Hot-dip zn-mg-al-based coated steel sheet and method of manufacturing same

Abstract

One aspect of the present invention is to provide hot-dip Zn-Mg-Al-based coated steel sheet and a manufacturing method thereof. A preferred aspect of the present invention is to provide a hot-dip Zn-Mg-Al-based coated steel sheet having excellent plating adhesiveness, and a manufacturing method thereof.

Description

ZN-MG-AL type hot-dip galvanized steel sheet and its manufacturing method

[0001] The present invention relates to a Zn-Mg-Al system hot-dip galvanized steel sheet and a method for manufacturing the same.

[0002] In line with the recent trend toward carbon neutrality, the need for steel with superior corrosion resistance that can better withstand harsh corrosive environments is increasing, particularly from an energy-saving perspective. To this end, zinc-alloy-based galvanized steel sheets have been developed, which enhance the corrosion resistance of steel by adding elements such as aluminum (Al) and magnesium (Mg) to the zinc plating bath. A representative example is Zn-Mg-Al hot-dip galvanized steel sheets.

[0003] In addition, in response to the continuous demand for lightweight vehicle bodies, it is common to add large amounts of elements such as Si, Mn, and Al to steel to increase the strength of automobile steel sheets. However, these elements have the problem of generating oxides on the surface of the steel sheet during the annealing heat treatment process, thereby reducing the plating properties and adhesion during hot-dip galvanizing.

[0004] A representative technology for addressing these issues is Patent Document 1. Patent Document 1 proposes a technique for improving plating properties by controlling the composition and ratio of surface oxides to favor plating by controlling annealing conditions to control surface oxide formation. However, Patent Document 1 suffers from a problem in that mechanical properties, such as fatigue strength, are reduced due to the soft layer formed through dew point control.

[0005] Therefore, there is an urgent need for a technology that can improve the plating adhesion of zinc-plated steel sheets for automobiles with excellent corrosion resistance.

[0006] [Prior Art Literature]

[0007] (Patent Document 1) Korean Patent Publication No. 2012-7004749

[0008] One aspect of the present invention is to provide a Zn-Mg-Al system hot-dip galvanized steel sheet and a method for manufacturing the same.

[0009] A preferred aspect of the present invention is to provide a Zn-Mg-Al hot-dip galvanized steel sheet having excellent plating adhesion and a method for manufacturing the same.

[0010] One embodiment of the present invention provides a Zn-Mg-Al hot-dip galvanized steel sheet comprising, by weight %, Si: 0.10% or more and less than 0.50%, Mn: 0.80 to 3.0%, the remainder Fe and other unavoidable impurities; and a Zn-Mg-Al hot-dip galvanized layer formed on at least one surface of the cold-rolled steel sheet; wherein an Fe-Al intermetallic compound layer is formed at an interface between the cold-rolled steel sheet and the hot-dip galvanized layer, and an Fe reduction layer having an average thickness of 50 to 250 nm is formed directly under the surface of the cold-rolled steel sheet.

[0011] The above cold rolled steel sheet may additionally include at least one of C: 0.050 to 0.30%, P: 0.10% or less (excluding 0%), S: 0.010% or less (excluding 0%), Al: 0.010 to 0.10%, N: 0.0080% or less (excluding 0%), and B: 0.00010 to 0.0030%.

[0012] The above Zn-Mg-Al system hot-dip galvanized layer may contain, in wt%, Mg: 1.0 to 13.0%, Al: 1.0 to 36.0%, the remainder Zn, and other unavoidable impurities.

[0013] The above Zn-Mg-Al system molten plating layer may additionally include at least one of the following groups (a) to (h).

[0014] (a) Si: 0.5% or less, Ni: 0.5% or less, at least one of these

[0015] (b) At least one of the following: Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less

[0016] (c) Ti: 0.1% or less

[0017] (d) W: 0.5% or less

[0018] (e) Cu: 2.0% or less

[0019] (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these

[0020] (g) B: 0.1% or less, P: 0.1% or less, at least one of these

[0021] (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these

[0022] The above Fe-Al intermetallic compound layer may have an area ratio of 70% or more compared to the total area of ​​the surface of the steel plate.

[0023] Another embodiment of the present invention comprises the steps of: heating a slab containing, by weight %, Si: 0.10% or more and less than 0.50%, Mn: 0.80 to 3.0%, the remainder Fe and other unavoidable impurities; finishing hot-rolling the heated slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 600 to 680°C; cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; performing an oxidation heat treatment on the cold-rolled steel sheet by passing it through a DFF (Direct Fired Furnace) facility so that the exit temperature becomes 630 to 670°C; performing a reduction heat treatment on the oxidation-heat-treated cold-rolled steel sheet in a reducing atmosphere having a dew point temperature of -60°C or more and less than -45°C; And a step of immersing the cold-rolled steel sheet subjected to the reduction heat treatment in a Zn-Mg-Al-based hot-dip galvanizing bath to obtain a Zn-Mg-Al-based hot-dip galvanizing steel sheet; the DFF equipment includes four or more zones, and provides a method for manufacturing a Zn-Mg-Al-based hot-dip galvanizing steel sheet controlled to satisfy the following relational expression 1.

[0024] [Relationship 1] (Air ratio of the last zone - 1) × (Air ratio of the (last-1)th zone - 1) ≥ 0

[0025] The above slab may additionally contain one or more of C: 0.050 to 0.30%, P: 0.10% or less (excluding 0%), S: 0.010% or less (excluding 0%), Al: 0.010 to 0.10%, N: 0.0080% or less (excluding 0%), and B: 0.00010 to 0.0030%.

[0026] Heating of the above slab can be performed at 1100 to 1300°C.

[0027] The above finishing hot rolling can be performed at 800 to 1000°C.

[0028] The above reducing atmosphere may contain 3 to 25% hydrogen by volume and the remainder nitrogen.

[0029] Each of the above four or more zones may have an air ratio of 1.0 or greater.

[0030] The above Zn-Mg-Al system plating bath may contain, in wt%, Mg: 1.0 to 13.0%, Al: 1.0 to 36.0%, the remainder Zn, and other unavoidable impurities.

[0031] The above Zn-Mg-Al system plating bath may additionally include at least one of the following groups (a) to (h).

[0032] (a) Si: 0.5% or less, Ni: 0.5% or less, at least one of these

[0033] (b) At least one of the following: Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less

[0034] (c) Ti: 0.1% or less

[0035] (d) W: 0.5% or less

[0036] (e) Cu: 2.0% or less

[0037] (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these

[0038] (g) B: 0.1% or less, P: 0.1% or less, at least one of these

[0039] (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these

[0040] According to one aspect of the present invention, a Zn-Mg-Al system hot-dip galvanized steel sheet and a method for manufacturing the same can be provided.

[0041] According to a preferred aspect of the present invention, a Zn-Mg-Al hot-dip galvanized steel sheet having excellent plating adhesion and a method for manufacturing the same can be provided.

[0042] Figure 1 is a schematic diagram showing a Zn-Mg-Al system hot-dip galvanized steel sheet according to one embodiment of the present invention.

[0043] Figure 2 is a GDS (Glow Discharge Spectroscopy) depth profile showing the degree of Si, Mn, and B concentrated on the surface of a steel sheet after reduction heat treatment of Comparative Example 1, which deviates from the embodiment of the present invention.

[0044] FIG. 3 is a GDS (Glow Discharge Spectroscopy) depth profile showing the degree of Si, Mn, and B concentrated on the surface of a steel sheet after reduction heat treatment of Invention Example 1 according to an embodiment of the present invention.

[0045] Figure 4 is a photograph of Comparative Example 1, which deviates from the embodiment of the present invention, observed using TEM.

[0046] Figure 5 is an EDS (Energy Dispersive Spectroscopy) mapping image for Comparative Example 1, which deviates from the embodiment of the present invention.

[0047] Figure 6 is a photograph of Invention Example 1 according to an embodiment of the present invention observed using TEM.

[0048] Figure 7 is an EDS (Energy Dispersive Spectroscopy) mapping image for Invention Example 1 according to an embodiment of the present invention.

[0049] Figure 8 is a photograph after evaluation of plating adhesion for Comparative Example 1, which deviates from the embodiment of the present invention.

[0050] Figure 9 is a photograph after evaluation of plating adhesion for Invention Example 1 according to an embodiment of the present invention.

[0051] Hereinafter, a Zn-Mg-Al hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0052] One embodiment of the present invention provides a Zn-Mg-Al hot-dip galvanized steel sheet comprising, by weight %, Si: 0.10% or more and less than 0.50%, Mn: 0.80 to 3.0%, the remainder Fe and other unavoidable impurities; and a Zn-Mg-Al hot-dip galvanized layer formed on at least one surface of the cold-rolled steel sheet; wherein an Fe-Al intermetallic compound layer is formed at an interface between the cold-rolled steel sheet and the hot-dip galvanized layer, and an Fe reduction layer having an average thickness of 50 to 250 nm is formed directly under the surface of the cold-rolled steel sheet.

[0053] The cold-rolled steel sheet of the present invention preferably contains, in wt%, Si: 0.10% or more and less than 0.50%, Mn: 0.8 to 3.0%, the remainder Fe, and other unavoidable impurities.

[0054] Si: 0.10% or more and less than 0.50%

[0055] Si is an important element that contributes to strength improvement through solid solution strengthening, and plays a role in improving strength while suppressing deterioration of workability. When the Si content is less than 0.10%, it is difficult to sufficiently obtain the above-mentioned effect. When the Si content is 0.50% or more, a layer of Si or Mn oxide may be formed directly under the Fe oxide layer during oxidation heat treatment, which may reduce plating adhesion. Therefore, the Si content is preferably in the range of 0.10% or more and less than 0.50%. The lower limit of the Si content is more preferably 0.12%, and the upper limit of the Si content is more preferably 0.45%, and the upper limit of the Si content is more preferably 0.4%.

[0056] Mn: 0.80~3.0%

[0057] Mn is an element that contributes to the improvement of strength through solid solution strengthening and at the same time improves the hardenability of the austenite phase, and effectively contributes to the stabilization of strength. When the content of Mn is less than 0.80%, it is difficult to sufficiently obtain the above-mentioned effect. When the content of Mn exceeds 3.0%, workability may deteriorate. Therefore, the content of Mn is preferably in the range of 0.80 to 3.0%. The lower limit of the content of Mn is more preferably 1.50%. The upper limit of the content of Mn is more preferably 2.50%.

[0058] The cold rolled steel sheet of the present invention is not particularly limited in terms of alloy composition other than Si and Mn, and the alloy composition of all cold rolled steel sheets used in the relevant technical field can be applied. However, as an example, the cold rolled steel sheet of the present invention may additionally include at least one of C: 0.050 to 0.30%, P: 0.10% or less (excluding 0%), S: 0.010% or less (excluding 0%), Al: 0.010 to 0.10%, N: 0.0080% or less (excluding 0%), and B: 0.00010 to 0.0030%.

[0059] The remaining components of the cold-rolled steel sheet of the present invention are iron (Fe), and may include some unintended and inevitable impurities introduced during the manufacturing process. Since these impurities are readily apparent to anyone skilled in the art of manufacturing, their full details are not specifically mentioned in this specification.

[0060] Meanwhile, the cold-rolled steel sheet of the present invention can have a thickness of 0.6 to 2.3 mm, and thus can be preferably used as an automobile steel sheet for impact structural members.

[0061] Figure 1 is a schematic diagram showing a Zn-Mg-Al system hot-dip galvanized steel sheet according to one embodiment of the present invention.

[0062] It is preferable that the Zn-Mg-Al based hot-dip galvanized steel sheet (10) of the present invention includes a Zn-Mg-Al based hot-dip galvanized layer (2) formed on at least one surface of the aforementioned cold-rolled steel sheet (1). In the present invention, there is no particular limitation on the type of the Zn-Mg-Al based hot-dip galvanized layer, and all types of Zn-Mg-Al based hot-dip galvanized layers commonly used in the relevant technical field can be applied. However, as an example, the Zn-Mg-Al based hot-dip galvanized layer may include, in wt%, Mg: 1.0 to 13.0%, Al: 1.0 to 36.0%, the remainder Zn, and other unavoidable impurities.

[0063] The above Zn-Mg-Al system molten plating layer may additionally include at least one of the following groups (a) to (h).

[0064] However, since the elements in each group below are not essential for achieving the tasks of the present invention, their lower limits of content are not limited. Therefore, even if not specifically mentioned below, the lower limit of the content of each element may be 0%.

[0065] (a) Si: 0.5% or less, Ni: 0.5% or less, at least one of these

[0066] (b) At least one of the following: Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less

[0067] (c) Ti: 0.1% or less

[0068] (d) W: 0.5% or less

[0069] (e) Cu: 2.0% or less

[0070] (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these

[0071] (g) B: 0.1% or less, P: 0.1% or less, at least one of these

[0072] (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these

[0073] (a) Si: 0.5% or less, Ni: 0.5% or less, at least one of these

[0074] Si has the effect of preventing Fe-Zn alloying due to the formation of interface Mg2Si, and can prevent excessive formation of Fe-Al alloy phase. However, if its content exceeds 0.5%, there is a concern that the melting point of the plating bath will increase, and brittleness will increase due to the excessive formation of Mg2Si. Ni has the effect of preventing Fe diffusion by forming an Al-Ni alloy phase, but if its content exceeds 0.5%, there may be a problem that the cost of auxiliary materials increases excessively.

[0075] (b) At least one of the following: Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less

[0076] Ca, La, Ce, Y and Sr have the effect of preventing oxidation of Mg in the plating bath by forming an oxide film, but if their contents exceed 1.0%, 0.1%, 0.1%, 0.1% and 1.0%, respectively, Ca may have the problem of increased dross due to increased oxides, and La, Ce, Y and Sr may have the problem of reduced plating properties due to increased viscosity of the plating bath.

[0077] (c) Ti: 0.1% or less

[0078] Ti acts as a nucleation site for Ti-Al intermetallic compounds and has the effect of refining crystal grains (spangles), but if its content exceeds 0.1%, the melting point of the plating bath may increase and there may be a problem of increased dross.

[0079] (d) W: 0.5% or less

[0080] W forms W oxide on the surface, which improves corrosion resistance, but if its content exceeds 0.5%, there may be a problem of the melting point of the plating bath increasing.

[0081] (e) Cu: 2.0% or less

[0082] Cu has the effect of forming an Al-Cu process structure and lowering the hardness of the plating layer, but if its content exceeds 2.0%, there may be a problem of the spangles becoming coarser.

[0083] (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these

[0084] Fe, Cr, Mn, and V have the effect of preventing electrode deterioration by suppressing alloying between zinc and the welding electrode due to rapid liquid loss, but if their contents exceed 1.0%, 0.5%, 0.5%, and 0.5%, respectively, there may be a problem in that the melting point of the plating bath increases excessively.

[0085] (g) B: 0.1% or less, P: 0.1% or less, at least one of these

[0086] B and P have the effect of suppressing LME cracks in welds, but if their contents exceed 0.1% each, there may be a problem of increased dross generation.

[0087] (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these

[0088] Sn, Sb, and Bi have the effect of uniformizing spangles and improving pot durability by lowering the plating bath temperature, but if their contents exceed 1.0% each, there may be a problem of coarsening of spangles.

[0089] It is preferable that an Fe-Al intermetallic compound layer (3) is formed at the interface of the cold-rolled steel sheet (1) and the Zn-Mg-Al-based hot-dip galvanized layer (2). The Fe-Al intermetallic compound layer helps to improve plating adhesion and suppress the phenomenon of the plating layer being peeled off. The Fe-Al intermetallic compound layer may have an area ratio of 70% or more with respect to the total area of ​​the steel sheet surface. If the area ratio of the Fe-Al intermetallic compound layer is less than 70%, the plating adhesion may be reduced. The area ratio of the Fe-Al intermetallic compound layer is more preferably 80% or more, more preferably 90% or more, and theoretically or if possible in terms of the manufacturing process, the area ratio of the Fe-Al intermetallic compound layer is most preferably 100%. The present invention does not specifically limit the type of the Fe-Al intermetallic compound layer, but may be, for example, an FeAl3 layer.

[0090] As an example, the above Fe-Al intermetallic compound layer may have a film shape when viewed from the side of the steel sheet, thereby improving plating adhesion.

[0091] An Fe reduction layer (4) having an average thickness of 50 to 250 nm may be formed directly beneath the surface of the cold-rolled steel sheet (1). The Fe reduction layer is formed as the Fe oxide layer formed by the oxidation heat treatment undergoes a reduction heat treatment process again. The Fe oxide layer is widely known to reduce plating adhesion. In the present invention, by reducing the Fe oxide layer to form an Fe reduction layer, the plating adhesion can be improved, thereby suppressing the phenomenon of the plating layer being peeled off. The Fe reduction layer preferably has an average thickness of 50 to 250 nm. When the average thickness of the Fe reduction layer is less than 50 nm, the above-described effect cannot be sufficiently obtained. When the average thickness of the Fe reduction layer exceeds 250 nm, since the Fe reduction layer is not completely reduced, Si or Mn oxide may be formed directly beneath it, which may result in reduced plating adhesion.

[0092] As described above, the Zn-Mg-Al hot-dip galvanized steel sheet of the present invention can secure excellent plating adhesion.

[0093] Hereinafter, a method for manufacturing a Zn-Mg-Al-based hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0094] First, a slab satisfying the above-described alloy composition is heated. The present invention does not specifically limit the slab heating process, and conventional conditions applicable in the relevant technical field can be utilized. However, as an example, the slab may be heated at 1100 to 1300°C.

[0095] Thereafter, the heated slab is subjected to final hot rolling to obtain a hot-rolled steel sheet. The present invention does not specifically limit the final hot rolling process, and typical conditions applicable in the relevant technical field may be utilized. However, as an example, the final hot rolling may be performed at 800 to 1000°C.

[0096] Thereafter, the hot-rolled steel sheet is coiled at 600 to 680°C. If the coiling temperature is lower than 600°C, a problem of deterioration in shape may occur. If the coiling temperature exceeds 680°C, a problem of scale dust generation may occur. Therefore, the coiling temperature is preferably in the range of 600 to 680°C. The lower limit of the coiling temperature is more preferably 610°C, and even more preferably 620°C. The upper limit of the coiling temperature is more preferably 670°C, and even more preferably 650°C.

[0097] Thereafter, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The present invention does not specifically limit the cold-rolling process, and typical conditions applicable in the relevant technical field can be used.

[0098] Thereafter, the cold-rolled steel sheet is passed through a DFF (Direct Fired Furnace) facility and subjected to oxidation heat treatment at an exit temperature of 630 to 670°C. The oxidation heat treatment is intended to form an internal oxide layer while simultaneously oxidizing iron present on the surface of the cold-rolled steel sheet to form an iron oxide layer. The iron oxide layer can serve as a diffusion barrier that can suppress the diffusion of elements such as Si or Mn to the surface of the steel sheet during the heat treatment process. When the exit temperature of the DFF facility is less than 630°C, there is a problem that the iron oxide layer is not formed or is formed very thinly, thereby minimizing the effect of suppressing the concentration of the target alloy element. When the exit temperature of the DFF facility exceeds 670°C, the excessively formed iron oxide layer may be picked up by the rolls as it is shed by friction with the rolls, which may cause a problem of causing a defect such as a dent on the surface of the steel sheet. Therefore, the exit temperature of the DFF facility is preferably in the range of 630 to 670°C. The lower limit of the outlet temperature of the above DFF facility is more preferably 635°C, and more preferably 640°C. The upper limit of the outlet temperature of the above DFF facility is more preferably 665°C, and more preferably 660°C.

[0099] At this time, it is preferable that the DFF facility includes four or more zones and is controlled to satisfy the following relational expression 1. The purpose of the relational expression 1 below is to ensure that all of the supplied fuel is combusted and the remaining excess oxygen oxidizes the Fe present in the surface layer of the cold-rolled steel sheet. In addition, the purpose is to control so that the oxide is not concentrated on the surface of the steel sheet but is trapped directly under the surface layer of the steel sheet. The Fe oxide layer formed through the above process is subsequently transformed into an Fe reduced layer through a reduction process, and when the Fe oxide layer is completely reduced, the oxide present in the surface layer of the steel sheet is removed, so that the Fe-Al intermetallic compound layer can be uniformly formed. If the relational expression 1 below is not satisfied, the distribution of the Fe-Al intermetallic compound layer may become non-uniform, and as a result, it may be difficult for the Fe-Al intermetallic compound layer to have an area ratio of 70% or more compared to the total area of ​​the steel sheet surface. In addition, some particles within the Fe-Al intermetallic compound layer may grow coarsely. Meanwhile, in the present invention, the purpose of the present invention can be achieved as long as the following relational expression 1 is satisfied, and therefore, there is no particular limitation on the upper limit of the value of the following relational expression 1.

[0100] [Relationship 1] (Air ratio of the last zone - 1) × (Air ratio of the (last-1)th zone - 1) ≥ 0

[0101] Meanwhile, each of the above four or more zones may have an air ratio of 1.0 or more, and by controlling it in this way, Fe oxidation can be made easier.

[0102] Thereafter, the cold-rolled steel sheet subjected to the oxidation heat treatment is subjected to a reduction heat treatment in a reducing atmosphere having a dew point temperature of -60°C or higher and less than -45°C. The reduction heat treatment is intended to reduce the Fe oxide layer formed by the oxidation heat treatment to form an Fe reduction layer. Accordingly, when the dew point temperature is lower than -60°C, the plating property may deteriorate. When the dew point temperature is higher than -45°C, the amount of oxide may increase due to a selective oxidation transition zone, which may cause a problem of deterioration in the plating property. Therefore, the dew point temperature is preferably in the range of -60°C or higher and less than -45°C. The lower limit of the dew point temperature is more preferably -55°C. The upper limit of the dew point temperature is more preferably -46°C, and even more preferably -47°C.

[0103] Meanwhile, the present invention does not specifically limit the conditions of the reducing atmosphere, and conventional conditions applicable in the relevant technical field may be utilized. However, as an example, it may be a gas atmosphere containing 3 to 25% hydrogen by volume and the remainder nitrogen.

[0104] Thereafter, the cold-rolled steel sheet subjected to the reduction heat treatment is immersed in a Zn-Mg-Al based hot-dip galvanizing bath to obtain a Zn-Mg-Al based hot-dip galvanized steel sheet. The present invention does not specifically limit the hot-dip galvanizing, and typical conditions applied in the relevant technical field can be used. However, as an example, the Zn-Mg-Al based hot-dip galvanizing bath may contain, in wt%, Mg: 1.0 to 13.0%, Al: 1.0 to 36.0%, and the remainder Zn. An Fe-Al intermetallic compound layer may be formed by the hot-dip galvanizing process. The present invention does not specifically limit the type of the Fe-Al intermetallic compound layer. However, as an example, it may be formed by a reaction between Al contained in the plating bath and Fe eluted from the base steel sheet.

[0105] Hereinafter, the present invention will be described in more detail through examples. However, the examples described below are intended only to explain the present invention in more detail and do not limit the scope of the present invention.

[0106] (Example)

[0107] A slab (C: 0.08%, P: 0.02%, S: 0.003%, Al: 0.03%, N: 0.004%, B: 0.0005%) having Si and Mn contents as described in Table 1 below was heated at 1200°C, and then the heated slab was final hot-rolled at 900°C to obtain a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was coiled under the conditions described in Table 1 below, and the coiled hot-rolled steel sheet was cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.2 mm. Thereafter, the cold-rolled steel sheet was subjected to oxidation heat treatment by passing it through a DFF (Direct Fired Furnace) facility (line speed: 90 mpm) under the conditions described in Table 2 below. Thereafter, the oxidation-heat-treated cold-rolled steel sheet was subjected to reduction heat treatment in a reducing atmosphere (hydrogen: 5 vol% and the remainder nitrogen) under the conditions described in Table 1 below, and the reduction-heat-treated cold-rolled steel sheet was immersed in a Zn-Mg-Al-based hot-dip galvanizing bath (Al: 2.5%, Mg: 3.0%, the remainder Zn) to manufacture a Zn-Mg-Al-based hot-dip galvanized steel sheet having the composition described in Table 2 below.

[0108] For the Zn-Mg-Al system hot-dip galvanized steel sheet manufactured in this manner, the presence or absence of formation and area fraction of the Fe-Al intermetallic compound layer, the presence or absence of formation and average thickness of the Fe reduction layer were measured, and the results are shown in Table 2 below.

[0109] The presence or absence of formation of the Fe-Al intermetallic compound layer and the area fraction were determined by photographing the surface of the Fe-Al intermetallic compound layer using a scanning electron microscope, and then measuring 10 random locations using an image analyzer, and calculating the average value.

[0110] The presence or absence of the formation of the Fe reduction layer and its average thickness were measured at 10 random locations on the side of the Zn-Mg-Al system hot-dip galvanized steel sheet using a transmission electron microscope, and the average value was calculated.

[0111] In addition, the plating adhesion was measured for the Zn-Mg-Al system hot-dip galvanized steel sheet, and the results are shown in Table 2 below.

[0112] In order to evaluate the plating adhesion and the presence or absence of plating layer peeling, a 30 mm x 80 mm sized specimen was taken from the hot-dip galvanized steel sheet, a structural adhesive was applied to the specimen, and it was cured in an oven at 170°C for 20 minutes. Then, it was clamped to a bending jig and bent 90°. Thereafter, the presence or absence of plating layer peeling was determined by visually checking whether the plating layer was attached to the surface to which the structural adhesive was applied.

[0113] Classification Alloy composition (weight %) Coiling temperature (℃) DFF outlet temperature (℃) Dew point temperature (℃) Relationship 1 Satisfaction SiMn Invention example 10.24 0.89 60 2648-50 Satisfied Invention example 20.27 0.816 15652-48 Satisfied Invention example 30.25 0.916 13666-47 Satisfied Invention example 40.29 1.35 666656-48 Satisfied Comparative example 10.30 1.31 625 45-46 Unsatisfied Comparative example 20.21 1.12 624653-51 Unsatisfied Comparative example 30.24 1.30 632667-49 Unsatisfied Comparative example 40.25 1.16 648 610-52 Satisfied Comparative example 50.31 0.926 367 03-48 Satisfied [Relationship 1] (air ratio of the last zone - 1) × (air ratio of the (last-1)th zone - 1) ≥ 0

[0114] ClassificationFe-Al intermetallic compound layerFe reduction layerPlating layerPeelingFormationPresence / absenceArea fraction (%)FormationPresence / absenceAverage thickness (nm)Invention example 1Yu96Yu115Non-peelingInvention example 2Yu98Yu152Non-peelingInvention example 3Yu97Yu187Non-peelingInvention example 4Yu95Yu106Non-peelingComparative example 1Yu11None0PeelingComparative example 2None0None0PeelingComparative example 3None0NonePeelingComparative example 4Yu40Yu30PeelingComparative example 5Yu84Yu370Peeling

[0115] As can be seen from Tables 1 and 2, in the case of invention examples 1 to 4 that satisfy the manufacturing conditions proposed by the present invention, an Fe-Al intermetallic compound layer with an appropriate area fraction and an Fe reduction layer with an appropriate thickness are formed, and thus the plating properties are good.

[0116] On the other hand, in the case of Comparative Examples 1 to 3, since relational expression 1 is not satisfied, it can be seen that the Fe-Al intermetallic compound layer is not formed or is formed in small amounts, and the Fe reduction layer is not formed, resulting in poor plating properties.

[0117] In the case of Comparative Example 4, it can be seen that the Fe-Al intermetallic compound layer and the Fe reduction layer were not sufficiently formed due to the low DFF exit temperature, resulting in poor plating properties.

[0118] In the case of Comparative Example 5, it can be seen that the Fe-Al intermetallic compound layer and the Fe reduction layer were excessively formed as the DFF exit temperature was high, resulting in poor plating properties.

[0119] Fig. 2 is a GDS (Glow Discharge Spectroscopy) depth profile showing the degree of Si, Mn, and B concentrated on the surface of the steel sheet after the reduction heat treatment of Comparative Example 1, and Fig. 3 is a GDS (Glow Discharge Spectroscopy) depth profile showing the degree of Si, Mn, and B concentrated on the surface of the steel sheet after the reduction heat treatment of Inventive Example 1. As can be seen from Figs. 2 and 3, in the case of Inventive Example 1, the formation of surface oxides was suppressed, whereas in the case of Comparative Example 1, a large amount of surface oxides was formed.

[0120] Fig. 4 is a TEM photograph of Comparative Example 1, and Fig. 5 is an EDS (Energy Dispersive Spectroscopy) mapping image for Comparative Example 1. Fig. 6 is a TEM photograph of Inventive Example 1, and Fig. 7 is an EDS (Energy Dispersive Spectroscopy) mapping image for Inventive Example 1. As can be seen from Figs. 4 to 7, in the case of Comparative Example 1, not only is there no Fe reduction layer, but also an Fe-Al intermetallic compound layer is formed non-uniformly. On the other hand, in the case of Inventive Example 1, not only is there an Fe reduction layer in the cold-rolled steel sheet, but in particular, referring to Fig. 7, it can be seen that an Fe-Al intermetallic compound layer is densely formed in the form of a film between the cold-rolled steel sheet and the Zn-Mg-Al hot-dip galvanized layer.

[0121] Fig. 8 is a photograph after evaluation of plating adhesion for Comparative Example 1, and Fig. 9 is a photograph after evaluation of plating adhesion for Inventive Example 1. As can be seen from Figs. 8 and 9, in the case of Comparative Example 1, peeling of the plating layer occurred, whereas in the case of Inventive Example 1, the plating layer did not peel.

[0122] [Explanation of symbols]

[0123] 1: Cold rolled steel sheet

[0124] 2: Zn-Mg-Al system hot-dip galvanized layer

[0125] 3: Fe-Al intermetallic compound layer

[0126] 4: Fe reduction layer

[0127] 10: Zn-Mg-Al system hot-dip galvanized steel sheet

Claims

1. Cold rolled steel sheet containing, by weight%, Si: 0.10% or more and less than 0.50%, Mn: 0.80 to 3.0%, the remainder Fe and other unavoidable impurities; and Including a Zn-Mg-Al based hot-dip galvanized layer formed on at least one surface of the cold rolled steel sheet; An Fe-Al intermetallic compound layer is formed at the interface between the cold rolled steel sheet and the hot-dip galvanized layer. A Zn-Mg-Al hot-dip galvanized steel sheet having an Fe reduction layer having an average thickness of 50 to 250 nm formed directly beneath the surface of the cold rolled steel sheet.

2. In claim 1, The above cold rolled steel sheet is a Zn-Mg-Al system hot-dip galvanized steel sheet additionally containing at least one of C: 0.050 to 0.30%, P: 0.10% or less (excluding 0%), S: 0.010% or less (excluding 0%), Al: 0.010 to 0.10%, N: 0.0080% or less (excluding 0%), and B: 0.00010 to 0.0030%.

3. In claim 1, The above Zn-Mg-Al system hot-dip galvanized steel sheet contains, in weight %, Mg: 1.0 to 13.0%, Al: 1.0 to 36.0%, the remainder Zn, and other unavoidable impurities.

4. In claim 3, A Zn-Mg-Al hot-dip galvanized steel sheet, wherein the Zn-Mg-Al hot-dip galvanized layer further comprises at least one of the following groups (a) to (h). (a) Si: 0.5% or less, Ni: 0.5% or less, or at least one of these (b) Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less, or at least one of these (c) Ti: 0.1% or less (d) W: 0.5% or less (e) Cu: 2.0% or less (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these (g) B: 0.1% or less, P: 0.1% or less, at least one of these (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these 5. In claim 1, A Zn-Mg-Al hot-dip galvanized steel sheet, wherein the Fe-Al intermetallic compound layer has an area ratio of 70% or more of the total area of ​​the steel sheet surface.

6. A step of heating a slab containing, by weight%, Si: 0.10% or more and less than 0.50%, Mn: 0.80 to 3.0%, the remainder Fe and other unavoidable impurities; A step of obtaining a hot-rolled steel sheet by final hot-rolling the above heated slab; A step of coiling the above hot-rolled steel plate at 600 to 680°C; A step of cold rolling the above-mentioned hot-rolled steel sheet to obtain a cold-rolled steel sheet; A step of passing the cold rolled steel sheet through a DFF (Direct Fired Furnace) facility and performing oxidation heat treatment so that the exit temperature becomes 630 to 670°C; A step of performing a reduction heat treatment on the above-mentioned oxidation-heat-treated cold-rolled steel sheet in a reducing atmosphere having a dew point temperature of -60℃ or higher and -45℃ or lower; and A step of immersing the cold-rolled steel sheet subjected to the reduction heat treatment in a Zn-Mg-Al system molten galvanizing bath to obtain a Zn-Mg-Al system molten galvanized steel sheet; The above DFF equipment includes four or more zones and is a method for manufacturing a Zn-Mg-Al system hot-dip galvanized steel sheet controlled to satisfy the following relational expression 1. [Relationship 1] (air ratio of the last zone - 1) × (air ratio of the (last-1)th zone - 1) ≥ 0 7. In claim 6, The above slab further contains at least one of C: 0.050 to 0.30%, P: 0.10% or less (excluding 0%), S: 0.010% or less (excluding 0%), Al: 0.010 to 0.10%, N: 0.0080% or less (excluding 0%), and B: 0.00010 to 0.0030%, which is a method for manufacturing a Zn-Mg-Al system hot-dip galvanized steel sheet.

8. In claim 6, A method for manufacturing a Zn-Mg-Al system hot-dip galvanized steel sheet, wherein the heating of the above slab is performed at 1100 to 1300°C.

9. In claim 6, The above finishing hot rolling is a method for manufacturing a Zn-Mg-Al system hot-dip galvanized steel sheet, which is performed at 800 to 1000°C.

10. In claim 6, The above reducing atmosphere is a method for manufacturing a Zn-Mg-Al system hot-dip galvanized steel sheet containing 3 to 25% by volume of hydrogen and the remainder of nitrogen.

11. In claim 6, A method for manufacturing a Zn-Mg-Al hot-dip galvanized steel sheet, wherein each of the above four or more zones has an air ratio of 1.0 or higher.

12. In claim 6, A method for manufacturing a Zn-Mg-Al system hot-dip galvanized steel sheet, wherein the above Zn-Mg-Al system galvanizing bath contains, in wt%, Mg: 1.0 to 13.0%, Al: 1.0 to 36.0%, the remainder Zn, and other unavoidable impurities.

13. In claim 6, A method for manufacturing a Zn-Mg-Al system hot-dip galvanized steel sheet, wherein the Zn-Mg-Al system hot-dip galvanizing bath further comprises at least one of the following groups (a) to (h). (a) Si: 0.5% or less, Ni: 0.5% or less, or at least one of these (b) Ca: 1.0% or less, La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less, or at least one of these (c) Ti: 0.1% or less (d) W: 0.5% or less (e) Cu: 2.0% or less (f) Fe: 1.0% or less, Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less, at least one of these (g) B: 0.1% or less, P: 0.1% or less, at least one of these (h) Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less, or at least one of these

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