JPS62170340A - Organic coating steel plate having excellent baking hardenability - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a highly corrosion-resistant, organic-coated steel sheet for drawing that has bake hardenability and is intended for use in automobiles.

Here, bake hardenability refers to the hardenability that occurs during the paint baking process applied to steel plates after press forming in the automobile manufacturing process, etc., and is usually 2% pre-strain followed by 170°C x 20
It is evaluated based on the degree of increase in yield strength after a baking heat treatment of electrodeposition coating for 30 minutes.

<Conventional technology and its problems> In recent years, increasing the corrosion resistance of automobile steel sheets has attracted attention as a social demand, and various rust-proof steel sheets have been proposed to meet this issue, and are gradually becoming established. be.

Some of these antirust steel sheets are surface-treated by hot-dip Zn plating, hot-dip Zn-based alloy plating, electrolytic Zn-based plating, electrolytic Zn-based alloy plating, and organic coating 11q-based zinc-rich coating.

Furthermore, in addition to these, composite coated steel sheets have also been developed in which a perforated coating is applied to a plated steel sheet. This composite type coated steel plate is currently known as the most widely used rust-proof steel plate.

On the other hand, the amount of high-strength steel plates used in automobiles is increasing in order to save energy and improve driving performance.
In order to compensate for the Punt resistance that accompanies the decrease in plate thickness, a steel plate is desired that has a low yield strength on the press-formed surface and increases in yield strength when baked and painted. In addition, even in general steel sheets for drawing, bake hardenability after press forming is a favorable phenomenon for Punt resistance especially when used for automobile exterior panels, and steel sheets that have both deep drawability and bake hardenability are desired.

As described above, there is a desire for a rust-proof steel sheet that meets a variety of requirements, including not only corrosion resistance but also weight reduction, safety, and rigidity.

Conventionally, it has been possible to manufacture Zn or Zn-based alloy coated steel sheets that meet these requirements, but in conventional organic coated steel sheets, the organic coating is a hardened film, so these coated steel sheets cannot be manufactured. The baking temperature of the organic coating on the top is over 150°C, and as a result, the organic coated steel sheet is hardened before press forming, so there is a problem that it is not suitable for drawing purposes.

In particular, conventional automotive organic coated steel sheets (or painted steel sheets)
Synchrometal ■ (Japanese Patent Publication No. 47-6882, Patent Publication No. 52-904, No. 53-44887, etc.), Japanese Patent Application Publication No. 57-189842, No. 60-174879, No. 60-86281, No. 60-86281, No. 60-105535, etc., but all of these have the above-mentioned problem because the organic coating is a hardened coating, and the baking temperature is over 150°C, and the baking of each steel sheet itself is difficult. Curability could not be used effectively.

<Object of the Invention> The present invention has been made in view of the above-mentioned points, and an object of the present invention is to provide an organic coated steel sheet that has bake hardenability even after being treated with an organic film and has excellent workability.

<Structure of the Invention> The present invention is a highly corrosion-resistant organic film in which a drawing steel plate made of ultra-low carbon steel and having bake hardenability is plated with a Zn-based alloy, and then an organic film is applied to a surface-treated steel plate that has been subjected to chromate treatment. A coated steel sheet, wherein the baking temperature of the organic film is 150
By setting the temperature below ℃ as an essential condition, the bake hardenability of the steel plate is maintained until after press forming.

As mentioned above, conventional organic coated (painted) steel sheets for automobiles are
In order to make all organic films into hardened films, it was required that they be baked at a temperature of over 150°C.

The reason for this is that Synchrometal ■, which paints cold-rolled steel sheets, must be baked at high temperatures in order to cause chromate containing zinc dust to react with the steel sheet, and because the resin is a polymer, a high boiling point solvent is used. This is due to the fact that baking treatment cannot be performed at temperatures below ℃.

Furthermore, even when using plated steel sheets, for example, JP-A-57
In a zinc-rich type organic composite steel sheet such as No. 189842, the organic film must be baked at a high temperature similar to the cold-rolled steel sheet described above.

Furthermore, thin-film type organic coated steel sheets other than the zinc-rich type are currently baked at high temperatures in order to add a high temperature cross-linking type curing agent such as melamine resin.

In other words, none of these conventional organic coatings is designed to take advantage of the characteristics of bake-hardenable steel sheets as in the present invention. Therefore, there was a problem that the processability was inferior.

As described above, in order to take advantage of the characteristics of bake-hardened steel sheets and maintain the conventional high corrosion resistance, the present inventors have investigated an organic coating that has sufficient corrosion resistance when baked at 150°C or less, and has developed an ultra-low carbon film. An organic rust-preventive coating is applied to a bake-hardenable cold-rolled steel plate made of steel by heat treatment at 150°C or less so as not to impair the bake-hardenability, and this rust-preventive coating is different from the conventional highly corrosion-resistant organic coating ( In order to have corrosion resistance, workability, and weldability equivalent to or better than that of steel sheets (painted), we propose an optimal range of comprehensive coatings consisting of ten coatings and ten organic coatings.

According to the present invention, Zn-based alloy plating is applied to the surface of an ultra-low carbon steel sheet having bake hardenability at 10 to 40 g/d, and after a chromate treatment of 10 mg/m2 or more in terms of metallic chromium is applied thereto, the baking temperature is Provided is an organic coated steel sheet with excellent bake hardenability, characterized in that an organic film is formed at 150° C. or lower.

Further, according to the present invention, a Zn-based alloy plating of 10 to 40 g/m2 is applied to the surface of an ultra-low carbon steel sheet having bake hardenability,
An organic coating with excellent bake hardenability, characterized in that an organic coating containing silica is formed at a baking temperature of 150°C or less after chromate treatment of 10 mg/rn' or more in terms of metallic chromium. Steel plate is provided.

The present invention will be explained in detail below.

The reason why the baking temperature of the organic film for maintaining the bake hardenability of the steel sheet was limited to 150° C. or lower is as follows.

C: 0.003wt%, Si; 0
．． 01wt%, Mn; 0.16wL%, 81
; 0.04wt%, P H0,070wt
%, Nb; 0.026 wt%, Nb/(:
6.5, remainder: A steel plate substantially made of Fe was made into a plate thickness of 0.7mo+ with a cold reduction rate of 80%, heat treated in a continuous annealing furnace at 850°C for 30 seconds, and then heated to 650°C.
The mechanical properties of the ultra-low carbon bake-hardenable steel sheet cooled at 5°C/sec and further temper-rolled at a reduction rate of 1.0% are as follows: YS (yield strength): 20 gf/+wd, TS (tensile strength) ;3
5 gf7yd, EiL (elongation); 45%, 81 ((
Bake hardenability): 5Kgf/-.

When this steel plate was coated with an organic film and baked for 1 minute at various temperatures, its roundness was as shown in Figure 1. As is clear from this result, when the baking temperature exceeds 150° C., the BH value of the steel sheet decreases rapidly and the bake hardenability is impaired.

As a bake hardenable cold rolled steel sheet, c; o, oot ~ 0
．． 008 wt%, Si; 0.5 wt% or less, M
n; 0.0! i~1.2wt%, P; 0.1w
t% or less, Afi; 0.01 to 0.08wL% and Nwt%X8 or more, Nb; Cwt at Cwt%x3 or more
, %x E3 + 0.02wt% or less, and the steel plate contains approximately 1゛i and 0.05wt% or less, and is continuously annealed to remove the old! The amount was controlled at 3-6 Kgf/- [lH
Cold rolled steel plate is suitable. By using ultra-low carbon steel as the material, we were able to obtain properties that maintain the original properties without hardening when baked at low temperatures below 150°C.

The reason for this is not necessarily certain, but BH raw steel plate made of low carbon steel. Old! made of ultra-low carbon steel! Even if the apparent Bl [is the same as that of a steel plate, the solid solute C in the grains
It is expected that the distribution of will be different.

Here, in the bake-hardenable cold-rolled steel sheet (ultra-low carbon steel sheet), the content of each element was limited to the following ranges.

C is preferably 0.001 to 0.008 wt%. 0.
If it is less than 0.001 wt%, there will be insufficient solid solution C that contributes to bake hardening, and if it is more than 0.008 wt%, the yield strength will be high and the ductility and lower value will deteriorate.

The content of Si is preferably 0.5 wt% or less. This is because if it exceeds 0.5 wt%, an oxide film will be formed and the chemical conversion properties will be impaired.

Mn is preferably 0.05 to 1.2 wL%. 0.05
This is because if it is less than 1.2 wt%, the red heat brittleness deteriorates, and if it exceeds 1.2 wt%, the lower value deteriorates.

Dl, + n f + t' M to I T h< b1 Main 1.
This is because the steel plate becomes brittle if it is introduced at -0.1wt:%.

82 is 0.01 to 0.08 wt% and Nwt%X8
The above is preferable. 81 fixes N, so 0.01w
It is necessary to use 1 t% or more, but if it exceeds 0.08 wt%, inclusions will occur frequently, which is not preferable. Also, to fix N
wL%X8 or more is required.

Nb is CwL%x3 or more and Cat%x8+0.
The content is preferably 0.02 wt% or less. If the Cwt% is less than 3, a large amount of solid solute C remains, and a texture with excellent drawability cannot be formed during cold rolling recrystallization. On the other hand, CwL%x 8 + 0
．． If it exceeds 0.02 wt%, the ductility of the steel sheet will be impaired.

Ti is preferably 0.05 wt% or less. 0.05wt%
This is because in super, the old oneness is lost.

Further, it is preferable to set the Bl amount to 3 to 6 gf/-. If it is less than 3 gF/-, there is a substantial shortage of all;
This is because if it exceeds 6 Kgf/-, aging deterioration is severe and stretcher strain is likely to occur during processing.

The Zn-based alloy plating applied to the ultra-low carbon steel sheet mentioned above is Zn-Ni alloy plating (preferably Ni content 5-1Z).
n-Fe alloy plating (preferably Fe content 8-25wt)
%), 1n-GO-AjlL203-Cr20
3 alloy plating (preferably Go content 1 to 5 wt%), - Zn-A1 alloy plating (preferably AI!. content 1 to 1
5wL%), Zn-Ni/Fe-P two-layer alloy plating (preferably F
P-containing fil O. 0003 ~5 wt%
), Zn-Fe/Fe-P two-layer alloy plating, etc. can be used. These Zn-based alloy platings are different from conventional pure Z
Since it has corrosion resistance several times higher than that of n-plating, it further improves the object of the present invention. Also, the plating size is 10
-40 g/m'' is preferable. If it is less than 10 g/m1, the corrosion resistance is insufficient, and if it exceeds 40 g/m2, there is no further significant improvement in corrosion resistance and it is not economical.

The reason why the content of each element in the Zn-based alloy is limited as described above is as follows.

The Ni content of the Zn-Ni alloy is preferably 5 to 13 wt%. 5wt. If it is less than %, corrosion resistance will be insufficient and f3wL
This is because if it exceeds %, the plating layer will become too hard.

The Fe content of Zn-Fe alloy is 8-25 wt%
is preferred. If it is less than 8 wt%, corrosion resistance will be insufficient, and 25
This is because red rust is likely to occur if the content exceeds t%.

7, Co of 41203-Cr203 alloy to n-(:o-
The content (2) is preferably 1 to 5 wt%. This is because if it is less than 1 wt%, corrosion resistance will be insufficient, and if it exceeds 5 wt%, it will be economically disadvantageous.

The ^2 content of the Zn-A 1 alloy is preferably 1 to 15 w[%. If it is less than 1wt%, corrosion resistance will be insufficient, and if it is less than 15wt%.
This is because the sacrificial anti-corrosion property is lost when the content is too high.

Zn-Ni/Fe-P alloy and Zn-Fe/Fe-P
The P content in the Fe-P alloy is 0.0003 to 5 w
L% is preferred. 0.0003wt. If it is less than 5 wt %, the chemical conversion treatment will be poor, and if it exceeds 5 wt %, the current efficiency during plating will be poor and it will be uneconomical.

In order to improve the adhesion of the below-mentioned organic film on the Zn-based alloy plating and further improve its corrosion resistance,
Perform chromate treatment. This chromate treatment requires a coating amount of 10mg/rn'' or more as metallic chromium, and for this purpose, coating type chromate or electrolytic chromate is effective in controlling the coating amount.If it is less than 10mg/m2, corrosion resistance will simply be insufficient. Moreover, the adhesion with the organic film is also poor.

On the chromate-treated film, heat was applied at 150°C as described above.
An organic film treatment is performed at the following baking temperature to improve rust prevention. As the resin for this organic film, any one of the following (1) to (4) is used, either alone or in combination.

■Water-dispersible resin Acrylic, polyethylene, epoxy, alkyd, etc.

■Solvent-based resin: epoxy, polyester, etc.

■Ultraviolet or electron beam curable resin Acrylic, epoxy, polyurethane, etc.

For resin treatment using the above ■ or ■, 150℃
In order to obtain sufficient curing with the following low 7H treatment, examples include a method of using a metal salt catalyst such as Go naphthenate as a low-temperature curing agent, and a method of using an amine-based curing agent such as diethylenetriamine in combination. can give. Although curing proceeds even at room temperature, these can be easily formed by adopting a method that is compatible with the organic resin used in forming the organic film.

On the other hand, regarding (2), as mentioned above, there are already known acrylic resin films using benzoin ether as a photopolymerization initiator, and epoxy resins using aromatic diazonium salts as a photopolymerization initiator. Examples of resins include acrylic and epoxy resins that use an epoxy-acid addition reaction, and polyurethane resin films that use an isocyanate-hydrogen addition reaction. Bake hardenability can be maintained sufficiently.

Any one of these (1) to (4) is coated on the chromate-treated film at a thickness of 0.5 to 3 μs. Film thickness is 0.5
If it is less than -, corrosion resistance will be insufficient, and if it exceeds 3 houses, weldability will deteriorate.

Note that the corrosion resistance is further improved by adding up to 60% of silica sol to the organic resin film. 60wt
If it exceeds %, the viscosity of the treatment liquid will increase, making it easier to gel.

<Example> Hereinafter, the present invention will be explained based on Examples.

[Examples and comparative examples]

First, the composition and mechanical properties of the bake-hardenable cold-rolled steel sheet to be used are determined.
Shown in the table.

Using the bake-hardenable steel plates (Material No. 1 to 6) shown in Table 1 as raw materials, Zn-based alloy plating was subjected to chromate treatment and organic coating treatment, and these were used as test steel plates. The treatment steps are shown in Table 2.

Table 3 shows the results of corrosion resistance, weldability, workability, and bake hardenability of each of the test steel plates.

The evaluation of each test was conducted according to the following method.

[Test method and its evaluation]

(1) Corrosion resistance (1-a) SST Cross-cuts were made on the perforated film formed on each test steel plate, and 5% NaCf) was sprayed onto these at 35°C, and the time until red rust appeared was measured. . (1-b) CGT Each test steel plate was sprayed with 5% NaCl at 35°C for 4 hours under the following conditions ↓ Dry at 60°C for 2 hours ↓ 50°C and left in 95% RH (humidity) for 2 hours for 1 cycle. A cyclic corrosion test was conducted and the number of cycles until red rust appeared was counted.

(2) Weldable electrode R type (40R), pressing force 170g, welding time 1
Two sheets of each test steel plate were stacked alternately under the conditions of 0~,
The number of continuous welding points was investigated.

(3) Workability Each test steel plate (90mmφ) has a diameter of 50mtnφ and a depth of 25mm.
+nm cylindrical drawing processing (B liver 1 tor) The processed portion was peeled off with cellophane tape, and the amount of film peeled off (mg/circumference) was measured.

(4) Bake hardenability (B11 value) After pre-straining each test steel plate by 2%, the increase in YS (yield strength) after baking addition heat treatment at 170°C for 20 minutes (kgf/
m) was measured.

As shown in Table 3, by the treatment method of the present invention, a highly corrosion-resistant bake-hardenable organic coated steel sheet having high corrosion resistance, weldability, workability, and bake-hardenability can be obtained.

<Effects of the Invention> As detailed above, according to the present invention, the baking temperature is 150°C.
By carrying out the organic coating treatment described below, it is possible to provide an organic coated steel sheet with excellent bake hardenability as well as high corrosion resistance, weldability, workability, and bake hardenability.

[Brief explanation of drawings]

Figure 1 shows the baking temperature of the perforator film and the baking hardenability of the steel plate (B1
1 value). Patent Applicant Kawasaki Steel Co., Ltd. Agent Patent Attorney Nozomu Watanabe Nejiri Patent Attorney Yo Ishii -FIG, 1 Baking temperature of Kobinho Rebellion C)

Claims (2)

[Claims] (1) After applying 10 to 40 g/m^2 of Zn-based alloy plating to the surface of an ultra-low carbon steel sheet with bake hardenability, and performing chromate treatment of 10 mg/m^2 or more in terms of metallic chromium, then baking. An organic coated steel sheet with excellent bake hardenability, characterized in that an organic film is formed at a temperature of 150°C or less. (2) After applying 10 to 40 g/m^2 of Zn-based alloy plating to the surface of an ultra-low carbon steel sheet with bake hardenability, and performing chromate treatment of 10 mg/m^2 or more in terms of metallic chromium, then baking. An organic coated steel sheet with excellent bake hardenability, characterized in that an organic film containing silica is formed at a temperature of 150°C or less.