JP5375246B2 - Corrosion-resistant steel for crude oil tank and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a shape steel for crude oil tank which has excellent general corrosion resistance not only when being used for a crude oil tank in a bare state but also when being used in a primer-coated state, and in which the elongation effect of a coating life is remarkable, and also to provide a method for producing the same, and a crude oil tank using the shape steel. <P>SOLUTION: The corrosion resistant shape steel member for a crude oil tank contains, by mass, 0.001 to 0.16% C, &le;1.5% Si, 0.1 to 2.5% Mn, &le;0.03% P, &le;0.01% S, 0.005 to 0.1% Al, 0.001 to 0.008% N, 0.008 to 0.35% Cu, &gt;0.1 to 0.5% Cr, &le;0.01% Mo and 0.005 to 0.3% Sn, wherein the above components are contained in such a manner that the index of corrosion resistance reaches a predetermined value, and has microstructure composed of ferrite including deformed ferrite by &ge;10 area% to the whole structure and pearlite. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  TECHNICAL FIELD The present invention relates to a shape steel suitable for use in an oil tank of a crude oil tanker or a tank for transporting or storing crude oil (hereinafter collectively referred to as “crude oil tank”), and specifically, a ceiling portion of the crude oil tank. The present invention relates to a section material for a crude oil tank and a method for producing the same by hot rolling, which can reduce the overall corrosion that occurs on the surface of the section steel material on the side wall and the bottom.

It is known that the steel used for the inner surface of the tanker's crude oil tank, particularly the back of the upper deck and the upper part of the side wall, is subject to full corrosion. As a cause of general corrosion,
(1) Dew condensation on the steel surface due to temperature difference between day and night, and repeated wet and dry,
(2) inert gas sealed in explosion-proof into the oil tank _{(O 2} to about 5 vol%, CO ₂ about 13 vol%, SO ₂ about 0.01 vol%, gas boiler or engine typified composition the balance _{N 2)} in Of O ₂ , CO ₂ , SO ₂ into condensed water,
(3) Dissolution of corrosive gas such as H ₂ S volatilized from crude oil into condensed water,
(4) Residual seawater used for cleaning crude oil tanks,
Etc. These can also be known from the fact that strongly acidic dew condensation water, sulfate ions, and chloride ions were detected during the actual dock inspection.

Furthermore, H ₂ S is oxidized using iron rust generated by corrosion as a catalyst, and solid S is formed in layers in the iron rust, and these corrosion products easily peel off and fall off. Deposit on the bottom. For this reason, in the dock inspection every 2.5 years, the current situation is that repair of the upper part of the tank and collection of deposits at the bottom of the tank are performed with great expense.

  The most effective method for suppressing the corrosion is a method in which the steel material surface is heavily coated to shield the steel material from the corrosive environment. However, the coating area of the crude oil tank is enormous. In addition, since the coating film needs to be repainted once every 10 years due to deterioration of the coating film, a large cost is incurred for inspection and painting. Furthermore, it has been pointed out that corrosion is promoted on the damaged part of the heavy-painted coating film in the crude oil tank environment.

  Therefore, several corrosion resistant steels that improve the corrosion resistance of the steel material itself and have corrosion resistance in a crude oil tank environment have been proposed. For example, Patent Document 1 adds appropriate amounts of Si, Mn, P, S and Ni: 0.05 to 3% to steel containing C: 0.01 to 0.3% by mass. In addition, a corrosion resistant steel having improved resistance to general corrosion and local corrosion by selectively adding Mo, Cu, Cr, W, Ca, Ti, Nb, V, and B is disclosed.

  Further, Patent Document 2 discloses that steel containing C: 0.001 to 0.2% by mass% and appropriate amounts of Si, Mn, P, S and Cu: 0.01 to 1.5%, Al: 0.001 to 0.3%, N: 0.001 to 0.01% is added, and at least one of Mo: 0.01 to 0.2% or W: 0.01 to 0.5% Has been disclosed, which is excellent in overall corrosion resistance and local corrosion resistance, and further suppresses the formation of corrosion products containing solid S.

  Patent Document 3 discloses that steel containing C: 0.01 to 0.2% by mass% and appropriate amounts of Si, Mn, P, Ni: 0.01 to 2%, Cu: 0 Add 0.05 to 2%, W: 0.01 to 1%, selectively add Cr, Al, N, and O, and further define the addition amount of Cu, Ni, and W with a parameter formula Discloses a corrosion-resistant steel with improved overall corrosion and local corrosion.

  Patent Document 4 discloses that steel containing C: 0.01 to 0.2% by mass% with appropriate amounts of Si, Mn, P, Cr, Al, and Ni: 0.01 to 1%. Cu: 0.05-2%, Sn: 0.01-0.2% are added, and Mo, W, Ti, Zr, Sb, Ca, Mg, Nb, V, and B are selectively added. Thus, a corrosion-resistant steel having improved resistance to general corrosion and local corrosion is disclosed.

Japanese Patent Laid-Open No. 2003-082435 JP 2004-204344 A JP 2005-325439 A JP 2007-270196 A

  However, the corrosion-resistant steels disclosed in the above Patent Documents 1 to 4 are devised on the assumption of thick steel plates, and when applied to the manufacture of shape steel materials, the microstructure differs depending on the manufacturing method. It was difficult to say that sufficient corrosion resistance was expressed.

  Furthermore, the shape steel in crude oil tanks is used as a structural member, and due to its shape, corrosive substances are likely to accumulate on the surface, and moreover, it is more corrosive than thick steel plates because it is subject to corrosion from both sides of the steel. Is required.

  The present invention was developed in order to solve the above-mentioned problems, and its purpose is not only to have excellent overall corrosion resistance when used in a crude oil tank in a bare state, but also on the surface of a shaped steel material. An object of the present invention is to provide a shape material for a crude oil tank and a method for producing the same, which exhibits a remarkably excellent overall corrosion resistance and a coating life extending effect even when used in an existing state.

  In order to solve the above problems, the inventors first extracted factors that are considered to be involved in the overall corrosion in the crude oil tank, conducted a corrosion test in which these factors were combined in various ways, and conducted the overall corrosion that occurred in the crude oil tank. Was successfully reproduced. And through the corrosion test, the following knowledge was acquired about the governing factor and corrosion mechanism of a general corrosion.

The inert gas enclosed in the crude oil tank for explosion protection contains water vapor. Therefore, condensation occurs on the surface of the shape steel material on the inner wall of the tank due to the temperature difference between day and night during voyage. In this condensed water, inert gas components such as CO ₂ (carbon dioxide), O ₂ (oxygen), SO ₂ (sulfur dioxide), and volatile components from crude oil such as H ₂ S (hydrogen sulfide) dissolve and sulfate ions. A corrosive acidic solution containing is produced. It is also necessary to consider the chloride ions brought in by washing seawater in crude oil tanks. The corrosive acidic solution in which these components are dissolved concentrates in the process of increasing the temperature of the shape steel material, and causes overall corrosion on the surface of the shape steel material. Furthermore, S (sulfur) is precipitated from H ₂ S by using iron rust formed on the surface of the shaped steel material as a catalyst to form a rust layer in which the iron rust and sulfur are layered. The corrosion will continue continuously.

  Therefore, the inventors investigated the influence of various alloy elements on the overall corrosion of the shape steel material surface in an environment where condensed water containing sulfate ions and chloride ions is present. As a result, the addition of Cu, Cr and Sn has the effect of densifying the rust layer on the surface of the shape steel material formed in the environment used as the shape steel material for crude oil tanks and improving the overall corrosion resistance, W and It was confirmed that the addition of Sb promotes the formation of the dense rust layer and further improves the overall corrosion resistance. In addition, in an acidic corrosion environment where both chloride ions and sulfate ions exist simultaneously, it was confirmed that addition of Mo deteriorates the corrosion resistance. That is, by adding mainly Cu, Cr and Sn as elements for improving corrosion resistance, in addition to adding appropriate amounts of W and Sb, and limiting the Mo content, excellent overall corrosion resistance in the bare state. It was found that a section for a crude oil tank having

  Furthermore, the inventors have found that the above-described shaped steel material with optimized Cu, Cr, Sn, W and Sb contents has excellent corrosion resistance even in an unpainted state, but the surface contains a metal Zn or Zn compound. It has been found that the coating life can be greatly extended and the overall corrosion resistance is remarkably improved when used with the coating. The present invention has been completed based on the above findings and further studies.

That is, the present invention includes C: 0.001 to 0.16 mass%, Si: 1.5 mass% or less, Mn: 0.1 to 2.5 mass%, P: 0.03 mass% or less, S: 0.01 mass%. Hereinafter, Al: 0.005 to 0.1 mass%, N: 0.001 to 0.008 mass%, Cu: 0.008 to 0.35 mass%, Cr: more than 0.1 mass%, and 0.5 mass% or less, Mo: 0.01 mass% or less, Sn: 0.005-0.3mass% is contained, Furthermore, the said component is following (1) Formula:
B1 = 28 × C + 2000 × P ² + 27000 × S ² + 0.0083 × (1 / Cu) + 0.027 × (1 / Cr) + 95 × Mo + 0.00098 × (1 / Sn) −6
... (1)
Here, each element symbol in the above formula is the content of each element (mass%)
A crude oil having a microstructure of ferrite and pearlite containing B1 as defined in (1) or less, the balance being Fe and inevitable impurities, and containing processed ferrite in an area ratio of 10% or more with respect to the entire structure Corrosion-resistant steel for tanks.

In addition to the above component composition, the corrosion resistant steel for crude oil tank of the present invention further contains Ni: 0.005 to 0.4 mass%, and the following formula (2):
B2 = 28 × C + 2000 × P ² + 27000 × S ² + 0.0083 × (1 / Cu) + 2 × Ni + 0.027 × (1 / Cr) + 95 × Mo + 0.00098 × (1 / Sn) −6
... (2)
Here, each element symbol in the above formula is the content of each element (mass%)
The value of B2 defined in (1) is 0 or less.

In addition to the above component composition, the corrosion resistant steel for a crude oil tank according to the present invention may be one or more selected from W: 0.001 to 0.5 mass% and Sb: 0.005 to 0.3 mass%. Contains 2 types, the following formula (3):
B3 = 28 × C + 2000 × P ² + 27000 × S ² + 0.0083 × (1 / Cu) + 2 × Ni + 0.027 × (1 / Cr) + 95 × Mo + 0.00098 × (1 / Sn) + 0.0019 × (1 / (Sb + W))-6.5 (3)
Here, each element symbol in the above formula is the content of each element (mass%)
The value of B3 defined in (1) is 0 or less.

  In addition to the above component composition, the corrosion resistant steel for crude oil tanks of the present invention further includes Nb: 0.002-0.1 mass%, V: 0.002-0.1 mass%, Ti: 0.001-0. .1 mass% and B: containing one or more selected from 0.01 mass% or less.

  In addition to the above component composition, the corrosion resistant steel for a crude oil tank of the present invention is further selected from Ca: 0.0002 to 0.005 mass% and REM: 0.0005 to 0.015 mass%, It contains two types.

  Moreover, the corrosion-resistant steel material for crude oil tank of the present invention is characterized in that the yield stress is 315 MPa or more and the tensile strength is 440 MPa or more.

  Moreover, the corrosion-resistant steel material for crude oil tank of the present invention is characterized in that a coating film containing metal Zn or a Zn compound is formed on the surface of the steel material.

Moreover, the corrosion-resistant shape steel material for crude oil tanks of the present invention is characterized in that the Zn content in the coating film is 1.0 g / m ² or more.

Further, the present invention is the cumulative rolling reduction after heating a steel material having a composition as set forth in any one of the above in 1,000-1,350 ° C., a process for the preparation of shape steel by hot rolling, Ar ₃ or less transformation point Is a corrosion-resistant shape steel for a crude oil tank, characterized by being allowed to cool after hot rolling with a rolling finish temperature of (Ar ₃ transformation point −30 ° C.) to (Ar ₃ transformation point −180 ° C.). We propose a manufacturing method.

  The method for producing a corrosion-resistant steel material for a crude oil tank according to the present invention is characterized in that the hot rolling is performed by controlling the surface temperature difference of the whole material to be rolled within 70 ° C. during rolling.

  According to the present invention, it is possible to provide a shape steel material having excellent overall corrosion resistance and suitable for use in an oil tank of a crude oil tanker or a tank for transporting or storing crude oil. There is a remarkable effect.

It is a schematic diagram of the general corrosion test apparatus used in the Example of this invention.

The reason for limiting the component composition of the crude steel tank shape steel of the present invention to the above range will be described.
C: 0.001 to 0.16 mass%
C is an element that enhances the strength of the steel material. In the present invention, it is necessary to contain 0.001 mass% or more in order to obtain a desired strength. On the other hand, C not only deteriorates the corrosion resistance as the content increases, but addition exceeding 0.16 mass% deteriorates the weldability and the toughness of the heat affected zone. Therefore, C is in the range of 0.001 to 0.16 mass%. In addition, from the viewpoint of balancing strength and toughness, a range of 0.01 to 0.15 mass% is preferable.

Si: 1.5 mass% or less Si is an element that acts as a deoxidizer and increases the strength, but addition exceeding 1.5 mass% reduces the toughness of the steel. Therefore, in this invention, Si is limited to the range of 1.5 mass% or less. In order to obtain the above-described effect of Si reliably, addition of 0.01 mass% or more is desirable. Further, since Si contributes to the improvement of corrosion resistance by forming a corrosion prevention film in an acidic environment, it is added in the range of 0.3 to 1.5 mass% from the viewpoint of improving the corrosion resistance in an acidic environment. preferable.

Mn: 0.1 to 2.5 mass%
Mn is an element that increases the strength of the steel material. In the present invention, it is necessary to add 0.1 mass% or more in order to obtain a desired strength. On the other hand, addition exceeding 2.5 mass% reduces the toughness and weldability of the steel and promotes segregation, leading to non-uniform steel composition. Therefore, Mn is set to a range of 0.1 to 2.5 mass%. In addition, from the viewpoint of suppressing the formation of inclusions that maintain high strength and deteriorate corrosion resistance, the range of 0.5 to 1.6 mass% is preferable, and the range of 0.8 to 1.4 mass% is more preferable. preferable.

P: 0.03 mass% or less P is a harmful element that segregates at the grain boundaries to lower the toughness of the steel and also reduces the corrosion resistance, and is desirably reduced as much as possible. In particular, when the content exceeds 0.03 mass%, the central segregation is promoted to cause non-uniform steel composition, and the toughness and corrosion resistance are remarkably lowered. Therefore, P is set to 0.03 mass% or less. . Note that reducing P to less than 0.005 mass% causes an increase in production cost, so the lower limit of P is preferably about 0.005 mass%, and from the viewpoint of improving the general corrosion resistance in an acidic environment. 0.020 mass% or less is preferable.

S: 0.01 mass% or less S is a harmful element that forms MnS, which is a non-metallic inclusion, and serves as a starting point for corrosion and lowers the overall corrosion resistance, and is desirably reduced as much as possible. In particular, when the content exceeds 0.01 mass%, the overall corrosion resistance is significantly reduced. Therefore, in the present invention, the upper limit of S is set to 0.01 mass%. In addition, from the viewpoint of further improving the corrosion resistance, 0.005 mass% or less is desirable. However, since extreme reduction leads to an increase in manufacturing cost, a practical preferable range of S is 0.0010 to 0.0050 mass%. It is.

Al: 0.005 to 0.1 mass%
Al is an element which acts as a deoxidizing agent, and in the present invention, it is necessary to contain 0.005 mass% or more. On the other hand, if added in excess of 0.1 mass%, the toughness of the steel decreases. Therefore, Al is made into the range of 0.005-0.1 mass%. Preferably, it is the range of 0.01-0.05 mass%.

N: 0.001 to 0.008 mass%
N needs to be added in an amount of 0.001 mass% or more in order to improve the toughness of the steel and the mechanical properties of the welded joint. However, addition exceeding 0.008 mass% results in an increase in solute N, and the toughness of the joint is significantly reduced depending on the welding conditions. Therefore, N is set to a range of 0.001 to 0.008 mass%.

Cu: 0.008 to 0.35 mass%
Cu has an action of suppressing the overall corrosion by forming an anticorrosion film, and is an essential element in the present invention. However, if it is less than 0.008 mass%, the above effect cannot be obtained. On the other hand, when Cu is added in combination with Sn, the overall corrosion resistance is remarkably improved. However, when Cu is added in an amount exceeding 0.35 mass%, the hot workability is lowered and the manufacturability is impaired. Therefore, Cu is set to a range of 0.008 to 0.35 mass%. In addition, since the effect of Cu addition is saturated as the addition amount increases, the range of 0.008 to 0.15 mass% is preferable from the viewpoint of cost effectiveness.

Cr: more than 0.1 mass% and less than 0.5 mass% Cr forms a protective film on the steel material surface together with Cu, improves the overall corrosion resistance in an acidic environment, and has the effect of increasing the strength of the steel material. Is an essential element. In particular, in an acidic environment containing sulfate ions and chloride ions, Cr has an effect of forming an oxide film to cover the steel material surface and reducing the overall corrosion rate. In addition, since Cr densifies the rust layer together with Cu, the Zn compound remains in the rust layer for a long time even when the zinc primer is applied, and thus greatly contributes to the improvement of the corrosion resistance, including the corrosion resistance after coating. Furthermore, since the addition amount of Cu can be suppressed by the effect of improving the corrosion resistance by adding Cr, there is an effect of reducing a decrease in hot workability caused in the presence of Cu and Sn. However, when Cr is added in an amount of 0.1 mass% or less, the above-described effect cannot be obtained. On the other hand, when the content exceeds 0.5 mass%, the above effect is saturated, and the cost is increased and weldability is deteriorated. . Therefore, Cr is added in a range of more than 0.1 mass% and 0.5 mass% or less.

Mo: 0.01 mass% or less Mo is generally considered to be an element having the same action as W and improving the corrosion resistance. However, the inventors have formed an insoluble salt in an acidic salt water environment, whereas Mo forms a soluble salt in an acidic salt water environment and does not exhibit a barrier effect. It has been newly found that when the Mo content exceeds 0.01 mass%, the corrosion resistance in an acidic salt water environment deteriorates. Therefore, in the present invention, the Mo content is limited to 0.01 mass% or less.

Sn: 0.005-0.3 mass%
Sn has the effect of suppressing the overall corrosion in an acidic environment by forming a dense rust layer due to the combined effect with Cu, and is an element that is essential for addition in the present invention. However, if the amount is less than 0.005 mass%, the above-described addition effect is not obtained. On the other hand, the addition exceeding 0.3 mass% causes deterioration of hot workability and toughness. Therefore, Sn is set to a range of 0.005 to 0.3 mass%.

The above elements are basic components of the shape steel according to the present invention. However, in order for the shaped steel material of the present invention to exhibit excellent overall corrosion resistance, not only the above components are in the above composition range, but also the following formula (1):
B1 = 28 × C + 2000 × P ² + 27000 × S ² + 0.0083 × (1 / Cu) + 0.027 × (1 / Cr) + 95 × Mo + 0.00098 × (1 / Sn) −6
... (1)
Here, each element symbol in the above formula is the content of each element (mass%)
It is necessary to contain so that the value of B1 defined by (1) may be 0 or less.
The above formula (1) is an empirical formula representing an index of corrosion resistance obtained by the corrosion test conducted in the present invention and summarizing the influence of each element on the overall corrosion resistance, and the value of B1 exceeds 0. It is known that the general corrosion resistance cannot be secured. In the above formula (1), regarding the influence of each element on the corrosion resistance, the element of the primary and secondary terms shows that the overall corrosion resistance decreases as the element is added, while the reciprocal number. The element of the term which has shown has shown that a general corrosion resistance improves, so that it adds. That is, C and Mo are corrosion resistance decreasing elements, P and S are corrosion resistance decreasing elements that are affected by the square of the content, and Cu, Cr, and Sn are corrosion resistance improving elements.

In addition to the above basic components, Ni can be added to the structural steel material of the present invention in the following range.
Ni: 0.005-0.4 mass%
Ni is added in combination with Cu, thereby suppressing the deterioration of hot workability. However, if the addition is less than 0.005 mass%, the above effect cannot be obtained. On the other hand, the addition exceeding 0.4 mass% causes an increase in cost. Therefore, Ni is preferably added in the range of 0.005 to 0.4 mass%. In addition, from a cost-effective viewpoint, the range of 0.005-0.15 mass% is more preferable.

In addition, when adding Ni, it replaces with the value of said B1, and the following (2) formula:
B2 = 28 × C + 2000 × P ² + 27000 × S ² + 0.0083 × (1 / Cu) + 2 × Ni + 0.027 × (1 / Cr) + 95 × Mo + 0.00098 × (1 / Sn) −6
... (2)
Here, each element symbol in the above formula is the content of each element (mass%)
It is necessary to contain each component so that the value of B2 defined in (1) is 0 or less. Here, as can be seen from the equation (2), Ni is an element that reduces the corrosion resistance.

Moreover, in addition to the said component, the steel material of this invention can further add 1 type or 2 types chosen from W and Sb in the following range.
W: 0.001 to 0.5 mass%
W inhibits the progress of corrosion by forming an insoluble FeWO ₄ while WO ₄ ^2- ions formed in a corrosive environment exert a barrier effect against anions such as chloride ions. Furthermore, there is also an effect of densifying the rust layer formed on the steel material surface. And, W has an effect of suppressing the progress of the general corrosion in the corrosive environment where H ₂ S and Cl ⁻ exist due to these chemical and physical effects. However, if the amount is less than 0.001 mass%, a sufficient addition effect cannot be obtained. On the other hand, addition exceeding 0.5 mass% not only saturates the effect but also causes an increase in cost. Therefore, when adding W, it is preferable to set it as the range of 0.001-0.5 mass%.

Sb: 0.005 to 0.3 mass%
Sb, like Sn, has a function of suppressing corrosion in an acidic environment by forming a dense rust layer due to the combined effect of Cu and W, and can be added when it is desired to further improve this property. However, if the addition is less than 0.005 mass%, there is no effect. On the other hand, if the addition exceeds 0.3 mass%, the effect is saturated and the workability is lowered. Therefore, when adding Sb, it is preferable to set it as the range of 0.005-0.3 mass%.

When adding W and / or Sb in addition to Ni, the following formula (3) is used instead of the value of B1 or B2:
B3 = 28 × C + 2000 × P ² + 27000 × S ² + 0.0083 × (1 / Cu) + 2 × Ni + 0.027 × (1 / Cr) + 95 × Mo + 0.00098 × (1 / Sn) + 0.0019 × (1 / (Sb + W))-6.5 (3)
Here, each element symbol in the above formula is the content of each element (mass%)
It is necessary to contain each element so that the value of B3 defined in (1) is 0 or less. Here, as can be seen from the equation (3), W and Sb are elements that improve the corrosion resistance.

  Furthermore, in order to improve the strength and toughness of the structural steel material of the present invention, in addition to the above components, one or more selected from Nb, V, Ti and B should be added in the following range. Can do.

Nb: 0.002 to 0.1 mass%
Nb is an element added for the purpose of improving the strength and toughness of steel. However, if it is less than 0.002 mass%, the effect is not obtained. On the other hand, if it exceeds 0.1 mass%, the above effect is saturated. Therefore, when adding Nb, it is preferable to set it as the range of 0.002-0.1 mass%.

V: 0.002-0.1 mass%
V is an element added for the purpose of improving the strength of steel. However, if it is less than 0.002 mass%, there is no effect of improving the strength. On the other hand, addition exceeding 0.1 mass% causes a decrease in toughness. Therefore, when adding, it is preferable to set it as the range of 0.002-0.1 mass%.

Ti: 0.001 to 0.1 mass%
Ti is an element added for the purpose of improving the strength and toughness of steel. However, if it is less than 0.001 mass%, the effect is not obtained. On the other hand, if it exceeds 0.1 mass%, the effect is saturated. Therefore, when adding, it is preferable to set it as the range of 0.001-0.1 mass%.

B: 0.01 mass% or less B is an element added for the purpose of improving the strength of steel, and the effect is obtained by addition of 0.0003 mass% or more. When added, 0.0003 mass% or more is added. It is preferable to do this. However, since addition exceeding 0.01 mass% reduces toughness, when adding, it is preferable to make it 0.01 mass% or less.

Furthermore, in order to improve ductility and toughness, the structural steel material of the present invention may further contain one or two selected from Ca and REM in the following ranges in addition to the above components.
Ca: 0.0002 to 0.005 mass%
Ca has an effect of improving ductility and toughness by controlling the form of inclusions, and also has an effect of improving corrosion resistance in a painted state, and therefore can be added for the purpose of improving these properties. However, if the amount is less than 0.0002 mass%, the effect is not obtained. On the other hand, addition exceeding 0.005 mass% causes a decrease in toughness. Therefore, when adding, it is preferable to set it as the range of 0.0002-0.005 mass%. In addition, from the viewpoint of improving the corrosion resistance, a range of 0.001 to 0.005 mass% is more preferable.

REM: 0.0005 to 0.015 mass%
REM (Rare Earth Metal) means a rare earth element having an atomic number of 57 to 71, and can be generally added using misch metal which is a mixture containing La, Ce, Pr, Nd and the like. This REM has the effect | action which controls the form of an inclusion and improves ductility and toughness. However, if it is less than 0.0005 mass%, the effect is not obtained. On the other hand, addition exceeding 0.015 mass% reduces toughness. Therefore, when adding, it is preferable to set it as the range of 0.0005-0.015 mass%. In addition, from the viewpoint of improving the corrosion resistance, a range of 0.005 to 0.015 mass% is more preferable.

  In the structural steel material of the present invention, the balance other than the above components is composed of Fe and inevitable impurities. However, the shape steel material of the present invention does not refuse the inclusion of other elements as long as the effects of the present invention are not impaired. For example, if it is O, it may contain 0.008 mass% or less. it can.

Next, the microstructure of the steel tank shape steel according to the present invention will be described.
In general, steel sheets used in ships, especially high-strength thick steel sheets with yield stress YP: 315 MPa or more and tensile strength TS: 440 MPa or more, are controlled rolling of steel materials with low carbon equivalents and high weldability. By adopting TMCP combined with the control cooling and introducing hard bainite into the steel sheet structure, high strength is achieved. And the case where low temperature toughness is calculated | required and the request | requirement to thickening respond | correspond by optimizing the conditions of the said controlled rolling and controlled cooling. Therefore, in this case, the microstructure of the steel sheet is usually a ferrite + bainite structure.

  On the other hand, in the case of a section steel, the width and thickness of the short side and the long side are often different.For example, in the case of an unequal side unequal thick angle steel with a non-rectangular cross section, inevitably during hot rolling or During subsequent cooling, temperature non-uniformity occurs. Here, when further controlled cooling (accelerated cooling) is applied, the residual stress becomes non-uniform, causing torsion, bending, and warping, resulting in a decrease in dimensional accuracy, thus increasing the shape correction load after rolling. Therefore, it is difficult to apply TMCP, which has a high strength by introducing a hard bainite structure as the second phase, to the shape steel.

  Therefore, the shape steel used for ships has a high strength of yield stress YP: 315 MPa or more and tensile strength TS: 440 MPa or more with a ferrite + pearlite structure which is a normal hot rolled structure without accelerated cooling. It is required to achieve. Means to achieve this include a method of increasing the pearlite fraction of the second phase, a method of refining the ferrite structure, a method of hardening the ferrite by solid solution strengthening or precipitation strengthening, or a (γ + α) two-phase region. A method of hot rolling and converting a part of the ferrite into processed ferrite is conceivable.

  Of the above methods, the method of refining ferrite is an effective means for increasing YP. However, since the increase in TS is small, sufficient strength cannot be achieved by this method alone. The method for increasing the pearlite fraction requires addition of a large amount of C, but excessive addition of C is not preferable because it causes a decrease in weldability. In addition, the method of strengthening ferrite by adding a solid solution strengthening element or a precipitation strengthening element causes a decrease in weldability or an increase in material cost due to the addition of a large amount of alloy elements.

  On the other hand, the method of utilizing processed ferrite can increase YP and TS in a state where the addition of C and alloy elements is minimized and weldability is maintained. In other words, since this method can achieve high strength without hot-rolling and controlled cooling (accelerated cooling), the occurrence of bending and warping during rolling and cooling, which is an inherent problem during shape steel production It is possible to increase the strength while suppressing. Therefore, in the present invention, as a means for increasing the strength of the shape steel for the crude oil tank, a method is adopted in which the microstructure of the steel is made of ferrite containing processed ferrite + pearlite structure.

The processed ferrite needs to be 10% or more of the entire steel structure in terms of area ratio. If the processed ferrite is less than 10%, the steel cannot be sufficiently strengthened. The upper limit is not particularly specified, but if it exceeds 70%, the increase in strength is saturated and the risk of roll breakage accompanying an increase in load during two-phase rolling of (α + γ) increases, so the upper limit is set to 70%. Is preferred. Here, the processed ferrite is ferrite having a high dislocation density formed by hot rolling in the (α + γ) two-phase region below the Ar ₃ transformation point, and the fraction is flattened processed ferrite. Can be obtained by quantifying the area ratio of the entire microstructure by image analysis. The measurement position of the microstructure is preferably a ¼ part thickness at the thickest part. The balance is ferrite (other than processed ferrite) and pearlite structure. The pearlite structure is preferably 20% or less in terms of area ratio. Note that a structure other than ferrite and pearlite, such as bainite, may exist if the area ratio is 20% or less.

Next, a method for producing a crude steel tank shape steel having a ferrite + pearlite structure containing the processed ferrite will be described.
In producing the steel tank for crude oil tank of the present invention, first, steel having the above component composition is melted by a generally known method such as a converter or an electric furnace, and then by a generally known method such as a continuous casting method or an ingot forming method. It is preferable to use a steel material such as bloom or billet. In addition, after melting molten steel, you may add processes, such as ladle refining and vacuum degassing.

  Next, the steel material is reheated in a heating furnace and hot-rolled to obtain a crude steel tank shape having desired dimensions, desired microstructure and mechanical properties. At this time, the reheating temperature of the steel material needs to be in the range of 1000 to 1350 ° C. When the heating temperature is less than 1000 ° C., the deformation resistance is large and hot rolling becomes difficult. On the other hand, heating exceeding 1350 ° C. causes generation of surface marks, or increases scale loss and fuel consumption rate. Preferably, it is the range of 1100-1300 degreeC.

In the subsequent hot rolling, it is necessary to set the cumulative rolling reduction below the Ar ₃ transformation point to 10 to 80%. If the rolling temperature is not lower than the Ar ₃ transformation point, the microstructure of the steel does not contain processed ferrite, and the required strength and toughness cannot be ensured. Similarly, when the cumulative rolling reduction below the Ar ₃ transformation point is less than 10%, the toughening effect is small because the amount of processed ferrite produced is small. Conversely, when the rolling reduction exceeds 80%, the rolling load increases and rolling becomes difficult, or the number of rolling passes increases, leading to a decrease in productivity. Therefore, the cumulative rolling reduction below the Ar ₃ transformation point is 10 to 80%. Preferably, it is 10 to 60% of range. Note that the rolling below the Ar ₃ transformation point may be performed at least one pass or more, and may be a plurality of passes. Here, the cumulative rolling reduction below Ar ₃ transformation point, Ar ₃ in cross-section area reduction rate of the cross-sectional area of the rolled material after rolling completion to the cross-sectional area of the rolled material (S _f) in the transformation point (S _a) It is expressed by the following formula.
(Cumulative rolling reduction [Ar] below Ar ₃ transformation point) = 100 × (S _f −S _a ) / S _f

Further, the hot rolling must be performed under conditions in which the rolling finishing temperature and (Ar ₃ transformation point -30 ℃) ~ (Ar ₃ transformation point -180 ° C.). When the rolling finishing temperature exceeds (Ar ₃ transformation point −30 ° C.), the effect of toughening due to the introduction of processed ferrite having a high dislocation density by two-phase rolling cannot be obtained sufficiently, while (Ar ₃ transformation point −180 ° C.). This is because the rolling load increases due to an increase in deformation resistance, and it becomes difficult to perform rolling.

Further, the above hot rolling is performed before the rolling at an Ar ₃ transformation point or less, and a temperature difference (that is, heat during rolling) due to a portion of the shape steel (long side, short side, web, flange, etc.) during rolling. It is preferable that the temperature difference in the whole of the rolled steel material is within 70 ° C., more preferably 50 ° C. or less. For example, for an unequal side unequal thick angle steel that has a difference in wall thickness between the long side and the short side, water-cool the thick side of the short side before and after the rolling mill. The temperature difference between the long side and the short side is preferably kept within 70 ° C., more preferably within 50 ° C. When the temperature difference exceeds 70 ° C., not only the variation in strength and toughness characteristics between the short side and the long side, but also bending and warping in the cooling process after rolling increase, and the burden required for correction is large. This is because productivity decreases. Furthermore, if it is 50 degrees C or less, since production stability is improved, it is more preferable. In addition, the temperature difference of each part of a shape steel measures the surface temperature in each part of the shape steel in the middle of rolling with a radiation thermometer, and calculates | requires from the difference of the obtained maximum temperature and minimum temperature.

As a means for suppressing the temperature difference between each part of the shape steel (for example, the short side and the long side) within 70 ° C., more preferably within 50 ° C., cooling equipment disposed before and after the rough rolling mill is used. The control method is preferable. Specifically, a method of eliminating the temperature difference by intensively water-cooling the thicker short side with the cooling facility is preferable. Water cooling at this time may be performed only on the front and back of the rolling mill, only on the rear surface, or both on the front and back, and may be performed in multiple times depending on the dimensions and required accuracy of the shape steel to be hot rolled. Also good. The water density at the time of water cooling is preferably 1 m ³ / m ² · min or more.

  Cooling subsequent to hot rolling is air cooling (cooling). Thereby, shape changes such as bending and warping caused by uneven cooling after rolling can be reduced, and the correction burden on the product after rolling can be reduced. The cooling rate during cooling is about 0.4 to 1.0 ° C./sec, although it depends on the plate thickness. Applying measures for accelerating / decelerating the cooling within the range of the cooling rate (forced cooling, heat retention, etc.) is substantially the same as that for cooling, so this is not particularly excluded.

  In general, the shape steel used for crude oil tanks of tankers is improved in overall corrosion resistance by applying a coating such as a primer containing metal Zn or a Zn compound (hereinafter collectively referred to as “zinc primer”). Have been used. Since these shape steel materials are subjected to a shot blast treatment on the surface and then coated with a zinc primer, depending on the surface condition such as the roughness of the steel material, the base may not be completely covered. Therefore, in order to completely cover the entire surface of the steel material, a coating thickness of a certain level or more is necessary. Preferably, the coating thickness of the zinc primer is set to an average thickness of 15 μm or more, and the overall corrosion resistance is remarkably increased. Can be improved.

In this respect, the crude steel tank shape steel of the present invention produced by the above method using a steel material having the above component composition is not only excellent in overall corrosion resistance even in a non-painted state, but also painted. It is characterized by its excellent later corrosion resistance. In particular, the shape steel material for crude oil tanks of the present invention has a remarkable overall corrosion resistance by making the coating amount of the primer containing metal Zn or Zn compound 1.0 g / m ² or more in terms of Zn content. In addition, the paint life can be greatly extended. Furthermore, if it is 2.5 g / m ² or more, more excellent overall corrosion resistance can be obtained. In addition, from the viewpoint of overall corrosion resistance, there is no upper limit on the amount of zinc primer applied. However, if the coating film of the zinc primer becomes thicker, the cutting property and weldability deteriorate, so the upper limit thickness is about 100 μm. Is preferred.

The relation between the coating thickness of the zinc primer and the Zn content on the surface of the shaped steel material depends on the Zn content in the zinc primer, but generally, if the average coating thickness is 15 μm or more, the entire surface of the shaped steel material The coating amount of 1.0 g / m ² or more can be ensured in terms of Zn content regardless of the type of zinc primer.
The Zn content on the surface of the steel material is, for example, a plurality of (for example, 10) pieces of 30 mm square pieces cut out from the steel material, and all the coating film or rust layer on the surface is dissolved and recovered, and the amount of Zn contained therein Can be obtained by analyzing the above.
As described above, the structural steel of the present invention includes components for various containers (primers or paints used in combination) constituting oil tanks of oil tanks, tanks for transporting crude oil, or tanks for storing crude oil. ) Can be widely used.

No. shown in Table 1-1 and Table 1-2. Steel having a component composition of 1 to 35 was melted in a vacuum melting furnace or converter to form a bloom, and this bloom was heated to a temperature of 1300 ° C. in a heating furnace, and then shown in Tables 2-1 and 2-2. After hot rolling with a cumulative reduction of 40% below the Ar ₃ transformation point and a rolling finishing temperature of 730 ° C., air cooling is performed, and the unequal side unequal thickness angle steel with a cross-sectional dimension of 350 × 100 × 11/17 mm ( NAB) was produced. About some steel, the manufacturing method outside the range of this patent was performed, and it was set as the comparative example.
The following various test pieces were collected from the short side portion of the shaped steel obtained as described above, and each test was performed. Specifically, refer to the description of the ship classification standard of the Japan Maritime Association (so-called NK standard, 2007 edition) [Figure K3.1 (2) in Chapter 3 of K], and in accordance with this, the specimen collection position It was determined. First, a JIS No. 1A tensile test piece was collected and measured for tensile properties (yield stress YP, tensile strength TS, elongation El). Also, a sample for observing the structure was collected, the structure having a thickness of ¼ part was observed with a microscope at a magnification of 200 times, and the flattened processed ferrite generated by the two-phase region rolling was traced, and the microstructure was The area occupied was quantified by image analysis, and the area ratios of ferrite, processed ferrite and pearlite were determined. In the steel having processed ferrite, the main phases other than processed ferrite were pearlite or bainite and unprocessed ferrite generated after hot rolling.

  Similarly, the above-mentioned No. Cut out a small piece of length 50mm x width 25mm x thickness 4mm from the short side of 1 to 35 shaped steel, and after shot blasting the surface, bare zinc coating with no inorganic zinc primer A total of four types of corrosion test pieces were prepared: a test piece (coating thickness of 0 μm) and a test piece applied to three levels of coating thicknesses of 5 to 10 μm, 15 to 25 μm, and 50 to 70 μm. In addition, in order to accelerate corrosion, the test piece coated with zinc primer was subjected to X-shaped cutting with a coating film damage area ratio of 1.0% on the surface to reach the steel surface, and this was simulated damage It was a place.

About the said corrosion test piece, the general corrosion resistance was evaluated using the corrosion test apparatus shown in FIG. 1 which can reproduce the full corrosion environment which simulated the environment of the upper deck of a crude oil tank.
This corrosion test apparatus is composed of a corrosion test tank 2 and a temperature control plate 3. Water 6 is injected into the corrosion test tank 2 in order to maintain a saturated vapor pressure, and the temperature is maintained at 30 ° C. Yes. Further, inside the corrosion test tank 2, in order to simulate the corrosive environment in the crude oil tank, CO ₂ : 13 vol%, O ₂ : 5 vol%, SO ₂ : 0.01 vol%, H ₂ S: 0.3 vol% The remaining part is N ₂ mixed gas 4 and filled under saturated water vapor pressure. The test piece 1 is attached below the temperature control plate 3 installed in the upper part of the corrosion test tank, and is heated and cooled by a heater and a cooling device at 25 ° C. × 1 hour / 50 ° C. × 5 hours: 1 hour each. Was set to 1 cycle (8 hours), and this was applied for 28 days to simulate the general corrosion caused by condensed water.
In addition, in order to give sulfate ion and chloride ion to the surface of the test piece (surface to be tested), 500 μL of an aqueous solution mixed with sodium sulfate and sodium chloride corresponding to 1000 massppm of sulfate ion and 10,000 massppm of chloride ion is applied and dried. The test was conducted. In addition, after the start of the test, sulfate ions and chloride ions were supplied every week.

After the above corrosion test is completed, for unpainted specimens, the rust generated on the specimen surface is removed, and the weight loss due to corrosion is determined from the mass change before and after the test. In terms of the above, the general corrosion resistance was evaluated according to the following criteria.
<Evaluation of overall corrosion resistance of unpainted materials>
○: Corrosion rate of less than 0.2 mm / year Δ: Corrosion rate of 0.2 mm / year or more and less than 0.8 mm / year ×: Corrosion rate of 0.8 mm / year or more For the primer coating material, the surface of each test piece and The area ratio of rust that progressed under the coating film was measured, and the overall corrosion resistance was evaluated according to the following criteria.
<Evaluation of corrosion resistance of primer coating material>
○: Rust area ratio of less than 25% △: Rust area ratio of 25% or more and less than 50% ×: Rust area ratio of 50% or more

The results of the tensile test, the microstructure observation result, and the overall corrosion test are shown together in Tables 2-1 and 2-2. From the above table, the composition and production conditions of No. The shape steels 1 to 25 have a general corrosion resistance of ○ under any condition, that is, the overall corrosion resistance is good even in an uncoated bare state, and the entire surface is resistant even when a zinc primer is applied. It was confirmed that it has excellent overall corrosion resistance.
On the other hand, even if the composition of the present invention is satisfied, the shape steel no. The shape steels of Nos. 9-3, 23-2, and 23-3 are shaped steel Nos. The shape steels 9-3 and 23-2 cannot secure the target strength because of the low processed ferrite fraction. As for the shape steel of 23-3, the temperature difference in each part of the shape steel became large by water cooling, and the shape defect occurred.
In addition, the composition of the components deviating from the scope of the present invention. It can be seen that the shape steels 26 to 35 are inferior in overall corrosion resistance because the corrosion resistance evaluation is x or Δ not only when the inorganic zinc primer is not applied but also when it is applied.
Furthermore, it can be seen that a shape steel having a microstructure containing processed ferrite within the scope of the present invention is a high strength shape steel having a yield stress of 315 MPa or more and a tensile strength of 440 MPa or more.

  Since the steel tank for crude oil tank of the present invention exhibits excellent corrosion resistance in a corrosive environment caused by seawater, it is effective for extending the life of the ship itself by extending the repair period of the ship, but is used in a similar corrosive environment. It can also be used as a shape steel used in other fields.

1: Corrosion test piece 2: Test tank 3: Temperature control plate 4: Introduction gas 5: Exhaust gas 6: Water

Claims (10)

C: 0.001 to 0.16 mass%,
Si: 1.5 mass% or less,
Mn: 0.1 to 2.5 mass%,
P: 0.03 mass% or less,
S: 0.01 mass% or less,
Al: 0.005 to 0.1 mass%,
N: 0.001 to 0.008 mass%,
Cu: 0.008 to 0.35 mass%,
Cr: more than 0.1 mass% and 0.5 mass% or less,
Mo: 0.01 mass% or less,
Sn: 0.005-0.3mass% is contained,
Furthermore, the above components are contained so that the value of B1 defined by the following formula (1) is 0 or less,
A corrosion-resistant steel for a crude oil tank, the balance of which consists of Fe and inevitable impurities, and has a microstructure composed of ferrite and pearlite containing processed ferrite in an area ratio of 10% or more with respect to the entire structure.
B1 = 28 × C + 2000 × P ² + 27000 × S ² + 0.0083 × (1 / Cu) + 0.027 × (1 / Cr) + 95 × Mo + 0.00098 × (1 / Sn) −6
... (1)
Here, each element symbol in the above formula is the content of each element (mass%) The crude oil according to claim 1, further comprising Ni: 0.005 to 0.4 mass% in addition to the component composition, wherein the value of B2 defined by the following formula (2) is 0 or less. Corrosion-resistant steel for tanks.
B2 = 28 × C + 2000 × P ² + 27000 × S ² + 0.0083 × (1 / Cu) + 2 × Ni + 0.027 × (1 / Cr) + 95 × Mo + 0.00098 × (1 / Sn) −6
... (2)
Here, each element symbol in the above formula is the content of each element (mass%) In addition to the above component composition, it further contains one or two selected from W: 0.001-0.5 mass% and Sb: 0.005-0.3 mass%, and is defined by the following formula (3) The value of B3 to be reduced is 0 or less, and the corrosion resistant steel for a crude oil tank according to claim 1 or 2.
B3 = 28 × C + 2000 × P ² + 27000 × S ² + 0.0083 × (1 / Cu) + 2 × Ni + 0.027 × (1 / Cr) + 95 × Mo + 0.00098 × (1 / Sn) + 0.0019 × (1 /(Sb+W))−6.5 (3)
Here, each element symbol in the above formula is the content of each element (mass%) In addition to the above component composition, Nb: 0.002 to 0.1 mass%, V: 0.002 to 0.1 mass%, Ti: 0.001 to 0.1 mass%, and B: 0.01 mass% or less The corrosion-resistant shape steel material for crude oil tanks according to any one of claims 1 to 3, comprising one or more selected from the group consisting of: 2. In addition to the above component composition, it further contains one or two selected from Ca: 0.0002 to 0.005 mass% and REM: 0.0005 to 0.015 mass%. The corrosion-resistant steel material for crude oil tanks according to any one of -4. The yield strength is 315 MPa or more, and the tensile strength is 440 MPa or more, the corrosion-resistant steel for a crude oil tank according to any one of claims 1 to 5. The corrosion resistant steel for a crude oil tank according to any one of claims 1 to 6, wherein a coating film containing metal Zn or a Zn compound is formed on the surface of the steel. The corrosion resistant shaped steel for a crude oil tank according to claim 7, wherein the content of Zn in the coating film is 1.0 g / m ² or more. Cumulative reduction of a steel material having a composition as set forth in claim 1 after heating to from 1000 to 1350 ° C., in a method for producing a shape steel by hot rolling, the following Ar ₃ transformation point Corrosion-resistant form for crude oil tank, characterized by cooling after hot rolling with a rate of 10 to 80% and a rolling finishing temperature of (Ar ₃ transformation point −30 ° C.) to (Ar ₃ transformation point −180 ° C.) Steel manufacturing method. The method for producing a corrosion-resistant steel for a crude oil tank according to claim 9, wherein the hot rolling is performed while controlling a difference in surface temperature of the whole material to be rolled during rolling within 70 ° C.

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