JP2005290479A - Steel material for bottom plate of crude oil tank - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material for the bottom plate of a crude oil tank having excellent local corrosion resistance even in the case of use in a state where no rust-preventive coating is applied. <P>SOLUTION: The steel material for the bottom plate of a crude oil tank having excellent local corrosion resistance and used for the bottom plate of a crude-oil storage tank or a crude-oil transport tank has a chemical composition which consists of, by mass, 0.001 to 0.20% C, 0.01 to 1.0% Si, 0.1 to 1.5% Mn, ≤0.03% P, ≤0.01% S, further one or more kinds among 0.1 to 1% Cu, 0.01 to 2% Ni, 0.1 to 4% Cr and 0.001 to 1% Mo, and the balance Fe with inevitable impurities and in which the value of Pcm represented by equation Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B is made to ≤0.22. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an oil tanker depot, a tank for transporting crude oil, or a bottom plate for storing crude oil (hereinafter collectively referred to as “crude oil tank”) (hereinafter collectively referred to as “crude oil tank bottom plate”). In particular, the present invention relates to a “steel material for a crude oil tank bottom plate” that can reduce local corrosion occurring in the bottom plate of a crude oil tank when used in a primer-coated state. In addition, the steel material for crude oil tank bottom plates as referred to in the present invention includes thick steel plates, thin steel plates, and shaped steels.

Conventionally, corrosion has occurred in the steel used for the bottom plate of crude oil tanks due to the corrosion inhibition action of crude oil itself and the protective action of a protective film (hereinafter referred to as “crude oil protection film”) produced on the inner surface of the crude oil tank. It was thought not. Recently, however, it has become clear that saddle-shaped local corrosion occurs in steel on the tank bottom plate. As a cause of such local corrosion,
(1) Removal of crude oil protective film due to excessive washing, (2) High concentration of sulfide in crude oil,
(3) In an inert gas (engine exhaust gas whose representative composition is approximately ₂ vol. Of O ₂ , approximately 13 vol% of CO ₂ , approximately 0.01 vol% of SO ₂ , and the balance of N ₂ ) enclosed in a crude oil tank for explosion protection, Items such as high concentrations of O ₂ , CO ₂ , and SO ₂ and (4) microbe involvement are listed, but none of them are inferred, and no clear cause has yet been found.

  The most effective way to suppress corrosion is to coat the steel surface with heavy coating to shield the steel from the corrosive environment, but the coating on the crude oil tank has an enormous area of application, and once every 10 years. Since repainting is necessary, there is a problem that a large initial cost and repair cost are required, and in heavy coating, corrosion is promoted on the damaged part of the coating film.

On the other hand, as a countermeasure from the steel material side, corrosion-resistant steel for shipbuilding having corrosion resistance in a use environment such as a ship outer plate, a ballast tank, a cargo oil tank, and a coal carrier cargo hold has been proposed. The corrosion-resistant steel for shipbuilding is C: 0.01 to 0.25% by weight, and after adjusting Si, Mn, P and S to appropriate amounts, Cu: 0.01 to 2.00% , Al: 0.005 to 0.10%, Mg: 0.0002 to 0.0150%, and the corrosion resistance and local corrosion resistance of the steel material are improved (for example, see Patent Document 1) ).
Japanese Unexamined Patent Publication No. 2000-17371 (page 3-4, Table 1)

  However, when the corrosion-resistant steel for shipbuilding is used for a crude oil tank bottom plate, there is a problem that resistance to local corrosion (hereinafter referred to as “local corrosion resistance”) is not stably and sufficiently exhibited. For this reason, there is a demand for the development of a steel material that is further improved in local corrosion resistance even when used in a crude oil tank bottom plate.

  An object of the present invention is to provide a steel material for a bottom plate of a crude oil tank that has excellent local corrosion resistance when used in a primer-coated state.

In order to achieve the above object, the present inventors first extracted factors involved in local corrosion of the bottom plate of the crude oil tank, and conducted a laboratory level corrosion test using a combination of these factors. As a result, we succeeded in reproducing local corrosion of the same form as that generated in the actual crude oil tank bottom plate, and clarified the controlling factors and corrosion mechanism of local corrosion occurring in the crude oil tank bottom plate.
That is, it became clear that the bowl-shaped local corrosion generated in the actual bottom plate of the crude oil tank has O ₂ and H ₂ S contained in the liquid particularly acting as a controlling factor of the local corrosion.

Specifically, oxygen (O ₂ ) and H ₂ S (hydrogen sulfide) coexist, and both the O ₂ partial pressure and the H ₂ S partial pressure are low (O ₂ partial pressure 2 to 8%) and O ₂ gas, it was revealed that occurs in an aqueous solution) containing the H ₂ S partial pressure 5-20% of H ₂ S gas. That is, in a low O ₂ partial pressure and low H ₂ S partial pressure environment, a strong corrosion-generated film is first formed on the steel surface, and this corrosion-generated film is partially destroyed in the presence of crawl (Cl). As a result, local corrosion occurs.

On the other hand, when O ₂ and H ₂ S coexist and both contents in the test solution are high, large overall corrosion occurs but local corrosion does not occur.
Also, the test solution containing one of O ₂ or H ₂ S only, for example, test solution containing no H ₂ S comprises O ₂ (O ₂ min pressure of about 21% aqueous solution containing only the gas), or free of O ₂ among laden test solution (H ₂ aqueous solution containing only the S partial pressure of 100% gas) H ₂ S and no local corrosion is not generated.

Therefore, the present inventors have investigated the influence of various alloy elements on the occurrence of local corrosion under the low O ₂ partial pressure and low H ₂ S partial pressure environments, and as a result, applied primer. When used in a state, it was found that a steel material for a crude oil tank bottom plate excellent in local corrosion resistance can be obtained mainly by optimizing the contents of Cu, Ni and Mo.

The present invention has been completed based on the above findings and further studies. That is,
(1) The steel material for a crude oil tank bottom plate of the present invention is a steel material for a crude oil tank bottom plate used in a primer application state in mass%, C: 0.001 to 0.16%, Si: 0.01 to 1.5. %, Mn: 0.1 to 2.5%, P: 0.05% or less, S: 0.01% or less, Cu: 0.15 to 1.4%, the balance from Fe and inevitable impurities Having the composition
The Pcm value represented by the following formula (1) is 0.24 or less, and is excellent in resistance to a base material and a welded portion used for a bottom plate of a crude oil transport tank or a crude oil storage tank. Steel for bottom plate of crude oil tank that has local corrosivity.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B Formula (1)
However, an element symbol shows mass% of each element.

  (2) Moreover, in addition to the chemical component, it may contain at least one of Ni: 0.01 to 0.7% and Mo: 0.001 to 0.5% in mass%. Steel material for crude oil tank bottom plate.

  (3) Further, in addition to the chemical component of (1) or (2), further, in mass%, Al: 0.005 to 0.5%, Sn: 0.005 to 0.3%, Sb : 0.005-0.3%, Nb: 0.005-0.1%, V: 0.005-0.1%, Ti: 0.005-0.1%, W: 0.001-0 .5%, B: Steel material for crude oil tank bottom plate characterized by containing one or more of 0.01% or less.

  (4) Further, the steel material for a crude oil tank bottom plate, wherein the thickness of the zinc primer applied to the steel material of any one of the chemical components (1), (2) to (3) is 15 μm or more .

  The reason for limiting the chemical composition of the steel plate for crude oil tank bottom plate of the present invention will be described. Hereinafter, mass% is simply referred to as%.

[C: 0.001 to 0.16%]
C is an element that increases the strength of the steel material. In the present invention, it is necessary to contain 0.001% or more in order to obtain a desired strength. On the other hand, the content exceeding 0.16% deteriorates the weldability and the toughness of the weld heat affected zone. For this reason, C was limited to 0.001 to 0.16% of range. From the viewpoint of strength and toughness, it is preferably 0.01 to 0.15%.

[Si: 0.01 to 1.5%]
Si is an element that acts as a deoxidizer and increases the strength. In the present invention, it is necessary to contain 0.01% or more, but the content exceeding 1.5% deteriorates the toughness of steel. . For this reason, Si was limited to the range of 0.01 to 1.5%.

[Mn: 0.1 to 2.5%]
Mn is an element that increases the strength of the steel material, and in the present invention, it is necessary to contain 0.1% or more in order to obtain a desired strength. On the other hand, the content exceeding 2.5% lowers the toughness and weldability of steel. For this reason, Mn was limited to the range of 0.1 to 2.5%. In addition, Preferably it is 0.5 to 1.5%, More preferably, it is 0.8 to 1.2%.

[P: 0.05% or less]
P is a harmful element that segregates at the grain boundaries and lowers the toughness of the steel, and is preferably reduced as much as possible, but if it exceeds 0.05%, the toughness significantly decreases. For this reason, P was limited to 0.05% or less. In addition, since the reduction of less than 0.005% causes an increase in manufacturing cost, P is preferably 0.005 to 0.05%.

[S: 0.01% or less]
S is a harmful element which forms MnS of non-metallic inclusions and becomes a starting point of local corrosion and lowers the local corrosion resistance, and is preferably reduced as much as possible. This causes a significant decrease in local corrosivity. For this reason, S was limited to 0.01% or less. In addition, since reduction of less than 0.003% causes increase in manufacturing cost, S is preferably 0.003 to 0.01%.

[Cu: 0.15-1.4%]
Cu suppresses the growth of local corrosion and has the effect of suppressing the occurrence of local corrosion when used in combination with a primer, and can be contained as necessary. However, if it is less than 0.15%, there is no effect, and if it exceeds 1.4%, the effect is saturated, so Cu is limited to the range of 0.15 to 1.4%.

[Ni: 0.01 to 0.7%]
Ni improves the low-temperature toughness and has the effect of densifying the corrosion products formed on the surface of the steel material when used in combination with the primer to strengthen the protection, and can be added as necessary. If 0.01% or more is included, the effect of reducing the amount of corrosion in an acidic environment can be expected, but if it exceeds 0.7%, the effect is saturated and the cost increases, so Ni is 0.01 to 0.7%. Limited to range.

[Mo: 0.001 to 0.5%]
Mo is added together with Cu and Ni to improve local corrosion resistance, and has the effect of densifying the corrosion products formed on the surface of the steel material when the primer is used together to strengthen the protection, and is added as necessary. be able to. If 0.001% or more is included, the corrosion amount reduction effect in an acidic environment can be expected, but if it exceeds 0.5%, the effect is saturated.
Mo was limited to the range of 0.001 to 0.5%.

[Al: 0.005 to 0.5%]
Al is an element that acts as a deoxidizing agent, and in the present invention, it is necessary to contain 0.005% or more. On the other hand, if the content exceeds 0.5%, the toughness of the steel decreases. For this reason, Al was limited to the range of 0.005 to 0.5%. In addition, Preferably, it is 0.01 to 0.05%.

[Sn: 0.005-0.3%]
The addition of Sn is effective for improving the corrosion resistance. However, the addition of 0.005% or less has no effect, and the addition of 0.3% or more saturates the effect, so Sn was limited to 0.005 to 0.3%.

[Sb: 0.005 to 0.3%]
The addition of Sb is effective for improving the corrosion resistance. However, when 0.005% or less is added, there is no effect, and when 0.3% or more is added, workability deteriorates, so Sb is limited to 0.005 to 0.3%.

[Nb: 0.005 to 0.1%]
Nb is an element added for the purpose of improving the strength as required. However, Nb is 0% because 0.005% or less has no effect on improving strength, and 0.1% or more deteriorates toughness. Limited to 0.005 to 0.1%.

[V: 0.005 to 0.1%]
V is an element added for the purpose of improving the strength as necessary. However, if 0.005% or less, there is no effect on improving the strength, and if 0.1% or more, the toughness deteriorates. Limited to 0.005 to 0.1%.

[Ti: 0.005 to 0.1%]
Ti is an element added for the purpose of improving strength and toughness as necessary. However, Ti is not effective at 0.005% or less, and the effect is saturated at 0.1% or more. It was limited to 005 to 0.1%.

[W: 0.001 to 0.5%]
W has the effect of improving local corrosion resistance, and can be added as necessary. If 0.001% or more is included, the effect of reducing the corrosion amount in an acidic environment can be expected, but if it exceeds 0.5%, the effect is saturated, so W is limited to a range of 0.001 to 0.5%.

[B: 0.01% or less]
B is an element added for the purpose of improving the strength as necessary. However, since the toughness deteriorates at 0.01% or more, B is limited to 0.01% or less.
In the steel material of the present invention, the balance other than the above components is Fe and inevitable impurities. Inevitable impurities include Cr: 0.05% or less, N: 0.007% or less, and 0: 0.008% or less.

[Pcm value: 0.24 or less]
With restrictions on each of the above elements,
Pcm = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B Formula (1)
Is defined as 0.24 or less. This is a characteristic value indicating the sensitivity to weld cracking. If this value exceeds 0.24, the rate of occurrence of low-temperature cracking during welding becomes extremely high, so it is necessary to keep it below 0.24.

  Of the chemical components in the present invention, the balance other than the chemical components is substantially Fe. Here, “the balance is substantially Fe” means that, as long as the effects of the present invention are not lost, those containing inevitable impurities and other trace elements can be included in the scope of the present invention. Means. For example, Cr: 0.05% or less, N: 0.007% or less, and 0: 0.008% or less are acceptable as inevitable impurities.

  The steel having the above chemical components can be produced in the same manner as ordinary steel. For example, in steel melting, the main five elements C, Si, Mn, P, and S are adjusted to the scope of the invention by a converter or the like, and other alloy elements are added as necessary.

Thereafter, the slab obtained by continuous casting or the like is rolled as it is or after cooling. The rolling conditions are not particularly limited as the corrosion resistant steel, but it is necessary to ensure an appropriate reduction ratio from the viewpoint of mechanical properties.
By controlling the cooling rate after hot rolling during rolling, a high-strength steel material having a tensile strength of 490 N / mm ² or higher can be obtained. For example, the hot rolling finishing temperature is set to 750 ° C. or higher, and then cooled to 600 ° C. or lower at a cooling rate of 2 ° C./sec or higher. If the finishing temperature is less than 750 ° C., deformation resistance increases and shape defects tend to occur. If the cooling rate is less than 2 ° C / sec or the cooling stop temperature exceeds 600 ° C, a tensile strength of 490 N / mm class ² or higher cannot be obtained.

  Pitting corrosion resistance is improved by applying a zinc primer. A coating amount of 15 μm or more is preferable because the pitting corrosion resistance is remarkably improved. From the viewpoint of pitting corrosion resistance, an upper limit is not provided, but when the zinc primer is thick, the effect is saturated, and the cutting property and economical efficiency are deteriorated, so the upper limit is preferably in the range of 100 μm.

[Corrosion test]
FIG. 1 is a configuration diagram of a corrosion test apparatus used for confirming local corrosion resistance of a steel material for a bottom plate of a crude oil tank according to an embodiment of the present invention. In FIG. 1, the corrosion test apparatus is a double type apparatus of a corrosion test tank 2 and a thermostat 3, and the corrosion test tank 2 generates local corrosion similar to the local corrosion that occurs in the bottom plate of an actual crude oil tank. The test liquid capable of being injected is injected (hereinafter referred to as “test liquid 6”).

As the test liquid 6, artificial seawater specified in ASTM D141 was used as a test mother liquid, and a mixed gas adjusted to a partial pressure ratio of 5% O ₂ + 10% H ₂ S was introduced into the test mother liquid. N ₂ gas was used as an inert gas for adjusting the balance of the mixed gas. The mixed gas adjusted with the inert gas is referred to as an introduction gas 4.
Moreover, the temperature of the test solution 6 was maintained at 50 ° C. by adjusting the temperature of the water 7 placed in the thermostat 3.

Molten steels 1 to 47 having chemical components shown in Table 1 were melted in a converter and made into a slab having a thickness of 200 mm by a continuous casting method. These slabs were heated to 1200 ° C. and subjected to hot rolling at a finish rolling end temperature of 800 ° C. to form a 15 mm thick steel plate. Steel plates corresponding to the molten steels 1 to 47 are referred to as steel plates 1 to 47, respectively.
Then, test pieces 1 to 47 (15 mm thickness × 50 mm width × 50 mm length) were cut out from the steel plates 1 to 47 (having the respective chemical components). The cutout test pieces 1 to 47 were evaluated for weldability. The results are also shown in Table 1. When the steel plate has a Pcm value of 0.24 or less, the weldability is good without cracking during welding.

In addition, the test piece No. used for the evaluation of weldability was used. 1 to 47, steel plate No. 1 which is a test piece satisfying the chemical composition of the present invention and the Pcm formula (0.24 or less). Change the thickness of the inorganic zinc primer to 3 types without applying the inorganic zinc primer and the surface of the test pieces of 2, 7, 16, 21, 23, 28, 34, 37, 39 and 43. A coated one was created.
And the corrosion test which immerses these test pieces in the test liquid 6 of a corrosion test apparatus for one month was done. At this time, since the introduction gas 4 is continuously supplied, the test solution 6 is not stationary. And after the test, the rust produced | generated on the test piece surface was removed, the corrosion form was observed visually, and the local corrosion depth was measured with the dip meter. The results are shown in Table 2.

In Table 2, the occurrence of local corrosion was evaluated as ◎ when no pitting occurred, pitting occurred, ○ when the pitting depth was less than 0.5 mm, and x when the pitting depth was 1 mm or more. did.
That is, the steel plate for crude oil tank bottom plate according to the present invention to which an inorganic zinc primer is applied has a good pitting corrosion resistance evaluation of ◯ or more. In addition, when the coating is 15 μm or more, the pitting corrosion resistance evaluation is marked by “◎” and is markedly improved. Compared with it, the thing which has not apply | coated the inorganic type zinc primer is inferior to pitting corrosion resistance by x. Therefore, it has been confirmed that the steel of the present invention has good local corrosion resistance by applying an inorganic zinc primer.
On the other hand, steel plate No. which does not contain Cu as a steel plate component (same as the chemical component of the test piece) as a comparative steel is also provided. Although No. 37, 39 and 43 are coated with an inorganic zinc primer, the pitting corrosion resistance is inferior with x, and therefore local corrosion resistance due to coating of the inorganic zinc primer is not recognized.

  From the above, excellent local corrosion resistance is recognized in the steel plate for crude oil tank bottom plate of the present invention coated with inorganic zinc primer, and even when local corrosion occurs, the depth is 0.5 mm at the maximum. It turns out that it is suppressed to the following. On the other hand, when the inorganic zinc polymer is not applied, local corrosion at a depth of 1 mm or more is observed. Therefore, it can be said that the local corrosion resistance of the steel for crude oil tank bottom plate of the present invention is improved by the combined use with the inorganic zinc primer.

  As described above, according to the present invention, it is widely inexpensive as an oil tanker depot, a tank for transporting crude oil or a tank for storing crude oil, etc. Can be used.

It is a block diagram of the corrosion test apparatus used in order to confirm the local corrosion resistance of the steel materials for crude oil tank bottom plates.

Explanation of symbols

1: Test piece 2: Corrosion test tank 3: Constant temperature bath 4: Introduced gas 5: Exhaust gas 6: Test liquid 7: Water

Claims (4)

In the steel material for the bottom plate of the crude oil tank used in the primer application state, as chemical components, mass%, C: 0.001 to 0.16%, Si: 0.01 to 1.5%, Mn: 0.1 to 2 0.5%, P: 0.05% or less, S: 0.01% or less, Cu: 0.15 to 1.4%, the balance is composed of Fe and inevitable impurities,
The Pcm value represented by the following formula (1) is 0.24 or less, and is excellent in resistance to a base material and a welded portion used for a bottom plate of a crude oil transport tank or a crude oil storage tank. Steel for bottom plate of crude oil tank that has local corrosivity.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B Formula (1)
However, an element symbol shows mass% of each element. In addition to the chemical component according to claim 1, in mass%, Ni: 0.01 to 0.7%, Mo: any one or two of 0.001 to 0.5%, The balance has a composition consisting of Fe and inevitable impurities,
The Pcm value represented by the following formula (1) is 0.24 or less, and is excellent in resistance to a base material and a welded portion used for a bottom plate of a crude oil transport tank or a crude oil storage tank. Steel for bottom plate of crude oil tank that has local corrosivity.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B Formula (1)
However, an element symbol shows mass% of each element.   In addition to the chemical components according to claim 1 and 2, further, in mass%, Al: 0.005-0.5%, Sn: 0.005-0.3%, Sb: 0.005-0. 3%, Nb: 0.005-0.1%, V: 0.005-0.1%, Ti: 0.005-0.1%, W: 0.001-0.5%, B: 0 A steel material for a crude oil tank bottom plate, comprising any one or more of 0.01% or less. Furthermore, the steel primer for crude oil tank bottom plates in any one of Claims 1 thru | or 3 characterized by the thickness of the zinc primer to apply | coat 15 micrometers or more.

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