KR100543514B1 - Manufacturing method for bright cold rolled steel sheet - Google Patents

Abstract

The purpose of the present invention is to remove the mixed structure by managing more than FGS (Ferrite Grain Size) No7 in hot rolling operation, to solve the problem of poor formability such as orange peel caused by high drawing in the metal texture of large grain size and high pressure, high pressure The present invention provides a method for manufacturing a bright cold rolled steel sheet that can satisfy the demanded surface glossiness and surface roughness of a consumer by only one temper rolling by improving the transfer rate by tension.

The present invention for achieving the above object is cold rolled steel plate and electrolytically clean it, then heat treated and temper rolled surface glossiness of 50 or more. In the bright cold rolled steel sheet manufacturing method to ensure the roughness 0.3 ~ 0.6㎛,

The roll to be temper rolled using a chrome coated bright roll having a roughness of 0.25 to 0.4 μm with only one high elongation temper rolling with a rolling reduction of 250 to 300 tons, shear tension of 4.5 to 5.0 tons and post shear strength of 5.2 to 5.7 tons. will be.

Temper Rolling, Cold Rolled Steel, Work Roll, Glossiness, Roughness

Description

Manufacturing method for bright cold rolled steel sheet

1 is a schematic view showing a conventional bright cold rolled steel sheet manufacturing method process,

2 is an excerpt of a work roll used in conventional cold rolling and temper rolling;

3 is a schematic view showing a process of manufacturing a bright cold rolled steel sheet according to the present invention;

Figure 4 is an excerpt of the work roll used for cold rolling, temper rolling according to the present invention,

※ Code explanation for main part of drawing ※

10: cold rolling mill

20: electrolytic cleaning line

30: continuous annealing furnace

40: primary temper rolling equipment

50: 2nd temper rolling mill

The present invention relates to a method for manufacturing a bright cold rolled steel sheet, and more particularly, a bright cold rolled steel sheet used for automobiles, home appliances, and furniture products by one temper rolling to satisfy the required surface glossiness and surface roughness of the consumer. It relates to a cold rolled steel sheet manufacturing method.

In general, there are three main reasons for temper rolling a steel sheet after heat treatment. First, in the case of low-carbon steel and medium-low carbon steel having a carbon content of 0.01wt% or more, In order to suppress the aging phenomenon generated by the second, the second is to correct the distortion of the steel sheet generated in the process such as heat treatment, and the third is to give the desired surface roughness to the steel sheet.

In general, the surface roughness of the steel sheet after temper rolling is obtained by transferring the roughness applied to the work roll to the steel sheet through temper rolling, and subject to process conditions such as rolling force of the roll, tension applied to the steel sheet, use of rolling oil, and the like. Depending on the degree of transfer.

At this time, the bright work roll used to produce the bright cold rolled steel sheet is manufactured by grinding the roll with grinding stone, and roughness is subdivided according to the grinding degree. It is used only for the production of bright cold rolled steel sheets that require gloss, and the surface quality standards of the bright cold rolled steel sheets require that the steel plate surface roughness (roughness) after temper rolling be 0.3 to 0.6 m.

This is because the glossiness varies according to the degree of surface roughness, and the glossiness increases as the surface roughness is fine, and the glossiness also increases as the maximum surface roughness (Rmax) value is smaller in the same roughness. That is, as the roughness is uniform and low, the glossiness increases, and if the average surface roughness value is lowered in order to suppress the glossiness decrease, problems occur in the post process and the processing of the consumer.

In the conventional bright cold rolled steel sheet manufacturing process, as shown in FIG. 1, a hot rolled steel sheet manufactured at a hot rolling mill is supplied to a cold rolling mill 10 to be cold rolled, and then supplied to an electrolytic clean line 20 to remove the rolling oil and other contaminants. After the cleaning process, supply it to the continuous annealing furnace (30), and heat treatment, and after the temper rolling is carried out sequentially through the first rough rolling facility (40) and the second rough rolling facility (50), the required roughness and gloss It will produce a bright cold rolled steel sheet having a degree.

The reason for producing bright cold rolled steel sheet by two temper rolling as mentioned above is to produce bright cold rolled steel sheet by performing only one temper rolling. (Orange feel: defects such as tangerine peel on the surface of steel sheet during processing) The moldability is poor due to the occurrence of temper rolling. Since the above method has to be performed at least two times of temper rolling, there is a problem that it is a factor of lowering productivity and rising cost.

Therefore, in order to solve the above problems, the first temper rolling is carried out at 0.5% elongation to make temper rolling at the target of 1.2% elongation, and then the second temper rolling is performed without affecting the mechanical properties. Patent Application No. 1996-69647 is provided which allows a finer control of the surface roughness. However, the method also needs to be subjected to temper rolling twice or more, which causes a problem of a decrease in productivity and a cost increase.

 The present invention has been invented to solve the above problems, the purpose is to remove the mixed tissue by the management of FGS (Ferrite Grain Size) No7 or more in hot rolling operations, orange paper caused by high stretching in a large grain size metal texture It is to provide a bright cold rolled steel sheet manufacturing method that can solve the problem of poor formability, such as peeling and improve the transfer rate with high pressure, high tension to satisfy the required surface gloss and surface roughness of the consumer with only one temper rolling.

In order to achieve the above object, the technical configuration of the bright cold rolled steel sheet manufacturing method of the present invention will be described.

The present invention uses a bright work roll having a roughness of 0.25 ~ 0.4㎛ Cr-coated (Coated) in the temper rolling process after annealing the steel sheet worked in FGS (Ferrite Grain Size) No7 in the hot rolling process and SPM (Skin Pass Mill) By adjusting the reduction amount and tension before and after SPM and performing temper rolling work under high drawing condition,

1.122mm thickness, 923mm width, low-carbon low-strength steel cold rolled steel sheet is produced by primary tempered rolling to produce bright cold rolled steel sheet that has formability, surface roughness and glossiness. It is to be able to produce products that meet the requirements.

The present invention as described above is cold rolled steel plate and electrolytically clean it, then heat treated and temper rolled surface glossiness of 50 or more. In the method of manufacturing a bright cold rolled steel sheet having a roughness of 0.3 to 0.6 µm, the tempered roll is rolled using a chromium coated bright roll having a roughness of 0.25 to 0.4 µm, and has a rolling reduction of 250 to 300 tons, shear tension of 4.5 to 5.0 tons, It is made by performing temper rolling only once so that elongation is more than 3% with post tension of 5.2 ~ 5.7 ton.

In the present invention as described above, hot coils produced in the hot rolling process are required to be managed over FGS No7 in order to prevent the generation of orange peel and moldability during processing by surface coarse grains even after the temper rolling high stretching operation by removing the mixed structure. The relationship between the grain size (GRAIN SIZE) is shown in Table 1 below.

FGS No 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10 10.5 Size (μm) 55 45 38 32 27 22 19 16 13 11 9.4

In the above method, the reason for the high drawing operation of more than 3% is to increase the transfer rate in order to secure the target roughness in one temper rolling, but the high drawing operation is required to improve the transfer rate.

In Table 1, FGS No 7.0 or more means grain size of 32 μm or less, and FGS No 7 or more is required to secure moldability after high stretching. On the other hand, FGS according to the hot-rolling operation conditions of cold-rolled medium-low carbon steel ([C] 0.02%, [Mn] 0.25%, [B] 10 ppm) is shown in Table 2 below.

           CT temperature FDT temperature 650 ℃ 580 ℃ 890 ℃ 7.0 to 7.5 7.5-8.5 870 ℃ 7.5-8.0 8.5-9.0

As a result, operating conditions for hot rolling to secure the FGS No 7 or more is required CT temperature of 650 ℃ or less, FDT temperature of 890 ℃ or less, which is a condition possible in general hot rolling operations. As a result of the bending test after the yarn working under the above conditions, the moldability did not occur.

If the relationship between the yield stress and grain size during temper rolling after controlling to the grain size 32㎛ or less (FGS No 7 or more) is explained by using Hall-Petch formula

σ _{0 =} σ _i + k'D ^-1/2

σ ₀ = tensile yield stress

σ _i = frictional stress that impedes the movement of dislocations in the mouth

k '= constant indicating the relative strengthening contribution of grain boundaries

D = diameter of grain

As a result, the yield strength of most crystalline materials increases as the grain size decreases. The Hall-Petch equation holds between the flow stress and grain size at any plastic deformation until ductile fracture occurs, and this relationship holds for the brittle fracture stress, grain size, fatigue strength and grain size.

It can be seen that when the grain is smaller in the above formula, the yield point elongation is increased, so that a higher stretching operation is possible.

The present invention uses a Cr-coated bright work roll to secure more than FGS No7 in the hot rolling process, using the work roll of 0.25 ~ 0.4㎛ in the tempered rolling stand as shown in Figure 4, the second SPM reduced 250 ~ 300 TON, SPM shear tension 4.5 ~ 5.0 TON, post-tension tension 5.2 ~ 5.7 TON is required and the result of one high elongation (more than 3%) temper rolling resulted in the following Table 3.

Table 3 below compares the conventional ones with those of the present invention.

division Roughness Glossiness Remarks Bright Regular ROLL 0.35 ~ 0.48 30 to 35 Low gloss Bright Cr-Coating ROLL 0.38 ~ 0.48 50 to 80 Improved gloss

On the other hand, for the high stretching operation pursued in the present invention, a rolling load of 250 tons or more is required, which is a bright roll having a roughness of 0.25 to 0.4 μm, which requires an increase in the rolling load by a large friction between the workpieces, When working below 250 TON, there is a problem that SLIP is generated between work roll and the workpiece because the yield stress of the material cannot be controlled. If the rolling load exceeds 300 TON, high pressure drop against the yield stress of the material occurs. Due to the rapid change in tensile stress occurring at the center of the workpiece and compressive stress occurring at the edges, it was confirmed in the manufacturing industry that quality defects such as pinch tree generation occurred momentarily at the edge of the workpiece.

In addition, the tension before and after SPM is also a large variable in temper rolling. When the tension is large, the elongation is increased and it is advantageous for shape control. However, when excessive tension is used, there is a problem that the shape defect due to aging after temper rolling is generated.

The SPM shear tension less than 4.5 TON setting does not control the yield stress of the material for the high stretching operation pursued by the present invention can not prevent the slip occurs between the work roll and the workpiece, and the setting more than 5.0 TON after the temper rolling This is because shape defects due to aging may be a concern.

The SPM shear and post tension are generally designed at a constant ratio of 0.8: 1 to 0.9: 1. This is because rolling force is higher when the rear end tension is higher in relation to the rolling force and the geometrical shape.The speed of SPM rear bridal roll is faster than the speed of front bright roll according to elongation after the temper rolling of the workpiece. To maintain, the front and rear tension ratios are the main variables.

In the high-stretching operation pursued by the present invention, the result of continuous testing of the most efficient tension ratio that improves the transfer rate without the occurrence of slip of the bright roll through the yarn operation, yields a tension ratio of 0.86: 1 before and after which It is recommended to use 5.2 ~ 5.7 TON considering the ratio with shear tension.

Referring to the embodiment of the above-described bright cold rolled steel sheet production example as follows.

(Example)

As shown in FIGS. 3 and 4, a cold rolled steel sheet having FGS No 7 or more secured in a hot rolling process is used to improve the transfer rate during the temper rolling by using a Cr-Coating work roll of 0.35 μm in a tempered rolling stand. In order to meet the demanded surface roughness of 0.3 ~ 0.6㎛ with one SPM, the elongation target was set to 3% or more.

* CAL tempered rolling seal work results *

-Representative coil NO: CP 3619 (STRIP thickness 1.122 mm, width 923 mm, material medium and low carbon steel)

-Work roll used: Roughness 0.35㎛, Cr-Coating

Annealing Temperature HS 816, SS 813, RCS 430, OAS 429, SST 44, SS Hit Ratio 100%

-SPM Rolling Force 254.2 Ton, SPM Shear Tension 4.5 TON, SPM Back Tension 5.2TON

Elongation 3.98%

1) Thickness change of actual test results

-Material entrance thickness: 1.122mm, Outgoing product thickness: 1.068mm, FCE shrinkage: 0.012mm

-SPM side thickness = material side thickness Fce shrinkage = 1.122 0.012 = 1.110 mm

-SPM thickness = product thickness = 1.068 mm

2) Theoretical elongation calculation formula = ((SPM exit speed-SPM exit speed) / SPM exit speed) * 100 (%) = (1-SPM exit thickness / SPM exit thickness) * 100 (%) = (1- 1.068 / 1.110) * 100 = 3.78%

▶ Compared to theoretical elongation of 3.78% and actual elongation of 3.98%, it is possible to confirm that high stretching work is performed under the above working conditions by showing similar results considering the amount of LOSS.

3) temper rolling ratio by high drawing operation

-Surface roughness of STRIP after cold rolling: 0.81 ~ 1.01㎛

CAL SPM work roll roughness: 0.35㎛

-Surface roughness of STRIP after 1 time of temper rolling: Average 0.42㎛

-Temper rolling ratio by high drawing: 120%

▶ As the result of the actual work, high drawing work is performed when working with the above working method, so that the target roughness can be secured by one SPM work by improving the temper rolling ratio.

In the present invention as described above, in the production of bright cold-rolled steel sheet, in order to secure formability, after the annealing of the hot-rolled coil produced in FGS No 7 or more in the temper rolling process Cr-Coated bright work roll of 0.25 ~ 0.4㎛ roughness It is possible to shorten the process, increase productivity, and improve the surface by requiring only one high-strength temper rolling that applies SPM pressing 210 ~ 250 TON, SPM shear tension 4.0 ~ 4.5 TON, and post tension 4.5 ~ 5.0 TON. There is a distinctive effect of ensuring the roughness and glossiness.

Claims (2)

Cold rolled steel plate, electrolytically clean it, heat treatment and temper rolling to make surface gloss more than 50. In the bright cold rolled steel sheet manufacturing method to ensure the roughness 0.3 ~ 0.6㎛, The roll to be temper rolled using a chrome coated bright roll having a roughness of 0.25 to 0.4 μm with only one high elongation temper rolling with a rolling reduction of 250 to 300 tons, shear tension of 4.5 to 5.0 tons and post shear strength of 5.2 to 5.7 tons. Bright cold rolled steel sheet manufacturing method characterized in that. The method of claim 1, wherein the elongation of the temper rolling is a bright cold rolled steel sheet manufacturing method characterized in that more than 3%.

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