JPH03243756A - Externally rust preventive steel sheet for automobile use excellent in low temperature chipping resistance and pitting corrosion resistance - Google Patents

Abstract

PURPOSE:To produce an externally rust preventive steel sheet for automobile use excellent in low temp. chipping resistance and pitting corrosion resistance by successively coating a steel sheet with internal-surface-side and external-surface-side alloying hot dip galvanizing layers in which respective coating weights, Fe contents, gamma -phase thicknesses in the interface between ferrite and plating, and main peaks of X-ray diffraction are specified and an organic film of specific thickness. CONSTITUTION:In a differential coated alloying hot dip galvanized steel sheet for rust preven tive steel sheet for automobile use, two alloying hot dip galvanizing layers where coating weight, Fe content, and gamma -phase in the interface between ferrite and plating are regulated to 20-60 g/m<2>, 7-13%, and <=1.0mu, respectively, on the internal surface side and to 40-100 g/m<2>, 5-11%, and <=1.0mu, respectively, on the external surface side and also the main peak of X-ray diffraction is delta1-phase on the internal surface side and zeta-phase on the external surface side, respectively, are formed on respective surfaces of a steel sheet. Further, an organic film (epoxy resin, etc.) is formed to 0.2-3.0mu thickness at least on the plating layer on the internal surface side. By this method, the steel sheet excellent in low temp. chipping resistance and bare corrosion resistance and useful for externally rust preventive steel sheet for automobile use can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an exterior rust-proof steel plate for automobiles that satisfies low-temperature chipping resistance and pitting corrosion resistance.

Alloyed hot-dip galvanized steel sheet is produced by heating the hot-dip galvanized steel sheet after plating to diffuse the iron in the base steel sheet into the coating layer.
Although it is an iron-zinc alloy, it has superior corrosion resistance compared to galvanized steel sheets, so it is widely used as a material for automobiles, building materials, home appliances, etc.

(Prior Art) In recent years, due to the demand for improved corrosion resistance, there has been a strong demand for thicker alloyed hot-dip galvanized steel sheets.

However, since alloyed hot-dip galvanized steel sheets are manufactured by thermal diffusion treatment, the iron concentration gradient in the coating layer increases as the coating layer thickens, and a brittle r-phase with a high Fe concentration is formed at the interface with the base iron. It becomes easier to do.

If the r-phase is thick, powdering, which causes the plating layer to peel off during press processing, is likely to occur, resulting in scratches caused by plating peeling powder on the product, resulting in negative effects such as a decrease in yield and efficiency due to increased frequency of mold cleaning. Get out. This kind of thick coating amount (45
Alloyed hot-dip galvanized steel sheets with coating weights of g/rd or more are required to satisfy balladulation resistance during the pressing process.

Moreover, such poor powdering properties are the biggest cause of poor low temperature chipping properties. Since this low-temperature chipping property is an important characteristic required for exterior rust-preventing steel sheets for automobiles, it is essential to improve the low-temperature chipping resistance in order to apply it to exterior rust prevention.

On the other hand, alloyed hot-dip galvanized steel sheets have excellent corrosion resistance after painting, but even with thick coatings, bare corrosion resistance is not sufficient, and this occurs because the electrodeposition coating is not sufficiently covered on the hems of car body doors, hoods, etc. There is a problem with pitting corrosion, and there is a strong demand for improvement.

(Problems to be Solved by the Invention) In the conventional manufacturing method of alloyed hot-dip galvanized steel sheets, the effective AQ amount (AQ%-Fe%) is set to 0.0, for example, in a hot-dip zinc bath.
The steel strip is plated by passing it through a bath containing an additive of 9 to 0.15%, and after adjusting the basis weight by gas wiping etc., the strip is passed through an alloying furnace until the metallic luster on the plated surface disappears, that is, until the surface is alloyed. The manufacturing process was carried out by heat-treating until the alloying was completed and immediately cooling to control the degree of alloying (Japanese Patent Application Laid-open No. 1983-
223174). The composition of this plating layer is Fe
: 8 to 13%, Al: 0.25 to 0.35%, and the remainder Zn.

However, when a hot-dip galvanized steel sheet with a basis weight of 45 g/rd or more is alloyed in such a process, the thickness of the r-phase formed at the interface between the base metal is, for example, about 1 to 3 μm, and the powdering resistance is not sufficient.

Therefore, the effective amount of AQ in the bath was reduced to about 0.10% or less, and a thin alloy layer was applied to the Fe-^ formed in the bath.
By making the formation of the Zn alloy phase relatively easy, an alloyed hot-dip galvanized steel sheet can be produced by heat treatment at a lower temperature. The composition of this plating layer is Fe: 6-1
1%, AQ: 0.05 to 0.25%, and the balance is Zn. Even when the basis weight is 45 g/rrr or more, there is a condition that the thickness of the F phase is 1 μm or less. Also,
In order to ensure press workability, the formation of the r phase generated in the lower layer of the plating layer (boundary with the base steel) is suppressed as much as possible, and the upper layer is made of a plating layer consisting of the η phase, δ1 phase, and ζ phase, and the molten alloy It is also disclosed that iron plating is applied on the layer (Japanese Unexamined Patent Application Publication No. 1983-1999)
-228662), both have low Fe% and high sacrificial anticorrosion ability, so their corrosion resistance in a humid atmosphere such as the inner surface of an automobile is poor.

That is, in order to satisfy low-temperature chipping (powdering property), which is important for external rust-preventing steel sheets such as automobiles, lowering the plating Fe% will result in poorer internal corrosion caused by a humid atmosphere. If it is increased, the workability deteriorates, and it is currently difficult to achieve both performances at the same time.

(Means for Solving the Problems) As a result of intensive studies, the present invention is required for such an exterior rust-preventing steel plate for automobiles. We have found an alloyed hot-dip galvanized steel sheet that has satisfactory low-temperature chipping resistance and pitting corrosion resistance.

In order to solve the above problems, the present invention provides alloyed hot-dip galvanized steel sheets for automotive rust-preventing steel sheets of different thicknesses, the inner surface of the steel sheet has a basis weight of Jt 20 to 60 g/rrf, Fe 7 to 13%, and has a base metal-plating interface. The r-phase is less than 1.0μ, the main peak of the X-ray diffraction of the plating layer is 61 phase, and the outer surface has a basis weight of 40 to 100 g/rd and 5 to 11% Fe, and the F phase at the base steel-plating interface. is 1.0 μm or less, and the main peak of the X-ray diffraction of the plating layer is the ζ phase, and an organic film is applied to at least the inner surface of the steel sheet by 0.2 to 3 μm.
This is an exterior rust-preventing steel sheet for automobiles that is coated with a 0μ■ coating and has excellent low-temperature chipping resistance and puncture corrosion resistance.

(Function) The ζ phase has a low Fe% and is inferior in sacrificial corrosion protection ability, and is inferior in internal corrosion in a humid atmosphere, but has good corrosion resistance in an environment where the external surface is relatively dry. In addition, the ζ phase has extensibility and does not easily cause re-powdering, and also has the effect of preventing the propagation of cracks generated in the lower layer, and has good low-temperature chipping.

Taking advantage of the advantages of this ζ phase, it is placed on the outer surface, and in order to increase the sacrificial anticorrosion ability on the inner surface, Fe% is increased and δl is mainly used.However, the naked corrosion resistance, which is still insufficient, is improved by applying an organic film coated on the upper layer. This is a feature of the present invention.

By plating the base plating of the present invention with a difference in thickness, it is possible to easily make the outer surface a ζ phase with a low Fe% and the inner surface a δ1 phase with a high Fe%.
It can be done. The base-plated steel sheet on the outer surface side is, for example, plated in a hot-dip galvanizing bath containing AM: 0.003 to 0.13%, and then heat-treated at a temperature of:
By heating within 15 seconds at 520-470℃, the lower the amount of AQ in the bath, the lower the temperature, ensuring that ζ
Can be manufactured mainly in phases. To confirm that the ζ phase is the main one,
Although there are electrolytic stripping methods and cross-sectional etching methods, they are not preferred because the results obtained vary depending on the conditions. In the present invention, the term ζ-phase mainly refers to the main peak of X-ray diffraction, which is relatively easy to understand and allows the structure of the plating layer to be determined.

Next, the limited range of each component will be explained.

[Outside side]

Fe%: If Fe is less than 5%, the η phase tends to remain on the surface of the plating layer.

When Fe exceeds 11%, the phase exceeds 1 μm, and δ1
I don't like it because it tends to make me the main character.

F phase: It is preferable that the r phase has a thickness of 1 μm or less in order to improve powdering resistance.
If it exceeds 5 g/m, the powdering resistance deteriorates, causing problems in press formability.

Plating deposition amount: The applicable area weight is 30 to 100 g/rrr.

If it is less than 30g/nr, there is a problem in corrosion resistance. 1
If it exceeds 00 g/rf, it is practically difficult to plate the r phase with a thickness of 1 μm or less.

[Inner side]

Fe%: If the Fe content is less than 7%, the plating layer tends to become a ζ phase.

If Fe exceeds 13%, the weight of the r phase tends to exceed 1 μm, which is not preferable.

F phase: It is preferable that the r phase has a thickness of 1 μm or less in order to improve powdering resistance. Especially when it exceeds 1 μL, the basis weight is 4.
If it exceeds 5 g/rd, the powdering resistance will deteriorate and the press formability will be adversely affected.

Organic film: The presence of an organic film dramatically improves the bare corrosion resistance of the inner surface, which has a high Fe% but a low amount of adhesion, and
Adhesion of plated metal to the mold can be suppressed, and press workability can also be improved. The thickness of the organic film is preferably 0.2 to 3.0 μm. If the thickness is less than 0.2 μm, it is difficult to completely cover the lower plating layer, and corrosion resistance may deteriorate from exposed portions of the lower layer, which is not preferable. If it exceeds 3.0 μm, it is undesirable because it tends to deteriorate the electrocoatability, which is an important characteristic required for anticorrosion steel plates for car bodies.The resin for the organic coating should be an epoxy resin that has excellent adhesion to the steel plate. Although it is preferable, the surface condition of the alloyed hot-dip galvanized steel sheet is rich in irregularities, and there is no problem in adhesion with any resin. Moreover, it does not particularly matter whether it is solvent-based or water-based. The presence of various pigments and additives in the organic film is effective as long as the lubricity is not impaired. Further, addition of wax is preferable because it improves lubricity. Furthermore, chemical conversion treatments such as phosphate treatment and chromate treatment are effective as pre-painting treatments to improve the adhesion of the organic film, and these also fall within the scope of the present invention. The method of coating the organic film is not particularly limited, but commonly used methods such as a roll coater method are easy. Also.

This organic coating is required at least on the inner surface, but it is also possible to coat the outer surface.

Plating amount: The applicable area weight is 20 to 60 g/nf.

If it is less than 20g/rd, there is a problem in corrosion resistance. 60g1
If it exceeds rd, it is practically difficult to plate the δl-based material and the r phase with a thickness of 1 μm or less.

Although only Fa was specified as the composition of the alloyed hot-dip galvanized layer, other components such as AQ, Pb, Cd, Sn
, In, LL%Sb, As, Bi, Mg, La
, Ce, Ti, Zr, Ni.

Even if a small amount of C01Cr, Mn, P, S, 0, etc. is added or unavoidably mixed, the effects of the present invention essentially remain unchanged. In particular, regarding AQ, in current processes, around 0.1% is added to the plating bath to control plating and alloying, and it is inevitably mixed into the plating layer. As long as the plating layer is mainly composed of ζ phase or δ1, there is no effect of such standing on the present invention.Furthermore, even in the thermal diffusion alloying material of electrogalvanized material where Ajl does not exist, the plating layer is mainly composed of ζ phase or δ1. If it becomes an alloy phase, the effects of the present invention can be essentially achieved.

(Example) Next, Examples of the present invention are listed in Table 1 along with comparative examples.

The plating material is CG-Al1- and steel (0.8X1
Immediately after plating in a non-oxidation furnace-type continuous hot-dip galvanizing line, continuous heating and alloying treatment was performed in an alloying treatment furnace using 000w x c).

The effective AQ in the plating bath is 0.10%, and the F in the plating layer is
The e concentration was manufactured by appropriately selecting the heating conditions of the alloying furnace.

Plating was carried out under the conditions that the plate passing speed was 40 to 70 m/min and the immersion time was 2 to 5 seconds. In addition, 30 m of coated chromate (chromic anhydride 30 g/Q) was applied on the painting line.
g/rrf, an organic film (solvent type epoxy paint: bisphenol type) was applied to a thickness of 0.1 μm to 5 μm.

Next, the method for testing the workability of the plating layer will be described.

(1) Powdering resistance test Before processing, apply vinyl tape to the bent part, perform bending with the tape side inside (2T bending), redeploy and peel off the tape, and check that the plating layer adheres to the tape. Judgment was made based on the degree of blackening.

(Good) O-0-Δ-× (Poor) (0,0 means no problem in practical use) (2) Flaking resistance test Evaluation was performed by tension molding with square beads. 2.0kgf/cd between punch and die (plug size 0.7
75 x 280 am) and then pull the test piece to pass through the bead. 200 pieces were repeatedly molded, and the degree of deposition of the plating layer metal on the steel plate or bead portion was relatively evaluated.

(Good) 0-0-Δ-x (Poor) (0,0 means no problem in practical use) (3) Actual press test A fender part of an ordinary passenger car was molded using an actual press. 300 sheets were repeatedly molded, and the degree of adhesion and accumulation of plating metal on the steel plate or press mold was evaluated relative to the degree of adhesion. The evaluation was based on the degree of blackening of the metal powder transferred to the tape after attaching tape to each area and removing it.

(Good) O-0-Δ-x (Poor) (0, 0 means no problem in practical use) The test results for each of the above are shown in Table 1 along with comparative examples.

(4) Electrodeposition coating test After standard chemical conversion treatment of Bt3020 (manufactured by Nippon Barker),
PT-U2O5 (manufactured by Nippon Paint) was applied by electrodeposition at 200V for 3 minutes with a 30μ coating, and the appearance after painting was evaluated.

(Good) 0-0-Δ-X (Poor) (0,0 means no practical problem) (5) Low-temperature chipping test - 20℃, 100g of No. 7 crushed stone, pressure 2kg/aJ
After the gravel chipping test, the peeled area when the tape was peeled off was measured, and a score was given according to the following criteria.

(Good) O (5%) - 0 (15%) - Δ (30%) -x
(50%<) (Poor) (0,0 means no practical problem) (6) Bare corrosion resistance test Cycle corrosion test (5% salt water immersion 10 minutes → 5
0℃, RH95% or more humidity 10 minutes → 60℃, RH40
% or less for 10 minutes of drying for 1 cycle) After 1000 cycles, the plate thickness was measured and scored based on the following criteria.

(Good) O(0,1+am)-0(0,2M)-Δ(0
, 4 mm) - Demonstrates excellent performance as an exterior rust-proof steel plate for automobiles.

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Claims (1)

[Claims] In alloyed hot-dip galvanized steel sheets for automobile rust prevention steel sheets of different thicknesses, the inner surface of the steel sheet has a basis weight of 20 to 60 g/m^2, F
e7~13% and the Γ phase at the base metal-plating interface is 1.0
Below μm, the main peak of the X-ray diffraction of the plating layer is the δ1 phase, and the outer surface side is Fe with a basis weight of 40 to 100 g/m^2.
5 to 11%, and the Γ phase at the substrate-plating interface is 1.0μ
m or less, the main peak of the X-ray diffraction of the plating layer is the ζ phase, and at least the inner surface of the steel sheet is coated with an organic film of 0.2 to 3.0 μm,
Automotive exterior rust-proof steel plate with excellent low-temperature chipping resistance and puncture corrosion resistance.