JPWO2017098700A1 - Steel for crude oil tanks and crude oil tanks with excellent corrosion resistance - Google Patents

Abstract

The composition of the steel material is, by mass%, C: 0.03-0.18%, Si: 0.03-1.50%, Mn: 0.1-2.0%, P: 0.025% or less, S: 0.010% or less, Al: 0.015-0.049%, N: 0.008% or less, W: 0.005-0.5%, and Nd: 0.0001-0.010%, the balance being Fe and inevitable impurities, and the dislocation density α of the steel material satisfies the following formula (1) By setting the range, the steel material for a crude oil tank is excellent in both the general corrosion resistance on the top plate of the crude oil tank such as the tanker oil tank and the local corrosion resistance on the bottom plate of the crude oil tank. α (/ m2) ≦ {1020 × [% W] × [% Nd]} / {5 × ([% Al] −0.01)} (1) where [% M] is element M in steel Content (mass%)

Description

The present invention relates to an oil tank of a crude oil tanker formed by welding steel materials or a tank for transporting or storing crude oil (hereinafter collectively referred to as “crude oil tank”). The present invention relates to a steel material for a crude oil tank that reduces the overall corrosion that occurs at the ceiling and side walls and the local corrosion that occurs at the bottom of the crude oil tank, and a crude oil tank that is composed of the steel material.
In addition, the steel material for crude oil tanks of the present invention includes thick steel plates, thin steel plates, and shaped steels.

It is known that the steel used on the inner surface of a tanker's crude oil tank, particularly on the back of the upper deck and the upper part of the side wall, is totally corroded. As a cause of this total corrosion,
(1) Repeated condensation and drying (wet and dry) on the steel sheet surface due to temperature difference between day and night,
(2) Inert gas enclosed in crude oil tanks for explosion protection (O ₂ approx. 4 vol%, CO ₂ approx. 13 vol%, SO ₂ approx. 0.01 vol%, remaining N ₂ as representative composition boiler or engine exhaust gas, etc.) Of O ₂ , CO ₂ , SO ₂ into condensed water,
(3) Dissolution of corrosive gas such as H ₂ S volatilized from crude oil into condensed water,
(4) Residual seawater used for cleaning crude oil tanks.
These can be recognized from the fact that sulfate ions and chloride ions are detected in strongly acidic condensed water during dock inspections of actual ships that are usually conducted every 2.5 years.

In addition, when H ₂ S is oxidized using iron rust generated by corrosion as a catalyst, solid S is formed in layers in the iron rust, but these corrosion products easily peel off and fall off, and the crude oil tank Deposit at the bottom of the. For this reason, in the dock inspection, the current situation is that repair of the upper part of the tank and collection of deposits at the bottom of the tank are performed with great expense.

On the other hand, steel materials used as bottom plates for tankers' crude oil tanks, etc., have been affected by the corrosion of the crude oil itself and the protective action of oil-derived protective coat (oil coat) formed on the inner surface of the crude oil tank. It was thought not to occur. However, recent research has revealed that bowl-shaped local corrosion (pitting corrosion) occurs in the steel of the tank bottom plate.
As a cause of such local corrosion,
(1) Presence of condensed water in which salts represented by sodium chloride are dissolved at a high concentration,
(2) Oil coat detachment due to excessive cleaning,
(3) Increase the concentration of sulfides contained in crude oil,
(4) High concentration of O ₂ , CO ₂ , SO _2, etc. in the explosion-proof inert gas dissolved in the condensed water
Etc.
Actually, when the water stayed in the crude oil tank was analyzed during the dock inspection of the actual ship, high concentrations of chloride ions and sulfate ions were detected.

  By the way, the most effective method for preventing the above-described general corrosion and local corrosion is to apply heavy coating on the surface of the steel material to shield the steel material from the corrosive environment. However, the painting operation of the crude oil tank not only has an enormous application area, but also requires repainting once every 10 years due to the deterioration of the coating film, resulting in an enormous cost for inspection and painting. Furthermore, it has been pointed out that corrosion is promoted in the damaged part of the heavy-painted coating film in the corrosive environment of the crude oil tank.

For the corrosion problem as described above, several techniques for improving the corrosion resistance of the steel material itself in the corrosive environment of the crude oil tank have been proposed.
For example, Patent Document 1 discloses a marine vessel that has improved corrosion resistance in an environment that is exposed to constant temperature and high humidity including salt and an environment that includes sulfur, and that has excellent toughness without being subjected to painting or cathodic protection. For the purpose of providing steel materials, C: 0.01 to 0.30%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, Al: 0.005 to 0.10%, Bi: 0.0005 to 0.40%, P: 0.003 to 0.050 %, And the balance: Fe and inevitable impurities, and a technology related to marine steel satisfying the following formulas (1) and (2) is disclosed.
[P] × 7 + [Bi] <0.50% (1)
0.050 ≤ [P] / [Bi] ≤ 5.0 (2)

  Patent Document 2 discloses a steel material for an upper tank of a crude oil tanker or a cargo ship of a bulk carrier, and is exposed to a severe corrosive environment in which sulfur-containing substances such as sulfur, sulfur oxides and sulfides are present. In order to provide a steel material that exhibits excellent corrosion resistance and has weldability and hot workability equal to or higher than that of ordinary marine steel materials, C: 0.01 to 0.30%, Si : 0.20 to 1.0%, Mn: 0.50 to 1.60%, P: 0.005 to 0.040%, S: 0.005 to 0.020%, Al: 0.050 to 0.100%, Cu: 0.20 to 1.0%, Ni: 0.03% or less (0% Including: Cr: 0.05 to 0.30%, Zn: 0.001 to 0.50%, Sn: 0.005 to 0.050% and Ca: 0.0005 to 0.0050%, and the balance is made of Fe and inevitable impurities. ing.

JP 2007-197763 JP 2013-028830

In order to protect the marine environment and operate the crude oil tanker safely, it is important to manage the crude oil tank so that the crude oil does not leak. Especially in the crude oil tank, the occurrence of through holes due to corrosion must be prevented. I must. Therefore, the corrosion of the bottom plate of the crude oil tank is investigated at the time of docking every 2.5 years, and pitting corrosion exceeding 4mm in depth is repaired.
In view of this situation, in order to reduce the maintenance cost of crude oil tankers, the application of corrosion resistant steel to tankers has been proposed as one of the means for suppressing the occurrence of pitting corrosion with a depth of more than 4 mm.

  However, with the technique described in Patent Document 1, it is difficult to suppress local corrosion (pitting corrosion) generated in the tanker bottom plate and the welded joint to 4 mm or less in 2.5 years. This is because, in recent years, corrosion surveys of actual ships have revealed that the pH of the solution inside the pitting corrosion generated on the tanker bottom plate and welds is 1.0 or less. In general, steel material corrosion in an acidic solution is rate-determined by a hydrogen reduction reaction, and it is well known that the corrosion rate dramatically increases as the pH decreases. Therefore, it can be said that the combined cycle test in the neutral range in which salt water is sprayed and the wet and dry repeated test is performed as described in the example of Patent Document 1 sufficiently reflects the corrosive environment in an actual ship. Because there is no.

  Moreover, in the steel material of patent document 2, the satisfactory effect cannot be acquired about suppression of the general corrosion generate | occur | produced in a tanker upper board. Because the actual crude oil tanker has a service life of 25 years and the design corrosion allowance of the tanker upper plate is about 2mm on one side, the corrosion rate of the corrosion-resistant steel applied to the upper plate is 0.08mm / y or less. This is because, even in the invention example described in Patent Document 2, even when the corrosion rate is the lowest, it is only about 0.11 mm / y. In particular, longages welded to the tanker upper plate are exposed to the corrosive environment inside the tanker, so repair is required when applying corrosion-resistant steel with a corrosion rate exceeding 0.1 mm / y. Therefore, the technique described in Patent Document 2 cannot be desired to omit painting.

  The present invention was developed in view of the above situation, and a steel material for a crude oil tank that is excellent in both general corrosion resistance in a top plate of a crude oil tank such as a tanker oil tank portion and local corrosion resistance in a bottom plate of a crude oil tank, It aims at providing with the crude oil tank comprised from this steel material.

Now, the inventors have intensively studied to solve the above problems.
As a result, it has been found that the above-mentioned overall corrosion and local corrosion can be remarkably reduced by strictly controlling the component composition of steel, particularly W and Nd, within the proper amount range and strictly controlling the dislocation density of steel. Obtained.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.03-0.18%
Si: 0.03-1.50%,
Mn: 0.1-2.0%
P: 0.025% or less,
S: 0.010% or less,
Al: 0.015-0.049%,
N: 0.008% or less,
W: 0.005-0.5% and
Nd: 0.00002-0.010%
A steel material for a crude oil tank that has excellent corrosion resistance with the dislocation density α of the steel material satisfying the following formula (1):
α (/ m ² ) ≦ {10 ²⁰ × [% W] × [% Nd]} / {5 × ([% Al] −0.01)} (1)
However, [% M] is the content of M element in steel (mass%)

2. The steel material is further in mass%,
Cu: 0.05-0.4%,
Ni: 0.005-0.4%,
Mo: 0.005-0.5%
Sn: 0.005-0.4% and
Sb: 0.005-0.4%
2. The steel material for crude oil tanks according to 1 above, which contains one or more selected from the above, and the dislocation density β of the steel material satisfies the following formula (2) and is excellent in corrosion resistance.
β (/ m ² ) ≦ {10 ²⁰ × [% W] × [% Nd] +10 ¹⁵ × ([% Cu] + [% Ni] + [% Mo] + 3 × [% Sn] + 3 × [% Sb] )} / {5 × ([% Al] −0.01)} (2)
However, [% M] is the content of M element in steel (mass%)

3. The steel material is further in mass%,
Cr: 0.01-0.2%
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.002 to 0.2%,
Mg: 0.0002 to 0.01%
Ca: 0.0002 to 0.01% and
REM: 0.0002 to 0.015%
3. The steel material for a crude oil tank having excellent corrosion resistance according to 1 or 2 above, which contains one or more selected from among the above.

4). The crude oil tank comprised with the steel materials for crude oil tanks in any one of said 1-3.

  ADVANTAGE OF THE INVENTION According to this invention, the general corrosion and local corrosion which generate | occur | produce in the oil tank of a crude oil tanker, the tank which conveys or stores crude oil, etc. can be suppressed effectively, and it is very useful industrially.

It is the figure which showed the relationship between the value of {10 < ²⁰ > * [% W] * [% Nd]} / {5 * ([% Al] -0.01)}} and dislocation density of the steel material in a dew condensation test. {10 ²⁰ × [% W] × [% Nd] +10 ¹⁵ × ([% Cu] + [% Ni] + [% Mo] + 3 × [% Sn] + 3 × [% Sb]) It is the figure which showed the relationship between the value of} / {5x ([% Al] -0.01)}, and a dislocation density. It is the figure which showed the relationship between the value of {10 < ²⁰ > * [% W] * [% Nd]} / {5 * ([% Al] -0.01)}} and dislocation density of steel materials in an acid resistance test. {10 ²⁰ × [% W] × [% Nd] +10 ¹⁵ × ([% Cu] + [% Ni] + [% Mo] + 3 × [% Sn] + 3 × [% Sb]) of steel in acid resistance test) It is the figure which showed the relationship between the value of} / {5x ([% Al] -0.01)}, and a dislocation density. In the Example of this invention, it is a figure explaining the test apparatus used for the general corrosion test. In the Example of this invention, it is a figure explaining the test apparatus used for the pitting corrosion test.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel material for crude oil tank of the present invention is limited to the above range will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.03-0.18%
C is an element that enhances the strength of steel. In the present invention, C is added in an amount of 0.03% or more in order to ensure a desired strength (490 to 620 MPa). However, addition of C exceeding 0.18% lowers the weldability and the toughness of the heat affected zone. Therefore, the C content is in the range of 0.03 to 0.18%. Preferably it is 0.06 to 0.16% of range.

Si: 0.03-1.50%
Si is an element added as a deoxidizer, but is also an effective element for increasing the strength of steel. Therefore, in the present invention, 0.03% or more of Si is added to ensure a desired strength. However, addition of Si exceeding 1.50% reduces the toughness of the steel. Therefore, the Si content is in the range of 0.03 to 1.50%. Preferably it is 0.05 to 0.40% of range.

Mn: 0.1-2.0%
Mn is an element that increases the strength of steel, and in the present invention, 0.1% or more is added to obtain a desired strength. However, Mn addition exceeding 2.0% decreases the toughness and weldability of steel. Therefore, the Mn content is in the range of 0.1 to 2.0%. Preferably it is 0.80 to 1.60% of range.

P: 0.025% or less P is a harmful element that segregates at the grain boundaries and lowers the toughness of the steel, so it is desirable to reduce it as much as possible. In particular, when P exceeds 0.025%, the toughness is greatly reduced. Moreover, when P is contained exceeding 0.025%, it will also have a bad influence on the corrosion resistance in a tank oil tank. Therefore, the P content is 0.025% or less. Preferably it is 0.015% or less.

S: 0.010% or less S is a harmful element that forms MnS, which is a non-metallic inclusion, and serves as a starting point for local corrosion and reduces local corrosion resistance. Therefore, it is desirable to reduce S as much as possible. In particular, when S exceeds 0.010%, the local corrosion resistance is significantly reduced. Therefore, the S amount is set to 0.010% or less. Preferably it is 0.005% or less.

Al: 0.015-0.049%
Al is an element added as a deoxidizer, and in the present invention, 0.015% or more is added. However, when Al is added in excess of 0.049%, not only the toughness of the steel is reduced, but also the aluminum oxide formed on the surface of the steel material is preferentially dissolved in the acid and the corrosion resistance is also lowered. The upper limit is 0.049%.

N: 0.008% or less Since N is a harmful element that lowers toughness, it is desirable to reduce it as much as possible. In particular, if N is contained in excess of 0.008%, the toughness is greatly reduced, so the upper limit of N content is 0.008%.

W: 0.005-0.5%
W not only suppresses pitting corrosion in the tanker tank bottom plate, but can also suppress overall corrosion of the tanker upper deck, and is an extremely effective element for improving corrosion resistance. The effect of W appears when 0.005% or more is added, but when it exceeds 0.5%, the effect reaches saturation. Therefore, the W amount is in the range of 0.005 to 0.5%. Preferably it is 0.01 to 0.3%, More preferably, it is 0.02 to 0.2% of range.
The reason why W has the above-described effect of improving corrosion resistance is that WO ₄ ^2- is produced in the rust produced as the steel sheet corrodes, and the presence of this WO ₄ ^2- This is because sulfate ions are prevented from entering the steel sheet surface. Further, it is considered that corrosion of the steel material is also suppressed by the inhibitor action by adsorption of WO ₄ ^{2- on} the steel material surface.

Nd: 0.00002-0.010%
Nd forms on the steel surface by reacting with crude oil-derived hydrogen sulfide dissolved in the water film formed on the steel material surface by condensation on the upper plate of the tanker oil tank section to form neodymium disulfide and dineodymium trisulfide. It has the effect of reinforcing the protection of the rust layer. Nd is an extremely effective element for securing the toughness of the welded joint at low temperature because neodymium oxide generated during high heat input welding prevents the structure of the heat-affected zone from becoming coarse. These effects of Nd are manifested when 0.00002% or more is added, but when it exceeds 0.010%, the effects reach saturation. Therefore, the Nd content is set in the range of 0.00002 to 0.010%. Preferably it is 0.0001 to 0.005%, more preferably 0.0002 to 0.002%. In addition, since the effect of Nd exerted on the corrosion resistance is remarkably exhibited when used in combination with W, in the present invention, it is particularly important to use W and Nd as a corrosion resistant element in a predetermined amount.

  Although the basic components have been described above, in the present invention, the following elements can be appropriately contained in addition to the above-described components.

Cu: 0.05-0.4%
Cu not only increases the strength of the steel, but also exists in the rust generated by the corrosion of the steel, and has the effect of increasing the corrosion resistance by suppressing the diffusion of Cl ^- ions that promote the corrosion. These effects of Cu cannot be sufficiently obtained with addition of less than 0.05%, while addition of more than 0.4% saturates the effect of improving corrosion resistance and may cause problems such as surface cracking during hot working. . Therefore, the Cu content is in the range of 0.05 to 0.4%. Preferably it is 0.06 to 0.35% of range.

Ni: 0.005-0.4%
Ni has the effect of refining the generated rust particles to improve the corrosion resistance in the bare state and the corrosion resistance in the state where the epoxy primer is applied to the zinc primer. Therefore, Ni is added when it is desired to further improve the corrosion resistance. The effect of Ni described above is manifested when 0.005% or more is added. On the other hand, even if Ni exceeds 0.4%, the effect is saturated. Therefore, Ni is preferably added in the range of 0.005 to 0.4%. More preferably, it is 0.08 to 0.35% of range.

Mo: 0.005-0.5%
Mo is an element effective in improving corrosion resistance, which not only suppresses pitting corrosion in the tanker tank bottom plate but also can suppress overall corrosion of the tanker upper deck. The effect of Mo is manifested when 0.005% or more is added, but when it exceeds 0.5%, the effect reaches saturation. Therefore, the Mo amount is preferably in the range of 0.005 to 0.5%. More preferably, it is 0.01 to 0.3% of range, still more preferably 0.02 to 0.2% of range.
The reason why Mo has the above-described effect of improving corrosion resistance is that MoO ₄ ^2- is generated in the rust generated as the steel sheet corrodes, and the presence of WO ₄ ^2- This is because sulfate ions are prevented from entering the steel sheet surface. Further, it is considered that corrosion of the steel material is also suppressed by the inhibitor action due to the adsorption of MoO ₄ ^{2- on} the steel material surface.

Sn: 0.005-0.4%
Sn is a useful element that contributes to the suppression of local corrosion and overall corrosion of steel by being taken into the rust layer during corrosion and forming a dense rust layer. The effect of Sn is manifested by addition of 0.005% or more. However, if added over 0.4%, not only the low temperature toughness is lowered, but also defects are generated during welding. Therefore, the Sn content is in the range of 0.005 to 0.4%. Preferably it is 0.01 to 0.2% of range, More preferably, it is 0.01 to 0.1% of range.

Sb: 0.005-0.4%
Sb not only suppresses pitting corrosion at the tanker tank bottom plate, but also has the effect of suppressing overall corrosion at the tanker upper deck. The effect of the above Sb is manifested by addition of 0.005% or more, but the effect is saturated even if it exceeds 0.4%. Therefore, the Sb content is in the range of 0.005 to 0.4%.

Cr: 0.01-0.2%
When Cr is added to a steel material that is used as it is blackened or blasted, there is no significant effect on the corrosion resistance improvement in the environment in the tank. However, when a Zn-containing primer is applied to the steel surface, it can form a complex oxide of Cr and Zn centering on Fe, and can keep Zn on the surface of the steel for a long period of time. Can be improved. The effect of Cr described above is particularly remarkable in a portion that comes into contact with a liquid containing high-concentration salinity separated from crude oil, such as a bottom plate portion of a tanker oil tank, and the above steel containing Cr contains Zn. By applying the primer treatment, the corrosion resistance can be remarkably improved as compared with a steel material not containing Cr. If the Cr content is less than 0.01%, it is not sufficient. On the other hand, if it exceeds 0.2%, the toughness of the weld is deteriorated. Therefore, the Cr content is in the range of 0.01 to 0.2%. Preferably it is 0.05 to 0.2% of range.

Nb: 0.001 to 0.1%, Ti: 0.001 to 0.1%, V: 0.002 to 0.2%
Nb, Ti and V are all elements that increase the strength of the steel material, and can be appropriately selected and added according to the required strength. In order to obtain the above effects, it is preferable to add Nb and Ti to 0.001% or more and V to 0.002% or more, respectively. However, if Nb and Ti are added in excess of 0.1% and V is added in excess of 0.2%, the toughness is lowered. Therefore, Nb, Ti and V are preferably added in the above ranges.

Mg: 0.0002 to 0.01%
Mg not only contributes to improving the toughness of the weld heat-affected zone, but also has an effect of increasing the corrosion resistance by being present in rust generated by corrosion of steel. These effects of Mg cannot be sufficiently obtained when the addition amount is less than 0.0002%. On the other hand, if the addition amount exceeds 0.01%, the toughness is reduced, so the Mg amount is in the range of 0.0002 to 0.01%.

Ca: 0.0002 to 0.01%, REM: 0.0002 to 0.015%
Both Ca and REM are effective in improving the toughness of the weld heat-affected zone, and can be added as necessary. The above effect can be obtained with addition of Ca: 0.0002% or more, REM: 0.0002% or more, but if Ca exceeds 0.01% and REM exceeds 0.015%, it causes a decrease in toughness. Ca and REM are preferably added within the above ranges.

Next, the dislocation density of the steel material defined in the present invention will be described.
In the corrosion resistant steel of the present invention, as described above, by adding a predetermined amount of various corrosion resistant elements to the steel material, the various corrosion resistant elements are concentrated in the rust layer on the steel material surface formed in the corrosive environment of the tanker tank bottom plate and top plate. It suppresses the diffusion of various corrosion factors and reduces the corrosion rate of steel materials.

  On the other hand, in steel materials, the formation of dislocations derived from the manufacturing process cannot be avoided. However, since the dislocations are thermodynamically unstable, they function as anode sites in which iron dissolves in a corrosive environment. Although the rust layer formed on the surface of the corrosion-resistant steel has protective properties and has the effect of reducing the corrosion rate of the steel material, its function is not perfect, and changes depending on the density of dislocations on the steel material surface under the rust layer . That is, the allowable dislocation density of the steel material varies depending on the degree of protection of the rust layer formed on the steel material surface.

Therefore, the inventors investigated the relationship between the protection of the rust layer and the dislocation density.
As a result, in steel materials having a predetermined amount of W and Nd as corrosion resistant elements, when the dislocation density α is lower than the value specified by the right side of the following equation (1), it is good in the environment in the tank of the crude oil tanker. It became clear that corrosion resistance was obtained.
α (/ m ² ) ≦ {10 ²⁰ × [% W] × [% Nd]} / {5 × ([% Al] −0.01)} (1)
However, [% M] is the content of M element in steel (mass%)

Further, when a predetermined amount of one or more of Cu, Ni, Mo, Sn and Sb is contained as a corrosion resistant element, the protection of the rust layer formed on the surface is achieved by the effect of these corrosion resistant elements. In order to further improve, it has been clarified that the upper limit of dislocation density β allowed from the viewpoint of corrosion resistance can be relaxed to the value specified by the right side of the following equation (2).
β (/ m ² ) ≦ {10 ²⁰ × [% W] × [% Nd] +10 ¹⁵ × ([% Cu] + [% Ni] + [% Mo] + 3 × [% Sn] + 3 × [% Sb] )} / 5 × ([% Al] −0.01)} (2)
However, [% M] means the content (mass%) of M element in steel materials.

Hereinafter, the background of finding the relationship between the protection of the rust layer and the dislocation density will be described.
Steel materials having the component compositions shown in Table 1 were rolled under the conditions described in Table 2. Thereafter, for the purpose of controlling the dislocation density, 1%, 3%, 5%, and 7% of pre-strain was applied to some test pieces, and then each of 25 corrosion test pieces having the dimensions described in the examples described later. Collected. As the prestrain increases, the dislocation density increases regardless of the steel type. These test pieces were subjected to a general corrosion test (condensation test) simulating the back of the upper deck described in the examples and a local corrosion test (acid resistance test) simulating the tanker bottom plate environment, respectively. Each test was evaluated according to the criteria described in the examples, and then a part of the test piece was cut out, and the dislocation density on the steel material surface was measured by the method described in the examples. The obtained results are also shown in Table 2.

1 and 2, the horizontal axis shows {10 ²⁰ × [% W] × [% Nd]} / {5 × ([% Al] −0.01)} or {10 ²⁰ × [% W] × [ % Nd] +10 ¹⁵ × ([% Cu] + [% Ni] + [% Mo] + 3 × [% Sn] + 3 × [% Sb])} / {5 × ([% Al] −0.01)} And the measured dislocation density of each steel material is plotted on the vertical axis. In the figure, ● indicates a case where the predicted amount of wear after 25 years when the method described in the examples is used in the dew condensation test is 2 mm or less, and × indicates a case where it exceeds 2 mm.
As shown in FIGS. 1 and 2, it was found that only when the dislocation density of the steel material satisfies the above formula (1) or (2), the target performance is satisfied in the dew condensation test. Furthermore, it was confirmed that the upper limit of the allowable dislocation density increases as the addition amount of Mo, Sn, Cu, Ni and Sb effective for the formation of protective rust increases.

Similarly, FIG. 3 and FIG. 4 are the results in the acid resistance test. In the figure, ● indicates the case where the corrosion rate determined by the method described in the examples is 1.0 mm / y or less, and × indicates the case where it exceeds 1.0 mm / y.
As shown in FIGS. 3 and 4, it was found that only when the dislocation density of the steel material satisfies the above expression (1) or (2), the target performance is satisfied in the acid resistance test.

Next, the suitable manufacturing method of the steel material for crude oil tanks of this invention is demonstrated.
The steel material of the present invention is prepared by melting the steel adjusted to the above-mentioned preferred component composition using a known refining process such as a converter, electric furnace, vacuum degassing, etc., and a continuous casting method or ingot-bundling rolling method Thus, a steel material (slab) is obtained, and then the material is reheated and then hot-rolled to obtain a thick steel plate, a thin steel plate, a shaped steel, or the like.

  Here, the reheating temperature before hot rolling is preferably 900 to 1200 ° C. If the heating temperature is less than 900 ° C, the deformation resistance is large and it is difficult to perform hot rolling.On the other hand, if the heating temperature exceeds 1200 ° C, the austenite grains are coarsened and the toughness is reduced. This is because the above becomes remarkable and the yield decreases. A more preferable heating temperature is in the range of 1000 to 1150 ° C.

In addition, when rolling into a steel material having a desired shape and size by hot rolling, the finish rolling finishing temperature is preferably 700 ° C. or higher. If the finish rolling finish temperature is less than 700 ° C, the deformation resistance of the steel increases, the rolling load increases and rolling becomes difficult, or there is a waiting time until the rolled material reaches a predetermined rolling temperature. Efficiency decreases. In addition, by performing finish rolling at a temperature significantly below the Ar ₃ transformation point, the dislocation density of the steel material is increased, and the corrosion resistance is deteriorated.

  The steel material after hot rolling may be cooled by either air cooling or accelerated cooling. However, accelerated cooling is preferred when higher strength is desired. In addition, when performing accelerated cooling, it is preferable that a cooling rate shall be 2-80 degree-C / s, and cooling stop temperature shall be 650-400 degreeC. If the cooling rate is less than 2 ° C / s and the cooling stop temperature exceeds 650 ° C, the effect of accelerated cooling is small and sufficient strength cannot be achieved, while the cooling rate exceeds 80 ° C / s and the cooling stop temperature is 400 When the temperature is lower than 0 ° C., the toughness of the obtained steel material is lowered and the shape of the steel material is not only distorted, but also the dislocation density of the steel material is increased and the corrosion resistance is lowered.

Steels having various compositions shown in Table 3 as Nos. 1 to 37 are melted in a vacuum melting furnace to form a steel ingot, or melted in a converter and formed into a steel slab by continuous casting. After reheating to 1150 ° C, hot rolling was performed at the finish rolling finish temperature shown in Table 4 to obtain a steel plate with a thickness of 25 mm, and then the water cooling rate: 10 ° C / s to the cooling stop temperature shown in Table 4 Cooled down.
The thick steel plates No. 1 to 37 thus obtained were subjected to a dew condensation test and an acid resistance test to evaluate their corrosion resistance. In addition, the dislocation density of the steel was also measured.

That is, in the following manner, a general corrosion test (condensation test) simulating the back of the upper deck and a local corrosion test (acid resistance test) simulating the tanker bottom plate environment were performed.
(1) Full-surface corrosion test (condensation test) simulating the tanker upper deck environment
In order to evaluate the corrosion resistance against overall corrosion on the back of the tanker upper deck, a rectangular piece of width 25mm x length 60mm x thickness 5mm was set for each of the thick steel plates No. 1 to 37 above from the position of the surface 1mm. (21 days, 49 days, 77 days, 98 days), a total of 20 sheets were cut out to 5 sheets, and the surface was polished with 600th emery paper. Next, the back surface and the end surface were sealed with tape so as not to corrode, and a full corrosion test was conducted using a corrosion test apparatus shown in FIG.

This corrosion test apparatus includes a corrosion test tank 2 and a temperature control plate 3. Water 6 having a temperature maintained at 30 ° C. is injected into the corrosion test tank 2, and 13 vol% CO ₂ , 4 vol% O ₂ , 0.01 vol are introduced into the water 6 through the introduction gas pipe 4. A mixed gas consisting of% SO ₂ , 0.05 vol% H ₂ S and the balance N ₂ is introduced to fill the corrosion test tank 2 with supersaturated steam, and the corrosive environment of the upper deck of the crude oil tank is reproduced. And the corrosion test piece 1 is set on the upper and lower surfaces of this test tank, and 25 ° C. × 1.5 hours + 50 ° C. × 22.5 hours with respect to the corrosion test piece 1 through the temperature control plate 3 incorporating a heater and a cooling device. The temperature change with 1 cycle was repeatedly applied for 21 days, 49 days, 77 days, and 98 days to generate dew condensation water on the surface of the test piece 1 to cause general corrosion. In FIG. 5, 5 indicates an exhaust gas pipe from the test tank.

  After the above corrosion test, the rust on the surface of each test piece was removed, the mass loss due to corrosion was determined from the mass change before and after the test, and this value was converted into a reduction in plate thickness. Then, the predicted amount of wear after 25 years is calculated from the value of the test period by the least square method using an exponential function. When the corrosion amount is 2 mm or less, the overall corrosion resistance is good (○), and when it exceeds 2 mm Evaluated that the overall corrosion resistance was poor (x).

(2) Local corrosion test (acid resistance test) simulating the tanker tank bottom plate environment
In order to evaluate the corrosion resistance against pitting corrosion in the tanker tank bottom plate, five rectangular pieces of width 25mm x length 60mm x thickness 5mm were cut out from each of the No. 1 to 37 thick steel plates. The surface was polished with 600th emery paper.
Next, a test solution prepared with a 10% NaCl aqueous solution using concentrated hydrochloric acid to have a Cl ion concentration of 10% and pH of 0.85 was prepared, suspended through a 3 mmφ hole in the upper part of the test piece, and suspended from each test piece. Was subjected to a corrosion test by immersing in a 2 L test solution for 168 hours. The test solution was preheated and maintained at 30 ° C. and replaced with a new test solution every 24 hours.
The apparatus used for the corrosion test is shown in FIG. This corrosion test apparatus is a dual structure apparatus consisting of a corrosion test tank 8 and a constant temperature bath 9, and the test solution 10 is put in the corrosion test tank 8, and the test piece 7 is suspended and immersed in the teg 11 therein. Has been. The temperature of the test solution 10 is maintained by adjusting the temperature of the water 12 placed in the thermostatic chamber 9.

  After removing the rust generated on the surface of the test piece after the above corrosion test, the mass difference before and after the test is obtained, the difference is divided by the total surface area, and the reduction in thickness (corrosion rate on one side) per year is calculated. Asked. As a result, when the corrosion rate was 1.0 mm / y or less, the local corrosion resistance was evaluated as good (◯), and when the corrosion rate was higher than 1.0 mm / y, the local corrosion resistance was evaluated as poor (×).

(3) Measurement of dislocation density of steel material Each 20 × 20 × 5mmt test piece was cut out from each of No.1-37 test pieces after acid resistance test and 98-day dew condensation test, and 1mm side of the original steel surface The surface was used as the measurement surface. The diffraction peaks of the (110), (211) and (220) planes of the steel material were measured using an X-ray diffractometer, and the diffraction angle 2θ and the half width βm were determined for each test piece.
The horizontal axis represents sin θ / λ, and the vertical axis represents β cos θ / λ, and the measurement results of the above crystal planes are plotted.
Here, λ represents the X-ray wavelength of 1.789 mm, β represents the true half-value width of the diffraction peak, and was obtained from the measured half-value width βm and the unstrained half-value width βs by the equation (3).
Note that a Si powder standard sample was used as the unstrained standard sample (βs at the peak position was obtained from interpolation calculation by parabolic approximation).
β = (βm ² -βs ² ) ^0.5 (3)
An approximate curve was drawn for the above three points by the least square method, and strain ε was obtained from the slope as shown in equation (4), and dislocation density ρ and its average value were obtained from equation (5).
β ・ cosθ / λ = 0.9 / D + 2ε ・ sinθ / λ (4)
ρ = 14.4 ε ² / b ² (5)
Where b is Burgers vector 0.25nm,
D represents the crystallite size.
The obtained results are also shown in Table 4.

As shown in Table 4, the thick steel plates Nos. 1, 2, and 5 to 36 that satisfy the conditions of the present invention are all in the full corrosion test that simulates the upper deck back and the local corrosion test that simulates the tanker bottom plate environment. Also showed good corrosion resistance.
On the other hand, the thick steel plates No. 3, 4, and 37 that do not satisfy the conditions of the present invention could not obtain good results in any of the corrosion resistance tests.

DESCRIPTION OF SYMBOLS 1,7 Corrosion test piece 2,8 Corrosion test tank 3 Temperature control plate 4 Introducing gas pipe 5 Exhaust gas pipe 6,12 Water 9 Constant temperature bath
10 Test solution
11 Teguz

Claims (4)

% By mass
C: 0.03-0.18%
Si: 0.03-1.50%,
Mn: 0.1-2.0%
P: 0.025% or less,
S: 0.010% or less,
Al: 0.015-0.049%,
N: 0.008% or less,
W: 0.005-0.5% and
Nd: 0.00002-0.010%
A steel material for a crude oil tank that has excellent corrosion resistance with the dislocation density α of the steel material satisfying the following formula (1), the balance being Fe and the balance consisting of Fe and inevitable impurities.
α (/ m ² ) ≦ {10 ²⁰ × [% W] × [% Nd]} / {5 × ([% Al] −0.01)} (1)
However, [% M] is the content of M element in steel (mass%) The steel material is further in mass%,
Cu: 0.05-0.4%,
Ni: 0.005-0.4%,
Mo: 0.005-0.5%
Sn: 0.005-0.4% and
Sb: 0.005-0.4%
The steel material for a crude oil tank having excellent corrosion resistance according to claim 1, wherein the steel material contains one or more selected from the above, and the dislocation density β of the steel material satisfies the following formula (2).
β (/ m ² ) ≦ {10 ²⁰ × [% W] × [% Nd] +10 ¹⁵ × ([% Cu] + [% Ni] + [% Mo] + 3 × [% Sn] + 3 × [% Sb] )} / {5 × ([% Al] −0.01)} (2)
However, [% M] is the content of M element in steel (mass%) The steel material is further in mass%,
Cr: 0.01-0.2%
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.002 to 0.2%,
Mg: 0.0002 to 0.01%
Ca: 0.0002 to 0.01% and
REM: 0.0002 to 0.015%
The steel material for crude oil tanks having excellent corrosion resistance according to claim 1 or 2, comprising one or more selected from among the above. The crude oil tank comprised with the steel material for crude oil tanks in any one of Claims 1-3.

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