1. Preface
High-strength steel bars are considered the backbone and skeleton of the construction industry. Currently, there are five main areas of development of high-strength reinforcement materials:
- Emphasizing research, development, promotion and implementation of high-strength steel bars with strength of 500MPa and above.
- Focusing on the production and use of seismic reinforcement.
- Emphasizing the research, development, promotion and implementation of corrosion-resistant reinforcement.
- Strengthen research, development, promotion and application of high-performance and cost-effective reinforcements.
- Focusing on research into technology for applying high-resistance reinforcement.
This article provides a brief overview of the properties and production process of high-strength steel bars and seismic steel bars with a grade of 500MPa and above for civil construction.
2. Production process of 500 MPa and above high strength reinforcement
2.1 Production process of 500MPa high strength reinforcement
The main production process of 500 MPa high-strength steel bars involves adding the microalloy element vanadium to 20MnSi low-alloy steel and using cheap nitrogen to obtain precipitation strengthening. This allows the steel to reach a strength of 500MPa.
Vanadium microalloy technology has several advantages, including economical and reasonable composition design, stable reinforcement performance, high strength-yield ratio, and excellent welding and low-temperature performance.
This process is considered an ideal method to produce high strength reinforcement of 500MPa.
2.1.1 Composition design and mechanical properties
GB1499.2 (revised 2016) specifies that the chemical composition and carbon equivalent of HRB500 must meet the requirements listed in Table 1. In addition, elements such as vanadium, niobium and titanium can be added to the steel as needed.
Table 1 in GB1499.2 (revised 2016) describes the chemical composition and mechanical property requirements for 500MPa high strength reinforcement.
| Chemical composition, % by mass | Brand | HRB500 | HRBFS00 | HRBSODE | HRBFSOOE |
| W | 0.25 | ||||
| Yes | 0.8 | ||||
| Mn | 1.6 | ||||
| P | 0.045 | ||||
| s | 0.045 | ||||
| Here | 0.S5 | ||||
| Mechanical property | Yield limit RtL, MPa | 500 | |||
| Tensile strength R, MPa | 630 | ||||
| Stretching after fracture A% | 15 | – | |||
| Ratio of total secondary length to maximum force A% | 7.5 | 9 | |||
2.1.2 Technical route
The technical processes for producing 500 MPa high-strength steel bars include heat treatment of post-rolling waste, ultrafine grains and microalloys.
The first two methods utilize the low alloy steel composition 20MnSi, while the microalloying process involves adding microalloying elements such as vanadium, niobium and titanium to the 20MnSi.
1) Microalloy
Microalloy technology improves the mechanical properties of steel by adding microalloy elements to 20MnSi steel through metallurgical methods. The strengthening mechanism involves the formation of high-melting, high-hardness carbides and nitrides from the microalloying elements and carbon and nitrogen atoms of the steel.
On the one hand, the precipitation of these carbides and nitrides at the austenite grain boundary hinders the growth of the austenite grains during heating and results in the strengthening of the fine grains.
On the other hand, the precipitation of these carbides and nitrides during or after the transformation of austenite into ferrite hinders the dislocation movement in the iron lattice and leads to the strengthening of precipitation.
2) Ultrafine grain technology
Ultrafine grain technology is a modern production process that combines controlled rolling and controlled cooling, and does not require the addition of microalloying elements. The implementation of this process requires computerized temperature control throughout the steel rolling production line, and the specific steel rolling process system must be adapted to the steel variety and specifications.
This technology utilizes a combination of recrystallization-controlled rolling, non-recrystallization-controlled rolling, strain-induced ferrite transformation, and dynamic ferrite recrystallization mechanisms to control grain size and microstructure, ultimately achieving grain reinforcement. thin steel.
3) Residual heat treatment after rolling
Post-lamination waste heat treatment technology is a process that does not require the addition of microalloying elements. It integrates hot rolling and heat treatment processes, where steel bars are quenched online after hot rolling for surface cooling, and then the residual heat from the steel core is used to quench the surface layer of steel bars. This transforms the surface structure of the steel bars into tempered sorbite, which maintains the martensitic orientation, while the core becomes a refined structure of ferrite and pearlite with a higher relative pearlite content. Ultimately, this results in the 20MnSi steel achieving a strength level of 500MPa through microstructural reinforcement.
Although post-rolling and ultrafine grain heat treatment technologies do not require the addition of microalloying elements, they have high equipment costs and a low strength-to-yield ratio, as well as being prone to aging. As a result, these methods are not suitable for mechanical connections that utilize welding or surface damage.
Microalloy technology has the lowest equipment cost, as it does not require temperature control equipment in the steel rolling production line. It also has a high strength-to-yield ratio, low aging sensitivity and good welding performance.
Based on the comparison between product performance and production cost, it can be concluded that the best technical method to produce 500MPa high strength steel bars is through the microalloying process.
Table 2 in GB1499.2 (revised 2016) describes the chemical composition and mechanical property requirements for 600MPa high strength reinforcement.
| Chemical composition, % by mass | Spleen number | HRB600 |
| W | 0.28 | |
| Yes | 0.8 | |
| Mn | 1.6 | |
| P | 0.045 | |
| s | 0.045 | |
| Cr | 0.58 | |
| Mechanical property | Yield limit RL, MPa | 600 |
| Tensile strength Rm/MPa | 730 | |
| Stretching after fracture% | 14 | |
| Maximum strength total stretch A% | 7.5 |
2.2 Production process of 600MPa high strength reinforcement
2.2.1 Composition design and mechanical properties
Currently, steel mills such as Shagang, Chenggang and Jigang in China have a proven track record of successfully producing 600MPa hot-rolled deformed bars.
Table 2 in GB1499.2 (revised 2016) describes the requirements for the chemical composition and mechanical properties of 600MPa HRB600 high strength reinforcement.
2.2.2 Technical route
Currently, many steel mills in China can produce high-strength steel bars with a grade of 600 MPa that are used in construction projects. However, there is limited research on the chemical composition, phase transformation and microstructure evolution of these steel bars and their relationship to the rolling and cooling production processes. This results in the inappropriate combination of microalloy technology and controlled rolling and cooling processes, leading to the waste of expensive alloying elements and the failure to meet the required mechanical properties of steel bars.
Domestic steel mills such as Shagang, Chenggang and Jigang, which have successfully achieved the production of HRB600, mainly adopt the vanadium alloying technique, which involves adding vanadium to significantly improve the strength. The production of 600 MPa high-strength steel bars through niobium, titanium and process control is still rare.
Vanadium alloy technology is the main technical route for the development of high-strength weldable steel bars worldwide. Process control can be achieved through controlled rolling and controlled cooling or post-rolling heat treatment. High-strength steel bars are produced through controlled rolling and cooling, mainly through low-temperature rolling and rapid cooling, to reduce grain size and improve strength.
Using the same medium and low strength steel bar production process to produce 600 MPa high strength steel bars through alloying brings several benefits. Firstly, it avoids the transformation of the production line and the problems associated with it, including the entry of costs for modifying equipment. Secondly, it helps in the rapid production and large-scale promotion of new HRB600 products.
However, relying on alloy alone to improve strength increases the cost of alloys, and higher alloy content can also cause structural abnormalities.
In conclusion, the current process route for the production of 600 MPa high strength reinforcement is mainly alloying, supplemented by process control. During the initial phase, the production process of 600 MPa high-strength reinforcement should be as close as possible to that of medium and low-strength reinforcement to facilitate its wide adoption and application.
3. Production process of high-strength anti-seismic reinforcement
Due to the Chinese construction industry's increasing demands for high-performance steel bars, there is widespread concern about the safety and seismic resistance of building structures.
3.1 Composition design and mechanical properties
In standard GB 1499.2-2007, the seismic performance index of reinforcement is included for the first time as a national standard. Three representative seismic reinforcement indices were specified: the strength-yield ratio (R ˚ m /R ˚ eL), the superflexure ratio (R ˚ eL/ReL) and the total elongation at maximum force (Agt).
Tables 3 and 4 show the chemical composition and mechanical property indices for the HRB400E and HRB500E seismic reinforcement of a national steel mill. These indices were obtained from multi-sample inspection.
Table 3 Chemical Composition of % High Strength Seismic Reinforcement HRB400E and HRB500E
| Brand | W | Yes | Mn | V |
| HRB400E | 0.19-0.25 | 0.36-0.57 | 0.27-1.52 | 0.035-0.056 |
| HRB500E | 0.20-0.25 | 0.36-0.57 | 1.38-1.58 | 0.082-0.113 |
Table 4 Mechanical Property Inspection of High Strength Seismic Reinforcement HRB400E and HRB500E
| Brand | RpL, MPa | Rm, MPa | A,% | Agt,% | R 0 sir 0 pL | R 0 pL/RpL |
| HRB400E | 425-485 | 570-625 | 21.5-30.5 | 10.5-18.5 | 1.28-1.41 | 1.06-1.21 |
| HRBS00E | 515-595 | 665-725 | 19.5-26.5 | 10.0-17.5 | 1.26-1.39 | 1.03-1.19 |
3.2 Technical route
3.2.1 Microalloy technology
High deformation and low cycle fatigue performance is the main seismic index for steel bars.
The main method to improve the high-stress, low-cycle fatigue performance of seismic steel bars is through microalloying. This technology is widely used both nationally and internationally to improve the comprehensive properties of steel bars, refining grains and strengthening precipitation.
In China, vanadium is preferred as a microalloying element, and a small amount of nitrogen is added at the same time to increase the number of precipitated phases V (C, N). This increases the role of precipitation strengthening and fine-grain reinforcement and significantly improves the seismic performance of the steel.
Some researchers have also successfully developed 600MPa grade fine-grained high-strength anti-seismic reinforcement using a Cr+V microalloy process. Vanadium is used to form V(C,N) compounds in steel, which significantly improves its strength. In addition, a certain amount of chromium is added to improve the seismic performance of the reinforcement. The final mechanical properties meet the high-strength and fine-grain seismic resistance requirements of 600MPa.
The metallographic structure of the armor is composed of “ferrite+pearlite” at the edge and center, without bainite structure or edge tempering that would negatively impact its performance in service.
3.2.2 Fine crystallization technology
Japan has a long history of studying fine crystallization technology, which involves combining large-strain rolling with dynamic recrystallization to refine grain structure. This has led to the development of ultra-high-strength seismic reinforcement with a strength range of 685-980MPa, which is considered internationally advanced.
In contrast, China is focusing on combining deformation and phase transformation to achieve grain refinement.
Fine-grained steel bars are known for their wide range of cyclic plastic deformation and low probability of cracking during material deformation. Furthermore, these bars have higher cyclic toughness and shorter fatigue life compared to heat-treated steel bars. Furthermore, ultrafine grain steel has better weldability than pearlite ferrite steel.
However, there are still some limitations in the practical application of fine-grained steel bars. These include stringent requirements for equipment and part size, uneven microstructure and properties due to deformation and uneven cooling of large bars, and a decrease in the strength yield rate due to a greater increase in yield strength than in yield strength. traction when the grain size is very small. Fine-grained steel also has low corrosion resistance due to its fine-grained structure and the increased number of grain contours.
Therefore, further development of fine crystallization technology is necessary.
4. Conclusion
Three common methods for producing high-strength steel bars are microalloying, fine crystallization and residual heat treatment.
Compared with the other two processes, microalloyed steel bars have the advantages of stable performance, low sensitivity to deformation aging and good welding performance.
Heat-treated steel bars are produced by quenching hot-rolled steel bars, resulting in increased strength. This process is resource and energy efficient, leading to lower production costs.
Fine-grained reinforcement is capable of meeting the strength and toughness requirements for seismic reinforcement.
Despite these advances, there are still some challenges in the above processes, including:
- The high production cost of microalloy technology;
- The low ductility, weldability, mechanical connection performance and construction adaptability of waste heat treatment reinforcement;
- The complexity of fine crystallization technology and the low strength yield rate of reinforcement.
Therefore, to produce high-strength steel bars, it is crucial to effectively combine microalloying, fine crystallization and waste heat treatment technologies based on actual application needs and cost-effectiveness. This will not only reduce the addition of alloying elements and lower production costs, but also significantly increase the mechanical properties of steel bars.
