WO2015099455A1 - Molten zinc plated steel sheet with excellent crack resistance due to liquid metal bromide - Google Patents

Molten zinc plated steel sheet with excellent crack resistance due to liquid metal bromide Download PDF

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WO2015099455A1
WO2015099455A1 PCT/KR2014/012824 KR2014012824W WO2015099455A1 WO 2015099455 A1 WO2015099455 A1 WO 2015099455A1 KR 2014012824 W KR2014012824 W KR 2014012824W WO 2015099455 A1 WO2015099455 A1 WO 2015099455A1
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Prior art keywords
steel sheet
alloy layer
layer
hot
dip galvanized
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PCT/KR2014/012824
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French (fr)
Korean (ko)
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WO2015099455A8 (en
Inventor
이주연
진광근
전선호
김종상
김명수
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주식회사 포스코
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Priority to JP2016543078A priority Critical patent/JP6317453B2/en
Priority to US15/108,263 priority patent/US20160319415A1/en
Priority to EP14875617.4A priority patent/EP3088557B1/en
Priority to CN201480071104.XA priority patent/CN105849304A/en
Publication of WO2015099455A1 publication Critical patent/WO2015099455A1/en
Publication of WO2015099455A8 publication Critical patent/WO2015099455A8/en

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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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Definitions

  • the present invention relates to a hot-dip galvanized steel sheet excellent in crack resistance due to liquid metal embrittlement.
  • the steel sheet for use as automotive parts need to secure high strength, ductility and corrosion resistance.
  • Patent Document 1 is a representative technology for this.
  • the technology is 0.15 to 0.30 wt% of carbon (C), 0.01 to 0.03 wt% of silicon (Si), 15 to 25 wt% of manganese (Mn), 1.2 to 3.0 wt% of aluminum (Al), and 0.020 wt% of phosphorus (P).
  • sulfur (S) 0.001 to 0.002% by weight
  • balance iron (Fe) and other unavoidable impurities and the TWIP (Twin Induced Plasticity) type ultra high strength steel sheet, characterized in that the steel microstructure consists of austenite phase.
  • TWIP Twin Induced Plasticity
  • hot-dip galvanized steel sheet is excellent in corrosion resistance and widely used in building materials, structures, home appliances and automobile bodies.
  • Hot-dip galvanized steel sheet which is used most recently can be divided into hot-dip galvanized steel sheet (hereinafter referred to as 'GI steel sheet') and alloyed hot-dip galvanized steel sheet (hereinafter referred to as 'GA steel sheet').
  • GI steel sheet is a steel plate in which molten zinc is plated on a steel plate, and is easily used for automobile body due to its easy plating and excellent corrosion resistance.
  • a GI steel sheet is a steel plate which formed the plating layer by immersing in the zinc plating bath which added 0.16-0.25 weight% of Al.
  • the plating layer is mostly composed of zinc, but the alloying inhibitor layer capable of suppressing alloying of iron and zinc is present at a thickness of 1 ⁇ m or less at the base iron and the zinc plated layer interface, so that the adhesion between the base iron and the plating layer is reduced. great.
  • the alloying inhibiting layer is usually made of Fe 2 Al 5-x Zn x .
  • the alloying inhibiting layer formed on the GI steel sheet generates liquid zinc while melting by welding heat.
  • the welded portion rises to about 1500 ° C. or more within about 1 second, whereby the base iron and the plating layer are melted and welded.
  • the temperature of the plating layer is raised to 600 ⁇ 800 °C in the welding heat affected (HAZ) portion, whereby Fe is diffused in the plating layer, a portion of the plating layer is alloyed with Fe-Zn alloy layer, the rest is liquid It becomes zinc.
  • liquid zinc penetrates into the grain boundary of the surface of the body, and when the tensile stress is operated on the HAZ, cracks having a size of about 10 to 100 ⁇ m are generated, causing brittle fracture. This is called liquid metal embrittlement (hereinafter referred to as 'LME').
  • 'LME' liquid metal embrittlement
  • the metal has a higher resistance value than other steel grades, resulting in a higher temperature, and the grain boundary due to the high coefficient of thermal expansion is expanded.
  • thermal stress may occur because it has a higher coefficient of thermal expansion than other steel grades such as ferritic steel sheets, which may cause liquid metal embrittlement because the thermal stress is applied to the weld part without external tensile stress. This is very high.
  • FIG. 1 is a photograph observing the GI TWIP steel generated LME crack in the weld. As shown in FIG. 1, when the LME crack is generated, it causes breakage of the steel sheet, and thus it is difficult to be used as an automotive part.
  • Patent Document 1 Korean Unexamined Patent Publication No. 2007-0018416
  • the present invention is to provide a hot-dip galvanized steel sheet excellent in crack resistance due to liquid metal embrittlement.
  • One embodiment of the present invention is a steel sheet having a microstructure having a fraction of austenite of 90 area% or more; And a hot dip galvanized layer formed on the base steel sheet, wherein the hot dip galvanized layer comprises: a Fe—Zn alloy layer; And a Zn layer formed on the Fe—Zn alloy layer, wherein the Fe—Zn alloy layer is hot-dip galvanized steel sheet having excellent crack resistance due to liquid metal embrittlement having a thickness of [(3.4 ⁇ t) / 6] ⁇ m or more.
  • the present invention it is possible to provide a hot-dip galvanized steel sheet in which the plating layer peeling phenomenon easily occurring under ordinary automotive welding and molding conditions is not only prevented, but also the occurrence of cracks due to embrittlement of liquid metal is suppressed.
  • FIG. 2 (a) is a schematic diagram showing a cross section of a conventional GI TWIP steel
  • Figure 2 (b) is a schematic diagram showing a cross section of a hot-dip galvanized steel sheet according to an embodiment of the present invention.
  • FIG 3 is a cross-sectional view of a welded part of Inventive Example 1 according to an embodiment of the present invention.
  • Figure 4 is a cross-sectional photograph of the welded portion of Comparative Example 1 outside the scope of the present invention.
  • the inventors of the present invention while researching a method for effectively suppressing the occurrence of cracks due to liquid metal embrittlement in the manufacturing of the aforementioned GI TIWP steel, the surface oxide and Fe-Al or Fe-Al-Zn to suppress the diffusion of Fe
  • the present invention has been completed under the knowledge that formation of an alloy layer can be suppressed and formation of a Fe-Zn alloy layer having a sufficient thickness can prevent crack generation by LME.
  • FIG. 2 is merely a representation of one embodiment of the present invention in order to explain the present invention, and does not limit the scope of the present invention.
  • the existing conventional GI TWIP steel is a Fe-Al or Fe-Al-Zn alloy layer (2) on the base steel sheet (1), the Fe-Al or Fe-Al-Zn alloy It can be seen that the Zn layer 3 is formed on the layer 2, and that a surface oxide 4 such as MnO is present between the base steel sheet 1 and the Zn layer 3. .
  • the Fe-Al or Fe-Al-Zn alloy layer 2 generates liquid zinc during spot welding, causing LME cracks.
  • the hot-dip galvanized layer 20 is formed on the holding steel sheet 10 and the holding steel sheet, and at this time, the molten zinc plated steel sheet
  • the galvanized layer 20 has a structure in which the Fe—Zn alloy layer 21 and the Zn layer 22 are sequentially formed to secure not only the plating adhesion but also the crack generation resistance due to excellent LME.
  • the hot dip galvanized layer 20 of the present invention formed on the base steel sheet 10 preferably has a structure in which the Fe—Zn alloy layer 21 and the Zn layer 22 are sequentially formed.
  • the base steel sheet 100 to be applied to the present invention targets TWIP steel which is severely cracked by LME as mentioned above. Accordingly, the present invention has a microstructure having an austenitic fraction of 90 area% or more. It is preferable.
  • the base steel sheet used in the hot-dip galvanized steel sheet of the present invention is a weight% as an embodiment, C: 0.10 ⁇ 0.30%, Mn: 10 ⁇ 30 %, Si: 0.01 to 0.03%, Ti: 0.05 to 0.2%, Mn: 10 to 30%, Al: 0.5 to 3.0%, Ni: 0.001 to 10%, Cr: 0.001 to 10%, N: 0.001 to 0.05% , P: 0.020% or less, S: 0.001% to 0.005%, residual Fe, and other unavoidable impurities.
  • the Fe-Zn alloy layer 21 is formed to a sufficient thickness.
  • the Fe—Zn alloy layer 21 is effective in suppressing the occurrence of cracks by LME by reducing the formation of liquid zinc.
  • the thickness of the said Fe-Zn alloy layer is [(3.4xt) / 6] micrometer or more.
  • the thickness of the Fe—Zn alloy layer is less than [(3.4 ⁇ t) / 6] ⁇ m, the effect of suppressing crack generation by LME cannot be sufficiently obtained.
  • t mentioned above means the thickness of the hot dip galvanized layer.
  • the thicker the Fe-Zn alloy layer the thicker the preferred effect, so that the upper limit of the Fe-Zn alloy layer thickness is not particularly limited.
  • the Fe-Zn alloy layer 21 preferably contains 3 to 15% by weight of Fe.
  • the Fe content in the Fe-Zn alloy layer is less than 3% by weight, the LME is the same as the existing GI steel sheet. There may be a disadvantage in that cracks are generated, and if it exceeds 15% by weight may cause a problem that workability is lowered.
  • the Fe-Zn alloy layer 21 does not react with Fe and Zn remains as a Zn layer.
  • the formation of the Fe—Al or Fe—Al—Zn alloy layer 23 formed under the hot dip galvanized layer 30, that is, between the base steel sheet 10 and the Fe—Zn alloy layer 21 is performed. It is desirable to suppress as much as possible.
  • the Fe-Al or Fe-Al-Zn alloy layer 23 forms liquid zinc during welding, causing cracks by LME, so that the Fe-Al or Fe-Al-Zn alloy layer 23 is formed as thin as possible in the present invention.
  • the component content of the Fe-Al and Fe-Al-Zn alloy layer is not particularly limited, but as an example, the Fe-Al alloy layer may be Fe 2 Al 5 , the Fe-Al The Zn alloy layer may be Fe 2 Al 5 Znx.
  • the alloy layer 23 preferably contains less than 0.3% by weight of Al, when the Al content contained in the alloy layer 23 exceeds 0.3% by weight Fe diffusion is suppressed to a sufficient thickness It may be difficult to secure the Fe—Zn alloy layer.
  • the Fe-Ni alloy layer 30 is further included directly below the surface of the base steel sheet.
  • the Fe-Ni alloy layer 30 suppresses surface oxides such as MnO formed by concentrating oxidative elements such as Mn on the surface of the Fe-Ni alloy layer 30, such as conventional GI TWIP steels. It ensures the excellent plating adhesion by being present.
  • the Fe-Ni alloy layer may be formed by a Ni coating layer having an adhesion amount of 300 ⁇ 1000mg / m 2 , the thickness can be different under the influence of the manufacturing conditions.
  • the thickness of the Fe-Ni alloy layer may have a range of 0.05 ⁇ 5 ⁇ m.
  • the Fe—Ni alloy layer is formed to be less than 0.05 ⁇ m, the zinc wettability may deteriorate, resulting in unplating or plating adhesion.
  • the thickness of the Fe-Ni alloy layer exceeds 5 ⁇ m may cause a problem that the amount of Fe diffused from the base steel plate to the plating layer is reduced, there is a disadvantage that the manufacturing cost increases rapidly.
  • the base steel sheet and the hot-dip galvanized layer 1 selected from the group consisting of Fe-X alloy layer, Fe-Al-X alloy layer, Fe-Al-Zn-X alloy layer and Fe-Zn-X alloy layer. More than one species may be further included.
  • the alloy layer it is possible to secure not only plating adhesion but also crack generation resistance due to excellent LME.
  • X mentioned above may be one of Ni and Cr, for example, as a material capable of having a cation in the electroplating solution.
  • the hot-dip galvanized steel sheet of the present invention provided as described above can not only secure crack resistance by excellent LME, but also can secure a good level of plating adhesion, which is usually required for hot-dip galvanized steel sheet.
  • the hot-dip galvanized steel sheet of the present invention can be produced by a variety of methods, preferably after forming the Ni coating layer on the steel plate, 700 ⁇ 900 in a reducing atmosphere furnace H 2 -N 2 mixed gas is charged After heating to °C, the heated steel sheet can be cooled using, and then immersed in a hot dip galvanizing bath of 440 ⁇ 460 °C containing Al of 0.13% by weight or less, the technique If the person skilled in the art other than the conditions can be easily controlled without additional repetitive experiments can be produced hot-dip galvanized steel sheet proposed by the present invention.
  • a base steel sheet having a microstructure having a fraction of austenite of 90 area% or more is prepared.
  • the steel sheet is a TWIP steel, and has a high austenite fraction.
  • the steel sheet includes a large amount of oxidizing elements Mn, Al, and Ni, it is necessary to clean the surface of the steel sheet in advance. For example, in order to remove foreign substances, oxide films, etc. on the surface, it is preferable to perform a pickling or washing process. If the pickling or washing process is not performed, the coating layer or the plating layer may be uneven, and the appearance and adhesion of the plating may deteriorate.
  • the Ni coating layer is formed on the base steel sheet prepared as above.
  • the Ni coating layer may be formed by electroplating, thereby forming a coating layer having a uniform thickness.
  • the Ni coating layer is preferably having an adhesion amount of 300 ⁇ 1000mg / m 2, when the adhesion amount of the Ni coating layer is less than 300mg / m 2 Fe-Ni alloy layer of sufficient thickness is not formed, the surface concentration of Mn It may not be sufficiently inhibited and the zinc wettability may also deteriorate, resulting in unplating or deterioration of plating adhesion.
  • the steel sheet on which the Ni coating layer is formed is heated to 700 to 900 ° C. in a reducing atmosphere in which the H 2 -N 2 mixed gas is charged.
  • Ni of the Ni coating layer may be penetrated into the base steel sheet to form a Fe—Ni alloy layer.
  • the heating temperature is less than 700 ° C there is a problem that the steel sheet structure does not transform into austenite phase after cold rolling, and when the temperature exceeds 900 ° C, the possibility of deformation and breakage in the steel sheet increases.
  • the fraction of the H 2 -N 2 mixed gas used for forming the reducing atmosphere may be used as commonly used in the art, the present invention specifically refers to the fraction of the H 2 -N 2 mixed gas. I never do that.
  • the holding steel sheet is preferably maintained for 20 seconds or more in the heating temperature range.
  • the holding time is less than 20 seconds, a Fe-Ni alloy layer having a sufficient thickness is not formed, thereby sufficiently increasing the surface concentration of Mn. It may not be suppressed.
  • the heated steel sheet is cooled to a cooling rate of 5 ° C / s or more to 400 ⁇ 500 ° C. If the cooling rate is less than 5 ° C / s it is difficult to secure austenite of more than 90 area%.
  • the plating bath inlet temperature of the cooled steel sheet is controlled to have a range of (hot dip galvanizing bath -40 °C) ⁇ (hot dip galvanizing bath +10 °C).
  • the plating bath inlet temperature is lower than (hot dip galvanizing bath -40 °C)
  • Fe contained in the steel sheet is less eluted to suppress the formation of Fe-Zn alloy phase, and exceeds (hot dip galvanizing bath +10 °C)
  • the holding plate is cooled when the cooling stop temperature is higher than the plating bath inlet temperature, and when the cooling stop temperature is the same as the plating bath inlet temperature When the cooling stop temperature is lower than the plating bath inlet temperature, the holding steel sheet may be heated.
  • the plated steel sheet controlled to the plating bath inlet temperature range is immersed in a hot dip galvanizing bath at 440 to 460 ° C. containing 0.13% by weight or less of Al to apply a plating solution to the surface of the plated steel sheet.
  • a hot dip galvanizing bath at 440 to 460 ° C. containing 0.13% by weight or less of Al to apply a plating solution to the surface of the plated steel sheet.
  • the Al content of the hot-dip galvanizing bath exceeds 0.13% by weight, diffusion of Fe may be suppressed and it may be difficult to secure a Fe—Zn alloy layer having a sufficient thickness.
  • the temperature of the hot dip galvanizing bath is less than 440 °C, it is difficult to secure the fluidity of the plating solution and plating may not be performed smoothly, if the temperature exceeds 460 °C, problems such as volatilization of the plating solution occurs.
  • the base steel plate coated with the plating liquid is slowly cooled at a cooling rate of 4 to 20 ° C./s to form a hot dip galvanized layer. If the slow cooling rate is less than 4 °C / s uncoagulated zinc buried in the equipment, such as a roll causes a secondary defect of the product, if the temperature exceeds 20 °C / s Fe-Zn alloy layer grows to a sufficient thickness There is a drawback to not doing it.
  • an Ni coating layer was applied to the base steel sheet by the deposition amount shown in Table 1 below by electroplating (Comparative Examples 2 to 4 were not performed). Subsequently, the base steel sheet was heated under the conditions shown in Table 1 in a reducing atmosphere loaded with a 5% H 2 -N 2 mixed gas, cooled to 400 ° C., and the plating bath inlet temperature was controlled. The plating liquid was applied by immersion in a hot dip galvanizing bath.
  • the plating adhesion evaluation was carried out to check whether the plating is buried on the tape after bending the hot-dip galvanized steel sheet 180 °, peeled when the plating is smeared, it was represented as non-peeled.

Abstract

The objective of the present invention is to provide a molten zinc plated steel sheet with excellent crack resistance due to liquid metal bromide. One aspect of the present invention provides a molten zinc plated steel sheet with excellent crack resistance due to liquid metal bromide, the steel sheet comprising: a base steel sheet having a microstructure in which the fraction of austenite is greater than or equal to 90 area%; and a molten zinc plated layer formed on the base steel sheet, wherein the molten zinc plated layer comprises: a Fe-Zn alloy layer; and a Zn layer formed on the Fe-Zn alloy layer, and the Fe-Zn alloy layer provides a molten zinc plated steel sheet having a thickness of [(3.4×t)/6]㎛ or more with excellent crack resistance due to liquid metal bromide (where t is the thickness of the molten zinc plated steel sheet). The present invention can prevent a plated layer peeling phenomenon which easily occurs in common car welding and molding conditions, and can also provide a molten zinc plated steel sheet in which crack occurrence due to liquid metal bromide is suppressed.

Description

액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판Hot-dip galvanized steel sheet with excellent crack resistance due to liquid metal embrittlement
본 발명은 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판에 관한 것이다.The present invention relates to a hot-dip galvanized steel sheet excellent in crack resistance due to liquid metal embrittlement.
일반적으로 자동차용 부품은 차체경량화와 안정성이 요구되며, 이를 위해 자동차용 부품으로 사용하기 위한 강판은 높은 강도와 연성 및 내식성을 확보하는 것이 필요하다.In general, automotive parts are required to reduce the weight and stability of the body, for this purpose, the steel sheet for use as automotive parts need to secure high strength, ductility and corrosion resistance.
이를 위한 대표적인 기술로는 특허문헌 1이 있다. 상기 기술은 탄소(C) 0.15∼0.30 중량%, 실리콘(Si) 0.01∼0.03 중량%, 망간(Mn) 15∼25 중량%, 알루미늄(Al) 1.2∼3.0 중량%, 인(P) 0.020 중량% 이하, 황(S) 0.001∼0.002 중량%, 잔부 철(Fe) 및 기타 불가피한 불순물로 이루어지며, 강의 미세조직이 오스테나이트 상으로 이루어진 것을 특징으로 하는 TWIP(TWin Induced Plasticity)형 초고강도 강판에 관한 것으로서, 초고장력 및 고연신율을 확보함으로써 차체 경량화 요구에 대응하고 있다. Patent Document 1 is a representative technology for this. The technology is 0.15 to 0.30 wt% of carbon (C), 0.01 to 0.03 wt% of silicon (Si), 15 to 25 wt% of manganese (Mn), 1.2 to 3.0 wt% of aluminum (Al), and 0.020 wt% of phosphorus (P). Hereinafter, sulfur (S) 0.001 to 0.002% by weight, balance iron (Fe) and other unavoidable impurities, and the TWIP (Twin Induced Plasticity) type ultra high strength steel sheet, characterized in that the steel microstructure consists of austenite phase. As a result, ultra high tensile strength and high elongation are secured to meet the demand for weight reduction.
한편, 용융도금강판은 내식성이 우수하여 건축자재, 구조물, 가전제품 및 자동차 차체 등에 널리 사용되고 있다. 최근에 가장 많이 사용되고 있는 용융도금강판은 용융아연도금강판(이하, 'GI강판'이라고 함)과 합금화 용융아연도금강판(이하, 'GA강판'이라고 함)으로 나눌 수 있다.On the other hand, hot-dip galvanized steel sheet is excellent in corrosion resistance and widely used in building materials, structures, home appliances and automobile bodies. Hot-dip galvanized steel sheet which is used most recently can be divided into hot-dip galvanized steel sheet (hereinafter referred to as 'GI steel sheet') and alloyed hot-dip galvanized steel sheet (hereinafter referred to as 'GA steel sheet').
GI강판은 강판에 용융아연을 도금한 강판으로서, 도금이 쉽고 내식성이 우수하여 자동차 차체로 많이 사용되고 있다. 통상 GI강판은 Al을 0.16~0.25중량% 첨가한 아연도금욕에 침지하여 도금층을 형성한 강판이다. 상기 GI강판은 도금층이 대부분 아연으로 구성되어 있으나, 소지철과 아연도금층 계면에는 철과 아연의 합금화를 억제할 수 있는 합금화 억제층이 1㎛이하의 두께로 존재함으로서, 소지철과 도금층의 밀착성이 우수하다. 상기 합금화 억제층은 통상적으로 Fe2Al5-xZnx으로 이루어져 있다.GI steel sheet is a steel plate in which molten zinc is plated on a steel plate, and is easily used for automobile body due to its easy plating and excellent corrosion resistance. Usually, a GI steel sheet is a steel plate which formed the plating layer by immersing in the zinc plating bath which added 0.16-0.25 weight% of Al. In the GI steel sheet, the plating layer is mostly composed of zinc, but the alloying inhibitor layer capable of suppressing alloying of iron and zinc is present at a thickness of 1 μm or less at the base iron and the zinc plated layer interface, so that the adhesion between the base iron and the plating layer is reduced. great. The alloying inhibiting layer is usually made of Fe 2 Al 5-x Zn x .
한편, 상기 GI강판을 자동차용 부품으로 사용하기 위해 일반적으로 스팟 용접을 행하게 되는데, 이 때 상기 GI강판에 형성된 합금화 억제층은 용접열에 의해 용융되면서 액체 아연을 발생시키게 된다. 보다 상세하게는, 상기 스팟 용접시 용접부는 약 1초 이내에 약 1500℃이상까지 상승하게 되고, 이로 인해 소지철과 도금층이 용융되어 용접된다. 이 때, 용접 열영향(HAZ)부에서는 도금층의 온도가 600~800℃까지 상승하게 되는데, 이에 의해 상기 도금층에 Fe가 확산되어 상기 도금층의 일부는 Fe-Zn 합금층으로 합금화되고, 나머지는 액체 아연이 된다. 상기 액체 아연은 소지 표면의 결정립계에 침투해 들어가게 되는데, 이 때 HAZ에 인장응력이 작동하게 되면 약 10~100㎛의 크기를 갖는 크랙을 발생시켜 취성파괴를 일으키게 된다. 이를 액체금속취화(Liquid Metal Embrittlement, 이하, 'LME'라도고 함)라고 한다. 특히 오스테나이트 분율이 많은 TWIP강 등의 경우에는 타 강종 대비 높은 저항값을 가져 보다 고온의 상태가 되며, 높은 열팽창 계수에 의한 결정립계가 확장되기 때문에, 액체금속취화 문제가 심하게 일어난다. 또한, TWIP강의 경우에는 페라이트계 강판 등 타 강종에 비해 높은 열팽창 계수를 갖기 때문에 열적 응력이 발생할 수 있는데, 이로 인해 외부의 인장응력이 없어도 상기 열적 응력이 용접부에 가해지게 되어 액체금속취화가 발생할 가능성이 매우 높다.On the other hand, in order to use the GI steel sheet as an automotive part, spot welding is generally performed. At this time, the alloying inhibiting layer formed on the GI steel sheet generates liquid zinc while melting by welding heat. In more detail, during the spot welding, the welded portion rises to about 1500 ° C. or more within about 1 second, whereby the base iron and the plating layer are melted and welded. At this time, the temperature of the plating layer is raised to 600 ~ 800 ℃ in the welding heat affected (HAZ) portion, whereby Fe is diffused in the plating layer, a portion of the plating layer is alloyed with Fe-Zn alloy layer, the rest is liquid It becomes zinc. The liquid zinc penetrates into the grain boundary of the surface of the body, and when the tensile stress is operated on the HAZ, cracks having a size of about 10 to 100 μm are generated, causing brittle fracture. This is called liquid metal embrittlement (hereinafter referred to as 'LME'). In particular, in the case of TWIP steel having a high austenite fraction, the metal has a higher resistance value than other steel grades, resulting in a higher temperature, and the grain boundary due to the high coefficient of thermal expansion is expanded. In addition, in the case of TWIP steel, thermal stress may occur because it has a higher coefficient of thermal expansion than other steel grades such as ferritic steel sheets, which may cause liquid metal embrittlement because the thermal stress is applied to the weld part without external tensile stress. This is very high.
도 1은 용접부에서 LME 크랙이 발생한 GI TWIP강을 관찰한 사진이다. 도 1에 나타난 바와 같이 LME 크랙이 발생하는 경우에는 강판의 파단 원인이 되므로, 자동차용 부품 등으로 사용되기 곤란하다.1 is a photograph observing the GI TWIP steel generated LME crack in the weld. As shown in FIG. 1, when the LME crack is generated, it causes breakage of the steel sheet, and thus it is difficult to be used as an automotive part.
이러한 기술적인 문제로 인해 오스테나이트 상분율이 많은 GI TWIP강판에 대해서는 용접 후 액체금속취화에 의한 크랙 저항성을 향상시키는 기술의 개발이 요구된다. Due to these technical problems, the development of a technique for improving crack resistance due to liquid metal embrittlement after welding is required for GI TWIP steel sheets having a high austenite phase fraction.
[선행기술문헌][Preceding technical literature]
(특허문헌 1) 한국 공개특허공보 제2007-0018416호(Patent Document 1) Korean Unexamined Patent Publication No. 2007-0018416
본 발명은 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판을 제공하고자 하는 것이다.The present invention is to provide a hot-dip galvanized steel sheet excellent in crack resistance due to liquid metal embrittlement.
본 발명의 일 실시형태는 오스테나이트의 분율이 90면적%이상인 미세조직을 갖는 소지강판; 및 상기 소지강판 상에 형성된 용융아연도금층을 포함하고, 상기 용융아연도금층은, Fe-Zn 합금층; 및 상기 Fe-Zn 합금층 상에 형성된 Zn층을 포함하며, 상기 Fe-Zn 합금층은 [(3.4×t)/6]㎛이상의 두께를 갖는 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판을 제공한다.One embodiment of the present invention is a steel sheet having a microstructure having a fraction of austenite of 90 area% or more; And a hot dip galvanized layer formed on the base steel sheet, wherein the hot dip galvanized layer comprises: a Fe—Zn alloy layer; And a Zn layer formed on the Fe—Zn alloy layer, wherein the Fe—Zn alloy layer is hot-dip galvanized steel sheet having excellent crack resistance due to liquid metal embrittlement having a thickness of [(3.4 × t) / 6] μm or more. To provide.
(단, 상기 t는 상기 용융아연도금층의 두께임.)(Wherein t is the thickness of the hot dip galvanized layer.)
본 발명에 따르면, 통상적인 자동차 용접 및 성형 조건에서 쉽게 일어나는 도금층 박리현상이 방지될 뿐만 아니라, 액체금속취화에 의한 크랙 발생이 억제된 용융아연도금강판을 제공할 수 있다.According to the present invention, it is possible to provide a hot-dip galvanized steel sheet in which the plating layer peeling phenomenon easily occurring under ordinary automotive welding and molding conditions is not only prevented, but also the occurrence of cracks due to embrittlement of liquid metal is suppressed.
도 1은 용접부에서 LME 크랙이 발생한 GI TWIP강을 관찰한 사진이다.1 is a photograph observing the GI TWIP steel generated LME crack in the weld.
도 2 (a)는 기존의 GI TWIP강의 단면을 나타내는 모식도이며, 도 2 (b)는 본 발명의 일 실시형태에 따른 용융아연도금강판의 단면을 나타내는 모식도이다.2 (a) is a schematic diagram showing a cross section of a conventional GI TWIP steel, Figure 2 (b) is a schematic diagram showing a cross section of a hot-dip galvanized steel sheet according to an embodiment of the present invention.
도 3은 본 발명의 일 실시예에 따른 발명예 1의 용접부 단면 사진이다.3 is a cross-sectional view of a welded part of Inventive Example 1 according to an embodiment of the present invention.
도 4는 본 발명의 범위를 벗어나는 비교예 1의 용접부 단면 사진이다.Figure 4 is a cross-sectional photograph of the welded portion of Comparative Example 1 outside the scope of the present invention.
본 발명자들은 앞서 언급한 GI TIWP강의 제조시 액체금속취화에 의한 크랙 발생을 효과적으로 억제할 수 있는 방안에 대해서 연구를 행하던 중, Fe의 확산을 억제하는 표면산화물과 Fe-Al 또는 Fe-Al-Zn 합금층의 형성을 억제하고, 충분한 두께의 Fe-Zn 합금층을 형성시킴으로써 LME에 의한 크랙 발생을 방지할 수 있다는 식견하에 본 발명을 완성하게 되었다.The inventors of the present invention, while researching a method for effectively suppressing the occurrence of cracks due to liquid metal embrittlement in the manufacturing of the aforementioned GI TIWP steel, the surface oxide and Fe-Al or Fe-Al-Zn to suppress the diffusion of Fe The present invention has been completed under the knowledge that formation of an alloy layer can be suppressed and formation of a Fe-Zn alloy layer having a sufficient thickness can prevent crack generation by LME.
도 2 (a)는 기존의 GI TWIP강의 단면을 나타내는 모식도이며, 도 2 (b)는 본 발명의 일 실시형태에 따른 용융아연도금강판의 단면을 나타내는 모식도이다. 이하, 도 2를 참조하여 본 발명을 설명한다. 다만, 도 2는 본 발명을 설명하기 위하여 본 발명의 일 실시형태를 모식적으로 표현한 것일 뿐, 본 발명의 권리범위를 한정하지 않는다. 2 (a) is a schematic diagram showing a cross section of a conventional GI TWIP steel, Figure 2 (b) is a schematic diagram showing a cross section of a hot-dip galvanized steel sheet according to an embodiment of the present invention. Hereinafter, the present invention will be described with reference to FIG. 2. 2 is merely a representation of one embodiment of the present invention in order to explain the present invention, and does not limit the scope of the present invention.
도 2 (a)에 나타난 바와 같이, 기존의 일반적인 GI TWIP강은 소지강판(1)상에 Fe-Al 또는 Fe-Al-Zn 합금층(2), 상기 Fe-Al 또는 Fe-Al-Zn 합금층(2) 상에 Zn층(3)이 형성되어 있음을 알 수 있고, MnO 등과 같은 표면산화물(4)이 상기 소지강판(1)과 Zn층(3) 사이에 존재하고 있음을 알 수 있다. 이러한 구조의 도금층을 갖는 GI TWIP강의 경우 스팟 용접시 상기 Fe-Al 또는 Fe-Al-Zn 합금층(2)이 액체 아연을 발생시켜 LME 크랙을 유발하게 된다.As shown in Figure 2 (a), the existing conventional GI TWIP steel is a Fe-Al or Fe-Al-Zn alloy layer (2) on the base steel sheet (1), the Fe-Al or Fe-Al-Zn alloy It can be seen that the Zn layer 3 is formed on the layer 2, and that a surface oxide 4 such as MnO is present between the base steel sheet 1 and the Zn layer 3. . In the case of GI TWIP steel having a plating layer having such a structure, the Fe-Al or Fe-Al-Zn alloy layer 2 generates liquid zinc during spot welding, causing LME cracks.
그러나, 도 2 (b)에 나타난 바와 같이, 본 발명의 일 실시형태에 따른 용융아연도금강판은 소지강판(10)과 상기 소지강판 상에는 용융아연도금층(20)이 형성되도록 하고, 이 때, 용융아연도금층(20)은, Fe-Zn 합금층(21) 및 Zn층(22)이 순차적으로 형성되는 구조를 갖도록 함으로써 도금밀착성 뿐만 아니라 우수한 LME에 의한 크랙 발생 저항성을 확보하게 된다.However, as shown in Fig. 2 (b), the hot-dip galvanized steel sheet according to an embodiment of the present invention, the hot-dip galvanized layer 20 is formed on the holding steel sheet 10 and the holding steel sheet, and at this time, the molten zinc plated steel sheet The galvanized layer 20 has a structure in which the Fe—Zn alloy layer 21 and the Zn layer 22 are sequentially formed to secure not only the plating adhesion but also the crack generation resistance due to excellent LME.
상기와 같이, 소지강판(10) 상에 형성되는 본 발명의 용융아연도금층(20)은 Fe-Zn 합금층(21) 및 Zn층(22)이 순차적으로 형성되는 구조를 갖는 것이 바람직하다. As described above, the hot dip galvanized layer 20 of the present invention formed on the base steel sheet 10 preferably has a structure in which the Fe—Zn alloy layer 21 and the Zn layer 22 are sequentially formed.
본 발명에 적용되는 소지강판(100)은 앞서 언급한 바와 같이 LME에 의한 크랙 문제가 심하게 발생하는 TWIP강을 대상으로 하며, 이에 따라, 본 발명은 오스테나이트 분율이 90면적%이상인 미세조직을 갖는 것이 바람직하다. 또한, 상기 미세조직 확보와 더불어 우수한 기계적 물성 등을 확보하기 위해서, 본 발명의 용융아연도금강판에 이용되는 소지강판은 일 실시형태로서 중량%로, C: 0.10~0.30%, Mn: 10~30%, Si: 0.01∼0.03%, Ti: 0.05~0.2%, Mn: 10∼30%, Al: 0.5∼3.0%, Ni: 0.001~10%, Cr: 0.001~10%, N: 0.001~0.05%, P: 0.020%이하, S: 0.001∼0.005%, 잔부 Fe 및 기타 불가피한 불순물을 포함할 수 있다.The base steel sheet 100 to be applied to the present invention targets TWIP steel which is severely cracked by LME as mentioned above. Accordingly, the present invention has a microstructure having an austenitic fraction of 90 area% or more. It is preferable. In addition, in order to secure the microstructure and excellent mechanical properties, etc., the base steel sheet used in the hot-dip galvanized steel sheet of the present invention is a weight% as an embodiment, C: 0.10 ~ 0.30%, Mn: 10 ~ 30 %, Si: 0.01 to 0.03%, Ti: 0.05 to 0.2%, Mn: 10 to 30%, Al: 0.5 to 3.0%, Ni: 0.001 to 10%, Cr: 0.001 to 10%, N: 0.001 to 0.05% , P: 0.020% or less, S: 0.001% to 0.005%, residual Fe, and other unavoidable impurities.
본 발명에서는 상기 Fe-Zn 합금층(21)를 충분한 두께로 형성시키는 것을 특징으로 한다. 상기 Fe-Zn 합금층(21)은 액체 아연의 형성을 저감시켜 LME에 의한 크랙 발생을 억제하는데 효과적이다. LME에 의한 크랙 발생을 억제하기 위해서는 용접시 Fe가 빨리 확산하여 상기 Fe가 Zn와 반응하여 Fe-Zn 합금층을 형성하도록 하는 것이 유리하다. 이는 Zn이 Fe와 우선적으로 반응함으로써 상기 Zn가 용접에 의한 열영향을 받아 액체 아연이 되는 것을 억제시킬 수 있기 때문이다. 따라서, 본 발명에서는 상기 Fe-Zn 합금층(21)을 미리 충분한 두께로 형성시킴으로써 상기 효과를 보다 향상시키고자 한다. 이를 위해, 상기 Fe-Zn 합금층의 두께가 [(3.4×t)/6]㎛이상인 것이 바람직하다. 상기 Fe-Zn 합금층의 두께가 [(3.4×t)/6]㎛미만인 경우에는 LME에 의한 크랙 발생을 억제 효과를 충분히 얻을 수 없다. 한편, 상기 언급한 t는 상기 용융아연도금층의 두께를 의미한다. 본 발명에서는 상기 Fe-Zn 합금층의 두께는 두꺼울수록 바람직한 효과를 구현하므로, 상기 Fe-Zn 합금층 두께의 상한에 대해서는 특별히 한정하지 않는다.In the present invention, the Fe-Zn alloy layer 21 is formed to a sufficient thickness. The Fe—Zn alloy layer 21 is effective in suppressing the occurrence of cracks by LME by reducing the formation of liquid zinc. In order to suppress crack generation by LME, it is advantageous that Fe diffuses quickly during welding so that Fe reacts with Zn to form a Fe—Zn alloy layer. This is because the Zn reacts with Fe preferentially, thereby suppressing the Zn from becoming a liquid zinc due to heat effect by welding. Therefore, in the present invention, the Fe-Zn alloy layer 21 is formed to a sufficient thickness in advance to further improve the effect. For this purpose, it is preferable that the thickness of the said Fe-Zn alloy layer is [(3.4xt) / 6] micrometer or more. When the thickness of the Fe—Zn alloy layer is less than [(3.4 × t) / 6] μm, the effect of suppressing crack generation by LME cannot be sufficiently obtained. Meanwhile, t mentioned above means the thickness of the hot dip galvanized layer. In the present invention, the thicker the Fe-Zn alloy layer, the thicker the preferred effect, so that the upper limit of the Fe-Zn alloy layer thickness is not particularly limited.
나아가, 상기 Fe-Zn 합금층(21)은 3~15중량%의 Fe를 포함하는 것이 바람직한데, 상기 Fe-Zn 합금층 내 Fe 함량이 3중량%미만일 경우에는 기존 GI강판과 동일한 함량으로서 LME에 의한 크랙이 발생하는 단점이 있을 수 있으며, 15중량%를 초과하는 경우에는 가공성이 저하되는 문제점이 발생할 수 있다.Furthermore, the Fe-Zn alloy layer 21 preferably contains 3 to 15% by weight of Fe. When the Fe content in the Fe-Zn alloy layer is less than 3% by weight, the LME is the same as the existing GI steel sheet. There may be a disadvantage in that cracks are generated, and if it exceeds 15% by weight may cause a problem that workability is lowered.
상기 Fe-Zn 합금층(21) 상에는 Fe와 반응하지 못하여 Zn이 Zn층으로서 잔류하게 된다. The Fe-Zn alloy layer 21 does not react with Fe and Zn remains as a Zn layer.
한편, 본 발명에서는 용융아연도금층(30)의 하부 즉, 소지강판(10)과 Fe-Zn 합금층(21) 사이에 형성되는 Fe-Al 또는 Fe-Al-Zn 합금층(23)의 형성을 가능한 억제하는 것이 바람직하다. 상기 Fe-Al 또는 Fe-Al-Zn 합금층(23)은 용접시 액체 아연을 형성하여 LME에 의한 크랙을 유발하게 되므로, 본 발명에서는 가능한 얇은 두께로 형성되도록 한다. 한편, 본 발명에서는 상기 Fe-Al 및 Fe-Al-Zn 합금층의 구성 요소 함량에 대해서는 특별히 한정하지 않으나, 일례로는, Fe-Al 합금층은 Fe2Al5일 수 있고, 상기 Fe-Al-Zn 합금층은 Fe2Al5Znx일 수 있다.Meanwhile, in the present invention, the formation of the Fe—Al or Fe—Al—Zn alloy layer 23 formed under the hot dip galvanized layer 30, that is, between the base steel sheet 10 and the Fe—Zn alloy layer 21 is performed. It is desirable to suppress as much as possible. The Fe-Al or Fe-Al-Zn alloy layer 23 forms liquid zinc during welding, causing cracks by LME, so that the Fe-Al or Fe-Al-Zn alloy layer 23 is formed as thin as possible in the present invention. In the present invention, the component content of the Fe-Al and Fe-Al-Zn alloy layer is not particularly limited, but as an example, the Fe-Al alloy layer may be Fe 2 Al 5 , the Fe-Al The Zn alloy layer may be Fe 2 Al 5 Znx.
더불어, 상기 합금층(23)은 0.3중량%이하의 Al을 포함하는 것이 바람직한데, 상기 합금층(23) 내에 포함되는 Al 함량이 0.3중량%를 초과하는 경우에는 Fe의 확산이 억제되어 충분한 두께의 Fe-Zn 합금층을 확보하기 곤란할 수 있다. In addition, the alloy layer 23 preferably contains less than 0.3% by weight of Al, when the Al content contained in the alloy layer 23 exceeds 0.3% by weight Fe diffusion is suppressed to a sufficient thickness It may be difficult to secure the Fe—Zn alloy layer.
한편, 상기 소지강판의 표면 직하에는 Fe-Ni 합금층(30)이 추가로 포함되는 것이 바람직하다. 보다 상세하게는, 상기 Fe-Ni 합금층(30)은 기존의 GI TWIP강과 같이 Mn 등의 산화성 원소가 표면에 농화되어 형성되는 MnO 등의 표면산화물을 억제함으로써 상기 MnO 등이 내부산화물(40)로 존재하게 하여 우수한 도금밀착성을 확보하게 한다. 상기 효과를 확보하기 위해서는, 상기 Fe-Ni 합금층은 300~1000mg/m2의 부착량을 갖는 Ni 코팅층에 의해 형성될 수 있으며, 제조조건의 영향에 지배를 받아 그 두께가 상이해질 수 있다. 일례로는, 상기 Fe-Ni 합금층의 두께가 0.05~5㎛의 범위를 가질 수 있다. 만일, 상기 Fe-Ni 합금층이 0.05㎛미만으로 형성되는 경우에는 아연 젖음성이 악화되어 미도금이 되거나 도금밀착성이 저하될 수 있다. 반면, 상기 Fe-Ni 합금층의 두께가 5㎛를 초과하는 경우에는 소지강판으로부터 도금층으로 확산되는 Fe의 양이 줄어들게 되는 문제가 발생할 수 있을 뿐만 아니라, 제조원가가 급격히 상승하게 되는 단점이 있다.On the other hand, it is preferable that the Fe-Ni alloy layer 30 is further included directly below the surface of the base steel sheet. In more detail, the Fe-Ni alloy layer 30 suppresses surface oxides such as MnO formed by concentrating oxidative elements such as Mn on the surface of the Fe-Ni alloy layer 30, such as conventional GI TWIP steels. It ensures the excellent plating adhesion by being present. In order to secure the effect, the Fe-Ni alloy layer may be formed by a Ni coating layer having an adhesion amount of 300 ~ 1000mg / m 2 , the thickness can be different under the influence of the manufacturing conditions. As an example, the thickness of the Fe-Ni alloy layer may have a range of 0.05 ~ 5㎛. If the Fe—Ni alloy layer is formed to be less than 0.05 μm, the zinc wettability may deteriorate, resulting in unplating or plating adhesion. On the other hand, when the thickness of the Fe-Ni alloy layer exceeds 5㎛ may cause a problem that the amount of Fe diffused from the base steel plate to the plating layer is reduced, there is a disadvantage that the manufacturing cost increases rapidly.
또한, 상기 소지강판과 상기 용융아연도금층의 사이에는 Fe-X 합금층, Fe-Al-X 합금층, Fe-Al-Zn-X 합금층 및 Fe-Zn-X 합금층으로 이루어지는 그룹으로부터 선택된 1종 이상이 추가로 포함될 수 있다. 상기 합금층이 형성됨으로써 도금밀착성 뿐만 아니라 우수한 LME에 의한 크랙 발생 저항성을 확보할 수 있다. 상기 언급한 X는 예를 들면 전기도금 용액 내에서 양이온을 가질 수 있는 물질로서 Ni 및 Cr 중 하나일 수 있다.In addition, between the base steel sheet and the hot-dip galvanized layer 1 selected from the group consisting of Fe-X alloy layer, Fe-Al-X alloy layer, Fe-Al-Zn-X alloy layer and Fe-Zn-X alloy layer. More than one species may be further included. By forming the alloy layer, it is possible to secure not only plating adhesion but also crack generation resistance due to excellent LME. X mentioned above may be one of Ni and Cr, for example, as a material capable of having a cation in the electroplating solution.
전술한 바와 같이 제공되는 본 발명의 용융아연도금강판은 우수한 LME에 의한 크랙 저항성을 확보할 수 있을 뿐만 아니라, 용융아연도금강판에 통상적으로 요구되는 물성인 도금밀착성 또한 우수한 수준으로 확보할 수 있다.The hot-dip galvanized steel sheet of the present invention provided as described above can not only secure crack resistance by excellent LME, but also can secure a good level of plating adhesion, which is usually required for hot-dip galvanized steel sheet.
한편, 본 발명의 용융아연도금강판은 다양한 방법에 의해 제조될 수 있으나, 바람직하게는 소지강판 상에 Ni 코팅층을 형성시킨 후, H2-N2 혼합가스가 장입된 환원성 분위기로에서 700~900℃까지 가열한 뒤, 상기 가열된 소지강판을 냉각하고, 그 뒤, 0.13중량%이하의 Al을 포함하는 440~460℃의 용융아연도금욕에 침지하는 방법을 이용하여 제조될 수 있으며, 당해 기술분야에서 통상의 지식을 가진 자라면 이외의 조건은 별도의 반복적인 실험없이 용이하게 제어함으로써 본 발명이 제안하는 용융아연도금강판을 제조할 수 있다.On the other hand, the hot-dip galvanized steel sheet of the present invention can be produced by a variety of methods, preferably after forming the Ni coating layer on the steel plate, 700 ~ 900 in a reducing atmosphere furnace H 2 -N 2 mixed gas is charged After heating to ℃, the heated steel sheet can be cooled using, and then immersed in a hot dip galvanizing bath of 440 ~ 460 ℃ containing Al of 0.13% by weight or less, the technique If the person skilled in the art other than the conditions can be easily controlled without additional repetitive experiments can be produced hot-dip galvanized steel sheet proposed by the present invention.
우선, 오스테나이트의 분율이 90면적%이상인 미세조직을 갖는 소지강판을 준비한다. 상기 소지강판은 TWIP강으로서, 높은 오스테나이트의 분율을 가지며, 이를 위해 다량의 산화성 원소인 Mn, Al 및 Ni 등을 포함하게 되므로, 사전에 상기 소지강판의 표면을 청정화할 필요가 있다. 예를 들면 표면의 이물질이나 산화막 등을 제거하기 위하여, 산세 혹은 세정 공정을 행하는 것이 바람직하다. 만일 상기 산세 혹은 세정 공정을 행하지 않는 경우에는 코팅층 또는 도금층이 불균일하게 되고, 도금 외관이나 밀착성이 악화될 수 있다. First, a base steel sheet having a microstructure having a fraction of austenite of 90 area% or more is prepared. The steel sheet is a TWIP steel, and has a high austenite fraction. For this purpose, since the steel sheet includes a large amount of oxidizing elements Mn, Al, and Ni, it is necessary to clean the surface of the steel sheet in advance. For example, in order to remove foreign substances, oxide films, etc. on the surface, it is preferable to perform a pickling or washing process. If the pickling or washing process is not performed, the coating layer or the plating layer may be uneven, and the appearance and adhesion of the plating may deteriorate.
상기와 같이 준비된 상기 소지강판 상에 Ni 코팅층을 형성시킨다. 상기 Ni 코팅층의 형성은 전기도금에 의해 행하여질 수 있는데 이를 통해 균일한 두께를 갖는 코팅층을 형성시킬 수 있다. 한편, 상기 Ni 코팅층은 300~1000mg/m2의 부착량을 갖는 것이 바람직한데, 상기 Ni 코팅층의 부착량이 300mg/m2미만인 경우에는 충분한 두께의 Fe-Ni 합금층이 형성되지 않아 Mn의 표면농화량을 충분히 억제하지 못하고 아연 젖음성 또한 악화되어 미도금이 되거나 도금밀착성이 저하될 수 있다. 1000mg/m2를 초과하는 경우에는 Ni 함량이 높은 Fe-Ni 합금층의 형성으로 인해 소지강판으로부터 도금층으로 확산되는 Fe의 양이 줄어들 게 되어 충분한 두께의 Fe-Zn 합금층을 얻을 수 없을 뿐만 아니라, 제조원가가 급격히 상승하게 되는 단점이 있다. The Ni coating layer is formed on the base steel sheet prepared as above. The Ni coating layer may be formed by electroplating, thereby forming a coating layer having a uniform thickness. On the other hand, the Ni coating layer is preferably having an adhesion amount of 300 ~ 1000mg / m 2, when the adhesion amount of the Ni coating layer is less than 300mg / m 2 Fe-Ni alloy layer of sufficient thickness is not formed, the surface concentration of Mn It may not be sufficiently inhibited and the zinc wettability may also deteriorate, resulting in unplating or deterioration of plating adhesion. When the amount exceeds 1000 mg / m 2 , the amount of Fe diffused from the base steel plate to the plating layer is reduced due to the formation of a high Ni-containing Fe-Ni alloy layer, and thus a Fe-Zn alloy layer having a sufficient thickness cannot be obtained. However, there is a disadvantage in that the manufacturing cost rises sharply.
이후, 상기 Ni 코팅층이 형성된 소지강판을 H2-N2 혼합가스가 장입된 환원성 분위기로에서 700~900℃까지 가열한다. 상기 가열 공정을 통해 상기 Ni 코팅층의 Ni가 소지강판 내부로 침투되도록 함으로써 Fe-Ni 합금층을 형성시킬 수 있다. 상기 가열온도가 700℃미만인 경우에는 냉간압연 후 강판 조직이 오스테나이트 상으로 변태되지 않는 문제가 있으며, 900℃를 초과하는 경우에는 강판에 변형 및 파단이 발생할 가능성이 높아지게 된다. Subsequently, the steel sheet on which the Ni coating layer is formed is heated to 700 to 900 ° C. in a reducing atmosphere in which the H 2 -N 2 mixed gas is charged. Through the heating process, Ni of the Ni coating layer may be penetrated into the base steel sheet to form a Fe—Ni alloy layer. When the heating temperature is less than 700 ° C there is a problem that the steel sheet structure does not transform into austenite phase after cold rolling, and when the temperature exceeds 900 ° C, the possibility of deformation and breakage in the steel sheet increases.
한편, 상기 환원성 분위기 형성을 위하여 사용되는 H2-N2 혼합가스의 분율은 당해 기술분야에서 통상적으로 이용되는 것을 이용하면 되므로, 본 발명에서는 상기 H2-N2 혼합가스의 분율에 대해서 특별히 언급하지 않는다.On the other hand, since the fraction of the H 2 -N 2 mixed gas used for forming the reducing atmosphere may be used as commonly used in the art, the present invention specifically refers to the fraction of the H 2 -N 2 mixed gas. I never do that.
상기 가열 후에는 상기 소지강판을 상기 가열온도 범위에서 20초 이상 유지하는 것이 바람직한데, 상기 유지시간이 20초 미만일 경우에는 충분한 두께의 Fe-Ni 합금층이 형성되지 않아 Mn의 표면농화량을 충분히 억제하지 못할 수 있다.After the heating, the holding steel sheet is preferably maintained for 20 seconds or more in the heating temperature range. When the holding time is less than 20 seconds, a Fe-Ni alloy layer having a sufficient thickness is not formed, thereby sufficiently increasing the surface concentration of Mn. It may not be suppressed.
이어서, 상기 가열된 소지강판을 400~500℃까지 5℃/s이상의 냉각속도로 냉각한다. 상기 냉각속도가 5℃/s미만인 경우에는 90면적%이상의 오스테나이트를 확보하기 곤란하다. Subsequently, the heated steel sheet is cooled to a cooling rate of 5 ° C / s or more to 400 ~ 500 ° C. If the cooling rate is less than 5 ° C / s it is difficult to secure austenite of more than 90 area%.
상기 냉각 후에는 상기 냉각된 소지강판의 도금욕 인입온도가 (용융아연도금욕-40℃)~(용융아연도금욕+10℃)의 범위를 갖도록 제어한다. 상기 도금욕 인입온도가 (용융아연도금욕-40℃)미만일 경우에는 소지강판에 함유된 Fe가 적게 용출되어 Fe-Zn 합금상의 형성을 억제하고, (용융아연도금욕+10℃)를 초과하는 경우에는 Fe-Al 또는 Fe-Al-Zn 합금층이 두껍게 형성되어 Fe의 확산을 방해하는 문제가 있다. 한편, 상기 소지강판의 도금욕 인입온도 제어는, 상기 냉각정지온도가 상기 도금욕 인입온도보다 높을 경우 상기 소지강판을 냉각하고, 상기 냉각정지온도가 상기 도금욕 인입온도와 같을 경우 상기 소지강판을 유지시키고, 상기 냉각정지온도가 상기 도금욕 인입온도보다 낮을 경우에는 상기 소지강판을 가열하는 방식으로 이루어질 수 있다. After the cooling, the plating bath inlet temperature of the cooled steel sheet is controlled to have a range of (hot dip galvanizing bath -40 ℃) ~ (hot dip galvanizing bath +10 ℃). When the plating bath inlet temperature is lower than (hot dip galvanizing bath -40 ℃), Fe contained in the steel sheet is less eluted to suppress the formation of Fe-Zn alloy phase, and exceeds (hot dip galvanizing bath +10 ℃) In this case, there is a problem that the Fe-Al or Fe-Al-Zn alloy layer is formed thick to prevent the diffusion of Fe. On the other hand, the plating bath inlet temperature control of the holding steel sheet, the holding plate is cooled when the cooling stop temperature is higher than the plating bath inlet temperature, and when the cooling stop temperature is the same as the plating bath inlet temperature When the cooling stop temperature is lower than the plating bath inlet temperature, the holding steel sheet may be heated.
상기 도금욕 인입온도 범위로 제어된 소지강판을 0.13중량%이하의 Al을 포함하는 440~460℃의 용융아연도금욕에 침지하여 도금액을 상기 소지강판의 표면에 도포한다. 상기 용융아연도금욕의 Al 함량이 0.13중량%를 초과하는 경우에는 Fe의 확산이 억제되어 충분한 두께의 Fe-Zn 합금층을 확보하기 곤란할 수 있다. 상기 용융아연도금욕의 온도가 440℃미만인 경우에는 도금용액의 유동성을 확보하기 곤란하여 도금이 원활히 이루어지지 않을 수 있으며, 460℃를 초과하는 경우에는 도금용액의 휘발 등과 문제가 발생한다.The plated steel sheet controlled to the plating bath inlet temperature range is immersed in a hot dip galvanizing bath at 440 to 460 ° C. containing 0.13% by weight or less of Al to apply a plating solution to the surface of the plated steel sheet. When the Al content of the hot-dip galvanizing bath exceeds 0.13% by weight, diffusion of Fe may be suppressed and it may be difficult to secure a Fe—Zn alloy layer having a sufficient thickness. If the temperature of the hot dip galvanizing bath is less than 440 ℃, it is difficult to secure the fluidity of the plating solution and plating may not be performed smoothly, if the temperature exceeds 460 ℃, problems such as volatilization of the plating solution occurs.
이후, 상기 도금액이 도포된 소지강판을 4~20℃/s의 냉각속도로 서냉하여 용융아연도금층을 형성시킨다. 상기 서냉속도가 4℃/s미만인 경우에는 미응고 아연이 롤과 같은 설비에 묻어 제품의 2차 결함을 유발하게 되고, 20℃/s를 초과하는 경우에는 Fe-Zn 합금층이 충분한 두께로 성장하지 못하는 단점이 있다.Thereafter, the base steel plate coated with the plating liquid is slowly cooled at a cooling rate of 4 to 20 ° C./s to form a hot dip galvanized layer. If the slow cooling rate is less than 4 ℃ / s uncoagulated zinc buried in the equipment, such as a roll causes a secondary defect of the product, if the temperature exceeds 20 ℃ / s Fe-Zn alloy layer grows to a sufficient thickness There is a drawback to not doing it.
상기와 같은 공정을 통해, 소지강판의 직하에 Fe-Ni합금층을 형성함과 동시에, 소지강판 내에 함유된 Fe가 도금층으로 확산되도록 하여 상기 소지강판 상에 본 발명이 얻고자 하는 구조의 용융아연도금층을 형성시킬 수 있다.Through the above process, while forming a Fe-Ni alloy layer directly below the base steel sheet, and the Fe contained in the base steel sheet to be diffused into the plating layer by the molten zinc of the structure to obtain the present invention on the base steel sheet The plating layer can be formed.
이하, 실시예를 통해 본 발명을 보다 상세히 설명한다. 다만, 하기 실시예는 본 발명을 보다 상세히 설명하기 위한 예시일 뿐, 본 발명의 권리범위를 한정하지 않는다.Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only examples for describing the present invention in more detail, and do not limit the scope of the present invention.
(실시예)(Example)
냉연 TWIP 소지강판을 알칼리 탈지 및 산세하여 청정화 처리한 후, 전기도금을 통해 상기 소지강판 상에 하기 표 1의 부착량으로 Ni 코팅층을 하였다(비교예 2 내지 4는 실시하지 않음). 이어서, 상기 소지강판을 5%H2-N2 혼합가스가 장입된 환원성 분위기로에서 하기 표 1의 조건으로 가열한 뒤, 400℃까지 냉각하고, 도금욕 인입온도를 제어한 뒤, 460℃의 용융아연도금욕에 침지하여 도금액을 도포하였다. 상기 도금액이 도포된 소지강판을 에어나이핑하여 도금 부착량을 제어한 뒤, 하기 표 1의 조건으로 서냉하여 소지강판의 표면 직하에 Fe-Ni합금층이 형성되고, 용융아연도금층이 Fe-Al 또는 Fe-Al-Zn 합금층, Fe-Zn 합금층, Zn층으로 구성되는 용융아연도금강판을 제조하였다. 상기 용융아연도금강판의 Fe-Zn 합금층의 두께를 측정하고, 도금밀착성을 평가한 뒤, 그 결과를 하기 표 1에 나타내었다. 또한, 상기 용융아연도금강판을 5.8kA의 용접전류로 스팟 용접한 뒤, LME에 의한 크랙 크기를 측정하고, 그 결과를 하기 표 1에 나타내었다. 한편, 상기 도금밀착성 평가는 용융아연도금강판을 180°벤딩한 후, 도금이 테이프에 묻어나오는지 확인하는 것으로 진행되었으며, 도금이 묻어나올 경우에는 박리, 묻어나오지 않을 경우에는 비박리로 나타내었다.After the cold-rolled TWIP base steel sheet was subjected to alkali degreasing and pickling to purify it, an Ni coating layer was applied to the base steel sheet by the deposition amount shown in Table 1 below by electroplating (Comparative Examples 2 to 4 were not performed). Subsequently, the base steel sheet was heated under the conditions shown in Table 1 in a reducing atmosphere loaded with a 5% H 2 -N 2 mixed gas, cooled to 400 ° C., and the plating bath inlet temperature was controlled. The plating liquid was applied by immersion in a hot dip galvanizing bath. After controlling the plating amount by air-knitting the plated steel sheet coated with the plating solution, a slow cooling was performed under the conditions shown in Table 1 below to form a Fe-Ni alloy layer directly under the surface of the plated steel sheet. A hot-dip galvanized steel sheet consisting of a Fe—Al—Zn alloy layer, a Fe—Zn alloy layer, and a Zn layer was prepared. After measuring the thickness of the Fe-Zn alloy layer of the hot-dip galvanized steel sheet, and evaluated the plating adhesion, the results are shown in Table 1 below. In addition, after spot-welding the hot-dip galvanized steel sheet with a welding current of 5.8 kA, crack size by LME was measured, and the results are shown in Table 1 below. On the other hand, the plating adhesion evaluation was carried out to check whether the plating is buried on the tape after bending the hot-dip galvanized steel sheet 180 °, peeled when the plating is smeared, it was represented as non-peeled.
표 1
구분 Ni부착량(mg/m2) 가열온도(℃) 도금욕인입온도(℃) 도금욕Al함량(중량%) 도금부착량(g/m2) 서냉속도(℃/s) Fe-Zn합금층두께(㎛) 도금밀착성 LME 크랙길이(㎛)
비교예1 300 760 480 0.2 60 10 0 미박리 24.5
비교예2 - 760 420 0.12 60 10 1.6 박리 37.0
비교예3 - 760 460 0.12 60 10 1.8 박리 25.5
비교예4 - 760 500 0.12 60 10 0.8 박리 40.0
비교예5 300 760 420 0.12 60 10 2.3 미박리 6.3
비교예6 300 760 460 0.12 60 10 2.3 미박리 7.3
비교예7 300 760 500 0.12 60 10 2.0 미박리 23.0
비교예8 500 760 420 0.12 60 10 2.9 미박리 6.3
비교예9 500 760 460 0.12 60 10 3.7 미박리 2.8
발명예1 780 760 450 0.12 60 10 4.8 미박리 0
발명예2 500 760 460 0.12 45 10 3.8 미박리 0
Table 1
division Ni adhesion amount (mg / m 2 ) Heating temperature (℃) Plating bath inlet temperature (℃) Plating bath Al content (wt%) Coating Weight (g / m 2 ) Slow cooling rate (℃ / s) Fe-Zn alloy layer thickness (㎛) Plating adhesion LME crack length (㎛)
Comparative Example 1 300 760 480 0.2 60 10 0 Unpeeled 24.5
Comparative Example 2 - 760 420 0.12 60 10 1.6 Peeling 37.0
Comparative Example 3 - 760 460 0.12 60 10 1.8 Peeling 25.5
Comparative Example 4 - 760 500 0.12 60 10 0.8 Peeling 40.0
Comparative Example 5 300 760 420 0.12 60 10 2.3 Unpeeled 6.3
Comparative Example 6 300 760 460 0.12 60 10 2.3 Unpeeled 7.3
Comparative Example 7 300 760 500 0.12 60 10 2.0 Unpeeled 23.0
Comparative Example 8 500 760 420 0.12 60 10 2.9 Unpeeled 6.3
Comparative Example 9 500 760 460 0.12 60 10 3.7 Unpeeled 2.8
Inventive Example 1 780 760 450 0.12 60 10 4.8 Unpeeled 0
Inventive Example 2 500 760 460 0.12 45 10 3.8 Unpeeled 0
상기 표 1에서 알 수 있듯이, 본 발명이 제안하는 Fe-Zn 합금층의 두께를 만족하는 용융아연도금층을 갖는 발명예 1 및 2의 경우에는 도금밀착성이 우수할 뿐만 아니라, LME에 의한 크랙이 전혀 발생하지 않았음을 알 수 있다.As can be seen from Table 1, in the case of Inventive Examples 1 and 2 having a hot-dip galvanized layer that satisfies the thickness of the Fe-Zn alloy layer proposed by the present invention, not only the plating adhesion is excellent, but also no cracks due to LME It can be seen that it did not occur.
반면, 비교예 1의 경우에는 Al도금욕의 함량이 과도하여 Fe-Zn합금층이 형성되지 않았으며, 이로 인해 LME에 의한 크랙이 24.5㎛ 수준으로 발생하였음을 알 수 있다.On the other hand, in the case of Comparative Example 1, the Fe-Zn alloy layer was not formed because the content of the Al plating bath is excessive, and it can be seen that cracks due to LME occurred at a level of 24.5 μm.
비교예 2 내지 4의 경우에는 Ni 코팅층이 형성되지 않음에 따라 도금이 모두 박리되었음을 알 수 있고, 본 발명이 제안하는 Fe-Zn합금층 두께를 만족하지 않음에 따라 LME에 의한 크랙이 심하게 발생하였음을 확인할 수 있다.In the case of Comparative Examples 2 to 4 it can be seen that the plating is all peeled off as the Ni coating layer is not formed, and the crack caused by LME was severely generated as it did not satisfy the Fe-Zn alloy layer thickness proposed by the present invention. can confirm.
비교예 4 내지 9의 경우에는 본 발명이 제안하는 충분한 두께의 Fe-Zn합금층이 형성되지 않아 LME에 의한 크랙이 발생하였음을 확인할 수 있다.In Comparative Examples 4 to 9, the Fe-Zn alloy layer having a sufficient thickness proposed by the present invention was not formed, and it can be confirmed that cracks occurred by LME.
[부호의 설명][Description of the code]
1 : 소지강판1: steel sheet
2 : 합금화 억제층2: alloying inhibitory layer
3 : Zn층3: Zn layer
4: 표면산화물4: surface oxide
10 : 소지강판10: steel sheet
20 : 용융아연도금층20: hot dip galvanized layer
21 : Fe-Zn 합금층21: Fe-Zn alloy layer
22 : Zn층22: Zn layer
23 : Fe-Al 또는 Fe-Al-Zn 합금층23 Fe-Al or Fe-Al-Zn alloy layer
30 : Fe-Ni합금층30: Fe-Ni alloy layer
40 : 내부산화물40: internal oxide

Claims (9)

  1. 오스테나이트의 분율이 90면적%이상인 미세조직을 갖는 소지강판; 및A steel sheet having a microstructure having an austenitic fraction of at least 90 area%; And
    상기 소지강판 상에 형성된 용융아연도금층을 포함하고,It includes a hot dip galvanized layer formed on the base steel sheet,
    상기 용융아연도금층은, The hot dip galvanized layer,
    Fe-Zn 합금층; 및Fe—Zn alloy layer; And
    상기 Fe-Zn 합금층 상에 형성된 Zn층을 포함하며,It includes a Zn layer formed on the Fe-Zn alloy layer,
    상기 Fe-Zn 합금층은 [(3.4×t)/6]㎛이상의 두께를 갖는 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판.The Fe-Zn alloy layer is a hot-dip galvanized steel sheet excellent in crack resistance by embrittlement of liquid metal having a thickness of [(3.4 × t) / 6] μm or more.
    (단, 상기 t는 상기 용융아연도금층의 두께임.)(Wherein t is the thickness of the hot dip galvanized layer.)
  2. 청구항 1에 있어서,The method according to claim 1,
    상기 Fe-Zn 합금층은 3~15중량%의 Fe를 포함하는 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판.The Fe-Zn alloy layer is hot-dip galvanized steel sheet excellent crack resistance by embrittlement of liquid metal containing 3 to 15% by weight of Fe.
  3. 청구항 1에 있어서,The method according to claim 1,
    상기 소지강판은 중량%로, C: 0.10~0.30%, Mn: 10~30%, Si: 0.01∼0.03%, Ti: 0.05~0.2%, Mn: 10∼30%, Al: 0.5∼3.0%, Ni: 0.001~10%, Cr: 0.001~10%, N: 0.001~0.05%, P: 0.020%이하, S: 0.001∼0.005%, 잔부 Fe 및 기타 불가피한 불순물을 포함하는 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판.The steel sheet is in weight%, C: 0.10 to 0.30%, Mn: 10 to 30%, Si: 0.01 to 0.03%, Ti: 0.05 to 0.2%, Mn: 10 to 30%, Al: 0.5 to 3.0%, Crack resistance by liquid metal embrittlement containing Ni: 0.001-10%, Cr: 0.001--10%, N: 0.001-0.05%, P: 0.020% or less, S: 0.001-0.005%, balance Fe and other unavoidable impurities This excellent hot dip galvanized steel sheet.
  4. 청구항 1에 있어서,The method according to claim 1,
    상기 용융아연도금층은 상기 Fe-Zn 합금층 하부에 Fe-Al 또는 Fe-Al-Zn 합금층이 추가로 포함되는 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판.The hot dip galvanized layer is a hot dip galvanized steel sheet having excellent crack resistance by embrittlement of the liquid metal in which Fe-Al or Fe-Al-Zn alloy layer is further included below the Fe-Zn alloy layer.
  5. 청구항 4에 있어서,The method according to claim 4,
    상기 합금화 억제층은 0.6중량%이하의 Al을 포함하는 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판.The alloying inhibiting layer is hot-dip galvanized steel sheet excellent in crack resistance by embrittlement of liquid metal containing Al of 0.6% by weight or less.
  6. 청구항 1에 있어서,The method according to claim 1,
    상기 소지강판의 표면 직하에는 Fe-Ni 합금층이 추가로 포함되는 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판.Hot-dip galvanized steel sheet excellent crack resistance by embrittlement of the liquid metal containing a Fe-Ni alloy layer is further included directly below the surface of the base steel sheet.
  7. 청구항 1에 있어서,The method according to claim 1,
    상기 Fe-Ni 합금층은 300~1000mg/m2의 부착량을 갖는 Ni 코팅에 의해 형성되는 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판.The Fe-Ni alloy layer is hot-dip galvanized steel sheet excellent crack resistance by embrittlement of liquid metal formed by Ni coating having an adhesion amount of 300 ~ 1000mg / m 2 .
  8. 청구항 1에 있어서,The method according to claim 1,
    상기 Fe-Ni 합금층은 0.05~5㎛의 두께를 갖는 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판.The Fe-Ni alloy layer is hot-dip galvanized steel sheet excellent crack resistance by embrittlement of liquid metal having a thickness of 0.05 ~ 5㎛.
  9. 청구항 1에 있어서,The method according to claim 1,
    상기 소지강판과 상기 용융아연도금층의 사이에는 Fe-X 합금층, Fe-Al-X 합금층, Fe-Al-Zn-X 합금층 및 Fe-Zn-X 합금층으로 이루어지는 그룹으로부터 선택된 1종 이상을 추가로 포함하는 액체금속취화에 의한 크랙 저항성이 우수한 용융아연도금강판.At least one member selected from the group consisting of a Fe-X alloy layer, a Fe-Al-X alloy layer, a Fe-Al-Zn-X alloy layer, and a Fe-Zn-X alloy layer between the base steel sheet and the hot dip galvanized layer. Hot-dip galvanized steel sheet excellent in crack resistance by embrittlement of liquid metal further comprising.
    (단, 상기 X는 Ni 및 Cr 중 하나임.)(Wherein X is one of Ni and Cr)
PCT/KR2014/012824 2013-12-25 2014-12-24 Molten zinc plated steel sheet with excellent crack resistance due to liquid metal bromide WO2015099455A1 (en)

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