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The Importance of Temperature Control in Converter Steelmaking

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This article describes the importance of temperature on converter steelmaking and its impact on various aspects.

Keywords: converter steelmaking; temperature control

The importance of temperature to converter steelmaking

When smelting steel, the temperature of the steel is an important parameter. Temperature control is mainly process temperature control and end point temperature control. The quality of end-point temperature control will directly affect technical and economic indicators such as energy in the smelting process, yield of alloy elements, service life of the furnace lining and quality of finished steel. Scientific and reasonable control of the molten pool temperature is an important technological means to regulate the direction and limit of metallurgical reactions. If a suitably low temperature is conducive to dephosphorization, a higher temperature is conducive to carbon oxidation, etc. In summary, the impact of molten pool temperature on converter steelmaking production is mainly reflected in the smelting operation, composition control, pouring process and ingot quality. ​

Factors affecting the temperature of molten steel

There are many factors that affect the temperature of molten steel under production conditions, and the amount of coolant added must be determined after comprehensive consideration.

Molten iron composition

Si and P in molten iron are strong exothermic elements. If their content is too high, it can increase heat, but it will also bring many problems to smelting. Therefore, if possible, pretreatment of molten iron should be carried out to remove Si and P. According to the measurement of the 30t converter, when increasing ω (si) = 0.1%, the furnace temperature can be increased by 15°C.

Molten iron temperature

The temperature of the molten iron is related to the amount of physical heat, so when other conditions remain unchanged, the temperature of the molten iron entering the furnace affects the end temperature. When the temperature of molten iron increases by 10℃, the end temperature of molten steel can increase by 6℃.

Loading amount of molten iron

As the amount of molten iron charged increases or decreases, the physical heat and chemical heat will change. If other conditions are constant, the higher the molten iron ratio, the higher the end temperature will be. For every 1 ton increase in molten iron volume in the 30t converter, the end point temperature can increase by 8°C.

Furnace age

The new lining temperature of the converter is low and the tapping opening is small. Therefore, the end temperature in the early stage of the furnace operation is 20~30℃ higher than that of the normal blowing heat in order to obtain the same pouring temperature. Therefore, the amount of coolant should be reduced accordingly. In the later stages of furnace service, the furnace lining is thin, the furnace mouth is large, and heat loss is large. Therefore, in addition to appropriately reducing the amount of coolant, the auxiliary time should also be shortened as much as possible.

End point carbon content

Carbon is an important heating element in converter steelmaking. According to the experience of a certain factory, when the end point carbon is below 0.24%, for every 0.01% increase or decrease in carbon, the tapping temperature will also decrease and increase by 2~3°C accordingly. Therefore, this impact should be considered when blowing low carbon steel.

The interval between furnaces

The longer the interval, the more heat the furnace lining dissipates. Under normal circumstances, the interval between furnaces is 4 to 10 minutes. If the interval is within 10 minutes, the amount of coolant does not need to be adjusted; if the interval exceeds 10 minutes, the amount of coolant should be reduced accordingly. In addition, when the furnace is empty due to repairing, consider reducing the cooling utilization based on the amount of filling materials and the empty furnace time. According to the measurement of a 30t converter, an empty furnace can reduce the end point temperature by 30°C for 1 hour.

Gun position

If the low gun position is used, the chemical reaction speed in the furnace will be accelerated, especially the decarburization speed, the oxygen supply time will be shortened, the heat released per unit time will increase, and the heat loss will be reduced accordingly.

Splash

Splashing will increase heat loss, so for furnaces with severe splashing, special attention should be paid to adjusting the amount of coolant.

Lime dosage

The cooling effect of lime is similar to that of scrap steel. A large amount of lime will increase the amount of slag, resulting in a long blowing time and affecting the end temperature. Therefore, when the amount of lime is too large, the amount of other coolants should be reduced accordingly. According to the calculation of the 30t converter, adding 100kg more lime reduces the end temperature by 5.7℃. The amount of coolant in this furnace can be adjusted according to the tapping temperature of the previous furnace.

Temperature control of converter steelmaking process

During the blowing process, the temperature is adjusted based on the judgment of the furnace conditions. In the early stage of blowing, if the carbon flame comes up early, it means that the temperature of the melt pool is too high. It can be controlled by adding the second batch of materials appropriately in advance; on the contrary, if the carbon flame comes up late, it indicates that the early temperature is low, and the gun should be lowered to increase the temperature of the melt pool. In the middle stage of blowing, the temperature of the molten pool can be judged based on the flame at the furnace mouth. If the temperature is too high, ore should be added to adjust it.

By adding coolant through calculation and adjustment results, it can basically ensure that the temperature of the molten steel at the end point reaches the temperature required for tapping. However, during the blowing process, the furnace conditions should be carefully observed, the temperature in the furnace should be accurately judged, and corresponding measures should be taken. Adjustments such as increasing or decreasing the amount of coolant, adjusting the gun position, etc. can be made to meet the needs of the metallurgical reactions in the furnace at each stage, while accurately controlling the temperature at the end point.

(1)Initial stage of blowing

If the carbon flame comes up early (previously it was the flame of silicon and manganese oxidation, and the color was red), it means that the temperature in the furnace is already high and the first batch of slag has been melted. The second batch of slag can be added in advance. On the contrary, if the carbon flame does not come up for a long time, it means that the temperature has been low since the blowing started. The gun should be pressed appropriately to strengthen the oxidation of various elements, increase the temperature of the molten pool, and then add two batches of slag.

(2) Middle stage of blowing

Usually, the temperature in the furnace is judged based on the brightness of the flame at the furnace mouth and the temperature of the cooling water (water entering and exiting the oxygen lance). If the temperature of the molten pool is high, a small amount of ore can be added. Otherwise, the lance should be pressed to increase the temperature. . If splashing occurs during the blowing process, a large amount of heat will be lost. The amount of ore should be appropriately reduced depending on the degree of splashing. If necessary, a temperature-increasing agent must be added to increase the temperature.

(3) End of blowing

When approaching the end point (judged based on oxygen consumption and oxygen blowing time), stop blowing and measure the temperature, and perform corresponding adjustment operations to bring the temperature of the molten steel into the tapping temperature range required by the steel type. If the temperature is high, an appropriate amount of lime or raw dolomite can be added to cool it down.

Effect of temperature on pouring operation and ingot quality

The main influence on the pouring operation and ingot quality is the oxidation end temperature, which is the tapping temperature of the converter steelmaking method.

If the tapping temperature is too high, it not only increases the energy consumption in smelting, but also the molten steel can easily absorb gas during the tapping and pouring processes. The secondary oxidation is serious, and the corrosion of the refractory materials of the ladle and pouring system is intensified, thereby increasing foreign inclusions; at the same time, the temperature adjustment time after the furnace and before continuous casting is increased. ​

If the tapping temperature is low, the killing time will be forced to be shortened, and the inclusions in the steel will not be able to fully float, affecting the inherent quality of the steel. In severe cases, the pouring temperature may be too low, causing steel quality problems, and even pouring accidents such as cold steel in the ladle and bonding of the nozzle may occur, causing the entire furnace of steel to be scrapped.

Effect of temperature on ingredient control

In steelmaking production, if the chemical composition of the finished steel is unqualified, the steel grade will be changed in some cases, or it will be directly scrapped in serious cases. There are many factors that cause substandard ingredients, but temperature conditions are one of the main factors. The influence of temperature on composition control is mainly reflected in the following three aspects:

(1) Affects the yield of alloy elements. At different temperatures, the yields of alloying elements are also different. For example, when easily oxidizable elements aluminum, titanium, and boron are added at higher temperatures, their burning losses are large and the yield is low. If the temperature of the molten pool is low, some alloys with high melting point and high density elements such as tungsten and molybdenum may not be completely melted and deposited on the bottom of the furnace, which will also cause a decrease in yield. These will affect the accuracy of the composition control of molten steel. .

(2) Affect the removal of harmful elements phosphorus and sulfur. When the temperature is too high, the thermodynamic conditions for dephosphorization are poor, which not only fails to dephosphorize, but may cause phosphorization back. When the temperature is too low, the kinetic conditions of dephosphorization and desulfurization will be worsened, which can easily lead to excessive sulfur and phosphorus content in finished steel.

(3) Affects the order of oxidation of elements in the molten pool. Normally, blowing oxygen at higher temperatures is beneficial to decarburization but inhibits phosphorus removal. On the contrary, if the temperature is too low, chromium and vanadium will be oxidized before carbon; conversely, when oxygen is blown at a higher temperature, carbon will be oxidized before chromium and vanadium.

Effect of temperature on smelting operations

Proper temperature is the primary condition for all converter steelmaking reactions in the molten bath, so temperature will have a direct impact on the smelting operation.

The operation of starting a new converter furnace requires rapid temperature rise to sinter the furnace lining. If the operation is improper and the temperature rise is slow, not only the smelting time will be long, but in serious cases, the normal smelting operation will be affected due to the cracking of the furnace lining. During the converter blowing process, due to the heat released by oxidation of elements, the temperature in the furnace will rise too fast and affect the dephosphorization operation. If a large amount of coolant is added to cool down, it will easily cause splashing.

During electric arc furnace smelting, the temperature is controlled throughout the entire smelting period, but the control of the slag removal temperature at the end of the oxidation period is particularly important. Since the slag surface is calm during the reduction period and arc light exposure makes it difficult and costly to heat the molten pool, the temperature of the slag removal determines the temperature during the reduction period. If the temperature during the reduction period is too high, it will easily lead to poor deoxidation of the molten steel, unstable white slag and easy yellowing, thin slag, and serious air absorption by the molten steel. At the same time, the corrosion of the furnace lining will be intensified, which will not only affect the furnace age but also easily increase foreign matter and debris. . When the temperature is too low, the fluidity of the slag is poor, the physical and chemical reactions such as deoxidation and desulfurization between the steel and the slag cannot proceed normally, and inclusions in the steel are not easy to float. At the same time, in order to adjust the temperature to the tapping temperature, high power must be sent during the reduction period, and the temperature rise after the reduction period will not only make the molten pool temperature uneven. That is, the temperature of the upper layer is high and the temperature of the lower layer is low, which will seriously damage the furnace wall and furnace cover, and prolong the smelting time. ​

During the vacuum refining process, if the temperature of the molten steel is too high, the stability of the oxides in the refractory material will be weakened under low pressure conditions, and the furnace lining will be easily corroded by the molten steel and slag, thus affecting the refining operation.

Determination of tapping temperature

No matter which converter steelmaking method or smelting process is used, one of the tasks of temperature control is to ensure that the temperature of the molten steel reaches the temperature required by the steel type at the end of smelting. The tapping temperature depends on the melting point of the steel, the superheat required for pouring and the temperature drop of the molten steel during tapping and pouring: T out = t melt + △ t superheat + △ t drop

Measurement of molten pool temperature

During the smelting process, the temperature of the molten pool should be appropriately measured and adjusted accordingly to meet the needs of the reaction in the furnace. Therefore, accurate measurement of the temperature of the molten pool is a necessary condition for temperature control. There are many methods for measuring molten pool temperature, which can be roughly divided into two categories: instrument measurement and visual temperature estimation.

Control of end point temperature

The way to control the end point temperature is to add a certain amount of coolant to consume the excess heat generated during blowing, so that when the blowing process reaches the end point, the temperature of the molten steel just reaches the temperature range required for tapping. The end point molten steel temperature is mainly determined based on the following principles: (1) The solidification temperature of the type of steel being made. The solidification temperature depends on the chemical composition of the steel type. (2) Suitable pouring temperature. Generally, it is required that the pouring temperature should be 50~100℃ higher than the solidification temperature of the steel type, with the upper limit for small furnaces and the lower limit for large furnaces. (3) During the tapping process, the molten steel is sedated in the steel drum until the molten steel cools down during pouring. These values are mainly determined based on the production conditions and experience of each factory. Generally speaking, it is related to factors such as tapping time, energy-calming time, size and condition of the steel rod, baking temperature and other factors. (4) The size of the steel ingot to be poured. If pouring small steel ingots, the tapping temperature should be higher. (5) Pouring method. Mold injection or continuous casting, upper injection or lower injection, and tapping temperature are all different.

Summary

For the converter steelmaking process, temperature is life. Temperature is not only the basis of the steelmaking process, but also the basis for obtaining good casting quality. The temperature control level of the converter steelmaking system is related to the quality of many indicators such as the consumption of steel and steel materials, the consumption of alloy materials, the consumption of refractory materials, etc., and directly determines the level of steelmaking costs. Low-temperature balanced and effective system temperature control is of great significance to ensure smooth production and improve product quality.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

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