JPS63143270A - Highly corrosion resistant organic coated steel sheet having excellent baking hardenability - Google Patents

Abstract

PURPOSE:To develop a highly corrosion resistant org. coated steel sheet having excellent baking hardenability by forming a specifically composed Zn-Ni composite plating layer, Zn-Ni alloy plating layer, chromate treatment layer and org. resin film on the surface of an extremely low carbon steel sheet. CONSTITUTION:The Zn-Co composite plating layer contg. 0.1-6% Co and further 0.01-3% at least one kind among chromium, alumina, silica and titania is formed at >=3g/m<2> deposition on the surface of the extremely low carbon steel sheet contg. 0.001-0.008% C. The Zn-Ni alloy plating layer contg. 3-15% Ni is formed at 3-30g/m<2> deposition thereon and further, the chromate treatment layer is formed to >=10mg/m<2> in terms of metal chromium by a chromate treatment thereon. Finally, the org. resin film consisting of acryl, PE, epoxy or others is baked at <=150 deg.C. The corrosion resistance is additionally improved if <=60% silica sol is added into the org. resin film in this case.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Field of Application> The present invention relates to a highly corrosion-resistant organic coated steel sheet for drawing that has bake hardenability and is used 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 applied after 2% prestrain and subsequent 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 have been surface-treated by hot-dip Zn plating, hot-dip Zn alloy plating, electric 2N plating, electric Zn alloy plating, organic film-based zinc-rich coating, and the like.

Furthermore, in addition to these, composite coated steel sheets have also been developed in which organic coatings are applied to plated steel sheets. This composite type coated copper plate is currently known as an excellent 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 steel sheets plated with 7, n or Zn alloys that have bake hardenability to meet these requirements, but in conventional organic coated steel sheets, the organic coating is a hardened film, so these The baking temperature of the organic coating on the brushed steel sheet is over 150° C., which causes the organic coated steel sheet to harden before press forming, making it unsuitable for drawing purposes.

In particular, conventional automotive organic coated steel sheets (or painted copper 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.

<Objective of the Invention> The present invention has been made in view of the above-mentioned points, and an object thereof 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 drawing steel plate made of ultra-low carbon steel and having bake hardenability, the first layer of which is coated with Zn-Co composite plating, and the second layer of which is coated with Zn-Ni alloy. Later, organic skin II was applied to the chromate-treated surface-treated copper plate! 2, the baking temperature of the organic film is 15
By making the temperature below 0°C 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 react with the steel sheet with chromate containing lead powder, 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 150°C.

Also, even when using a steel plate with a grain size of 7, for example, JP-A-57
In a zinc-rich organic composite steel sheet such as No. 189842, the organic film must be baked at a high temperature as in the case of the cold-rolled steel sheet.

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.

That is, 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.

The present inventors have made the following efforts to take advantage of the characteristics of bake-hardened steel sheets as described above and to provide them with higher corrosion resistance than before.
We investigated organic coatings and base plating treatments that have sufficient corrosion resistance when baked at temperatures below 50°C, and applied 150°C to a bake-hardenable cold-rolled steel sheet made of ultra-low carbon steel without impairing the bake-hardenability. An organic anti-rust coating is applied by heat treatment below ℃, and ten pre-plating treatments + This paper proposes an optimal range of comprehensive coverage consisting of organic films.

According to the first aspect of the present invention, on at least one side of an ultra-low carbon steel plate having bake hardenability, in order from the surface side of the steel plate,
The first layer has a plating weight of 3 g/m2 or more and a Co content of 0.
1 to 6 wt%, and 0.0% of at least one selected from the group consisting of chromium, alumina, silica, and titania.
Zn-C containing 1 to 3 wt%.

, V-composite plating, second layer plating weight is 3-30g
/m2, Zn-N with a Ni content of 3 to 15 wt%
i, alloy plating, third layer is 11011 in terms of metallic chromium
I/IT+! A highly corrosion-resistant organic coated steel sheet with excellent bake hardenability is provided, which is characterized in that the above chromate treatment layer and the fourth layer have an organic film formed at a baking temperature of 150° C. or lower.

Further, according to the second aspect of the present invention, on at least one side of the ultra-low carbon steel sheet having bake hardenability, the first layer is coated in order from the surface side of the steel sheet with a plating R amount of 3 g/ll + 2 or more and a Co content of 0. Zn-Co composite plating containing 0.01 to 3 wt% of at least one selected from the group consisting of chromium, alumina, silica, and titania, with a second layer plating weight of 3 to 6 wt%. 3037m2, Ni
Zn-Ni alloy plating with a content of 3 to 157 L, the third layer is a chromate treatment layer of 10 B/m2 or more in terms of metal chromium, and the fourth layer is formed at a baking temperature of 150°C or less and contains silica. A highly corrosion-resistant organic coated steel sheet with excellent bake hardenability characterized by having an organic coating is provided.

The present invention will be explained in detail below.

Organic film (4th layer) to maintain bake hardenability of steel plate
The reason why the baking temperature was limited to 150° C. or lower is as follows.

C; 0.003wt%, Si; 0.0
1wt%, Mn; 0. l6wt%, An;
Q, 04 wt%, P, 0.070 gastric t%,
Nb; 0.026wt%, Nb/C; 6.5
, remainder: a copper plate consisting essentially of Fe at a cold reduction rate of 80
%, the plate thickness is 0.7 mm, and soaked at 850℃ in a continuous annealing furnace.
× 30 seconds heat treatment, then 45℃/sec to 650℃
The mechanical properties of the ultra-low carbon bake-hardenable steel sheet, which was cooled at
0 gf/ml, TS (tensile strength); 35 Kgf
/wf, Eu (elongation): 45%, B) + (bake hardenability); gf/- in 5.

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

As a bake hardenable cold rolled steel sheet, C; 0.001 to 0.
008 wt%, Si; 0.5 wt% or less, Mn
(L, 05~I, 2wt%, P;
0.1wt% or less, 8fl; 0.01~
0.08wt% and Nwt%×8 or more, Nb; Cwt
%x3 or more and Cat%x8 + 0.02wt% or less, and the steel sheet further contains Ti at 0.05wt% or less, and the OH amount is controlled to 3 to 6 gf/- by continuous annealing. Steel plate is suitable. By using ultra-low carbon steel as the material, we were able to obtain properties that maintain BH properties without hardening at low temperatures of 150°C or lower.

The reason for this is not necessarily certain, but the distribution of solid solute C in the grains is different between the BH steel sheet made of low carbon steel and the old 1 steel sheet made of ultra-low carbon steel, even if the apparent ottH is the same. It is expected that.

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 at%, 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. [), if it exceeds 5 wt%, an oxide film will be formed and the chemical conversion properties will be impaired.

Mn is preferably 0.05 to 1.2 wt%). 0.0
If it is less than 5tvt%, red brittleness deteriorates and 1.2wj%
This is because if it exceeds, the lower price will deteriorate.

P is preferably 0-1 wL% or less. This is because if it exceeds 0.1 wt%, the steel plate becomes brittle.

Aλ is 0.01 to 0.08 wt% Nwt%X8
The above is preferable. l is 0.01wt% to fix N
The above amount is necessary, but if it exceeds 0.08 wt%, inclusions will occur frequently, which is not preferable. Also, to fix N, Nw
L%×8 or more is required.

Nb is Cwt%×3 or more, Cwt%X 8 + 0.0
It is preferably 2 wt% or less. If CWL%x3 is less than solid solution C
remains in large quantities, and a texture with excellent drawability cannot be formed during cold rolling recrystallization. On the other hand, Cwt%x 8 + 0.02t
If it exceeds vt%, the ductility of the steel plate will decrease.

Ti is preferably 0.05wt% or less. 0.05wt
%, BH properties are lost.

Further, it is preferable to set the amount of B11 to 3 to 6 gf/-. If it is less than 3 Kgf77, BH is practically insufficient,
This is because if it exceeds 6 Kgf/-, aging deterioration is severe and stretcher strain is likely to occur during processing.

As the Zn-based alloy plating applied to the ultra-low carbon steel sheet, it is preferable to form a Zn-Co 4 composite plating on the first layer and a Zn-Ni alloy plating on the second layer in order from the steel sheet surface side. . The purpose of forming the Zn-Co composite plating on the first layer is to improve the adhesion of the plating to the base steel, and the purpose of forming the ZrrNi alloy plating on the second layer is to improve the adhesion of the plating to the base steel. This is to uniformly form the chromate treatment layer.

Co content rate of first layer Zn-Co composite plating 0.1
~6 wt% is preferred. This is because if the content is less than 0.1 wt%, no effect on corrosion resistance will be observed, and if it exceeds 6 wt%, the plating adhesion will decrease.

This 7. Corrosion resistance can be further improved by further containing 0.01 to 3 wt% of at least one selected from the group consisting of chromium, alumina, silica, and titania to the n-Co composite plating. If it is less than 0.01 wt%, no effect of improving corrosion resistance will be observed, while if it exceeds 3 wt%, plating adhesion will be deteriorated, so the above range is preferable.

The adhesion amount of Zn-C:O-based composite plating is 3g/+
It is preferable that n2 or more. The reason for this is that if it is less than 3 g/m2, no corrosion resistance improvement effect is observed. There is no upper limit to the amount of plating deposited, but from the viewpoint of cost, it is desirable that the amount be 60 g/m2 or less.

Ni content of second layer Zn-Ni alloy plating 3 to 15
wt% is preferred. This is because if it is less than 3 wt%, no effect of improving corrosion resistance will be observed, and if it exceeds 15 wt%, the plating layer will become hard and the plating adhesion will decrease. 7. n-N
The amount of deposited i-alloy plating is preferably 3 to 30 g/m2. This is because if it is less than 3 g/m2, no effect of improving corrosion resistance will be observed, while if it exceeds 30 g/m2, weldability and workability will deteriorate, and the cost will also increase, which is undesirable.

On top of the Zn-based alloy plating (layer 1.2), a third layer is added to improve the adhesion of the mechanical coating (fourth layer), which will be described later, and further improve its corrosion resistance. Chromate treatment is performed.This chromate treatment requires a coating amount of 10 mg/rn' or more as metallic chromium, and for this purpose, coating type 5 chromate or electrolytic chromate is effective for controlling the coating amount.

If it is less than 10 rng/rrf, not only the corrosion resistance is insufficient but also the adhesion with the organic film is poor.

As a fourth layer, an organic film is baked on the chromate-treated film at a baking temperature of 150°C or less in order to maintain the baking effect of the steel sheet and to improve rust prevention. As the organic film-forming resin, one of the following resins (1) to (2) is used, either alone or in combination.

■Permanently 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 in the following low-temperature treatment, examples include a method of using a metal salt catalyst such as Co naphthenate as a low-temperature curing agent, and a method of using an amine-based curing agent such as diethylenetriamine. It will be done. 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. Existing 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. The thickness of the film may be changed as appropriate depending on the application.

Note that the corrosion resistance is further improved by adding up to 60 wt % of perilla sol into 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]

Table 1 shows the composition, heat treatment conditions, and mechanical properties of the bake-hardenable cold-rolled steel sheets used.

Using the bake-hardenable steel plates (Materials Nos. 001 to 6) shown in Table 1 as raw materials, Nos. 011 to 61 shown in Table 2 were subjected to chromate treatment and organic coating treatment, and these were used as test steel sheets. The treatment steps are also 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 A cross cut was made on the organic film formed on each test steel plate, and 5% NaCJl was sprayed onto the film at 35° C., and the time until red rust appeared was measured.

(1-b) (:CT) Each test steel plate was sprayed with 5% NaCl at 35℃ for 4 hours under the following conditions ↓ Dry at 60℃ for 2 hours ↓ Leave in 50℃ and 95% RH (fflfa) for 2 hours 1
A cyclic corrosion test was conducted as a cycle, and the number of cycles until red rust appeared was counted.

(2) Weldable electrode R type (40R), pressure 170g, welding time 1
Two of each test steel plate was stacked alternately under 0 cycle conditions, and the number of continuous welding points was examined.

(3) Workability Each test steel plate (90mmφ) has a diameter of 50mmφ and a depth of 25m.
Cylindrical drawing processing of m (BHF I Lor) L/, the processed part was peeled off with cellophane tape, and the amount of film peeled off (mg/circumference) was measured.

(4) Bake hardenability (B11 value) After each test steel plate was pre-strained by 2% and subjected to baking heat treatment at 170°C for 20 minutes, the increase in ys (yield strength) (kgf/
wd) was measured.

As shown in Table 3, by the treatment method of the present invention, a highly corrosion-resistant, bake-hardenable, punch-coated steel sheet that has not only high corrosion resistance, weldability, workability, but also bake hardenability can be obtained.

Table 3 (Part 1) Table 3 (Part 2) <Effects of the invention> As detailed below, according to the present invention, Zn-Co is used in the lower layer.
Composite plating: 5 layers are coated with Zn-Ni alloy, and the 7th layer is chromate treated and then subjected to percussion film treatment at a baking temperature of 150℃ or below, resulting in high corrosion resistance, weldability, and loadability. It is possible to provide an organic coated steel sheet that also has hardenability and has excellent bake hardenability.

[Brief explanation of the drawing]

Figure 1 shows the baking temperature of the organic film and the baking hardenability of the steel sheet (B1
1 value). [Stocks] 3 Mi J-Sat, ″; r 1 box also FIG, 1

Claims (2)

[Claims] (1) On at least one side of an ultra-low carbon steel plate with bake hardenability, in order from the surface side of the steel plate, a first layer is coated with a coating weight of 3 g/m^2 or more and a Co content of 0.
．． 1 to 6 wt%, and at least one selected from the group consisting of chromium, alumina, silica, and titania.
Zn-Co composite plating containing 01 to 3 wt%, second layer Zn-Ni alloy plating with a coating weight of 3 to 30 g/m^2 and Ni content of 3 to 15 wt%, and third layer metal chromium. A highly corrosion-resistant organic coated steel sheet with excellent bake hardenability, characterized by having a chromate treatment layer of 10 mg/m^2 or more in terms of conversion, and an organic film in which the fourth layer is formed at a baking temperature of 150° C. or less. (2) On at least one side of an ultra-low carbon steel plate with bake hardenability, the first layer is coated in order from the surface side of the steel plate with a coating weight of 3 g/m^2 or more and a Co content of 0.
．． 1 to 6 wt%, and at least one selected from the group consisting of chromium, alumina, silica, and titania.
Zn-Co composite plating containing 01 to 3 wt%, second layer Zn-Ni alloy plating with a coating weight of 3 to 30 g/m^2 and Ni content of 3 to 15 wt%, and third layer metal chromium. A highly corrosion-resistant organic coated steel sheet with excellent bake hardenability, characterized by having a chromate treatment layer of 10 mg/m^2 or more in terms of conversion, and an organic film containing silica in which the fourth layer is formed at a baking temperature of 150°C or less. .