As can be seen from Table 9, the specimens’ yield strength, tensile strength, and total elongation at maximum force fully meet both internal control requirements and the new standard.
5.2 Microstructural Results and Analysis
The results of the metallographic examination are shown in Table 10. None of the HRB600E specimens of different specifications exhibited a quenched layer; the microstructure consisted of ferrite and pearlite, with some specimens showing core segregation. All microstructures met the standard requirements.
6 Conclusions
1) The HRB600E reinforcing bars in sizes ranging from 18 to 32 mm, developed using a low-niobium + vanadium-nitrogen composite strengthening alloy composition, effectively address the technical challenges of low strength-to-yield ratio and low first-pass yield in high-strength reinforcing bars.
2) The addition of a composite alloy and the use of niobium-phosphorus iron in place of niobium iron can effectively reduce the production costs of high-strength seismic-resistant reinforcing bars.
Table 9 Mechanical Properties of HRB600E
Table 10 Microstructure of HRB600E
References: Shanxi Metallurgy; Research, Development, and Application of Economical Niobium-Vanadium Microalloyed HRB600E Reinforcing Bars; Li He
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