KR20150074975A - Zinc alloy plating steel sheet having excellent weldability and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a Zn-Mg-Al ternary alloy coated steel sheet having improved weldability. The steel sheet comprises a base material and a Zn-Al-Mg ternary alloy coated layer. The coated layer has an interface layer containing a compound in Fe-Al metal of Fe_2Al_5 or Fe_3Al on an interface. The interface layer has a thickness of 20-100 nm. Moreover, the present invention relates to a method for improving weldability of a Zn-Mg-Al ternary alloy coated steel sheet. According to the method, a cold-rolled steel sheet comprises aluminum, magnesium, zinc, and inevitable impurities, and is immersed into a hot dip zinc alloy plating bath at a temperature of 400-700°C for a plating solution to be attached to a surface of the steel sheet, or the cold-rolled steel sheet is processed by a plasma or an excimer laser to activate a surface of the steel sheet, and then, the activated steel sheet comprises aluminum, magnesium, zinc, and inevitable impurities, and is immersed into a hot dip zinc alloy plating bath for a plating solution to be attached to a surface of the steel sheet.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a galvannealed steel sheet having excellent weldability,

The present invention relates to a method of improving the weldability of a Zn-Mg-Al ternary alloy-plated steel sheet and a steel sheet improved in weldability.

The fusion-bonded gold-plated steel sheet of Zn-Al-Mg has high corrosion resistance. Therefore, a Zn-Al-Mg hot-dip coated steel sheet is used in the manufacture of products requiring high corrosion resistance.

In general, welding may cause cracking of the material in the welded portion due to shrinkage or constraining stress when the material is solidified after melting. In the case where the molten plated layer penetrates into the crack part of the base material, the plated layer that has penetrated into the cracked part becomes a brittle phase, which may cause cracks due to LME (liquid metal embrittlement).

However, conventionally, cracking of the plating layer in such a ternary molten alloy plated steel sheet has not been studied.

The present invention attempts to suppress LME (liquid metal embrittlement) cracks caused by penetration of a molten plated layer into cracks occurring in a base material due to a low melting point of a Zn-Mg-Al ternary plated layer.

Accordingly, the present invention provides a Zn-Al-Mg alloy hot dip galvanized steel sheet capable of suppressing LME generation as described above,

Further, it is intended to provide a method of manufacturing a Zn-Al-Mg alloy hot dip galvanized steel sheet capable of suppressing the occurrence of LME.

The present invention is to provide a ternary alloy coated steel sheets of excellent weldability Zn-Al-Mg, the steel sheet comprises a ternary alloy plated layer of the base material, and Zn-Al-Mg, the plated layer at the interface Fe ₂ Al ₅ or an Fe-Al intermetallic compound of Fe ₃ Al, wherein the interface layer has a thickness of 20 to 100 nm.

The interface layer preferably contains 10 to 80% by weight of Fe.

The plated layer may comprise 1-15 wt% Al, 1-5 wt% Mg, and the balance zinc and inevitable impurities.

The plating solution may be one or more selected from the group consisting of As, Rb, Tc, Ru, Rh, Pd, Ag, Te, Cs, Re, Os, Ir, Pt, Au, Hg, Tl, Po, , And the like.

According to another aspect of the present invention, there is provided a method of manufacturing a hot-dip galvanized steel sheet having excellent weldability, comprising the steps of: annealing a heat treated steel sheet comprising aluminum, magnesium and zinc and unavoidable impurities, A plating step of immersing the surface of the steel sheet in a molten zinc alloy plating bath at 700 ° C to adhere the plating solution to the surface of the steel sheet; And a cooling step of cooling the steel sheet to which the plating solution is adhered by gas wiping to adjust the amount of coating and cooling.

As another embodiment of the method, an activation step of activating a steel sheet surface by plasma or excimer laser treatment of the annealed heat treated steel sheet; A plating step of immersing the activated steel sheet in a molten zinc alloy plating bath containing aluminum, magnesium and zinc and unavoidable impurities to adhere the plating solution to the surface of the steel sheet; And cooling the steel sheet with the plating solution adhered thereto by gas wiping to adjust the amount of coating and cooling to obtain a coated steel sheet.

Further, the plating bath may contain 0.5 to 5.0% of aluminum, 1 to 5% of magnesium, and the balance of zinc (Zn) and unavoidable impurities in weight%.

The plating solution may be one or more selected from the group consisting of As, Rb, Tc, Ru, Rh, Pd, Ag, Te, Cs, Re, Os, Ir, Pt, Au, Hg, Tl, Po, And one or more components selected from the above.

According to the present invention, a large amount of Fe-Al intermetallic compound having a high melting point is formed at the interface between the base steel sheet and the plating layer, thereby reducing and preventing the LME cracking phenomenon that may occur during welding.

By preventing welding cracking as described above, it is possible to prevent the fatigue breakdown starting from such minute cracks in the LME and to prolong the life of the product.

FIG. 1 is a photograph of a section of a plated layer before welding of a Zn-Al-Mg alloy alloy hot-dip galvanized steel sheet, in which (a) is a photograph of a section of a plated layer, (b) (C) is an enlarged photograph of the interface surface after dissolution.
FIG. 2 is a graph showing the measurement of the composition ratio according to the depth of the interfacial layer in a plated steel sheet plated in a plating bath at a temperature of 450 ° C., and is a graph showing the results of measurement of the interface thickness at 12 nm.
FIG. 3 is a graph showing the measurement of the composition ratio according to the depth of the interfacial layer in a plated steel sheet plated in a plating bath at 480 ° C. temperature, and is a graph showing a result of measuring the interfacial thickness of the plating layer at 14 nm.
FIG. 4 is a graph showing the measurement of the composition ratio according to the depth of the interfacial layer in a plated steel sheet plated in a plating bath at a temperature of 600 ° C., and is a graph showing the results of measurement of the interfacial thickness of the plating layer at 26 nm.
FIG. 5 is a graph showing the composition ratio according to the depth of the interfacial layer in a plated steel sheet plated in a plating bath at 450 ° C after argon plasma treatment of a base steel sheet before immersion in a plating bath, wherein the interface thickness of the plating layer is 36nm. ≪ / RTI >
FIG. 6 is a graph showing the composition ratio according to the depth of an interfacial layer in a plated steel sheet plated in a plating bath at 480 ° C. after argon plasma treatment of a base steel sheet before immersion in a plating bath, wherein the interface thickness of the plating layer is 38 nm. Fig.
FIG. 7 is a graph showing the composition ratio according to the depth of the interfacial layer in a plated steel sheet plated in a plating bath at 600 ° C after argon plasma treatment of a base steel sheet before immersion in a plating bath, wherein the interface thickness of the plating layer is 87 nm. Fig.
Fig. 8 is a photograph of a cross-sectional shape of a welded portion and a micro crack occurring in a welded portion when a Zn-Al-Mg alloy hot-dip coated steel sheet is welded according to Embodiment 2;
FIG. 9 is a photograph showing cross-sectional photographs of welds of the steel sheets in Comparative Examples 1 to 2 and Examples 1 to 4, showing whether or not a material crack occurred during welding according to plating conditions.

The welding is performed by melting the base material and then solidifying the two materials. However, when the Zn-Al-Mg based alloy hot-dip coated steel sheet is welded, the plating structures of the Zn-Mg-Al ternary alloy plating layer generally have a low melting point. As a result, the plating layer is dissolved by welding, and the shape of the tissue is changed during the solidification process.

For example, as shown in Fig. 1, the surface of the plated layer before welding is composed of alloy structures formed between Zn, Al and Mg, but by dissolving it by welding and then re-solidifying, Disappear, and a brittle crystal phase remains.

Meanwhile, when the base material is melted and then solidified by welding, cracks may be generated in the base material. At this time, when the melted plating layer component penetrates into the crack region, as described above, . Fig. 2 shows the welded portion where the base material crack was generated. The possibility of cracks is further increased by the LME (liquid metal embrittlement) generated by the penetration of the molten plated layer on the cracks generated in the base material during the welding.

Therefore, in order to suppress the cracking of the base material due to the LME due to the melting of the plating layer, it is preferable that plating structures having a high melting point are formed at the time of forming the alloy plating layer of the Zn-Mg-Al ternary system. In particular, when the plating structure having such a high melting point is formed at the interface between the base material and the plating layer, cracks caused by the LME can be more effectively suppressed.

Examples of the structure having a higher melting point that can be produced from the alloy plating of the Zn-Mg-Al ternary system include Fe-Al intermetallic compounds and the like. The Fe-Al intermetallic compound is not particularly limited, and examples thereof include Fe-Al intermetallic compounds such as Fe ₂ Al ₅ and Fe ₃ Al.

In order to effectively suppress the LME crack through the formation of the Fe-related alloy phase, the Fe content of the interface layer formed between the base steel sheet and the plating layer is preferably in the range of 10 to 80% by weight. When the content of iron is less than 10% by weight, the iron-related alloy phase is not sufficiently generated in the interface layer, so that the LME crack due to melting of the plating layer can not be sufficiently suppressed, and when it exceeds 80% by weight, the hardness of the plating layer interface increases And the plating layer is disadvantageously dropped off during processing.

Such an intermetallic compound is formed by the alloying of the iron and the plating component of the base material, and is formed in the interface layer between the plating layer and the base material. As the Fe-related alloy phase having a high melting point is formed thicker, It is possible to suppress the change to the brittle crystal structure, and it is possible to inhibit the molten plated layer from penetrating into the cracked portion of the base material and causing the LME crack to occur.

To this end, the interface layer preferably has a thickness in the range of 20 nm or more, for example, 20 to 100 nm. When the thickness of the interface layer is less than 20 nm, cracking of the base material can not be sufficiently prevented at the time of welding. Such a thick interface layer is preferable for preventing the LME crack in the base material, but if it is about 100 nm, it provides a sufficient effect to prevent the LME crack, and if it exceeds 100 nm, it causes an excessive cost increase in the treatment for the interface layer formation But the interface becomes rather brittle and a problem occurs that the plating layer falls off during processing. Furthermore, it is more preferable that the interface layer has a thickness of 25 to 90 nm.

In order to form the Fe-related alloy layer having such a high melting point, it is preferable to induce alloying between the iron and the plating composition of the base material by further activating the surface of the base material when forming the alloy plating layer of the Zn-Mg-Al ternary system. For this purpose, it is preferable to set the temperature of the plating bath at a high level during plating.

Preferably, the plating bath performs the plating at a temperature in the range of 540-700 占 폚. When the temperature is lower than 540 占 폚, sufficient Fe-related alloy phase can not be formed. On the other hand, although the plating bath temperature is limited to 700 캜 or lower, even if the plating bath is managed at a higher temperature, there is no problem in forming the iron-related alloy phase, but the cost may be increased to maintain the temperature of the plating bath. Also, if the plating bath temperature is too high, the plating apparatus such as a sink roll in the plating bath may be damaged and its service life may be shortened. More preferably 600-700 < 0 > C.

As another embodiment in which the above-described iron-related alloy phase is thickly formed, the surface of the steel sheet may be activated and then immersed in a plating bath. Activation of the surface of the steel sheet can be exemplified by plasma treatment or excimer laser treatment. The treatment conditions of the steel plate surface activation treatment are not particularly limited as long as the surface of the plating layer can be activated uniformly. By activating the surface of the steel sheet in this manner, the formation of the alloy phase of the plating solution component and the base material iron can be improved.

When the plasma treatment or the excimer laser treatment is performed as described above, the interface layer having the iron-related alloy phase can be thickened even if the plating treatment is performed at a temperature lower than the plating bath required when the activation treatment is not performed. At this time, the plating bath temperature is not particularly limited, but may be in the range of 400 to 700 占 폚.

In the present invention, the Zn-Al-Mg alloy hot-dip coating solution containing Fe and Al and the remainder Zn and inevitable impurities may be suitably applied to the present invention in forming the plating layer. For example, the plating solution may be a plating solution containing 1-15% by weight of Al and 1-5% by weight of Mg.

In addition, it may further include an additive that is conventionally added for suppressing the generation of dross of the plating bath, preventing oxidation of the plating layer, and the like. As the additive, for example, As, Rb, Tc, Ru, Rh, Pd, Ag, Te, Cs, Re, Os, Ir, Pt, Au, Hg, Ca and the like, and oxides thereof may be added. Furthermore, they may be used alone or in combination.

Example

Hereinafter, the present invention will be described more specifically with reference to examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

Example  One

A Zn-Mg-Al alloy melt containing 2.5% by weight of Al, 3% by weight of Mg and the balance of zinc and other trace inevitable impurities was maintained at the temperature shown in Table 1, and then a cold- Immersed in a plating bath, held for 3 seconds, and plated.

At this time, in the case of Examples 2 to 4, the cold-rolled steel sheet was surface-treated with Ar plasma and dipped in the plating bath.

Before the plasma formation, the chamber had a degree of vacuum of 7.5 x 10 < ^-7 > Torr, and a plasma cell (ALPHA PLUS AP-Plsma300) was used for plasma generation. 12 sccm of Ar was injected to maintain a working pressure of 10 mTorr. The RF power of the plasma generator was set at 100 W.

Subsequently, the plating bath was removed from the plating bath, and the adhesion amount of the plating solution adhered to the surface of the steel sheet by gas wiping was adjusted to 70 g / m 2, followed by cooling to solidify the plating solution.

For the steel sheet of Comparative Example 1, the microstructure of the plating layer was photographed by FE-SEM and is shown in Fig.

The interfacial layer between the plated layer and the base material was analyzed by GDS (Glow discharge spectrometer) for each of the thus obtained coated steel sheets. The results are shown in Table 1 and Figs. 2 to 7.

The GDS analysis of the interfacial layer was performed by immersing the surface layer in a nitric acid solution (95 ml of distilled water + 5 ml of distilled water + 20 g of CrO ₃ + ₄ g of ZnSO ₄ ) to remove the coating layer formed on the base steel sheet, The results were measured. The results are shown in Table 1 and Figs. 2 to 7.

Further, two specimens (210 x 300 mm 2) of the obtained plated steel sheet were prepared. The prepared specimens were welded under the following conditions.

The welded portion of the welded steel sheet was observed with an FE-SEM, and the surface state thereof was photographed. The results are shown in Fig.

On the other hand, for the steel sheet of Comparative Example 1, microstructures were taken by FE-SEM, and the results are also shown in Fig.

division plasma
Whether or not to process Plating bath
Temperature (℃) Plating interfacial layer
Thickness (nm) Material crack
Occurrence Comparative Example One × 450 12 ○ 2 × 480 14 ○ Example One × 600 26 × 2 ○ 450 36 × 3 ○ 480 38 × 4 ○ 600 87 ×

It can be seen from Table 1 that when the plating is performed by setting the plating bath at a high temperature (Example 1) or when the plating is performed after activating the surface of the base material by the plasma treatment (Examples 2 to 4) It can be seen that the thickness of the interfacial layer is increased from FIG. 2 to FIG. 7 as well.

In addition, when the thickness of the plating interface layer is large as described above, cracks are not generated in the material after welding, and this can be confirmed from FIG.

Claims (8)

A base material and a ternary alloy plating layer of Zn-Al-Mg,
The plating layer has an interfacial layer including an Fe-Al intermetallic compound of Fe ₂ Al ₅ or Fe ₃ Al on the interface, and the interfacial layer has a thickness of 20 to 100 nm, Alloy coated steel sheet.
The ternary alloy-plated steel sheet according to claim 1, wherein the interfacial layer contains 10 to 80 wt% Fe.
The plating method according to claim 1 or 2, wherein the plating layer comprises a ternary alloy plating of Zn-Al-Mg excellent in weldability including 1-15% by weight of Al, 1-5% by weight of Mg and residual zinc and inevitable impurities Steel plate.
The plating solution as claimed in claim 3, wherein the plating solution is at least one selected from the group consisting of As, Rb, Tc, Ru, Rh, Pd, Ag, Te, Cs, Re, Os, Ir, Pt, Au, Hg, , Ca, or an oxide of any of these elements, and further has excellent weldability.
A plating step of immersing the steel sheet in a molten zinc alloy plating bath containing aluminum, magnesium and zinc and inevitable impurities at a temperature of 400-700 DEG C to adhere the plating solution to the surface of the steel sheet; And
The steel plate to which the plating solution is adhered is gas-wiped to adjust the amount of deposition, and a cooling step
Wherein the Zn-Al-Mg-based alloy-coated steel sheet has excellent weldability.
An activation step of activating the surface of the steel sheet by plasma or excimer laser treatment of the steel sheet;
A plating step of immersing the activated steel sheet in a molten zinc alloy plating bath containing aluminum, magnesium and zinc and unavoidable impurities to adhere the plating solution to the surface of the steel sheet; And
A cooling step of regulating the deposition amount by gas wiping the steel plate with the plating solution attached thereto and cooling the steel sheet to obtain a coated steel sheet
Wherein the Zn-Al-Mg-based alloy-coated steel sheet has excellent weldability.
The plating solution according to claim 5 or 6, wherein the plating solution contains 0.5 to 5.0% of aluminum (Al) and 1 to 5% of magnesium (Mg), and the balance contains zinc and inevitable impurities Wherein the method comprises the steps of: preparing a Zn-Al-Mg ternary alloy-plated steel sheet having excellent weldability.
8. The method as claimed in claim 7, wherein the plating solution is at least one selected from the group consisting of As, Rb, Tc, Ru, Rh, Pd, Ag, Te, Cs, Re, Os, Ir, Pt, Au, Hg, , Ca, or an oxide of any of the above-mentioned metals and / or oxides thereof.

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