JP2016074028A - Equipment and method for producing hot rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide equipment for producing a hot rolled steel sheet having an excellent surface quality.SOLUTION: The equipment for producing a hot rolled steel strip is equipment for producing a hot rolled steel sheet which reduces thickness of a slab cast in a continuous casting line, without heating the slab in a heating furnace and winds the slab in a coil shape or heats the slab to a predetermined temperature in a heating furnace provided in the hot rolling line then reduces the thickness of the slab to the prescribed thickness and winds the slab in the coil shape. A rolling process is comprised of a rough rolling process and a finish rolling process, a heater which can heat a surface temperature of a sheet bar to 1300°C or higher, and descaling equipment for removing an oxidized scale on a surface thereof are provided in this order between the rough rolling process and the finish rolling process.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a production facility and a production method for producing a hot-rolled steel sheet having excellent surface properties.

  Generally, a hot rolled steel sheet is a coil in which a slab having a thickness of about 200 to 300 mm cast in a continuous casting line is directly fed to a hot rolling line without being heated in a heating furnace, and is reduced to a predetermined thickness. HDR (Hot Direct Rolling) to be wound around and slab whose temperature has been lowered to 1000 ° C. or less is charged into a heating furnace, reheated to about 1100 to 1250 ° C. in a short time, and then hot rolled to a predetermined thickness DHCR (Direct Hot Charged Rolling) or HCR (Hot Charged Rolling), which is reduced in thickness and wound on a coil, and a slab that has been cooled to room temperature once heated in a heating furnace for a long time and then heated to a predetermined temperature in a hot rolling line CCR (Cold Charged Rolli) reduced to thickness and wound on coil ng).

  In continuous casting, clusters of alumina inclusions adhering to the immersion nozzle are agglomerated on the surface of the slab due to the flow of molten steel. In addition, powder used from the viewpoints of lubrication between the solidified molten steel and the mold (mold), heat retaining action of the molten steel, and trapping action of floating inclusions may be caught in the slab surface layer. Many of these inclusions are present in the range of several millimeters on the surface of the slab during the solidification process, particularly in the range of 1 to 2 mm. The powder contains a lot of calcium as a component.

  Inclusions due to steelmaking such as alumina and calcium oxide existing in the surface layer of the slab are gradually removed by removing the surface oxide layer generated during the hot rolling process by descaling and stretching by thickness reduction by rolling. Move to the surface side. And the inclusion extended | stretched to the longitudinal direction turns into a surface flaw called the linear shaving which appears on the surface of a rolling material.

  Such balding is present between the surrounding materials, so it is difficult to remove by descaling with high-pressure water, and it remains in the product as a surface defect. Such surface defects not only cause appearance problems, but also cause problems such as opening the mouth and damaging the mold during press molding, so the surface defects are cut from the coil after appearance inspection. After that, the coil is shipped as a product. For this reason, the occurrence of lashes has become a serious problem in terms of manufacturing cost, such as a decrease in production efficiency and a decrease in yield.

  In order to deal with such problems, for the orders with a demanding surface quality level strict, the range of the surface layer number mm of the continuously cast slab is removed by a scarf (flamming by flame) or a grinder (hereinafter, Hot rolling is performed after the surface treatment).

  However, in these surface care processes, it takes time to handle the slab and remove the surface layer, and the slab temperature is lowered. Therefore, the slab is usually subjected to CCR, that is, hot rolling after long-time heating in a heating furnace. There is a problem that the manufacturing time becomes long.

  In addition, even if the surface is cleaned, if the cutting surface remains on the surface of the slab, or if the maintenance marks become uneven, they may become surface defects after being rolled. It is difficult to completely eliminate surface defects caused by surface inclusions.

  Various proposals have been made to solve the above-described problems.

  Patent Document 1 discloses a scale-off control that intentionally increases the amount of scale generated and removes surface defects for a steel type that has a strict requirement for surface properties such as for automotive outer panels. In Patent Document 1, in such scale-off control, the heating furnace temperature is set in accordance with the slab that requires the smallest amount of scale-off among a plurality of slabs that are charged at the same time. For a slab that is heated at a constant temperature and the scale-off amount is insufficient at this heating furnace temperature, a technique for growing the scale by heating the sheet bar after the rough rolling process has been proposed.

  In Patent Document 2, after performing water cooling before performing descaling before finish rolling and increasing the growth rate of the scale on the surface of the material to be rolled, the scale is regrown to a peelable thickness, Next, it is disclosed that descaling before finish rolling is performed on the material to be rolled.

  In Patent Document 3, the slab is heated to 1170 to 1300 ° C. in a heating furnace, the scale of the surface layer is removed by descaling, heated again at 1170 to 1300 ° C. for 30 minutes, It is disclosed to remove surface scale by scaling.

JP 2006-297481 A JP 2002-28713 A JP-A-6-269841

  However, each of the above conventional techniques has the following problems.

  In the technique disclosed in Patent Document 1, as shown in the examples, the temperature rise by heating the sheet bar is as low as 25 ° C., and the oxidation is performed when the normal sheet bar temperature is about 1000 to 1100 ° C. Considering that the conditions are slow, the heating of the sheet bar is insufficient to incorporate and remove surface layer inclusions into the oxide scale.

  In the technique disclosed in Patent Document 2, the secondary scale is self-destructed by water-cooling the surface layer of the sheet bar, and then the secondary scale is generated again on the surface of the sheet bar and descaling is performed. By doing this, a hot-rolled steel sheet having excellent surface properties can be produced. However, since the temperature of the sheet bar is a temperature range where the growth rate of the secondary scale is slow, the secondary scale can be sufficiently grown. However, it is quite insufficient to remove even the steelmaking inclusions present on the surface of the slab.

  In the technique disclosed in Patent Document 3, an oxide scale layer is positively formed on the slab surface layer in a heating furnace, and after removing the oxide scale layer by descaling, heating is performed again in the heating furnace. This produces an oxide scale layer on the surface of the slab, but it is insufficient to remove the steel-making inclusions present at a depth of several millimeters at the slab stage, preventing steel-making-induced galling. The effect to do was small.

  The present invention was made to overcome the above-described problems of the prior art, and was present in the slab surface layer by heating the surface layer to a temperature of 1300 ° C. or higher in the state of the sheet bar after the rough rolling process. The present invention relates to a production facility and a production method for producing a hot-rolled steel sheet having excellent surface properties by incorporating steelmaking-derived inclusions into an oxide scale layer and removing the inclusion by descaling before the finish rolling step.

In order to solve the above-mentioned problems, the present inventors have intensively studied and conceived an equipment and a manufacturing method for manufacturing a hot-rolled steel sheet having excellent surface properties by efficiently removing inclusions on the slab surface layer. did. The present invention has been made based on such an idea, and has the following features.
[1] The slab cast in the continuous casting line is reduced in thickness in the hot rolling line without being heated in a heating furnace and wound on a coil, or the slab is provided in the hot rolling line. It is a manufacturing facility for hot-rolled steel sheets that are heated to a predetermined temperature in a furnace and then reduced to a predetermined thickness in a hot rolling line and wound on a coil.
The hot rolling line has a rough rolling facility and a finish rolling facility, and a heating device capable of heating the surface temperature of the sheet bar to 1300 ° C. or more between the rough rolling facility and the finish rolling facility, A hot-rolled steel plate manufacturing facility comprising a descaling facility for removing oxide scale in this order.
[2] The equipment for producing a hot-rolled steel sheet according to [1], wherein the heating device is an induction heating type heating device having a heating frequency of 50 kHz or more.
[3] The slab cast in the continuous casting line is reduced in thickness in the hot rolling line without being heated in a heating furnace and wound on a coil, or the slab is provided in the hot rolling line. It is a method for producing a hot-rolled steel sheet that is heated to a predetermined temperature in a furnace and then reduced to a predetermined thickness in a hot rolling line and wound on a coil.
After reducing the thickness of the slab to a predetermined sheet bar thickness by rough rolling, the sheet bar surface temperature is heated to 1300 ° C. or higher and 1370 ° C. or lower for 2 seconds or longer, and then subjected to high-pressure water before finish rolling. A method of manufacturing a hot-rolled steel sheet that is descaled by the above method.
[4] The method for producing a hot-rolled steel sheet according to [3], wherein the sheet bar surface is heated by an induction heating method with a heating frequency of 50 kHz or more.

  According to the present invention, it is possible to remove the steel-making inclusions present in the surface layer of the slab before the finish rolling step, and it is possible to produce a hot-rolled steel sheet having excellent surface properties.

FIG. 3 shows an HDR process with a seat bar surface care facility according to the present invention. FIG. 2 shows a DHCR process with a sheet bar surface care facility according to the present invention. It is a figure which shows the relationship between the distance from plate | board thickness center, and current density ratio at the time of changing a heating frequency. It is a figure which shows the relationship between oxidation time at the time of changing surface temperature and scale thickness. It is a schematic diagram which shows the state of the steelmaking origin inclusion which exists in the surface layer of a slab and a sheet bar.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  As described above, steel-derived inclusions such as alumina and calcium oxide are present on the surface layer of the slab after continuous casting. FIG. 5 schematically shows the form of the steelmaking inclusion 14 existing on the surface layer of the slab 2 after continuous casting and on the surface layer of the sheet bar 6 after the rough rolling process. Many steel-making inclusions 14 are present at a depth of about 1 to 2 mm on the surface layer of the slab 2.

  In the production of a hot-rolled steel sheet, since the sheet width is larger than the sheet thickness, the sheet is rolled in substantially the same state in the sheet width direction, that is, the thickness is reduced in the sheet thickness direction in a plane strain state. When rolled in a plane strain state, it deforms almost uniformly in the plate thickness direction in the steady rolling state. For example, when a slab having a thickness of 250 mm is reduced to a sheet bar having a thickness of 30 mm, the surface layer of the slab 2 has a depth of 1 to 2 mm. The steel-making inclusions 14 at this position are present at a depth of 0.12 to 0.24 mm (= 30 mm / 250 mm × 1 to 2 mm) in the seat bar 6. That is, in the state of the seat bar 6, the removal depth necessary for removing the steel-making inclusions 14 may be considerably shallower than that of the slab 2.

  Based on the above knowledge, the present inventors actively formed an oxide scale layer on the surface layer at the sheet bar stage and incorporated steelmaking inclusions therein, and then oxidized containing steelmaking inclusions. It was conceived that a hot-rolled steel sheet having excellent surface properties can be produced by removing the scale layer using a descaling apparatus. FIG. 4 is a diagram showing the relationship between the oxidation time and the scale thickness when the surface temperature is changed. In general, it is known that the growth rate of oxidized scale of steel greatly depends on time and temperature. The higher the surface temperature, the faster the growth rate. As described above, at the stage of the sheet bar, there are many steel-making inclusions in the region up to about 0.2 mm from the surface layer. Therefore, the surface temperature is about 1300 ° C. or higher and the heating is about 2 to 4 seconds from FIG. It can be seen that most of the oxide is taken into the oxide scale layer. When water is sprayed on the surface of the oxide scale in the high temperature range, it easily breaks due to heat shrinkage. Therefore, after heating at a surface temperature of about 1300 ° C. or more for about 2 to 4 seconds, most of the steelmaking-related intervention is performed by removing the oxide scale layer by performing descaling using high-pressure water that is generally used Things can be removed.

  Incidentally, the structure of the oxide scale of steel is FeO (wustite) at the initial stage of production, and its melting point is 1372 ° C. That is, when the surface temperature is heated to 1372 ° C. or higher, the surface oxide scale layer melts and solidifies rapidly after heating. In this case, even if the descaling with high-pressure water is subsequently performed, it is difficult to remove the steel-making inclusions existing in the surface layer. For this reason, it is preferable to carry out heating at a sheet bar surface layer temperature of 1300 ° C. or higher and 1370 ° C. or lower for 2 seconds or longer. The heating for a long time leads to the coarsening of the structure inside the sheet bar and deteriorates the material, and most of the steelmaking inclusions can be taken into the oxide scale by heating for about 4 seconds. The heating time of the surface layer of the sheet bar is preferably within about 4 seconds.

  In the present invention, since the purpose is to remove the steel-making inclusions existing in a very small area of the surface layer of the sheet bar, only the temperature of the surface layer of the sheet bar needs to be rapidly increased. In the manufacture of a hot-rolled steel sheet, a heat treatment for securing a desired material is performed, and the temperature before finish rolling, the temperature after finish rolling, and the temperature when winding the coil are strictly controlled. For this reason, it is not preferable to raise the sheet bar temperature excessively. For this reason, the present invention is characterized in that only the extreme surface layer of the sheet bar is heated. For this purpose, the induction heating method is suitable as the sheet bar surface layer heating device.

  FIG. 3 is a diagram showing the relationship between the distance from the center of the plate thickness and the current density ratio for each frequency when performing induction heating on a sheet bar having a plate thickness of 30 mm. Regarding the current density ratio, the current density on the plate surface was 1. The induced current flowing in the sheet bar varies greatly in the thickness direction depending on the heating frequency, and the higher the heating frequency, the more the current flowing flows concentrate on the surface layer of the sheet bar. What is necessary is just to determine the output of an induction heating apparatus so that the surface temperature of a sheet bar may be 1300 degreeC or more and 1370 degrees C or less. Then, considering the range of the moving speed of the sheet bar determined by the finish rolling conditions according to the steel type and the finish plate thickness, the longitudinal dimension of the induction heating device that enables heating for 2 seconds or more may be determined.

  FIG.1 and FIG.2 is a figure which shows the manufacturing equipment of the hot rolled sheet steel for implementing this invention. FIG. 1 shows the HDR process, and FIG. 2 shows the DHCR process and the CCR process.

  FIG. 1 shows that a slab 2 having a thickness of 200 to 300 mm cast in a continuous casting line 1 is directly fed to a hot rolling line without being heated in a heating furnace 3, and is reduced to a predetermined thickness to be a coil. Winding up (HDR). FIG. 2 shows that the slab 2 whose temperature has been lowered to 1000 ° C. or less is charged into the heating furnace 3 and reheated to about 1100 to 1250 ° C. in a short time, and then reduced to a predetermined thickness in a hot rolling line. And then winding it on a coil (DHCR, CCR).

  In either case, the slab 3 is subjected to a predetermined width adjustment by the width reduction device 4 and then subjected to a predetermined thickness reduction process by the rough rolling equipment 5. The thickness of the sheet bar 6 after this rough rolling step is set according to the thickness of the steel plate 12 after the finish rolling equipment 10, but in the production of a hot-rolled steel plate having excellent surface properties in the present invention. Usually, the thickness is often about 30 mm to 40 mm.

In the present invention, a sheet bar surface heating device 7 for heating the surface layer of the sheet bar 6 and a descaling device 9 for removing the scale of the surface layer are installed in this order between the rough rolling facility 5 and the finish rolling facility 10. It is characterized by that. In the example of FIGS. 1 and 2, a sheet bar overall heating device 8 for heating the entire sheet bar is installed between the sheet bar surface heating device 7 and the descaling device 9.
In the manufacturing equipment shown in FIG. 1 and FIG. 2, after the scale of the surface layer is removed by the descaling device 9, the sheet bar 6 is reduced in thickness in the finishing rolling equipment 10 to become a steel plate 12 and cooled by the runout cooling device 11. The coiler 13 takes up the coil.

  The whole sheet bar heating device 8 is installed for the purpose of ensuring the pre-finishing temperature necessary for securing a desired material, whereas the sheet bar surface heating device 7 heats only the sheet bar surface layer. It is possible to heat up to the inside. The whole sheet bar heating device 8 may be an induction heating device having a heating frequency of about 1 to 10 kHz. The heating temperature of the whole sheet bar heating device 8 is about several tens of degrees Celsius. The descaling device 9 is preferably of a high pressure water type for the purpose of removing the oxide scale layer.

  In the present invention, the oxidized scale layer is positively generated on the sheet bar surface layer using the sheet bar surface layer heating device 7, and the scale layer is removed by the descaling device 9, resulting in steelmaking existing in the slab surface layer. Inclusions can be removed. Thereby, generation | occurrence | production of the lashes by the inclusion resulting from steelmaking is reduced, and manufacture of the hot-rolled steel plate excellent in surface property is possible. In particular, the present invention is also characterized in that it is possible to remove steelmaking inclusions even in an HDR process in which slab surface maintenance is difficult.

  In addition, by sufficiently generating an oxidized scale layer on the surface of the sheet bar using the sheet bar surface heating device 7, it is possible to significantly reduce the scale biting and the like in the rough rolling process that has been conventionally generated.

  The effects of the present invention will be described below based on examples.

A hot-rolled steel sheet having a thickness of 2.0 mm was manufactured from a slab having a thickness of 250 mm using the hot-rolled steel sheet manufacturing facility shown in FIG. At this time, the slab was directly fed to the hot rolling line and rolled (HDR). Further, in the hot-rolled steel sheet manufacturing equipment shown in FIG. 2, the slab was charged into a heating furnace at an average slab temperature of 700 to 800 ° C., heated to 1220 ° C. by heating for 1 hour, and then rolled. (DHCR). In the hot-rolled steel sheet manufacturing equipment shown in FIG. 2, the slab is cooled to room temperature in a slab yard, and then the slab is charged into a heating furnace and heated up to 1220 ° C. by heating for 3 hours. (CCR).
At this time, in HDR and DHCR, the surface maintenance of the slab was not performed, and in the CCR, the surface maintenance of the slab was performed, and the comparison was made under both conditions. When the surface of the slab was cleaned, the amount removed by welding was 2 mm.

And in all of HDR, DHCR, and CCR, the presence or absence of each heating of a sheet bar surface layer and execution of descaling was performed. In the following description, the method according to the present invention (with sheet bar surface heating and descaling performed) is referred to as “sheet bar surface care”.
The surface heating of the sheet bar in the sheet bar surface heating device 7 was performed under the conditions of a heating frequency of 50 kHz, a slab surface temperature of 1330 ° C., and a heating time of 3 seconds. The effect of the present invention was made by comparing the number of linear defects detected by a CCD camera type surface defect meter (individually detected on both sides of the steel sheet) installed just before the coiler 13. At that time, the number of defects in the conventional example in the HDR process (no surface treatment for both the slab and the sheet bar) was used as a reference (1.0), and the ratio of the number of defects to the reference condition was arranged for each condition.

  Table 1 shows a comparison result of the ratio of the number of defects under each condition. In any of the processes of HDR, DHCR, and CCR, in the present invention example in which the sheet bar was subjected to surface care, the number of linear defects was reduced to 0.18 or less with respect to the reference condition, It was confirmed that the linear defects generated by the scale biting in the rough rolling process can be greatly reduced.

DESCRIPTION OF SYMBOLS 1 Continuous casting line 2 Slab 3 Heating furnace 4 Width reduction device 5 Rough rolling equipment 6 Sheet bar 7 Sheet bar surface layer heating device 8 Sheet bar whole heating device 9 Descaling device 10 Finishing rolling equipment 11 Runout cooling device 12 Steel plate 13 Coiler 14 Steel making Caused inclusions

Claims (4)

The slab cast in the continuous casting line is reduced in thickness in the hot rolling line without being heated in the heating furnace and wound on a coil, or the slab is placed in a heating furnace provided in the hot rolling line. Is a hot rolled steel plate manufacturing facility that is heated to a predetermined temperature and then reduced to a predetermined thickness in a hot rolling line and wound on a coil.
The hot rolling line has a rough rolling facility and a finish rolling facility, and a heating device capable of heating the surface temperature of the sheet bar to 1300 ° C. or more between the rough rolling facility and the finish rolling facility, A hot-rolled steel plate manufacturing facility comprising a descaling facility for removing oxide scale in this order.   The hot-rolled steel sheet manufacturing equipment according to claim 1, wherein the heating device is an induction heating type heating device having a heating frequency of 50 kHz or more. The slab cast in the continuous casting line is reduced in thickness in the hot rolling line without being heated in the heating furnace and wound on a coil, or the slab is placed in a heating furnace provided in the hot rolling line. Is a method for producing a hot-rolled steel sheet that is heated to a predetermined temperature and then reduced to a predetermined thickness in a hot rolling line and wound on a coil.
After reducing the thickness of the slab to a predetermined sheet bar thickness by rough rolling, the sheet bar surface temperature is heated to 1300 ° C. or higher and 1370 ° C. or lower for 2 seconds or longer, and then subjected to high-pressure water before finish rolling. A method of manufacturing a hot-rolled steel sheet that is descaled by the above method.   It is a manufacturing method of the hot rolled sheet steel of Claim 3, Comprising: The heating of a sheet-bar surface is a manufacturing method of the hot rolled sheet steel made into the induction heating system whose heating frequency is 50 kHz or more.

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