KR101382009B1 - Steel for ship having excellent corrosion resistance - Google Patents

Abstract

SUMMARY OF THE INVENTION An object of the present invention is to provide marine steels exhibiting excellent long-term corrosion resistance in a harsh corrosive environment and various structures constructed using such marine steels. The steel materials for ships of the present invention are C: 0.04 to 0.30%, Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, P: 0.010 to 0.040%, S: 0.011 to 0.025%, Al: 0.010 to 0.10%, Cu : 0.10 to 1.0%, Cr: 0.01 to 0.3% and N: 0.0030 to 0.010%, respectively, the remainder being made of iron and unavoidable impurities, and the content of S [S] and the content of N [N] The ratio ([S] / [N]) is 1.50 to 6.0.

Description

Corrosion-resistant marine steel {STEEL FOR SHIP HAVING EXCELLENT CORROSION RESISTANCE}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel materials for ships used as major structural members in ships such as crude oil tankers, container ships, LNG ships, bulk carriers, cargo ships, passenger ships, sundries, warships, and the like. The present invention relates to a ship steel material exhibiting excellent corrosion resistance under many environments and in an environment containing corrosive gas and oil such as SO ₂ and SO _{3 in a} crude oil tank.

Steel materials used as main structural members in various ships are exposed to harsh corrosive environments by salts caused by seawater, high temperature and high humidity, moisture contained in crude oil, and corrosive gas components. For example, in a ballast tank for loading seawater in order to prevent the ship from becoming unstable, steel corrosion caused by high temperature, high humidity and high salinity becomes remarkable, and it is indispensable to apply an anticorrosive method for securing the ship's safety and extending its life. Do.

As an anticorrosive method of a ship, the method by coating (anticorrosion painting) and the electric system (sacrificial system) is common, or may use both methods together. As the anticorrosive method of the said ballast tank, the electric system which installs so that the zinc which is an active metal rather than a steel material and short circuits with a steel material is employ | adopted in combination with the coating by an epoxy resin coating material is used in many cases. In addition, in the crude oil tank of the tanker, corrosion by hydrogen sulfide, sulfur oxides (SOx), etc. is remarkable, and coating with an epoxy resin paint is generally applied.

Anticorrosive coating is an anticorrosive means generally employed in ships. However, due to external factors, deterioration, or the like, damage to the coating film or peeling of the coating may result in failure to maintain anticorrosive performance. Maintenance is required. In particular, there are many places where the scaffold needs to be assembled in the tank for inspection and maintenance of the coating on the back of the deck, such as ballast tanks and crude oil tanks, and there is a problem that the time and cost required for maintenance are increased.

On the other hand, in the case of applying the electric method by a sacrificial anode such as zinc or an external electrode, it is necessary to form an electric circuit in a state of being immersed in an electrolyte solution such as seawater, but the gas phase part in the tank not immersed in the electrolyte solution ( In the gap portion), there is a problem that the electric system effect is not sufficiently exhibited.

In order to improve the safety of a ship and to prolong the life of a ship, more effective anticorrosion means are required until now. In such a point, the technique which improves the corrosion resistance of steel itself by adjusting the chemical component of steel materials is also proposed. For example, Patent Document 1 assumes that the surface treatment of forming an inorganic zinc rich primer layer containing 30% by mass or more of metal zinc powder on the surface of a steel material and an electric system are used in combination. A technique for adjusting this is disclosed. Patent Literature 2 also discloses a technique in which a chemical component composition is precisely defined and controlled so that each element satisfies a predetermined relationship. By these techniques, it can be said that corrosion resistance superior to the conventional anticorrosive means is ensured.

However, under severe corrosive environments such as ballast tanks and crude oil tanks, it is not possible to say that sufficient corrosion resistance is still exhibited, and further improvement of corrosion resistance is desired. In addition, although each said technique describes that it may contain Sb, Sn, etc. as a corrosion resistance improvement element, these elements are not recommended from a viewpoint of environmental load.

Japanese Patent Application Publication No. 2008-144204 Japanese Patent Application Publication No. 2010-222701

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to provide a ship steel material exhibiting excellent long-term corrosion resistance in a harsh corrosive environment, and various structures constructed using such ship steel materials.

The steel materials for ships of the present invention, which were able to achieve the above object, are C: 0.04 to 0.30% (mean of mass%, hereinafter the same), Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, and P: 0.010 to 0.040%, S: 0.011 to 0.025%, Al: 0.010 to 0.10%, Cu: 0.10 to 1.0%, Cr: 0.01 to 0.3%, and N: 0.0030 to 0.010%, respectively, with the balance being iron and inevitable. It consists of an impurity and has a summary that the ratio ([S] / [N]) of the content [S] of S and the content [N] of N is 1.50 to 6.0.

In the ship steel of the present invention, if necessary, (a) Co: 2.0% or less (does not contain 0%) and / or Ni: 2.0% or less (does not contain 0%), (b) Mg : 0.005% or less (without 0%) and / or Ca: 0.005% or less (without 0%), (c) Ti: 0.1% or less (without 0%), Zr: 0.1% One or more selected from the group consisting of the following (does not contain 0%) and Hf: 0.1% or less (does not contain 0%), (d) Mo: 0.5% or less (does not contain 0%), and / Or W: 0.5% or less (does not contain 0%), (e) B: 0.005% or less (does not contain 0%), V: 0.1% or less (does not contain 0%), and Nb: 0.1 It is also effective to contain one or more selected from the group consisting of% or less (not containing 0%), and the like, and by including them, the characteristics of the steel for ships are further improved depending on the type of the component.

By using the steel materials for ships mentioned above, various structures, such as a ship tank, a tanker crude oil tank upper deck, and the gas phase part of a tanker crude oil tank, which exhibit the outstanding corrosion resistance, can be obtained.

In the present invention, the chemical composition is strictly defined, and by controlling the value of the ratio ([S] / [N]) of the contents of S and N to an appropriate range, excellent corrosion resistance is exhibited even in a harsh corrosive environment and for a long time. The steel materials for ships which can ensure the good corrosion resistance over can be realized. Such ship steels are extremely useful as materials for structures such as ship tanks, tanker crude oil tank upper decks, and meteorological sections of tanker crude oil tanks.

BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which shows the external appearance shape of the coating test piece used for the corrosion resistance test.
Explanatory drawing which shows the external appearance of the unpainted test piece used for the corrosion resistance test.
3 is a schematic explanatory diagram illustrating a method for measuring a brister width.
4 is a schematic explanatory diagram showing a test apparatus that simulates a gas phase part in a crude oil tank;
5 is an explanatory diagram schematically showing a cross section of a tanker;

The present inventors earnestly researched in order to solve the said subject. As a result, while precisely adjusting elements such as C, Si, Mn, P, S, Al, Cu, Cr, N, and the like, the value of the ratio ([S] / [N]) of S and N is 1.50. When it controlled in the range of -6.0, it discovered that the ship steel material excellent in corrosion resistance was obtained, and completed this invention. Moreover, according to the steel material of this invention, there also exists an advantage that it does not need to use special additive elements, such as Sb and Sn, which are not recommended from the viewpoint of environmental load.

In a corrosive environment, it is well known from the past that what is called rust, such as iron oxide and iron oxyhydroxide, is formed as a corrosion product on the steel surface, and the protection by these corrosion products exhibits corrosion resistance. The present inventors examined the relationship between the corrosion mechanism in the gaseous-phase corrosion environment, such as in a ballast tank or a crude oil tank, and the corrosion resistance of steel materials.

As a result, in the corrosion environment of the gas phase part as described above, it was clarified that corrosion products other than rust are also exhibiting corrosion resistance of steel materials. Specifically, in a steel material in which the chemical composition is appropriately adjusted, a complex sulfide containing Cu and Cr is formed on the steel surface as a compound film by precipitation, and the presence of this film greatly suppresses the corrosion reaction of the steel material. It turned out.

The complex sulfide acting on the corrosion resistance is formed by the complex formation of Cu ₂ S, which is a sulfide of Cu, and Cr ₂ S _{3, which} is a sulfide of CuS and Cr, which is caused by Cu, Cr, and S contained in steel materials. Is generated. Composite sulfides containing Cu and Cr are effectively produced by the action of stabilizing the compound by P in the steel, the action of catalyzing the production of the compound by N in the steel and the like. Therefore, in order to produce a composite sulfide that is effective in suppressing corrosion of steel, not only the contents of Cu, Cr and S, but also the ratios of the contents of P, N and the contents of S and N ([S] / It is also necessary to appropriately adjust the value of [N]).

[Value of Ratio ([S] / [N]) of Content [S] and Content [N] of S: 1.50 to 6.0]

In order to effectively exhibit the corrosiveness improvement effect by the said complex sulfide, it is necessary to adjust the value of ratio ([S] / [N]) of content of S and N (unit: mass%) suitably. When the value of this ratio ([S] / [N]) is less than 1.50, the catalysis (compound formation catalysis) of N to the sulfide formation reaction becomes insufficient, and the effect of improving the corrosion resistance becomes difficult to be exhibited. Moreover, when the value of ratio ([S] / [N]) exceeds 6.0, content of S will become small, a complex sulfide formation reaction will not progress easily, and sufficient corrosion resistance is not exhibited. For this reason, the value of the ratio [S] / [N] needs to be in the range of 1.50 to 6.0. Moreover, the minimum with preferable value of ratio ([S] / [N]) is 1.6 (more preferably 1.8 or more), and a preferable upper limit is 5.9 (more preferably 5.5 or less).

In the steel of the present invention, in order to satisfy the basic characteristics (mechanical characteristics and weldability) and corrosion resistance as the steel, it is necessary to appropriately adjust components such as C, Si, Mn, P, S, Al, Cu, Cr, and N. have. The reason for range limitation of these components is as follows.

[C: 0.04 to 0.30%]

C is a basic additive element necessary for securing the strength of the steel. In order to exhibit such an effect, it is necessary to contain 0.04% or more. However, when C is excessively contained, in addition to deterioration of corrosion resistance, toughness also deteriorates. In order to avoid such adverse influence of C, it is necessary to make C content into 0.30% or less. Moreover, the minimum with preferable C content is 0.045%, More preferably, it is good to set it as 0.05% or more. The upper limit of the C content is preferably 0.29%, more preferably 0.28% or less.

[Si: 0.05-1.0%]

Si is an element necessary for deoxidation and securing strength, and if it is less than 0.05%, Si cannot secure the minimum strength required as a structural member. However, when Si is contained in excess of 1.0%, weldability will deteriorate. Moreover, the minimum with preferable Si content is 0.08%, More preferably, it is good to set it as 0.10% or more. In addition, the upper limit with preferable Si content is 0.95%, More preferably, it is 0.90% or less.

[Mn: 0.1 to 2.0%]

Mn, like Si, is an element necessary for deoxidation and securing strength, and if it is less than 0.1%, Mn cannot secure the minimum strength required as a structural member. However, when it contains exceeding 2.0% excessively, toughness will deteriorate. Moreover, the minimum with preferable Mn content is 0.15%, More preferably, it is good to set it as 0.20% or more. In addition, the upper limit with preferable Mn content is 1.9%, More preferably, it is good to set it as 1.8% or less.

[P: 0.010 to 0.040%]

P has a function of stabilizing a complex sulfide containing Cu and Cr by forming phosphate when dissolved in a corrosive environment, and is therefore an indispensable addition element in the steel of the present invention. Such an effect is particularly great in a gaseous-phase corrosion environment in which corrosion by thin water film formation proceeds. In order to exhibit such an effect, it is necessary to contain P 0.010% or more. However, when P is excessively contained, the toughness and weldability deteriorate, so it is up to 0.040%. Moreover, the minimum with preferable P content is 0.011%, More preferably, it is good to set it as 0.012% or more. Moreover, the upper limit with preferable P content is 0.038%, More preferably, it is good to set it as 0.035% or less.

[S: 0.011 to 0.025%]

S dissolves in a corrosive environment and, together with Cu and Cr dissolved in the same manner, forms a film (coating formed by sedimentation) on the surface of the steel to reduce the corrosion dissolution reaction (dissolution reaction due to corrosion). It is an element necessary for improvement. Such an effect is particularly great in a gaseous-phase corrosion environment in which corrosion by thin water film formation proceeds. In order to exhibit such an effect, it is necessary to contain S 0.011% or more. However, similarly to P, however, when excessively contained, the toughness and weldability of steel materials deteriorate, so that the allowable content is up to 0.025%. Moreover, the minimum with preferable S content is 0.012%, More preferably, it is recommended to set it as 0.013% or more. In addition, the upper limit with preferable S content is 0.024%, More preferably, it is good to set it as 0.023% or less.

[Al: 0.010 to 0.10%]

When Al dissolves in a corrosive environment, it becomes a stable Al oxide, forms a film on the steel surface, and reduces the corrosion dissolution reaction, and is an element necessary for improving corrosion resistance. Al, like Si and Mn, is also an element necessary for deoxidation and securing strength. In order to exhibit these effects, Al needs to be contained 0.010% or more. However, when Al is excessively contained, weldability deteriorates, so it is made into 0.10%. Moreover, the minimum with preferable Al content is 0.011%, More preferably, it is 0.012% or more. Moreover, the upper limit with preferable Al content is 0.095%, More preferably, it is good to set it as 0.090% or less.

[Cu: 0.10 to 1.0%]

After dissolving in a corrosive environment, Cu forms a dense film (coating formed by sedimentation) on the surface of the steel together with the dissolved S and Cr in the same manner, thereby reducing the corrosion reaction and is an element necessary for improving corrosion resistance. In order to exhibit such an effect, it is necessary to contain Cu 0.1% or more. However, when Cu is excessively contained, it deteriorates the weldability and hot workability of the steel material, and therefore it is necessary to set it to 1.0%. Moreover, the minimum with preferable Cu content is 0.11%, More preferably, it is 0.12% or more. Moreover, the upper limit with preferable Cu content is 0.95%, More preferably, it is good to set it as 0.90% or less.

[Cr: 0.01 to 0.3%]

After dissolving in a corrosive environment, Cr forms a dense film (coating formed by sedimentation) on the surface of the steel together with the dissolved S and Cu in the same manner, thereby reducing the corrosion reaction, and is an element necessary for improving corrosion resistance. In order to exert such an effect, it is necessary to contain Cr 0.1% or more. However, when Cr is excessively contained, the pH of the corrosion tip is lowered and the corrosion resistance is deteriorated. In addition, the weldability and hot workability are deteriorated. Therefore, the Cr content needs to be 0.3%. Moreover, the minimum with preferable Cr content is 0.02%, More preferably, it is 0.03% or more. The upper limit of the Cr content is preferably 0.28%, more preferably 0.26% or less.

[N: 0.0030% to 0.010%]

In order to stably produce | generate complex sulfide (film | membrane formed by sedimentation) containing Cu and Cr, N needs to adjust the content. It is believed that N acts catalytically on the production of complex sulfides comprising Cu and Cr. In order to exhibit such an effect, it is necessary to contain N 0.0030% or more. However, when N is excessively contained, the solid solution N increases, which adversely affects the ductility and toughness of the steel, so the upper limit needs to be 0.010%. Moreover, the minimum with preferable N content is 0.0033%, More preferably, it is 0.0035% or more. In addition, the upper limit with preferable N content is 0.0095%, More preferably, it is good to set it as 0.0090% or less.

The basic component in the ship steel of this invention is as above-mentioned, and remainder consists of iron and an unavoidable impurity. Examples of unavoidable impurities include O, H, and the like, and these elements may be contained to such an extent that they do not interfere with the characteristics of the steel. However, these unavoidable impurities are preferably suppressed to 0.1% or less (more preferably 0.09% or less) in total, thereby maximizing the corrosion resistance expression effect in the present invention.

In addition, in the ship steel of this invention as needed other than the said component, (a) Co: 2.0% or less (it does not contain 0%) and / or Ni: 2.0% or less (it does not contain 0%) (b) Mg: 0.005% or less (0% not included) and / or Ca: 0.005% or less (0% not included), (c) Ti: 0.1% or less (0% not included) At least one selected from the group consisting of Zr: 0.1% or less (not containing 0%) and Hf: 0.1% or less (not containing 0%), and (d) Mo: 0.5% or less (0% And / or W: 0.5% or less (does not contain 0%), (e) B: 0.005% or less (does not contain 0%), V: 0.1% or less (does not contain 0%) ) And Nb: 0.1% or less (not including 0%), it is also effective to contain one or more selected from the group consisting of, and the characteristics of the steel for ships is further improved depending on the components to be contained.

[Co: 2.0% or less (does not contain 0%) and / or Ni: 2.0% or less (does not contain 0%)]

Co and Ni have the effect of improving the corrosion resistance by increasing the protective effect of the rust coating. Ni also exhibits the effect of improving the toughness of steel materials. However, when it contains excessively, it is preferable to set it as 2.0% or less (more preferably 1.9% or less) from the point that weldability and hot workability deteriorate. Moreover, in order to exhibit such an effect, it is preferable to contain Co or Ni 0.01% or more (more preferably 0.02% or more).

[Mg: 0.005% or less (0% not included) and / or Ca: 0.005% or less (0% not included)]

Mg and Ca have the effect of raising the pH in rust in a chloride environment and an acid furnace environment, and are effective elements for suppressing a cathode reaction and improving corrosion resistance. However, when excessively contained, deterioration of workability and weldability, it is preferable to make it 0.005% or less, respectively. Moreover, the minimum with preferable for exhibiting such an effect is 0.0003% or more (more preferably, respectively, 0.0004% or more, More preferably, it is 0.0005% or more). Moreover, a more preferable upper limit is 0.0045% (more preferably, 0.004% or less), respectively.

[Ti: 0.1% or less (does not contain 0%), Zr: 0.1% or less (does not contain 0%) and Hf: 1% or less selected from the group consisting of More than]

Ti, Zr, and Hf have the effect of improving the protective effect by the rust film formed on the steel surface. However, when it contains excessively, since weldability and hot workability deteriorate, it is preferable to set it as 0.1% or less, respectively. Moreover, the minimum with preferable content in these elements is 0.003% (more preferably, 0.005% or more), respectively, and a more preferable upper limit is 0.09% (more preferably, 0.08% or less), respectively.

[Mo: 0.5% or less (0% not included) and / or W: 0.5% or less (0% not included)]

Mo and W are elements having a function of generating molybdate ions and tungstate ions and improving corrosion resistance by the inhibitor effect, particularly in chloride environments (environments in which chlorides are present). However, when it contains excessively, since weldability and toughness deteriorate, it is preferable to make content of Mo or W into 0.5% or less, respectively. In addition, it is preferable to make content of Mo or W into 0.001% or more, respectively. The minimum with more preferable content in these elements is 0.003% or more (more preferably 0.005% or more), and a more preferable upper limit is 0.48% or less (more preferably 0.46% or less), respectively.

(B: 0.005% or less (not including 0%), V: 0.1% or less (without 0%) and Nb: 0.1% or less (without 0%) More than]

B, V, and Nb are effective elements for improving strength. However, when contained excessively, since the toughness of steel materials deteriorates, it is preferable to set it as 0.005% or less in B, and 0.1% or less in V or Nb, respectively. Moreover, it is preferable to make content of B, V, or Nb into 0.0001% or more in B, and 0.001% or more in V or Nb, respectively. The minimum with more preferable content in these elements is 0.0002% (more preferably 0.0003% or more) in B, and 0.002% (more preferably 0.003% or more) in V or Nb, respectively. The upper limit is more preferably 0.0045% (more preferably 0.004% or less) in B and 0.095% (more preferably 0.09% or less) in V or Nb.

The ship steel of this invention can be manufactured by the following method, for example. First, secondary refining including component adjustment and temperature adjustment is performed on the molten steel withdrawn from the converter or electric furnace by ladle using a vacuum circulation degassing apparatus (RH apparatus). Subsequently, it is made into a steel ingot by the usual casting methods, such as a continuous casting method and an ingot method. As a deoxidation type at this time, it is preferable to use a kicked steel from a mechanical characteristic or weldability viewpoint, More preferably, an Al kicked steel is recommended.

Next, after heating the obtained ingot to the temperature range of 1000-1300 degreeC, it is preferable to hot-roll and to make it a desired shape. Predetermined strength characteristics are obtained by controlling the hot rolling finish temperature at this time to 650-850 degreeC, and controlling the cooling rate from completion of hot rolling to 500 degreeC in the range of 0.1-15 degreeC / sec.

The steel material for ships of the present invention basically exhibits excellent corrosion resistance even though the steel material itself does not necessarily have anticorrosion means such as coating or electric system, but, if necessary, tar epoxy resin paint or other representative heavy-duty coating It can also be combined with other methods such as zinc rich paint and shop primer. Moreover, in ship steel materials, the chemical composition of a ship is adjusted suitably, and also the effect that weldability and hot workability are also equivalent to or more than normal ship steel is exhibited.

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not restrict | limited by the following example originally, Of course, it implements by changing suitably in the range which may be suitable for the meaning of the previous and the later. It is possible and all of them are included in the technical scope of the present invention.

Example

The steel materials (test No. 1-test No. 55) of the chemical composition shown in following Table 1 and Table 2 were melted with the electric furnace, and it was set as 40 kg steel ingot. After heating the obtained ingot at 1150 degreeC, hot rolling was performed and it was set as steel material of 10 mm of plate | board thickness. The hot rolling end temperature at this time was made into the range of 650-850 degreeC, and the cooling rate from after completion | finish of hot rolling to 500 degreeC was adjusted suitably in the range of 0.1-15 degreeC / sec. All the test pieces TP provided for the following test were 50x30x4 (mm) finally, and were cut out from the said steel raw material.

In the simulation test of the environment in a ballast tank, the coating test piece with a cut | wound scratch was used. The coating test piece with this cut | wound scratch was produced by the following procedure. First, one test surface (coated surface) was shot blasted, acetone washed and dried, and the inorganic zinc rich primer was applied to have a thickness of 15 µm. Then, the modified epoxy resin coating material was apply | coated to 300 micrometers in thickness by air spray, and it dried. In order to prevent corrosion except the test surface, coating was performed by applying a silicone sealant in addition to the painted surface. After the coating film and the silicone sealant were dried, one cut scratch reaching the base having a length of 100 mm and a width of 0.5 mm was formed with a cutter knife on the test surface side. The external shape of a coating test piece is shown in FIG.

An unpainted test piece was used for the environmental simulation test of the tanker crude oil tank gas phase part. This unpainted test piece was used for the test after the entire surface was polished up to SiC # 6O with a wet rotary polishing machine, washed with water and acetone, and dried.

About the test materials of each chemical component composition shown in the said Table 1, Table 2, various test pieces (coated test piece and uncoated test piece) were used for the corrosion test. The corrosion test method at this time is as follows.

[Corrosion test method]

Corrosion tests were conducted to simulate the environment in the ballast tanks and the meteorological part of the tanker crude oil tanks. As a simulation test of the environment in a ballast tank, the composite cycle test which repeats a 1-day artificial seawater immersion test and a 6-day wet-wetting cycle test was performed using the coating test piece with a cut scratch (corrosion test A). In the one-day artificial seawater immersion test, the coated test piece with cut scratches was immersed in artificial seawater kept at 30 ° C for one day. In the subsequent wet-and-dry cycle test, 5 hours of holding | maintenance (1st process) at temperature: 50 degreeC and humidity: 40% RH (25 degreeC), temperature: 30 degreeC, humidity: 95% RH (25 degreeC) The maintenance of time (second process) was repeated (transition time to mutual process is 1 hour each). The test period was 3 months.

In this corrosion test, three test pieces of the steel materials of Test No. 1 to Test No. 55 shown in Table 1 and Table 2 were prepared, respectively, and the blister width of the coating film flaws after the test was shown in Fig. 3 (Brister width measuring method). Measured by the method shown in (measured from the cut | disconnected flaw part of the "top part of a blister generating part"), and the largest one of the blister widths of the three test pieces provided for each test was made into the largest brister width.

The environmental test of the gaseous phase of the tanker crude oil tank is a simulated gas in the tank, the gas containing 0.1 vol% H ₂ S, 0.01vol% SO ₂ , 10vol% CO ₂ , 5vol% O ₂ (remaining part: N2) Using it, the gas corrosion test was done and corrosion resistance was evaluated (corrosion test B). The corrosion test apparatus used at this time is typically shown in FIG. In addition, the gas phase part assumed by this corrosion test is typically shown in FIG. 5 (FIG. 5 is explanatory drawing which shows the tanker's cross section typically).

The simulated gas was vented to distilled water maintained at 40 ° C, and introduced at a flow rate of 50 ml / min into a test tank provided with an uncoated test piece (TP) (see FIG. 4). The test tank was given the temperature cycle which repeats 20 hours of holding | maintenance at 50 degreeC, and 2 hours of holding | maintenance (30 hours of implementation time each) at 30 degreeC. In this temperature cycle, the test piece became a dry state at 50 ° C, and a wet state at 30 ° C, and repeated drying and wetting. The test period was 3 months. In this corrosion test, three test pieces of the steel materials of Test No. 1-Test No. 55 shown in Table 1 and Table 2 were prepared, respectively, and the amount of corrosion was computed as an average value of three test pieces. Corrosion amount was made into the mass change before and behind a test. In addition, the mass after a test was measured after removing a corrosion product by the negative electrode electrolysis in 10% of ammonium hydrogen citrate aqueous solution at room temperature (25 degreeC), washing with water, acetone, and drying.

[Test result]

The measurement results of the maximum blister width of the coating film according to the corrosion test A and the corrosion amount according to the corrosion test B are as shown in Tables 3 and 4 below, and the relative values when the steel materials in Test No. 1 were set to 100, respectively. It is shown. As a comprehensive evaluation, it is "rank 2" that the largest blister width (100 of test No. 1) of the coating film by corrosion test A is 75 or less, and corrosion amount (100 of test No. 1 100) is 50 or less. In addition, it was set as "rank 3" that the maximum blister width is 75 or less, and the corrosion amount is 40 or less, and "rank 4" that both were 40 or less was shown as "rank 5", respectively. In addition, it indicated with the "rank 1" that the largest brister width exceeds 75 and the corrosion amount exceeds 50.

 From these results, it can consider as follows. Test No. 1 to Test No. 12 are comparative examples which deviate from the requirements (chemical composition or ratio [S] / [N]) specified in the present invention, and the maximum film thickness of the coating film exceeds 75. In addition, the relative value of the corrosion amount also exceeds 50, and corrosion resistance is not enough (rank 1). Among them, the test No. 2 is insufficient in P content, and thus, generation of phosphate in a corrosive environment is insufficient, and sulfides of Cu and Cr are not produced stably, and thus corrosion resistance is considered to be insufficient.

In Test No. 3 to Test No. 6, since the S content is too small, the amount of composite sulfide containing Cu and Cr is reduced, and the corrosion inhibitive effect is considered to be insufficient. In Test No. 7 and Test No. 8, since the content of Cu or Cr is too small, respectively, the amount of production of complex sulfides containing Cu and Cr is reduced, and the corrosion inhibitory effect is considered to be insufficient.

In Test No. 9, since the N content is too small, the catalytic action against the sulfide formation of Cu or Cr is not sufficient, and the amount of the precipitated film generated is too small, and the corrosion inhibitory effect is considered to be insufficient. Since the ratio ([S] / [N]) is out of the range prescribed | regulated by this invention, the thing of test No. 10-the test No. 12 has too little generation amount of the complex sulfide precipitation film containing Cu and Cr, and it is corrosion-resistant It is thought that no improvement effect was obtained.

On the other hand, the steel materials of Test Nos. 13 to 55 which satisfy the requirements specified in the present invention have a maximum blister width of 75 or less and a corrosion amount of 50 or less, and exhibit excellent corrosion resistance. It is a result. These corrosion resistance anticorrosive effect | action of the sulfide precipitation film containing Cu and Cr obtained by controlling content of P, S, Cu, Cr, and N suitably, and controlling ratio ([S] / [N]) to an appropriate range. It is apparent that the addition of elements such as Co, Ni, Mg, Ca, and the like is effective for suppressing corrosion.

As described above, the steel material exhibiting excellent corrosion resistance is extremely useful as a structural member such as a ship tank, because it is excellent in corrosion resistance at a site where an electrical system is insufficient, such as a wet environment in a ballast tank or an environment of a gas phase of a tanker crude oil tank. Do.

Claims (5)

C: 0.04 to 0.30% (meaning of mass%, the same below), Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, P: 0.010 to 0.040%, S: 0.011 to 0.025%, Al: 0.010 to 0.10 %, Cu: 0.10 to 1.0%, Cr: 0.01 to 0.3%, and N: 0.0030 to 0.010%, respectively, the remainder being made of iron and unavoidable impurities, and the content of S [S] and the content of N [ N] ratio ([S] / [N]) is 1.50-6.0, The ship steel material excellent in corrosion resistance. The ship steel material of Claim 1 which contains at least 1 group from the following (a)-(e) groups.
(a) at least one member selected from the group consisting of Co: 2.0% or less (not containing 0%) and Ni: 2.0% or less (not containing 0%),
(b) at least one member selected from the group consisting of Mg: 0.005% or less (not containing 0%) and Ca: 0.005% or less (not containing 0%),
(c) Ti: 0.1% or less (does not contain 0%), Zr: 0.1% or less (does not contain 0%), and Hf: 0.1% or less (does not contain 0%) 1 or more species,
(d) at least one member selected from the group consisting of Mo: 0.5% or less (not containing 0%) and W: 0.5% or less (not containing 0%),
(e) B: 0.005% or less (does not contain 0%), V: 0.1% or less (does not contain 0%) and Nb: 0.1% or less (does not contain 0%) 1 or more types The ship tank comprised using the steel materials of Claim 1 or 2. A tanker crude oil tank upper deck, constructed using the steel as set forth in claim 1. The tanker crude oil tank vapor phase part comprised using the steel materials of Claim 1 or 2.

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