Description：From the perspective of reducing energy consumption of steel rolling process, effectively reducing carbon emission and reducing production cost, a low-carbon and efficient production process of rebar is proposed based on direct rolling hot delivery and rolling and cooling control technology.
Keyword：Hot rolling, rebar rolling
In this paper, in terms of reducing the energy consumption of steel rolling process, effectively reducing the carbon emission and reducing the production cost, under the multi-partition production mode of rebar, with the technology of direct rolling and cold rolling and control as the research direction, it proposes a low carbon and efficient rebar production process.
This production process has been applied in the production line of a steel mill, and the daily production day is stable between 5500 and 5800 t. The gas consumption of the production line is about 70% less than the traditional production line, the CO2 emission of 63.2kg, and the production of 400E rebar (φ 25mm specification below) without microalloy elements, which has achieved good economic and social benefits.
Rebar is the common name of hot-rolled ribbed steel bar, which is one of the basic materials widely used in the national economic construction. According to the website of the Ministry of Industry and Information Technology, China’s steel output in 2020 is 1.325 billion tons, of which rebar output is about 260 million tons, accounting for 19.6% of the country’s steel output. At present, the country has clearly proposed to strive to reach the peak of carbon dioxide emissions by 2030, and strive to achieve the goal of carbon neutrality by 2060. Therefore, carbon emission reduction is the biggest challenge facing the steel industry to achieve sustainable development.
In addition, due to the fierce competition of domestic steel industry, the key to the survival of steel enterprises is to control the product production cost. Therefore, from the perspective of the production cost of the steel plant and the macro national policy of carbon emission reduction, it is necessary to develop a low-carbon and efficient rebar production process.
At present, China’s rod and wire production energy saving and emission reduction and efficient production technology is mainly to reduce energy consumption and reduce the amount of alloys, the technologies involved include: multi-line cutting and dividing rolling technology, direct rolling technology, hot loading technology, hot delivery technology, low temperature rolling technology, headless rolling technology and other. Direct rolling technology is not widely used in at this stage due to the low direct rolling rate and low annual production line. However, in the low temperature rolling technology, it still can not solve the controlled rolling technology under the high yield production.
This paper presents a new production process of low carbon and efficient rebar to meet the requirement of high productivity. This process reduces energy consumption and realizes carbon emission reduction by direct rolling and heat delivery, and uses multi-line cutting production method to reduce the amount of billet alloy and realize cost reduction and efficiency increase.
1. Multiline cutting process with high yield
At present, the production of rebar mainly includes: multi-line cutting, single-line high-speed bar and double-line high-speed bar. Multi-wire cutting process is through the cutting (five, four, three and two) process to produce rebar. Table 1 is the comparison table of the three rebar production methods. It can be seen from Table 1 that the multi-line cutting process has the characteristics of high yield.
| Three kinds of rebar production mode comparison table | |||
| 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 |
| Mode of production | Single line production | Double production | Five line, four line, three line, second line and single line combination production |
| Final rolling speed (Max)/m·s-1 | 45 | 45 | 18 |
| Hourly tonnage /t·h-1 | 90-150 | 180-250 | 230-300 |
| Annual output/102t | 80 | 130 | 160 |
2. Rebar direct rolling and heat delivery process
Studies have shown that in the typical rebar production process, the energy consumption of steel rolling only accounts for the total energy consumption
16.9% of the consumption, while the billet heating energy consumption accounts for 80% of [7]. Therefore, direct rolling can eliminate the heating process without gas consumption; the heat delivery process can reduce gas consumption. Both technologies can significantly reduce energy consumption and carbon dioxide emissions in the rolling process, achieving the goal of low cost production and carbon reduction.
2.1 Problems in direct rolling process
There are three main problems to be solved in the continuous casting hot billet rolling process [8]: the mismatch affects the rolling rate and yield of the continuous casting process; the temperature difference of the continuous casting process affects the performance stability of the carbon and tail; the direct casting process.
| hourly yield of average pulling speed of continuous casting machine with different flow numbers /t·h-1 | ||||
| Even cast billet type | 5 | 6 | 7 | 8 |
| 150mm*150mm*12000mm | 169.1 | 202.9 | 236.8 | 270.6 |
| 165mm*165mm*12000mm | 191.4 | 229.6 | 268.0 | 306.2 |
The raw materials of rebar production are mostly 150mm 150mm and 165mm 165mm billet, and the supporting continuous casting machine is 6~8 flow, and the pulling speed of continuous casting machine is 2.5~4m / min. Taking the pulling speed of continuous casting machine at 3.3m / min, the hourly output of continuous casting machine under different blanks and different flow numbers is estimated, as shown in Table 2. Compared with Table 1 and Table 2, the hourly production rate of the production line under the multiline cutting process is relatively matched, which can effectively increase the direct rolling rate.
In the real production, due to the different connection and layout of each steel mill, the inconsistency between the replacement of the refrigerator and the cycle of rolling roll, and the different production mode and efficiency of continuous casting and rolling line, the offline cold billet will inevitably occur.
2.2 Direct rolling and heat delivery process and Improvement measures
2.2.1 Direct rolling and hot delivery process
Therefore, in order to meet the production needs, it is necessary to combine the direct rolling + hot delivery + cold billet production process. When the hot billet is about 950℃, when the casting temperature drops to 300-850℃, it can enter the heating furnace and the temperature reaches about 1000℃. When the cold billet caused by production rhythm and failure are concentrated to a certain amount, it can be produced by heating furnace heating.
2.2.2 Improvement measures
(1) In order to solve the temperature drop of the billet in the transportation process and the temperature difference between the head and tail of the continuous casting billet, measures are taken to put a single billet into the straight roll channel to reduce the transportation time of the billet from the outlet channel to the straight rolling channel, and at the same time, the insulation cover is installed on all the transport rollers to reduce the temperature drop in the transportation process.
(2) In order to solve the problem of billet bending in the transportation of arc roll, a straightening machine is added before the billet enters the heating furnace, and the side bending of the billet is controlled within 0.5% by straightening the straightening machine.
(3) in order to solve the straight rolling process billet without heating, micro alloy carbon nitride precipitation and dissolution process reinforcement effect is not obvious, developed in multi-line cutting production mode of the control rolling cooling technology, make the billet reduce or no microalloy elements, so as to avoid the billet by heating the micro alloy carbon nitride precipitation reinforcement effect.
2.3 Production Practice of Direct Rolling and Hot Delivery Process
This direct rolling and hot delivery process has been put into use in a plant. The production line is located in the north of China. In spring, summer and autumn, the direct rolling process is used in winter, the direct rolling rate is about 75%, the hot delivery rate is about 20% (the hot delivery process is used in winter), and the cold billet heating is about 5%.
| The corresponding CO2 emission per ton of steel under different production modes | |||
| order number | Straight rolling | Hot send | Cold billet |
| Blast furnace gas consumption | 0 | 173 | 37.3 |
| CO2 Emissions per ton of steel | 4.64 | 54.656 | 117.84 |
The calorific value of blast furnace gas is 3763kJ / m3, equivalent to 0.1286kgce / m3 of standard coal; the carbon dioxide emission of standard coal is 2.456tce / t (recommended by National Development and Reform Commission); the power consumption of one ton of steel is 5kWh / t higher than heating rolling; CO20.928kg/kWh. Through calculation, the emission of CO2 per ton of steel in the production line is 23.036kg, 94.8kg is less than the emission of CO2, and 31.62kg is less than that of CO2. Comprehensive (compared with the ratio of cold billet and heat transmission is 1:1) is 63.2kg per ton of steel, with significant energy saving and emission reduction effect.
