Hot-dip galvanized steel sheet and method of manufacturing the same

Abstract

An aspect of the present application aims to provide a hot-dip galvanized steel sheet and a method of manufacturing the same. A preferred aspect of the present application aims to provide a high-strength hot-dip galvanized steel sheet having excellent plated appearance and plated adhesion and a method of manufacturing the same.

Description

Technical Field

[0001] This invention relates to a hot-dip galvanized steel sheet and its manufacturing method. Background Technology

[0002] In recent years, with increasingly stringent environmental regulations, the demand for ultra-high-strength steel sheets is rising sharply as a solution to meet stringent fuel efficiency and collision safety regulations for automobiles. To achieve national carbon reduction targets, improved fuel efficiency is required; on the other hand, vehicle weight is continuously increasing due to the rise in high performance and various convenience features. To address these issues, the demand for ultra-high-strength steel sheets is steadily increasing, prompting steel companies to focus on developing high-strength steel sheets such as dual-phase (DP) steel, transformation-induced plasticity (TRIP) steel, and complex-phase (CP) steel.

[0003] To increase the strength of automotive steel sheets, large amounts of elements such as Si, Mn, and Al are usually added to the steel. However, these elements will generate oxides on the surface of the steel sheet during the annealing heat treatment process, which will reduce the coating and adhesion during hot-dip galvanizing.

[0004] A representative technology for solving this problem is Patent Document 1. Patent Document 1 describes a technique for reducing annealing steel plates containing large amounts of Si and Al in a furnace using a direct-fire reduction heating method at an air-fuel ratio of 0.7-1.2 to improve plating quality. It discloses that through this technique, the Fe oxides on the surface of the steel plate are reduced to reduced iron, and the Fe-Al intermetallic compound layer is well-developed, ensuring an excellent plating surface. However, during the reduction annealing process, the Si, Mn, or Al oxides formed at the interface between the Fe oxides and the base steel plate are in a layered form, which can lead to plating peeling after galvanizing.

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

[0006] [Existing Technical Documents] (Patent Document 1) Japanese Patent Publication No. 2005-154856 Summary of the Invention

[0007] (a) Technical problems to be solved One aspect of the present invention aims to provide a hot-dip galvanized steel sheet and a method for manufacturing the same.

[0008] A preferred aspect of the present invention aims to provide a high-strength hot-dip galvanized steel sheet with excellent coating appearance and coating adhesion, and a method for manufacturing the same.

[0009] (II) Technical Solution One embodiment of the present invention provides a hot-dip galvanized steel sheet, comprising: a cold-rolled steel sheet, which, by weight percent, contains: Si: 0.10% or more and less than 0.50%, Mn: 1.0-3.0%, with the balance being Fe and other unavoidable impurities; and a hot-dip galvanized layer formed on at least one side of the cold-rolled steel sheet, wherein 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 50-250 nm is formed directly below the surface of the cold-rolled steel sheet, and an internal oxide layer is formed directly below the Fe reduction layer.

[0010] Another embodiment of the present invention provides a hot-dip galvanized steel sheet comprising: a cold-rolled steel sheet, which, by weight percent, comprises: Si: 0.10% or more and less than 0.50%, Mn: 1.0-3.0%, with the balance being Fe and other unavoidable impurities; and a hot-dip galvanized layer formed on at least one side of the cold-rolled steel sheet, wherein a porous Fe-Al intermetallic compound layer is formed at the interface between the cold-rolled steel sheet and the hot-dip galvanized layer, and an Fe reduction layer having an average thickness of 50-250 nm is formed directly below the surface of the cold-rolled steel sheet, wherein the pores have an area ratio of less than 10% relative to the total surface area of ​​the hot-dip galvanized steel sheet.

[0011] The cold-rolled steel sheet may further comprise, by weight percent, one or more of the following: C: 0.050-0.30%, P: less than 0.10% (excluding 0%), S: less than 0.010% (excluding 0%), Al: 0.010-0.10%, and N: less than 0.0080% (excluding 0%).

[0012] The Fe-Al intermetallic compound layer can have an area ratio of more than 90% relative to the total area of ​​the hot-dip galvanized steel sheet surface.

[0013] The Fe reduction layer can have a grain morphology.

[0014] The internal oxide layer can have an average thickness of 0.5-3 μm.

[0015] Another embodiment of the present invention provides a method for manufacturing hot-dip galvanized steel sheet, comprising the following steps: heating a slab, wherein the slab comprises, by weight, Si: 0.10% or more and less than 0.50%, Mn: 1.0-3.0%, with the balance being Fe and other unavoidable impurities; hot-rolling the heated slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 600-680°C; cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and passing the cold-rolled steel sheet through a direct-fired furnace. A Furnace (DFF) device is used to perform oxidative heat treatment at an outlet temperature of 630-670°C; the cold-rolled steel sheet subjected to the oxidative heat treatment is then subjected to reduction heat treatment in a reducing atmosphere with a dew point temperature above -60°C and below -45°C; and the cold-rolled steel sheet subjected to the reduction heat treatment is then subjected to hot-dip galvanizing to obtain a hot-dip galvanized steel sheet. The DFF device includes four or more zones and is controlled to satisfy the following relationship 1.

[0016] [Formula 1] (Air ratio of the last segment - 1) × (Air ratio of the second to last segment - 1) ≥ 0 The slab may further contain one or more of the following: C: 0.050-0.30%, P: less than 0.10% (excluding 0%), S: less than 0.010% (excluding 0%), Al: 0.010-0.10%, and N: less than 0.0080% (excluding 0%).

[0017] The slab can be heated at 1100-1300℃.

[0018] The hot finishing rolling can be carried out at 800-1000℃.

[0019] The reducing atmosphere may contain 3-25% hydrogen and the balance nitrogen by volume.

[0020] The air ratio of each of the four or more sections can be 1.0 or higher.

[0021] (III) Beneficial Effects According to one aspect of the present invention, a hot-dip galvanized steel sheet and a method for manufacturing the same can be provided.

[0022] According to a preferred aspect of the present invention, a high-strength hot-dip galvanized steel sheet with excellent coating appearance and coating adhesion, and a method for manufacturing the same, can be provided. Attached Figure Description

[0023] Figure 1 This is a schematic diagram illustrating a hot-dip galvanized steel sheet according to an embodiment of the present invention.

[0024] Figure 2 This is a photograph of Invention Example 1, which is observed using TEM according to an embodiment of the present invention.

[0025] Figure 3 These are photographs of Comparative Example 1, which are derived from embodiments of the present invention, observed using TEM.

[0026] Figure 4 This is a photograph of the surface of the Fe-Al intermetallic compound layer according to an embodiment of the present invention, as observed by SEM.

[0027] Figure 5 This is a photograph of the surface of the Fe-Al intermetallic compound layer of Comparative Example 1, which is detached from the embodiment of the present invention, observed by SEM. Best practice

[0028] The following describes a hot-dip galvanized steel sheet according to one embodiment of the present invention.

[0029] According to one embodiment of the present invention, a hot-dip galvanized steel sheet may include: a cold-rolled steel sheet, which, by weight percent, comprises: Si: 0.10% or more and less than 0.50%, Mn: 1.0-3.0%, with the balance being Fe and other unavoidable impurities; and a hot-dip galvanized layer formed on at least one side of the cold-rolled steel sheet, wherein an Fe-Al intermetallic compound layer may be 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 50-250 nm may be formed directly below the surface of the cold-rolled steel sheet, and an internal oxide layer may be formed directly below the Fe reduction layer.

[0030] Another embodiment of the present invention provides a hot-dip galvanized steel sheet comprising: a cold-rolled steel sheet, which, by weight percent, comprises: Si: 0.10% or more and less than 0.50%, Mn: 1.0-3.0%, with the balance being Fe and other unavoidable impurities; and a hot-dip galvanized layer formed on at least one side of the cold-rolled steel sheet, wherein a porous Fe-Al intermetallic compound layer is formed at the interface between the cold-rolled steel sheet and the hot-dip galvanized layer, and an Fe reduction layer having an average thickness of 50-250 nm is formed directly below the surface of the cold-rolled steel sheet, wherein the pores have an area ratio of less than 10% relative to the total surface area of ​​the hot-dip galvanized steel sheet.

[0031] The cold-rolled steel sheet of the present invention preferably contains, by weight percent: Si: 0.10% or more and less than 0.50%, Mn: 1.0-3.0%, with the balance being Fe and other unavoidable impurities.

[0032] Si: ≥0.10% and <0.50% Si is an important element that contributes to strength improvement through solid solution strengthening, playing a role in increasing strength while suppressing processability degradation. When the Si content is less than 0.10%, it is difficult to fully obtain the above-mentioned effect. When the Si content is 0.50% or more, a layer of Si or Mn oxide may form directly below the Fe oxide layer during oxidative 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 even more preferably 0.15%. The upper limit of the Si content is more preferably 0.45%, and even more preferably 0.4%.

[0033] Mn: 1.0-3.0% Mn (metallurgical element) contributes to strength improvement through solid solution strengthening while also enhancing the hardenability of the austenite phase, effectively contributing to strength stabilization. When the Mn content is less than 1.0%, it is difficult to fully achieve the aforementioned effects. When the Mn content is greater than 3.0%, it may reduce workability. Therefore, the Mn content is preferably in the range of 1.0-3.0%. The lower limit of the Mn content is more preferably 1.50%. The upper limit of the Mn content is more preferably 2.50%.

[0034] The cold-rolled steel sheet of the present invention is not particularly limited in terms of alloy composition other than Si and Mn, and can be applied to the alloy composition of all cold-rolled steel sheets used in this technical field. However, as an example, the cold-rolled steel sheet of the present invention may further contain, by weight percent, one or more of the following: C: 0.050-0.30%, P: less than 0.10% (except 0%), S: less than 0.010% (except 0%), Al: 0.010-0.10%, and N: less than 0.0080% (except 0%).

[0035] The remaining component of the cold-rolled steel sheet of the present invention is iron (Fe), and may contain some unintentional, unavoidable impurities introduced during the manufacturing process. These impurities are well known to those skilled in the art of manufacturing, and therefore their contents are not specifically mentioned in this specification.

[0036] In addition, the cold-rolled steel sheet of the present invention can have a thickness of 1.0-2.5 mm, and thus can be preferably used as automotive steel sheet for impact structural components.

[0037] Figure 1 This is a schematic diagram illustrating a hot-dip galvanized steel sheet according to an embodiment of the present invention.

[0038] The hot-dip galvanized steel sheet 10 of the present invention preferably includes a hot-dip galvanized layer 2 formed on at least one side of the aforementioned cold-rolled steel sheet 1. 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 this art can be applied. However, as an example, the hot-dip galvanized layer may contain less than 0.5% Al and the balance Zn by weight.

[0039] A Fe-Al intermetallic compound layer 3 is preferably formed at the interface between the cold-rolled steel sheet 1 and the hot-dip galvanized layer 2. This Fe-Al intermetallic compound layer helps improve plating adhesion and inhibits plating peeling. The Fe-Al intermetallic compound layer can have an area ratio of 90% or more relative to the total surface area of ​​the hot-dip galvanized steel sheet. Furthermore, when viewed from above, the Fe-Al intermetallic compound layer contains pores in a scanning electron microscope (SEM) image, and these pores can have an area ratio of less than 10% relative to the total surface area 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 is greater than 10%, unplated or plating peeling may occur. More preferably, the area ratio of the Fe-Al intermetallic compound layer is 95% or more, and even more preferably 98% or more. More preferably, the area ratio of the pores is 5% or less, and even more preferably 2% or less. The present invention does not particularly limit the type of Fe-Al intermetallic compound layer; for example, it can be an Fe2Al5 layer.

[0040] As an example, the Fe-Al intermetallic compound layer can have a film morphology when the steel plate is viewed from the side, thereby suppressing the occurrence of uncoated areas.

[0041] An Fe reduction layer 4 with an average thickness of 50-250 nm can be formed directly beneath the surface of the cold-rolled steel sheet 1. This Fe reduction layer is formed by further reducing a Fe oxide layer created through an oxidation heat treatment process. The Fe oxide layer is widely recognized as reducing plating adhesion. In this invention, the Fe reduction layer is formed by reducing the Fe oxide layer, thereby improving plating adhesion and suppressing plating peeling. The Fe reduction layer preferably has an average thickness of 50-250 nm. When the average thickness of the Fe reduction layer is less than 50 nm, the above-mentioned effects cannot be fully achieved. When the average thickness of the Fe reduction layer is greater than 250 nm, the Fe reduction layer cannot be completely reduced, and Si or Mn oxides may form directly beneath it, potentially reducing plating adhesion. Furthermore, the morphology of the Fe reduction layer is not particularly limited in this invention; as an example, it can have a granular morphology and can exist intermittently.

[0042] An internal oxide layer 5 can be formed directly beneath the Fe reduction layer 4. This internal oxide layer prevents the diffusion of Si or Mn present in the cold-rolled steel sheet to the surface layer, thus preventing the formation of Si or Mn oxides on the surface and improving plating performance. The internal oxide layer can have an average thickness of 0.5-3 μm. When the average thickness of the internal oxide layer is less than 0.5 μm, it may be difficult to achieve the above-mentioned effect. When the average thickness of the internal oxide layer is greater than 3 μm, a decrease in fatigue strength may occur. Furthermore, when viewed from the side, the internal oxide layer can be seen to form along grain boundaries in a transmission electron microscope (TEM) image.

[0043] The hot-dip galvanized steel sheet of the present invention, as described above, ensures excellent coating appearance and coating adhesion.

[0044] The following describes a method for manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present invention.

[0045] First, the slab satisfying the above alloy composition is heated. The heating process for the slab is not particularly limited in this invention, and commonly used conditions in this technical field can be used. However, as an example, the heating of the slab can be performed at 1100-1300°C.

[0046] The heated slab is then hot-finished to obtain a hot-rolled steel sheet. The hot-finishing process is not particularly limited in this invention, and commonly used conditions in this technical field can be used. However, as an example, the hot-finishing can be performed at 800-1000°C.

[0047] The hot-rolled steel sheet is then coiled at 600-680°C. If the coiling temperature is below 600°C, shape deterioration may occur. If the coiling temperature is above 680°C, oxide scale dust may be generated. Therefore, the coiling temperature is preferably in the range of 600-680°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.

[0048] The hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling process is not particularly limited in this invention, and commonly used conditions in this technical field can be employed.

[0049] Subsequently, the cold-rolled steel sheet is subjected to an oxidation heat treatment in a direct combustion furnace (DFF) at an exit-side temperature of 630-670°C. This oxidation heat treatment aims to form an internal oxide layer while simultaneously oxidizing the Fe oxide layer present on the surface of the cold-rolled steel sheet to form an Fe oxide layer. This Fe oxide layer acts as a diffusion-preventing film, inhibiting the diffusion of elements such as Si or Mn to the steel sheet surface during heat treatment. When the exit-side temperature of the DFF is below 630°C, the Fe oxide layer may not form or may form very thinly, resulting in minimal inhibition of the enrichment of target alloying elements. When the exit-side temperature of the DFF is above 670°C, the excessively formed Fe oxide layer may detach and adhere to the rollers due to friction, potentially causing defects such as dents on the steel sheet surface. Therefore, the exit-side temperature of the DFF is preferably in the range of 630-670°C. The lower limit of the exit-side temperature of the DFF is more preferably 635°C, and even more preferably 640°C. The upper limit of the outlet temperature of the DFF device is more preferably 665°C, and even more preferably 660°C.

[0050] At this time, the DFF equipment includes four or more sections, and is preferably controlled to satisfy the following relationship 1. Relationship 1 is used to ensure complete combustion of the supplied fuel and to oxidize the Fe present on the surface of the cold-rolled steel sheet with residual excess oxygen. Furthermore, it is used to control the oxides from accumulating on the steel sheet surface, instead trapping them directly beneath the steel sheet surface. The Fe oxide layer formed by this process is transformed into a Fe reduced layer through a subsequent reduction process. When the Fe oxide layer is completely reduced, the oxides present on the surface of the steel sheet are removed, thereby uniformly forming an Fe-Al intermetallic compound layer. When Relationship 1 is not satisfied, the distribution of the Fe-Al intermetallic compound layer may become uneven, thus making it difficult for the Fe-Al intermetallic compound layer to have an area ratio of more than 70% relative to the total area of ​​the steel sheet surface. Furthermore, some particles within the Fe-Al intermetallic compound layer may grow coarsely. In addition, the purpose of this invention can be achieved as long as Relationship 1 is satisfied; therefore, no particular upper limit is imposed on the value of Relationship 1.

[0051] [Formula 1] (Air ratio of the last segment - 1) × (Air ratio of the second to last segment - 1) ≥ 0 Furthermore, the air ratio of each of the four or more sections can be 1.0 or higher, and by controlling it as described above, Fe oxidation can be made easier.

[0052] Subsequently, the cold-rolled steel sheet that has undergone the oxidation heat treatment is subjected to reduction heat treatment in a reducing atmosphere with a dew point temperature of -60°C or higher and below -45°C. This reduction heat treatment is used to reduce the Fe oxide layer formed by the oxidation heat treatment to form a reduced Fe layer. Therefore, when the dew point temperature is below -60°C, plating performance may be reduced. When the dew point temperature is above -45°C, the amount of oxide increases due to the selected oxidation transformation range, which may also lead to reduced plating performance. Therefore, the dew point temperature preferably has a range of -60°C or higher and below -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.

[0053] Furthermore, the conditions of the reducing atmosphere are not particularly limited in this invention, and common conditions used in this art can be used. However, as an example, it can be a gaseous atmosphere containing 3-25% hydrogen and the balance nitrogen by volume.

[0054] Subsequently, the cold-rolled steel sheet subjected to the reduction heat treatment is hot-dip galvanized to obtain a hot-dip galvanized steel sheet. The hot-dip galvanizing process is not particularly limited in this invention, and commonly used conditions in this technical field can be used. Through the hot-dip galvanizing process, an Fe-Al intermetallic compound layer can be formed. The type of Fe-Al intermetallic compound layer is not particularly limited in this invention. However, as an example, it can be formed by the reaction of trace amounts of Al contained in the plating bath with Fe dissolved from the base steel sheet. Detailed Implementation

[0055] The present invention will now be described in more detail through embodiments. However, the embodiments described below are merely for illustrating the present invention in more detail and do not limit the scope of the invention.

[0056] (Example) A slab with the Si and Mn contents listed in Table 1 (C: 0.08%, P: 0.02%, S: 0.003%, Al: 0.03%, N: 0.004%) was heated at 1200°C, and then the heated slab was hot-finished at 900°C to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was then coiled under the conditions listed in Table 1, and then cold-rolled to obtain a cold-rolled steel sheet with a thickness of 2.0 mm. The cold-rolled steel sheet was then subjected to anodizing heat treatment under the conditions listed in Table 2 in a direct-fired furnace (DFF) with four sections (linear speed: 60 m / min). Subsequently, the cold-rolled steel sheet that has undergone the oxidation heat treatment is subjected to reduction heat treatment in a reducing atmosphere (5% hydrogen by volume and the balance nitrogen) under the conditions specified in Table 1 below, and the cold-rolled steel sheet that has undergone the reduction heat treatment is immersed in a hot-dip galvanizing bath (Al: 0.2%, balance Zn) to produce hot-dip galvanized steel sheet.

[0057] For the hot-dip galvanized steel sheet manufactured as described above, the results of measuring whether an Fe-Al intermetallic compound layer was formed, whether an Fe reduction layer was formed and its average thickness, whether an internal oxide layer was formed and its average thickness, and the area fraction of pores are shown in Table 2 below.

[0058] Whether an Fe-Al intermetallic compound layer has formed and the area ratio of pores were determined by taking a picture of the surface of the Fe-Al intermetallic compound layer with a scanning electron microscope, measuring 10 random locations with an image analyzer, and calculating the average value.

[0059] Whether an Fe reduction layer is formed and its average thickness, and whether an internal oxide layer is formed and its average thickness, are determined by measuring 10 arbitrary points on the side of the hot-dip galvanized steel sheet using a transmission electron microscope and then calculating the average value.

[0060] Furthermore, the coating properties of the hot-dip galvanized steel sheet were measured, and the results are shown in Table 2 below.

[0061] As part of the evaluation of coating performance, whether or not there is no coating is confirmed by visually inspecting the surface of the hot-dip galvanized steel sheet to identify any uncoated areas.

[0062] As part of the evaluation of coating performance, to assess whether the coating has peeled off, a 30mm × 80mm test piece is taken from the hot-dip galvanized steel sheet. A structural adhesive is applied to the test piece, which is then cured in an oven at 170°C for 20 minutes. The piece is then clamped in a bending fixture and bent at 90°. Afterward, visual inspection is performed to confirm whether the coating remains on the surface coated with the structural adhesive, thus determining whether the coating has peeled off.

[0063] [Table 1] [Table 2] As can be seen from Tables 1 and 2, in Invention Examples 1 to 4, which meet the conditions proposed in this invention, the plating performance is good.

[0064] In addition, in the case of Comparative Example 1, since the Si and Mn content, winding temperature and Equation 1 of the cold-rolled steel sheet were not satisfied, the Fe reduction layer and the internal oxide layer were not formed, and no pores were generated, it can be seen that uncoated and coating peeling occurred.

[0065] In Comparative Example 2, since the winding temperature and Equation 1 are not satisfied, no Fe reduction layer and internal oxide layer are formed, and no pores are generated, indicating that no coating has occurred.

[0066] In Comparative Example 3, since the Si content of the cold-rolled steel sheet, the DFF exit side temperature and Equation 1 are not satisfied, the Fe reduction layer is not formed, the internal oxide layer is not fully formed, and no pores are generated, it can be seen that the coating is not applied.

[0067] In Comparative Example 4, due to the failure to meet the DFF outlet temperature, an excessive Fe reduction layer and internal oxide layer were formed, and excessive porosity was also generated, indicating that uncoated and coating peeling occurred.

[0068] In Comparative Example 5, since the DFF outlet temperature and Equation 1 were not satisfied, the Fe reduction layer was not formed, the internal oxide layer was not fully formed, and no pores were generated, indicating that no coating occurred.

[0069] Figure 2 The photograph of Invention Example 1 was observed using a TEM. (Through...) Figure 2As can be seen, 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 the Fe reduction layer has a discontinuous granular morphology.

[0070] Figure 3 These are photographs of Comparative Example 1 observed using TEM. Figure 3 It can be seen that in the case of Comparative Example 1, the Fe-Al intermetallic compound layer was not formed uniformly, and it can be confirmed that no Fe reduction layer was formed.

[0071] Figure 4 This is a photograph of the surface of the Fe-Al intermetallic compound layer in Invention Example 1, observed using SEM. According to... Figure 4 As can be seen, in the case of Invention Example 1, the Fe-Al intermetallic compound layer is formed densely and uniformly. It is also known that during the reduction of the Fe oxide layer, pores still exist due to volume shrinkage.

[0072] on the other hand, Figure 5 These are SEM images of the Fe-Al intermetallic compound layer surface of Comparative Example 1. Figure 5 It can be seen that, in the case of Comparative Example 1, the Fe-Al intermetallic compound layer is distributed in a grain-like morphology and is not densely formed.

[0073] [Explanation of reference numerals in the attached figures] 1: Cold-rolled steel sheet 2: Hot-dip galvanized layer 3: Fe-Al intermetallic compound layer 4: Fe reduction layer 5: Internal oxide layer 10: Hot-dip galvanized steel sheet

Claims

1. A hot-dip galvanized steel sheet, comprising: The cold-rolled steel sheet, by weight percent, comprises: Si: ≥0.10% and <0.50%, Mn: 1.0-3.0%, with the balance being Fe and other unavoidable impurities; and A hot-dip galvanized layer, said hot-dip galvanized layer being formed on at least one side 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 with an average thickness of 50-250 nm is formed directly beneath the surface of the cold-rolled steel sheet. An internal oxide layer is formed directly beneath the Fe reduction layer.

2. A hot-dip galvanized steel sheet, comprising: The cold-rolled steel sheet, by weight percent, comprises: Si: ≥0.10% and <0.50%, Mn: 1.0-3.0%, with the balance being Fe and other unavoidable impurities; and A hot-dip galvanized layer is formed on at least one side of the cold-rolled steel sheet. In this process, a porous 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 with an average thickness of 50-250 nm is formed directly beneath the surface of the cold-rolled steel sheet. The pores have an area ratio of less than 10% relative to the total area of ​​the hot-dip galvanized steel sheet surface.

3. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein, The cold-rolled steel sheet further comprises, by weight percent, one or more of the following: C: 0.050-0.30%, P: less than 0.10% and excluding 0%, S: less than 0.010% and excluding 0%, Al: 0.010-0.10%, and N: less than 0.0080% and excluding 0%.

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

5. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein, The Fe reduction layer has a granular morphology.

6. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein, The internal oxide layer has an average thickness of 0.5-3 μm.

7. A method for manufacturing hot-dip galvanized steel sheet, comprising the following steps: The slab is heated, and the slab contains, by weight %, Si: more than 0.10% and less than 0.50%, Mn: 1.0-3.0%, with the balance being Fe and other unavoidable impurities; The heated slab is hot-rolled to obtain hot-rolled steel sheet; The hot-rolled steel sheet is coiled at 600-680℃; The hot-rolled steel sheet that has been coiled is cold-rolled to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is subjected to oxidative heat treatment by passing it through a direct combustion furnace (DFF) to achieve an outlet temperature of 630-670°C. The cold-rolled steel sheet that has undergone the oxidation heat treatment is subjected to reduction heat treatment in a reducing atmosphere with a dew point temperature above -60°C and below -45°C. as well as The cold-rolled steel sheet that has undergone the reduction heat treatment is then hot-dip galvanized to obtain hot-dip galvanized steel sheet. The DFF device comprises four or more sections and is controlled to satisfy the following relationship 1. [Formula 1] (air ratio of the last segment - 1) × (air ratio of the second to last segment - 1) ≥ 0.

8. The method for manufacturing hot-dip galvanized steel sheet according to claim 7, wherein, The slab further comprises, by weight percent, one or more of the following: C: 0.050-0.30%, P: less than 0.10% and excluding 0%, S: less than 0.010% and excluding 0%, Al: 0.010-0.10%, and N: less than 0.0080% and excluding 0%.

9. The method for manufacturing hot-dip galvanized steel sheet according to claim 7, wherein, The slab is heated at 1100-1300℃.

10. The method for manufacturing hot-dip galvanized steel sheet according to claim 7, wherein, The hot finishing rolling is carried out at 800-1000℃.

11. The method for manufacturing hot-dip galvanized steel sheet according to claim 7, wherein, The reducing atmosphere contains 3-25% hydrogen and the balance nitrogen by volume.

12. The method for manufacturing hot-dip galvanized steel sheet according to claim 7, wherein, The air ratio of each of the four or more sections is 1.0 or higher.

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