JPH08232025A - Production of cold-rolled bright steel strip - Google Patents

Abstract

PURPOSE: To remove the excess moisture stuck to a coil and a furnace refractory, to prevent the development of an oxide layer on the width edge parts of coiled strip at the time of executing finish-annealing and to improve the surface quality of the bright steel strip and the product yield. CONSTITUTION: At the time of annealing the tight coil, an annealing is executed at the temp. of 450-580 deg.C which is lower than a prescribed annealing temp. in the initial stage and successively the finish-annealing is executed. The finish- annealing temp. is desirably set at 600-720 deg.C continuously before reaching the dew point of <=0 deg.C after being raised once with the dew point of the atmosphere gas at the time of annealing in the initial stage and then lowered again. By this method, the oxide coloration of the width edge parts of the tight coiled strip is prevented, and besides, since the finish-annealing is executed within the suitable temp. range, the recrystallization and the growth of crystal grain in the steel strip are sufficiently developed and the excellent workability is given and the mutual burning between the steel strips can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing cold rolled bright steel strip, and more particularly to a tight coil annealing method.

[0002]

2. Description of the Related Art Cold rolled bright steel strip with excellent surface gloss is
It is used in a variety of applications, such as stove reflectors, fog lamp reflectors, binder back fittings, battery jackets, etc.It is made of low carbon steel for cold rolling, electrolytic cleaning, tight coil annealing, temper rolling. Each step is manufactured in this order.

The electrolytic cleaning step is a step for cleaning the surface of the steel strip by removing rolling oil, iron powder and the like which have adhered to the steel strip in the cold rolling step. The tight coil annealing process is a process of softening the steel strip hardened by cold rolling to give workability, and the tight coil charged in the box furnace is recrystallized at a temperature not lower than the recrystallization temperature in the reducing atmosphere gas. Heat treated. The tight coil is a coil wound in a so-called tight state with no gap between the steel strips. The temper rolling step is a step of imparting surface gloss to the cold rolled bright steel strip, and the dry rolling is performed on the cold rolled bright steel strip by a temper rolling machine having a work roll having a small surface roughness. . Dry temper rolling is a temper rolling that does not use a temper rolling liquid, and the gloss of a work roll is reliably transferred to a cold rolled bright steel strip as compared with wet temper rolling that uses a temper rolling liquid. The cold rolled bright steel strip obtained through the above manufacturing process has the following problems conventionally.

FIG. 3 is a characteristic diagram showing the temporal transition of the furnace top temperature, the coil temperature, and the dew point of the atmospheric gas in the conventional tight coil annealing. The furnace top temperature Tf, which indicates the furnace temperature of the box furnace, rises with the start of heating, reaches a predetermined temperature, for example 700 ° C., and then is kept constant at that temperature. The coil temperature Tc rises later than the furnace top temperature Tf,
The temperature approaches a predetermined temperature with the passage of time. The dew point Td of the atmospheric gas once rises as the furnace top temperature Tf and the coil temperature Tc rise, and then falls again after passing the peak dew point. The peak dew point reaches, for example, + 10 ° C or higher. This increase in the dew point is due to the evaporation of water contained in the tight coil and the furnace material. This dew point Td and coil temperature T
Due to the rise of c, an oxide layer may be formed on the surface of the steel strip at the widthwise end of the tight coil.

FIG. 4 is a cross-sectional view showing a laminated state of steel strips at the widthwise end of the tight coil. In the strip width direction strip thickness distribution of the steel strip, since there is an edge drop region E where the strip thickness sharply decreases at the strip width end portion, even in a tight coil wound with no gap between the strips, A gap G is formed between the steel strips at the width end of the coil plate. Atmosphere gas can freely flow in and out of the gap G, so that when the dew point of the atmosphere gas is high, an oxide layer is likely to be formed on the surface of the steel strip at the end of the coil width.

Component analysis of the oxide layer is performed by IMA (Ion Micr
o Mn and Si are strongly detected when it is performed by an Analyzer). Mn and Si are easily oxidizable elements contained in the raw material low carbon steel, and react with the moisture in the atmospheric gas on the surface of the steel strip to oxidize, and as the oxidation progresses, they concentrate on the surface, It was detected as a Mn, Si concentrated layer. The surface thickened layer accompanying this oxidation will be simply referred to as an oxidized layer hereinafter. In the temper rolling step, which is the next step, when the oxide layer is temper rolled, it adheres to the work rolls to cloud the work rolls, thereby lowering the gloss of the work rolls. Further, when temper rolling is continued with the work roll in which cloudiness has occurred, that portion is transferred to the steel strip, and uneven luster occurs in the cold rolled bright steel strip. In order to solve such a problem of the conventional art, the following prior art has been disclosed conventionally.

In the prior art disclosed in Japanese Patent Laid-Open No. 57-75212, a brush roll having abrasive grains is disposed on the work roll entrance side during mirror finishing in temper rolling, whereby the work roll is mounted. This is a method of polishing to maintain the gloss of the work roll surface.

In the prior art disclosed in Japanese Patent Laid-Open No. 4-301096, an appropriate amount of silica of sodium silicate is electrodeposited on the surface of a steel strip during electrolytic cleaning, and the oxide layer of the steel strip is directly worked during temper rolling. This is a method of preventing the work roll surface from fogging by preventing it from coming into contact with the roll.

[0009]

However, each of the above prior arts has the following problems. JP-A-57
In the prior art disclosed in Japanese Laid-Open Patent Application No. 75212, installation cost of the brush rolls is high, and in order to uniformly polish the steel strip in the plate width direction, it is necessary to maintain fine equipment. However, if the polishing is not uniform, there is a problem that a hairline pattern or the like is formed on the steel strip and the surface appearance is impaired. Further, in the prior art disclosed in Japanese Patent Application Laid-Open No. 4-301096, the silica adhesion amount may be out of the proper range at a position where the plate passing speed fluctuates, such as near the coil joint portion, and the silica adhesion amount is small. If it is too much, the oxide layer adheres to the work roll. On the other hand, if the amount of silica adhered is too large, a milky white temper color is generated and the appearance is impaired. For this reason, in this prior art, the surface gloss at the head-to-tail portion of the coil is likely to decrease.

An object of the present invention is to solve the above problems and to provide a method for producing a cold rolled bright steel strip having excellent surface gloss over the entire area.

[0011]

The present invention provides a method for producing a cold-rolled bright steel strip produced through the steps of cold rolling, electrolytic cleaning, tight coil annealing and temper rolling in this order, A method for producing a cold-rolled bright steel strip, comprising performing initial annealing at 450 to 580 ° C. lower than a predetermined annealing temperature during tight coil annealing, and subsequently performing finish annealing at a predetermined temperature. Further, the present invention is characterized in that the finish annealing temperature is 600 to 720 ° C. Further, the initial annealing of the present invention is characterized in that the dew point of the atmospheric gas is once raised and then is lowered again until the dew point reaches 0 ° C. or lower.

[0012]

According to the present invention, the cold-rolled bright steel strip is manufactured by performing the initial annealing at a temperature lower than the temperature of the final annealing and then the final annealing during the tight coil annealing.
As a result, excess water adhering to the tight coil and the furnace material is removed in the initial annealing, so that the generation of an oxide layer at the widthwise end of the tight coil plate is prevented during finish annealing. Therefore, a cold-rolled bright steel strip having an excellent surface gloss over the entire region can be manufactured in the temper rolling of the next step.

Further, according to the present invention, since the finish annealing is performed in an appropriate temperature range, the recrystallization of the steel strip and the growth of crystal grains sufficiently occur, and the cold rolled bright steel strip has excellent workability. You can Further, since the upper limit temperature of the finish annealing is set appropriately, the occurrence of seizure between the steel strips of the tight coil during the finish annealing can be prevented.

Further, according to the present invention, the initial annealing is continued until the dew point of the atmospheric gas once rises and then decreases again to reach 0 ° C. or lower. As a result, the removal of water in the initial annealing is surely performed, so that the generation of an oxide layer at the widthwise end of the tight coil plate during the final annealing is surely prevented. Therefore, in the temper rolling of the next step, the work roll is prevented from being fogged.

[0015]

EXAMPLE A cold-rolled bright steel strip is manufactured through the steps of cold rolling, electrolytic cleaning, tight coil annealing and temper rolling as described above in this order. FIG. 1 is a cross-sectional view showing a simplified structure of a tight coil annealing equipment capable of suitably performing the tight coil annealing according to the present invention.

The tight coil annealing equipment 1 comprises a furnace body 2 for heating a tight coil 7 such as a low carbon steel strip, and a protective atmosphere gas space for covering the tight coil 7 in the furnace to prevent oxidation of the steel strip. And an inner cover 3 that forms a base gas, and a base fan 4 that enhances heat transfer by forcibly circulating a protective atmosphere gas (hereinafter sometimes abbreviated as atmosphere gas). A plurality of combustion burners 5 are provided in the lower part of the furnace body 2 at intervals along the circumferential direction, and a thermocouple 6 for measuring the furnace top temperature is provided in the upper part of the furnace body 2.

When carrying out the tight coil annealing, the tight coils 7 are loaded in a stack of 2 to 4 steps. Between each tight coil 7, a steel disc having a radial passage for inserting an atmosphere gas called a convector plate 8 to increase heat transfer by convection is inserted. A coil temperature measuring thermocouple 9 is brought into contact with the lower surface of the plate width end portion of the lowermost tight coil 7. After covering the inner coil 3 with the tight coil 7 and sealing the bottom of the inner cover 3 with sand or mineral fibers 10, a protective atmosphere gas is fed into the inner cover 3 to purge air.
Then, the furnace body 2 is covered over the inner cover 3, and the combustion burner 5 is ignited to start heating. The protective atmosphere gas is continuously fed into the inner cover 3 even during heating, and is discharged from the discharge pipe 11 to the outside. By providing the dew point meter 12 on the discharge pipe 11, the dew point of the atmospheric gas can be measured.

The dew point meter 12 is composed of, for example, aluminum as a base material and aluminum oxide and gold as coating layers.
It is a sensor that uses an impedance element consisting of a layered structure as a detection part, and the measurement principle is to adsorb moisture in atmospheric gas to many pore walls formed in the aluminum oxide layer and detect the impedance change at that time. The dew point is measured. As the protective atmosphere gas, so-called HN gas in which a decomposition product gas obtained by thermally decomposing liquid ammonia and high-purity nitrogen from an air separation device are blended is preferably used. The composition of the HN gas used in this example is, for example, 1
A _2% H 2 -N 2. As the steel type of the cold rolled bright steel strip, low carbon steel, that is, low carbon aluminum killed steel, low carbon rimmed steel, and the like are preferably used. The low carbon steel is softened by applying appropriate annealing and exhibits excellent workability. Typical low carbon aluminum killed steel has the chemical composition, for example, C: 0.04%, Si: 0.02%, M.
n: 0.30%, P: 0.018%, S: 0.015
%, Al: 0.02%.

FIG. 2 is a timing chart for explaining a tight coil annealing method which is an embodiment of the present invention. The steel type of the tight coil used in the example shown in FIG.
It was a low carbon aluminum killed steel, the number of loading stacks was 3, and the total weight of loading was 37.3 tons. The heating of the tight coil annealing equipment 1 is started by the method as described above, and the furnace top temperature Tf, the coil temperature Tc, and the atmosphere are controlled by the furnace top temperature measuring thermocouple 6, the coil temperature measuring thermocouple 9, and the dew point meter 12. The dew point Td of the gas was measured. After the heating is started, the furnace top temperature Tf, the coil temperature Tc, and the dew point Td all rise. The heating rate of the furnace top temperature Tf is higher than the heating rate of the coil temperature Tc.

At time t1, the furnace top temperature Tf reaches a preset initial annealing setting value, for example, 550 ° C., and thereafter the combustion of the combustion burner 5 is controlled so as to keep the furnace top temperature Tf constant. . The initial annealing is performed in order to dry the water adhering to the tight coil 7, the inner cover 3, the convector plate 8, etc., that is, to remove the excess water. Dew point Td and coil temperature Tc
Continues to rise after time t1. At time t2, the dew point T
d reaches the peak dew point during initial annealing, for example, + 6 ° C.

The set temperature of the initial annealing is 450 to 580 ° C.
It is preferable to set the temperature range to. This is 580
This is because the peak dew point during the initial annealing becomes too high at a temperature higher than 0 ° C, and the above-mentioned oxide layer may be generated at the plate width end portion of the tight coil 7, and at a temperature lower than 450 ° C, it is removed during the initial annealing. Because the amount of water that is taken is too small,
This is because the amount of water vaporized in the subsequent finish annealing becomes excessively large, and as a result, the dew point becomes too high and the oxide layer may be formed. On the other hand, when the temperature range of the initial annealing is 450 to 580 ° C., water is appropriately removed, so that the formation of the oxide layer in the final annealing is reliably prevented. After time t2, the dew point Td of the atmospheric gas starts decreasing. Further, the furnace top temperature Tf is maintained at the set value for the initial annealing, and the coil temperature Tc continues to rise so as to approach the furnace top temperature Tf.

At time t3, the initial annealing ends. The initial annealing is performed until the dew point Td of the atmospheric gas once rises, passes through the peak dew point of the initial annealing, then decreases again, and reaches 0 ° C. This is because when the dew point of the atmosphere gas at the end of the initial annealing exceeds 0 ° C, the amount of water removed in the initial annealing is too small, and the dew point increases during the subsequent finish annealing, and This is because the oxide layer may be generated. When actually performing the tight coil annealing, the holding time of the initial annealing is set in advance so as to satisfy this condition. In this example, the holding time of the initial annealing was set to 5 hours, and the measured value of the dew point Td of the atmospheric gas at the time of finishing the initial annealing was −2 ° C. After time t3, the furnace top temperature Tf starts to increase again.
Further, since the evaporation amount of water increases accordingly, the dew point Td of the atmospheric gas starts to rise again. The coil temperature Tc continues to increase.

At time t4, the furnace top temperature Tf reaches a preset finish annealing setting value, for example, 650 ° C., and thereafter the combustion of the combustion burner 5 is controlled so as to keep the furnace top temperature Tf constant. . Dew point Td and coil temperature T
c continues to rise even after time t4. The setting value of finish annealing is preferably set in a temperature range of 600 to 720 ° C. This is because the crystal grains of the cold rolled bright steel strip are not sufficiently grown at a temperature lower than 600 ° C., so that the mechanical properties of the cold rolled bright steel strip become hard and sufficient workability cannot be obtained. If the temperature exceeds 720 ° C., the steel strips of the stacked tight coils 7 may be seized, and
This is because, since the heating temperature exceeds the A _c1 transformation point, carbides are coarsened and the workability of the cold rolled bright steel strip deteriorates. In the finish annealing temperature range of 600 to 720 ° C., recrystallization and growth of crystal grains are sufficiently performed, so that the cold rolled bright steel strip can obtain excellent workability.

At time t5, the dew point Td of the atmospheric gas reaches the peak dew point during finish annealing, for example, + 2 ° C.
The peak dew point Td during finish annealing is preferably kept below + 10 ° C. in order to prevent the formation of the oxide layer.
This is the reason why the initial annealing temperature and the dew point Td of the atmospheric gas at the end of the initial annealing are limited as described above. After time t5, the dew point Td of the atmospheric gas starts to decrease again. Further, the furnace top temperature Tf is maintained at the set value for finish annealing, and the coil temperature Tc continues to rise so as to approach the furnace top temperature Tf.

At time t6, the coil temperature Tc reaches a predetermined soaking temperature, for example, 630 ° C., and thereafter the combustion of the combustion burner 5 is controlled so as to keep the coil temperature Tc constant. The coil temperature Tc is kept at that temperature for a predetermined time, for example, 15 hours, so that the tight coil 7 is soaked. After the soaking is completed, the combustion burner 5 is extinguished and the furnace body 2 is removed. The tight coil 7 is cooled while the inner cover 3 is covered. Inner cover 3
Is removed at a time when the coil temperature Tc is cooled to a predetermined temperature, for example, 140 ° C., in order to prevent the tight coil 7 from being oxidized. The supply of the atmospheric gas is continued until the inner cover 3 is removed, and the tight coil annealing is completed by removing the inner cover 3.

Next, the excellent surface quality of the cold-rolled bright steel strip produced according to this example will be described by a specific example in comparison with a comparative example. The raw steel grades of the cold rolled bright steel strips used in this example and the comparative example are both low carbon aluminum killed steels, and cold rolling, electrolytic cleaning, tight coil annealing and temper rolling processes are performed in this order to cool. Rolled bright steel strip was manufactured. The manufacturing conditions of Examples and Comparative Examples were the same except for the tight coil annealing step. In the tight coil annealing process, the tight coil 7 is tight coil annealed equipment 1
2 to 4 stacks and charged into HN gas (12% H _2-
Tight coil annealing was performed in N ₂ ) atmosphere gas. Table 1 shows the tight coil annealing conditions and the tight coil charging weights of Examples and Comparative Examples, the atmospheric gas dew point at the end of the initial annealing and the peak dew point of the atmospheric gas at the time of finish annealing, which were measured during the tight coil annealing. . In the temper rolling process, the surface roughness (Ra) of the example and the comparative example was 0.
Dry temper rolling was performed at a reduction rate of 1 to 3% using a bright work roll of 20 μm or less. Table 1 shows the surface qualities of the examples and the comparative examples after the dry temper rolling.

In each of the examples from Table 1, the peak dew point of the atmospheric gas during finish annealing is low, and the generation of an oxide layer at the widthwise end of the tight coil 7 is prevented. It can be seen that the surface quality of the rolled bright steel strip is extremely good and that it exhibits excellent surface gloss over the entire area. Further, from Table 1, in all of the comparative examples, the peak dew point of the atmospheric gas during finish annealing is high, and an oxide layer is generated at the strip width end portion of the tight coil 7, so during dry temper rolling,
It can be seen that fogging occurs on the bright work roll, and uneven luster occurs on the surface of the cold rolled bright steel strip after the dry temper rolling.

As described above, since the cold-rolled bright steel strip manufactured according to this embodiment can suppress the increase in dew point during the finish annealing that is subsequently performed by removing the water during the initial annealing in the tight coil annealing step, It is possible to prevent the generation of an oxide layer at the end portion of the tight coil plate width, which is a problem in the conventional technique. Therefore, by cold temper rolling in the temper rolling step, it is possible to stably produce a cold rolled bright steel strip having excellent surface gloss over the entire region.

[0029]

[Table 1]

[0030]

As described above, according to the present invention, it is possible to produce a cold-rolled bright steel strip having excellent luster over the entire area by removing moisture from the atmospheric gas. Product yield is greatly improved.

Further, according to the present invention, since the cold-rolled bright steel strip has excellent workability, the quality competitiveness of the cold-rolled bright steel strip is significantly improved. Further, since the steel strips of the tight coil do not seize during the finish annealing, the defective rate of the cold rolled bright steel strip is significantly reduced.

Further, according to the present invention, since the generation of the oxide layer at the end portion of the width of the tight coil plate is surely prevented, the work roll does not fog in the temper rolling of the next step.
This reduces the number of work roll changes, so
The efficiency of temper rolling and the basic unit of work roll are greatly improved.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a simplified configuration of a tight coil annealing equipment capable of suitably performing the tight coil annealing according to the present invention.

FIG. 2 is a timing chart for explaining a tight coil annealing method that is an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing a temporal transition of a furnace top temperature, a coil temperature, and an atmospheric gas dew point in tight coil annealing according to a conventional technique.

FIG. 4 is a cross-sectional view showing a laminated state of steel strips at the plate width end portion of the tight coil.

[Explanation of symbols]

 1 Tight coil annealing equipment 2 Furnace body 3 Inner cover 6 Thermocouple for furnace top temperature measurement 7 Tight coil 8 Convector plate 9 Thermocouple for coil temperature measurement 12 Dew point meter

Claims (3)

[Claims] 1. A method for producing a cold-rolled bright steel strip produced through the steps of cold rolling, electrolytic cleaning, tight coil annealing, and temper rolling in this order, wherein predetermined annealing is performed during tight coil annealing. Lower than temperature 45
A method for producing a cold-rolled bright steel strip, comprising performing initial annealing at 0 to 580 ° C. and subsequently performing finish annealing at a predetermined temperature. 2. The finish annealing temperature is 600 to 720 ° C.
The method for producing a cold-rolled bright steel strip according to claim 1, wherein 3. The cold-rolled bright steel strip according to claim 1, wherein the initial annealing is continued until the dew point of the atmospheric gas once rises and then decreases again to reach 0 ° C. or lower. Manufacturing method.