JP2012153933A - Steel product for ship, excellent in corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel products for a ship, exhibiting excellent long term corrosion resistance in a severe corrosive environment, and various structures configured using the steel products for a ship.SOLUTION: The steel products for a ship includes 0.04-0.30% of C, 0.05-1.0% of Si, 0.1-2.0% of Mn, 0.010-0.040% of P, 0.011-0.025% of S, 0.010-0.10% Al, 0.10-1.0% of Cu, 0.01-0.3% of Cr, and 0.0030-0.010% of N, respectively, with the balance composed of iron and unavoidable impurities, and a ratio ([S]/[N]) of S content [S] to N content [N] is 1.50-6.0.

Description

The present invention relates to marine steel materials used as main structural members in ships such as crude oil tankers, container ships, LNG ships, bulk carriers, cargo ships, passenger ships, miscellaneous ships, warships, etc. The present invention relates to a marine steel material that exhibits excellent corrosion resistance in a high-temperature and high-humidity environment with a high amount of salt, or in an environment containing a corrosive gas or oil such as SO ₂ or SO ₃ in a crude oil tank.

  Steel materials used as main structural members in various ships are exposed to severe corrosive environments due to salt content from seawater, hot and humid atmosphere, moisture contained in crude oil, corrosive gas components, and the like. For example, in a ballast tank for loading seawater to prevent the ship from becoming unstable, corrosion of steel due to high temperature and humidity and high salinity becomes significant, and anticorrosion methods are applied to ensure ship safety and extend the life of the ship. Is indispensable.

  As a corrosion prevention method for a ship, a method by painting (corrosion prevention coating) and an electric corrosion prevention (sacrificial corrosion prevention) are generally used, or both methods may be used in combination. As the anti-corrosion method for the ballast tank, there is often adopted an anti-corrosion method in which zinc, which is a base metal rather than steel, is installed so as to be short-circuited with the steel in combination with painting with an epoxy resin-based paint. Further, the inside of a tanker crude oil tank is significantly corroded by hydrogen sulfide, sulfur oxide (SOx) or the like, and is generally coated with an epoxy resin paint to be anticorrosive.

  Anti-corrosion coating is a general anti-corrosion measure used in ships, but the anti-corrosion performance cannot be maintained due to wrinkles on the coating film or peeling off of the coating due to external factors or deterioration over time. In some cases, maintenance for the inspection and repair is required. In particular, there are many places where it is necessary to build a scaffolding in the tank, such as ballast tanks and crude oil tanks, for inspection and repair of the coating on the back of the deck, and there is a problem that the time and cost required for maintenance become large. is there.

  On the other hand, when applying anti-corrosion with 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 aqueous electrolyte solution such as seawater, but the air in the tank that is not immersed in the aqueous electrolyte solution is required. In the phase part (void part), there is a problem that the anticorrosive effect is not sufficiently exhibited.

  In order to improve the safety of ships and extend their life, more effective anticorrosion means are required than before. For these reasons, a technique for improving the corrosion resistance of the steel material itself by adjusting the chemical composition of the steel material has also been proposed. For example, in Patent Document 1, on the premise that a surface treatment for forming an inorganic zinc rich primer layer containing a metal zinc content of 30% by mass or more on the surface of a steel material and an anticorrosion are used together, the chemical components of the base steel material are described. A technique for adjusting is disclosed. Patent Document 2 discloses a technique in which the chemical component composition is strictly defined and controlled so that each element satisfies a predetermined relationship. It can be said that the corrosion resistance superior to the conventional anti-corrosion means was ensured by these techniques.

  However, in the severe corrosive environment like the backside of decks such as ballast tanks and crude oil tanks, it cannot be said that sufficient corrosion resistance is still being demonstrated, and the fact is that further improvement of corrosion resistance is desired. . In addition, each of the above technologies describes that Sb or Sn can be included as an element for improving corrosion resistance, but these elements are not recommended in terms of environmental load.

JP 2008-144204 A JP 2010-222701 A

  The present invention has been made paying attention to the circumstances as described above, and its purpose is to use a marine steel material exhibiting excellent long-term corrosion resistance in a severe corrosive environment, and such a marine steel material. It is to provide various structures.

  The marine steel material of the present invention that has achieved the above object is C: 0.04 to 0.30% (meaning of mass%, the same shall apply hereinafter), Si: 0.05 to 1.0%, Mn: 0.1-2.0%, P: 0.010-0.040%, S: 0.011-0.025%, Al: 0.010-0.10%, Cu: 0.10-10. 0%, Cr: 0.01 to 0.3%, and N: 0.0030 to 0.010%, respectively, the balance is made of iron and inevitable impurities, and S content [S] and N The content [N] ratio ([S] / [N]) is 1.50 to 6.0.

  In the marine steel material of the present invention, if necessary, (a) Co: 2.0% or less (not including 0%) and / or Ni: 2.0% or less (not including 0%), ( b) Mg: 0.005% or less (not including 0%) and / or Ca: 0.005% or less (not including 0%), (c) Ti: 0.1% or less (not including 0%) ), Zr: 0.1% or less (excluding 0%) and Hf: 0.1% or less (not including 0%), (d) Mo: 0.5% Or less (excluding 0%) and / or W: 0.5% or less (not including 0%), (e) B: 0.005% or less (not including 0%), V: 0.1% The following may be included (not including 0%) and Nb: not less than 0.1% (not including 0%), etc. Is effective, the characteristics of marine steel according to the type of components by incorporating them can be further improved.

  By using the marine steel as described above, various structures such as a marine tank, a tanker crude oil tank upper deck, and a gas phase part of the tanker crude oil tank exhibiting excellent corrosion resistance can be obtained.

  In the present invention, the chemical composition is strictly defined, and the ratio of S and N content ([S] / [N]) is controlled within an appropriate range, so that it is excellent even in severe corrosive environments. It is possible to realize a marine steel material that exhibits high corrosion resistance and can ensure good corrosion resistance over a long period of time. Such marine steel materials are extremely useful as materials for structures such as marine tanks, tanker crude oil tank upper decks, and gas phase parts of tanker crude oil tanks.

It is explanatory drawing which shows the external appearance shape of the coating test piece used for the corrosion resistance test. It is explanatory drawing which shows the external appearance shape of the bare test piece used for the corrosion resistance test. It is a schematic explanatory drawing which shows the coating-film swelling width measuring method. It is a schematic explanatory drawing which shows the test device which simulated the gas phase part in a crude oil tank. It is explanatory drawing which showed the cross section of the tanker typically.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, elements such as C, Si, Mn, P, S, Al, Cu, Cr, and N are strictly adjusted, and the ratio of S and N content ([S] / [N]) is set. It has been found that a marine steel material excellent in corrosion resistance can be obtained by controlling in the range of 1.50 to 6.0, and the present invention has been completed. Moreover, according to the steel material of this invention, there also exists an advantage that it is not necessary to use special additive elements, such as Sb and Sn which are not recommended from the surface of an environmental load.

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

  As a result, it was clarified that corrosion products other than iron rust also exhibit the corrosion resistance of steel materials in the corrosive environment of the gas phase as described above. Specifically, in steel materials with appropriately adjusted chemical composition, 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. Turned out to be.

The composite sulfide that acts to develop corrosion resistance is formed by compositely forming Cu ₂ S and CuS, which are Cu sulfides, and Cr ₂ S _3, which is a sulfide of Cr. The product is generated due to Cu, Cr and S contained in the steel material. The composite sulfide containing Cu and Cr is effectively produced by the action of stabilizing the compound by P in the steel and the action of catalyzing the production of the compound by N in the steel. Therefore, in order to produce a composite sulfide effective for inhibiting corrosion of steel materials, not only the contents of Cu, Cr and S, but also the contents of P and N which are usually regarded as inevitable impurities, and S The value of the N content ratio ([S] / [N]) also needs to be adjusted appropriately.

[Value of ratio of S content [S] to N content [N] ([S] / [N]): 1.50 to 6.0]
In order to effectively exert the corrosive improvement effect by the composite sulfide, it is necessary to appropriately adjust the value of the ratio of S and N (unit: mass%) ([S] / [N]). is there. If the value of this ratio ([S] / [N]) is less than 1.50, the catalytic action of N (sulfide formation catalytic action) for the sulfide formation reaction becomes insufficient, and the effect of improving corrosion resistance is exhibited. It becomes difficult to be done. On the other hand, if the value of the ratio ([S] / [N]) exceeds 6.0, the content of S decreases, the composite sulfide formation reaction is difficult to proceed, 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. The preferable lower limit of the ratio ([S] / [N]) is 1.6 (more preferably 1.8 or more), and the preferable upper limit is 5.9 (more preferably 5.5 or less).

  In the steel material of the present invention, components such as C, Si, Mn, P, S, Al, Cu, Cr, and N are added to satisfy the basic properties (mechanical properties and weldability) and corrosion resistance of the steel material. It is necessary to adjust appropriately. The reasons for limiting the ranges of these components are as follows.

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

[Si: 0.05-1.0%]
Si is an element necessary for deoxidation and securing strength, and the minimum strength required as a structural member cannot be secured unless it is less than 0.05%. However, if the Si content exceeds 1.0%, the weldability deteriorates. In addition, the minimum with preferable Si content is 0.08%, More preferably, it is good to set it as 0.10% or more. Moreover, the upper limit with preferable Si content is 0.95%, It is good to set it as 0.90% or less more preferably.

[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%, the minimum strength required as a structural member cannot be secured. However, if the content exceeds 2.0%, the toughness deteriorates. In addition, the minimum with preferable Mn content is 0.15%, More preferably, it is good to set it as 0.20% or more. Moreover, 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 is an indispensable additive element in the steel material of the present invention because it has an effect of stabilizing a composite sulfide containing Cu and Cr when it is dissolved in a corrosive environment. Such an effect is particularly significant in a gas phase corrosive environment where corrosion due to the formation of a thin water film proceeds. In order to exhibit such an effect, P needs to be contained 0.010% or more. However, if P is contained excessively, toughness and weldability deteriorate, so the content is limited to 0.040%. In addition, 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, after dissolving in a corrosive environment, has the effect of reducing the corrosion dissolution reaction (dissolution reaction due to corrosion) by producing a film (film formed by precipitation) on the steel material surface together with the dissolved Cu and Cr. It is an element necessary for improving corrosion resistance. Such an effect is particularly significant in a gas phase corrosive environment where corrosion due to the formation of a thin water film proceeds. In order to exert such an effect, S needs to be contained by 0.011% or more. However, as in the case of P, when S is excessively contained, the toughness and weldability of the steel material deteriorate, so the allowable content is limited to 0.025%. In addition, the minimum with preferable S content is 0.012%, More preferably, it is recommended to set it as 0.013% or more. Moreover, the upper limit with preferable S content is 0.024%, It is good to set it as 0.023% or less more preferably.

[Al: 0.010 to 0.10%]
Al, when dissolved in a corrosive environment, becomes a stable Al oxide and forms a film on the steel surface to reduce the corrosion dissolution reaction, and is an element necessary for improving corrosion resistance. Al is also an element necessary for deoxidation and securing strength, like Si and Mn. In order to exhibit these effects, Al needs to be contained 0.010% or more. However, if Al is contained excessively, weldability is impaired, so the content is limited to 0.10%. In addition, 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%]
Cu dissolves in a corrosive environment and then forms a dense film (film formed by precipitation) on the steel material surface together with the dissolved S and Cr to reduce the corrosion reaction, and is necessary for improving corrosion resistance. It is an element. In order to exert such an effect, it is necessary to contain 0.10% or more of Cu. However, if Cu is excessively contained, the weldability and hot workability of the steel material are deteriorated, so that it is necessary to be up to 1.0%. In addition, 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%, It is good to set it as 0.90% or less more preferably.

[Cr: 0.01 to 0.3%]
Cr dissolves in a corrosive environment and then forms a dense film (film formed by precipitation) on the steel material surface together with the dissolved S and Cu to reduce the corrosion reaction and is necessary for improving corrosion resistance. It is an element. In order to exhibit such an effect, it is necessary to contain Cr 0.01% or more. However, if Cr is excessively contained, in addition to deteriorating the corrosion resistance due to a decrease in pH at the corrosion tip, weldability and hot workability are also degraded. is there. In addition, the minimum with preferable Cr content is 0.02%, More preferably, it is 0.03% or more. Moreover, the upper limit with preferable Cr content is 0.28%, It is good to set it as 0.26% or less more preferably.

[N: 0.0030 to 0.010%]
The content of N needs to be adjusted in order to stably produce a composite sulfide containing Cu and Cr (a film formed by precipitation). N is thought to act catalytically on the formation of complex sulfides containing Cu and Cr. In order to exert such an effect, N needs to be contained by 0.0030% or more. However, when N is contained excessively, the solute N increases, which adversely affects the ductility and toughness of the steel material. Therefore, the upper limit thereof needs to be 0.010%. In addition, the minimum with preferable N content is 0.0033%, More preferably, it is 0.0035% or more. Moreover, 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 components in the marine steel of the present invention are as described above, and the balance consists of iron and inevitable impurities. Examples of inevitable impurities include O and H, and these elements may be contained to the extent that they do not impair the properties of the steel material. However, these inevitable impurities are preferably suppressed to a total of 0.1% or less (more preferably 0.09% or less), so that the effect of developing corrosion resistance in the present invention can be maximized.

  In addition to the above components, the marine steel of the present invention may further include (a) Co: 2.0% or less (excluding 0%) and / or Ni: 2.0% or less (0 %), (B) Mg: 0.005% or less (not including 0%) and / or Ca: 0.005% or less (not including 0%), (c) Ti: 0.1% 1 or more selected from the group consisting of: (not including 0%), Zr: not more than 0.1% (not including 0%), and Hf: not more than 0.1% (not including 0%), (d ) Mo: 0.5% or less (not including 0%) and / or W: 0.5% or less (not including 0%), (e) B: 0.005% or less (not including 0%) V: 0.1% or less (not including 0%) and Nb: 0.1% or less (not including 0%) It is also effective to the characteristics of marine steel according to the components to be contained is further improved.

[Co: 2.0% or less (not including 0%) and / or Ni: 2.0% or less (not including 0%)]
Co and Ni have the effect | action which raises the protection effect by a rust film and improves corrosion resistance. Ni also exhibits the effect of improving the toughness of the steel material. However, since excessive weldability deteriorates weldability and hot workability, the content is preferably 2.0% or less (more preferably 1.9% or less). In order to exert such effects, it is preferable to contain Co or Ni in an amount of 0.01% or more (more preferably 0.02% or more).

[Mg: 0.005% or less (not including 0%) and / or Ca: 0.005% or less (not including 0%)]
Mg and Ca have an effect of increasing pH in rust in a chloride environment or an acid condensation environment, and are effective elements for improving the corrosion resistance by suppressing the cathode reaction. However, since it will degrade workability and weldability if it is contained excessively, it is preferable to make it 0.005% or less respectively. In addition, the preferable minimum for exhibiting such an effect is 0.0003% or more (more preferably 0.0004% or more, and further preferably 0.0005% or more), respectively. Further, the more preferable upper limit is 0.0045% (more preferably 0.004% or less).

[Ti: 0.1% or less (not including 0%), Zr: 0.1% or less (not including 0%), and Hf: 0.1% or less (not including 0%) One or more
Ti, Zr, and Hf have the effect | action which improves the protective effect by the rust film | membrane produced | generated on the steel material surface. However, if it is excessively contained, weldability and hot workability deteriorate, so it is preferable to make each 0.1% or less. In addition, the preferable minimum of 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. It is.

[Mo: 0.5% or less (not including 0%) and / or W: 0.5% or less (not including 0%)]
Mo and W are elements having an action of generating molybdate ions and tungstate ions and improving the corrosion resistance by an inhibitor effect, and the effect is particularly great in a chloride environment (an environment where chloride exists). However, since the weldability and toughness deteriorate when contained in excess, the content of Mo or W is preferably 0.5% or less. The Mo or W content is preferably 0.001% or more. The more preferable lower limit of the content in these elements is 0.003% or more (more preferably 0.005% or more), respectively, and the more preferable upper limit is 0.48% or less (more preferably 0.46% or less). ).

[B: selected from the group consisting of 0.005% or less (not including 0%), V: 0.1% or less (not including 0%), and Nb: 0.1% or less (not including 0%) One or more
B, V and Nb are effective elements for improving the strength. However, since an excessive content deteriorates the toughness of the steel material, it is preferable that the B content is 0.005% or less and the V or Nb content is 0.1% or less. The B, V or Nb content is preferably 0.0001% or more for B and 0.001% or more for V or Nb. The more preferable lower limit of the content in these elements is 0.0002% (more preferably 0.0003% or more) for B, and 0.002% (more preferably 0.003% or more) for V or Nb, respectively. Further, more preferable upper limit is 0.0045% (more preferably 0.004% or less) for B, and 0.095% (more preferably 0.09% or less) for V or Nb, respectively.

  The marine steel material of the present invention can be produced, for example, by the following method. First, secondary refining including component adjustment and temperature adjustment is performed on the molten steel discharged from the converter or electric furnace to the ladle using a vacuum circulation degassing device (RH device). Then, it is made into a steel ingot by a normal casting method such as a continuous casting method or an ingot-making method. As a deoxidation type at this time, it is preferable to use killed steel from the viewpoint of mechanical properties and weldability, and Al killed steel is more preferable.

  Next, after heating the obtained steel ingot to 1000-1300 degreeC temperature range, it is preferable to perform hot rolling and to make a desired shape. By controlling the hot rolling end temperature at this time to 650 to 850 ° C., and controlling the cooling rate from the end of hot rolling to 500 ° C. in the range of 0.1 to 15 ° C./second, the predetermined strength characteristics Is obtained.

  The marine steel material of the present invention basically exhibits excellent corrosion resistance without requiring anticorrosion means such as painting or electrocorrosion. However, depending on necessity, the tar epoxy resin paint or It can also be used in combination with other anticorrosion methods such as heavy duty anticorrosion coating, zinc rich paint, shop primer and the like. Further, in the marine steel material, the chemical component composition is appropriately adjusted, so that the weldability and hot workability are also equal to or higher than those of ordinary marine steel.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steel materials (test Nos. 1 to 55) having the chemical composition shown in Tables 1 and 2 were melted in an electric furnace to obtain a 40 kg steel ingot. The obtained steel ingot was heated to 1150 ° C. and then hot-rolled to obtain a steel material having a plate thickness of 10 mm. At this time, the hot rolling end temperature was in the range of 650 to 850 ° C., and the cooling rate from the end of hot rolling to 500 ° C. was appropriately adjusted in the range of 0.1 to 15 ° C./second. All the test pieces (TP) subjected to the following tests were finally 50 × 30 × 4 (mm) and cut out from the steel material.

  For the simulation test of the environment in the ballast tank, a paint specimen with cut scratches was used. The cut scratched coating test piece was produced by the following procedure. First, the entire test surface (painted surface) was shot blasted, washed with acetone and dried, and an inorganic zinc rich primer was applied to a thickness of 15 μm. Thereafter, the modified epoxy resin paint was applied by air spray at a thickness of 300 μm and dried. In order to prevent corrosion on the surfaces other than the test surface, a silicon sealant was applied on the surfaces other than the painted surface and coated. After the coating film and the silicone sealant were dried, one cut wound reaching a substrate having a length of 100 mm and a width of 0.5 mm was formed on the test surface side with a cutter knife. The appearance shape of the coating test piece is shown in FIG.

  Bare (unpainted) test pieces (bare test pieces) were used for the environmental simulation test of the gas phase part of the tanker crude oil tank. This bare test piece was polished on the entire surface up to SiC # 600 with a wet rotary polishing machine, washed with water and acetone, dried and then used for the test.

  About the test material of each chemical composition shown in the said Table 1, 2, it used for the corrosion test using various test pieces (a coating test piece and a bare test piece). The corrosion test method at this time is as follows.

[Corrosion test method]
Corrosion tests were performed simulating the environment in the ballast tank and the gas phase environment of the tanker crude oil tank. As a simulation test of the environment in the ballast tank, a combined cycle test in which a one-day artificial seawater immersion test and a six-day wet and dry cycle test were repeated using a coated test piece with cut scratches (corrosion test A). In the one-day artificial seawater immersion test, the cut specimen with cut scratches was immersed in artificial seawater maintained at 30 ° C. for one day. In the subsequent wet / dry cycle test, temperature: 50 ° C., humidity: 40% RH (25 ° C.) held for 5 hours (first step), temperature: 30 ° C., humidity: 95% RH (25 ° C.) 5 The time retention (second step) was repeated (the transition time to each step was 1 hour each). The test period was 3 months.

  In this corrosion test, the test numbers shown in Tables 1 and 2 were used. Three test pieces of steel materials 1 to 55 were prepared for each three pieces, and after the test, the coating film swelling width of the coating film scratched part was measured in the manner shown in FIG. 3 (coating film swelling width measuring method). The distance from the cut flaw portion of the “top portion” was measured), and the largest of the three test specimens that were tested was defined as the maximum swelling width.

The environmental test of the gas phase part of the tanker crude oil tank was conducted using a gas containing 0.1 vol% H ₂ S, 0.01 vol% SO ₂ , 10 vol% CO ₂ , and 5 vol% O ₂ as the simulated gas in the tank (remainder: N ₂ ) Was used to conduct a gas corrosion test to evaluate the corrosion resistance (corrosion test B). The corrosion test apparatus used at this time is schematically shown in FIG. Moreover, the gas phase part assumed in this corrosion test is schematically shown in FIG. 5 (FIG. 5 is an explanatory diagram schematically showing the cross section of the tanker).

  The simulated gas was passed through distilled water maintained at 40 ° C. and introduced into a test tank in which a bare test piece (TP) was installed at a flow rate of 50 mL / min (see FIG. 4). The test tank was provided with a temperature cycle that repeated holding at 50 ° C. for 20 hours and holding at 30 ° C. for 2 hours (transition time was 1 hour each). In this temperature cycle, the test piece was dried at 50 ° C. and wet at 30 ° C., and drying and wetting were repeated. The test period was 3 months. In this corrosion test, the test numbers shown in Tables 1 and 2 were used. Three test pieces of 1 to 55 steel materials were prepared, and the corrosion amount was calculated as an average value of the three test pieces. The amount of corrosion was the change in mass before and after the test. The mass after the test was measured after removing the corrosion products by cathodic electrolysis in a 10% aqueous solution of ammonium hydrogen citrate at room temperature (25 ° C.), washing with water and acetone, and drying.

[Test results]
The measurement results of the maximum swollen width of the coating film by the corrosion test A and the corrosion amount by the corrosion test B are as shown in Tables 3 and 4 below. It represents with the relative value when 1 steel material is set to 100. As a comprehensive evaluation, the maximum swollen width (100 for the test No. 1) of the coating film by the corrosion test A is 75 or less and the corrosion amount (100 for the test No. 1) is 50 or less. “Rank 2”, the maximum swollen width of 75 or less and the corrosion amount of 40 or less are “rank 3”, both are 40 or less “rank 4”, and both are 30 or less “ "Rank 5" is displayed. In addition, the case where the maximum swelling width exceeds 75 and the corrosion amount exceeds 50 is indicated as “rank 1”.

  From these results, it can be considered as follows. Test No. Samples 1 to 12 are comparative examples in which the requirements (chemical component composition or ratio [S] / [N]) specified in the present invention are removed. The maximum swollen width of the coating film exceeds 75, and the amount of corrosion The relative value of also exceeds 50, and the corrosion resistance is not sufficient (rank 1). Of these, Test No. In the case of No. 2, the P content was insufficient, so that phosphate was not sufficiently produced in a corrosive environment, and Cu and Cr sulfides were not stably produced, resulting in insufficient corrosion resistance. it is conceivable that.

  Test No. In the case of 3-6, since the S content is too small, the amount of composite sulfides containing Cu and Cr is reduced, and it is considered that the corrosion inhibiting effect is insufficient. Test No. 7 and test no. In No. 8, since the content of Cu or Cr is too small, the amount of composite sulfide containing Cu and Cr is reduced, and it is considered that the corrosion inhibiting effect is insufficient.

  Test No. In No. 9, the N content is too small, so the catalytic action for the formation of sulfides of Cu or Cr is not sufficient, and the amount of precipitate film formation is too small, and the corrosion inhibition effect is insufficient. Conceivable. Test No. In the case of 10 to 12, the ratio ([S] / [N]) is out of the range specified in the present invention, so that the amount of the composite sulfide precipitation film containing Cu and Cr is too small, and the corrosion resistance improving effect is obtained. It is thought that it was not obtained.

  On the other hand, Test No. satisfying the requirements defined in the present invention. In each of the 13 to 55 steel materials, the maximum swollen width of the coating film is 75 or less, and the corrosion amount is 50 or less, which results in excellent corrosion resistance. These corrosion resistances include Cu and Cr obtained by appropriately controlling the contents of P, S, Cu, Cr and N and controlling the ratio ([S] / [N]) to an appropriate range. It is manifested by the anticorrosive action of the sulfide precipitate film, and it is clear that the addition of elements such as Co, Ni, Mg, and Ca is effective for inhibiting corrosion.

  Steel materials that exhibit excellent corrosion resistance as described above have excellent corrosion resistance in areas where electrocorrosion is insufficient, such as the wet environment in ballast tanks and the environment in the gas phase of tanker crude oil tanks. It is extremely useful as a structural member.

Claims (9)

  C: 0.04 to 0.30% (meaning of mass%, the same shall apply hereinafter), Si: 0.05 to 1.0%, Mn: 0.1 to 2.0%, P: 0.010 to 0. 040%, S: 0.011-0.025%, Al: 0.010-0.10%, Cu: 0.10-1.0%, Cr: 0.01-0.3%, and N : 0.0030 to 0.010% each, the balance being iron and inevitable impurities, and the ratio of S content [S] to N content [N] ([S] / [N] ) Is 1.50 to 6.0, a marine steel material excellent in corrosion resistance.   The marine steel material according to claim 1, further comprising Co: 2.0% or less (excluding 0%) and / or Ni: 2.0% or less (not including 0%).   The marine steel material according to claim 1 or 2, further comprising Mg: 0.005% or less (excluding 0%) and / or Ca: 0.005% or less (not including 0%). .   Further, Ti: 0.1% or less (excluding 0%), Zr: 0.1% or less (not including 0%), and Hf: 0.1% or less (not including 0%) The marine steel material according to any one of claims 1 to 3, wherein the marine steel material contains one or more selected.   Furthermore, Mo: 0.5% or less (not including 0%) and / or W: 0.5% or less (not including 0%) are contained. Marine steel.   Furthermore, from the group consisting of B: 0.005% or less (not including 0%), V: 0.1% or less (not including 0%), and Nb: 0.1% or less (not including 0%) The steel material for ships according to any one of claims 1 to 5, which contains at least one selected from the above.   The ship tank comprised using the steel materials in any one of Claims 1-6.   The tanker crude oil tank upper deck comprised using the steel materials in any one of Claims 1-6.   A tanker crude oil tank gas phase part constituted using the steel materials according to any one of claims 1 to 6.

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