Steel material for crude oil storage tank

Abstract

To provide a steel material for crude oil storage tank capable of exhibiting superior corrosion resistance under weak acid conditions.SOLUTION: A steel material for crude oil storage tank having a chemical composition consisting, in mass%, of C: 0.05 to 0.20%, Si: 0.10 to 1.00%, Mn: 0.05 to 2.00%, P: 0.030% or less, S: 0.010% or less, Cu: 0.01 to 1.50%, Al: 0.002 to 0.050%, In: 0.005 to 0.200%, N: 0.0010 to 0.0060%, and O: 0.0005 to 0.0060%, with the balance being Fe and inevitable impurities, the composition satisfying 50≤(0.7×In+0.9×Si+1.1×Al) / O≤300.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel material for crude oil tanks. [Background technology]

[0002] It has been known that pitting corrosion (hereinafter also referred to as "pits") occurs in steel materials used in the bottoms of crude oil tanks. Pits are formed by corrosion in the initiation stage and corrosion in the progression stage. The causes of pit formation and progression are as follows:

[0003] Crude oil contains saltwater called brine, which has a concentration of about 10%, and the brine, which is heavier in specific gravity than crude oil, accumulates on the bottom plate of the oil tank during transport. Normally, steel materials are covered with a highly viscous oil layer (hereinafter referred to as "oil coat"). The oil coat has the same corrosion prevention effect as paint, so corrosion by brine does not occur.

[0004] However, defects or areas where the oil coat does not provide sufficient corrosion protection may occur. These defects in the oil coat cause direct contact between the brine and the steel. As a result, the brine dissolves the iron, causing pits. The dissolved iron ions then hydrolyze and the chloride ions concentrate, causing corrosion and the pits to grow.

[0005] Patent Document 1 discloses a steel material for a crude oil tanker containing Sn, and a crude oil tanker. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2018 / 066019 [Patent Document 2] Japanese Patent Application Publication No. 2023-112979 Summary of the Invention [Problem to be solved by the invention]

[0007] Incidentally, to prevent explosions, ship engine exhaust gas, known as inert gas, is pumped into crude oil tanks. This inert gas dissolves in the brine, resulting in a brine pH of approximately 2-4. Therefore, pits occur in a weakly acidic environment with a pH of approximately 2-4. Meanwhile, it is known that the inside of a pit in the progression stage becomes a strongly acidic environment with a pH of 0.85 due to the hydrolysis of dissolved iron ions and the concentration of chloride ions.

[0008] Although the corrosion resistance of the steel material for crude oil tankers and the crude oil tanker disclosed in Patent Document 1 in a strong acid environment has been studied, no study has been made on the corrosion resistance in a weak acid environment with a pH of about 2 to 4.

[0009] An object of the present invention is to solve the above problems and to provide a steel material for crude oil tanks that exhibits excellent corrosion resistance in a weakly acidic environment. [Means for solving the problem]

[0010] The present invention has been made to solve the above-mentioned problems, and is summarized as the following steel material for crude oil tanks.

[0011] (1) Chemical composition, in mass%, C: 0.05~0.20%, Si: 0.10 to 1.00%, Mn: 0.05 to 2.00%, P: 0.030% or less, S: 0.010% or less, Cu: 0.01 to 1.50% Al: 0.002 to 0.050%, In: 0.005~0.200%, N: 0.0010~0.0060%, O: 0.0005 to 0.0060%, The balance is Fe and impurities. Satisfy the following formula (i): Steel materials for crude oil tanks. 50≦(0.7×In+0.9×Si+1.1×Al) / O≦300 (i) In the above formula, the element symbols represent the content (mass%) of each element contained in the steel material for crude oil tanks.

[0012] (2) Chemical composition, in mass %, C: 0.05~0.20%, Si: 0.10 to 1.00%, Mn: 0.05 to 2.00%, P: 0.030% or less, S: 0.010% or less, Cu: 0.01 to 1.50% Al: 0.002 to 0.050%, In: 0.005~0.200%, N: 0.0010~0.0060%, O: 0.0005 to 0.0060%, and further containing one or more selected from the group consisting of the following Group A and Group B: The balance is Fe and impurities. Satisfy the following formula (i): Steel materials for crude oil tanks. 50≦(0.7×In+0.9×Si+1.1×Al) / O≦300 (i) In the above formula, the element symbols represent the content (mass%) of each element contained in the steel material for crude oil tanks. [Group A] One or more selected from the group consisting of Ni: 1.00% or less, Cr: 0.10% or less, Mo: 0.20% or less, W: 0.50% or less, Sn: 0.30% or less, Sb: 0.30% or less, and Bi: 0.30% or less [Group B] One or more selected from the group consisting of Ti: 0.030% or less, Nb: 0.030% or less, V: 0.050% or less, B: 0.0030% or less, Ta: 0.20% or less, Zr: 0.20% or less, Ca: 0.0080% or less, Mg: 0.0080% or less, and REM: 0.0080% or less

[0013] (3) A steel material for crude oil tanks according to (2) above, wherein the chemical composition contains one or more elements selected from Group A.

[0014] (4) The steel material for crude oil tanks according to (2) above, wherein the chemical composition contains one or more elements selected from the B group. [Effects of the Invention]

[0015] According to the present invention, a steel material for crude oil tanks that exhibits excellent corrosion resistance in a weakly acidic environment can be obtained. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram for explaining the corrosion test method. DETAILED DESCRIPTION OF THE INVENTION

[0017] Through research conducted by the present inventors, it has been found that by adding In to steel materials, an excellent corrosion inhibitory effect can be obtained in a weak acid environment of pH 2 to 4 (see Patent Document 2). Specifically, In exhibits the following properties in a weak acid environment of pH 2 to 4: 3+ It dissolves as a fluorine-containing compound and acts as an inhibitor in weak acid solutions to suppress corrosion. 3+ has the effect of significantly suppressing the anodic dissolution reaction of Fe through underpotential deposition (UPD), so even a small amount can significantly improve corrosion resistance.

[0018] As a result of further research, the inventors found that while increasing the amount of solute In in a steel improves corrosion resistance, the segregation of In to grain boundaries and the like becomes significant, resulting in a deterioration in toughness. Therefore, they conducted extensive research to improve toughness while maintaining corrosion resistance. As a result, they found that by making a portion of the In in the steel exist as In oxide, the segregation of solute In to grain boundaries and the like can be suppressed. This makes it possible to improve the toughness of the steel. Furthermore, In oxide, like solute In, acts as a catalyst for the segregation of In in a weak acid environment. 3+Therefore, indium oxide exposed on the steel surface contributes to ensuring the corrosion resistance of steel through the same mechanism as that of solute indium.

[0019] However, it was found that when the ratio of solute In in the steel is excessive, corrosion resistance deteriorates. This is thought to be due to the following mechanism. Normally, solute In is present almost uniformly in the steel, but when In oxides are formed, In is locally concentrated. This results in the formation of areas with high and low In concentrations, resulting in a variation in the In concentration distribution in the steel. As a result, the areas with low In concentrations are unable to exhibit sufficient corrosion resistance in a weak acid environment, which is thought to result in a deterioration of corrosion resistance. For these reasons, it was found that maintaining an appropriate ratio of solute In and In oxide is important for achieving both corrosion resistance and toughness. In other words, it is necessary to strictly control the ratio of solute In and In oxide in the steel.

[0020] O forms oxides not only with In but also with Si and / or Al. Therefore, the present inventors investigated the effects on corrosion resistance and toughness using steel materials with variously changed O, In, Si, and Al contents. As a result, they found that the ratio of solute In and In oxides in the steel material can be controlled by adjusting the O, In, Si, and Al contents so as to satisfy the following formula (i). By satisfying the following formula (i), it is possible to reduce deterioration in toughness while ensuring corrosion resistance. 50≦(0.7×In+0.9×Si+1.1×Al) / O≦300 (i)

[0021] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0022] (A) Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0023] C: 0.05 to 0.20% C is an element necessary to ensure the strength of the material. However, excessive C content significantly reduces weldability. Furthermore, as the C content increases, the amount of cementite that acts as a cathode and promotes corrosion in a pH-decreasing environment increases, reducing corrosion resistance. Therefore, the C content is set to 0.05 to 0.20%. The C content is preferably 0.07% or more, and more preferably 0.09% or more. The C content is preferably 0.18% or less, and more preferably 0.16% or less.

[0024] Si: 0.10 to 1.00% Silicon is an element necessary for deoxidation. However, excessive content of silicon impairs the toughness of the base material and welded joints. Therefore, the silicon content is set to 0.10 to 1.00%. The silicon content is preferably 0.30% or more, and more preferably 0.50% or more. The silicon content is preferably 0.80% or less, and more preferably 0.60% or less.

[0025] Mn: 0.05 to 2.00% Mn is a low-cost element that increases the strength of steel. However, excessive Mn content deteriorates weldability and the toughness of the joint and base metal. Therefore, the Mn content is set to 0.05 to 2.00%. The Mn content is preferably 0.10% or more, and more preferably 0.50% or more. The Mn content is preferably 1.50% or less, and more preferably 1.00% or less.

[0026] P:0.030% or less P is an element present as an impurity in steel. It also reduces the acid resistance of steel and reduces corrosion resistance in chloride-rich corrosive environments, which lower the pH of the corrosion interface. It also reduces weldability and toughness in the weld heat-affected zone. Therefore, the P content is set to 0.030% or less. The P content is preferably 0.020% or less, and more preferably less than 0.010%. There is no need to specify a lower limit for the P content; in other words, the P content may be 0%, but excessive reductions increase steelmaking costs. Therefore, the P content may be set to 0.0001% or more.

[0027] S: 0.010% or less S is an element present as an impurity in steel. S forms MnS in steel, which is the starting point for corrosion, and excessive S content significantly reduces corrosion resistance. Therefore, the S content is set to 0.010% or less. The S content is preferably 0.008% or less, and more preferably 0.005% 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 reduction leads to increased steelmaking costs. Therefore, the S content may be set to 0.0001% or more.

[0028] Cu: 0.01 to 1.50% Cu is an element that improves corrosion resistance by suppressing the anodic dissolution of steel in low pH environments. However, excessive addition not only saturates this effect, but can also cause embrittlement. Therefore, the Cu content is set to 0.01 to 1.50%. The Cu content is preferably 0.03% or more, and more preferably 0.05% or more. Furthermore, the Cu content is preferably 1.20% or less, and more preferably 1.00% or less.

[0029] Al: 0.002 to 0.050% Al is an effective element for deoxidizing steel. However, excessive Al content not only deteriorates corrosion resistance in corrosive environments with low pH and high chloride content, but also causes nitrides to coarsen, resulting in a deterioration in toughness. Therefore, the Al content is set to 0.002 to 0.050%. The Al content is preferably 0.005% or more, and more preferably 0.010% or more. The Al content is preferably 0.045% or less, and more preferably 0.040% or less.

[0030] In: 0.005 to 0.200% In in a corrosive environment 3+ It dissolves as a fluorine-containing compound and acts as an inhibitor in acidic chloride solutions to suppress corrosion. 3+ Since In has the effect of significantly suppressing the anodic dissolution reaction of steel through UPD, even a small amount can significantly improve corrosion resistance. However, if it is added in excess, not only does this effect saturate, but the toughness of the steel material deteriorates. Therefore, the In content is set to 0.005 to 0.200%. The In content is preferably 0.010% or more, and more preferably 0.020% or more. Furthermore, the In content is preferably 0.150% or less, and more preferably 0.100% or less.

[0031] N: 0.0010~0.0060% N is an element that forms nitrides and contributes to improving mechanical properties by refining crystal grains. However, excessive N content causes deterioration of mechanical properties due to nitrides. Therefore, the N content is set to 0.0010 to 0.0060%. The N content is preferably 0.0020% or more, and more preferably 0.0030% or more. Furthermore, the N content is preferably 0.0050% or less, and more preferably 0.0040% or less.

[0032] O: 0.0005 to 0.0060% In the present invention, O is an element that contributes to improving toughness by forming an oxide with In. However, excessive content of O results in excess In oxide, which deteriorates corrosion resistance. Furthermore, excess In oxide results in localized distribution of solute In on the steel surface, which deteriorates corrosion resistance. Therefore, the O content is set to 0.0005 to 0.0060%. The O content is preferably 0.0010% or more, and more preferably 0.0020% or more. Furthermore, the O content is preferably 0.0050% or less, and more preferably 0.0040% or less.

[0033] In the chemical composition of the steel material of the present invention, the balance is Fe and impurities. Here, the impurities refer to components that are mixed in due to raw materials such as ores and scraps or other factors during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material of the present invention.

[0034] 50≦(0.7×In+0.9×Si+1.1×Al) / O≦300 (i) As described above, in order to reduce deterioration in toughness while maintaining corrosion resistance, it is necessary to control the contents of O, In, Si, and Al. We have also found that the median value of equation (i) serves as an index for estimating the ratio of solute In and In oxide. When the median value of equation (i) is less than 50, excessive In oxide is formed, resulting in a higher ratio of In oxide than solute In. As a result, areas with low In concentration are formed, which cannot exhibit sufficient corrosion resistance in a weak acid environment and result in deterioration of corrosion resistance. On the other hand, when the median value of equation (i) is more than 300, In oxide is less likely to form, resulting in a higher ratio of solute In than In oxide. As a result, In segregates at grain boundaries, etc., resulting in deterioration of toughness. Therefore, it is necessary to satisfy equation (i).

[0035] The value in equation (i) is preferably 70 or more, and more preferably 90 or more. The value in equation (i) is preferably 280 or less, and more preferably 260 or less.

[0036] In order to improve corrosion resistance in an acid corrosion environment, the chemical composition of the steel of the present invention may further contain one or more elements selected from Ni, Cr, Mo, W, Sn, Sb, and Bi within the ranges shown below. Note that these elements are not necessarily essential for the steel material, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0037] Ni: 1.00% or less Ni has the effect of improving corrosion resistance by suppressing the anodic dissolution of steel in high-chloride environments where protective rust formation cannot be expected, so it can be added as needed. However, excessive addition not only saturates the effect but also leads to increased costs. Therefore, the Ni content is set to 1.00% or less. The Ni content is preferably 0.80% or less, and more preferably 0.50% or less. To stably obtain the above effect, the Ni content is preferably 0.01% or more, and more preferably 0.05% or more.

[0038] Cr:0.10% or less Cr has the effect of improving corrosion resistance, so it can be added as needed. However, excessive addition may deteriorate acid resistance, and corrosion resistance may deteriorate in chloride-rich environments. Therefore, the Cr content is set to 0.10% or less. The Cr content is preferably 0.08% or less, and more preferably 0.05% or less. To stably obtain the above effects, the Cr content is preferably 0.01% or more, and more preferably 0.02% or more.

[0039] Mo: 0.20% or less Mo dissolves and forms oxyanion MoO4 2-Mo is an element that adsorbs to rust in the form of , and has the effect of inhibiting the permeation of chloride ions through the rust layer, so it can be added as needed. However, if added in excess, not only will the effect saturate, but the cost of the steel will increase significantly. Therefore, the Mo content is set to 0.20% or less. The Mo content is preferably 0.18% or less, and more preferably 0.15% or less. To stably obtain the above effect, the Mo content is preferably 0.01% or more, and more preferably 0.02% or more.

[0040] W: 0.50% or less W, like Mo, dissolves and forms oxyanion WO4 2- W exists in the form of tungsten and is an element that has the effect of suppressing the permeation of chloride ions through rust layers, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the cost of the steel will increase significantly. Therefore, the W content is set to 0.50% or less. The W content is preferably 0.45% or less, and more preferably 0.40% or less. To stably obtain the above effect, the W content is preferably 0.01% or more, and more preferably 0.02% or more.

[0041] Sn: 0.30% or less Sn is in a corrosive environment 2+ It dissolves as a tin oxide and acts as an inhibitor in acidic chloride solutions to suppress corrosion. 2+ Since Sn has the effect of significantly suppressing the anodic dissolution reaction of steel through UPD, even a small amount can significantly improve corrosion resistance. Furthermore, since Sn exerts the above effect in environments with a lower pH than In, when pits progress and the pH drops, Sn can slow the pit growth rate. Therefore, Sn can be added as needed.

[0042] However, if Sn is added in excess, not only will the above-mentioned effects saturate, but the toughness of the base material and high-heat-input welded joints will deteriorate. Therefore, the Sn content is set to 0.30% or less. Furthermore, the Sn content is preferably 0.25% or less, and more preferably 0.20% or less. To stably obtain the above-mentioned effects, the Sn content is preferably 0.005% or more, and more preferably 0.010% or more.

[0043] Sb: 0.30% or less Sb is an element that improves corrosion resistance in acidic environments. It suppresses the anodic dissolution reaction of steel in low pH environments and also inhibits the hydrogen gas generation reaction and Fe 3+ Sb can be added as needed because it improves corrosion resistance in chloride environments by suppressing the reduction reaction of Sb. However, excessive Sb content significantly deteriorates toughness. Therefore, the Sb content is set to 0.30% or less. The Sb content is preferably set to 0.25% or less, and more preferably set to 0.20% or less. To stably obtain the above effects, the Sb content is preferably set to 0.05% or more, and more preferably set to 0.08% or more.

[0044] Bi:0.30% or less Although the effect of Bi is not as significant as that of Sb and Sn, it is an element that improves corrosion resistance in acidic environments, so it can be added as needed. However, excessive addition of Bi reduces hot workability. Therefore, the Bi content is set to 0.30% or less. The Bi content is preferably set to 0.10% or less. To stably obtain the above effects, the Bi content is preferably set to 0.002% or more, and more preferably 0.005% or more.

[0045] In order to improve mechanical properties, the chemical composition of the steel of the present invention may further contain one or more elements selected from Ti, Nb, V, B, Ta, Zr, Ca, Mg, and REM within the ranges shown below. Note that these elements are not necessarily essential for the steel material, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0046] Ti: 0.030% or less Ti is an element that produces precipitates and has the effect of increasing the strength of steel, so it can be added as needed. However, excessive addition may cause a deterioration in toughness. Therefore, the Ti content is set to 0.030% or less. The Ti content is preferably 0.025% or less, and more preferably 0.020% or less. To stably obtain the above effects, the Ti content is preferably 0.005% or more, and more preferably 0.010% or more.

[0047] Nb: 0.030% or less Nb is an element that increases the strength of steel, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the toughness of the HAZ and base material will decrease. Therefore, the Nb content is set to 0.030% or less. The Nb content is preferably 0.025% or less, and more preferably 0.020% or less. To stably obtain the above effects, the Nb content is preferably 0.001% or more, and more preferably 0.003% or more.

[0048] V:0.050% or less Like Nb, V is an element that increases the strength of steel. Like Mo and W, V dissolves and exists in the form of oxygen ions, inhibiting the permeation of chloride ions through rust layers. Therefore, V can be added as needed. However, excessive V content not only saturates the effect but also significantly increases costs. Therefore, the V content is set to 0.050% or less. The V content is preferably set to 0.040% or less, and more preferably set to 0.030% or less. To consistently achieve the above effects, the V content is preferably set to 0.005% or more, and more preferably set to 0.010% or more.

[0049] B: 0.0030% or less B is an element that improves hardenability and increases strength, so it can be added as needed. However, if added in excess, the strength-enhancing effect saturates, and the toughness of both the base metal and the HAZ tends to deteriorate significantly. Therefore, the B content is set to 0.0030% or less. The B content is preferably set to 0.0025% or less, and more preferably set to 0.0020% or less. To stably obtain the above effects, the B content is preferably set to 0.0003% or more, and more preferably set to 0.0005% or more.

[0050] Ta: 0.20% or less Ta is an element that contributes to improving the strength of steel. It has also been found that Ta contributes to improving corrosion resistance, although the mechanism is not entirely clear. Therefore, it can be added as needed. However, excessive Ta content not only saturates the effect but also increases costs. Therefore, the Ta content is set to 0.20% or less. The Ta content is preferably 0.18% or less, and more preferably 0.15% or less. To stably obtain the above effects, the Ta content is preferably 0.001% or more, and more preferably 0.005% or more.

[0051] Zr: 0.20% or less Like Ti, Zr has the effect of suppressing the formation of MnS, which is the starting point for corrosion, by forming sulfides, so it can be added as needed. However, if it is added in excess, not only will the effect saturate, but the cost of the steel will increase. Therefore, the Zr content is set to 0.20% or less. The Zr content is preferably 0.18% or less, and more preferably 0.15% or less. To stably obtain the above effect, the Zr content is preferably 0.001% or more, and more preferably 0.005% or more.

[0052] Ca:0.0080% or less Ca is an element used mainly to control the morphology of sulfides and can be added as needed. It also has the effect of suppressing the decrease in pH at the interface in the corrosion reaction zone and suppressing the acceleration of corrosion. However, if it is added in excess, the effect saturates. Therefore, the Ca content is set to 0.0080% or less. The Ca content is preferably 0.0070% or less, and more preferably 0.0060% or less. To stably obtain the above effects, the Ca content is preferably 0.0002% or more, and more preferably 0.0005% or more.

[0053] Mg: 0.0080% or less Like Ca, Mg suppresses the decrease in pH at the interface in the corrosion reaction zone, so it can be added as needed. However, if added in excess, the effect saturates. Therefore, the Mg content is set to 0.0080% or less. The Mg content is preferably 0.0070% or less, and more preferably 0.0060% or less. To stably obtain the above effect, the Mg content is preferably 0.0002% or more, and more preferably 0.0005% or more.

[0054] REM: 0.0080% or less REM (rare earth elements) have the effect of improving the weldability of steel, and can be added as needed. However, if added in excess, the effect saturates, so the REM content is set to 0.0080% or less. The REM content is preferably 0.0070% or less, and more preferably 0.0060% or less. In order to stably obtain the above effect, the REM content is preferably 0.0002% or more, and more preferably 0.0005% or more.

[0055] Here, REM is a general term for 17 elements in total: Sc, Y, and lanthanides, and the REM content refers to the total amount of the above elements. Note that lanthanides are industrially added in the form of misch metals.

[0056] (B) Anticorrosion coating The steel material of the present invention described above exhibits good corrosion resistance even when used as is. However, when its surface is subjected to a corrosion prevention treatment, specifically when the surface is covered with a corrosion-resistant coating made of an organic resin or metal, the durability of the corrosion-resistant coating improves compared to conventional steel materials, and the corrosion resistance is further improved.

[0057] Examples of corrosion-resistant coatings made of organic resins include vinyl butyral-based, epoxy-based, urethane-based, and phthalic acid-based resin coatings. Examples of corrosion-resistant coatings made of metals include plated coatings of Zn, Al, Zn-Al, etc., and thermally sprayed coatings of Zn, Al, Al-Mg, etc.

[0058] The improved durability of the corrosion-protective coating is thought to be due to the fact that corrosion of the underlying steel material of the present invention is significantly suppressed, thereby suppressing swelling or peeling of the corrosion-protective coating due to corrosion of the underlying steel material from defective areas in the corrosion-protective coating.

[0059] (C) Manufacturing method There are no particular limitations on the method for producing the steel material according to the present invention. For example, the steel material includes steel plates, steel pipes, etc., which are produced by hot rolling an ingot having the above-mentioned chemical composition, and further cold rolling it as needed. There are no particular limitations on the heating conditions when hot rolling, and ordinary conditions may be used.

[0060] When producing steel materials, steel is melted in a conventional manner, and after adjusting the components, the resulting cast steel billet is hot-rolled and, if necessary, cold-rolled. After hot-rolling, the steel may be water-cooled directly or air-cooled, and then reheated and quenched. After hot-rolling, the steel may be wound into a coil. After hot-rolling, the steel may be cold-rolled and then further heat-treated.

[0061] When producing steel pipes, steel plates may be formed into a tubular shape and welded, and can be used to produce UO steel pipes, electric resistance welded steel pipes, forged steel pipes, spiral steel pipes, etc. Seamless steel pipes produced by hot extrusion or piercing rolling of steel billets are also included in the steel materials of the present invention.

[0062] The treatment for covering with the above-mentioned anticorrosion coating may be carried out by a conventional method. Furthermore, it is not necessarily required to apply an anticorrosion coating to the entire surface of the steel material, and it is sufficient to apply an anticorrosion treatment to only one side of the steel material that is exposed to a corrosive environment, or in the case of a steel pipe, only the outer or inner surface, i.e., only at least a part of the steel material surface.

[0063] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0064] Steel having the chemical composition shown in Table 1 was melted and formed into a 50 kg ingot, which was then hot forged in a conventional manner to produce a 60 mm thick block. The block was then heated at 1120°C for 1 hour, hot rolled, and finished at 850°C to a thickness of 20 mm. The block was then allowed to cool to room temperature in the air to produce a steel plate.

[0065] [Table 1]

[0066] <Corrosion test> Corrosion resistance was evaluated by the maximum corrosion depth. The maximum corrosion depth was determined as follows. Figure 1 illustrates the corrosion test method. (a) is a perspective view of a test piece 10, and (b) is a schematic cross-sectional view of a test apparatus 50. The method for preparing the test piece 10 will now be described. After removing the surface of each steel plate to a depth of 1 mm, small pieces measuring 25 mm in width, 50 mm in length, and 4 mm in thickness were collected. Artificial sludge was then prepared. The artificial sludge was prepared by mixing 25 wt% crude oil and 75 wt% artificial rust. The artificial rust was mixed so that the weight ratio of α-FeOOH:Fe3O4 was 2:1. The artificial sludge prepared in this manner was applied to one surface 12 of the small piece in the thickness direction. When applying the artificial sludge, a 5 mm diameter exposed metal portion 14 was formed in the center of the surface 12 of the small piece to simulate an oil coat defect, as shown in Figure 1(a). In Figure 1(a), the areas where the artificial sludge was applied are indicated by dots. In addition, the other surface of the small piece in the thickness direction and the side surfaces other than surface 12 were coated with an epoxy-based paint to form test piece 10.

[0067] As shown in FIG. 1(b), the test apparatus 50 includes an NaCl aqueous solution 52, a test chamber 54, a thermostatic chamber 56, an inlet gas pipe 58, and an outlet gas pipe 60. The thermostatic chamber 56 houses a glass test chamber 54. The test chamber 54 houses the NaCl aqueous solution 52 and multiple test pieces 10 (three in FIG. 1(b)). The NaCl aqueous solution 52 is adjusted to 10% NaCl and is maintained at 40°C by the thermostatic chamber 56. The inlet gas pipe 58 introduces a mixed gas into the NaCl aqueous solution 52. The outlet gas pipe 60 discharges the mixed gas from the test chamber 54. The mixed gas, which simulates an inert gas, has a composition of 13% CO2, 5% O2, 0.01% SO2, 0.2% H2S, and the balance N2. The reason why the NaCl aqueous solution 52 is kept at 40°C is that in crude oil tanks, the inside of the tank is sometimes heated to ensure the fluidity of the crude oil. Also, this is to prevent the acceleration of corrosion.

[0068] The corrosion test method will be described. First, the test piece 10 was placed at the bottom of the test tank 54 with the surface 12 of the test piece 10 facing upward in the drawing. Next, an NaCl aqueous solution 52 was poured into the test tank 54, and the test tank 54 with the test piece 10 placed therein was placed in a thermostatic chamber 56. Then, a mixed gas was blown into the NaCl aqueous solution 52 from a gas inlet pipe 58, and it was confirmed that the pH of the NaCl aqueous solution 52 was 2 to 4 one hour after the start of the mixed gas blowing. In this way, an environment simulating the bottom plate of a crude oil tank was created. The mixed gas was continuously blown into the NaCl aqueous solution 52 for 28 days from the start of the mixed gas blowing.

[0069] After the 28-day immersion test, the artificial sludge was wiped off with a cloth soaked in ethanol. The corrosion products formed on the exposed metal portion 14 were then removed, and the test specimen 10 was immersed in a citric acid solution for descaling. The height difference between the portion of the surface 12 where the artificial sludge had been applied and the pitting corrosion formed on the exposed metal portion 14 was measured using a micrometer. The depth of the deepest pitting corrosion formed on the exposed metal portion 14 was used as the maximum corrosion depth. Table 2 shows the maximum corrosion depth for each test specimen.

[0070] <Toughness evaluation> Toughness was evaluated by the fracture transition temperature. The fracture transition temperature was determined as follows. V-notch test specimens as specified in JIS Z 2242:2023 were taken from the center at the 1 / 4 position of the plate thickness so that the plate width direction of the steel plate and the longitudinal direction of the test specimen were aligned. Using such test specimens, the fracture transition temperature was determined in accordance with JIS Z 2242:2023. Table 2 shows the fracture transition temperatures of each test specimen.

[0071] [Table 2]

[0072] As is clear from the results in Table 2, in Test No. 29, a comparative example, the corrosion depth exceeded 0.80 mm because it did not contain In. In Test No. 30, the median value of equation (i) was low, so the corrosion depth exceeded 0.80 mm. In Test No. 31, the median value of equation (i) was high, so the fracture transition temperature was high.

[0073] On the other hand, in Test Nos. 1 to 28, which are examples of the present invention, all of the component contents specified in the present invention were satisfied, and therefore the corrosion depth was 0.80 mm or less, and the fracture transition temperature was low at 0°C or less. [Industrial Applicability]

[0074] According to the present invention, a steel material for crude oil tanks that exhibits excellent corrosion resistance in a weakly acidic environment can be obtained. [Explanation of symbols]

[0075] 10 test specimens 12 One side of the piece in the thickness direction 14 Exposed bare metal 50 Test Equipment 52 Artificial seawater 54 Test Tank 56 Temperature bath 58 Inlet gas pipe 60 Exhaust gas pipe

Claims

1. The chemical composition, in mass%, is C: 0.05-0.20%, Si: 0.10-1.00%, Mn: 0.05-2.00%, P: 0.030% or less, S: 0.010% or less, Cu: 0.01 to 1.50%, Al: 0.002-0.050%, In: 0.005 to 0.200%, N: 0.0010-0.0060%, O: 0.0005-0.0060%, The balance is Fe and impurities. The following formula (i) is satisfied: Steel materials for crude oil tanks. 50≦(0.7×In+0.9×Si+1.1×Al) / O≦300...(i) In the above formula, the element symbols represent the content (mass%) of each element contained in the steel material for crude oil tanks.

2. The chemical composition, in mass%, is C: 0.05-0.20%, Si: 0.10-1.00%, Mn: 0.05-2.00%, P: 0.030% or less, S: 0.010% or less, Cu: 0.01 to 1.50%, Al: 0.002-0.050%, In: 0.005 to 0.200%, N: 0.0010-0.0060%, O: 0.0005-0.0060%, and further containing one or more selected from the group consisting of the following Group A and Group B: The balance is Fe and impurities. The following formula (i) is satisfied: Steel materials for crude oil tanks. 50≦(0.7×In+0.9×Si+1.1×Al) / O≦300...(i) In the above formula, the element symbols represent the content (mass%) of each element contained in the steel material for crude oil tanks. [Group A] One or more selected from the group consisting of Ni: 1.00% or less, Cr: 0.10% or less, Mo: 0.20% or less, W: 0.50% or less, Sn: 0.30% or less, Sb: 0.30% or less, and Bi: 0.30% or less [Group B] One or more selected from the group consisting of Ti: 0.030% or less, Nb: 0.030% or less, V: 0.050% or less, B: 0.0030% or less, Ta: 0.20% or less, Zr: 0.20% or less, Ca: 0.0080% or less, Mg: 0.0080% or less, and REM: 0.0080% or less

3. 3. The steel material for crude oil tanks according to claim 2, wherein the chemical composition contains one or more elements selected from Group A.

4. 3. The steel material for crude oil tanks according to claim 2, wherein the chemical composition contains one or more elements selected from the B group.

Images