CN100494452C - Steel for ships with excellent corrosion resistance - Google Patents

Abstract

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. 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.

Description

Steel for ships with excellent corrosion resistance

technical field

The present invention relates to a corrosion-resistant steel for ships used as a main structural material in ships such as oil tankers, cargo ships, passenger ships, and warships, and more particularly to steels that are excellent in corrosion resistance when exposed to salt in seawater and in environments with constant temperature and humidity, and A steel material for ships that is also excellent in corrosion resistance required as a material for petroleum-based liquid fuel tanks.

Background technique

In each of the above-mentioned ships, steel materials used as main structural materials (for example, outer plates, ballast tanks, oil tanks, etc.) suffer from corrosion damage due to exposure to salt in seawater and constant temperature and humidity. Such corrosion may cause marine accidents such as flooding or sinking, so it is necessary to implement certain anti-corrosion measures for steel materials. Conventionally known anticorrosion means include (a) painting or (b) electrical corrosion protection.

Among them, in the painting represented by multi-pass painting, the possibility of defects in the coating film is high, and the coating film may also be damaged by collisions in the manufacturing process, so that the base steel material is often exposed. In such an exposed portion of the steel material, the steel material is corroded locally or intensively, leading to early leakage of the stored petroleum-based liquid fuel.

In addition, in electrical corrosion protection, it is very effective for parts completely immersed in seawater, but parts that are splashed with seawater in the atmosphere, etc., may not be able to form circuits required for corrosion protection, and the corrosion protection effect cannot be fully exerted. In addition, when the anti-corrosion sacrificial anode is abnormally worn out, falls off, and disappears, accelerated corrosion may start immediately.

In addition to the above-mentioned techniques, for example, the technique of Patent Document 1 has also been proposed as a technique for improving the corrosion resistance of the steel material itself. In this technology, corrosion-resistant steel for shipbuilding is disclosed, which can be used without painting even if its corrosion resistance is improved by appropriately adjusting the chemical composition of the steel material. In addition, Patent Document 2 discloses a steel material for ships in which the lifetime of a coating film is improved by appropriately designing the chemical composition of the steel material. In the above technique, it can be said that a certain level of corrosion resistance can be ensured compared to conventional ones.

However, the corrosion resistance in a more severe corrosion environment cannot be said to be very good yet, and further improvement of the corrosion resistance is required. In particular, the corrosion (that is, crevice corrosion) at the contact part of the foreign matter and the steel material, the "crack" part formed by the structural reason or the damaged part of the anti-corrosion coating, etc. is remarkable, and the service life may be shortened, but it has been proposed in the past In the prior art, the corrosion resistance on such a part is insufficient.

However, the corrosion on the tank body (petroleum-based liquid fuel tank) of an oil tanker spreads remarkably at the defective part of the oil film formed on the surface of the steel plate, and it is considered that the defective part is repaired by movement of crude oil during shipping or deformation of the ship's hull, etc. or reformed. Therefore, the corrosion site does not concentrate in one place, but occurs on almost the entire surface of the steel material. Therefore, steel materials used as base materials for petroleum-based liquid fuel tanks are required to have good corrosion resistance under a special corrosion environment in which localized corrosion spreads across the board. In addition, even in such petroleum-based liquid fuel tanks, the above-mentioned "crevice corrosion" remarkably occurs, reducing the life of the tank body, and therefore, improvement in crevice corrosion resistance is also required.

As a base material of the above-mentioned petroleum liquid fuel tank, for example, the technology of Patent Document 3 has been proposed as a base material for improving the corrosion resistance thereof. In this technique, a corrosion-resistant steel that can be used as a base material for a tank storing liquid fuel has been proposed by appropriately adjusting the chemical composition. In this technique, localized corrosion such as "crevice corrosion" is also taken into consideration in addition to general corrosion, and it can be said that the corrosion resistance has been improved. However, even such steel materials cannot be said to have corrosion resistance that satisfies the level required in recent years.

Patent Document 1: Scope of Protection Claimed in JP-A-2001-17381, etc.

Patent Document 2: Scope of Protection Claimed in JP-A-2002-26605, etc.

Patent Document 3: Scope of Protection Claimed in JP-A-2001-214236, etc.

Contents of the invention

The present invention was made in view of the above facts, and its object is to provide a steel for shipbuilding that can improve the corrosion resistance that can be used without painting or electrical corrosion protection, and specifically to provide a steel for shipbuilding. A steel material that can improve durability against crevice corrosion, and at the same time exhibit excellent durability against corrosion caused by salt adhesion in seawater and a humid environment, and can also exhibit excellent performance when used in petroleum-based liquid fuel tanks corrosion resistance.

The shipbuilding steel material of the present invention capable of achieving the above object is characterized in that, in addition to containing C: 0.01-0.30% (indicated by mass %, the same below), Si: 0.01-2.0%, Mn: 0.01-2.0%, Al: In addition to 0.005-0.10%, it contains Se: 0.005-0.50%, and the balance is composed of Fe and unavoidable impurities.

In addition, in the steel material for ships of the present invention, if necessary, (1) Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, Co: 0.01 to 5.0%, Ni: 0.01 to 5.0%, and Ti: One or more selected from the group consisting of 0.005 to 0.20%, (2) 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 0.15% More than one selected, (3) Mo: 0.01-5.0%, (4) from Sb: 0.01-0.5%, As: 0.01-0.5%, Sn: 0.01-0.5%, Bi: 0.01-0.5%, Te : one or more selected from the group consisting of 0.01 to 0.5%, (5) 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%, (6) Zn: 0.001 to 0.10%, etc. are also effective, and can further improve the properties of shipbuilding steel materials depending on the type of components contained.

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

In the steel for shipbuilding of the present invention, by containing a predetermined amount of Se and adjusting the chemical composition appropriately, even without painting or electrical corrosion protection, it is possible to realize a steel for shipbuilding with improved corrosion resistance that can be used practically. Specifically It is possible to realize a steel for shipbuilding that can improve the durability against crevice corrosion, and at the same time exhibit excellent durability against corrosion caused by salt adhesion in seawater and a humid environment, and can be used in petroleum liquid fuel tanks Even in this corrosive environment, it can exhibit excellent corrosion resistance when used as a base material. Such steel materials for ships are used not only as outer plates in ships such as oil tankers, cargo ships, passenger ships, and warships, but also as base materials for ballast tanks, oil tanks, and the like.

Description of drawings

FIG. 1 is an explanatory view showing the external appearance of a test piece A used in a corrosion resistance test I of Example 1. FIG.

FIG. 2 is an explanatory view showing the external appearance of a test piece B used in the corrosion resistance test I of Example 1. FIG.

FIG. 3 is an explanatory view showing the appearance of a test piece C used in the corrosion resistance test II of Example 1. FIG.

FIG. 4 is an explanatory view showing the external appearance of a test piece D used in a corrosion resistance test of Example 2. FIG.

FIG. 5 is an explanatory view showing the external appearance of the test piece E used in the corrosion resistance test of Example 2. FIG.

FIG. 6 is an explanatory view showing the external appearance of the test piece F used in the corrosion resistance test of Example 2. FIG.

FIG. 7 is an explanatory view showing the external appearance of the test piece G used in the corrosion resistance test of Example 3. FIG.

FIG. 8 is an explanatory view showing the external appearance of the test piece H used in the corrosion resistance test of Example 3. FIG.

FIG. 9 is an explanatory view showing the external appearance of the test piece I used in the corrosion resistance test of Example 3. FIG.

Detailed ways

The inventors of the present invention conducted intensive studies to solve the above problems. As a result, they found that a steel material for shipbuilding capable of solving the above-mentioned problems can be realized by appropriately adjusting the chemical composition while containing a predetermined amount of Se, and completed the present invention.

In the steel material of the present invention, it is important to contain a predetermined amount of Se, and various functions and effects of this component will be described later, but the reason why the corrosion resistance is improved by containing Se is considered to be as follows.

Se is an element that suppresses the lowering of the pH at the site of the dissolution reaction that causes corrosion, suppresses the corrosion reaction, and improves the corrosion resistance. By containing such Se, since it is difficult to cause a local pH change, it has the effect of improving the uniformity of corrosion. On the one hand, when the generated rust is densified and stabilized, the general corrosion resistance is improved, but on the other hand, the local corrosion is high, and the pH is lowered at the corrosion starting point, and there is a tendency to corrode at the local pH lowered part ( localized corrosion) tends to increase. Regarding such a tendency, it is considered that by containing Se, the Se tends to concentrate on rust defect parts that are likely to be the starting point of localized corrosion, and therefore has a large effect of suppressing the pH drop for such a local pH drop. For this reason, the uniformity of corrosion and the localized corrosion resistance are improved by containing Se, but such effects are achieved by combining Cu, Ni, and Ti, which are contained as necessary and have the functions of densifying and stabilizing rust formation. Coexistence can improve by leaps and bounds.

Also in the steel material of the present invention, in order to satisfy the basic properties of the steel material, it is necessary to appropriately adjust basic components such as C, Si, Mn, and Al. The reasons for limiting the ranges of these components will be described below together with the effect of Se described above.

C: 0.01 to 0.30%

C, is an element required to ensure the strength of the material. In order to obtain the minimum strength as a structural part of a ship, which is generally about 400 MPa (but it also depends on the thickness of the steel used), it needs to contain more than 0.01%. However, if it is contained in excess of 0.30%, the toughness will deteriorate. Therefore, the range of C content is specified at 0.01 to 0.30%. In addition, the lower limit of the C content is preferably 0.02%, more preferably 0.04% or more. In addition, the preferable upper limit of the C content is 0.28%, more preferably 0.26% or less.

Si: 0.01 to 2.0%

Si is an element necessary for deoxidation and securing strength, and if it is less than 0.01%, the minimum strength as a structural member cannot be secured. However, if it is contained excessively exceeding 2.0%, weldability will deteriorate. In addition, the lower limit of the Si content is preferably 0.02%, more preferably 0.05% or more. In addition, the preferable upper limit of the Si content is 1.80%, more preferably 1.60% or less.

Mn: 0.01 to 2.0%

Like Si, Mn is an element necessary for deoxidation and securing strength, and if it is less than 0.01%, the minimum strength as a structural member cannot be secured. However, if it is contained in excess of 2.0%, the toughness will deteriorate. In addition, the preferable lower limit of the Mn content is 0.05%, more preferably 0.10% or more. In addition, the preferable upper limit of the Mn content is 1.80%, more preferably 1.60% or less.

Al: 0.005～0.10%

Al, like Mn and Si, is an element necessary for deoxidation and securing strength, and if it is less than 0.005%, deoxidation has no effect. However, if it is added in excess of 0.10%, weldability will be impaired, so the range of the amount of Al added is defined as 0.005 to 0.10%. In addition, the lower limit of the Al content is preferably 0.010%, more preferably 0.015% or more. In addition, the preferable upper limit of the Al content is 0.040%, more preferably 0.050% or less.

Se: 0.005～0.50%

As described above, Se has the effect of suppressing the drop in pH at the point of the dissolution reaction that causes corrosion, suppressing the corrosion reaction, and improving the corrosion resistance. By containing such Se, since it is difficult to cause a local pH change, it has the effect of improving the uniformity of corrosion. In addition, the effect (local corrosion inhibitory effect) is effectively exerted on the (cracks) where the movement of substances is restricted and where a local pH drop is likely to occur, for the above-mentioned reason. In order to secure the corrosion resistance required under such an environment, the Se content needs to be set at 0.005% or more. However, if it is contained in excess of 0.50%, workability and weldability will deteriorate. Therefore, the Se content is specified at 0.005 to 0.50%. In addition, the preferred lower limit of the Se content is 0.006%, more preferably 0.008% or more. In addition, the preferable upper limit of the Se content is 0.45%, more preferably 0.40% or less.

The basic components of the steel for ships of the present invention are as described above, and the balance is composed of iron and unavoidable impurities (such as P, S, O, etc.), but other than these, components to the extent that they do not hinder the properties of the steel (such as , Zr, N, etc.). However, if the content of these allowable components is too large, the toughness will deteriorate, so the level should be controlled to 0.1% or less.

In addition, in addition to the above components, the steel material for ships of the present invention contains (1) Cu: 0.01 to 5.0%, Cr: 0.01 to 5.0%, Co: 0.01 to 5.0%, Ni: 0.01 to One or more selected from the group consisting of 5.0% and Ti: 0.005 to 0.20%, (2) 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% One or more selected from the group consisting of (3) Mo: 0.01-5.0%, (4) Sb: 0.01-0.5%, As: 0.01-0.5%, Sn: 0.01-0.5%, Bi: 0.01 One or more selected from the group consisting of ～0.5%, Te: 0.01～0.5%, (5) selected from the group consisting of B: 0.0001～0.010%, V: 0.01～0.5%, and Nb: 0.003～0.50% One or more kinds of (6) Zn: 0.001 to 0.10%, etc. are also effective, and depending on the type of the contained components, the properties of the shipbuilding steel can be further improved.

From Cu: 0.01～5.0%, Cr: 0.01～5.0%, Co: 0.01～5.0%, Ni: 0.01～ One or more selected from the group consisting of 5.0% and Ti: 0.005 to 0.20%

Cu, Cr, Co, Ni, and Ti are all effective elements for improving corrosion resistance. Among them, Cu, Cr, and Co are effective elements for forming a dense surface rust film that greatly contributes to the improvement of corrosion resistance. In addition, Co is an effective element in a high-salt environment. In order to exhibit the effects of these elements, all of them are preferably contained at 0.01% or more, but if contained in excess, weldability or hot workability will deteriorate, so it is preferably specified at 5.0% or less. When Cu, Cr, and Co are contained, the more preferable lower limit is 0.05%, and the more preferable upper limit is 4.50%.

Ni is an effective element 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 preferable to contain 0.01% or more. However, if the Ni content is excessive, the weldability or hot workability will be deteriorated, so it is preferable to make it 5.0% or less. When Ni is contained, the more preferable lower limit is 0.05%, and the more preferable upper limit is 4.50%.

Ti is an element that densifies the rust coating on the surface, which greatly contributes to the improvement of corrosion resistance, improves its environmental barrier properties, suppresses corrosion inside crevices, and also 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, but if it contains more than 0.20%, workability and weldability will deteriorate. Therefore, when Ti is contained, the more preferable lower limit is 0.008%, and the more preferable upper limit is 0.15%.

From La: 0.0005～0.15%, Ce: 0.0005～0.15%, Ca: 0.0005～0.015% and Mg: 0.0005 to 0.15% of one or more selected from the group

These elements have the function of suppressing the pH drop caused by the hydrolysis of corrosively dissolved Fe ions, and also have the function of promoting the densification of rust formed by Cu, etc. contained as needed, and further enhancing the function of suppressing the local pH drop of Se formation. Such an action can be effectively exhibited by making one or more of these elements contain 0.0005% or more. However, if La and Ce are contained in excess of more than 0.15%, and if Ca and Mg are contained in excess of more than 0.15%, both workability and weldability will deteriorate. In addition, when La and Ce are contained, the more preferable lower limit is 0.0010%, and the more preferable upper limit is 0.10%. In addition, when Ca and Mg are contained, the more preferable lower limit is 0.0010%, and the more preferable upper limit is 0.010%.

Mo: 0.01 to 5.0%

Mo has the effect of improving the uniformity of corrosion and suppressing the perforation formed by localized corrosion. In particular, when it is contained together with Cu, Cr, Co, etc., it exerts the effect of significantly improving the uniform corrosion resistance. In order to exert such an effect, Mo is preferably contained in an amount of 0.01% or more, but if contained in excess, the weldability deteriorates, so it is preferably made 5.0% or less. When these elements are contained, the more preferable lower limit is 0.02%, and the more preferable upper limit is 4.50%.

From Sb: 001～0.5%, As: 0.01～0.5%, Sn: 0.01～0.5%, Bi: 0.01～ One or more selected from the group consisting of 0.5%, Te: 0.01 to 0.5%

These elements are elements that promote the rust densification action by Cu or the like or the pH lowering action by La or the like to improve the corrosion resistance. In order to exert such an effect, it is preferable to contain 0.01% or more of both, but if it is contained in excess, since workability and weldability deteriorate, it is preferable to make it 0.5% or less. When these elements are contained, the more preferable lower limit is 0.02%, and the more preferable upper limit is 0.40%.

Composed of B: 0.0001-0.010%, V: 0.01-0.50%, and Nb: 0.003-0.50% Choose more than one of the groups

In marine steel materials, higher strength is sometimes required depending on the application site, and these elements are necessary to increase strength. Among them, B is effective for improving hardenability and strength by containing 0.0001% or more, but excessive content exceeding 0.010% is not preferable because the toughness of the base material deteriorates. V is effective for improving the strength by containing 0.01% or more, but excessive content exceeding 0.50% is not preferable because it leads to deterioration of the toughness of the steel material. Nb is effective in improving the strength by containing 0.003% or more, but if it is contained in excess of 0.50%, the toughness of the steel material deteriorates. In addition, the more preferable lower limits of these elements are 0.0003% for B, 0.02% for V, and 0.005% for Nb. Further, more preferable upper limits are 0.0090% for B, 0.45% for V, and 0.45% for Nb.

Zn: 0.001～0.10%

Zn has the effect of reacting with salt or sulfur to form a zinc chloride or zinc sulfide precipitation film on the steel surface, separating the steel matrix from the water in the environment, and inhibiting corrosion. Zinc chloride or zinc sulfide is easy to deposit on the surface of steel without splashing in the coating film or cracks where the movement of substances is restricted, so it is particularly effective in inhibiting corrosion under the coating film or cracks.

In order to achieve such an effect and ensure the required corrosion resistance, the Zn content needs to be set at 0.001% or more. However, if it is contained in excess of 0.10%, workability and weldability will deteriorate. Therefore, the Zn content is specified at 0.001 to 0.10%. In addition, the more preferable lower limit of the Zn content is 0.003%, more preferably 0.005% or more. In addition, the preferable upper limit of the Zn content is 0.09%, more preferably 0.08% or less.

When welding the steel material of the present invention to form a welded structure, if the usual welding conditions or welding materials are used, the concentration of the above-mentioned effective elements changes at the welded joint, so corrosion resistance may not be found at the welded part. In particular, in the case where the ratio of the Se content to the deposited metal and the base material (Se content of the deposited metal/Se content of the base material) is less than 0.3, the effect of inhibiting the corrosion reaction that suppresses the formation of a pH drop was not found, and the weld Corrosion resistance of metallized parts is insufficient. Moreover, if this ratio exceeds 3.0, since it will cause the toughness of a welded part to deteriorate, it is unpreferable from the viewpoint of mechanical strength. Therefore, it is recommended to adjust this ratio within the range of 0.3 to 3.0, more preferably within the range of 0.5 to 2.0.

In addition, effective elements for improving corrosion resistance other than Se, namely Cu, Cr, Co, Ni, Ti, La, Ce, Ca, Mg, Mo, Sb, As, Sn, Bi, Te, Zn, are also added in In the case of these elements, it is recommended to adjust the content ratio of the deposited metal to the base material (the content of the deposited metal/the content of the base material) within the range of 0.3 to 3.0, more preferably within the range of 0.5 to 2.0.

The steel for shipbuilding of the present invention basically exhibits the excellent corrosion resistance of the steel body without painting. The representative multiple anti-corrosion paint, zinc-rich paint, shop primer (shopprime), electric anti-corrosion and other anti-corrosion methods are used together. When such an anti-corrosion paint is applied, the corrosion resistance of the paint film itself (paint corrosion resistance) is also good as shown in Examples described later.

In addition, in the steel material of the present invention, it is possible to provide a steel material for shipbuilding that has excellent durability even against corrosion caused by adhesion of salt in seawater and a humid environment, even when used as a base material for a petroleum-based liquid fuel tank , Even in its corrosive environment, it can also exert excellent corrosion resistance.

Hereinafter, the present invention is described in more detail by giving examples, but the present invention is not limited by the following examples, and certainly within the scope of meeting the purpose before and after, it can be implemented with changes, which are all included in the technology of the present invention within range.

[Example]

Example 1

Steel materials with the chemical compositions shown in Tables 1 and 2 below were smelted in a converter, and various steel plates were produced by continuous casting and hot rolling. The obtained steel plate was cut, the surface was ground, and finally a test piece (test piece A) having a size of 100×100×25 (mm) was prepared. FIG. 1 shows the external appearance of the test piece A.

In addition, as shown in Figure 2, four small test pieces of 20×20×5 (mm) are brought into contact with a large test piece of 100×100×25 (mm) (same as the above-mentioned test piece A) to form cracks. Part B of the test piece. The small test piece and the large test piece for crack formation are steel materials with the same chemical composition, and the surface finish is the same as that of the above-mentioned test piece A, which is defined as surface grinding. Then, a hole of Φ5 mm was made in the center of the small test piece, a screw hole was made on the base material side (large test piece side), and it was fixed with M4 plastic bolts.

The steel material of the present invention may be used in combination with an anti-corrosion paint, but if the paint is damaged for some reason and the base steel material is exposed, corrosion may be prominent in the cracks between the coating film and the base steel material. Therefore, in order to verify the corrosion resistance improvement effect of the combined use of anti-corrosion paints, a test piece C (Fig. 3) was also used which fully applied a tar epoxy resin paint (primer: zinc-rich paint) with an average thickness of 250 μm. On one side of the test piece C, a cut (length: 100 mm, width: about 0.5 mm) reaching the substrate was formed with a dicing blade.

For the samples of the respective chemical compositions shown in Tables 1 and 2, five test pieces A, B, and C were used for the corrosion test. The corrosion test method at this time is as follows.

[Corrosion test method]

First, simulate the marine environment, and conduct repeated repeated seawater spray, dry and wet compound cycle corrosion tests. In the seawater spray test, the samples (each test piece A to C) were set in the test tank so as to be inclined at 60° horizontally, and artificial seawater (salt water) at 35° C. was sprayed in mist form. The spraying of salt water is usually carried out continuously. At this time, in the test tank, 1.5 ± 0.3 mL of artificial seawater is collected at any position every hour in a circular dish with an area of 80 cm ² installed horizontally, and the spray volume is adjusted in advance in this way. During the drying process, the temperature in the test tank was kept at 50° C., and the humidity was kept at 50% RT. During the wetting process, the temperature in the test tank was kept at 60° C., and the humidity was kept at 98%. The seawater spray process takes 2 hours as a cycle, the drying process takes 3 hours as a cycle, and the wet process takes 3 hours as a cycle. These cycles are repeated to promote the corrosion of the sample. The total trial duration was set at 6 months. In the corrosion test [hereinafter referred to as "corrosion test I"], evaluation was performed using five test pieces A and B each.

However, in the ballast tank, when seawater is injected at empty load, it is immersed in seawater for electrical corrosion protection, but when crude oil is loaded (without seawater), it is exposed to a corrosive environment under high temperature and humidity. In addition, even in the vicinity of the sea surface of the outer plate, electrical corrosion protection can be used for corrosion protection when immersed in seawater, but in the case of exposure to the sea surface, electrical corrosion protection does not work and is exposed to the humid and corrosive environment of the atmosphere. In this way, in order to simulate the corrosion environment formed by repeating the electric corrosion protection and atmospheric protection atmosphere in seawater, corrosion tests were also carried out by repeating the composite cycle of cathodic electrolysis and wetting in artificial seawater.

In the electrolysis in artificial seawater, the electrode potential of each sample immersed in artificial seawater at a temperature of 30° C. was maintained at −800 mV (based on a silver/silver chloride electrode) using a potentiostat. At this time, platinum was used as the counter electrode, and silver/silver chloride electrode was used as the reference electrode. As the atmospheric protective atmosphere, it is kept under a constant temperature and constant humidity protective atmosphere with a temperature of 60° C. and a humidity of 95% RT. One day of cathodic electrolysis in artificial seawater is regarded as one cycle, and one day of atmospheric protection atmosphere is regarded as one cycle. These cycles are repeated to promote the corrosion of the sample. The total trial duration was set at 6 months. In this corrosion test [hereinafter referred to as "corrosion test II"], five test pieces A and C were respectively used for evaluation.

(1) Regarding the test piece A, the weight change before and after the test was converted into the average plate thickness decrease D-ave (mm), and the average value of five test pieces was calculated, and the overall corrosion resistance of each sample was evaluated. In addition, using a stylus-type 3D shape measuring device, the maximum erosion depth D-max (mm) of the test piece A was obtained, and normalized according to the average plate thickness reduction [D-ave (mm)] (that is, the calculation of D-max/ D-ave), to evaluate the uniformity of corrosion. In addition, the weight measurement and plate thickness measurement after the test were carried out after removing corrosion products such as rust by cathodic electrolysis [JIS K8284] in diammonium hydrogen citrate aqueous solution.

(2) Regarding the test piece B, visually observe the crevice portion (contact surface) to investigate whether crevice corrosion occurs. 3-dimensional shape measurement device to measure the maximum crevice corrosion depth D-crev (mm).

(3) Regarding the test piece C (with a cut) subjected to the painting process, the ratio of the area of the coating film swelled on the surface where the cut was formed after the test (protruded area ratio) was measured. The bulging area ratio was determined by the grid point method (grid interval: 1 mm). That is, the value obtained by dividing the total number of grid points by the number of grid points confirmed to bulge is defined as the bulge area ratio, and the average value of 5 test pieces is obtained. In addition, the area perpendicular to the cut is measured with a caliper. For the bulging width of the coating film, the maximum value of five test pieces was defined as the maximum bulging width.

In the general corrosion resistance (D-ave), corrosion uniformity (D-max/D-ave), crevice corrosion resistance (D-crev), paint corrosion resistance (bulging area ratio and maximum bulging The evaluation criteria of range) are shown in Table 3 below. Tables 4 and 5 below show the corrosion test results.

Based on the above results, it can be considered as follows. In the No. 2 sample of the conventional corrosion-resistant steel (equivalent to JISSMA490), the No. 3 sample with the Se content less than the lower limit specified by the present invention, the same as the conventional steel (C-Si-Mn steel) Compared with the No.1 sample, the overall corrosion resistance is slightly improved, but the other corrosion resistance has not reached a satisfactory level.

On the other hand, it was found that in the samples (No. 4 to 41) containing an appropriate amount of Se, due to the corrosion resistance improvement effect of Se, all corrosion resistances were superior to those of the conventional steel (No. 1). It has excellent crevice corrosion resistance and is preferably used as a corrosion-resistant steel for shipbuilding.

In addition, it has been found that the corrosion resistance of steel materials is further improved by including various corrosion resistance-improving elements. In particular, in the samples containing Ca or Mg (No. 10, 13, 31, etc.), it was found that the uniformity of corrosion was further improved, and it is considered that the inhibitory effect of the local pH drop of these elements works synergistically. In addition, in the samples to which Cu, Cr, Ni, or Ti were added, the effect of reducing the maximum bulging amplitude of the painted samples was found (No. 7, 8, 9, etc.), and it was judged that the rust densification of these elements was the cause. As a result of being used for rust stabilization of the cutting part, and suppressing progress of corrosion. In addition, it was found that the corrosion resistance can be greatly improved by including Sb, Sn, etc. (No. 21, 22, etc.).

Example 2

Steel materials with the chemical compositions shown in Tables 6 to 8 below were smelted in a converter, and various steel plates were produced by continuous casting and hot rolling. The obtained steel plate was cut, the surface was ground, and finally a test piece (test piece D) having a size of 300×300×25 (mm) was produced. FIG. 4 shows the external appearance of the test piece D. As shown in FIG.

In addition, as shown in Figure 5, four small test pieces of 60 x 60 x 5 (mm) were brought into contact with a large test piece of 300 x 300 x 25 (mm) (the same as the above-mentioned test piece D) to form cracks. Part of the test piece E. The small test piece and the large test piece for crack formation are steel materials with the same chemical composition, and the surface finishing is also the same as that of the above-mentioned test piece D, which is specified as surface grinding. Then, a hole of Φ10 mm was drilled in the center of the small test piece, a screw hole was drilled on the base material side (large test piece side), and it was fixed with M8 plastic bolts.

In addition, the test piece F (FIG. 6) to which the tar epoxy resin coating (primer: zinc-rich coating material) of average thickness 250 micrometers was applied to the whole surface was also used. In addition, in order to understand the progress of corrosion when the base steel material is exposed due to damage to the anti-corrosion coating film, a cut (length: 300 mm, width: about 0.5 mm) reaching the base was formed on one side of the test piece F with a dicing blade.

For the samples of the respective chemical composition shown in Tables 6 to 8, five test pieces D, E, and F were used for the corrosion test. The corrosion test method (real ship exposure test) at this time is as follows.

[Corrosion test method]

The prepared samples (each test piece D to F) were installed on the bottom plate, wall surface and upper deck back of the inner surface of the VLCC crude oil tank, and the corrosion status of each sample was investigated after normal shipping for 5 years. 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 5 years of exposure, the test piece D was subjected to cathodic electrolysis [JIS K8284] in diammonium hydrogen citrate aqueous solution to remove corrosion products such as rust. In addition, for the test piece E, the small test piece for crack formation was removed, and the corrosion products were removed by the same method.

(1) Regarding the test piece D, the weight change before and after the test was converted into the average plate thickness decrease D-ave (mm), and the average value of five test pieces was calculated to evaluate the overall corrosion resistance of each sample. In addition, the maximum erosion depth D-max (mm) of the test piece D was obtained by using a stylus-type 3D shape measuring device, and normalized according to the average plate thickness reduction [D-ave (mm)] (that is, the calculation of D-max/ D-ave), to evaluate the uniformity of corrosion.

(2) Regarding the test piece E, the maximum crevice corrosion depth D-crev (mm) on the large test piece side was measured using a stylus-type three-dimensional shape measuring device.

(3) Regarding the test piece F (with a cut) subjected to painting treatment, the bulge width of the coating film in the direction perpendicular to the cut was measured with a caliper, and the maximum value of the five test pieces was defined as the maximum bulge width.

In the general corrosion resistance (average plate reduction: D-ave), corrosion uniformity (D-max/D-ave), crevice corrosion resistance (D-crev), paint corrosion resistance (maximum swelling The evaluation criteria of width) are shown in Table 9 below. Tables 10 to 12 below show the corrosion test results. Based on the above results, it can be considered as follows. In conventional corrosion-resistant steels (No. 2 to 4), general corrosion resistance and corrosion uniformity were slightly improved compared with conventional steel No. 1 samples, but in crevice corrosion resistance or paint corrosion resistance In terms of performance, it is at the level of conventional steel, and it is not enough as a corrosion-resistant steel. In addition, in the No. 5 sample containing a small amount of Se, the crevice corrosion resistance was slightly improved, but since the Se content was less than the lower limit specified in the present invention, a good effect could not be exhibited.

On the other hand, it was found that in the samples (No. 6-65) containing an appropriate amount of Se, due to the effect of improving the corrosion resistance of Se, all corrosion resistances were superior to those of the conventional steel (No. 1), and especially excellent Excellent crevice corrosion resistance, preferably used as corrosion-resistant steel for shipbuilding. It is known that in addition to general corrosion resistance and corrosion uniformity, crevice corrosion resistance or paint corrosion resistance can be greatly improved by containing various corrosion resistance improving elements (Cu, Cr, Co, Ni, Ti, etc.) . Such an effect of improving the corrosion resistance is judged to be due to the synergistic effect of the formation of the above-mentioned elements on the effect of suppressing the local pH drop of the formation of Se.

In addition, it was found that in samples (No. 14, 15, 18-27, etc.) containing La, Ce, etc. in appropriate amounts, the crevice corrosion resistance of the base plate was found to be further improved, and samples containing Ca or Mg in appropriate amounts (No. 16, 17, 19-27, etc.), greatly improving the paint corrosion resistance of the wall. Such an effect is presumed to be that La, Ce, Ca, Mg, etc. promote the densification of rust formed by Cu, Cr, Ni, and Ti, and promote the action of suppressing the local pH drop by promoting the formation of Se.

Containing an appropriate amount of Mo also contributes to improving the uniformity of corrosion on the wall surface (for example, No. 31, 34, 35, etc.), and it is considered that this element contributes to the uniformity of the rust densification effect formed by this element.

In addition, by containing Sb, As, Sn, Bi, or Te, etc., it is obvious that the general corrosion resistance is greatly improved (No. 37-40, etc.), and these elements are presumed to contribute to the rust densification of the above-mentioned elements or the pH relaxation effect. the result of.

In addition, in samples (Nos. 51, 52, 53, etc.) to which Zn was added, paint corrosion resistance and crevice corrosion resistance were also improved as a result. For example, in addition to Cu, Ni, and Se, No. 52 with an appropriate amount of Zn added compared with No. 9 with only Cu, Ni, and Se added, the maximum bulging range of the test piece F on the wall surface was reduced, and as a result, the paint was improved. Corrosion resistance. The above-mentioned improvement in corrosion resistance by adding Zn is presumed to be the result of the formation of a zinc chloride or zinc sulfide deposition film on the steel surface to isolate the steel substrate from moisture in the environment and inhibit corrosion.

Example 3

Steel materials having the chemical compositions shown in Table 13 below were smelted in a converter, and various steel plates were produced by continuous casting and hot rolling. The obtained steel plate was cut, the surface was ground, and finally a test piece G' having a size of 300×150×25 (mm) was produced. Subcede arc welding was performed using welding consumables having the chemical composition shown in Table 14, and a joint test piece G ( FIG. 7 ) shown in the figure was produced using two D's. In addition, the wire diameter of all welding materials is 4.8 mm, and the groove shape is V-shaped. The input heat is properly adjusted within the range of 1-10kJ/mm.

In addition, two small test pieces of 60×60×5 (mm) were brought into contact with the welded portion of the test piece G to make contact with the test piece H in which a crack was formed ( FIG. 8 ). The chemical composition of the small test piece for crack formation is the same as that of the base material of test piece D, and the surface finishing is also the same as that of the above-mentioned test piece G, which is defined as surface grinding. Then, a hole of Φ10 mm was drilled in the center of the small test piece, a screw hole was drilled on the base material side (large test piece side), and it was fixed with M8 plastic bolts.

In addition, the test piece I (FIG. 9) to which the tar epoxy resin paint (primer: zinc-rich paint) of average thickness 250 micrometers was applied to the whole surface was also used. And, in order to understand the degree of corrosion progress when the base steel is exposed due to the damage of the anti-corrosion coating film, on one side of the test piece 1, use a cutting blade to form a cut (length : 200mm, width: about 0.5mm).

Corrosion tests were performed on the joint test pieces produced using the base metals and welding materials shown in Tables 13 and 14, using five test pieces G, H, and I, respectively. The corrosion test method is as follows.

[Corrosion test method]

The prepared samples, five test pieces G to I each, were mounted on the bottom plate inside the tank body of the VLCC crude oil tank, and the corrosion status of each sample was investigated after normal shipping for 5 years. After five years of exposure, the test piece G was subjected to cathodic electrolysis [JIS K8284] in diammonium hydrogen citrate aqueous solution to remove corrosion products such as rust. In addition, with respect to the test piece H, the small test piece for crack formation was removed, and the corrosion product was removed by the same method.

(1) Regarding the test piece G, the weight change before and after the test was converted into the average plate thickness decrease D-ave (mm), and the average value of five test pieces was calculated, and the overall corrosion resistance of each sample was evaluated. In addition, using a stylus-type 3D shape measuring device, the maximum erosion depth D-max (mm) of the test piece G was obtained, and normalized according to the average plate thickness reduction [D-ave (mm)] (that is, the calculation of D-max/ D-ave), to evaluate the uniformity of corrosion.

(2) Regarding the test piece H, the maximum crevice corrosion depth D-crev (mm) on the large test piece side was measured using a stylus-type three-dimensional shape measuring device.

(3) With regard to the test piece I (with cut) subjected to painting treatment, measure the bulging width (mm) of the coating film in the direction perpendicular to the cut with a caliper, and define the maximum value of the 5 test pieces as the maximum bulging width .

In the general corrosion resistance (average plate reduction: D-ave), corrosion uniformity (D-max/D-ave), crevice corrosion resistance (D-crev), paint corrosion resistance (maximum swelling The evaluation criteria of the range) are exactly the same as the evaluation criteria in Example 2 shown in Table 9 above. Table 15 below shows the corrosion test results. However, in Table 15, I(Se) represents the Se content of the deposited metal/Se content of the base material.

The Se content in the welded portion does not satisfy No. 66 of the relational expression (1), and is not satisfactory especially in terms of crevice corrosion resistance. This is a result of the progress of corrosion on parts of the weld metal. On the other hand, among No. 67 to No. 75 whose ratio satisfies the relational expression (1), it was found that the corrosion resistance was improved in all corrosion characteristics, and as a result, corrosion resistance which is preferable as a welded structure was shown.

In addition, in this embodiment, the welded portion by the hidden arc welding method was used as the evaluation object, but the same effect can be obtained by other welding methods such as the chemical coating welding method and the electroslag welding method. In addition, the solder material used is not limited to Table 14, either.

In summary, the steel of the present invention can exhibit excellent corrosion resistance against localized corrosion on oil film defects, crevice corrosion on structural cracks, or corrosion on damaged coatings, and can be very suitable for corrosion resistance of oil tankers. steel.

Claims (2)

1. the oil of the boats and ships of a corrosion resistance excellent is a liquid-fuel tank, it is characterized in that, form by following steel, the composition of these steel is counted C:0.01～0.30%, Si:0.01～2.0%, Mn:0.01～2.0%, Al:0.005～0.10%, Se:0.005～0.50%, Cu:0.01～5.0%, Cr:0.05～5.0%, Ni:0.01～5.0%, Ti:0.005～0.20%, Ca:0.0005～0.015% with quality %, and surplus is made of Fe and unavoidable impurities.

2. the oil of the boats and ships of a corrosion resistance excellent is a liquid-fuel tank, it is characterized in that, form by following steel, the composition of these steel is counted C:0.01～0.30%, Si:0.01～2.0%, Mn:0.01～2.0%, Al:0.005～0.10%, Se:0.005～0.50%, Cu:0.01～5.0%, Cr:0.05～5.0%, Ni:0.01～5.0%, Ti:0.005～0.20%, Ca:0.0005～0.015% with quality %

And be selected from by in Co:0.01～5.0%, La:0.0005～0.15%, Ce:0.0005～0.15%, Mg:0.0005～0.015%, Mo:0.01～5.0%, Sb:0.01～0.5%, As:0.01～0.5%, Sn:0.01～0.5%, Bi:0.01～0.5%, Te:0.01～0.5%, B:0.0001～0.010%, V:0.01～0.50%, Nb:0.003～0.50%, Zn:0.001～0.10% group that constitutes more than one

Surplus is made of Fe and unavoidable impurities.

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