JP6736255B2 - Corrosion resistant steel for ballast tanks - Google Patents

Description

TECHNICAL FIELD The present invention relates to a corrosion-resistant steel material for use in a ballast tank or the like that is particularly in a severe corrosive environment due to seawater, and relates to a corrosion-resistant steel material for a ballast tank having an epoxy coating film on its surface.

When a cargo ship such as a tanker is empty, the hull floats during navigation and becomes unstable with respect to crosswinds and transverse waves.Therefore, when there is no cargo, seawater is loaded on the ballast tank to prevent the draft from decreasing. ing. Ballast tanks are exposed to severe corrosive environments because seawater is repeatedly injected and discharged as cargo is loaded and unloaded. Therefore, the surface of the steel material used for the ballast tank is protected by an epoxy-based paint.

However, near the top of the ballast tank, especially on the back side of the upper deck, the temperature rises during the daytime and the nighttime decreases with the splash of seawater, resulting in a very severe corrosive environment. In addition, even if the ballast tank is subjected to cathodic protection, it does not function because seawater is not injected when cargo is loaded, and is severely corroded by the action of residual adhering salt.

Although the ballast tank is required to be painted, the life of the paint film is said to be about 15 years in a very severe corrosive environment. On the other hand, since the life of the ship is 25 years, it is necessary to repair the painting and change the steel plate. Since repairing ballast tanks increases repair costs and periods at the time of docking, it is desired to develop a steel material having excellent corrosion resistance that does not lead to pitting corrosion for about 10 years after deterioration of the coating.

Corrosion-resistant steel materials that exhibit excellent coating corrosion resistance have been proposed to meet such requirements (see, for example, Patent Documents 1 and 2). These are steel materials in which one or both of W and Mo are added, and one or both of Sn and Sb are added to reduce the blister of the coating film generated from the defective portion of the coating film.

JP, 2009-046749, A JP, 2009-046750, A

W, Mo, Sn, and Sb are effective elements for suppressing the corrosion of coating defects, but as a result of the study by the present inventors, MnS formed in steel deteriorates the corrosion resistance of coating defects. It turned out that there are cases where it is necessary to do so. As a method of reducing MnS, there is a method of reducing the amount of S in steel, but the load on steelmaking becomes large, and there is a problem that productivity is reduced and cost is increased.

In view of the above situation, the present invention is for a ballast tank that has an S content of about 0.003% to suppress an increase in manufacturing cost, and has excellent coating corrosion resistance in a corrosive environment in the ballast tank. It provides a corrosion resistant steel material.

The present inventors have studied in order to improve the corrosion resistance of a coating defect portion of a steel material containing S in an amount that does not impose a load on steelmaking, specifically, a steel material having an S content of 0.003% or more. went. As a result, by limiting the product of the S addition amount [S] and the Mn addition amount [Mn] to 0.020 or less, and further adding one or both of Sn and Sb, corrosion of the coating film defective portion can be prevented. We have found that progress is greatly curtailed. Further, they have found that it is necessary to limit the upper limit of the amount of Mn to 1.20% or less in order to suppress the generation of MnS, and to add Nb in order to secure the strength of the base material.

The present invention has been made based on such findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.03 to 0.25%,
Si: 0.05 to 0.50%,
Mn: 0.10 to 1.20%,
S: 0.003 to 0.020%,
Al: 0.001 to 0.100%, and
Nb: 0.001-0.020%
Containing
Sb: 0.010 to 0.300%, and
Sn: 0.010 to 0.300%
Containing at least one of the
Cu: 0.05 to 0.40%,
Ni: 0.05 to 0.40%,
Cr: 0.06 to 0.40%,
Containing one or more of
P: 0.025% or less, the balance consisting of Fe and unavoidable impurities, and the content of Mn [Mn] and the content of S [S] are 0.005≦ [Mn]×[S] A corrosion-resistant steel material for ballast tanks, which satisfies ≦0.020 and has an epoxy coating film on the surface.
(2) Furthermore,
Mo: 0.50% or less, and
W: Corrosion-resistant steel material for ballast tanks according to the above (1), characterized by containing 1.00% or less of one or two kinds .
(3) Furthermore,
Ti: 0.100% or less,
Zr: 0.10% or less, and
V: The corrosion-resistant steel material for ballast tanks according to the above (1) or (2), which contains 0.20% or less of one kind or two or more kinds.
(4) Further, the
B: 0.0030% or less is contained, The corrosion-resistant steel material for ballast tank ships in any one of said (1)-(3) characterized by the above-mentioned.
(5) Furthermore,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
REM: 0.015% or less, and
Y: 0.100% or less of one type or two or more types of corrosion resistant steel materials for ballast tanks according to any one of the above (1) to (4).
(6) The corrosion resistant steel material for ballast tanks according to any one of the above (1) to (5), which has a zinc primer coating film as a base of the epoxy coating film.

The ballast tank corrosion-resistant steel material of the present invention suppresses an increase in manufacturing cost, and exhibits excellent coating corrosion resistance under a corrosive environment in the ballast tank, so when applied to a ballast tank exposed to a severe corrosive environment, Initial costs and maintenance costs such as repair and repainting can be significantly reduced.

The present inventors conducted the following investigations on corrosion of defective portions of epoxy coating films using various corrosion resistant steel materials coated with epoxy coating materials.

Steels to which various alloying elements were added were melted and hot-rolled to produce steel plates having a plate thickness of 5 mm, and test pieces having a length of 150 mm and a width of 70 mm were collected. The scale on the surface of the test piece was removed by shot brass, and the epoxy-based paint was applied twice so that the coating film thickness was 300 to 400 μm. Then, in order to evaluate the corrosion resistance of the defective portion of the coating film, a 50 mm-long flaw reaching the surface of the base metal was formed in one letter in the center of the test piece with an end mill having a width of 2 mm.

The corrosion resistance was evaluated by a combined cycle test. In order to match the environment of the ballast tank, artificial seawater was used as the corrosive liquid instead of the 5% NaCl aqueous solution. The cycle conditions were 1 hour for spraying a corrosive liquid (temperature: 35° C.), 2 hours for drying (60° C., 20 to 30% humidity), and 1 hour for wetting (50° C., 95% or higher humidity). After performing this cycle 300 times, the maximum length of coating blistering on the applied flaw was measured.

As a result, it was found that when one or both of Sn and Sb were added, a test piece in which coating film swelling and corrosion in the coating defect part were suppressed and a test piece in which coating film swelling and corrosion were not suppressed were observed. As a result of detailed investigation, it was found that a large amount of MnS was generated in the case where the bulging of the coating defect portion was not suppressed.

Next, the surface was mirror-polished, marking was performed around the observed MnS with a Vickers hardness meter, and corrosion resistance was evaluated using a test piece with a flaw in that position. As a result, it was revealed that the swelling of the coating film was not suppressed and that MnS adversely affected the corrosion resistance of the coating defect portion. Even in a corrosion-resistant steel material containing Sn or Sb, when a large amount of MnS is produced, the reason why the corrosion of the coating film defect portion is not suppressed may be that sulfuric acid is generated by the hydrolysis of MnS and the pH is greatly lowered. I'm estimating.

Next, the relationship between the Mn content and S content of the corrosion-resistant steel material to which one or both of Sn and Sb was added and the corrosion resistance of the coating film defect portion was arranged. As a result, even if the amount of S is 0.003% or more, if the product of the amount of Mn and the amount of S ([Mn]×[S]) is 0.020 or less, corrosion of the coating film defect portion is suppressed. It has been found. Further, it is necessary to limit the amount of Mn in order to suppress the formation of MnS while allowing the content of S to be 0.003% or more.

As a result of studies by the present inventors, it was found that the upper limit of the amount of Mn needs to be limited to 1.2% or less in order to suppress the generation of MnS. However, if the amount of Mn is reduced, the strength of the base material decreases. Therefore, it is necessary to add Nb so as to prevent the mechanical properties of the base material from being reduced as much as possible, and to supplement the lack of strength due to the decrease in Mn.

Based on the results of such examination, in the corrosion resistant steel material for ballast tanks of the present invention (hereinafter referred to as “corrosion resistant steel material of the present invention”), Nb is added, the S amount is 0.003 to 0.02%, and the Mn amount is Is 0.10 to 1.20%, the product [Mn]×[S] of the Mn content [Mn] and the S content [S] is limited to 0.020 or less, and one of Sb and Sn or It was decided to make the component composition containing both. Further, if necessary, one or more of Cu, Ni, Cr, Mo, and W are added to obtain further excellent corrosion resistance.
Next, the component composition of the corrosion resistant steel material of the present invention will be specifically described. In addition, "%" means "mass %" unless otherwise specified.

(C: 0.03 to 0.25%)
C is an element effective in increasing the strength of the steel material, and needs to be contained in an amount of 0.03% or more to obtain the desired strength. It is preferably at least 0.05%, more preferably at least 0.08%. On the other hand, the content of C exceeding 0.25% lowers the toughness of HAZ (welding heat affected zone), so the upper limit of the content of C is 0.25%. The upper limit of the C content is preferably 0.20%, more preferably 0.17%.

(Si: 0.05 to 0.50%)
Si is an element added as a deoxidizing agent and for increasing the strength of the steel material, and contains 0.05% or more. The content is preferably 0.10% or more, more preferably 0.15% or more. However, addition of more than 0.50% deteriorates the toughness of steel, so the upper limit of the Si content is made 0.50%. The upper limit of the Si content is preferably 0.40%, more preferably 0.30%.

(Mn: 0.10 to 1.20%)
Mn is an element that enhances the strength of the steel material and is added in an amount of 0.10% or more. It is preferably 0.50% or more, more preferably 0.70% or more. However, the addition of Mn in excess of 1.20% increases MnS and reduces the corrosion resistance of coating film defects, so the content is made 1.20% or less. It is preferably 1.10% or less, more preferably 1.00% or less.

(S: 0.003 to 0.020%)
Since S produces MnS and reduces the corrosion resistance of the coating film defective portion, the content of S is set to 0.020% or less. The content is preferably 0.015 or less, more preferably 0.010% or less. The content of S is preferably reduced from the viewpoint of corrosion resistance, but if it is less than 0.003%, the load on steelmaking is large and the cost becomes high, so the lower limit is made 0.003%.

(Al: 0.001 to 0.100%)
Al is an element added as a deoxidizer, and is added in an amount of 0.001% or more. The content is preferably 0.005% or more, more preferably 0.010% or more. However, if Al is contained in excess of 0.100%, the toughness of the base material decreases, so the upper limit is made 0.100%. The upper limit of the Al content is preferably 0.080%, more preferably 0.060%.

(Nb: 0.001-0.020%)
Nb is an element necessary to secure the strength and toughness of the base material, and 0.001% or more is added. The content is preferably 0.003% or more, more preferably 0.005% or more. However, the addition of more than 0.020% lowers the HAZ toughness, so the Nb content is 0.020% or less. Preferably, the Nb content is 0.018% or less.

(Sb:0.010~0.300%)
(Sn: 0.010 to 0.300%)
Sb and Sb have an effect of improving the corrosion resistance of the coating film defective portion, and one or both of them are added. In order to obtain the effect, both Sn and Sb must be contained in an amount of 0.010% or more, preferably 0.020% or more, more preferably 0.030% or more. On the other hand, if the content of both Sn and Sb exceeds 0.300%, the toughness of the base material and HAZ is deteriorated. Therefore, the upper limit of the Sb and Sn contents is 0.300%, preferably 0.200%, and more preferably 0.150%.

(P: 0.025% or less)
P is an impurity and deteriorates the base metal toughness, weldability, and weld zone toughness of steel, so it is preferable to reduce P as much as possible. In particular, if the P content exceeds 0.025%, the toughness of the base metal and the toughness of the welded portion will be significantly reduced, so the content is limited to 0.025% or less. The P content is preferably 0.020% or less, more preferably 0.015% or less.

([Mn]×[S]≦0.020)
The contents of Mn and S are set in the above ranges, and the product ([Mn]×[S]) of the numerical values of the Mn content [Mn] and the S content [S] is 0.020 or less. By setting [Mn]×[S] to 0.020 or less, even if S is contained in an amount of 0.003% or more, generation of MnS is suppressed, and corrosion of the coating film defective portion is suppressed. [Mn]×[S] is preferably 0.010 or less, more preferably 0.007 or less.

The corrosion-resistant steel material of the present invention contains, in addition to the above-mentioned basic components (essential elements), one or more of Cu, Ni, Cr, Mo, and W in order to further improve the corrosion resistance of the coating defect portion. You may add as a selective element.

(Cu: 0.40% or less)
(Ni: 0.40% or less)
(Cr: 0.40% or less)
(Mo: 0.50% or less)
(W: 1.0% or less)
When one or more of Cu, Ni, Cr, Mo and W are added at the same time as one or both of Sn and Sb, the effect of further enhancing the corrosion resistance of the coating defect portion is exhibited. In order to obtain the effect of improving the corrosion resistance of the coating defect portion, it is preferable to add 0.01% or more of each of Cu, Ni, Cr, Mo and W. However, if Cu, Ni, Cr, Mo, and W are excessively added, the HAZ toughness may deteriorate. It is preferable that Cu is 0.40%, Ni is 0.40%, Cr is 0.40%, Mo is 0.50%, and W is 1.0%. More preferably, Cu is 0.30%, Ni is 0.30%, Cr is 0.20%, Mo is 0.20%, and W is 0.5%.

The corrosion-resistant steel material of the present invention contains, in addition to the above-mentioned essential elements, one of Ti, Zr, V, B, Ca, Mg, REM, and Y in order to further improve the mechanical properties of the base material and HAZ. Alternatively, two or more kinds may be added.

(Ti: 0.100% or less)
(Zr: 0.10% or less)
(V: 0.20% or less)
Each of Ti, Zr, and V is an element that produces a precipitate to enhance the strength of the steel material, and can be contained if necessary. It is preferable to add 0.001% or more of Ti, Zr, and V. On the other hand, if Ti, Zr, and V are excessively added, the toughness may decrease. Therefore, it is preferable to add Ti with 0.100%, Zr with 0.10%, and V with 0.20% as the upper limits. More preferably, Ti is 0.020%, Zr is 0.02%, and V is 0.03%.

(B: 0.0030% or less)
B is an element that enhances the strength of the steel by adding a trace amount, and can be contained as necessary. The B content is preferably 0.0003% or more. More preferably, it is 0.0005% or more. On the other hand, if added in excess of 0.0030%, the toughness may deteriorate, so the upper limit of the B content is preferably 0.0030%. More preferably, it is 0.0020%.

(Ca: 0.0100% or less)
(Mg: 0.0100% or less)
(REM: 0.015%)
(Y: 0.100% or less)
Each of Ca, Mg, REM, and Y is an element effective in improving the toughness of the weld heat affected zone, and can be selected and contained if necessary. It is preferable to add 0.0001% or more of each of Ca, Mg, REM, and Y. More preferably, the contents of Ca, Mg, REM, and Y are each set to 0.0005% or more. On the other hand, excessive addition of these may lower the toughness, so Ca is preferably 0.0100% or less, Mg is 0.0100% or less, REM is 0.015% or less, and Y is preferably 0.100% or less. .. More preferably, the contents of Ca, Mg, REM and Y are each set to 0.0030% or less.

In the corrosion-resistant steel material of the present invention, the components other than the above are Fe and inevitable impurities, but the inclusion of components other than the above is acceptable as long as it is within the range that does not impair the effects of the present invention.

The corrosion resistant steel material of the present invention has an epoxy coating film on the surface of the base steel material having the above composition. The epoxy coating film is not particularly limited as long as it satisfies the coating performance standards set by the International Maritime Organization (IMO), and is formed by coating the epoxy coating material and drying it. Good.

Further, the zinc-rich primer coating film can be formed on the surface of the base steel material having the above composition, and then the epoxy coating film can be provided. The zinc-rich primer coating film is not particularly limited and may be formed by applying a zinc-rich primer and drying.

The corrosion resistant steel material of the present invention can be manufactured by a conventional method.
For example, molten steel is melted by a known method such as a converter or an electric furnace, and made into a steel material such as a slab or billet by a known method such as a continuous casting method or an ingot making method, and is subjected to hot rolling. The molten steel may be subjected to ladle refining and vacuum degassing.

Then, the steel material is preferably heated to a temperature of 1050 to 1250° C. and hot rolled into a desired size and shape. The steel material after casting or ingot may be hot-rolled as it is. In the hot rolling, in order to secure the strength, it is preferable to optimize the hot finish rolling finish temperature and the cooling rate after the hot finish rolling finish, and the hot finish rolling finish temperature is 700° C. or higher, The cooling after the hot finish rolling is preferably air cooling or accelerated cooling at a cooling rate of 100° C./s or less. Moreover, you may re-heat-process after cooling.

Then, an epoxy-based paint is applied to the surface of the steel material and dried to form an epoxy-based coating film. A zinc rich primer coating may be formed before applying the epoxy paint. Also, shot blasting or pickling may be performed before applying the epoxy paint or the zinc rich primer.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the conditions in the Examples are one condition example adopted for confirming the feasibility and effects of the present invention. It is not limited to the example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steel having the composition shown in Table 1 is melted in a vacuum melting furnace or a converter to form an ingot or a steel slab, which are heated to 1150° C. in a heating furnace and hot-rolled to a thickness of 25 mm. It was a steel plate. The tensile properties and impact properties of the base metal of the obtained thick steel plate were investigated. The tensile test was performed at room temperature according to JIS Z 2241, and the Charpy impact test was performed at -40°C according to JIS Z 2242.

Next, a test piece having a length of 150 mm, a width of 70 mm, and a thickness of 5 mm was taken from each thick steel plate, the scale on the surface of the test piece was removed by shot brass, and then an epoxy paint was applied twice to form a coating film. A test piece having a thickness of 300 to 400 μm was produced. In addition, some steel plates were prepared by applying a zinc primer of 10 μm before applying the epoxy paint.

A 50 mm long flaw reaching the surface of the base metal was formed in a letter shape in the lateral direction in the end mill having a width of 2 mm at the center of these test pieces, and a combined cycle test was performed using artificial seawater. The cycle conditions were 1 hour for spraying a corrosive liquid (temperature: 35° C.), 2 hours for drying (60° C., 20 to 30% humidity), and 1 hour for wetting (50° C., 95% or higher humidity). After performing this cycle 300 times, the maximum length of coating blistering on the applied flaw was measured. Corrosion resistance is No. 1 which does not particularly include the corrosion resistance improving element. The ratio of the maximum length of paint blisters was calculated and evaluated using 27 steels as base steel (100).

Table 2 shows the results of the corrosion test, tensile test and impact test. No. of the invention examples or reference examples satisfying the component composition of the present invention. The steels Nos. 1 to 26 have a ratio of the maximum length of paint swelling to the base steel (No. 27) of 50% or less, and it is understood that the steels have good corrosion resistance. In addition, the steel material coated with the zinc primer in the invention example or the reference example has a ratio of the maximum length of coating swelling to the base steel (No. 27) of 25% or less, and it is understood that the steel material has good corrosion resistance. ..
On the other hand, No. 1 which does not satisfy the condition of the composition of the present invention. In each of the 28 to 32 steels, the ratio of the maximum coating blistering length to the base steel (No. 27) exceeds 50%. No. The steel No. 33 has a ratio of the maximum length of coating swelling to the base steel (No. 27) of 50% or less, but does not contain Nb, so that the mechanical properties of the base material are low.

INDUSTRIAL APPLICABILITY The present invention is a corrosion resistant steel material for ballast tanks having an epoxy coating film on its surface, and includes thick steel plates, thin steel plates, shaped steel and steel bars. The corrosion-resistant steel material for a ballast tank of the present invention is, for example, a coal ship, an ore ship, a coal-bearing ship, a crude oil tanker, an LPG ship, an LNG ship, a chemical tanker, a container ship, a bulk carrier, a timber carrier, a chip carrier, and a frozen container. It can be suitably used as a material for a ballast tank of a carrier ship, a car carrier, a heavy carrier, a RORO carrier, a limestone carrier, a cement carrier, and the like. Since the corrosion-resistant steel material of the present invention exhibits excellent coating corrosion resistance in a corrosive environment due to seawater, it can be used not only in a ballast tank of a ship, but also in other similar corrosive environment due to seawater. .. Therefore, the present invention has high industrial applicability.

Claims (6)

In mass %,
C: 0.03 to 0.25%,
Si: 0.05 to 0.50%,
Mn: 0.10 to 1.20%,
S: 0.003 to 0.020%,
Al: 0.001 to 0.100%, and
Nb: 0.001-0.020%
Containing
Sb: 0.010 to 0.300%, and
Sn: 0.010 to 0.300%
Containing at least one of the
Cu: 0.05 to 0.40%,
Ni: 0.05 to 0.40%,
Cr: 0.06 to 0.40%,
Containing one or more of
P: 0.025% or less, the balance consisting of Fe and unavoidable impurities, and the content of Mn [Mn] and the content of S [S] are 0.005≦ [Mn]×[S] A corrosion-resistant steel material for ballast tanks, which satisfies ≦0.020 and has an epoxy coating film on the surface. Furthermore,
Mo: 0.50% or less, and
W: 1.00% or less of 1 type or 2 types is contained, The corrosion-resistant steel material for ballast tanks of Claim 1 characterized by the above-mentioned. Furthermore,
Ti: 0.100% or less,
Zr: 0.10% or less, and
V: 0.20% or less of 1 type or 2 or more types is contained, The corrosion-resistant steel material for ballast tanks of Claim 1 or 2 characterized by the above-mentioned. Furthermore,
B: 0.0030% or less is contained, The corrosion-resistant steel material for ballast tank ships of any one of Claims 1-3 characterized by the above-mentioned. Furthermore,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
REM: 0.015% or less, and
Y: 0.100% or less of 1 type or 2 or more types is contained, The corrosion-resistant steel material for ballast tanks in any one of Claims 1-4 characterized by the above-mentioned. The corrosion resistant steel material for ballast tanks according to any one of claims 1 to 5, further comprising a zinc primer coating film as a base of the epoxy coating film.