JPS6250530B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing cold-rolled steel sheets, in particular, directly cold-rolling a hot-rolled steel sheet with scale still attached, and performing open box annealing in a non-reducing atmosphere to improve the surface of the steel sheet during or after cooling. The present invention relates to a method of manufacturing a cold rolled steel sheet by removing scale by shot blasting and temper rolling. Conventional cold-rolled steel sheets are manufactured through the following steps: hot rolling → pickling descaling → cold rolling → electrolytic cleaning → box or continuous annealing → temper rolling, but some processes do not involve electrolytic cleaning. There is also. The pickling descaling process removes a thick surface oxide film (scale) with a thickness of approximately 5 to 15 μm that is generated during finishing and winding of hot rolling, and this scale FeO,
It has a higher hardness than base steel whose main component is Fe ₃ O ₄ , and if it is cold rolled as is, scale fragments will dig into the surface of the steel plate.
It is essential to descale the product before cold rolling, as this significantly impairs the quality of the finished product. A pickling method is often used as a descaling method. Because this scale adheres strongly to the steel base, descaling takes 20 to 60 seconds even though pickling involves contact with inorganic acids such as hydrochloric acid or sulfuric acid at high temperatures of 80 to 95°C. It takes time. For this reason, there are inherent problems such as the need for extensive equipment and the need for waste acid treatment. 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. A number of proposals have been made, such as a method of suppressing the amount of scale by reducing the amount of scale, and a method of applying light pressure reduction such as a skin pass prior to pickling, but these methods are still insufficient and only a few have been put into practical use. be. In recent years, as a new descaling method to replace pickling, attempts have been made to remove scale by spraying high-pressure water containing grains on the hot-rolled sheet as it is or after preliminary treatment under light rolling. However, as mentioned above, the scale is firmly attached to the steel substrate and the scale thickness is as thick as 5 to 15 μm.
Considerable equipment and descaling energy are required. In the electrolytic cleaning process, electrolytic cleaning is performed in an alkaline bath to remove surface stains such as rolling oil attached during cold rolling and iron powder generated during rolling. A large amount of equipment and running costs are required. With the aim of omitting this electrolytic cleaning process, we have developed a cold rolling oil that easily decomposes and volatilizes during the annealing process, a method of applying a chemical solution at the final stage of cold rolling to clean the steel sheet surface, a method of reducing the reduction ratio during cold rolling, and Many attempts have been made to adjust the rolling oil concentration, improve annealing methods, etc. It is still far from sufficient to obtain excellent steel sheet surface properties. In the annealing process, especially box annealing, an expensive reducing atmosphere gas (usually
H ₂ , N ₂ gas), it takes a long time for heating, soaking and cooling, and if it comes into contact with moist air during cooling to the temper rolling start temperature (usually below 40℃) after de-furnacing. , Since rust occurs on the surface of the steel plate, it cannot be used as a product, so rust prevention treatment equipment using dehumidified air or _N2 gas is required, etc., which reduces equipment costs.
There are inherent problems such as manufacturing costs and manufacturing time losses. The present invention has been made to solve the above-mentioned problems, and in the manufacturing process of cold rolled steel sheets, the descaling process and electrolytic cleaning process before cold rolling are omitted, and the annealing process is greatly reduced. The purpose is to provide a low-cost method for manufacturing cold-rolled steel sheets through simplification and simple surface adjustment. The gist of the present invention is to directly cold-roll a hot-rolled steel plate with scales still attached to it without descaling, finish it to a predetermined thickness, and then use a non-reducing atmospheric gas (for example, N ₂ gas) to finish the steel plate. This is a method for producing cold-rolled steel sheets characterized by box annealing in an open coil state and simple removal of scale adhering to the surface of the steel sheets before temper rolling. Next, the present invention will be explained. When a steel plate with hot rolling scale still attached is directly cold rolled under good rolling lubrication, 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. 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 sheet surface decreases due to the increase in surface area due to the stretching effect as the cold rolling reduction increases. Table 1 shows a hot-rolled steel plate with a scale thickness of 9 μm, with the scale still attached.
This is a result of cold rolling and actual measurement of the rolling reduction ratio and scale thickness after rolling. For example, at 70% rolling, the scale thickness is reduced to 2.1 μm. Depending on the method of rolling, part of the fine scale may be mixed into the rolling oil, but
The amount of scale mixed in is small.

[Table] As mentioned above, the scale thickness of 2.1 μm is the average scale thickness due to an aggregate of fine powder scales. More specifically, the scale has been 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 satin-finished cold-rolled plate, and there are no scratches or the like. The black scale coated steel sheet thus obtained is then brought into an open coil state and box annealed. As the atmospheric gas, a non-reducing gas such as N ₂ gas is sufficient, and an expensive reducing gas is not necessary. Black scale-coated steel sheets have excellent heat absorption, shorten heating time, and the scale-coated portion has anti-corrosion and anti-corrosion effects, so the de-covering temperature can be higher than that of normal scale-free steel sheets, and the de-covering process is easier. No rust will occur even if exposed to humid air in the back atmosphere. For ordinary scaleless steel plates,
During annealing, the steel plate surfaces may come into metal contact with each other and cause seizure trouble, but in the present invention, the steel plate surface is coated with a fine powder scale with a high melting point, so seizure does not occur. As mentioned above,
The improvements in annealing efficiency are extremely large, such as shortening heating time, shortening cooling time and improving equipment efficiency through high-temperature de-furnacing, and eliminating the need for dehumidification equipment. Annealed steel sheets (steel strips) are made into good steel sheets by removing fine scale, rolling oil decomposition products (carbon), etc. remaining on the surface of the steel sheets at once during cooling or prior to the temper rolling process after cooling. Perform surface conditioning to impart surface texture. As mentioned above, the residual scale is separated from the base iron and is in the form of fine powder and pressed, and strong energy is not required for descaling. Therefore, a simple method is sufficient as a surface conditioning method, and mild dry blasting or liquid honing is advantageous. Generally, when mechanically disposing annealed steel sheets, there are concerns that the abrasive grains biting into the surface of the steel sheet may affect the surface properties of the steel sheet, and that the quality of the material deteriorates due to surface hardening. The quality is comparable to that of ordinary cold-rolled steel sheets. As described above, the method according to the present invention achieves process saving, labor saving, and energy saving in the manufacturing process of cold rolled steel sheets, and its industrial advantages are truly great. Next, an example will be described. Example 1 Hot-rolled steel plate with a thickness of 3.0 mm and a scale thickness of 9 μm (capped steel, component C: 0.060%, Su: 0.001
%, Mn: 0.30%, P: 0.007%, S: 0.010%)
was directly cold-rolled at a reduction rate of 70% with the scale still attached to obtain a black fine scale-coated steel sheet. Next, it was box annealed in an open coil state in a _N2 gas atmosphere (dew point -40°C). The heat treatment conditions are
The temperature was raised to 700°C at 100°C/hour, held at 700°C for 4 hours, cooled in the furnace, and de-furnaced at 300°C and 100°C.
As-cold-rolled sample, 300℃ de-furnaced sample, 100℃
The residual scale thickness was actually measured for the demolded samples. As a result, they were all the same at 2.1μm, and 300μm.
Even in the case of the 100°C de-heated sample, the scale thickness did not increase and the appearance was similar to that of the 100°C de-heated sample, indicating that the scale has oxidation resistance and can be de-heated at high temperatures. Example 2 The sample obtained in Example 1 was evaluated for rust resistance. Test conditions are temperature 40℃ and relative humidity.
The test was carried out in 90% high temperature and humid air, the samples were stacked (stack test), the number of test days was 14 days, and the rust test was expressed as the percentage of rusted area. In addition, according to the conventional manufacturing process, pickling → cold rolling → electrolytic cleaning → box annealing (electrolyzed material) and pickling → cold rolling → box annealing (unelectrolyzed material) were obtained. The steel plate used was used as a comparison material. The material and cold rolling reduction box annealing conditions of the comparison material are the same as in Example 1, except that the material is in a tight coil state and a reducing atmosphere gas (H ₂ :5
%, _N2 : 95%, dew point -40°C or lower), and the de-furnacing temperature was 100°C. An alkaline bath was used for electrolytic cleaning. Table 2 shows the evaluation results. It was found that the samples obtained by the method of the present invention did not generate rust and the black scale had sufficient rust resistance.

[Table] Example 3 Surface conditioning experiments were conducted on the 100°C demolished sample of Example 2 by dry blasting and liquid honing. For the dry blast method, glass powder with a particle size of #100 is used, and the blowing pressure is 2~
At 4 kg/cm ² , complete descaling was achieved within 2 to 3 seconds of spraying time. Regarding the liquid honing method, complete descaling was achieved using alundum (aluminum oxide) with a particle size of #100 at a spraying pressure of 3 to 5 kg/cm ² and a spraying time of 2 to 3 seconds. The surface of the steel plate after descaling was in excellent condition with no scratches or residual abrasive grains. Example 4 Regarding the descaled sample according to the present invention in Example 3 and the comparative material sample in Example, the surface cleanliness (cellotape test) and phosphate treatment coating of steel plates subjected to 1% temper rolling The subsequent paint adhesion and paint corrosion resistance were evaluated using a salt spray test. These results are shown in Table 3. 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】

[Table] Example 5 Table 4 shows the results of examining the mechanical properties of the sample according to the present invention of Example 4 and the comparative material sample (electrochemical material). The mechanical properties of the cold-rolled steel sheet manufactured by the method of the present invention were equivalent to those of the steel sheet manufactured using the conventional process.

[Table] Although the explanation has been given above mainly on ordinary steel cold-rolled steel sheets, the method of the present invention can also be applied to the production of silicon steel sheets and high-strength cold-rolled steel sheets, for which descaling is more difficult than for ordinary steel.

Claims (1)

[Claims] 1 After cold-rolling a hot-rolled steel plate with scale still attached, it is box-annealed in an open coil state in a non-reducing atmosphere, and the surface of the steel plate is subjected to dry blasting or liquid honing during or after cooling. 1. A method for producing a cold-rolled steel sheet, which comprises removing scale and surface dirt by heat rolling.