KR20200073343A - The steel plate for excellent impact-toughness in welded joint, method for manufacturing thereof, and steel pipe using thereof - Google Patents

Abstract

The present invention is to provide a steel material having a strength equivalent to API standard 5ST CT90 while having excellent impact toughness in a welded joint, a method for manufacturing the same, and a steel pipe obtained using the steel material.

Description

Steel with excellent impact toughness in welding joints and manufacturing method thereof, steel pipe using the same {THE STEEL PLATE FOR EXCELLENT IMPACT-TOUGHNESS IN WELDED JOINT, METHOD FOR MANUFACTURING THEREOF, AND STEEL PIPE USING THEREOF}

The present invention relates to a steel material used for oil or gas mining, and more particularly, to a steel material excellent in impact toughness of a welded joint, a manufacturing method thereof, and a steel pipe obtained using the steel material.

Recently, the environment for developing oil wells or gas wells (hereinafter, collectively referred to as'oil wells') is becoming increasingly severe, and efforts to lower production costs in order to improve profitability have continued.

On the other hand, coiled tubes used in the oil and gas industry, etc., manufactured tubes with a diameter of 1 to 3.25 inches and a small diameter of several kilometers, and wound them on a reel, reel spooling during work. It is a form that is supplied through.

These coiled tubes can be used for various purposes, such as fluid circulation, pumping, drilling, drilling, drilling, and drilling in oil and gas wells.

The coiled tube needs to repeat reeling/unreeling every time it is used, and as a result, repeated bending stress accumulates, causing an early fracture of the tube.

Particularly, when a shock is applied to a welded part in which fatigue stress is concentrated, a problem occurs that shortens the product life due to fracture. Therefore, the development of a material that improves the impact toughness and the fatigue resistance of the welded joint can be improved. Development is required.

Korea Patent Publication No. 2014-0104497

One aspect of the present invention is to provide a steel material having a strength equivalent to API standard 5ST CT90 while having excellent impact toughness of a welded joint, a method of manufacturing the same, and a steel pipe obtained using the steel.

The subject of this invention is not limited to the above-mentioned content. Anyone having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding additional problems of the present invention from the contents throughout the present specification.

One aspect of the present invention, by weight, carbon (C): 0.10 to 0.15%, silicon (Si): 0.10 to 0.25%, manganese (Mn): 1.0 to 2.0%, phosphorus (P): 0.025% or less, Sulfur (S): 0.005% or less, Niobium (Nb): 0.05 to 0.10%, Nickel (Ni): 0.1 to 0.3%, Chromium (Cr): 0.15 to 0.30%, Molybdenum (Mo): 0.05 to 0.15%, Titanium (Ti): 0.01 to 0.05%, Vanadium (V): 0.05 to 0.10%, Nitrogen (N): 0.008% or less, the balance includes Fe and unavoidable impurities,

A microstructure is composed of ferrite, pearlite, and residual bainite having an average grain size of 15 µm or less and a maximum grain size of 30 µm or less, and provides a steel material having excellent weld joint impact toughness characterized by satisfying the following relational expression 1. .

[Relationship 1]

70 <3×10 ^-2 (5.19[F]+4.46[P]-0.7[B])-0.36[Dmax]-0.3[YS] + 4.7[EL] + 0.755[Hv] <82

(In equation 1, [F] is ferrite, [P] is pearlite, [B] is the fraction of bainite phase, [Dmax] is the maximum grain size of ferrite (㎛), [YS] is yield strength (MPa), [EL ] Means elongation, Hv means Vickers hardness.)

Another aspect of the present invention, preparing a steel slab having the above-described alloy composition; Heating the steel slab in a temperature range of 1100 to 1300°C; Preparing a hot rolled steel sheet by finishing hot rolling the heated steel slab in a temperature range of 750 to 850°C; And cooling the hot-rolled steel sheet to 500 to 600° C. and then winding it up.

Another aspect of the present invention provides a steel pipe having excellent impact toughness of a welded joint obtained by molding and welding the steel.

According to the present invention, it is possible to provide a steel material having not only the strength equivalent to API standard 5ST CT90, but also excellent in the impact toughness of the welded joint even after molding and welding with a steel pipe.

Further, the steel pipe obtained by molding and welding the steel material of the present invention can be suitably applied as a coiled tubing.

The present inventors have studied in depth to develop a material capable of improving physical properties suitable for the demand for a coiled tubing used in a gas well or an oil well environment. In particular, after manufacturing the material as a steel pipe, it was intended to provide a steel material with excellent impact toughness at the welded joint, having properties equivalent to CT90 required within the PAI standard 5ST standard.

As a result, it was confirmed that the intended steel can be provided by optimizing the relationship between the composition and mechanical properties of the steel, as well as the alloy composition and manufacturing conditions of the steel, and the present invention has been completed.

Hereinafter, the present invention will be described in detail.

The weld joint according to an aspect of the present invention has excellent impact toughness in weight%, carbon (C): 0.10 to 0.15%, silicon (Si): 0.10 to 0.25%, manganese (Mn): 1.0 to 2.0%, Phosphorus (P): 0.025% or less, sulfur (S): 0.005% or less, niobium (Nb): 0.05 to 0.10%, nickel (Ni): 0.1 to 0.3%, chromium (Cr): 0.15 to 0.30%, molybdenum ( Mo): 0.05 to 0.15%, titanium (Ti): 0.01 to 0.05%, vanadium (V): 0.05 to 0.10%, nitrogen (N): 0.008% or less.

Hereinafter, the reason for limiting the alloy composition of the steel material provided in the present invention as described above will be described in detail.

On the other hand, unless specifically stated in the present invention, the content of each element is based on weight, and the proportion of tissue is based on area.

Carbon (C): 0.10~0.15%

Carbon (C) is an element added to secure the strength of the steel, and can improve the quenching property to secure the strength. If the content of C is less than 0.10%, it is difficult to improve strength by combining with precipitation-enhancing elements such as Nb, V, Ti, etc., whereas when the content exceeds 0.15%, yield strength is excessively increased. You will exceed your target level of strength.

Therefore, in the present invention, C may be included in 0.10 to 0.15%.

Silicon (Si): 0.10~0.25%

Silicon (Si) is added for deoxidation during steel production and is an element that is advantageous for securing strength. If the content of Si is less than 0.10%, the deoxidation effect is insufficient, whereas when the content exceeds 0.25%, the MA phase is formed, impact toughness deteriorates, and embrittlement may occur during tempering.

Therefore, in the present invention, Si may be included at 0.10 to 0.25%.

Manganese (Mn): 1.0-2.0%

Manganese (Mn) is added to secure the strength of steel, but if its content is less than 1.0%, it is difficult to secure the target level of strength. On the other hand, when the content of Mn exceeds 2.0%, there is a problem that a central segregation is generated during performance, thereby deteriorating impact toughness and inhibiting fatigue property resistance.

Therefore, in the present invention, Mn may be included in an amount of 1.0 to 2.0%.

Phosphorus (P): 0.025% or less

Phosphorus (P) is an imperatively added impurity in steel and inhibits toughness, so it is preferable to contain it as low as possible. When the content of P exceeds 0.025%, there is a problem that the impact toughness is inhibited by inducing central segregation when playing, so the P may be included in an amount of 0.025% or less.

Sulfur (S): 0.005% or less

Since sulfur (S) is an imperatively added impurity in steel, it is preferable to contain it as low as possible. When the content of S exceeds 0.005%, MnS is formed by combining with Mn in steel, which is a major cause of deteriorating the toughness of steel. Therefore, the S may be included in 0.005% or less.

Niobium (Nb): 0.05 to 0.10%

Niobium (Nb) is an element that is advantageous for forming a precipitate to secure the strength of the steel. In particular, it is possible to obtain a precipitation strengthening effect by generating an NbC precipitate. If the content of Nb is less than 0.05%, a precipitation strengthening effect cannot be sufficiently obtained, whereas when the content exceeds 0.10%, there is a problem of inhibiting toughness by promoting formation of coarse precipitates and MA phases.

Therefore, in the present invention, Nb may be included in 0.05 to 0.10%.

Nickel (Ni): 0.1 to 0.3%

Nickel (Ni) serves to improve toughness by lowering the stacking fault energy (SFE) in the lattice to lower the transition temperature. In addition, there is an effect of suppressing the generation of a low melting point compound to suppress the occurrence of cracks during hot working. In order to sufficiently obtain the above-described effect, the Ni may be contained in 0.1% or more, but if the content exceeds 0.3%, there is a problem that the Ni compound is generated and the toughness deteriorates.

Therefore, in the present invention, the Ni may be included in 0.1 to 0.3%.

Chromium (Cr): 0.15~0.30%

Chromium (Cr) is an effective element for improving the hardenability and corrosion resistance of steel. If the content is less than 0.15%, the above-described effect cannot be sufficiently obtained. On the other hand, if the content exceeds 0.30%, there is a problem that causes defects in the weld or causes brittleness.

Therefore, in the present invention, Cr may be included in an amount of 0.15 to 0.30%.

Molybdenum (Mo): 0.05~0.15%

Molybdenum (Mo) is an element having a large hardenability, and is advantageous not only for strength improvement but also for fatigue resistance improvement. If the content of Mo is less than 0.05%, the above-described effect cannot be sufficiently obtained, whereas when the content exceeds 0.15%, carbonized precipitates are generated to inhibit toughness.

Therefore, in the present invention, the Mo may be included as 0.05 to 0.15%.

Titanium (Ti): 0.01~0.05%

Titanium (Ti) is an element added to improve the strength and toughness of steel. In particular, TiC precipitates can be produced to obtain a precipitation strengthening effect, and TiN precipitates are generated to suppress austenite grain growth, thereby reducing strength and toughness by grain refinement. Improvement effect can be obtained. If the content of Ti is less than 0.01%, the above-described effect cannot be sufficiently obtained, whereas when the content exceeds 0.05%, coarse Ti precipitates are generated, thereby inhibiting toughness.

Therefore, in the present invention, Ti may be included in an amount of 0.01 to 0.05%.

Vanadium (V): 0.05~0.10%

Vanadium (V) is an element that is advantageous for improving the strength of steel from the precipitation strengthening effect. When the content of V is less than 0.05%, the above-described effect cannot be sufficiently obtained, whereas when the content exceeds 0.10%, the manufacturing cost may increase and the toughness may be rapidly reduced by producing carbide.

Therefore, in the present invention, V may be included as 0.05 to 0.10%.

Nitrogen (N): 0.008% or less (excluding 0%)

Nitrogen (N) is mainly combined with Ti in the steel to generate nitrides, which may impair the function of other alloying elements. In particular, when the content exceeds 0.008%, coarse precipitates are generated, thereby inhibiting toughness.

Therefore, in the present invention, the N may be limited to 0.008% or less, and 0% may be excluded in consideration of the load of the steelmaking process.

The remaining component of the invention is iron (Fe). However, in the normal manufacturing process, impurities that are not intended from the raw material or the surrounding environment may be inevitably mixed, and therefore cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, they are not specifically mentioned in this specification.

The steel material of the present invention that satisfies the above-described alloy composition may include some bainite together with a ferrite and pearlite composite structure as a microstructure.

Specifically, the steel material of the present invention includes ferrite of an area fraction of 60 to 80%, pearlite of 20 to 40%, and may include some bainite, wherein the bainite phase may include an area fraction of 15% or less. have.

Since the ferrite is easy to form a slip surface of a lattice structure and exhibits excellent shock absorption, it is preferable to include an area fraction of 60% or more. However, if the fraction exceeds 80%, the target level of strength cannot be secured.

In addition, the ferrite preferably has an average grain size of 15 μm or less and a maximum grain size of 30 μm or less. When the average grain size of the ferrite exceeds 15 µm, the grain boundary absorbing the impact is reduced and the toughness decreases. When the maximum grain size exceeds 30 µm, non-uniform grains exist and the impact toughness rapidly decreases. have. Here, the average grain size means an average diameter corresponding to a circle.

On the other hand, the steel material of the present invention that satisfies the above-described alloy composition and microstructure, it is preferable that the relationship between the structure phase (phase) fraction and mechanical properties (YS, El, Hv) satisfies the following relational expression 1.

[Relationship 1]

70 <3×10 ^-2 (5.19[F]+4.46[P]-0.7[B])-0.36[Dmax]-0.3[YS] + 4.7[EL] + 0.755[Hv] <82

(In equation 1, [F] is ferrite, [P] is pearlite, [B] is the fraction of bainite phase, [Dmax] is the maximum grain size of ferrite (㎛), [YS] is yield strength (MPa), [EL ] Means elongation, Hv means Vickers hardness.)

In the present invention, the tissue phase fraction and grain size are important factors in securing the intended physical properties, and by optimizing the relationship between the material properties, the toughness of the weld targeted in the present invention can be excellently secured.

Specifically, the yield strength determines the elastic limit of the material (material), and the higher the yield strength and elongation, the higher the material's impact absorption capacity, that is, toughness. However, even if the yield strength is improved, the higher the hardenability (Vickers hardness) of the material, the shorter the fracture life of the material and the lower the toughness.

Therefore, in the present invention, there is a technical significance in controlling the relationship between yield strength, elongation and Vickers hardness along with the fraction and grain size on the tissues by the above equation (1).

Hereinafter, another aspect of the present invention will be described in detail with respect to a method for manufacturing a steel material having excellent weld joint impact toughness.

The steel material according to the present invention can be prepared through a series of processes of [heating-hot rolling-cooling-winding] after preparing a steel slab that satisfies the alloy composition proposed in the present invention. It will be described in detail.

[Slab heating process]

Since the heating process of the steel slab is a process of smoothly performing the subsequent rolling process and heating the steel so as to sufficiently obtain the properties of the target steel sheet, the temperature is maintained within a suitable austenite region to obtain homogeneous initial microstructure and precipitates. Should be controlled.

In the present invention, a heating process may be performed in a temperature range of 1100 to 1300°C. If the temperature during heating is less than 1100°C, high-melting-point compounds such as Nb and Ti are difficult to be completely dissolved, and thus there is a high possibility of remaining as a segregation zone. On the other hand, when it exceeds 1300°C, the initial crystal grain becomes too coarse and fine in the final product. Organizations cannot be obtained.

[Hot rolling process]

The steel slab heated according to the above can be hot-rolled to produce a hot-rolled steel sheet.

Finishing hot rolling at the time of the hot rolling may be performed in a temperature range of 750 ~ 850 ℃. If the finish hot rolling temperature is less than 750°C, MnS may be generated, and thus impact toughness and fatigue resistance may be deteriorated. On the other hand, when the temperature exceeds 850°C, secondary phase (carbide) formation and microstructural crystallites are unstable. It is difficult to secure impact toughness and strength due to intensification of homogenization.

[Cooling and coiling process]

After cooling the hot-rolled steel sheet produced according to the above-described can be wound.

The cooling may be performed to a temperature range of 500 to 600° C., and then coiled at that temperature. At this time, when the coiling temperature is less than 500°C, a mild low-temperature transformation phase such as a bainite phase is locally formed, and thus there is a fear that fatigue resistance is deteriorated. On the other hand, when the temperature exceeds 600°C, a coarse pearlite phase is easily formed, and thus, fatigue resistance may be deteriorated as fatigue propagation is facilitated. More advantageously, winding can be performed in a temperature range of 500 to 550°C.

The cooling rate when cooling to the above-described temperature range is not particularly limited, but can be performed at 95° C./s or less in terms of promoting grain refinement while limiting formation of a low-temperature transformation structure.

However, since the ferrite phase is excessively formed when the cooling rate is too slow, it may be performed at 40° C./s or more in consideration of this.

It can be manufactured into a steel pipe using a hot-rolled steel sheet manufactured according to the above. As an example, the prepared hot-rolled steel sheet may be formed into a pipe shape after removing the scale of the surface from the pickling treatment process, and welded abutted portions to obtain a steel pipe having excellent fatigue resistance. As another example, the hot rolled steel sheet may be slit to a predetermined width and welded and fabricated.

The method for manufacturing the steel pipe is not particularly limited, but can be prepared using an electric resistance welding having excellent economic efficiency. Any welding method can be used for the electric resistance welding, so the welding method is not particularly limited.

The steel pipe obtained by using the steel material of the present invention has a yield strength of 620 to 689 MPa, a tensile strength of 669 MPa or more, and the impact toughness of the welded joint is 40 J or more, and the target strength as well as the impact toughness of the welded portion can be excellently secured. . Therefore, the steel pipe of the present invention can be suitably used as a coiled tubing.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the items described in the claims and the items reasonably inferred therefrom.

( Example )

The steel slab having the alloy composition of Table 1 was heated-finished hot rolled-cooled-wound under the conditions shown in Table 2 below to prepare a hot-rolled steel sheet having a thickness of 5.2 mm. At this time, heating was performed for 2 hours, and cooling was performed at 70°C/s.

Then, microstructures were observed for each of the hot-rolled steel sheets prepared, and the results are shown in Table 3 below.

On the other hand, the prepared hot-rolled steel sheet was subjected to a tensile test and an impact test according to ASTM A370 standard after the electric resistance welding pipe, and the results are shown in Table 3 below. At this time, the tensile test was measured using a MTS 100kN class tensile tester at a rate of 10mm/min yield strength (YS), tensile strength (TS), elongation (El), and the impact test was a welding heat affected zone (HAZ). Was measured using a Charpy impact tester.

Then, the hardness was measured using a Vickers hardness tester at the center of the thickness of each steel pipe, and it was expressed as an average value after 15 measurements.

division Alloy composition (% by weight) C Si Mn P S Nb Ni Cr Mo Ti V N Invention Steel 1 0.13 0.24 1.04 0.007 0.001 0.058 0.13 0.20 0.05 0.02 0.05 0.005 Invention Steel 2 0.11 0.25 1.06 0.011 0.002 0.063 0.11 0.17 0.13 0.02 0.06 0.005 Invention Steel 3 0.11 0.24 1.01 0.007 0.002 0.066 0.10 0.20 0.09 0.01 0.06 0.004 Comparative Steel 1 0.12 0.25 1.06 0.006 0.002 0.060 0.10 0.17 0.11 0.02 0.06 0.005 Comparative Steel 2 0.12 0.25 1.04 0.009 0.001 0.057 0.11 0.16 0.11 0.02 0.06 0.004 Comparative Steel 3 0.12 0.30 1.04 0.006 0.001 0.060 0.18 0.16 0.14 0.02 0.05 0.005 Comparative steel 4 0.12 0.29 1.04 0.005 0.001 0.060 0.19 0.16 0.14 0.02 0.06 0.004 Comparative Steel 5 0.11 0.26 1.01 0.005 0.001 0.067 0.01 0.19 0.09 0.01 0.06 0.005 Comparative steel 6 0.10 0.27 0.99 0.007 0.001 0.063 0.02 0.20 0.10 0.01 0.05 0.004

division Heating temperature
(℃) Finish hot rolling
Temperature (℃) Winding temperature
(℃) Invention Steel 1 1203 773 524 Invention Steel 2 1160 781 518 Invention Steel 3 1185 775 504 Comparative steel 1 1138 777 504 Comparative steel 2 1230 784 512 Comparative steel 3 1200 761 565 Comparative River 4 1200 789 565 Comparative steel 5 1207 784 502 Comparative steel 6 1180 780 500

division
Microstructure Mechanical properties Ceq HAZ
Impact toughness
(J@0℃) Relation
One F P B F average
Crystal grain
(㎛) F max
Crystal grain
(㎛) YS
(MPa) TS
(MPa) El
(%) Hardness
(Hv) Invention Steel 1 71 29 0 8.5 25 671 696 23 209.3 0.37 42.6 70.8 Invention Steel 2 68 32 0 8.4 21 642 714 23 204.3 0.37 73.9 77.1 Invention Steel 3 70 30 0 7.8 15 654 703 24 206 0.36 63.5 81.6 Comparative Steel 1 67 24 9 11 25 672 737 22 208.3 0.38 21.3 63.5 Comparative Steel 2 67 25 8 6.8 41 712 770 23 213.3 0.37 18.6 54.4 Comparative Steel 3 67 26 7 15 35 705 761 21 194.5 0.38 9.3 35.2 Comparative steel 4 68 26 6 20 20 690 753 20 206 0.38 18.6 49.3 Comparative Steel 5 63 37 0 8.5 23 610 667 25 198.3 0.35 47 90.7 Comparative steel 6 64 36 0 6.7 20 615 669 24 194.3 0.35 34.6 82.6

(In Table 3, F is ferrite, P is pearlite, and B is bainite, and each is expressed as an area fraction. The carbon equivalent (Ceq) value is [Ceq = C + Mn/6 + Si/24 + Ni/40 + Cr/ 5 + Mo/4 + V/14].)

As shown in Tables 1 to 3, the invention steels 1 to 3 satisfying both the alloy composition and the manufacturing conditions proposed in the present invention can be confirmed that the weld heat-affected zone has an impact toughness of 40 J or more after manufacturing the steel pipes. , It can be confirmed that it has the target strength.

On the other hand, it can be seen that the comparative alloys 1 to 6 in which the alloy composition deviates from the present invention are poor in the impact toughness of the welding heat-affected zone as the structure is coarsely formed or the precipitate or low-temperature transformation phase is excessively formed.

In particular, all of Comparative Steels 1 to 6 did not satisfy the relationship 1 suggested in the present invention.

Claims (9)

In weight percent, carbon (C): 0.10 to 0.15%, silicon (Si): 0.10 to 0.25%, manganese (Mn): 1.0 to 2.0%, phosphorus (P): 0.025% or less, sulfur (S): 0.005% Hereinafter, niobium (Nb): 0.05 to 0.10%, nickel (Ni): 0.1 to 0.3%, chromium (Cr): 0.15 to 0.30%, molybdenum (Mo): 0.05 to 0.15%, titanium (Ti): 0.01 to 0.05 %, Vanadium (V): 0.05 to 0.10%, Nitrogen (N): 0.008% or less, including residual Fe and unavoidable impurities,
The microstructure is composed of ferrite, pearlite, and residual bainite having an average grain size of 15 μm or less and a maximum grain size of 30 μm or less,
Steel with excellent weld toughness impact toughness, characterized in that it satisfies the following relational expression 1.

[Relationship 1]
70 <3×10 ^-2 (5.19[F]+4.46[P]-0.7[B])-0.36[Dmax]-0.3[YS] + 4.7[EL] + 0.755[Hv] <82
(In equation 1, [F] is ferrite, [P] is pearlite, [B] is the fraction of bainite phase, [Dmax] is the maximum grain size of ferrite (㎛), [YS] is yield strength (MPa), [EL ] Means elongation, Hv means Vickers hardness.)
According to claim 1,
The ferrite includes an area fraction of 60 to 80%, and the pearlite comprises an area fraction of 20 to 40%.
According to claim 1,
The bainite is a steel material having excellent impact toughness in the folded part, which includes an area fraction of 15% or less.
In weight percent, carbon (C): 0.10 to 0.15%, silicon (Si): 0.10 to 0.25%, manganese (Mn): 1.0 to 2.0%, phosphorus (P): 0.025% or less, sulfur (S): 0.005% Hereinafter, niobium (Nb): 0.05 to 0.10%, nickel (Ni): 0.1 to 0.3%, chromium (Cr): 0.15 to 0.30%, molybdenum (Mo): 0.05 to 0.15%, titanium (Ti): 0.01 to 0.05 %, Vanadium (V): 0.05 to 0.10%, Nitrogen (N): 0.008% or less, preparing a steel slab containing the balance Fe and unavoidable impurities;
Heating the steel slab in a temperature range of 1100 to 1300°C;
Preparing a hot rolled steel sheet by finishing hot rolling the heated steel slab in a temperature range of 750 to 850°C; And
After cooling the hot-rolled steel sheet to 500 ~ 600 ℃ winding up
Method for producing a steel material having excellent weld toughness impact welding.
The method of claim 4,
The winding is a method of manufacturing a steel joint having excellent impact toughness that is performed in a temperature range of 500 to 550°C.
The method of claim 4,
The cooling method is performed at a cooling rate of 95°C/s or less, and a method of manufacturing a steel with excellent weld toughness impact toughness.
The method of claim 4,
The steel is a method of manufacturing a steel joint having excellent impact toughness that satisfies the following relational expression 1.

[Relationship 1]
70 <3×10 ^-2 (5.19[F]+4.46[P]-0.7[B])-0.36[Dmax]-0.3[YS] + 4.7[EL] + 0.755[Hv] <82
(In equation 1, [F] is ferrite, [P] is pearlite, [B] is the fraction of bainite phase, [Dmax] is the maximum grain size of ferrite (㎛), [YS] is yield strength (MPa), [EL ] Means elongation, Hv means Vickers hardness.)
A steel pipe excellent in impact toughness of a welded joint obtained by molding and welding a steel material according to any one of claims 1 to 3.
The method of claim 8,
The welded steel pipe has a yield strength of 620 to 689 MPa, a tensile strength of 669 MPa or higher, and a weld joint having an impact toughness of 40 J or more, which is excellent in impact toughness of a welded joint.