KR102551615B1 - Electric resistance steel pipe, manufacturing method thereof, and steel pipe pile - Google Patents

Abstract

An electric resistance welded pipe having a welded portion in the axial direction of the base metal portion, wherein the component composition of the base metal portion has a specific composition, and when the plate thickness of the base metal portion is t, a depth of 1/4t of the plate thickness t from the outer surface of the electric resistance welded pipe The steel structure at the position is 70% or more of bainite in terms of area ratio, the average effective grain size of bainite is 10.0 µm or less in terms of average equivalent circle diameter, and the average aspect ratio of bainite is 0.1 to 0.8 in the tube axial direction. Tensile strength of 590 MPa or more, 0.2% yield strength of 450 MPa or more, yield ratio of 85 to 95%, Charpy absorbed energy at -30 ° C. with the tube axial direction in the base material portion being the test piece longitudinal direction is 70 J or more An electric resistance welded steel pipe having a residual stress in the pipe axial direction of the outer surface of the steel pipe in a base material portion of 250 MPa or less, a manufacturing method thereof, and a steel pipe pile.

Description

Electric resistance steel pipe, manufacturing method thereof, and steel pipe pile

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 manufacturing method thereof, and a steel pipe pile. In particular, the present invention uses a hot-rolled steel sheet (hot rolled steel strip) as a material, and the high-strength, high-toughness, optimization of yield ratio, and buckling resistance ( buckling resistance) performance.

In recent years, as a response to large-scale earthquakes, there has been a strong demand for higher strength and improvement in strain energy absorption capacity also for steel pipe piles used as foundations of structures. Generally, in order to improve the strain energy absorption capacity of a steel pipe, it is effective to use a steel material having a high tensile strength and a low yield ratio. However, it is difficult for steel pipe piles to excessively lower the yield ratio in the pipe axis direction from the viewpoint of suppressing deformation of the steel pipe during pile driving. In addition, high low-temperature toughness is also required for steel pipe piles used particularly in cold regions. In addition, high buckling resistance is required in order to withstand deformation due to an earthquake or the like.

Patent Literature 1 describes a method for producing an earthquake-resistant welded steel pipe having excellent internal buckling properties. In Patent Literature 1, by weight, C: 0.03 to 0.15%, Mn: 1.0 to 2.0% are contained, 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 hot rolling a steel having a composition such that Pcm is 0.10 to 0.25, and after completion of rolling A steel sheet obtained by cooling to 600°C or less at a cooling rate of 5°C/s or more is cold formed to obtain a steel pipe. As a result, it is possible to obtain a steel pipe with excellent deformation performance having a strain hardening index of 0.10 or more in a tensile test in the pipe axial direction, resulting in local buckling due to external force acting on the steel pipe from the side, and brittleness caused therefrom. It is said that occurrence of brittle cracking or fracture can be prevented.

In Patent Literature 2, in weight%, C: 0.02 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.50 to 2.00% are included, and Cu: 0.10 to 1.5%, Ni: 0.10 to 0.50%, Nb : 0.005 to 0.10% and V: 0.005 to 0.10%, including one or two or more selected from the group consisting of Ceq: 0.38 to 0.45, the reduction ratio per pass in the temperature range of 900 ° C. or higher is Hot-rolling is performed so that it is 4% or less to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is reheated in a dual-phase temperature region of Ac1 point or higher and Ac3 point or lower, and quenching from the dual-phase temperature region A method for manufacturing a steel pipe is described in which a steel pipe is further subjected to tempering and then a pipe-making process is performed. The steel pipe thus obtained is a high tensile steel pipe with a low yield ratio of 0.2% yield strength: 440 MPa or more, tensile strength: 590 to 700 MPa, yield ratio: 80% or less, and is said to be suitable for use in steel structures such as buildings, bridges, and tanks. .

In Patent Literature 3, in manufacturing a steel pipe having a composition containing, in mass%, C: 0.10 to 0.18%, Si: 0.1 to 0.5%, and Mn: 1 to 2%, a step of heating to Ac3 or higher and then rapidly cooling And, after heating to the two-phase temperature range of the Ac1 point to the Ac3 point, the step of air cooling, the step of forming into a tubular shape in cold, and the step of reheating at 500 to 600 ° C. A method for producing a steel pipe is described. Accordingly, it is claimed that steel pipes for building structures having a tensile strength of 590 MPa or more can be manufactured without using expensive alloy elements.

Patent Literature 4 contains C: 0.11 to 0.20%, Si: 0.05 to 0.50%, Mn: 1.00 to 2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.01 to 0.08%, in terms of mass%. In addition, the ferrite phase is the main phase, the second phase other than the main phase is pearlite and/or pseudo pearlite with an area ratio of 8 to 30%, and the average particle diameter including the main phase and the second phase is 4.0 to 10 μm A low yield ratio high-strength electric resistance welded steel pipe for steel pipe piles having a phosphorus structure, a 0.2% proof stress YS: 450 MPa or more, a tensile strength TS: 590 MPa or more, and a yield ratio: 90% or less in the pipe circumferential direction and the pipe axial direction is described. .

Japanese Laid-Open Patent Publication No. 11-6032 Japanese Patent No. 2687841 Japanese Unexamined Patent Publication No. 2004-300461 Japanese Patent No. 6123734

However, in the steel pipe manufactured by the technique described in Patent Document 1, the yield ratio in the pipe axial direction is excessively lowered. For this reason, when the obtained steel pipe is applied as a steel pipe pile, there is a possibility of causing problems such as buckling due to driving in at the time of pile driving.

The technology described in Patent Literature 2 requires a heat treatment step for tempering. Further, in the technique described in Patent Literature 3, a large-sized pipe heat treatment apparatus is required, and a heat treatment step is required after canning. In technologies requiring these heat treatments, there is a problem that the yield ratio becomes too low. In addition, there is also a problem that the productivity is reduced due to the complexity of the process. In addition, the production cost increases, making it difficult to provide at low cost.

In the technique described in Patent Literature 4, after hot rolling, cooling from the finish rolling end temperature to a temperature range of 550 to 700 ° C. in 10 to 100 s is performed to obtain a structure mainly composed of ferrite and pearlite, but the desired structure is not obtained. there is. In addition, equipment with a very long cooling zone is required, making it difficult to provide inexpensive high-strength, high-toughness electric resistance welded steel pipes for steel pipe piles.

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

Further, in the present invention, when a hot-rolled steel sheet having a sheet thickness of 16 mm or more is mainly used as a raw material, an electric resistance welded steel pipe capable of achieving the above object, a manufacturing method thereof, and a steel pipe pile are also provided.

"High strength" as used herein means 0.2% yield strength (YS): 450 MPa or more, tensile strength (TS): 590 MPa in the tube axial direction in the base metal zone of the electric resistance welded steel pipe. refers to the case of more than "High toughness" as used herein refers to the case where the Charpy absorbed energy at -30 ° C is 70 J or more, with the tube axial direction in the base material portion of the electric resistance welded steel pipe being the longitudinal direction of the test piece. In other words, the above-described high toughness is satisfied in both the pipe circumferential direction and the pipe axial direction of the electric resistance welded steel pipe. The term "optimal yield ratio" as used herein refers to a ratio (YR) of 80 to 90% of the 0.2% yield strength to the tensile strength described above. "High buckling resistance" as used herein refers to the case where the residual stress in the tube axial direction of the outer surface of the steel pipe in the base material portion of the electric resistance welded steel pipe is 250 MPa or less, and the yield ratio is 90% or less.

In order to achieve the above object, the present inventors intensively studied the influence of various alloying elements and manufacturing conditions on yield ratio, 0.2% proof stress, tensile strength, and Charpy impact properties. In addition, the buckling resistance of the obtained steel pipe (electrically welded steel pipe) was also intensively studied. As a result, it was recognized that there is an appropriate component composition, steel structure, and manufacturing conditions capable of achieving both high strength and high toughness while maintaining a relatively low yield ratio and having high buckling resistance.

That is, in the cold roll forming process by cold roll forming, a hot-rolled steel sheet manufactured under specific component composition and hot rolling conditions is subjected to diameter reduction rolling under specific conditions after welding. As a result, the steel structure at a depth of 1/4t of the plate thickness t from the outer surface of the steel pipe of the base material portion of the electric resistance welded steel pipe is 60% or more in area ratio of bainite, and the average effective grain size of bainite is equivalent to an average circle The diameter is 20.0 μm or less, and the average aspect ratio of bainite is 0.1 to 0.8. As a result, the 0.2% proof stress is relatively low at 450 MPa or more, the tensile strength is as high as 590 MPa or more, the yield ratio is 80 to 90%, the Charpy absorbed energy at -30°C is 70 J or more, and It was found that an electric resistance welded steel pipe having a low yield ratio, high strength, high toughness, and high buckling resistance in which the residual stress in the pipe axial direction of the outer surface of the steel pipe is 250 MPa or less can be obtained.

The present invention was completed by further examination based on such recognition, and the gist of the present invention is as follows.

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

The component composition of the base material portion is 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

, the remainder being Fe and unavoidable impurities,

When the plate thickness of the base material is t, the steel structure at a depth of 1/4t of the plate thickness t from the outer surface of the electric resistance welded pipe is

Bainite is 60% or more in area ratio,

The average effective grain size of the bainite is 20.0 μm or less in average equivalent circle diameter, and the average aspect ratio of the bainite is 0.1 to 0.8,

The tensile strength in the tube axial direction is 590 MPa or more, the 0.2% yield strength is 450 MPa or more, and the yield ratio is 80 to 90%,

The Charpy absorbed energy at -30 ° C with the tube axial direction in the base material portion as the longitudinal direction of the test piece is 70 J or more,

An electric resistance welded pipe having a residual stress in the pipe axial direction of the outer surface of the steel pipe in the base material portion of 250 MPa or less.

[2] The electric resistance welded pipe according to [1], which further contains, in mass%, B: 0.008% or less in addition to the above component composition.

[3] In addition to the above component composition, in mass%,

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 to 0.010%

The electric resistance welded steel pipe according to [1] or [2], containing one or two or more selected from among.

[4] A method for producing an electric resistance welded steel pipe in which a hot rolled steel sheet is obtained by subjecting a steel material to a hot rolling process and a cooling process in this order, and further, a cold roll forming process is performed on the hot rolled steel sheet to obtain an electric resistance welded steel pipe,

The steel material has the component composition described in any one of [1] to [3],

In the hot rolling process, after heating the steel material at a heating temperature of 1100 to 1280° C., a rough rolling end temperature of 850 to 1150° C. and a finish rolling end temperature of 750 to 850° C., furthermore, in the rough rolling and the finish rolling The total reduction ratio at 930 ° C. or less in

The cooling step is a step of cooling the hot-rolled sheet at an average cooling rate from the start of cooling to the stop of cooling at a central temperature of the sheet thickness: 5 to 25 ° C./s, and a cooling stop temperature: 450 to 650 ° C.,

In the cold roll forming step, a steel pipe material subjected to roll forming processing is welded to the hot-rolled steel sheet, and a diameter reduction ratio of 0.2 to 0.5% is performed with respect to the circumferential length of the outer surface of the steel pipe after welding. A manufacturing method of an electric resistance welded steel pipe.

[5] It has the component composition described in any one of [1] to [3], and when the plate thickness is t, the steel structure at a depth of 1/4t of the plate thickness t from the outer surface has bainite as an area The hot-rolled steel sheet having a ratio of 60% or more, the average effective grain diameter of the bainite being 20.0 μm or less in terms of the average equivalent circle diameter, and the average aspect ratio of the bainite being 0.1 to 0.8 is subjected to a cold roll forming process to obtain an electric resistance welded steel pipe. As a method of manufacturing an electric resistance welded pipe,

In the cold roll forming step, a steel pipe material subjected to roll forming processing is welded to the hot-rolled steel sheet, and a diameter reduction ratio of 0.2 to 0.5% is performed with respect to the circumferential length of the outer surface of the steel pipe after welding. A manufacturing method of an electric resistance welded steel pipe.

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

According to the present invention, it is possible to provide an electric resistance welded steel pipe, a manufacturing method thereof, and a steel pipe pile, which are suitably used as steel pipe piles, have an optimum yield ratio and high buckling resistance, and further have high strength and high toughness. The electric resistance welded pipe of the present invention can be easily manufactured, bringing about a remarkable industrial effect.

(Mode for implementing the invention)

Hereinafter, the present invention will be described in detail.

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

The electric resistance welded pipe of the present invention has a base metal portion and a welded portion, and the base metal portion 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: It has a component composition containing 0.010 to 0.060%, Nb: 0.010 to 0.080%, Ti: 0.010 to 0.050%, and N: 0.006% or less, the remainder being Fe and unavoidable impurities.

In addition, the electric resistance welded pipe of the present invention has a welded portion in the pipe axial direction. The "hot-rolled steel sheet" described later shall include a hot-rolled steel sheet and a hot-rolled steel strip.

C: 0.12 to 0.20%

C is an element involved in the formation of steel structures such as bainite while increasing the strength of steel pipes (electrically welded steel pipes) by solid solution strengthening. In addition, C is an element effective for optimizing the yield ratio. A steel pipe having a relatively large plate thickness (for example, a steel pipe having a plate thickness of 16 mm or more) has a large difference between the outer diameter and the inner diameter, so the degree of workability at the time of manufacturing the steel pipe is large and the yield ratio is likely to increase. For this reason, it is necessary to contain a large amount of C. Therefore, in order to obtain the above effect, the content of 0.12% or more of C is required. On the other hand, if the content of C exceeds 0.20%, martensite tends to be formed, and the target steel structure in the present invention cannot be obtained. As a result, it becomes impossible to ensure the high toughness targeted in the present invention. Therefore, C is 0.12 to 0.20%. C is preferably 0.13% or more, more preferably 0.14% or more. C is preferably 0.19% or less, more preferably 0.18% or less.

Si: 0.60% or less

Si is an element that can increase the strength of a steel pipe while acting as a deoxidizer. However, when Si is contained excessively, toughness will fall. From these points, Si is made 0.60% or less. Si is preferably 0.50% or less, more preferably 0.45% or less. In addition, the lower limit of Si is not particularly specified, but from the viewpoint of electric resistance weldability, it is preferably set to 0.01% or more. More preferably, it is 0.02% or more.

Mn: 0.50 to 1.70%

Mn is an element that increases the strength of a steel pipe through solid solution strengthening. In order to obtain these effects and to ensure high strength, which is the purpose of the present invention, Mn content of 0.50% or more is required. On the other hand, when Mn is contained exceeding 1.70%, the steel structure is refined, the yield strength increases, and the target yield ratio in the present invention cannot be secured. For this reason, Mn is made into 0.50 to 1.70%. Mn is preferably 0.55% or more, more preferably 0.60% or more. Mn is preferably 1.65% or less, more preferably 1.60% or less.

P: 0.030% or less

P is an element that reduces toughness by segregating at grain boundaries, and it is desirable to reduce P as much as possible as an impurity, but in the present invention, up to 0.030% is permissible. From these points, P is made 0.030% or less. P is preferably 0.025% or less, more preferably 0.020% or less. However, excessive reduction of P causes an increase in refining cost, so P is preferably set to 0.002% or more. More preferably, it is 0.003% or more.

S: 0.015% or less

S exists as MnS in steel when producing a hot-rolled steel sheet, which is a material for steel pipes, and adversely affects the ductility and toughness of steel pipes by being thinly stretched in a hot rolling process. For this reason, in the present invention, it is desirable to reduce S as an impurity as much as possible, but the inclusion of S is acceptable up to 0.015%. For this reason, S is made into 0.015% or less. S is preferably 0.010% or less, more preferably 0.008% or less. However, since excessive reduction of S causes an increase in refining cost, S is preferably set to 0.0002% or more. More preferably, it is 0.001% or more.

Al: 0.010 to 0.060%

Al, while acting as a deoxidizer, combines with N to form AlN, contributing to refinement of crystal grains. In order to obtain such an effect, it is necessary to contain 0.010% or more of Al. On the other hand, the inclusion of a large amount of Al exceeding 0.060% lowers the cleanliness of steel materials (hot-rolled steel sheet as a raw material of steel pipes) and lowers the ductility and toughness of steel pipes. For this reason, Al is 0.010 to 0.060%. Al is preferably 0.015% or more, more preferably 0.020% or more. Al is preferably 0.055% or less, more preferably 0.050% or less.

Nb: 0.010 to 0.080%

Nb combines with carbon or nitrogen to form fine precipitates, and increases the strength of steel pipes by precipitate strengthening. In order to obtain such an effect, it is necessary to contain 0.010% or more of Nb. On the other hand, when Nb is contained in excess of 0.080%, when manufacturing a hot-rolled steel sheet, which is a raw material for a steel pipe, it becomes difficult to form a solid solution by heating in a hot rolling step. As a result, it remains as a coarse precipitate and the toughness decreases. For this reason, Nb is made into 0.010 to 0.080%. Nb is preferably 0.015% or more, more preferably 0.020% or more. Nb is preferably 0.075% or less, more preferably 0.070% or less.

Ti: 0.010 to 0.050%

Ti combines with carbon or nitrogen to form fine precipitates, and increases the strength of steel pipes 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 exceeding 0.050%, precipitates coarsen and toughness decreases. For this reason, Ti is made into 0.010 to 0.050%. Ti is preferably 0.012% or more, more preferably 0.015% or more. Ti is preferably 0.045% or less, more preferably 0.040% or less.

N: 0.006% or less

N has an effect of increasing the strength of a steel pipe when it is in a small amount, but when it is contained in a large amount, it forms coarse precipitates at high temperatures and lowers the toughness. For this reason, N is made into 0.006% or less. Since an excessive reduction of N causes an increase in refining cost, it is preferably 0.001% or more, more preferably 0.002% or more. N is preferably 0.005% or less, more preferably 0.004% or less.

The balance is Fe and unavoidable impurities. Incidentally, as an unavoidable impurity, inclusion of O: 0.0050% or less is permissible within a range that does not impair the effects of the present invention.

The components described above are the basic component composition of the electric resistance welded pipe in the present invention. Although the characteristics targeted in the present invention are obtained from the above essential elements, in addition to this basic component composition, the following elements may be further contained as needed.

B: 0.008% or less

B is an element that contributes to refinement of the steel structure by lowering the ferrite transformation start temperature, and may be contained as necessary. However, when the content of B exceeds 0.008%, it tends to segregate at grain boundaries and there is a possibility that toughness may decrease. Therefore, when B is contained, it is preferable to make B 0.008% or less. More preferably, it is 0.006% or less. In addition, B is preferably 0.0003% or more.

One or two or more 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%, and Ca: 0.0005 to 0.010%

Cr: 0.01 to 1.0%

Cr is an element that increases the strength of the steel pipe by increasing hardenability, and can be contained as necessary. In order to obtain such an effect, it is preferable to contain 0.01% or more of Cr. On the other hand, since there exists a possibility of reducing toughness and weldability when Cr is contained exceeding 1.0 %, it is preferable to set it as 1.0 % or less. Therefore, when it contains Cr, it is preferable to make Cr into 0.01 to 1.0%. Cr is more preferably 0.02% or more, still more preferably 0.03% or more. Cr is more preferably 0.8% or less, and still more preferably 0.6% or less.

V: 0.010 to 0.060%

V is an element that bonds with carbon or nitrogen to form fine precipitates and increases the strength of steel pipes by precipitation strengthening, and can 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 exceeding 0.060%, the precipitate coarsens, and the above-described effect tends to be saturated. For this reason, V is made into 0.010 to 0.060%. V is preferably 0.012% or more, more preferably 0.015% or more. V is preferably 0.055% or less, more preferably 0.050% or less.

Mo: 0.01 to 1.0%

Mo is an element that increases the strength of the steel pipe by increasing hardenability, and can be contained as necessary. In order to obtain such an effect, it is preferable to contain 0.01% or more of Mo. On the other hand, since there exists a possibility of reducing toughness when Mo is contained exceeding 1.0 %, it is preferable to set it as 1.0 % or less. Therefore, when containing Mo, it is preferable to make Mo into 0.01 to 1.0%. Mo is more preferably 0.02% or more, still more preferably 0.03% or more. Mo is more preferably 0.8% or less, still more preferably 0.6% or less.

Cu: 0.01 to 0.50%

Cu is an element that increases the strength of the steel pipe by solid solution strengthening, and can be contained as necessary. In order to obtain such an effect, it is preferable to contain 0.01% or more of Cu. On the other hand, since there exists a possibility of reducing toughness when Cu is contained exceeding 0.50 %, it is preferable to set it as 0.50 % or less. Therefore, when containing Cu, it is preferable to make Cu into 0.01 to 0.50%. Cu is more preferably 0.02% or more, 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 to 1.0%

Ni is an element that increases the strength of a steel pipe by solid solution strengthening, and may be contained as necessary. In order to obtain such an effect, it is preferable to contain 0.01% or more of Ni. On the other hand, since there exists a possibility of reducing toughness when Ni is contained exceeding 1.0 %, it is preferable to set it as 1.0 % or less. Therefore, when containing Ni, it is preferable to make Ni into 0.01 to 1.0%. Ni is more preferably 0.02% or more, and still more preferably 0.03% or more. Ni is more preferably 0.8% or less, and still more preferably 0.6% or less.

Ca: 0.0005 to 0.010%

Ca is an element that contributes to improving the toughness of steel by sphericizing sulfides such as MnS, which are thinly stretched in the hot rolling process, when producing hot-rolled steel sheets, which are materials for steel pipes, and can be contained as necessary. In order to acquire such an effect, when containing Ca, it is preferable to contain 0.0005% or more. However, when the Ca content exceeds 0.010%, there is a possibility that Ca oxide clusters are formed in the steel and the toughness deteriorates. Therefore, when containing Ca, it is preferable to make Ca 0.0005% - 0.010%. Ca is more preferably 0.0010% or more, still more preferably 0.0015% or more. Ca is more preferably 0.005% or less, still more preferably 0.004% or less.

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

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

Here, the 1/4t depth position of the sheet thickness t is important in controlling the steel structure, and the outermost layer where the cooling rate in the hot rolling process at the time of manufacturing a hot-rolled steel sheet, which is a material for a steel pipe, is the largest, and This is the middle position of the smallest 1/2t depth position. In the present invention, the cross section parallel to the rolling direction at the position of 1/4W of the sheet width W in hot rolling is used as the 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 and the structure of the steel pipe (base material) become the same.

Area ratio of bainite: 60% or more

In the present invention, in order to achieve both high strength and high toughness, it is important to contain bainite in an area ratio of 60% or more. If bainite is less than 60%, it becomes difficult to obtain the target strength 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, the area ratio of bainite is 60% or more. Preferably it is 65% or more. In addition, since the yield ratio becomes too high when the area ratio of bainite is excessive, it is preferable that the area ratio of bainite is 98% or less. More preferably, it is 95% or less.

As the structure (residual structure) other than bainite, ferrite, pearlite, martensite, austenite, or the like can be considered. When the sum of the area ratios of these structures is 40% or more with respect to the entire steel structure, lack of strength or toughness, and an increase or excessive decrease in yield ratio are caused. Therefore, it is preferable to set it as less than 40%. More preferably, it is less than 35%. In consideration of obtaining the target yield ratio in the present invention, the lower limit of the sum of the area ratios of the remaining structures is preferably more than 2%, and more preferably more than 5%.

In the present invention, the area ratio of each tissue described above can be measured by the method described in Examples described later.

Average effective particle diameter of bainite: 20.0 μm or less as an average equivalent circle diameter

In the present invention, in order to achieve both high strength and high toughness, it is important that the average equivalent circle diameter of the average effective grain size of bainite be 20.0 µm or less. If the average effective grain size of bainite exceeds 20.0 µm in terms of average equivalent circular diameter, the target toughness in the present invention cannot be obtained. In addition, in the present invention, the target strength cannot be obtained. Preferably, it is 15.0 μm or less. Also, if the bainite is too fine, the yield ratio becomes too high. Therefore, the average effective average grain diameter of bainite is preferably set to 1.0 μm or more, and more preferably 2.0 μm or more.

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

Average aspect ratio of bainite: 0.1 to 0.8

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

In the present invention, the average of the lengths of the bainite crystal grains in the plate thickness direction and the average of the lengths in the rolling direction can be measured by the method described in Examples described later.

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

In the electric resistance resistance welded pipe of the present invention, for example, a hot-rolling step, a cooling step, and a winding step are performed in this order on a steel material having the above-described component composition to obtain a hot-rolled steel sheet, and further, cold-roll forming the hot-rolled steel sheet. The process is performed to make an electric resistance welded pipe.

Incidentally, in the description of the manufacturing method below, unless otherwise specified, "°C" for temperature is the surface temperature of a steel material or steel sheet (hot-rolled steel sheet). These surface temperatures can be measured with a radiation thermometer or the like. In addition, the temperature at the center of the sheet thickness of the steel sheet can be obtained by calculating the temperature distribution in the cross section of the steel sheet by electrothermal analysis and correcting the result with the surface temperature of the steel sheet. In addition, "hot-rolled steel sheet" shall include a hot-rolled steel sheet and a hot-rolled steel strip.

In the present invention, the melting method of the steel material (steel slab) does not need to be particularly limited. The molten steel having the above component composition is melted by a commercial melting method such as a converter, an electric furnace, or a vacuum melting furnace, and then is melted by a commercial casting method such as a continuous casting method. It is preferable to use cast steel as a slab or the like from the viewpoints of quality, productivity, and the like. In addition, there is no problem even if an ingot casting-slabbing process is applied instead of the continuous casting method. Secondary refining such as ladle refining may be further applied to the molten steel.

Next, a hot rolling process is performed on the obtained steel raw material (steel slab). In the hot rolling process, after heating the steel material at a heating temperature of 1100 to 1280°C, rough rolling is performed at a rough rolling end temperature of 850 to 1150°C, and finish rolling is performed at a finish rolling end temperature of 750 to 850°C. It is a step of performing hot rolling with a total reduction ratio of 65% or more at 930°C or less in rough rolling and finish rolling to obtain a hot-rolled sheet.

Heating temperature: 1100 to 1280°C

When the heating temperature is less than 1100°C, coarse carbides present in the steel material formed during casting cannot be dissolved into solid solution. As a result, the effects of the contained carbide-forming elements cannot be sufficiently obtained. On the other hand, when the heating temperature exceeds 1280 ° C., crystal grains significantly coarsen, and the structure of the hot-rolled steel sheet, which is the material of the steel pipe, coarsens, making it difficult to secure the desired characteristics in the present invention. For this reason, the heating temperature of the raw material steel needs to be 1100 to 1280°C. Preferably it is 1120-1230 degreeC. In addition, let this temperature be set temperature in the furnace of a heating furnace.

Rough rolling end temperature: 850 to 1150 ° C.

When the rough rolling end temperature is less than 850°C, recovery of the structure during hot rolling does not occur, and crystal grains excessively elongated in the rolling direction tend to be generated. As a result, the average aspect ratio of bainite tends to be less than 0.1. On the other hand, when the rough rolling end temperature exceeds 1150°C, the amount of reduction in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the average effective grain size of bainite targeted in the present invention It becomes difficult to secure For this reason, the rough rolling completion|finish temperature is made into 850-1150 degreeC. Preferably it is 860-1000 degreeC.

Finish rolling end temperature: 750 to 850 ° C.

When the finish rolling end temperature is less than 750°C, recovery of the structure during hot rolling does not occur, and crystal grains excessively elongated in the rolling direction tend to be generated. As a result, the average aspect ratio of bainite tends to be less than 0.1. On the other hand, when the finish rolling end temperature exceeds 850°C, the amount of reduction in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the average effective grain size of bainite targeted in the present invention It becomes difficult to secure For this reason, the finish rolling end temperature is set to 750 to 850°C. Preferably it is 770-830 degreeC.

Total reduction ratio at 930 ° C. or less in rough rolling and finish rolling: 65% or more

In the present invention, by refining austenite in the hot rolling process, bainite and the remaining structure formed in the subsequent cooling process and winding process are refined, which is suitable as a material for an electric resistance welded steel pipe having strength and toughness aimed at in the present invention. A hot-rolled steel sheet can be obtained. In order to refine austenite in the hot rolling process, it is necessary to increase the reduction ratio in the austenite non-recrystallization temperature region and introduce sufficient processing strain. In order to obtain this effect, in the present invention, the total rolling reduction in the temperature range up to the finish rolling end temperature of 930°C or less is set to 65% or more. Here, the total reduction ratio refers to the total reduction ratio of each rolling pass in the temperature range up to the finish rolling end temperature of 930°C or less.

When the total rolling reduction in the temperature range up to the finish rolling end temperature of 930 ° C. or less is less than 65%, since sufficient processing strain cannot be introduced in the hot rolling process, the average effective grain size of bainite targeted in the present invention A strong structure with is not obtained. The total reduction ratio in the temperature range up to the finish rolling end temperature of 930°C or less is more preferably 70% or more. There is no particular upper limit, but if it exceeds 80%, the effect of improving toughness against the increase in reduction ratio will be small, and the equipment load will only increase. For this reason, the total rolling reduction in the temperature range up to the finish rolling end temperature of 930°C or less is preferably 80% or less. More preferably, it is 75% or less.

The reason why the temperature is 930°C or lower in the present invention is that at a temperature higher than 930°C, austenite recrystallizes in the hot rolling step, dislocations introduced by rolling are lost, and refined austenite is not obtained.

Further, in the present invention, when hot rolling a steel material, in both the above-described rough rolling and finish rolling, the total reduction ratio up to the finish rolling end temperature of 930 ° C. or less may be hot rolling in which the total reduction ratio is 65% or more, It is good also as hot rolling which makes the total rolling reduction up to 930 degreeC or less finish-rolling end temperature of 65% or more only by finish-rolling. In the latter case, when the total reduction ratio up to the finish rolling end temperature of 930°C or less cannot be made 65% or more only by finish rolling, the slab is cooled in the middle of rough rolling to bring the temperature to 930°C or less, and then rough rolling The total reduction ratio up to the finish rolling end temperature of 930° C. or less in both the finish rolling and the finish rolling may be 65% or more.

Next, a cooling process is given to the hot-rolled sheet after a hot rolling process. In the cooling step, the hot-rolled sheet is cooled at an average cooling rate from the start of cooling to the stop of cooling from the center temperature of the sheet thickness: 5 to 25°C/s, and the cooling stop temperature: 450 to 650°C.

Average cooling rate from start of cooling to stop of cooling: 5 to 25°C/s

When the average cooling rate in the temperature range from the center temperature of the sheet thickness of the hot-rolled sheet to the cooling start temperature to the cooling stop temperature described later is less than 5 ° C./s, the area ratio of bainite decreases due to the formation of ferrite, In the present invention, the target strength cannot be obtained. On the other hand, when the average cooling rate exceeds 25° C./s, the average aspect ratio of bainite exceeds 0.8. As a result, the yield ratio easily exceeds 90%. The average cooling rate is preferably 10°C/s or more, and preferably 20°C/s or less.

In the present invention, unless otherwise specified, the average cooling rate is a value (cooling average speed). The cooling method includes, for example, water cooling by spraying water from a nozzle or cooling by spraying a cooling gas. In the present invention, it is preferable to perform a cooling operation (treatment) on both surfaces of the hot-rolled sheet so that both surfaces of the hot-rolled sheet are cooled under the same conditions.

Cooling stop temperature: 450 to 650°C

When the cooling stop temperature is less than 450°C at the center temperature of the sheet thickness of the hot-rolled sheet, the average aspect ratio of bainite exceeds 0.8, and as a result, the yield ratio tends to exceed 90%. On the other hand, if the cooling stop temperature exceeds 650°C, the area ratio of bainite cannot be set to 60% or more because it exceeds the bainite transformation start temperature. The cooling stop temperature is preferably 480°C or higher, and preferably 620°C or lower.

Next, a coiling process in which the hot-rolled steel sheet after the cooling process is wound up and then allowed to cool is performed.

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

Next, a cold roll forming step is applied to the hot-rolled steel sheet after the coiling step. In the cold roll forming process, a hot-rolled steel sheet is formed into a cylindrical open pipe by cold-rolling it, and both ends of the steel pipe material (that is, the abutted portion of the open pipe) are electrically welded, and the circular steel pipe after welding is welded. Diameter reduction rolling is performed with a reduction in diameter of 0.2 to 0.5% with respect to the circumferential length of the outer surface of the steel pipe.

Diameter reduction ratio in diameter reduction rolling: 0.2 to 0.5%

When the diameter reduction ratio in diametral reduction rolling is less than 0.2%, reduction of residual stress due to plastic deformation becomes insufficient in the steel material for the steel pipe of the present invention described above. As a result, the residual stress in the pipe axial direction on the outer surface of the steel pipe exceeds 250 MPa. In addition, the yield ratio becomes less than 80% due to lack of workability. On the other hand, when the diameter reduction ratio in diameter reduction rolling exceeds 0.5%, the yield ratio exceeds 90% due to work hardening. As a result, desired plastic deformation capacity, that is, buckling resistance cannot be obtained. In addition, even if the residual stress exceeds 250 MPa, the buckling resistance deteriorates.

As a result of the above, the electric resistance welded pipe of the present invention is manufactured. According to the present invention, the tensile strength in the longitudinal direction of the pipe is 590 MPa or more, the 0.2% proof stress is 450 MPa or more, the yield ratio is 80 to 90%, the Charpy absorbed energy at -30 ° C is 70 J or more, and the outer surface of the steel pipe is An electric resistance welded steel pipe having a residual stress in the pipe axial direction of 250 MPa or less is obtained. Accordingly, it is possible to easily manufacture an electric resistance welded pipe having high strength, high toughness, optimal yield ratio and excellent buckling resistance. Since this electric resistance welded pipe can be used suitably for steel pipe piles used as foundations of structures in particular, it brings significant industrial effects.

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

The steel pipe pile of the present invention is composed of an electric resistance welded steel pipe having a plate thickness of 16 mm or more, an outer diameter of 300 mm or more and 700 mm or less, and having the above-described component composition and steel structure. By defining the component composition and steel structure of the electric resistance welded steel pipe as described above, the tensile strength in the longitudinal direction of the pipe is 590 MPa or more, the 0.2% proof stress is 450 MPa or more, the yield ratio is 80 to 90%, and the Charpy absorption at -30 ° C. A steel pipe pile having an energy of 70 J or more and a residual stress in the pipe axial direction on the outer surface of the steel pipe is 250 MPa or less. The steel pipe pile of the present invention is driven into the ground and, if necessary, is connected to each other by welding or screw joints or other connection means during driving, and is constructed as a long pile on site. According to the steel pipe pile of the present invention, since it has the above characteristics, it is possible to reduce the possibility of causing problems such as buckling in driving the pile.

Example

Hereinafter, the present invention will be further described in detail based on examples. In addition, this invention is not limited to the following example.

Molten steel having the component composition shown in Table 1 was smelted in a converter, and a slab (steel material: thickness 250 mm) was obtained by a continuous casting method. The obtained slab was subjected to a hot rolling process, a cooling process, a winding process, and a cold roll forming process under the manufacturing conditions shown in Tables 2-1 and 2-2, and the outer diameters and plates shown in Tables 2-1 and 2-2. An electric resistance welded pipe of thickness was manufactured. In addition, in the cold roll forming process, the butted portion of the open pipe was electrically welded.

A test piece was taken from the obtained electric resistance welded pipe, and structure observation, tensile test, Charpy impact test, measurement of residual stress, and member compression test were conducted by the methods shown below.

[Tissue observation]

A test piece for structure observation was prepared by taking an end surface in the axial direction of the tube at a position of 90° in the circumferential direction when the angle of the electric resistance weld was set to 0°, polishing, and performing nital etching. For tissue observation, observe the tissue at a depth of 1/4t of the plate thickness t from the outer surface of the electric resistance welded pipe using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) and photographed. From the obtained optical microscope image and SEM image, the area ratio of bainite was determined. The area ratio of bainite was observed in five or more fields of view, and was calculated as an average value of values obtained in each field of view.

In addition, the average effective particle diameter (average equivalent circular diameter) of bainite was measured using the SEM/EBSD method. For the effective grain size, the orientation difference between adjacent crystal grains was obtained, and when the region surrounded by the boundary with the orientation difference of 15 ° or more was regarded as the effective crystal grain, the diameter of a circle equal in area to the effective crystal grain was used as the effective grain size of bainite. The arithmetic average of the obtained effective particle diameters was determined, and the average equivalent circle diameter was obtained. The measurement area was 500 μm × 500 μm, and the measurement step size was 0.5 μm. In addition, in the crystal grain size analysis, those having an effective grain size of 2.0 µm or less were excluded from the analysis target as measurement noises.

In addition, the average aspect ratio of bainite was obtained by measuring the length in the sheet thickness direction and the length in the tube axial direction of each effective crystal grain measured by the above method, and calculating the average of each. The length in the sheet thickness direction and the tube axial direction were the maximum lengths of each effective crystal grain in the sheet thickness direction and the tube axial direction, respectively.

[Tensile test]

In the tensile test, a JIS No. 5 tensile test piece was taken so that the tensile direction was parallel to the pipe axial direction at a position of 90 ° in the circumferential direction when the electric resistance welded portion of the obtained electric resistance welded pipe was 0 °. A tensile test was conducted in accordance with the provisions of JIS Z 2241. The 0.2% yield strength (yield strength YS) and the tensile strength TS were measured, and the yield ratio defined by (0.2% yield strength)/(tensile strength) was calculated.

[Charpy impact test]

In the Charpy impact test, at the position of 90 ° in the circumferential direction when the electric resistance welded portion of the obtained electric resistance welded pipe is 0 °, from the plate thickness t / 2 position, the longitudinal direction of the test piece is parallel to the tube axial direction, V notch test piece (V -notch test specimen) was collected. In accordance with the provisions of JIS Z 2242, a Charpy impact test was conducted at a test temperature: -30°C, and absorbed energy (J) was determined. In addition, the number of test pieces was set to 3 each, and the average value was calculated to determine the absorbed energy (J).

[Measurement of residual stress]

The residual stress was measured by the X-ray diffraction cosα method using µ-X360 manufactured by Pulsetech. The measurement position of the residual stress was the outer surface of the center of the pipe length of the obtained electric resistance welded pipe, and when the electric resistance welded part was 0 °, the 90 ° position, the 180 ° position, and the 270 ° position were three locations. The average value of the obtained three measured values was made into the residual stress. In addition, the stress measurement direction was the pipe axis direction.

[Member Compression Test]

In the present invention, in order to evaluate performance as a steel pipe pile, a member compression test was conducted to obtain a buckling strength ratio σcr/σy (where σcr is the buckling stress and σy is the material yield strength). If the buckling strength ratio is greater than the reduction coefficient R = 0.8 + 2.5 × t / r (where t is the plate thickness and r is the radius), it can be judged that the buckling strength, which is important as the performance of the steel pipe pile, is sufficient.

The obtained results are shown in Table 3-1 and Table 3-2, respectively.

(Table 1)

(Table 2-1)

(Table 2-2)

(Table 3-1)

(Table 3-2)

As shown in Tables 1 to 3-2, all electric resistance welded steel pipes within the scope of the present invention have a tensile strength of 590 MPa or more in the tube axial direction, a 0.2% proof stress of 450 MPa or more, and a yield ratio of 80 to 90%, - The Charpy absorbed energy at 30°C was 70 J or more, and the residual stress in the tube axial direction on the outer surface of the tube was 250 MPa or less. In addition, it was found that electric resistance welded steel pipes having these characteristics are sufficient in buckling strength, which is important as the performance of steel pipe piles.

On the other hand, steel pipes outside the scope of the present invention in terms of component composition, steel structure, and manufacturing conditions have tensile strength in the longitudinal direction of the pipe, 0.2% proof stress, yield ratio, Charpy absorbed energy at -30 ° C, and pipe outer surface Among the residual stresses in the axial direction, any one or more could not obtain the value targeted in the present invention.

From the above points, by setting the component composition, steel structure, and manufacturing conditions of the electric resistance welded steel pipe within the scope of the present invention, it has an optimal yield ratio and high buckling resistance suitable for use in steel pipe piles, and further achieves both high strength and high toughness. An electric welded steel pipe can be provided.

Claims (6)

An electric resistance welded pipe having a welded portion in the axial direction of the base material and the pipe,
The component composition of the parent material part is in mass%,
C: 0.12 to 0.20%;
Si: more than 0% and 0.60% or less;
Mn: 0.50 to 1.70%;
P: more than 0% and 0.030% or less;
S: more than 0% and 0.015% or less;
Al: 0.010 to 0.060%;
Nb: 0.010% to 0.080%;
Ti: 0.010 to 0.050%;
N: More than 0% and 0.006% or less
It contains, the balance consists of Fe and unavoidable impurities,
When the plate thickness of the base material is t, the steel structure at a depth of 1/4t of the plate thickness t from the outer surface of the electric resistance welded pipe is
Bainite is 60% or more in area ratio,
The average effective grain size of the bainite is 20.0 μm or less in average equivalent circle diameter, and the average aspect ratio of the bainite is 0.1 to 0.8,
The tensile strength in the tube axial direction is 590 MPa or more, the 0.2% yield strength is 450 MPa or more, and the yield ratio is 80 to 90%,
The Charpy absorbed energy at -30 ° C with the tube axial direction in the base material portion as the longitudinal direction of the test piece is 70 J or more,
An electric resistance welded pipe having a residual stress in the pipe axial direction of the outer surface of the steel pipe in the base material portion of 250 MPa or less. According to claim 1,
An electric resistance welded pipe characterized by further containing at least one of the following (A) and (B) in terms of mass% in addition to the above component composition.
(A) B: 0.008% or less
(B) 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 to 0.010%, or 2 or more delete A hot-rolled steel sheet is obtained by subjecting a steel material to a hot-rolling process and a cooling process in this order, and further, the hot-rolled steel sheet is subjected to a cold roll forming process to obtain an electric resistance welded steel pipe according to claim 1 or 2. As a method for manufacturing a steel pipe,
The steel material has the component composition according to claim 1 or 2,
In the hot rolling process, after heating the steel material at a heating temperature of 1100 to 1280° C., a rough rolling end temperature of 850 to 1150° C. and a finish rolling end temperature of 750 to 850° C., furthermore, in the rough rolling and the finish rolling The total reduction ratio at 930 ° C. or less in
The cooling step is a step of cooling the hot-rolled sheet to a sheet thickness center temperature at an average cooling rate from start of cooling to stop of cooling: 5 to 25° C./s and stop temperature of cooling: 450 to 650° C.,
In the cold roll forming step, a steel pipe material subjected to a roll forming process is welded to the hot-rolled steel sheet, and a diameter reduction ratio of 0.2 to 0.5% with respect to the circumferential length of the outer surface of the steel pipe after welding is performed. manufacturing method. delete A steel pipe pile using the electric resistance steel pipe according to claim 1 or 2.