JP3411442B2 - Manufacturing method of steel sheet with excellent surface quality - Google Patents

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a method for producing a steel sheet having a good surface color tone and excellent surface quality.
The purpose of the present invention is to suitably prevent deterioration of the surface quality due to a surface condensate such as Mn generated during annealing of a cold-rolled steel strip. 2. Description of the Related Art Generally, in a method of manufacturing a steel sheet such as an original tin plate, a steel strip rolled to a target thickness by cold rolling is washed with water or degreased, and then subjected to a reducing atmosphere. In order to improve workability by annealing treatment below, and subsequently to improve mechanical properties, it is usual to apply a temper rolling to give a low elongation and finish. Therefore, major items such as sheet surface roughness, color tone, and shape that determine the surface quality of a cold rolled steel strip are mostly determined by temper rolling. [0004] However, in the above method for producing a steel sheet, the smudge (rolling wear powder, rolling oil, etc.) adhered to the surface of the steel strip after cold rolling is washed before annealing. Although it is removed in the process, there is a problem that the smudge that has entered the minute concave portion such as a scratch is not removed. That is, when the smudge remaining in the minute concave portions is reduced in the annealing stage, the partial pressure of oxygen at the interface of the steel strip surface is increased, whereby Mn, Si, Al and the like in the steel are preferentially produced. This is one of the causes of the formation of concentrated substances (oxides) on the surface. [0006] When temper rolling is carried out in a state where the above-mentioned concentrates are present on the surface of the steel strip, these concentrates adhere and deposit on the work roll surface, and the deposits are transferred to the subsequent steel strip surface. In addition, there has been a problem in that the surface color of the steel sheet is impaired due to the occurrence of unevenness of the mottled pattern or a decrease in the overall gloss on the steel strip surface after the temper rolling. Japanese Patent Application Laid-Open No. 6-108160 discloses that a steel strip after annealing is immersed in a 0.001 to 1% aqueous solution of an organic acid to remove concentrated substances such as Mn generated during annealing. However, a method for improving the surface quality of a steel strip is disclosed.However, it is ineffective for the poor surface tone caused by the minute recesses described above, and when there is a large amount of the residual smudge, a concentrated product after sufficiently annealing. May not be removed. The present invention has been made in view of the above-mentioned problems, and is intended to remove the concentrated material on the surface of the steel strip before temper rolling while suppressing the formation of the concentrated material during annealing. It is an object of the present invention to provide a method for manufacturing a steel sheet having excellent surface color tone. In order to solve the above-mentioned problems, a method for producing a steel sheet according to the present invention comprises the steps of:
In the method for producing a steel sheet to be subjected to annealing and temper rolling, a cold-rolled steel strip is pickled with an acid aqueous solution having a pH of 2 or more and 3 or less before annealing. And then temper rolling after pickling with an acid aqueous solution. [0010] In the present invention, the pH before annealing is performed.
Is subjected to an acid pickling treatment by dipping in an aqueous acid solution between 2 and 3 or applying the aqueous acid solution, thereby removing smudges such as iron powder remaining on the surface of the steel strip, particularly on fine concave portions before annealing. Thus, the dew point in the atmosphere gas in the annealing furnace is reduced. As a result, during annealing, Mn, S
The concentration of i, S, and the like on the surface is suppressed preferentially, and the formation of a surface concentrate is suppressed. Further, the steel strip after annealing is again subjected to p
By performing an acid washing treatment such as immersion in an aqueous acid solution in which H is between 2 and 3, the concentrate generated during annealing is removed. At this time, the amount of the condensate generated at the time of annealing is smaller than in the prior art, as described above, and thus can be sufficiently removed. [0012] Thereby, temper rolling is performed in a state in which the concentrate on the surface of the steel strip has been completely removed, and accumulation of the concentrate on the surface of the work roll in the temper rolling is prevented. The reason why the pH of the acid aqueous solution is limited to 2 or more and 3 or less is that if the pH is less than 2, intergranular corrosion occurs in a peracid washing state that generates etch pits. If it exceeds, it is because it was confirmed that the smudge was not removed before annealing, and that the concentrate was not sufficiently removed after annealing. Next, an embodiment of the present invention will be described with reference to the drawings. The production of a tin plate as a steel plate will be described as an example. FIG. 1 is an equipment configuration diagram showing an outline of a process for showing an example of an embodiment of the present invention. Reference numeral 1 denotes a payoff reel, on which a coil of a cold-rolled steel strip 2 rolled to a target thickness by a cold rolling mill (not shown) is installed. Downstream of the payoff reel 1, along a pass line, a degreasing tank 3, an pickling tank 4, a washing tank 5, an annealing furnace 6, a water cooling tank 7 for final cooling, a washing tank 8, and a refining tank A rolling mill 9 is arranged, and a tension reel 10 for winding the steel strip 2 is arranged at the most downstream side. The pickling tank 4 is filled with a sulfuric acid solution whose pH has been adjusted to 2-3. In addition, the water cooling tank 7
Is filled with an aqueous solution whose pH has been adjusted to 2 to 3 by adding sulfuric acid, and passing through the water-cooling tank 7 enables pickling after annealing. In the above facility, the payoff reel 1
After the steel strip 2 is unwound from the degreasing tank 3, the steel strip 2 is immersed in a sulfuric acid solution in a pickling tank 4 and subjected to pickling treatment. The smudge that has entered the recess is removed. Subsequently, the steel strip 2 is passed through a washing tank 5 to remove the acid, and is subjected to an annealing treatment in an annealing furnace 6. At this time, since the steel sheet to be annealed has completely removed the smudge on the surface of the steel sheet by the above pickling treatment, the dew point in the atmosphere gas in the annealing furnace 6 is reduced as compared with the conventional steel sheet, and the steel generated by the annealing treatment is reduced. The amount of surface concentration of the middle component decreases. That is, the amount of enrichment generated on the steel sheet surface after annealing is significantly reduced as compared with the conventional case. Next, immediately after the annealing, the steel strip 2 is immersed in an aqueous solution in a water cooling tank 7 and cooled. At this time, in the present embodiment, since sulfuric acid is added to the aqueous solution in the water cooling tank 7, the pickling treatment is performed simultaneously with the cooling, and the concentrated product such as Mn generated on the steel sheet surface during annealing. Dissolved in sulfuric acid solution and completely removed. here,
Since the amount of the generated concentrate is much smaller than in the past, it can be sufficiently removed by the pickling treatment in the water cooling tank 7. Subsequently, after being washed in the washing tank 8, the steel strip 2 is continuously sent to the temper rolling mill 9, where the steel strip 2 is wound on a tension reel 10 after the temper rolling is performed. It becomes an original plate product. As described above, in the present embodiment, since the smudge is completely removed by pickling before annealing, the concentration of components in the steel on the steel sheet surface during annealing is suppressed, and further, in the annealing step, The generated concentrate is completely removed in the water cooling tank 7 to which sulfuric acid has been added. As a result, in temper rolling subsequent to annealing, accumulation of concentrate on the roll surface of temper rolling mill 9 is avoided. [0022] This makes it possible to always provide a rough surface with a sound roll surface, to stably produce a steel plate having stable plate surface roughness and excellent glossiness with little change over time in color tone. Can be. Further, as described above, the smudge such as iron powder remaining after degreasing is completely removed by pickling before annealing, so that the smudge is prevented from adhering to the roll surface in the annealing furnace 6. You. As a result, the surface of the furnace roll is maintained in a healthy state, and the indentation defect caused by pickup on the surface of the furnace roll, which appears particularly during annealing of a silicon steel sheet, is also resolved. Here, in the above embodiment, sulfuric acid has been described as an example of an acid, but an acid used in an aqueous acid solution may be an inorganic acid such as hydrochloric acid or sulfuric acid, or an organic acid such as formic acid, acetic acid or citric acid. Any of acids may be used. Further, in the above embodiment, the pickling after annealing is performed in the water cooling tank 7, but a pickling tank may be provided separately from the water cooling tank 7. Further, in the above embodiment, immersion is described as an example of the pickling treatment. However, the pickling treatment is not limited to this, and the pickling treatment is performed by a known method such as application or spraying of the above acid aqueous solution. Is also good. In the above description, an original tin plate was described as an example of a steel plate. However, it is needless to say that the present invention can be applied to a steel plate for automobiles, a general cold rolled steel plate, and the like, and is particularly suitable for a steel plate having a severe surface quality. The steel sheet 2 immediately before temper rolling treated by the above equipment according to the present invention was measured for the state of enrichment on the surface of the steel sheet by GDS (glow discharge optical emission spectroscopy).
The result as shown in FIG. 2 was obtained. The horizontal axis represents the distance in the thickness direction from the surface of the steel sheet, and the vertical axis represents the amount of the concentrate. For comparison, the same applies to the steel strip 2 immediately before temper rolling by equipment based on the conventional production method without the pickling tank 4 and without adding acid to the water cooling tank 7. As a result, the result shown in FIG. 3 was obtained. As can be seen from FIGS. 2 and 3, when the method for manufacturing a steel sheet according to the present invention is employed, the concentrate on the steel sheet surface is completely removed before the temper rolling. Further, in both of the above facilities, the steel strip 2 after temper rolling is used.
When the plate surface roughness was measured, the results shown in FIG. 4 were obtained. Open circles are cases according to the present invention, and solid circles are cases according to the conventional method. As can be seen from FIG. 4, it is understood that the plate surface roughness can be obtained stably by applying the present invention.
That is, by applying the present invention, during temper rolling, the accumulation of concentrates on the temper rolling rolls is avoided, and stable surface roughness can always be performed using sound rolls. It turns out that it is possible. Further, when the color tone (brightness) of the steel strip 2 after the temper rolling was measured in both facilities, the result shown in FIG. 5 was obtained. From FIG. 5, it can be seen that the application of the present invention stably provides a high-luminance plate surface. As described above, in the method for manufacturing a steel sheet according to the present invention, the smudge that has entered the minute recesses on the steel sheet surface is also removed, and the components in the steel during the annealing are concentrated on the steel sheet surface. In addition, since the concentrate generated after annealing is removed by pickling again, there is an effect that stable roughness can be obtained by temper rolling. As a result, it is possible to manufacture a steel sheet having stable plate surface roughness and excellent glossiness. In addition, the pickling prior to the annealing removes the smudge that has entered the minute recesses, so that the smudge is prevented from being deposited on the rolls in the annealing furnace. Thereby, for example, a flaw caused by a pick-up on the roll surface which is conspicuous during annealing of a silicon steel sheet is eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic configuration diagram showing an example of a manufacturing process according to an embodiment of the present invention. FIG. 2 is a diagram showing a state of a thickened material on a steel sheet surface after annealing according to the present invention. FIG. 3 is a comparative diagram showing a state of a thickened material on a steel sheet surface after annealing based on a conventional method. FIG. 4 is a diagram showing a relationship between a rolling distance in temper rolling and sheet surface roughness. FIG. 5 is a diagram illustrating a relationship between a rolling distance and a color tone in the temper rolling. [Description of Signs] 1 Payoff reel 2 Steel strip 3 Degreasing tank 4 Pickling tank 5 Washing tank 6 Annealing furnace 7 Water cooling tank 8 Washing tank 9 Temper rolling mill 10 Tension reel

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-3-97878 (JP, A) JP-A-55-58334 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23G 1/08 C21D 9/52 101

Claims (1)

(1) A method for producing a steel sheet in which a cold-rolled steel strip is annealed and temper-rolled, wherein the cold-rolled steel strip has a pH of 2 to 3 before annealing. A method for producing a steel sheet having excellent surface quality, comprising pickling with an acid aqueous solution described below, tempering a steel strip after annealing with an acid aqueous solution having a pH of 2 to 3, and then temper rolling.