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Bar and rebar production line cost reduction production practice

This paper introduces the difficulties faced by the cost control of the bar production line in the rolling mill of Honghe Iron and Steel Co., Ltd. The equipment level of the bar production line is not high, the supply of billets is insufficient, and the technical management is inefficient. By using special billets to increase the yield rate and control cooling and fine graining Through measures such as process optimization and spare parts function contracting, the technical and economic indicators of bars have been further improved, and costs have been further reduced. The yield index has increased by 0.14% year-on-year, and the cost of using spare parts has been reduced by 4.15 yuan/ton, reducing costs and increasing efficiency by 18.2149 million yuan/year.

 

Bar Production, cost reduction

 

1. Introduction

 

Bar products are one of the important products in the steel industry and are mainly used in the construction of houses, bridges, roads and other civil engineering projects. With the decrease in domestic crude steel production, low-cost manufacturing has become a key factor in winning market competitiveness. Most companies have adopted methods such as improving billet quality, increasing hot feeding and charging rates, and split rolling to reduce production costs. Wuhan Iron and Steel Group Kunming Iron and Steel Co., Ltd. Honghe Iron and Steel Co., Ltd. (referred to as Honggang) is located in a small town in southern Yunnan. The cost of raw material transportation is higher than that of similar production lines. The equipment level of the bar production line is not high, the supply of blanks is insufficient, and the technical management is inefficient. These problems restrict the cost of bar production. At the same time, the bar production lines for the construction industry have shown large losses, and better companies have also entered the era of meagre profits. Analyze the key controllable reasons that restrict cost reduction, and how to reduce raw material consumption and cost investment. It is particularly important to achieve low-cost production of rods.

 

2. Factors affecting the cost of domestic bar and rebar production lines

 

Straight-threaded steel bars (commonly known as “bars”) are mainly produced using the process route of converter → continuous casting → bar rolling mill. Steel is made through converter smelting, nitrogen blowing (argon) refining or LF furnace refining to obtain steel with stable composition and high cleanliness. The molten steel is then passed through a billet casting machine to produce cast billets with good external and internal quality for steel rolling. In the rolling process, a “shot blasting, ultrasonic flaw detection” or “magnetic particle flaw detection, grinding” production line is set up before entering the heating furnace to assist in appearance inspection, improve the quality of the billet entering the furnace, and reduce intermediate rolling losses; in terms of heating, currently, the billet heating furnace Most use double regenerative side-in and side-out full-beam walking furnaces, which can preheat air and gas to about 1000°C at the same time, and can use low calorific value fuels such as blast furnace gas. Regenerative heating furnaces have the advantages of low fuel consumption, high efficiency, a high degree of automation, and uniform temperature distribution in the furnace. The step length is adjustable, and the temperature difference between the head and tail of the billet can be controlled at about 30°C. It only takes more than 50 minutes to heat the hot billet to reach the rolling temperature, which can greatly reduce the oxidation and burning loss of the steel billet, increase the metal yield, and reduce the need for intermediate steps. Metal loss, increased production capacity. The main factors that affect the cost during the bar rolling process are the burning of the steel billet during the heating process, head and tail cutting during the rolling process, rolling scrap, short-length scrap steel, and the control of negative deviation rate. In addition, the rolling or controlled cooling process affects the composition. Control also has a certain impact on cost control.

At present, various steel mills reduce production costs by increasing material production capacity based on different equipment and production process conditions. In some enterprises, labour costs are low, which has also become an important measure to reduce costs. Enterprises with convenient transportation conditions can significantly increase investment in advanced equipment, ultimately increasing production capacity and reducing labour costs.

 

3. Cost reduction control measures for red steel bars and rebars

 

3.1 Improve yield and reduce costs

 

The Honggang threaded steel bar production line is based on the existing production equipment and technology, and based on the L12 mm-L28 mm different specifications and product scale solutions produced by the bar production line as well as production practical experience, the blanks are supplied in fixed weights to minimize The non-fixed length caused by the non-uniform weight of the billet better solves the problem of difficulty in increasing the yield rate of straight threaded steel bars, breaks through the traditional way of thinking that relies on controlling the negative weight deviation to improve the bar yield rate, and achieves the goal of controlling the weight of the billet. A major change in improving the yield rate. The unsuitability and serious cutting damage caused by the difference in billet weight have been effectively controlled, and the yield rate has been steadily improved. Red Steel has merged the material codes that were more in the past and were not in response to production needs and transformed them into ones that can be actually operated. The fixed length is currently rolled using 4 specifications of blanks. Fine adjustments have been made based on production practices. The stability rate of special blanks has reached more than 75%, which has a direct impact on the yield index of Φ12~Φ32 mm straight-threaded steel bars. , every 0.1% increase in yield rate can reduce the cost per ton of steel by 2.36 yuan. Based on a total of 694,300 tons of bar products rolled in 2016, the cost is reduced by 229,300 yuan.

3.2 Process optimization to reduce costs

 

3.2.1 Optimization of slit rolling technology The original design of the red steel bar production line is composed of Φ550×3, Φ500×3, Φ400×6, and Φ320×6 (three of which are flat and vertical convertible) high-stiffness short-stress line rolling mills. The current main products of the fully continuous rolling production line are Φ12~Φ32 mm straight threaded steel bars. Among them, Φ12 and Φ14 rebars are produced by the four-slit rolling method (Φ12 is reserved for four-slit rolling), and Φ16 is produced by the three-slit rolling method. The rebars of Φ18, Φ20 and Φ22 are produced by the two-slit method. Rebars of other specifications are single-rolled. The proportion of slit rolling is 67%. The bar products produced by slicing account for 76~86% of the product. The output of the slitting rolling mill increased from 155.51 t/h to 156.39 t/h. Calculated based on the profit of 130 yuan per ton of steel, the annual profit can be 919,200 yuan, which is also a cost reduction.

The application of the split rolling process has the advantages of high productivity and large output increase. The bar split rolling technology is optimized and process accidents are reduced. Among the split rolling types currently used in the red steel bar production line, four split The rolling technology is relatively difficult. The bar production line has optimized the number of stands to 16, reducing the use of two stands. Tracking and recording of key pass pass processing (slitting and pre-slitting), and monitoring of the rolls The pass size accuracy was strictly inspected, and the process of each split rolling was summarized. The steel stacking accident was gradually reduced by 20%, and the steel stacking tonnage was reduced by 95.48 tons. Calculated based on the cumulative yield rate of 101.17% in 2016, it directly reduced production costs by 386,300 yuan.

 

3.2.2 Optimization of rough and medium rolling pass patterns and guide and guard sharing technology

 

The pass system of the rolling mill adopts an oval-round pass system. The red steel bar production line combines the current process requirements and cost needs, adopts the integrated bar production process, and has successively organized and implemented the application of rough rolling grooveless rolling technology while achieving 165 square meters. The billet process is shared with the 150 square billets, and the roll input and processing volume are greatly reduced. The annual consumption of rough rolling rolls is reduced from 72 to 48. Calculated at 15,639.28 yuan per roll, the cost reduction is 375,300 yuan. For the three different pass series of the intermediate rolling mill, the intermediate rolling pass types have been merged. Only the outlet pass adopts two types of round holes, which can meet the needs of L12, L14 four-cutting technology, L16 three-cutting technology, and L22 two-cutting technology. In terms of technical transformation and application, the degree of process sharing has been significantly enhanced. The maximum rolling speed has been reduced from 15.8 m/s to 13.5 m/s, improving rolling stability. The design capacity of the bar production line is 800,000 tons, and the actual maximum output has reached 1,127 217.909 tons. The integrated optimization process reduces the investment in spare parts, reduces the use of two 650KW motors, reduces the electricity bill by 2.3503 million yuan, reduces the use of 6 intermediate rolling rollers, reduces the cost by 236,800 yuan, and reduces the number of guide assemblies by 20. The cost is reduced by 160,000 yuan, and the cumulative cost reduction is 2.4771 million yuan per year.

3.2.3 Controlled rolling and controlled cooling process optimization

 

The main content of the controlled rolling of Red Steel is to control the opening rolling temperature of the heating furnace to meet the requirements of various specifications, to control the average elongation coefficient of the rolling passes to about 1.25, and to control the recrystallization process by dividing the deformation passes to no less than 14 passes. Adding an appropriate amount of micro-alloying elements can increase the recrystallization starting temperature of the steel a lot, and at the same time appropriately reduce the rolling temperature, so that the effects of multiple passes of deformation can be superimposed, allowing recrystallization to proceed at a larger amount of deformation and at a lower temperature. , to achieve the purpose of grain refinement.

The controlled cooling process after rolling changes the austenite structure before phase transformation increases the nucleation position and nucleation rate of ferrite grains when austenite transforms into ferrite, and reduces the ferrite grain size. , thereby refining the ferrite grains. The use of a post-rolling controlled cooling process can further improve the strength of the steel without reducing the toughness of the rolled material. The red steel bar production line controls the matching of the rolling speed and cooling water volume. The speed is not higher than 14.5 m/s and the water pressure is Not less than 1.98 MPa, to achieve accelerated cooling control of the rolling stock and achieve the opening cooling temperature, final cooling temperature and cooling rate required for the rolling stock.

Through the optimization of the controlled cooling process, the current ferrite grain size of Red Steel HRB400E steel bar has been increased from 9.0 before process optimization to the current 10.5~11.0, and steel with microstructure and process mechanical properties that meet the seismic requirements has been obtained. By optimizing the rolling and controlled cooling processes, the manganese content of Red Steel rebar bar blanks has been reduced by 0.08 to 0.10% year-on-year, and the cost per ton of steel has been reduced by approximately 10 yuan. Based on the 2016 output of 694,300 tons, the cumulative cost reduction is 6.943 million yuan.

 

3.3 Spare parts contracting reduces costs

 

The red steel bar production line has increased the scope of contracting projects with functional conditions and reasonably reduced production costs. The steel rolling mill has organized and completed the functional contracting of some rolling stock. After the contracting, the original day lathes were filled in each production shift, which made up for the loss to a certain extent. Previously, various production teams were experiencing a shortage of personnel. At the same time, the bar production line implements comprehensive contracting of functional spare parts through functional contracting of some production spare parts, such as bar flying shear blades, bar 1#~12# rack grooveless rolling rollers, guides, etc. The unit price is 10.63 yuan/ton of material, which is 4.15 yuan/ton of material lower than the consumption before functional contracting of 14.48 yuan/ton of material, which can reduce the consumption of production spare parts by more than 7.26 million yuan per year.

 

4. Application results

 

Before the implementation of various measures, the bar and rebar production line indicators lagged behind other production bases of Kunming Iron & Steel Co., Ltd. Through the implementation of special billet supply, the yield index increased by 0.14% and the cost was reduced by 229,300 yuan; the implementation of process optimization reduced the cost by 10.7256 million yuan; the implementation of functional contracting of spare parts reduced the cost by 7.26 million yuan.

 

5. Conclusion

 

1) The Honggang bar and rebar production line has achieved a stability rate of 75% through the implementation of special billet supply, and the comprehensive yield rate has increased from 101.17% to 101.31%, reducing production costs by 229,300 yuan;

2) Through process optimization, the hourly output of the red steel bar rebar machine increased by 0.88 t/h, the number of steel stacks was reduced by 30.4 times, and the cost was reduced by 1.3055 million yuan; by sharing the rough and medium rolling pass patterns and the use of guides, reducing 20 guides; through the implementation of post-rolling controlled cooling process, the Mn content in steel was reduced by 0.08-0.10% year-on-year, and the total bar production cost was reduced by 10.7256 million yuan;

3) The Red Steel Bar and Rebar Production Line has reduced costs and increased efficiency by 18.2149 million yuan through three main measures: improving yield indicators, optimizing processes, and adopting spare parts function contracting, and the technical and economic indicators of steel rolling have been further improved.

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