KR101086326B1 - Method for manufacturing ferrite stainless steel sheet with excellent surface property - Google Patents

Abstract

The present invention relates to a manufacturing method for reducing the white scratches caused by the oil pit generated during cold rolling of a ferritic stainless steel sheet.

Technical composition of the present invention for achieving the above object, by weight% C: 0.080% or less (excluding 0%), N: 0.08% or less (excluding 0%), C + N: 0.10% or less (excluding 0%) ), Si: 0.7% or less (excluding 0%), S: 0.010% or less (excluding 0%), Cr: 16.0 to 17.0%, Ni: 1.0% or less (excluding 0%), Mo: 0.5% or less (0% Cu: When cold rolling a ferritic stainless steel sheet having a composition range of 0.5% or less (excluding 0%), the surface roughness (Ra) of the work roll is set to 0.4 to 0.75 µm, and is applied to the strip (S). It is comprised so that a maximum tension may be 26 tons or less, and cold rolling may be carried out with the reduction ratio at the 1st pass as 7 to 10%.

Ferritic, oil pit, white crack, surface roughness, maximum tension, rolling reduction

Description

Manufacturing method of ferritic stainless steel plate with excellent surface quality {METHOD FOR MANUFACTURING FERRITE STAINLESS STEEL SHEET WITH EXCELLENT SURFACE PROPERTY}

The present invention relates to a method for manufacturing a ferritic stainless steel sheet having excellent surface quality, and to a method for reducing white defects caused by oil pits generated during cold rolling of a ferritic stainless steel sheet.

Stainless steel sheet is largely divided into austenitic, ferritic, martensitic, and precipitation hardening stainless steel sheets.

Austenitic stainless steel plate (representative steel grade: STS 304) has a crystal structure of non-magnetic face-centered cubic lattice (FCC), which has a high nickel content, which is excellent in corrosion resistance, and hardened only by processing without being hardened by heat treatment. Has characteristics.

Ferritic stainless steel plate (representative steel grade: STS 430) has a ferromagnetic body-centered cubic lattice (BCC), which is less corrosion resistant than austenitic but resistant to stress corrosion cracking, and is not hardened by heat treatment and has excellent cold workability. .

Martensitic stainless steel plate (STS 410) is a stable austenitic structure at high temperature, but martensite transformation by air cooling or oil cooling, and has a martensite structure with ferromagnetic body-centered cubic lattice (BCC) at room temperature. will be. Corrosion resistance is relatively low but its strength is excellent, so it is widely used as a high strength structural steel.

Precipitation hardening stainless steel sheet (representative steel grade: STS 631) is an aging treatment of an alloy of chromium 17% by weight-7% by weight of nickel to precipitate intermetallic compounds such as Cu, Al, Ti, and Nb to improve strength. It has both characteristics of high austenitic corrosion resistance and high martensitic strength.

Cold rolled coils of stainless steel sheet are typically a hot rolling process for stretching the slabs produced in the steelmaking process in the longitudinal direction above the recrystallization temperature, an annealing pickling process for removing the surface scale of the hot rolled coils and reducing the internal stress, It is manufactured through a cold rolling process that makes the surface more beautiful and produces steel plates with the correct dimensions. In the cold rolling process, the surface quality of the cold rolled coil is greatly influenced by the roll roughness, tension, and reduction ratio.

As described above, the ferritic stainless steel plate represented by STS 430 is widely used in home appliances and kitchen appliances because it satisfies both corrosion resistance and economical efficiency. In order to be used for this purpose, not only the mechanical properties of the cold rolled coil but also excellent surface quality are required. .

However, in the conventional waste-light stainless steel sheet, an oil pit is generated by rolling oil which is inevitably supplied during rolling in the cold rolling process, and the oil pit causes defects such as white scratches on the surface of the final product. There was a problem that is generated to reduce the surface quality. The problem of white defects is becoming more important in the bright annealing ferritic stainless steel sheet which requires strict surface quality.

The present invention was developed to solve the above problems, by optimizing the surface roughness of the work roll, the maximum tension of the cold rolling mill, the rolling reduction rate, etc. in the cold rolling conditions of ferritic stainless steel sheet can minimize the occurrence of white defects The purpose is to provide a manufacturing method.

Technical composition of the present invention for achieving the above object, by weight% C: 0.080% or less (excluding 0%), N: 0.08% or less (excluding 0%), C + N: 0.10% or less (excluding 0%) ), Si: 0.7% or less (excluding 0%), S: 0.010% or less (excluding 0%), Cr: 16.0 to 17.0%, Ni: 1.0% or less (excluding 0%), Mo: 0.5% or less (0% Cu: When cold rolling a ferritic stainless steel sheet having a composition range of 0.5% or less (excluding 0%), the surface roughness (Ra) of the work roll is set to 0.4 to 0.75 µm, and is applied to the strip (S). It is comprised so that a maximum tension may be 26 tons or less, and cold rolling may be carried out with the reduction ratio at the 1st pass as 7 to 10%.

In addition, the surface roughness of the work roll is configured to work through an abrasive of 80 mesh.

In addition, the cold rolling is configured to be made for a stainless steel sheet subjected to bright annealing.

According to the method for producing a ferritic stainless steel sheet having excellent surface quality according to the present invention configured as described above, it is possible to significantly reduce surface defects such as white defects appearing during cold rolling. As a result, the other properties of surface quality, such as glossiness and stiffness, are also improved, thereby greatly improving the surface quality of the final product.

Hereinafter, a method of manufacturing a ferritic stainless steel sheet having excellent surface quality according to the present invention with reference to the accompanying drawings in more detail.

1 schematically shows a cold rolling process of a stainless steel sheet. The white coil which has undergone the annealing pickling process is gradually mailed from the first tension reel 10 and wound around the first tension reel 50 in a state of being pressed by about 7 to 8 m. The plate strip S is subjected to the first pass rolling under a constant tension, and after the first pass is completed, the end portion of the strip is wound again by the first tension reel 10 in the opposite direction. It progresses and 2nd pass rolling is performed. As such, the strip of the stainless steel sheet reaches the final thickness after cold rolling of several passes while reciprocating between the first tension reel 10 and the second tension reel 50.

In the center of the cold rolling facility, a 20-stage rolling mill 40 capable of applying a high reduction force to roll a high strength stainless steel sheet is installed, and in this 20-stage rolling mill 40, sprayed rolling oil is applied to increase the lubricity of the strip. Rolled oil injector 45 is installed. Then, a thickness gauge 20 for measuring the thickness of the strip after one pass and a plurality of wiper rolls 30 for preventing leakage of the rolling oil are provided on both sides of the 20-stage rolling mill 40, respectively.

As such, the rolling oil sprayed during the cold rolling process causes surface deformation between the work roll and the strip of the 20-stage rolling mill 40 to generate white defects. Referring to Figure 2 briefly described the behavior of the rolling oil between the work roll and the strip during the cold rolling and the cause of the white scratches due to this.

In the cold rolling, the rolling oil 70 sprayed for cooling and lubrication forms an oil film between the work roll 60 and the strip S. The oil film is formed on the surface of the strip not completely flat. The amount of oil will also be uneven. In particular, the oil finely present in the concave portion of the strip forms an uneven oil pit 80 on the surface of the strip due to the hydrostatic pressure applied by the work roll 60 at the time of rolling, and the oil pit 80 Due to the nature of the strip, fine cracks are generated along slip bands that easily occur in the perpendicular direction of rolling, which is left as white spots in the final product. 3 shows a series of white flaws formed in the direction perpendicular to the rolling.

The inventors have optimized the cold rolling conditions of the ferritic stainless steel sheet through several experiments in terms of surface roughness, maximum tension, and reduction ratio of the first pass to minimize white defects.

Ferritic stainless steel sheet is weight% C: 0.080% or less (excluding 0%), N: 0.08% or less (excluding 0%), C + N: 0.10% or less (excluding 0%), Si: 0.7% or less (0 S: 0.010% or less (excluding 0%), Cr: 16.0 to 17.0%, Ni: 1.0% or less (excluding 0%), Mo: 0.5% or less (excluding 0%), Cu: 0.5% or less ( Except 0%), the present invention is directed to this ferritic stainless steel sheet.

According to the present invention, first, the surface roughness Ra of the work roll of the 20-stage rolling mill is controlled to 0.4-0.75 µm. Since the work roll is the part in direct contact with the surface of the strip, its surface roughness is a very important factor in determining the surface quality of the strip. If the surface roughness of the work roll is less than 0.4 μm, the surface of the work roll is too flat, making it difficult to apply an appropriate reduction to the surface of the strip. It is not possible to effectively prevent the formation of non-uniform oil pits.

The surface roughness of the work roll is processed by an abrasive having a constant mesh, and the mesh of the abrasive is varied such as # 80, # 120, # 240, # 400, and the optimum surface roughness range that can be processed for each mesh Is fixed. Therefore, in order to process a work roll with the surface roughness of 0.4-0.75 micrometer, it is preferable to use an abrasive of 80 mesh.

Further, according to the present invention, the maximum tension acting on the strip between the first and second tension reels in the cold rolling process is controlled to 26 tons or less. In the cold rolling process, the tension applied to the strip is changed according to the cold rolling process (initial, middle and final stages), so the minimum tension cannot be limited, but the maximum tension applied to the entire cold rolling process can be controlled. .

When cold rolling a ferritic stainless steel sheet was conventionally controlled so that the maximum tension of 38 to 40 tons acts. However, if the maximum tension is too high, the rolling speed in the work roll will be increased, so that uniform rolling to prevent the formation of oil pits across the strip width direction is not guaranteed, so controlling the maximum tension during cold rolling to 26 tons or less is not recommended. desirable.

Cold rolling of ferritic stainless steel sheet is made through several passes. Conventionally, it is known that the higher the reduction ratio of the first pass, the better the surface quality, and the first pass was performed at a reduction ratio of 20% or more. However, the inventors confirm that the oil pit is saturated when the third pass or more progresses, and it is more effective to suppress the generation of the oil pit at the initial pass stage, thereby controlling the reduction ratio of the first pass to 7 to 10%. It was. If the reduction rate of the first pass is less than 7%, the reduction rate to be handled in the subsequent pass becomes too large, and if the reduction rate is greater than 10%, it will not effectively suppress the formation of the initial oil pit.

The annealing treatment of the stainless steel sheet includes a general annealing (Cold Annealing and Pickling Line, CAPL) and a bright annealing line (BAL) for the surface gloss, the manufacturing method according to the present invention is a bright annealing that places importance on the surface quality It is more effective when applied to treated materials.

Table 1 below illustrates the scheduling of cold rolling passes to be carried out in accordance with the present invention. According to this, cold rolling is performed by controlling the reduction ratio of the first pass to 9.8%, and controlling the maximum tension to 26 tons in all passes except the entry tension in the first pass.

pass Exit thickness (mm) Rolling speed (mpm) Rolling reduction (%) Input tension (ton) Outgoing tension (ton) One 3.680 250 9.8 10 26 2 3.150 450 14.4 26 26 3 2.668 650 15.3 26 26 4 2.246 650 15.8 26 26 5 1.894 650 15.7 26 26 6 1.600 650 15.5 26 26 7 1.357 650 15.2 26 26 8 1.156 450 14.8 26 26 9 1.001 300 13.4 26 26

In order to examine the effect of cold rolling according to the present invention, after performing cold rolling in each of the two conditions shown in Table 2 below, the surface quality of the final product was divided into white blemish, glossiness, and stiffness.

division Work Roll Surface Roughness (㎛) Tonnage First pass reduction rate (%) Comparative example # 120 (Ra: 0.15-0.4) 40 18-20 Example # 80 (Ra: 0.4-0.75) 26 7 to 10

Referring to FIG. 4, which shows a comparison of the surface photograph of the final product, it can be seen that in Example (b), the white defects on the surface of the steel sheet were significantly reduced than in Comparative Example (a). In the two cases, the results of measuring glossiness and lightness are shown in Table 3 below.

location direction Glossiness (GS20) Sundo-do (DOI) W / S C D / S Average W / S C D / S Average Comparative example Top L 709 664 939 771 90.35 90.20 94.45 91.87 C 793 567 882 747 88.62 88.84 92.31 89.92 End L 796 689 922 802 90.02 90.26 95.41 91.90 C 678 631 834 714 95.35 86.23 91.06 87.55 Example Top L 1194 1184 1186 1188 96.38 95.87 97.25 94.85 C 1097 972 1066 1045 94.38 94.14 96.03 94.85 End L 964 844 928 912 91.41 83.54 91.28 88.74 C 873 779 864 839 86016 85.86 89.73 87.25

Of the positions, Top means the leading end of the strip and End means the rear end. In the direction, L (Length) means the rolling direction and C (Cross) means the right angle direction of rolling. Among the widths of the strip, W / S stands for Work Side, D / S stands for Drive Side, and C stands for Center. For example, as a result of measuring three points of W / S, C, and D / S in the right angle direction of rolling (C) of the top of the comparative example, the average glossiness is 747 and the average degree of stiffness is 89.92. On the other hand, the three points of W / S, C, and D / S were measured in the perpendicular direction (C) of the rolling in the top of the embodiment. appear. In all other cases, the glossiness and lightness of the examples were higher than those of the comparative examples.

1 is a view showing a cold rolling process of a common stainless steel sheet.

2 is a view showing a white scratch generation process of the stainless steel sheet.

3 is a photograph of the surface of the steel plate with white scratches.

Figure 4 is a photograph showing a comparison of the steel plate surface of the comparative example and the embodiment.

[Description of Reference Numerals]

10: first tension reel 20: thickness gauge

30: wiper roll 40: 20-speed rolling mill

45: rolling oil injector 50: second tension reel

60: work roll 70: rolling oil

80: oil pit S: strip

Claims (3)

By weight% C: 0.080% or less (excluding 0%), N: 0.08% or less (excluding 0%), C + N: 0.10% or less (excluding 0%), Si: 0.7% or less (excluding 0%), S : 0.010% or less (excluding 0%), Cr: 16.0 to 17.0%, Ni: 1.0% or less (excluding 0%), Mo: 0.5% or less (excluding 0%), Cu: 0.5% or less (excluding 0%) In cold rolling a ferritic stainless steel sheet having a composition range, The surface roughness Ra of the work roll is 0.4 to 0.75 µm, the maximum tension applied to the strip S is 26 tons or less, and the cold rolling is performed at a rolling reduction of 7 to 10% during the first pass. Method for producing a ferritic stainless steel sheet having an excellent surface quality. The method according to claim 1, The surface roughness of the work roll is a method of manufacturing a ferritic stainless steel sheet having excellent surface quality, characterized in that the processing through the abrasive of 80 mesh. The method according to claim 1, The cold rolling is a method of manufacturing a ferritic stainless steel sheet having excellent surface quality, characterized in that made for a stainless steel sheet that is bright annealing treatment.

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