Hot rolling method

Abstract

To provide a method where, in the production of a tramp element-containing steel material, the positive addition of Ni is not required, temperature restriction is reduced, and the effect of suppressing red heat embrittlement is higher.SOLUTION: A hot-rolling method comprising a step of heating a steel containing, by mass%, 0 to 0.5% of C, 0 to 3.0% of Si, 0 to 3.0% of Mn, 0 to 0.05% of P, 0 to 0.05% of S, 0 to 1.0% of Al, 0.01 to 1.0% of Cu, 0 to 1.0% of Ni, 0 to 1.0% of Cr, 0.001 to 1.0% of Sn, and 0 to 0.01% of N, with the balance being Fe and an impurity, wherein a steel surface temperature satisfies the following formula in a temperature range of 950 °C or higher and 1200 °C or lower: In the formula, t1 is the time (minute) when the surface temperature of the steel product exceeds 950 °C, t2 is the earlier one (minute) of the time when the surface temperature of the steel product reaches 1200 °C and the time when the steel product is extracted from the heating oven, and T (t) is the surface temperature (°C) of the steel product at the time t.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hot rolling method for steel materials, and more particularly to a hot rolling method capable of suppressing 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 general mechanism by which red embrittlement occurs is thought to be as follows. Generally, in the production of steel by hot rolling, the steel is loaded into a heating furnace prior to hot rolling, heated with combustion gas to the desired temperature for hot rolling, and then extracted from the heating furnace. The oxide scale is then removed (descaled) with high-pressure water before hot rolling. 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 metals more noble than iron, such as Cu and Sn, are present, they do not oxidize 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, and if the concentration of Cu or Sn exceeds this level, they appear as metallic phases. The melting points of Cu and Sn are approximately 1080°C and 232°C, respectively. Steel is typically heated to temperatures above these levels before hot rolling. Liquid phases of molten Cu and Sn form at the oxide scale / base steel interface, penetrate the grain boundaries of the base steel, and are unable to withstand the shear and tensile stresses during hot rolling, resulting in 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. In 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 material), and the concentrated Cu melts and penetrates into the grain boundaries of the base material, which is thought to cause red embrittlement. Patent Document 2 is 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 relates to a method for producing a Cu-containing steel material, and describes how red embrittlement can be avoided by increasing the temperature at an average heating rate of 50°C / h or more between 1000 and 1100°C, and maintaining the maximum heating temperature at 1200 to 1350°C for 1 hour or more. Patent Document 3 specifies the average heating rate between 1000 and 1100°C, but does not describe or suggest the heating rate in the temperature range of 950 to 1200°C. However, as will be described in detail later, the inventors have conducted research and found that heating in the temperature range of 950 to 1200°C in a short period of time is important for suppressing red embrittlement cracking. Patent Document 3 also describes a technology intended to prevent red embrittlement in Cu-containing steel material that is substantially free of Sn. [Prior art documents] [Patent documents]

[0009] [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 Publication No. 2011-168843 Summary of the Invention [Problem to be solved by the invention]

[0010] Various methods have been investigated to roll steel containing tramp elements without impairing the surface quality, but further improvements are needed.

[0011] 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.

[0012] Patent Document 2 discloses that, since the melting point of Cu is around 1100°C, red embrittlement can be avoided by setting the upper heating temperature limit for Cu-containing steel to 1050°C or lower. However, in many cases, heating to 1050°C or higher is required 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.

[0013] Patent Document 3 specifies an average heating rate of 1000°C to 1100°C in order to avoid red embrittlement of Cu-containing steel. However, the inventors have conducted research and found that red embrittlement cracking cannot be sufficiently suppressed by simply managing the heating rate specified in Patent Document 3. As will be described in detail later, the inventors have found that the residence time outside the temperature range of 1000°C to 1100°C is also important; in other words, raising the temperature in the temperature range of 950°C to 1200°C in a short time is important in order to suppress red embrittlement cracking. Furthermore, Patent Document 3 does not take Sn into consideration.

[0014] To summarize the above situation, scrap contains tramp elements such as Cu or Sn, which can cause red-embrittlement cracking, and it is desirable to suppress such red-embrittlement cracking. Patent Document 1 discloses that Ni has the effect of suppressing red-embrittlement cracking, but it is expensive. Patent Document 2 discloses that red-embrittlement cracking can be suppressed by setting the heating temperature to 1050°C or less without adding Ni, but actual steel production often requires heating to 1050°C or higher. Patent Document 3 also discloses controlling the average heating rate from 1000°C to 1100°C, but temperature control only in this temperature range does not sufficiently suppress red-embrittlement cracking.

[0015] Therefore, the inventors set out to provide a method for producing steel material 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, steel material that contains large amounts of Cu and Sn is referred to as tramp element-containing steel, and a concentrated layer that contains large amounts of Cu and Sn is referred to as a tramp element-concentrated layer. [Means for solving the problem]

[0016] The present inventors have found that the temperature history during heating of a steel material (slab) for hot rolling is closely correlated with red embrittlement, and that the occurrence of red embrittlement can be prevented by shortening the time the steel is held in the temperature range of 950°C to 1200°C in particular. They have also found that if the time held in this temperature range is short, red embrittlement will not occur in subsequent rolling even if the steel material is held in a temperature range where the surface temperature is 1200°C or higher for a long period of time. Based on these findings, the present inventors have completed the present invention, which is summarized as follows. [1] 1. A hot rolling method comprising: a step of heating a steel material 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: 0.001-1.0%, N: 0-0.01%, and the balance being Fe and impurities; wherein the steel material surface temperature satisfies the following formula in a temperature range of 950°C or higher and 1200°C or lower:

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[0017] 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 add excessive Ni or to limit the temperature (typically, the heating temperature may be 1050°C or higher). [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram showing an example of the temperature history of a steel material and an index ST. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention will be described.

[0020] One embodiment of the present invention comprises: 1. A hot rolling method comprising: a step of heating a steel material 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: 0.001-1.0%, N: 0-0.01%, and the balance being Fe and impurities; wherein the steel material surface temperature satisfies the following formula in a temperature range of 950°C or higher and 1200°C or lower:

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[0021] (Applicable steel type) The steel type targeted in this embodiment is a tramp element-containing steel, and the steel type in which the effects of the present invention are most pronounced is one 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: 0.001-1.0%, N: 0-0.01%, with the balance being Fe and impurities.

[0022] (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 saturates, 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 may be 0.001 mass% or more.

[0023] (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 0.1% 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 an excessive amount of Si is added, 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.

[0024] (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.

[0025] (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.

[0026] (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.

[0027] (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.

[0028] (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.

[0029] (Sn: 0.001 to 1.0%) Sn is generally a component contained in scrap. It prevents 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. To achieve this effect, the lower limit of the Sn content may be set to 0.001% by mass or more, preferably 0.010% by mass or more, and more preferably 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 the upper limit is set to 1.0% by mass. 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, depending on the scrap properties and the desired properties.

[0030] (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 0.10 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. Even in a composition with a low Ni content, as in this embodiment, cracking due to red embrittlement can be prevented.

[0031] (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. The lower limit may be 0% by mass. Depending on the scrap properties, desired properties, and scrap blending ratio, the content may be set to 0.10% by mass or more, more preferably 0.20% by mass or more, and even more preferably 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.

[0032] (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.

[0033] (balance: Fe and impurities) The balance of the chemical composition of the tramp element-containing steel material 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 material, and are acceptable within a range that does not adversely affect the steel material of this embodiment.

[0034] (Optionally added elements) Material strengthening elements such as Ti, Nb, Mo, and V are known, and the composition of the tramp element-containing steel of this embodiment may contain these. Therefore, the content of each material strengthening element such as Ti, Nb, Mo, and V may 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.

[0035] (Regarding the indicator ST) In this embodiment, in the step of heating the steel material for hot rolling, the index ST calculated from the temperature history of the steel material in a temperature range in which the surface temperature of the steel material is 950°C or higher and 1200°C or lower is 7500 or lower. The index ST is expressed by the following formula (1).

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[0036] The reason why limiting the index ST to a specific range can suppress cracking on the steel surface due to red embrittlement is not entirely clear, but the inventors have discovered that the temperature history of the process in which the steel (slab) obtained by continuous casting is heated in a heating furnace before hot rolling is the most important factor in hot rolling steel containing tramp elements without surface cracking due to red embrittlement. The heating process for hot rolling involves placing the steel (slab) cooled to room temperature to approximately 700°C after continuous casting into a heating furnace and heating it again to 1200-1300°C, up to the temperature required for hot rolling. The atmosphere in the heating furnace often contains oxygen and water vapor, which cause oxide scale to form on the surface of the steel (slab). In the case of steel containing tramp elements, as scale is formed by oxidation of Fe, the Cu and Sn contained in the steel are concentrated because they are more noble than Fe and are not oxidized, and this concentrated Cu and Sn penetrate the grain boundaries of the steel, causing red embrittlement and resulting in surface cracking during subsequent hot rolling. After detailed studies by the inventors, it was found that when a steel material (slab) is held for a long period of time in a temperature range where the surface temperature of the steel material (slab) is 950°C to 1080°C in a heating furnace, Cu and Sn are concentrated at the grain boundaries of the steel, and when a slab is held for a long period of time in a temperature range where the surface temperature is 1080°C to 1200°C, Cu and Sn are concentrated at the interface between the oxide scale and the steel (base metal), and it was found that in both cases, surface cracking due to red embrittlement occurs during subsequent hot rolling. Conventionally, as in Patent Document 2, the concentration of Cu below approximately 1080°C, the melting point of Cu, was not considered important. However, according to the investigations leading to the present invention, when Sn is included, the melting point of Sn is lower than that of Cu, so Sn concentration and melting occur even in the temperature range below 1080°C. It is believed that molten Sn-Cu is formed when solid metallic Cu dissolves in molten Sn, and penetrates into the grain boundaries of the steel. It has been found that red-hot embrittlement cracking occurs even in the temperature range below 1080°C. Because metallic Cu also melts at temperatures above 1080°C, the proportion of Cu contained in molten Sn-Cu increases. It was presumed that the higher the proportion of Sn in molten Sn-Cu, the better the wettability with steel and the easier it is to penetrate into the grain boundaries.On the other hand, it was also found that when the temperature is raised within the range of 950°C to 1200°C in a short period of time, even if the steel (slab) is held at 1200°C or higher for a long period of time, the concentration of Cu and Sn during this period is slight, and surface cracks do not occur during the subsequent hot rolling. Therefore, it was concluded that the concentration of Cu and Sn during heating occurs significantly when the steel is held at a temperature range of 950°C to 1200°C for a long period of time, and that this concentrated Cu and Sn causes cracks on the steel surface due to red embrittlement. Based on this conclusion, the inventors conceived the idea that surface cracks due to red embrittlement can be avoided by rearranging the index ST as in the above formula (1) and satisfying a value of 7500 or less, and confirmed this to be the case through various experiments.

[0037] Furthermore, in one embodiment of the present invention, following the step of heating the steel material (slab) in hot rolling (limiting the index ST to a specific range in the temperature range of 950°C to 1200°C), the surface temperature of the steel material (slab) may be held in a temperature range of more than 1200°C and less than 1350°C for 30 minutes or more, and the temperature at which the steel material (slab) is subsequently extracted from the heating furnace may be more than 1200°C and less than 1350°C. This embodiment can further suppress surface cracking due to red embrittlement. As described above, it has been confirmed that even if the steel material (slab) is held at 1200°C or higher for a long period of time, the concentration of Cu and Sn during this period is slight, and surface cracking does not occur during subsequent hot rolling. Further detailed investigations have confirmed that by holding the steel material (slab) in a temperature range of 1200°C or higher, the amount of Cu and Sn present at the scale / steel interface and at the steel grain boundaries accumulated during the temperature rise (typically 950°C to 1200°C) is reduced. Although not wishing to be bound by a particular theory, this is thought to be because Cu and Sn are more easily diffused when the steel is held at a high temperature of 1200°C or higher, which reduces the amount of Cu and Sn present at the scale / steel interface and steel grain boundaries, further suppressing surface cracking due to red embrittlement.

[0038] (Hot rolling conditions) The hot rolling conditions (rolling temperature, reduction, etc.) may be adjusted as appropriate within the range of commonly employed hot rolling conditions depending on the final desired properties of the steel material. Typical hot rolling conditions or preferred conditions will be described below, but the embodiments of the present invention are not limited to these.

[0039] (heating) The steel material (slab) to be subjected to hot rolling may be produced by continuous casting. The steel material (slab) produced by continuous casting is inserted into a heating furnace during the hot rolling process and heated within the range specified in the present invention. That is, to prevent surface cracking due to red embrittlement during the production of tramp element-containing steel, the steel material (slab) is heated in the heating process so that the index ST is 7500 or less. If the index ST does not satisfy formula (1), i.e., if the index ST exceeds 7500, Cu and Sn contained in the steel material will concentrate at the scale / steel (base metal) interface or the steel grain boundaries during heating, causing surface cracking due to red embrittlement. The shorter the time in the temperature range of 950°C to 1200°C, the less Cu and Sn will be concentrated, and the more likely the occurrence of red embrittlement will be suppressed. From this perspective, the index ST is preferably 7000 or less, and more preferably 6000 or less. Although a lower limit for the index ST is not specified, further reduction of ST would require equipment with high heating capacity, which is not realistic. Therefore, the index ST may be set to 1000 or more, 3000 or more, or 5000 or more. The time from t1 to t2, i.e., the time during which the temperature is in the range of 950°C to 1200°C, or the time from 950°C to exiting the heating furnace, whichever is earlier, is not particularly limited as long as the index ST is within a predetermined range (7500 or less). However, from the viewpoint of reducing fuel costs, it may be set to 120 minutes or less, preferably 100 minutes or less, and more preferably 80 minutes or less.

[0040] (hold) After heating, the steel material (slab) is preferably held at a temperature between 1200°C and 1350°C for at least 30 minutes, and is removed from the heating furnace at the same temperature range. Because Cu and Sn are likely to concentrate at temperatures below 1200°C and are less likely to dissipate due to diffusion, holding at temperatures below 1200°C is undesirable. Heating at temperatures above 1350°C is also undesirable from the viewpoint of material quality and fuel costs. Holding times of less than 30 minutes may result in insufficient dissipation of Cu and Sn. Holding times are preferably 40 minutes or more, more preferably 50 minutes or more, and the upper limit of the holding time may be 60 minutes or less, 90 minutes or less, 120 minutes or less, or 180 minutes or less.

[0041] (rolling) The steel material (slab) extracted from the heating furnace may be subjected to high-pressure water descaling to remove surface scale, and then rolled to a predetermined thickness. General 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 the following, a high-pressure water descaling discharge force of 5 to 20 MPa and a final rolling temperature in the range of 800 to 1000°C are 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.

[0042] (Surface texture verification method) The surface quality of hot-rolled steel sheets after steel rolling is evaluated by removing scale by pickling and visually inspecting for flaws. Furthermore, three of the largest flaws are selected and the depth of the cracks is measured by cross-sectional observation to determine the degree of surface flaws. According to an embodiment of the present invention, the maximum crack depth of the cracks observed is 5 μm or more and less than 10 μm, and preferably less than 5 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 refers to meeting the above-mentioned crack depth standard (maximum crack depth less than 10 μm). [Example]

[0043] 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.

[0044] Steel samples 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 remainder consisting of Fe and other impurities, were hot-rolled using a laboratory rolling mill consisting of a heating furnace, high-pressure water descaling unit, and rolling mill. The resulting steel sheets were then examined for red-hot embrittlement cracking. Test steel samples, 30 mm thick, were heated and extracted under the heating conditions shown in Table 1. They were then subjected to high-pressure water descaling at a discharge pressure of 15 MPa and rolled five times to a thickness of 3 mm. After rolling, the samples were allowed to cool to room temperature in the air, then pickled to remove surface scale, and visually inspected for surface cracks and defects. If defects were found, three locations were selected, including large defects, 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 ×, those between 5 μm and 10 μm as ○, and those less than 5 μm as ◎. The crack judgment results for each heating condition are also shown in Table 1.

[0045] [Table 1]

[0046] In Example A, the index ST was 7500 or less, and the holding time at 1200 to 1350°C was 30 minutes or more, and no surface cracks were observed (cracking evaluation: ◎). In Examples B to D, the index ST was 7500 or less, and surface cracks were observed, but the maximum depth was confirmed to be less than 10 μm (cracking evaluation: ○). In Example B, the holding time at 1200 to 1350°C was less than 30 minutes, and in Examples C and D, the extraction temperature was less than 1200°C. Therefore, it is presumed that the Cu and Sn concentrated during the temperature rise were not sufficiently dispersed, and slight surface cracks occurred as a result of rolling. However, since the surface cracks of the hot-rolled steel sheet were less than 10 μm, there was no problem with product quality. On the other hand, in Comparative Examples E and F, the index ST exceeded 7500, and cracks exceeding a maximum depth of 10 μm were observed (cracking evaluation: ×). As described above, by setting the index ST during heating to 7500 or less, it is possible to suppress surface cracking due to red embrittlement even in steel containing tramp elements.

Claims

1. 1. A hot rolling method comprising: a step of heating a steel material containing, by 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: 0.001 to 1.0%, and N: 0 to 0.01%, with the balance being Fe and impurities; wherein the steel material surface temperature satisfies the following formula in a temperature range of 950°C or higher and 1200°C or lower: [Equation 1] however t1: the time (minutes) when the steel surface temperature exceeded 950 ° C. t2: the time when the steel surface temperature reaches 1200 ° C. or the time when the steel is extracted from the heating furnace, whichever occurs first (minutes); T(t): The surface temperature (°C) of the steel material at time t.

2. 2. The hot rolling method according to claim 1, wherein, following the heating step, the surface temperature of the steel material is maintained in a temperature range of more than 1200°C and not more than 1350°C for 30 minutes or more, and thereafter, the temperature at which the steel material is extracted from the heating furnace is more than 1200°C and not more than 1350°C.

3. The hot rolling method according to claim 1 or 2, wherein the steel material contains, by mass%, Ni: 0.01 to 0.04%.

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