JPS60138052A - Cold rolled steel sheet having superior corrosion resistance after coating - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet having superior corrosion resistance after coating by forming a proper amount of an iron layer on the surface of a cold rolled steel sheet contg. specified amounts of C, Si, Mn, Nb and Ti. CONSTITUTION:A cold rolled steel sheet contg., by weight, 0.08-0.15% C, 0.1- 1.0% Si, 0.5-3.0% Mn and Nb satisfying Nb/C>2 and/or Ti satisfying Ti/C>2 is well cleaned by electrolytic cleaning or other method, and an iron layer is formed on the surface of the steel sheet by 20-1,500mg/m<2> by electroplating, the application of a compound, vapor deposition or other method. The suitability of the steel sheet to phosphating is improved without deteriorating the corrosion resistance, so a cold rolled steel sheet having superior corrosion resistance after coating is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cold-rolled steel sheet with excellent corrosion resistance after painting.

In recent years, automobile manufacturers have been actively promoting the use of cold-rolled steel plates such as various high-strength steel plates and deep-drawn steel plates in order to reduce the weight of car bodies and improve safety due to social demands. At the same time, as the wall thickness of steel plates becomes thinner, the amount of corrosion is reduced, so strengthening the rust prevention ability to prevent pitting has become a major issue.

When developing and manufacturing cold-rolled steel sheets, the steel composition, rolling conditions, heat treatment conditions, etc. are important deciding factors, but these also affect the reactivity of the steel sheet surface, especially the phosphate treatment properties, and the corrosion resistance after painting. Needless to say, it did not have a big impact on the situation.

Factors that do not affect the phosphate treatment properties of cold-rolled steel sheets have been extensively researched, and the relationship between contaminants on the steel sheet surface, oxide film properties, surface saponification elements, etc. is as follows. I understand that.

First, regarding surface contaminants that are unavoidable in the manufacturing process, carbon adhesion on the surface of a copper plate due to annealing carbonization of rolling oil residue or surface concentration of carbon in steel has the greatest influence. This significantly inhibits the formation of a phosphate film and produces a phosphate film that is not dense, which greatly deteriorates the corrosion resistance after painting.

When the surface of a steel sheet is sufficiently clean, the properties of the oxide film on the surface of the steel sheet have a large effect on the rate and distribution of nucleation at the initial stage of phosphate film formation. In general, when the oxide film is poorly soluble, nucleation occurs rate-limiting and sparsely, resulting in the formation of a coarse phosphate film, which often results in poor corrosion resistance after painting. In particular, for high strength, deep drawing, etc., Mn is added to steel.
, Si, C.

If a large amount of Nb, Ti, etc. is actively added, these will be exposed to the surface as a selective oxidation layer concentrated on the surface layer during annealing heating, and will have a great effect on the reactivity of the oxide film. .

In addition, when pickling is performed to prevent these effects, the oxide film layer containing these alloying elements is removed, but instead, hydrated oxides are formed on the surface of the steel sheet, which will This inhibits phosphate processing. Furthermore, recent research has revealed that the surface crystal orientation, which is controlled by rolling reduction, annealing heating conditions, etc., has no small effect on phosphate treatability.

As mentioned above, there are various factors that affect the phosphating properties of cold-rolled steel sheets, and in reality, these factors intertwine and have complex effects, so it is important to develop new materials. However, even if a manufacturing method was established, it was extremely difficult to maintain a stable quality level during actual operation.

In particular, 0,08~0.1' 5 w which is the subject of the present invention
t.

Chi C, O-1~1 wt% Si, sail 5~3 wt,%
In the case of a cold-rolled steel sheet containing at least one of the following components: Nb with Nl)/C>2 and Ti with Ti/C>2, mere improvement of the manufacturing method does not lead to drastic improvement. It was found that no significant improvement in phosphate treatability could be expected.

In order to overcome these problems and more actively control the phosphate treatment properties, the following ideas have been made and proposed.

That is, in general, the procedure for phosphate treatment is to first completely degrease the steel plate and prepare the surface appropriately, and then react the surface of the steel plate with the phosphate treatment solution to form a film. In this case, the density of the phosphate film, which is an essential condition for obtaining good post-painting corrosion resistance, depends on the nucleation rate and distribution of phosphate crystals at the initial stage of film formation, that is, the relationship between the phosphate treatment solution and the steel plate surface. It is largely controlled by the formation rate and distribution of local batteries at the interface of
It is greatly influenced by the surface crystal orientation, etc., and the faster and denser the initial nucleation, the denser the phosphate film will be formed.

Therefore, in order to make this initial nucleation proceed smoothly, titanium-based colloids are adsorbed onto the surface of the steel sheet in the surface conditioning step of the phosphate treatment procedure, and Ni is added to the phosphate treatment solution.

Treatment methods such as adding heavy metal ions such as Co and displacing and precipitating them on the surface of steel sheets have been widely researched and improved.

On the other hand, on the steel plate side, as well, in order to make the local battery generation proceed smoothly,
6887, etc., Ti*Mn, Ni, Co, Cu, etc. are added to the surface of the steel sheet in advance during the manufacturing stage of the steel sheet.
.

A method has been proposed in which a small amount of a transition metal other than iron is deposited by electroplating or the like.

However, these methods have the following drawbacks.

First of all, since the surface reactivity of the steel sheet becomes extremely high due to the metal attached, the period from the time the steel sheet is manufactured until it reaches the user, or until the time when the user performs phosphate treatment after processing. 9, which caused rust in
There is a risk of oil stain occurring when the anti-rust oil blends in. However, if all metals with a large potential difference with Fe are attached, these risks are extremely high.

Next, it goes without saying that the adhesion between the deposited metal and the steel surface must be strong, but the adhesion between the deposited metal and the phosphate film that forms is also extremely important. If a certain metal is attached to the surface of the steel plate, as mentioned above, it is highly reactive, so the surface of the attached metal will be oxidized over time after the steel plate is manufactured, and as a result, The adhesion with the phosphate crystals formed on it deteriorates over time.

Furthermore, if a metal that is significantly more base than iron is deposited, that part will be eluted to the treatment bath interface as an anode during the phosphate treatment reaction. Hopeite Zn5(PO4)z・4H which is not desirable for the purpose
It is also clear that the composition of 2O has a considerable influence on the composition of the phosphate produced and on the components of the treatment solution, as shown in the example of dogfish.

Furthermore, when considering the tendency to corrode the paint after painting, even if a dense phosphate film is produced by such a method, corrosion under the paint film cannot be avoided because the phosphate film is inherently porous. Furthermore, because of the large potential difference between the deposited metal that still exists under the phosphate coating and the base steel, local corrosion between them increases, canceling out the effect of the phosphate coating. However, no significant effect on needle corrosion after painting can be obtained.

In view of the above-mentioned current situation, the present inventors have developed a steel plate that has been surface-treated to improve phosphate treatment properties and exhibit excellent post-painting needle corrosion resistance without impairing the corrosion resistance of the steel plate itself. As a result of extensive research aimed at developing this, we found the following.

In other words, there is a general perception that copper plates are prone to rust because they are made of iron9, but what has a greater influence on the rusting properties and the reactivity of the surface of the steel plates than the reactivity of the iron itself is that
This is a local battery reaction between the added elements in the steel concentrated on the surface of the steel sheet or foreign substances present on the steel surface and the base iron. Iron oxide is originally chemically stable, and iron oxide and copper sheet The potential of the oxide film formed on the surface is approximately the same level.

Therefore, on the surface of a sufficiently clean cold-rolled steel plate, 20 to
It has been found that when 1500 mf/- is deposited, the corrosion resistance of the cold rolled steel sheet itself remains the same as when no iron is deposited, and the phosphate treatability is greatly improved, and the needle corrosion resistance after painting is dramatically improved.

This invention was made based on the above knowledge, and C:
0.08~O,'l 5 wt, %, Si: 0
．． 1-1.0wt, %, Mn: 0.5-3. O
wt, %, Nb: Nb/C>2 and Ti:
It is characterized in that an iron coating layer in an amount of 20 to 1,500 times 27 is formed on the surface of a cold rolled steel sheet containing at least one component of Ti/C>2.

In this invention, the reasons why the phosphating properties of the steel sheet were improved have not yet been completely elucidated, but the iron coating facilitates the formation of local batteries, and the surface conditioning process before phosphating. The reason for this is presumed to be that the adsorption of titanium-based colloids increases.

In addition, the reason why the needle corrosion resistance improved dramatically after painting is that a phosphate film is formed on a pure iron coating layer that does not contain impurities that form local batteries such as added elements in steel. The corrosion reaction under the salt film is rate-limiting, and the constituent phosphate is difficult to dissolve in phosphoferrite (P) during electrodeposition coating.
phosphophyllite) Zn2Fe (PO4
) 2-4H+0 is the main cause.

0.5~. 3. Owt% Mn and Sails 1-1. Ow
A sample of high Mn-Si high-strength steel containing t% QSt was colored due to poor annealing heating conditions, and as a result of strong pickling, the phosphatability was significantly deteriorated.
Figure 1 shows the relationship between the iron coating treatment and the number of phosphate crystal nuclei at the initial stage of phosphate film formation according to this invention. The relationship between the amount of Fe deposited and the corrosion resistance when a salt spray test was performed after electrodeposition using a coating material was
FIG. 3 shows the relationship between the amount of Fe deposited and the amount of peeling of the iron coating layer when a drawbead test was conducted on the steel plate subjected to the iron coating treatment. In Fig. 2, the post-painting puncture resistance was evaluated based on the width of rust formed on the steel plate and the number of blisters.

The iron coating treatment was performed by a known electric iron plating method. The amount of coated iron was determined by performing thick plating by extending the plating time under the same electrolytic conditions as for sample preparation, assuming that the plating layer was pure Fe, and assuming that the plating time and the amount of precipitated Fe were in a proportional relationship. The relationship between the measured plating thickness and plating time was calculated from the plating time at the time of writing the sample lyrics.

As is clear from Figures 1 to 3, in order to increase the number of strands at the initial stage of phosphate film formation and form a dense phosphate film, it is necessary to deposit 20 mftIn1 or more of Fe. In other words, in order to greatly improve the corrosion resistance after painting, it is effective to have a thickness of 100 mη or more. On the other hand, the amount of Fe attached increased to 300 mt/n? When it exceeds 1500 mW/n?, slight peeling of the iron coating layer is observed in the drawbead test, and furthermore, when it exceeds 1500 mW/n? It can be seen that this becomes more noticeable when the value exceeds .

0.08-0.15 wt% C, 0.1-l wt%
Si.

0.5-3 wt% (7) Mn, Nb/C>2N
According to the present invention, a steel plate containing at least one component of Ti with Ti/C>2 and having poor phosphate treatment properties and corrosion resistance after painting is coated by an electroplating method different from the above. As a result of repeating similar tests with iron-coated gold, it was found that although there were slight variations in the number of crystal nuclei, corrosion resistance after painting, and processing removability of the iron-coated layer, the first
~ It was found that almost the same tendency as in Fig. 3 was obtained.

From the above test results, in this invention, the optimum Fe deposition amount is 20 to 1500 +l'/m'', preferably 100 to
300 mff/wt.

The iron coating method is not limited to the electroplating method as described above, but may also be a compound coating method, a vapor deposition method, etc., and similar effects can be obtained. In addition, heating, temper rolling, etc. may be performed after the iron coating is performed, and the effect will not be affected in any way.

It should be noted that a single-sided zinc-based electroplated steel sheet has been proposed, in which at least an iron-zinc-based alloy electroplating layer containing at least 10 wt% of iron or iron is adhered to the non-plated surface of a single-sided zinc-based electroplated steel sheet. Iruga (Unexamined Japanese Patent Publication 1983-1981)
No. 991), the iron or iron-zinc alloy plating on this electroplated steel sheet is intended to prevent the negative effects of trace amounts of zinc caused by the zinc-based electroplating solution adhering to the non-plated surface. After that, iron or iron-zinc alloy plating is performed by simply washing the steel plate with water without actively removing the attached zinc.

In this invention, O,OS~0.15wt% C,
0.1-1 wt% Si, 0.5-3 wt% M
n.

The target is a cold-rolled steel sheet containing at least one component of Nb with Nb/C>2 and Ti with Ti/C)2, and the surface of such a cold-rolled steel sheet is coated with iron. ,
Since the purpose is to improve the surface quality that has deteriorated due to the ingredients of the material, even if the steel sheet surface is coated with iron while impurities are present, its performance cannot be fully exhibited. Therefore, when coating a steel plate with iron according to the present invention,
Immediately before this, it is preferable to clean the surface of the steel plate by pickling or the like.

Next, this fiA will be explained using an example.

Example 1 A steel plate A-F'e having the chemical composition shown in Table 1 was cold rolled and then heated in a nitrogen gas atmosphere containing 5% hydrogen for 20 hours, further cooled to room temperature in the same atmosphere, and then Iron was coated according to the present invention, and its phosphate treatment properties and post-painting corrosion resistance were investigated.

In addition, in Table 1, steel plates A to E are steel plates that are subject to this invention, steel plate F is a steel plate that is not subject to this invention in terms of composition, and steel plate F is in a state where no iron treatment is applied. Now, steel plate A
-E also has excellent phosphate treatment properties and post-painting corrosion resistance. The iron coating treatment is done by electroplating. After pickling for 3 seconds at room temperature in a 100 y/l H2SO bath as a pretreatment, after washing with water, the iron is plated by applying electricity for 3 seconds under the electrolytic conditions shown below. I did this.

Bath composition: ferrous sulfate 300 t/l, sodium sulfate 30 fl/l citric acid 5 l/l pH: 2.2 Bath temperature: 35 ± 2°C Current density: 3 A/dm" Phosphate treatment property is Bt' manufactured by Nippon Parkerizing Co., Ltd.
Using the 137 treatment solution, the initial number of phosphate crystals after 5 seconds of treatment and the crystal size of the completed phosphate film after 2 minutes of treatment were examined using a travel electron microscope.

Corrosion resistance after painting is Elekron N170 manufactured by Kansai Paint Co., Ltd.
After coating with 20 μm of 00 electrodeposition paint, the rust width and number of blisters at the cross-cut portion were evaluated after 500 hours of salt water spray test.

Table 2 shows the test results along with comparative examples. Here, in Comparative Examples 1 to 6, steel plates A to F were subjected to phosphate treatment without iron coating, and in Comparative Example 7, steel plates F
This is a case where iron coating treatment and phosphate treatment, etc. are performed on the steel.

As is clear from Table 2, in Invention Examples 1 to 5, the initial number of coats during coating formation increased in the phosphate treatment, regardless of the chemical components and chemical component contents of the steel sheets. It can also be seen that the crystal size has become smaller and the finished film has become denser, and that the corrosion resistance after painting has improved significantly.

Example 2 Si-Mn-based high-strength steel plate (C: 0.116 wt%
IV+ln: 0.40 wt% Si: 0.91
wt%) and further electrolytically cleaned, they were subjected to iron coating treatment in the Senzo method shown in Table 3, and their phosphate treatability and coating were evaluated in the same manner as in Example 1. Post-corrosion resistance was investigated. In Table 3, Comparative Examples 1 to 5 are cases in which the above steel sheets were cold rolled, electrolytically cleaned, and then subjected to phosphate treatment, etc. without iron coating treatment, or by box annealing, etc. It is.

The iron coating treatment was carried out by electroplating, and the electrolytic conditions used were different from those in Example 1 as described below.

Bath composition: 350 f/l of ferrous ammonium sulfate to 120 f/l of ammonium sulfate pH: 3.0 Bath temperature: 30±2°C Current density: 2'OA/d Current application time = 0.5 seconds Evidently from Table 3 As shown, in Examples 1 to 7 of the present invention, even after undergoing steps such as annealing, pickling, and pressure adjustment before and after the iron coating treatment,
The effects of improving phosphate treatment properties and post-painting resistance remain unchanged, and both exhibit good performance.

[Brief explanation of drawings]

Fig. 1 is a graph showing the relationship between the Fe adhesion amount and the initial number of crystal nuclei in phosphate treatment, Fig. 2 is a graph showing the relationship between the Fe adhesion amount and post-painting corrosion resistance, and Fig. 3 is the graph showing the relationship between the Fe adhesion amount and the number of initial crystal nuclei in phosphate treatment. It is a graph showing the relationship between the amount of adhesion and the amount of processed peeling of the iron coating layer. Applicant Nippon Kokan Co., Ltd. Agent Natsuo Shioya (and 2 others)

Claims (1)

[Claims] c: o, o s ~ 0.15 wt, %, Si:
0.1~1, Owt. %, Ivin: 0.5-3. Owt, %, N
b: Nb/C>2 and Ti: Ti/C>2
on the surface of a cold rolled steel sheet containing at least one component of 20 to 15 oomg/rr? A cold-rolled steel sheet with excellent corrosion resistance after painting, characterized by having an iron coating layer formed therein.