How much do you know about the process flow of hot rolled strip steel?

 --- Heating process: raw material preparation → feeding → pushing steel → heating → steel pusher  →

The continuous casting billet sent hot from the continuous casting workshop or the cold continuous casting billet hoisted by a crane is hoisted to the loading table, and then sent from the loading table to the loading roller table, and then sent to the furnace roller table via the loading roller table. The hydraulic pusher pushes the continuous casting billet into the heating furnace for heating. The heating furnace is an end-in and end-out double regenerative push steel heating furnace, which is vaporized and cooled.

The billet is heated to about 1050°C-1250°C in the heating furnace, and the billet is carried from the heating furnace to the tapping roller table by the steel supporting machine, and the primary oxide scale on the surface of the billet is removed by the high-pressure water descaling device; The steel stripping roller table returns the raw material span.

---Rough rolling process: high-pressure water descaling→rough rolling (continuous rolling with 3 stands, 5 flats and 8 stands)→

The steel billet after descaling is sent from the work roller table before the rough rolling to the rough rolling unit consisting of three vertical rolls and five two-high flat-roll mills. The roughing mill is composed of 8 rolling mills. The layout is "one vertical, two leveling, one vertical, two leveling, one vertical, one leveling", there is no looper between the stands, and it adopts micro tension rolling. In order to ensure the width accuracy of the strip steel, the vertical roll adopts hydraulic pressure and is controlled by AWC+SSC. The pressing method of the flat-roll two-roll mill is hydraulic pressing. According to the thickness of the finished strip, the rough rolling mill rolls the billet to an intermediate billet with a thickness of 28-35mm. Head-off arrangement between rough rolling and finishing rolling. A width gauge is installed at the rough rolling outlet.

In order to ensure the quality of finishing rolling, the temperature of the intermediate billet entering the finishing mill should be between 950-1050℃, and the strip with unqualified temperature shall not enter the finishing rolling. The middle roller table is equipped with a rejection device to reject unqualified middle billets to the outside of the rolling line.

---Finish rolling process: flying shear→finishing rolling (continuous rolling with 1 stand, 8 flat and 9 stands)→twist→

The qualified intermediate billet is sent to the flying shear head through the intermediate roller table. Then it is sent to the finishing mill to be rolled to the thickness of the finished strip.

The finishing mill is composed of one stand and eight flats. A descaling ring is installed after finishing rolling one stand to remove the secondary scale produced during the rough rolling and transportation process; the first two of the eight flat-roll mills It is a two-high rolling mill to increase the reduction. The last 6 flat-roll rolling mills are a four-high rolling mill to improve the strip shape and dimensional tolerances; 7 electric loop devices are installed between the stands to ensure the belt The steel is rolled with constant micro-tension; in order to ensure the dimensional accuracy of the strip steel, all the adjustment devices of the rolling mill adopt hydraulic pressing. The vertical roll is equipped with AWC+SSC control system, and the flat roll is equipped with AGC thickness control system. The exit of the finishing rolling mill is equipped with a width gauge and a thickness gauge to implement closed-loop control of the thickness and width of the strip.

The final rolling temperature of the strip is controlled between 850℃～950℃, and the maximum final rolling speed can reach 15m/s.

---Finishing process: cooling → inspection → coiling → small bundle → collection, big bundle, spray number → storage

The rolled steel strip is sent to the flat chain conveyor for cooling through the torsion guide groove, pinch roller, three-way guide plate, water-cooled guide groove (reserved for forced cooling) and oscillator.

At the exit end of the flat chain, the strip head is manually clamped and sent to the exit pinch roller, and then sent to the vertical coiler (shrink core type) through a five-roller tension leveler for coiling, and the coiling ends through the dense roller The belt tail is manually welded by the winding machine and the winding machine, and then the steel coil is collected by the steel coil transportation chain, the hydraulic tumbling machine and the steel coil collection device.

It is not only the quality of the refractory lining that affects the life of the converter

There are many factors that affect the damage of converter bricks, which are related to factors such as matte grade, refractory material quality, masonry technology, blowing system and actual operation.

 

1. Masonry and lining damage of converter

There are 2 60t converters in our factory. The masonry structure of the converter is: blasthole bricks 520mm, above the blasthole, there are 9 layers of 520mm and 14 layers of 460mm transition zone, below the blasthole zone is 380mn, and the furnace mouth is built with refractory Material, masonry is thicker in the wind eye area and above, in order to enhance the corrosion resistance.

Production practice shows that the vulnerable parts of the converter lining are: furnace mouth, wind eye, end wall. During the blowing process, it is subject to severe mechanical erosion of high-temperature melt, severe erosion of slag and quartz flux, periodic fluctuations of furnace temperature, mechanical collision and abrasion during furnace mouth cleaning and wind eye maintenance, and the operating conditions are extremely harsh, especially for furnaces. The three parts of the mouth, wind eye, and end wall slag line are not only the vulnerable parts of refractory materials, but also the weakest link of the masonry structure, and the parts that require the highest technical content in road construction. The synchronous life of these three parts largely represents the age of the converter.

According to production practice, when the thickness of the bricks in the blast hole area of ​​the converter is less than 90mm, they can no longer be used, and the furnace needs to be stopped for digging. When the remaining parts of the masonry are below 150mm, the furnace needs to be shut down for overhaul.

 

2. Analysis of factors affecting converter life

There are many reasons for the damage of the converter lining. In summary, they are mainly the result of mechanical force, thermal stress and chemical corrosion.

2.1 The influence of mechanical force

2. 1.1 Damage to brick lining caused by the energy of stirring the melt

Due to the impact force of the blown gas and the rise and expansion of the air flow, a large amount of stirring energy is brought to the melt. When the gas-liquid two-phase mixed fluid impacts the surface of the melt, the melt is sprayed onto the brick lining by the gas-liquid two-phase fluid It causes strong mechanical impact on the furnace lining and creates conditions for chemical erosion. Therefore, choosing a reasonable blast intensity is an important part of improving the life of the converter. A relatively suitable air supply intensity and air supply system will help weaken the melt’s impact on the furnace lining. The impact force extends the life of the converter.

2. 1. 2 Clean up the damage of the wind eye to the wind eye brick

In the blowing process, magnetic iron is inevitably generated. When the wind eye is operated, the melt in the tuyere area is recharged, and nodules are easily formed at the tuyere. The tuyere needs to be cleaned continuously, and the damage effect of mechanical vibration on the brick lining in the tuyere area It is very large, causing the surface of the brick lining in the tuyere area to deteriorate under the action of melt erosion. When the metamorphic layer expands to a certain extent, the brick body will peel off, which seriously affects the furnace life.

2.2 The influence of thermal stress

The resistance of refractory materials to damage caused by temperature changes during heating and cooling is called thermal shock resistance, which is an important indicator of the quality of refractory materials. Most refractory materials are damaged due to poor thermal shock resistance at temperatures much lower than their refractoriness. The thermal damage of refractory materials is mainly related to the thermal stress produced by refractory materials during the production process.

The converter is a periodic operation, and it is inevitable that the temperature of the converter will fluctuate due to the failure of the material, the repair of the furnace port and the failure of the equipment during the production.

2.3 Effects of chemical attack

Chemical attack mainly has two forms: melt erosion (slag, metal solution) and gas erosion. It is manifested in the dissolution, combination and penetration of magnesia refractory materials, which changes the structure of refractory materials and weakens their performance.

2. 3. 1 Melt erosion

The melt contacts and penetrates through the pores, cracks and the interface between the refractory materials. During the contact process, the refractory material dissolves into the melt, and the surface of the refractory material forms an easily soluble compound whose bulk density changes greatly with the raw material. When it dissolves to a certain extent, the infiltration occurs. When the melt penetrates the refractory material to a certain depth, it will produce The metamorphic layer with completely different properties of the raw material changes in volume due to the different structure of the metamorphic layer and the raw material, resulting in structural stress, which leads to cracks in the production of raw materials. Serious cracks cause the metamorphic layer to peel off or crack, and new materials will be generated under the erosion of the melt. Refractory material is seriously damaged by this cycle.

2. 3. 2 Gas erosion

Gas erosion generally refers to the reaction of the so 2 and o 2 in the copper matte with the alkali oxides in the refractory during the blowing process to form metal sulfates, and its density is lower than that of the alkali oxides. The difference in phase bulk density produces stress, which makes the refractory material loose and peels off, and aggravates the damage of the refractory material.

 

3. Measures to extend converter life

3. 1. Change the masonry method and improve the process standard:

3.1.1 Under normal circumstances, wet masonry will cause the brick body to become damp, which is not conducive to constant temperature dehydration at 400 ℃. The masonry of the converter adopts a combination of dry and wet, that is, the upper and lower 4 layers of the tuyere area and the furnace mouth area are made of wet masonry, and the rest are dry-laid.

3.1.2 The masonry of the tuyere bricks was changed from one end to the middle to both ends to avoid triangle joints and dislocation of the tuyere combination bricks.

3.1.3 Changed from laying on one end and lower furnace mouth reverse arch bricks to masonry from the center to both ends, and proceeded symmetrically, which is conducive to closing and locking on both sides, and preventing the uneven and tight gap between the two bricks from falling off .

3.1.4 The distribution of the brick joints is substantial, uniform, and the inside and outside are consistent. The expansion joints meet the requirements of 2-3 mm. The joints of each part of the brick body shall be locked, and the processed brick body shall not exceed one-third. The processed brick Body is not less than two-thirds of itself.

3.1.5 Magnesium fillers are required to be kneaded into a mass by an expert hand, and scattered from a height of one meter. The thickness of the filler is uniform and the firmness is uniform.

3.1.6 Damaged, broken corners and damp chrome-magnesium bricks must not be used.

3.2. Control the converter cold material to prevent high temperature corrosion

The test proves that when the chrome-magnesia brick has thermal vibration resistance at 850 ℃, it will break and break 18 times, resulting in damage to the brick lining. Therefore, it is necessary to prevent the furnace temperature from rising and falling and violent fluctuations, and to reduce and eliminate the damage to the brick lining caused by thermal stress. In production, the method of controlling the amount of cold material added is used to stabilize the furnace temperature.

3.3. Reasonably control the silicon content of converter slag, reduce chemical corrosion, neutral or weak alkaline slag, and protect the brick lining. Ferroolivine corrodes severely, and it can not only dissolve the surface of the magnesia refractory, but also penetrate into the interior to dissolve. The higher the temperature, the greater the solubility of M g O in converter slag, and the formation of forsterite with a lower softening temperature under load at high temperatures, which reduces the working performance of the magnesia brick. Iron oxides can also saturate periclase and chromite crystal grains, cause crystal grain damage, and cause magnesia bricks to be damaged too quickly. The converter slag contains less than 18% silicon and is alkaline, while the converter slag contains more than 28% silicon and is acidic. Both of them seriously corrode the lining of magnesia bricks. The converter slag contains between 19% and 24% silicon, which is neutral or weakly alkaline and does not corrode the lining of the magnesia brick. In production, the silicon content of converter slag is strictly controlled to stabilize it between 19% and 24%.

3.4. Improve personnel quality

Improve the quality and ability of furnace building, converter operations, and production managers to ensure the quality of furnace building. Improve the ability to respond to emergencies, scientifically and strictly supervise and manage production.

3.5. Reasonable selection of air supply intensity and oxygen enrichment concentration

It is inevitable that the furnace body and the fan do not match during the production process. It is strictly forbidden to use a large fan to supply air to the small furnace body to prevent the tuyere area from being washed out and the melt is severely sprayed. The oxygen-enriched concentration of the converter should not be higher than 27%, and the oxygen-enriched concentration should be greater than 27%, which will wash the brick lining more.

 

4. Issues that should be paid attention to

The following aspects should also be paid attention to in production:

(1) Formulate scientific standards for shutdown, repair and start-up, such as brick lining removal standards, heating standards, etc., and strictly implement them. (2) When the newly repaired furnace body is started, the operations of "hanging the furnace" and "copperizing" should be carried out to protect the furnace body.

(3) Strict process operation, the control of the furnace temperature at each stage and the judgment of the end point must be accurate. Prevent the occurrence of "overblowing", especially the second-cycle overblowing, which will cause serious damage to the furnace body.

(4) Attach importance to the training of employees and improve the quality of all employees and the level of copper smelting technology.

 

5. Summary

Through the implementation of the above measures, the energy consumption per ton of copper bricks is well controlled, costs are reduced, and annual benefits are created. As long as attention is paid to masonry quality, process conditions, and elimination of the thermal stress, mechanical force and chemical corrosion factors that damage the chrome-magnesium bricks, the life of the furnace bricks can be prolonged.

The Turkish steel industry, which has 24 electric arc steel plants, has made another big move! Erdemir acquires Kümaş

Focus on:

·Kümaş, a leading company in magnesium and refractory materials, was acquired

·Kümaş joins Almatis and belongs to OYAK Group

·As end users have requirements for supply security, the vertical integration strategy begins

·The purchase price of 340 million US dollars

Turkey’s Kümaş Manyezit Sanayi A.Ş. (Kümaş), one of the world’s major magnesia producers, was acquired by Turkish steel leader Ereğli Demir ve Çelik Fabrikaları T.A.Ş. (Erdemir) for US$340 million.

The equity conversion agreement was officially signed on January 4, 2021. The current share ratio is

Erdemir’s owner Gözde Girişim Sermayesi Yatırım Ortaklığı A.Ş. holds 51% and Kümaş’s owner Yıldız Holding A.Ş. holds 49%. The agreement will come into effect after the law is approved.

Erdemir is a member of the metallurgical mining group OYAK, which is also Turkey's largest supplementary occupational pension fund with total assets of approximately US$16 billion and is headquartered in Ankara.

The OYAK Group has a wide-ranging industrial and service portfolio, including its chemical products group (the group acquired Almatis, the world's leading specialty alumina producer in 2015). Almatis and Kümaş are both major material suppliers for the global refractory market.

This advancement also optimizes the group’s market share, strengthens the general ownership trend, and reduces competition among magnesia producers, because they are grouped into a larger group of magnesia refractories and directly face end users group. A similar incident occurred a year ago when Sibelco sold QMAG, a leading manufacturer of magnesium oxide, to Refreshnik, a major German refractory manufacturer.

At present, magnesia suppliers other than China are mainly distributed in Mexico, Brazil, the Netherlands, Ireland, Spain, Greece, Turkey, Austria, Slovakia, Russia, Japan and Australia. The resources emerging in recent years mainly come from Greece, Turkey, Saudi Arabia, Jordan, Serbia and Turkey.

In a statement issued on January 5, 2021, OYAK stated that the acquisition is mainly aimed at achieving cost control and efficiency by ensuring the vertical integration of its activities in the steel and cement industries. It is also to protect the value of national resources. There is no doubt that the synergy after integration will increase.

On August 11, 2020, OYAK Group also integrated and acquired 60% of the shares of Haznedar Durer refractories from Imerys, a global industrial mineral producer that was headquartered in Paris.

As we all know, RHI Magnesita (RHIM), a global manufacturer of magnesium and refractories based in Vienna, has established a magnesium oxide plant (MAS, Eskişehir) in Turkey. In the past two and a half years, Kümaş has been closely monitored, and he participated in the acquisition in May last year, but it was not successful.

Kümaş’s main mineral products include heavy burnt magnesia (annual production of 300,000 tons); fused magnesia (annual production of 40,000Tons); light burned magnesia (annual production of 100,000 tons) and heavy burned dolomite (annual production of 20,000 tons). Refractory materials

It mainly produces refractory bricks and mortar based on magnesium, dolomite and alumina, with an annual output of approximately 150,000 tons. OYAK mainly produces steel products, which require the use of magnesia refractory materials and chemicals.

The steel industry in Turkey is in good condition, including 24 electric arc furnace plants, 7 induction furnace plants and three alkaline gas plants. In 2020, when the epidemic is pandemic, Turkey is also bucking the trend, and steel production is expected to exceed 2019.

Turkey is currently the second largest steel producer in Europe and is expected to surpass Germany in the future. OYAK continues to invest overseas, with the goal of becoming the world's top 5 in the next five years. For the circular economy, the group also has specific plans.

OYAK not only owns the steel and refractory business, but also covers the fields of agriculture, chemistry, cement and paper making.

According to our survey, Kümaş is the largest producer of magnesite products in Turkey. It has a wealth of magnesite and dolomite deposits in Turkey, and is the second largest producer of magnesite after China. In terms of magnesia production, it is second only to China and Russia, ranking third in the world and fourth in export volume.

Figure 1: 2019 global magnesite reserves and production

Figure 2: 2019 global magnesia supply and transactions

It is understood that Kümaş has 163 million tons of magnesite resources, of which high-quality cryptocrystalline magnesite reserves account for about 20% of the world. It is characterized by very fine magnesite crystals, usually 1-10μm, with high surface area and High reactivity. In addition, there are about 96 million tons of dolomite resources.

The other two major magnesite producers in Turkey are Magnesit AŞ and Eskişehir under RHIM, with an annual capacity of 260,000 tons of heavy burned magnesia, and Konya Selçuklu Krom Magnezit Tuğla Sanayii AŞ, Konya, with an annual capacity of 45,000 tons of heavy burned magnesia. .

Some suggestions on reducing energy consumption during converter steelmaking

Today’s article is a special article on energy saving and consumption reduction in converter steelmaking at the request of our back-end fans. For reference.

The iron and steel industry is a high energy consumption industry, which consumes about 10% of the national energy consumption. In the process of steel production, the converter steelmaking process accounts for a very high proportion of energy loss, and it is also the link that generates the most secondary energy. Therefore, reducing the energy loss of the converter steelmaking process is the fundamental promotion of the steel industry. The foundation of sustainable development.

Many scholars and practitioners have proposed energy-saving and consumption-reducing measures in the converter steelmaking process, including the renewal of energy-saving equipment, the optimization of smelting technology, the improvement of product process structure, energy conversion, recycling, and so on.

Before talking about this, let us first understand the process of converter steelmaking. Converter steelmaking uses molten iron, scrap, and ferroalloy as the main raw materials. It does not rely on external energy sources, but is produced by the physical heat of the molten iron itself and the chemical reaction between the components of the molten iron. The heat is used to complete the steelmaking process in the converter, which is mainly used to produce carbon steel, alloy steel, copper and nickel smelting.

The picture below shows the evolution of the converter steelmaking method.

The energy consumption in the steelmaking production process is mainly divided into direct energy consumption, indirect energy consumption and energy recovery. Direct energy consumption is the energy consumption in the converter steelmaking process, and the consumption of auxiliary equipment in the converter steelmaking process becomes indirect Consumption, operation process, equipment and technology also affect energy consumption. The main purpose of energy consumption calculation analysis and calculation is to strengthen the control and planning of energy, reduce energy consumption, improve production efficiency, and determine that the converter steelmaking process can achieve The consumption level of this product provides a basis for energy management and energy saving, and makes full use of energy while improving processes and reducing raw materials.

The energy media consumed in the converter steelmaking process mainly includes oxygen, coal gas, nitrogen, argon, coke, electricity, water, steam, etc. Electricity, water and gas are the main energy consumption, especially in the cooling process, the energy consumption is very large, and a lot of production Steam carries a lot of energy. Improper control and management are prone to escape, resulting in waste of water and steam resources. The annual energy consumption of China's iron and steel industry accounts for 10% of the country's total energy consumption, while electricity consumption accounts for 11% of industrial electricity consumption; new water consumption accounts for 9% of total industrial water consumption.

For the entire energy consumption structure of the steel industry, coal is the first consumable, followed by electricity.

In the converter steelmaking production process, the energy actually consumed by electric energy and various gas forms negative energy steelmaking. Negative energy steelmaking is the largest energy consumption affecting the entire process. Strengthen the research on the recovery of converter gas and analyze the impact of converter productivity. The impact of energy consumption.

The realization of negative energy steelmaking in the converter is an important indicator to measure the production technology level of modern steel mills. Through the calculation of the heat balance of the converter steelmaking, the energy consumption and recycling situation in the converter process are analyzed; there are many energy consumption factors that affect the converter process. The converter process can greatly reduce energy consumption, reduce oxygen and electricity consumption, and improve the steam recovery efficiency of the converter, especially the recovery amount of converter gas.

Oxygen and power consumption account for the largest proportion of energy consumption in the converter steelmaking process. By adopting relevant technical measures to control oxygen and power consumption, the converter steelmaking energy consumption can be effectively reduced. The main methods are: use sub-lance to dynamically control steel tapping, improve the end-point hit rate, and strictly control the tapping temperature; increase the composition and temperature compliance rate of molten steel to the refining furnace, optimize the operating process system, carry out standardized operations, and control the refining power consumption; Advanced steelmaking process equipment to achieve high quality, high yield, low cost and low energy consumption; improve production management level, improve production personnel's technical operation level; implement full-process insulation measures, control temperature drop, and achieve stable process operation to achieve a low-temperature system throughout the process run.

Main measures to achieve energy saving and emission reduction

(1) The converter gas recovery is completed by the cooperation between the intersections and converter blowing controlled by PLC. The computer improves the gas recovery program, controls the lifting time, determines the best recovery time, and improves the recovery volume and heating value of steelmaking gas. . Positive pressure control is adopted for the furnace mouth, the pressure is controlled at about 5pa during production, and segmented control is adopted to ensure the smooth progress of gas recovery. In addition, it is necessary to control the timing of lifting, distribute the pressure of the system point, and shorten the recovery time.

(2) The oxygen consumption of the converter steelmaking process is about 19% of the total consumption. The oxygen consumption can be reduced by lowering the tapping temperature, controlling the quantity of high-temperature steel, increasing the carbon pulling rate, and prohibiting the use of oxygen blowing hoods. This measure reduces the oxygen consumption of the converter gradually, reducing the energy consumption of the converter steelmaking process. Converter gas is the main energy source in the steelmaking process. Due to the imperfect technical equipment level of converter gas, the recovery volume fluctuates greatly. During converter operation, most operators control according to experience and do not implement falling hood recovery, resulting in a large amount of air in the pipeline. , The reduction of CO content can ensure the reduction of the gas recovery. The recovered gas is provided to the steel-making electric furnace, and part of the gas is released, resulting in a waste of resources.

(3) Improve the recovery of converter steam and reduce power consumption. When recovering coal gas, the coal gas can generate more steam, and the converter steam recovery fluctuates greatly. Using converter steam in the lime rotary kiln can reduce the influence of restrictive factors on the converter residual energy recovery.

(4) The waste heat boiler can recover the steam after the converter flue gas is cooled to realize the repeated use of the high-temperature sensible heat in the converter flue gas. Based on the high and low pressure organic fusion of the evaporator, the steam is recovered.

(5) The frequency conversion transformation of the dust removal system, using frequency conversion dust removal, can reduce the probability of failure, increase the stability of the equipment, and reduce the energy loss caused by the circulation between equipment.

(6) Pay attention to the selection and application of slagging methods. Less slag smelting is to complete the converter smelting work by reducing the amount of slag. Based on the characteristics that the low temperature in the early stage of converter smelting is conducive to the dephosphorization reaction, the dephosphorization slag is poured out, and an appropriate amount of slag is added to carry out the blowing and decarbonization work. The slag is convenient for the dephosphorization of the next furnace, and the slag can be reused, which can reduce the amount and consumption of raw materials such as dolomite and lime. Since the final process has a small amount of slag and no slag dumping, it can improve the yield of molten steel and oxygen. Utilization rate. To carry out this method of smelting with less slag, it is particularly important to prepare dephosphorization slag in the early stage. This is mainly determined by the fluidity of the slag. If the alkalinity is high, the fluidity of the slag will become worse. If the alkalinity is low, the phosphorus will be difficult to remove. Therefore, the fluidity of the slag can be better only when the alkalinity is appropriate. Refer to the ternary phase diagram, the content of the slag is generally controlled within the range of 11%-16%, in order to make the slag become a homogeneous liquid phase. State, the alkalinity is strictly required to be controlled at about 1.6. In this case, the melting performance of the slag can be improved.

(7) Control the splash phenomenon. There are three types of splashes: metal splashes, explosive splashes and foam splashes. No matter which method of spraying, it will bring losses to the metal. The factor leading to the spraying is the unscientific control of the position of the oxygen lance. Reducing metal loss is the primary goal of converter production, which is equivalent to increasing the amount of steel. Output and splashing will not only cause accidents such as burning gun sticking to the gun, but also impact on the furnace lining and hinder the removal of phosphorus. Therefore, it is very important to scientifically control the gun position to reduce splashing. In the initial stage, avoid slag re-drying In the case of circumstance, the slag foaming should be avoided in the middle stage, and the temperature in the furnace should be controlled scientifically.

(8) Reduce the number of tapping downs and waiting time. In the process of converter steelmaking, once the operation is wrong, the position of the oxygen lance will be too high during the blowing process, causing the slag to foam, so that the furnace needs to be poured before the tapping, resulting in no tapping. In the process of dumping the furnace and slag, the temperature of the converter steelmaking will drop accordingly, breaking the original plan. Extend the tapping time, so that the procedures that should be performed after tapping cannot be carried out in time, causing economic losses. Therefore, when the slag foaming is serious, slag pressing materials should be used to treat the slag in time to avoid adverse effects such as temperature reduction. In the process of converter tapping, the waiting time is the primary reason for the temperature drop, and the reasons for the long waiting time include: equal sample composition and equal rhythm. Waiting for the sample composition means that the waiting time for the sample is too long, so the best time to send the sample is delayed, and there will be some deviations in the sample inspection process, resulting in the need to produce special steel grades, unable to achieve timely tapping.

In addition, in terms of resources, catalysts can also be added to the converter to promote combustion, so as to maximize the rate of energy obtained from burning; strengthen the secondary utilization of scrap steel; in terms of operation, pay attention to the loading of each furnace in the converter smelting Too much loading will cause poor mixing of the molten pool, failure to normalize the slag, and eventually residual steel. If the loading is too small, the output will decrease or the furnace bottom will be damaged.

Production Practice of Tundish Sizing Nozzle

 1 Introduction

The continuous casting process is an important process in the steelmaking production. It acts as a link between the converter and the steel rolling process, that is, the qualified molten steel made by the converter is poured into qualified billets and supplied to the rolling mill in time.

Refractory materials for continuous casting play an important role in the quality of steel and the normal production and performance of continuous casting. With the development of modern high-efficiency continuous casting technology, it is necessary to further improve the performance of refractory materials for continuous casting and develop new materials. The optimization and development direction of refractory materials for continuous casting is to increase the service life of refractory materials, reduce the pollution of molten steel, and meet the requirements of longer-life continuous casting and other special functions. As a key refractory material for continuous casting, the tundish sizing nozzle plays a crucial role in continuous casting production.

The tundish sizing nozzle is widely used in the tundish non-stop bar control flow pouring system for the production of continuous casting small-section billets, which plays a role in controlling the flow of molten steel and stabilizing the drawing speed. The molten steel flows into the mold evenly and stably through the sizing nozzle, which is a necessary condition to ensure the normal operation of continuous casting.

The main raw material for the sizing nozzle is a zirconium core. The zirconium sizing nozzle is made of stabilized zirconia and natural oblique zircon after being stabilized by a special process, and then formed by high pressure molding and high temperature firing. The sizing nozzle has the characteristics of high refractoriness, good thermal shock resistance, corrosion resistance, erosion resistance, small diameter change, and long service life.

The sizing nozzle is divided into an upper nozzle and a lower nozzle. The performance of the sizing nozzle directly affects the improvement of the life of the continuous casting tundish, which is of great significance for stabilizing the number of continuous casting furnaces and ensuring the normal production organization. Domestic steel mills currently have an average life of about 40 hours for sizing nozzles. With the continuous advancement of technology, the service life of sizing nozzles of domestic steel mills has reached more than 100 hours, which not only improves the production rate of casting machines, but also increases the output. Cost reduction is of great significance.

2. Current situation analysis

The steelmaking plant has 3 billet continuous casters and 1 special-shaped billet continuous caster, mainly casting HRB400E, HRB400K, Q235B, Q345B, HPB300D, MG500, HRB5000E and other steel grades. The main process equipment parameters are shown in Table 1.

Table 1 Main process equipment parameters of continuous casting machine

Casting machine model	shape	Slab section	Arc radius	Tundish capacity	crystallizer length	Casting machine flow number	Working pulling speed
5	camber	150×150	10000	23	900	5	1.8～2.8
6	camber	430×300×85	10000	23	700	4	0.75～0.85
7	camber	150×150	8000	25	900	6	1.8～2.8
8	camber	150×150	10280	25	1000	6	1.8～2.8

In recent years, reducing iron ratio and increasing output has become a common goal pursued by all companies in the steel industry. Our plant's production capacity has been further improved. The average casting speed of the caster has increased from about 2.2m/min in 2018 to about 2.6m/mim in 2019. The normal life of the tundish is 36 hours, the life of the upper nozzle is synchronized with the life of the tundish, and the life of the lower nozzle is 6-8 hours.

The sizing nozzle is mainly affected by chemical erosion, mechanical erosion and stress spalling in use. Starting from May 2019, the continuous casting process has started to occur intermittently in the middle and late stages of the use of the tundish. The nozzle has expanded diameter, dropped blocks, and reddened the nozzle. Such problems became more and more serious in July and August. During this period, there were two incidents of steel running during the delivery nozzle, which seriously threatened normal production, and the continuous caster was forced to shut down several times in advance or to heat up the package. It not only has a great impact on the safe and stable production of the continuous casting machine, but also disrupts the original production organization plan, and the cost of tundish refractory materials has increased sharply.

Through the technical personnel's follow-up inspection and analysis of the water inlet, the water outlet and the tundish from installation to use, and offline, the reasons for the problems of the sizing nozzle are as follows;

 

2.1 The quality of the zirconium core at the upper water inlet has decreased

The sizing nozzle must have good erosion resistance, corrosion resistance and thermal stability, and is generally made of high-purity zircon, zirconia and CaO.

Zirconia exists as three different types of isoisomers at different temperatures, namely: monoclinic system and tetragonal system

And cubic crystal system. The densities of the three crystal systems are: 5.65 g/cm3, 6.10 g/cm3, and 6.27 g/cm3. The density of the three crystal systems is quite different, especially between the monoclinic system and the tetragonal system.

From the thermodynamic analysis, when the temperature is lower than 1170℃, the monoclinic phase of pure zirconia is stable. When the temperature exceeds 117 0℃, ZrO2 changes from a monoclinic phase to a tetragonal phase. When the temperature exceeds 2370℃, ZrO2 changes from a tetragonal phase to a vertical phase.

Square phase, melting until 2680-2700℃. The entire phase change process is reversible.

When changing from a monoclinic phase to a tetragonal phase, it is accompanied by a volume shrinkage of about 7%. When the reverse phase transition occurs from a high-temperature cooling process, the temperature drops by about 100°C and the volume expands by about 3-5%. This volume change is sufficient to cause microcracks or obvious cracking of the material. Therefore, an effective solution is to add stabilizers to zirconia to prevent volume changes and stress generation during the phase change process.

Therefore, the lower quality of the zirconium core at the upper water port is the main reason for the lump drop at the water port.

 

2.2 The mud gap between the zirconium core and the outer skin penetrates the steel

The upper nozzle of the tundish used in our factory is an inlaid sizing nozzle, the body is made of high aluminum, and the inner core of the nozzle is made of zircon and zirconia composite. The nozzle zirconium core and the nozzle body are made separately, and then the two are glued together with refractory mud. The nozzle has low cost and good thermal stability.

After follow-up inspection of the off-line tundish, it was found that the redness of the upper nozzle is also directly related to the penetration of steel between the zirconium core and the outer skin. Sludge penetration of steel causes the body of the upper and lower nozzle working face to be corroded by molten steel, and cold steel is entrained, which is very easy to cause steel running accidents during the replacement of the nozzle; on the other hand, the mud joints penetrate the steel to cause the upper nozzle to become red, and it is easy to crack accident.

If the zirconium core of the nozzle and the nozzle body are not well bonded, the nozzle core will fall off and the steel will be penetrated by the mud gap after long time use. Its safety depends on the properties of the mud and the bonding process.

 

2.3 Increased pulling speed increases erosion of mud joints in the upper water mouth

The erosion of the zirconium core by molten steel is very serious. Due to the influence of the earth's gravity, the molten steel flows downward in a spiral vortex in the upper nozzle of the tundish, and the flow velocity can reach 14m/s. Due to the high specific gravity, high temperature, and fast flow rate of molten steel, severe heat scouring of the upper nozzle occurs. Since the inlaid top nozzle is a weak link in the mud joints, it is easily washed away by the high-speed rotating molten steel, causing the steel to penetrate to the top and bottom nozzle working surface.

Tracking found that when the continuous casting speed is low, the phenomenon of steel penetration in the mud seam basically does not occur, and there are more accidents of steel penetration in the mud seam when the drawing speed is increased.

 

2.4 Misalignment of the upper and lower nozzles

The service life of the drain is about 6-8 hours. During the quick change process, the drain is not in place due to various reasons, and the upper and lower nozzles are misaligned, which causes the high aluminum refractory material outside the zirconium core of the lower slider to be corroded by molten steel to produce pits After the pit is enlarged, the outer side of the zirconium core of the upper nozzle is eroded, resulting in serious steel strip on the upper and lower nozzle working face. The lighter ones cause the water outlet to become red and block the flow, and the more serious ones cause steel wear accidents in the process of replacing the water outlet.

 

2.5 Uneven working surface of upper and lower nozzles

Uneven working faces of the upper and lower runners will cause cold steel to be clamped on the working faces of the upper and lower runners during production, and the working faces will be scratched during the replacement of the runners, resulting in accidents such as steel running on the working faces.

 

3. Take measures

3.1 Improve the quality of the zirconium core at the water inlet

In order to improve the quality of the zirconium cores in Sheung Shui, on the one hand, manufacturers are required to adjust the formula and improve the production process, and on the other hand, find other manufacturers to conduct tests. After testing the water inlets of different manufacturers and different formulations, a new formula and manufacturer were finally determined. The expansion of the zirconium core and the phenomenon of zirconium core lumps in the water inlet were fundamentally controlled.

 

3.2 Change the block

After continuous tracking by the technicians, due to the direct erosion of the steel flow to the upper nozzle, it was decided to change the nozzle block, from the open upper mouth to the upper mouth capped type to protect the upper nozzle mud joints Not directly washed by molten steel.

3.3 Improve the quality of mud joints inlaid with zirconium cores

Improve the quality of refractory mud for mud joints and use zirconium fire mud that is more resistant to erosion. Improve the inlay process and minimize the mud gap to less than 0.5mm.

 

3.4 Ensure that the nozzle is centered

Train workers who install quick-change institutions to achieve standardized operations.

Before installing the upper and lower nozzles, pick out and return the upper and lower nozzles with obvious eccentricity.

Before installing the quick-change mechanism each time, measure the pressure of the metal parts, the nozzle bracket, and the spring to ensure that the cylinder stroke error is less than 1mm, the spring pressure is within the specified range, and the slides with obvious wear are replaced in time. Before each tundish baking, the captain will check the quick-change mechanism and measure the stroke of the quick-change cylinder to ensure that the nozzle is centered during production.

 

3.5 Return of nozzles with poor working surface flatness

After the arrival of each batch of nozzles, randomly sample the smoothness of the nozzle working surface, and return the nozzles with poor flatness.

3.6 Improved slider zirconium core

The zirconium core of the slider is thickened, especially the area of ​​the zirconium core of the working surface part of the slider is enlarged to ensure that the high-aluminum skin is not affected by the erosion of molten steel in normal use when there is a centering error in the nozzle.

 

4 Conclusion

After nearly half a year of follow-up, improvement, and testing, accidents such as nozzle blockage and redness have been thoroughly controlled. The disorder of production organization caused by the nozzle problem of the tundish was solved.

With the rapid development of continuous casting technology, there has been a billet caster capable of reaching above 5m/min or even 6-7 m/min, and the continuous casting time reached more than 100 hours. The development of these technologies is inseparable from the support of new refractory materials. All in all, refractory materials for continuous casting are developing in the direction of multi-function, high performance and long life. The development of new refractory materials for continuous casting should be accelerated, and the upgrading of products should be accelerated.