Steel plate and method for manufacturing the same

Abstract

A steel plate, the chemical composition of which, by mass%, is: C: 0.030–0.200%, Si: 0.050–0.500%, Mn: 0.50–2.00%, P: less than 0.030%, S: less than 0.010%, Al: 0.001–0.100%, N: 0.0005–0.0080%, O: 0.0005–0.0080%, Ti: 0.001–0.050%, Nb: 0.001–0.050%, Cu: 0.01–0.50%, Mo: 0. 0.01-0.10%, Sn: 0.01-0.30%, balance: Fe and impurities; the total content of dissolved Mo and dissolved Sn in the surface layer of the steel plate is above 0.005%; the metallographic structure at the 1 / 4t position is pearlite: 5-30%, bainite: less than 10%, balance: ferrite; the metallographic structure at the 1 / 10t position is pearlite: 1-20%, bainite: less than 5%, balance: ferrite; the average grain size of ferrite at the 1 / 10t position is 5-50μm; the average grain size of pearlite at the 1 / 10t position is less than 30μm.

Description

Technical Field

[0001] This invention relates to steel plates and methods for manufacturing the same. Background Technology

[0002] Steel tanks used for transporting or storing crude oil, such as crude oil tankers or above-ground or underground crude oil tanks (hereinafter collectively referred to as "crude oil tanks"), use welded structural steel with excellent strength and weldability. In addition, the steel used as crude oil tanks is required to have excellent corrosion resistance to corrosive gas components, salts, etc. contained in crude oil (see, for example, Patent Document 1).

[0003] Patent document 1 discloses a crude oil tank steel for welded structures, a method for manufacturing crude oil tank steel, a crude oil tank, and a method for preventing corrosion of crude oil tanks. The crude oil tank steel for welded structures is excellent in suppressing overall corrosion caused by crude oil corrosion in steel tanks, both uniform corrosion on the steel plate surface and localized corrosion caused by localized corrosion concentrated on the steel plate surface. In addition, it can suppress the formation of corrosion products (sludge) containing solid S.

[0004] Existing technical documents

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2004-204344 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] The crude oil tank steel described in Patent Document 1 contains a specified amount of Mo and W in a solid solution state, thus exhibiting excellent corrosion resistance. However, the results of research conducted by the inventors indicate that there is room for further improvement in corrosion resistance.

[0009] The purpose of this invention is to solve the above-mentioned problems and provide a steel plate with excellent corrosion resistance to corrosive gas components, salts, etc. contained in crude oil, and a method for manufacturing the same.

[0010] Solution for solving the problem

[0011] The inventors have conducted a detailed study on the above-mentioned issues and have obtained the following insights.

[0012] As a method to improve the corrosion resistance of steel plates, it is possible to consider including Cu, Sn, and Mo. However, in steels containing these elements, especially those with a pearlitic structure that is a mixture of ferrite and cementite, as well as a bainitic structure, there is a problem that localized cells caused by the C concentration difference can form between the ferrite and cementite, leading to corrosion.

[0013] If the metallographic structure of steel is made into a single phase of ferrite, the above problems will not occur, but there is a problem that sufficient strength cannot be guaranteed.

[0014] Therefore, the inventors conducted research and found that by increasing the area ratio of ferrite in the surface region of steel and forming a multiphase structure containing ferrite and pearlite in the inner region of steel, both corrosion resistance and strength can be achieved.

[0015] The present invention is based on the above insights, and its main purpose is the steel plate described below and its manufacturing method.

[0016] (1) A steel plate, wherein the chemical composition of the steel plate, in mass % is:

[0017] C: 0.030~0.200%

[0018] Si: 0.050~0.500%

[0019] Mn: 0.50~2.00%

[0020] P: below 0.030%

[0021] S: less than 0.010%

[0022] Al: 0.001~0.100%

[0023] N: 0.0005~0.0080%

[0024] O: 0.0005~0.0080%

[0025] Ti: 0.001~0.050%

[0026] Nb: 0.001~0.050%

[0027] Cu: 0.01–0.50%

[0028] Mo: 0.01–0.10%

[0029] Sn: 0.01~0.30%

[0030] Balance: Fe and impurities,

[0031] The total content of dissolved Mo and dissolved Sn in the surface layer of the steel plate is 0.005% or more by mass.

[0032] In the cross-section along the rolling direction of the steel plate, when the thickness of the steel plate is set as t,

[0033] The metallographic structure at a distance of 1 / 4t from the surface of the steel plate, expressed as an area percentage, is:

[0034] Pearlite: 5-30%

[0035] Bainite: less than 10%

[0036] Balance: Ferrite

[0037] The metallographic structure at a distance of 1 / 10t from the surface of the steel plate, expressed as an area percentage, is:

[0038] Pearlite: 1-20%

[0039] Bainite: less than 5%

[0040] Balance: Ferrite

[0041] The average grain size of ferrite at a distance of 1 / 10t from the surface of the steel plate is 5–50 μm.

[0042] The average grain size of the pearlite at a position 1 / 10t away from the surface of the steel plate is less than 30μm.

[0043] (2) A steel plate, wherein the chemical composition of the steel plate, in mass % is:

[0044] C: 0.030~0.200%

[0045] Si: 0.050~0.500%

[0046] Mn: 0.50~2.00%

[0047] P: below 0.030%

[0048] S: less than 0.010%

[0049] Al: 0.001~0.100%

[0050] N: 0.0005~0.0080%

[0051] O: 0.0005~0.0080%

[0052] Ti: 0.001~0.050%

[0053] Nb: 0.001~0.050%

[0054] Cu: 0.01–0.50%

[0055] Mo: 0.01–0.10%

[0056] Sn: 0.01~0.30%

[0057] W: 0-0.20%

[0058] Sb: 0-0.30%

[0059] Pb: 0–0.30%

[0060] As: 0-0.30%

[0061] Bi: 0-0.30%

[0062] Ni: 0-0.50%

[0063] Cr: 0–0.10%

[0064] V: 0~0.100%

[0065] B: 0~0.0050%

[0066] Ta: 0-0.50%

[0067] Zr: 0~0.50%

[0068] Ca: 0–0.0080%

[0069] Mg: 0–0.0080%

[0070] REM: 0~0.0080%

[0071] Balance: Fe and impurities,

[0072] The total content of dissolved Mo and dissolved Sn in the surface layer of the steel plate is 0.005% or more by mass.

[0073] In the cross-section along the rolling direction of the steel plate, when the thickness of the steel plate is set as t,

[0074] The metallographic structure at a distance of 1 / 4t from the surface of the steel plate, expressed as an area percentage, is:

[0075] Pearlite: 5-30%

[0076] Bainite: less than 10%

[0077] Balance: Ferrite

[0078] The metallographic structure at a distance of 1 / 10t from the surface of the steel plate, expressed as an area percentage, is:

[0079] Pearlite: 1-20%

[0080] Bainite: less than 5%

[0081] Balance: Ferrite

[0082] The average grain size of ferrite at a distance of 1 / 10t from the surface of the steel plate is 5–50 μm.

[0083] The average grain size of the pearlite at a position 1 / 10t away from the surface of the steel plate is less than 30μm.

[0084] (3) The steel plate according to (2) above, wherein the chemical composition, in mass %, contains selected from

[0085] W: 0.01~0.20%

[0086] Sb: 0.03~0.30%

[0087] Pb: 0.01~0.30%

[0088] As: 0.01~0.30%, and

[0089] Bi: 0.01% to 0.30% of one or two of the group consisting of the Fe group, replacing a portion of the Fe.

[0090] (4) The steel plate according to (2) or (3) above, wherein the chemical composition, in mass %, contains selected from

[0091] Ni: 0.05~0.50%

[0092] Cr: 0.01~0.10%

[0093] V: 0.010~0.100%

[0094] B: 0.0003~0.0050%

[0095] Ta: 0.005~0.50%, and

[0096] Zr: 0.005% to 0.50% of at least one of the group consisting of the Fe group to replace a portion of the Fe.

[0097] (5) The steel plate according to any one of (2) to (4) above, wherein the chemical composition contains a total of 0.0005 to 0.0080% by mass of at least one of the group consisting of Ca, Mg and REM to replace a portion of the Fe.

[0098] (6) A method for manufacturing a steel plate, comprising the following steps:

[0099] The refining process for producing molten steel;

[0100] A continuous casting process for producing steel billets having any of the chemical compositions described in (1) to (5) above by continuously casting the molten steel;

[0101] A heating process for heating the obtained steel billet;

[0102] The hot rolling process involves hot rolling heated steel billets to produce steel plates.

[0103] A natural cooling process for naturally cooling the hot-rolled steel sheet; and

[0104] The steel plate, after natural cooling, undergoes an accelerated cooling process involving water cooling.

[0105] In the heating process, the steel billet is heated to a temperature of 950–1300°C.

[0106] In the hot rolling process, the surface temperature of the steel billet is Ar3~T rex Rolling is completed within the specified temperature range.

[0107] In the natural cooling process, under the condition that the average cooling rate from the start to the end of natural cooling is less than 3°C / second, the surface temperature of the steel billet is naturally cooled to the natural cooling end temperature of Ar3-100 to Ar3-30°C.

[0108] In the accelerated cooling process, under the condition that the average cooling rate from the start to the end of accelerated cooling is greater than 3°C / second and less than 30°C / second, the surface temperature of the steel billet is water-cooled to an accelerated cooling end temperature of 350-650°C.

[0109] Ar3 is obtained from the following equation (i), T rex It can be obtained from equation (ii) below. It should be noted that the element symbols in the following equations represent the content (mass%) of each element.

[0110] Ar3=910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (i)

[0111] T rex =-91900[Nb*] 2 +9400[Nb*]+770 (ii)

[0112] Wherein, when the amount of Nb dissolved in solid solution (mass%) obtained by equation (iii) below is set as sol.Nb,

[0113] When Nb ≥ sol.Nb, [Nb*] = sol.Nb

[0114] When Nb < sol.Nb, [Nb*] = Nb.

[0115] sol.Nb=(10 (-6770 / (T+273)+2.26) (iii) / (C+12×N / 14)

[0116] It should be noted that T in the above formula represents the heating temperature (°C) of the steel billet.

[0117] (7) The steel plate manufacturing method according to (6) above, wherein after the accelerated cooling process, a tempering process of heating to a temperature range of 350 to 650°C is also performed.

[0118] The effects of the invention

[0119] According to the present invention, it is possible to obtain a steel plate with excellent corrosion resistance to corrosive gas components, salts, etc. contained in crude oil. Detailed Implementation

[0120] The features of the present invention will now be described in detail.

[0121] (A) Chemical composition

[0122] The reasons for the limitations of each element are as follows. It should be noted that in the following description, "%" for content refers to "mass %". Furthermore, in this specification, unless otherwise specified, "~" indicating a numerical range is used to mean that the values ​​before and after it are the lower and upper limits.

[0123] C: 0.030~0.200%

[0124] C is an effective element for forming pearlite and improving strength. On the other hand, when the C content is too high, it is difficult to ensure weldability and joint toughness. Therefore, the C content is set to 0.030 to 0.200%. The C content is preferably 0.050% or more, 0.070% or more, or 0.100% or more, and preferably 0.180% or less or 0.160% or less.

[0125] Si: 0.050~0.500%

[0126] Si is effective as an inexpensive deoxidizing and strengthening element. However, excessive Si content deteriorates weldability and joint toughness. Therefore, the Si content is set to 0.050 to 0.500%. The Si content is preferably 0.100% or more, more preferably 0.150% or more. Furthermore, the Si content is preferably 0.450% or less, more preferably 0.400% or less.

[0127] Mn: 0.50~2.00%

[0128] Mn is effective as an element for improving the strength and toughness of the base material. On the other hand, excessive Mn content deteriorates weldability and joint toughness. Therefore, the Mn content is set to 0.50 to 2.00%. The Mn content is preferably 0.80% or more, more preferably 0.90% or more. Furthermore, the Mn content is preferably 1.60% or less, more preferably 1.50% or less.

[0129] P: below 0.030%

[0130] Phosphorus (P) is an element contained in steel as an impurity. To ensure corrosion resistance, its content is set to below 0.030%. Furthermore, to ensure toughness, a lower P content is preferred, ideally below 0.015%. It should be noted that there is no lower limit to the P content; it can be 0%, but excessive reduction will increase costs, so it can be set to 0.003% or higher.

[0131] S: below 0.010%

[0132] Sulfur (S) is an element contained in steel as an impurity, and its content is set to below 0.010% to ensure corrosion resistance. Furthermore, to ensure toughness, a lower S content is preferred, ideally below 0.003%. It should be noted that there is no lower limit to the S content; it can be 0%, but excessive reduction will increase costs, so it can be set to 0.001% or higher.

[0133] Al: 0.001~0.100%

[0134] Al is an important deoxidizing element. On the other hand, when the Al content is too high, it will damage the surface quality of the steel billet and form inclusions that are detrimental to toughness. Therefore, the Al content is set to 0.001 to 0.100%. The Al content is preferably 0.005% or more or 0.010% or more, and more preferably 0.080% or less or 0.050% or less.

[0135] N: 0.0005~0.0080%

[0136] N forms nitrides with Al to improve joint toughness. On the other hand, when the N content is too high, embrittlement caused by solid-solution N will occur. Therefore, the N content is set to 0.0005 to 0.0080%. The N content is preferably 0.0010% or more or 0.0020% or more, preferably 0.0070% or less, and more preferably 0.0060% or less.

[0137] O: 0.0005~0.0080%

[0138] O, along with Ca, Mg, and REM (described later), forms oxides. Excessive O content leads to coarsening of the oxides and reduced toughness. On the other hand, while lower O content is generally better, excessive reduction is impractical, as it would lengthen the reflux operation time in an RH vacuum degassing unit. Therefore, the O content is set at 0.0005–0.0080%.

[0139] Ti: 0.001~0.050%

[0140] Ti, by being present in trace amounts, helps improve toughness by refining the microstructure of the base material and the weld. On the other hand, when the Ti content is too high, it will harden the weld and significantly deteriorate the toughness. Therefore, the Ti content is set to 0.001 to 0.050%. The Ti content is preferably 0.003% or more or 0.005% or more, and more preferably 0.040% or less or 0.030% or less.

[0141] Nb: 0.001~0.050%

[0142] Nitrogen (Nb), when added in trace amounts, helps to refine the microstructure and is an effective element for ensuring the strength of the base material. On the other hand, when the Nb content is too high, it will harden the weld and significantly deteriorate the toughness. Therefore, the Nb content is set to 0.001 to 0.050%. The Nb content is preferably 0.003% or more or 0.005% or more, and more preferably 0.040% or less or 0.030% or less.

[0143] Cu: 0.01–0.50%

[0144] Cu is an element that is effective in improving resistance to both general and localized corrosion. Furthermore, it also has the effect of suppressing the formation of solid sulfur from corrosive gas components. On the other hand, when the Cu content is too high, adverse effects such as increased surface cracking in the steel billet and deterioration of joint toughness become apparent. Therefore, the Cu content is set to 0.01–0.50%. The Cu content is preferably 0.03% or more, preferably 0.40% or less, and more preferably less than 0.20%.

[0145] Mo: 0.01–0.10%

[0146] Mo is an effective element for improving resistance to localized corrosion. On the other hand, when the Mo content is too high, resistance to localized corrosion decreases, and weldability and toughness deteriorate. Therefore, the Mo content is set to 0.01 to 0.10%. The Mo content is preferably 0.02% or more, more preferably 0.03% or more. Furthermore, the Mo content is preferably 0.08% or less, more preferably 0.07% or less.

[0147] Sn: 0.01~0.30%

[0148] Sn has the effect of further inhibiting the aggravation of local corrosion. On the other hand, even if the Sn content exceeds 0.30%, the effect is saturated and may have adverse effects on other properties. Therefore, taking into account economic efficiency, the Sn content is set to 0.01-0.30%. The Sn content is preferably 0.03% or more or 0.05% or more, and more preferably 0.25% or less or 0.20% or less.

[0149] W: 0–0.20%

[0150] W is an effective element for improving resistance to localized corrosion, and therefore can be included as needed. On the other hand, when the W content is too high, resistance to localized corrosion decreases, and weldability and toughness deteriorate. Therefore, the W content is set to 0.20% or less. The W content is preferably 0.15% or less, more preferably 0.10% or less, and even more preferably less than 0.05%. When the above-mentioned effects are desired more reliably, the W content is preferably 0.01% or more.

[0151] Sb: 0~0.30%

[0152] Sb has a further effect in inhibiting the aggravation of localized corrosion, and therefore can be included as needed. On the other hand, even if the Sb content exceeds 0.30%, its effect is saturated and may adversely affect other properties. Therefore, considering economic factors, the Sb content is set to 0.30% or less. The Sb content is preferably 0.25% or less or 0.20% or less. When it is desired to obtain the above-mentioned effects more reliably, the Sb content is preferably 0.03% or more or 0.05% or more.

[0153] Pb: 0–0.30%

[0154] As: 0-0.30%

[0155] Bi: 0~0.30%

[0156] Pb, As, and Bi further inhibit the aggravation of localized corrosion, and therefore can be included as needed. On the other hand, even if the content of any one of them exceeds 0.30%, its effect is saturated, and it may also have an adverse effect on other properties. Therefore, considering economic factors, the content of Pb, As, and Bi is all set to 0.30% or less. In addition, the content of any one element is preferably 0.15% or less. When the above-mentioned effects are desired, it is preferable to include one or more elements selected from Pb: 0.01% or more, As: 0.01% or more, and Bi: 0.01% or more.

[0157] Ni: 0-0.50%

[0158] Ni is effective in ensuring strength and improving toughness, and therefore can be included as needed. On the other hand, excessive Ni content increases costs. Therefore, the Ni content is set to 0.50% or less. When the above effects are desired more reliably, the Ni content is preferably 0.05% or more.

[0159] Cr: 0–0.10%

[0160] Cr improves hardenability and is effective for increasing strength, so it can be included as needed. On the other hand, when the Cr content is too high, the hardness of the joint sometimes increases while the toughness decreases. Therefore, the Cr content is set to 0.10% or less. When the above-mentioned effects are desired more reliably, the Cr content is preferably 0.01% or more.

[0161] V: 0~0.100%

[0162] V contributes to increased strength through precipitation strengthening, and therefore can be included as needed. On the other hand, excessive V content can sometimes impair joint toughness. Therefore, the V content is set to 0.100% or less. For more reliable achievement of the above-mentioned effects, the V content is preferably 0.010% or more.

[0163] B: 0~0.0050%

[0164] Boron (B) improves hardenability and thus contributes to increased base metal strength by being added in trace amounts, and can therefore be included as needed. On the other hand, excessive B content can degrade joint toughness. Therefore, the B content is set to 0.0050% or less. For more reliable results, a B content of 0.0003% or more is preferred.

[0165] Ta: 0~0.50%

[0166] Zr: 0~0.50%

[0167] Ta and Zr are trace elements that are effective in improving the strength of steel, mainly by adjusting strength, and therefore can be included as needed. On the other hand, when the content of either exceeds 0.50%, the toughness deteriorates significantly. Therefore, the content of both Ta and Zr is set to 0.50% or less. When the above-mentioned effect is desired, it is preferable to include one or both of Ta: 0.005% or more and Zr: 0.005% or more.

[0168] Ca: 0–0.0080%

[0169] Mg: 0–0.0080%

[0170] REM: 0~0.0080%

[0171] Ca, Mg, and REM all improve toughness by inhibiting the formation of coarse inclusions (such as extended MnS) through the formation of sulfides, and therefore can be included as needed. On the other hand, even if the content of any one of them exceeds 0.0080%, its effect is saturated, and coarse oxides or sulfides will form, degrading toughness. Therefore, the content of Ca, Mg, and REM is set to 0.0080% or less.

[0172] To reliably achieve the aforementioned effects, the total content of these elements is preferably set to 0.0005% or more. Furthermore, from the viewpoint of preventing deterioration of toughness properties caused by coarse oxides or sulfides, the total content of these elements is preferably set to 0.0080% or less. More preferably, the total content is 0.0010% or more, and even more preferably 0.0015% or more. Furthermore, the total content is more preferably 0.0060% or less, and even more preferably 0.0040% or less.

[0173] In this invention, REM refers to a total of 17 elements, including Sc, Y, and the lanthanides, and the aforementioned REM content refers to the total content of these elements. It should be noted that the lanthanides are industrially added in the form of a mixture of rare earth elements.

[0174] In the chemical composition of the steel plate of the present invention, the balance is Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial manufacturing of steel plates due to various factors such as raw materials (ore, waste, etc.) and manufacturing processes, and are permissible components within the range that will not adversely affect the present invention.

[0175] The total content of dissolved Mo and dissolved Sn in the surface layer of the steel plate is ≥0.005%.

[0176] Since Mo and Sn are more effective in corrosion resistance when present in a solid solution state, the amount of dissolved Mo and Sn in the surface layer of the steel plate must be at least a specified value. Specifically, the total content of dissolved Mo and Sn in the surface layer of the steel plate is set to 0.005% or more by mass. Preferably, the total content of dissolved Mo and Sn in the surface layer of the steel plate is 0.010% or more, more preferably 0.020% or more. No upper limit is set for the total content of dissolved Mo and Sn, but 0.40% is the practical upper limit of the total content of Mo and Sn contained in the steel.

[0177] It should be noted that, in this invention, the surface layer of the steel plate refers to the area extending 1 mm from the surface of the steel plate along the depth direction. Furthermore, the total content (mass%) of dissolved Mo and dissolved Sn is determined according to the following steps: First, two 1 mm thick test pieces are cut from the surface of the steel plate. Then, for one of the test pieces, the content of Mo and Sn in the test piece is determined using a known chemical analysis method (e.g., ICP emission spectrometry).

[0178] Additionally, for another test piece, 10% acetylacetone - 1% tetramethylammonium chloride / methanol was used at 20 mA / cm². 2 Electrolysis was performed at a current density of 0.4 g. The solution used in this electrolysis was filtered through a filter with a pore size of 0.2 μm, and the content of Mo and Sn in the extraction residue captured on the filter was determined by using known chemical analysis methods (e.g., ICP emission spectrometry).

[0179] The Mo and Sn in the test sample are considered as Mo precipitates and Sn precipitates, as well as dissolved Mo and dissolved Sn. The Mo and Sn in the extraction residue are also considered as Mo precipitates and Sn precipitates. Then, the contents of dissolved Mo and dissolved Sn are determined by subtracting the contents of Mo and Sn in the extraction residue from the contents of Mo and Sn in the test sample.

[0180] In addition, in the chemical composition of the steel plate involved in this invention, Ceq, as defined by the following formula (iv), can be set to a range of 0.20% to 0.50%.

[0181] Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 (iv)

[0182] In the above formula, the element symbols represent the content (mass%) of each element contained in the steel plate. If the element is not present, 0 is substituted.

[0183] By ensuring that the Ceq value is 0.20% or higher, the required strength of the steel plate can be easily ensured. On the other hand, by ensuring that the Ceq value is 0.50% or lower, excellent toughness can be ensured. Ceq is preferably 0.22% or higher, more preferably 0.24% or higher, and even more preferably 0.26% or higher. Furthermore, Ceq is preferably 0.48% or lower, more preferably 0.46% or lower, and even more preferably 0.45% or lower.

[0184] (B) Metallographic structure

[0185] In this invention, the steel plate has metallographic structures as shown below in both the inner and outer layers. The metallographic structures of the inner and outer layers of the steel plate will be described separately.

[0186] In the following explanation, "%" refers to "area %". It should be noted that the metallographic structure of the inner layer of the steel plate refers to the structure at a distance of 1 / 4t from the surface of the steel plate when the thickness of the steel plate is set to t. Conversely, the metallographic structure of the surface layer of the steel plate refers to the structure at a distance of 1 / 10t from the surface of the steel plate.

[0187] (B-1) Metallographic structure of the inner layer of the steel plate

[0188] Pearlite: 5-30%

[0189] To ensure the yield stress and tensile strength, which are key strength properties, the area ratio of pearlite is set to 5% to 30%. Preferably, the area ratio of pearlite is 10% to 20%.

[0190] Bainite: less than 10%

[0191] In this invention, the metallographic structure is mainly ferrite and contains a specified amount of pearlite. Even if it contains less than 10% bainite, the aforementioned effects are not hindered; however, if the area ratio of bainite is too high, the toughness will deteriorate. Therefore, the area ratio of bainite is set to 10% or less, preferably 5% or less. Bainite may also be absent, i.e., the area ratio of bainite can be 0%.

[0192] Balance: Ferrite

[0193] Ferrite is a microstructure with excellent toughness. Ferrite is the microstructure excluding pearlite and bainite. That is, the area fraction of ferrite is 60% or more. On the other hand, from the viewpoint of ensuring strength properties, the area fraction of ferrite is preferably 90% or less, more preferably less than 80%.

[0194] (B-2) Metallographic structure of the surface layer of the steel plate

[0195] Pearlite: 1-20%

[0196] Bainite: less than 5%

[0197] Balance: Ferrite

[0198] Pearlite is inevitably present in the metallographic structure. Additionally, bainite may also be incorporated. However, as mentioned above, in corrosive environments, when a large amount of pearlite and bainite is present in the surface region of the steel plate, localized galvanic cells will form between ferrite and cementite, leading to corrosion. Therefore, it is necessary to reduce the area fraction of pearlite and bainite at the surface. From this perspective, the area fraction of pearlite is set to 1–20%, and the area fraction of bainite is set to less than 5%.

[0199] Ideally, the area ratios of pearlite and bainite should be as low as possible. Specifically, the area ratio of pearlite is preferably 10% or less, more preferably 5% or less. Furthermore, the area ratio of bainite is preferably 3% or less, more preferably 1% or less. Bainite may also be absent, i.e., the area ratio of bainite may be 0%.

[0200] In the surface metallographic structure, the remainder is ferrite. That is, the area fraction of ferrite is 75% or more. Preferably, the area fraction of ferrite is greater than 85%, more preferably greater than 95%. The practical upper limit of the area fraction of ferrite is 99%.

[0201] The average grain size of ferrite is 5–50 μm.

[0202] In the surface metallographic structure, toughness can be improved by refining the ferrite grains. Therefore, the average grain size of the ferrite is set to 50 μm or less. Furthermore, while finer ferrite grains are preferred, grains smaller than 5 μm are difficult to achieve industrially; therefore, a lower limit of 5 μm is set. The average grain size of the ferrite is preferably 40 μm or less, and more preferably 30 μm or less.

[0203] The average particle size of pearlite is less than 30 μm.

[0204] In the surface metallographic structure, the finer the average grain size of pearlite, the finer the cementite on the cathode side, thus reducing localized corrosion. Therefore, the average grain size of pearlite is set to be below 30 μm.

[0205] (B-3) Relationship between the metallographic structure of the inner layer and the surface layer

[0206] As described above, in this invention, the area ratio of ferrite is increased in the surface layer of the steel plate, while a multiphase structure comprising ferrite and pearlite is formed in the inner layer, thereby achieving a balance between corrosion resistance and strength. If the metallographic structures of the inner and surface layers of the steel plate respectively satisfy the above conditions, a balance between corrosion resistance and strength can be achieved. Therefore, the relationship between the metallographic structures of the inner and surface layers is not particularly limited, but to further improve both corrosion resistance and strength, it is preferable that the area ratio of ferrite in the surface layer is higher than that in the inner layer.

[0207] (B-4) Methods for determining metallographic structure

[0208] In this invention, the area ratio of the metallographic structure is calculated as follows. As described above, samples are first collected at positions 1 / 4t and 1 / 10t away from the surface of the steel plate. Then, the rolling direction section (the so-called L-direction section) of the sample is observed. It should be noted that the "rolling direction" mentioned above refers to the rolling direction in finish rolling.

[0209] Specifically, the sample was etched with a nitric acid-alcohol solution, and then observed using an optical microscope at 500x magnification within a 300μm × 300μm field of view. The resulting microstructure images were then analyzed; white areas were considered ferrite, and black areas were considered pearlite, and their respective area ratios were calculated. It should be noted that in this invention, pearlite also includes pseudo-pearlite. Furthermore, in the steel plate of this invention, bainite is present in addition to ferrite and pearlite; therefore, the area ratio of bainite was calculated based on the area ratio of the remaining material. It should be noted that under the above conditions, bainite appears gray.

[0210] Furthermore, the average grain size of ferrite and pearlite at the surface was measured during the aforementioned microscopic observation. Specifically, the area of ​​each ferrite and pearlite grain in the field of view was calculated through image analysis, and the grain diameter of ferrite and pearlite was determined by calculating the diameter of a circle with the same area. Then, the average grain size of ferrite and pearlite was calculated by averaging the diameters of all ferrite and pearlite within the field of view. It should be noted that when calculating the average grain size of ferrite and pearlite, the minimum grain size of the analyzed object was set to 1 μm.

[0211] (C) Mechanical properties

[0212] There are no particular limitations on the mechanical properties, but the steel plates involved in this invention preferably have the strength required for use as crude oil tanks. Specifically, a yield stress (YS) of 235 MPa or higher and a tensile strength (TS) of 400 to 620 MPa are preferred. It should be noted that an upper limit is set for the preferred range of tensile strength because excessive tensile strength can sometimes degrade toughness.

[0213] It should be noted that the tensile strength (TS) and yield stress (YS) are based on JIS Z 2241:2011 and were determined using tensile test specimen No. 1B collected in a direction perpendicular to the rolling direction. Specifically, the yield stress (YS) is the conditional yield strength of the permanent elongation method at a permanent elongation of 0.2%.

[0214] (D) Manufacturing method

[0215] There are no particular restrictions on the manufacturing conditions of the steel plate involved in this invention, but it can be manufactured by sequentially performing the refining process, continuous casting process, heating process, hot rolling process, natural cooling process, and accelerated cooling process, which will be described later. Each process will be explained.

[0216] (a) Refining process

[0217] In the refining process, molten steel is produced. For the refining process, well-known methods can be used, and there are no particular restrictions.

[0218] (b) Continuous casting process

[0219] In the continuous casting process, molten steel is continuously cast to produce a steel billet with the aforementioned chemical composition. Known methods can be used for the continuous casting process, and there are no particular restrictions.

[0220] (c) Heating process

[0221] To perform hot rolling on the steel billet, the billet is heated. In the heating process, the steel billet having the above-mentioned chemical composition is heated to a heating temperature of 950–1300°C. The heating process can be carried out in a heating furnace. It should be noted that heating the steel billet to 950–1300°C means heating it so that the average temperature of the billet's total thickness when it is removed from the heating furnace is within the range of 950–1300°C. In this specification, this average temperature of the billet's total thickness is referred to as the billet's heating temperature. Furthermore, the average temperature of the total thickness can be calculated based on the temperature inside the heating furnace, the heating time, and the surface temperature of the billet.

[0222] Hot rolling is difficult to perform when the heating temperature is below 950°C. On the other hand, by setting the heating temperature to 1300°C or below, it is possible to suppress the coarsening of ferrite and pearlite grains at the surface layer and to appropriately optimize the area ratio of ferrite and pearlite at the inner layer. The heating temperature is preferably set to 1200°C or below, and more preferably to 1100°C or below.

[0223] Furthermore, there are no particular restrictions on the holding time when heating the steel billet; for example, it can be set to less than 120 minutes. The holding time is preferably set to less than 80 minutes or less, or less than 60 minutes. It should be noted that in this specification, the holding time refers to the total time when the steel billet is heated to a temperature range of 950–1300°C.

[0224] (d) Hot rolling process

[0225] In the hot rolling process, steel billets are hot-rolled to produce steel plates. At this time, the surface temperature of the steel billet is Ar3~T. rex Rolling is completed within a temperature range specified in the text. By ending rolling above Ar3, the formation of stretched ferrite can be suppressed. Furthermore, by stopping rolling at T... rex Ending rolling in the non-recrystallized region below can suppress the coarsening of ferrite and pearlite grains at the surface and make the area ratio of pearlite at the inner layer appropriate.

[0226] Here, Ar3 is the ferrite phase transformation initiation temperature when the steel is cooled, which is obtained by the following equation (i).

[0227] Ar3=910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (i)

[0228] In the above formula, the element symbols represent the content (mass%) of each element.

[0229] In addition, T rex This refers to the recrystallization initiation temperature at which the formation / growth of new austenite grains begins, calculated by equation (ii) below. Equation (ii) is an empirical formula. Since undissolved Nb still exists during low-temperature heating, [Nb*] in equation (ii) is derived by considering the amount of Nb in the steel and correcting for the theoretical dissolved Nb amount (mass%) calculated using the amount of Nb in the steel and the heating temperature. Using [Nb*], T is calculated... rex .

[0230] T rex =-91900[Nb*] 2 +9400[Nb*]+770 (ii)

[0231] Regarding [Nb*], when the amount of Nb dissolved in solid solution calculated by equation (iii) below is set as sol.Nb,

[0232] When Nb ≥ sol.Nb, [Nb*] = sol.Nb

[0233] When Nb < sol.Nb, [Nb*] = Nb.

[0234] sol.Nb=(10 (-6770 / (T+273)+2.26) (iii) / (C+12×N / 14)

[0235] It should be noted that T in the above formula represents the heating temperature (°C) of the steel billet.

[0236] (e) Natural cooling process

[0237] In the natural cooling process, the rolled steel sheet is naturally cooled. At this time, under the condition that the average cooling rate from the start to the end of natural cooling is less than 3°C / second, the surface temperature of the steel billet is naturally cooled to the natural cooling end temperature of Ar3-100 to Ar3-30°C. By setting the average cooling rate to less than 3°C / second, pearlite and bainite phase transformations can be suppressed in the surface layer of the steel sheet. It should be noted that in this invention, the surface temperature of the steel billet at the end of natural cooling is managed as the natural cooling end temperature.

[0238] Furthermore, by naturally cooling the billet to a surface temperature of Ar3-30°C or lower, the area ratio of ferrite in the surface metallographic structure can be adequately ensured. On the other hand, by setting the natural cooling end temperature in the natural cooling process to Ar3-100°C or higher, the temperature of the inner layer of the steel plate can be prevented from falling below Ar3 during natural cooling, thus generating the required pearlite in the inner layer metallographic structure.

[0239] (f) Accelerated cooling process

[0240] In the accelerated cooling process, the naturally cooled steel sheet is water-cooled. At this time, under the condition that the average cooling rate from the start to the end of accelerated cooling is greater than 3°C / second and less than 30°C / second, the accelerated cooling is completed at a temperature of 350–650°C. By water-cooling to the accelerated cooling completion temperature of 350–650°C at an average cooling rate greater than 3°C / second and less than 30°C / second, a specified area percentage of pearlite can be generated in the metallographic structure of the inner layer. It should be noted that in this invention, the surface temperature of the steel billet at the point where water cooling ends and the surface temperature of the billet has finished reheating is managed as the accelerated cooling completion temperature.

[0241] (g) Tempering process

[0242] Following the accelerated cooling process, a tempering process can be performed, heating the material to a temperature range of 350–650°C. It should be noted that if the cooling stop temperature in the accelerated cooling process is high, a self-tempering effect can be achieved, thus the tempering process may not be necessary.

[0243] The present invention will be described in more detail below through embodiments, but the present invention is not limited to these embodiments.

[0244] Example

[0245] Using steel billets with the chemical composition shown in Table 1, trial-produced steel plates with a thickness of 5 to 50 mm were manufactured according to the manufacturing conditions shown in Table 2.

[0246] [Table 1]

[0247]

[0248] [Table 2]

[0249] Table 2

[0250]

[0251] The metallographic structure of the obtained steel plate was observed, and the area ratio of each structure was measured. Specifically, firstly, in the section of the steel plate along the rolling direction, with the thickness of the steel plate set as t, test pieces for metallographic observation were cut from positions 1 / 4t and 1 / 10t away from the surface of the steel plate, respectively.

[0252] Then, the rolling direction section (the so-called L-direction section) of the above test piece was etched with a nitric acid-alcohol solution. After etching, it was observed using an optical microscope at 500x magnification within a 300μm × 300μm field of view. Image analysis of the obtained microstructure photographs was performed to determine the area ratios of ferrite, pearlite, and bainite. More specifically, white microstructures were considered ferrite, and black microstructures were considered pearlite; their respective area ratios were calculated, and the area ratio of bainite was determined based on the area ratio of the remaining material.

[0253] In addition, the average grain size of ferrite and pearlite at the surface location was determined using the following steps. The area of ​​each ferrite and pearlite grain in the field of view was calculated through image analysis. The grain diameter of ferrite and pearlite was then determined by finding the diameter of a circle with the same area. The average grain size of ferrite and pearlite was then calculated by averaging the diameters of all ferrite and pearlite grains in the field of view. It should be noted that the minimum grain size of the ferrite and pearlite sample was set to 1 μm when determining the average grain size.

[0254] Furthermore, the total content (mass%) of dissolved Mo and dissolved Sn in the surface layer of the steel plate was determined according to the following steps. First, two test pieces with a thickness of 1 mm were cut from the surface of the steel plate. For one of the test pieces, the content of Mo and Sn in the test piece was determined by ICP emission spectroscopy.

[0255] Additionally, for another test piece, 10% acetylacetone-1% tetramethylammonium chloride / methanol was used at 20 mA / cm. 2 Electrolysis was performed at a current density of 0.4 g, and the solution used in the electrolysis was filtered through a filter with a pore size of 0.2 μm. The content of Mo and Sn in the extraction residue captured on the filter was determined by ICP emission spectroscopy.

[0256] Then, the contents of dissolved Mo and dissolved Sn are determined by subtracting the contents of Mo and Sn in the extraction residue from the contents of Mo and Sn in the test piece.

[0257] Furthermore, tensile strength (TS) and yield stress (YS) were determined based on JIS Z 2241:2011. The test specimens were measured using tensile test specimen No. 1B, collected along the width direction perpendicular to the rolling direction. The yield stress (YS) was the conditional yield strength obtained by the permanent elongation method at 0.2% permanent elongation.

[0258] Then, to evaluate the corrosion resistance of the steel plate, three corrosion tests were performed as shown below. It should be noted that corrosion tests 1 and 2 were conducted in accordance with the International Maritime Organization (IMO) Resolution MSC.289(87).

[0259] <Corrosion Test 1>

[0260] Test pieces measuring 60 mm in the rolling direction, 25 mm in the width direction, and 5 mm in the thickness direction were collected from the surface of steel plates. All six surfaces of the test pieces were ground with 600-grit steel abrasive paper to expose the iron matrix. The test pieces were then immersed in a 10% (w / w) NaCl aqueous solution adjusted to pH 0.85 with hydrochloric acid. The immersion conditions were 30°C and 72 hours. It should be noted that the test solution was replaced every 24 hours. The volume of the test solution was 25 cc / cm² based on the surface area of ​​the test piece. 2 .

[0261] The corrosion weight loss was measured to evaluate the corrosion rate. The composition of the corrosive solution simulated the environmental conditions of localized corrosion in actual steel structures. As the corrosion rate in this corrosion test decreased, the corresponding rate of localized corrosion intensification in the actual environment also decreased. It should be noted that the corrosion weight loss was calculated by subtracting the mass of the test piece after the corrosion test (after acid washing to remove corrosion products) from the mass of the test piece before the corrosion test.

[0262] <Corrosion Test 2>

[0263] Test pieces were collected from the surface of the steel plate, measuring 60 mm in length along the rolling direction, 25 mm in length along the width direction, and 5 mm in length along the thickness direction. The surface of the test pieces was ground with 600-grit steel abrasive paper. The cross-section (excluding the surface) was coated with a paint to prepare test pieces measuring 60 mm × 25 mm, exposing only the iron matrix on the surface of the steel plate. It should be noted that test pieces were prepared for measurements after 21, 49, 77, and 98 cycles.

[0264] Prepare a glass container with distilled water, placing the lower third of it inside. Seal the top of the glass container's opening with an acrylic cap containing a gas supply port, onto which the collected test piece is mounted. Next, place the sealed glass container in a thermostat and apply a temperature cycle of 21, 49, 77, and 98 cycles at four levels: distilled water at 30°C, test piece at 50°C for 19 hours → cooling for 1 hour → 25°C for 3 hours → heating for 1 hour → [the cycle is repeated here]. During this time, a gas with the following composition is blown into the gas phase of the glass container through the gas supply port: CO2: 13 vol%, H2S: 500 ppm, O2: 4 vol%, SO2: 100 ppm, N2: balance.

[0265] Then, the corrosion weight loss was measured after 21, 49, 77, and 98 cycles, and the corrosion rate was evaluated based on their relationship. The composition of the corrosive solution simulated the environmental conditions of general corrosion in actual steel structures. As the corrosion rate in this corrosion test decreased, the corresponding rate of acceleration of general corrosion in the actual environment also decreased. It should be noted that the corrosion weight loss was calculated by subtracting the mass of the test piece after the corrosion test (after acid washing to remove corrosion products) from the mass of the test piece before the corrosion test.

[0266] <Corrosion Test 3>

[0267] Test pieces with lengths of 40 mm along the rolling direction, 40 mm along the width direction, and 4 mm along the thickness direction were collected from the surface of the steel plate. The cross-section (excluding the surface) was coated with a paint, and the surface was wet-ground with 600 grit to remove the iron oxide (scale) from the steel plate surface, resulting in a 40 mm × 40 mm test piece exposing only the iron matrix on the surface of the steel plate. Then, using this test piece, the corrosion rate and the formation rate of sludge mainly composed of solid sulfur were evaluated according to the following steps.

[0268] First, before the corrosion test, ensure that the NaCl adhesion level is 1000 mg / m³. 2 The NaCl aqueous solution was coated onto the surface of the test piece and allowed to dry. The piece was then placed horizontally on a constant-temperature heating plate inside the test chamber. Gas adjusted to a constant dew point (30°C) was then introduced into the test chamber. The gas used had a composition of CO2: 12 vol%, H2S: 500 ppm, O2: 5 vol%, and N2: balance.

[0269] Then, a temperature cycle of 2 hours / cycle at 20℃×1 hour and 40℃×1 hour was applied, resulting in alternating wet and dry conditions on the test piece surface. After 720 cycles, the corrosion rate was evaluated based on corrosion weight loss, and the sludge formation rate was evaluated based on the mass of the products formed on the test piece surface. It should be noted that the products were identified as iron hydroxide (rust) and solid sulfur through preliminary chemical and X-ray analysis. The mass of the products was calculated based on the difference in mass before and after acid washing to remove the corrosion products. Furthermore, the corrosion weight loss was calculated by subtracting the mass of the test piece after acid washing from the mass of the test piece before the corrosion test.

[0270] Based on the results of corrosion tests 1, 2, and 3, the corrosion rate and sludge formation rate for test number 45 were set to 100. For each corrosion test, the relative values ​​for each test number were calculated. That is,

[0271] Relative corrosion rate = (Corrosion rate of each test number / Corrosion rate of test number 45) × 100

[0272] Relative sludge formation rate = (sludge formation rate of each test number / sludge formation rate of test number 45) × 100.

[0273] Table 3 shows the relative corrosion rate and relative sludge formation rate for each corrosion test. It should be noted that in this embodiment, when both the relative corrosion rate and relative sludge formation rate are below 40%, the corrosion resistance is considered excellent.

[0274] [Table 3]

[0275] Table 3

[0276]

[0277] As shown in Table 3, in the examples of the present invention (test numbers 1 to 26) that meet the requirements of the present invention, the results have suitable strength and show excellent corrosion resistance in all corrosion tests.

[0278] In contrast, the comparative examples showed poor corrosion resistance in tests No. 28, 31-38, and 40. Specifically, in test No. 28, the corrosion resistance deteriorated due to excessive C content, resulting in a pearlite area ratio exceeding the specified range. In tests No. 31 and 32, corrosion resistance deteriorated due to excessive P and S content, respectively. In test No. 33, corrosion resistance deteriorated due to excessive Mo content. In test No. 34, corrosion resistance deteriorated because it lacked Sn and Sb.

[0279] In Experiment No. 35, due to excessively high heating temperatures during the heating process, the ferrite and pearlite grains at the surface layer became coarse, and the area ratios of ferrite and pearlite at the inner layer were outside the specified range. In Experiment No. 36, due to excessively low rolling end temperatures during the hot rolling process, a ferrite phase transformation occurred before sufficient dislocations were introduced, preventing the refinement of ferrite and pearlite grains at the surface layer. On the other hand, in Experiment No. 37, due to excessively high rolling end temperatures during the hot rolling process, the number of dislocations decreased through recrystallization, failing to ensure sufficient dislocations during the ferrite phase transformation, resulting in coarsening of ferrite and pearlite grains at the surface layer.

[0280] In Experiment No. 38, the average cooling rate during the natural cooling process was too high. In Experiment No. 40, the natural cooling end temperature during the natural cooling process was too high. As a result, the area ratio of pearlite and bainite in the surface layer was too large.

[0281] It should be noted that in the comparative examples, the contents of C, Si, and Mn in tests No. 27, 29, and 30 were lower than the specified values. Furthermore, in test No. 39, the natural cooling end temperature in the natural cooling process was too low; in test No. 41, the average cooling rate in the accelerated cooling process was too low; and in test No. 44, the accelerated cooling end temperature in the accelerated cooling process was too high. Therefore, in these examples, although the corrosion resistance, which is the subject of this invention, was good, the tensile strength was lower.

[0282] On the other hand, in Test No. 42, the average cooling rate in the accelerated cooling process was too high; and in Test No. 43, the accelerated cooling termination temperature in the accelerated cooling process was too low, resulting in an excessively large area ratio of bainite in the inner layer. Therefore, in these examples, although the corrosion resistance, which is the subject of this invention, is good, the excessive strength does not meet the suitable conditions.

[0283] Industrial availability

[0284] According to the present invention, a steel plate with excellent resistance to corrosive gas components, salts, etc. contained in crude oil can be obtained. Therefore, the steel plate of the present invention can be suitably used for crude oil tanks.

Claims

1. A steel plate, the chemical composition of which, in mass percent, is: C：0.030~0.200%、 Si: 0.050~0.500% Mn: 0.50~2.00% P: below 0.030% S: Below 0.010% Al：0.001~0.100%、 N：0.0005~0.0080%、 O：0.0005~0.0080%、 Ti: 0.001~0.050%, Nb: 0.001~0.050%, Cu: 0.01~0.50%, Mo: 0.01~0.10% Sn: 0.01~0.30% Balance: Fe and impurities, The total content of Mo and Sn in the surface layer of the steel plate is 0.005% or more by mass. In the cross-section along the rolling direction of the steel plate, when the thickness of the steel plate is set as t, The metallographic structure at a distance of 1 / 4t from the surface of the steel plate, expressed as an area percentage, is: Pearlite: 5~30%, Bainite: less than 10% Balance: Ferrite The metallographic structure at a distance of 1 / 10t from the surface of the steel plate, expressed as an area percentage, is: Pearlite: 1~20%, Bainite: less than 5% Balance: Ferrite The average grain size of ferrite at a distance of 1 / 10t from the surface of the steel plate is 5~50μm. The average grain size of the pearlite at a position 1 / 10t away from the surface of the steel plate is less than 30μm.

2. A steel plate, the chemical composition of which, in mass percent, is: C：0.030~0.200%、 Si: 0.050~0.500% Mn: 0.50~2.00% P: below 0.030% S: Below 0.010% Al：0.001~0.100%、 N：0.0005~0.0080%、 O：0.0005~0.0080%、 Ti: 0.001~0.050%, Nb: 0.001~0.050%, Cu: 0.01~0.50%, Mo: 0.01~0.10% Sn: 0.01~0.30% W：0~0.20%、 Sb: 0~0.30%, Pb: 0~0.30%, As: 0~0.30% Bi: 0~0.30% Ni: 0~0.50% Cr：0~0.10%、 V：0~0.100%、 B：0~0.0050%、 Ta: 0~0.50%, Zr：0~0.50%、 Ca: 0~0.0080%, Mg: 0~0.0080%, REM: 0~0.0080%, Balance: Fe and impurities, The total content of Mo and Sn in the surface layer of the steel plate is 0.005% or more by mass. In the cross-section along the rolling direction of the steel plate, when the thickness of the steel plate is set as t, The metallographic structure at a distance of 1 / 4t from the surface of the steel plate, expressed as an area percentage, is: Pearlite: 5~30%, Bainite: less than 10% Balance: Ferrite The metallographic structure at a distance of 1 / 10t from the surface of the steel plate, expressed as an area percentage, is: Pearlite: 1~20%, Bainite: less than 5% Balance: Ferrite The average grain size of ferrite at a distance of 1 / 10t from the surface of the steel plate is 5~50μm. The average grain size of the pearlite at a position 1 / 10t away from the surface of the steel plate is less than 30μm.

3. The steel plate according to claim 2, wherein, The chemical composition, expressed in mass%, contains selected free radicals. W：0.01~0.20%、 Sb: 0.03~0.30% Pb: 0.01~0.30%, As: 0.01~0.30%, and Bi: 0.01-0.30% of one or two of the group consisting of the Fe group to replace a portion of the Fe.

4. The steel plate according to claim 2 or claim 3, wherein, The chemical composition, expressed in mass%, contains selected free radicals. Ni: 0.05~0.50% Cr：0.01~0.10%、 V：0.010~0.100%、 B：0.0003~0.0050%、 Ta: 0.005~0.50%, and Zr: at least one of the group consisting of 0.005 to 0.50% is used to replace a portion of the Fe.

5. The steel plate according to any one of claims 2 to 4, wherein, The chemical composition contains, by mass percent, a total of 0.0005 to 0.0080% of at least one selected from the group consisting of Ca, Mg and REM to replace a portion of the Fe.

6. A method for manufacturing a steel plate, comprising the following steps: The refining process for producing molten steel; A continuous casting process for continuously casting the molten steel to produce a steel billet having the chemical composition described in any one of claims 1 to 5; A heating process for heating the obtained steel billet; The hot rolling process involves hot rolling heated steel billets to produce steel plates. A natural cooling process for naturally cooling the hot-rolled steel plate; and The steel plate, after natural cooling, undergoes an accelerated cooling process involving water cooling. In the heating process, the steel billet is heated to a heating temperature of 950~1300℃. In the hot rolling process, the surface temperature of the steel billet is Ar3~T rex Rolling is completed within the specified temperature range. In the natural cooling process, under the condition that the average cooling rate from the start to the end of natural cooling is less than 3°C / second, the surface temperature of the steel billet is naturally cooled to the natural cooling end temperature of Ar3-100~Ar3-30°C. In the accelerated cooling process, under the condition that the average cooling rate from the start to the end of accelerated cooling is greater than 3°C / second and less than 30°C / second, the surface temperature of the steel billet is water-cooled to an accelerated cooling end temperature of 350~650°C. Ar3 is obtained from the following equation (i), T rex It is determined by equation (ii) below. It should be noted that the element symbols in the following equations represent the content of each element in terms of mass percentage. Ar3=910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (i) T rex =-91900[Nb*] 2 +9400[Nb*]+770 (ii) Wherein, when the amount of Nb dissolved in solid solution obtained by equation (iii) below is set as sol.Nb, When Nb ≥ sol.Nb, [Nb*] = sol.Nb When Nb < sol.Nb, [Nb*] = Nb sol.Nb=(10 （-6770 / （T+273）+2.26） ) / (C+12×N / 14) (iii) It should be noted that the unit of the above-mentioned solid solution Nb amount is mass%, and T in the above formula represents the heating temperature of the steel billet, in °C.

7. The method for manufacturing a steel plate according to claim 6, wherein, After the accelerated cooling process, a tempering process is performed, in which the temperature is heated to a range of 350 to 650°C.

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