KR20030089955A - The method of decreasing nitrogen in deoxidized molten steel - Google Patents

Abstract

PURPOSE: A method for reducing nitrogen in deoxidized molten steel is provided to reduce cracks of slab, coil and plate by reducing the content of nitrogen that is a cause of cracks when continuously casting molten steel passing through the refining process. CONSTITUTION: In a method for refining a steel containing 0.04 to 1.00 wt.% of carbon (C), 0.005 to 0.050 wt.% of aluminum (Al), 0.010 wt.% of sulfur (S), 0.50 to 1.60 wt.% of manganese (Mn) and 0.10 to 0.40 wt.% of silicon (Si), the method comprises the steps of injecting flux for second refining into converter at a time point corresponding to 10% of tapping in the tapping process after completing blowing operation in converter process; injecting manganese based ferro alloy into the converter after adding the flux for second refining to the converter; and injecting aluminum and silicon based ferro alloy into the converter at a time point corresponding to 70 to 80% of tapping.

Description

Nitrogen reduction method in deoxidation molten steel {THE METHOD OF DECREASING NITROGEN IN DEOXIDIZED MOLTEN STEEL}

The present invention relates to a method for reducing nitrogen in deoxidized molten steel, and more particularly, to a method for reducing nitrogen that causes cracks in continuous casting of molten steel that has undergone refining.

In the steel manufacturing process for producing steel from raw materials, the raw materials are blown into a furnace and subjected to a steelmaking process to reduce iron ore, and then the necessary alloying elements are added from the iron ore from which the impurities are removed, and the temperature is raised for the subsequent process. Steelmaking process is performed.

This steelmaking process usually consists of molten iron pretreatment, converter refining, and secondary refining. In each of these processes, the molten iron pretreatment removes sulfur and phosphorus using various fluxes, and in the refining, the pure oxygen is deodorized. In this case, the secondary refining removes oxygen and oxides due to the oxygen injection during the converter refining.

0.04 to 1.00 wt% carbon (C), 0.005 to 0.050 wt% aluminum (Al), 0.010 wt% sulfur (S), 0.50 to 1.60 wt% manganese (Mn) and 0.10 Steel containing ~ 0.40% by weight of silicon (Si) is used for structural steel in shipbuilding and construction, steel for pressure vessels such as low temperature LNG storage tanks or vessels, and pipe steel for transporting petroleum, crude oil and natural gas. Used. In particular, the steel containing the above-mentioned components needs to contain as little gas components as possible, especially nitrogen, contained in the molten steel from the refining stage in order to be manufactured into a product having excellent surface quality satisfactory to the consumer.

Since nitrogen in molten steel causes such a problem, a secondary refining process of the steelmaking process is performed to reduce not only nitrogen but also other impurities. Secondary refining is a post-treatment of molten steel produced in the converter, and is a series of processes in which the molten steel is pulled out to the ladle and various metallurgical operations are performed to suit the demands of the consumer and continuous casting. In this secondary refining, not only the chemical composition in molten steel is adjusted, but also deoxidizer and ferroalloy are added, the molten steel is heated up, and the temperature of molten steel is adjusted. It also improves the cleanliness of molten steel, controls non-metallic inclusions, and removes impurities such as phosphorus, sulfur, nitrogen and hydrogen in the steel. Therefore, it is possible to equalize the composition and temperature in the molten steel and have the advantage of adjusting the operating time between tapping and casting time.

Specifically, 0.04 to 1.00% by weight of carbon (C), 0.005 to 0.050% by weight of aluminum (Al), 0.010% by weight of sulfur (S), 0.50 to 1.60% by weight of manganese (Mn) and 0.10 to 0.40% by weight of The method for refining steel containing silicon (Si) is as follows. First, the molten steel containing 0.03 to 0.08% by weight of carbon, which has been refined in the converter or the electric furnace, is removed by ladle. In the tapping process, aluminum (Al) is added as a molten steel deoxidizer, and at the same time, silicon (Si), manganese (Mn), carburizing agent (C) and other alloying elements are added to adjust the components in the molten steel. Subsequently, the ladle containing the molten steel is transferred to a ladle furnace (LF), and arc heating is performed to adjust the molten steel temperature to an appropriate range. Then, the ladle is transferred to a vacuum degassing apparatus such as RH (Rheinstahl Huttenwerke) or VTD (vacuum tank degasser) to vacuum molten steel at a reduced pressure of several torr, and hydrogen (H ₂ ) and nitrogen (N ₂ ) in the molten steel. Remove the gas. Finally, vacuum refining is performed to finish molten steel refining, followed by continuous casting.

In such a conventional refining method, there is a problem that it is difficult to control the nitrogen (N ₂ ) concentration in the molten steel within a predetermined range. That is, in the process of tapping molten steel in a converter or electric furnace, in the conventional tapping method, aluminum (Al) is added at the same time as tapping starts, and then silicon (Si) alloy iron and manganese (Mn) alloy iron are added. It is added as a secondary refining flux such as quicklime and fluorspar.

In such a tapping method is aluminum, the total amount introduced into the molten steel in a state that is not tapped in the fully molten steel ray molten steel is completely deoxidized, so that the complete deoxidizing the molten steel in contact with an atmosphere of nitrogen corresponding to 80% of the atmosphere (N ₂ ) Has a problem that the component is dissolved in molten steel.

5 is a graph showing the nitrogen content in the molten steel in this general steelmaking process from the process immediately after the converter refining to the end of the second refining. As shown in FIG. 5, in general, the nitrogen content in the molten steel at the converter termination point is about 10 ppm, and in the conventional deoxidation method, the nitrogen content in the molten steel after tapping is about 30 to 50 ppm, about 20 to 40 ppm during tapping. Nitrogen in the atmosphere is dissolved into molten steel. Nitrogen in molten steel rises by about 5ppm through bubbling process and LF process after tapping in the converter, and the nitrogen content in molten steel before degassing is about 35 ~ 60ppm. When a high vacuum degassing treatment is performed in a vacuum degassing apparatus such as RH or VTD, nitrogen of about 40 to 60 ppm is generally present in molten steel, and it is difficult to obtain about 40 ppm of nitrogen.

This content of nitrogen forms precipitates such as AlN, VCN, NbCN inside the slab during continuous casting, and during rolling, these precipitates increase the local brittleness within the slab. There is a problem of causing surface cracks during rolling into the product.

Korean Patent Application Publication No. 1990-7443 provides a method for preventing the absorption of molten steel in order to solve this problem. The present invention provides a method for preventing absorption of molten steel used as a cold working continuous annealing material and a material having excellent impact resistance and corrosion resistance. In this method, iron ore is introduced at the end of converter conversion to control the amount of oxygen in the molten steel at 0.04wt% or more during the tapping, and TiO ₂ ore is added at the end of tapping to adjust the amount of TiO ₂ in the ladle slag to 10-40wt% or more. In the first process of adjusting the ladle slag thickness to 80 ~ 120mm with a stopper and a thickness measuring device, the amount of 75% Mn alloy iron containing Mn during the RH treatment is reduced to 0.27 ~ 0.8kg per ton of molten steel. Immediately after the decarburization is completed, a first step of 0.9 to 1.5 kg of Al deoxidizer is added per molten steel, followed by a second step of dividing 0.4 to 0.6 kg.

However, such a method of preventing absorption of molten steel is not only complicated because the process is divided into two types, but also many parts such as slag thickness and amount of additives have to be controlled.

The present invention aims to reduce the nitrogen in the molten steel during the refining process while adding aluminum to complete deoxidation.

The present invention is to provide a process consisting of a specific step to prevent the incorporation of nitrogen during the refining process for this purpose, to reduce the surface cracks generated in the slab, coil and plate due to nitrogen.

1 is a graph showing molten steel absorption mass according to deoxidation time point during converter tapping.

2 is a graph showing the content of the total iron in the slag after the tapping according to the deoxidation time during the converter tapping.

3 is a graph showing the dehydration rate during refining and the absorption mass during tapping according to the flux input point during tapping of the converter.

4 is a graph showing the nitrogen content in the lumen of Examples of the present invention and a conventional Comparative Example.

5 is a graph showing the nitrogen content in each molten steel in each general steelmaking process.

The present invention is 0.04 to 1.00% by weight of carbon (C), 0.005 to 0.050% by weight of aluminum (Al), 0.010% by weight of sulfur (S), 0.50 to 1.60% by weight of manganese (Mn) and 0.10 to 0.40% by weight A method for refining steel containing silicon (Si), comprising: injecting a secondary refining flux at a 10% point of tapping during tapping after completing a smelting in a converter process; After adding flux, manganese-based ferroalloy is added, aluminum and silicon-based ferroalloy are added at 70-80% of tapping time, and the ladle is transferred to a vacuum degassing apparatus and vacuum-reduced to below 2 torr vacuum. Characterized in that it comprises a step. In another aspect, the present invention is characterized by the molten steel produced by the molten steel refining method described above.

Hereinafter, with reference to the accompanying drawings will be described in more detail the present invention.

The molten steel after the converter refining contains 0.03 to 0.08 wt% carbon and 400 to 800 ppm dissolved oxygen [O]. In the case of such deoxidized molten steel, aluminum is added as a deoxidizer during tapping to not only completely deoxidize but also to lower the content of sulfur in the molten steel. Sulfur contained in steels causes anisotropy and red-brittle brittleness of the material, so it is necessary to control the concentration as low as possible except for some special-purpose steels such as sulfur-added free-cutting steels. Therefore, deoxidizer is added to remove sulfur during the steelmaking process. The reason for adding the deoxidizer to remove sulfur can be seen in the following formula (1).

[S] + (O ^-2 ) ⇔ (S ^-2 ) + [O]

Sulfur dissolved in the ionic state in molten iron is oxidized or reduced in accordance with the oxygen partial pressure of the atmosphere. When the oxidizing power of the atmosphere is strong, an oxidation reaction occurs, and when it is weak, a reduction reaction occurs. In the formula (1), [S] and (O- ² ) represent sulfur in slag and oxygen in molten steel, respectively, and (S- ² ) and [O] represent sulfur ions in molten steel and oxygen in slag, respectively.

The equilibrium constant equation according to the desulfurization reaction equation is obtained as in Equation 1 below.

hereK _s Is the equilibrium constant,aRepresents activity coefficient. Therefore, in order to promote the desulfurization reaction equilibriumally, sulfur activity coefficient of sulfur in slag and oxygen activity in molten steel are kept low and oxygen activity coefficient in slag is kept high in order to improve sulfur efficiency. It is desirable to.

In this way, in order to promote the desulfurization reaction by keeping the oxygen activity in the molten steel low, aluminum which is a deoxidizer is added during tapping after converter refining. Aluminum added during tapping is introduced at the beginning of tapping (at the time of completion of tapping 10%), so molten steel is completely deoxidized and nitrogen in the atmosphere is mixed into the molten steel. In order to prevent the absorption of molten steel by nitrogen in the air, it is desirable to completely block the contact between the atmosphere and the molten steel, but there are many difficulties in the current facilities and technical conditions. The method was developed.

As a method for preventing absorption of molten steel, oxygen or sulfur components may be used as described above. Oxygen and sulfur components exist in the molten steel as the surface active element, so the greater the amount present in the molten steel during tapping, the better the absorption. However, when oxygen or sulfur is present in the molten steel in a large amount, the molten steel is brittle, so the amount needs to be adjusted within a certain range.

First, the case of using sulfur to prevent the absorption of molten steel will be described. Since the desulfurization rate during the secondary refining is about 50%, the sulfur content in the molten steel is increased, so if more than 200% of the sulfur content required by the steel species is present in the molten steel immediately after tapping, it is less than the content required for the steel species due to the excessive amount of sulfur. It is difficult to control the sulfur component.

Second, the case of using oxygen to prevent the absorption of molten steel will be described. In the case of oxygen, deoxidation is required during tapping in steel grades requiring low sulfur content in molten steel, and the deoxidation process is usually performed at the beginning of tapping. As described above, since there is a certain limit in the method of preventing the absorption by containing a large amount of sulfur in the molten steel, the present invention intends to prevent the absorption by using a deoxidation process.

As described above, aluminum is used for desulfurization in the deoxidation process. 1 is a graph showing the absorption mass of molten steel according to the deoxidation time due to the addition of aluminum during the tapping of the converter.

Nitrogen absorbent mass in molten steel is increased when aluminum deoxidizer is added to molten steel before tapping 70%, and when the aluminum deoxidizer is added to molten steel after tapping 80%, molten steel absorbing mass due to delay in feeding time is 70 ~ 80 % No difference from point in time. In addition, the molten steel cannot be uniformly deoxidized due to the delay in the addition of the aluminum deoxidizer, and it is difficult to adjust the sulfur component. Therefore, in the present invention, the input time of the aluminum deoxidizer during tapping is limited to the tapping time 70-80%. Thereby, the water absorption during tapping can be reduced.

As can be seen from the above formula (1), in order to adjust the sulfur component, in general, the oxygen ion concentration in the ladle slag after tapping is low, and in order to lower the oxygen ion concentration in the ladle slag, the amount of the total iron oxide in the slag is reduced. Should be lowered.

In this regard, Figure 2 shows a graph of the total iron (total Fe) content in the slag after the tapping in accordance with the deoxidation time point during the converter tapping. As shown in Figure 2, when the aluminum deoxidizer is added after 70 to 80% of the tapping time, the iron oxide content in the slag is increased to more than 3% level, it is difficult to adjust the sulfur component by increasing the oxygen ion concentration Lose.

In consideration of this point, when the aluminum deoxidizer is added after 70 to 80% of the tapping time, the iron oxide content in the slag increases to a level of 3% or more, thus delaying the molten iron dehydration reaction in the secondary refining process. Wants to limit the amount of aluminum deoxidizer to 70 ~ 80%. Therefore, delay in molten iron deflow reaction can be prevented.

3 is a graph showing the desorption rate during refining and the absorption mass during tapping according to the flux input time during tapping of the converter. As shown in Fig. 3, the secondary refining flux introduced during tapping promotes the dehydration reaction during the secondary refining since the goods by the molten steel drop energy and the molten steel temperature are promoted as they are added at the initial tapping point. The secondary refining flux also forms a product in molten steel, as shown in the following formula (2).

On the other hand, the secondary refining flux (flux) in the tap after the completion of the converter blown is melted by the following reaction.

αCaO + βAl ₂ O ₃ + γFeO → α (CaO) β (Al ₂ O ₃ ) γ (FeO)

In Formula 2, α, β, and γ are arbitrary reaction coefficients, and according to each reaction, 12 (CaO) 7 (Al ₂ O ₃ ), 2 (CaO) (FeO), 3 (CaO) (Al ₂ O ₃ ), etc. Can form the product of. As such, the secondary refining flux reduces the absorption of molten steel by preventing the contact between molten steel and the atmosphere as much as possible, but when the secondary refining flux is injected before the start of the converter, it is fused to the ladle bottom and What has been fused can rise and cause slag overflow. Therefore, in the present invention, by injecting the secondary refining flux at the time of initial tapping 10%, this problem is eliminated.

In addition, in the present invention, by absorbing the manganese-based ferroalloy, it is possible to prevent the absorption. This is because even if the air is minutely sucked into the molten steel, the atmosphere can be removed and discharged from the interface of the molten steel by decarburized CO bubbles. If manganese-based ferroalloy is added before the secondary refining flux is added to molten steel, ladle bottom fusion may occur. If manganese-alloy is added after the addition of aluminum deoxidizer, It may cause undissolved and abnormal component between processes during secondary refining. Therefore, in the present invention, by introducing a manganese-based ferroalloy between the secondary refining flux into the molten steel and before the aluminum deoxidizer injection time, to eliminate the above problems.

Hereinafter, the present invention will be described in more detail with reference to Examples. The embodiments of the present invention described below are merely for illustrating the present invention, and the present invention is not limited thereto.

Example

Reducing nitrogen in molten steel when producing steels containing 0.04 to 1.00 wt% carbon, 0.005 to 0.050 wt% aluminum, 0.010 wt% sulfur, 0.50 to 1.60 wt% manganese and 0.10 to 0.40 wt% silicon The experiment to perform was performed.

Table 1 shows the experimental conditions of the Example according to the present invention and the conventional comparative example.

As shown in Table 1, first, in the embodiment of the present invention, the first refining of the molten iron is finished in the 350 ton converter, and 6 to 7 kg / TS of quicklime and 0.6 to 0.8 kg / TS of fluorite are added at the time of 1 minute after the start of tapping. Input. The total leaving time depends on the life of the exit, and the average leaving time for the experiment was about 5 minutes. After that, the manganese ferroalloy was added 17 ~ 19kg / TS at 2 minutes of tapping, and aluminum was added at 4 minutes of tapping.

In contrast, in the conventional comparative example, aluminum was added at the time of tapping 1 minute after the start of tapping, and 17-19kg / TS of manganese alloy iron was added at the time of tapping 3 minutes, and quick lime 6-7kg / TS at the time of tapping 3 minutes. Add 0.6 ~ 0.8kg / TS of fluorspar.

That is, in the comparative example, the order of loading is made in the order of aluminum → manganese alloy iron → flux for secondary refining, whereas in the examples, the flux for secondary refining → manganese alloy iron → aluminum is reversed.

Table 2 shows the input subsidiary materials and the types of ferroalloy for each tapping time of the examples of the present invention and the conventional comparative examples.

As a result, the result shown in Table 3 was obtained.

In Table 3, the molten steel sample was taken at the converter end point in the experiment of the present invention, the molten steel and the ladle slag sample immediately after completion of tapping were taken, and the result compared with the conventional comparative example is shown.

As shown in Table 3, in the examples of the present invention and the conventional comparative example, the nitrogen level at the converter end point (end point) was 12 to 13 ppm, but there was no difference, but the nitrogen content in the molten steel after the tapping was changed according to the change of the tapping method. 22 ppm of Examples and 35 ppm of the Comparative Example, the nitrogen content in the molten steel was reduced by about 37% based on the Comparative Example.

In addition, the nitrogen content in molten steel at the time of continuous casting was twenty-five ppm and the comparative example was 41 ppm, and the nitrogen content was lowered by about 39% based on the comparative example. 4 is a graph showing the nitrogen content in the lumen of the examples of the present invention and the conventional comparative example.

In addition, in order to investigate the effect on the dewatering behavior during the secondary refining according to the deoxidation method during the tapping, after the tapping and bubbling, the sulfur content in the molten steel at the tundish, the continuous casting point, was investigated. The ratio was 63% in the Examples and 64% in the Comparative Example, and it was found that there was no significant difference under the same secondary refining process and conditions.

According to the present invention, when producing a steel containing 0.04-1.00% by weight carbon, 0.005-0.050% by weight aluminum, 0.010% by weight sulfur, 0.50-1.60% by weight manganese and 0.10-0.40% by weight silicon, By providing a method for stably reducing the nitrogen content of the conventional, it is possible to reduce the surface cracks of the slab, coil, and plate.

Although the present invention has been described above, it will be readily understood by those skilled in the art that various modifications and variations are possible without departing from the spirit and scope of the claims set out below.

Claims (1)

0.04-1.00 wt% carbon (C), 0.005-0.050 wt% aluminum (Al), 0.010 wt% sulfur (S), 0.50-1.60 wt% manganese (Mn) and 0.10-0.40 wt% silicon ( As a method of refining steel containing Si), Injecting the second refining flux at the time of tapping 10% during tapping after completion of the blow in the converter process, Adding the secondary refining flux, and then introducing manganese-based ferroalloy and Step to inject aluminum and silicon-based ferroalloy at 70 ~ 80% of tapping time Molten steel refining method comprising a.