Thick steel plate

Abstract

To provide a high strength thick steel plate excellent in toughness and sulfuric acid and hydrochloric acid corrosion resistance in a weld heat-affected zone.SOLUTION: 0.08 to 0.18% of C, 0.01 to 0.09% of Si, 0.10 to 1.60% of Mn, P ≤ 0.030%, S ≤ 0.020%, 0.005 to 0.060% of Al, 0.10 to 0.50% of Cu, 0.10 to 0.50% of Ni, 0.26 to 1.00% of Cr, 0.01 to 0.20% of Sb, and N ≤ 0.006%, optionally containing at least one selected from Mo, Nb, V, Ti, B, Sn, W, Co, Ca, and Mg, with the balance being Fe and inevitable impurities, (Cu + Co + W + 2Sn + 2Sb ≥ 0.30) and (Ni ≥ Cu / 3 + 3Sn + Sb / 2) are satisfied, and Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 is in a range of 0.331 to 0.400.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to thick steel plates, particularly thick steel plates having excellent toughness in the heat-affected zone of welds, excellent resistance to sulfuric acid corrosion and hydrochloric acid corrosion, and a tensile strength of 440 MPa or more. [Background technology]

[0002] Boiler smoke exhaust systems and gasification melting furnaces that burn various fuels, waste materials, sewage sludge, etc. are exposed to combustion exhaust gas atmospheres, which are environments that are prone to sulfuric acid dew-point corrosion, hydrochloric acid dew-point corrosion, or aqueous solutions of sulfuric acid or hydrochloric acid. Steel plates exposed to such severe corrosive environments are required to have excellent acid resistance over a long period of time.

[0003] To address these problems, various steel materials have been disclosed that have excellent sulfuric acid dew-point corrosion resistance and that have excellent sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance. For example, there are steel materials that have excellent sulfuric acid dew-point corrosion resistance by adding Cu and Sb in combination to the steel, or by adding Sb and Mg in addition to Cu to the steel. There are also steel materials that have excellent sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance by adding one or more of Sn and Sb in addition to Cu while limiting the S content in the steel. There are also steel materials that have excellent sulfuric acid dew-point corrosion resistance by adding Cu and Co to the steel. There are also steel materials that have excellent sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance by adding Cu, Sb, and Co to the steel, or by adding Cu, Sb, and W to the steel.

[0004] On the other hand, adding large amounts of elements such as Cu, Sn, and Sb is known to increase the likelihood of surface cracking in steel and reduce weldability. Therefore, restrictions on the amount of addition or other measures are required. Furthermore, Co and W are expensive elements, and adding them significantly reduces economic efficiency.

[0005] For this reason, technologies have been disclosed for achieving both weldability and weld heat-affected zone toughness using steel materials with excellent sulfuric acid dew-point corrosion resistance or steel materials with excellent sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance. For example, Patent Document 1 discloses a steel material that contains Cu and one or more elements, such as Sb and Sn, that improve sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance, while maintaining a carbon equivalent Ceq of 0.330 or less. In addition to sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance, this steel material has a tensile strength of 400 MPa or more and excellent weldability.

[0006] Patent Document 2 discloses a steel material to which Cu is added and in which the interrelationships between the contents of Cu and S, Si and Al, and Cr and N are controlled within optimum ranges. In addition to sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance, the steel material has a tensile strength of 400 MPa or more and excellent weldability. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2019-196539 [Patent Document 2] Japanese Patent Publication No. 2020-111791 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the above-mentioned conventional techniques have the following problems. Even if elements such as Cu, Sn, and Sb are added to ensure excellent sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance, no method has been shown to simultaneously achieve the weldability and weld heat-affected zone toughness required for use as thick steel plates. Furthermore, even if the weld zone hardness, which is related to weldability and weld heat-affected zone toughness, is reduced while ensuring excellent sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance by reducing C, it is difficult to ensure a tensile strength of 440 MPa or more. Furthermore, while the steel materials disclosed in Patent Documents 1 and 2 can simultaneously achieve excellent sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance and weldability, they do not disclose a method for ensuring the weld heat-affected zone toughness required for use as thick steel plates. Furthermore, the technologies disclosed in Patent Documents 1 and 2 do not disclose a method for suppressing surface defects, which are a problem when manufacturing thick steel plates.

[0009] Steel plates used in sulfuric acid dew-point corrosion environments, such as those found in boilers, incineration facilities, and chimneys and flues for exhaust gas from ship engines, must have excellent corrosion resistance. These steel plates must have a tensile strength of 440 MPa or more, sufficient for use as structural members, and excellent toughness in the weld heat-affected zone (HAZ). Furthermore, they must have excellent weldability to prevent cold cracking in the HAZ during welding.

[0010] On the other hand, when hot-rolling steel plates containing Cu and Sb, or Sn, to ensure excellent sulfuric acid and hydrochloric acid dew-point corrosion resistance, the Cu and Sb, or Sn, can form low-melting-point compounds on the steel surface during heating before hot rolling. Hot-rolling such steel plates without further treatment can result in surface defects. This requires grinding to remove these defects, and the plate must be rolled thicker to accommodate grinding, resulting in significant yield and productivity losses. While adding W or Co instead of Sb or Sn is possible, these elements are very expensive. Therefore, the challenge is to produce steel plates that suppress surface defects after rolling without adding these elements.

[0011] An object of the present invention is to solve the problems of the conventional art and to provide a thick steel plate that has excellent toughness in the weld heat-affected zone, excellent sulfuric acid corrosion resistance and hydrochloric acid corrosion resistance, ensures tensile strength, and suppresses the occurrence of surface defects after rolling. [Means for solving the problem]

[0012] The inventors conducted extensive research into technologies for simultaneously achieving tensile strength of 440 MPa or greater, excellent weld heat-affected zone toughness, and excellent weldability while maintaining excellent sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance. As a result, they found that in order to ensure excellent sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance, it is necessary to control the content of the contained elements in the relational formula Cu + Co + W + 2Sn + 2Sb to 0.30 mass% or greater. The element symbols in the formula represent the content of each element expressed in mass%. Next, they found that to ensure excellent weldability while satisfying tensile strength of 440 MPa or greater, it is necessary to control the carbon equivalent Ceq to 0.331 to 0.400 mass%. Increasing Cu + Co + W + 2Sn + 2Sb or Ceq degrades the toughness of the weld heat-affected zone of thick steel plates. After studying methods for achieving even better weld heat-affected zone toughness while satisfying these prerequisites, they found that the Si content must be controlled to 0.09 mass% or less.

[0013] Next, we investigated technologies to suppress surface defects that occur when manufacturing steel plates containing Cu and Sb, or Sn. As a result, we found that the occurrence of surface defects can be suppressed by satisfying the relationship Ni≧Cu / 3+3Sn+Sb / 2. The element symbols in the above formula represent the content of each element expressed in mass%. This relationship revealed that adding Ni to low-melting-point compounds containing Cu, Sb, and Sn can increase the melting point.

[0014] The present invention was completed after further investigation based on the above-mentioned novel findings, and the gist and configuration of the present invention are as follows. [1] In mass%, C: 0.08~0.18%, Si: 0.01 to 0.09%, Mn: 0.10 to 1.60% P: 0.030% or less, S: 0.020% or less, Al: 0.005 to 0.060%, Cu: 0.10 to 0.50% Ni: 0.10 to 0.50% Cr: 0.26~1.00%, Sb: 0.01 to 0.20%, and N: 0.006% or less Contains Optionally, Mo: 0.30% or less Nb: 0.05% or less V:0.08% or less Ti: 0.030% or less, B: 0.0020% or less, Sn: 0.20% or less, W: 0.050% or less, Co: 0.020% or less, Ca: 0.0050% or less, and Mg: 0.0020% or less Contains at least one selected from the balance being Fe and unavoidable impurities; The steel plate has a chemical composition that satisfies the relationships expressed by the following formulas (1) and (2), and has a Ceq calculated by the following formula (3) in the range of 0.331 to 0.400 mass %. [Formula (1)] Cu+Co+W+2Sn+2Sb≧0.30 [Formula (2)] Ni≧Cu / 3+3Sn+Sb / 2 [Formula (3)] Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 The element symbols in the above formulas represent the content of each element expressed in mass %. [Effects of the Invention]

[0015] The steel plate of the present invention exhibits excellent corrosion resistance in acid corrosion environments, such as sulfuric acid corrosion environments such as those found in exhaust gas stacks of boilers and incineration facilities, and sulfuric acid corrosion environments such as those found in exhaust gas stacks of waste incinerators, and hydrochloric acid corrosion environments. Furthermore, the occurrence of surface defects during steel plate production can be suppressed. Therefore, it is possible to provide a steel plate that is excellent in manufacturability, weldability, and weld heat-affected zone toughness, and that has sufficient strength when used as a structural material, even when thick. Therefore, the present invention makes a significant contribution to industry. DETAILED DESCRIPTION OF THE INVENTION

[0016] A preferred embodiment of the present invention will be described in detail below. First, the reasons for limiting the component composition of the steel material according to this embodiment will be described. In this specification, "%" representing the content of each component element means "mass %" unless otherwise specified.

[0017] C: 0.08 to 0.18% C is an element that improves strength very inexpensively, and in order to stably ensure a strength of 440 MPa or more even in thick-walled steel materials, it is necessary to include 0.08% or more. On the other hand, if the C content exceeds 0.18%, the weld heat-affected zone hardens when welding, deteriorating the weldability and toughness of the weld heat-affected zone. Therefore, the C content is set to 0.18% or less. Preferably, the C content is set to 0.10% or more. Preferably, the C content is set to 0.16% or less. More preferably, the C content is set to 0.15% or less.

[0018] Si: 0.01 to 0.09% Si is an element that contributes to deoxidation and improving strength, and must be contained in an amount of 0.01% or more in order to control the morphology of oxides. On the other hand, Si is an element that deteriorates the toughness of the weld heat-affected zone. In this embodiment, the C content is set to 0.08% or more to ensure strength, so if the Si content exceeds 0.09%, the toughness of the weld heat-affected zone will deteriorate. Therefore, the upper limit of the Si content is set to 0.09%. Preferably, the Si content is set to 0.08% or less. More preferably, the Si content is set to 0.02% or more, and more preferably, the Si content is set to 0.07% or less.

[0019] Mn: 0.10 to 1.60% Mn is an element that improves strength and toughness, and this effect is manifested by including 0.10% or more. On the other hand, if the Mn content exceeds 1.60%, the weldability and the toughness of the weld heat affected zone deteriorate, so the upper limit is set to 1.60%. Preferably, the Mn content is set to 0.20% or more. Preferably, the Mn content is set to 1.50% or less. More preferably, the Mn content is set to 0.30% or more. More preferably, the Mn content is set to 1.40% or less.

[0020] P:0.030% or less P is an impurity that deteriorates the toughness of the weld heat-affected zone, so the P content is set to 0.030% or less. There is no lower limit for the P content, but because it is difficult to achieve a P content of less than 0.001% in industrial-scale production, a P content of 0.001% or more is permitted. More preferably, the P content is set to 0.005% or more. More preferably, the P content is set to 0.025% or less.

[0021] S: 0.020% or less S is an impurity that deteriorates the toughness of the weld heat-affected zone, so the S content is set to 0.020% or less. There is no lower limit for the S content, but a S content of less than 0.001% would result in an excessive refining load, so a content of 0.001% or more is allowed. More preferably, the S content is set to 0.002% or more. More preferably, the S content is set to 0.015% or less.

[0022] Al: 0.005 to 0.060% Al is a deoxidizer and must be contained in an amount of 0.005% or more. However, excessive Al content deteriorates the toughness of the weld heat affected zone, so the Al content is set to 0.060% or less. Preferably, the Al content is set to 0.010% or more. More preferably, the Al content is set to 0.015% or more. More preferably, the Al content is set to 0.050% or less.

[0023] Cu: 0.10 to 0.50% Cu is an extremely important element that, when contained together with Sb, significantly improves corrosion resistance against sulfuric acid and hydrochloric acid. To ensure corrosion resistance in an acidic environment, the Cu content must be 0.10% or more. On the other hand, if the Cu content exceeds 0.50%, surface defects occur during steel plate production, making surface treatment necessary after rolling. Therefore, the Cu content is set to 0.50% or less. Preferably, the Cu content is set to 0.20% or more. Preferably, the Cu content is set to 0.40% or less. More preferably, the Cu content is set to 0.25% or more. More preferably, the Cu content is set to 0.35% or less.

[0024] Ni: 0.10 to 0.50% When Ni is added to a steel plate containing Cu, Sb, or Sn, the low-melting-point compounds containing Cu, Sb, or Sn become composite compounds containing Ni, thereby increasing the melting point. This effect can prevent low-melting-point compounds from forming on the surface of the steel plate and causing surface defects. To achieve this effect, the Ni content must be 0.10% or more. However, excessive addition of Ni can cause surface defects due to Ni itself, so the upper limit of the Ni content is set to 0.50%. Preferably, the Ni content is 0.10% or more. Preferably, the Ni content is 0.40% or less. More preferably, the Ni content is 0.15% or more.

[0025] Cr: 0.26 to 1.00% Cr is the third most inexpensive element after C, Mn, and Si in increasing tensile strength, and since it does not degrade the toughness of the weld heat-affected zone compared to C and Si, it is actively added when high strength steel is desired. To obtain a sufficient effect in increasing the strength of steel, the Cr content must be 0.26% or more. On the other hand, if the Cr content exceeds 1.00%, the toughness of the weld heat-affected zone and weldability deteriorate, so the upper limit of the Cr content is set to 1.00%. Preferably, the Cr content is set to 0.80% or less, and more preferably, the Cr content is set to 0.60% or less.

[0026] Sb: 0.01 to 0.20% As mentioned above, Sb is an extremely important element that improves acid resistance when contained together with Cu, and in order to ensure corrosion resistance in an acidic environment, it is necessary to contain 0.01% or more. On the other hand, if the Sb content exceeds 0.20%, the occurrence of surface defects in the steel sheet cannot be suppressed, so the upper limit of the Sb content is set to 0.20%. Preferably, the Sb content is set to 0.03% or more. Preferably, the Sb content is set to 0.15% or less.

[0027] N: 0.006% or less N is an impurity that deteriorates the mechanical properties of steel and reduces manufacturability by causing surface defects. Therefore, the N content is set to 0.006% or less. There is no lower limit for the N content, but an excessive reduction increases the refining load, so the N content may be set to 0.001% or more. Preferably, the N content is set to 0.005% or less.

[0028] The basic components of this embodiment have been described above. The balance other than the above components is Fe and inevitable impurities. Inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are allowed to be included to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap. In addition to the above basic components, the steel plate according to this embodiment may contain at least one of the following optional elements to improve strength, sulfuric acid dew-point corrosion resistance, and hydrochloric acid dew-point corrosion resistance.

[0029] Mo: 0.30% or less Mo is an element that improves corrosion resistance in acidic environments when contained together with Cu, Sb, and Cr. In particular, to improve corrosion resistance to hydrochloric acid, the Mo content can be set to 0.05% or more. On the other hand, since Mo is an expensive element, from the viewpoint of cost, when Mo is contained, the Mo content is set to 0.30% or less. Preferably, the Mo content is set to 0.25% or less, and more preferably, the Mo content is set to 0.20% or less.

[0030] Nb: 0.05% or less Like Ti and V, Nb is an element that forms carbides and nitrides, and when present in a solid solution state in steel, it has the effect of refining crystal grains during hot rolling. Therefore, for the purpose of improving strength and toughness, Nb may be contained in an amount of 0.01% or more. On the other hand, if the Nb content exceeds 0.05%, the toughness of the weld heat-affected zone may deteriorate. Therefore, when Nb is contained, the Nb content is set to 0.05% or less. Preferably, the Nb content is set to 0.04% or less, and more preferably, the Nb content is set to 0.03% or less.

[0031] V:0.08% or less Like Ti and Nb, V is an element that forms carbides and nitrides, and may be added primarily to improve strength through precipitation strengthening. To achieve this effect, V may be added in an amount of 0.01% or more. However, if V is added in an amount exceeding 0.08%, the toughness of the weld heat-affected zone may deteriorate, so if V is added, the V content is set to 0.08% or less. Preferably, the V content is set to 0.07% or less, and more preferably, the V content is set to 0.05% or less.

[0032] Ti: 0.030% or less Ti is an element that forms nitrides and contributes to refining crystal grains and improving strength, and may be contained in an amount of 0.002% or more to achieve this effect. On the other hand, if the Ti content exceeds 0.030%, the nitrides become coarse and the toughness of the weld heat-affected zone may deteriorate. Therefore, when Ti is contained, the Ti content is set to 0.030% or less. Preferably, the Ti content is set to 0.005% or more. Preferably, the Ti content is set to 0.020% or less.

[0033] B: 0.0020% or less B is an element that improves hardenability and increases strength. To achieve this effect, B may be contained in an amount of 0.0002% or more. On the other hand, if the B content exceeds 0.0020%, the effect saturates and the toughness of the weld heat affected zone may decrease. Therefore, when B is contained, the B content is set to 0.0020% or less. Preferably, the B content is set to 0.0003% or more. More preferably, the B content is set to 0.0005% or more.

[0034] Sn: 0.20% or less Sn is an element that improves corrosion resistance in acidic environments, and may be contained in an amount of 0.01% or more. However, excessive Sn content reduces the toughness of the weld heat-affected zone, so when Sn is contained, the Sn content is set to 0.20% or less. Preferably, the Sn content is set to 0.02% or more. Preferably, the Sn content is set to 0.18% or less. More preferably, the Sn content is set to 0.05% or more. More preferably, the Sn content is set to 0.15% or less.

[0035] W: 0.050% or less Like Mo, W is an element that, when contained together with Cu, Sb, and Cr, improves corrosion resistance in an acidic environment. In particular, to improve corrosion resistance to hydrochloric acid, 0.001% or more of W may be contained. On the other hand, since W is also an expensive element, when W is contained, the W content is set to 0.050% or less from the viewpoint of cost. Preferably, the W content is set to 0.030% or less, and more preferably, the W content is set to 0.020% or less.

[0036] Co:0.020% or less Co is an element that improves corrosion resistance in acidic environments, and may be contained in an amount of 0.002% or more to exert this effect. On the other hand, Co is a very expensive element, and if contained in excess, economic efficiency decreases. Therefore, when Co is contained, the Co content is set to 0.020% or less. Preferably, the Co content is set to 0.015% or less, and more preferably, the Co content is set to 0.010% or less.

[0037] Ca:0.0050% or less Ca is an element used mainly to control the morphology of sulfides and also to form fine oxides. To achieve these effects, Ca may be contained in an amount of 0.0002% or more. On the other hand, if the Ca content exceeds 0.0050%, mechanical properties may be impaired. Therefore, when Ca is contained, the Ca content is set to 0.0050% or less. Preferably, the Ca content is set to 0.0005% or more. Preferably, the Ca content is set to 0.0040% or less.

[0038] Mg: 0.0020% or less Mg may be contained in an amount of 0.0001% or more to form fine oxides. On the other hand, when Mg is contained, the Mg content is set to 0.0020% or less from the viewpoint of production costs. Preferably, the Mg content is set to 0.0002% or more. More preferably, the Mg content is set to 0.0005% or more.

[0039] Note that even if the optional elements are contained in an amount less than the lower limit of the preferred content of each element, the properties of the steel plate according to this embodiment are not affected. Therefore, the inclusion of an optional element in an amount less than the lower limit of the preferred content is permitted as an unavoidable impurity. Furthermore, in this embodiment, the compositions of the basic components and optional elements must satisfy the following relationship:

[0040] [Formula (1)] Cu+Co+W+2Sn+2Sb≧0.30% The element symbols in the formulas represent the content of the element in %. The combined addition of Cu and Sb improves sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance. Furthermore, the inclusion of Co, W, or Sn can further improve sulfuric acid dew-point corrosion resistance and hydrochloric acid dew-point corrosion resistance. This effect is achieved when the left side of the above formula (1), i.e., Cu + Co + W + 2Sn + 2Sb, is 0.30% or more. Therefore, the lower limit of the left side of formula (1) is set to 0.30%. While there is no particular upper limit, from the viewpoints of preventing cost increases and suppressing the occurrence of surface defects, the left side of formula (1) is preferably set to 0.80% or less. More preferably, the left side of formula (1) is set to 0.60% or less.

[0041] [Formula (2)] Ni≧Cu / 3+3Sn+Sb / 2 The element symbols in the formulas represent the content of the element in %. When Ni is added, low-melting-point compounds containing Cu, Sb, or Sn become composite compounds containing Ni, resulting in a high melting point. This effect prevents liquefied low-melting-point compounds from forming on the surface of the steel plate, penetrating grain boundaries, and causing surface defects during subsequent rolling. To achieve this effect, the Ni content must be within the above range and must be added in an amount equal to or greater than the right-hand side of formula (2), i.e., Cu / 3 + 3Sn + Sb / 2. Although there is no particular upper limit for the Ni content, because Ni is an expensive element, it is preferable to set the upper limit to the value obtained by adding 0.30% to the right-hand side of formula (2), at which the effect saturates. More preferably, the upper limit for the Ni content is set to the value obtained by adding 0.20% to the right-hand side of formula (2).

[0042] Carbon equivalent Ceq: 0.331 to 0.400% [Formula (3)] Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 The element symbols in the formulas represent the content of the element in %. The carbon equivalent Ceq is an index showing the deterioration of weldability due to an increase in hardness, and also has a significant effect on the tensile strength and the toughness of the weld heat-affected zone. In order to stably ensure a tensile strength of 440 MPa or more in thick steel plates, it must be 0.331% or more. On the other hand, if it exceeds 0.400%, the weldability and the toughness of the weld heat-affected zone deteriorate, so the upper limit is set to 0.400% or less. Preferably, the carbon equivalent Ceq is set to 0.395% or less. More preferably, the carbon equivalent Ceq is set to 0.390 or less.

[0043] Next, a method for manufacturing a steel plate according to this embodiment will be described. The steel plate according to this embodiment is manufactured by melting molten steel having the above-described composition in a known furnace such as a converter or an electric furnace, and then forming it into a steel material such as a slab by a known method such as a continuous casting method or an ingot casting method. Note that vacuum degassing refining or the like may be carried out during the melting. The method for adjusting the composition of the molten steel may follow a known steel refining method.

[0044] Next, when the above steel material is hot rolled into a thick steel plate of the desired dimensions and shape, it is reheated to reduce deformation resistance during rolling. The reheating temperature of the steel material is preferably 1000°C or higher and 1250°C or lower, and the final pass of hot rolling is preferably performed at a temperature between 700°C and 1000°C. After hot rolling, accelerated cooling, water cooling or air cooling after reheating, or a combination thereof may be performed as needed.

[0045] The steel plate according to this embodiment preferably has a thickness t in the range of 3 mm to 60 mm, more preferably 6 mm or more, and even more preferably 40 mm or less. [Example]

[0046] The technical content of the present invention will be further explained below with reference to examples of the present invention. Note that the conditions in the examples shown below are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Furthermore, various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0047] Steels having the chemical compositions shown in Table 1 were melted using a converter furnace and then made into steel materials using a continuous casting method. The steel materials were heated at 1200°C for 2 hours, and then hot-rolled under conditions where the final rolling pass was carried out within a temperature range of 750 to 1000°C. The steel plates obtained by hot rolling were air-cooled to produce steel plates with thicknesses ranging from 4 to 60 mm. The remainder of the chemical compositions of each steel shown in Table 1 was iron (Fe) and unavoidable impurities.

[0048] [Table 1]

[0049] A 25mm x 25mm x 4mm test piece was taken from the center of the thickness of each steel plate obtained and finished with wet #400 polishing to prepare a test piece for evaluating corrosion resistance. Corrosion resistance was evaluated by a sulfuric acid immersion test and a hydrochloric acid immersion test. In the sulfuric acid immersion test, the steel plate was immersed in a 40 mass% sulfuric acid aqueous solution at 60°C for 6 hours. In the hydrochloric acid immersion test, the steel plate was immersed in a 20 mass% hydrochloric acid aqueous solution at 80°C for 6 hours. Corrosion weight loss was then evaluated for each test piece. The corrosion weight loss was determined by measuring the difference between the weight of the test piece before immersion and the weight of the test piece after the test. In the sulfuric acid immersion test, the corrosion weight loss was 400g / (m 2 ·hr) or less, and the hydrochloric acid immersion test is 400g / (m 2 These evaluation criteria correspond to a corrosion weight loss of 1 / 6 or less in a sulfuric acid immersion test and a corrosion weight loss of 1 / 3 or less in a hydrochloric acid immersion test, respectively, for ordinary SS400 with no added alloying elements.

[0050] The tensile test was performed using No. 5 tensile test pieces according to JIS Z 2241:2022, taken from a direction perpendicular to the rolling direction of the thick steel plate, and evaluation was performed. Those with a tensile strength of 440 MPa or more were judged to have passed the test.

[0051] Weldability was evaluated by a y-type weld cracking test at a test temperature of 20°C under the welding conditions for a covered electrode in Appendix A of JIS Z 3158:2016. The weld cracking test was carried out on three specimens, and if cracking occurred in even one specimen, it was deemed a failure and marked with an "X", and if no cracking occurred in any of the three specimens, it was deemed a pass and marked with an "O".

[0052] To evaluate the toughness of the weld heat-affected zone, a 30° V-groove was machined in the steel plate. For steel plates less than 12 mm thick, welded joints were fabricated using a single or multiple pass shielded arc welding (welding consumable: JKW KS-76K) with a heat input of 0.7 kJ / mm. For steel plates 12 mm or thicker, welded joints were fabricated using a single or multiple pass submerged arc welding (welding consumable: Kobe Steel USW52×MF38) with a heat input of 4.5 kJ / mm. For steel plates 12 mm or thicker, three 10 mm × 10 mm × 55 mm impact test specimens were taken from the weld heat-affected zone of the welded joints in the direction perpendicular to the weld line, in accordance with JIS Z 2242:2023. Charpy impact tests were conducted at 0°C, and a test was deemed to have passed if the average absorbed energy of the three specimens was 27 J or greater. For thick steel plates with a thickness of 8.5 mm or greater but less than 12 mm, test specimens measuring 7.5 mm x 10 mm x 55 mm were taken. For thick steel plates with a thickness of 6 mm or greater but less than 8.5 mm, test specimens measuring 5 mm x 10 mm x 55 mm were taken. For thick steel plates with a thickness less than 6 mm, test specimens measuring 1 mm less than the plate thickness x 10 mm x 55 mm were taken.

[0053] To check for the occurrence of surface defects on the steel plates, a 0.5g / L fluorescent wet magnetic particle inspection test was carried out on the entire front and back surfaces of the steel plates using the electrode gap method specified in JIS Z 2320-1: 2017. If even one cracked magnetic particle pattern exceeding 1mm was found, the plate was deemed a failure and marked with an "X", while anything smaller was deemed a pass and marked with an "O".

[0054] The evaluation results of these tests are shown in Table 2. The inventive examples were within the target range in all tests, whereas the comparative examples, which are outside the scope of the present invention, had test results that were outside the target range in one of the tests.

[0055] [Table 2]

[0056] The unit of volume, "L", is 10 -3 m3 "x~y" representing a range of numerical values ​​means from x to y, inclusive, and includes the boundary value. [Industrial Applicability]

[0057] The steel plate of the present invention can be used in smoke exhaust systems for boilers that burn fossil fuels such as heavy oil and coal, gas fuels such as liquefied natural gas, general waste such as municipal solid waste, industrial waste such as waste oil, plastics, and exhaust tires, and sewage sludge, as well as ships, etc. Specifically, it can be suitably used in flue ducts, casings, and heat exchangers of smoke exhaust systems, gas-gas heaters consisting of two heat exchangers (a heat recovery unit and a reheater), desulfurization equipment, electrostatic precipitators, induced draft fans, basket materials and heat transfer element plates of rotary regenerative air preheaters, etc.

Claims

[Claim 1] In mass%, C: 0.08-0.18%, Si: 0.01-0.09%, Mn: 0.10 to 1.60%, P: 0.030% or less, S: 0.020% or less, Al: 0.005-0.060%, Cu: 0.10-0.50%, Ni: 0.10-0.50%, Cr: 0.26-1.00%, Sb: 0.01 to 0.20%, and N: 0.006% or less Contains Optionally, Mo: 0.30% or less, Nb: 0.05% or less, V: 0.08% or less, Ti: 0.030% or less, B: 0.0020% or less, Sn: 0.20% or less, W: 0.050% or less, Co: 0.020% or less, Ca: 0.0050% or less, and Mg: 0.0020% or less and containing at least one selected from the balance being Fe and unavoidable impurities; A thick steel plate having a component composition that satisfies the relationships represented by the following formulas (1) and (2) and has a Ceq calculated by the following formula (3) in the range of 0.331 to 0.400 mass%. [Formula (1)] Cu+Co+W+2Sn+2Sb≧0.30% by mass [Formula (2)] Ni≧Cu / 3+3Sn+Sb / 2 [Formula (3)] Ceq=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 The element symbols in the above formulas represent the content of each element expressed in mass %, and if the element is not contained, it is set to 0.

Images