Process and equipment for micropattern forming on roll surface, metal sheets for pressworking prepared by the roll and methods of preparing same

Abstract: A process and an equipment for micro-pattern forming on the surface of a rolling roll (1), and a metallic thin sheet and preparation thereof by transfering the micro-pattern on the surface by use of the roll are provided. Such a rolling roll can be attained by a method wherein resin film (28) mixed with optical absorbing agent is formed on the surface, a Q switch YAG laser having an output of 5 to 100 W is applied to make a marking of this resin film, a part of the coated film is removed in strictly accordance with the pattern, then an etching process is applied to the roll surface. The metallic thin sheet skin pass rolled by this roll is used as a car panel being superior in pressworkability and sharpness of reflections.

Description: The present invention relates to a process and equipment for micro-pattern forming on the surface of a rolling roll in forming a precise pattern having a number of projections and indentations in the same surface, metal sheets obtained through the rolling roll which are to be pressed and painted, such as outer panels of automobiles for example, and a method of preparing the metal sheets. The beautiful appearance of car bodies has recently become keenly demanded. Actually, a car body indistinctly reflecting a surrounding scene in the surface thereof is becoming no longer appealing to car owners; that is, the car owners demand such a car that sharply reflects a surrounding scene in the body surface. The sharpness of reflection of the scene thus reflected in the body surface is evaluated by a DOI value by measuring the rate of reflection of a slit light that has entered the surface of the car body at a given angle. The larger the value thus obtained, the higher the sharpness of reflection in the car body surface. Nowadays, there is adopted a method of forming a number of projections and indentations in the surface of a rolling roll by projecting pulses of a laser beam directly to the surface of the roll, and transferring a pattern of the projections and indentations from the roll surface to the surface of a cold-rolled steel sheet by the use of the roll having the projections and indentations formed therein, in a skin pass rolling process after the annealing of the steel sheet. A dull-surface skin pass rolling roll with the surface roughened by forming projections and indentations by shot blasting has been popularly used in skin pass rolling, thereby transferring the pattern of projections and indentations to the surface of the steel sheet, and steel sheets thus obtained are used as a material for pressworking. The pattern with projections and indentations formed in the roll surface by the use of the laser beam differs from that formed through a conventional shot blasting process or an electrical-discharge dulling process. The projections and indentations formed are of uniform size and are at a fixed pitch throughout the roll surface. Therefore, steel sheets processed by this process and painted have such an advantage that the surface thereof has higher sharpness of reflection, and has excellent resistance to die galling at the time of pressing. Prior arts disclosed in Japanese Patent Laid-Open Nos. 55-94790 and 56-119687 and Japanese Patent Publication No. 58-25557 are the processes described above or variations thereof. Conventional laser-beam machining technology has the following problems. (1) Since the laser beam is directly applied to the roll surface to make indentations, a laser-beam oscillator is required to output a high power of 1 kW or more for efficient machining of a work, such as a rolling roll, having a wide surface area. It is, therefore, natural to adopt a CO₂ laser. This CO₂ laser, however, is of a large size and needs larger maintenance costs and labour. (2) In a pattern of projections and indentations thus produced by such a laser-beam machining equipment, each indentation is likely to become ring-like or of a similar form with molten metal deposited around it. It is therefore impossible to form a pattern of projections and indentations into a free shape. (3) The pattern of projections and indentations formed by melting the metal with the laser beam is composed of austenite in the projection-indentation section. A roll having projections and indentations thus formed has a low wear resistance when used in rolling. (4) The diameter of each indentation of the pattern of projection and indentation is determined by the diameter of a laser beam converged by a condenser. This diameter can not physically be decreased below about 100 µm because of a long (10.6 µm) wavelength of the CO₂ laser beam. (5) To ensure efficient machining of a wide surface area, it is necessary to produce a laser beam of very high-frequency pulse wave. In the case of the CO₂ gas laser, however, this can not be realized by electrical means such as Q switching. Accordingly, a mechanical chopper has been used for this purpose. This method, however, also has a problem that the high mechanical chopper and roll speeds and phases do not necessarily match each other and a pattern obtained has projections and indentations irregularly arranged. As a means to solve the above-mentioned problems, a photo-etching method used in pattern forming on roll surface in a gravure printing machine is adopted. This photo-etching process is adopted to produce a rolling roll having a roughened surface with a uniform projection and indentation pattern. Steel plates manufactured by use of the roll have excellent pressworking and decorative properties as long known, for example as disclosed in Japanese Patent Publication Nos. 41-14973 and 46-19535. However, this process has not yet been put into fully practical use because of its low roll-processing efficiency and very high cost. The photo-etching process, as well known, may be broken down into the following steps. (1) Activation of roll surface (2) Application of resist (photosensitive corrosion-resistant agent) (3) Drying (4) Film affixing (5) Exposure to light (6) Removal of film (7) Development (8) Drying (9) Etching (10) Removal of resist (11) Post-treatment (washing, neutralization, etc.) This photo-etching process can be used to form a pattern of fine projections and indentations on a rolling roll. Particularly in processing a large roll, such as the rolling roll, however, the aforementioned process requires not only many manhours, but operation in a darkroom, and moreover since it is difficult to adopt automatic operations, the process is extremely disadvantageous in manufacturing costs as compared with the aforesaid shot-blasting process or laser-beam machining process. Japanese Patent Publication No. 62-11922 discloses a process for covering a sheet surface with an acid corrosion-resistant material, locally destroying the coated surface with a laser beam, and chemically etching the locally uncovered spots. This process, however, is still premature to be practically adopted as a substantial process. Next, steel sheets provided with projections and indentations will be described. In Japanese Patent Publication No. 62-11922 and Japanese Patent Laid-Open No. 62-168602 is disclosed a laser-beam dulling process wherein pulses of a laser beam are projected onto the surface of a roll to form projections and indentations at a fixed pitch in the surface of the roll. According to the process, since regular projections and indentations can be formed at an arbitrary spacing on a bright roll, steel sheets skinpass rolled by the use of this roll are provided with level sections and projections and indented sections alternately and regularly. It is known that, in the level sections, no irregular reflection takes place; and higher sharpness of reflection is obtained rather in the skinpass-rolled sheets than in sheets rolled with the aforementioned shot-blasted roll. The sharpness of reflection of a steel sheet surface before painting has been explained above. Next, the sharpness of reflection after painting will be described. After chemical conversion treatment and electrodeposition painting, the sheets undergo painting by dip coating or spray washing. Generally, the painting process provides level projections and indentations on the steel sheet surface, thereby improving DOI as compared with the sheet before painting. In vertical painting, such as the dip coating, the paint liquid applied to the sheet flows down on the sheet surface with the gravity thereof. In this case, when there exist fixed paths along which the paint can flow down, the paint flows uniformly; if, however, there is any separated independent indentation, the flow of the paint will stop thereat, resulting in an uneven film thickness of the coating. As stated above, a steel sheet with projections and indentations having a large ratio of flat area in the surface thereof and having fixed paths that allow the uniform flow of the paint liquid is suitable for use because of the sharpness of reflection. A steel sheet manufactured by the laser-dulled roll satisfies at least the former, while a steel sheet manufactured by the shot-blasted roll can not satisfy both, resulting in a low DOI value. Next the press workability will be described. Press working is done by first holding edges of a sheet between a die and a blank holder, applying a specific blank hold-down pressure to the sheet, and then punching the sheet at the center into a predetermined shape. Commonly, material flows from the blank holding position into the die section immediately after pressing. This becomes an inflow characteristic, which is used as an index indicating the difficulty or easiness of press working. For example, a material having a great frictional resistance flows little from the die, resulting in the fracture thereof. The surface of the steel sheet, when pressed, is commonly coated with a lubricating oil. It is important to obtain a substantial effect of this lubricating oil to prevent the fracture of the sheet. In the meantime, there is a problem of galling, in that a part of the steel sheet surface is galled by the die and coheres on a tool in case of a high frictional resistance when the steel material is in contact with, or slides on, the tool. A steel sheet with a large level surface area has superior sharpness of reflection, but does not necessarily have a satisfactory press working function. It is an object of the present invention to provide a process for forming a micro-pattern on a roll surface and industrial equipment that can positively realize such rolls that have regularly or arbitrarily arranged projections and indentations. It is a further object of the present invention to provide steel sheets having high sharpness of reflection which are provided with flat areas on the steel sheet surface and with paint paths for uniform flow of the paint liquid. It is a further object of the present invention to provide metal sheets which can secure a lubricating oil fed at the time of pressing and efficiently supply the lubricating oil to their boundary surface in contact with the tool, and also have a projection-indentation structure having indentations in which a chafed stock of the sheet can be trapped so as not to cohere on the tool. It is still another object of the present invention to provide a roll of such a surface structure that can manufacture steel sheets having complicated projections and indentations on the surface as described above. Accordingly, one aspect of the present invention provides a process for forming a micro-pattern on the surface of a rolling roll, characterised in that the process comprises the steps of: (i) forming a film of acid corrosion-resistant material on the surface of the rolling roll by applying a mixture comprising an acid corrosion-resistant resin solution and a light absorbing agent comprising a pigment or carbon black, (ii) irradiating the material in a desired pattern with a YAG laser in combination with a Q-switch, thereby to remove the resin and to expose the surface of the rolling roll in the irradiated portions in accordance with the desired pattern, the laser being of insufficient power to melt the surface of the roll, (iii) etching the exposed portions with an etching solution, and (iv) removing the remaining resin by dissolution in a solvent. If the thickness of resin film to be formed on the surface of the roll is 1 to 10 µm, the resin is evaporated into an accurate pattern with a laser of low output and at the same time the resin may be endured against an etching process, resulting in that the process is quite preferable. The apparatus of the present invention for preferentially carrying out the above-mentioned process is an apparatus for use in making a roughened surface of the rolling roll comprises: (a) means for supporting and rotating the rolling roll, (b) means for coating the acid corrosion-resistant material onto the rolling roll, (c) a laser plotter comprising a laser oscillator for irradiating the coating in a pulse-like manner in response to an electrical signal, and means for guiding and focusing the laser onto the coating, (d) means for applying an etching solution to the surface of the rolling roll, (e) means for moving, independently or simultaneously, the laser plotter, the coating means and the etching means in the axial direction of the rolling roll. (f) an image processing device for providing a pattern to be formed on the roll surface, reducing the pattern to a plurality of dots to make a binary encoding and to make a series of plot data, and (g) a control means for controlling the pulse emitting time of the laser plotter in response the plot data and to a signal sent from a means for detecting the rotational angle of the rolling roll. The apparatus of the present invention is further added with the following constituents in addition to the above arrangement. That is to say, the laser oscillator of the laser plotter is an YAG laser, a first optical modulator is placed within the laser oscillator to make a Q switch oscillator, a second optical modulator is placed in the midway of a laser optical path ranging from the oscilaltor to the machining part out of the laser oscillator, and a series of optical units composed of a mirror and a lens for guiding the laser beam to the surface of the roll and collecting the laser beam, wherein the detector for detecting a rotational angle of the machined roll is a rotary encoder connected to a rotating shaft of the roll rotating device, and the control device is comprised of an optical modulator driver for driving the above-mentioned first optical modulator in response to a clock signal from the rotary encoder and an optical modulator driver for driving the above-mentioned second optical modulator in response to a signal taking out a clock of a part corresponding to a dot required for forming the above-mentioned image from the clock signal in reference to the plotted data. In addition, the present invention relates to a metallic sheet for use in press forming which has a minute pattern composed of independent pond-like indentations and a minute pattern composed of continuous groove-like concave portions at its surface. The independent pond-like indentations are of a large number of arranged indentations, they may have a surface pattern in which a continuous net-like indentation encloses a circumference of each of the indentations, and the portion where the pond-like indentations are arranged and another portion where a net-like indentation is arranged may be placed at separate positions. The roll for use in press forming such a metallic sheet as described above has independently or commonly a surface pattern composed of independent trapezoid projections with a flat top surface and a surface pattern composed of continuous net-like projections with a top flat surface. It is possible to manufacture the above-mentioned metallic sheet for use in press forming by using a roll having an independent trapezoid projection and another roll having a continuous net-like projection of these rolls as above and skin pass rolling each of the metallic sheets once. BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is an entire perspective view taken from the front side of a preferred embodiment of the device of the present invention. Fig. 2 is a perspective view taken from the rear surface of Fig. 1. Fig. 3 is an illustrative view for showing the operation of the device of the present invention. Fig. 4 is an enlarged front elevational view for showing an example of a pattern of the present invention. Fig. 5 is a three-dimensional roughness chart for showing a shape of the surface of the roll actually manufactured by the method of the present invention. Fig. 6 is a block diagram for showing the device of the present invention. Fig. 7 is a view for comparing and illustrating actions between the present invention and the prior art. Fig. 8 is a block diagram for showing another example of the device of the present invention. Fig. 9 is a pattern of irregular projections and indentations. Fig. 10 is a pattern of regular projections and indentations. Fig. 11 is a schematic perspective view for showing a surface of a metallic steel sheet for press machining work of the present invention. Fig. 12 is a graph for showing influence of a skin pass rolling rate against a transferring rate. Fig. 13 is a graph for showing the influence of a skin pass rolling rate against a mechanical property. Fig. 14 is a schematic view for schematically showing a press machining operation. Fig. 15 is a view for showing projections and indentations at the surface in the preferred embodiment of the present invention. Fig. 16 is a view of patterns for showing surfaces of rolls experimentally formed and condition of projections and indentations in the surface of material made by the experimentally formed rolls. Fig. 17 is a diagram of steps for showing a roll manufacturing process. Fig. 18 is an illustrative view for showing a testing method for evaluating a press machining operation. Fig. 19 is a graph for showing a result of a sliding test. Fig. 20 is a graph for showing a result of sharpness of reflection. Figs. 21 and 22 are a partial section for showing a steel sheet and a roll for indicating a double skin pass. An important feature in the present invention is how to make an efficient evaporation and removal of a resin film formed on a surface of a rolling roll. That is to say, the resin film where a laser beam is radiated is evaporated and removed without having any remained molten film and further a surplus heat must be prevented from being spread over and around the formed fine holes (dots) to form a thermally influenced part and to make a vague contour of each of the dots. In view of the fact described above, the present invention has a feature that as a laser, a Q switched YAG laser is used and an optical absorbing agent is mixed with the resin used in order to increase a rate of absorption of the laser beam. The Q switch is means for making a pulse in which a radiation time per one pulse is made quite short and a peak power is increased by a value corresponding to the shortened time in case of making pulsation of the laser. With this arrangement, a heating time of the irradiation part is quite short and its thermal conducting time is short, resulting in that a fed optical energy is not dispensed through a thermal conduction, but locally concentrated. As a result, it is possible to make an efficient evaporation of the resin and further to reduce the expansion of the thermally influenced part around it. In general, since a quite high frequency of pulse is required in view of the efficiency of the forming operation, the laser is required to provide a certain high output. So, the laser to be used in this case is most preferably an YAG laser of a so-called continuous exciting Q-switch type. The above-mentioned laser has an advantage that a high frequency pulse of several 10 kHz at a high output of 5 to 100 W can be attained and further the pulse is easily made a short pulse through a Q switch. In addition, the wave length of the YAG laser is 1.06 µm which is 1/10 of the wave length of a CO₂ laser and the minimum spot diameter which can be metered by a condenser can be reduced to about 10 µm, resulting in that it has an effect that a fine pattern of projections and indentations can be freely formed. Provided that this wave length is near that of visible light and its absorbing rate in a normal resin is low, resulting in that the resin may not be machined. In view of this fact, the desired object can be easily attained by mixing the resin with optical absorbing material and applying them. In this case, as the optical absorbing material, either carbon black or a pigment is preferable. According to the experiments performed by the inventors of the present invention, in the case that a resin mixed with the above-mentioned optical absorbing material is used, it is preferable to have a film thickness of about 1 to 10 µm when the YAG laser of 100 W or so is used. This is due to the fact that a through-pass hole in the resin film is formed in a reverse conical shape by the laser, resulting in that a too thick film thickness may not cause a hole (dot) having a sufficient diameter to be formed and in turn if a spraying method is applied, a film thickness less than 1 µm is hardly formed in a uniform thickness. It is preferable to use a coating of normal temperature drying type due to the fact that a drying and baking step in case of performing a coating operation is eliminated. In the method of the present invention, either carbon black or a pigment, as an absorbing material, is added to the anti-oxidization corrosion-resistant resin film, so that it is possible to evaporate this resin film with the YAG laser beam having a low output and further it is also possible to expose the surface of the roll strictly in accordance with the pattern. So, it is also possible to form a pattern quite easily and accurately. Referring now to Fig. 3, the method of the present invention will be described. This method is composed of the following three steps: At first, a first step will be described. Resin coating mixed with optical absorbing material is at first sprayed by a spray gun 6 onto a surface of the roll, the resin shows evaporation of its solvent agent at the surface of the roll and is solidified there, resulting in the formation of a thin film 28 of resin at the roll surface. If the roll 1 is rotated and the spray gun is moved in an axial direction of the roll, the thin film 28 is formed over an entire circumference of the roll 1 as the spray gun 6 is moved. A second step of the operation will now be described: Also in this case, the roll is kept rotating and a laser radiating head 10 is moved in an axial direction of the roll and a pulse laser beam 29 is concentrated through a lens 30 onto a predetermined location on the roll surface having the thin film 28 formed thereon and then irradiated. The thin film 28 at a portion where the beam is irradiated, through this energy, is evaporated to expose the surface of the roll 1. A row of minute holes is formed helically around the circumference of the roll 1 under rotation of the roll 1 and movement of the laser radiation head 10. A third step of the operation of the present invention will be described: As etching liquid 31 is sprayed through a moving spray gun 11, a portion not covered by the thin film 28, i.e. only the portion radiated by a laser beam is corroded and then a crater 32 is formed. In Fig. 3 is illustrated a continuous operation from the first step to the third step as described above. However, it is optional to make an independent operation unless an order of each of the operations is varied. One example of the device for practically realizing the method of the present invention will be described in reference to Figs. 1 and 2. Fig. 1 is a perspective view taken from a front side of the device of the preferred embodiment and Fig. 2 is also a perspective view taken from a rear surface of the same. Figs. 1 and 2 are schematic views for illustrating a technical concept of the device of the present invention. Accompanying devices which are installed actually but of which description is not required are eliminated in the illustrations. A roll rotating device comprises roll supporting blocks 2 and 3, a chuck 4 and a spindle 5, and a rotating motor is provided in the supporting block 3. Both ends of the rolling roll of a workpiece are supported respectively by the chuck 4 and the spindle 5. The roll is rotated by a motor (not shown). Then, as shown in Fig. 2, as a coating device, a spray gun 6 for spraying resin coating mixed with optical absorbing material against a surface of the roll is arranged at a side part of the roll 1. To this spray gun 6 is supplied resin coating from a resin tank 7 through a tub 8. As shown in fig. 1, a laser plotter 9 is arranged at a side opposite to the spray gun 6. This laser plotter 9 is composed of a Q switch for producing a pulse beam in response to an electric singal sent from a control device 24 to be described later, an YAG laser oscillator 9a, and a laser radiation head 9b including an optical unit such as a mirror for guiding the laser beam to the surface of the roll and a lens for focusing the laser beam onto the surface of the roll and the like. The conventional type of the system uses a mechanical chopper. However, the number of rotations and the phase of a mechanical chopper rotated at a high speed and those of the roll are do not necessarily coincide with each other and the produced pattern of projections and indentations is irregularly arranged as shown in Fig. 9. In Fig. 9, the direction indicated by an arrow shows the rotational direction of the roll and another direction perpendicular to the former direction indicates an axial direction of the roll. The steel sheet produced through this rolling with this pattern shows a stripe pattern in a rotating direction of the roll and this is not preferable. In order to resolve this problem, it is necessary to make a regular pattern as shown in Fig. 10. In case of a method using the mechanical chopper, a quite close accuracy is required in the mechanical control such as a motor and the like, resulting in that there are many hard technical problems. As shown in Fig. 2, in this example, there is provided a spray gun 11 of an etching device for use in spraying etching liquid adjacent to the coating device. The etching liquid is supplied from the tank 12 through the pipe 13. A spray gun 6, a laser plotter 9 and a spray gun 11 are mounted on XY tables 14, 15 and 16, respectively. The XY tables 14, 15 and 16 have a similar structure to each other. For example, the XY table 14 can be moved in an axial direction of the roll and a direction perpendicular to the axial direction under rotations of the ball screws 19 and 20 caused by rotations of motors 21 and 22 along the guide rails 17 and 18. This table can be moved longitudinally in particular in an axial direction of the roll and the device on the XY table can act on an entire barrel of the roll 1. The example shown in Figs. 1 and 2 is constructed such that the laser plotter is mounted on one side of the roll and each of the coating device and the etching device is mounted at the opposite side on each of the independent XY tables. This arrangement may be optional and any XY table can be used in common. However, it is preferable to mount the spray gun 6 at a side opposite to the laser plotter 9 in order to protect the laser plotter 9 having optical equipment therein against a mist of etching liquid. The coating and etching liquids sprayed from the spray guns 6 and 11 and dropped are recovered into a waste liquid tank 47. An image processing device 23 may produce a pattern to be etched on the surface of the roll 1, make a binary coding from it and then generate a series of plot data. A control device 24 may receive a pulse signal from a detector of a rotational angle, through a cable 25, which is composed of a rotary encoder (not shown) assembled in the roll supporting block 3, transmit a signal to said laser resonator 9a through a cable 26 and a cable bare 27 in order to radiate a laser onto a predetermined one point on the surface of the roll 1 specified by the above-mentioned plot data in response to the received pulse signal and then form a predetermined pattern. Since the present invention is constructed as above, it has less number of steps than that of the above-mentioned photo-etching process and further the manufacturing cost can be substantially reduced. In addition, it is a feature of the present invention to enable the above-mentioned series of operations to be easily and automatically carried out. Since the thin film 28 upon completion of the etching process is not needed, it is possible to remove it easily with a treatment such as washing with solvent. However, this removing step may easily be carried out continuously by mounting it on the XY table in the same manner as described above. Referring now to Figs. 4 and 5, a process for forming a pattern of projections and indentations to be formed on the surface of the roll 1 will be described. Fig. 4 shows one example of an enlarged front elevational view of a pattern of minute projections and indentations constructed on the surface of the roll, wherein a white background is a part not radiated with laser, i.e. a part not etched. The part indicated by hatched lines corresponds to a part irradiated with laser, i.e. a part to be etched. In Fig.4, vertical and horizontal lines are drawn only for a sake of convenience and so they are not actually present. In Fig. 4, the rotational direction of the roll 1 is set in a vertical arrow direction 33 and the moving direction of the XY table 15 toward the axial direction of the roll is set in a lateral arrow direction 34. As described above, since the punching holes in the thin film by the laser plotter are helically formed on the circumference of the roll, regions indicated by hatched lines in Fig. 4 are formed in sequence from the left vertical row to the right vertical row. In case of forming this pattern, it is necessary to make a highly accurate coincidence with a rotational speed of the roll and an ejecting timing of the pulse laser in order to eliminate a displacement in position of plotted point in the adjoining rows. In the apparatus of the present invention, the rotational angle of the roll is detected by a rotary encoder and a pulse beam ejecting timing is defiend by electrical means in correspondence with the detection, so that the present invention can be realized without requiring a substantial accuracy in the motor for the roll rotating device. In the present invention, it is constructed as a system in which the power of the laser oscillator does not directly melt metal on the surface of the roll, but evaporates only the resin of low melting point formed on the surface of the roll, so that the present invention does not require any high power laser such as a CO₂ laser. In view of this fact, the present invention is based upon that a laser capable of oscillating a quite high frequency pulse beam through electric means under a superior dependability, i.e. a Q switch YAG laser, in this case can be used. Fig.5 is a three-dimensional roughness chart for showing one example of a profile of a roughened surface roll formed by the method and apparatus of the present invention. Of course, in accordance with the present invention, it is not restricted to the pattern shown in fig. 5, but it is possible to etch a universal pattern, wherein its figure is automatically formed by the image processing device 23 and then it can be copied to the surface of the roll automatically by the laser plotter. An important advantage of the present invention consists in the fact that steps such as development, drying and the like are eliminated and made in an automatic operation as compared with that of the conventional type of photo-etching process, so that an operation of the prior art requiring eight hours per one roll is finished approximately in sixty minutes. It has a superior feature in mass production and a labour saving. In addition, in accordance with the method of the present invention, it is one of the features that work in a dark room which was required in the above-mentioned photo-etching process can be eliminated. Now, a practical configuration of the above-mentioned laser plotter and its control device will be described. As already described above, the method of the present invention is carried out such that short light pulses through a Q switch are struck onto the roll side by side to form a pattern. In this case, it is necessary to make a highly accurate coincidence between the rotational speed of the roll and the ejection timing of the pulse laser. Further, the above-mentioned process has a problem in that the power value is irregular one in respect of certain pulses. As is apparent from the arrangement of dots in each of the rows in Fig. 4, it is due to the fact that a portion having a dot in one row and a portion having no dot are mixed to each other. Since the scanning speed is kept constant, an irregular time interval is generated between the dots. Since the energy per pulse in case of a continuous energization Q switch is dependent upon the time interval of the pulse, this results in that the pulse energy just after the portion having no dot becomes quite high and the sizes of the dots become irregular and so the accuracy of the formed pattern is decreased. Referring now to Fig. 6, an example of a practical apparatus of the present invention will be described. A laser oscillator 35 is an YAG rod in case of a Nd-YAG laser. The laser oscillator 35 is energized by light from a discharging lamp not shown. A well-known laser resonator is composed of these elements, a full reflection mirror 36 and a half-mirror 36a. A first optical modulator 37 is arranged between the resonator mirrors 36 and 36a and an acoustic optical element or the like is normally used as this modulator. A Q switch oscillation is carried out by turning ON or OFF of the first optical modulator 37. A second optical modulator 37a is arranged on an optical path of a laser beam 30 out of the resonator and this is similar to the above-mentioned first optical modulator 37. The laser beam 30 passed through the second optical modulator 37a is properly changed in its optical path by a mirror 38 and the like, focused by a condenser 39 and then radiated onto the roll 1, which is a workpiece. A method of controlling the optical modulators 37 and 37a will be described. A high frequency pulse signal acting as a reference signal for getting a timing when a pulse is generated is produced by a rotary encoder 40 connected to a rotating shaft of the roll 1. Since this pulse is produced not by the rotational speed of the roll, but generated in response to the rotational angle of the roll, this is a signal which is accurately followed by the rotation of the roll. This reference signal is reduced by a counter 41, for example, to a fraction of integer as required and then converted into the number of dots for every circumference of the roll 1. At this time, the higher the high frequency of the signal from the rotary encoder 40, the higher the degree of freedom of the frequency of the signal reduced by the counter 41, resulting in that it is preferable to provide the rotary encoder 40 having a resolution as high as possible. A drive 42 for driving the optical modulator 37 is controlled directly by using a signal produced from the counter 41. The driver 42 may generate a high frequency voltage in the case that the optical modulator 37 is an acoustic optical element; this signal is turned ON or OFF in response to the signal from the counter 41 to turn ON or OFF the optical modulator. Since the driver 42 is controlled by using an input signal from the counter 41, the optical modulator 37 may act to turn ON a laser at all dot positions in the surface of the workpiece 1. Then, the driver 42a for use in driving the optical modulator 37a may drive the optical modulator 37a in response to a timing signal sent from the counter 41 on the basis of data (binary encoding data) which is arranged in the form of presence of non-presence of dots for making an image stored in the image memory 43 and formed. That is to say, the optical modulator 37a is constructed such that the laser is turned ON only at the position of a picture element at such a dot position as one required for forming an image or some images in the surface of the workpiece 1. Practically, the data is read out from the image memory 43 in response to a signal from the frequency converter 41 and then an optical modulator driver 42a is controlled by a signal having a product of the read-out data and the former data itself. The action of the apparatus of the present invention will be described in reference to Fig. 7. For the sake of convenience of description, a modulating process in the prior art and a wave-form of a laser pulse in the prior art modulation process are indicated in Figs. 7(a) and (b). In Fig. 7(c) is illustrated the process of the present invention and a wave-form got through the process of the present invention. Fig. 7(a) illustrates a case in which the optical modulator 37 is arranged between the resonator mirrors 36 and 36a which is a so-called Q switch type oscillation method. In this case, there is a certain picture element which does not require any dots for the sake of convenience of picture element itself. If the time interval of the pulse becomes an irregular one, a pulse just after the above-mentioned long interval shows a high peak power as shown in Fig. 7(a), resulting in the production of irregularity. As shown in Fig. 7(b), in the case that an optical modulator 37a is placed outside the resonant mirrors 36 and 36a, the value of each of the pulses if regularly arranged even if there is an irregular pulse spacing. However, the peak power value is quite low, resulting in that its radiation time is extended thereby to generate irregularity as described above. To the contrary, the process of the present invention shown in Fig. 7(c) is performed such that the optical modulator 37 turns ON the laser for all picture elements, resulting in that the laser pulse wave-form just after the mirror 36a becomes a wave-form having a linear line of Fig. 7(c) added with a dotted line, i.e. a wave-form having an equal spacing and then the peak power values are equal to each other at all pulses. In addition, non-required pulses are shielded by the optical modulator 37a, thereby a series of pulse waves having irregular spacing and equal peak power values as indicated by a solid line of Fig. 7(c) can be attained. Fig. 7(d) illustrates schematically a row of dots 44 formed by this laser pulse. In the process shown in Fig. 7(a), dots 45 have different sizes after a certain spacing is left in respect to a size of each of dots 44. However, in accordance with the process of the present invention, dots 46 which are equal in size to other dots 44 are formed. With the above-mentioned arrangement, it is possible to make an accurate control of a dot position during a process for machining the surface of the roll with a Q switch and a pulse of YAG laser, thereby to eliminate the irregular size of dots and so accurate pattern forming can be achieved. As an alternative process of the present invention, as shown in Fig.8, it can be considered to provide such an arrangement as one in which a laser beam outputted from a resonator 9a is divided into a plurality of beams through the spectroscopic mirrors 47a, 47b and 47c, each of the divided laser beams is turned ON-OFF independently through optical modulators 37a, 37b and 37c and at the same time the beam is radiated onto the surface of the roll 1. According to this process, each of the optical modulators 37a, 37b and 37c is controlled through separate data. This process is effective as means for improving the machining speed. The apparatus of the present invention may generate the following operations. (a) Since the laser oscillator may evaporate a thin resin film only, it is possible to apply a high frequency pulse beam of a low output with a short wave-length and further a finer pattern can be formed; in addition no mechanical chopper is used, with the result that the ON-OFF of the laser pulse can freely be controlled and a free pattern can be formed. (b) Since the roll rotation and the pulse beam ejecting timing are electrically coupled, it is possible to make an accurate pattern of projections and indentations without having any pattern displacement, not being dependent upon the accuracy of speed control of a motor or the like. (c) Since the pattern of projections and indentations is formed by an etching process, there is no austenite layer through melting at the formed projections and indentations of the roll and so the rolling roll having projections and indentations has a superior anti-wear characteristic. (d) It has a simple step, requires no dark room work such as a photo-etching or the like, and an automatic operation can be realized and its manufacturing cost is less expensive due to its labour saving. According to the present invention, a rolling roll having a pattern of various fine projections and indentations which could not be manufactured by the conventional type of shot-blasting process or a laser processing can be manufactured efficiently and less expensively than by a photo-etching process. In this case, an accurate and more fine pattern forming can be performed with the Q switch and YAG laser. Accordingly, it is possible to provide a less-expensive large amount of steel sheets which have superior surface characteristics in press forming or ornamental characteristics, by using this rolling roll. Next, steel sheets provided with a surface pattern by applying the above-mentioned roll and its manufacturing method will be described. As described above, the conventional roll for use in manufacturing a steel sheet having micro-patterns on its surface is given a rough surface through a shot blasting or a laser beam. The conventional type of roll may not accomplish the desired object of the present invention. In view of this fact, an etching work is applied in order to adopt it to the above-mentioned object and this etching process is applied to the rolling roll. That is, as shown in a process figure of Fig. 17, a resist is applied to the roll surface and coated thereon, a film having a desired pattern, for example, is applied to cover the surface, then the roll is exposed to light and developed and thereafter an etching process is applied, or, a laser beam is radiated against the resist in a desired pattern to remove the resist, and then etching work is performed to apply a pattern of projections and indentations on the roll as shown in Fig. 16. This roll is used to perform skin pass rolling for a cold rolled steel sheet. The pattern of projections and indentations thus obtained on the steel sheet is also shown in Fig. 16. Checking of these shapes of projections and indentations of the roll surface apparently shows that a quite superior accurate motif with flat top projections is formed notwithstanding of a comparison with Fig. 5 of Japanese Patent Publication No.62-11922 and Fig. 1 of Japanese Patent Laid-Open No.62-168602. Lubricant oil was applied to the surfaces of these steel sheets and the press machining characteristics were evaluated. The evaluation method was carried out such that as shown in Fig. 18, 100 kgf of a pushing force was added to a tool 61 having a spherical surface with an extremity end of 20 mmR from both sides of a test piece 60 in advance, and then a sliding resistance was measured by a sliding test method for pulling out the test piece in a direction 63 perpendicular to the pushing force 62. Of course, as described above, a test piece having a low resistance is superior in a press forming characteristics. In Fig. 19 is shown a result of an experiment on the sliding resistance. As shown in Fig. 19, it is a feature of the present invention that the sliding resistance of a steel sheet having pond-like indentations is low and further a substantially constant sliding resistance appears without having any relation with a surface roughness Ra. In turn, in case of a pattern having groove-like indentations, a sliding resistance is quite high and this sliding resistance is increased as Ra is increased. The steel sheet manufactured by the conventional type of shot blast or a roll provided with projections and indentations through a laser dulling process is illustrated in the figure as an example of comparison (the material processed by the prior art). These are positioned at an intermediate part of the steel sheet having surface indentations such as the abovementioned pond-like or groove like ones. This mechanism can be construed as follows. When the tool and the steel sheet slide with respect to each other, the lubricant oil held in the indentations is applied with a pressure through the tool and is thus discharged to the sliding surface by that pressure so as to realize an effect of lubrication. In turn, in case of groove-like indentations, the oil is not enclosed in them, so that the lubricant is simply discharged together with the tool along the groove-like indentations, resulting in that no supplying of oil to the most important part is carried out. Accordingly, the sliding resistance is high and it is not suitable for the press forming work. As regards influence of the steel sheet manufactured by the shot blast roll in the conventional type of system against the sliding resistance, this can be explained if the projections and indentations in the surface are random in shape and the structure of the projections and indentations is considered as one in which the pond-like indentations and the groove-like indentations are mixedly present to each other. In the case of a steel sheet manufactured by the laser dulling roll, its indentation shape is sharp and an edge of the indentation is cut and raised, so that the lubricant is hardly discharged if any pressure occurs. Accordingly, the conventional type of material shows a sliding resistance at the intermediate part of the steel sheet having groove-like indentations and pond-like indentations. Now, sharpness of reflection characteristics will be described. DOI before and after the coating of these steel sheets is indicated in Fig. 20. A coating condition is set such that after a background part is processed, then an electro-deposition is applied to perform a vertical coating once. As shown in Fig. 20, the material having a high DOI before coating operation shows a high DOI value also after the coating operation is carried out. The material having a high DOI value before the coating operation has a high rate of flat areas and has a low surface roughness. In Fig. 20, a checking of an improved margin of DOI after coating from the same DOI before coating shows that an improved margin of the steel sheet having groove-like indentations is high and the projections and indentations in the surface are easily made uniform. In case of a steel sheet made by a shot blast roll, DOI values before and after coating operations are low and its sharpness of reflection characteristic is deteriorated. In case of the steel sheet obtained through a laser dulling process, an improved margin of DOI value through a coating process is low. In view of the knowledge obtained through the basic experiment as described above, it is made apparent that the pattern having the pond-like indentations and the continuous groove-like indentations arranged on the surface of the steel sheet is the superior one for the steel sheet satisfying both press machining characteristic and sharpness of reflection. As the arranging form of this pattern, two cases can be provided. One of them (a case 1) is a pattern in which pond-like indentations 51 and groove-like indentations 52 are alternately arranged as shown schematically in Fig. 11. In case of performing a press machining operation, the lubricant held within the pond-like indentations 51 in Fig. 11 receives pressure from the tool and is discharged to a flat part 53 where the tool is contacted with the material so as to realize an effective lubricating characteristic. In turn, during a coating operation, the groove-like indentations 52 may become a coating flow path, resulting in the making of a uniform coating and the sharpness of reflection is improved. Another case (a case No. 2) is made such that the pond-like indentations and groove-like indentations are formed on the separate positions in the steel sheet in response to a shape of the press formed product. In case that the steel sheet 54 is press formed, the most important location is a part 54a of the steel sheet contacting with the die 57 shown in Fig. 14 and in turn a location where a sharpness of reflection characteristic is required is a part 54b of the steel sheet corresponding to a punch surface 55. Accordingly, in case that the shape of the formed product is well-known one, a pond-like pattern is formed at the steel sheet 54a and the groove-like pattern is formed at the part 54b. These steel sheets are obtained through a skin pass rolling by rolls having projections and identations opposite to those of the steel sheets. In this case, a desired steel sheet can be manufactured through a skin pass rolling shown in Fig. 16 by rolls having both indentations of the pond-like and groove-like types. Otherwise, the skin pass rolling is applied with one roll formed with either the pond-like indentations or groove-like indentations and then with another roll formed with either indentations opposite to the former. A method for applying a preferable pattern to the steel sheet during a reskin pass rolling will be described as follows. For example, a steel sheet skin pass rolled once with a roll having a rectangular indentation with one side length (a) is transferred with groove-like indentations so as to enclose the projections with one side (a). Under this condition, although the sharpness reflection characteristic is superior, a press rolling workability is not always superior. Accordingly, it is necessary to form only the pond-like indentations separated through the second skin pass at the projections with one side (a). Due to this fact, during second skin pass operation, the roll formed with the groove-like indentations (in other words, the projections with one side (b)) is applied, it is necessary to fulfill the following equation (1). In this equation, ℓ denotes a pitch of the projection and indentation in the second skin pass roll. ℓ + b < a (1) In Fig. 21 is illustrated schematically a positional relationship of these elements. In Fig.21, the reference numeral 71 denotes a sectional surface of the surface part of the steel sheet obtained through the first skin pass rolling in which a projection 73 with a side length (a) is formed together with the groove-like indentations 72. That is, the first time skin pass roll is used with a roll formed with a rectangular indentation with a side length (a). In the case that this steel sheet is provided with pond-like indentations 76 with a pitch ℓ and a width (b) spaced apart, by using a second skin pass roll 75 having the groove-like projections 74 with a pitch ℓ and a width (b), the above-mentioned equation (1) must be fulfilled. Fig. 22 indicates a case in which the steel sheet 71 obtained through the first skin pass rolling has pond-like indentations 76 spaced apart with a pitch ℓ and a width (b). As shown in Fig. 22, the spaced-apart pond-like indentations 76 with a pitch ℓ and a side (b) are applied to the steel sheet 71 through a first skin pass rolling and then a roll having spaced-apart pond-like indentations 77 with a side (a) so as to fulfill the above-mentioned equation (1) through the second skin pass rolling may also be used. Then, an appropriate size of these minimum surface pattern will be described. In order to prevent a trouble such as cracks during a press forming work, it is necessary to keep lubricant in the pond-like indentations. In order to prevent the galling of the die as described above, it is necessary to provide indentations to trap the worn powders generated during press working operation. The lubricant oil stored in the pond-like indentations 51 shown in Fig. 11 flows out to the flat part 53 so as to prevent a metallic contact between the flat part 53 and the tool. During this operation, it is assumed that the minimum film thickness of the lubricant oil required at its contacting surface is about 1 µm, and further it is assumed that surface areas of the pond-like indentations 51 and the flat part 53 are defined as Sa and Sb, a depth d (µm) of the pond-like indentations 1 can be expressed as follows, d ≧ (Sb/Sa) That is to say, when the area of the flat portion 53 is larger than that of the pond-like indentations 51, the depth of the pond-like indentations is deep and in turn in case that the former is opposite to the latter one, it may be shallow. Provided that if a value of Sb/Sa is too high, as described above in reference to the laser dulling roll process, there is apt to be a situation where a sufficient amount of lubricant oil is not supplied to the flat part 53 and in this case it may be expressed as (Sb/Sa) ≦ 5. A decreasing of the value (Sb/Sa) may result in an increasing of the space for keeping lubricant oil in the pond-like indentations and this is effective in performing a press forming work. However, in turn the area of the flat surface 53 is decreased and the sharpness of reflection is deteriorated. Provided that if the work passes through the coating process, the sharpness of reflection after coating is improved more under an effect of side grooves. In this case, it is assumed that this problem is understood as a wide one and even if the coating is not applied, a better sharpness of reflection is assured. According to this technical concept, a relation of (Sb/Sa) ≧ 0.5 is appropriate. So, the following relation of 0.5 ≦ (Sb/Sa) ≦ 5.0 is appropriate. As regards the depth (d), as described above, a relation of d ≧ (Sb/Sa) can be applied. However, if it is assumed that a trap function for iron powder caused by the galling of die is considered, a relation of d >2.0 (µm) is preferable. Provided that the above foregoing is a mere example, and in case that excessive drawing work is applied for a bath tub, for example, or when a part of which die galling becomes a quite problem, a relation of d >10.0 (µm) is also generated and in case of a low amount of formation, the value (d) is low and a case that a relation of (Sb/Sa) ≧ 10 is generated. As regards a shape of the pond-like indentations, a pattern shown in Fig. 11 is naturally one example of it and so it is not necessary to restrict it to a rectangular shape. Although the plane surface as described above can be accomplished by using the roll manufactured by an etching process of which one example is illustrated in Fig. 17, its depth is necessarily varied in response to a skin pass rolling condition. Now, the influence of the skin pass rolling rate will be described. As the skin pass reduction rate is increased, as shown in Fig. 12, the projections and indentations of the roll are deeply transferred to the steel sheet, with the result that a predetermined shape of projections and indentations of the steel sheet can easily be attained. If the rate of rolling is low, the rate of transfer is low and a predetermined shape may not be attained. Accordingly, it is preferable to have a rate more than 0.3 %. Provided that if the rate of reduction is increased, as shown in Fig.13, the mechanical characteristics of the steel sheet itself (a yield stress Ys and an elongation Eℓ) deteriorate and the press workability is deteriorated. That is, Ys is increased and Eℓ is decreased. With foregoing reasons, it is preferable to perform the skin pass rolling within the range of reduction rate shown in Fig. 12, i.e. within a reduction rate of 2.0 %. Now, the characteristics of the steel sheet of the preferred embodiment of the present invention will be described in reference to its example. One example of a result of the present invention in which a structure of projections and indentations of the present invention are applied to a low carbon aluminum killed steel sheet with a thickness of 0.6 mm is illustrated in Fig. 15. The roll at this time is one in which a cold rolling bright roll (3% Cr steel) is provided with projections and indentations opposite to those of Fig. 15 through the above-mentioned etching. Rz of this roll (a maximum difference in steps of projections and indentations) is 20 µm. As a result of rolling with a reduction rate of 0.6 to 1.5 %, a value of Rz of the steel sheet is 4 to 10 µm. A deterioration of the mechanical characteristic was also of a level having no problem at all. Each of the test results of a sliding test and a coating test by using this steel sheet is indicated in Figs. 19 and 20, respectively. A sliding resistance for showing the performance of the press forming characteristic is the same level as that of the pond-like indentations, showing a quite low sliding resistance under a preliminary experiment. Also as regards the sharpness of reflection, a result of the same level as that of the groove-like indentations could be attained. The thin steel sheet of the present invention has a structure of projections and indentations which is superior in sharpness of reflection after coating and it may easily be manufactured by using the rolling roll transferring the pattern of projections and indentations through a skin pass rolling process.

Claims: A process for forming a micro-pattern on the surface of a rolling roll, characterised in that the process comprises the steps of: (i) forming a film of acid corrosion-resistant material on the surface of the rolling roll by applying a mixture comprising an acid corrosion-resistant resin solution and a light absorbing agent comprising a pigment or carbon black, (ii) irradiating the material in a desired pattern with a YAG laser in combination with a Q-switch, thereby to remove the resin and to expose the surface of the rolling roll in the irradiated portions in accordance with the desired pattern, the laser being of insufficient power to melt the surface of the roll, (iii) etching the exposed portions with an etching solution, and (iv) removing the remaining resin by dissolution in a solvent. A process as claimed in claim 1 wherein the film has a thickness of from 1 to 10µm. An apparatus for carrying out the method of claim 1 or 2 comprising: (a) means for supporting and rotating the rolling roll, (b) means for coating the acid corrosion-resistant material onto the rolling roll, (c) a laser plotter comprising a laser oscillator for irradiating the coating in a pulse-like manner in response to an electrical signal, and means for guiding and focusing the laser onto the coating, (d) means for applying an etching solution to the surface of the rolling roll, (e) means for moving, independently or simultaneously, the laser plotter, the coating means and the etching means in the axial direction of the rolling roll. (f) an image processing device for providing a pattern to be formed on the roll surface, reducing the pattern to a plurality of dots to make a binary encoding and to make a series of plot data, and (g) a control means for controlling the pulse emitting time of the laser plotter in response the plot data and to a signal sent from a means for detecting the rotational angle of the rolling roll. An apparatus as claimed in claim 3 wherein: the laser plotter is provided with a YAG laser oscillator, a first optical modulator disposed between two resonant mirrors in the laser oscillator and a second optical modulator disposed in the midway of the laser beam path out of the laser oscillator; the roll rotating means is provided with a device, comprising a rotary encoder, for detecting the rotational angle of the rolling roll; the control device includes a first optical modulator driver for driving the first optical modulator in response to a clock signal from the rotary encoder and a second optical modulator driver for driving the second optical modulator in response to portions of the clock signal, provided with reference to the plot data, corresponding to dots for the required micro-pattern. An apparatus as claimed in claim 4 wherein the laser plotter is provided with a plurality of second optical modulators and is further provided with a spectroscope, disposed between the laser oscillator and the second optical modulators, adapted to divide the laser beam into a plurality of lines corresponding to the number of second optical modulators. A rolling roll produced by means of a process according to any preceding claim and comprising a surface pattern including flat topped independent trapezoid projections and/or flat topped containers net-like projections. A method of preparing a metallic sheet for press forming comprising preparing first and second rolling rolls by the method of any of claims 1 to 5 and having, respectively, a surface comprising flat topped independent trapezoid projections and flat topped continuous net-like projections and skin pass rolling the metallic sheet successively with the first and second rolls. A method as claimed in claim 7 wherein the rolling roll comprises flat topped continuous net like projections having an aperture larger than the sum of a width the a pitch of the independent trapezoid projections of the other rolling roll. A method according to claim 7 or 8 wherein the metallic sheet is skin pass rolled at a reduction rate of 0.3 to 2%. A metallic sheet prepared by the method of any of claims 7 to 9 comprising independent pond-like indentations arranged at specific locations and continuous net-like groove-like indentations enclosing the circumferences of the pond-like indentations. A metallic sheet prepared by the method of any of claims 7 to 9 comprising independent pond-like indentations arranged at first regions of the sheet surface and continuous net-like groove-like indentations arranged at second regions of the sheet surface.

Inventors: ABE HIDEO RESEARCH LABORATORIE, HIRA TAKAAKI RESEARCH LABORATO, IGUCHI TAKAAKI RESEARCH LABORA, ISOBE KUNIO RESEARCH LABORATOR, TAMARI TAKANORI RESEARCH LABOR, YARITA IKUO RESEARCH LABORATOR, ABE, HIDEO RESEARCH LABORATORIES, HIRA, TAKAAKI RESEARCH LABORATORIES, IGUCHI, TAKAAKI RESEARCH LABORATORIES, ISOBE, KUNIO RESEARCH LABORATORIES, TAMARI, TAKANORI RESEARCH LABORATORIES, YARITA, IKUO RESEARCH LABORATORIES
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1989
Main classification: C23F1
IPC codes: C23F1, B21B27, B23K26, B41C1, B21B1