JP2009214173A - Linear mark prevention method in hot dip galvannealed steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a linear mark prevention method for producing a hot dip galvannealed steel sheet having excellent surface appearance, and capable of withstanding use as an automotive body outer panel in which an appearance grade is extremely severe. <P>SOLUTION: The surface of a slab for a hot dip galvannealed steel sheet is subjected to scarfing by a hot scarfer in a range where scarfing oxygen pressure p(kPa), scarfing velocity V(mpm) and the distance a(mm) between the hot scarfer and the tip of an upper block combustion nozzle satisfy the four conditional equations of V≥K√P(K is 139, preferably, 193), p≤250, V≤40 and 8≤a≤20, and a welding scull generated by the scarfing is removed from the surface of the slab. A linear mark caused by the welding scull can be eliminated. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for preventing linear marks in an alloyed hot-dip galvanized steel sheet for producing an alloyed hot-dip galvanized steel sheet having an excellent surface appearance.

  Conventionally, alloyed hot dip galvanized steel sheets that are alloyed with hot dip galvanizing on ultra-low carbon steel sheets with a C content of 0.0050% by mass or less are used for automotive outer panels and home appliance parts that require corrosion resistance and workability Plated steel sheet is used. In particular, when it is used for a part used on a part that can be seen by human eyes, such as a body skin of an automobile, the appearance as well as the corrosion resistance and workability are regarded as important.

  However, the alloyed hot-dip galvanized steel sheet obtained by subjecting the above-mentioned ultra-low carbon steel sheet having a C content of 0.0050% by mass or less to alloy hot-dip galvanizing has a streak pattern called a linear mark when formed. It may be raised and the appearance quality may be impaired, which is a problem.

  In order to prevent such a linear mark of the alloyed hot-dip galvanized steel sheet, Patent Document 1 focuses on a slab oscillation mark generated when continuously casting a slab. That is, a method of cutting the slab oscillation mark or reducing P has been proposed from the viewpoint that P is concentrated at the valley portion of the slab oscillation mark and this affects the surface properties.

  Further, Patent Document 2 finds out that the unevenness of Ti addition IF steel is caused by uneven nitriding phenomenon that occurs on the surface when heating a continuously cast slab, and responds by enriching oxygen as an oxidation treatment during slab heating. It has been proposed.

Further, in Patent Document 3, it is proposed that S in the steel affects the linear mark and that a threshold value is determined and dealt with.
JP-A-6-336666 Japanese Patent Laid-Open No. 10-330846 JP 2005-2363 A

  However, in alloyed hot-dip galvanized steel sheets used for automobile body outer plates with extremely strict appearance quality, the occurrence of linear marks cannot be suppressed even if the above-mentioned measures are taken. A linear mark that seems to have a different cause from that which has been revealed has become obvious, and the response has been an urgent need.

  Accordingly, the object of the present invention is to solve the above-mentioned conventional problems, and to provide an alloyed hot-dip galvanized steel sheet excellent in surface appearance that can withstand use as a car body outer plate with extremely severe appearance quality as well as workability. It is providing the linear mark prevention method for manufacturing this.

  As a result of intensive investigation and research in order to solve this problem, the present inventor conducted slab maintenance at the slab stage after continuous casting to satisfy strict appearance requirements of the galvannealed steel sheet. Although the objects and wrinkles have been removed, it has been surprisingly found that the larger the amount of hot scurfing, the more linear marks are generated.

  That is, FIG. 1 shows the retention rate due to the linear mark according to the amount of hot scurfer slicing in an alloyed hot-dip galvanized ultra-low carbon steel sheet having a C content of 0.0050% by mass or less used for an automobile body outer plate. It is a graph, and it can be seen that the retention rate increases remarkably as the amount of cutting increases. This fact indicates that the slab surface cutting by the hot scurfer, which has been performed for improving the surface quality, is the cause of the generation of the linear mark. However, the slab surface is indispensable for improving the surface quality and cannot be eliminated.

  Therefore, as a result of further investigation, the present inventor has found that the welding metal (noro) generated by the welding adheres to the surface of the slab in a streak-like manner under the conventional hot scurfing, and the slab is rolled. In this case, a steel plate that is sound in appearance but has different properties (oxygen-rich portions in the steel) remains linear, and local differences in alloying speed occur when hot dip galvanizing is performed. The mechanism that was previously unknown was clarified. In addition, this phenomenon appears remarkably in the alloyed hot-dip galvanized ultra-low carbon steel sheet having a C content of 0.0050% by mass or less. Since ultra-low carbon steel has high viscosity, the welding metal is slabded under conventional conditions. It is assumed that it is difficult to blow away completely from the surface.

The present invention was made in order to prevent linear marks by optimizing the cutting conditions by hot scurfers based on the above findings, and the surface of the slab for galvannealed steel sheets Hot scar in a range where the cutting oxygen pressure p (kPa), the cutting speed V (mpm), and the distance a (mm) from the tip of the upper block combustion nozzle of the hot scarfer satisfy the following four conditional expressions: It is characterized by performing welding with a fur and removing the welding metal generated by the welding from the slab surface.
V ≧ K√P (K is 139)
p ≦ 250
V ≦ 40
8 ≦ a ≦ 20
As shown in claim 2, it is more preferable that K in the conditional expression of V ≧ K√P is 193.

  ADVANTAGE OF THE INVENTION According to this invention, the welding metal which arises when cleaning a slab surface with a hot scurf is blown off from a slab surface, and a surface property is uniform and a slab with favorable surface quality can be obtained. For this reason, when the alloying hot dip galvanizing is performed, plating unevenness does not occur, and the generation rate of the linear marks can be greatly reduced. According to the conditions of the invention of claim 2, this effect can be further enhanced.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
FIG. 2 is a view showing the surface of a slab subjected to hot scarf, and a welding metal called Noro adheres to the welded portion. The steel component of the weld metal is basically the same as that of the slab, but the oxide content is high and hardened. The present inventor performed hot rolling, pickling, cold rolling, and galvannealed hot dip galvanization in a laboratory experiment using a cut sample to which noro was attached, and the same position as the noro was attached as shown in FIG. It was confirmed that a linear mark was generated at the position.

  The presumed mechanism by which adhesion of Noro generates a linear mark is shown in FIG. If a part of the welding ingot adhering to the surface layer of the slab remains without dropping even after the processes of descaling, pickling and cold rolling during hot rolling and hot rolling, the part will be removed during hot dip galvanization. The Fe—Al—Zn layer becomes thinner. Therefore, in the alloying process, in the plating layer of the weld metal part, the diffusion of zinc from the surrounding plating layer and the diffusion of iron from the steel plate proceed locally, resulting in a thickening of the basis weight of the part and generation of linear marks. It is estimated that

  In order to prevent the occurrence of linear marks due to such a mechanism, it is necessary to optimize the welding conditions by the hot scurfer so that the welding metal does not adhere to the slab. Therefore, the cutting conditions by hot scurfer were examined. FIG. 5 is a cross-sectional view of the main part of the hot scarfer, in which 1 is an upper block, 2 is a lower block, and 3 is a shoe facing a slab. Fuel gas (for example, propane gas) and oxygen are injected from the combustion nozzle 4 between the upper block 1 and the lower block 2, and the surface of the slab is cut by a high-temperature and high-pressure frame.

  The cutting oxygen pressure p (kPa) has been conventionally set to a constant value of 130 kPa. In the present invention, the cutting oxygen pressure p is variously changed in relation to the cutting speed V (mpm), An experiment was conducted to see whether the weld metal could be completely removed from the slab surface under such conditions. The result is shown in FIG. FIG. 6 shows the results when the distance a (mm) between the tip of the combustion nozzle 4 of the upper block 1 of the hot scurfer and the slab surface is set to 10-20. Judgment of the quality is done by visual inspection. × indicates that a large amount of welding metal remains, black triangle indicates that some welding metal remains, and the surface of the welding metal is slightly rough although the slab surface is slightly rough. □ indicates that there is no slab surface and no weld metal, ♦ indicates that the slab surface is insufficient, ♦ indicates that the slab surface is insufficiently welded, and □ indicates that the slab surface is tiger-cut. Among these, ○ was most preferable, and □ was also determined to be acceptable.

  From FIG. 6, it was confirmed that the quality of the surface of the slab passed in the range of V ≧ 139√P. Moreover, if it is set as the range of V> = 193 (root) P, it will become still more preferable quality. However, when the cutting oxygen pressure p (kPa) exceeds 250, the tiger cutting state is reached, so p ≦ 250. Although not shown, if the cutting oxygen pressure p is less than 30, stable cutting cannot be performed, which is not preferable for operation.

  Furthermore, if the cutting speed V (mpm) exceeds 40, insufficient cutting occurs and the surface quality is not stable, so V ≦ 40. Although not shown, if the cutting speed V is less than 5, slipping or the like occurs between the transport roll and the slab, and the movement of the slab is not stable. When a (mm) is set to 10 to 20 in this way, V ≧ K√P (K is 139, preferably 193), p ≦ 250, V ≦ 40, and hot cutting with hot scurfer. It was confirmed that the welding ingot does not adhere to the slab when performing the above.

  Next, FIG. 7 is a similar graph when the distance a (mm) between the tip of the combustion nozzle 4 of the upper block 1 of the hot scurfer and the slab surface is set to 8. FIG. 8 is a similar graph when the distance a is set to 24. In the case of FIG. 6 in which a is set to 10 to 20, a satisfactory result was obtained. Even when V ≧ K√P, when a is 8, the slab surface is too close to the slab. It is considered that the slab surface is deteriorated by the hoisting of the bare metal that has been cut by combustion gas. Conversely, when a is 24, it is considered that the distance to the slab surface is too large and the collision pressure of the combustion gas of the hot scurfer becomes weak and the slab surface cannot be sufficiently cut.

  As a result of the above, if surface cutting is performed under the conditions of V ≧ K√P (K is 139, preferably 193), p ≦ 250, V ≦ 40, and 8 ≦ a ≦ 20, the surface quality is free from adhesion of welding metal. It was confirmed that a good slab can be obtained, and the linear marks when an alloyed hot-dip galvanized steel sheet is produced can be reliably prevented. In addition, the unevenness of the surface of the slab that has been cut by the conditions of the present invention has been halved from about 1 mm to 0.5 mm, and the retention rate by the linear mark has sometimes reached 10%. By adopting the method, it was possible to reduce it to almost 0%. Thus, it was confirmed that the effect of preventing the linear mark of the galvannealed steel sheet according to the present invention is enormous.

It is a graph which shows the conventional retention rate resulting from the linear mark according to the amount of hot skar fur cutting. It is explanatory drawing of the slab surface which performed the hot scarf. It is a figure which shows the relationship between the adhesion position of a welding metal, and the generation | occurrence | production position of a linear mark. It is explanatory drawing of the mechanism in which a weld metal produces | generates a linear mark. It is principal part sectional drawing of a hot scarf. It is a graph which shows the influence which the cutting oxygen pressure p (kPa) and the cutting speed V (mpm) have on slab surface quality when a (mm) is set to 10-20. FIG. 7 is a graph similar to FIG. 6 when a (mm) is set to 8. FIG. FIG. 7 is a graph similar to FIG. 6 when a (mm) is set to 24. FIG.

Explanation of symbols

1 Upper block 2 Lower block 3 Shoe 4 Combustion nozzle

Claims (3)

The surface of the slab for alloyed hot-dip galvanized steel sheet has a cutting oxygen pressure p (kPa), a cutting speed V (mpm), and a distance a (mm) from the tip of the upper block combustion nozzle of the hot scarfer. The prevention of linear marks on alloyed hot-dip galvanized steel sheets, characterized by performing hot-scratching with a hot scurfer within a range that satisfies the following four conditional expressions, and removing the welding ingot generated from the hot-scraping from the slab surface Method.
V ≧ K√P (K is 139)
p ≦ 250
V ≦ 40
8 ≦ a ≦ 20   The method of preventing linear marks in an alloyed hot-dip galvanized steel sheet according to claim 1, wherein K in the conditional expression of V ≧ K√P is 193.   The slab having a C content of 0.0050% by mass or less is used. The method for preventing linear marks in an alloyed hot-dip galvanized steel sheet according to claim 1 or 2.

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