JP4581251B2 - Manufacturing method of steel sheet with good surface quality - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a steel sheet having good surface quality, and particularly to a method for suitably preventing deterioration of surface quality due to a surface concentrate such as Mn generated during annealing of a cold-rolled steel strip.
[0002]
[Prior art]
In recent years, when manufacturing cold-rolled thin sheets, a continuous annealing furnace is used instead of a batch furnace in the annealing process, and a coil-shaped material (steel strip) is continuously fed into the furnace from the furnace inlet to a predetermined temperature. The continuous annealing method in which the heat treatment is applied is employed for improving productivity and reducing quality variation. In this case, the final cooling is performed on the steel strip that has just been removed from the furnace through a recrystallization process while being mechanically fed into the continuous annealing furnace. In particular, when manufacturing a tin plate or the like, the cooling capacity is improved by responding to the increase in operation speed by performing final cooling by water cooling. Here, in the final cooling, purified water (industrial water) and soft water (water with reduced Ca and Mg contents) are used as cooling water.
[0003]
And recently, a rolling mill is directly connected after the continuous annealing apparatus, and the annealed steel strip is temper-rolled or second cold-rolled (hereinafter simply referred to as temper rolling), that is, light cold working is performed. A method of improving productivity and yield by applying continuously is adopted. For example, for a tin plate that has been temper-rolled, the roughness, color, and shape of the plate surface are extremely important qualities, and the surface quality is almost determined by the temper rolling after annealing.
[0004]
However, during annealing, Mn, Si, S, and the like, which are steel components, diffuse in the steel structure and collect on the surface to produce a concentrate. These concentrates such as Mn are deposited on the work rolls during temper rolling, and the deposits are transferred to the plate surface of the steel strip that is continuously fed. Therefore, the lightness of the surface of the steel strip after temper rolling is lowered, and there is a problem that the surface quality such as the color tone, the appearance, and the surface roughness is impaired.
[0005]
Concentrates such as Mn are easily peeled off by water cooling performed after annealing, and are unevenly deposited on the work roll during temper rolling, and this is transferred to the steel strip plate surface, resulting in uneven color tone in the plate width direction. There was also a problem that so-called two-tone color occurred.
[0006]
[Problems to be solved by the invention]
In order to solve the problem caused by the concentrate such as Mn, the applicant of the present invention has applied the cold-rolled steel strip to a concentration by weight in the final cooling step after annealing as described in Japanese Patent No. 2932883. While cooling by immersing in a 0.01% to 1% organic acid aqueous solution at the time of annealing, remove the concentrate such as Mn formed on the surface of the steel strip during annealing, and then perform temper rolling or second cold rolling continuously. developed. According to this method, the cold-rolled steel strip after annealing is immersed in an organic acid aqueous solution in the final cooling step to uniformly remove surface concentrates such as Mn generated during annealing. The surface quality of the product is assured with no decrease in lightness or mottled pattern on the steel strip after subsequent temper rolling or second cold rolling.
[0007]
However, the above-mentioned invention of Japanese Patent No. 2982883 has a problem in terms of running cost because it requires frequent concentration control in order to set an organic acid aqueous solution of a predetermined concentration, and the used organic acid has a problem. Since a waste water treatment facility for treating the aqueous solution is also required, there is a problem in terms of facility cost.
The present invention has been made in view of the above circumstances, and solves the problem of surface quality caused by the surface concentrate of a cold rolled steel strip at a relatively low cost without running costs and equipment costs. An object of the present invention is to provide a method for producing a steel sheet with good surface quality.
[0008]
[Means for Solving the Problems]
The inventors of the present invention focused on the fact that the method of performing the final cooling by water cooling is superior in terms of running cost and equipment cost as compared with the conventional method of immersing in an organic acid aqueous solution. Here, in the final cooling by conventional water cooling, purified water and soft water were used as cooling water, but these purified water and soft water contain a lot of impurities, so concentrates such as Mn generated on the surface of the steel strip. Is less reactive. Therefore, intensive research has been conducted on a method of selecting a cooling water with a small amount of impurities, that is, low electrical conductivity, and uniformly removing the concentrated product formed on the surface of the steel strip by increasing the reactivity.
[0009]
As a result, the steel strip temperature during final cooling of the steel strip after annealing is strictly controlled, and the steel strip is immersed in pure water with an electric conductivity of 0.5 × 10 ^-4 Ω ^-1 · m ^-1 or less. It was found that it can be uniformly removed by reaction with a concentrate such as Mn while cooling the band, and the present invention has been made.
According to this method, the concentrate does not accumulate unevenly on the work roll of temper rolling, and the surface roughness and color tone of the steel strip change over time when temper rolled continuously after annealing. There is no. Therefore, a steel plate having a good surface quality can be manufactured stably. In addition, since pure water is used, running costs are hardly required, and wastewater treatment equipment is not required, so that equipment costs can be reduced.
[0010]
Here, the intrusion temperature of the steel strip immediately before being immersed in the pure water is set to 120 to 150 ° C.
That is, the concentrate such as Mn formed on the surface of the steel strip after annealing is an atomic level deposit having a thickness of about 5 to 10 nm, and stabilization by oxidation proceeds at a high dew point immediately before entering the water tank. Therefore, it is necessary to lower the temperature of the infiltration plate immediately before being immersed in water, and the infiltration temperature of the steel strip immediately before being immersed in pure water is set to 150 ° C. or less. On the other hand, if the infiltration plate temperature becomes too low, the reactivity is lowered and the operation speed cannot be increased, so the infiltration plate temperature of the steel strip is set to 120 ° C. or higher.
[0011]
Therefore, in this invention, the penetration temperature of a steel strip is made into the low temperature of 120-150 degreeC, and the removal from the surface of a steel strip is made easy by making a concentrate into an unstable state.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows an outline of a process for carrying out the present invention. The steel strip 2 unwound from the cold-rolled coil of the payoff reel (POR) 1 is conveyed to the annealing furnace 4 through the degreasing device 3, and an annealing process is performed. Immediately thereafter, it is passed through a final cooling water cooling device 5 filled with 5 A of pure water having an electric conductivity of 0.5 × 10 ⁻⁴ Ω ⁻¹ · m ⁻¹ or less, and is immersed and cooled. And the steel strip 2 which passed the water-cooling apparatus 5 is sent to the temper rolling mill 7 through the dryer 6, temper-rolled, and is wound up with the tension reel (TL) 8, and a tinplate original plate product is manufactured.
[0013]
Here, the temperature of the steel strip 2 (the steel strip 2 at the position indicated by the symbol C in FIG. 1) immediately before being immersed in the water cooling device 5 is set to 120 to 150 ° C.
According to the process of the above configuration, when the steel strip 2 that has passed through the annealing furnace 4 is immersed in pure water having an electric conductivity of 0.5 × 10 ⁻⁴ Ω ⁻¹ · m ⁻¹ or less, it is applied to the surface of the steel strip 2. The produced concentrate such as Mn is easily removed due to increased reactivity with pure water with few impurities. And by setting the temperature of the steel strip 2 just before being immersed in the water-cooling apparatus 5 to 120-150 degreeC, the concentrate produced | generated in the steel strip 2 becomes low temperature, becomes an unstable state, and the steel strip 2 It becomes easier to remove the concentrated product on the surface.
[0014]
Moreover, since the temperature of the steel strip 2 immediately before being immersed in the water cooling device 5 is set to 120 to 150 ° C., the temperature of the steel strip 2 entering the dryer 6 is not lowered, and the drying efficiency of the dryer 6 is prevented from being lowered. can do.
For this reason, in the process of the present embodiment, the concentrated product does not deposit non-uniformly on the work roll of the temper rolling mill 7, and the surface roughness and color tone of the steel strip 2 when temper rolled are time-dependent. Therefore, the steel strip 2 having high brightness and good surface quality can be manufactured stably.
[0015]
In this embodiment, since pure water is used, the running cost is hardly required, and the waste water treatment facility is not required as in the conventional case. Therefore, the facility cost can be reduced. And the pure water used by this embodiment also has the effect that it can also be reused with the degreasing apparatus 3. FIG.
[0016]
【Example】
The process of the present invention (the temperature of the steel strip 2 immediately before being immersed in the water cooling device 5 is filled with pure water 5A having an electric conductivity of 0.5 × 10 ⁻⁴ Ω ⁻¹ · m ⁻¹ or less in the water cooling device 5). A tin plate was manufactured in two steps, a step set to 150 ° C.) and a conventional step using a water cooling device 5 in which only purified water was filled for comparison.
[0017]
After the removal of the concentrate such as Mn is performed by the water cooling device 5, the steel strip 2 immediately before being sent to the temper rolling mill 7 is sampled for each of the conventional process and the process of the present invention. Then, the surface concentration state of Mn or the like is measured by GDS (glow discharge emission spectrometer), FIG. 2 (a) shows the measurement result of the conventional process, and FIG. 2 (b) shows the measurement result of the process of the present invention. . The horizontal axis represents the depth from the steel sheet surface, and the vertical axis represents the amount of concentrate.
[0018]
Compared with the conventional process of FIG. 2A, the condensate such as Mn is clearly removed in the process of the present invention of FIG. 2B.
Moreover, the result of having measured the relationship between the rolling distance in the temper rolling mill 7 and the color tone (lightness) of the product tin plate in the conventional process and the process of the present invention is shown in FIG. From these contrasts, it is clear that the process of the present invention has higher brightness and superior glossiness, and the process of the present invention does not cause uneven color tone, so-called two-tone problems. Further, in the process of the present invention, even when the steel strip temperature just before dipping in the water cooling device 5 was set to 170 ° C., the same investigation as in FIG. 3 was performed, but only a lightness of about 1.1 was obtained. It was.
[0019]
In addition, although a present Example targets tinplate original plate, this invention is applicable not only to the steel plate for surface treatment like a tinplate original plate but to the steel plate for motor vehicles, and a general cold-rolled steel plate.
[0020]
【The invention's effect】
As described above, according to the present invention, when the steel strip that has passed through the annealing furnace is immersed in pure water having an electrical conductivity of 0.5 × 10 ⁻⁴ Ω ⁻¹ · m ⁻¹ or less, Concentrates such as Mn generated on the surface are easily removed due to increased reactivity with pure water with few impurities. Moreover, since the temperature of the steel strip immediately before being immersed in pure water is set to 120 to 150 ° C., the concentrated product generated in the steel strip is lowered in temperature and becomes unstable. Removal of the concentrate is promoted. Therefore, the brightness of the steel strip surface after the temper rolling or the second cold rolling can be increased, and the surface quality of the product is guaranteed.
[0021]
In this embodiment, since pure water is used, the running cost is hardly required, and the waste water treatment facility is not required as in the conventional case. Therefore, the facility cost can be reduced.
[Brief description of the drawings]
FIG. 1 is a diagram showing an outline of a manufacturing process according to an embodiment of the present invention.
FIG. 2 is a graph showing the results of measuring the state of the concentrate formed on the steel strip immediately before being sent to the temper rolling mill by GDS, according to the present invention and the conventional method.
FIG. 3 is a graph showing a comparison between the method of the present invention and the conventional method with respect to the color tone after constant-distance temper rolling.
[Explanation of symbols]
1 Payoff reel (POR)
2 Steel strip 4 Annealing furnace 5 Water cooling device 5A Pure water 7 Temper rolling mill 8 Tension reel (TR)

Claims (1)

The cold-rolled steel strip is cooled by immersing it in pure water having an electric conductivity of 0.5 × 10 ⁻⁴ Ω ⁻¹ · m ⁻¹ or less in the final cooling step after annealing. The intrusion temperature of the steel strip immediately before dipping is set to 120 to 150 ° C. to increase the reactivity between the concentrated product generated on the surface of the steel strip and the pure water, and then continuously the steel strip. A method for producing a steel sheet with good surface quality, characterized in that temper rolling or second cold rolling is performed.

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