JP4119941B2 - Marine steel with excellent crevice corrosion resistance in humid air - Google Patents

Description

  The present invention relates to marine corrosion resistant steel used as a main structural material in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships and the like, and is particularly excellent in crevice corrosion resistance in a humid atmospheric atmosphere. It relates to marine steel.

  Steel materials used as main structural materials (for example, outer plates, ballast tanks, crude oil tanks, etc.) in the above various ships are often corroded because they are exposed to salt from seawater and high temperature and humidity. Since such corrosion may cause marine accidents such as inundation and sinking, it is necessary to apply some anticorrosion means to the steel. Conventionally, (a) coating, (b) cathodic protection, and the like are well known as anticorrosion means used so far.

  Of these, in coatings represented by heavy coating, there is a high possibility that coating film defects exist, and the coating film may be damaged due to a collision or the like in the manufacturing process, so that the base steel material is often exposed. In such a steel exposed portion, the steel material corrodes locally and intensively, leading to early leakage of the petroleum-based liquid fuel contained therein.

  On the other hand, in the anti-corrosion, it is very effective for the part completely immersed in the seawater. However, in the part that receives the seawater splash in the atmosphere, the electric circuit necessary for the anticorrosion is not formed, and the anticorrosion. The effect may not be fully demonstrated. In addition, when the galvanic anode for anticorrosion disappears due to abnormal consumption or dropping, severe corrosion may immediately proceed.

  In addition to the above technique, for example, a technique as disclosed in Patent Document 1 has been proposed as a means for improving the corrosion resistance of the steel material itself. This technology discloses a corrosion-resistant steel for shipbuilding that has excellent corrosion resistance by appropriately adjusting the chemical composition of the steel material and can be used even without coating. Patent Document 2 discloses a marine steel material having an improved coating film life by making the chemical composition of the steel material appropriate. With these technologies, it can be said that a certain degree of corrosion resistance can be ensured as compared with the prior art.

However, the corrosion resistance in a more severe corrosive environment is still not sufficient, and further improvement in corrosion resistance is required. In particular, corrosion (so-called crevice corrosion) in the “clearance” portion formed at the contact portion between the foreign material and the steel material, the structural reason, or the damaged portion of the anticorrosion coating film, etc., becomes prominent and may reduce the service life. So far, the proposed techniques have insufficient corrosion resistance in these areas.
JP, 2000-17381, A Claims etc. JP, 2002-266052, A Claims etc.

  The present invention has been made paying attention to the circumstances as described above, and its purpose is to provide a marine steel material excellent in corrosion resistance that can be put into practical use without being subjected to painting or anticorrosion, in particular, ballast that does not act on anticorrosion. An object of the present invention is to provide a marine steel material that exhibits excellent durability against crevice corrosion in a humid atmospheric atmosphere such as an upper part of a tank or an upper deck of a crude oil tank.

  The marine steel material of the present invention that has achieved the above object is C: 0.04 to 0.18% (meaning of mass%, the same shall apply hereinafter), Si: 0.10 to 0.50%, Mn: In addition to containing 0.10 to 1.80%, Al: 0.05 to 0.50%, Cu: 0.05 to 4.00%, Cr: 0.05 to 4.50%, Ni: 0 0.05% to 5.0%, Co: 0.01% to 5.0%, and Ti: 0.008% to 0.20% or more selected from the group consisting of Ca: 0.0010-0 .015% and / or Mg: 0.0005 to 0.020%, P: 0.020% or less (including 0%) and S: 0.010% or less (including 0%), respectively The balance is Fe and inevitable impurities, and the ratio of Cr content [Cr] to Al content [Al] ([Cr / [Al]) is one having the gist in that 1 to 50.

  In the marine steel material of the present invention, (1) Se: 0.005-0.50%, (2) Sb: 0.01-0.50% and / or Sn: 0.01-0. 50%, (3) one or more selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50% and Nb: 0.003 to 0.50%, etc. It is also effective to contain it, and the characteristics of the marine steel will be further improved according to the kind of the component to be contained.

  In the steel material for shipbuilding of the present invention, a predetermined amount of Al and Cr are used together and contained, and by appropriately adjusting the chemical composition, it has excellent corrosion resistance that can be put into practical use without coating and cathodic protection. Ship steel can be realized, and in particular, the durability against crevice corrosion is improved, and it is excellent against crevice corrosion in wet air atmosphere such as the upper part of the ballast tank and the upper deck of the crude oil tank where no anticorrosion acts. A marine steel material that demonstrates durability was realized. In addition to the above-mentioned uses, such marine steel materials are useful as materials for outer plates and the like in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships and the like.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, the present inventors have found that a marine steel material capable of solving the above-described problems can be realized if a predetermined amount of Al and Cr are contained in combination and the chemical composition is appropriately adjusted, thereby completing the present invention.

  In the steel material of the present invention, it is important to contain Al and Cr in combination, and the object of the present invention cannot be achieved without any of these components. Although each effect of these components will be described later, the reason why the corrosion resistance is improved by using these components in combination can be considered as follows.

Al has the effect of forming a stable oxide anticorrosion film on the steel surface. Al ³⁺ ions corroded and dissolved in the steel are combined with dissolved oxygen or the like to form Al oxide, which is deposited on the surface to form an anticorrosion film. The anticorrosive effect of this film is not sufficient in a high chloride environment on ships. On the other hand, Cr, like Al, forms a stable oxide film on the surface and exhibits an effect of preventing corrosion of the steel material. However, Cr oxide alone cannot be said to have sufficient corrosion protection effect.

The Al oxide film has a very high stability in a substantially neutral range where the pH is about 5 to 8.5, but the solubility becomes high when the pH exceeds 8.5. Seawater to which marine steel materials are exposed has a pH of about 8 when it is clean, but may be alkalized to a pH of about 9.5 in sea areas where seaweed and the like are breeding. Further, at the site where the cathodic reaction of corrosion occurs, the pH tends to increase due to OH ⁻ ions generated by the reduction of dissolved oxygen. For these reasons, Al oxides in a marine environment cannot always exist stably, but rather are more easily dissolved to obtain their protective properties. In contrast, Cr oxide has high stability in the alkaline region, and also has the effect of lowering the pH by hydrolysis equilibrium of a very small amount of dissolved Cr ions. Inhibiting the dissolution of the resin and exerting the effect of securing its protective property. Therefore, it is considered that the anticorrosive effect of the steel material is synergistically enhanced by the coexistence of Cr oxide and Al oxide in appropriate amounts.

  Such an effect is exhibited by controlling to an appropriate amount described later, but the value of the ratio of these contents ([Cr] / [Al]: mass ratio) is also appropriately controlled. is required. That is, if this value ([Cr] / [Al]) is less than 1, corrosion uniformity tends to be insufficient, and if it exceeds 50, crevice corrosion resistance becomes insufficient. The value of [Cr] / [Al] is more preferably about 10 to 35.

  In the steel material of the present invention, it is necessary to appropriately adjust components such as C, Si, Mn, Cu, Ni, Co, Ti, Ca, Mg, P, and S in order to satisfy the basic characteristics as the steel material. is there. The reasons for limiting the ranges of these components will be described below together with the effects of the above-described Al and Cr elements.

[C: 0.04 to 0.18%]
C is an element necessary for ensuring the strength of the material. In order to obtain the minimum strength as a structural member of a ship, that is, about 400 MPa (depending on the thickness of the steel material used), it is necessary to contain 0.04% or more. However, if the content exceeds 0.18%, the toughness deteriorates. For these reasons, the C content range was 0.04 to 0.18%. In addition, the minimum with preferable C content is 0.08%. Moreover, the upper limit with preferable C content is 0.16%.

[Si: 0.10 to 0.50%]
Si is a necessary element for deoxidation and securing strength, and the minimum strength as a structural member cannot be secured unless it is less than 0.10%. However, if the content exceeds 0.50%, the weldability deteriorates. In addition, the upper limit with preferable Si content is 0.40%, More preferably, it is good to set it as 0.30% or less.

[Mn: 0.10 to 1.80%]
Mn is also necessary for deoxidation and securing strength in the same manner as Si, and if it is less than 0.10%, the minimum strength as a structural member cannot be secured. However, if the content exceeds 1.80%, the toughness deteriorates. In addition, the preferable upper limit of Mn content is 1.60%.

[Al: 0.05 to 0.50%]
As described above, Al has an effect of forming a stable oxide anticorrosive film on the surface. When the Al content decreases, the corrosion-dissolved Al ³⁺ ions are scattered in the seawater and are not deposited on the surface of the steel material, so that an anticorrosion film is not formed. In order to exhibit a sufficient anticorrosion effect in the presence of Cr oxide, Al needs to be contained in an amount of 0.05% or more. In the case of a normal steel material, if the Al content exceeds 0.10%, there is a problem in weldability such as a decrease in the toughness of the welded portion, but C, Si, P like the steel material of the present invention. By setting S and S within the proper range, weldability equivalent to that of conventional steel can be ensured even when the Al content is in the range of more than 0.1% to 0.50%. However, when the Al content exceeds 0.50% and becomes excessive, weldability is impaired. For these reasons, the range of Al content is set to 0.05 to 0.50%. In addition, the minimum with preferable Al content is 0.08%, More preferably, it is good to set it as 0.10% or more. Moreover, the upper limit with preferable Al content is 0.45%, More preferably, it is good to set it as 0.40% or less.

[Cu: 0.05 to 4.00%]
Cu is an element effective for forming a dense surface rust film that greatly contributes to the improvement of corrosion resistance. In addition, the dense rust film formed by containing Cu and the stable oxide anticorrosive film in which Al oxide and Cr oxide coexist synergistically enhance the protection of the base material, and have excellent corrosion resistance. Will be demonstrated. In order to exert such effects, it is necessary to contain 0.05% or more, but if it is contained excessively, weldability and hot workability deteriorate, so it is necessary to make it 4.00% or less. is there.

[Cr: 0.05 to 4.50%]
Cr, like Al, exhibits the effect of forming a stable oxide film on the surface and preventing corrosion of the steel material. In the present invention, as described above, the coexistence of Al oxide and Cr oxide will drastically improve the corrosion resistance of the steel material. It is necessary to contain above. However, if it is contained excessively, weldability deteriorates, so it is necessary to be 4.50% or less.

[2 or more selected from the group consisting of Ni: 0.05 to 5.0%, Co: 0.01 to 5.0% and Ti: 0.008 to 0.20%]
Ni, Co and Ti are all effective elements for improving corrosion resistance. Among these, Ni and Co are effective elements for forming a dense surface rust film that greatly contributes to the improvement of corrosion resistance. In order to exert such an effect, it is necessary to contain 0.05% or more of Ni and 0.01% or more of Co. However, if contained excessively, weldability and hot workability deteriorate. 0.0% or less is necessary. A preferable lower limit when Co is contained is 0.05%, and a preferable upper limit of Ni and Co is 4.50%.

  Ti is an element that densifies the surface rust film, which greatly contributes to the improvement of corrosion resistance, improves its environmental barrier properties, suppresses corrosion inside the crevice, and improves crevice corrosion resistance. In order to ensure the corrosion resistance required in such an environment, it is preferable to contain 0.008% or more, but if it exceeds 0.20%, workability and weldability are deteriorated. . A preferable upper limit of Ti is 0.15%.

[Ca: 0.0010 to 0.015% and / or Mg: 0.0005 to 0.020%]
Ca and Mg are elements that are effective in improving corrosion resistance by exhibiting a pH raising action when dissolved, suppressing a pH drop due to a hydrolysis reaction at a local anode where iron is dissolved, and suppressing a corrosion reaction. Such an effect is effectively exerted by adding 0.0010% or more of Ca and 0.0005% or more of Mg, but it exceeds 0.015% for Ca and exceeds 0.020% for Mg. When it is contained, the workability and weldability are deteriorated. A preferable lower limit of Mg is 0.0010%, and a more preferable upper limit is 0.015%.

[P: 0.020% or less (including 0%)]
P is an element that deteriorates toughness and weldability, and the content is preferably suppressed as much as possible. The allowable upper limit of the P content is 0.020%, and if it exceeds this, weldability as marine steel cannot be ensured. For these reasons, the P content is set to 0.020% or less. In addition, the preferable upper limit of P content is 0.015%.

[S: 0.010% or less (including 0%)]
S, like P, is an element that deteriorates toughness and weldability, and the content is preferably suppressed as much as possible. The allowable upper limit of the S content is 0.010%, and if it exceeds this, the weldability as a marine steel material cannot be ensured. For these reasons, the S content is set to 0.010% or less. In addition, the upper limit with preferable S content is 0.008%.

  The basic components in the marine steel of the present invention are as described above, and the balance is composed of iron and unavoidable impurities (for example, O, etc.). For example, Zr, N, etc.) are acceptable. However, these allowable components should be suppressed to about 0.1% or less because their toughness deteriorates when the amount is excessive.

  Further, in the marine steel material of the present invention, (1) Se: 0.005 to 0.50%, (2) Sb: 0.01 to 0.50% and / or Sn: It is also effective to contain 0.01 to 0.50%, (3) Nb: 0.003 to 0.50%, etc., and the characteristics of marine steel materials are further improved according to the type of components to be contained. Will be.

[Se: 0.005 to 0.50%]
Se is an element that exerts the action of suppressing the corrosion reaction by suppressing the pH drop of the site where the corrosion dissolution reaction occurs, and improving the corrosion resistance. Inclusion of such Se makes it difficult for local pH changes to occur, and thus has an effect of improving corrosion uniformity. In addition, in the “gap portion” where the mass transfer is restricted and the pH is likely to decrease, the effect (local corrosion inhibition effect) is effectively exhibited for the reason described above. In order to ensure the corrosion resistance required in such an environment, the Se content is preferably 0.005% or more. However, if the content exceeds 0.50%, workability and weldability deteriorate. For these reasons, the Se content is set to 0.005 to 0.50%. A more preferable lower limit of the Se content is 0.008%, and more preferably 0.010% or more. Moreover, a preferable upper limit from the Se content is 0.45%, and more preferably 0.40% or less.

[Sb: 0.01 to 0.50% and / or Sn: 0.01 to 0.50%]
Sb and Sn are elements that enhance the corrosion resistance by promoting the effect of densification of rust produced by Cu, Ni, Ti, etc., and the effect of lowering the pH by Se, Ca, Mg, etc. In order to exert such an effect, it is preferable to contain 0.01% or more in any case. However, if excessively contained, workability and weldability deteriorate, so 0.50% or less is preferable. The more preferable lower limit when these elements are contained is 0.02%, and the more preferable upper limit is 0.40%.

[B: one or more selected from the group consisting of 0.0001 to 0.010%, V: 0.01 to 0.50% and Nb: 0.003 to 0.50%]
In marine steel materials, higher strength may be required depending on the site to be applied, but these elements are elements necessary for strength improvement. Of these, B is contained in an amount of 0.0001% or more, which improves the hardenability and is effective in improving the strength. However, if it is contained in excess of 0.010%, the base material toughness deteriorates, which is not preferable. . V is effective for improving the strength by containing 0.01% or more, but if it exceeds 0.50%, it is not preferable because it causes toughness deterioration of the steel material. Nb is effective for improving the strength by containing 0.003% or more, but if it exceeds 0.50% and it is contained excessively, the toughness of the steel will be deteriorated. More preferable lower limits of these elements are 0.0003% for B, 0.02% for V, and 0.005% for Nb. The more preferable upper limit is 0.0090% for B, 0.45% for V, and 0.45% for Nb.

  The marine steel material of the present invention basically exhibits excellent corrosion resistance even if it is not coated, but if necessary, the tar epoxy resin paint shown in the examples below, or other than that It can also be used in combination with other anticorrosion methods such as representative heavy anticorrosion coating, zinc rich paint, shop primer. When such anticorrosion coating is applied, the corrosion resistance of the coating film itself (coating corrosion resistance) is also good as shown in the examples described later.

  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 having chemical composition shown in Tables 1 and 2 below were melted in a converter, and various steel plates were produced by continuous casting and hot rolling. The obtained steel plate was cut and subjected to surface grinding to finally produce a test piece having a size of 100 × 100 × 25 (mm) (test piece A). The external shape of the test piece A is shown in FIG.

  Further, as shown in FIG. 2, four small test pieces of 20 × 20 × 5 (mm) are brought into contact with a large test piece of 100 × 100 × 25 (mm) (the same as the test piece A), A test piece B having a clearance was formed. The small test piece and the large test piece for forming the gap were steel materials having the same chemical composition, and the surface finish was the same as that of the test piece A. Then, a hole of 5 mmφ was formed in the center of the small test piece, and a screw hole was made on the base material side (large test piece side), and fixed with an M4 plastic screw.

  Further, a test piece C (FIG. 3) on which an entire thickness of 250 μm thick tar epoxy resin coating (undercoating: zinc rich primer) was applied was used. Then, in order to investigate the degree of corrosion progress when the base steel material is exposed due to scratches on the anticorrosion coating film, the cut surface reaching the base on one side of the test piece C (length: 100 mm, width: about 0) 0.5 mm) was formed with a cutter knife.

  About the test material of each chemical component composition shown in the said Table 1, 2, the test piece A, the test piece B, and five test pieces C were used for the corrosion test, respectively. The corrosion test method at this time is as follows.

[Corrosion test method]
A corrosion test was conducted in which a moist air atmosphere such as the upper part of the ballast tank where the cathodic protection does not act was simulated and the sea salt particles were adhered and kept in a wet state (corrosion test A). In addition, 7.5 mL of real seawater collected in Kakogawa City, Hyogo Prefecture, was dripped almost uniformly onto the test surface, and the dried test piece was placed in a constant temperature and humidity test tank with a temperature of 50 ° C. and a humidity of 95% RH. Installed horizontally and corroded. The test time was 6 months, and 5.0 mL of actual seawater was added dropwise to the test surface every month. In this test, the test piece A and the test piece B were used to evaluate the general corrosion resistance, the corrosion uniformity, and the crevice corrosion resistance.

In addition, by simulating the corrosive environment of the upper deck in the crude oil tank, a test piece was horizontally installed in a test tank maintained at a temperature of 50 ° C., and the composition was 5 vol% O ₂ -10 vol% CO ₂ -0.01 vol. The test piece was corroded by passing 1 L / min of corrosive gas of% SO ₂ -0.30 vol% H ₂ S (corrosion test B). At this time, the humidity was controlled to 98% RH or higher so that the inside of the test tank was always saturated with water vapor, and the wet state was maintained. The test time is 6 months. In this test, 5.0 mL of actual seawater was additionally dropped on the test surface every month. In this test, the test piece A and the test piece C were used to evaluate the general corrosion resistance, the corrosion uniformity, and the paint corrosion resistance.

(1) For test piece A, the weight change before and after the test is converted into the average thickness reduction D-ave (mm), the average value of the five test pieces is calculated, and the overall corrosivity of each specimen is calculated. Evaluated. Further, the maximum erosion depth D-max (mm) of the test piece A is obtained using a stylus type three-dimensional shape measuring apparatus, and normalized by the average thickness reduction amount [D-ave (mm)] (that is, D-max / D-ave was calculated) and corrosion uniformity was evaluated. In addition, the weight measurement and the plate thickness measurement after the test were performed after removing corrosion products such as iron rust by the cathodic electrolysis method [JIS K8284] in an aqueous solution of diammonium hydrogen citrate.
(2) For test piece B, the crevice portion (contact surface) was visually observed to check for crevice corrosion. If crevice corrosion was observed, the corrosion product was removed by the cathodic electrolysis method, The maximum crevice corrosion depth D-crev (mm) was measured using a stylus type three-dimensional shape measuring apparatus.
(3) About the test piece C (with cut flaws) which performed the coating process, the coating film swollen width of the perpendicular | vertical direction to a cut flaw was measured with calipers, and the maximum value of five test pieces was defined as the maximum swollen width.

  The evaluation criteria for the above general corrosion resistance (D-ave), corrosion uniformity (D-max / D-ave), crevice corrosion resistance (D-crev), and coating corrosion resistance (blowing area ratio and maximum swollen width) are as follows: As shown in Table 3. The corrosion test results are shown in Tables 4 and 5 below.

  These results can be considered as follows. First, in any corrosion test, when the content of Al, Cu and Cr is not satisfied within the appropriate range specified in the present invention (No. 2 to 6), compared with the conventional ordinary steel (No. 1). Overall corrosion resistance is slightly improved. However, no improvement effect is observed with respect to corrosion uniformity and coating corrosion resistance.

  On the other hand, those containing appropriate amounts of Al, Cu and Cr (Nos. 7 to 43) have greatly improved any corrosion resistance due to the synergistic effect due to the addition of these elements. It can be seen that the corrosion resistance and paint corrosion resistance are also improved. It is thought that the protective action of the stable oxide corrosion protective film in which Al oxide and Cr oxide coexist and the dense rust film formed by containing Cu synergistically contribute to such corrosion resistance improvement. It was.

  Among these, in addition to the combined use of Al, Cu and Cr, by further containing an element for improving corrosion resistance such as Ni, Co, Ti, Ca, Mg, etc. (No. 11-15), the overall corrosion resistance of the steel material is greatly increased. It can be seen that it has improved. In particular, inclusion of Ca and Mg has been confirmed to improve corrosion uniformity and crevice corrosion resistance (No. 14 to 15), and local corrosion is prevented by the action of suppressing the local pH drop of these elements. Inferred to have been suppressed.

  Also, by containing Ni, Co or Ti, the effect of improving the coating corrosion resistance is recognized (No. 11 to 13 etc.), and the progress of corrosion at the scratches on the coating film is prevented by the synergistic effect of the rust densification action of these elements. It is inferred that

  Furthermore, it is clear that the corrosion resistance is greatly improved by containing Se (No. 35, 36, etc.), and the effect of suppressing local pH change by Se is improved in corrosion resistance against local corrosion such as crevice corrosion. It was thought that it contributed to. No. As is apparent from the results of 7, 8, 9, 10, etc., it can be seen that by appropriately adjusting the value of ([Cr] / [Al]), various corrosion resistances are greatly improved.

It is explanatory drawing which shows the external appearance shape of the test piece A used for the corrosion resistance test. It is explanatory drawing which shows the external appearance shape of the test piece B used for the corrosion resistance test. It is explanatory drawing which shows the external appearance shape of the test piece C used for the corrosion resistance test.

Claims (4)

C: 0.04 to 0.18% (meaning of mass%, the same applies hereinafter), Si: 0.10 to 0.50%, Mn: 0.10 to 1.80%, Al: 0.05 to 0. 50%, Cu: 0.05 to 4.00%, Cr: 0.05 to 4.50%, Ni: 0.05 to 5.0%, Co: 0.01 to 5.0 % And Ti: two or more selected from the group consisting of 0.008 to 0.20%, Ca: 0.0010 to 0.015% and / or Mg: 0.0005 to 0.020% P: 0.020% or less (including 0%) and S: 0.010% or less (including 0%), respectively, with the balance being Fe and inevitable impurities, and Cr content The ratio of [Cr] to Al content [Al] ([Cr] / [Al]) is 1 to 50. Crevice corrosion resistance excellent marine steel atmosphere.   Furthermore, the steel materials for ships of Claim 1 containing Se: 0.005-0.50%.   The marine steel material according to claim 1 or 2, further comprising Sb: 0.01 to 0.50% and / or Sn: 0.01 to 0.50%.   Furthermore, it contains at least one selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50% and Nb: 0.003 to 0.50%. 4. The marine steel material according to any one of 3 above.

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