Plated steel sheet

Abstract

A plated steel sheet, according to one embodiment of the present invention, comprises: a base steel sheet including, in weight %, Si in an amount of more than 0% and 1.0% or less; and a plating layer provided on the surface of the base steel sheet, wherein the base steel sheet includes a reduction layer formed directly under the surface of the base steel sheet, and in a cross-section taken in a thickness direction, an average L / C value of crystal grains present in the reduction layer is 2.0 or more, where L (μm) is a maximum length from among lengths of transverse axes of the crystal grains, and C (μm) is a maximum length from among lengths of longitudinal axes of the crystal grains.

Description

galvanized steel sheet

[0001] The present invention relates to plated steel sheets.

[0002] Generally, steel sheets for automobiles or construction materials typically contain large amounts of elements such as Si, Mn, and Cr to increase strength; however, steel sheets containing these elements have a problem in that they form oxides on the surface of the steel sheet during the annealing heat treatment process, thereby reducing plating adhesion.

[0003] In this regard, a technology has been proposed to improve plating adhesion by suppressing the formation of Si, Mn oxides, etc. on the surface through the control of the content of Si, Mn, P, and Cr in steel (Patent Document 1). However, there is still a need for the development of technology to ensure plating adhesion in the plating process.

[0004] In addition, when steel plates for automobiles or construction materials are welded and used, parts are assembled by welding them using spot welding or arc welding after processing. However, when zinc-containing galvanized steel plates are spot welded, the plating layer in the heat-affected zone (HAZ) melts due to the welding heat and remains as molten zinc. This molten zinc penetrates into the grain boundaries on the surface of the substrate, causing cracks and resulting in liquid metal embrittlement, a type of brittle fracture. This can lead to reduced durability and part lifespan.

[0005] [Prior Art Literature]

[0006] [Patent Literature]

[0007] (Patent Document 1) Japanese Patent Publication No. 2002-115039

[0008] [Non-patent literature]

[0009] (Non-patent literature 1) Iron and Steel, 89(11), 1158 (2003)

[0010] One embodiment of the present invention provides a plated steel sheet with improved plating adhesion.

[0011] One embodiment of the present invention provides a plated steel sheet with improved spot welding crack resistance.

[0012] One embodiment of the present invention provides a plated steel sheet with excellent plating adhesion and improved spot welding crack resistance.

[0013] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0014] A plated steel sheet according to one embodiment of the present invention comprises a base steel sheet containing more than 0% and less than or equal to 1.0% Si in weight%, and a plating layer provided on the surface of the base steel sheet, wherein the base steel sheet comprises a reduction layer formed directly below the surface of the base steel sheet, and in a cross-section cut in the thickness direction, when the longest length of the transverse axis of the crystal grain is L (μm) and the longest length of the longitudinal axis of the crystal grain is C (μm), the average L / C value of the crystal grains present in the reduction layer is 2.0 or more.

[0015] Let A (㎛) be the average circle equivalent diameter of the crystal grains present in the reduction layer, and let B (㎛) be the average circle equivalent diameter of the crystal grains distributed at a point t / 10 from the surface of the base steel sheet. Then, the value of A / B may be 0.20 or less. Here, t represents the thickness of the base steel sheet.

[0016] It may further include an interfacial alloy layer disposed between the above-mentioned base steel plate and the above-mentioned plating layer.

[0017] The area fraction of the interfacial alloy layer occupying the surface of the above-mentioned base steel plate may be 70% or more in area % relative to the total surface area of ​​the above-mentioned base steel plate.

[0018] The above interface alloy layer may include one or more alloy phases among FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al.

[0019] The above plating layer may be a zinc-based plating layer or an aluminum-based plating layer.

[0020] The above plating layer may contain, in weight percent, Mg: 1.0~20.0%, Al: 1.0~40.0%, the remainder being Zn and other unavoidable impurities.

[0021] The above plating layer may further include one or more elements selected from the following groups i) to iii).

[0022] i) Total content of elements derived from the base steel sheet: 1.000% or less

[0023] ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1.000% or less

[0024] iii) Total content of elements added to control the surface quality of galvanized steel sheets: 1,000% or less

[0025] The elements belonging to i) above are one or more of Si, Cr, Mn, Co, Ti, Ni, Fe, Cu, V, Nb, Mo, P, W, and B, the elements belonging to ii) above are one or more of Y, Zr, La, Ce, Ca, Sr, and Be, and the elements belonging to iii) above may be one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In.

[0026] The thickness of the above-mentioned base steel plate may be 0.6 to 2.3 mm.

[0027] One embodiment of the present invention can provide a plated steel sheet with improved plating adhesion.

[0028] More specifically, one embodiment of the present invention can provide a plated steel sheet with excellent plating adhesion and improved spot welding crack resistance.

[0029] Figure 1 is a TEM-observed photograph of Example 1 of the present invention.

[0030] Figure 2 is a TEM photograph of Comparative Example 1 of the present invention.

[0031] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0032] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0033] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0034] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0035] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0036] Unless otherwise specifically defined in the specification of the present invention, the % unit of content means weight %.

[0037] In this specification, terms such as 'top', 'upper', 'upper surface', 'lower', 'lower surface', 'lower surface', and 'side surface' are based on the drawings and may actually vary depending on the direction in which the elements or components are arranged.

[0038] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0039] A plated steel sheet according to exemplary embodiments may include a base steel sheet and a plating layer.

[0040] According to exemplary embodiments, the substrate steel sheet may contain Si in a weight percent of greater than 0% and less than or equal to 1.0%.

[0041] Si: Greater than 0% and less than or equal to 1.0%

[0042] Si is an important element that contributes to the improvement of strength through solid solution strengthening and can contribute to suppressing the deterioration of processability. However, if the Si content exceeds 1.0%, it may form film-shaped Si oxides, which can cause plating peeling. In this regard, the upper limit of the Si content may be 1.0% or less, and preferably 0.8% or less. More preferably, it may be 0.5% or less. In terms of preventing the formation of Si oxides, a lower Si content is preferable, but it may be included as an essential element (i.e., 0%) in terms of contributing to the improvement of strength.

[0043] Any components other than Si are not particularly limited. As a non-limiting example, the base steel sheet may additionally include one or more of C: 0.02 to 0.10%, Mn: 0.2 to 1.80%, Nb: 0.05% or less, and Ti: 0.02% or less.

[0044] C: 0.02% to 0.10%

[0045] The above C is an element that contributes to the stabilization of the austenite structure, and as its content increases, it is advantageous for securing the austenite structure. To obtain the above effect, the C content may be 0.02% or more. However, if the C content exceeds 0.10%, there is a risk that defects may occur in the cast billet and weldability may also decrease. Therefore, the C content may have a range of 0.02 to 0.10%. More specifically, the C content may be 0.02 to 0.08%.

[0046] Mn: 0.2% to 1.8%

[0047] Mn is an element that contributes to the improvement of strength through solid solution strengthening and simultaneously improves the hardenability of the austenite phase, and effectively contributes to the stabilization of strength. To stably obtain the desired strength, the Mn content can be set to 0.2 to 1.8%. However, if the Mn content exceeds 1.8%, there is a concern that it may be difficult to secure the target ductility due to the formation of martensite resulting from the delayed phase transformation during the secondary annealing heat treatment process. More specifically, the Mn content may be 0.2% to 1.5%.

[0048] Nb: 0.05% or less

[0049] Nb plays a role in refining grain size and increasing strength by forming carbonitrides, but if added in excess of 0.05%, it can reduce hot ductility and impair slab quality, so it can be added in an amount of 0.05% or less. Since Nb is an optional component that does not significantly affect the technical concept of the present invention, its lower limit may include 0%.

[0050] Ti: 0.02% or less

[0051] Ti improves the strength of steel by reacting with carbon in the steel to form carbides. In order to obtain this effect in the present invention, the titanium content may be 0.02% or less. However, if the content exceeds 0.02%, precipitates may be excessively formed, which may deteriorate the fatigue properties of the steel. Since Ti is an optional component that does not significantly affect the technical concept of the present invention, its lower limit may include 0%.

[0052] The remaining component of the base steel sheet of the present invention is iron (Fe), and it may contain some unintended and unavoidable impurities introduced during the manufacturing process. Since these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification. Furthermore, in addition to the components described above, the base steel sheet may include additional components within the scope that does not depart from the technical spirit of the present invention. As one example, the base steel sheet may further include one or more of B, Al, P, S, N, V, Cr, Ni, Cu, Mo, W, Ca, Zr, REM, and Hf, but the present invention is not necessarily limited thereto.

[0053] As a non-limiting example, the thickness of the base steel plate may be 0.5 mm to 2.3 mm, and thereby it can be used as an automotive steel plate for impact structural members.

[0054] The base steel plate may include a reduction layer disposed directly beneath the surface of the base steel plate.

[0055] Generally, silicon can contribute to improving the quality of steel sheets by acting as a deoxidizer during the manufacturing process. Furthermore, silicon can be added to improve the strength of steel sheets through solid solution strengthening. However, silicon forms a thin oxide film on the surface of the steel sheet during the manufacturing process. Consequently, when a steel sheet containing silicon is used as a substrate for plated steel sheets, problems such as plating peeling may occur. In contrast, according to exemplary embodiments, a reduction layer is provided on the surface layer of the substrate steel sheet. During the process of forming this reduction layer, the fine oxide films formed on the surface of the steel sheet are removed, thereby preventing problems such as plating peeling caused by the components of the substrate steel sheet. Therefore, the performance of the substrate steel sheet can be further improved by adding silicon, and consequently, the quality of the plated steel sheet can be enhanced. Furthermore, the diffusion of iron and plating bath components within the substrate steel sheet can be facilitated more easily during the plating process, thereby further improving plating adhesion.

[0056] According to exemplary embodiments, in a cross-section of the substrate steel plate in the thickness direction, when the longest length of the transverse axis of the grain is L (μm) and the longest length of the longitudinal axis is C (μm), the average L / C value of the grains in the reduction layer may be 2.0 or higher. As a result, the stress applied due to the pressure of the spot welding tip during spot welding is uniformly distributed, and the penetration of molten zinc along the transverse grain boundaries can delay the penetration of zinc in the thickness direction of the substrate steel plate. Consequently, spot welding crack resistance can be improved.

[0057] The longest axis among the transverse axes of the grain may be the longest segment among the segments overlapping the grain boundaries formed by drawing a line downward in the transverse direction, but the present invention is not necessarily limited thereto. The longest axis among the longitudinal axes of the grain may be the longest segment among the segments overlapping the grain boundaries formed by drawing a line moving left and right in the longitudinal direction, but the present invention is not necessarily limited thereto. The transverse axes of the grain may be substantially parallel to the rolling direction (RD) or the transverse direction (TD) of the base steel sheet, but the present invention is not necessarily limited thereto. The longitudinal axes of the grain may be substantially parallel to the thickness direction (Normal Direction, ND) of the base steel sheet.

[0058] According to exemplary embodiments, the longest length (L) of the transverse axis of the grains in the reduction layer described above may be 0.30 to 5.00 μm. If the longest length (L) of the transverse axis of the grains is less than 0.30 μm, there is a concern that the stress generated during the spot welding tip pressure may not be sufficiently dispersed, and that molten zinc may easily penetrate into the interior of the base steel sheet. If the longest length (L) of the transverse axis of the grains exceeds 5.00 μm, the grains of the base steel sheet become excessively coarse, and cracks may easily occur during spot welding. Therefore, the longest length (L) of the transverse axis of the grains may satisfy the range described above, and more preferably may be 0.50 to 4.00 μm.

[0059] According to exemplary embodiments, when the average circle equivalent diameter of the crystal grains in the reduction layer is A (μm) and the average circle equivalent diameter of the crystal grains distributed at a point t / 10 from the surface of the base steel sheet is B (μm), the value of A / B may be 0.20 or less. Here, t represents the thickness of the base steel sheet.

[0060] If the ratio of average particle sizes (A / B) exceeds 0.20, it may cause liquid metal embrittlement (LME) during spot welding, which may lead to reduced durability and part life. Therefore, the ratio of average particle sizes (A / B) may be 0.20 or less, and more preferably 0.17 or less.

[0061] As a non-limiting example, the average thickness of the reduction layer may be 0.05 to 1.00 μm. If the average thickness of the reduction layer is less than 0.05 μm, there is a concern that the effect of the reduction layer may not be sufficiently secured. If the average thickness of the reduction layer exceeds 1.00 μm, the plating adhesion may be reduced due to the Fe oxide layer remaining immediately beneath it as the Fe reduction layer is not completely reduced.

[0062] The plating layer can be placed on the surface of the base steel sheet. As one example, the plating layer can be placed on one side of the base steel sheet. As another example, the plating layer can be placed on both sides of the base steel sheet.

[0063] In the present invention, the type of plating layer is not specifically limited, and any type of plating layer commonly used in the relevant technical field may be applied. As a non-limiting example, the plating layer may be a zinc-based plating layer or an aluminum-based plating layer.

[0064] In the present invention, a zinc-based plating layer refers to a plating layer that mainly contains zinc. In the present invention, "mainly containing" means that it contains more than 40% by weight. If the zinc-based plating layer mainly contains zinc, there are no particular limitations on the remaining additional components. The zinc-based plating layer may further contain aluminum. The zinc-based plating layer may further contain magnesium. The zinc-based plating layer may further contain aluminum and magnesium. Additionally, the zinc-based plating layer may be provided by any one of the hot-dip plating method, the electroplating method, or a combination thereof, but the present invention is not necessarily limited thereto. An aluminum-based plating layer refers to a plating layer that mainly contains aluminum. If the aluminum-based plating layer mainly contains zinc, there are no particular limitations on the remaining additional components. The aluminum-based plating layer may further contain zinc. The aluminum-based plating layer may further contain magnesium. The aluminum-based plating layer may further contain zinc and magnesium.

[0065] As one example, the plating layer may contain, in weight percent, Mg: 1.0 to 20.0%, Al: 1.0 to 40.0%, and the remainder being Zn and other unavoidable impurities. Thus, the plating layer may be provided as a zinc-based plating layer, and corrosion resistance can be further improved by controlling magnesium and aluminum to the ranges described above. As another example, the plating layer may contain, in weight percent, Mg: 1.0 to 13.0%, Al: 1.0 to 36.0%, and the remainder being Zn and other unavoidable impurities.

[0066] Optionally, the Zn-Al-Mg-based plating layer may further include one or more elements selected from the following groups i) to iii). However, since the elements of each of the following groups are not essential for achieving the objectives of the present invention, the lower limit of their content is not restricted. Accordingly, the lower limit of the content of each element may be 0%, even if not specifically mentioned below. The elements of each of the following groups may be intentionally added to the plating bath or derived from the base steel sheet, but the present invention is not necessarily limited thereto.

[0067] i) Total content of elements derived from the base steel sheet: 1.000% or less

[0068] ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1.000% or less

[0069] iii) Total content of elements added to control the surface quality of galvanized steel sheets: 1,000% or less

[0070] The elements of i) above may contribute to the formation of corrosion resistance or contribute to grain stabilization. However, if the total content of these elements exceeds 1.000%, the surface properties of the plated steel sheet may deteriorate. As a specific example, the elements belonging to group i) above may be one or more of Si, Cr, Mn, Co, Ti, Ni, Fe, Cu, V, Nb, Mo, P, W, and B.

[0071] The elements of ii) above can suppress the formation of Mg oxide in the plating bath and contribute to the formation of corrosion resistance of the plated steel sheet. However, if the total content of these elements exceeds 1.000%, the brittleness of the plating layer may increase. As a specific example, the elements belonging to group ii) above may be one or more of Y, Zr, La, Ce, Ca, Sr, and Be.

[0072] The elements of iii) above are elements that affect the size of spangles, which are solidification bodies of the Zn-Al-Mg plating layer. However, if the total content of these elements exceeds 1.000%, there is a risk of discoloration when the Zn-Al-Mg plating layer is exposed to a humid environment. As a specific example, the elements belonging to group iii) above may be one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In.

[0073] Furthermore, the Zn-Al-Mg-based plating layer may include additional elements other than the alloying elements described above within the scope of the technical concept of the present invention, and the route of addition of these elements (addition to the plating bath, origin from the base steel sheet, etc.) is not particularly limited.

[0074] According to exemplary embodiments, the plated steel sheet may further include an interfacial alloy layer interposed between the base steel sheet and the plating layer. The interfacial alloy layer may include alloy components of the Fe component of the base steel sheet and the component constituting the plating layer. This further improves plating adhesion, thereby suppressing the occurrence of plating peeling.

[0075] According to exemplary embodiments, the area occupied by the interfacial alloy layer on the surface of the substrate steel sheet may be 70% or more in area percentage relative to the total surface area of ​​the substrate steel sheet. If the area percentage of the region occupied by the interfacial alloy layer is less than 70%, the effect of improving plating adhesion may not be sufficient. Therefore, the area percentage of the region occupied by the interfacial alloy layer may be 70% or more, and more preferably 80% or more in terms of improving plating adhesion. Theoretically, or if possible in the manufacturing process, it is most desirable for the area percentage of the region occupied by the interfacial alloy layer to be 100%.

[0076] The interfacial alloy layer may include one or more interfacial alloy phases among FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al.

[0077] Hereinafter, a method for manufacturing a plated steel sheet according to one embodiment of the present invention will be described.

[0078] A method for manufacturing a plated steel sheet according to one embodiment of the present invention may include the steps of heating a slab; hot rolling; coiling; oxidizing heat treatment; reducing heat treatment; and plating. Additionally, the method for manufacturing a plated steel sheet of the present invention may optionally further include the step of cold rolling after coiling and before oxidizing heat treatment.

[0079] As a non-limiting example, the step of heating the slab may utilize ordinary conditions applicable in the relevant technical field, provided that the slab containing more than 0% and less than or equal to 1% Si is heated. As one example, the heating of the slab may be performed at 1100°C to 1300°C.

[0080] In the hot rolling step, a hot-rolled steel sheet can be obtained by finishing hot rolling the heated slab. The present invention does not specifically limit the finishing hot rolling process, and ordinary conditions applicable in the relevant technical field may be used. However, as an example, the finishing hot rolling may be performed at 800°C to 1000°C.

[0081] The coiling step may be a step of coiling a hot-rolled steel sheet at 600°C to 680°C. If the coiling temperature is below 600°C, a problem of shape degradation may occur. If the coiling temperature exceeds 680°C, a problem of scale dust generation may occur. Therefore, it is preferable that the coiling temperature be in the range of 600°C to 680°C. It is more preferable that the lower limit of the coiling temperature be 610°C, and more preferable that it be 620°C. It is more preferable that the upper limit of the coiling temperature be 670°C, and more preferable that it be 650°C.

[0082] Optionally, a cold-rolled steel sheet can be obtained by cold-rolling the coiled hot-rolled steel sheet. The present invention does not specifically limit the cold-rolling process and may utilize ordinary conditions applicable in the relevant technical field.

[0083] The oxidation heat treatment step may be a step of heat treating the hot-rolled steel sheet or the cold-rolled steel sheet by passing it through a Direct Fired Furnace (DFF) facility. The oxidation heat treatment is intended to oxidize Fe present on the surface layer of the base steel sheet to form an Fe oxide layer. The Fe oxide layer can serve as a diffusion barrier to inhibit the diffusion of elements such as Si or Mn to the surface of the steel sheet during the heat treatment process.

[0084] The above oxidation heat treatment can be performed such that the output temperature is 630℃ to 670℃.

[0085] The above oxidation heat treatment step can control the air ratio of the DFF section to 1.0 or higher. In this way, when the air ratio of the DFF section is controlled to 1.0 or higher, the surface of the steel sheet is oxidized by excess oxygen to form an Fe oxide layer. Subsequently, by heat treatment in a reducing atmosphere, the surface of the hot-rolled or cold-rolled steel sheet is reduced to pure Fe, forming a reduction layer. At the same time, oxidizing elements such as Si and Mn in the steel that diffuse to the surface can react with the oxygen in the Fe oxide layer and be captured in the form of oxides directly beneath the reduction layer. As a result, the oxide on the steel sheet surface is minimized, thereby improving plating adhesion. On the other hand, if the air ratio of the DFF section is less than 1.0 in the above oxidation heat treatment step, the Fe oxide layer is not sufficiently formed, which may lead to a decrease in the subsequent capture effect of oxidizing elements.

[0086] In addition, the oxidation heat treatment step can control the heating rate from the DFF section to the cracking stage to 1.3℃ / s or higher. If the heating rate is less than 1.3℃ / s, the grains of the reduction layer may coarsen as the residence time in the heating section increases. In this case, when pressurizing the spot welding tip later, the stress distribution effect becomes insignificant, and the spot welding LME crack resistance may be inferior.

[0087] According to exemplary embodiments, when the heating rate immediately after the DFF section is denoted as R (°C / s) and the Si content in the slab is denoted as S (%), the R / S value may be 2.6 or higher. If the Si content in the slab is excessively high, Si may excessively diffuse toward the surface of the hot-rolled steel sheet or cold-rolled steel sheet during the heating section immediately after the DFF section, which may cause plating peeling due to surface oxides. As a result, Si may become excessively concentrated directly beneath the Fe oxide layer and diffuse into the Fe oxide layer. Therefore, it is necessary to minimize the residence time of the hot-rolled steel sheet or cold-rolled steel sheet during the heating section immediately after the DFF section. According to exemplary embodiments, when the heating rate immediately after the DFF section is denoted as R (°C / s) and the Si content in the slab is denoted as S (%), the above-described phenomenon can be minimized or suppressed by controlling the R / S value to 2.6 or higher.

[0088] The reduction heat treatment step may be a step of heat-treating the oxidized heat-treated substrate steel sheet in a reducing atmosphere with a dew point temperature of -60°C or higher and -45°C or lower. The reduction heat treatment is intended to reduce the Fe oxide layer formed by the oxidation heat treatment to form a reduction layer. Consequently, if the dew point temperature is below -60°C, the Fe oxide layer is not sufficiently reduced, which may lead to a decrease in plating performance. If the dew point temperature is above -45°C, the amount of oxide may increase due to the selective oxidation transition region, which may result in a problem where plating performance actually deteriorates. Therefore, it is desirable for the dew point temperature to have a range of -60°C or higher and -45°C or lower. It is more desirable for the lower limit of the dew point temperature to be -55°C. It is more desirable for the upper limit of the dew point temperature to be -46°C, and even more desirable for it to be -47°C.

[0089] At this time, as an example, the cracking temperature in the above reduction heat treatment step may be 750 to 850℃.

[0090] Meanwhile, the conditions of the reducing atmosphere in the present invention are not specifically limited, and ordinary conditions applicable in the relevant technical field may be used. However, as an example, it may be a gas atmosphere comprising hydrogen: 3 to 25% by volume and the remainder being nitrogen.

[0091] Subsequently, the above-mentioned reduced steel sheet can be immersed in a molten plating bath to obtain a plated steel sheet. The present invention does not specifically limit the above-mentioned molten plating and may use ordinary conditions applicable in the relevant technical field. However, as an example, the above-mentioned molten plating bath may contain, in weight percent, Mg: 1.0~20.0%, Al: 1.0~40.0%, and the remainder Zn. An interfacial alloy layer may be formed by the above-mentioned molten plating process.

[0092] As the above description applies equally to the plating layer and the interfacial alloy layer, a detailed description is omitted.

[0093] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0094] Examples

[0095] A slab having the weight of the Si content in Table 1 below was heated at 1200°C, and then the heated slab was finished hot-rolled at 900°C to obtain a hot-rolled steel sheet. Afterwards, the hot-rolled steel sheet was coiled at 620°C to 650°C, and then the coiled hot-rolled steel sheet was cold-rolled to produce a cold-rolled steel sheet with a thickness of 1.0 mm.

[0096] Subsequently, for annealing heat treatment, the air ratio in the DFF section was controlled according to the conditions in Table 1 in a plating line equipped with a Direct Fired Furnace. Immediately after the DFF section, the temperature was raised according to the conditions in Table 1 in a reducing atmosphere of 5% H2-N2 with a dew point temperature of -60°C or higher and less than -45°C until reaching the cracking stage of 790°C.

[0097] Subsequently, a plating layer was formed on the surface through plating treatment, with a plating adhesion amount of 150 g / m² on one side. At this time, the Zn-Al-Mg plating layer was composed of, in weight percent, aluminum (Al): 30.5%, magnesium (Mg): 8.7%, the remainder being zinc (Zn) and unavoidable impurities.

[0098] Classification Si Content Air Ratio Heating Rate Immediately After DFF Section (°C / s) R / S Inventive Example 10.16 1.05 1.38.1 Inventive Example 20.35 1.11 2.36.6 Inventive Example 30.29 1.21.75.9 Comparative Example 10.30.9 1.55.0 Comparative Example 20.5 1.07 12.0 Comparative Example 31.5 1.03 1.6 1.1 * R: Heating rate immediately after DFF section (°C / s) S: Si content in slab S(%)

[0099] For the manufactured plated steel sheets, the presence or absence of an interfacial alloy layer and its area fraction, the presence or absence of a reduction layer, the ratio of average grain sizes (A / B), and the ratio of longitudinal / transverse axis lengths (L / C) were measured and are shown in Table 2 below.

[0100] The presence or absence of an interfacial alloy layer and its area fraction were determined by removing the plating layer of the plated steel sheet by immersing it in a dichromate solution containing ZnSO4 to expose the interfacial alloy layer, and then calculating the surface area of ​​the steel sheet with the exposed interfacial alloy layer by multiplying the length and width of the steel sheet. Subsequently, 10 random locations were selected on the surface of the exposed interfacial alloy layer and photographed using a Scanning Electron Microscope (SEM). Afterward, elemental mapping data of Fe and Al was obtained from the photographed data using Electron Probe Micro Analyzer (EPMA), and the distribution of the interfacial alloy layer was confirmed by comparing the SEM images with the elemental mapping data. Finally, the area fraction was measured using an image analyzer (Clemex) and averaged.

[0101] The presence or absence of a reduction layer was confirmed on the cross-section of the plated steel sheet using Scanning Electron Microscopy (SEM) and Electron Probe Micro Analyzer (EPMA). More specifically, a cross-section of the plated steel sheet was taken, and the plating layer and the surface layer of the substrate steel sheet were imaged together using SEM. After obtaining elemental mapping data of Fe and O from the imaged data via EPMA, the presence or absence of a reduction layer was confirmed by comparing the SEM image with the elemental mapping data.

[0102] The average circle equivalent diameter A (μm) of the crystal grains present in the reduction layer was calculated by photographing the cross-section of the reduction layer using a transmission electron microscope (TEM), determining the area of ​​each crystal grain through image analysis (Clemex, Image analyzer), and converting it into a circle equivalent diameter. At this time, the value B was defined as the average circle equivalent diameter of the crystal grains distributed at a depth of 100 μm, which is 1 / 10 of the thickness of the base steel plate.

[0103] The ratio of the longitudinal and transverse axis lengths (L / C) was calculated by averaging the lengths of virtual longitudinal and transverse axes passing through the centerlines of the crystal grains in the reduction layer through image analysis of the above TEM image (Clemex, Image analyzer).

[0104] In addition, the plating adhesion of the above-mentioned plated steel sheet was measured, and the results are shown in Table 2 below.

[0105] In order to evaluate whether the plating layer peeled off as an evaluation of plating adhesion, a specimen measuring 30mm x 80mm was taken from the plated steel plate, a structural adhesive was applied to the specimen, and after curing in an oven at 170°C for 20 minutes, it was clamped in a bending jig and bent at 90°. Afterward, whether the plating layer peeled off was determined by visually inspecting whether the plating layer was attached to the surface where the structural adhesive was applied.

[0106] To evaluate spot weldability, welding was performed using a Cu-Cr electrode with a tip diameter of 6 mm, applying a welding current with a pressure of 2.6 kN, under conditions of 16 cycles of current flow and 15 cycles of holding time. The welding current at the point where spatter occurred was set as the upper limit (expulsion current), and spot welding was performed at a current value 0.2 kA lower than the upper limit. The center of the weld indentation was cut, and the occurrence of spot weld LME cracks was determined by observing the cross-sectional micrographs at four points on the weld indentation boundary (upper left, lower left, upper right, and lower right relative to the center of the weld indentation) under an optical microscope at 100x magnification, and the results are recorded in Table 2.

[0107] Classification Interface Alloy Layer Reduced Fe Layer A / BL Diameter (㎛) C Diameter (㎛) L / C Adhesion LME Presence / Absence of Crack Formation Presence / Absence of Distribution Area Fraction (%) Formation of Reduced Fe Layer Presence / Absence Invention Example 1 Formed 98 Formed 0.1 00.6 70.2 33.3 Good Not Occurring Invention Example 2 Formed 95 Formed 0.1 00.8 50.2 53.5 Good Not Occurring Invention Example 3 Formed 96 Formed 0.0 50.3 60.1 92.2 Good Not Occurring Comparative Example 1 Formed 17 Not Formed 1.0 05.3 3.5 1.6 Delamination Occurred Comparative Example 2 Formed 74 Formed 0.2 50.2 10.1 41.5 Not Delamination Occurred Comparative Example 3 Formed 95 Formed 0.1 20.7 20.2 13.4 Delamination Not Occurring

[0108] Referring to Tables 1 and 2, it can be seen that in the case of Invention Examples 1 to 3 satisfying the conditions proposed by the present invention, the plating adhesion is good and no LME cracks occur.

[0109] On the other hand, in the case of Comparative Examples 1 to 3, which do not satisfy the conditions proposed by the present invention, it can be seen that the plating adhesion is poor or LME cracks occur.

[0110] Figure 1 is a TEM image of Inventive Example 1, and Figure 2 is a TEM image of Comparative Example 1. Referring to Figures 1 and 2, in the case of Inventive Example 1, it can be seen that a reduction layer (3) is formed on the surface layer of the base steel plate (4), and an interfacial alloy layer (2) is formed between the base steel plate (4) and the plating layer (1). However, in the case of Comparative Example 1, a reduction layer is not formed, and it can be seen that alloy phases (30) are formed between the plating layer (10) and the base steel plate (20) in an uneven and non-densified form.

[0111] [Explanation of the symbol]

[0112] Plating layer (1, 10), interface alloy layer (2), reduction layer (3), base steel sheet (4, 20), alloy phases (30)

Claims

1. A base steel sheet containing more than 0% and less than or equal to 1.0% Si by weight; and It includes a plating layer provided on the surface of the above-mentioned base steel plate, and The above-mentioned base steel plate comprises a reduction layer provided directly beneath the surface of the above-mentioned base steel plate, and A plated steel sheet having an average L / C value of the grains present in the reduction layer of a cross-section cut in the thickness direction, wherein L (μm) is the longest length of the transverse axis of the grains and C (μm) is the longest length of the longitudinal axis of the grains.

2. In Paragraph 1, Let A (㎛) be the average circle equivalent diameter of the crystal grains present in the reduction layer, and let B (㎛) be the average circle equivalent diameter of the crystal grains distributed at a point t / 10 from the surface of the base steel sheet, then the value of A / B is 0.20 or less, and A plated steel sheet, where t is the thickness of the base steel sheet.

3. In Paragraph 1, A plated steel sheet further comprising an interfacial alloy layer interposed between the above-mentioned base steel sheet and the above-mentioned plating layer.

4. In Paragraph 3, A plated steel sheet in which the area occupied by the interface alloy layer on the surface of the above-mentioned base steel sheet is 70% or more in area % relative to the total surface area of ​​the above-mentioned base steel sheet.

5. In Paragraph 3, The above-mentioned interfacial alloy layer comprises one or more alloy phases selected from FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al, forming a plated steel sheet.

6. In Paragraph 1, A plated steel sheet in which the above plating layer is a zinc-based plating layer or an aluminum-based plating layer.

7. In Paragraph 1, A plated steel sheet in which the plating layer comprises, in weight percent, Mg: 1.0~20.0%, Al: 1.0~40.0%, the remainder being Zn and other unavoidable impurities.

8. In Paragraph 7, The above plating layer is a plated steel sheet further comprising one or more elements selected from the following groups i) to iii). i) Total content of elements derived from the base steel sheet: 1.000% or less ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1,000% or less iii) Total content of elements added to control the surface quality of galvanized steel sheets: 1,000% or less 9. In Paragraph 8, The elements belonging to i) above are one or more of Si, Cr, Mn, Co, Ti, Ni, Fe, Cu, V, Nb, Mo, P, W, and B, and The elements belonging to ii) above are one or more of Y, Zr, La, Ce, Ca, Sr, and Be, and A plated steel sheet in which the element belonging to iii) above is one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In.

10. In Paragraph 1, A plated steel sheet having a thickness of 0.6 to 2.3 mm.

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