WO2015087531A1

WO2015087531A1 - Steel for crude oil tank and crude oil tank

Info

Publication number: WO2015087531A1
Application number: PCT/JP2014/006097
Authority: WO
Inventors: 釣　之郎; 務小森
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2013-12-12
Filing date: 2014-12-05
Publication date: 2015-06-18
Also published as: TWI563100B; CN105745347A; JP6149943B2; TW201529865A; KR20160075717A; CN105745347B; KR101786409B1; JPWO2015087531A1

Abstract

Steel for crude oil tanks such as tanker oil tanks excelling in both resistance to uniform corrosion on the upper plate of the crude oil tank and resistance to local corrosion on the bottom plate of the crude oil tank is provided by: the steel having a component composition containing, in 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: 0.005-0.10%, N: 0.008% or less and Cu: 0.05-0.4% with the remainder being obtained from Fe and unavoidable impurities; and the steel dislocation density being in a range that satisfies formula (1) in relation to Cu content α ≤ 4 × 10¹⁶ × [%Cu]^2.8 (1) wherein [%Cu] is the Cu content (mass%) in the steel.

Description

Steel for crude oil tank and crude oil tank

The present invention relates to an oil tank of a crude oil tanker formed by welding steel materials and a tank for transporting or storing crude oil (hereinafter collectively referred to as “crude oil tank”). Specifically, 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 of the crude oil tank and the local corrosion that occurs at the bottom of the crude oil tank, and a crude oil tank that includes 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, in Patent Document 1, in mass%, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, P: 0.03% or less, S: 0.02% or less, Cu: 0.01 to 1.5% , Al: 0.001 to 0.3%, N: 0.001 to 0.01%, Mo: 0.01 to 0.5% and W: 0.01 to 1%, one or two, with the balance being Fe and inevitable impurities A technique for forming a welded joint so that the contents of Cu, Mo, and W in the weld metal satisfy the following three expressions when welding the steel materials to form a welded joint is disclosed.
3 ≧ Cu content of weld metal (mass%) / Cu content of steel (mass%) ≧ 0.15
3 ≧ (Mo content of weld metal + W content (mass%)) / (Mo content of steel + W content (mass%)) ≧ 0.15
−0.3 ≦ Cu content of weld metal (mass%) − Cu content of steel (mass%) ≦ 0.5

Further, in Patent Document 2, by mass, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, P: 0.03% or less, S: 0.02% or less, Cu: 0.01 to 1.5 %, Al: 0.001 to 0.3%, N: 0.001 to 0.01%, Mo: 0.01 to 0.5%, and W: 0.01 to 1%, one or two, with the balance being Fe and inevitable impurities A technique for forming a welded joint so that the contents of Cu, Mo, and W in the weld metal satisfy the following two equations when welding a steel material made of the above to form a crude oil tank is disclosed.
3 ≧ Cu content of weld metal (mass%) / Cu content of steel (mass%) ≧ 0.15
3 ≧ (Mo content of weld metal + W content (mass%)) / (Mo content of steel material + W content (mass%)) ≧ 0.15

JP 2005-21981 Japanese Unexamined Patent Publication No. 2005-23421

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 it does not leak, and it is necessary to prevent the occurrence of through holes due to corrosion in the crude oil tank. . Therefore, when the docking is done every 2.5 years, the corrosion status of the bottom plate of the crude oil tank is investigated, and pitting corrosion over 4 mm deep is repaired, in order to reduce the maintenance cost of the crude oil tanker. Thus, 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 techniques described in Patent Documents 1 and 2, it is difficult to suppress local corrosion (pitting corrosion) generated on 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, the immersion test at pH 2.0 as described in the examples of Patent Documents 1 and 2 cannot be said to sufficiently reflect the corrosive environment in an actual ship.

On the other hand, regarding the suppression of the overall corrosion generated on the upper plate of the tanker, it is about 0.11 mm / y even in the case of the lowest corrosion rate among the invention examples described in Patent Documents 1 and 2. On the other hand, the actual crude oil tanker has a service life of 25 years, and the design corrosion allowance of the tanker upper plate is about 2 mm on one side. y or less is required. 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 techniques described in Patent Documents 1 and 2 cannot be desired to omit the 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, we obtained knowledge that the overall corrosion and local corrosion described above can be significantly reduced by appropriately controlling the dislocation density in relation to the steel component composition and the dislocation density of the steel, especially the Cu content and Sn content. .
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: 0.005-0.10%,
N: 0.008% or less and Cu: 0.05-0.4%
A steel material for crude oil tanks, the balance of which is composed of Fe and inevitable impurities, and the dislocation density α of the steel material satisfies the following formula (1) in relation to the Cu content.
α ≦ 4 × 10 ¹⁶ × [% Cu] ^2.8 --- (1)
However, [% Cu] is the Cu content (% by mass) in the steel.

2. The steel material is further in mass%,
Sn: 0.005-0.4%
The steel material for a crude oil tank according to 1 above, wherein the dislocation density α of the steel material satisfies the following formula (2) in relation to the Cu and Sn contents.
α ≦ 4 × 10 ¹⁶ × ([% Cu] + [% Sn]) ^2.8 --- (2)
However, [% Cu] and [% Sn] are the Cu and Sn contents (% by mass) in the steel materials, respectively.

3. The steel material is further in mass%,
Ni: 0.005-0.4%,
Cr: 0.01-0.2%
Mo: 0.005-0.5%
W: 0.005-0.5%
Sb: 0.005 to 0.4%,
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.002 to 0.2%
Ca: 0.0002 to 0.01%,
Mg: 0.0002 to 0.01% and REM: 0.0002 to 0.015%
The steel material for crude oil tanks according to 1 or 2 above, which contains one or more selected from among the above.

4). A crude oil tank manufactured using the steel material for a crude oil tank as described in any one of 1 to 3 above.

According to the present invention, it is possible to effectively suppress the general corrosion and local corrosion generated in an oil tank of a crude oil tanker, a tank for transporting or storing crude oil, and the like, which is extremely useful industrially.

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 increases the strength of steel. In the present invention, C is added in an amount of 0.03% or more 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. In the present invention, Mn is added in an amount of 0.1% or more in order 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 allowable upper limit of the S amount is 0.010%. Preferably it is 0.005% or less.

Al: 0.005-0.10%
Al is an element added as a deoxidizer, and 0.005% or more is added in the present invention. However, if Al is added in excess of 0.10%, the toughness of the steel decreases, so the upper limit of Al content is 0.10%.

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%.

Cu: 0.05-0.4%
Cu is an essential additive element that not only increases the strength of the steel but also exists in the rust generated by the corrosion of the steel, and suppresses the diffusion of Cl- ions that promote the corrosion, thus improving the corrosion resistance. These effects cannot be fully obtained with Cu addition of less than 0.05%. On the other hand, addition of Cu exceeding 0.4% saturates the effect of improving corrosion resistance and may cause problems such as surface cracking during hot working. is there. Therefore, the Cu content is set in the range of 0.05 to 0.4%. Preferably it is 0.06 to 0.35% of range.

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. This effect is manifested when Sn is added in an amount of 0.005% or more. However, when Sn is added in excess of 0.4%, not only the low-temperature toughness is lowered, but also defects are generated during welding. Therefore, the Sn content is set in the range of 0.005 to 0.4%. Preferably it is in the range of 0.01 to 0.2%, more preferably in the range of 0.01 to 0.1%.

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.
Cr: 0.01-0.2%
Cr is with the progress of corrosion proceeds to rust layer, Cl ^- of by blocking entry into rust layers, Cl to interface rust layer and base iron ^- suppressing concentration of, whereby corrosion resistance It contributes to the improvement. In addition, 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 sheet for a long period of time. Can be improved. The above-mentioned effect is remarkable especially 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 a Zn-containing primer treatment is applied to the steel material in the above-mentioned portion containing Cr. As a result, the corrosion resistance can be remarkably improved as compared with a steel material not containing Cr. The effect of Cr is not sufficient if the Cr content is less than 0.01%, while 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 0.20% of range.

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 cannot be obtained sufficiently if the Mg content is less than 0.0002%, while if added over 0.01%, the toughness is reduced, so the Mg content is in the range of 0.0002 to 0.01%.

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 above effect is manifested by adding 0.005% or more of Ni. 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%. Preferably it is 0.08 to 0.35% 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 above effect is manifested when 0.005% or more of Sb is added, but the effect is saturated even if Sb is added in excess of 0.4%. Therefore, Sb is preferably added in the range of 0.005 to 0.4%.

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, when Nb and Ti are added in excess of 0.1% and V is added in excess of 0.2%, the toughness decreases. Therefore, it is preferable to add Nb, Ti and V within the above ranges.

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 effects can be obtained by adding Ca: 0.0002% or more and REM: 0.0002% or more. However, if Ca is added in excess of 0.01% and REM is added in excess of 0.015%, the toughness is reduced. Therefore, Ca and REM are preferably added within the above ranges.

Mo: 0.005-0.5%, W: 0.005-0.5%
Mo and W not only suppress pitting corrosion in the tanker tank bottom plate, but also have an effect of suppressing overall corrosion of the tanker upper deck. The effects of Mo and W are manifested when 0.005% or more is added, but when the content exceeds 0.5%, the effect reaches saturation. Therefore, the Mo and W contents are each preferably in the range of 0.005 to 0.5%. More preferably, it is 0.01 to 0.3%, and still more preferably 0.02 to 0.2%.
The reason why Mo and W have the effect of improving the corrosion resistance as described above is that MoO ₄ ^2- and WO ₄ ^2- are generated in the rust generated as the steel sheet corrodes, and this MoO ₄ ^2- and This is because the presence of WO ₄ ^2- suppresses chloride ions 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 MoO ₄ ^2- and WO ₄ ^{2- on} the steel material surface.

Next, the transition 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 a transition derived from the manufacturing process cannot be avoided. However, since this transition is thermodynamically unstable, it functions as an anode site in which iron dissolves in a corrosive environment. The rust layer formed on the surface of the corrosion-resistant steel is protective and has the effect of reducing the corrosion rate of the steel material, but its function is not perfect, and the density of the transition on the steel surface under the rust layer is large In this case, sufficient protection of the rust layer, and hence satisfactory corrosion resistance, cannot be obtained.

The protection of the rust layer is mainly determined by the Cu concentration in the steel, or the concentration of Cu and Sn when Sn is contained. The higher the Cu and Sn concentration, the better the protection. For this reason, the allowable dislocation density also changes according to the amount of Cu and the amount of Sn.
Therefore, the inventors investigated the relationship between the protection of the rust layer and the amount of Cu and Sn. The dislocation density α was determined according to the following formulas (1) and (2) according to the amount of Cu and Sn in the steel. Thus, it has been determined that good protection of the rust layer can be obtained by controlling to the range given by.
α ≤ 4 × 10 ¹⁶ × [% Cu] ^2.8 --- (1)
α ≦ 4 × 10 ¹⁶ × ([% Cu] + [% Sn]) ^2.8 --- (2)
However, [% Cu] and [% Sn] are the Cu and Sn contents (% by mass) in the steel materials, respectively.

The steel material for crude oil tank of the present invention is preferably produced by the following method.
That is, the steel material of the present invention is obtained by melting steel adjusted to the above-described component composition using a known refining process such as a converter, electric furnace, vacuum degassing, etc., and continuously casting or ingot-bundling rolling. It is preferable to use a steel material (slab) by the method, and then reheat this material and then hot-roll it to obtain a thick steel plate, a thin steel plate, a shaped steel, and the like.

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.

Also, when rolling into a steel material having a desired shape and size by hot rolling, the finish rolling finish 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. This is because the efficiency is lowered.

The steel material after hot rolling may be cooled by either air cooling or accelerated cooling, but accelerated cooling is preferable when higher strength is desired. When accelerated cooling is performed, it is preferable that the cooling rate is 2 to 80 ° C./s and the cooling stop temperature is 650 to 400 ° C. 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 This is because if the temperature is lower than 0 ° C., the toughness of the obtained steel material is lowered or the shape of the steel material is distorted.

Steels with various compositions shown in Tables 1 to 37 in Table 1 are melted in a vacuum melting furnace into steel ingots, or melted in a converter and continuously cast into steel slabs. After reheating to 1150 ° C, hot rolling was performed at the finish rolling finish temperature shown in Table 2 to obtain a steel plate with a thickness of 25 mm, and then cooled to the cooling stop temperature shown in Table 2 at a water cooling rate of 10 ° C / s. did.
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 the overall corrosion on the back of the tanker upper deck, a rectangular piece of width 25mm x length 60mm x thickness 5mm was cut out from each of the thick steel plates No. 1 to 37 above the surface 1mm. 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 surface corrosion test was conducted using a corrosion test apparatus shown in FIG.

This corrosion test apparatus is composed of a corrosion test tank 2 and a temperature control plate 3, and water 6 having a temperature maintained at 30 ° C. is injected into the corrosion test tank 2, and Introduces a mixed gas consisting of 13 vol% CO ₂ , 4 vol% O ₂ , 0.01 vol% SO ₂ , 0.05 vol% H ₂ S and the balance N ₂ through the introduction gas pipe 4 and enters the inside of the corrosion test tank 2. Filled with supersaturated steam, the corrosive environment on 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. Was repeatedly applied for 21, 49, 77, and 98 days to generate condensed water on the surface of the test piece 1 to cause general corrosion. In FIG. 1, 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 sheet thickness per year (corrosion rate on one side). Then, the predicted amount of wear after 25 years is calculated from the values of the four test periods. When the corrosion amount is 2 mm or less, the general corrosion resistance is good (○), and when it exceeds 2 mm, the general corrosion resistance is 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 bottom plate of the tanker oil tank part, rectangular pieces each having a width of 25 mm, a length of 60 mm and a thickness of 5 mm were cut out from the position of the surface of 1 mm of the thick steel plates No. 1 to 37 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, obtain the mass difference before and after the test, divide this difference by the total surface area, and calculate the reduction in thickness (corrosion rate on both sides) per year. 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 Cut out a 20 × 20 × 5mmt test piece from No.1-37 test pieces after the acid resistance test, and use the surface on the 1mm side of the original steel as the measurement surface. did. 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 from the three points of the above plot by the least square method, strain ε was determined from the slope as shown in equation (4), and dislocation density α was determined from equation (5).
β ・ cosθ / λ = 0.9 / D + 2ε ・ sinθ / λ --- (4)
α = 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 2.

As shown in Table 2, the thick steel plates Nos. 1 to 4, 7 to 10, and 13 to 36 that satisfy the conditions of the present invention are subjected to the full corrosion test that simulates the upper deck and the local corrosion test that simulates the tanker bottom plate environment. In any case, good corrosion resistance was exhibited.
On the other hand, the thick steel plates No. 5, 6, 11, 12, and 37 that do not satisfy the conditions of the present invention could not obtain good results in any corrosion resistance test.

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 Thermostatic bath 10 Test solution 11 Tegs

Claims

% 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: 0.005-0.10%,
N: 0.008% or less and Cu: 0.05-0.4%
A steel material for crude oil tanks, the balance of which is composed of Fe and inevitable impurities, and the dislocation density α of the steel material satisfies the following formula (1) in relation to the Cu content.
α ≤ 4 × 10 16 × [% Cu] 2.8 --- (1)
However, [% Cu] is the Cu content (% by mass) in the steel.
The steel material is further in mass%,
Sn: 0.005-0.4%
The steel material for a crude oil tank according to claim 1, wherein the dislocation density α of the steel material satisfies the following formula (2) in relation to the Cu and Sn contents.
α ≦ 4 × 10 16 × ([% Cu] + [% Sn]) 2.8 --- (2)
However, [% Cu] and [% Sn] are the Cu and Sn contents (% by mass) in the steel materials, respectively.
The steel material is further in mass%,
Ni: 0.005-0.4%,
Cr: 0.01-0.2%
Mo: 0.005-0.5%
W: 0.005-0.5%
Sb: 0.005 to 0.4%,
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
V: 0.002 to 0.2%
Ca: 0.0002 to 0.01%,
Mg: 0.0002 to 0.01% and REM: 0.0002 to 0.015%
The steel material for crude oil tanks of Claim 1 or 2 containing 1 type, or 2 or more types chosen from these.
A crude oil tank produced using the steel material for a crude oil tank according to any one of claims 1 to 3.