JP3485713B2 - Aluminum-plated steel sheet excellent in brazing property and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum-plated steel sheet having excellent brazing properties and a method for producing the same.

[0002]

2. Description of the Related Art Aluminum-plated steel sheets are used in various fields such as pipes, vehicle parts, and building materials by taking advantage of the excellent corrosion resistance, heat resistance, oxidation resistance, and surface properties of aluminum plating layers. In particular, in an environment exposed to a high temperature oxidizing atmosphere, high temperature oxidation resistance and durability that cannot be obtained with other surface-treated steel sheets are exhibited. When using an aluminized steel sheet as a structural member or the like, it is necessary to join it with other members. Normally, an aluminum-plated steel plate member is joined to another dissimilar member by mechanical fastening such as bolts and rivets, but mechanical fastening requires a large number of steps and it is difficult to obtain the required joining strength. Then, for example, JP-A-62-238066
In the gazette, an aluminum plated steel material is joined to an aluminum material by brazing.

[0003]

When brazing an aluminum-plated steel sheet, the brazing temperature is set to a temperature slightly lower than the melting point of normal Al, specifically, a temperature range of 580 to 630 ° C. However, due to the heating during brazing, the interdiffusion reaction of Al and Fe progresses between the aluminum plating layer and the base steel, and the original excellent properties of the aluminum plating layer may be lost after brazing. For example, when Fe diffuses from the base material into the aluminum plating layer and reaches the surface of the plating layer, the silver white color peculiar to the aluminum plating layer changes to gray black. The gray-blackened plating layer not only significantly impairs the appearance, but also deteriorates the corrosion resistance and heat resistance. In addition, no brazing reaction occurs between the aluminum plating layer and the brazing material, which may prevent brazing. Even if it is brazed, the Al—Fe-based intermetallic compound layer grows thickly, and it is often impossible to obtain a joint having the bonding strength required for the application. As a result of detailed study on this point, the thickly grown Al—Fe-based intermetallic compound layer was found to show that the Al ₅ FeSi intermetallic compound layer grown in the initial stage of heating and the newly crystallized Al ₅ Fe _{2 layer.} It was found to be formed from two layers of the intermetallic compound layer. As the mutual diffusion reaction of Al-Fe progresses, Al ₅ F
Al ₅ crystallized between the eSi intermetallic compound layer and the base steel
Since the Fe ₂ intermetallic compound layer is extremely hard and brittle, it causes the strength of the brazed joint to be reduced. The present invention has been devised to solve such a problem, paying attention to the fact that the aluminum plated steel sheet is inferior in brazing property due to the diffusion reaction between the aluminum plated layer and the base steel, The purpose of the present invention is to improve the brazability of an aluminized steel sheet by forming an N-enriched layer that suppresses the reaction at the interface between the aluminized layer and the base steel.

[0004]

The aluminum-plated steel sheet of the present invention is 0.0075 to 0.0 in order to achieve its object.
A steel plate containing 200% by weight of N and 0.015 to 0.030% by weight of Al was used as a base material, and an aluminum plating layer containing Si: 5 to 15% by weight and having a film thickness of 7 μm or more was formed on the surface of the steel plate. , N: 3.0 at the interface between the steel plate and the aluminum plating layer
It is characterized in that an N-concentrated layer of atomic% or more is formed. The N enriched layer is based on a steel plate containing 0.0075 to 0.0200 wt% N and 0.015 to 0.030 wt% Al, and contains Si: 5 to 15 wt% and has a film thickness of 7 μm or more. After forming the aluminum plating layer on the surface of the steel sheet, when the lower limit temperature is 0.0075 wt ≤ N wt% ≤ 0.0200 wt%, T = -1414 x N ^1/2 + 500-35log (t / 5
0) [where t is the heating time (hours) and N is the N (nitrogen) content (%) of the steel sheet] regulated by temperature T (° C) to 570
It is formed by performing a heat treatment of heating in the temperature range of ° C for 0.5 to 50 hours.

The original plating plate used in the present invention must contain 0.0020% by weight or more of N in order to form an N concentrated layer at the interface between the aluminum plating layer and the base steel by the heat treatment after hot dip aluminum plating. Is. N content is 0.0020
If it is less than wt%, a sufficient N-enriched layer is not formed even by heat treatment, and the diffusion reaction easily occurs between the aluminum plating layer and the base steel due to heating during brazing. However, when the N content exceeds 0.0200% by weight, the steel plate itself becomes hard,
The forming process performed before brazing becomes difficult. Note that N
As long as the content is 0.0020% by weight or more, there is no restriction on the type of steel that can be used, and various types of steel plates such as ordinary steel, low alloy steel, high alloy steel, and stainless steel can be used as the original plating plate. However, Al existing in the steel reacts with N due to the heat history of the steel material and precipitates in the steel as AlN, which is an element that inhibits the formation of the N-enriched layer.
A steel material having an Al content of 0.030% by weight or less is preferable. In this case, 0.0075 to 0.0200% by weight of N
And a steel sheet containing 0.015 to 0.030% by weight of Al.

The aluminum plating layer formed on the surface of the steel sheet is
It has the effect of improving the affinity with the brazing material and suppressing an increase in the brazing temperature. That is, the brazing filler metal usually used for brazing aluminum contains several% of Si, and the brazing temperature is lowered by the inclusion of this Si. When brazing an aluminum-plated steel sheet using such a brazing material, if the aluminum plating layer does not contain Si, the temperature at which Si diffuses from the brazing material to the aluminum plating layer and the brazing material softens and melts There is no choice but to raise it.
As a result, the thermal deformation of the braze material increases,
Graying of the aluminum plating layer progresses due to Fe diffusion from the base material. Furthermore, since the Al-Fe-based intermetallic compound layer grows thick during hot-dip aluminum plating in the aluminum-plated layer that does not contain Si, if the molding process is performed before brazing and bonding, large cracks will occur in the plated layer, making brazing difficult. Become.

In this respect, in the present invention, S of the aluminum plating layer is
By setting the i content to 5% by weight or more, the diffusion of Si from the brazing material to the aluminum plating layer is suppressed and the brazing temperature is lowered. Therefore, even after brazing, the surface exhibiting a silver-white color peculiar to aluminum plating is maintained. Moreover, since the Al—Fe based intermetallic compound layer does not grow thick during hot dipping, it has good workability. However, when the Si content exceeds 15% by weight, Si diffuses from the aluminum plating layer to the brazing material, and the Si concentration in the brazing material increases,
The brazing material itself reduces mechanical properties, especially strength. As long as such characteristics are not impaired, in order to improve other characteristics, Mg, Zn, Cr, Mn, Sr, Sb, S
You may include n, Ti, etc. in a plating layer.

The aluminum plating layer also functions as a part of the brazing material, and enables brazing with a relatively low consumption of the brazing material. The aluminum plating layer has a film thickness of 7 μm or more and exhibits good wettability with respect to the brazing material. Therefore, in order to enable good brazing, it is necessary to form the aluminum plating layer with a thickness of 7 μm or more. On the other hand, the upper limit of the film thickness is not particularly specified. At the interface between the aluminum plating layer and the base steel, an N enriched layer of 3.0 atomic% or more is formed. The N-enriched layer acts as a barrier against Fe that tends to diffuse from the base steel to the aluminum plating layer, and suppresses the Al-Fe interdiffusion reaction that occurs between the aluminum plating layer and the base steel. As a result, the aluminum plating layer can be brazed like a normal aluminum material. N
The effect of the concentrated layer on the improvement of brazing property has been found by the present inventors, and the effect of the N concentrated layer becomes remarkable at a concentration of 3.0 atomic% or more.

In order for the N-enriched layer to effectively act as a barrier, the N-enriched layer having a concentration of 3.0 atomic% or more should be present at the interface between the aluminum plating layer and the base steel with a thickness of 50 Å or more. Is preferred. If the thickness of the N enriched layer is less than 50Å, the effect of suppressing diffusion of Fe from the base steel into the aluminum plating layer becomes weak. The N-concentrated layer has a lower limit temperature of 0.0075% by weight ≦ N% by weight ≦ 0.0200 when the heating time is t (hour)
T = -1414 x N ^1/2 + 500-35log in weight%
(T / 50) [where t is the heating time (hours) and N is the N (nitrogen) content (%) of the steel sheet] Temperature T (° C) regulated
It is formed by performing a heat treatment at 570 ° C. for 0.5 to 50 hours. The lower limit temperature is N as the N content increases.
Since the concentrated layer is easily generated, the concentrated layer shifts to the low temperature side according to the N content. However, when the heat treatment temperature is lower than the lower limit temperature, long-time heating of more than 50 hours is required to form the N-concentrated layer that suppresses the Al-Fe mutual diffusion reaction.
Considering the production on an industrial scale, the cost will increase significantly. On the other hand, if heat treatment is performed at a temperature above the lower limit temperature,
By heating for 50 hours or less, a 3.0 atomic% or more N-concentrated layer effective in suppressing the Al—Fe diffusion reaction is formed.

Further, when the heat treatment temperature exceeds the Al-Si eutectic temperature of 577 ° C., the plating layer itself begins to partially melt, resulting in coil adhesion during heat treatment, non-uniformity of plating layer thickness,
Various troubles such as local adhesion between the heating equipment and the aluminum-plated steel sheet are likely to occur. Therefore, in the present invention, the upper limit of the heat treatment temperature is set to 570 ° C. If the heating temperature is in the range of the lower limit temperature to 570 ° C.,
The N concentration layer of 3.0 atomic% or more is formed at the interface between the aluminum plating layer and the base steel, so that it is set in the range of 0.5 to 50 hours. The heat treatment atmosphere is not particularly limited, and when heating under the heat treatment conditions described above, H ₂ , N
₂ , regardless of the atmosphere such as Ar, vacuum, etc., an N-concentrated layer having a desired function is formed. The aluminum-plated steel sheet on which the N-enriched layer is formed in this way can be brazed in the same manner as ordinary aluminum material, and the surface after brazing has the original silver-white luster, and the original corrosion resistance of aluminum plating ，
Heat resistance and oxidation resistance are maintained.

[0011]

[Examples] Steel materials A to C whose compositions are shown in Table 1 were used as original plating plates, and soaking was performed at 720 ° C for 30 seconds in a 50% H ₂ -N ₂ atmosphere with a dew point of -40 ° C. Then, Si: 9.2 wt% and Fe: kept under the same atmosphere.
2 in an aluminum plating bath with a bath temperature of 650 ° C containing 1.7% by weight
Soaked for seconds, the Si content on the steel plate surface is 9.0% by weight, and the film thickness is 2
An aluminum plating layer of 0 μm was formed.

[0012]

The steel sheet plated with hot dip aluminum has a size of 25 mm ×
It was adjusted to a size of 100 mm, heated at a temperature rising rate of 100 ° C./hour, and heat-treated at 520 ° C. for 6 hours. In order to investigate the growth state of the alloy layer during brazing and heating of the heat-treated aluminized steel sheet, the temperature was raised at 10 ° C / sec,
Hold at 570-610 ° C for 5 minutes. When the effect of the heating temperature on the growth of the alloy layer was investigated, as shown in FIG. 1, the alloy layer grew thicker in Comparative Steel A with the increase in the heating temperature, whereas in Steel B according to the present invention. , C, the thickness of the alloy layer was substantially constant between 4 and 5 μm. Comparative steel A
For Steel C according to the present invention, the holding time at 600 ° C. was changed to 2, 5 and 20 minutes, and the cross section of the plating layer was observed.
The growth process of alloy layer was investigated. As can be seen from the investigation result of FIG. 2, in the comparative steel A, the Al—Fe-based intermetallic compound layer grows thickly in a short time, whereas in the steel C according to the present invention, the Al—Fe-based intermetallic compound layer grows even after heating for 20 minutes. It can be seen that the growth of the Fe-based intermetallic compound layer is suppressed.

The present inventors presumed that the cause of such a difference in the growth of the alloy layer was the surface condition of the base steel. Therefore, after removing the aluminum plating layer from the surface of the original plating plate of the present invention example, the concentration distribution of various elements in the depth direction from the steel surface was obtained by Auger spectroscopic analysis. In Steel B, it was found that N was concentrated in the surface layer as seen in the analysis result of FIG. On the other hand, in the same elemental analysis of Comparative Steel A, no N enrichment was detected. Also,
Even with Steel B, the N-enriched layer was not detected unless the heat treatment at 520 ° C. for 6 hours was performed. From this, it was confirmed that the alloying reaction of the aluminum plating layer was suppressed by the N concentrated layer. In FIG. 3, a sputtering time of 10 minutes corresponds to a thickness of 500Å in terms of Fe. Further, in order to investigate the influence of the heat treatment on the formation of the N-enriched layer, assuming that the aluminum-plated steel sheet having the steel B as a plating original plate is brazed, heat treatment is performed at 400 to 560 ° C. for 6 hours, and then 570 A heat resistance test was conducted at ˜600 ° C. for 5 minutes. The thickness of the alloy layer generated at this time is shown in FIG. As is clear from FIG. 4, in the case where the heat treatment was performed at 500 to 560 ° C. for 6 hours, the growth of the intermetallic compound layer was suppressed even after the heat resistance test at 570 to 600 ° C.

Steels A to H shown in Table 1 were aluminized under the same conditions to form an aluminized layer in which the thickness of the plated layer and the Si concentration in the plating bath were changed. The heat treatment conditions after aluminum plating were also changed. Then, in order to investigate the brazing properties of the aluminum-plated steel sheets, the aluminum-plated steel sheets were overlaid and brazed as shown in FIG. In brazing, a fluoride-based flux was applied to the mating surfaces, JIS A4045 alloy was used as a brazing material, and heating was performed in the atmosphere according to the heating pattern shown in FIG. The joint strength of the brazed material was measured by a tensile test. Good tensile strength of 10 kgf / mm ² or more was evaluated as ◯, and tensile strength was 10 kgf.
/ Less than mm ² △, no bonding at all ×
Was evaluated as 3 levels. As can be seen in Table 2 which shows the brazeability study results, the braze joint according to the invention is
It was a sound joint having a good joint strength with a tensile strength of 10 kgf / mm ² or more. Further, the growth of the alloy layer was not observed, and the aluminum plating layer had the original silver white color.

[0016]

Further, when the influence of the N content on the temperature of the N-concentration heat treatment is investigated, the upper limit of the temperature range of the heat treatment in which the growth of the Al--Fe intermetallic compound layer can be suppressed is 57.
Although it was a constant value of 0 ° C, as shown in Fig. 7, the lower limit temperature shifted to the lower temperature side as the N content increased. The relationship between the lower limit temperature T (° C.) and the N content (% by weight) was determined by multiple regression analysis. When the heating time was t (hour), 0.002% by weight ≦ N% by weight <0.005 In% by weight, T = -3848 × N ^1/2 + 672-35 log (t / 5
0), 0.005 wt% ≤ N wt% ≤ 0.020 wt%
Then, T = −1414 × N ^1/2 + 500−35 log (t /
The relationship of 50) was obtained. Therefore, the lower limit temperature of the N-concentration heat treatment is determined from the N content of the original plating plate, and the heat treatment is performed at a temperature not lower than this lower limit temperature and not higher than 570 ° C.
It was confirmed that an aluminum-plated steel sheet with good brazability was obtained.

[0018]

As described above, the aluminum-plated steel sheet of the present invention has the N-enriched layer formed at the interface between the aluminum-plated layer and the base steel, and therefore has excellent brazing properties and is equivalent to the aluminum material. You can braze. Further, the aluminum plating layer maintains a beautiful silver-white luster without losing the initial surface state even when heated during brazing.
Further, since the diffusion of Fe into the aluminum plating layer is suppressed,
Excellent corrosion resistance is maintained even after brazing. In this way, the aluminum-plated steel sheet of the present invention has a wide range of applications as a structural material for a portion where corrosion resistance, heat resistance, oxidation resistance, designability, etc. are required, particularly as a structural material suitable for joining different materials. Used in.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of heating temperature during brazing on the growth of an alloy layer.

FIG. 2 is a photograph showing a metallographic structure of a cross section of an aluminized steel sheet showing a process in which an alloy layer grows with the passage of heating time.

FIG. 3 is a graph showing the concentration distributions of Fe, Al and N when the steel material surface after heat treatment is subjected to elemental analysis in the thickness direction.

FIG. 4 is a graph showing the effect of heat treatment temperature on the degree of growth of an alloy layer grown at a brazing temperature (the numbers attached to each broken line indicate the heat resistance test temperature).

FIG. 5 is a perspective view of a brazing joint used in the embodiment.

FIG. 6 is a graph showing a heating pattern at the time of brazing adopted in the example.

FIG. 7: Effect of N content of original plating plate on temperature range of N concentration heat treatment

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 62-50454 (JP, A) JP 59-177355 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C23C 2/00-2/40 B23K 1/20

Claims (2)

(57) [Claims] 1. 0.0075-0.0200 wt% N
And a steel plate containing 0.015 to 0.030% by weight of Al as a base material, and an aluminum plating layer containing Si: 5 to 15% by weight and having a film thickness of 7 μm or more is formed on the steel plate surface. An aluminum-plated steel sheet having an excellent brazing property in which an N-concentrated layer of N: 3.0 atomic% or more is formed at the interface with the layer. 2. 0.0075-0.0200 wt% N
When, the steel containing 0.015 to 0.030 wt% of Al and matrix, Si: After the aluminum plating layer over a thickness of 7μm comprises 5 to 15 wt% is formed on the surface of the steel sheet, the lower limit temperature
0.0075 wt% ≤ N wt% ≤ 0.0200 wt%
Is T = -1414 × N ^1/2 + 500-35log (t / 5
0) [however, t is the heating time (hours), N is the N of the steel sheet (nitriding)
Elementary) content (%) ] regulated by temperature T (° C) to 570
A method for producing an aluminum-plated steel sheet excellent in brazability, which comprises performing N-concentration heat treatment of heating in a temperature range of ° C for 0.5 to 50 hours.