JPS633010B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing cold-rolled steel sheets, in particular, directly cold-rolling a steel sheet that has been hot-rolled in a normal process with scale still attached, and acid-treating the resulting cold-rolled steel sheet with scale still attached. The present invention relates to a method of manufacturing a cold-rolled steel sheet by removing scale from the surface of the steel sheet and then annealing it. Conventional cold rolled steel sheets are manufactured through the following steps: hot rolling → pickling and descaling → cold rolling → electrolytic cleaning → box or continuous annealing → temper rolling. In some cases, electrolytic cleaning is not necessary. The pickling descaling process removes a thick surface oxide film (scale) with a thickness of about 5 to 10 microns that is generated during hot rolling finishing and winding. Because this scale is firmly attached to the steel base, pickling is also difficult with inorganic acids such as hydrochloric acid or sulfuric acid.
Despite contacting at a high temperature of ~95°C, descaling takes a long time, 20 to 60 seconds. For this reason,
Requires long equipment, production speed is slower than the steps before and after hot rolling and cold rolling, and requires a large amount of acid.
It has inherent problems such as. In the conventional pickling method, in hopes of shortening the descaling time, methods such as leveling or bending the hot-rolled steel sheet prior to pickling and creating cracks in the scale layer to encourage the infiltration of acid solution are also used. It has been implemented. In addition, as measures to improve pickling efficiency, there are methods to modify the scale composition by adjusting the hot rolling winding temperature and cooling conditions, methods to modify the scale by applying chemicals, and adjusting the atmosphere in which scale is generated. Although there have been many proposals, such as a method of suppressing the amount of scale by reducing the amount of scale, and a method of applying a light reduction such as a skin pass prior to pickling, these methods are still insufficient and only a few have been put into practical use. The electrolytic cleaning process removes rolling oil that adheres during cold rolling.
Electrolytic cleaning is performed in an alkaline bath to remove surface stains such as iron powder generated during rolling.
As with the pickling and descaling process, a large amount of equipment and running costs are required. In order to omit this electrolytic cleaning process, we have developed a cold rolling oil composition that easily decomposes and volatilizes during the annealing process, a method for cleaning the steel plate surface by applying a chemical solution at the final stage of cold rolling, and a reduction rate distribution during cold rolling. Although many attempts have been made to improve the annealing method, adjusting the rolling oil concentration, etc., the current situation is still insufficient to obtain the excellent surface properties of steel sheets that can be obtained by electrolytic cleaning. be. The present invention aims to greatly simplify the descaling process and omit the electrolytic cleaning process in the manufacturing process of cold-rolled steel sheets, and to provide a method for manufacturing low-cost cold-rolled steel sheets with excellent steel sheet surface properties. It is something. The gist of the present invention is to directly cold-roll a steel plate manufactured in a normal hot rolling process with scale still attached to it and finish it to a predetermined thickness, and then acid-treat the surface of the steel plate to remove scale, rolling oil, etc. This is a method for producing cold-rolled steel sheets, which is characterized by removing all the particles all at once and annealing them to give a clean surface. Next, the present invention will be explained. Previously, the present inventors proposed in Japanese Patent Application Laid-Open No. 134810/1983 a method of directly cold rolling a hot rolled steel plate with scale still attached.
Other similar proposals are published in Japanese Patent Application Laid-Open No. 52-21245,
No. 52-21246, No. 52-52157, No. 52-
It is in Publication No. 52158. These proposals were based on the premise of modifying the scale composition of hot rolled steel sheets. However, in these methods, the rolling speed decreases due to modification of the scale composition in the hot rolling process, and in the cold rolling process, the scale peels off and scatters, or gets mixed into the rolling oil, which must be treated. Therefore, the equipment becomes complicated, and there are problems with reduced workability, ease of maintenance, and economy.
In order to solve the above-mentioned problems, the method of the present invention was developed by taking these proposals one step further through subsequent research. It has been found that direct cold rolling is possible for the hot rolled steel sheet having the scale composition obtained in , and that the surface scale after cold rolling can be easily removed by simple acid treatment. That is, when a steel plate with hot rolling scale still attached is directly cold rolled, a steel plate with a black surface coated with fine powder scale is obtained. Hot-rolled scale has poorer plasticity than base steel, so it is crushed by compressive and tensile stress within the rolling roll bite. The scale form after rolling is a fine powder aggregate that loosely adheres to the surface of the steel sheet together with rolling oil. That is, since scale hardly peels off from the steel sheet during cold rolling, rolling oil is not contaminated with scale. This fine powder scale can also be removed by rubbing it with your fingers. At the same time, the thickness of the black scale covering the steel plate surface decreases due to the increase in surface area due to the stretching effect as the cold rolling reduction increases and the peeling effect of the scale due to rolling. Table 1 shows the results of cold rolling a hot-rolled steel plate with a scale thickness of 9 μm with the scale still attached, and measuring the reduction ratio and scale thickness after rolling. For example, at 70% reduction, the scale thickness decreases to 2.1 μm. are doing.

[Table] As mentioned above, the scale thickness of 2.1 μm is the average scale thickness due to the aggregate of fine powder scale. Furthermore, in detail, the scale is peeled off and separated from the base iron, as if it had been loosely crimped with a roll. This is a situation in which fine powder scale is crimped onto a steel plate, and when the fine powder scale is removed, it has an appearance similar to a cold-rolled plate with a satin finish, and there are no scratches or the like. However, if this black scale remains attached, it is difficult to obtain a sufficiently beautiful product, and it is necessary to remove it. Utilizing acid is a simple and effective method for removing black scale. The black scale after cold rolling is thinner than the hot rolled scale and is finely powdered, so that the acid solution can penetrate therethrough very easily. That is, in the descaling process by acid treatment, the acid solution instantly penetrates into the fine powder scale layer and the boundary between the scale and the steel base, and the scale can be removed in an extremely short period of time. Even in the conventional manufacturing process of hot rolling → pickling → cold rolling, we expect to shorten the pickling and descaling time.
Leveling or bending prior to pickling,
Alternatively, skin pass rolling with a rolling reduction of about 3 to 5% may be performed, but this does not directly lead to a drastic reduction in the pickling time as in the method of the present invention. In other words, with bending or leveling processing, the scale layer is destroyed macroscopically, the number of cracks per unit area on the scale-covered surface is small, and the penetration of the pickling solution is localized, so it takes a long time to remove the scale. cannot be shortened to Under light rolling such as a skin pass, cracks appear in the hot-rolled scale layer over the entire surface in the width direction and rolling direction, and the cross-sectional direction of the scale layer is also destroyed to some extent.
Penetration of pickling liquid is promoted more than bending or leveling processing. However, the method of the present invention does not result in pulverized scale, and the boundary between the base iron and the scale, which plays an important role in the pickling descaling effect, is still in close contact under the skin pass level of reduction, and the pickling time There is a limit to the shortening of the width. Figure 1 shows scale cross-sectional forms and acid solution infiltration conditions for A: bending such as bending or leveling, B: light reduction such as skin pass, and C: cold rolling according to the present invention. This is a schematic diagram showing the difference between the two. In FIG. 1, 1 is the steel base, 2 is the boundary between the base steel and the scale, 3 is the scale, 4 is the crack, and 5 is the infiltrated acid solution. Next, the descaling method by acid treatment of the present invention will be explained. The acid used may be an inorganic acid such as hydrochloric acid or sulfuric acid, or an organic acid such as formic acid. Descaling methods include immersion in a bathtub as in the conventional pickling process, brushing while spraying an acid solution onto the scaled surface,
Liquid honing, which involves spraying a granular acid solution, is effective. In the present invention, in the descaling process by this acid treatment, the black fine powder scale is removed, and at the same time, all dirt present on the surface of the steel sheet, such as residual rolling oil, is also removed. Therefore, the electrolytic cleaning step of the conventional method can be omitted. Furthermore, since there is no residue on the surface of the steel sheet after acid treatment, the product has superior surface properties compared to conventional methods. The annealing step after the acid treatment can be performed in a non-oxidizing or reducing atmosphere in the same manner as conventional methods such as box annealing, continuous annealing, open coil annealing, etc. By directly cold-rolling a steel plate with hot-rolled scale attached to it, pulverizing the scale, and reducing the thickness of the scale, the descaling time was reduced compared to the conventional method, as shown in the example. It can be drastically shortened to less than /5 to 1/10. Therefore, it is possible to shorten the line length of acid treatment equipment, improve the line threading speed,
The advantages are extremely large, such as reducing the amount of acid used and reducing waste acid processing costs. Furthermore, it is an excellent method for producing cold rolled steel sheets that has the advantage of omitting the electrolytic cleaning step and producing products with clean surface properties. Example 1 A hot-rolled steel plate with a thickness of 3.0 m/m and a scale thickness of 9 μm was rolled at a rolling reduction rate of 3 to 85% with the scale still attached.
After directly cold rolling the rolled material, the descaling time required to completely remove the scale on the surface of the rolled material was measured using acid. The results are shown in FIG. It was found that the descaling time of cold-rolled materials can be significantly shortened by crushing the scale and reducing the thickness of the scale. The acid solution composition was 5% HCl, 0.3% commercially available organic inhibitor, and 29 g/FeCl ₂ .6H ₂ O, and the solution temperature was 90°C. The descaling method involved immersion in liquid. As the cold rolling roll, a mirror-finished bright roll was used. Example 2 The descaled rolled sheet with a cold rolling reduction of 70% shown in Example 1 was placed in a reducing atmosphere (H ₂ : 5%, N ₂ : 95%).
%, dew point below -40°C). Heat treatment conditions: temperature increase 100°C/hour up to 700°C, 700°C x 2
It was maintained for a certain period of time and was released after cooling in the furnace at 40℃. The steel plate after 1% temper rolling was evaluated for surface cleanliness (cellotape test), paint adhesion after phosphate treatment, and paint corrosion resistance by salt spray test. In addition, according to the conventional manufacturing process, pickling → cold rolling → electrolytic cleaning → box annealing → pressure adjustment (electrocleaned material) and pickling → cold rolling → box annealing → pressure adjustment (unelectrolyzed material) The steel plate obtained using the above method was used as a comparison material. The material, cold rolling reduction, box annealing conditions, and temper rolling ratio of the comparison material were the same as those described above, and an alkaline bath was used for electrolytic cleaning. These results are shown in Table 2. The surface properties of the cold-rolled steel sheets obtained by the method of the present invention were equivalent to or superior to those obtained by the conventional method.

[Table] (Notes) (1) Surface cleanliness The reflectance was measured using Sellotape. (2) Paint adhesion Adhesion was evaluated by burlap, bending, and impact tests. ◎Peeling rate 10% or less, 〇20% or less, △
Good if the peeling rate is less than 30%. (3) Painting corrosion resistance This is the result of the 10th salt spray test (JIS-Z2371) after phosphate treatment and painting. The evaluation is good in the order of ×, △, 〇, and ◎. Although the above description has mainly been given to ordinary cold-rolled steel sheets, the method of the present invention can also be applied to the production of silicon steel sheets, stainless steel sheets, and high-strength cold-rolled steel sheets, which are more difficult to descale than ordinary steel.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the cross-sectional form of scale and the infiltration of acid solution when hot-rolled steel sheets are descaled by bending, skin pass rolling, and cold rolling. FIG. 2 is a diagram showing the relationship between descaling time and cold rolling reduction ratio when a hot rolled steel sheet is directly cold rolled and then treated with an acid.

Claims (1)

[Claims] 1 A hot-rolled steel sheet with scale produced in a normal hot-rolling process still attached is cold-rolled, and after the cold-rolling, most of the hot-rolled scale is attached to the surface as a fine powder aggregate. A method for producing a cold-rolled steel sheet, which comprises cleaning the cold-rolled steel sheet by removing fine powder scale and dirt from the surface by acid treatment, and then annealing the sheet.