JP3753088B2 - Steel material for cargo oil tanks - Google Patents

Description

【００６５】
【表２】

【００６６】
次いで、上記鍛造によって得た厚さが１２０ｍｍのブロックを、１１５０℃で２時間加熱してから熱間圧延し、厚さ２０ｍｍの鋼板にした。なお、上記２０ｍｍの鋼板の製造条件は下記の「製造法１」又は「製造法２」である。
【００６７】
「製造法１」：厚さ１２０ｍｍのブロックを１１５０℃で２時間加熱後、熱間圧延して９５０℃で厚さ２０ｍｍに仕上げ、その後室温まで大気中放冷する製造方法。
【００６８】
「製造法２」：厚さ１２０ｍｍのブロックを１１５０℃で２時間加熱後、熱間圧延して８５０℃で厚さ２０ｍｍに仕上げ、その後８００℃から５００℃までの温度域を水冷し、その後室温まで大気中放冷する製造方法。
【００６９】
厚さが２０ｍｍの各鋼板から、JIS G 0555に準じて、圧延方向に平行に、その中心線をとおって切断したミクロ試験片を作製し、光学顕微鏡による組織と介在物の調査を行った。
【００７０】
すなわち、上記の圧延方向に平行に切断した面（Ｌ断面）を被検面とし、鏡面研磨した後、倍率４００倍で６０視野の光学顕微鏡写真を撮影し、その写真から粒径が３０μｍを超える介在物の個数を測定した。
【００７１】
又、鏡面研磨した面をナイタルで腐食し、倍率１００倍で１０視野光学顕微鏡観察して、組織に占めるパーライトの％単位での割合Ａｐを測定し、前述の「Ａｐ／Ｃ」の値を算出した。
【００７２】
更に、前記厚さが２０ｍｍの各鋼板から、幅が２５ｍｍ、長さが５０ｍｍ、厚さが４ｍｍの試験片を採取し、そのまま次に述べる原油がＨ ₂ Ｓを含む実船のデッキ裏環境を模擬した腐食試験に供した。
【００７３】
すなわち、０．１質量％ＮａＣｌ水溶液を下部１／３部分に入れたガラス容器を準備する一方、採取した試験片を下面に取り付けたガス供給口を有するアクリル製の蓋によって上記ガラス容器の開口上端を密閉した。次いで、密閉後のガラス容器を恒温槽内に設置し、５０℃×２０時間→２５℃×４時間の温度サイクルを４ヶ月間付与した。その際、ガラス容器内の気相部には、バラスト時とフルロード時をシミュレートし、前記のガス供給口より下記２種類のガスＡとガスＢを２週間間隔で交互に吹き込んだ。
【００７４】
ガスＡ：体積％で、５％Ｏ₂−１３％ＣＯ₂−０．０２％ＳＯ₂−残Ｎ₂、
ガスＢ：体積％で、５％Ｏ₂−１３％ＣＯ₂−０．０２％ＳＯ₂−０．２５％Ｈ₂Ｓ−残Ｎ₂。
【００７５】
４ヶ月の腐食試験の後、各試験片の減少質量から「ｍｍ／年」単位での腐食速度（全面腐食速度）を求めた。表３に、上記の各試験結果を厚さ２０ｍｍ鋼板の製造条件とともに示す。
【００７６】
【表３】

【００７７】
表３に示す結果からわかるように、化学組成、介在物規定及び「Ａｐ／Ｃ」の値が本発明で規定する範囲内の試番B1〜B20の鋼材は、全面腐食速度が０．１３ｍｍ／年以下と小さい。これに対し、化学組成、介在物規定及び「Ａｐ／Ｃ」の値の少なくともいずれかが本発明で規定する範囲を外れる試番B21〜B26の鋼材は、全面腐食速度が０．２９ｍｍ／年以上と大きい。
【００７８】
（実施例２）
上記実施例１で述べた厚さが２０ｍｍの各鋼板から、幅が１００ｍｍ、長さが１００ｍｍ、厚さが４ｍｍの試験片を採取し、次に述べる原油がＨ ₂ Ｓを含む実船のカーゴオイルタンク底板の環境を模擬した腐食試験に供した。
【００７９】
すなわち、上記の幅が１００ｍｍ、長さが１００ｍｍ、厚さが４ｍｍの試験片の幅、長さ方向それぞれについて、中央部３ｍｍをシールして残りの部分に原油を１ｍｍの厚さで塗布した後、中央部のシールを除去して十字の形で鋼面を露出させた試験片の上に固体Ｓを０．２ｇ／ｃｍ²の割合で付着させて腐食試験に供した。
【００８０】
図２に、このようにして作製した腐食試験片を示す。
【００８１】
次に、人工海水を入れたガラス容器の底に上記の腐食試験片を取り付け、ガス供給口を有するアクリル製の蓋でガラス容器の開口上端を密閉した。次いで、密閉後のガラス容器を恒温槽内に設置し、温度４０℃で１ヶ月間試験を行なった。その際、ガラス容器内の人工海水に、バラスト時とフルロード時をシミュレートし、前記のガス供給口より下記２種類のガスＡとガスＢを２週間間隔で交互に吹き込んだ。
【００８２】
ガスＡ：体積％で、５％Ｏ₂−１３％ＣＯ₂−０．０２％ＳＯ₂−残Ｎ₂、
ガスＢ：体積％で、５％Ｏ₂−１３％ＣＯ₂−０．０２％ＳＯ₂−０．２５％Ｈ₂Ｓ−残Ｎ₂。
【００８３】
１ヶ月の腐食試験の後、試験片の最大孔食深さから孔食進展速度を求めた。表３には、上記の試験結果を併せて示した。
【００８４】
表３から明らかなように、化学組成が本発明で規定する範囲内にあり、しかも、前記(1)式及び介在物の規定を満たす試番B1〜B20の鋼材は、孔食進展速度が０．１５ｍｍ／年以下と小さい。これに対し、化学組成、介在物規定及び「Ａｐ／Ｃ」の値の少なくとのいずれかが本発明で規定する範囲を外れる試番B21〜B26の鋼材は、孔食進展速度が０．６１ｍｍ／年以上と大きい。
【００８５】
（実施例３）
表４に示す化学組成を有する１４種類の鋼を真空溶解炉を用いて溶製し５０ｋｇ鋼塊とした後、通常の方法で熱間鍛造して厚さが１２０ｍｍのブロックを作製した。
【００８６】
【表４】

【００８７】
次いで、上記鍛造によって得た厚さが１２０ｍｍのブロックを、１１５０℃で２時間加熱してから熱間圧延し、厚さ２０ｍｍの鋼板にした。なお、上記２０ｍｍの鋼板の製造条件は前記実施例１における「製造法１」又は「製造法２」である。
【００８８】
実施例１の場合と同様の方法で、厚さが２０ｍｍの各鋼板からミクロ試験片を作製し、光学顕微鏡による組織と介在物の調査を行った。
【００８９】
又、上記の厚さが２０ｍｍの各鋼板から幅が２５ｍｍ、長さが５０ｍｍ、厚さが４ｍｍの試験片を採取し、その表面に通常の方法で厚さ２００μｍのタールエポキシ樹脂の防食被膜を施し、実施例１の場合と同じ条件の腐食試験に供した。
【００９０】
４ヶ月の腐食試験の後、各試験片の減少質量から「ｍｍ／年」単位での全面腐食速度を求めた。
【００９１】
表５に、上記の各試験結果を厚さ２０ｍｍ鋼板の製造条件とともに示す。
【００９２】
【表５】

【００９３】
表５から明らかなように、化学組成が本発明で規定する範囲内にあり、しかも、前記(1)式及び介在物の規定を満たす試番B27〜B35の鋼材は、全面腐食速度が０．０４ｍｍ／年以下と小さい。これに対し、化学組成、介在物規定及び「Ａｐ／Ｃ」の値の少なくとのいずれかが本発明で規定する範囲を外れる試番B36〜B40の鋼材は、全面腐食速度が０．０８ｍｍ／年以上と大きい。
【発明の効果】
【００９４】
本発明の鋼材によれば、イナートガス及びＨ ₂ Ｓを含むカーゴオイルタンクの腐食環境における耐食性が向上し、メンテナンス費用の大幅な削減が可能である。
【図面の簡単な説明】
【図１】
酸性水による乾湿繰り返しの環境におけるＣｕ含有量と全面腐食速度との関係を示す図である。
【図２】
実施例２で用いた腐食試験片の形状を示す図である。[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a steel material for a cargo oil tank which is a crude oil tank in a tanker.
[0002]
[Prior art]
  At present, bare steel is used as a material for tanker cargo oil tanks.
[0003]
  The gas phase part of the cargo oil tank is N which uses engine exhaust for explosion-proof purposes.₂It is filled with the main gas (hereinafter referred to as inert gas), but in a severe corrosive environment with repeated wet and dry conditions.And moreover, Hydrogen sulfide (H₂When loading crude oil containing S), H contained in crude oil₂Since a part of S moves into the gas phase, the corrosive environment becomes extremely severe. Of course, some of the hydrocarbons contained in the crude oil are also vaporized and mixed with the inert gas, but it has little effect on corrosion and can be ignored. An example of the inert gas is 13% CO in volume%.₂-5% O₂-0.1% SO₂-Remaining N₂The gas of the composition is mentioned.
[0004]
  In the corrosive environment as described above, there is a case where the entire surface of the deck which is the ceiling of the cargo oil tank is corroded, and the corrosion rate exceeds 0.1 mm / year. A case of general corrosion with a very high corrosion rate of 0.3 mm / year or more has been reported. In addition, pitting corrosion occurs on the bottom plate of the cargo oil tank, and there are cases where the pitting corrosion progressing speed is as high as several mm / year.
[0005]
  For these reasons, some parts of the cargo oil tank are painted, but the cost of initial painting and repainting every 10 years is high. For this reason, for example, taking a corrosion allowance of 2 mm for 20 years of use, it is the actual situation that it is taken as a countermeasure against general corrosion and local corrosion.
[0006]
  However, if the corrosion allowance is taken, the thickness of the steel material increases, so there are disadvantages such as an increase in the manufacturing cost of the tank and a reduction in the amount of crude oil loaded. Accordingly, there is a strong demand for the development of a steel material for cargo oil tanks that can reduce the corrosion allowance and that can prevent an increase in cost and has excellent corrosion resistance.
[0007]
  In addition, as steel for cargo oil tanks, for example, steel including Cu and Mg as essential components in Japanese Patent Laid-Open No. 2000-17181, and steel including Cr and Al as essential components in Japanese Patent Laid-Open No. 2001-107180 are disclosed. Each has been proposed. However, in the steels disclosed in these publications, crude oil is H₂When S is included, H₂The influence of S on corrosion is not taken into consideration at all, and for this reason, sufficient corrosion resistance may not be obtained in a cargo oil tank of an actual ship.
[0008]
[Problems to be solved by the invention]
  The present invention has been made in view of the above situation, and its purpose is as follows.Inert gas and H ₂ In an environment containing SIt is to provide a steel material for a cargo oil tank that has excellent resistance to general corrosion and local corrosion.
[Means for Solving the Problems]
[0009]
  The gist of the present invention is the following (1) to(4)In steel materials for cargo oil tanks shown inThe
[0010]
  (1)In mass%, C: 0.01 to 0.3%, Si: 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: 0 to 1%, W: 0 to 1%, Ca: 0 to 0.01%, Sb: 0 to 0.5%, Sn : 0-0.5%, Ti: 0-0.1%, Nb: 0-0.1%, V: 0-0.1%, B: 0-0.05%, Cr: 0-0. 05%, Al: 0 to 0.07%, the balance is Fe and impurities, and inclusions with a particle size exceeding 30 μm are 1 cm.²Less than 30 and satisfy the following formula (1)It is characterized bySteel for cargo oil tanks. However, Ap in the following formula (1) is the percentage of pearlite in the structure in% units, and C represents the content in mass% of carbon.
[0011]
  Ap / C ≦ 130 (1).
[0012]
  (2)Mo content is 0.01 to 1%, W content is 0.01 to 1%, Sb content is 0.01 to 0.5%, Sn content is 0.01 to 0.5%. % At leastIt is characterized bythe above(1)The steel for cargo oil tanks described in 1.
[0013]
  (3)The Ti content is 0.005 to 0.1%, the Nb content is 0.002 to 0.1%, the V content is 0.01 to 0.1%, and the B content is 0.0002. Satisfies at least one of -0.05%It is characterized bythe above(1)Or(2)The steel for cargo oil tanks described in 1.
[0014]
  (4) Above(1)From(3)It is a steel material for cargo oil tanks described in any of the above, and at least one surface is subjected to anticorrosion treatmentIt is characterized bySteel material.
[0015]
  The above (1) to(4)The inventions related to the steel materials of "the present invention (1)" to "the present invention", respectively.(4)" Also, it may be collectively referred to as “the present invention”.
[0016]
  “Inclusion” in the present invention refers to A-type inclusions or B-type inclusions described in JIS G 0555, and “particle size” refers to the case where the shape is circular, the diameter of which is flat. Indicates a value defined by the average of the major axis and the minor axis.
[0017]
  In order to achieve the above-described problems, the present inventors have conducted many experiments by simulating the corrosive environment of an actual ship.
[0018]
  SnowINarto gas and H₂A number of experiments were conducted in a dry and wet environment containing S. As a result, H₂Succeeded in reproducing the corrosion product layer found on the back of an actual ship loaded with crude oil containing S. Furthermore, regarding the corrosion mechanism in the gas phase and the pitting corrosion in the bottom plate,(A)~(F)I got the knowledge.
[0019]
  (A)SO contained in inert gas₂And H moving from crude oil to gas phase₂Both S and O contained in inert gas₂Or H₂O reacts with H₂SO_ThreeProduces. In addition, H₂SO_ThreeIs O₂H generated by oxidation by₂SO_FourAlso generate.
[0020]
  (B)When condensation occurs, the above H₂SO_ThreeAnd H₂SO_FourIs contained in moisture. Therefore, the inert gas and H₂The wet and dry repeated environment containing S is a dry and wet repeated corrosive environment with acidic water.
[0021]
  (C)Corrosion on the deck of the actual ship progresses at night when the temperature drops and condensation occurs, and a large amount of CO is not loaded without loading crude oil.₂, SO₂Or O₂As a corrosion product in the presence of slag, α-FeOOH (hereinafter referred to as “rust”) is mainly used.
[0022]
  (D)On the other hand, H₂H when loading crude oil containing S₂S is rust surface and O₂Is oxidized to produce solid S.
[0023]
  (E)By repeating the case where the crude oil is not loaded and the case where the crude oil is loaded, a layered structure of “rust / S / rust / S...” Is formed on the back of the deck of the actual ship.
[0024]
  (F)The bottom of the cargo oil tank retains drain water, and the surface is coated with an oil film, but the coating with the oil film is peeled off due to sludge movement or crude oil washing, etc. S falls and adheres. The S then acts as an oxidant, resulting in a noble corrosion potential,₂S and Cl^-Pitting corrosion occurs in the presence of In addition, since the rock salt existing in the oil well is mixed with crude oil during mining and brought into the tank, Cl^-Will exist.
[0025]
  Based on the above-mentioned corrosion mechanism of the gas phase part and the pitting corrosion generation mechanism of the bottom plate part, the present inventors conducted further experiments.(G)~(M)The matter became clear.
[0026]
  (G)By adding Cu, it is possible to suppress the occurrence of general corrosion in a dry and wet environment with acidic water and the occurrence of pitting corrosion in the presence of S. As an example,FIG.Shows the relationship between the Cu content and the overall corrosion rate in a dry and wet environment with acidic water. In addition, thisFIG.smellAnd“Overall corrosion rate” was simply expressed as “corrosion rate”.FIG.As can be seen from the above, the corrosion rate decreases significantly with increasing Cu content.
[0027]
  (H)By containing Cu and Ni in combination, the overall corrosion resistance and pitting resistance are further improved.
[0028]
  (I)When one or more of Mo, W, Ca, Sb and Sn are added in addition to Cu and Ni, the overall corrosion resistance and pitting corrosion resistance are further improved.
[0029]
  (J)If the above Cu to Sn are added, the coating life will be longer than before.
[0030]
  (K)By limiting the contents of Cr and Al, it is possible to suppress overall corrosion in an environment of repeated wet and dry with acidic water.
[0031]
  ( l )The overall corrosion in the environment of repeated wet and dry with acidic water is also related to the percentage Ap of pearlite in the structure and the C content in mass%, and with the reduction of the value represented by "Ap / C" Overall corrosivity is improved.
[0032]
  (M)The particle size of inclusions in steel and the abundance per unit area affect the occurrence of general corrosion in the environment of repeated wet and dry conditions with acidic water and the occurrence of pitting corrosion in the presence of S.
[0033]
  Present invention (1) to present invention(4)Are the above (a) to(M)It was completed based on the knowledge ofThe
[0034]
  NaThe present invention(1)In each of the above elements constituting the steel materialMo, W, Ca, Sb, Sn, Ti, Nb, V, B, Cr, and Al, it is not always necessary to add positively, and the addition is optional. Of the above elements, the lower the Cr and Al content, the better. The lower the S content, the better.
DETAILED DESCRIPTION OF THE INVENTION
[0035]
  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".
[0036]
  (A) Chemical composition of steel
  C:
  C is an element necessary for securing strength as a material, and a content of 0.01% or more is necessary. However, if the content exceeds 0.3%, the weldability decreases. Also, as the C content increases, the amount of cementite produced that promotes corrosion in the environment of repeated wet and dry with acidic water increases, and particularly when the C content exceeds 0.30%, cementite is produced. Increases corrosion. Therefore, the content of C is set to 0.01 to 0.3%. A preferable range of the C content is 0.01 to 0.2%, and a more preferable range is 0.01 to 0.15%.
[0037]
  Si:
  Si is an element necessary for deoxidation, and in order to obtain a sufficient deoxidation 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 set to 0.02 to 1%. A preferable range of the content is 0.02 to 0.8%, and a more preferable range is 0.02 to 0.5%.
[0038]
  Mn:
  Mn is an element having an effect of increasing the strength of steel at a low cost, and a content of 0.05% or more is necessary to obtain this effect. However, if the content exceeds 1.5%, the weldability deteriorates. Therefore, the Mn content is set to 0.05 to 1.5%.
[0039]
  P:
  P is an impurity element contained in the steel and reduces weldability. In particular, when the content exceeds 0.05%, the weldability is significantly lowered. Therefore, the P content is set to 0.05% or less. In addition, since P has the effect | action which improves a general corrosion resistance, while reducing weldability, in order to improve a general corrosion resistance, you may contain 0.01% or more. The upper limit with preferable content of P is 0.04%, and a more preferable upper limit is 0.03%.
[0040]
  S: 0.01% or less
  S is an impurity element contained in the steel. When the content exceeds 0.01%, a large amount of MnS is formed in the steel, and MnS becomes a starting point of corrosion, resulting in overall corrosion and pitting corrosion. Therefore, the S content is set to 0.01% or less. The upper limit with preferable S content is 0.008%, and a more preferable upper limit is 0.005%. In addition, the lower the S content, the better.
[0041]
  Ni:
  NiThe wetThe effect of improving corrosion resistance and improving pitting corrosion resistance by forming a corrosion-resistant sulfide film in a hydrogen sulfide environmentButis there. However, in order to obtain these effects, it is necessary to contain Ni by 0.01% or more, and if it contains 0.05% or more, a more remarkable effect can be obtained. However, even if Ni is contained in excess of 3%, the effect is saturated and the cost is increased. ThereforeNiContent of0.01-3%.
[0042]
  Main departureClearlySuch steel is the above chemical group.FinallyIn addition, Cu must be included as an essential element. In addition, this departureClearlySteelThe material isAs needed, you may include 1 or more types chosen from the elements from Mo to Al described below.
[0043]
  Cu:
  CuThe acidEffective in improving the general corrosion resistance in the presence of hydrogen sulfide and remarkably improving the general corrosion resistance in the presence of hydrogen sulfide, and also in suppressing the occurrence of pitting corrosion in the presence of S In order to obtain these effects, Cu needs to be contained in an amount of 0.01% or more. If 0.1% or more is contained, a more reliable effect can be obtained. OnlyCuEven if it contains more than 2%, the effect is saturated. In addition, the steel may become brittle. In addition, from the viewpoint of preventing embrittlement of steel, the upper limit of the Cu content is more preferably 1%. ThereforeCuThe content of was made 0.01 to 2%.
[0044]
  Mo:
  The addition of Mo is optional. AddDampThe effect of improving corrosion resistance and improving pitting corrosion resistance by forming a corrosion-resistant sulfide film in a hydrogen sulfide environmentButis there. Furthermore, Mo also has the effect | action which raises acid resistance. In order to obtain these effects remarkably, the Mo content is preferably 0.01% or more. However, even if Mo is contained in an amount exceeding 1%, the effect is saturated and weldability is impaired, and the cost is increased. Therefore, the content of Mo is set to 0 to 1%. In addition, when adding, the lower limit of the Mo content is more preferably 0.1%, and more preferably 0.3%.
[0045]
  Cr:
  Cr is H₂SIncludeIt reduces the overall corrosion resistance in a dry and wet repeated environment, that is, in an environment of repeated wet and dry with acidic water, and also reduces the pitting corrosion resistance in the presence of S. In particular, when its content exceeds 0.05%, Decrease in overall corrosion resistance and pitting resistance in the above environment becomes significant. ThereforeCrThe content was 0 to 0.05%. NaOh, CrThe content is preferably 0 to 0.04%.
[0046]
  W:
  The addition of W is optional. AddDampThe effect of improving corrosion resistance and improving pitting corrosion resistance by forming a corrosion-resistant sulfide film in a hydrogen sulfide environmentButis there. Furthermore, W also has an effect of increasing acid resistance. In order to obtain these effects remarkably, the W content is preferably 0.01% or more. However, even if W is contained in excess of 1%, the above effect is saturated and the cost is increased. Therefore, the content of W is set to 0 to 1%. In addition, when adding, the lower limit of the W content is more preferably 0.1%, and even more preferably 0.3%.
[0047]
  Ca:
  The addition of Ca is optional. AddCaIs present in the form of oxides in steel and becomes alkaline when dissolved in water, and suppresses the pH drop at the steel interface during the corrosion reaction.₂Action to suppress general corrosion when S is includedTheHave.SuchIn order to obtain the effect remarkably, the Ca content is preferably 0.0002% or more. However, even if Ca is contained in excess of 0.01%, the above effect is saturated and the cost is increased. Therefore, the content of Ca is set to 0 to 0.01%. In addition, when adding, the lower limit of Ca content is more preferably 0.0005%, and even more preferably 0.001%.
[0048]
  Ti:
  The addition of Ti is optional. If added, it has the effect of increasing the strength of the steel. Ti also has the effect of improving the toughness of steel and the effect of suppressing the generation of MnS, which is the starting point of corrosion by forming TiS, and enhancing the resistance to general corrosion and pitting corrosion. In order to obtain these effects remarkably, the Ti content is preferably 0.005% or more. However, even if Ti is contained more than 0.1%, the above effect is saturated and the cost is increased. Therefore, the content of Ti is set to 0 to 0.1%. In addition, it is more preferable that the lower limit value of the Ti content when added is 0.01%, and more preferable is 0.05%.
[0049]
  Nb:
  Addition of Nb is optional. If added, it has the effect of increasing the strength of the steel. In order to obtain this effect remarkably, the Nb content is preferably 0.002% or more. However, when Nb exceeds 0.1%, toughness is reduced. Therefore, the Nb content is set to 0 to 0.1%. In addition, it is preferable that the upper limit of content of Nb in the case of adding is 0.05%, and if it is 0.03%, it is still more preferable.
[0050]
  V:
  The addition of V is optional. If added, it has the effect of improving the strength of the steel. In order to obtain this effect remarkably, the V content is preferably 0.01% or more. However, when V exceeds 0.1%, toughness and weldability are reduced. Therefore, the content of V is set to 0 to 0.1%. In addition, when adding, it is preferable that the upper limit of content of V is 0.05%, and if it is 0.03%, it is still more preferable.
[0051]
  B:
  The addition of B is optional. If added, it has the effect of increasing the strength of the steel. In order to obtain this effect remarkably, the B content is preferably 0.0002% or more. However, when B exceeds 0.05%, toughness is reduced. Therefore, the content of B is set to 0 to 0.05%. In addition, when adding, it is preferable that the upper limit of content of B is 0.02%, and if it is 0.01%, it is still more preferable.
[0052]
  Sb:
  The addition of Sb is optional. If added, it has the effect of improving the overall corrosion resistance in a wet and dry repeated environment and increasing the acid resistance. In order to obtain these effects remarkably, the Sb content is preferably 0.01% or more. However, the above effect is saturated even if Sb is contained in excess of 0.5%. ThereforeSbThe content of was made 0 to 0.5%. In addition, when adding, it is preferable that the lower limit of content of Sb is 0.01%, and if it is 0.05%, it is still more preferable.
[0053]
  Sn:
  The addition of Sn is optional. If added, it has the effect of improving the overall corrosion resistance in a wet and dry repeated environment and increasing the acid resistance. In order to obtain these effects remarkably, the Sn content is preferably 0.01% or more. However, the effect is saturated even if Sn is contained in excess of 0.5%. ThereforeSnThe content of was made 0 to 0.5%. In addition, it is preferable that the lower limit of content of Sn in the case of adding is 0.1%.
[0054]
  Al:
  Al is an element effective for deoxidation of steel, but in the present invention, since it contains the amount of Si already described, it can be deoxidized with Si. Therefore, since it is not particularly necessary to deoxidize with Al, addition of Al is optional. On the other hand, if Al is positively added, the deoxidizing effect is enhanced, and nitride is formed to make the austenite grains fine, so that the strength is improved. Furthermore, the effect of improving toughness is also obtained. In order to reliably obtain these effects, the Al content is preferably 0.001% or more. However, even if Al is contained in excess of 0.07%, not only the deoxidation effect is almost saturated, but also the toughness is lowered due to coarsening of the nitride. In addition, when there is much content of Al, the production amount of an alumina inclusion increases, and H₂Reduces the overall corrosion resistance in a dry and wet repeated environment containing S, that is, in an environment of repeated wet and dry with acidic water. . Therefore, the content of Al is set to 0 to 0.07%. In addition, it is preferable that the lower limit of content of Al in the case of adding is 0.05%.
[0055]
  (B) Steel structure
  The total corrosion in the environment of repeated wet and dry with acidic water is because the cementite forming pearlite accelerates the hydrogen ion reduction of the cathode reaction, so the percentage Ap of pearlite in the structure and the C content in mass% In relation to this, when the value of “Ap / C” increases, the overall corrosion resistance decreases. In particular, when the value of “Ap / C” is 130 or more, even if the steel material has a chemical composition adjusted to the above-described range, the overall corrosion resistance is significantly reduced. Therefore, this departureClearlyIn this case, it is defined so as to satisfy the expression (1), that is, “Ap / C ≦ 130”.
[0056]
  In addition, inclusions in steel serve as a starting point for corrosion and affect the occurrence of pitting corrosion in the presence of S in the presence of S and the overall corrosion in a dry and wet environment with acidic water. However, as the particle size of inclusions is smaller and the abundance per unit area is smaller, there is less influence on the occurrence of general corrosion and pitting corrosion, and inclusions with a particle size exceeding 30 μm are 1 cm.²If it is less than 30 per sheet, the overall corrosion resistance and pitting corrosion resistance will not be reduced. Therefore, this departureClearlyIn this case, inclusions having a particle size exceeding 30 μm are 1 cm.²It was defined as less than 30 per one.
[0057]
  Inclusions with a particle size exceeding 30 μm are 1 cm.²For example, a steel material satisfying “Ap / C ≦ 130” is less than 30 per slab, and, for example, a slab subjected to electromagnetic stirring in the steelmaking stage while keeping the S content low is usually selected according to the steel composition. In this method, hot rolling is performed under the conditions that the heating temperature is about 1150 ° C., the rolling reduction per pass of rolling is 3% or more, and the rolling finishing temperature is about 800 to 900 ° C._ThreeIt can be produced by water-cooling a temperature range from the temperature above the point to at least about 570 ° C. and then allowing to cool in the atmosphere. In addition, all the above-mentioned temperature is the temperature in the surface part of steel materials.
[0058]
  The present invention described above(1), the present invention (2) and the present invention (3)This steel material shows good corrosion resistance even if it is used as it is, and can reduce the corrosion allowance. However, when the surface is covered with an anticorrosion coating made of organic resin or metal, the durability of the anticorrosion coating is improved and the corrosion resistance is improved. Will further improve.
[0059]
  Here, examples of the anticorrosion coating made of an organic resin include vinyl butyral, epoxy, urethane, and phthalic acid resin coatings, and examples of the anticorrosion coating made of metal include a plating coating and a thermal spray coating of Zn or Al. be able to.
[0060]
  In addition, the durability of the anticorrosive coating is improved because the corrosion of the steel material of the present invention as a base is remarkably suppressed, and the swelling and peeling of the anticorrosive coating due to corrosion of the base steel material from the defective portion of the anticorrosive coating is suppressed. This is probably because of this.
[0061]
  What is necessary is just to perform the process covered with said anti-corrosion film by a normal method. Further, it is not always necessary to apply the anticorrosion coating on the entire surface of the steel material, and only one surface of the steel material as the surface exposed to the corrosive environment may be anticorrosive.
[0062]
  Therefore, the steel material according to the present invention (4) is a steel material for a cargo oil tank described in any of the present invention (1) to the present invention (3), and at least one surface of which is subjected to anticorrosion treatment. It was stipulated that
[0063]
【Example】
  (Example 1)
  Table 1as well asTable 2After melting 26 types of steel having the chemical composition shown in FIG. 5 using a vacuum melting furnace to form a 50 kg steel ingot, a block having a thickness of 120 mm was produced by hot forging by a normal method.
[0064]
[Table 1]

[0065]
[Table 2]

[0066]
  Subsequently, the block having a thickness of 120 mm obtained by the forging was heated at 1150 ° C. for 2 hours and then hot-rolled to obtain a steel plate having a thickness of 20 mm. In addition, the manufacturing conditions of the said 20 mm steel plate are the following "manufacturing method 1" or "manufacturing method 2".
[0067]
  “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 allowed to cool to room temperature.
[0068]
  “Manufacturing method 2”: A 120 mm thick block was heated at 1150 ° C. for 2 hours, then hot-rolled to finish at 850 ° C. to a thickness of 20 mm, then water-cooled in a temperature range from 800 ° C. to 500 ° C., and then room temperature Manufacturing method that cools to the air.
[0069]
  From each steel plate having a thickness of 20 mm, a micro test piece cut along the center line in parallel with the rolling direction was prepared according to JIS G 0555, and the structure and inclusions were examined with an optical microscope.
[0070]
  That is, the surface (L cross section) cut parallel to the rolling direction is used as a test surface and mirror-polished, and then an optical microscope photograph of 60 fields of view is taken at a magnification of 400 times, and the grain size exceeds 30 μm from the photograph. The number of inclusions was measured.
[0071]
  In addition, the mirror-polished surface is corroded with nital, observed with a 10-field optical microscope at a magnification of 100 times, the ratio Ap of% of pearlite in the tissue is measured, and the above-mentioned “Ap / C” value is calculated. did.
[0072]
  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 steel plate having a thickness of 20 mm and described as it is next.Crude oil is H ₂ Including SIt was subjected to a corrosion test simulating the environment behind the deck of an actual ship.
[0073]
  That is, while preparing a glass container in which a 0.1% by weight NaCl aqueous solution was placed in the lower third part, the upper end of the glass container was opened by an acrylic lid having a gas supply port attached to the lower surface of the collected specimen. Was sealed. Next, the sealed glass container was placed in a thermostatic bath, and a temperature cycle of 50 ° C. × 20 hours → 25 ° C. × 4 hours was applied for 4 months. At that time, ballast time and full load time were simulated into the gas phase portion in the glass container, and the following two types of gas A and gas B were alternately blown from the gas supply port at intervals of two weeks.
[0074]
  Gas A:% by volume, 5% O₂-13% CO₂-0.02% SO₂-N remaining₂,
  Gas B: 5% O in volume%₂-13% CO₂-0.02% SO₂-0.25% H₂S-remaining N₂.
[0075]
  After the 4-month corrosion test, the corrosion rate (total corrosion rate) in units of “mm / year” was determined from the reduced mass of each specimen.Table 3Each of the above test results are shown together with the manufacturing conditions for a steel plate having a thickness of 20 mm.
[0076]
[Table 3]

[0077]
  Table 3As can be seen from the results shown in Table 1, the steel compositions of trial numbers B1 to B20 within the range defined by the present invention in terms of chemical composition, inclusion definition, and "Ap / C" have a general corrosion rate of 0.13 mm / year or less. And small. On the other hand, the steel materials of trial numbers B21 to B26 in which at least one of the chemical composition, the inclusion definition, and the value of “Ap / C” is outside the range specified in the present invention have a total corrosion rate of 0.29 mm / year or more. And big.
[0078]
  (Example 2)
  the aboveExample 1From each steel plate having a thickness of 20 mm described in the above, a test piece having a width of 100 mm, a length of 100 mm, and a thickness of 4 mm is collected.Crude oil is H ₂ Including SIt was subjected to a corrosion test that simulated the environment of the cargo oil tank bottom plate of an actual ship.
[0079]
  That is, after each of the width and length direction of the test piece having a width of 100 mm, a length of 100 mm, and a thickness of 4 mm, the central portion 3 mm is sealed and the crude oil is applied to the remaining portion with a thickness of 1 mm. The solid S was 0.2 g / cm on the test piece with the steel surface exposed in a cross shape by removing the seal at the center.²It was made to adhere in the ratio and was used for the corrosion test.
[0080]
  FIG.Shows a corrosion test piece produced in this manner.
[0081]
  Next, the above-mentioned corrosion test piece was attached to the bottom of the glass container containing artificial seawater, and the upper end of the glass container was sealed with an acrylic lid having a gas supply port. Next, the sealed glass container was placed in a thermostatic bath, and a test was conducted at a temperature of 40 ° C. for one month. At that time, ballast time and full load time were simulated into the artificial seawater in the glass container, and the following two kinds of gas A and gas B were alternately blown from the gas supply port at intervals of two weeks.
[0082]
  Gas A:% by volume, 5% O₂-13% CO₂-0.02% SO₂-N remaining₂,
  Gas B: 5% O in volume%₂-13% CO₂-0.02% SO₂-0.25% H₂S-remaining N₂.
[0083]
  After the 1 month corrosion test, the pitting corrosion growth rate was determined from the maximum pitting depth of the test piece.Table 3The above test results are also shown.
[0084]
  Table 3As is clear from the above, the steel composition Nos. B1 to B20 having a chemical composition within the range defined by the present invention and satisfying the above-mentioned formula (1) and the definition of inclusions has a pitting corrosion growth rate of 0.15 mm. / Smaller than a year. On the other hand, the steel composition of trial numbers B21 to B26, in which any one of the chemical composition, the inclusion definition, and the “Ap / C” value is outside the range defined in the present invention, has a pitting corrosion growth rate of 0.61 mm. / Greater than a year.
[0085]
  (Example 3)
  Table 414 types of steel having the chemical composition shown below were melted into a 50 kg steel ingot using a vacuum melting furnace, and then hot forged by a normal method to produce a block having a thickness of 120 mm.
[0086]
[Table 4]

[0087]
  Subsequently, the block having a thickness of 120 mm obtained by the forging was heated at 1150 ° C. for 2 hours and then hot-rolled to obtain a steel plate having a thickness of 20 mm. The manufacturing conditions for the 20 mm steel plate are “Production Method 1” or “Production Method 2” in Example 1.
[0088]
  In the same manner as in Example 1, micro test pieces were prepared from each steel plate having a thickness of 20 mm, and the structure and inclusions were examined using an optical microscope.
[0089]
  In addition, a test piece having a width of 25 mm, a length of 50 mm, and a thickness of 4 mm is taken from each steel plate having a thickness of 20 mm, and a 200 μm-thick tar epoxy resin anticorrosive coating is applied to the surface of the test piece. Application, Examples1The sample was subjected to a corrosion test under the same conditions as in.
[0090]
  After the 4-month corrosion test, the overall corrosion rate in units of “mm / year” was determined from the reduced mass of each specimen.
[0091]
  Table 5Each of the above test results are shown together with the manufacturing conditions for a steel plate having a thickness of 20 mm.
[0092]
[Table 5]

[0093]
  Table 5As is clear from the above, the steel composition Nos. B27 to B35 satisfying the above-mentioned formula (1) and the inclusions in the chemical composition is within the range defined in the present invention, and the overall corrosion rate is 0.04 mm / Less than a year. On the other hand, the steel materials of trial numbers B36 to B40 in which any one of the chemical composition, the inclusion definition, and the value of “Ap / C” is outside the range specified in the present invention have a total corrosion rate of 0.08 mm / Greater than a year.
【The invention's effect】
[0094]
  According to the steel material of the present invention,Inert gas and H ₂ Including SCorrosion resistance of the cargo oil tank in the corrosive environment is improved, and maintenance costs can be significantly reduced.
[Brief description of the drawings]
[FIG.]
  It is a figure which shows the relationship between Cu content and the general corrosion rate in the environment of the wet and dry repetition by acidic water.
[FIG.]
  Example 2It is a figure which shows the shape of the corrosion test piece used by.

Claims (4)

In mass%, C: 0.01 to 0.3%, Si: 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: 0 to 1%, W: 0 to 1%, Ca: 0 to 0.01%, Sb: 0 to 0.5%, Sn : 0-0.5%, Ti: 0-0.1%, Nb: 0-0.1%, V: 0-0.1%, B: 0-0.05%, Cr: 0-0. 05%, Al: 0 to 0.07%, the balance is Fe and impurities, the number of inclusions with a particle size exceeding 30 μm is less than 30 per 1 cm ² , and the following formula (1) is satisfied A steel material for cargo oil tanks.
Ap / C ≦ 130 (1)
Here, Ap in the above formula (1) represents the percentage of pearlite in the structure in% units, and C represents the content in mass% of carbon. Mo content is 0.01 to 1%, W content is 0.01 to 1%, Sb content is 0.01 to 0.5%, Sn content is 0.01 to 0.5%. The steel material for a cargo oil tank according to claim 1 , wherein at least one of the following values is satisfied. The Ti content is 0.005 to 0.1%, the Nb content is 0.002 to 0.1%, the V content is 0.01 to 0.1%, and the B content is 0.0002. The steel material for cargo oil tanks according to claim 1 or 2 , wherein at least one of -0.05% is satisfied. A cargo oil tank steel material according to any one of claims 1 to 3, steel, characterized in that at least one surface anticorrosion treatment is performed.

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