LUBRICANT COMPOSITION FOR HOT-ROLLING OF STEEL

Abstract: The title lubricant composition comprises a base oil or base grease and a specified amount of a heat insulating agent comprising (A) an inorganic compound undergoing endothermic melting at temperatures of 1200°C or below and (B) an inorganic powder which neither melts nor decomposes at temperatures of 1200°C or below and which has a thermal conductiviry of 0.01 cal/cm.s.°C or less and a friction coefficient of 0.7 or less at room temperature added alone or as a combination. This lubricant composition has an excellent heat-insulating effect and is effective in preventing occurrence of thermal crown of working rolls.

Description: TECHNICAL FIELD The present invention relates to a lubricating composition for hot-rolling steel. More particularly, the present invention relates to a lubricating composition by which a transfer of heat to a work roll from a material to be rolled is prevented and the effect of reducing the thermal crown of the work roll is attained by incorporating a specific heat-insulating agent into a base oil or base grease. BACKGROUND ART In the conventional hot-rolling method, only roll-cooling water is used for protecting a roll, but now a rolling oil is used for reducing the rolling load and decreasing wear of the roll, and an excellent effect is attached thereby. The main object of the conventional lubricant for hot rolling is to reduce wear of a work roll and improve the roll surface, because the requirement for the quality of a rolled product is relatively moderate and the thermal crown of the work roll is not regarded as important. Nevertheless, recently, an increased of the quality of the product has been demanded, and the effect of reducing the thermal crown of the work roll, which has an direct adverse influence on the quality of the product, has become important. EP-A-135,932 discloses a lubrication composition for metal forming, which comprises a condensed phosphoric acid; a sulfur- or chlorine-containing organic compound as extreme pressure additive; and a solid lubricant such as boron nitride as an additive when the mold temperature is 300°C or higher. US-A-4,107,058 discloses a composition containing a lithium soap grease and an insoluble phosphorus composition such a tricalcium phosphate. DISCLOSURE OF THE INVENTION The present invention relates to a lubricant composition for hot-rolling steel, which is characterized in that a heat-insulating agent is incorporated into a base oil or base grease. More specifically, it was found that if two specific kinds of heat-insulating agents, i.e., (A) an inorganic compound which is melted by an absorption of heat at a temperature lower than 1200°C, and (B) an inorganic powder, selected from silicon dioxide or silicon nitride, or powder of amorphous carbon, which is not melted or decomposed at a temperature lower than 1200°C and has a heat conductivity lower than 0.01 cal/cm.s.°C at room temperature and a friction coefficient smaller than 0.7, are incorporated in combination in specific amounts in the composition, and excellent heat-insulating effect can be attained and the thermal crown of a work roll can be effectively prevented. The present invention was completed based on this finding. According to the present invention a lubricating composition for hot-rolling steel, which consists essentially of a base oil or base grease; a combination of heat-insulating agents (A) and (B), the heat-insulating agent (A) being an inorganic compound which is melted by an adsorption of heat at a temperature lower than 1200°C, the heat-insulating agent (B) being an inorganic powder, selected from silicon dioxide or silicon nitride, or powder of amorphous carbon, which is not melted or decomposed at a temperature lower than 1200°C and has a heat conductivity lower than 0.01 cal/cm. s. °C at room temperature and a friction coefficient smaller than 0.7; an extreme pressure additive and a solid lubricant, said combination of heat-insulating agents (A) and (B) being selected from the group consisting of a condensed phosphoric acid salt and silicon nitride; a condensed phosphoric acid salt and amorphous carbon; a condensed phosphoric acid salt and silicon dioxide; sodium chloride and silicon nitride; and sodium silicate and silicon nitride, said extreme pressure additive being selected from the group consisting of zinc dialkyldithiophosphate, tricresyl phosphate and sulfurized lard, said solid lubricant being selected from the group consisting of graphite, molybdenum disulfide, mica and MCA, said heat-insulating agents (A) and (B) being contained at a weight ratio of from 49/1 to 1/49 and in a total amount of 5 to 50% by weight of the composition. When the components (A) and (B) are incorporated in combination, a highest effect can be attained due to the synergistic action of the two components. In the above-mentioned lubricating composition for hot-rolling, by incorporating an extreme pressure additive and/or a solid lubricant together with the heat-insulating agents (A) and (B), the lubricating property of the base oil or grease can be further improved, and the lubricating property, heat-insulating property, storage stability, working property, and water washing resistance of the base grease can be further improved. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a diagram illustrating the method of measuring the contact heat transfer ratio between metals. BEST MODE OF CARRYING OUT THE INVENTION The surface temperature of a work roll is elevated to about 800°C by contact with a material to be rolled. Most of the conventional lubricants for hot rolling comprise a mineral oil, an oiliness agent, an extreme pressure additive, and a solid lubricating agent, in combination, and although the lubricating property is taken into consideration, an insulation of heat (prevention of transfer of heat to the work roll from the material to be rolled) is not considered. Japanese Unexamined Patent Publication No. 60-6211 teaches that a roll can be protected by adding a fine powder of an inorganic compound having a melting point lower than 1200°C under atmospheric pressure, an average particle size smaller than 1 µm, and no corrosive action on iron and steel including cast iron and cast steel and other metals, and acting as a substance having a poor heat conductivity to a commercially available hot-rolling oil (liquid). The base oil disclosed in this patent publication is a commercially available hot-rolling oil (liquid) and is different from the base grease used in the second embodiment of the present invention. Furthermore, the powder used in the invention of the above-mentioned patent publication is a powder of an inorganic compound which melts at a temperature lower than 1200°C, and the heat transfer-preventing effect is drastically reduced after melting. In the present invention an inorganic compound melting at a temperature lower than 1200°C and an inorganic powder not melting or decomposing at a temperature lower than 1200°C are used in combination, The heat-insulating agent used in the present invention includes (A) an inorganic compound which is melted by an absorption of heat at a temperature lower than 1200°C and (B) an inorganic powder which is not melted or decomposed at a temperature lower than 1200°C, is stable against oxidation and has a heat conductivity lower than 0.01 cal/cm·s·°C at room temperature (20°C) and a friction coefficient smaller than 0.7. The inorganic compound (A) which is melted by an absorption of heat at a temperature lower than 1200°C is at least one of condensed phosphoric acid salts such as (KPO₃) n , (NaPO₃) n and K₄P₂O₇ , sodium silicate, chromic acid salts such as K₂Cr₂O₇⁻, and halides such as NaCl, KCl, KF, KBr and KI. Condensed phosphoric acid salts and sodium silicate, which have no corrosive action on a rolling mill or a material to be rolled, are especially preferable. The inorganic powder (B), which is not melted or decomposed at a temperature lower than 1200°C, is stable against oxidation and has a heat conductivity lower than 0.01 cal/cm·s·°C at room temperature and a friction coefficient smaller than 0.7, is at least one of silicon nitride, amorphous carbon, K₃PO₄ , bentonite, SiO₂ and ZnO. The friction coefficient referred to herein is determined by the pin-on-disk method (a rod having a diameter of 3 mm and a flat top end is pressed under a load of 1 kgf against a disk having a diameter of 11 mm, and the disk is slid at a speed of 0.01 m/s). A heat-insulating agent having an average particle size smaller than 50 µm can be used, but in view of the clearance between the roll and the material to be rolled, preferably the average particle size of the heat-insulating agent is smaller than 10 µm. The reason why better results are obtained when the inorganic compound (A), which is melted by an absorption of heat at a temperature lower than 1200°C and the inorganic powder which is not melted or decomposed at a temperature lower than 1200°C and has a heat conductivity lower than 0.01 cal/cm·s·°C and a friction coefficient smaller than 0.7 are used in combination, has not been completely elucidated, but it is considered that the reason is probably as follows. At the rolling step, the temperature and pressure become high in a roll bite (higher than 600°C and higher than 2000 kgf/cm²). At this point, the heat-insulating agent (A) is promptly melted by absorption of heat and prevents heat from being transferred to the work roll from the material to be rolled. It is known that the heat conductivity of a liquid is, in general, increased more than that of a powder. Accordingly, it is considered that the heat transfer-reducing effect of the heat-insulating agent (A) is abruptly decreased by melting. On the other hand, since the heat-insulating agent (B) is not melted or decomposed even under high-temperature and high-pressure conditions, the heat-insulating agent (B) is present in the form of a powder in the roll bite and prevents the work roll from falling in contact with the material to be rolled, and it is considered that since the powder per se has a lubricating property, a generation of heat by friction in the roll bite is reduced by the powder of the heat-insulating agent (B). Namely, although the heat-insulating agent (A) has an excellent heat-insulating property, when the heat-insulating agent (A) is melted at a high temperature, the heat-insulating property is drastically reduced. On the other hand, since the heat-insulating agent (B) is not melted or decomposed at a temperature lower than 1200°C, the heat-insulating agent (B) has a heat-insulating property over a broad temperature range. Accordingly, if the heat-insulating agent (A) and the heat-insulating agent (B) are made present at a specific ratio, a lubricating agent having the excellent effects of both heat-insulating agents (A) and (B) can be obtained. Preferably, the heat-insulating agent is added in an amount of 5 to 50% by weight, especially 10 to 40% by weight. If the amount of the heat-insulating agent is smaller than 5% by weight, the heat-insulating effect is too low, and if the amount of the heat-insulating agent is larger than 50% by weight, the viscosity of the lubricant becomes too high and the oil-supplying property is degraded. Preferably, the ratio of the heat-insulating agent (A) to the heat-insulating agent (B) is in the range of 49/1 to 1/49, especially 19/1 to 1/4. This is because, if the proportion of the heat-insulating agent (A) is reduced, the heat-insulating property is lowered by an absorption of heat in the roll bite, and if the proportion of the heat-insulating agent (B) is reduced, the heat-insulating property at high temperature is lowered. As the base oil that can be used in the present invention, there can be mentioned medium and heavy mineral oils such as spindle oil, machine oil, dynamo oil, motor oil, cylinder oil and bright stock, animal and vegetable oils such as beef tallow, lard, sperm oil, palm oil, coconuts oil, linseed oil, rice bran oil and soybean oil, synthetic oils such as esters of fatty acids having 8 to 22 carbon atoms with monohydric or polyhydric alcohols, α-olefins, polybutene, silicone oils and fluorine oils, and mixtures of these oils. As the base grease that can be used in the present invention, there can be mentioned lithium soap grease, calcium soap grease, sodium soap grease, aluminum soap grease, calcium complex grease, polyurea grease and organo-clay grease. Lithium soap grease, calcium complex grease, polyurea grease and organo-clay grease, which have an excellent heat resistance, are preferable. As the solid lubricant that can be used in the present invention, there can be mentioned inorganic solid lubricants such as graphite (natural graphite and artificial graphite), molybdenum disulfide, mica (natural mica and artificial mica), fluorinated graphite, boron nitride, soft metals (such as gold, silver and copper) and talc, and organic solid lubricants such as PTFE (polytetrafluoroethylene), MCA (melamine/cyanuric acid adduct) and phthalocyanine. Graphite (natural graphite and artificial graphite), mica (natural mica and artificial mica), boron nitride and talc, which have an excellent heat resistance and oxidation stability at a high temperature and have no substantial influence on a material to be rolled, are preferably. Preferably the amount added of the solid lubricant is 0 to 40% by weight, especially 5 to 15% by weight. If the amount added of the solid lubricant exceeds 40% by weight, the viscosity of the lubricant becomes too high and the oil-supplying property is reduced. As the extreme pressure additive that can be used in the present invention there can be mentioned sulfur compounds, phosphorus compounds, chlorine compounds and organic metal compounds. Preferably the amount added of the extreme pressure additive is 0 to 20% by weight, especially 0.5 to 10% by weight. If the amount added of the extreme pressure additive exceeds 20% by weight, undesired side effects such as an appearance of a corrosive action and reduction of the stability of the micell structure of the grease occur. The present invention will now be described in detail with reference to the following examples and comparative examples. Examples 1 through 16 and Comparative Examples 1 through 11 A base oil, a base grease, a heat-insulating agent, an extreme pressure additive, and a solid lubricant were mixed at a mixing ratio shown in Table 1, whereby lubricating agents of Examples 1 through 16 and Comparative Examples 1 through 11 were prepared. With respect to each of the so-obtained compositions, the performances were evaluated according to the test methods described below. The results are shown in Table 1. A) Lubricating Property Test by Hot Lubricating Property Tester Model E-12 According to the principle of the hot lubricating property tester Model E-12, both ends of a test piece were fixed and the test piece induction-heated gripped between rolls while supplying an oil to the test piece, and a slip lubrication effected. The friction coefficient and seizure resistance of each lubricant were examined to evaluate the lubricating property. Friction coefficient µ = T/R·W in which T represents a shaft torque, R represents a roll radius, and W represents a load. The outlines of the tester and the test conditions are as follows. a) Type: lubricity tester of two-high type for slip lubricating b) Roll dimension: 124 mm (diameter ) x 80 mm (length) c) Roll material: high chromium roll (Hs = 70 - 75) d) Test piece material: SS-41 [20 mm (height) x 20 mm (width) x 580 mm (length)] e) Test piece temperature: 400°C, 600°C and 800°C (automatically adjusted) f) Revolution: 200 rpm g) Rolling load: 500 to 3000 kgf (the load is increased by 500 kgf at every time) h) Method of supplying lubricant: applying B) Rust Prevention Test a) The test piece used at the test (A) was cut to a size of 20 mm x 20 mm x 100 mm. b) The test piece prepared at a) above has hung under the eaves and allowed to stand for 2 weeks, and the state of rusting was checked. o: no rusting x: extreme rusting C) Water Washing Resistance Test a) A defatted and weighed steel sheet (SPCC-SD 100 mm x 100 mm x 0.8 mm) was uniformly coated with 30 ± 3 mg of the lubricant. b) The steel sheet prepared at a) above was washed with water under the following conditions, and the weight was measured after the water washing and the residual oil ratio is determined. a) Nozzle model number: 1/4 KBF 0865 b) Extrusion rate: 6.4 ℓ/min (extrusion pressure = 2.0 kgf/cm²) c) Water washing time: 5 seconds (water temperature = 25°C) d) Distance between steel sheet and nozzle: 200 mm Residual oil or grease ratio (%) = [(amount of residual oil or grease)/(amount coated of oil or grease)] x 100 D) Measurement of Contact Heat Transfer Ratio between Metals a) Material of test piece: WT-60 [25 mm (diameter) x 50 mm (length)] b) Temperature: 780°C (high-temperature material), 22 to 30°C (low-temperature material) c) Thermocouple: CA (0.5 mm) sheath (attachment position = 1.5 mm, 3.0 mm) d) Heat-insulating material: kao wool e) Compressive force: 500 kgf/cm² f) Method of filling sample and thickness: As shown in Fig. 1, a high-temperature material 3 was pressed against a low-temperature material 2 coated with a sample 1, and the contact interface temperature of each sample and the heat flow flux were reckoned backward from the change of the temperatures of both materials with a lapse of time after the contact. The cooling law of Newton was applied in an extended manner to determine the heat transfer coefficient between the metals. The obtained coefficient was compared with the coefficient obtained when the sample is not coated, and the heat transfer ratio determined. Note, in Fig. 1, reference numeral 4 represents a heat-insulating material and reference numeral 5 represents a thermocouple. E) Roll Wearing Quantity Ratio in Actual Rolling Mill Eight air spray nozzles (20 mℓ/min·nozzle) were attached to a work roll on the inlet side of F5 stand (6-stand mill), and about 300 metric tons of an ordinary material rolled by using nickel grain rolls. The wear quantity was measured and compared with the wear quantity observed when an oiling agent now available was used. Explanation of Reference Numerals in the Drawing 1: sample 2: low-temperature material 3: high-temperature material 4: heat-insulating material 5: thermocouple

Claims: A lubricating composition for hot-rolling steel, which consists essentially of a base oil or base grease; a combination of heat-insulating agents (A) and (B), the heat-insulating agent (A) being an inorganic compound which is melted by an adsorption of heat at a temperature lower than 1200°C, the heat-insulating agent (B) being an inorganic powder, selected from silicon dioxide or silicon nitride, or powder of amorphous carbon, which is not melted or decomposed at a temperature lower than 1200°C and has a heat conductivity lower than 0.01 cal/cm. s. °C at room temperature and a friction coefficient smaller than 0.7; an extreme pressure additive and a solid lubricant, said combination of heat-insulating agents (A) and (B) being selected from the group consisting of a condensed phosphoric acid salt and silicon nitride; a condensed phosphoric acid salt and amorphous carbon; a condensed phosphoric acid salt and silicon dioxide; sodium chloride and silicon nitride; and sodium silicate and silicon nitride, said extreme pressure additive being selected from the group consisting of zinc dialkyldithiophosphate, tricresyl phosphate and sulfurized lard, said solid lubricant being selected from the group consisting of graphite, molybdenum disulfide, mica and MCA (melamine/cyanuric acid adduct), said heat-insulating agents (A) and (B) being contained at a weight ratio of from 49/1 to 1/49 and in a total amount of 5 to 50% by weight of the composition. A lubricating composition for hot-rolling steel according to claim 1, wherein the base oil is a member selected from the group consisting of spindle oil, machine oil, dynamo oil, motor oil, cylinder oil, bright stock, beef tallow, lard, sperm oil, palm oil, coconut oil, linseed oil, rice bran oil, soybean oil, esters of fatty acids having 8 to 22 carbon atoms with monohydric and polyhydric alcohols, α-olefins, polybutene, silicone oils and fluorine oils. A lubricating composition for hot-rolling steel according to claim 1, wherein the base grease is a member selected from the group consisting of lithium soap grease, calcium soap grease, sodium soap grease, aluminum soap grease, calcium complex grease, polyurea grease and organo-clay grease. A lubricating composition for hot-rolling steel according to claim 1, wherein the solid lubricant is a member selected from the group consisting of natural graphite, artificial graphite molybdenum disulfide, natural mica, artificial mica, boron nitride, soft metals, talc, polytetrafluoroethylene, melamine/cyanuric acid adducts and phthalocyanine. A lubricating composition for hot-rolling steel according to claim 1, wherein the extreme pressure additive is a member selected from the group consisting of sulfur compounds, phosphorus compounds, chlorine compounds and organic metal compounds.

Inventors: KAMIYASHIKI HIROSHI, KOUGA T C O NIPPON STEEL CORP, KURAHASHI R C O NIPPON STEEL C, SAKAKIBARA MASAYOSHI, SHIRATA MASATAKA, KAMIYASHIKI, HIROSHI, KOUGA, T. C/O NIPPON STEEL CORP. MURORAN WORKS, KURAHASHI, R. C/O NIPPON STEEL CORP. MURORAN WORKS, SAKAKIBARA, MASAYOSHI, SHIRATA, MASATAKA
Applicants: KYODO YUSHI, NIPPON STEEL CORP, KYODO YUSHI CO., LTD., NIPPON STEEL CORPORATION
Application year: 1989
Main classification: C10M141
IPC codes: C10M141, C10M177, C10M169, C10M125