KR20240090929A - Steel materials for crude oil tanks - Google Patents

Abstract

The chemical composition of steel materials for crude oil tanks is, in mass%, C: 0.03 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.60 to 2.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.001 to 0.001%. Contains 0.050%, N: 0.001 to 0.010%, Cu: 0.01 to 0.60%, Mo: 0.01 to 0.20%, W: 0.01 to 0.20%, Sn: 0.01 to 0.20%, and Sb: 0.01 to 0.20%. Contains one or two or more types selected from, the balance: Fe and impurities, [Cu+6×Mo+2×W+0.5×Sn+0.5×Sb] is 0.50 or more, and in the rolling direction and thickness direction In the metal structure in a parallel cross section, the total area ratio of bainite and/or acular ferrite is 20 to 95%, the area ratio of martensite is 2.0% or less, and the Vickers hardness is 210HV10 or less. Steel for oil tanks.

Description

Steel materials for crude oil tanks

The present invention relates to steel materials for crude oil tanks.

Although welded structural steel with excellent strength and weldability is used in crude oil tankers, many pitting-shaped localized corrosion (called pits) with a relatively high growth rate are generated. The diameter of these pits is approximately 10 to 30 mm, and their progress rate is 2 to 3 mm/year. This value far exceeds the average wasting rate due to corrosion, which is 0.1 mm/year, which is considered when designing the hull.

Therefore, in the 2010 SOLAS Convention (International Convention for the Safety of Life at Sea), in the oil tank of a crude oil tanker with a cargo weight of 5,000 tons or more for which a construction contract was concluded after January 1, 2013, Anti-corrosion measures by painting or corrosion-resistant steel are now mandatory. The reason why corrosion-resistant steel is applied is that the depth of the pit, which develops at intervals of about 2 and a half years when tankers are regularly docked, makes repairs by welding and touch-up by painting unnecessary. This is because it is suppressed to such an extent that subsequent corrosion stops.

The causes of pits are as follows. Crude oil contains brine with a concentration of about 10%, and brine, which has a heavier specific gravity than crude oil, is deposited on the bottom of the oil tank during transportation. Normally, the oil tank bottom plate is covered with a high-viscosity oil layer (hereinafter also referred to as “oil coat”) attached to the steel plate and has the same anti-corrosion effect as painting, so corrosion caused by brine does not occur. However, if a part of this oil coat has a defective part or a part that does not have a sufficient anti-corrosive effect, the part is corroded by brine. In addition, hydrolysis due to corrosion occurs in the corroded area and becomes acidic, so corrosion is further accelerated and corrosion occurs in the form of pitting pits. In other words, the cause is that defective parts of the oil coat are corroded by brine.

In addition, the reason why the corrosion of the pit that had occurred until then due to drydock does not progress again and stops is as follows. Upon docking, the oil tank is cleaned to inspect the occurrence of cracks and pits within the tank. Additionally, during pit inspection, the corrosive liquid in the pit is also removed in order to determine the pit level or measure the depth. Then, the inside of the pit is taken out empty and an oil coat is formed when crude oil is loaded. In particular, the area where the pit was formed is covered with an oil coat thicker than the surrounding area, so it is corrosion-resistant and subsequent corrosion does not progress.

In other words, by using corrosion-resistant steel, corrosion progress is suppressed even when pits are formed in defective areas of the oil coat, and deep repairs that require welding repair and painting touch-up are performed until docking where pit growth stops. You can prevent it from becoming pitiful.

However, in cleaning at the time of docking, there are parts that are not cleaned and parts that are not sufficiently cleaned depending on the cleaning method and differences in the hull structure. If pits occurring in such locations are missed during pit inspection, the corrosive liquid remains in the pits. In this case, these pits continue to develop corrosion and become deep pits that require repair by welding and touch-up by painting, and there are also cases in which corrosive liquid continues to remain, which is undesirable.

Next, the proposed technologies for corrosion-resistant steel for crude oil tanker bottom plates and the problems of these proposed technologies will be described.

Patent Document 1 proposes a steel for downstream oil pipes that is used in an environment where crude oil and seawater are exposed alternately or simultaneously. Patent Document 2 proposes a corrosion-resistant steel plate for a cargo oil tank that takes corrosion resistance at the weld zone into consideration.

Patent Document 3 proposes a corrosion-resistant steel containing Cu, Ni, Cr, Mo, Sb, and Sn that is used in crude oil and heavy oil storage. Patent Document 4 proposes a corrosion-resistant steel used in tanks for transporting and storing crude oil.

Japanese Patent Publication No. 50-158515 Japanese Patent Publication No. 2003-105487 Japanese Patent Publication No. 2001-214236 Japanese Patent Publication No. 2002-173736

However, since the steel for downstream oil pipes described in Patent Document 1 contains Cr in excess of 0.1% in a crude oil tank environment, there remains room for improvement in terms of local corrosion resistance, weldability, and economic efficiency.

In the corrosion-resistant steel sheet for cargo oil tanks described in Patent Document 2, there was a problem that the effect of suppressing the development of local corrosion could not be obtained when Mo addition was excessive.

In the corrosion-resistant steel for crude oil and heavy oil storages described in Patent Document 3, the addition of a large amount of alloy elements is necessary to obtain excellent corrosion resistance, so there remains room for improvement from the viewpoints of economic efficiency and weldability.

The corrosion-resistant steel for crude oil transportation and storage tanks described in Patent Document 4 contains Cu: 0.5 to 1.5%, Ni: 0.5 to 3.0%, and Cr: 0.5 to 2.0% as basic components, so a large amount of alloy is required to achieve the effect. There was a problem that elements had to be added, and economic efficiency and weldability were poor. In addition, since Cr is excessively contained in the crude oil tank bottom plate environment, the speed of local corrosion occurring in the bottom plate is not reduced, and there is a problem that corrosion resistance commensurate with the total amount of alloy addition cannot be obtained.

In this regard, it is necessary to further improve the corrosion resistance of existing corrosion-resistant steels and to improve the corrosion resistance inside the plate thickness in order to suppress further corrosion even when pits advance and reach the inside of the plate thickness. .

The present invention was made to solve the above problems, and its purpose is to provide a steel material for a crude oil tank that exhibits excellent corrosion resistance in the bottom plate environment of a crude oil tank.

The present invention was made to solve the above problems, and its main focus is the following steel materials for crude oil tanks.

(1) Chemical composition of steel for crude oil tank, in mass%,

C：0.03~0.20%,

Si: 0.05~0.50%,

Mn: 0.60~2.00%,

P：0.030% or less,

S: 0.030% or less,

Al: 0.001~0.050%,

N: Contains 0.001 to 0.010%, and

Cu: 0.01~0.60%,

Mo：0.01~0.20%,

W：0.01~0.20%,

Sn: 0.01 to 0.20%, and

Sb: 0.01~0.20%

Contains one or two or more types selected from,

Remainder: Fe and impurities,

The CRI value defined by formula (i) below is 0.50 or more,

In the metal structure in a cross section parallel to the rolling direction and thickness direction of the crude oil tank steel, the total area ratio of bainite and/or asymmetric ferrite is 20 to 95%,

The area ratio of martensite is 2.0% or less, and

Vickers hardness of 210HV10 or less,

Steel materials for crude oil tanks.

CRI=Cu+6×Mo+2×W+0.5×Sn+0.5×Sb···(i)

However, each element symbol in the formula represents the content (mass %) of each element contained in the steel material, and is set to 0 when not contained.

(2) The chemical composition is in mass%, instead of part of the Fe,

Cr: less than 0.10%

Containing,

Steel material for crude oil tank as described in (1) above.

(3) The chemical composition is in mass%, instead of part of the Fe,

Ni：0.05~0.50%, and

Co：0.05~0.50%

Containing one or two types selected from,

Steel material for crude oil tank according to (1) or (2) above.

(4) The chemical composition is in mass%, instead of part of the Fe,

Nb: 0.002~0.200%,

V：0.005~0.500%,

Ti：0.002~0.200%,

Ta：0.005~0.500%,

Zr: 0.005~0.500%, and

B：0.0002~0.0050%

Containing one or two or more types selected from,

Steel material for crude oil tank according to any one of (1) to (3) above.

(5) The chemical composition is in mass%, instead of part of the Fe,

Mg: 0.0001~0.01%,

Ca: 0.0005~0.01%,

Y：0.0001~0.1%,

La：0.005~0.1%, and

Ce：0.005~0.1%

Containing one or two or more types selected from,

Steel material for crude oil tank according to any one of (1) to (4) above.

According to the present invention, it is possible to provide a steel material for a crude oil tank that exhibits excellent corrosion resistance in the bottom plate environment of a crude oil tank.

1 is a graph showing the relationship between CRI value and relative corrosion rate.

In order to solve the above problem, the present inventors, in a corrosion test for a COT (crude oil tank) bottom plate according to IMO (International Maritime Organization) regulations simulating the environment in the pit, first selected Cu, Mo, and W, which are effective as corrosion resistance elements. , Sn, and Sb were used to investigate the optimal conditions using steel materials containing them in a usable concentration range as shipbuilding steel.

Specifically, the C content is 0.06 to 0.16%, the Si content is 0.10 to 0.30%, the Mn content is 0.60 to 1.60%, the P content is 0.005 to 0.020%, the S content is 0.004 to 0.030%, and the Al content is 0.01 to 0.05%. % and N content in the range of 0.0015 to 0.0050% were melted and rolled to form steel sheet TP1 with a thickness of 6 mm. In addition to the above elements, one or two or more types selected from Cu: 0.01 to 0.60%, Mo: 0.01 to 0.20%, W: 0.01 to 0.20%, Sn: 0.01 to 0.20%, and Sb: 0.01 to 0.20%. The steel materials shown in Table 1 containing were melted and rolled to form steel plates TP2 to TP24 with a thickness of 6 mm. The chemical composition of TP1 to TP24 is shown in Table 1.

After that, test pieces for IMO testing were prepared and tested. As a result, as shown in the white circle plot in FIG. 1, it was first found that the relationship between these alloy elements and corrosion rate can be summarized by the CRI value expressed by the following equation.

CRI=Cu+6×Mo+2×W+0.5×Sn+0.5×Sb···(i)

Additionally, it has been found that when the CRI value is 0.50 or more, excellent corrosion resistance can be stably exhibited.

So, next, a steel ingot with a thickness of 120 mm was melted from the steel materials shown in Table 1 with a CRI value of 0.50 or more. Additionally, after heating and holding at 1150°C for 90 minutes and rolling to a thickness of 30 mm, air-cooled and water-cooled products were prepared. Then, the sampling location was changed, a test piece with a thickness of 5 mm was sampled, and the corrosion rate according to IMO regulations was examined.

As a result, in the case of air cooling, in a cross section parallel to the rolling direction and thickness direction of the steel material (hereinafter also referred to as “L cross section”), when the thickness of the steel material is t, the 1/4t position is the plate thickness of the test piece. In the test piece taken at the center, the corrosion rate was approximately twice that of the test piece taken near the surface. On the other hand, in the case of water cooling, the corrosion rate of the test specimens collected at the 1/4t position and the test specimens collected near the surface layer was 0.3 to 0.5 times lower than the test specimens collected near the surface layer when air cooled. It turned out. Additionally, even in test pieces taken so that the 1/2t position was at the center of the test piece thickness, it was found that the corrosion rate was 0.4 to 0.6 times lower when cooled with water than when cooled with air.

As a result of detailed examination of the metal structure of these, the air-cooled test pieces were found to have slightly flat ferrite and pearlite, and the ferrite grains were larger and granular at the center of the plate thickness. On the other hand, the water-cooled test piece with good corrosion resistance was found to contain bainite and/or asymmetric ferrite in total area ratio of 20% or more at any depth.

As a result of a more detailed examination, it was found that by stopping water cooling at 750 to 500°C after finish rolling, the total area ratio of bainite and/or asymmetric ferrite was 20 to 95%, and the area ratio of martensite was 2.0%. It has been revealed that the following metal structure can be obtained and thus good corrosion resistance can be secured. Additionally, it was found that when the Vickers hardness exceeds 210HV10, corrosion resistance deteriorates.

As a result of the above review, the CRI value is set to 0.50 or more, the metal structure has a total area ratio of bainite and/or asymmetric ferrite of 20 to 95% throughout the entire plate thickness, and the area ratio of martensite is 2.0% or less. In addition, it was found that when the Vickers hardness is 210HV10 or less, the corrosion resistance of the steel sheet is improved across the entire thickness.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition

The reasons for limitation of each element are as follows. In addition, in the following description, “%” relative to content means “mass%.”

C：0.03~0.20%

C is an element effective in increasing strength. Additionally, if the C content is less than 0.03%, industrial economic feasibility is significantly impaired. On the other hand, if the C content is excessive, weldability and joint toughness deteriorate, so it is not preferable as a steel material for welded structures. Therefore, the C content is set to 0.03 to 0.20%. The C content is preferably 0.05% or more, or 0.08% or more, and is preferably 0.18% or less, or 0.16% or less.

Si：0.05~0.50%

Si is necessary as a deoxidizing element. On the other hand, if the Si content is excessive, the base metal toughness deteriorates. Therefore, the Si content is set to 0.05 to 0.50%. When the requirements for weldability and joint toughness are strict, the Si content is preferably 0.10% or more, or 0.20% or more, and preferably 0.40% or less, or 0.30% or less.

Mn: 0.60~2.00%

Mn is effective as an element that improves the strength of steel materials. On the other hand, if the Mn content is excessive, the base metal toughness deteriorates. Therefore, the Mn content is set to 0.60 to 2.00%. The Mn content is preferably 0.75% or more, or 0.90% or more. Additionally, when the requirements for weldability and joint toughness are strict, the Mn content is preferably 1.80% or less, or 1.60% or less.

P：0.030% or less

P is an impurity element, and in order to reduce the rate of local corrosion growth and improve weldability, the P content is set to 0.030% or less. In order to ensure corrosion resistance and weldability, the P content is preferably 0.020% or less. In order to further improve corrosion resistance, it is more preferable that the P content is 0.010% or less. There is no need to specifically specify the lower limit of the P content, that is, the P content may be 0%, but extreme reduction will result in an increase in steelmaking costs. Therefore, the P content may be 0.001% or more, or 0.005% or more.

S: 0.030% or less

S is an impurity element, and in order to reduce the rate of local corrosion growth and improve mechanical properties, especially ductility, the S content is set to 0.030% or less. Moreover, in order to ensure corrosion resistance and mechanical properties, the smaller the S content, the more preferably 0.020% or less, or 0.010% or less. There is no need to specifically specify the lower limit of the S content, that is, the S content may be 0%, but extreme reduction will result in an increase in steelmaking costs. Therefore, the S content may be 0.001% or more, or 0.003% or more.

Al: 0.001~0.050%

Al is an element effective in refining the austenite grain size of the base material by forming AlN. On the other hand, if the Al content is excessive, coarse oxides are formed and ductility and toughness are deteriorated. Therefore, the Al content is set to 0.001 to 0.050%. The Al content is preferably 0.005% or more, or 0.010% or more, and is preferably 0.040% or less, or 0.030% or less.

N：0.001~0.010%

N is effective in austenite grain refinement and precipitation strengthening in combination with V, Al, and Ti. Additionally, it is impossible to completely remove N in steel industrially, and reducing the N content more than necessary is undesirable because it imposes an excessive load on the manufacturing process. On the other hand, if the N content is excessive, it adversely affects ductility and toughness in the solid solution state. Therefore, the N content is set to 0.001 to 0.010%. The N content is preferably 0.002% or more, or 0.003% or more, and is preferably 0.008% or less, or 0.006% or less.

Cu: 0.01~0.60%

Mo：0.01~0.20%

W：0.01~0.20%

Sn: 0.01~0.20%

Sb: 0.01~0.20%

Cu, Mo, W, Sn, and Sb are elements that improve total corrosion resistance. On the other hand, if the content of these elements is excessive, it not only impairs economic efficiency but also causes a decrease in mechanical properties. Therefore, one or two or more types selected from Cu: 0.01 to 0.60%, Mo: 0.01 to 0.20%, W: 0.01 to 0.20%, Sn: 0.01 to 0.20%, and Sb: 0.01 to 0.20% are contained. The Cu content is preferably 0.05% or more, or 0.08% or more, and is preferably 0.55% or less, or 0.50% or less. Additionally, the contents of Mo, W, Sn, and Sb are preferably 0.02% or more, or 0.03% or more, and 0.18% or less, or 0.15% or less, respectively.

Also, as mentioned above, if the CRI value expressed by formula (i) is less than 0.50, sufficient corrosion resistance cannot be obtained. On the other hand, when it is 0.50 or more, excellent corrosion resistance can be stably exhibited. Therefore, the CRI value is set to 0.50 or more. The CRI value is preferably 0.55 or more, and more preferably 0.60 or more. There is no particular upper limit to the CRI value, but in the chemical composition of the present invention, the upper limit of the CRI value is 2.40.

CRI=Cu+6×Mo+2×W+0.5×Sn+0.5×Sb···(i)

However, each element symbol in the formula represents the content (mass %) of each element contained in the steel material, and is set to 0 when not contained.

In the chemical composition of the steel material for crude oil tank according to the present invention, the balance is Fe and impurities. In addition, “impurities” are components that are mixed when industrially manufacturing steel materials due to raw materials such as ore and scrap and various factors in the manufacturing process, and are allowed as long as they do not adversely affect the present invention.

In the chemical composition of the steel material for a crude oil tank according to the present invention, instead of part of Fe, one or more types selected from the following elements may be contained in the range shown below. In addition, since these elements are not necessarily essential in steel materials for crude oil tanks, the lower limit of their content is 0%. The reason for limitation of each element is explained.

Cr: less than 0.10%

Since Cr is effective in increasing strength, it may be contained as needed to adjust strength. On the other hand, Cr accelerates the rate of local corrosion development and deteriorates local corrosion resistance in a crude oil environment. Additionally, Cr may promote the production of solid S. Therefore, the Cr content is set to less than 0.10%. The Cr content is preferably 0.08% or less, or 0.05% or less. The lower limit is not particularly limited and may be 0%, but when the above effect is desired, the Cr content is preferably 0.01% or more, or 0.03% or more.

Ni：0.05~0.50%

Co：0.05~0.50%

Ni and Co improve the toughness of the base material and HAZ. Additionally, since it is an element effective in improving corrosion resistance, it may be contained as needed. On the other hand, if the Ni and Co contents are excessive, weldability is reduced. Additionally, Ni and Co are expensive elements, and containing them in excessive amounts is economically disadvantageous. Therefore, it is preferable to contain one or two types selected from Ni: 0.05 to 0.50% and Co: 0.05 to 0.50%. The Ni and Co contents are preferably 0.08% or more, or 0.10% or more, respectively, and are preferably 0.40% or less, or 0.30% or less.

Nb: 0.002~0.200%

V：0.005~0.500%

Ti：0.002~0.200%

Ta：0.005~0.500%

Zr: 0.005~0.500%

B：0.0002~0.0050%

Nb, V, Ti, Ta, Zr, and B are elements effective in increasing the strength of steel in trace amounts, and may be contained as needed to adjust the strength. On the other hand, when the content of the above elements is excessive, the deterioration of toughness becomes significant. Therefore, one type selected from Nb: 0.002 to 0.200%, V: 0.005 to 0.500%, Ti: 0.002 to 0.200%, Ta: 0.005 to 0.500%, Zr: 0.005 to 0.500%, B: 0.0002 to 0.0050% or It is preferable to contain two or more types.

The contents of Nb and Ti are preferably 0.005% or more, 0.010% or more, or 0.020% or more, respectively, and are preferably 0.180% or less, 0.150% or less, or 0.130% or less. Moreover, the contents of V, Ta, and Zr are preferably 0.010% or more, 0.020% or more, or 0.050% or more, and are preferably 0.400% or less, 0.300% or less, or 0.200% or less, respectively. Moreover, the B content is preferably 0.0005% or more, or 0.0010% or more, and is preferably 0.0040% or less, or 0.0030% or less.

Mg: 0.0001~0.01%

Ca: 0.0005~0.01%

Y：0.0001~0.1%

La：0.005~0.1%

Ce: 0.005~0.1%

Mg, Ca, Y, La, and Ce are elements effective in controlling the shape of inclusions, improving ductility characteristics, and improving HAZ toughness of high heat input weld joints, so they may be contained as needed. On the other hand, if the content of the above elements is excessive, inclusions coarsen and have a detrimental effect on mechanical properties, especially ductility and toughness. Therefore, it is recommended to contain one or two or more types selected from Mg: 0.0001 to 0.01%, Ca: 0.0005 to 0.01%, Y: 0.0001 to 0.1%, La: 0.005 to 0.1%, and Ce: 0.005 to 0.1%. desirable.

The Mg content is preferably 0.0005% or more, 0.0010% or more, or 0.0020% or more, and is preferably 0.0090% or less, 0.0080% or less, and 0.0070% or less. The Ca content is preferably 0.0008% or more, 0.0010% or more, or 0.0020% or more, and is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less. The Y content is preferably 0.0005% or more, 0.0010% or more, or 0.0020% or more, and is preferably 0.09% or less, 0.08% or less, or 0.07% or less. The content of La and Ce is preferably 0.008% or more, 0.010% or more, or 0.015% or more, and is preferably 0.09% or less, 0.08% or less, or 0.07% or less, respectively.

(B) Metal structure of steel for crude oil tank

In the metal structure of the steel material for a crude oil tank of the present invention in a cross section parallel to the rolling direction and thickness direction, the total area ratio of bainite and/or asymmetric ferrite is 20 to 95%, and the area ratio of martensite is 20 to 95%. It is less than 2.0%. The reasons for the limitations of each organization are explained.

Total of bainite and/or asymmetric ferrite: 20-95%

As described above, the corrosion resistance of the steel material of the present invention is exhibited by the alloy elements contained therein. Examples of this mechanism include these alloy elements being eluted in the pit environment and introduced into corrosion products, or being precipitated and coated as metal on the surface of the steel material. Here, bainite and asymmetric ferrite generate a slight potential difference between them and ferrite. In addition, it moderately promotes the corrosion of the alloy elements in the early stages of corrosion, promotes the introduction of the alloy elements into corrosion products and their precipitation as metal onto the surface of the steel material, thereby demonstrating the effect of improving corrosion resistance. Therefore, the total area ratio of bainite and/or asymmetric ferrite is set to 20% or more.

On the other hand, if the total area ratio of bainite and/or asymmetric ferrite is too large, the contact area with ferrite decreases, and corrosion of the alloy elements in the early corrosion stage becomes insufficient. Therefore, the expression of the above-described effects is delayed, and corrosion resistance decreases. Therefore, the total area ratio of bainite and/or asymmetric ferrite is set to 95% or less, preferably less than 90%.

Here, in the present invention, bainite may also include tempered bainite, but no distinction is made in the present specification. In addition, “arcular ferrite” refers to ferrite that transforms in a low temperature range, and refers to needle-shaped ferrite or ferrite that does not exhibit granular shapes. It does not refer to granular ferrite with polygonal grain boundaries, i.e. the polygonal ferrite seen in micrographs.

Martensite: 2.0% or less

In the pit environment, martensite forms a battery between ferrite and acts as a cathode. As a result, the corrosion progress of ferrite is accelerated, causing uneven corrosion. Therefore, the area ratio of martensite is set to 2.0% or less. In addition, in the present invention, martensite does not include island-shaped martensite.

As described above, ferrite has the effect of improving corrosion resistance by generating a slight potential difference between bainite and asymmetric ferrite and moderately promoting corrosion of alloy elements in the early stages of corrosion. Therefore, it is preferable that the area ratio of ferrite is 10% or more. However, if the area ratio of ferrite is too large, the contact area between bainite and asymmetric ferrite decreases, and corrosion of the alloy elements in the early stages of corrosion becomes insufficient. As a result, corrosion resistance deteriorates. Therefore, it is preferable that the area ratio of ferrite is 75% or less.

The remainder other than the above is pearlite and island-like martensite. Pearlite deteriorates corrosion resistance. Therefore, it is preferable that the area ratio of pearlite is 5% or less. Moreover, it is preferable that the area ratio of island-shaped martensite is 5% or less.

Metal structure is measured by the following method. In the L cross section of the steel material, the metal structure is observed at a 1 mm depth position, a 1/4t position, and a 1/2t position from the surface. The observation surface is mirror polished, etched with Nital, and evaluated from photographs taken at a magnification of 200 times using an optical microscope. In the photograph taken, the portion excluding the monochromatic white granular ferrite and the black granular pearlite is painted in an arbitrary color such as bainite or asymmetric ferrite, and image analysis is performed. Additionally, martensite is formed in a region surrounded by ferrite, asymmetric ferrite, bainite, and/or pearlite structure, and island-shaped martensite is a structure formed between laths of the bainite structure.

Then, the ratio of the portion painted in an arbitrary color to the total is calculated as the total area ratio of bainite and/or asymmetric ferrite. Additionally, the ratio of ferrite to the total is calculated as the area ratio of ferrite, and the ratio of martensite to the total is calculated as the area ratio of martensite.

In addition, in the present invention, the total area ratio of bainite and/or asymmetric ferrite is 20 to 95%, meaning that bainite and/ Alternatively, it refers to the total area ratio of asymmetric ferrite being 20 to 95%. In addition, in the present invention, the area ratio of martensite is 2.0% or less, meaning that the area ratio of martensite is 2.0% or less at all 1 mm depth position, 1/4t position, and 1/2t position from the surface. says In addition, the shape of the steel material for a crude oil tank according to the present invention is not particularly limited, but is typically a thick steel plate with a thickness of 10 to 40 mm.

(C) hardness

The steel material for a crude oil tank according to the present invention has a Vickers hardness of 210HV10 or less. When the Vickers hardness exceeds 210HV10, corrosion resistance deteriorates, although the detailed mechanism has not been elucidated. Moreover, it is preferable that the Vickers hardness is 160HV10 or more. In addition, “HV10” means the “hardness symbol” when a Vickers hardness test was performed with a test force of 98 N (10 kgf) (refer to JIS Z 2244-1: 2020).

Here, the Vickers hardness is measured at 5 points at 1 mm intervals in the direction parallel to the rolling direction for each of the 1 mm depth position, 1/4 t position, and 1/2 t position from the surface of the steel material in the L cross section of the steel material. , calculate the average value of Vickers hardness at each depth position. Vickers hardness is measured with a test force of 98N (10kgf).

In addition, in the present invention, Vickers hardness of 210HV10 or less means that the average value of Vickers hardness is 210HV10 or less at all 1 mm depth position, 1/4t position, and 1/2t position from the surface.

(D) Manufacturing method

A suitable manufacturing method for the steel material for a crude oil tank of the present invention will be described. Molten steel with the above-described chemical composition is melted in a known furnace such as a converter or electric furnace, and used as a steel material such as a slab or billet by a known method such as continuous casting or ingot. Additionally, vacuum degassing refining, etc. may be performed during solvent preparation. In addition, the method for adjusting the components of molten steel may be according to a known steel smelting method.

Next, the steel material described above is hot rolled into the desired size and shape. It is preferable to heat the steel material to a temperature of 1020°C or higher and maintain it for 20 min or more before performing hot rolling.

As the heating temperature decreases, the rolling load increases, so the heating temperature is set to 1020°C or higher. The heating temperature is preferably 1030°C or higher, and more preferably 1040°C or higher.

However, if the heating temperature exceeds 1350°C, it may cause surface flaws or increase scale loss and fuel intensity. Therefore, the heating temperature is preferably 1350°C or lower, and more preferably 1300°C or lower.

Additionally, from the viewpoint of productivity etc., the cracking time is set to 20 to 120 min. The cracking time is preferably 50 min or more. Here, “crack time” means the time for isothermal maintenance after the temperature of the slab reaches the above-mentioned heating temperature.

Hot rolling includes rough rolling and finish rolling, and the time from the end of rough rolling to the start of finish rolling (hereinafter also referred to as “finish rolling waiting time”) is preferably 40 seconds or less. The surface layer of the steel material has a faster cooling rate than the inside, but the temperature difference between the surface layer and the inside of the steel material can be reduced by shortening the finish rolling waiting time. As a result, the deviation of the metal structure at the 1 mm depth position, 1/4t position, and 1/2t position from the surface layer is suppressed, and at all of the 1 mm depth position, 1/4t position, and 1/2t position from the surface layer, A metal structure can be obtained in which the total area ratio of bainite and/or asymmetric ferrite is 20 to 95%, and the area ratio of martensite is 2.0% or less.

Additionally, the finish rolling temperature is set to 830 to 930°C. When the finish rolling temperature is lower than 830°C, many strains that serve as nuclei for ferrite transformation exist in austenite. As a result, the transformation from austenite to ferrite is promoted, and the area ratio of ferrite becomes excessive. On the other hand, when the finish rolling temperature is higher than 930°C, the strain in austenite is small, so transformation into ferrite, bainite, and asymmetric ferrite is suppressed, and the area ratio of martensite becomes excessive.

In addition, as described above, the steel materials after hot rolling are cooled by water cooling. By water cooling after hot rolling, a metal structure containing bainite and/or asymmetric ferrite in a total area ratio of 20% or more can be obtained.

The water cooling start temperature is 900 to 800°C. At all of the 1 mm depth position, 1/4t position, and 1/2t position from the surface layer, cooling is started in a state in which austenite is single phase, and 1 mm depth position, 1/4t position, and 1/2t position from the surface layer. Deviations in metal structure can be suppressed. Additionally, when the water cooling start temperature is less than 800°C, water cooling begins after transformation to ferrite begins, so the total area ratio of bainite and/or asymmetric ferrite decreases.

Then, after water cooling to 700~500℃, water cooling is stopped. By setting the water cooling stop temperature to 700 to 500°C, a metal structure with a total area ratio of bainite and/or asymmetric ferrite of 20 to 95% and an area ratio of martensite of 2.0% or less can be obtained. When water cooling is stopped below 500°C, martensite is generated in excess of 2.0%, and corrosion resistance deteriorates. On the other hand, when water cooling is stopped above 700°C, since it is substantially the same as air cooling, the metal structure becomes a ferrite/pearlite structure, and corrosion resistance deteriorates.

In addition, after hot rolling, if necessary, pickling and cold rolling may be performed to obtain a cold rolled steel sheet of a predetermined thickness. Additionally, there is no particular limitation on manufacturing conditions other than those mentioned above, and they may be in accordance with commercial law.

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

<Example>

A steel ingot having the chemical composition shown in Table 2 and having a thickness of 120 mm was melted. Additionally, it was heated and held at 1150°C for 90 minutes, and hot rolled to a thickness of 30 mm under the conditions shown in Table 3. After that, as shown in Table 3, water cooling or air cooling was performed to obtain a hot rolled steel sheet.

In the L cross section of the obtained steel plate, the metal structure was observed at a 1 mm depth position, a 1/4t position, and a 1/2t position from the surface. The observation surface was mirror polished and corroded with Nital, and then evaluated from photographs taken at a magnification of 200 times using an optical microscope. In the photograph taken, the portion excluding the solid white granular ferrite and black granular pearlite was colored with bainite or asymmetric ferrite, and image analysis was performed. Additionally, martensite is formed in a region surrounded by ferrite, asymmetric ferrite, bainite, and/or pearlite structure, and island-shaped martensite is formed between the laths of the bainite structure.

Then, the ratio of the part painted in an arbitrary color to the total was calculated as the total area ratio of bainite and/or asymmetric ferrite. In addition, the ratio of ferrite to the total was calculated as the area ratio of ferrite, and the ratio of martensite to the total was calculated as the area ratio of martensite.

Vickers hardness is measured at 5 points at 1 mm intervals in the direction parallel to the rolling direction for each of the 1 mm depth position, 1/4 t position, and 1/2 t position from the surface of the steel material in the L cross section of the steel material, respectively. The average value of Vickers hardness at the depth position was calculated. Vickers hardness was measured with a test force of 98 N (10 kgf).

Corrosion resistance test is IMO regulation corrosion test for COT floor plate (SOLAS Chapter II-I, Part A-1, Reg. 3-11, as amended by resolution MSC. 291 (87), APPENDIX, Test Procedures for Qualification of Corrosion A test was conducted based on Resistant Steel for Cargo Tanks in Crude Oil Tankers. However, the pH of the solution was adjusted from 0.85 to 0.5 to make it more stringent.

First, test pieces of 25 mm x 60 mm x 5 mm (thickness) were taken from the 1 mm depth position, the 1/4t position, and the 1/2t position from the surface. The sampling method is to collect surface 1mm so that one side of the test piece has 1mm of the surface, and for the 1/4t position and 1/2t position, take the sample so that the center of the plate thickness of the test piece is at the 1/4t and 1/2t positions, respectively. did. To perform a hanging immersion test, a hole with a diameter of 2 mm was drilled near the longitudinal end. The surface of the test piece was polished with Emery polishing paper No. 600. Before the test, it was degreased and the weight and dimensions of the test piece were measured.

The test solution used was a 10% weight concentration NaCl aqueous solution whose pH was adjusted to 0.5 with hydrochloric acid. Put 800 ml of this solution (specific liquid volume of 20 ml/cm ² or more) into a beaker, and keep the temperature of the test solution at 30°C. After that, the test piece was hung using a nylon No. 4 fishing line and immersed in this beaker. The solution was tested for 3 days with changes every 24 hours. After the test, it was washed to remove corrosion products, dried, and weighed.

The corrosion rate was calculated from the weight loss before the test and the surface area and density before the test (7.87 g/cm ³ was used here). The number of tests for each steel plate was 5, and it was checked whether the average value was 0.5 mm or less. In addition, the evaluation standard for corrosion-resistant steel in the corrosion test for COT floor plates according to IMO regulations is 1 mm or less.

Table 4 summarizes the results of the area ratio of bainite and/or acular ferrite, Vickers hardness, and corrosion rate of the metal structure at the 1 mm depth position, 1/4t position, and 1/2t position from the surface. In addition, regarding the results of the corrosion rate, ○ was written when the corrosion rate was 0.5 mm/y or less in all parts, and × was written when the corrosion rate was more than 0.5 mm/y in any part.

As shown in Table 4, in tests Nos. 1 to 21 and 26 that all satisfied the provisions of the present invention, excellent results were obtained in all performances. On the other hand, Test Nos. 22 to 25, 27, and 28, which are comparative examples, resulted in deterioration in at least one of Vickers hardness and corrosion resistance.

[Industrial applicability]

According to the present invention, it is possible to obtain a steel material for a crude oil tank that exhibits excellent corrosion resistance against localized corrosion of the pitting form that occurs in the oil tank of a crude oil tanker, especially in the pitting form with a relatively high growth rate.

Claims (5)

The chemical composition of steel materials for crude oil tanks is in mass%,
C：0.03~0.20%,
Si: 0.05~0.50%,
Mn: 0.60~2.00%,
P: 0.030% or less,
S: 0.030% or less,
Al：0.001~0.050%,
N: Contains 0.001 to 0.010%, and
Cu：0.01~0.60%,
Mo：0.01~0.20%,
W：0.01~0.20%,
Sn: 0.01~0.20%, and
Sb: 0.01~0.20%
Contains one or two or more types selected from,
The remainder: Fe and impurities,
The CRI value defined by formula (i) below is 0.50 or more,
In the metal structure in a cross section parallel to the rolling direction and thickness direction of the crude oil tank steel, the total area ratio of bainite and/or asymmetric ferrite is 20 to 95%,
The area ratio of martensite is 2.0% or less, and
Steel for crude oil tanks with a Vickers hardness of 210HV10 or less.
CRI=Cu+6×Mo+2×W+0.5×Sn+0.5×Sb···(i)
However, each element symbol in the formula represents the content (mass %) of each element contained in the steel material, and is set to 0 when not contained. In claim 1,
The chemical composition is in mass%, instead of a portion of the Fe,
Cr: less than 0.10%
A steel material for a crude oil tank containing. In claim 1 or claim 2,
The chemical composition is in mass%, instead of a portion of the Fe,
Ni：0.05~0.50%, and
Co: 0.05~0.50%
A steel material for a crude oil tank containing one or two types selected from the following. The method according to any one of claims 1 to 3,
The chemical composition is in mass%, instead of a portion of the Fe,
Nb: 0.002~0.200%,
V：0.005~0.500%,
Ti：0.002~0.200%,
Ta：0.005~0.500%,
Zr: 0.005~0.500%, and
B：0.0002~0.0050%
A steel material for crude oil tank containing one or two or more types selected from. The method according to any one of claims 1 to 4,
The chemical composition is in mass%, instead of a portion of the Fe,
Mg: 0.0001~0.01%,
Ca: 0.0005~0.01%,
Y：0.0001~0.1%,
La：0.005~0.1%, and
Ce: 0.005~0.1%
A steel material for crude oil tank containing one or two or more types selected from.