JPH09176812A - Production of galvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for inexpensively producing a galvannealed steel sheet improving its adhesion at the time of press working only by the change of producing conditions without remodeling the existing equipment. SOLUTION: A steel strip subjected to hot rolling is coiled round a coil, is cooled, is thereafter recoiled, is subjected to pickling and cold rolling and is galvannealed in a continuous galvanizing line to produce a galvannealed steel sheet. In this case, the temp. of the steel strip at the time of being coiled is regulated to 600 to 750 deg.C, and the subsequent cooling rate is regulated to <=3 deg.C/min.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a galvannealed steel sheet, and in particular, a hot-rolled steel sheet having an oxide formed on a grain boundary in the black skin of the steel strip surface and directly below It is related to the manufacturing technology of galvannealed steel sheets with excellent plating adhesion.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheet (commonly known as G
A) is mainly used for automobile body materials because it is inexpensive and has excellent corrosion resistance. In addition to corrosion resistance, the performance required as a steel sheet for automobile bodies is required to have plating adhesion during press working. When the plating adhesion of the steel sheet deteriorates, the plating layer peels off from the steel sheet in powder or lump form (referred to as powdering), which causes so-called die galling, or the corrosion resistance of the peeled portion deteriorates, or peeling occurs. However, there is a problem that the plated metal or alloy pieces may cause flaws on other parts of the steel sheet.

Then, as a technique for improving this adhesion, Japanese Patent Laid-Open No. 61-276961 has proposed to hot-dip galvanize a steel strip and then alloy it at a high temperature of 700 to 850.degree. However, alloying at such a high temperature not only involves an increase in manufacturing cost, but also has a problem of increasing the load on equipment such as rolls. Further, JP-A-3-232926 discloses that Zr, La and C are contained in steel.
e, Y, contains at least one or more of Ca elements,
Further, a technique is disclosed in which cooling from recrystallization annealing to plating work is performed at a cooling rate of 50 ° C./s or more. However, since the Zr element or the like is added to the steel, this technique also has a high manufacturing cost, and the stripping speed has to be slowed due to the problem of the cooling capacity, so that it has a drawback of low productivity. .

Further, in Japanese Patent Laid-Open No. 2-163356, the O, Al, and N components of the steel material are 0.0045 each.
wt% or less, (25 × N wt%) to 0.15 wt%,
Method of limiting to 0.0030 wt% or less, JP-A-6-8
Japanese Patent No. 1101 discloses that the amount of Ti, Si and P is limited and Si (w
A method of simultaneously satisfying t%) + P (wt%) ≧ Ti (wt%) has been disclosed. However, regulation of these steel material components cannot always impart desired steel plate performance such as strength and drawability to the plated steel plate, and there is a risk of deterioration due to powdering due to deviation of the components.

In addition, Japanese Patent Laid-Open No. 4-333552 discloses a method of improving plating adhesion by performing so-called pre-plating before the actual hot dip galvanizing.
However, such a hot dip galvanizing line usually does not have such a pre-plating facility, and a large amount of investment is required to improve the facility, so that it has not been realized yet.

[0006]

SUMMARY OF THE INVENTION In view of such circumstances, the present invention is an inexpensive alloy galvanized steel sheet which improves adhesion during press working only by changing manufacturing conditions without modifying existing equipment. It is intended to provide a manufacturing method of.

[0007]

Means for Solving the Problems The inventor has conducted extensive studies in order to achieve the above-mentioned object, and has focused on producing an oxide in the black skin of a steel strip before plating and the crystal grain boundary just below the black skin,
The present invention has been completed. That is, the present invention, the hot-rolled steel strip is wound into a coil, cooled, then rewound, pickled, cold-rolled, and galvannealed steel sheet in a continuous hot-dip galvanizing line. In manufacturing the above, the temperature of the steel strip when wound into the coil (hereinafter referred to as the winding temperature) is set to 600 ° C. or higher and 750 ° C. or lower, and the subsequent cooling rate is set to 3 ° C./min or lower. It is a method for manufacturing a galvannealed steel sheet. In the present invention, since the winding temperature of the hot-rolled steel strip and the cooling rate of the coil are limited as described above, an oxide is generated in the black skin on the surface of the steel strip and the crystal grain boundary immediately thereunder, and Since the gaps between the oxides are plated, the adhesion is greatly improved.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The contents of the present invention will be described below with reference to the drawings. The embodiment of the present invention is to limit the winding temperature and the cooling rate of the hot rolled steel sheet as described in the "Claims", and as a result, the black scale (scale layer) of the hot rolled steel sheet surface. Oxides were generated at the grain boundaries directly under the black skin. A cross-sectional photograph of the steel sheet having oxides on the black skin and crystal grain boundaries is shown in FIG. 1, and it can be seen that the oxides are formed up to a depth of about 10 μm on the crystal grain boundaries of the steel sheet directly below the black skin.

This oxide is formed of an oxide such as Mn, P, Si and Al, and is produced by diffusion of oxygen into the steel when the coil winding temperature is high. Therefore, even if the amount of Mn, P, Si, etc. added to the steel is small, if the coiling temperature and cooling rate disclosed in the present invention are satisfied,
It is possible to form these oxides. vice versa,
If the coiling temperature is lower than 600 ° C., or even if the temperature is higher than 600 ° C. and the subsequent cooling rate is higher than 3 ° C./minute, no oxide is observed at the grain boundaries. Further, when the winding temperature exceeds 750 ° C, the amount of black skin increases, and it takes time to remove the black skin in the subsequent pickling. The coiling temperature is preferably 750 ° C. or lower because the peel cannot be completely removed and becomes defective.

The oxide thus formed just below the black skin remains almost even after the subsequent steps such as pickling, cold rolling, annealing, plating, etc., and alloying melt having the oxide directly below the plating layer. It will provide galvanized steel sheet. Usually, pickling is performed to remove the black skin, and even if pickling is carried out, most of the oxides at the crystal grain boundaries can remain, and therefore the conditions are not limited at all. Further, the surface may be cleaned by grinding or the like as long as the oxide of the crystal grain boundaries remains. The cold rolling is not limited at all, because the oxide remains regardless of the conditions such as reduction ratio.

Next, the reason for improving the plating adhesion will be described. It is known that the galvannealed steel sheet is mainly subjected to compressive stress during press working to cause peeling of the plated layer. Further, during the alloying treatment, a zinc-iron alloy is produced by thermal diffusion of zinc and iron, but zinc also diffuses into the steel sheet grain boundaries to form a zinc-iron alloy layer. The alloyed hot-dip galvanized steel sheet produced by the production method according to the present invention has an oxide immediately below the plating layer, so that there is a gap between crystals as compared to a conventional crystal grain boundary where no oxide is present. Easy to penetrate. As a result, the unevenness of the interface between the plating layer and the steel sheet substrate becomes severe, and the plating layer can firmly adhere to the steel sheet. That is, the galvannealed steel sheet produced by the production method according to the present invention,
The adhesiveness of the plating can be maintained in a good state even if press working is performed. For example, the result of observing by SEM with the plating layer of the plated steel sheet forcibly melted between the iron potentials by the low potential method to expose the steel sheet substrate is shown in FIG. Is shown in FIG. Comparing FIG. 2 and FIG. 3, in comparison with the conventional oxide-free steel plate base (FIG. 3), it is apparent that in FIG. 2 of the present invention, the unevenness of the interface between the plating layer and the steel plate base is obviously severe. Recognize.

In the present invention, the characteristics of the plating layer are not particularly limited, but from the viewpoint of corrosion resistance and the like, the adhesion amount of the zinc-iron alloy as a steel sheet for automobiles is usually 25 to 90.
It is suitable that g / m ² and the iron content in the plating layer be 8 to 13 wt%. The conditions of the zinc bath are also not particularly limited, but the Al concentration in the zinc bath is 0.13 to 0.
It is considered appropriate that the Fe concentration is about 15 wt% and the Fe concentration is 0.01 wt% to saturation, and the bath may further contain component elements such as Pb, Mg and Mn.

[0013]

EXAMPLES Examples of carrying out the manufacturing method according to the present invention will be shown below. The test materials of the low carbon steel (test steel A) and the ultra low carbon steel (test steel B) were melted in a converter and then made into slabs by continuous casting. Heating temperature of this slab (SR
T) 1100 to 1250 ° C, finishing temperature 850 to 950 ° C
The coil winding temperature and cooling rate were changed as shown in Table 1, and hot rolling was performed on a steel strip having a thickness of 35 mm.

[0014]

[Table 1]

After that, the coil was rewound and the scale layer (black scale) on the surface of the steel strip was removed by pickling, and then ordinary cold rolling was performed to reduce the thickness to 0.7 mm. This cold-rolled steel sheet was placed on a continuous hot-dip galvanizing line (CGL) for 7
After recrystallization annealing at 50 to 880 ° C., 470 to 48
Hot dip galvanizing at 0 ° C, then 480-530 ° C
The alloying treatment was performed for 15 to 30 seconds. The plating adhesion of the obtained alloyed hot-dip galvanized steel sheet was determined by conducting a commonly used "press workability evaluation test". That is,
A cellophane tape was attached to one surface of the above galvannealed steel sheet, bent in the plane direction at an angle of 90 degrees, bent back and peeled off the pressure-bonded tape, and the amount of zinc peeled at that portion was converted to fluorescent X-rays. Measured. The evaluation criteria are as follows.

The powdering resistance rank is determined with respect to the number of counts by the fluorescent X-ray. And The test results are collectively shown in Table 2. From Table 2, it can be seen that when the manufacturing method according to the present invention is applied, there is clearly powdering resistance and good plating adhesion.

[0017]

[Table 2]

[0018]

As described above, according to the present invention, the present equipment is used as it is, and the alloying excellent in the plating adhesion during the press working is limited only by the coiling temperature and the cooling rate of the hot rolled steel strip. Hot-dip galvanized steel sheets can now be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a cross-sectional photograph showing that an oxide is present under the black skin of a hot-rolled steel sheet.

FIG. 2 is an SEM photograph of a steel plate surface in which a plating layer of a steel plate is melted by a manufacturing method according to the present invention.

FIG. 3 is an SEM photograph of a steel plate surface in which a plating layer of a steel plate according to a conventional method is melted.

[Explanation of symbols]

 1 Steel plate substrate 2 Black skin (oxidized scale) 3 Oxide

Claims (1)

[Claims] 1. A hot-rolled steel strip is wound into a coil, cooled, then rewound, pickled, and cold-rolled.
When manufacturing an alloyed hot-dip galvanized steel sheet in a continuous hot-dip galvanizing line, the steel strip temperature at the time of coiling is 600 ° C. or more 75
A method for producing an alloyed hot-dip galvanized steel sheet, which is set to 0 ° C. or lower and a cooling rate thereafter is set to 3 ° C./min or lower.