Method for suppressing red embrittlement cracking and scale indentation of steel material containing cu and ni

Abstract

To provide a method for suppressing red heat embrittlement cracking in the production of a steel containing Cu and Ni, particularly in a hot rolling stage, and also suppressing the generation of scale indentation flaws caused by the degradation in descaling properties.SOLUTION: A method for suppressing red heat embrittlement cracking and scale indentation when producing a steel containing Cu and Ni, the Cu-containing steel having a Cu equivalence CCuEq (mass%) of a base metal of 0.15% or more and 3.00% or less and a Ni concentration CNi (mass%) of 0.05% or more and half or less of the Cu equivalence CCuEq (mass%), the method comprising: mixing a Si-containing material having a Si amount per surface area of the steel of 200 to 1200g / m2 with an iron oxide or iron hydroxide having a Fe amount of 0.3 to 20.0% of the Si amount to obtain a mixture; The mixture is disposed on a steel material surface of the steel material, the steel material is heated in a temperature range of 1170 °C or higher, descaling is performed, and then hot rolling is performed.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for suppressing red embrittlement cracking and scale indentation defects in steel materials containing Cu and Ni. [Background technology]

[0002] In the long term, global demand for steel has been expanding, and this trend is expected to continue. As a result, the prices of raw materials such as iron ore are also rising. Therefore, it is more important than ever to utilize scrap steel as a source of iron for steel. Generally, when steel is produced from recycled scrap, the scrap contains impurities that cannot be separated and are harmful and difficult to remove, so-called tramp elements. Among tramp elements, metals more noble than Fe, such as Cu and Sn, are known to cause red embrittlement cracking during hot rolling.

[0003] The mechanism by which red embrittlement occurs is roughly as follows. Generally, in the production of steel by hot rolling, the steel is loaded into a heating furnace and heated with combustion gas for 1 to 5 hours to a temperature of approximately 1000 to 1300°C before being removed from the heating furnace. The oxide scale is then removed (descaling) with high-pressure water before the hot rolling begins. Because the combustion gas supplied to the heating furnace typically contains oxidizing gases such as oxygen, water vapor, and carbon dioxide, an oxide scale layer forms on the surface of the steel heated to high temperatures in the heating furnace. This oxide scale layer is primarily composed of iron oxides and generally consists of three layers: hematite (Fe2O3), magnetite (Fe3O4), and wüstite (FeO), from the surface down. When iron is oxidized by the oxidizing gases in the combustion gas at high temperatures, if it contains metals more noble than iron, such as Cu or Sn, these metals do not oxidize but concentrate at the interface between the oxide scale and the base steel. Cu only has a solubility of a few percent in gamma iron, and if the Cu concentration exceeds this level, Cu appears as a metallic phase. The melting point of Cu is approximately 1080°C, and steel materials are typically heated to temperatures above this before hot rolling. This causes a liquid phase of molten Cu to form at the oxide scale / base steel interface, penetrate the grain boundaries of the base steel, and become unable to withstand the shear and tensile stresses that occur during hot rolling, resulting in surface cracking, or red-hot embrittlement cracking.

[0004] It is known that adding Ni in an amount roughly half the amount of Cu is effective in preventing red embrittlement cracking caused by Cu. This is because adding Ni increases the solid solubility limit of Cu in γ-iron and also raises the melting point of the Cu-enriched phase, thereby suppressing the appearance of Cu at the oxide scale / base steel interface. However, because Ni is a rare and expensive element, it is preferable to add as little Ni as possible from the perspective of manufacturing costs. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2017-087282 [Patent Document 2] Japanese Patent Application Publication No. 6-297025 [Patent Document 3] Japanese Patent Application Publication No. 2017-19013 [Patent Document 4] Japanese Patent Application Publication No. 2018-94624 Summary of the Invention [Problem to be solved by the invention]

[0006] In order to promote the production of steel that recycles scrap, as described above, methods for producing Cu-containing steel that can prevent red brittleness during hot rolling without actively adding Ni are also being investigated. As an example, Patent Document 1 discloses coating the surface of a steel sheet with an antioxidant (a mixture of refractory powder such as SiC, a binder such as silica, and a solvent such as water) during hot rolling. It describes how this antioxidant reduces the amount of Cu enrichment at the interface between the oxide scale and the base metal, thereby suppressing red brittleness. Furthermore, as the term "antioxidant" is mentioned, Patent Document 1 also teaches avoiding heating in an oxidizing atmosphere.

[0007] Patent Document 2 discloses that in order to prevent surface cracking of Cu-containing steel, a flux containing SiO2 is applied to the surface of a continuously cast slab, followed immediately by hot rolling at a temperature of 1150°C or higher. The flux reacts with the FeO scale on the surface of the steel (slab) to form an oxide liquid, which then absorbs molten Cu, which is thought to be the cause of cracking, preventing cracking. Patent Document 2 also proposes not adding Ni at all, as Ni increases costs.

[0008] On the other hand, Ni is sometimes contained in scrap and is sometimes added to steel to improve its strength, hardness, corrosion resistance, and toughness. As mentioned above, the addition of Ni is also known to suppress red embrittlement cracking. However, while the addition of Ni can suppress red embrittlement cracking, Ni also has the effect of increasing the adhesion between scale and steel (base metal), which may reduce the descaling ability of the steel and lead to concerns about the occurrence of scale indentation marks during rolling.

[0009] Patent Document 3 discloses that in order to improve the descaling properties of Ni-containing steel and suppress the occurrence of surface defects, Si-containing substances are placed on the surface of the steel before and / or during heating, the steel is heated to a temperature of 1170°C or higher, and then high-pressure water descaling is performed. The Si-containing substances react with Fe in the steel to produce Si-Fe liquid-phase oxides in the scale, which facilitates descaling.

[0010] Furthermore, Patent Document 4 discloses that in order to further improve the descaling properties of Ni-containing steel, an Fe-containing material is added to an Si-containing material, and the steel is placed on the surface of the steel, heated at a temperature of 1170°C or higher, and then subjected to high-pressure water descaling. It is disclosed that the method of Patent Document 4 makes it possible to further shorten the heating time.

[0011] However, Patent Documents 3 and 4 neither describe nor suggest application to steel materials containing Cu, nor do they describe or suggest red embrittlement cracking.

[0012] To summarize the above situation, scrap contains Cu, Ni, and the like. Cu can cause red embrittlement cracking, while Ni can reduce descaling ability and cause scale indentation defects. Therefore, a means for solving both of these problems is desired.

[0013] Therefore, the inventors set out to provide a method for suppressing red embrittlement cracking and also suppressing the occurrence of scale indentation defects due to a decrease in descaling ability during the production of steel material containing Cu and Ni, particularly during the hot rolling process. [Means for solving the problem]

[0014] The gist of the present invention completed by the present inventors is as follows.

[0015] [1] A method for suppressing red embrittlement cracking and scale indentation defects when producing a steel material containing Cu and Ni, comprising: The steel material has a Cu equivalent of C CuEq (mass%) is 0.15% or more and 3.00% or less, and the Ni concentration C Ni (mass%) is 0.05% or more of the Cu equivalent C CuEq (mass%) is less than half of The amount of Si per surface area of ​​the steel material is 200 to 1200 g / m 2and an iron oxide or iron hydroxide having an Fe content of 0.3 to 20.0% of the Si content to obtain a mixture, A method for suppressing red embrittlement cracking and scale indentation defects, comprising placing the mixture on the surface of the steel material, heating the steel material at a temperature range of 1170°C or higher, then descaling, and then hot rolling. [2] The method according to [1], wherein the Si-containing substance is any one of metallic Si, SiC, SiN, and SiO2, or a combination thereof. [3] The method according to [1] or [2], wherein the iron oxide is wustite, magnetite, hematite, margemite, or a mixture thereof, and the iron hydroxide is iron (II) hydroxide, iron (III) hydroxide, goethite, akaganite, lepidocrocite, ferroxite, or a mixture thereof. [Effects of the Invention]

[0016] According to the present invention, a high tramp element material, specifically, a Cu equivalent C CuEq (mass%) is 0.15% or more and 3.00% or less, and the Ni concentration C Ni (mass%) is 0.05% or more of the Cu equivalent C CuEq When hot rolling a steel containing Cu at a content of not more than half (mass %) of the Cu content of the steel, red embrittlement cracking and scale indentation defects can be suppressed. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described.

[0018] One embodiment of the present invention is a method for suppressing red embrittlement cracking and scale indentation defects when producing a steel material containing Cu and Ni, comprising: The steel material has a Cu equivalent of C CuEq (mass%) is 0.15% or more and 3.00% or less, and the Ni concentration C Ni (mass%) is 0.05% or more of the Cu equivalent C CuEq(mass%) is less than half of The amount of Si per surface area of ​​the steel material is 200 to 1200 g / m 2 and an iron oxide or iron hydroxide having an Fe content of 0.3 to 20.0% of the Si content to obtain a mixture, and the mixture is placed on the surface of the steel material, and the steel material is heated at a temperature of 1170°C or higher, and then descaling is performed, followed by hot rolling.

[0019] First, the general mechanism of how red embrittlement cracking occurs in steel materials containing Cu will be described. Generally, in the production of steel by hot rolling, the steel is loaded into a heating furnace and heated with combustion gas for 1 to 5 hours to a temperature of approximately 1000 to 1300°C before being removed from the heating furnace. The oxide scale is then removed (descaling) with high-pressure water before the hot rolling begins. Because the combustion gas supplied to the heating furnace typically contains oxidizing gases such as oxygen, water vapor, and carbon dioxide, an oxide scale layer forms on the surface of the steel heated to high temperatures in the heating furnace. This oxide scale layer is primarily composed of iron oxides and generally consists of three layers: hematite (Fe2O3), magnetite (Fe3O4), and wüstite (FeO), from the surface down. When iron is oxidized by the oxidizing gases in the combustion gas at high temperatures, if it contains metals more noble than iron, such as Cu or Sn, these metals do not oxidize but concentrate at the interface between the oxide scale and the base steel. Cu only has a solubility of a few percent in gamma iron, and if the Cu concentration exceeds this level, Cu appears as a metallic phase. The melting point of Cu is approximately 1080°C, and steel is typically heated to temperatures above this before hot rolling. This causes a liquid phase of molten Cu to form at the oxide scale / base steel interface, penetrate the grain boundaries of the base steel, and become unable to withstand the shear and tensile stresses that occur during hot rolling, resulting in surface cracking, or red-hot embrittlement cracking.

[0020] In addition, we will explain the general mechanism by which descaling ability decreases and scale indentation defects occur in steel materials containing Ni, which is another premise. As described above, an oxide scale layer is formed on the surface of the steel material heated to a high temperature in a heating furnace. Generally, the oxide scale can be distinguished into an outer layer scale and an inner layer scale. The outer layer scale exists on the atmosphere side, and the inner layer scale exists on the steel material (base metal) side. Further, the outer layer scale is a scale mainly grown by the outward diffusion of iron and has a dense structure, while the inner layer scale is a scale mainly grown by the inward diffusion of oxygen and has many voids inside.

[0021] In a steel material containing Ni, since Ni is nobler than Fe, Fe is selectively oxidized, and Ni is left behind and gradually concentrated in the inner layer scale. The inner layer scale produces an effect (anchor effect) in which the Ni-concentrated part holds the scale to the steel like a wedge. As a result, it becomes difficult to remove the scale, and even when descaling with high-pressure water is performed, it is difficult to remove the entire scale. The scale that could not be removed causes scale-pressed-in defects in subsequent rolling.

[0022] <Regarding the component composition of the base metal of the steel material containing Cu and Ni> The component composition of the base metal of the Cu-containing steel material used in the present embodiment will be described. The Cu equivalent C CuEq (mass%) is 0.15% or more and 3.00% or less, and the Ni concentration C Ni (mass%) is 0.05% or more and half or less of the above Cu equivalent C CuEq (mass%).

[0023] (Cu equivalent C CuEq (mass%): 0.15% or more and 3.00% or less) The steel material containing Cu and Ni targeted by the present embodiment has a Cu equivalent C of the base metal CuEq (mass%) of 0.15% or more and 3.00% or less. The Cu equivalent C CuEq (mass%) is obtained by multiplying the Cu concentration C Cu (mass%) by a predetermined coefficient and adding the Sn concentration C Sn (mass%), and is represented by the following formula. C CuEq (mass%) = C Cu (mass%) + 4·CSn (mass%) When Sn is contained in steel containing Cu, the susceptibility to surface red shortness increases, resulting in more severe surface cracking. This is because the inclusion of Sn lowers the solid solubility limit of Cu in Fe, and also lowers the melting point of Cu alone, which is approximately 1080°C. Therefore, when examining whether red shortness cracking will occur, the effect of Sn as well as Cu must be taken into account. If the Cu equivalent is less than 0.15% by mass, red shortness due to Cu will not occur even when heated in a normal heating furnace, so the lower limit of the Cu equivalent is set to 0.15% or more. Furthermore, if the Cu equivalent exceeds 3.00% by mass, red shortness during rolling is almost unavoidable, so the upper limit of the Cu equivalent is set to 3.00% or less. Here, C Sn (mass%) may be 0.00%, in which case the Cu equivalent C CuEq (mass%) is essentially the Cu concentration C Cu (% by mass).

[0024] (Cu concentration C Cu (mass%): May be 0.15% or more and 3.00% or less) In addition, the steel material containing Cu and Ni that is the subject of this embodiment has a Cu concentration C Cu The Cu concentration C (mass%) of the base material of the steel may be 0.15% or more and 3.00% or less. Cu The Cu content (mass%) may be limited because, if the Cu concentration is 0.15 mass% or more, red brittleness tends to occur regardless of whether Sn is present, and if the Cu concentration exceeds 3.00 mass%, red brittleness during rolling is almost unavoidable regardless of whether Sn is present.

[0025] (Ni concentration C Ni (mass%): 0.05% or more 0.5 C CuEq (mass%) or less) Furthermore, the steel material containing Cu and Ni that is the subject of this embodiment has a Ni concentration C Ni (mass%) is the Cu equivalent C CuEqThe upper limit of the Ni concentration is set to half of the Cu equivalent. In other words, C Ni (mass%)≦0.5·C CuEq (% by mass). Since Ni is an expensive element, C is used from the viewpoint of low-cost production. Ni (mass%) may be lowered. Ni (mass%)≦0.3·C CuEq (mass%), preferably C Ni (mass%)≦0.2·C CuEq (% by mass) may also be used. On the other hand, Ni is sometimes contained in scrap and is sometimes added to steel materials to improve their strength, hardness, corrosion resistance, and toughness. Therefore, the steel materials containing Cu and Ni that are the subject of this embodiment have a Ni concentration C Ni (mass%) is 0.05% or more. Ni If the Ni concentration (mass%) is less than 0.05%, the decrease in descaling ability due to Ni is slight, and the problem of scale indentation may not occur. Ni (% by mass) may be preferably 0.10% or more, and more preferably 0.20% or more.

[0026] The balance may be Fe and unavoidable impurities. Elements for modifying the properties of the steel material may be further added within a range that does not affect the effects of the present invention.

[0027] In one embodiment of the present invention, the amount of Si per surface area of ​​the steel material is 200 to 1200 g / m 2 and an iron oxide or iron hydroxide having an Fe content of 0.3 to 20.0% of the Si content to obtain a mixture, The mixture is placed on the surface of the steel material, and the steel material is heated to a temperature range of 1170°C or higher, then descaling is performed, and then hot rolling is performed.

[0028] This allows the scale, which has been made less descalable by the Ni-enriched layer, to be transformed into a structure that is easier to descale, and since the scale can be easily peeled off by descaling, it also helps to suppress scale indentation scratches.Furthermore, not only the scale but also the Cu-enriched layer can be easily removed from the steel surface, which also helps to suppress red embrittlement cracking.

[0029] The mechanism is explained below. First, when a mixture of Si inclusions and iron oxide or iron hydroxide arranged on the steel surface is heated, a composite oxide (Si-Fe oxide) is formed. More specifically, the Si inclusions arranged on the steel surface are oxidized to form silica (SiO2) during heating. Meanwhile, the oxidation of Fe in the iron oxide or iron hydroxide forms wüstite (FeO). Silica (SiO2) and wüstite (FeO) react to form Si-Fe oxide. Since the eutectic point of FeO and Fe2SiO4 is 1170°C, the composite oxide (Si-Fe oxide) becomes liquid at temperatures above 1170°C. This liquid phase component penetrates into the scale, particularly the inner scale layer, which has many voids inside. The inner scale layer, which has been penetrated by the liquid phase component, can be easily peeled off from the steel (base metal) using high-pressure water or other methods, making descaling easy. The inner scale layer is formed on the steel material (base metal) side, and the outer scale layer formed thereon (on the atmosphere side) is also easily descaled. Therefore, both the inner and outer scale layers are easily removed by descaling with high-pressure water or the like, and no scale remains on the steel material surface (base metal surface), which suppresses the occurrence of indentation defects in the subsequent hot rolling process.

[0030] The atmosphere in which the mixture of the Si-containing material and the iron oxide or iron hydroxide is heated is not particularly limited as long as it causes an oxidation reaction, and may be an oxidizing atmosphere such as air or combustion gas. Normally, the combustion gas supplied to the heating furnace contains oxidizing gases such as oxygen, water vapor, and carbon dioxide, and therefore causes an oxidation reaction.

[0031] Furthermore, at temperatures above 1170°C, a Cu-enriched layer that causes red embrittlement cracking exists at the interface between the inner scale layer and the steel material, but because the melting point of Cu is 1080°C, the Cu-enriched layer is also in a liquid phase. Therefore, the Cu-enriched layer is easily descaled along with the inner scale layer, preventing red embrittlement cracking in the subsequent hot rolling process.

[0032] The upper limit of the heating temperature is not particularly limited as long as the Si-Fe oxide and Cu-enriched layer are in a liquid phase. Generally, a higher heating temperature is preferable because it reduces the viscosity of the liquid phase and facilitates descaling. However, if the heating temperature is too high, the viscosity reduction effect saturates, causing increased scale loss and resulting in a loss of thermal energy. Therefore, the upper limit of the heating temperature may be set to 1300°C.

[0033] The amount of Si contained in the steel surface is 200 to 1200 g / m2 in terms of the amount of Si per surface area of ​​the steel. 2 Here, unless otherwise specified, the term "surface of a steel material" refers to the top surface (front surface) of the steel material when placed horizontally, and a mixture of Si-containing materials and iron oxide or iron hydroxide is disposed on the surface of the steel material. Furthermore, unless otherwise specified, the term "surface area of ​​a steel material" refers to the area of ​​the top surface (front surface) of the steel material when placed horizontally.

[0034] The amount of Si is the amount of Si contained (g / m 2 ) × (total atomic weight of Si per molecule of Si inclusion ÷ molecular weight of Si inclusion). 2If the Si content is less than 400 g / m, even if the Si-Fe composite oxide becomes liquid, it may not penetrate sufficiently into the entire scale and into the scale / steel (base metal) interface, and descaling performance may not improve. From the viewpoint of improving descaling performance, a higher Si content is preferable because it generates more composite oxide and increases the amount of liquid phase. Therefore, the amount of Si content placed on the steel surface should be 400 g / m 2 From the viewpoint of improving descaling properties, the upper limit of the amount to be disposed is not particularly limited. However, the amount to be disposed is not limited to 1200 g / m 2 If the amount exceeds 1200 g / m, it may be difficult to arrange the Si inclusions on the steel surface, or the Si inclusions may not be in sufficient contact with the atmosphere, making it difficult for the reaction to form the complex oxide to proceed. 2 is the upper limit.

[0035] The Si-containing material is not particularly limited as long as it contains Si, and any one of metallic Si, SiC, SiN, SiO2, or a combination thereof can be used. Silica (SiO2) may also be used because it is stable, easy to handle, readily available, and inexpensive. Furthermore, the Si-containing material is preferably in a paste form. Being in a paste form provides a moderate viscosity, making it easy to increase the amount of Si-containing material and the mixed Fe-containing material without spilling out of the steel material, and also facilitating application to the steel surface.

[0036] The Si-containing material may be mixed with iron oxide or iron hydroxide in an amount of Fe of 0.3 to 20.0% of the Si amount to form a mixture, and the mixture may then be placed on the surface of the steel material.

[0037] When heated, Si-containing materials, together with Fe, form a complex oxide (Si-Fe oxide), which becomes liquid and improves descaling properties. By premixing Si-containing materials with iron oxide or iron hydroxide and then placing the mixture on the steel surface, the Si-containing materials can react with the Fe contained in the mixed iron oxide or iron hydroxide. This promotes the formation of the complex oxide (Si-Fe oxide) and its subsequent liquid phase. Because the Si compound is premixed with iron oxide or iron hydroxide, the liquid complex oxide quickly penetrates the entire scale. This further improves descaling properties. As a result, the occurrence of scale indentation defects and red-hot embrittlement cracking are further suppressed.

[0038] When Si-containing materials are mixed with iron oxide or iron hydroxide, the Si content of the Si-containing materials must be 400 g / m 2 It is preferable that the density is 400 g / m or more. 2 By mixing the above-mentioned Si-containing material with iron oxide or iron hydroxide at a concentration corresponding to the Si-containing material, a sufficient amount of composite oxide is generated, and the amount of liquid phase increases, which is preferable. Regarding the upper limit of the amount of Si-containing material to be disposed, even when mixed with iron oxide or iron hydroxide, the amount of Si-containing material to be disposed is 1200 g / m 2 If the amount exceeds 1200 g / m, it may be difficult to arrange the Si inclusions on the steel surface, or the Si inclusions may not be in sufficient contact with the atmosphere, making it difficult for the reaction to form the complex oxide to proceed. 2 is the upper limit.

[0039] The iron oxide may be wustite, magnetite, hematite, margemite, or a mixture thereof. Also, the iron hydroxide (Fe-containing substance) may be iron(II) hydroxide, iron(III) hydroxide, goethite, akaganite, lepidocrocite, ferroxite, or a mixture thereof.

[0040] Silicon-containing materials oxidize during the heating process, forming silica (SiO2), which then reacts with wüstite (FeO) to form Si-Fe oxides, which readily form a liquid phase. Therefore, among iron oxides, wüstite is desirable because it readily forms a liquid phase with silicon-containing materials. However, other iron oxides can also achieve the same effect by becoming wüstite when placed on the steel surface in an oxidizing atmosphere at temperatures above 1170°C. Iron hydroxides can also be dehydrated and become wüstite when heated at high temperatures. Furthermore, iron oxides or iron hydroxides (Fe-containing materials) are preferably in powder form to facilitate mixing with silicon-containing materials.

[0041] The mixture of Si-containing material and iron oxide or iron hydroxide may contain other components such as binders, paints, and various additives, as long as the effects of the present invention are not impaired. It is preferable to limit the amount of these other components if they may affect the scale properties. Typically, B (boron), P (phosphorus), and S (sulfur) can lower the eutectic point of FeO and Fe2SiO4, so these elements and the components containing them may be limited. As a guideline, B content should be 4 g / m2 in terms of the amount of B per surface area of ​​the steel material. 2 The P content may be less than 8 g / m2 in terms of P amount per steel surface area. 2 , and the amount of S contained in the steel material is less than 8 g / m 2 It may be less than. [Example]

[0042] The present invention will be described in detail below based on examples, but the technical scope of the present invention is not limited to the following examples.

[0043] As examples and comparative examples, an experiment was conducted in which a steel material containing Cu and Ni was cast and hot-rolled to produce a steel sheet, and the presence or absence of red embrittlement cracking and scale indentation defects on the surface of the obtained steel sheet was investigated. Prior to hot rolling, the steel sheet was heated in an air atmosphere at a constant temperature of 1000 to 1300°C for a predetermined heating time, followed by descaling, and then hot rolling.

[0044] Table 1 shows the Cu concentration (mass%) of the base material, Cu equivalent (mass%) of the base material, Ni concentration (mass%) of the base material, heating atmosphere, heating temperature (°C), heating time (hours), and Si content (g / m 2 ) and Fe content / Si content (%). Red embrittlement cracking and scale indentation defects on the surface of the hot-rolled steel sheets were evaluated using the following indices, and the results are also shown in Table 1.

[0045] (Evaluation index for red embrittlement cracking and scale indentation) ◎: Neither cracks nor indentations were observed ○: Fine indentation scratches (depth or length 200 μm or less) were confirmed in cross-sectional observation. ×: Cracks or indentations (depth or length exceeding 200 μm) were observed in cross-section observation.

[0046] [Table 1]

[0047] In the present invention, the Si content is 400 to 1200 g / m 2 When the Si-containing material and iron oxide or iron hydroxide with an Fe content of 0.3 to 20.0% of the Si content were used (Nos. 1 to 10), no cracks or indentations that would cause problems in terms of quality or appearance occurred.

[0048] Si content: 200 to 400 g / m 2In the cases where Si-containing materials were used (Nos. 11 and 12) and where no iron oxide or iron hydroxide was used (Nos. 13 and 14), no cracks occurred, but cross-sectional observation confirmed fine indentation scratches (depth or length of 200 μm or less). In these cases, it is presumed that sufficient liquid Si-Fe oxide had formed to suppress cracking.

[0049] On the other hand, the amount of Si content is 200 g / m 2 When the heating temperature was less than 1170°C (Nos. 15, 17, 19, 20, 21, and 24), or when the heating temperature was less than 1170°C (Nos. 16, 18, 22, and 23), cracks and / or indentations (depth or length exceeding 200 μm) were confirmed by cross-sectional observation. It is thought that when the amount of Si inclusions is small, the amount of liquid Si-Fe oxides formed is insufficient to incorporate Cu into the scale or to improve descaling ability, and that when the heating temperature is low, liquid Si-Fe oxides themselves are not formed.

[0050] It is clear that the present invention can suppress red embrittlement cracking and scale indentation defects during hot rolling even in steel materials containing Cu and Ni.

Claims

1. A method for suppressing red embrittlement cracking and scale indentation defects when producing a steel material containing Cu and Ni, comprising: The steel material has a Cu equivalent of C ＣｕＥｑ (mass%) is 0.15% or more and 3.00% or less, and Ni concentration C Ｎｉ (mass%) is 0.05% or more ＣｕＥｑ (mass%) or less, The amount of Si per surface area of ​​the steel material is 200 to 1200 g / m ２ and an iron oxide or iron hydroxide having an Fe content of 0.3 to 20.0% of the Si content to obtain a mixture, A method for suppressing red embrittlement cracking and scale indentation defects, comprising placing the mixture on a surface of the steel material, heating the steel material in a temperature range of 1170°C or higher, then descaling, and then hot rolling.

2. The Si-containing substance is selected from metal Si, SiC, SiN, and SiO ２ 2. The method of claim 1, wherein the method is any one of the following or a combination thereof:

3. 3. The method according to claim 1, wherein the iron oxide is wustite, magnetite, hematite, margemite, or a mixture thereof, and the iron hydroxide is iron (II) hydroxide, iron (III) hydroxide, goethite, akaganite, lepidocrocite, ferroxhyte, or a mixture thereof.

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