JP2022065798A - Seamless steel pipe - Google Patents

Abstract

To provide a seamless steel pipe that can yield excellent low temperature toughness of HAZ.SOLUTION: A seamless steel pipe of the present disclosure has a specific chemical composition and includes a composite inclusion in which MnS exists around Ti oxide. In a cross section of the seamless steel pipe, an area ratio of the MnS to the composite inclusion is 10-90%, a ratio of a length of an interface between the Ti oxide and MnS of the composite inclusion to a length of an entire periphery of the Ti oxide is 10% or greater, and a number density of the composite inclusion for which an equivalent circle diameter of the Ti oxide is 0.5-5.0 μm is 10-100 pieces/mm2. In the cross section, an average effective crystal particle diameter of a metallographic structure is greater than 10 μm.SELECTED DRAWING: None

Description

The present invention relates to a seamless steel pipe.

For seamless steel pipes for line pipes, it is required to secure the low temperature toughness of the heat-affected zone (HAZ) from the viewpoint of enhancing the reliability of the circumferential welded portion against fracture.

In HAZ, the heating temperature during welding becomes higher as it approaches the melting line, and austenite (γ) becomes significantly coarser especially in the region heated to 1400 ° C. or higher near the melting line, and the HAZ structure after cooling becomes coarser. And the toughness deteriorates. This tendency becomes more remarkable as the amount of heat input to welding increases.

The conventional techniques for improving the toughness of HAZ can be broadly classified based on two basic techniques. One of them is a technique to prevent the coarsening of austenite by utilizing the pinning effect of the particles in the steel. Fine particles that contribute to the prevention of grain coarsening of HAZ are called pinning particles. The other is a technique for refining the effective crystal grain size by utilizing the intragranular ferrite transformation of austenite.

For example, Patent Document 1 discloses a high-strength steel plate containing a composite inclusion in which MnS is present around a Ti oxide and having an average effective crystal grain size of a metal structure of 10 μm or less.

Japanese Unexamined Patent Publication No. 2019-56148

However, Patent Document 1 mainly targets thick plates, and it is not easy to apply the technique of Patent Document 1 to seamless steel pipes. That is, in a thick plate, the average crystal grain size can be made finer by low-temperature rolling by TMCP (Thermo Mechanical Control Process: processing heat treatment or heat processing control). Therefore, Patent Document 1 is for improving the toughness of HAZ. As a countermeasure, the thick plate contains the above-mentioned composite inclusions. On the other hand, seamless steel pipes are manufactured at a higher temperature than thick plates because surface defects occur when they are manufactured at a low temperature. Recrystallization occurs during the manufacture of seamless steel pipes, and the average effective crystal grain size of seamless steel pipes is larger than the average effective crystal grain size of thick plates. It is preferable to be done.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a seamless steel pipe capable of obtaining excellent low temperature toughness of HAZ.

As a result of diligent studies by the present inventors, the present inventors have found that excellent low temperature toughness of HAZ can be obtained even in a seamless steel pipe by having a composite inclusion containing a specific amount of MnS around the Ti oxide. Found. The gist of the present invention is as follows.
<1> The seamless steel pipe according to one aspect of the present invention has a mass% of C: 0.03 to 0.08%, Si: 0.05 to 0.25%, and Mn: 1.0 to 2.5. %, P: 0.05% or less, S: 0.001 to 0.005%, N: 0.0150% or less, O: 0.0010 to 0.0050%, Ti: 0.005 to 0.050% , Which is a seamless steel pipe containing a composite inclusion in which MnS is present around the Ti oxide, the balance of which is composed of Fe and impurities, and the MnS with respect to the composite inclusion in the cross section of the seamless steel pipe. The area ratio of the above is 10 to 90%, and the ratio of the length of the interface between the Ti oxide and the MnS of the composite inclusions to the total perimeter of the Ti oxide is 10% or more. Moreover, the number density of the composite inclusions in which the equivalent circle diameter of the Ti oxide is 0.5 to 5.0 μm is 10 to 100 pieces / mm ² , and the average effective crystal grain size of the metal structure in the cross section is Is over 10 μm.
<2> The seamless steel pipe according to <1> may contain Al: 0.007% or less in mass% instead of a part of Fe.
<3> The seamless steel pipe according to <1> or <2> is replaced with a part of Fe in terms of mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: It may contain one or more selected from the group consisting of 0.3% or less, V: 0.08% or less, B: 0.010% or less, and Ca: 0.005% or less.
<4> The seamless steel pipe according to any one of <1> to <3> is replaced with a part of the Fe in mass%, Mo: 0.3% or less, and Nb: 0. It may contain one or more of 05% or less.
<5> The seamless steel pipe according to any one of <1> to <4> may have a tensile strength of 758 MPa or less.
<6> The seamless steel pipe according to any one of <1> to <5> may have a yield ratio YR of 0.75 or more.
<7> The seamless steel pipe according to any one of <1> to <6> may have a wall thickness of less than 50 mm.
<8> The seamless steel pipe according to any one of <1> to <7> may be used for a line pipe.

According to the above aspect of the present invention, it is possible to provide a seamless steel pipe capable of obtaining excellent low temperature toughness of HAZ.

Hereinafter, the seamless steel pipe according to the present embodiment will be described.

<Seamless steel pipe>
As a means for ensuring the low temperature toughness of HAZ in seamless steel pipes, as a result of diligent studies by the present inventors, it was found that a method of growing ferrite in the old austenite grains to make the crystal grains finer is effective. rice field. When the present inventors diligently studied the growth of intragranular ferrite, the following was found.
(A) In the cooling process during circumferential welding of the seamless steel pipe, when MnS is compound-precipitated around the Ti oxide in the seamless steel pipe, a gradient of Mn concentration is generated in the seamless steel pipe, thereby causing a gradient of Mn concentration in the seamless steel pipe. A driving force is generated in which Mn is diffused inside the Ti oxide.
(B) Mn is absorbed by the atomic vacancies existing inside the Ti oxide.
(C) A Mn-deficient layer having a low Mn concentration is formed around the Ti oxide, and the ferrite growth start temperature in this region rises.
(D) As a result, ferrite preferentially grows from the periphery of the inclusions in which the Mn-deficient layer exists, and the crystal grains become finer.

As a result of diligent studies by the present inventors based on the above findings, the following findings were obtained.
(1) In a composite inclusion in which MnS exists around the Ti oxide, if the equivalent circle diameter of the Ti oxide is 0.5 to 5.0 μm, an appropriate amount of Mn can be absorbed.
(2) In a composite inclusion in which MnS is present around the Ti oxide, if the area ratio of MnS to the composite inclusion is 10 to 90%, an appropriate amount of Mn can be absorbed.
(3) In a composite inclusion in which MnS exists around the Ti oxide, the length of the interface between the Ti oxide and MnS of the composite inclusion is 10% or more of the total circumference of the Ti oxide. Then, the Mn-deficient layer is likely to be formed.
(4) When the number density of the composite inclusions in which MnS is present around the Ti oxide is 10 to 100 / mm ² , ferrite can be stably produced in the crystal grains.
Furthermore, the present inventors have obtained the following findings.
(5) Even if the average effective crystal grain size of the metal structure of the base metal of the seamless steel pipe is more than 10 μm, the toughness of the base metal may be improved by the composite inclusions in which MnS is present around the Ti oxide. be.

In the seamless steel pipe according to the present embodiment, the configuration of the seamless steel pipe was determined based on the above findings. The seamless steel pipe according to the present embodiment can obtain excellent low temperature toughness of HAZ due to the synergistic effect of each configuration, and preferably secures the toughness of the base metal without performing low temperature rolling that causes surface defects. be able to. Hereinafter, each configuration of the seamless steel pipe according to the present embodiment will be described. In the present specification, the numerical range represented by using "-" means a range including the numerical values before and after "-" as the lower limit value and the upper limit value. Numerical values indicated as "less than" and "greater than" do not include the value in the numerical range. All% of the chemical composition indicate mass%.

(Chemical composition)
The chemical composition of the seamless steel pipe according to the present embodiment is, in terms of mass%, C: 0.03 to 0.08%, Si: 0.05 to 0.25%, Mn: 1.0 to 2.5%, P: 0.05% or less, S: 0.001 to 0.005%, N: 0.0150% or less, O: 0.0010 to 0.0050%, Ti: 0.005 to 0.050%. It is contained and the balance consists of Fe and impurities.

"C: 0.03 to 0.08%"
Carbon (C) is an element that enhances the hardenability of steel and improves the strength of steel. If the C content is less than 0.03%, the hardenability is insufficient and it becomes difficult to secure high strength. On the other hand, when the C content exceeds 0.08%, the toughness of the steel decreases. Therefore, the C content is 0.03 to 0.08%. The C content is preferably more than 0.03%, more preferably 0.04% or more. The C content is preferably less than 0.08%, more preferably 0.07% or less, still more preferably 0.06% or less.

"Si: 0.05-0.25%"
Silicon (Si) is an element that deoxidizes steel. When the Si content exceeds 0.25%, the toughness of the steel decreases. Therefore, the Si content is 0.25% or less. In order to obtain the above-mentioned deoxidizing effect, the Si content is 0.05% or more, more preferably 0.08% or more, and further preferably 0.10% or more. The Si content is preferably less than 0.25%, more preferably 0.20% or less, still more preferably 0.15% or less.

"Mn: 1.0-2.5%"
Manganese (Mn) is an element that enhances the hardenability of steel and enhances the strength of steel. In addition, Mn improves the hot workability of steel. If the Mn content is less than 1.0%, the above effect cannot be sufficiently obtained. On the other hand, when the Mn content exceeds 2.5%, Mn segregates in the steel and the toughness decreases. Therefore, the Mn content is 1.0% to 2.5%. The Mn content is preferably 1.1% or more, more preferably 1.3% or more. From the viewpoint of the upper limit, the Mn content is preferably 2.4% or less, more preferably 2.0% or less, still more preferably 1.8% or less.

"P: 0.05% or less"
Phosphorus (P) is an impurity and is an element that reduces the toughness of steel. Therefore, it is preferable that the P content is as low as possible. Therefore, the P content is limited to 0.05% or less. The P content is preferably less than 0.05%, more preferably 0.04% or less, still more preferably 0.02% or less.

"S: 0.001 to 0.005%"
Sulfur (S) is an element that combines with Mn to form MnS. When S is excessively contained, coarse single MnS is deposited, which lowers the toughness of the steel. Therefore, the S content is set to 0.005% or less. The S content is preferably 0.004% or less, more preferably 0.003% or less. From the viewpoint of complex precipitation of MnS and ensuring low temperature toughness of HAZ, the S content is 0.001% or more, more preferably 0.002% or more.

"N: 0.0150% or less"
Nitrogen (N) is an element that affects the refinement of crystal grains. In order to obtain the above effects, the N content is preferably 0.0020% or more. If the N content is higher than 0.0150%, the billet may crack. Therefore, the N content is 0.0150% or less. The N content is preferably less than 0.0150%, more preferably 0.0120% or less, still more preferably 0.0100% or less.

"O: 0.0010-0.0050%"
O is an element that affects the formation of Ti oxide. In order to obtain a sufficient inclusion density, the O content is 0.0010% or more. On the other hand, if O is excessively contained, it becomes easy to form a coarse oxide that can be a fracture starting point. Therefore, the O content is set to 0.0050% or less. The O content is preferably 0.0015% or more, and more preferably 0.0020% or more. The O content is preferably 0.0045% or less, more preferably 0.0040% or less.

"Ti: 0.005 to 0.050%"
Ti is an element that affects the formation of Ti oxide. The Ti content should be 0.005% or more in order to obtain a sufficient inclusion density. On the other hand, if Ti is excessively contained, carbides such as TiC are likely to be generated, so that the toughness of HAZ is lowered. Therefore, the Ti content is set to 0.050% or less. From the viewpoint of ensuring sufficient inclusion density and ensuring the toughness of HAZ, the Ti content is preferably 0.009% or more, and preferably 0.020% or less.

"Remaining"
The rest of the chemical composition of the seamless steel pipe according to this embodiment is Fe and impurities. The term "impurity" as used herein means an element mixed from ore or 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 according to the present embodiment may contain Al: 0.007% or less in mass% instead of a part of Fe.

"Al: 0.007% or less"
Aluminum (Al) is an element that deoxidizes steel. If the Al content exceeds 0.007%, cracks may occur in the billet. Therefore, the Al content is 0.007% or less. In order to obtain the above-mentioned deoxidizing effect, the Al content is preferably 0.001% or more, more preferably 0.002% or more.
Further, Al has a larger oxide-forming ability than Ti, and when the Al content is high, the formation of Ti oxide is suppressed. The Al content is preferably less than 0.007%, more preferably 0.006% or less, still more preferably 0.005% or less. The Al content in the present specification means the content of acid-soluble Al (so-called sol.Al).

Further, the chemical composition of the seamless steel pipe according to the present embodiment is, in mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.3% or less, instead of a part of Fe. , V: 0.08% or less, B: 0.010% or less, and Ca: 0.005% or less, which may contain one or more selected from the group.

"Cu: 1.0% or less"
Since Cu is an element having an effect of increasing the strength, it may be contained. However, if Cu is excessively contained, hot embrittlement occurs, which leads to deterioration of the quality of the billet surface. Therefore, the Cu content is 1.0% or less. The Cu content is preferably 0.01% or more in order to increase the strength. Since it is not necessary to contain Cu, the lower limit of the Cu content is 0%. The C content is preferably 0.3% or less.

"Ni: 1.0% or less"
Since Ni is an element that increases the strength without lowering the toughness, it may be contained. However, since Ni is an austenite stabilizing element, if it is contained in an excessive amount, it becomes difficult to form intragranular ferrite. Therefore, the Ni content is set to 1.0% or less. The Ni content is preferably 0.9% or less, more preferably 0.8% or less, still more preferably 0.7% or less. Since it is not necessary to contain Ni, the lower limit of the Ni content is 0%. The Ni content is preferably 0.01% or more.

"Cr: 0.3% or less"
Chromium (Cr) is an element that enhances the hardenability of steel and improves the strength of steel, and may be contained. When the Cr content exceeds 0.3%, the toughness of the steel decreases. Therefore, the Cr content is 0.3% or less. The Cr content is less than 0.3%, more preferably 0.2% or less. Since it is not necessary to contain Cr, the lower limit of the Cr content is 0%. The Cr content is preferably 0.05% or more, more preferably 0.1% or more.

"V: 0.08% or less"
Vanadium (V) is an element that combines with C in steel to form V carbides and enhances the strength of steel, and may be contained. V further dissolves in Mo carbide to form a carbide. By including V, the carbide is less likely to be coarsened. If the V content is higher than 0.08%, the carbides will be coarse. Therefore, the V content is 0.08% or less. The V content is preferably less than 0.08%, more preferably 0.07% or less. Since it is not necessary to contain V, the lower limit of the V content is 0%. The V content is preferably 0.02% or more, more preferably 0.04% or more.

"B: 0.010% or less"
Boron (B) is an element whose hardenability is improved by adding a small amount, and may be contained. However, even if a large amount of B is contained, in addition to the above-mentioned effect being saturated, coarse borides such as Mo ₂ B and Fe ₂ B are formed. Therefore, the B content is 0.010% or less. The B content is preferably 0.009% or less, more preferably 0.008% or less. Since it is not necessary to contain B, the lower limit of the B content is 0%. The B content is preferably 0.001% or more, more preferably 0.002% or more.

"Ca: 0.005% or less"
Calcium (Ca) is an element that spheroidizes MnS and improves toughness, and may be contained. If the Ca content is higher than 0.005%, the cleanliness of the steel is reduced and the toughness of the steel is reduced. Therefore, the Ca content is 0.005% or less. The Ca content is preferably less than 0.005%, more preferably 0.004% or less, still more preferably 0.003% or less. Since it does not have to contain Ca, the lower limit of the Ca content is 0%. The Ca content is preferably 0.0004% or more, more preferably 0.0008% or more.

Further, the chemical composition of the seamless steel pipe according to the present embodiment contains at least one of Mo: 0.3% or less and Nb: 0.05% or less in mass% instead of a part of Fe. May be.

"Mo: 0.3% or less, Nb: 0.05% or less at least one type"
Since Mo and Nb are elements that improve the strength, one or more types of Mo and Nb may be contained. However, if Mo is contained in excess, the toughness of HAZ is significantly reduced. Therefore, the Mo content is set to 0.3% or less. The Mo content is preferably 0.2% or less. Since it is not necessary to contain Mo, the lower limit of the Mo content is 0%. The Mo content is preferably 0.01% or more.

Further, if Nb is excessively contained, carbides such as NbC are likely to be generated, which leads to a decrease in toughness. Therefore, the Nb content is set to 0.05% or less. The Nb content is preferably 0.03% or less. Since it is not necessary to contain Nb, the lower limit of the Nb content is 0%. The Nb content is preferably 0.01% or more.

The chemical composition of the seamless steel pipe described above may be measured by a general analytical method. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) may be used for measurement. In addition, C and S may be measured by using the combustion-infrared absorption method, and N may be measured by using the inert gas melting-thermal conductivity method. Al may be measured by ICP-AES using a filtrate obtained by heat-decomposing the sample with an acid.

(Composite inclusions in which MnS is present around the Ti oxide)
The seamless steel pipe according to the present embodiment contains a composite inclusion in which MnS is present around the Ti oxide. Hereinafter, the composite inclusions will be described.

"The area ratio of MnS to the composite inclusions is 10% to 90%"
Area ratio of MnS to composite inclusions (cross-sectional area ratio of MnS in cross-sectional area of composite inclusions) in a cross section obtained by cutting a seamless steel pipe according to the present embodiment in an arbitrary direction (for example, thickness direction). Is 10% to 90%. If the area ratio of MnS to the composite inclusions in the cross section of the seamless steel pipe is less than 10%, the amount of Mn that can be absorbed from the periphery of the composite inclusions is small, and it is difficult to form the Mn-deficient layer required for intragranular ferrite formation. become. When the ratio of MnS to the composite inclusions in the cross section of the seamless steel pipe exceeds 90%, the ratio of Ti oxide decreases. As a result, the Mn absorption capacity of the composite inclusions during welding is reduced, and it becomes difficult to form the Mn-deficient layer required for the formation of intragranular ferrite. The area ratio of MnS to the composite inclusions in the cross section of the seamless steel pipe is preferably 20% or more, more preferably 30% or more. The area ratio of MnS to the composite inclusions in the cross section of the seamless steel pipe is preferably 80% or less, more preferably 70% or less, still more preferably 60% or less.

"The ratio of the length of the interface between the Ti oxide and MnS of the composite inclusions to the total perimeter of the Ti oxide is 10% or more."
In order for the Ti oxide to efficiently absorb Mn of MnS, MnS needs to be present around the Ti oxide. In a cross section obtained by cutting a seamless steel pipe according to the present embodiment in an arbitrary direction (for example, in the thickness direction), the Ti oxide and MnS of the composite inclusions have a length with respect to the entire perimeter of the Ti oxide. The ratio of the length of the interface is 10% or more. When the ratio of the length of the interface between the Ti oxide and MnS of the composite inclusions to the total perimeter of the Ti oxide is less than 10%, the formation of the Mn-deficient layer required for the formation of intragranular ferrite is formed. It will be difficult. The ratio of the length of the interface between the Ti oxide and MnS of the composite inclusions to the total perimeter of the Ti oxide is preferably 20% or more. The upper limit of the ratio of the length of the interface between the Ti oxide and MnS of the composite inclusion to the total perimeter of the Ti oxide is, for example, 100%. The ratio of the length of the interface between the Ti oxide and MnS of the composite inclusions to the total perimeter of the Ti oxide is preferably 90% or less.

"Composite inclusions in which the equivalent circle diameter of Ti oxide is 0.5 to 5.0 μm"
When the equivalent circle diameter of the Ti oxide in the composite inclusions is less than 0.5 μm, the amount of Mn that can be absorbed from the periphery of the Ti oxide is small, and as a result, it is difficult to form the Mn-deficient layer required for the formation of intragranular ferrite. Will be. Therefore, in the cross section obtained by cutting the seamless steel pipe according to the present embodiment in an arbitrary direction (for example, in the thickness direction), the equivalent circle diameter of the Ti oxide in the composite inclusions is 0.5 μm or more. .. The equivalent circle diameter of the Ti oxide in the composite inclusions is preferably 1.0 μm or more. Further, when the equivalent circle diameter of the Ti oxide in the composite inclusions is larger than 5.0 μm, the composite inclusions become the starting point of fracture. Therefore, the equivalent circle diameter of the Ti oxide in the composite inclusions is 5.0 μm or less. The equivalent circle diameter of the Ti oxide in the composite inclusions is preferably 4.5 μm or less. The equivalent diameter of a circle is the diameter of a circle having an area equal to the projected area of the particles.

"Number density of composite inclusions is 10 to 100 / mm ² "
For stable intragranular ferrite formation, at least one composite inclusion is required in the old austenite. Therefore, in the cross section obtained by cutting the seamless steel pipe according to the present embodiment in an arbitrary direction (for example, in the thickness direction), the area ratio of MnS is 10% to 90%, and the area ratio of MnS is 10% to 90%, and the entire circumference of the Ti oxide is used. The ratio of the length of the interface between the Ti oxide and MnS of the composite inclusions to the length is 10% or more, and the number density of the composite inclusions in which the equivalent circle diameter of the Ti oxide is 0.5 to 5.0 μm. Is 10 pieces / mm ² or more. The number density of the composite inclusions is preferably 20 pieces / mm ² or more. If there are too many complex inclusions, it is likely to be the starting point of rupture. Therefore, the number density of the composite inclusions is 100 pieces / mm ² or less. The number density of the composite inclusions is preferably 90 pieces / mm ² or less.

In a cross section obtained by cutting a seamless steel tube according to the present embodiment in an arbitrary direction (for example, in the thickness direction), the area ratio of MnS of the composite inclusions, the length of the interface between Ti oxide and MnS, and the circle. In order to measure the equivalent diameter and the number density, it is preferable to measure with a wide field of view using a scanning electron microscope (SEM).

More specifically, for example, in a cross section obtained by cutting a seamless steel pipe in the thickness direction, about 1/2 of the wall thickness in the thickness direction is observed by SEM from the outer surface of the seamless steel pipe. The measurement field of view is 1 mm × 1 mm, the composite inclusions are identified by the backscattered electron image, and the composition is analyzed by EDS (Energy Dispersive X-ray Spectrometer) contained in the SEM to identify the composite inclusions. Further, the composite inclusions were photographed, and the equivalent circle diameter, the area ratio of MnS, the length of the entire circumference of the Ti oxide and the length of the interface between the Ti oxide and MnS were obtained by image analysis, and the above conditions were satisfied. By counting the number of composite inclusions to be formed, the number density of the composite inclusions is obtained. The measurement of this composite inclusion may be performed using software attached to the SEM, which can be automatically and continuously measured. The Ti oxide can be identified as having the main composition of Ti and O, and the MnS can be identified as having the main composition of Mn and S, respectively.

(Average effective crystal grain size of metal structure)
In the case of a seamless steel pipe, surface defects are likely to occur when low-temperature rolling is performed in order to reduce the average effective crystal grain size of the metal structure to 10 μm or less. In addition, in the case of a seamless steel pipe, if the average effective crystal grain size of the metal structure is 10 μm or less, the manufacturing cost becomes high. Therefore, the average effective crystal grain size of the metal structure of the seamless steel pipe according to the present embodiment is more than 10 μm. If the production cost is not considered, the average effective crystal grain size of the metal structure is preferably finer, but in the present embodiment, it is, for example, 15 μm or more. The upper limit of the average effective crystal grain size of the metal structure is not particularly set, but it may be set to 200 μm. Here, the effective crystal grain means a region surrounded by grain boundaries having an orientation difference of 15 ° or more.

The average effective crystal grain size of the metal structure may be measured as follows. For example, a seamless steel pipe is cut in the thickness direction, and a cross section near 1/2 of the wall thickness is observed by electron backscatter diffraction (EBSD). The equivalent circle diameter (diameter) of the effective crystal grains in the field of view (800 μm in the wall thickness direction) of 800 μm × 1000 μm is obtained, and the average value thereof is taken as the average effective crystal grain size of the metal structure.
Here, since the seamless steel pipe according to the present embodiment has Ti oxide, it has excellent toughness even if the average effective crystal grain size of the metal structure is more than 10 μm, and satisfies the required performance of the seamless steel pipe. Contribute to. That is, in addition to the composite inclusions improving the toughness of HAZ, the Ti oxide contributes to the improvement of the toughness of the base metal.

(Tensile strength: 758 MPa or less)
The tensile strength of the seamless steel pipe according to this embodiment is preferably 758 MPa or less. The tensile strength is more preferably 745 MPa or less. The tensile strength of the seamless steel pipe is preferably 513 MPa or more. More preferably, it is 545 MPa or more. The tensile strength of the seamless steel pipe is measured according to JIS Z 2241 (2011). The test piece is collected from the steel pipe so that the longitudinal direction of the test piece is the L direction.

(Yield ratio YR: 0.75 or more)
It is preferable that the strength of the peripheral welded portion of the seamless steel pipe is higher than the strength of the base metal of the seamless steel pipe. Therefore, it is better to keep the tensile strength of the base material of the seamless steel pipe smaller than that of the weld metal. Therefore, the yield ratio YR of the seamless steel pipe according to the present embodiment is preferably 0.75 or more. The yield ratio YR of the seamless steel pipe is more preferably 0.80 or more. The yield ratio of the seamless steel pipe is measured according to JIS Z 2241 (2011).

(Thickness)
The wall thickness of the seamless steel pipe according to this embodiment is preferably less than 50 mm. When the wall thickness of the seamless steel pipe is less than 50 mm, it is preferable as the seamless steel pipe for line pipes. The wall thickness of the seamless steel pipe is preferably 25 mm or more.

<Manufacturing method>
Hereinafter, an example of a method for manufacturing a seamless steel pipe according to the present embodiment will be described. However, the method for manufacturing a seamless steel pipe according to the present embodiment is not limited to this.

Steel with the above chemical composition is melted and refined. Subsequently, billets are produced from molten steel by a continuous casting method. A slab or bloom may be produced from molten steel, and the slab or bloom may be hot-processed to produce a billet.

Subsequently, the billet is hot-made to manufacture a seamless steel pipe. Specifically, the billet is heated in a heating furnace, and the billet extracted from the heating furnace is hot-worked to manufacture a seamless steel pipe. Specifically, a raw pipe is manufactured by performing drilling rolling based on the Mannesmann method. A seamless steel pipe is manufactured by performing draw rolling and constant diameter rolling on the manufactured raw pipe with a mandrel mill, reducer, sizing mill or the like.

Quenching and tempering are carried out on the seamless steel pipes that have been manufactured. For quenching and tempering, either in-line QT or offline QT described below may be adopted.

[Inline QT]
In-line QT refers to a process in which a seamless steel pipe is hot-made and then immediately quenched and tempered, or a seamless steel pipe is hot-made and then heated in a heating furnace and then quenched and tempered. .. Compared with the offline QT described later, the in-line QT can be quenched by utilizing the heat of the hot pipe making, which is advantageous in terms of energy efficiency.

The hot-made seamless steel pipe is cooled from a temperature of (Ar3 points + 50 ° C.) to 1100 ° C. at a cooling rate of 5 ° C./sec or more. At this time, a heating furnace may be used to heat up to the quenching start temperature before cooling. If the quenching start temperature is less than (Ar3 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 keep the temperature below 1100 ° C.

[Offline QT]
Off-line QT is a process in which a seamless steel pipe is once cooled after being hot-made, then reheated to Ac 3 points or more after cooling, and then quenched and tempered.

Cool the seamless steel pipe after hot pipe production to room temperature. The cooled seamless steel pipe is heated to a temperature of Ac 3 points or more. The heated seamless steel pipe is cooled from a temperature of (Ar 3 points + 50 ° C.) to 1100 ° C. at a cooling rate of 5 ° C./sec or more. If the quenching start temperature is less than (Ar3 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 keep the temperature below 1100 ° C.

Next, an example of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one condition example. Not limited. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

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

<Manufacturing of seamless steel pipe>
A plurality of molten steels having the chemical compositions shown in Table 1 were produced. "-" In Table 1 indicates that it was not intentionally added. Billets were manufactured from molten steel by a continuous casting method. Each billet produced was heated in a heating furnace, and the heated billet was drilled and rolled by a drilling machine to make a raw pipe. Subsequently, each raw tube was stretched and rolled by a mandrel mill. Subsequently, each raw pipe was rolled to a fixed diameter with a sizer to produce a seamless steel pipe having a wall thickness shown in Table 2.

<Calculation of the equivalent circle diameter of Ti oxide, MnS area ratio, and interface length ratio in the cross section of the composite inclusions>
As the test piece for the analysis of the composite inclusions, the one collected from 1 / 2t part was used, where t is the wall thickness of the seamless steel pipe produced above. As the composite inclusions, the EDX attached to the SEM was used, and the equivalent circle diameter of the Ti oxide, the MnS area ratio, and the proportion of MnS at the interface of the composite inclusions were measured from the mapping image obtained by surface analysis of the composite inclusions.

<Calculation of the number density of composite inclusions>
The number of composite inclusions is determined by an analyzer that combines SEM and EDX, and the circle-equivalent diameter of the Ti oxide is in the range of 0.5 to 5.0 μm from the detected shape measurement data of the composite inclusions. Calculate the number of composite inclusions in which the area ratio of MnS is 10 to 90% and the ratio of the interface average length between Ti oxide and MnS to the total perimeter of Ti oxide is 10% or more. As a result, the number density of the composite inclusions was calculated. The results are shown in Table 2.

<Average effective crystal grain size of metal structure>
A seamless steel pipe was cut in the thickness direction, and a cross section near 1/2 of the wall thickness was observed by electron backscatter diffraction (EBSD). The equivalent circle diameter (diameter) of the effective crystal grains in the field of view (800 μm in the wall thickness direction) of 800 μm × 1000 μm was determined, and the average value thereof was taken as the average effective crystal grain size of the metal structure. The results are shown in Table 2.

<Tensile test>
No. 12 test piece (width 2) specified in JIS Z 2241 (2011) from each seamless steel pipe.
5 mm, reference point distance 50 mm) was sampled in the longitudinal direction (L direction) of the steel pipe. Tensile tests conforming to JIS Z 2241 were carried out in the atmosphere at room temperature (25 ° C.) using the collected test pieces, and the yield stress and the tensile strength were determined. The yield stress was determined by the 0.5% total elongation method. Moreover, the yield ratio YR was obtained from the yield stress and the tensile strength. The results are shown in Table 2.

<CTOD test>
A CTOD (Crac Tip Opening Displacement) test was performed to evaluate the low temperature toughness of HAZ. Hereinafter, the test method will be described. A test piece for the CTOD test was collected from each seamless steel pipe at n = 3. Groove processing was applied to each test piece, and multi-layer welding was performed by gas metal arc welding (GMAW) at an input heat of 2.0 kJ / mm. The HAZ of the prepared welded joint was notched, and a CTOD test was performed at a test temperature of −20 ° C. in accordance with BS7448 standard. A CTOD value of 0.50 mm or more was regarded as acceptable. The results are shown in Table 2.

No. Since the seamless steel pipes (1) to (3), (5), (8), and (10) to (18) satisfy all the conditions specified in the present invention, the metal structure of the base metal is not satisfied. Even if the average effective crystal grain size was more than 10 μm, the CTOD test was passed.

No. In the seamless steel pipe of (4), the O content was higher than the range of the present invention, and the number density of the composite inclusions in the seamless steel pipe was higher than the range of the present invention. Therefore, No. The seamless steel pipe of (4) failed the CTOD test.

No. In the seamless steel pipe of (6), the O content was below the range of the present invention, and the number density of the composite inclusions in the seamless steel pipe was lower than the range of the present invention. Therefore, No. The seamless steel pipe of (6) failed the CTOD test.

No. In the seamless steel pipe of (7), the Ti content was higher than the range of the present invention, and the number density of the composite inclusions in the seamless steel pipe was higher than the range of the present invention. Therefore, No. The seamless steel pipe of (7) failed the CTOD test.

No. In the seamless steel pipe of (9), the Ti content was below the range of the present invention, and the number density of the composite inclusions in the seamless steel pipe was lower than the range of the present invention. Therefore, No. The seamless steel pipe of (9) failed the CTOD test.

As described above, the seamless steel pipe according to the present embodiment is excellent in low temperature toughness of HAZ. Therefore, it is suitable for line pipe applications.

Since the seamless steel pipe of the present invention is excellent in low temperature toughness of HAZ, it has high industrial applicability.

Claims (8)

By mass%,
C: 0.03 to 0.08%,
Si: 0.05-0.25%
Mn: 1.0-2.5%,
P: 0.05% or less,
S: 0.001 to 0.005%,
N: 0.0150% or less,
O: 0.0010-0.0050%,
Ti: 0.005 to 0.050%,
Containing, the balance consists of Fe and impurities,
A seamless steel pipe containing a composite inclusion in which MnS is present around the Ti oxide.
In the cross section of the seamless steel pipe,
The area ratio of the MnS to the composite inclusions is 10 to 90%, and the ratio of the length of the interface between the Ti oxide and the MnS of the composite inclusions to the total perimeter of the Ti oxide is The number density of the composite inclusions is 10% or more and the equivalent circle diameter of the Ti oxide is 0.5 to 5.0 μm, and the number density is 10 to 100 / mm ² .
In the cross section, the average effective crystal grain size of the metal structure is more than 10 μm.
A seamless steel pipe characterized by that. The seamless steel pipe according to claim 1, wherein instead of a part of the Fe, it contains Al: 0.007% or less in mass%. In place of a part of Fe, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.3% or less, V: 0.08% or less, B: 0.010 in mass%. The seamless steel pipe according to claim 1 or 2, characterized in that it contains one or more selected from the group consisting of% or less and Ca: 0.005% or less. The first to third claims are characterized in that, in place of a part of the Fe, one or more of Mo: 0.3% or less and Nb: 0.05% or less are contained in mass%. The seamless steel pipe described in any one of the items. The seamless steel pipe according to any one of claims 1 to 4, wherein the tensile strength is 758 MPa or less. The seamless steel pipe according to any one of claims 1 to 5, wherein the yield ratio YR is 0.75 or more. The seamless steel pipe according to any one of claims 1 to 6, wherein the wall thickness is less than 50 mm. The seamless steel pipe according to any one of claims 1 to 7, characterized in that it is for a line pipe.