JP2012135817A - Welded joint and welded structure excellent in corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welded joint and a welded structure which are excellent in corrosion resistance under environment where they are exposed to salt caused by seawater and high temperature and high humidity and environment which contains sulfur which comes from oil or the like without applying painting and electric corrosion protection especially the welded joint and the welded structure which are raised in the corrosion resistance to local corrosion and crevice corrosion.SOLUTION: In this welded joint, mutual base metal is welded and the welded joint satisfies under formula (5), (6) and (7): 0.30≤[A]/[B]≤3.0...(5), 0.30≤[A]/[B]≤3.0...(6) and 0.30≤[A]/[B]≤3.0...(7). In the formula, [A] means the content (mass%) of Al which is contained in the weld metal in the welded part, [B] means the content (mass%) of Al which is contained in the base metal, [A] means the content (mass%) of Cu which is contained in the weld metal in the welded part, [B] means the content (mass%) of Cu which is contained in the base metal, [A] means the content (mass%) of Cr which is contained in the weld metal in the welded part, and [BCr] means the content (mass%) of Cr which is contained in the base metal.

Description

  The present invention relates to a welded joint having excellent corrosion resistance and a welded structure including the welded joint. The welded structure of the present invention is suitably used for, for example, ships such as oil tankers, cargo ships, cargo passenger ships, passenger ships, warships, and oil tanks used as containers for storing and transporting oils.

  Generally, various types of ships such as petroleum tankers and petroleum tanks such as crude oil tankers (petroleum liquid fuel tanks) are welded structures that include welded joints in which the base materials (steel materials) are welded together with welding materials. It is composed of

  Welded structures used for these applications are mainly exposed to salt in seawater and high temperature and humidity (in the case of ships), or in environments containing sulfur derived from petroleum (oils) Therefore, it is required to have excellent corrosion resistance. So far, various methods have been adopted from the viewpoint of enhancing the corrosion resistance of steel materials used in ships and petroleum tanks.

  For example, it is known to paint or perform anticorrosion on steel materials used for main structures (for example, outer plates, ballast tanks, crude oil tanks, etc.) of various ships and petroleum tanks.

  Of these, when coatings represented by heavy coating are performed, there is a high possibility that a coating film defect is generated, and the coating film may be damaged due to a collision or the like in the manufacturing process, so the base steel material is often exposed. . In such a steel exposed portion, since local corrosion (local corrosion) proceeds intensively, for example, petroleum-based liquid fuel stored in a petroleum tank leaks early.

  On the other hand, although the anticorrosion method is very effective for a part completely immersed in seawater, an electrical circuit necessary for anticorrosion is not formed in a part that receives seawater splashes in the atmosphere. The anticorrosion effect may not be fully exhibited. In addition, if the anticorrosive galvanic anode is consumed or lost due to dropping off, severe corrosion may proceed immediately.

  In addition to the above, techniques for improving the corrosion resistance of the steel material itself have also been proposed (Patent Document 1, Patent Document 2, etc.). Patent Document 1 describes a technique for corrosion-resistant steel for shipbuilding in which the contents of Mg and Cu are appropriately controlled and has excellent corrosion resistance even without coating. Further, Patent Document 2 relates to a technology for marine steel materials in which the contents of Ni and Cu are appropriately controlled, and exhibits excellent coating damage resistance even when applied to a ballast tank exposed to a severe corrosive environment. It is described to do.

  However, any of the above methods is inferior in the effect of improving the corrosion resistance under a more severe corrosive environment. In particular, the corrosion resistance against local corrosion is insufficient, and in particular, the deterioration of the corrosion resistance against crevice corrosion poses a serious problem. Crevice corrosion is corrosion that occurs at the `` crevice '' part such as the contact part between steel and foreign matter or the damaged part of the anticorrosion coating film, and because of the high corrosion rate, it is the main cause of shortening the life of ships etc. Yes.

  Such a local corrosion and crevice corrosion are remarkably observed in petroleum tanks such as the above-described petroleum-based liquid fuel tanks. In the case of petroleum tanks, local corrosion proceeds remarkably at the defective part of the oil coat formed on the steel plate surface. This defective part is thought to be repaired or newly formed by the movement of crude oil, deformation of the hull, etc. during operations of crude oil tankers, etc., so local corrosion does not concentrate in one place, Progresses to almost the entire surface of steel. In addition, since crevice corrosion also occurs remarkably in petroleum tanks, it is desired to improve steel materials having excellent local corrosion resistance and crevice corrosion resistance.

  Among these, as a technique for improving corrosion resistance against local corrosion, for example, Patent Document 3 to Patent Document 5 are cited. These all relate to a technique in which chemical components in steel are appropriately controlled. For example, Patent Document 3 discloses a crude oil in which the contents of Cu, Ni, Cr, Mo, Sb, and Sn are appropriately controlled. Corrosion resistant steel for heavy oil storage, Patent Document 4 discloses a steel material for a cargo oil tank in which the contents of Cu, Ni, Cr, and Al are appropriately controlled. Patent Document 5 discloses Cu, Ni, Mo, and Cr. Steel materials for crude oil tank bottom plates whose contents are appropriately controlled are described respectively. Although these techniques increase the corrosion resistance against general corrosion and local corrosion, further improvement is desired. In addition, none of the above-mentioned patent documents pays sufficient attention to the corrosion resistance against crevice corrosion. Therefore, improvement of crevice corrosion resistance is strongly desired.

Japanese Patent Laid-Open No. 2000-17371 JP 2002-266052 A JP 2001-214236 A JP 2003-82435 A JP 2004-2948 A

  As described above, all of the above-mentioned patent documents are techniques for improving corrosion resistance against general corrosion and local corrosion, and no consideration is given to improving the corrosion resistance against crevice corrosion.

  In addition, including the above-mentioned patent documents, until now, mainly to improve the corrosion resistance by controlling the chemical components of the steel material, in order to increase the corrosion resistance of the welded structure, not only steel materials, It is also necessary to pay attention to the welded part (welded joint part).

  The present invention has been made in view of the above circumstances, and the purpose thereof is sulfur derived from an environment exposed to salt or high temperature / humidity from seawater, petroleum, etc., without applying coating or cathodic protection. It is an object of the present invention to provide a welded joint and a welded structure having improved corrosion resistance in an environment including water, and in particular, improved corrosion resistance against local corrosion and crevice corrosion.

The welded joint of the present invention is a welded joint in which base materials are welded to each other, and there is a gist in satisfying the following formula (5), the following formula (6), and the following formula (7).
0.30 ≦ [A _Al ] / [B _Al ] ≦ 3.0 (5)
0.30 ≦ [A _Cu ] / [B _Cu ] ≦ 3.0 (6)
0.30 ≦ [A _Cr ] / [B _Cr ] ≦ 3.0 (7)
Where
[A _Al ] is the content (% by mass) of Al contained in the weld metal of the welded portion,
[B _Al ] is the content (% by mass) of Al contained in the base material,
[A _Cu ] is the content (% by mass) of Cu contained in the weld metal of the welded part,
[B _Cu ] is the content (% by mass) of Cu contained in the base material,
[A _Cr ] is the content (% by mass) of Cr contained in the weld metal of the welded part,
[B _Cr ] is the content (% by mass) of Cr contained in the base material.
Respectively.

  In a preferred embodiment, the weld metal contains C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: 0.01 to 2.0%, Al: 0.05 to 0. .50%, Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, P: 0.020% or less (including 0%), and S: 0.010 % Or less (including 0%), and the balance: Fe and inevitable impurities.

  In preferable embodiment, the component in steel of the said base material is C: 0.01-0.20%, Si: 0.01-0.50%, Mn: 0.01-2.0%, Al: 0 0.05 to 0.50%, Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, P: 0.020% or less (including 0%) and S : 0.010% or less (including 0%), and the balance: Fe and inevitable impurities.

  In a preferred embodiment, the ratio ([Cr] / [Al]) of the Cr content [Cr] and the Al content [Al] is in the range of 1-50.

  In a preferred embodiment, the steel component of the base material further includes (i) Ni: 0.01 to 5.0%, Co: 0.01 to 5.0%, and Ti: 0.005 to 0.00. At least one selected from the group consisting of 20%, (ii) Ca: 0.0005-0.020% and / or Mg: 0.0005-0.020%, (iii) Se: 0.005-0.50 %, (Iv) Sb: 0.01 to 0.50% and / or Sn: 0.01 to 0.50%, (v) B: 0.0001 to 0.010%, V: 0.01 to 0 .50% and Nb: at least one selected from the group consisting of 0.003 to 0.50%.

  The welded structure of the present invention includes any one of the above-described welded joints.

  In a preferred embodiment, the welded structure is used for a ship or a crude oil tank.

  Since the present invention has the above-described configuration, it is possible to provide a welded joint and a welded structure with improved overall corrosion resistance, corrosion uniformity, crevice corrosion resistance, and paint corrosion resistance. It was.

  The welded structure of the present invention is excellent in corrosion resistance in an environment exposed to seawater salinity and high temperature and humidity, and therefore is suitably used for ships such as petroleum tankers, cargo ships, cargo passenger ships, passenger ships, warships, and the like. . In addition, the welded structure of the present invention is excellent in corrosion resistance under an environment containing a sulfur content derived from petroleum or the like, and therefore is suitably used for petroleum tanks used as containers for petroleum storage or transportation. .

FIG. 1 is an explanatory view showing the external shape of a joint test piece B used in Example 1. FIG. FIG. 2 is an explanatory view showing the external shape of the test piece C for crevice corrosion resistance measurement used in Example 1. FIG. FIG. 3 is an explanatory view showing the appearance of the test piece D for measuring coating corrosion resistance used in Example 1. FIG.

  The present inventor provides a welded structure with improved overall corrosion resistance, corrosion uniformity, crevice corrosion resistance, and paint corrosion resistance. The investigation was conducted while paying attention to the composition of the weld metal in the welded part (welded joint part). Here, the focus is on the weld metal of the welded joint, when the base metals are welded under normal welding conditions or via welding materials, even if the composition of the base materials is controlled, This is because the composition of the weld metal in the welded joint portion where the metal is welded changes, and the corrosion resistance (particularly, crevice corrosion resistance) at the welded portion may be reduced.

  As a result, the ratio ([A] / [B]) of the content [A] of the anticorrosive film forming component contained in the weld metal of the welded portion and the content [B] of the anticorrosive film forming component contained in the base material. The inventors have found that the intended purpose can be achieved by controlling the value within a predetermined range, thereby completing the present invention.

  In the present specification, the “corrosion-preventing film-forming component” means a component that improves the corrosion resistance in an environment exposed to salt from seawater, high temperature and high humidity, or an environment containing sulfur derived from petroleum. Specifically, (1) an oxide anticorrosive film forming element such as Al and Cr, a dense rust forming element such as Cu, (2) an element for suppressing pH decrease at a corrosion site such as Se, and (3 ) Zinc sulfide film forming elements such as Zn. Hereinafter, the welded joints or welded structures containing the anticorrosive film forming components (1) to (3) above are referred to as “first to third welded joints or welded structures according to the present invention”, respectively.

  Hereinafter, the configuration of each welded joint will be described in detail.

(First welded joint according to the present invention)
The first welded joint according to the present invention satisfies the following formula (5), the following formula (6), and the following formula (7). As a result, it is possible to provide a welded structure with improved overall corrosion resistance, corrosion uniformity, crevice corrosion resistance, and paint corrosion resistance.
0.30 ≦ [A _Al ] / [B _Al ] ≦ 3.0 (5)
0.30 ≦ [A _Cu ] / [B _Cu ] ≦ 3.0 (6)
0.30 ≦ [A _Cr ] / [B _Cr ] ≦ 3.0 (7)
Where
[A _Al ] is the content (% by mass) of Al contained in the weld metal of the welded portion,
[B _Al ] is the content (% by mass) of Al contained in the base material,
[A _Cu ] is the content (% by mass) of Cu contained in the weld metal of the welded part,
[B _Cu ] is the content (% by mass) of Cu contained in the base material,
[A _Cr ] is the content (% by mass) of Cr contained in the weld metal of the welded part,
[B _Cr ] is the content (% by mass) of Cr contained in the base material.
Respectively.

In the above formulas (5), (6), and (7), [A _Al ] / [B _Al ] (hereinafter referred to as S value), [A _Cu ] / [B _Cu ] (hereinafter referred to as T value) ), [A _Cr ] / [B _Cr ] (hereinafter referred to as U value) is less than 0.30, respectively, as shown in the examples described later, corrosion protection of Al, Cu, and Cr Since the effect of improving the corrosion resistance by the film-forming component is not effectively exhibited, the above characteristics are deteriorated. On the other hand, the upper limit of the S value, the T value, and the U value is not particularly limited from the viewpoint of improving the corrosion resistance, but if it exceeds 3.0 (strictly 3.00), the toughness of the welded portion deteriorates. However, a predetermined mechanical strength cannot be ensured (see the examples described later). For example, as shown in Table 5 below. 12 had an S value of 3.07 and Table 5 No. 13 has a T value of 3.08 and Table 5 The U value of 14 is 3.07, both of which exceed the above range, so the HAZ toughness is reduced. Taking these into account, it is preferable to control the ranges of the S value, the T value, and the U value to be 0.5 or more and 2.0 or less, respectively.

Here, [A _Al ], [A _Cu ], and [A _Cr ] included in the weld metal of the welded portion are [B _Al ], [B _Cu ], and [B _Cr ] included in the base material, respectively. And the welding material used.

  In order to control the above S value, the above T value, and the above U value within the range of the above formula (5) to the above formula (7), respectively, Each is preferably controlled as follows.

(Components in steel of base material in first welded joint according to the present invention)
As shown below, the base material used in the present embodiment contains Al, Cu, and Cr as essential components in addition to the basic components of C, Si, and Mn, and contains harmful components of P and S. Suppressed. This base material is useful as a marine steel material having excellent corrosion resistance, and the present applicant has filed an application earlier (filed on July 29, 2004, Japanese Patent Application No. 2004-222372).

  First, Al, Cu, and Cr that characterize this embodiment will be described.

  First, the action of Al and Cr will be described.

Al is an element which forms a stable oxide anticorrosion film on the surface of the steel material and enhances the corrosion resistance. Specifically, Al ³⁺ ions dissolved by corrosion of the steel material are combined with dissolved oxygen or the like to become Al oxide, and are deposited on the surface of the steel material to form an anticorrosion film. However, the anticorrosion action by the above-mentioned film may not be exhibited effectively in a high salt environment such as a ship. In addition, the Al oxide film is very stable in a substantially neutral region having a pH of about 5 to 8.5. However, when the pH exceeds 8.5, the solubility becomes high. In particular, in the case of seawater to which a ship is exposed, the pH of clean seawater is about 8, but the pH of the sea area where seaweed is bred rises to about 9.5 and becomes alkaline. Further, at the site where the cathodic reaction of corrosion occurs, the pH tends to further increase due to OH ⁻ ions generated by the reduction of dissolved oxygen. For these reasons, Al oxides in a marine environment do not necessarily exist stably, but rather dissolve easily, and thus corrosion resistance due to Al oxides is often not fully exhibited.

  On the other hand, Cr, like Al, forms a stable oxide film on the surface of the steel material and exhibits an anticorrosive action. However, Cr oxide alone is not sufficient for the above action, and is remarkably exhibited in the presence of Al oxide. In addition, Cr oxide is excellent in stability in the alkaline region and lowers pH by hydrolysis of a very small amount of dissolved Cr ions. Therefore, dissolution of Al oxide due to seawater pH increase can be prevented. It becomes possible to secure the anti-corrosion action by things.

Therefore, in this embodiment, Cr and Al are used in combination.
Hereinafter, the contents of Al and Cr will be described.

Al: 0.05 to 0.50%
In order to effectively exhibit the above-described action by Al, 0.05% or more of Al is added. When the Al content is less than 0.05%, Al ³⁺ ions are not deposited on the surface of the steel material due to scattering in seawater, and a desired anticorrosion film is not formed. However, if Al is added in excess of 0.1%, the weldability may be adversely affected, for example, the toughness of the welded portion may be slightly lowered. By controlling P and S (described later) in an appropriate range in addition to C and Si in the base material, weldability can be secured even if Al is added in a range exceeding 0.1%. . However, when the Al content exceeds 0.50% and becomes excessive, the weldability decreases. The Al content is preferably 0.08% or more and 0.45% or less, and more preferably 0.10% or more and 0.40% or less.

Cr: 0.01-5.0%
In order to effectively exhibit the anticorrosive action due to Cr, 0.01% or more of Cr is added. However, if Cr is added excessively, weldability deteriorates, so the content is made 5.0% or less. The Cr content is 0.05% or more and 4.50% or less.

  The above-described action of Cr and Al is effectively exhibited by controlling the ratio between the amount of Cr contained in the steel material and the amount of Al ([Cr] / [Al], mass ratio) to 1 or more and 50 or less. . When the above ratio is less than 1, the corrosion uniformity is insufficient. On the other hand, when it exceeds 50, the crevice corrosion resistance decreases. The ratio is more preferably 10 or more and 35 or less.

  Furthermore, in this embodiment, in addition to said Al and Cr, Cu is contained in the following range.

Cu: 0.01 to 5.0%
Cu forms a dense rust film on the surface and greatly contributes to the improvement of corrosion resistance. Further, when the rust film is coexisted with the above-mentioned Al oxide and Cr oxide described later, the protection of the base material is increased synergistically, and further excellent corrosion resistance is exhibited. In order to exhibit such an action effectively, Cu is added by 0.01% or more. However, if Cu is added excessively, weldability and hot workability deteriorate, so 5.0% or less is preferable. The Cu content is 0.05% or more and 4.00% or less.

C: 0.01 to 0.20%
C is added to ensure strength. In order to ensure the minimum level of strength required for the structural members of the ship (approximately 400 MPa, depending on the thickness of the steel used), 0.01% or more is added. However, when C is added excessively exceeding 0.20%, toughness deteriorates. The content of C is preferably 0.04% or more and 0.18% or less, and more preferably 0.08% or more and 0.16% or less.

Si: 0.01 to 0.50%
Si is added for deoxidation and ensuring strength. When the Si content is less than 0.01%, the minimum strength required for the structural member cannot be ensured. However, when Si is added excessively exceeding 0.50%, weldability deteriorates. The Si content is preferably 0.05% or more and 0.40% or less, and more preferably 0.10% or more and 0.30% or less.

Mn: 0.01 to 2.0%
Mn, like Si, is added for deoxidation and ensuring strength. If the Mn content is less than 0.01%, the minimum level of strength required for the structural member cannot be ensured. However, when Mn is added excessively exceeding 2.0%, toughness deteriorates. The Mn content is preferably 0.05% or more and 1.80% or less, and more preferably 0.10% or more and 1.60% or less.

P: 0.020% or less (including 0%)
Since P deteriorates toughness and weldability, it is preferable to suppress the content as much as possible. If the P content exceeds 0.020%, the weldability required for ships and the like cannot be ensured. The P content is preferably 0.015% or less.

S: 0.010% or less (including 0%)
S, like P, degrades toughness and weldability, so it is preferable to suppress the content as much as possible. If the S content exceeds 0.010%, the weldability required for ships and the like cannot be ensured. The S content is preferably 0.008% or less.

  The basic components of the base material used in the first welded joint according to the present invention are as described above, and the balance is iron and inevitable impurities (for example, O). In addition, other permissible components (for example, Zr, N, etc.) may be further added to such an extent that the action of the present invention is not inhibited. Since these tolerable components are deteriorated in toughness when the content thereof is excessive, it is preferable to suppress them to about 0.1% or less in total.

  Further, the above-mentioned base material may include (i) Ni: 0.01 to 5.0%, Co: 0.01 to 5.0%, and Ti: 0.005 to 0.20% as necessary. (Ii) Ca: 0.0005 to 0.020% and / or Mg: 0.0005 to 0.020%, (iii) Se: 0.01 to 0.50%, (ii) iv) Sb: 0.01 to 0.50% and / or Sn: 0.01 to 0.50%, (v) B: 0.0001 to 0.010%, V: 0.01 to 0.50% Nb: At least one selected from the group consisting of 0.003 to 0.50% may be further added, and the characteristics of the welded joint are further improved according to the type of the added component.

(I) At least one selected from the group consisting of Ni: 0.01 to 5.0%, Co: 0.01 to 5.0%, and Ti: 0.005 to 0.20%. However, a dense rust film is formed on the surface to improve corrosion resistance. Furthermore, Ti suppresses corrosion inside the crevice and improves crevice corrosion resistance. However, when these elements are added excessively, weldability and hot workability deteriorate. Taking these into consideration, the contents of the above elements are set in the above ranges, respectively. The contents of Ni and Co are more preferably 0.05% or more and 4.50% or less, respectively. Further, the Ti content is more preferably 0.008% or more and 0.15% or less.

(Ii) Ca: 0.0005 to 0.020% and / or Mg: 0.0005 to 0.020%
Ca and Mg exhibit a pH increasing action by dissolution, suppress a pH decrease due to a hydrolysis reaction of a local anode where iron is dissolved, suppress a corrosion reaction, and improve corrosion resistance. Such an effect is effectively exhibited by adding 0.0005% or more of each of the above elements. However, if the above element is added in excess of 0.020%, workability and weldability deteriorate. The content of the elements is more preferably 0.0010% or more and 0.015% or less.

(Iii) Se: 0.005 to 0.50%
Se suppresses the pH reduction at the site where the corrosion dissolution reaction occurs to suppress the corrosion reaction and improve the corrosion uniformity. The above-described action by Se is particularly effective in suppressing local corrosion of the “clearance” formed in the welded joint. This is because, in the gap portion, since the movement of the substance is restricted, the pH is likely to decrease locally. In order to effectively exhibit such an action, 0.005% or more of Se is added. However, when Se is added excessively exceeding 0.50%, workability and weldability deteriorate. The Se content is preferably 0.008% or more and 0.45%, and more preferably 0.010% or more and 0.40% or less.

(Iv) Sb: 0.01 to 0.50% and / or Sn: 0.01 to 0.50%
Sb and Sn are rust densification by Cu, which is an essential component of this embodiment, rust densification by (i) elements (Ni, Ti, etc.), and (iii) Se and ( ii) Promotes the effect of inhibiting pH reduction by the elements (Ca, Mg) and improves the corrosion resistance. In order to effectively exhibit such an action, it is preferable to add 0.01% or more of any of the above elements. However, if the above elements are added excessively, the workability and weldability deteriorate, so each content is preferably 0.50% or less. The content of the elements is more preferably 0.02% or more and 0.40% or less, respectively.

(V) at least one selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50%. It is useful for improvement. However, when these elements are added excessively, the toughness of the base material deteriorates. Considering these points, the preferable content of the above elements is set in the above range. The content of B is more preferably 0.0003% or more and 0.0090% or less. The content of V is more preferably 0.02% or more and 0.45% or less. The Nb content is more preferably 0.005% or more and 0.45% or less.

  The steel material of this embodiment may be further coated as needed. For example, as shown in the examples described later, a coating film may be formed using a tar epoxy resin paint. Or you may perform the typical heavy-duty anticorrosion coating methods other than the said coating, for example, the coating by a zinc rich paint and a shop primer. Furthermore, it is possible to use an anticorrosion method such as cathodic protection in combination. By these methods, the corrosion resistance of coating is further improved (see Examples described later).

(Composition of weld metal in the first weld joint according to the present invention)
The weld metal is C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: 0.01 to 2.0%, Al: 0.05 to 0.50%, Cu : 0.01-5.0%, Cr: 0.01-5.0%, P: 0.020% or less (including 0%) and S: 0.010% (0%) And the remainder is preferably Fe and inevitable impurities. That is, the composition of the weld metal is preferably substantially the same as that of the base material.

  Next, a preferred method for producing the first welded joint according to the present invention will be described.

  The first welded joint according to the present invention can be obtained, for example, by appropriately controlling the composition of the base material and the welding material. The preferable steel components of the base material are as described above.

  In particular, Al, Cu, and the welding material are used so that the S value, the T value, and the U value satisfy the ranges of the above formula (5), the above formula (6), and the above formula (7), respectively. It is preferable to appropriately control the Cr content. Specifically, although the composition of the welding material varies depending on the welding conditions and the like, the Al amount of the welding material is generally in the range of more than 0.3 times and less than 3.1 times compared to the Al amount of the base material. Strictly, it is more preferable to control within the range of more than 0.30 times and less than 3.10 times (see the examples described later). Further, it is preferable to control the amount of Cu of the welding material within a range of more than 0.1 times and less than 2.7 times compared to the amount of Cu of the base material. It is more preferable to control within a range of less than 70 times (see Examples described later). Further, it is preferable to control the Cr amount of the welding material within a range of more than 0.2 times and less than 3.0 times compared to the Cr amount of the base material. It is more preferable to control within a range of less than 00 times (see Examples described later). For example, as shown in Table 5 below. Since 1 is less than the above-mentioned preferable range, that is, the amount of Cu in the welding material / the amount of Cu in the base material≈0.096, the coating corrosion resistance is reduced. In Table 5, No. 3 is less than the above-mentioned preferable range, ie, Cr amount in the welding material / Cr amount in the base material≈0.186, and therefore, the coating corrosion resistance is lowered. In Table 5, No. No. 8 is excellent in corrosion resistance because the amount of Cr in the welding material / the amount of Cr in the base metal≈2.966, which is within the above preferred range. No. 14 exceeds the above preferable range, ie, the amount of Cr in the welding material / the amount of Cr in the base metal≈3.051, so the HAZ toughness is reduced. The other components of the welding material are not particularly limited as long as the first welded joint according to the present invention is obtained. For example, C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: Control within the range of 0.01 to 2.0%, and suppress P to 0.020% or less (including 0%) and S to 0.010% or less (including 0%) It is recommended.

  The type of welding conditions is not particularly limited, and generally used welding methods can be appropriately selected and used so as to improve the mechanical properties (such as strength and toughness) of the welded joint. Typical examples include arc welding. For example, submerged arc welding (automatic welding) described in the examples described later, semi-automatic arc welding such as solid wire or flux-cored wire, covered arc welding, TIG welding (TIG), etc. Including manual arc welding. In addition, gas welding may be performed.

(Second welded joint according to the present invention)
The second welded joint according to the present invention satisfies the following expression (4). As a result, it is possible to provide a welded structure with improved overall corrosion resistance, corrosion uniformity, crevice corrosion resistance, and paint corrosion resistance.
0.30 ≦ [A _Se ] / [B _Se ] ≦ 3.0 (4)
Where
[A _Se ] is the Se content (% by mass) contained in the weld metal of the welded part,
[B _Se ] is the Se content (% by mass) contained in the base material.
Respectively.

In the above formula (4), when [A _Se ] / [B _Se ] (hereinafter referred to as R value) is less than 0.30, as shown in the examples described later, an anticorrosive film of Se Since the effect of improving the corrosion resistance by the forming component is not effectively exhibited, these characteristics are deteriorated. On the other hand, the upper limit of the R value is not particularly limited from the viewpoint of improving the corrosion resistance. However, if it exceeds 3.0 (strictly 3.00), the toughness of the welded portion deteriorates and a predetermined mechanical strength is ensured. Can not do it. Considering these comprehensively, the R value is preferably 0.5 or more and 2.0 or less.

Here, [A _Se ] included in the weld metal of the welded part can be determined mainly by [B _Se ] included in the base material and the welding material to be used.

  In order to control the R value within the range of the above formula (4), it is preferable to control the components of the base metal in the steel and the composition of the weld metal as follows.

(Components in steel of base material in second welded joint according to the present invention)
As shown below, the base material used in the present embodiment contains Se as an essential component in addition to the basic components of C, Si, and Mn. This base material is useful as a marine steel material having excellent corrosion resistance, and the applicant of the present application has already filed an application (filed on June 29, 2004, Japanese Patent Application No. 2004-191759).

  First, Se that characterizes the present embodiment will be described.

Se: 0.005-0.50%
Se has the action of suppressing the corrosion reaction by suppressing the pH drop at the site where the corrosion dissolution reaction occurs, and improving the corrosion resistance. As a result, local changes in pH are less likely to occur, and corrosion uniformity is improved.

  This point will be explained in a little more detail. For example, when an element that densifies or stabilizes rust such as Cu, Ni, Ti (described later) is added, the overall corrosion resistance is improved as shown in the examples described later. The pH tends to decrease at the starting point, and local corrosion tends to increase. Se has a tendency to concentrate in a defective portion of rust that is likely to be a starting point of local corrosion, and is therefore considered to exert a pH lowering suppressing action against a local pH decrease. In particular, since the movement of the substance is restricted in the gap portion, a pH drop occurs locally and local corrosion occurs. However, the addition of Se can effectively suppress such local corrosion. Such an effect of improving the corrosion uniformity and local corrosion resistance by Se is drastically improved when used in combination with Cu, Ni, Ti or the like.

  In order to effectively exhibit the above action, 0.005% or more of Se is added. However, when Se is added excessively exceeding 0.50%, workability and weldability deteriorate. The Se content is preferably 0.006% or more and 0.45% or less, and more preferably 0.008% or more and 0.40% or less.

C: 0.01 to 0.30%
C is necessary for securing the strength of the material. C is added in an amount of 0.01% or more in order to ensure the minimum level of strength required for the structural members of the ship (approximately 400 MPa depending on the thickness of the steel used). However, when C is added excessively exceeding 0.30%, toughness deteriorates. The C content is preferably 0.02% or more and 0.28% or less, and more preferably 0.04% or more and 0.26% or less.

Si: 0.01 to 2.0%
Si is added for deoxidation and ensuring strength. If the Si content is less than 0.01%, the minimum strength required for the structural member cannot be ensured. However, when Si is added excessively exceeding 2.0%, weldability deteriorates. The Si content is preferably 0.02% or more and 1.80% or less, and more preferably 0.05% or more and 1.60% or less.

Mn: 0.01 to 2.0%
Mn is added for deoxidation and securing of strength, similar to Si. If the Mn content is less than 0.01%, the minimum level of strength required for the structural member cannot be ensured. However, when Mn is added excessively exceeding 2.0%, toughness deteriorates. The Mn content is preferably 0.05% or more and 1.80%, and more preferably 0.10% or more and 1.60% or less.

Al: 0.005-0.10%
Al, like Si and Mn, is added for deoxidation and securing of strength. When the Al content is less than 0.005%, the deoxidation action is not effectively exhibited. However, if Al is added in excess of 0.10%, the weldability decreases. The Al content is preferably 0.010% or more and 0.050% or less, and more preferably 0.015% or more and 0.040% or less.

  The basic components of the base material used in the second welded joint according to the present invention are as described above, and the balance is iron and inevitable impurities (for example, P, S, O, etc.). In addition, other permissible components (for example, Zr, N, etc.) may be further added to such an extent that the action of the present invention is not inhibited. Since the toughness deteriorates when the content of these permissible components is excessive, it is preferable to keep the total to about 0.1% or less.

  Furthermore, the above-mentioned base material includes (i) Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, Co: 0.01 to 5.0%, Ni as necessary. : At least one selected from the group consisting of 0.01 to 5.0% and Ti: 0.005 to 0.20%, (ii) La: 0.0005 to 0.15%, Ce: 0.0005 One or more selected from the group consisting of 0.15%, Ca: 0.0005 to 0.015%, and Mg: 0.0005 to 0.015%, (iii) Mo: from 0.01 to 5.0% At least one selected from the group consisting of: (iv) Sb: 0.01 to 0.5%, As: 0.01 to 0.5%, Sn: 0.01 to 0.5%, Bi: 0.01 to 0.5% and Te: at least one selected from the group consisting of 0.01 to 0.5%, (v) B: 0.000 At least one selected from the group consisting of -0.010%, V: 0.01-0.50%, and Nb: 0.003-0.50%, (vi) Zn: 0.001-0.10% May be further added, and the characteristics of the welded joint are further improved depending on the kind of the added component.

(I) Cu: 0.01-5.0%, Cr: 0.01-5.0%, Co: 0.01-5.0%, Ni: 0.01-5.0%, and Ti: One or more elements selected from the group consisting of 0.005 to 0.20% These elements greatly contribute to improving corrosion resistance by forming a dense rust film on the surface. Furthermore, Co is useful for inhibiting corrosion in a high salinity environment. In addition to the above effects, Ti suppresses corrosion inside the crevice and improves crevice corrosion resistance. However, when these elements are added excessively, weldability and hot workability deteriorate. Considering these points, the preferable content of the above elements is set in the above range. The contents of Cu, Cr, Co, and Ni are more preferably 0.05% or more and 4.50% or less, respectively. The Ti content is more preferably 0.008% or more and 0.15% or less.

(Ii) A group consisting of La: 0.0005 to 0.15%, Ce: 0.0005 to 0.15%, Ca: 0.0005 to 0.015%, and Mg: 0.0005 to 0.015% One or more selected from these elements, in addition to suppressing the pH drop due to hydrolysis of Fe ions dissolved by corrosion, promotes the rust densification action of the element (i) (Cu, etc.) added as necessary. , Se has the effect of further enhancing the effect of suppressing pH reduction by Se. However, when these elements are added excessively, workability and weldability are lowered. Taking these into consideration, the preferable content of the above elements is set in the above range. The contents of La and Ce are more preferably 0.0010% or more and 0.10% or less, respectively. The Ca and Mg contents are more preferably 0.0010% or more and 0.010% or less, respectively.

(Iii) Mo: 0.01-5.0%
Mo improves corrosion uniformity and suppresses perforation due to local corrosion. In particular, by using Mo together with the element (i) (Cu, Cr, Co, etc.), the corrosion uniformity is further improved. However, when Mo is added excessively, weldability deteriorates. Taking these into consideration, the preferable content of Mo is set to the above range. The Mo content is more preferably 0.02% or more and 4.50%.

(Iv) Sb: 0.01-0.5%, As: 0.01-0.5%, Sn: 0.01-0.5%, Bi: 0.01-0.5%, and Te: At least one selected from the group consisting of 0.01 to 0.5% These elements include rust densification by the element (i) (Cu, etc.) and pH by the element (ii) (La, etc.). Promotes lowering action and improves corrosion resistance. In order to effectively exhibit such an action, it is preferable to add 0.01% or more of any of the above elements. However, if these elements are added excessively, the workability and weldability are deteriorated. Therefore, it is preferable that both elements be 0.5% or less. The content of the elements is more preferably 0.02% or more and 0.40% or less, respectively.

(V) at least one selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50%. Contributes to improvement. However, when added excessively, the toughness of the base material decreases, so the content of the above elements was set within the above range. The content of B is more preferably 0.0003% or more and 0.0090% or less. The content of V is more preferably 0.02% or more and 0.45% or less. The Nb content is more preferably 0.005% or more and 0.45% or less.

  The steel material of this embodiment may be further coated as needed. For example, as shown in the examples described later, a coating film may be formed using a tar epoxy resin paint. Or you may perform the typical heavy-duty anticorrosion coating methods other than the said coating, for example, the coating by a zinc rich paint and a shop primer. Furthermore, it is possible to use an anticorrosion method such as cathodic protection in combination. By these methods, the corrosion resistance of coating is further improved (see Examples described later).

(Composition of weld metal in the second welded joint according to the present invention)
The weld metal contains C: 0.01 to 0.30%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, Al: 0.005 to 0.10%, respectively. In addition to containing, it is preferable that Se: 0.005 to 0.50% is contained, and the balance is Fe and inevitable impurities. That is, the composition of the weld metal is preferably substantially the same as that of the base material.

  Next, a preferred method for producing the second welded joint according to the present invention will be described.

  The second welded joint according to the present invention can be obtained, for example, by appropriately controlling the composition of the base material and the welding material. The preferable steel components of the base material are as described above.

  In particular, it is preferable to appropriately control the Se content of the welding material so that the R value satisfies the range of the above formula (4). Specifically, although the composition of the welding material varies depending on the welding conditions and the like, the Se amount of the welding material is generally more than 0.35 times and less than 4.0 times compared to the Se amount of the base material (strictly Is preferably controlled within a range of less than 4.00 times. Other components of the welding material are not particularly limited as long as the second welded joint according to the present invention is obtained. For example, C: 0.01 to 0.30%, Si: 0.01 to 2.0%, It is recommended to control within the range of Mn: 0.01 to 2.0% and Al: 0.005 to 0.10%.

  The type of welding conditions is not particularly limited, and generally used welding methods can be appropriately selected and used so as to improve the mechanical properties (such as strength and toughness) of the welded joint. Typical examples include arc welding. For example, submerged arc welding (automatic welding) described in the examples described later, semi-automatic arc welding such as solid wire or flux-cored wire, covered arc welding, TIG welding (TIG), etc. Including manual arc welding. In addition, gas welding may be performed.

(Third welded joint according to the present invention)
The third welded joint according to the present invention satisfies the following expression (8). As a result, it is possible to provide a welded structure with improved overall corrosion resistance, corrosion uniformity, crevice corrosion resistance, and paint corrosion resistance.
0.30 ≦ [A _Zn ] / [B _Zn ] ≦ 3.0 (8)
Where
[A _Zn ] is the Zn content (mass%) contained in the weld metal of the welded part,
[B _Zn ] is the content (mass%) of Zn contained in the base material,
Mean,

In the above formula (8), when [A _Zn ] / [B _Zn ] (hereinafter referred to as V value) is less than 0.30, the corrosion resistance due to the anticorrosive film forming component of Zn as shown in the examples described later. Since the improving action is not effectively exhibited, the above characteristics are deteriorated. On the other hand, the V value is not particularly limited from the viewpoint of improving the corrosion resistance, but if it exceeds 3.0 (strictly 3.00), the toughness of the welded portion deteriorates and a predetermined mechanical strength is ensured. I can't. Taking these into account, it is preferable to control the V value to be 0.5 or more and 2.0 or less.

Here, [ _AZn ] contained in the weld metal of the welded portion can be determined mainly by [ _BZn ] contained in the base material and the welding material to be used.

  In order to control the V value within the range of the above formula (8), it is preferable to control the components of the base metal in the steel and the composition of the weld metal as follows.

(Components in steel of base material in third welded joint according to the present invention)
As shown below, the base material used in the present embodiment contains Zn as an essential component in addition to the basic components of C, Si, and Mn. This base material is useful as a steel material for petroleum tanks having excellent corrosion resistance, and the present applicant has filed an application earlier (filed on October 21, 2004, Japanese Patent Application No. 2004-307130).

  First, Zn that characterizes this embodiment will be described.

Zn: 0.001 to 0.10%
Zn is useful for enhancing the corrosion resistance in an environment where sulfur (such as elemental sulfur and hydrogen sulfide gas) derived from petroleum exists. In general, the reaction in which steel dissolves in water and corrodes is relatively slow, and the corrosion rate is very slow and is not a problem. However, in an environment where sulfur exists to some extent, sulfur promotes the dissolution reaction. Corrosion is thought to progress significantly. Since Zn in steel has a lower potential than Fe and other alloy elements, it tends to selectively dissolve when placed in the environment. The dissolved Zn reacts with sulfur to produce zinc sulfide (ZnS), which precipitates on the surface of the steel material to form a zinc sulfide coating. Since zinc sulfide is low in water solubility, the formation of the coating film protects the base steel material from the moisture environment and suppresses the progress of corrosion. In particular, in the crevice portion where the movement of the substance is restricted, zinc sulfide does not scatter offshore but easily settles on the surface of the steel material. Therefore, the addition of Zn further enhances crevice corrosion resistance. The above-mentioned action by Zn is drastically improved by using in combination with elements such as Cu, Ni, Cr, Ti, etc. that exert a densifying action or stabilizing action of rust added as necessary.

  In order to exhibit such an action effectively, 0.001% or more of Zn is added. However, when Zn is added excessively exceeding 0.10%, workability and weldability are deteriorated. The Zn content is preferably 0.003% or more and 0.09% or less, and more preferably 0.005% or more and 0.08% or less.

C: 0.01 to 0.30%
C is added to ensure the strength of the material. When the welded joint of the present embodiment is applied as a structural member such as a petroleum tank, for example, in order to ensure the lowest level of strength (depending on the thickness of the steel material used, approximately 400 MPa), C is used. Add 0.01% or more. However, when C is added excessively exceeding 0.30%, toughness deteriorates. The C content is preferably 0.02% or more and 0.28% or less, and more preferably 0.04% or more and 0.26% or less.

Si: 0.01 to 2.0%
Si is added for deoxidation and ensuring strength. When the Si content is less than 0.01%, the minimum strength required for the structural member cannot be ensured. However, when Si is added excessively exceeding 2.0%, weldability deteriorates. The Si content is preferably 0.02% or more and 1.80% or less, and more preferably 0.05% or more and 1.60% or less.

Mn: 0.01 to 2.0%
Mn, like Si, is added for deoxidation and ensuring strength. If the Mn content is less than 0.01%, the minimum level of strength required for the structural member cannot be ensured. However, if Mn is added excessively exceeding 2.0%, the toughness deteriorates. The Mn content is preferably 0.05% or more and 1.80% or less, and more preferably 0.10% or more and 1.60% or less.

Al: 0.005-0.10%
Al, like Si and Mn, is added for deoxidation and securing of strength. If the Al content is less than 0.005%, the desired deoxidizing action cannot be obtained. However, when Al is added in excess of 0.10%, the weldability is lowered. The Al content is preferably 0.010% or more and 0.050% or less, and more preferably 0.015% or more and 0.040% or less.

  The basic components of the base material used for the third welded joint according to the present invention are as described above, and the balance is iron and inevitable impurities (for example, P, S, O, etc.). In addition, other permissible components (for example, Zr, N, etc.) may be further added to such an extent that the action of the present invention is not inhibited. Since these tolerable components are deteriorated in toughness when the content thereof is excessive, it is preferable to keep the total to about 0.1% or less.

  Further, the base material may include (i) Cu: 0.01 to 5.0%, Ni: 0.01 to 5.0%, Cr: 0.01 to 5.0%, and At least one selected from the group consisting of Ti: 0.005 to 0.20%, (ii) Ca: 0.0005 to 0.015%, Mg: 0.0005 to 0.015%, and Se: 0.005 At least one selected from the group consisting of ˜0.50%, (iii) Mo: 0.01-5.0%, (iv) Sb: 0.01-0.5%, (v) B: 0.0001 -0.010%, V: 0.01-0.50%, and Nb: At least one selected from the group consisting of 0.003-0.50% may be further added. Accordingly, the properties of the welded joint are further improved.

(I) The group consisting of Cu: 0.01 to 5.0%, Ni: 0.01 to 5.0%, Cr: 0.01 to 5.0%, and Ti: 0.005 to 0.20% At least one of these elements selected from the above forms a dense rust film on the surface of the steel material and contributes to an improvement in corrosion resistance. As described above, in this embodiment, a zinc sulfide film is formed on the surface of the steel material by the addition of Zn. However, since this film does not have good adhesion to the base steel material, it is a fluid component such as petroleum. There is a risk of peeling. Since the rust film formed by the element (i) is formed so as to incorporate the zinc sulfide film, it acts as a protective film against fluid components such as petroleum and can prevent a decrease in corrosion resistance. Further, Ti suppresses corrosion inside the crevice and improves crevice corrosion resistance in addition to the above-described effects. The content of Cu, Ni, and Cr is more preferably 0.05% or more and 4.50% or less. The Ti content is more preferably 0.008% or more and 0.15% or less.

(Ii) At least one selected from the group consisting of Ca: 0.0005 to 0.015%, Mg: 0.0005 to 0.015%, and Se: 0.005 to 0.50%. Has the effect of suppressing pH reduction due to hydrolysis of Fe ions dissolved by the above, and has the effect of suppressing corrosion promotion due to pH reduction. As a result, local changes in pH are less likely to occur, and corrosion uniformity is improved. As described above, in this embodiment, a zinc sulfide film is formed on the surface of the steel material by the addition of Zn. However, since zinc sulfide has a relatively high solubility in an acidic solution, a decrease in pH locally occurs. In this case, the zinc sulfide film dissolves and the desired protective action is not exhibited. On the other hand, when the above elements are added, local acidification is prevented by the pH lowering suppressing action, so that the dissolution of zinc sulfide is suppressed, and as a result, the corrosion resistance is further enhanced. However, when the above elements are added excessively, workability and weldability are reduced. Considering these points, the preferable content of the above elements is set in the above range. The Ca and Mg contents are more preferably 0.0010% or more and 0.010% or less, respectively. The Se content is more preferably 0.008% or more and 0.40% or less.

(Iii) Mo: 0.01-5.0%
Mo has an effect of increasing corrosion uniformity and suppressing perforation due to local corrosion. In particular, by using Mo together with the element (i) (Cu, Cr, etc.), the effect of improving the corrosion uniformity is further enhanced. In order to effectively exhibit such an action, it is preferable to add Mo by 0.01% or more. However, since the weldability deteriorates when Mo is added excessively, the Mo content is preferably 5.0% or less. The Mo content is more preferably 0.02% or more and 4.50% or less.

(Iv) Sb: 0.01 to 0.5%
Sb is an element that further enhances the corrosion resistance by promoting the rust densification action by the element (i) (Cu etc.) and the pH lowering suppression action by the element (Ca) (ii). In order to exert such effects, it is preferable to add 0.01% or more of Sb. However, if Sb is added excessively, the workability and weldability deteriorate, so the Sb content is preferably 0.5% or less. The Sb content is preferably 0.02% or more and 0.40% or less.

(V) at least one selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50%. It is effective for improvement. However, when added excessively, the toughness of the base material is lowered. Taking these into consideration, the preferable content of the above elements is set in the above range. The content of B is more preferably 0.0003% or more and 0.0090% or less. The content of V is more preferably 0.02% or more and 0.45% or less. The Nb content is more preferably 0.005% or more and 0.45% or less.

  The steel material of this embodiment may be further coated as needed. For example, as shown in the examples described later, a coating film may be formed using a tar epoxy resin paint. Or you may perform the typical heavy-duty anticorrosion coating methods other than the said coating, for example, the coating by a zinc rich paint and a shop primer. Furthermore, it is possible to use an anticorrosion method such as cathodic protection in combination. By these methods, the corrosion resistance of coating is further improved (see Examples described later).

(Composition of weld metal in the third weld joint according to the present invention)
The weld metal contains C: 0.01 to 0.30%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, Al: 0.005 to 0.10%, respectively. In addition to Zn, 0.001 to 0.10% is contained, and the balance is preferably Fe and inevitable impurities. That is, the composition of the weld metal is preferably substantially the same as that of the base material.

  Next, a preferred method for producing the third welded joint according to the present invention will be described.

  The third welded joint according to the present invention can be obtained, for example, by appropriately controlling the composition of the base material and the welding material. The preferable steel components of the base material are as described above.

  In particular, it is preferable to appropriately control the Zn content so that the welding material satisfies the range of the above formula (8). Specifically, although the composition of the welding material varies depending on the welding conditions and the like, the Zn amount of the welding material is generally in the range of more than 0.4 times and less than 3.5 times the Zn amount of the base material. (Strictly speaking, it is preferable to control to 0.40 times and less than 3.50 times). The other components of the welding material are not particularly limited as long as the third welded joint according to the present invention is obtained. For example, C: 0.01 to 0.30%, Si: 0.01 to 2.0%, It is recommended to control within the range of Mn: 0.01 to 2.0% and Al: 0.005 to 0.10%.

  The type of welding conditions is not particularly limited, and generally used welding methods can be appropriately selected and used so as to improve the mechanical properties (such as strength and toughness) of the welded joint. Typical examples include arc welding. For example, submerged arc welding (automatic welding) described in the examples described later, semi-automatic arc welding such as solid wire or flux-cored wire, covered arc welding, TIG welding (TIG), etc. Including manual arc welding. In addition, gas welding may be performed.

  In this embodiment, in order to control the above-described V value within the range of the above formula (8) and to control the preferred composition of the weld metal within the above range, it is preferable to appropriately control the welding conditions. . Details of the welding conditions are as described in the first weld joint described above.

  The welded structure of the present invention exhibits excellent corrosion resistance against the adhesion of salt caused by seawater and the like and corrosion due to a wet environment, but is not limited thereto, and is applied to, for example, a petroleum liquid fuel tank. However, excellent corrosion resistance can be exhibited in the corrosive environment.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and may be implemented with appropriate modifications within a range that can meet the gist of the preceding and following description of the present specification. Included in the technical scope.

Example 1 (Examination of the first welded joint according to the present invention)
In this example, in the first welded joint according to the present invention, the S value ([A _Al ] / [B _Al ]), the T value ([A _Cu ] / [B _Cu ]), and the U value ([A _Cr ]). / [B _Cr ]) and the corrosion resistance were investigated. In addition, in the present Example and the Example described later, the following corrosion tests were performed and evaluated using the test pieces D prepared from the test pieces B as follows.

(Preparation of joint specimen B)
The joint test piece B shown in FIG. 1 was produced as follows.

  First, steel materials (base materials) M1 to M8 having various component compositions shown in Table 1 (remainder: Fe and inevitable impurities) are melted in a converter, and various steel plates (size: about 300 mm) are obtained by continuous casting and hot rolling. X about 300 mm x about 25 mm). The obtained steel sheet was cut into two and the surface was ground to produce two test pieces A (size: about 300 mm × about 150 mm × about 25 mm) shown in FIG.

  The two test pieces A thus obtained were subjected to submerged arc welding using welding materials W1 to W14 having various component compositions (remainder: Fe and inevitable impurities) shown in Table 2, and joint tests were performed. Piece B was obtained. The wire diameters of the welding materials W1 to W14 were all about 4.8 mm, and the groove shape was V-shaped. The amount of heat input was appropriately adjusted within the range of 1 kJ / mm to 10 kJ / mm. Table 3 shows the composition of the weld metal in the welded portion thus obtained. No. shown in Table 3 The details of the base materials 1 to 14 and the welded joint material are as shown in Table 5 to be described later.

(Preparation of test piece C for crevice corrosion resistance measurement)
The test piece C shown in FIG. 2 was produced as follows.

  First, two gap forming test pieces of about 60 mm × about 60 mm × about 5 mm are brought into contact with the welded portion of the test piece B obtained as described above to form a gap portion as shown in FIG. A test specimen was prepared. Next, as shown in FIG. 2, a hole of about 10 mmφ is provided at the center of the test piece C, and a screw hole (not shown) is opened on the base material side (large test piece side) and an M8 plastic screw is used. By fixing, a test piece C for crevice corrosion resistance measurement was obtained.

(Preparation of test specimen D for coating corrosion resistance measurement)
In order to examine the coating corrosion resistance when the steel material to which the anticorrosion coating film was applied was damaged and the base steel material was exposed, a test piece D shown in FIG. 3 was prepared as follows.

  First, undercoating of zinc rich primer (average thickness of about 15 μm) and tar epoxy resin (average thickness) were applied to the entire surface of a steel plate (size of about 300 mm × about 300 mm × about 25 mm) prepared in the same manner as the above-mentioned test piece B. About 250 μm) was applied in order. Next, using a cutter knife on one side of this test piece, as shown in FIG. 3, cut scratches (length: about 200 mm, width: about 0.5 mm) reaching the substrate are perpendicular and horizontal to the weld line. The test piece D for coating corrosion resistance measurement was obtained.

(Measurement method for corrosion test)
The following corrosion tests were performed on the thus obtained joint specimen B, crevice corrosion resistance test specimen C, and paint corrosion resistance measurement specimen D. This corrosion test is set on the assumption that a ship composed of a welded structure including a welded joint is corroded.

  First, five test pieces are attached to the bottom plate on the inner surface of the tank of a VLCC (Very Large Crude Carrier) crude oil tanker, and after normal operation for five years, the degree of corrosion of each test piece is as follows. I investigated.

(Corrosiveness of specimen B)
The corrosivity (overall corrosivity and corrosion uniformity) of the test piece B was evaluated as follows.

  First, in order to remove corrosion products such as iron rust formed on the test piece B, cathodic electrolysis was performed in a diammonium hydrogen citrate aqueous solution based on JIS K8284.

  Next, the weight of the test piece B before and after the corrosion test was measured and converted into an average reduction amount D-ave (mm) of the plate thickness. The average reduction amount D-ave of five test pieces was calculated in the same manner, and the overall corrosivity was evaluated.

  Moreover, the maximum erosion depth D-max (mm) of the test piece B was calculated | required using the stylus type three-dimensional shape measuring apparatus. The maximum erosion depth D-max (D-max / D-ave) with respect to the average reduction amount D-ave of the plate thickness calculated as described above was calculated, and the corrosion uniformity was evaluated.

(Crevice corrosion resistance of specimen C)
The crevice corrosion resistance was evaluated by measuring the maximum crevice corrosion depth D-crev (mm) as follows.

  First, after removing the gap forming test piece attached to the test piece C, the corrosion products such as iron rust were removed in the same manner as the test piece B.

  Next, the maximum crevice corrosion depth D-crev (mm) on the large test piece side was measured using the stylus type three-dimensional shape measuring apparatus used for the evaluation of corrosivity.

(Coating corrosion resistance of test piece D)
The coating corrosion resistance was evaluated by measuring the maximum swollen width as follows.

  First, about 5 test pieces D, the coating film swollen width (mm) in the direction perpendicular to the cut wound was measured with a caliper. The maximum measured value was defined as the maximum swollen width.

  Table 4 shows the evaluation criteria for the corrosion resistance (total corrosion resistance and corrosion uniformity) of the test piece B, the crevice corrosion resistance of the test piece C, and the coating corrosion resistance of the test piece D. In Table 4, when all the evaluation items were “◯” or “◎”, the evaluation was accepted, and the overall judgment was “△” or “x” was rejected. The results of evaluation according to the evaluation criteria shown in Table 4 are shown in Table 5.

(Toughness evaluation of welds)
A Charpy impact test piece (JIS Z3111-4) was taken so that the 1/4 part of the plate thickness was the center line, and the Charpy impact test was conducted at 0 ° C to absorb the absorbed energy (Charpy impact) Value, vE ₀ ) was calculated. Three Charpy impact test specimens were collected, and the average value thereof was taken as the Charpy impact value (vE ₀ ). In this example, those having a vE ₀ of 100 J or more were evaluated as “excellent in HAZ toughness”. These results are also shown in Table 5.

  From Table 5, it is considered as follows.

  First, the S value, the T value, and the U value satisfy the ranges of the above formula (5), the above formula (6), and the above formula (7), respectively. 4 to No. No. 11 shows excellent corrosion resistance in any of the evaluation items, has excellent HAZ toughness, and has good characteristics as a welded structure.

  On the other hand, since the amount of Cu in the welding material is smaller than the amount of Cu in the base material, the S value falls below the range of the above formula (5). 1. Since the Al amount in the welding material is smaller than the Al amount in the base material, the T value is less than the range of the above formula (6). 2, and the amount of Cr in the welding material is smaller than the amount of Cr in the base metal, so the U value falls below the range of the above formula (7). Although all 3 were excellent in overall corrosion resistance, any of corrosion uniformity, coating corrosion resistance, and crevice corrosion resistance was slightly lowered or lowered.

  On the other hand, since the amount of Al in the welding material is larger than the amount of Al in the base metal, the S value exceeds the range of the above formula (5). No. 12, since the amount of Cu in the welding material is larger than the amount of Cu in the base metal, the T value exceeds the range of the above formula (6). No. 14, because the amount of Cr in the welding material is larger than the amount of Cr in the base metal, the U value exceeds the range of the above equation (7). No. 15 had a HAZ toughness of less than 100 J, and the toughness of the welded portion decreased.

  In addition, in the present Example, although the joint test piece B was produced by the submerged arc welding method, the welding method is not limited to this. For example, it has been confirmed by experiments that the same effect can be obtained even if the joint specimen B is produced by a welding method usually used for producing a welded structure such as a coated arc welding method or an electroslag welding method. ing. Moreover, the welding material to be used is not limited to the composition of Table 2, and various welding materials can be used as long as the S value, the T value, and the U value satisfy the scope of the present invention.

Example 2 (Examination of second welded joint according to the present invention)
In this example, the relationship between the R value ([A _Se ] / [B _Se ]) and the corrosion resistance was examined in the second welded joint according to the present invention.

  Specifically, steel materials (base materials) M1 to M8 having various component compositions (remainder: Fe and inevitable impurities) shown in Table 6 and various component compositions (remainder: Fe and inevitable impurities) shown in Table 7 are included. A joint test piece B was obtained in the same manner as in Example 1 using the welding materials W1 to W9. Table 8 shows the composition of the weld metal in the welded portion thus obtained. No. shown in Table 8 The details of the base materials 1 to 9 and the welded joint material are as shown in Table 9 to be described later.

  Next, in the same manner as in Example 1, a test piece C for crevice corrosion resistance measurement and a test piece D for paint corrosion resistance measurement were prepared and subjected to a corrosion resistance test. evaluated.

  These results are shown in Table 9.

  From Table 9, it is considered as follows.

  First, the R value satisfies the range of the above formula (4). 2 to No. No. 9 shows excellent corrosion resistance in any of the evaluation items, and it is understood that the welded structure has good characteristics.

  On the other hand, since the Se amount in the welding material is smaller than the Se amount in the base material, the R value is less than the range of the above equation (4). In No. 1, the overall corrosion resistance, the corrosion uniformity, and the coating corrosion resistance were slightly lowered, and the crevice corrosion resistance was lowered.

  In addition, in the present Example, although the joint test piece B was produced by the submerged arc welding method, the welding method is not limited to this. For example, it has been confirmed by experiments that the same effect can be obtained even if the joint specimen B is produced by a welding method usually used for producing a welded structure such as a coated arc welding method or an electroslag welding method. ing. Further, the welding material to be used is not limited to the composition shown in Table 7, and various welding materials can be used as long as the R value satisfies the range of the above formula (4).

Example 3 (Examination of third welded joint according to the present invention)
In this example, the relationship between the V value ([A _Zn ] / [B _Zn ]) and the corrosion resistance was examined in the third welded joint according to the present invention.

  Specifically, steel materials (base materials) M1 to M8 having various component compositions (remainder: Fe and inevitable impurities) shown in Table 10 and various component compositions (remainder: Fe and inevitable impurities) shown in Table 11 are included. A joint test piece B was obtained in the same manner as in Example 1 using the welding materials W1 to W9. Table 12 shows the composition of the weld metal in the welded portion thus obtained. No. shown in Table 12 Details of the base metals 1 to 9 and the welded joint material are as shown in Table 13 to be described later.

  Next, in the same manner as in Example 1, a test piece C for crevice corrosion resistance measurement and a test piece D for paint corrosion resistance measurement were prepared and subjected to a corrosion resistance test. evaluated.

  These results are shown in Table 13.

  From Table 13, it is considered as follows.

  First, No. 1 in which the V value satisfies the range of the above equation (8). 2 to No. No. 9 shows excellent corrosion resistance in any of the evaluation items, and it is understood that the welded structure has good characteristics.

  On the other hand, since the amount of Zn in the welding material is smaller than the amount of Zn in the base metal, the V value falls below the range of the above formula (8). No. 1 was excellent in overall corrosion resistance, but the corrosion uniformity and coating corrosion resistance were slightly lowered, and crevice corrosion resistance was lowered.

  In addition, in the present Example, although the joint test piece B was produced by the submerged arc welding method, the welding method is not limited to this. For example, it has been confirmed by experiments that the same effect can be obtained even if the joint specimen B is produced by a welding method usually used for producing a welded structure such as a coated arc welding method or an electroslag welding method. ing. Further, the welding material to be used is not limited to the composition shown in Table 11, and various welding materials can be used as long as the V value satisfies the range of the above formula (8).

Claims (12)

It is a welded joint where the base materials are welded together,
A welded joint excellent in corrosion resistance, characterized by satisfying the following formula (5), the following formula (6), and the following formula (7).
0.30 ≦ [A _Al ] / [B _Al ] ≦ 3.0 (5)
0.30 ≦ [A _Cu ] / [B _Cu ] ≦ 3.0 (6)
0.30 ≦ [A _Cr ] / [B _Cr ] ≦ 3.0 (7)
Where
[A _Al ] is the content (% by mass) of Al contained in the weld metal of the welded portion,
[B _Al ] is the content (% by mass) of Al contained in the base material,
[A _Cu ] is the content (% by mass) of Cu contained in the weld metal of the welded part,
[B _Cu ] is the content (% by mass) of Cu contained in the base material,
[A _Cr ] is the content (% by mass) of Cr contained in the weld metal of the welded part,
[B _Cr ] is the content (% by mass) of Cr contained in the base material.
Respectively.   The weld metal is C: 0.01 to 0.20% (meaning mass%, hereinafter the same for weld metal), Si: 0.01 to 0.50%, Mn: 0.01 to 2.0% Al: 0.05 to 0.50%, Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, P: 0.020% or less (0% 2) and S: 0.010% or less (including 0%), and the balance: Fe and unavoidable impurities. The steel component of the base metal is
C: 0.01 to 0.20% (meaning mass%, hereinafter the same for the components in steel), Si: 0.01 to 0.50%, Mn: 0.01 to 2.0%, Al: 0 0.05 to 0.50%, Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, P: 0.020% or less (including 0%) and S The weld joint according to claim 1, wherein the weld joint is suppressed to 0.010% or less (including 0%) and the balance is Fe and inevitable impurities.   The weld joint according to claim 3, wherein a ratio ([Cr] / [Al]) of the Cr content [Cr] and the Al content [Al] is in the range of 1-50.   Furthermore, at least one selected from the group consisting of Ni: 0.01 to 5.0%, Co: 0.01 to 5.0%, and Ti: 0.005 to 0.20% or 4. The welded joint according to 4.   Furthermore, the welded joint in any one of Claims 3-5 containing Ca: 0.0005-0.020% and / or Mg: 0.0005-0.020%.   Furthermore, the welded joint according to any one of claims 3 to 6, containing Se: 0.005 to 0.50%.   Furthermore, the welded joint in any one of Claims 3-7 containing Sb: 0.01-0.50% and / or Sn: 0.01-0.50%.   Furthermore, it contains at least one selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50%. The weld joint according to any one of 8.   A welded structure including the welded joint according to claim 1.   The welded structure according to claim 10, which is used for a ship.   The welded structure according to claim 10, which is used in a crude oil tank.