KR20210079054A - Galvanized steel sheet with excellent liquid metal embrittlement resistance and hydrogen embrittlement resistance and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a galvanized steel sheet and a method for manufacturing the same, wherein the galvanized steel sheet comprises: a base steel sheet; and a galvanized layer formed on at least one surface of the base steel sheet, wherein the galvanized layer comprises: 0.01-0.3 wt% in total of one or more types selected from a group consisting of Ti, V, Nb, Ni, Cu, Zr, Y, and La; 0.01-0.15 wt% of Al; and balance Zn and other unavoidable impurities. The galvanized layer has a size of 0.01-10 ㎛ and comprises an (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound having a polygonal shape.

Description

Galvanized steel sheet with excellent resistance to liquefied metal embrittlement and hydrogen embrittlement and manufacturing method thereof

The present invention relates to a galvanized steel sheet, and more particularly, to a galvanized steel sheet having excellent resistance to liquefied metal embrittlement and hydrogen embrittlement and a method for manufacturing the same.

Existing hot-dip galvanized steel sheet is subjected to a pre-process to improve plating properties or wettability before the plating process, and is then subjected to a pickling process and a reducing atmosphere in an annealing furnace to remove Fe- and Mn-based oxides generated on the surface of the strip. However, it is known that an embrittlement phenomenon in which mechanical properties are lowered occurs due to hydrogen present in the atmosphere. Hydrogen diffused into the steel material due to the concentration gradient lowers the bonding force between Fe-Fe atoms of the base material and at the same time is concentrated in a specific area to generate pressure and cause internal cracks, which in turn adversely affects the mechanical properties of the base material. is known This trend occurs more frequently as the tensile strength of steel becomes higher than 1 GPa, and it is a problem that is receiving a lot of attention in the current automobile industry, which uses high-strength steel to lighten the car body and reduce the weight to increase fuel efficiency.

In addition, when the existing hot-dip galvanized steel sheet is accompanied by plastic deformation, the effect of deterioration of properties due to hydrogen already introduced into the base steel sheet appears, and at the same time, the hydrogen embrittlement phenomenon due to the additional hydrogen introduced from the external corrosive environment is further accelerated. , the biggest problem is that it is impossible to predict the break time due to this phenomenon, so it is known that there are many restrictions in practical use. In order to overcome the hydrogen embrittlement phenomenon of hot-dip galvanizing of high-strength steel, various attempts have been made so far, such as controlling the microstructure, mechanical properties, and alloying elements of the base material. It is known that the influence of hydrogen diffusion and permeability depending on the crystal lattice is a more important factor.

In addition, in the case of hot-dip galvanized steel sheet, liquefied zinc penetrates into the grain boundary of the material and weakens the bonding force between Fe atoms, so that the main physical properties of the material cannot be secured and fracture occurs early (Liquid Metal Embrittlement, LME). ) is a phenomenon. As with the hydrogen embrittlement phenomenon mentioned above, the deterioration of physical properties due to this phenomenon occurs more frequently as the tensile strength of the material is increased by 1 GPa or more, and is a problem that is also receiving a lot of attention in the current automobile industry.

Accordingly, there is a need to develop a technique for overcoming the limitations of hydrogen embrittlement and liquefied metal embrittlement existing in hot-dip galvanizing of existing high-strength steel.

Korean Publication No. 2013-0133358

One aspect of the present invention is the hydrogen embrittlement phenomenon in which the mechanical properties of steel are deteriorated due to hydrogen that has already been introduced in the plating process and hydrogen that is additionally introduced during the use environment, and liquefied metal embrittlement that occurs during resistance welding of hot-dip galvanized materials. An object of the present invention is to provide a method for reducing the phenomenon by reforming the hot-dip galvanizing layer.

The subject of the present invention is not limited to the above. Those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the contents throughout the present specification.

One aspect of the present invention is

Comprising a galvanized layer formed on at least one surface of the base steel plate and the base steel plate,

The galvanized layer is at least one selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La by weight%: 0.01 to 0.3% in total, Al: 0.01 to 0.15%, balance Zn and others containing unavoidable impurities;

The galvanized layer has a size of 0.01 to 10 μm and includes a (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound having a polygonal shape, to provide a galvanized steel sheet.

In addition, another aspect of the present invention,

Preparing a base steel plate;

At least one selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La: 0.01 to 0.3% in total, Al: 0.01 to 0.15%, balance galvanizing bath containing Zn and other unavoidable impurities to prepare;

hot-dip galvanizing by immersing the base steel sheet in the galvanizing bath in an inert atmosphere;

A first cooling step of cooling the hot-dip galvanized base steel sheet to a temperature range within 230 ~ 270 ℃ at a cooling rate of 0.01 ~ 20 ℃ / s; and

It provides a method for manufacturing a galvanized steel sheet, comprising a second cooling step of cooling to room temperature at a cooling rate that is in the range of 0.01 to 10 °C/s and slower than the cooling rate of the first cooling step.

According to one aspect of the present invention, it is possible to provide a galvanized steel sheet having excellent resistance to hydrogen embrittlement with reduced sensitivity to hydrogen embrittlement under the same hydrogen charge condition as compared to conventional hot-dip galvanizing materials, and a method for manufacturing the same.

In addition, according to another aspect of the present invention, it is possible to provide a galvanized steel sheet having excellent resistance to liquefied metal embrittlement and a method for manufacturing the same, having reduced sensitivity to cracking in welds under resistance welding conditions.

1 is a photograph showing cross-sections of an invention material and a comparative material according to an embodiment of the present invention observed at a magnification of 1000 using a back-sacattered image (BSI) in an electron scanning microscope.
2 shows the chemical composition results obtained by 2D scanning with Electron-Probe Micro-Analysis (EPMA) for the inventive material and the comparative material according to an embodiment of the present invention.
3 shows the results of measuring the amount of hydrogen released over time by heating the invention material and the comparative material according to an embodiment of the present invention after charging them in a hydrogen atmosphere.
Figure 4 shows a photograph (12.5 magnification) of observing the LME cracks generated after resistance welding of the invention material and the comparative material according to an embodiment of the present invention.
5 is a result of measuring the size of the LME crack generated after resistance welding the invention material and the comparative material according to an embodiment of the present invention.
6 shows a photograph of the LME crack region of the inventive material according to an embodiment of the present invention observed at high magnification (1500 magnification) and the result of component analysis.

As a method for reducing embrittlement caused by hydrogen and liquefied zinc, the present inventors have developed a specific element in a galvanized steel sheet, more specifically, aluminum and intermetallic compounds in a temperature atmosphere at which hot-dip galvanizing process is performed. The addition of an element that can form a material and increase the surface energy of the material has been deeply studied.

As a result, by optimizing the alloy composition and plating conditions in the hot-dip galvanizing bath, and forming a plating layer capable of forming an intermetallic compound of a certain size in the hot-dip galvanizing process atmosphere, a galvanized steel sheet having intended physical properties can be produced. It was confirmed that it can be provided, and the present invention has been completed.

Hereinafter, a galvanized steel sheet having excellent hydrogen embrittlement resistance and/or liquid metal embrittlement resistance will be described in detail. In the present invention, when the content of each element is indicated, it should be noted that, unless otherwise specified, it means weight %.

The galvanized steel sheet excellent in hydrogen embrittlement resistance and/or liquid metal embrittlement resistance according to an aspect of the present invention may include a base steel sheet and a galvanized layer formed on at least one surface of the base steel sheet.

In the present invention, the type of the holding steel sheet may not be particularly limited. For example, it may be a Fe-based base steel sheet used as a base of a conventional galvanized steel sheet, that is, a hot-rolled steel sheet or a cold-rolled steel sheet, but is not limited thereto. In addition, the base steel sheet may include, for example, carbon steel, ultra-low carbon steel, high manganese steel, etc. used as a material for automobiles, but is not limited thereto.

However, in the case of a hot-rolled steel sheet, there may be a problem of having a large amount of oxide scale on the surface, and such oxide scale deteriorates plating adhesion and deteriorates plating quality. Therefore, when using a hot-rolled steel sheet as a base steel sheet, it can be made into a hot-rolled steel sheet from which oxide scale has been previously removed by an acid solution.

On the other hand, the galvanized layer may be formed on one side or both sides of the base steel sheet.

The galvanized layer is at least one selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La by weight%: 0.01 to 0.3% in total, Al: 0.01 to 0.15%, balance Zn and others It may contain unavoidable impurities. Such a galvanized layer may be formed from a plating bath containing Ti, V, Nb, Ni, Cu, Zr, Y, La, Al, balance Zn and other unavoidable impurities in the above-described content.

On the other hand, the other unavoidable impurities may include Fe, which is likely to be introduced by diffusion from the base steel sheet to the plating layer. Specifically, since the diffusion of Fe from the base steel sheet to the plating layer is restricted by the inhibition layer formed at the interface between the base steel sheet and the zinc plated layer, the amount in the plating layer is insignificant, so it is included at the level of impurities. In some cases, the Fe content can be ignored.

[At least one selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La: 0.01 to 0.3% in total]

The zinc plating layer of the present invention is from the group consisting of titanium (Ti), vanadium (V), neobium (Nb), nickel (Ni), copper (Cu), zirconium (Zr), yttrium (Y) and lanthanum (La). It may include one or more selected types, and the content may be 0.01 to 0.3% in total.

The elements of Ti, V, Nb, Ni, Cu, Zr, Y, and La are elements that play a very important role in improving the hydrogen embrittlement resistance in the zinc plating layer. It is an element capable of forming an intermetallic compound with Al and increasing the surface tension of the base material.

Specifically, the elements of Ti, V, Nb, Ni, Cu, Zr, Y, and La are (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic secondary compounds inside the plating layer. By creating a phase, it is possible to trap by dispersing hydrogen already introduced into the base steel sheet and additional hydrogen from the usage environment, thereby lowering the sensitivity of hydrogen embrittlement compared to conventional hot-dip galvanizing when exposed to the same amount of hydrogen. can

In addition, these (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compounds improve the surface tension of the base material, so that they are melted during the resistance welding process and, at the same time, liquid zinc enters the grain boundary of the base material. Since it serves to suppress the penetration phenomenon (liquid metal embrittlement phenomenon), the sensitivity to liquid metal embrittlement can also be lowered compared to the existing hot-dip galvanizing.

Meanwhile, in the present invention, the meaning of the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound is Ti, V, Nb, Ni, Cu, Zr, Y and La It refers to a case in which one or more elements selected from the group consisting of Al and Al form an intermetallic compound. For example, when only Ti is added as the one or more elements, a Ti-Al-based intermetallic compound may be formed in the zinc plating layer, or when Ti and V are added as the one or more elements, the zinc plating layer is TiV-Al-based intermetallic compounds may also be formed.

If the content of the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound is less than 0.01%, (Ti, V, Nb, Ni, Cu, Zr, Y, La)- The effect of improving the hydrogen embrittlement resistance due to the generation of the Al-based intermetallic compound cannot be sufficiently obtained.

On the other hand, if the content of the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound exceeds 0.3%, rather excessive growth of the compound and Fe of the base material during hot-dip galvanizing may be present in the plating layer. Since it becomes difficult to form an inhibition layer that inhibits the diffusion phenomenon (Fe-burst), an incidental problem that is different from the object of the present invention occurs.

Therefore, in the present invention, the total content of one or more selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in the galvanized layer may be 0.01 to 0.3%, preferably 0.01 to 0.25% may be, and more preferably 0.05 to 0.25%.

[Al: 0.01~0.15%]

The aluminum (Al) in the galvanizing layer generates an Fe-Al-based intermetallic compound at the interface between the plating layer and the base steel sheet in the hot-dip zinc alloy plating bath, thereby preventing Fe diffusion into the plating layer. However, in the present invention, there is also a purpose for the formation of (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound, which is an essential requirement for exhibiting the desired effect. .

Therefore, if the Al content in the galvanized layer is less than 0.01%, the continuous formation of the aforementioned suppression layer becomes difficult, and the effect of the suppression layer for preventing Fe diffusion into the plating layer cannot be sufficiently obtained. On the other hand, when the Al content in the zinc plating layer exceeds 0.15%, an excessive Zn/Al binary eutectic phase is formed in the plating layer, thereby reducing the sacrificial anticorrosive effect of zinc plating on the cross section and the painted portion. Therefore, in the present invention, it is preferable to include 0.01 to 0.15% of Al in the galvanized layer, and more preferably 0.1 to 0.15%.

In addition, in addition to the composition of the above-described zinc plating layer, the remainder may be Zn and other unavoidable impurities. Here, other unavoidable impurities may be unintentionally mixed in the manufacturing process of a conventional hot-dip galvanized steel sheet and cannot be completely excluded, and those skilled in the art can easily understand the meaning, so it is not defined does not

Meanwhile, according to one aspect of the present invention, a suppression layer mainly made of an Fe-Al-based intermetallic compound may be formed between the base steel sheet and the galvanized layer. The suppression layer may be formed by diffusion of Fe of the steel sheet in the initial stage of plating and Al in the plating bath, and serves to improve the adhesion between the steel sheet and the zinc plating layer, and at the same time, the amount of Fe from the steel sheet to the zinc plating layer. It acts as a barrier to spread. On the other hand, in the present invention, that the suppression layer is made of the Fe-Al-based intermetallic compound means that it may include other unavoidable impurities, and does not exclude the possibility that such impurities may be included.

Meanwhile, according to one aspect of the present invention, the zinc plating layer includes (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound, and specifically, the size inside the zinc plating layer is A (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound having a range of 0.01 to 10 μm and having a polygonal shape may be formed.

Oxide usually has a spherical shape, whereas in the present invention, (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based metal having a polygonal shape by performing in an inert atmosphere excluding oxygen in the process liver compounds can be obtained. Specifically, the shape of the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound varies depending on the addition ratio of each component. In general, in the case of a 50:50 ratio, It has rock-salt cubic, and as the Al ratio increases, the polygon increases. When the ratio is 25:75, it has a hexagonal wurtzite structure. After the hexagonal nucleus is generated, it grows in the vertical direction of the edge connecting the edge.

In the present invention, the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound is generated in the hot-dip galvanized layer, and is introduced from the hydrogen previously introduced into the steel sheet and the external use environment. It can serve to lower the hydrogen embrittlement sensitivity by dispersing and trapping the hydrogen that is used.

In addition, the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound can cause deterioration of wettability through melting of the intermetallic compound during welding, and liquid zinc It can also play a role in lowering the embrittlement sensitivity of liquefied metals penetrating into the parent material grain boundary.

In particular, the hydrogen embrittlement phenomenon is called 'diffusive hydrogen' in lattice structure: hydrogen capable of diffusion movement within interstitial sites, and reversible hydrogen capable of diffusion in a temperature region of less than 300 °C (in the present invention, these two are collectively referred to as 'diffusible hydrogen'). ), and this diffusible hydrogen causes hydrogen embrittlement and hydrogen-induced cracking, which is one of the main causes of lowering mechanical properties.

However, by trapping a part of this diffusible hydrogen in a hydrogen trapping site in a high temperature region, it can be converted into irreversible hydrogen, thereby lowering the hydrogen embrittlement sensitivity. The (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound may act as a hydrogen trapping zone in the high-temperature region described above.

Meanwhile, according to an aspect of the present invention, the size of the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound may be in the range of 0.01 to 10 μm. In the present invention, the size of the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound is measured to penetrate the center of the intermetallic compound when the intermetallic compound is observed. It can be defined as the arithmetic mean value of the major axis and minor axis, that is, '(major axis + minor axis)/2', and the same applies hereafter.

In the present invention, when the size of the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound exceeds 10 μm, the region to collect hydrogen is uniformly dispersed Not only is it difficult to be, but it is highly likely to act as a hydrogen-induced cracking point by itself, so it is difficult to obtain the effect of improving hydrogen embrittlement resistance, which is the object of the present invention. Moreover, it is difficult to expect effective melting of the intermetallic compound at the same amount of heat input during the resistance welding process, so it may have an adverse effect.

In addition, the finer the size of the (Ti, V, Ni, Nb, Cu, Zr, Y, La)-Al-based intermetallic compound is, the more effective it is to achieve the object of the present invention, so the lower limit of the size will not be separately limited. However, in consideration of the temperature conditions and cooling process of hot-dip galvanizing, the lower limit of the size of the compound may be limited to 0.01 μm.

According to one aspect of the present invention, the content of one or more elements selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in the zinc plating layer may be divided into regions having different contents.

Specifically, according to one aspect of the present invention, with respect to the central region in the thickness direction of the galvanized layer,

The sum of one or more elements selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound a first region having a content of 20% or more;

The sum of one or more elements selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound and a second region having a content of less than 20%.

Meanwhile, the meaning of the total content of at least one element selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La is (Ti, V, Nb, Ni, Cu, Zr, Y, La )- means the content ratio of one or more elements selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in the Al-based intermetallic compound. That is, the first region means that the total content of one or more elements selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in the intermetallic compound is 20% or more and less than 80% Al, and , the second region means that the total content of at least one element selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in the intermetallic compound is less than 20% and 80% or more of Al.

In addition, in this specification, the central part means a central part when it is divided into three with respect to the thickness direction of the galvanizing layer. More specifically, when the thickness of the galvanized layer is t, it may refer to a region from the surface at 1/3t to 2/3t.

In other words, in the present invention, in the central region in the thickness direction of the galvanized layer, the first region and the second region are from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La. It can be distinguished by the difference in the content of one or more selected elements.

In addition, according to one aspect of the present invention, the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound having a size of 0.01 to 10 μm and a polygonal shape is In the region of the central portion in the thickness direction, it may be present in the first region, and through this, the intended object of the present invention can be preferably achieved.

That is, in the case of the second region in which the total content of one or more elements selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La is less than 20% and Al exceeds 80%, the above ( It is difficult to nucleate Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compounds.

Therefore, at the interface between the galvanized layer and the base steel sheet, the total content of one or more elements selected from the group consisting of V, Nb, Ni, Cu, Zr, Y and La is less than 20%, and Al exceeds 80%. Although some (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compounds may be present in the , Cu, Zr, Y, La)-Al-based intermetallic compounds are present in the second region.

According to one aspect of the present invention, when the composition ratio in the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound corresponds to the first region, a nucleus of the compound is generated, growth becomes possible On the other hand, when the composition ratio in the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound corresponds to the second region, nucleation in the zinc plating layer is impossible, so that the secondary phase (this In the present invention, precipitation into a polygonal shape) is not possible.

On the other hand, according to one aspect of the present invention, the first region and the second region are separately formed because of non-uniform/non-equilibrium element diffusion of elements immediately after passing through the molten metal, that is, supercooling during plating operation. It is directly related to the charging time/cooling speed, and the area of the plating layer can be divided by intentionally applying accelerated cooling during the primary cooling during the second stage cooling during this plating operation.

Meanwhile, according to an aspect of the present invention, the galvanized layer may have a thickness of 5 to 20 μm.

According to an aspect of the present invention, a galvanized steel sheet including a galvanized layer having the above-described configuration includes (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound in the galvanized layer. Compared with the conventional galvanized steel sheet that does not contain it, the excellent effect of hydrogen embrittlement resistance and liquid metal embrittlement resistance is exhibited.

That is, according to an aspect of the present invention, the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound having the above-mentioned size of 0.01 to 10 μm and a polygonal shape in the zinc plating layer Due to the presence of this, the amount of hydrogen emitted in a temperature range of 300 ° C or less was measured by the thermal desorption analysis test method compared to the conventional galvanized steel sheet that does not contain the intermetallic compound in the galvanized layer. There is an effect that the emission amount can be reduced by 10% or more. In addition, in the test method, the peak temperature to be analyzed may also be shifted to a high temperature region. Therefore, according to the present invention, it is possible to induce an improvement in the hydrogen embrittlement resistance compared to the prior art.

In addition, according to an aspect of the present invention, the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound having the above-mentioned size of 0.01 to 10 μm and a polygonal shape in the zinc plating layer Due to this existence, not only the above-mentioned hydrogen embrittlement resistance, but also by raising the surface tension of the material compared to the conventional galvanized steel sheet, it causes a decrease in the wettability of liquid zinc with iron, so that zinc permeates into the grain boundary of the base steel sheet. It can lead to a decrease in the generated mechanical properties, that is, an improvement in the resistance to embrittlement of liquefied metal that causes cracking.

Hereinafter, a method for manufacturing a galvanized steel sheet having excellent resistance to hydrogen embrittlement and liquefied metal embrittlement according to another aspect of the present invention will be described in detail.

A method for manufacturing a galvanized steel sheet according to another aspect of the present invention,

Preparing a base steel plate;

At least one selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La: 0.01 to 0.3% in total, Al: 0.01 to 0.15%, balance galvanizing bath containing Zn and other unavoidable impurities to prepare;

hot-dip galvanizing by immersing the base steel sheet in the galvanizing bath in an inert atmosphere;

A first cooling step of cooling the hot-dip galvanized base steel sheet to a temperature range within 230 ~ 270 ℃ at a cooling rate of 0.01 ~ 20 ℃ / s; and

It provides a method for manufacturing a galvanized steel sheet, comprising a second cooling step of cooling to room temperature at a cooling rate that is in the range of 0.01 to 10 °C/s and slower than the cooling rate of the first cooling step.

First, prepare a steel plate. The type of the base steel sheet is not particularly limited, and it may be a Fe-based base steel sheet used as a base steel sheet of a conventional galvanized steel sheet, that is, a hot-rolled steel sheet or a cold-rolled steel sheet, but is not limited thereto. In addition, the base steel sheet may be, for example, carbon steel, ultra-low carbon steel, or high manganese steel used as a material for automobiles, but is not limited thereto.

Next, a galvanizing bath is prepared, and the above-described base steel sheet is immersed in the galvanizing bath in an inert atmosphere to perform hot-dip galvanizing. In this case, the zinc plating layer formed by the hot-dip galvanizing may be formed on one or both surfaces of the base steel sheet.

Meanwhile, according to one aspect of the present invention, the zinc plating bath is at least one selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La: 0.01 to 0.3% in total, Al: 0.01 to 0.15%, balance Zn and other unavoidable impurities.

In addition, according to an aspect of the present invention, in some cases, the Al content in the galvanizing bath may be 0.10 to 0.15%, and/or Ti, V, Nb, Ni, Cu, Zr, Y and La The total content of one or more selected from the group consisting of may be 0.05 to 0.25%.

In addition, according to one aspect of the present invention, in order to obtain the configuration of the galvanized layer described above, one or more components selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La are sufficient in the plating bath. It is very important to be molten. By the way, the commonly known melting point of Ti is about 1668°C, the melting point of V is about 1910°C, the melting point of Nb is about 2477°C, the melting point of Ni is about 1455°C, the melting point of Cu is about 1085°C, and the melting point of Zr is about 1855° C., the melting point of Y is 1526° C., and the melting point of La is about 920° C. In general, considering that the temperature of the plating bath during hot-dip galvanizing does not exceed the zinc melting point (about 419.5° C.) and up to 500° C., it may be difficult to thermodynamically melt even if the above components are added to the plating bath.

However, in the present invention, the method of melting one or more components selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in the plating bath may not be specifically limited, but one embodiment The components can be sufficiently melted in the plating bath by adding the above components to the plating bath as a method and maintaining the components at a normal plating bath temperature for a long time (eg, about 24 hours or more) for erosion melting. More specifically, if the reaction is carried out at the galvanizing bath temperature for a long time, erosion may occur due to the flow of liquid zinc in the molten metal, and due to such erosion, Ti, V, Nb, Ni, Cu, Zr, Y and La One or more components selected from may be melted.

According to one aspect of the present invention, the temperature of the galvanizing bath may be greater than 450 °C and less than 550 °C. That is, conventionally, in the case of manufacturing a galvanized steel sheet, the temperature of the plating bath is controlled to a temperature not exceeding 500°C above the melting point, whereas in the present invention, the temperature of the galvanizing bath is relatively high, exceeding 450°C and less than 550°C. By controlling it, the structure of the intended plating layer can be obtained.

In this way, when the temperature of the galvanizing bath is controlled to be high, one or more components selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in the plating bath are easily eroded during preparation of the galvanizing bath. When plating is performed by immersing the steel sheet in a zinc plating bath at the same time, (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound can be formed. Sufficient thermal energy may be provided.

That is, according to one aspect of the present invention, when plating is performed by immersing the base steel sheet in the galvanizing bath, preferentially selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in the galvanizing bath. At the same time as one or more components are melted through solid solution with zinc, sufficient thermal energy is provided to enable the formation of (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compounds. There is a need. This is made possible by controlling the above-mentioned temperature of the galvanizing bath to be more than 450°C and less than 550°C, and cannot be achieved when the temperature of the galvanizing bath is set to 450°C or lower or 550°C or higher.

Specifically, according to one aspect of the present invention, when the temperature of the galvanizing bath is 450° C. or less, the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound is not sufficiently formed. A galvanized layer having the intended resistance to hydrogen embrittlement cannot be obtained. On the other hand, if the temperature of the galvanizing bath is 550° C. or higher, there is a high possibility that the base steel sheet and the equipment inside the plating bath are eroded, resulting in shortening the life of the equipment.

On the other hand, according to one aspect of the present invention, the lead-in temperature when immersing the base steel sheet in the galvanizing bath during galvanizing may be higher than the temperature of the galvanizing bath by 5 ℃ or more, and in some cases, it may be 20 ℃ or more. .

In addition, according to one aspect of the present invention, preferably, the pull-in temperature of the base steel sheet when immersed in the galvanizing bath may be more than 565 ° C. and less than 600 ° C.

According to one aspect of the present invention, by controlling the pull-in temperature of the base steel sheet to be higher than the temperature of the plating bath by 5° C. or more, (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al in the galvanized layer Sufficient thermal energy may be provided to enable the formation of the based intermetallic compound. That is, the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound is sufficiently generated to obtain a zinc plating layer having the intended resistance to hydrogen embrittlement.

In addition, according to one aspect of the present invention, by controlling the pull-in temperature of the base steel sheet to less than 600 ℃, by preventing excessive generation of the Fe-burst region, it is possible to secure the corrosion resistance of zinc plating and the gloss of the plating material at the same time. have.

On the other hand, by controlling the lead-in temperature of the base steel sheet to more than 565° C., there is an effect that plating can proceed in a line that does not affect the temperature condition of the plating port set above as much as possible.

In addition, according to one aspect of the present invention, one or more components selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La, which are elements with high oxygen affinity in the temperature atmosphere of the plating bath, are aluminum ( In order to form a stable intermetallic compound with Al), plating is preferably performed in an inert atmosphere for the purpose of suppressing the formation of oxides or nitrides of the elements, and it is more preferable to use argon gas in an inert gas.

That is, according to one aspect of the present invention, in order to block or remove an active gas containing oxygen during plating, plating may be performed in an inert atmosphere using argon gas, which is an inert gas. More specifically, it can be performed by spraying argon gas at the time of plating.

On the other hand, according to one aspect of the present invention, with respect to the steel sheet having a plating layer selectively formed before cooling, it may further include the step of wiping the gas. The gas wiping is a process for adjusting the plating adhesion amount, and the method is not particularly limited.

At this time, according to one aspect of the present invention, air or an inert gas may be used as the gas to be used, but preferably, an inert gas such as nitrogen or argon may be sprayed to prevent additional oxide or nitride formation in the zinc plating layer. have. Specifically, air may be used as the gas used in the process, but an inert gas such as nitrogen or argon is preferably used, and more preferably argon gas should be used. This is because, when an active gas containing oxygen is used, coarse oxides or films of (Ti, V, Ni, Nb, Cu, Zr, Y, La) are generated on the surface of the plating layer, thereby causing surface defects of the plating layer.

According to one aspect of the present invention, in performing plating, the plating amount per side ^{may be in the range of 30 to 150 g/m 2} , and through this, a galvanized layer having a thickness of 5 to 20 μm can be obtained.

Thereafter, the hot-dip galvanized base steel sheet is cooled. At this time, cooling may be performed in stages. Specifically, according to an aspect of the present invention, a first cooling step of cooling the above-described hot-dip galvanized base steel sheet to a temperature range within 230 ~ 270 ℃ at a cooling rate of 0.01 ~ 20 ℃ / s; and a second cooling step of cooling to room temperature at a cooling rate in the range of 0.01 to 10°C/s and slower than the cooling rate of the first cooling step.

Meanwhile, according to an aspect of the present invention, in order to obtain a zinc plating layer having the size of the above-described (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound, step-wise cooling is performed. can do.

Specifically, in the present invention, the solid-liquid phase of zinc is properly formed while sufficiently solidifying the zinc single phase through the first cooling process, and (Ti, V, Nb, Ni, Cu, After shortening the time for coarsening and growth of the Zr, Y, La)-Al-based intermetallic compound, the cooling rate during the second cooling is lowered relative to the first cooling rate, thereby causing thermal shock ( It can be solidified by minimizing thermal shock.

Further, according to one aspect of the present invention, in the cooling process, gas may be sprayed to both the front and back surfaces of the plating material, and preferably, an inert gas such as argon may be sprayed to prevent further oxidation.

More specifically, the present invention can control the size of the first region through the cooling step. During cooling, (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound growth proceeds in the first region, and the cooling end temperature range of the first cooling step is set to 230 to 270 ° C. By controlling as , a (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound having a size of 0.1 to 10 μm can be formed in the first region. On the other hand, in the second cooling step, since the cooling is already completed, it does not significantly affect the size and distribution of (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compounds, but only plating In consideration of the thermal shock that may occur on the surface layer, weak cooling may be performed.

In addition, according to one aspect of the present invention, in order to obtain a galvanized layer having the size and shape of the above-described (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound, moisture is removed during cooling. It is preferable to exclude the method of cooling including. In consideration of this, in the present invention, a gas may be sprayed to both the front and back surfaces of the plating material to be cooled, and preferably, an inert gas such as nitrogen or argon may be sprayed for cooling, but the present invention is not limited thereto.

By completing the above-described series of processes, the (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound has a size of 0.01 to 10 μm and a multi-layer galvanized steel sheet can get

The galvanized steel sheet according to the present invention is not only excellent in resistance to hydrogen embrittlement, but also in liquid phase during the resistance welding process due to the presence of (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compounds. Zinc permeates into the grain boundary of the base steel sheet and has the effect of inducing improvement in resistance to liquid metal embrittlement (LME), which causes deterioration of mechanical properties, that is, cracks.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

(Example)

As a test piece for plating, a cold-rolled steel sheet of TRansformation Induced Plasticity (TRIP) steel having a thickness of 1.0 mm, a width of 110 mm, and a length of 200 mm, having the composition shown in Table 1 below was prepared as a base steel sheet, and then plated under the conditions of Tables 2 and 3 to obtain each A galvanized steel sheet was manufactured. On the other hand, all processes including annealing, plating and cooling processes were carried out in a state in which all active gases such as oxygen were blocked or removed, and (Ti, V, Nb, Ni, Cu, Zr, Y, La) related oxides were produced. An experiment was conducted through atmosphere control using argon gas, which is an inert gas, so as to be suppressed.

The TRIP steel is known as a steel that frequently cracks due to liquefied metal embrittlement when resistance welding is performed after hot-dip galvanizing. At this time, cooling after plating was performed to room temperature by spraying nitrogen on the front and back surfaces of the steel sheet.

ingredient C Mn Si P S Sol. Al Content (wt%) 0.18 2.45 1.40 0.01 0.003 0.03

division Plating bath composition (wt%) Total content of (at least one selected from the group consisting of Ti, V, Ni, Nb, Cu, Zr, Y and La) Al Mg invention 0.17 0.14 Tr. comparative goods - 0.13 Tr.

(The remainder of the plating bath composition in Table 2 is Zn and unavoidable impurities.)

division Plating conditions plating layer
Thickness (㎛) Plating bath temperature (℃) Inlet temperature (℃) first cooling
Rate (℃/s) second cooling
Rate (℃/s) invention 500 567 15.0 8.0 12.80 comparative goods 485 565 15.0 8.0 9.10

The galvanized steel sheets of the inventive and comparative materials prepared according to the above were evaluated for resistance to tissue analysis, hydrogen embrittlement and liquefied metal embrittlement.

First, in order to observe the structure of each galvanized steel sheet, it was observed using a scanning electron microscope (SEM), and it is shown in FIG. 1 that the thickness direction of the plating layer is not limited to a specific area.

Figure 1 (a) shows the invention material, Figure 1 (b) shows the comparative material, it can be seen that the invention material is divided into three phases according to the degree of contrast. 1 is taken with a back-scattered image (BSI) in an electron scanning microscope, and the contrast is different depending on the density.

That is, it can be confirmed that it is divided into a zinc plating layer region, (Ti, V, Ni, Nb, Cu, Zr, Y, La)-Al-based intermetallic compound region, and a base material region. In addition, the size of the intermetallic compound can be confirmed in the corresponding Figure 1, and it was confirmed that the size is 0.01 ~ 10 ㎛ or less.

In addition, in order to confirm the distribution of each element, the chemical composition was 2D-scanned by electron-backscattered diffraction (EBSD), and the results are shown in FIG. 2 . Figure 2 (a) shows the invention material, Figure 2 (b) shows a comparative example, in the case of the invention material through Figure 2 (a), (Ti, V, Ni, Nb, Cu, Zr in the plating layer) It can be confirmed that the ,Y,La)-Al-based intermetallic compound exists in the form of a polygon in the hot-dip galvanizing layer.

In addition, in order to confirm the degree of hydrogen embrittlement and resistance of the present invention material and the comparative material, the same amount of hydrogen is charged by applying the hydrogen charging condition through atmospheric corrosion, and then the amount of hydrogen released by applying a constant temperature increase rate is measured did. This test method is called thermal desorption analysis (TDA), and the measurement results are shown in FIG. 3 .

It is known that the greater the amount of hydrogen emitted in the region below 300°C, the greater the amount of diffusible hydrogen, so it is known to be sensitive to hydrogen embrittlement. When the present invention material has an oxide distribution, it can be seen that the influence of hydrogen is reduced by 13% compared to the comparative material, which means that when exposed to a hydrogen atmosphere, the hydrogen embrittlement resistance is relatively improved compared to the comparative material. In addition, it can be confirmed that the temperature region (maximum peak temperature) in which the amount of hydrogen emitted is high has moved to the high temperature region, and it can be seen that not only the absolute amount of hydrogen is reduced, but also hydrogen diffused in the steel is stably trapped.

In addition, after resistance welding of the invention material and the comparative material, resistance spot welding was performed under the same conditions to check the degree of embrittlement and starvation of liquefied metal, and then, the microstructure of the cross-section of the weld was observed with an optical microscope, and the degree of crack generation is shown in Figs. indicated.

4(a) shows the invention material, and FIG. 4(b) shows the comparative material. In the case of the comparative material, it can be seen that a coarse liquefied metal embrittlement crack of 220 μm occurred in the upper left corner in the low magnification optical photograph, In the case of the present invention material, it can be confirmed that the crack is not generated.

In addition, since the embrittlement phenomenon may have variations between evaluation materials, a total of 8 samples were collected and compared, and the size of each crack is statistically shown in FIG. 5 . The average size of the LME material cracks when the comparison 100.8㎛, but also reduced the size of the present invention the absolute crack 25.7㎛ case member, when the coating weight material prior invention over the plating layer cross-section analysis of about 90g / m ^2, compared 65g / m when material ² , it can be seen that the effect of improving the liquefied metal embrittlement resistance is even greater. (average LME crack size per unit coating weight, average LME crack size/plating amount, invention material: 0.29, comparative material: 1.55)

To confirm the improved resistance to embrittlement of liquefied metals of the present invention, the periphery of the LME crack was observed and analyzed at high magnification through a scanning electron microscope, and the results are shown in FIG. 6 . Elements (at least one selected from the group consisting of Ti, V, Ni, Nb, Cu, Zr, Y, and La) that are expected to inhibit the penetration of liquefied zinc by increasing the surface energy around and inside the crack It has been proven by the analysis that the element exists, and when it is seen that the element does not exist in the case of the base material, it can be seen that the presence of the element in the plating layer can suppress the propagation of the liquefied metal embrittlement crack.

Claims (11)

Comprising a galvanized layer formed on at least one surface of the base steel plate and the base steel plate,
The galvanized layer is at least one selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La by weight%: 0.01 to 0.3% in total, Al: 0.01 to 0.15%, balance Zn and others containing unavoidable impurities;
The galvanized layer includes a (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound having a size of 0.01 to 10 μm and a polygonal shape.
The method of claim 1,
With respect to the central region in the thickness direction of the galvanized layer,
The sum of at least one element selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound a first region having a content of 20% or more;
The sum of at least one element selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La in (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound a second region having a content of less than 20%;
A galvanized steel sheet having a size of 0.01 to 10 μm and a (Ti, V, Nb, Ni, Cu, Zr, Y, La)-Al-based intermetallic compound having a polygonal shape is present in the first region.
The method of claim 1,
A galvanized steel sheet having a suppression layer formed of an intermetallic compound of Fe and Al between the base steel sheet and the galvanized layer.
The method of claim 1,
The total content of at least one selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La of the galvanized layer is 0.05 to 0.25%, galvanized steel sheet.
The method of claim 1,
The Al content of the galvanized layer is 0.1 to 0.15%, galvanized steel sheet.
Preparing a base steel plate;
At least one selected from the group consisting of Ti, V, Nb, Ni, Cu, Zr, Y and La: 0.01 to 0.3% in total, Al: 0.01 to 0.15%, balance galvanizing bath containing Zn and other unavoidable impurities to prepare;
hot-dip galvanizing by immersing the base steel sheet in the galvanizing bath in an inert atmosphere;
A first cooling step of cooling the hot-dip galvanized base steel sheet to a temperature range of 230 ~ 270 ℃ at a cooling rate of 0.01 ~ 20 ℃ / s; and
A method of manufacturing a galvanized steel sheet comprising a second cooling step of cooling to room temperature at a cooling rate in the range of 0.01 to 10 °C/s and slower than the cooling rate of the first cooling step.
7. The method of claim 6,
The plating bath temperature of the base steel sheet is more than 450 ℃ less than 550 ℃, the method for producing a galvanized steel sheet.
8. The method of claim 7,
The inlet temperature of the base steel sheet is 5 ℃ higher than the temperature of the galvanizing bath, the method for producing a galvanized steel sheet.
7. The method of claim 6,
The inlet temperature of the base steel sheet is more than 565 ℃ less than 600 ℃, the method of manufacturing a galvanized steel sheet.
7. The method of claim 6,
The inert atmosphere is to use argon gas, a method of manufacturing a galvanized steel sheet.
7. The method of claim 6,
The cooling is performed while spraying an inert gas on the front and back surfaces of the hot-dip galvanized steel sheet, the method of manufacturing a galvanized steel sheet.

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