JP2012148286A - Hot rolled steel plate having superior scale adhesiveness, method for manufacturing the same, and manufacturing equipment line for hot rolled steel plate having superior scale adhesiveness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel plate imparted with superior scale adhesiveness at an edge of the steel plate without using a large-scale apparatus and/or chemicals, a method for manufacturing the same, and a manufacturing equipment line for the hot rolled steel plate.SOLUTION: In the method for manufacturing a hot rolled steel plate having excellent scale adhesiveness in which steel material is hot-rolled, and coiled in a coil shape, a roughly rolled steel plate is subjected to the finish rolling at the temperature of 850-1,050°C, and the finish-rolled steel plate is coiled in a coil shape at the coiling temperature of 500-650°C while both end faces of the hot-rolled steel plate are cooled so that the temperature at the end faces reaches ≤480°C within 5 minutes after starting the coiling. Thereafter, the temperature at the end faces is maintained at ≤480°C. Then, the steel plate in a coil shape is gradually cooled from the time when the temperature at the end faces is 400-480°C.

Description

  The present invention relates to a hot rolled steel sheet in which the full width of the hot rolled coil, in particular, the adhesiveness of the scale is required at the edge portion, a manufacturing method thereof, It relates to the manufacturing equipment line.

  A scale having a thickness of about 10 μm is formed on the surface of the steel sheet after hot rolling. A hot-rolled steel sheet may be used as an automotive part or a steel for construction without being descaled such as pickling.

  When processing is performed with a scale attached to a hot-rolled steel sheet as an automotive part or an architectural steel material (as it is black), indentation wrinkles may occur due to scale peeling during processing. In addition, when a chemical conversion treatment such as pickling is performed, a portion where the scale is peeled off before the treatment appears as a pattern after the treatment, and the appearance quality is remarkably impaired.

  In order to solve these problems, a scale having excellent adhesion is required. In particular, a portion within 30 mm from the end face of the hot-rolled steel sheet (hereinafter referred to as “edge portion”) is in contact with the atmosphere even after winding, and therefore, compared with a portion on the inner side (hereinafter referred to as “center portion”), Scale peeling is likely to occur. Therefore, when shipping a hot-rolled steel sheet, or when performing cold working on a hot-rolled steel sheet, it is necessary to cut | disconnect an edge part. That is, the scale peeling at the edge portion is one of the causes of yield deterioration.

  The scale becomes thicker due to oxidation. When the thickness of the scale is approximately 15 μm or more, the scale is easily broken due to internal stress regardless of what kind of structure it is made of, and thus easily peeled. Therefore, in order to increase the adhesion of the scale, it is first necessary to suppress the scale from becoming thick. And when the thickness of a scale is thin, the adhesiveness of a scale is decided by the structure | tissue which comprises a scale.

For example, in Patent Documents 1 and 2, the thickness of the scale is reduced, and further, generation of hematite (Fe ₂ O ₃ ) containing a large amount of oxygen on the surface layer is suppressed. Of magnetite (Fe ₃ O ₄ ) (hereinafter referred to as “magnetite seam”) and an eutectoid structure (stripe structure) of iron and magnetite in which wustite has undergone eutectoid transformation, and is generated from the scale surface layer. In addition, it is shown that the adhesion of the scale is increased by suppressing the formation of hematite.

  However, the manufacturing method of the steel sheet with high scale adhesion described in Patent Documents 1 and 2 does not consider the manufacturing conditions in the edge portion. At the edge portion, the steel plate is in contact with the atmosphere even after the steel sheet is wound in a coil shape, so that oxidation proceeds as compared with the center portion. Therefore, wustite in the scale is oxidized to magnetite and hematite. As a result, the eutectoid structure of iron and magnetite cannot be obtained from wustite, and the transformation to magnetite at the interface between scale and ground iron does not proceed.

  That is, the scale of the edge portion is likely to be thick, and further, a structure having a hematite layer on the surface and the remainder becoming magnetite is formed, and a scale with low adhesion is likely to be formed. Therefore, peeling of the scale is likely to occur, and it is necessary to cut the edge portion of the steel plate before cold working.

  In the invention disclosed in Patent Document 3, the steel sheet is coiled at a temperature of 600 to 700 ° C., and then inserted into a slow cooling cover within 20 minutes to keep the scale oxidized by keeping it in a non-oxidizing atmosphere. Suppression suppresses the formation of magnetite from the surface layer of the scale, and further transforms wustite into a magnetite seam and a eutectoid structure of iron and magnetite to improve the adhesion of the scale.

  However, the technique described in Patent Document 3 has a drawback in that a large-scale dedicated facility is required to keep the coil in a non-oxidizing atmosphere, and the number of processes is increased. Further, as a result of the study by the present inventors, when the winding temperature is 650 to 700 ° C., the edge portion is oxidized and the scale becomes thick in the winding process, and the adhesion of the scale may be lowered. I understood.

  In the invention described in Patent Document 4, an oxide heating material is deposited on the end face of a hot-rolled steel sheet wound in a coil shape, and oxygen is shielded, thereby reducing the thickness of the scale and reducing the magnetite from the scale surface. The formation is suppressed, the formation of magnetite seam and the eutectoid transformation of wustite are advanced to improve the adhesion.

  However, with the technique described in Patent Document 4, it is practically difficult to completely shut off oxygen, and further, it is necessary to collect and treat the oxidation heat generating material, which is not preferable for safety and environmental health. There is a problem.

Japanese Patent Laid-Open No. 11-19702 JP 11-36050 A JP 2001-198604 A JP-A 62-289314

  When the hot-rolled steel sheet is wound in a coil shape, the temperature drop is slow because the high-temperature steel sheet is wound. In the state where the steel plate is being wound, the temperature drop at the edge portion of the steel plate is usually about 30 ° C./hr, and while the steel plate is being wound, it is under a high temperature (about 500 to 600 ° C.). Become.

  Therefore, according to the conventional manufacturing method, as shown in FIG. 1A, oxidation of wustite in the scale proceeds particularly at the edge portion of the hot-rolled steel sheet in contact with the atmosphere. As a result, generation of magnetite from the scale surface proceeds, and the thickness of the scale increases. Wustite can be transformed into magnetite by magnetite seam, transformation of iron and magnetite into a eutectoid structure, and oxidation, but their growth competes with each other.

  Therefore, when the production of magnetite by oxidation from the surface layer or the end face proceeds, the magnetite seam with good scale adhesion and the eutectoid structure of iron and magnetite are not produced, and the scale adhesion deteriorates.

  Therefore, in order to improve the scale adhesion at the edge portion of the steel sheet, it is necessary to suppress the formation of magnetite due to oxidation at the edge portion and to proceed the transformation from wustite to magnetite seam and the eutectoid structure of iron and magnetite. However, conventionally, for this purpose, it has been necessary to use a large-scale facility such as a non-oxidizing atmosphere at the edge portion or a chemical that blocks oxygen.

  In view of the above-described circumstances, the present invention advances the transformation of wustite to a magnetite seam and a eutectoid structure of iron and magnetite at the steel plate edge without using large-scale equipment and chemicals. It is an object of the present invention to provide a hot-rolled steel sheet having excellent scale adhesion across the entire width of the steel sheet as shown in 1 (b), a method for producing the hot-rolled steel sheet, and a production equipment line for the hot-rolled steel sheet.

  The present inventors have made extensive studies to solve the above problems. As a result, when the hot-rolled steel sheet is wound in a coil shape, the edge part of the wound steel sheet is rapidly cooled, and the temperature of the edge part is lowered, so that the scale grows thick even at the edge part in contact with the atmosphere. In addition, the present inventors have found that it is possible to transform the wustite in the scale into an iron-magnetite eutectoid structure without oxidizing it and leave almost no untransformed wustite remaining. It was.

  As described above, in order to increase the adhesion of the scale, it is necessary to generate a magnetite seam layer at the interface between the ground iron and the scale, and to generate an eutectoid structure of iron and magnetite in the upper layer.

  The center portion of the hot-rolled steel sheet wound up in a coil shape loses contact with the atmosphere by being wound up, so that there is little oxygen present in the periphery. Therefore, oxidation of the scale (wustite) on the surface of the hot-rolled steel sheet hardly proceeds even at high temperatures while being wound. Therefore, most wustite transformed into a magnetite seam and a eutectoid structure of iron and magnetite, and a scale having good adhesion was obtained at the center of the hot-rolled steel sheet.

  However, at the edge portion of the hot-rolled steel sheet, the steel sheet continues to be in contact with the atmosphere even after being wound in a coil shape, so that oxygen is abundant in the periphery. Therefore, if it is at a high temperature while being coiled, the scale (wustite) is likely to be oxidized at the edge of the hot-rolled steel sheet.

  This will be described with reference to an Fe—O phase diagram shown in FIG. The wustite in the scale formed on the steel sheet after hot rolling is stable at 570 ° C. or more, and when the temperature is lowered, it transforms into a eutectoid structure of iron and magnetite (arrow A). However, when oxygen is present, wustite changes to magnetite or hematite (arrow B). When wustite is oxidized, it changes to magnetite or hematite, so that even if the temperature is lowered thereafter, a eutectoid structure of iron and magnetite cannot be obtained (arrow C).

  Since the change of wustite to magnetite or hematite (arrow B) is an oxidation reaction, the rate decreases as the temperature decreases. However, the change of wustite from iron and magnetite into a eutectoid structure (arrow A) is a eutectoid transformation, and the transformation is likely to proceed as the temperature decreases.

  Therefore, the edge of the steel sheet wound in a coil shape is quenched, the temperature is lowered faster than usual, and the rate of oxidation of wustite is reduced to suppress the formation of magnetite due to oxidation. Proceed with the formation and transformation of iron and magnetite into a eutectoid structure.

  The rapid cooling of the edge part means that if the steel sheet is not rapidly cooled, the time for which the steel sheet is held at a high temperature becomes long, so that the oxidation of the wustite easily proceeds and the thickness of the scale also increases when the edge part comes into contact with the atmosphere. Because.

  This invention is made | formed based on said knowledge, Comprising: The summary is as follows.

(1) A method for producing a hot-rolled steel sheet produced by rolling a steel material hot and winding it into a coil.
The roughly rolled steel sheet is subjected to finish rolling at 850 to 1050 ° C., and then
While the finish-rolled hot-rolled steel sheet is wound in a coil shape at a coiling temperature of 500 to 650 ° C., both ends of the hot-rolled steel sheet have a temperature of 480 ° C. or less within 5 minutes from the start of winding. To cool and then
Maintaining the temperature at the end face below 480 ° C., then
A method for producing a hot-rolled steel sheet excellent in scale adhesion, characterized in that it is gradually cooled from the time when the temperature at the end face is 400 to 480 ° C. while still in a coil shape.

(2) A hot-rolled steel sheet having a scale,
The scale in the region within 30 mm from the end face of the coil is
There is a magnetite layer in contact with the ground iron at an area ratio of 90% or more at the interface between the ground iron and the scale,
In the upper layer of the magnetite layer in contact with the ground iron, it has a eutectoid structure layer of iron and magnetite,
The upper layer of the eutectoid structure layer of iron and magnetite has a magnetite layer,
A hematite layer on the magnetite layer;
The sum of the thickness of the upper magnetite layer of the eutectoid structure layer of iron and magnetite and the hematite layer is 30% or less of the total thickness of the scale, and
A hot-rolled steel sheet having excellent scale adhesion, wherein the difference between the thickness of the scale at 30 mm from the end face of the coil and the thickness of the scale at the center of the coil is 2 μm or less.

  (3) The hot-rolled steel sheet having excellent scale adhesion according to (2) above, wherein the scale has a thickness of 5 to 12 μm at a portion within 30 mm from the end face of the coil.

(4) In the winding equipment for rolling the steel material hot and winding it into a coil shape,
A winding device that winds the hot-rolled steel sheet that has been finish-rolled into a coil at a winding temperature of 500 to 650 ° C .;
A facility for manufacturing a hot-rolled steel sheet excellent in scale adhesion, comprising an end face cooling device capable of cooling both end faces of a hot-rolled steel sheet being wound to 480 ° C. or less within 5 minutes from the start of winding. .

  (5) The end face cooling device is a device for cooling both end surfaces of a hot-rolled steel sheet wound in a coil shape by water cooling. Equipment column.

(6) The end face cooling device is a device that cools both end faces of a hot-rolled steel sheet wound in a coil shape with one or more gases selected from N ₂ , H ₂ , Ar, He, and water vapor. (4) A hot-rolled steel sheet manufacturing facility line having excellent scale adhesion.

  According to the present invention, it is possible to obtain a hot-rolled steel sheet that is excellent in the full width of the hot-rolled steel sheet, particularly in the scale adhesion at the edge portion. Therefore, without cutting off the edge of the hot-rolled steel sheet, it is possible to suppress the generation of patterns after pressing and pickling due to scale peeling during cold working, resulting from cutting off the edge of the hot-rolled steel sheet. Yield drop can be improved.

It is a figure which shows the outline of the scale structure of the width direction of a hot-rolled steel plate, (a) is the hot-rolled steel plate manufactured by the prior art, (b) is the scale structure of the hot-rolled steel plate manufactured by this invention. It is a Fe-O type phase diagram. It is a figure which shows the outline of the temperature change of the end surface of a hot-rolled steel plate in the manufacturing method of this invention. It is a figure which shows the outline of the winding apparatus and end surface cooling apparatus of this invention. In the manufacturing method of the hot-rolled steel sheet of this invention, it is a figure which shows the outline of the temperature history of the position of 40 mm from a center part, an edge part, and an end surface, and the scale structure of the manufactured hot-rolled steel sheet.

  Embodiments of the present invention will be described below.

  First, the hot-rolled steel sheet that has been roughly rolled is finish-rolled at a finishing temperature of 850 to 1050 ° C. By performing finish rolling under these conditions, a scale having a thickness of 5 to 12 μm is formed over the entire width of the hot-rolled steel sheet. In order to increase the adhesion of the scale and ensure the material of the scale, the scale thickness is preferably 5 to 12 μm.

  In normal rolling, a scale having a thickness of about 10 μm is often formed at the center of the coil. When the scale is thick, the scale adhesion is reduced regardless of the structure constituting the scale. Therefore, the scale thickness is preferably suppressed to 12 μm or less. On the other hand, in order to make the scale thinner, it is necessary to largely change the finish rolling conditions and the cooling conditions, and the material is not secured. Therefore, it is desirable that the scale has a thickness of 5 μm or more even if it is thin.

  The exit side temperature of finish rolling is desirably in the range of 850 to 1050 ° C. As the finishing temperature increases, the scale grows thicker and the ratio of hematite and magnetite layers on the scale surface also increases. As a result, the magnetite seam layer and the eutectoid structure layer of iron and magnetite are less likely to be generated. On the other hand, since rolling will become difficult if too low, a minimum is made into 850 degreeC.

  Next, the hot-rolled steel sheet subjected to finish rolling is wound in a coil shape at a winding temperature of 500 to 650 ° C. at the edge portion of the steel sheet. The winding temperature refers to the temperature of the steel sheet at the start of winding.

  In order to improve the scale adhesion, it is necessary to sufficiently generate a magnetite seam layer having high adhesion at the interface between the ground iron and the scale when the steel sheet wound in a coil shape is gradually cooled. The magnetite seam layer in which the wustite is transformed and precipitates from the interface between the scale and the ground iron has an effect of improving the adhesion of the scale, and the magnetite seam layer is more easily deposited in the coil. Therefore, it is desirable that the scale structure at the contact point with the ground iron is all magnetite deposited from the ground iron, and it is necessary that at least 90% or more of the area ratio is magnetite.

  As wustite in the scale oxidizes and changes to magnetite and hematite, the magnetite and magnetite seam layers are connected. Since magnetite is hard, if the layer becomes too thick, the adhesion is rather reduced. That is, in order to improve the scale adhesion, an eutectoid structure of iron and magnetite needs to exist between the magnetite seam layer and the magnetite of the scale surface layer.

  The area ratio of the magnetite seam is obtained by observing a cross section with an electron microscope and determining the ratio of the length of the interface magnetite layer (magnetite seam) in contact with the ground iron to the length of the scale in contact with the steel. It is preferable to perform measurement at three or more locations and obtain an average value.

  The coiling temperature is preferably in the range of 500 to 650 ° C. at the edge of the steel plate. When the coiling temperature is higher, magnetite is remarkably precipitated from the interface between the base iron and the scale, and eutectoid transformation is liable to occur. However, if the coiling temperature is too high, strength is lowered. Therefore, the upper limit of the coiling temperature is 650 ° C. Further, when the coiling temperature is low, temperature control is difficult, and strength variation and shape deterioration are caused. Therefore, the coiling temperature needs to be 500 ° C. or higher.

  The coiling temperature is preferably 570 to 650 ° C. If the winding temperature is 570 ° C. or higher, most of the scale becomes wustite at the start of winding, and it becomes easy to obtain a magnetite seam and a eutectoid structure of iron and magnetite. More preferably, it is 600-650 degreeC. After the winding is completed, it is possible to effectively transform the wustite in the scale into a magnetite seam and a eutectoid structure of iron and magnetite by slow cooling from a temperature higher than the eutectoid temperature of wustite. is there.

  Here, the coiling temperature refers to the temperature at the edge part of the steel sheet. At the start of coiling, the temperature difference between the center part and the edge part of the steel sheet is small. Also good.

  And while winding the hot-rolled steel sheet in a coil shape, both ends of the steel sheet are cooled so that the temperature of the end face becomes 480 ° C. or less within 5 minutes from the start of winding, thereby suppressing oxidation of the edge portion. To do.

  As described above, in normal rolling, a scale having a thickness of about 10 μm is often formed at the center of the coil. However, since the end face of the coil is in contact with the atmosphere while being wound around the coil, oxidation proceeds and the scale tends to grow thick. When the scale becomes too thick, the adhesion of the scale decreases regardless of the structure constituting the scale.

  Even if the scale thickness of the end is relatively thin, such as about 12 μm, if the difference in scale thickness between the center and end of the coil is large, the scale of the edge portion is caused by the stress difference inside the scale. It may be easily destroyed or cause uneven pickling during pickling.

  In the present invention, by rolling both ends of the steel sheet while winding the hot-rolled steel sheet in a coil shape, by suppressing oxidation of the edge part, the growth of the scale of the edge part is suppressed, and further, the coil center The difference in scale thickness between the part and the end is reduced.

  When the end surface of the steel sheet is cooled, how much time the end surface temperature can be lowered from the start of winding of the coil greatly affects the progress of the scale transformation at the edge portion. The cooling rate of the end face of the steel sheet is very slow, about 30 ° C./hr, when winding a normal hot-rolled steel sheet. In order to suppress the oxidation of the edge portion, it is necessary to quickly reduce the temperature of the end face to a temperature at which the progress of the oxidation can be suppressed.

  As a result of the study by the present inventors, it is possible to suppress the growth of the scale thickly by setting the temperature of the end face of the steel sheet to 480 ° C. or less within 5 minutes from the start of winding of the coil. It was found that the transformation of wustite to the eutectoid structure of iron and magnetite proceeded predominately before the oxidation progressed, and the transformation of wustite proceeded to obtain a highly adherent scale. The cooling rate of the end face at this time is approximately 10 to 100 ° C./min, preferably about 10 to 30 ° C./min, although it depends on the coiling temperature.

  The faster the cooling rate, the greater the effect of suppressing oxidation. However, if the cooling rate is too fast, there is a possibility that the shape deteriorates and the cost increases due to the amount of water required for cooling and the increase in water pressure. Therefore, it is preferable that the time from the start of winding of the coil until the temperature of the end surface becomes 480 ° C. or less is 1 minute or more. A more preferable time range is 1 to 3 minutes.

  When the temperature of the end face becomes 480 ° C. or lower and the temperature returns to a high temperature range by recuperation, the oxidation of wustite proceeds and the oxidation of the edge scale is not suppressed. Therefore, after the temperature of the end surface becomes 480 ° C. or lower, it is necessary to maintain the temperature at 480 ° C. or lower until the start of slow cooling described later.

  The temperature history while maintaining the end face temperature at 480 ° C. or lower does not matter as long as the end face temperature does not exceed 480 ° C. FIG. 3 shows an example of the cooling pattern. The example shown in FIG. 3 is an example in which winding is started at 0 minutes for both solid and broken lines, and slow cooling is started at 25 minutes.

  For example, as shown by the solid line in FIG. 3, the end face temperature is set to 480 ° C. or lower within 5 minutes after the start of winding, then supercooled to a temperature of 300 ° C. or lower, and then raised to 400 to 480 ° C. by recuperation. Thereafter, it may be gradually cooled. However, if the temperature at the time of supercooling becomes too low, the coil shape and material may be deteriorated. Therefore, the temperature at the time of supercooling is preferably 250 ° C. or higher.

  Also, as shown by the broken line in FIG. 3, the end surface temperature is controlled to be 480 ° C. or less within 5 minutes after the start of winding, and then controlled so that the end surface temperature falls within a certain range, and then from 400 to 480 ° C. It may be gradually cooled.

  In the actual manufacturing process, for example, if the recuperation magnitude is obtained in advance experimentally or by calculation and the temperature at which cooling is stopped is determined, a cooling pattern as shown by the solid line in FIG. 3 can be obtained. Further, for example, if the controlled cooling is performed until the temperature rise due to recuperation is eliminated while measuring the temperature of the coil end face using a radiation thermometer or the like, the cooling pattern as shown by the broken line in FIG. 3 can be obtained.

  On the other hand, in order to sufficiently transform the wustite in the scale into a magnetite seam and a eutectoid structure of iron and magnetite, it is necessary to slowly cool in a temperature range of 450 ° C. or lower. In the manufacturing method of the present invention, when the temperature of the coil end face is not increased due to recuperation and the temperature starts to decrease without quenching the coil end face, the slow cooling is started, and the temperature at that time is started gradually. Let it be temperature.

  When the wustite is transformed at 500 ° C. or lower, first, magnetite is precipitated, and then a eutectoid structure of iron and magnetite appears, and the untransformed wustite remains as untransformed wustite. When the annealing start temperature is less than 400 ° C., the temperature is lowered without sufficiently transforming the wustite, so that the untransformed residual wustite increases. The eutectoid structure of iron and magnetite contributes to improving the adhesion of the scale, while untransformed wustite reduces the adhesion.

  Therefore, it is desirable that the wustite is completely transformed to have a eutectoid structure of iron and magnetite, and the residual wustite is preferably 5% or less in terms of volume fraction. Therefore, the annealing start temperature is 400 to 480 ° C, preferably 450 to 480 ° C. That is, the temperature of the end face at the end of cooling needs to be 400 to 480 ° C. When cooled to less than 400 ° C., the temperature rises to a temperature range of 400 to 480 ° C. at the start of slow cooling due to recuperation. Need to be.

  For example, as shown in FIG. 4, the cooling method of the end face of the coil is provided with water cooling nozzles on both sides of the winder and an end face cooling device that discharges water to both end faces of the steel sheet while winding the hot-rolled steel sheet in a coil shape. Can be realized. According to this method, there is a cooling effect from the end surface to the inside of about 50 mm, and the inside of the inside from about 50 mm from the end surface has almost the same temperature as the end surface of the steel plate.

  If the cooling start temperature exceeds 480 ° C with only water cooling during rewinding, water cooling is continued after winding, and during that time, the steel material that is subsequently passed through another winding machine can be wound. That's fine.

  Although the number of nozzles and the installation angle are not particularly specified, it is necessary to appropriately adjust the cooling so that the entire end face has a cooling effect.

  The installation of the nozzle is very simple and does not require extensive equipment. Moreover, even when producing a steel type to which the production method of the present invention is not applied, it is possible to produce a steel plate as in the conventional case by stopping water discharge from the nozzle. That is, the method of installing the water-cooled nozzle in the winder is very effective in that it does not hinder the production of other steel types.

The cooling method is not limited to water cooling. For _{example, N 2, H 2, Ar} , He, may be cooled using a gas such as water vapor.

  After the end face temperature reaches 400-480 ° C and the temperature rise due to recuperation disappears, the hot-rolled steel sheet is gradually cooled in the form of a coil to progress to the formation of magnetite seam and the transformation of wustite to iron and magnetite eutectoid structure. Let

  For the slow cooling, for example, a cooling floor may be added to the production equipment row, and air cooling may be performed while the coil is in the cooling floor. The method of air-cooling and the direction of placing are not limited as long as they are not limited by the application. The cooling floor only needs to be air-cooled and does not need to be a special device. Here, the slow cooling means that the cooling rate at the start of the slow cooling is 10 to 100 ° C./hr, preferably 10 to 30 ° C./hr, and thereafter cooling without actively increasing the cooling rate.

  The scale at the edge portion of the hot-rolled steel sheet produced by the production method of the present invention is composed of a magnetite seam layer having an area ratio of 90% or more, an eutectoid structure layer of iron and magnetite, a magnetite layer, and hematite in order from the interface between the scale and the ground iron. It becomes a layer and becomes a structure with high adhesion.

  At this time, the total thickness of the magnetite layer and hematite layer is 30% or less of the total thickness of all scales, and the wustite remaining without eutectoid transformation is 5% or less in volume fraction. It becomes. The volume fraction of wustite is obtained by observing a cross section with an electron microscope and determining the area ratio of wustite with respect to the scale cross-sectional area of the observed portion. It is preferable to perform measurement at three or more locations and obtain an average value.

  Since the manufacturing method of the present invention is not a method of blocking contact with oxygen at the end face, the hematite layer remains at a certain level without changing into a magnetite layer at the edge portion. However, if the total thickness of the magnetite layer and the hematite layer is 30% or less of the total thickness of all scales, there is no effect on the adhesion of the scales.

  Furthermore, the scale at the edge portion of the hot-rolled steel sheet produced by the production method of the present invention does not grow thick because it is rapidly cooled, and the difference between the scale thickness at the edge portion and the scale thickness at the center portion is 2 μm. It is also possible to make it 1 μm or less. This difference in scale thickness cannot be obtained by conventional techniques, particularly the techniques described in Patent Documents 3 and 4, except when trimming a few mm of edge portions.

  By the method of manufacturing a hot-rolled steel sheet according to the present invention, the hot-rolled steel sheet is wound into a coil shape, the end face is rapidly cooled, and then air-cooled, from the center part, the edge part, and the end face of the coil from finish rolling to air cooling. FIG. 5 shows changes in temperature history and scale inside 40 mm (region where the cooling capacity reaches).

  In the hot-rolled steel sheet after finish rolling, hematite, magnetite, and wustite are formed in this order from the surface layer of the base iron.

  The scale structure of the center part of the hot-rolled steel sheet manufactured according to the method for manufacturing a hot-rolled steel sheet of the present invention has a slight magnetite layer on the surface layer, and a magnetite seam layer precipitated at the interface between the scale and the ground iron Exists, and the remainder is composed of a eutectoid structure of iron and magnetite. The surface hematite layer present in the hot-rolled steel sheet after finish rolling changes to magnetite by the diffusion of Fe from the base iron. In the center portion, basically a similar structure can be obtained even if it is not the method for producing a hot-rolled steel sheet of the present invention.

  The scale structure of the edge part of the hot-rolled steel sheet manufactured according to the method of manufacturing the hot-rolled steel sheet of the present invention has a slightly thick hematite layer and magnetite layer on the surface layer, and a slight amount of wustite remaining untransformed. To do. A deposited magnetite seam layer is present at the interface between the scale and the ground iron, and the remainder consists of a eutectoid structure of iron and magnetite.

  The scale structure at a position of 40 mm from the end face of the hot-rolled steel sheet manufactured according to the method for manufacturing a hot-rolled steel sheet of the present invention is almost the same as the scale structure at the edge portion, but the thickness of the surface hematite layer and magnetite layer Is less than the edge.

  In the hot-rolled steel sheet manufactured according to the method for manufacturing a hot-rolled steel sheet of the present invention, the center part, the edge part, and the scale at a position of 40 mm from the end face all have an area ratio of the magnetite seam layer at the interface with the ground iron. Over 90%, the upper layer of the magnetite seam layer is a eutectoid structure of iron and magnetite, and the total thickness of the upper magnetite layer and hematite layer of the eutectoid structure of iron and magnetite is the total thickness of the scale. 30% or less. Preferably, residual wustite has a volume fraction of 5% or less and a scale thickness of 5 to 12 μm.

  Furthermore, the difference between the thickness of the scale at the center portion and the thickness of the scale at the edge portion is 2 μm or less. Thereby, the destruction of the scale due to the stress difference in the scale between the center portion and the edge portion can be suppressed, and the scale peeling at the edge portion can be suppressed.

  The present invention is made by paying attention to the point that the center part does not contact the atmosphere and the edge part contacts the atmosphere when the hot-rolled steel sheet is wound in a coil shape. However, it is particularly suitable for the production of hot-rolled steel sheets having a width of 1000 to 2000 mm and a thickness of 2.5 to 13 mm.

In mass%, C: 0.002%, Si: 0.002%, Mn: 0.08%, P: 0.004%, S: 0.004%, Al: 0.032%, and N: The ultra-low carbon steel containing 0.0022% was subjected to finish rolling, winding, and cooling under the conditions shown in Table 1, and the scale structure at the edge portion and the presence or absence of scale peeling were evaluated. The presence or absence of scale peeling was confirmed by visual observation, and the case where the ground iron was visible was judged as “with peeling”.
The manufactured hot-rolled steel sheet coil has a width of 1700 mm and a thickness of 6 mm.

  Table 2 shows the evaluation results of scale peeling under each condition.

  In A to D within the temperature condition range shown in the present invention, it was confirmed that the scale of the edge portion had a structure advantageous for improving adhesion, and no scale peeling occurred at the edge portion.

  The thickness of the scale became thicker as the finishing temperature was higher. Further, according to the manufacturing method of the present invention, the cooling rate after winding is fast, so the scale thickness at the edge portion is the same as the scale thickness at the center portion. The area ratio of the magnetite seam increased as the coiling temperature increased, but it was confirmed that sufficient magnetite seam was generated within the temperature condition range of the present invention.

  E was manufactured by cooling the coil end face without air cooling. Due to air cooling, the cooling rate of the coil end face was slow, and as a result, the scale of the edge portion became thicker than the center portion. Moreover, since the winding temperature was low, the ratio of the hematite layer and the magnetite layer on the scale surface layer was increased. Furthermore, since the surface magnetite layer was thick, it became difficult for the magnetite seam to precipitate, and the area ratio decreased.

  F was manufactured under the conditions that the finishing temperature and the coiling temperature were high, the time until the end surface temperature was 480 ° C. or less was long, and the temperature returned to 500 ° C. by reheating after cooling. Since the finishing temperature was high, the scale was thick overall, and the winding temperature was high, so the scale thickness at the edge was even thicker. Moreover, since the cooling rate was slow, the ratio of the hematite layer and magnetite layer on the scale surface at the edge portion increased. Furthermore, since the magnetite layer on the surface layer is thick at the edge portion, the magnetite seam is less likely to precipitate and the area ratio is reduced.

  G was produced under conditions where the annealing start temperature was low. Since the annealing start temperature was too low, it became difficult to form a magnetite seam at the edge portion, and the area ratio was reduced.

  In addition, although the said Example described ultra-low carbon steel, since the manufacturing method of this invention is widely applicable with respect to the steel plate which does not perform a descaling process, it is not limited to said Example. In particular, the production method of the present invention is C: 0.001 to 0.18%, Si: 0.001 to 0.045%, Mn: 0.01 to 0.6%, P: 0.008 by mass%. -0.022%, S: 0.001-0.02%, the balance being suitable for hot-rolled steel sheets consisting of Fe and inevitable impurities.

  INDUSTRIAL APPLICABILITY The present invention can be applied as a steel material that requires excellent surface properties at the time of processing or painting and a method for manufacturing the same, and is suitable for manufacturing automobile parts and architectural steel materials.

1 Scale 2 Ground Iron 11 Hematite 12 Magnetite 13 Magnetite (Magnetite Seam) Deposited at the Interface between the Ground Iron and Scale
14 Eutectoid structure of iron and magnetite 15 Residual wustite 16 Wustite 21 Water-cooled nozzle 22 Hot rolled coil

Claims (6)

A method of manufacturing a hot-rolled steel sheet produced by rolling a steel material hot and winding it into a coil.
The roughly rolled steel sheet is subjected to finish rolling at 850 to 1050 ° C., and then
While the finish-rolled hot-rolled steel sheet is wound in a coil shape at a coiling temperature of 500 to 650 ° C., both ends of the hot-rolled steel sheet have a temperature of 480 ° C. or less within 5 minutes from the start of winding. To cool and then
Maintaining the temperature at the end face below 480 ° C., then
A method for producing a hot-rolled steel sheet excellent in scale adhesion, characterized in that it is gradually cooled from the time when the temperature at the end face is 400 to 480 ° C. while still in a coil shape. A hot-rolled steel sheet having a scale,
The scale in the region within 30 mm from the end face of the coil is
There is a magnetite layer in contact with the ground iron at an area ratio of 90% or more at the interface between the ground iron and the scale,
In the upper layer of the magnetite layer in contact with the ground iron, it has a eutectoid structure layer of iron and magnetite,
The upper layer of the eutectoid structure layer of iron and magnetite has a magnetite layer,
A hematite layer on the magnetite layer;
The sum of the thickness of the upper magnetite layer of the eutectoid structure layer of iron and magnetite and the hematite layer is 30% or less of the total thickness of the scale, and
A hot-rolled steel sheet having excellent scale adhesion, wherein the difference between the thickness of the scale at 30 mm from the end face of the coil and the thickness of the scale at the center of the coil is 2 μm or less.   The hot-rolled steel sheet with excellent scale adhesion according to claim 2, wherein the scale has a thickness of 5 to 12 µm at a portion within 30 mm from the end face of the coil. In the winding equipment which rolls steel materials hot and winds up and manufactures in the shape of a coil,
A winding device that winds the hot-rolled steel sheet that has been finish-rolled into a coil at a winding temperature of 500 to 650 ° C .;
A facility for manufacturing a hot-rolled steel sheet excellent in scale adhesion, comprising an end face cooling device capable of cooling both end faces of a hot-rolled steel sheet being wound to 480 ° C. or less within 5 minutes from the start of winding. .   The said end surface cooling apparatus is an apparatus which cools the both end surfaces of the hot-rolled steel sheet wound by the coil shape by water cooling, The manufacturing equipment row | line | column of the scale adhesiveness excellent in scale adhesiveness of Claim 4 characterized by the above-mentioned. . The end face cooling device is a device that cools both end faces of a hot-rolled steel sheet wound in a coil shape with one or more gases selected from N ₂ , H ₂ , Ar, He, and water vapor. The manufacturing equipment row | line | column of the hot-rolled steel plate excellent in the scale adhesiveness of Claim 4.

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