JPS60199508A - Roll for rolling mill and its manufacture - Google Patents

Abstract

PURPOSE:To improve the bending strength of a roll by changing the material of ceramics from the central axis of a roll to its outer circumference and making the coefficient of thermal expansion of the outer-peripheral side smaller than that of the central-axis side. CONSTITUTION:Silicon nitride and sialon, etc. different in chemical compositions are used for a central part 1, an intermediate layer 2, and an outer peripheral layer 3, to form the ceramic layers of three kinds into a concentrically circular body. The ceramic roll of said construction is provided with a cold hydrostatic press, and formed body is calcined in the atmosphere of 1,750 deg.C nitrogen, and further, provided with a hot hydrostatic press of 1,700 deg.C, etc. Successively, it is machined into a roll of ceramic concentrically-circular construction, the coefficient of thermal expansion of its outer layer 3 is smaller than that of its central part 1. Accordingly, a residual compressive stress caused by the difference of coefficient of thermal expansion, is produced in the roll outer layer 3, and the roll durable for ending load is obtained.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a roll for a rolling mill and a method for manufacturing the same, and particularly relates to a composite structure suitable for use as a work roll that requires high strength against bending loads. The present invention relates to a ceramic rolling mill roll and a method for manufacturing the same.

[Background of the invention]

It has already been proposed to make work rolls for rolling mills from ceramics, taking advantage of the high hardness and high wear resistance that ceramics inherently have.

For example, Japanese Unexamined Patent Publication No. 49-43862 describes that a roll for a rolling mill is a composite roll having a three-layer structure, and the outermost layer is formed of a ceramic ring. Further, US Pat. No. 3,577,619 describes that the work roll of a rolling mill is a mating roll consisting of a core and a sleeve, and the sleeve is made of ceramics.

However, in these techniques, ceramics are only used on a part of the roll surface, and the material and manufacturing method of the ceramics are not disclosed at all.

Furthermore, in general, K, which increases the bonding strength at the interface between metal and ceramics, is technically difficult and
In the case of an e-gold alloy, the difference in thermal expansion coefficient between the ceramic and the Fe alloy is usually too large, depending on the material of the ceramic, so peeling at the interface is likely to occur due to heat history during use of the work roll, etc.

Furthermore, ceramics are generally strong against compressive loads, but weak against tensile loads.This is described, for example, in Ceramics, 11 (1976), 6, Pbo2. A bending load acts on the rolling roll during rolling,
As a result, higher tensile stress occurs closer to the surface layer of the roll, so ceramics that are weak against tensile loads may not be able to be used depending on the rolling conditions.

[Purpose of the invention]

An object of the present invention is to provide a roll for a rolling mill that is entirely made of ceramic and is resistant to bending loads, and a method for manufacturing the roll.

[Summary of the invention]

The present invention changes the ceramic material stepwise or continuously on concentric circles from the center axis of the roll toward the outer circumference, and makes the outer circumference of the roll a ceramic material with a smaller coefficient of thermal expansion than the center axis of the roll. Compressive residual stress is generated on the outer peripheral side of the roll, and tensile residual stress is generated on the central side of the roll.

In the production of ceramics, a common method is to mold raw material powder and sinter it by heating it to a high temperature. Sintering progresses by reducing the gaps between powders while the raw material powders are bonded together, and sintering is completed. At this point, almost no internal stress exists. After sintering is completed, the sintered body is cooled to room temperature. However, in the roll of the present invention, since the portion near the outer periphery is made of a ceramic material with a small fin, after cooling, the difference in thermal expansion coefficient This causes internal strain, compressive residual stress is generated on the outer peripheral side of the roll, and tensile residual stress is generated on the center side. When a bending load is applied to a roll for a rolling mill, the tensile stress at the outer periphery of the roll is reduced by the compressive stress caused by the difference in thermal expansion.

Therefore, a roll that is resistant to bending loads can be obtained.

If the difference in thermal expansion coefficient between the roll outer periphery and the roll center is too large, the tensile residual stress at the roll center becomes too large, causing excessive strain near the boundary between the roll outer periphery and the roll center, resulting in a decrease in roll strength. tends to decline. Therefore, the above-mentioned difference in Mr expansion coefficient should be selected within a range that does not cause such distortion. In this case, it is desirable to provide one or more intermediate ceramic layers having an intermediate thermal tensile coefficient between the outer circumferential portion of the roll and the center portion of the roll in order to prevent thermal expansion of these ceramic materials. When the intermediate ceramic layer is multi-layered, it is desirable to gradually increase the coefficient of thermal expansion from the ceramic layer closer to the outer circumference of the roll to the ceramic layer closer to the center of the roll.

In the present invention, the coefficient of thermal expansion! It is essential that the roll is made of ceramic material A, but in order to achieve a roll with high strength, each of the component ceramic materials must have 12 strength. be.

Examples of ceramic materials that can provide high strength include:
Suitable materials include silicon carbide, silicon nitride, sialon, alumina, zirconia, and composite materials thereof.

Next, a method for manufacturing the roll of the present invention will be explained.

Ceramic powder is formed into a round bar shape using a cold isostatic pressing method, a slurry casting method, etc., and if necessary, drying, pre-calcination and machining are performed to produce a round bar-shaped porous body.

When this round rod-shaped porous body is immersed in ceramic slurry having a different composition, the moisture in the slurry is absorbed into the porous body at the interface between the porous body and the slurry. Mud sticks to the surface of the mass and hardens.

This solidified layer has a uniform thickness and increases in thickness as the immersion time progresses. Therefore, the thickness of the solidified layer can be controlled to a predetermined thickness by removing the porous body from the mud drain after a predetermined time. In this case, there is naturally a limit to the amount of water absorbed by the porous material, so if necessary, remove the porous material from the mud bath, dry it, and then soak it in the mud bath again to form a thicker solidified layer. can be formed.

The molded body taken out from the slurry was dried, and the same process as above was repeated using a slurry with a different composition from the slurry described above. As a result, the composition of the ceramic changed in stages in a concentric pattern. A molded body is obtained. If this molded body is dried, sintered, and machined, a roll having a composite structure can be obtained.

In the manufacturing process of such rolls, the materials of the porous body and slurry in the center are changed, and the parts near the outer periphery are made of ceramic material that has a small coefficient of thermal expansion after firing. During the cooling process after firing, compressive residual stress is generated on the outer periphery of the roll, resulting in a roll that is resistant to bending loads.

In the method of forming a porous ceramic body and then immersing it in slurry, the slurry infiltrates into the pores near the surface layer of the porous ceramic body, so that the concentric circles of the final roll are The bonding strength between the layers formed is high. Furthermore, the ceramic roll after firing has some pores, but if these are subjected to hot isostatic pressing (pressing and heat treatment), the porosity of the roll surface layer will be reduced, and the strength of the roll surface layer will be increased. improves.

In addition to the method using slurry as described above, the method for manufacturing the rolling mill roll of the present invention may be the following method.

The first method is to use ceramic green sheets instead of plaster. That is, cold isostatic pressing method,
Ceramic powder is formed into a round rod shape using a slurry casting method, etc., and if necessary, it is dried, calcined, and machined, and the resulting round rod-shaped ceramic porous body has a composition different from that of this porous body. A desired roll is obtained by winding a ceramic green sheet of a predetermined thickness, sintering and machining. In this case, by applying ceramic slurry to the green sheet and winding and bonding the coated surface to a round bar-shaped ceramic porous body, a roll with high bonding strength can be obtained.

The second method is to use a ceramic pipe manufactured by extrusion molding or the like. That is, a round bar-shaped ceramic porous body is formed by the method described above or by a method such as extrusion molding. This round bar-shaped ceramic porous body is inserted into an undried, poorly shaped pipe manufactured by a method such as extrusion molding, the pipe is plastically deformed by a rubber press method, and then dried and sintered to form a desired roll. is obtained.

In the first and second methods, the coefficients of thermal expansion of the materials of the round rod-shaped porous ceramic body and the green sheet or pipe are different, and the coefficient of thermal expansion of the green sheet or pipe is smaller. Become. Furthermore, a plurality of green sheets made of different materials may be wound and joined together, and a number of pipes made of different materials may be used.

[Embodiments of the invention]

Example 1 An example using a total of three types of ceramic materials, silicon nitride and two types of sialon having different compositions, will be described below.

Using the above three types of ceramic materials, the diameter is 20+m.
, made a round bar-shaped roll of 100 degrees. FIG. 1 shows a sectional view perpendicular to the axis of the roll.

In Figure 1, 1 is silicon nitride in the center, 2 is a middle layer similar to sialon, and its chemical composition is S16-zALzO.
zNB-z, Z = 0.4. 3 is the outer peripheral layer, which is also Sialon, but in the above chemical composition formula, Z =
It has a composition of 1.5. These three types of ceramics are structured concentrically, and the outer layer of sialon (
The thickness of Z=1.5) is 1 inch and the coefficient of thermal expansion is 3.02X1
0-'1/130~1000t)'), the thickness of the middle layer Sialon (Z=0.4) is 3 squares, and the coefficient of thermal expansion is 3.1.
6X10-'1/C (30~1000C), the silicon nitride in the middle has a diameter of 1211B and a thermal tensile coefficient of 3.25X
l O-61/C (30-1000C).

The roll uses raw material powder of 0.7 to 5 μm, and after arranging each ceramic material in a concentric structure, cold isostatic pressing is performed, and the resulting molded body is fired in a 1750C1 nitrogen gas atmosphere. After applying a high temperature isostatic press at 1700 tons, the surface was machined and finished. As a result, it has a density of 99.8% of the theoretical density. Four concentric rolls were obtained.

Using this roll as a work roll and a backup roll made of an alloy with a Vickers hardness of 33000, a stainless steel 5U plate with a plate width of 50++ m and a plate thickness of 0.3 rtrm was used.
Rolling of S304 was performed. The surface hardness of this work roll is Pinkers hardness 3100. stainless steel 5U
Although S304 was rolled to 0.1 wg, it was possible to obtain a good rolled material with high gloss without causing any damage to the roll.

For comparison, rolls made of a single material of silicon nitride and sialon (Z=0.4 and Z=1.5) were manufactured in the same manner as above, and rolled in the same manner as above. As a result, it was possible to roll the material to 0.1511111 with the silicon roll, but surface cracks occurred when the number of reduction blades was further increased. Sialon (Z=0.4 and Z=
In the roll of No. 1.5), a crack occurred in the roll at a thickness of 0.13. From this result, it was found that the roll of the present invention can withstand even higher reduction blades.

Next, Table 1 shows the results of the bending test for the rolls of the four dC plants. Each roll had a diameter of 20 mm and a length of 100 m, and a three-point bending fracture test was conducted on each of these rolls with a fulcrum circle of 70 fins.

, J So'F:#ri The roll of the present invention is jfi 20Kyf/
(2) It is higher than 2, and the effect of the pongee residual stress imparted to the surface layer by the roll of the present invention is clearly recognized.

Example 2 Silicon nitride powder with an average particle size of 0.7 μm and 3% by weight of ittria powder as a sintering aid and 12% heavy weight of polvinyl butyral as a binder were added, and after mixing these, 9f/cJ was added to 900. After hydrostatic pressing with pressure, a roll-shaped molded body (porosity 40 cm) with a body diameter of 50 cm, a body length of 100 cm, and a total length of 200 cm is machined.
I got it.

On the other hand, 48% by weight of silicon nitride powder with an average particle size of 0.7 μm
A slurry was obtained by adding and mixing water and a deflocculant to a powder consisting of 48 grams of alumina powder with an average particle size of 1.0 μm and 4 layers of ittria powder.

The roll-shaped molded product was immersed in the slurry, and when a solidified layer of 51III had adhered to the surface of the molded product, the molded product was taken out, exposed to the atmosphere for 24 hours, and then placed in a drying oven at 50C for 5 hours. After drying for 2 hours in a 1oor drying oven, each sample was fired at 1750C for 3 hours in a nitrogen gas atmosphere. The size shrunk by about 20 inches during firing. By machining and finishing the surface layer of this fired product, the diameter of the body is 3
A composite ceramic roll having a body length of 8+ms and a total length of 157 m was obtained. As a result of the ultrasonic flaw detection test and cutting investigation, no defects were observed on the surface or inside, and a sound roll was obtained. In this example, the thickness of the outer layer formed by the slurry after firing and machining was about 4 mm. The porosity of the fired roll was 1.7 inches. As a result of heating the roll after baking 1 pattern processing at 1700C for 2 hours in an argon atmosphere of 9f/cJ, the porosity was 0.
．． It was reduced to less than 1 inch, and I was able to make a roll with a high altitude.

〔Effect of the invention〕

As described above, according to the present invention, compressive residual stress is generated on the roll outer circumferential side, so even if a bending load is applied to the roll for a rolling mill, the tensile stress on the roll outer circumferential side is generated only by the compressive residual stress valve. becomes smaller. Therefore, it is possible to provide a roll that is resistant to bending loads and the seventh manufacturing method.

[Brief explanation of the drawing]

FIG. 1 is a new view showing an embodiment of the present invention at right angles to the center l1tIIVC of a rolling surface roll. l...center, 2...middle 1st layer, 3...outer layer. Agent Patent Attorney Tatsuyuki Unuma 1 2 ゛3

Claims (1)

[Claims] 1. The ceramic material is changed stepwise or continuously on a concentric circle from the center axis of the roll toward the outer periphery, so that the outer periphery of the roll has a smaller coefficient of thermal expansion than the center axis of the roll. A roll for a rolling mill characterized by being made of ceramic material. 2. A porous ceramic body is formed in the center of a round rod, and at least one layer is formed on the outer peripheral surface of the porous ceramic body, so that the coefficient of thermal expansion of the final ceramic material becomes smaller toward the outer layer. 1. A method for producing a roll for a rolling mill, which comprises forming a porous outer peripheral ceramic body, followed by firing and machining. 3. The peripheral ceramic porous body is obtained by applying and drying ceramic slurry to the central ceramic porous body at least once or more. A method for manufacturing rolls for rolling mills. 4. The method for manufacturing a roll for a rolling mill according to claim 2 or 3, characterized in that the fired ceramic roll is subjected to pressure and heat treatment using a hot isostatic press.