CN113614268B

CN113614268B - Electric resistance welded steel pipe, method for producing same, and steel pipe pile

Info

Publication number: CN113614268B
Application number: CN201980094635.3A
Authority: CN
Inventors: 井手信介; 松本昌士; 松本晃英
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2019-03-29
Filing date: 2019-03-29
Publication date: 2022-12-02
Anticipated expiration: 2039-03-29
Also published as: WO2020202334A1; TW202039884A; KR102551615B1; KR20210132698A; JP6690788B1; JPWO2020202334A1; CN113614268A; TWI762881B

Abstract

An electric resistance welded steel pipe having a parent material portion and a welded portion in a pipe axial direction, wherein the parent material portion has a specific composition, and when the thickness of the parent material portion is t, in a steel structure at a depth of 1/4t from the outer surface of the electric resistance welded steel pipe, bainite is 70% or more in terms of an area ratio, the average effective grain diameter of bainite is 10.0 [ mu ] m or less in terms of an average equivalent circle diameter, the average aspect ratio of bainite is 0.1 to 0.8, the tensile strength in the pipe axial direction is 590MPa or more, the 0.2% yield strength is 450MPa or more, the yield ratio is 85 to 95%, the charpy absorption energy of the parent material portion at-30 ℃ in the pipe axial direction as a test piece length direction is 70J or more, and the residual stress of the outer surface of the steel pipe in the pipe axial direction is 250MPa or less.

Description

Electric resistance welded steel pipe, method for producing same, and steel pipe pile

Technical Field

The present invention relates to an electric resistance welded steel pipe suitable for a steel pipe pile used as a foundation of a structure, a method for manufacturing the same, and a steel pipe pile. In particular, the present invention relates to an electric resistance welded steel pipe obtained by cold roll forming and pipe forming a material made of a hot-rolled steel sheet (hot-rolled steel strip), which has improved strength, improved toughness, optimized yield ratio, and improved buckling resistance.

Background

In recent years, as a countermeasure against a large-scale earthquake, a steel pipe pile used as a foundation of a structure is also strongly desired to have high strength and improved deformation energy absorbing ability. In general, in order to improve the deformation energy absorbing ability of a steel pipe, it is effective to produce a steel material having a high tensile strength and a low yield ratio. However, in the steel pipe pile, it is difficult to excessively reduce the yield ratio in the pipe axial direction from the viewpoint of suppressing deformation of the steel pipe during driving. In addition, a steel pipe pile used particularly in cold regions is also required to have high low-temperature toughness. In addition, high buckling resistance is also required to resist deformation due to an earthquake or the like.

Patent document 1 describes a method for producing a shock-resistant welded steel pipe excellent in local buckling resistance. In patent document 1, a composition containing, in wt%: 0.03 to 0.15%, mn:1.0 to 2.0%, and contains Cu:0.05 to 0.50%, ni:0.05 to 0.50%, cr:0.05 to 0.50%, mo:0.05 to 0.50%, nb:0.005 to 0.10%, V:0.005 to 0.10%, ti:0.005 to 0.080% and Pcm 0.10 to 0.25, cooling the steel sheet to 600 ℃ or lower at a cooling rate of 5 ℃/sec or more after the rolling, and cold forming the resulting steel sheet to produce a steel pipe. Thus, a steel pipe excellent in deformation performance with a work hardening index of 0.10 or more in a tensile test in the pipe axial direction can be obtained, and occurrence of local buckling due to an external force acting on the steel pipe in the lateral direction and occurrence of brittle cracks or fractures due to the local buckling can be prevented.

Patent document 2 describes a method for producing a steel pipe containing, in terms of wt%, C:0.02 to 0.20%, si:0.02 to 0.50%, mn:0.50 to 2.00%, and contains a metal selected from the group consisting of Cu:0.10 to 1.5%, ni:0.10 to 0.50%, nb: 0.005-0.10% and V:0.005 to 0.10% and a steel sheet having a Ceq of 0.38 to 0.45 are hot-rolled so that a reduction per 1 pass in a temperature range of 900 ℃ or higher is 4% or less to produce a hot-rolled steel sheet, the hot-rolled steel sheet is reheated to a two-phase temperature range of Ac1 point or higher and Ac3 point or lower, quenched from the two-phase temperature range, further tempered, and then subjected to pipe-making. The steel pipe thus obtained is a low yield ratio high tension steel pipe having 0.2% yield strength of 440MPa or more, tensile strength of 590 to 700MPa, and yield ratio of 80% or less, and is suitable for use as steel structures such as buildings, bridges, tanks, and the like.

Patent document 3 describes a method for producing a high-tension steel pipe for building structures having a low yield ratio, which comprises, in mass%, C:0.10 to 0.18%, si:0.1 to 0.5%, mn:1 to 2% by weight, a step of heating to a temperature of Ac3 point or more and then quenching the steel, a step of heating to a two-phase temperature range of Ac1 point to Ac3 point and then cooling by air, a step of cold-forming the steel into a tubular shape, and a step of reheating to 500 to 600 ℃. Thus, a steel pipe for building structures having a tensile strength of 590MPa or more can be produced without using expensive alloying elements.

Patent document 4 describes a low yield ratio high strength electric resistance welded steel pipe for a steel pipe pile, which contains, in mass%, C:0.11 to 0.20%, si:0.05 to 0.50%, mn: 1.00-2.00%, P:0.030% or less, S:0.010% or less, al:0.01 to 0.08%, and has pearlite and/or pseudopearlite in an area ratio of 8 to 30% in a ferrite phase as a main phase and a second phase other than the main phase, and a structure having an average particle diameter of 4.0 to 10 [ mu ] m including the main phase and the second phase, and has a 0.2% yield strength YS of 450MPa or more, a tensile strength TS of 590MPa or more, and a yield ratio of 90% or less in the circumferential direction and the axial direction of the pipe.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 11-6032

Patent document 2: japanese patent No. 2687841

Patent document 3: japanese patent laid-open publication No. 2004-300461

Patent document 4: japanese patent No. 6123734

Disclosure of Invention

Problems to be solved by the invention

However, the yield ratio in the pipe axial direction of the steel pipe manufactured by the technique described in patent document 1 is excessively lowered. Therefore, when the obtained steel pipe is used as a steel pipe pile, there is a problem that buckling or the like may occur due to driving during pile driving.

In the technique described in patent document 2, a heat treatment step for tempering is required. In addition, in the technique described in patent document 3, not only a large-sized heat treatment apparatus for a pipe but also a heat treatment step after pipe production is required. In these techniques requiring heat treatment, there is a problem that the yield ratio is excessively lowered. Further, there is a problem that the process becomes complicated and the productivity is lowered. In addition, the production cost increases, and it is difficult to provide the product at low cost.

In the technique described in patent document 4, after hot rolling, the steel sheet is cooled from the finish rolling temperature to a temperature range of 550 to 700 ℃ for 10 to 100 seconds, and a structure mainly composed of ferrite and pearlite is obtained, and a desired structure cannot be obtained. Further, facilities having a very long cooling zone are required, and it is difficult to provide an inexpensive high-strength high-toughness electric resistance welded steel pipe for a steel pipe pile.

The present invention has been made in view of the above problems, and an object thereof is to provide an electric resistance welded steel pipe having an optimum yield ratio and high buckling resistance, and having high strength and high toughness, a method for producing the same, and a steel pipe pile.

In the present invention, an electric resistance welded steel pipe, a method for manufacturing the same, and a steel pipe pile are mainly provided which can solve the above-described problems when a hot-rolled steel sheet having a thickness of 16mm or more is used as a material.

The term "high strength" as used herein means a case where the 0.2% proof stress (YS) in the pipe axial direction of the parent metal portion of the electric resistance welded steel pipe is 450MPa or more and the Tensile Strength (TS) is 590MPa or more. The term "high toughness" as used herein means that the Charpy absorption energy at-30 ℃ is 70J or more when the tube axial direction of the matrix portion of the electric resistance welded steel pipe is the test piece longitudinal direction, and the high toughness is satisfied in both the circumferential direction and the tube axial direction of the electric resistance welded steel pipe. The "optimum yield ratio" referred to herein means that the ratio (YR) of 0.2% yield strength to the tensile strength is 80 to 90%. The term "high buckling resistance" as used herein means that the residual stress in the pipe axial direction of the outer surface of the steel pipe in the parent metal portion of the electric resistance welded steel pipe is 250MPa or less and the yield ratio is 90% or less.

Means for solving the problems

In order to achieve the above object, the present inventors have intensively studied the influence of various alloying elements and manufacturing conditions on yield ratio, 0.2% yield strength, tensile strength and charpy impact characteristics. In addition, the buckling resistance of the obtained steel pipe (electric resistance welded steel pipe) was also studied intensively. As a result, they have found that there are appropriate composition, steel structure and production conditions that can achieve both high strength and high toughness and high buckling resistance while maintaining the yield ratio low.

That is, a hot-rolled steel sheet manufactured by limiting to a specific composition and hot rolling conditions is subjected to diameter reduction rolling under specific conditions after welding in a cold roll forming step using cold roll forming. In this way, in the steel structure at a depth of 1/4t from the thickness t of the outer surface of the steel pipe in the parent metal portion of the electric resistance welded steel pipe, bainite is 60% or more in area ratio, the average effective grain diameter of bainite is 20.0 μm or less in average equivalent circle diameter, and the average aspect ratio of bainite is 0.1 to 0.8. As a result, it has been found that an electric resistance welded steel pipe having a low yield ratio, a high strength, a high toughness and a high buckling strength can be obtained, the electric resistance welded steel pipe having a low 0.2% yield strength of 450MPa or more, a tensile strength of 590MPa or more and a yield ratio of 80 to 90%, a Charpy absorption energy at-30 ℃ of 70J or more, and a residual stress in the pipe axial direction of the outer surface of the steel pipe in the parent material portion of 250MPa or less.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.

[1] An electric resistance welded steel pipe having a base material portion and a welded portion extending in the steel axial direction,

the above-described base material portion has a composition containing, in mass%, C:0.12 to 0.20%, si:0.60% or less, mn:0.50 to 1.70%, P:0.030% or less, S:0.015% or less, al:0.010 to 0.060%, nb:0.010 to 0.080%, ti:0.010 to 0.050%, N: less than 0.006%, the balance being Fe and unavoidable impurities,

when the thickness of the parent metal portion is defined as t, bainite in an area ratio of 60% or more in a steel structure at a depth of 1/4t from the thickness t of the outer surface of the electric resistance welded steel pipe, an average effective grain diameter of the bainite is 20.0 μm or less in terms of an average equivalent circle diameter, and an average aspect ratio of the bainite is 0.1 to 0.8,

a tensile strength in the pipe axial direction of 590MPa or more, a 0.2% yield strength of 450MPa or more, a yield ratio of 80 to 90%,

the matrix part has a Charpy absorption energy at-30 ℃ of 70J or more in the direction of the tube axis as the longitudinal direction of the test piece,

the residual stress in the pipe axial direction of the outer surface of the steel pipe in the parent metal part is 250MPa or less.

[2] The electric resistance welded steel pipe according to [1], which further comprises, in mass%, in addition to the above-mentioned composition: less than 0.008 percent.

[3] The electric resistance welded steel pipe according to [1] or [2], further comprising, in addition to the above-described composition, a component selected from the group consisting of Cr:0.01 to 1.0%, V:0.010 to 0.060%, mo:0.01 to 1.0%, cu:0.01 to 0.50%, ni:0.01 to 1.0%, ca: 0.0005-0.010% of one or more than two.

[4] A method for manufacturing an electric resistance welded steel pipe, wherein a hot-rolled steel sheet is manufactured by subjecting a steel material to a hot-rolling step and a cooling step in this order, and further an electric resistance welded steel pipe is manufactured by subjecting the hot-rolled steel sheet to a cold-roll forming step,

in the manufacturing method,

the steel material has the composition of any one of [1] to [3],

the hot rolling step is as follows: heating the steel material to a heating temperature of 1100 to 1280 ℃, then performing rough rolling and finish rolling at a rough rolling finishing temperature of 850 to 1150 ℃, a finish rolling finishing temperature of 750 to 850 ℃, and a total reduction ratio of 930 ℃ or lower in the rough rolling and the finish rolling of 65% or more to produce a hot rolled sheet,

the cooling step is as follows: the hot-rolled sheet is cooled at an average cooling rate of 5 to 25 ℃/sec from the start of cooling to the stop of cooling by a central thermometer for sheet thickness and at a cooling stop temperature of 450 to 650 ℃,

in the cold roll forming step, a steel pipe material obtained by subjecting the hot-rolled steel sheet to roll forming is welded, and diameter reduction rolling is performed so that the diameter reduction ratio of the welded steel pipe material to the circumferential length of the outer surface is 0.2 to 0.5%.

[5] A method for producing an electric resistance welded steel pipe, comprising subjecting a hot-rolled steel sheet having a composition of any one of [1] to [3], wherein bainite is present in an area ratio of 60% or more in a steel structure at a depth of 1/4t from an outer surface of the steel sheet to the thickness t, an average effective grain diameter of the bainite is 20.0 [ mu ] m or less in terms of an average equivalent circle diameter, and an average aspect ratio of the bainite is 0.1 to 0.8,

in the manufacturing method,

[6] A steel pipe pile using the electric resistance welded steel pipe according to any one of [1] to [3 ].

Effects of the invention

According to the present invention, it is possible to provide an electric resistance welded steel pipe having an optimum yield ratio and high buckling resistance, and having high strength and high toughness, which is suitable for use as a steel pipe pile, a method for producing the same, and a steel pipe pile. The electric resistance welded steel pipe of the present invention can be easily manufactured, and industrially exhibits significant effects.

Detailed Description

The present invention will be described in detail below.

First, the reasons for the limitations of the composition of the electric resistance welded steel pipe of the present invention will be explained. Hereinafter, "mass%" in the component composition is abbreviated as "%" unless otherwise specified.

The electric resistance welded steel pipe of the present invention has a base material portion and a welded portion, the base material portion having the following composition: contains C:0.12 to 0.20%, si:0.6% or less, mn:0.50 to 1.70%, P:0.030% or less, S:0.015% or less, al:0.010 to 0.060%, nb:0.010 to 0.080%, ti: 0.010-0.050%, N:0.006% or less, and the balance of Fe and inevitable impurities.

The electric resistance welded steel pipe of the present invention has a welded portion in the pipe axial direction. The "hot rolled steel sheet" described later includes a hot rolled steel sheet and a hot rolled steel strip.

C：0.12～0.20％

C is an element that increases the strength of a steel pipe (electric resistance welded steel pipe) by solid solution strengthening and participates in the formation of a steel structure such as bainite. Further, C is an element effective for optimizing the yield ratio. Since a steel pipe having a large thickness (for example, a steel pipe having a thickness of 16mm or more) has a large difference between the outer diameter and the inner diameter, the degree of working at the time of manufacturing the steel pipe is large, and the specific yield ratio tends to increase. Therefore, it is necessary to contain a large amount of C. Therefore, in order to obtain the above effect, it is necessary to contain 0.12% or more of C. On the other hand, if C is contained in an amount exceeding 0.20%, martensite is easily generated, and the steel structure targeted in the present invention cannot be obtained. As a result, the high toughness aimed at in the present invention cannot be ensured. Therefore, C is set to 0.12 to 0.20%. C is preferably 0.13% or more, more preferably 0.14% or more. C is preferably set to 0.19% or less, more preferably 0.18% or less.

Si: less than 0.60%

Si is an element that acts as a deoxidizer and can increase the strength of the steel pipe. However, when Si is contained excessively, toughness is lowered. Therefore, si is set to 0.60% or less. Si is preferably set to 0.50% or less, more preferably 0.45% or less. The lower limit of Si is not particularly limited, but is preferably set to 0.01% or more from the viewpoint of resistance weldability. More preferably, it is set to 0.02% or more.

Mn：0.50～1.70％

Mn is an element that increases the strength of the steel pipe by solid solution strengthening. In order to obtain such an effect and ensure the high strength aimed at in the present invention, it is necessary to contain 0.50% or more of Mn. On the other hand, when Mn is contained in an amount exceeding 1.70%, the steel structure becomes finer, the yield strength becomes high, and the yield ratio targeted in the present invention cannot be secured. Therefore, mn is set to 0.50 to 1.70%. The Mn is preferably 0.55% or more, more preferably 0.60% or more. The Mn is preferably 1.65% or less, more preferably 1.60% or less.

P: less than 0.030%

P is an element which segregates to grain boundaries to lower toughness, and is preferably reduced as much as possible as an impurity, but in the present invention, it is allowable to be 0.030%. Therefore, P is set to 0.030% or less. P is preferably set to 0.025% or less, more preferably 0.020% or less. However, since an excessive reduction in P leads to an increase in refining cost, it is preferable to set P to 0.002% or more. More preferably, it is set to 0.003% or more.

S: less than 0.015%

S exists as MnS in steel when manufacturing a hot rolled steel sheet as a material of a steel pipe and exerts an adverse effect on ductility and toughness of the steel pipe by being thinly drawn in a hot rolling process. Therefore, in the present invention, S is preferably reduced as much as possible as an impurity, but the content of S may be allowed to be 0.015%. Therefore, S is set to 0.015% or less. S is preferably set to 0.010% or less, more preferably 0.008% or less. However, since an excessive reduction in S leads to an increase in refining cost, it is preferable to set S to 0.0002% or more. More preferably, it is set to 0.001% or more.

Al：0.010～0.060％

Al functions as a deoxidizer, and bonds with N to form AlN, contributing to grain refinement. In order to obtain such an effect, it is necessary to contain 0.010% or more of Al. On the other hand, if Al is contained in a large amount exceeding 0.060%, the cleanliness of the steel material (hot-rolled steel sheet as a material of the steel pipe) is reduced, and the ductility and toughness of the steel pipe are reduced. Therefore, al is set to 0.010 to 0.060%. Al is preferably 0.015% or more, and more preferably 0.020% or more. Al is preferably set to 0.055% or less, and more preferably set to 0.050% or less.

Nb：0.010～0.080％

Nb bonds with carbon and nitrogen to form fine precipitates, and increases the strength of the steel pipe by precipitation strengthening. In order to obtain such an effect, 0.010% or more of Nb needs to be contained. On the other hand, if Nb is contained in excess of 0.080%, it is difficult to make a solid solution by heating in the hot rolling step when manufacturing a hot-rolled steel sheet as a material of a steel pipe. As a result, coarse precipitates remain, and the toughness is lowered. Therefore, nb is set to 0.010 to 0.080%. Nb is preferably 0.015% or more, and more preferably 0.020% or more. Nb is preferably set to 0.075% or less, and more preferably 0.070% or less.

Ti：0.010～0.050％

Ti combines with carbon and nitrogen to form fine precipitates, and increases the strength of the steel pipe by precipitation strengthening. In order to obtain such an effect, it is necessary to contain 0.010% or more of Ti. On the other hand, when Ti is contained in an amount exceeding 0.050%, precipitates become coarse, and toughness is lowered. Therefore, ti is set to 0.010 to 0.050%. Ti is preferably 0.012% or more, and more preferably 0.015% or more. Ti is preferably set to 0.045% or less, more preferably 0.040% or less.

N: less than 0.006%

When N is a small amount, it has an effect of increasing the strength of the steel pipe, but if it is contained in a large amount, coarse precipitates are formed at high temperature, and the toughness is lowered. Therefore, N is set to 0.006% or less. Since an excessive reduction in N leads to an increase in refining cost, it is preferably set to 0.001% or more, and more preferably 0.002% or more. N is preferably set to 0.005% or less, more preferably 0.004% or less.

The balance being Fe and unavoidable impurities. As inevitable impurities, those containing O:0.0050% or less.

The above-described composition is a basic composition of the electric resistance welded steel pipe of the present invention. The essential elements described above can provide the characteristics aimed at in the present invention, but the following elements may be further contained as necessary in addition to the basic composition.

B: less than 0.008%

B is an element that contributes to the refinement of the steel structure by lowering the ferrite transformation start temperature, and may be contained as necessary. However, if the content of B exceeds 0.008%, segregation tends to occur in the grain boundary, and the toughness may be lowered. Therefore, when B is contained, it is preferable to set B to 0.008% or less. More preferably, it is set to 0.006% or less. B is preferably set to 0.0003% or more.

Is selected from Cr:0.01 to 1.0%, V:0.010 to 0.060%, mo:0.01 to 1.0%, cu:0.01 to 0.50%, ni:0.01 to 1.0%, ca: 0.0005-0.010% of one or more than two

Cr：0.01～1.0％

Cr is an element that increases the strength of the steel pipe by increasing the hardenability, and may be contained as needed. In order to obtain such an effect, 0.01% or more of Cr is preferably contained. On the other hand, when Cr is contained in an amount exceeding 1.0%, toughness and weldability may be deteriorated, and therefore, it is preferably set to 1.0% or less. Therefore, when Cr is contained, cr is preferably set to 0.01 to 1.0%. Cr is more preferably set to 0.02% or more, and still more preferably set to 0.03% or more. Cr is more preferably set to 0.8% or less, and still more preferably set to 0.6% or less.

V：0.010～0.060％

V is an element that forms fine precipitates by bonding with carbon and nitrogen and increases the strength of the steel pipe by precipitation strengthening, and may be contained as necessary. In order to obtain such an effect, it is necessary to contain 0.010% or more of V. On the other hand, when V is contained in an amount exceeding 0.060%, precipitates become coarse, and the above effects are easily saturated. Therefore, V is set to 0.010 to 0.060%. V is preferably set to 0.012% or more, and more preferably 0.015% or more. V is preferably set to 0.055% or less, more preferably 0.050% or less.

Mo：0.01～1.0％

Mo is an element that increases the strength of the steel pipe by increasing the hardenability, and may be contained as needed. In order to obtain such an effect, 0.01% or more of Mo is preferably contained. On the other hand, when Mo is contained in an amount exceeding 1.0%, toughness may be lowered, and therefore, it is preferably set to 1.0% or less. Therefore, when Mo is contained, mo is preferably set to 0.01 to 1.0%. Mo is set to more preferably 0.02% or more, and still more preferably 0.03% or more. Mo is more preferably set to 0.8% or less, and still more preferably set to 0.6% or less.

Cu：0.01～0.50％

Cu is an element that increases the strength of the steel pipe by solid solution strengthening, and may be contained as necessary. In order to obtain such an effect, cu is preferably contained in an amount of 0.01% or more. On the other hand, when Cu is contained in an amount exceeding 0.50%, toughness may be lowered, and therefore, it is preferably set to 0.50% or less. Therefore, when Cu is contained, it is preferable to set Cu to 0.01 to 0.50%. Cu is more preferably 0.02% or more, and still more preferably 0.03% or more. Cu is more preferably 0.45% or less, and still more preferably 0.40% or less.

Ni：0.01～1.0％

Ni is an element that increases the strength of the steel pipe by solid-solution strengthening, and may be contained as necessary. In order to obtain such an effect, 0.01% or more of Ni is preferably contained. On the other hand, when Ni is contained in an amount exceeding 1.0%, toughness may be lowered, and therefore, it is preferably set to 1.0% or less. Therefore, when Ni is contained, ni is preferably set to 0.01 to 1.0%. Ni is more preferably set to 0.02% or more, and still more preferably set to 0.03% or more. Ni is more preferably set to 0.8% or less, and still more preferably set to 0.6% or less.

Ca：0.0005～0.010％

Ca is an element that contributes to the improvement of toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in a hot rolling process when manufacturing a hot-rolled steel sheet that is a material of a steel pipe, and may be contained as necessary. In order to obtain such an effect, when Ca is contained, 0.0005% or more is preferably contained. However, if the content of Ca exceeds 0.010%, ca oxide clusters may be formed in the steel, and the toughness may deteriorate. Therefore, when Ca is contained, it is preferable to set Ca to 0.0005% to 0.010%. Ca is more preferably 0.0010% or more, and still more preferably 0.0015% or more. Ca is more preferably set to 0.005% or less, and still more preferably set to 0.004% or less.

Next, the reason for limiting the steel structure of the electric resistance welded steel pipe of the present invention will be described.

When the thickness of the base material portion of the electric resistance welded steel pipe of the present invention is denoted by t, the steel structure at a depth of 1/4t of the thickness t from the outer surface of the electric resistance welded steel pipe has the following steel structure: the bainite is 60% or more in terms of area ratio, the average effective grain diameter of the bainite is 20.0 μm or less in terms of average equivalent circle diameter, and the average aspect ratio of the bainite is 0.1 to 0.8.

Here, the 1/4t depth position of the sheet thickness t is an intermediate position between the maximum surface layer where the cooling rate is the maximum and the minimum 1/2t depth position in the hot rolling step in manufacturing a hot-rolled steel sheet as a material of a steel pipe, which is important in controlling the steel structure. In the present invention, a cross section parallel to the rolling direction at a 1/4W position of the sheet width W during hot rolling is used as an evaluation plane of the steel structure. In the present invention, since heat treatment or the like is not performed after hot rolling, the structure of the hot-rolled steel sheet is the same as the structure of the steel pipe (base material portion).

Area ratio of bainite: more than 60 percent

In the present invention, in order to achieve both high strength and high toughness, it is important to contain 60% or more of bainite in terms of area ratio. When the bainite is less than 60%, it is difficult to obtain the strength aimed at in the present invention. Therefore, in the steel structure at a depth of 1/4t of the plate thickness t from the outer surface of the steel pipe, bainite is set to 60% or more in area ratio. Preferably 65% or more. Since the yield ratio becomes excessively high when the area ratio of bainite is excessive, bainite is preferably set to 98% or less in terms of area ratio. More preferably, it is set to 95% or less.

The structure other than bainite (residual structure) may be ferrite, pearlite, martensite, austenite, or the like. If the total area ratio of these structures is 40% or more based on the entire steel structure, the strength and toughness are insufficient, and the yield ratio is increased or excessively decreased. Therefore, it is preferably set to less than 40%. More preferably, it is set to less than 35%. In consideration of obtaining the yield ratio targeted in the present invention, the lower limit of the total of the area ratios of the excess microstructure is preferably more than 2%, more preferably more than 5%.

In the present invention, the area ratio of each tissue can be measured by the method described in the examples described below.

Average effective grain size of bainite: 20.0 μm or less in terms of average equivalent circle diameter

In the present invention, it is important to set the average equivalent circle diameter of the average effective grain diameter of bainite to 20.0 μm or less in order to achieve both high strength and high toughness. When the average effective grain size of bainite exceeds 20.0 μm in terms of average equivalent circle diameter, the toughness aimed at in the present invention cannot be obtained. In addition, the strength targeted in the present invention cannot be obtained. Preferably 15.0 μm or less. Since the yield ratio becomes too high when bainite becomes excessively fine, the average equivalent circle diameter of the average effective grain diameter of bainite is preferably set to 1.0 μm or more, more preferably 2.0 μm or more.

Here, when a difference in orientation between adjacent crystals is determined and a region surrounded by a boundary where the difference in orientation between adjacent crystals (difference in crystal orientation) is 15 ° or more is defined as a crystal grain, the diameter of a circle having an area equal to that of the crystal grain is defined as the effective grain size of bainite. The arithmetic mean of the particle diameters was obtained from the obtained effective particle diameters and was defined as an average circle-equivalent diameter (average effective particle diameter). In the present invention, the crystal orientation difference, the effective particle diameter, and the average equivalent circle diameter can be measured by the methods described in the examples described later.

Average length-diameter ratio of bainite: 0.1 to 0.8

In the present invention, in order to control the yield ratio in the pipe axial direction to 80 to 90%, it is necessary to set the average aspect ratio of bainite to 0.1 to 0.8. Here, for the crystal grains of bainite, (average of length in the plate thickness direction)/(average of length in the pipe axis direction) was calculated as the average aspect ratio of bainite. When the average aspect ratio of bainite exceeds 0.8, the plastic deformability in the pipe axis direction is reduced, and the yield ratio easily exceeds 90%. On the other hand, if the average aspect ratio of bainite is less than 0.1, the strength in the pipe axial direction decreases, and the strength targeted in the present invention cannot be obtained.

In the present invention, the average length of the bainite crystal grains in the plate thickness direction and the average length in the rolling direction can be measured by the methods described in the examples described later.

Next, a method for manufacturing an electric resistance welded steel pipe according to an embodiment of the present invention will be described.

The electric resistance welded steel pipe of the present invention is produced, for example, by subjecting a steel material having the above-described composition to a hot rolling step, a cooling step, and a winding step in this order to produce a hot-rolled steel sheet, and further subjecting the hot-rolled steel sheet to a cold roll forming step.

In the following description of the manufacturing method, unless otherwise specified, "° c" with respect to the temperature is set to the surface temperature of the steel material or steel sheet (hot-rolled steel sheet). These surface temperatures can be measured by a radiation thermometer or the like. The temperature at the center of the thickness of the steel sheet can be obtained by calculating the temperature distribution in the cross section of the steel sheet by heat transfer analysis and correcting the result by the surface temperature of the steel sheet. The "hot-rolled steel sheet" includes a hot-rolled steel sheet and a hot-rolled steel strip.

In the present invention, the method of melting the steel material (billet) is not particularly limited. From the viewpoint of quality, productivity, and the like, it is preferable that the molten steel having the above-described composition is melted by a usual melting method such as a converter, an electric furnace, or a vacuum melting furnace, and cast pieces such as billets are produced by a usual casting method such as a continuous casting method. Note that, even if the ingot-cogging-rolling method is applied instead of the continuous casting method, there is no problem. The molten steel may be further subjected to secondary refining such as ladle refining.

Next, the obtained steel material (billet) is subjected to a hot rolling process. The hot rolling process comprises the following steps: the steel material is heated to a heating temperature of 1100-1280 ℃, then rough rolling is performed so that the rough rolling finishing temperature is 850-1150 ℃, finish rolling is performed so that the finish rolling finishing temperature is 750-850 ℃, and hot rolling is performed so that the total reduction ratio at 930 ℃ or lower in the rough rolling and the finish rolling is 65% or more, thereby producing a hot rolled sheet.

Heating temperature: 1100-1280 DEG C

When the heating temperature is less than 1100 ℃, coarse carbides existing in the steel material generated during casting cannot be dissolved in solid solution. As a result, the effect of the carbide-forming element contained therein cannot be sufficiently obtained. On the other hand, when the heating temperature exceeds 1280 ℃ and becomes high, the crystal grains are significantly coarsened, the structure of the hot-rolled steel sheet as a material of the steel pipe is coarsened, and it is difficult to ensure the characteristics targeted in the present invention. Therefore, the heating temperature of the steel material needs to be set to 1100 to 1280 ℃. Preferably 1120 to 1230 ℃. The temperature is a furnace internal set temperature of the heating furnace.

Rough rolling finishing temperature: 850-1150 deg.C

When the rough rolling finishing temperature is less than 850 ℃, the structure is not restored during hot rolling, and crystal grains excessively elongated in the rolling direction are easily generated. As a result, the average aspect ratio of bainite becomes easily smaller than 0.1. On the other hand, when the rough rolling finishing temperature exceeds 1150 ℃, the reduction amount in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained, and as a result, it is difficult to secure the average effective grain size of bainite as the target in the present invention. Therefore, the rough rolling finishing temperature is set to 850 to 1150 ℃. Preferably, the temperature is set to 860 to 1000 ℃.

Finish rolling finish temperature: 750-850 deg.C

When the finish rolling temperature is less than 750 ℃, the structure is not restored during hot rolling, and grains excessively elongated in the rolling direction are easily generated. As a result, the average aspect ratio of bainite becomes easily smaller than 0.1. On the other hand, when the finish rolling temperature exceeds 850 ℃, the rolling reduction in the unrecrystallized temperature range of austenite is insufficient, and fine austenite grains cannot be obtained, and as a result, it is difficult to secure the average effective grain size of bainite as the target in the present invention. Therefore, the finish rolling finishing temperature is set to 750 to 850 ℃. Preferably set at 770 to 830 ℃.

The total reduction ratio at 930 ℃ or lower in the rough rolling and the finish rolling: over 65 percent

In the present invention, by making austenite finer in the hot rolling step, bainite and a residual structure formed in the subsequent cooling step and coiling step can be made finer, and a hot-rolled steel sheet suitable as a material for electric resistance welded steel pipes having the strength and toughness targeted in the present invention can be obtained. In order to refine austenite in the hot rolling step, it is necessary to increase the reduction ratio in the austenite non-recrystallization temperature range and introduce sufficient working strain. In order to obtain this effect, in the present invention, the total reduction ratio in the temperature range of 930 ℃ or lower to the finish rolling temperature is set to 65% or higher. The total reduction ratio is the total of the reduction ratios of the respective rolling passes in the temperature range of 930 ℃ or less to the finish rolling temperature.

When the total reduction ratio in the temperature range of 930 ℃ or lower to the finish rolling temperature is less than 65%, a sufficient working strain cannot be introduced in the hot rolling step, and therefore a steel structure having an average effective grain size of bainite as a target in the present invention cannot be obtained. The total reduction ratio in the temperature range of 930 ℃ or lower to the finish rolling temperature is more preferably 70% or higher. In particular, the upper limit is not specified, but if it exceeds 80%, the effect of improving toughness with respect to the increase in reduction ratio becomes small, and only the equipment load becomes large. Therefore, the total reduction ratio in the temperature range of 930 ℃ or lower to the finish rolling temperature is preferably 80% or lower. More preferably 75% or less.

In the present invention, the reason why the temperature is set to 930 ℃ or lower is that, when the temperature exceeds 930 ℃, austenite is recrystallized in the hot rolling step, and dislocations introduced by rolling disappear, and refined austenite cannot be obtained.

In the present invention, when the steel material is hot-rolled, hot rolling may be performed such that the total reduction ratio from 930 ℃ or lower to the finish rolling end temperature is 65% or more in both the rough rolling and the finish rolling, or hot rolling may be performed such that the total reduction ratio from 930 ℃ or lower to the finish rolling end temperature is 65% or more only in the finish rolling. In the latter case, when the total reduction ratio from 930 ℃ or lower to the finish rolling temperature cannot be made 65% or higher by only the finish rolling, the slab may be cooled to 930 ℃ or lower in the middle of the rough rolling, and then the total reduction ratio from 930 ℃ or lower to the finish rolling temperature in both the rough rolling and the finish rolling may be made 65% or higher.

Next, the hot-rolled sheet after the hot-rolling step is subjected to a cooling step. The cooling process comprises the following steps: the hot-rolled sheet is cooled under the conditions that the average cooling rate from the start of cooling to the stop of cooling is5 to 25 ℃/sec and the cooling stop temperature is 450 to 650 ℃.

Average cooling rate from start of cooling to stop of cooling: 5-25 ℃/second

When the average cooling rate in the temperature range from the start of cooling to the stop temperature of cooling described later is less than 5 ℃/sec, ferrite is formed, and the area ratio of bainite decreases, and the strength aimed at in the present invention cannot be obtained. On the other hand, when the average cooling rate exceeds 25 ℃/sec, the average aspect ratio of bainite exceeds 0.8. As a result, the yield ratio easily exceeds 90%. The average cooling rate is preferably set to 10 ℃/sec or more, and preferably 20 ℃/sec or less.

In the present invention, unless otherwise specified, the average cooling rate is set to be the average of the values (cooling rates) obtained by ((the thickness center temperature of the hot-rolled sheet before cooling-the thickness center temperature of the hot-rolled sheet after cooling)/the cooling time). Examples of the cooling method include water cooling by spraying water from a nozzle, and cooling by spraying a cooling gas. In the present invention, it is preferable to perform the cooling operation (treatment) on both sides of the hot-rolled sheet in such a manner that both sides of the hot-rolled sheet are cooled under the same conditions.

Cooling stop temperature: 450-650 deg.C

When the cooling stop temperature of the hot-rolled sheet is less than 450 ℃ by the sheet thickness center thermometer, the average aspect ratio of bainite exceeds 0.8, and as a result, the yield ratio easily exceeds 90%. On the other hand, when the cooling stop temperature exceeds 650 ℃, the area ratio of bainite cannot be made 60% or more because the bainite transformation start temperature is exceeded. The cooling stop temperature is preferably set to 480 ℃ or higher, and preferably 620 ℃ or lower.

Next, a winding step of winding the hot-rolled steel sheet after the cooling step and then naturally cooling the wound sheet is performed.

In the coiling step, it is preferable to coil at a coiling temperature of 450 to 650 ℃ from the viewpoint of the steel sheet structure of the hot-rolled steel sheet as a material of the steel pipe. When the coiling temperature is less than 450 ℃, the average aspect ratio of bainite exceeds 0.8, and as a result, the yield ratio may exceed 90%. On the other hand, when the coiling temperature exceeds 650 ℃, the area ratio of bainite cannot be made 60% or more in some cases because the bainite transformation start temperature is exceeded. The coiling temperature is more preferably 480 to 620 ℃.

Next, the hot-rolled steel sheet after the winding step is subjected to a cold roll forming step. In the cold roll forming step, the hot-rolled steel sheet is cold roll formed into a cylindrical open pipe, both ends of the steel pipe material (i.e., the butted portion of the open pipe) are resistance welded, and the steel pipe is subjected to diameter reduction rolling at a diameter reduction ratio of 0.2 to 0.5% with respect to the circumferential length of the outer surface of the steel pipe of the circular steel pipe after welding.

Reduction ratio in reducing rolling: 0.2 to 0.5 percent

When the reduction ratio in the reduction rolling is less than 0.2%, the reduction of the residual stress due to the plastic deformation in the steel material of the steel pipe of the present invention is insufficient. As a result, the residual stress in the pipe axial direction of the outer surface of the steel pipe exceeded 250MPa. In addition, the yield ratio is less than 80% due to insufficient workability. On the other hand, when the reduction ratio in the reduction rolling exceeds 0.5%, the yield ratio exceeds 90% due to work hardening. As a result, the desired plastic deformability, i.e., buckling resistance, cannot be obtained. In addition, even if the residual stress exceeds 250MPa, the buckling resistance is lowered.

The electric resistance welded steel pipe of the present invention is produced as described above. According to the present invention, an electric resistance welded steel pipe having a tensile strength of 590MPa or more in the longitudinal direction of the pipe, a 0.2% yield strength of 450MPa or more, a yield ratio of 80 to 90%, a Charpy absorption energy at-30 ℃ of 70J or more, and a residual stress of 250MPa or less in the pipe axial direction of the outer surface of the steel pipe can be obtained. Thus, it is possible to easily produce an electric resistance welded steel pipe excellent in high strength, high toughness, an optimum yield ratio, and buckling resistance. The electric resistance welded steel pipe is particularly suitable for a steel pipe pile used as a foundation of a structure, and therefore industrially exhibits a remarkable effect.

Next, the steel pipe pile of the present invention will be explained.

The steel-pipe pile of the present invention has a plate thickness of 16mm or more and an outer diameter of 300mm or more and 700mm or less, and is composed of an electric resistance welded steel pipe having the above-described composition and steel structure. By specifying the composition and steel structure of the electric resistance welded steel pipe as described above, a steel pipe pile can be obtained in which the tensile strength in the pipe longitudinal direction is 590MPa or more, the 0.2% yield strength is 450MPa or more, the yield ratio is 80 to 90%, -the charpy absorption energy at 30 ℃ is 70J or more, and the residual stress in the pipe axial direction of the outer surface of the steel pipe is 250MPa or less. The steel pipe piles of the present invention are driven into the ground, and if necessary, the steel pipe piles can be connected to each other by connecting means such as welding or a screw joint during driving, and long piles can be produced by on-site construction. According to the steel pipe pile of the present invention, since the steel pipe pile has the above-described characteristics, the possibility of problems such as buckling occurring in the driven pile can be reduced.

Examples

The present invention will be described in further detail below with reference to examples. It should be noted that the present invention is not limited to the following examples.

Molten steel having a composition shown in Table 1 was melted in a converter and made into billets (steel material: 250mm in wall thickness) by a continuous casting method. The obtained steel slabs were subjected to the hot rolling step, cooling step, coiling step and cold roll forming step under the production conditions shown in tables 2-1 and 2-2, to thereby produce electric resistance welded steel pipes having the outer diameters and thicknesses shown in tables 2-1 and 2-2. In the cold roll forming step, the butted portion of the open pipe is resistance-welded.

Test pieces were cut out from the obtained electric resistance welded steel pipes, and structure observation, tensile test, charpy impact test, measurement of residual stress, and member compression test were performed by the following methods.

[ tissue Observation ]

The test piece for tissue observation was prepared as follows: the test piece for structure observation was produced by cutting and polishing so that the cross section in the tube axis direction at the 90 ° position in the circumferential direction when the electric resistance welded portion was set to 0 ° was an observation surface, and then etching with a nital solution. The structure observation is carried out by observing and imaging the structure at a depth of 1/4t of the plate thickness t from the outer surface of the electric resistance welded steel pipe using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times). The area ratio of bainite was determined from the obtained optical microscope image and SEM image. The area ratio of bainite was observed in 5 visual fields or more, and calculated as an average value of values obtained in each visual field.

The average effective grain size (average equivalent circle diameter) of bainite was measured by the SEM/EBSD method. Regarding the effective grain size, the misorientation between adjacent grains is determined, and when a region surrounded by a boundary where the misorientation is 15 ° or more is taken as an effective grain, the diameter of a circle having an area equal to that of the effective grain is taken as the effective grain size of bainite. The arithmetic mean of the obtained effective particle diameters was obtained as the average circle-equivalent diameter. The measurement region was set to 500. Mu. M.times.500. Mu.m, and the measurement step size was set to 0.5. Mu.m. In the crystal grain size analysis, crystal grains having an effective grain size of 2.0 μm or less are excluded from the analysis object as measurement noise.

The average aspect ratio of bainite was determined as follows: the length of each effective crystal grain measured by the above method in the thickness direction and the length of each effective crystal grain in the pipe axis direction are measured, and the average of the lengths is calculated to determine the average aspect ratio of bainite. The length in the thickness direction and the length in the tube axis direction are set to the maximum length of each effective crystal grain in the thickness direction and the tube axis direction.

[ tensile test ]

In the tensile test, JIS5 tensile test pieces were cut out at a position of 90 ° in the circumferential direction so that the tensile direction was parallel to the tube axial direction, with the electric resistance welded portion of the obtained electric resistance welded steel tube set at 0 °. Tensile test was carried out in accordance with JIS Z2241. The 0.2% yield strength (yield strength YS) and the tensile strength TS were measured to calculate a yield ratio defined by (0.2% yield strength)/(tensile strength).

[ Charpy impact test ]

In the charpy impact test, when the electric resistance welded portion of the obtained electric resistance welded steel pipe was set to 0 °, V-notch test pieces were cut from the position of 90 ° in the circumferential direction at the position of t/2 of the plate thickness so that the longitudinal direction of the test pieces was parallel to the pipe axis direction. The absorption energy (J) was determined by conducting a Charpy impact test at a test temperature of-30 ℃ in accordance with JIS Z2242. The number of test pieces was three, and the average value was calculated to obtain the absorption energy (J).

[ measurement of residual stress ]

The residual stress was measured by X-ray diffraction cos α method using μ -X360 manufactured by Pulstec. The measurement positions of the residual stress were set to the outer surface of the center of the tube length of the obtained electric resistance welded steel tube, and when the electric resistance welded portion was set to 0 °, the residual stress was set to three positions, i.e., the 90 ° position, the 180 ° position, and the 270 ° position. The average of the three obtained measurement values was taken as the residual stress. The stress measurement direction is set to the tube axis direction.

[ compression test of Member ]

In the present invention, in order to evaluate the performance as a steel pipe pile, a member compression test is performed to determine a buckling strength ratio σ cr/σ y (note that σ cr is a buckling stress, and σ y is a material yield strength). When the buckling strength ratio is larger than the reduction coefficient R =0.8+2.5 × t/R (note that t is the plate thickness and R is the radius), it can be determined that the buckling strength, which is important as the performance of the steel pipe pile, is sufficient.

The results are shown in tables 3-1 and 3-2.

As shown in tables 1 to 3-2, the electric resistance welded steel pipes in the range of the present invention were all electric resistance welded steel pipes having a tensile strength of 590MPa or more in the pipe axial direction, a 0.2% yield strength of 450MPa or more, a yield ratio of 80 to 90%, -Charpy absorption energy at 30 ℃ of 70J or more, and a residual stress of 250MPa or less in the pipe axial direction on the outer surface of the pipe. It is also known that the electric resistance welded steel pipe having these characteristics has sufficient buckling strength which is important as the performance of the steel pipe pile.

On the other hand, in the steel pipe having a composition, a steel structure and production conditions outside the ranges of the present invention, at least one of the tensile strength in the pipe longitudinal direction, the 0.2% yield strength, the yield ratio, the charpy absorption energy at-30 ℃ and the residual stress in the pipe axial direction of the outer surface of the pipe is not the value targeted in the present invention.

As described above, by setting the composition, steel structure and production conditions of the electric resistance welded steel pipe within the range of the present invention, it is possible to provide an electric resistance welded steel pipe which is suitable as a steel pipe pile, has an optimum yield ratio and high buckling resistance, and has both high strength and high toughness.

Claims

1. An electric resistance welded steel pipe having a base material portion and a welded portion in a pipe axial direction, wherein,

the composition of the matrix part in mass% C:0.12 to 0.20%, si:0.60% or less, mn:0.50 to 1.70%, P:0.030% or less, S:0.015% or less, al:0.010 to 0.060%, nb:0.010 to 0.080%, ti:0.010 to 0.050%, N:0.006% or less, and the balance consisting of Fe and unavoidable impurities,

when the thickness of the parent metal portion is defined as t, bainite is present in an area ratio of 60% or more in the steel structure at a depth of 1/4t of the thickness t from the outer surface of the electric resistance welded steel pipe, the average effective grain diameter of the bainite is 20.0 μm or less in terms of the average equivalent circle diameter, and the average aspect ratio of the bainite (the average of the length in the thickness direction)/(the average of the length in the pipe axis direction) is 0.1 to 0.8,

the residual stress of the outer surface of the steel pipe of the parent metal part in the pipe axial direction is 250MPa or less.

2. An electric resistance welded steel pipe as claimed in claim 1, characterized by further comprising, in mass%, in addition to said composition, B: less than 0.008 percent.

3. An electric resistance welded steel pipe as claimed in claim 1 or 2, wherein said electric resistance welded steel pipe further comprises, in mass%, a component selected from the group consisting of Cr:0.01 to 1.0%, V:0.010 to 0.060%, mo:0.01 to 1.0%, cu:0.01 to 0.50%, ni:0.01 to 1.0%, ca: 0.0005-0.010% of one or more than two.

4. A method for manufacturing an electric resistance welded steel pipe, wherein a hot-rolled steel sheet is manufactured by sequentially performing a hot-rolling step and a cooling step on a steel material, and a cold-roll forming step is further performed on the hot-rolled steel sheet to manufacture an electric resistance welded steel pipe,

in the manufacturing method,

the steel material has the composition according to any one of claims 1 to 3,

the hot rolling process comprises the following steps: heating the steel material to a heating temperature of 1100 to 1280 ℃, then performing rough rolling and finish rolling at a rough rolling finishing temperature of 850 to 1150 ℃, a finish rolling finishing temperature of 750 to 850 ℃, and a total reduction ratio of 930 ℃ or lower in the rough rolling and the finish rolling of 65% or more to produce a hot rolled sheet,

the cooling process comprises the following steps: cooling the hot-rolled sheet under the conditions that the average cooling rate from the start of cooling to the stop of cooling by a central thermometer of the sheet thickness is 5-25 ℃/s and the temperature at which cooling is stopped is 450-650 ℃,

in the cold roll forming step, a steel pipe material obtained by subjecting the hot-rolled steel sheet to roll forming is welded, and diameter reduction rolling is performed so that the diameter reduction ratio is 0.2 to 0.5% with respect to the circumferential length of the outer surface of the steel pipe after welding.

5. A method for producing an electric resistance welded steel pipe, wherein a cold roll forming step is performed on a hot-rolled steel sheet having a composition as defined in any one of claims 1 to 3, wherein, assuming that the sheet thickness is t, in a steel structure at a depth of 1/4t of the sheet thickness t from an outer surface, bainite is 60% or more in area ratio, an average effective grain diameter of the bainite is 20.0 μm or less in average equivalent circular diameter, and an average aspect ratio of the bainite is 0.1 to 0.8,

in the manufacturing method,

6. A steel pipe pile using the electric resistance welded steel pipe according to any one of claims 1 to 3.