JP5569186B2 - Production line for hot rolled steel sheet and method for producing hot rolled steel sheet - Google Patents

Description

The present invention relates to a manufacturing method and manufacturing equipment for manufacturing a hot-rolled steel material having excellent surface properties in a process of hot-rolling and manufacturing a steel material.

After being cast, the steel material is heated by a combustion gas in a heating furnace, extracted at a temperature of 900 to 1300 ° C., descaled with high-pressure water, and rolled by a hot rolling mill. Usually, a scale layer is generated on the surface of a steel material heated to a high temperature in a heating furnace. This scale layer is mainly made of iron oxide, and generally _three layers of hematite (Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ), and wustite (FeO) are formed from the surface layer. Thus, the scale produced | generated with a heating furnace is called a primary scale.

  In order to prevent surface flaws from occurring when the steel material is hot-rolled, high-pressure water descaling is performed in which high-pressure water is sprayed before rolling to remove the primary scale. However, even if the primary scale is removed, since the hot rolling is performed in the atmosphere, the steel surface is again covered with the scale. Thus, the scale produced | generated in a hot rolling process is called a secondary scale.

  Usually, the primary scale generated during heating is removed by high-pressure water descaling in which high-pressure water is sprayed before the first hot rolling. The secondary scale generated during hot rolling is removed by high-pressure water descaling continuously performed during rolling, or a beautiful steel surface is formed by being uniformly stretched together with the steel during rolling. .

    However, if high pressure water descaling is not sufficient to remove the primary and secondary scales, a partially thick scale remains. When the remaining scale is pushed into the steel during hot rolling, surface defects and patterns are generated. Further, when the scale grows thick and peeling occurs immediately before hot rolling, the peeled scale is pushed into the steel material during hot rolling, and surface flaws and patterns are generated.

  Furthermore, when the scale generated before hot rolling is relatively thick, even if the scale does not peel off, the scale cracks greatly during hot rolling, and they are pushed into the steel material. Patterns may occur.

  That is, when high-pressure water descaling before hot rolling is insufficient, when the scale peels off immediately before hot rolling, or when a thick scale cracks during hot rolling, in these cases surface defects or patterns It is shown in Non-Patent Document 1 that this occurs. These surface wrinkles and patterns not only impair the appearance, but in any case, since the scale is pushed into the steel material, irregularities are formed on the surface of the steel material, which becomes a starting point for corrosion and destruction of the steel material. Therefore, it is essential to prevent such surface flaws from occurring. For this purpose, it is necessary to identify the process in which surface defects have occurred and to improve the process.

  From the descaling to the start of finish rolling in the finish rolling stage of hot rolling, or between each rolling of continuous hot finish rolling (hereinafter referred to as “pass” for each rolling of continuous rolling unless otherwise specified) During the rolling, it is referred to as “between passes”), and the scale peels off hot, and the peeled scale is pushed in the subsequent rolling to cause wrinkles. Such scale peeling between heats generated in a short time after descaling is called blistering. A cross-sectional photograph of the steel material surface when blistering occurs is shown in FIG. It can be seen that the scale is locally swollen and peeled off.

  Non-Patent Document 2 shows the generation behavior of blistering. According to this, as oxidation progresses, stress is applied in the scale and blistering occurs. It is shown that blistering occurs in a shorter time as the temperature is higher, and that it is effective to lower the rolling temperature.

  In Patent Document 1, it is proposed to supply an aqueous solution containing a chemical solution having an oxidation-inhibiting action for the purpose of preventing hot scale peeling. Since the chemical is used, it is necessary to collect the chemical in the cooling water or to treat the waste water.

  Patent Document 2 proposes a rolling method for starting rolling before occurrence of blistering after descaling. Although this method is effective when a product is manufactured by one-pass rolling after descaling, there is a problem that blistering cannot be suppressed when rolling twice or more after final descaling.

JP 2006-272410 A JP-A-11-129016

“Mika Okada, Scale Behavior in Hot Rolling” Plasticity and Processing (Journal of the Japan Society for Technology of Plasticity), Vol. 44, No. 505 (2003-2), p. 12-17 “Effects of steel composition and temperature on high temperature adhesion of scales by Kizu Taro” Materials and Processes (Japan Iron and Steel Institute), Vol. 13, p. 1088-1091

  If the rolling temperature is lowered to suppress the occurrence of blistering when the steel surface is oxidized as in the technique described in Non-Patent Document 2, the temperature in the final rolling may become too low. In addition, as in the technique described in Patent Document 1, the use of a medicine has a problem in that the cost of collecting the medicine and wastewater treatment is high. Furthermore, with the technique described in Patent Document 2, it is not possible to suppress the occurrence of blistering in rolling after the second pass. Furthermore, when correction is performed after finish rolling, it is necessary to suppress the occurrence of blistering after finish rolling.

The object of the present invention is to suppress the occurrence of blistering without adjusting the temperature of the steel material or using chemicals when rolling multiple times after final descaling or when correcting after finishing rolling, and has an excellent surface condition The object is to propose a production facility and a production method for obtaining hot rolled steel.

  In order to solve the above-mentioned problems, the present inventors have made extensive studies and analyzed the mechanism of blistering. That is, after removing the primary scale due to descaling by hot rolling, the surface of the steel sheet is activated and the oxidation proceeds very quickly because of the high temperature. When iron is oxidized, it generally has a three-layered scale structure of hematite, magnetite, and wustite, which is produced when oxygen in the atmosphere is sufficiently supplied to the steel surface. Is defined as the high oxygen concentration atmosphere condition. That is, it is considered that oxygen (Fe) ions are rate-limiting although oxygen is sufficient.

  On the other hand, when the amount of oxygen in the gas phase near the steel surface is small, the diffusion of oxygen becomes rate limiting, and no hematite or magnetite is produced, resulting in a scale structure made of wustite. A condition that satisfies such a state is defined as a low oxygen concentration atmosphere condition.

  It is known that the oxidation rate under high oxygen concentration atmosphere conditions is inversely proportional to the thickness of the scale. This is because the growth of the scale is mainly controlled by the diffusion of iron ions in the scale. As can be seen from this, a very fast oxidation rate can be obtained at the very beginning of oxidation immediately after descaling. However, since the oxygen concentration in the atmosphere is actually about 21%, the initial oxidation stage must be in a low oxygen concentration atmosphere condition. Subsequent growth of the scale shifts to high oxygen concentration atmospheric conditions.

The present inventors have found from the above analysis that the transition from a low oxygen concentration atmosphere condition to a high oxygen concentration atmosphere condition is necessary for the occurrence of blistering. Under low oxygen concentration atmosphere conditions, it has a wustite single-layer scale structure, and shifts to a high oxygen concentration atmosphere condition and changes to a three-layer scale structure of hematite, magnetite, and wustite. It was found that a large compressive stress was generated in the scale when the scale had a three-layer structure. First, in the case of a wustite monolayer, no new stress is generated in the scale because a new oxide is generated on the surface of the wustite.
Next, when a three-layer scale of hematite, magnetite, and wustite is generated from the surface, a new oxide is formed as hematite on the hematite surface and the hematite / magnetite interface, but at the latter hematite / magnetite interface. When magnetite is oxidized to produce hematite, a compressive stress is generated due to an increase in volume.
Further, the hematite layer on the surface is changed to magnetite or wustite sequentially, so that iron (Fe) ions are sent toward the outer layer of the scale like a relay, and the scale continues to grow. Here, hematite changes to magnetite at the hematite / magnetite interface, and magnetite changes to wustite at the magnetite / wustite interface.

  Since all of these reactions occur in the solid and increase in volume, compressive stress is generated in the scale. When a scale having a three-layer structure is generated in this way, a large compressive stress is generated in the scale.

  That is, it has been found that the occurrence of blistering cannot be explained by the critical points of oxidation time and scale thickness as in Non-Patent Document 2 and Patent Document 2. No matter how thick the scale, blistering does not occur in a low oxygen concentration atmosphere. It occurs only when transitioning from a low oxygen concentration atmosphere condition to a high oxygen concentration atmosphere condition. Therefore, the present inventors have found that the occurrence of blistering does not occur if the scale continues to grow under low oxygen concentration atmosphere conditions.

  That is, after removing the scale by descaling, the occurrence of blistering can be suppressed by setting the steel sheet surface to a low oxygen concentration atmosphere condition. It was also found that the low oxygen concentration atmosphere condition can be achieved by blowing an atmosphere gas having an oxygen concentration in the atmosphere lower than that of the atmosphere onto the steel surface. Based on these findings, the inventors have made the present invention. The gist of the present invention is as follows.

(1) In hot rolling of a steel sheet, a steel rolling hot rolling equipment row equipped with descaling and finishing rolling mills in this order, and before or after the finishing rolling mill with respect to the steel sheet conveying direction or the finishing rolling mill If it is composed of a plurality of rolling mill between them rolling mill, at least one position, the steel plate having a low oxygen concentration gas than air (except those containing Cr) scale structures sprayed on the surface A hot-rolling equipment line for steel sheets, comprising a gas spraying equipment with a scale made of wustite.

  (2) The hot rolling equipment row of steel sheets according to (1), wherein the finish rolling mill is a continuous finish rolling mill composed of a plurality of finish rolling mills.

  (3) The hot rolling equipment row of steel sheets according to (1), wherein the finish rolling mill is a reverse rolling mill.

  (4) The hot rolling equipment row according to (3), wherein a straightening machine is disposed after the reverse rolling mill.

  (5) The gas having a lower oxygen concentration than the atmosphere uses any one of nitrogen gas, inert gas, water vapor and carbon dioxide, or a mixture of two or more thereof (1) to (4) The hot-rolling equipment row | line | column of the steel plate of any one of these.

  (6) The gas having a lower oxygen concentration than the atmosphere uses any one or more of nitrogen gas, inert gas, water vapor and carbon dioxide mixed with the atmosphere (1) to The hot-rolling equipment row | line | column of the steel plate of any one of (4).

(7) In the method of manufacturing a hot-rolled steel sheet that performs finish rolling after descaling, at least one point before or after finish rolling or when the finish rolling is composed of a plurality of rollings in method of hot-rolled steel sheet, characterized in that the scale structure formed by blowing low gas oxygen concentration than the atmosphere (excluding those containing Cr) steel plate surface scale structures from wustite.

  (8) The method for producing a hot-rolled steel sheet according to (7), wherein the finish rolling is continuous finish rolling by a continuous finish rolling mill constituted by a plurality of continuous finish rolling mills.

  (9) The method for producing a hot-rolled steel sheet according to (7), wherein the finish rolling is rolling performed by a reverse rolling mill.

  (10) The method for producing a hot-rolled steel sheet according to (9), wherein correction is performed after the finish rolling.

  (11) The gas having a lower oxygen concentration than the atmosphere uses any one of nitrogen gas, inert gas, water vapor, and carbon dioxide, or a mixture of two or more thereof. (7) to (10) The manufacturing method of the hot-rolled steel plate of any one of these.

  (12) The gas having a lower oxygen concentration than the atmosphere uses any one or more of nitrogen gas, inert gas, water vapor and carbon dioxide mixed with the atmosphere (7) to (7) The method for producing a hot rolled steel sheet according to any one of (10).

(13) The production of the hot-rolled steel sheet according to any one of (7) to (12), wherein the gas having a lower oxygen concentration than the atmosphere is sprayed at a steel plate temperature of 900 ° C. or higher. Method.

ADVANTAGE OF THE INVENTION According to this invention, generation | occurrence | production of the blistering which is scale peeling at the high temperature which causes a surface flaw can be suppressed, and the hot-rolled steel materials excellent in surface property can be obtained.

It is a schematic diagram which shows the relationship between the oxygen concentration of blowing gas required in order to set it as low oxygen concentration atmospheric conditions, steel material temperature, and the time after descaling. It is a schematic diagram which shows the equipment of the hot rolling of the thin steel plate which implements this invention. It is a schematic diagram which shows the equipment of the hot rolling of the thick steel plate which implements this invention. It is a figure which shows the cross-sectional photograph of the surface of the steel material of the site | part which the blistering generate | occur | produced.

Embodiments of the present invention will be described below.
FIG. 1 shows the oxygen concentration of the blowing gas necessary to obtain the low oxygen concentration atmosphere condition obtained by simulation and experiment in relation to the steel material temperature and the time after descaling. It can be seen that in order to obtain the low oxygen concentration atmosphere condition, it is more effective to blow the gas having a low oxygen concentration after a certain amount of time has passed since descaling than immediately after descaling.

  Immediately after descaling, the scale thickness is very thin and the steel plate temperature is high, so the growth rate of iron oxide is very fast (A region in FIG. 1). Therefore, in order to maintain a three-layer scale composed of hematite, magnetite, and wustite, a large amount of oxygen must be supplied from the gas phase. However, since there is only about 21% oxygen in the atmosphere, This is because the supply rate is limited, the scale of the three-layer structure is difficult to maintain, and the atmosphere condition is low. Accordingly, in the region A of FIG. 1, the blistering suppression effect is low even when low oxygen gas is blown.

  Next, as time elapses even at the same temperature, the iron oxide layer becomes thicker, and the supply of iron ions is controlled at the outermost surface of the iron oxide layer, resulting in a high oxygen concentration atmosphere condition. Similarly, if the temperature decreases even with the same time passage, that is, the same thickness of the iron oxide layer, the growth rate of the iron oxide decreases, the rate of supply of iron ions is limited, and the atmosphere condition is high. For this reason, in order to suppress blistering, it is necessary to lower the oxygen concentration (B region). Therefore, in the region B of FIG. 1, blistering can be suppressed by creating a low oxygen concentration atmosphere condition by blowing a gas having a lower oxygen concentration than the atmosphere to the steel sheet.

  Further, as time passes and the iron oxide layer becomes thicker, or when the temperature drops even with the same iron oxide film thickness, the outermost surface of the iron oxide becomes increasingly oxygen-limited (C in FIG. 1). region). Therefore, in order to suppress blistering, it is necessary to blow a gas whose oxygen concentration is significantly lower than that of the atmosphere. For example, the oxygen concentration must be 10% or less. However, the lower the temperature, the more difficult the scale peels off, and blistering does not occur below 900 ° C. Therefore, the object of the present invention is a temperature range in which the steel material temperature exceeds 900 ° C.

In order for such blistering to become surface defects, rolling at a stage close to the product is the most important. Therefore, the present invention is applied between the rolling passes of finish rolling performed after descaling and between finishing rolling and correction. Depending on the temperature and rolling speed, it may be performed after descaling and before rolling.
For example, in hot continuous rolling, if low oxygen atmosphere conditions are set between each rolling pass, from descaling to the first rolling (that is, before the first rolling), and at one or more places after the final rolling. Good. That is, it is preferable to provide equipment for creating a low oxygen atmosphere condition before the first rolling mill of the continuous rolling mill, behind the last rolling mill, and between the rolling mills.

  In addition, for example, in hot rolling with a reverse rolling mill, one or both of between descaling and reverse rolling, or between reverse rolling and straightening may be a low oxygen atmosphere condition. That is, it is preferable to provide equipment for creating a low oxygen atmosphere before and after the reverse rolling mill.

In order to create a low oxygen atmosphere, a gas having a lower oxygen concentration than the atmosphere may be blown onto the steel plate surface. The gas spraying requires 100 L / min or more per 1 m ^{2 of the} surface area of the steel material. This is a flow rate necessary for the gas supply to the steel surface to effectively suppress the intrusion of oxygen from the atmosphere.
As an example, the manufacture of a thin steel plate will be described as an example. The thin steel sheet is descaled after the end of the rough rolling, and is subjected to continuous hot rolling (called continuous hot rolling or continuous finish rolling) through finishing mills arranged in several rows, and then cooled with water. There are a plurality of finish rolling mills in succession, and there are many cases of 6 to 7 stages. In this case, the occurrence of blistering can be suppressed by spraying a gas having a low oxygen concentration between the finishing mills. Depending on the rolling temperature and rolling speed, it is necessary to carry out from the descaling device to the finishing mill. Whether this is necessary or not can be determined from FIG.

  The broken line in FIG. 1 shows an example of a temperature-time path from descaling after rough rolling to the final pass value of continuous finish rolling when a thin steel plate is produced by continuous hot rolling. In the example shown in this figure, the descaling is finished at around 1060 ° C., and the first stage at around 1050 ° C., the second stage at around 1010 ° C., and the third stage at around 980 ° C. are applied. In this example, since it is in the A region until immediately after the first stage, it is not necessary to spray low oxygen gas, but since the second and subsequent stages are in the B area, low oxygen gas must be sprayed and the fifth and subsequent stages. It has been shown that an oxygen concentration of 10% or less is necessary because of the C region.

  In addition, when a plurality of rolling operations are continuously performed, blistering hardly occurs immediately after each rolling pass. This is because the scale is strongly pressed by rolling to increase the adhesion of the scale, and the scale is deformed by rolling to release the stress in the scale. The occurrence of blistering becomes a problem after about 2 seconds or more have passed since the stress is generated by the change of the scale to the three-layer structure again after the rolling pass. Therefore, in the case of rolling a plurality of times, the present invention is effective when the passing time between the rolling mills (between rolling passes) exceeds about 2 seconds.

  When rolling continuously like a thin steel plate, the passage time between each rolling pass becomes shorter in the later stages. In normal continuous hot rolling, the occurrence of blistering becomes a problem between the first and second stages and between the second and third stages. Here, since the passing time between the finishing mills (between each rolling pass) is long, the change to the three-layer structure of the scale proceeds, and the stress is generated again on the scale, so that blistering occurs. Therefore, in this region, it is most effective to blow a gas having a low oxygen concentration to suppress blistering.

  After the third and fourth stages, the rolling speed increases and the next rolling starts before the stress is generated due to the change of the scale to the three-layer structure. Will disappear. In addition, the scale is strongly pressed by being rolled, and thereby the adhesion of the scale is improved, and the scale is deformed by rolling, so that the stress in the scale is released. As a result, between the third stage and the fourth stage and later, even if the oxygen concentration is lowered, a high oxygen concentration atmosphere tends to be obtained. Blistering is less likely to occur than between the third stage.

  Next, the manufacture of thick steel plates will be described as an example. In the case of a thick steel plate, finish rolling is performed after rough rolling. Normally, finish rolling is performed by reverse rolling. At the time of finish rolling, descaling is performed immediately before or after the rolling pass as necessary. After finishing rolling, the product is air-cooled or water-cooled and corrected as necessary to obtain a product. Blistering is likely to occur in the temperature range up to about 900 ° C. during finish rolling or after finish rolling. As described above, blistering hardly occurs at 900 ° C. or lower. Therefore, in the region where the steel material stays during the finish rolling and the region until the steel material after the finish rolling reaches 900 ° C., it is effective to blow the gas with a low oxygen concentration for suppressing the occurrence of blistering. Since it is reverse rolling, it is effective to spray a low oxygen concentration gas before or after the rolling mill.

  In the present invention, it is not necessary to completely suppress oxidation. The scale may be thicker if the change to a three-layer structure does not begin. That is, it is sufficient that the steel material surface is in a low oxygen concentration atmosphere until it is cooled to 900 ° C. Therefore, it is not necessary to completely partition the atmosphere, and it is sufficient to blow a gas whose oxygen concentration is lower than that of the atmosphere. Of course, the effect of the present invention is exhibited even if there is a partition of the atmosphere.

  The gas to be sprayed may be a gas having a lower oxygen concentration than the atmosphere, but it is desirable that the oxygen concentration be 10% or less when no atmosphere partition is provided. When the first to third rolling steps are performed in the region B of FIG. 1 and the atmosphere partition is not provided, it is more preferable to ensure the atmosphere by blowing a gas having an oxygen concentration of 10% or less. Naturally, further reducing the oxygen concentration enhances the effect of the present invention.

A non-oxidizing gas may be used to lower the oxygen concentration. For example, it is preferable to use an inert gas such as nitrogen, argon, or helium. Moreover, even if it is oxidizing, water vapor or carbon dioxide can be used. This is because oxidation with these gases is very slow although it is oxidizing to water vapor and carbon dioxide. Accordingly, a gas such as steam or combustion exhaust gas can be used as the gas having a reduced oxygen concentration. One or more of these gases may be mixed and sprayed onto the surface of the steel sheet, or one or more of these gases and the atmosphere may be mixed and sprayed onto the surface of the steel sheet.

Application to thin steel sheets In the continuous hot rolling facility for thin steel sheets as shown in Fig. 2, between the descaling and continuous finish rolling mill before continuous finish rolling, the finish rolling of the first and second stages of continuous finish rolling. A gas spraying device for the steel plate was provided between the machines. Using this equipment, a steel material containing 0.09% C, 0.014% Si, 0.40% Mn, 0.44% P, 0.004% S and 0.003% S by mass% as main components. Rolled. Before continuous finish rolling, a steel material having a plate thickness of 30 mm and a steel material temperature of 1080 ° C. was subjected to descaling and subjected to seven-stage continuous finish rolling to a plate thickness of 3.2 mm. At this time, a thin steel sheet was produced by changing the gas blowing conditions on the steel sheet surface before the continuous finish rolling and in the continuous finish rolling mill. The surface condition of the steel material obtained at that time is shown in Table 1. The generation of surface flaws was suppressed.

Application to Thick Steel Plate In the hot rolling equipment for thick steel plates as shown in FIG. 3, a gas spraying device for the steel plates was provided before and after the finish rolling mill (reverse rolling mill). A steel material containing 0.15% C, 0.07% Si, 0.7% Mn, 0.7% Mn, 0.008% P and 0.008% S in mass% as main components using this equipment. Rolled. A steel material having a plate thickness of 120 mm and a steel material temperature of 930 ° C. was subjected to 11 passes of finish rolling with a finish rolling mill before the finish rolling. Rolling was performed by reverse rolling and rolled to a thickness of 22 m. The first to tenth passes were descaled before rolling, and the eleventh pass was rolled without descaling. Thick steel plates were produced by changing the gas blowing conditions and rolling conditions before and after finish rolling at this time, and Table 2 shows the surface states of the steel materials obtained at that time. By implementing the present invention, generation of surface flaws could be suppressed.

INDUSTRIAL APPLICABILITY The present invention can be used as a manufacturing facility or a manufacturing method that suppresses the generation of surface defects when hot rolling a steel material.

a Rough rolling mill
b Finish rolling mills
c Descaling
d Low oxygen concentration gas spray
e Finishing mill
f Water cooling equipment g Hot straightening machine

Claims (13)

In hot rolling of steel plates, descaling, a hot rolling equipment row of steel plates equipped with a finish rolling mill in this order,
When the finish rolling mill is composed of a plurality of rolling mills before or after the finishing mill with respect to the steel sheet conveying direction, a gas having a lower oxygen concentration than the atmosphere is provided at least at one location between the rolling mills. hot rolling mill train of steel sheet characterized by having a gas blowing equipment and scale features (except those containing Cr) steel plate by spraying the surface becomes the scale structure from wustite.   The hot rolling equipment row of steel sheets according to claim 1, wherein the finish rolling mill is a continuous finish rolling mill composed of a plurality of rolling mills.   The hot rolling equipment row of steel sheets according to claim 1, wherein the finish rolling mill is a reverse rolling mill.   The hot rolling equipment train according to claim 3, wherein a straightening machine is disposed after the reverse rolling machine.   The gas having a lower oxygen concentration than the atmosphere uses any one of nitrogen gas, inert gas, water vapor, and carbon dioxide, or a mixture of two or more thereof. The hot-rolling equipment row | line | column of the steel plate as described in.   5. The gas according to claim 1, wherein the gas having a lower oxygen concentration than the atmosphere uses one or more of nitrogen gas, inert gas, water vapor, and carbon dioxide mixed with the atmosphere. The hot rolling equipment row | line | column of the steel plate of Claim 1. In the method for manufacturing a hot rolled steel sheet, which is subjected to finish rolling after descaling, before or after finish rolling, or when the finish rolling is constituted by a plurality of rollings, at least at one place between those rollings, method for producing a hot rolled steel sheet, characterized in that the scale structure comprising a scale structures from wustite by blowing low oxygen concentration gas (excluding those containing Cr) steel plate surface than.   The method for producing a hot-rolled steel sheet according to claim 7, wherein the finish rolling is continuous finish rolling by a continuous finish rolling mill constituted by a plurality of continuous rolling mills.   The method for producing a hot-rolled steel sheet according to claim 7, wherein the finish rolling is rolling performed by a reverse rolling mill.   The method for producing a hot-rolled steel sheet according to claim 9, wherein straightening is performed after the finish rolling.   The gas having a lower oxygen concentration than the atmosphere uses any one of nitrogen gas, inert gas, water vapor, and carbon dioxide, or a mixture of two or more thereof. A method for producing a hot-rolled steel sheet according to 1.   The gas having a lower oxygen concentration than the atmosphere uses any one or two or more of nitrogen gas, inert gas, water vapor, and carbon dioxide mixed with the atmosphere. A method for producing a hot-rolled steel sheet according to claim 1.   The method for producing a hot-rolled steel sheet according to any one of claims 7 to 12, wherein a gas having a lower oxygen concentration than the atmosphere is sprayed at a steel plate temperature of 900 ° C or higher.