METHOD FOR PRODUCING MILLING ROLL

Abstract

A smooth roll (1) having a surface layer with water-retaining function is obtained by a method for producing a milling roll, comprising: a roughening process for blasting a surface (2a) of a roll substrate (2) constituting the smooth roll (1); and a coating process for spraying a thermal spray material over the roughened surface (2a) of the roll substrate (2) so as to form a thermal sprayed coating (3) having pores (5) that retain water. Satisfactory cereal powder is obtained by preventing drying of cereals in a milling process and retaining an adequate water content.

Claims

1. A method for producing a milling roll, comprising: a roughening process for blasting a surface of a substrate; and a coating process for spraying a thermal spray material over a roughened surface of the substrate so as to form a thermal sprayed coating having pores that retain water.

2. The method for producing a milling roll according to claim 1, wherein a surface roughness Ra of the milling roll after the coating process is 2-15 m.

3. The method for producing a milling roll according to claim 1, wherein a surface roughness Ra of the substrate after the roughening process is within the range of 2 m to +8 m with respect to a surface roughness Ra of the milling roll after the coating process.

4. The method for producing a milling roll according to claim 1, wherein Vickers hardness HV of a surface of the milling roll after the coating process is greater than 1000.

5. The method for producing a milling roll according to claim 1, wherein the thermal spray material is a carbide cermet.

6. The method for producing a milling roll according to claim 1, wherein an average thickness of the thermal sprayed coating is 10-150 m.

7. The method for producing a milling roll according to claim 1, comprising: an adjusting process A for shot blasting after the coating process so as to smooth fine unevenness in undulation of a surface of the thermal sprayed coating.

8. The method for producing a milling roll according to claim 1, comprising: an adjusting process B for peak cutting after the coating process so as to planarize a tip of a convex portion on a surface of the thermal sprayed coating.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0022] FIG. 1 is a perspective view of a smooth roll produced by the method for producing a milling roll according to a first embodiment of the present invention.

[0023] FIG. 2 is an enlarged sectional view of a surface layer of the smooth roll in FIG. 1.

[0024] FIG. 3 is a graph of time until a surface temperature of each test material reaches a predetermined temperature.

[0025] FIG. 4 is a schematic diagram of blasting in the roughening process.

[0026] FIG. 5(a) is an enlarged sectional view of a surface layer before conducting the shot blasting, and FIG. 5(b) is an enlarged sectional view of a surface layer after conducting the shot blasting.

[0027] FIG. 6 is a sectional view of a surface layer before and after conducting the peak cutting.

[0028] FIG. 7(a) is a perspective view of a part of a brake roll produced by the method for producing a milling roll according to a second embodiment of the present invention, and FIG. 7(b) is an enlarged sectional view of a surface portion of the brake roll.

[0029] FIG. 8 is an enlarged sectional view of a surface layer of a circled portion of the brake roll in FIG. 7.

DESCRIPTION OF EMBODIMENTS

[0030] Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view of a smooth roll (milling roll) 1 produced by the method for producing a milling roll according to the first embodiment of the present invention, and FIG. 2 is an enlarged sectional view of a surface layer of the smooth roll 1. In FIG. 1, two smooth rolls 1, 1 are arranged in parallel. The smooth roll 1 of the present embodiment is provided with a roll substrate 2 (substrate) having a circular cross section, and a thermal sprayed coating 3 is formed on a surface 2a of the roll substrate 2. The milling roll of the present invention is applied to rolls of all aspects used in the milling process, in addition to smooth rolls and brake rolls. In one example of an actual milling process, cereals are supplied between rolls in which the smooth rolls 1, 1 shown in FIG. 1 are arranged side by side and are rotated inward in opposite directions to each other, and the cereals are crushed and classified.

[0031] In the smooth roll 1 of this embodiment, any cereal can be milled. Specific examples of cereals include wheat, barley, oat, pearl barley, corn, rye, buckwheat, barnyard millet, foxtail millet, Chinese millet, sorghum, sorghum bicolor, macomo, and the like. These cereals may be used each singly or two or more of them may be used in combination. Application to wheat and barley, among these cereals, is particularly preferred. Cereal flour derived from wheat includes hard flour, semi-hard flour, medium flour, soft flour, whole cereal flour, and durum semolina.

[0032] The size and shape of each part such as a roll diameter and a length of the smooth roll 1 are not limited. A material constituting the roll substrate 2 on which the thermal sprayed coating 3 is formed may be any material as long as it can be applied to a milling roll and is capable of forming various thermal sprayed coatings. As the material constituting the roll substrate 2, a metal material is suitably used. Specific examples of the metal material include metals selected from, for example, Fe, Cr, Ni, Mo, Co, Cu, Mn, Zn, Ta, W, Al, Ti, and Mg, or alloys such as chilled steel, stainless steel and the like, containing one or more of these metals. Such a metal material is formed by extrusion molding, cutting processing, plastic processing, forging, or the like.

[0033] The thermal sprayed coating 3 is formed by collision of softened or molten various thermal spray materials with the surface 2a of the roll substrate 2 at a high speed and deposition of these materials thereon. The thermal spray material is not limited, but ceramics or cermets having high hardness are preferable.

[0034] Specific examples of ceramics as a thermal spray material include oxide-based ceramics, nitride-based ceramics, carbide-based ceramics and boride-based ceramics, containing at least one element selected from the group consisting of Ni, Cr, Co, Al, Ta, Y, W, Nb, V, Ti, B, Si, Mo, Zr, Fe, Hf and La, and mixtures thereof.

[0035] The oxide-based ceramics include Al.sub.2O.sub.3, Cr.sub.2O.sub.3, HfO.sub.2, La.sub.2O.sub.3, TiO.sub.2, Y.sub.2O.sub.3, ZrO.sub.2, Al.sub.2O.sub.3.SiO.sub.2, NiO, ZrO.sub.2.SiO.sub.2, SiO.sub.2, MgO, and CaO. The nitride-based ceramics include TiN, TaN, AlN, BN, Si.sub.3N.sub.4, HfN, NbN, YN, ZrN, Mg.sub.3N.sub.2, and Ca.sub.3N.sub.2. The carbide-based ceramics include TiC, WC, TaC, B.sub.4C, SiC, HfC, ZrC, VC, and Cr.sub.3C.sub.2. The boride-based ceramics include TiB.sub.2, ZrB.sub.2, HfB.sub.2, VB.sub.2, TaB.sub.2, NbB.sub.2, W.sub.2B.sub.5, CrB.sub.2, and LaB.sub.6.

[0036] Cermet materials which are composites of metal materials and ceramic materials may be used as the thermal spray material. The cermet materials include a composite material of the ceramic material selected from the group consisting of Cr.sub.3C.sub.2, TaC, WC, NbC, VC, TiC, B.sub.4C, SiC, CrB.sub.2, WB, MoB, ZrB.sub.2, TiB.sub.2, FeB.sub.2, CrN, Cr.sub.2N, TaN, NbN, VN, TiN and BN, and the metal material selected from the group consisting of Ni, Cr, Co, Al, Ta, Y, W, Nb, V, Ti, B, Si, Mo, Zr, Fe, Hf and La; and the like. Among them, carbide cermets are particularly preferable because a coating with high hardness is easily obtained.

[0037] The surface of the thermal sprayed coating 3 is not sealed and a large number of pores 5 are present inside the thermal sprayed coating 3. By retaining water inside these pores 5, the thermal sprayed coating 3 has high water retentivity. Thus, water retentivity is given to the surface layer of the smooth roll 1, and the cereal is prevented from getting heated in the milling process, and simultaneously, cooling property for the cereal is obtained and drying of the cereal can be suppressed. Then, the cereal is crushed by the surface unevenness of the smooth roll 1 while retaining an appropriate water content in the cereal, whereby satisfactory cereal powder can be obtained.

[0038] An average porosity of the thermal sprayed coating 3 may be about 0.5-15%, and preferably 2.0-10%. Adjustment of the average porosity is made by selection of thermal spraying methods and thermal spraying conditions. The average porosity at which the pores 5 inside the thermal sprayed coating 3 can well retain water is 2.0% or more. However, if the average porosity is increased, although the water retentivity increases, there is a concern that the wear resistance will decrease. Hence, from the viewpoint of maintaining the wear resistance, the average porosity is preferably 10% or less.

[0039] The following methods are effective for adjusting the porosity in thermal spraying construction. That is, effective are selection of particle diameter of the thermal spray material, adjustment of particle velocity during thermal spraying, and adjustment of spraying distance, which are used for coating formation. For example, in the case where the porosity of the thermal sprayed coating is reduced to form a dense coating, it is preferable to use powder having a fine particle diameter as the thermal spray material, and furthermore, it is preferable that the particle velocity is higher. It is also possible to select a method for setting the thermal spraying distance to be short so that heat flux is obtained at which the coating does not break due to heat effect. On the other hand, in order to increase the porosity and form a porous coating, forming of a coating by a method opposite to the above method is effective. However, the above adjustments need to be changed only to the extent that does not impair other coating properties such as hardness, wear resistance, and surface shape retention.

[0040] In order to verify heat transfer characteristics based on the water content of the thermal sprayed coating, the following experiment was conducted. For the experiment, two samples of each of the following test materials A to C and one sample of the following test material D were firstly prepared.

Substrate: Stainless steel material
Substrate size: 5 cm square, 5 mm thickness
Blasting conditions: Alumina particles (#60), 0.4 MPa pressure
Thermal spray material: WCCrNi
Thermal spraying method: HVOF

Porosity: 2-4%

[0041] Cross section was observed by using SEM with 200 times. There was variation depending on measurement place. From sectional SEM-BEI image, black parts inside the coating were considered as pores and the ratio of the pores to the entire coating was calculated. Thermal sprayed coating thickness:

[0042] 50 m (test materials A1, A2)

[0043] 100 m (test materials B1, B2)

[0044] 150 m (test materials C1, C2)

[0045] No thermal sprayed coating (test material D)

[0046] Tap water was dropped on the surface of each of test materials A1, B1, and C1, and the thermal sprayed coating was sufficiently moistened. The water remaining on the surface was lightly wiped off with Kimtowel. The test materials A1 to C1 containing water were placed on a plate heater kept at 100 C., and the surface temperature of each test material was measured with a contact thermometer. The time until the surface temperature reached 40 C., 60 C., and 80 C. was measured and recorded.

[0047] The test materials A2, B2, C2, and D were placed on the plate heater kept at 100 C., and the surface temperature of each test material was measured with the contact thermometer. The time until the surface temperature reached 40 C., 60 C., and 80 C. was measured and recorded.

[0048] From these measurement records, heat transfer characteristics based on the coating thickness difference and the water content difference were verified. As shown in Table 1 and FIG. 3, there was hardly any difference in temperature rise up to 40 C. regardless of the presence or absence of water in any of the samples. However, the thermal sprayed coating containing water reached the target temperature in a longer time at 80 C. From the above, it was found that the temperature rise can be more suppressed for the thermal sprayed coating containing water, regardless of the coating thickness.

TABLE-US-00001 TABLE 1 Test material Presence or absence Before test Reached time (sec) No of water (24.4 C.) 40 C. 60 C. 80 C. A1 Presence 0 19 116 223 A2 Absence 0 14 96 212 B1 Presence 0 15 99 222 B2 Absence 0 15 94 200 C1 Presence 0 16 110 240 C2 Absence 0 18 105 214 D Absence 0 15 100 182

[0049] An average thickness t of the thermal sprayed coating 3 is appropriately set, and is preferably 10-150 m, more preferably 20-100 m. When the average thickness t of the thermal sprayed coating 3 is too small, the durability is concerned, while when too large, heat amount to be accumulated in the roll substrate 2 is increased due to frictional heat, to promote drying of the cereals in the milling process. Especially, since ceramics and cermets generally have lower thermal conductivity as compared with metals, when ceramics or cermets are thermally sprayed on metallic materials, this point should be noted.

[0050] The surface roughness Ra of the roll substrate 2 is adjusted so as to be in the range of 2 m to +8 m with respect to the surface roughness Ra of the smooth roll 1 finally aimed at. In other words, the surface roughness Ra of the smooth roll 1 reflects the surface roughness Ra of the roll substrate 2. The thermal sprayed coating 3 of the present embodiment is formed so as to have uniform thickness t so that the surface roughness Ra of the roll substrate 2 is reflected in the surface roughness Ra of the smooth roll 1. Here, the uniform means that the maximum thickness and the minimum thickness of the same coating are included within 30% of the average thickness, respectively.

[0051] Since the thermal sprayed coating 3 is formed on the surface 2a of the roll substrate 2 having surface area and surface roughness increased by adjusting the surface roughness, peeling of the thermal sprayed coating 3 can be prevented. An undercoat layer may be provided between the roll substrate 2 and the thermal sprayed coating 3 for the purpose of improving adhesion and the like.

[0052] The surface roughness Ra of the smooth roll 1 is set to 5-15 m from the viewpoint of crushing and classification of cereals. Thus, it is possible to obtain satisfactory cereal powder. The surface hardness of the surface 1a of the smooth roll 1 is high, and the surface hardness thereof is adjusted to a value greater than 1000 in terms of Vickers hardness HV. Thereby, the wear resistance of the smooth roll 1 can be improved.

[0053] An example of a method for producing the smooth roll 1 in which the thermal sprayed coating 3 is formed on the surface will be described. There are conducted in this order, a roughening process for roughening the surface 2a of the roll substrate 2, a cleaning treatment of the surface 2a of the roll substrate 2, and a coating process for spraying the thermal spray material onto the roll substrate 2 to form the thermal sprayed coating 3 having the pores 5 for retaining water. Other processes such as a preheating process and the like may be included depending on the type of the substrate and the kind of the thermal spray material.

[0054] In the roughening process, unevenness is formed on the surface 2a of the roll substrate 2 by blasting for causing a shot material 21 to collide with the surface 2a of the roll substrate 2 from a blast nozzle 20, as shown in FIG. 4. The blasting refers to a technique for spraying particles (grit) having a rough surface onto a surface of a substrate with compressed air or the like to roughen the surface of the substrate. The roughness of the surface 2a of the roll substrate 2 is within the range of 2 m to +8 m with respect to the surface roughness Ra of a milling roll after the coating process as a post process. As processing conditions, the kind and grain size of a blast material, injection pressure, blasting time, and the like are appropriately set. The blasting can be carried out one by one against the roll substrates. In conventional techniques, a medium is poured into between two rolls rotating in opposite directions to each other, and a similar surface roughness is imparted to each of a pair of rolls. However, according to the blasting, since the surface roughness Ra of individual milling roll can be uniquely defined, it is possible to flexibly change the design according to the purpose of use, for example, by making the surface roughness Ra different between a pair of milling rolls, and the like.

[0055] A thermal spraying method for obtaining the thermal sprayed coating 3 in the coating process includes an atmospheric plasma thermal spraying method, a low-pressure plasma thermal spraying method, a high-speed flame thermal spraying method, a gas flame thermal spraying method, an arc thermal spraying method, a detonation thermal spraying method, and the like. In order to make it possible for the thermal sprayed coating 3 to retain water, these thermal spraying methods are appropriately selected, and further appropriately set according to the thermal spraying method are thermal spraying conditions such as the kind of the thermal spray material, heat source temperature, thermal spraying angle, thermal spraying distance, and the like.

[0056] In the case of thermal spraying the cermets, the high-speed flame thermal spraying method (HVOF) is particularly suitable. This thermal spraying method is a thermal spraying method using combustion energy of combustion gas as a heat source. By raising pressure in a combustion chamber, high-speed flame comparable to explosive combustion flame is generated, and a thermal spray material is supplied to the center of this combustion flame jet. The thermal spray material is accelerated, molten or semi-molten, and continuously sprayed at a high speed. Since the molten thermal spray particles collide with the substrate at supersonic speed, the thermal sprayed coating 3 having high adhesion can be formed. For fuels used as the heat source, used are kerosene, and as combustion gas, acetylene, ethylene, propane and the like, which are mainly composed of carbon and hydrogen, in addition to H.sub.2 gas.

[0057] In the case of thermal spraying the ceramics, the plasma thermal spraying methods are particularly suitable. The plasma thermal spraying method is a thermal spraying method in which a thermal spray material is heated by plasma and molten into liquid thermal spray particles, and the thermal spray particles are caused to collide with coating-forming surface of the substrate at high speed by plasma jet. The plasma thermal spraying method using electric energy as a heat source is a method in which a coating is formed using argon, hydrogen, nitrogen, and the like as a source of plasma. Since the method has a high heat source temperature and a high flame rate, it is possible to form a coating having a high melting point.

[0058] As described above, after the thermal sprayed coating 3 is formed on the surface 2a of the roll substrate 2, an adjustment process for adjusting the surface shape of the thermal sprayed coating 3 may be performed. As a result, it is possible to obtain surface properties according to the intended use.

[0059] After the thermal sprayed coating 3 is formed, the shot blasting is preferably carried out (adjusting process A). The shot blasting refers to a technique for spraying spherical particles onto the surface of a substrate with compressed air or the like to adjust the surface shape of the substrate. In the undulation on the surface of the thermal sprayed coating 3, fine unevenness as shown in FIG. 5(a) is present. By the shot blasting, the fine unevenness in the undulation on the surface of the thermal sprayed coating 3 can be smoothed, and it is possible to prevent adhesion and clogging of the crushed cereal in the undulation, as shown in FIG. 5(b).

[0060] After the thermal sprayed coating 3 is formed, the peak cutting may be carried out as necessary (adjusting process B). As a method for peak cutting, buffing or the like can be exemplified. By the peak cutting, the tip of the convex portion on the surface of the thermal sprayed coating 3 can be planarized as shown in FIG. 6, and the wear resistance can be improved. The peak cutting may be carried out after conducting the shot blasting.

[0061] Such shot blasting and peak cutting can be carried out one by one against the milling rolls. As a result, since the surface shape of each milling roll can be adjusted uniquely, the design can be flexibly changed according to the purpose of use. Depending on the peak cutting, the surface roughness Ra of the thermal sprayed coating 3 will change. However, it is preferable to keep the surface roughness Ra within the numerical range as described above even after the peak cutting so that crushing performance for the cereal is not imparted.

[0062] According to the method for producing a milling roll of the present embodiment, since the thermal sprayed coating 3 is formed on the surface 2a of the roll substrate 2 by the thermal spraying, unevenness of the surface layer can be kept even if the smooth roll 1 is used for a long period of time and the crushing performance is not impaired. In addition, according to the thermal spraying, it is possible to control the size and presence ratio of the pores 5 in the coating to be formed by adjusting the thermal spraying conditions.

[0063] Since the thermal sprayed coating 3 formed by this method for producing a milling roll has pores 5 for retaining water, it is possible to make it hard to transmit heat to cereals and the water retentivity in the surface layer of the smooth roll 1 is enhanced, so that the cooling property for the cereals can be obtained. Further, since the thermal sprayed coating 3 is formed by thermal spraying on the roll substrate 2 having surface area and surface roughness increased by the blasting, peeling of the thermal sprayed coating 3 can be prevented. Hence, it is possible to eliminate the problems of wear on the surface layer of the smooth roll 1 and the peeling of the thermal sprayed coating 3, and the water retentivity is improved and the cooling property for the cereals is enhanced. This high cooling property prevents the cereals from getting heated in the milling process, and it can suppress drying of the cereals. This makes it possible to retain an adequate water content in the cereals, and to obtain satisfactory cereal powder.

[0064] The method for producing a milling roll of the above embodiment is an example and is not restrictive. In the first embodiment, the method for producing a milling roll was applied to a smooth roll, but the method may be also applied to a brake roll. FIG. 7(a) is a perspective view of a part of a brake roll 10 produced by the method for producing a milling roll according to a second embodiment of the present invention. FIG. 7(b) is an enlarged sectional view of the surface portion of the brake roll 10. On the surface portion of the brake roll 10, a plurality of grooves 11 extending axially are formed over the entire circumference by machining. Each groove 11 has a V-shaped cross section consisting of two inclined surfaces having different inclination angles, and a top 12 as the outer periphery is present between the adjacent grooves 11, 11.

[0065] FIG. 8 is an enlarged sectional view of the surface layer of the circled portion M of the brake roll 10 in FIG. 7(b). A thermal sprayed coating 14 is formed on a surface 13a of a roll substrate 13 of the brake roll 10. Appropriately set within the above-described ranges according to function of the brake roll are materials constituting the roll substrate 13, thermal spray materials for forming the thermal sprayed coating 14, average porosity of a lot of pores 15 present inside the thermal sprayed coating 14, adjustment of the porosity in thermal spray construction, and thickness t of the thermal sprayed coating 14.

[0066] Also appropriately set within the above-described ranges are surface hardness of the surface 10a of the brake roll 10, and adjustment of surface roughness Ra of the roll substrate 13 to be within the range of 2 m to +8 m with respect to surface roughness Ra of the brake roll 10 finally aimed at. The surface roughness Ra of the brake roll 10 is set to 2-8 m from the viewpoint of removal of the skin of cereals, which is a pre-process of the process using a smooth roll.

[0067] The method for producing the brake roll 10 in which the thermal sprayed coating 14 is formed on the surface thereof is the same as in the first embodiment. That is, carried out in this order are a roughening process for roughening the surface 13a of the roll substrate 13, a cleaning treatment of the surface 13a of the roll substrate 13, and a coating process for spraying a thermal spray material on the roll substrate 13 to form the thermal sprayed coating 14 having pores 15 for retaining water. Other processes such as a preheating process and the like may be included depending on the type of the substrate and the kind of the thermal spray material. Also contained in the processes of this embodiment can be adjusting processes A and B for adjusting surface shape of the thermal sprayed coating 14 after the thermal sprayed coating 14 is formed on the surface 13a of the roll substrate 13.

[0068] Depending on specification and construction embodiment of the milling roll, other processes may be included in the method for producing the milling roll. The configurations and processes described in the above embodiments can be changed as long as effects of the present invention are not impaired, and other configurations and processes to be provided as necessary are not limited. The scope of the present invention is defined by the claims, and it is intended that all modifications within meaning and scope equivalent to the claims are included.

DESCRIPTION OF REFERENCE CHARACTERS

[0069] 1 Smooth roll [0070] 1a Surface of smooth roll [0071] 2 Roll substrate [0072] 2a Surface of roll substrate [0073] 3 Thermal sprayed coating [0074] 5 Pores [0075] t Thickness [0076] 10 Brake roll [0077] 10a Surface of brake roll [0078] 11 Groove [0079] 12 Top [0080] 13 Roll substrate [0081] 13a Surface of roll substrate [0082] 14 Thermal sprayed coating [0083] 15 Pores [0084] 20 Blast nozzle [0085] 21 Shot material