JP2017008362A - Seamless steel pipe for line pipe and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a seamless steel pipe for linepipe excellent in toughness.SOLUTION: A linepipe seamless steel pipe has a chemical composition containing, by mass%, C:0.02 to 0.15%, Si:0.05 to 1.0%, Mn:0.30 to 2.5%, P:0.030% or less, S:0.006% or less, Al:0.001 to 0.100%, N:0.008% or less, O:0.004% or less, Ca:0.0005 to 0.0040%, Cr:0.05 to 1.0%, Mo:0.02 to 0.5%, V:0.02 to 0.20%, Ti:0% or more and less than 0.007%, Nb:0 to 0.05%, Cu:0 to 1.5%, Ni:0 to 1.5% and the balance:Fe with impurities and satisfying the following formula (1). Ca×O<0.325×10(1). The content of corresponding element in mass% is substituted in each symbol of the elements in the formula (1).SELECTED DRAWING: None

Description

  The present invention relates to a seamless steel pipe suitable for a line pipe and a method for manufacturing the same.

  In recent years, the development of oil wells and gas wells in a harsher environment than the past, represented by deep seas and cold regions, has been progressing. A line pipe laid in such a harsh environment is required to have higher strength and toughness than before.

Japanese Patent Application Laid-Open No. 2004-124158 discloses a seamless steel pipe excellent in toughness and a method for manufacturing the same. This document describes that a seamless steel pipe excellent in toughness can be obtained by limiting the maximum diameter and the number of oxide inclusions. Specifically, this document specifies that the number of oxide inclusions having a diameter exceeding 300 μm is 1 or less per 1 cm ² and the number of oxide inclusions of 5 to 300 μm is 200 or less per 1 cm ^2. Yes.

  Japanese Patent Application Laid-Open No. 2004-143593 discloses a high-strength seamless steel pipe having a yield stress of 483 MPa or more and excellent resistance to hydrogen-induced cracking, and a method for producing the same.

Japanese Patent Application Laid-Open No. 9-31525 discloses a method for producing a high-strength steel excellent in resistance to hydrogen-induced cracking. This document describes that CaO inclusions and CaO—Al ₂ O ₃ -based composite inclusions that cause hydrogen-induced cracking are reduced in melting point to form spheroidized inclusions that do not cluster.

JP 2004-124158 A JP 2004-143593 A JP-A-9-31525

  Japanese Patent Application Laid-Open No. 2004-124158 discloses that the fracture surface transition temperature vTrs of the seamless steel pipe of the same document is −40 ° C. or lower. However, even if the fracture surface transition temperature vTrs is −40 ° C. or lower, the toughness at −30 ° C. or −40 ° C. may not be sufficient.

  The objective of this invention is obtaining the seamless steel pipe for line pipes excellent in toughness, and its manufacturing method.

The seamless steel pipe for line pipe according to an embodiment of the present invention has a chemical composition of mass%, C: 0.02 to 0.15%, Si: 0.05 to 1.0%, Mn: 0.30. ~ 2.5%, P: 0.030% or less, S: 0.006% or less, Al: 0.001 to 0.100%, N: 0.008% or less, O: 0.004% or less, Ca : 0.0005 to 0.0040%, Cr: 0.05 to 1.0%, Mo: 0.02 to 0.5%, V: 0.02 to 0.20%, Ti: 0% or more. Less than 007%, Nb: 0 to 0.05%, Cu: 0 to 1.5%, Ni: 0 to 1.5%, balance: Fe and impurities, and the chemical composition is represented by the following formula (1): Fulfill.
Ca × O <0.325 × 10 ⁻⁵ (1)
In the element symbol in the formula (1), the content of the corresponding element is substituted by mass%.

In the method for producing a seamless steel pipe for line pipe according to an embodiment of the present invention, the chemical composition is mass%, C: 0.02 to 0.15%, Si: 0.05 to 1.0%, Mn: 0.30 to 2.5%, P: 0.030% or less, S: 0.006% or less, Al: 0.001 to 0.100%, N: 0.008% or less, O: 0.004% Hereinafter, Ca: 0.0005 to 0.0040%, Cr: 0.05 to 1.0%, Mo: 0.02 to 0.5%, V: 0.02 to 0.20%, Ti: 0% Or more, less than 0.007%, Nb: 0 to 0.05%, Cu: 0 to 1.5%, Ni: 0 to 1.5%, balance: Fe and billet as impurities and the billet A process of producing a steel pipe by hot working, and after hot working the steel pipe, a temperature of (Ar ₃ points + 50 ° C.) to 1100 ° C. to 5 ° C. / It comprises a step of cooling and quenching at a cooling rate of at least 2 seconds, and a step of tempering the quenched steel pipe at a temperature of 550 ° C. or more and Ac ₁ point or less, and the chemical composition satisfies the following formula (1).
Ca × O <0.325 × 10 ⁻⁵ (1)
In the element symbol in the formula (1), the content of the corresponding element is substituted by mass%.

The method of manufacturing a seamless steel pipe for line pipe according to another embodiment of the present invention has a chemical composition of mass%, C: 0.02 to 0.15%, Si: 0.05 to 1.0%, Mn : 0.30 to 2.5%, P: 0.030% or less, S: 0.006% or less, Al: 0.001 to 0.100%, N: 0.008% or less, O: 0.004 % Or less, Ca: 0.0005 to 0.0040%, Cr: 0.05 to 1.0%, Mo: 0.02 to 0.5%, V: 0.02 to 0.20%, Ti: 0 % To less than 0.007%, Nb: 0 to 0.05%, Cu: 0 to 1.5%, Ni: 0 to 1.5%, balance: Fe and billet as impurities, a step of producing a steel pipe by processing between the billet hot, the step of cooling the steel pipe to a temperature of below ₃ points Ar, the cooled steel Ac ₃ And heating to a temperature above, a step of hardening by cooling at 5 ° C. / sec or more cooling rate said heated steel tube from (Ar ₃ point + 50 ° C.) or higher 1100 ° C. or less of the temperature, the hardened steel pipe Tempering at a temperature of 550 ° C. or more and Ac ₁ point or less, and the chemical composition satisfies the following formula (1).
Ca × O <0.325 × 10 ⁻⁵ (1)
In the element symbol in the formula (1), the content of the corresponding element is substituted by mass%.

  ADVANTAGE OF THE INVENTION According to this invention, the seamless steel pipe for line pipes excellent in toughness is obtained.

FIG. 1 is a diagram for explaining cluster-like inclusions.

  The present inventors examined a method for increasing the toughness of a seamless steel pipe for a line pipe. The inventors focused on reducing the number density of fine inclusions as well as coarse inclusions.

Japanese Patent Application Laid-Open No. 2004-124158 described above describes that when an appropriate amount of Ca is added to molten steel, it reacts with Al ₂ O ₃ inclusions to produce low melting point Ca—Al—O inclusions. . The document further describes that these aggregates become large inclusions, which are effective for reduction by floating separation from the molten steel. Therefore, it is described in the same document that the value of Ca / O is preferably 0.5 to 2.0.

However, in the seamless steel pipe of the same document, although there are few coarse oxide inclusions having a diameter exceeding 300 μm, there are relatively many oxide inclusions having a diameter of 300 μm or less. According to Table 4 of the document, the maximum diameter of the oxide inclusions is 60 to 180 μm, and the number density of the oxide inclusions having a diameter of 5 to 300 μm is 60 to 179 pieces / cm ² .

The present inventors have found that it is effective to reduce the value of Ca × O to less than 0.325 × 10 ⁻⁵ in order to reduce oxide inclusions having a diameter of 300 μm or less. .

Preferably, the number density of oxide inclusions having a diameter exceeding 50 μm is set to 2 pieces / cm ² or less, and the number density of oxide inclusions having a diameter of 5 to 50 μm is set to 50 pieces / cm ² or less. As a result, excellent toughness can be obtained even at low temperatures such as −30 ° C. and −40 ° C.

  Furthermore, according to the investigation by the present inventors, it has been found that it is important to limit Ti in order to ensure toughness at a low temperature.

  Ti produces TiC and reduces the toughness of the steel. In order to obtain excellent toughness even at low temperatures, it is necessary to strictly limit the Ti content in addition to reducing the number of oxide inclusions. Specifically, the Ti content is less than 0.007%.

Furthermore, under the condition of Ca × O <0.325 × 10 ⁻⁵ , the oxide inclusions are made to have a high melting point with a Ca / O value of less than 0.5, so that aggregation and coarsening easily occur in high-temperature molten steel. Thus, floating separation becomes easy, and it is considered effective for further reduction of inclusions. Therefore, more preferably, in addition to setting the value of Ca × O to less than 0.325 × 10 ⁻⁵ , the value of Ca / O is set to less than 0.5.

  Based on the above findings, the present invention has been completed. Hereinafter, a seamless steel pipe for a line pipe according to an embodiment of the present invention will be described in detail with reference to the drawings.

[Chemical composition]
The seamless steel pipe for line pipes according to this embodiment has a chemical composition described below. In the following description, “%” of the element content means mass%.

C: 0.02-0.15%
Carbon (C) increases the hardenability of the steel and improves the strength of the steel. If the C content is less than 0.02%, the hardenability is insufficient and it is difficult to ensure high strength. On the other hand, if the C content exceeds 0.15%, the toughness of the steel decreases. Therefore, the C content is 0.02 to 0.15%. From the viewpoint of the lower limit, the C content is preferably higher than 0.02%, and more preferably 0.04% or more. In view of the upper limit, the C content is preferably less than 0.15%, more preferably 0.12% or less, and further preferably 0.08% or less.

Si: 0.05-1.0%
Silicon (Si) deoxidizes steel. If the Si content is 0.05% or more, the above-described effect is remarkably obtained. On the other hand, if the Si content exceeds 1.0%, the toughness of the steel decreases. Therefore, the Si content is 0.05 to 1.0%. From the viewpoint of the lower limit, the Si content is preferably higher than 0.05%, more preferably 0.08% or more, and further preferably 0.10% or more. From the viewpoint of the upper limit, the Si content is preferably less than 1.0%, more preferably 0.50% or less, and even more preferably 0.25% or less.

Mn: 0.30 to 2.5%
Manganese (Mn) increases the hardenability of the steel and increases the strength of the steel. Mn further improves the hot workability of the steel. If the Mn content is less than 0.30%, the above effect cannot be obtained sufficiently. On the other hand, if the Mn content exceeds 2.5%, Mn is segregated in the steel and the toughness is lowered. Therefore, the Mn content is 0.30 to 2.5%. From the viewpoint of the lower limit, the Mn content is preferably higher than 0.30%, more preferably 1.0% or more, and further preferably 1.3% or more. From the viewpoint of the upper limit, the Mn content is preferably less than 2.5%, more preferably 2.0% or less, and even more preferably 1.8% or less.

P: 0.030% or less Phosphorus (P) is an impurity. P decreases the toughness of the steel. Therefore, the P content is preferably as low as possible. Therefore, the P content is limited to 0.030% or less. The P content is preferably less than 0.030%, more preferably 0.015% or less, and still more preferably 0.012% or less.

S: 0.006% or less Sulfur (S) is an impurity. S combines with Mn to form coarse MnS and lowers the toughness of the steel. Accordingly, the S content is preferably as low as possible. Therefore, the S content is limited to 0.006% or less. The S content is preferably less than 0.006%, more preferably 0.003% or less, and still more preferably 0.002% or less.

Al: 0.001 to 0.100%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.001%, deoxidation is insufficient, and surface flaws may occur in the billet. On the other hand, if the Al content exceeds 0.100%, the billet may be cracked. Therefore, the Al content is 0.001 to 0.100%. From the viewpoint of the lower limit, the Al content is preferably higher than 0.001%, and more preferably 0.020% or more. In view of the upper limit, the Al content is preferably less than 0.100%, more preferably 0.080% or less, and further preferably 0.060% or less. The Al content in the present specification means the content of acid-soluble Al (so-called Sol. Al).

N: 0.008% or less Nitrogen (N) is an impurity. If the N content is higher than 0.008%, the billet may be cracked. Therefore, the N content is limited to 0.008% or less. The N content is preferably less than 0.008%, more preferably 0.006% or less, and still more preferably 0.005% or less.

O: 0.004% or less Oxygen (O) is an impurity. O forms coarse oxides or oxide clusters to lower the toughness of the steel. Therefore, it is preferable that the O content is as low as possible. Therefore, the O content is limited to 0.004% or less. The O content is preferably 0.003% or less, and more preferably 0.002% or less. The O content indicates the total content of dissolved oxygen in the steel and oxygen in the oxide inclusions. However, in steel that has been sufficiently deoxidized, the O content is substantially equal to the oxygen content in the oxide inclusions.

Ca: 0.0005 to 0.0040%
Calcium (Ca) improves the toughness by spheroidizing MnS. If the Ca content is less than 0.0005%, the above effect cannot be obtained sufficiently. On the other hand, if the Ca content is higher than 0.0040%, the cleanliness of the steel decreases and the toughness of the steel decreases. Therefore, the Ca content is 0.0005 to 0.0040%. From the viewpoint of the lower limit, the Ca content is preferably higher than 0.0005%, and more preferably 0.0008% or more. From the viewpoint of the upper limit, the Ca content is preferably less than 0.0040%, more preferably 0.0020% or less, and further preferably 0.0013% or less.

Cr: 0.05-1.0%
Chromium (Cr) increases the hardenability of the steel and improves the strength of the steel. Cr further increases the temper softening resistance of the steel. If the Cr content is less than 0.05%, the above effects cannot be obtained sufficiently. On the other hand, if the Cr content exceeds 1.0%, the toughness of the steel decreases. Therefore, the Cr content is 0.05 to 1.0%. From the viewpoint of the lower limit, the Cr content is preferably higher than 0.05%, and more preferably 0.20% or more. From the viewpoint of the upper limit, the Cr content is preferably less than 1.0%, more preferably 0.8% or less, and further preferably 0.5% or less.

Mo: 0.02 to 0.5%
Molybdenum (Mo) improves the strength of steel by transformation strengthening and solid solution strengthening. If the Mo content is less than 0.02%, the above effects cannot be obtained sufficiently. On the other hand, if the Mo content exceeds 0.5%, the toughness of the steel decreases. Therefore, the Mo content is 0.02 to 0.5%. From the viewpoint of the lower limit, the Mo content is preferably higher than 0.02%, and more preferably 0.05% or more. From the viewpoint of the upper limit, the Mo content is preferably less than 0.5%, more preferably 0.3% or less, and further preferably 0.1% or less.

V: 0.02 to 0.20%
Vanadium (V) combines with C in the steel to form a V carbide and increases the strength of the steel. V further dissolves in Mo carbides to form carbides. By including V, the carbide is less likely to be coarsened. If the V content is less than 0.02%, the above effect cannot be obtained effectively. On the other hand, if the V content is higher than 0.20%, the carbides become coarse. Therefore, the V content is 0.02 to 0.20%. From the viewpoint of the lower limit, the V content is preferably higher than 0.02%, and more preferably 0.04% or more. From the viewpoint of the upper limit, the V content is preferably less than 0.20%, and more preferably 0.10% or less.

  The balance of the chemical composition of the seamless steel pipe for line pipe according to the present embodiment is Fe and impurities. The impurity here refers to an element mixed from ore and scrap used as a raw material for steel, or an element mixed from the environment of the manufacturing process.

  The chemical composition of the seamless steel pipe for line pipe according to the present embodiment may contain Ti, Nb, Cu, and Ni instead of a part of Fe. Ti, Nb, Cu, and Ni are all selective elements. In other words, the chemical composition of the seamless steel pipe for line pipe according to the present embodiment may not contain part or all of Ti, Nb, Cu, and Ni.

Ti: 0% or more and less than 0.007% Titanium (Ti) suppresses cracking of billets. On the other hand, when the Ti content is 0.007% or more, TiC is generated and the toughness of the steel is lowered. Therefore, the Ti content is 0% or more and less than 0.007%. From the viewpoint of the lower limit, the Ti content is preferably 0.001% or more, and more preferably 0.002% or more. From the viewpoint of the upper limit, the Ti content is preferably 0.006% or less, and more preferably 0.005% or less.

  Nb, Cu, and Ni are all elements that improve the strength of steel. Therefore, the chemical composition of the seamless steel pipe for line pipe according to the present embodiment may contain one or more elements selected from the group consisting of Nb, Cu, and Ni.

Nb: 0 to 0.05%
Niobium (Nb) combines with C in the steel to form fine Nb carbides and increases the toughness of the steel. On the other hand, if the Nb content is higher than 0.05%, the carbides become coarse. Therefore, the Nb content is 0 to 0.05%. If the Nb content is 0.005% or more, the above-described effect is remarkably obtained. From the viewpoint of the lower limit, the Nb content is preferably higher than 0.005%, more preferably 0.010% or more, further preferably 0.015% or more, and further preferably 0.02% or more. It is. The Nb content is preferably 0.04% or less, more preferably 0.035% or less, from the viewpoint of the upper limit.

Ni: 0 to 1.5%
Nickel (Ni) increases the hardenability of the steel and increases the strength of the steel. On the other hand, if the Ni content is higher than 1.5%, the SSC resistance decreases. Therefore, the Ni content is 0 to 1.5%. If the Ni content is 0.03% or more, the above effect is remarkably obtained. From the viewpoint of the lower limit, the Ni content is preferably higher than 0.03%, more preferably 0.04% or more, further preferably 0.08% or more, more preferably 0.10% or more. It is. From the viewpoint of the upper limit, the Ni content is preferably less than 1.5%, more preferably 0.7% or less, and further preferably 0.5% or less.

Cu: 0 to 1.5%
Copper (Cu) increases the hardenability of the steel and increases the strength of the steel. On the other hand, if the Cu content is higher than 1.5%, the weldability of steel decreases. If the Cu content is too high, the grain boundary strength of the steel at a high temperature is further lowered, and the hot workability of the steel is lowered. Therefore, the Cu content is 0 to 1.5%. If Cu content is 0.02% or more, the said effect will be acquired notably. From the viewpoint of the lower limit, the Cu content is preferably higher than 0.04%, more preferably 0.08% or more, and further preferably 0.10% or more. In view of the upper limit, the Cu content is preferably less than 1.5%, more preferably 0.7% or less, and further preferably 0.5% or less.

The chemical composition of the seamless steel pipe for line pipes according to the present embodiment satisfies the following formula (1).
Ca × O <0.325 × 10 ⁻⁵ (1)
In the element symbol in the formula (1), the content of the corresponding element is substituted by mass%.

  The chemical composition of seamless steel pipes for line pipes satisfies the formula (1), thereby reducing the number density of oxide inclusions having a diameter of 300 μm or less as well as coarse oxide inclusions having a diameter exceeding 300 μm. can do.

More preferably, the chemical composition of the seamless steel pipe for line pipe according to the present embodiment further satisfies the following formula (2).
Ca / O <0.5 (2)
In the element symbol in the formula (2), the content of the corresponding element is substituted by mass%.

Under the condition of Ca × O <0.325 × 10 ⁻⁵ , the oxide inclusions are made to have a high melting point with a Ca / O value of less than 0.5, which facilitates agglomeration and coarsening in high-temperature molten steel. Since floating separation becomes easy, it is considered effective for further reduction of inclusions. By satisfying the formula (2), the number density of oxide inclusions having a diameter exceeding 50 μm can be further reduced. Thereby, excellent low temperature toughness can be obtained more stably.

[Oxide inclusions]
In the seamless steel pipe for line pipe according to this embodiment, the number density of oxide inclusions having a diameter of more than 50 μm is preferably 2 pieces / cm ² or less, and the number density of oxide inclusions having a diameter of 5 to 50 μm. Is 50 pieces / cm ² or less.

The oxide inclusions are CaO, Al ₂ O ₃ , SiO ₂ , and complex oxides thereof. As the form of the oxide inclusions, the so-called cluster inclusions in which the granular inclusions are arranged discontinuously in a rolling direction like Al ₂ O ₃ and the CaO—Al ₂ O ₃ type Thus, it is divided into so-called granular inclusions that are irregularly dispersed without viscous deformation.

The diameter and number of oxide inclusions are quantified with an optical microscope. The inclusion quantification specimen is collected so that the polished surface is parallel to the rolling direction and includes the thickness center of the steel pipe. The magnification of the optical microscope is, for example, 200 to 1000 times, and the area of the visual field is, for example, 100 mm ² .

The number of oxide inclusions is counted separately for those having a diameter of more than 50 μm and those having a diameter of 5 to 50 μm. Here, the lower limit of the diameter of the oxide inclusions to be counted is set to 5 μm because it is difficult to count inclusions having a diameter of less than 5 μm. Divide the measured number by the area of the field of view to find the number density. This measurement is performed with two visual fields, and the larger one of the number densities obtained with the two visual fields is defined as the number density (pieces / cm ² ) of oxide inclusions in each seamless steel pipe. More specifically, the larger one of the number density of oxide inclusions having a diameter exceeding 50 μm measured in two fields of view is defined as the number density of oxide inclusions having a diameter exceeding 50 μm. Similarly, the larger one of the number density of oxide inclusions having a diameter of 5 to 50 μm measured in two visual fields is defined as the number density of oxide inclusions having a diameter of 5 to 50 μm.

In the case of cluster-like inclusions, as shown in FIG. 1, whether or not two individual inclusions of lengths l ₁ and l ₂ are straight or not, the interval d is 40 μm or less and the inclusions When the center-to-center distance s is 10 μm or less, it is regarded as one inclusion.

[Production method]
Hereinafter, an example of the manufacturing method of the seamless steel pipe for line pipes by this embodiment is demonstrated. However, the manufacturing method of the seamless steel pipe for line pipes by this embodiment is not limited to this.

  The steel having the above chemical composition is melted and refined. Subsequently, billets are produced from the molten steel by a continuous casting method. A billet may be manufactured by manufacturing a slab or bloom from molten steel and hot working the slab or bloom.

  As a method for reducing inclusions in the steel, it is possible to prevent oxide contamination from the refractories on the furnace wall during melting, to increase the cooling rate of 1500 to 1000 ° C. immediately after melting, It is known that a coarse inclusion is floated by heating or stirring with a heater, and these are preferably employed as necessary.

  Then, a billet is hot-tube-made and a seamless steel pipe is manufactured. Specifically, the billet is heated in a heating furnace, and hot working is performed on the billet extracted from the heating furnace to produce a seamless steel pipe. Specifically, piercing and rolling based on the Mannesmann method is performed to manufacture a raw pipe. A seamless steel pipe is produced by subjecting the produced raw pipe to stretching and constant diameter rolling using a mandrel mill, reducer, sizing mill or the like.

  By carrying out piercing and rolling, inclusions that are segregated in the center are removed. Therefore, in the seamless steel pipe, inclusions can be further reduced as compared with the ERW pipe.

  Quenching and tempering are performed on the seamless steel pipes that have been made. Quenching and tempering may employ either inline QT or offline QT described below.

[Inline QT]
In-line QT is a process of immediately quenching and tempering a seamless steel pipe after hot pipe making, or a process of quenching and tempering after a hot steel pipe is hot-heated and then supplemented in a reheating furnace. . Inline QT is advantageous in terms of energy efficiency because it can be quenched using the heat of hot pipe making, compared to offline QT described later.

The seamless steel pipe that has been hot-formed is cooled from a temperature of (Ar ₃ points + 50 ° C.) to 1100 ° C. at a cooling rate of 5 ° C./second or more. At this time, an auxiliary heating furnace may be used to heat to the quenching start temperature before cooling. If the quenching start temperature is less than (Ar ₃ points + 50 ° C.), the strength varies. On the other hand, if the quenching start temperature is increased, the toughness deteriorates, so it is necessary to set the temperature to 1100 ° C. or lower. When the cooling rate is less than 5 ° C./second, a structure containing martensite or bainite cannot be secured. The cooled seamless steel pipe is tempered at a temperature of 550 ° C. or higher and Ac ₁ point or lower. Thereby, a structure containing tempered martensite or tempered bainite is obtained.

[Offline QT]
Off-line QT is a process in which a seamless steel pipe is once cooled after hot pipe making, and after cooling, is reheated to Ac ₃ points or higher and then quenched and tempered. The off-line QT is advantageous in that the crystal grains of the steel can be made finer than the inline QT described above.

The seamless steel pipe after hot pipe making is cooled to room temperature. The cooled seamless steel pipe is heated to a temperature of Ac ₃ point or higher. The heated seamless steel pipe is cooled at a cooling rate of 5 ° C./second or more from a temperature of (Ar ₃ points + 50 ° C.) to 1100 ° C. If the quenching start temperature is less than (Ar ₃ points + 50 ° C.), the strength varies. On the other hand, if the quenching start temperature is increased, the toughness deteriorates, so it is necessary to set the temperature to 1100 ° C. or lower. When the cooling rate is less than 5 ° C./second, a structure containing martensite or bainite cannot be secured. The cooled seamless steel pipe is tempered at a temperature of 550 ° C. or higher and Ac ₁ point or lower. Thereby, a structure containing tempered martensite or tempered bainite is obtained.

  The seamless steel pipe for line pipe according to the present embodiment has excellent toughness.

  The seamless steel pipe for line pipe according to the present embodiment is not limited to this, but can be suitably used as a seamless steel pipe having a wall thickness of 20 to 55 mm. The wall thickness of the seamless steel pipe is more preferably 20 to 40 mm from the viewpoint of rationalization of the alloy.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to these examples.

  Seamless pipes for line pipes with various chemical compositions were manufactured, and the relationship between the number density of inclusions and mechanical properties was investigated.

[Investigation method]
A plurality of molten steels having chemical compositions shown in Table 1 were produced. Billets were produced from molten steel by continuous casting. In Table 1, “-” indicates that the content of the element is at the impurity level.

  Each manufactured billet was heated in a heating furnace, and the heated billet was pierced and rolled into a raw pipe by a piercing machine. Subsequently, each raw tube was stretched and rolled by a mandrel mill. Subsequently, each raw pipe was subjected to constant diameter rolling with a sizer to produce a seamless steel pipe having an outer diameter and a wall thickness shown in Table 2. The manufactured seamless steel pipe was quenched and tempered under the conditions shown in Table 2.

  In the column “temperature (° C.)” of “Q furnace” in Table 2, the quenching start temperature is described. In the “T furnace”, the “Temperature (° C.)” column describes the holding temperature for tempering, and in the “T Furnace”, the “Holding (min)” column describes the holding time for tempering. In-line QT used an auxiliary heating furnace, and the seamless steel pipe after pipe making was heated to the indicated temperature and then cooled. All quenching was cooled to room temperature with a water cooling device at a cooling rate of 5 ° C./second or more.

[Tensile test]
A No. 12 test piece (width 25 mm, gauge distance 50 mm) defined in JIS Z 2241 (2011) was collected from each seamless steel pipe in the longitudinal direction (L direction) of the steel pipe. Using the collected test pieces, a tensile test based on JIS Z 2241 was performed in the air at normal temperature (25 ° C.), and yield stress and tensile strength were determined. Yield stress was determined by the 0.5% total elongation method.

[Charpy impact test]
A V-notch test piece (width 10 mm, height 10 mm, length 55 m, notch depth 2 mm) defined in JIS Z 2242 was taken from each seamless steel pipe. Using the collected specimens, the ductile fracture surface ratio at −40 ° C., the ductile fracture surface ratio at −30 ° C., and the fracture surface transition temperature vTrs were determined according to JIS Z 2242. The ductile fracture surface ratio was measured with three specimens at each temperature, and the target was that the ductile fracture area ratio was 75% or more for all specimens.

  From each seamless steel pipe, a specimen for inclusion quantification was sampled so that the polished surface was parallel to the rolling direction and included the thickness center of the steel pipe. The collected specimen was observed at a magnification of 200 times. What was clustered was measured at 200 to 1000 times to determine whether it was a cluster. The number of counts was divided by the area of the field of view to obtain the number density, and the larger of the number density obtained in two fields of view was taken as the number density of each inclusion in each seamless steel pipe.

[Investigation result]
The results are shown in Table 3. In Table 3, “YS” indicates yield stress, and “TS” indicates tensile strength. "-" In the column of ductile fracture surface ratio and vTrs indicates that the data is not measured.

  From Table 3, it can be seen that the ductile fracture surface ratio at −30 ° C. and −40 ° C. cannot be simply inferred from the fracture surface transition temperature. For example, the fracture surface transition temperature of steel E is lower than the fracture surface transition temperature of steel G. However, in Steel E, there is a test piece having a ductile fracture surface ratio of less than 75% in a Charpy impact test at −30 ° C. On the other hand, in Steel G, the ductile fracture surface ratio is 75% or more in all test pieces even in the -40 ° C Charpy impact test.

The seamless steel pipe manufactured from steel B, D, G, H, J, K, and M to Q had an appropriate chemical composition and satisfied the formula (1). In these seamless steel pipes, the number density of oxide inclusions having a diameter of more than 50 μm is 2 pieces / cm ² or less, and the number density of oxide inclusions having a diameter of 5 to 50 μm is 50 pieces / cm ² or less. there were. These seamless steel pipes had a ductile fracture surface ratio of 75% or more in all test pieces in a Charpy impact test at -30 ° C and -40 ° C.

Among the seamless steel pipes described above, the chemical composition of the seamless steel pipes manufactured from the steels B, D, G, H, J, and K further satisfied the formula (2). In these seamless steel pipes, the number density of oxide inclusions having a diameter exceeding 50 μm was 1 piece / cm ² or less. These seamless steel pipes showed stable and excellent low temperature toughness. Specifically, in the Charpy impact test at −40 ° C., the ductile fracture surface ratio was 100% with three test pieces.

The seamless steel pipe manufactured from Steel A had a test piece having a ductile fracture surface ratio of less than 75% in a Charpy impact test at -30 ° C. This is thought to be because the number density of oxide inclusions having a diameter of more than 50 μm exceeded ² / cm ² , or the number density of oxide inclusions having a diameter of 5 to 50 μm exceeded 50 / cm ^2. It is done. And this is considered because the chemical composition of the steel A did not satisfy Formula (1).

The seamless steel pipe manufactured from Steel C had a test piece having a ductile fracture surface ratio of less than 75% in a Charpy impact test at -30 ° C. This is presumably because the number density of oxide inclusions having a diameter of 5 to 50 μm exceeded 50 / cm ² . And this is considered because the chemical composition of the steel C did not satisfy Formula (1).

  The seamless steel pipe manufactured from the steel E had a test piece having a ductile fracture surface ratio of less than 75% in a Charpy impact test at -30 ° C. This is probably because the Ti content of Steel E was too high.

The seamless steel pipe manufactured from the steel F had a test piece having a ductile fracture surface ratio of less than 75% in a Charpy impact test at -30 ° C. This is thought to be because the number density of oxide inclusions having a diameter of more than 50 μm exceeded ² / cm ² , or the number density of oxide inclusions having a diameter of 5 to 50 μm exceeded 50 / cm ^2. It is done. And this is considered because the chemical composition of the steel F did not satisfy Formula (1).

The seamless steel pipe manufactured from Steel I had a test piece having a ductile fracture surface ratio of less than 75% in a Charpy impact test at -40 ° C. This is thought to be because the number density of oxide inclusions having a diameter of more than 50 μm exceeded ² / cm ² , or the number density of oxide inclusions having a diameter of 5 to 50 μm exceeded 50 / cm ^2. It is done. And this is considered because the chemical composition of the steel I did not satisfy the formula (1).

  The seamless steel pipe manufactured from the steel L had a test piece having a ductile fracture surface ratio of less than 75% in a Charpy impact test at -30 ° C. This is probably because the Ti content of the steel L was too much.

Claims (7)

Chemical composition is mass%,
C: 0.02 to 0.15%,
Si: 0.05 to 1.0%,
Mn: 0.30 to 2.5%,
P: 0.030% or less,
S: 0.006% or less,
Al: 0.001 to 0.100%,
N: 0.008% or less,
O: 0.004% or less,
Ca: 0.0005 to 0.0040%,
Cr: 0.05 to 1.0%,
Mo: 0.02 to 0.5%,
V: 0.02 to 0.20%,
Ti: 0% or more and less than 0.007%,
Nb: 0 to 0.05%,
Cu: 0 to 1.5%,
Ni: 0 to 1.5%,
Balance: Fe and impurities,
The said chemical composition is the seamless steel pipe for line pipes which satisfy | fills following formula (1).
Ca × O <0.325 × 10 ⁻⁵ (1)
In the element symbol in the formula (1), the content of the corresponding element is substituted by mass%. It is a seamless steel pipe for line pipes according to claim 1,
The number density of oxide inclusions having a diameter exceeding 50 μm is 2 pieces / cm ² or less,
A seamless steel pipe for a line pipe, wherein the number density of oxide inclusions having a diameter of 5 to 50 µm is 50 pieces / cm ² or less. A seamless steel pipe for a line pipe according to claim 1 or 2,
The chemical composition is a seamless steel pipe for line pipes that satisfies the following formula (2).
Ca / O <0.5 (2)
In the element symbol in the formula (2), the content of the corresponding element is substituted by mass%. A seamless steel pipe for a line pipe according to any one of claims 1 to 3,
The chemical composition is mass%,
Ti: 0.001% or more and less than 0.007%,
Containing seamless steel pipes for line pipes. A seamless steel pipe for a line pipe according to any one of claims 1 to 4,
The chemical composition is mass%,
Nb: 0.005 to 0.05%,
Cu: 0.02-1.5%, and Ni: 0.03-1.5%,
A seamless steel pipe for line pipes, containing one or more elements selected from the group consisting of: Chemical composition is mass%, C: 0.02-0.15%, Si: 0.05-1.0%, Mn: 0.30-2.5%, P: 0.030% or less, S : 0.006% or less, Al: 0.001 to 0.100%, N: 0.008% or less, O: 0.004% or less, Ca: 0.0005 to 0.0040%, Cr: 0.05 -1.0%, Mo: 0.02-0.5%, V: 0.02-0.20%, Ti: 0% or more and less than 0.007%, Nb: 0-0.05%, Cu: 0 to 1.5%, Ni: 0 to 1.5%, balance: Fe and a step of preparing billets which are impurities,
Manufacturing the steel pipe by hot working the billet;
After the steel pipe is hot worked, it is cooled and quenched at a cooling rate of 5 ° C./second or more from a temperature of (Ar ₃ points + 50 ° C.) or higher and 1100 ° C. or lower;
Tempering the quenched steel pipe at a temperature of 550 ° C. or more and Ac ₁ point or less,
The said chemical composition is a manufacturing method of the seamless steel pipe for line pipes which satisfy | fills following formula (1).
Ca × O <0.325 × 10 ⁻⁵ (1)
In the element symbol in the formula (1), the content of the corresponding element is substituted by mass%. Chemical composition is mass%, C: 0.02-0.15%, Si: 0.05-1.0%, Mn: 0.30-2.5%, P: 0.030% or less, S : 0.006% or less, Al: 0.001 to 0.100%, N: 0.008% or less, O: 0.004% or less, Ca: 0.0005 to 0.0040%, Cr: 0.05 -1.0%, Mo: 0.02-0.5%, V: 0.02-0.20%, Ti: 0% or more and less than 0.007%, Nb: 0-0.05%, Cu: 0 to 1.5%, Ni: 0 to 1.5%, balance: Fe and a step of preparing billets which are impurities,
Manufacturing the steel pipe by hot working the billet;
Cooling the steel pipe to room temperature;
Heating the cooled steel pipe to a temperature of Ac ₃ point or higher;
Cooling and quenching the heated steel pipe at a cooling rate of 5 ° C./second or more from a temperature of (Ar ₃ points + 50 ° C.) or higher and 1100 ° C. or lower;
Tempering the quenched steel pipe at a temperature of 550 ° C. or more and Ac ₁ point or less,
The said chemical composition is a manufacturing method of the seamless steel pipe for line pipes which satisfy | fills following formula (1).
Ca × O <0.325 × 10 ⁻⁵ (1)
In the element symbol in the formula (1), the content of the corresponding element is substituted by mass%.

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