JP5928408B2 - Method and apparatus for preventing tip warpage in hot finish rolling - Google Patents

Description

  The present invention relates to a tip warpage prevention method and apparatus that enables stable controlled rolling by preventing the occurrence of tip warp in finish rolling in a hot rolling process for producing a hot-rolled steel sheet.

  In recent years, demand for large-diameter, thick-walled, high-strength pipe materials of API standard X70 to 100 grade has been increasing mainly for the purpose of improving transportation efficiency of resources such as crude oil and natural gas in continental pipelines.

  In order to transport these resources efficiently, high internal pressure is applied to the inside of the pipe, and the toughness and high strength characteristics are extremely important for pipe materials in consideration of crustal deformation due to use in cold regions and earthquakes. It is important to

  The high-strength pipes used in these pipelines have a large diameter of about 15 to 25 mm and an outer diameter of about 20 inches or more, and conventionally, the short side of a thick steel plate that is vertically long in the longitudinal direction. UOE steel pipes are often used in which the side is formed into a circular shape and then the butt portion is welded in the longitudinal direction to form a pipe. Usually, a thick steel plate is manufactured in a substantially rectangular shape by multi-pass rolling with a hot plate slab having one or two rolling mills, and the product length is about 30 m at the maximum. .

  On the other hand, in recent years, instead of UOE steel pipes from thick steel plates rolled by a thick plate mill, a hot rolled steel plate having a predetermined dimension is rolled in a normal hot rolling line for thin sheet rolling as shown in FIG. And coiled into a hot-rolled steel sheet coil, then unwound the hot-rolled steel sheet coil and cut it to a predetermined length, and then formed into an electric-welded tube, or unwound the hot-rolled steel sheet coil There is an increasing demand for producing a pipe (spiral steel pipe) while forming a spiral shape in the longitudinal direction and welding the butted portion at the end of the plate width.

  In FIG. 2, 9 is a final rough rolling mill, 1 is a sheet bar, 6 is a table roll, 7 is a roller table, 3 is a thermometer, 8 is a crop shear, 5 is a finish rolling mill, and 5a is the first. A finish rolling mill 4 is a finish rolling mill group.

  Such a hot-rolled steel sheet coil can be manufactured up to a maximum of about 45 tons. For example, if the thickness is 20 mm and the width is about 1900 mm, the length of the hot-rolled steel sheet is about 151 m and the spiral steel pipe has a diameter of 28 inches. The pipe length when molded into is about 128 m. Thus, by using a hot-rolled steel sheet coil as the base material before forming the pipe, it is possible to manufacture a long pipe continuously as compared with the case of manufacturing from a thick steel sheet, and thus an improvement in productivity can be expected.

  One of the characteristics required for a hot-rolled steel sheet used as a material for such a pipe is toughness, and it is effective to make crystal grains fine as a means for increasing the toughness. For this reason, in the manufacture of a hot-rolled steel sheet used as a pipe material, controlled rolling (hereinafter referred to as CR) is generally performed. The principle of CR is as follows. When finish rolling is performed by adjusting the temperature of the sheet bar after rough rolling to the non-recrystallized region on the low-temperature side of the austenite phase, the dislocation density significantly increases during finish rolling, and the rolled material is cooled in that state. Thus, a fine ferrite transformation structure can be obtained.

  In controlled rolling, the temperature at which finish rolling is started and the rolling reduction in finish rolling are important parameters. Normally, the temperature at which the finish rolling is started is adjusted in FIG. 2 between the rough rolling sheet bar 1 between the roughing mill (final roughing mill 9) and the finishing mill (first finishing mill 5a). This is performed by waiting on the roller table 7 located in the center.

  However, at this time, there is a difference in cooling rate between the upper surface side of the sheet bar 1 exposed to the atmosphere and the lower surface side facing the roller table 7, and the temperature in the thickness direction of the sheet bar 1 is increased. Distribution occurs. That is, on the lower surface side of the sheet bar 1 facing the roller table 7, the temperature of the roller table 7 rises due to the radiant heat from the sheet bar 1 and the ambient temperature becomes higher. Is relaxed and the temperature of the sheet bar 1 becomes higher on the lower surface side.

  Such temperature distribution in the sheet thickness direction greatly affects the distribution of deformation resistance of the rolled material, and the deformation resistance becomes asymmetric rolling in the sheet thickness direction. There was a problem that the finish rolling equipment was damaged and the windability of the tip to the coiler deteriorated.

  Conventionally, as a technique for preventing the tip warpage of a rolled material in hot rolling including thick plate rolling, a technique for improving the above-described asymmetric temperature distribution of the rolled material (for example, Patent Document 1 and Patent Document 2), a rolling mill A technique (for example, Patent Document 3) or the like that mechanically suppresses the tip warp of the rolled material with a guide installed on the outlet side is disclosed.

  That is, the technique disclosed in Patent Document 1 measures the temperature of the upper and lower surfaces of the rolled material with a thermometer installed on the exit side of the rolling mill, and considers the measured upper and lower surface temperature difference and transformation behavior of the rolled material. Thus, water cooling is performed on the upper surface or the lower surface, or the difference in rotational speed between the upper and lower rolls is controlled so that tip warpage does not occur during rolling.

  In addition, the technique disclosed in Patent Document 2 is based on the conditions such as the heating zone in the heating furnace and the set temperatures of the upper and lower surfaces of the soaking slab, the residence time of the slab in the furnace, and the occurrence of warpage in finish rolling. The correlation between behaviors is examined and converted into a function, and the upper and lower temperatures in the heating zone and the soaking zone in the heating furnace are set so that the upper end of the rolled material does not warp during finish rolling.

  In addition, the technique disclosed in Patent Document 3 is to install a guide in the upper part immediately adjacent to the exit side of the rolling mill and mechanically press down the warp of the rolling material tip, and provide appropriate rigidity at an appropriate position. If it is possible to arrange the guide having the tip warp, it is possible to correct the tip warp with a small load by utilizing plastic deformation during rolling.

JP-A-5-237530 JP-A-8-243615 JP-A-62-104606

  However, the above-described tip warpage prevention techniques in hot rolling (Patent Documents 1 to 3) each have the following problems.

  First, for the implementation of the technique disclosed in Patent Document 1, it is essential to measure the upper and lower surface temperatures of the rolled material with high accuracy on a regular basis. Radiation thermometers are used, and in particular, thermometers that measure the bottom surface temperature are very difficult to maintain temperature measurement accuracy because the light-receiving surface becomes dirty due to the fall of the oxide scale. In addition, there is a problem in equipment cost such as special water cooling equipment for cooling the upper surface or the lower surface of the rolled material.

  In addition, in the technique disclosed in Patent Document 2, it takes a long time due to thermal inertia to change the temperature distribution in the heating furnace, so time is required for temperature adjustment before and after rolling the pipe material. There was a decline in productivity.

  Normally, in hot rolling, operations are performed in units of cycles in which several tens of slabs are combined into one cycle. As the number of slabs rolled in one cycle increases, the dead time decreases and the operation becomes highly productive. However, in the technique disclosed in Patent Document 2, a special heating furnace temperature setting specialized for controlled rolling is used. Therefore, it is necessary to organize one cycle with only pipe materials, so the production timing is delayed until the number of pipe materials ordered is reduced (delay of delivery date), or the number of rolling is small. There is a problem that it must be rolled in a cycle.

  In addition, in the technique disclosed in Patent Document 3, since there are various incidental facilities in addition to a stripper guide that drains the roll cooling water in the immediate vicinity of the exit side of the rolling mill, Installation of guides is difficult and very expensive equipment.

  The present invention has been intensively studied to overcome the above-mentioned problems of the prior art, and effectively prevents the occurrence of tip warpage in finish rolling in the hot rolling process for producing hot-rolled steel sheets. Thus, an object of the present invention is to provide a tip warpage prevention method and apparatus in hot finish rolling that enables stable controlled rolling.

  In order to solve the above problems, the present invention has the following features.

[1] In the hot rolling process for producing a hot-rolled steel sheet, in order to prevent tip warpage in finish rolling when performing control rolling in which standby cooling of the sheet bar is performed after rough rolling to adjust the finish rolling start temperature. A method for preventing tip warpage in hot finish rolling,
Until the temperature of the sheet bar after the rough rolling is lowered to a predetermined temperature, it is necessary to keep only the upper surface of the sheet bar in the rolling direction on the table between the rough rolling mill and the finish rolling mill. A feature of preventing tip warpage in hot finish rolling.

  [2] The region that retains heat only on the upper surface side of the front end of the sheet bar in the rolling direction is a region starting from the leading edge of the sheet bar and ending at a position of 500 mm to 1,000 mm from the leading edge of the sheet bar. The method for preventing tip warpage in hot finish rolling according to [1] above.

[3] A tip warpage preventing device in hot finish rolling for performing the tip warp preventing method in hot finish rolling according to [1] or [2],
Tip warpage prevention in hot finish rolling, characterized by having a heat retaining cover that keeps heat only on the upper surface of the front end of the sheet bar in the rolling direction above the table between the roughing mill and the finishing mill. apparatus.

  By using this invention, in the hot rolling process which manufactures a hot-rolled steel plate, generation | occurrence | production of the tip curvature in finish rolling can be prevented efficiently, and stable control rolling can be performed.

It is a figure which shows one Embodiment of this invention. It is a figure which shows the normal hot rolling line for sheet metal rolling. It is a figure which shows transition of the amount of temperature fall of the upper and lower surfaces during sheet bar cooling in the normal hot rolling line for sheet rolling. It is a figure which shows the curvature shape of the rolling-material front-end | tip part in a rolling mill delivery side. It is another figure which shows the curvature shape of the rolling material front-end | tip part in a rolling mill delivery side.

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

  Usually, in the hot rolling of line pipe material, from the viewpoint of suppressing rolling load in the rough rolling process, from the viewpoint of dissolving alloy elements such as niobium added to increase material strength, a heating furnace Heating is performed until the slab temperature reaches 1200 ° C. or higher. After the slab heated to a high temperature is extracted from the heating furnace, it is subjected to width reduction by a sizing press or an edger and horizontal reduction by a roughing mill, and formed into a sheet bar having a predetermined thickness and a predetermined width. At this time, the cross-sectional average temperature of the sheet bar is lowered to about 1000 to 1100 ° C.

  In controlled rolling (CR), in order to obtain desired toughness and strength characteristics, it is necessary to secure a finish reduction ratio of a predetermined amount or more in the untransformed temperature region of the austenite region. Is called CR rate. For example, if the finishing plate thickness is 20 mm and the CR rate required to obtain desired characteristics is 50%, the sheet bar thickness must be 40 mm or more. In recent years, the thickness of a line pipe for transporting energy resources, which is in high demand, is about 25 mm at the maximum. In this case, the sheet bar thickness is 50 mm or more at a CR rate of 50%, and 62.5 mm or more at a CR rate of 60%.

  Thus, in the rough rolling process, the sheet bar is formed to a thickness determined by the CR ratio required for finish rolling, but in controlled rolling, it is necessary to perform rolling in the untransformed temperature region of the austenite region. Until the average cross-sectional temperature of the sheet bar drops to around 900 ° C., it is placed on a roller table installed between the rough rolling process (rolling with a roughing mill) and the finishing rolling process (rolling with a finishing mill). Cooling treatment is carried out.

  At that time, since the thickness of the sheet bar is large, the cooling rate of the sheet bar by cooling is slow, and it takes several minutes to start the finish rolling. In this case, if the sheet bar is made to stand still on the roller table and is put on standby, the temperature of the portion in contact with the table roll on the lower surface of the sheet bar is locally lowered, and the desired characteristics as a product cannot be satisfied. In order to prevent this, the table roll is rotated at a low speed, and the sheet bar is moved back and forth (hereinafter referred to as oscillation) by performing reverse cooling in the forward and reverse directions with respect to the rolling direction. doing.

  However, at this time, since the upper surface side of the seat bar is exposed to the atmosphere, the temperature is lowered by heat transfer and radiation, but the lower surface side of the seat bar is the heat transfer from the hot seat bar. Since the table roll and the ambient temperature rise due to radiation, it is inevitable that the temperature decrease rate on the lower surface side of the seat bar is slower than that on the upper surface side.

  FIG. 3 shows the upper and lower surfaces of the sheet bar 1 that is oscillating on the roller table 7 after the final rough rolling by the thermometer 3 installed on the roller table 7 in the hot rolling line shown in FIG. It is the result of examining the temperature transition. As shown in FIG. 3, the temperature lowering speed on the upper surface side is faster than the lower surface temperature of the sheet bar 1, and a temperature difference of about 30 ° C. occurs about 300 seconds after the final rough rolling.

  As described above, in the controlled rolling of the pipe material, the lower surface side temperature of the sheet bar is higher than the upper surface side temperature. As a result, the end of the sheet bar tends to be warped in finish rolling. This is because a deformation resistance distribution is generated corresponding to the temperature distribution in the thickness direction of the sheet bar generated during air cooling, and the hardness on the lower surface side of the seat bar is softer than that on the upper surface side.

  In such a situation, as shown in FIG. 4, the tip of the rolled material 10 is warped in horizontal rolling (for example, rolling in the first finish rolling mill 5a). This is because, in horizontal rolling, the load applied to the upper and lower rolls becomes equal in principle, and as a result, the reduction amount and elongation on the lower surface side, which is lower in hardness than the upper surface side, increase, resulting in an upward warping shape.

  In addition, when the top warp shape of the rolling material 10 is formed in the finish rolling mill group 4, it becomes a disturbance in the next stand rolling, and depending on the angle at which the leading end of the rolling material 10 enters the rolling roll bite, the top warping is performed. Not only the shape but also the downward warping shape may occur.

  That is, once warping has occurred at the tip of the rolled material 10 in the finish rolling mill group 4, it is difficult to secure a stable plate at the tip of the rolled material 10, so the first finish. It is very important to prevent the rolling material tip warpage from occurring in the rolling mill 5a.

  Therefore, the present inventors conducted extensive studies on the subject of suppressing the occurrence of rolling of the rolled material tip from the first finish rolling mill when performing controlled rolling, and as a result, the temperature of the sheet bar after rough rolling. Until the temperature drops to a predetermined temperature, the inventors have conceived that only the upper surface side of the front end of the sheet bar is kept on the table between the roughing mill and the finishing mill.

  FIG. 1 is a diagram showing a hot rolling line in an embodiment of the present invention. The basic configuration is the same as in FIG. 2 described above. In FIG. 1, 9 is a final roughing mill, 1 is a sheet bar, 6 is a table roll, 7 is a roller table, 3 is a thermometer, and 8 is a crop shear. Reference numeral 5 denotes a finish rolling mill, 5a denotes a first finish rolling mill, and 4 denotes a finish rolling mill group.

  In FIG. 1, a slab (not shown) heated to a predetermined temperature in a heating furnace (not shown) is rolled a plurality of times by a roughing mill such as the final roughing mill 9 and the sheet bar. 1 until the average cross-sectional temperature of the sheet bar 1 reaches a predetermined temperature, and waits while oscillating on the roller table 7 between the final roughing mill 9 and the finishing mill group 4. It shows the state.

  In addition, in this embodiment, the leading end of the sheet bar 1 is located above the vicinity of the leading end of the sheet bar 1 that is oscillating on the roller table 7 between the final roughing mill 9 and the finishing mill group 4. A heat insulating cover 2 is installed to suppress a temperature drop due to radiation on the upper surface side.

  Usually, the heat insulating cover used in the hot rolling line is made of a metal reflector and is arranged in a range covering the entire length of the sheet bar for the purpose of suppressing the temperature drop of the latter half of the sheet bar during finish rolling. ing. In general, the heat insulating cover is composed of a plurality of units in the longitudinal direction of the seat bar, and has a mechanism that can be opened and closed by a retractable method, and the presence or absence of use is determined according to the manufacturing conditions of each rolled material.

  However, in the controlled rolling targeted by this embodiment, it is necessary to actively lower the temperature of the sheet bar, and thus, conventionally, the heat insulating cover is manufactured under conditions that do not use it.

  On the other hand, in this embodiment, as means for reducing the cooling rate difference between the upper and lower surfaces of the sheet bar tip, only the upper surface side of the sheet bar tip is heated by the heat retaining cover 2, thereby The cooling rate difference between the top and bottom surfaces of the part is greatly reduced, so that the upper warp of the rolled material generated during finish rolling is almost eliminated.

  In this embodiment, as shown in FIG. 1, a heat insulation cover 2 for newly keeping only the front end of the sheet bar is newly installed, and the oscillation of the sheet bar 1 is performed at that position. However, in a hot rolling line having a heat insulation cover composed of a plurality of units corresponding to conventional heat insulation purposes, only the heat insulation cover unit closest to the finishing mill group is used. The oscillation of the seat bar may be performed in accordance with the position.

  The set height of the heat insulating cover 2 or the heat insulating cover unit is preferably set to a height position at which the temperature lowering speeds at the upper and lower surfaces of the seat bar front end are substantially equal, and are determined by adjustment in actual operation. That's fine.

  Next, the length of the heat retaining area on the top surface of the front end of the sheet bar will be described.

  FIG. 5 shows a state where the horizontal rolling is further advanced from the state where the tip of the rolled material 10 is warped in the horizontal rolling shown in FIG. The warp height is almost the same. This is because a bending moment is generated by the weight of the rolled material on the roll bite exit side of horizontal rolling, and this bending moment acts on the stress state in the roll bite, thereby suppressing the warping state in horizontal rolling. This is because it occurs.

  The inventors of the present invention have studied the warp shape of the leading end of the rolled material 10 on the exit side of the first finish rolling mill 5a at the time of performing the control rolling, and as a result, the length of the upper warped portion of the leading end of the rolled material 10 is determined. It was found that there was not much variation at about 500 mm. It is estimated that the variation in the length of the warped part is a result of the above-described action due to its own weight.

  For this reason, it is desirable that the length in the rolling direction of the heat retaining region on the top surface of the front end of the sheet bar 1 by the heat retaining cover 2 is about 500 mm or more. On the other hand, the heat retention area has a higher cross-sectional average temperature than the other areas, so that a desired material cannot be ensured in the product and there is a risk of yield reduction.

  For this reason, in this embodiment, taking into consideration the oscillation accuracy of the sheet bar 1 due to the rotation of the table roll 6, the heat retention area on the top surface of the sheet bar 1 is set to 500 mm or more and 1000 mm or less from the leading edge of the sheet bar 1. . In other words, the heat insulating cover is such that the region where the starting point is the leading edge of the seat bar 1 and the end point is a position of 500 mm or more and 1,000 mm or less rearward from the leading edge of the seat bar 1 is the heat insulating region on the top surface of the leading end of the sheet bar 1. 2 is installed.

  In the above description, the leading edge of the sheet bar 1 refers to the portion that becomes the leading edge of the sheet bar 1 after cutting the unsteady end portion of the sheet bar 1 with a crop shear (that is, the first finishing mill 5a). It means the leading edge of the seat bar 1 when biting.

  As a result, it is possible to perform stable controlled rolling with a minimum risk of causing a decrease in yield because a desired material cannot be secured in the product in the heat retaining region.

  Examples of the present invention will be described.

  The target material in this example is API standard X80 grade, and a slab (weight: 33 tons) having a thickness of 250 mm, a width of 1850 mm, and a length of 9090 mm was heated to 1250 ° C. in a heating furnace of a hot rolling line. Thereafter, the sheet was formed into a sheet bar having a thickness of 60 mm through 9 passes of rough rolling. Under the present circumstances, the cross-sectional average temperature of the sheet bar immediately after the final rough rolling was about 1070 ° C. as a result of conversion from the measurement result of the thermometer installed on the delivery side of the final rough rolling mill.

  And when this sheet bar is allowed to stand by and cool on a roller table between the roughing mill and the finishing mill, the average cross-sectional temperature at the center position in the longitudinal direction of the sheet bar is a desired temperature (900 ° C.). The time to become 4 minutes and 40 seconds from the calculation result. After the sheet bar was allowed to cool at this time, the tip unsteady portion was cut with a crop shear, and the sheet was reduced to a thickness of 25.4 mm with a finishing mill group. Note that the finishing rolling mill group used in this example is composed of seven horizontal rolling mills. In this example, five rolling mills are used to reduce the rolling from 60 mm to 25.4 mm. The coil was wound around a coiler after predetermined cooling on the run-out table.

  In this case, based on the above-described embodiment of the present invention (FIG. 1), the case where the seat bar is cooled by performing heat retention only on the upper surface of the front end of the sheet bar is used as an example of the present invention, and for comparison. As shown in FIG. 2, the conventional case is a case where standby cooling of the seat bar is performed without heat retention of the seat bar.

  And about each, the curvature condition of the rolling-material front-end | tip part in a finishing mill group was observed. In addition, the observation of the state of warping of the rolled material tip was confirmed by a video image that was set up between the rolling mills.

  As a result, in the example of the present invention, the warp of the rolling material tip hardly occurred in the finish rolling mill group, extremely stable rolling was possible, and the winding form in the coiler was excellent. And as a result of collecting and evaluating samples for property evaluation from the vicinity of the tip, center, and tail end in the rolling direction from the product, predetermined strength and toughness were obtained.

  On the other hand, in the conventional example, the first finishing mill is on the outlet side with a height of about 250 mm, and then the vertical warping is reversed on the finishing mill outlet side, and the finishing mill group outlet side. Then, the warped shape was about 200 mm. As a result, the winding shape of the coiler became a large kink defect that was bent on the inner diameter side.

  This confirmed the effectiveness of the present invention.

DESCRIPTION OF SYMBOLS 1 Sheet bar 2 Heat insulation cover 3 Thermometer 4 Finishing mill group 5 Finishing mill 5a First finishing mill 6 Table roll 7 Roller table 8 Crop shear 9 Final rough rolling mill 10 Rolling material

Claims (3)

In the hot rolling process for producing hot-rolled steel sheets, the temperature distribution in the thickness direction at the front end of the sheet bar in the rolling direction is performed when performing control rolling to adjust the finishing rolling start temperature by performing standby cooling of the sheet bar after rough rolling. In order to prevent the tip warp in the finish rolling mill group based on , a tip warp prevention method in hot finish rolling,
Until the temperature of the sheet bar after the rough rolling is lowered to a predetermined temperature, only the upper surface of the sheet bar in the rolling direction is kept on the table between the rough rolling mill and the finish rolling mill . A tip warpage prevention method in hot finish rolling, characterized by preventing tip warpage in a finish rolling mill group based on a plate thickness direction temperature distribution at a tip portion in the rolling direction of a sheet bar .   The region that retains heat only on the upper surface side in the rolling direction front end of the sheet bar is a region that starts from the leading edge of the sheet bar and ends at a position of 500 mm or more and 1,000 mm or less from the leading edge of the sheet bar. Item 2. A method for preventing tip warpage in hot finish rolling according to Item 1. A tip warpage prevention device in hot finish rolling for performing the tip warpage prevention method in hot finish rolling according to claim 1 or 2,
Tip warpage prevention in hot finish rolling, characterized by having a heat retaining cover that keeps heat only on the upper surface of the front end of the sheet bar in the rolling direction above the table between the roughing mill and the finishing mill. apparatus.

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