JP2016020007A - Manufacturing method of hot-rolled steel sheet and manufacturing device of hot-rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a hot-rolled steel sheet which can secure biting performance of a tip part of material to be rolled even when performing rolling under high pressure by using a small-size work roller of which a surface layer part is made of hard alloy on a later stage of thermal finish rolling.SOLUTION: A manufacturing method of a hot-rolled steel sheet which includes a process of performing finish rolling by means of a hot finish rolling mill train is characterized in that a work roller of at least one rolling mill in the later stage of rolling mills provided in the hot finish rolling mill train has a diameter of 600 mm or less, is formed with such hard alloy that surface elastic modulus is 300 GPa or more, surface roughness Ra of the work roller is 0.3 to 2.0 μm, a back-up roller arranged so as to bring the outer circumference into contact with the work roller has a diameter of 1350 mm or more, draft rate by means of the rolling mill provided with the work roller is 25% or more and the rolling is performed without supplying lubricant to the work roller.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a hot-rolled steel sheet and an apparatus for producing a hot-rolled steel sheet, and more particularly to a method for producing a hot-rolled steel sheet that performs hot rolling at a high reduction rate, and an apparatus for producing a hot-rolled steel sheet.

  Steel materials used for automobiles and structural materials are required to have excellent mechanical properties such as strength, workability, and toughness. To improve these mechanical properties comprehensively, the structure of the steel material is refined. It is effective. Therefore, many methods for obtaining a steel material having a fine structure have been sought. Further, according to the refinement of the structure, it is possible to produce a high-strength hot-rolled steel sheet having excellent mechanical properties even if the addition amount of the alloy element is reduced.

  As a method for refining the structure, particularly in the latter stage of the hot finish rolling, the austenite grains are refined by rolling at a high reduction rate (hereinafter sometimes referred to as “high pressure rolling”), and the steel sheet is made fine. It is known to accumulate rolling strain and to refine the ferrite grains obtained after rolling.

  On the other hand, since the rolling load becomes high in high-pressure rolling, the equipment cost increases because the equipment specifications of the rolling mill are becoming larger, such as requiring a large load resistance. In order to prevent this, measures such as reducing the diameter of the work roll or reducing the friction coefficient by lubrication are taken.

  In Non-Patent Document 1, the smaller the work roll diameter when rolling under high pressure, and the lower the friction coefficient by performing lubrication, the smaller the rolling load, for example, the work roll diameter of the final stand is 500 mm to 600 mm, It is disclosed that the above-described high-pressure rolling can be realized if the rolling friction coefficient μ is 0.2 or less. Further, in Non-Patent Document 1, even if the diameter of the work roll is reduced, seizure becomes a problem because the surface pressure of the work roll is high, or seizure occurs if a work roll whose surface layer is made of cemented carbide is used. It has also been shown that this can be reduced.

  Further, when the work roll having a reduced diameter is used, the contact opportunity with the material to be rolled increases, so that the wear of the work roll is likely to progress. Therefore, even if the diameter of the work roll is reduced, there is a problem that wear is excessive when a work roll similar to the conventional one is used. Patent Document 1 discloses that when a work roll made of a cemented carbide is used for the surface of the rolled part with respect to the work roll of a hot finish rolling mill, the amount of wear can be greatly reduced as compared with a conventional work roll. Yes.

JP 2001-321804 A

Fukushima et al., “Manufacturing technology of ultrafine grained steel by ultra-short multi-pass rolling”, Proceedings of 2007 Spring Meeting of Plastic Working, P.A. 49

  However, as described in Non-Patent Document 1 and Patent Document 1, in the technique using a small-diameter work roll whose surface layer portion is made of cemented carbide after the hot finish rolling, the work roll at the tip of the material to be rolled is used. Occasional troubles in operation become easier and troubles may occur.

  Therefore, in view of the above problems, the present invention is capable of biting the tip of the material to be rolled even when high-pressure rolling is performed using a small-diameter work roll whose surface layer portion is made of cemented carbide after the hot finish rolling. It is an object of the present invention to provide a method for manufacturing a hot-rolled steel sheet and a manufacturing apparatus for the hot-rolled steel sheet capable of ensuring the above.

  The present invention will be described below.

The invention according to claim 1 is a method of manufacturing a hot-rolled steel sheet including a step of performing finish rolling with a hot finish rolling mill row, and is a subsequent rolling mill among the rolling mills provided in the hot finish rolling mill row. The work roll of at least one rolling mill is formed of a cemented carbide having a diameter of 600 mm or less and a surface elastic modulus of 300 GPa or more. The backup roll that is 3 μm or more and 2.0 μm or less and that is arranged so that the outer periphery is in contact with the work roll has a diameter of 1350 mm or more, the rolling reduction by the rolling mill equipped with the work roll is 25% or more, and the work roll is lubricated It is the manufacturing method of the hot-rolled steel plate characterized by rolling without supplying an agent.
Here, the “rear stage” means a rolling mill disposed on the process side downstream of the half among a plurality of rolling mills provided in the hot finish rolling mill row. That is, for example, when seven rolling mills are arranged, the downstream three are downstream.

  According to a second aspect of the present invention, in the method for manufacturing a hot-rolled steel sheet according to the first aspect, the at least one rolling mill includes a final rolling mill in a finish rolling mill row.

  Invention of Claim 3 has a hot finish rolling mill row | line | column which comprises several rolling mills, and several rolling mills make the outer periphery contact a work roll which rolls a to-be-rolled material, and a work roll And a work roll of at least one rolling mill in the latter stage among a plurality of rolling mills, comprising a cemented carbide in the outer shell, A hot rolled steel sheet manufacturing apparatus having a surface elastic modulus of 300 GPa or more, a diameter of 600 mm or less, and a surface roughness Ra of 0.3 μm or more and 2.0 μm or less.

  According to the present invention, it is possible to perform rolling at a high rolling reduction by a rolling mill in the subsequent stage of the finish rolling mill row while securing biting performance and maintaining operational stability.

It is the figure which showed typically a part of manufacturing apparatus of the hot-rolled steel plate concerning one embodiment. FIG. 2 is an enlarged view paying attention to a final stand and its periphery in FIG. 1.

  The above-mentioned operation and gain of the present invention will be clarified from the following embodiments for carrying out the invention. However, the present invention is not limited to these embodiments.

FIG. 1 is a diagram for explaining one embodiment, and is a diagram schematically showing a part of a hot-rolled steel sheet manufacturing apparatus 10. In FIG. 1, the steel sheet 1 is conveyed from the left side (upstream side, upper process side) to the right side (downstream side, lower process side) of the paper surface, and the top and bottom of the paper surface is the vertical direction. The direction from the upstream side (upper process side) to the downstream side (lower process side) may be referred to as the passing plate direction, and the direction of the width of the steel plate to be passed in the direction perpendicular to this is the plate width direction. May be described. For ease of viewing in the drawings, repeated reference numerals may be omitted.
FIG. 2 is an enlarged view of the final stand 11g and its periphery in FIG.

  As shown in FIG. 1, a hot rolled steel sheet manufacturing apparatus 10 includes a hot finish rolling mill row 11, a cooling device 20, a transport roll 12, and a pinch roll 13. Although illustration and explanation are omitted, a heating furnace, a rough rolling mill row, and the like are arranged upstream from the hot finish rolling mill row 11 to adjust the conditions of the steel sheet for entering the hot finish rolling mill row 11. ing. On the other hand, another cooling device and a winder are provided on the downstream side of the pinch roll 13, and various facilities for shipping as a steel plate coil are arranged.

In the hot finish rolling mill row 11 in this embodiment, as shown in FIG. 1, seven stands 11a,... 11f, 11g are arranged in parallel along the sheet passing direction. Each of the stands 11a,... 11f, 11g is equipped with a rolling mill, and the reduction rate so as to satisfy the requirements of the steel sheet thickness, mechanical properties, surface quality, etc. required for the final product. Such rolling conditions are set.
Here, the rolling reduction of the rolling mill of each stand is set so as to satisfy the performance that the steel sheet to be manufactured should have, but in the manufacturing apparatus 10, high-pressure rolling is performed to refine the austenite grains and roll the steel sheet From the viewpoint of accumulating strain and reducing the size of ferrite grains obtained after rolling, at least one rolling mill in the latter stage (stands 11e, 11f, 11g in the present embodiment) in the finish rolling mill row 11 has a rolling reduction of 25. % Or more and 60% or less is preferable. It is further preferable that the at least one rolling mill includes a rolling mill having a final stand 11g. “Rolling ratio” means that the sheet thickness of the rolled material before entering a certain rolling mill is T, and the sheet thickness of the rolled material after leaving the rolling mill is t,
{(T−t) / T} × 100%
It is represented by

  When cooling is performed by the cooling device 20 on the downstream side of the final stand 11g, the ferrite transformation during the rapid cooling is induced as the reduction rate of the final stand 11g increases. Here, when the rolling reduction is less than 25%, the ferrite transformation may not be sufficiently performed. On the other hand, if the rolling reduction is too high and exceeds 60%, heat generation during rolling increases, and the cooling effect by the cooling device 20 may not be obtained.

  The rolling mills in each stand are a pair of work rolls 11aw, 11aw,..., 11fw, 11fw, 11gw, 11gw that are actually rolled down with the steel plate 1 interposed therebetween, and a pair of outer rolls arranged in contact with the work rolls. Backup rolls 11ab, 11ab,..., 11fb, 11fb, 11gb, 11gb. The rolling mill includes a work roll and a backup roll, and includes housings 11ah,..., 11fh, and 11gh that form an outer shell of the rolling mill and support the work roll and the backup roll. The housing has two upright portions (upright portions 11gr in the final stand 11g, see FIG. 2) which are erected opposite to each other. That is, the two standing portions of the housing are erected so as to sandwich the steel plate 1 (pass line) in the plate width direction.

  In the present embodiment, the work roll 11gw of the final stand 11g is made of steel that is usually used for hot rolling as a base material, and the outer periphery of the base material is covered with a cemented carbide containing tungsten carbide (WC) as a main component. It is the roll formed in. Thereby, the outer shell of the work roll 11gw is made of a cemented carbide, the elastic coefficient on the surface thereof can be 300 GPa or more, and the surface flat deformation of the work roll can be suppressed. The coating thickness of the cemented carbide is preferably 5 mm to 20 mm. If the thickness is less than 5 mm, there is a high possibility that flatness will be insufficiently suppressed. Moreover, when it becomes thicker than 20 mm, the cost which manufactures a work roll will become large. In addition, cemented carbide is often produced by powder molding, sintering, and the like, and particularly when the thickness is increased with a large roll diameter, it tends to be easily damaged.

The kind of the cemented carbide is not particularly limited, and as a bonding material for bonding tungsten carbide (WC), for example, cobalt (Co), nickel (Ni), and various alloys including the same can be used. The ratio of tungsten carbide to the bonding material is preferably 70% by mass to 90% by mass of tungsten carbide with respect to the mass of the cemented carbide. When the proportion of tungsten carbide is reduced, it becomes difficult to crack, but the wear resistance tends to be reduced. On the other hand, when the proportion of tungsten carbide is increased, the wear resistance is improved, but cracking tends to occur.
Further, any of carbides of Group IVa, Va, and VIa group metals may be used instead of tungsten carbide.

  When high-pressure rolling is performed so that the rolling reduction is 25% or more at the final stand, it is assumed that the edge drop is at least about 1.5 times larger than that of normal rolling. If the edge drop becomes too large, the JIS standard that guarantees the thickness accuracy of the steel sheet in the region of the end in the sheet width direction of the steel sheet may be deviated. On the other hand, in this invention, it becomes easy to ensure such plate | board thickness precision by making the elastic modulus of a work roll into 300 GPa or more.

Moreover, the diameter of the work roll 11gw using the cemented carbide shown by D1 in FIG. 2 is 600 mm or less. That is, considering that the diameter of a normal work roll is about 700 mm to 800 mm, the diameter is small. Thereby, the load to the work roll 11gw in high pressure rolling can be reduced. That is, when high-pressure rolling at a rolling reduction of 25% or more and less than 60% as described above is performed with a normal work roll having a roll diameter of 700 mm to 800 mm, the rolling load (line load) per unit width of the material to be rolled is 30 MN / It may be larger than mm and reach 45 MN / mm. When such a high rolling load is applied, the roll surface is deformed and cracks are generated starting from the roll surface. As a result, the work roll may be damaged. On the other hand, if the diameter of the work roll is 600 mm or less, the linear load can be suppressed to 30 MN / mm or less, so that the possibility that the work roll is damaged can be significantly reduced.
On the other hand, when the work roll diameter is less than 450 mm, when driving the upper and lower work rolls, the strength of the drive system parts is reduced, so that the torque cannot be sufficiently transmitted and high-pressure rolling is difficult. There is. Therefore, the diameter of the work roll is preferably 450 mm or more.

  Further, by reducing the diameter of the work roll, the amount of the cemented carbide can be reduced, and the work roll can be easily manufactured and its cost can be reduced.

  The distance between the shaft center of the work roll 11gw and the downstream end face of the housing standing portion 11gr, indicated by L1 in FIG. 2, is preferably larger than the radius (D1 / 2) of the work roll 11gw. Therefore, a part of the cooling device 20 can be disposed at a portion corresponding to L1- (D1 / 2). That is, by reducing the diameter of the work roll, when inserting a part of the cooling device 20 into the housing 11gh and arranging it, the cooling device 20 can be arranged close to the reduction point by the work roll 11gw. Become.

  Furthermore, the surface of the work roll 11gw, that is, the surface of the cemented carbide has a roughness Ra in the range of 0.3 μm to 2.0 μm. Here, Ra of the surface roughness means arithmetic average roughness, and is as per JIS B 0601-2001. The present inventor bites the tip of the material to be rolled by setting the surface roughness of the work roll to Ra ≧ 0.3 μm when rolling under high pressure using a work roll having a small diameter using a cemented carbide. It has been found that defects can be largely suppressed, and that when Ra ≧ 0.5 μm, biting defects hardly occur. On the other hand, when Ra> 2.0 μm, no biting failure occurs, but it has been found that the unevenness due to the roughness of the work roll is transferred to the surface of the material to be rolled, which adversely affects the surface quality of the material to be rolled.

  Conventionally, as a means for polishing a work roll using a cemented carbide in the surface layer portion, a diamond grindstone suitable for polishing with a small surface roughness is often used, and wear during rolling is often caused. Since it is difficult to proceed, the surface roughness was made smaller than that of a normal work roll in order to suppress the transfer of the polished skin. That is, even when using a work roll with a small diameter having a cemented carbide for rolling under high pressure, the surface roughness is reduced by the fine grinding as described above, so that the tip of the material to be rolled is caught. A defect occurred and the operation could not be stabilized. However, this has not been recognized in the past.

On the other hand, in the present invention, unlike the conventional case, in the present invention, in the work roll using the cemented carbide, the surface of the work roll is roughened within a predetermined range to suppress the biting failure and to stabilize the operation. Made possible.
Therefore, it is refined by rolling with a rolling mill having a work roll formed of a cemented carbide having a diameter of 600 mm or less and an elastic modulus of 300 GPa or more on the surface, with a rolling reduction of 25% or more. When obtaining a steel sheet having a structure, the surface roughness of the work roll needs to be 0.3 μm or more and 2.0 μm or less in terms of Ra, and the surface roughness is 0.5 μm or more and 2.0 μm or less in terms of Ra. Is more preferable.

  In addition, when the surface roughness of the work roll is made larger than usual in order to improve the biting property, when the work roll made of the conventional material is rolled at a high reduction rate, the surface roughness during rolling is low. It increases rapidly and may adversely affect the surface quality of the material being rolled. On the other hand, since the cemented carbide is excellent in wear resistance, the change in surface roughness during rolling is very small even when rolled at a high rolling reduction. Therefore, even during rolling, the surface roughness of the work roll is unlikely to increase rapidly, and special roll roughness maintenance work such as in-line roll polishing is frequently performed without adversely affecting the surface quality of the material to be rolled. It is possible to operate a plurality of rolling stably without any problems.

  The backup roll 11gb can be a normal roll, that is, a roll having a diameter of about 1200 mm to 1650 mm. However, when the elastic modulus in the vicinity of the surface of the work roll 11gw is increased, the Hertz pressure acting on the portion where the work roll and the backup roll come into contact with each other increases. From the viewpoint of reducing the Hertz pressure, it is advantageous that the diameter of the backup roll is large, and the diameter of the backup roll is preferably 1350 mm or more.

  A well-known thing can be used for the cooling device 20, the conveyance roll 12, and the pinch roll 13. FIG.

  In the method for manufacturing a hot-rolled steel sheet according to one embodiment, for example, the hot-rolled steel sheet is manufactured as follows. Here, although the case where the said manufacturing apparatus 10 is used is demonstrated as an example, it does not necessarily need to be based on the manufacturing apparatus 10, and may be based on another apparatus.

The rough bar extracted from the heating furnace and rolled to a predetermined thickness by the rough rolling mill is continuously rolled to the predetermined thickness by the hot finish rolling mill row 11 while the temperature is controlled. In the final stand 11g of the hot finish rolling mill 11, the roll diameter of the work roll 11gw is 600 mm or less, and the surface elastic modulus of the work roll 11gw is 300 GPa or more. The roughness Ra is polished and maintained at 0.3 μm or more and 2.0 μm or less. And the rolling reduction at that time is 25% or more. As a result, sufficient ferrite transformation is possible and the biting property is secured. The rolling reduction is preferably 60% or less. Thereby, the cooling effect by the cooling device 20 is sufficiently obtained.
In the present embodiment, an example in which the surface roughness of the work roll is maintained and managed by polishing has been described. However, the method is not particularly limited as long as the surface roughness is maintained and managed to be a predetermined roughness. Here, polishing is described as a specific example because polishing is efficient as a method for maintaining and managing the surface roughness within a predetermined range.

Thus, while the diameter of a work roll is small, it becomes possible to shorten contact length with a to-be-rolled material, and the flat amount at the time of load load can be restrained small because the surface elastic modulus is high. Therefore, rolling at a high pressure reduction is possible, and crystal grain refinement of the steel sheet can be performed. And by defining the surface roughness of the work roll in the above range, it is possible to ensure the biting property of the tip of the material to be rolled, despite the high rolling reduction.
Therefore, in the method for manufacturing a hot-rolled steel sheet that obtains a steel sheet having crystal grains refined by rolling under a high pressure ratio, the biting property can be ensured and smooth production can be achieved.

  Moreover, since the surface of the work roll 11gw is a cemented carbide having a high modulus of elasticity, the amount of flatness of the work roll 11gw can be kept small during rolling. It is also possible to ensure flatness. Moreover, since it is excellent in abrasion resistance, what is necessary is just to replace | exchange a work roll with the same frequency as before even if it is rolling under high pressure.

Furthermore, since the metal adhesion can be reduced if a cemented carbide containing a large amount of carbide is used, a low friction coefficient can be ensured even when rolling without lubrication without using a lubricant.
Therefore, according to the present invention, since a lubricant is not used, the biting failure is further prevented, stable operation can be performed, and rolling is also performed without lubrication. Rolling under high pressure can be performed.

  The steel sheet 1 exiting the hot finish rolling mill row 11 is cooled by the cooling device 20 and conveyed downstream via the pinch roll 13.

  According to the method for producing a hot-rolled steel sheet including the steps as described above, the final stand 11g of the finish rolling mill row 11 can be rolled at a high reduction rate, and the structure refined without damaging the work roll 11gw. In addition, edge drops can be reduced. In spite of such a high pressure reduction rate, it is possible to operate without impairing the production stability because of its excellent biting property.

  Although the manufacturing method of the hot-rolled steel sheet described above is an example in which high-pressure rolling is applied only to the final stand 11g of the hot finish rolling mill row 11, it is not limited thereto, and the hot finish rolling mill is not limited thereto. The above-described high-pressure rolling may be applied to at least one subsequent stage of the row.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to this.
In this example, rolling was performed by changing the material of the work roll, the diameter of the work roll, and the like. Table 1 shows the conditions and the results.

“Roll outer member quality” is the material of the material forming the outer portion of the work roll. “High Alloy Glen” is a work roll material generally used conventionally in the latter stage of a hot finish rolling mill, and its elastic modulus was 200 GPa. On the other hand, “Cemented Carbide” has a thickness of 20 mm of a cemented carbide containing 80% by mass of WC in the outer part of the work roll. The elastic modulus of the cemented carbide was 400 GPa.
“Surface roughness Ra” represents the arithmetic average roughness of the work roll according to JIS B 0601-2001, and the work roll contacts the material to be rolled before rolling the material to be rolled using a stylus type roughness meter. The part to be scanned was scanned in a plurality of locations in the circumferential direction, and the average was taken.

“Lubrication” indicates the presence or absence of a lubricant when the material to be rolled is caught.
The “tip bite” is indicated by “◯” when the material to be rolled is good and “x” when the material is not bitten.
“Edge drop E75-15” is the thickness (central plate thickness) of the central portion in the plate width direction of the material after rolling, and the plate thickness (width direction) at a position 15 mm away from the plate width direction end in the 75 mm plate width direction. The thickness deviation is calculated by subtracting the thickness in the width direction from the center thickness, and the thickness deviation in the width direction at 75 mm from the thickness deviation in the width direction at 15 mm. It is the value which subtracted.
“Roll breakage” represents whether the work roll was damaged between the biting of the material to be rolled and the end of rolling.
“Line load” was determined by measuring the rolling load with a load cell and dividing by the sheet width.
Note that “edge drop E75-15”, “roll breakage”, and “line load” were evaluated for those that could be bitten, and were indicated by “−” if biting was not possible.

  The material to be rolled was a steel material having a plate width of 700 mm containing 0.1% by mass of C and 1.0% by mass of Mn, and the rolling temperature was 850 ° C.

In either case, rolling was attempted at a high rolling reduction of 25% or more.
No. In No. 1, since the roll diameter was large, high alloy grain was used, and rolling was performed without lubrication, the edge drop was large and the work roll was damaged.
No. In No. 2, the line load decreased due to lubrication and there was no breakage. However, since high alloy grain was used at a high pressure reduction rate, the edge drop was large.
No. 3 is No.3. Since the roll diameter was reduced with respect to 1, the line load was reduced and the roll was not damaged, but the drop of the edge drop was insufficient.
No. No. 4 is No.4. In this example, the roll diameter was made smaller than 2, but the tip could not be bitten.
No. In No. 5, a cemented carbide was used as the material of the roll outer member, and its surface roughness was Ra = 0.3 μm. However, since the roll diameter was large, the line load was large and the work roll was damaged.
No. 6 is No.6. The roll diameter was smaller than 5, the tip biting and edge drop were good, and the work roll was not damaged.
No. No. 7 For example, the surface roughness of the cemented carbide was set to Ra = 0.2 μm and the rolling reduction was increased to 50%, but the tip could not be bitten.
No. 8 is No.8. When the surface roughness of the cemented carbide was set to Ra = 0.3 μm with respect to 7, the tip biting and the edge drop were good, and the roll was not damaged.

DESCRIPTION OF SYMBOLS 1 Steel plate 10 Manufacturing apparatus 11 Rolling machine row 11g Final stand 11gw Work roll of final stand 12 Conveyance roll 13 Pinch roll 20 Cooling device

Claims (3)

A method for producing a hot-rolled steel sheet including a step of performing finish rolling by a hot finish rolling mill,
Among the rolling mills provided in the hot finish rolling mill row, the work roll of at least one rolling mill of the subsequent rolling mill has a diameter of 600 mm or less and a cemented carbide having a surface elastic modulus of 300 GPa or more. The surface roughness of the work roll is Ra and is 0.3 μm or more and 2.0 μm or less,
The backup roll arranged so that the outer periphery contacts the work roll has a diameter of 1350 mm or more,
A method for producing a hot-rolled steel sheet, comprising rolling at a rolling reduction by a rolling mill having the work roll of 25% or more without supplying a lubricant to the work roll.   The method for producing a hot-rolled steel sheet according to claim 1, wherein the at least one rolling mill includes a final rolling mill in the finish rolling mill row. Having a hot finish rolling mill row comprising a plurality of rolling mills,
The plurality of rolling mills include a work roll for rolling the material to be rolled, and a backup roll having a diameter of 1350 mm or more arranged so as to contact the outer periphery of the work roll.
The work roll of at least one subsequent rolling mill among the plurality of rolling mills is configured to include a cemented carbide in the outer shell, the elastic modulus of the surface of the work roll is 300 GPa or more, and the diameter is 600 mm. An apparatus for producing a hot-rolled steel sheet having a surface roughness Ra of 0.3 μm or more and 2.0 μm or less as well as below.