TWI593803B - Melting method of high cleanness steel - Google Patents

Description

High-definition clarity steel melting method

The present invention relates to a method of melting high-definition clarity steel which can be used to produce steel having high-definition clarity using a circulating vacuum degassing apparatus.

In recent years, with the expansion of the use of steel materials, the demand for high quality of internal quality has been increasing in terms of improvement in characteristics such as strength and toughness and quality assurance. Therefore, it is strongly desired to improve the cleanliness of steel.

In the steel making stage of the molten steel, a deoxidizing agent such as Al or Si is usually added to the molten steel to reduce the amount of dissolved oxygen in the molten steel. In many cases, a deoxidation product such as Al ₂ O ₃ or SiO ₂ which is formed at this time is stirred by an Ar gas or the like, and is floated, separated, and removed to the outside of the molten steel, but a part remains in the molten steel. Among them, it exists as an oxide-type inclusion in steel. When the amount of the oxide-based inclusions is small and minute, it is practically harmless. However, if the amount and size exceed a certain value, the internal quality or surface quality of the final product may be seriously adversely affected. Therefore, such harmful oxide-based inclusions are desirably completely removed during the steelmaking stage.

As a technique for the harmful oxide-based inclusions as described above, for example, Patent Document 1 discloses a technique in which CaO-SiO _{2 is} used in deoxidation of molten steel using Al, an Al alloy, and/or a Si alloy as a deoxidizer. A flux is added to the molten steel together with the deoxidizer, so that the Al ₂ O ₃ formed by the deoxidation reaction is absorbed by the CaO-SiO ₂ -based flux, and the morphology is controlled to CaO-SiO ₂ -Al ₂ O ₃ system. The ductile inclusions thereby detoxifying the deoxygenated product.

Further, Patent Document 2 discloses a technique in which a low-carbon, non-deoxidized steel is tapped into a steel ladle, and a deoxidizing agent (Al source) is introduced into the slag in the ladle, and the slag is placed in the slag. After the lower-order oxide is reduced and the T.Fe concentration is 5% or less, the degassing treatment is performed while blowing oxygen into the vacuum degassing tank in the vacuum degassing apparatus shown in Fig. 1, so that the C content becomes 0.006. Below %, then Al deoxidation is performed, thereby preventing reoxidation from the slag and improving the cleanliness. [Prior Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. Hei 6-33132 (Patent Document 2) Japanese Patent Laid-Open No. Hei 2-30711

However, the technique of adding a flux or reforming slag disclosed in Patent Document 1 and Patent Document 2 described above not only causes an increase in manufacturing cost, but also does not expect much effect on cleaning of steel in the final stage of steel making. The reason for this is that the above prior art is aimed at a step until a deoxidizing agent is added to the molten steel, and the influence is small when added to the molten steel in the subsequent step.

The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide a method for melting high-definition clarity steel, which can reduce the content contained in steel in a method of melting steel using a circulation type vacuum degassing device. Inclusions can also significantly reduce defects in steel products.

The inventors of the present invention have repeatedly conducted active research in order to solve the above problems. As a result, it was found that in order to melt the high-definition-purity steel using the circulation type vacuum deaerator, when the cooling material is added after the addition of the deoxidizer, it is important to set the flow rate of the circulating gas after the cooling material is put into The present invention has been developed by specifying the circulation time of the predetermined time and ensuring the circulation time for a predetermined period of time, and when the cooling material is introduced before the addition of the deoxidizer, the limitation in the above-described circulation treatment can be alleviated.

That is, the present invention is a method for melting high-definition clarity steel, in which a molten steel in a steel ladle is subjected to decarburization while circulating a molten steel in a circulation type vacuum degassing device, and a deoxidizing agent is added for quietness ( The method for melting the treated steel, wherein the method for melting the high-definition clarity steel is characterized in that the flow rate of the circulating gas in the net static treatment is 4 L (normal pressure)/min·t or more, and the circulation is melted. The steel is charged with a cooling material to adjust the temperature of the molten steel, and the relationship between the circulation time T after the cooling material is introduced and the amount W of cooling material added per ton of molten steel is set to satisfy the following formula: T(min) ≧ 0.25W (kg/t) + 2.

Further, the above circulation time is preferably set to a time accurate to the minute unit.

The method for melting a high-definition clarity steel according to the present invention is characterized in that the slag composition in the steel ladle is set to a CaO-SiO ₂ -Al ₂ O ₃ -MgO system, and further (T.Fe in the slag) The total concentration of (MnO) is adjusted to 4% by mass or less.

In addition, the present invention is a method for melting high-definition clarity steel, in which a molten steel in a steel ladle is subjected to decarburization while circulating a molten steel in a circulation type vacuum degassing device, and a deoxidizing agent is added for static treatment. In the method for melting steel, the method for melting high-definition clarity steel is characterized in that the temperature of the molten steel at the end of the converter blowing, the dissolved oxygen in the molten steel, and the subsequent RH treatment are input before the above-mentioned deoxidizing agent is charged. The amount of the cooling material determined by the time and the processing time until casting, thereby adjusting the temperature of the molten steel. [Effects of the Invention]

According to the present invention, the cleanness of the steel can be improved by a simple method using the circulation type vacuum deaerator, so that not only the manufacturing cost can be reduced, but also the steel product which does not cause defects due to inclusion properties can be provided to the market.

The deoxidation of the molten steel in the steel making step is usually carried out by adding a deoxidizing agent containing a deoxidizing element such as Al or Si to the molten steel to reduce the dissolved oxygen. When a deoxidizing agent is added to the molten steel, Al ₂ O ₃ or SiO ₂ is produced as a primary deoxidation product by the reaction of the following formulas (1) and (2), and is suspended in the molten steel. 2[Al]+3[O]=Al ₂ O ₃ (1) [Si]+2[O]=SiO ₂ (2)

The deoxidized products suspended in the molten steel are blown into the molten steel by an inert gas such as Ar gas, stirred, floated, separated, and discharged to the outside of the molten steel.

For example, RH treatment using a cycloflow degasser (Ruhrstahl-Hausen (RH) degasser) as shown in Figure 1 does not have other ladle refining equipment such as ladle furnace (LF) In the refining stage, the final steps are mostly directly connected to casting steps such as continuous casting. Therefore, in the RH process, since the temperature management of the molten steel in the casting step such as continuous casting is important, the molten steel temperature is adjusted to a predetermined range.

Here, the RH degasser shown in FIG. 1 includes: a steel ladle 1 storing a molten steel 2 having an upper layer formed with a slag layer 3; and a vacuum degassing tank 4 having a lower portion immersed in the molten steel 2 The immersion tube 5 is provided with an exhaust port 9 connected to the exhaust device and an input port for adding a cooling material stored in the hopper 7 on the upper side surface of the vacuum degassing tank 4 (shooter) 8. One of the two dip tubes 5 is connected to a pipe for blowing a circulating gas 6 such as Ar gas into the dip tube, and the inside of the vacuum degassing tank 4 is evacuated by means of an exhaust device to be melted. The steel 2 is introduced into the vacuum degassing tank 4, and the circulating gas 6 is supplied into the dip tube 5, and the molten steel 2 flows into the vacuum degassing tank 4 as the circulating gas 6 rises, and descends from the other dip tube. A molten steel stream returned to the ladle 1 is produced, and the molten steel is subjected to vacuum degassing.

The temperature adjustment of the molten steel is usually carried out by adding a deoxidizing agent to the molten steel while circulating the molten steel, and then introducing a cooling material such as chopper scraps or cutting chips. However, in the added cooling material, surface oxidation is also present in a large amount. Therefore, a deoxidizing agent such as Al is added, and in the case where the molten steel is in a static state, the reaction in the following formula (3) is used in the molten steel. [Al] is oxidized to form Al ₂ O ₃ , and the cleanliness of the molten steel is greatly reduced. Fe ₂ O ₃ +2[Al]=2[Fe]+Al ₂ O ₃ (3)

In order to remove Al ₂ O ₃ generated by the introduction of the cooling material, it is necessary to sufficiently ensure the circulation time after the cooling material is supplied in the RH treatment. The inventors of the present invention investigated the circulation time necessary for purifying the molten steel which was deteriorated by the introduction of the above-described cooling material to the extent that the quality of the steel product was not adversely affected. As a result, it was found that the circulation time T necessary for the purification is changed in accordance with the amount W of the cooling material added, and it is necessary to set the following formula (4): T (min) ≧ 0.25 W (kg / t) + 2 ... ( 4). The above circulation time is preferably set to a time accurate to the minute unit.

Further, in order to prevent a casting failure or the like caused by a drop in the temperature of the molten steel, the upper limit of the circulation time after the cooling material is charged is preferably set to 6 min.

In addition, the inventors of the present invention have found that the flow rate of the circulating gas (Ar) after the cooling material is supplied is set to a predetermined flow rate or more, specifically, 4 L (normal pressure)/min. Above t, the generation rate of inclusions can be more stably reduced.

Further, the inventors of the present invention have found that in the above RH treatment, it is preferable to add a slag-modified material containing CaO as a main component, and to set the slag in the ladle to CaO-SiO ₂ -Al ₂ O. The composition of the _3- MgO system further adjusts the total concentration of (T.Fe) and (MnO) in the slag to 4% by mass or less. This is because the composition of the slag is CaO-SiO ₂ -Al ₂ O ₃ -MgO system, and inclusions such as Al ₂ O ₃ formed in the molten steel can be efficiently absorbed.

In addition, the above-mentioned (T.Fe) and (MnO) are the total amount of iron and Mn contained in the slag as an oxide, and the smaller the total concentration, the more the oxidation of Al or the like in the molten steel can be suppressed. The resulting oxide-based inclusions increase. In order to obtain the above effects, the total concentration of (T.Fe) and (MnO) is preferably reduced to 4% by mass or less, preferably to 2% by mass or less.

Further, the inventors of the present invention have studied the following method: Even if a cooling material for adjusting the temperature of the molten steel is supplied, the high-purity steel is melted without restricting the circulation time or the circulating gas flow rate as described above.

As a result, it was found that the amount of the cooling material input was determined by the temperature of the molten steel at the end of the refining of the converter, the dissolved oxygen in the molten steel, the subsequent RH treatment time, and the processing time until casting, before the deoxidizing agent was charged. Further, even if the circulation time after the cooling material is supplied is shortened to the minimum time necessary for the quiet treatment, the cleanliness of the steel can be ensured.

The reason why the dissolved oxygen is determined in consideration of the amount of the cooling material input is that the amount of the deoxidizer added varies depending on the amount of dissolved oxygen, and the amount of temperature rise also differs. Further, the reason for the subsequent RH processing time and the processing time until casting is considered to be the temperature drop in the elapsed time from the RH process and the casting. [Example 1]

When the steel was refined using the RH degasser shown in Fig. 1, the following experiment was carried out in order to investigate the RH treatment conditions in the case where the deoxidizer was added and the cooling material was introduced.

After the molten iron coming out of the blast furnace is subjected to molten iron preparation treatment such as desulfurization and dephosphorization, the converter is blown to obtain about 250 tons of molten steel, and about 250 tons of molten steel is tapped in an undeoxidized state to In the steel barrel. The composition of the molten steel is: [C]: 0.03 mass% to 0.04 mass%, [Si]: 0.03 mass% to 0.06 mass%, [Mn]: 0.3 mass% to 0.5 mass%, [P]: 0.007 mass % or less, [S]: 0.0030% by mass or less.

Then, the RH degasser was used for the molten steel in the non-deoxidized state, and the circulating gas (Ar) was blown from the immersion pipe to circulate the molten steel for about 15 minutes to make the carbon concentration in the molten steel [C]. When the temperature is reduced to 30 ppm by mass or less, the Al source is added from the hopper via the ejector in the same manner, so that the Al concentration [Al] in the molten steel is 0.02% by mass to 0.04% by mass, and deoxidation is performed. Then, after the cooling material is supplied to adjust the temperature, the degree of vacuum in the vacuum degassing tank is controlled to be in the range of 0.5 torr to 2 torr (67 Pa to 266 Pa) to carry out the static treatment.

In the RH treatment, an Al source is added to the molten steel in which the dissolved oxygen [O] at the end of the decarburization treatment is variously changed in the range of 200 ppm by mass to 800 ppm by mass. In addition, the amount of the cooling material input and the circulation time after the cooling material is introduced are also variously changed, and the flow rate of the circulating gas (Ar) after the cooling material is introduced is 500 L (normal pressure) / min to 3000 L (normal pressure). ) / min range changes. Further, in the above RH treatment, a slag reforming material containing CaO as a main component is added, and the slag in the ladle is made into a composition of CaO-SiO ₂ -Al ₂ O ₃ -MgO system, and further, slag is added. The total concentration of (T.Fe) and (MnO) in the middle is adjusted to 4% by mass or less.

Then, the steel subjected to the above RH treatment was continuously cast to form a cast piece, which was subjected to hot rolling, cold rolling, and final annealing to obtain a cold-rolled steel sheet (steel product), and then the cleanness of the steel product was examined. In the investigation of the cleanliness, a part of the cold-rolled steel sheet is cut out, and the thickness profile is observed by an optical microscope or an electron microscope, and the number of inclusions of 100 μm or more which are confirmed in the section of 1 cm ² is measured, and the number of inclusions is generated. When the rate was 0.01/cm ² or less, it was evaluated that the degree of cleanliness was good (○), and the case where the ratio was more than 0.01/cm ² was evaluated as a poor cleansing degree (×).

2 is a view showing a cooling material input amount per ton of molten steel when the flow rate of the circulating gas (Ar) in the static cooling treatment is 8 L (normal pressure)/min·t, and a molten steel after the cooling material is charged. A graph of the effect of circulation time T on the cleanliness of steel products. As can be seen from the figure, the smaller the cooling material input amount W is, the shorter the circulation time T is, and the finer degree is improved. Specifically, the circulation time T after the cooling material is supplied according to the cooling material input amount W is set to satisfy. According to the relationship of the following formula (4), steel having excellent cleanliness is obtained. T(min)≧0.25W(kg/t)+2...(4)

In addition, FIG. 3 shows the flow rate of the circulating gas (Ar) in the net static treatment to the inclusions in the steel product when the cooling material input amount is 4 kg/t and the circulation time after the cooling material is supplied is 6 min. The graph of the impact of the rate. As can be seen from the figure, the flow rate of the inclusions in the steel product can be reduced to 0.01/cm ² or less by setting the flow rate of the circulating gas (Ar) to 4 L (normal pressure)/min·t or more. [Example 2]

In order to investigate the influence of the cooling material input timing on the cleanliness (inclusion generation rate) of the steel product, the undeoxidized molten steel having the same composition as that of the example 1 was subjected to an RH degassing apparatus. The experiment of RH treatment was carried out under the following conditions.

In the above RH treatment, a clean surface treatment in which the molten steel is circulated in a state of not deoxidizing is performed for about 15 minutes to reduce the carbon concentration [C] in the molten steel to 30 ppm by mass or less, and then the molten steel at this time is measured. The temperature and the dissolved oxygen [O] in the molten steel, and the amount of the cooling material input is determined to be 2 kg/t to 12 kg/t according to the subsequent RH treatment time and the processing time until casting, and after the cooling material is introduced, The Al source is added to deoxidize so that the Al concentration [Al] in the molten steel is 0.02% by mass to 0.04% by mass, and the degree of vacuum in the degassing tank is controlled to 0.5 torr to 2 torr (67 Pa to 266 Pa). In the range of the circulating gas (Ar), the flow rate of the circulating gas (Ar) is controlled within a range of 8 L (normal pressure) / min · t to 12 L (normal pressure) / min · t, and is cooled according to the reference shown in Fig. 2 . The amount of material added is such that the circulation time varies from 1 min to 6 min to carry out a net static treatment.

Further, as a comparative example, an experiment in which the RH treatment was carried out under the same conditions as described above was also carried out after the addition of the Al source (after deoxidation).

The molten steel thus obtained was continuously cast to form a cast piece, and then hot rolled, cold rolled, and finally annealed to obtain a cold-rolled steel sheet (steel product), and then the cleaned steel product was measured in the same manner as in Example 1. Degree (incidence of inclusions), and the results are shown in Fig. 4. As can be seen from the figure, in the comparative example in which the cooling material was supplied after the addition of the Al source (after deoxidation), the generation rate of the inclusions largely fluctuated due to the change in the circulation time after the cooling material was introduced, and the cooling material was introduced. In the example of the present invention before the addition of the Al source (before deoxidation), the generation rate of the inclusions can be stably reduced to 0.01/cm ² or less even if the circulation time is changed. [Example 3]

In order to investigate the effect of slag upgrading by the cleanliness of the steel product (inclusion rate), the undeoxidized molten steel of the converter having the same composition as that of Example 1 was subjected to the following conditions using an RH degassing apparatus. The experiment of RH treatment was carried out.

In the above RH treatment, the surface treatment of circulating the molten steel in a state of not deoxidizing is performed for about 15 minutes to reduce the carbon concentration [C] in the molten steel to 30 ppm by mass or less, and to control the dissolved oxygen [O]. In the range of 100 ppm by mass to 300 ppm by mass, the Al source is added to deoxidize so that the Al concentration [Al] in the molten steel is 0.02% by mass to 0.04% by mass in the same manner. After adjusting the temperature by cooling the material 8 kg/t, the vacuum in the degassing tank is controlled within the range of 0.5 torr to 2 torr (67 Pa to 266 Pa), and the flow rate of the circulating gas (Ar) is controlled to 10 L. 6 minutes of net static treatment was carried out within the range of (normal pressure) / min · t.

Further, in the above RH treatment, a slag reforming material containing CaO as a main component is added, and the slag in the ladle is made into a composition of CaO-SiO ₂ -Al ₂ O ₃ -MgO system, and further, slag is added. The total concentration of (T.Fe) and (MnO) in the middle is adjusted to 4% by mass or less.

Further, the total concentration of (T.Fe)+(MnO) in the slag is controlled by inserting an iron rod from the upper portion of the slag layer according to the red hot portion of the rod tip. The slag thickness, the average slag composition, and the specific gravity of the length are calculated as the amount of (T.Fe)+(MnO) of the lower oxide, and the equivalent amount of Al and CaO necessary for the reduction is obtained. It is put into the upper part of the steel drum as a slag upgrading material. Moreover, the addition amount of Al and CaO in this example is: 50: 100 kg to 100 kg (0.2 kg/t to 0.4 kg/t), and CaO: 25 kg to 50 kg (0.1 kg/) with respect to 250 t of molten steel. t ~ 0.2 kg / t).

Further, as a comparative example, the slag composition was not modified, and the slag composition was set to CaO-SiO ₂ -Al ₂ O ₃ -MgO-Fe _t O-MnO system, and (T.Fe) in the slag was used. The total concentration of (MnO) was adjusted to 5 mass% to 8 mass%, and an experiment for performing RH treatment was also performed.

The molten steel thus obtained was continuously cast to form a cast piece, and then hot rolled, cold rolled, and finally annealed to obtain a cold-rolled steel sheet (steel product), and then the cleaned steel product was measured in the same manner as in Example 1. Degree (production rate of inclusions), and the results are shown in Table 1. From this table: in the case of slag without modification, resulting in significant changes in the rate of inclusions was 0.005 / cm ² ~ 0.007 th / cm ^2, the other hand, is performed by the addition of Al + CaO In the case of the present invention in which the slag is modified, the rate of occurrence of inclusions is stabilized at 0.002/cm ² or less. The reason for this is considered to be that reoxidation of [Al] dissolved in the molten steel in Al deoxidation is prevented by performing slag reforming.

[Table 1] [Industrial availability]

The technique of the present invention is also applicable to secondary refining of molten steel subjected to vacuum circulation treatment such as vacuum oxygen decarburization (VOD) or ladle refining (ASEA-SKF).

1‧‧‧Steel drum
2‧‧‧Fused steel
3‧‧‧ slag layer
4‧‧‧vacuum degassing tank
5‧‧‧Dip tube
6‧‧‧Circulating gas (Ar)
7‧‧‧Washing material hopper
8‧‧‧Injector for cooling material addition
9‧‧‧Exhaust port

1 is a schematic cross-sectional view of a cycloid vacuum degassing apparatus. 2 is a graph showing the influence of the amount of cooling material input and the circulation time after the cooling material is introduced on the rate of inclusion formation in steel products. 3 is a graph showing the influence of the flow rate of the circulating gas after the cooling material is applied on the rate of inclusion formation in the steel product. 4 is a graph comparing the incidence of inclusions in a steel product in a case where a deoxidizing agent is added and then a cooling material is introduced, and a cooling material is introduced before a deoxidizing agent is added.

Claims (1)

A method for melting high-definition clarity steel, which is a method for melting a steel in which a molten steel in a steel ladle is circulated while being subjected to decarburization in a circulating vacuum degasser, and a deoxidizing agent is added to perform a static treatment The method for melting a high-definition clarity steel is characterized in that the temperature of the molten steel at the end of the converter blowing, the dissolved oxygen in the molten steel, the subsequent RH treatment time, and the casting are introduced before the introduction of the deoxidizing agent. The amount of the cooling material determined by the processing time, thereby adjusting the temperature of the molten steel, and the circulation time after the cooling material is supplied satisfies T≧0.25W+2, and the circulation time after the above-mentioned deoxidizing agent is input is set to 1 min to 6 min. Within the scope of the above, wherein T is the circulation time (min) after the cooling material is put in, and W is the amount of cooling material added per kg of molten steel (kg/t).