JP7207245B2 - Steel pipe pile joint, steel pipe pile and construction method of steel pipe pile - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel pipe pile joint for mechanically connecting steel pipe piles that support a structure. Furthermore, the present invention relates to a steel pipe pile having the steel pipe pile joint and a construction method for the steel pipe pile.

For example, when the pile foundation is required to have a large bearing capacity in soft ground, the tip of the pile needs to reach the bearing layer at a depth of 50 m or more underground. When steel pipe piles are used as pile bodies, the maximum length per steel pipe pile is limited to about 15m due to transportation and construction restrictions. The tip of the pile must reach the supporting layer by joining.

In general, steel pipe piles are welded on-site, but this involves difficulties in securing welders, quality control of the welds, and work in the event of rain and wind, and it can take a long time to inspect the welds. was a problem. Therefore, several methods have been proposed for mechanically connecting steel pipe piles without on-site welding.

That is, in Patent Document 1, it is composed of a male cylinder having a male screw and a female cylinder having a female screw. There has been proposed a joining joint for steel pipe piles, which is characterized by having a cylindrical portion having an outer diameter that fits loosely in the joint. Patent Literature 2 proposes a steel pipe pile joint structure in which a first steel pipe pile and a second steel pipe pile are joined in series.

All of the conventional joints described above (hereinafter sometimes referred to as mechanical joints) have a structure in which paired joints are mechanically joined together. Mechanical joints are required to have tensile, compressive and bending strengths equal to or higher than those of the pile body (steel pipe) excluding joints.

Mechanical joints are usually thinner than the pile body due to machining. Therefore, it is common to manufacture a joint part separately using a material having a higher strength than that of the pile body, and weld it to the pile body at a factory to use it as a steel pipe pile. Therefore, a forged ring manufactured by hot forging a steel material is generally used as a material for the mechanical joint.

Since the forged ring has a low roundness, it requires extra cutting such as threading when forming a mechanical joint, which leads to a poor product yield and an increase in the production cost of the joint. In addition, there is a problem that the material cost is high because the heating cost is high at the time of manufacturing and many alloying elements need to be added in order to obtain the desired strength after hot forging.

In order to solve the above cost problem, Patent Document 3 proposes to use a straight-seam steel pipe. Since straight-seam steel pipes are manufactured using the plate winding method, they can be provided at a lower cost than forged rings. Here, in the plate winding method, a hot-rolled steel sheet is formed into a cylindrical shape by cold pressing or the like and welded to produce the product, and the production cost can be reduced. Hereinafter, the steel pipe manufactured by this plate winding process is also referred to as a plate-wrapped steel pipe.

Japanese Patent No. 3747594 Japanese Patent No. 6079933 Japanese Patent Application Laid-Open No. 2017-115368

As described above, the sheet-wrapped steel pipe has a lower material cost than the forged ring, but has a low roundness similar to the forged ring, resulting in a poor yield during cutting. In addition, since there are welds extending in the axial direction of the pipe, the material in the circumferential direction of the pipe is not uniform. Machining failures such as tool breakage may occur. In addition, it is necessary to make holes to provide joints with retaining mechanisms and rotation prevention mechanisms around the pipe axis. must. In addition, since the compressive residual stress in the pipe axial direction around the welded portion is high, the compressive rigidity is low, and there is a concern that buckling is likely to occur when a compressive load is applied.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength steel pipe pile joint and a steel pipe pile at a low cost. Another object of the present invention is to propose a steel pipe pile construction method for driving a pile using the steel pipe pile.

The term "high strength" as used in the present invention means that the yield stress is 685 MPa or more. For example, when the strength of the pile body is a general yield stress of 235 MPa or more, the strength required for the joint is desirably a yield stress of 685 MPa or more from the balance with the strength of the pile body.

The present inventors conducted intensive studies to solve the above problems, and obtained the following findings (A) and (B).
(A) When a steel pipe pile joint is produced using a steel pipe having a welded portion as a material, the width of the welded portion and its heat-affected zone is sufficiently wide by setting the circumferential width of the melt-solidified portion of the welded portion to 1000 μm or less. It becomes smaller, and it is possible to suppress the deterioration of machinability and the occurrence of cutting troubles due to the difference in hardness from the non-welded portion. Further, even if the position of the hole necessary for providing a retaining mechanism, a rotation preventing mechanism around the tube axis, etc. overlaps with the welded portion, if the width is 1000 μm or less, the rigidity does not significantly decrease.

(B) In addition to (A) above, by setting the compressive residual stress in the pipe axis direction on the surface of the steel pipe pile joint to 250 MPa or less, the amount of decrease in compressive rigidity is reduced, and when a compressive load is applied, early Buckling can also be suppressed.

As a result of further studies based on these findings, a steel pipe pile joint manufactured using an electric resistance welded steel pipe as a raw material, which is manufactured by appropriately controlling the upset amount when electric resistance welding is performed on a steel pipe, is the above (A). It was found that the characteristics were satisfied. Further, in addition to the control of the upset amount, the steel pipe pile joint manufactured by appropriately controlling the diameter reduction rate in the sizing process after the electric resistance welding is manufactured using the electric resistance welded steel pipe as the material. In addition to the characteristics of ), it was found that the characteristics of (B) were also satisfied.

Furthermore, when the electric resistance welded steel pipe according to the present invention is used as a joint material, it was found that a desired joint strength can be obtained with less alloying element addition than in the case of using a conventional forged ring or plate-wrapped steel pipe as a raw material.
Furthermore, it has been found that the electric resistance welded steel pipe according to the present invention has a higher circularity than seamless steel pipes and plate-wound steel pipes, and therefore has the advantage of remarkably improving the cutting yield.

The present invention has been completed based on the above findings, and the gist thereof is as shown in [1] to [4] below.
[1] A tubular joint provided with mechanical means attached to the ends of steel pipes and interconnecting said steel pipes, having a weld extending in the direction of the pipe axis, defined by the following formula (1): A steel pipe pile joint having a chemical composition with Ceq of 0.20 or more and 0.60 or less, and a width of the molten solidified portion in the welded portion in the pipe circumferential direction of 1.0 μm or more and 1000 μm or less over the entire pipe thickness.
Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 (1)
However, each element display in the formula indicates the content (% by mass) of the element.

[2] The steel pipe pile joint according to [1], wherein the compressive residual stress in the pipe axial direction on the surface is 250 MPa or less.

[3] A steel pipe pile having the steel pipe pile joint according to [1] or [2] at the end of the steel pipe.

[4] The steel pipe pile construction method of connecting the steel pipe piles according to the above [3] with the steel pipe pile joint and driving the steel pipe piles.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide a high-strength steel pipe pile joint and a steel pipe pile at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a steel pipe pile with a multi-threaded joint according to one embodiment of the present invention; FIG. 4 is a schematic diagram of a longitudinal cross-section of the steel pipe pile after joint connection. It is a schematic diagram of the pipe circumferential direction cross section of a welded part.

The steel pipe pile joint of the present invention includes mechanical means for interconnecting steel pipe piles, further has a welded portion extending in the direction of the pipe axis, and has a Ceq of 0.20 defined by equation (1) below. It is characterized by having a component composition of not less than 0.60 and not less than 1.0 μm and not more than 1000 μm over the entire thickness of the pipe in the circumferential direction of the melted and solidified portion of the welded portion.

First, the steel pipe pile joint has the mechanical means as a joint, but the structure is not particularly limited as long as it functions as a joint. For example, a joint structure such as a screw type or an insertion type described in Japanese Patent No. 4600407 can be applied. Here, one example of a threaded joint structure is shown in FIG. That is, as shown in FIG. 1, the steel pipe pile joints are attached to the ends of the upper steel pipe 1 and the lower steel pipe 2, respectively. Specifically, a multi-start threaded joint for joining an upper steel pipe 1 and a lower steel pipe 2, which includes a pin joint 3 having a parallel thread and a multi-start thread of 3 or more threads, and a parallel thread and a multi-start thread joint of 3 or more threads. It consists of a box joint 4 consisting of a screw. Typically, a pin joint 3 is attached to one end of the steel pipe and a box joint 4 is attached to the other end of the same steel pipe.

As described later, the steel pipe pile joint of the present invention is an electric resistance welded steel pipe obtained by making a hot-rolled steel strip wound into a coil into a cylindrical shape and welding the butt portions of the side end surfaces with electric resistance welding. It has a welded portion extending in the pipe axial direction along the butted portion. It is essential for this welded portion to satisfy the following conditions in order to ensure high joint strength.

The pipe circumferential width of the melt-solidified portion in the welded portion is 1.0 μm or more and 1000 μm or less over the entire pipe thickness. It reduces the buckling strain in the joint compression test. On the other hand, if the width of the melt-solidified portion in the pipe circumferential direction is larger than 1000 μm, the difference in hardness between the welded portion and the non-welded portion tends to cause a decrease in machinability and a cutting trouble. In addition, the compressive residual stress in the pipe axis direction on the surface of the steel pipe pile joint increases, the compressive rigidity decreases, and the buckling strain in the joint compression test decreases. In addition, in order to prevent a drastic drop in rigidity when the position to make a hole, which is necessary for providing a retaining mechanism or a mechanism to prevent rotation around the pipe axis, overlaps with the welded part, The width is preferably 800 μm or less, more preferably 500 μm or less. Further, in order to ensure the joining of the welded portion over the entire joint surface, the width of the molten solidified portion is preferably 2.0 μm or more, more preferably 5.0 μm or more.

Further, the steel pipe pile joint of the present invention has a chemical composition in which Ceq defined by the following formula (1) is 0.20 or more and 0.60 or less. That is, the chemical composition of the hot-rolled steel strip, which is the material for the steel pipe pile joint, has a Ceq defined by the following formula (1) of 0.20 or more and 0.60 or less in order to ensure mechanical properties and weldability. It is essential that In addition, the display of each element in the following formula (1) is the content (% by mass) of each element. However, in the formula (1), an element not contained is set to 0 (zero).
Further, in this specification, unless otherwise specified, "%" regarding steel composition means "% by mass".
Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 (1)

Ceq defined by the above formula (1) is a carbon equivalent and serves as an index of the hardness of the base metal part, weld zone and heat affected zone in the steel pipe pile joint. If this Ceq is less than 0.20%, the amount of solid-solution strengthening and hardenability decrease, so that the strength required for a joint cannot be obtained. On the other hand, when Ceq exceeds 0.60%, the ductility is lowered, the buckling resistance performance is lowered, and the welded portion is excessively hardened, which causes cutting failure.

Furthermore, the steel pipe pile joint of the present invention preferably has a compressive residual stress of 250 MPa or less in the pipe axial direction on the surface of the steel pipe pile joint. That is, it is possible to provide a steel pipe pile joint that reduces the amount of decrease in compressive rigidity, suppresses early buckling when a compressive load is applied, and has excellent buckling resistance. Here, "excellent in buckling resistance" means that the buckling strain in the joint compression test is 0.75% or more. The compressive residual stress in the pipe axial direction of 250 MPa or less can be realized, for example, by regulating the diameter reduction of the steel pipe in the sizing process after electric resistance welding, as described later.

A method for manufacturing a steel pipe pile joint of the present invention will be described below.
A hot-rolled steel strip wound into a coil is continuously discharged and cold-roll-formed into a cylindrical open pipe. An electric resistance welded steel pipe is made by electric resistance welding that is pressure welded by upsetting.

Here, for the purpose of adjusting the hardness of the electric resistance welded portion, heat treatment may be applied to the electric resistance welded portion as necessary. The heat treatment is preferably an induction heating method from the viewpoint of productivity, but furnace heating may also be used. Subsequently, the diameter of the steel pipe is reduced by the sizing rolls in the latter stage to improve the roundness and reduce the residual stress in the axial direction of the pipe introduced during the roll forming and electric resistance welding in the former stage.
The diameter-reduced steel pipe is cut online, or the electric resistance welded steel pipe cut to a fixed length is further cut off-line to a predetermined length, and then an appropriate mechanical joint means is provided by cutting to form a joint of the desired shape. make.
The above is the basic form of the steel pipe pile joint in the present invention.

Further, for example, the pin joint and the box joint thus produced are welded and joined to a steel pipe serving as a pile body to form a steel pipe pile. Generally, a pin joint is joined to one end of a steel pipe pile, and a box joint is joined to the other end.

The shape of the joint may have a structure in which the joints that form a pair are mechanically joined together. For example, the threaded joint structure described above is conceivable. The joint shown in FIG. 1 is driven into the ground so that the upper end side of the steel pipe becomes the box joint 4 and the lower end side becomes the pin joint 3 as shown in FIG. 1 when constructing the steel pipe pile. When connecting the upper steel pipe 1 to the lower steel pipe 2 driven into the ground, the upper steel pipe 1 is hung by a crane or the like so that the lower end becomes the pin joint 3, and the upper steel pipe 1 is lowered. The pin joint 3 at the lower end is inserted into the box joint 4 at the upper end of the lower steel pipe 2, and in this state, the upper steel pipe 1 is rotated and the pin joint 3 is screwed into the box joint 4 to connect both steel pipes.

Furthermore, the method for manufacturing the steel pipe pile joint and the steel pipe pile of the present invention will be described in detail.
First, the chemical composition of the hot-rolled steel strip, which is the material for the steel pipe pile joint, has a Ceq defined by the above formula (1) of 0.20 or more and 0.60 or less in order to ensure mechanical properties and weldability. In addition, it is preferable that Pcm defined by the following formula (2) is 0.25 or less. In addition, the display of each element in the following formula (2) is the content (% by mass) of each element. However, in formula (2), elements not contained are set to 0 (zero).
Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B (2)

Pcm defined by the above formula (2) is an index indicating weld cracking susceptibility, and when Pcm exceeds 0.25%, cold cracking is likely to occur in the melt-solidified zone and the heat-affected zone.

The chemical composition of the hot-rolled steel strip is, in terms of % by mass, C: 0.02% or more and 0.25% or less, Si: 2.0% or less, Mn: 0.2% or more and 3.0% or less. , P: 0.10% or less, S: 0.050% or less, Al: 0.005% or more and 0.10% or less, N: 0.010% or less, and Nb: 0.005% or more and 0 .150% or less, V: 0.005% or more and 0.150% or less, Ti: 0.005% or more and 0.150% or less, Nb + V + Ti: 0.010% or more and 0.200% or less and the balance being Fe and unavoidable impurities.

Further, in addition to the above component composition, in mass%, Cr: 0.01% to 2.0%, Mo: 0.01% to 1.0%, Cu: 0.01% to 1.0% % or less, Ni: 0.01% or more and 1.0% or less, Ca: 0.0005% or more and 0.010% or less, and B: one or more selected from 0.0003% or more and 0.010% or less may contain.
The reasons for limiting the content of each component will be described in detail below.

C: 0.02% or more and 0.25% or less C is an element that increases the strength of steel by contributing to solid-solution strengthening, formation of hard phases, and precipitation of carbides. In order to obtain such effects, it is preferable to contain 0.02% or more of C. However, if the C content exceeds 0.25%, the temperature range in which the solid-liquid two phases coexist widens, and since the liquid phase remains at low temperatures during electric resistance welding, solidification cracking is likely to occur. Therefore, the C content is preferably 0.02% or more and 0.25% or less. More preferably, the lower limit of the C content is 0.04% and the upper limit of the C content is 0.20%.

Si: 2.0% or less Si is an element that increases the strength of steel by solid-solution strengthening, and can be contained as necessary. In order to obtain such effects, it is desirable to contain 0.01% or more of Si. However, if the Si content exceeds 2.0%, oxides tend to form in the electric resistance welded portion, and the toughness of the welded portion decreases. Therefore, the Si content is preferably 2.0% or less. More preferably, the lower limit of Si content is 0.08% and the upper limit of Si content is 1.5%.

Mn: 0.2% to 3.0% Mn is an element that increases the strength of steel through solid solution strengthening. Moreover, Mn is an element that contributes to improvement in strength and toughness by lowering the ferrite transformation start temperature and refining the structure. In order to obtain such effects, it is preferable to contain 0.2% or more of Mn. However, if the Mn content exceeds 3.0%, oxides tend to form in the electric resistance welded portion, and the toughness of the welded portion decreases. Therefore, the Mn content is preferably 0.2% or more and 3.0% or less. More preferably, the lower limit of the Mn content is 0.8% and the upper limit of the Mn content is 2.5% or less.

P: 0.10% or less P is an unavoidable impurity that segregates at grain boundaries and lowers toughness. In order to avoid this, it is desirable to set the P content to 0.10% or less. More preferably, it is 0.08% or less.

S: 0.050% or less S is an unavoidable impurity and usually exists as MnS in steel. In order to avoid this, it is desirable to set the S content to 0.05% or less. More preferably, it is 0.020% or less.

Al: 0.005% to 0.10% Al is an element that acts as a strong deoxidizing agent. In order to obtain such effects, it is preferable to contain 0.005% or more of Al. However, if the Al content exceeds 0.10%, the amount of alumina-based inclusions increases, which deteriorates the weldability and the toughness of the weld zone. Therefore, the Al content is preferably 0.005% or more and 0.10% or less. More preferably, the lower limit of Al content is 0.008% and the upper limit of Al content is 0.08%.

N: 0.010% or less N is an unavoidable impurity, and is an element that has the effect of lowering the toughness by firmly fixing the movement of dislocations. In order to avoid this, it is desirable to set the N content to 0.010% or less. More preferably, it is 0.008% or less.

Nb: 0.005% or more and 0.150% or less, V: 0.005% or more and 0.150% or less, and Ti: one or more of 0.005% or more and 0.150% or less Ni is also an element that contributes to the strength improvement of steel by forming fine carbides and nitrides in the steel. In order to obtain such an effect, when one or more of Nb, Ti and V are contained, it is necessary to make each content 0.005% or more and Nb + V + Ti: 0.010% or more. be. On the other hand, when each content exceeds 0.150% or Nb+V+Ti exceeds 0.200%, the effect of increasing the strength is saturated, and a strength increase corresponding to the added amount cannot be obtained. Also, toughness is reduced. More preferably, Nb+V+Ti: 0.020% or more and 0.150% or less.

The balance is Fe and unavoidable impurities. However, 0.005% or less of O may be contained as an unavoidable impurity.

The above ingredients are the basic ingredient composition of the steel material of the steel pipe in the present invention. Furthermore, the following elements can be contained as necessary.
Cr: 0.01% to 2.0%, Mo: 0.01% to 1.0%, Cu: 0.01% to 1.0%, Ni: 0.01% to 1.0% Below, one or more selected from Ca: 0.0005% or more and 0.010% or less and B: 0.0003% or more and 0.010% or less

Cr: 0.01% or more and 2.0% or less, Mo: 0.01% or more and 1.0% or less Cr and Mo are elements that increase the hardenability of steel and increase the strength of steel. can contain In order to obtain the above effects, when Cr and Mo are contained, it is preferable that Cr: 0.01% or more and Mo: 0.01% or more, respectively. On the other hand, an excessive content may lead to deterioration in toughness and weldability. Therefore, when Cr and Mo are contained, it is preferable that Cr: 1.0% or less and Mo: 1.0% or less, respectively. Therefore, when Cr and Mo are contained, it is preferable that Cr: 0.01% or more and 1.0% or less and Mo: 0.01% or more and 1.0% or less, respectively. More preferably, the lower limit of Cr is 0.02% and the upper limit of Cr is 0.8%. More preferably, the lower limit of Mo is 0.02% and the upper limit of Mo is 0.8%.

Cu: 0.01% or more and 1.0% or less, Ni: 0.01% or more and 1.0% or less Cu and Ni are elements that increase the hardenability of steel and increase the strength of steel through solid-solution strengthening. Yes, and can be included if desired. In order to obtain the above effects, when Cu and Ni are contained, it is preferable that Cu: 0.01% or more and Ni: 0.01% or more, respectively. On the other hand, an excessive content may lead to deterioration in toughness and weldability. Therefore, when Cu and Ni are contained, it is preferable that Cu: 1.0% or less and Ni: 1.0% or less, respectively. Therefore, when Cu and Ni are contained, it is preferable that Cu: 0.01% or more and 1.0% or less and Ni: 0.01% or more and 1.0% or less, respectively. More preferably, the lower limit of Cu is 0.02% and the upper limit of Cu is 0.8%. More preferably, the lower limit of Ni is 0.02% and the upper limit of Ni is 0.8%.

Ca: 0.0005% or more and 0.010% or less Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process. can be contained. In order to obtain such an effect, when Ca is contained, it is preferable to contain 0.0005% or more of Ca. However, when the Ca content exceeds 0.010%, Ca oxide clusters are formed in the steel, which may deteriorate the toughness. Therefore, when Ca is contained, the Ca content is preferably 0.0005% or more and 0.010% or less. More preferably, the lower limit of Ca content is 0.0008% and the upper limit of Ca content is 0.008%.

B: 0.0003% or more and 0.010% or less B is an element that contributes to improvement in strength and toughness by lowering the ferrite transformation start temperature and refining the structure. In order to obtain such an effect, when containing B, it is preferable to contain 0.0003% or more of B. However, when the B content exceeds 0.010%, the ductility may decrease. Therefore, when B is contained, it is preferably 0.0003% or more and 0.010% or less. More preferably, the lower limit of the B content is 0.0005% and the upper limit of the B content is 0.008%.

Furthermore, the details of producing a steel pipe from the hot-rolled steel strip having the chemical composition described above will be described below. In the following description, "°C" regarding temperature indicates the surface temperature of the hot-rolled steel strip unless otherwise specified. This surface temperature can be measured with a radiation thermometer or the like.
In the cooling process after the finish rolling of the hot-rolled steel strip, in order to suppress the generation of residual stress due to phase transformation during cooling, the average cooling rate to the cooling stop temperature is 60 ° C./s or less, and the cooling stop temperature is preferably 350° C. or higher.
When the average cooling rate to the cooling stop temperature is more than 60°C/s or the cooling stop temperature is less than 350°C, the volume fraction of martensite generated near the surface during cooling of the steel strip increases, and the steel strip surface compressive residual stress of

In electric resistance welding, in order to satisfy the pipe circumferential width of the molten solidified portion required in the present invention, the amount of upset during electric resistance welding is set to the plate thickness so that the molten steel generated during electric resistance welding can be sufficiently discharged. It should be 20% or more. If the amount of upset is less than 20% of the plate thickness, the width of the melt-solidified portion in the pipe circumferential direction is greater than 1000 μm, resulting in reduced machinability and increased compressive residual stress. If the upset amount is more than 100% of the plate thickness, the squeeze roll load increases, and the circumferential width of the molten solidified portion becomes smaller than 1.0 μm, resulting in insufficient joining of the welded portion and reduced compression rigidity. It reduces the buckling strain in the joint compression test. Therefore, the upset amount is preferably 20% or more and 100% or less of the plate thickness. More preferably, it is 40% or more and 80% or less.
Here, the upset amount is (L1-L2)/L1×100 (% ).

Other conditions in electric resistance welding may be in accordance with general electric resistance welding, and are not particularly limited.

Next, in the sizing process after electric resistance welding, in order to satisfy the roundness (0.5% or less) and residual stress in the pipe axial direction required by the present invention, the steel pipe circumference is set to 0.30 in total. It is necessary to reduce the diameter of the steel pipe so that the However, if the diameter of the steel pipe is reduced so that the total circumferential length of the steel pipe is reduced by more than 2.0%, the amount of bending in the axial direction of the pipe when passing through the rolls will increase, and the residual stress in the axial direction of the pipe after diameter reduction will increase. On the contrary, it rises. Furthermore, since the steel pipe is greatly work-hardened, the ductility is lowered and the buckling resistance is lowered. For this reason, it is preferable to reduce the diameter of the steel pipe so that the circumferential length of the steel pipe is reduced at a rate of 0.30% or more and 2.0% or less.

In the sizing process, in order to minimize the amount of bending in the tube axis direction when passing through the rolls and to suppress the generation of residual stress in the tube axis direction, it is preferable to perform multistage diameter reduction using multiple stands. The diameter reduction in the stand is preferably performed so that the pipe circumference is reduced at a rate of 1.0% or less.

In addition, in the sizing process, by reducing the diameter by the above method, the electric resistance welded steel pipe is work-hardened by an appropriate amount, and the yield stress can be increased to a desired value without significantly reducing the ductility.

The present invention will be further described in detail below based on examples. In addition, the present invention is not limited to the following examples.
Molten steel having the chemical composition shown in Table 1 was melted in a converter and made into a slab (steel material: thickness 250 mm) by a continuous casting method. A part of the obtained slab was hot-rolled and then subjected to a cooling process under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. A part of the hot-rolled steel sheet was subjected to a coiling process to form a coil, thereby forming a hot-rolled steel sheet for electric resistance welded steel pipes. The remainder was used as hot-rolled steel sheets for plate-wrapped steel pipes.

The hot-rolled steel sheet was subjected to the following pipe-making process.
The hot-rolled steel sheets for electric resistance welded steel pipes were formed into cylindrical open pipes by roll forming, and their butt portions were electric resistance welded. At the time of electric resistance welding, the amount shown in Table 2 was upset by rolls arranged on the left and right to discharge the molten steel. After that, sizing rolls arranged vertically and horizontally were used to reduce the diameter as shown in Table 2 to obtain an electric resistance welded steel pipe having an outer diameter of 610 mm and a plate thickness of 25 mm.

The remaining hot-rolled steel sheets for plate-wound steel pipes were formed into cylindrical open pipes by press bending, and the butt portions were submerged arc-welded from the inner and outer surfaces to form plate-wound steel pipes with an outer diameter of 610 mm and a plate thickness of 30 mm. got Submerged arc welding was performed under general butt conditions.

The remainder of the slab was hot rolled into a billet, then reheated to 1150° C. or higher for hot forging to obtain a forged ring with an outer diameter of 610 mm or more, an inner diameter of 550 mm or less, and a length of 220 mm or more. The obtained forged ring was heated to 900° C. or higher and 1050° C. or lower in a heating furnace and water-cooled, then reheated to 500° C. or higher and 700° C. or lower and air-cooled.

The obtained electric resistance welded steel pipe, plate-wound steel pipe and forged ring were cut, and as shown in FIG. A box joint 6 having a diameter of 600 mm was produced. The length of the joint in the pipe axis direction at the time of joining was set to 200 mm.

[Tensile test]
In order to measure the strength of the joint material, a JIS No. 5 tensile test piece was taken from the welded portion of the material steel pipe at a position 90 degrees in the pipe circumferential direction so that the tensile direction was parallel to the pipe axial direction. Using this, a tensile test was performed in accordance with JIS Z 2241 to determine the yield stress. The number of test pieces was two for each, and the average value of the yield stresses of them was taken as the yield stress of the joint material.

[Cutting test]
In order to evaluate the machinability of the joint material, the material steel pipe was placed on a lathe, and a P10 class carbide cuboid tip was used under the conditions of a cutting speed of 100 m/min, a feed of 0.1 mm/rev, and a depth of cut of 0.5 mm. The outer circumference was cut with After cutting 1000 m, the cutting was stopped and the wear width of the flank of the tool tip was measured.

[Specification of the welded part and measurement of the pipe circumferential width of the melted and solidified part]
The weld zone was identified by polishing the circumferential cross-section of the joint and corroding it by an appropriate method, visually observing the polished surface, and discriminating it as a region consisting of a melt-solidified zone and a heat-affected zone. After that, a small piece including the welded portion was cut out, and the circumferential width of the melt-solidified portion was measured at intervals of 1 mm in the plate thickness direction from the outer surface to the inner surface by observation with an optical microscope. Here, an appropriate corrosive liquid may be selected according to the steel composition and the type of steel pipe. Further, as the cross section after corrosion is schematically shown in FIG. 3, the melt-solidified portion can be visually recognized as a region 11 having a different structural form and contrast from the base material portion 9 and the heat-affected zone 10 in FIG. For example, a molten solidified portion of an electric resistance welded steel pipe of carbon steel and low alloy steel can be identified as a white region observed with an optical microscope in the cross section corroded with nital. Further, the melt-solidified portion of the UOE steel pipe of carbon steel and low alloy steel can be identified as a region containing a cellular or dendrite-like solidified structure under an optical microscope in the cross section corroded with nital.

[Compressive residual stress measurement]
The compressive residual stress was measured by the X-ray diffraction method on the inner surface of the pin joint and the outer surface of the box joint which were electropolished by 100 μm. The direction of residual stress to be measured was the tube axis direction. Measurements were made at 12 points per joint and 24 points per set of a pin joint and a box joint at each position at intervals of 30 degrees in the circumferential direction of the pipe with respect to the welded part. From the measurement results at these 24 points, the maximum magnitude of compressive stress was determined.

[Joint compression test (buckling strain)]
A compression test of the joint was carried out by connecting a pin joint and a box joint and applying a load in the axial direction of the pipe to the combined joint. The compression test was performed with the pin joint on the upper side and the box joint on the lower side. In FIG. 2, the load is applied vertically from above and the compressive force is transmitted at the abutting portion 7 of the pin joint 5 and the box joint 6 . At this time, since the compressive stress acting on the joint is highest at the portion where the cross-sectional area of the box joint 6 is the smallest, the compression test result is greatly affected by the thickness of the box joint 6 excluding the thread height. In this example, the joint compression test was performed by setting the plate thickness of the produced box joint 6, excluding the thread height, to 6 mm. The buckling strain (%) was obtained by dividing the displacement under the maximum compressive load by the initial total length of the joint, 200 mm, and multiplying it by 100. If the buckling strain was 0.75% or more, it was determined that the properties were good.
Table 3 shows the results obtained.

In Table 3, joint No. Joint Nos. 1, 3, 5, 6, 8, 10 and 11 are examples of the present invention. 2, 4, 7 and 9 are comparative examples. In addition, joint No. Joint Nos. 1, 3 and 5 to 11 are joints made from electric resistance welded steel pipes. 2 and 4 are joints made from plate-wrapped steel pipes.

All of the joints of the present invention have a Ceq of 0.20 or more and 0.60 or less, a melt-solidified width of the welded portion in the pipe circumferential direction width of 1.0 μm or more and 1000 μm or less over the entire pipe thickness, and a pressure of 685 MPa or more. showed the yield stress. Moreover, the wear width of the flank of the tool tip after the cutting test was 0.10 mm or less, indicating good machinability.

Joint No. of the comparative example. In Nos. 2 and 4, the melt-solidification width of the weld exceeded the range of the present invention, so the wear width and compressive residual stress of the tool tip flank did not reach the desired values. Moreover, since the compressive residual stress did not reach the desired value, the buckling strain did not reach the desired value. Also, the yield stress did not reach the desired value.

Joint No. of the comparative example. In No. 7, the buckling strain did not reach the desired value because the Ceq exceeded the range of the present invention.

Comparative example No. In No. 9, the melt-solidification width of the weld exceeded the range of the present invention, so the wear width and compressive residual stress of the tool tip flank did not reach the desired values. Moreover, since the compressive residual stress did not reach the desired value, the buckling strain did not reach the desired value.

Furthermore, among the examples of the present invention, No. In 1, 3, 5, 8, 10 and 11, the total diameter reduction rate after electric resistance welding was within an appropriate range, so the compressive residual stress was 250 MPa or less, and the buckling strain was 0.75% or more. Indicated.

REFERENCE SIGNS LIST 1 Upper steel pipe 2 Lower steel pipe 3 Pin joint 4 Box joint 5 Longitudinal section of pin joint 6 Longitudinal section of box joint 7 Butt part for transmitting compressive force 8 Central axis of pipe 9 Base metal part 10 Welding heat affected zone 11 Melt solidification Department

Claims (4)

A tubular joint provided with mechanical means for attaching to the ends of steel pipes and interconnecting said steel pipes, having an electric resistance weld extending in the direction of the pipe axis, defined by the following formula (1) Ceq is 0.20 or more and 0.60 or less, the width of the molten solidified portion in the electric resistance welded portion in the pipe circumferential direction is 1.0 μm or more and 1000 μm or less over the entire pipe thickness, and the yield stress is 685 MPa or more .
Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 (1)
However, each element display in the formula indicates the content (% by mass) of the element. The steel pipe pile joint according to claim 1, wherein the compressive residual stress in the pipe axial direction on the surface is 250 MPa or less. A steel pipe pile having the steel pipe pile joint according to claim 1 or 2 at an end of a steel pipe. A steel pipe pile construction method for driving steel pipe piles by connecting the steel pipe piles according to claim 3 with the steel pipe pile joint.

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