KR20220088030A - Hot-dip galvanized steel sheet with excellent resistance to welding LME, and method of manufacturing the same - Google Patents

Abstract

The present invention is a hot-dip galvanized steel sheet comprising a base iron and a zinc plated layer plated on the surface of the base iron, wherein the base iron is weight%, C: 0.15 to 0.3%, Al: 0.5 to 2%, Mn: 1.5 to 3.5%, Si: 0.01 to 0.5%, Cr: 0.1 to 1%, Mo: 0.1 to 0.5%, and the remainder including Fe and unavoidable impurities, having a thickness of 5 to 100 μm in the direction of the thickness of the substrate from the surface, To a hot-dip galvanized steel sheet including a ferrite layer made of ferrite having an area fraction of 90% or more, and a method for manufacturing the same.

Description

Hot-dip galvanized steel sheet with excellent resistance to welding LME, and method of manufacturing the same}

The present invention relates to a hot-dip galvanized steel sheet having excellent welding liquid brittle crack resistance, and a method for manufacturing the same.

Conventionally, a high-strength hot-dip galvanized steel sheet using a metamorphic structure has been widely used for automobiles, but during spot welding, the galvanized layer is melted and penetrates along the grain boundaries of the base iron to generate cracks. This is called LME (Liquid Metal Embrittlement Crack), and in order to be used as an automobile part, such liquid brittle cracks must be minimized.

Japanese Patent No. 2864960 discloses a wear-resistant steel containing C: 0.05-0.20 wt%, Mn: 0.50-2.50 wt%, Al: 0.02-2.00 wt% and excellent workability and weldability, and martensite in base iron It was disclosed that the workability and weldability were excellent as the area fraction was 5-50%.

However, when a general-purpose hot-dip galvanizing technology is applied to a steel having such a composition, there is a problem in that liquefied brittle cracks occur during spot welding because the base iron has martensite and ferrite phases. In addition, when a large amount of Mn and Al are included, there is a problem in that non-plating occurs during hot-dip galvanizing due to the formation of annealing oxide on the surface layer during annealing.

Japanese Patent Registration No. 2864960 (December 18, 1998)

An object of the present invention is to provide a hot-dip galvanized steel sheet having excellent plating properties while improving liquefied brittle cracks in welding generated when hot-dip galvanizing of high-strength steel and a method for manufacturing the same.

However, the object of the present invention is not limited to the above-mentioned purpose, and other objects not mentioned will be clearly understood by those of ordinary skill in the art to which the present invention belongs from the following description.

One aspect of the present invention for achieving the above object is a hot-dip galvanized steel sheet comprising a base iron and a galvanized layer plated on the surface of the base iron, wherein the base iron is weight%, C: 0.15 to 0.3%, Al: 0.5~2%, Mn: 1.5~3.5%, Si: 0.01~0.5%, Cr: 0.1~1%, Mo: 0.1~0.5%, and the remainder of Fe and unavoidable impurities. It relates to a hot-dip galvanized steel sheet having a thickness of 5 to 100 μm and comprising a ferrite layer made of ferrite having an area fraction of 90% or more.

In one aspect of the present invention, the base iron may be prepared by recrystallization annealing the steel slab in an annealing furnace controlled to a dew point temperature of -20 to 25 ℃.

In one aspect of the present invention, the base iron may include retained austenite of 1 to 15% of the area fraction and martensite of 5 to 20%.

In one aspect of the present invention, the base iron may have a tensile strength of 980 MPa or more.

In one aspect of the present invention, the hot-dip galvanized steel sheet may have a length of liquefied brittle cracks during spot welding of 20 μm or less.

In addition, another aspect of the present invention is by weight %, C: 0.15 to 0.3%, Al: 0.5 to 2%, Mn: 1.5 to 3.5%, Si: 0.01 to 0.5%, Cr: 0.1 to 1%, Mo: Reheating the steel slab containing 0.1-0.5% and the balance Fe and unavoidable impurities; manufacturing a hot-rolled steel sheet by hot-rolling the reheated steel slab; manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet; manufacturing the base iron by recrystallization annealing the cold-rolled steel sheet in an annealing furnace controlled to a dew point temperature of -20 to 25°C; and immersing the base iron in a hot-dip galvanized bath to prepare a hot-dip galvanized steel sheet plated with a galvanized layer.

The hot-dip galvanized steel sheet according to the present invention has an alloy composition and content of C: 0.15 to 0.3%, Al: 0.5 to 2%, Mn: 1.5 to 3.5%, Si: 0.01 to 0.5%, Cr: 0.1 to 1%, Mo : As the base iron is manufactured by controlling the dew point temperature to -20 to 25 ° C during recrystallization annealing, while containing 0.1 to 0.5% and the remainder of Fe, the surface layer of the base iron has a thickness of 5 to 100 μm, the area fraction A ferrite layer made of 90% or more of ferrite can be formed.

Through this, excellent characteristics such as a tensile strength of 980 MPa or more and a length of liquefied brittle cracks of 20 μm or less during spot welding can be secured. This has the advantage that it does not peel off easily.

1 is a photograph of the microstructure of the substrate prepared in Example 1 and Comparative Example 9, respectively, measured with a scanning electron microscope (SEM).
2 is a photograph of the LME crack length measured by SEM after spot welding the hot-dip galvanized steel prepared in Example 1 and Comparative Example 9, respectively.
3 is a photograph of evaluating plating properties by hot-dip galvanizing the base iron prepared in Example 1 and Comparative Example 9, respectively.

Hereinafter, a hot-dip galvanized steel sheet having excellent welding liquid brittle crack resistance and a method for manufacturing the same according to the present invention will be described in detail. The drawings introduced below are provided as examples so that the spirit of the present invention can be sufficiently conveyed to those skilled in the art. Accordingly, the present invention is not limited to the drawings presented below and may be embodied in other forms, and the drawings presented below may be exaggerated to clarify the spirit of the present invention. At this time, if there is no other definition in the technical terms and scientific terms used, it has the meaning commonly understood by those of ordinary skill in the art to which this invention belongs, and the gist of the present invention in the following description and accompanying drawings Descriptions of known functions and configurations that may be unnecessarily obscure will be omitted.

Throughout the specification, when a part "includes" a certain element, it means that other elements may be further included, rather than excluding other elements, unless otherwise stated.

One aspect of the present invention is a hot-dip galvanized steel sheet including a base iron and a zinc plated layer plated on the surface of the base iron, wherein the base iron is weight %, C: 0.15 to 0.3%, Al: 0.5 to 2%, Mn: 1.5 to 3.5%, Si: 0.01 to 0.5%, Cr: 0.1 to 1%, Mo: 0.1 to 0.5%, and the remainder of Fe and unavoidable impurities. It has a thickness and relates to a hot-dip galvanized steel sheet comprising a ferrite layer made of ferrite with an area fraction of 90% or more.

As such, the hot-dip galvanized steel sheet according to the present invention has an alloy composition and content of C: 0.15 to 0.3%, Al: 0.5 to 2%, Mn: 1.5 to 3.5%, Si: 0.01 to 0.5%, Cr: 0.1 to 1 %, Mo: 0.1 to 0.5% and the balance of Fe is included, and at the same time as the dew point temperature during recrystallization annealing is controlled to -20 to 25 ° C. , a ferrite layer made of ferrite having an area fraction of 90% or more can be formed.

Through this, excellent characteristics such as a tensile strength of 980 MPa or more and a length of liquefied brittle cracks of 20 μm or less during spot welding can be secured. This has the advantage that it does not peel off easily.

On the other hand, even if the alloy composition and content are satisfied, when the dew point temperature is out of the range of -20 to 25 ° C, the ferrite layer is not formed or has a thickness of less than 5 µm, so that the LME crack length during spot welding may exceed 20 µm. .

Conversely, even if it has a thickness of 5 to 100 μm and a ferrite layer made of ferrite having an area fraction of 90% or more, the alloy composition and content are C: 0.15 to 0.3%, Al: 0.5 to 2%, Mn: 1.5 to 3.5%, When Si: 0.01~0.5%, Cr: 0.1~1%, Mo: 0.1~0.5% and the remainder of Fe are not satisfied, the tensile strength is lowered to less than 980 MPa, but the LME crack length may exceed 20 μm. Alternatively, during hot-dip galvanizing, an unplated area may be generated, or plating adhesion may be lowered, so that the galvanized layer may be easily peeled off.

Hereinafter, the reason for numerical limitation of the alloy component content in an example of the present invention will be described. Hereinafter, unless otherwise specified, the unit is % by weight.

First, as described above, the hot-dip galvanized steel sheet according to the present invention may be manufactured by plating a base iron prepared by recrystallization annealing a steel slab with hot-dip zinc.

In an example of the present invention, the steel slab is, as described above, in wt%, C: 0.15 to 0.3%, Al: 0.5 to 2%, Mn: 1.5 to 3.5%, Si: 0.01 to 0.5%, Cr: 0.1 to 1%, Mo: 0.1 to 0.5%, and the remainder of Fe and unavoidable impurities.

C: 0.15-0.3 wt%

Carbon (C) is an essential element added to secure the strength of steel, and in order to secure sufficient strength, it is preferably added in the range of 0.15 to 0.3 wt %, more preferably 0.18 to 3 wt % do. When the content is added to less than 0.15% by weight, the strength is reduced, and when it exceeds 0.3% by weight, there is a problem in that weldability is deteriorated and press forming and roll workability are deteriorated.

Al: 0.5-2 wt%

Aluminum (Al) is an element for improving the strength of steel and expanding the ferrite area fraction. For this purpose, Al is preferably added in an amount of 0.5 to 2 wt%, more preferably in an amount of 1.5 to 2 wt%. do. When the content is less than 0.5% by weight, strength may be lowered, and it may be difficult to secure a ferrite layer having an area fraction of 90% or more. Conversely, if the content exceeds 2% by weight, an unplated area may occur during hot-dip galvanizing due to the formation of annealing oxide in the surface layer during annealing. There is a problem in that it is difficult to prevent cracking due to zinc.

Mn: 1.5-3.5 wt%

Manganese (Mn) is well known as a hardenability enhancing element that stabilizes austenite. In addition, as an element effective for improving the strength of steel, it is preferable to add Mn in an amount of 1.5 wt% or more in order to secure the tensile strength targeted in the present invention, and more preferably, add it in an amount of 2.5 to 3.5 wt%. . The higher the Mn content, the easier it is to secure strength. However, when Mn is added in excess of 3.5 wt%, an annealing oxide is formed in the surface layer during annealing and fixing, so unplated areas may occur during hot-dip galvanizing. During martensite, there is a problem in that it is difficult to prevent cracks caused by molten zinc in the liquid phase penetrating into the martensite phase interface.

Si: 0.01-0.5 wt%

Silicon (Si) is an element effective for improving the strength of steel and stabilizing ferrite, and it is preferable to add Si in an amount of 0.01 wt % or more in order to secure the tensile strength targeted in the present invention. However, when Si is added in excess, an oxide layer is formed on the surface of the base iron to suppress decarburization of the surface layer, so it is preferable that Si is added in an amount of 0.5 wt % or less. More preferably, it is more preferable to add 0.1 to 0.5 wt% of Si in order to secure a tensile strength of 1200 MPa or more and a liquefied brittle crack length of 10 μm or less.

Cr: 0.1 to 1 wt%

Chromium (Cr) is an element that has a large effect on improving the strength of steel, and in order to secure a tensile strength of 980 MPa or more and a liquefied brittle crack length of 20 μm or less, which is targeted in the present invention, it is preferably added in an amount of 0.1 wt % or more. , when the content exceeds 1% by weight, there is a problem in that the amount of alloy input is excessive, causing a cost increase. More preferably, adding 0.5 to 1% by weight of Cr is more preferable in order to secure a tensile strength of 1200 MPa or more and a liquefied brittle crack length of 10 μm or less.

Mo: 0.1-0.5 wt%

Molybdenum (Mo) is an element having a large effect of contributing to the improvement of the strength of steel, similar to Cr, and is an element advantageous for securing strength without impairing the wettability of molten zinc. However, since it is economically undesirable if the content of such Mo is excessive, it is preferable to limit the content to 0.1 to 0.5% by weight. More preferably, it is more preferable to add 0.3 to 0.5 wt% of Mo in order to secure a tensile strength of 1200 MPa or more and a liquefied brittle crack length of 10 μm or less.

balance of Fe

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

In addition, as described above, the base iron according to the present invention has a thickness of 5 to 100 µm in the thickness direction of the base iron from the surface, and a ferrite layer made of ferrite having an area fraction of 90% or more may be formed on the surface layer. More preferably, the ferrite layer may be made of ferrite having an area fraction of 95% or more, more preferably of 99% or more, and most preferably, it may be made of only ferrite.

Such a ferrite layer has the advantage of reducing liquefied brittle cracks during spot welding, and is good because it can improve the non-plating problem during hot-dip plating. On the other hand, when the thickness of the ferrite layer is less than 5 μm, the length of the liquefied brittle crack after spot welding may exceed 20 μm, and when the thickness of the ferrite layer exceeds 100 μm, there may be a problem in that the fatigue strength decreases.

As such, in order to form a ferrite layer having a thickness of 5 to 100 μm on the surface layer of the iron substrate, it is necessary to decarburize the surface layer of the substrate iron by increasing the oxygen concentration in the annealing furnace during recrystallization annealing. Decarburization is a phenomenon in which carbon in steel is evacuated from the surface layer of the base iron, and the carbon content of the surface layer is lowered, thereby promoting the transformation from austenite to ferrite. In the case of high-strength steel, Mn or Si is added to form a transformed structure and improve strength. These elements form an oxide layer on the surface during annealing, so the decarburization of the surface layer tends to be suppressed.

Therefore, in order to promote decarburization, it is necessary to appropriately control the annealing conditions as well as control the composition and content of the steel component. Specifically, the base iron may be prepared by recrystallization annealing a steel slab in an annealing furnace controlled to a dew point temperature of -20 to 25 °C. In the case of recrystallization annealing of a steel slab satisfying the above composition and content in an annealing furnace controlled at a dew point temperature of -20 to 25° C., it has a thickness of 5 to 100 μm in the direction of the thickness of the base iron from the surface, and ferrite with an area fraction of 90% or more It is possible to form a ferrite layer made of On the other hand, even if a base iron satisfying the above composition and content is used, if the dew point temperature is not controlled to -20 to 25° C. during recrystallization annealing, the LME crack length may become long or a problem may occur in that some areas are not plated during hot-dip galvanizing. Not good. In addition, when the dew point temperature exceeds 25 ℃ may cause a problem that the thickness of the ferrite layer of the surface layer of the iron substrate exceeds 100 ㎛.

Next, the structure of So Ji-cheol will be described.

The base iron of this hot-dip galvanized steel sheet includes retained austenite and martensite, and the area fraction of retained austenite may be 5 to 15%, and the area fraction of martensite may be 5 to 30%. In this range, it is possible to achieve the effect of reducing liquefaction cracks during spot welding while having excellent strength.

On the other hand, the galvanized layer according to an embodiment of the present invention may be formed by a method commonly used in the art, and the galvanized layer may be plated on the surface of the base iron by immersing the base iron in a hot-dip galvanizing bath.

In addition, another aspect of the present invention relates to the above-described method for manufacturing a hot-dip galvanized steel sheet, after preparing a steel slab, [reheating - hot rolling - cold rolling - reheating annealing - galvanizing] through the process of hot-dip galvanized steel sheet can be manufactured.

Specifically, the method for manufacturing a hot-dip galvanized steel sheet according to the present invention comprises:

In weight %, C: 0.15 to 0.3%, Al: 0.5 to 2%, Mn: 1.5 to 3.5%, Si: 0.01 to 0.5%, Cr: 0.1 to 1%, Mo: 0.1 to 0.5%, and the remainder of Fe reheating the steel slab containing unavoidable impurities; manufacturing a hot-rolled steel sheet by hot-rolling the reheated steel slab; manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet; manufacturing the base iron by recrystallization annealing the cold-rolled steel sheet in an annealing furnace controlled to a dew point temperature of -20 to 25°C; and immersing the base iron in a hot-dip galvanizing bath to prepare a hot-dip galvanized steel sheet plated with a galvanized layer.

reheating process

First, it is preferable to reheat the steel slab satisfying the above-mentioned composition to a certain temperature range.

At this time, it is preferable that the reheating temperature range is 1100 to 1300 ° C. If the temperature is less than 1100 ° C, there is a problem in that the hot rolling load rapidly increases, whereas if it exceeds 1300 ° C, the reheating cost increases as well as the surface This is not preferable because there is a problem in that the amount of scale becomes excessive.

hot rolling process

It is preferable to produce a hot-rolled steel sheet by hot-rolling the reheated steel slab according to the above, specifically, finish hot-rolling.

At this time, it is preferable that the finish rolling temperature during the finish hot rolling is Ar ₃ or higher. If the finish rolling temperature is less than Ar ₃ , it is not preferable because there is a risk of malfunction due to variations in the hot rolling load.

Thereafter, the hot-rolled steel sheet may be wound. At this time, the winding is preferably carried out at 700 ° C. or less, and more advantageously, it is preferably carried out at 400 to 700 ° C. If the coiling temperature exceeds 700℃, there is a risk that an oxide film is excessively formed on the surface of the steel sheet to cause defects. Therefore, it is not preferable

cold rolling process

Thereafter, it is preferable to cold-roll the hot-rolled steel sheet to manufacture a cold-rolled steel sheet.

There is no particular limitation on the cold rolling reduction during the cold rolling, but it is preferably carried out at 60% or less in consideration of the rolling process load and the like.

reheat annealing process

Thereafter, it is preferable to perform annealing heat treatment by charging the cold-rolled steel sheet manufactured according to the above into an annealing furnace.

In the present invention, it is necessary to appropriately control the dew point in terms of manufacturing a base iron having excellent strength and improving plating properties during hot-dip galvanizing.

Specifically, as described above, the cold-rolled steel sheet may be recrystallized annealing in an annealing furnace controlled to a dew point temperature of -20 to 25° C. to manufacture base iron.

Even if the alloy composition and content are satisfied, ferrite may not be formed on the surface layer of the base iron when the dew point temperature is out of the range of -20 to 25 ℃, or the tensile strength is lowered to less than 980 MPa, but the LME crack length is 20 ㎛ Alternatively, during hot-dip galvanizing, an unplated area may occur or plating adhesion may be lowered to cause a problem in which the galvanized layer is easily peeled off. In addition, when the dew point temperature exceeds 25 ℃ may cause a problem that the thickness of the ferrite layer of the surface layer of the iron substrate exceeds 100 ㎛.

At this time, the temperature of the annealing furnace is 750 ~ 950 ℃, the annealing furnace gas atmosphere can be controlled with 3 to 70% by volume of hydrogen (H ₂ ) and the balance nitrogen (N ₂ ), the annealing time is 10 seconds to 100 seconds It is preferable to perform

Next, it is possible to prepare the base iron by cooling in a conventional way.

Hot-dip galvanizing process

Thereafter, the base iron may be immersed in a hot-dip galvanizing bath to manufacture a hot-dip galvanized steel sheet.

At this time, after reheating or re-cooling the base iron to 450 ~ 500 ℃, it can be immersed in a hot-dip galvanizing bath consisting of 0.13 to 0.3 wt% of Al, the balance of Zn and inevitable impurities, and the hot-dip galvanizing bath is It can be maintained at 440 ~ 500 ℃. During plating, a hot-dip galvanized steel sheet can be manufactured by taking out the base iron immersed in the plating bath, adjusting the plating adhesion amount, and cooling it.

If the temperature of the plating bath is less than 440° C., the viscosity of zinc increases and thus the drivability of the roll in the plating bath is deteriorated. On the other hand, when the temperature of the plating bath exceeds 500° C., it is not preferable because the evaporated zinc increases.

In addition, when the Al content in the plating bath is less than 0.13% by weight, the formation of the Fe-Al alloy phase formed at the interface between the base iron and the plating layer is suppressed and there is a risk of plating peeling, whereas when it exceeds 0.3% by weight, the Al in the plating layer is suppressed It is not preferable because there is a problem that the content is increased to impair weldability.

Hereinafter, a hot-dip galvanized steel sheet excellent in welding liquid brittle crack resistance and a method for manufacturing the same according to the present invention through Examples will be described in more detail. However, the following examples are only a reference for describing the present invention in detail, and the present invention is not limited thereto, and may be implemented in various forms.

Also, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein is for the purpose of effectively describing particular embodiments only and is not intended to limit the invention. In addition, the % unit of additives not specifically described in the specification is weight %.

[Examples 1 to 8, and Comparative Examples 1 to 12]

Each steel slab was prepared after dissolving the steel having the component composition shown in Table 1 below. Each of the steel slabs was reheated at 1200° C. for 1 hour and then finished hot rolled at 900° C. to prepare a hot-rolled steel sheet.

Thereafter, each hot-rolled steel sheet was cooled to 650° C. and then maintained for 1 hour in a heating furnace maintained at 650° C., followed by furnace cooling. After visually observing whether hot-rolled cracks occurred in the cooled hot-rolled steel sheet, cold rolling was performed on the cooled steel sheet at a reduction ratio of 50% to manufacture a cold-rolled steel sheet.

Each of the cold-rolled steel sheets was subjected to recrystallization annealing under the dew point temperature conditions of Table 1 below, and at this time, heat treatment was performed at an annealing temperature of 870° C. and an annealing time of 50 seconds, and the gas atmosphere in the annealing furnace was 95% by volume N ₂ , 5% by volume H ₂ was adjusted with

In addition, the recrystallization annealed specimen was immersed in a hot-dip galvanizing bath at 460° C. for plating, and the plating amount was kept constant at 60 g/m 2 per side. After plating, it was cooled using nitrogen.

content (wt%) dew point temperature
(℃) C Mn Al Si Cr Mo Example 1 0.18 2.7 1.5 0.3 0.5 0.3 -20 Example 2 0.18 2.7 1.5 0.3 0.5 0.3 25 Example 3 0.18 2.7 1.5 0.3 0.5 0.3 5 Example 4 0.15 2.7 1.5 0.5 0.5 0.3 5 Example 5 0.15 1.5 0.5 0.01 0.1 0.1 -20 Example 6 0.15 1.5 0.5 0.01 0.1 0.1 25 Example 7 0.3 3.5 2.0 0.5 One 0.5 -20 Example 8 0.3 3.5 2.0 0.5 One 0.5 25 Comparative Example 1 0.3 3.5 2.0 0.5 One 0.5 30 Comparative Example 2 0.18 2.7 2.2 0.3 0.5 0.3 20 Comparative Example 3 0.18 2.7 2.2 0.3 0.5 0.3 30 Comparative Example 4 0.18 3.6 1.0 0.3 0.5 0.3 10 Comparative Example 5 0.18 3.6 0.4 0.3 0.5 0.3 -40 Comparative Example 6 0.18 2.7 1.5 0.7 0.5 0.3 -40 Comparative Example 7 0.18 2.7 1.5 0.3 0.5 0.3 30 Comparative Example 8 0.18 2.7 1.5 0.3 0.5 0.3 -40 Comparative Example 9 0.14 2.7 1.5 0.3 0.5 0.3 -40 Comparative Example 10 0.32 2.7 1.5 0.5 0.1 0.1 5 Comparative Example 11 0.18 2.7 1.5 0.3 0.5 0.05 5 Comparative Example 12 0.18 2.7 0.4 0.3 0.5 0.3 5

[Characteristic evaluation]

The ferrite thickness was measured using a recrystallization annealed specimen before hot-dip galvanizing, and the tensile strength, spot welding test (LME crack) and plating properties of the plated hot-dip galvanized steel specimen were evaluated based on the following evaluation criteria. Table 2 shows.

1) Ferrite thickness (㎛): The ferrite thickness of the recrystallization annealed specimen was measured using an electron microscope.

2) Tensile strength (MPa): A tensile test was performed according to JIS 5.

3) Spot welding test: Spot welding was performed according to ISO 18278-2, and the length (㎛) of LME cracks generated at the shoulder of the weld after spot welding was measured with a scanning electron microscope (SEM).

4) Plating properties: After plating, the plating adhesion was evaluated after visually observing whether non-plating was performed. If non-plating did not occur and plating adhesion was good, it was evaluated as O, and in case of non-plating or plating peeling, X was evaluated. .

Ferrite thickness (㎛) Tensile strength (㎫) LME crack length (μm) Plating Example 1 30 1230 15 O Example 2 100 1230 5 O Example 3 60 1230 7 O Example 4 50 1130 10 O Example 5 5 1050 12 O Example 6 100 1050 3 O Example 7 5 1390 18 O Example 8 100 1390 9 O Comparative Example 1 120 1390 14 X Comparative Example 2 90 1255 12 X Comparative Example 3 90 1255 12 X Comparative Example 4 70 1270 11 X Comparative Example 5 70 940 73 O Comparative Example 6 100 1270 91 X Comparative Example 7 110 1230 5 X Comparative Example 8 0 1230 56 X Comparative Example 9 0 890 35 X Comparative Example 10 50 1310 25 O Comparative Example 11 3 1230 22 O Comparative Example 12 110 910 O X

As shown in Tables 1 and 2, in the case of Examples 1 to 8 in which the dew point temperature during recrystallization annealing was controlled to -20 to 25° C. while satisfying the composition and content of the base iron alloy presented in the present invention, the surface layer of the base iron had a thickness It was composed of ferrite with a thickness of 1 to 100 μm, and showed characteristics such as a tensile strength of 980 MPa or more and a length of 20 μm or less of LME cracks.

On the other hand, in the case of Comparative Example 1, although slab steel having the same alloy composition as in Examples 7 and 8 was used, the dew point temperature was controlled to be slightly higher at 30° C. during recrystallization annealing, so there was a problem in that some areas were not plated during hot-dip galvanizing. occurred.

In Comparative Example 2, as Al was added in a rather large amount at 2.2 wt%, there was a problem in that some areas were not plated during hot-dip galvanizing.

Even in Comparative Example 3, Al was added somewhat at 2.2 wt%, and the dew point temperature during recrystallization annealing was controlled to be somewhat high at 30° C., so there was a problem in that some areas were not plated during hot-dip galvanizing.

In the case of Comparative Example 4, as Mn was added in a rather large amount (3.6 wt%), there was a problem in that some areas were not plated during hot-dip galvanizing.

In the case of Comparative Example 5, Mn was added in a rather large amount at 3.6 wt%, and Al was added in a rather small amount at 0.4 wt%, and the dew point temperature during recrystallization annealing was also controlled to be low at -40 ° C. There was a problem with the length of the

In the case of Comparative Example 6, Si was added in a rather excessive amount at 0.7 wt%, and the dew point temperature during recrystallization annealing was also controlled to be low at -40 ° C. A non-plating problem occurred.

In Comparative Example 7, although slab steel having the same alloy composition as in Examples 1 to 3 was used, the dew point temperature during recrystallization annealing was controlled to be somewhat high at 30° C., so there was a problem in that some areas were not plated during hot-dip galvanizing. .

In the case of Comparative Example 8, too, slab steel of the same alloy composition as in Examples 1 to 3 was used, but as the dew point temperature during recrystallization annealing was controlled as low as -40° C., ferrite was not formed on the surface layer of the base iron, molten zinc There was a problem that some areas were not plated during plating.

In the case of Comparative Example 9, C was added in a rather small amount at 0.14 wt %, and as the dew point temperature during recrystallization annealing was controlled as low as -40 ° C, ferrite was not formed on the surface layer of the base iron, the tensile strength was lowered, and LME cracking length was increased, and there was a problem that some areas were not plated during hot-dip galvanizing, so all the measured properties were not good.

In the case of Comparative Example 10, a problem occurred that the length of the LME crack was increased to 20 μm or more as C was added in a rather large amount at 0.32 wt%, and even in Comparative Example 11, Mo was added in a rather small amount at 0.05 wt%. Accordingly, there was a problem in that the length of the LME crack was increased to 20 μm or more.

In the case of Comparative Example 12, as Al was added in a rather small amount (0.4 wt%), the thickness of the ferrite layer exceeded 100 µm, and there was a problem in that the fatigue strength was reduced, and the problem that some areas were not plated during hot-dip galvanizing occurred. occurred.

Although the present invention has been described with reference to specific matters and limited examples as described above, these are only provided to help a more general understanding of the present invention, and the present invention is not limited to the above examples, and the present invention belongs to Various modifications and variations are possible from these descriptions by those of ordinary skill in the art.

Therefore, the spirit of the present invention should not be limited to the described embodiments, and not only the claims to be described later, but also all those with equivalent or equivalent modifications to the claims will be said to belong to the scope of the spirit of the present invention. .

Claims (6)

A hot-dip galvanized steel sheet comprising a base iron and a galvanized layer plated on the surface of the base iron,
The base iron is in wt%, C: 0.15 to 0.3%, Al: 0.5 to 2%, Mn: 1.5 to 3.5%, Si: 0.01 to 0.5%, Cr: 0.1 to 1%, Mo: 0.1 to 0.5% and A hot-dip galvanized steel sheet containing the remainder of Fe and unavoidable impurities, having a thickness of 5 to 100 μm in the direction of the thickness of the base iron from the surface, and comprising a ferrite layer made of ferrite having an area fraction of 90% or more.
The method of claim 1,
The base iron is one manufactured by recrystallization annealing of a steel slab in an annealing furnace controlled to a dew point temperature of -20 to 25 °C, hot-dip galvanized steel sheet.
The method of claim 1,
The base iron is a hot-dip galvanized steel sheet comprising retained austenite of 1 to 15% of the area fraction and martensite of 5 to 20%.
The method of claim 1,
The base iron has a tensile strength of 980 MPa or more, hot-dip galvanized steel sheet.
The method of claim 1,
In the hot-dip galvanized steel sheet, the length of the liquid brittle crack during spot welding is 20 μm or less, the hot-dip galvanized steel sheet.
By weight%, C: 0.15 to 0.3%, Al: 0.5 to 2%, Mn: 1.5 to 3.5%, Si: 0.01 to 0.5%, Cr: 0.1 to 1%, Mo: 0.1 to 0.5%, and the remainder of Fe reheating the steel slab containing unavoidable impurities;
manufacturing a hot-rolled steel sheet by hot rolling the reheated steel slab;
manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet;
manufacturing the base iron by recrystallization annealing the cold-rolled steel sheet in an annealing furnace controlled to a dew point temperature of -20 to 25°C; and
manufacturing a hot-dip galvanized steel sheet plated with a galvanized layer by immersing the base iron in a hot-dip galvanized bath;
A method of manufacturing a hot-dip galvanized steel sheet comprising a.

Images