WO2019182056A1 - Method for manufacturing high-purity steel - Google Patents

Abstract

Provided is a method for manufacturing high-purity steel wherein clogging of an immersion nozzle in continuous casting equipment can be prevented and more excellent sulfide stress cracking resistance (SSC resistance) can be achieved. This method for manufacturing high-purity steel is characterized by having a deoxidation processing step in which Al is added to molten steel in a converter furnace after Si has been added, a ladle refining step performed by means of a ladle furnace, a vacuum degassing processing step, a step in which a Ca-containing metal is added to the molten steel, and a step in which the molten steel is continuously cast, and is characterized in that the Si is not added to the molten steel in the ladle refining step; or, when additional Si is added to adjust the components in the molten steel, it is added in the first half of the processing period of the ladle refining step, and is not added in the latter half of the processing period of the ladle refining step nor is added during the vacuum degassing processing period.

Description

Manufacturing method of high cleanliness steel

The present invention relates to a method for producing a steel having a small amount of oxide-based non-metallic inclusions, that is, a high cleanliness steel, and particularly to a method for producing a calcium-added steel.

Demands for high cleanliness steels with reduced amounts of oxide-based non-metallic inclusions in steel materials are increasing due to stricter product characteristics and demand for more sophisticated materials. In addition, high-strength steel pipes used in applications such as oil well pipes are used in acidified severe environments (sour environments) containing the corrosive gas hydrogen sulfide, so hydrogen-induced crack resistance (HIC resistance) ) And sulfide stress corrosion cracking resistance (SSC resistance).

In order to improve HIC resistance and SSC resistance, not only reducing the amount of oxide-based non-metallic inclusions in the molten steel stage, but also sulfides represented by MnS that precipitate and crystallize during solidification of the molten steel. Reduction and detoxification are necessary. In particular, MnS has a high extensibility, and it is known that it is detrimental to HIC resistance and SSC resistance because it is stretched when a subsequent steel is rolled and becomes a hydrogen storage site.

As this countermeasure, it is generally known that it is effective to change MnS to CaS by adding a Ca-containing metal at the molten steel stage. Regarding the addition method and addition amount of the Ca-containing metal, the following techniques are known.

In Patent Document 1, Ca or a Ca-containing substance is added to molten steel between the time when the steel is discharged from the converter and before casting, and 0.0005 to 0.005 mass% or more of Ca is contained in the molten steel. Medium S, Al, Ca and T. There is described a method for producing steel for oil wells having excellent resistance to sulfide stress corrosion cracking, wherein [O] (total oxygen) is controlled to satisfy the following formula.
-0.005 ≦ (Ca / 40-S / 32) × sol.Al × T. [O] × 1000000 ≦ 0.0042

Patent Document 2 describes the T. of molten steel after the completion of secondary refining. High strength, which controls [O], and controls the inclusion by adding an amount of Ca calculated based on the measured value before injecting the molten steel into the tundish of the continuous casting machine. A method for melting a steel material for high corrosion resistance oil well pipes is described.

In Patent Document 3, Al is added to the molten steel at the time of or after the steel from the converter to the ladle, and the molten steel is deoxidized. First, a flux containing CaO is added to the molten steel in the ladle. In this desulfurization process, a Ca-containing metal is added, and then the molten steel in the ladle is vacuum degassed, and further, the Ca-containing metal is added to the molten steel in the ladle after the vacuum degassing process. Is added, and then, in the process of casting the molten steel, the amount of pure Ca contained in the Ca-containing metal during the desulfurization treatment is adjusted according to the Al concentration and the total oxygen concentration in the molten steel. A method for producing clean steel having excellent resistance to sulfide corrosion cracking is described.

JP 2002-60893 A JP 2011-89180 A JP 2010-209372 A

By adding a Ca-containing metal to the molten steel, it is possible not only to suppress the formation of MnS as described above, but also to change the Al ₂ O ₃ -based inclusions to CaO—Al ₂ O ₃ -based inclusions. The techniques of Patent Documents 1 to 3 define the addition amount of the Ca-containing metal for the purpose of improving the HIC resistance and the SSC resistance from this viewpoint. That is, in the techniques of Patent Documents 1 to 3, it is assumed that only Al ₂ O ₃ inclusions exist before Ca addition, and Ca reacts with the Al ₂ O ₃ inclusions so that appropriate CaO— This is a technique for defining the addition method and the addition amount based on the idea of changing to Al ₂ O ₃ inclusions.

However, according to the study by the present inventors, when Ca is added based on such a concept, nozzle clogging becomes a problem, particularly with an immersion nozzle having a small inner diameter in a continuous casting facility, or high strength and strictness of 110 psi (760 MPa) or more. It has been found that large inclusions exceeding 200 μm cannot be completely generated and suppressed in steel types that require high SSC resistance, and cannot meet such strict SSC resistance requirements.

Therefore, in view of the above-mentioned problems, the present invention provides a method for producing a high cleanliness steel that enables both prevention of clogging of an immersion nozzle of a continuous casting facility and superior resistance to sulfide stress corrosion cracking (SSC resistance). The purpose is to provide.

The present inventors investigated in detail the inclusion composition such as steel for high-strength seamless pipe used in sour environment. Since this steel generally requires extremely low S, P component, and low O component, it is generally produced by the following process. First, Si and Al are added to the molten steel in the converter or the subsequent ladle to perform deoxidation treatment. Next, a flux containing CaO is added to the molten steel in the ladle, and a ladle refining process (desulfurization treatment) using a ladle furnace (LF) is performed. Next, vacuum degassing processing is performed using an RH vacuum degassing apparatus. Next, a Ca treatment in which a Ca-containing metal is added to the molten steel is performed (also simply referred to as “Ca addition” in this specification). Then, the molten steel is transferred from the ladle to the tundish, and continuous casting is performed to obtain a slab.

Regarding inclusions in the molten steel, Al ₂ O ₃ inclusions are mainly used immediately after the deoxidation treatment. Here, since high strength steel is required for high-strength seamless pipes and line pipes used in a sour environment, the Si content is generally 0.1% or more. When manufacturing such steel, the Si component is added in a large amount at the same time as the deoxidizer Al, and then in a ladle that receives the molten steel discharged from the converter, or the subsequent LF. In the process and the vacuum degassing process, it is common to add the FeSi alloy to the molten steel in several times so as to achieve the target Si content. About 1% of the Ca component is unavoidably mixed in the FeSi alloy. In the ladle refining process, Mg penetrates into the molten steel due to the reaction between the CaO—Al ₂ O ₃ —SiO ₂ flux added for the purpose of desulfurization and the refractory having the MgO—C composition. For this reason, the composition of inclusions at the end of ladle refining is often changed to CaO—MgO—Al ₂ O ₃ inclusions containing CaO and MgO instead of the Al ₂ O ₃ inclusions alone. It was confirmed.

According to the study by the present inventors, when the CaO concentration varies between a plurality of inclusions in the molten steel before addition of Ca within one charge, a predetermined amount of Ca-containing metal is added during subsequent Ca treatment. Even when added, it has been found that the composition of oxide inclusions in the molten steel in the tundish stage also varies. And when the dispersion | variation in the composition of the inclusion mentioned above arises, it turned out that nozzle obstruction | occlusion generate | occur | produces or it cannot respond to the request | requirement of more stringent SSC resistance.

As a result of detailed investigations by the present inventors, the higher the Ca concentration in the molten steel after the vacuum degassing treatment and before the Ca treatment, the higher the Ca content during the subsequent Ca treatment, the final intervention. It was confirmed that the composition composition was likely to vary, and the probability that giant inclusions of 200 μm or more were observed on the slab increased.

As for the manufacturing process, in contrast to the killed steelmaking process in which a deoxidizer such as Si or Al is added in the converter, a rimmed steel that adds a deoxidizer such as Si or Al to the ladle after the converter. The above-mentioned problem is more likely to occur in the case of the steel process. Furthermore, the above problem is noticeable when the FeSi alloy is added to adjust the Si component in the latter half of the ladle refining (LF) or vacuum degassing treatment. Was confirmed to occur. In these cases, analysis results were obtained in which the Ca concentration in the molten steel after the vacuum degassing treatment and before the Ca treatment was increased to about 5 to 10 ppm.

On the other hand, the present inventors have (1) performing deoxidation treatment not with rimmed steel but with killed steel, and (2) adding a deoxidizer and adding Al after adding Si. (3) When additional Si is added to adjust the ingredients, it should be done by the first half of ladle refining and not in the latter half of ladle refining and vacuum degassing process. By satisfying (A) the Ca concentration in the molten steel from the converter to the vacuum degassing step can be continuously maintained at a low concentration of 4 ppm or less, (B) the variation in inclusion composition after the addition of Ca is suppressed, The inclusion composition can be controlled within the liquid phase composition range of 1600 ° C., and (C) there can be obtained an effect that there are few large inclusions having a diameter of 5 μm or more in the molten steel after the addition of Ca. Prevents nozzle clogging and provides better SSC resistance It found that it becomes possible to produce a high cleanliness steel.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
[1] A step of adding Al after adding Si to the molten steel in a converter and subjecting the molten steel to deoxidation treatment;
A ladle refining step of adding a flux containing CaO to the molten steel and subjecting the molten steel to a desulfurization treatment using a ladle furnace;
Then, a step of vacuum degassing the molten steel with a vacuum degassing device,
Thereafter, a step of adding a Ca-containing metal to the molten steel;
Then, the step of continuously casting the molten steel,
Have
Do not add Si to the molten steel in the ladle refining process,
When adding additional Si for adjusting the composition of the molten steel, it is added to the first half of the ladle refining process, and the second half of the ladle refining process and the vacuum degassing process. A method for producing high cleanliness steel, characterized in that it is not added during the period.

[2] The method for producing a high cleanliness steel according to [1], wherein the addition of the additional Si is performed within 10 minutes from the start of the ladle refining process.

[3] The method for producing a high cleanliness steel according to the above [1] or [2], wherein an interval between Si addition and Al addition in the deoxidation treatment is 1 minute or more and 10 minutes or less.

[4] The Ca concentration in the molten steel after the vacuum degassing treatment and before the addition of the Ca-containing metal is 0.0004 mass% or less,
The method for producing a high cleanliness steel according to any one of claims 1 to 3, wherein an addition amount of the Ca-containing metal is set so as to satisfy the following formula (1).
1.00 ≦ {[% Ca] − (0.18 + 130 × [% Ca]) × [% O]}
/1.25/[% S] ≦ 2.00 (1)
Here, [% Ca], [% O], [% S]: Concentration (mass%) of each element in the molten steel in the tundish
It is.

According to the present invention, it is possible to prevent clogging of the immersion nozzle of the continuous casting equipment, and to manufacture a high cleanliness steel having superior resistance to sulfide stress corrosion cracking (SSC resistance).

(A) is a manufacturing process flowchart of the manufacturing method of the high cleanliness steel by one Embodiment of this invention, (B) is a manufacturing process flowchart of the manufacturing method of the high cleanliness steel by a comparative example. It is an example of Ca concentration transition in the molten steel in the manufacturing processes of Comparative Methods 1 and 2 and the method of the present invention. (A) is an investigation of the average composition of CaO—MgO—Al ₂ O ₃ inclusions in a plurality of charges in molten steel samples taken after RH treatment and before Ca addition in Comparative Methods 1 and 2 and the method of the present invention. The results (B) are the results of investigating the average composition of CaO—MgO—Al ₂ O ₃ inclusions in molten steel samples collected after the addition of Ca in each charge of (A). In Comparative Methods 1 and 2 and the method of the present invention, the number of CaO—MgO—Al ₂ O ₃ inclusions having a diameter of 5 μm or more in the molten steel sample collected after the addition of Ca was investigated. It is a graph which shows the relationship between an atomic concentration ratio (ACR) parameter | index and a stress corrosion cracking (SSC) test rejection rate.

In one embodiment of the present invention, a method for producing a high cleanliness steel includes a step of adding a deoxidizer to molten steel in a converter and deoxidizing the molten steel, and a flux containing CaO in the molten steel. And a ladle refining step for desulfurizing the molten steel using a ladle furnace, a step for subjecting the molten steel to vacuum degassing using a vacuum degassing device, and then a Ca-containing metal in the molten steel. And a step of continuously casting the molten steel.

As the deoxidation treatment, for example, as shown in FIG. 1A, a killed steel treatment in which a deoxidizer such as Si or Al is added in a converter, and for example, as shown in FIG. There is a rimmed steel treatment in which a deoxidizer such as Si or Al is added during ladle refining after furnace refining or during vacuum degassing. In this embodiment, killed steel treatment is employed as the deoxidation treatment. In the case of the rimmed steel treatment, as described later, the Ca concentration in the molten steel cannot be reduced to 0.0004% or less between the converter and the vacuum degassing treatment, and the inclusion composition after the addition of Ca is 1600. The liquid phase composition range cannot be controlled within the temperature range, and many large inclusions having a diameter of 5 μm or more are generated. This causes a problem of nozzle clogging and a problem that sufficient SSC resistance cannot be obtained. The deoxidation treatment can be performed by a general method of adding a deoxidizer such as Si or Al to molten steel. The deoxidation product formed by the deoxidation treatment is Al ₂ O ₃ (alumina).

In this embodiment, it is important to add Al after the addition of Si as the order of addition of the deoxidizer during the deoxidation treatment. When Si is added after the addition of Al, or when Al and Si are added simultaneously, the Ca concentration in the molten steel cannot be reduced to 0.0004% or less between the converter and the vacuum degassing treatment. Further, the inclusion composition after addition of Ca cannot be stably controlled within the 1600 ° C. liquid phase composition range, and many large inclusions having a diameter of 5 μm or more are generated. This causes a problem of nozzle clogging and a problem that sufficient SSC resistance cannot be obtained.

The interval between Si addition and Al addition in the deoxidation treatment is not particularly limited, but is preferably 1 minute or more and 10 minutes or less. If the interval is less than 1 minute, the effects of the present invention may not be sufficiently obtained, and if the interval exceeds 10 minutes, it may grow into a giant SiO ₂ —MnO (—CaO) oxide. Because there is.

The ladle refining process includes a heating and stirring process in which gas is introduced into the molten steel while the molten steel is heated by arc discharge using a ladle furnace (LF). A flux containing CaO is added to the molten steel to perform a desulfurization treatment. As the flux, quick lime (CaO) alone or a mixture of quick lime and Al ₂ O ₃ or SiO ₂ which is a hatching accelerator of CaO can be used. The vacuum degassing treatment can be performed using a general apparatus such as an RH vacuum degassing apparatus, for example. The processing time for the ladle refining process and the vacuum degassing process is not particularly limited, and may be appropriately determined according to the O and S contents before the treatment with respect to the target O and S contents. The processing time of the pot refining process is about 30 to 60 minutes, and the processing time of the vacuum degassing process is about 10 to 40 minutes.

The composition of the molten steel is finally adjusted to the target component composition by adding the alloy in the vacuum degassing process, but a large amount of Mn and Si components are added at the same time as the deoxidizer Al. Thereafter, it is generally added in several times so as to become a target component until ladle refining or vacuum degassing treatment. On the other hand, in this embodiment, when additional Si is added for component adjustment, it is performed before the first half of the ladle refining process and not performed in the second half of the ladle refining process and the vacuum degassing process. is important. It is also preferable not to add additional Si after the ladle refining process so that the target Si content is satisfied only by Si addition performed before Al addition in the converter. Thereby, the Ca concentration in the molten steel from the converter to the vacuum degassing step can be continuously reduced to a low concentration of 4 ppm or less, and it becomes possible to produce a high cleanliness steel having better SSC resistance.

From the viewpoint of obtaining the effect of the present invention more reliably, when adding additional Si, it is preferable to carry out within 10 minutes from the start of the ladle refining process.

After the vacuum degassing treatment, add Ca-containing metal to the molten steel. Although the Ca addition method is not particularly defined, a method of adding a massive alloy having a content of Ca: 70% by mass and Si: 30% by mass or a wire wrapped with Fe hoop into molten steel is generally used. Yes. Since the Ca alloy reacts violently with the molten steel, it is easy to form molten steel re-oxide during the addition, and it is preferable to complete the argon seal during the addition.

In addition, Ca addition after the vacuum degassing treatment may be performed subsequently to the vacuum degassing treatment in the ladle of the RH vacuum degassing apparatus, but after transferring the molten steel to a ladle dedicated for Ca treatment separately, It is preferable to add Ca to molten steel in the ladle.

Hereinafter, experimental examples that have led to the completion of the present invention will be described.
(Method of the present invention)
In the process shown in FIG. 1 (A), the chemical composition C of the molten steel in the tundish C: 0.2-0.3%, Si: 0.22-0.27%, Mn: 0.4-0.6% , P: 0.005-0.009%, S: 0.0005-0.002%, sol. Steel with Al: 0.03-0.1%, Ca: 0-0.003%, O: 0.0010-0.0020%, balance: Fe and inevitable impurities was melted. The converter treatment time was 60 minutes. When 50 minutes passed, 2.2 kg / ton-steel FeSi alloy was added, and after 5 minutes, 3.5 kg / ton-steel Al was added. The treatment time of the LF process was 30 minutes, and after 10 minutes, 1.8 kg / ton-steel FeSi alloy was added as additional Si for component adjustment. Si was not added in the latter half of the LF process and in the RH process. After the RH process, Ca was added to the molten steel.

(Comparison method 1)
Steel was melted in the same manner as in the method of the present invention except that the order of Al addition and Si addition in the converter was reversed. That is, in Comparative Method 1, the converter processing time was set to 60 minutes, 3.7 kg / ton-steel Al was added after 50 minutes, and 2.2 kg / ton-steel FeSi alloy was added 3 minutes later. .

(Comparison method 2)
In the process of rimmed steel as illustrated in FIG. 1 (B), the chemical composition of molten steel in the tundish C: 0.2-0.3%, Si: 0.22-0.27%, Mn: 0 4-0.6%, P: 0.005-0.009%, S: 0.0005-0.002%, sol. Steel with Al: 0.03-0.1%, Ca: 0-0.003%, O: 0.0010-0.0020%, balance: Fe and inevitable impurities was melted. That is, Si and Al were not added as deoxidizers in the converter. Thereafter, the processing time of the LF process was 45 minutes, and after 5 minutes, 2.2 kg / ton-steel FeSi alloy as deoxidizer Si was charged simultaneously with 3.5 kg / ton-steel Al. Further, an additional FeSi alloy was added for component adjustment at a timing of 2 minutes from the start of the RH treatment. After the RH process, Ca was added to the molten steel.

The inventors of the present invention collected molten steel samples for each of the manufacturing processes and investigated the molten steel composition, the amount of inclusions, and the inclusion composition. The component analysis of the molten steel was performed by cantback rapid analysis. The inclusion investigation was carried out using a PSEM apparatus manufactured by ASPEX. Specifically, first, a molten steel sample was taken from a depth position of 2 m or more from the bath surface, and resin embedding and polishing were performed to prepare a sample for SEM observation. The sample was subjected to SEM observation, and the composition was determined by EDX for all inclusions having an inclusion diameter of 5 μm or more in a 15 × 15 mm visual field, and the average was calculated. In addition, when the inclusion cross-sectional shape has anisotropy, the square diameter of the product of the major axis and the minor axis of the ellipse surrounding the section was taken as the inclusion diameter.

Regarding the inclusion composition, the reaction of the oxide and the element entering from the slag with the deoxidizer (Al, Si, Mn, etc.), and the strong deoxidation element (Ca, Mg, Ti, etc.) contained in the alloy Changes under the influence. In addition, Ca treatment is finally performed for the purpose of suppressing MnS inclusions generated during solidification, and an oxide or CaS-based sulfide having a high CaO content is formed.

The findings investigated by the present inventors have confirmed that the inclusion composition largely changes as follows.
(1) Before Al addition: Fe ₂ O ₃ (+ MnO + SiO ₂ + CaO...)
(2) After Al addition: Al ₂ O ₃ inclusions (3) During desulfurization treatment by adding CaO flux: MgO—Al ₂ O ₃ inclusions (4) Si addition: CaO—Al ₂ O ₃ inclusions increase (5) After ladle refining (LF), after vacuum degassing (RH): CaO—MgO—Al ₂ O ₃ inclusions (6) After Ca treatment: CaO—Al ₂ O ₃ inclusions + CaS

Regarding the above inclusion composition, (3) MgO / Al ₂ O ₃ inclusions are formed by reacting Mg melted in the slag from refractories during the desulfurization treatment by adding CaO-based flux. It has been known.

(4) Addition of Si is generally performed by adding an FeSi alloy to adjust the Si component. A general FeSi alloy inevitably contains about 0.3 to 1.5% of the Ca component, and a small amount of Ca component is added to the molten steel due to the addition of Si. CaO—Al ₂ O ₃ System inclusions will be generated. Further, as another Si addition method, it is also possible to add an alloy such as a SiMn alloy or Si scrap within a range not exceeding the allowable amount of other components including Mn.

In addition, although the Ca addition process is not required, in the production of high-tensile steel (High Ten) that implements the LF process to reduce S, for example, converter → LF process → RH process → tundish → mold Through a refining process. Thus, by controlling similarly to the present invention the timing of addition of FeSi alloy, it is possible to suppress the generation huge CaO · Al ₂ O ₃ inclusions in the steel after continuous casting.

(5) after ladle refining (LF), inclusions after vacuum degassing (RH) is, MgO · _Al 2 _{O 3} inclusions and CaO · _Al 2 _{O 3} inclusions described above Irimajiri CaO-MgO-Al ₂ O ₃ -based inclusions were present, and as described later, it was confirmed that the composition had considerable variations.

(6) The Ca treatment is carried out mainly by introducing a CaSi alloy into the ladle after vacuum degassing, and is generally introduced so that Ca becomes 10 ppm or more in the molten steel. The CaO—MgO—Al ₂ O ₃ inclusions described above become CaO—Al ₂ O ₃ inclusions or CaS sulfides with a small MgO content.

In order to avoid Ca contamination when adding the FeSi alloy described above, it is effective to use an alloy having a low Ca content of 0.1 to 0.2% called high-purity FeSi, but it is very expensive. In other words, the alloy types that can be used in production are limited. In the present invention, a method having a great effect can be provided without using high-purity FeSi.

FIG. 2 shows the transition of Ca concentration in molten steel in Comparative Methods 1 and 2 and the method of the present invention. For Comparative Method 1, the average value of 22 charges was plotted, and for the method of the present invention and Comparative Method 2, the average value of 5 charges was plotted. As is clear from FIG. 2, in the method of the present invention, the Ca analysis value before the addition of Ca is as low as 4 ppm or less, whereas in Comparative methods 1 and 2, the Ca analysis value varies greatly from 5 to 15 ppm. We were able to confirm that

The reason why the Ca concentration is low when adding a deoxidizer in the order of Si and Al in killed steel is not clear, but when FeSi is added in a high oxygen concentration state that is hardly deoxidized, it is oxidized. It is thought that Ca, which has a strong property and easily evaporates, is oxidized on the surface of the molten steel at the moment of addition and remains on the surface of the molten steel or is evaporated and discharged out of the system. On the other hand, when FeSi is added after Al addition or simultaneously with Al, oxygen in the steel is rapidly reduced by Al deoxidation, and Ca becomes Al ₂ in a state where Al ₂ O ₃ inclusions are generated. This is considered to be due to reaction with O ₃ inclusions and stable existence as CaO · Al ₂ O ₃ inclusions. In addition, FeSi is often added several times during rim refining (LF) and vacuum degassing (RH) for component adjustment, and a small amount of Ca component is mixed into the molten steel each time. It seems to be a thing.

Furthermore, the present inventors compared the molten steel after RH processing and before Ca addition with the molten steel after Ca addition regarding the average composition of all inclusions having a diameter of 5 μm or more. FIG. 3 (A) is a result of investigating the average composition of CaO—MgO—Al ₂ O ₃ inclusions in molten steel samples collected after RH treatment and before Ca addition by multiple charges, and FIG. 3 (B) FIG. 3A is a result of investigating the average composition of CaO—MgO—Al ₂ O ₃ inclusions in a molten steel sample collected by tundish after adding Ca in each charge of FIG.

In any charge, the amount of Ca added was determined so that the inclusion composition after Ca addition would be in the liquid phase range at 1600 ° C. in the tundish stage, and Ca addition was performed. However, as shown in FIGS. 3 (A) and 3 (B), in the case of rimmed steel (Comparative Method 1), the average composition of inclusions before Ca treatment is caused by Ca that is considered to be caused by the FeSi alloy. It can be seen that the inclusion composition already contains many CaO—Al ₂ O ₃ . In addition, even when Si is added after addition of Al in killed steel (Comparative Method 2), the change to CaO—Al ₂ O ₃ inclusions progresses before Ca treatment, and the inclusion composition has a large variation. It could be confirmed.

On the other hand, in the case of killed steel obtained by adding Al after Si addition according to the method of the present invention, the inclusion composition before Ca addition is 10 to 20 wt% CaO mainly composed of MgO—Al ₂ O ₃ component. It was confirmed that the composition was uniform with very little variation. As a result, the inclusion composition of the sample collected from the tundish after the Ca treatment could be controlled to the 1600 ° C. liquid phase range. On the other hand, in Comparative Methods 1 and 2, it was found that CaO—Al ₂ O ₃ inclusions having a high CaO composition with a large compositional variation and a high melting point were generated.

Here, the purpose of setting the average composition of inclusions in the tundish stage to the 1600 ° C. liquid phase range is as follows.
(1) When CaO—Al ₂ O ₃ inclusions (3CaO · Al ₂ O ₃ to CaO + CaS) with high CaO concentration accompanied by CaS precipitation at the molten steel stage, the temperature drops in the subsequent tundish to mold immersion nozzle Sometimes nozzle clogging due to CaS is likely to occur. In addition, the inclusions that have become large due to aggregation drop off from the nozzle adhering location and are taken into the slab, and the deterioration of HIC resistance and SSC resistance becomes significant.
(2) CaO—Al ₂ O ₃ inclusion composition (especially CaO · 6Al ₂ O ₃ to CaO) in which the average composition of inclusions in the molten steel stage is lower than the liquid phase inclusion composition (1600 ° C. liquid phase range)・ When 2Al ₂ O ₃ ), nozzle clogging easily occurs. Further, harmful MnS is likely to precipitate during solidification, and the deterioration of HIC resistance and SSC resistance becomes remarkable.
Therefore, it is important to control the inclusion composition of CaO · Al ₂ O ₃ to 3CaO · Al ₂ O ₃ , preferably 12CaO · 7Al ₂ O ₃ inclusion.

Moreover, the result of investigating the inclusion cleanliness of the sample collected with the tundish used in FIG. 3 (B) is shown in FIG. It was confirmed that the number of inclusions having a diameter of 5 μm or more was significantly improved in the method of the present invention compared to Comparative Methods 1 and 2. In the method of the present invention, the average composition of inclusions after the addition of Ca could be controlled within the liquid phase range of 1600 ° C., which is considered to be due to the progress of inclusion floating removal.

Next, the appropriate range of the Ca addition amount during the Ca treatment was determined by investigating the Ca addition conditions and the results of the sulfide stress corrosion cracking (SSC) test in advance.

In the method of the present invention, the relationship between the atomic concentration ratio (ACR value) in the tundish when Ca is added after the vacuum degassing treatment (RH) and the rejection rate of the SSC test is shown in FIG. In the SSC test, a test piece having a hardness of HRC = 27 was given a stress of 85% of the minimum yield stress in a NACE test solution saturated with 1 atm of hydrogen sulfide, and a uniaxial tensile test was performed for 720 hours. Carried out. In the SSC test, the test piece that was broken in the middle of 720 hours was regarded as a failure. In the case of the above failure, breakage in a relatively short time from the start of the test to several tens of hours (short-time breakage type) is the main, and when the fracture surface is confirmed, a huge CaO-Al extended to several hundred μm ₂ O ₃ inclusions and CaS inclusions were observed.
The atomic concentration ratio (ACR) was defined by the following formula.
ACR = {[% Ca] − (0.18 + 130 × [% Ca]) × [% O]}
/1.25/[%S]
[% Ca], [% O], [% S]: Concentration (mass%) of each element in the molten steel in the tundish

The ACR value is an index used to control the composition of MnS sulfide crystallized during solidification, CaS sulfide generated when Ca is excessively added, CaO oxide, and calcium aluminate inclusion (CaO—Al ₂ O ₃ ). It is. In general, it is known that ACR ≧ 1.0 is effective in suppressing MnS sulfide generation, and that ACR ≦ 3.0 can suppress CaO—CaS inclusion generation caused by excessive Ca addition. Yes.

However, when the present inventors advanced a detailed evaluation with a pipe having a strength of 110 psi (760 MPa) or more, as shown in FIG. 5, the SSC test rejection rate rapidly increases in the range of ACR> 2.00. It was confirmed. This result also confirms that stress corrosion cracking (SSC) occurs due to CaO—Al ₂ O ₃ inclusions and CaS having a melting point higher than the liquid phase at the 1600 ° C. molten steel stage described above. Thus, the effectiveness of setting the Ca treatment condition in the range of ACR = 1.00 to 2.00 could be confirmed.

According to the present invention described above, the composition of the MgO—CaO—Al ₂ O ₃ inclusions before Ca addition is controlled to a state with less variation, and the subsequent oxide composition and sulfide composition are controlled more accurately. It becomes possible. It is also possible to prevent clogging due to inclusions in the tundish immersion nozzle and to sufficiently suppress the formation of inclusions such as oxides and sulfides that are harmful to the SSC resistance. By applying the present invention, it is possible to manufacture a steel pipe excellent in SSC resistance without blocking due to inclusions in the immersion nozzle, and it is possible to achieve manufacturing cost reduction and yield stabilization.

Chemical composition of molten steel in tundish C: 0.2-0.3%, Si: 0.22-0.27%, Mn: 0.4-0.6%, P: 0.005-0.009 %, S: 0.0005-0.002%, sol. Al: 0.03-0.1%, Ca: 0-0.003%, O: 0.0010-0.0020%, balance: Fe and unavoidable impurities are melted, and the slab size is 210Φ. Casting was performed with a round billet continuous casting machine.

Table 1 shows the form of steel exit (killed steel and rimmed steel), the time of FeSi alloy addition, the Ca concentration in the molten steel before Ca treatment, the molten steel composition in the tundish after Ca treatment, and the ACR value. The converter processing time was 60 minutes. In the case of killed steel, Si and Al were added to the molten steel in the converter for deoxidation treatment. The order of addition is shown in Table 1. In the example in which Al was added after FeSi was added, 2.2 kg / ton-steel FeSi alloy was added after 50 minutes from the start of the converter process, and 3.5 kg / ton-steel Al was added 5 minutes later. did. In the example in which FeSi was added after the addition of Al, 3.7 kg / ton-steel of Al was added 50 minutes after the start of the converter process, and 2.2 kg / ton-steel of FeSi alloy was added 3 minutes later. . In the case of rimmed steel, a deoxidizer was not added in the converter, and Si and Al were added 5 minutes after the start of the LF treatment to perform a deoxidation treatment.

Next, a CaO—Al ₂ O ₃ —SiO ₂ flux was added to the molten steel, and a ladle refining process (desulfurization treatment) using LF was performed. The processing time of the LF process was 45 minutes. In FIG. 1, in the example where Si was added in the “first half of LF”, Si was added 5 minutes after the start of the LF treatment. Further, in the example in which Si was added in the “LF second half”, Si was added 30 minutes after the start of the LF treatment.

Next, vacuum degassing processing was performed using an RH vacuum degassing apparatus. Next, the molten steel was transferred to another ladle, and Ca was added to the molten steel. Thereafter, the molten steel was transferred from the ladle to the tundish, and continuous casting was performed to obtain a slab.

<SSC resistance evaluation>
In the SSC resistance test, a uniaxial tensile test was performed for 720 hours by applying a stress of 85% of the minimum yield stress to the sample in a NACE test solution saturated with hydrogen sulfide at 1 atm. The sample used for the SSC test was heat treated to have a hardness of HRC = 27. The SSC test was carried out with 6 samples for each condition, and the ratio of the number of samples that could pass the test without breaking with respect to the expiration time of 720 hours is shown in Table 1 as the pass rate. When the pass rate is 100%, the SSC resistance is judged good.

<Nozzle blockage determination>
As a method for determining nozzle clogging, the clogging situation was determined from the opening (hereinafter referred to as SN opening) of the sliding nozzle above the immersion nozzle for pouring molten steel into the mold from the tundish. That is, when the cross-sectional area of the flow path of the immersion nozzle becomes smaller due to the blockage, the SN opening degree approaches 100% by the automatic control function of the mold surface level. Under the present casting conditions, the operation with the SN opening of 60 to 70% is a stable casting state, but when the nozzle clogging occurs, the SN opening rapidly increases to 80 to 100%. Therefore, it was determined that the nozzle blockage occurred when the SN opening was 80% or more.

Levels A, B, and C satisfied all the conditions of the present invention, and the SSC resistance and the degree of blockage of the immersion nozzle were good. Level D is an invention example where ACR value is below the lower limit of the preferred range, the clogging of the immersion nozzle due to the inclusions of CaO weight ratio lower high melting _{CaO · 6Al 2 O 3 ~ CaO} · 2Al 2 O 3 composition The SSC test results were slightly worsened. Level E is an invention example in which the ACR value exceeded the upper limit of the preferred range, and the SSC test result decreased to 50% (3 out of 6 samples were broken) due to an increase in CaO-CaS inclusions.

Level F is a comparative example in which the FeSi addition timing does not satisfy the conditions of the present invention, and the Ca concentration before Ca treatment also exceeded the upper limit of the preferred range, so the SSC test result was 33% (4 out of 6 samples were Rupture). Level G was the same as Level F. Levels H to L are rimmed steel (non-deoxidized steel), a comparative example that does not satisfy the FeSi addition timing, and the Ca concentration before Ca treatment is high, so the SSC test result was low. Level M is a comparative example in which the order of introduction of FeSi and Al does not satisfy the conditions of the present invention, and the Ca concentration before Ca treatment is also high, so the results of the SSC test did not reach levels A, B, and C.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to prevent obstruction | occlusion of the immersion nozzle of a continuous casting installation, and to manufacture the high cleanliness steel which has the more excellent SSC resistance.

Claims (4)

Adding Al after adding Si to the molten steel in the converter, and deoxidizing the molten steel;
A ladle refining step of adding a flux containing CaO to the molten steel and subjecting the molten steel to a desulfurization treatment using a ladle furnace;
Then, a step of vacuum degassing the molten steel with a vacuum degassing device,
Thereafter, a step of adding a Ca-containing metal to the molten steel;
Then, the step of continuously casting the molten steel,
Have
Do not add Si to the molten steel in the ladle refining process,
When adding additional Si for adjusting the composition of the molten steel, it is added to the first half of the ladle refining process, and the second half of the ladle refining process and the vacuum degassing process. A method for producing high cleanliness steel, characterized in that it is not added during the period. The method for producing a high cleanliness steel according to claim 1, wherein the addition of the additional Si is performed within 10 minutes from the start of the ladle refining process. The method for producing high cleanliness steel according to claim 1 or 2, wherein an interval between Si addition and Al addition in the deoxidation treatment is 1 minute or more and 10 minutes or less. The Ca concentration in the molten steel after the vacuum degassing treatment and before the Ca-containing metal addition is 0.0004 mass% or less,
The method for producing a high cleanliness steel according to any one of claims 1 to 3, wherein an addition amount of the Ca-containing metal is set so as to satisfy the following formula (1).
1.00 ≦ {[% Ca] − (0.18 + 130 × [% Ca]) × [% O]} / 1.25 / [% S] ≦ 2.00 (1)
Here, [% Ca], [% O], [% S]: Concentration (mass%) of each element in the molten steel in the tundish
It is.

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