JPH10265901A - Hot dip zn-al coated steel sheet for heat resistant member, excellent in workability, and its use - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip Zn-Al coated steel sheet in which the occurrence of high temp. embrittlement is remarkably reduced particularly in a plastic- worked part and also to provide its use where the progress of its high temp. embrittlement is reduced. SOLUTION: The steel sheet, excellent in workability, can be obtained by forming a hot dip Zn-Al coating layer containing 0.1 to 75 mass% Al on the surface of a steel sheet as a base material which has a composition consisting of, by mass, <=0.01% C, <=0.05% Si, <=0.5% Mn, <=0.030% P, <=0.0040% N, 0.0007-0.0050% B, and the balance Fe with inevitable impurity elements and further containing, if necessary, one or more elements among Ti, Nb, and Zr by amount, in total, >=4 times the total amount of C and N (by mass%). This steel sheet is formed into a heat resistant member by means, e.g. of plastic working and used in a high temp. environment as high as 200 to 500 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a Zn-A excellent in workability for heat-resistant members used in a high-temperature environment of 200 to 500 ° C.
1. Field of the Invention The present invention relates to an l-type hot-dip coated steel sheet and a method of using the same to suppress the progress of high-temperature embrittlement.

[0002]

2. Description of the Related Art Zn-Al-based hot-dip coated steel sheets exhibit excellent corrosion resistance due to the sacrificial corrosion protection effect of Zn.
It has been widely used for building materials such as roofs and walls.
However, in recent years, the balance between cost and quality has been emphasized,
The use of members has been subdivided. As a result, Zn-A
l-type hot-dip coated steel sheet is not only used for conventional corrosion resistance,
It has also been used for heat-resistant applications. Above all, Zn
Since the −55% Al alloy plated steel sheet is excellent in corrosion resistance and heat resistance of the plated layer itself, it is a plated steel sheet having good overall durability and is expected to be applied to various uses.

[0003] However, when the steel material coated with the Zn-based plating layer is used at a high temperature of about 200 ° C or higher, the Zn content in the plating layer is reduced.
Is known to diffuse into crystal grain boundaries of the base steel sheet and cause a phenomenon that the base steel sheet itself is embrittled (herein referred to as “high-temperature embrittlement”). For this reason, it is not preferable to use a Zn-based plated steel material for a member that is heated to a temperature of about 200 ° C. or more and to which external force due to vibration or the like is applied, such as an exhaust system member mounted on an automobile, for example. Al for various applications
Expensive materials such as plated steel sheets and stainless steel had to be used.

To cope with such a problem of high-temperature embrittlement of Zn-based plated steel material, Japanese Patent Publication No. 1-35071 discloses a method in which carbon steel as a plating base material contains at least 0.039% by weight of P. A method for preventing high-temperature embrittlement is disclosed. According to the data, tensile elongation data when heating at 900 ° F (482 ° C) for up to 600 hours is shown. For example, in the case of 55% Al-Zn alloy coated steel, P is added to the base metal.
For those containing 0.042% or more, the elongation after heating for 600 hours exceeded the elongation before heating.

[0005]

[Problems to be solved by the invention]
In the method described in JP-A-35071, a large amount of P is added, so that the base steel sheet itself is hardened, thereby impairing the workability that carbon steel originally has. For this reason, it is difficult to produce a heat-resistant member having a complicated shape, and it is not possible to sufficiently respond to the recent various needs. Also, when the present inventors performed additional tests,
According to the above-mentioned P addition means, although the sample heated without processing after plating shows excellent high-temperature embrittlement resistance,
When heating is performed after plastic working, the hot brittleness resistance in the processed part is not necessarily sufficient. For example, when the heating temperature is 400 ° C., even if the heating time exceeds 600 hours, the progress of embrittlement is still observed in the processed portion, and further improvement in durability is desired particularly in applications requiring processing of a complicated shape.

As described above, the prior art has not sufficiently solved the problem of high-temperature embrittlement in heat-resistant members made by processing Zn-Al-based plated steel sheets. In addition, it is difficult to evaluate the degree of embrittlement of a processed portion only by a tensile test, and it is necessary to consider introducing a new evaluation method. Therefore, the present invention is a Zn-Al-based plated steel sheet exhibiting excellent high-temperature embrittlement resistance even in a heat-resistant member manufactured by imparting plastic working to a Zn-Al-based plated steel sheet, and in particular 1000 hours or more. It is an object of the present invention to provide a material capable of maintaining stable durability even when used for a long time under heating. Further, the present invention provides a method for using a Zn-Al-based hot-dip galvanized steel sheet for a heat-resistant member in which the progress of high-temperature embrittlement is suppressed.

[0007]

In order to achieve the above-mentioned object, the invention according to claim 1 is characterized in that, in mass%, C: 0.01% or less, Si: 0.05% or less, Mn: 0.5% or less, P: 0.020%
Below, N: 0.0040% or less, B: 0.0007-0.0050%
On the surface of a base steel sheet composed of Fe and unavoidable impurity elements, with the balance being Al: 0.1 to 75% by mass.
An object of the present invention is to provide a Zn-Al-based hot-dip steel sheet for heat-resistant members used in a high-temperature environment of 200 to 500 ° C, which is a steel sheet having excellent workability and having an Al-based hot-dip coating layer formed thereon.

According to a second aspect of the present invention, there is provided the base steel sheet according to the first aspect, wherein C: 0.01% or less, Si: 0.05% or less, Mn: 0.5%.
Below, P: 0.030% or less, N: 0.0040% or less,
B: contains 0.0007 to 0.0050%, and further contains Ti, Nb, Z
r is a base steel sheet containing at least one kind of r in total four times or more of C + N (mass%) and the balance being Fe and unavoidable impurity elements. Where C + N (% by mass)
Means the total value of the C content and the N content expressed in mass%.

A third aspect of the present invention provides the base steel sheet according to the first or second aspect, wherein the B content is particularly 0.00
The content is in the range of 13 to 0.0050% by mass.

According to a fourth aspect of the present invention, the hot-dip coating layer according to the first to third aspects of the present invention,
Al: 0.1-75%, Si: 5% or less, Mg:
Z containing at least one of 3% or less, Ti: 1% or less, and Cr: 1% or less, with the balance being Zn and unavoidable impurities.
This is an n-Al-based hot-dip plating layer. Examples of the inevitable impurities include elements such as Mn, Sn, and Fe, for which it is difficult to avoid mixing during production.

In a fifth aspect of the present invention, in the first to fourth aspects of the present invention, the thickness of the hot-dip plating layer is particularly set to 5 to 60 μm.

According to a sixth aspect of the present invention, in the Zn-Al-based hot-dip coated steel sheets of the first to fourth aspects, particularly in the embrittlement property, the inner radius of the steel sheet is twice the thickness t (mm) of the steel sheet. so
When the sample subjected to 90 ° bending is heated in the air at 400 ° C. and the embrittlement depth D ₆₀₀ (mm) after heating for 600 hours and the embrittlement depth D ₁₀₀₀ (mm) after heating for 1000 hours are measured. , And the following formulas (1) and (2) are established, and high durability is obtained. D ₁₀₀₀ ≦ 0.20 × t ----- (1) 1.2 × D ₆₀₀ ≧ D ₁₀₀₀ ----- (2) The embrittlement depth referred to here is a value obtained by heating a 90 ° bent portion after heating. Furthermore, close bending is performed, and the microstructure of the cross section perpendicular to the sheet thickness direction of the bent portion is observed with a microscope, and the maximum depth of the crack generated outside the bent portion is determined by the plating layer and the base material outside the bent portion. It means a value measured in mm with reference to the interface. Therefore, this can directly evaluate the degree to which cracks actually occur.

A seventh aspect of the present invention is a Zn-Al based molten steel containing 0.0007 to 0.0050% by mass of B and 0.1 to 75% by mass of Al on the surface of a low carbon steel sheet containing less than 0.020% by mass of P. A Zn-Al-based hot-dip steel sheet having a plating layer formed thereon is processed into a heat-resistant member and used in a high-temperature environment of 200 to 500 ° C.
An object of the present invention is to provide a method for using an l-type hot-dip coated steel sheet.

According to an eighth aspect of the present invention, the working in the seventh aspect of the invention includes a plastic working.
Here, the plastic working is processing involving plastic deformation, and examples thereof include bending, flare, overhang, drawing, and burring.

According to a ninth aspect of the present invention, in the invention of the seventh or eighth aspect, the heat-resistant member is an automobile exhaust system member.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors are concerned with overcoming high-temperature embrittlement of Zn-Al-based hot-dip coated steel sheets, and in particular, impart excellent high-temperature embrittlement resistance even to heat-resistant members obtained by processing the steel sheets into complicated shapes. We studied ways to do this. As a result, it has been found that by adding an appropriate amount of B to the base steel sheet, the high-temperature embrittlement resistance in a portion processed after plating is remarkably improved, and the present invention has been completed based on this finding. Hereinafter, matters specifying the present invention will be described.

[Base Material Steel Plate] In the present invention, it is assumed that a steel plate based on inexpensive low-carbon ordinary steel is used. As the content of C increases in the base steel sheet, the steel sheet itself becomes harder. Therefore, as the base steel of the present invention requiring good workability, a lower C content is more desirable. However, reducing the C content more than necessary only increases the load in the steel making process and only raises the cost. In the present invention, considering the workability of heat-resistant members having complicated shapes, C
It is desirable that the content is 0.01% by mass or less.

When the content of Si in the base steel sheet increases, the steel sheet itself becomes harder, and at the same time, it becomes easier to induce plating defects such as non-plating, which causes troubles such as poor appearance and reduced corrosion resistance. For this reason, it is necessary to keep the Si content low, and in the present invention, it is desirable to set the content to 0.05% by mass or less. When the content of Mn also increases in the base steel sheet, the steel sheet itself is hardened, so it is necessary to keep the content in the base steel sheet low. In the present invention, the Mn content is desirably 0.5% by mass or less.

P is an element that causes the steel sheet itself to harden as its content increases in the base steel sheet, similarly to the above elements. In addition, it has been found that when the content of P is high, it is difficult to sufficiently suppress high-temperature embrittlement in a portion processed after the Zn-Al-based hot-dip plating. As a result of our experiments, the P content of the base steel sheet was 0.030% by mass.
If it exceeds, the high-temperature embrittlement suppression effect by the addition of B described below cannot be sufficiently exhibited, and for example, it is difficult to prevent the progress of embrittlement in the processed portion when heating is performed at 400 ° C. for a long time exceeding 600 hours. It became. Therefore, in the present invention, it is important to reduce the P content of the base steel sheet as much as possible, and the P content should not exceed 0.030% by mass.
In particular, when it is assumed to be used for a heat-resistant member having a large workability, the P content of the base steel sheet is desirably less than 0.020% by mass.

When the content of N in steel increases, not only does the mechanical property of the plated material itself deteriorate, but also the ability to suppress embrittlement in a high-temperature environment decreases. Therefore,
The lower the N content of the base steel sheet, the better. In the present invention, the N content is 0.0040.
It is desirable that the content be not more than mass%.

B is an element that plays a very important role in the present invention. That is, B is Zn in the present invention.
-A steel component indispensable for imparting high-temperature embrittlement resistance to an Al-based hot-dip coated steel sheet. As a result of detailed studies, the present inventors have reduced P as described above and set B to 0.
[0007] When a Zn-Al-based hot-dip galvanized layer is formed on a base steel sheet containing not less than mass%, not only when the Zn-Al-based hot-dip coated steel sheet is heated as it is, but also when a plastically processed portion is formed on the steel sheet. Even if heated after
It has been found that high-temperature embrittlement of the base steel sheet is significantly suppressed. In other words, when a heat-resistant member obtained by processing a Zn-Al-based hot-dip coated steel sheet into a complex shape is used by exposing it to a high-temperature environment, the progress of embrittlement of the processed part is significantly suppressed even by heating for a long time, which is extremely high. It exhibits excellent durability.
Specifically, for example, when heating is performed at a temperature of 500 ° C. or less and for a long time exceeding 600 hours after processing, the progress of embrittlement becomes very slow even in the processed portion, and long-term durability is maintained. From the above, in the present invention, the B content of the base steel sheet is specified to be 0.0007% by mass or more. In particular, in order to more remarkably reduce the progress of embrittlement in the processed portion, the B content of the base steel sheet is preferably set to 0.0013% by mass or more. Such an effect of B will be demonstrated in Examples described later.

At present, there are many unknowns about the mechanism by which B suppresses the high-temperature embrittlement of the base steel sheet itself.
The following can be considered. That is, Zn-Al
The high-temperature embrittlement that occurs in hot-dip galvanized steel sheet is considered to be caused by Zn in the plating layer preferentially diffusing and penetrating into the crystal grain boundaries of the base steel sheet while using the coated steel sheet at a high temperature, This is a phenomenon common to plated steel sheets having a plated layer containing Zn. Therefore, this Z
It is considered that high-temperature embrittlement is suppressed if the diffusion and penetration of n can be prevented. In a steel sheet containing an appropriate amount of B in steel, when it is used in a temperature range of about 200 to 500 ° C., B having a high diffusion rate is preferentially segregated at the crystal grain boundary of the base steel sheet, and Zn Suppress field diffusion. In addition, since B can sufficiently segregate even with minute grain boundary defects generated by processing, B is particularly effective in suppressing high-temperature embrittlement in a processed portion. On the other hand, it is considered that P, which has been conventionally considered to be effective in improving high-temperature embrittlement, cannot sufficiently perform such segregation at the grain boundaries of the processed portion. That is, in the case of P whose atomic diameter is larger than that of B, it is difficult to cover all the fine grain boundary defects caused by the processing only by the grain boundary segregation of P. As a result, the grain boundary of Zn in the processed portion is difficult. It is presumed that diffusion cannot be completely suppressed. Further, it is considered that in a Zn-based plated steel sheet to which P and B are added in a complex manner, the grain boundary segregation of P interferes with the function of B, so that the original function of B cannot be exhibited.

It is not necessary to set an upper limit on the range of the B content as long as the above-mentioned effects are exerted. However, if too much B is added, the steel sheet itself may be hardened. Therefore, the B content of the base steel sheet is 0.0050% by mass.
It is desirable to make the following. In consideration of the balance between the effect of adding B and the cost in actual operation, the content is more preferably 0.0030% by mass or less.

Ti, Nb and Zr are effective elements for fixing Free-C and Free-N and further improving the mechanical properties and the like of the base steel sheet itself. The effect is Ti, Nb, Z
It becomes remarkable when the total content of r is 4 times or more of C + N (% by mass), and it may be added alone or in combination of two or more. Therefore, when Ti, Nb, and Zr are contained, it is desirable that at least one of them is contained at least 4 times as much as C + N (mass%).

[Plating Layer] The corrosion resistance and heat resistance of a Zn-based hot-dip galvanized layer containing Al are improved as the Al content in the plated layer is increased. However, a material containing 0.1% by mass or more of Al is used as a practical material. In view of the importance of providing a plated steel sheet having improved high-temperature embrittlement resistance even in the use of these relatively inexpensive practical materials, in the present invention, the content of Al in the plating layer is 0.1%. Those whose mass% or more were used. However, considering the actual operation of hot-dip plating, the plating bath temperature rises as the Al concentration in the plating bath increases, causing plating defects due to Zn evaporation in the snout and increasing dross in the plating bath. The problem of poor appearance of the plated steel sheet accompanying the problem is likely to occur. Therefore, as a result of examining the relationship between the frequency of occurrence of defective products and the composition of the plating bath at the time of product production, it was decided that those having an Al content of 75% by mass or less in the plating layer were targeted.

The Zn—Al-based hot-dip coating layer is made of Si, M
Even if g, Ti, Cr, etc. are added in an appropriate amount, it does not essentially affect the embrittlement behavior of the base steel sheet and the function of suppressing high-temperature embrittlement due to the addition of B. The effects of adding these elements are generally as follows.

Si is Zn-containing at least 10% by mass of Al.
It has the effect of suppressing the growth of the alloy layer formed on the Al-based hot-dip coated steel sheet, and is effective in improving the workability of the plated steel sheet. Its content is generally allowed up to 5% by mass. Mg has an effect of improving the corrosion resistance of the plating layer,
It can be contained up to about 3% by mass. Ti has an effect of making spangles fine, and can be contained up to approximately 1% by mass. C
r has the effect of improving the corrosion resistance of the plating layer itself,
It can be contained up to approximately 1% by mass. As described above, in the present invention, in order to impart characteristics depending on the application, Si: 5% by mass or less, Mg: 3% by mass or less, Ti: 1% by mass or less, Cr: 1% by mass.
It is possible to form a Zn-Al-based hot-dip plating layer containing at least one of the following mass%.

The thickness of the Zn—Al-based hot-dip coating is 5 to 60 μm in order to maintain excellent properties such as corrosion resistance.
It is desirable that

[Characteristics of Zn-Al-based hot-dip coated steel sheet] The Zn-Al-based hot-dip coated steel sheet of the present invention exhibits particularly excellent high-temperature embrittlement resistance when processed into a heat-resistant member having a complicated shape. Is what you do. Therefore, the steel sheet must have good workability in a state where the Zn-Al-based hot-dip plating has been performed. Although “elongation” is most commonly used as an index indicating the workability of a steel sheet, it is necessary to consider the effect of the thickness of the steel sheet when assuming various types of plastic working including bending. As a result of the study by the present inventors, Z
When the workability of the n-Al-based hot-dip coated steel sheet itself is represented by “elongation” by a tensile test, elongation (%) ≧ 35 × 0.9
× It is desirable to use a Zn-Al-based hot-dip coated steel sheet that satisfies the relationship of sheet thickness (mm).

Further, the Zn-Al-based hot-dip galvanized steel sheet provided by the present invention has a performance in which the progress of high-temperature embrittlement in a processed portion after processing the coated steel sheet is suppressed to a certain level or less by long-term heating. It is desirable that the steel sheet has the following. Such a steel plate is specified as follows.

In order to objectively specify the performance of a steel sheet, it is necessary to follow a certain evaluation method. Therefore, the present inventors have introduced the following method as a method for appropriately evaluating the performance when the processed member is actually used. That is, when a sample obtained by subjecting the steel plate to a 90 ° bend with an inner radius twice as thick as the plate thickness t (mm) is heated in the air at 400 ° C., the degree of embrittlement in the bent portion is determined. An investigative technique was used. The degree of the progress is whether the embrittlement depth after heating for 1000 hours is less than or equal to the value according to the plate thickness, and the rate of change of the embrittlement depth after heating for 600 hours and after heating for 1000 hours is constant The evaluation is based on whether it is less than the value. Specifically, the former can be determined based on whether or not the following equation (1) is satisfied, and the latter can be determined based on whether or not the following equation (2) is satisfied. D ₁₀₀₀ ≦ 0.20 × t ----- (1) 1.2 × D ₆₀₀ ≧ D ₁₀₀₀ ----- (2)

Here, D ₆₀₀ and D ₁₀₀₀ are each 6
This is a value in mm showing the embrittlement depth after heating for 00 hours and after heating for 1000 hours. As described above, the measurement of the embrittlement depth is performed by heating the part to which the 90 ° bend has been applied, and further performing close bending after heating, and observing a metal structure of a cross section perpendicular to the sheet thickness direction of the bent part with a microscope. The maximum depth of the crack generated outside the bent portion is measured based on the interface between the plating layer outside the bent portion and the base material. When the embrittlement depth obtained in this way satisfies the relationship of the above equations (1) and (2), the Zn
-It is determined that the Al-based hot-dip coated steel sheet shows stable durability after long-term use by heating. This evaluation method estimates the progress of high-temperature embrittlement in long-term use exceeding 1000 hours by examining the change in the embrittlement depth after heating for 600 hours and after heating for 1000 hours. As a result of these experiments, it was found that the durability of the Zn-Al-based hot-dip coated steel sheet in actual use can be evaluated without any practical problem by this method.

[Operating Environment] High-temperature embrittlement of a Zn—Al-based hot-dip galvanized steel sheet causes almost no problem in practical use at a temperature lower than about 200 ° C. Further, when used in an environment exceeding 500 ° C., regardless of the Al concentration in the plating layer, an intermetallic compound with Fe is generated in the plating layer, and the original excellent properties of the Zn—Al-based hot-dip coated steel cannot be exhibited. . Therefore, the Zn-Al-based hot-dip galvanized steel sheet of the present invention having the performance of suppressing the progress of high-temperature embrittlement in
Its performance can be maximized when it is used for a long time in the temperature range of ~ 500 ° C.

The Zn—Al-based hot-dip coated steel sheet of the present invention
It is suitably used in applications such as automotive exhaust system members, home appliance members, and combustion device members.

[0035]

【Example】

Example 1 A cold-rolled steel sheet having a thickness of 1.0 mm was prepared from steel having the chemical composition shown in Table 1 by a conventional method.

[0036]

[Table 1]

These steel sheets were passed through a NOF type hot-dip coating line as a base steel sheet, recrystallized and annealed in an atmosphere of 50% by volume H ₂ -N ₂ , and then continuously immersed in a plating bath to obtain a Zn- An Al-based hot-dip coated steel sheet was obtained. The thickness of the plating layer is
It was in the range of 10 to 15 μm. The plating conditions are as follows.・ Reducing atmosphere: 50% by volume H ₂ —N ₂ , DP = -40 ° C. ・ Plating bath composition: 4.0% by mass Al-0.1% by mass Mg—Zn ・ Plating bath temperature: 450 ° C. Zn-Al-based hot-dip obtained Cut out a test piece of 40 × 100mm size from a steel plate and leave it as a flat plate (unprocessed) for 200 ~ 4
Heating was performed for 1000 hours in air at various temperatures of 50 ° C. Ten
The plated steel sheet after heating for 00 hours is subjected to 0t close contact bending as it is, and the metal structure of the cross section perpendicular to the thickness direction of the bent portion is observed with a microscope, and the maximum depth of a crack generated outside the bent portion is determined by the bent portion. The measured value is based on the interface between the outer plating layer and the base metal, and the measured value is the embrittlement depth (mm) of the unprocessed part of the steel sheet.
And Separately, a tensile test was performed using a tensile test piece cut out from each steel sheet after plating, and the workability of the steel sheet was evaluated by measuring the total elongation. Table 2
Shows the test results.

[0038]

[Table 2]

From Table 2, it can be seen that 4% A using the base steel sheet according to the present invention in which the P content was reduced and the B content was adjusted to an appropriate range.
The 1-0.1% Mg-Zn hot-dip coated steel sheet (No.2,7,8,9) has very little embrittlement at each temperature, and the elongation of the coated steel sheet exceeds 35%, which shows sufficient workability for practical use. Had.
On the other hand, those using a base steel sheet containing a large amount of P (N
o.3,4,10,11), although the embrittlement depth was kept low, the elongation of the plated steel sheet was low and the workability was poor. In addition, those using base steel sheets having low contents of P and B (Nos. 1, 5, and 6) were significantly embrittled.

Example 2 In the same manner as in Example 1, a base steel sheet having a thickness of 1.0 mm was prepared for each of the steels in Table 1, and Zn-A
An l-type hot-dip coated steel sheet was obtained. However, the plating conditions were as follows.・ Reducing atmosphere: 50% by volume H ₂ —N ₂ , DP = -40 ° C. ・ Plating bath composition: 55% by mass Al-1.5% by mass Si—Zn ・ Plating bath temperature: 600 ° C. The thickness of the plating layer is 15 to 18 μm Was in the range. The same test as in Example 1 was performed on the obtained Zn-Al-based hot-dip coated steel sheet. Table 3 shows the results.

[0041]

[Table 3]

As can be seen from Table 3, 55% Al-1.5%
The same results as in Example 1 were obtained for the unprocessed portion of the Si-Zn hot-dip coated steel sheet.

Example 3 Same as in Example 2 with 55% Al-1.
This time, the degree of embrittlement in the processed part of the 5% Si-Zn hot-dip coated steel sheet was examined. In order to simulate the actual use environment, each steel plate after plating has an inner radius (2
A 90 ° bend in mm R) was applied and the samples were heated at various temperatures from 200 to 450 ° C. for 1000 hours. A further 180 ° contact bending was performed on the bent portion of the sample after heating for 1000 hours, and the same microscopic observation method as in Example 1 was used.
The embrittlement depth of the processed part was determined. Table 4 shows the results. FIG. 1 shows an example of the result of the microscopic observation of the sample after heating at 400 ° C. for 1000 hours. Separately, 90
° After bending, heat the sample to 400 ° C,
The embrittlement depth after heating for 0 hours, 600 hours, and 1000 hours was determined by the same method as described above. FIG. 2 shows the change over time in the embrittlement depth for some samples.

[0044]

[Table 4]

From Table 4, it can be seen that 55% A using the base steel sheet according to the present invention in which the P content was reduced and the B content was adjusted to an appropriate range.
1-1.5% Si-Zn hot-dip coated steel sheet (No.2,7,8,9)
Shows that the embrittlement depth is kept low even in the processed part. On the other hand, in the case of using a base steel sheet containing a large amount of P (Nos. 3, 4, 10, and 11), the embrittlement of the unprocessed portion was suppressed low in Examples 1 and 2 described above. Nevertheless,
Remarkable embrittlement was observed in the processed part. Also, from FIG. 2, in the case of using the No. 2 and No. 7 base steel sheets according to the present invention, the progress of the embrittlement depth after 600 hours was hardly observed, and the heating for a long time exceeding 1000 hours was observed. It can be expected that the durability can be maintained stably in use.
On the other hand, in the case of using the base material steel sheet of No. 10 containing a large amount of P, embrittlement progressed even after 600 hours.

Example 4 From the results of Examples 1 to 3, it was found that a Zn-Al-based hot-dip coated steel sheet containing B in a base metal sheet was excellent in high-temperature embrittlement resistance even in a processed portion. , This time affecting the high temperature embrittlement resistance
AK steel and TK
A base steel sheet (cold rolled steel sheet having a thickness of 1.0 mm) with a varied B content was prepared by a conventional method based on steel. Table 5 shows their chemical compositions.

[0047]

[Table 5]

These steel sheets were recrystallized and annealed in a 50% by volume H ₂ —N ₂ atmosphere using a gas reduction type lab plating tester, and then immersed in a plating bath under the same atmosphere to obtain Zn—Al
A hot-dip galvanized steel sheet was obtained. The plating conditions are as follows.・ Reducing atmosphere: 50% by volume H ₂ —N ₂ , DP = −40 ° C. ・ Plating bath composition: 55% by mass Al-1.5% by mass Si—Zn ・ Plating bath temperature: 600 ° C. The thickness of the plating layer is 22 to 25 μm Was in the range. The obtained Zn-Al-based hot-dip coated steel sheet was subjected to 90 ° bending with an inner radius (2 mmR) twice as large as the sheet thickness, and the samples were subjected to 400 times.
Heated at a temperature of 1000C for 1000 hours. Further subjected to adhesion Bending the bent portion of the sample after heating 1000 hours, by a technique based on the same microscope as in Example 1 to obtain embrittlement depth of the processing member (corresponding to D _1000). The result is shown in FIG.

As can be seen from FIG. 3, when the B content of the base steel sheet is 0.0007% by mass or more, a high embrittlement suppressing effect that the TK steel-based plated steel sheet does not break after the close bending test even in the processed portion of the plated steel sheet. Admitted. Further, when the B content is 0.0013% by mass or more, the embrittlement depth becomes remarkably shallow, and Zn-
It was found that an Al-based hot-dip coated steel sheet was obtained.

[0050]

As described above, according to the present invention, Zn-
The phenomenon of high-temperature embrittlement, which had been a problem when using an Al-based hot-dip coated steel sheet at a high temperature for a long time, could be overcome without adding P to the base steel sheet. Since P is not added to the base steel sheet, the Zn-Al-based hot-dip galvanized steel sheet according to the present invention has excellent workability, and has a performance of remarkably suppressing the progress of high-temperature embrittlement even in the processed part. Therefore, the present invention enables a Zn-Al-based hot-dip coated steel sheet to be processed into a heat-resistant member having a complicated shape and used for a long time.

[Brief description of the drawings]

FIG. 1 is an optical micrograph showing a metal structure of a cross section perpendicular to the thickness direction of a contact bending portion.

FIG. 2 is a graph showing the relationship between the heating time and the embrittlement depth when a processed portion of a 55% Al-1.5% Si-Zn hot-dip coated steel sheet is heated at 400 ° C.

FIG. 3 is a graph showing the relationship between the B content and the embrittlement depth of a base steel sheet when a processed part of a 55% Al-1.5% Si—Zn hot-dip coated steel sheet is heated at 400 ° C. for 1000 hours.

Claims (9)

[Claims] 1. In mass%, C: 0.01% or less, S
i: 0.05% or less, Mn: 0.5% or less, P: 0.020% or less,
N: 0.0040% or less, B: 0.0007% to 0.0050%, Zn-Al containing 0.1 to 75% by mass of Al on the surface of a base steel sheet composed of Fe and inevitable impurity elements.
Excellent workability steel sheet with a hot-dip galvanized layer, Zn for heat-resistant members used in a high temperature environment of 200 to 500 ° C
-Al-based hot-dip coated steel sheet. 2. In mass%, C: 0.01% or less, S
i: 0.05% or less, Mn: 0.5% or less, P: 0.030% or less,
N: 0.0040% or less, B: 0.0007% to 0.0050%, and one or more of Ti, Nb and Zr in total C
+ N (mass%) at least 4 times as much as + N (mass%), with the balance being Al: 0.
A Zn-Al-based hot-dip galvanized steel sheet for a heat-resistant member used in a high-temperature environment of 200 to 500 ° C. 3. The Zn-Al-based hot-dip galvanized steel sheet for a heat-resistant member according to claim 1, wherein the B content of the base steel sheet is 0.0013 to 0.0050% by mass. 4. The hot-dip plating layer contains A
l: 0.1 to 75%, Si: 5% or less, Mg: 3
% Or less, Ti: 1% or less, Cr: 1% or less, with the balance being Zn and unavoidable impurities.
The Zn-Al-based hot-dip galvanized steel sheet for a heat-resistant member according to any one of claims 1 to 3, wherein the Zn-Al-based hot-dip steel sheet is an -Al-based hot-dip layer. 5. The Zn—Al-based hot-dip galvanized steel sheet for a heat-resistant member according to claim 1, wherein the thickness of the hot-dip coating layer is 5 to 60 μm. 6. The Zn-Al according to claim 1, wherein:
-Based hot-dip galvanized steel sheet having a thickness t (m
m) A sample that has been bent 90 ° with an inner radius twice as large as that of m) is heated in the air at 400 ° C and the embrittlement depth D ₆₀₀ (mm) after heating for 600 hours
And a hot-dip galvanized steel sheet for a heat-resistant member that satisfies the following expressions (1) and (2) when the embrittlement depth D ₁₀₀₀ (mm) after heating for 1000 hours is measured. D ₁₀₀₀ ≦ 0.20 × t ----- (1) 1.2 × D ₆₀₀ ≧ D ₁₀₀₀ ----- (2) 7. A composition containing B in an amount of 0.0007 to 0.0050% by mass,
A on the surface of low carbon steel sheet with a content of 0.020 mass% or less
A Zn-Al-based hot-dip galvanized steel sheet having a Zn-Al-based hot-dip coating layer containing 0.1 to 75% by mass is processed and used as a heat-resistant member in a high-temperature environment of 200 to 500 ° C. A method for using a Zn-Al-based hot-dip galvanized steel sheet for a heat-resistant member in which progress is suppressed. 8. The method according to claim 7, wherein the working includes plastic working. 9. The method of using a Zn—Al-based hot-dip coated steel sheet according to claim 7, wherein the heat-resistant member is an automobile exhaust system member.