WO2020202334A1

WO2020202334A1 - Electric resistance welded steel pipe and method for manufacturing same, and steel pipe pile

Info

Publication number: WO2020202334A1
Application number: PCT/JP2019/014232
Authority: WO
Inventors: 井手　信介; 昌士松本; 晃英松本
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2019-03-29
Filing date: 2019-03-29
Publication date: 2020-10-08
Also published as: TW202039884A; CN113614268B; KR102551615B1; KR20210132698A; JP6690788B1; JPWO2020202334A1; CN113614268A; TWI762881B

Abstract

An electric resistance welded steel pipe having a base material part and also having a welded part in the tube axis direction, wherein the base material part has a specified component composition, a steel structure at a position corresponding to the depth 1/4t from the outer surface of the electric resistance welded steel pipe contains bainite at an area ratio of 70% or more wherein t represents the thickness of the base material part, the average effective grain diameter of the bainite is 10.0 μm or less in terms of average equivalent circle diameter, the average aspect ratio of the bainite is 0.1 to 0.8, the tensile strength in the tube axis direction is 590 MPa or more, the 0.2% proof stress is 450 MPa or more, the yield ratio is 85 to 95%, the Charpy absorbed energy at -30ºC as determined employing the tube axis direction in the base material part as the length direction of a test specimen is 70 J or more, and the residual stress of the outer surface of the steel pipe in the base material part as observed in the tube axis direction is 250 MPa or less; a method for manufacturing the electric resistance welded steel pipe; and a steel pipe pile.

Description

Electric pipe and its manufacturing method, and steel pipe pile

The present invention relates to an electrosewn 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 uses a hot-rolled steel sheet (hot-rolled steel strip) as a material, and cold-rolls the material to form a pipe, resulting in high strength, high toughness, and optimization of the yield ratio. , And the improvement of buckling resistance performance.

In recent years, as a response to large-scale earthquakes, there has been a strong demand for higher strength and improved deformation energy absorption capacity for steel pipe piles used as the foundation of structures. In general, in order to improve the deformation 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 have an excessively low yield ratio in the pipe axis direction from the viewpoint of suppressing deformation of the steel pipe during pile driving. Furthermore, high temperature toughness is also required for steel pipe piles used especially in cold regions. In addition, high buckling resistance is also required to withstand deformation due to earthquakes and the like.

Patent Document 1 describes a method for manufacturing an earthquake-resistant welded steel pipe having excellent local buckling resistance. In Patent Document 1, C: 0.03 to 0.15%, Mn: 1.0 to 2.0%, Cu: 0.05 to 0.50%, Ni: 0.05 in weight%. ~ 0.50%, Cr: 0.05 ~ 0.50%, Mo: 0.05 ~ 0.50%, Nb: 0.005 ~ 0.10%, V: 0.005 ~ 0.10%, Ti: Steel containing at least one of 0.005 to 0.080% and having a Pcm of 0.10 to 0.25 is hot-rolled, and after the rolling is completed, the temperature is 5 ° C./s to 600 ° C. or lower. The steel sheet obtained by cooling at the above cooling rate is cold-formed to obtain a steel pipe. As a result, it is possible to obtain a steel pipe having excellent work hardening index of 0.10 or more in the tensile test in the pipe axial direction, and the occurrence of local buckling due to the external force acting on the steel pipe from the side and the brittleness caused by the occurrence. It is said that it can prevent the occurrence of specific cracks and breaks.

Patent Document 2 contains C: 0.02 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.50 to 2.00% by weight, and Cu: 0. 1 or 2 selected from the group consisting of 10 to 1.5%, Ni: 0.10 to 0.50%, Nb: 0.005 to 0.10% and V: 0.005 to 0.10%. A hot-rolled steel sheet is hot-rolled so that the reduction rate per pass in a temperature range of 900 ° C. or higher is 4% or less on a steel piece containing seeds or more and having a Ceq: 0.38 to 0.45. A method for producing a steel pipe, in which the hot-rolled steel sheet is reheated to a two-phase temperature range of 1 point or more and 3 points or less of Ac, quenched from the two-phase temperature range, tempered, and then pipe-made. Is described. The steel pipe obtained thereby is a low-yield ratio high-strength steel pipe having a 0.2% proof stress: 440 MPa or more, a tensile strength: 590 to 700 MPa, and a yield ratio: 80% or less, and is a steel structure such as a building, a bridge, or a tank. It is said that it is suitable for use.

Patent Document 3 describes Ac 3 points in producing a steel pipe having a composition containing C: 0.10 to 0.18%, Si: 0.1 to 0.5%, and Mn: 1 to 2% in mass%. A step of heating and then quenching, a step of heating to a two-phase temperature range of Ac1 to Ac3 and then air-cooling, a step of forming into a tubular shape in a cold state, and a step of reheating to 500 to 600 ° C. A manufacturing method is described in which the pipes are sequentially applied to obtain a high-tensile steel pipe for a building structure having a low yield ratio. As a result, it is possible to manufacture steel pipes for building structures with a tensile strength of 590 MPa or more without using expensive alloying elements.

Patent Document 4 describes in terms of mass%, 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%, and in addition, the ferrite phase is the main phase, and the second phase other than the main phase is pearlite with an area ratio of 8 to 30%. / Or pseudo-pearlite, having a structure having an average particle size of 4.0 to 10 μm including the main phase and the second phase, and 0.2% proof stress YS: 450 MPa or more in the pipe circumferential direction and the pipe axial direction. , A low yield strength high-strength pearlite steel pipe for steel pipe piles having a tensile strength TS: 590 MPa or more and a yield ratio: 90% or less is described.

JP-A-11-6032 Japanese Patent No. 2678441 Japanese Unexamined Patent Publication No. 2004-300461 Japanese Patent No. 6123734

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 applied as a steel pipe pile, there is a risk that problems such as buckling may occur due to the driving during pile driving.

The technique described in Patent Document 2 requires a heat treatment step for tempering. Further, the technique described in Patent Document 3 requires a large-scale heat treatment apparatus for pipes, and also requires a heat treatment step after making the pipe. Techniques that require these heat treatments have the problem that the yield ratio is too low. Further, there is a problem that the process becomes complicated and the productivity is lowered. In addition, the production cost increases, and it becomes difficult to provide the product at low cost.

In the technique described in Patent Document 4, after hot rolling, the structure is cooled from the finish rolling end temperature to a temperature range of 550 to 700 ° C. in 10 to 100 s to obtain a structure mainly composed of ferrite and pearlite, which is desired. No organization has been obtained. In addition, equipment having a very long cooling zone is required, and it becomes difficult to provide inexpensive high-strength and high-toughness electric resistance welded steel pipes for steel pipe piles.

The present invention has been made in view of the above-mentioned problems, and is an electrosewn steel pipe having an optimum yield ratio and high buckling resistance, and further having high strength and high toughness, a method for manufacturing the same, and a steel pipe pile. The purpose is to provide.

The present invention also provides an electrosewn steel pipe, a method for manufacturing the same, and a steel pipe pile that can achieve the above problems when a hot-rolled steel sheet having a plate thickness of 16 mm or more is mainly used as a material.

The term "high strength" as used herein means a case where 0.2% proof stress (YS): 450 MPa or more and tensile strength (TS): 590 MPa or more in the pipe axial direction in the base material portion of the electrosewn steel pipe. The term "high toughness" as used herein means a case where the Charpy absorption energy at −30 ° C. is 70 J or more, with the pipe axis direction in the base material portion of the electric resistance pipe as the longitudinal direction of the test piece, and the pipe of the electric resistance steel pipe. The above-mentioned high toughness shall be satisfied in both the circumferential direction and the pipe axis direction. The "optimal yield ratio" as used herein means that the ratio (YR) of 0.2% proof stress to the above-mentioned tensile strength is 80 to 90%. The term "high buckling resistance" as used herein means a case where the residual stress of the outer surface of the steel pipe in the base metal portion of the electric resistance pipe in the pipe axial direction is 250 MPa or less and the yield ratio is 90% or less.

In order to achieve the above object, the present inventors have diligently studied the effects of various alloying elements and manufacturing conditions on the yield ratio, 0.2% proof stress, tensile strength, and Charpy impact characteristics. In addition, the buckling resistance performance of the obtained steel pipe (electric pipe) was also diligently examined. As a result, it was found that while maintaining a relatively low yield ratio, both high strength and high toughness can be achieved, and there are appropriate composition, steel structure and manufacturing conditions having high buckling resistance.

That is, the hot-rolled steel sheet manufactured with a specific composition and hot rolling conditions is subjected to diameter reduction rolling under specific conditions after welding in the cold roll forming process by cold roll forming. As a result, bainite has an area ratio of 60% or more, and the average effective particle size of bainite is an average circle in the steel structure at a depth of 1/4 t of the plate thickness t from the outer surface of the steel pipe base material of the electrosewn steel pipe. The equivalent 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 high at 590 MPa or more, the yield ratio is 80 to 90%, the charpy absorption energy at −30 ° C. is 70 J or more, and the mother. It has been found that an electrosewn steel pipe having a low yield ratio, high strength, high toughness, and high buckling resistance performance in which the residual stress of the outer surface of the steel pipe in the material portion in the pipe axis direction is 250 MPa or less can be obtained.

The present invention has been completed by further studying based on such findings, and the gist of the present invention is as follows.
[1] An electric resistance pipe having a base metal portion and a welded portion in the steel axis direction.
The component composition of the base material portion is 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: Contains 0.006% or less, the balance consists of Fe and unavoidable impurities,
When the plate thickness of the base metal portion is t, the steel structure at a depth of 1/4 t of the plate thickness t from the outer surface of the electrosewn steel pipe is
Bainite has an area ratio of 60% or more,
The average effective particle size of the bainite is 20.0 μm or less in the diameter equivalent to the average circle, and the average aspect ratio of the bainite is 0.1 to 0.8.
The tensile strength in the tube axis direction is 590 MPa or more, the 0.2% proof stress is 450 MPa or more, and the yield ratio is 80 to 90%.
The Charpy absorption energy at −30 ° C. with the tube axis direction in the base metal portion as the longitudinal direction of the test piece is 70 J or more.
An electrosewn steel pipe having a residual stress of 250 MPa or less in the pipe axial direction on the outer surface of the steel pipe in the base material portion.
[2] The electric resistance welded steel pipe according to [1], which further contains B: 0.008% or less in mass% in addition to the component composition.
[3] In addition to the above component composition, in mass%,
Cr: 0.01-1.0%,
V: 0.010 to 0.060%,
Mo: 0.01-1.0%,
Cu: 0.01-0.50%,
Ni: 0.01-1.0%,
Ca: 0.0005-0.010%
The electric resistance pipe according to [1] or [2], which contains one or more selected from the above.
[4] A method for manufacturing an electric resistance-sewn steel pipe in which a hot-rolling process and a cooling process are applied to a steel material in this order to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is further subjected to a cold roll forming process to obtain an electric-sewn steel pipe. And
The steel material has the component composition according to any one of [1] to [3].
In the hot rolling step, after heating the steel material to a heating temperature of 1100 to 1280 ° C., rough rolling end temperature: 850 to 1150 ° C., finish rolling end temperature: 750 to 850 ° C., and rough rolling and finish rolling. Total rolling reduction at 930 ° C. or lower: 65% or more in rough rolling and finish rolling to obtain a hot-rolled plate.
The cooling step is a step of cooling the hot-rolled plate at a plate thickness center temperature at an average cooling rate of 5 to 25 ° C./s from the start of cooling to the stop of cooling and a cooling stop temperature of 450 to 650 ° C.
In the cold roll forming step, a steel pipe material that has been roll-formed is welded to the hot-rolled steel sheet, and the diameter reduction ratio is 0.2 to 0.5% with respect to the peripheral length of the outer surface of the steel pipe after welding. A method for manufacturing an electrosewn steel pipe for diameter rolling.
[5] When the composition has the component composition according to any one of [1] to [3] and the plate thickness is t, the steel structure at a depth of 1/4 t from the outer surface to the plate thickness t is For hot-rolled steel sheets in which bainite has an area ratio of 60% or more, the average effective particle size of the bainite is 20.0 μm or less in the equivalent circle diameter, and the average aspect ratio of the bainite is 0.1 to 0.8. This is a method for manufacturing a bainite steel pipe that is subjected to a cold roll forming process to form a bainite steel pipe.
In the cold roll forming step, a steel pipe material that has been roll-formed is welded to the hot-rolled steel sheet, and the diameter reduction ratio is 0.2 to 0.5% with respect to the peripheral length of the outer surface of the steel pipe after welding. A method for manufacturing an electrosewn steel pipe for diameter rolling.
[6] A steel pipe pile using the electrosewn steel pipe according to any one of [1] to [3].

According to the present invention, an electrosewn steel pipe having an optimum yield ratio and high buckling resistance, and further having high strength and high toughness, which is suitably used as a steel pipe pile, a method for manufacturing the same, and a steel pipe pile are provided. can do. The electrosewn steel pipe of the present invention can be easily manufactured, and has a remarkable effect in industry.

Hereinafter, the present invention will be described in detail.

First, the reason for limiting the component composition of the electrosewn steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, "mass%" in the component composition is simply described as "%".

The electrosewn steel pipe of the present invention has a base material portion and a welded portion, and the base metal portion has 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 to 0.050% , N: 0.006% or less, and has a component composition in which the balance is composed of Fe and unavoidable impurities.

The electrosewn steel pipe of the present invention has a welded portion in the pipe axial direction. The "hot-rolled steel sheet" described later includes hot-rolled steel sheets and hot-rolled steel strips.

C: 0.12 to 0.20%
C is an element that increases the strength of steel pipes (electrically sewn steel pipes) by solid solution strengthening and is involved in the formation of steel structures such as bainite. Further, 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 that the degree of processing when manufacturing the steel pipe is large and the yield ratio tends to increase. Therefore, it is necessary to contain a large amount of C. Therefore, in order to obtain the above-mentioned effect, the content of C of 0.12% or more is required. On the other hand, if the content of C exceeds 0.20%, martensite is likely to be formed, and the steel structure desired in the present invention cannot be obtained. As a result, the high toughness intended by the present invention cannot be ensured. Therefore, C is set to 0.12 to 0.20%. C is preferably 0.13% or more, and more preferably 0.14% or more. C is preferably 0.19% or less, and more preferably 0.18% or less.

Si: 0.60% or less Si is an element that can act as an antacid and increase the strength of steel pipes. However, if Si is contained in excess, the toughness decreases. For this reason, Si is set to 0.60% or less. Si is preferably 0.50% or less, and more preferably 0.45% or less. Although the lower limit of Si is not particularly specified, it is preferably 0.01% or more from the viewpoint of electric sewing weldability. More preferably, it is 0.02% or more.

Mn: 0.50 to 1.70%
Mn is an element that increases the strength of steel pipes through solid solution strengthening. In order to obtain such an effect and secure the high strength desired in the present invention, the content of Mn of 0.50% or more is required. On the other hand, if Mn is contained in excess of 1.70%, the steel structure becomes finer and the yield strength becomes high, so that the yield ratio desired in the present invention cannot be secured. Therefore, Mn is set to 0.50 to 1.70%. Mn is preferably 0.55% or more, and more preferably 0.60% or more. Mn is preferably 1.65% or less, and more preferably 1.60% or less.

P: 0.030% or less P is an element that segregates at grain boundaries and lowers toughness, and it is desirable to reduce it as an impurity as much as possible, but in the present invention, up to 0.030% is acceptable. For this reason, P is set to 0.030% or less. P is preferably 0.025% or less, and more preferably 0.020% or less. However, since excessive reduction of P causes an increase in refining cost, P is preferably 0.002% or more. More preferably, it is 0.003% or more.

S: 0.015% or less S exists as MnS in steel when manufacturing a hot-rolled steel sheet, which is a material for steel pipes, and is thinly stretched in a hot rolling process, which adversely affects the ductility and toughness of steel pipes. To exert. Therefore, in the present invention, it is desirable to reduce S as an impurity as much as possible, but the content of S can be up to 0.015%. Therefore, S is set to 0.015% or less. S is preferably 0.010% or less, and more preferably 0.008% or less. However, since excessive reduction of S causes an increase in refining cost, it is preferable that S is 0.0002% or more. More preferably, it is 0.001% or more.

Al: 0.010 to 0.060%
Al acts as an antacid and combines with N to form AlN, which contributes to the 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 content of a large amount of Al exceeding 0.060% lowers the cleanliness of the steel material (hot-rolled steel sheet which is the material of the steel pipe), and lowers the ductility and toughness of the steel pipe. 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 0.055% or less, and more preferably 0.050% or less.

Nb: 0.010 to 0.080%
Nb combines with carbon and nitrogen to form fine precipitates, and the strength of the steel pipe is increased by strengthening the precipitation. In order to obtain such an effect, it is necessary to contain 0.010% or more of Nb. On the other hand, if Nb is contained in an amount of more than 0.080%, it becomes difficult to dissolve the hot-rolled steel sheet, which is a material of the steel pipe, by heating in the hot rolling step. As a result, it remains as a coarse precipitate 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 0.075% or less, and more preferably 0.070% or less.

Ti: 0.010 to 0.050%
Ti combines with carbon and nitrogen to form fine precipitates, which increase the strength of the steel pipe by strengthening the precipitates. In order to obtain such an effect, it is necessary to contain 0.010% or more of Ti. On the other hand, if Ti is contained in excess of 0.050%, the precipitate becomes coarse and the toughness decreases. Therefore, Ti is set to 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, and more preferably 0.040% or less.

N: 0.006% or less N has the effect of increasing the strength of the steel pipe if it is in a small amount, 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. Excessive reduction of N causes an increase in refining cost, so it is preferably 0.001% or more, more preferably 0.002% or more. N is preferably 0.005% or less, and more preferably 0.004% or less.

The rest is Fe and unavoidable impurities. As long as the effect of the present invention is not impaired, the content of O: 0.0050% or less can be allowed as an unavoidable impurity.

The above-mentioned components are the basic component composition of the electrosewn steel pipe in the present invention. Although the above-mentioned essential elements can obtain the properties desired in the present invention, the following elements can be further contained, if necessary, in addition to this basic composition.

B: 0.008% or less B is an element that contributes to the miniaturization of the steel structure by lowering the ferrite transformation start temperature, and can be contained as necessary. However, if the B content exceeds 0.008%, segregation is likely to occur at the grain boundaries, and the toughness may decrease. Therefore, when B is contained, it is preferable that B is 0.008% or less. More preferably, it is 0.006% or less. B is preferably 0.0003% or more.

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 One or more selected from 1.0%, Ca: 0.0005 to 0.010% Cr: 0.01 to 1.0%
Cr is an element that increases the strength of steel pipes by enhancing hardenability, and can be contained as needed. In order to obtain such an effect, it is preferable to contain Cr in an amount of 0.01% or more. On the other hand, if Cr is contained in excess of 1.0%, the toughness and weldability may be lowered, so the content is preferably 1.0% or less. Therefore, when Cr is contained, it is preferable that Cr is 0.01 to 1.0%. Cr is more preferably 0.02% or more, and even more preferably 0.03% or more. Cr is more preferably 0.8% or less, and even more preferably 0.6% or less.

V: 0.010 to 0.060%
V is an element that combines with carbon and nitrogen to form fine precipitates and increases the strength of the steel pipe by strengthening the precipitation, and can be contained as needed. In order to obtain such an effect, it is necessary to contain 0.010% or more of V. On the other hand, if V is contained in excess of 0.060%, the precipitate becomes coarse and the above-mentioned effect tends to be saturated. Therefore, V is set to 0.010 to 0.060%. V is preferably 0.012% or more, and more preferably 0.015% or more. V is preferably 0.055% or less, and more preferably 0.050% or less.

Mo: 0.01-1.0%
Mo is an element that increases the strength of steel pipes by enhancing hardenability, and can be contained as needed. In order to obtain such an effect, it is preferable to contain Mo in 0.01% or more. On the other hand, if Mo is contained in excess of 1.0%, the toughness may be lowered, so the content is preferably 1.0% or less. Therefore, when Mo is contained, it is preferable that Mo is 0.01 to 1.0%. Mo is more preferably 0.02% or more, and even more preferably 0.03% or more. Mo is more preferably 0.8% or less, and even more preferably 0.6% or less.

Cu: 0.01-0.50%
Cu is an element that increases the strength of steel pipes by solid solution strengthening, and can be contained as needed. In order to obtain such an effect, it is preferable to contain 0.01% or more of Cu. On the other hand, if Cu is contained in excess of 0.50%, the toughness may be lowered, so the content is preferably 0.50% or less. Therefore, when Cu is contained, it is preferable that Cu is 0.01 to 0.50%. Cu is more preferably 0.02% or more, and even more preferably 0.03% or more. Cu is more preferably 0.45% or less, and even more preferably 0.40% or less.

Ni: 0.01-1.0%
Ni is an element that increases the strength of steel pipes by solid solution strengthening, and can be contained as needed. In order to obtain such an effect, it is preferable to contain 0.01% or more of Ni. On the other hand, if Ni is contained in excess of 1.0%, the toughness may be lowered, so the content is preferably 1.0% or less. Therefore, when Ni is contained, it is preferable that Ni is 0.01 to 1.0%. Ni is more preferably 0.02% or more, and even more preferably 0.03% or more. Ni is more preferably 0.8% or less, and even more preferably 0.6% or less.

Ca: 0.0005-0.010%
Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process when manufacturing hot-rolled steel sheets that are the material of steel pipes. It can be contained accordingly. In order to obtain such an effect, when Ca is contained, it is preferably contained in an amount of 0.0005% or more. However, if the Ca content 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 that Ca is 0.0005% to 0.010%. Ca is more preferably 0.0010% or more, and even more preferably 0.0015% or more. Ca is more preferably 0.005% or less, and even more preferably 0.004% or less.

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

When the plate thickness of the base metal portion of the electrosewn steel pipe of the present invention is t, bainite has an area ratio of 60% or more in the steel structure at a depth of 1/4 t of the plate thickness t from the outer surface of the electrosewn steel pipe. It has a steel structure in which the average effective particle size of bainite is 20.0 μm or less in the equivalent circle diameter and the average aspect ratio of bainite is 0.1 to 0.8.

Here, the 1 / 4t depth position of the plate thickness t has the highest cooling rate in the hot rolling process when manufacturing a hot-rolled steel sheet, which is a material for steel pipes, which is important for controlling the steel structure. It is a position between the outermost layer and the smallest 1 / 2t depth position. In the present invention, the cross section parallel to the rolling direction at the 1/4 W position of the plate width W in hot rolling is used as the evaluation surface 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 metal portion) are the same.

Area ratio of bainite: 60% or more In order to achieve both high strength and high toughness in the present invention, 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 strength desired in the present invention. Therefore, in the steel structure at a depth of 1/4 t of the plate thickness t from the outer surface of the steel pipe, bainite is set to 60% or more in area ratio. It is preferably 65% or more. If the area ratio of bainite is excessive, the yield ratio becomes too high. Therefore, the area ratio of bainite is preferably 98% or less. More preferably, it is 95% or less.

The structure other than bainite (remaining structure) may be ferrite, pearlite, martensite, austenite, etc. When the total area ratio of these structures is 40% or more of the total steel structure, the strength and toughness are insufficient, and the yield ratio is increased or excessively decreased. Therefore, it is preferably less than 40%. More preferably, it is less than 35%. Considering that the desired yield ratio can be obtained in the present invention, the lower limit of the total area ratio of the residual structure is preferably more than 2%, 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 size of bainite: 20.0 μm or less in average effective particle diameter In order to achieve both high strength and high toughness in the present invention, the average effective particle size of bainite may be 20.0 μm or less. is important. If the average effective particle size of bainite exceeds 20.0 μm in the diameter equivalent to an average circle, the toughness intended by the present invention cannot be obtained. In addition, the strength desired in the present invention cannot be obtained. It is preferably 15.0 μm or less. If the bainite becomes too fine, the yield ratio becomes too high. Therefore, the average effective particle size of bainite is preferably 1.0 μm or more, and more preferably 2.0 μm or more. ..

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

Bainite average aspect ratio: 0.1-0.8
In the present invention, in order to control the yield ratio in the tube axis direction to 80 to 90%, it is necessary to set the average aspect ratio of bainite to 0.1 to 0.8. Here, in the above-mentioned bainite crystal grains, (average length in the plate thickness direction) / (average length in the tube axis direction) was calculated and used as the average aspect ratio of bainite. When the average aspect ratio of bainite exceeds 0.8, the plastic deformability in the tube axis 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 axis direction decreases, and the strength desired in the present invention cannot be obtained.

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

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

In the following description of the manufacturing method, the "° C" indication regarding temperature shall be the surface temperature of the steel material or steel plate (hot-rolled steel plate) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. Further, 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. Further, the "hot-rolled steel sheet" shall include a hot-rolled steel sheet and a hot-rolled steel strip.

In the present invention, the method for melting the steel material (steel slab) does not need to be particularly limited. Molten steel having the above-mentioned composition can be melted by a common melting method such as a converter, an electric furnace, or a vacuum melting furnace, and made into a slab or the like by a common casting method such as a continuous casting method. It is preferable from the viewpoint of quality, productivity and the like. There is no problem even if the ingot-block rolling method is applied instead of the continuous casting method. The molten steel may be further subjected to secondary refining such as ladle refining.

Next, the obtained steel material (steel slab) is subjected to a hot rolling process. In the hot rolling step, the steel material is heated to a heating temperature of 1100 to 1280 ° C., then rough rolled to a rough rolling end temperature of 850 to 1150 ° C., and a finish rolling end temperature of 750 to 850 ° C. This is a step of hot rolling to obtain a hot-rolled plate, which has a total reduction ratio of 65% or more at 930 ° C. or lower in rough rolling and finish rolling.

Heating temperature: 1100-1280 ° C
If the heating temperature is less than 1100 ° C., the coarse carbides present in the steel material produced during casting cannot be solid-solved. As a result, the effect of the contained carbide-forming element cannot be sufficiently obtained. On the other hand, when the heating temperature exceeds 1280 ° C. and becomes high, the crystal grains become remarkably coarse, and the structure of the hot-rolled steel sheet, which is the material of the steel pipe, becomes coarse, and it becomes difficult to secure the characteristics intended by the present invention. .. Therefore, the heating temperature of the steel material needs to be 1100 to 1280 ° C. The temperature is preferably 1120 to 1230 ° C. This temperature is the set temperature inside the heating furnace.

Rough rolling end temperature: 850 to 1150 ° C
When the rough rolling end temperature is less than 850 ° C., the structure during hot rolling does not recover and crystal grains that are excessively elongated in the rolling direction are likely 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 unrecrystallized temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the average effective grain of bainite, which is the object of the present invention, is obtained. It becomes difficult to secure the diameter. Therefore, the rough rolling end temperature is set to 850 to 1150 ° C. The temperature is preferably 860 to 1000 ° C.

Finish rolling end temperature: 750 to 850 ° C
When the finish rolling end temperature is less than 750 ° C., the structure during hot rolling does not recover and crystal grains that are excessively elongated in the rolling direction are likely 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 rolling in the austenite unrecrystallized temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the average effective grain of bainite, which is the object of the present invention, is obtained. It becomes difficult to secure the diameter. Therefore, the finish rolling end temperature is set to 750 to 850 ° C. The temperature is preferably 770 to 830 ° C.

Total rolling reduction at 930 ° C or lower in rough rolling and finish rolling: 65% or more In the present invention, by refining austenite in the hot rolling process, bainite and the residual structure generated in the subsequent cooling process and winding process are produced. It is possible to obtain a hot-rolled steel sheet that is miniaturized and has the strength and toughness desired in the present invention and is suitable as a material for bainite pipes. In order to miniaturize austenite in the hot rolling process, it is necessary to increase the rolling reduction in the austenite unrecrystallized temperature range and introduce sufficient processing strain. In order to obtain this effect, in the present invention, the total reduction rate in the temperature range up to the finish rolling end temperature of 930 ° C. or lower is set to 65% or more. Here, the total rolling reduction rate refers to the total rolling reduction rate of each rolling path in the temperature range up to the finish rolling end temperature of 930 ° C. or lower.

When the total reduction rate in the temperature range up to the finish rolling end temperature of 930 ° C. or lower is less than 65%, sufficient machining strain cannot be introduced in the hot rolling process, and therefore, the average effective grain size of bainite, which is the object of the present invention. A steel structure with a diameter cannot be obtained. The total reduction rate in the temperature range up to the finish rolling end temperature of 930 ° C. or lower is more preferably 70% or more. The upper limit is not specified, but if it exceeds 80%, the effect of improving the toughness on the increase in the reduction rate becomes small, and the equipment load only increases. Therefore, the total reduction rate in the temperature range up to the finish rolling end temperature of 930 ° C. or lower is preferably 80% or less. More preferably, it is 75% or less.

The reason why the temperature was set to 930 ° C. or lower in the present invention is that above 930 ° C., austenite recrystallizes in the hot rolling process, dislocations introduced by rolling disappear, and finely divided austenite cannot be obtained.

In the present invention, when hot rolling a steel material, hot rolling may be performed in which the total rolling reduction ratio up to the finish rolling end temperature of 930 ° C. or lower is 65% or more in both the rough rolling and the finish rolling described above. Hot rolling may be performed in which the total rolling reduction up to the finish rolling end temperature of 930 ° C. or lower is 65% or more only by finish rolling. In the latter case, if the total rolling reduction to the finish rolling end temperature cannot be 65% or more only by finish rolling, the slab is cooled during rough rolling to bring the temperature to 930 ° C or less. The total reduction rate up to the finish rolling end temperature of 930 ° C. or lower in both rough rolling and finish rolling may be 65% or more.

Next, a cooling process is performed on the hot-rolled plate after the hot rolling process. The cooling step is a step of cooling the hot-rolled plate at the center temperature of the plate thickness at an average cooling rate of 5 to 25 ° C./s from the start of cooling to the stop of cooling and a cooling stop temperature of 450 to 650 ° C.

Average cooling rate from the start of cooling to the stop of cooling: 5 to 25 ° C / s
At the center temperature of the thickness of the hot-rolled plate, when the average cooling rate in the temperature range from the start of cooling 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, and in the present invention. The desired 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 tends to exceed 90%. The average cooling rate is preferably 10 ° C./s or higher, and preferably 20 ° C./s or lower.

In the present invention, the average cooling rate is a value obtained by ((center temperature of the thickness of the hot-rolled plate before cooling-center temperature of the thickness of the hot-rolled plate after cooling) / cooling time) unless otherwise specified. It is the average of (cooling rate). Examples of the cooling method include water cooling by injecting water from a nozzle, cooling by injecting cooling gas, and the like. In the present invention, it is preferable to perform a cooling operation (treatment) on both sides of the hot-rolled plate so that both sides of the hot-rolled plate are cooled under the same conditions.

Cooling stop temperature: 450-650 ° C
At the center temperature of the thickness of the hot-rolled plate, when the cooling stop temperature is less than 450 ° C., the average aspect ratio of bainite exceeds 0.8, and as a result, the yield ratio tends to exceed 90%. On the other hand, when the cooling stop temperature exceeds 650 ° C., the bainite transformation start temperature is exceeded, so that the area ratio of bainite cannot be 60% or more. The cooling stop temperature is preferably 480 ° C. or higher, and preferably 620 ° C. or lower.

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

In the winding process, it is preferable to wind at a winding temperature of 450 to 650 ° C. from the viewpoint of the steel plate structure of the hot-rolled steel sheet which is the material of the steel pipe. If the take-up temperature is less than 450 ° C., the average aspect ratio of bainite may exceed 0.8, resulting in a yield ratio of more than 90%. On the other hand, if the winding temperature exceeds 650 ° C., the bainite transformation start temperature is exceeded, so that the area ratio of bainite may not be 60% or more. The take-up temperature is more preferably 480 to 620 ° C.

Next, a cold roll forming process is performed on the hot-rolled steel sheet after the winding process. In the cold roll forming process, a hot-rolled steel sheet is cold-rolled to form a cylindrical open pipe, and both ends of the steel pipe material (that is, the butt portion of the open pipe) are welded and welded. The diameter of the round steel pipe is reduced by 0.2 to 0.5% with respect to the peripheral length of the outer surface of the steel pipe.

Diameter reduction ratio in reduced diameter rolling: 0.2 to 0.5%
When the diameter reduction ratio in the reduced diameter rolling is less than 0.2%, the steel material of the steel pipe of the present invention described above is insufficient in reducing the residual stress due to plastic deformation. 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 is less than 80% due to insufficient processing. 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, the desired plastic deformability, that is, the buckling resistance performance cannot be obtained. Further, even if the residual stress exceeds 250 MPa, the buckling resistance performance deteriorates.

From the above, the electric resistance welded steel 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 absorption energy at −30 ° C. is 70 J or more, and the steel pipe. An electro-sewn steel pipe having a residual stress on the outer surface in the pipe axis direction of 250 MPa or less can be obtained. As a result, it is possible to easily manufacture an electrosewn steel pipe having excellent strength, high toughness, optimum yield ratio and buckling resistance. Since this electric pipe is particularly suitable for steel pipe piles used as the foundation of a structure, it is extremely effective in industry.

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

The steel pipe pile of the present invention is made of an electrosewn 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-mentioned composition and steel structure. By defining the composition and steel structure of the electrosewn 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 -30. A steel pipe pile having a charpy absorption energy at ° C. of 70 J or more and a residual stress of the outer surface of the steel pipe in the pipe axial direction of 250 MPa or less can be obtained. The steel pipe pile of the present invention is driven into the ground, and if necessary, the steel pipe piles are connected to each other by welding or a connecting means such as a screw joint during the driving to be constructed on-site to a long pile. Become. 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 with respect to pile driving.

Hereinafter, the present invention will be described in more detail based on Examples. The present invention is not limited to the following examples.

Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material: wall thickness 250 mm) by a continuous casting method. The obtained slab was subjected to a hot rolling step, a cooling step, a winding step, and a cold roll forming step under the manufacturing conditions shown in Tables 2-1 and 2-2, and then subjected to a hot rolling step, a cooling step, and a cold roll forming step. An electro-sewn steel pipe having an outer diameter and a plate thickness shown in the above was manufactured. Further, in the cold roll forming step, the butt portion of the open pipe was welded by electric stitching.

A test piece was collected from the obtained electric resistance welded steel pipe, and a microstructure observation, a tensile test, a Charpy impact test, a residual stress measurement, and a member compression test were carried out by the methods shown below.

[Tissue observation]
The test piece for observing the structure was prepared by collecting the test piece so that the cross section in the pipe axis direction at the position of 90 ° in the circumferential direction becomes the observation surface when the electric stitch welded portion was set to 0 °, polishing it, and then corroding it with nital. For microstructure observation, use an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) to observe the structure at a depth of 1/4 t of the plate thickness t from the outer surface of the electrosewn steel tube. And imaged. The area ratio of bainite was determined from the obtained optical microscope image and SEM image. The area ratio of bainite was calculated as the average value of the values obtained in each field of view by observing in 5 or more fields of view.

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

The average aspect ratio of bainite was obtained by measuring the length of each effective crystal grain measured by the above method in the plate thickness direction and the length in the tube axis direction, and calculating the average of each. The length in the plate thickness direction and the length in the tube axis direction were set to the maximum lengths in the plate thickness direction and the tube axis direction for each effective crystal grain.

[Tensile test]
In the tensile test, a tensile test piece of JIS No. 5 was collected so that the tensile direction was parallel to the pipe axis direction at a position of 90 ° in the circumferential direction when the electric resistance welded portion of the obtained electric resistance pipe was 0 °. did. Tensile tests were carried out in accordance with JIS Z 2241. 0.2% proof stress (yield strength YS) and tensile strength TS were measured, and the yield ratio defined by (0.2% proof stress) / (tensile strength) was calculated.

[Charpy impact test]
In the Charpy impact test, the longitudinal direction of the test piece is parallel to the pipe axis direction from the plate thickness t / 2 position at the position of 90 ° in the circumferential direction when the power stitch welded portion of the obtained power-sewn steel pipe is 0 °. As described above, a V-notch test piece was collected. The Charpy impact test was carried out at a test temperature of −30 ° C. in accordance with JIS Z 2242 to determine the absorbed energy (J). The number of test pieces was 3 each, and the average value was calculated to obtain 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 Pulstec. The residual stress was measured at three positions, 90 °, 180 °, and 270 °, when the electric resistance welded portion was 0 °, with the outer surface of the obtained electric resistance pipe at the center of the length of the pipe. The average value of the obtained three measured values was taken as the residual stress. The stress measurement direction was the pipe axis direction.

[Member compression test]
In the present invention, in order to evaluate the performance of the steel pipe pile, a member compression test was performed to determine the buckling strength ratio σcr / σy (where σcr is the buckling stress and σy is the material yield strength). If the buckling strength ratio is larger than the reduction coefficient R = 0.8 + 2.5 × t / r (t is the plate thickness and r is the radius), the buckling strength, which is important for the performance of steel pipe piles, is sufficient. I can judge.

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

As shown in Tables 1 to 3-2, all the electrosewn steel pipes within the range of the present invention have a tensile strength of 590 MPa or more in the pipe axial direction, a 0.2% proof stress of 450 MPa or more, and a yield ratio of 80 to 90. The Charpy absorption energy at −30 ° C. was 70 J or more, and the residual stress on the outer surface of the pipe in the pipe axial direction was 250 MPa or less. It was also found that the electric resistance sewn steel pipe having these characteristics has sufficient buckling strength, which is important for the performance of the steel pipe pile.

On the other hand, steel pipes that are outside the scope of the present invention in terms of composition, steel structure and manufacturing conditions include tensile strength in the longitudinal direction of the pipe, 0.2% proof stress, yield ratio, Charpy absorption energy at -30 ° C, and pipe on the outer surface of the pipe. Of the residual stresses in the axial direction, any one or more of them did not obtain the value desired in the present invention.

Based on the above, by setting the composition, steel structure, and manufacturing conditions of the electrosewn steel pipe within the scope of the present invention, it has an optimum yield ratio and high buckling resistance suitable for steel pipe piles. Further, it is possible to provide an electrosewn steel pipe having both high strength and high toughness.

Claims

An electrosewn steel pipe having a base metal part and a welded part in the pipe axis direction.
The composition of the base material is 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: Contains 0.006% or less, the balance consists of Fe and unavoidable impurities,
When the plate thickness of the base metal portion is t, the steel structure at a depth of 1/4 t of the plate thickness t from the outer surface of the electrosewn steel pipe is
Bainite has an area ratio of 60% or more,
The average effective particle size of the bainite is 20.0 μm or less in the diameter equivalent to the average circle, and the average aspect ratio of the bainite is 0.1 to 0.8.
The tensile strength in the tube axis direction is 590 MPa or more, the 0.2% proof stress is 450 MPa or more, and the yield ratio is 80 to 90%.
The Charpy absorption energy at −30 ° C. with the tube axis direction in the base metal portion as the longitudinal direction of the test piece is 70 J or more.
An electrosewn steel pipe having a residual stress of 250 MPa or less in the pipe axial direction on the outer surface of the steel pipe in the base material portion.
The electric resistance steel pipe according to claim 1, further containing B: 0.008% or less in mass% in addition to the component composition.
In addition to the above component composition, in% by mass,
Cr: 0.01-1.0%,
V: 0.010 to 0.060%,
Mo: 0.01-1.0%,
Cu: 0.01-0.50%,
Ni: 0.01-1.0%,
Ca: 0.0005-0.010%
The electric resistance welded steel pipe according to claim 1 or 2, which contains one or more selected from the above.
This is a method for manufacturing a hot-rolled steel pipe in which a hot-rolling process and a cooling process are applied to a steel material in this order to obtain a hot-rolled steel sheet, and further, a cold roll forming process is applied to the hot-rolled steel sheet to obtain an electric-sewn steel pipe. ,
The steel material has the component composition according to any one of claims 1 to 3.
In the hot rolling step, after heating the steel material to a heating temperature of 1100 to 1280 ° C., rough rolling end temperature: 850 to 1150 ° C., finish rolling end temperature: 750 to 850 ° C., and rough rolling and finish rolling. Total rolling reduction at 930 ° C. or lower: 65% or more in rough rolling and finish rolling to obtain a hot-rolled plate.
The cooling step is a step of cooling the hot-rolled plate at a plate thickness center temperature at an average cooling rate of 5 to 25 ° C./s from the start of cooling to the stop of cooling and a cooling stop temperature of 450 to 650 ° C.
In the cold roll forming step, a steel pipe material that has been roll-formed is welded to the hot-rolled steel sheet, and the diameter reduction ratio is 0.2 to 0.5% with respect to the peripheral length of the outer surface of the steel pipe after welding. A method for manufacturing an electrosewn steel pipe for diameter rolling.
When the composition has the component composition according to any one of claims 1 to 3 and the plate thickness is t, bainite is the area ratio of the steel structure at the depth position of 1/4 t of the plate thickness t from the outer surface. A cold roll forming step on a hot-rolled steel sheet having 60% or more, an average effective particle size of the bainite of 20.0 μm or less in an average circle equivalent diameter, and an average aspect ratio of the bainite of 0.1 to 0.8. This is a method for manufacturing bainite steel pipes that are made into bainite steel pipes.
In the cold roll forming step, a steel pipe material that has been roll-formed is welded to the hot-rolled steel sheet, and the diameter reduction ratio is 0.2 to 0.5% with respect to the peripheral length of the outer surface of the steel pipe after welding. A method for manufacturing an electrosewn steel pipe for diameter rolling.
A steel pipe pile using the electric pipe sewn steel pipe according to any one of claims 1 to 3.