JP7769263B2 - Steel materials for crude oil tanks - Google Patents

Description

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

Crude oil tankers are made of welded structural steel, which has excellent strength and weldability, but they suffer from numerous localized pitting-like corrosion (known as pits) that progress at a relatively high rate. These pits are approximately 10-30 mm in diameter and progress at a rate of 2-3 mm per year. This is far greater than the average corrosion wear rate of 0.1 mm per year that is taken into account when designing a ship's hull.

For this reason, in 2010, the SOLAS Convention (International Convention for the Safety of Life at Sea) made it mandatory for oil tanks of crude oil tankers with a deadweight of 5,000 tonnes or more, for which construction contracts were concluded after January 1, 2013, to be protected from corrosion by painting or using corrosion-resistant steel. The reason for applying corrosion-resistant steel is that the depth of pits that develop over the approximately two-and-a-half-year intervals when a tanker is regularly docked is limited to a level that makes repairs by welding or touch-ups by painting unnecessary, thereby halting further corrosion.

The causes of pitting are as follows: Crude oil contains brine, a saltwater solution with a concentration of approximately 10%. The brine, which is heavier in density than crude oil, accumulates on the bottom plate of the oil tank during transport. Normally, the bottom plate of an oil tank is covered with a highly viscous oil layer (hereinafter referred to as "oil coat") that adheres to the steel plate and provides the same level of corrosion protection as paint, so corrosion from brine does not occur. However, if there is a defect in this oil coat or an area that does not provide sufficient corrosion protection, that area will be corroded by brine. Then, in the corroded area, hydrolysis occurs due to corrosion, causing acidity, which further accelerates corrosion and leads to pitting-like corrosion. In other words, the cause is corrosion of defects in the oil coat by brine.

The reason why docking stops any corrosion in pits that has already developed is as follows: When docking, the oil tank is cleaned to check for cracks and pitting inside the tank. Furthermore, during the pit inspection, the corrosive fluid inside the pit is also removed to determine the pit level and measure its depth. The pit is then unloaded with the pit empty, and an oil coat is formed when crude oil is loaded. The area where the pit had developed is particularly covered with a thicker oil coat than the surrounding area, preventing corrosion and preventing further corrosion from progressing.

In other words, by using corrosion-resistant steel, even if a pit forms in a defective area of the oil coat, the progression of corrosion is suppressed, preventing the pit from becoming a deep pit that would require repair by welding and touch-up by painting before the vessel can be docked, at which point the pit's progression will stop.

However, when cleaning during docking, there are areas that are not cleaned, or areas that are not cleaned sufficiently, depending on the cleaning method and differences in the hull structure. If pits that occur in such areas are overlooked even during pit inspections, the corrosive liquid in the pits will remain. In this case, the corrosion will continue to progress, turning these pits into deep pits that require repair by welding and touch-up by painting, and in some cases the corrosive liquid will continue to remain, which is undesirable.

Next, we will discuss the technologies proposed for corrosion-resistant steel for crude oil tanker bottom plates and the challenges associated with these proposed technologies.

Patent Document 1 proposes steel for cargo oil pipes used in environments where crude oil and seawater are alternately or simultaneously exposed. Patent Document 2 proposes corrosion-resistant steel plate for cargo oil tanks that takes into account corrosion resistance at welds.

Patent Document 3 proposes corrosion-resistant steel containing Cu, Ni, Cr, Mo, Sb, and Sn for use in crude oil and heavy oil storage tanks. Patent Document 4 proposes corrosion-resistant steel for use in tanks that transport and store crude oil.

Japanese Unexamined Patent Publication No. 158515/1983 Japanese Patent Application Laid-Open No. 2003-105487 Japanese Patent Application Laid-Open No. 2001-214236 Japanese Patent Application Laid-Open No. 2002-173736

However, the steel for cargo oil pipes described in Patent Document 1 contains more than 0.1% Cr in crude oil tank environments, leaving room for improvement in terms of localized corrosion resistance, weldability, and economic efficiency.

The corrosion-resistant steel plate for cargo oil tanks described in Patent Document 2 had the problem that if excessive Mo was added, it was not effective in suppressing the progression of localized corrosion.

The corrosion-resistant steel for crude oil and heavy oil storage tanks described in Patent Document 3 requires the addition of large amounts of alloying elements to achieve excellent corrosion resistance, leaving room for improvement in terms of economy and weldability.

The corrosion-resistant steel for crude oil transport and storage tanks described in Patent Document 4 contains the basic components Cu: 0.5-1.5%, Ni: 0.5-3.0%, and Cr: 0.5-2.0%, which necessitates the addition of large amounts of alloying elements to achieve its intended effect, resulting in issues such as poor economic efficiency and poor weldability. Furthermore, because it contains excessive Cr in the crude oil tank bottom plate environment, the rate of progression of localized corrosion that occurs on the bottom plate does not decrease, and corrosion resistance commensurate with the total amount of alloying added is not achieved.

For these reasons, it is necessary to further improve the corrosion resistance of existing corrosion-resistant steels, as well as to improve the corrosion resistance within the plate thickness in order to suppress the progression of subsequent corrosion even if pits progress into the plate thickness.

The present invention has been made to solve the above problems and aims to provide a steel material for crude oil tanks that exhibits excellent corrosion resistance in the bottom plate environment of a crude oil tank.

The present invention has been made to solve the above problems and is centered around the following steel material for crude oil tanks.

(1) The chemical composition of the steel material 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: 0.001 to 0.010%, and further
Cu: 0.01 to 0.60%,
Mo: 0.01-0.20%,
W: 0.01-0.20%,
Sn: 0.01 to 0.20%, and Sb: 0.01 to 0.20%
Contains one or more selected from
The balance is Fe and impurities.
The CRI value defined by the following formula (i) is 0.50 or more,
In a metal structure in a cross section parallel to the rolling direction and thickness direction of the steel material for a crude oil tank, a total area ratio of bainite and/or acicular ferrite is 20 to 95%,
The area fraction of martensite is 2.0% or less, and
Vickers hardness is 210HV10 or less.
Steel materials for crude oil tanks.
CRI=Cu+6×Mo+2×W+0.5×Sn+0.5×Sb...(i)
In the formula, each element symbol represents the content (mass%) of each element contained in the steel material, and is set to 0 if the element is not contained.

(2) The chemical composition is, in mass%, replacing a part of the Fe,
Cr: less than 0.10%
The steel material for crude oil tanks according to (1) above.

(3) The chemical composition is, in mass%, replacing a part of the Fe,
Ni: 0.05 to 0.50%, and Co: 0.05 to 0.50%
It contains one or two selected from
A steel material for crude oil tanks according to (1) or (2) above.

(4) The chemical composition is, in mass%, replacing a part of the Fe,
Nb: 0.002-0.200%,
V: 0.005-0.500%,
Ti: 0.002 to 0.200%,
Ta: 0.005-0.500%,
Zr: 0.005 to 0.500%, and B: 0.0002 to 0.0050%
It contains one or more selected from
A steel material for crude oil tanks according to any one of (1) to (3) above.

(5) The chemical composition is, in mass%, replacing a part of the Fe,
Mg: 0.0001-0.01%,
Ca: 0.0005-0.01%,
Y: 0.0001-0.1%,
La: 0.005 to 0.1%, and Ce: 0.005 to 0.1%
It contains one or more selected from
A steel material for crude oil tanks according to any one of (1) to (4) above.

The present invention makes it possible to provide steel materials for crude oil tanks that exhibit excellent corrosion resistance in the bottom plate environment of crude oil tanks.

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

In order to solve the above problem, the inventors first investigated the optimal conditions for corrosion testing of COT (crude oil tank) bottom plates, which is regulated by the IMO (International Maritime Organization) and simulates the environment inside a pit, using steel containing Cu, Mo, W, Sn, and Sb, which are effective corrosion-resistant elements, in concentration ranges that are usable for shipbuilding steel.

Specifically, steel material having a C content of 0.06-0.16%, a Si content of 0.10-0.30%, a Mn content of 0.60-1.60%, a P content of 0.005-0.020%, a S content of 0.004-0.030%, an Al content of 0.01-0.05%, and a N content of 0.0015-0.0050% was melted and rolled to form a 6 mm thick steel plate TP1. In addition to the above elements, the steel materials shown in Table 1 containing one or more 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% were melted and rolled to form 6 mm thick steel plates TP2 to TP24. The chemical compositions of TP1 to TP24 are shown in Table 1.

Then, test pieces for the IMO test were prepared and tested. As a result, as shown by the plot of white circles in Figure 1, it was revealed that the relationship between these alloying elements and corrosion rate can be expressed by the CRI value shown in the following formula.
CRI=Cu+6×Mo+2×W+0.5×Sn+0.5×Sb...(i)

Furthermore, it was found that excellent corrosion resistance can be consistently achieved if the CRI value is 0.50 or higher.

Next, 120 mm thick steel ingots were produced using the steel materials listed in Table 1, which have a CRI value of 0.50 or higher. These were then heated and held at 1150°C for 90 minutes, rolled to a thickness of 30 mm, and then air-cooled and water-cooled specimens were prepared. 5 mm thick test specimens were then taken from different locations, and the corrosion rate according to IMO regulations was investigated.

As a result, when the steel was air-cooled, the corrosion rate of test specimens taken at the 1/4t position in the cross section parallel to the rolling direction and thickness direction of the steel (hereinafter also referred to as "L cross section"), where t is the thickness of the steel, was approximately twice as fast as that of test specimens taken near the surface. On the other hand, when the steel was water-cooled, the corrosion rates of both test specimens taken at the 1/4t position and those taken near the surface were found to be 0.3 to 0.5 times slower than those of test specimens taken near the surface when air-cooled. Furthermore, even when the 1/2t position was taken at the center of the thickness of the steel, the corrosion rate of water-cooled test specimens was found to be 0.4 to 0.6 times slower than that of air-cooled test specimens.

A detailed investigation of the metal structure of these specimens revealed that the air-cooled specimens consisted of slightly flattened ferrite and pearlite, with the ferrite grain size becoming larger and more granular at the center of the plate thickness. On the other hand, the water-cooled specimens, which had good corrosion resistance, were found to contain bainite and/or acicular ferrite at a total area ratio of 20% or more at all depth positions.

Further detailed investigation revealed that by stopping water cooling at 750-500°C after finish rolling, a metal structure can be obtained in which the total area ratio of bainite and/or acicular ferrite is 20-95% and the area ratio of martensite is 2.0% or less, thereby ensuring good corrosion resistance. It was also found that corrosion resistance deteriorates when the Vickers hardness exceeds 210 HV10.

As a result of the above investigations, it was found that the corrosion resistance of a steel plate can be improved throughout its thickness if the CRI value is 0.50 or higher, the metal structure has a total area fraction of bainite and/or acicular ferrite of 20 to 95% throughout the plate thickness, an area fraction of martensite of 2.0% or less, and the Vickers hardness is 210 HV10 or less.

The present invention was made based on the above findings. Each of the requirements of the present invention will be explained in detail below.

(A) Chemical Composition The reasons for limiting the content of each element are as follows: In the following description, "%" for the content means "% by mass."

C: 0.03-0.20%
C is an element effective in increasing strength. Furthermore, if the C content is less than 0.03%, it significantly impairs industrial economic efficiency. On the other hand, an excessive C content deteriorates weldability and joint toughness, making it undesirable for steel materials 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, excessive Si content deteriorates the toughness of the base material. Therefore, the Si content is set to 0.05 to 0.50%. When strict requirements are placed on weldability and joint toughness, the Si content is preferably 0.10% or more, or 0.20% or more, and is preferably 0.40% or less, or 0.30% or less.

Mn: 0.60-2.00%
Mn is an effective element for improving the strength of steel. However, excessive Mn content deteriorates the toughness of the base material. 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. Furthermore, when strict requirements are placed on weldability and joint toughness, the Mn content is preferably 1.80% or less, or 1.60% or less.

P: 0.030% or less P is an impurity element. To reduce the local corrosion rate and improve weldability, the P content is set to 0.030% or less. To ensure corrosion resistance and weldability, the P content is preferably 0.020% or less. To further improve corrosion resistance, the P content is more preferably 0.010% or less. There is no need to specify a lower limit for the P content; in other words, the P content may be 0%, but an extreme reduction will increase steelmaking costs. Therefore, the P content may be set to 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 localized corrosion progression and improve mechanical properties, particularly ductility, the S content is set to 0.030% or less. Furthermore, in order to ensure corrosion resistance and mechanical properties, the lower the S content, the better, and it is preferably 0.020% or less, or 0.010% or less. There is no need to specify a lower limit for the S content; in other words, the S content may be 0%, but excessive reductions lead to increased steelmaking costs. Therefore, the S content may be set to 0.001% or more, or 0.003% or more.

Al: 0.001-0.050%
Al is an element that is effective in refining the austenite grain size of the base material by forming AlN. On the other hand, excessive Al content forms coarse oxides, which deteriorates ductility and toughness. 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 combines with V, Al, and Ti to effectively refine austenite grains and strengthen precipitation. It is industrially impossible to completely remove N from steel, and reducing the N content more than necessary is undesirable because it places excessive strain on the manufacturing process. On the other hand, excessive N content 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 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 general corrosion resistance. However, excessive content of these elements not only impairs economic efficiency but also leads to deterioration of mechanical properties. Therefore, one or more elements selected from 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% are added. The Cu content is preferably 0.05% or more or 0.08% or more, and 0.55% or less or 0.50% or less. The Mo, W, Sn, and Sb contents are preferably 0.02% or more or 0.03% or more, and 0.18% or less or 0.15% or less, respectively.

Furthermore, as described above, if the CRI value represented by formula (i) is less than 0.50, sufficient corrosion resistance cannot be obtained. On the other hand, if the CRI value 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 for the CRI value, but in the chemical composition of the present invention, the upper limit for the CRI value is 2.40.
CRI=Cu+6×Mo+2×W+0.5×Sn+0.5×Sb...(i)
In the formula, each element symbol represents the content (mass%) of each element contained in the steel material, and is set to 0 if the element is not contained.

In the chemical composition of the steel material for crude oil tanks according to the present invention, the balance is Fe and impurities. Note that "impurities" refer to components that are mixed in during the industrial production of steel due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and are acceptable within the scope of the present invention, as long as they do not adversely affect the present invention.

In the chemical composition of the steel material for crude oil tanks according to the present invention, one or more elements selected from the following may be contained in place of a portion of the Fe, within the ranges shown below. Note that these elements are not necessarily essential for steel materials for crude oil tanks, and therefore the lower limit of their content is 0%. The reasons for limiting each element are explained below.

Cr: Less than 0.10% Cr is effective in increasing strength, so it may be added as needed to adjust strength. On the other hand, Cr accelerates the rate of localized corrosion and deteriorates localized corrosion resistance in crude oil environments. Cr may also promote the formation 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. There is no particular lower limit and it may be 0%, but if 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. They are also effective elements for improving corrosion resistance, so they may be included as needed. However, excessive Ni and Co contents result in reduced weldability. Furthermore, Ni and Co are expensive elements, and excessive inclusion is economically disadvantageous. Therefore, it is preferable to include one or two elements 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 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 that are effective in increasing the strength of steel in small amounts, and may be added as needed to adjust strength. On the other hand, excessive contents of these elements significantly deteriorate toughness. Therefore, it is preferable to add one or more elements 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%, and B: 0.0002 to 0.0050%.

The Nb and Ti contents are preferably 0.005% or more, 0.010% or more, or 0.020% or more, and preferably 0.180% or less, 0.150% or less, or 0.130% or less. The V, Ta, and Zr contents are preferably 0.010% or more, 0.020% or more, or 0.050% or more, and preferably 0.400% or less, 0.300% or less, or 0.200% or less. The B content is preferably 0.0005% or more, or 0.0010% or more, and 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 morphology of inclusions, improving ductility, and improving HAZ toughness in high-heat-input welded joints, and may be added as needed. On the other hand, excessive contents of these elements cause inclusions to coarsen, adversely affecting mechanical properties, particularly ductility and toughness. Therefore, it is preferable to add one or more elements 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%.

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

(B) Metallographic Structure of Steel Material for Crude Oil Tanks In the metallographic structure of the steel material for crude oil tanks of the present invention in a cross section parallel to the rolling direction and thickness direction, the total area ratio of bainite and/or acicular ferrite is 20 to 95%, and the area ratio of martensite is 2.0% or less. The reasons for limiting each structure will be explained.

Sum of bainite and/or acicular ferrite: 20 to 95%
As described above, the corrosion resistance of the steel material of the present invention is exhibited by the alloying elements contained therein. Examples of mechanisms for this include the dissolution of these alloying elements in the pit environment and their incorporation into corrosion products, or their deposition as metals onto the steel surface to form a coating. Here, bainite and acicular ferrite create a slight potential difference with ferrite. This promotes moderate corrosion of the alloying elements in the early stages of corrosion, promoting their incorporation into corrosion products and their precipitation as metals onto the steel surface, thereby improving corrosion resistance. Therefore, the total area ratio of bainite and/or acicular ferrite is set to 20% or more.

On the other hand, if the total area ratio of bainite and/or acicular ferrite is too high, the contact area with ferrite decreases, and corrosion of the alloying elements in the early stages of corrosion becomes insufficient. As a result, the above effects are not realized as quickly as expected, and corrosion resistance decreases. Therefore, the total area ratio of bainite and/or acicular ferrite should be 95% or less, preferably less than 90%.

In this specification, bainite can also include tempered bainite, but the two are not distinguished in this specification. Furthermore, "acicular ferrite" refers to ferrite that transforms at low temperatures, and refers to acicular ferrite or non-granular ferrite. It does not refer to granular ferrite, in which the grain boundaries are polyhedral, i.e., polygonal ferrite as seen in micrographs.

Martensite: 2.0% or less In the pit environment, martensite forms a battery with ferrite and acts as a cathode. As a result, it accelerates the corrosion of ferrite and induces uneven corrosion. Therefore, the area ratio of martensite is set to 2.0% or less. In the present invention, martensite does not include island martensite.

As mentioned above, ferrite creates a slight potential difference between bainite and acicular ferrite, which moderately promotes the corrosion of alloying elements in the early stages of corrosion, thereby improving corrosion resistance. Therefore, it is preferable that the area ratio of ferrite be 10% or more. However, if the area ratio of ferrite is too high, the contact area with bainite and acicular ferrite will decrease, resulting in insufficient corrosion of alloying elements in the early stages of corrosion. As a result, corrosion resistance will deteriorate. Therefore, it is preferable that the area ratio of ferrite be 75% or less.

The remainder is pearlite and island martensite. Pearlite reduces corrosion resistance. Therefore, it is preferable that the area fraction of pearlite be 5% or less. It is also preferable that the area fraction of island martensite be 5% or less.

Metal structure is measured using the following method. The metal structure is observed at a depth of 1 mm from the surface, at the 1/4t position, and at the 1/2t position on the L-section of the steel material. The observation surface is mirror-polished and etched with nital, and then photographed at 200x magnification using an optical microscope. The photograph is evaluated based on the areas excluding the solid white granular ferrite and black granular pearlite, and the remaining areas are filled in with a color of your choice to represent bainite or acicular ferrite, and image analysis is performed. Furthermore, martensite is considered to have formed in areas surrounded by ferrite, acicular ferrite, bainite, and/or pearlite structures, and island martensite is considered to have formed between the laths of the bainite structure.

The percentage of the area filled in with any color is calculated as the total area ratio of bainite and/or acicular ferrite. The percentage of ferrite in the total is calculated as the area ratio of ferrite, and the percentage of martensite in the total is calculated as the area ratio of martensite.

In addition, in the present invention, a total area ratio of bainite and/or acicular ferrite of 20 to 95% means that the total area ratio of bainite and/or acicular ferrite is 20 to 95% at all positions 1 mm deep from the surface, 1/4 t position, and 1/2 t position.In addition, in the present invention, an area ratio of martensite of 2.0% or less means that the area ratio of martensite is 2.0% or less at all positions 1 mm deep from the surface, 1/4 t position, and 1/2 t position.The shape of the steel material for crude oil tanks 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 crude oil tanks according to the present invention has a Vickers hardness of 210 HV10 or less. If the Vickers hardness exceeds 210 HV10, corrosion resistance deteriorates, although the detailed mechanism is not clear. Furthermore, the Vickers hardness is preferably 160 HV10 or more. Note that "HV10" refers to the "hardness symbol" when a Vickers hardness test is conducted with a test force of 98 N (10 kgf) (see JIS Z 2244-1:2020).

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

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

(D) Manufacturing Method A preferred manufacturing method for the steel material for crude oil tanks of the present invention will now be described. Molten steel having the above-described composition is melted in a known furnace such as a converter or an electric furnace, and is then formed into a steel material such as a slab or billet by a known method such as a continuous casting method or an ingot casting method. Note that vacuum degassing refining or the like may be performed during the melting process. The method for adjusting the composition of the molten steel may follow known steel smelting methods.

Then, the above steel material is hot-rolled to the desired dimensions and shape. It is preferable to heat the steel material to a temperature of 1020°C or higher and hold it for 20 minutes or more before hot-rolling.

Because a lower heating temperature increases the rolling load, the heating temperature should be 1020°C or higher. A heating temperature of 1030°C or higher is preferred, and a temperature of 1040°C or higher is even more preferred.

However, heating temperatures above 1350°C can cause surface marks and increase scale loss and fuel consumption. Therefore, it is preferable to keep the heating temperature below 1350°C, and more preferably below 1300°C.

From the standpoint of productivity, the soaking time is set to 20 to 120 minutes. It is preferable that the soaking time be 50 minutes or more. Here, "soaking time" refers to the time for which the slab temperature is maintained isothermally after reaching the heating temperature.

Hot rolling includes rough rolling and finish rolling, and the time between the end of rough rolling and the start of finish rolling (hereinafter also referred to as the "finish rolling wait time") is preferably 40 seconds or less. The surface layer of a steel material cools faster than the interior, but shortening the finish rolling wait time reduces the temperature difference between the surface layer and the interior of the steel material. As a result, variation in the metal structure at the 1 mm depth position from the surface, the 1/4 t position, and the 1/2 t position is reduced, resulting in a metal structure in which the total area fraction of bainite and/or acicular ferrite is 20 to 95%, and the area fraction of martensite is 2.0% or less at all of the 1 mm depth position from the surface, the 1/4 t position, and the 1/2 t position.

The finish rolling end temperature is set to 830 to 930°C. If the finish rolling end temperature is below 830°C, there will be a lot of strain in the austenite that will act as the nucleus for ferrite transformation. As a result, the transformation from austenite to ferrite is accelerated, resulting in an excessive ferrite area ratio. On the other hand, if the finish rolling end temperature is above 930°C, there will be little strain in the austenite, which will suppress the transformation to ferrite, bainite, and acicular ferrite, resulting in an excessive martensite area ratio.

Furthermore, as mentioned above, the steel material is cooled by water after hot rolling. Water cooling after hot rolling allows for a metal structure containing bainite and/or acicular ferrite in a total area ratio of 20% or more.

The water cooling start temperature is 900 to 800°C. By starting cooling from a state where the austenite phase is present at all positions 1 mm deep from the surface, 1/4 t position, and 1/2 t position, it is possible to reduce the variation in the metal structure at the positions 1 mm deep from the surface, 1/4 t position, and 1/2 t position. Furthermore, if the water cooling start temperature is below 800°C, water cooling will begin after the transformation to ferrite has begun, reducing the total area ratio of bainite and/or acicular ferrite.

Then, water cooling is stopped after water cooling to 700-500°C. By setting the water cooling stop temperature to 700-500°C, a metal structure can be obtained in which the total area ratio of bainite and/or acicular ferrite is 20-95%, and the area ratio of martensite is 2.0% or less. If water cooling is stopped below 500°C, more than 2.0% of martensite will be formed, resulting in a deterioration of corrosion resistance. On the other hand, if water cooling is stopped above 700°C, the metal structure will become a ferrite-pearlite structure, which is essentially the same as air cooling, and corrosion resistance will be deteriorated.

After hot rolling, if necessary, the steel sheet may be pickled and cold-rolled to a specified thickness. Furthermore, there are no particular restrictions on manufacturing conditions other than those described above, and conventional methods may be used.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

A steel ingot with a thickness of 120 mm and the chemical composition shown in Table 2 was produced. It was then 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. It was then water-cooled or air-cooled as shown in Table 3 to produce a hot-rolled steel sheet.

The metallographic structure of the L-section of the obtained steel plate was observed at a depth of 1 mm from the surface, at the 1/4t position, and at the 1/2t position. The observation surface was mirror-polished and etched with nital, and then photographed at 200x magnification using an optical microscope. The photographs were then colored as bainite or acicular ferrite, excluding the solid white granular ferrite and black granular pearlite, and image analysis was performed. Furthermore, martensite was considered to have formed in areas surrounded by ferrite, acicular ferrite, bainite, and/or pearlite structures, and island martensite was considered to have formed between the laths of the bainite structure.

The proportion of the area filled with a given color was calculated as the total area ratio of bainite and/or acicular ferrite. The proportion of ferrite in the total was calculated as the area ratio of ferrite, and the proportion of martensite in the total was calculated as the area ratio of martensite.

Vickers hardness was measured at five points, 1 mm apart, parallel to the rolling direction on the L-section of the steel material: 1 mm deep from the surface, at the 1/4 t position, and at the 1/2 t position. The average Vickers hardness at each depth position was calculated. Vickers hardness was measured with a test force of 98 N (10 kgf).

The corrosion resistance test was conducted in accordance with the IMO-specified corrosion test for COT bottom plates (SOLAS Chapter II-I, Part A-1, Reg. 3-11, as amended by resolution MSC. 291 (87), APPENDIX, Test Procedures for Qualification of Corrosion Resistant Steel for Cargo Tanks in Crude Oil Tankers.). However, the pH of the solution was adjusted from 0.85 to 0.5 to meet more stringent conditions.

First, test pieces measuring 25 mm x 60 mm x 5 mm (thickness) were taken from a position 1 mm deep from the surface, the 1/4 t position, and the 1/2 t position. The test pieces were taken so that one side of the test piece was at the 1 mm surface, and the test pieces at the 1/4 t and 1/2 t positions were taken so that the center of the test piece's thickness was at the 1/4 t and 1/2 t positions, respectively. A 2 mm diameter hole was drilled near the longitudinal end to allow for hanging and immersion testing. The test piece surface was polished with 600-grit emery paper. The test pieces were degreased before testing, and the pre-test weight and dimensions were measured.

The test solution used was a 10% NaCl aqueous solution with a pH adjusted to 0.5 using hydrochloric acid. 800 ml of this solution (specific liquid volume of 20 ml/ ^cm² or more) was placed in a beaker, and the temperature of the test solution was maintained at 30°C. The test specimen was then suspended using a nylon No. 4 fishing line and immersed in the beaker. The solution was replaced every 24 hours, and the test was conducted for three days. After the test, the specimen was washed to remove corrosion products, dried, and then weighed.

The corrosion rate was calculated from the weight loss before the test, the surface area, and the density before the test (7.87 g/ ^cm3 was used here). Five steel plates were tested, and it was investigated whether the average corrosion rate was 0.5 mm or less. The evaluation standard for corrosion-resistant steel in the corrosion test for COT bottom plates specified by the IMO is 1 mm or less.

Table 4 summarizes the results for the area ratio of bainite and/or acicular ferrite in the metal structure, Vickers hardness, and corrosion rate at a depth of 1 mm from the surface, the 1/4th t position, and the 1/2th t position. Regarding the corrosion rate results, a "good" was marked if the corrosion rate was 0.5 mm/y or less at all locations, and an "x" was marked if even one location exceeded 0.5 mm/y.

As shown in Table 4, Tests Nos. 1 to 21 and 26, which satisfied all of the specifications of the present invention, achieved excellent results in all performance aspects. In contrast, Tests Nos. 22 to 25, 27, and 28, which are comparative examples, showed deterioration in at least one of Vickers hardness and corrosion resistance.

According to the present invention, it is possible to obtain steel materials for crude oil tanks that exhibit excellent corrosion resistance, particularly against pitting-type localized corrosion, which occurs in the oil tanks of crude oil tankers and has a relatively high rate of progression.

Claims (7)

The chemical composition of steel 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: 0.001 to 0.010%, and further
Cu: 0.01 to 0.60%,
Mo: 0.01-0.20%,
W: 0.01-0.20%,
Sn: 0.01 to 0.20%, and Sb: 0.01 to 0.20%
Contains one or more selected from
The balance is Fe and impurities.
The CRI value defined by the following formula (i) is 0.50 or more,
In a cross section parallel to the rolling direction and thickness direction of the steel material for a crude oil tank , in all metallographic structures at a depth position of 1 mm from the surface of the steel material for a crude oil tank, a 1/4 thickness position, and a 1/2 thickness position , a total area ratio of bainite and/or acicular ferrite is 20 to 95%,
The total area ratio of bainite, acicular ferrite, and ferrite is 82% or more,
The area fraction of martensite is 2.0% or less, and
In a cross section parallel to the rolling direction and thickness direction of the steel material for crude oil tanks, the Vickers hardness at a depth of 1 mm from the surface of the steel material for crude oil tanks, a thickness 1/4 position, and a thickness 1/2 position is 210 HV10 or less.
Steel materials for crude oil tanks.
CRI=Cu+6×Mo+2×W+0.5×Sn+0.5×Sb...(i)
In the formula, each element symbol represents the content (mass%) of each element contained in the steel material, and is set to 0 if the element is not contained. The chemical composition contains, in mass %, replacing a part of the Fe,
Cr: less than 0.10%
The steel material for crude oil tanks according to claim 1. The chemical composition contains, in mass %, replacing a part of the Fe,
Ni: 0.05 to 0.50%, and Co: 0.05 to 0.50%
It contains one or two selected from
The steel material for crude oil tanks according to claim 1. The chemical composition contains, in mass %, replacing a part of the Fe,
Ni: 0.05 to 0.50%, and Co: 0.05 to 0.50%
It contains one or two selected from
The steel material for crude oil tanks according to claim 2. The chemical composition contains, in mass %, replacing a part of the Fe,
Nb: 0.002-0.200%,
V: 0.005-0.500%,
Ti: 0.002 to 0.200%,
Ta: 0.005-0.500%,
Zr: 0.005 to 0.500%, and B: 0.0002 to 0.0050%
It contains one or more selected from
The steel material for crude oil tanks according to any one of claims 1 to 4. The chemical composition contains, in mass %, replacing a part of the Fe,
Mg: 0.0001-0.01%,
Ca: 0.0005-0.01%,
Y: 0.0001-0.1%,
La: 0.005 to 0.1%, and Ce: 0.005 to 0.1%
It contains one or more selected from
The steel material for crude oil tanks according to any one of claims 1 to 4. The chemical composition contains, in mass %, replacing a part of the Fe,
Mg: 0.0001-0.01%,
Ca: 0.0005-0.01%,
Y: 0.0001-0.1%,
La: 0.005 to 0.1%, and Ce: 0.005 to 0.1%
It contains one or more selected from
The steel material for crude oil tanks according to claim 5.