ELECTRIC RESISTANCE WELDED STEEL PIPE FOR MECHANICAL STRUCTURAL COMPONENTS, AND METHOD FOR PRODUCING SAME

Abstract

An ERW steel pipe for a mechanical structural part, the pipe including a straight pipe portion includes a base metal portion and an ERW portion, wherein the base metal portion has a chemical composition including, in terms of % by mass: 0.30 to 0.38% of C, 0.05 to 0.40% of Si, 0.50 to 2.00% of Mn, 0.010 to 0.060% of Al, 0.005 to 0.050% of Ti, and 0.0003 to 0.0050% of B, wherein a microstructure of a wall thickness central portion of the base metal portion is tempered martensite, wherein, a de-C layer in which a concentration of C is 90% or less with respect to the concentration of C in the chemical composition has a thickness of less than 0.20 mm, and a de-B layer in which a concentration of B is 90% or less with respect to the concentration of B in the chemical composition has a thickness of less than 0.10 mm, at each of inner and outer surface sides of the base metal portion, and wherein each of a Vickers hardness at a position at a depth of 0.5 mm from each of the inner and outer surface sides is 420 Hv or more and less than 510 Hv.

Description

TECHNICAL FIELD

The present disclosure relates to an electric resistance welded steel pipe for a mechanical structural part, and a method of producing the same.

BACKGROUND ART

In recent years, various examinations have been done on electric resistance welded steel pipes (hereinafter, each also referred to as “electric resistance welded steel pipe for a mechanical structural part”) which are used as mechanical structural parts such as automotive parts, or as materials of the mechanical structural parts.

For examples, Patent Document 1 discloses an electric resistance welded steel pipe for use in a heat treatment, which can be formed into a member having an excellent durability even when subjected to a rapid heat quenching treatment, such as one used for producing a hollow stabilizer or the like. This electric resistance welded steel pipe has a composition consisting of, in terms of % by mass: from 0.15 to 0.40% of C, from 0.05 to 0.50% of Si, from 0.30 to 2.00% of Mn, from 0.01 to 0.10% of Al, from 0.001 to 0.04% of T, from 0.0005 to 0.0050% of B and from 0.0010 to 0.0100% of N, wherein Ti and N satisfy the relation (N/14)<(Ti/47.9), and a balance consisting of Fe and unavoidable impurities. In this electric resistance welded steel pipe, a bond width of an electric resistance welded portion is 25 μm or less.

Patent Document 2 discloses a steel for an electric resistance welded steel pipe for use in a hollow stabilizer. This steel is composed of: 0.35% or less of C, 0.25% or less of Si, from 0.30 to 1.20% of Mn, 0.60% or less of Cr, 0.0020% or less of P, 0.0020% or less of S, 0.10% or less of sol. Al, 0.0200% or less of N+O, Ti in an amount of from 4 to 12 times the amounts of (N+O) in the steel, from 0.0005 to 0.009% of B, and the balance consisting of Fe and unavoidable impurities. This steel is composed of components in which the contents of C, Si, Mn and Cr are adjusted so as to achieve an ideal critical diameter DI (in) as determined by a predetermined formula of 1.0 (in) or more, and a carbon equivalent Ceq as determined by a predetermined formula of 0.60% or less.

Patent Document 3 discloses a method of producing a hollow stabilizer. This method includes: preparing a slab of a steel, the steel being composed of: 0.35% or less of C, 0.20% or less of Si, from 0.30 to 1.20% of Mn, 0.60% or less of Cr, 0.0020% or less of P, 0.0020% or less of S, 0.10% or less of sol. Al, Ti in an amount of from 4 to 12 times the amounts of (N+O) in the steel, from 0.0005 to 0.009% of B, and the balance consisting of Fe and unavoidable impurities, wherein the contents of C, Si, Mn and Cr are adjusted so as to achieve an ideal critical diameter DI (in) as determined by a predetermined formula of 1.0 (in) or more, and a carbon equivalent Ceq as determined by a predetermined formula of 0.48% or less; subjecting the thus prepared slab to hot rolling; coiling the resultant at a coiling temperature controlled within the range of from 570 to 690° C.; and using the resulting steel sheet or steel band, to producing the electric resistance welded steel pipe for use in a hollow stabilizer.

Patent Document 4 discloses a method of producing a high-strength and high-ductility electric resistance welded steel pipe for a mechanical structure, which is used as a steel pipe for reinforcing an automobile door, or the like. This method includes: performing a normalizing treatment at a temperature of from 850 to 950° C., on an electric resistance welded steel pipe which contains: from 0.18 to 0.28% of C, from 0.10 to 0.50% of Si, from 0.60 to 1.80% of Mn, from 0.020 to 0.050% of Ti and from 0.0005 to 0.0050% of B, and which further contains one or more selected from the group consisting of: from 0.20 to 0.50% of Cr, 0.5% or less of Mo, and 0.015 to 0.050% of Nb; and then subjecting the treated electric resistance welded steel pipe to quenching.

Patent Document 5 discloses a high-strength electric resistance welded steel pipe for use in an automobile, which has an excellent tensile strength and shock absorption performance as well as excellent impact properties at low temperature. This electric resistance welded steel pipe is formed from a steel sheet, the steel sheet containing: from 0.2 to 0.4% of C, from 0.05 to 0.5% of Si, from 0.5 to 2.5% of Mn, 0.025% or less (excluding 0%) of P, 0.01% or less (excluding 0%) of S, 0.15% or less (excluding 0%) of Al, 2% or less (excluding 0%) of Cu, 2% or less (excluding 0%) of Cr, 0.2% or less (excluding 0%) of Ti and 0.005% or less (excluding 0%) of B, and the balance consisting of Fe and unavoidable impurities; wherein the steel has a tensile strength of 1,750 N/mm² or more, a 0.1% proof stress of 1,320 N/mm² or more, and a Charpy impact value at a test temperature of −40° C. of 50 J/cm² or more.

Patent Document 6 discloses an electric resistance welded steel pipe having an excellent fatigue durability after being subjected to a rapid short-time heat quenching treatment . . . . This electric resistance welded steel pipe includes: a base metal portion composed of a steel sheet having a specific component composition; and an electric resistance welded portion having a bond width of from 40×10⁻⁶ m to 120×10⁻⁶. In this electric resistance welded steel pipe, the value of C₀-C₁, which is the difference between the lowest C content: C₁ (% by mass) of the electric resistance welded portion and the C content: C₀ (% by mass) of the steel sheet, is 0.05% by mass or less; and the depth of a total de-C layer in each of an inner surface layer and an outer surface layer of the electric resistance welded steel pipe is 50×10⁻⁶ m or less.

• • Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2013-147751
  • Patent Document 2: JP-A No. S58-123858
  • Patent Document 3: JP-A No. S57-126917
  • Patent Document 4: JP-A No. H6-93339
  • Patent Document 5: JP-A No. 2008-261049
  • Patent Document 6: WO 2019/131813

SUMMARY OF INVENTION

Technical Problem

However, there is a case in which a further improvement in fatigue strength is required for an electric resistance welded steel pipe for a mechanical structural part (namely, an electric resistance welded steel pipe which is used as a mechanical structural part such as an automotive part, or as a material of the mechanical structural part), by means different from those disclosed in Patent Document 1 to Patent Document 6.

An object of one embodiment of the present disclosure is to provide: an electric resistance welded steel pipe for a mechanical structural part, the pipe including a straight pipe portion having an excellent fatigue strength; and a method of producing the electric resistance welded steel pipe for a mechanical structural part.

Solution to Problem

Means for solving the problem described above includes the following embodiments.

• • <1> An electric resistance welded steel pipe for a mechanical structural part, the pipe comprising a straight pipe portion,
    • wherein the straight pipe portion comprises a base metal portion and an electric resistance welded portion,
    • wherein the base metal portion has a chemical composition consisting of, in terms of % by mass:
  • from 0.30 to 0.38% of C,
  • from 0.05 to 0.40% of Si,
  • from 0.50 to 2.00% of Mn,
  • from 0.010 to 0.060% of Al,
  • from 0.005 to 0.050% of Ti,
  • from 0.0003 to 0.0050% of B,
  • from 0.0005 to 0.0040% of Ca,
  • from 0 to 0.0060% of N,
  • from 0 to 0.020% of P,
  • from 0 to 0.0200% of S,
  • from 0 to 0.0050% of O,
  • from 0 to 0.50% of Cu,
  • from 0 to 0.50% of Ni,
  • from 0 to 0.50% of Cr,
  • from 0 to 0.20% of V,
  • from 0 to 0.10% of Nb,
  • from 0 to 0.50% of Mo,
  • from 0 to 0.0500% of Mg,
  • from 0 to 0.0500% of REM, and
  • a balance consisting of Fe and impurities,
    • wherein a microstructure of a central portion in a wall thickness direction of the base metal portion is tempered martensite,
    • wherein, in a case in which a layer in which a concentration of C is 90% or less with respect to the concentration of C in the chemical composition of the base metal portion is defined as a de-C layer, and a layer in which a concentration of B is 90% or less with respect to the concentration of B in the chemical composition of the base metal portion is defined as a de-B layer, the de-C layer has a thickness of less than 0.20 mm and the de-B layer has a thickness of less than 0.10 mm, at each of an inner surface side and an outer surface side of the base metal portion, and
    • wherein each of a Vickers hardness at a position at a depth of 0.5 mm from an inner surface of the base metal portion and a Vickers hardness at a position at a depth of 0.5 mm from an outer surface of the base metal portion is 420 Hv or more and less than 510 Hv.
  • <2> The electric resistance welded steel pipe for a mechanical structural part according to <1>, wherein the chemical composition of the base metal portion comprises one or more selected from the group consisting of, in terms of % by mass:
  • from 0.01 to 0.50% of Cu,
  • from 0.05 to 0.50% of Ni,
  • from 0.05 to 0.50% of Cr, and
  • from 0.01 to 0.50% of Mo.
  • <3> The electric resistance welded steel pipe for a mechanical structural part according to <1> or <2>,
    • wherein the straight pipe portion has an outer diameter of from 10 to 50 mm, and
    • wherein a value obtained by dividing a wall thickness of the base metal portion by the outer diameter of the straight pipe portion is from 0.04 to 0.25.
  • <4> The electric resistance welded steel pipe for a mechanical structural part according to any one of <1> to <3>, wherein, in the chemical composition of the base metal portion, F1 represented by the following Formula (1) is 0.50 or more:

$\begin{array}{cc}F1=\mathrm{Ca}\times \left(1-124\times O\right)/\left(1.25\times S\right)& \mathrm{Formula}\left(1\right)\end{array}$

• • wherein each element symbol in Formula (1) represents a content of each element in terms of % by mass.
  • <5> A method of producing the electric resistance welded steel pipe for a mechanical structural part according to any one of <1> to <4>, the method comprising:
    • a preparation step of preparing an as-rolled electric resistance welded steel pipe, wherein:
      • the as-rolled electric resistance welded steel pipe comprises a base metal portion A and an electric resistance welded portion A; and
      • the base metal portion A has a chemical composition consisting of, in terms of % by mass:
  • from 0.30 to 0.38% of C,
  • from 0.05 to 0.40% of Si,
  • from 0.50 to 2.00% of Mn,
  • from 0.010 to 0.060% of Al,
  • from 0.005 to 0.050% of Ti,
  • from 0.0003 to 0.0050% of B,
  • from 0.0005 to 0.0040% of Ca,
  • from 0 to 0.0060% of N,
  • from 0 to 0.020% of P,
  • from 0 to 0.0200% of S,
  • from 0 to 0.0050% of O,
  • from 0 to 0.50% of Cu,
  • from 0 to 0.50% of Ni,
  • from 0 to 0.50% of Cr,
  • from 0 to 0.20% of V,
  • from 0 to 0.10% of Nb,
  • from 0 to 0.50% of Mo,
  • from 0 to 0.0500% of Mg,
  • from 0 to 0.0500% of REM, and
  • a balance consisting of Fe and impurities;
    • a quenching step of subjecting the as-rolled electric resistance welded steel pipe to quenching; and
    • a tempering step of subjecting the as-rolled electric resistance welded steel pipe that has been subjected to quenching to tempering, to obtain the electric resistance welded steel pipe for a mechanical structural part;
    • wherein, in the quenching step, an oxygen content in an atmosphere in which the quenching is performed is 1,000 volume ppm or less, and a cooling rate in the quenching is 10° C./sec or more.
  • <6> The method of producing the electric resistance welded steel pipe for a mechanical structural part, according to <5>, further comprising a pipe-drawing step of subjecting the as-rolled electric resistance welded steel pipe to pipe drawing, after the preparation step and before the quenching step,
    • wherein the as-rolled electric resistance welded steel pipe that has been subjected to pipe drawing is subjected to quenching, in the quenching step.
  • <7> The method of producing the electric resistance welded steel pipe for a mechanical structural part, according to <5> or <6>, further comprising a step of subjecting the as-rolled electric resistance welded steel pipe to shot blasting, after the preparation step and before the quenching step.
  • <8> The method of producing the electric resistance welded steel pipe for a mechanical structural part, according to any one of <5> to <7>,
    • wherein a heating temperature in the quenching is from 900 to 1,050° C., and
    • wherein a heating temperature in the tempering is from 100 to 500° C.

Advantageous Effects of Invention

One embodiment of the present disclosure provides: an electric resistance welded steel pipe for a mechanical structural part, the pipe including a straight pipe portion having an excellent fatigue strength; and a method of producing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a Vickers hardness profile in a depth direction showing the relationship between the depth from the inner surface of the base metal portion (namely, a distance in the wall thickness direction) and the Vickers hardness (Hv), in a cross section (hereinafter, also referred to as “C cross section”) perpendicular to the pipe axis direction of a conventional electric resistance welded steel pipe.

FIG. 2 is a C concentration profile in the depth direction showing the relationship between the depth from the inner surface of the base metal portion and the C concentration (% by mass), in the C cross section of the electric resistance welded steel pipe from which the profile shown in FIG. 1 was obtained.

FIG. 3 is a B concentration profile in the depth direction showing the relationship between the depth from the inner surface of the base metal portion (namely, the distance from the inner surface in the wall thickness direction) and the B concentration (% by mass), in the C cross section of the electric resistance welded steel pipe from which the profile shown in FIG. 1 was obtained.

FIG. 4 is a conceptual diagram showing the relationship between the depth from the inner surface of the base metal portion and the Vickers hardness, in the C cross section of the electric resistance welded steel pipe.

DESCRIPTION OF EMBODIMENTS

In the present disclosure, any numerical range indicated using the expression “from to” represents a range in which numerical values described before and after the “to” are included in the range as a lower limit value and an upper limit value.

In the present disclosure, the symbol “%” indicating the content of a component (element) refers to “% by mass”.

In the present disclosure, the content of C (carbon) may be referred to as “C content”. The same may apply to the contents of other elements.

Further, in the present disclosure, the definition of the term “step” includes not only an independent step, but also a step which is not clearly distinguishable from another step, as long as the intended purpose of the step is achieved.

In the present disclosure, the term “straight pipe portion” of an electric resistance welded steel pipe refers to a straight portion (namely, a straight portion in a state as it is, as having been subjected to pipe-making or to pipe drawing) that has not been subjected to bending, of the electric resistance welded steel pipe. The straight pipe portion is preferably a portion that constitutes 70% or more of the length in the pipe axis direction of the electric resistance welded steel pipe.

In the present disclosure, the term “base metal portion” refers to a portion other than the electric resistance welded portion and a heat affected zone, of the electric resistance welded steel pipe. The “heat affected zone (hereinafter, also referred to as “HAZ”)” as used herein refers to a zone which is present in the vicinity of the electric resistance welded portion and which has been affected by heat caused by electric resistance welding and a seam heat treatment.

In the present disclosure, the term “as-rolled electric resistance welded steel pipe” refers to an electric resistance welded steel pipe that has not been subjected to a heat treatment other than the seam heat treatment, after pipe-making.

The term “pipe-making” refers to a process of roll-forming a hot-rolled steel sheet that has been uncoiled from a hot coil to form an open pipe, and subjecting abutting portions of the resulting open pipe to electric resistance welding to form an electric resistance welded portion.

The term “hot coil” refers to a hot-rolled steel sheet that has been produced using a hot strip mill, and coiled in the form of a coil.

The term “roll forming (to roll-form)” refers to subjecting a hot-rolled steel sheet that has been uncoiled from a hot coil to bending continuously, to form the sheet in the form of an open pipe.

The hot-rolled steel sheet produced using a hot strip mill is different from a steel plate produced using a plate mill in that the hot-rolled steel sheet is a continuous steel sheet.

A steel plate cannot be used in roll forming which is a continuous bending, since the steel plate is not a continuous steel sheet.

An electric resistance welded steel pipe is clearly distinguished from a welded steel pipe (such as a UOE steel pipe) produced using a steel plate, in the points described above.

[Electric Resistance Welded Steel Pipe for Mechanical Structural Part]

The electric resistance welded steel pipe for a mechanical structural part according to the present disclosure (hereinafter, also simply referred to as “electric resistance welded steel pipe”) is an electric resistance welded steel pipe for a mechanical structural part, the pipe includes a straight pipe portion,

• • wherein the straight pipe portion includes a base metal portion and an electric resistance welded portion,
  • wherein the base metal portion has a chemical composition consisting of, in terms of % by mass:
  • from 0.30 to 0.38% of C,
  • from 0.05 to 0.40% of Si,
  • from 0.50 to 2.00% of Mn,
  • from 0.010 to 0.060% of Al,
  • from 0.005 to 0.050% of Ti,
  • from 0.0003 to 0.0050% of B,
  • from 0.0005 to 0.0040% of Ca,
  • from 0 to 0.0060% of N,
  • from 0 to 0.020% of P,
  • from 0 to 0.0200% of S,
  • from 0 to 0.0050% of O,
  • from 0 to 0.50% of Cu,
  • from 0 to 0.50% of Ni,
  • from 0 to 0.50% of Cr,
  • from 0 to 0.20% of V,
  • from 0 to 0.10% of Nb,
  • from 0 to 0.50% of Mo,
  • from 0 to 0.0500% of Mg,
  • from 0 to 0.0500% of REM, and
  • the balance consisting of Fe and impurities,
    • wherein the microstructure of the central portion in the wall thickness direction of the base metal portion is tempered martensite,
    • wherein, when a layer in which the concentration of C is 90% or less with respect to the concentration of C in the chemical composition of the base metal portion is defined as a de-C layer, and a layer in which the concentration of B is 90% or less with respect to the concentration of B in the chemical composition of the base metal portion is defined as a de-B layer, the de-C layer has a thickness of less than 0.20 mm and the de-B layer has a thickness of less than 0.10 mm, at each of an inner surface side and an outer surface side of the base metal portion, and
    • wherein each of the Vickers hardness at a position at a depth of 0.5 mm from an inner surface of the base metal portion and the Vickers hardness at a position at a depth of 0.5 mm from an outer surface of the base metal portion is 420 Hv or more and less than 510 Hv.

The electric resistance welded steel pipe according to the present disclosure is an electric resistance welded steel pipe which satisfies the combination of:

• • the above-described chemical composition of the base metal portion in the straight pipe portion;
  • the above-described microstructure of the base metal portion, which is tempered martensite; and
  • the fact that each of the Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion and the Vickers hardness at a position at a depth of 0.5 mm from the outer surface of the base metal portion is 420 Hv or more and less than 510 Hv; and which also has an excellent fatigue strength.

The excellent fatigue strength is an effect provided by the fact that the de-C layer has a thickness of less than 0.20 mm and the de-B layer has a thickness of less than 0.10 mm, at each of the inner surface side and the outer surface side of the base metal portion.

The electric resistance welded steel pipe for a mechanical structural part according to the present disclosure is an electric resistance welded steel pipe used as a mechanical structural part or as a material of the mechanical structural part.

Examples of the mechanical structural part for which the electric resistance welded steel pipe for a mechanical structural part according to the present disclosure is used, include: mechanical structural parts for which high strength and fatigue resistance properties are required, such as automotive parts (for example, drive shafts, axle beams, hollow springs, and stabilizers).

The electric resistance welded steel pipe according to the present disclosure is based on the following findings obtained by the present inventors.

In general, the fatigue strength of steel and the hardness thereof shows a positive correlation. Therefore, the inventors have studied the hardness of the inner surface layer of the base metal portion of the electric resistance welded steel pipe. As a result, the profile shown in FIG. 1 was obtained.

The “inner surface layer of the base metal portion” as used herein refers to a region from the inner surface to a predetermined depth, of the base metal portion. The “predetermined depth” refers, for example, to the region from the inner surface to a position at a depth of 0.50 mm.

FIG. 1 is a Vickers hardness profile in the depth direction showing the relationship between the depth from the inner surface of the base metal portion (namely, the distance in the wall thickness direction) and the Vickers hardness (Hv), in a cross section (hereinafter, also referred to as “C cross section”) perpendicular to the pipe axis direction of a conventional electric resistance welded steel pipe.

As shown in FIG. 1, it has been found out that, in the inner surface layer (for example, the region from the inner surface to a position at a depth of 0.50 mm) of the base metal portion of a conventional electric resistance welded steel pipe, the hardness of the region from the inner surface to a position at a depth of 0.20 mm is lower than the hardness of the region deeper than a position at a depth of 0.20 mm from the inner surface.

A region having a low hardness is more likely to be an origin of a fatigue crack. When the region having a low hardness in the inner surface layer is thick, the fatigue strength of the electric resistance welded steel pipe is decreased.

Thus, the inventors devised a way to decrease the thickness of the region having a low hardness in the inner surface layer of the base metal portion of an electric resistance welded steel pipe, in order to increase the fatigue strength of the electric resistance welded steel pipe.

First, the inventors investigated and examined the reason for a decrease in the hardness, in the inner surface layer of the base metal portion of the electric resistance welded steel pipe. The hardness correlates with the element concentrations of steel. Therefore, the inventors focused on C (carbon) which affects the hardness.

The inventors measured the C concentration profile in the depth direction showing the relationship between the depth from the inner surface (namely, the distance from the inner surface in the wall thickness direction) and the C concentration (% by mass), in the conventional electric resistance welded steel pipe (C content: 0.38% by mass) from which the profile shown in FIG. 1 was obtained, by the method to be described later.

FIG. 2 shows the C concentration profile in the depth direction described above.

A dashed line 1 in FIG. 2 indicates the C concentration (=0.342%) which corresponds to 90% with respect to the C concentration in the base metal portion.

The “C concentration in the base metal portion” refers to the C concentration (% by mass) in the interior of the base metal portion. The “C concentration in the interior of the base metal portion” refers specifically to the C concentration (% by mass) in the central portion in the wall thickness direction, at a position (hereinafter, also referred to as “base material 90° position”) 90 degrees away from the electric resistance welded portion in the pipe circumferential direction of the electric resistance welded steel pipe.

As shown in FIG. 2, the C concentration in the region from the inner surface to a depth position of 0.20 mm of the base metal portion of the electric resistance welded steel pipe is lower than 90% (namely, 0.342%) of the C concentration in the base metal portion.

Based on the above results, the inventors thought that the reason for a decrease in the hardness in the region from the inner surface to a depth position of 0.20 mm of the base metal portion of the electric resistance welded steel pipe is because the C concentration in the inner surface layer is decreased.

A phenomenon in which the C concentration is decreased is hereinafter also referred to as “de-C”.

However, the distribution of Vickers hardness shown in FIG. 1 is markedly decreased in the region from the inner surface to a depth position of 0.10 mm of the base metal portion, relative to the decreased proportion of the C concentration in the inner surface layer of the base metal portion, which is predictable from the C concentration profile shown in FIG. 2.

Thus, the inventors considered that there is an element other than C which affects the hardness of the inner surface layer of the base metal portion. The inventors measured the concentration profiles of various elements other than C, by the method described above. As a result, it has been revealed that not only the C concentration but also the B concentration is decreased in the inner surface layer of the base metal portion.

FIG. 3 is a graph showing the relationship (namely, the B concentration profile in the depth direction) between the depth from the inner surface of the base metal portion (namely, the distance from the inner surface in the wall thickness direction) and the B concentration (% by mass), in the C cross section of the base metal portion (B content: 0.00273%) of the electric resistance welded steel pipe from which the profile shown in FIG. 1 was obtained. FIG. 3 was obtained by measuring the B concentration by the same method as that used for measuring the C concentration described above, using the electric resistance welded steel pipe from which the profiles shown in FIG. 1 and FIG. 2 were obtained.

In FIG. 3, a dashed line 2 indicates the B concentration (0.00246%) which corresponds to 90% with respect to the B concentration in the base metal portion, and a dashed line 20 indicates the B concentration (0.00273%) in the base metal portion.

The “B concentration in the base metal portion” refers to the B concentration in the interior of the base metal portion. The B concentration in the interior of the base metal portion refers specifically to the B concentration (% by mass) in the central portion in the wall thickness direction at the base material 90° position in the electric resistance welded steel pipe.

As shown in FIG. 3, the B concentration is decreased in the region from the inner surface to a depth position of 0.10 mm of the base metal portion of the electric resistance welded steel pipe, and the B concentration in this region is lower than 90% with respect to the B concentration in the base metal portion.

Based on the above-described results shown in FIG. 1 to FIG. 3, the inventors thought that the reason for a decrease in the hardness in the region from the inner surface to a depth position of 0.10 mm of the base metal portion of the electric resistance welded steel pipe is not only because the C concentration is decreased but also the B concentration is decreased, in the inner surface layer of the base metal portion of the electric resistance welded steel pipe.

A phenomenon in which the B concentration is decreased is hereinafter also referred to as “de-B”

As a result of the examinations described above, the inventors have found out that it is effective not only to reduce a decrease in the C concentration (de-C) but also to reduce a decrease in the B concentration (de-B), in order to reduce a decrease in the hardness of the inner surface layer of the base metal portion of the electric resistance welded steel pipe.

FIG. 4 is a conceptual diagram showing the relationship between the depth from the inner surface of the base metal portion and the Vickers hardness, in the C cross section of the electric resistance welded steel pipe.

A solid line 4 in FIG. 4 is a line that schematically represents the Vickers hardness shown in FIG. 1.

A dashed line 3 in FIG. 4 is a line that indicates the Vickers hardness when it is thought that the de-C alone is occurring.

When it is assumed that the de-C alone is occurring in the inner surface layer of the base metal portion, the Vickers hardness should decrease gently as it gets closer toward the inner surface as shown by the dashed line 3, in the region from the inner surface to a depth position of 0.20 mm of the base metal portion of the electric resistance welded steel pipe.

Actually, however, as shown by the solid line 4 in FIG. 4, the Vickers hardness is gently decreased from a depth position of 0.20 mm from the inner surface of the base metal portion to a depth position of 0.10 mm thereof, and markedly decreased as it gets closer toward the inner surface, in the region from a depth position of 0.10 mm of the base metal portion of the electric resistance welded steel pipe to the inner surface thereof.

As described above, it is thought that not only the de-C but also the de-B is occurring in the region from the inner surface to a depth position of 0.10 mm of the base metal portion of the electric resistance welded steel pipe.

Therefore, it is necessary to reduce not only a decrease in the C concentration (de-C) but also a decrease in the B concentration (de-B), in the inner surface layer of the base metal portion of the electric resistance welded steel pipe.

In the electric resistance welded steel pipe according to the present embodiment, not only the de-C but also the de-B is reduced, in the inner surface layer of the base metal portion of the electric resistance welded steel pipe. This makes it possible to reduce not only the thickness of the de-C layer, but also the thickness of the de-B layer. As a result, the thickness of the region having a low hardness can be reduced in the inner surface layer. As a result, the hardness of the inner surface layer of the base metal portion is increased, and thus the fatigue strength of the electric resistance welded steel pipe is increased. Specifically, in the electric resistance welded steel pipe according to the present embodiment, the thickness of the de-C layer in the inner surface layer of the base metal portion of the electric resistance welded steel pipe is set to less than 0.20 mm. The “de-C layer” refers to a region in which the C concentration is 90% or less with respect to the C concentration in the base metal portion, and a region extending from the inner surface. When the thickness of the de-C layer is set to less than 0.20 mm, the thickness of the region having a low hardness can be reduced. As a result, the hardness of the inner surface layer of the base metal portion is increased, and thus the fatigue strength of the electric resistance welded steel pipe is increased.

Further, in the electric resistance welded steel pipe according to the present embodiment, the thickness of the de-B layer in the inner surface layer of the base metal portion of the electric resistance welded steel pipe is set to less than 0.10 mm. The “de-B layer” refers to a region in which the B concentration is 90% or less with respect to the B concentration in the base metal portion, and a region extending from the inner surface. When the thickness of the de-B layer is set to less than 0.10 mm, the thickness of the region having a low hardness can be reduced. As a result, the hardness of the inner surface layer of the base metal portion is increased, and thus the fatigue strength of the electric resistance welded steel pipe is increased.

While the effect of improving the fatigue strength which can be obtained when the thickness of each of the de-C layer and the de-B layer on the inner surface side of the base metal portion is reduced, has been decried above, the same effect of improving the fatigue strength can be obtained when the thickness of each of the de-C layer and the de-B layer on the outer surface side of the base metal portion is reduced.

The electric resistance welded steel pipe according to the present disclosure has been completed based on the findings described above.

<Chemical Composition of Base Metal Portion>

The content of each element in the chemical composition of the base metal portion will be described below.

C: from 0.30 to 0.38%

C (carbon) is an element that dissolves in steel or precipitates as carbides, to increase the fatigue strength of the steel. In a case in which the C content is less than 0.30%, it may result in a failure to sufficiently obtain this effect. In a case in which the C content is more than 0.38%, on the other hand, a decrease in workability may occur.

Accordingly, the C content is from 0.30 to 0.38%.

The lower limit of the C content is preferably 0.31%, more preferably 0.32%, and still more preferably 0.33%.

The upper limit of the C content is preferably 0.37%.

Si: from 0.05 to 0.40%

Si is an element that increases the fatigue strength of steel by solid solution strengthening. In a case in which the Si content is less than 0.15%, it may result in a failure to sufficiently obtain this effect. In a case in which the Si content is more than 0.30%, on the other hand, Si—Mn-based inclusions may be more likely to be formed.

Accordingly, the Si content from 0.05 to 0.40%.

The lower limit of the Si content is preferably 0.10%, and more preferably 0.15%.

The upper limit of the Si content is preferably 0.35%, and more preferably 0.30%.

Mn: from 0.50 to 2.00%

Mn is an element that increases the hardenability of steel, and thus increases the fatigue strength of the steel. In a case in which the Mn content is less than 0.50%, it may result in a failure to sufficiently obtain this effect. In a case in which the Mn content is more than 2.00%, on the other hand, coarse inclusions such as MnS may be formed, possibly resulting in a decrease in the fatigue life of the steel.

Accordingly, the Mn content is from 0.50 to 2.00%.

The lower limit of the Mn content is preferably 0.60%, more preferably 0.80%, still more preferably 1.00%, yet still more preferably 1.10%, and yet still more preferably 1.20%.

The upper limit of the Mn content is preferably 1.80%, and more preferably 1.70%.

Al: from 0.010 to 0.060%

Al is an element that deoxidizes steel. Further, Al is also an element that fixes N and ensures the effective amount of dissolved B for improving the hardenability. In a case in which the Al content is less than 0.010%, it may result in a failure to sufficiently obtain this effect. In a case in which the Al content is more than 0.060%, on the other hand, inclusions may be more likely to be formed, possibly resulting in a decrease the fatigue strength of the steel.

Accordingly, the Al content is from 0.010 to 0.060%.

The lower limit of the Al content is preferably 0.015%, and more preferably 0.020%.

The upper limit of the Al content is preferably 0.050%, and more preferably 0.045%.

Ti: from 0.005 to 0.050%

Ti is an element that fixes N and ensures the effective amount of dissolved B for improving the hardenability. Further, Ti is also an element that precipitates as fine carbides, reduces the coarsening of crystal grains during heat treatment by a pinning effect, and as a result, increases the toughness of steel. In a case in which the Ti content is less than 0.005%, it may result in a failure to sufficiently obtain the effects described above. In a case in which the Ti content is more than 0.050%, on the other hand, inclusions may be coarsened, possibly resulting in a decrease in the toughness and the fatigue strength of the steel.

Accordingly, the Ti content is from 0.005 to 0.050%.

The lower limit of the Ti content is preferably 0.007%, and more preferably 0.010%.

The upper limit of the Ti content is preferably 0.025%, and more preferably 0.020%.

B: from 0.0003 to 0.0050%

B (boron) is an element that increases the hardenability of steel. Further, B is also an element that increases the fatigue strength of the steel by grain boundary strengthening. In a case in which the B content is less than 0.0003%, it may result in a failure to sufficiently obtain the effects described above. In a case in which the B content is more than 0.0050%, on the other hand, coarse B inclusions may be formed, possibly resulting in a decrease in the toughness of the steel. Further, in a case in which the B content is more than 0.0050%, the thickness of the de-B layer on each surface side may be excessively increased.

Accordingly, the B content is from 0.0003 to 0.0050%.

The lower limit of the B content is preferably 0.0005%, and more preferably 0.0008%.

The upper limit of the B content is preferably 0.0025%, and more preferably 0.0020%.

Ca: from 0.0005 to 0.0040%

Ca is an element that reduces the formation of MnS by fixing S as CaS, and as a result, provides the effect of reducing a decrease in the fatigue strength due to MnS. In a case in which the Ca content is less than 0.0005%, it may result in a failure to obtain these effects. In a case in which the Ca content is more than 0.0040%, on the other hand, coarse Ca inclusions may be formed, possibly resulting in a decrease in the toughness and the fatigue strength of the steel.

Accordingly, the Ca content is from 0.0005 to 0.0040%.

The lower limit of the Ca content is preferably 0.0005%, more preferably 0.0010%, and still more preferably 0.0012%.

The upper limit of the Ca content is preferably 0.0038%, more preferably 0.0035%, and still more preferably 0.0030%.

N: from 0 to 0.0060%

N (nitrogen) is an impurity. The N content may be 0%, or may be more than 0%.

N is an element that precipitates as BN. The precipitation of BN may cause a decrease in the effect of improving the hardenability provided by dissolved N. Further, the precipitation of BN may cause a decrease in the toughness, due to coarsening of nitrides and age hardening.

Accordingly, the N content is 0.0060% or less.

The upper limit of the N content is preferably 0.0040%, and more preferably 0.0030%. On the other hand, N increases the strength of steel by forming nitrides and carbonitrides. From the viewpoint of more effectively obtaining such an effect, the lower limit of the N content is preferably 0.0010%, and more preferably 0.0015%.

P: from 0 to 0.020%

P (phosphorus) is an impurity. The P content may be 0%, or may be more than 0%.

P is an element that reduces the weld crack resistance and the toughness of steel.

Accordingly, the P content is from 0 to 0.020%.

The upper limit of the P content is preferably 0.015%, and more preferably 0.012%.

The P content is preferably as low as possible. However, an attempt to excessively reduce the P content may lead to a higher production cost. Therefore, the P content may be more than 0%, 0.001% or more, 0.002% or more, or 0.005% or more, from the viewpoint of reducing the production cost.

S: from 0 to 0.0200%

S (sulfur) is an impurity. The S content may be 0%, or may be more than 0%.

S is an element that forms non-metallic inclusions. The non-metallic inclusions cause decreases in the bendability, the fatigue life and the workability of electric resistance welded steel pipe. Further, S is also an element that causes decreases in the toughness, anisotropy, and reheat cracking sensitivity.

Accordingly, the S content is from 0 to 0.0200%.

The upper limit of the S content is preferably 0.0100%, and more preferably 0.0050%. The S content is preferably as low as possible. However, an attempt to excessively reduce the S content may lead to a higher production cost. Therefore, the S content may be more than 0%, 0.0001% or more, 0.0002% or more, or 0.0005% or more, from the viewpoint of reducing the production cost.

O: from 0 to 0.0050%

O (oxygen) is an impurity. The O content may be 0%, or may be more than 0%.

O is an element that forms CaO and compromises the effect of Ca (namely, the effect of reducing the formation of MnS by fixing S as CaS).

Accordingly, the O content is from 0 to 0.0050%.

The upper limit of the O content is preferably 0.040%, and more preferably 0.030%.

The O content is preferably as low as possible. However, an attempt to excessively reduce the O content may lead to a higher production cost. Therefore, the O content may be more than 0%, 0.0001% or more, or 0.0005% or more, from the viewpoint of reducing the production cost.

Cu: from 0 to 0.50%

Cu is an optional element. In other words, the Cu content may be 0%, or may be more than 0%.

Too high a Cu content may cause a decrease in the workability of steel.

Accordingly, the Cu content is from 0 to 0.50%.

The upper limit of the Cu content is preferably 0.40%, and more preferably 0.30%.

On the other hand, Cu is an element that increases the hardenability of steel, and thus increases the strength of the steel. From the viewpoint of obtaining such effects, the lower limit of the Cu content is preferably 0.01%, more preferably 0.02%, still more preferably 0.05%, and yet still more preferably 0.10%.

Ni: from 0 to 0.50%

Ni is an optional element. In other words, the Ni content may be 0%, or may be more than 0%.

Too high a Ni content may lead to a higher material cost.

Accordingly, the Ni content is from 0 to 0.50%.

The upper limit of the Ni content is preferably 0.40%, and more preferably 0.30%.

On the other hand, Ni is an element that increases the hardenability of steel, and thus increases the strength of the steel. From the viewpoint of obtaining such effects, the lower limit of the Ni content is preferably 0.05%, and more preferably 0.10%.

Cr: from 0 to 0.50%

Cr is an optional element. In other words, the Cr content may be 0%, or may be more than 0%.

In a case in which the Cr content is more than 0.50%, inclusions may be formed to cause the occurrence of cracks.

Accordingly, the Cr content is from 0 to 0.50%.

The upper limit of the Cr content is preferably 0.35%, and more preferably 0.20%.

On the other hand, Cr is an element that increases the hardenability of steel, and thus increases the fatigue strength of the steel. From the viewpoint of obtaining such effects, the lower limit of the Cr content is preferably 0.05%, more preferably 0.10%, and still more preferably 0.13%.

V: from 0 to 0.20%

V (vanadium) is an optional element. In other words, the V content may be 0%, or may be more than 0%.

Too high a V content may cause a decrease in the toughness of steel.

Accordingly, the V content is from 0 to 0.20%.

The upper limit of the V content is preferably 0.15%, and more preferably 0.10%.

On the other hand, V is an element that increases the strength of steel. From the viewpoint of obtaining such an effect, the lower limit of the V content is preferably 0.01%.

Nb: from 0 to 0.10%

Nb is an optional element. In other words, the Nb content may be 0%, or may be more than 0%.

Too high a Nb content may cause a decrease in the toughness of steel.

Accordingly, the Nb content is from 0 to 0.10%.

The upper limit of the Nb content is preferably 0.08%, and more preferably 0.05%. On the other hand, Nb is an element that increases the strength of steel, and inhibits grain growth to increase low temperature toughness. From the viewpoint of obtaining such effects, the lower limit of the Nb content is preferably 0.001%, and more preferably 0.003%.

Mo: from 0 to 0.50%

Mo is an optional element. In other words, the Mo content may be 0%, or may be more than 0%.

Too high a Mo content may cause the formation of coarse carbides, possibly resulting in a decrease in the toughness of steel.

Accordingly, the Mo content is from 0 to 0.50%.

The upper limit of the Mo content is preferably 0.40%, and more preferably 0.30%.

On the other hand, Mo is an element that increases the hardenability of steel, and thus increases the strength of the steel. Further, Mo is also an element that increases the strength of the steel by solid solution strengthening. From the viewpoint of obtaining these effects, the lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, and still more preferably 0.03%.

Mg: 0 to 0.0500%

Mg is an optional element. In other words, the Mg content may be 0%, or may be more than 0%.

Too high a Mg content may cause the coarsening of oxides in steel, possibly resulting in a decrease in the toughness of the steel.

Accordingly, the Mg content is from 0 to 0.0500%.

The upper limit of the Mg content is preferably 0.0400%, more preferably 0.0300%, and still more preferably 0.0200%.

On the other hand, Mg is an element that detoxify S in steel by converting S into sulfides, and increases the toughness of the steel. From the viewpoint of obtaining such effects, the lower limit of Mg content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and yet still more preferably 0.0005%.

REM: from 0 to 0.0500%

REM is an optional element. In other words, the REM content may be 0%, or may be more than 0%.

The term “REM” as used herein refers to a rare earth element(s), namely, at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The “REM content” refers to the total content of the rare earth element(s).

Too high a REM content may cause the coarsening of oxides in steel, possibly resulting in a decrease in the toughness of the steel.

Accordingly, the REM content is from 0 to 0.0500%.

The upper limit of the REM content is preferably 0.0400%, more preferably 0.0300%, and still more preferably 0.0200%.

On the other hand, REM is an element that controls the form of sulfides in steel, and thus increases the toughness of the steel. From the viewpoint of obtaining such effects, the lower limit of the REM content is preferably 0.0001%, more preferably 0.0003%, and still more preferably 0.0005%.

Balance: Fe and impurities

In the chemical composition of the base metal portion in the straight pipe portion, the balance excluding the respective elements described above is Fe and impurities.

The term “impurities” as used herein refers to components which are contained in raw materials (such as ores and scraps), or components which are mixed during production steps and are not intentionally incorporated into the steel.

Examples of the impurities include all elements other than the elements described above. Only one kind, or two or more kinds of elements may be contained as the impurities.

Examples of the impurities include Sb, Sn, W, Co, As, Pb, Bi, and H.

Among the elements described above, Sb, Sn, Co and As can be contained, for example, in a content of 0.1% or less, Pb and Bi can be contained, for example, in a content of 0.005% or less, and H can be contained, for example, in a content of 0.0004% or less, as the impurities.

The contents of other elements need not be particularly controlled, as long as the contents are within usual ranges.

From the viewpoint of obtaining the effect(s) provided by at least one of the following elements, the chemical composition of the base metal portion may contain one or more selected from the group consisting of:

• • from 0.01 to 0.50% of Cu,
  • from 0.05 to 0.50% of Ni,
  • from 0.05 to 0.50% of Cr, and
  • from 0.01 to 0.50% of Mo.

A more preferred range of the content of each of these elements is as described above.

• • (F1)

In the chemical composition of the base metal portion, F1 represented by the following Formula (1) is preferably 0.50 or more. This further improves the fatigue strength of the steel.

$\begin{array}{cc}F1=\mathrm{Ca}\times \left(1-124\times O\right)/\left(1.25\times S\right)& \mathrm{Formula}\left(1\right)\end{array}$

Each element symbol in Formula (1) represents the content of each element in terms of % by mass.

In a case in which F1 is 0.50 or more, the fatigue strength of the steel is further improved.

The reason for this is thought to be as follows: in a case in which F1 is 0.50 or more, the amount of Ca that functions effectively (namely, the amount of Ca which is not in the form of CaO) is secured to some extent, and thus the above-described effect provided by Ca (namely, the effect of reducing the formation of MnS that causes a decrease in the fatigue strength of the electric resistance welded steel pipe, by fixing S as CaS) is more effectively obtained.

The lower limit of F1 is more preferably 0.60, and still more preferably 0.70, from the viewpoint of further improving the fatigue strength of the steel.

The upper limit of F1 is not particularly limited. However, the upper limit of F1 is preferably 3.00, more preferably 2.50, and still more preferably 2.00, from the viewpoint of further reducing the production cost for reducing the O content and the S content, but not particularly limited thereto.

<Microstructure>

In the electric resistance welded steel pipe according to the present disclosure, the microstructure of the central portion in the wall thickness direction of the base metal portion is tempered martensite.

The microstructure of the central portion in the wall thickness direction of the base metal portion is determined as follows.

In the central portion in the wall thickness direction at the base material 90° position in the C cross section of the electric resistance welded steel pipe, an observation surface is etched with Nital. The etched observation surface is observed with a light microscope, to examine the microstructure.

The visual field of the observation surface is in the form of a rectangle having a size of 200 μm in a rolling direction and 500 μm in the wall thickness direction. The observation is carried out at a magnification of 500 times.

In the present disclosure, the expression “the microstructure of the central portion in the wall thickness direction of the base metal portion is tempered martensite” means that the microstructure uniformly appears to be tempered martensite, as a result of the observation under the above-described conditions.

<De-C Layer>

In the electric resistance welded steel pipe according to the present disclosure, the de-C layer (namely, a layer in which the concentration of C is 90% or less with respect to the concentration of C in the chemical composition of the base metal portion) has a thickness of less than 0.20 mm, at each of the inner surface side and the outer surface side of the base metal portion. This improves the fatigue strength of the electric resistance welded steel pipe.

In a case in which the thickness of the de-C layer on the inner surface side of the base metal portion is 0.20 mm or more, the hardness of the inner surface layer of the base metal portion is decreased. As a result, the fatigue strength of the electric resistance welded steel pipe is decreased.

Accordingly, the thickness of the de-C layer on the inner surface side of the base metal portion is less than 0.20 mm.

The lower limit of the thickness of the de-C layer on the inner surface side of the base metal portion is not particularly limited. The thickness of the de-C layer on the inner surface side of the base metal portion may be 0 mm. The thickness of the de-C layer on the inner surface side of the base metal portion is the thinner the better. The upper limit of the thickness of the de-C layer on the inner surface side of the base metal portion is preferably 0.19 mm, more preferably 0.18 mm, still more preferably 0.16 mm, yet still more preferably 0.14 mm, yet still more preferably 0.12 mm, and yet still more preferably 0.10 mm.

In a case in which the thickness of the de-C layer on the outer surface side of the base metal portion is 0.20 mm or more, the hardness of the outer surface layer of the base metal portion of the electric resistance welded steel pipe is decreased. As a result, the fatigue strength of the electric resistance welded steel pipe is decreased.

Accordingly, the thickness of the de-C layer on the outer surface side of the base metal portion is less than 0.20 mm.

The lower limit of the thickness of the de-C layer on the outer surface side of the base metal portion is not particularly limited. The thickness of the de-C layer on the outer surface side of the base metal portion may be 0 mm. The thickness of the de-C layer on the outer surface side of the base metal portion is the thinner the better.

The upper limit of the thickness of the de-C layer on the outer surface side of the base metal portion which is preferred, is the same as the preferred upper limit of the thickness of the de-C layer on the inner surface side of the base metal portion.

The “outer surface layer of the base metal portion” refers to a region from the outer surface to a predetermined depth of the base metal portion. The “predetermined depth” refers, for example, to the region from the outer surface to a depth position of 0.50 mm.

The “C concentration in the base metal portion” as used herein refers to the C concentration in the interior of the base metal portion. The “C concentration in the interior of the base metal portion” refers specifically to the C concentration (% by mass) in the central portion in the wall thickness direction at the base material 90° position.

The C concentration in the base metal portion is measured in accordance with JIS G 1253 (2013), by a known check analysis. Specifically, a sample is collected from the central portion in the wall thickness direction at the base material 90° position. The collected sample is processed such that the surface to be analyzed of the sample has a diameter of 20 mm or more and a thickness of 3 mm or more. The surface to be analyzed of the processed sample is adjusted to be planar by grinding. The adjusted sample is set to a spark discharge emission spectrometer to measure the C concentration (% by mass). The thus obtained C concentration is defined as the C concentration in the base metal portion (% by mass).

The thickness of the de-C layer on the inner surface side of the base metal portion is measured as follows.

Using glow discharge optical emission spectrometry (GD-OES), the C concentration profile in the depth direction (namely, the wall thickness direction) showing the relationship between the depth from the inner surface of the base metal portion (namely, the distance from the inner surface in the wall thickness direction) and the C concentration (% by mass), is measured.

Specifically, the C concentration is measured from the inner surface of the base metal portion toward the depth direction (namely, the wall thickness direction) at intervals of 0.03 μm in the depth direction, while performing sputtering with argon ions, to obtain the C concentration profile described above. GD-OES is carried out at a measurement diameter of 4 mm.

The resulting C concentration profile is subjected to smoothing, taking into consideration measurement errors. Specifically, for each measured depth position, the arithmetic mean value of the C concentration values measured within the range of +1.50 μm of the measured depth position is calculated. The thus obtained arithmetic mean value of the C concentration is defined as the C concentration of the corresponding measured depth. The above-described C concentration is determined for each measured depth position. For example, the C concentration at a measured position at a depth position of 1.50 μm from the surface is defined as the arithmetic mean value of the C concentration values at 101 measured depth positions (at 0.03 μm intervals) located at depth positions from 0 to 3.00 μm from the surface.

The C concentration at each measured depth position is determined by the smoothing described above, to obtain the C concentration profile. When the smoothing is performed, the C concentration profile is shown as a curve starting from a depth position of 1.50 μm from the surface.

In the resulting C concentration profile, a depth region extending from the inner surface in which the C concentration is 90% or less of the C concentration in the base metal portion is identified, and the identified depth region is defined as the thickness of the de-C layer (mm).

The thickness of the de-C layer on the outer surface side of the base metal portion is measured in the same manner as in the measurement of the “thickness of the de-C layer on the inner surface side of the base metal portion” described above, except for replacing the “inner surface” in the above description with the “outer surface”.

<De-B Layer>

In the electric resistance welded steel pipe according to the present disclosure, the de-B layer (namely, a layer in which the concentration of B is 90% or less with respect to the concentration of B in the chemical composition of the base metal portion) has a thickness of less than 0.10 mm, at each of the inner surface side and the outer surface side of the base metal portion. This improves the fatigue strength of the electric resistance welded steel pipe.

In a case in which the thickness of the de-B layer on the inner surface side of the base metal portion is 0.10 mm or more, the hardness of the inner surface layer of the base metal portion is decreased. As a result, the fatigue strength of the electric resistance welded steel pipe is decreased.

Accordingly, the thickness of the de-B layer on the inner surface side of the base metal portion is less than 0.10 mm.

The lower limit of the thickness of the de-B layer on the inner surface side of the base metal portion is not particularly limited.

The thickness of the de-B layer on the inner surface side of the base metal portion may be 0 mm. The thickness of the de-B layer on the inner surface side of the base metal portion is the thinner the better.

The upper limit of the thickness of the de-B layer on the inner surface side of the base metal portion is preferably 0.09 mm, and more preferably 0.08 mm.

In a case in which the thickness of the de-B layer on the outer surface side of the base metal portion is 0.10 mm or more, the hardness of the outer surface layer of the base metal portion of the electric resistance welded steel pipe is decreased. As a result, the fatigue strength of the electric resistance welded steel pipe is decreased.

Accordingly, the thickness of the de-B layer on the outer surface side of the base metal portion is less than 0.10 mm.

The lower limit of the thickness of the de-B layer on the outer surface side of the base metal portion is not particularly limited. The thickness of the de-B layer on the outer surface side of the base metal portion may be 0 mm. The thickness of the de-B layer on the outer surface side of the base metal portion is the thinner the better.

The upper limit of the thickness of the de-B layer on the outer surface side of the base metal portion which is preferred, is the same as the preferred upper limit of the thickness of the de-B layer on the inner surface side of the base metal portion.

The “B concentration in the base metal portion” as used herein refers to the B concentration in the interior of the base metal portion. The “B concentration in the interior of the base metal portion” refers specifically to the B concentration (% by mass) in the central portion in the wall thickness direction at the base material 90° position.

The B concentration in the base metal portion is measured in the same manner as in the measurement of the C concentration in the base metal portion as described above.

The thickness of the de-B layer on the inner surface side of the base metal portion is measured in the same manner as in the measurement of the “thickness of the de-C layer on the inner surface side of the base metal portion” described above”, except for replacing “C” (carbon) in the above description with “B” (boron).

The thickness of the de-B layer on the outer surface side of the base metal portion is measured in the same manner as in the measurement of the “thickness of the de-C layer on the inner surface side of the base metal portion” described above”, except for replacing “C” (carbon) in the above description with “B” (boron) and replacing the “inner surface” therein with the “outer surface”.

<Vickers Hardness>

In the electric resistance welded steel pipe according to the present disclosure, each of the Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion (hereinafter, also referred to as the “Vickers hardness on the inner surface side”) and the Vickers hardness at a position at a depth of 0.5 mm from the outer surface of the base metal portion (hereinafter, also referred to as the “Vickers hardness on the outer surface side”) is 420 Hv or more and less than 510 Hv.

A Vickers hardness within the range of 420 Hv or more and less than 510 Hv is a hardness corresponding to a tensile strength within the range of 1,370 MPa or more and less than 1,750 MPa.

The fact that each of the Vickers hardness on the inner surface side and the Vickers hardness on the outer surface side is 420 Hv or more contributes to ensuring the strength as an electric resistance welded steel pipe for a mechanical structural part. The Vickers hardness described above is preferably 450 Hv or more.

The fact that each of the Vickers hardness on the inner surface side and the Vickers hardness on the outer surface side is less than 510 Hv contributes to the ease of producing an electric resistance welded steel pipe for a mechanical structural part (for example, the ease of producing a hot-rolled steel sheet as the material thereof, the ease of roll forming the hot-rolled steel sheet when roll forming the sheet to perform pipe-making, and the like). The Vickers hardness as described above is preferably 509 MPa or less.

In the present disclosure, the Vickers hardness (Hv) refers to a Vickers hardness as measured in accordance with JIS Z 2244 (2009) with a test force of 0.98 N.

The Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion is determined as follows.

In the C cross section of the electric resistance welded steel pipe, the positions (three locations in total) which are 90 degrees, 180 degrees and 270 degrees away (namely, “base material 90° position”, “base material 180° position” and “base material 270° position”, respectively) from the electric resistance welded portion in the circumferential direction in clockwise rotation, when the electric resistance welded portion is defined as 0 degrees, and which are each at a depth of 0.5 mm from the inner surface, are determined as measurement positions.

A Vickers hardness test in accordance with JIS Z 2244 (2009) is carried out at each of the above-described three locations, to obtain the Vickers hardness (Hv) at each position. The test force is set to 0.98 N.

The arithmetic mean value of the thus obtained three Vickers hardness values (measured values) is defined as the Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion (Hv).

The Vickers hardness at a position at a depth of 0.5 mm from the outer surface of the base metal portion was measured in the same manner as in the measurement of the “Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion” described above, except for replacing the “inner surface” in the above description with the “outer surface”.

<Size of Electric Resistance Welded Steel Pipe>

The size of the electric resistance welded steel pipe according to the present disclosure is not particularly limited.

The straight pipe portion of the electric resistance welded steel pipe according to the present disclosure has, for example, an outer diameter of from 10 to 50 mm.

In the electric resistance welded steel pipe according to the present disclosure, the value (t/D value) obtained by dividing the wall thickness (t) of the base metal portion in the straight pipe portion by the outer diameter (D) of the straight pipe portion is, for example, from 0.04 to 0.25.

In the electric resistance welded steel pipe according to the present disclosure, the base metal portion in the straight pipe portion has, for example, a wall thickness of from 2.0 to 8.0 mm.

[One Example of Method of Producing Electric Resistance Welded Steel Pipe for Mechanical Structural Part (Production Method X)]

One example of the method of producing the electric resistance welded steel pipe according to the present disclosure (hereinafter, referred to as “production method X”) will be described below.

The production method X to be described below is a method of producing an electric resistance welded steel pipe of each Example to be described later.

The production method X is a method of producing an electric resistance welded steel pipe including:

• • a preparation step of preparing an as-rolled electric resistance welded steel pipe, wherein:
    • the as-rolled electric resistance welded steel pipe includes a base metal portion A and an electric resistance welded portion A; and
    • the base metal portion A has a chemical composition consisting of, in terms of % by mass:
  • from 0.30 to 0.38% of C,
  • from 0.05 to 0.40% of Si,
  • from 0.50 to 2.00% of Mn,
  • from 0.010 to 0.060% of Al,
  • from 0.005 to 0.050% of Ti,
  • from 0.0003 to 0.0050% of B,
  • from 0.0005 to 0.0040% of Ca,
  • from 0 to 0.0060% of N,
  • from 0 to 0.020% of P,
  • from 0 to 0.0200% of S,
  • from 0 to 0.0050% of O,
  • from 0 to 0.50% of Cu,
  • from 0 to 0.50% of Ni,
  • from 0 to 0.50% of Cr,
  • from 0 to 0.20% of V,
  • from 0 to 0.10% of Nb,
  • from 0 to 0.50% of Mo,
  • from 0 to 0.0500% of Mg,
  • from 0 to 0.0500% of REM, and
  • the balance consisting of Fe and impurities;
    • a quenching step of subjecting the as-rolled electric resistance welded steel pipe to quenching; and
    • a tempering step of subjecting the as-rolled electric resistance welded steel pipe that has been subjected to quenching to tempering, to obtain the electric resistance welded steel pipe for a mechanical structural part;
    • wherein, in the quenching step, the oxygen content in the atmosphere in which the quenching is performed is 1,000 volume ppm or less, and the cooling rate in the quenching is 10° C./sec or more.

According to the production method X, the electric resistance welded steel pipe according to the present disclosure (namely, the electric resistance welded steel pipe for a mechanical structural part according to the present disclosure) can be produced.

The respective steps in the production method X will be described below.

<Preparation Step>

The preparation step is a step of preparing the as-rolled electric resistance welded steel pipe described above.

The present step may be a step of simply preparing the as-rolled electric resistance welded steel pipe that has been produced in advance, or may be a step of producing the as-rolled electric resistance welded steel pipe.

An example of the method of producing the as-rolled electric resistance welded steel pipe will be described later as “production method A”.

In the production method X, the as-rolled electric resistance welded steel pipe corresponds to the material of the electric resistance welded steel pipe for a mechanical structural part to be produced.

In the production method X, a portion of the as-rolled electric resistance welded steel pipe may be subjected to bending, as long as the finally obtained electric resistance welded steel pipe for a mechanical structural part includes a straight pipe portion (namely, a portion that has not been subjected to bending, and that is a straight portion in a state as it is, as having been subjected to pipe-making or to pipe drawing). In this case, the portion which has not been subjected to bending corresponds to the straight pipe portion.

Further, in the production method X, a portion or the entirety of the as-rolled electric resistance welded steel pipe may be subjected to pipe drawing to be described later.

In the production method X, the as-rolled electric resistance welded steel pipe is subjected to quenching under the above-described conditions, and to tempering, to obtain the electric resistance welded steel pipe for a mechanical structural part.

At this time, at least one portion of the base metal portion A and at least one portion of the electric resistance welded portion A of the as-rolled electric resistance welded steel pipe are converted to the base metal portion and the electric resistance welded portion of the straight pipe portion in the electric resistance welded steel pipe for a mechanical structural part, respectively.

The respective steps of the production method X do not affect the chemical composition of the resulting steel.

Therefore, the chemical composition of the base metal portion in the straight pipe portion of the electric resistance welded steel pipe to be produced by the production method X can be regarded as the same as the chemical composition of the base metal portion A of the as-rolled electric resistance welded steel pipe as the material.

(Size of as-Rolled Electric Resistance Welded Steel Pipe)

The size of the as-rolled electric resistance welded steel pipe is not particularly limited.

The outer diameter of the as-rolled electric resistance welded steel pipe is, for example, from 10 to 50 mm.

In the as-rolled electric resistance welded steel pipe, the value (t/D value) obtained by dividing the wall thickness (t) of the base metal portion by the outer diameter (D) of the as-rolled electric resistance welded steel pipe is, for example, from 0.04 to 0.25.

The wall thickness of the base metal portion of the as-rolled electric resistance welded steel pipe is, for example, from 2.0 to 8.0 mm.

<Quenching Step>

The quenching step is a step of subjecting the as-rolled electric resistance welded steel pipe to quenching.

In the quenching step, the oxygen content in the atmosphere in which the quenching is performed is 1,000 volume ppm or less. This allows for preventing B and C diffused into the inner surface layer and the outer surface layer from reacting with oxygen in the atmosphere. As a result, the de-B and the de-C can be reduced.

In the quenching step, the cooling rate in the quenching is 10° C./sec or more. This allows for reducing the residence time of the as-rolled electric resistance welded steel pipe in the temperature range in which B and C are more easily diffused. Therefore, it is possible to prevent B and C from diffusing into the inner surface layer and the outer surface layer, and from reacting with the diffused oxygen. As a result, the de-B and the de-C can be reduced.

The quenching in the quenching step is performed, for example, in a heat treatment furnace.

In the quenching step, the oxygen content in the atmosphere can be adjusted to 1,000 volume ppm or less, for example, by a method in which one or more selected from the group consisting of an inert gas, CO and CO₂ is used, as an atmosphere gas.

The lower limit of the cooling rate in the quenching is preferably 13° C./sec, and more preferably 15° C./sec.

The upper limit of the cooling rate in the quenching step is not particularly limited. The upper limit is, for example, 30° C./sec or less.

The heating temperature (hereinafter, also referred to as “quenching temperature”) in the quenching is preferably from 900 to 1,050° C.

In a case in which the quenching temperature is within the range described above, the above-described Vickers hardness (namely, a hardness of 420 Hv or more and less than 510 Hv) is more likely to be achieved in the finally obtained electric resistance welded steel pipe for a mechanical structural part.

Further, in a case in which the quenching temperature is from 900 to 1,050° C., in general, there is a tendency that the diffusion coefficient of B in an original pipe is increased to allow B to be more easily diffused into a surface layer, making the de-B more likely to occur. In the production method X, however, the de-B is reduced despite the fact that the quenching temperature is from 900 to 1,050° C., by adjusting the oxygen content in the atmosphere in the quenching step to 1,000 volume ppm or less.

The lower limit of the quenching temperature is preferably 910° C., and more preferably 920° C.

The upper limit of the quenching temperature is preferably 1000° C., and more preferably 970° C.

<Tempering Step>

The tempering step is a step of subjecting the as-rolled electric resistance welded steel pipe that has been subjected to quenching (hereinafter, also referred to as the “electric resistance welded steel pipe after the quenching and before the tempering”) to tempering, to obtain the electric resistance welded steel pipe for a mechanical structural part.

The heating temperature (hereinafter, also referred to as “tempering temperature”) in the tempering step is preferably from 100 to 500° C.

In a case in which the tempering temperature is 100° C. or higher, the fatigue strength of the electric resistance welded steel pipe is further increased.

In a case in which the tempering temperature is 500° C. or lower, the coarsening of precipitates can be reduced, and cracks due to hydrogen can be further reduced.

The retention time at the tempering temperature is, for example, from 1 to 60 minutes.

The method of performing the tempering step is not particularly limited.

The method of performing the tempering step may be, for example, a method of using an electric furnace or an atmospheric furnace in which temperature control can be carried out easily.

<Preferred Embodiment of Electric Resistance Welded Steel Pipe after Quenching and Before Tempering>

A preferred embodiment of the electric resistance welded steel pipe after the quenching and before the tempering will be described.

In a case in which the electric resistance welded steel pipe after the quenching and before the tempering has the following preferred embodiment, the electric resistance welded steel pipe for a mechanical structural part according to the present disclosure which satisfies the above-described requirements is more likely to be obtained, by the subsequent tempering.

(Microstructure)

In the electric resistance welded steel pipe after the quenching and before the tempering, the microstructure of the central portion in the wall thickness direction of the base metal portion is preferably martensite.

The expression “the microstructure of the central portion in the wall thickness direction of the base metal portion is martensite” as used herein means that the microstructure uniformly appears to be martensite, as a result of observing the microstructure in the electric resistance welded steel pipe after the quenching and before the tempering, under the same conditions as the conditions described above for observing the microstructure in the electric resistance welded steel pipe for a mechanical structural part according to the present disclosure (namely, the electric resistance welded steel pipe after the tempering).

(Martensite Fraction Based on Hardness)

In the electric resistance welded steel pipe after the quenching and before the tempering, each of a martensite fraction based on the Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion (hereinafter, also referred to as “martensite fraction on the inner surface side of the base metal portion”) and a martensite fraction based on the Vickers hardness at a position at a depth of 0.5 mm from the outer surface of the base metal portion (hereinafter, also referred to as “martensite fraction on the outer surface side of the base metal portion”) is preferably 90% or more.

Whether or not the martensite fraction on the inner surface side of the base metal portion is 90% or more was determined by whether or not the measured value of the Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion is equal to or higher than a 90% martensite hardness to be calculated by Formula (2) and Formula (3) described later.

Whether or not the martensite fraction on the outer surface side of the base metal portion is 90% or more was determined by whether or not the measured value of the Vickers hardness at a position at a depth of 0.5 mm from the outer surface of the base metal portion is equal to or higher than the 90% martensite hardness to be calculated by Formula (2) and Formula (3) described later.

At this time, the Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion and the Vickers hardness at a position at a depth of 0.5 mm from the outer surface of the base metal portion were measured in the same manner as in the measurement of the Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion and the Vickers hardness at a position at a depth of 0.5 mm from the outer surface of the base metal portion in the above-described electric resistance welded steel pipe for a mechanical structural part according to the present disclosure, respectively.

Since the Vickers hardness distribution in the region from the “position at a depth of 0.5 mm from the inner surface of the base metal portion” to the “position at a depth of 0.5 mm from the outer surface of the base metal portion” is almost flat, the average of the Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion and the Vickers hardness at a position at a depth of 0.5 mm from the outer surface of the base metal portion roughly coincides with the average Vickers hardness of the entire wall thickness.

The “90% martensite hardness” described above refers to a value calculated by Formula (2) and Formula (3).

$\begin{array}{cc}90\%\mathrm{Martensite}\mathrm{hardness}(\mathrm{Vickers}\mathrm{hardness})=107.61+6.177\times HRC\left(90\%M\right)\times \exp \left(2.089\times {10}^{-6}\times HR{C\left(90\%M\right)}^{3.008}\right)& \mathrm{Formula}\left(2\right)\end{array}$ $\begin{array}{cc}HRC\left(90\%M\right)=30+50\times C\left(\%\right)& \mathrm{Formula}\left(3\right)\end{array}$

(Thickness of Region Having Less than 90% Martensite Hardness)

In the base metal portion of the electric resistance welded steel pipe after the quenching and before the tempering, the thickness of a region having less than the 90% martensite hardness at each of the inner surface side and the outer surface side is preferably less than 0.20 mm.

The thickness of the region having less than the 90% martensite hardness on the inner surface side is measured as follows.

When the wall thickness of the base metal portion is defined as t mm, in the C cross section of the electric resistance welded steel pipe after the quenching and before the tempering, the Vickers hardness is measured at every 0.02 mm from a depth position of t/8 mm from the inner surface of the base metal portion, toward the inner surface of the base metal portion, to obtain a Vickers hardness profile in the depth direction. The conditions for measuring the Vickers hardness are the same as the conditions for measuring the Vickers hardness in the electric resistance welded steel pipe for a mechanical structural part described above.

The thickness of the region having less than the 90% martensite hardness on the inner surface side is measured, based on the thus obtained Vickers hardness profile.

The thickness of the region having less than the 90% martensite hardness on the outer surface side is measured in the same manner as in the measurement of the thickness of the region having less than the 90% martensite hardness on the outer surface side, except for replacing the “inner surface” in the above description with the “outer surface”.

<Pipe-Drawing Step>

The production method X preferably further includes a pipe-drawing step of subjecting the as-rolled electric resistance welded steel pipe to pipe drawing, after the preparation step and before the quenching step.

In this case, the as-rolled electric resistance welded steel pipe that has been subjected to pipe drawing is subjected to quenching, in the quenching step described above.

In a case in which the production method X includes the pipe-drawing step, and even when the de-B layer and the de-C layer have been formed in the hot-rolled steel sheet as the material of the as-rolled electric resistance welded steel pipe, on each surface side, the de-B layer and the de-C layer are physically drawn by pipe drawing. This allows for reducing the thicknesses of the de-B layer and the de-C layer, and thus, the above-described ranges of the de-B layer and the de-C layer (namely, a thickness of the de-C layer of less than 0.20 mm and a thickness of the de-B layer of less than 0.10 mm) are more easily achieved, in the resulting electric resistance welded steel pipe for a mechanical structural part.

The pipe drawing is carried out, for example, by cold drawing or stretch reducer rolling.

A cross-sectional area reduction rate in the pipe drawing is preferably from 10 to 40%. The “cross-sectional area reduction rate (%) as used herein refers to a value obtained by subtracting the area of the C cross section of the as-rolled electric resistance welded steel pipe after the pipe drawing from the area of the C cross section of the as-rolled electric resistance welded steel pipe before the pipe drawing, dividing the difference by the area of the C cross section of the as-rolled electric resistance welded steel pipe before the pipe drawing, and then multiplied by 100.

<Step of Subjecting to Shot Blasting>

The production method X preferably further includes a step of subjecting the as-rolled electric resistance welded steel pipe to shot blasting, after the preparation step and before the quenching step.

In a case in which the production method X includes the pipe-drawing step described above, shot blasting is preferably performed after the pipe-drawing step and before the quenching step.

In a case in which the production method X includes the step of subjecting the as-rolled electric resistance welded steel pipe to shot blasting, and even when the de-B layer and the de-C layer have been formed in the hot-rolled steel sheet as the material of the as-rolled electric resistance welded steel pipe, on each surface side, the de-B layer and the de-C layer can be physically removed by shot blasting. This allows for reducing the thicknesses of the de-B layer and the de-C layer, and thus, the above-described ranges of the de-B layer and the de-C layer (namely, a thickness of the de-C layer of less than 0.20 mm and a thickness of the de-B layer of less than 0.10 mm) are more easily achieved, in the resulting electric resistance welded steel pipe for a mechanical structural part.

[One Example of Method of Producing as-Rolled Electric Resistance Welded Steel Pipe (Production Method A)]

The preparation step of preparing the as-rolled electric resistance welded steel pipe in the above-described production method X (namely, one example of the method of producing the electric resistance welded steel pipe for a mechanical structural part according to the present disclosure) may be a step of producing the as-rolled electric resistance welded steel pipe.

One example of the method of producing the as-rolled electric resistance welded steel pipe will be described below as the production method A.

The production method A includes:

• • a slab preparation step of preparing a slab having the same chemical composition as the chemical composition of the base metal portion A of the as-rolled electric resistance welded steel pipe;
  • a hot rolling step of rolling the prepared slab to obtain a hot-rolled steel sheet;
  • a cooling step of cooling the hot-rolled steel sheet obtained in the hot rolling step until a coiling temperature CT is reached;
  • a coiling step of coiling the hot-rolled steel sheet after cooling, at the coiling temperature CT, to obtain a hot coil composed of the hot-rolled steel sheet; and
  • a pipe-making step of uncoiling the hot-rolled steel sheet from the hot coil, roll-forming the uncoiled hot-rolled steel sheet to form an open pipe, and subjecting the abutting portions of the resulting open pipe to electric resistance welding to form an electric resistance welded portion, thereby obtaining the electric resistance welded steel pipe.

The hot rolling step, the cooling step and the coiling step as described above are performed using a hot strip mill.

The respective steps in the production method A will be described below.

<Slab Preparation Step>

The slab preparation step is a step of preparing a slab.

The present step may be a step of simply preparing a slab that has been produced in advance, or may be a step of producing a slab.

The chemical composition of the slab to be prepared is the same as the chemical composition of the base metal portion A of the as-rolled electric resistance welded steel pipe produced by the production method A, and preferred ranges of the respective components are also the same.

The respective steps of the production method A do not affect the chemical composition of the resulting steel. Therefore, the chemical composition of the base metal portion A of the as-rolled electric resistance welded steel pipe to be produced by the production method A can be regarded as the same as the chemical composition of the slab as the raw material.

In the case of producing a slab in the present step, first, a molten steel having the above-described chemical composition is produced, and the resulting molten steel is used to produce a slab. The chemical composition of the molten steel can be regarded as the same as the chemical composition of the slab.

At this time, the slab may be produced by a continuous casting method, or alternatively, the slab may be produced by forming an ingot using the molten steel, and slabbing the ingot.

<Hot Rolling Step>

The hot rolling step is a step of heating the slab, for example, to a slab heating temperature of from 1,100 to 1,300° C., and hot-rolling the heated slab to obtain a hot-rolled steel sheet.

The hot rolling is preferably carried out by performing a rough rolling step using a roughing mill and a finish rolling step using a finishing mill in the order mentioned.

In the rough rolling step, the prepared slab is heated and subjected to rough rolling, to produce a rough rolled plate (rough bar).

The hot roughing mill may be a reverse mill, or a tandem mill including a plurality of rolling stands arranged in a row.

In the finish rolling step, the rough rolled plate is subjected to finish rolling using a finishing mill, to obtain a hot-rolled steel sheet.

In the finish rolling step,

• • a finish rolling using a tandem finishing mill including a plurality of rolling stands arranged in
  • a row (each rolling stand includes a pair of work rolls) may be performed, or
  • a finish rolling using a reverse rolling mill including a pair of work rolls.

In a case in which the surface temperature of the steel sheet on the exit side of the finishing mill final stand is defined as “finish rolling temperature (° C.)”, in the finish rolling step, the finish rolling temperature (° C.) is, for example, from 900 to 1,200° C.

The sheet thickness of the steel sheet after the finish rolling step is, for example, from 2.0 to 20.0 mm, but not particularly limited thereto.

<Cooling Step>

The cooling step is a step of cooling the hot-rolled steel sheet obtained in the hot rolling step to a coiling temperature CT.

The “coiling temperature CT” as used herein refers to the surface temperature of the hot-rolled steel sheet when coiling the steel sheet.

The coiling temperature T1 is preferably 800° C. or lower. In a case in which the coiling temperature T1 is 800° C. or lower, the formation of scales can be further reduced. The upper limit of the coiling temperature T1 is preferably 650° C.

The lower limit of the coiling temperature T1 is preferably 500° C., and more preferably 550° C., but not particularly limited thereto.

<Coiling Step>

The coiling step is a step of coiling the hot-rolled steel sheet after cooling, at the coiling temperature CT, to obtain a hot coil composed of the hot-rolled steel sheet.

The coiling temperature CT is preferably within the range described above.

<Pipe-making Step>

The pipe-making step is a step of uncoiling the hot-rolled steel sheet from the hot coil, roll-forming the uncoiled hot-rolled steel sheet to form an open pipe, and subjecting the abutting portions of the resulting open pipe to electric resistance welding to form an electric resistance welded portion, thereby obtaining the electric resistance welded steel pipe.

The pipe-making step can be carried out in accordance with a known method.

The production method A may include other steps if necessary.

Examples of the other steps include:

• • a step of performing a seam heat treatment on the electric resistance welded portion, after the pipe-making step;
  • a step of reducing the outer diameter of the electric resistance welded steel pipe with a sizer, after the pipe-making step (after the step of performing a seam heat treatment described above, in a case in which the step of performing a seam heat treatment is included); and
  • a step of removing scales by washing the hot-rolled steel sheet with an acid, before the pipe-making step.

The respective steps of the production method A described above do not affect the chemical composition of the resulting steel.

Therefore, the chemical composition of the base metal portion A of the as-rolled electric resistance welded steel pipe to be produced by the production method A can be regarded as the same as the chemical composition of the raw material (molten steel or slab).

EXAMPLES

The present invention will now be described in specific detail with reference to Examples. However, the invention is in no way limited to these Examples.

Underlined values in Table 1 to Table 4 indicate that the corresponding values fall outside the range of the present disclosure, or outside the range of preferred production conditions.

<Preparation Step>

As the preparation step, as-rolled electric resistance welded steel pipes were prepared (produced) in accordance with the production method A described above.

The details will be described below.

Molten steels (Steels A to AR) having the chemical compositions shown in Table 1 and Table 2 were subjected to continuous casting, to produce respective slabs (slab preparation step).

The numerical values shown in Table 1 and Table 2 represent the contents (% by mass) of the corresponding elements.

Empty cells in Table 1 and Table 2 indicate that the contents of the corresponding elements are less than a detection limit (namely, that the corresponding elements are not contained).

The balance excluding the elements shown in Table 1 and Table 2 is Fe and impurities. Underlined values in Table 1 and Table 2 indicate that the corresponding values fall outside the range of the present disclosure.

REM in Steel Y represents Y (yttrium).

REM in Steel Z represents La.

REM in Steel AA represents Ce.

REM in Steel AB represents Nd.

REM in Steel AE represents Y.

REM in Steel AF represents Y (0.0037%) and Ce (0.0052%).

TABLE 1
	Element (% by mass)
Steel	C	Si	Mn	Al	Ti	B	Ca	N	P	S	O
A	0.34	0.23	1.26	0.025	0.033	0.0025	0.0030	0.0036	0.010	0.0020	0.0015
B	0.36	0.25	1.32	0.041	0.029	0.0029	0.0028	0.0022	0.012	0.0025	0.0011
C	0.33	0.20	1.20	0.042	0.024	0.0022	0.0019	0.0029	0.011	0.0019	0.0012
D	0.35	0.23	1.29	0.037	0.028	0.0025	0.0019	0.0040	0.011	0.0017	0.0012
E	0.35	0.23	1.25	0.029	0.027	0.0024	0.0025	0.0034	0.008	0.0019	0.0013
F	0.37	0.24	1.29	0.029	0.037	0.0027	0.0019	0.0043	0.009	0.0019	0.0014
G	0.37	0.25	1.30	0.028	0.020	0.0020	0.0030	0.0045	0.012	0.0020	0.0015
H	0.35	0.20	1.29	0.035	0.025	0.0028	0.0025	0.0035	0.008	0.0018	0.0022
I	0.35	0.25	1.23	0.050	0.013	0.0016	0.0022	0.0017	0.004	0.0025	0.0015
J	0.32	0.15	0.74	0.021	0.044	0.0032	0.0028	0.0055	0.011	0.0019	0.0012
K	0.37	0.22	1.50	0.013	0.033	0.0022	0.0018	0.0025	0.007	0.0023	0.0016
L	0.34	0.22	1.62	0.021	0.015	0.0026	0.0027	0.0018	0.006	0.0018	0.0014
M	0.36	0.26	1.94	0.055	0.047	0.0036	0.0031	0.0054	0.007	0.0024	0.0016
N	0.33	0.25	0.98	0.014	0.018	0.0034	0.0018	0.0060	0.006	0.0021	0.0011
O	0.32	0.26	0.90	0.024	0.022	0.0012	0.0021	0.0020	0.005	0.0020	0.0016
P	0.33	0.24	1.28	0.020	0.037	0.0008	0.0031	0.0025	0.002	0.0019	0.0016
Q	0.33	0.18	1.64	0.055	0.040	0.0048	0.0027	0.0049	0.012	0.0020	0.0012
R	0.37	0.16	1.70	0.018	0.047	0.0017	0.0027	0.0045	0.011	0.0018	0.0013
S	0.35	0.19	1.45	0.033	0.046	0.0011	0.0031	0.0059	0.017	0.0020	0.0015
T	0.39	0.24	0.95	0.049	0.040	0.0012	0.0032	0.0037	0.012	0.0019	0.0012
U	0.34	0.26	1.86	0.037	0.024	0.0010	0.0017	0.0055	0.001	0.0018	0.0010
V	0.36	0.21	1.80	0.054	0.016	0.0006	0.0022	0.0031	0.013	0.0018	0.0014
W	0.38	0.19	1.01	0.043	0.027	0.0012	0.0028	0.0024	0.012	0.0020	0.0017
X	0.36	0.22	1.11	0.032	0.035	0.0050	0.0027	0.0016	0.004	0.0025	0.0015
Y	0.38	0.18	0.99	0.045	0.016	0.0005	0.0017	0.0056	0.016	0.0017	0.0015
Z	0.38	0.25	0.53	0.056	0.033	0.0038	0.0029	0.0059	0.002	0.0022	0.0016
AA	0.39	0.19	1.57	0.030	0.034	0.0041	0.0020	0.0041	0.019	0.0025	0.0010
AB	0.35	0.26	0.71	0.038	0.025	0.0027	0.0029	0.0033	0	0.0025	0.0017
AC	0.33	0.28	1.74	0.023	0.049	0.0037	0.0016	0.0023	0.014	0.0019	0.0011
AD	0.35	0.28	1.68	0.017	0.048	0.0006	0.0025	0.0019	0.017	0.0019	0.0011
AE	0.35	0.30	1.69	0.051	0.022	0.0019	0.0026	0.0058	0.009	0.0022	0.0010
AF	0.37	0.19	1.93	0.056	0.044	0.0045	0.0030	0.0032	0.012	0.0023	0.0017
AG	0.28	0.22	1.52	0.045	0.027	0.0041	0.0024	0.0053	0.006	0.0018	0.0013
AH	0.40	0.22	0.95	0.043	0.048	0.0042	0.0025	0.0049	0.017	0.0021	0.0018
AI	0.35	0.02	1.77	0.056	0.037	0.0008	0.002	0.004	0.004	0.0019	0.0016
AJ	0.32	0.45	0.66	0.019	0.044	0.0019	0.0019	0.0039	0.011	0.0022	0.0011
AK	0.32	0.29	0.42	0.023	0.032	0.0018	0.0031	0.0047	0.020	0.0024	0.0016
AL	0.33	0.25	2.27	0.054	0.022	0.0046	0.0028	0.0054	0.002	0.0020	0.0014
AM	0.35	0.27	1.32	0.011	0.003	0.0035	0.0018	0.0026	0.016	0.0019	0.0011
AN	0.32	0.19	1.44	0.026	0.052	0.0012	0.0024	0.0029	0.011	0.0024	0.0013
AO	0.34	0.20	1.36	0.055	0.048	0.0001	0.0024	0.0032	0.011	0.0023	0.0010
AP	0.37	0.28	1.37	0.045	0.018	0.0065	0.0023	0.0025	0.005	0.0021	0.0014
AQ	0.33	0.22	1.98	0.03	0.024	0.0039	0.0002	0.0055	0.004	0.0023	0.0017
AR	0.34	0.17	1.25	0.019	0.036	0.003	0.0045	0.0036	0.001	0.0022	0.0017

TABLE 2
	Element (continuation of Table 1: % by mass)
Steel	Cu	Ni	Cr	V	Nb	Mo	Mg	REM	F1
A									0.98
B									0.77
C									0.68
D									0.76
E									0.88
F									0.66
G									0.98
H									0.81
I									0.57
J									1.00
K									0.50
L	0.10								0.99
M		0.49							0.83
N			0.13						0.59
O	0.28	0.08							0.67
P		0.31	0.10						1.05
Q				0.04					0.92
R					0.04				1.01
S						0.27			1.01
T				0.03	0.04				1.15
U					0.01	0.27			0.66
V				0.02		0.23			0.81
W				0.04	0.09	0.07			0.88
X							0.0267		0.70
Y								0.0063	0.65
Z								0.0055	0.85
AA								0.0071	0.56
AB								0.0066	0.73
AC			0.12	0.03					0.58
AD					0.02		0.0209		0.91
AE	0.49						0.0312	0.0099	0.83
AF		0.16	0.24	0.08	0.09			0.0089	0.82
AG									0.89
AH									0.74
AI									0.68
AJ									0.60
AK									0.83
AL									0.93
AM									0.65
AN									0.67
AO									0.73
AP									0.72
AQ									0.05
AR									1.29

Each of the slabs obtained as described above was heated to a slab heating temperature of 1,250° ° C., and the heated slab was hot-rolled (specifically, each slab was subjected to rough rolling and finish rolling in the order mentioned) to obtain a hot-rolled steel sheet (hot rolling step). At this time, the finish rolling temperature was set within the range of from 900° C. to 1,000° C.

Each hot-rolled steel sheet obtained in the hot rolling step was cooled until the corresponding coiling temperature CT shown in the following Table 3 and Table 4 was reached (cooling step).

The hot-rolled steel sheet after cooling was coiled at the coiling temperature CT, to obtain a hot coil composed of a hot-rolled steel sheet having a sheet thickness of 4.9 mm (coiling step).

The hot rolling step, the cooling step and the coiling step as described above were performed using a hot strip mill.

Thereafter, the hot-rolled steel sheet was uncoiled from each hot coil obtained above, the uncoiled hot-rolled steel sheet was roll-formed to form an open pipe, and the abutting portions of the resulting open pipe was subjected to electric resistance welding to form an electric resistance welded portion. Subsequently, bead removal and a seam heat treatment were performed on the electric resistance welded portion, in the order mentioned, to obtain an as-rolled electric resistance welded steel pipe having an outer diameter of 24.0 mm and a wall thickness of 4.9 mm (pipe-making step).

At this time, in each Test No. in which the column of “Acid washing” in Table 3 and Table 4 is indicated as “Yes”, an acid-washing treatment as a scale removal treatment was performed on the hot-rolled steel sheet after being uncoiled and before being roll-formed.

The respective as-rolled electric resistance welded steel pipes were prepared by the preparation step described above.

<Optional Step>

In the Test No. in which the column of “Optional step” in Table 3 and Table 4 is indicated as “Drawn once”, the as-rolled electric resistance welded steel pipe before the quenching step was subjected once to pipe drawing at a cross-sectional area reduction rate of 16%. The as-rolled electric resistance welded steel pipe after being subjected once to pipe drawing had a size of an outer diameter of 22.0 mm and a wall thickness of 4.5 mm.

In the Test No. in which the column of “Optional step” in Table 3 and Table 4 is indicated as “Drawn twice”, the as-rolled electric resistance welded steel pipe before the quenching step was subjected twice to pipe drawing so as to achieve a cross-sectional area reduction rate of 16% in total. The as-rolled electric resistance welded steel pipe after being subjected twice to pipe drawing had a size of an outer diameter of 22.0 mm and a wall thickness of 4.5 mm.

In the Test No. in which the column of “Optional step” in Table 3 and Table 4 is indicated as “Shot blasting”, the as-rolled electric resistance welded steel pipe before the quenching step was subjected shot blasting.

<Quenching Step and Tempering Step>

Each resulting as-rolled electric resistance welded steel pipe was subjected to the quenching step and the tempering step under the corresponding conditions shown in the following Table 3 and Table 4, in the order mentioned, to produce the electric resistance welded steel pipe for a mechanical structural part of each Test No.

The quenching was carried out in a heat treatment furnace.

In each Test No. in which the column of “Atmosphere” in Table 3 and Table 4 is indicated as “CO₂”, the quenching was carried out under the conditions in which CO₂ was used as the atmosphere in the heat treatment furnace, and the oxygen content in the atmosphere was set to 1,000 volume ppm or less.

In the Test No. in which the column of “Atmosphere” in Table 3 and Table 4 is indicated as “Air”, the quenching was carried out under the conditions in which air was used as the atmosphere in the heat treatment furnace.

The quenching temperature and the cooling rate were set as shown in Table 3 and Table 4.

The tempering temperature was set as shown in Table 3 and Table 4.

The retention time at the tempering temperature was set within the range of from 1 to 60 minutes.

The entire pipe in a length direction consists of a straight pipe portion (namely, a straight portion), in the electric resistance welded steel pipe for a mechanical structural part of each of all Test Nos.

However, the electric resistance welded steel pipe for a mechanical structural part according to the present disclosure is not limited to an embodiment in which the entire pipe in the length direction consists of the straight pipe portion, and an embodiment in which the pipe includes the straight pipe portion and also includes a portion (such as a bent portion) other than the straight pipe portion may also be used.

<Measurement of Electric Resistance Welded Steel Pipe after Quenching and Before Quenching>

The martensite fraction (%) based on hardness and the thickness (mm) of the region having less than the 90% martensite hardness were measured for each of the inner surface side and the outer surface side of each electric resistance welded steel pipe after the quenching and before the quenching, by the methods described above.

The results are shown in Table 3 and Table 4.

As shown in Table 3 and Table 4, it has been confirmed that the thickness of the region having less than the 90% martensite hardness at each of the inner surface side and the outer surface side was less than 0.20 mm, in all of the Examples.

<Measurement of Electric Resistance Welded Steel Pipe for Mechanical Structural Part>

The thickness of the de-B layer at each of the inner surface side and the outer surface side, the thickness of the de-C layer at each of the inner surface side and the outer surface side, and the hardness of the base metal portion at each of the inner surface side and the outer surface side (specifically, each of the Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion and the Vickers hardness at a position at a depth of 0.5 mm from the outer surface of the base metal portion) were measured, for each electric resistance welded steel pipe for a mechanical structural part, by the methods described above.

The results are shown in Table 3 and Table 4.

As shown in Table 3 and Table 4, it has been confirmed that the thickness of the de-B layer was less than 0.10 mm, the thickness of the de-C layer was less than 0.20 mm and the Vickers hardness was 420 Hv or more and less than 510 Hv, at each of the inner surface side and the outer surface side, in all of the Examples.

Further, when the microstructure of the central portion in the wall thickness direction of the base metal portion of each electric resistance welded steel pipe for a mechanical structural part was examined by the method described above, the microstructure of the central portion in the wall thickness direction of the base metal portion was tempered martensite, in all of the Examples.

<Fatigue Strength Test (Number of Fractures) in Electric Resistance Welded Steel Pipe for Mechanical Structural Part>

From the base material 90° position of each electric resistance welded steel pipe for a mechanical structural part (hereinafter, also simply referred to as “electric resistance welded steel pipe”) that had been produced, a fatigue test specimen including the inner surface of the electric resistance welded steel pipe and having a sheet thickness of 2 mm was collected.

The fatigue test specimen was collected such that the longitudinal direction thereof is parallel to the pipe axis direction of the electric resistance welded steel pipe and the length thereof is 60 mm.

Each collected fatigue test specimen was used to carry out a fatigue strength test.

The fatigue strength test was carried out in accordance with JIS Z 2273 (1978).

The fatigue strength test was carried out under the conditions of a load stress of 350 MPa, and double swing at a stress ratio R (minimum stress/maximum stress) of −1.

The above-described fatigue strength test was carried out to obtain the number of fractures.

In a case in which the thus obtained number of fractures was 80,000 times or more, the corresponding electric resistance welded steel pipe was evaluated as having a high fatigue strength.

The results are shown in Table 3 and Table 4.

TABLE 3
											ERW steel pipe
											after quenching and
										90%	before tempering
										marten-	(inner surface side)
									Tem-	site		Thickness
			Preparation		Quenching	pering	hard-	Marten-	of region
			step		step	step	ness	site	having less
			Coiling				Quen-		Tem-	(Calcu-	fraction	than 90%
			temp.				ching	Cooling	pering	lated	based on	martensite
Test			T1	Acid	Optional	Atmos-	temp.	rate	temp.	value)	hardness	hardness
No.	Steel	F1	(° C.)	washing	step	phere	(° C.)	(° C./s)	(° C.)	(Hv)	(%)	(mm)
1	A	0.98	600	No	—	CO2	950	10 ≤	300	471	94	0.10
2	B	0.77	650	No	Drawn	CO2	960	10 ≤	350	484	92	0.12
					once
3	C	0.68	570	No	—	CO2	940	10 ≤	250	464	93	0.15
4	D	0.76	680	Yes	—	CO2	930	10 ≤	250	477	94	0.10
5	E	0.88	550	Yes	Drawn	CO2	940	10 ≤	270	477	93	0.08
					Twice
6	F	0.66	600	No	Shot	CO2	900	10 ≤	300	491	95	0.05
					blasting
7	B	0.77	630	No	—	CO2	980	10 ≤	280	484	94	0.09
8	C	0.68	670	No	—	CO2	970	10 ≤	240	464	96	0.12
9	F	0.66	610	Yes	—	CO2	970	10 ≤	300	491	94	0.15
10	C	0.68	630	No	—	Air	950	10 ≤	260	464	93	0.25
11	D	0.76	590	No	—	CO2	850	10 ≤	340	477	79	>0.50
12	G	0.98	600	No	—	CO2	930	10 ≤	350	491	91	0.15
13	A	0.98	600	No	—	CO2	950	3	280	471	75	>0.50
14	H	0.81	610	No	—	CO2	940	10 ≤	300	477	94	0.11
15	I	0.57	610	No	—	CO2	950	10 ≤	300	477	95	0.16
16	J	1.00	610	No	—	CO2	950	10 ≤	300	458	93	0.07
17	K	0.50	610	No	—	CO2	950	10 ≤	300	484	90	0.08
18	L	0.99	610	No	—	CO2	950	10 ≤	300	464	92	0.15
19	M	0.83	610	No	—	CO2	950	10 ≤	300	477	95	0.08
20	N	0.59	610	No	—	CO2	950	10 ≤	300	464	96	0.14
21	O	0.67	610	No	—	CO2	950	10 ≤	300	458	91	0.12
		ERW steel pipe
	ERW steel pipe for	after quenching and
	mechanical structural	before tempering	ERW steel pipe for
	part (inner surface side)	(outer surface side)	mechanical structural part
			Hard-		Thickness	(outer surface side)
	Thick-	Thick-	ness		of region			Hardness	Number
	ness	ness	of	Martensite	having less			of	of times
	of	of	base	fraction	than 90%	Thickness	Thickness	base	of
	de-B	de-C	metal	based on	martensite	of de-B	of de-C	metal	break-
	layer	layer	portion	hardness	hardness	layer	layer	portion	age
	(mm)	(mm)	(Hv)	(%)	(mm)	(mm)	(mm)	(Hv)	(× 104)	Note
1	0.05	0.05	492	92	0.14	0.05	0.05	493	12.0	Examples
2	0.06	0.06	495	90	0.13	0.06	0.06	507	45.2
3	0.08	0.08	479	91	0.18	0.08	0.08	491	13.0
4	0.05	0.05	500	94	0.05	0.05	0.05	500	34.0
5	0.04	0.04	496	93	0.15	0.04	0.04	501	62.3
6	0.03	0.03	505	95	0.06	0.03	0.03	509	30.5
7	0.05	0.05	506	91	0.09	0.05	0.05	507	11.0
8	0.06	0.06	495	96	0.13	0.06	006	491	16.0
9	0.08	0.08	509	95	0.09	0.08	0.08	509	21.5
10	0.13	0.25	479	92	0.15	0.13	0.25	491	7.2	Comparative
11	0.09	0.15	373	95	0.16	0.09	0.15	385	7.6	Examples
12	0.08	0.08	496	93	0.12	0.08	0.08	509	12.0	Example
13	0.08	0.09	393	90	0.14	0.08	0.09	415	7.9	Comparative
										Example
14	0.06	0.09	498	94	0.13	0.06	0.09	499	12.0	Examples
15	0.05	0.09	492	93	0.10	0.06	0.04	489	17.0
16	0.08	0.09	476	91	0.16	0.06	0.11	485	38.9
17	0.05	0.09	498	91	0.11	0.04	0.05	500	18.2
18	0.03	0.09	481	96	0.06	0.07	0.06	490	75.4
19	0.03	0.09	492	94	0.06	0.04	0.02	493	30.2
20	0.09	0.09	481	92	0.18	0.04	0.11	481	13.2
21	0.03	0.09	476	91	0.07	0.05	0.04	481	13.9

TABLE 4
								ERW steel pipe
								after quenching and
							90%	before tempering
							marten-	(inner surface side)
						Tem-	site		Thickness
			Preparation		Quenching	pering	hard-	Marten-	of region
			step		step	step	ness	site	having less
			Coiling				Quen-		Tem-	(Calcu-	fraction	than 90%
			temp.				ching	Cooling	pering	lated	based on	martensite
Test			T1	Acid	Optional	Atmos-	temp.	rate	temp.	value)	hardness	hardness
No.	Steel	F1	(° C.)	washing	step	phere	(° C.)	(° C./s)	(° C.)	(Hv)	(%)	(mm)
22	P	1.05	610	No	—	CO2	950	10 ≤	300	464	91	0.08
23	Q	0.92	610	No	—	CO2	950	10 ≤	300	464	89	0.11
24	R	1.01	610	No	—	CO2	950	10 ≤	300	491	94	0.07
25	S	1.01	610	No	—	CO2	950	10 ≤	300	471	96	0.08
26	T	1.15	610	No	—	CO2	950	10 ≤	300	484	95	0.11
27	U	0.66	610	No	—	CO2	950	10 ≤	300	464	93	0.11
28	V	0.81	610	No	—	CO2	950	10 ≤	300	471	94	0.10
29	W	0.88	610	No	—	CO2	950	10 ≤	300	484	94	0.06
30	X	0.70	610	No	—	CO2	950	10 ≤	300	477	96	0.09
31	Y	0.65	610	No	—	CO2	950	10 ≤	300	484	96	0.14
32	Z	0.85	610	No	—	CO2	950	10 ≤	300	484	91	0.16
33	AA	0.56	610	No	—	CO2	950	10 ≤	300	484	94	0.18
34	AE	0.73	610	No	—	CO2	950	10 ≤	300	471	89	0.08
35	AC	0.58	610	No	—	CO2	950	10 ≤	300	464	96	0.15
36	AD	0.91	610	No	—	CO2	950	10 ≤	300	471	95	0.07
37	AE	0.83	610	No	—	CO2	950	10 ≤	300	471	91	0.14
38	AF	0.82	610	No	—	CO2	950	10 ≤	300	477	94	0.12
39	AG	0.89	610	No	—	CO2	950	10 ≤	300	434	90	0.07
40	AH	0.74	610	No	—	CO2	950	10 ≤	300	512	91	0.07
41	AI	0.68	610	No	—	CO2	950	10 ≤	300	477	90	0.05
42	AJ	0.60	610	No	—	CO2	950	10 ≤	300	458	90	0.14
43	AK	0.83	610	No	—	CO2	950	10 ≤	300	458	91	0.72
44	AL	0.93	610	No	—	CO2	950	10 ≤	300	464	95	0.10
45	AM	0.65	610	No	—	CO2	950	10 ≤	300	477	95	0.11
46	AN	0.67	610	No	—	CO2	950	10 ≤	300	458	96	0.17
47	AC	0.73	610	No	—	CO2	950	10 ≤	300	471	94	0.48
48	AP	0.72	610	No	—	CO2	950	10 ≤	300	491	90	0.07
49	AQ	0.05	610	No	—	CO2	950	10 ≤	300	464	90	0.10
50	AR	1.29	610	No	—	CO2	950	10 ≤	300	471	89	0.05
		ERW steel pipe
	ERW steel pipe for	after quenching and
	mechanical structural	before tempering	ERW steel pipe for
	part (inner surface side)	(outer surface side)	mechanical structural part
			Hard-		Thickness	(outer surface side)
	Thick-	Thick-	ness		of region			Hardness	Number
	ness	ness	of	Martensite	having less			of	of times
	of	of	base	fraction	than 90%	Thickness	Thickness	base	of
	de-B	de-C	metal	based on	martensite	of de-B	of de-C	metal	break-
	layer	layer	portion	hardness	hardness	layer	layer	portion	age
	(mm)	(mm)	(Hv)	(%)	(mm)	(mm)	(mm)	(Hv)	(× 104)	Note
22	0.06	0.02	484	92	0.12	0.05	0.09	490	58.2	Examples
23	0.06	0.11	484	95	0.13	0.05	0.12	485	17.6
24	0.07	0.02	506	89	0.14	0.07	0.08	508	17.1
25	0.04	0.03	489	90	0.08	0.09	0.09	495	55.1
26	0.06	0.08	501	93	0.13	0.04	0.09	502	50.8
27	0.02	0.06	484	92	0.05	0.08	0.06	487	27.7
28	0.03	0.06	489	92	0.07	0.08	0.07	491	51.5
29	0.07	0.05	501	91	0.15	0.05	0.10	504	53.8
30	0.08	0.08	495	93	0.16	0.08	0.04	499	33.0
31	0.06	0.07	501	92	0.12	0.08	0.04	504	26.0
32	0.06	0.11	501	96	0.13	0.08	0.04	505	59.4
33	0.03	0.03	501	90	0.08	0.08	0.10	502	13.6
34	0.03	0.09	489	94	0.07	0.02	0.06	492	28.4
35	0.05	0.08	484	93	0.10	0.04	0.03	489	21.2
36	0.06	0.02	489	89	0.12	0.04	0.05	494	50.7
37	0.08	0.02	489	89	0.16	0.03	0.09	495	49.7
38	0.03	0.04	495	91	0.07	0.09	0.04	500	56.7
39	0.06	0.02	456	89	0.13	0.06	0.05	461	8.5	Comparative
40	0.07	0.11	523	96	0.15	0.03	0.09	528	N.D.	Examples
41	0.03	0.07	430	93	0.08	0.03	0.04	432	9.5
42	0.05	0.02	478	89	0.11	0.03	0.03	485	7.8
43	0.02	0.09	421	94	0.76	0.03	0.09	423	5.3
44	0.07	0.10	484	94	0.14	0.05	0.04	486	7.7
45	0.04	0.06	423	92	0.08	0.04	0.02	426	6.7
46	0.07	0.07	478	92	0.15	0.05	0.03	482	3.2
47	0.09	0.12	427	96	0.47	0.03	0.11	425	7.5
48	0.05	0.09	506	94	0.10	0.05	0.09	507	8.1
49	0.04	0.03	484	89	0.08	0.09	0.05	488	3.0
50	0.02	0.02	489	89	0.05	0.02	0.02	496	4.8

As described above (for example, as shown in Table 3 and Table 4), in the electric resistance welded steel pipe for a mechanical structural part of each Example,

• • the chemical composition of the base metal portion in the straight pipe portion was the chemical composition in the present disclosure,
  • the microstructure of the central portion in the wall thickness direction of the base metal portion was tempered martensite,
  • the de-C layer had a thickness of less than 0.20 mm and the de-B layer had a thickness of less than 0.10 mm, at each of the inner surface side and the outer surface side of the base metal portion, and
  • the hardness of the base metal portion at each of the inner surface side and the outer surface side (namely, each of the Vickers hardness at a position at a depth of 0.5 mm from the inner surface of the base metal portion and the Vickers hardness at a position at a depth of 0.5 mm from the outer surface of the base metal portion) was 420 Hv or more and less than 510 Hv.

In the electric resistance welded steel pipe for a mechanical structural part of each Example, the number of fractures in the fatigue strength test was high, indicating an excellent fatigue strength.

As described above, an electric resistance welded steel pipe for a mechanical structural part having an excellent tensile strength and fatigue strength was obtained in each Example.

In contrast to the respective Examples, the results of Comparative Examples were as follows.

In Test No. 10, the chemical composition of the steel was adequate, but air was used as the atmosphere in the heat treatment furnace in the quenching step, and the oxygen content in the atmosphere was more than 1,000 volume ppm. Consequently, the thickness of the de-B layer was 0.10 mm or more, and the thickness of the de-C layer was more than 0.20 mm. As a result, the number of fractures in the fatigue strength test was less than 80,000 times, resulting in a low fatigue strength.

In Test No. 11, the chemical composition of the steel was adequate, but the quenching temperature was less than 900° C. As a result, the hardness of the base metal portion at each of the inner surface side and the outer surface side was insufficient.

In Test No. 13, the chemical composition of the steel was adequate, but the cooling rate after the quenching step was less than 10° C./sec. As a result, the hardness of the base metal portion at each of the inner surface side and the outer surface side was insufficient, resulting in an insufficient fatigue strength.

The C content was too low in test No. 39. As a result, the fatigue strength was insufficient.

The C content was too high in Test No. 40. As a result, the hardness of the base metal portion at each of the inner surface side and the outer surface side exceeded the upper limit. Further, weld cracks occurred and it was unable to perform the fatigue strength test in the Test No. 40. Therefore, the result of the fatigue strength was indicated as “N.D.” (No data).

The Si content was too low in Test No. 41. As a result, the fatigue strength was insufficient.

The Si content was too high in Test No. 42. As a result, the fatigue strength was insufficient.

The Mn content was too low in Test No. 43. As a result, the fatigue strength was insufficient.

The Mn content was too high in Test No. 44. As a result, the fatigue strength was insufficient.

The Ti content was too low in Test No. 45. As a result, the fatigue strength was insufficient.

The Ti content was too high in Test No. 46. As a result, the fatigue strength was insufficient.

The B content was too low in Test No. 47. As a result, the fatigue strength was insufficient.

The B content was too high in Test No. 48. As a result, the fatigue strength was insufficient.

The Ca content was too low in Test No. 49. As a result, the fatigue strength was insufficient.

The Ca content was too high in Test No. 50. As a result, the fatigue strength was insufficient.

Claims

1. An electric resistance welded steel pipe for a mechanical structural part, the pipe comprising a straight pipe portion,

wherein the straight pipe portion comprises a base metal portion and an electric resistance welded portion,

wherein the base metal portion has a chemical composition consisting of, in terms of % by mass:

from 0.30 to 0.38% of C,

from 0.05 to 0.40% of Si,

from 0.50 to 2.00% of Mn,

from 0.010 to 0.060% of Al,

from 0.005 to 0.050% of Ti,

from 0.0003 to 0.0050% of B,

from 0.0005 to 0.0040% of Ca,

from 0 to 0.0060% of N,

from 0 to 0.020% of P,

from 0 to 0.0200% of S,

from 0 to 0.0050% of O,

from 0 to 0.50% of Cu,

from 0 to 0.50% of Ni,

from 0 to 0.50% of Cr,

from 0 to 0.20% of V,

from 0 to 0.10% of Nb,

from 0 to 0.50% of Mo,

from 0 to 0.0500% of Mg,

from 0 to 0.0500% of REM, and

a balance consisting of Fe and impurities,

wherein a microstructure of a central portion in a wall thickness direction of the base metal portion is tempered martensite,

wherein, in a case in which a layer in which a concentration of C is 90% or less with respect to the concentration of C in the chemical composition of the base metal portion is defined as a de-C layer, and a layer in which a concentration of B is 90% or less with respect to the concentration of B in the chemical composition of the base metal portion is defined as a de-B layer, the de-C layer has a thickness of less than 0.20 mm and the de-B layer has a thickness of less than 0.10 mm, at each of an inner surface side and an outer surface side of the base metal portion, and

wherein each of a Vickers hardness at a position at a depth of 0.5 mm from an inner surface of the base metal portion and a Vickers hardness at a position at a depth of 0.5 mm from an outer surface of the base metal portion is 420 Hv or more and less than 510 Hv.

2. The electric resistance welded steel pipe for a mechanical structural part according to claim 1, wherein the chemical composition of the base metal portion comprises one or more selected from the group consisting of, in terms of % by mass:

from 0.01 to 0.50% of Cu,

from 0.05 to 0.50% of Ni,

from 0.05 to 0.50% of Cr, and

from 0.01 to 0.50% of Mo.

3. The electric resistance welded steel pipe for a mechanical structural part according to claim 1,

wherein the straight pipe portion has an outer diameter of from 10 to 50 mm, and

wherein a value obtained by dividing a wall thickness of the base metal portion by the outer diameter of the straight pipe portion is from 0.04 to 0.25.

4. The electric resistance welded steel pipe for a mechanical structural part according to claim 1, wherein, in the chemical composition of the base metal portion, F1 represented by the following Formula (1) is 0.50 or more: F ⁢ 1 = Ca × ( 1 - 124 × O ) / ( 1.25 × S ) Formula ⁢ ( 1 )

wherein each element symbol in Formula (1) represents a content of each element in terms of % by mass.

5. A method of producing the electric resistance welded steel pipe for a mechanical structural part according to claim 1, the method comprising:

a preparation step of preparing an as-rolled electric resistance welded steel pipe, wherein: the as-rolled electric resistance welded steel pipe comprises a base metal portion A and an electric resistance welded portion A; and the base metal portion A has a chemical composition consisting of, in terms of % by mass:

from 0.30 to 0.38% of C,

from 0.05 to 0.40% of Si,

from 0.50 to 2.00% of Mn,

from 0.010 to 0.060% of Al,

from 0.005 to 0.050% of Ti,

from 0.0003 to 0.0050% of B,

from 0.0005 to 0.0040% of Ca,

from 0 to 0.0060% of N,

from 0 to 0.020% of P,

from 0 to 0.0200% of S,

from 0 to 0.0050% of O,

from 0 to 0.50% of Cu,

from 0 to 0.50% of Ni,

from 0 to 0.50% of Cr,

from 0 to 0.20% of V,

from 0 to 0.10% of Nb,

from 0 to 0.50% of Mo,

from 0 to 0.0500% of Mg,

from 0 to 0.0500% of REM, and

a balance consisting of Fe and impurities;

a quenching step of subjecting the as-rolled electric resistance welded steel pipe to quenching; and

a tempering step of subjecting the as-rolled electric resistance welded steel pipe that has been subjected to quenching to tempering, to obtain the electric resistance welded steel pipe for a mechanical structural part;

wherein, in the quenching step, an oxygen content in an atmosphere in which the quenching is performed is 1,000 volume ppm or less, and a cooling rate in the quenching is 10° C./sec or more.

6. The method of producing the electric resistance welded steel pipe for a mechanical structural part, according to claim 5, further comprising a pipe-drawing step of subjecting the as-rolled electric resistance welded steel pipe to pipe drawing, after the preparation step and before the quenching step,

wherein the as-rolled electric resistance welded steel pipe that has been subjected to pipe drawing is subjected to quenching, in the quenching step.

7. The method of producing the electric resistance welded steel pipe for a mechanical structural part, according to claim 5, further comprising a step of subjecting the as-rolled electric resistance welded steel pipe to shot blasting, after the preparation step and before the quenching step.

8. The method of producing the electric resistance welded steel pipe for a mechanical structural part, according to claim 5,

wherein a heating temperature in the quenching is from 900 to 1,050° C., and

wherein a heating temperature in the tempering is from 100 to 500° C.

9. An electric resistance welded steel pipe for a mechanical structural part, the pipe comprising a straight pipe portion,

wherein the straight pipe portion comprises a base metal portion and an electric resistance welded portion,

wherein the base metal portion has a chemical composition comprising, in terms of % by mass:

from 0.30 to 0.38% of C,

from 0.05 to 0.40% of Si,

from 0.50 to 2.00% of Mn,

from 0.010 to 0.060% of Al,

from 0.005 to 0.050% of Ti,

from 0.0003 to 0.0050% of B,

from 0.0005 to 0.0040% of Ca,

from 0 to 0.0060% of N,

from 0 to 0.020% of P,

from 0 to 0.0200% of S,

from 0 to 0.0050% of O,

from 0 to 0.50% of Cu,

from 0 to 0.50% of Ni,

from 0 to 0.50% of Cr,

from 0 to 0.20% of V,

from 0 to 0.10% of Nb,

from 0 to 0.50% of Mo,

from 0 to 0.0500% of Mg,

from 0 to 0.0500% of REM, and

a balance comprising Fe and impurities,

wherein a microstructure of a central portion in a wall thickness direction of the base metal portion is tempered martensite,

wherein, in a case in which a layer in which a concentration of C is 90% or less with respect to the concentration of C in the chemical composition of the base metal portion is defined as a de-C layer, and a layer in which a concentration of B is 90% or less with respect to the concentration of B in the chemical composition of the base metal portion is defined as a de-B layer, the de-C layer has a thickness of less than 0.20 mm and the de-B layer has a thickness of less than 0.10 mm, at each of an inner surface side and an outer surface side of the base metal portion, and

wherein each of a Vickers hardness at a position at a depth of 0.5 mm from an inner surface of the base metal portion and a Vickers hardness at a position at a depth of 0.5 mm from an outer surface of the base metal portion is 420 Hv or more and less than 510 Hv.

10. The electric resistance welded steel pipe for a mechanical structural part according to claim 9, wherein the chemical composition of the base metal portion comprises one or more selected from the group consisting of, in terms of % by mass:

from 0.01 to 0.50% of Cu,

from 0.05 to 0.50% of Ni,

from 0.05 to 0.50% of Cr, and

from 0.01 to 0.50% of Mo.

11. The electric resistance welded steel pipe for a mechanical structural part according to claim 9,

wherein the straight pipe portion has an outer diameter of from 10 to 50 mm, and

wherein a value obtained by dividing a wall thickness of the base metal portion by the outer diameter of the straight pipe portion is from 0.04 to 0.25.

12. The electric resistance welded steel pipe for a mechanical structural part according to claim 9, wherein, in the chemical composition of the base metal portion, F1 represented by the following Formula (1) is 0.50 or more: F ⁢ 1 = Ca × ( 1 - 124 × O ) / ( 1.25 × S ) Formula ⁢ ( 1 )

wherein each element symbol in Formula (1) represents a content of each element in terms of % by mass.

13. A method of producing the electric resistance welded steel pipe for a mechanical structural part according to claim 9, the method comprising:

a preparation step of preparing an as-rolled electric resistance welded steel pipe, wherein: the as-rolled electric resistance welded steel pipe comprises a base metal portion A and an electric resistance welded portion A; and the base metal portion A has a chemical composition comprising, in terms of % by mass:

from 0.30 to 0.38% of C,

from 0.05 to 0.40% of Si,

from 0.50 to 2.00% of Mn,

from 0.010 to 0.060% of Al,

from 0.005 to 0.050% of Ti,

from 0.0003 to 0.0050% of B,

from 0.0005 to 0.0040% of Ca,

from 0 to 0.0060% of N,

from 0 to 0.020% of P,

from 0 to 0.0200% of S,

from 0 to 0.0050% of O,

from 0 to 0.50% of Cu,

from 0 to 0.50% of Ni,

from 0 to 0.50% of Cr,

from 0 to 0.20% of V,

from 0 to 0.10% of Nb,

from 0 to 0.50% of Mo,

from 0 to 0.0500% of Mg,

from 0 to 0.0500% of REM, and

a balance comprising Fe and impurities;

a quenching step of subjecting the as-rolled electric resistance welded steel pipe to quenching; and

a tempering step of subjecting the as-rolled electric resistance welded steel pipe that has been subjected to quenching to tempering, to obtain the electric resistance welded steel pipe for a mechanical structural part;

wherein, in the quenching step, an oxygen content in an atmosphere in which the quenching is performed is 1,000 volume ppm or less, and a cooling rate in the quenching is 10° C./sec or more.

14. The method of producing the electric resistance welded steel pipe for a mechanical structural part, according to claim 13, further comprising a pipe-drawing step of subjecting the as-rolled electric resistance welded steel pipe to pipe drawing, after the preparation step and before the quenching step,

wherein the as-rolled electric resistance welded steel pipe that has been subjected to pipe drawing is subjected to quenching, in the quenching step.

15. The method of producing the electric resistance welded steel pipe for a mechanical structural part, according to claim 13, further comprising a step of subjecting the as-rolled electric resistance welded steel pipe to shot blasting, after the preparation step and before the quenching step.

16. The method of producing the electric resistance welded steel pipe for a mechanical structural part, according to claim 13, wherein:

a heating temperature in the quenching is from 900 to 1,050° ° C., and

a heating temperature in the tempering is from 100 to 500° C.