Method for producing cu-containing steel material

Abstract

To provide a new method for producing a Cu-containing steel material in which red heat embrittlement is suppressed, which does not require the positive addition of Ni, in which the limitation of heating temperature is less than that in the conventional method, and which does not require scale suppression by a coating agent or the like, in the production of the Cu-containing steel material.SOLUTION: Mixing a Si-containing material and at least one of a B-containing material, a P-containing material, and a S-containing material to obtain a mixture, the steel material having a Cu equivalent of 0.15% or more and 3.00% or less; Here, the Si-containing material is mixed with a Si amount of 400 to 1200g / m2 per surface area of the steel, the B-containing material is mixed with a B amount of 4 to 40g / m2 per surface area of the steel, the P-containing material is mixed with a P amount of 8 to 80g / m2 per surface area of the steel, and the S-containing material is mixed with a S amount of 8 to 80g / m2 per surface area of the steel, the mixture is disposed on the surfaces of the steel, heating is performed in a temperature range of 1000 °C to 1300 °C, and then descaling is performed.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a manufacturing method for suppressing red embrittlement cracking of Cu-containing steel material, and in particular to a novel manufacturing method for Cu-containing steel material that does not require the active addition of Ni, strict temperature control, or suppression of scale formation that have been used in the past. [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 by recycling scrap, the scrap contains impurities, so-called tramp elements, which are difficult to separate and which are harmful and difficult to remove. 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 furnace. The oxide scale is then removed (descaled) with high-pressure water before being hot rolled. 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 (Fe) oxide 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 noble metals are less likely to oxidize and concentrate at the interface between the oxide scale and the base steel. Cu only has a solubility of a few percent in gamma iron, but 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 temperature 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 that 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 Cu, thereby suppressing the concentration 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. 6-297025 [Patent Document 2] Japanese Patent Application Publication No. 6-297026 [Patent Document 3] Japanese Patent Application Publication No. 2017-087282 [Patent Document 4] Japanese Patent Application Publication No. 05-222447 [Patent Document 5] Japanese Patent Application Publication No. 05-237509 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, a method for producing Cu-containing steel that can prevent red shortness during hot rolling without actively adding Ni has been investigated.

[0007] As an example, the inventors discovered that by placing Si, an element other than Ni, on the surface of a Cu-containing steel material and then heating it to 1170°C or higher and then descaling it, it is possible to suppress red embrittlement caused by Cu during hot rolling.

[0008] Although we do not wish to be bound by any particular theory, the mechanism is thought to be as follows. First, when Si inclusions located on the steel surface and Fe in the steel are heated in an oxidizing atmosphere, a composite oxide (Si-Fe oxide) is formed. More specifically, the Si inclusions located on the steel surface are oxidized to form silica (SiO2) during heating. Meanwhile, the oxidation of Fe in the steel forms scale containing wüstite (FeO). Silica (SiO2) reacts with wüstite (FeO) to form Si-Fe oxides. Since the eutectic point of FeO and Fe2SiO4 is 1170°C, the composite oxide (Si-Fe oxide) becomes a liquid phase component at temperatures above 1170°C. This liquid phase component penetrates into the scale. Scale penetrated by the liquid phase component can be easily peeled off from the steel (base steel) using high-pressure water, making descaling easy. At temperatures of 1170°C or higher in an oxidizing atmosphere, a Cu-enriched phase that causes red embrittlement cracking exists at the interface between the scale and the steel, but because the melting point of Cu is approximately 1080°C, the Cu-enriched phase is also a liquid phase. Therefore, the Cu-enriched phase is easily descaled along with the scale, preventing red embrittlement cracking in the subsequent hot rolling process.

[0009] The above method requires heating at 1170°C or higher. However, depending on the structure and composition of the desired steel material, such a temperature restriction may not be desirable. In other words, a method with fewer temperature control restrictions is desirable.

[0010] As an example of a method for preventing red embrittlement, Patent Document 1 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), forming an oxide liquid. This oxide liquid then absorbs Cu molten liquid, which is thought to be the cause of cracking, preventing the occurrence of cracking. Patent Document 1 also proposes not adding Ni at all, as Ni increases costs. Furthermore, the method of Patent Document 1 requires heating to 1150°C or higher.

[0011] Patent Document 2 proposes that, instead of applying a flux containing SiO2 as in Patent Document 1, Si is added as an alloy component to generate a SiO2-FeO-based low-melting-point oxide liquid scale, which then traps molten Cu and suppresses cracking. However, like Patent Document 1, Patent Document 2 requires heating to 1150°C or higher.

[0012] Patent Document 3 discloses that an antioxidant (a mixture of refractory powder such as SiC, Al2O3, ZrO2, CaO, MgO, etc., a binder such as silica, and a solvent such as water) is applied to the surface of the steel sheet during hot rolling. It describes that by suppressing oxidation of the steel sheet surface with this antioxidant, the amount of Cu concentration at the interface between the oxide scale and the base steel is reduced, and red embrittlement is suppressed. Note that the antioxidant in Patent Document 3 cracks when the temperature exceeds 1250°C, and oxidation progresses.

[0013] Patent Document 4 proposes that a coating agent containing a heat-resistant glass component (preferably having a melting point of 1300°C or higher) is applied to a slab before heating, thereby suppressing the precipitation of metallic Cu-Sn alloys on the surface during hot working, thereby producing steel products free of cracks and surface defects. However, Patent Document 4 requires heating to a temperature at which the glass component melts, typically 1300°C or higher. In addition, glassy coating agents may adhere to continuous casting rolls or rolling rolls, preventing the desired surface quality from being achieved, which can result in a reduced yield of shipped products.

[0014] Patent Document 5 proposes reducing the content of glassy binder and using a coating agent containing 50% by weight of MgO, CaO, and Al2O3, either alone or in combination, to suppress the precipitation of metallic Cu and Sn alloy on the surface during hot working, thereby producing steel material free from cracks and surface defects. Patent Document 5 does not disclose details about the temperature, and the heating temperature in the examples is only 1200°C.

[0015] To summarize the above situation, scrap contains Cu, which can cause red-hot embrittlement cracking, and it is desirable to suppress such red-hot embrittlement cracking. Ni is effective in suppressing red-hot embrittlement cracking, but is expensive. Patent Documents 1 and 2 propose using Si to suppress red-hot embrittlement cracking, but require heating to 1150°C or higher. Patent Documents 3 to 5 also propose using a coating agent to prevent scale growth and Cu concentration and suppress red-hot embrittlement cracking, but each coating agent has its own temperature limit and has disadvantages such as the coating agent adhering to the roll.

[0016] Therefore, the inventors have set it as an object of the present invention to provide a method for producing Cu-containing steel (sometimes referred to as Cu-containing steel), particularly in the hot rolling process, that is unprecedented, i.e., new, does not require the active addition of Ni, has fewer heating temperature restrictions than conventional methods (typically, it is not necessary to heat the steel to 1150°C or higher as in conventional methods, but rather 1000°C or higher is sufficient), does not require measures to suppress scale formation using coating agents or the like, and suppresses red embrittlement. [Means for solving the problem]

[0017] The gist of the present invention completed by the present inventors is as follows.

[0018] [1] In the production of Cu-containing steel, the steel is heated in a heating furnace and then hot-rolled. The steel material has a Cu equivalent of C CuEq (mass%) is 0.15% or more and 3.00% or less, Mixing a Si-containing material with at least one of a B-containing material, a P-containing material, and a S-containing material to obtain a mixture; Here, the Si content of the steel material is 400 to 1200 g / m 2 , The B content of the steel material is 4 to 40 g / m 2 , The amount of P contained in the steel material is 8 to 80 g / m 2 , The amount of S contained in the S-containing material is 8 to 80 g / m2 per surface area of ​​the steel material. 2 The mixture is placing the mixture on the surface of the steel material; The heating is carried out in a temperature range of 1000°C to 1300°C, This is a manufacturing method for Cu-containing steel, followed by descaling. In the present invention, unless otherwise specified, "Cu-containing steel" refers to the final product manufactured by the manufacturing method of the present invention, and "steel" refers to an intermediate product or raw material that is heated in a heating furnace and hot-rolled to manufacture "Cu-containing steel." [2] The method for producing a Cu-containing steel material according to [1], wherein the Si-containing substance is any one of metal Si, SiC, SiN, and SiO2, or a combination thereof, the B-containing substance is B2O3, the P-containing substance is P2O5, and the S-containing substance is any one of FeS and MnS, or a combination thereof. [Effects of the Invention]

[0019] According to the present invention, a high tramp element steel material, specifically, a steel material having a Cu equivalent of C CuEq When a Cu-containing steel material having a Cu content (mass%) of 0.15% to 3.00% is hot-rolled, it can be produced while suppressing red embrittlement cracking. Furthermore, the present invention does not require the active addition of expensive Ni, has fewer heating temperature restrictions than conventional steels (typically, it is not necessary to heat the steel to 1150°C or higher as in conventional steels, but rather 1000°C or higher is sufficient), and does not require measures to suppress scale formation using coating agents or the like. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, an embodiment of the present invention will be described.

[0021] One embodiment of the present invention relates to a method for producing a Cu-containing steel material by heating the steel material in a heating furnace and then hot rolling the steel material, The steel material has a Cu equivalent of C CuEq (mass%) is 0.15% or more and 3.00% or less, Mixing a Si-containing material with at least one of a B-containing material, a P-containing material, and a S-containing material to obtain a mixture; Here, the Si content of the steel material is 400 to 1200 g / m 2 , The B content of the steel material is 4 to 40 g / m 2 , The amount of P contained in the steel material is 8 to 80 g / m 2 , The amount of S contained in the S-containing material is 8 to 80 g / m2 per surface area of ​​the steel material. 2 It is mixed with placing the mixture on the surface of the steel material; The heating is carried out in a temperature range of 1000°C to 1300°C, Thereafter, descaling is performed.

[0022] 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 raw 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 extracted 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 (Fe) oxide 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 noble metals are less likely to oxidize and concentrate at the interface between the oxide scale and the base steel. Cu only has a solubility of a few percent in gamma iron, but 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 temperature 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.

[0023] <About the composition of the base steel material> The composition of the base material of the steel material used in this embodiment will be described.CuEq (mass%) is 0.15% or more and 3.00% or less.

[0024] (Cu equivalent C CuEq (mass%): 0.15% or more, 3.00% or less) The steel material targeted in this embodiment is a steel material having a Cu equivalent of C CuEq (mass%) is 0.15% or more and 3.00% or less. Cu equivalent C CuEq (mass%) is the Cu concentration C Cu (mass%) multiplied by a predetermined coefficient Sn (% by mass) and is expressed by the following formula: C CuEq (mass%)=C Cu (mass%)+4×C Sn (mass%) When Sn is contained in the base material of a steel, 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 the melting point of Cu alone, which is approximately 1080°C, is lowered by the inclusion of Sn. Therefore, when examining whether red shortness cracking will occur, the effect of Sn as well as Cu must be taken into consideration. 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, the Sn concentration 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).

[0025] (Ni concentration C Ni (mass%): 0.02% or more, 0.5×C CuEq (may be less than % by mass) Furthermore, in one embodiment of the present invention, the Ni concentration C of the base material of the steel material Ni (mass%) is the Cu equivalent C CuEqNi has the effect of suppressing surface red shortness caused by Cu, and in order to prevent surface red shortness, Ni may be contained in an amount about half the normal Cu content. In steel materials containing Ni at a concentration about half the Cu equivalent, red shortness caused by Cu does not occur even when heated in a normal heating furnace, so the upper limit of the Ni concentration may be set to half the Cu equivalent. That is, C Ni (mass%)≦0.5×C CuEq It may also be expressed as (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 (mass%). As mentioned above, the Cu equivalent C CuEq (mass%) is 0.15% or more, so as a guideline, C Ni (mass%) may be 0.05% or less, or 0.03% or less. In addition, Ni may be contained in scrap, and it is not necessary to remove all of this Ni, and it may be used to suppress red shortness. 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 base material of the steel material targeted in one embodiment of the present invention has a Ni concentration of C Ni (mass%) may be 0.02% or more. Ni (% by mass) may be preferably 0.05% or more, more preferably 0.10% or more, and even more preferably 0.20% or more.

[0026] The balance of the base material of the steel may be Fe and unavoidable impurities. Elements for modifying the properties of the Cu-containing steel obtained by hot rolling may be further added to the base material of the steel, as long as they do not affect the effects of the present invention.

[0027] In one embodiment of the present invention, a mixture is obtained by mixing a Si-containing material with at least one of a B-containing material, a P-containing material, and an S-containing material. Here, the Si content of the steel material is 400 to 1200 g / m 2 , The B content of the steel material is 4 to 40 g / m 2 , The amount of P contained in the steel material is 8 to 80 g / m 2 , The amount of S contained in the S-containing material is 8 to 80 g / m2 per surface area of ​​the steel material. 2 are mixed. placing the mixture on the surface of the steel material; The heating is carried out in a temperature range of 1000°C to 1300°C, Then descaling is performed.

[0028] According to this embodiment, the scale can be easily peeled off by descaling, and not only the scale but also the Cu-enriched layer can be easily removed from the steel surface, which results in the suppression of red embrittlement cracking.

[0029] While not wishing to be bound by any particular theory, the mechanism is thought to be as follows. First, when Si inclusions located on the steel surface and Fe in the steel are heated in an oxidizing atmosphere, a composite oxide (Si-Fe oxide) is formed. More specifically, the Si inclusions located on the steel surface are oxidized to form silica (SiO2) during the heating process. Meanwhile, the oxidation of Fe in the steel forms wüstite (FeO). Silica (SiO2) and wüstite (FeO) react to form Si-Fe oxides, primarily Fe2SiO4. Because 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, penetrated by the liquid phase component, can be easily peeled off from the steel (base steel) using high-pressure water or other methods, making descaling easy. The inner scale layer is formed on the steel (base steel) side, and the outer scale layer formed on top of it (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. If a Cu-enriched layer that causes red embrittlement cracking is present at the interface between the inner scale layer and the steel, the Cu-enriched layer is also easily descaled together with the inner scale layer, thereby preventing red embrittlement cracking in the subsequent hot rolling process.

[0030] In this embodiment, at least one of a B-containing material, a P-containing material, and an S-containing material is mixed with a Si-containing material. The B-containing material, the P-containing material, and the S-containing material lower the eutectic temperature of FeO and Fe2SiO4, and the liquid phase generation reaction proceeds from an early stage during heating. In other words, the eutectic temperature of FeO and Fe2SiO4 can be lowered below 1170°C. It should be noted that the melting point of Cu, which is the cause of red embrittlement cracking, is approximately 1080°C, and that this melting point can be further reduced by, for example, the inclusion of Sn in the steel composition. In other words, in principle, red embrittlement cracking can occur at temperatures of approximately 1080°C or lower. Meanwhile, conventional techniques (e.g., Patent Documents 1 to 5) require heating to at least 1150°C or higher to suppress red embrittlement cracking. In contrast, in this embodiment, by mixing B-containing, P-containing, and S-containing materials, the eutectic point can be lowered, descaling can be easily performed at heating temperatures of 1000°C or higher, and red embrittlement cracking can be prevented at relatively low temperatures. Furthermore, even at the same heating temperature (e.g., 1000°C), when at least one of B, P, and S is contained, the amount of liquid phase generated is greater than when these are not contained, making descaling easier and increasing the effect of suppressing red embrittlement cracking.

[0031] (Si-containing material) The amount of Si contained in the steel surface is 400 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 in a horizontal position, and the Si-containing material (or a mixture of the Si-containing material and at least one of the B-containing material, the P-containing material, and the S-containing material, which will be described in detail later) 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 in a horizontal position.

[0032] 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). 2 If the Si content is less than 600 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 steel) 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 600 g / m2 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 The upper limit of the amount of the Si-containing material to be disposed may be adjusted as appropriate, and is, for example, 1100 g / m 2 , or 1000g / m 2 It may also be possible to use the following.

[0033] 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. The Si-containing material is preferably in a paste form. Being in a paste form provides an appropriate viscosity, making it easy to increase the amount of the Si-containing material and the mixed B-containing, P-containing, and S-containing materials without spilling out of the steel material, and also facilitating application to the steel surface.

[0034] (B-containing materials, P-containing materials, S-containing materials) At least one of a B-containing material, a P-containing material, and a S-containing material is mixed with a Si-containing material to form a mixture, which is then placed on the surface of a steel material.

[0035] When expressing the amount of B, P, and S inclusions per steel surface area, the amount of B per steel surface area is 4 to 40 g / m 2 , P content per steel surface area is 8 to 80 g / m 2 , S amount per steel surface area is 8 to 80 g / m 2 are mixed.

[0036] The lower limits for the amount of B-containing, P-containing, and S-containing materials are set based on the following considerations. B, P, and S have the effect of lowering the eutectic point of FeO and Fe2SiO4. In the case of B, the inventors have found that in order to obtain this effect, the concentration of B in the range of 1 cm deep from the surface of the steel material needs to be 0.005 mass% or more. They have also found that in the case of P and S, the concentrations need to be 0.01 mass% or more. Based on this finding, B, S, and P in the range of 1 cm deep from the surface of the steel material are supplied from B-containing materials, P-containing materials, and S-containing materials. In this case, when the density of Fe, the main component of the steel material, is 7.9 g / cm 3 Based on this, the lower limit of the amount of each ingredient mixed is set as follows. 7.9g / cm 3 ×1cm×0.005%=3.95×10 -4 g / cm 2 ≒4g / m 2 For P, S: 7.9g / cm 3 ×1cm×0.01%=7.9×10 -4 g / cm 2 ≒8g / m 2

[0037] On the other hand, the upper limit is set based on the following idea: Generally, the greater the amount of each of these elements added, the greater the effect of lowering the eutectic point. However, if too much is added, it may affect the properties of the steel, so the upper limit is set within a concentration range that is not thought to affect the properties of the steel. B is said to be an element effective in strengthening grain boundaries and increasing the strength of steel, but if the content is too high, workability may decrease, so the upper limit of the concentration is set at 0.05 mass%. If the S content is too high, workability may decrease, and the upper limit of the S content is set at 0.1% by mass. P tends to cause grain boundary embrittlement and deteriorates workability, so the upper limit of its concentration is set at 0.1 mass %. These effects are thought to extend to a depth of 1 cm from the surface of the steel material, and the density of Fe, the main component of steel, is 7.9 g / cm 3Based on this, the upper limits for the amount of B-containing materials, P-containing materials, and S-containing materials mixed are set as follows: For B: 7.9g / cm 3 ×1cm×0.05%=39.5×10 -4 g / cm 2 ≒40g / m 2 For , P, S: 7.9g / cm 3 ×1cm×0.1%=79×10 -4 g / cm 2 ≒80g / m 2

[0038] The B-containing substance is not particularly limited as long as it is a compound containing B, but in consideration of availability and ease of handling, B2O3 (boron oxide) may also be used. There are no particular limitations on the P-containing substance as long as it is a compound containing P, but in consideration of availability and ease of handling, P2O5 (diphosphorus pentoxide) may also be used. The S-containing substance is not particularly limited as long as it is a compound containing S, but in consideration of availability and ease of handling, either FeS (iron sulfide), MnS (manganese sulfide), or a combination thereof may be used.

[0039] The method for mixing the Si-containing material with at least one of the B-containing material, the P-containing material, and the S-containing material is not particularly limited as long as it can homogeneously mix these materials. For example, they may be mixed batchwise using a mixer, a shooter, a shovel, or the like, or they may be continuously mixed by being poured into a hopper, or the like.

[0040] Furthermore, the mixture of the Si-containing material and at least one of the B-containing material, the P-containing material, and the S-containing material may contain other substances, as long as the effects of the present invention are not affected. Typically, the amount of the other substances may be 10% or less, 8% or less, 6% or less, 4% or less, or 2% or less by mass, based on the total mass of the mixture.

[0041] Methods for applying the mixture include dipping, rolling, spraying, and bar coating. The solid mixture may be applied directly to the steel surface, or may be dispersed in a solvent and applied as a paint or slurry. The mixture may also be applied as an aqueous solution. A liquid paint is suitable for the application methods listed above, as it allows the mixture to be applied more uniformly on the steel surface and can more uniformly suppress red embrittlement cracking on the steel surface. The mixture may be dried after being applied on the steel. Since the mixture is dried and present in a solid state on the steel, it is preferable because the mixture is less likely to fall off the steel when the steel moves in a heating furnace.

[0042] The heating temperature is 1000° C. to 1300° C. The reason for specifying the temperature range is as follows.

[0043] The upper limit of the heating temperature is not particularly limited as long as the Si-Fe oxide and Cu-enriched layer become liquid phases. Generally, a higher heating temperature is preferable because it reduces the viscosity of the liquid phase, facilitates descaling, and enhances the effect of suppressing red embrittlement cracking. However, if the heating temperature is too high, the effect of reducing viscosity saturates, causing increased scale loss and resulting in a loss of thermal energy. Therefore, the upper limit of the heating temperature is set to 1300°C. The upper limit of the heating temperature may be adjusted appropriately depending on the steel type being handled and the properties required of the final product. For example, it may be 1280°C, 1260°C, 1240°C, 1220°C, 1200°C, 1190°C, 1180°C, 1170°C, 1160°C, 1150°C, or lower or less than these.

[0044] The lower limit of the heating temperature is not particularly limited as long as it is equal to or higher than the eutectic point of the Si-Fe oxides. Generally, the lower the heating temperature, the more energy-saving it is. However, if the heating temperature is too low, the viscosity of the liquid phase increases, making descaling difficult. Therefore, the lower limit of the heating temperature is set to 1000°C. The lower limit of the heating temperature may be adjusted appropriately depending on the type of steel being handled and the properties required of the final product, and may be, for example, 1020°C, 1040°C, 1060°C, 1080°C, 1100°C, or higher or exceeding these temperatures.

[0045] The above temperature range (1000°C to 1300°C) covers the heating temperature of steel for general hot rolling. In other words, the heating temperature in this embodiment is not particularly limited as long as it is within the heating temperature range for general hot rolling, and there are fewer restrictions on temperature control, which is advantageous compared to conventional methods with temperature restrictions.

[0046] Descaling is an operation in which water is sprayed onto the surface of a steel material from a nozzle to peel off and remove oxide scale formed on the surface of the steel material, and generally adopted operating conditions can be applied. Typically, the water pressure of the descaling water may be 3 MPa or more and 100 MPa or less. Descaling not only removes the oxide scale, but also removes the Cu liquid phase formed at the interface between the oxide scale and the steel substrate, thereby preventing subsequent red embrittlement cracking. [Example]

[0047] 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.

[0048] As examples of the present invention and comparative examples, an experiment was conducted in which Cu-containing steel materials were cast and hot-rolled to produce Cu-containing steel materials, and the presence or absence of red embrittlement cracking on the surface of the obtained steel materials was investigated. Prior to hot rolling, the steel materials were 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. In addition, in the sample steel, a mixture of a Si-containing material and one of B, P, and S-containing materials was placed under the conditions shown in Table 1 before heating.

[0049] Table 1 shows the Si content (g / m 2 ), Si content type, mixture type, B, P, S amount (g / m 2), Cu equivalent (mass%) of the steel, Cu concentration (mass%) of the steel, Sn concentration (mass%) of the steel, Ni concentration (mass%) of the steel, heating temperature (°C), and heating time (hours). Red embrittlement cracking on the surface of the steel obtained through hot rolling was evaluated using the following index, and the results are also shown in Table 1.

[0050] (Evaluation index for red embrittlement cracking) 〇: No cracks that affect quality or appearance ×: A crack with a depth or length of more than 200 μm was observed in the cross section.

[0051] [Table 1]

[0052] In the inventive examples (Nos. 1 to 16), no cracks that would cause problems in quality or appearance occurred. By adding Si and mixing any of B, P, or S-containing substances that have the effect of lowering the eutectic point of FeO and Fe2SiO4, liquid Si-Fe oxides are formed in the scale regardless of the heating temperature, and liquid Cu is absorbed into this, which is presumably the result of suppressing red embrittlement cracking. A sufficient crack suppression effect was obtained even without the active addition of expensive Ni.

[0053] In Comparative Examples 1 to 6 (Nos. 17 to 22), the Si content was sufficiently distributed, but the B, P, and S content was not sufficiently distributed, and it was confirmed that cracks occurred in the obtained steel sheets, which caused problems in terms of quality and appearance. It is presumed that the eutectic point of FeO and Fe2SiO4 was higher than the heating temperature, and liquid-phase Si-Fe oxides that could absorb liquid Cu were not formed.

[0054] In Comparative Examples 7 to 12 (Nos. 23 to 28), the B, P, and S inclusions were sufficiently distributed, but the Si inclusions were not sufficiently distributed, and it was confirmed that cracks occurred in the obtained steel sheets, which were problematic in terms of quality and appearance. It is presumed that the amount of liquid-phase Si-Fe oxide that absorbs liquid Cu was not sufficient.

[0055] It is clear that the present invention can suppress red embrittlement cracking during hot rolling even in steel materials containing Cu. It is also clear that the present invention does not require the active addition of Ni (typically more than half the Cu equivalent (mass%)), that the heating temperature can be 1000°C to 1300°C and does not need to be 1150°C or higher as in the prior art, and that heating can be done in the air without taking any special measures to suppress scale formation using coating agents or the like.

Claims

1. In the production of Cu-containing steel material, the steel material is heated in a heating furnace and then hot-rolled, The steel material has a Cu equivalent of C ＣｕＥｑ (mass%) is 0.15% or more and 3.00% or less, A mixture is obtained by mixing a Si-containing material with at least one of a B-containing material, a P-containing material, and a S-containing material; Here, the Si content of the steel material is 400 to 1200 g / m ２ , The B content per surface area of ​​the steel material is 4 to 40 g / m ２ , The amount of P contained in the steel material is 8 to 80 g / m ２ , The amount of S contained in the S-containing material is 8 to 80 g / m2 per surface area of ​​the steel material. ２ The mixture is placing the mixture on the surface of the steel material; The heating is carried out at a temperature in the range of 1000°C to 1300°C, This method for producing a Cu-containing steel material includes subsequently performing descaling.

2. The Si-containing substance is selected from metal Si, SiC, SiN, and SiO ２ or a combination thereof, and the B-containing substance is B ２ O ３ and the P-containing material is P ２ O ５ 2. The method for producing a Cu-containing steel material according to claim 1, wherein the S-containing substance is either FeS or MnS, or a combination thereof.

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