JP2976845B2 - Galvannealed steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvannealed steel sheet having excellent adhesion between a plating film and a base steel sheet, and particularly suitable as a coated steel sheet for household appliances and a steel sheet for automobiles.

[0002]

2. Description of the Related Art In recent years, zinc-based plated steel sheets have been used in large quantities in the fields of home appliances, building materials, and automobiles. Excellent alloyed hot-dip galvanized steel sheets are widely used.

[0003] When a small amount of Fe is alloyed in a Zn coating film, the alloyed hot-dip galvanized steel sheet has an improved paint adhesion and has a corrosion potential noble and has a moderate sacrificial corrosion prevention effect as compared with Zn. It is something that takes advantage of what it becomes. In particular, they are widely used in automobiles because their weldability and corrosion resistance after painting are remarkably improved, and they can be alloyed at low cost in a process that is continuous with hot-dip plating.

[0004] This alloyed hot-dip galvanized steel sheet is usually
Continuously hot-dip galvanized steel sheet is continuously heated and maintained in a heat treatment furnace for alloying at a steel sheet temperature of 480 to 600 ° C. for 3 to 30 seconds to diffuse the Fe of the mother plate into the adhered Zn plating film, It is manufactured by changing to Fe-Zn alloy. The plated coating of the finished steel sheet is made of an intermetallic compound of Fe-Zn, and the average Fe concentration in the coating layer is often about 7 to 12% by weight.

[0005] The coating amount of Zn on the plating film is about 25 to 70 g / m ² per side, but if the amount is less than this range, it is difficult to manufacture by ordinary means. Degrades the powdering resistance. Powdering is a phenomenon in which a part of a coating film becomes powdery and peels off at the time of molding, and not only causes a decrease in rust prevention ability but also causes a surface defect of a product at the time of press molding. To prevent powdering, Al in the plating film
It is also effective to slightly increase. A small amount of Al, usually about 0.08 to 0.11%, is added to the hot dip galvanizing bath to improve the adhesion of the hot dip galvanized coating and to suppress dross generation in the bath during operation. You. Al in the bath tends to be concentrated about 1.5 to 3 times in the plating film, and is about 0.12 to 0.2%.

[0006] The alloyed hot-dip galvanized steel sheet is excellent in paintability and corrosion resistance as described above. However, since the plated film is an intermetallic compound having poor workability, depending on the manner of deformation and the situation in which stress is applied, Peeling may occur at the interface between the plating film and the steel sheet (hereinafter referred to as “plating film / steel interface”). In particular, when an alloyed hot-dip coating is brought on the outside of an automobile outer panel, a total of about 100 μm of paint is applied on it. As the coating film is damaged, peeling occurs at the plating coating / steel interface, which not only deteriorates the appearance but also impairs the corrosion resistance. That is, the chipping resistance is inferior.

[0007] Assuming that P in steel influences the adhesion deterioration of the coating film / steel plate interface of such a galvannealed steel sheet, for example, JP-A-6-41707 and JP-A-6-81099 Japanese Patent Application Laid-Open Publication No. H11-15064 discloses an invention in which the content of P in a steel sheet is limited to improve the content. That is, P in the steel greatly deteriorates the adhesion.

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, the need for deep drawability has increased, and the use of extremely low carbon steel called IF steel (Interstitial Free steel) has been increasing. And, for the purpose of increasing the strength and weight of the vehicle body, in order to improve the strength of the steel sheet, a steel sheet added with P, which is cost-effective, has come to be used in large quantities.

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

[0010]

As described above, in an alloyed hot-dip galvanized steel sheet, if a large amount of P is present in the steel, the adhesion strength of the plating film is reduced. And the adhesion of the plating film was inferior. On the other hand, the present invention intends to provide an alloyed hot-dip galvanized high-strength steel sheet containing P, which is an economic strengthening element, and having excellent adhesion of a coating film.

[0011]

Means for Solving the Problems The present inventors have found that the alloyed hot-dip galvanized steel sheet has a P content of 0.02% in the base steel sheet.
If it is less than 30, the adhesion strength of the plating film can be ensured, but 0.
Various investigations were performed on the cause of the significant decrease in the adhesion strength of the coating when the content became 02% or more, especially 0.03% or more. As a result, an increase in the P content not only lowers the adhesive strength by flattening the interface between the plated film and the steel after the alloying treatment, but also lowers the strength of the interface itself and lowers the adhesive strength of the film. It was estimated.

During the alloying process, when a Γ phase of an intermetallic compound is formed at the plating film / steel interface, each of the metal crystals of the steel present on the surface of the base steel sheet is eroded in a concave shape. The rate of the Γ phase formation reaction differs depending on the crystal orientation, and tends to be small in the {111} plane and large in the {100} or {110} plane in the α phase. In addition to these typical low index planes, crystals having various plane orientations exist on the steel sheet surface.
The shape of the steel interface, that is, the surface of the steel sheet from which the plating film has been removed, is in a state of large irregularities and large surface roughness. However, an increase in the P content reduces such a difference in erosion rate depending on the crystal orientation and flattens the shape of the plating film / steel interface.

Therefore, in the case of a base steel sheet having a high P content, various conditions for increasing the adhesion strength of the coating were examined, and the shape of the plating coating / steel interface was made to show a severely uneven state. It was found that film adhesion was obtained, and the present invention was reached. The gist of the present invention is as follows.

On the surface of a steel sheet containing 0.02 to 0.20% by weight of P, there is provided an alloyed hot-dip galvanized coating containing 0.2 to 0.5% of Al, and the roughness R of the steel surface after removing the plated film _A galvannealed steel sheet wherein _z satisfies 12 ≧ R _z ≧ 0.0075 · S _m + 6.7. Here, 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 the plating base metal is a composition containing 0.02 to 0.20% of P as a strengthening element, and other steel components are not particularly limited. However, a desirable chemical composition as a highly versatile steel is C: 0.01% by weight.
%, 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 remainder consists of unavoidable impurities and Fe.

Further, in order to improve the friction characteristics of the surface of the alloyed hot-dip galvanized steel sheet and the electrodeposition coating property, an upper plating such as Fe plating or Zn-Ni plating is further applied on the alloyed plating film. However, it is needless to say that the effects of the present invention can also be exhibited in these.

[0017]

In the plated steel sheet of the present invention, the steel sheet as the base material contains at least 0.02% of P, but the strength is particularly high and the effect of improving the coating adhesion is exhibited at at least 0.03%. . If it is 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 the plating film is removed.
Does not satisfy the formula, the adhesion strength of the coating can be ensured. Moreover, if it is less than 0.02%, there is almost no effect of improving the strength of the steel by the addition. 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 also the steel sheet itself becomes brittle. Therefore, the content range of P in the base steel sheet targeted in the present invention is set to 0.02 to 0.2%.

The degree of alloying of the plating film is not particularly limited as long as the performance required for the film, such as paintability, corrosion resistance after painting, and powdering resistance, is not particularly limited. Preferably, the concentration is 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 so that the ten-point average roughness _Rz of the steel surface after the removal of the plating film satisfies the expression within the range of the required degree of alloying. Al in the Zn plating film has a function to make the alloying reaction at the interface between Fe and Zn microscopically nonuniform. 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 _Rz of the steel surface after the removal of the plating film after the alloying is represented by:
It is to be regulated within the range shown by the following equation.

12 ≧ R _z ≧ 0.0075 · S _m + 6.7 ・ ・ ・ ・ ・ ・ ・ ・ ・ The right side of this equation R _z ≧ 0.0075 · S _m + 6.7 ・ ・ ・ is high P The results obtained by investigating the coating adhesion strength of a steel sheet which has been subjected to alloyed hot-dip galvanizing on a base metal having a content, and if this condition is satisfied, a coating equivalent to that obtained when a low-P steel sheet is used as the base metal is obtained. Adhesion strength is obtained. This formula also the smaller the average distance S _m namely irregularities, the roughness R _z means that the adhesion can be ensured even smaller.

R _z is an index mainly indicating the height of the peak and the depth of the valley of the unevenness, while S _m is an index indicating the average interval of the unevenness. Therefore, the mountain of per unit length the smaller the S _m - a large number of repetitions of the valley. That is, if geometric area are the same, the more R _z is large, and the area of the plated coating / steel interface viewed as microscopically S _m is less increased. What is the essential controlling factor of the adhesion strength of the coating of the alloyed hot-dip galvanized steel sheet is not sufficiently clear at present, but the interface is complicated and the adhesion strength of the coating increases as the microscopic contact area increases. There will be.

However, if the steel surface roughness _Rz after the removal of the plating film after alloying becomes too large, the unevenness becomes remarkable even on the surface of the plating, and the sharpness of the coating surface when coated thereon is also reduced. Therefore, the upper limit is set to R _z ≦ 12. The expression is a combination of this expression and the expression.
In order to remove only the coating layer from the alloyed hot-dip galvanized steel sheet without damaging the shape of the coating film / steel interface, immersion in a solution containing about 10% by weight of hydrochloric acid and an appropriate inhibitor for hydrochloric acid added. Can be easily implemented.

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

The C content of the base material is desirably 0.01% or less.
This means that the lower the C content, the more microscopically the alloying reaction at the interface between Fe and Zn becomes uneven, and the plating film /
This is because the steel interface can be roughened. In general, if the C content is high, it is not necessary to increase the P content for the purpose of improving the strength. However, in the annealing process of rapid heating and quenching in the hot-dip galvanizing process, if good press workability of the base material steel sheet such as deep drawability is obtained, and if aging resistance is to be improved, 0.
In order to increase the strength of the ultra-low carbon steel without impairing the press workability, the addition of P is extremely effective.

Si is effective in improving the strength of ultra-low carbon steel, and may be added as necessary. However, since it deteriorates the surface properties of the base steel sheet and generates unplated portions, it is desirable that the content be at most 0.25% or less when added.

Mn is preferably contained in an amount of 0.08% or more in order to suppress hot embrittlement during production due to S, which is one of the inevitable impurities. In addition, it can be used to increase strength with ultra-low carbon steel. The addition of a certain amount has the effect of increasing the adhesion at the plating film / steel interface and 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 preferable range of Mn content 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 sound slabs. However, if the content is too large, non-plating is likely to be induced. Therefore, a desirable content range is 0.005 to 0.05%.

Ti and Nb are carbons present in steel,
Bonding with S, N, etc., these elements are fixed, and in the annealing process of rapid heating and quenching in the hot-dip galvanizing line, the press workability of the steel sheet is enhanced, and the steel is effective for non-aging. In addition, there is an effect that the interface between Fe and Zn is microscopically roughened. This is considered to be because the same effect as reducing the C content was obtained because solid solution C was fixed. To obtain such an effect, the content of both elements is required to be 0.003% or more. However, if the content is too large, the ductility of the steel sheet is deteriorated.
03 to 0.1%. Ti or Nb may be added to either one or both.

The addition of P tends to embrittle the steel sheet. To suppress this embrittlement, the addition of B is preferable. The desirable content range for exerting the effect of the addition is 0.0003 to
0.003%.

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

The ten-point average roughness _Rz of the plated film / steel interface after alloying according to the present invention is _defined as follows:
Controlling within the range of the equation cannot be easily realized by ordinary methods. Examples of desirable plating process conditions for obtaining this will be described.

(1) In the continuous process of hot-dip galvanizing, after the steel sheet passes through the reduction zone and immediately before entering the plating bath, 4
In a temperature range of 50 to 700 ° C., stay for 20 to 120 s.

Thus, even if the P content is high, _Rz on the steel surface after the removal of the plating film can be increased. Although the reason is not clear, since Mn segregates on the steel surface, it is considered that the presence of Mn at the interface between Fe and Zn after plating affects the alloying reaction. Next, (2) The concentration of Al in the Zn plating bath was 0.12 to 0.1% by weight.
Control to 0.17%. As a result, Al in the plating film
The content can be controlled to 0.2-0.5%. Al concentration 0.12%
If it is less than 3, the content in the plating film becomes less than 0.2%, and _Rz cannot satisfy the expression. Further, when the Al concentration in the bath exceeds 0.17%, _Rz of the steel surface after the removal of the plating film does not satisfy the expression, that is, it becomes too coarse. (3) When performing alloying after plating, the alloying temperature
550 ° C or higher, and the temperature range from 430 ° C to 550 ° C is 30 ° C /
Rapidly heats at a heating rate of s or more. This rapid heating to a high temperature makes the alloying reaction at the interface between Fe and Zn microscopically nonuniform even if the P in the steel is high, making it possible to roughen the interface. When the heating temperature for alloying is less than 550 ° C or the rate of temperature rise is less than 30 ° C / s, R _z satisfying the formula is satisfied.
Can not be obtained. The alloying temperature is not particularly limited,
If it gets too high, the powdering becomes so severe that it can't be too high. Although there is no upper limit for the heating rate, there is a limit naturally in terms of equipment and economic restrictions for heating. It is considered that the rapid heating to high temperature is effective because the alloying reaction proceeds preferentially at the defects and grain boundaries in the crystal grains of the steel, and the unevenness of the plating film / steel interface is increased.

[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. And then cooled to 460 ° C under the conditions shown in Table 2,
The residence time in the temperature range from 700 ° C to 460 ° C was varied. Then, it was immersed in a molten zinc bath at 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 alloy was heated to a required temperature while changing the heating rate within a range of 15 to 65 ° C./s, subjected to an alloying treatment by holding for a predetermined time, and cooled to obtain an alloyed galvanized steel sheet.
Table 3 summarizes the manufacturing conditions from cooling of each steel sheet after this reduction annealing and staying at 700 ° C or lower to completion of alloying.
Shown in

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

A test piece was cut out from the obtained alloyed hot-dip galvanized steel sheet, and a solution prepared by adding 0.5% by volume of an inhibitor (manufactured by Asahi Chemical: Ivit 710N) 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. After removing the plating film, the steel plate was measured for 55 d by a stylus type surface roughness meter (Tokyo Seimitsu: Surfcom 554A).
eg Scanning distance 8m using a stylus with conical shape, tip diameter 1μm
The surface roughness was measured under the conditions of m and a cutoff of 0.8 mm, and the roughness parameters _Rz and _Sm were determined. Table 3 shows the results.

Further, a chipping test was performed to evaluate the adhesion of the coating of the obtained steel sheet. 70mm width, length
A 150mm steel sheet was cut out and treated with phosphate (Chemifil: CF
168), followed by a 30μm-thick cationic electrodeposition coating (PPG: Uniprime used), a 15μm-thick intermediate coating (Epoxy-based coating used), and a 45μm-thick topcoat (Made by the company: using acrylic / enamel paint). A crushed stone for roads specified in JIS-A5001 was sprayed on the coated test piece using a gravure meter based on SAE-J400, and the tape was peeled off to observe the peeling mark of the coating. In this case, five peeling traces were selected in order from the largest one in one test piece, and the trace diameters were measured and averaged. Table 3 also shows these results.

As can be seen from the comparison of these results, the steel sheet whose coating / steel interface roughness after removing the plating film satisfies the range specified by the present invention is out of the range. The peeling diameter in the chipping test is small,
Excellent adhesion of plating film. In the case of reference numbers 15 and 16, the peeling diameter in the chipping test was small and the film adhesion was good, but the roughness of the film / 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 comprises:
The base material is a high-strength steel sheet containing P, and has excellent coating adhesion. This alloyed hot-dip galvanized steel sheet can be utilized for the above-mentioned wide range of uses, particularly for automobiles.

Claims (1)

(57) [Claims] An alloyed hot-dip galvanized coating containing 0.2 to 0.5% of Al is provided on the surface of a steel sheet containing 0.02 to 0.20% by weight of P and the roughness of the steel surface after removing the plated coating. 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)