EP0032583B1 - Process for continuously annealing of a cold rolled steel strip - Google Patents
Process for continuously annealing of a cold rolled steel strip Download PDFInfo
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- EP0032583B1 EP0032583B1 EP80108148A EP80108148A EP0032583B1 EP 0032583 B1 EP0032583 B1 EP 0032583B1 EP 80108148 A EP80108148 A EP 80108148A EP 80108148 A EP80108148 A EP 80108148A EP 0032583 B1 EP0032583 B1 EP 0032583B1
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- Prior art keywords
- steel strip
- temperature
- cooling
- overaging
- process according
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
Definitions
- the present invention relates to a process for continuously annealing a cold rolled steel strip. More particularly, the present invention relates to a process for continuously annealing a cold rolled steel strip, which process is capable of completing the annealing operation within a short time and; also, capable of obtaining a cold rolled steel strip having an excellent workability, especially formability, and an excellent surface quality, at a low cost.
- the continuous annealing method can exhibit an extremely high efficiency in comparison with the conventional batch type annealing method. However, it is strongly desired to increase the efficiency of the continuous annealing method to such an extent that the continuous annealing operation is completed within a few minutes.
- the heating and cooling operations result in the formation of a considerably large thickness of the oxide layer, and this large thickness causes the time necessary for completing the elimination of the oxide layer to be undesirably long.
- the sheet surface is coarsened due to the pickling for removing the oxide layer.
- the elimination of the oxide layer from the steel strip is carried out during the reheating overaging operation at a relatively low temperature. Therefore, in order to effectively attain the elimination of the oxide layer, the reducing operation should be carried out by using a strictly controlled reducing atmosphere having a special concentration of hydrogen and dew point. Also, in the processes (1) and (2), in order to overage the steel strip after the rapid cooling, it is necessary to reheat the steel strip to an overaging temperature thereof.
- the combustion air ratio is adopted as a parameter of the combustion condition in the direct fired furnace.
- the oxidation tendency of the sheet surface is dependent upon the combustion air ratio.
- DE-B 2 133 411 discloses a continuous cooling treatment for an annealed steel strip by application of quenching water which is jetted with high pressure in sheet form onto the strip, the cooling rate of latter being in the range of from 500°C/s to 2000°C/s. This quenching with water is very rapid, and it is difficult to obtain controlled moderate cooling rates. According to DE-B 2 133 411 1 the quenching is carried out until a temperature of about 100°C is reached. For the following shelfing treatment the strip has to be reheated until the desired temperature in the range of from 400°C to 500°C is reached.
- DE-B 2 165 049 discloses a method of quench hardening a metal plate, comprising spraying a water fog onto a hot metal plate, said fog being generated by means of special fog nozzles in which water and pressurized air are mixed. Separate jet nozzles for gas and liquid respectively are used.
- the present invention involves a concept of combining the following:
- the process without the overaging step of the present invention can be applied to cold-rolled non- aging low carbon steel strips, for example, cold-rolled, extremely low carbon aluminium killed steel strips, and cold-rolled non- or retarded-aging extremely low carbon steel strips containing a small amount of Ti, Nb, V or B, which are capable of forming carbonitrides.
- the process of the present invention can be applied to various cold-rolled low carbon steel strips which include the usual type of cold-rolled low carbon steel strips having a drawing quality and a deep drawing quality, for example, bodies of automobiles, high tensile strength cold-rolled low carbon steel strips and other types of cold-rolled low carbon steel strips suitable for various surface-treating processes, for example, metal plating and coating processes. Since, there is no necessity of overaging in the non- or retarded-aging steels, the steel strips are directly pickled after cooling, so as to remove the oxide layer.
- the primary cooling operation is followed by an overaging operation.
- the overaging operation is usually applied to them.
- the cooling operation must be terminated when the temperature of the steel strip reaches a level near an overaging temperature of the steel strip, the cooled steel strip may be overaged and, then, the overaged steel strip may be additionally cooled to a desired temperature.
- the peripheral surface of the cold-rolled steel strip may be cleaned to remove grease or rolling oil therefrom by a conventional surface-cleaning method. Otherwise, the process of the present invention may be applied to the cold rolled steel strip without surface-cleaning- it.
- radiation heating means such as a radiant-tube heating and other appropriate heating means, are employed for heating the steel strip to a temperature within the range of from the recrystallization temperature to the Ac 3 point of the steel strip and also for holding the steel strip within the temperature range of from the recyrstallization temperature to the A C3 point over a period of at least 10 seconds, preferably from 10 to 60 seconds in a non-oxidizing or reducing atmosphere.
- the present invention is characterized as compared with the prior arts in the combination use of the radiation heating furnace for heating steel strip to and holding it within the requisite annealing temperature range, cooling the steel strip by spraying a specifically generated cooling medium consisting of a mixture of an inert gas and a liquid, a pickling removal of the oxide layer formed by the cooling and overaging without reheating, if overaging is necessary.
- the holding temperature depends on the composition and the other properties of the steel strip, as well as the objective properties of the final product.
- the holding temperature is not always constant but may be varied in the direction of the holding zone so as to adapt the heat cycle for recrystallization and growth of crystal grains.
- the heating rate is lower than that of the direct fired heating, but the surface deterioration due to the direct contact of flame with the sheet surface is prevented.
- the pickling time in the final step is shortened as compared with that of steel strips heated in the direct fired furnace, and also a surface quality problem involved in the pickling is mitigated.
- the non-oxidizing or reducing atmosphere inert to the steel is employed for the furnace atmosphere and is maintained during the heating and holding zones, the oxidation of the sheet surface in these zones is prevented.
- the pickling at the final step is carried out at an extent sufficient to remove only a thin oxide layer formed at this cooling step or the primary cooling step.
- the fact that no oxide layer is formed during the heating step advantageously leads to the prevention of the oxide pick up on the hearth rollers.
- the reducing atmosphere preferably comprises a mixture of 4% by volume or more of hydrogen gas, with the balance consisting of nitrogen gas, and preferably exhibits a dew point of 10°C or less.
- the steel strip which is subjected to the holding in a radiation heating furnace, is then rapidly cooled at a cooling rate from 10 to 300°C/second which is preferable from the view points of the metallurgical properties of the annealed steel strip.
- the steel strip may be directly rapid-cooled from the holding temperature, or may be slow-cooled from the holding temperature to a temperature of preferably 600°C at the lowest and then rapid-cooled, if necessary.
- the rapid cooling must start at a temperature between the above holding temperature and 600°C from the view points of shortening the overaging time period.
- gas-liquid cooling medium means a fluid which is produced through such a process that a high speed inert gas stream and a liquid stream of a predetermined pressure are injected from their respective nozzles as jet streams and these streams are then mixed with each other by intersecting with each other so that the liquid (e.g., water) reduces itself to fine particles mixed in the gas in the form of a mist, or in a form almost equivalent to spray.
- the liquid is preferably water and the gas is selected from inert gases, such as nitrogen gas, and mixtures of nitrogen and hydrogen.
- the gas-liquid cooled medium consists of a mixture of nitrogen gas with water.
- the overaging operation is carried out for the purpose of precipitating carbon in the steel matrix which has been super-saturated with a solid solution carbon.
- the overaging operation is preferably carried out in a temperature range of from 300 to 550°C, more preferably, from 350 to 450°C, for 3 minutes or less, more preferably 2 minutes or less. It is not always necessary that the steel strip be maintained at a constant temperature throughout the overaging operation. That is, the overaging temperature in an initial stage of the overaging operation may be higher than that in a final stage of the overaging operation.
- the temperature profile from the entry to withdrawal ends of the overaging equipment may be gradual or may decrease stepwise,
- the peripheral surface portion of the steel strip cannot be prevented from oxidation. That is, the resultant layer of oxides causes the appearance of the steel strip surface to be unsatisfactory, and the surface property of the steel strip to be unsuitable to the surface treatments. Therefore, it is necessary to eliminate the layer of oxides from the peripheral surface of the steel strip.
- the elimination of the oxide layer is effected by any conventional pickling methods effective for eliminating various oxides.
- the oxide layer can be removed by treating the peripheral surface of the steel strip with, for example, an acid aqueous solution of an inorganic acid, such as hydrochloric acid, sulfuric acid or phosphoric acid, or an organic acid, such as formic acid or oxalic acid.
- the treatment may be effected by immersing the steel strip in an acid aqueous solution, by spraying the acid aqueous solution onto a peripheral surface of the steel strip, or by subjecting the steel strip to an electrolytic pickling with an acid aqueous solution.
- the oxide layer formed in the cooling and , optionally, overaging operations is very thin. Therefore, the oxide layer can be readily eliminated by the above-mentioned methods.
- the pickled steel strip is rinsed with water.
- the rinsed steel strip be neutralized with a diluted alkali aqueous solution. This neutralization is effective for preventing rust and discoloration of the peripheral surface of the steel strip.
- the cold rolled steel strip for example, to be used for producing an automobile body, is coated before the working process.
- the surface of steel strip is treated with zinc phosphate.
- the quality of the zinc phosphate film formed on the surface of the steel strip can be improved by applying the following treatment to the steel strip after the pickling operation.
- an aqueous suspension containing water-insoluble phosphate for example, Zn,(P0 4 ) 21 is sprayed onto the surface of the pickled steel strip, or a thin film of Ni, Zn or Mn is flash-coated on the pickled steel strip surface by means of electroplating. Thereafter, as a precoating operation the surface of steel strip is treated with the zinc phosphate.
- the above-mentioned surface pre-treatment is effective for promoting the formation of crystal nuclei of the zinc phosphate and for providing a dense film of the zinc phosphate. Therefore, the above-mentioned surface pre-treatment is very effective for enhancing the bonding strength of the zinc phosphate layer to the coating layer and for increasing the resistance of the coating layer to corrosion.
- the process of the present invention can exhibit the following advantages.
- 0.8 mm thick cold rolled steel strips were processed under the conditions given in Table 1.
- the oxide film thickness in the comparative examples 1 and 2 was determined by preliminary experiments, in which the steel strips were heated under the same condition as in these examples by a direct fired furnace followed by a rapid cooling means of blowing nitrogen gas on the strips in the next zone. Samples were cut from the strips and pickled in a 5% hydrochloric acid aqueous solution. The weight decrease of samples due to pickling was reduced to the thickness of the oxide layer of FeO.
- the 0.8 mm thick cold rolled steel strips were processed under the conditions given in Table 2.
- the oxide layer thickness was determined by the same procedure as described above.
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- Heat Treatment Of Strip Materials And Filament Materials (AREA)
Description
- The present invention relates to a process for continuously annealing a cold rolled steel strip. More particularly, the present invention relates to a process for continuously annealing a cold rolled steel strip, which process is capable of completing the annealing operation within a short time and; also, capable of obtaining a cold rolled steel strip having an excellent workability, especially formability, and an excellent surface quality, at a low cost.
- It is known that a cold rolled steel strip having a deep drawing quality can be produced by tightly or loosely coiling a cold rolled steel strip and, then, by annealing it batchwise in a box type annealing furnace. This type of method needs several days to complete the entire process thereof and, therefore, is extremely inefficient. In order to avoid the above-mentioned disadvantage, various attempts have been made to continuously carry out the annealing process, and some of the attempts have been practically used in steel industry.
- The continuous annealing method can exhibit an extremely high efficiency in comparison with the conventional batch type annealing method. However, it is strongly desired to increase the efficiency of the continuous annealing method to such an extent that the continuous annealing operation is completed within a few minutes.
- In order to accelerate the continuous annealing operation, it has been attempted to rapidly heat the steel strip by using a direct fired furnace or to rapidly cool the heated steel strip with water or gas in the primary stage of the cooling operation. However, both rapid heating operation and rapid cooling. operation in the above-mentioned processes cause an oxide layer to be formed on the peripheral surface of the steel strip. Therefore, it is necessary to eliminate the oxide layer from the annealed steel strip. Examples of the accelerated continuous annealing processes are as follows.
- (1) Japanese Patent Application Laying-open (Kokai) No. 52-14431 (1977) discloses an annealing process in which a steel strip is rapidly heated to a predetermined temperature in a direct fired furnace and, then, rapidly cooled with water, reheated, overaged and, finally, subjected to a pickling operation to remove an oxide layer formed on the pripheral surface of the steel strip.
- (2) Japanese Patent Application Laying-open (Kokai) No. 53-17518 (1978) discloses a process wherein a steel strip is rapidly heated to a predetermined temperature and maintained at the temperature in the direct fired furnace, rapidly cooled with water and, reheated, overaged while the oxide layer on the peripheral surface thereof is removed by reducing it.
- Especially, in the above-mentioned process (1), the heating and cooling operations result in the formation of a considerably large thickness of the oxide layer, and this large thickness causes the time necessary for completing the elimination of the oxide layer to be undesirably long. In addition, the sheet surface is coarsened due to the pickling for removing the oxide layer.
- In the above-mentioned process (2), the elimination of the oxide layer from the steel strip is carried out during the reheating overaging operation at a relatively low temperature. Therefore, in order to effectively attain the elimination of the oxide layer, the reducing operation should be carried out by using a strictly controlled reducing atmosphere having a special concentration of hydrogen and dew point. Also, in the processes (1) and (2), in order to overage the steel strip after the rapid cooling, it is necessary to reheat the steel strip to an overaging temperature thereof.
- When the electrolytic cleaning is omitted in the processing line of the cold rolled steel strip including the direct fired furnace, iron powder, which is adhered on the surface of the cold rolled steel strip and remains on the steel strip conveyed into the direct fired furnace, is oxidized in this furnace. When the quantity of so oxidized iron powder is large, the oxide layer is partly peeled off from the sheet surface as the steel travels through a direct fired holding furnace. The peeled oxide layer is picked up by and firmly bonded around the hearth rollers of the holding furnace due to sintering phenomenon thereof, which causes generation of flaws on the high-temperature steel strips conveyed by the hearth rollers.
- As is well known, the combustion air ratio is adopted as a parameter of the combustion condition in the direct fired furnace. The oxidation tendency of the sheet surface is dependent upon the combustion air ratio.
- DE-B 2 133 411 discloses a continuous cooling treatment for an annealed steel strip by application of quenching water which is jetted with high pressure in sheet form onto the strip, the cooling rate of latter being in the range of from 500°C/s to 2000°C/s. This quenching with water is very rapid, and it is difficult to obtain controlled moderate cooling rates. According to DE-B 2 133 411 1 the quenching is carried out until a temperature of about 100°C is reached. For the following shelfing treatment the strip has to be reheated until the desired temperature in the range of from 400°C to 500°C is reached.
- DE-B 2 165 049 discloses a method of quench hardening a metal plate, comprising spraying a water fog onto a hot metal plate, said fog being generated by means of special fog nozzles in which water and pressurized air are mixed. Separate jet nozzles for gas and liquid respectively are used.
- It is an object of the present invention to provide a continuously annealing process of a cold rolled steel strip, capable of eliminating the problems of a surface-quality caused by conventional processes.
- It is another object of the present invention to provie a continuously annealing process of a cold rolled steel strip, capable of eliminating the surface-quality problems caused by conventional processes and simultaneously omitting the reheating step for overaging of the steel strip.
- The present invention involves a concept of combining the following:
- using instead of a direct fired furnace a radiation heating furnace, thereby preventing the formation of an appreciable oxide layer during the stages for heating to and holding within the requisite annealing temperature range;
- cooling the annealed steel strip by spraying a cooling medium being specifically generated and consisting of a mixture of inert gas and a liquid, and
- removing the oxide film formed during the cooling stage, by means of pickling at the final step.
- In accordance with the objects of the present invention, there is provided a process, comprising the steps of:
- introducing a cold rolled steel strip into at least one radiation heating furnace, in which said steel strip is heated to a temperature within the range of from the recrystallization temperature to the Ac3 point of said steel strip and held within said temperature range over a period of at least 10 seconds wherein a non-oxidizing or reducing atmosphere is maintained at the heating and holding zones of said at least one radiation heating furnace;
- cooling said steel strip by spraying a cooling medium consisting of a mixture of an inert gas and a liquid, which cooling medium is provided by injecting a high speed gas stream and a liquid stream of a predetermined pressure from their respective nozzles as jet streams and mixing these streams with each other by intersecting them with each other, and
- pickling said steel strip.
- In accordance with the objects of the present invention there is further provided a modified process for another type of steel strips, comprising the steps of:
- introducing a cold rolled steel strip into at least one radiation heating furnace, in which said steel strip is heated to a temperature within the range of from the recrystallization temperature to the AC3 point of said steel strip and held at said temperature range over a period of at least 10 seconds wherein a non-oxidizing or reducing atmosphere is maintained at the heating and holding zones of said at least one radiation heating furnace;
- cooling said steel strip by spraying a cooling medium consisting of a mixture of an inert gas and a liquid, which cooling medium is produced by injecting a high speed gas stream and a liquid stream of a predetermined pressure from their respective nozzles as jet streams and mixing these streams with each other by intersecting them with each other;
- said cooling being conducted over a primary-cooling temperature range of from 600°C at the lowest to a temperature near to an overaging treatment temperature;
- overaging said primary-cooled steel strip;
- secondary-cooling said overaged steel strip, and
- pickling said secondary-cooled steel strip.
- The process without the overaging step of the present invention can be applied to cold-rolled non- aging low carbon steel strips, for example, cold-rolled, extremely low carbon aluminium killed steel strips, and cold-rolled non- or retarded-aging extremely low carbon steel strips containing a small amount of Ti, Nb, V or B, which are capable of forming carbonitrides. In other words, the process of the present invention can be applied to various cold-rolled low carbon steel strips which include the usual type of cold-rolled low carbon steel strips having a drawing quality and a deep drawing quality, for example, bodies of automobiles, high tensile strength cold-rolled low carbon steel strips and other types of cold-rolled low carbon steel strips suitable for various surface-treating processes, for example, metal plating and coating processes. Since, there is no necessity of overaging in the non- or retarded-aging steels, the steel strips are directly pickled after cooling, so as to remove the oxide layer.
- In the process with the overaging step of the present invention, the primary cooling operation is followed by an overaging operation. In the cases of usual cold rolled low carbon steel strips having an aging property, the overaging operation is usually applied to them. In this case, the cooling operation must be terminated when the temperature of the steel strip reaches a level near an overaging temperature of the steel strip, the cooled steel strip may be overaged and, then, the overaged steel strip may be additionally cooled to a desired temperature.
- The present invention is hereinafter explained in detail in the sequence of the production steps.
- Steel is processed in a conventional manner until the cold rolling step. Before applying the processes of the present invention, the peripheral surface of the cold-rolled steel strip may be cleaned to remove grease or rolling oil therefrom by a conventional surface-cleaning method. Otherwise, the process of the present invention may be applied to the cold rolled steel strip without surface-cleaning- it.
- In the process of the present invention, radiation heating means, such as a radiant-tube heating and other appropriate heating means, are employed for heating the steel strip to a temperature within the range of from the recrystallization temperature to the Ac3 point of the steel strip and also for holding the steel strip within the temperature range of from the recyrstallization temperature to the AC3 point over a period of at least 10 seconds, preferably from 10 to 60 seconds in a non-oxidizing or reducing atmosphere. Although a radiation heating surface is well known the present invention is characterized as compared with the prior arts in the combination use of the radiation heating furnace for heating steel strip to and holding it within the requisite annealing temperature range, cooling the steel strip by spraying a specifically generated cooling medium consisting of a mixture of an inert gas and a liquid, a pickling removal of the oxide layer formed by the cooling and overaging without reheating, if overaging is necessary.
- The holding temperature depends on the composition and the other properties of the steel strip, as well as the objective properties of the final product. The holding temperature is not always constant but may be varied in the direction of the holding zone so as to adapt the heat cycle for recrystallization and growth of crystal grains.
- In the processes of the present invention, the heating rate is lower than that of the direct fired heating, but the surface deterioration due to the direct contact of flame with the sheet surface is prevented. As a result, the pickling time in the final step is shortened as compared with that of steel strips heated in the direct fired furnace, and also a surface quality problem involved in the pickling is mitigated. The non-oxidizing or reducing atmosphere inert to the steel is employed for the furnace atmosphere and is maintained during the heating and holding zones, the oxidation of the sheet surface in these zones is prevented. As a result, the pickling at the final step is carried out at an extent sufficient to remove only a thin oxide layer formed at this cooling step or the primary cooling step. The fact that no oxide layer is formed during the heating step advantageously leads to the prevention of the oxide pick up on the hearth rollers. The reducing atmosphere preferably comprises a mixture of 4% by volume or more of hydrogen gas, with the balance consisting of nitrogen gas, and preferably exhibits a dew point of 10°C or less.
- The steel strip, which is subjected to the holding in a radiation heating furnace, is then rapidly cooled at a cooling rate from 10 to 300°C/second which is preferable from the view points of the metallurgical properties of the annealed steel strip. In the process without the overaging step of the present invention, the steel strip may be directly rapid-cooled from the holding temperature, or may be slow-cooled from the holding temperature to a temperature of preferably 600°C at the lowest and then rapid-cooled, if necessary. In the process with the overaging step of the present invention, the rapid cooling must start at a temperature between the above holding temperature and 600°C from the view points of shortening the overaging time period.
- The spraying of the cooling medium consisting of a mixture of gas and liquid hereinafter referred to as a gas-liquid cooling medium not only realizes the rapid cooling but also provides such advantages as, good shape of the cooled steel strip, and easiness in controlling the cooling rate. The term "gas-liquid cooling medium" means a fluid which is produced through such a process that a high speed inert gas stream and a liquid stream of a predetermined pressure are injected from their respective nozzles as jet streams and these streams are then mixed with each other by intersecting with each other so that the liquid (e.g., water) reduces itself to fine particles mixed in the gas in the form of a mist, or in a form almost equivalent to spray.
- The liquid is preferably water and the gas is selected from inert gases, such as nitrogen gas, and mixtures of nitrogen and hydrogen. In a preferable example, the gas-liquid cooled medium consists of a mixture of nitrogen gas with water. One of the advantages of the gas-liquid cooling medium is to control the cooling rate in the range of 10 to 300°C/second in such a manner as to control the temperature near to the overaging temperature and then to overage without reheating. That is, when the present invention is applied to a cold-rolled low carbon steel strip having an aging property, the primary cooling operation can be advantageously terminated when the temperature of the steel strip reaches a level near the overaging temperature of the steel strip, the cooled steel strip is overaged and, then, additionally cooled to a desired temperature.
- The overaging operation is carried out for the purpose of precipitating carbon in the steel matrix which has been super-saturated with a solid solution carbon. The overaging operation is preferably carried out in a temperature range of from 300 to 550°C, more preferably, from 350 to 450°C, for 3 minutes or less, more preferably 2 minutes or less. It is not always necessary that the steel strip be maintained at a constant temperature throughout the overaging operation. That is, the overaging temperature in an initial stage of the overaging operation may be higher than that in a final stage of the overaging operation. The temperature profile from the entry to withdrawal ends of the overaging equipment may be gradual or may decrease stepwise,
- When the gas-liquid cooling medium contains water, the peripheral surface portion of the steel strip cannot be prevented from oxidation. That is, the resultant layer of oxides causes the appearance of the steel strip surface to be unsatisfactory, and the surface property of the steel strip to be unsuitable to the surface treatments. Therefore, it is necessary to eliminate the layer of oxides from the peripheral surface of the steel strip.
- The elimination of the oxide layer is effected by any conventional pickling methods effective for eliminating various oxides. For example, the oxide layer can be removed by treating the peripheral surface of the steel strip with, for example, an acid aqueous solution of an inorganic acid, such as hydrochloric acid, sulfuric acid or phosphoric acid, or an organic acid, such as formic acid or oxalic acid. The treatment may be effected by immersing the steel strip in an acid aqueous solution, by spraying the acid aqueous solution onto a peripheral surface of the steel strip, or by subjecting the steel strip to an electrolytic pickling with an acid aqueous solution.
- In the processes of the present invention, the oxide layer formed in the cooling and , optionally, overaging operations, is very thin. Therefore, the oxide layer can be readily eliminated by the above-mentioned methods. After the pickling operation is completed, the pickled steel strip is rinsed with water. However, since the peripheral surface of the acid-cleaned steel strip is reactive to oxygen and easily rusts, it is preferable that the rinsed steel strip be neutralized with a diluted alkali aqueous solution. This neutralization is effective for preventing rust and discoloration of the peripheral surface of the steel strip.
- Usually, the cold rolled steel strip, for example, to be used for producing an automobile body, is coated before the working process. In-this case, the surface of steel strip is treated with zinc phosphate. The quality of the zinc phosphate film formed on the surface of the steel strip can be improved by applying the following treatment to the steel strip after the pickling operation.
- That is, as a surface pre-treatment an aqueous suspension containing water-insoluble phosphate, for example, Zn,(P04)21 is sprayed onto the surface of the pickled steel strip, or a thin film of Ni, Zn or Mn is flash-coated on the pickled steel strip surface by means of electroplating. Thereafter, as a precoating operation the surface of steel strip is treated with the zinc phosphate. The above-mentioned surface pre-treatment is effective for promoting the formation of crystal nuclei of the zinc phosphate and for providing a dense film of the zinc phosphate. Therefore, the above-mentioned surface pre-treatment is very effective for enhancing the bonding strength of the zinc phosphate layer to the coating layer and for increasing the resistance of the coating layer to corrosion.
- The process of the present invention can exhibit the following advantages.
- (1) By utilizing the cooling operation with a mixture of a gas and a liquid, the cooling rate of the steel strip can be easily controlled. For example, the steel strip can be easily cooled to a temperature near to the overaging temperature of the steel strip. Therefore, the overaging operation can be directly applied to the cooled steel strip without heating the cooled steel strip to the overaging temperature. A reheating step for overaging treatment is not necessary, if such treatment is required for having an aging -property.
- (2) Since no appreciable oxide layer is formed in the heating and holding stages, the oxide layer to be removed by the pickling is merely that formed at the cooling step, especially the primary cooling step. The pickling operation can therefore be simplified.
- (3) It is possible to prevent the oxide adhesion on the hearth rollers.
- (4) Surface quality of the final product is excellent.
- The following specific examples are presented for the purpose of clarifying the present invention. However, it should be understood that these examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention in any way.
- In these examples, 0.8 mm thick cold rolled steel strips were processed under the conditions given in Table 1. The oxide film thickness in the comparative examples 1 and 2 was determined by preliminary experiments, in which the steel strips were heated under the same condition as in these examples by a direct fired furnace followed by a rapid cooling means of blowing nitrogen gas on the strips in the next zone. Samples were cut from the strips and pickled in a 5% hydrochloric acid aqueous solution. The weight decrease of samples due to pickling was reduced to the thickness of the oxide layer of FeO.
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Claims (11)
Applications Claiming Priority (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP166827/79 | 1979-12-24 | ||
JP16682679A JPS5933171B2 (en) | 1979-12-24 | 1979-12-24 | Continuous annealing method for cold rolled steel sheets |
JP16682779A JPS5933172B2 (en) | 1979-12-24 | 1979-12-24 | Continuous annealing treatment method for cold rolled steel sheets |
JP166826/79 | 1979-12-24 |
Publications (2)
Publication Number | Publication Date |
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EP0032583A1 EP0032583A1 (en) | 1981-07-29 |
EP0032583B1 true EP0032583B1 (en) | 1984-08-01 |
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Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
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EP80108148A Expired EP0032583B1 (en) | 1979-12-24 | 1980-12-22 | Process for continuously annealing of a cold rolled steel strip |
Country Status (4)
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EP (1) | EP0032583B1 (en) |
BR (1) | BR8008511A (en) |
CA (1) | CA1152870A (en) |
DE (1) | DE3068827D1 (en) |
Families Citing this family (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPS5767134A (en) * | 1980-10-09 | 1982-04-23 | Nippon Steel Corp | Method and installation for continuous annealing method of cold-rolled steel strip |
FR2562563B1 (en) * | 1984-04-05 | 1988-09-23 | Stein Heurtey | PROCESS FOR DEGREASING A COLD-ROLLED METAL STRIP |
DE3580055D1 (en) * | 1984-04-05 | 1990-11-15 | Stein Heurtey | DEGREASING METHOD FOR COLD ROLLED STEEL TAPE. |
CN112111638B (en) * | 2020-09-27 | 2022-05-24 | 攀钢集团研究院有限公司 | Grading production method of low-carbon cold-rolled steel plate |
CN112593051B (en) * | 2020-12-07 | 2022-01-25 | 山东钢铁集团日照有限公司 | Method for preventing generation of iron scale and achieving rapid furnace ignition and temperature rise of annealing furnace |
CN115058582B (en) * | 2022-07-14 | 2024-06-11 | 上海曙佳科技发展有限公司 | Method for visualization and workpiece temperature management in continuous annealing furnace |
Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
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DE2165049B2 (en) * | 1970-12-28 | 1973-11-15 | Nippon Kokan K.K., Tokio | Method and apparatus for quenching |
Family Cites Families (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPS4923727B1 (en) * | 1969-12-16 | 1974-06-18 | ||
JPS535607B2 (en) * | 1973-01-11 | 1978-03-01 |
-
1980
- 1980-12-18 CA CA000367063A patent/CA1152870A/en not_active Expired
- 1980-12-22 DE DE8080108148T patent/DE3068827D1/en not_active Expired
- 1980-12-22 EP EP80108148A patent/EP0032583B1/en not_active Expired
- 1980-12-23 BR BR8008511A patent/BR8008511A/en not_active IP Right Cessation
Patent Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
DE2165049B2 (en) * | 1970-12-28 | 1973-11-15 | Nippon Kokan K.K., Tokio | Method and apparatus for quenching |
Non-Patent Citations (2)
Title |
---|
Patents Abstracts of Japan Vol. 4, No. 22, 23 February 1980, page 128C74 * |
Patents Abstracts of Japan, Vol. 1, No. 64, 22 June 1977, page 995C77 * |
Also Published As
Publication number | Publication date |
---|---|
BR8008511A (en) | 1981-07-21 |
DE3068827D1 (en) | 1984-09-06 |
CA1152870A (en) | 1983-08-30 |
EP0032583A1 (en) | 1981-07-29 |
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