JPS5974290A - Surface treated steel sheet - Google Patents

Abstract

PURPOSE:To obtain a steel sheet having weldability, press workability, etc. in a well-balanced state as well as superior corrosion resistance by heat-treating layers of a Zn-Fe mixture contg. a specified amount of Fe deposited on the surfaces of a steel sheet by electroplating so as to form a stable alloy phase. CONSTITUTION:A coiled steel sheet manufactured through a thermal cycle in a cold rolling stage is uncoiled with an uncoiler 1, and after washing the surfaces of the plate in a degreasing bath 2 and a washing bath 3, the plate is fed to electroplating baths 4. In the baths 4, layers of a Zn-Fe mixture contg. >=6.5% Fe are formed on the surfaces of the sheet by electroplating. The plated steel sheet is fed to a heater 7 through a washing bath 5 and a water removing bath 6, and it is heated to about 500 deg.C to provide a stable alloy phase to the Zn-Fe layers. The steel sheet is then coiled with a coiler 8.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface-treated steel sheet for use in automobiles, home appliances, etc.

Among the surface-treated steel sheets conventionally used to improve the corrosion resistance of painted products such as automobiles and home appliances, this product has excellent paint corrosion resistance and is the best in meeting processing requirements such as press workability and weldability. As a material, alloyed galvanized steel sheets are made by heat-treating galvanized steel sheets to diffuse iron from the steel sheets into the lead plating layer, creating an alloy phase of zinc and iron. be. Most of the surface-treated steel sheets in this case are alloyed hot-dip galvanized steel sheets that are alloyed by placing the base steel sheet in a molten zinc bath at around 450°C and then heat-treating it at around 550°C for around 10 seconds. Some of them are alloyed electroplated steel sheets that are alloyed by soaking them at around 350°C for several hours and heating and cooling them for more than 10 hours each in an annealing furnace during the cold rolling process. Although both galvanized steel sheets and alloyed electrogalvanized steel sheets have a good balance of paint corrosion resistance, galvanizing workability, and welding, they both require a heat treatment process in the alloying process. This results in a thermal cycle that deviates from the thermal cycle that optimizes the workability of cold-rolled steel sheets, and it is difficult to obtain a cold-rolled steel sheet with the best press workability.

On the other hand, it is possible to simultaneously attach zinc and iron to a steel sheet by electroplating, but in that case, the press workability is the best because the plating is performed at room temperature on a cold-rolled steel sheet that has gone through an optimal thermal cycle. You can get something similar to a cold-rolled steel plate.

However, even in this case, it is difficult to stably obtain an alloy phase in an equilibrium state between zinc and iron in the plating layer. In other words, in an equilibrium state, the alloy phases such as ↓ phase, g phase, etc. should exist in the region where the iron pressure ratio is small, but pure zinc phase (η phase) is mixed in, or the iron pressure ratio is low. In a region with a high value, a strained crystal containing α-Fe in the phase is observed. Therefore, all of these various conditions are considerably inferior to the above-mentioned alloyed galvanized steel sheets in terms of paint compatibility and corrosion resistance after painting.

The present invention solves these drawbacks seen in electroplating.In order to obtain the best press workability for cold rolled steel sheets, the present invention performs almost complete thermal cycling and also reduces the temperature of galvanized steel sheets after painting. The objective is to provide a steel plate that has a better balance of corrosion resistance, weldability, press workability, etc. after painting than conventional steel plates by making it compatible with painting. That is, the present invention uses zinc-iron mixed N containing 6.5% or more of iron precipitated on the surface of a steel sheet.
, @, relates to a surface-treated steel sheet in which the plating layer is heat-treated to form a stable alloy phase.

In the surface-treated steel sheet of the present invention, by setting the iron pressure ratio in the electroplating layer to 6.5% or more, the alloy phase formed by heat treatment is in an equilibrium state of zinc-iron, and ζ
The η phase is not mixed in the phase, g, and phase, and the T' phase with a high iron pressure is also finely distributed. Therefore, it has excellent corrosion resistance and is particularly excellent in the balance of weldability, press workability, etc. It is a steel plate.

An example of manufacturing a surface-treated steel sheet according to the present invention is described below in F.
This will be explained in more detail.

FIG. 1 illustrates an example of the W manufacturing process. The coiled steel sheet produced through the heat cycle of the R skin in the cold rolling process is returned to the restoring device 1, and then its surface is cleaned in the degreasing tank 2 and cleaning tank 3, and then zinc is removed in the electroplating tank 4. −
Plating iron mixing tank. In this case, electroplating in the electroplating tank may be carried out under sufficient conditions to obtain a plating layer in which the required amount of iron is evenly mixed and the iron pressure ratio is usually 6.5% or more. For example, FeSO4-7H20
100g/l, ZnSO4-7H20120+z/n
.

(NH4) 280430 g/l, KOl 20
g/l, ferric ammonium citrate 811 g/13
By setting the pH to 4.0 and plating at a current density of A/dm2, a plated layer with an iron ratio of ~(9)% and a basis weight of about 20 g/in2 can be obtained.

Further, the electroplating in this case can be carried out in two or more baths having different iron pressure ratios, and in that case, the electroplating temperature may be sufficient as long as it is sufficient to obtain the desired iron pressure ratio. Next, the plated steel sheet obtained as described above is passed through a cleaning tank 5 and a draining furnace 6.
A heating device after passing through it? After heat treatment at t7, the winding device 8
It is manufactured by winding it up.

The heat treatment conditions after electroplating are such that the diffusion of iron from the substrate to the plated layer is kept to one-third or less of the total thickness of the plated layer in order to prevent the η phase from being mixed into the K1 phase and K1 phase. For example, heat treatment using a high-frequency induction heating device for thermal insulation and cooling at around 500°C may be used.In this case, high-frequency induction heat treatment when the steel plate is plated on only one side is particularly suitable for non-oxidizing It is best to carry out the process in an atmosphere. Furthermore, this heat treatment process after electroplating does not necessarily have to be done in-line. Any device can be used as long as the heating and holding time can be selected such that the heating and holding time stays within the following range. You can get steel plates.

According to the present invention, since iron is evenly mixed in the required amount in the plating layer, the base iron is diffused throughout the entire thickness of the plating layer, as in conventional alloyed galvanized steel sheets. There is no need for temperature and time for this process, and the heat treatment is sufficient to turn the plating layer into a stable alloy phase, so deterioration in press formability can be kept to a minimum. Further, the electroplating conditions are not intended to achieve a specific alloy state, but may be conditions that allow only the iron pressure ratio to be controlled. Therefore, the alloy phase obtained is in the most stable state, and is homogeneous over the entire length of the steel plate, especially when using high-frequency induction heating.

The surface-treated steel sheet U of the present invention obtained as described above is a steel sheet with excellent corrosion resistance, weldability, press workability, etc. EXAMPLES The composition of a surface-treated steel sheet and a surface-treated steel sheet of the present invention, which is subjected to heat treatment after electroplating, was investigated by X-ray diffraction of the plating layer.

First of all, MvI: iron = 55:45 (heavy JIJt, )
Nausea plating was performed in a TV mixed electroplating tank to obtain a surface-treated steel sheet A with a coating weight of 24mm2. On the other hand, the surface-treated steel sheet obtained in this way was heat-treated in a heating device under the conditions of 350°C x 10 minutes, and the surface-treated steel sheet B of the present invention was heated.
I got it. The compositions of the plating layers of these steel plates according to X-ray diffraction are shown in Figure 2 (#II plate) and Figure 3 (steel plate B).Similarly, zinc:iron = 65:35
(weight ratio) in a mixed electroplating tank with a coating weight of 26
g/in" surface treated steel plate 0 and this steel plate C.
The surface-treated steel sheet of the present invention was heat-treated under the conditions of 0°0XIO minutes, and the composition of the plating layer was examined by X-ray diffraction. That's right.

As is clear from these figures, the treated steel sheet of the present invention has a stable alloy phase when the iron ratio is 6.5% or more. It is also well understood that iron is evenly mixed in the plating layer in the T phase as well. This is specifically caused by the fact that the iron pressure ratio in the electroplated layer is 6.5% or more. In other words, an iron pressure ratio of 6.5% is the lower limit of the iron content ratio of the phase, which is a stable alloy phase, and if it is less than this, the pure zinc phase (η phase) will remain during plating, resulting in a favorable equilibrium state. This is because you cannot hope for it.

In consideration of this, even in the r-phase with a high iron pressure, which was thought to be impossible with conventional steel sheets, iron is plated evenly in the plating layer. As a result, the inconveniences that existed during conventional press working and welding are completely eliminated, and the corrosion resistance is also excellent.

In order to confirm this corrosion resistance, the rust occurrence ratios of the steel plates A to D obtained above were compared using the salt spray test method, and the results are shown in Table 1.

Table 1 Cation ED 20 μm Salt water spray As is clear from the results of 750 hours or more, the surface-treated steel sheet of the present invention is a steel sheet with extremely excellent properties.

[Brief explanation of the drawing]

Figure 1 shows one manufacturing process of the surface-treated steel sheet of the present invention, and Figures 2 to 5 show the composition distribution of the plating layer by X-ray diffraction. Equipment, 2... Degreasing tank, 3... Cleaning tank, 4... Electroplating tank, 5... Cleaning tank, 6... Water cutting furnace, 7... Heat treatment device, 8... Winding device, (
P! or 1 person) 2vA h a: η8 (zn) b : < phase (Fe2!n13) C: α-Fe 3 Figure O: j1 phase (FeZn7) b: ζ phase C: α-Fe d: r phase Figure 5 Q: Self phase b: η phase (Zn) C; α-Fe Figure 5 q: Phase b = 61 phase C: 4 phase d: Goo Fe

Claims (1)

[Claims] (1) A surface-treated steel sheet in which a zinc-iron mixed electroplated layer containing 6.55X or more of iron is precipitated on the steel plate surface and is heat-treated to form a stable alloy phase.