JP2003082435A - Steel for cargo oil tank - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide inexpensive steel for a cargo oil tank which has excellent corrosion resistance. SOLUTION: (1) The steel has a composition containing 0.01 to 0.3% C, 0.02 to 1% Si, 0.05 to 2% Mn, <=0.05% P, <=0.01% S and 0.03 to 3% Ni, and the balance Fe with impurities, and, Mo, Cu, Cr, W, Ca, Ti, Nb, V, B, Sb, Sn and Al can be contained therein. (2) The steel has a composition containing 0.01 to 0.3% C, 0.02 to 1% Si, 0.05 to 2% Mn, <=0.05% P, <=0.01% S, 0.01 to 3% Ni, 0.01 to 2% Cu, <=0.05% Cr and <=0.07 Al, and the balance Fe with impurities, and in which the number of inclusion particles with a grain diameter of >30 μm is <30 pieces per cm<2> , and, provided that Ap denotes the ratio in the % unit of pearlite occupied in the structure, and C denotes the content of carbon in mass%, A/C<=130 is satisfied. The steel can contain Mo, Cu, W, Ca, Ti, Nb, V, B, Sb and Sn.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a steel material for a cargo oil tank which is a crude oil tank in a tanker.

[0002]

2. Description of the Related Art At present, bare steel is used as a material for cargo oil tanks of tankers.

The gas phase portion of the cargo oil tank is used for explosion-proof purposes.
Using N engine exhaust gas_TwoMain gas (hereinafter,
This is called inert gas), but dry and wet
It is a repetitive and severe corrosive environment. Furthermore, hydrogen sulfide
(H_TwoS) is included in the crude oil when loaded.
H_TwoSince a part of S 2 is transferred to the gas phase,
Then it will be extremely severe. Carbonization contained in crude oil
It is of course not possible to vaporize some of the hydrogen and mix it with inert gas.
However, since it has almost no effect on corrosion, it can be ignored.
It As an example of the inert gas, volume%
And 13% CO _Two-5% O_Two-0.1% SO_Two -The balance
N_Two A gas having the composition of

In the corrosive environment as described above, there is a case where the entire surface of the back of the deck, which is the ceiling of the cargo oil tank, is corroded, and the corrosion rate exceeds 0.1 mm / year.
Cases of general corrosion with a very high corrosion rate of 0.3 mm / year or more have also been reported. In addition, pitting corrosion may occur on the bottom plate of the cargo oil tank, resulting in a large pitting corrosion growth rate of several mm / year.

Under these circumstances, the material of the cargo oil tank is partially coated, but the cost of the initial coating and repainting approximately every 10 years is high. Therefore, for example, by taking a corrosion allowance of 2 mm for 20 years of use, the actual situation is that it is a countermeasure against general corrosion or local corrosion.

However, if the corrosion allowance is taken, the thickness of the steel material increases, so that the manufacturing cost of the tank rises, and the crude oil load decreases, which is also a demerit. Therefore, it is strongly desired to develop a steel material for a cargo oil tank having excellent corrosion resistance, which can reduce the corrosion allowance and prevent the cost from increasing.

As the steel for the cargo oil tank,
For example, Japanese Patent Laid-Open No. 2000-17381 discloses Cu and Mg.
A steel containing as an essential component is also disclosed in JP-A-2001-107.
No. 180 discloses a steel containing Cr and Al as essential components,
Each has been proposed. However, in the steels disclosed in these publications, when the crude oil contains H ₂ S, no consideration is given to the effect of H ₂ S on corrosion, and therefore, the cargo oil of the actual ship is not considered. In some cases, sufficient corrosion resistance could not be obtained in the tank.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel material for a cargo oil tank having excellent resistance to general corrosion and local corrosion.

[0009]

The gist of the present invention resides in the steel materials for cargo oil tanks shown in the following (1) to (7).

(1)% by mass, C: 0.01 to 0.3
%, Si: 0.02-1%, Mn: 0.05-2%,
P: 0.05% or less, S: 0.01% or less, Ni: 0.
05-3%, Mo: 1% or less, Cu: 1% or less, Cr:
2% or less, W: 1% or less, Ca: 0.01% or less, T
i: 0.1% or less, Nb: 0.1% or less, V: 0.1%
Hereinafter, a steel material for a cargo oil tank containing B: 0.05% or less and the balance Fe and impurities.

(2) The steel material for cargo oil tank according to the above (1), the surface of which is covered with an anticorrosion coating.

(3)% by mass, C: 0.01 to 0.3
%, Si: 0.02-1%, Mn: 0.05-2%,
P: 0.05% or less, S: 0.01% or less, Ni: 0.
01 to 3%, Mo: 1% or less, Cu: 2% or less, Cr:
2% or less, W: 1% or less, Ca: 0.01% or less, T
i: 0.1% or less, Nb: 0.1% or less, V: 0.1%
Below, B: 0.05% or less, Sb: 0.5% or less, S
A steel material for a cargo oil tank containing n: 0.5% or less, Al: 0.07% or less, and the balance being Fe and impurities.

(4) C: 0.01 to 0.3 in mass%
%, Si: 0.02-1%, Mn: 0.05-2%,
P: 0.05% or less, S: 0.01% or less, Ni: 0.
01 to 3%, Cu: 0.01 to 2%, Mo: 1% or less,
W: 1% or less, Ca: 0.01% or less, Sb: 0.5%
Below, Sn: 0.5% or less, Ti: 0.1% or less, N
b: 0.1% or less, V: 0.1% or less, B: 0.05%
Hereafter, Cr: 0.05% or less and Al: 0.07% or less are contained, the balance is Fe and impurities, and the particle size is 30 μm.
A steel material for a cargo oil tank that has less than 30 inclusions per 1 cm ² and that satisfies the following formula (1).
However, in the following formula (1), Ap represents the ratio of pearlite in the structure in%, and C represents the content of carbon in mass%.

Ap / C ≦ 130 (1).

(5) Mo content of 0.01 to 1%, W
Content of 0.01 to 1%, Sb content of 0.01 to 1%
The steel material for a cargo oil tank according to (4) above, wherein the content of 0.5% and the content of Sn satisfy at least any one of 0.01 to 0.5%.

(6) The content of Ti is 0.005 to 0.1
%, Nb content is 0.002-0.1%, V content is 0.01-0.1%, B content is 0.0002-
The steel material for cargo oil tank according to the above (4) or (5), which satisfies at least one of 0.05%.

(7) The steel material for a cargo oil tank according to any one of the above (3) to (6), the steel material having at least one surface subjected to anticorrosion treatment.

Hereinafter, the inventions relating to the steel materials (1) to (7) are referred to as inventions (1) to (7), respectively.

In the present invention, "inclusion" means JIS G 05
Refers to the A type inclusions or B type inclusions described in 55, and the "particle size" is the value defined by the diameter of a circular shape and the average of the major axis and the minor axis when the shape is flat. Refers to.

In order to achieve the above-mentioned objects, the present inventors conducted a number of experiments by simulating the corrosive environment of an actual ship.

That is, first, H containing an inert gas
Numerous experiments were conducted in a dry and wet repeated environment containing no ₂ S. As a result, the following findings (a) to (c) were obtained.

(A) In a dry-wet repeated environment containing an inert gas but not H ₂ S, the resistance to general corrosion (hereinafter referred to as general corrosion resistance) is remarkably improved by adding Ni. FIG. 1 shows the relationship between the Ni content and the general corrosion rate. In FIG. 1, the “general corrosion rate” is simply referred to as the “corrosion rate”. As is clear from FIG. 1, the corrosion rate remarkably decreases as the Ni content increases.

(B) In addition to Ni, Mo, Cu, Cr,
Addition of one or more of W, Ca, Sb and Sn further improves general corrosion resistance.

(C) Further, when the above elements of Mo to Sn are added in combination, not only the general corrosion resistance is further improved, but also the durability of the anticorrosion coating applied on the surface is improved.

Next, the present inventors conducted a number of experiments in a dry-wet cyclic environment containing inert gas and H ₂ S. As a result, we succeeded in reproducing the corrosion product layer found on the backside of the deck of an actual ship loaded with crude oil containing H ₂ S. Furthermore, regarding the corrosion mechanism of the vapor phase part and the pitting corrosion mechanism of the bottom plate part, the following (d )-(I).

(D) SO contained in inert gas_Two 
And H that migrates from crude oil to the gas phase_TwoBoth S and Ina
O contained in gas_Two Or H_TwoReacts with O
H _TwoSO_ThreeIs generated. Furthermore, H_TwoSO_ThreeIs O_Two To
More oxidized H_TwoSO _FourAlso generate.

(E) At the time of dew condensation, the above H ₂ SO ₃ and H
₂ SO ₄ is contained in water. Therefore, the environment of repeated dry and wet conditions containing the inert gas and H ₂ S is a corrosive environment of repeated dry and wet with acidic water.

(F) Corrosion on the backside of the deck of an actual ship progresses at night when the temperature drops and dew condensation occurs, and when a large amount of CO ₂ , SO ₂ or O ₂ exists without loading crude oil, corrosion products Is mainly α-FeOOH (hereinafter referred to as “rust”).

(G) On the other hand, when the crude oil containing H ₂ S is loaded, H ₂ S is oxidized by O _{2 on} the surface of rust to produce solid S.

(H) When the crude oil is not loaded and the crude oil is loaded repeatedly, "rust /
A layered structure of "S / rust / S ..." is formed.

(I) Although the drain water is retained at the bottom of the cargo oil tank and the surface is coated with an oil film, the oil film coating is partly peeled off due to sludge movement or crude oil washing. The generated S drops on the back of the deck and adheres to it. The S then acts as an oxidant, leading to a noble corrosion potential, and pitting occurs in the presence of H ₂ S and Cl ⁻ . In addition, since rock salt existing in the oil well is mixed into crude oil and brought into the tank at the time of mining, Cl at the bottom of the tank.
^-Is present.

Based on the above corrosion mechanism of the vapor phase portion and the pitting corrosion generation mechanism of the bottom plate portion, further experiments by the present inventors revealed the following items (j) to (p). .

(J) By adding Cu, it is possible to suppress the general corrosion in the environment of repeated dry and wet with acidic water and the occurrence of pitting corrosion in the presence of S. As an example, in FIG.
The relationship between the Cu content and the general corrosion rate in the environment of repeated dry and wet with acidic water is shown. Note that, also in FIG. 2, the "general corrosion rate" is simply referred to as "corrosion rate".
As is clear from FIG. 2, the corrosion rate remarkably decreases as the Cu content increases.

(K) By including Cu and Ni in combination, general corrosion resistance and pitting corrosion resistance are further improved.

(L) In addition to Cu and Ni, Mo, W,
Addition of one or more of Ca, Sb and Sn further improves general corrosion resistance and pitting corrosion resistance.

(M) If Cu to Sn are added, the coating life will be longer than in the conventional case.

(N) By limiting the contents of Cr and Al, it is possible to suppress general corrosion in an environment of repeated dry and wet conditions with acidic water.

(O) The general corrosion in an environment of repeated dry and wet with acidic water is also related to the ratio Ap of the pearlite in the structure in% unit and the C content in mass%.
The general corrosion resistance improves as the value represented by "C" decreases.

(P) The grain size and the amount of inclusions per unit area of the inclusions in steel influence the occurrence of general corrosion and pitting corrosion in the presence of S in the environment of repeated dry and wet conditions with acidic water.

The inventions (1) to (3) are the same as the above (a) to
It was completed based on the knowledge of (c).

In the inventions of (1) and (2), among the above-mentioned elements constituting the steel material, Mo, Cu and C are included.
The nine elements of r, W, Ca, Ti, Nb, V and B do not necessarily have to be positively added, and the content thereof may be an impurity level. The lower the S content, the better.

Also in the invention of (3), Mo, Cu, C
r, W, Ca, Ti, Nb, V, B, Sb, Sn and A
The 12 elements of 1 do not necessarily have to be positively added, and the content thereof may be an impurity level. The lower the S content, the better.

The inventions of (4) to (7) are (d) to (7) above.
It was completed based on the knowledge of (p).

In the invention of (4), among the elements constituting the steel material, Mo, W, Ca, Sb and S are included.
The 11 elements of n, Ti, Nb, V, B, Cr, and Al do not necessarily have to be positively added, and the content thereof may be an impurity level. Among the above elements, the lower the content of Cr and Al, the more preferable. The lower the S content, the better.

[0045]

BEST MODE FOR CARRYING OUT THE INVENTION Each requirement of the present invention will be described in detail below. The “%” display of the content of each element means “mass%”. (A) Chemical composition C of steel material: C is an element necessary for securing the strength as a material, and the content of 0.01% or more is necessary. However, if the content exceeds 0.3%, the weldability decreases.
Further, as the C content increases, the amount of cementite produced which becomes a cathode and promotes corrosion in an environment of repeated wet and dry with acidic water increases, and in particular, when the C content exceeds 0.30%, cementite is produced. Corrosion increases and corrosion becomes significant. Therefore, the content of C is set to 0.01 to 0.3%. The preferable range of the C content is 0.01 to 0.2%, and the more preferable range is 0.01 to 0.15%.

Si: Si is an element necessary for deoxidation,
In order to obtain a sufficient deoxidizing effect, it is necessary to contain 0.02% or more. However, if the content exceeds 1%, the toughness is impaired. Therefore, the Si content is 0.02-1
%. The preferred content range is 0.02-0.8%
And a more preferable range is 0.02 to 0.5%.

Mn: Mn is an element having the effect of enhancing the strength of steel at low cost, and the content of 0.05% or more is necessary to obtain this effect. However, if the content exceeds 2%, the weldability deteriorates. Therefore, Mn
Content of 0.05 to 2%. The preferable range of the content is 0.05 to 1.8%, and the more preferable range is 0.05 to 1.5%.

P: P is an impurity element contained in steel,
Reduces weldability. In particular, if the content exceeds 0.05%, the weldability deteriorates significantly. Therefore, the P content is set to 0.05% or less. Since P has the effect of improving the general corrosion resistance while decreasing the weldability, it may be contained in an amount of 0.01% or more in order to increase the general corrosion resistance. The preferable upper limit of the P content is 0.04.
%, And a more preferable upper limit is 0.03%.

S: 0.01% or less S is an impurity element contained in steel, and its content is 0.1%.
If it exceeds 01%, a large amount of MnS is produced in the steel, and MnS becomes the starting point of corrosion, causing general corrosion and pitting corrosion. Therefore, the content of S is set to 0.01% or less. The preferable upper limit of the S content is 0.008%, and the more preferable upper limit thereof is 0.0.
It is 05%. The lower the S content, the better.

Ni: Ni is an element that does not contain H ₂ S and improves the general corrosion resistance in a dry and wet repeated environment.
Ni also has the effect of forming a corrosion-resistant sulfide film in a wet hydrogen sulfide environment to enhance general corrosion resistance and the effect of improving pitting corrosion resistance. However, in order to obtain these effects, it is necessary to contain Ni in an amount of 0.01% or more.
If the content is 5% or more, a more remarkable effect can be obtained. However, even if Ni is contained in an amount of more than 3%, the effect is saturated and the cost is increased. Therefore, the content of Ni is set to 0.01 to 3%. A preferable content range is 0.1 to 3%, and a more preferable range is 0.5 to 3%.

It is sufficient for the steel materials according to the inventions (1) to (3) and (7) to satisfy the above chemical composition. But,
In addition to the above component elements, Cu described below may be added if necessary.
One or more elements selected from the elements from to Al may be included.

On the other hand, in the case of the steel materials according to the inventions (4) to (6), it is necessary to contain Cu as an essential element in addition to the above chemical composition. Even in the case of the steel materials according to the inventions of (4) to (6), one or more elements selected from the elements from Mo to Al described below may be included if necessary.

Cu: Cu is an element that does not contain H ₂ S and improves the general corrosion resistance in a dry and wet repeated environment.
This effect can be obtained even if the content is at the impurity level, but the effect becomes remarkable when the content is 0.01% or more, and the effect becomes more remarkable when the content is 0.1% or more. Cu has the effect of enhancing the general corrosion resistance in the environment of repeated dry and wet with acidic water and significantly improving the general corrosion resistance in the presence of hydrogen sulfide, and further suppressing the occurrence of pitting corrosion in the presence of S. Is also effective, and in order to obtain these effects, the content of Cu needs to be 0.01% or more, and if it is 0.1% or more, a more reliable effect can be obtained. However, in any case, the effect is saturated even if Cu is contained in excess of 2%. Furthermore, the steel may become brittle. From the viewpoint of preventing the embrittlement of steel, the upper limit of the Cu content is more preferably 1%. Therefore,
In the inventions of (1) to (3), the content of Cu in the case of steel not containing Cu as an essential constituent element is 1% or less in the inventions of (1) and (2), and in the invention of (3). 2%
Below. Further, the content of Cu in the case of steel containing Cu as an essential constituent element is set to 0.01 to 2%.

Mo: Mo is an element which does not contain H ₂ S and improves the general corrosion resistance in a dry and wet repeated environment.
Mo also has the effect of forming a corrosion-resistant sulfide film in a wet hydrogen sulfide environment to enhance general corrosion resistance and the effect of improving pitting corrosion resistance. Furthermore, Mo also has the effect of increasing acid resistance. These effects can be obtained even at the content of the impurity level, but in order to obtain the effect more remarkably, M
The content of o is preferably 0.01% or more. However, if Mo is contained in excess of 1%, not only the effect is saturated, but also the weldability is impaired and the cost is increased. Therefore, when Mo is added, its content is preferably 1% or less. The lower limit of the Mo content when added is more preferably 0.1%, and even more preferably 0.3%.

Cr: Cr has a function of forming a rust layer having a protective property in a dry and wet repeated environment containing no H ₂ S to enhance general corrosion resistance. This effect can be obtained even when the content is at the impurity level, but in order to obtain the effect more significantly, it is preferable that the content of Cr be 0.01% or more. However, if the content exceeds 2%, the weldability decreases. Therefore, when Cr is added to enhance the general corrosion resistance in the above environment, its content is preferably 2% or less. In this case, the more preferable range of the Cr content is 0.1 to 2%, and the more preferable range is 0.5 to 2%.

On the other hand, Cr reduces the general corrosion resistance in a dry and wet repeated environment containing H ₂ S, that is, the environment of a dry and wet repeated with acidic water, and also reduces the pitting corrosion resistance in the presence of S. If the content exceeds 0.05%, the general corrosion resistance and the pitting corrosion resistance in the above-mentioned environment are significantly deteriorated. Therefore, Cr when used in the above environment
The content was set to 0.05% or less. In this case, Cr
The content is preferably 0.04% or less.

W: W is an element that does not contain H ₂ S and improves the general corrosion resistance in a dry and wet repeated environment. W also has an effect of forming a corrosion-resistant sulfide film in a wet hydrogen sulfide environment to enhance general corrosion resistance and an effect of improving pitting corrosion resistance. Further, W also has the effect of increasing acid resistance. These effects can be obtained even with the content of the impurity level, but in order to obtain the effect more remarkably, W is 0.0
The content is preferably 1% or more. However, even if W is contained in an amount of more than 1%, the above effect is saturated and the cost is increased. Therefore, when W is added, its content is preferably 1% or less. The lower limit of the W content when added is more preferably 0.1% and even more preferably 0.3%.

Ca: Ca improves general corrosion resistance in a dry and wet repeated environment containing no H ₂ S. Note that Ca
Exists in the form of an oxide in the steel, dissolves in water, becomes alkaline, and suppresses the pH decrease at the steel material interface at the time of a corrosion reaction, and therefore also has the effect of suppressing general corrosion when H ₂ S is contained. These effects can be obtained even when the content is at the impurity level, but in order to obtain the effect more remarkably, Ca is less than 0.
The content is preferably 0002% or more. However, even if Ca is contained in an amount of more than 0.01%, the above effect is saturated and the cost is increased. Therefore, Ca
When adding, the content is preferably 0.01% or less. The lower limit of the content of Ca when added is more preferably 0.0005%, and 0.0
More preferably, it is 01%.

Ti: Ti has the function of increasing the strength of steel. Ti has the effect of improving the toughness of steel and TiS.
The formation of MnS also suppresses the formation of MnS, which is the starting point of corrosion, and also has the effect of enhancing general corrosion resistance and pitting corrosion resistance.
These effects can be obtained even if the content is at the impurity level, but in order to obtain the effect more significantly, Ti is 0.00
The content is preferably 5% or more. However, Ti
If the content of Al exceeds 0.1%, the above effect is saturated and the cost is increased. Therefore, when Ti is added, its content is preferably 0.1% or less.
The lower limit of the Ti content when added is 0.01
% Is more preferable, and 0.05% is even more preferable.

Nb: Nb is an element having the action of increasing the strength of steel. This effect can be obtained even if the content is at the impurity level, but in order to obtain the effect more significantly, N
The content of b is preferably 0.002% or more.
However, if the Nb content exceeds 0.1%, the toughness is lowered. Therefore, when Nb is added, its content is preferably 0.1% or less. The upper limit of the Nb content when added is preferably 0.05%, and more preferably 0.03%.

V: V is an element having an action of improving the strength of steel. This effect can be obtained even with the content of the impurity level, but in order to obtain the effect more remarkably, V
Is preferably 0.01% or more. However, if V exceeds 0.1%, toughness and weldability are deteriorated. Therefore, when V is added, its content is preferably 0.1% or less. The upper limit of the V content when added is preferably 0.05%, and more preferably 0.03%.

B: B has the effect of increasing the strength of the steel. This effect can be obtained even when the content is at the impurity level, but in order to obtain the effect more markedly, B is 0.000.
The content is preferably 2% or more. However, if B is contained in excess of 0.05%, the toughness is lowered. Therefore, when B is added, its content should be 0.0
It is preferable to be 5% or less. When added, the upper limit of the B content is preferably 0.02%,
More preferably, it is 01%.

Sb: Sb has the functions of improving the general corrosion resistance in a dry and wet repeated environment and also increasing the acid resistance. These effects can be obtained even if the content is at the impurity level, but in order to obtain the effect more remarkably, it is preferable that the content of Sb is 0.01% or more. However, even if Sb exceeds 0.5%, the above effect is saturated. Therefore, when Sb is added, its content is preferably 0.5% or less. The lower limit of the Ti content when added is preferably 0.01%, and more preferably 0.05%.

Sn: Sn has the functions of improving the general corrosion resistance in a dry-wet repeated environment and enhancing the acid resistance. These effects can be obtained even with the content of the impurity level, but in order to obtain the effect more remarkably, it is preferable that the content of Sn is 0.01% or more. However, even if Sn is contained in an amount of more than 0.5%, the above effect is saturated. Therefore, when Sn is added, its content is preferably 0.5% or less. The lower limit of the Sn content when added is preferably 0.1%.

Al: Al is an element effective for deoxidizing steel, but since it contains Si in an amount already described in the present invention, it can be deoxidized with Si. Therefore, A
Since deoxidation treatment with 1 is not particularly necessary, Al may not be added. On the other hand, if Al is positively added,
As the deoxidizing effect is enhanced, the nitride is formed and the austenite grains are made finer, so that the strength is improved. Further, an effect of improving toughness can be obtained. These effects are surely obtained when the Al content is 0.001% or more. Therefore, in order to obtain the deoxidizing effect and the effect of improving strength and toughness, A
You may add 1 and make it contain 0.001% or more. However, even if Al is contained in an amount of more than 0.07%, not only the deoxidizing effect is almost saturated, but also the toughness is rather deteriorated due to the coarsening of the nitride. In addition, when the content of Al is large, the amount of alumina-based inclusions is increased to reduce the general corrosion resistance in a dry and wet repeated environment containing H ₂ S, that is, an environment of repeated dry and wet with acidic water. 0.
If it exceeds 07%, the general corrosion resistance is remarkably lowered in the environment of repeated dry and wet with acidic water. Therefore, when Al is added, its content is preferably 0.07% or less. The lower limit of the Al content when added is preferably 0.05%.

(B) The general corrosion of the steel material in the environment of repeated dry and wet with acidic water causes the cementite forming pearlite to accelerate the hydrogen ion reduction of the cathodic reaction, so that% of pearlite in the structure is occupied.
Related to the proportion Ap in units and the C content in mass%,
When the value of "Ap / C" becomes large, the general corrosion resistance decreases. In particular, when the value of "Ap / C" is 130 or more, even in the case of a steel material whose chemical composition has been adjusted to the range already described, the general corrosion resistance is significantly reduced. Therefore,
The formula (1), that is, “Ap / C ≦ 130” is specified.

Further, the inclusions in the steel serve as the starting point of corrosion, which affects the general corrosion in the environment of repeated dry and wet with acidic water and the occurrence of pitting corrosion in the presence of S. However, the smaller the particle size of inclusions and the smaller the amount of inclusions per unit area, the less the effect on the occurrence of general corrosion and pitting corrosion, and the number of inclusions having a particle size of more than 30 μm per cm ² is less than 30. Therefore, general corrosion resistance and pitting corrosion resistance do not decrease.
Therefore, inclusions with a particle size exceeding 30 μm are 1 cm ²
It was defined as less than 30 per item.

It should be noted that the number of inclusions having a particle size exceeding 30 μm is 1
Less than 30 per cm ² , and “Ap / C ≦ 13
The steel material satisfying “0” is, for example, a slab in which the content of S is kept low and electromagnetic stirring is performed in the steelmaking stage,
Depending on the steel composition, hot rolling is carried out by a usual method under the conditions that the heating temperature is about 1150 ° C, the rolling reduction per pass is 3% or more, and the rolling finishing temperature is about 800 to 900 ° C.
After the rolling is completed, the temperature should be at least 57 after the temperature of Ar ₃
It can be produced by water-cooling a temperature range up to about 0 ° C. and then allowing it to cool in the atmosphere. The above temperatures are all temperatures at the surface of the steel material.

The above-described steel material of the present invention exhibits good corrosion resistance even when used as it is and can reduce the corrosion allowance. However, when the surface thereof is covered with an anticorrosion coating made of organic resin or metal, The durability of the coating is improved and the corrosion resistance is further improved.

Here, the organic resin anticorrosion coating is a vinyl butyral type, epoxy type, urethane type, phthalic acid type resin coating or the like, and the metal anticorrosion coating is a plated coating such as Zn or Al or a thermal spray coating. Mention may be made of coatings.

The durability of the anticorrosion coating is improved by
It is considered that this is because the corrosion of the steel material of the present invention as the base is remarkably suppressed, and as a result, the swelling and peeling of the anticorrosion coating due to the corrosion of the base steel material from the defective portion of the anticorrosion coating is suppressed.

The treatment for covering with the anticorrosion coating may be carried out by a usual method. Further, it is not always necessary to apply an anticorrosion coating to the entire surface of the steel material, and only one surface of the steel material that is exposed to the corrosive environment may be anticorrosion-treated.

[0073]

Examples (Example 1) 28 kinds of steel materials having the chemical compositions shown in Table 1 and Table 2 were prepared by a usual method,
25mm wide, 50mm long, 4m thick from each steel
A test piece of m was sampled and directly subjected to a corrosion test simulating an actual ship described below.

That is, while preparing a glass container in which a 0.1% by mass NaCl aqueous solution is placed in the lower 1/3 portion, the above glass container is covered by an acrylic lid having a gas supply port on which a sample piece is attached. The upper end of the opening was sealed. Then, the hermetically sealed glass container was placed in a constant temperature bath, and a temperature cycle of 50 ° C. × 20 hours → 25 ° C. × 4 hours was applied for 4 months. At that time, the following gas A was blown into the gas phase portion in the glass container through the gas supply port.

Gas A: volume% 5% O ₂ -13% CO
₂ -0.02% SO ₂ - residual _N 2.

After the corrosion test for 4 months, the corrosion rate in "mm / year" unit, that is, the general corrosion rate was obtained from the reduced mass of each test piece, and the results are shown in Table 1 and Table 2 together. It was

[0077]

[Table 1]

[0078]

[Table 2]

As can be seen from the results shown in Tables 1 and 2,
Trial Nos. A1 to A20 and A whose chemical composition is within the range specified in the present invention
The steel materials of 37 to A40 have a small general corrosion rate of 0.13 mm / year or less. On the other hand, the steel materials of the trial numbers A21 to A24, whose chemical compositions are out of the range specified in the present invention, have a large general corrosion rate of 0.27 mm / year or more.

(Example 2) 16 kinds of steel materials having the chemical compositions shown in Table 3 were prepared by a usual method, and a test piece having a width of 25 mm, a length of 50 mm and a thickness of 4 mm was taken from each steel material. And the surface is 200 μm thick in the usual way.
A tar-epoxy resin anticorrosion coating was applied and subjected to a corrosion test under the same conditions as in Example 1.

After the corrosion test for 4 months, the general corrosion rate in "mm / year" was obtained from the reduced mass of each test piece.

The durability of the anticorrosion coating was also investigated. That is,
Prepare a test piece with a crosscut-shaped notch that reaches the steel surface in the anticorrosion coating using a cutter knife, subject it to a corrosion test under the same conditions as above, and swell the anticorrosion coating from the crosscut portion after the test. The durability of the anticorrosion coating was evaluated by measuring (mm). Table 3 also shows the results of the above tests.

[0083]

[Table 3]

As can be seen from the results shown in Table 3, sample numbers A25 to A33 and A41 to A whose chemical compositions are within the range specified in the present invention.
The steel materials of No. 44 have a small general corrosion rate of 0.04 mm / year or less and a swelling width of 1.0 mm or less. On the other hand, a trial number whose chemical composition is outside the range specified in the present invention
The steel materials of A34 to A36 have a large general corrosion rate of 0.08 mm / year or more and an extremely large blistering width of 10.0 mm or more.

Example 3 26 kinds of steels having the chemical compositions shown in Tables 4 and 5 were melted by using a vacuum melting furnace and 50
After making into a steel ingot, the thickness is 1 by hot forging by the usual method.
A 20 mm block was made.

[0086]

[Table 4]

[0087]

[Table 5]

Then, the thickness obtained by the above forging is 12
The 0 mm block was heated at 1150 ° C. for 2 hours and then hot-rolled into a steel plate having a thickness of 20 mm. In addition, the above 2
The production conditions for a 0 mm steel plate are the following "Production Method 1" or "Production Method 2".

[Manufacturing method 1]: A manufacturing method in which a block having a thickness of 120 mm is heated at 1150 ° C. for 2 hours, hot-rolled to a thickness of 20 mm at 950 ° C., and then left to cool to room temperature in the atmosphere.

[Manufacturing method 2]: A block having a thickness of 120 mm was heated at 1150 ° C for 2 hours, hot-rolled to a thickness of 20 mm at 850 ° C, and then water-cooled in a temperature range from 800 ° C to 500 ° C. Then, a manufacturing method of allowing to cool to room temperature in the atmosphere.

From each steel plate having a thickness of 20 mm, JIS G 0555
A micro test piece was cut in parallel with the rolling direction through the center line in accordance with the above, and the structure and inclusions were examined by an optical microscope.

That is, the surface cut in parallel to the rolling direction (L section) was used as the surface to be inspected, and after mirror polishing, the magnification was 4
An optical microscope photograph of 60 fields of view was taken at 00 times, and the number of inclusions having a particle size of more than 30 μm was measured from the photograph.

Also, the mirror-polished surface is corroded with nital,
It was observed with a 10-field optical microscope at a magnification of 100 times, the ratio Ap of the pearlite in the tissue in% was measured, and the value of "Ap / C" was calculated.

Further, from each steel plate having the thickness of 20 mm,
A test piece having a width of 25 mm, a length of 50 mm, and a thickness of 4 mm was sampled and directly subjected to a corrosion test simulating the deck back environment of an actual ship described below. The corrosion test is based on the assumption that the crude oil contains H ₂ S.

That is, while preparing a glass container containing a 0.1% by mass NaCl aqueous solution in the lower 1/3 part, the above-mentioned glass container is covered with an acrylic lid having a gas supply port on which a sample piece is attached The upper end of the opening was sealed. Then, the hermetically sealed glass container was placed in a constant temperature bath, and a temperature cycle of 50 ° C. × 20 hours → 25 ° C. × 4 hours was applied for 4 months. At that time, the gas phase portion in the glass container was simulated at the time of ballast and at the time of full load, and the following two kinds of gas A and gas B were alternately blown at two-week intervals from the gas supply port.

Gas A: volume% 5% O ₂ -13% CO
₂ -0.02% SO ₂ - residual _N 2, gas B: _{volume%, 5% O 2 -13%} CO 2 -0.02
_{% SO} 2 -0.25% _H 2 S- residual _N 2.

After the corrosion test for 4 months, the corrosion rate in units of "mm / year" from the reduced mass of each test piece (general corrosion rate)
I asked. Table 6 shows the results of each of the above tests together with the manufacturing conditions for a 20 mm thick steel plate.

[0098]

[Table 6]

As can be seen from the results shown in Table 6, the steel materials of trial Nos. B1 to B20 having chemical compositions within the range specified by the present invention are
The general corrosion rate is as small as 0.13 mm / year or less. On the other hand, trial number B2 whose chemical composition is outside the range specified in the present invention
Steel materials 1 to B26 have a large general corrosion rate of 0.29 mm / year or more.

(Example 4) From each steel plate having a thickness of 20 mm described in Example 3 above, a width of 100 mm and a length of 100
A test piece having a thickness of 4 mm and a thickness of 4 mm was sampled and subjected to a corrosion test simulating the environment of the cargo oil tank bottom plate of the actual ship described below. This corrosion test is also based on the assumption that the crude oil contains H ₂ S, as in the case of Example 3 above.

That is, with respect to each of the width and length directions of the test piece having the width of 100 mm, the length of 100 mm, and the thickness of 4 mm, the central portion 3 mm is sealed and the remaining portion is filled with the crude oil with a thickness of 1 mm. After coating, remove the seal at the center and apply solid S onto the test piece with the steel surface exposed in the shape of a cross.
Was attached at a rate of 0.2 g / cm ² and subjected to a corrosion test.

FIG. 3 shows the corrosion test piece thus produced.

Next, the above corrosion test piece was attached to the bottom of a glass container containing artificial seawater, and the opening upper end of the glass container was sealed with an acrylic lid having a gas supply port. Then, the sealed glass container is placed in a constant temperature bath, and the temperature is kept at 40
The test was performed at 1 ° C for 1 month. At that time, artificial seawater in a glass container was simulated at the time of ballast and at the time of full load, and the following two kinds of gas A and gas B were supplied from the gas supply port.
Were alternately blown in at intervals of 2 weeks.

Gas A: volume% 5% O ₂ -13% CO
₂ -0.02% SO ₂ - residual _N 2, gas B: _{volume%, 5% O 2 -13%} CO 2 -0.02
_{% SO} 2 -0.25% _H 2 S- residual _N 2.

After the corrosion test for one month, the pitting growth rate was determined from the maximum pitting depth of the test piece. Table 6 also shows the above test results.

As is clear from Table 6, the steel materials of the trial numbers B1 to B20 having the chemical composition within the range specified by the present invention and satisfying the above-mentioned formula (1) and the inclusions had pitting corrosion progress. The speed is as small as 0.15 mm / year or less. On the other hand, the steel materials of trial Nos. B21 to B26 in which any one of the chemical composition, the inclusion definition, and the small value of "Ap / C" is out of the range defined by the present invention, have the pitting corrosion rate of 0.61 mm. / Larger than a year.

Example 5 14 kinds of steels having the chemical compositions shown in Table 7 were melted in a vacuum melting furnace into a 50 kg steel ingot, which was then hot forged by a usual method to a thickness of 120 mm.
Block was prepared.

[0108]

[Table 7]

Then, the thickness obtained by the above forging is 12
The 0 mm block was heated at 1150 ° C. for 2 hours and then hot-rolled into a steel plate having a thickness of 20 mm. In addition, the above 2
The manufacturing conditions for the 0 mm steel plate are the "manufacturing method 1" or the "manufacturing method 2" in the first embodiment.

In the same manner as in Example 1, the thickness is 2
A micro test piece was prepared from each 0 mm steel plate, and the structure and inclusions were examined by an optical microscope.

Further, a test piece having a width of 25 mm, a length of 50 mm and a thickness of 4 mm was taken from each of the steel plates having a thickness of 20 mm and the surface thereof was coated with a tar-epoxy resin having a thickness of 200 μm by a usual method. After applying an anticorrosion coating, it was subjected to a corrosion test under the same conditions as in Example 3.

After the 4-month corrosion test, the general corrosion rate in "mm / year" unit was determined from the reduced mass of each test piece.

Table 8 shows the results of each of the above tests with a thickness of 20 mm.
Shown together with the steel plate manufacturing conditions.

[0114]

[Table 8]

As is clear from Table 8, the steel materials of trial Nos. B27 to B35 whose chemical compositions are within the range specified by the present invention and which satisfy the above-mentioned formula (1) and inclusions have the general corrosion rate. Is as small as 0.04 mm / year or less. On the other hand, the steel materials of sample numbers B36 to B40 in which any one of the chemical composition, the inclusion definition, and the small value of "Ap / C" is out of the range defined by the present invention, the general corrosion rate is 0.08 mm / Big as over a year.

[0116]

According to the steel material of the present invention, the corrosion resistance of the cargo oil tank in the corrosive environment is improved, and the maintenance cost can be greatly reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a Ni content and a general corrosion rate.

FIG. 2 Cu in an environment of repeated dry and wet with acidic water
It is a figure which shows the relationship between content and general corrosion rate.

FIG. 3 is a view showing the shape of a corrosion test piece used in Example 4.

Claims (7)

[Claims] 1. In mass%, C: 0.01 to 0.3%, S
i: 0.02 to 1%, Mn: 0.05 to 2%, P: 0.
05% or less, S: 0.01% or less, Ni: 0.05 to 3
%, Mo: 1% or less, Cu: 1% or less, Cr: 2% or less, W: 1% or less, Ca: 0.01% or less, Ti: 0.
1% or less, Nb: 0.1% or less, V: 0.1% or less,
B: A steel material for a cargo oil tank containing 0.05% or less and the balance Fe and impurities. 2. The steel material for a cargo oil tank according to claim 1, wherein the surface is covered with an anticorrosion coating. 3. In mass%, C: 0.01 to 0.3%, S
i: 0.02 to 1%, Mn: 0.05 to 2%, P: 0.
05% or less, S: 0.01% or less, Ni: 0.01 to 3
%, Mo: 1% or less, Cu: 2% or less, Cr: 2% or less, W: 1% or less, Ca: 0.01% or less, Ti: 0.
1% or less, Nb: 0.1% or less, V: 0.1% or less,
B: 0.05% or less, Sb: 0.5% or less, Sn: 0.
5% or less, Al: 0.07% or less, balance Fe
Steel material for cargo oil tanks, which contains impurities. 4. C: 0.01 to 0.3%, S in mass%
i: 0.02 to 1%, Mn: 0.05 to 2%, P: 0.
05% or less, S: 0.01% or less, Ni: 0.01 to 3
%, Cu: 0.01 to 2%, Mo: 1% or less, W: 1%
Below, Ca: 0.01% or less, Sb: 0.5% or less, S
n: 0.5% or less, Ti: 0.1% or less, Nb: 0.1
% Or less, V: 0.1% or less, B: 0.05% or less, C
Contains r: 0.05% or less, Al: 0.07% or less,
The balance consists of Fe and impurities, the number of inclusions having a particle size of more than 30 μm is less than 30 per 1 cm ² , and
Steel material for cargo oil tank that satisfies the formula (1). Ap / C ≦ 130 (1) Here, Ap in the above formula (1) represents the ratio of pearlite in the structure in%, and C represents the content of carbon in mass%. 5. The Mo content is 0.01 to 1%, the W content is 0.01 to 1%, and the Sb content is 0.01 to 0.5.
%, Sn content satisfies at least any one of 0.01-0.5%, The steel material for cargo oil tanks of Claim 4. 6. A Ti content of 0.005 to 0.1%, N
b content is 0.002-0.1%, V content is 0.
01-0.1%, B content 0.0002-0.05
The steel material for cargo oil tank according to claim 4 or 5, which satisfies at least any one of the percentages. 7. A steel material for a cargo oil tank according to any one of claims 3 to 6, wherein at least one surface thereof is subjected to anticorrosion treatment.