JP2016084489A - Weld joint for ship excellent in corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a weld joint for a ship excellent in corrosion resistance capable of exhibiting good corrosion resistance even under an environment with severe corrosion of various ships.SOLUTION: A weld joint for a ship made by butt welding thick steel sheets having a predetermined component composition, contents of Cu and Cr of a weld metal satisfy the larger of [Cu]b×0.5 and 0.10%≤[Cu]w≤[Cu]b×1.5 and the larger of [Cr]b×0.5 and 0.01%≤[Cr]w≤[Cr]b×1.5 and the difference between the maximum width Wmax and the minimum width Wmin of the weld metal on a cross section at 1/2 position of a sheet thickness parallel to a rolling face of the welded thick sheet is 5 mm or less.SELECTED DRAWING: None

Description

  The present invention relates to a welded joint for ships used as a structural member of various ships such as oil tankers, bulk carriers, car carriers, container ships, LNG ships, passenger ships, warships and the like, and particularly in tanker crude oil tanks and bulker holds. The present invention relates to a marine welded joint that has excellent corrosion resistance and can be used for parts that are used without coating, such as inside.

  Steel plates are often used as structural members for various types of ships such as oil tankers, bulk carriers, car carriers, container ships, LNG ships, passenger ships, warships, etc., and these ships are built by welding thick steel plates. Yes. However, there are many steel materials that are used unpainted such as in tanker crude oil tanks and in bulks of bulker ships. In such cases, the base metal and welded joints of ship structural members are exposed to harsh environments such as the uniform corrosion of the bulker hold by sulfuric acid. In addition, since ship structural members are exposed to severe corrosive environments such as salt derived from seawater and dew condensation caused by temperature changes during the day and night, some kind of corrosion protection is required.

  As anticorrosion means for ships, painting or cathodic protection is generally applied, and both are often used in combination. For example, in ballast tanks that are exposed to salty water and high-temperature and high-humidity environments, painting with epoxy resin paint is often applied, and in combination with painting, zinc, which is a base metal rather than steel, is short-circuited with steel. In many cases, the anti-corrosion (sacrificial anti-corrosion) to be installed is applied.

  However, in the anticorrosion coating, the coating film may be peeled off from the steel material due to external damage or aging deterioration, and the corrosion and wear of the exposed thick steel plate may progress. Electrocorrosion is effective when immersed in an aqueous electrolyte solution such as seawater.However, in the absence of seawater, such as in a ballast tank at the time of loading, the anticorrosion action cannot be obtained, so the anticorrosion property is not good. There is a problem that it may be sufficient.

  Thick steel plates are concerned that the strength of structural members will be insufficient if corrosion wear progresses to some extent. In particular, when corrosion progresses locally, there is a possibility that it may lead to leakage accidents due to holes. It is done. For this reason, it is a fact that ships spend a great deal of time and money on periodic inspections of corrosion conditions and painting repairs.

  From the viewpoint of improving ship safety, extending the service life and reducing maintenance costs, more effective anti-corrosion measures are currently required. In addition to improving the anti-corrosion properties of coating and cathodic protection, the steel itself There is still a high demand for improvement in corrosion resistance. With respect to improving the corrosion resistance of the steel material itself, for example, Patent Document 1 proposes a technique for suppressing corrosion by adjusting the chemical composition and structure of the steel material.

  In the welded joint, depending on the combination of thick steel plate and welding material used, significant dissimilar metal contact corrosion may occur when the paint is peeled off due to the difference in chemical composition between the two. Corrosion and wear may be accelerated more than the part. In addition, concentrated parts of inevitable impurity elements that promote corrosion such as P and S due to solidification segregation during welding are formed in the weld metal, and local corrosion may occur starting from these concentrated parts. It is required to improve its corrosion resistance. For improving the corrosion resistance of a welded joint in a ship, for example, the techniques of Patent Document 2 and Patent Document 3 are known, but the corrosion resistance of the joint itself is still insufficient, and more effective techniques for improving the corrosion resistance of welded joints are available. It is hoped that it will be developed.

JP 2014-1450 A JP 2010-43342 A JP 2012-117139 A

  The present invention has been made as a solution to the above-mentioned conventional problems, and as a structural member for various ships such as oil tankers, bulk carriers, car carriers, container ships, LNG ships, passenger ships, warships, etc. It is an object of the present invention to provide a marine welded joint with excellent corrosion resistance that can exhibit good corrosion resistance even when used in more severe environments such as crude oil tanks and bulk tankers that are often used for painting. Is.

In the present invention, by mass, C: 0.04 to 0.30%, Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, P: 0.04% or less (0 %), S: 0.04% or less (not including 0%), Al: 0.010 to 0.10%, Cu: 0.10 to 1.0%, Cr: 0.01 to 0 .5%, N: 0.010% or less (excluding 0%), a marine welded joint obtained by butt welding a thick steel plate made of Fe and inevitable impurities, The content of Cu and Cr satisfies the following formulas (1) and (2), and the content of chemical components other than Cu and Cr in the weld metal is mass%, and C: 0.04 to 0.30%. , Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, P: 0.04% or less (excluding 0%), S: 0.04 The following (not including 0%), Al: 0.010 to 0.10%, N: 0.010% or less (not including 0%), the balance being Fe and inevitable impurities, Corrosion resistance is characterized in that the difference between the maximum width Wmax and the minimum width Wmin of the weld metal in the cross section at a half position of the plate thickness parallel to the rolling surface of the welded thick steel plate satisfies the following formula (3). It is an excellent marine weld joint.
The larger of [Cu] b × 0.5 and 0.10% ≦ [Cu] w ≦ [Cu] b × 1.5 (1)
The larger of [Cr] b × 0.5 and 0.01% ≦ [Cr] w ≦ [Cr] b × 1.5 (2)
Wmax−Wmin ≦ 5 mm (3)
However, in Formula (1), [Cu] w is the Cu content (% by mass) in the weld metal, and [Cu] b is the Cu content (% by mass) in the base metal. In Formula (2), [Cr] w is the Cr content (mass%) in the weld metal, and [Cr] b is the Cr content (mass%) in the base metal.

  The thick steel plate is further in mass%, Co: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Mo: 0.01 to 1.0%, W: 0.01 to It is preferable to contain 1.0% of one kind or two or more kinds.

  Moreover, the said thick steel plate is 1% or 2 of Ti: 0.001-0.05%, Zr: 0.001-0.05%, Nb: 0.001-0.05% further by the mass%. It is preferable to contain seeds or more.

  Moreover, it is preferable that the said thick steel plate contains 1 type or 2 types of Mg: 0.0003-0.005% and Ca: 0.0003-0.005% by the mass% further.

  Moreover, the said thick steel plate is 1 type or 2 of Sn: 0.005-0.20%, Sb: 0.005-0.20%, Se: 0.005-0.20% by mass% further. It is preferable to contain seeds or more.

  Moreover, it is preferable that the said thick steel plate contains 1 type or 2 types of B: 0.0001-0.005% and V: 0.001-0.1% further by the mass%.

  According to the marine welded joint with excellent corrosion resistance according to the present invention, it is used as a structural member of various ships such as oil tankers, bulk carriers, car carriers, container ships, LNG ships, passenger ships, warships, etc. Even when used in a more severely corrosive environment such as a crude oil tank or a bulker hold, good corrosion resistance can be exhibited.

  In addition, even if the anticorrosion coating is peeled off due to some reason and the welded joint is exposed to a corrosive environment, plate thickness wear due to corrosion can be suppressed, and the period until switching of structural members including welded joints That is, the lifetime as a structure can be extended.

In an Example, it is explanatory drawing which shows the position which extract | collects a test piece from a welded joint, and the shape of a test piece.

  The present inventor has eagerly obtained a welded joint of a thick steel plate that can exhibit good corrosion resistance even when used in a severely corrosive environment such as a crude oil tank or a bulk tanker which is often used without painting. , Repeated experiments and research. As a result, in addition to appropriately adjusting the chemical component content of the thick steel plate to be butt welded, the content of a specific element among the chemical components of the weld metal depends on the content of the same specific element in the base metal. It was found that a marine welded joint having excellent corrosion resistance can be obtained by adjusting the width of the weld metal and satisfying the conditions that define the width of the weld metal.

  Hereinafter, the present invention will be described in detail based on embodiments.

<Thick steel plate>
The thick steel plate used for producing the welded joint of the present invention is required to satisfy mechanical properties and weldability required as a structural member in addition to being excellent in corrosion resistance because it is used as a structural member of a ship. From these viewpoints, it is necessary to appropriately adjust the contents of various additive elements. Below, the reason for limitation of the component range of these various additive elements is demonstrated. All units are described as%, but indicate mass%.

(Chemical composition)
・ C: 0.04 to 0.30%
C is a basic additive element necessary for securing the strength of the thick steel plate. In order to obtain the strength characteristics normally required for a thick steel plate, it is necessary to contain at least 0.04% or more. However, when C is contained excessively, in addition to deterioration of corrosion resistance, toughness is also deteriorated. In order not to cause such an adverse effect due to excessive addition of C, the C content needs to be suppressed to 0.30% at most. For these reasons, the C content range is set to 0.04 to 0.30%. In addition, the minimum with preferable content of C is 0.045%, It is good to set it as 0.05% or more more preferably. Moreover, the upper limit with preferable content of C is 0.29%, It is good to set it as 0.28% or less more preferably.

・ Si: 0.05-1.0%
Si is also an element necessary for deoxidation and securing strength, and the minimum strength as a structural member cannot be secured unless it is contained at least 0.05% or more. However, if the content exceeds 1.0%, weldability deteriorates. In addition, the minimum with preferable content of Si is 0.08%, More preferably, it is good to set it as 0.10% or more. Moreover, the upper limit with preferable content of Si is 0.95%, More preferably, it is good to set it as 0.90% or less.

Mn: 0.1 to 2.0%
Like Si, Mn is an element necessary for deoxidation and securing strength, and if it is less than 0.1%, the minimum strength as a structural member cannot be secured. However, if the content exceeds 2.0%, the toughness deteriorates. In addition, the minimum with preferable content of Mn is 0.15%, More preferably, it is good to set it as 0.2% or more. Moreover, the upper limit with preferable Mn content is 1.9%, More preferably, it is good to set it as 1.8% or less.

・ P: 0.04% or less (excluding 0%)
P is an element that deteriorates toughness and weldability when contained excessively, and the upper limit of the allowable content of P is 0.04%. The content of P is preferably as small as possible, and the more preferable upper limit of the content of P is 0.038%, and more preferably 0.035% or less. However, it is difficult industrially to make P in a thick steel plate 0%.

S: 0.04% or less (excluding 0%)
S, like P, is an element that promotes corrosion in addition to degrading toughness and weldability as the content increases. The upper limit of the allowable content of S is 0.04%. The upper limit with more preferable content of S is 0.038%, More preferably, it is good to set it as 0.035% or less. However, it is difficult to make S in a thick steel plate industrially 0%.

・ Al: 0.010-0.10%
In addition to improving the corrosion resistance by forming an oxide film on the surface, Al is an element that also exhibits the effects of deoxidation and ensuring strength in the same manner as Si and Mn described above. In order to exhibit such an action effectively, it is necessary to contain 0.010% or more. However, if the content exceeds 0.10%, the weldability is impaired, so the range of Al content is set to 0.010 to 0.10%. In addition, the minimum with preferable content of Al is 0.011%, More preferably, it is good to set it as 0.012% or more. Moreover, the upper limit with preferable content of Al is 0.095%, More preferably, it is good to set it as 0.090% or less.

Cu: 0.10 to 1.0%
Cu is an element necessary for improving corrosion resistance because it has a function of reducing a corrosion reaction by forming a dense rust film on the surface of a thick steel plate in an environment where corrosion is severe. In order to exhibit such an action effectively, it is necessary to contain at least 0.10% or more. However, if contained excessively, weldability and hot workability deteriorate, so the Cu content needs to be 1.0% or less. The minimum with preferable content of Cu is 0.11%, and a more preferable minimum is 0.12%. Moreover, the upper limit with preferable Cu content is 0.95%, and a more preferable upper limit is 0.90%.

・ Cr: 0.01-0.5%
Cr is an element necessary for improving corrosion resistance by forming a dense rust film on the surface of a thick steel plate and reducing the corrosion reaction in an environment where corrosion is severe like Cu. In order to effectively exhibit such an action, it is necessary to contain at least 0.01% or more. However, if excessively contained, the corrosion resistance is deteriorated due to a decrease in pH at the corrosion tip, and also weldability and hot workability are deteriorated. Therefore, the Cr content needs to be 0.5% or less. There is. A preferable lower limit of the Cr content is 0.02%, and a more preferable lower limit is 0.03%. Moreover, the upper limit with preferable content of Cr is 0.48%, and a more preferable upper limit is 0.46%.

・ N: 0.01% or less (excluding 0%)
N, like P and S, is an element that deteriorates toughness and weldability as the content increases. The upper limit of the allowable content of N is 0.01%. A more preferable upper limit of the N content is 0.009%, and further preferably 0.008% or less. However, it is difficult to make N in a thick steel plate industrially 0%.

  The above is the reason for limiting the component range of the essential additive elements of the thick steel plate used for producing the welded joint of the present invention, and the balance is Fe and inevitable impurities. Inevitable impurities include O, H, and the like, and these elements may be contained to the extent that they do not impair various properties of the thick steel plate. However, by suppressing the total content of these inevitable impurities to 0.1% or less, preferably 0.09% or less, the corrosion resistance effect according to the present invention can be maximized.

  Further, it is more effective if the thick steel plate used for producing the welded joint of the present invention contains the following elements. The reason for limiting the component range when these elements are contained will be described below.

One or two types of Co: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Mo: 0.01 to 1.0%, W: 0.01 to 1.0% As described above, Co, Ni, Mo, and W are all elements that have an action of improving the corrosion resistance by improving the protection of rust generated on the surface of the thick steel plate. Such an effect is effectively exhibited by containing any one of Co, Ni, Mo and W by 0.01% or more. However, if Co, Ni, Mo, and W are contained in excess of 1.0%, weldability and hot workability are deteriorated. For these reasons, when Co, Ni, Mo, and W are contained, the content is in the range of 0.01 to 1.0%. The more preferable lower limit in the case of containing Co, Ni, Mo, and W is 0.02%, more preferably 0.03% or more. On the other hand, the more preferable upper limit in the case of containing Co, Ni, Mo, W is 0.95%, and more preferably 0.9% or less.

・ Ti: 0.001 to 0.05%, Zr: 0.001 to 0.05%, Nb: 0.001 to 0.05%, one or more types Ti, Zr, and Nb are thick steel plates It has the effect of refining rust generated on the surface and improving corrosion resistance. In order to exert such an effect, it is necessary to contain at least 0.001% of Ti, Zr, or Nb. However, if Ti, Zr, and Nb are contained in amounts exceeding 0.05%, weldability and hot workability are deteriorated. For these reasons, when Ti, Zr, and Nb are contained, the content is in the range of 0.001 to 0.05%. The more preferable lower limit in the case of containing Ti, Zr, and Nb is 0.003%, more preferably 0.005% or more. On the other hand, the more preferable upper limit in the case of containing Ti, Zr, and Nb is 0.045%, and more preferably 0.04% or less.

-Mg: 0.0003-0.005%, Ca: 0.0003-0.005% 1 type or 2 types Mg, Ca has the effect of increasing the pH of the surface of the thick steel plate, cathode It is an element effective in suppressing reaction and improving corrosion resistance. In order to exhibit such an action, it is necessary to contain 0.0003% or more of either Mg or Ca. However, if both Mg and Ca are contained in excess of 0.005%, workability and weldability are deteriorated. For these reasons, when Mg and Ca are contained, the content is in the range of 0.0003 to 0.005%. The more preferable lower limit in the case of containing Mg and Ca is 0.0004%, and more preferably 0.0005% or more. On the other hand, the more preferable upper limit in the case of containing Mg and Ca is 0.0045%, more preferably 0.004% or less.

・ Sn: 0.005 to 0.20%, Sb: 0.005 to 0.20%, Se: 0.005 to 0.20%, one or more kinds Sn, Sb, and Se are thick steel plates. It has the effect of suppressing the corrosion reaction at the site where the pH is locally lowered on the surface and improving the corrosion resistance. In order to exhibit such an action, it is necessary to contain 0.005% or more of Sn, Sb, or Se. However, when all Sn, Sb, and Se are contained in amounts exceeding 0.20%, weldability and toughness deteriorate. For these reasons, when Sn, Sb, and Se are contained, the content is in the range of 0.005 to 0.20%. The more preferable lower limit in the case of containing Sn, Sb, and Se is 0.006%, and more preferably 0.007% or more. On the other hand, the more preferable upper limit in the case of containing Sn, Sb, and Se is 0.19%, and more preferably 0.18% or less.

-B: 0.0001 to 0.005%, V: 0.001 to 0.1%, 1 type or 2 types B and V are effective elements for improving the strength of thick steel plates. In order to exert such an effect, it is necessary to contain 0.0001% or more of B and 0.001% or more of V. However, if it is excessively contained, the base material toughness deteriorates. Therefore, when B is contained, it is necessary to be 0.005% or less, and when V is contained, it is necessary to be 0.1% or less. In addition, the more preferable minimum in the case of containing B is 0.0002%, and it is still more preferable to set it as 0.0003% or more. Further, a more preferable upper limit in the case of containing B is 0.0045%, and further preferably 0.004% or less. On the other hand, the more preferable lower limit in the case of containing V is 0.002%, and more preferably 0.003% or more. Moreover, the more preferable upper limit in the case of containing V is 0.095% respectively, and it is still more preferable to set it as 0.09% or less.

(Production method)
The method for producing the thick steel plate used for the production of the welded joint of the present invention is not particularly limited, but for reliable production, for example, it is preferably produced by the method described below.

  First, with respect to the molten steel discharged from the converter or electric furnace to the ladle, the RH vacuum degassing apparatus is used to adjust the composition of the thick steel plate specified in the present invention and to adjust the temperature. Next refining. Thereafter, the steel ingot may be formed by a normal casting method such as a continuous casting method or an ingot-making method. As a deoxidation type, it is preferable to use killed steel from the viewpoint of mechanical properties and weldability, and it is recommended to use Al killed steel more preferably. Next, the obtained steel ingot is heated to a temperature range of 1000 to 1300 ° C., and then hot rolled to obtain a desired size and shape. At this time, the hot rolling end temperature is controlled to 650 to 850 ° C., and the cooling rate from the end of hot rolling to 500 ° C. is controlled in the range of 0.1 to 15 ° C./second, whereby a predetermined strength characteristic is obtained. can get.

<Welded metal>
The present invention relates to a marine welded joint obtained by butt welding the thick steel plates, and depending on the combination of the thick steel plate and the welding material used in the welded joint part, the coating peels off due to the difference in chemical composition between the two. In some cases, significant dissimilar metal contact corrosion may occur. Dissimilar metal contact corrosion is caused by the potential difference between the weld metal and the base metal (thick steel plate). To achieve a potential difference that does not cause dissimilar metal contact corrosion, welding of Cu and Cr, which has a large effect on the potential, is performed. It is necessary to adjust the content in the metal so as to satisfy the expressions (1) and (2). Moreover, in order to satisfy the mechanical characteristics of the welded joint, C, Si, Mn, P, S, Al, and N in the weld metal also need to have an appropriate component range. Below, the reason for limitation of the component range of each element including content of Cu and Cr in a weld metal is demonstrated. All units are described as%, but indicate mass%.

(Chemical composition)
[Cu] b × 0.5 and 0.10%, whichever is greater ≦ [Cu] w ≦ [Cu] b × 1.5 Formula (1)
(However, [Cu] w is the Cu content in the weld metal, and [Cu] b is the Cu content in the base metal)
Cu contained in the weld metal has the action of reducing the corrosion reaction by forming a dense rust film on the surface of the weld metal in the corrosive environment, as well as the action in the base metal. It is an element necessary for improvement. In order to exert such effects, it is necessary to contain 0.10% or more of Cu in the weld metal. However, when the Cu content of the weld metal is less than ½ of the Cu content of the base metal, the weld metal becomes lower than the base metal, and the corrosion of the weld metal is promoted by the different metal contact corrosion. Therefore, the lower limit of the Cu content of the weld metal needs to be ½ of the Cu content of the base metal or more than the larger value of 0.10%. Further, when the Cu content of the weld metal exceeds 1.5 times the Cu content of the base metal, the weld metal becomes noble than the base metal, and the corrosion of the base metal is promoted by the contact corrosion of different metals. Therefore, the upper limit of the Cu content of the weld metal needs to be 1.5 times or less the Cu content of the base metal.

  The more preferable lower limit of the Cu content of the weld metal is [Cu] b × 0.55 or 0.11%, whichever is larger, and the more preferable lower limit is [Cu] b × 0.6. The larger of 0.12%. Moreover, the upper limit with preferable Cu content of a weld metal is [Cu] b * 1.45, and a more preferable upper limit is [Cu] b * 1.4.

[Cr] b × 0.5 and 0.01%, whichever is larger ≦ [Cr] w ≦ [Cr] b × 1.5 (2)
(However, in formula (2), [Cr] w is the Cr content in the weld metal, and [Cr] b is the Cr content in the base metal)
Cr contained in the weld metal has the effect of reducing the corrosion reaction by forming a dense rust film on the surface of the weld metal in a corrosive environment, similar to the action in the base metal. It is an element necessary for improvement. In order to exert such an effect, it is necessary to contain 0.01% or more in the weld metal. However, when the Cr content of the weld metal is less than [Cr] b × 0.5, the weld metal becomes baser than the base metal, and the corrosion of the weld metal is promoted by the different metal contact corrosion. Therefore, the lower limit of the Cr content of the weld metal needs to be ½ of the Cr content of the base metal, or 0.01% or more, whichever is larger. Further, when the Cr content of the weld metal exceeds [Cr] b × 1.5%, the weld metal becomes nobler than the base metal, and the corrosion of the base metal is promoted by the different metal contact corrosion. Therefore, the upper limit of the Cr content of the weld metal needs to be 1.5 times or less the Cr content of the base metal.

  The more preferable lower limit of the Cr content of the weld metal is [Cr] b × 0.55 or 0.02%, whichever is larger, and the more preferable lower limit is [Cr] b × 0.6 or 0.8. 03%, whichever is greater. Moreover, the upper limit with preferable Cr content of a weld metal is [Cr] b * 1.45, and a more preferable upper limit is [Cr] b * 1.4%.

・ C: 0.04 to 0.30%
C is a basic additive element necessary for ensuring the strength of the weld metal part. In order to obtain the required strength characteristics, it is necessary to contain at least 0.04% or more. However, when C is contained excessively, corrosion resistance is deteriorated, crack sensitivity is increased, and toughness is also deteriorated. In order not to cause such an adverse effect due to excessive addition of C, the C content needs to be suppressed to 0.30% at most. For these reasons, the C content range is set to 0.04 to 0.30%. In addition, the minimum with preferable content of C is 0.045%, It is good to set it as 0.05% or more more preferably. Moreover, the upper limit with preferable content of C is 0.29%, It is good to set it as 0.28% or less more preferably.

・ Si: 0.05-1.0%
Si is also an element necessary for securing the strength of the weld metal part, and the minimum strength as a structural member cannot be secured unless it is contained at least 0.05% or more. However, if the content exceeds 1.0%, the toughness deteriorates. In addition, the minimum with preferable content of Si is 0.08%, More preferably, it is good to set it as 0.10% or more. Moreover, the upper limit with preferable content of Si is 0.95%, More preferably, it is good to set it as 0.90% or less.

Mn: 0.1 to 2.0%
Like Si, Mn is an element necessary for improving the strength of the weld metal part, and if it is less than 0.1%, the minimum strength as a structural member cannot be secured. However, if the content exceeds 2.0%, the toughness deteriorates. In addition, the minimum with preferable content of Mn is 0.15%, More preferably, it is good to set it as 0.2% or more. Moreover, the upper limit with preferable Mn content is 1.9%, More preferably, it is good to set it as 1.8% or less.

・ P: 0.04% or less (excluding 0%)
P is an element that deteriorates the toughness of the weld metal part, and the upper limit of the allowable content of P is 0.04%. The content of P is preferably as small as possible, and the more preferable upper limit of the content of P is 0.038%, and more preferably 0.035% or less. However, it is difficult to reduce the P content in the weld metal part to 0%.

S: 0.04% or less (excluding 0%)
S, like P, is an element that promotes corrosion in addition to degrading the toughness of the weld metal when the content increases. The upper limit of the allowable content of S is 0.04%. The upper limit with more preferable content of S is 0.038%, More preferably, it is good to set it as 0.035% or less. However, it is difficult to reduce the S content in the weld metal part to 0%.

・ Al: 0.010-0.10%
In addition to improving the corrosion resistance by forming an oxide film on the surface, Al is an element that also exhibits the effect of improving the strength of the weld metal part in the same manner as Si and Mn described above. In order to exhibit such an action effectively, it is necessary to contain 0.010% or more. However, if the content exceeds 0.10%, the weldability is impaired, so the range of Al content is set to 0.010 to 0.10%. In addition, the minimum with preferable content of Al is 0.011%, More preferably, it is good to set it as 0.012% or more. Moreover, the upper limit with preferable content of Al is 0.095%, More preferably, it is good to set it as 0.090% or less.

・ N: 0.01% or less (excluding 0%)
N, like P and S, is an element that deteriorates the toughness of the weld metal part as the content increases. The upper limit of the allowable content of N is 0.01%. A more preferable upper limit of the N content is 0.009%, and further preferably 0.008% or less. However, it is difficult to reduce the N content of the weld metal part to 0%.

  Other chemical components of the weld metal are Fe and unavoidable impurities. Inevitable impurities must be limited to a content that does not impair corrosion resistance and mechanical properties, and the specific content should generally be 1% or less in total.

In addition, the content [X] w of each element of Cu, Cr, C, Si, Mn, P, S, Al, and N in the weld metal is the content [X] of each element of the base material (thick steel plate). ], The dilution rate a determined by the welding method and welding conditions to be employed, and the content [X] r of each element of the welding material can be obtained from the following formula (4). In other words, the content [X] r of each element of the welding material used for welding of the thick steel plate depends on the content [X] b of each element of the base material (thick steel plate), the welding method employed and the welding conditions. It can be determined by calculating using the determined dilution ratio a and the target content of each element in the molten metal.
[X] w = [X] b × a + [X] r × (1−a) (4)

  Although the dilution rate a varies depending on the welding method and welding conditions, many studies have been made on the dilution rate a in various welding methods and welding conditions. For example, the Japan Welding Society Hen: Welding Handbook (3rd revised edition), Maruzen Co., Ltd., p. 494, Shinko Welding General Catalog (2013 edition), p. 257 and the like. For reference, if the welding conditions are the welding speed 70 cm / min and the welding heat input 110 kJ / cm in the submerged arc welding method employed in the following examples, the dilution rate is 50%.

  Note that N may be mixed into the weld metal to some extent from the atmosphere during welding. In that case, 0.02% or more from the [N] r value calculated using the above equation (4). What is necessary is just to produce a welded joint using the reduced welding material.

(Width of weld metal)
・ Wmax−Wmin ≦ 5 mm (3)
In the weld metal, concentration of inevitable impurity elements that promote corrosion such as P and S occurs due to solidification segregation during welding, and local corrosion may be promoted. The present inventor has found that such solidification segregation is particularly noticeable when the heat input fluctuation during welding is large, and decided to investigate the suppression measures. As a result of the examination, the base metal melts in response to the heat input of the weld during welding, so the width of the weld metal varies, but by controlling the variation in the width of the weld metal within a specified value, excellent corrosion resistance It has been found that a welded joint that expresses can be obtained.

  If the difference between the maximum width Wmax and the minimum width Wmin (Wmax-Wmin) of the weld metal in the cross section at 1/2 position of the plate thickness parallel to the rolled surface of the welded thick steel plate exceeds 5 mm, solidification segregation during welding occurs. A concentrated portion of P or S is formed remarkably, and local corrosion of the weld metal is promoted. Therefore, Wmax−Wmin must be suppressed to 5 mm or less. The upper limit with more preferable Wmax-Wmin is 4 mm, and 3 mm or less is still more preferable.

  There is no particular limitation on the method of keeping Wmax-Wmin within the specified value. For example, the welding current I, the welding voltage E, and the welding speed V are constantly monitored, and E × I related to the amount of heat input. It can be mentioned that / V is always controlled to be constant. Further, since the welding wire feeding speed also affects the width of the weld metal, Wmax-Wmin can be reduced by controlling the welding wire feeding speed with high accuracy. Moreover, when using the alternating current power supply used for welding, the arc stability at the time of welding improves by using a high frequency alternating current power supply, and Wmax-Wmin can be reduced. As the waveform of the AC power source, a rectangular wave is preferable to a normal sin wave.

  In addition, even when a conventional welding method is performed conventionally, if welding is stable, Wmax-Wmin may be 5 mm or less as a result. However, even if Wmax-Wmin is simply set to 5 mm or less, a target welded joint with excellent corrosion resistance cannot be obtained. Furthermore, by optimizing the chemical composition of the thick steel plate and the weld metal as described above, it is possible to obtain a weld joint having excellent target corrosion resistance for the first time.

  About Wmax-Wmin, it is possible to make a test piece having a weld line length of about 200 to 300 mm in advance by the above-described welding method and confirm that the specified range is satisfied.

(Welding method)
The welding method for obtaining the welded joint of the present invention includes various welding methods such as a covering arc welding method, a mag welding method, a TIG welding method, a self-shielding welding method, a submerged arc welding method, an electroslag welding method, and an electrogas arc welding method. Can be used.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Production of test materials]
Steel materials having various chemical composition compositions shown in Tables 1 and 2 were melted in a vacuum melting furnace to form a 100 kg steel ingot. The obtained steel ingot was heated to 1150 ° C. and then hot-rolled to obtain a steel plate having a thickness of 20 mm, and a steel material made of a steel plate having a size of 500 mm × 100 mm × 20 mm was cut out.

  Using welding materials having chemical composition compositions shown in Tables 3 and 4, 500 mm × 100 mm × 20 mm two cut steel materials are butt welded by 500 mm sides using the submerged arc welding method, and welded joints shown in Tables 5 and 6 Was made. The groove shape at the time of welding of all the welded joints was V-shaped with a groove angle of 50 °, the welding speed was 70 cm / min, and the welding heat input was 110 kJ / cm. No. In order to stabilize the width of the weld metal, the AC power source used for welding is a rectangular wave with a frequency of 100 Hz, and the welding current, voltage, welding wire feed speed, and welding speed are constantly monitored. The parameters affecting the width of these weld metals were controlled so as to be always constant. No. In the welding of No. 6, a conventional welding method using a sin wave AC power source with a frequency of 50 Hz was adopted.

  As shown in FIG. 1, ten test piece materials each having a size of 60 mm × 25 mm × 5 mm were cut out from the obtained welded joint. In addition, it cut out so that the single side | surface of these test piece raw materials might become a cross section of 1/2 position of the plate | board thickness parallel to the rolling surface of the welded thick steel plate (steel raw material). Further, the entire surface of the cut test piece material was wet-polished to # 600 with emery paper to obtain a test piece (TP).

[Measurement of weld metal width]
As described above, the surface of the TP cut out from the thick steel plate was etched with a copper ammonium chloride aqueous solution, and the width of the weld metal on the surface corresponding to the cross section at a half position of the plate thickness was measured with a digital caliper. The maximum width and the minimum width of the weld metal are respectively determined for 10 TPs collected from each weld joint, and the maximum value among the obtained maximum widths of 10 points is Wmax, and the minimum value among the minimum widths is Wmin. As a result, Wmax-Wmin of each welded joint was obtained.

[Corrosion test]
It can be estimated that severe acid corrosion is promoted in the pit of the bottom plate of the tanker crude oil tank and in the bulk of the bulk carrier that carries coal and iron ore. Therefore, as a corrosion test for evaluating the corrosion resistance of each welded joint used for marine structural members, an immersion corrosion test in an aqueous HCl solution adjusted to pH 1 was conducted to reduce the corrosion weight, local corrosion status, and dissimilar metal contact corrosion. We decided to evaluate the corrosion resistance from the situation. The solution temperature was controlled to be constant at 30 ° C. using a water bath, and the immersion time was 72 hours. The used TP was the TP used for the measurement of the weld metal width described above, and 10 pieces of each welded joint were subjected to the corrosion test. Each TP was wet-polished to # 600 again with emery paper after measuring the width of the weld metal, washed with water and acetone, dried, and then used for the corrosion test.

  After TP is immersed in the test solution for 72 hours, it is taken out from the test solution, washed with water, subjected to ultrasonic cleaning in acetone, measured for TP mass after drying, and corroded from the mass change before and after the test for each TP. The weight loss value obtained was determined, and the average value of 10 sheets was taken as the corrosion weight loss.

  Further, the appearance of the TP after the mass measurement was observed using a loupe, and the presence or absence of local corrosion such as pitting corrosion was investigated. Thereafter, resin embedding of the TP cross section was performed, and the presence or absence of dissimilar metal contact corrosion between the base material and the weld metal was investigated. As for dissimilar metal contact corrosion, it was judged that there was no dissimilar metal contact corrosion when the difference between the base metal and the weld metal when viewed from the cross section was 30 μm or less, and that there was dissimilar metal contact corrosion when the step exceeded 30 μm .

The corrosion resistance of welded joints was comprehensively evaluated based on the above weight loss, local corrosion, and dissimilar metal contact corrosion. The evaluation criteria were as follows, and ◎◎, ◎, ○ to ◎, ○ was evaluated as acceptable, and × was evaluated as unacceptable.
◎◎: Corrosion weight loss is less than 20, no local corrosion, and no contact corrosion between weld metal and base material ◎: Corrosion weight loss is less than 40, no local corrosion, and between weld metal and base material No contact corrosion ○ to ◎: Corrosion weight loss less than 60, no local corrosion, no contact corrosion between weld metal and base metal ○: Corrosion weight loss less than 80, no local corrosion, weld metal and base material No contact corrosion between: No: Corrosion weight loss of 80 or more, or local corrosion, or contact corrosion between weld metal and base metal

[Test results]
Tables 5 and 6 show the results of comprehensive evaluation of corrosion weight loss, local corrosion status, dissimilar metal contact corrosion status, and corrosion resistance of welded joints by immersion corrosion test in HCl aqueous solution of pH1. “Corrosion weight loss” is “No. 1 shows the relative value when the corrosion weight loss (average value of 10 TPs) of one welded joint TP is 100. In addition, “local corrosion” was indicated by “◯” when no local corrosion was observed in all 10 TPs, and “No” was observed when any one of the TPs was recognized as local corrosion. In addition, x (b) indicates that local corrosion has occurred on the base metal side, and x (w) indicates that local corrosion has occurred on the weld metal side. In addition, “contact corrosion” was marked with “◯” when no different metal contact corrosion was observed in all of the 10 TPs, and “X” when any one of the TPs was recognized with different metal contact corrosion. In addition, x (b) shows that the corrosion on the base metal side is accelerated than the weld metal, and x (w) shows that the weld metal side is accelerated than the base metal.

  According to Tables 5 and 6, No. 1 does not satisfy any of the provisions of the present invention. In Comparative Examples 1 to 8, the corrosion weight loss was relatively large, and at least one of local corrosion and dissimilar metal contact corrosion occurred. As a result, the corrosion resistance of the welded joint was insufficient. No. 1, 2, and 3 are comparative examples that do not satisfy the component range defined by any of Al, Cu, and Cr of the base material. In any of these, no dissimilar metal contact corrosion is observed, but local corrosion occurs on the base metal side, and the overall weight loss of the welded joint is relatively large. No. In Nos. 4 and 5, the amount of Cu or Cr of the weld metal was too small relative to the base metal, so that the corrosion of the weld metal was accelerated by the contact corrosion of different metals, and local corrosion also occurred. No. In Nos. 7 and 8, since the amount of Cu or Cr of the weld metal is too much with respect to the base material, the corrosion of the base material was accelerated by the contact corrosion of different metals. No. by conventional welding method from the past. No. 6 has a relatively unstable weld metal width, and as a result, Wmax-Wmin exceeds the specified value. However, inevitable impurity elements that promote corrosion such as P and S by solidification segregation during welding. Concentrated portions of these were formed on the weld metal, and local corrosion occurred starting from these concentrated portions.

  On the other hand, No. 1 satisfying all the provisions of the present invention. All of the welded joints of 9 to 62 had a corrosion weight loss of less than 80, and neither local corrosion nor dissimilar metal contact corrosion was observed, resulting in excellent corrosion resistance. These corrosion resistances include a dense rust film obtained by adding appropriate amounts of Al, Cu, Cr, etc., suppression of dissimilar metal contact corrosion by optimizing the amount of Cu and Cr between the base material and the weld metal, and P, S, etc. This is obtained by suppressing local corrosion by suppressing concentration. As described above, each of the welded joints of the present invention is excellent in corrosion resistance under severe acid conditions, and is suitable as a welded joint for a crude oil tank of a tanker or a bulk cargo hold that is often used without coating.

  Even in a corrosive environment such as a ballast tank in which a thick steel plate is used for coating specifications, the anticorrosion coating is peeled off for some reason, and the welded joint is exposed to the corrosive environment, and corrosion often proceeds. Therefore, even when the welded joint of the present invention is used with its surface coated, it has an effect on inhibiting corrosion when the paint is peeled off, so it can be effectively used as a marine welded joint regardless of the presence or absence of coating. it can.

Claims (6)

In mass%, C: 0.04 to 0.30%, Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, P: 0.04% or less (excluding 0%) ), S: 0.04% or less (excluding 0%), Al: 0.010 to 0.10%, Cu: 0.10 to 1.0%, Cr: 0.01 to 0.5%, N: 0.010% or less (not including 0%), a marine welded joint obtained by butt welding a thick steel plate made of Fe and inevitable impurities,
The content of Cu and Cr in the weld metal satisfies the following formulas (1) and (2), and the content of chemical components other than Cu and Cr in the weld metal is C%: 0.04-0. .30%, Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, P: 0.04% or less (excluding 0%), S: 0.04% or less (0 %), Al: 0.010 to 0.10%, N: 0.010% or less (not including 0%), the balance being Fe and inevitable impurities,
Furthermore, the difference between the maximum width Wmax and the minimum width Wmin of the weld metal in a cross section at a half position of the plate thickness parallel to the rolled surface of the welded thick steel plate satisfies the following formula (3). Marine welded joint with excellent corrosion resistance.
The larger of [Cu] b × 0.5 and 0.10% ≦ [Cu] w ≦ [Cu] b × 1.5 (1)
The larger of [Cr] b × 0.5 and 0.01% ≦ [Cr] w ≦ [Cr] b × 1.5 (2)
Wmax−Wmin ≦ 5 mm (3)
However, in Formula (1), [Cu] w is the Cu content (% by mass) in the weld metal, and [Cu] b is the Cu content (% by mass) in the base metal. In Formula (2), [Cr] w is the Cr content (mass%) in the weld metal, and [Cr] b is the Cr content (mass%) in the base metal.   The thick steel plate is further in mass%, Co: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Mo: 0.01 to 1.0%, W: 0.01 to The marine welded joint with excellent corrosion resistance according to claim 1, which contains 1.0% or more of 1.0%.   The thick steel plate is further one or more of mass%, Ti: 0.001 to 0.05%, Zr: 0.001 to 0.05%, Nb: 0.001 to 0.05%. A marine welded joint having excellent corrosion resistance according to claim 1 or 2.   The thick steel plate according to any one of claims 1 to 3, further comprising one or two of Mg: 0.0003 to 0.005% and Ca: 0.0003 to 0.005% in mass%. Ship welded joint with excellent corrosion resistance.   Further, the thick steel plate is one or more of Sn: 0.005 to 0.20%, Sb: 0.005 to 0.20%, Se: 0.005 to 0.20% in mass%. The marine welded joint with excellent corrosion resistance according to any one of claims 1 to 4.   The said thick steel plate contains the 1 type (s) or 2 types of B: 0.0001-0.005% and V: 0.001-0.1% further by the mass%. Ship welded joint with excellent corrosion resistance.

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