JP5402259B2 - Method for producing ultra-low carbon steel - Google Patents

Description

  The present invention relates to a high-efficiency manufacturing method of ultra-low carbon steel excellent in cleanliness.

  Extremely low carbon steel is used for automotive exterior steel sheets that require few surface defects and excellent formability. And high cleaning measures are taken.

  In the case of melting ultra-low carbon steel, a method of causing a decarburization reaction of undeoxidized molten steel using a vacuum processing apparatus is common. That is, undeoxidized molten steel having a carbon content of 0.02% by mass or more and 0.10% by mass or less from a steelmaking furnace such as a converter is put into a ladle, and then oxygen in the molten steel and The carbon content is decarburized to 0.001 mass% or more and 0.005 mass% or less by reaction with carbon. It is known that the oxygen content necessary for obtaining a sufficient decarburization rate during the above reaction is 0.04% by mass or more. When such molten steel with a high oxygen content is obtained in a steelmaking furnace such as a converter, the total content of FeO and MnO, which are lower oxides in the slag, is as high as 15% by mass or more and 20% by mass or less. .

In the molten steel of ultra-low carbon steel that has been deoxidized with Al after the vacuum decarburization treatment, Al in the ladle molten steel reacts with the lower oxide in the slag during the continuous casting after the vacuum treatment. By this reaction, an oxide of Al (Al ₂ O ₃ ) is generated. This oxide is not removed from the molten steel in the tundish or mold during continuous casting, but remains in the slab and becomes non-metallic inclusions, deteriorating the quality of the final product.

The Al ₂ O ₃ inclusions are likely to accumulate near the surface of the slab, which may cause surface defects in automotive exterior steel sheets, or may cause clogging of the immersion nozzle during continuous casting. There is. When the immersion nozzle is closed, casting cannot be performed continuously and productivity is hindered, and drift occurs in the molten steel passing through the nozzle to change the flow state in the mold and cause surface defects. Furthermore, in order to prevent this nozzle clogging, it is necessary to increase the flow rate of an inert gas such as Ar blown from the upper part of the nozzle. This blown inert gas also contributes to surface defects if it remains near the surface of the slab and is captured. In order to prevent such surface defects, when the surface of a slab or hot-rolled steel sheet material is cared for, there is a problem from the viewpoint of economy and productivity.

Therefore, in the production of ultra-low carbon steel, measures for reducing Al ₂ O ₃ inclusions in the molten steel subjected to continuous casting are required. Generally, as a method for reducing Al ₂ O ₃ inclusions, securing of the final reflux time after the addition of Al in the RH treatment using the floating separation of inclusions can be mentioned. For example, in Patent Documents 1 and 2, a reflux treatment for 10 minutes is performed after the addition of the deoxidizer.
JP-A-9-49011 JP 2000-129338 A JP 2002-328125 A JP-A-10-311782

  However, as shown in FIG. 1, a sample of molten steel immediately after the addition of Al in the RH treatment for melting ultra-low carbon steel was taken and T. When [O] was investigated, there was a variation of about 10 to 100 ppm. Therefore, when the reflux time is constant, the T.O. In order to deal with the variation in [O], an operation loss due to excessive reflux was generated.

  The object of the present invention is to analyze the total oxygen concentration and Al concentration in molten steel after completion of RH deoxidation before the end of RH final reflux in RH treatment, and determine the optimum RH final reflux time based on the values. It is providing the manufacturing method of the ultra-low carbon steel for motor vehicle exteriors characterized by the above-mentioned.

  By implementing the present invention, it is possible to achieve rationalization of RH operation while maintaining good product quality of ultra-low carbon steel sheets including automobile exterior steel sheets.

In the production of ultra-low carbon steel, in order to increase the cleanliness of the molten steel, it is necessary to take measures for reducing Al ₂ O ₃ inclusions in the vacuum degassing tank. Generally, as a method for reducing Al ₂ O ₃ inclusions, securing the final reflux time after adding deoxidation Al using the floating separation of inclusions can be mentioned.

Conventionally, it has been known that the reflux time at which a sufficient floating separation effect is obtained depends on the amount of Al ₂ O ₃ inclusions in the molten steel. However, in the molten steel after addition of Al in RH, which is a base for appropriately determining the reflux time, the T.V. There was no way to know the [O] concentration accurately.

  Therefore, first, the present inventors have described that We started to establish a molten steel analysis method to know the [O] concentration accurately. At the same time, the molten steel after the addition of Al for deoxidation is sampled during RH treatment for the purpose of melting ultra-low carbon steel and analyzed off-line, and the total oxygen concentration, Al concentration and required recirculation time of the sample are analyzed. Investigation and analysis of the relationship was conducted. As a result, it has been found that it is effective to determine the necessary reflux time using the following formulas (1 ') and (2') in order to produce a steel sheet for an automobile exterior. Here, “required reflux time” means the T.D. [O] Refers to the time required to bring the concentration to 30 ppm or less.

  In addition, regarding the “constant (1 ≦ α ≦ 2)”, various factors affect the “relationship between the T. [O] concentration, Al concentration and required reflux time of the sample after deoxidation” in actual operation. T. Even when the [O] concentration and Al% are accurately known, it is necessary to determine the timing for stopping the reflux operation assuming that there is some variation. The cause of this variation has not been completely clarified, but the influence of slag existing on the molten steel during the RH treatment can be considered. Moreover, the influence of a molten steel component and molten steel reflux conditions is also considered. However, the variation range is about 1 minute as shown in FIGS. Therefore, in actual operation, considering the end point [C]% in the converter, the amount of slag modifier that may be added when steel is output from the converter, and the treatment conditions in RH, etc. The necessary recirculation treatment time is determined within the range of the expressions (1 ′) and (2 ′), and the recirculation process is terminated.

When 0.065 ≧ [% Al]> 0.030%:
t = 0.043 × T. [O] (ppm) + α (1 ')
When 0.005 ≦ [% Al] ≦ 0.030%:
t ′ = 0.043 × T. [O] (ppm) + α (2 ')
Here, [% Al]: Al concentration in molten steel (mass%)
T. T. et al. [O]: Total oxygen concentration (ppm)
t: T. [O] Time (minutes) from when the sample for analysis is collected until the molten steel reflux is stopped
t ′: Correction required recirculation time (minutes) = t−100 × (0.03-[% Al])
α: Constant (1.0 ≦ α ≦ 2.0)
Furthermore, the present inventors based on the above-mentioned knowledge, the T. Efforts were made to develop a method for applying the rapid analysis method of [O] to RH online operation, and it was established as a highly efficient method for producing clean ultra-low carbon steel including automobile exterior steel plates. In order to implement the present invention, T.W. A method for analyzing [O] in a short time and with high accuracy is indispensable. As a specific method thereof, the following analysis method was used.

  (I) A method in which a steel sample is put in a graphite crucible, heated and melted in an inert gas, and the oxygen concentration in the sample is measured from the infrared absorption of either one or both of generated carbon monoxide and carbon dioxide. Use.

  (Ii) Vacuum arc plasma treatment as a pretreatment for removing and cleaning the oxide film on the surface of the sample, the degree of vacuum at the start of arc plasma discharge being 5 Pa to 35 Pa, and the arc plasma output current being 15 A to 55 A Apply under conditions.

  (Iii) Machined so that the steel ingot collected from the molten steel has a height of 1.5 mm to 7 mm and a ratio of surface area S to volume V (S / V) of 1.05 to 1.30. A small piece obtained in this way is used as a sample.

(Iv) After the arc plasma discharge is applied to the sample for a total of 4 times or less and a total treatment time of 0.2 seconds or more and 1.2 seconds or less,
The sample is directly heated and cleaned at a temperature higher than the temperature at the time of analysis without being brought into contact with the atmosphere, and then put into a graphite crucible that has been lowered to the temperature to be analyzed and placed on standby.
By applying this analytical method online, the T.O. It became possible to determine the [O] concentration before the completion of the final RH reflux and to determine the optimal RH final reflux time.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) By mass%, C: 0.0030% or less, Si: 0.5% or less, Mn: 0.5% or less, P: 0.05% or less, S: 0.03% or less, N: 0 .0040% or less, Sol. A manufacturing method using RH of an ultra-low carbon steel containing Al: 0.005% or more and 0.065% or less, Ti: 0.01% or more and 0.06% or less, using an RH vacuum processing apparatus After performing vacuum decarburization in the range of the Ar flow rate for recirculation of 0.24 to 0.45 Nm ³ / (hr · t) for 200 to 300 t of molten steel, the Al content in the molten steel is set to 0.005. The T. content in the molten steel sample after the deoxidation adjustment was adjusted to not less than 0.06 mass% and not more than 0.065 mass%. In analyzing the [O] and Al concentrations in the molten steel reflux, the T.O. As an analysis method of [O], a steel sample is put in a graphite crucible and heated and melted in an inert gas, and the oxygen concentration in the sample is determined from the infrared absorbance of one or both of the generated carbon monoxide and carbon dioxide. As a pretreatment for removing and cleaning the oxide film on the sample surface, the vacuum arc plasma treatment is performed at a vacuum degree of 5 Pa or more and 35 Pa or less at the start of arc plasma discharge, and the arc plasma output current is 15 A or more. Under the condition of 55A or less, the height of 1.5 mm to 7 mm and the ratio of surface area S to volume V (S / V) is 1.05 to 1.30 with respect to the steel ingot collected from the molten steel A small piece obtained by machining in this manner was used as a sample, and the arc plasma discharge was applied to the sample a total of 4 times or less, and the total processing time was 0.2 seconds or more and 1.2 seconds or less. Then, without contacting the sample with the atmosphere, after directly heating and cleaning at a temperature higher than the temperature at the time of analysis, the method for analyzing oxygen in steel is put into a graphite crucible that is lowered to the temperature to be analyzed and placed on standby And T. An ultra-low carbon steel manufacturing method characterized by adjusting the molten steel recirculation within the range of the following formulas (1) and (2) based on [O] and the Al concentration value.

When 0.065 ≧ [% Al]> 0.030 mass%:
0.043 × T. [O] + 1.0 ≦ t ≦ 0.043 × T. [O] +2.0 (1)
When 0.005 ≦ [% Al] ≦ 0.030 mass%:
0.043 × T. [O] + 100 × (0.03-[% Al]) + 1.0 ≦ t
≦ 0.043 × T. [O] + 100 × (0.03-[% Al]) + 2.0 (2)
Here, [% Al]: Al concentration in molten steel after deoxidation adjustment (mass%)
T. T. et al. [O]: T. in molten steel after deoxidation adjustment [O] (ppm)
t: T. after deoxidation adjustment [O] Time (minutes) from when the sample for analysis is collected until the molten steel reflux is stopped

  According to the present invention, in the RH treatment, the total oxygen concentration and Al concentration in the molten steel after completion of RH deoxidation are analyzed before the end of the RH final reflux, and the optimum RH final reflux time is determined based on these values. Further, it is possible to reduce the excess RH reflux time, and it is possible to manufacture an ultra-low carbon steel for automobile exterior with high efficiency.

A sample of molten steel immediately after the addition of Al in the RH treatment for melting ultra-low carbon steel was taken and T. It is a graph which shows the result of having analyzed and investigated [O]. It is a graph which shows the relationship between T. [O] after RH process, and a product disqualification rate. It is a graph which shows the relationship between recirculation | reflux time and T. [O] in molten steel. It is a graph which shows the relationship between T. [O] after a deoxidation process, and excess reflux time. It is a graph which shows the relationship between T. [O] after deoxidation, and required reflux time. It is a graph which shows the relationship between T. [O] after deoxidation, and correction | amendment required reflux time. It is a figure which shows typically the oxygen analysis equipment in the steel which concerns on this invention.

  Hereinafter, the best mode of a method for producing an ultra-low carbon steel according to the present invention will be described with reference to the drawings. In the present specification, “%” indicating the chemical composition of steel or slag is “% by mass” unless otherwise specified.

  C concentration melted in a converter or the like: 0.02% to 0.1% of undeoxidized or weakly deoxidized molten steel is taken out into a ladle. At this time, the furnace slag inevitably flows out into the ladle. This outflow slag contains lower oxides (FeO, MnO) and reacts with the deoxidizing element after deoxidation to generate inclusions, which may deteriorate the cleanliness of the molten steel. Therefore, when the concentration of the lower oxide in the slag is extremely high, the lower oxide in the slag is reduced in advance and the concentration is reduced in order to reduce the concentration of the lower oxide in the slag. A slag modifier containing metal Al or an Al alloy may be added. The management target of FeO + MnO% in the slag is desirably 2% or more and 10% or less in total.

A slag modifier having a composition such as Al: 40% + CaCO ₃ : 60% or Al: 50% + Al ₂ O ₃ : 40% + CaO: 10% may be used. Al may be metal Al or Al alloy. The metal Al or Al alloy in the slag modifier is contained as a reducing agent for lower oxides in the slag. Metal Al or Al alloy has a strong reducing power and is effective in reducing lower oxides in the slag.

  In the method of the present invention, the entire ladle is placed in a vacuum vessel, and a two-leg dip tube is dipped in a tank degassing device that blows inert gas from a porous plug at the bottom of the ladle or molten steel in the ladle. The vacuum decarburization process is performed using an apparatus capable of vacuum decarburization of molten steel, such as an RH degasser that blows inert gas for reflux from one dip tube, and then improves the cleanliness of the steel. Therefore, a predetermined deoxidizer is added to this molten steel, deoxidation treatment is performed, and the optimum RH final reflux time is determined from the total oxygen concentration and Al concentration of the molten steel sample after deoxidation, and the treatment is performed To do.

  As shown in FIG. 2, as a result of investigating the relationship between T. [O] after RH treatment and the rate of product deterioration for an automotive exterior steel plate, the T.O. It has been found that when [O] exceeds 30 ppm, the rate of downgrade increases. Table 1 shows the component ranges of the automobile exterior steel plates targeted in this investigation. In addition, the unit of the chemical composition of steel shown in Table 1 is mass%, and the balance is Fe and impurities.

  Conventionally, in the RH treatment of ultra-low carbon steel, as shown in FIG. It has been known that the [O] concentration decreases monotonously according to the molten steel reflux time. Therefore, the initial T.I. [O] When the variation in concentration is large, T.I. It is easily considered that the molten steel reflux time required for reducing the [O] concentration below a certain value is also greatly different.

  Conventionally, however, the T.V. in the molten steel after adding deoxidizing Al with RH. A specific method for accurately knowing the value of [O] concentration during the molten steel reflux treatment has not been known. Therefore, the circulation time after the addition of Al must be determined by relying on experience and intuition, and the circulation time has to be taken for a long time in consideration of the existence of this variation.

As a result, as shown in FIG. In many cases, the reflux time at [O] ≦ 30 ppm was excessive.
However, in the molten steel after addition of Al for deoxidation, T.I. If the value of the [O] concentration can be accurately known during the molten steel recirculation treatment, T.W. It is possible to stop the molten steel recirculation by accurately obtaining the recirculation time necessary for setting [O] ≦ 30 ppm.

  First, the inventors of the present invention focused on establishing a rapid analysis method for the oxygen concentration in molten steel. As the analysis method was established, the technology of the present invention was developed so that the analysis method could be used for optimizing the RH reflux time for melting ultra-low carbon steel.

  The investigation conditions are shown in Table 2, and the investigation results are shown in FIGS. In addition, the unit of the chemical composition of steel shown in Table 2 is mass%, and the balance is Fe and inevitable impurities. Moreover, the unit of the chemical composition of slag is also mass%.

  The range of the Ar flow rate for reflux in the table is common for 200 to 300 t of molten steel. Further, the ring flow rate can be calculated using the following calculation formula (3). If the calculation condition is applied to the formula, the ring flow rate corresponds to a range of 90 to 100 t / min.

FIG. 5 is a graph showing the relationship between T. [O] after deoxidation and the required reflux time.
FIG. 6 is a graph showing the relationship between T. [O] after deoxidation and the required reflux time.
As shown in FIGS. 5 and 6, if the RH final reflux time is shorter than the time determined by the equations (1 ′) and (2 ′), the total oxygen in the molten steel after the circulation will exceed 30 ppm. , It turns out that the rate of disqualification of the product will increase.

  As described above, the final reflux time after the addition of Al in the RH treatment is TD after the addition of the Al. [O] It is appropriate to add the time required to collect the molten steel sample for analysis and the necessary recirculation time after sampling determined by the equations (1 ′) and (2 ′). If it becomes longer than this time, T.P. In order to set [O] ≦ 30 ppm, excessive reflux treatment is performed, and the RH treatment time is meaninglessly extended, which greatly impedes productivity.

  In view of the above, the final reflux time after the addition of Al in the RH treatment is TD after the addition of the Al. [O] It is necessary to be within the time range obtained by adding the necessary recirculation time after sampling determined by the equations (1) and (2) to the time until the molten steel sample for analysis is taken.

When 0.065 ≧ [% Al]> 0.030%:
0.043 × T. [O] + 1.0 ≦ t ≦ 0.043 × T. [O] +2.0 (1)
When 0.005 ≦ [% Al] ≦ 0.030%:
0.043 × T. [O] + 100 × (0.03-[% Al]) + 1.0 ≦ t
≦ 0.043 × T. [O] + 100 × (0.03-[% Al]) + 2.0 (2)
Here, [% Al]: Al concentration in molten steel (%)
T. T. et al. [O]: Total oxygen concentration in molten steel (ppm)
t: T. [O] Time (minutes) from when the sample for analysis is collected until the molten steel reflux is stopped

  In order to reflect the above-described knowledge in online operation, the present inventors have described T.C. As a method for analyzing [O] in a short time and with high accuracy, the following analysis method was used.

The analysis method will be described in detail below with reference to the drawings.
FIG. 7 schematically shows an oxygen analyzer in steel for carrying out the analysis method according to the present invention.

  In order to achieve a short time and high accuracy analysis required for the analysis method according to the present invention, vacuum arc plasma treatment was selected as a rapid and reproducible sample pretreatment method among the elemental technologies combined in the present invention. For example, the preparation method and apparatus for a sample for analyzing an in-metal component disclosed in Patent Document 3 may be applied. A sample can be inserted into the sample pretreatment apparatus 1 that has been previously kept in vacuum through the isolation valve 4 through the isolation valve 4 with almost no change in the degree of vacuum. Thereafter, the oxide film on the sample surface is removed in a few seconds by vacuum arc plasma treatment. In this apparatus, since the sample is automatically conveyed, the sample shape is limited to a cylinder or a block (a rectangular parallelepiped). Since the sample is placed on the sample stage and processed, the surface in contact with the sample stage is not processed. Therefore, it is necessary to invert the sample for processing. That is, it is necessary to discharge at least twice for one sample. When the number of discharges increases, the sample is heated for a long time, and once the oxide film is removed, the sample surface is oxidized again. Therefore, in order to remove the oxide film on the sample surface reliably, accurately and with good reproducibility, and to ensure the analysis accuracy required for the refining operation, it is necessary to perform an arc plasma treatment under the following conditions.

(a) Degree of vacuum: 5 Pa or more and 35 Pa or less. The sample surface oxide film removal reaction by vacuum arc plasma is promoted as the degree of vacuum increases, but if it exceeds 35 Pa, the reoxidation reaction accompanying the increase in the sample temperature becomes remarkable, which is not preferable. On the other hand, if it is lower than 5 Pa, the oxide film removal reaction itself does not proceed, which is not preferable. There is therefore an optimum degree of vacuum.
It is more preferable to have a pressure control mechanism for controlling the opening and closing of the vacuum exhaust valve and the gas introduction valve so that the degree of vacuum is maintained at a constant value during processing.
(b) Arc plasma output current: 15A or more and 55A or less.
(c) Processing time: The total processing time is 0.2 second or more and 1.2 seconds or less for one sample.
(d) Number of treatments: The total number of treatments per sample is 4 or less.

  The treated sample is finally put into the graphite crucible through the pretreated sample inlet 5 arranged in the analyzer 2 without being brought into contact with the atmosphere. The sample pretreatment chamber and the sample inlet of the analyzer are connected by a connecting tube 8 whose inside is replaced with vacuum or an inert gas. As the inert gas species, Ar is preferable from the viewpoint of reliably replacing the gas in the connecting pipe in consideration of the specific gravity difference with air and preventing reoxidation of the sample after processing, and from an economical viewpoint. . In the apparatus configuration disclosed in Patent Document 3, the pretreated sample is dispensed and then transferred to a separate oxygen analyzer. However, since the object of the present invention requires quickness, the sample pretreatment device 1 and the oxygen analyzer 2 are arranged vertically above and below, and freely fall in the connecting tube 8 to transfer the sample. That is, an apparatus configuration as shown in FIG. 7 was adopted.

  In the apparatus configuration of the present invention, the oxygen analyzer 2 is disposed at a position close to the floor surface, and cleaning inside the analyzer 2 hinders maintenance work of the apparatus such as replacement of the impurity adsorbent in the gas. Therefore, the entire apparatus incorporated in the gantry 6 is placed on the lifter 7 so that it can be raised and lowered, and during the operation, the entire apparatus is raised to ensure workability. The driving method of the lifter 7 is not particularly limited. However, since the weight of the entire apparatus is considerable, an automatic hydraulic type is preferable from the viewpoint of operability. Moreover, it is desirable to cover the movable part of the lifter 7 with a stretchable material and to have a structure in consideration of safety so that an operator is not caught.

  Further, the sample pretreatment device 1 and the oxygen analyzer 2 are connected in preparation for the case where the connected oxygen analyzer 2 cannot be used due to a failure or when a preprocessed sample waiting for analysis is analyzed by another oxygen analyzer. An outlet 9 for a pretreated sample is provided in the middle of the tube 8.

  Among the elemental technologies combined in the present invention, as a method for obtaining an analysis sample more easily and quickly than a steel ingot collected from molten steel, the height (thickness) produced by cutting the steel ingot collected from molten steel is 1.5 mm or more. A punched cylindrical piece is used as a sample for a slice of 7 mm or less. Specifically, for example, an analysis sample adjustment method and apparatus disclosed in Patent Document 4 may be applied. In order to remove the oxide film on the sample surface reliably, accurately and with good reproducibility, the ratio S / V of the surface area S to the volume V calculated from the diameter and height of the sample bottom is “1.05 ≦ S / V It is necessary to ensure a shape that satisfies “≦ 1.30”.

  The reason for this is not fully understood at this time, but it corresponds to the fact that the position suitable for efficient processing is limited in the spatial distribution of the arc plasma depending on the shape of the arc processing part such as the electrode shape. It is guessed.

  Among the elemental technologies combined in the present invention, an oxygen analyzer based on the operating principle of heating and melting in an inert gas-infrared absorption method was selected as a highly accurate method for analyzing oxygen in steel. In this analysis method, a graphite crucible serving as a sample holder and a sample deoxidation reagent (carbon) supply source is used.

  Prior to analysis, in order to remove oxygen and contamination adsorbed on the surface of the crucible, a so-called “empty baking” process is performed in which only the crucible is preheated at a temperature slightly higher than at the time of analysis. The “blank” treatment can reduce the influence of oxygen, carbon monoxide or carbon dioxide generated from the graphite crucible changing the analytical value. When analyzing oxygen in steel with a commercially available oxygen analyzer, a crucible, that is, a sample is usually heated to a temperature of about 1800 ° C. to 2200 ° C. In order to realize the high analysis accuracy required in the present invention, for example, the heating may be performed at a temperature that is 100 ° C. or more higher than the temperature at the time of analysis and for 15 seconds or more.

  In addition, in a commercially available oxygen analyzer, first, a sample is taken into the analyzer, and the atmosphere around the sample is replaced with helium gas as a carrier gas. carry out. Therefore, it takes a relatively long time until the analytical value is determined after the sample is introduced. Replacing the crucible, cleaning the electrodes, and performing the “blank” process in advance, and loading the sample that has been pre-cleaned in a state where the analyzer can analyze, according to the required analysis time. Speeding up can be realized.

  Usually, in the oxygen analysis, in order to convert the detected gas amount into the oxygen concentration in the sample, it is necessary to accurately weigh the sample weight. As a result of evaluating the change in the sample weight before and after the vacuum arc plasma treatment, although there was some variation depending on the shape of the sample and the degree of surface oxidation, the weight loss was about 1 mg at most, so the sample weight was reduced to 0.5 to 1.0 g. On the other hand, it has been found that the error is negligible for practical use. Therefore, when carrying out the present invention, the analysis sample obtained by machining and previously weighed was subjected to vacuum arc plasma treatment and inserted into the oxygen analyzer as it was without being brought into contact with the atmosphere.

(Basic conditions)
Using a converter and an RH vacuum processing apparatus, 270 t of ultra-low carbon steel for automobile exterior (component range is as shown in Table 1) was melted. In the converter, the C concentration was refined to 0.02% by mass or more and 0.10% by mass or less, and molten steel having a temperature of 1640 ° C. or more and 1690 ° C. or less was taken out into a ladle. When steeling out, slag outflow from the converter was suppressed as much as possible. To the molten slag in the ladle immediately after steeling, quick lime, Al ₂ O ₃ flux, CaO flux as a slagging agent, and Al ash, Al-CaO flux as slag modifiers are added as appropriate, It adjusted so that the FeO + MnO density | concentration in slag might be 2 mass% or more and 10 mass% or less.

Next, vacuum decarburization was performed using a RH vacuum processing apparatus until the carbon content in the molten steel was 0.0050 mass% or less. Thereafter, Al was added to the vacuum chamber for deoxidation, and the Al content in the molten steel was adjusted to 0.005 mass% or more and 0.065 mass% or less. As an example of the present invention, the T. [O] and Al concentrations in the molten steel sample after the deoxidation adjustment were rapidly analyzed by the above method, and the final reflux time was determined using the values. Further, as a comparative example, the treatment was performed with a reflux time of 10 minutes. The RH dip tube diameter was 660 mm, the reflux Ar gas flow rate was 1.2 Nm ³ / min, and the degree of vacuum during processing was 1 to ³ Torr. As the deoxidizer, metal Al was used.
Next, slabs were formed by continuous casting and product coils were formed by rolling, and product defects were investigated by ultrasonic flaw detection.

The results are shown in Table 3. In Invention Examples 1 and 2, after deoxidation, [% Al] = 0.038 mass% and 0.058 mass%, respectively, the final reflux time was determined using equation (1). The final reflux time was 5 to 6 minutes shorter than that of Comparative Example 1 having the same level of [% Al] after deoxidation, and the failure rate was almost the same value. From this, it can be seen that, in the example of the present invention, necessary and sufficient adequate reflux time is secured.
In addition, since Inventive Examples 3 and 4 have [% Al] = 0.025 mass% and 0.008 mass% after deoxidation, respectively, the final reflux time was determined using the formula (2). The final reflux time was 3 to 6 shorter than that of Comparative Example 2 having the same level of [% Al] after deoxidation, and the rate of disqualification was almost the same value. From this, it can be seen that, in the example of the present invention, even when [% Al] is relatively low, a sufficient adequate reflux time is ensured.

DESCRIPTION OF SYMBOLS 1 Pretreatment apparatus 2 Oxygen analyzer 3 Pretreatment sample inlet 4 Isolation valve 5 Pretreatment specimen inlet 6 Base 7 Lifter 8 Connection pipe 9 Pretreatment specimen intermediate outlet

Claims (1)

In mass%, C: 0.0030% or less, Si: 0.5% or less, Mn: 0.5% or less, P: 0.05% or less, S: 0.03% or less, N: 0.0040% Hereinafter, Sol. A production method using RH of an ultra-low carbon steel containing Al: 0.005% or more and 0.065% or less, Ti: 0.01% or more and 0.06% or less,
After performing vacuum decarburization in the range of Ar flow rate for recirculation of 0.24 to 0.45 Nm ³ / (hr · t) for 200 to 300 t of molten steel using an RH vacuum processing apparatus , The Al content is adjusted to 0.005 mass% or more and 0.065 mass% or less,
T. in the molten steel sample after the deoxidation adjustment. [O] and when analyzing the Al concentration in the molten steel reflux,
T. in the molten steel sample. As an analysis method of [O],
A method of measuring an oxygen concentration in a sample from an infrared absorption of one or both of carbon monoxide and carbon dioxide generated by placing a steel sample in a graphite crucible and heating and melting in an inert gas,
As a pretreatment for removing and cleaning the oxide film on the surface of the sample, vacuum arc plasma treatment is performed under the conditions of a vacuum degree at the start of arc plasma discharge of 5 Pa to 35 Pa and an arc plasma output current of 15 A to 55 A.
It was obtained by machining a steel ingot collected from molten steel so that the height was 1.5 mm or more and 7 mm or less, and the ratio of surface area S to volume V (S / V) was 1.05 or more and 1.30 or less. Using a small piece as a sample,
After the arc plasma discharge is applied to the sample a total of 4 times or less and a total treatment time of 0.2 seconds or more and 1.2 seconds or less,
Without using the sample in contact with the atmosphere, directly after heating and cleaning at a temperature higher than the temperature at the time of analysis, using a method for oxygen analysis in steel that is put into a graphite crucible that is lowered to the temperature to be analyzed and placed on standby,
The T.A. Based on [O] and the Al concentration value, an ultra-low carbon steel production method is characterized in that the molten steel reflux is adjusted within the range of the following formulas (1) and (2).
When 0.065 ≧ [% Al]> 0.030 mass%:
0.043 × T. [O] + 1.0 ≦ t ≦ 0.043 × T. [O] +2.0 (1)
When 0.005 ≦ [% Al] ≦ 0.030 mass%:
0.043 × T. [O] + 100 × (0.03-[% Al]) + 1.0 ≦ t
≦ 0.043 × T. [O] + 100 × (0.03-[% Al]) + 2.0 (2)
Here, [% Al]: Al concentration in molten steel after deoxidation adjustment (mass%)
T. T. et al. [O]: T. in molten steel after deoxidation adjustment [O] (ppm)
t: T. after deoxidation adjustment [ O ] Time (minutes) from when the sample for analysis is collected until the molten steel reflux is stopped

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