JPS6056424B2 - Galvanized steel sheet with excellent cavity resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface-treated high-strength steel sheet with excellent spot weldability, and particularly to a galvanized steel sheet that can prevent the cavitation phenomenon that occurs during spot welding.

In recent years, from the viewpoint of durability and safety of automobiles, there has been a demand for surface-treated high-strength steel sheets with excellent corrosion resistance. There are many types of surface-treated steel sheets, but alloy galvanized steel sheets, which have excellent paint adhesion and corrosion resistance, are particularly popular for their high strength because they have excellent paint film adhesion during press forming. Steel sheets tend to be used more in the automotive industry.

As is well known, automobile steel sheets are required to have excellent spot weldability, but surface-treated steel sheets sometimes have poor spot weldability, which is different from ordinary steel sheets.

This tendency is particularly noticeable with hot-dip zinc alloy plated steel sheets, where the current value for dust generation is low, and when a current exceeding this value is applied, dust generation is intense and a large cavity is created in the nugget. This cavity sometimes occupies more than half of the nugget diameter, resulting in a cross tensile strength (C) of the spot weld.
(abbreviated as TS) may be 60% or less of the base material strength. This phenomenon is called a "cavity" and is empirically estimated to be 1.2
Thick material of Tfrfn or higher and tensile strength of 40kg/
It is known that this is common in one or more high-strength steel plates. Therefore, materials that are required to have high strength as surface-treated steel sheets for automobiles are required to have excellent cavity resistance properties. According to the knowledge obtained by the present inventors so far, it has been revealed that galvanized steel sheets that are not subjected to alloying treatment do not form cavities.

Therefore, it is presumed that the cavitation phenomenon depends on the structure and structure of the galvanized layer. Therefore, the present inventors conducted various experiments on the influence of the phase composition of the alloy plating layer on the generation of cavities in order to analyze the factors that cause the generation of cavities, using hot-dip zinc alloy plated steel sheets. As a result, the cavity generation phenomenon is caused by Fe-Zn intermetallic compound r with high Fe concentration.
phase, and by extension, Fe-Zn-Ae intermetallic compounds (Fe
, Zn) was found to be deeply related to XAfy. The present invention has been made based on the above knowledge, and its gist is that in galvanized steel sheets,
A Fe-Zn layer with a thickness of 0.1 to 1.0μ is placed at the boundary between the normal zinc-iron alloy plating layer consisting of ξ phase, δ1 phase, etc. and the steel substrate.
- A galvanized steel sheet with excellent cavity resistance characterized by the presence of an A'-based compound layer and an r-phase with a thickness of 0.3 μm or less. The present invention will be explained in detail below.

The layered structure of the galvanized steel sheet according to the present invention is shown in FIG.

That is, in Figure 1, an r layer and a Fe-Zn-Ae compound layer (Fe-Zn), Ae layer exist at the boundary between the steel substrate and the zinc-iron alloy plating layer consisting of the δ1 phase. This is a schematic diagram showing the current state. As seen in the figure, the r layer and (
The Fe-Zn)XAe, layers do not necessarily overlap each other, but rather often have a mixed form. ~1.0 .mu.m and the thickness of the r layer 0.3 .mu.m or less means that in the mixed form as shown in the figure, the thicknesses of both layers intermittent intermittently. Next, the present inventors conducted the following research to investigate the relationship between the intermetallic compounds of the Fe-Zn system consisting of F and the (Fe-Zn)XAey(7)Fe-Zn-A' system and the cavity generation phenomenon. I conducted an experiment.

That is, galvanized. At 460 to 500 degrees Celsius (partly at 700 degrees C
Heating for ~600 seconds (alloying treatment) in the temperature range (including °C)
A sample was prepared, spot welded, and the cavity occurrence rate was measured. Established. The steel tested was 4 with a plate thickness of 1.4TT0fL.
This is a 5 kg class high strength steel plate, and the amount of Ae in the hot dip galvanizing bath is 0.15%. Spot welding was carried out by stacking two or three samples, inserting a spacer between the first plate and the second plate, applying an electrode pressure of 400 k9, a current value of 13 kA, and a conduction time of 15 minutes. The cavity was determined by visually observing the inside of the nugget after the beer test, and determining the proportion (%) of the cavity diameter to the nugget diameter (■R) of 112 in the entire sample (n number = 25). . FIG. 2 shows the relationship between the alloying treatment conditions and the microstructure/structure of the alloy galvanized layer obtained under each of these conditions and the cavity generation rate. The phase structure of the plating layer is shown on the horizontal axis of the figure. As is clear from the figure, the occurrence of cavities depends on the amount of precipitation of the r-phase in the alloy plating layer, and the thickness of the r-phase is 0.3μ.
It can be seen that the incidence increases rapidly when the number of cases exceeds . Furthermore, as can be seen from the phase structure of the plating layer shown on the horizontal axis of FIG. 2, the process of forming the intermetallic compound constituting the alloy plating layer is extremely complicated. In particular, it is known that at the initial stage of "phase formation", the state of r-phase nucleus growth is controlled by the Ae% in the molten zinc bath. The Ae% in the bath is usually about 0.15%, but even at this level of Ae%, an intermetallic compound of (Fe, Zn) In order to suppress the diffusion of Fe(7) from Zn into the Zn plating layer, alloying of the galvanized layer is delayed and the formation of the r phase is suppressed. On the other hand, when a zinc alloy plated steel sheet is subjected to chemical conversion treatment and used, it is known that when the Fe%1 in the zinc alloy plated layer is 11%, that is, the δ1 phase, it has excellent properties such as paint corrosion resistance. There is.

Therefore, by proceeding with alloying until most of the zinc alloy plating layer becomes the δ1 phase, chemical conversion treatment properties can be ensured, and in order to prevent the formation of cavities, the generation of the r phase should be minimized. It must be suppressed. Therefore, the following experiment was conducted to confirm the relationship between the intermetallic compounds (Fe, Zn) and A' and the cavity generation rate. In other words, a 45 kg class high strength thin steel plate is
The copper plate was immersed in a n-bath, then passed through an alloying furnace maintained at 460°C, and after the surface temperature of the copper plate reached a constant temperature, it was blast cooled. In addition, Ae% in the Zn bath was set to 0.10, 0.15, 0.
20.Instead of 0.25%, various thicknesses of (Fe-Zn)XAe were grown on the boundary between the galvanized layer and the steel sheet base. The effect of the thickness of (Fe-Zn)XAe on the cavity occurrence rate was investigated. The results obtained are shown in Table 1. In this case, (Fe-Zn), Aey is (Fe-Zn)2A
'5, and the state of coexistence with the r phase was as shown in the schematic diagram of FIG. In addition, the identification results of the zinc alloy plating layer by X-ray diffraction are also described. The alloy plating layer made in the Zn bath without the addition of Ae had a cavity generation rate of 20% due to alloying. On the other hand, other alloy plating layers made using Zn baths containing Ae are coated on steel plate bases with a thickness of 0.1 to 1.0μ (
When Fe--Zn) In this way, the generation of r-phase is suppressed by (Fe-Zn) (Fe-Zn) of appropriate thickness, A
The existence of 'tsu is necessary. As shown in Table 1, if the thickness is 0.1μ, the effect will begin to be exerted.
If the thickness exceeds 1.0μ, the effect will be saturated, and if it is too thick, it will actually be disadvantageous. That is, the Zn alloy plated copper plate of the present invention has a thickness of 0.1μ to 1.0μ at the boundary of the steel plate substrate.
(Fe-Zn) with a thickness of This is an iron alloy layer, thereby suppressing the generation of cavities during spot welding and making it possible to obtain a zinc alloy plated steel sheet having excellent chemical conversion treatment properties. The effects of the present invention will be illustrated in more detail with reference to Examples below.

Example 1 A 45 kg class high-tensile steel sheet with a thickness of 1.5 wrm was
After being cut to 00x150d and degreased and washed, it was cut using the Sendzimier method. Hot-dip Zn plating was performed using a simulator.

The amount of Ae in the Zn bath is 0.01 and 0.2%. The thickness of the plating layer was 6 μm. The obtained Zn-plated steel plate was cut into strips of 30 x 150 d and alloyed.
Alloying heat treatment was performed so that the n-plated layer consisted of (Fe.Zn)2Ae5 of various thicknesses and a δ1 phase with an r phase. The spot welding conditions are as follows.

Electrode pressure 400k9, welding current 13kA, energizing time 1
5~, holding time 6=-, three steel plates were stacked, and a spacer was inserted between the top plate and the middle plate. The presence of cavities was determined by visually observing the inside of the nugget after the beer was broken. Table 2 shows the judgment results.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a cross-sectional example of a galvanized copper plate of the present invention, and FIG. 2 is a diagram showing the influence of structural elements of the plating layer on the cavity occurrence rate.

Claims (1)

[Claims] 1. In a galvanized copper plate, a Fe-Zn-Al compound with a thickness of 0.1 to 1.0 μm and a 0.1 μm thick Fe-Zn-Al compound and a 0.1 μm to 1.0 μm thick layer are coated on the boundary between the normal zinc-iron alloy plating layer consisting of ξ phase, δ_1 phase, etc. and the steel sheet base. A galvanized steel sheet with excellent cavity resistance characterized by the presence of a Γ phase of 3μ or less.