Ultra-high-strength steel pipe pile and method for manufacturing the same

Abstract

To provide an ultra high strength steel pipe pile and a producing method thereof.SOLUTION: An ultra-high strength steel pipe pile has a host material portion and a weld portion formed by submerged arc welding in a pipe axial direction. The host material portion has a specific component composition, and the component of the host material portion has a Pcm of Equation (1) of 0.170-0.220, and a weld metal of the weld portion has a specific component composition. The component of the weld metal has a Pcm of Equation (1) of 0.200-0.300 of and a CS of Equation (2) of 0.0 or more. A yield strength in a tube axial direction of the host material portion is 700 MPa or more. A tensile strength in the tube axial direction of the host material portion is 780-930 MPa. A tensile strength in a circumferential direction of a welded joint is 780 MPa or more. A Charpy absorption energy at -10°C of the host material portion and the weld metal is 27 J or more. Pcm=C+Mn / 20+Si / 30+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B...(1) CS=5.1-36.3C-0.6Mn-Ni+1.4Mo...(2)SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an ultra-high strength steel pipe pile in which the yield strength in the pipe axis direction of the base material part is 700 MPa or more, and a method for manufacturing the same.

Background Art

[0002] In recent years, in response to large-scale earthquakes, there has been a strong demand for higher strength of steel pipe piles used as the foundation of structures. Especially in the case of designing structures where plasticization of the steel pipe pile itself is not allowed, when using steel pipe piles with a tensile strength of 570 MPa class that have been used so far, it is difficult to suppress plasticization only by simply burying the pile, and it is necessary to adopt a special structure such as installing a support pile. As a result, it is often impossible to achieve from the viewpoints of construction period, cost, and installation space. Furthermore, especially for steel pipe piles used in cold regions, high low-temperature toughness may also be required, making it difficult to apply centrifugally cast steel pipes, etc., which are relatively easy to increase in strength, to steel pipe piles.

[0003] Since steel pipe piles are a type of product that requires mass production, it is desirable to apply the TMCP process to the production of the base material rather than the quenching and tempering process.

[0004] On the other hand, for example, Patent Documents 1 and 2 disclose a method for manufacturing an electric resistance welded steel pipe for a steel pipe pile that achieves both high strength and high toughness by welding a steel pipe material obtained by subjecting a hot-rolled steel sheet to a cold roll forming process.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the manufacturing methods described in Patent Documents 1 and 2 describe steel pipe piles with a yield strength of 450 MPa or more and 624 MPa or less, but they cannot cope with the manufacture of even higher-strength steel pipe piles.

[0007] In the case of steel pipe piles used for landslide prevention and soil retention in port areas, stress is applied in the pipe axis direction, and it is usually designed within the elastic range. Therefore, the yield strength becomes an important strength parameter. If the yield strength in the pipe axis direction is 700 MPa or more, it can withstand deformation within the elastic range even during a large earthquake or tsunami of the magnitude of the Great East Japan Earthquake, and it is possible to suppress the subsidence and uplift of the surrounding ground caused by the deformation of the steel pipe pile due to the earthquake. The yield strength and tensile strength in the pipe axis direction using thick steel plates are often lower than those in the pipe circumference direction, and it is also a problem that there are cases where the ultra-high strength in the pipe axis direction cannot be achieved with conventional welded steel pipes with an ultra-high strength in the pipe axis direction.

[0008] Also, when manufacturing welded steel pipes by submerged arc welding, suppressing the occurrence of cold cracking, which is a problem with high-strength materials, becomes an important issue.

[0009] The present invention has been made in view of the above problems, and an object thereof is to provide an ultra-high-strength steel pipe pile having excellent toughness and excellent cold crack resistance, and a manufacturing method thereof.

[0010] Here, "ultra-high strength" in the present invention means that the yield strength in the pipe axis direction of the base material part is 700 MPa or more, the tensile strength in the pipe axis direction of the base material part is 780 to 930 MPa, and the tensile strength in the pipe circumference direction of the welded joint is 780 MPa or more.

[0011] Also, "excellent toughness" in the present invention means that the Charpy absorption energy at -10 °C in the base material part and the weld metal is 27 J or more.

[0012] Also, "excellent cold crack resistance" in the present invention means a case where no cracks are detected when an ultrasonic flaw detection test is performed on the welded part in accordance with JIS Z3060.

Means for Solving the Problem

[0013] As a result of studying the technology for manufacturing an ultra-high strength steel pipe pile with a yield strength of 700 MPa or more in the pipe axis direction of the base material part at high productivity, the present inventors obtained the following findings.

[0014] First, the manufacturing method of the thick steel plate used as the material of the steel pipe was TMCP (Thermo-Mechanical Control Process) or DQT (Direct Quenching and Tempering) in order to maintain high productivity. Furthermore, the welding method in the steel pipe axis direction was submerged arc welding.

[0015] The most problematic in the above premise is ensuring weldability. Therefore, in the present invention, further studies were conducted to ensure excellent low-temperature crack resistance even for ultra-high strength steel pipes. As a result, it was clarified that by ensuring Pcm≤0.220 as the component composition of the base material part and Pcm≤0.300 and CS≥0.0 as the component composition of the weld metal, no low-temperature cracks occur even when welding construction in the steel pipe axis direction is performed without preheating. It was also clarified that it is necessary to control the component composition of the base material part to Pcm≥0.170 and the component composition of the weld metal to Pcm≥0.200 in order to ensure the joint strength of the welded joint.

[0016] The present invention was completed by further studying the above findings, and the gist of the present invention is as follows. [1] An ultra-high strength steel pipe pile having a base material part and a welded part formed by submerged arc welding in the pipe axis direction, The component composition of the base material part is, by mass%, C: 0.04 to 0.11%, Si: 0.55% or less, Mn: 1.30 to 2.00%, P: 0.030% or less, S: 0.006% or less, Al: 0.060% or less, Nb: 0.01 to 0.10%, N: Below 0.006% contains, and further, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 0.10% or less, Ti: 0.05% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.100% or less contains one or more selected from among them, with the balance being composed of Fe and inevitable impurities, the components of the base metal part have a Pcm defined by formula (1) of 0.170 to 0.220, the component composition of the weld metal of the welded part is, by mass%, C: 0.04 to 0.11%, Si: 0.20 to 0.55%, Mn: 1.30 to 2.00%, Ni: 0.50 to 5.00%, Mo: 0.30 to 5.00%, N: 0.010% or less, O: 0.060% or less, P: 0.030% or less, S: 0.010% or less, Al: 0.100% or less contains, and further, Cu: 1.00% or less, Cr: 1.00% or less, Nb: 0.08% or less, V: 0.10% or less, Ti: 0.05% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.100% or less contains one or more selected from among them, with the balance being composed of Fe and inevitable impurities, The components of the weld metal have a Pcm defined by formula (1) of 0.200 to 0.300 and a CS defined by formula (2) of 0.0 or more. The yield strength of the base metal part in the pipe axis direction is 700 MPa or more, and the tensile strength of the base metal part in the pipe axis direction is 780 to 930 MPa. The tensile strength of the welded joint in the circumferential direction of the pipe is 780 MPa or more. The base metal part and the weld metal are ultra-high strength steel pipe piles with a Charpy impact energy of 27 J or more at -10°C. Pcm = C + Mn / 20 + Si / 30 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ··· (1) CS = 5.1 - 36.3C - 0.6Mn - Ni + 1.4Mo ··· (2) Here, each element in formula (1) and formula (2) represents the content (mass%), and for elements not contained, the content is taken as 0. [2] The manufacturing method of the ultra-high strength steel pipe pile according to [1], The steel having the component composition of the base metal part is heated to 1000 to 1250°C, then hot rolling is performed with a rolling end temperature of 650 to 950°C, then accelerated cooling is performed to a cooling stop temperature of 500°C or less, and then air cooling is used to cool to room temperature to form a thick steel plate. The thick steel plate is cold-formed into a tubular shape, and the butted part of the tubular shape is subjected to submerged arc welding in the pipe axis direction using a welding wire and a flux having a component composition of one electrode or an average component composition of a plurality of electrodes containing, in mass%, C: 0.01 to 0.14%, Si: 0.20 to 0.70%, Mn: 0.70 to 2.30%, Ni: 1.0 to 10.0%, and Mo: 0.5 to 5.0% to form the weld metal. The manufacturing method of the ultra-high strength steel pipe pile. [3] The manufacturing method of the ultra-high strength steel pipe pile according to [1], The steel having the component composition of the base metal part is heated to 1000 to 1250°C, then hot rolling is performed with a rolling end temperature of 650 to 950°C, then accelerated cooling is performed to a cooling stop temperature of less than 300°C, then reheated to a tempering temperature of 300 to 650°C, and then cooled to room temperature to form a thick steel plate. The thick steel plate is cold-formed into a tubular shape, and the butt joint of the tubular shape is subjected to submerged arc welding in the pipe axis direction using a welding wire and a flux whose composition of one electrode or the average composition of a plurality of electrodes contains, by mass%, C: 0.01 to 0.14%, Si: 0.20 to 0.70%, Mn: 0.70 to 2.30%, Ni: 1.0 to 10.0% and Mo: 0.5 to 5.0% to form the weld metal, a method for manufacturing an ultra-high strength steel pipe pile.

Effect of the Invention

[0017] According to the present invention, an ultra-high strength steel pipe pile having a yield strength of 700 MPa or more in the pipe axis direction of the base material portion used for, for example, a tsunami protection structure can be manufactured with high productivity, which is extremely effective industrially. Further, according to the present invention, an ultra-high strength steel pipe pile having excellent toughness, excellent low temperature crack resistance, and ultra-high strength can be provided.

Embodiments for Carrying Out the Invention

[0018] First, the ultra-high strength steel pipe pile of the present invention will be described. The ultra-high strength steel pipe pile of the present invention has a base material portion and a weld portion formed by submerged arc welding in the pipe axis direction. The reasons for limiting each constituent element of the present invention will be described below.

[0019] 1. Regarding the component composition of the thick steel plate (base material portion) First, the reason for defining the component composition of the thick steel plate of the present invention will be explained. In the component composition, all “%” means “mass%”.

[0020] C: 0.04 to 0.11% C is the most effective element for increasing the strength of the thick steel plate manufactured by accelerated cooling. However, if the C content is less than 0.04%, sufficient strength cannot be ensured, and if the C content exceeds 0.11%, the toughness and weld crack susceptibility deteriorate. Therefore, the C content is in the range of 0.04 to 0.11%. The C content is preferably 0.05% or more and preferably 0.08% or less.

[0021] Si: 0.55% or less Si is added for deoxidation, but when the Si content exceeds 0.55%, toughness and weldability deteriorate. Therefore, the Si content should be within the range of 0.55% or less. The Si content is preferably 0.35% or less. To obtain the effect of deoxidation, the Si content is preferably 0.03% or more.

[0022] Mn: 1.30 - 2.00% Mn is inexpensive and is actively added to improve the strength and toughness of thick steel plates. However, when the Mn content is less than 1.30%, the effect is not sufficient, and when the Mn content exceeds 2.00%, weldability deteriorates. Therefore, the Mn content should be within the range of 1.30 - 2.00%. The Mn content is preferably 1.60 - 2.00%.

[0023] P: 0.030% or less P is an inevitable impurity element, and it deteriorates toughness and weldability by increasing the hardness of the center segregation part. This tendency becomes significant when the P content exceeds 0.030%. Therefore, the P content should be 0.030% or less. The P content is preferably 0.015% or less. However, excessive reduction of P leads to an increase in refining cost, so the P content is preferably 0.001% or more.

[0024] S: 0.006% or less S generally becomes MnS - based inclusions in the steel plate, deteriorating toughness and weldability. This tendency becomes significant when the S content exceeds 0.006%. Therefore, the S content should be 0.006% or less. The S content is preferably 0.003% or less. However, excessive reduction of S leads to an increase in refining cost, so the S content is preferably 0.0001% or more.

[0025] Al: 0.060% or less Al is added for deoxidation, but when the Al content exceeds 0.060%, toughness and weldability deteriorate. Therefore, the Al content should be within the range of 0.060% or less. The Al content is preferably 0.010% or more and preferably 0.050% or less.

[0026] Nb: 0.01 - 0.10% Nb is an element that suppresses the grain growth of crystal grains during hot rolling and improves toughness through fine grain formation. To obtain this effect, it is necessary to contain Nb at 0.01% or more. On the other hand, when Nb is contained in excess of 0.10%, the weldability deteriorates. Therefore, the Nb content is in the range of 0.01 - 0.10%. The Nb content is preferably 0.02% or more and preferably 0.06% or less.

[0027] N: 0.006% or less N is an inevitable impurity element, and solid solution deteriorates toughness and weldability. This tendency becomes significant when the N content exceeds 0.006%. Therefore, the N content is 0.006% or less. However, for reducing refining costs, it is preferable that the N content is 0.001% or more.

[0028] The above components are the basic components of the thick steel plate (the base material part of the ultra-high strength steel pipe pile) used in the ultra-high strength steel pipe pile of the present invention. However, in order to obtain the desired strength and toughness, in addition to this basic component, one or more selected from the following alloy elements may be contained as necessary. Note that each component of Cu, Ni, Cr, Mo, V, Ti, B, Ca, Mg, and REM described below can be contained as necessary, so these components may be 0%.

[0029] Cu: 1.00% or less Cu is an element effective for improving toughness and increasing strength and can be contained as necessary. If Cu is not contained at 0.01% or more, there is no effect, and when Cu is contained in excess of 1.00%, surface defects occur. Therefore, when Cu is contained, it is preferable that the Cu content is 1.00% or less. The Cu content is more preferably 0.50% or less. The Cu content is preferably 0.01% or more.

[0030] Ni: 1.00% or less Ni is an element effective for improving toughness and increasing strength and can be contained as required. If Ni is not contained at 0.01% or more, there will be no effect, and if Ni is contained exceeding 1.00%, surface defects will occur. Therefore, when Ni is contained, it is preferable that the Ni content is 1.00% or less. The Ni content is more preferably 0.50% or less. The Ni content is preferably 0.01% or more.

[0031] Cr: 1.00% or less Cr is an element effective for increasing strength by enhancing hardenability and can be contained as required. If Cr is not contained at 0.01% or more, there will be no effect, and if Cr is contained exceeding 1.00%, weldability will deteriorate. Therefore, when Cr is contained, it is preferable that the Cr content is 1.00% or less. The Cr content is more preferably 0.50% or less. The Cr content is preferably 0.01% or more.

[0032] Mo: 1.00% or less Mo is an element effective for increasing strength by enhancing hardenability and can be contained as required. If Mo is not contained at 0.01% or more, there will be no effect, and if Mo is contained exceeding 1.00%, weldability will deteriorate. Therefore, when Mo is contained, it is preferable that the Mo content is 1.00% or less. The Mo content is more preferably 0.50% or less. The Mo content is preferably 0.01% or more.

[0033] V: 0.10% or less V is an element that increases strength by precipitation strengthening and can be contained as required. If V is not contained at 0.01% or more, there will be no effect, and if V is contained exceeding 0.10%, toughness will deteriorate. Therefore, when V is contained, it is preferable that the V content is 0.10% or less. The V content is more preferably 0.05% or less. The V content is preferably 0.01% or more.

[0034] Ti: 0.05% or less Ti not only forms TiN to suppress the grain growth of crystal grains during slab heating, but also suppresses the grain growth of crystal grains in the heat-affected zone of welding, and improves toughness through the refinement of crystal grains in the thick steel plate and the heat-affected zone of the thick steel plate welding. Ti can be contained as required. If the Ti content is less than 0.01%, there will be no such effect, and if the Ti content exceeds 0.05%, the toughness will deteriorate. Therefore, when Ti is contained, it is preferably that the Ti content is 0.05% or less. The Ti content is more preferably 0.03% or less. The Ti content is preferably 0.01% or more.

[0035] B: 0.0050% or less B is an element that significantly improves hardenability and increases strength, and can be contained as required. If the B content is less than 0.0001%, there will be no such effect, and if the B content exceeds 0.0050%, the toughness will deteriorate. Therefore, when B is contained, the B content should be 0.0050% or less. The B content is preferably 0.0001% or more.

[0036] Ca: 0.0100% or less Ca is an element effective for controlling the morphology of sulfide inclusions and improving ductility, and can be contained as required. If the Ca content is less than 0.0010%, there will be no such effect, and even if the Ca content exceeds 0.0100%, the effect will be saturated, and rather, the toughness and internal quality will deteriorate due to the decrease in cleanliness. Therefore, when Ca is contained, it is preferably that the Ca content is 0.0100% or less. The Ca content is preferably 0.0010% or more.

[0037] Mg: 0.0100% or less Mg is an element effective for controlling the morphology of sulfide inclusions and improving ductility, and can be contained as required. If the Mg content is less than 0.0010%, there will be no such effect, and even if the Mg content exceeds 0.0100%, the effect will be saturated, and rather, the toughness and internal quality will deteriorate due to the decrease in cleanliness. Therefore, when Mg is contained, it is preferably that the Mg content is 0.0100% or less. The Mg content is preferably 0.0010% or more.

[0038] REM: Below 0.100% REM (Rare Earth Metal) is an element effective for controlling the form of sulfide inclusions and improving ductility, and can be contained as necessary. If the REM content is less than 0.001%, there is no such effect. Even if REM is contained in excess of 0.100%, the effect is saturated, and rather, toughness and internal quality are deteriorated due to a decrease in cleanliness. Therefore, when REM is contained, it is preferable that the REM content be 0.100% or less. The REM content is preferably 0.001% or more.

[0039] The balance other than the above-described components is Fe and inevitable impurities.

[0040] Pcm: 0.170 - 0.220 Pcm is widely known as an index of welding crack susceptibility. When Pcm defined by the following formula (1) in the components of the base metal part is less than 0.170, the hardness of the heat-affected zone of welding decreases, causing a decrease in joint strength. Further, when Pcm in the components of the base metal part exceeds 0.220, the hardness of the heat-affected zone of welding increases, and as a result, cold cracking occurs after welding. Therefore, Pcm in the components of the base metal part is controlled to 0.170 - 0.220. Pcm in the components of the base metal part is preferably 0.180 or more and preferably 0.210 or less.

[0041] Pcm = C + Mn / 20 + Si / 30 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ···(1) Here, each element in formula (1) represents the content (mass%), and for elements not contained, the content is taken as 0.

[0042] 2. Regarding the component composition of the weld metal The reason for defining the component composition of the weld metal of the welded part formed by the submerged arc welding in the present invention will be explained. Note that all “%” in the component composition means “mass%”.

[0043] C: 0.04 - 0.11% The C content of the weld metal needs to be within the range of 0.04 to 0.11%. If the C content is less than 0.04%, the strength of the weld metal is insufficient and hot cracking occurs. If the C content exceeds 0.11%, carbides increase in the weld metal and toughness deteriorates. The C content is preferably 0.05% or more and preferably 0.08% or less.

[0044] Si: 0.20 - 0.55% Since Si has the function of promoting the segregation of P and S, it not only promotes the occurrence of cracks but also slows down the diffusion of C. Therefore, although Si is a ferrite-stabilizing element, it stabilizes austenite and promotes the formation of martensite, resulting in deterioration of the toughness of the weld metal. Therefore, the Si content needs to be 0.55% or less. If the Si content is too low, the oxygen content in the weld metal increases and there is a risk of impairing toughness, so the Si content needs to be 0.20% or more. Therefore, the Si content of the weld metal is within the range of 0.20 to 0.55%. The Si content is preferably 0.25% or more and preferably 0.50% or less.

[0045] Mn: 1.30 - 2.00% The Mn content of the weld metal needs to be within the range of 1.30 to 2.00%. Mn promotes the solidification segregation of P, not only promotes the occurrence of cracks but also increases the lamination defect energy, so the austenite stabilization effect at 800°C or lower is remarkable. As a result, bainite transformation is suppressed and martensite is likely to occur, so adding a large amount of Mn deteriorates the toughness of the weld metal. Therefore, the Mn content needs to be 2.00% or less. However, when the Mn content is less than 1.30%, the oxygen content in the weld metal increases and there is rather a concern of impairing toughness, so it is necessary to contain 1.30% or more of Mn. The Mn content is preferably 1.50% or more and preferably 1.80% or less.

[0046] Ni: 0.50 - 5.00% Ni is an important element for improving the low-temperature toughness of high-strength steel. Different from Mn, the addition of Ni lowers the stacking fault energy, making it difficult to mechanically stabilize austenite and ensuring ductility. Therefore, it is necessary to contain 0.50% or more of Ni for toughness improvement. However, when a large amount of Ni is added to chemically stabilize austenite, the ferrite phase does not crystallize in the final solidification phase, and low-temperature cracking occurs. Therefore, it is necessary to contain Ni within the range where CS is 0.0 or more while taking the balance with Mo, C, and Mn. The upper limit of the Ni content is 5.00%. Therefore, the Ni content of the weld metal shall be within the range of 0.50 to 5.00%. The Ni content is preferably 1.00% or more. The Ni content is preferably 4.00% or less.

[0047] Mo: 0.30 - 5.00% Mo is an extremely important element for controlling the solidification morphology of weld metal as a ferrite-stabilizing element, and has an extremely important function of destabilizing austenite to produce bainite in the weld metal microstructure and improving toughness. Therefore, it is necessary to make the Mo content at least 0.30% or more. On the other hand, when the Mo content exceeds 5.00%, the toughness of the outer surface weld metal is particularly impaired. Therefore, the Mo content of the weld metal needs to be within the range of 0.30 to 5.00%. The Mo content is preferably 0.40% or more and preferably 4.00% or less.

[0048] N: 0.010% or less Reducing the amount of solid-solution N in the weld metal improves toughness. In particular, when the N content is 0.010% or less, the toughness of the weld metal is significantly improved, so the upper limit is set at 0.010%. To suppress the increase in refining cost during the manufacture of welding materials, the N content is preferably 0.001% or more.

[0049] O: 0.060% or less Reducing the oxygen content in the weld metal improves toughness. In particular, when the content of O (oxygen) is 0.060% or less, the toughness of the weld metal is significantly improved. Therefore, the upper limit is set at 0.060%. To suppress the increase in refining cost during the production of welding materials, the O content is preferably 0.001% or more.

[0050] P: 0.030% or less P is an inevitable impurity element that deteriorates the toughness and weldability of the weld metal. This tendency becomes prominent when the P content exceeds 0.030%. Therefore, the P content is set at 0.030% or less. The P content is preferably 0.015% or less. However, excessive reduction of P leads to an increase in refining cost during the production of welding materials. Therefore, the P content is preferably 0.001% or more.

[0051] S: 0.010% or less In the weld metal, S generally becomes MnS-based inclusions, deteriorating toughness and weldability. This tendency becomes prominent when the S content exceeds 0.010%. Therefore, the S content is set at 0.010% or less. The S content is preferably 0.008% or less. However, excessive reduction of S leads to an increase in refining cost during the production of welding materials. Therefore, the S content is preferably 0.0001% or more.

[0052] Al: 0.100% or less Al is added to reduce the oxygen content in the weld metal. However, when the Al content exceeds 0.100%, it deteriorates toughness and weldability. Therefore, the Al content is within the range of 0.100% or less. The Al content is preferably 0.080% or less. The Al content is preferably 0.010% or more.

[0053] The above components are the basic components of the weld metal in the ultra-high strength steel pipe pile of the present invention. However, in order to obtain the desired strength and toughness, in addition to this basic component, one or more selected from the following alloy elements may be contained as necessary. Note that each of the components of Cu, Cr, Nb, V, Ti, B, Ca, Mg, and REM described below can be contained as necessary, so these components may be 0%.

[0054] Cu: Below 1.00% Cu not only widens the temperature range between the liquidus and solidus, promotes the occurrence of high-temperature cracking, but also increases the low-temperature cracking susceptibility. Cu can be contained as needed. Therefore, when containing Cu, the Cu content in the weld metal should be 1.00% or less. The Cu content is preferably 0.01% or more.

[0055] Cr: Below 1.00% Cr is an element that controls the solidification morphology of the weld metal as a ferrite-stabilizing element, and destabilizes austenite to generate bainite in the weld metal microstructure, improving toughness. Cr can be contained as needed. When the Cr content exceeds 1.00%, it particularly impairs the toughness of the outer surface weld metal. Therefore, when containing Cr, the Cr content in the weld metal should be 1.00% or less. The Cr content is preferably 0.01% or more.

[0056] Nb: Below 0.08% Nb contributes to the high strength of the weld metal, but when containing more than 0.08% Nb, it impairs the toughness of the weld metal. Nb can be contained as needed. Therefore, when containing Nb, the Nb content in the weld metal should be 0.08% or less. The Nb content is preferably 0.01% or more.

[0057] V: Below 0.10% V contributes to the high strength of the weld metal, but when containing more than 0.10% V, it impairs the toughness of the weld metal. V can be contained as needed. Therefore, when containing V, the V content in the weld metal should be 0.10% or less. The V content is preferably 0.01% or more.

[0058] Ti: Below 0.05% Ti acts as a deoxidizing element in the weld metal, is effective in reducing oxygen in the weld metal, and can be contained as needed. To obtain this effect, a Ti content of 0.01% or more is required. However, when the Ti content exceeds 0.05%, the excess Ti forms carbides and deteriorates the toughness of the weld metal. Therefore, the upper limit of the Ti content is set at 0.05%. Accordingly, when Ti is contained, the Ti content of the weld metal shall be 0.05% or less. The Ti content is preferably 0.01% or more.

[0059] B: 0.0050% or less B can be added to ensure the strength of the weld metal. B can be contained as needed. To obtain the effect, it is effective to set the B content at 0.0001% or more. However, when the B content in the weld metal exceeds 0.0050%, a martensite structure with low toughness is formed. Therefore, when B is contained, the B content of the weld metal shall be 0.0050% or less. The B content is preferably 0.0001% or more.

[0060] Ca: 0.0100% or less Ca is an element effective in controlling the morphology of sulfide inclusions and improving ductility, and can be contained as needed. When the Ca content is less than 0.0010%, there is no such effect. Even if Ca is contained in excess of 0.0100%, the effect is saturated, and rather, the toughness and internal quality deteriorate due to a decrease in cleanliness. Therefore, when Ca is contained, the Ca content of the weld metal shall be 0.0100% or less. The Ca content is preferably 0.0010% or more.

[0061] Mg: 0.0100% or less Mg is an element effective in controlling the morphology of sulfide inclusions and improving ductility, and can be contained as needed. When the Mg content is less than 0.0010%, there is no such effect. Even if Mg is contained in excess of 0.0100%, the effect is saturated, and rather, the toughness and internal quality deteriorate due to a decrease in cleanliness. Therefore, when Mg is contained, the Mg content of the weld metal shall be 0.0100% or less. The Mg content is preferably 0.0010% or more.

[0062] REM: Below 0.100% REM is an element effective for controlling the morphology of sulfide inclusions and improving ductility, and can be contained as necessary. If the REM content is less than 0.001%, there is no such effect. Even if REM is contained in excess of 0.100%, the effect saturates, and rather, toughness and internal quality deteriorate due to a decrease in cleanliness. Therefore, when REM is contained, the REM content of the weld metal shall be 0.100% or less. The REM content is preferably 0.001% or more.

[0063] The balance other than the above-described components is Fe and inevitable impurities.

[0064] Pcm: 0.200 - 0.300 Pcm is an index of weld cracking susceptibility and has a high correlation with the tensile strength of the weld metal. When Pcm defined by the above formula (1) in the components of the weld metal is less than 0.200, the hardness of the weld metal decreases, and as a result, the desired tensile strength of the weld metal cannot be obtained. Also, when Pcm in the components of the weld metal exceeds 0.300, the hardness of the weld metal increases, and as a result, cold cracking of the weld metal occurs. Therefore, Pcm in the weld metal is controlled to 0.200 - 0.300. Pcm in the weld metal is preferably 0.210 or more and preferably 0.290 or less.

[0065] CS ≧ 0.0 CS is a parameter for quantifying the occurrence of transverse cracks in the weld metal. By controlling the component composition of the weld metal within the range where CS ≧ 0.0, it is possible to prevent the formation of coarse crystal grains during solidification of the weld metal and prevent transverse cracks in high-strength weld metal. Therefore, CS defined by the following formula (2) in the weld metal shall be 0.0 or more. To prevent the formation of coarse crystal grains during solidification of the weld metal, CS is preferably 0.5 or less.

[0066] CS = 5.1 - 36.3C - 0.6Mn - Ni + 1.4Mo ···(2) Here, each element in formula (2) represents the content (mass %), and for an element not contained, the content is set to 0.

[0067] For a weld metal that satisfies the above numerical ranges of Pcm and CS, as described above, it will be a welded part that has excellent low-temperature crack resistance and can ensure the joint strength of the welded joint.

[0068] 3. Performance of Steel Pipe Piles The ultra-high-strength steel pipe piles of the present invention will have the following ultra-high-strength base material parts and weld metals. The reasons for specifying the performance of the steel pipe piles are shown below.

[0069] Yield strength in the pipe axis direction of the base material part: 700 MPa or more In the present invention, it conforms to the requirement values of JIS SBHS700, which has a lot of application records as an actual steel plate for welded structures. Note that since the steel pipe pile receives stress in the pipe axis direction when an earthquake or tsunami occurs, the yield strength in the pipe axis direction is specified.

[0070] Tensile strength in the pipe axis direction of the base material part: 780 - 930 MPa Similar to the above, it conforms to the requirement values of JIS SBHS700, which has a lot of application records as an actual steel plate for welded structures. Note that since the steel pipe pile receives stress in the pipe axis direction when an earthquake or tsunami occurs, the tensile strength in the pipe axis direction is specified.

[0071] Tensile strength in the circumferential direction of the welded joint: 780 MPa or more The tensile strength of the welded joint specifies the same lower limit of tensile strength as that of the base material part. Since the reasons for the specification are the same, the explanation is omitted.

[0072] Charpy impact energy of the base material part at -10°C: 27 J or more Charpy impact energy of the weld metal at -10°C: 27 J or more In the present invention, the burial of steel pipe piles in cold regions is assumed, and -10°C is defined as the specified temperature. For the Charpy test specimens, for the base metal part, a 2-mm V-notch test specimen is taken in the pipe axis direction and at the position of 1 / 4t (1 / 4 of the plate thickness), and for the weld metal, a 2-mm V-notch test specimen is taken in the circumferential direction of the pipe and at the position of 1 / 2t (1 / 2 of the weld thickness).

[0073] Note that the yield strength, tensile strength, and Charpy absorbed energy described above can be measured by the method described in the examples described later.

[0074] Next, an embodiment of the manufacturing method of the ultra-high-strength steel pipe pile of the present invention will be described.

[0075] First, a thick steel plate (weldment) manufactured under the manufacturing conditions described later is prepared. Next, this thick steel plate is cold-formed into a tubular shape, and submerged arc welding is performed in the pipe axis direction on the butting portions at both ends in the width direction of the thick steel plate formed into a tubular shape using the welding wire and flux described later to form a weld portion that becomes the weld metal, thereby manufacturing a steel pipe pile welded and joined via the weld metal.

[0076] The submerged arc welding is either single electrode (submerged arc welding with a single welding wire) or multiple electrodes (submerged arc welding with multiple welding wires). Grooving is performed on the weld planned portion of the butting portion to form a groove (for example, X-groove, J-groove), and submerged arc welding is performed in this groove to form the weld metal having the above component composition. The number of layers is either one-layer welding on each of the inner and outer surfaces or multi-layer welding on each of the inner and outer surfaces according to the pipe thickness.

[0077] 4. Manufacturing method of thick steel plate In the present invention, the thick steel plate can be manufactured by the TMCP process (First Embodiment) or the DQT process (Second Embodiment). Note that, from the viewpoint of application to steel pipe piles such as tsunami protection structures, the plate thickness of the thick steel plate is preferably 6 to 60 mm.

[0078] [First Embodiment] When manufacturing a thick steel plate by the TMCP process, steel (slab) having the component composition of the base material part described above is heated to 1000 to 1250°C, then hot rolling is performed with the finish rolling temperature being 650 to 950°C, then accelerated cooling is performed to a cooling stop temperature of 500°C or lower, and then air cooling is performed to cool to room temperature to manufacture a thick steel plate.

[0079] The reasons for the regulations of the manufacturing method of the thick steel plate used as the material of the steel pipe are shown below. Regarding the conditions common to the first and second embodiments, the explanations are omitted as appropriate.

[0080] Heating temperature: 1000 to 1250°C At the start of hot rolling, in order to completely austenitize the steel structure, the lower limit temperature when heating the slab is set to 1000°C. On the other hand, when the slab is heated to a temperature exceeding 1250°C, the austenite grain growth is remarkable and the toughness of the base material deteriorates, so the upper limit temperature when heating the slab is set to 1250°C. The above heating temperature is preferably 1050°C or higher and preferably 1200°C or lower.

[0081] Finish rolling temperature: 650 to 950°C The finish temperature of hot rolling is most correlated with the properties of the base material. When the finish rolling temperature is less than 650°C, the effect of subsequent accelerated cooling decreases and the desired strength cannot be satisfied. On the other hand, when the finish rolling temperature exceeds 950°C, the crystal grains become coarse and the toughness of the base material deteriorates. Therefore, the finish rolling temperature is set to 650 to 950°C. The above heating temperature is preferably 700°C or higher and preferably 900°C or lower.

[0082] Cooling stop temperature: 500°C or lower When the cooling stop temperature exceeds 500°C, the transformation strengthening is insufficient and the desired strength cannot be achieved, so the upper limit of the cooling stop temperature is set to 500°C. Therefore, the cooling stop temperature is 500°C or lower. Although there is no particular lower limit for the cooling stop temperature, since the toughness of the base material tends to deteriorate when the cooling stop temperature is low, the cooling stop temperature is more preferably 300 to 500°C.

[0083] [Second Embodiment] When manufacturing a thick steel plate by the DQT process, steel (slab) having the component composition of the base material part described above is heated to 1000 - 1250 °C, then hot rolling is performed with the finish rolling temperature being 650 - 950 °C, then accelerated cooling is performed to a cooling stop temperature of less than 300 °C, then reheated to a tempering temperature of 300 - 650 °C, and then cooled to room temperature to manufacture the thick steel plate. Note that conditions different from those of the first embodiment will be described below.

[0084] Cooling stop temperature: Less than 300 °C When the cooling stop temperature is less than 300 °C, the strength before tempering increases due to transformation strengthening, and the desired strength can be ensured even after tempering. Therefore, the cooling stop temperature is set to less than 300 °C. Although no particular lower limit for the cooling stop temperature is provided, since the toughness of the base material tends to deteriorate when the cooling stop temperature is low, it is preferably 10 °C or higher.

[0085] Note that in the present invention, when the stop temperature of the accelerated cooling is made less than 300 °C, tempering can be performed under the following conditions thereafter.

[0086] Tempering temperature: 300 - 650 °C When the tempering temperature is less than 300 °C, the fixation of dislocations cannot be sufficiently realized and the tempering effect cannot be obtained, so the tempering temperature is set to 300 °C or higher. Also, when the tempering temperature exceeds 650 °C, the disappearance of dislocations becomes large and the desired strength cannot be obtained, so the upper limit of the tempering temperature is set to 650 °C. Therefore, the tempering temperature is 300 - 650 °C. The above tempering temperature is preferably 350 °C or higher and preferably 600 °C or lower. After reheating to the tempering temperature, it is cooled to room temperature by, for example, air cooling or water cooling.

[0087] Subsequently, the welding method will be described.

[0088] In the present invention, submerged arc welding is used. In order to form a weld metal having the above-described performance (strength, toughness, and low-temperature crack resistance) at the welded portion, the component composition of one electrode or the average component composition of a plurality of electrodes shall contain, by mass%, C: 0.01 to 0.14%, Si: 0.20 to 0.70%, Mn: 0.70 to 2.30%, Ni: 1.0 to 10.0%, and Mo: 0.5 to 5.0% using a welding wire.

[0089] In order to form the weld metal of the present invention, it is preferable to control the heat input of welding to 2.0 to 10.0 kJ / mm.

[0090] 5. Regarding the component composition of the welding wire The reason for defining the component composition of the welding wire used in the submerged arc welding of the present invention will be explained. Note that all “%” in the component composition means “mass%”. In addition, submerged arc welding may be performed using a plurality of electrodes (a plurality of welding wires). In that case, the average component composition is used. It is only necessary that this average component composition be within the following numerical range.

[0091] C: 0.01 to 0.14% The C content of the welding wire is in the range of 0.01 to 0.14% in consideration of the dilution of the base material and the amount entering from the atmosphere in order to obtain the range of the C amount required for the above-described weld metal. The C content of the welding wire is preferably 0.04% or more and preferably 0.12% or less.

[0092] Si: 0.20 to 0.70% The Si content of the welding wire is in the range of 0.20 to 0.70% in consideration of the dilution of the base material and the reduction from SiO2 in the flux in order to obtain the range of the Si amount required for the above-described weld metal.

[0093] Mn: 0.70 to 2.30% The Mn content of the welding wire is in the range of 0.70 to 2.30% in consideration of the dilution of the base material and the consumption loss due to deoxidation in order to obtain the range of the Mn amount required for the above-described weld metal.

[0094] Ni: 1.0 to 10.0% The Ni content of the welding wire is in the range of 1.0 to 10.0% in order to obtain the range of the Ni amount required in the above welding metal.

[0095] Mo: 0.5 to 5.0% The Mo content of the welding wire is in the range of 0.5 to 5.0% in order to obtain the range of the Mo amount required in the above welding metal.

[0096] In addition to the above component composition, the welding wire of the present invention preferably contains one or more selected from the following alloy elements.

[0097] P: 0.050% or less P is at least 0.050% or less in order to satisfy the upper limit in the welding metal.

[0098] S: 0.010% or less S is at least 0.010% or less in order to satisfy the upper limit in the welding metal.

[0099] Al: 0.100% or less Al is at least 0.100% or less in order to satisfy the upper limit in the welding metal.

[0100] N: 0.0100% or less N is at least 0.0100% or less in order to satisfy the upper limit in the welding metal.

[0101] Cr: 3.0% or less Cr is at least 3.0% or less in order to satisfy the upper limit in the welding metal.

[0102] V: 0.10% or less V is at least 0.10% or less in order to satisfy the upper limit in the welding metal.

[0103] Ti: 0.10% or less Ti is at least 0.10% or less in order to satisfy the upper limit in the welding metal.

[0104] In the present invention, as long as the flux used for welding can satisfy the component composition of the desired weld metal, the type and component composition such as a melting type or a baking type can be freely selected.

Example

[0105] The present invention will be specifically described based on examples. Note that the following examples show a preferred example of the present invention, and the present invention is not limited to these examples.

[0106] A slab having the component composition shown in Table 1 was manufactured by continuous casting, and the obtained slab was subjected to a TMCP process or a DQT process under the conditions shown in Table 2 to manufacture a thick steel plate. The obtained thick steel plate was cold-formed into a tubular shape, and then after performing temporary welding by gas shielded arc welding, submerged arc welding was performed on the butted portion of the tubular shape in the pipe axis direction to manufacture a welded steel pipe.

[0107] Note that the blanks in Tables 1, 3, and 4 represent that elements are not intentionally added, and include not only the case where no element is contained (0%) but also the case where elements are unavoidably contained. The blank in the "tempering temperature" column in Table 2 indicates that the TMCP process was performed as the manufacturing method of the thick steel plate.

[0108] For submerged arc welding, two electrodes (two welding wires) shown in Table 4 were used, and the heat input per pass was set to 3.5 kJ / mm. The welding planned portion of the butted portion was processed to have an X groove with a groove angle of 30°, and multi-layer welding was performed with the number of layers changed according to the pipe thickness. The component composition of the welding wire used in submerged arc welding is shown in Table 3. All the fluxes used for submerged arc welding were CaO-CaF2-SiO2-based highly basic melting type fluxes. The diffusible hydrogen amount of this flux based on JIS Z 3118 was 4.6 ml / 100 g.

[0109]

Table 1

[0110]

Table 2

[0111]

Table 3

[0112] The performance of the manufactured welded steel pipe (welded joint) is shown in Table 4. Here, although the following evaluations were carried out using the welded steel pipe, it may be regarded as equivalent to the evaluation of the steel pipe pile of the present invention.

[0113] The yield strength (YS) in the pipe axis direction of the base metal part, the tensile strength (TS) in the pipe axis direction of the base metal part, and the tensile strength in the circumferential direction of the welded joint were evaluated by taking No. 12 test pieces of JIS Z 2241. The tensile test was carried out based on JIS Z 2241 (2011). The tensile test pieces of the base metal part were taken at the 90° position in the circumferential direction when the welding part of the welded joint was 0°, so that the tensile direction was parallel to the pipe axis direction. The tensile test pieces of the welded joint were taken from the center position of the plate thickness and the center position of the width of the weld metal of the welded joint, so that the tensile direction was parallel to the circumferential direction.

[0114] Also, the component composition of the weld metal was analyzed by taking chips from the position of 1 / 4t in the pipe thickness direction.

[0115] Also, for the Charpy test pieces of the base metal part and the weld metal, for the base metal part, at the 90° position in the circumferential direction when the welding part of the welded joint was 0°, a 2mm V-notch 10×10 test piece was taken from the position of 1 / 4t in the pipe axis direction, and for the weld metal, a 2mm V-notch 10×10 test piece was taken from the position of 1 / 2t in the circumferential direction so that the notch position was at the center of the weld metal. The Charpy impact test was carried out based on JIS Z 2242 (2018). The test temperature was -10°C. The number of test pieces was 3 for each, and the average value was taken as the absorbed energy (J) of the base metal part and the weld metal respectively. Note that the above "pipe axis direction" means that "the longitudinal direction of the test piece is parallel to the pipe axis direction".

[0116] In addition, for the evaluation of low-temperature crack resistance, the presence or absence of defects in the welded part was investigated. Specifically, an ultrasonic flaw detection test was conducted in accordance with JIS Z3060 (2015). When cracks were observed in the weld metal, it was evaluated that there were defects in the welded part, and when no cracks were observed in the weld metal, it was evaluated that there were no defects in the welded part.

[0117]

Table 4

[0118] As shown in Table 4, within the scope of the inventive examples of the present invention, the base material part and the weld metal satisfied the target performance, but outside the scope of the inventive examples, one or more of the target performances were not satisfied.

Claims

1. A super high strength steel pipe pile having a base material part and a welded part formed by submerged arc welding in the pipe axis direction, wherein the component composition of the base material part is, by mass%, C: 0.04 to 0.11%, Si: 0.55% or less, Mn: 1.30 to 2.00%, P: 0.030% or less, S: 0.006% or less, Al: 0.060% or less, Nb: 0.01 to 0.10%, N: 0.006% or less is contained, and further, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 0.10% or less, Ti: 0.05% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.100% or less is contained, and the balance consists of Fe and unavoidable impurities, the components of the base material part have a Pcm defined by formula (1) of 0.170 to 0.220, wherein the component composition of the weld metal of the welded part is, by mass%, C: 0.04 to 0.11%, Si: 0.20 to 0.55%, Mn: 1.30 to 2.00%, Ni: 0.50 to 5.00%, Mo: 0.30 to 5.00%, N: 0.010% or less, O: 0.060% or less, P: 0.030% or less, S: 0.010% or less, Al: 0.100% or less is contained, and further, Cu: 1.00% or less, Cr: 1.00% or less, Nb: 0.08% or less, V: 0.10% or less, Ti: 0.05% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.100% or less is contained, and the balance consists of Fe and unavoidable impurities, the components of the weld metal have a Pcm defined by formula (1) of 0.200 to 0.300 and a CS defined by formula (2) of 0.0 or more, the yield strength in the pipe axis direction of the base material part is 700 MPa or more, and the tensile strength in the pipe axis direction of the base material part is 780 to 930 MPa, the tensile strength in the pipe circumferential direction of the welded joint is 780 MPa or more, the base material part and the weld metal are super high strength steel pipe piles having a Charpy impact energy at -10°C of 27 J or more. Pcm = C + Mn / 20 + Si / 30 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B... (1) CS = 5.1 - 36.3C - 0.6Mn - Ni + 1.4Mo... (2) Here, each element in formula (1) and formula (2) represents the content (mass %), and for elements not contained, the content is taken as 0.

2. A method for manufacturing the ultra-high strength steel pipe pile according to claim 1, wherein steel having the component composition of the base material part is heated to 1000 to 1250 °C, then hot rolling is performed with the rolling finish temperature being 650 to 950 °C, then accelerated cooling is performed to a cooling stop temperature of 500 °C or lower, and then air cooling is performed to cool to room temperature to obtain a thick steel plate. The thick steel plate is formed into a tubular shape by cold working, and submerged arc welding is performed on the butt joint part of the tubular shape in the pipe axis direction using a welding wire and a flux in which the component composition of one electrode or the average component composition of a plurality of electrodes contains, in mass %, C: 0.01 to 0.14%, Si: 0.20 to 0.70%, Mn: 0.70 to 2.30%, Ni: 1.0 to 10.0% and Mo: 0.5 to 5.0% to form the weld metal. A method for manufacturing an ultra-high strength steel pipe pile.

3. A method for manufacturing the ultra-high strength steel pipe pile according to claim 1, wherein steel having the component composition of the base material part is heated to 1000 to 1250 °C, then hot rolling is performed with the rolling finish temperature being 650 to 950 °C, then accelerated cooling is performed to a cooling stop temperature of less than 300 °C, then reheated to a tempering temperature of 300 to 650 °C, and then cooled to room temperature to obtain a thick steel plate. The thick steel plate is formed into a tubular shape by cold working, and submerged arc welding is performed on the butt joint part of the tubular shape in the pipe axis direction using a welding wire and a flux in which the component composition of one electrode or the average component composition of a plurality of electrodes contains, in mass %, C: 0.01 to 0.14%, Si: 0.20 to 0.70%, Mn: 0.70 to 2.30%, Ni: 1.0 to 10.0% and Mo: 0.5 to 5.0% to form the weld metal. A method for manufacturing an ultra-high strength steel pipe pile.

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