JP5453727B2 - Hot rolling method for steel strip - Google Patents

Description

  The present invention relates to a steel strip hot rolling method in which a steel strip is manufactured by roughly rolling a heated steel slab into a sheet bar and then finish rolling with a continuous multi-stand finish rolling mill.

  In general, “hot rolling” of steel strip (hereinafter sometimes abbreviated as “hot rolling”) refers to a pair of work rolls incorporated in a rolling mill after the steel material is heated to about a few hundred degrees below its melting point. Rolling by rotating the work roll while pinching with, and finishing to a thin steel strip. In the steel strip hot rolling line, the material to be rolled (slab) heated to a predetermined temperature as described above is roughly rolled into a sheet bar having a thickness of 20 to 50 mm, which is an intermediate product. The steel strip is supplied to a finish rolling mill usually composed of a continuous stand row of 6 to 7 stands, and finish-rolled to form a steel strip having a predetermined dimension, for example, a thickness of 1 to 25 mm.

  The quality of the hot-rolled steel strip includes the plate thickness accuracy in the longitudinal and width directions (particularly, the plate thickness distribution in the width direction is referred to as “plate crown”), flatness such as belly stretch (also referred to as mid-stretch), and ear stretch. (These are referred to as “shapes”), surface textures (also referred to as surface quality) such as surface wrinkles and rough skin, etc., but the following items are listed as factors that reduce or degrade these shapes and quality It is done.

  Regarding the surface properties, it is known that the control of the oxide film in the front stage stand of the finish rolling mill (usually the first to third stands in the stand row of 6 to 7 stands) is important. In the first stage, the temperature of the material to be rolled is high and the rolling speed is lower than that in the latter stage, so that an oxide layer is formed on the surface of the material to be rolled, and this oxide layer is It peels from the surface and moves to the surface of the work roll, and the surface of the work roll gradually becomes rough. As the rolling progresses and the amount of transferred oxide film accumulates, the oxide layer transferred to the roll is re-peeled and transferred to the rolled material along with the transferred material from the rolled material to the roll. The phenomenon of being embedded in the surface of the material to be rolled also occurs, and these are repeated, and the surface of the material to be rolled becomes rough simultaneously with the surface of the work roll. This is usually called “surface roughness scale 疵”.

  In recent years, demands on materials have become stricter for each use of steel materials (specifically, steel types), and various alloying elements have been added to the materials in order to meet this demand. As a result, the frequency and extent of surface roughness scale wrinkles due to the transfer of oxide films has deteriorated, and the stand that is prone to transfer of such oxide film among the previous stage stands, It is recognized that it varies depending on the steel type.

  On the other hand, regarding the sheet crown and flatness, it is known that control at the rear stage of the finish rolling mill (usually the last two to three stands) is important. In the latter stage stand, since the surface of the work roll is scraped by the oxide film present on the surface of the material to be rolled, the profile of the work roll changes with the progress of rolling, and the accuracy of the plate crown and flatness is lowered.

In order to cope with such a problem, an online roll grinder has been proposed in which a grindstone is pressed against a work roll being rolled to grind the surface, and an example thereof is described in Patent Document 1. Further, Patent Document 2 has a detailed consideration as to which stand of the finishing rolling mill row the online roll grinder should be installed.
On the other hand, as a method for fundamentally solving the above problems, a steel roll whose surface layer is made of a cemented carbide (hereinafter referred to as “carbide roll”) is used as a work roll of a finish rolling mill in hot rolling of steel. The use is described in Patent Document 3. Carbide rolls are composed of ceramics whose surface layer is typified by tungsten carbide, so it is great for “seizure”, which is an adhesion phenomenon in which the roll and steel plate, which is the material to be rolled, are in direct metal contact, and for the aforementioned transfer. In addition to the prevention effect, there is an advantage that the roll is hardly worn due to its high hardness.
JP 7-88514 A JP 2005-262286 A JP 2001-321804 A

However, in recent years, as the demand for quality has become stricter, it has become an unacceptable situation even for slight roughness on the surface of a cemented carbide roll and a slight profile change in a product.
When online roll grinders are used for carbide rolls, raw materials are expensive because a large amount of carbide rolls are ground using a high-hardness grindstone such as a diamond grindstone or CBN (cubic boron nitride) grindstone. Therefore, the basic unit of roll rises remarkably. Also, there are problems such as excessive load on the grinder and uneven grinding.

  Further, as will be described later, the offset as shown in Patent Document 1, that is, the method of providing a distance in the grinding surface between the rotation center of the online roll grinder and the work roll rotation axis, Although there is a tendency to leave grinding marks inclined with respect to the rotating direction, the hardness of the roll itself is high with carbide rolls, and these grinding marks grind the surface of the steel strip that is the material to be rolled, resulting in rough surfaces and profiles. The problem cannot be solved.

Further, such grinding traces have a problem that the friction coefficient between the roll and the material to be rolled is increased, and the rolling load is increased.
Carbide rolls are used for materials with a high rolling load such as ultra-high-strength steel called “HITEN” and materials that are prone to roughening of the roll surface, such as stainless steel. When the friction coefficient and the rolling load are increased, the effect of using the cemented carbide roll is reduced.

Thus, when using a cemented carbide roll in the hot rolling of a steel strip, development of an appropriate usage method of an on-line roll grinder has been desired.
An object of this invention is to implement | achieve the hot rolling method of the steel strip by the usage method of an on-line roll grinder which eliminates said various problems.

The reference invention is a method of hot rolling a steel strip that performs finish rolling in a continuous multi-stand finish rolling mill row, and provides an online roll grinder for the work roll of at least one stand of the finish rolling mill, A work roll whose surface layer is made of cemented carbide is used for at least one of the stands using this online roll grinder, and the abrasive hardness of the grindstone of the online roll grinder of this stand is equal to the cemented carbide hardness of the work roll surface layer. It is a hot rolling method of a steel strip characterized by being 2 times or less.

The present invention described in claim 1 is a method for hot rolling a steel strip in which finish rolling is performed in a series of continuous multi-stand finish rolling mills, and is on-line with respect to a work roll of at least one stand of the finish rolling mill. A roll grinder is provided, and at least one of the stands using the online roll grinder uses a work roll whose surface layer is made of cemented carbide, and the abrasive hardness of the cup-shaped grindstone of the online roll grinder of the stand The hardness of the roll surface layer is higher than twice the hardness of the cemented carbide, and the distance between the rotation center of the cup-shaped grindstone and the contact position with the work roll, that is , the inner diameter r of the cup-shaped grindstone and the rotation tangent on the inner diameter side. The offset amount H shown in the following equation (1) consisting of the direction angle θ is set to approximately zero with θ being 3 degrees or less , A steel strip hot rolling method characterized by grinding the work roll itself simultaneously with the oxide film peeled off from the surface of the material to be rolled and transferred to the work roll surface.
Record
H = r · sinθ (1)

According to the reference invention, wherein the grindstone hardness of the grindstone of the online roll grinder is less than twice the hardness of the cemented carbide on the surface of the work roll, the amount of grinding by the online roll grinder is minimized. The basic unit of the cemented carbide roll is improved, and at the same time, the surface property of the hot-rolled steel strip can be improved.
Further, the abrasive hardness of the grindstone of the online roll grinder is higher than twice the hardness of the cemented carbide on the surface of the work roll, and the distance between the rotation center of the grindstone and the position of contact with the work roll, that is, the offset According to the present invention as set forth in claim 1 , the amount of grinding is almost minimized, and the generation of grinding marks by an on-line roll grinder is minimized, so that the surface quality, sheet crown, flatness, etc. are excellent. It has an excellent effect of being able to produce high quality products.

  First, an example of a method for manufacturing a cemented carbide roll will be described. The sleeve member is cold isostatic press molding (CIP), also called rubber press, made of a powder of 20 mass% cobalt added to tungsten carbide (WC), and the resulting WC-Co alloy is 70 mm thick and 400 mm wide. Six sleeve materials are arranged in the axial direction of the roll, and joined by HIP (hot isostatic pressing) at 1260 ° C. and 10 atm to form a sleeve, which is then machined. This is shrink-fitted on a 5 mass% chromium forged steel shaft member, and finished to complete a carbide roll having a diameter of 650 mm and a barrel length of 2050 mm. In addition to adding 20 mass% of Co, adding some nickel further improves the wear resistance.

In addition, the Vickers hardness of the tungsten carbide powder that is a material of the cemented carbide roll is about 2000.
1 and 2 are schematic views of a hot finish rolling mill suitable for applying the present invention. In these examples, four rows and seven stands are shown, but the present invention is not limited to this example in terms of the number of stages and the number of stands. 1a and 1b are backup rolls, 2a and 2b are work rolls, and 3a and 3b are online roll grinders.

In the rolling method of the present invention, if the stand using a cemented carbide roll is a plurality of stands, it is desirable that the work rolls of the stand have the same size and shape within the range of common sense dimensional accuracy. And in this invention, in the stand which uses a cemented carbide roll, an on-line roll grinder shall be used with respect to a work roll.
The inventors first investigated the rough skin of the roll (transfer of the oxide film) generated in the front stand. As a result, it was found that in the case of a carbide roll, not only the transfer amount of the oxide film is small compared to a normal steel roll, but also the adhesion between the transferred oxide film and the roll surface is very low. .

On the other hand, in the latter stage stand, the thickness of the material to be rolled decreases and the rotation speed of the roll increases, so the wear of the roll becomes a problem in the case of a normal steel roll, but almost no wear is found in the case of a carbide roll. Rather, as with the previous stage stand, it was found that a slight transfer of oxide film occurred.
In other words, it has been clarified that when a cemented carbide roll is used, it is not necessary to grind the cemented carbide roll itself with an online roll grinder, and the emphasis should be on removing the transferred oxide film.

From such knowledge, in the reference invention, diamond or CBN is not used as a grindstone for an on- line roll grinder, but a Vickers hardness of about 2000 to 3000, which is twice or less the cemented carbide hardness of the work roll surface layer. It was decided to use a grindstone such as alumina (molten alumina), silicon carbide, or boron carbide. As described above, the cemented carbide hardness of the work roll surface layer is approximately 2000 in terms of Vickers hardness, and therefore, 2 times or less means 4000 or less in terms of Vickers hardness.

In this case, as described above, it is difficult to suppress the rough surface and the profile change of the cemented carbide roll itself, but the carbide roll itself is hardly ground and the oxide film transferred to the roll surface is removed. Therefore, there is an advantage that the roll basic unit can be remarkably improved (the numerical value is lowered), and the grindstone itself is also inexpensive.
In the reference invention, the offset amount H to be described later is not particularly limited, but it is desirable to make it as large as possible within the range of the inner diameter of the cup-shaped grindstone. The larger the offset amount, the better the grinding efficiency of the grinder, and the oxide film transferred to the surface of the carbide roll can be efficiently removed. In this invention, since the abrasive hardness of the grindstone is limited to a low level, there are very few grinding marks on the surface of the work roll, and there is almost no adverse effect on the quality of the material to be rolled.

Next, in the present invention described in claim 1 , as a grindstone, a diamond grindstone or a CBN grindstone that is usually used for grinding a cemented carbide roll is used as it is. These grindstones have a Vickers hardness of about 7000 to 10,000, which is higher than twice the hardness of the cemented carbide on the surface of the work roll described above. In this case, since the roll itself can be ground simultaneously with the oxide film transferred to the roll surface, the surface state of the roll can be kept constant.

Furthermore, in the present invention described in claim 1 , the offset amount is set to be almost zero in order to suppress the grinding amount of the online roll grinder. This will be described in detail with reference to the drawings.
3 is FIG. 5 of Patent Document 1 cited above, FIG. 4 is also FIG. 6 of Patent Document 1, the former is a front view showing a work roll and an on-line roll grinder, and the latter is a side view of the AA arrow, 2 is a work roll, 3 is an online roll grinder, 31 is a grindstone, 32 is a headstock, and 33 is a guide surface of the headstock. The difference H between the height of the rotation center of the grindstone 31 and the height of the contact position with the roll (the center height of the roll if the grindstone axis is horizontal) is called an offset.

FIG. 5 is an explanatory diagram for explaining FIG. 4 in more detail. R is the inner diameter of the cup-shaped grindstone, and θ is the rotational tangential angle on the inner diameter side. The offset H with respect to θ is
H = r · sinθ
It is a relationship. In general, the offset amount is about 20 mm.
According to the experiments by the present inventors, in order to make H almost zero, the angle θ should be set to 3 degrees or less. If the mechanical accuracy of a normal roll grinder, it is easy to set so that θ is 3 degrees or less. By setting the offset H to be almost zero, the grinding traces remaining on the roll surface are almost only in the circumferential direction, and the trouble due to the inclined grinding traces is solved.

  The present invention can be referred to as a method of applying a carbide roll to a stand where an online roll grinder is installed in a finish rolling mill, but by applying the carbide roll in order from the stand having the greatest effect, The maximum effect can be demonstrated. In addition, it is completely arbitrary to apply carbide rolls within the range allowed for roll cost and roll operation for stands without online roll grinders, and it does not impair the effect of the carbide rolls in those stands. .

The steel strip was hot-rolled with a 4-stage 7-stand finish rolling mill shown in FIG. The backup roll is made of steel and has a diameter of 1600 mm and a work roll diameter of 650 mm. On-line roll grinders 3a and 3b are installed in the final seventh stand, and the cup-type grindstone has an outer diameter of 250 mm and an inner diameter of 120 mm. The work roll of the seventh stand uses a carbide roll.
The material to be rolled is a general hot rolled steel strip SPHC, and the finished thickness is 1.5 mm. 20 coils were rolled at the conditions shown in Table 1, and the rollability and the coil surface after rolling were compared and observed. The weight per coil was 18 tons, and the rolling speed in the final stand was in the range of 800 to 1400 m / min.

  In Table 1, 1 to 9 use cemented carbide rolls as work rolls, and 1 to 7 set the Vickers hardness of the grindstone to 2 times or less of these cemented carbide rolls, and the offset amount is changed from 100 mm to almost zero. 8 and 9 are comparative examples in which a CBN grindstone or a diamond grindstone is used to increase the hardness of the grindstone to at least twice that of the carbide roll, and 10 is a conventional example in which a steel roll is used as a work roll.

Although the rolling conditions and rolling results in each example are described in Table 1, in the reference examples 1 to 7, there are significant improvements in rolling properties such as rolling load in the seventh stand, surface quality of the steel strip after rolling, etc. In the comprehensive evaluation, 1 to 7 were acceptable, 8 and 9 were slightly poor, and 10 was poor.

The steel strip was hot-rolled with a 4-stage 7-stand finish rolling mill shown in FIG. The backup roll is made of steel and has a diameter of 1600 mm and a work roll diameter of 650 mm. On-line roll grinders 3a and 3b are installed in the final seventh stand, and the cup-type grindstone has an outer diameter of 250 mm and an inner diameter of 120 mm. The work roll of the seventh stand uses a carbide roll.
The material to be rolled is 590 MPa class high tensile steel, and the finished thickness is 2.4 mm. 20 coils were rolled at the conditions shown in Table 2, and the rollability and the coil surface after rolling were compared and observed. The weight per coil was 24 tons, and the rolling speed in the final stand was in the range of 800 to 1400 m / min.

In Table 2, 1 to 3 use carbide rolls as work rolls, 1 and 3 use diamond as a grindstone, and 2 and 4 use alumina. 1 is an embodiment of the present invention in which the offset amount is almost zero , 2 is a reference example in which the offset amount is almost zero, 3 is a comparative example in which the offset amount is increased, and 4 is a conventional work roll using a steel roll. It is an example.
The rolling conditions and rolling results in each example are shown in Table 2. In Example 1 and Reference Example 2, the rolling properties such as the rolling load in the seventh stand, the surface quality of the steel strip after rolling, and the flatness Significant improvement was recognized in all items such as degree, and in the overall evaluation, 1 and 2 passed, and 3 and 4 became poor.

The steel strip was hot-rolled by a 4-stage 7-stand finish rolling mill shown in FIG. The roll dimensions, grindstone dimensions, and the like are the same as in the second embodiment, but a carbide roll is used as the work roll of the second stand, and online roll grinders 3a and 3b are installed.
The material to be rolled is ferritic stainless steel, SUS430, and the finished thickness is 1.2 mm. 20 coils were rolled at the conditions shown in Table 3, and the rollability and the coil surface after rolling were compared and observed. The weight per coil was 15 tons, and the rolling speed in the final stand was in the range of 600 to 800 m / min.

Also in Table 3, 1 to 3 use carbide rolls as work rolls, 1 and 3 use CBN as a grindstone, and 2 and 4 use silicon carbide. 1 is an embodiment of the present invention in which the offset amount is substantially zero, 2 is a reference example in which the offset amount is substantially zero, 3 is a comparative example in which the offset is increased, and 4 is a conventional example in which a steel roll is used as a work roll. It is.
The rolling conditions and rolling results in each example are shown in Table 3. In Example 1 and Reference Example 2, rolling properties such as rolling load in the second stand, surface quality of the steel strip after rolling, flatness Significant improvement was recognized in all items such as degree, and in the overall evaluation, 1 and 2 passed, and 3 and 4 became poor.

It is a schematic diagram of the hot finish rolling mill concerning this invention. It is a schematic diagram of a hot finish rolling mill according to the present invention. It is a front view which shows the vicinity of a conventionally well-known online roll grinder. It is a side view by the AA arrow of FIG. FIG. 5 is an explanatory diagram for explaining FIG. 4 in more detail.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a, 1b Backup roll 2, 2a, 2b Work roll 3, 3a, 3b On-line roll grinder 31 Grinding wheel 32 Spindle head 33 Guide surface S Steel strip (rolled material)
H Offset amount

Claims (1)

In the hot rolling method of a steel strip in which finish rolling is performed by a plurality of continuous finish rolling mill rows, an online roll grinder is provided for a work roll of at least one stand of the finish rolling mill, and the online roll grinder A work roll whose surface layer is made of a cemented carbide is used for at least one of the stands using the hard roll, and the abrasive hardness of the cup-shaped grindstone of the online roll grinder of this stand is twice the hardness of the cemented carbide of the work roll surface layer. And the distance between the rotation center of the cup-shaped grindstone and the contact position with the work roll, that is, the following formula (1) consisting of the inner diameter r of the cup-shaped grindstone and the rotational tangential angle θ on the inner diameter side. The amount of offset H shown in Fig. 2 is approximately zero when θ is 3 degrees or less, and is peeled off from the surface of the material to be rolled. A method for hot rolling a steel strip, comprising grinding the work roll itself simultaneously with the oxide film transferred to the surface of the work roll.
Record
H = r · sinθ (1)

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