JP5453835B2 - Corrosion resistant steel for ships - Google Patents

Description

The present invention includes a coal ship, an ore ship, a coal mine ship, a crude oil tanker, an LPG ship, an LNG ship, a chemical tanker, a container ship, a bulk carrier, a timber ship, a chip ship, a refrigerated carrier ship, an automobile ship, a heavy article The present invention relates to a steel material for ships such as a ship, a RORO ship, a limestone ship and a cement ship, particularly a marine corrosion resistant steel material suitable for use in a ballast tank in a severe corrosive environment caused by seawater.
In the present invention, the marine corrosion resistant steel material includes a thick steel plate, a thin steel plate, a shape steel, a bar steel, and the like.

The ship's ballast tank plays a role of enabling stable navigation of the ship by injecting seawater when there is no cargo, and is placed in an extremely severe corrosive environment. For this reason, the corrosion protection of steel materials used in ballast tanks is usually performed in combination with coating of an anticorrosion coating film using an epoxy-based paint and cathodic protection.
However, even if these anticorrosion measures are taken, the corrosion state of the ballast tank is still severe.

  That is, when seawater is injected into the ballast tank, the progress of corrosion can be suppressed in the portion completely immersed in seawater if the anticorrosion is functioning. However, the vicinity of the uppermost portion of the ballast tank, particularly the back side of the upper deck, is not immersed in seawater and is in a state where it is exposed to the splash of seawater. Furthermore, since the temperature of the steel plate is increased by sunlight, this part becomes a more severe corrosive environment and is severely corroded. Further, when seawater is not injected into the ballast tank, since the anti-corrosion does not work at all in the entire ballast tank, it is severely corroded by the action of residual adhered salt.

The life of the anticorrosion coating film of the ballast tank under such severe corrosive environment is generally said to be about 10 to 15 years, and is about half of the life of the ship (20 to 25 years). Therefore, in the remaining 10 years, the actual situation is that the corrosion resistance is maintained by repair painting. However, since the ballast tank is in a severe corrosive environment as described above, it is difficult to maintain the effect for a long time even if repair coating is performed. In addition, since repair painting is performed in a narrow space, it is not preferable as a work environment.
Therefore, it is desired to develop a steel material with excellent corrosion resistance that can extend the period until repair coating as much as possible and can reduce the repair coating work as much as possible.

Therefore, several techniques for improving the corrosion resistance of the steel material itself used in a part having a severe corrosive environment such as a ballast tank have been proposed.
For example, Patent Document 1 discloses a corrosion-resistant low alloy steel in which Cu: 0.05 to 0.50 mass% and W: less than 0.01 to 0.05 mass% are added to steel having C: 0.20 mass% or less as corrosion resistance improving elements. .
In Patent Document 2, Cu: 0.05 to 0.50 mass% and W: 0.05 to 0.5 mass% are added to steel materials having C: 0.20 mass% or less as corrosion resistance improving elements, and Ge, Sn, Pb, As, A corrosion-resistant low alloy steel to which 0.01 to 0.2 mass% of one or more of Sb, Bi, Te, and Be is added is disclosed.
Furthermore, Patent Document 3 discloses a corrosion-resistant low alloy steel obtained by adding Cu: less than 0.05 to 0.15 mass% and W: 0.05 to 0.5 mass% to steel having C: 0.15 mass% or less.

In addition, Patent Document 4 discloses a low alloy in which P: 0.03 to 0.10 mass%, Cu: 0.1 to 1.0 mass%, and Ni: 0.1 to 1.0 mass% are added to steel having C: 0.15 mass% or less as corrosion resistance improving elements. A ballast tank is disclosed in which an anticorrosion paint such as a tar epoxy paint, a pure epoxy paint, a solventless epoxy paint, and a urethane paint is applied to a corrosion-resistant steel material and is coated with a resin. This technology is intended to extend the life of the anticorrosion coating by improving the corrosion resistance of the steel material itself, and to realize maintenance-free for 20 to 30 years, which is the use period of the ship.
Patent Document 5 proposes to improve the corrosion resistance by adding Cr: 0.2 to 5 mass% as a corrosion resistance improving element to steel of C: 0.15 mass% or less to realize maintenance-free ship. .
In Patent Document 6, steel having C: 0.15 mass% or less added with Cr: 0.2-5 mass% as a corrosion resistance improving element is used as a constituent material, and the oxygen gas concentration in the ballast tank is a value in the atmosphere. In contrast, a ballast tank anticorrosion method characterized by a ratio of 50% or less is proposed.

Further, Patent Document 7 proposes to improve the corrosion resistance by adding Cr: 0.5 to 3.5 mass% to steel of C: 0.1 mass% or less, thereby realizing a maintenance-free ship. .
Patent Document 8 describes marine steel materials that improve coating film damage resistance by adding Ni: 0.1-4.0 mass% to C: 0.001-0.025 mass% steel and reduce maintenance costs such as repair coating. Is disclosed.

Furthermore, in Patent Document 9, C: 0.01 to 0.25 mass%, Cu: 0.01 to 2.00 mass%, Mg: 0.0002 to 0.0150 mass% are added to the steel of C: 0.01 to 0.25 mass%, so that the ship outer plate, ballast tank, cargo oil tank And marine steel with improved corrosion resistance in use environments such as coal ore cargo hold.
In Patent Document 10, C: 0.001 to 0.2 mass% steel, Mo, W and Cu are added together, and the amount of P and S which are impurities is limited to limit the total corrosion that occurs in a crude oil tank. Steels with reduced local corrosion are disclosed.

However, in the above Patent Documents 1 to 3, the corrosion resistance in the presence of a coating film such as an epoxy paint generally applied to a steel material constituting a ballast tank or the like has not been studied, Therefore, it is necessary to separately examine the improvement of the corrosion resistance in the presence of the coating film as described above.
Moreover, in order to improve the corrosion resistance of the base metal, the steel material of Patent Document 4 contains P in a relatively large amount of 0.03 to 0.10 mass%, so that problems remain in terms of weldability and weld toughness.
Furthermore, since the steel materials of Patent Document 5 and Patent Document 6 contain Cr in a relatively large amount of 0.2 to 5 mass% and the steel material of Patent Document 7 in a relatively large amount of Cr of 0.5 to 3.5 mass%, both of them have weldability and weld toughness. In addition to the above problems, there are problems that the manufacturing cost is high. Moreover, since the steel material of patent document 8 has comparatively low C content and comparatively high Ni content, there existed a problem that manufacturing cost became high.

Moreover, although the steel material of patent document 9 requires addition of Mg, there existed a problem that the mechanical characteristic of steel materials was not stabilized, since Mg does not stabilize the steelmaking yield. Furthermore, since the steel material of Patent Document 10 is a corrosion-resistant steel used in an environment where H ₂ S exists in a crude oil tank, the corrosion resistance in a ballast tank without H ₂ S is unknown, and further, the ballast tank Since corrosion resistance in a state where an epoxy-based paint generally used for steel is applied has not been studied, it has been necessary to separately examine it for application to a ballast tank.

JP 48-50921 A JP-A-48-50922 JP-A-48-50924 JP-A-7-34197 JP-A-7-34196 Japanese Unexamined Patent Publication No. 7-34270 JP 7-310141 A JP 2002-260552 A JP 2000-17381 A Japanese Patent Laid-Open No. 2004-204344

  The present invention advantageously solves the above-mentioned problems, exhibits excellent coating corrosion resistance even in severe seawater corrosive environments such as ship ballast tanks, and can extend the period until repair coating, The objective is to propose a marine corrosion resistant steel that can reduce the work of repair painting.

Generally, a ship is constructed by welding steel materials such as thick steel plates, thin steel plates, shape steels, and steel bars, and the surface of the steel materials is used with anticorrosion coating. This anticorrosion coating is generally applied with a zinc primer as a primary rust prevention, and after a small assembly or a large assembly, an epoxy coating is applied as a secondary coating (main coating). Therefore, most of the steel surface of the ship has a two-layer structure of zinc primer and epoxy coating.
However, since the zinc primer is burned away by welding heat, the weld primer is repainted as a touch-up for rust prevention between the time after welding and the main coating. However, if the period until the main coating is short, the zinc primer may not be repainted.

  The most severely corroded part in a ship is a ballast tank, and the deterioration of the coating film in the ballast tank is caused by the progress of corrosion from the damaged part of the coating film, the coating film pinhole, and the coating film thin film part. In the two-layer structure of zinc primer + epoxy coating, the corrosion progress is slow and the coating deterioration is slight due to the action of the zinc primer for several years after the ship enters service. It disappears gradually, and the coating film deterioration becomes remarkable. Therefore, it is desired to develop a steel material that exhibits paint corrosion resistance in the absence of a zinc primer (without the assistance of a zinc primer).

Therefore, in order to meet the above requirements, the inventors have earnestly researched and studied the improvement of the coating corrosion resistance, particularly the improvement of the coating corrosion resistance in the absence of the zinc primer. It was. Here, the coating corrosion resistance is a performance of reducing the swelling of a coating film generated from a coating film defect portion existing on the surface of a steel material on which a coating film is formed by applying a coating material.
(1-1) It is effective that the rust layer at the defective portion of the coating film becomes a protective coating against chloride ions contained in seawater.
(1-2) It is effective to suppress the progress of corrosion at the low pH local anode part of the coating film defect part.
(1-3) Adsorption of ions eluted from steel due to corrosion to the surface of the steel is effective in suppressing the progress of corrosion.
(1-4) It is effective to reduce the content of elements in the steel that promote the lowering of the pH of the steel surface of the defective part of the coating film or elements that have a small hydrogen overvoltage and promote the swelling of the coating film.

Further, suitable elements for the above (1-1) to (1-4) are as follows.
(1-5) For (1-1) above, as the steel material corrodes, the ions eluted from the steel material become oxygen acid, and the selection of the alloy element taken into the rust layer is effective. W and Mo are effective.
(1-6) For (1-2) above, the inclusion of Sn and Sb in the steel material is effective. Further, W that forms FeWO ₄ that is a stable corrosion product in a low pH environment is effective. Furthermore, reduction of P and S, especially reduction of S is effective.
(1-7) For the above (1-3), W adsorbed on the steel material surface as WO ₄ ^2- is effective.
(1-8) For (1-4), reduction of Cr, Cu, Ni, Co is effective.

FIG. 1 shows the results of experimental verification of the coating corrosion resistance of each element based on the above concepts (1-1) to (1-8).
As an experimental method, a test piece of 3 mmt × 50 mmW × 150 mmL was collected, and then the surface of the test piece was shot blasted to remove the scale and oil, and then the tar epoxy resin coating (thickness) : About 100 μm) to prepare a test piece coated with a single layer coating.
Next, an 80mm long scratch ridge that reaches the surface of the iron bar with a cutter knife from the top of the paint film is applied in a single letter, and after the corrosion test under the following conditions, the swelling area of the paint film that occurs around the scratch moth is measured. (See FIG. 2).
The corrosion test simulated a corrosive environment corresponding to the upper deck of the actual ballast tank. That is, 132 cycles of a test in which (35 ° C., 5% NaCl solution spray, 2 hr) → (60 ° C., RH 25%, 4 hr) → (50 ° C., RH 95%, 2 hr) were performed as one cycle were performed.
The coating corrosion resistance was evaluated based on the relative ratio of the swollen area of the coating film when each element was added to the area, with the swollen area of the test piece having the basic composition as 1.0.

  As shown in Fig. 1, Sn, W, Mo, and Sb are effective in improving paint corrosion resistance, while Cr, Cu, Ni, and Co have deteriorated paint corrosion resistance. Therefore, from the viewpoint of improving paint corrosion resistance. It can be seen that the addition of Sn, W, Mo, Sb into the steel and the reduction of Cr, Cu, Ni, Co are effective.

Further, the inventors have studied and studied the compounding of these elements from the viewpoint of improving the coating corrosion resistance, and as a result, have found an experimental coating corrosion resistance index formula represented by the following formula (1).
(1-9) ACP = {1− (0.8 × W + 0.5 × Mo) ^0.3 } × {1− (Sn + 0.4 × Sb) ^0.3 }
× (1 + Cr) × (1 + 0.7 × Cu) × (1 + 0.5 × Ni) × (1 + 0.5 × Co)
X {1-40 x (0.015-P)} x {1-200 x (0.003-S)} --- (1)
However, W, Mo, Sn, Sb, Cr, Cu, Ni, Co, P, and S are component contents (mass%) of each element, respectively.
And it was investigated that desired coating corrosion resistance can be obtained by adjusting elements in steel so that this ACP value is 0.20 or less.

(1-10) However, the inclusion of Sn, W, Mo, Sb and Cr, Cu, Ni, Co may have to satisfy the relationship of the following formula (2) from the viewpoint of securing weld toughness. It was investigated.
WI = C + Mn / 6 + Cr / 5 + Mo / 5 + V / 5 + Ni / 15 + Cu / 15 + W / 10 + Co / 15 + Sn / 2 + Sb / 2
≦ 0.50 --- (2)
However, C, Mn, Cr, Mo, V, Ni, Cu, W, Co, Sn, and Sb are each component content (mass%) of each element.

The present invention was completed after further investigation based on the above findings, and the gist of the present invention is as follows.
1. C: 0.01-0.25 mass%,
Si: 0.05-0.50mass%,
Mn: 0.1-2.0mass%,
P: 0.010 mass% or less,
S: 0.0010 mass% or less,
Al: 0.10 mass% or less,
Ti: 0.005-0.030 mass%,
N: 0.0010 to 0.0070 mass%,
O: 0.0030 mass% or less and
Ca: 0.0005 to 0.0030 mass%
And W: 0.01 to 0.5 mass% and
Mo: 0.02-0.5mass%
Containing one or two selected from
Sn: 0.001 ~ 0.2mass% and
Sb: 0.01-0.2mass%
Contains one or two selected from the above, and contains Cu, Ni, Cr, and Co, respectively.
Cu: less than 0.20mass%,
Ni: less than 0.20mass%,
Cr: less than 0.20mass% and
Co: Suppressed to less than 0.20 mass%, the balance is Fe and inevitable impurities composition, ACP value represented by the following formula (1) is 0.20 or less, and WI value represented by the following formula (2) is 0.50 or less Corrosion-resistant steel for marine vessels, characterized by satisfaction.
Record
ACP = {1− (0.8 × W + 0.5 × Mo) ^0.3 } × {1− (Sn + 0.4 × Sb) ^0.3 }
× (1 + Cr) × (1 + 0.7 × Cu) × (1 + 0.5 × Ni) × (1 + 0.5 × Co)
X {1-40 x (0.015-P)} x {1-200 x (0.003-S)} --- (1)
However, W, Mo, Sn, Sb, Cr, Cu, Ni, Co, P, and S are component contents (mass%) of each element, respectively.
WI = C + Mn / 6 + Cr / 5 + Mo / 5 + V / 5 + Ni / 15 + Cu / 15 + W / 10 + Co / 15 + Sn / 2 + Sb / 2
--- (2)
However, C, Mn, Cr, Mo, V, Ni, Cu, W, Co, Sn, and Sb are each component content (mass%) of each element.

2. Steel material,
Nb: 0.001 to 0.1 mass%,
Zr: 0.001 to 0.1 mass% and V: 0.002 to 0.2 mass%
The marine corrosion-resistant steel material according to 1 above, comprising one or more selected from among the above.

3. Steel material,
B: 0.0002 ～ 0.003mass%
3. The marine corrosion resistant steel material according to 1 or 2 above, which contains

4). Steel material,
REM: 0.0001 ~ 0.015mass%,
Mg: 0.0001-0.01mass% and Y: 0.0001-0.1mass%
The marine corrosion resistant steel material according to any one of the above items 1 to 3, which contains one or more selected from among the above.

5. Steel material,
Se: 0.0005 ～ 0.50mass%
The marine corrosion-resistant steel material according to any one of the above 1 to 4, characterized by comprising:

6). The marine corrosion-resistant steel material according to any one of the above 1 to 5, wherein an epoxy-based coating film is coated on the surface of the steel material.

7). The marine corrosion-resistant steel material according to any one of the above 1 to 5, wherein a zinc primer coating is applied to the surface of the steel material.

8). The marine corrosion-resistant steel material according to any one of 1 to 5, wherein a zinc primer coating and an epoxy coating are coated on the surface of the steel.

  According to the present invention, a ship capable of exhibiting excellent paint corrosion resistance in a severe seawater corrosive environment such as a ship's ballast tank, extending the period until repair painting, and reducing the work of repair painting. Corrosion resistant steel material can be obtained.

It is the graph which showed the influence which W, Mo, Sn, Sb, Cu, Ni, Cr, and Co which are elements in steel exert on coating corrosion resistance. It is the figure which showed the evaluation test point of painting corrosion resistance.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel material is limited to the above range in the present invention will be described.
C: 0.01-0.25mass%
C is an element effective for increasing the strength of the steel material. In the present invention, it is necessary to contain 0.01 mass% or more in order to obtain a desired strength. On the other hand, the content exceeding 0.25 mass% lowers the toughness of the weld heat affected zone. Therefore, C is set to a range of 0.01 to 0.25 mass%. Preferably it is the range of 0.03-0.20 mass%, More preferably, it is the range of 0.05-0.16 mass%.

Si: 0.05-0.50mass%
Si is an element added as a deoxidizer and to increase the strength of the steel material. In the present invention, it contains 0.05 mass% or more. However, since addition exceeding 0.50 mass% degrades the toughness of steel, the upper limit of Si is 0.50 mass%.

Mn: 0.1-2.0mass%
Mn is an element that has the effect of preventing hot brittleness and increasing the strength of the steel material, and is added by 0.1 mass% or more. However, addition of Mn exceeding 2.0 mass% decreases the toughness and weldability of the steel, so it is set to 2.0 mass% or less. Preferably, it is in the range of 0.9 to 1.6 mass%.

P: 0.010 mass% or less P is a harmful element that deteriorates not only the base metal toughness of steel but also the weldability and weld zone toughness. Therefore, it is desirable to reduce as much as possible. Also, from the viewpoint of corrosion resistance, the reduction of P is effective, and when the P content is 0.010 mass% or less, the improvement in corrosion resistance becomes significant. Therefore, P is set to 0.010 mass% or less. Preferably it is 0.008 mass% or less.

S: 0.0010 mass% or less Since S is a harmful element that deteriorates the toughness and weldability of steel, it is desirable to reduce it as much as possible. Also, from the viewpoint of corrosion resistance, reduction of S is effective, and the corrosion resistance is dramatically improved when the S content is 0.0010 mass% or less. Therefore, the amount of S shall be 0.0010 mass% or less. Preferably it is 0.0008 mass% or less.

Al: 0.10 mass% or less
Al is added as a deoxidizer, but if it exceeds 0.10 mass%, it adversely affects the toughness of the welded part, so it was limited to 0.10 mass% or less.

Ti: 0.005-0.030 mass%
Ti has a strong affinity with N and precipitates as TiN, thereby suppressing the coarsening of austenite grains in the weld heat affected zone, or contributes to increasing the toughness of the weld heat affected zone as a ferrite nucleus. Such an effect is recognized when the content is 0.005 mass% or more. However, if the content exceeds 0.030 mass%, the TiN particles become coarse and the above effect cannot be expected. For this reason, Ti shall be contained in the range of 0.005 to 0.030 mass%.

N: 0.0010 to 0.0070 mass%
N combines with Ti and precipitates as TiN to suppress coarsening of austenite grains in the welding heat-affected zone, or contribute to high toughness of the welding heat-affected zone as a ferrite formation nucleus. In order to secure the necessary amount of TiN having such an effect, N needs to be contained in an amount of 0.0010 mass%. On the other hand, when it contains exceeding 0.0070 mass%, the amount of solid solution N will increase in the area | region heated to the temperature which TiN melt | dissolves with welding heat, and the remarkable fall of toughness will be caused. For this reason, N shall be contained in the range of 0.0010 to 0.0070 mass%.

O: 0.0030 mass% or less O is mixed as an unavoidable impurity, exists as an oxide in steel, and lowers cleanliness. Therefore, in the present invention, it is preferable to reduce it as much as possible. If the O content exceeds 0.0030 mass%, CaO inclusions are coarsened, which adversely affects toughness. Therefore, it is preferable to reduce O in molten steel to 0.0030 mass% or less.

Ca: 0.0005 to 0.0030 mass%
Ca is an element that contributes to improving the toughness of steel by controlling the form of sulfide. In order to exhibit such an effect, it is necessary to contain at least 0.0005 mass%. On the other hand, even if it contains exceeding 0.0030 mass%, the effect is saturated. For this reason, Ca content was restrict | limited to the range of 0.0005-0.0030 mass%.

1 type or 2 types selected from W: 0.01 to 0.5 mass% and Mo: 0.02 to 0.5 mass% W has corrosion resistance in the presence of an epoxy coating film even in the absence of a zinc primer, as described above. Remarkably improved. Therefore, in the steel material of the present invention, it is one of the most important elements for improving corrosion resistance. The above-described effect is manifested when W: 0.01 mass% or more is contained. However, when the amount of W exceeds 0.5 mass%, the effect is saturated. Therefore, the amount of W was limited to a range of 0.01 to 0.5 mass%. Preferably it is the range of 0.02-0.3 mass%.
The reason why W exhibits the above-described effect of improving corrosion resistance is that, as the steel sheet corrodes, WO ₄ ^2- is generated in the rust generated, and the presence of this WO ₄ ^2- causes chloride ions to be generated on the surface of the steel sheet. Intrusion into the steel plate surface is suppressed, and in addition, the poorly soluble FeWO ₄ is formed at the site where the pH is lowered, such as the anode portion on the steel plate surface, and the presence of this FeWO ₄ also prevents chloride ions from entering the steel plate surface. This is because the corrosion of the steel sheet is effectively suppressed by suppressing the penetration of chloride ions into the steel sheet surface. Moreover, corrosion of steel is also suppressed by the inhibitor action by adsorption of WO ₄ ^{2- on} the steel surface.

Mo improves the corrosion resistance in the presence of an epoxy coating even in the absence of a zinc primer. Therefore, in the steel material of this invention, it is one of the important corrosion-resistance improvement elements. The above effect is manifested when Mo is contained in an amount of 0.02 mass% or more. However, when the amount of Mo exceeds 0.5 mass%, the effect is saturated. Therefore, the amount of Mo was limited to the range of 0.02 to 0.5 mass%. Preferably it is the range of 0.03-0.35 mass%.
The reason why Mo has the above-described effect of improving the corrosion resistance is that, like W, MoO ₄ ^2- is generated in the rust generated as the steel sheet corrodes, and the presence of MoO ₄ ^2- This is because the penetration of chloride ions into the surface of the steel sheet is suppressed, and the penetration of chloride ions into the steel sheet surface is suppressed, whereby the corrosion of the steel sheet is effectively suppressed.

Since W and Mo coincide with each other in forming oxygen acid, both elements can be selected or used in combination.
In addition to W, W has the advantage that it is easy to produce poorly soluble FeWO ₄ even in a low pH environment and has a high inhibitor effect due to adsorption to the steel surface. Therefore, W has a content higher than that of Mo. At least, it exhibits excellent corrosion resistance.

One or two selected from Sn: 0.001 to 0.2 mass% and Sb: 0.01 to 0.2 mass%
Both Sn and Sb have the effect of improving corrosion resistance even in the absence of a zinc primer. The effect of Sn and Sb is to suppress corrosion at sites where the pH is lowered, such as the anode portion on the steel sheet surface. This effect is manifested when Sn is contained in an amount of 0.001 mass% or more and Sb is contained in an amount of 0.01 mass% or more. However, if the content exceeds 0.2 mass%, the base material toughness and the weld heat affected zone toughness are deteriorated. Therefore, Sn is limited to the range of 0.001 to 0.2 mass%, and Sb is limited to the range of 0.01 to 0.2 mass%.

Cu: less than 0.20 mass%, Ni: less than 0.20 mass%, Cr: less than 0.20 mass% and Co: less than 0.20 mass%
Since Cu, Ni, Cr and Co all deteriorate coating corrosion resistance in the absence of a zinc primer, it is preferable to reduce their content as much as possible from the viewpoint of coating corrosion resistance. However, it is an element that cannot be avoided as an inevitable impurity when scrap is used. Therefore, the inventors examined the allowable ranges of these elements, and found that Cu, Ni, Cr, and Co were all acceptable if they were less than 0.20 mass% with little adverse effect on coating corrosion resistance. More preferably, all are 0.15 mass% or less, More preferably, it is 0.10 mass% or less.

As described above, the basic component and the suppression component have been described. However, in the present invention, it is not sufficient to satisfy the above component composition range, and the ACP value and the WI value represented by the following formulas (1) and (2) are within a predetermined range. It is necessary to satisfy.
ACP value: 0.20 or less
ACP = {1− (0.8 × W + 0.5 × Mo) ^0.3 } × {1− (Sn + 0.4 × Sb) ^0.3 }
× (1 + Cr) × (1 + 0.7 × Cu) × (1 + 0.5 × Ni) × (1 + 0.5 × Co)
X {1-40 x (0.015-P)} x {1-200 x (0.003-S)} --- (1)
However, W, Mo, Sn, Sb, Cr, Cu, Ni, Co, P, and S are component contents (mass%) of each element, respectively.
This ACP value is an index of coating corrosion resistance. The more the content of W, Mo, Sn, and Sb effective for coating corrosion resistance, the more harmful Cr, Cu, Ni, Co, and P are. , S content decreases, coating corrosion resistance improves, and desired coating corrosion resistance can be obtained with an ACP value of 0.20 or less.

WI value: 0.50 or less
WI = C + Mn / 6 + Cr / 5 + Mo / 5 + V / 5 + Ni / 15 + Cu / 15 + W / 10 + Co / 15 + Sn / 2 + Sb / 2
--- (2)
However, C, Mn, Cr, Mo, V, Ni, Cu, W, Co, Sn, and Sb are each component content (mass%) of each element.
The WI value serves as an index of weld zone toughness. If the WI value contains each element within the range of 0.50 or less, desired weld zone toughness can be obtained.

In the present invention, in addition to the basic components described above, the components described below can be appropriately contained as necessary.
One or more selected from Nb: 0.001 to 0.1 mass%, Zr: 0.001 to 0.1 mass%, and V: 0.002 to 0.2 mass%
Nb, Zr, and V are all elements that increase the strength of the steel material, and can be selected and contained according to the required strength. In order to obtain such an effect, it is preferable to contain Nb and Zr in an amount of 0.001 mass% or more and V in an amount of 0.002 mass% or more. However, if Nb, Zr exceeds 0.1 mass% and V exceeds 0.2 mass%, the toughness decreases. Therefore, it is preferable that Nb, Zr, V is contained with the above value as the upper limit.

B: 0.0002 ～ 0.003mass%
B is an element that increases the strength of the steel material, and can be contained as necessary. In order to acquire such an effect, it is preferable to contain 0.0002 mass% or more, but when added exceeding 0.003 mass%, toughness will deteriorate. Therefore, it is preferable to contain B in the range of 0.0002 to 0.003 mass%.

REM: 0.0001 to 0.015 mass%, Mg: 0.0001 to 0.01 mass%, Y: One or more selected from 0.0001 to 0.1 mass%
REM, Mg, and Y are all elements effective in improving the toughness of the weld heat affected zone, and can be selected and contained as necessary. This effect is obtained when REM: 0.0001 mass% or more, Mg: 0.0001 mass% or more, Y: 0.0001 mass% or more, REM exceeds 0.015 mass%, Mg exceeds 0.01 mass%, Y If the content exceeds 0.1 mass%, the toughness is deteriorated. Therefore, REM, Mg, and Y are preferably included in the above ranges.

Se: 0.0005 ～ 0.50mass%
Se is an element that increases the strength of the steel material, and can be contained as necessary. In order to acquire this effect, it is preferable to contain 0.0005 mass% or more, but when it contains exceeding 0.50 mass%, toughness will deteriorate. Therefore, Se is preferably contained in the range of 0.0005 to 0.50 mass%.

  In the steel material of the present invention, components other than those described above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

Next, the suitable manufacturing method of the corrosion-resistant steel material which concerns on this invention is demonstrated.
The molten steel having the preferred component composition described above is melted in a known furnace such as a converter or an electric furnace, and is made into a steel material such as a slab or billet by a known method such as a continuous casting method or an ingot forming method. In addition, when melting, treatment such as ladle refining or vacuum degassing refining may be performed.
The component adjustment method of molten steel should just follow a well-known steel smelting method.

Next, when hot rolling the steel material to a desired size and shape, it is preferable to heat the steel material to a temperature of 1050 to 1250 ° C. from the viewpoint of preventing grain coarsening. In addition, when the temperature of the steel material is high enough to allow hot rolling, the hot rolling may be performed as it is. Moreover, when the temperature distribution of a high-temperature steel raw material is large, it is preferable to heat-roll and then hot-roll.
In hot rolling, in order to ensure strength, it is preferable to optimize the hot finish rolling end temperature, the cooling rate after the hot finish rolling end, and the cooling stop temperature, and the hot finish rolling end temperature is Cooling after completion of hot finish rolling at 700 ° C. or higher is preferably air cooling or accelerated cooling at 150 ° C./s or lower. The cooling stop temperature for accelerated cooling is preferably in the range of 400 to 600 ° C. Note that, after cooling, reheating treatment may be performed.

The molten steel having the composition shown in Table 1 was made into a slab by continuous casting after melting or converter melting in a vacuum melting furnace. Next, the slab was charged into a heating furnace, heated to 1150 ° C., and hot rolled to obtain a 30 mm thick steel plate. For these steel plates, the tensile properties (YS, TS, EL) and impact properties (vE (−40 ° C.)) of the base material were investigated. In addition, as a high heat input weld zone toughness, a reproducible thermal cycle equivalent to 1 mm of weld heat affected zone with heat input of 200 kJ / cm (1 mm from the fusion line on the base metal side) was given, and -20 by Charpy impact test Absorption energy vE (−20 ° C.) at a temperature was measured.
Further, a test piece of 3 mmt × 50 mmW × 150 mmL is taken from the above steel plate, the surface of the test piece is shot blasted, the surface scale and oil are removed, and the tar epoxy resin paint (about approx. A test piece coated with a single layer coating of 100 μm) was produced.
Corrosion resistance is obtained by applying a scratch 長 of 80mm length that reaches the surface of the iron bar with a cutter knife from the top of the coating in a single character, and after the corrosion test under the following conditions, the swelling of the coating that occurred around the scratch 疵The area was evaluated.
・ Corrosion test: Simulating a corrosive environment equivalent to the upper deck of a real ballast tank, (35 ℃, 5% NaCl solution spray, 2 hours) → (60 ℃, RH25%, 4 hours) → (50 ℃, RH95 %, 2 hr) was 1 cycle, and 132 cycles were conducted.
Table 2 shows the corrosion test results and mechanical property investigation results.

In Table 1, No. 25 exemplifies steel having a conventional general level composition used in this field as the base steel.
The film swell area was expressed as a relative ratio with the film swell area of No. 25 being base steel being 100%. If the film swelling area shown by this relative ratio is 20% or less, it can be said that it is excellent in coating corrosion resistance.

From Table 2, the steel of No. 1-24 of the invention example which satisfy | fills the component composition and ACP value of this invention has a coating film swelling area of 20% or less with respect to the steel of No. 25 which is base steel, It can be seen that it has good paint corrosion resistance.
In contrast, the coating film swollen area of No. 26 to 36, 38, and 39 steels that do not satisfy the component composition and ACP value of the present invention is smaller than that of No. 25 steel that is the base steel. However, the area is over 20% of the base steel, and it cannot be said that it has sufficient paint corrosion resistance.
In addition, No. 37 steel does not satisfy the composition of the present invention and has a very large ACP value, so that the swollen area of the coating film is remarkably as large as 119% of the base steel.
Steel No. 40 satisfies the composition of the present invention, but the ACP value does not meet the requirements, so the coating swollen area is 27% of the base steel and has sufficient paint corrosion resistance. Not.
No. 41 and No. 42 steels satisfy the corrosion resistance improving components (W, Mo, Sn and Sb) and ACP values of the present invention, so that the film swelling area is 20% or less of the base steel. Although the coating corrosion resistance was sufficient, the impact characteristics of the welded portion were 30 J or less, and sufficient impact characteristics were not obtained. The reasons are as follows.
-No. 41 steel: WI value exceeds the upper limit (0.50).
-No.42 steel: N amount exceeds the upper limit (0.007 mass%).

  The marine corrosion resistant steel material of the present invention exhibits excellent paint corrosion resistance in a corrosive environment in a ballast tank, and when applied to a ballast tank placed in a severe corrosive environment, its excellent paint corrosion resistance allows repair repainting, etc. The maintenance cost can be greatly reduced. For this reason, the contribution to the industry is extremely large. In addition, since this steel material shows the outstanding corrosion resistance in the corrosive environment by seawater, it can be used not only for the ballast tank of a ship but for the use used by the corrosive environment by other similar seawater.

Claims (8)

C: 0.01-0.25 mass%,
Si: 0.05-0.50mass%,
Mn: 0.1-2.0mass%,
P: 0.010 mass% or less,
S: 0.0010 mass% or less,
Al: 0.10 mass% or less,
Ti: 0.005-0.030 mass%,
N: 0.0010 to 0.0070 mass%,
O: 0.0030 mass% or less and
Ca: 0.0005 to 0.0030 mass%
And W: 0.01 to 0.5 mass% and
Mo: 0.02-0.5mass%
Containing one or two selected from
Sn: 0.001 ~ 0.2mass% and
Sb: 0.01-0.2mass%
Contains one or two selected from the above, and contains Cu, Ni, Cr, and Co, respectively.
Cu: less than 0.20mass%,
Ni: less than 0.20mass%,
Cr: less than 0.20mass% and
Co: Suppressed to less than 0.20 mass%, the balance is Fe and inevitable impurities composition, ACP value represented by the following formula (1) is 0.20 or less, and WI value represented by the following formula (2) is 0.50 or less Corrosion-resistant steel for marine vessels, characterized by satisfaction.
Record
ACP = {1− (0.8 × W + 0.5 × Mo) ^0.3 } × {1− (Sn + 0.4 × Sb) ^0.3 }
× (1 + Cr) × (1 + 0.7 × Cu) × (1 + 0.5 × Ni) × (1 + 0.5 × Co)
X {1-40 x (0.015-P)} x {1-200 x (0.003-S)} --- (1)
However, W, Mo, Sn, Sb, Cr, Cu, Ni, Co, P, and S are component contents (mass%) of each element, respectively.
WI = C + Mn / 6 + Cr / 5 + Mo / 5 + V / 5 + Ni / 15 + Cu / 15 + W / 10 + Co / 15 + Sn / 2 + Sb / 2
--- (2)
However, C, Mn, Cr, Mo, V, Ni, Cu, W, Co, Sn, and Sb are each component content (mass%) of each element. Steel material,
Nb: 0.001 to 0.1 mass%,
Zr: 0.001 to 0.1 mass% and V: 0.002 to 0.2 mass%
The marine corrosion resistant steel material according to claim 1, comprising one or more selected from among the above. Steel material,
B: 0.0002 ～ 0.003mass%
The marine corrosion-resistant steel material according to claim 1 or 2, characterized by comprising: Steel material,
REM: 0.0001 ~ 0.015mass%,
Mg: 0.0001-0.01mass% and Y: 0.0001-0.1mass%
The marine corrosion-resistant steel according to any one of claims 1 to 3, comprising one or more selected from among the above. Steel material,
Se: 0.0005 ～ 0.50mass%
The marine corrosion-resistant steel material according to claim 1, comprising:   The corrosion-resistant steel for ships according to any one of claims 1 to 5, wherein an epoxy-based coating is applied to the surface of the steel.   The marine corrosion resistant steel material according to any one of claims 1 to 5, wherein a zinc primer coating is applied to the surface of the steel material.   The corrosion resistant steel material for ships according to any one of claims 1 to 5, wherein the surface of the steel material is coated with a zinc primer coating film and an epoxy coating film.

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