Description: From the perspective of reducing energy consumption in the steel rolling process, effectively decreasing carbon emissions, and lowering production costs, a low-carbon and high-efficiency rebar production process is proposed based on direct rolling, hot delivery, and controlled rolling and cooling technology.
Keywords: Hot rolling, rebar rolling
Practice of Low-Carbon and High-Efficiency Rebar Production Process
This paper proposes a low-carbon and high-efficiency rebar production process under a multi-strand cutting production mode, focusing on direct rolling and controlled cooling technology to reduce rolling energy consumption, cut carbon emissions, and decrease production costs.
This process has been implemented in the production line of a steel mill, achieving a stable daily output between 5,500 and 5,800 tons. The gas consumption of the line is reduced by about 70% compared to conventional lines, with CO₂ emissions of 63.2 kg per ton of steel. It enables the production of 400E rebar (specifications below φ25 mm) without adding microalloy elements, yielding significant economic and social benefits.
Rebar, the common name for hot-rolled ribbed steel bar, is one of the fundamental materials widely used in national economic construction. According to the website of the Ministry of Industry and Information Technology, China’s crude steel output in 2020 reached 1.325 billion tons, of which rebar output accounted for about 260 million tons, or 19.6% of the total. With China’s clear national goals of striving to peak carbon dioxide emissions by 2030 and achieve carbon neutrality by 2060, carbon emission reduction has become the most critical challenge for the sustainable development of the steel industry.
Furthermore, due to fierce competition in the domestic steel industry, controlling production costs is essential for the survival of steel enterprises. Therefore, considering both production costs and national carbon reduction policies, it is imperative to develop a low-carbon and high-efficiency rebar production process.
Currently, the main energy-saving, emission-reduction, and high-efficiency production technologies for rod and wire rolling in China focus on reducing energy consumption and alloy usage. These include multi-strand cutting and divided rolling, direct rolling, hot charging, hot delivery, low-temperature rolling, headless rolling, and others. However, direct rolling technology has not been widely adopted due to low direct rolling rates and limited annual production capacity. Similarly, existing low-temperature rolling technologies still face challenges in achieving controlled rolling under high-yield conditions.
This paper introduces a new low-carbon, high-efficiency rebar production process designed for high productivity. The process reduces energy consumption and carbon emissions through direct rolling and hot delivery and uses a multi-strand cutting production method to reduce the use of billet alloys, thereby lowering costs and improving efficiency.
High-Yield Multi-Strand Cutting Process
Current rebar production primarily uses multi-strand cutting, single-strand high-speed bar, and double-strand high-speed bar processes. The multi-strand cutting process produces rebar through cutting (five-, four-, three-, and two-strand) operations. Table 1 compares these three production methods, showing that the multi-strand cutting process offers high yield.
Table 1. Comparison of Three Rebar Production Methods
| Content | Single-Line High-Speed Bar Process | Double-Line High-Speed Bar Process | Multi-Line Cutting Process |
|---|---|---|---|
| Production Specifications/mm | φ10–25 | φ10–25 | φ10–50 |
| Production Mode | Single-line production | Double-line production | Combination of five-, four-, three-, two-, and single-line production |
| Max. Finishing Speed / m·s⁻¹ | 45 | 45 | 18 |
| Hourly Output / t·h⁻¹ | 90–150 | 180–250 | 230–300 |
| Annual Output / 10²t | 80 | 130 | 160 |
Rebar Direct Rolling and Hot Delivery Process
Research shows that in a typical rebar production process, rolling energy consumption accounts for only 16.9% of total energy use, while billet heating consumes about 80%. Therefore, direct rolling eliminates the heating process and saves gas consumption, while hot delivery reduces gas use. Both technologies significantly lower energy consumption and CO₂ emissions in the rolling process, supporting low-cost production and carbon reduction.
2.1 Challenges in Direct Rolling
The continuous casting hot billet direct rolling process faces three main challenges: mismatch between continuous casting and rolling affecting direct rolling rate and yield; temperature variations in continuous casting billets affecting head-tail performance stability; and hourly yield coordination.
Table 2. Hourly Output of Continuous Casting Machine with Different Strands and Billet Sizes / t·h⁻¹
| Billet Type | 5 Strands | 6 Strands | 7 Strands | 8 Strands |
|---|---|---|---|---|
| 150mm×150mm×12000mm | 169.1 | 202.9 | 236.8 | 270.6 |
| 165mm×165mm×12000mm | 191.4 | 229.6 | 268.0 | 306.2 |
Rebar production typically uses 150 mm × 150 mm and 165 mm × 165 mm billets, with continuous casters having 6 to 8 strands and a casting speed of 2.5–4 m/min. Assuming a casting speed of 3.3 m/min, Table 2 estimates the hourly output for different billet sizes and strand numbers. Compared with Table 1, the multi-strand cutting process offers better production rate matching, which can effectively increase the direct rolling rate.
In actual production, offline cold billets are inevitable due to differences in mill layout, roll replacement schedules, and production efficiency between continuous casting and rolling lines.
2.2 Direct Rolling and Hot Delivery Process and Improvements
2.2.1 Direct Rolling and Hot Delivery Process
To meet production needs, a combined process of direct rolling + hot delivery + cold billet reheating is adopted. Hot billets around 950°C can be directly rolled. Billets that cool to 300–850°C can be reheated to about 1,000°C in a furnace. Cold billets resulting from production rhythm issues or failures are reheated in batches.
2.2.2 Improvement Measures
(1) To reduce temperature drop during transportation and minimize head-to-tail temperature difference in continuous casting billets, single billets are placed in direct rolling channels to shorten transfer time. Insulation covers are installed on all transport rollers to reduce heat loss.
(2) To address billet bending during curved roller transport, a straightening machine is added before the furnace entrance, controlling side bending within 0.5%.
(3) To compensate for the lack of microalloy carbonitride precipitation strengthening in direct rolling (without reheating), a controlled rolling and cooling technology under multi-strand cutting production has been developed. This reduces or eliminates the need for microalloy elements, avoiding dependence on reheating for precipitation strengthening.
2.3 Production Practice of Direct Rolling and Hot Delivery
This process has been implemented in a plant in northern China. Direct rolling is used in spring, summer, and autumn, with a direct rolling rate of about 75%. Hot delivery is used in winter, accounting for about 20%, and cold billet reheating makes up about 5%.
Table 3. CO₂ Emissions per Ton of Steel under Different Production Modes
| Item | Direct Rolling | Hot Delivery | Cold Billet |
|---|---|---|---|
| Blast Furnace Gas Consumption / m³ | 0 | 173 | 373 |
| CO₂ Emission per Ton of Steel / kg | 4.64 | 54.656 | 117.84 |
Note: Calorific value of blast furnace gas is 3,763 kJ/m³, equivalent to 0.1286 kgce/m³; CO₂ emission factor of standard coal is 2.456 kgce/t (NDRC recommendation); extra electricity consumption per ton of steel compared to hot rolling is 5 kWh/t; CO₂ emission factor of electricity is 0.928 kg/kWh.
The overall CO₂ emission per ton of steel is 63.2 kg, demonstrating significant energy-saving and emission-reduction effects.
