How to Choose Electric Arc Furnace?

Electric furnace is the abbreviation of an electric arc furnace. It is a kind of process equipment with scrap as the main raw material, some molten iron or pig iron as the additional part, electricity as the main energy and chemical energy as the auxiliary energy for smelting. Because there is no coking, sintering and iron making process in the process flow, it is also called a short process, which is different from the long process of converter smelting.

Due to the characteristics of low greenhouse gas emission and low energy consumption, the short process has been widely concerned by metallurgical workers and has experienced rapid development in the historical development process. At present, the annual output of converter steel accounts for about 2 / 3 of the world's steel output, and that of electric furnace steel accounts for about 1 / 3 of the world's steel output.

Classification of electric furnaces
According to the power supply mode, the electric furnace can be divided into DC electric furnace and AC electric furnace.

Direct current furnace

DC electric furnace uses DC power as the main energy to provide heat source for the furnace. Generally, there is only one anode on the top of the furnace, forming a circuit with the cathode at the bottom of the furnace. It is characterized by very low electrode consumption, generally less than 1kg / ton steel, and stable molten pool. Because only one electrode and molten pool form arc column, the radiation damage to furnace lining is low, the consumption of refractory materials is relatively low, and the damage of water-cooled plate on the furnace wall is relatively small. The disadvantage is that the bottom electrode is relatively complex, there are several forms of water-cooled, conductive bottom type and needle-like distribution in the bottom refractory, but the service life is relatively short, the maintenance cost is high, resulting in high smelting cost. In addition, the rectifier equipment investment is relatively high, resulting in higher one-time investment.

The direct current electric furnace is not the main equipment in the electric furnace plant. In recent years, new progress has been made in the research of the bottom electrode, which basically solves the problem of short bottom life of the bottom electrode furnace. IThe low consumption of the graphite electrode shows a great advantage in smelting cost. It is expected that the DC electric furnace will be put into production in the new round of electric furnace construction.

Alternating current electric arc furnace

The three-phase alternating current furnace has been used in industrial production for more than 100 years. Its equipment is characterized by the use of three-phase alternating current power supply. When the three-phase graphite electrode contacts with scrap steel, an arc is generated under the action of huge current. The high temperature of the arc is used to melt scrap steel and molten steel. Compared with the direct current furnace, because there are three-phase electrodes, the electrode consumption is higher than that of the direct current furnace. The three-phase electrode has a polar circle, which is relatively close to the furnace wall, and the refractory consumption is relatively high, but the furnace bottom is relatively safer and has a long service life. 

At the beginning of this century, the EAF steelmaking technology has made great progress. On the basis of electric energy as the main energy, auxiliary energy and chemical energy have been introduced. With the development of automatic control technology, power input has increased from ordinary power to high power and ultra-high power, consumption has decreased and productivity has increased. In the case of all scrap steel as raw material, the smelting cycle of this furnace is less than 60 minutes, the power consumption is less than 280kwh / ton steel, the electrode consumption is less than 1.2kg/ton steel, and the annual steel output per nominal capacity has exceeded 10000 tons (i.e. the furnace with a nominal capacity of 100 tons, with an annual steel output of more than 1 million tons), the productivity is greatly improved, and at the same time, the process changes have also derived a variety of furnace types. 

  

 
Common type 3 AC electric furnace

Traditional electric furnace
The traditional electric furnace is the early mainstream electric arc furnace. Scrap steel is added from the top of the furnace, that is, the crown block basket is used to open the furnace cover and scrap steel is added. Each furnace needs to be charged twice to three times or more. The power consumption is relatively high, the electrode consumption is high, the noise is large when melting, the amount of flue gas at the moment of charging is large, the overflow is very obvious, the heat loss is large, the capacity of transformer is large, and the impact on the power grid is relatively large. Generally, mining In addition to the three-electrode holes, the fourth hole of the furnace cover is used for dust removal. The dust removal effect is relatively poor. In areas with strict dust and noise requirements, the way of increasing smoke hood for dust removal or building isolation hood, commonly known as doghouse, is used for secondary dust collection and noise collection. The early electric furnaces in China belong to the traditional type. However, the furnace technology is relatively mature, low failure rate, wide application, strong adaptability of raw materials, but the main technical and economic indicators are general.

Continuous charge furnace

In recent years, the continuous charging type electric furnace is one of the more furnace types newly put into operation. The biggest feature is that the scrap steel is continuously added in the smelting process, the furnace cover can be kept open, the energy input can be kept uninterrupted, the huge energy loss can be avoided, and the smoke and dust can not overflow. The high-temperature flue gas extracted from the furnace passes through the continuous charging tunnel, which also plays the role of preheating the scrap steel The consumption can be reduced by 30kwh-100kwh, and the molten pool is relatively stable, which reduces the huge noise and impact on the power grid produced by the traditional electric furnace during the electrode well penetration period, and the electrode consumption per ton of steel can be reduced by 0.1kg-0.3kg. In addition, in the smelting process, the molten pool is stable, the preheating temperature of scrap is generally not more than 400 ℃, the smelting cycle is shortened, the productivity is improved, the main technical and economic indicators are superior to the traditional electric furnace, but the one-time investment is higher than the traditional electric furnace.

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What Are the Main Types of Continuous Caster?

Vertical continuous caster

The vertical continuous caster is the main model before the 1950s-1960s. The steel pouring system, mold, cutting device and billet outlet position of the caster are all arranged on the vertical axis of the caster, and the whole equipment occupies a large space in the workshop.

During the production of the vertical continuous caster, the slab solidifies in the mold and secondary cooling section. Because of the vertical arrangement, the non-metallic inclusions in the liquid metal are easy to float up, the heat dissipation around the slab is relatively uniform, and the slab is not affected by other bending forces during the drawing process. The quality of the slab is good and it is not easy to crack, so it is more suitable for the production of alloy steel and other crack sensitive products steel grade of the sensor. However, the height of the caster is too high, the investment of infrastructure and equipment is large, and the maintenance is also difficult. Because of the high head casting, the lower section of the slab is prone to bulging deformation under the hydrostatic pressure of the molten steel, so it is only suitable for the production of small section slab.

Vertical bending caster
The vertical bending caster is a transitional type of caster developed on the basis of the vertical caster. The difference between the upper part and the vertical continuous caster is that the vertical bending continuous caster uses the top bending device to bend the slab 90°after it is completely solidified, so that the slab can be drawn out and cut in the horizontal direction. The vertical bending caster reduces the height of the whole machine in part, but because it is bent up after the slab is completely solidified, it is easy to crack and other defects in production, which is mainly used in the production of small section billet.

Arc caster

Arc caster is the most widely used type in the world. Its mold, secondary cooling section and billet straightener are all arranged on 1 / 4 circumference of the same radius. The billet is completely solidified in 1 / 4 circumference. After a point of straightening at the horizontal tangent line, it must be pulled out in the horizontal direction, and then cut to a certain length.

1. Full arc caster

Full arc continuous casting machine is also called single-point straightening arc anchor machine. The secondary cooling device of the mold and the tension straightening device of the full arc caster is all arranged on around 1 / 4 arc. The billets form an arc in the mold, move along the arc track after pulling out, and receive water spray cooling until complete solidification. After full solidification, the billet is straightened to the horizontal cutting point, and then the continuous casting machine is pulled out to cut into a fixed length, which is output from the horizontal direction. The height of the continuous caster is basically equal to the arc radius. Generally, the external arc radius of the continuous casting machine is called the arc radius of the arc continuous casting machine.

The main features of full arc caster are summarized as follows:

Main advantages:

(1) Because it is arranged in the range of 1 / 4 arc, its height is lower than the vertical and vertical bending type, the equipment quality is lighter, the investment cost is lower, the equipment installation and maintenance are convenient, so it is widely used;

(2) Because of the low height of the equipment, the static pressure on the billet during solidification is relatively small, which can reduce the internal crack and segregation caused by bulging deformation, and improve the casting speed and quality.

Main disadvantages:

(1) During the solidification process of molten steel, the non-metallic inclusions tend to accumulate in the inner arc, which is easy to form the defect of inclusions enrichment at about 1 / 4 of the side of the inner arc;

(2) In order to prevent internal crack, it is required that the slab should be completely solidified before straightening, which limits the improvement of drawing speed and affects the production capacity.

2. Multiple points straightening arc caster

The schematic diagram of multiple points straightening arc caster is shown in the figure below. With the increase of casting speed, the slab can't be completely solidified when it reaches the straightening point. When the slab with the liquid core is straightened at a single point, due to a large amount of solid-liquid interface deformation, the central area of the slab is prone to crack. Therefore, the multiple point straightening technology is used to disperse the total strain into the strain of each straightening point, so as to reduce the solid-liquid interface deformation rate. In this way, the billet can be straightened with a liquid core without internal cracks, which is conducive to improving the drawing speed.

3. Arc continuous caster with direct mold

The straight mold arc caster (straight arc Caster) adopts the straight mold, which is equipped with a straight section of 2.5-3.5m below the mold. After the straight section, the slab with the liquid core is gradually bent into an arc, and the subsequent process is exactly the same as that of the multiple point straightening arc caster.

Main advantages:

(1) The advantages of the vertical continuous caster are retained. During the solidification process of molten steel in the straight mold and its lower straight section, it is beneficial to the floatation and uniform distribution of large inclusions in the molten steel. The defect of inclusion enrichment at 1 / 4 of the inner arc side of the slab is avoided, and the effect of high-purity steel production is obvious;

(2) Because the billet is curved into an arc with liquid core, it still has the advantages of low equipment height and low construction cost.

Main disadvantages: the straight arc caster has one more bending process, which increases the possibility of cracks on the outer arc side for crack sensitive steel.

It should be said that the straight mold arc caster is a new type of caster integrating the advantages of vertical and arc caster. At present, more and more slab casters in steel plants adopt this type of caster, because it can better meet the requirements of slab quality and improve production efficiency.

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300,000 Ton TPY Re-bar Production Line

General Description

1. Design for a Re-bar production line with 300,000MT annual capacity.

2. Finished product size: TMT Re-bar φ10mm—φ38mm, Rolling speed:12-16m/ s.

3. Type of steel: plain steel, carbon constructional quality steel, low-alloy steel, tray screw thread steel, and so on.

4. Raw material: Billet 150mm×150mm×6000mm continuous casting.

5. The Re-bar Production Line Capacity: About 1000MT per 20 hours/day (About 50MT per Hour), 6000 hours/year.

Product scheme

Finished Product: φ10mm~φ38mm TMT Re-bar

Product	Specification（mm）	Type of steel	Annual output（T）
Re-bar	Φ10~φ38	TMT500	300,000
Total			300,000

Brief description of the process

The qualified billet was transported to rolling plant by roller bed, and was heated by the pusher-type furnace to 1050 ~ 1150 ℃, pushed out by discharging machine. Then the billet was fed to roughing rolling millφ550 group by furnace delivery table, after roll 5 passes, cutting by 1# flying shear, then enter into intermediate rolling mill group φ420 mm×8, after sheared by 2# flying shear, then enter into the φ320 mm intermediate mill group(6 stands), after water treatment，enter into conveyor roller bed, and then sheared by dividing shear, enter into rolling piece conveyor roller bed, then enter into cooling bed, cutting by cut-to-length shear, then banded, then delivery or enter into storage. The production process is as follows:

The Composition of rolling mills

The whole production line will be equipped with 20 rolling passes all together, among which, φ550mm 41 rougher mill is the shuttle rolling type mill, and after taking off ends and the semi-finished materials will enter other 20 intermediate(φ420mm×6+φ320mm×6 +φ285×4) finishing rolling mills, which are all the continuous rolling type (DC controlled).

Supply goods scope for Main rolling area

No	Equipment Name	Quantity	W (T)	Remarks
1	φ550 Horizontal rough mill group (2-Hi)	6 groups	300	The complete equipment of the rougher area
2	φ450 Horizontal pre-stressed rolling mill	3groups	150	（neutral gear surface)
3	φ320 Horizontal pre-stressed rolling mill	3 groups	120.7	（neutral gear surface)
4	φ285 2H2V finishing mill group	1 group	44
5	Pneumatic Loop	4 sets	2	Betweenφ320 Horizontal pre-stressed rolling mill andφ285 finishing mill
6	1#cropping shear (cut head)	1 set	3
7	2#Rotary flying shear	1 set	15	（hard gear surface)
8	Divided Shearing	1 set	12	（hard gear surface)
9	Water cooling system after finishing mill (36m)	1 set	12	1 segments water-cooled(with slow cooling according to the drawings)

If you are interested in the 300,000 Ton TPY Re-bar production line, please mail at marketing2@hanrm.com.

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What Is Continuous Casting Machine?

Continuous casting is the process of continuously casting high-temperature molten steel into billets with certain section shape and size.

The equipment needed to complete this process is called continuous casting complete equipment. The mechanical and electrical hydraulic integration of steel casting equipment, caster body equipment, cutting area equipment, dummy bar collection and conveying equipment constitutes the core equipment of continuous casting steel, which is commonly called continuous caster.

The high-temperature molten steel is continuously poured into one or a group of water-cooled copper crystallizers, and the molten steel gradually solidifies into the shell along the periphery of the crystallizer. When the liquid level of the steel rises to a certain height and the shell solidifies to a certain thickness, the tension leveler will pull out the billet, and the billet will be completely solidified by spray cooling in the secondary cooling area. The cutting device will cut the billet to a certain length according to the rolling requirements. The process of making high-temperature molten steel directly cast into billet is called continuous casting.

Its appearance has fundamentally changed the ingot rolling process, which has been dominant for a century. Because it simplifies the production process, improves the production efficiency and metal yield, and saves energy consumption, the production cost is greatly reduced, and the billet quality is good. At present, in steel-making enterprises, whether it is long process steel-making or short process steel-making, the configuration of continuous casting machine is almost inevitable.

Classification of the continuous casting machine

Continuous casters can be classified in many forms. According to the structure and shape, the caster can be divided into vertical caster, vertical bending caster, arc caster with straight section, arc caster, multi radius elliptical caster and horizontal caster. With the development of continuous casting technology, the research of wheel caster, especially thin slab caster, has been carried out.

According to the size and shape of the section cast by the caster, the caster can be divided into slab caster, billet caster, bloom caster, round billet caster, special section caster and thin slab caster. The rectangular billet caster is also included in the square billet caster. Generally, the billets with casting section or equivalent sectional area greater than 200 × 200 mm are called bloom, the billets with section or equivalent sectional area less than 160 × 160 mm are called billet, and the rectangular billets with width thickness ratio greater than 3 are called slab.

If the continuous casting machine can be divided into single flow, double flow or multi flow continuous casting machines according to the number of billet flow that can be cast under the same ladle.

Composition of continuous caster

Generally, the continuous caster is composed of liquid steel carrier (ladle, rotary table), tundish and its replacement device, mould and its vibration device, secondary cooling area clamping roller and cooling water system, pulling straightener, cutting equipment, dummy bar device, etc.

The tundish plays the role of buffering and purifying the molten steel. The capacity of tundish is generally 20% - 40% of the capacity of ladle. The more the flow number of caster is, the larger its capacity is. The mold is the "heart" of the continuous casting machine, which requires good thermal conductivity, structural rigidity, wear resistance and easy to manufacture and maintain. It is generally made of forged copper or cast brass, and its outer wall is forced to cool evenly by water.

The function of the mould vibration device is to make the mould vibrate periodically, so as to prevent the bonding between the primary shell and the mould wall from being broken. The vibration curve generally changes according to the sinusoidal law to reduce the impact. Its amplitude and frequency should be closely matched with the pulling speed to ensure the quality and output of the slab.

The secondary cooling device is installed at the outlet of the mould, which can accelerate the solidification of the slab and control the temperature of the slab by means of water spray or atomization cooling. The pinch roll and guide roll support the high-temperature billet with liquid core to prevent bulging deformation or internal crack. At the same time, the liquid core reduction technology can be carried out in this section to improve the quality and output of the billet. The water pressure and water volume of the secondary cooling device are required to be adjustable to meet the needs of different steel grades and different pulling speeds.

The function of the straightening machine is to provide the power of drawing billet and straighten the bent billet, and to push the cutting device to move. The speed of drawing billet has a great influence on the output and quality of the continuous casting. The function of the dummy device is to plug the lower mouth of the mould with the dummy head before the start of continuous casting, pull the billet out of the caster after the solidification of the molten steel, then disconnect the dummy head, and put the dummy bar into the storage device.

The billet cutting equipment can cut the continuously moving billet into a fixed length. The commonly used cutting equipment is flame cutter or hydraulic shear and swing shear.

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How Does Electric Arc Furnace Work?

1. Feeding
The operation of adding raw materials such as molten iron or scrap steel to the electric furnace is the first step of the steelmaking operation of the electric arc furnace.

2. Slagging
Slag making is an operation to adjust the composition, basicity, viscosity and reaction ability of slag in steel and iron production. For example, the purpose of oxygen blowing operation is to generate slag with enough fluidity and basicity, which can transfer enough oxygen to the metal surface, so as to reduce the sulfur and phosphorus below the upper limit of planned steel grade, and minimize the amount of splashing and slag overflowing during oxygen blowing.

3. Slag out
According to different smelting conditions and purposes, the operation of slag discharging or raking is adopted in the process of EAF steelmaking. For example, when smelting with the single slag method, the oxidation slag must be removed at the end of oxidation; when reducing slag is made with the double slag method, the original oxidation slag must be completely discharged to prevent phosphorus return, etc.

4. Pool stirring
Energy is supplied to the molten metal pool to make the molten metal and slag move, so as to improve the dynamic conditions of the metallurgical reaction. Pool stirring can be realized by means of gas, mechanical, electromagnetic induction and other methods.

5. Dephosphorization
Chemical reaction to reduce phosphorus content in molten steel. Phosphorus is one of the harmful impurities in the steel. The steel with more phosphorus is easy to crack when it is used at room temperature or lower, which is called "cold brittleness". The higher the carbon content in steel, the more serious the brittleness caused by phosphorus. Generally, the phosphorus content of ordinary steel is not more than 0.045%, and the phosphorus content of high-quality steel is less.

6. Bottom blowing
N2, Ar, CO2, Co, CH4, O2 and other gases are blown into the molten pool of the furnace through the nozzle at the bottom of the furnace according to the process requirements to accelerate the melting and promote the metallurgical reaction process. The bottom blowing process can shorten the smelting time, reduce the power consumption, improve the dephosphorization and desulfurization operation, increase the amount of residual manganese in the steel, and improve the metal and alloy yield. It can make the composition and temperature of molten steel more uniform, so as to improve the quality of steel, reduce the cost and improve productivity.

7、 Melting period
The melting period of steelmaking is mainly for open-hearth and electric furnace steelmaking. The melting period of EAF steelmaking is from the beginning of power on to the end of the melting of all the trace materials in the furnace, and from the end of molten iron blending to the end of the melting of all the materials in the open hearth steelmaking. The task of the melting period is to melt and heat up the charge as soon as possible, and make the slag in the melting period.

8. Oxidation and decarbonization
The oxidation period of EAF steel-making usually refers to the process stage from the melting and cleaning of furnace burden, sampling and analysis to the completion of slag removal. It is also believed that it starts from blowing oxygen or adding ore to decarbonize. The main tasks of the oxidation period are to oxidize the carbon and phosphorus in the molten steel, to remove the gas and inclusions, and to heat the molten steel uniformly. Decarburization is an important process in the oxidation stage. In order to ensure the purity of steel, the amount of decarburization is required to be more than 0.2%. With the development of off furnace refining technology, the oxidation refining of EAF mostly moves to ladle or refining furnace.

9. Refining period
In the process of steelmaking, some elements and compounds harmful to the quality of steel are selected into the gas phase or discharged or floated into the slag through chemical reaction through slagging and other methods, so that they can be excluded from the molten steel.

10. Reduction period
In the operation of EAF, the period from the end of oxidation to the end of tapping is usually called reduction period. Its main task is to make reducing slag for diffusion, deoxidization, desulfurization, chemical composition control and temperature adjustment. The reduction period has been canceled in high power and superpower EAF steelmaking operations.

11. Off furnace refining
The steelmaking process in which the primary molten steel in a steelmaking furnace (converter, electric furnace, etc.) is transferred to another vessel for refining, also called secondary metallurgy. Therefore, the steelmaking process is divided into two steps: primary refining and refining. Primary smelting: the charge is melted, dephosphorized, decarburized and main alloyed in an oxidizing atmosphere. Refining: degassing, deoxidizing, desulfuration, removal of inclusions and fine-tuning of the composition of primary molten steel in vacuum, inert gas or reducing atmosphere. The advantages of steelmaking in two steps are: improving the quality of steel, shortening the smelting time in steelmaking workshop, simplifying the process and reducing the production cost. There are many kinds of off furnace refining, which can be roughly divided into two categories: off furnace refining under atmospheric pressure and off furnace refining under vacuum. According to the different treatment methods, it can be divided into ladle treatment and ladle refining.

12. Liquid steel mixing
The stirring of molten steel in the refining process outside the furnace. It can homogenize the composition and temperature of molten steel and promote the metallurgical reaction. Most metallurgical reactions are phase interface reactions, and the diffusion rate of reactants and products is the limiting link of these reactions. In the static state, the metallurgical reaction speed of molten steel is very slow, for example, it takes 30-60 minutes to desulfurize the static molten steel in the electric furnace, while it only takes 3-5 minutes to desulfurize the molten steel by stirring the molten steel in the furnace refining. When the molten steel is in static state, the removal rate of inclusions floats upward and is slow; when the molten steel is stirred, the removal rate of inclusions increases exponentially and is related to the stirring strength, type, characteristics and concentration of inclusions.

13. Ladle wire feeding
By feeding deoxidizing, desulfurizing and fine-tuning powder wrapped with iron sheet, such as Ca Si powder, or directly feeding aluminum wire, carbon wire, etc. into the ladle through wire feeder, the method of deep desulfurizing, calcium treatment and fine-tuning of carbon and aluminum in the steel can be carried out. It also has the functions of cleaning molten steel and improving the morphology of non-metallic inclusions.

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