JPH0520512B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Field of Application> The present invention relates to a method for producing a highly corrosion-resistant organic composite plated steel sheet that is mainly press-formed and used for automobile bodies. <Prior Art> In recent years, high corrosion resistance has been required for automobile bodies, and surface-treated steel sheets in which conventional cold-rolled steel sheets are plated with Zn or Zn-based alloys are being used.
Furthermore, in regions with severe corrosive environments, bare corrosion resistance in terms of hardness is required for inner plate bag structures and bent parts where coating is not sufficiently applied. As disclosed in Publication No. 58-224174, etc.
Organic composite plated steel sheets have been developed, which are Zn-based plated steel sheets coated with chromate and organic coatings. On the other hand, steel sheets for automobiles are required to have low yield strength, no yield elongation, and excellent drawability before being press-formed into a car body. However, in the above-mentioned method, no consideration is given to the materials required for these automobile bodies, and during heat treatment during steel sheet manufacturing, solid solution C in the steel sticks to movable dislocations and hardens the steel sheet. There was a problem that moldability was impaired. As a method for achieving both corrosion resistance and material quality of organic composite plated steel sheets, Japanese Patent Application Laid-open No. 58-45322,
It is disclosed in 58-96821 and 58-96822. JP-A No. 58-45322 provides the same quality of material as before baking by subjecting it to temper rolling after baking the paint, but the organic coating layer is damaged by temper rolling after organic coating. The drawback was that corrosion resistance was significantly impaired. In JP-A-58-96821, recrystallization annealing is performed after cold rolling, and then
The purpose of this method is to precipitate carbides by performing an over-aging treatment at 300 to 500℃ and adjust the amount of solute C to obtain the required material quality without skin-pass rolling after organic coating. In order to carry out overaging treatment after annealing, it is necessary to install special heating equipment in the line, and this cannot be carried out in an annealing line that does not have such equipment. Furthermore, Japanese Patent Application Laid-Open No. 58-96822 attempts to secure the required material quality without skin-pass rolling after organic coating, taking into account the heat treatment conditions during coating for box-annealed steel sheets. However, since box annealing requires a long time for annealing, there is a problem in that productivity is significantly reduced and costs are increased. <Problems to be Solved by the Invention> The present invention was made to solve the above-mentioned problems, and it maintains excellent press formability even after being coated with an organic coating, and has good post-coating water resistance and secondary adhesion and resistance. The present invention provides a method for producing a highly corrosion-resistant organic composite plated steel sheet having chromium leaching properties. <Means for solving the problem> The present invention contains C: 0.008wt% or less and Nb:
Cwt%×2~(Cwt%×8+0.02wt%) and/
Or ultra-low carbon cold rolled steel sheet containing Ti: 0.05wt% or less, or B: 0.0030wt% in addition to these components
An ultra-low carbon cold-rolled steel sheet containing the following is subjected to continuous annealing, then zinc-based alloy plating is applied, a chromate layer is applied on the top layer, baked at 60-130℃, and then 10-60wt of silica sol is applied per total weight. % of organic film is coated as a solid content of 0.3 to 2.0 g/ ^m2 ,
Temper rolling is performed at a reduction rate of (t+0.5) to 3%, where the thickness of the steel plate is tmm, after baking at a temperature above 150℃ and below 200℃, and after continuous annealing and coating with an organic film. This is a method for producing an organic composite plated steel sheet having excellent press formability and adhesion after coating. <Function> The reasons for limiting the components and manufacturing conditions in the present invention will be explained in detail below. If C exceeds 0.008 wt%, the yield strength will be high and the ductility and r value will be reduced. Therefore, it is limited to 0.008wt% or less. If Nb is less than (C x 2) wt%, a large amount of solid solution C remains, and a texture with excellent drawability does not develop during recrystallization annealing after cold rolling. On the other hand, (C×8+0.02)wt
If it exceeds %, the ductility of the steel plate will be impaired. Therefore, the Nb content is limited to Cwt%×2 to (Cwt%×8+0.02wt%). Here, C represents the content. Furthermore, Ti is added to obtain good ductility. However, if it exceeds 0.05wt%, ductility is lost, so it is limited to 0.05wt% or less. Further, in order to obtain good ductility, Nb and Ti may be added in combination. More preferably, B is added. B is added to prevent secondary processing brittleness. However, if it exceeds 0.0030wt%, the r value will drop rapidly and press formability will deteriorate, so it is limited to 0.0030wt% or less. The content of impurities and other elements is not particularly limited, but is preferably as small as possible. However, it is particularly preferable to limit the following elements to the following ranges. Si is preferably 0.5 wt% or less. This is because if it exceeds 0.5 wt%, oxides will be generated on the surface of the steel sheet during annealing, impairing phosphating properties. Mn is preferably 0.05 to 1.20 wt%. If it is less than 0.05wt%, brittleness deteriorates, and if it exceeds 1.20wt%, the r value deteriorates. P is preferably 0.1 wt% or less. This is because if it exceeds 0.1wt%, the steel plate will become brittle. Al is preferably 0.01 to 0.08 wt% and at least (N×8) wt%. Al is 0.01wt because N is fixed
% or more is necessary, but if it exceeds 0.08 wt%, inclusions will occur frequently, which is not preferable. In addition, in order to fix N, more than (N×8)wt% is required. Like C, N impairs workability, so it is desirable that its content in steel be as low as possible. Next, continuous annealing is performed to recrystallize the structure of the cold rolled steel sheet, but in the present invention, the C content in the steel is 0.001wt.
% to less than 0.008wt%, so almost no cementite precipitation is observed during annealing, so special heat treatment such as overaging treatment is not required. It is necessary to perform temper rolling at least once between continuous annealing and coating with the organic film. This is to correct the shape of the steel plate and introduce appropriate movable dislocations into the steel plate to suppress the occurrence of yield elongation and prevent stretcher strain during press forming. When the thickness of the steel plate is tmm, if the rolling reduction is less than (t + 0.5)%, the effect will not be sufficient because the density of mobile dislocations introduced into the steel plate will be small, and if the rolling reduction is more than 3.0%, the effect will be insufficient due to work hardening. Significant deterioration of the material. Therefore, the range of rolling reduction is (t+0.5) to 3.0%.
Further, this temper rolling may be performed once or more than once in the step after continuous annealing and before organic coating. Zn-based plating applied to steel sheets is Zn, which has traditionally been used to improve corrosion resistance.
-Ni alloy plating (Ni content 8-16wt%), Zn-
Fe alloy plating (Fe content: 5 to 30 wt%) can be used, and is mainly performed by electroplating. These platings are applied to impart corrosion resistance to the steel plate, and the coating weight is preferably 10 to 40 g/m ² . This is because if the basis weight is less than 10 g/m ² , corrosion resistance is insufficient, and if it exceeds 40 g/m ² , there is no further significant improvement in corrosion resistance and it is not economical. On top of these Zn-based alloy plating layers, chromate treatment is applied, firstly to impart high corrosion resistance, and secondly to fix the upper layer film and give it adhesion by bonding it with the organic film described below. I do. The main component of the chromate treatment solution is a water-soluble chromium compound, in addition to which appropriate amounts of phosphate radicals, anions such as fluorine ions, metal ions such as Zn, Ni, Co, etc., and organic substances such as starch and methanol are required. Add accordingly. Furthermore, by adding silica sol, corrosion resistance is further improved. The amount of chromate deposited is preferably 10 to 150 mg/m ² in terms of chromium. If the adhesion amount is less than 10 mg/m ² , the adhesion with the organic film layer will not be sufficient, and if it exceeds 150 mg/m ² , the amount of soluble components in the chromate layer will increase, resulting in poor chromium elution during degreasing. This is because it deteriorates. Baking after applying the chromate layer is done at 60-130℃.
This is because the chromate film does not dry sufficiently at temperatures below 60°C, resulting in chromium elution.
This is because at temperatures exceeding 0.degree. C., the chromium hydrate in the chromate layer bonded to the organic film layer dehydrates, and the adhesion between the organic film layer and the chromate layer is rapidly lost. Next, a composite organic film containing 10 to 60 wt% of silica sol based on the total amount is applied to the upper layer of the chromate film at a solid content of 0.3 to 2.0 g/m ² and baked at a temperature of 150 to 200°C to form a composite organic film layer. . Here, the organic resin in the composite organic film is as follows:
For example, acrylic resin, epoxy resin, polyethylene resin, alkyd resin, polyester resin,
It is a resin such as urethane resin, and these resins are used alone or in combination. Silica sol is added to improve the corrosion resistance of an organic film, and its forms include water-dispersed colloidal silica sol, organic solvent-substituted colloidal silica sol, and fumed silica, all of which have a primary particle size of several to several tens of nanometers. In some cases, it is used by dispersing it in water or an organic solvent together with the above-mentioned organic resin. The silica sol content is 10 to 60 wt%. 10wt
This is because corrosion resistance is insufficient if it is less than %.
This is because if it exceeds 60 wt%, the viscosity of the paint increases and it becomes more likely to gel. The amount of organic composite film deposited is 0.3 to 2.0g/
Let it be ^m2 . If the coating weight is less than 0.3 g/m ² , corrosion resistance will be poor, and if it exceeds 2.0 g/m ² , the coating resistance will increase, resulting in poor spot weldability and electrodeposition coating properties. The organic composite film is baked at temperatures above 150℃ and below 200℃. The reasons for limiting the baking temperature will be explained below. As mentioned above, the adhesion between the organic composite film and the chromate layer is provided by the bond between the hydroxyl groups in the resin in the organic composite film and the chromium hydrated oxide in the chromate layer. On the other hand, when silica sol is present in the organic composite film, the hydroxyl groups in the resin react with the silanol groups of the silica sol, so the number of hydroxyl groups in the organic film that should react with the chromate layer decreases, and the adhesion deteriorates. Therefore, in order to provide good adhesion between the chromate layer and the organic composite film even if silica sol is present in the resin, it is necessary to increase the baking temperature after coating the paint on the chromate layer. The inventors of the present invention discovered this as a result of intensive research. If the baking temperature of the organic composite film is 150°C or lower, the chromate layer and the organic composite film will not bond sufficiently, resulting in poor secondary waterproof adhesion after painting, so the baking temperature must be higher than 150°C. On the other hand, when the baking temperature exceeds 200℃, solid solution C diffuses and fixes to the movable dislocations introduced by temper rolling before painting, resulting in yield elongation of the steel plate and stretcher strain during press working. occurs. Therefore, the upper limit of the organic composite film baking temperature is determined from the viewpoint of press formability.
The temperature shall be 200℃. <Example> Hereinafter, the effects of the present invention will be specifically explained based on Examples. Steel with the chemical composition shown in Table 1 was cold-rolled to a thickness of 0.7 mm, then continuously annealed at 830°C for 1 minute. An amount of 20.6 g/m ² ) was applied, and a coating type chromate treatment was performed using a roll coater. After baking, an organic composite film was applied using a roll coater and baked. The manufacturing conditions for each are shown in Table 2. The coating type chromate treatment liquid has Cr ³⁺ /Cr ⁶⁺ =
50/50, solid content 100g/, PH2.5. The composite organic silicate treatment liquid is an organic solvent-based paint with a colloidal silica sol:epoxy resin ratio of 60/40 and a solid content of 15%. Table 3 shows the results of the corrosion resistance, water resistant secondary adhesion, chromium elution resistance, and material properties of each steel plate. The details of each performance evaluation method are as follows. ◎ Corrosion resistance Salt spray 4 hours 5% NaCl solution 35°C Dry 2 hours 60°C Wet 2 hours 95% RH A cycle corrosion test was conducted with 50°C as one cycle, and the evaluation was based on the number of cycles at which red rust occurs. ◎ Water resistant secondary adhesion after painting After the sample was treated with phosphate (PB L3020, manufactured by Nippon Parker Co., Ltd.), cationic electrodeposition painting (Power Top
U100, manufactured by Nippon Paint Co., Ltd.) applied to 20μm, 170℃
x 20 minutes of baking, then topcoat on top (Olga
G65 B326, manufactured by Nippon Paint Co., Ltd.) 35μm coated, 140
After baking for 30 minutes at ℃, immersion in pure warm water at 40℃ for 10 days, peeling off a 2 mm 10 x 10 grid tape using a cutter knife, and measuring the coating film residual rate to evaluate secondary water adhesion. . ◎ Chromium elution resistance Phosphate treatment solution (PB L3020, Nippon Parker Co., Ltd.)
The Cr coating was carried out through 4 steps of degreasing, water washing, and chemical formation using a commercially available commercially available product (manufactured by J.D. Co., Ltd.), and the amount of Cr deposited before and after the treatment was measured by fluorescent X-ray analysis to calculate the elution amount. As shown in Table 3, organic composite plated steel sheets (No. 3, 4, 7, 8,
9, 13, 14 and 16) all have higher water resistance secondary adhesion and Cr elution resistance than the comparative example steel plates (Nos. 1, 2, 5, 6, 10, 11, 12 and 15). , products with excellent yield elongation have been obtained.

【table】

[Table] * Examples of the present invention

【table】

[Table] * Examples of the present invention <Effects of the invention> According to the production method of the present invention, a highly corrosion-resistant organic composite plated steel sheet with excellent press formability and good post-coating water-resistant secondary adhesion and chromium elution resistance can be obtained. It will be done.

Claims (1)

[Claims] 1 C: Contains 0.008wt% or less, Nb: Cwt% x 2
~(Cwt%×8+0.02wt%) and/or Ti:
Ultra-low carbon cold-rolled steel sheets containing 0.05wt% or less are subjected to continuous annealing and then plated with zinc-based alloy.A chromate layer is applied on top of this, baked at 60-130℃, and then silica sol is applied per total weight. 10~60wt
% containing organic film as solid content 0.3-2.0g/
When the thickness of ^the steel plate is tmm, the rolling reduction rate is between (t+0.5) and 3%. A method for producing an organic composite plated steel sheet having excellent press formability and adhesion after coating, which comprises performing temper rolling. 2 Contains C: 0.008wt% or less, Nb: Cwt% x 2
~(Cwt%×8+0.02wt%) and/or Ti:
Ultra-low carbon cold-rolled steel sheets containing 0.05wt% or less and B: 0.0030wt% or less are subjected to continuous annealing, then zinc-based alloy plating is applied, a chromate layer is applied on the top layer, and the plate is heated at 60 to 130℃. Baked, then coated with an organic film containing 10 to 60 wt% of silica sol based on the total weight, with a solid content of 0.3 to 2.0 g/ ^m2 , and heated to temperatures above 150℃.
It is characterized by performing temper rolling at a reduction rate of (t+0.5) to 3%, where the thickness of the steel plate is tmm, during baking at 200℃ or less and continuous annealing, and then coating with an organic film. A method for producing an organic composite plated steel sheet with excellent press formability and adhesion after coating.