処理中

しばらくお待ちください...

設定

設定

出願の表示

1. IN315/DELNP/2013 - ANTIOXIDANT AGENT PROCESS FOR PRODUCTION OF ANTIOXIDANT AGENT AND PROCESS FOR PRODUCING METALLIC MATERIAL

注意: このテキストは、OCR 処理によってテキスト化されたものです。法的な用途には PDF 版をご利用ください。

[ EN ]
ORIGINALDESCRIPTIONANTIOXIDANT AGENT, PROCESS FOR PRODUCING ANTIOXIDANT AGENT, AND PROCESS FOR PRODUCING METALLIC MATERIALTechnical Field [0001]The present invention relates to an antioxidant agent, a process for producing the antioxidant agent, and a process for producing a metallic material. More particularly, it relates to an antioxidant agent intended to be applied to the surface of a metallic material to be heated, a process for producing the antioxidant agent, and a process for producing a metallic material.Background Art [0002]JP2007-314780 (Patent Document 1) discloses a lubricant composition for hot extrusion working, and WO2007/122972 (Patent Document 2) discloses a lubricant composition for hot plastic working. The lubricant compositions disclosed in these Patent Documents contain a plurality of glass frits having different softening points, and are applied to the surface of a starting material to be subjected to hot plastic working.Disclosure of the Invention O- [0003]The lubricant compositions disclosed in Patent Documents 1 and 2 prevent oxides (hereinafter, referred to as scale) from being formed on the surface of a heated starting material to some extent. However, even if these lubricant compositions are used, scale is still produced on the surface of the heated starting material. [0004]An objective of the present invention is to provide an antioxidant agent for preventing the production of scale on the surface of a heated metallic starting material more effectively than the conventional antioxidant agents. [0005]The antioxidant agent according to the present invention contains a plurality of glass frits having different softening points and an inorganic compound having a melting point not higher than 600°C. The antioxidant agent according to the present invention is intended to be applied to the surface of a metallic starting material to be heated. [0006]For the antioxidant agent according to the present invention, the inorganic compound and the glass frits are softened in that order with the increase in temperature of the metallic starting material, and the softened inorganic compound and glass frits cover the surface of3 the metallic starting material. Therefore, the antioxidant agent according to the present invention prevents scale from being produced on the surface of the metallic starting material. [0007]Preferably, the inorganic compound is an inorganic salt and/or an oxide each having a melting point of 400°C to 600°C. Or, preferably, the inorganic compound is boric acid and/or boron oxide. [0008]Preferably, the plurality of glass frits contain high-temperature glass frits and medium-temperature glass frits. The viscosity at 1200°C of the high-temperature glass frits is 2 x 10^ to 10^ dPa-s. The viscosity at 700°C of the medium-temperature glass frits is 2 x 10^ to 10^ dPa-s. [0009]In this case, the high-temperature glass frits, the medium-temperature glass frits, and inorganic compound soften in different temperature ranges. Therefore, the antioxidant agent covers the surface of metallic starting material in a temperature range broader than that of the conventional antioxidant agent. For this reason, scale is less liable to be produced on the surface of metallic starting material. The "viscosity" in this description means so-called "static viscosity". [0010]^ Preferably, the antioxidant agent further contains an alkali metal salt. [0011]In this case, the secular change of viscosity of the antioxidant agent is restrained. [0012]Preferably, the antioxidant agent further contains an insoluble group 2 metal salt. Preferably, the insoluble group 2 metal salt is magnesium carbonate and/or calcium carbonate.In this case, the secular change of viscosity of antioxidant agent is restrained. [0013]The process for producing the antioxidant agent according to the present invention includes a step of producing a mixed composition by grinding and mixing the plurality of glass frits having different softening points, the boric acid and/or boron oxide, and water by using a grinding device, and a step of producing the antioxidant agent by mixing water having a temperature not higher than normal temperature with the mixed composition. [0014]In this case, the boric acid or boron oxide dissolved in water is less liable to crystallize. [0015]^ The process for producing a metallic material according to the present invention includes a step of applying the above-described antioxidant agent to the surface of a metallic starting material, and a step of heating the metallic starting material to which the lubricant composition has been applied. The "heating" in this description includes heating for heat-treating (quenching, tempering, etc.) the metallic starting material, and heating for hot-working the metallic starting material. The hot working includes hot extrusion working, hot piercing rolling, hot rolling, and hot forging. [0016]In this case, scale is less liable to be produced on the heated metallic starting material.Brief Description of the Drawings [0017]Figure 1 is a diagram showing the relationship between the viscosity and temperature of a component contained in an antioxidant agent according to an embodiment of the present invention;Figure 2 is a flowchart showing one example of a process for producing the antioxidant agent according to an embodiment of the present invention;6 Figure 3 is a flowchart showing one example of a process for producing a metallic material according to an embodiment of the present invention;Figure 4 is a chart showing cross-sectional images of specimens in Example 1;Figure 5A is a photograph showing a cross-sectional image of a specimen to which an antioxidant agent is applied in Example 2;Figure 5B is a photograph showing a cross-sectional image of a round billet to which an antioxidant agent different from that in Figure 5A is applied;Figure 6 is a diagram showing the relationship between the viscosity of a specimen and the content of potassium carbonate in an antioxidant agent in Example 3; andFigure 7 is a diagram showing secular changes of viscosities of specimens in Example 4.Best Mode for Carrying Out the Invention [0018]Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the figures, the same symbols are applied to the same or equivalent parts, and the explanation of the parts is not repeated. [0019]7 The present inventors examined the cause for some amount of scale produced on the surface of a heated metallic starting material even if a lubricant composition disclosed in Patent Document 1 or 2 is used. As the result of examination, the present inventors obtained the findings described below. [0020](1) Not only in the medium-temperature range andhigh-temperature range in which the heating temperatureis higher than 600°C but also in the low-temperaturerange in which the heating temperature is not higher than600°C, scale is produced on the surface of the metallic starting material. Hereinafter, the temperature range ofnot higher than 600°C is referred to as a "low-temperature range". [0021](2) The lubricant compositions disclosed in PatentDocuments 1 and 2 contain the plurality of glass fritshaving different softening points. The plurality ofglass frits soften in the medium-temperature range andhigh-temperature range, and cover the metal surface. Inthe low-temperature range, however, the glass frits areless liable to soften. Therefore, in the low-temperaturerange, in some cases, the surface of the metallicstarting material is not protected sufficiently by thelubricant composition, and the surface is partiallyexposed. Since the exposed portion comes into contactg with the outside air, the exposed portion oxidizes, andscale is liable to be produced.[0022](3) The inorganic compound having a melting pointnot higher than 600°C softens in the low-temperaturerange, and covers the metal surface. Therefore, in thelow-temperature range, scale can be prevented from beingproduced on the surface of metallic starting material.If the antioxidant agent contains the plurality of glassfrits having different softening points and the inorganiccompound having a melting point not higher than 600°C,the antioxidant agent softens in a broad temperaturerange of low-, medium-, and high-temperature ranges, andcovers the surface of metallic starting material. Forthis reason, scale can be prevented from being producedon the surface of metallic starting material.[0023](4) In the case where the antioxidant agent isslurry at normal temperature, and contains the pluralityof glass frits having different softening points and theabove-described inorganic compound, the viscosity ofantioxidant agent at normal temperature sometimes changeswith time. If the antioxidant agent at normaltemperature contains an alkali metal salt, the viscosityof antioxidant agent is prevented from changing with time,[0024]^ (5) If the antioxidant agent contains an insoluble group 2 metal salt, the long-term secular change of viscosity of antioxidant agent is prevented. Herein, the group 2 metal salt is a salt of group 2 metal in the periodic table. Also, "insoluble" means insoluble in water, and "insoluble in water" means that the solubility in water of 25°C is not higher than 1000 ppm. [0025]The antioxidant agent according to this embodiment is based on the above-described findings. Hereunder, the details of the antioxidant agent are explained. [0026] [Constitution of antioxidant agent]The antioxidant agent according to this embodiment contains the plurality of glass frits having different softening points and the inorganic compound having a melting point not higher than 600°C. Hereinafter, the inorganic compound having a melting point not higher than 600°C is referred to as a "low-temperature inorganic compound". The details of the glass frits and inorganic compound are explained. [0027] [Glass frits]The plurality of glass frits are produced by the process described below. A plurality of well-known inorganic components constituting glass are mixed with each other. The mixed plurality of inorganic componentsfO are melted to produce molten glass. The molten glass is rapidly cooled in water or air and is solidified. The solidified glass is ground as necessary. The glass frits are produced by the steps described above. [0028]The glass frits are of a flake form or a powder form. As described above, the glass frits contain the plurality of well-known inorganic components. Therefore, the melting point of glass frits is not identified definitely. In the case where each of inorganic components in the glass frits is heated singly, each inorganic component liquefies at its melting point. However, in the case of glass frits, as the temperature rises, the inorganic components in the glass frits begin to liquefy at temperatures different from each other. For this reason, with the increase in temperature, the glass frits soften gradually. Therefore, as compared with the case where the inorganic components are used singly as an antioxidant agent, the glass frits produced by melting the plurality of inorganic components are liable to adhere stably to the surface of the heated metallic starting material. The glass frits can be regulated so as to have a viscosity suitable for coating the surface of metallic starting material. [0029]The plurality of glass frits contain high-temperature glass frits and medium-temperature glass frits. The high-temperature glass frits have a softeningpoint higher than that of the medium-temperature glassfrits. Hereunder, the details of the high-temperatureglass frits and medium-temperature glass frits areexplained.[0030][High-temperature glass frits]The high-temperature glass frits have a high softening point. The antioxidant agent has a proper viscosity in a high-temperature range of not lower than 1000°C on account of the plurality of high-temperature glass frits. The antioxidant agent can wettingly spread on the surface of metallic starting material, and can cover the metal surface at the high-temperature range ofnot larger than 1000°C. At this time, the antioxidant agent adheres to the surface of metallic starting material. [0031]In effect, due to the high-temperature glass frits, the antioxidant agent prevents the surface of metallic starting material from coming into contact with the outside air in the high-temperature range. Therefore, the antioxidant agent can prevent scale from being produced on the surface of metallic starting material in the high-temperature range. [0032]12, -X-If the antioxidant agent does not contain high-temperature glass frits, in the high-temperature range, the viscosity of the antioxidant agent becomes too low. Therefore, the antioxidant agent becomes less liable to adhere stably to the surface of metallic starting material, and becomes liable to flow down from the surface. If the antioxidant agent flows down, the surface of metallic starting material is partially exposed. The exposed surface portion comes into contact with the outside air, and scale is produced. [0033]The preferable viscosity at 1200°C of the high-temperature glass frits is 2 x 10^ to 10^ dPa-s. If the viscosity at 1200°C of the high-temperature glass frits is too low, in the high-temperature range, the antioxidant agent is less liable to adhere to the surface of metallic starting material, and is liable to flow down from the surface of metallic starting material. On the other hand, if the viscosity at 1200°C of the high-temperature glass frits is too high, in the high-temperature range, the antioxidant agent is liable to peel off the surface of metallic starting material. If the viscosity at 1200°C of the high-temperature glass frits is 2 X 10^ to 10^ dPa-s, in a high-temperature range of 1000 to 1400°C, the high-temperature glass frits soften, and becomes liable to adhere to the surface of metallic starting material. Therefore, in the high-li -X-temperature range, the antioxidant agent is liable to cover the surface of metallic starting material, and is liable to adhere stably to the surface of metallic starting material. The preferable upper limit of the viscosity at 1200°C of the high-temperature glass frits is 10^ dPa-s, and the preferable lower limit thereof is 10^ dPa-s. [0034]In the case where the high-temperature glass frits are of a spherical powder form, the preferable particle diameter is not larger than 25 ^m. The particle diameter herein is a volume mean particle diameter D50. The volume mean particle diameter D50 is determined by the method described below. By using a particle counter, the volume particle size distribution of the high-temperature glass frits is determined. By using the obtained volume particle size distribution, the particle diameter at which the cumulative volume becomes 50% from the small particle diameter side in a cumulative volume distribution is defined as a volume mean particle diameter D50. [0035]If the particle diameter is not larger than 25 \xra, at normal temperature, the high-temperature glass frits are liable to disperse in a liquid. [0036]1^ As described above, the high-temperature glass frits contain the plurality of well-known inorganic components. For example, the high-temperature glass frits contain 60 to 70 mass% of silicon dioxide (SiOa) , 5 to 20 mass% of aluminum oxide (AI2O3) , and 0 to 20 mass% of calcium oxide (CaO). Calcium oxide is an optional compound, and need not be contained. Further, the high-temperature glass frits contain one or more kinds of magnesium oxide (MgO), zinc oxide (ZnO), and potassium oxide (K2O). The inorganic components constituting the high-temperature glass frits are not limited to the above-described examples. In effect, the high-temperature glass frits can be produced by the well-known inorganic components constituting the glass. [0037] [Medium-temperature glass frits]The medium-temperature glass frits have a softening point lower than that of the high-temperature glass frits, The antioxidant agent has a proper viscosity in a medium-temperature range of 600 to 1000°C on account of medium-temperature glass frits. Therefore, the antioxidant agent wettingly spreads on the whole surface of the metallic starting material not only in the high-temperature range but also in the medium-temperature range, and covers the surface. Further, in the medium-temperature range, the antioxidant agent adheres stably to the surface of metallic starting material. Therefore,15- in the medium-temperature range, the surface of metallic starting material is prevented from coming into contact with the outside air, and the production of scale is prevented. [0038]If the antioxidant agent does not contain the medium-temperature glass frits, the antioxidant agent in the medium-temperature range is less liable to adhere to the surface of metallic starting material. Therefore, in the medium-temperature range, the antioxidant agent flows down from the surface of metallic starting material, or peels off, and thereby the surface of metallic starting material is partially exposed. The exposed portion comes into contact with the outside air, and scale is liable to be produced. [0039]The preferable viscosity at 700°C of the medium-temperature glass-frits is 2 x 10^ to 10^ dPa-s. If the viscosity of the medium-temperature glass-frits is too low, in the medium-temperature range, the antioxidant agent is less liable to adhere to the surface of metallic starting material, and is liable to run down from the surface of metallic starting material. On the other hand, if the viscosity of the medium-temperature glass-frits is too high, the antioxidant agent does not soften sufficiently in the medium-temperature range. Therefore, the antioxidant agent becomes liable to peel off theiJb surface of metallic starting material. If the viscosity at 700°C of the medium-temperature glass-frits is 2 x 10^ to 10^ dPa-s, in the medium-temperature range of 600 to 1000°C, the medium-temperature glass-frits soften, and becomes liable to adhere to the surface of metallic starting material. Therefore, in the medium-temperature range, the antioxidant agent becomes liable to cover the surface of metallic starting material. The preferable upper limit of the viscosity at 700°C of the medium-temperature glass frits is 10^ dPa-s, and the preferable lower limit thereof is 10^ dPa-s. [0040]In the case where the medium-temperature glass frits are of a spherical powder form, the preferable particle diameter of the medium-temperature glass frits is not larger than 25 |Jin. The definition of the particle diameter of the medium-temperature glass frits is the same as that of the above-described particle diameter of the high-temperature glass frits. That is, the particle diameter of the medium-temperature glass frits is a volume mean particle diameter D50. If the particle diameter is not larger than 25 \xm, the medium-temperature glass frits disperse stably in a liquid. Therefore, when the antioxidant agent is applied to the surface of metallic starting material, the medium-temperature glass frits are liable to disperse substantially uniformly to the whole surface of the metallic starting material.7 [0041]For example, the medium-temperature glass frits contain 40 to 60 mass% of Si02, 0 to 10 mass% of AI2O3, 20 to 40 mass% of B2O3, 0 to 10 mass% of ZnO, and 5 to 15 mass% of Na20, Further, the medium-temperature glass frits may contain one or more kinds of MgO, CaO, and K2O. The inorganic components constituting the medium-temperature glass frits are not limited to the above-described examples. The medium-temperature glass frits can be produced by the well-known inorganic components constituting the glass. [0042] [Low-temperature inorganic compound]The antioxidant agent according to this embodiment further contains the inorganic compound having a meltingpoint not higher than 600°C (low-temperature inorganic compound). The low-temperature inorganic compound preferably has a melting point of 400 to 600°C. On account of the low-temperature inorganic compound, the antioxidant agent wettingly spreads on the whole surface of the metallic starting material in the low-temperature range of not higher than 600°C, and is liable to adhere to the surface of metallic starting material. That is, in the low-temperature range, the low-temperature inorganic compound prevents the surface of metallic starting material from coming into contact with theI& outside air, and prevents scale from being produced inthe low-temperature range.[0043]If the antioxidant agent does not contain the low-temperature inorganic compound, in the low-temperature range, the antioxidant agent does not wettingly spread sufficiently on the surface of metallic starting material, Therefore, the surface of metallic starting material comes into partial contact with the outside air, and scale is produced in the portion contacting with the outside air. [0044]The preferable low-temperature inorganic compound is an inorganic salt and/or an oxide having a melting point of 400 to 600°C. The oxide having a melting point not higher than 600°C is, for example, boric acid (H3BO3) or boron oxide (B2O3) . If being heated, boric acid turns to boron oxide. The melting point of boron oxide is about 450°C. The inorganic salt having a melting point not higher than 600°C is, for example, phosphate, thallium bromide (TlBr), or silver metaphosphate (AgOsP) . Themelting point of thallium bromide is about 480°C, and the melting point of silver metaphosphate is about 480°C. Further preferably, the low-temperature inorganic compound is boric acid and/or boron oxide. [0045]!<] [Relationship between viscosities of high-temperature and medium-temperature glass frits and viscosity of low-temperature inorganic compound]Figure 1 is a diagram showing the relationship between the viscosities of high-temperature and medium-temperature glass frits and the viscosity of low-temperature inorganic compound. Figure 1 was obtained by the process described below. High-temperature glass frits HTl and HT2, medium-temperature glass frits LTl and LT2, and low-temperature inorganic compound LL given in Table 1 were prepared.[Table 1]Table 1 Chemical composition (wt%) Si02 AI2O3 B2O3 CaO MgO ZnO Na20 K2OHigh-temperature glass frits HTl 66.1 9.6 - 13.1 1.6 3.0 - 6.4Medium-temperature glass frits LTl 51.7 2.6 28.4 0.2 - 6.3 8.7 2.0High-temperature glass frits HT2 65-70 5-10 1-3 10-15 0-3 - - 5-10Medium-temperature glass frits LT2 50-55 0-5 20-25 5-10 0-3 - 10-15 0-5Low-temperature inorganic compound LL - - 100 - - - - -[0046]Referring to Table 1, the low-temperature inorganic compound LL was boron oxide. By heating the components (HTl, HT2, LTl, LT2, and LL), the viscosities at respective temperatures were measured. For theao measurement of viscosity, the well-known platinum ball pulling-up method was used. Specifically, a platinum ball submerged in molten glass was pulled up. Based on the load applied to the platinum ball at this time and the pulling-up speed, the viscosity of molten glass was determined. [0047]Referring to Figure 1, the symbol "•" in the figure denotes the viscosity of the high-temperature glass frits HTl. The symbol "O" denotes the viscosity of the high-temperature glass frits HT2. The symbol "■" denotes the viscosity of the medium-temperature glass frits LTl. The symbol "D" denotes the viscosity of the medium-temperature glass frits LT2. The symbol "A" denotes the viscosity of the viscosity of the low-temperature inorganic compound LL. [0048]Referring to Figure 1, the viscosity of the low-temperature inorganic compound LL was 2 x 10^ to 10^ dPa-s in the temperature range of 400 to 800°C, and was not lower than 10'^ dPa-s in the temperature range of not higher than 600°C. The viscosities of the medium-temperature glass frits LTl and LT2 were 2 x 10^ to 10^ dPa-s in the temperature range of 600 to 1200°C. That is, at 700°C, the viscosities of the medium-temperature glass frits LTl and LT2 were in the range of 2 x 10^ to 10^ dPa-s. The viscosities of the high-temperature glass■21 frits HTl and HT2 were 2 x 10^ to 10^ dPa-s in the temperature range of 1000 to 1550°C. That is, at 1200°C, the viscosities of the high-temperature glass frits HTl and HT2 were in the range of 2 x 10^ to 10^ dPa-s. [0049]As described above, with the increase in temperature, the viscosity lowers in the order of low-temperature inorganic compound, medium-temperature glass frits, and high-temperature glass frits, and softening occurs. The antioxidant agent contains the high-temperature glass frits, the medium-temperature glass frits, and the low-temperature inorganic compound. Therefore, the antioxidant agent is capable of having a viscosity of a degree such as to be able to adhere stably to the surface of metallic starting material in a broad temperaturerange (400 to 1550°C).[0050][Other constitutions of antioxidant agent]As described above, the antioxidant agent contains the high-temperature glass frits, the medium-temperature glass frits, and the low-temperature inorganic compound. Further, the antioxidant agent may contain one or more kinds of water, a suspending agent, an antislipping agent (friction coefficient increasing agent), and a gluing agent. [0051] [Water]a^ Water is mixed with the high-temperature glass frits, the medium-temperature glass frits, and the low-temperature inorganic compound to produce slurry. If water is mixed, the antioxidant agent turns to slurry. Therefore, the antioxidant agent is liable to be applied substantially uniformly to the surface of metallic starting material before being heated. [0052] [Suspending agent]The suspending agent causes the high-temperature and medium-temperature glass frits and the like to disperse substantially uniformly in a solution (water). The suspending agent is, for example, clay. The clay is less liable to generate gas even if being heated. Further, the clay is not destroyed by fire. Therefore, the clay prevents the glass frits (the high-temperature and medium-temperature glass frits) from coming off the surface of metallic starting material. [0053]The clay contains, for example, 50 to 60 mass% of Si02 and 10 to 40 mass% of AI2O3, and further contains, as other minor components, one or more kinds selected from a group consisting of Fe203, CaO, MgO, Na20, and K2O. [0054]One example of clay contains about 55 mass% of Si02, and about 30 mass% of AI2O3, and Fe203, CaO, MgO, Na20, K2O, and the like as other minor components. Another example■23 of clay contains about 60 mass% of Si02 and about 15 mass% of AI2O3, and FeaOs, CaO, MgO, Na20, K2O, and the like as other minor components. [0055]As described above, the suspending agent causes the glass frits to disperse substantially uniformly in a solution. Therefore, when the antioxidant agent is applied to the surface of metallic starting material before being heated, the glass frits disperse substantially uniformly to the surface of metallic starting material. Further, the suspending agent causes the applied glass frits to bond to the surface of metallic starting material, and prevents the glass frits from coming off the surface of metallic starting material, [0056] [Antislipping agent]The heated metallic starting material is sometimes hot-worked. In this case, the metallic starting material is rolled by a rolling roll to produce a metal plate or a metal bar. Also, the metallic starting material is piercing-rolled by the plug and inclined rolls of a piercing machine to produce a metal pipe. Therefore, the metallic starting material is preferably liable to be caught by the rolling rolls or the inclined rolls. If the friction coefficient of metallic starting material against a hot-working roll such as the rolling roll and^^ the inclined roll is high, the metallic starting materialis liable to be caught by the hot-working rolls.[0057]Therefore, the antioxidant agent may contain the antislipping agent to increase the friction coefficient. The antislipping agent is, for example, an oxide having a high melting point. The antislipping agent is, for example, alumina or silica. When the metallic starting material to which the antioxidant agent is applied comes into contact with the rolls, the antislipping agent such as alumina or silica comes into contact with the rolls. At this time, since the friction coefficient of metallic starting material against the rolls becomes high, the metallic starting material becomes liable to be caught by the rolls. [0058] [Gluing agent]The antioxidant agent may further contain the gluing agent to improve the adhering force to the surface of metallic starting material. The gluing agent is, for example, an organic binder. The organic binder is, for example, an acrylic resin. [0059]Further, the antioxidant agent may contain an alkali metal salt or an insoluble (that is, insoluble in water) group 2 metal salt. These components prevent the^r viscosity of the antioxidant agent from changing withtime.[0060][Alkali metal salt]As described above, the antioxidant agent containing water is slurry (a fluid) at normal temperature. In the case where the antioxidant agent contains less than 50 wt% of water, at normal temperature, the antioxidant agent sometimes sets to gel with the elapse of time. The gelation increases the viscosity of the antioxidant agent, Also, gel lumps are sometimes produced. [0061]It is preferable that the secular change of viscosity of the antioxidant agent be restrained. The alkali metal salt peptizes the antioxidant agent having set to gel. Therefore, the antioxidant agent fluidizes again, and the increase in viscosity is prevented. The alkali metal salt is, for example, potassium carbonate (KCO3) , sodium hexametaphosphate, or the like. [0062] [Insoluble group 2 metal salt]In the case where the antioxidant agent contains not less than 55 wt% of water, at normal temperature, the viscosity of antioxidant agent sometimes decreases with the elapse of time. Such a secular change of viscosity is preferably restrained. [0063]26 - Z