JP2006009129A - Steel for vessel having excellent corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for shipbuilding having excellent corrosion resistance which can be put to practical use even without applying coating and electrical protection, particularly steel for shipbuilding in which the improvement of durability to crevice corrosion can be attained, and which exhibits excellent durability even to corrosion owing to the sticking of salt caused by seawater and owing to a wetting environment, and further can exhibit excellent corrosion resistance even when applied to a petroleum based liquid fuel tank. <P>SOLUTION: The corrosion resistant steel for shipbuilding has a composition comprising 0.01 to 0.30% C, 0.01 to 2.0% Si, 0.01 to 2.0% Mn and 0.005 to 0.10% Al, and further comprising 0.005 to 0.50% Se, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2006,JPO&NCIPI

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, etc., particularly in environments exposed to salt and constant temperature and humidity due to seawater. The present invention relates to a marine steel having excellent corrosion resistance and excellent corrosion resistance required as a material for petroleum liquid fuel tanks.

  Steel materials used as main structural materials (for example, outer plates, ballast tanks, crude oil tanks, etc.) in the above-mentioned various vessels are often corroded because they are exposed to seawater salt and constant 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.

  By the way, corrosion in crude oil tanker tanks (petroleum liquid fuel tanks) proceeds remarkably at the defective part of the oil coat formed on the surface of the steel plate, and this defective part is caused by the movement of crude oil and deformation of the hull during operation. Therefore, it is considered that it is repaired or newly formed. For this reason, a corrosion location does not concentrate on one certain location, but generate | occur | produces over the substantially whole surface of steel materials. Therefore, the steel material used as the raw material for the petroleum-based liquid fuel tank is required to have good corrosion resistance in a special corrosive environment in which local corrosion progresses to the entire surface. Also, in such a petroleum-based liquid fuel tank, the above-mentioned “crevice corrosion” occurs remarkably and the life of the tank may be shortened, so that it is required to have excellent crevice corrosion resistance.

As a material for the above-described petroleum-based liquid fuel tank, for example, a technique as disclosed in Patent Document 3 has been proposed as a material having improved corrosion resistance. In this technique, corrosion-resistant steel useful as a material for a tank that stores liquid fuel has been proposed by appropriately adjusting the chemical composition. In this technique, local corrosion such as “crevice corrosion” is considered in addition to overall corrosion, and it can be said that the corrosion resistance is improved. However, these steel materials cannot be said to have sufficient corrosion resistance to withstand recent demands.
JP, 2001-17381, A Claims etc. JP, 2002-26605, A Claims etc. JP, 2001-214236, A Claims etc.

  The present invention has been made by paying attention to the above-mentioned circumstances, and its purpose is to provide a shipbuilding steel excellent in corrosion resistance that can be put into practical use without being subjected to painting or anticorrosion, particularly durability against crevice corrosion. Shipbuilding capable of improving, exhibiting excellent durability against salt adhesion caused by seawater and corrosion due to wet environment, and also exhibiting excellent corrosion resistance when applied to petroleum liquid fuel tanks It is to provide steel for construction.

  The steel for shipbuilding of the present invention that has achieved the above object is C: 0.01 to 0.30% (meaning of mass%, the same shall apply hereinafter), Si: 0.01 to 2.0%, Mn: In addition to containing 0.01 to 2.0%, Al: 0.005 to 0.10%, and Se: 0.005 to 0.50%, the balance being Fe and inevitable impurities. It has a gist.

  Further, in the steel material for shipbuilding of the present invention, (1) Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, Co: 0.01 to 5.0%, Ni if necessary. : One or more selected from the group consisting of 0.01 to 5.0% and Ti: 0.005 to 0.20%, (2) La: 0.0005 to 0.15%, Ce: 0.0005 One or more selected from the group consisting of 0.15%, Ca: 0.0005 to 0.015% and Mg: 0.0005 to 0.015%, (3) Mo: 0.01 to 5.0%, (4) Sb: 0.01 to 0.5%, As: 0.01 to 0.5%, Sn: 0.01 to 0.5%, Bi: 0.01 to 0.5%, Te: 0 .01-0.5%, one or more selected from the group consisting of: (5) B: 0.0001-0.010%, V: 0.01-0.50% and Nb At least one member selected from the group consisting of from 0.003 to 0.50%, and the like is also effective to contain, so that the characteristics of shipbuilding steel materials according to the type of components to be contained are further improved.

  Even when the steel material for shipbuilding of the present invention is used as a material for a petroleum-based liquid fuel tank, it exhibits excellent corrosion resistance in a corrosive environment.

  In the steel for shipbuilding of the present invention, a steel for shipbuilding having excellent corrosion resistance that can be put into practical use without coating and cathodic protection is realized by containing a predetermined amount of Se and appropriately adjusting the chemical composition. In particular, it is possible to improve the durability against crevice corrosion, and to realize a steel material for shipbuilding that exhibits excellent durability against adhesion of salt caused by seawater and corrosion caused by a wet environment. Even when used as a raw material, excellent corrosion resistance is exhibited even in the corrosive environment. Such steel materials for ships are useful not only as outer plates in ships such as crude oil tankers, cargo ships, cargo passenger ships, passenger ships, warships, but also as materials for ballast tanks, crude oil tanks, and the like.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, it has been found that if a predetermined amount of Se is contained and the chemical composition is appropriately adjusted, a steel material for shipbuilding capable of solving the above-described problems can be realized, and the present invention has been completed.

  In the steel material of the present invention, it is important to contain a predetermined amount of Se, and each effect of this component will be described later. The reason why the corrosion resistance is improved by containing Se can be considered as follows. did it.

  Se exerts the action of suppressing the pH drop at the site where the dissolution reaction of corrosion occurs to suppress the corrosion reaction and improve 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. Simply, when the generated rust is densified and stabilized, the overall corrosion resistance is increased, but on the other hand, the local corrosion is increased and the pH is lowered at the corrosion starting point, and the corrosion tendency of the local pH lowered part ( There is a tendency for local corrosion) to increase. In contrast to such a tendency, by containing Se, Se tends to concentrate in a defective portion of rust that tends to be a starting point of local corrosion. It will be big. For these reasons, the inclusion of Se improves corrosion uniformity and local corrosion resistance. However, these functions and effects include Cu and Ni, which are contained if necessary and have the function of densifying and stabilizing the generated rust. , Ti can be improved dramatically by coexisting with Ti.

  In the steel material of the present invention, basic components such as C, Si, Mn, and Al need to be appropriately adjusted in order to satisfy the basic characteristics as the steel material. The reasons for limiting the ranges of these components will be described below together with the effects of Se.

C: 0.01 to 0.30%
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 0.01% or more. However, if the content exceeds 0.30%, the toughness deteriorates. For these reasons, the C content range was set to 0.01 to 0.30%. In addition, the minimum with preferable C content is 0.02%, More preferably, it is good to set it as 0.04% or more. Moreover, the upper limit with preferable C content is 0.28%, More preferably, it is good to set it as 0.26% or less.

Si: 0.01 to 2.0%
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 0.01%. However, if the content exceeds 2.0%, the weldability deteriorates. In addition, the minimum with preferable Si content is 0.02%, More preferably, it is good to set it as 0.05% or more. Moreover, the upper limit with preferable Si content is 1.80%, It is good to set it as 1.60% or less more preferably.

Mn: 0.01 to 2.0%
Mn is also necessary for deoxidation and securing strength in the same manner as Si, and if it is less than 0.01%, the minimum strength 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 0.05%, It is good to set it as 0.10% or more more preferably. Moreover, the upper limit with preferable Mn content is 1.80%, More preferably, it is good to set it as 1.60% or less.

Al: 0.005-0.10%
Al is also necessary for deoxidation and securing of strength in the same manner as Si and Mn, and if less than 0.005%, there is no effect on deoxidation. However, if added over 0.10%, the weldability is impaired, so the range of the amount of Al added is set to 0.005 to 0.10%. In addition, the minimum with preferable Al content is 0.010%, It is good to set it as 0.015% or more more preferably. Moreover, the upper limit with preferable Al content is 0.040%, It is good to set it as 0.050% or less more preferably.

Se: 0.005-0.50%
As described above, Se exerts an action of suppressing corrosion reduction by suppressing the pH drop of the site where the corrosion dissolution reaction occurs to improve 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 needs to be 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%. In addition, the minimum with preferable Se content is 0.006%, More preferably, it is good to set it as 0.008% or more. Moreover, the upper limit with preferable Se content is 0.45%, More preferably, it is good to set it as 0.40% or less.

  The basic components in the marine steel of the present invention are as described above, and the balance is composed of iron and inevitable impurities (for example, P, S, O, etc.), but does not impair the properties of the steel other than these. Some components (eg, 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, the marine steel of the present invention has (1) Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, Co: 0.01 to 5 depending on the necessity in addition to the above components. One or more selected from the group consisting of 0.0%, Ni: 0.01 to 5.0% and Ti: 0.005 to 0.20%, (2) La: 0.0005 to 0.15%, Ce : One or more selected from the group consisting of 0.0005 to 0.15%, Ca: 0.0005 to 0.015% and Mg: 0.0005 to 0.015%, (3) Mo: 0.01 to One or more selected from the group consisting of 5.0%, (4) Sb: 0.01 to 0.5%, As: 0.01 to 0.5%, Sn: 0.01 to 0.5%, Bi: 0.01 to 0.5%, Te: 0.01 to 0.5%, one or more selected from the group consisting of: (5) B: 0.0001 to 0.010 , V: 0.01 to 0.50% and Nb: 0.001 to 0.50% selected from the group consisting of one or more, etc. are also effective, depending on the type of component to be included The characteristics of the steel for shipbuilding will be further improved.

Cu: 0.01-5.0%, Cr: 0.01-5.0%, Co: 0.01-5.0%, Ni: 0.01-5.0% and Ti: 0.005- One or more selected from the group consisting of 0.20% Cu, Cr, Co, Ni and Ti are all effective elements for improving corrosion resistance. Among these, Cu, Cr and Co are effective elements for forming a dense surface rust film that greatly contributes to the improvement of corrosion resistance. Co is an effective element in a high salinity environment. In order to exert the effect of these elements, it is preferable to contain 0.01% or more of all, but if contained excessively, weldability and hot workability deteriorate, so 5.0% or less. It is preferable to do. A more preferable lower limit when Cu, Cr and Co are contained is 0.05%, and a more preferable upper limit is 4.50%.

  Ni is an element effective for stabilizing a dense surface rust film that greatly contributes to the improvement of corrosion resistance. In order to exert such an effect, it is preferably contained in an amount of 0.01% or more. However, if the Ni content is excessive, weldability and hot workability deteriorate, so 5.0% or less is preferable. The more preferable lower limit when Ni is contained is 0.05%, and the more preferable upper limit is 4.50%.

  Ti is an element that densifies the surface rust coating, 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 0.005% or more. However, if it exceeds 0.20%, workability and weldability are deteriorated. . The more preferable lower limit when Ti is contained is 0.008%, and the more preferable upper limit is 0.15%.

1 selected from the group consisting of La: 0.0005 to 0.15%, Ce: 0.0005 to 0.15%, Ca: 0.0005 to 0.015% and Mg: 0.0005 to 0.015% These elements more than seeds have the effect of suppressing pH decrease due to hydrolysis of Fe ions dissolved by corrosion, and promote rust densification by Cu or the like if necessary, and local pH by Se It has the function of further increasing the reduction suppressing effect. Such an effect is effectively exhibited by containing at least 0.0005% of one or more of these elements. However, if La and Ce are contained in excess of 0.15% and Ca and Mg in excess of 0.015%, workability and weldability are deteriorated. In addition, a more preferable lower limit when containing La and Ce is 0.0010%, and a more preferable upper limit is 0.10%. Moreover, a more preferable lower limit when Ca and Mg are contained is 0.0010%, and a more preferable upper limit is 0.010%.

Mo: 0.01-5.0%
Mo has the effect of increasing the uniformity of corrosion and suppressing perforations due to local corrosion. In particular, by containing Cu, Cr, Co, etc. at the same time, a remarkable uniform corrosion improvement effect is exhibited. In order to exhibit these effects, Mo is preferably contained in an amount of 0.01% or more. However, if excessively contained, the weldability is deteriorated, so 5.0% or less is preferable. The more preferable lower limit when Mo is contained is 0.02%, and the more preferable upper limit is 4.50%.

Sb: 0.01 to 0.5%, As: 0.01 to 0.5%, Sn: 0.01 to 0.5%, Bi: 0.01 to 0.5%, Te: 0.01 to One or more elements selected from the group consisting of 0.5% are elements that enhance corrosion resistance by promoting rust densification by Cu or the like and pH lowering action by La or the like. 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 0.5% 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%.

In one or more marine steel materials 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% , depending on the portion to be applied In some cases, higher strength is required, but these elements are 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, excessively soliciting exceeding 0.010% deteriorates the base material toughness, 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 steel material for shipbuilding 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 be used in combination with other anticorrosion methods such as heavy duty anticorrosion coating, zinc rich paint, shop primer, and anticorrosion. 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.

  In addition, the steel material of the present invention can realize a steel material for shipbuilding that exhibits excellent durability against salt adhesion caused by seawater and corrosion due to a moist environment, but when used as a material for petroleum liquid fuel tanks. Even if it exists, it will exhibit excellent corrosion resistance even in the corrosive environment.

  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.

Example 1
Steel materials having the chemical composition shown in the following Tables 1 and 2 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.

  In the steel material of the present invention, anti-corrosion coating may be used in combination, but if the coating is damaged for some reason and the base steel material is exposed, corrosion may be significant in the gap between the coating film and the base steel material. is there. Therefore, in order to verify the effect of improving the corrosion resistance when used together with the anticorrosion coating, a test piece C (FIG. 3) on which the tar epoxy resin coating (undercoat: zinc rich primer) having an average thickness of 250 μm was applied to the entire surface was also used. Then, cut scratches (length: 100 mm, width: about 0.5 mm) reaching the substrate were formed on one side of the test piece C 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]
First, a marine environment was simulated, and a combined cycle corrosion test was performed by sequentially repeating seawater spraying, drying and wetting. In the seawater spray test, the specimen (each test piece A to C) was tilted at an angle of 60 ° from the horizontal, and was placed in a test tank, and 35 ° C artificial seawater (salt water) was sprayed in the form of a mist. Spraying of salt water was continuously performed. At this time, in the test tank, the spray amount was adjusted in advance so that 1.5 ± 0.3 mL of artificial seawater was collected at an arbitrary position per hour on a horizontally installed circular dish having an area of 80 cm ² . In the drying process, the temperature in the test tank was maintained at 50 ° C. and the humidity at 50% RT. In the wetting process, the temperature in the test chamber was maintained at 60 ° C. and the humidity at 98%. Seawater spraying process: 2 hours, drying process: 3 hours, wetting process: 3 hours as one cycle, these were repeated to promote corrosion of the specimen. The total test time was 6 months. In this corrosion test [hereinafter referred to as “corrosion test I”], evaluation was performed using five test pieces A and five test pieces B, respectively.

  By the way, the ballast tank is in a corrosive environment in which it is immersed in seawater when it is in an unloaded state and is seawater-immersed, and is exposed to high temperatures and humidity when crude oil is loaded (without seawater). Further, even near the sea surface of the outer plate, it is protected by electrocorrosion when immersed in seawater. However, when it is exposed to the sea, it is exposed to the wet and corrosive environment of the atmosphere without being acted on by the anticorrosion. In order to simulate such a corrosive environment caused by repeated anti-corrosion in seawater and atmospheric atmosphere, a corrosion test was also conducted by a combined cycle in which cathodic electrolysis and wetting in artificial seawater were repeated.

In electrolysis in artificial seawater, the electrode potential of each test material immersed in artificial seawater at a temperature of 30 ° C. was kept at −800 mV (silver / silver chloride electrode standard) using a potentiostat. At this time, platinum was used for the counter electrode, and a silver / silver chloride electrode was used for the reference electrode. The air atmosphere was maintained in a constant temperature and humidity atmosphere of temperature: 60 ° C. and humidity: 95% RT. The cathodic electrolysis in artificial seawater was repeated for 1 day and the atmospheric atmosphere for 1 day, which was repeated to promote corrosion of the specimen. The total test time was 6 months. In this corrosion test [hereinafter referred to as “corrosion test II”], the test piece A and the test piece C were evaluated using five pieces each.
(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 ratio (bulging area rate) of the coating film swollen area in the surface which formed the cut flaw after a test was measured. The swollen area ratio was determined by a lattice point method (lattice interval 1 mm). That is, an average value of five test pieces was obtained by defining a swelling area ratio by dividing the number of lattice points where swelling was observed by the total number of lattice points. In addition, the swollen width of the coating film in the direction perpendicular to the 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. No. 1 which is a conventional corrosion resistant steel equivalent material (equivalent to JIS SMA490). No. 2 or No. in which the Se content is less than the lower limit specified in the present invention. No. 3 of the conventional steel (C—Si—Mn steel) No. 3. Although the overall corrosion resistance is slightly improved as compared with the one, the other satisfactory levels of corrosion resistance are not achieved.

  On the other hand, those containing Se in an appropriate amount (Nos. 4 to 41) are superior in corrosion resistance to conventional steel (No. 1) due to the effect of improving the corrosion resistance by Se, and particularly excellent in crevice corrosion resistance. The results are shown, and it can be seen that it is preferable as a corrosion-resistant steel for shipbuilding.

  Moreover, it turns out that the corrosion resistance of steel materials is further improving by containing various corrosion resistance improvement elements. In particular, in the case of containing Ca or Mg (No. 10, 13, 31 etc.), further improvement in corrosion uniformity was observed, and the local pH lowering inhibitory action of these elements acted synergistically. it was thought. In addition, in the test materials to which Cu, Cr, Ni or Ti is added, the effect of reducing the maximum swollen width of the painted test materials is recognized (No. 7, 8, 9 etc.), and rust densification of these elements. It is presumed that this acts on the rust stabilization of the cut part and suppresses the progress of corrosion. Furthermore, it turns out that corrosion resistance improves significantly by containing Sb, Sn, etc. (No. 21, 22, etc.).

Example 2
Steel materials having the chemical composition shown in Tables 6 and 7 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 300 × 300 × 25 (mm) (test piece D). The external shape of the test piece D is shown in FIG.

  Further, as shown in FIG. 5, four small test pieces of 60 × 60 × 5 (mm) are brought into contact with a large test piece of 300 × 300 × 25 (mm) (the same as the test piece D), A test piece E in which a clearance was formed was produced. 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 D. Then, a 10 mmφ hole 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 M8 plastic screw.

  Further, a test piece F (FIG. 6) on which the entire surface was coated with a tar epoxy resin having an average thickness of 250 μm (undercoat: zinc rich primer) was also used. Then, in order to investigate the degree of corrosion progression when the base steel material is exposed due to scratches on the anticorrosion coating film, cut scratches (length: 100 mm, width: about 0) reaching the base on one side of the test piece F 0.5 mm) was formed with a cutter knife.

  About the test material of each chemical component composition shown in the said Table 6, 7, the test piece D, the test piece E, and the test piece F were used for the corrosion test using 5 pieces each, respectively. The corrosion test method (actual ship exposure test) at this time is as follows.

[Corrosion test method]
The prepared specimens (each test piece D to F) were attached to the inner bottom plate, wall surface and upper deck of the VLCC crude oil tanker, and after 5 years of normal operation, the corrosion status of each specimen was investigated. . Five test pieces D and E were exposed on the bottom plate and the back of the deck, and five test pieces D and F were exposed on the wall surface.

After exposure for 5 years, the test piece D was subjected to removal of corrosion products such as iron rust by a cathodic electrolysis method in diammonium hydrogen citrate aqueous solution [JIS K8284]. For the test piece E, the small test piece for forming the gap was removed, and the corrosion products were removed by the same method.
(1) For test piece D, the weight change before and after the test is converted to 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 D is obtained by 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.
(2) For specimen E, the maximum crevice corrosion depth D-crev (mm) on the large specimen side was measured using a stylus type three-dimensional shape measuring device.
(3) For test piece F (with cut flaws) that had been subjected to coating treatment, the swollen width (mm) of the coating film perpendicular to the cut flaw was measured with a caliper, and the maximum value of the five test pieces was the maximum swollen width Defined.

  Evaluation criteria for the above general corrosion resistance (average plate reduction: D-ave), corrosion uniformity (D-max / D-ave), crevice corrosion resistance (D-crev), and coating corrosion resistance (maximum swollen width) It is as shown in Table 8 below. The results of the corrosion test are shown in Tables 9 and 10 below.

  These results can be considered as follows. In conventional corrosion resistant steels (Nos. 2 to 4), conventional steel Nos. Although the overall corrosion resistance and the corrosion uniformity are slightly improved as compared with those of No. 1, the crevice corrosion resistance and paint corrosion resistance are at the conventional steel level, which is insufficient as a corrosion resistant steel. In addition, No. containing a small amount of Se. In the case of No. 5, the crevice corrosion resistance is slightly improved, but it can be seen that a sufficient effect is not exhibited because the Se content is less than the lower limit defined in the present invention.

  On the other hand, in the case of containing an appropriate amount of Se (No. 6 to 50), any corrosion resistance is superior to that of the conventional steel (No. 1) due to the effect of improving the corrosion resistance by Se, and particularly excellent in crevice corrosion resistance. The results are shown, and it can be seen that it is preferable as a corrosion-resistant steel for shipbuilding. In particular, the inclusion of various corrosion resistance improving elements (Cu, Cr, Co, Ni, Ti, etc.) greatly improves crevice corrosion resistance and paint corrosion resistance in addition to overall corrosion resistance and corrosion uniformity. I understand that. Such a corrosion resistance improvement effect is presumed that the effect of the above elements acts synergistically on the local pH lowering suppression effect of Se.

  In addition, in the test materials (No. 14, 15, 18 to 27, etc.) containing an appropriate amount of La, Ce, etc., an effect was found to further improve the crevice corrosion resistance of the bottom plate, and an appropriate amount of Ca, Mg was contained. It can be seen that the test materials (No. 16, 17, 19 to 27, etc.) have significantly improved coating corrosion resistance on the wall surface. Such an effect is presumed that La, Ce, Ca, Mg, etc. promoted the rust densification by Cu, Cr, Ni, Ti and promoted the local pH lowering suppressing action by Se.

  Inclusion of an appropriate amount of Mo is also effective for improving the corrosion uniformity of the wall surface (for example, No. 31, 34, 35, etc.), and it is considered that the rust densification action by this element was made uniform. It is done.

  Further, it is clear that the inclusion of Sb, As, Sn, Bi, Te or the like significantly improves the overall corrosion resistance (No. 37 to 40, etc.), and these elements are the above-mentioned elements. This is presumed to be the result of promoting rust densification and pH relaxation.

  As described above, the steel of the present invention exhibits excellent corrosion resistance against local corrosion at oil coating defects, crevice corrosion at structural gaps, or corrosion at coating scratches. It can be suitably used as tank corrosion resistant steel.

3 is an explanatory view showing an external shape of a test piece A used in a corrosion resistance test I of Example 1. FIG. 3 is an explanatory view showing an external shape of a test piece B used in a corrosion resistance test I of Example 1. FIG. 3 is an explanatory view showing an external shape of a test piece C used in a corrosion resistance test II of Example 1. FIG. 6 is an explanatory view showing an external shape of a test piece D used in a corrosion resistance test of Example 2. FIG. It is explanatory drawing which shows the external appearance shape of the test piece E used for the corrosion resistance test of Example 2. FIG. 6 is an explanatory view showing an appearance shape of a test piece F used in a corrosion resistance test of Example 2. FIG.

Claims (7)

  C: 0.01-0.30% (meaning of mass%, the same applies hereinafter), Si: 0.01-2.0%, Mn: 0.01-2.0%, Al: 0.005-0. A marine steel material having excellent corrosion resistance, characterized by containing 10% each, Se: 0.005 to 0.50%, and the balance being Fe and inevitable impurities.   Furthermore, Cu: 0.01-5.0%, Cr: 0.01-5.0%, Co: 0.01-5.0%, Ni: 0.01-5.0% and Ti: 0.0. The marine steel material according to claim 1, comprising at least one selected from the group consisting of 005 to 0.20%.   Further, selected from the group consisting of La: 0.0005 to 0.15%, Ce: 0.0005 to 0.15%, Ca: 0.0005 to 0.015%, and Mg: 0.0005 to 0.015% The marine steel material of Claim 1 or 2 containing 1 or more types.   Furthermore, the marine steel material in any one of Claims 1-3 containing Mo: 0.01-5.0%.   Sb: 0.01 to 0.5%, As: 0.01 to 0.5%, Sn: 0.01 to 0.5%, Bi: 0.01 to 0.5%, Te: 0.01 to The marine steel material according to any one of claims 1 to 4, comprising at least one selected from the group consisting of 0.5%.   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%. 5. The marine steel material according to any one of 5 above.   The marine steel material according to any one of claims 1 to 6, which is used as a material for a petroleum liquid fuel tank.

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