JP2003247047A - Electric resistance welded tube and production method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric resistance welded tube which has excellent toughness in an electric resistance weld zone. <P>SOLUTION: (1) The electric resistance welded tube has a composition mainly containing C, Si, Mn and Al, and the balance Fe with impurities of ≤0.020% P, ≤0.006% S, ≤0.0150% N and ≤0.0010% Ca, and in which Mn/Si lies within the range of 3 to 15, and the content of a CaO component included in CaO-Al<SB>2</SB>O<SB>3</SB>based inclusions is ≤30 mass% on the average. (2) In the method of producing an electric resistance welded tube, a cast slab subjected to continuous casting in such a manner that a slab continuous casting machine in which the length of a vertical part below a meniscus is ≥2 m is used, the casting rate is controlled to 0.6 to 1.5 m/min, the mean molten steel residence time in a tundish is controlled to ≥3 min, the flow rate of gaseous Ar blown into an immersion nozzle is controlled to ≤10 L/min, and the content of S contained in mold powder is controlled to ≤1.0 mass%, is hot-rolled, and, if required, is subjected to cold rolling, and the obtained steel sheet is subjected to electric resistance welding. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric resistance welded steel pipe and a method for manufacturing the same, and more particularly to an electric resistance welded steel pipe having excellent toughness at an electric resistance welded portion and a method for manufacturing the same.

[0002]

2. Description of the Related Art Electric resistance welded steel pipes are used for members requiring extremely high safety such as automobile parts. However, if the toughness of the electric resistance welded portions is insufficient, cracks occur at the electric resistance welded portions. There is a risk. Conventionally, the following techniques for improving the toughness of an electric resistance welded portion have been disclosed.

Japanese Patent Laid-Open No. 54-112369 discloses S and O.
Content of 0.002% or less and 0.004% or less, respectively.
A high toughness electric resistance welded steel pipe limited to 009% or less is disclosed. Japanese Unexamined Patent Publication No. 61-221354 discloses high-strength electric resistance welding having excellent chemical resistance and low temperature toughness of the electric resistance welded portion in which Ti oxide having a size of 0.01 to 50 μm is uniformly dispersed and contained. A hot rolled steel material for steel pipe is disclosed. Further, in Japanese Patent Laid-Open No. 63-97371, during the electric resistance welding of steel in which the Al content is limited to 0.025% or less, an oxidation that may become an intragranular ferrite generation nucleus with a particle size of 0.001 to 50 μm at the electric resistance abutting portion. There is described a method for producing an electric resistance welded steel pipe having excellent toughness at an electric resistance welded portion in which the material powder is blown together with a carrier gas to remain in the electric resistance welded portion.

However, in the inventions described in these publications, the SiO ₂ and MnO oxides of welding defects (penetrator), and CaO − which can become plate-like inclusions during welding.
Since the generation of Al ₂ O ₃ -based inclusions cannot be suppressed, the toughness of the electric resistance welded portion deteriorates due to these oxides and inclusions.

Japanese Unexamined Patent Publication (Kokai) No. 9-31595 discloses that the chemical composition and [C
Disclosed is a steel for corrosion-resistant electric resistance welded steel pipe, characterized in that a] / [O] is in the range of 1.5 to 2.0, and a method for producing the same.
However, inclusions are unevenly distributed, so if Ca is added, [Ca]
The value of / [O] varies depending on the position even within the same heat. This is because [Ca] is a value calculated from the concentration of inclusions such as CaO and CaS, and [O] is a value calculated from the concentration of inclusions such as Al ₂ O ₃ and CaO. .
Therefore, even in the same heat, the whole [Ca] /
It is difficult to control [O] within the range of 1.5 to 2.0.

[0006]

As described above, even if the size and the number of inclusions in the base metal are specified, depending on the chemical composition contained in the steel and the manufacturing conditions, heating or upsetting during welding may occur. , Penetrator is generated at the electric resistance weld. This penetrator is mainly present in the base metal
MnO-based and SiO formed by Mn and Si being oxidized during welding
A inclusions ₂ system, unless reduced penetrator mainly these MnO system and inclusions SiO ₂ system, it is impossible to have excellent toughness in electric resistance welding unit.

Generally, Ca treatment is performed in the steelmaking stage for the purpose of reducing the Al ₂ O ₃ type inclusions and improving the cleanliness of the steel material. , CaO-Al ₂ O ₃ type inclusions having a low melting point composition remain in the steel material. This is, for example, without Ca treatment,
It is a phenomenon that occurs when a steel plate is obtained by melting scraps containing Ca.

FIG. 1 shows the low melting point Ca remaining in the steel sheet.
O-Al ₂ O ₃ inclusions is a schematic view showing a process of deformation of.
(a) is a schematic view of a general electric resistance welded steel pipe manufacturing apparatus, (b) is an enlarged view of the vicinity of the welding point of the electric resistance welded steel pipe, (c) is an unwelded portion, a minute upset portion and a welding completed portion. Low melting point Ca
The form of the O-Al ₂ O ₃ inclusions is a diagram schematically showing.

As shown in FIG. 1 (c), the CaO-Al ₂ O ₃ -based inclusions having a low melting point composition in the base metal are deformed into a plate shape by heating during welding or upsetting, so that electric resistance welding is performed. It deteriorates the soundness (safety) such as the toughness of the part. Such a phenomenon occurs only in CaO—Al ₂ O ₃ inclusions having a low melting point composition. Therefore, even if inclusions other than the low melting point composition are present in the welded portion, this inclusion remains spherical even during welding and does not deform into a plate shape. It does not degrade soundness.

The present invention is characterized in that (1) a penetrator formed during electric resistance welding and (2) CaO-Al ₂ O ₃ type inclusions having a low melting point composition in the base metal are deformed by heating and upsetting during welding, An object of the present invention is to provide an electric resistance welded steel pipe having excellent toughness of the electric resistance welded portion by reducing the plate-like inclusions remaining in the sewn welded portion and the vicinity thereof. In addition, the present invention is concerned with an increase in Al ₂ O, which may be caused by reducing the Ca content as much as possible.
It is an object of the present invention to provide a method for manufacturing an electric resistance welded steel pipe capable of reducing ₃ type inclusions.

[0011]

DISCLOSURE OF THE INVENTION The gist of the present invention is an electric resistance welded steel pipe shown in the following and and a method for manufacturing the electric resistance welded steel pipe shown in the following.

% By mass, C: 0.04 to 0.25%, Si: 0.05
~ 0.40%, Mn: 0.30-2.00% and Al: 0.020-0.100%
Of which P is 0.020% or less, S is 0.006% or less, N is 0.0150% or less, Ca is 0.0010% or less, and Mn /
An electric resistance welded steel pipe characterized in that Si is in the range of 3 to 15 and the content of the CaO component contained in the CaO—Al ₂ O ₃ -based inclusion is 30 mass% or less on average.

% By mass, C: 0.04 to 0.25%, Si: 0.05
~ 0.40%, Mn: 0.30-2.00% and Al: 0.020-0.100%
In addition, Nb: 0.10% or less, Ti: 0.10% or less, C
u: 0.50% or less, Ni: 0.50% or less, V: 0.10% or less, M
o: 1.50% or less, Cr: 3.00% or less, and B: 0.0050% or less, and the balance is Fe and impurities. The balance is 0.020% of P in steel.
Below, S: 0.006% or less, N 0.015% or less, Ca 0.0
010% or less, Mn / Si is in the range of 3 to 15, and the content of CaO component contained in CaO-Al ₂ O ₃ -based inclusions is 30 mass% or less on average. Characteristic ERW steel pipe

In producing the steel having the chemical composition described above or in, a slab continuous casting machine in which the length of the vertical portion below the meniscus is 2 m or more is used, and the casting speed is 0.6 to 1.5 m / min. The average molten steel residence time in the tundish was 3 minutes or longer, the flow rate of Ar gas blown into the dipping nozzle was 10 liters / minute or less, and the S content in the mold powder was 1.0 mass% or less for continuous casting. The method for producing an electric resistance welded steel pipe as described in the above item 1 or 2, wherein the slab is hot-rolled and the obtained steel sheet is electric resistance welded.

If necessary, the steel sheet obtained by hot rolling and then cold rolling may be subjected to electric resistance welding.

[0016]

First, the reasons for limiting the chemical composition and inclusion composition of the electric resistance welded steel pipe of the present invention will be described. In addition,
In the following description,% of each component means mass%.

C: 0.04 to 0.25% C is effective in improving the strength of the steel pipe, and its content must be 0.04% or more. However, if the content exceeds 0.25%, the toughness of the electric resistance welded portion decreases.
Therefore, the content of C is set to 0.04 to 0.25%.

Si: 0.05 to 0.40% Si is an element which is effective and important as a deoxidizing agent, and is also an element which is effective for improving strength, like Al described later. To obtain these effects, it is necessary to contain Si in an amount of 0.05% or more. However, if the content exceeds 0.40%, deterioration of weldability and welding defects such as SiO ₂
A system penetrator is likely to occur. Therefore, the content of Si is set to 0.05 to 0.40%.

Mn: 0.30 to 2.00% Mn is an element effective for improving the strength of the steel pipe.
To obtain this effect, its content must be 0.30% or more. However, if the content exceeds 2.00%, deterioration of weldability and welding defects such as MnO-based penetrators are likely to occur. Therefore, the content of Mn is 0.30 ~
It was set to 2.00%. When the Mn content exceeds 1.40%, it is desirable to perform electric resistance welding in a state of being sealed with an inert gas such as nitrogen, Ar, or He from the viewpoint of preventing the MnO-based penetrator.

Al: 0.020 to 0.100% Al is an Al formed in steel when the content is small.
_{Since the} amount of ₂ O ₃ becomes insufficient, the content of CaO component contained in CaO-Al ₂ O ₃ -based inclusions, which will be described later, cannot be reduced to 30 mass% or less on average, and Deteriorates toughness. From this viewpoint, Al is sufficient if it is contained at 0.003% or more, but if the content is less than 0.020%,
Since the amount of AlN produced in steel decreases and the microstructure becomes coarser, the toughness of the base material decreases. Therefore, Al is 0.02
It is necessary to contain 0% or more. On the other hand, its content is 0.1
If it exceeds 00%, the weldability is adversely affected. Therefore, the content of Al is set to 0.020 to 0.100%.

Mn / Si: 3 to 15 The main type of penetrator generated during welding is MnO formed by the oxidation of Mn and Si present in the base metal during welding.
And SiO ₂ -based inclusions. These inclusions are
Usually, it is harmless because it is discharged as bead scraps by upset during electric resistance welding. However, among these inclusions, those having a high melting point are difficult to be discharged, and if they remain in the electric resistance welded portion, they become defects and deteriorate the toughness.

FIG. 2 is a quasi-binary phase diagram of the MnO-SiO ₂ system. As shown in the figure, due to the eutectic reaction, when the SiO ₂ content ratio is less than 26%, the melting point becomes higher as the ratio becomes lower, and when it exceeds 43%, the ratio becomes higher. The higher the melting point, the higher the melting point. Therefore, if the eutectic composition is in the vicinity, that is, if the SiO ₂ content ratio is within the range of 26 to 43%, the melting point of the oxide is 1,350 ° C.
The following low melting point can be maintained, and the penetrator generated during electric resistance welding can be reduced. So Mn
As a result of repeated research aimed at lowering the melting point of O-based and SiO ₂ -based oxides, if the Mn / Si value is 3 to 15, the content ratio of SiO ₂ in the above oxides is 26 to 43. It was found that it can be kept within the range of%.

The electric resistance welded steel pipe of the present invention has the above chemical composition, and the balance consisting of Fe and impurities. P existing in the steel as impurities is 0.020% or less, S is 0.006% or less, and N is N.
Must be limited to 0.0150% or less and Ca to 0.0010% or less.

P: 0.020% or less, S: 0.006% or less Both P and S are usually present in the steel as impurities. However, if their contents are large, the toughness of the base material deteriorates and the pipe is formed. At times, cracks starting from inclusions tend to progress. Therefore, the content of P is set to 0.020% or less. Desirably, it is 0.015% or less. Also, the S content is 0.006%
Below. Desirably, it is 0.004% or less.

N: 0.0150% or less N is present in steel as an impurity, but its content is 0.
If it exceeds 0150%, the toughness of the base material deteriorates. Therefore, N
Content was 0.0150% or less.

Ca: 0.0010% or less Ca is not added in the present invention from the viewpoint of preventing the formation of CaO—Al ₂ O ₃ type plate inclusions during electric resistance welding. Therefore, its content is preferably 0%. However, since Ca may remain in the steel as an impurity, the allowable upper limit of its content was set to 0.0010%. In particular, 0.
It is preferably less than 0005%.

The electric resistance welded steel pipe of the present invention has Nb, Ti, Cu, Ni, V and M for the purpose of improving the strength in addition to the above chemical composition.
One or more selected from o, Cr and B may be contained. However, even if these elements are contained excessively, the effect is saturated. Therefore, the content when containing Nb 0.10% or less, the content when containing Ti 0.10% or less, the content when containing Cu 0.50% or less, the content when containing Ni 0.50% or less, V content 0.10% or less, Mo content 1.50% or less, Cr content 3.00% or less, B content B The amount was 0.0050% or less. In addition, in order to exert these effects remarkably,
Nb is 0.005% or more, Ti is 0.005% or more, Cu is 0.05% or more,
Ni is 0.05% or more, V is 0.01% or more, Mo is 0.05% or more, Cr
Is preferably 0.05% or more and B is 0.0003% or more.

Content of CaO component contained in CaO-Al ₂ O ₃ -based inclusions: 30% or less on average FIG. 3 is a diagram showing a quasi-binary phase diagram of the CaO-Al ₂ O ₃ system. . As shown in the figure, the CaO-Al ₂ O ₃ -based inclusions have a CaO content of around 50% and undergo a eutectic reaction at 1,380 ° C.
It has a low melting point composition. CaO-Al _{2 with} such a low melting point
The O ₃ inclusions are deformed into a plate shape with a large surface area by heating and upsetting during electric resistance welding. The inclusions present in the portion which is melted by heating among the inclusions deformed in a plate shape as described above are discharged as bead scraps thereafter, and thus there is no problem. However, the portion that does not melt, that is, the portion that exists in the heat-affected zone, remains as bead scraps without being discharged, and thus reduces the toughness of the electric resistance welded portion of the steel pipe. Therefore, it is necessary to keep the content of the CaO component contained in the CaO-Al ₂ O ₃ -based inclusions within the range of about 50%.

By increasing the content of CaO component, CaO
The melting point of -Al ₂ O ₃ -based inclusions can be increased, but in this case, the abundance ratio of cluster-like coarse inclusions increases. Therefore, in the present invention, means for reducing the content of the CaO component was adopted.

As shown in FIG. 3 above, when the content of CaO component is within the range of about 40% or less, the CaO—Al ₂ O ₃ -based inclusions can have a high melting point composition exceeding 1,500 ° C. .
However, when the content exceeds 40%, the melting point sharply decreases. Therefore, in the present invention, in order to stably maintain the melting point of the CaO-Al ₂ O ₃ -based inclusion at a high level, the content of the CaO component is changed. We decided to limit the average to 30% or less.

In addition, "the content of CaO component is 30% or less on average" means the following method (for details, "Analytical technology in the Japanese iron and steel industry" [page 165, issued by Japan Iron and Steel Institute (1982)] or Iron and Steel vol.82 No.12 ”[Kuraho et al., Published by The Iron and Steel Institute of Japan (1996)]), and the average content of CaO components is 30% or less.

That is, bromine-methanol, which is an organic solvent mixed with halogen, is used to react with steel to decompose a steel matrix, carbides, sulfides, etc., and oxide inclusions that are hardly decomposed by halogen are left as they are. The method is generally called the halogen organic solvent method). The CaO-Al ₂ O ₃ -based inclusions thus obtained are filtered, separated and extracted as a residue. The Ca content and Al content of the extracted residue were analyzed, and converted into the content of CaO component and Al ₂ O ₃ component, and the content of CaO component contained in the CaO-Al ₂ O ₃ inclusions was analyzed. Find the amount. This operation is repeated for 10 inclusions per one test piece, and the content of each CaO component obtained is averaged.

As the ratio of the CaO component contained in the CaO-Al ₂ O ₃ -based inclusions increases, there is a risk that the oxide will decompose due to the reaction with water in methanol or the atmosphere. The filtration and extraction operations of the residue shall be performed in a glove box in an Ar gas atmosphere with the atmosphere cut off.

Next, the reasons for limiting each condition of the method for manufacturing an electric resistance welded steel pipe of the present invention will be described. This is mainly for the purpose of reducing inclusions which may be increased by not adding Ca.

Length of vertical part below meniscus: If the vertical part of 2 m or more and below meniscus does not have a sufficient length, the floating removal of inclusions in the mold and below the mold will be insufficient. The cleanliness of the steel sheet obtained from such cast pieces deteriorates. Therefore, the length of the vertical portion below the meniscus is set to 2 m or more. The upper limit of the length of the vertical portion below the meniscus is not particularly limited, but may be within the range allowed in the equipment design of the continuous casting machine.

Casting speed: 0.6 to 1.5 m / min When the casting speed is less than 0.6 m / min, the molten steel temperature in the meniscus is lowered, equiaxed crystal nuclei are generated in the meniscus, and the cast piece is accompanied by inclusions. Since the frequency of sedimentation inside the steel sheet increases, the cleanliness of the steel sheet obtained from such a slab deteriorates. On the other hand, when the casting speed exceeds 1.5 m / min, since the inclusions that are brought into the slab deep and are not floated and removed due to the discharge flow from the dipping nozzle, the steel plate obtained from such slab is cleaned. Sex deteriorates. Therefore, the casting speed is set to 0.6 to 1.5 m / min.

Average molten steel residence time in the tundish: 3 minutes or more If the average molten steel residence time in the tundish is short,
The floatation of inclusions in the tundish is not sufficiently removed, so that the cleanliness of the steel sheet obtained from such a cast piece deteriorates. Therefore, the average molten steel residence time in the tundish was set to 3 minutes or longer. The upper limit of the average molten steel retention time in the tundish is not particularly limited, but if the time is long, the molten steel temperature decreases and casting becomes difficult, so it is preferable to be about 15 minutes.

Flow rate of Ar gas blown into the immersion nozzle: 10 liter (L) / min or less If the flow rate of Ar gas blown into the immersion nozzle is large, many Ar bubbles flow into the cast slab. After the bubbles move in the slab and are captured by the solidified shell to form pinholes, they trap inclusions in the molten steel, so that the cleanliness of the steel plate obtained from such slab deteriorates. . Therefore, blow
The Ar gas flow rate was set to 10 liters / minute or less. The flow rate of Ar gas blown into the immersion nozzle may be 0 liter / min depending on the conditions such as the chemical composition of molten steel, but if the gas flow rate is too low, inclusions will adhere to the nozzle and casting will occur. Therefore, it is desirable to set it to 1 liter / min or more.

In the present invention, the flow rate of Ar gas blown into the immersion nozzle means the total flow rate of Ar gas blown through the nozzle, the stopper and the sliding gate.

S content in the mold powder:
1.0 mass% or less If the content of S contained in the mold powder is large, S moves to the interface between the powder dissolution layer and the meniscus in the mold and lowers the interfacial tension. Deteriorates cleanliness. Further, when S is transferred from the powder into the steel, the S content in the steel is increased and the toughness of the steel sheet is reduced. In order to prevent this, it is not desirable because in the refining stage, it is necessary to perform S removal in anticipation of such an increase in S content.
Therefore, the S content contained in the mold powder should be as low as possible, and the allowable upper limit was set to 1.0% by mass.

In the method of manufacturing the electric resistance welded steel pipe of the present invention,
It is excellent in cleanliness and is electric resistance welded by hot-rolling a continuously cast slab under the conditions that satisfy the above-mentioned conditions and further electric-welding a steel plate obtained by cold-rolling if necessary. It is possible to obtain an electric resistance welded steel pipe having excellent toughness of the portion. In addition, cold rolling, when producing a thin steel plate that is difficult to produce only by hot rolling, when producing a steel plate that requires high wall thickness accuracy that cannot be obtained only by hot rolling, there are restrictions on the components. This is effective when manufacturing a steel sheet that needs to have strength by plastic working by cold working.

[0042]

EXAMPLE Molten steel having the chemical composition shown in Table 1 was continuously cast under the conditions shown in Table 2, and a hot-rolled coil or a cold-rolled coil was produced from the obtained slab under normal conditions. These coils were electric resistance welded under normal conditions to prepare test steel pipes having the dimensions shown in Table 2. Table 2 shows the results of the Charpy test and the ultrasonic flaw detection test performed on the electric resistance welded portions of the respective test steel pipes.

The Charpy test was carried out according to the metal material impact test method specified in JIS Z 2242, and the transition temperature (vTrs) was investigated. The ultrasonic flaw detection test is JIS G 0582.
According to the method specified in 1), it was performed in the flaw detection sensitivity category UC.
This test was conducted in order to investigate the detergency which may be deteriorated by not adding Ca.

[0044] Here, the average value of the content of the CaO component contained in the CaO-Al ₂ O ₃ inclusions, using a halogen organic solvent method of the, CaO-Al ₂ O ₃ from each test steel pipe The system inclusions as a residue are separated by filtration in a glove box in an Ar gas atmosphere and extracted, and the Ca content and Al content of the residue are analyzed and converted into CaO and Al ₂ O ₃ content. Then, the content of the CaO component contained in the CaO-Al ₂ O ₃ inclusions is calculated, and this work is repeated for 10 inclusions per test steel pipe, and the calculated content of the CaO component is averaged. Sought by doing.

[0045]

[Table 1]

[0046]

[Table 2]

For "vTrs" in the table, a Charpy test was performed on three test steel pipes under each condition, and the condition where the vTrs value was -20 ° C or less was "◎", and -20 ° C was selected. The condition of exceeding 0 ° C was evaluated as "○", and the condition of exceeding 0 ° C was evaluated as "X". In addition, the "ultrasonic inspection result" is the number of steel pipes that have been rejected by the judgment method stipulated in JIS G 0582 for ultrasonic deep scratch inspection for 1000 or more test steel pipes for each condition. Defective rate is calculated and the defective rate is 0.1
% Is less than "◎", defective rate is over 0.1% and 0.2
If the condition was less than or equal to%, "○", the defect rate exceeds 0.2% and 0.3
The condition where the percentage was less than or equal to "%" was evaluated, and the condition where the defective rate exceeded 0.3% was evaluated as "x".

As shown in Table 2, some of Examples 1 to 9 of the present invention do not satisfy the continuous casting conditions specified in the present invention, but all have good toughness with vTrs of 0 ° C. or less. Then
It has good cleanliness with a defect rate of 0.3% or less as a result of ultrasonic inspection. Here, in Inventive Examples 6 and 7, the continuous casting conditions are out of the range specified in the present invention, but since P is 0.015% or less and S is 0.004% or less, the defective rate is reduced to 0.2% or less. did it. Further, in Inventive Examples 1 to 5, all of the chemical composition conditions, inclusion conditions and continuous casting conditions defined in the present invention were satisfied, and the defective rate was 0.2% or less. Particularly, in Inventive Examples 1 and 2, P is 0.015% or less,
Moreover, S was 0.004% or less, and the defect rate could be reduced to 0.1% or less.

In Comparative Example 10, since the content of C is out of the range specified by the present invention, vTrs is high, and the content of P is out of the range specified by the present invention. The defective rate has risen. In Comparative Example 11, the Si content was out of the range specified by the present invention, so that the SiO ₂ type penetrator in the weld zone was generated and the vTrs was increased. Comparative Example 12
In the case of S, the content of S was out of the range specified by the present invention, and thus the defective rate of the ultrasonic inspection result was increased. Comparative Examples 13 and 18, the Ca content is out of the range defined by the present invention,
Further, the content of the CaO component contained in the CaO-Al ₂ O ₃ -based inclusions also falls outside the range specified by the present invention, so that vTrs was increased.

In Comparative Example 14, the Mn content exceeded the range specified in the present invention, so that an MnO-based penetrator was generated in the electric resistance welded portion and vTrs increased. Comparative Example 15, the content of Ca is within the range specified in the present invention, the content of Al is
Not only is 0.001% below the range specified in the present invention, but A
It is a range of less than 0.003% where the amount of l ₂ O ₃ produced cannot be secured. In addition, C contained in CaO-Al ₂ O ₃ -based inclusions
Since the content of the aO component is also outside the range specified by the present invention, vTrs is increased.

In Comparative Examples 16 and 17, the respective contents of Si and Mn are within the range specified by the present invention, but in Comparative Example 16, the value of Mn / Si is within the range specified by the present invention. VTrs is increased due to the generation of the MnO-based penetrator, and in Comparative Example 17, the value of Mn / Si is below the range specified by the present invention, and therefore the vTrs is generated due to the generation of the SiO ₂ -based penetrator.
s became higher.

[0052]

According to the present invention, CaO-Al ₂ O _{3 having} a low melting point composition in the penetrator and the base metal produced during electric resistance welding.
Deforms from system inclusions by heating during welding and upsetting,
Since all the plate-like inclusions remaining in the electric resistance welded portion and in the vicinity thereof can be reduced, an electric resistance welded steel pipe having excellent toughness in the electric resistance welded portion can be obtained.

[Brief description of drawings]

FIG. 1 Low-melting-point composition CaO-Al ₂ O ₃ remaining in the steel sheet
It is a schematic diagram which shows the process of deformation | transformation of a system inclusion. (a) is a schematic diagram of a general ERW pipe manufacturing apparatus, (b) is an enlarged view of the vicinity of the welding point of the ERW pipe, (c) is a low melting point composition in the unwelded portion, the minute upset portion and the welding completed portion. FIG. 3 is a diagram schematically showing the morphology of CaO—Al ₂ O ₃ type inclusions in FIG.

FIG. 2 is a quasi-binary phase diagram of the MnO-SiO ₂ system.

FIG. 3 is a quasi-binary phase diagram of the CaO—Al ₂ O ₃ system.

[Explanation of symbols]

1. Formed steel strip; 2. Welding point, 3. Work roll,
4. Squeeze roll

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B22D 11/108 B22D 11/108 F 11/11 11/11 B 11/20 11/20 A C21C 7/076 C21C 7/076 P C22C 38/06 C22C 38/06 38/58 38/58 F term (reference) 4E004 HA01 JA10 MB05 MB14 MC05 NA01 NA02 NB01 NC01 4K013 AA09 BA05 EA00

Claims (6)

[Claims] 1. In mass%, C: 0.04 to 0.25%, Si: 0.05 to
0.40%, Mn: 0.30 to 2.00% and Al: 0.020 to 0.100%, the balance consisting of Fe and impurities, P existing in the steel as impurities 0.020% or less, S: 0.006% or less,
N is 0.0150% or less, Ca is 0.0010% or less, and Mn / Si is
An electric resistance welded steel pipe which is in the range of 3 to 15 and in which the content of the CaO component contained in the CaO-Al ₂ O ₃ -based inclusions is 30 mass% or less on average. 2. In mass%, C: 0.04 to 0.25%, Si: 0.05 to
0.40%, Mn: 0.30 to 2.00% and Al: 0.020 to 0.100%, and further Nb: 0.10% or less, Ti: 0.10% or less, Cu:
0.50% or less, Ni: 0.50% or less, V: 0.10% or less, Mo: 1.
50% or less, Cr: 3.00% or less, and B: 0.0050% or less, and the balance is Fe and impurities. The balance of P present in steel as impurities is 0.020% or less,
S: 0.006% or less, N 0.015% or less, Ca 0.0010%
Below, Mn / Si is in the range of 3 to 15, and CaO
-The electric resistance welded steel pipe characterized in that the content of CaO component contained in the Al ₂ O ₃ -based inclusions is 30 mass% or less on average. 3. In mass%, C: 0.04 to 0.25%, Si: 0.05 to
0.40%, Mn: 0.30 to 2.00% and Al: 0.020 to 0.100%, the balance consisting of Fe and impurities, P existing in the steel as impurities 0.020% or less, S: 0.006% or less,
N is 0.0150% or less, Ca is 0.0010% or less, and Mn /
When manufacturing steel with Si in the range of 3 to 15, a slab continuous casting machine with a vertical length of 2m or more below the meniscus is used and the casting speed is 0.6 to 1.5m / min. The average molten steel residence time in the interior is 3 minutes or more, the flow rate of Ar gas blown into the immersion nozzle is 10 liters / minute or less, and the content of S contained in the mold powder is 1.0 mass% or less. The method for producing an electric resistance welded steel pipe according to claim 1, wherein the steel sheet obtained by hot rolling is electric resistance welded. 4. The method for producing an electric resistance welded steel pipe according to claim 3, wherein the steel sheet obtained by hot rolling and then cold rolling is subjected to electric resistance welding. 5. In mass%, C: 0.04 to 0.25%, Si: 0.05 to
0.40%, Mn: 0.30 to 2.00% and Al: 0.020 to 0.100%, and further Nb: 0.10% or less, Ti: 0.10% or less, Cu:
0.50% or less, Ni: 0.50% or less, V: 0.10% or less, Mo: 1.
50% or less, Cr: 3.00% or less, and B: 0.0050% or less, one or more selected from the rest, Fe and impurities are the balance, and P present in the steel as impurities is 0.020% or less,
S: 0.006% or less, N 0.015% or less, Ca 0.0010%
The length of the vertical part below the meniscus is 2 m when manufacturing steel having the following values and Mn / Si in the range of 3 to 15:
Using the slab continuous casting machine that is above, the casting speed is 0.6
~ 1.5 m / min, average molten steel residence time in the tundish is 3 minutes or more, Ar gas flow rate into the dipping nozzle is 10 liters / min or less, and the S content in the mold powder is 1.0% by mass. The method for producing an electric resistance welded pipe according to claim 2, wherein the continuously cast slab is hot-rolled as described below, and the obtained steel sheet is electric resistance welded. 6. The method for manufacturing an electric resistance welded steel pipe according to claim 5, wherein the steel sheet obtained by performing hot rolling and then cold rolling is subjected to electric resistance welding.