Technical field
[0001]The present invention, API5L X60 ~ X70 grade strength (413 MPa specified minimum yield strength, respectively, 482MPa) having relates submerged arc welded steel pipe from the inner and outer surfaces in the longitudinal direction.
BACKGROUND
[0002]As long-distance transport method of crude oil and natural gas, the importance of line pipe is increasing. Long distance as the transport trunk line pipes for the American Petroleum Institute (API) 5L standard X65 (448MPa specified minimum yield strength of each) are the basis for the design, actual usage is large.
[0003]Steel pipe for line pipe generally by forming a steel sheet is manufactured seam welding from both the inner and outer surfaces of the butted portion of the steel plate in the longitudinal direction. Seam welding is usually a part of the groove after welding with temporary gas shielded arc welding, the submerged arc welding, and welding by layer from the inner surface and the outer surface of the steel pipe is completed. Tack welding is completely erased by submerged arc welding is performed so as to follow.
[0004]Examples of such a steel pipe to be manufactured, UOE steel pipe, and a JCOE steel. Welded joint portion of the line pipe, from the viewpoint of transportation efficiency improvement by cold reduction and high pressure mining areas, high toughness of is required.
[0005]Patent Document 1 relates to a welded steel pipe of API standard X65 ~ X70 grade, the weld metal multiplicity of TiO and transformation product was fine acicular ferrite structure as a core, it discloses possible to achieve both excellent toughness and high strength there.
CITATION
Patent Document
[0006]
Patent Document 1: JP 2013-49895 JP
Summary of the Invention
Problems that the Invention is to Solve
[0007]
　For line pipe steel pipe, it is also often used in deep-sea wells and cold climates, thickening is progressing. The welding of steel plate, it is necessary high heat input welding such as submerged arc welding. In large heat input welding, generally a problem decrease in toughness to be solved of the weld heat affected zone (hereinafter referred to as [HAZ]).
[0008]
　The present invention has a strength of API standard X60 ~ X70 grade, by forming a thick steel sheet having a thickness of 6 ~ 40 mm, intended for vertical seam welded steel pipe having a weld, which is seam welded from the inner and outer surfaces in the longitudinal direction, also the steel plate a case of the welded to the steel pipe by welding heat input 15 ~ 110kJ / cm, it is an object to obtain a steel pipe excellent in toughness of the weld metal portion at a low temperature.
Means for Solving the Problems
[0009]
　In submerged arc welding, the components of the weld metal of the longitudinal seam is influenced by the base material dilution, must the component design of the weld metal portion commensurate with the components of the base material. Specifically, Al quantity, Ti amount, O amount, control of the N content is important.
[0010]
　Microstructure of the weld metal is substantially determined by alloy weight. Considering the components of the base material, if the base material has a strength of X60 ~ X70 grade, the weld metal becomes a tissue around the acicular ferrite. Incidentally, when the strength of the base material becomes higher, the weld metal becomes bainite structure. In the strength of the base material to improve the toughness of the weld metal in the case of X60 ~ X70 grade, for example it is necessary to original study of different design concept as in the strength of the base material is X80 grade.
[0011]
　We refer to components of the weld metal in consideration of the influence of the base material diluted as appropriate, further Al, O, Ti, and is determined based on the stoichiometric ratio of N, valid acicular ferrite formation parameters α'showing ability and the ratio of the Al content and O content, found that can improve the toughness of the weld metal portion by an appropriate value in accordance with the O content in the weld metal, further studying, None of the present invention. Its gist is as follows.
[0012]
　(1) A steel pipe having a weld inner and outer surfaces are longitudinally welded, the chemical composition of the base material, in mass%, C: 0.01 ~ 0.1%, Si: 0.03 ~ 0.5%, Mn: 0.5 ~ 2.0%, P: 0.015% or less, S: 0.01% or less, Al: 0.01 ~ 0.05%, Ti: 0.005 ~ 0 .03%, N: 0.002 ~ 0.006%, O: 0.005% or less, Mg: 0 ~ 0.01%, Ca: 0 ~ 0.03%, Ni: 0 ~ 0.6%, cr: 0 ~ 0.5%, Cu: 0 ~ 0.5%, Mo: 0 ~ 0.4%, Nb: 0 ~ 0.06%, B: 0 ~ 0.002%, and V: 0 ~ containing 0.06%, balance being Fe and impurities, the tensile strength of the base material is the 480 ~ 620 MPa, the chemical composition of the weld metal, in mass%, C: 0.03 ~ 0.1%, Si: 0.03 ~ .5%, Mn: 0.5 ~ 2.0%, P: 0.015% or less, S: 0.01% or less, Al: 0.001 ~ 0.03%, Ti: 0.005 ~ 0. 04%, N: 0.002 ~ 0.006%, B: 0 ~ 0.035%, O: 0.015 ~ 0.055%, Ni: 0 ~ 0.6%, Cr: 0 ~ 0.5 %, Cu: 0 ~ 0.5%, Mo: 0 ~ 0.4%, V: 0 ~ 0.06%, Ca: 0 ~ 0.005%, Mg: 0 ~ 0.01%, and Nb: contains 0 to 0.06%, balance being Fe and impurities,% when X represents the content of the weld metal of the element X, Pcm =% C +% Si / 30 + (% Mn +% Cu +% Cr) / 20 +% Ni / 60 +% Mo / 15 +% V / 10 + 5% Pcm defined by B is not more than 0.2%, Ceq =% C +% Mn / 6 + (% Cr +% Mo +% V / 5 + (% Ni +% Cu) / 15 Ceq defined by is 0.35 ~ 0.45%, α'= (1.5 × (% O-0.89% Al) + 3.4 ×% and wherein the [alpha] 'defined by N-% Ti) × 1000 satisfies the 1000 ×% O-10 ≦ α'≦ 1000 ×% O + 1,% Al /% O satisfies 0.3-0.8 vertical seam welded steel pipe to be.
[0013]
　(2) the weld metal of tissue, an area ratio, acicular ferrite 70% or more, 15% grain boundary ferrite or less, the longitudinal seam welding of said, characterized in that it comprises the following 3% island martensite (1) steel pipe.
[0014]
　(3) vertical seam welded steel pipe according to (1) or (2) the tensile strength of the weld metal is characterized in that at least 1.05 times the tensile strength of the base material.
[0015]
　(4) one of the longitudinal seam welded steel pipe of the said Charpy absorbed energy at -10 ° C. of the weld metal is characterized in that at least 100 J (1) ~ (3).
[0016]
　(5) the longitudinal seam welded steel pipe of any said that Charpy absorbed energy at 1300 ×% of the weld metal O-60 (° C.) is characterized in that at least 100 J (1) ~ (4).
Effect of the invention
[0017]
　According to the present invention, has a strength of API standard X60 ~ X70 grade, excellent in toughness of the weld metal portion at a low temperature, it is possible to obtain UOE steel pipe, the longitudinal seam welded steel pipe such as JCOE steel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018]
[Figure 1] is a view for explaining the amount and low temperature toughness of the weld metal, (a) shows the relationship between O content and [alpha] ', (b) is a diagram showing the relationship between absorbed energy at [alpha]' and -10 ° C. is there.
[Figure 2] is an example of a structure of the weld metal, (a), (b) the of the present invention the weld metal of the structure of the longitudinal seam welded steel pipe, (c), (d) is a longitudinal seam welded steel pipe in the prior art weld metal is of the organization.
DESCRIPTION OF THE INVENTION
[0019]
　Hereinafter, with embodiments of the present invention will be described in detail.
[0020]
　　First, a description will be given component composition of the weld metal. Hereinafter, "%" regarding the component composition is intended to represent "% by mass".
[0021]
　C: 0.03 ~ 0.10%
　C is an element necessary for ensuring strength of steel, it is necessary to contain at least 0.03%. Since weld hot cracking in the weld seam C amount is large is likely to occur, the upper limit is made 0.10%. C is preferably 0.05% or more and 0.065% or less.
[0022]
　Si: 0.03 ~
　0.50% Si is required content of not less than 0.03% for blowhole prevention. Si content is likely to form the island-shaped martensite and often, since significantly degrade the low temperature toughness, the upper limit is made 0.50%. Si is preferably 0.15% or more, 0.25% or less.
[0023]
　Mn: 0.5 ~
　2.0% Mn acts as a hardenability improving element. Weld metal is required content of 0.5% or more to the acicular ferrite mainly tissues. If Mn amount is large, coarse MnS is formed, to become a starting point of fracture, the upper limit is made 2.0%. Mn is preferably 1.2% or more, 1.5% or less.
[0024]
　P: 0.015% or less (including%
　0) S: 0.010% or less (including
　0%) P, S are both impurity is an element to deteriorate the toughness of the joint. P 0.015% is less, S is limited to 0.010% or less. These content is preferably as small as possible. Preferably, P is 0.008% or less. Preferably, S is 0.003% or less.
[0025]
　Al: 0.001 ~
　0.030% Al acts as a deoxidizing element, is required for the oxygen quantity control for dispersing an effective Ti oxides as acicular ferrite nucleation sites. Considering the base material dilution is required content of 0.001% or more. If Al content exceeds 0.030%, the inhibition of formation of oxides, can not be ensured toughness, the upper limit is 0.030%. Preferably 0.010% or more and 0.015% or less.
[0026]
　Ti: 0.005 ~
　0.040% of Ti reacts with oxygen in the weld metal, to form a Ti oxide as a nucleus of acicular ferrite. Number in order to finely disperse the weld metal the oxides, it is necessary to contain 0.005% or more. When Ti content is excessive, and the aggregation and coarsening Ti oxide, it is capable of generating a nucleus of acicular ferrite decreases, and since Ti oxide can not be secured to be toughness starting points of fracture, the upper limit is 0. and 040%. Preferably 0.009% or more, 0.015% or less.
[0027]
　N: 0.002 ~ 0.006%
　N is to be effective element for adjusting the effective Ti amount for the acicular ferrite structure formed, it is necessary to contain at least 0.002%. However, when it exceeds 0.006%, the solute N which did not react with the Ti reduces significantly toughness, preferably the upper limit 0.006%. Preferably 0.003% or more and 0.004% or less.
[0028]
　B: 0 ~ 0.035% or less
　B is B solid solution state, by inhibiting grain boundary ferrite formation in the weld metal, promotes the formation of acicular ferrite. B may not contain, but in order to obtain this effect, preferably contains at least 0.0001%. And the amount of B is too high strength exceeds 0.035, the toughness is reduced, so the upper limit is made 0.035%. B added to the weld metal can be added plank matrix, flux, or from any wire even. For example, if the base material is B-free steels, B oxides may be used a flux containing. B is preferably 0.0005% or 0.010% or less.
[0029]
　O: 0.015 ~ 0.055%
　O is an element required for oxide formation at the core of acicular ferrite. Therefore it is necessary to contain at least 0.015%. When O content exceeds 0.055% excess oxide formation, since the toughness is reduced due to aggregation and coarsening, the upper limit is 0.055%. Preferably 0.020% or more, 0.030% or less.
[0030]
　Ni: 0 ~ 0.60% Ni
　without lowering the toughness, an element capable of improving the strength of the weld metal. Content of Ni is not essential. The effect is saturated if it exceeds 0.60% and the upper limit is 0.60%.
[0031]
　Cr: 0 ~
　0.50% Cr is an element capable of improving the strength of the weld metal. Content of Cr is not essential. The effect is saturated if it exceeds 0.50%, the upper limit is made 0.50%.
[0032]
　Cu: 0 ~
　0.50% Cu is an element capable of improving the strength of the weld metal. Content of Cu is not essential. The effect is saturated if it exceeds 0.50%, the upper limit is made 0.50%.
[0033]
　Mo: 0 ~
　0.40% Mo is an element capable of improving the strength of the weld metal. Content of Mo is not essential. The effect exceeds 0.40 percent saturated, so the upper limit 0.40%.
[0034]
　V: 0 ~ 0.06%
　V is an element which can improve the strength of the weld metal. Content of V is not required. The effect is saturated if it exceeds 0.06%, the upper limit is made 0.06%.
[0035]
　Ca: 0 ~
　0.005% Ca is an element effective for improvement and refinement of the structure of ductility due to morphology control. Content of Ca is not essential. When Ca amount is large, resulting coarsening of the sulfide or oxide, the ductility and toughness are deteriorated, the upper limit is made 0.005%.
[0036]
　Mg: 0 ~
　0.010% Mg is MgS or MgAl 2 O 4 is formed and acts as pinning particles. Content of Mg is not essential. To suppress the austenite grain growth in the weld metal is preferably contained in 0.001% or more. The effect is saturated if it exceeds 0.010%, the upper limit is made 0.010%. Preferably 0.0015% or more, 0.0025% or less.
[0037]
　Nb: 0 ~
　0.06% Nb strength increase, it is an effective element in order to present an effective solid solution B at the grain boundary ferrite suppressed. Content of Nb is not essential. Nb amount is likely to form an island-shaped martensite exceeds 0.06%, since the toughness is reduced, so the upper limit is made 0.06%. Desirably, it is 0.02%.
[0038]
　The remainder of the weld metal is Fe and impurities. The impurities in the course of welding, the welding wire, a component mixed flux, steel, from the surrounding atmosphere or the like, refers to not intended to intentionally containing component.
[0039]
　Specifically, P, S, N, Sb, Sn, W, Co, As, Pb, Bi, and H and the like. Among, P and S, as described above, respectively, P: 0.015% or less, S: it is necessary to control so that 0.01% or less.
[0040]
　For other elements, typically, Sb, Sn, W, Co, and As 0.1% is less, Pb and Bi 0.005% below, H is the incorporation of a 0.0005% or less of unavoidable impurities There may however, if the normal range, there is no particular need to control.
[0041]
　Components of the weld metal in the present embodiment, furthermore, it is necessary to satisfy the relationship described below.
[0042]
　Pcm: 0.2% or less
　component composition of the weld metal needs to Pcm represented by the following formula is 0.2% or less. The content of the weld metal of% X is an element X in the formula (wt%) refers to (the same in the following description). Moreover, elements not added to the weld metal (in the same or later described) for calculating as a zero.
[0043]
　Pcm =% C +% Si / 30 + (% Mn +% Cr +%
　　　　Cu) / 20 +% Ni / 60 +% Mo / 15 +% V / 10 + 5% B
[0044]
　Pcm is called welding sensitivity is obtained by quantitatively evaluating the effects of chemical components of the steel to cold cracking. Since Pcm is cold cracking is likely to occur when it exceeds 0.2%, the upper limit is made 0.2%.
[0045]
　Ceq: 0.35 to 0.45%
　component composition of the weld metal needs to Ceq represented by the following formula is 0.35 to 0.45%.
[0046]
　Ceq =% C +% Mn / 6 + (% Cr
　　　　+% Mo +% V) / 5 + (% Ni +% Cu) / 15
[0047]
　Ceq about hardenability due to welding heat affected the base material is a hardenability of each alloy element in the sum in terms of C content, respectively. To weld metal to achieve the desired tensile strength, to control the Ceq to 0.35 to 0.45%. Preferably with 0.40 to 0.43 percent the Ceq.
[0048]
　α': -20 ≦ α'≦ 40
　component composition of the weld metal of the welded joint has to [alpha] 'represented by the following formula is -20 to 40.
[0049]
　α'= (1.5 × (% 0.89% Al) + 3.4 ×% n-%) × 1000
[0050]
　α', based Al, O and Ti, the stoichiometric ratio of N, is a parameter showing the effective acicular ferrite formation potential, acicular ferrite by controlling the range of -20 to 40 α' to improve the nucleation ability.
[0051]
　If α'is less than -20, Al, or Ti amount is either the excess, or N, since the O amount is too small, decreases markedly acicular ferrite nucleation ability. If α'of 40 greater, Al, or Ti amount is either is too small, or N, since the O amount becomes excessive, decreasing significantly acicular ferrite nucleation ability.
[0052]
　Al% /% O: 0.30
　~ 0.80 % Al /% O is the ratio of the Al content and O content, which is an index showing the oxygen potential after completion of the aluminum deoxidation. % Al /% O to by controlling 0.3 to 0.80 can be improved production of acicular ferrite.
[0053]
　% Al /% when O ratio is less than 0.30, O amount becomes excessive, dissolved oxygen did not form a Ti oxide is lowered toughness to lower the cleanliness of the steel. On the other hand,% Al /% when O is 0.80 greater, becomes Al amount is excessive, O amount is reduced to bind to Ti, reed Ti oxide having a Molecular ferrite nucleus is reduced, toughness is reduced. Therefore,% Al /% O is a 0.30 to 0.80.
[0054]
　It is described next metal structure of the weld metal.
[0055]
　The components and parameters of the weld metal is within the above range, the steel sheet having a strength of X60 ~ X70 grade, when the submerged arc welding at heat input 15 ~ 110 kJ / cm, the metal structure of the weld metal of acicular ferrite main It will become the organization. UO steel pipe to which the present invention is directed, a plate thickness of about 6 ~ 40 mm, when the submerged arc welding steel sheets having such a thickness is in a range of heat input 15 ~ 110kJ / cm. Then, Sadamari cooling rate of weld metal is subjected, the weld metal of the metal structure of the final pass becomes the tissue as follows. Proportions indicated below are the area ratio.
[0056]
　Acicular ferrite: 70%
　acicular ferrite is the acicular ferrite structure in which the Ti-based oxides as nuclei, as the rate is larger, breaking unit of the weld metal portion is miniaturized. To obtain the effect, it is preferable to set the acicular ferrite and 70% or more.
[0057]
　Grain boundary ferrite at 15% or less
　grain boundary ferrite is one of brittle phase, becomes a starting point of fracture, the decrease in toughness factor. Therefore, grain boundary ferrite is preferably set to 15% or less.
[0058]
　Island martensite: 3% or less
　in one of the island martensite brittle phase, becomes very fracture origin for high hardness, a decrease in toughness factor. Therefore, it is preferably 3% or less island martensite.
[0059]
　EBSD particle size: 10 [mu] m or less
　EBSD (Electron Backscatter Diffraction) particle size is the grain diameter size which is a measure of the destruction unit. EBSD particle size breakdown units if 10μm or less is fine, preferred from the viewpoint of securing the toughness at low temperatures.
[0060]
　The weld metal and satisfies components described above, by welding with heat input 15 ~ 110 kJ / cm, tensile strength of the weld metal is 480 ~ 620 MPa, measured according to JIS Z2242 of the weld metal -10 Charpy absorbed energy at ℃ can be obtained submerged arc welded joint is at least 100 J.
[0061]
　Low temperature toughness depends parameters α'showing valid acicular ferrite forming ability. α'exists more preferred range by the concentration of oxygen in the weld metal. Specifically, it is preferable to make a 1000 ×% O-10 ≦ α'≦ 1000 ×% O + 1 (Fig. 1). By adjusting the α'in this range, further improves the acicular ferrite nucleation ability, improved low temperature toughness, submerged arc welding Charpy absorbed energy at 1300 ×% O-60 (℃) is not less than 100J it can be obtained joint.
[0062]
　Also, the weld metal and satisfies components described above, by welding with heat input 15 ~ 110 kJ / cm, hardness of the weld metal becomes larger than the hardness of the base material, preferably, the difference is the Vickers hardness equal to or greater than the 10Hv in of. Furthermore, the tensile strength of the weld metal is preferably equal to or greater than 1.05 times the tensile strength of the base material.
[0063]
　If the base material is steel sheet having strength of X60 ~ X70 grade (steel tensile strength of 480 ~ 620 MPa for base metal), no particular tissue is limited. Hereinafter, a component of the steel sheet having a strength suitable X60 ~ X70 grade as a base material of the longitudinal seam welded steel pipe of the present invention.
[0064]
　C: 0.01 ~ 0.1%
　C is effective for improving the strength of the steel, is contained more than 0.01%. C amount is too large, the base material and the HAZ of the low temperature toughness is degraded sushi, further, since the weldability is deteriorated, the amount of C is 0.1% or less. Preferably from 0.03 to 0.07%.
[0065]
　Si: less than 0.5%
　Si is an element necessary for deoxidation. Si content is likely to form the island-shaped martensite and often, since significantly degrade the low temperature toughness, the Si content is less than 0.5%. Preferably less than 0.35%. Deoxidation is, Al, Si addition is not essential because performed even Ti.
[0066]
　Mn: 0.5 ~
　2.0% Mn acts as a hardenability improving element, is contained 0.5% or more for achieving its effect. Mn amount is large and increases the hardenability of steel, deteriorates HAZ toughness and weldability. Further, promotes center segregation in continuously cast billets, so degrades the low temperature toughness of the base metal, Mn content is set to 2.0% or less. Preferably, a 1.0 to 1.8 percent.
[0067]
　P: 0.015% or less
　S: 0.01% or less
　P, S are both impurity is an element to deteriorate the toughness of the joint. These content is preferably better as low as possible, P is 0.015% or less, S is 0.01% or less. Preferably, P is 0.008% or less. Preferably, S is 0.003% or less.
[0068]
　Al: 0.01 ~
　0.05% Al is an element contained in the steel in as a deoxidizer. Al further, AlN was formed by combining the N, suppresses grain coarsening of hardened portion of the steel material. If the content of Al is too low, since this effect can not be obtained, it is contained more than 0.01%. When Al content is too high, since the high-frequency hardenability of the steel is lowered, Al content is set to 0.05% or less. Preferably, from 0.02 to 0.04%.
[0069]
　Ti: 0.005 ~
　0.03% Ti forms fine TiN in steel, the single or Mg (MgAl, 2 O 4 compound inclusions with) oxide acts as pinning particles. As a result, the microstructure coarsening of the HAZ austenite grains is suppressed finer and improve the low temperature toughness. To obtain this effect, Ti can be contained 0.005% or more. When Ti content is increased, Ti oxide are agglomerated, coarsened, since the toughness is deteriorated, Ti content is set to 0.03% or less. Preferably, from 0.01 to 0.02%.
[0070]
　N: 0.002 ~ 0.006%
　N is an element to form a TiN combines with Ti, it is contained more than 0.002%. When the amount of N is large, solute N which did not bind to Ti because lowering the toughness, the N content is set to 0.006% or less. Preferably, it is 0.003 to 0.005%.
[0071]
　O: 0.005% or less
　O is an element which forms a pinning particles. However, the smaller are preferred because cleanliness of the steel to contain the O decreases, and 0.005% or less. Preferably not more than 0.003%.
[0072]
　Mg: 0 ~
　0.01% Mg is MgAl 2 O 4 is an element which forms inclusions, such as, MgS. MgAl 2 O 4 is deposited on the TiN. These inclusions act as pinning particles, the microstructure finer and suppresses the coarsening of the HAZ austenite grains, improving the low temperature toughness. When Mg content is increased, the effect is saturated. Mg is the base material of the longitudinal seam welded steel pipe is not necessarily to be contained, the preferred Mg content is 0 to 0.01%.
[0073]
　Ca: 0 ~
　0.03% Ca controls the form of sulfide inclusions, is an element improving the low temperature toughness. Furthermore, phosphides, to form a sulfide, substantially reduce the concentration of P and S, to improve the sulfide stress cracking resistance. When Ca amount is large, CaO-CaS becomes large clusters and inclusions, it can adversely affect the toughness. Ca is the base material of the longitudinal seam welded steel pipe is not necessarily to be contained, the preferred Ca content is 0 to 0.03%.
[0074]
　Ni: 0 ~ 0.6% Ni
　without lowering the toughness, an element capable of improving the strength of the base material. When the amount of Ni increases, the effect is saturated. Ni is the base material of the longitudinal seam welded steel pipe is not necessarily to be contained, the preferred Ni content is 0 to 0.6%.
[0075]
　Cr: 0 ~
　0.5% Cr is an element capable of improving the strength of the base material. When the Cr content is increased, the effect is saturated. Cr is the base material of the longitudinal seam welded steel pipe is not necessarily to be contained, the preferred Cr content is 0 to 0.5%.
[0076]
　Cu: 0 ~
　0.5% Cu is an element capable of improving the strength of the base material. When the Cu content is increased, the effect is saturated. Cu is the base material of the longitudinal seam welded steel pipe is not necessarily to be contained, the preferred Cu content is 0 to 0.5%.
[0077]
　Mo: 0 ~
　0.4% Mo is an element capable of improving the strength of the base material. When Mo content is increased, the effect is saturated and further, toughness is reduced. Mo is the base material of the longitudinal seam welded steel pipe is not necessarily to be contained, the preferred amount of Mo is 0 to 0.4%.
[0078]
　Nb: 0 ～ 0.060
　Pasento Nb is an element that improves the strength of the base material. When the Nb content increases, island martensite is easily formed, toughness is reduced. Nb is the base material of the longitudinal seam welded steel pipe is not necessarily to be contained, the preferred amount of Nb is 0 to 0.40%.
[0079]
　B: 0 ~ 0.002%
　B is improving hardenability of the base material, it is an effective element in the grain boundary ferrite formation suppressing. When the amount of B increases, the effect is saturated. B is the base material of the longitudinal seam welded steel pipe is not necessarily to be contained, the preferred amount of B is from 0 to 0.002%.
[0080]
　V: 0 ～ 0.06
　Pasento Nb is an element that improves the strength of the base material. When the amount of V is increased, the yield ratio by the precipitation hardening may rise. V is the base material of the longitudinal seam welded steel pipe is not necessarily to be contained, the preferred V content is from 0 to 0.06%.
[0081]
　Balance other than those described above is Fe and impurities. The impurities contained in the raw material, or a component mixed in the manufacturing process, intentionally refer to components not intended to be contained in the steel.
[0082]
　Specifically, P, S, O, Sb, Sn, W, Co, As, Pb, Bi, and H and the like. Among, P, S, and O are preferably controlled to be suitable range described above.
[0083]
　For other elements, typically, Sb, Sn, W, Co, and As 0.1% is less, Pb and Bi 0.005% below, H is the incorporation of a 0.0005% or less of unavoidable impurities There may however, if the normal range, there is no particular need to control.
[0084]
　Method for producing a steel sheet as the base material is not particularly limited, and may according to the general manufacturing method of a steel sheet having a strength of X60 ~ X70 grade. Vertical seam welded pipe can be obtained by joining a steel plate which is the thickness of 6 ~ 40 mm approximately preform in submerged arc welding. UOE steel pipe and steel pipe JCOE are examples.
[0085]
　Welding method will be described in detail.
[0086]
　First, the above thick steel sheet, subjected to edge preparation of a predetermined shape. Groove shape is not particularly limited. Vertical seam welded steel pipe is weldable groove shape from both the front and back surfaces to the end of the steel plate, for example, processed into X mold opening destination to complete the submerged arc welding from the inner side against the end after it is prepared by performing the submerged arc welding from the outer surface side in the longitudinal direction.
[0087]
　Then, in the GMA sprayed flux, using the submerged arc welding steel wire, they are bonded by high heat input submerged arc welding of heat input 15 ~ 110kJ / cm. Flux and the steel wire is not limited in particular, it may be used known ones. When using steel wire, flux, known sintered type flux, it can be used such as fused type flux, if we can thereby obtaining the weld metal components described above, resulting excellent weld metal toughness. Further, if necessary, may be subjected to flux preheating prior to welding.
[0088]
　The method of submerged arc welding is not limited in particular, comprises a submerged arc welding of multi-electrode, any known welding methods can be applied, the welding conditions are also not particularly limited.
Example
[0089]
　Next, a description will be given of an embodiment of the present invention. Conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to this single example of conditions. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0090]
　Smelted steel of various chemical composition, the slab molten steel is refined by a continuous casting method, after heating to 1100 ° C., subjected to hot rolling, the finishing temperature of hot rolling as 780 ° C., cooled to 750 ° C. after, water cooled from 750 ° C. to room temperature to prepare a steel sheet having a strength of X60 ~ X70 grade various component compositions. Thickness of the steel sheet in Table 1 shows the chemical composition, and tensile strength.
[0091]
[Table 1]

[0092]
　Then, the X mold opening destination formed on the steel sheet thus produced, formed into a tubular, using known wires and flux, the inner surface of the tube, subjected to submerged arc welding in the order of outer surface, and a UO pipe. During welding, so heat input is about 65 kJ / cm, and adjusting the welding speed. Steel sheet used in Tables 2-4 show the chemical composition of the weld metal.
[0093]
[Table 2]

[0094]
[table 3]

[0095]
[Table 4]

[0096]
　After submerged arc welding, weld metal structure area ratio of (acicular ferrite, the sum of the grain boundary ferrite and island martensite) (%), EBSD grain size of the weld metal, the tensile strength of the weld metal, weld metal and the base metal hardness difference, and to measure the energy absorbed Charpy impact test. Table 5-7 shows the results. AF ratio in Tables 5-7, show GBF rate, respectively MA rate, acicular ferrite in the weld metal structure, grain boundary ferrite, the area ratio of island martensite.
[0097]
[table 5]

[0098]
[Table 6]

[0099]
[Table 7]

[0100]
　Absorption energy Charpy impact test was measured as follows.
[0101]
　In the thickness cross section parallel to a direction including HAZ and weld metal, the Charpy test pieces were taken from the weld metal center from the bottom surface 2mm steel plate according to JIS Z2242, performed Charpy impact test at -10 ° C., the absorption energy It was measured. Absorbed energy, three times a Charpy impact test, and the average value, toughness of less than 100J is determined to be defective. For some samples, it was also measured Charpy absorbed energy at temperatures other than -10 ° C..
[0102]
　Area of ​​the tissue was measured as follows.
[0103]
　1/2 parts of the weld bead width of the wall thickness t / 4 position from the surface layer of the second pass to specimen collection, after polishing, subjected to nital corrosion and Repera corrosion was revealing tissue by an optical microscope, 1000 .mu.m × and 10 perimetry tissue targeting observed in the range of 1000 .mu.m, the resulting image is an image analysis, was determined by calculating the average area ratio of each organization.
[0104]
　EBSD particle size is 20 fields EBSD analysis in a range of 500 [mu] m × 500 [mu] m, and the average crystal grain size when separated by crystal orientation difference 15 °.
[0105]
　As shown in Tables 5-7, Inventive Examples satisfying the welded joint component composition of the present invention are both the Charpy absorbed energy at -10 ° C. is not less than 100 J, be one having a good weld metal toughness It was.
[0106]
　In contrast, comparative examples not satisfying the welded joint component composition of the present invention, the Charpy absorbed energy at -10 ° C. is less than 100 J, the weld metal and the welded joint toughness is lowered.
[0107]
　In the invention example, depending on the O content in the weld metal, -20 ° C., -30 ° C., was obtained a high Charpy absorbed energy at -45 ° C..
[0108]
　It shows an example of a structure of the weld metal in FIG. (A), (b) the structure of Invention Example, (c), is (d) shows a comparative example of a tissue. In the invention examples it can be seen that the structure of the weld metal is fine.
Industrial Applicability
[0109]According to the present invention, even when the bonding was carried out large heat input welding thick steel plates, it is possible to provide a longitudinal seam welded steel pipe superior in toughness of the weld metal. Accordingly, the present invention has high industrial applicability.

The scope of the claims
[Requested item 1]A steel pipe having a weld inner and outer surfaces are longitudinally welded,
　the chemical composition of the base material, in
　　mass%, C: 0.01
　　~ 0.1%, Si: less than
　　0.5%, Mn : 0.5
　　~ 2.0%, P: 0.015% or
　　less, S: 0.01% or
　　less,
　　Al: 0.01 ~ 0.05%, Ti: 0.005 ~
　　0.03%, N: ~ 0.006% 0.002,
　　O: 0.005% or
　　less,
　　Mg: 0 ~
　　0.01%, Ca: 0 ~ 0.03%, Ni: 0 ~
　　0.6%, Cr: 0 ~ 0.
　　% 5,

　　Cu: 0 ~ 0.5%, Mo: 0 ~ 0.4%,
　　Nb: 0 ~ 0.06%, B: 0 ~ 0.002%, and
　　V: 0 ~ 0.06%
containing and the balance is Fe and impurities,
　the tensile strength of the base material is the 480 ~ 620 MPa,
　the chemical composition of the weld metal, in
　　mass%, C: 0.03 ~ 0.10%,
　　Si:
　　0.03 ~ 0.50%, Mn: 0.5
　　~ 2.0%, P: 0.015% or
　　less, S: 0.010% or
　　less, Al: 0.001 ~
　　0.030%, Ti :
　　0.005
　　~ 0.040%, N: 0.002
　　~ 0.006%, B: 0 ~ 0.035%, O: 0.015
　　~ 0.055%, Ni: 0 ~ 0.60%, 　　cr:
　　0 ~ 0.50%, 　　Cu: 0 ~ 0.50%, Mo: 0 ~ 　　0.40%, V: 0 ~ 0.06%, Ca: 0 ~ 　　0.005%, Mg: 0 ~ 0 .010%, and 　　Nb: 0 ~ 0.060% containing the balance is Fe and 　impurities, % when X represents the content of the weld metal of the element 　X, Pcm =% C +% Si / 30 + ( % Mn +% Cu +% Cr ) / 20 +% Ni / 60 +% Mo / 15 +% V / 10 + 5% Pcm defined by B is 0.2% or less , 　Ceq =% C +% Mn / 6 + (% Cr +% Mo +% V) / 5 + (% Ni +% Cu) / 15 Ceq defined by a is from 0.35 to 0.45%,

　α'= (1.5 × (% O -0.89% Al) + 3.4 ×% N-% Ti) α' defined by × 1000 is ~ 40
　-20, % Al /% O is is 0.3-0.8
longitudinal seam welded steel pipe, characterized in that.
[Requested item 2]Wherein [alpha] 'is the longitudinal seam welded steel pipe according to claim 1, characterized in that meet 1000 ×% O-10 ≦ α'≦ 1000 ×% O + 1.
[Requested item 3]Claim wherein the weld metal of the tissue, an area ratio, the acicular ferrite of 70% or more, 15% grain boundary ferrite or less, comprises more than 3% island martensite, wherein the EBSD grain size is 10μm or less vertical seam welded steel pipe according to 1 or 2.
[Requested item 4]Vertical seam welded steel pipe according to any one of claims 1 to 3, the tensile strength of the weld metal is characterized in that at least 1.05 times the tensile strength of the base material.
[Requested item 5]The weld hardness of the metal is greater than the hardness of the base material, the longitudinal seam welded steel pipe according to any one of claims 1 to 4, characterized in that the difference is not less than 10HV.
[Requested item 6]Vertical seam welded steel pipe according to any one of claims 1 to 5, Charpy absorbed energy at -10 ° C. of the weld metal is characterized in that at least 100 J.
[Requested item 7]Vertical seam welded steel pipe according to any one of claims 1 to 6, Charpy absorbed energy at 1300 ×% O-60 (℃) of the weld metal is characterized in that at least 100 J.