JP3383122B2 - Hot-dip aluminized steel sheet with excellent heat resistance, high temperature strength, and corrosion resistance, and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is mainly applied to automobile exhaust system materials, home appliances, various heat appliances, building materials, etc.
The present invention relates to a hot-dip aluminized steel sheet excellent in high temperature strength and corrosion resistance and a method for producing the same.

[0002]

2. Description of the Related Art A hot-dip aluminized steel sheet is an aluminum-plated layer mainly composed of aluminum (hereinafter referred to as a plated layer), and a layer composed of an intermetallic compound which is a reaction product of the steel sheet to be plated and aluminum (hereinafter referred to as an alloy layer). It is known that the steel sheet having a) has excellent heat resistance and corrosion resistance. Taking advantage of these features, it is widely used in automobile exhaust system materials, home appliances such as heat appliances, and building materials such as roofs and walls. Stainless steel sheets are similarly excellent in heat resistance and corrosion resistance, but hot-dip aluminized steel sheets are cheaper than stainless steel sheets, and their applications are spreading in recent years. However, the need for products having further improved heat resistance and high-temperature strength has expanded, and many inventions have been made to add various elements to an aluminum-plated original plate. For example, the applicant of the present invention added Ti to a plating original plate to increase heat resistance (Japanese Patent Publication No. 5-21
No. 978), and Ti, M for increasing high temperature strength.
Those to which elements such as n and P are added (Japanese Patent Publication No. 2-61541)
Gazette) has already been disclosed. On the other hand, there are many examples in which elements are added to the aluminum plating bath to improve the corrosion resistance. For example, Japanese Examined Patent Publication No. 63-23264 discloses Si: 3.
%, Mn: 0.5 to 4%, and an example of a steel sheet having a plating layer, and Japanese Patent Publication No. 6-11906 discloses C.
There is a method for producing a plated steel sheet in which r: 0.01 to 2% is added to the plating bath.

[0003]

However, the above-mentioned invention has the following problems. For example, T
Although the heat resistance improving effect by i is effective against plating defects such as non-plating, it was not different from ordinary aluminum plated steel sheets from the viewpoint of heat resistance of plating itself. Further, Ti has been added to the steel having increased high temperature strength in order to ensure heat resistance, and the same problem has been encountered in terms of heat resistance. On the other hand, even in the invention in which the element is added to the plating bath, the addition amount of the element to the plating bath needs to be limited to a certain amount in order not to raise the temperature of the plating bath, and the addition is sufficient to sufficiently enhance the corrosion resistance. There is a problem that it is difficult to add the amount.

[0004]

The present invention provides a hot-dip aluminized steel sheet having excellent heat resistance, high temperature strength and corrosion resistance, which overcomes the above-mentioned drawbacks. The present inventors have repeatedly conducted various experiments on the properties of the steel composition, the plating layer, and the alloy layer that affect various properties of the hot-dip aluminized steel sheet, and have obtained the following findings. That is, M in the aluminum plating bath
It has been found that when n and Cr are added in combination, these elements are not uniformly dispersed in the plating layer but are significantly concentrated in the alloy layer. This is a phenomenon observed remarkably when the elements are added in combination. Specifically, the concentration of these elements in the plating layer is about 1/5 to 1/10 of the addition amount,
The rest is concentrated in the alloy layer. These elements are particularly concentrated in the upper part of the alloy layer, that is, at the interface between the plating layer and the alloy layer. Therefore, it becomes possible to create a layer with an increased concentration of additional elements that are effective in corrosion resistance and heat resistance, and a hot-dip aluminized steel sheet with a harder composition has significantly better corrosion resistance and heat resistance than conventional steel sheets. I found it to have.

Further, both Sn and Zn in the plating bath are elements which significantly impair the corrosion resistance of the aluminized steel sheet. Therefore, the sum of these impurity elements needs to be limited to a certain amount or less. As described above, the effect of improving corrosion resistance and heat resistance by adding Mn and Cr is M at the interface of the plating layer-alloy layer.
It is speculated that this is because the n, Cr concentrated layer significantly contributes to the heat resistance of the steel sheet or the corrosion resistance when corrosion progresses.

The heat resistance improving effect of the Mn, Cr concentrated layer described above is due to the plating layer itself. However, by using Ti-added steel as a plating base plate for a plating bath that becomes difficult, abnormal oxidation due to plating defects can be suppressed, and overall excellent heat resistance can be obtained. That is, when a steel containing Ti as a plating base plate and containing Mn, P, B, etc. and having excellent high temperature strength and a Mn, Cr addition bath as a plating bath are used, the heat resistance of the plating layer itself is improved. In addition, it becomes possible to obtain the heat resistance of the non-plated portion and at the same time obtain a hot dip aluminum plated steel sheet having excellent high temperature strength. Thus, the above-mentioned plated steel sheet contains a specified amount of C, Mn, Si, Ti, N, Al, B as a plating original plate, and a specified amount of a specified amount of Mn, Cr, Fe, Si was added to the plating bath. In addition, it can be manufactured by limiting the sum of impurities Sn and Zn to a specific amount.

The present invention will be specifically described below. According to the present invention, as a plating original plate, C: 0.02% or less, Mn:
0.6-2%, Si: 0.1-1.5%, Ti: 0.1
~ 0.5%, N: 0.004% or less, Al: 0.01 ~
A steel sheet containing 0.08% and B: 0.003% or less, and the balance being substantially Fe and impurity elements,
The surface has an average composition of Fe: 25 to 50%, Si: 3 to
18%, Mn: 0.1 to 5%, Cr: 0.05 to 0.8
%, The balance being substantially Al, and having an alloy layer having a thickness of 5 μm or less, and further having Si: 2 to 12 on the surface thereof.
%, Fe: 1% or less, Mn: 0.005-0.3%, C
There is provided a hot-dip aluminized steel sheet having a plating layer in which r: 0.002 to 0.05%, the balance substantially consisting of Al, and the total of Sn and Zn in impurities is 1% or less. Further, as a method for producing such a hot dip aluminum plated steel sheet, the above steel sheet is Si: 3 to 12%, Fe: 0.5 to
2.5%, Mn: 0.05 to 1.0%, Cr: 0.02
Provided is a manufacturing method of plating with a plating bath in which the sum of Zn and Sn in impurities is 1% or less, the balance being ˜0.15% and the balance substantially consisting of Al.

The reasons for limitation of the present invention will be described below.
First, the composition of the original plating plate will be described. C: As the C content increases, the grain boundary precipitated carbides generally increase,
Promotes intergranular corrosion of steel. Further, C is an element that impairs heat resistance. From these meanings, it is desirable that C is small, and in the present invention, it is limited to 0.02% or less. Mn: Mn is an element that contributes to the normal temperature and high temperature strength of the steel sheet, and is required to be 0.6% or more to secure the strength at 600 ° C. or higher. On the other hand, Mn is an element that impairs the formability of steel at the same time, and if it is contained in excess of 2%, the workability is significantly deteriorated. Therefore, the upper limit is set to this amount.

Ti: Ti is an element that reacts with C, N in steel or O invading from the outside to improve the heat resistance of the aluminum-plated steel sheet. For this effect, the amount of Ti is C
And N are required to be about 20 times the total amount, and the lower limit is 0.1% as the necessary content corresponding to the industrially reducible value of C and N (C + N: 0.003 to 0.004%). did. On the other hand, the effect of Ti on heat resistance is saturated, so the upper limit is 0.5%.
And N: N is an element that hinders the workability of steel and is an element that combines with Ti to increase the required amount of Ti, so its lower content is desirable. Therefore, it is limited to 0.004% or less. Al: Al is added in order to adjust oxygen in the steel in the refining process of molten steel, but if it is too much, it impairs aluminum plating property and causes non-plating, and it also hinders workability of steel plate. Therefore, the lower limit is 0.01% and the upper limit is 0.
It was set to 08%.

In the present invention, one or more of the following components may be added in addition to the above components. Si: Si is an element that increases the strength at room temperature and high temperature. Although the strength increases as the added amount increases, Si is also an element that forms Si oxides on the surface of the steel sheet in the plating process and impedes the wettability of the plating. Therefore, the upper limit is 1.
Limited to 5%. P: P also increases the normal temperature and high temperature strength like Si. The greater the amount added, the greater the strength, but the addition of 0.1
When it exceeds%, the weldability is deteriorated, for example, the spot weld nugget portion is cracked. Therefore, the upper limit is made 0.1%. B: B precipitates as a B compound at the crystal grain boundaries, suppresses crystal grain coarsening at high temperatures, and has an effect of improving high temperature strength. However, if the added amount is large, quenching is performed due to welding heat, etc., and it is excessively hardened and the ductility of the welded portion is impaired. Therefore, the upper limit is limited to 0.3%.

Next, the composition of the plating layer and the bath composition of the manufacturing method will be described. Si: As described above, a very hard and brittle alloy layer is formed on the hot dip aluminized steel sheet in addition to the aluminum plating layer, and this layer hinders the plating adhesion. In order to reduce this effect, about 10% of Si is usually added to the plating bath to suppress the thickness of the alloy layer. Also in the present invention, Si is added for the same purpose. For this purpose, the minimum amount of Si in the plating bath is 3%.
The i amount is 2% or more. On the other hand, if Si is added too much, coarse primary crystal Si is generated in the plated layer and adversely affects the corrosion resistance, so the upper limit is made 12%. Si in the plating layer at this time
The amount is about 12%.

Fe: Fe is eluted from the plating original plate or the equipment in the bath and is not particularly positively added in the present invention. Usually, the plating layer also contains about 0.3 to 0.8%. Fe is preferable because it has a bad effect on the corrosion resistance, and the upper limit is 1% in the plating layer. Originally, the smaller the amount, the better. However, as described above, it is difficult to completely remove the element that is inevitably mixed. It is an unavoidable element even in the bath, and it is almost impossible to remove it. If it is forcibly reduced, the equipment in the bath is likely to be melted, so the lower limit value in the bath is set to 0.5%. The upper limit value in the bath is set to 2.5% because appearance stains due to corrosion resistance inhibition or dross appear.

Mn: This element is of particular importance in the present invention. It is an element that has a remarkable effect on corrosion resistance, heat resistance, etc. when it is concentrated in the alloy layer, and at least 0.05% is necessary in the plating bath to exert its effect. When plating is performed in this plating bath, 0.005% is contained in the plating layer, so this concentration is the lower limit value in the plating layer. On the other hand, the solubility of Mn in the plating bath is about 0.6% at a normal plating temperature of 650 ° C. A1-Mn Binary phase diagram M
Although the solubility of n is about 1%, it seems that the solubility decreases in a bath containing about 10% Si. M
To dissolve n by 0.6% or more, it is necessary to raise the bath temperature,
Then, the alloy layer becomes thick and easy to grow, which causes a problem that the plating adhesion is deteriorated. Therefore, the upper limit of the Mn concentration in the bath is 1%. The maximum Mn concentration in the plating layer when plating is performed in this bath is about 0.3% , which is the upper limit of Mn in the plating layer.

Along with Cr: Mn, it is also an important element in the present invention. Cr has a particularly large effect on the corrosion resistance and has the effect of concentrating Mn in the alloy layer. In order to expect the effect of improving corrosion resistance, 0.002% or more is required in the plating layer. For this purpose, it is necessary to add 0.02% or more to the bath. Similar to Mn, Cr also has a low solubility in a plating bath, which is about 0.1% at 650 ° C., and if it is attempted to dissolve Cr further, the bath temperature must be raised. Then, the alloy layer grows thickly, so 0.15% is made the upper limit of the Cr content in the bath. Since the Cr content in the plating layer at this time is about 0.05%, this value is set as the upper limit of the Cr content in the plating layer. In the Al-Cr binary phase diagram, the solubility of Cr in Al is 0.
Although it is 4%, it seems that the solubility is lowered for the same reason as Mn.

The reason why both elements are concentrated in the alloy layer when Cr and Mn are added in combination is not yet clear at present, but a stable metal of the Cr--Mn--Fe (--Al--Si) system is used. It is considered that Cr and Mn move to the original plate side with a high Fe concentration due to the formation of intermetallic compounds. Zn, Sn: All of these are elements that significantly impair the corrosion resistance of Al and accelerate the occurrence of white rust. Therefore, the sum of these elements is limited to 1% or less in both the plating layer and the bath.

Next, the reasons for limiting the composition of the alloy layer will be described. Si: As described above, Si: 3 to 12% is added to the plating bath for the purpose of suppressing alloy layer growth. At this time, the Si concentration in the alloy layer is 3 to 18%. Therefore S in the alloy layer
i is limited within this range. The Fe: alloy layer is formed mainly by the reaction between Al of the plating bath and Fe of the original plate. At this time, the Fe concentration in the alloy layer becomes 25 to 50%. Therefore, Fe in the alloy layer is limited to this range.

Mn: Mn added to the bath is concentrated in the alloy layer by the effect of Cr as described above. Due to this effect, various properties such as corrosion resistance, oxidation resistance, and adhesion are greatly improved.
A minimum of 0.1% Mn is required for these effects to appear. On the other hand, since the Mn concentration in the bath has an upper limit as described above, the Mn concentration in the alloy layer also has an upper limit. This is 5
%. Like Cr: Mn, Cr also concentrates in the alloy layer. Cr is also considered to have an effect on corrosion resistance, and the effect is exerted at 0.05% or more. The upper limit of Cr also depends on the amount of Cr that can be added to the plating bath and is 0.8%.

The upper limit of the alloy layer thickness is 5 μm because if it is too thick, the plating adhesion is impaired. Since the alloy layer hinders the plating adhesion, it is preferable that the alloy layer be thin. Under normal operating conditions, the alloy layer thickness is 2 to
It is 3 μm. The aluminum-plated steel sheet according to the present invention has excellent corrosion resistance. The reason for this is considered to be that the concentrated Mn and Cr in the upper part of the alloy layer (near the interface with the plating layer) have a great influence, as described above. A feature of the product of the present invention is that the propagation of corrosion from the end faces and scratches is greatly suppressed, and high corrosion resistance can be expected in a molded product including a flaw during molding and a spot welded portion. The product of the present invention may be subjected to zero spangle treatment to make its appearance beautiful. Also, for the purpose of initial rust prevention, it is not detrimental to the gist of the present invention to use chromate containing chromic acid as a main component for use.

[0019]

EXAMPLES Next, the present invention will be described in more detail by way of examples. Hot-rolled and cold-rolled steel sheets having a thickness of 0.8 mm and having several types of steel components as shown in Table 1 are used as plating base plates, and hot dip aluminum plating is performed in a non-oxidizing furnace-reducing furnace type line. It was After plating, the coating weight was adjusted to about 120 g / m ² on both sides by the gas wiping method, and after cooling, it was wound. At this time, Si, Mn, and Cr were added as plating bath components to perform plating, and plating with a good appearance was possible. The aluminum plated steel sheet thus obtained was evaluated. The evaluation method is shown below. The manufacturing conditions are summarized in Table 2 and the evaluation results are summarized in Table 3.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

(1) Method for analyzing composition of plating layer and alloy layer Plating layer: 3% NaOH + 1% AlCl ₃ .4H ₂ O
Only the plating layer was peeled off by electrolytic peeling, and each element was quantitatively analyzed as a plating layer composition analysis liquid. Alloy layer: After the above electrolytic stripping, the alloy layer was stripped with chemical conversion soda to obtain an alloy layer composition analysis liquid, and quantitative analysis of each element was performed.

(2) Corrosion resistance test: The following three tests were conducted. JIS Z 23 for salt spray test size 70 × 150 mm sample
The salt spray test according to No. 71 was performed for 30 days, and the corrosion weight loss was measured. Automobile exhaust system simulated condensed water immersion test A sample having a size of 70 × 150 mm was immersed in the solution of Table 4 for 30 minutes and dried at 70 ° C. for 30 minutes. 1000 this cycle
The cycle was repeated, and the corrosion weight loss after the test was measured.

[0025]

[Table 4]

(3) Plating adhesion: The following two types of tests were conducted. Reverse Bend Test A test piece was subjected to impact bending in a shape as shown in FIG. 1 and the state of plating peeling at the bent portion was observed to give a rating. The criteria for rating are shown below. Rating Criteria 1 No abnormality 2 Cracks in plating layer 3 Spot plating peeling 4 Foil plating peeling 5 Full surface plating peeling Cup drawing test Blank diameter: 50 mm Drawing depth: 10 mm Die shoulder radius: 2 mm Punch diameter: 33 mm Drawing was performed under the above conditions, and the state of plating peeling on the side surface was observed. The scoring criteria are the same as in the reverse bend test.

(4) Heat resistance test A sample having a size of 100 × 100 mm was held at 800 ° C. for 48 hours, and then cooled. One cycle was repeated, and this cycle was repeated 5 times to measure the increase in oxidation after the test. (5) Press-moldability Moldability was evaluated by forming a crack with a diameter of 80 mm and a depth of 40 mm. ◯: No cracking ×: Cracking occurred

(6) Weldability I Weld butt TIG welding was performed, and the Erichsen test was performed on the welded portion to evaluate ductility. Welding conditions: Current 95A, voltage 11V, welding speed 300mm
/ min Arc length 1.5 m (7) High temperature strength test A high temperature tensile test was performed at a temperature of 600 ° C to evaluate the tensile strength and elongation.

[0029]

The aluminized steel sheet obtained by the present invention has excellent heat resistance and high temperature strength. Furthermore, the plate thickness phenomenon due to corrosion is small even in severe environments such as salt spray and immersion environment of automobile exhaust system condensed water. Therefore, this material is considered to be extremely promising as an automobile exhaust system material that can replace stainless steel.

[Brief description of drawings]

FIG. 1 is an explanatory view of a forming shape and a procedure of a reverse bend method which is a plating adhesion evaluation method.

─────────────────────────────────────────────────── --- Continuation of the front page (56) Reference JP-A-5-287492 (JP, A) JP-A-62-199759 (JP, A) JP-A-60-165366 (JP, A) JP-A-6- 330274 (JP, A) JP-A-8-311629 (JP, A) JP-A-8-319549 (JP, A) JP-A-8-319550 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] 1. A steel composition in% by weight, C: 0.02% or less Mn: 0.6 to 2% Ti: 0.1 to 0.5% N: 0.004% or less Al: 0.01 to In addition to 0.08%, Si: 1.5% or less P: 0.1% or less B: 0.0003% or less One or more kinds are contained, and the balance is substantially Fe and unavoidable. On the surface of the steel sheet which is an impurity element, the average composition is% by weight, Fe: 25-50% Si: 3-18% Mn: 0.1-5% Cr: 0.05-0.8% The balance is substantially Has an intermetallic compound coating layer that is made of Al and has a thickness of 5 μm or less, and further, on the surface of the intermetallic compound coating layer, by weight%, Si: 2 to 12% Fe: 1% or less Mn: 0.005-0.3% Cr: 0.002-0.005% The balance consists essentially of Al and Z in the impurities , S
A hot-dip aluminized steel sheet having excellent heat resistance and corrosion resistance, which has a coating layer having a total n content of 1% or less. 2. A steel composition in% by weight, C: 0.02% or less Mn: 0.6 to 2% Ti: 0.1 to 0.5% N: 0.004% or less Al: 0.01 to In addition to 0.08%, Si: 1.5% or less P: 0.1% or less B: 0.0003% or less One or more kinds are contained, and the balance is substantially Fe and unavoidable. In the steel sheet which is an impurity element, Si: 3-12% Fe: 0.5-2.5% Mn: 0.05-1.0% Cr: 0.02-0.15% The balance is substantially more than Al. And Zn, S in impurities
A method for producing a hot-dip aluminized steel sheet having excellent heat resistance and corrosion resistance, which comprises hot-dip aluminum plating in a plating bath having a total n content of 1% or less.