JP2014176864A - Manufacturing method for hot rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method for a hot rolled steel sheet with where rupture of a plate can be prevented in a complete continuous hot rolling by improving bondability of a periphery of a plate width direction edge part of a sheet bar junction without increasing a size of a joint device.SOLUTION: A manufacturing method for a hot rolled steel sheet in which when a hot rolled steel plate is manufactured by use of a perfect continuous hot rolling method, at least one junction end surface of a preceding sheet bar and a subsequent sheet bar is formed into a predetermined concaved-shape and after a predetermined induction heating is applied in such a state that the preceding sheet bar and the subsequent sheet bar do not contact each other, induction heating current is once switched off. In such a state, the preceding sheet bar and the subsequent sheet bar are brought into contact with each other, then the predetermined induction heating is again applied, and thereafter, upset bonding is carried out at a specified upset quantity.

Description

  The present invention relates to a method for producing a hot-rolled steel sheet capable of preventing sheet breakage in complete continuous hot rolling by improving the bondability in the vicinity of the edge part in the sheet width direction of the sheet bar joint part. .

  In the manufacturing process of hot-rolled steel sheets, the thinner the finished sheet thickness, the more likely problems occur during the finish rolling, such as bumping at the front end and drawing or folding at the tail end. Since it takes time to remove the cut material, it is a factor that greatly reduces the efficiency of the line.

  This is because in plate rolling, there are hard asymmetries such as looseness in each part of the rolling mill and roll roll wear, asymmetric temperature distribution of the material, local bending called nose bending of the sheet bar, and large bending over the entire length called camber. The main cause is that the rolling mill is deformed asymmetrically with respect to the rolling direction. And it becomes easy to be in an unstable rolling state especially at the tip end portion where no tension is applied, and is more susceptible to influence as the finished plate thickness becomes thinner. In addition, the thinner the finished plate, the more easily the flying phenomenon of the tip (lifting phenomenon due to air resistance) occurs on the cooling table after finish rolling, and it is necessary to reduce the rolling speed. It has become.

  For this reason, in a batch rolling process in which one hot-rolled steel sheet coil is sequentially manufactured from one conventional slab, the minimum sheet thickness that can be manufactured is limited to about 1.2 mm.

  As measures for overcoming such a situation, for example, hot-rolled steel sheet rolling equipment (heating furnace 10, rough rolling mill 11, coil box 12, joining device 13, finishing rolling mill 14, water cooling device 15, shown in FIG. After roughly rolling the hot slab using a coiler 16), the tail end portion of the preceding sheet bar and the tip end portion of the succeeding sheet bar are joined and finish-rolled, so that a plurality of continuous slabs are provided. A fully continuous hot rolling method for producing a hot rolled steel sheet coil has been put into practical use.

  In the fully continuous hot rolling method, except for the tip of the most advanced coil and the tail end of the last continuous coil, the finish rolling is almost steady with a tension applied during finish rolling. The plate in the machine is very stable and rolling is possible with almost no troubles such as drawing. And combined with running sheet thickness change technology, running coil cutting, winding technology, etc., it is also possible to continuously manufacture hot rolled steel sheet coils with different finishing plate thickness, including thin hot rolled steel sheets of 1.0 mm or less. Yes.

  As a joining method of the sheet bar which has been put into practical use, the joining surface of the sheet bar is heated to the vicinity of the melting point and the joining surface is upset to join.

  At this time, heating in the vicinity of the joint surface is performed by inducting the magnetic flux in the thickness direction of the sheet bar by the induction heating method, rapidly raising the temperature by Joule heat due to the induction current, heating and upsetting in just a few seconds, and joining The technique which complete | finishes is proposed (for example, patent document 1).

  Further, in order to increase the temperature around the edge in the plate width direction of the joint and improve the bondability, it has been proposed to install a high-frequency coil dedicated to edge heating outside both edges of the joint (for example, Patent Documents). 2).

  Similarly, as a technique for improving the bondability around the edge in the plate width direction, an alternating magnetic field penetrating in the plate thickness direction is applied to heat and raise the temperature of the joint over the entire plate width direction, and the temperature variation is large. A technique for improving the temperature distribution of the edge portion in the plate width direction by generating an alternating magnetic flux in a direction opposite to the alternating magnetic flux in the region has been proposed (for example, Patent Document 3).

  And the technique which improves the temperature distribution of a board width direction edge part by arrange | positioning a magnetic body to the board width direction edge part periphery of a junction part, and applying the alternating magnetic field which penetrates a board thickness direction is proposed (for example, Patent Document 4).

JP 62-234679 A JP-A-7-164018 Japanese Patent Laid-Open No. 8-1203 Japanese Patent Laid-Open No. 8-1202

“Theory and Practice of Sheet Rolling” The Japan Iron and Steel Institute, p. 73

  However, the conventional techniques (Patent Documents 1 to 4) related to the above-described sheet bar bonding heating method using induction heating have the following problems.

  First, in the technique disclosed in Patent Document 1, a phenomenon occurs in which the circulating current generated by the alternating magnetic field bypasses the vicinity of the edge portion in the plate width direction of the joint portion, and thus the temperature in the vicinity of the edge portion in the plate width direction does not increase. For this reason, the edge portion in the plate width direction has a higher hardness at low temperatures than the plate width center portion in a semi-molten state. For this reason, when upsetting by joining the joining surfaces, the unmelted portion around the edge in the plate width direction becomes resistance, increasing the upset load, resulting in a bonding state in the entire plate width direction such as insufficient upset amount. It is inevitable to have an adverse effect, and there is a problem that the probability of plate breakage from the joint portion increases during finishing rolling.

  On the other hand, the techniques disclosed in Patent Documents 2 to 4 are techniques devised as a method for improving the plate width direction edge portion temperature, which is a problem in Patent Document 1, but the following problems are present. There was a point.

  Patent Document 2 is intended to improve the plate width direction edge portion temperature by arranging an induction heating coil exclusively for the plate width direction edge portion different from the induction heating coil for applying the alternating magnetic flux to the entire joint portion, Although the effect of reliably improving the temperature of the edge portion in the plate width direction was recognized, the increase in equipment size and the construction cost was inevitable.

  In Patent Document 3, although an improvement in the temperature in the edge direction in the sheet width is recognized, a temperature rise is hardly obtained due to the detour current in the vicinity of the edge part, so that there remains a problem in the bondability in the edge part in the sheet width direction. It was.

  Further, in Patent Document 4, a magnetic material is arranged around the edge portion in the plate width direction to improve the temperature rise amount around the edge portion in the plate width direction by increasing the magnetic flux density. Although the effect of improving the temperature is recognized, it is necessary to adjust the vertical position of the heating coil for the arrangement of the magnetic body, and the enlargement of the equipment is inevitable.

  The present invention has been intensively studied to overcome the above-described problems of the prior art, and does not involve an increase in the size of the joining device, and the joining property around the edge portion in the sheet width direction of the seat bar joining portion. It is an object of the present invention to provide a method for producing a hot-rolled steel sheet capable of preventing sheet breakage in complete continuous hot rolling by improving the above.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive studies and prevented the sheet breakage in complete continuous hot rolling by improving the bondability around the sheet width direction edge part of the sheet bar joint part. The inventors have devised a method for producing a hot-rolled steel sheet that can be used.

  That is, the present invention has the following features.

[1] Immediately before finish rolling on the hot rolling line, the tail end of the preceding sheet bar and the leading end of the succeeding sheet bar are sheared with a shear, heated, upset, joined, and continuously finished By producing a hot-rolled steel sheet using a complete continuous hot rolling method for producing a plurality of hot-rolled steel sheet coils continuously from a plurality of slabs by rolling,
After forming the shape of the joining end surface of at least one of the preceding sheet bar or the following sheet bar into a predetermined concave shape, and applying predetermined induction heating in a state where the preceding sheet bar and the following sheet bar do not contact, A state in which the preceding seat bar and the succeeding seat bar are brought into contact with each other with the induction heating current turned off, and then upset joining is performed with a predetermined upset amount after applying predetermined induction heating again. A method for producing a hot-rolled steel sheet.

  [2] Adjust the amount of width reduction by the sizing press and / or edger in the rough rolling process to adjust the planar shape of one or both of the tail end portion of the preceding sheet bar and the leading end portion of the succeeding sheet bar. After that, the fishtail portion is cut with a shear so that the planar shape becomes a concave shape with a dent amount at the central portion of the plate width of 10 mm or less. A method of manufacturing a steel sheet.

  [3] A width reduction amount by a sizing press and / or an edger in the rough rolling process so that one or both of the leading end portion of the preceding sheet bar and the leading end portion of the succeeding seat bar have a fishtail shape. To determine the cutting position on the shear by measuring the planar shape of either the tail end of the preceding sheet bar or the leading end of the succeeding sheet bar or both at the rough rolling final pass exit side. The method for producing a hot-rolled steel sheet according to the above [1] or [2].

  [4] The amount of upset between the preceding seat bar and the succeeding seat bar is 10 to 30 mm, and the center of the plate width formed at either the tail end portion of the preceding seat bar or the leading end portion of the succeeding seat bar The method for producing a hot-rolled steel sheet according to any one of [1] to [3], wherein the amount is at least twice the amount of the dent.

  According to the method for producing a hot-rolled steel sheet according to the present invention, it is possible to improve the bondability in the vicinity of the edge portion in the sheet width direction of the sheet bar joint, and it is possible to prevent the sheet breakage in complete continuous hot rolling and stabilize Rolling becomes possible.

It is a figure which shows an example of the fishtail shape of a seat bar front-end | tip part or tail end part. It is a figure which shows an example of the concave shape after shear cutting | disconnection of the sheet bar front-end | tip part or tail end part by this invention. It is a figure which shows the other example of the concave shape after shear cutting | disconnection of the sheet bar front-end | tip part or tail end part by this invention. It is a figure which shows the temperature distribution after the 1st induction heating process by this invention. It is a figure which shows the state which made the joining surface of a preceding sheet bar and a succeeding sheet bar contact after the 1st induction heating process by this invention. It is a figure which shows the temperature distribution after the 2nd induction heating process by this invention. It is a figure which shows the flow of the induction current in the 2nd induction heating process by this invention. It is a figure which shows the joining method of the conventional seat bar. It is a figure which shows the flow of the induction current by the induction heating of a sheet bar. It is a figure which shows a crop cutting position. It is a figure which shows the crop shape measuring method. It is a figure which shows the manufacturing equipment of a hot-rolled steel plate.

  Embodiments of the present invention will be described with reference to the drawings.

  FIG. 8 is a top view of a conventional sheet bar joining method, in which the tail end portion of the preceding sheet bar 1 and the leading end portion of the succeeding sheet bar 2 are each cut in a straight line by a shear in the plate width direction. In this state, alternating magnetic flux is applied by the induction heating coil 3 while keeping the joint surfaces of the sheet bars that have been (sheared) in a rectangular shape separated by several mm (transverse induction heating method).

  At this time, as shown in FIG. 9, an induced current 5 is generated in the vicinity of the leading end of each sheet bar, and the temperature in the vicinity of the joint surface is rapidly increased. Usually, the sheet thickness of the sheet bar is about 25 mm to 50 mm, and the temperature before finish rolling is about 1000 ° C. to 1100 ° C. The end of the sheet bar is heated from this temperature for joining, but the temperature at which the melting of the steel begins (solidus) varies depending on the amount of carbon contained, but the steel grade of the component used for thin plate applications is joined in a semi-molten state. In order to do so, it is necessary to heat to at least about 1450 ° C. to 1500 ° C. In complete continuous hot rolling, it is necessary to perform joining with the succeeding sheet bar while finishing rolling the preceding sheet bar, so that a heating capability of at least about 200 ° C./sec is required from the viewpoint of equipment space and heating efficiency. It is desirable to complete the joining within a few seconds from induction heating to upset using an induction heating device.

  However, since the induced current generated by this method bypasses the corners of the rectangular sheet bar due to the characteristics (induced current bypass region 8) as shown in FIG. Even if it can be heated up to this temperature, it is difficult to greatly increase the temperature around the edge portion in the plate width direction. If the output of the induction heating device is extremely increased and the temperature in the vicinity of the edge in the plate width direction is equal to or higher than the solidus line, the temperature near the center of the plate width further rises and melts over the liquidus line. Usually, the heating conditions are set in a narrow range so that the temperature at the center of the plate width is between the solidus and below the liquidus.

  As described above, FIG. 8 is a diagram showing a situation in which the vicinity of the joining surface is heated by the conventional sheet bar joining method, and both the sheet bars are upset, and the hatching area 4 is a semi-molten area. Represents. Usually, from the edge in the plate width direction to about 50 mm is the region 6 where the temperature rise is insufficient due to the detour current, and it is in a completely solid state even during upset. Due to the upset, the semi-molten portion 4 is deformed while being pushed out in the plate thickness direction and becomes a joined state, but the edge portion in the plate width direction is in strong contact with a complete solid state (strong contact portion 7). Plastic deformation occurs in the direction in which both edges protrude toward the outside in the plate width direction. In such a situation, the strong contact portion 7 at the edge portion in the plate width direction is a resistance against the upset deformation, and an upset force larger than necessary is required.

  For this reason, especially with hard materials and wide materials such as high-strength steel, the upset load becomes high and exceeds the equipment capacity. It becomes difficult to add deformation.

  In particular, in recent years, the need for reducing the weight of automobiles has rapidly expanded due to global environmental problems, and the demand for increasing the tension of thin plates is rapidly increasing. To increase the strength, alloy components such as Si, Cr, and Mo are frequently used. . Since the melting point of the oxides of these strengthened alloys is higher than the melting point of steel, it tends to remain as a solid at the bonding interface when the sheet bar is bonded, which causes a decrease in bonding strength.

  For this reason, it is desirable to discharge the oxide near the joint interface from the joint interface by the material flow in the plate thickness direction due to the upset deformation, and it is important to realize a necessary and sufficient upset amount. The amount of upset here is defined as the amount of movement in the compression direction in the plate longitudinal direction of both seat bars.

  Therefore, the present inventors have solved the unbonded state of the sheet width direction edge low-temperature part, and as a means for ensuring the bonding strength in the entire sheet width region, the leading end of the preceding sheet bar or the leading end of the succeeding sheet bar The idea was to raise the temperature of the region until reaching the edge of the sheet bar in the plate width direction by carrying out induction heating in two stages by making any of the planar shapes concave.

  That is, as shown in FIG. 4, in the first induction heating step, the sheet bar whose concave shape is concave (in this example, the succeeding sheet bar 2) and the rectangular sheet bar (in this example, the preceding sheet bar 1). An alternating magnetic flux is applied by the induction heating coil 3 in a state where no contact is made. At this time, due to the induced current generated by the magnetic flux penetrating the sheet bars 1 and 2, the temperature rapidly rises at the outer peripheral portion of both the sheet bars 1 and 2 excluding the edge corners in the plate width direction. At this stage, the heating output and the heating time are set so that the temperature in the vicinity of the joining end surfaces of the sheet bars 1 and 2 facing each other becomes a predetermined temperature that is not lower than the solidus temperature and not higher than the liquidus temperature.

  And after predetermined | prescribed heating is complete | finished, as shown in FIG. 5, the joining end surface of both the sheet | seat bars 1 and 2 is made to contact in the following step. At this time, since the tip of one sheet bar 2 is formed in a concave shape, the contact portion of both sheet bars 1 and 2 becomes a parallel portion of the tip of the recess. In the second induction heating process, an alternating magnetic flux is applied to the coil 3 in this state to increase the plate width direction edge portion temperature.

  That is, as shown in FIG. 6, in the second induction heating step, the induced current flows through the contact interface between the sheet bars 1 and 2 to form a large loop, and the induced current flows in the outer peripheral side. Therefore, as shown in FIG. 7, the temperature in the vicinity of the edge portion in the plate width direction rapidly increases.

  At this time, the induction current hardly flows in the region in the solid-liquid coexistence state in the first induction heating process, but heat is dissipated to the surrounding atmosphere when solidifying from the solid-liquid coexistence state. Since the solidification latent heat is generated, the semi-solid state is maintained for a while and the temperature change becomes very small. For this reason, it is possible to maintain a state equal to or higher than the solidus temperature over the entire plate width direction by performing the first heating to the end of the second heating within several seconds.

  And after implementing the 2nd induction heating process, it becomes possible to obtain a strong joined state over the whole board width direction by setting up both the sheet bars 1 and 2 immediately.

  In the present invention, it is preferable that the planar shape of either the leading end portion of the preceding sheet bar or the leading end portion of the succeeding sheet bar is positively formed into a fishtail shape in the rough rolling process.

  Although not shown in FIG. 12, width reduction is performed by a sizing press and an edger (vertical rolling mill) in the rough rolling process, and FIG. 1 is performed by a sizing press and an edger (vertical rolling mill) in the rough rolling process. It is a schematic diagram of a planar shape (fish tail) formed by adjusting the width reduction amount. In the roughing process of hot rolling, a sizing press device capable of drastically reducing the slab is installed for the purpose of separately producing products having different sheet widths from the slab having the same width, and the width can be changed up to about 300 mm. Further, in the rough rolling process, width reduction of about 10 to 30 mm is performed by an edger immediately before each horizontal rolling for the purpose of compensating for the amount of width spread generated in horizontal rolling. In any width reduction step, deformation tends to concentrate on the end portion of the plate width, so that a convex thickening shape called a dog bone is easily formed in the plate thickness section around the end portion of the plate width. And since this dogbone is deformed in the sheet width direction by horizontal rolling and also extends in the rolling direction, the end of the sheet bar after rough rolling has a sheet width end compared to the center of the sheet width. A planar shape called a fish tail having a long periphery is easily formed.

  Usually, this fishtail is cut by a shear before finish rolling so that the planar shape of the cut sheet bar is rectangular. This is because in finish rolling where the plate thickness is thin, if the planar shape of the tip and tail of the sheet bar is asymmetric, bending and meandering are likely to occur due to rolling, and the plate is likely to become unstable. This is because the passage plate is stabilized by forming it into a rectangular shape.

  However, in the present invention, the fishtail portion is not completely cut and removed, and as shown in FIG. 2, both tip portions of the fishtail of FIG. 1 are cut into a concave shape.

  Usually, in joining of the sheet bar by the butt method, the upset amount for joining is set to 10 to 30 mm. This is to establish a stable joint and protect the joining apparatus by preventing an excessive increase in joining load. The upset amount is an approach distance between both seat bars. For this reason, it is desirable that the dent depth of the dent-shaped sheet bar be 10 mm or less in consideration of subsequent joining by upset.

  Therefore, in the present invention, the plane shape of the tail end portion of the preceding sheet bar and the tip end portion of the succeeding sheet bar is accurately measured by the crop shape measuring device on the final rolling pass exit side of the rough rolling process, and the fish tail It is preferable to determine the cutting position on the shear by setting the longitudinal direction to 10 mm or less on the basis of the most valley portion of the crop recess formed in the shape (FIG. 10). This can be realized by accurately tracking the position of the leading end of the sheet bar with a sensor and precisely setting the shear timing by the shear at the previously determined cutting position.

  Note that, as described above, in order to measure the planar shape of the tail end portion of the preceding sheet bar and the leading end portion of the succeeding sheet bar, a crop shape is formed on the exit side of the rough rolling final rolling mill 10 as shown in FIG. What is necessary is just to install the measuring apparatus 11 and to measure a planar shape.

  In the crop shape measuring apparatus 11 in FIG. 11, the light projected linearly in the plate width direction from the upper light projecting unit to the upper surface of the sheet bar is blocked by the sheet bar. The linear sensor camera system recognizes the crop shape by connecting the ends in the longitudinal direction, but it may be a system that electronically processes a digital image taken by a CCD camera.

  And it is preferable that the upset amount after a 2nd induction heating process shall be 2 times or more of the dent depth of the sheet bar edge part after a cutting | disconnection. This is because the recess at the end of the recessed sheet bar is completely bonded to the other end of the sheet bar and the oxide at the bonding interface is discharged.

  In FIGS. 1 and 2, the left and right lengths and widths of the fishtail are shown due to the uneven heat in the heating furnace and the asymmetric rolling in the edger in the actual operation. May be asymmetric.

  However, in the method according to the present invention, some asymmetry is not a problem. As shown in FIG. 3, the second induction heating is performed even when the left and right convex widths of the concave shape after cutting by the shear are different. In the process, the induced current has a property of collecting and flowing on the outer peripheral side, and therefore the temperature rise region in the vicinity of the edge portion in the plate width direction hardly changes. Further, due to this property, there is almost no influence of the concave shape of the concave shape formed on the sheet bar, and in the second induction heating process, it is a necessary and sufficient condition that there is a concave portion, that is, a non-contact portion at the center portion of the plate width.

  It is desirable that the maximum recess amount of the concave portion at the center of the plate width of the fish tail portion after cutting by the shear is 10 mm or less, and the upset amount is at least twice the maximum recess amount of the concave portion at the central portion of the plate width. preferable.

  The effect of this invention is demonstrated based on an Example.

  The target material is a high-tensile steel with a tensile strength of 590 MPa at room temperature. Both the leading and trailing materials are 260 mm thick and 1750 mm wide slabs, which are subjected to a roughing process and are 28 mm thick and 1500 mm wide sheet. Molded into a bar. At this time, the width reduction amount by the sizing press is 300 mm, and the width reduction amount by edging before each horizontal rolling in the rough rolling process is expressed by the Siebel formula (for example, refer to Non-Patent Document 1). The width reduction amount ΔW by horizontal rolling calculated in this way was set in consideration of the width reduction efficiency.

          ΔW = C · Ld · Δh / H (1)

  In equation (1), C is a constant (= 0.35), Ld is the contact length between the rolling roll and the material during rolling determined from the rolling amount and the rolling roll diameter, Δh is the rolling amount, and H is the rolling amount before rolling. Thickness.

  The heating temperature of the slab in the heating furnace was set so that the sheet bar temperature before bonding heating was about 1050 ° C. under all conditions. The induction heating conditions for heating the sheet bar joint are a frequency of 1 kW and an input power of 1060 kW. By changing the shape of the shearing blade of the shear before the joining device, the planar shape of the tail end portion of the preceding sheet bar or the leading end portion of the succeeding sheet bar was formed into a rectangular shape or a concave shape. The planar shape of the sheet bar after cutting with a shear was confirmed by a CCD camera installed at the upper part of the line immediately after the shear.

Then, (2) calculating the ratio of the sum Wt and sheet bar overall width W of the concave lateral tip parallel part width at equation was organized as interfacial contact width ratio W _R.

W _R = Wt / W × 100 (%) (2)

  The amount of upset was two levels of 15 mm and 25 mm. Then, after joining the sheet bar, using another shear installed before finish rolling, cut out a sample with a length of about 500 mm across the joint in the longitudinal direction, and observe the state of the joint after cooling to room temperature did.

  Table 1 shows a comparison between the edge unjoined part length (average value of both edges) and the upset load at one end in the plate width direction of the joined part under each joining condition.

  As shown in Table 1, in the conventional example (Nos. 9 and 10) in which the sheet bars having a rectangular cross section having a dent amount of 0 mm, which is the conventional joining method shown in FIG. In the present invention examples (Nos. 1 to 8) in which the sheet bars were joined based on the present invention, the edge unjoined part length was 0 mm at the stage of the sheet bar. It was good. Moreover, the upset load was also reduced in the present invention examples (Nos. 1 to 8) compared to the conventional examples (Nos. 9 and 10).

  Further, after the sheet bar was joined under exactly the same conditions as in Table 1, finish rolling was performed. In that case, the joint part after finishing rolling was observed with the CCD system camera installed on the cooling table after finishing rolling. If the unjoined portion length from the plate width direction edge or the total unjoined portion length at the plate width central portion is less than 50 mm, ◎, if it is 50 mm or more and less than 100 mm, ○, 100 mm or more and less than 200 mm, Δ And when the unjoined part length was 200 mm or more or the joint part was broken, it was evaluated as x, and the evaluation after finish rolling is shown in Table 1.

  As shown in Table 1, in the conventional examples (Nos. 9 and 10), the evaluation after finish rolling was x. That is, when the upset amount was 15 mm (No. 9), the unjoined portion length was 200 mm or more. When the upset amount was 25 mm (No. 10), the joint was broken in the finishing mill.

  On the other hand, the examples of the present invention (Nos. 1 to 8) were evaluated as ◎, ○, or Δ after finish rolling. In particular, the present invention example (No. 1, No. 1 in which the ratio A / Δmax between the upset amount A and the maximum recess depth Δmax in the center portion of the plate width is 2 or more is the maximum recess depth Δmax in the center portion of the plate width is 10 mm or less. 2, 4, 5, 6) were all evaluated as ◎ or ○, and it was confirmed that stable finish rolling was possible. Other invention examples (Nos. 3, 7, and 8) were evaluated as Δ, but stable continuous hot rolling was possible.

DESCRIPTION OF SYMBOLS 1 Predecessor sheet bar 2 Subsequent sheet bar 3 Induction heating coil 4 Semi-molten part 5 Inductive current 6 Insufficient temperature rise area 7 Area | region strongly contacted in solid state 8 Induction current detour area | region of sheet bar corner 10 Heating furnace 11 Rough rolling 12 Coil box 13 Joining device 14 Finishing mill 15 Water cooling device 16 Coiler 17 Coarse rolling final rolling mill 18 Crop shape measuring device

Claims (4)

Immediately before the finish rolling of the hot rolling line, the tail end of the preceding sheet bar and the leading end of the succeeding sheet bar are sheared with a shear and then heated, upset, joined, and continuously finish rolled. By manufacturing a hot-rolled steel sheet using a complete continuous hot rolling method for manufacturing a plurality of hot-rolled steel sheet coils continuously from a plurality of slabs,
After forming the shape of the joining end surface of at least one of the preceding sheet bar or the following sheet bar into a predetermined concave shape, and applying predetermined induction heating in a state where the preceding sheet bar and the following sheet bar do not contact, A state in which the preceding seat bar and the succeeding seat bar are brought into contact with each other with the induction heating current turned off, and then upset joining is performed with a predetermined upset amount after applying predetermined induction heating again. A method for producing a hot-rolled steel sheet.   Adjust the amount of width reduction by the sizing press and / or edger in the rough rolling process, and the planar shape of one or both of the tail end portion of the preceding sheet bar and the leading end portion of the succeeding sheet bar is made into a fish tail shape, Then, a fishtail part is cut | disconnected by a shear, and it is made for a planar shape to become a concave shape whose dent amount of a plate width center part is 10 mm or less, The manufacturing method of the hot-rolled steel plate of Claim 1 characterized by the above-mentioned. .   Adjust the amount of width reduction by the sizing press and / or edger in the rough rolling process so that the planar shape of one or both of the tail end of the preceding seat bar and the leading end of the succeeding seat bar becomes a fishtail shape. The cutting position at the shear is determined by measuring the planar shape of one or both of the tail end portion of the preceding sheet bar and the leading end portion of the succeeding sheet bar at the final rolling final pass exit side. The manufacturing method of the hot rolled sheet steel of Claim 1 or 2.   The amount of upset between the preceding seat bar and the succeeding seat bar is 10 to 30 mm, and the amount of dent at the center of the plate width formed at either the tail end portion of the preceding seat bar or the leading end portion of the succeeding seat bar. The method for producing a hot-rolled steel sheet according to any one of claims 1 to 3, wherein the production method is twice or more.

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