Hot-dip galvanized steel sheet and method of manufacturing same

Abstract

One aspect of the present invention is to provide a hot-dip galvanized steel sheet and a method of manufacturing same. One preferred aspect of the present invention is to provide a high-strength hot-dip galvanized steel sheet having excellent plated appearance and plating adhesiveness, and a manufacturing method therefor.

Description

Hot-dip galvanized steel sheet and its manufacturing method

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

[0002] Demand for ultra-high-strength steel is rapidly increasing in response to stringent automotive fuel economy and crash safety regulations, driven by increasingly stringent environmental regulations. While improved fuel efficiency is essential to meet national carbon emission reduction targets, increased performance and convenience features are steadily increasing vehicle weight. To address these challenges, demand for ultra-high-strength steel is steadily increasing, and steelmakers are focusing on developing high-strength steels, such as Dual Phase (DP) steel, Transformation Induced Plasticity (TRIP) steel, and Complex Phase (CP) steel.

[0003] In order to increase the strength of automobile steel plates, it is common to add large amounts of elements such as Si, Mn, and Al to the steel, but these elements have the problem of generating oxides on the surface of the steel plate during the annealing heat treatment process, which reduces the plating properties and adhesion during hot-dip galvanizing.

[0004] A representative technology for solving this problem is Patent Document 1. Patent Document 1 describes a technology for performing reduction annealing on a steel sheet containing a large amount of Si and Al using a direct-fired reduction heating furnace at an air-fuel ratio of 0.7 to 1.2 to improve the plating quality. Through this, it is disclosed that the Fe oxide on the surface of the steel sheet is reduced to reduced Fe, and the Fe-Al intermetallic compound layer is well developed, so that an excellent plating surface can be secured. However, there is a problem in that the Si, Mn, or Al oxides formed at the interface between the Fe oxide and the base steel sheet during the reduction annealing process take the form of a layer, which causes a plating peeling phenomenon in which the plating layer is removed after zinc plating.

[0005] Therefore, there is an urgent need for technology that can secure the plating appearance and plating adhesion of high-strength hot-dip galvanized steel sheets.

[0006] [Prior Art Literature]

[0007] (Patent Document 1) Japanese Patent Publication No. 2005-154856

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

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

[0010] One embodiment of the present invention provides a hot-dip galvanized steel sheet comprising, by weight %, Si: 0.10% or more and less than 0.50%, Mn: 1.0 to 3.0%, the remainder Fe and other unavoidable impurities; and a 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, 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, and an internal oxidation layer is formed directly under the Fe reduction layer.

[0011] Another embodiment of the present invention provides a hot-dip galvanized steel sheet comprising: a cold-rolled steel sheet comprising, by weight %, Si: 0.10% or more and less than 0.50%, Mn: 1.0 to 3.0%, the remainder Fe and other unavoidable impurities; and a hot-dip galvanized layer formed on at least one surface of the cold-rolled steel sheet; wherein an Fe-Al intermetallic compound layer including pores 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 beneath the surface of the cold-rolled steel sheet, and wherein the pores have an area ratio of 10% or less relative to the total area of ​​the surface of the hot-dip galvanized steel sheet.

[0012] The above cold rolled steel sheet may additionally include one or more of the following elements in weight %: 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%).

[0013] The above Fe-Al intermetallic compound layer may have an area ratio of 90% or more compared to the total area of ​​the surface of the hot-dip galvanized steel sheet.

[0014] The above Fe reduction layer may have a grain form.

[0015] The above internal oxidation layer may have an average thickness of 0.5 to 3 μm.

[0016] 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: 1.0 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 obtaining a hot-dip galvanized steel sheet by hot-dip galvanizing the cold-rolled steel sheet subjected to the reduction heat treatment; the DFF equipment includes four or more zones, and provides a method for manufacturing a hot-dip galvanized steel sheet controlled to satisfy the following relational expression 1.

[0017] [Relationship 1] (Air ratio of the last zone - 1) × (Air ratio of the (last-1)th zone - 1) ≥ 0

[0018] 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%, and N: 0.0080% or less (excluding 0%).

[0019] Heating of the above slab can be performed at 1100 to 1300°C.

[0020] The above finishing hot rolling can be performed at 800 to 1000°C.

[0021] The above reducing atmosphere may contain 3 to 25% hydrogen by volume and the remainder nitrogen.

[0022] Each of the above four or more zones may have an air ratio of 1.0 or greater.

[0023] According to one aspect of the present invention, a hot-dip galvanized steel sheet and a method for manufacturing the same can be provided.

[0024] According to a preferred aspect of the present invention, a high-strength hot-dip galvanized steel sheet having excellent plating appearance and plating adhesion and a method for manufacturing the same can be provided.

[0025] Figure 1 is a schematic diagram showing a hot-dip galvanized steel sheet according to one embodiment of the present invention.

[0026] Figure 2 is a photograph of Invention Example 1 according to an embodiment of the present invention observed using TEM.

[0027] Figure 3 is a photograph of Comparative Example 1, which deviates from the embodiment of the present invention, observed using TEM.

[0028] Figure 4 is a photograph of the surface of the Fe-Al intermetallic compound layer of Invention Example 1 according to an embodiment of the present invention observed using SEM.

[0029] Figure 5 is a photograph of the surface of the Fe-Al intermetallic compound layer of Comparative Example 1, which is different from the embodiment of the present invention, observed using SEM.

[0030] Hereinafter, a hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0031] According to one embodiment of the present invention, a hot-dip galvanized steel sheet comprises: a cold-rolled steel sheet containing, by weight %, Si: 0.10% or more and less than 0.50%, Mn: 1.0 to 3.0%, the remainder Fe and other unavoidable impurities; and a 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, 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, and an internal oxidation layer may be formed directly under the Fe reduction layer.

[0032] Another embodiment of the present invention provides a hot-dip galvanized steel sheet comprising: a cold-rolled steel sheet comprising, by weight %, Si: 0.10% or more and less than 0.50%, Mn: 1.0 to 3.0%, the remainder Fe and other unavoidable impurities; and a hot-dip galvanized layer formed on at least one surface of the cold-rolled steel sheet; wherein an Fe-Al intermetallic compound layer including pores 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 beneath the surface of the cold-rolled steel sheet, and wherein the pores have an area ratio of 10% or less relative to the total area of ​​the surface of the hot-dip galvanized steel sheet.

[0033] The cold-rolled steel sheet of the present invention preferably contains, in wt%, Si: 0.10% or more and less than 0.50%, Mn: 1.0 to 3.0%, the remainder Fe, and other unavoidable impurities.

[0034] Si: 0.10% or more and less than 0.50%

[0035] 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%.

[0036] Mn: 1.0~3.0%

[0037] 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 1.0%, 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 1.0 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%.

[0038] 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 one or more of the following in weight %: 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%).

[0039] 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.

[0040] Meanwhile, the cold-rolled steel sheet of the present invention can have a thickness of 1.0 to 2.5 mm, and thus can be preferably used as an automobile steel sheet for impact structural members.

[0041] Figure 1 is a schematic diagram showing a hot-dip galvanized steel sheet according to one embodiment of the present invention.

[0042] The hot-dip galvanized steel sheet (10) of the present invention preferably includes a hot-dip galvanized layer (2) formed on at least one surface of the cold-rolled steel sheet (1) described above. The present invention does not particularly limit the type of the hot-dip galvanized layer, and all types of hot-dip galvanized layers commonly used in the relevant technical field may be applied. However, as an example, the hot-dip galvanized layer may include, by weight %, 0.5% or less of Al and the remainder of Zn.

[0043] It is preferable that an Fe-Al intermetallic compound layer (3) is formed at the interface between the cold-rolled steel sheet (1) and the hot-dip galvanized layer (2). The Fe-Al intermetallic compound layer improves plating adhesion and helps suppress the phenomenon of the plating layer being peeled off. The Fe-Al intermetallic compound layer may have an area ratio of 90% or more with respect to the total area of ​​the surface of the hot-dip galvanized steel sheet. In addition, the Fe-Al intermetallic compound layer includes pores in a SEM (Scanning Electron Microscopy) image when viewed from above, and the pores may have an area ratio of 10% or less with respect to the total area of ​​the surface of the hot-dip galvanized steel sheet. When the area ratio of the Fe-Al intermetallic compound layer is less than 90% or the area ratio of the pores exceeds 10%, non-plating or plating peeling may occur. The area ratio of the Fe-Al intermetallic compound layer is more preferably 95% or more, and more preferably 98% or more. The area ratio of the pores is more preferably 5% or less, and more preferably 2% or less. In the present invention, the type of the Fe-Al intermetallic compound layer is not particularly limited, but may be, for example, an Fe2Al5 layer.

[0044] As an example, the above Fe-Al intermetallic compound layer may have a film shape when the steel sheet is viewed from the side, thereby suppressing the occurrence of under-plating.

[0045] 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. Meanwhile, the present invention does not specifically limit the shape of the Fe reduction layer, but as an example, it may have a grain shape and may exist intermittently.

[0046] An internal oxide layer (5) may be formed directly beneath the above Fe reduction layer (4). The internal oxide layer prevents Si or Mn present in the cold-rolled steel sheet from diffusing into the surface layer of the steel sheet, thereby preventing Si or Mn oxides from being formed on the surface layer, thereby improving plating properties. The internal oxide layer may have an average thickness of 0.5 to 3 μm. When the average thickness of the internal oxide layer is less than 0.5 μm, it may be difficult to sufficiently obtain the above-described effect. When the average thickness of the internal oxide layer exceeds 3 μm, a problem of reduced fatigue strength may occur. Meanwhile, the internal oxide layer may be formed along grain boundaries in a TEM (Transmission Electron Microscopy) image when viewed from the side.

[0047] As described above, the hot-dip galvanized steel sheet of the present invention can secure excellent plating appearance and plating adhesion.

[0048] Hereinafter, a method for manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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 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 following relational expression 1 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.

[0055] [Relationship 1] (Air ratio of the last zone - 1) × (Air ratio of the (last-1)th zone - 1) ≥ 0

[0056] 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.

[0057] 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.

[0058] 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.

[0059] Thereafter, the cold-rolled steel sheet subjected to the reduction heat treatment is subjected to hot-dip galvanization to obtain a hot-dip galvanized steel sheet. The present invention does not specifically limit the hot-dip galvanizing process, and typical conditions applicable in the relevant technical field can be used. An Fe-Al intermetallic compound layer can 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 can be formed by a reaction between Al contained in a trace amount in the plating bath and Fe eluted from the base steel sheet.

[0060] 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.

[0061] (Example)

[0062] A slab (C: 0.08%, P: 0.02%, S: 0.003%, Al: 0.03%, N: 0.004%) having Si and Mn contents as shown 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 as shown 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 2.0 mm. Thereafter, the cold-rolled steel sheet was subjected to oxidation heat treatment by passing it through a DFF (Direct Fired Furnace) facility having four zones (line speed: 60 mpm) under the conditions as shown in Table 2 below. Thereafter, the cold-rolled steel sheet subjected to oxidation heat treatment 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 cold-rolled steel sheet subjected to reduction heat treatment was immersed in a hot-dip galvanizing bath (Al: 0.2%, the remainder Zn) to manufacture a hot-dip galvanized steel sheet.

[0063] For the hot-dip galvanized steel sheet manufactured in this manner, the presence or absence of the formation of an Fe-Al intermetallic compound layer, the presence or absence of the formation and average thickness of an Fe reduction layer, the presence or absence of the formation and average thickness of an internal oxidation layer, and the area fraction of pores were measured, and the results are shown in Table 2 below.

[0064] The presence or absence of formation of the Fe-Al intermetallic compound layer and the area fraction of pores 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.

[0065] The presence or absence of formation of an Fe reduction layer and its average thickness, and the presence or absence of formation of an internal oxidation layer and its average thickness were measured at 10 random locations on the side of the hot-dip galvanized steel sheet using a transmission electron microscope, and the average value was calculated.

[0066] In addition, the plating properties of the above-mentioned hot-dip galvanized steel sheet were measured, and the results are shown in Table 2 below.

[0067] As one of the evaluations of plating properties, the presence or absence of plating was checked by visually observing the surface of the hot-dip galvanized steel sheet to see if there were any unplated areas.

[0068] In order to evaluate the peeling of the plating layer as one of the evaluations of the plating properties, 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 after curing in an oven at 170°C for 20 minutes, it was clamped to a bending jig and bent 90°. Thereafter, the presence 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.

[0069] Classification Alloy composition (weight%) Coiling temperature (℃) DFF outlet temperature (℃) Relationship 1 Satisfaction SiMn Invention example 10.21.69608651 Satisfied Invention example 20.11.79605650 Satisfied Invention example 30.31.59623665 Satisfied Invention example 40.251.64676660 Satisfied Comparative example 11.40.49585655 Unsatisfied Comparative example 20.51.39574650 Unsatisfied Comparative example 30.71.19637677 Unsatisfied Comparative example 40.31.59646700 Satisfied Comparative example 50.21.69656625 Unsatisfied [Relationship 1] (Air ratio of the last zone - 1) × (Air ratio of the (last-1)th zone - 1) ≥ 0

[0070] ClassificationFe-Al intermetallic compound layer formationFe reduction layerInternal oxidation layerPore area fraction (area%)Plating propertyFormation propertyAverage thickness (nm)Formation propertyAverage thickness (㎛)Non-platingPlating layer peelingpropertyInvention example 1YesYes130Yes2.02No peeling ...

[0071] As can be seen from Tables 1 and 2, in the case of invention examples 1 to 4 that satisfy the conditions proposed by the present invention, it can be seen that the plating properties are good.

[0072] On the other hand, in the case of Comparative Example 1, it can be seen that the Fe reduction layer and internal oxidation layer were not formed and pores did not occur due to the Si and Mn contents of the cold-rolled steel sheet, the coiling temperature, and the non-coating equation 1 not being satisfied, resulting in the occurrence of unplated and plating layer peeling.

[0073] In the case of Comparative Example 2, it can be seen that the Fe reduction layer and internal oxidation layer were not formed and pores did not occur because the coiling temperature and equation 1 were not satisfied, resulting in non-plating.

[0074] In the case of Comparative Example 3, it can be seen that the Fe reduction layer was not formed, the internal oxidation layer was not sufficiently formed, and pores were not generated, resulting in non-plating, as the Si content of the cold-rolled steel sheet, the DFF exit temperature, and the relationship 1 were not satisfied.

[0075] In the case of Comparative Example 4, it can be seen that the Fe reduction layer and internal oxidation layer were excessively formed and pores were also excessively generated as the DFF exit temperature was not satisfied, resulting in non-plating and plating layer peeling.

[0076] In the case of Comparative Example 5, it can be seen that the Fe reduction layer was not formed, the internal oxidation layer was not sufficiently formed, and pores were not generated, resulting in non-plating, as the DFF exit temperature and relational equation 1 were not satisfied.

[0077] Fig. 2 is a photograph of Invention Example 1 observed using a TEM. As can be seen from Fig. 2, in the case of Invention Example 1, an Fe-Al intermetallic compound layer, an Fe reduction layer, and an internal oxide layer are formed sequentially, and it can be seen that the Fe reduction layer has a continuous grain shape.

[0078] Figure 3 is a photograph of Comparative Example 1 observed using TEM. As can be seen from Figure 3, in the case of Comparative Example 1, it can be confirmed that the Fe-Al intermetallic compound layer is formed unevenly and the Fe reduction layer is not formed.

[0079] Fig. 4 is a photograph of the surface of the Fe-Al intermetallic compound layer of Invention Example 1 observed using SEM. As can be seen from Fig. 4, in the case of Invention Example 1, the Fe-Al intermetallic compound layer is formed densely and uniformly, and it can be seen that pores also exist due to volume shrinkage during the reduction process of the Fe oxide layer.

[0080] On the other hand, Fig. 5 is a photograph of the surface of the Fe-Al intermetallic compound layer of Comparative Example 1 observed using SEM. As can be seen from Fig. 5, in the case of Comparative Example 1, the crystals of the Fe-Al intermetallic compound layer are distributed in the form of particles and are not formed densely.

[0081] [Explanation of symbols]

[0082] 1: Cold rolled steel sheet

[0083] 2: Hot-dip galvanized layer

[0084] 3: Fe-Al intermetallic compound layer

[0085] 4: Fe reduction layer

[0086] 5: Internal oxidation layer

[0087] 10: 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: 1.0 to 3.0%, the remainder Fe and other unavoidable impurities; and Including a 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. An Fe reduction layer having an average thickness of 50 to 250 nm is formed directly beneath the surface of the cold rolled steel plate. A hot-dip galvanized steel sheet having an internal oxide layer formed directly beneath the above Fe reduction layer.

2. Cold rolled steel sheet containing, by weight%, Si: 0.10% or more and less than 0.50%, Mn: 1.0 to 3.0%, the remainder Fe and other unavoidable impurities; and Including a hot-dip galvanized layer formed on at least one surface of the cold-rolled steel sheet; An Fe-Al intermetallic compound layer containing pores is formed at the interface between the cold-rolled steel sheet and the hot-dip galvanized layer. An Fe reduction layer having an average thickness of 50 to 250 nm is formed directly beneath the surface of the cold rolled steel plate. The above-mentioned pores are a hot-dip galvanized steel sheet having an area ratio of 10% or less of the total area of ​​the surface of the hot-dip galvanized steel sheet.

3. In claim 1 or 2, The above cold rolled steel sheet is a hot-dip galvanized steel sheet additionally containing at least one of the following in weight %: 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%).

4. In claim 1 or 2, A hot-dip galvanized steel sheet, wherein the Fe-Al intermetallic compound layer has an area ratio of 90% or more of the total area of ​​the surface of the hot-dip galvanized steel sheet.

5. In claim 1 or 2, The above Fe reduction layer is a hot-dip galvanized steel sheet having a grain shape.

6. In claim 1 or 2, The above internal oxidation layer is a hot-dip galvanized steel sheet having an average thickness of 0.5 to 3 ㎛.

7. A step of heating a slab containing, by weight%, Si: 0.10% or more and less than 0.50%, Mn: 1.0 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 obtaining a hot-dip galvanized steel sheet by hot-dip galvanizing the cold-rolled steel sheet subjected to the reduction heat treatment; A method for manufacturing a hot-dip galvanized steel sheet, wherein the above DFF equipment includes four or more zones and is 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 8. In claim 7, A method for manufacturing a hot-dip galvanized steel sheet, wherein the above slab additionally contains at least one of, in weight %, 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%, and N: 0.0080% or less (excluding 0%).

9. In claim 7, A method for manufacturing a hot-dip galvanized steel sheet, wherein the heating of the above slab is performed at 1100 to 1300°C.

10. In claim 7, The above finishing hot rolling is a method for manufacturing a hot-dip galvanized steel sheet, which is performed at 800 to 1000°C.

11. In claim 7, A method for manufacturing a hot-dip galvanized steel sheet, wherein the reducing atmosphere contains 3 to 25% by volume of hydrogen and the remainder of nitrogen.

12. In claim 7, A method for manufacturing a hot-dip galvanized steel sheet in which each of the above four or more zones has an air ratio of 1.0 or higher.

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