Causes and reduction measures of oxide scale in continuous casting billets

Through the analysis of the causes of iron oxide scale in the casting slab, measures to reduce the amount of iron oxide scale in the casting slab were formulated and achieved certain results.

Keywords: continuous casting billet; iron oxide scale; technical measures

In 2012, a certain company’s steelmaking plant discovered this through internal benchmarking. Compared with the original 1* and 2* continuous casting machines, the newly put into production 3* and 4* continuous casting machines produce higher iron oxide scale during the continuous casting production process. Seriously affects the recovery rate of molten steel in continuous casting. Therefore, the General Steel Plant organized continuous casting-related production technicians to tackle the problem of excessive iron oxide scale during the production process of 3* and 4* continuous casting machines, and achieved certain results.

The formation mechanism of iron oxide scale during continuous casting process

The iron oxide scale produced by the continuous casting slab is caused by the chemical reaction between the iron element on the surface of the casting slab and the oxidizing gas in the environment to generate iron oxides, and the oxygen further diffuses inward from the surface of the casting slab to form the iron oxide scale. During continuous casting production, the high-temperature slab must be cooled by the crystallizer and water spray in the secondary cooling zone before it reaches the tension and leveling machine. Then it is straightened by the tension leveler and sent to the cooling bed of the continuous casting machine from the roller table. The H₂O (water vapor) and O₂ in the continuous casting cooling environment have strong oxidizing properties. The oxidation reactions that occur on the surface of the cast slab are mainly as follows:

2Fe+O₂=2Fe03Fe+2O₂=Fe₃O₄

4Fe+3O₂=2Fe₂O₃

Fe+H₂O=FeO+H₂

3Fe+H₂O=Fe₃O₄+H₂

Reasons for the production of iron oxide scale in continuous casting billets

Practice has shown that the main factors that affect the production of iron oxide scale on the slabs of 3* and 4″ continuous casting machines include the amount of water distribution for secondary cooling in continuous casting, the temperature of the slab during the continuous casting process, the temperature of the secondary cooling water in continuous casting, the composition of molten steel, etc.

Effect of continuous casting secondary cooling water distribution on cast slab iron oxide scale

The water distribution volume of the crystallizer of 3″ and 4″ continuous casting machines is generally controlled at 120~140t/h. After the molten steel contacts the copper tube wall in the crystallizer, it is forced to cool and condenses into a billet shell of fine equiaxed crystals. The primary billet The thickness of the shell is greater than 10mm. When the billet pulled out from the mold enters the secondary cooling zone, the liquid core volume is large, the billet shell is thin, and the thermal resistance is small. Increasing the secondary cooling intensity can quickly increase the thickness of the billet. When the slab reaches a certain thickness, the atomized cooling water forms a steam film of a certain thickness on the surface of the slab, which affects the cooling effect of the cooling water and causes the slab to warm up. Therefore, the secondary cooling water is based on the principle from strong to weak. Evenly cooled. When distributing water for secondary cooling, first determine the temperature of each section of the slab in the secondary cooling zone according to different steel types, and determine the amount of water distribution in each section. It is necessary to avoid excessive reheating of the local slab due to uneven water distribution in each section of the secondary cooling zone. The large thermal stress produced causes cracks inside the billet. At the same time, the temperature difference inside the slab is too large, which greatly increases the formation of iron oxide scale on the surface of the slab. Therefore, the requirements for secondary cold water are:

(1) The surface of the slab must be cooled evenly to avoid clogging of the nozzles on the secondary cooling water strip;

(2) Because atomized cooling water can break the steam film on the surface of the cast slab to enhance the cooling effect, in order to ensure the maximum cooling effect, the higher the evaporation rate of the atomized cooling water, the better;

(3) Because reducing the thickness of the vapor film formed on the surface of the cast slab can effectively reduce the oxidation intensity on the surface of the cast slab, the shorter the residence time of unevaporated water on the surface of the cast slab, the better.

Due to design reasons, the number of nozzles on each secondary cooling water strip of the 3* and 4″ continuous casting machines is 4 less than that of the original 1* and 2* continuous casting machines. The flow rate of the secondary cooling water for continuous casting is insufficient. The cold water cooling intensity is weak, resulting in a high temperature return of the slab and a large amount of iron oxide scale produced in the slab.

In addition, because the filter screen of the secondary cold water main pipe is not cleaned in time, the secondary cold water contains many impurities, and the spray nozzle is often clogged during the continuous casting production process, resulting in uneven spraying of the secondary cold water, poor atomization cooling effect, and poor casting quality. The unevaporated water on the surface stays for a long time, so the cast slab produces more iron oxide scale.

Effect of temperature on the production of iron oxide scale in cast slabs

The straightening temperature of the billet of 3* and 4″ continuous casting machines is generally controlled above 900℃, because 700℃~900℃ is the sensitive temperature zone for straightening cracks in the billet.

That is to say, the quality of the cast slab can only be guaranteed when the cast slab is at around 1000°C. According to data, when the slab straightening temperature is around 700°C, the generation of iron oxide scale is not obvious. When the slab straightening temperature is around 900°C, the production of iron oxide scale increases significantly. At about 1000°C, the amount of iron oxide scale produced by the cast slab increases sharply, which is about twice the amount of iron oxide scale produced when the cast slab is at about 900°C. At about 1100°C, the amount of iron oxide scale produced by the cast slab is about 4 times the amount of iron oxide scale produced by the cast slab at about 900°C. Therefore, the longer the cast slab stays in the high temperature section above 1000°C, the more serious the oxidation of the cast slab will be. The molten steel for the 3* and 4″ continuous casters is supplied by the newly put into operation 3* and 4* converters.Due to improper control of the end-point temperature of the converter, the temperature of the molten steel supplied to the 3* and 4″ continuous casters was higher than the upper limit.Adding cold material to the tundish of the continuous casting machine to adjust the temperature is limited, resulting in a high superheat of the molten steel in the tundish. This is also one of the reasons for the high iron oxide scale produced in the slabs of 3* and 4″ continuous casting machines.

Effect of secondary cooling water temperature on iron oxide scale of cast slab

The temperature of the secondary cooling water during continuous casting production of the 3* and 4″ continuous casting machines is too high, which cannot achieve the normal cooling effect, causing the temperature of the slab to be too high.Especially during summer continuous casting production, due to insufficient cooling capacity of the cooling tower of the integrated water pump station, the inlet temperature of the secondary cooling water for continuous casting even exceeds 40°C, and the return water temperature exceeds 48°C, seriously affecting the cooling effect of the cast slab.

Figure 1 Trend chart of the change in the amount of iron oxide scale produced in the slab with the temperature of the slab

Effect of molten steel composition on iron oxide scale produced in cast slab

Continuous casting production practice shows that in the same steel type, the formation of iron oxide scale with relatively high content of carbon, silicon, manganese and other elements will also be reduced. Because the carbon in the steel combines with the oxygen in the air to form carbon monoxide, which prevents further diffusion of oxygen in the cast slab; in addition, the high content of silicon and manganese elements has poor peelability of iron oxide scale, so the amount of iron oxide scale generated in the cast slab is relatively small.

Measures to reduce the amount of iron oxide scale produced in cast slabs

(1) By adding a new continuous casting turbid ring water cooling tower to the comprehensive water pumping station, the cooling area of the continuous casting turbid ring return water is increased and the temperature of the continuous casting secondary cold water is reduced. Ensure that the temperature of the secondary cold water inlet for continuous casting during summer production is not greater than 35°C.

(2) Transform the water strips in the secondary cooling zone of continuous casting, increase the number of spray nozzles in the secondary cooling water strips of continuous casting according to the principle from strong to weak, increase the flow rate of secondary cooling water in continuous casting, and at the same time improve the quality of the spray nozzles , improve the atomization effect of the secondary cold water in continuous casting, thereby increasing the cooling intensity and achieving a uniform water spray cooling effect from strong to weak.

(3) Strengthen the maintenance of the supporting rollers and water strips in the secondary cooling zone of the 3* and 4” continuous casting machines to align the spray water with the slab during continuous casting production to ensure uniform cooling of the slab and avoid local regression of the slab surface. Warming too quickly.

(4) Strengthen the inspection and cleaning of the filter screen of the main pipe of the secondary cold water in continuous casting to ensure the quality of the secondary cold water, and backflush and clean the nozzles of the secondary cold water strips of each flow every week. During continuous casting production, ensure that the surface of the slab is evenly reheated to avoid There is a phenomenon of uneven cooling of the slab caused by the blockage of the water nozzle.

(5) Control the production of high-temperature steel by controlling the end temperature of the 3* and 4* converters, adding cold materials to the molten steel ladle on the argon station platform, blowing argon and stirring, etc., and reducing the amount of steel supplied to the 3* and 4″ continuous casting machines. The superheat of the molten steel fluctuates too much to avoid excessive temperature return of the cast slab due to excessive superheat of the molten steel.

(6) Strictly control the composition of the molten steel supplied to the converter for continuous casting. During the converter smelting operation, on the premise of ensuring the quality of the molten steel and reducing the cost, the carbon content of the molten steel should be maximized and the amount of oxide scale produced in the cast slab should be reduced.

The effect after optimization

After two years of continuous optimization of continuous casting production operating conditions, the amount of oxide scale produced in the 3* and 4* continuous casting machines has been reduced from the original 4.5% to 2.5%, which has greatly increased the recovery rate of molten steel in continuous casting. The annual output of 3″ and 4″ continuous casting machines is based on 2.5 million tons of slabs and the price of steel billets is based on 2,100 yuan/t. The annual production cost will be reduced by about 10.5 million yuan (2,100 yuan/t × 2.5 million t × 2% = 10.5 million yuan). ), achieving higher economic benefits.

Conclusion

Practice has proved that in the actual production process of continuous casting, the production of iron oxide scale in the continuous casting billet is inevitable, but the amount of iron oxide scale generated in the billet can be controlled through certain technical means. The 3* and 4″ continuous casting machines of the General Steel Plant have continuously improved and optimized the continuous casting operating system based on their own process and equipment conditions, which has greatly reduced the amount of oxide scale generated in the casting slab.While providing high-quality cast slabs for the subsequent steel rolling process, it also reduces its own production costs and contributes to the quality improvement and consumption reduction activities carried out by the company.

Figure 2 Schematic diagram of the year-by-year decline of iron oxide scale

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