Hot shrinkage electric resistance welded steel pipe

Abstract

A hot shrinkage diameter electric resistance welded steel pipe has a base material portion and a weld portion. The base material portion has a prescribed chemical composition, and a value obtained by dividing a Ti content by an N content, that is, Ti / N is 3.0 or more. In a microstructure of the weld portion, an average grain size of the microstructure is 10.0 μm or less, an area ratio of ferrite is 20% or more, and a remaining portion of the microstructure contains at least one of pearlite and bainite-martensite. In a texture of the weld portion, an accumulation degree of a {001} plane is 6.0 or less, and a critical cooling speed Vc90 of the base material portion is 5°C / s to 90°C / s.

Description

Technical Field

[0001] This invention relates to heat-shrinkable welded steel pipes. This application claims priority based on Japanese Patent Application No. 2021-065833, filed on April 8, 2021, the contents of which are incorporated herein by reference. Background Technology

[0002] For example, in components subjected to alternating stress (repeated stress) such as parts of automobiles (fatigue-resistant components), steel bars were previously used, but due to the need for lightweighting, there is a trend towards hollowing out from solid steel.

[0003] For such components, fatigue properties are required. However, in the case of hollowing, it is difficult to obtain the same fatigue properties as solid materials when the ratio of wall thickness t to outer diameter D (t / D) of the steel pipe is small. To ensure fatigue properties, t / D needs to be increased. To meet this requirement, steel pipes with a high ratio of wall thickness t to outer diameter D (t / D) are needed. As a high t / D steel pipe, a heat-shrink welded steel pipe manufactured by heat-shrinking is suitable.

[0004] For high-t / D heat-shrink welded steel pipes manufactured by heat shrinking, excellent fatigue characteristics are required when used as components, i.e., after being processed into components and heat-treated. On the other hand, for welded steel pipes used in fatigue-durable components, high toughness is not required because they are subjected to fewer impact loads during use.

[0005] For example, as a steel pipe for automobiles, Patent Document 1 discloses a steel pipe with excellent formability, characterized in that the average value of r is 1.5 or more and / or the minimum value of r is 1.0 or more in the range of 0° to ±25° relative to the length direction of the steel pipe.

[0006] However, in recent years, with the increasing demand for high strength in welded steel pipes, excellent toughness is also required for welded steel pipes used in fatigue durability components. This is because when using welded steel pipes to manufacture components, plastic deformation is sometimes performed on the welded steel pipes. Therefore, if the toughness of the welded steel pipe deteriorates due to the increase in strength, brittle failure may sometimes occur during plastic deformation.

[0007] Against this backdrop, in recent years, there has been a demand for welded steel pipes with excellent flatness properties to prevent brittle fracture during plastic deformation in component manufacturing. In welded steel pipes, grain refinement is an effective means to improve both strength and flatness properties.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2002-20841 Summary of the Invention

[0011] However, the inventors' research revealed that while the average grain size of ferrite in the aforementioned technology can be refined to below 4–5 μm, the developed texture makes it prone to cracking in the welded section (hereinafter referred to as the welded section), thus deteriorating the flattening performance of the welded steel pipe. In particular, it was determined that high t / D heat-shrinkable welded steel pipes are more susceptible to the effects of texture due to the increased strain during the flattening test.

[0012] The present invention was made in view of the above facts, and its object is to provide a heat-shrink welded steel pipe with excellent flatness, excellent fatigue characteristics after heat treatment, and high strength (high hardness).

[0013] The inventors have conducted research on a method for suppressing cracks in the welded portion of hot-shrink welded steel pipes during plastic deformation. As a result, the inventors discovered that by refining the ferrite after hot shrinking and suppressing the development of texture, it is possible to suppress the occurrence of cracks in the welded portion and improve the flatness performance of the hot-shrink welded steel pipe.

[0014] The main idea of ​​the present invention, based on the above knowledge, is as follows.

[0015] (1) A heat-shrinkable welded steel pipe according to one aspect of the present invention is characterized in that it has a base material portion and a welded portion, wherein the chemical composition of the base material portion comprises, in mass %

[0016] C: 0.210~0.400%

[0017] Si: 0.05-0.50%

[0018] Mn: 0.50–1.70%

[0019] P: below 0.100%

[0020] S: less than 0.010%

[0021] N: below 0.0100%

[0022] Al: 0.010~0.100%

[0023] Ti: 0.010~0.060%

[0024] B: 0.0005~0.0050%

[0025] Cr: 0–0.500%

[0026] Mo: 0–0.500%

[0027] Cu: 0~1.000%

[0028] Ni: 0~1.000%

[0029] Nb: 0~0.050%

[0030] W: 0~0.050%

[0031] V: 0~0.500%

[0032] Ca: 0–0.0050%, and

[0033] REM: 0~0.0050%,

[0034] The balance consists of Fe and impurities.

[0035] The value obtained by dividing the Ti content by the N content, i.e., the Ti / N ratio, is 3.0 or higher.

[0036] In the microstructure of the welded portion

[0037] The average grain size of this microstructure is less than 10.0 μm.

[0038] The ferrite area fraction is more than 20%, and the remaining microstructure contains at least one of pearlite and bainite-martensite.

[0039] In the texture of the welded portion, the density of the {001} surface is 6.0 or less.

[0040] The critical cooling rate Vc90 of the base material is 5℃ / s to 90℃ / s.

[0041] The critical cooling rate Vc90, when the C content (mass%) is denoted as [C], the Si content (mass%) as [Si], the Mn content (mass%) as [Mn], the Cr content (mass%) as [Cr], the Mo content (mass%) as [Mo], and the Ni content (mass%) as [Ni], is expressed by the following formula (1) when the B content exceeds 0.0004%, and by the following formula (3) when the B content is less than 0.0004%.

[0042] log 10 Vc90=2.94-0.75×β…(1)

[0043] β=2.7×[C]+0.4×[Si]+[Mn]+0.8×[Cr]+2×[Mo]+0.45×[Ni]…(2)

[0044] log 10Vc90=2.94-0.75(β'-1)…(3)

[0045] β'=2.7×[C]+0.4×[Si]+[Mn]+0.8×[Cr]+[Mo]+0.45×[Ni]…(4)

[0046] (2) The heat-shrinkable welded steel pipe according to (1) above, wherein the chemical composition, in mass percent, contains components selected from...

[0047] Mo: 0.010~0.500%

[0048] Cu: 0.010~1.000%

[0049] Ni: 0.010~1.000%

[0050] Nb: 0.005~0.050%

[0051] W: 0.010~0.050%

[0052] V: 0.010~0.500%

[0053] Ca: 0.0001–0.0050%, and

[0054] REM: 0.0001~0.0050%

[0055] One or more of them.

[0056] According to the above-mentioned method of the present invention, it is possible to provide a heat-shrink welded steel pipe with excellent flatness, excellent fatigue characteristics after heat treatment, and high hardness.

[0057] The heat-shrinkable welded steel pipe described above can be appropriately applied to components of the running gear of automobiles, such as stabilizers, drive shafts, and rack bars. Attached Figure Description

[0058] Figure 1 This is a graph showing the relationship between the average grain size of the microstructure in the weld and the crack incidence rate.

[0059] Figure 2 This is a graph showing the relationship between the density of the {001} surface in the texture of the weld and the crack incidence rate.

[0060] Figure 3 This is a graph showing the relationship between the average grain size of the microstructure of the welded part and the rolling time of the heat-shrinked diameter.

[0061] Figure 4This is a graph showing the relationship between the cumulative diameter reduction rate and the accumulation degree of the {001} surface in the texture of the welded part in the temperature range below 850°C.

[0062] Figure 5 It is a diagram used to illustrate the welding mating surfaces. Detailed Implementation

[0063] The following is a detailed description of the electric welded steel pipe (hereinafter referred to as the heat-shrinkable electric welded steel pipe) according to this embodiment. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the spirit of the present invention.

[0064] Hot-shrink welded steel pipes are steel pipes manufactured by heating welded steel pipes to achieve a reduced diameter. After hot-shrinking, they are not cold-formed and become finished products. In contrast, welded steel pipes obtained through cold forming (generally, the cold-formed steel pipe is called a welded steel pipe) become finished products after cold forming. Therefore, in the tensile test along the length direction, the welded steel pipe obtained through cold forming undergoes work hardening due to strain caused by cold, resulting in a higher yield strength. Consequently, the yield ratio (yield strength / tensile strength) of the welded steel pipe is higher than that of the hot-shrink welded steel pipe. Therefore, the hot-shrink welded steel pipe and the cold-formed welded steel pipe according to this embodiment can be distinguished using the results of the tensile test along the length direction. Specifically, in the tensile test along the length direction, the yield strength is 95% or more for cold-formed pipes and less than 95% for hot-shrink welded steel pipes.

[0065] For the numerical ranges specified below, indicated by “~”, both the lower and upper limits are included in the range. Values ​​expressed as “less than” or “more than” are not included in the range. All “%” values ​​related to chemical composition refer to “mass %”.

[0066] The chemical composition of the base material of the heat-shrinkable welded steel pipe according to this embodiment, by mass%, comprises: C: 0.210–0.400%, Si: 0.05–0.50%, Mn: 0.50–1.70%, P: 0.100% or less, S: 0.010% or less, N: 0.0100% or less, Al: 0.010–0.100%, Ti: 0.010–0.060%, B: 0.0005–0.005%, and the balance: Fe and impurities. The following describes each element.

[0067] Furthermore, in this embodiment, the welded portion (sometimes referred to as the electric welding portion) refers to the mating surface and its surrounding portion, while the base material portion refers to the area outside the welded portion.

[0068] C: 0.210~0.400%

[0069] Carbon (C) is an element that contributes to increasing the hardness of steel. If the C content is less than 0.210%, the desired hardness cannot be obtained after heat treatment. Therefore, the C content is set to 0.210% or more. Preferably, it is 0.230% or more, more preferably 0.240% or more. The C content is even more preferably greater than 0.300%.

[0070] On the other hand, if the carbon content exceeds 0.400%, a large amount of cementite will be generated, deteriorating the flatness characteristics of the hot-shrink welded steel pipe. Therefore, the carbon content is set to 0.400% or less. Preferably, it is 0.380% or less, and more preferably 0.360% or less.

[0071] Si: 0.05–0.50%

[0072] Si is an element that strengthens steel through solid solution strengthening, thereby improving the fatigue properties of steel. If the Si content is less than 0.05%, the fatigue properties of the steel deteriorate. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.25% or more.

[0073] On the other hand, if the Si content exceeds 0.50%, the flatness and fatigue characteristics of the hot-shrink welded steel pipe deteriorate due to the formation of Mn and / or Si-based oxides in the welded portion. Therefore, the Si content is set to 0.50% or less. Preferably, it is 0.45% or less, and more preferably 0.40% or less.

[0074] Mn: 0.50–1.70%

[0075] Mn is an important element for solid solution strengthening and improving hardenability. If the Mn content is less than 0.50%, the desired hardness cannot be obtained after quenching. Therefore, the Mn content is set to 0.50% or more. Preferably, it is 0.70% or more, and more preferably 0.90% or more.

[0076] On the other hand, if the Mn content exceeds 1.70%, sulfides such as MnS will form, leading to deterioration of fatigue characteristics, especially those of welded joints. Therefore, the Mn content is set to 1.70% or less. Preferably, it is 1.50% or less, and more preferably 1.50% or less.

[0077] P: below 0.100%

[0078] Phosphorus (P) is an element that provides solid solution strengthening, but if the P content exceeds 0.100%, it can cause grain boundary embrittlement and other defects, thus deteriorating the flatness performance of the heat-shrink welded steel pipe. Therefore, the P content is set to 0.100% or less. Preferably, it is 0.080% or less, and more preferably 0.060% or less.

[0079] A lower phosphorus (P) content is preferred, ideally 0%, but excessively reducing the P content significantly increases the cost of P removal. Therefore, the P content can also be set to 0.001% or higher.

[0080] S: below 0.010%

[0081] Sulfur (S) is an element that deteriorates the fatigue properties of hot-shrink welded steel pipes by forming sulfides. If the S content exceeds 0.010%, the fatigue properties of the hot-shrink welded steel pipe, especially the fatigue properties of the welded joint, are significantly deteriorated. Therefore, the S content is set to 0.010% or less. Preferably, it is 0.008% or less, and more preferably 0.006% or less.

[0082] The lower the sulfur content, the better; 0% is preferred. However, excessively reducing the sulfur content significantly increases the cost of sulfur removal. Therefore, the sulfur content can also be set to 0.0001% or higher.

[0083] N: below 0.0100%

[0084] Nitrogen (N) is an element that reduces the hardenability of steel by causing BN (Nitrogen Nitrogen) to precipitate. If the N content exceeds 0.0100%, the desired hardness cannot be obtained after heat treatment, and fatigue properties deteriorate. Therefore, the N content is set to 0.0100% or less. Preferably, it is 0.0080% or less, and more preferably 0.0060% or less.

[0085] A lower nitrogen content is preferred, ideally 0%, but excessively reducing the nitrogen content significantly increases the cost of nitrogen removal. Therefore, the nitrogen content can also be set to 0.0005% or higher.

[0086] Al: 0.010~0.100%

[0087] Al is an effective element for deoxidation. If the Al content is less than 0.010%, the flatness performance of the heat-shrink welded steel pipe deteriorates. Therefore, the Al content is set to 0.010% or more. Preferably, it is 0.030% or more, and more preferably 0.050% or more.

[0088] On the other hand, if the Al content exceeds 0.100%, a large amount of Al oxides will be generated, which will deteriorate the flatness performance of the welded joint of the heat-shrink welded steel pipe. Therefore, the Al content is set to 0.100% or less. Preferably, it is 0.090% or less, and more preferably 0.080% or less.

[0089] Ti: 0.010~0.060%

[0090] Ti is an element that refines grain size and helps improve the flatness of heat-shrink welded steel pipes. If the Ti content is less than 0.010%, the flatness of the heat-shrink welded steel pipe deteriorates. Therefore, the Ti content is set to 0.010% or more. Preferably, it is 0.015% or more, and more preferably 0.020% or more.

[0091] On the other hand, if the Ti content exceeds 0.060%, the flattening performance deteriorates by forming coarse Ti carbonitrides. Therefore, the Ti content is set to 0.060% or less. Preferably, it is 0.050% or less, and more preferably 0.045% or less.

[0092] Furthermore, the addition of Ti also has the effect of forming TiN, thereby reducing the amount of dissolved N, and preventing the reduction of dissolved B, which is beneficial to hardenability, due to the precipitation of BN. In this case, it is preferable to set Ti ≥ 3.4N.

[0093] B: 0.0005~0.0050%

[0094] Boron (B) is an element that segregates at grain boundaries and contributes to the hardenability of steel. If the B content is less than 0.0005%, the desired hardness cannot be obtained after heat treatment, and fatigue characteristics deteriorate. Therefore, the B content is set to 0.0005% or more. Preferably, it is 0.0010% or more, and more preferably 0.0020% or more.

[0095] On the other hand, if the B content exceeds 0.0050%, then it will be detected through B. 23 The precipitation of boron-containing precipitates such as (CB)6 actually reduces hardenability, preventing the achievement of desired hardness after heat treatment and deteriorating fatigue properties. Therefore, the boron content is set to 0.0050% or less, preferably 0.0040% or less.

[0096] The remaining portion (balance) of the chemical composition of the base material of the heat-shrinkable welded steel pipe according to this embodiment may be Fe and impurities. In this embodiment, impurities mean substances mixed in from the ore used as raw material, waste, or the manufacturing environment, or substances that are permissible within a range that do not adversely affect the characteristics of the heat-shrinkable welded steel pipe according to this embodiment. Examples of impurities include Sn, Pb, Co, Sb, and As.

[0097] The base material of the heat-shrinkable welded steel pipe according to this embodiment may also contain any of the following elements instead of a portion of Fe. The lower limit of the content of any element is 0%. The chemical composition of the base material may also contain, by mass%, one or more elements selected from Mo: 0.010–0.500%, Cu: 0.010–1.000%, Ni: 0.010–1.000%, Nb: 0.005–0.050%, W: 0.010–0.050%, V: 0.010–0.500%, Ca: 0.0001–0.0050%, and REM: 0.0001–0.0050%. Hereinafter, each arbitrary element will be described.

[0098] Cr: 0~0.500%

[0099] Cr is an element that increases the hardness of steel through precipitation strengthening and improved hardenability. Therefore, it may be included as needed. To reliably achieve the above effects, the Cr content is preferably 0.010% or more. Preferably, it is 0.030% or more, and more preferably 0.100% or more. Since it may also be absent, the lower limit for the Cr content is 0%.

[0100] On the other hand, if the Cr content exceeds 0.500%, Cr oxides will be generated in the weld, deteriorating the flatness and fatigue properties of the hot-shrink welded steel pipe. Therefore, the Cr content is set to 0.500% or less. Preferably, it is 0.260% or less, and more preferably 0.240% or less.

[0101] Mo: 0–0.500%

[0102] Mo is an element that improves hardenability while simultaneously contributing to increased hardness after heat treatment by forming carbonitrides. Therefore, it may be included as needed. To reliably achieve the above effects, the Mo content is preferably set to 0.010% or more. Since it may also be absent, the lower limit for the Mo content is 0%.

[0103] Even if the Mo content exceeds 0.500%, the above effect will saturate, so the Mo content is set below 0.500%.

[0104] Cu: 0~1.000%

[0105] Cu is an element that improves the hardenability of steel and thus increases its hardness after heat treatment. Therefore, it can be included as needed. To reliably achieve the above effects, the Cu content is preferably set to 0.010% or more. Since it can also be omitted, the lower limit for the Cu content is 0%.

[0106] On the other hand, if the Cu content exceeds 1.000%, the steel will become brittle due to Cu precipitation. Therefore, the Cu content is set to be below 1.000%.

[0107] Ni: 0~1.000%

[0108] Ni is an element that improves the hardenability of steel while suppressing the brittleness of Cu. Therefore, it can be included as needed. To reliably achieve the above effects, the Ni content is preferably set to 0.010% or more. Since it can also be omitted, the lower limit for Ni content is 0%.

[0109] On the other hand, if the Ni content exceeds 1.000%, the weldability of the hot-shrink welded steel pipe decreases. Therefore, the Ni content is set to below 1.000%.

[0110] Nb: 0~0.050%

[0111] Nitrogen (Nb) is an element that improves the toughness of heat-shrink welded steel pipes by refining the grain size. Therefore, it can be included as needed. To reliably achieve the above-mentioned effects, the Nb content is preferably set to 0.005% or more. Since it can also be omitted, the lower limit for the Nb content is 0%.

[0112] On the other hand, if the Nb content exceeds 0.050%, the flatness performance of the hot-shrink welded steel pipe deteriorates by forming coarse Nb carbonitrides. Therefore, the Nb content is set below 0.050%.

[0113] W: 0~0.050%

[0114] W is an element that forms carbides in steel and contributes to increasing the hardness of the steel. Therefore, it can be included as needed. To reliably obtain the above-mentioned effects, the W content is preferably set to 0.010% or more. Since it can also be omitted, the lower limit of the W content is 0%.

[0115] On the other hand, if the W content exceeds 0.050%, the flatness of the hot-shrink welded steel pipe will decrease due to the excessive formation of carbides. Therefore, the W content is set below 0.050%.

[0116] V: 0~0.500%

[0117] V is a precipitation strengthening element. Therefore, it can be included as needed. To reliably obtain the above-mentioned effects, the V content is preferably set to 0.010% or more. Since it can also be omitted, the lower limit for the V content is 0%.

[0118] On the other hand, if the V content exceeds 0.500%, the flatness performance of the hot-shrink welded steel pipe deteriorates by forming coarse V carbides. Therefore, the V content is set below 0.500%.

[0119] Ca: 0–0.0050%

[0120] Ca is an element that inhibits the formation of elongated MnS by generating sulfides, thus contributing to improved flatness properties of hot-shrink welded steel pipes. Therefore, it may be included as needed. To reliably achieve the above-mentioned effects, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more. Since it may also be absent, the lower limit for Ca content is 0%.

[0121] On the other hand, if the Ca content exceeds 0.0050%, a large amount of CaO will be generated, which will deteriorate the flatness performance of the hot-shrink welded steel pipe. Therefore, the Ca content is set to below 0.0050%.

[0122] REM: 0~0.0050%

[0123] Like Ca, REM is an element that inhibits the formation of elongated MnS by generating sulfides, thus contributing to improved flatness performance of hot-shrink welded steel pipes. Therefore, it may be included as needed. To reliably achieve the above-mentioned effects, the REM content is preferably 0.0001% or more, more preferably 0.0005% or more. Since it may also be absent, the lower limit for REM content is 0%.

[0124] On the other hand, if the REM content exceeds 0.0050%, the number of REM oxides increases, and the flatness performance of the heat-shrink welded steel pipe deteriorates. Therefore, the REM content is set to below 0.0050%.

[0125] In this embodiment, REM refers to a total of 15 lanthanide elements, and the content of REM means the total content of these elements.

[0126] The value obtained by dividing the Ti content by the N content, i.e., the Ti / N ratio, is 3.0 or higher.

[0127] If the nitrogen (N) content is too high, the hardenability improvement effect brought by boron (B) cannot be fully obtained due to BN precipitation. As a result, the desired hardness cannot be achieved after heat treatment. In order to obtain the hardenability improvement effect brought by boron by fixing N in the form of TiN, the Ti / N ratio is set to 3.0 or higher. Preferably, it is 3.4 or higher, and more preferably 5.0 or higher.

[0128] The upper limit is not specifically specified, but Ti / N can be set to below 30.0.

[0129] In the shrink - diameter electric - welded steel pipe of the present embodiment, ensuring hardenability is important. As an index of hardenability, for example, the critical cooling rate Vc90 (°C / s) known from "Iron and Steel, 74(1988) P.1073" is used. The critical cooling rate Vc90, when the C content (mass%) is denoted as [C], the Si content (mass%) is denoted as [Si], the Mn content (mass%) is denoted as [Mn], the Cr content (mass%) is denoted as [Cr], the Mo content (mass%) is denoted as [Mo], and the Ni content (mass%) is denoted as [Ni], is expressed by the following formula (1) when the boron (B) content exceeds 0.0004 mass%, and is expressed by the following formula (3) when the B content is 0.0004 mass% or less. The critical cooling rate means the cooling rate at which the volume fraction of martensite becomes 90% or more. Therefore, the lower Vc90 is, the higher the hardenability is.

[0130] log 10 Vc90 = 2.94 - 0.75×β…(1)

[0131] β = 2.7×[C]+0.4×[Si]+[Mn]+0.8×[Cr]+2×[Mo]+0.45×[Ni]…(2)

[0132] log 10 Vc90 = 2.94 - 0.75(β’ - 1)…(3)

[0133] β’ = 2.7×[C]+0.4×[Si]+[Mn]+0.8×[Cr]+[Mo]+0.45×[Ni]…(4)

[0134] In the shrink - diameter electric - welded steel pipe of the present embodiment, the critical cooling rate Vc90 of the base - metal part is 90°C / s or less. The critical cooling rate Vc90 is preferably 70°C / s or less. If the critical cooling rate Vc90 is 90°C / s or less, excellent hardenability can be obtained. The lower limit of the critical cooling rate Vc90 is not particularly limited. The critical cooling rate Vc90 is 5°C / s or more. The critical cooling rate Vc90 is preferably 15°C / s or more.

[0135] Furthermore, the chemical composition of the electric - welded part of the shrink - diameter electric - welded steel pipe according to the present embodiment, although the C content is slightly reduced due to decarburization, is basically the same as that of the base - metal part. By satisfying the above - mentioned chemical composition, fatigue characteristics can be obtained while ensuring the hardness after the specified heat treatment.

[0136] Next, the welded portion (sometimes referred to as the welded portion) of the hot-shrink welded steel pipe according to this embodiment will be described in detail. The welded portion of the hot-shrink welded steel pipe according to this embodiment has an average grain size of 10.0 μm or less in its microstructure, a ferrite area fraction of 20% or more, and the remaining microstructure (the remaining microstructure) includes at least one of pearlite and bainite-martensite (bainite and martensite). Furthermore, in the texture of the welded portion, the density of the {001} plane is 6.0 or less.

[0137] Average particle size of the welded part: below 10.0 μm

[0138] The inventors have discovered that ensuring the average grain size of the microstructure in the welded portion of a heat-shrink welded steel pipe is below 10.0 μm is one of the effective requirements for suppressing cracks in the welded portion and improving the flatness performance of the heat-shrink welded steel pipe. Figure 1 The relationship between the average grain size of the microstructure in the welded area and the crack incidence rate is shown. Furthermore, Figure 1 The example described herein is an example where the average grain size of the microstructure was changed by altering the manufacturing conditions using steel grade A from the embodiments described later. The presence or absence of cracks was evaluated using the same method as in the embodiments described later. Figure 1 In the example, the density of the {001} surface in the texture of the welded part is 4-5. According to... Figure 1 It can be seen that by making the average grain size of the microstructure in the welded part below 10.0 μm, the crack incidence rate can be reduced.

[0139] The average grain size of the microstructure in the welded part is preferably 8.0 μm or less, more preferably 7.0 μm or less, and even more preferably 6.0 μm or less.

[0140] The average grain size of the microstructure can be set to 1.0 μm or more, 2.0 μm or more, or 3.0 μm or more. The average grain size of the microstructure in the base material of the heat-shrink welded steel pipe is to the same extent as the average grain size of the microstructure in the welded part. Specifically, the average grain size of the microstructure in the base material is 50% to 200% of the average grain size of the welded part when the average grain size of the welded part is set to 100%.

[0141] The average grain size of the microstructure in the welded section was determined using the following method. The observation surface was set as the butt joint surface (weld mating surface) of the welded section of the heat-shrink welded steel pipe. A sample was prepared on a plane perpendicular to the pipe axis (length direction) in a manner that allowed observation of the weld line representing the butt joint surface. The surface of the prepared sample perpendicular to the pipe axis was ground and etched with nitric acid ethanol etching solution (Nital) to determine the weld line. Furthermore, the weld line is a decarburized area, turning whitish in color, thus making it easy to identify. The surface perpendicular to the circumferential direction containing this weld line is the butt joint surface (…). Figure 5The diagonal section (of the weld) is processed by cutting and machining to make it an observation surface within 50μm to the left and right of the weld line in order to be able to observe the surface. That is, the welded part is equivalent to the part within 50μm to the left and right of the weld mating surface.

[0142] After wet grinding to achieve a mirror finish, the observation surface was electrolytically ground to remove the strain layer. Using an EBSD apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector), crystal orientation information was obtained by electron backscattering diffraction at 0.3 μm intervals over a 500 μm × 500 μm region centered at half the tube thickness on the observation surface. The vacuum level within the EBSD apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage is set to 15kV, the irradiation current level is set to 13, and the electron beam irradiation level is set to 62.

[0143] The orientation difference between adjacent measurement points is calculated based on the obtained crystal orientation information. Boundaries with an orientation difference of 15° or more are defined as grain boundaries, and the regions enclosed by these grain boundaries are extracted as grains of the microstructure. The equivalent circle diameter of the extracted grains is determined using the "area fraction" method, and the average grain size of the microstructure is obtained by calculating their average value. However, grains with an equivalent circle diameter of 0.50 μm or less are excluded from the calculation of the average grain size. Furthermore, when observing the base material, a plane perpendicular to the pipe axis and pipe surface, located 90° away from the welded portion along the circumferential direction of the steel pipe, is observed. Samples are prepared in a manner that allows observation of the 90° position from the welded portion along the circumferential direction of the steel pipe. Other conditions are set the same as for the observation of the welded portion.

[0144] Ferrite area fraction: 20% or more

[0145] If the area fraction of ferrite in the microstructure of the welded part is less than 20%, the flatness performance of the hot-shrink welded steel pipe deteriorates. Therefore, the area fraction of ferrite is set to 20% or more. Preferably, it is 30% or more, and more preferably 40% or more.

[0146] The upper limit is not specifically limited, but it can be set to below 90% or below 80%.

[0147] Pearlite

[0148] In this embodiment, the welded portion of the heat-shrinkable welded steel pipe may contain pearlite. The area ratio of pearlite is preferably set to 80% or less, more preferably 70% or less or 60% or less, depending on its relationship with the area ratio of ferrite. Furthermore, if the area ratio of pearlite is set to 20% or more, the flatness performance of the welded steel pipe is improved, and therefore this is preferred.

[0149] In the welded portion of the heat-shrinkable welded steel pipe according to this embodiment, the microstructure, other than ferrite and pearlite, may contain, for example, bainite-martensite. The microstructure of the remaining portion other than ferrite may be at least one of pearlite and bainite-martensite. The area fraction of the microstructure other than ferrite and pearlite is preferably 2% or less.

[0150] The microstructure fraction in the welded section was determined using the following method. The observation surface was the same as that used for the texture, specifically the butt joint of the heat-shrink welded steel pipe. Sample preparation and observation surface treatment were performed using the same method as for determining the average grain size of the microstructure. An EBSD apparatus, consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector), was used to obtain crystal orientation information by measuring a 500 μm × 500 μm region at 0.3 μm intervals within the 1 / 2 thickness portion of the observation surface using electron backscatter diffraction. The vacuum level within the EBSD apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage is set to 15kV, the irradiation current level is set to 13, and the electron beam irradiation level is set to 62.

[0151] Based on the obtained crystal orientation information, using the "OIM Analysis" software (registered trademark) included with the EBSD analysis device, regions within grains surrounded by grain boundaries with an orientation difference of 15° or more and an orientation difference (GAM value) of less than 1° are extracted as ferrite and pearlite. Regions with a GAM value exceeding 1° are extracted as bainite-martensite. In this specification, bainite and martensite are extracted without distinction. By calculating the area ratio of each region, the total area ratio of ferrite and pearlite, as well as the area ratio of bainite-martensite, are obtained.

[0152] Next, the area ratio of pearlite was determined by observation under an optical microscope. The same observation surface as described above was mirror-finished and then etched using a nitric acid-ethanol etching solution. This etched pearlite until it turned black, thus distinguishing it from ferrite. Pearlite is a structure in which ferrite and cementite alternate in layers, but when observed under an optical microscope, it appears black due to the low resolution. However, when observed using a scanning electron microscope, the layered ferrite and cementite structure can be directly identified. The area ratio of pearlite was obtained by calculating the area ratio of the etched blackened region. Furthermore, the area ratio of ferrite was obtained by subtracting the area ratio of pearlite from the total area ratio of ferrite and pearlite obtained by measurement using an EBSD device.

[0153] Furthermore, the microstructure of the base material is not particularly limited, but it is preferably designed to achieve the desired hardness after heat treatment. For example, it can be designed to contain 20-80% ferrite and 20-80% pearlite. The total area ratio of ferrite and pearlite should be 98% or more. The area ratio can be measured using the same method as for the welded portion.

[0154] Texture of the welded part: The density of the {001} surface is 6.0 or less.

[0155] The inventors have discovered that, in the texture of the welded section, having a density of 6.0 or less on the {001} surface is one of the effective requirements for suppressing cracks in the welded section and improving the flatness performance of the hot-shrink welded steel pipe. Figure 2 The diagram shows the relationship between the density of the {001} surface in the texture of the weld and the crack incidence rate. Furthermore, Figure 2 The example described herein is an example in which the density of the {001} surface was changed by altering the manufacturing conditions using steel grade A of the embodiment described later. The presence or absence of cracks was evaluated using the same method as in the embodiment described later. Figure 2 In the example, the microstructure of the welded part satisfies the above-mentioned average grain size and microstructure fraction. According to Figure 2 It is evident that by reducing the density of the {001} surface in the texture of the welded portion to 6.0 or less, the crack incidence rate can be reduced. Furthermore, in the texture of the base material, the density of the {001} surface is lower than that of the welded portion. For example, the density can be 4.0 or less, a value lower than that of the welded portion. Additionally, sometimes the texture remains even after quenching and tempering.

[0156] The density of the {001} surface in the texture of the welded part is preferably set to 5.0 or less, more preferably 4.5 or less, and even more preferably 4.0 or less.

[0157] The lower limit is not specifically limited, but since it becomes 1.0 when the crystal orientation is random, it can be set to 1.0 or higher.

[0158] Texture measurement

[0159] The texture in the welded section was determined using the following method. The measurement surface was set as the butt joint of the heat-shrink welded steel pipe. The sample preparation and measurement surface (observation surface) treatment were performed using the same method as when determining the average grain size of the microstructure.

[0160] The measurements were performed using an EBSD apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector). The vacuum level within the EBSD apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage was set to 15 kV, the irradiation current level to 13, and the electron beam irradiation level to 62. For a 1 mm × 1 mm region of half the tube thickness of the measurement surface, electron backscatter diffraction was used at measurement intervals of 0.3 μm to obtain crystal orientation information.

[0161] The density of the {001} plane is the ratio of the {001} orientation to the random orientation. Specifically, for the obtained crystal orientation information, the density of the {001} plane parallel to the tube axis is calculated using the "OIM Data Collection" function and "OIM Analysis (registered trademark)" software included with the EBSD analysis device. Thus, the density of the {001} plane in the texture of the weld is obtained.

[0162] Fatigue characteristics after heat treatment: fatigue limit above 350 MPa

[0163] Heat-shrink welded steel pipes used in automotive running gear components are typically machined into component shapes and then heat-treated before use. Therefore, these pipes are required to possess excellent fatigue characteristics after heat treatment. Ideally, such heat-shrink welded steel pipes should have a fatigue limit of 350 MPa or higher in a torsional fatigue test after specified heat treatment. Furthermore, fatigue failure occurs at the weld joint.

[0164] The heat treatment for determining the fatigue limit of hot-shrink welded steel pipes is explained. In this embodiment, heat treatment refers to quenching and tempering. Quenching involves heating the hot-shrink welded steel pipe to a temperature range of 850–1000°C, holding it in this temperature range for 10–1800 seconds, and then cooling it to a temperature range of room temperature (around 25°C)–300°C at an average cooling rate of 10°C / s or higher. Tempering involves heating the pipe to a temperature range of 200–420°C and holding it in this temperature range for 5–60 minutes.

[0165] Furthermore, the average cooling rate mentioned here refers to the value obtained by dividing the difference between the temperature at the start of cooling and the temperature at the end of cooling by the elapsed time from the start to the end of cooling. Additionally, maintaining a specified temperature range can result in a constant temperature or allow the temperature to vary within that range.

[0166] Next, the method for determining the fatigue limit will be explained. After the above heat treatment, a torsional fatigue test was conducted on the hot-shrink welded steel pipe. The torsional fatigue test was carried out at a frequency of 10 Hz under the condition that the ratio of the minimum stress to the maximum stress (stress ratio) was -1. The fatigue limit was obtained by determining the maximum stress that would not cause failure after 2 million cycles.

[0167] Vickers hardness after heat treatment: above 450 Hv

[0168] Heat-shrink welded steel pipes used in automotive running gear components are typically processed into component shapes and then heat-treated before use. Therefore, high hardness is required after heat treatment. If the Vickers hardness after heat treatment is less than 450 Hv, it may not be suitable for use in automotive running gear components. Therefore, a Vickers hardness of 450 Hv or higher after heat treatment is preferred. Preferably, the Vickers hardness after heat treatment is 480 Hv or higher, or 500 Hv or higher.

[0169] The upper limit of Vickers hardness is not specifically limited, but it can be set to below 650 Hv or below 600 Hv.

[0170] The method for determining Vickers hardness is explained below. After heat treatment under the same conditions as the heat treatment used for determining fatigue limit, the Vickers hardness of the heat-shrink welded steel pipe is determined. Samples are prepared with a cross-section perpendicular to the pipe axis that allows observation of the heat-shrink welded steel pipe. Vickers hardness is measured at all 30 locations (45°, 90°, 135°, 180°, 225°, and 270°, 0.5 mm from the outer surface, 1 mm from the outer surface, half the pipe thickness, 0.5 mm from the inner surface, and 1 mm from the inner surface) with the weld joint butt joint angle set to 0°. The average Vickers hardness obtained after heat treatment is calculated. Furthermore, the load is set to 98 N.

[0171] The thickness (wall thickness) t of the heat-shrinkable welded steel pipe involved in this embodiment is not particularly limited, but can be set to 2mm to 15mm.

[0172] The outer diameter D of the heat-shrinkable welded steel pipe involved in this embodiment is 10mm to 45mm.

[0173] The ratio of the wall thickness t (mm) to the outer diameter D (mm) of the hot-shrink welded steel pipe involved in this embodiment is preferably 10% to 30%.

[0174] Next, a preferred manufacturing method for the heat-shrinkable welded steel pipe according to this embodiment will be described.

[0175] Firstly, in this invention, the manufacturing method of the hot-rolled steel sheet that becomes the billet for the hot-shrink welded steel pipe is not particularly limited, and commonly used methods can be applied. Preferably, the molten steel with the above-mentioned composition is melted in a smelting furnace such as a converter or electric furnace, and a steel billet such as a slab is prepared using a continuous casting method. The obtained steel billet is then subjected to a heating process, a hot rolling process, a cooling process, and a coiling process to manufacture a hot-rolled steel sheet. If the width of the hot-rolled steel sheet in the coiled state is too wide, it can also be slit in the width direction to obtain a narrower coil (also called strip steel).

[0176] The preferred manufacturing method of the heat-shrinkable welded steel pipe according to this embodiment includes a process of welding the butt joint while roll forming the hot-rolled steel plate, and a process of heat-shrinking the diameter. Each process will be described below.

[0177] First, the butt joint (the end of the steel plate) is electrically welded while the hot-rolled steel plate is being rolled. The electric welding can be either resistance welding or high-frequency welding. After electric welding, a sizing process is typically performed to improve roundness. This yields an electric welded steel pipe (hereinafter referred to as a steel pipe to distinguish it from the hot-shrink electric welded steel pipe described in this embodiment) that serves as a blank.

[0178] Next, the steel pipe undergoes hot shrinking. Hot shrinking is performed by heating the steel pipe to a temperature range below 1100°C, holding it in this temperature range for 10–300 seconds, and then using a shrinking mill. Furthermore, if the heating temperature exceeds 1100°C or the holding time exceeds 300 seconds, the austenite coarsens, the average grain size of the microstructure increases, and the flattening performance deteriorates, making this undesirable. The purpose of heating is to heat the steel pipe to the austenitic region; therefore, the temperature is set to 900°C or higher.

[0179] Hot shrinking is preferably performed using a 3-roll reducing mill, but it is not limited to this. The reducing mill is preferably a reducing mill with multiple stands arranged in series that can perform continuous rolling.

[0180] The number of passes for heat shrinking is not specifically specified, but is preferably set to 10 to 30 passes. To ensure that the average grain size of the welded microstructure is below 10.0 μm, the rolling time (the elapsed time from the start of the first pass to the end of the final pass) is preferably set to 10 seconds or less. If the rolling time is too long, strain recovery will develop, the number of nucleation sites during ferrite phase transformation will decrease, and the ferrite will coarsen.

[0181] exist Figure 3 The figure shows the relationship between the average grain size of the microstructure of the welded area and the rolling time of the hot-shrink diameter. Furthermore, Figure 3 The example described herein is an example in which the average grain size of the microstructure of the welded portion was changed by altering the rolling time of the hot-shrinking section using steel grade A in the embodiment described later. According to... Figure 3 It is known that the shorter the rolling time of the hot-shrink diameter, the finer the average grain size of the microstructure of the welded part. This is believed to be because: by shortening the rolling time and the time between passes, the recovery of dislocations in austenite is suppressed, and the ferrite after phase transformation is refined.

[0182] In heat shrinking, it is preferable to control the cumulative shrinkage rate in a temperature range of 650°C or higher and in a temperature range of 850°C or lower. Furthermore, the cumulative shrinkage rate is defined as a percentage (%) obtained by dividing the change in outer diameter before and after heat shrinking within a specified temperature range by the outer diameter before heat shrinking. In the temperature range of 650°C or higher, it is preferable to perform heat shrinking in a manner that results in a cumulative shrinkage rate of 40.0% or higher. By ensuring a cumulative shrinkage rate of 40.0% or higher in the temperature range of 650°C or higher, the grain size in the welded portion can be controlled.

[0183] The upper limit of the cumulative shrinkage rate in this temperature range is not specifically specified, but it is preferably set to 90.0% or less.

[0184] The cumulative diameter reduction rate in the temperature range below 850°C is preferably set to 40.0% or less. Figure 4 The diagram shows the relationship between the cumulative diameter reduction rate and the accumulation degree of the {001} surface in the texture of the welded section in the temperature range below 850°C. Furthermore, Figure 4 The example described herein is an example in which the density of the {001} surface is changed by altering the cumulative reduction ratio in the temperature range below 850°C using steel grade A of the embodiment described later. Figure 4 It can be seen that by making the cumulative diameter reduction rate below 40.0% in the temperature range below 850℃, the concentration of the {001} surface in the texture of the welded part becomes 6.0 or less.

[0185] The lower limit of the cumulative diameter reduction rate in the temperature range below 850℃ is not particularly limited, but it can be set to 0.0% or higher.

[0186] The final temperature of the heat shrinking process (the exit temperature of the final pass) is preferably set to 650°C or higher in order to control the cumulative shrinkage rate in the above temperature range.

[0187] After heat shrinking, it is preferable to cool to room temperature (around 25°C) at an average cooling rate of less than 5°C / s. If the average cooling rate exceeds 5°C / s, a low-temperature phase transformation structure is formed, and the ferrite area fraction becomes less than 20%.

[0188] Using the manufacturing method described above, the heat-shrinkable welded steel pipe involved in this embodiment can be manufactured stably.

[0189] Example

[0190] Next, examples will be used to further illustrate the effects of one aspect of the present invention. However, the conditions in the examples are merely examples used to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. The present invention can employ various conditions without departing from the spirit of the invention and to achieve the purpose of the invention.

[0191] Steel grades with the chemical compositions shown in Tables 1-1 and 1-2 were smelted and hot-rolled to obtain hot-rolled steel sheets. Then, steel pipes as shown in Tables 3-1 and 3-2 were obtained by electric welding the butt joints (ends of the steel sheets) while the hot-rolled steel sheets were being rolled. Hot-shrink welded steel pipes with wall thicknesses t, outer diameters D, and t / D as shown in Tables 3-1 and 3-2 were obtained by hot-shrinking the steel pipes under the conditions shown in Tables 2-1 and 2-2. Microstructure and texture observations were performed on the hot-shrink welded steel pipes using the methods described above. The results are shown in Tables 4-1 and 4-2. The average grain size of the base material portion of No. 1 is 4.5 μm. In the remaining microstructure columns of Tables 4-1 and 4-2, P represents pearlite, and B / M represents bainite-martensite.

[0192] The obtained heat-shrinkable welded steel pipe was cut into 150mm lengths and used as samples for a flattening test. The heat-shrinkable welded steel pipe was positioned with the welded portion and a 180° angle relative to the welded portion in contact with the die of the press. The heat-shrinkable welded steel pipe was pressed into a flattened shape, and the presence or absence of cracks was evaluated. Pressing continued until the distance between the welded portion and the inner surface at a 180° angle relative to the welded portion was half the diameter. Penetrant testing was applied to the inner surface of the steel pipe; cracks larger than 1mm were considered to have occurred.

[0193] A flatness test was conducted on 250 samples each. If no cracks occurred in any of the samples, it was regarded as having excellent flatness performance and judged as qualified, and recorded as "OK" in the table. On the other hand, if cracks occurred in at least one sample, it was regarded as not having excellent flatness performance and judged as unqualified, and recorded as "NG" in the table. The crack incidence rate is the value obtained by dividing the number of samples with cracks by 100 as the denominator. In the flatness test, a case where the crack incidence rate is 0% is regarded as qualified.

[0194] After heat treatment (quenching and tempering) of the obtained shrink-fit electric welded steel pipes under the conditions shown in Table 2-1 and Table 2-2, a torsional fatigue test was carried out. Furthermore, it was held at the quenching heating temperature for 300 to 600 seconds, and then cooled to the room temperature range at an average cooling rate of 10°C / s or more. The torsional fatigue test was carried out at a frequency of 10 Hz under the condition that the ratio of the minimum stress to the maximum stress (stress ratio) was -1. The fatigue limit was obtained by finding the maximum stress that did not break under 2 million cycles of repetition.

[0195] Even after heat treatment, although the stacking degree decreased, the texture still remained. In addition, the manufacturing conditions as described above also affected the properties of the shrink-fit electric welded steel pipes before heat treatment.

[0196] When the obtained fatigue limit is 350 MPa or more, it is regarded as having excellent fatigue characteristics and judged as qualified. On the other hand, when the fatigue limit is less than 350 MPa, it is regarded as not having excellent fatigue characteristics and judged as unqualified.

[0197] In addition, after the above heat treatment, the Vickers hardness was measured by the above method. The obtained results are shown in Table 4-1 and Table 4-2. Furthermore, it was held at the quenching heating temperature for 300 to 600 seconds, and then cooled to the room temperature range at an average cooling rate of 10°C / s or more.

[0198] When the obtained Vickers hardness is 450 Hv or more, it is regarded as having high hardness and judged as qualified. On the other hand, when the Vickers hardness is less than 450 Hv, it is regarded as not having high hardness and judged as unqualified.

[0199]

[0200]

[0201]

[0202]

[0203] Table 3-1

[0204]

[0205] Table 3-2

[0206]

[0207]

[0208]

[0209] As can be seen from Tables 4-1 and 4-2, the hot-shrink welded steel pipes involved in this invention have high hardness, excellent flatness, and fatigue characteristics.

[0210] On the other hand, it can be seen that the heat-shrinkable welded steel pipes involved in the comparative examples have one or more of the following characteristics that are poor.

[0211] Example No. 21 is an example where the flatness performance is degraded due to the high C content.

[0212] Example No. 22 is an example of hardness degradation due to low C content.

[0213] Example No. 23 is an example where the flatness and fatigue properties deteriorated due to the high Si content.

[0214] Example No. 24 is an example of deterioration in hardness and fatigue properties due to low Si content.

[0215] Example No. 25 is an example of fatigue characteristics deteriorated due to high Mn content.

[0216] Example No. 26 is an example of hardness degradation due to low Mn content.

[0217] Example No. 27 is due to the high P content, which deteriorated the flatness and fatigue properties.

[0218] Example No. 28 is an example where the flatness and fatigue properties deteriorated due to high sulfur content.

[0219] Example No. 29 is an example where the flattening performance was degraded due to the high Al content.

[0220] Example No. 30 is an example where the flattening performance and fatigue characteristics are deteriorated due to the high Cr content.

[0221] Example No. 31 is an example where the flatness performance is degraded due to the high Ti content.

[0222] Example No. 32 is an example where the flatness performance is degraded due to low Ti content.

[0223] Example No. 33 is an example where the hardness and fatigue properties deteriorated due to the high B content.

[0224] Example No. 34 is an example of deterioration in hardness and fatigue properties due to low B content.

[0225] No. 35 is an example of a condition where hardness and fatigue properties deteriorated due to high nitrogen content.

[0226] Example No. 36 is an example of hardness degradation due to high Ti / N ratio.

[0227] Examples No. 37 and No. 38 are due to the long rolling time of the heat-shrinked diameter and the large average grain size of the microstructure, which resulted in deterioration of the flattening performance.

[0228] Examples No. 39 to 43 are due to the large cumulative shrinkage rate and high density of {001} surfaces in the texture in the temperature range below 850°C, which degraded the flattening performance.

[0229] Examples No. 44 and No. 45 are due to the high average cooling rate and small ferrite area ratio after heat shrinking, which degraded the flatness performance.

[0230] Example No. 46 is an example where the flattening performance deteriorated due to the small cumulative shrinkage rate and large accumulation of {001} surfaces in the texture in the temperature range above 650°C.

[0231] No.47 Due to the high Vc90, even within the range of the above-mentioned heat shrinkage conditions, the ferrite fraction becomes high and cannot meet the aggregation requirement.

[0232] No. 48, due to a heating temperature exceeding 1100℃, resulted in an average grain size in the microstructure exceeding 10 μm. Consequently, the flattening properties deteriorated.

[0233] Industrial availability

[0234] According to the above-described method of the present invention, it is possible to provide a heat-shrink welded steel pipe with excellent flatness, excellent fatigue properties after heat treatment, and high hardness.

[0235] The heat-shrinkable welded steel pipe described above can be appropriately applied to components of the running gear of automobiles, such as stabilizers.

Claims

1. A heat-shrinkable welded steel pipe, characterized in that, It has a base material part and a welding part. The chemical composition of the base material, expressed in % by mass, includes C：0.210~0.400%、 Si: 0.05~0.50% Mn: 0.50~1.70%, P: Below 0.100% S: Below 0.010% N: below 0.0100% Al：0.010~0.100%、 Ti: 0.010~0.060%, B：0.0005~0.0050%、 Cr：0~0.500%、 Mo: 0~0.500%, Cu: 0~1.000%, Ni: 0~1.000% Nb: 0~0.050%, W：0~0.050%、 V：0~0.500%、 Ca: 0~0.0050%, and REM: 0~0.0050%, The balance consists of Fe and impurities. The value obtained by dividing the Ti content by the N content, i.e., the Ti / N ratio, is 3.0 or higher. In the microstructure of the welded portion The average grain size of this microstructure is less than 10.0 μm. The ferrite area fraction is more than 20%, and the remaining part contains at least one of pearlite and bainite-martensite. In the texture of the welded portion, the density of the {001} surface is 6.0 or less. The critical cooling rate Vc90 of the base material is 5℃ / s to 90℃ / s. The critical cooling rate Vc90, when the C content is denoted as [C], the Si content as [Si], the Mn content as [Mn], the Cr content as [Cr], the Mo content as [Mo], and the Ni content as [Ni] in mass percent, is expressed by the following formula (1). log 10 Vc90=2.94-0.75×β ⋯ (1), β=2.7×[C]+0.4×[Si]+[Mn]+0.8×[Cr]+2×[Mo]+0.45×[Ni] ⋯ (2).

2. The heat-shrinkable welded steel pipe according to claim 1, characterized in that, The chemical composition, expressed in % by mass, contains ingredients selected from... Mo: 0.010~0.500%, Cu: 0.010~1.000% Ni: 0.010~1.000% Nb: 0.005~0.050%, W：0.010~0.050%、 V：0.010~0.500%、 Ca: 0.0001~0.0050%, and REM: 0.0001~0.0050% One or more of them.

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