JP7472826B2 - Electric resistance welded steel pipe and its manufacturing method - Google Patents

Description

The present invention relates to electric resistance welded steel pipes with high strength and excellent low-temperature toughness that are particularly suitable for use in frame members for construction machinery (referred to as construction machinery frames in this invention), particularly for crane lattices and crane booms, and a method for manufacturing the same.

Steel pipes for construction machinery frames, particularly for crane lattices and crane booms, are required to have high strength and toughness in consideration of the increasing size of cranes and their use in extremely cold regions.
Recently, there has been a demand for steel pipes with a tensile strength (TS) of 780 MPa and excellent toughness at a low temperature of -20° C. Steel pipes used for these applications range widely from relatively thin ones with a wall thickness of about mm to thick ones with a wall thickness of over 10 mm, and so far, seamless steel pipes or electric resistance welded steel pipes have been used for thin ones, while seamless steel pipes have been used for thick ones, and in the case of thick ones, a method has been used in which the required strength is ensured by post-heat treatment (quenching and tempering) after pipe making.

For example, the technology disclosed in Patent Document 1 makes it possible to provide seamless steel pipes that have a tensile strength of 700 MPa or more and an elongation of 25% or more, which is suitable for use in mechanical structural components such as crane booms, and that have excellent low-temperature toughness, with a fracture surface transition temperature vTrs of -40°C or less as determined by a Charpy impact test.

JP 2019-112705 A

However, although the technology proposed in Patent Document 1 produces a steel pipe having both high strength and high toughness, the production costs of the seamless steel pipe are generally higher than those of electric resistance welded steel pipe, and therefore there are issues in terms of price competitiveness.
Furthermore, a decarburized layer is formed on the surface of the pipe during the pipe-making process (high-temperature heating of the round billet before piercing: approximately 1,200°C), which can result in insufficient hardness after quenching in the subsequent quenching process, and can also cause the grain size of the steel pipe structure in the decarburized layer to further coarsen during the heating process during quenching, resulting in a decrease in low-temperature toughness.

Although these problems can be avoided by grinding the surface of the pipe and removing the decarburized surface layer before the hardening process, grinding the inner surface of such a pipe is not only extremely difficult, but also requires the addition of an additional grinding process, which increases manufacturing costs.

In view of the current situation, the present invention aims to provide electric-resistance welded steel pipes with high strength and excellent low-temperature toughness for construction machinery frames, particularly steel pipes for crane lattices or crane booms, which require steel pipes with little surface decarburization on the inner pipe surface, which is the inside surface of the steel pipe, and the outer pipe surface, which is the outside surface of the steel pipe (hereinafter also referred to as the inner pipe surface and the outer pipe surface), and which have excellent welding quality and can also be used for thick-walled products, together with a method for manufacturing such electric-resistance welded steel pipes at low cost.

The inventors have conducted extensive research and discovered that by optimizing the steel components, in particular the amount of Cr, it is possible to effectively suppress surface decarburization, i.e., decarburization of the outer and inner surfaces of the tube, during the heating stage of the mother tube in the quenching process.
Cr is an element that stabilizes cementite, and therefore has the effect of preventing the cementite in the pearlite in the structure before quenching from dissolving and diffusing during heating during quenching, and being subjected to a decarburization reaction. In other words, it is possible to obtain uniform hardness (strength) after quenching throughout the entire thickness of the pipe. Furthermore, it was also discovered that by suppressing the coarsening of crystal grains on the pipe surface, it is possible to effectively prevent a decrease in low-temperature toughness near the surface of the steel pipe.

The present invention was completed based on these findings and through further investigation.
That is, the gist and configuration of the present invention are as follows.
1. An electric resistance welded steel pipe containing, by mass%, 0.20-0.30% C, 0.05-0.50% Si, 0.05-1.50% Mn, 0.0005-0.0300%, S: 0.0001-0.0300%, Al: 0.010-0.100%, Cr: 0.25-0.50%, Ti: 0.010-0.100%, B: 0.0001-0.0050%, N: 0.0005-0.0100% and Ca: 0.0001-0.0050%, with the balance being Fe and unavoidable impurities, and having a composition which satisfies the relationship Ti/N≧4.0, and which has a decarburized layer on both its inner surface and outer surface, and the maximum depth of each of the decarburized layers is 0.20 mm or less.

2. The electric welded steel pipe according to 1, further containing, by mass%, one or more selected from the following: Nb: 0.0100-0.1000%, Mo: 0.05-0.30%, Cu: 0.001-0.500%, Ni: 0.001-0.500%, W: 0.001-0.050%, V: 0.001-0.010%, REM: 0.020%.

3. A method for producing an electric resistance welded steel pipe as described in 1 above, comprising the steps of: C: 0.20-0.30%, Si: 0.05-0.50%, Mn: 0.05-1.50%, P: 0.0005-0.0300%, S: 0.0001-0.0300%, Al: 0.010-0.100%, Cr: 0.25-0.50%, Ti: 0.010-0.100%, B: 0.0001-0.0050%, N: 0. a hot rolling process for hot rolling a steel material having a composition containing Ti: 0.0005-0.0100% and Ca: 0.0001-0.0050%, the balance being Fe and unavoidable impurities, and satisfying the relationship of Ti/N≧4.0, to form a hot rolled sheet to produce a steel pipe material, a pipe-making process for forming the steel pipe material into a cylindrical open pipe, and a pipe-making process for electric resistance welding the open pipe to produce an electric resistance welded steel pipe, a process for heating the electric resistance welded steel pipe to an _Ac3 transformation point or higher, and a process for quenching by cooling to a martensite transformation point (Ms point) or lower at a cooling rate of 50°C/s or higher, and a tempering process.

4. A method for producing an electric-resistance welded steel pipe as described in 2 above, wherein the composition of the steel material further contains, by mass%, one or more selected from the following: Nb: 0.0100-0.1000%, Mo: 0.05-0.30%, Cu: 0.001-0.500%, Ni: 0.001-0.500%, W: 0.001-0.050%, V: 0.001-0.010%, REM: 0.020%.

5. The method for manufacturing an electric-resistance welded steel pipe described in 3 or 4 above, in which the tempering process is carried out under conditions that result in a tensile strength TS of the electric-resistance welded steel pipe after tempering in the range of 780 to 980 MPa.

6. The method for manufacturing an electric-resistance welded steel pipe described in any one of claims 3 to 5, in which shot blasting is performed on the outer surface of the electric-resistance welded steel pipe after the tempering process.

7. A method for producing an electric resistance welded steel pipe as described in 1 or 2 above, comprising the steps of: conducting a pipe-making process of electric resistance welding an open pipe having the composition as described in 1 or 2 above to obtain an electric resistance welded steel pipe; heating the electric resistance welded steel pipe to the _Ac3 transformation point or higher; quenching the electric resistance welded steel pipe by cooling it at a cooling rate of 50°C/s or higher to the martensite transformation point (Ms point) or lower; and then tempering the electric resistance welded steel pipe.

8. The method for manufacturing an electric-resistance welded steel pipe described in 7 above, in which the tempering is performed under conditions that result in a tensile strength TS of the electric-resistance welded steel pipe after tempering of 780 to 980 MPa.

9. The method for manufacturing an electric-resistance welded steel pipe described in 7 or 8, in which shot blasting is performed on the outer surface of the electric-resistance welded steel pipe after the tempering.

The present invention makes it possible to manufacture and provide electric resistance welded steel pipes that have a high tensile strength (TS) of 780 to 980 MPa and excellent low-temperature impact properties (absorbed energy ≧ 24 J, Charpy test piece size width 10 mm × height 5 mm, test temperature: -20°C) and are suitable for structural components of construction machinery, particularly crane lattices or crane booms.

Hereinafter, an embodiment of the present invention will be described.
[Component composition]
The construction machine frame member of the present invention, particularly the electric resistance welded steel pipe for crane lattice or crane boom (hereinafter, sometimes simply referred to as electric resistance welded steel pipe or steel pipe), has a specific chemical composition. The content of each component and the reason for limiting the content are explained below. Unless otherwise specified, "%" in the following steel pipe components refers to "mass %".
C: 0.20 to 0.30%
C is an element necessary to ensure the strength (hardness) required for a crane lattice or a crane boom, and must be contained at 0.20% or more. On the other hand, if the C content exceeds 0.30%, low-temperature toughness is deteriorated. Therefore, the C content is set to 0.30% or less, and preferably 0.26% or less.

Si: 0.05 to 0.50%
Silicon acts as a deoxidizer and also acts as a solid solution strengthening element. In order to obtain the above effect, a content of 0.05% or more is required. Therefore, the Si content is set to 0.05% or more. On the other hand, silicon is prone to forming oxides during electric resistance welding, and it is essential to keep the Si content at 0.50% or less, particularly in thick-walled products with a pipe thickness of 6 mm or more, where it is difficult to ensure the quality of electric resistance welding. Therefore, the Si content is set to 0.50% or less.

Mn: 0.05 to 1.50%
Mn is an element that contributes to improving the strength of steel by dissolving in solid solution and improves the hardenability of steel. In order to ensure the strength (hardness) required for a crane lattice or a crane boom, a content of 0.05% or more is required. Therefore, the Mn content is set to 0.05% or more, preferably 0.2% or more. On the other hand, if the content exceeds 1.50%, not only does the toughness decrease, but the quality becomes high and pipe manufacturing becomes difficult. Therefore, the Mn content is set to 1.50% or less, preferably 1.45% or less.

P: 0.0005 to 0.0300%
P also acts as a solid solution strengthening element. To obtain the above effect, a content of 0.0005% or more is required. On the other hand, excessive addition of P causes segregation at grain boundaries, etc., leading to reduced weld crack resistance and toughness. Therefore, in order to use it as a crane lattice or a crane boom, the P content must be reduced to 0.0300% or less. Preferably, it is 0.0250% or less.

S: 0.0001 to 0.0300%
S exists as sulfide-based inclusions in steel and is an element that reduces hot workability and toughness. In order to use the steel as an electric resistance welded steel pipe for crane lattices or crane booms, it is necessary to reduce the S content to 0.0300% or less. The S content is preferably 0.0100% or less. On the other hand, excessive reduction leads to an increase in steelmaking (desulfurization) costs, so the lower limit is set to 0.0001%.

Al: 0.010 to 0.100%
Al acts as a deoxidizer and also combines with N to precipitate as AlN, thereby increasing strength. In order to obtain this effect, an Al content of 0.010% or more is required. Therefore, the Al content is set to 0.010% or more. On the other hand, if the Al content is greater than 0.100%, the amount of oxide-based inclusions increases and workability decreases. Therefore, the Al content is set to 0.100% or less, preferably 0.050% or less.

Cr: 0.25 to 0.50%
Cr is an important element in the present invention. Cr is dissolved to contribute to improving the strength of steel, and at the same time, Cr can stabilize cementite in pearlite of the ferrite-pearlite phase, which is the constituent phase of the mother pipe, during the heating process during quenching, so that it is an element that suppresses the dissolution of cementite and the progress of the decarburization reaction due to the diffusion of carbon that accompanies the dissolution of cementite. In order to obtain the above effect, it is essential that the Cr content is 0.25% or more. On the other hand, if the Cr content exceeds 0.50%, oxides are easily formed, and Cr oxides remain in the electric resistance welded part, which deteriorates the quality of the electric resistance welded part. Therefore, the Cr content is set to 0.50% or less. In addition, it is preferably 0.45% or less.

Ti: 0.010 to 0.100%
Ti combines with N in steel to form TiN, which has the effect of ensuring dissolved boron that is not combined with N, which is effective in improving hardenability. To achieve this, it is necessary to add 0.010% or more. On the other hand, if the content exceeds 0.100%, ductility decreases. For this reason, the Ti content is limited to 0.100% or less. Preferably, it is 0.050% or less.

B: 0.0001 to 0.0050%
B can improve hardenability by existing in a solid solution state in steel. To achieve this effect, the addition of 0.0001% or more is necessary. On the other hand, if the content of B exceeds 0.0050%, the above effect is saturated and the toughness is reduced, so the upper limit is set to 0.0050%.

N: 0.0005 to 0.0100%
Although N is inevitably contained as an impurity, it combines with nitride-forming elements in the steel, suppresses the coarsening of crystal grains, and contributes to increasing strength after tempering. To achieve such effects, the N content is 0.0005% or more. On the other hand, a content exceeding 0.0100% reduces the toughness of the welded portion. Therefore, the N content is set to 0.0100% or less, preferably 0.0050% or less.

Ca: 0.0001 to 0.0050%
Ca is an element that has the effect of controlling the form of sulfide-based inclusions to make them fine, approximately spherical inclusions. Here, MnS present in the slab is easily stretched in hot rolling, which reduces workability and toughness, but by adding Ca, the solid solution S in the slab does not form MnS, but forms relatively spherical CaS, which is difficult to stretch in the hot rolling process. Therefore, it is possible to suppress the influence of MnS. In order to exert such an effect, the Ca content is 0.0001% or more, preferably 0.0010% or more. On the other hand, if the Ca content exceeds 0.0050%, the number of coarse CaS-based clusters becomes too large, which in turn reduces workability and toughness. Therefore, the Ca content is 0.0050% or less, preferably 0.0030% or less.

Ti/N≧4.0
Ti has the effect of securing dissolved boron that is not bound to N, which is effective in improving hardenability, by combining with N in the steel to form TiN. To achieve this, in addition to the above-mentioned Ti and N amounts, an amount of Ti added according to the amount of N in the steel is required. Therefore, in the present invention, the amount of Ti in the steel and the amount of N in the steel must satisfy the relationship Ti/N≧4.0 (where Ti means the mass% of the Ti amount in the steel, and N means the mass% of the N amount in the steel). There is no particular limit to the upper limit of the Ti/N value, but since an excessively large amount of Ti reduces workability, it is preferable that the upper limit of Ti/N is about 20.

Furthermore, as another embodiment of the present invention, the above-mentioned composition may further optionally contain one or more elements selected from the group consisting of Nb, Mo, Cu, Ni, W, V and REM in the amounts described below.
Nb: 0.0100 to 0.1000%
Nb is an element that combines with C in steel to form NbC and finely disperses, thereby preventing coarsening of austenite grains during heating in the quenching process and suppressing a decrease in toughness. In order to exert such effects, a content of 0.0100% or more is preferable. On the other hand, if the Nb content exceeds 0.1000%, the effect of addition becomes saturated and no effect commensurate with the content can be obtained, which is economically disadvantageous. Therefore, the Nb content is preferably 0.1000% or less, more preferably 0.0500% or less.

Mo: 0.05 to 0.30%,
Mo is an element that contributes to improving the strength of steel by forming a solid solution. In order to obtain such an effect, it is desirable to set the Mo content to 0.05% or more. On the other hand, if the Mo content exceeds 0.30%, the effect saturates and the cost of the material increases. Therefore, the Mo content is preferably 0.30% or less, and more preferably 0.20% or less.

Cu: 0.001 to 0.500%
Cu is an element that has the effect of improving corrosion resistance. To obtain such an effect, a content of 0.001% or more is desirable. On the other hand, since Cu is an expensive alloy element, if the Cu content exceeds 0.500%, the material cost will rise. Therefore, the Cu content is preferably 0.500% or less, and more preferably 0.300% or less.

Ni: 0.001 to 0.500%
Ni, like Cu, is an element that has the effect of improving corrosion resistance. To obtain such an effect, a content of 0.001% or more is desirable. On the other hand, since Ni is an expensive alloy element, if the Ni content exceeds 0.500%, the material cost will rise. Therefore, the Ni content is preferably 0.500% or less, and more preferably 0.300% or less.

W: 0.001 to 0.050%
Like Nb, W is an element that forms fine carbides and contributes to increasing strength (hardness). To obtain such an effect, a content of 0.001% or more is desirable. On the other hand, if the W content exceeds 0.050%, the effect of addition becomes saturated and no effect commensurate with the content can be obtained, which is economically disadvantageous. Therefore, the W content is preferably 0.050% or less, more preferably 0.030% or less.

V: 0.001 to 0.010%
V is an element that combines with C in steel to form carbides, similar to Nb, prevents coarsening of austenite grains during heating in the quenching process, and suppresses the deterioration of toughness. In order to exert such effects, a content of 0.001% or more is desirable. On the other hand, if the V content exceeds 0.010%, the effect of addition becomes saturated and no effect commensurate with the content can be obtained, which is economically disadvantageous. Therefore, the V content is preferably 0.010% or less, more preferably 0.008% or less.

REM: 0.020% or less Like Ca, REM (rare earth metal) is an element that has the effect of controlling the shape of sulfide-based inclusions to fine, approximately spherical inclusions. In order to complement the effect of Ca, REM can be added arbitrarily. On the other hand, if the REM content exceeds 0.020%, the amount of inclusions that become the starting point of fatigue cracks becomes excessive, and the corrosion fatigue resistance property is rather deteriorated. Therefore, the REM content is preferably 0.020% or less, more preferably 0.010% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of enhancing the effect of adding REM, the REM content is preferably 0.001% or more.
The remaining components other than the above-mentioned components in the present invention are Fe and unavoidable impurities.

The surface decarburization of the steel pipe in the present invention will be described below.
The steel pipe according to the present invention is surface decarburized, i.e., has a decarburized layer (also referred to as a total decarburized layer in this specification) as defined below on the inner and outer surfaces of the pipe. Note that, in the present invention, the decarburized layer means the distance from the surface of the steel material to the position where the difference in chemical or physical properties between the decarburized layer and the base material can no longer be visually distinguished when the cross section of the steel pipe is polished and then corroded with a nital solution in accordance with the method described in JIS G0558 and observed under the following microscope.
The depth of the total decarburized layer is determined by observing the structure of a test piece for microstructural observation (cross-sectional observation in the circumferential (C) direction of a pipe) under an optical microscope (magnification: 100 times) and determining the maximum depth point among 10 visual fields (each visual field has a length of about 1 mm).

Furthermore, if the depth of the total decarburized layer exceeds 0.20 mm on either the inner or outer surface of the tube, not only will sufficient strength (hardness) not be obtained at the surface after quenching, but the low-temperature toughness will decrease due to the coarsening of the crystal grains on the surface. Therefore, in the present invention, the upper limit of the depth of the total decarburized layer is set to 0.20 mm. Preferably, it is 0.15 mm. The lower limit is 0 mm, since there is no problem even if there is no total decarburized layer.

The mechanical properties (tensile strength) and low-temperature toughness (impact properties) of the steel pipe of the present invention will be described below.
Tensile strength TS: 780 to 980 MPa
The steel pipe of the present invention is required to have high strength and high toughness in consideration of the increase in size of cranes and use in cold regions. To achieve this, the tensile strength TS must be 780 MPa or more. On the other hand, if the strength exceeds 980 MPa, it becomes necessary to increase the carbon content in the steel or lower the tempering temperature in order to ensure the required strength, which leads to a decrease in low-temperature toughness. Therefore, the upper limit of the tensile strength TS is set to 980 MPa.

Charpy absorbed energy: 24 J or more (Charpy test piece size: width: 10 mm x height: 5 mm, test temperature: -20°C)
In consideration of use in cold regions, the steel pipe of the present invention needs to have a low-temperature impact property of 24 J or more (Charpy absorbed energy Charpy test piece size: 10 x 5 mm, test temperature: -20°C). If it is less than 24 J, the risk of brittle fracture increases when used in cold regions. There is no particular upper limit.

The method for producing a steel pipe according to the present invention will now be described.
In the present invention, a steel material is subjected to a hot rolling process to form a steel pipe material, and then the steel pipe material is subjected to a pipe-making process to form an electric resistance welded steel pipe (blank pipe).
For example, the steel material used is prepared by melting molten steel having the above-mentioned composition in a conventional melting method such as a converter, and forming the steel material into a slab or the like by a continuous casting method or an ingot casting-rolling method.
The steel material is hot-rolled to obtain a hot-rolled sheet, which is then used as a steel pipe material, and then the steel pipe material is made into an electric resistance welded steel pipe by a pipe-making process in which the steel pipe material is continuously formed into a substantially cylindrical open pipe, and the open pipe is electric resistance welded to obtain an electric resistance welded steel pipe (blank pipe).

In the present invention, the above-mentioned blank pipe is further subjected to a heat treatment. The heat treatment method for the steel pipe in the present invention will be described below.
Quenching: The raw pipe is heated to or above the _Ac3 transformation point (the quenching start temperature is the _Ac3 transformation point or higher), and then quenched by cooling to or below the martensite transformation point (Ms point) at a cooling rate of 50°C/s or higher.
If the quenching start temperature is lower than the _Ac3 transformation point, sufficient quenching hardness cannot be obtained, while excessive heating to a high temperature causes coarsening of austenite grains and reduces low-temperature toughness. Therefore, the temperature of the blank pipe is raised to the _Ac3 transformation point or higher. The upper limit of the quenching start temperature is industrially about the _Ac3 transformation point + 100°C.
Furthermore, if the cooling rate after the heating is less than 50° C./s, sufficient hardening hardness cannot be obtained. Therefore, the lower limit of the cooling rate is 50° C./s. On the other hand, there is no particular upper limit to the cooling rate, but industrially it is about 150° C./s.

Tempering: The conditions for tempering may be any conditions that do not deviate from the strength required after tempering (TS: 780 to 980 MPa). Specifically, the tempering temperature is desirably Ac ₁ point or lower, since the strength of the steel pipe is significantly reduced when it exceeds the Ac ₁ transformation point. Furthermore, the lower limit of the tempering temperature is desirably 300°C or higher, since low-temperature toughness is reduced when it is lowered below 300°C. More desirably, it is 400°C or higher.
The holding time is not particularly limited, but in order to ensure uniformity of quality, it is desirable for the holding time to be 10 minutes or more.
In the present invention, if an open pipe satisfying the component composition and the like of the present invention is prepared, the electric welded steel pipe of the present invention can be obtained by subjecting the subsequent pipe-making process and heat treatment to the conditions of the present invention described above.

Furthermore, after the tempering process, shot blasting may be performed on the outer surface of the tube. There are no particular restrictions on the conditions for shot blasting, but it is performed under conditions that leave no surface oxide scale, ensuring the beauty of the surface and subsequent paintability.

Molten steel having the composition shown in Table 1 was melted in a converter and made into slabs (steel materials) by continuous casting. These slabs (steel materials) were hot-rolled to obtain hot-rolled sheets (steel pipe materials) with a thickness of 12 mm, which were then rolled to obtain roughly cylindrical open pipes. Next, while pressing the butt joints of the open pipes with a squeeze roll, the butt joints were electric-sewn by high-frequency resistance welding to obtain electric-sewn steel pipes (size, outer diameter: 114.3 mmφ x wall thickness: 12 mm).
Thereafter, the electric resistance welded steel pipes obtained by carrying out the heat treatments (quenching and tempering) shown in Table 2 were subjected to measurement of the total decarburized layer (surface decarburization) depth, tensile tests, and Charpy tests. The test methods were as follows.

(1) Total decarburization depth The total decarburization depth was measured according to the method described in JIS G0558. That is, a test piece for microstructure observation (observation of the cross section in the circumferential (C) direction of the pipe) was taken from the obtained steel pipe, the cross section was polished, and then corroded with a nital solution. The structure was observed using an optical microscope (magnification: 100 times) to measure the total decarburization depth. Observation was performed in 10 fields of view, and the maximum decarburization depth among the total decarburization depth was adopted.

(2) Tensile Test In accordance with the provisions of JIS Z 2241, JIS No. 12B tensile test pieces (gauge length: 50 mm) were taken from the obtained steel pipes so that the tensile direction was the pipe axis (L) direction, and tensile tests were carried out in accordance with the provisions of JIS Z 2241 to determine the 0.2% yield strength: YS (MPa), tensile strength: TS (MPa), and elongation: El (%).

(3) Charpy test (low temperature impact test)
From the obtained steel pipe, a V-notch test piece (width: 10 mm × height: 5 mm × length: 55 mm, notch angle: 45°, notch depth: 2 mm, notch bottom radius: 0.25 mm) was taken from the center of the pipe thickness in the pipe longitudinal (L) direction in accordance with the provisions of JIS Z 2242, and the absorbed energy was measured at a test temperature of -20°C.
The results are shown in Table 2.

As shown in Table 2, all steel pipes according to the present invention have high strength and excellent low-temperature toughness. In contrast, steel pipes made with steel components or manufacturing conditions outside the scope of the present invention are shown to be inferior in either strength or low-temperature impact properties, or both.

For comparison, No. 17 in Table 2 shows an example of a seamless steel pipe made from the same steel composition and quenched and tempered under the same conditions. Compared to the steel pipe of the present invention, the tensile strength was the same, but the total decarburized layer depth was large, and the low-temperature impact properties and surface texture (pockmarked and less aesthetically pleasing) were inferior.

Claims (9)

The steel pipe has a composition containing, by mass%, C: 0.20-0.30%, Si: 0.05-0.50%, Mn: 0.05-1.50%, P: 0.0005-0.0300%, S: 0.0001-0.0300%, Al: 0.010-0.100%, Cr: 0.25-0.50%, Ti: 0.010-0.100%, B: 0.0001-0.0050%, N: 0.0005-0.0100% and Ca: 0.0001-0.0050%, with the balance being Fe and unavoidable impurities, and the composition satisfies the relationship Ti/N≧4.0, and has a decarburized layer on both the inner surface and the outer surface of the steel pipe, and the maximum depth of the decarburized layer is 0.20 mm or less (50× ¹⁰ Electric welded steel pipes that are 2.5m or less in diameter. Further, in mass%,
2. The electric welded steel pipe according to claim 1, containing one or more selected from the following: Nb: 0.0100 to 0.1000%, Mo: 0.05 to 0.30%, Cu: 0.001 to 0.500%, Ni: 0.001 to 0.500%, W: 0.001 to 0.050%, V: 0.001 to 0.010%, and REM: 0.020%. A method for producing the electric resistance welded steel pipe according to claim 1,
In mass%, C: 0.20-0.30%, Si: 0.05-0.50%, Mn: 0.05-1.50%, P: 0.0005-0.0300%, S: 0.0001-0.0300%, Al: 0.010-0.100%, Cr: 0.25-0.50%, Ti: 0.010-0.100%, B: 0.0001-0.0050%, N: 0.0005-0.0100 % and Ca: 0.0001-0.0050%, the balance being Fe and unavoidable impurities, and the composition satisfies the relationship of Ti/N≧4.0, a hot rolling process is performed in which a steel pipe material is hot rolled to form a hot rolled sheet to produce a steel pipe material, the steel pipe material is shaped to form a cylindrical open pipe, and the open pipe is electric resistance welded to produce an electric resistance welded steel pipe, a pipe making process is performed in which the electric resistance welded steel pipe is heated to or above the _Ac3 transformation point, cooled at a cooling rate of 50°C/s or more to or below the martensite transformation point (Ms point) to be quenched, and a tempering process is performed. A method for producing an electric resistance welded steel pipe according to claim 2,
4. The method for producing an electric welded steel pipe according to claim 3, wherein the composition of the steel material further contains, in mass%, one or more selected from the following: Nb: 0.0100 to 0.1000%, Mo: 0.05 to 0.30%, Cu: 0.001 to 0.500%, Ni: 0.001 to 0.500%, W: 0.001 to 0.050%, V: 0.001 to 0.010%, and REM: 0.020%. The method for manufacturing electric-resistance welded steel pipes according to claim 3 or 4, wherein the tempering process is carried out under conditions that result in the tensile strength TS of the electric-resistance welded steel pipe after tempering being in the range of 780 to 980 MPa. The method for manufacturing an electric-resistance welded steel pipe according to any one of claims 3 to 5, further comprising shot blasting the outer surface of the electric-resistance welded steel pipe after the tempering process. A method for producing an electric resistance welded steel pipe according to claim 1 or 2,
A method for producing an electric-resistance welded steel pipe, comprising the steps of: conducting a pipe-making process of electric-resistance welding an open pipe having the composition according to claim 1 or 2 to produce an electric-resistance welded steel pipe; heating the electric-resistance welded steel pipe to the _Ac3 transformation point or higher; quenching the electric-resistance welded steel pipe by cooling it at a cooling rate of 50°C/s or higher to the martensite transformation point (Ms point) or lower; and then tempering the electric-resistance welded steel pipe. The method for manufacturing electric-resistance welded steel pipes according to claim 7, wherein the tempering is performed under conditions that result in a tensile strength TS of the electric-resistance welded steel pipe after tempering of 780 to 980 MPa. The method for producing an electric resistance welded steel pipe according to claim 7 or 8, further comprising the step of subjecting the outer surface of the electric resistance welded steel pipe after the tempering to shot blasting.