Method for producing hot-rolled steel sheet

Abstract

To provide a method for producing a Cu-containing hot rolled steel sheet, particularly a method in which, in a hot rolling stage, the positive addition of Ni is not required, temperature restriction is reduced (typically, heating temperature may be ≥ 1050 °C), and the effect of suppressing red heat embrittlement is higher.SOLUTION: 0 to 0.5 mass% of C, 0 to 3.0 mass% of Si, 0 to 3.0 mass% of Mn, 0 to 0.05 mass% of P, 0 to 0.05 mass% of S, 0 to 1.0 mass% of Al, 0.01 to 1.0 mass% of Cu, 0 to 1.0 mass% of Ni, 0 to 1.0 mass% of Cr, 0 to 1.0 mass% of Sn, and 0 to 0.01 mass% of N, in the method for producing a hot rolled steel sheet, a slab having a composition in which the balance is composed of Fe and impurity is hot-rolled, and during a period from slab heating to rolling, the surface temperature of the slab is heated to ≥ 1200 °C or recuperated, and the amount of scale grown at ≥ 1200 °C is controlled to ≥ 75 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a hot-rolled steel sheet, and more particularly to a method for manufacturing a hot-rolled steel sheet that can suppress surface cracks due to red embrittlement. [Background technology]

[0002] Utilizing scrap to manufacture steel products contributes greatly to reducing the environmental impact. In addition, global demand for steel has been expanding over the long term, and this trend is expected to continue. As a result, the prices of raw materials such as iron ore are also rising. Therefore, utilizing scrap as a source of iron for steel is more important than ever.

[0003] Generally, when steel is produced from recycled scrap, the scrap often contains tramp elements such as Cu derived from motors and copper wires, or Sn derived from plated steel sheets, and when scrap containing such tramp elements is used as a raw material, the surface quality of the hot-rolled steel sheet can be degraded. This is because when steel containing tramp elements is rolled, a phenomenon called red embrittlement cracking occurs during hot rolling, causing cracks on the surface of the steel.

[0004] The mechanism by which red embrittlement occurs is thought to be roughly as follows. Generally, in hot rolling, slabs are loaded into a heating furnace prior to hot rolling, heated with combustion gases to the desired temperature (slab heating), and then removed from the heating furnace. The oxide scale is then removed (descaling) with high-pressure water before hot rolling. Because the combustion gases supplied to the heating furnace typically contain oxidizing gases such as oxygen, water vapor, and carbon dioxide, an oxide scale layer forms on the surface of the slabs 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 metals more noble than iron, such as Cu and Sn, are present in the base steel, these noble metals are not oxidized but concentrate at the interface between the oxide scale and the base steel. Cu and Sn only have a solubility of a few percent in gamma iron, but if the concentration of Cu or Sn exceeds this level, they appear as metallic phases. The melting points of Cu and Sn are approximately 1085°C and 232°C, respectively. Because slabs are typically heated to temperatures above these levels before hot rolling, molten Cu and Sn form liquid phases at the oxide scale / base steel interface. These liquid phases penetrate the grain boundaries of the base steel and are unable to withstand the shear and tensile stresses that occur during hot rolling, leading to surface cracking, or red-hot embrittlement cracking.

[0005] Rolling such tramp element-containing steel without impairing the surface quality is an important issue, and various methods for addressing this issue have been investigated.

[0006] Patent Document 1 states that red embrittlement can be suppressed by adding Ni so that Cu+3×Sn<4×Ni in mass %.

[0007] Patent Document 2 relates to a method for producing hot-rolled steel sheet by recycling iron scrap containing tramp elements, and describes how red embrittlement can be avoided by setting the upper heating temperature limit to 1050°C or less in the process of reheating a slab obtained by continuous casting, i.e., the heating process in the hot rolling process. During the slab heating process in the hot rolling process, an oxide scale grows on the slab surface during heating. This causes Cu contained in the steel to concentrate near the steel surface (the interface between the scale and the base steel), and the concentrated Cu melts and penetrates into the grain boundaries of the base steel, resulting in red embrittlement. Patent Document 2 describes a technology intended to avoid melting of concentrated Cu during heating, since the melting point of Cu is around 1100°C.

[0008] Patent Document 3 states that red embrittlement can be avoided by heating a steel slab at a predetermined heating rate, raising the maximum heating temperature to 1200 to 1350°C, and holding the temperature within this range for 1 hour or more.

[0009] Patent Document 4 describes a method of removing a Cu-enriched layer by applying high-pressure water descaling multiple times to a steel material extracted from a heating furnace. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-282174 [Patent Document 2] Japanese Patent Application Publication No. 9-296223 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-168843 [Patent Document 4] Japanese Patent Application Laid-Open No. 2005-29883 Summary of the Invention [Problem to be solved by the invention]

[0011] Various methods have been investigated to roll steel containing tramp elements without impairing the surface quality, but further improvements are needed.

[0012] The aforementioned Patent Document 1 discloses the addition of Ni. However, since Ni is an expensive element, if there is a method that is effective regardless of the Ni content, a significant cost advantage can be obtained.

[0013] Patent Document 2 discloses that, since the melting point of Cu is approximately 1085°C, red embrittlement can be avoided by setting the upper heating temperature limit for Cu-containing steel to 1050°C or lower, which is lower than the melting point. However, heating to above 1050°C is often necessary to ensure the properties of the steel, and if measures to prevent surface cracking could be taken without restricting the upper heating temperature limit, it would be possible to ensure the quality of the material and prevent surface cracking at the same time, increasing the flexibility of manufacturing conditions.

[0014] Patent Document 3 specifies that in order to avoid red embrittlement, a steel slab is heated at a predetermined heating rate, the maximum heating temperature is 1200 to 1350°C, and the slab is held within this temperature range for one hour or more. However, holding the slab within this temperature range for one hour or more promotes scale growth on the slab, which may result in a decrease in product yield.

[0015] Patent Document 4 describes a method for removing a Cu-enriched layer by applying high-pressure water descaling multiple times to steel material extracted from a heating furnace. This is a technology in which the Cu-enriched layer at the scale / base steel interface is incorporated into the scale during re-oxidation between descaling treatments, and the Cu-enriched layer is then removed together with the scale during descaling after the re-oxidation. However, as will be described in detail later, the inventors have found that if the temperature during re-oxidation between descaling treatments is low (approximately 1100°C to 1150°C in the example of Patent Document 4), a new Cu-enriched layer will be formed during re-oxidation, raising the concern that red-hot embrittlement cracking will occur.

[0016] To summarize the above, scrap contains tramp elements such as Cu and Sn, which can cause red-embrittlement cracking. Therefore, there is a need to suppress such red-embrittlement cracking. Patent Document 1 discloses that Ni suppresses red-embrittlement cracking, but it is expensive. Patent Document 2 discloses suppressing red-embrittlement cracking by heating at a temperature of 1050°C or less without adding Ni, but actual steel production often requires heating at 1050°C or higher. Patent Document 3 also discloses controlling the average heating rate from 1000°C to 1100°C, but temperature control within this temperature range alone is insufficient to suppress red-embrittlement cracking. Patent Document 4 discloses incorporating a Cu-enriched layer at the scale / base steel interface into the scale and then removing the Cu-enriched layer along with the scale (descaling), but there is room for further improvement in terms of suppressing red-embrittlement cracking. (As will be described in detail later, the inventors discovered an appropriate temperature range for scale growth and came up with the idea of ​​applying it.)

[0017] Therefore, the inventors set out to provide a method for manufacturing a hot-rolled steel sheet containing tramp elements, particularly in the hot rolling process, that is unprecedented, i.e., novel, does not require the active addition of Ni, has few temperature restrictions (typically, the heating temperature may be 1050°C or higher), and is more effective in suppressing red embrittlement. In the present invention, a steel material that contains a large amount of Cu and Sn is referred to as a tramp element-containing steel, and a concentrated layer that contains a large amount of Cu and Sn is referred to as a tramp element-concentrated layer. [Means for solving the problem]

[0018] The present inventors have focused on the growth behavior of the tramp element-enriched layer, which causes red embrittlement cracking, and the scale present in the vicinity thereof, and have come up with an idea for solving the above-mentioned problems.

[0019] The tramp element-enriched layer that causes red embrittlement cracking is typically formed during the slab heating process before hot rolling, when tramp elements concentrate at the scale / base steel interface and steel grain boundaries as scale grows. This phenomenon occurs when the slab surface temperature is held in the temperature range of 950 to 1200°C for a long period of time.

[0020] The inventors have found that the maximum depth (distance from the scale / base steel interface) of tramp elements concentrated at steel grain boundaries remains almost constant at about 50 μm, regardless of the amount of scale growth, except in the early stages of scale growth. This is thought to be because as the tramp element-enriched layer deepens, oxidation occurs from the steel surface to scale, changing the position of the scale / base steel interface.

[0021] Furthermore, the inventors have found that when heating and holding at 1200°C or higher, almost no layer containing concentrated tramp elements is observed even if the amount of scale growth is large. This is presumably because the diffusion coefficient of tramp elements is large in the temperature range of 1200°C or higher, so the tramp elements diffuse into the steel without being concentrated, thereby reducing the concentration of tramp elements.

[0022] The inventors have come up with the idea of ​​utilizing these phenomena to remove the tramp element-enriched layer by holding the steel material at a high temperature of 1200°C or higher for a certain period of time before hot rolling the slab, typically in the slab heating process, and by making the amount of scale growth at 1200°C or higher 75 μm or more.

[0023] That is, when a slab passes through the temperature range of 950 to 1200°C during the heating process, a tramp element-enriched layer of about 50 μm in depth is formed, but if the steel, including the tramp element-enriched layer, can be scaled to a depth of 50 μm by holding it at 1200°C or higher for a certain period of time, no new tramp element-enriched layer will be formed during scale growth at 1200°C or higher, making it possible to heat the tramp element-enriched layer in a manner that is harmless. The thickness of the scale formed by oxidizing 50 μm thick steel is 75 μm.

[0024] Furthermore, because scale growth follows the so-called parabolic law, the thicker the scale, the less subsequent scale growth there is. Therefore, if scale grows thick during the slab heating process, maintaining the temperature at 1200°C or higher to produce scale of 75 μm or more requires long-term or high-temperature soaking, which may be undesirable from the standpoint of fuel cost and material quality.

[0025] Therefore, the inventors came up with the idea that, as one form of the present invention, scale on a slab heated to 1200°C can be removed by high-pressure water descaling or the like, and the slab surface temperature can then be raised again to 1200°C or higher and held soaking, thereby efficiently generating scale of 75 μm or more in a temperature range of 1200°C or higher.

[0026] Based on these ideas, the present inventors have completed the present invention, which has the following gist.

[0027] [1] In the hot rolling of a slab having a composition containing, by mass%, C: 0-0.5%, Si: 0-3.0%, Mn: 0-3.0%, P: 0-0.05%, S: 0-0.05%, Al: 0-1.0%, Cu: 0.01-1.0%, Ni: 0-1.0%, Cr: 0-1.0%, Sn: 0-1.0%, N: 0-0.01%, with the balance being Fe and impurities, during the period from heating the slab to rolling, Heating or reheating the surface temperature of the slab to 1200°C or higher, The amount of scale growth at 1200℃ or higher is 75μm or more. A method for manufacturing a hot-rolled steel sheet. [2] [1] The method for manufacturing a hot-rolled steel sheet according to [1], characterized in that during the period, the surface temperature of the slab when removed from the heating furnace is 1200°C or higher, and the amount of scale growth when soaked at 1200°C or higher is 75 μm or higher. [3] [1] or [2], characterized in that during the period, after the slab is extracted from the heating furnace, scale is removed by high-pressure water descaling, the surface temperature of the slab thereafter is 1200°C or higher, and the amount of scale growth during the period when the surface temperature of the slab after descaling is 1200°C or higher is 75 μm or higher. [4] [3] A method for manufacturing a hot-rolled steel sheet, characterized in that after the scale removal process by high-pressure water descaling, the surface temperature of the slab is raised to 1200°C or higher by reheating, and the surface temperature of the slab is maintained at 1200°C or higher on a table roller, and the amount of scale growth at this time is 75 μm or more. [5] [3] A method for manufacturing a hot-rolled steel sheet according to [3], characterized in that after the scale removal process by high-pressure water descaling, the slab is returned to the heating furnace and reheated so that the surface temperature of the slab becomes 1200°C or higher, and the amount of scale growth during the soaking at 1200°C or higher is 75 μm or higher. [6] [3] A method for manufacturing a hot-rolled steel sheet according to [3], characterized in that after the scale removal process by high-pressure water descaling, the slab is induction reheated in an induction heating device so that the surface temperature of the slab is 1200°C or higher, and the amount of scale growth at this time is 75 μm or higher. [7] The method for producing a hot-rolled steel sheet according to any one of [1] to [6], wherein the composition contains 0.01 to 0.04 mass % of Ni. [Effects of the Invention]

[0028] According to the present invention, in the hot rolling of steel containing tramp elements, it is possible to produce a hot-rolled steel sheet with excellent surface quality and with only minor surface cracking due to red embrittlement. Furthermore, according to the present invention, there is no need to actively add Ni, and there are few temperature restrictions (typically, the heating temperature may be 1050°C or higher). [Brief explanation of the drawings]

[0029] [Figure 1] FIG. 10 is a diagram showing an example of the change in scale growth over time. DETAILED DESCRIPTION OF THE INVENTION

[0030] Hereinafter, an embodiment of the present invention will be described.

[0031] One embodiment of the present invention comprises: In the hot rolling of a slab having a composition containing, by mass%, C: 0-0.5%, Si: 0-3.0%, Mn: 0-3.0%, P: 0-0.05%, S: 0-0.05%, Al: 0-1.0%, Cu: 0.01-1.0%, Ni: 0-1.0%, Cr: 0-1.0%, Sn: 0-1.0%, N: 0-0.01%, with the balance being Fe and impurities, during the period from heating the slab to rolling, Heating or reheating the surface temperature of the slab to 1200°C or higher, The amount of scale growth at 1200℃ or higher is 75μm or more. The present invention relates to a method for producing a hot-rolled steel sheet.

[0032] (Applicable steel type) The steel type of the hot-rolled steel sheet targeted in this embodiment is a tramp element-containing steel, and the steel type that best exhibits the effects of the present invention has a composition containing, by mass%, C: 0-0.5%, Si: 0-3.0%, Mn: 0-3.0%, P: 0-0.05%, S: 0-0.05%, Al: 0-1.0%, Cu: 0.01-1.0%, Ni: 0-1.0%, Cr: 0-1.0%, Sn: 0-1.0%, N: 0-0.01%, with the balance being Fe and impurities. Note that the slab from which the above-mentioned hot-rolled steel sheet is obtained also has substantially the above-mentioned composition.

[0033] (C:0~0.5% by mass) C is an element that is effective in ensuring strength. However, if an excessive amount of C is added, the effect of the addition becomes saturated, so the upper limit of C is set to 0.5 mass%. On the other hand, to fully obtain the effect of adding C, the C content is preferably 0.001 mass% or more, and the C content may be 0.01 mass% or more.

[0034] (Si:0~3.0% by mass) Silicon (Si) is an element that contributes to increasing the strength of steel and also acts as a deoxidizer for molten steel, and is therefore added as needed. To fully obtain the effects of adding Si, the Si content is preferably 0.05% by mass or more, more preferably 0.10% by mass or more, and even more preferably 0.30% by mass or more, 0.50% by mass or more, 1.0% by mass or more, or 1.5% by mass or more. However, since the effects of adding Si become saturated when the Si content is excessively high, the Si content is set to 3.0% by mass or less, and may be set to 2.5% by mass or less or 2.0% by mass or less depending on the desired purpose.

[0035] (Mn:0~3.0% by mass) Mn is an element that contributes to improving strength through solid solution strengthening and quench strengthening, and is added as needed. Therefore, the Mn content may be 0% by mass or more, preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and even more preferably 0.3% by mass or more. If the Mn content exceeds 3.0% by mass, the effect of addition becomes saturated, so the upper limit of Mn is set to 3.0% by mass.

[0036] (P:0~0.05% by mass) P is generally contained as an impurity, but because it has the effect of inexpensively increasing the strength of steel, it may be intentionally added. However, if the P content is too high, secondary work embrittlement and deterioration of weldability are likely to occur. Therefore, the P content is set to 0.05% by mass or less. There is no particular lower limit for the P content, but from the perspective of production costs, the P content may be more than 0% by mass or 0.0001% by mass or more.

[0037] (S:0~0.05% by mass) S is an element that is generally contained as an impurity and has the effect of embrittling steel by segregating at grain boundaries. If the S content is too high, embrittlement is likely to become a problem, so the S content is set to 0.05% by mass or less. There is no particular lower limit for the S content, but from the perspective of desulfurization costs, the S content may be more than 0% by mass or 0.0001% by mass or more.

[0038] (Al: 0~1.0% by mass) Al has the effect of deoxidizing molten steel to improve the quality of the steel. Therefore, the Al content may be 0% by mass or more, but to obtain a sufficient deoxidizing effect, it may be preferably 0.0010% by mass or more, more preferably 0.0050% by mass or more, and even more preferably 0.0100% by mass or more. However, even if the Al content exceeds 1.0% by mass, the effect of this action saturates and it becomes uneconomical. Therefore, the Al content is set to 1.0% by mass or less.

[0039] (Cu:0.01~1.0% by mass) Cu is a component generally found in scrap and contributes to improving strength through solid solution strengthening and precipitation strengthening, so its lower limit is 0.01% by mass. Depending on the scrap properties and the desired properties, the Cu content may be set to 0.10% by mass or more, 0.20% by mass or more, or 0.50% by mass or more. On the other hand, Cu can cause cracks on the surface of the steel material due to a phenomenon known as red embrittlement. If Cu exceeds 1.0% by mass, the effect of suppressing cracks due to red embrittlement according to the present invention may not be fully realized, so the upper limit is set to 1.0% by mass. Depending on the scrap properties and the desired properties, the upper limit may also be set to 0.90% by mass or less, 0.80% by mass or less, or 0.70% by mass or less.

[0040] (Sn: 0 to 1.0%) Sn is generally contained in scrap and has the effect of preventing easily oxidizable elements in steel sheet, such as Mn, Si, and / or Al, from diffusing to the steel sheet surface and forming oxides, thereby improving the surface quality and galvanizability of the steel sheet. However, since it is not an essential element, its lower limit is 0%. Depending on the scrap properties, desired properties, and scrap blending ratio, it may be set to 0.001% by mass or more, 0.010% by mass or more, or 0.10% by mass or more. On the other hand, when Sn is contained, cracking due to red embrittlement is likely to occur due to the low melting point of Sn. If the Sn content exceeds 1.0% by mass, the effect of suppressing cracking due to red embrittlement according to the present invention may not be fully realized, so its upper limit is 1.0% by mass. Depending on the scrap properties and desired properties, the upper limit may be set to 0.90% by mass or less, 0.80% by mass or less, or 0.70% by mass or less.

[0041] (Ni:0~1.0% by mass) Ni is a component generally contained in scrap and contributes to improving corrosion resistance. Its lower limit may be 0 mass%, or 0.01 mass% or more, 0.10 mass% or more, 0.20 mass% or more, or 0.50 mass% or more, depending on the scrap properties, desired properties, and scrap blending ratio. On the other hand, because Ni is an expensive element, the upper limit of Ni is 1.0 mass%. The upper limit may also be 0.90 mass% or less, 0.80 mass% or less, or 0.70 mass% or less, depending on the scrap properties and desired properties. In one embodiment, Ni may not be actively added to prevent cost increases, and only Ni contained in raw materials such as scrap may be used. Therefore, the upper limit is preferably 0.04 mass% or less, and may also be 0.03 mass% or less, or 0.02 mass% or less. According to an embodiment of the present invention, cracking due to red embrittlement can be prevented even in a composition with such a low Ni content.

[0042] (Cr:0~1.0% by mass) Cr is a component generally contained in scrap and is an element that contributes to improving oxidation resistance at high temperatures. Its lower limit may be 0% by mass. Depending on the scrap properties, desired properties, and scrap blending ratio, it may be set to 0.01% by mass or more, 0.10% by mass or more, 0.20% by mass or more, or 0.50% by mass or more. However, if the content exceeds 1.0% by mass, the effect of addition becomes saturated, so the upper limit of Cr is 1.0% by mass. The upper limit may also be set to 0.90% by mass or less, 0.80% by mass or less, or 0.70% by mass or less depending on the scrap properties and desired properties.

[0043] (N:0~0.01% by mass) N is an element that is generally inevitably mixed into steel materials and may dissolve in the steel material, reducing the workability of the steel material, so its upper limit is 0.01% by mass. A preferred upper limit is 0.0090% by mass, and more preferably 0.0080% by mass. The N content is preferably as low as possible. However, excessive reduction of the N content increases production costs. Therefore, considering normal industrial production, the preferred lower limit of the N content is 0.0001% by mass, more preferably 0.0030% by mass, and even more preferably 0.0050% by mass.

[0044] (balance: Fe and impurities) The remainder of the chemical composition of the steel grade of the hot-rolled steel sheet or the slab in this embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of steel, and are acceptable within a range that does not adversely affect the hot-rolled steel sheet of this embodiment.

[0045] (Optionally added elements) Material-strengthening elements such as Ti, Nb, Mo, and V are known, and the steel grade of the hot-rolled steel sheet or the chemical composition of the slab of this embodiment may contain these. Therefore, the content of material-strengthening elements such as Ti, Nb, Mo, and V may each be 0 mass% or more, preferably 0.001 mass% or more, more preferably 0.002 mass% or more, and even more preferably 0.003 mass% or more. On the other hand, since the effects of the above actions saturate, the content may be 0.200 mass% or less, preferably 0.150 mass% or less, and even more preferably 0.100 mass% or less.

[0046] (Temperature control from slab heating to rolling)

[0047] Generally, when a slab is hot rolled, the slab is heated in a heating furnace (slab heating) before hot rolling and then subjected to the hot rolling process. Red embrittlement cracking of steel containing tramp elements is caused by a tramp element-enriched layer that is formed during the process from slab heating to rolling. For purposes of describing the present invention, unless otherwise specified, "slab heating" refers to raising the temperature of a slab in a heating furnace in order to hot roll the slab. The term "heating" refers to applying heat to an object (typically a slab) using an external heat source, and the type of heat source is not particularly limited. "Recuperation" refers to raising the surface temperature of a slab by heat retained within the slab without heat treatment. "Reheating" refers to the process of subjecting a slab that has been "slab heated" to a heat treatment again after descaling. The means of "reheating" is not particularly limited, and the slab may be "reheated" by returning it to a heating furnace, or may be "reheated" by an induction heating device. In particular, "reheating" by an induction heating device is referred to as "induction reheating."

[0048] This embodiment provides a method for preventing the formation of a tramp element thickened layer in a heating furnace or for removing the formed tramp element thickened layer, which is based on the following findings of the present inventors. The first finding is that during the slab heating process, the tramp element-enriched layer is formed when the slab surface temperature is in the range of 950 to 1200°C, and its thickness is approximately 50 μm at most. The second finding is that no tramp element-enriched layer is formed even when the slab is kept at temperatures above 1200°C for a long time.

[0049] Based on these findings, in the first embodiment of the present invention, During the period from slab heating to rolling, The surface temperature of the slab is heated or reheated to 1200°C or higher. The amount of scale growth at 1200°C or higher is set to 75 μm or more.

[0050] When the slab surface temperature passes through a temperature range of 950 to 1200°C during slab temperature increase (or slab heating), a tramp element-enriched layer of up to about 50 μm thick is formed at the interface between the scale layer and the base metal.

[0051] However, by subsequently heating or reheating the slab surface to 1200°C or higher and holding the temperature for a certain period of time, the approximately 50 μm thick tramp element-enriched layer that formed during the heating process is completely transformed into scale. When 50 μm of base steel is transformed into scale, the resulting scale thickness is 75 μm. Furthermore, although scale forms at temperatures above 1200°C, no new tramp element-enriched layer forms. Therefore, at the start of rolling, a heated slab is obtained that does not have any tramp element-enriched layer formed (remaining). As a result, red embrittlement cracking during hot rolling is suppressed.

[0052] The temperature at which scale of 75 μm or more is formed needs only to be 1200° C. or higher, and may be 1230° C. or higher, 1250° C. or higher, or 1280° C. or higher. However, if the temperature exceeds 1300° C., the amount of scale growth may become too large, or the desired properties may not be obtained in the product (hot-rolled steel sheet), so the temperature is preferably 1300° C. or lower.

[0053] In the first embodiment, the time for which the slab surface temperature is maintained at 1200° C. or higher is not particularly limited as long as it is adjusted so that the scale grows to 75 μm or more.

[0054] The amount of scale growth at 1200°C or higher should be 75 μm or more. If the amount of scale growth is 75 μm or more, the approximately 50 μm thick tramp element-enriched layer that previously existed will all be converted to scale. Therefore, the amount of scale growth may be 100 μm or more, or even 150 μm or more. However, if the amount of scale growth exceeds 200 μm, the steel yield will deteriorate, so the amount of scale growth is preferably 200 μm or less.

[0055] In a second embodiment, The surface temperature of the slab when extracted from the heating furnace is 1200°C or higher, and The amount of scale growth when soaked at 1200°C or higher is 75 μm or more.

[0056] Typically, the slab heating process is performed in a heating furnace, followed by soaking the heated slab in a soaking furnace. The slab surface temperature when removed from the heating furnace can be set to 1200°C or higher. The slab is then soaked at 1200°C or higher, and the amount of scale growth during soaking can be 75 μm or higher.

[0057] In the second embodiment, the time for soaking the slab surface temperature at 1200°C or higher is not particularly limited as long as it is adjusted so that scale grows to 75 μm or more. When the slab surface temperature is 1200°C or higher, scale can grow to 75 μm or more in about 10 minutes or more, so the soaking time may be 10 minutes or more, or 30 minutes or more. However, a soaking time of more than 60 minutes can significantly reduce production efficiency or result in excessive scale growth, so a soaking time of 60 minutes or less is preferred.

[0058] This embodiment is preferable in that it can be carried out using a general heating furnace or soaking furnace and does not require the addition of special equipment.

[0059] In a third embodiment, After extracting the slab from the furnace, the scale is removed by high-pressure water descaling. The subsequent surface temperature of the slab is 1200°C or higher, The amount of scale growth is 75 μm or more while the surface temperature of the slab after descaling is 1200°C or higher.

[0060] As described above, in the embodiment of the present invention, the tramp element-enriched layer having a maximum thickness of approximately 50 μm is entirely converted into scale, thereby suppressing red embrittlement cracking. On the other hand, if a scale layer exists on the tramp element-enriched layer and the thickness of the scale layer is large, a higher temperature and a longer holding time are required to convert the tramp element-enriched layer into scale. Heating the slab to a high temperature or holding it for a long time may result in the product (hot-rolled steel sheet) not being able to obtain the desired properties.

[0061] In a third embodiment, the slab is extracted from the heating furnace, and surface scale is removed by high-pressure water descaling. Then, a scale of 75 μm or more is grown at a temperature of 1200°C or higher, neutralizing the tramp element-enriched layer. The scale growth follows the so-called parabolic law. Figure 1 shows an example of the temporal change in scale growth. As shown in Figure 1, the slab temperature remains nearly constant until high-pressure descaling is performed, and the rate of scale growth slows as the scale thickens. Descaling removes scale once, allowing new scale to form in a short period of time. Generally, descaling causes the slab surface temperature to drop temporarily, but recuperation allows it to recover to almost the temperature before descaling. Therefore, the temperature change caused by descaling has almost no effect on scale growth.

[0062] In the third embodiment, the holding time in the temperature range of 1200°C or higher can be adjusted as appropriate so that the amount of scale growth is 75 μm or more. In this embodiment, since the scale is removed once by descaling, the holding time can be shorter than when the scale is not removed. Therefore, the holding time may be 30 seconds or more, 60 seconds or more, or 90 seconds or more. Since a holding time of more than 10 minutes may result in a decrease in production efficiency, the holding time may be 10 minutes or less.

[0063] The discharge pressure during high-pressure water descaling can be adjusted appropriately depending on the thickness of the scale to be removed, and general high-pressure water descaling conditions may be used. Typically, if the discharge pressure during descaling is too low, scale may not be sufficiently removed, so it may be 1 MPa or more, 5 MPa or more, or 10 MPa or more. There are no particular restrictions on the upper limit of the discharge pressure during descaling, and it may be set to 20 MPa or less, which is a general condition.

[0064] In a fourth embodiment, After the scale removal process using high-pressure water descaling, By recuperating, the surface temperature of the slab is raised to 1200°C or higher. The surface temperature of the slab is maintained at 1200°C or higher on the table roller, and the amount of scale growth is 75 μm or more.

[0065] In this embodiment, after descaling, the surface temperature of the slab is increased to 1200°C or higher by reheating the slab. Generally, the surface temperature of the slab temporarily drops due to descaling, but the interior of the slab retains sufficient heat, allowing the surface temperature of the slab to be reheated to 1200°C or higher. Reheating of the slab can be performed while the slab is placed on a table roller. The table roller may be located between the slab heating furnace and the hot rolling mill. The slab may also be moved back and forth on the table roller as appropriate.

[0066] In the fourth embodiment, the holding time in the temperature range of 1200°C or higher can be adjusted as appropriate so that the amount of scale growth is 75 μm or more. In this embodiment, since the scale is removed once by descaling, the holding time can be shorter than when the scale is not removed. Therefore, the holding time may be 30 seconds or more, 60 seconds or more, or 90 seconds or more. Since a holding time of more than 10 minutes may result in a decrease in production efficiency, the holding time may be 10 minutes or less.

[0067] In this embodiment, the discharge pressure of the high-pressure water descaling may be adjusted so that the slab temperature is 1200°C or higher by recuperation. If the discharge pressure exceeds 15 MPa, the slab may be cooled too much, making it difficult to recuperate to 1200°C or higher. Even if the slab is recuperated to 1200°C or higher, the temperature may drop to less than 1200°C before scale of 75 μm or larger is formed. Therefore, the discharge pressure may be set to 15 MPa or lower, 13 MPa or lower, or 12 MPa or lower. This embodiment is also preferable because it does not require the addition of special equipment. Furthermore, the slab temperature during heating in the heating furnace or extraction may be appropriately adjusted so that the recuperation temperature is 1200°C or higher. Typically, these temperatures may be 1250°C or higher and lower than 1300°C.

[0068] In a fifth embodiment, After the scale removal process using high-pressure water descaling, The slab is returned to the heating furnace and reheated so that the surface temperature of the slab reaches 1200°C or higher, and the amount of scale growth during the soaking at 1200°C or higher is 75µm or more.

[0069] In the fifth embodiment, the soaking time in the temperature range of 1200°C or higher can be adjusted as appropriate so that the amount of scale growth is 75 μm or more. In this embodiment, since the scale is removed once by descaling, the soaking time can be shorter than when the scale is not removed. Therefore, the soaking time may be 30 seconds or more, 60 seconds or more, or 90 seconds or more. Since a soaking time of more than 10 minutes may result in a decrease in production efficiency, the soaking time may be 10 minutes or less.

[0070] After descaling the slab, the surface temperature of the slab can be reheated to 1200°C or higher relatively quickly by returning the slab to the heating furnace. Furthermore, no additional special equipment is required. In these respects, this embodiment is preferable.

[0071] In a sixth embodiment, after the descaling step by high pressure water descaling, The slab is induction reheated using an induction heating device so that the surface temperature of the slab reaches 1200°C or higher, and the amount of scale growth at this time is 75 μm or more.

[0072] The induction heating device allows the slab to be rapidly heated by induction current. The sixth embodiment generally allows for more rapid induction reheating than the fifth embodiment, in which the slab is returned to the heating furnace, and is therefore preferred from the standpoint of increasing production speed.

[0073] In both the fifth embodiment using a heating furnace and the sixth embodiment using an induction heating device, the slab surface temperature at the time of removal from the heating furnace (before high-pressure descaling) does not necessarily have to be 1200°C or higher; after descaling, the slab can be reheated or induction reheated to 1200°C or higher in a heating furnace or induction heating device to grow a scale of 75 μm or more. In either embodiment, the slab is heated in a heating furnace for hot rolling, and the interior of the slab still has sufficient heat even after descaling, so the surface temperature of the slab can be raised to 1200°C or higher in a relatively short time, and can easily be maintained at 1200°C or higher.

[0074] In the sixth embodiment, the holding time in the temperature range of 1200°C or higher can be adjusted as appropriate so that the amount of scale growth is 75 μm or more. In this embodiment, since the scale is removed once by descaling, the holding time can be shorter than when the scale is not removed. Therefore, the holding time may be 30 seconds or more, 60 seconds or more, or 90 seconds or more. Since a holding time of more than 10 minutes may result in a decrease in production efficiency, the holding time may be 10 minutes or less.

[0075] Scaling of a base steel containing a tramp element-enriched layer is referred to as scaling off. As in the first embodiment, oxidizing a tramp element-enriched layer approximately 50 μm thick to transform it into a scale having a thickness of 75 μm or more also falls under the category of scaling off. As in the second embodiment, the tramp element-enriched layer may be scaled off before being extracted from the heating furnace. As in the third embodiment, the tramp element-enriched layer may be scaled off after high-pressure water descaling. As in the fourth embodiment, the tramp element-enriched layer may be scaled off during reheating after high-pressure water descaling. As in the fifth embodiment, the tramp element-enriched layer may be returned to the heating furnace after high-pressure water descaling and reheated, and then scaled off. As in the sixth embodiment, the tramp element-enriched layer may be scaled off by induction reheating using an induction heating device after high-pressure water descaling. These embodiments may be combined as appropriate.

[0076] (Hot rolling conditions) In a slab that has undergone the above-described temperature control during the period from slab heating to rolling in hot rolling, no tramp element-enriched layer is formed (remains). Therefore, when the slab is subjected to conventional hot rolling, red embrittlement cracking is suppressed in the resulting hot-rolled steel sheet. Note that the hot-rolling conditions (rolling temperature, reduction, etc.) may be appropriately adjusted within the range of commonly employed hot-rolling conditions depending on the final desired properties of the hot-rolled steel sheet. Typical or preferred hot-rolling conditions are described below, but the embodiments of the present invention are not limited to these.

[0077] In typical hot rolling, the steel material (slab) extracted from the heating furnace is subjected to high-pressure water descaling to remove surface scale, and may then be rolled to a predetermined thickness. Typical high-pressure water descaling and rolling conditions do not significantly affect the occurrence of surface cracks due to red embrittlement and can be selected appropriately. Therefore, although these conditions are not limited to those described below, the discharge force of the high-pressure water descaling may be 1 to 20 MPa, as explained above. Furthermore, a final rolling temperature in the range of 800 to 1000°C is preferred to ensure material quality. The reduction ratio in all rolling passes is preferably 5 to 50%, and the final plate thickness is preferably 1 to 25 mm.

[0078] (Surface texture verification method) The surface quality of the hot-rolled steel sheet after hot rolling is evaluated by removing scale by pickling and visually checking for the presence or absence of defects. Furthermore, three of the largest defects found are selected, and the depth of the cracks is measured by cross-sectional observation to determine the degree of surface defects. According to an embodiment of the present invention, the maximum crack depth of the cracks found is less than 10 μm. Generally, a crack depth of less than 10 μm does not pose a problem in terms of product quality, except in special cases. In this specification, unless otherwise specified, excellent surface quality means that the above-mentioned crack depth standard (maximum crack depth of less than 10 μm) is met. [Example]

[0079] 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.

[0080] Slabs containing, by mass, 0.08% C, 0.3% Si, 2.2% Mn, 0.015% P, 0.005% S, 0.03% Al, 0.3% Cu, 0.04% Ni, 0.03% Cr, 0.01% Sn, and 0.003% N, with the balance consisting of Fe and impurities, were hot-rolled using a laboratory rolling mill consisting of a heating furnace, high-pressure water descaling, and rolling mill. The resulting steel sheets were then examined for red embrittlement cracking. Specifically, 30 mm-thick test steel slabs were heated in a heating furnace and descaled using the descaling process conditions shown in Table 1. The tramp element-enriched layer was then descaled. The slabs were then subjected to high-pressure water descaling at a discharge pressure of 15 MPa and rolled to a 3 mm-thick steel sheet through five passes. The descaling process conditions were as follows:

[0081] In conditions A to D, the scale-off step is carried out immediately before extraction from the heating furnace, and the final temperature in the scale-off step corresponds to the extraction temperature from the heating furnace, which is 1180 to 1280°C.

[0082] In conditions E to G, after the steel was extracted from the heating furnace, high-pressure water descaling at a discharge pressure of 15 MPa was applied to remove the scale, and the steel surface was allowed to cool in the atmosphere to reheat, thereby carrying out the descaling process. In this case, the final temperature in the descaling process was the reheated temperature, which was 1150 to 1230°C.

[0083] Under conditions H to J, after extraction from the heating furnace, the scale was removed by high-pressure water descaling, and the material was then reinserted into the heating furnace and reheated to remove the scale. The final temperature in the descaling process was 1180 to 1220°C, the temperature at which the material was extracted from the heating furnace again.

[0084] In conditions K and L, after extraction from the heating furnace, the steel was descaled by high-pressure water descaling, and then the steel surface was reheated in an induction heater to remove the scale. The final temperature of the descaling process was 1220-1250°C, the temperature at which the steel was removed from the induction heater.

[0085] In all cases, after rolling, the specimens were allowed to cool to room temperature in the air, then pickled to remove surface scale, and the presence or absence of surface cracks was visually inspected. If defects were found, three locations, including large ones, were selected, and if no defects were found, three random locations were selected, and the crack depth was measured by cross-sectional observation. Of the cracks found, those with a maximum crack depth of 10 μm or more were judged as ×, and those with a maximum crack depth of less than 5 μm were judged as ○.

[0086] Additionally, the thickness of the scale that grows when the surface temperature of the slab is 1200°C or higher was determined by cutting the same steel as the slab used in the hot rolling test into a 30mm x 30mm x 5mm piece and conducting an oxidation test that simulates the surface temperature history up to the rolling process in the hot rolling test. In the oxidation test, the steel is placed on a hanging balance, and the scale thickness can be determined from the change in mass of the steel during the heat treatment.

[0087] Table 1 shows the thickness of the scale grown in the temperature range of 1200°C or higher, as well as the crack assessment results under each scale-off condition.

[0088] [Table 1] *1: Scale thickness (μm) that grows when the surface temperature of the slab is above 1200°C

[0089] In Examples A, B, E, H, and K, no surface cracks were observed.

[0090] In Example C, the amount of scale-off was insufficient, and in Example D, the heating furnace extraction temperature was low, so scale-off was not achieved at 1200°C or higher, and cracks were observed in both cases.

[0091] In Example F, the reheating after descaling was insufficient, and the scale did not grow sufficiently while the sample was held (cooled) at 1200°C or higher, resulting in cracking. In Example G, the reheating was not sufficient to reach 1200°C after descaling, resulting in cracking.

[0092] In Example I, when the sample was reinserted into the heating furnace and reheated, the amount of scale-off was insufficient, and cracks were observed. In Example J, the sample was reinserted into the heating furnace, but the temperature did not reach 1200°C, and cracks were observed.

[0093] In addition, in Example J, as in Example G, the temperature of the slab surface after descaling did not reach 1200°C or higher, and scale grew below 1200°C. This is thought to have resulted in the formation of a new tramp element-enriched layer during the descaling process, causing cracks.

[0094] In Example L, the amount of scale growth was small when induction reheated using an induction heating device, and cracks occurred.

[0095] From these results, it was confirmed that in the examples of the present invention (Examples A, B, E, H, and K), scale growth of 75 μm or more was achieved at 1200°C or higher, that is, scale-off was achieved, in which all of the base metal including the tramp element-enriched layer was scaled, and that a hot-rolled steel sheet in which surface cracking due to red embrittlement was suppressed during subsequent hot rolling, i.e., a hot-rolled steel sheet with excellent surface properties, could be obtained.

Claims

1. In the hot rolling of a slab having a composition containing, in mass %, C: 0 to 0.5%, Si: 0 to 3.0%, Mn: 0 to 3.0%, P: 0 to 0.05%, S: 0 to 0.05%, Al: 0 to 1.0%, Cu: 0.01 to 1.0%, Ni: 0 to 1.0%, Cr: 0 to 1.0%, Sn: 0 to 1.0%, N: 0 to 0.01%, with the balance being Fe and impurities, during the period from heating the slab to rolling, Heating or reheating the surface temperature of the slab to 1200°C or higher; The amount of scale growth at 1200°C or higher is 75 μm or more. A method for manufacturing a hot-rolled steel sheet.

2. 2. The method for manufacturing a hot-rolled steel sheet according to claim 1, wherein, during the period, the surface temperature of the slab when removed from the heating furnace is 1200°C or higher, and the amount of scale growth when soaked at 1200°C or higher is 75 μm or higher.

3. 2. The method for manufacturing a hot-rolled steel sheet according to claim 1, wherein, during the period, after the slab is extracted from the heating furnace, scale is removed by high-pressure water descaling, the surface temperature of the slab thereafter is 1200°C or higher, and the amount of scale growth during the period when the surface temperature of the slab after descaling is 1200°C or higher is 75 μm or higher.

4. 4. The method for manufacturing a hot-rolled steel sheet according to claim 3, characterized in that after the scale removal process by high-pressure water descaling, the surface temperature of the slab is raised to 1200°C or higher by reheating, and the surface temperature of the slab is maintained at 1200°C or higher on a table roller, and the amount of scale growth at this time is 75 μm or more.

5. 4. The method for producing a hot-rolled steel sheet according to claim 3, wherein after the scale removal step by high-pressure water descaling, the slab is returned to the heating furnace and reheated so that the surface temperature of the slab becomes 1200°C or higher, and an amount of scale growth during soaking at 1200°C or higher is 75 μm or higher.

6. 4. The method for producing a hot-rolled steel sheet according to claim 3, wherein after the scale removal step by high-pressure water descaling, the slab is induction reheated in an induction heating device so that the surface temperature of the slab becomes 1200°C or higher, and the amount of scale growth at this time is 75 μm or higher.

7. The method for producing a hot-rolled steel sheet according to any one of claims 1 to 6, characterized in that the composition contains 0.01 to 0.04 mass% of Ni.

Images