JP7534593B2 - Spiral Steel Pipe - Google Patents

Description

The present invention relates to a spiral steel pipe, and in particular to a spiral steel pipe with excellent weld metal toughness.

Spiral steel pipes have traditionally been used in buildings and other structures, such as bridges and steel towers. Spiral steel pipes have long been studied with emphasis on strength and productivity.

In recent years, there has been a demand for spiral steel pipes that have excellent toughness as well as strength. Specifically, for example, a toughness value of 27 J at 0°C in the Charpy impact test is required.

Furthermore, spiral steel pipes are manufactured by processing hot-rolled steel sheet (steel strip) into a spiral shape, and then applying submerged arc welding to the inner and outer surfaces of the butted ends in the width direction to form a pipe. Therefore, it is becoming increasingly important for spiral steel pipes to have good toughness not only in their base material, but also in their weld metal parts. In particular, it is considered important for spiral steel pipes used in the construction industry to have high toughness in their weld metal parts. Furthermore, it is becoming increasingly necessary for spiral steel pipes used in the civil engineering industry to have good toughness in their weld metal parts.

Patent Document 1 discloses a high-strength spiral steel pipe pile having a structure in which the main phase is a bainitic ferrite phase and a secondary phase containing one or more of the following phases in a volume fraction of 10% to less than 50% in total: martensite phase, bainite phase, and pearlite; a low yield ratio high strength of axial yield strength YS of 450 MPa or more, tensile strength TS of 570 MPa or more, and yield ratio YR of 90% or less; and a high toughness of absorbed energy vE0 of 27 J or more at a test temperature of 0°C in a Charpy impact test.

Patent Document 2 discloses a spiral steel pipe in which the combined ratio of tempered martensite and bainite in the weld metal structure is 80% or more.

JP 2016-47956 A JP 2011-161500 A

For SM570 material, which is commonly used for steel pipe piles, the Charpy absorbed energy at -5°C is specified as an indicator of toughness. However, the toughness of the weld metal is lower than that of the SM570 base material, and there is room for improvement.

The objective of the present invention is to provide a spiral steel pipe with excellent toughness of the weld metal.

The inventors conducted extensive research to solve the above problems and discovered that it is possible to improve the toughness of the weld metal by suppressing grain boundary ferrite and forming a weld metal structure that is mainly composed of acicular ferrite. The present invention was made as a result of further research, and the gist of the invention is as follows.

[1] A spiral steel pipe in which the widthwise end faces of a spirally wound steel strip are welded from the inside and outside, and the chemical composition of the weld metal is, in mass%, C: 0.030 to 0.150%, Si: 0.05 to 0.50%, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Al: 0.001 to 0.050%, Ti: 0.002% to 0.050%, B: more than 0%, 0.0050% or less, N: 0.0100% or less, O: 0.0150 to 0.0600%, balance: Fe and impurities, and the content (mass) of elements %) is expressed by the element symbol, the condition 0.300≦Al/O≦1.000 is satisfied, α' defined as α' = (1.5 x (O-0.89Al) + 3.4 x N-Ti) x 1000 is 0 to 60.0%, Ceq defined as C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 is 0.350 to 0.450%, and Pcm defined as Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B is 0.250% or less.

[2] The structure of the weld metal has an area ratio of acicular ferrite: 70.0% or more;
The spiral steel pipe according to [1], characterized in that it contains grain boundary ferrite: 20.0% or less, island martensite: 5.0% or less, and has an EBSD grain size of 15.0 μm or less.

[3] The spiral steel pipe of [1] or [2], characterized in that the weld metal further contains, in place of a portion of the Fe, one or more elements selected from the group consisting of Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.050% or less, Nb: 0.050% or less, Mg: 0.0100% or less, and Ca: 0.0060% or less.

[4] A spiral steel pipe according to any one of [1] to [3], characterized in that the chemical composition of the base material of the spiral steel pipe is, in mass%, C: 0.030-0.150%, Si: 0.55% or less, Mn: 0.50-2.00%, P: 0.020% or less, S: 0.010% or less, balance: Fe and impurities.

[5] A spiral steel pipe according to any one of [1] to [4], characterized in that the base material further contains, in place of a portion of the Fe, one or more elements selected from the group consisting of Al: 0.100% or less, Ti: 0.030% or less, N: 0.0060% or less, O: 0.0050% or less, Ca: 0.0050% or less, Ni: 0.50% or less, Cr: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, Nb: 0.100% or less, B: 0.0020% or less, V: 0.060% or less, and Mg: 0.0100% or less.

According to the present invention, a spiral steel pipe with excellent toughness of the weld metal can be obtained.

The present invention will be described in detail below.

The spiral steel pipe of the present invention has a weld metal part formed by submerged arc welding the widthwise end faces of a spirally wound steel strip from the inside and outside. The spiral steel pipe of the present invention can be manufactured by a manufacturing method including a heating process, a cooling process, a tempering process, and a pipe-making process. Hereinafter, "%" in relation to the chemical composition means "% by mass."

First, we will explain the chemical composition of the weld metal.

C: 0.030-0.150%
If the C content is less than 0.030%, the hardenability is low and the weld metal does not form a sufficiently hardened structure. In addition, the susceptibility to hot cracking increases. In particular, after welding to the inner surface of a spiral steel pipe, In the subsequent weld metal part formed by welding to the outer surface, the cooling rate is slowed down due to the preheating effect when welding to the inner surface, so there is a high possibility of hot cracks occurring. If the C content exceeds 150%, the hardenability becomes excessive, increasing the risk of cracking during cooling in the cooling process. Also, if the C content is high, there is a concern of high-temperature cracking. Therefore, the C content is set to 0.030 to 0.150%. Let it be 0.150%.

Si: 0.05-0.50%
If the Si content is less than 0.05%, deoxidation is insufficient and coarse oxides may be formed. If the Si content exceeds 0.50%, coarse island martensite ( As a result, the toughness of the weld metal decreases. Therefore, the Si content is set to 0.05 to 0.50%.

Mn: 0.50-2.00%
Mn is an element necessary for ensuring hardenability. If the Mn content is less than 0.50%, the hardenability is insufficient. If the Mn content is more than 2.00%, the hardenability is insufficient. This causes excessive stress, which may cause cracking during the cooling process. It is also difficult to restore the toughness of the weld metal after quenching. Furthermore, coarse MnS is formed, which becomes the starting point of fracture and reduces toughness. Therefore, The Mn content is 0.50 to 2.00%.

P: 0.020% or less,
P is an impurity, and its content is set to 0.020% or less. The content may be 0. P is an element that promotes solidification cracking. When the P content exceeds 0.020%, Since the risk of solidification cracking increases when the content is increased, the content is reduced in consideration of refining costs.

S: 0.010% or less S is an impurity, and its content is set to 0.010% or less. The content may be 0. S is an element that promotes solidification cracking together with P. If the S content exceeds 0.010%, the risk of solidification cracking increases, so the content is reduced in consideration of refining costs.

Al: 0.001 to 0.050% Al is an element necessary for controlling the amount of oxygen in order to disperse a large number of oxides that become acicular ferrite formation sites in the weld metal. Al is contained in the base metal, welding wire, and flux. If the Al content is less than 0.001%, the above oxides are hardly obtained. If the Al content exceeds 0.050%, large amounts of Al ₂ O ₃ are formed, which become the starting points of fracture and reduce toughness. Therefore, the Al content is set to 0.001% or more and 0.050% or less.

Ti: 0.002% to 0.050%
Ti is one of the constituent elements of oxides that become the sites for the formation of acicular ferrite, and promotes the refinement of the weld metal structure. If the Ti content is less than 0.002%, the above effect cannot be obtained. Ti content If the Ti content exceeds 0.050%, the amount of solute Ti increases, forming carbides during the tempering process, and the toughness of the weld metal part decreases. 050% or less.

B: More than 0% and 0.0050% or less B in a solid solution state has the effect of suppressing the formation of grain boundary ferrite in the weld metal, thereby promoting the formation of acicular ferrite. This effect can be obtained even with a very small amount of B. To obtain the effect more reliably, a content of 0.0001% or more is preferable. In order to prevent a decrease in toughness due to excessive strength, the content is set to 0.0050% or less.

N: 0.0100% or less N is an impurity. In order to prevent dissolved N that does not react with Ti and remains there, from reducing toughness, the content is made 0.0100% or less.

O: 0.0150-0.0600%
O is to be 0.0150% or more in order to form oxides that become the nuclei of acicular ferrite, and is to be 0.0600% or less in order to suppress a decrease in toughness due to excessive formation, aggregation and coarsening of oxides.

The weld metal may further contain the following elements as necessary:

Cu: 0.50% or less Cu is an element that can improve the strength of the weld metal and is added as necessary. The Cu content is preferably in the range of 0.50% or less.

Ni: 0.50% or less Ni is an element that can improve the strength of the weld metal without reducing the toughness, and also improves the hardenability. Although not essential, it is preferable to include Ni in the range of 0.50% or less.

Cr: 0.50% or less Cr is an element that can improve the strength of the weld metal and also enhances hardenability. Although not essential, it is preferable to include Cr in the range of 0.50% or less.

Mo: 0.50% or less Mo is an element that can improve the strength of the weld metal and also enhances hardenability. Although not essential, it is preferable to include Mo in an amount of 0.50% or less.

V: 0.050% or less V is an element that can improve the strength of the weld metal. Although not essential, it is preferable that V be contained in the range of 0.050% or less.

Nb: 0.050% or less Nb is an effective element for improving strength and for providing dissolved B, which is effective for suppressing grain boundary ferrite. The content of Nb is not essential. In order to prevent a decrease in toughness due to the formation of island martensite, the content is preferably 0.050% or less.

Mg: 0.0100% or less Mg is an element that acts as a deoxidizer to reduce the amount of oxygen in the weld metal and improve toughness. Although not essential, it is preferable to include Mg in an amount of 0.0100% or less.

Ca: 0.0060% or less Ca is an element effective for improving ductility and refining the structure by morphology control. The inclusion of Ca is not essential. In order to prevent a decrease in ductility and toughness due to coarsening of sulfides and oxides, it is preferable to set the content to 0.0060% or less.

The remainder of the weld metal is Fe and impurities. Impurities are components that are mixed in during the welding process from the welding wire, flux, steel plate, surrounding atmosphere, etc., and are not intentionally included.

Specific examples include P, S, N, Sb, Sn, W, Co, As, Pb, Bi, and H. Of these, P, S, and N must be controlled to P: 0.020% or less, S: 0.010% or less, and N: 0.0100% or less, as described above.

As for other elements, Sb, Sn, W, Co, and As may be present as unavoidable impurities at levels of 0.1% or less, Pb and Bi at levels of 0.005% or less, and H at levels of 0.0005% or less. However, within normal ranges, there is no need to control them.

The chemical composition of the weld metal must also satisfy the following conditions for Al/O, α', Ceq, and Pcm. In the following explanations of Al/O, α', Ceq, and Pcm, the element symbols represent the content (mass%) of the element.

Al/O: 0.300-1.000
Al/O is the ratio of the amount of Al to the amount of O, and is an index showing the oxygen potential after the completion of aluminum deoxidation. By controlling Al/O to 0.300 to 1.000, the formation of acicular ferrite is improved. The production yield can be improved.

If Al/O is less than 0.300, the amount of O will be excessive, and the dissolved oxygen that did not form Ti oxides will reduce the cleanliness of the steel, resulting in reduced toughness. On the other hand, if Al/O is more than 1.000, the amount of Al will be excessive, reducing the amount of O that bonds with Ti, decreasing the amount of Ti oxides that become acicular ferrite nuclei, and reducing toughness. Therefore, Al/O should be between 0.300 and 1.000.

α': 0-60.0%
α' is a parameter that indicates the effective acicular ferrite formation ability based on the stoichiometric ratios of Al, O, Ti, and N, and is defined as α' = (1.5 x (O - 0.89Al) + 3.4 x N - Ti) x 1000. By controlling α' to the range of 0 to 60.0%, the acicular ferrite nucleation ability is improved. Here, elements that are not contained in the weld metal are calculated as zero (the same applies in the following explanation).

When α' is less than 0%, either the amount of Al or Ti is excessive, or the amount of N and O is insufficient, so the nucleation ability of acicular ferrite is significantly reduced. When α' is more than 60.0%, either the amount of Al or Ti is insufficient, or the amount of N and O is excessive, so the nucleation ability of acicular ferrite is significantly reduced.

Ceq: 0.350-0.450%
The chemical composition of the weld metal must have a Ceq of 0.350 to 0.450%, defined as C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15.

Ceq is the sum of the hardening ability of each alloy element converted into the amount of C for the base metal due to the welding heat effect. In order for the weld metal to achieve the desired tensile strength, Ceq is controlled to 0.350-0.450%. Preferably, Ceq is 0.400-0.430%.

Pcm: 0.250% or less The chemical composition of the weld metal must have a Pcm value defined as C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B of 0.250% or less.

Pcm is called weld sensitivity, and is a quantitative evaluation of the effect of the chemical components of steel on cold cracking. If Pcm exceeds 0.250%, cold cracking becomes more likely to occur, so the upper limit is set at 0.250%.

The weld metal of the spiral steel pipe of the present invention preferably has the following structure by having the above chemical composition. Hereinafter, "%" in relation to the structure means "area %".

Acicular ferrite: 70.0% or more Acicular ferrite is an acicular ferrite structure with Ti-based oxides as its nucleus, and the greater the proportion of acicular ferrite, the finer the fracture units of the weld metal part. To obtain this effect, it is preferable that the acicular ferrite content is 70.0% or more. Most preferably, the proportion of acicular ferrite is 100%.

Grain boundary ferrite: 20.0% or less Grain boundary ferrite is one of the embrittlement phases, which becomes the starting point of fracture and causes a decrease in toughness. Therefore, it is preferable that the grain boundary ferrite content is 20.0% or less.

Island martensite: 5.0% or less
Island martensite is one of the embrittling phases, and because it has a very high hardness, it can become the starting point of fracture and cause a decrease in toughness. Therefore, it is preferable that the island martensite content is 5.0% or less.

The remainder of the structure may include intragranular ferrite, ferrite side plates, and upper bainite. These structures are the starting points of fracture and cause a decrease in toughness, so as described above, it is preferable that the fracture units are refined by acicular ferrite.

EBSD grain size: 15.0 μm or less Furthermore, in the weld metal structure, it is preferable that the EBSD grain size is 15.0 μm or less. The EBSD (Electron Back Scatter Diffraction) grain size is a crystal grain size size that serves as a guide for fracture units. If the EBSD grain size is 15.0 μm or less, the fracture units are fine, and toughness at low temperatures can be ensured, which is preferable.

In the spiral steel pipe of the present invention, the composition of the weld metal can be controlled as described above by combining an appropriate flux and wire, and the welding heat input can be appropriately controlled to obtain the above-mentioned structure.

This makes it possible to obtain a spiral steel pipe with excellent toughness even in the weld metal zone. Specifically, it is possible to obtain a spiral steel pipe with a weld metal zone with a Charpy absorbed energy of 100 J or more at -5°C.

Next, we will explain the preferred chemical composition of the base material.

C: 0.030-0.150%
C is effective in improving the strength of steel, and it is preferable to include 0.030% or more in order to obtain the desired strength. If the C content is too high, the hardenability increases too much, and the toughness of the base material decreases. Therefore, the C content is preferably 0.150% or less, and more preferably 0.060 to 0.080%.

Si: 0.55% or less Si is an element necessary for deoxidation. If the amount of Si is large, island martensite is easily formed, and low-temperature toughness is significantly deteriorated, so the amount of Si is preferably 0.55% or less. More preferably, it is less than 0.35%. Since deoxidation can be performed with Al and Ti, the addition of Si is not essential.

Mn: 0.50-2.00%
Mn acts as an element for improving hardenability, and in order to obtain this effect, it is preferable to include 0.50% or more. If the Mn content is too high, the hardenability of the steel increases, deteriorating the HAZ toughness and weldability, and further However, Mn promotes center segregation in continuously cast steel slabs and deteriorates the low-temperature toughness of the base material, so the Mn content is preferably 2.00% or less, and more preferably 1.00 to 1.80%. .

P: 0.020% or less S: 0.010% or less Both P and S are impurities and elements that deteriorate the toughness of joints. It is preferable that their contents are as low as possible, and it is preferable that P is 0.020% or less and S is 0.010% or less. More preferably, P is 0.010% or less and S is 0.003% or less.

The base material may further contain the following elements as necessary:

Al: 0.100% or less Al is usually used as a deoxidizer and is an element contained in steel materials. If the Al content is too high, the amount of Al-based nonmetallic inclusions increases, the cleanliness of the steel material decreases, and the toughness deteriorates, so it is preferable to keep the content at 0.100% or less.

Ti: 0.030% or less Ti forms fine TiN in steel, and the TiN alone or in combination with Mg (MgAl ₂ O ₄ ) oxide acts as a pinning particle. As a result, the coarsening of austenite grains in the HAZ is suppressed, the microstructure is refined, and low-temperature toughness is improved. Ti is not an essential element, but in order to obtain this effect, it is preferable to contain Ti at 0.005% or more. If the Ti content is too high, Ti oxides will aggregate and coarsen, deteriorating toughness, so the Ti content is preferably 0.030% or less. More preferably, it is 0.008 to 0.020%.

N: 0.0060% or less N is an element that bonds with Ti to form TiN. N is not an essential element, but in order to obtain the effect of TiN acting as pinning particles, it is preferable to contain 0.0020% or more. If the N content is large, the dissolved N that is not bonded with Ti reduces toughness, so the N content is preferably 0.0060% or less. More preferably, it is 0.0030 to 0.0050%.

O: 0.0050% or less O is an element that forms pinning particles. However, since the cleanliness of the steel decreases when O is contained, the less O the better, and the more preferable O content is 0.0050% or less. More preferably, the more preferable O content is 0.0030% or less.

Ca: 0.0050% or less Ca is an element that controls the morphology of sulfide-based inclusions and improves low-temperature toughness. If the Ca content is large, CaO-CaS may become large clusters or inclusions, which may adversely affect toughness. Ca does not necessarily need to be contained in the base material of the spiral steel pipe, and the suitable Ca content is 0.0050% or less.

Ni: 0.50% or less Ni is an element that can improve the strength of the base material without reducing the toughness. When the Ni content is large, the effect is saturated. Ni does not necessarily need to be contained in the base material of the spiral steel pipe, and the suitable Ni content is 0.50% or less.

Cr: 0.50% or less Cr is an element that can improve the strength of the base material. If the Cr content is large, the effect is saturated. Cr does not necessarily need to be contained in the base material of the spiral steel pipe, and the suitable Cr content is 0.50% or less.

Cu: 0.50% or less Cu is an element that can improve the strength of the base material. If the Cu content is large, the effect is saturated. Cu does not necessarily need to be contained in the base material of the spiral steel pipe, and the suitable Cu content is 0.50% or less.

Mo: 0.50% or less Mo is an element that can improve the strength of the base material. If the amount of Mo is too large, the effect is saturated and furthermore, the toughness decreases. Mo does not necessarily need to be contained in the base material of the spiral steel pipe, and the suitable amount of Mo is 0.50% or less.

Nb: 0.100% or less Nb is an element that improves the strength of the base material. If the Nb content is large, island martensite is more likely to form, and toughness decreases. Nb does not necessarily need to be contained in the base material of the spiral steel pipe, and the suitable Nb content is 0.100% or less. From the viewpoint of strength and toughness, the Nb content is more preferably 0.020 to 0.050%.

B: 0.0020% or less B is an element effective in improving the hardenability of the base material and suppressing the formation of grain boundary ferrite. When the B content is large, the effect saturates. B does not necessarily need to be contained in the base material of the spiral steel pipe, and the suitable B content is 0.0020% or less.

V: 0.060% or less V is an element that improves the strength of the base material. If the V content is large, the yield ratio may increase due to precipitation hardening. V does not necessarily need to be contained in the base material of the spiral steel pipe, and the suitable V content is 0.060% or less.

Mg: 0.0100% or less Mg is an element that forms inclusions such as MgAl ₂ O ₄ and MgS. MgAl ₂ O ₄ precipitates on TiN. These inclusions act as pinning particles, suppressing the coarsening of austenite grains in the HAZ, refining the microstructure, and improving low-temperature toughness. When the Mg content increases, the effect saturates. Mg does not necessarily need to be contained in the base material of the spiral steel pipe, and the suitable Mg content is 0.0100% or less.

The remainder, other than what has been explained above, is Fe and impurities. Impurities are components that are contained in the raw materials or mixed in during the manufacturing process, and are not intentionally included in the steel.

Specific examples include P, S, O, Sb, Sn, W, Co, As, Pb, Bi, and H. Of these, it is preferable that P, S, and O are controlled to be within the preferred ranges described above.

As for other elements, Sb, Sn, W, Co, and As may be present as unavoidable impurities at levels of 0.1% or less, Pb and Bi at levels of 0.005% or less, and H at levels of 0.0005% or less. However, within normal ranges, there is no need to control them.

An embodiment of the present invention will be described. The conditions in the embodiment are an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Steel materials with various chemical compositions were melted, and the refined molten steel was turned into slabs using the continuous casting method. After heating to 1200°C, the slabs were hot-rolled to produce steel strips with thicknesses of 5 to 25 mm, with a pre-hot-rolling finish temperature of 1000°C and a coiling temperature of 500 to 700°C. Table 1 shows the thickness, chemical composition, and tensile strength of the steel plates.

Next, the steel strip was processed into a spiral shape, the widthwise ends were butted together, and submerged arc welding was performed from the outer surface, and then the inner surface was submerged arc welding. The heat input was controlled by adjusting the current, voltage and welding speed according to the plate thickness to a level sufficient to fill the groove.

The toughness of the weld metal on the inner and outer surfaces can be achieved by controlling the weld metal composition of the present invention. Furthermore, because the outer weld metal can have better toughness than the inner surface even with the same composition due to the tempering effect caused by the heat of the inner welding, the outer weld metal composition is shown as an example in Tables 2 and 3 in the examples.

After submerged arc welding, the area ratio (%) of the weld metal structure (total of acicular ferrite, grain boundary ferrite and island martensite), the EBSD grain size of the weld metal, the tensile strength of the weld metal, and the absorbed energy in the Charpy impact test were measured.

The results are shown in Table 4. The AF rate, GBF rate, and MA rate in Table 4 respectively indicate the area ratios of acicular ferrite, grain boundary ferrite, and island martensite in the weld metal structure.

The absorbed energy in the Charpy impact test was measured as follows:

In a cross section of the plate thickness parallel to the direction including the weld metal, a Charpy test piece was taken from the center of the weld metal part 2 mm below the surface layer of the steel plate, and a Charpy impact test was performed at 0°C in accordance with JIS Z2242 to measure the absorbed energy. The absorbed energy was calculated as the average value of three Charpy impact tests, and a value of less than 100 J was determined to have poor toughness.

The tissue area ratio was measured as follows:

A test piece was taken from the surface of the second pass, half the width of the weld bead at the t/4 position of the wall thickness, and after polishing, it was subjected to nital etching and Repera etching. The exposed structure was measured with an optical microscope in 10 fields of view for the structure observed in an area of 1000 μm x 1000 μm, and the obtained images were subjected to image analysis, and the average area ratio of each structure was calculated.

The EBSD grain size was calculated by EBSD analysis of 20 fields of view in an area of 500 μm x 500 μm, and the average grain size was calculated when the crystal orientation difference was divided by 15°.

As shown in Table 3, all of the examples of the invention that satisfied the chemical composition of the welded joint of the present invention had a Charpy absorbed energy of 100 J or more at 0°C and had excellent weld metal toughness.

Claims (4)

A spiral steel pipe in which the width direction end faces of a spirally wound steel strip are welded from the inside and outside,
The chemical composition of the weld metal is, in mass percent,
C: 0.030-0.150%,
Si: 0.05-0.50%,
Mn: 0.50-2.00%,
P: 0.020% or less,
S: 0.010% or less,
Al: 0.001-0.050%,
Ti: 0.002% to 0.050%,
B: more than 0%, less than 0.0050%,
N: 0.0100% or less,
O: 0.0150 to 0.0600%,
The balance is Fe and impurities. The content (mass%) of each element is expressed by the element symbol:
0.300≦Al/O≦1.000 is satisfied,
α' defined as α' = (1.5 × (O-0.89Al) + 3.4 × N-Ti) × 1000 is 0 to 60.0%,
Ceq defined as C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 is 0.350 to 0.450%,
Pcm defined as Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B is 0.250% or less;
The structure of the weld metal is, in terms of area ratio,
Acicular ferrite: 70.0% or more,
Grain boundary ferrite: 20.0% or less,
Island martensite: 5.0% or less
Contains
A spiral steel pipe having an EBSD grain size of 15.0 μm or less . The weld metal further contains, in place of a portion of the Fe,
Cu: 0.50% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Mo: 0.50% or less,
V: 0.050% or less,
Nb: 0.050% or less,
2. The spiral steel pipe according to claim 1, further comprising one or more elements selected from the group consisting of Mg: 0.0100% or less, and Ca: 0.0060% or less. The chemical composition of the base material of the spiral steel pipe is, in mass%,
C: 0.030-0.150%,
Si: 0.55% or less,
Mn: 0.50-2.00%,
P: 0.020% or less,
S: 0.010% or less,
3. The spiral steel pipe according to claim 1, characterized in that the balance is Fe and impurities. The base material further contains, in place of a portion of the Fe,
Al: 0.100% or less,
Ti: 0.030% or less,
N: 0.0060% or less,
O: 0.0050% or less,
Ca: 0.0050% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Cu: 0.50% or less,
Mo: 0.50% or less,
Nb: 0.100% or less,
B: 0.0020% or less,
The spiral steel pipe according to claim 3 , characterized in that it contains one or more elements selected from the group consisting of V: 0.060% or less, and Mg: 0.0100% or less.