KR100358287B1 - Manufacturing Method of Cold Rolled Steel Sheet having Good Surface Property for Automobile Use - Google Patents

Abstract

The present invention relates to a method for reducing white streak spot defects appearing on cold rolled steel sheets of general low and medium carbon steel and Ti-added ultra low carbon steel. In the present invention, it has been found that the cause of such defects originates from the pickling process. In other words, if the pickling condition is more severe during pickling, if there is a part that is locally overpicked, the surface irregularities become severe, and the recesses that do not come into contact with the roll during rolling remain as they are. It will have a white striped stain. Therefore, the surface of pickling steel plate under pickling condition is evaluated as "pickling degree" by -3 ~ +3 according to pickling degree according to pickling condition and the pickling condition is required to fall within the range where such defect does not occur. It relates to a method for producing a cold rolled steel sheet having excellent properties.

Description

Manufacturing Method of Cold Rolled Steel Sheet with Excellent Surface Property {Manufacturing Method of Cold Rolled Steel Sheet having Good Surface Property for Automobile Use}

The present invention relates to a method for manufacturing a cold rolled steel sheet for removing surface defects that appear as white speckles on the surface of the cold rolled steel sheet for automobiles. To a method for removal. FIG. 1 of the accompanying drawings shows a scanning electron microscope (SEM) surface photograph of surface speckles. Generally, the speckles are present as fine grooves invisible from the normal region. The inventors found that the cause of such defects originated from the pickling process, and the surface according to the pickling conditions was designated as "pickling degree" by numerical values from -3 to +3, and within the range of pickling degree where the defect does not occur. In order to provide a method for manufacturing a cold rolled steel sheet without surface defects in the form of white streaks, the cold rolled steel sheet is generally subjected to pickling and rolling under constant conditions in a continuous pickling rolling equipment and then degreased in a continuous annealing equipment. It is manufactured through processes such as continuous annealing and temper rolling. That is, as shown in Table 1, the concentration of hydrochloric acid is 4-20%, the temperature is 75-90 ° C, and the immersion time is pickled under the condition of 15-200 seconds according to the line speed, and the rolling is about 65-80%. The steel sheet produced through the continuous pickling and rolling mill is called Full Hard. Since this steel sheet is very hard to use, it must be heat treated in the continuous annealing plant. That is, as shown in Table 1, in order to remove the rolling oil contaminated on the surface of the steel sheet during rolling, the steel sheet was passed through 4 ~ 5% NaOH solution and degreased at 70 ~ 85 ℃, and then 700 ~ 850 over 20 ~ 30 seconds. The soft nitriding annealing heat treatment is performed at 캜. Then, in order to prevent the yield point stretching of the steel sheet, the crude rolling is usually performed at a reduced elongation of 0.5 to 1.5%, and then, the rust preventive oil is applied at 1 to 2 g / m 2 to complete the manufacture of the cold rolled product. The manufacturing conditions are clearly shown, and FIGS. 2 and 3 show the layout of the continuous pickling rolling equipment and the continuous annealing equipment. In this case, the surface condition of the cold rolled steel sheet is a very important quality factor in evaluating the value of the product, and thus the white streaks formed on the surface of the cold rolled steel sheet are one of the important defects that degrade the quality. Since the shape appears after coating during automobile manufacturing, it is difficult to use for automotive steel sheets requiring high surface properties.

Therefore, in the present invention, the purpose of the present invention is to find that the white streaks of cold rolled products can be improved by controlling the appropriate pickling conditions, and to provide a solution.

1 is a photograph obtained by observing the surface shape of a white streak spot with a scanning electron microscope of 700 times.

2 is a layout view of a continuous pickling rolling equipment as an embodiment of the present invention.

3 is a layout view of a continuous annealing facility as an embodiment of the present invention.

4 is a scanning electron microscope tissue photograph of the surface texture of the medium-low carbon steel before and after the test rolling observed with a scanning electron microscope when the pickling concentration, the pickling temperature, and the pickling time were treated at 18%, 90 ° C, and 50 seconds, respectively.

FIG. 5 is a scanning electron microscope photograph showing the surface texture of the medium and low carbon steel before and after the test rolling observed with a scanning electron microscope. The pickling concentration, the pickling temperature and the pickling time were respectively 12%, 85 ° C. and 150 seconds.

FIG. 6 is a scanning electron microscope tissue photograph obtained by treating the pickling concentration, pickling temperature, and pickling time at 18%, 90 ° C, and 150 seconds, respectively, before and after test rolling observed with a scanning electron microscope.

EMBODIMENT OF THE INVENTION Below, this invention is demonstrated in detail.

Hot rolled steel is usually used as a raw material for the hot rolled steel sheet. Since the surface of the hot rolled steel sheet is covered with a thick iron oxide film, the oxide film must be removed by a pickling process prior to the rolling process. The schematic layout of the line and continuous annealing equipment is shown. Here, the white streaks on the surface of the cold rolled steel sheet are one of the fatal defects degrading the appearance quality of the product, and this defect is also present after the coating of the automobile surface, which provides a reason for making it difficult to use as an automotive steel sheet.

As an embodiment of the present invention, as shown in FIG. 2, the pickling facility is composed of three tanks, and the pickling method of the Turbulence Type is selected as described in Table 1. During pickling, first, a tension leveler, that is, a load is applied to the roll through a flattener to form fine cracks on the surface of the oxide film perpendicular to the direction, and then through a pickling tank for a predetermined time at a constant hydrochloric acid concentration. Pickling is performed by spraying the pickling solution (hydrochloric acid) at high pressure through a plurality of nozzles in the roll direction and the direction perpendicular to the roll direction. At this time, depending on the pickling conditions, that is, the concentration, time, and temperature, the surface of the pickling plate may vary according to each part of the steel sheet. That is, fine scales (oxides) remain in areas where less pickling is performed locally, and surface irregularities are severe in areas where many picklings are performed. At this time, in the case where the pickling occurs a lot, that is, in the case of the overpickling part, the part which does not come into contact with the roll remains as it is, and this shape is locally generated as a stain. It will appear in the form of blobs.

In order to prove this, pickling was performed under various conditions and test rolling was performed. 4 shows the results of this test rolling. That is, in FIG. 4, which shows the result of observing the surface state after the test rolling by scanning electron microscope according to the irregularities of the pickling surface in the medium and low carbon steel, the grain boundary is over-acidified, and the uneven portions of the surface have fine grooves after rolling I can see that. From this, pickling is severe in the actual process, if the unevenness is severe, it can be seen that the white streaks stains are left in the grooves in a certain form even after rolling.

Example

Table 3 shows pickling by hydrochloric acid concentration, time and temperature for pickling of low and low carbon steels with chemical composition as shown in Table 2, and then observed the surface by scanning electron microscope (SEM). The degree of pickling is called "pickling degree" and is arbitrarily evaluated from -3 to +3. The closer to (-) side, the less acidic surface, and the (+) side is super acidic, which means that the surface irregularities are large. "0" indicates the most appropriate surface condition. The surface condition can be evaluated according to the treatment conditions. In particular, the surface condition changes when the pickling surface is observed by scanning electron microscope according to the pickling time, concentration and temperature. In particular, as the concentration, time and temperature were higher, partial surface irregularities became more severe and pickling was increased.

TABLE 2

TABLE 3

Table 4 shows the defective rate of white streaks after rolling according to the 'pickling degree' in the case of pickling as shown in Table 3.

Table 4

As a result, it was confirmed that pickling that is properly pickled or slightly pickled does not cause white streak spot defects along with other defects. In the case of -1 or less, it is impossible to use as a product due to an oxide film defect due to an acid pickling condition, and a white streaked stain defect occurs as a super acid wash until 2 to 3, and thus it was not determined as a proper product.

For example, in the case of medium and low carbon steel, the pickling time should be picked up to 100 ~ 150 seconds for the hydrochloric acid concentration of 12% and the temperature to 85 ~ 90 ℃, while if the concentration is increased to 18%, the pickling temperature will be increased for 25 seconds. At 90 ° C and immersion time of 150 seconds, a proper pickling surface can be obtained even if the temperature is only 75 ° C. Table 3 shows that the lower the hydrochloric acid concentration, the shorter the immersion time, the lower the immersion temperature, the more the fine pickling. On the contrary, it is shown that there is an overtaxing tax. Therefore, the sum of the above results shows that the appropriate pickling conditions are as follows. However, the complete miscellaneous tax, miscellaneous tax and peracid tax, and the complete superacid tax are excluded as adversely affecting the quality of the cold rolled steel sheet. In the case of medium and low carbon steels, the hydrochloric acid concentration used varies depending on the type of steel, but 15% to 18% is suitable. In this case, the pickling temperature in pickling conditions is 80 ~ 90 ℃ and the immersion time. It has been found that 50 to 100 seconds is particularly stable and good. In addition, in the case of titanium (Ti) -added ultra low carbon steel, the range of hydrochloric acid concentration used is further widened, so that the concentration of hydrochloric acid used is 12%, 15%, and 18% in the pickling temperature of 75 to 90 ° C. in the pickling tank. The soaking time for the pickling temperature of 50 seconds was suitable for the pickling temperature in the pickling tank, and for the pickling time of 100 seconds, all of them were good regardless of the pickling temperature. However, when the immersion time in the pickling tank was 150 seconds, the Ti-added ultra low carbon steel was relatively good even when pickling at a low temperature of 75 ° C., but the pickling failure was higher at higher temperatures. Under pickling conditions, when the concentration of hydrochloric acid is 15 to 18%, and the pickling temperature is fixed at 80 to 90 ° C, the immersion time is 100 to 150 seconds when the concentration of hydrochloric acid is 15% and 50 to 100 seconds when the concentration of hydrochloric acid is 18%. In addition, the Ti-added ultra low carbon steel has a wide range of pickling working conditions, and the hydrochloric acid concentration is 12-15%, and the pickling temperature and pickling bath immersion time are independent of the hydrochloric acid concentration. It is important to note that when the pickling temperature is fixed at 75 to 90 ° C., the immersion time is appropriate for 100 to 150 seconds. Here, the lower the hydrochloric acid concentration, the lower the immersion time, and the lower the pickling temperature, the better. Therefore, From the point of view, hydrochloric acid concentration of 15%, pickling temperature of 80 ~ 85 ℃ and immersion time of 100 ~ 150 seconds are the best conditions for medium and low carbon steels, and hydrochloric acid concentration of 12% and pickling temperature of 75 ~ 40 At 85 ° C., pickling solution immersion time of 100 to 150 seconds was found to be the best condition.

It was found that the pickling conditions, that is, pickling concentration, pickling temperature, and pickling time according to the present invention depend on whether or not white streak spot defects occur in the cold rolled steel sheet. It is possible to completely eliminate white streaks and spots.

Claims (6)

In order to prevent staining defects on the surface of the steel sheet, in the case of medium and low carbon steel, the hydrochloric acid concentration is 15-18% during pickling, the pickling temperature is 80-90 ° C, and the immersion time is 100-150 when the hydrochloric acid concentration is 15%. In case of 18% of hydrochloric acid, pickling is carried out at 50-100 seconds.In case of Ti-added ultra low carbon steel, the concentration of hydrochloric acid is 12-15%, pickling temperature is 75-90 ℃, and immersion time is 100-150. Method for producing a cold rolled steel sheet having excellent surface properties, characterized in that the pickling operation is carried out. The cold rolled sheet having excellent surface properties according to claim 1, wherein the medium to low carbon steel has a hydrochloric acid concentration of 15%, a pickling temperature of 80 ° C to 85 ° C, and a soaking time of 100 seconds to 150 seconds. Method of manufacturing steel sheet. delete [Claim 2] The cold rolled steel according to claim 1, wherein the Ti-added ultra low carbon steel is pickled at a hydrochloric acid concentration of 12%, a pickling temperature of 75 ° C to 85 ° C, and a pickling time of 100 to 150 seconds. Manufacturing method of steel sheet delete delete