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 0.40% of Cu, 0.26 to 1.00% of Cr, 0.01 to 0.20% of Sb, and 0.006% or less of N, optionally containing at least one selected from Ni, Mo, Nb, V, Ti, B, Sn, W, Co, Ca, and Mg, with the balance being Fe and an inevitable impurity, and satisfying (Cu + Co + W + 2Sn + 2Sb ≥ 0.30), in the thick steel plate, Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 is in a range of 0.331 to 0.400, and a compound mainly containing Cu, Sb, and Sn has a space factor of 5% or less at an interface between a steel surface and a scale.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, Sb, and Sn 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] Therefore, 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 each 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 conducted extensive research into technologies to suppress surface defects that occur when manufacturing steel plates containing Cu and Sb, or Sn. As a result, we found that it is effective to prevent low-melting-point compounds containing one or more of Cu, Sb, and Sn, which cause surface defects, from liquefying on the steel plate surface by diffusing them into the scale during reheating of the rolled material. We also found that diffusing low-melting-point compounds into the scale and preventing liquefaction on the steel plate surface can be achieved by optimizing the reheating conditions of the rolled material.

[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: 0.10-0.40% Cr: 0.26~1.00%, Sb: 0.01 to 0.10%, and N: 0.006% or less Contains Optionally, Ni: 0.50% or less, 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: 0.10% 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 composition satisfies the relationship expressed by the following formula (1), and the Ceq calculated by the following formula (2) is in the range of 0.331 to 0.400 mass%, This steel plate has a compound phase in which the sum of the contents of Cu, Sb and Sn is 50% or more, and the frequency at which this compound phase is formed at the interface between the steel plate surface and the scale adhering to the surface is 5% or less in terms of space factor. [Formula (1)] Cu + Co + W + 2Sn + 2Sb ≥ 0.30 mass% [Formula (2)] 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. [2] A method for producing a thick steel plate, comprising: a heating step of reheating a steel material having a chemical composition specified in [1] above; and a hot rolling step of hot rolling the reheated steel material to a desired dimension, In the heating step, the surface temperature of the steel material is raised at a rate of 1.5°C / min or more in the temperature range of 950°C to 1050°C during heating, and the reheating is terminated after the surface temperature of the steel material is maintained in a temperature range of over 1050°C and not exceeding 1250°C for 30 minutes or more. [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 0.40% 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 0.40%, surface defects occur during steel plate production, making surface treatment necessary after rolling. Therefore, the Cu content is set to 0.40% or less. Preferably, the Cu content is set to 0.20% or more. Preferably, the Cu content is set to 0.35% or less. More preferably, the Cu content is set to 0.25% or more.

[0024] 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.

[0025] Sb: 0.01 to 0.10% As mentioned above, Sb is an extremely important element that improves acid resistance when contained simultaneously 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 0.10%, 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 0.02% or more. More preferably, the Sb content is set to 0.04% or more.

[0026] 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.

[0027] 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 additional elements to improve strength, sulfuric acid dew-point corrosion resistance, and hydrochloric acid dew-point corrosion resistance.

[0028] Ni: 0.50% or less When Ni is added to a steel plate containing Cu, Sb, or Sn, the low-melting-point compound containing Cu, Sb, or Sn becomes a complex compound containing Ni, thereby increasing the melting point. This effect can prevent the low-melting-point compound from forming on the surface of the steel plate and causing surface defects. To achieve this effect, the Ni content is preferably 0.10% or more. On the other hand, excessive Ni content may cause surface defects due to Ni itself. Therefore, when Ni is contained, the upper limit of the Ni content should be 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.

[0029] Mo: 0.30% or less Mo is an element that improves corrosion resistance in an acidic environment when contained together with Cu, Sb, and Cr. In particular, to improve corrosion resistance to hydrochloric acid, Mo may be contained in an amount of 0.05% or more. On the other hand, since Mo is an expensive element, when Mo is contained, the Mo content is set to 0.30% or less from the viewpoint of cost. 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, 0.0002% or more of B may be contained. 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: 0.10% or less Sn is an element that improves corrosion resistance in acidic environments, and 0.01% or more may be contained to achieve this effect. However, excessive Sn content reduces the toughness of the weld heat-affected zone, so if Sn is contained, the Sn content is set to 0.10% or less. Preferably, the Sn content is set to 0.02% or more. More preferably, the Sn content is set to 0.05% or more.

[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 achieve this effect. However, Co is a very expensive element, and excessive Co content reduces economic efficiency. 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] Carbon equivalent Ceq: 0.331 to 0.400% [Formula (2)] 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.

[0042] In the present invention, the compound phase formed at the interface between the surface of the steel plate and the scale adhering to the surface is specified.

[0043] The frequency of compound phases with a total content of Cu, Sb, and Sn of 50% or more occurring at the interface between the steel plate surface and the scale adhering to the surface is 5% or less in terms of space factor. In steel plates containing one or more of Cu, Sb, and Sn, when the rolled steel material is reheated, a low-melting-point compound phase is formed at the interface between the steel plate surface and the scale, with the sum of the Cu, Sb, and Sn contents being 50% or more. This low-melting-point compound then liquefies and penetrates into the grain boundaries of the steel plate, causing cracks in the steel plate. If the frequency at which this compound phase is formed at the interface between the steel plate surface and the scale attached to the surface exceeds 5% in terms of space factor, surface defects will occur in the steel plate. Therefore, the upper limit is set to 5% in terms of space factor. No lower limit is specifically defined. The space factor is preferably 0 to 4%, more preferably 0 to 3%. In addition to Cu, Sb, and Sn, other elements such as C, Fe, Ni, and Mn are included, but these elements are not specifically defined because they do not affect the occurrence of surface defects. The space factor can be measured using the method described in the examples.

[0044] 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.

[0045] Next, when the above-mentioned steel material is hot-rolled into a steel plate of the desired size and shape, the steel material is reheated to reduce deformation resistance during rolling. In this embodiment, the low-melting-point compound phase between the surface of the steel plate and the scale adhering to the surface is controlled by the heating conditions during reheating.

[0046] The surface temperature of the steel material must be raised at a rate of 1.5°C / min or more in the temperature range of 950°C to 1050°C. In steel plates containing one or more of Cu, Sb, and Sn, a low-melting-point compound phase forms at the interface between the steel plate surface and the scale when the steel material used for rolling is reheated. This low-melting-point compound liquefies and penetrates the grain boundaries of the steel plate, causing cracks in the steel plate. The penetration of the liquefied low-melting-point compound into the grain boundaries of the steel plate occurs between 950°C and 1050°C. Therefore, it is preferable for the surface of the steel plate to remain in this temperature range for a short time, and a high heating rate is required for the steel plate surface. A heating rate of 1.5°C / min or higher significantly prevents the penetration of the liquefied low-melting-point compound into the grain boundaries, preventing surface defects during rolling. Therefore, the lower limit of the heating rate is set to 1.5°C / min. While there is no upper limit for the heating rate, a high heating rate promotes warpage during reheating of the steel material, so the heating rate is preferably set to 6.0°C / min or less. More preferably, the heating rate is set to 5.0°C / min or less.

[0047] The surface temperature of the steel material must be maintained at a temperature range of over 1050°C and below 1250°C for 30 minutes or more. In steel plates containing one or more of Cu, Sb, and Sn, a low-melting-point compound phase is formed at the interface between the steel plate surface and the scale when the rolled steel material is reheated. This low-melting-point compound liquefies and penetrates into the grain boundaries of the steel plate, causing cracks in the steel plate. This liquefied low-melting-point compound leaves the interface between the steel plate surface and the scale and diffuses into the scale at temperatures above 1050°C and below 1250°C. As a result, less liquefied low-melting-point compound remains at the interface between the steel plate surface and the scale, suppressing surface defects caused by the penetration of liquefied low-melting-point compounds into the grain boundaries. This effect is achieved by maintaining the surface temperature of the steel material at a temperature above 1050°C and below 1250°C for 30 minutes or more. Therefore, the lower limit of the holding time is set to 30 minutes. While there is no upper limit to the holding time, excessively long periods of time at high temperatures can cause the grains of the steel plate to coarsen, resulting in a deterioration in toughness. Therefore, the holding time is preferably set to 300 minutes or less. More preferably, the holding time is 180 minutes or less. Here, holding means that there is no problem even if the temperature rises or falls once or multiple times, as long as the surface temperature of the steel material is within a temperature range of more than 1050°C and not more than 1250°C.

[0048] After reheating, the steel material is hot-rolled. The hot-rolling temperature is not particularly specified, but for example, the final pass may be performed when the surface temperature is between 700 and 1000°C. Descaling is performed multiple times before and during hot rolling to remove scale. After hot rolling, accelerated cooling or air cooling may be performed, and the steel may then be reheated and subjected to heat treatment such as normalizing, quenching, and tempering.

[0049] 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]

[0050] 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.

[0051] Steels having the chemical compositions shown in Table 1 were melted using a converter furnace and made into steel materials by continuous casting. The steel materials were reheated under the conditions of the heating rate and holding time shown in Table 2, and hot-rolled under the conditions of a final pass rolling 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 in the range of 4 to 60 mm. The remainder of the chemical composition of each steel shown in Table 1 is iron (Fe) and unavoidable impurities.

[0052] [Table 1]

[0053] [Table 2]

[0054] The resulting steel plates were cut, and samples were taken, including the steel plate surface and its adjacent cross-section. The cross-sections were polished and subjected to observation. Specifically, the sum of the mass-based contents of Cu, Sb, and Sn was measured for compounds observed at the interface between the steel plate surface and the scale. Measurements were performed using an electron probe microanalyzer (EPMA) or a scanning electron microscope (SEM) and an energy dispersive X-ray analyzer (EDS). The volume fraction (the ratio of the compound to the interface line) of compounds where the sum of the mass-based contents of Cu, Sb, and Sn was 50% or greater was then measured from SEM images. Measurements were performed by taking 1000x magnification photographs of a 2-mm-long interface, and the volume fraction determined from that region was used; a value of 5% or less was considered acceptable.

[0055] 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.

[0056] 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.

[0057] 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".

[0058] 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.

[0059] 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".

[0060] 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.

[0061] The unit of volume, "L", is 10 -3 m 3 "x~y" representing a range of numerical values ​​means from x to y, inclusive, and includes the boundary value. [Industrial Applicability]

[0062] 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

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: 0.10-0.40%, Cr: 0.26-1.00%, Sb: 0.01 to 0.10%, and N: 0.006% or less Contains Optionally, Ni: 0.50% or less, 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: 0.10% 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; The composition satisfies the relationship represented by the following formula (1), and the Ceq calculated by the following formula (2) is in the range of 0.331 to 0.400 mass%, A thick steel plate in which a compound phase having a sum of the contents of Cu, Sb and Sn of 50% or more is formed at the interface between the surface of the thick steel plate and scale adhering to the surface at a frequency of 5% or less in terms of space factor. [Formula (1)] Cu+Co+W+2Sn+2Sb≧0.30% by mass [Formula (2)] 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.

2. A method for producing a thick steel plate, comprising: a heating step of reheating a steel material having a chemical composition defined in claim 1; and a hot rolling step of hot rolling the reheated steel material to a desired dimension, In the heating step, the surface temperature of the steel material is raised at a temperature rise rate of 1.5°C / min or more in a temperature range of 950°C to 1050°C during heating, and the surface temperature of the steel material is maintained in a temperature range of more than 1050°C and not more than 1250°C for 30 minutes or more, after which the reheating is terminated.

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