JP4935579B2 - 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 ship, RORO vessels, limestone dedicated ships and cement steel for ship of dedicated ships etc., particularly suitable for use in certain ballast tanks under severe corrosive environment due to seawater, moreover rafters steel for such vessels when thermal welding is relates to marine corrosion resistant steel materials with excellent weld toughness.
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.
In the present invention, high heat input welding refers to welding with a heat input of 60 kJ / cm or more, and REM refers to rare earth elements.

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, by adding Cu: 0.01-2.00 mass% and Mg: 0.0002-0.0150 mass% to steel of C: 0.01-0.25 mass%, ship outer plates, ballast tanks, cargo oil tanks 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

On the other hand, steel structures in the shipbuilding field are often assembled in a desired shape by generally joining steel materials by welding. Steel materials used for such welded structures are required to have excellent weld toughness as well as base metal toughness from the viewpoint of ensuring safety.
However, in recent years, with the increase in size of welded structures, improvement in welding efficiency has been demanded from the viewpoint of improving the construction efficiency of the structure and reducing the construction cost, and an increase in welding heat input has been directed. At that time, the most serious problem is the toughness of the weld bond. The weld bond is exposed to a high temperature just below the melting point during welding, the crystal grains are most likely to become coarse, and the cooling rate decreases as the welding heat input increases, so that a fragile upper bainite structure is likely to be formed. . Furthermore, in the welded bond portion, an embrittled structure such as a Widmanstatten structure or island martensite is likely to be generated, and the toughness is likely to be lowered.

In response to this problem, the following proposals have been made as means for improving the toughness of the welded heat affected zone or the welded heat affected zone in the vicinity thereof.
That is, in Patent Document 11 and Patent Document 12, Ti content, Ti content, and Ti / N ratio of Ti content and N content are defined, and TiN particles and MnS are combined to suppress austenite grain coarsening. ing.
In Patent Document 13, the amount of Ti and the amount of N are defined, and TiN particles and REM oxysulfide are combined to suppress austenite grain coarsening.
In Patent Document 14, REM and Ti are added in combination to suppress austenite grain growth.
In Patent Document 15 and Patent Document 16, Ti oxide is finely dispersed and used as a nucleation site for ferrite transformation.
In Patent Document 17, BN precipitated on TiN or the like in the cooling process during welding is used as the core of ferrite transformation.
In Patent Document 18 and Patent Document 19, the form control of sulfide is performed by adding Ca and REM.

However, with regard to weld zone toughness, although various improvement means have been proposed as described above, there are the following problems.
With the means using Ti oxide, it is difficult to disperse the oxide uniformly and finely, so that the variation in toughness of the welded portion becomes large.
In addition, with the means using TiN, attention must be paid to its distributed control. That is, when the amount of fine TiN produced is small, the grain refinement effect is lost and the weld toughness is not improved.
Furthermore, in the means using Al or Ca, oxides may be clustered and the toughness may be lowered as a starting point of fracture.
Moreover, in the conventional sulfide form control method using Ca or REM, there is a problem that it is difficult to ensure high toughness in a large heat input weld with a heat input of 60 kJ / cm or more.

JP-A-2-250917 JP-A-2-254118 Japanese Patent Publication No. 3-53367 JP-A-60-184663 JP-A-60-245768 JP 61-79745 JP 61-253344 JP Japanese Unexamined Patent Publication No. 60-204863 Japanese Examined Patent Publication No. 4-14180

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, it is possible to work the reduction of refinish, in addition, an object of the invention to propose a marine corrosion resistant steel excellent in high heat input weld toughness.

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 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.
(1-7) For the above (1-3), W adsorbed on the steel material surface as WO ₄ ^2- 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 (1-7).
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 simulates the corrosive environment equivalent to the upper deck of the ballast tank of an actual ship. (35 ° C, 5% NaCl solution spray, 2 hours) → (60 ° C, RH25%, 4 hours) → (50 ° C, RH95 %, 2 hr) was 1 cycle, and 132 cycles were conducted.
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, the coating corrosion resistance of Sn, W, Mo, and Sb is improved as the content increases. Therefore, from the viewpoint of improving the coating corrosion resistance, Sn, W, Mo, and Sb It can be seen that the addition to steel is effective.

Furthermore, the inventors have further studied and studied about the combination of these elements from the viewpoint of improving the coating corrosion resistance, and as a result, have found an experimental coating corrosion index formula represented by the following formula (1).
(1-8) ACP = {1- (0.8 × W + 0.5 × Mo) ^0.3 } × {1- (Sn + 0.4 × Sb) ^0.3 } --- (1)
However, W, Mo, Sn, and Sb are component contents of each element (mass%), respectively.
It was investigated that the desired coating corrosion resistance can be obtained by adjusting the elements in the steel so that the ACP value is 0.50 or less.
(1-9) However, it has also been found that the inclusion or addition of Sn, W, Mo, Sb must satisfy the relationship of the following formula (2) from the viewpoint of securing weld toughness.
WI = C + Mn / 6 + Mo / 5 + V / 5 + W / 10 + Sn / 2 + Sb / 2 ≦ 0.50 --- (2)
However, C, Mn, Mo, V, W, Sn, and Sb are component contents of each element (mass%), respectively.

On the other hand, as a result of repeated studies and examinations on various factors that affect the toughness of the high heat input weld toughness, the following knowledge was obtained.
(2-1) The toughness of high heat input welds, especially weld bonds, is greatly influenced by the presence or absence of embrittlement.
(2-2) The formation of the embrittlement structure can be prevented by suppressing the austenite coarsening in the region heated to a high temperature and by finely dispersing ferrite-forming nuclei that promote ferrite transformation during cooling. Conventionally, since these were insufficient, it is considered that high level stabilization of weld zone toughness could not be realized.
(2-3) Fine dispersion of TiN is effective for suppressing coarsening of austenite and promoting ferrite transformation during cooling, but the conventional Ti and N contents, and the ratio of Ti and N contents In the case of only Ti / N, the desired weld heat affected zone structure may not be obtained. In that case, the dispersion density of the fine TiN particles is small.
(2-4) The dispersion state of TiN in steel is influenced by the TiN precipitation process in addition to the Ti amount and N amount, and the slab cooling rate at the melting-casting stage. That is, the slab cooling rate changes due to the slab casting speed and slab size, but when the slab cooling rate is low, coarse TiN is generated, and when the slab cooling rate is high, fine TiN particles are dispersed and generated. .
(2-5) Therefore, a more direct index for controlling the heat affected zone structure is required. Ti: 0.005 to 0.030 mass%, N: 0.0015 to 0.0070 mass%, and further steel When fine TiN particles with an equivalent circle diameter of 50 nm or less are present at 5 × 10 ⁷ particles / cm ² or more per unit area, the microstructure of the weld heat affected zone becomes a fine structure and the weld heat affected zone toughness is also It becomes good. Moreover, as suitable manufacturing conditions for obtaining fine TiN particles of 50 nm or less of 5 × 10 ⁷ particles / cm ² or more, the slab cooling rate is set to 1400 after adjusting the Ti amount and the N amount to the above ranges. It is preferable to control to 0.05 ° C./s or more in the range of ˜1250 ° C.

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: 0.035 mass% or less,
S: 0.01 mass% or less,
Al: 0.10 mass% or less,
Ti: 0.005-0.030 mass% and N: 0.0015-0.0070 mass%
And W: 0.01 to 0.5 mass% and
Mo: 0.02-0.5mass%
Containing one or two selected from
Sb: 0.01 to 0.07 mass%, or
Sb: 0.01 to 0.07 mass% and Sn: 0.001 to 0.2 mass %
The balance is Fe and inevitable impurities, and there are more than 5 × 10 ⁷ TiN particles with an equivalent circle diameter of 10 to 50 nm / cm ^{2 in the} steel, and the following formula (1) A marine corrosion-resistant steel material having a high heat input weld toughness characterized by satisfying an ACP value of 0.50 or less and a WI value of 0.50 or less represented by the following formula (2):
Record
ACP = {1- (0.8 × W + 0.5 × Mo) ^0.3 } × {1- (Sn + 0.4 × Sb) ^0.3 } --- (1)
However, W, Mo, Sn, and Sb are component contents of each element (mass%), respectively.
WI = C + Mn / 6 + Mo / 5 + V / 5 + W / 10 + Sn / 2 + Sb / 2 --- (2)
However, C, Mn, Mo, V, W, Sn, and Sb are component contents of each element (mass%), respectively.

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 having excellent high heat input weld toughness as described in 1 above, comprising one or more selected from among the above.

3 . Steel material,
B: 0.0002 ～ 0.003mass%
The marine corrosion-resistant steel material having excellent high heat input weld toughness as described in 1 or 2 above.

4 . Steel material,
Ca: 0.0001 to 0.01 mass%,
REM: 0.0001 ~ 0.015mass%,
Mg: 0.0001-0.01mass% and Y: 0.0001-0.1mass%
The marine corrosion-resistant steel material having excellent high heat input weld toughness according to any one of the above 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 having excellent high heat input weld toughness according to any one of the above 1 to 4 , characterized by comprising:

6 . The marine corrosion-resistant steel material excellent in high heat input weld toughness according to any one of 1 to 5 above, wherein an epoxy-based coating film is coated on the surface of the steel material.

7 . The marine corrosion resistant steel material excellent in high heat input weld toughness according to any one of the above 1 to 5 , wherein a zinc primer coating film is coated on the surface of the steel material.

8 . The marine corrosion-resistant steel material having excellent high heat input weld toughness according to any one of 1 to 5 above, wherein the steel material is coated with a zinc primer coating and an epoxy coating.

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.
Moreover, according to this invention, in addition to said coating corrosion resistance, the corrosion-resistant steel material for ships excellent in the high heat input weld part toughness can be obtained.

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 for the purpose of improving the strength of steel materials, and in the present invention, 0.05 mass% or more is contained. 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 prevents hot brittleness and is useful for improving the strength of the steel material, so it is added in an amount of 0.1 mass% or more. However, since addition exceeding 2.0 mass% reduces the toughness and weldability of steel, Mn shall be 2.0 mass% or less. Preferably it is the range of 0.9-1.6 mass%.

P: 0.035 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. In particular, when the P content exceeds 0.035 mass%, the deterioration of the base metal toughness and weld zone toughness increases. Therefore, P is set to 0.035 mass% or less. Preferably it is 0.025 mass% or less.

S: 0.01 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. In the present invention, it is limited to 0.01 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.

N: 0.0010 to 0.0080 mass%
N is a component harmful to toughness, and is desirably reduced as much as possible in order to improve toughness. However, it is difficult to reduce to less than 0.0010 mass% industrially. On the other hand, when it contains exceeding 0.0080 mass%, the remarkable deterioration of toughness will be caused. Therefore, in this invention, N amount was limited to the range of 0.0010-0.0080 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.

Sb: 0.01 to 0.07 mass%, or Sb: 0.01 to 0.07 mass% and Sn: 0.001 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 the Sn content is 0.001 mass% or more and the Sb content is 0.01 mass% or more. However, when Sn exceeds 0.2 mass% and Sb exceeds 0.07 mass% , the toughness of the base metal and the weld heat affected zone Degradation of toughness. In the present invention, among these Sn and Sb, Sb is contained in the range of 0.01 to 0.07 mass% as an essential component. On the other hand, when Sn was contained, it was combined with Sb and contained in the range of 0.001 to 0.2 mass % .

In the present invention, it is not sufficient to satisfy the above component composition range, and it is necessary to satisfy a predetermined range for the ACP value and the WI value represented by the following formulas (1) and (2).
ACP value: 0.50 or less
ACP = {1- (0.8 × W + 0.5 × Mo) ^0.3 } × {1- (Sn + 0.4 × Sb) ^0.3 } --- (1)
However, W, Mo, Sn, and Sb are component contents of each element (mass%), respectively.
This ACP value is an index of coating corrosion resistance. The higher the content of W, Mo, Sn, and Sb that is effective for coating corrosion resistance, the better the coating corrosion resistance. The desired ACP value is 0.50 or less. Can be obtained.

WI value: 0.50 or less
WI = C + Mn / 6 + Mo / 5 + V / 5 + W / 10 + Sn / 2 + Sb / 2 --- (2)
However, C, Mn, Mo, V, W, Sn, and Sb are component contents of each element (mass%), respectively.
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.
Nb : One or more selected from 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 and Zr exceed 0.1 mass% and V exceeds 0.2 mass%, the toughness decreases. Therefore, it is preferable that Nb , Zr and V are included in the above ranges.

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 said effect, it is preferable to make it contain 0.0002 mass% or more, However, If it adds exceeding 0.003 mass%, toughness will deteriorate. Therefore, it is preferable to contain B in the range of 0.0002 to 0.003 mass%.

Ca: 0.0001 to 0.01 mass%, REM: 0.0001 to 0.015 mass%, Mg: 0.0001 to 0.01 mass%, and Y: 0.0001 to 0.1 mass%
Ca, REM, Mg, and Y are all effective elements for improving the toughness of the weld heat affected zone, and can be contained as necessary. This effect is obtained when Ca: 0.0001 mass% or more, REM: 0.0001 mass% or more, Mg: 0.0001 mass% or more, Y: 0.0001 mass% or more, Ca exceeds 0.01 mass%, and REM is 0.015. Exceeding mass%, Mg exceeding 0.01mass% and Y exceeding 0.1mass%, respectively, lead to a decrease in toughness, so Ca, REM, Mg and Y are within the above ranges. Is preferably contained.

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%.

By satisfying the above component composition range and the ranges of the above formulas (1) and (2), it is possible to obtain a marine corrosion resistant steel material excellent in coating corrosion resistance while ensuring desired strength.
In the present invention, in terms of having an excellent paint corrosion resistance described above, that provides even marine corrosion steel with excellent weld toughness as further subjected to high heat input welding.
In this case , it is necessary to contain Ti and N as essential elements, and to control the amount of Ti and the amount of N within the following ranges.

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 a desired effect cannot be expected. For this reason, Ti shall be contained in the range of 0.005 to 0.030 mass%.

N: 0.0015-0.0070mass%
N combines with Ti and precipitates as TiN to suppress the coarsening of austenite grains in the weld heat affected zone, or contribute to increasing the toughness of the weld 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 0.0015 mass% or more. 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 0.0015 to 0.0070 mass%.

As described above, after limiting the Ti amount and N amount as described above, the cooling rate in the temperature range of 1400 to 1250 ° C in the solidification stage is controlled to 0.05 ° C / s or more, particularly during the steel manufacturing process. Thus, fine TiN particles can be precipitated in the steel, thereby improving the weld heat affected zone toughness during high heat input welding.
In this case, the precipitation form of the fine TiN particles needs to be in the following range.

Circle equivalent diameter. 10 to 50 nm of TiN particles: 5 × 10 ⁷ / cm ² or more in the steel of. 10 to 50 nm in equivalent circle diameter of fine TiN particles, 5 × 10 ⁷ / cm ² or more in number In the case where it is contained, in the weld heat affected zone heated to a high temperature during welding, coarsening of austenite grains is suppressed, and ferrite transformation is promoted during cooling, so that the structure of the heat affected zone becomes fine and the toughness is improved. Even if the number of TiN particles exceeds 50 nm or the number is less than 5 × 10 ⁷ particles / cm ² even if the number is 10 to 50 nm, the effect of suppressing the coarsening of austenite grains is small, and the ferrite transformation Since the promoting effect is small, a satisfactory toughness improving effect cannot be obtained.
Note that the number of TiN particles having a particle diameter. 10 to 50 nm, particularly not intended to be defined upper limit, arbitrary preferred that the degree 1 × 10 ¹² / cm ² or less.

  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. It goes without saying that treatments such as ladle refining and vacuum degassing may be added to the molten steel.
Here, when the steel material is made by a method such as a continuous casting method or an ingot-making method, the cooling rate is cooling in the temperature range of at least 1400 to 1250 ° C in the solidification process from the viewpoint of dispersing and precipitating fine TiN particles. The speed should be 0.05 ° C / s or higher. This is because if the cooling rate in this temperature range is less than 0.05 ° C./s, the above-defined “number of TiN particles of 10 to 50 nm ≧ 5 × 10 ⁷ particles / cm ² ” cannot be satisfied. It is. A more preferable cooling rate is 0.10 ° C./s or more, and further preferably 0.15 ° C./s or more.

Next, from the viewpoint of preventing grain coarsening, the steel material is preferably heated to a temperature of 1050 to 1250 ° C. and then hot rolled to a desired size or shape, or the temperature of the steel material can be hot rolled. When the temperature is as high as possible, it is preferable to immediately hot-roll to a steel material having a desired size and shape without heating or soaking.
In hot rolling, in order to ensure strength, it is preferable to optimize the hot finish rolling end temperature and the cooling rate after the hot finish rolling end, and the hot finish rolling end temperature is 700 ° C. or higher. The cooling after the finish rolling is preferably air cooling or accelerated cooling with a cooling rate of 150 ° C./s or less. Note that, after cooling, reheating treatment may be performed.

As a reference example, the molten steel having the composition shown in Table 1 was made into a slab by continuous casting after melting in a vacuum melting furnace or melting in a converter. 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 and impact properties of the base material were investigated. In addition, as the weld zone toughness, a reproducible heat cycle equivalent to 1 mm of weld heat affected zone with heat input of 50 kJ / cm (1 mm from the fusion line on the base metal side) was given, and at -20 ° C by Charpy impact test Absorbed energy vE (−20 ° C.) was measured.
Further, a 3 mmt x 50 mmW x 150 mmL test piece is taken from the above steel plate, the surface of the test piece is shot blasted, the scale and oil are removed from the surface, 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 given by applying an 80mm-long scratch scissor that reaches the surface of the iron core with a cutter knife from the top of the paint film, and after the corrosion test under the following conditions, the swollen area of the paint film generated around the scratch scissors It was evaluated by.
・ 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.
FIG. 3 also shows the results of investigation on the relationship between the ACP value and the swollen area of the coating film, together with the results shown in Table 4 and Table 6 described later.

FIG. 3 shows that the smaller the ACP value, the smaller the swollen area of the coating film. Also, from Table 2 , the reference steel No.Al to A26 satisfying the ACP value has a paint swell area of 50% or less compared to the base steel No.A27, which has good paint corrosion resistance. It can be seen that Further, the steels of No. Al to A26 of the reference examples also show good values for the base metal mechanical properties and the welded portion impact properties.

On the other hand , the film swelling area of No. A28 to A33 steel that does not satisfy the ACP value is smaller than that of No. A27, which is the base steel, but the area is smaller than that of the base steel. Therefore, it cannot be said that it has sufficient coating corrosion resistance. In addition, No. A34 steel does not have sufficient coating corrosion resistance because the ACP value does not satisfy the regulations, and the swollen area of the coating film exceeds 50% of the base steel. Since No.A35 and No.A36 steels satisfy the ACP value, the swollen area of the coating film is 50% or less compared to the base steel, indicating sufficient paint corrosion resistance, but No.A35 steel. Since the WI value exceeds 0.50 and the N amount of the steel of No. A36 exceeds the upper limit, the impact characteristics of the weld zone are 50 J or less, and sufficient impact characteristics are not obtained.

The molten steel having the composition shown in Table 3 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 and impact properties of the base material were investigated. Also, as a high heat input weld toughness, a reproducible thermal cycle equivalent to 1 mm of the heat input zone with a heat input of 150 kJ / cm (1 mm from the fusion line to the base metal side) was given, and -20 by Charpy impact test. The absorption energy vE (−20 ° C.) at 0 ° C. was measured.
Further, a 3 mmt x 50 mmW x 150 mmL test piece is taken from the above steel plate, the surface of the test piece is shot blasted, the scale and oil are removed from the surface, 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 given by applying an 80mm-long scratch scissor that reaches the surface of the iron core with a cutter knife from the top of the paint film, and after the corrosion test under the following conditions, the swollen area of the paint film generated around the scratch scissors It was evaluated by.
・ 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.

Further, the observation of TiN particles in Table 3 and Table 5 described later and the density measurement thereof were performed according to the following procedure.
(1) A sample for microstructural observation was taken from the position where the thickness of the steel sheet was 1/4 t, and this was embedded in a conductive carbon resin and polished.
(2) Thereafter, electropolishing was performed under the following conditions, and after alcohol washing, drying was performed and Pt coating (about 10 seconds) was performed.
Electrolyte: 4% methyl salicylate-1% salicylic acid-1% TMAC-methanol Electrolytic potential: -300mV (vs. SCE)
・ Electrolysis amount: 0.5μm or more
(3) Thereafter, SEM (scanning electron microscope) observation was performed at an acceleration voltage of 30 kV. The magnification is determined by the distribution of TiN, but in this survey, observation was performed at 20000 times. The viewing field was 10 fields.
(4) Since TiN particles appear white in SEM photographs, the particle size distribution and density of these white particles were measured for the 10 visual fields observed using an image analyzer. The diameter distribution and density were calculated.
Note that the measurement target of this method is fine particles of 10 to 50 nm, and oxide and sulfide inclusions on the order of μm are excluded.
Table 3 shows the number density of TiN particles, and Table 4 shows the corrosion test results and mechanical property investigation results.
FIG. 4 also shows the results of examining the relationship between the TiN particle number density and the absorbed energy value at −20 ° C. in the reproduced thermal cycle Charpy impact test, together with the results shown in Table 6 described later.

As shown in Table 4 , No. Bl to B26 of the invention examples satisfying the component composition and ACP value of the present invention (however, No. B1 to B5, B7 to B9, B11, B13 to B15, B17 to B18, B21, It can be seen that the steel of B25 to B26 is a reference example) , and the coating swollen area is 50% or less compared to the steel of No. B27, which is the base steel, and has good coating corrosion resistance. In addition, the steels of No. Bl to B26 of the invention examples (however, No. B1 to B5, B7 to B9, B11, B13 to B15, B17 to B18, B21, B25 to B26 are reference examples) The characteristics and weld impact characteristics also show good values.
Furthermore, as shown in FIG. 4, when the number density of TiN particles is 5 × 10 ⁷ particles / cm ² or more, a good absorbed energy value is obtained at the weld.

On the other hand, the coating composition swell area of No. B28-B34 steel that does not satisfy the component composition and ACP value of the present invention is smaller than that of No. B27 steel, which is the base net, The area is over 50% of the base steel, and it cannot be said that it has sufficient paint corrosion resistance. In addition, although steel No. B35 satisfies the composition of the present invention, the ACP value does not meet the requirements, so the coating swell area exceeds 50% of the base steel, providing sufficient paint corrosion resistance. Does not have. No.B36 to B40 steel satisfy the ACP value, so the swollen area of the coating film is 50% or less of the base steel and shows sufficient paint corrosion resistance. Since the amount exceeds the upper limit, the No. B37 steel exceeds the upper limit, so the No. B38 steel has the Ti amount less than the lower limit and the TiN particle number density less than the lower limit. Since B39 steel has a TiN particle number density below the lower limit, No. B40 steel has a WI value of more than 0.50, so the impact characteristics of both welds are 50 J or less, and sufficient impact characteristics are obtained. Not.

The molten steel having the composition shown in Table 5 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 and impact properties of the base material were investigated. Also, as a high heat input weld toughness, a reproducible thermal cycle equivalent to 1 mm of the heat input zone with a heat input of 150 kJ / cm (1 mm from the fusion line to the base metal side) was given, and -20 by Charpy impact test. The absorption energy vE (−20 ° C.) at 0 ° C. was measured.
Further, a 3 mmt x 50 mmW x 150 mmL test piece is taken from the above steel plate, the surface of the test piece is shot blasted, the scale and oil are removed from the surface, 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 given by applying an 80mm-long scratch scissor that reaches the surface of the iron core with a cutter knife from the top of the paint film, and after the corrosion test under the following conditions, the swollen area of the paint film generated around the scratch scissors It was evaluated by.
・ 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 5 shows the results of examining the cooling rate in the temperature range of 1400 to 1250 ° C. and the number density of TiN particles in the steel during the solidification process of the steel.
Table 6 shows the corrosion test results and mechanical property investigation results.

As apparent from Table 6 , the steels of No. Cl to C14 of the invention satisfying the component composition and ACP value of the present invention (No. C1 to C7, C11, C13 to C14 are reference examples) are base steels. No. C15 steel, the swollen area of the coating film is 50% or less, indicating that it has good paint corrosion resistance. In addition, steels of No.Cl to C14 of the invention examples (however, No. C1 to C7, C11, and C13 to C14 are reference examples) show good values in the base material mechanical properties and welded portion impact properties. .
On the other hand, although the film swelling area of No. C16 steel that does not satisfy the ACP value of the present invention is smaller than that of No. C15 steel, which is the base steel, the area is the same as that of the base steel. On the other hand, it is over 50%, and it cannot be said that it has sufficient coating corrosion resistance. Further, the steels No. C15 to C17 have a TiN number particle density of less than the lower limit of the present invention. Therefore, the impact characteristics of the welded portion are 50 J or less, and sufficient impact characteristics are not obtained.

  The marine corrosion resistant steel material of the present invention exhibits excellent coating corrosion resistance in a corrosive environment in a ballast tank, so when applied to a ballast tank placed in a harsh corrosive environment, it is repaired and repainted due to its excellent coating corrosion resistance. Such maintenance costs can be greatly reduced. Furthermore, since it exhibits excellent high heat input weld toughness, high heat input welding can be applied during shipbuilding to improve the efficiency of welding work, and its contribution to the industry is extremely large. In addition, since the steel material of the present invention exhibits excellent corrosion resistance in a corrosive environment caused by seawater, it can be used not only for ship ballast tanks but also for other applications used in corrosive environments caused by seawater. Needless to say.

It is the graph which showed the influence which W, Mo, Sn, and Sb which are elements in steel exert on coating corrosion resistance. It is the figure which showed the evaluation test point of painting corrosion resistance. It is the figure which showed the relationship between an ACP value and a coating film swelling area. It is the figure which showed the relationship between the number density of TiN particle | grains, and the absorption energy [vE (-20 degreeC)] of -20 degreeC in the reproduction | regeneration thermal cycle Charpy impact test.

Claims (8)

C: 0.01-0.25 mass%,
Si: 0.05-0.50mass%,
Mn: 0.1-2.0mass%,
P: 0.035 mass% or less,
S: 0.01 mass% or less,
Al: 0.10 mass% or less,
Ti: 0.005-0.030 mass% and N: 0.0015-0.0070 mass%
And W: 0.01 to 0.5 mass% and
Mo: 0.02-0.5mass%
Containing one or two selected from
Sb: 0.01 to 0.07 mass%, or
Sb: 0.01 to 0.07 mass% and Sn: 0.001 to 0.2 mass %
The balance is Fe and inevitable impurities, and there are more than 5 × 10 ⁷ TiN particles with an equivalent circle diameter of 10 to 50 nm / cm ^{2 in the} steel, and the following formula (1) A marine corrosion-resistant steel material having a high heat input weld toughness characterized by satisfying an ACP value of 0.50 or less and a WI value of 0.50 or less represented by the following formula (2):
Record
ACP = {1- (0.8 × W + 0.5 × Mo) ^0.3 } × {1- (Sn + 0.4 × Sb) ^0.3 } --- (1)
However, W, Mo, Sn, and Sb are component contents of each element (mass%), respectively.
WI = C + Mn / 6 + Mo / 5 + V / 5 + W / 10 + Sn / 2 + Sb / 2 --- (2)
However, C, Mn, Mo, V, W, Sn, and Sb are component contents of each element (mass%), respectively. 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 having excellent high heat input weld toughness according to claim 1, comprising one or more selected from among them. Steel material,
B: 0.0002 ～ 0.003mass%
The marine corrosion resistant steel material having excellent high heat input weld toughness according to claim 1 or 2 . Steel material,
Ca: 0.0001 to 0.01 mass%,
REM: 0.0001 ~ 0.015mass%,
Mg: 0.0001-0.01mass% and Y: 0.0001-0.1mass%
The marine corrosion resistant steel material excellent in high heat input weld toughness 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 having excellent high heat input weld toughness according to any one of claims 1 to 4 . The corrosion resistant steel material for marine vessels excellent in high heat input weld toughness according to any one of claims 1 to 5 , wherein an epoxy-based coating film is coated on the surface of the steel material. 6. A marine corrosion resistant steel material having excellent high heat input weld toughness according to any one of claims 1 to 5 , wherein a zinc primer coating is applied to the surface of the steel material. The marine corrosion resistant steel material excellent in toughness of high heat input welds according to any one of claims 1 to 5 , wherein a zinc primer coating and an epoxy coating are coated on the surface of the steel. .

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