JPH08333669A - Galvannealed steel plate - Google Patents

Abstract

PURPOSE: To produce a galvannealed steel plate improved in the adhesion of a plating film using a steel plate contg. P as a base metal. CONSTITUTION: The surface of a steel plate contg., by weight, 0.02 to O.20% P is applied with a galvannealing film contg. 0.2 to 0.5% Al, and the roughness R2 of the steel surface after the removal of the plating film satisfies 12>=R2 >=0.0075.Sm +6.7 (where R2 : ten point average roughness and Sm : the average interval of ruggedness). Thus, the galvannealed steel plate using an inexpensive high P high tensile strength steel plate can be made the most of.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet which is excellent in adhesion between a coating film and a base steel sheet and is particularly suitable as a coated steel sheet for home appliances and a steel sheet for automobiles.

[0002]

2. Description of the Related Art In recent years, a large amount of zinc-based plated steel sheets have been used in the industrial fields of home appliances, building materials, and automobiles. Especially, zinc-plated steel sheets have excellent corrosion resistance, weldability, performance after painting, and economical efficiency. Excellent galvannealed steel sheets are widely used.

The alloyed hot-dip galvanized steel sheet is improved in its paint adhesion when a small amount of Fe is alloyed in the Zn coating film, so that the corrosion potential has a noble and moderate sacrificial anticorrosive action as compared with Zn. It is what is used. In particular, it is widely used in automobiles because it has significantly improved weldability and corrosion resistance after painting, and can be alloyed at a low cost in a process continuous with hot dip coating.

This galvannealed steel sheet is usually manufactured by
The hot-dip galvanized steel sheet is continuously heated and held at a steel sheet temperature of 480 to 600 ° C. for 3 to 30 seconds in a heat treatment furnace for alloying, and Fe of the mother plate is diffused in the attached Zn plating film, It is manufactured by changing to an Fe-Zn alloy. The plating film of the resulting steel sheet is composed of an Fe-Zn intermetallic compound, and the average Fe concentration in the coating layer is about 7 to 12% by weight in most cases.

The amount of Zn deposited on the plating film is about 25 to 70 g / m ^{2 on} one side, but if the amount is less than this range, it is difficult to produce by ordinary means. Deteriorates the powdering resistance of. Powdering is a phenomenon in which a part of the coating film becomes powdery and peels off during the molding process, which not only lowers the rust-preventive ability but also causes a flaw on the product surface during press molding. Al in the plating film to prevent powdering
It is also considered effective to increase a little. In the hot dip galvanizing bath, a small amount of Al (0.08 to 0.11%) is usually added to improve the adhesion of the hot dip galvanized coating and to suppress the generation of dross in the bath during operation. It In the plating film, Al in the bath tends to be concentrated by about 1.5 to 3 times, and becomes about 0.12 to 0.2%.

As described above, the galvannealed steel sheet is excellent in coatability and corrosion resistance, but since the plating film is an intermetallic compound having poor workability, it may be deformed or stressed. It may peel at the interface between the plating film and the steel sheet (hereinafter referred to as "plating film / steel interface"). In particular, if you bring an alloyed hot-dip coating to the outside of the outer skin of a car, a total coating of about 100 μm will be applied on it, but if you are shocked by pebbles while driving in cold regions, When the coating film is damaged, peeling occurs at the plating film / steel interface, which not only deteriorates the appearance but also deteriorates the corrosion resistance. That is, the chipping resistance is inferior.

It is known that, for example, P in the steel affects the deterioration of the adhesion of the coating film / steel sheet interface of the galvannealed steel sheet as described in JP-A-6-41707 and JP-A-6-81099. The publication discloses an invention in which the P content in the steel sheet is limited to improve the P content. That is, P in steel greatly deteriorates the adhesiveness.

Conventionally, low carbon rimmed steel or Al killed steel has often been used as the base material of the galvannealed steel sheet. However, in recent years, particularly as the application to automobile bodies has increased, deep drawability has been often required, and therefore, an extremely low carbon steel called IF steel (Interstitial Free steel) is also used more and more. For the purpose of increasing the strength and weight of the vehicle body, a large amount of steel sheets to which P is added, which is cost-effective in order to improve the strength of the steel sheets, has been used.

However, as described above, in the case of the galvannealed steel sheet, there is a problem that the P-added steel may not have sufficient chipping resistance or interfacial adhesion.

[0010]

As described above, in a galvannealed steel sheet, when a large amount of P is present in the steel, the adhesion strength of the plating film is reduced. Therefore, P-added high strength steel is used for the base plate. Then, there was a drawback that the adhesion of the plating film was poor. On the other hand, the present invention is intended to provide an alloyed hot-dip galvanized high-strength steel sheet containing P, which is an economical strengthening element, and having excellent coating adhesion.

[0011]

[Means for Solving the Problems] In the present inventors, in the galvannealed steel sheet, the P content in the base steel sheet is 0.02%.
If it is less than 0, the adhesion strength of the plating film can be secured, but it is 0.
Various investigations were carried out for the cause of the significant decrease in the adhesion strength of the coating when the content was 02% or more, particularly 0.03% or more. As a result, an increase in the P content not only flattens the plated coating / steel interface after alloying treatment to reduce its adhesive strength, but also lowers the strength of the interface itself and reduces the adhesive strength of the coating. Was estimated.

During the alloying process, when the Γ phase of the intermetallic compound is formed at the plating film / steel interface, each of the steel metal crystals present on the surface of the base steel sheet is eroded in a concave shape. The rate of this Γ-phase formation reaction depends on the crystal orientation, and the α phase tends to be small on the {111} plane and large on the {100} and {110} planes. In addition to these typical low-index planes, crystals with various plane orientations are present on the surface of the steel sheet.
The shape of the steel interface, that is, the surface of the steel sheet after the plating film is removed becomes a state where the unevenness is large and the surface roughness is large. However, the increase in the P content reduces such a difference in erosion rate due to the crystal orientation and flattens the shape of the plating film / steel interface.

Therefore, in the case of a base material steel sheet having a high P content, various conditions for increasing the adhesion strength of the coating film were investigated, and the shape of the plated coating film / steel interface was revealed to have a severe irregularity. It was found that the film adhesion was obtained, and the present invention was achieved. The gist of the present invention is as follows.

A steel sheet containing P: 0.02 to 0.20% by weight has a galvannealed coating film containing 0.2 to 0.5% of Al on the surface of the steel sheet, and the roughness R of the steel surface after removal of the coating film is R. galvannealed _steel sheet _z is characterized by satisfying the _{12 ≧ R z ≧ 0.0075 · S} m + 6.7 ···········. However, R _z (μm) is a ten-point average roughness, and S _m (μm) is an average interval of irregularities, both of which are defined in JIS-B-0601.

Here, the composition of the steel sheet as a plating base material is intended to contain 0.02 to 0.20% of P as a strengthening element, and other steel components are not particularly limited. However, the desirable chemical composition for highly versatile steel is C: 0.01% by weight.
% Or less, Si: 0.25% or less, Mn: 0.8% or less, Ti:
0.1% or less, Nb: 0.1% or less, B: 0.0030% or less,
The balance consists of inevitable impurities and Fe.

In order to improve the frictional properties of the surface of the galvannealed steel sheet and the electrodeposition coatability, an upper layer plating such as Fe plating or Zn-Ni plating is further applied on the alloyed plating film. However, it goes without saying that the effects of the present invention are exerted also in these.

[0017]

In the plated steel sheet of the present invention, the steel sheet as the base material is supposed to contain P in an amount of 0.02% or more, but it is particularly high in strength and the effect of improving the coating adhesion is 0.03% or more. . If less than 0.02%, the shape of the plating film / steel interface,
That is, the ten-point average roughness R _z of the steel surface after removal of the plating film
Even if does not satisfy the formula, the adhesion strength of the film can be secured. Moreover, if it is less than 0.02%, the effect of improving the strength of the steel by the addition is almost negligible. On the other hand, if it exceeds 0.2%, no matter how the shape of the plating film / steel interface is changed, not only the adhesion strength of the film is not sufficient, but the steel sheet itself becomes brittle. Therefore, the P content range of the base steel sheet targeted by the present invention is 0.02 to 0.2%.

The alloying degree of the plating film is not particularly limited as long as it satisfies the properties required for the film, such as coatability, corrosion resistance after coating, powdering resistance, etc. It is desirable that the concentration be in the range of 7.5 to 12.5%.

The content of Al in the plating film is 0.2 to 0.5
Regulate to%. This is important in order to bring the ten-point average roughness R _z of the steel surface after removal of the plating film to satisfy the formula within the range of the required alloying degree. Al in the Zn plating film has a function of making the alloying reaction at the interface between Fe and Zn microscopically nonuniform, and if it is less than 0.2%, the ten-point average roughness R _z of the formula tends to be insufficient, If it exceeds 0.5%, R _z tends to be too large.

The most characteristic feature of the present invention is that the ten-point average roughness R _z of the steel surface after removal of the plating film after alloying is
It is to regulate within the range shown by the following formula.

[0021] _{12 ≧ R z ≧ 0.0075 · S} m + 6.7 ········· the right-hand side _{R z ≧ 0.0075 · S m +} 6.7 ··········· high P of this formula Based on the results obtained by investigating the coating adhesion strength of a steel sheet obtained by alloying hot-dip galvanizing the base metal of the content, if this condition is satisfied, a coating film equivalent to that when a low P steel sheet is used as the base material is obtained. Adhesion strength can be obtained. This equation also means that if S _m, that is, the average spacing of the irregularities is small, the adhesion can be secured even if the roughness R _z is small.

R _z is an index showing mainly the height of the peaks and the depth of the valleys of the unevenness, while S _m is an index showing the average interval of the unevenness. Therefore, if S _m is small, the number of peak-valley repetitions per unit length is large. That is, when the geometric areas are the same, the larger the R _z and the smaller the S _m , the larger the area of the plating film / steel interface as seen on a microscopic scale. What is the essential controlling factor of the adhesion strength of the coating of the galvannealed steel sheet is not sufficiently clear at present, but since the interface is complicated and the adhesion strength of the coating increases as the micro contact area increases. Ah

However, if the steel surface roughness R _z after the removal of the plating film after alloying becomes too large, the irregularities on the surface of the plating will become excessive, and even the sharpness of the coated surface when coated on it will be noticeable. Since it lowers, the upper limit is set to R _z ≤12 .... The formula is a combination of this formula and the formula.
In addition, to remove only the plating layer from the galvannealed steel sheet without impairing the shape of the coating film / steel interface, dip it in a solution of hydrochloric acid with a concentration of about 10% by weight and an appropriate inhibitor for hydrochloric acid. It can be easily implemented.

Next, the chemical composition of the base steel sheet and the production conditions preferable for the steel of the present invention will be described.

The C content of the base material is preferably 0.01% or less.
This is because the lower the C content, the more non-uniform the alloying reaction at the interface between Fe and Zn becomes, and the plating film /
This is because the steel interface can be roughened. In general, if the C content may be high, it is not necessary to increase the P content for the purpose of improving strength. However, in the annealing process of rapid heating and quenching in the hot dip galvanizing process, if it is attempted to obtain good press workability of the base steel sheet such as deep drawability and to improve aging resistance,
The addition of P is extremely effective to increase the strength of the ultra-low carbon steel without impairing the press workability.

Si is effective in improving the strength of ultra-low carbon steel and may be added if necessary. However, since it deteriorates the surface properties of the base steel sheet and causes an unplated portion, it is desirable to keep the content of 0.25% or less at most when adding.

The content of Mn is preferably 0.08% or more in order to suppress hot embrittlement during production due to S which is one of the unavoidable impurities. Moreover, it can be used to increase the strength of ultra low carbon steel. Further, the inclusion of a certain amount has the effect of increasing the adhesion force at the plating film / steel interface and further suppressing the occurrence of non-plating due to the presence of Si. Especially Si
In the case of steel containing 0.1% or more, it is desirable to contain 0.3% or more. However, since the content exceeding 0.8% deteriorates the press workability of the product, the desirable Mn content range in the base steel is 0.08 to 0.8%.

Al in steel is inevitably present because it is added as a deoxidizing agent for obtaining a sound cast piece. However, if it is too large, non-plating is likely to occur, so the desirable content range is 0.005 to 0.05%.

Ti and Nb are C existing in steel,
It is effective in fixing these elements by combining with S, N, etc., improving the press workability of the steel sheet and deaging the steel in the annealing process of rapid heating and quenching in the hot dip coating line. It also has the effect of microscopically roughening the interface between Fe and Zn. It is considered that this is because the solid solution C was fixed and the same effect as that of reducing the C content was obtained. In order to obtain such an effect, the content of both elements must be 0.003% or more, but if the content is too large, the ductility of the steel sheet deteriorates, so the desirable Ti or Nb content range is 0.0
It is from 03 to 0.1%. Either Ti or Nb may be added, or both may be added.

The addition of P tends to make the steel sheet brittle. Addition of B is preferable for suppressing this embrittlement. The desirable content range to exert the effect of the addition is 0.0003 ~
It is 0.003%.

Typical examples of unavoidable impurities in steel are S and N. However, since these deteriorate the press workability of the steel sheet, the smaller the better, the better. Desirable is S
Is 0.02 or less and N is 0.007% or less.

The ten-point average roughness R _z of the plated coating / steel interface after alloying of the present invention is P in 0.02% or more.
Controlling within the range of the formula cannot be easily realized by a usual method. An example of desirable plating process conditions for obtaining this will be described.

(1) In the continuous hot-dip galvanizing process, the steel sheet is passed through the reduction zone and immediately before entering the plating bath.
Hold for 20 to 120 s in the temperature range of 50 to 700 ° C.

As a result, even if the P content is high, the R _z of the steel surface after removing the plating film can be increased. Although the reason is not clear, since Mn segregates on the steel surface, the presence of Mn at the interface between Fe and Zn after plating is considered to affect the alloying reaction. Next, (2) the Al concentration in the Zn plating bath is 0.12% by weight%.
Manage to 0.17%. As a result, the Al in the plating film
The content can be controlled to 0.2-0.5%. Al concentration is 0.12%
If it is less than 0.2%, the content in the plating film becomes less than 0.2%, and R _z cannot satisfy the formula. Further, if the Al concentration in the bath exceeds 0.17%, the R _z of the steel surface after removal of the plating film does not satisfy the formula, that is, becomes too rough. In addition, (3) set the alloying temperature when performing the alloying treatment after plating.
550 ℃ or above, temperature range from 430 ℃ to 550 ℃ is 30 ℃ /
Rapid heating occurs at a heating rate of s or more. This rapid heating to a high temperature makes it possible to make the alloying reaction of the interface between Fe and Zn non-uniform and roughen the interface even if P in the steel is high. If the heating temperature for alloying is less than 550 ° C or the heating rate is less than 30 ° C / s, R _z that satisfies the formula
Can't get The alloying temperature is not particularly limited,
If it becomes higher, powdering will become more serious, so it cannot be made too high. There is no upper limit to the rate of temperature rise, but naturally there is a limit in terms of facility and economic constraints for heating. It is considered that the rapid heating to high temperature is effective because the alloying reaction preferentially proceeds at the defects and grain boundaries in the crystal grains of steel, and the unevenness of the plating film / steel interface becomes large.

[0035]

[Example] Using five types of as-rolled cold-rolled steel sheets having the chemical compositions shown in Table 1, the surface was alkali-cleaned, and the maximum heating temperature was 820 ° C in a nitrogen atmosphere containing 15% hydrogen with a dew point of -35 ° C. After reduction annealing at, cool down to 460 ℃ under the conditions shown in Table 2,
The residence time in the temperature range from 700 ° C to 460 ° C was variously changed. Next, it was immersed in a molten zinc bath having an Al concentration of 0.11 to 0.18% and a temperature of 460 ° C. for 1 second to perform plating with an adhesion amount of 50 g / m ² . Subsequently, the temperature rising rate was changed in the range of 15 to 65 ° C./s, the temperature was raised to the required temperature, the alloying treatment was performed by holding for a predetermined time, and the alloyed galvanized steel sheet was cooled.
Table 3 summarizes the manufacturing conditions from the cooling of each steel sheet after this reduction annealing and the retention at 700 ° C or less to the end of alloying.
Shown in

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

A test piece was cut out from the obtained galvannealed steel sheet, and 0.5% by volume of an inhibitor (Ibit 710N, manufactured by Asahi Kagaku) was added to 10% by weight of hydrochloric acid.
Only the plating film was dissolved and removed, and the plating film was chemically analyzed to confirm the composition. The steel plate after removing the plating film is 55d with a stylus surface roughness meter (Tokyo Seimitsu: Surfcom 554A).
eg conical type, using a stylus with a tip diameter of 1 μm, scan distance 8 m
Surface roughness was measured under conditions of m and a cutoff of 0.8 mm to obtain roughness parameters R _z and S _m . The results are shown in Table 3.

Further, a chipping test was conducted in order to evaluate the adhesion of the coating film of the obtained steel sheet. Width 70 mm, length
Cut out a 150 mm steel plate and treat it with phosphate (CF made by Chemifil: CF
168), and then a 30 μm thick cation electrodeposition coating (made by PPG: Uniprime), a 15 μm thick intermediate coating (made by the same company: using an epoxy ester-based coating) and a 45 μm thick top coating. (Manufactured by the same company: using acrylic / enamel type paint). A crushed stone for road specified in JIS-A5001 was sprayed onto the coated test piece using a gravimeter that conformed to the SAE-J400 specification, and then tape peeling was performed to observe the peeling trace of the coating. In this case, in one test piece, five peel marks were selected in order from the largest, and the trace diameter was measured and averaged. The results are also shown in Table 3.

As can be seen from the comparison of these results, a steel sheet whose coating / steel interface roughness after removal of the plating coating satisfies the range defined by the present invention is compared with those out of the range, after coating. The peeling diameter in the chipping test is small,
Excellent adhesion of the plating film. It should be noted that in reference numbers 15 and 16, the peeling diameter in the chipping test was small and the coating adhesion was good, but the roughness of the coating / steel interface was extremely large and the sharpness of the surface after coating was poor.

[0042]

The alloyed hot-dip galvanized steel sheet of the present invention is
The base material is a high-strength steel sheet containing P, and the coating film has excellent adhesion. This alloyed hot-dip galvanized steel sheet can be utilized for the wide range of applications described above, particularly for automobiles.

Claims (1)

[Claims] 1. A steel sheet containing 0.02 to 0.20% by weight of P has an alloyed galvanized coating containing 0.2 to 0.5% of Al on the surface of the steel sheet, and the surface of the steel after the coating is removed is rough. galvannealed steel sheet, characterized in that the R _z satisfies _{12 ≧ R z ≧ 0.0075 · S} m + 6.7. (However, R _z (μm): 10-point average roughness, S _m (μ
m): average spacing of irregularities)