JPH0483859A - Galvannealed steel sheet excellent in adhesive strength against low temperature impact - Google Patents

Abstract

PURPOSE:To obtain a galvannealed steel sheet excellent in adhesive strength against low temp. impact after coating by specifying the thickness of an alloy layer in galvannealing and the thickness of GAMMA-phase in the alloy layer, respectively. CONSTITUTION:In a galvannealed steel sheet, the amount CW(g/m<2>) of alloy layer and the thickness tg (mum) of GAMMA-phase in the alloy layer are regulated to the values in the range represented by CW>46.tg+12, respectively. The above alloy layer thickness CW is a variable necessarily determined by plating thickness at the time of hot dipping. On the other hand, in order to control the thickness of GAMMA-phase, e.g., the regulation of the initiating temp. of the alloying reaction between the steel sheet as base material and the plating layer at the time of alloying treatment is effective. By this method, the thickness of the GAMMA-phase which is hard and becomes the origin of cracking can be controlled, and the plated steel sheet excellent in adhesive strength against low temp. impact can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Fields] The present invention is directed to low-temperature shock! II relates to an alloyed hot-dip galvanized steel sheet with excellent adhesion.

[Problems to be solved by conventional techniques and inventions Corrosion resistance has been rapidly gaining importance in recent years as a factor that determines the commercial value of various durable consumer products such as automobiles and home appliances. Among them, alloyed hot-dip galvanized steel sheets have excellent corrosion resistance after painting, so there is a remarkable growth in demand in such industrial fields where painting is the premise.

On the other hand, the alloy layer of the alloyed hot-dip galvanized steel sheet is hard with a micro-Vickers hardness: M-)1v>about 200,
Adhesion of the alloy layer during processing is an important quality control issue. In particular, when subjected to impact at low temperatures where the strength of the coating increases, the alloy layer may peel off or fall off along with the coating, which poses a problem from the viewpoint of appearance and rust prevention.

The present invention provides an alloyed hot-dip galvanized steel sheet that has excellent low-temperature impact adhesion after painting.

The low-temperature impact adhesion after coating of alloyed hot-dip galvanized steel sheets has been investigated as the chipping phenomenon caused by stone splashing, and the enamel hair phenomenon during shearing in pre-coated steel sheets. In these conventional studies, based on the correlation with the Fe concentration in the plating layer,
Controlling the concentration within a specific range is said to be effective in improving impact adhesion.

However, as a result of a detailed study of the low-temperature impact adhesion of alloyed hot-dip galvanized steel sheets after coating, the inventors found that low-temperature? #! No correlation was observed between I adhesion and Fe concentration, and new knowledge was obtained that it is controlled by the alloy layer thickness and the r Kashihara degree in the alloy layer. The present invention was completed based on this knowledge.

An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet that has excellent low-temperature impact adhesion after coating.

[Means for Solving the Problems] The present invention relates to an alloyed hot-dip galvanized steel sheet with excellent low-temperature impact adhesion, and the alloy layer amount CW (g/m2) and the r Kashiwara Sa11 (μff1) in the alloy layer. , CW>46・tg+
It is characterized by having a range expressed by 12.

The present inventors investigated in detail the mechanism of the low-temperature impact delamination phenomenon, and found that the starting point of a crack when subjected to impact is the r-phase (F
e5Zn2+) and that the impact energy given to the phase depends on the thickness of the alloy layer. ζ phase (FeZn+3): M-H
v~180, δ1 phase (FeZn7): M-HV~2
This is due to the fact that it is the most vulnerable, with M-Hv ~ 420 compared to 50. The latter is due to the fact that impact energy is absorbed by the ζ phase and δ1 phase, which are softer than the ζ phase, so the thicker the alloy layer is, the less impact energy reaches the r phase.

Based on this new knowledge, we examined the low-temperature impact adhesion and found that by setting the alloy layer amount CW (g/m2) and the r Kashihara depth 1g (μm) in the alloy layer to CW>46・tg+12, excellent It was discovered that the following characteristics can be obtained.

In other words, ``under certain conditions of phase thickness, ζ phase, δ1
Increasing the thickness of the alloy layer is effective in increasing the absorption of impact energy in the phase, and conversely, under conditions where the alloy layer thickness is constant, it is effective to reduce the r Kashihara stiffness, which is the starting point of cracks under impact. It means something.

The alloy layer thickness is a variable necessarily determined by the plating thickness during hot-dip plating. Any method can be used to control Kashihara-san, and there is no need to specify it in particular.
For example, (1) starting temperature of alloying reaction between base steel sheet and plating layer during alloying treatment: 470°C or higher, (2) maximum plate temperature during alloying reaction = 520°C or lower, (3
) After alloying to the surface is completed, it is effective to set the cooling rate to 350°C at 10°C/sec or more, and by optimally combining these manufacturing conditions, the above relational expression can be achieved. That's what you get.

The present invention is applicable to alloyed hot-dip galvanized steel sheets with an alloy layer amount of 20 to 150 g/m2. It goes without saying that the present invention can also be applied to products in which a part of the plating layer is alloyed.

Some alloyed hot-dip galvanized steel sheets contain (75 to 90
Weight%) Fe-(10~25% by weight) Zn alloy 2~
5 g/m2 electroplating, the impact release mechanism of such products is also similar to that described above and is included in the present invention.

[Example] A cold-rolled steel sheet and a hot-rolled steel sheet were passed through a continuous hot-dip galvanizing line to produce an alloyed hot-dip galvanized steel sheet. After recrystallizing the raw steel plate in a reducing furnace type annealing furnace, it is immersed in a metal bath at a temperature of 470 to 510°C, and 30 to 90 g
/m2, then alloyed in a gas-heated alloying furnace to obtain an alloyed hot-dip galvanized steel sheet. The composition of the plating bath is Afl: 0.11~0.1F1% by weight, Fe0902~0.04% by weight, the balance being Zn and unavoidable impurities, and the plating bath temperature is 450~465°C.
It is. Average plate temperature during alloying treatment is 490-510℃
The heating time is 15 to 30 seconds. After the alloying treatment, air-water cooling was carried out in a water flow density range of 0 to 30 IL/m2·min. After performing temper rolling of 0.8 to 1.2%, a general three-coat coating with a total coating thickness of 100 μm was applied, and a low-temperature impact peeling test was performed.

The low-temperature impact peel test was conducted using the gravelometer method.

That is, the test piece was cooled to -20°C, and 50g of No. 7 crushed stone was thrown at an angle of 90°C at a blowing pressure of 4 kgf/cm2.
More than 2.0mm in diameter! Number of lI separation parts (100mm x
(per 100 mm test piece area) was measured.

Table 1 shows the test results, and it is clear that the alloyed hot-dip galvanized steel sheets of the present invention all had extremely few peeled pieces and had excellent low-temperature impact adhesion. On the other hand, all of the comparative examples had a large number of peeled pieces and had poor low-temperature impact adhesion.

Table 1 Note: In the alloy layer, a phase is formed at the interface between the alloy plating layer and the steel sheet, and the upper part consists of the δ1 phase and the ζ phase.

[Effects of the Invention] By doing so, an alloy layer with excellent low-temperature impact resistance can be obtained, resulting in a plated steel sheet that is particularly suitable for automobile steel sheets, etc., and has great industrial effects.

Claims (1)

[Claims] 1. The alloy layer amount CW (g/m^2) and the Γ phase thickness t_g (μm) in the alloy layer are in the range expressed by CW>46・t_g+12. , alloyed hot-dip galvanized steel sheet with excellent low-temperature impact adhesion.