JP2765078B2 - Alloyed hot-dip coated steel sheet and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a surface-treated steel sheet excellent in corrosion resistance and film surface smoothness, and more specifically, rust-proof steel sheet for automobile bodies, building materials, appliances for home appliances and the like. The present invention relates to a method for producing an alloyed galvanized steel sheet and an alloyed galvanized steel sheet that are suitable as the method.

(Prior Art) Various types of surface-treated steel sheets are used in fields such as automobile bodies, building materials, home electric appliances and the like. In particular, hot-dip galvanized steel sheets, which are relatively inexpensive and have good corrosion resistance, are widely used. Also, a galvanized steel sheet obtained by alloying a galvanized film by subjecting this hot-dip galvanized steel sheet to thermal diffusion treatment, a so-called alloyed galvanized steel sheet, is relatively inexpensive and has excellent corrosion resistance, paintability, weldability, etc. Widely used in similar fields.

However, in these fields of use, the required level of rust prevention performance and surface quality for surface-treated steel sheets tends to increase year by year, and it is difficult for conventional hot-dip galvanized steel sheets and alloyed galvanized steel sheets to meet the required levels. It is becoming. Therefore, various new surface-treated steel sheets have been developed. For example, hot-dip zinc-based alloys such as Zn-Al, Zn-Mg, and Zn-Mn. However, steel sheets obtained by hot-dip coating these zinc-based alloys have the following problems in terms of quality or production, and their use is limited.

That is, Zn-Al alloy-plated steel sheets have already been put into practical use, and although corrosion resistance is superior to galvanized steel sheets, there is a problem that the coating surface is rough and the surface quality is inferior, so that alloying treatment cannot be performed. When this plated steel sheet is alloyed, the alloying reaction becomes non-uniform and the surface quality is further deteriorated.

For Zn-Mn alloy plated steel sheet, Mn is 0.05 to 15% by weight.
Japanese Patent Application Laid-Open No. 54-11836 discloses an alloyed steel sheet containing a hot-dip Zn-Mn alloy. In the case of Zn-Mn alloy plating, hot-dip plating is possible because Mn has a relatively high solubility in the liquid phase of Zn. However, dross composed of a ternary compound of Zn-Al-Mn in which Al is present in the plating bath is generated, and the fluidity of the plating solution is reduced, or the dross adheres to the film surface to deteriorate the surface quality. There is a problem.

Generally, in hot-dip galvanizing, a galvanizing bath containing 0.1 to 0.2% by weight of Al is used. This is because, in hot-dip galvanizing, a hard and brittle Fe-Zn alloy layer is generated at the interface between the base steel sheet and the plating layer during plating, but when this alloy layer is formed thickly, it becomes a poorly workable plating layer. In order to improve the performance, the formation of an alloy layer is suppressed as much as possible by adding Al,
It is thin. However, if Al is added to the Zn-Mn alloy plating bath for the same purpose, dross composed of a ternary compound is generated.

Also, if the amount of added Mn exceeds a threshold determined by the Al concentration, alloying is promoted during plating, which is preferable for the production of alloyed steel sheets, but elution of Fe from the base material during hot-dip plating is not preferable. It becomes intense, and a large amount of Fe-based dross is generated, thereby greatly reducing operability. Furthermore, the diffusion rate of Fe-Zn becomes excessive during the alloying treatment,
The film becomes easy to be powdered during press working.

Here, the threshold value [Mn ^* ] is a value represented by the following or an expression.

When the Al concentration is 0.3% or less: [Mn ^* ] (%) ≒ 3.5 [Al (%)] ^1.5 ... When the Al concentration is more than 0.3% and 5% or less: [Mn ^* ] (%) ≒ 0.6% ... However, all (%) are% by weight.

In the above-mentioned Zn-Mn alloy plating, generation of dross composed of the former ternary compound is a serious problem. The amount of dross generated increases as the Al concentration and Mn concentration in the bath increase. On the other hand, increasing the bath temperature tends to alleviate this problem, but increasing the bath temperature increases the generation of Fe-based dross. In addition, in the case of Zn-Mn alloy plating, if the amount of Mn added exceeds the above-mentioned threshold value, the alloy layer formed at the time of plating grows very thick, so that the workability of the film is extremely poor even with a non-alloyed material.

Regarding Zn-Mg alloy plated steel sheet, for example,
No. 120241 and Nos. 64-41359 have been reported, but the oxidation of the plating bath is severe, and zinc oxidation occurs even when plating in a low oxygen concentration atmosphere.
Since the operability is reduced, it is difficult to commercialize this Zn-Mg alloy plating.

(Problems to be Solved by the Invention) An object of the present invention is to provide a surface-treated steel sheet excellent in corrosion resistance, film surface smoothness and workability, specifically, an alloy which has been subjected to alloying treatment after hot-dip plating excellent in corrosion resistance. It is an object of the present invention to provide a method for efficiently producing a galvanized steel sheet and its alloyed galvanized steel sheet with good quality.

(Means for Solving the Problems) In the case of the above-mentioned Zn-Mn alloy plated steel sheet, Al is contained in the plating bath.
When coexisting, dross consisting of a ternary compound of Zn-Mn-Al is generated in the liquid phase, or the reaction between the base metal Fe and the plating bath is excessively promoted, resulting in the generation of Fe-based dross. There is.
However, the Zn-Mn-based alloy has a lower oxidation consumption rate of the plating bath than the Zn-Mg-based alloy, and is relatively easy to alloy as compared with the Zn-Al-based alloy. Thus, the present inventors have focused on Zn-Mn-based ones and have attempted to overcome the above problems.

That is, the present inventors, even if Al coexists in the Zn-Mn plating bath, or a Zn-Mn-Al-based bath in which Al is actively added for the purpose of enhancing corrosion resistance, Si It has been found that the above problems can be prevented by the addition of, and the present invention has been accomplished.

Here, the gist of the present invention includes the following (i) and (ii)
It is in.

(I) The steel sheet is passed through a plating bath containing Al: 0.05 to 5% by weight, Si: 0.005 to 0.8% by weight, Mn: 0.1 to 3% by weight, the balance being Zn and unavoidable impurities, and hot-dip plating. And then performing an alloying treatment.

(Ii) A hot-dip coated steel sheet which has been alloyed by a thermal diffusion process, wherein the coating composition of the coated steel sheet is A
l: 0.1 to 5% by weight, Si: 0.005 to 0.8% by weight, Mn: 0.1 to 3% by weight, Fe: 7 to 15% by weight, balance: Zn and inevitable impurities .

(Operation) Hereinafter, the present invention will be described in detail.

 First, the first invention of the present application will be described.

The first invention is an invention of a method for manufacturing an alloyed hot-dip coated steel sheet in which an alloying treatment is performed after hot-dip coating is performed. The feature of this method is that a plating bath having a composition consisting of Al: 0.05 to 5% by weight, Si: 0.005 to 0.8% by weight, Mn: 0.1 to 3% by weight, Zn and inevitable impurities: balance And hot-dip plating.

The plating system is not particularly limited, and a Zenzimer system, a flux system, or the like is applied. The steel sheet before plating is naturally subjected to pretreatment. For example,
In the case of the Sendzimer method, the steel sheet is passed through a plating bath after being subjected to a pretreatment such as an oxidation / reduction treatment.

In this method, the reasons for limiting the plating bath composition as described above are as follows.

Al is effective for improving the workability and corrosion resistance of the film.
However, if the content is less than 0.05% by weight, the processability of the film is reduced, and the generation of dross is promoted, resulting in a decrease in operability. on the other hand,
If the content exceeds 5% by weight, there is no effect of improving the corrosion resistance and the operability decreases.

Si suppresses abnormal growth of the Fe-Zn layer in the Mn added system,
It is effective for improving workability. In particular, in the present invention in which Mn coexists, Si is indispensable because Mn promotes the formation of the Fe-Zn layer. If the content of Si is less than 0.005% by weight, the above effect is not obtained. If the content is more than 0.8% by weight, non-plating points tend to occur.

Mn is effective in improving corrosion resistance in both non-alloyed and alloyed materials. However, if it is less than 0.1% by weight, the improvement in corrosion resistance is small, and if it exceeds 3% by weight, Fe-
Since the formation of the Zn alloy layer becomes excessive, the workability of the film is reduced.

The balance of the plating bath composition is Zn and unavoidable impurities.
It should be noted that Al has a property of being enriched in the film,
Even if Al is 0.05% by weight, 0.1% or more of Al can be precipitated in the coating, so that an alloyed hot-dip galvanized steel sheet can be manufactured.

In the alloying treatment after hot-dip plating, the Fe concentration in the coating is 7 ~
It is desirable to carry out so as to be 15% by weight. The alloying may be performed by subjecting the plated steel sheet to a heat treatment in a galvanile furnace provided immediately above the plating tank, or a heat treatment in a batch furnace outside the line. When the alloying treatment is performed in an in-line galvanic furnace, the heat treatment is preferably performed so that the temperature of the steel sheet material becomes 470 to 600 ° C, preferably 470 to 510 ° C. When using a batch furnace, the alloying treatment can be performed at a temperature of 300 to 400 ° C.

The second invention is an invention of a galvannealed steel sheet itself. The features of this alloyed hot-dip coated steel sheet are that the coating composition is as follows: Al: 0.1 to 5 wt%, Si: 0.005 to 0.8 wt%, Mn: 0.1
~ 3% by weight, Fe: 7 ~ 15% by weight, Zn and inevitable impurities:
The rest is to be.

The reasons for limiting the coating composition after alloying as described above are as follows.

Al is effective in improving the corrosion resistance of the plating film. However, if it is less than 0.1% by weight, a hard and brittle Fe-Zn alloy layer is formed thickly at the interface between the base steel sheet and the plating layer during plating, and the formation of a Γ phase having a high Fe concentration is particularly promoted during the alloying treatment, Decreases the workability of the coating, for example, the powdering resistance. On the other hand, if it exceeds 5% by weight, a long time is required for the alloying treatment. Al has a function of suppressing the alloying reaction, and Mn suppresses the alloying up to a specific concentration determined by the Al concentration as described above. However, when the threshold value is exceeded, the alloying has an effect of accelerating the alloying. Even if the content of Mn is in a range that promotes alloying, if the Al content exceeds 5% by weight, the effect of suppressing alloying of Al is stronger than the effect of promoting alloying of Mn, so that alloying is delayed.

Mn has the effect of improving the corrosion resistance like Al. But,
If it is less than 0.1% by weight, the improvement in corrosion resistance is small, and
Since it is less than the threshold value, alloying is delayed. On the other hand, if it exceeds 3% by weight, even in the coexistence of Si, the ternary compound of Al-Mn-Zn tends to precipitate in the liquid phase, and the fluidity of the plating bath and the surface quality of the plated steel sheet deteriorate.

Si is an important element in the present invention, and controls the influence of Mn on the alloying speed. Mn suppresses alloying up to a certain concentration as described above, and conversely, if this threshold is exceeded, alloying is promoted.However, excessive promotion of alloying lowers the workability of the plating layer, and during plating, The elution of Fe from the steel sheet into the plating bath is increased to generate Fe-based dross. Si has an effect of suppressing excessive alloying by Mn to an appropriate one and suppressing elution of Fe.

However, if the content is less than 0.005% by weight, the above effect is not obtained.
If the content exceeds 0.8% by weight, the effect will be saturated,
Metals are separated in the liquid phase, resulting in poor wet spots,
That is, micro-unplated points are easily formed. And if such a bear spot exists, since a steel base material is exposed, corrosion resistance will fall.

If the content of Al exceeds 5% by weight, a
In Al alloy plating, a small amount of Si may be added to the plating bath in order to suppress the formation of the Fe-Al alloy phase, but it is not necessary to add Al in a system with 5% by weight or less. Was thought. Assuming that Si is less than 5% by weight of Al
Even when added to the Zn-Al alloy plating bath, the solubility of Si is low,
Only dispersion increases the formation of bare spots, but no effect of Si is obtained. However, Mn and Si
When Mn or Si is added alone, the adverse effects caused when Mn or Si is added alone are offset, and a high quality plated steel sheet is obtained.

If the content of Fe is less than 7%, unalloyed zinc remains on the surface of the film after the alloying treatment, so that the blister resistance and the weldability after coating are reduced, and the content exceeds 15%. When this happens, the workability of the film is remarkably reduced, and the sacrificial corrosion protection of the film is also reduced, and red rust is easily generated. The balance of the plating layer is zinc and inevitable impurities.

 Hereinafter, the present invention will be further described with reference to examples.

(Example) C: 0.03%, Si: 0.01%, Mn: 0.23%, P: 0.008%, S: 0.0
08%, Sol.Al: 0.28%, from aluminum killed steel sheet (unannealed material) with thickness of 0.76mm, cut out 10mm wide x 250mm long test material, washed with organic solvent, and then vertical hot-dip plating simulator And annealed by heating to a temperature of 740 ° C. for 60 seconds in a mixed gas atmosphere of 26% H ₂ + N ₂ . Next, after immersing in a plating bath having a bath composition shown in Table 1 and having a bath temperature of 465 ° C. for 3 seconds to perform plating, wiping with N ₂ gas to reduce the adhesion amount to 60%.
After adjusting to g / m ² , it was further heated to a temperature of 500 ° C. to perform an alloying treatment.

The alloyed hot-dip galvanized specimen thus obtained was subjected to JIS
A salt spray test according to Z2867 was performed, and the elapsed time until the occurrence of red rust was measured. The results are shown in Table 1 together with the plating bath composition, the film composition after alloying, and the time required for alloying.

As is clear from Table 1, the examples of the present invention (No. 1 to No. 1)
In the case of 3), the time required for alloying is short, and the plated steel sheet after alloying is excellent in corrosion resistance.

On the other hand, Comparative Examples No. 14 to No. 17, which correspond to Zn-Al plating to which Mn and Si are not added, have lower corrosion resistance or longer alloying time than those of the present invention. Further, even if one or both of Mn and Si are contained as in Comparative Examples No. 18 to No. 24, if the amount of Mn, Si and Al is out of the range specified in the present invention, alloying is performed. Alloying cannot be performed because the required time is long or the appearance after hot-dip plating is poor.

(Effect of the Invention) As described above, according to the present invention, Zn-
It is possible to obtain an alloyed hot-dip galvanized steel sheet with less generation of Al-Mn ternary compounds and Fe-based dross, good operability, excellent corrosion resistance, and excellent film surface smoothness.

Continuation of the front page (56) References JP-A-60-52569 (JP, A) JP-A-63-134653 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 2 / 00-2/40

Claims (2)

(57) [Claims] 1. A steel sheet comprising Al: 0.05 to 5% by weight, Si: 0.005 to
Alloyed hot-dip coated steel sheet containing 0.8% by weight, Mn: 0.1 to 3% by weight, the remainder being passed through a plating bath composed of Zn and unavoidable impurities, subjected to hot-dip plating, and then subjected to alloying treatment. Manufacturing method. 2. A coated steel sheet which has been subjected to an alloying treatment by a heat diffusion treatment after hot-dip plating, wherein the coating composition of the plated steel sheet is: Al: 0.1 to 5% by weight, Si: 0.005 to 0.8% by weight, Mn: 0.1
An alloyed hot-dip galvanized steel sheet characterized by being composed of about 3% by weight, Fe: 7 to 15% by weight, and the balance: Zn and unavoidable impurities.