JP2972124B2 - Galvannealed steel sheet with excellent plating adhesion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an alloyed hot-dip galvanized steel sheet having excellent plating adhesion.

[0002]

2. Description of the Related Art Galvannealed steel sheets (GA) are
Since it is inexpensive and has excellent corrosion resistance, it is mainly used for automobile bodies. As the performance of a steel sheet for an automobile body, in addition to corrosion resistance, plating adhesion during press working is required. When the plating adhesion is deteriorated, the plating layer is peeled off in a powdery or massive form, so-called powdering is caused, causing mold seizure or deteriorating the corrosion resistance of the peeled portion. In addition, there is a problem that a scratch is caused by the peeled plating piece. As a conventional technique for improving adhesion, Japanese Patent Application Laid-Open No. 61-277661 requires that hot dip galvanizing is performed and then alloyed at a high temperature of 700 to 850 ° C. However, alloying at a high temperature not only increases the cost, but also increases the load on facilities such as rolls.

[0003] In Japanese Patent Application Laid-Open No. 3-232620, at least one of Zr, La, Ce, Y and Ca is contained in steel.
The cooling rate from recrystallization annealing to plating is set to 50 ° C./s or more. Addition of Zr or the like to steel increases costs, and there is a problem in that productivity is poor because the sheet passing speed has to be reduced due to the problem of cooling capacity.

In Japanese Patent Application Laid-Open No. Hei 2-163356, the components of O, Al and N are particularly 0.0045 wt%.
The following (25 × N wt%) to 0.15 wt%, 0.003
It is specified as 0 wt% or less. Also, JP-A-6-811
In Japanese Patent Publication No. 01, it is necessary to satisfy the restrictions on the amounts of Ti, Si, and P, and Si (wt%) + P (wt%) ≧ Ti (wt%). In any case, the regulation by the component does not always achieve the steel sheet performance such as the desired strength and drawability, and there is a great risk of deterioration of the powdering due to the component separation. In JP-A-4-333552, the plating adhesion is improved by performing Ni pre-plating before hot-dip galvanizing. However, such a facility is not usually provided in a hot-dip galvanizing line, and a large investment is required for improving the facility.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an alloyed hot-dip galvanized steel sheet having excellent adhesion during press working and a method for producing the same, overcoming the above problems. It is.

[0006]

SUMMARY OF THE INVENTION The present invention has been developed to solve the above-mentioned problems . Mn, P, and A formed at the grain boundaries of a steel sheet immediately below an alloyed hot-dip galvanized layer during hot rolling.
Disclosed is an alloyed hot-dip galvanized steel sheet having excellent plating adhesion, characterized by having an oxide.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the grain boundary oxide immediately below the plating layer in the present invention will be described. This grain boundary oxide
It is produced during hot rolling, and can be grown and formed particularly when the temperature during coil winding is high and the cooling rate thereafter is low. The oxide formed during the hot rolling is observed immediately below the black scale (scale) as shown in FIG. 6 (photograph). In addition, as shown in FIG. 7 (photograph), no oxide is found directly under the black scale in the conventional hot rolled sheet. FIGS. 6 and 7 are cross-sectional optical micrographs of grain boundary oxide immediately below the black scale (scale) in the hot-rolled sheet, and have a magnification of 1000 times. The black scale 2 is coated on the steel plate base 1, and in FIG. 6, the grain boundary oxide 3 is observed at the interface between the steel base and the black scale.

FIG. 1 shows the results of analysis of oxides observed during hot rolling of the steel sheet of the present invention by EPMA (X-ray microanalyzer). Peaks are observed in Mn, P, Al, and O, indicating that these oxides are generated. The oxide at the grain boundary immediately below the plating layer in the alloyed hot-dip galvanizing according to the present invention, the oxide immediately below the black scale formed at the hot rolling stage remains after the subsequent cold rolling, annealing, plating and other steps. Is what it is. FIG. 2 shows the results of elemental analysis in the depth direction from the surface layer to about 10 μm by glow discharge (GDS) of an unannealed sheet after cold rolling. FIG. 3 shows the results after cold rolling annealing. In FIG. 3, Mn that looks at a depth of about 0.3 to 4 μm from the surface layer,
The peaks of Al, P, and O correspond to grain boundary oxides.

Mn concentrated as these grain boundary oxides,
P and Al are not particularly present in the steel in a large amount. For example, the components in the steel of the steel sheets shown in FIGS.
1 wt%, P: 0.006 wt%, Al: 0.03 wt
%belongs to. The mechanism by which oxides are formed at the grain boundaries is due to the diffusion of oxygen into the steel, so that oxides are formed even if the components in the steel are minute.

Next, regarding the method of observing the oxide present at the grain boundary of the steel sheet immediately below the plating layer in the present invention, the grain boundary oxide immediately below the plating layer is removed by etching for several seconds to several tens of seconds with a 1% nital solution. Observation is possible. FIGS. 8 (photographs) and 9 (photographs) show observation examples of the conventional alloyed hot-dip galvanized steel sheet without oxides and the steel sheet with oxides according to the present invention. FIGS. 8 and 9 are cross-sectional optical microscope photographs of the galvannealed steel sheet at a magnification of 1000 times. The steel sheet substrate 1 is covered with the plating layer 4, and in FIG. 8, grain boundary oxides 5 indicated by arrows are observed at the interface. A black band-like thing (shown by an arrow) observed immediately below the plating layer is an oxide.

Next, the plating adhesion of the steel sheet of the present invention will be described. It is known that plating is peeled off mainly due to compressive stress during press working. In addition, during the alloying treatment, a zinc-iron alloy is generated by thermal diffusion of zinc and iron, but zinc is also diffused to the steel sheet grain boundaries to form a zinc-iron alloy phase. In the alloyed hot-dip galvanized steel sheet according to the present invention, the steel sheet grain boundary where the oxide exists directly below the coating layer has a gap between the oxide crystals as compared with the conventional grain boundary where the oxide does not exist, so that zinc permeates. It's easy to do. as a result,
The unevenness of the interface between the plating layer and the steel sheet becomes severe, and the plating layer firmly adheres to the steel sheet. As a result, in the alloyed hot-dip galvanized steel sheet of the present invention, the adhesion of the plating during the press working is improved.

FIGS. 10 and 11 (photographs) show the results obtained by forcibly melting the plating layer to the iron potential by the potentiostatic method to expose the steel plate and observing it with a scanning electron microscope (SEM). It can be seen that the unevenness of the interface between the plating layer and the steel sheet is clearly greater than that of the conventional steel sheet without grain boundary oxides.

The improvement of the adhesion of the steel sheet according to the present invention at the time of press working is effective when the cross section is polished, etched with 1% nital, and a small amount of oxide immediately below the plating layer is observed with an optical microscope. Was seen. In the present invention, the plating layer is not particularly limited,
From the viewpoint of corrosion resistance and the like, it is generally appropriate that the amount of zinc-iron alloy to be applied is 25 to 90 g / m ² and the iron content in the plating layer is 8 to 13 wt% as a steel sheet for automobiles. Similarly, the conditions of the zinc bath are not particularly limited, but it is appropriate that the Al concentration in the zinc bath is about 0.13 to 0.15 wt% and the Fe concentration is 0.01 wt% to saturation. In addition, P
b, Mg, Mn or the like may be contained.

[0014]

An example of the present invention will be described below. Test materials of low carbon steel (test steel A) and ultra-low carbon steel (test steel B) were melted in a converter, and then slab was formed by continuous casting. This slab is heated at a slab heating temperature (SRT) of 1,050 to 1,250 ° C.
Finishing temperature 850-950 ° C, coil winding temperature 60
The steel sheet having the grain boundary oxide of the example was manufactured by hot rolling at 0 to 750 ° C. and slowly cooling to a thickness of 35 mm. On the other hand, as a comparative example, a steel sheet having a winding temperature of less than 600 ° C. or a steel sheet rapidly cooled at a winding temperature of 600 to 750 ° C. and having no grain boundary oxides was manufactured. Thereafter, the scale layer was removed by pickling and cold rolling was performed to a thickness of 0.7 mm. This cold-rolled sheet was subjected to recrystallization annealing at 750 to 880 ° C. in a continuous hot-dip galvanizing line (CGL), and then to hot-dip galvanizing at 460 to 485 ° C.
Subsequently, an alloying treatment was performed at 480 to 530 ° C. for 15 to 30 seconds. The presence or absence of the grain boundary oxide was observed after etching with a 1% nital solution after polishing the cross section.

A press workability evaluation test was performed as follows. The alloyed hot-dip galvanized steel sheet is bent by 90 degrees, bent back, and the pressure-bonded side is tape-peeled to determine the amount of zinc peeling.
It was measured with a line. The number of counts by fluorescent X-ray And Table 1 shows the results. Although the powdering resistance changes depending on the change in the iron content and the change in the amount of adhesion, the effect of improving the powdering resistance in the examples of the present invention is sufficiently observed as compared with the comparative example.

[0016]

[Table 1]

[0017]

As explained above, the alloyed hot-dip galvanized steel sheet of the present invention has good adhesion in press working and supplies a high-quality hot-dip galvanized steel sheet. It is something to expand.

[Brief description of the drawings]

FIG. 1 is an EPMA analysis chart of oxides observed during hot rolling.

FIG. 2 shows a conventional example of glow discharge (GD) of an unannealed sheet after cold rolling.
It is a graph of the depth direction elemental analysis measurement result to about 10 micrometers from a surface layer by S).

FIG. 3 shows a glow discharge (GD) of an unannealed sheet after cold rolling in Example.
It is a graph of the depth direction elemental analysis measurement result to about 10 micrometers from a surface layer by S).

FIG. 4 is a graph of the results of elemental analysis in the depth direction from the surface layer to about 10 μm by glow discharge (GDS) after cold rolling annealing in a conventional example.

FIG. 5 is a graph of the results of elemental analysis in the depth direction from the surface layer to about 10 μm by glow discharge (GDS) after cold rolling annealing in Examples.

FIG. 6 is an optical micrograph (magnification: 1000) of grain boundary oxide immediately below the black scale of the hot-rolled sheet of the example.

FIG. 7 is an optical microscope photograph at 1000 times magnification of a grain boundary oxide immediately below the black scale of a hot-rolled sheet of a conventional example.

FIG. 8: Magnification 10 of the galvannealed steel sheet of the example
It is a cross-sectional optical microscope photograph of 00 times.

FIG. 9 is a cross-sectional optical microscope photograph of a conventional alloyed hot-dip galvanized steel sheet having no oxide at a magnification of 1000 times.

FIG. 10 is a magnification of 10 of a steel sheet in which a plating layer is melted in an example.
It is a SEM photograph of 00 times.

FIG. 11 is a magnification of 100 of a steel sheet in which a conventional plating layer is melted.
It is a SEM photograph of 0 time.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate base 2 Black scale 3 Grain boundary oxide 4 Plating layer 5 Grain boundary oxide

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-306461 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. An alloyed hot-dip galvanized steel sheet having excellent plating adhesion, characterized in that Mn, P, and Al oxides generated during hot rolling are present at the grain boundaries of a steel sheet immediately below an alloyed hot-dip galvanized layer.