JPS6146525B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for refining steel, and particularly to a method for actively dephosphorizing and desulfurizing molten steel using a solvent containing an alkali metal carbonate as a main component. In recent years, experiments have been underway to dephosphorize and desulfurize hot metal using a solvent containing an alkali metal carbonate as a main component, such as soda ash. However, the heat possessed by hot metal itself varies from 1300 to 1500℃, and if the soda ash consumption rate is constant, the dephosphorization rate decreases as the temperature rises, as shown by the broken line in Figure 1. be. Therefore, in order to increase the dephosphorization rate, a large amount of expensive soda ash must be used, which has been a major obstacle to practical application. Therefore, the present inventors have developed a method to dramatically improve the dephosphorization and desulfurization effects of solvents by adding solvents such as soda ash not to "hot metal" but to "molten steel in the process of being refined." was first developed (patent application filed on August 30, 1978), but as a result of further experimental research, it was discovered that the same effect could be obtained by adding a solvent to ``refined molten steel.'' . The present invention is based on the above findings, and the gist of the present invention is a steel refining method in which a solvent is added to the molten steel after refining the molten steel in a metal refining furnace until the carbon content in the molten steel becomes less than the target value for the finished product. . Specifically, a solvent is added at the time of tapping after refining or, for example, in a ladle or tundish after tapping, and after adding a portion of the total input amount of the solvent at the time of tapping, the remaining amount is The solvent may be added after tapping. First, the effects of adding soda ash to molten steel in general will be explained from the perspective of chemical reactions. When soda ash (Na ₂ CO ₃ ) is added to molten steel, the dephosphorization reaction of the steel proceeds according to equation (1). 4/5P + Na ₂ CO ₃ → Na ₂ O + 2/5P ₂ O ₅ +C ... (1) On the other hand, when soda ash is added to hot metal, as the hot metal temperature increases, the addition rate increases according to equations (2) and (3). Along with the reduction and evaporation loss caused by [C] in soda ash, (Na ₂ O) in the slag decreases according to equation (3), and the activity (α _P2O5 ) of (P ₂ O ₅ ) increases; Therefore, the reaction of formula (1) is suppressed, and the rate of rephosphorization due to the reduction reaction of (P ₂ O ₅ ) according to formula (4) increases, so it is thought that dephosphorization does not proceed as a result. . Na ₂ CO ₃ +C→Na ₂ O+2CO …(2) Na ₂ O+C→2Na+CO …(3) P ₂ O ₅ +5C→2P+5CO …(4) The solid line in Figure 1 and Figure 2 are undeoxidized molten steel. This shows the dephosphorization and desulfurization effects of soda ash in T.
This shows that dephosphorization using soda ash in low-carbon undeoxidized molten steel at T = 1650°C is comparable to dephosphorization in hot metal with a high carbon concentration at T = 1350°C. This is because the loss of (Na ₂ O) due to the reaction in equation (3) is small, the rephosphorization reaction in equation (4) is small, and the evaporation rate of soda ash itself is slower than previously thought (C Therefore, even at T=1650°C, the soda ash reacted sufficiently with the molten steel and became stable as Na ₂ O--P ₂ O ₅ -based slag. Next, if we pay attention to Fig. 3, it shows the case where soda ash is used according to the research conducted by the present inventors.
The relationship between the amount of [C] in molten steel and the dephosphorization rate is illustrated, and it shows that when the amount of [C] in molten steel becomes around 0.50% or less, the dephosphorization rate decreases significantly. In other words, this means that if the target value of the finished product for the amount of [C] is C = 0.50% or less, for example, if the amount of [C] is reduced to the target value in a converter and then molten steel is added to soda ash, the product will be very good. It is understood that the present invention is extremely effective in improving the dephosphorization effect. In addition, if the target amount of [C] in the finished product is 0.50% or more, for example, the amount of [C] should be reduced to 0.50% in the converter blowing process.
After lowering the amount to below, add soda ash to the molten steel to dephosphorize and desulfurize the steel, remove slag, add C, and finally adjust the [C] amount to the target value. The same effect can be obtained. Figure 4 shows the results of another investigation by the present inventors, showing the soda ash consumption rate when dephosphorization was caused by adding various amounts of soda ash to molten steel after refining;
The relationship between the dephosphorization rate and molten steel temperature is shown. This investigation involved blowing hot metal with P = 0.100 to 0.120% in a converter to produce molten steel with C = 0.10% or less and a temperature of 1650 to 1750℃, and after tapping it, it was made into molten steel with a consumption rate of 16 and 24 kg/T. This was done by treating each with ashes,
In all cases, although the basic unit of soda ash was extremely low, a very high dephosphorization rate of 80% or more was shown, and the effectiveness of the present invention can be understood more clearly. Furthermore, according to the same figure, it is clear that this dephosphorization effect is almost unaffected by the molten steel temperature, so soda ash can be added either at the time of tapping or after tapping as long as it is after refining. Almost the same stable effect can be achieved even if the steel is tapped, and adding it in portions during and after tapping is equally effective. Furthermore, Figure 5 shows the relationship between the soda ash unit required to convert P = 0.100% molten steel to P = 0.010% molten steel when C = 0.10% and the amount of [Si] in the molten steel. , after refining, the amount of [Si] in the molten steel is
Since the amount is 0.01% or less, the method of the present invention in which soda ash is added after refining allows efficient dephosphorization without adding any special desulfurization treatment to the hot metal. In the present invention, the solvent containing an alkali metal carbonate as a main component includes, in addition to the above-mentioned soda ash containing Na ₂ CO ₃ as a main component, those containing K ₂ CO ₃ as a main component. In addition, metal refining includes top-blown converters, bottom-blown converters, composite blowing furnaces with nozzles below the bath surface, AOD furnaces, and VOD furnaces.
The present invention is applicable to a wide range of steel refining methods, including furnaces, electric furnaces, etc. Furthermore, methods of adding the solvent of the present invention include adding it using an alloy adjustment hopper during tapping from a metal smelting furnace, and adding gas etc. from a refractory lance immersed under the bath surface during or after tapping. The method of addition is not particularly limited, and there are methods such as injecting the molten steel as a medium, or adding it from above using a hopper while stirring the molten steel by bubbling or the like after tapping. Examples of the present invention will be described below. <Example 1> 250 tons of hot metal with the following composition and temperature were charged into a converter.

【table】 Decarburized in a converter to the following composition.

[Table] During tapping, 18 kg/t of soda ash was continuously added for about 5 minutes. The composition and temperature of the obtained molten steel and the composition of the slag are as follows.

[Table] As above, C=0.05%, Mn=0.25%, P=
When soda ash with a basic unit of 18 kg/t is used in molten steel with 0.130% and S = 0.032%, P and S in the final molten steel can be extremely low at 0.015% and 0.010%, respectively, and the amount of soda ash added is 1/ of the conventional addition amount
This corresponds to 3 to 1/2, making it possible to significantly reduce the amount of soda ash added. Furthermore, it is possible to recover phosphorus, vanadium, etc. from the slag generated by the method of the present invention. <Example 2> 250T of hot metal with the following composition and temperature was charged into a converter.

[Table] The steel was decarburized and tapped in a converter to have the composition shown below.

[Table] After tapping, 15 kg/t of soda ash was added using the injection method using Ar gas as a medium from a refractory lance in the ladle. The composition and temperature of the obtained molten steel are as follows.

[Table] <Example 3> 250T of hot metal with the following composition and temperature was charged into a converter.

【table】 Decarburized in a converter to the following composition.

[Table] Dephosphorization and desulfurization are performed by adding 10 kg/t of soda ash to the following composition during tapping.

[Table] After tapping, pour soda ash into the refractory lance in the ladle.
15Kg/t was added using the injection method. The final composition and temperature of the molten steel are as follows.

【table】 [Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the temperature of hot metal and molten steel and the dephosphorization rate when the soda ash consumption rate is constant, and Figure 2 is a diagram showing the time course of the dephosphorization and desulfurization effects of undeoxidized molten steel by soda ash. Fig. 3 is a diagram showing the relationship between the amount of [C] in molten steel and the dephosphorization rate when soda ash is used, and Fig. 4 is a diagram showing the dephosphorization rate of low-carbon undeoxidized molten steel using soda ash. Figure 5 shows the relationship between the soda ash unit and the molten steel temperature using the soda ash unit as a parameter. FIG.

Claims (1)

[Claims] 1. A method for refining steel using a solvent containing an alkali metal carbonate as a main component, in which a solvent containing an alkali metal carbonate as a main component is used to refine steel in a metal refining furnace. A method of refining steel that is added to molten steel after refining. 2. The method for refining steel according to claim 1, wherein a solvent containing an alkali metal carbonate as a main component is added to molten steel at the time of tapping the steel after refining the steel in a metal refining furnace. 3. The steel according to claim 1, in which a solvent containing an alkali metal carbonate as a main component is added to molten steel in a ladle or tundish after refining the steel in a metal refining furnace and tapping the steel. Refining method. 4. The steel according to claim 3, wherein a part of the total input amount of a solvent mainly composed of an alkali metal carbonate is added to the molten steel at the time of tapping after refining the steel in a metal refining furnace. Refining method.