JP2021146372A - Method for manufacturing hot-rolled steel sheet and apparatus for manufacturing hot-rolled steel sheet - Google Patents

Abstract

To improve an inner quality of a hot-rolled steel sheet produced by a continuous casting machine.SOLUTION: There is provided a method for manufacturing a hot-rolled steel sheet using an apparatus for manufacturing a hot-rolled steel sheet in which a continuous casting line and a hot rolling line are directly connected to each other. The continuous casting line and the hot rolling line perform rolling at different peripheral velocities so as to give a peripheral velocity difference to upper and lower rolling rolls, in at least one rolling stand which forms a conveyance path of the hot-rolled steel sheet in this order and is arranged in the hot rolling line with a solid phase rate of a plate thickness center of a cast slab manufactured in the continuous casting line of 0.8-1.0.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for manufacturing a hot-rolled steel sheet and a hot-rolled steel sheet manufacturing apparatus.

As a continuous casting facility for thin steel sheets, for example, a line in which a continuous casting machine for producing slabs having a thickness of 50 mm to 150 mm and a rolling line are combined (hereinafter referred to as "TSCR: Thin Slab Casting and Rolling") is known. .. Since the TSCR is provided by directly connecting the continuous casting machine and the rolling line, it is more compact than the conventional process, and the hot-rolled steel sheet can be manufactured at low cost.

Patent Document 1 describes a method of continuously producing a steel sheet by such TSCR, in which a slab is formed at a temperature higher than 1100 ° C. after solidification of the continuously cast material, and the temperature is up to 1100 ° C. over the entire cross section of the slab. It is disclosed that the pressure is reduced after induction heating again. According to Patent Document 1, steel sheets can be continuously cast and finish-rolled at low equipment cost and low energy cost.

By the way, from the viewpoint of weight reduction and high strength of steel sheet as a product in recent years, alloying elements such as carbon (C), silicon (Si) and manganese (Mn) are added to the produced slab. However, with the increase in alloying of the slab component, deterioration of the internal quality of the slab has become an issue. It is known that the internal deterioration of the slab is caused by internal defects of the slab such as central segregation and porosity.

Patent Document 2 describes a continuous casting method for slabs that reduces central segregation. Immediately after the center of the slab in the thickness direction solidifies, a reduction is applied to refine the solidified structure and disperse the segregation. It is disclosed to reduce. Further, according to Patent Document 2, the amount of strain applied to the inside of the slab is increased by increasing the reduction ratio, which is the value obtained by dividing the thickness of the slab immediately before reduction by the thickness of the slab immediately after reduction. ..

Patent Document 3 discloses a continuous casting method for reducing center porosity, in which a temperature difference is provided between the central portion and the surface of a slab when the slab after solidification is completed during continuous casting. ing. According to Patent Document 3, by making a temperature difference in the slab, the amount of strain inside the slab can be increased and the center porosity can be reduced even with the same reduction ratio.

Japanese Patent No. 2726919 JP 2015-006680 Japanese Unexamined Patent Publication No. 2016-175104

As described in Patent Documents 1 and 2, by increasing the amount of strain applied to the inside of the slab, the occurrence of internal defects in the slab can be suppressed, that is, the internal quality can be improved.

However, when the hot-rolled steel sheet is manufactured by TSCR as described above, it is difficult to increase the reduction ratio as in the continuous casting method described in Patent Document 2 because the manufactured hot-rolled steel sheet is thin. Further, when a temperature difference is provided as in the continuous casting method described in Patent Document 3, the temperature at the center of the plate thickness is raised and the surface temperature is lowered in order to efficiently apply strain to the center portion of the plate thickness having many defects. It is necessary, but in such a case, surface cracks may occur due to a decrease in surface temperature.

In general, segregation and diffusion can be performed by soaking treatment (long-term heat treatment) of slabs, but in TSCR, continuously cast slabs are immediately hot-rolled, so soaking treatment is performed. I can't do that.

Therefore, the present inventors have focused on different peripheral speed rolling that can increase the amount of strain inside the slab that is continuously cast. Specifically, it was presumed that shear strain was applied to the inside of the slab by rotating the upper and lower work rolls at different peripheral speeds, thereby obtaining the effect of improving the internal quality of the hot-rolled steel sheet.

Patent Document 2 and Patent Document 3 do not describe center segregation as an internal defect and different peripheral speed rolling as a method for improving porosity as described above, and do not suggest this. Further, Patent Document 1 discloses a method for continuously casting a hot-rolled steel sheet, but there is also no description about performing different peripheral speed rolling in manufacturing the hot-rolled steel sheet. That is, there was room for improvement in the conventional method for improving the internal quality of hot-rolled steel sheets.

The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the internal quality of a hot-rolled steel sheet produced by continuous casting.

In order to achieve the above object, the present invention is a method for manufacturing a hot-rolled steel sheet using a hot-rolled steel sheet manufacturing apparatus in which a continuous casting line and a hot-rolling line are directly connected, wherein the continuous casting line and the hot-rolling are performed. The lines form a transport path for the hot-rolled steel sheet in this order, and in the hot-rolled line, the solid-state ratio at the center of the plate thickness of the slab produced by the continuous casting line is 0.8 to 1.0. At least one rolling stand arranged in the above range is characterized in that different peripheral speed rolling is performed so as to give a difference in peripheral speed to the upper and lower rolling rolls.

According to the present invention, the solid phase ratio of the slab is in the range of 0.8 to 1.0, that is, the hot-rolled steel sheet from the end of solidification to immediately after solidification is rolled at different peripheral speeds to form the inside of the slab. Strain can be applied appropriately.

It is desirable that the different peripheral speed rolling is performed at a rolling stand provided on the most upstream side of the hot rolling line in the transport path.

It is desirable that the different peripheral speed ratio χ obtained by the following formula (1) is larger than 0% and 8% or less.
χ = (| Va-Vb | / Vf) × 100 ... (1)
Va: Peripheral speed of the upper rolling roll Vb: Peripheral speed of the lower rolling roll Vf: Peripheral speed of the rolling roll on the higher peripheral speed side of Va and Vb

On the downstream side of the rolling stand that performs the different peripheral speed rolling in the transfer path, the warp prevention roll may prevent the warp of the slab caused by the different peripheral speed rolling.

The present invention according to another viewpoint is a hot-rolled steel sheet manufacturing apparatus in which a continuous casting line and a hot-rolled line are directly connected, and the continuous casting line and the hot-rolled line are in this order as a transport path for the hot-rolled steel sheet. At least one rolling stand in the hot rolling line, which is arranged in a range where the solid phase ratio at the center of the plate thickness of the slab produced in the continuous casting line is 0.8 to 1.0. It is characterized by being a different peripheral speed rolling stand that gives a difference in peripheral speed to the upper and lower rolling rolls.

It is desirable that the different peripheral speed rolling stand is provided on the most upstream side of the hot rolling line in the transport path.

On the downstream side of the different peripheral speed rolling stand in the transfer path, a warp prevention roll for preventing the warpage of the slab caused by the different peripheral speed rolling stand may be further provided.

According to the present invention, the internal quality of the hot-rolled steel sheet produced can be improved by performing different peripheral speed rolling during continuous casting of the hot-rolled steel sheet.

It is explanatory drawing which shows typically the structure of the manufacturing apparatus of a hot-rolled steel sheet.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the present specification and the drawings, elements having substantially the same functional configuration are designated by the same reference numerals, so that duplicate description will be omitted.

<Structure of hot-rolled steel sheet manufacturing equipment>
First, the configuration of the hot-rolled steel sheet manufacturing apparatus 1 according to the embodiment of the present invention will be described. FIG. 1 is an explanatory diagram schematically showing an outline of the configuration of a hot-rolled steel sheet manufacturing apparatus 1. In the following description, "hot-rolled steel sheet manufacturing equipment" corresponds to the above-mentioned TSCR.

As shown in FIG. 1, in the hot-rolled steel sheet manufacturing apparatus 1 according to the present embodiment, the continuous casting line 10 and the hot rolling line 20 are directly connected, and continuous casting to hot rolling are carried out continuously. The hot-rolled steel sheet is continuously manufactured by the continuous casting and rolling line. More specifically, a "hot-rolled steel sheet" is manufactured by rough-rolling the "slab" produced by the continuous casting line 10 and then finish-rolling it as a "rough bar". The manufactured hot-rolled steel sheet is cut to a predetermined cutting length and recovered as a product.

The continuous casting line 10 includes a tundish 11, a mold 12, and a continuous casting machine 13.

The tundish 11 is integrally formed with a dipping nozzle (not shown) provided at the bottom so as to be movable in the vertical direction, and the dipping nozzle can be pulled out to the mold 12.

The mold 12 has a rectangular shape for forming a slab into a predetermined cross-sectional shape. The mold 12 is made of, for example, a water-cooled copper plate.

In the continuous casting machine 13, the molten steel poured from the tundish 11 via the mold 12 is drawn out by the roll group of the continuous casting machine 13, and a slab having a plate thickness of, for example, 50 to 150 mm is produced.

The hot rolling line 20 includes a rough rolling mill row 21, a heating device 22, a finishing rolling mill row 23, a cooling device 24, a cutting machine 25, and a coiler (for example, a down coiler) 26. ..

The rough rolling machine row 21 is arranged on the downstream side in the plate-passing direction of the continuous casting machine 13, and hot-rolls (coarse-rolls) the slab after casting by the continuous casting machine 13. The rough rolling machine row 21 is provided with a plurality of, for example, three-stage rough rolling stands 21A, 21B, 21C. The rough rolling stands 21A, 21B, and 21C each have a pair of work rolls WR and a pair of backup rolls BUR. The number of rolling stands in the rough rolling mill row 21 is not particularly limited and can be arbitrarily designed.

The heating device 22 is arranged between the rough rolling mill row 21 and the finishing rolling mill row 23, and heats the slab before rolling in the finishing rolling mill row 23. This is because in the hot-rolled steel sheet manufacturing apparatus 1, the continuous casting line is directly connected to the hot rolling line, so that the continuous casting speed is the rate-determining rate for the plate passing speed. Therefore, since it is slower than the plate passing speed of the batch method hot rolling, the slab is heated by the heating device 22 so that the slab is not overcooled in the plate before rolling in the finish rolling mill row 23. ..

The configuration of the heating device 22 can be arbitrarily selected, and for example, an arbitrary configuration such as a heating zone, a holding furnace, or an induction heating device can be adopted.

The finish rolling mill row 23 is arranged on the downstream side of the rough rolling mill row 21 in the plate-passing direction, is cast by the continuous casting machine 13, and the rough bar after rough rolling is hot-rolled (finish-rolled). The finish rolling mill row 23 is provided with a plurality of, for example, five-stage finish rolling stands 23A, 23B, 23C, 23D, 23E. The finish rolling stands 23A, 23B, 23C, 23D, 23E each have a pair of work rolls WR and a pair of backup rolls BUR. The number of rolling stands in the finishing rolling mill row 23 is not particularly limited and can be arbitrarily designed.

The cooling device 24 is arranged on the downstream side in the plate-passing direction of the finish rolling mill row 23, and is a hot-rolled steel sheet that is conveyed to the runout table after hot rolling (finish rolling in this embodiment) by the final stage finish rolling stand 23E. , Cool with cooling water (water cooling). The hot-rolled steel sheet cooled by the cooling device 24 is cut to an arbitrary cutting length by the cutting machine 25, and then wound into a coil by the coiler 26.

A descaler (not shown) for removing the generated scale is provided at an arbitrary position in the hot-rolled steel sheet manufacturing apparatus 1, for example, on the upstream side of the rough rolling machine row 21 and the finishing rolling mill row 23. May be good.

The hot-rolled steel sheet manufacturing apparatus 1 according to the present embodiment is configured as described above, and the continuous casting machine 13, the rough rolling machine row 21, the heating device 22, the finishing rolling mill row 23, and the cooling device 24 are cast in this order. It forms a transport path for pieces, rough bars and hot-rolled steel sheets. Then, a cutting machine 25 and a coiler 26 are arranged at the end of the transport path, and the hot-rolled steel sheet cut to a predetermined cutting length is wound up and collected.

The slabs drawn from the tundish 11 to the transport path via the mold 12 are transported along the transport path, and are gradually cooled from the outer surface to the inside in the continuous casting machine 13 and the rough rolling mill row 21. , Coagulation progresses. That is, in the continuous casting machine 13 and the rough rolling machine row 21, the internal temperature of the slab is higher than the outer surface temperature. Then, as the slab moves downstream along the transport path, solidification inside the slab progresses, and as shown in FIG. 1, at the complete solidification position C (for example, in the rough rolling mill row 21 in the present embodiment). The entire surface of the slab in cross-sectional view is solidified, and the solid phase ratio is 1.0.

Here, in the hot-rolled steel sheet manufacturing apparatus 1 according to the present embodiment, as will be described later, the solid phase ratio of the slab is in the range of 0.8 to 1.0, for example, in the present embodiment, the rough rolling mill row. At 21, different peripheral speed rolling is performed in which the peripheral speeds of the upper and lower work rolls WR are changed. Different peripheral speed rolling is performed on at least one of the three rough rolling stands 21A, 21B, and 21C arranged side by side in the manufacturing direction. In the following description, of the rough rolling stands 21A, 21B, and 21C, the rough rolling stand that performs different peripheral speed rolling is the "different peripheral speed rolling stand", and the upper and lower work rolls of the different peripheral speed rolling stand are "upper". It may be called "work roll" or "lower work roll".

As shown in FIG. 1, a warp prevention roll 27 for absorbing the warp generated in the slab by the different peripheral speed rolling may be further provided on the downstream side of the different peripheral speed rolling stand.

<Manufacturing method of hot-rolled steel sheet>
Next, the flow of continuous casting performed by using the hot-rolled steel sheet manufacturing apparatus 1 will be described along the transfer path.

In continuous casting using the hot-rolled steel sheet manufacturing apparatus 1, first, molten steel whose composition has been investigated for the desired chemical composition is injected into the tundish 11 from a ladle (not shown). The tundish 11 also functions as a buffer portion for molten steel injected from the ladle.

The molten steel stored in the tundish 11 is then injected into the mold 12 via a dipping nozzle (not shown). The outer shell of the molten steel injected into the mold 12 is cooled by contact with the mold 12 and watering of cooling water, and a solidified shell is formed in accordance with the rectangular inner shape of the mold 12.

The molten steel in which the solidified shell is formed in the mold 12 is pulled out to the transport path as a slab by holding the outer shell portion, that is, the portion in which the solidified shell is formed on the support roll of the continuous casting machine 13 directly under the mold 12.

The slabs drawn out to the transport path are gradually transported to the downstream direction of the transport path by the support rolls arranged side by side in the manufacturing direction in the continuous casting machine 13. At this time, the transported slab is cooled from the outer surface side (outer shell portion), solidification gradually progresses inward, and the solid phase ratio increases.

The slabs transported along the transport path are subsequently transported to the rough rolling mill row 21 for rough rolling. In the rough rolling mill row 21 of the hot-rolled steel sheet manufacturing apparatus 1 according to the present embodiment, it is located in the range from the end of solidification of the slab to immediately after solidification, for example, the solid phase ratio of the slab is 0.8 to 1.0. Different peripheral speed rolling is performed on at least one rough rolling roll (different peripheral speed rolling stand). The solid phase ratio can be obtained by any method, but may be calculated by, for example, a solidification structure prediction simulation.

Specifically, the peripheral speed Va of the upper work roll and the lower work roll so that the different peripheral speed ratio χ in the different peripheral speed rolling stand obtained by the following equation (1) is larger than 0% and 9% or less. Determine the peripheral speed Vb. Note that Vf in the following equation (1) indicates the peripheral speed of the work roll on the high speed side of the upper work roll and the lower work roll. That is, the different peripheral speed ratio χ is the ratio (absolute value) of the difference (absolute value) between the peripheral speed Va and the peripheral speed Vb with respect to the roll peripheral speed Vf on the high speed side.

χ = (| Va-Vb | / Vf) × 100 ... (1)

Here, when different peripheral speed rolling is performed with a solid phase ratio of less than 0.8, that is, when different peripheral speed rolling is performed in a state where the inside of the slab is not sufficiently solidified, the different peripheral speed rolling stand is used. Rolling causes molten steel flow inside the slab, which worsens central segregation.

Further, when the different peripheral speed rolling is performed at a different peripheral speed ratio of more than 9%, that is, when the peripheral speed difference between the upper and lower work rolls WR is large, the warp generated in the slab becomes large and deviates from the transport path, that is, The slabs are not properly bitten into the rolling roll and the warp prevention roll 27 on the downstream side of the transport path, and the hot-rolled steel sheet cannot be manufactured.

After that, the slab is further conveyed and rolled to the downstream side in the conveying direction by the rough rolling mill row 21, and the entire surface of the slab (thin slab) is solidified in the cross-sectional view at the complete solidification position C, and a coarse bar is formed. Will be done.

The formed coarse bar is further conveyed, and then is conveyed to the heating device 22. The rough bar cooled by rolling in the rough rolling mill row 21 is heated to a temperature suitable for finish rolling in the heating device 22, for example, 1000 to 1200 ° C.

The coarse bar heated in the heating device 22 is subsequently conveyed to the finish rolling mill row 23. In the finish rolling mill row 23, the rough bar is finished rolled, specifically, the sheet is reduced to a thickness of 0.7 to 3.0 mm, which is the manufacturing thickness, to form a hot-rolled steel sheet.

The finish-rolled hot-rolled steel sheet is then conveyed to the cooling device 24. In the cooling device 24, the hot-rolled steel sheet is, for example, water-cooled, whereby the desired structure is controlled.

The structure-controlled hot-rolled steel sheet is cut to a predetermined cutting length by the cutting machine 25. After that, the hot-rolled steel sheet is wound into a coil by the coiler 26 to obtain a product, and a series of hot-rolled steel sheet manufacturing processes is completed.

According to the method for producing a hot-rolled steel sheet according to the present embodiment, shear strain can be appropriately applied to the inside of the slab by continuously casting the slab at different peripheral speeds. As described above, since the amount of strain applied to the inside of the slab increases, it is possible to appropriately suppress the occurrence of central segregation and porosity, that is, to improve the internal quality of the hot-rolled steel sheet to be manufactured.

Further, according to the present embodiment, the different peripheral speed rolling for improving the internal quality is performed in the range of the solid phase ratio of the slab in the range of 0.8 to 1.0, that is, for the slab from the end of solidification to the slab immediately after solidification. Is given. In other words, since the different peripheral speed rolling is performed in a state where the internal temperature of the slab is higher than the surface temperature, shear strain can be more appropriately applied to the inside of the slab.

Further, according to the present embodiment, different peripheral speed rolling is performed at a different peripheral speed ratio of more than 0% to 9% or less. As a result, the internal quality of the hot-rolled steel sheet to be manufactured can be improved, and the production of the hot-rolled steel sheet can be suppressed from being interrupted due to the warp generated in the slab due to the different peripheral speed rolling.

Although the embodiments of the present invention have been described above, the present invention is not limited to such examples. It is clear that a person skilled in the art can come up with various modifications or modifications within the scope of the technical ideas described in the claims, and these are naturally within the technical scope of the present invention. It is understood that it belongs.

Examples for confirming the effect of different peripheral speed rolling according to the above-described embodiment will be described, but the present invention is not limited to the following examples.

In this embodiment, slabs having a pre-rolling thickness of 100 mm and a width of 1100 m are cast at a casting speed of 4.8 m / min using a hot-rolled steel sheet manufacturing apparatus, and three rough-rolling stands in a row of rough-rolling machines are used. Of these, different peripheral speed rolling was performed at the first stand (upstream side). The rough-rolled slab was rolled hot at another rolling stand provided on the downstream side thereof, and rolled into a hot-rolled steel sheet having a plate thickness of 2.5 mmt.

The slab (hot-rolled steel sheet) contains C: 0.16 wt%, Si: 0.50 wt%, Mn: 2.10 wt%, P: 0.009 wt%, S: 0.002 wt%, Al as steel grade components. : Contains 0.03 wt%.

Further, as a method for confirming the effect of improving the internal quality by different peripheral speed rolling, EPMA (Electron Probe Micro Analyzer) was measured on the cross section of the hot-rolled steel sheet obtained by winding it on a coiler, and the segregation index was obtained. For the segregation index, a line analysis was performed on the Mn element using EPMA with a beam diameter of 50 μm, and the Mn concentration distribution of the sample was measured in the thickness direction to determine the maximum Mn concentration in the measurement range. Then, the value obtained by dividing the value of the maximum concentration of Mn by the initial content of Mn obtained from the chemical analysis at the molten steel stage was used as the segregation index. That is, it can be said that the effect of improving the internal quality of the hot-rolled steel sheet was obtained by reducing the segregation index.

Further, in the obtained hot-rolled steel sheet, a hole expansion test was further carried out. In the hole expansion test, a test sample is cut out from the obtained hot-rolled steel sheet, and the test is performed in accordance with JIS Z 2256 (hole expansion test method for metal materials) to determine the hole expansion limit value (λ [%]). Calculated. As an evaluation method, when the hole expansion ratio λ was 70% or more, it was positive, and when it was less than that, it was negative.

(Implementation condition 1)
In Implementation Condition 1, the segregation index was compared between the example of the present invention in which different peripheral speed rolling was performed in a state where the internal temperature of the slab at the end of solidification was high and the comparative example in which different peripheral speed rolling was not performed. Specifically, in the examples, rolling was performed with a different peripheral speed ratio of 5%, and in the comparative example, the different peripheral speed ratio was 0%. In Implementation Condition 1, the reduction ratios of the slabs were changed to 1.43, 1.66, and 2.00, respectively.

(Implementation result 1)
Table 1 is a table showing the experimental results under the implementation condition 1.

As shown in Table 1, for example, when rolling was performed at a rolling reduction ratio of 1.43, the segregation index was 1.31 and λ was 60% in the comparative example, whereas the segregation index was 1. 20 and λ became 70%. That is, even if the reduction ratio was the same, the internal quality of the hot-rolled steel sheet was improved by performing different peripheral speed rolling.

The effect of improving the internal quality of the hot-rolled steel sheet is not limited to the case where the reduction ratio is 1.43, and is enjoyed even when the reduction ratios are 1.66 and 2.00 as shown in Table 1. I was able to. It was also found that as the reduction ratio increases, the effect of improving the internal quality of the hot-rolled steel sheet increases. Specifically, the segregation index was 1.20 and λ was 70% when rolling at different peripheral speeds was performed at a rolling ratio of 1.43, whereas for example, rolling at different peripheral speeds was performed at a rolling ratio of 2.00. The segregation index was 1.05 and λ was 76%.

That is, according to the experimental results of Implementation Condition 1, it was found that the effect of improving the internal quality of the hot-rolled steel sheet can be obtained by performing different peripheral speed rolling at the end of solidification of the slab in the hot-rolled steel sheet manufacturing apparatus.

(Implementation condition 2)
In this Implementation Condition 2, only the different peripheral speed ratio is taken as a comparative example of Test No. 1-1 in Table 1, that is, the case where rolling is performed with a rolling reduction ratio of 1.43 and a different peripheral speed ratio of 0% with respect to the slab. The segregation index and the hole expansion ratio λ were obtained by changing. Specifically, different peripheral speed rolling was performed with different peripheral speed ratios of 1%, 2%, 5%, 9%, and 10%, respectively (Examples 1 to 5).

(Implementation result 2)
Table 2 is a table showing the experimental results under the implementation condition 2.

As shown in Table 2, when rolling was performed at a different peripheral speed ratio of 0%, the segregation index was 1.31 and the hole expansion ratio was 60%, whereas as the different peripheral speed ratio increased, the different peripheral speed ratio increased. That is, as the difference in peripheral speed between the upper roll and the lower roll of the rolling stand increased, the segregation index decreased and the hole expansion rate increased. Specifically, the segregation index at a different peripheral speed rate of 1% was 1.29 and λ was 62%, whereas the segregation index at a different peripheral speed rate of 9% was 1.20 and λ was 70%. It is presumed that this is because the shear strain applied to the inside of the slab increased by increasing the different peripheral speed ratio, and thus the effect of improving the internal quality of the hot-rolled steel sheet was obtained more appropriately.

On the other hand, when the slab was rolled at a different peripheral speed with a different peripheral speed ratio of 10%, the slab could not be appropriately rolled due to the warp generated in the slab due to the different peripheral speed rolling. Specifically, the slab could not be properly conveyed in the rough rolling mill row.

That is, according to the experimental results of Implementation Condition 2, the effect of improving the internal quality can be further enhanced by increasing the different peripheral speed ratio when rolling the slab, but on the other hand, if the different peripheral speed ratio becomes too large, rolling is performed appropriately. It turned out that I couldn't do it. From this, the different peripheral speed ratio when rolling the slab is at least greater than 0% and preferably 9% or less.

(Implementation condition 3)
In this Implementation Condition 3, only the solid phase ratio of the slab is obtained in the example of Test No. 2 in Table 1, that is, when rolling is performed with a rolling reduction ratio of 1.66 and a different peripheral speed ratio of 5% with respect to the slab. Was changed to obtain the segregation index. Specifically, as shown in Table 3 below, the solid phase ratio of the slab was changed within the range of 0.65 to 1.00, and different peripheral speed rolling was performed for each. As a reference example, the segregation index when different peripheral speed rolling was not performed when the rolling ratio was 1.66 and the solid phase ratio was 0.9 was 1,24 as shown in Table 1.

(Implementation result 3)
Table 3 is a table showing the experimental results under the execution condition 3.

As shown in Table 3, it was found that the segregation index became small in the range of the solid phase ratio of the slab in the range of 0.8 to 1.0, and the effect of improving the internal quality of the slab could be enjoyed (Examples 1 to 5). ). Specifically, the segregation index was 1.13 at a solid phase ratio of 0.8 and 1.11 at a solid phase ratio of 0.85 or higher. It is presumed that this is because the solidified structure inside the slab is appropriately miniaturized by applying shear strain to the inside of the slab immediately after solidification from the end of solidification, and diffusion is promoted by the dispersion of segregation. NS.

On the other hand, in the range where the solid phase ratio of the slab was less than 0.8, the segregation index did not improve, and the segregation index deteriorated with respect to the reference example (Comparative Examples 1 to 3). It is presumed that this is because the shear strain is applied to the inside of the slab in a state where the inside of the slab is not sufficiently solidified, so that molten steel flows inside the slab, and as a result, the central segregation deteriorates.

That is, according to the experimental results of Implementation Condition 3, the solid phase ratio inside the slab is different from the final stage of solidification of the slab in the hot-rolled steel sheet manufacturing apparatus to immediately after solidification, specifically in the range of 0.8 to 1.0. It was found that the effect of improving the internal quality of the hot-rolled steel sheet can be obtained by performing peripheral speed rolling.

(Implementation condition 4)
In the above implementation conditions 1 to 3, out of the three rough rolling stands in the rough rolling mill row, only the first stand (upstream side) was used as the different peripheral speed rolling stand for different peripheral speed rolling. In No. 4, the positions and numbers of different peripheral speed rolling stands were changed. Specifically, as shown in Table 4, the number of different peripheral speed rolling stands is changed from 1 to 3 in the three rough rolling stands in the rough rolling mill row, and the positions of the different peripheral speed rolling stands are changed respectively. changed.

In the present implementation condition 4, a slab having a thickness of 120 mm and a width of 1100 m before rolling is cast at a casting speed of 4.8 m / min, and the rolling ratio of the three rough rolling stands is 1.5 (plate) in order from the upstream side. The thickness was 120 mm to 80 mm (rolled down), 1.6 (rolled down from 80 mm to 50 mm thick), and 1.67 (rolled down from 50 mm to 30 mm thick). Further, the different peripheral speed ratio in the different peripheral speed rolling stand was set to 5%.

(Implementation result 4)
Table 4 is a table showing the experimental results under the implementation condition 4.

As shown in Table 4, by performing different peripheral speed rolling at the first stand, it is preferable regardless of whether or not different peripheral speed rolling is performed at the second stand (middle stage) and the third stand (downstream side). It was found that the effect of improving the internal quality can be enjoyed. On the other hand, it was found that when the different peripheral speed rolling was not performed at the first stand, the effect of improving the internal quality obtained was small even if the different peripheral speed rolling was performed at the second stand and the third stand. Specifically, the segregation index was 1.13 when the different peripheral speed rolling was performed at the first stand, whereas the segregation index was 1.13 when the different peripheral speed rolling was not performed at the first stand. However, all of them were 1.20 or more.

It is presumed that this is because the temperature of the slab during different peripheral speed rolling decreases as the different peripheral speed rolling stand is arranged on the downstream side, that is, toward the downstream side of the transport path. ..

For this reason, when different peripheral speed rolling is performed in the rough rolling mill row in order to enjoy the effect of improving the internal quality in the production of hot-rolled steel sheets, the different peripheral speed rolling stand is used for at least one of the rough rolling mill rows. It is desirable that the different peripheral speed rolling stand is arranged on the upstream side of the transport path. In other words, it is desirable that the different peripheral speed rolling is performed from the final stage of solidification of the slab to immediately after solidification.

The present invention is useful in improving the internal quality of hot-rolled steel sheets produced by continuous casting.

1 Hot-rolled steel sheet manufacturing equipment 10 Continuous casting line 11 Tundish 12 Mold 13 Continuous casting machine 20 Hot rolling line 21 Rough rolling machine row 21A, 21B, 21C Rough rolling stand 22 Heating equipment 23 Finishing rolling mill row 23A, 23B, 23C , 23D, 23E Finishing rolling stand 24 Cooling device 25 Cutting machine 26 Koyler 27 Warp prevention roll BUR Backup roll C Complete solidification position WR Work roll

Claims (8)

It is a method of manufacturing a hot-rolled steel sheet using a hot-rolled steel sheet manufacturing device in which a continuous casting line and a hot-rolled line are directly connected.
The continuous casting line and the hot rolling line form a transport path for the hot-rolled steel sheet in this order.
In the hot rolling line, upper and lower rolling is performed in at least one rolling stand arranged in a range in which the solid phase ratio at the center of the plate thickness of the slabs manufactured in the continuous casting line is in the range of 0.8 to 1.0. A method for manufacturing a hot-rolled steel sheet, which comprises performing different peripheral speed rolling that gives a difference in peripheral speed to rolls. The method for manufacturing a hot-rolled steel sheet according to claim 1, wherein the different peripheral speed rolling is performed at a rolling stand provided on the most upstream side of the hot rolling line in the transport path. The method for producing a hot-rolled steel sheet according to claim 1 or 2, wherein the different peripheral speed ratio χ obtained by the following formula (1) is larger than 0% and 9% or less.
χ = (| Va-Vb | / Vf) × 100 ... (1)
Va: Peripheral speed of the upper rolling roll Vb: Peripheral speed of the lower rolling roll Vf: Peripheral speed of the rolling roll on the higher peripheral speed side of Va and Vb 3. The method for manufacturing a hot-rolled steel sheet according to item 1. It is a hot-rolled steel sheet manufacturing equipment in which a continuous casting line and a hot rolling line are directly connected.
The continuous casting line and the hot rolling line form a transfer path for the hot-rolled steel sheet in this order.
In the hot rolling line, at least one rolling stand arranged in a range where the solid phase ratio at the center of the plate thickness of the slab manufactured in the continuous casting line is in the range of 0.8 to 1.0 is rolled up and down. A hot-rolled steel sheet manufacturing device characterized by a different peripheral speed rolling stand that gives a difference in peripheral speed to the rolls. The hot-rolled steel sheet manufacturing apparatus according to claim 5, wherein the different peripheral speed rolling stand is provided on the most upstream side of the hot rolling line in the transport path. The hot-rolled steel sheet manufacturing apparatus according to claim 5 or 6, wherein the different peripheral speed ratio χ obtained by the following formula (1) in the different peripheral speed rolling stand is larger than 0% and 9% or less. ..
χ = (| Va-Vb | / Vf) × 100 ... (1)
Va: Peripheral speed of the upper rolling roll Vb: Peripheral speed of the lower rolling roll Vf: Peripheral speed of the rolling roll on the higher peripheral speed side of Va and Vb Claims 5 to 7, wherein a warp prevention roll for preventing the warpage of the slab caused by the different peripheral speed rolling stand is further provided on the downstream side of the different peripheral speed rolling stand in the transport path. The hot-rolled steel sheet manufacturing apparatus described in any one direction.

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