High-strength hot-dip galvanized steel sheet with excellent surface quality and spot weldability, and method for manufacturing the same.

Abstract

According to one aspect of the present invention, a high-strength hot-dip galvanized steel sheet excellent in surface quality and spot weldability, and a method for manufacturing the same can be provided.

Description

Technical Field

[0001] The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in surface quality and spot weldability, and a method for manufacturing the same.

Background Art

[0002] Due to problems such as environmental pollution, regulations on automobile exhaust gas and fuel consumption are being strengthened day by day. Therefore, there is a strong demand for reducing fuel consumption by lightening automobile steel sheets. Accordingly, various types of high-strength steel sheets with high strength per unit thickness have been developed and sold.

[0003] High-strength steel generally means steel having a strength of 490 MPa or more, but is not necessarily limited thereto. Transformation Induced Plasticity (TRIP) steel, Twin Induced Plasticity (TWIP) steel, Dual Phase (DP) steel, Complex Phase (CP) steel, etc. may correspond thereto.

[0004] On the other hand, automobile steel materials are supplied in the form of galvanized steel sheets plated on the surface to ensure corrosion resistance. Among them, galvanized steel sheets (GI), high-corrosion-resistant galvanized steel sheets (ZM), or alloyed galvanized steel sheets (GA) are widely used as materials for automobiles because they have high corrosion resistance by utilizing the sacrificial anticorrosion characteristics of zinc.

[0005] However, when the surface of high-strength steel sheets is plated with zinc, there is a problem in that the spot weldability is weakened. In other words, in the case of high-strength steel, both the tensile strength and yield strength are high, so it is difficult to relieve the tensile stress generated during welding through plastic deformation, and there is a high possibility that microcracks will occur on the surface. When welding is performed on high-strength galvanized steel sheets, zinc, which has a low melting point, penetrates into the microcracks in the steel sheet, and as a result, a phenomenon called liquid metal embrittlement (LME) occurs, which can lead to the steel sheet breaking in a fatigue environment. This acts as a major obstacle to increasing the strength of steel sheets.

[0006] Furthermore, alloying elements such as Si, Al, and Mn, which are present in large quantities in high-strength steel sheets, diffuse onto the surface of the steel sheet during the manufacturing process, forming surface oxides. This can significantly reduce the wettability of zinc, potentially degrading surface quality and leading to unplated areas. [Overview of the project] [Problems that the invention aims to solve]

[0007] According to one aspect of the present invention, a high-strength hot-dip galvanized steel sheet with excellent surface quality and spot weldability, and a method for manufacturing the same, can be provided.

[0008] The problems that the present invention addresses are not limited to those described above. A person of ordinary skill should have no difficulty understanding further problems that the present invention addresses from the overall content of this specification. [Means for solving the problem]

[0009] A galvanized steel sheet according to one aspect of the present invention is a galvanized steel sheet comprising a base steel sheet and a zinc-based plating layer provided on the surface of the base steel sheet, wherein the base steel sheet includes a first surface region corresponding to a depth of 25 μm in the thickness direction of the base steel sheet from the interface between the base steel sheet and the zinc-based plating layer, and a second surface region adjacent to the first surface region and corresponding to a depth of 25 μm to 50 μm in the thickness direction of the base steel sheet, wherein the ferrite fraction of the first surface region is 55 area % or more, the average crystal grain size of the ferrite contained in the first surface region is 2 to 10 μm, the ferrite fraction of the second surface region is 30 area % or more, and the average crystal grain size of the ferrite contained in the second surface region is 1.35 to 7 μm.

[0010] The ratio of the average hardness of the first surface layer to the average hardness of the center of the base steel sheet can be 90% or less, and the ratio of the average hardness of the second surface layer to the average hardness of the center of the base steel sheet can be 95% or less.

[0011] The plating thickness of the above zinc-based plating layer is 30-70 g / m². 2 It can be.

[0012] The above-mentioned base steel sheet may contain, by weight %, C: 0.05-1.5%, Si: 2.5% or less, Mn: 1.5-20.0%, S-Al (acid-soluble aluminum): 3.0% or less, Cr: 2.5% or less, Mo: 1.0% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, Sb+Sn+Bi: 0.1% or less, N: 0.01% or less, with the remainder being Fe and unavoidable impurities.

[0013] The tensile strength of the above-mentioned galvanized steel sheet can be 900 MPa or more.

[0014] The surface layer of the above-mentioned base steel sheet may contain an oxide that includes at least one of Si, Mn, Al, and Fe.

[0015] The above-mentioned base steel sheet has a thickness of 1.0 to 2.0 mm and is galvanized steel.

[0016] A method for manufacturing a galvanized steel sheet according to one aspect of the present invention includes the steps of: reheating a steel slab to a temperature range of 950 to 1300°C; hot rolling the reheated slab at a finish rolling start temperature of 900 to 1150°C and a finish rolling end temperature of 850 to 1050°C to provide a hot-rolled steel sheet; winding the hot-rolled steel sheet at a temperature range of 590 to 750°C; heating the hot-rolled steel sheet in a heating zone at a heating rate of 1.3 to 4.3°C / s; and providing an atmosphere gas with a dew point temperature of -10 to +30°C, N2-5 to 10%H2, and 650 to 900°C. The process may include the steps of: annealing the hot-rolled steel sheet in a homogenized zone with a temperature range of °C; slowly cooling the annealed hot-rolled steel sheet in a slow-cooling zone with a temperature range of 550 to 700 °C; rapidly cooling the slowly cooled hot-rolled steel sheet in a rapid-cooling zone with a temperature range of 270 to 550 °C; reheating the rapidly cooled hot-rolled steel sheet and then immersing it in a zinc-based plating bath at an entry temperature of 420 to 550 °C to form a zinc-based plating layer; and selectively heating the steel sheet on which the zinc-based plating layer has been formed to a temperature range of 480 to 560 °C to alloy it.

[0017] The sheet metal feeding speed during the annealing process can be 40 to 130 mpm.

[0018] The above steel slab may contain, by weight %, C: 0.05-0.30%, Si: 2.5% or less, Mn: 1.5-10.0%, S-Al (acid-soluble aluminum): 1.0% or less, Cr: 2.0% or less, Mo: 0.2% or less, B: 0.005% or less, Nb: 0.1% or less, Ti: 0.1% or less, Sb+Sn+Bi: 0.05% or less, N: 0.01% or less, with the remainder being Fe and unavoidable impurities.

[0019] The means of solving the above problems do not enumerate all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail by referring to the following specific implementation examples. [Effects of the Invention]

[0020] According to one aspect of the present invention, in order to control the ferrite crystal grain size of the substrate surface layer directly under the plating layer within a certain range, even when tensile stress is applied during spot welding, the possibility of crack generation can be reduced, thereby effectively reducing the liquid metal embrittlement (LME) phenomenon in which the hot-dip galvanized layer penetrates along the crack and occurs.

[0021] According to one aspect of the present invention, since the formation of oxides on the surface of the steel sheet can be reduced, the deterioration of plating quality can be effectively suppressed.

[0022] The effects of the present invention are not limited to the above-described matters, and can be interpreted to include technical effects that can be inferred by an ordinary technician from the matters described below.

Mode for Carrying Out the Invention

[0023] The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in surface quality and spot weldability and a method for producing the same. Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified into various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. This embodiment is provided to explain the present invention in more detail to those having ordinary knowledge in the technical field to which the invention belongs.

[0024] Hereinafter, the galvanized steel sheet of the present invention will be described through several embodiments.

[0025] It should be noted that in the present invention, the galvanized steel sheet is a concept that includes not only a galvanized steel sheet (GI steel sheet) but also an alloyed galvanized steel sheet (GA), and all galvanized steel sheets on which a zinc-based plating layer mainly containing zinc is formed. The fact that zinc is mainly contained means that the ratio of zinc among the elements contained in the plating layer is the highest. However, in the case of an alloyed galvanized steel sheet, the ratio of iron may be higher than that of zinc, and the present invention can include steel sheets in which the ratio of zinc is the highest among the remaining components excluding iron within the scope of the present invention.

[0026] The inventors of the present invention focused on the fact that the liquid metal embrittlement (LME) occurring during welding is caused by microcracks generated from the surface of the steel plate, studied means for suppressing the microcracks on the surface, and found that it is necessary to particularly control the microstructure of the steel plate surface, thus arriving at the present invention.

[0027] Normally, in the case of high-strength steel, in order to ensure the hardenability and austenite stability of the steel, a large amount of elements such as carbon (C), manganese (Mn), and silicon (Si) can be contained. However, such elements play a role in increasing the crack sensitivity of the steel. Therefore, steel containing a large amount of such elements is prone to generating microcracks and ultimately causes liquid metal embrittlement during welding.

[0028] As a result of intensive research on a plan to reduce the crack sensitivity of high-strength steel, the inventors of the present invention found that the generation behavior of microcracks is closely related to the carbon (C) distribution in the steel plate. Therefore, when introducing ferrite with a relatively low carbon (C) concentration into the surface layer of the steel plate, the crack sensitivity of the steel plate can be effectively reduced. In particular, the inventors of the present invention investigated that there is a close correlation between the ferrite fraction or grain size in a specific region of the steel plate surface layer and the generation behavior of cracks, thus arriving at the present invention.

[0029] According to one embodiment of the present invention, a galvanized steel sheet includes a base steel sheet and a zinc-based plating layer provided on the surface of the base steel sheet, wherein the base steel sheet includes a first surface region corresponding to a depth of 25 μm in the thickness direction of the base steel sheet from the interface between the base steel sheet and the zinc-based plating layer, and a second surface region adjacent to the first surface region and corresponding to a depth of 25 μm to 50 μm in the thickness direction of the base steel sheet, wherein the ferrite fraction of the first surface region is 55 area % or more, the average crystal grain size of the ferrite included in the first surface region is 2 to 10 μm, the ferrite fraction of the second surface region is 30 area % or more, and the average crystal grain size of the ferrite included in the second surface region is 1.35 to 7 μm.

[0030] For example, the surface area of ​​the base steel sheet adjacent to the zinc-plated layer can be divided into a first surface area and a second surface area. The first surface area can be the region corresponding to a depth of 25 μm in the thickness direction of the base steel sheet, from the interface between the base steel sheet and the zinc-plated layer. The second surface area is adjacent to the first surface area and can be the region corresponding to a depth of 25 μm to 50 μm in the thickness direction of the base steel sheet.

[0031] The microstructure of the first surface region can consist of ferrite and secondary hard phases, and may include other unavoidable structures. Since the first surface region contains 55 area percent or more of ferrite, it can effectively reduce the crack susceptibility of the steel sheet. There is no specific upper limit for the ferrite fraction of the first surface region, but from the viewpoint of ensuring the strength of the steel sheet, it can be limited to 97 area percent. The secondary hard phase refers to a microstructure that is relatively harder than ferrite, and can be one or more selected from bainite, martensite, retained austenite, and pearlite.

[0032] The average grain size of the ferrite contained in the first surface region may be in the range of 2 to 10 μm. To suppress the crack susceptibility of the steel sheet, the average grain size of the ferrite contained in the first surface region can be limited to 2 μm or more. On the other hand, if the average grain size of the ferrite contained in the first surface region exceeds a certain level, it is disadvantageous in terms of ensuring the strength of the steel sheet, so the average grain size of the ferrite contained in the first surface region can be limited to 10 μm or less.

[0033] Not only the ferrite fraction and average grain size contained in the first surface region adjacent to the zinc-based plating layer, but also the ferrite fraction and average grain size contained in the second surface region separated from the zinc-based plating layer at a certain distance, are factors that greatly influence the crack susceptibility of steel sheets.

[0034] The microstructure of the second surface region can also consist of ferrite and a secondary hard phase, and may include other unavoidable structures. Since the second surface region contains 30 area percent or more of ferrite, it can effectively reduce the crack susceptibility of the steel sheet. There is no specific upper limit for the ferrite fraction of the second surface region, but from the viewpoint of ensuring the strength of the steel sheet, the upper limit can be restricted to 85 area percent. The secondary hard phase refers to a microstructure that is relatively harder than ferrite, and can be one or more selected from bainite, martensite, retained austenite, and pearlite.

[0035] The average grain size of the ferrite contained in the second surface region may be in the range of 1.35 to 7 μm. To suppress the crack susceptibility of the steel sheet, the average grain size of the ferrite contained in the second surface region can be limited to 1.35 μm or more. On the other hand, if the average grain size of the ferrite contained in the second surface region exceeds a certain level, it is disadvantageous in terms of ensuring the strength of the steel sheet, so the average grain size of the ferrite contained in the second surface region can be limited to 7 μm or less.

[0036] The average ferrite grain size in the first and second surface regions can be measured by observing three or more regions in the cross-section of the steel sheet using SEM (Scanning Electron Microscopy), and the ferrite fraction in the first and second surface regions can be measured using a phase map obtained using EBSD (Electron Back-Scattered Diffraction). An ordinary technician in this art can measure the ferrite fraction and average grain size contained in the first and second surface regions without any special technical difficulty.

[0037] To provide buffering force against tensile stress generated during spot welding, it is preferable that the first and second surface layers have lower hardness than the center of the base steel sheet. The ratio of the average hardness of the first surface layer to the average hardness of the center of the base steel sheet may be 90% or less, and the ratio of the average hardness of the second surface layer to the average hardness of the center of the base steel sheet may be 95% or less. The second surface layer can have a higher average hardness value than the first surface layer. There is no specific lower limit for the ratio of the average hardness of the first surface layer to the average hardness of the center of the base steel sheet or the ratio of the average hardness of the second surface layer to the average hardness of the center of the base steel sheet, but from the viewpoint of ensuring the strength and material uniformity of the steel sheet, the lower limit may be restricted to 70% in each case.

[0038] The average hardness of the first surface region refers to the average of Vickers hardness values ​​measured at points 5 μm, 10 μm, 15 μm, and 20 μm away from the interface in the cross-section of the steel plate. The average hardness of the second surface region refers to the average of Vickers hardness values ​​measured at points 30 μm, 35 μm, 40 μm, and 45 μm away from the interface in the cross-section of the steel plate. The average hardness of the center refers to the average of Vickers hardness values ​​measured at points 1 / 2t and 1 / 2t ± 5 μm, respectively, in the cross-section of the steel plate. Here, t represents the thickness of the steel plate (mm). Vickers hardness can be measured using a nanoindentation Vickers hardness tester under a load condition of 5 g, and an ordinary technician in the art can measure the average Vickers hardness of the first surface region, the second surface region, and the center without any special technical difficulty.

[0039] The present invention does not limit the type of high-strength steel sheet, as long as it has a strength of 900 MPa or more. However, although not necessarily limited thereto, the steel sheet covered by the present invention may contain, by weight ratio, C: 0.05~1.5%, Si: 2.5% or less, Mn: 1.5~20.0%, S-Al (acid-soluble aluminum): 3.0% or less, Cr: 2.5% or less, Mo: 1.0% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, Sb+Sn+Bi: 0.1% or less, N: 0.01% or less, with the remainder being Fe and unavoidable impurities. In some cases, elements that may be contained in steel but are not listed above may be further included in a total of 1.0% by weight or less. Unless otherwise specified, the content of each component element in the present invention is expressed on a weight basis. The composition described above refers to the bulk composition of the steel plate, that is, the composition at the 1 / 4 point of the steel plate's thickness (the same applies hereafter).

[0040] In some realizations of the present invention, the high-strength steel plates can be TRIP steel, DP steel, CP steel, and the like. When further classified, these steels can have the following compositions.

[0041] Steel composition 1: C: 0.05~0.30% (preferably 0.10~0.25%), Si: 0.5~2.5% (preferably 1.0~1.8%), Mn: 1.5~4.0% (preferably 2.0~3.0%), S-Al: 1.0% or less (preferably 0.05% or less), Cr: 2.0% or less (preferably 1.0% or less), Mo: 0.2% or less (preferably 0.1% or less), B: 0.005% or less (preferably 0.004% or less), Nb: 0.1% or less (preferably 0.05% or less), Ti: 0.1% or less (preferably 0.001~0.05%), Sb+Sn+Bi: 0.05% or less, N: 0.01% or less, the remainder being Fe and unavoidable impurities. In some cases, elements not listed above but that may be present in the steel may be further included in a total of 1.0% or less.

[0042] Steel composition 2: C: 0.05~0.30% (preferably 0.10~0.2%), Si: 0.5% or less (preferably 0.3% or less), Mn: 4.0~10.0% (preferably 5.0~9.0%), S-Al: 0.05% or less (preferably 0.001~0.04%), Cr: 2.0% or less (preferably 1.0% or less), Mo: 0.5% or less (preferably 0.1~0.35%), B: 0.005% or less (preferably 0.004% or less), Nb: 0.1% or less (preferably 0.05% or less), Ti: 0.15% or less (preferably 0.001~0.1%), Sb+Sn+Bi: 0.05% or less, N: 0.01% or less, the remainder being Fe and unavoidable impurities. In some cases, elements not listed above but that may be present in steel may be included in a total of up to 1.0%.

[0043] Furthermore, if the lower limit of the content of any of the component elements mentioned above is not specified, it means that these elements may be treated as arbitrary elements, and their content may be 0%.

[0044] Although not necessarily limited thereto, the thickness of the base steel plate in one embodiment of the present invention can be 1.0 to 2.0 mm.

[0045] Furthermore, a plated steel sheet according to one embodiment of the present invention can have improved surface quality by containing an internal oxide in the surface layer of the base steel sheet that contains at least one of Si, Mn, Al, and Fe. That is, the presence of the oxide in the surface layer suppresses the formation of oxides on the surface of the steel sheet, and as a result, wettability between the base steel sheet and the plating solution is ensured during plating, and good plating performance can be obtained.

[0046] According to one embodiment of the present invention, the surface of the steel sheet may contain one or more plating layers, and the plating layer may be a zinc-based plating layer containing a GI (Galvanized) or GA (Galva-annealed) layer. In the present invention, as described above, the ferrite fraction and average crystal grain size of the surface layer are controlled to an appropriate range, so even if a zinc-based plating layer is formed on the surface of the steel sheet, liquid metal embrittlement that occurs during spot welding can be effectively prevented.

[0047] In one embodiment of the present invention, when the zinc-based plating layer is a GA layer, the degree of alloying (meaning the Fe content in the plating layer) can be controlled to 8 to 13% by weight, preferably 10 to 12% by weight. If the degree of alloying is insufficient, zinc in the zinc-based plating layer may penetrate into microcracks, potentially causing liquid metal embrittlement. Conversely, if the degree of alloying is too high, problems such as powdering may occur.

[0048] Furthermore, the plating amount of the zinc-based plating layer is 30-70 g / m². 2 This is also acceptable. If the amount of plating is too little, sufficient corrosion resistance will be difficult to obtain, while if the amount of plating is too much, manufacturing costs will increase and problems with liquid metal embrittlement may occur, so it should be controlled within the range described above. A more preferable range for the amount of plating is 40-60 g / m². 2This is possible. The amount of plating deposited refers to the amount of plating layer attached to the final product. In the case of GA plating, the amount of plating deposited increases due to alloying, so its weight may decrease slightly before alloying. This varies depending on the degree of alloying and is not necessarily limited to this amount, but the amount deposited before alloying (i.e., the amount of plating deposited from the plating bath) can be about 10% less than this value.

[0049] The following describes one example of manufacturing the steel sheet of the present invention. However, it should be noted that the steel sheet of the present invention does not necessarily have to be manufactured by the following example, and that the following example is one preferred means of manufacturing the steel sheet of the present invention.

[0050] First, a hot-rolled steel sheet can be produced by reheating a steel slab of the above composition, performing rough rolling and finish rolling, followed by hot rolling, then ROT (Run Out Table) cooling, and finally winding. Subsequently, the produced steel sheet can be pickled and cold-rolled, and the resulting cold-rolled steel sheet can be annealed and plated. There are no particular restrictions on the hot-rolling conditions such as ROT cooling, but in one embodiment of the present invention, the slab heating temperature, the start and end temperatures of finish rolling, the winding temperature, the pickling conditions, the cold-rolling conditions, the annealing conditions, and the plating conditions can be limited as follows.

[0051] Slab heating temperature: 950~1300℃ Slab heating is performed before hot rolling to ensure rollability. During slab reheating, the surface layer of the slab combines with oxygen in the furnace to form scale, which is an oxide. During scale formation, a decarburization reaction occurs, in which the carbon in the steel also reacts to form carbon monoxide gas. The higher the slab reheating temperature, the greater the amount of decarburization. If the slab reheating temperature is too high, an excessive decarburized layer is formed, which can lead to softening of the material of the final product. If it is too low, hot rollability cannot be ensured, edge cracks may occur, and the hardness of the surface layer cannot be sufficiently reduced, resulting in insufficient improvement of LME (Long Metal Efficiency).

[0052] Finish rolling start temperature: 900~1150℃ If the finish rolling start temperature is excessively high, the hot-rolled scale on the surface may develop excessively, increasing the amount of surface defects caused by scale in the final product; therefore, the upper limit is restricted to 1150°C. Furthermore, if the finish rolling start temperature is below 900°C, the stiffness of the bar may increase due to the decrease in temperature, significantly reducing its hot-rollability; therefore, the finish rolling start temperature can be restricted to the range mentioned above.

[0053] Finishing rolling completion temperature: 850~1050℃ If the finish rolling completion temperature exceeds 1,050°C, the scale removed by descaling during finish rolling will excessively form again on the surface, increasing the amount of surface defects. Conversely, if the finish rolling completion temperature is below 850°C, the hot rollability will decrease. Therefore, the finish rolling completion temperature can be limited to the above range.

[0054] Winding temperature: 590~750℃ Hot-rolled steel sheets are then wound into coils for storage, but the wound steel sheets undergo a slow cooling process. This process removes hardening elements contained in the surface layer of the steel sheet. However, if the winding temperature of the hot-rolled steel sheet is too low, the coil will cool slowly at a temperature lower than the temperature required for the oxidation and removal of these elements, making it difficult to obtain a sufficient effect.

[0055] Pickling treatment: Performed at a feed speed of 180-250 mpm. After the above process, the hot-rolled steel sheets are subjected to pickling by being placed in a hydrochloric acid bath to remove the hot-rolled scale. During pickling, the hydrochloric acid concentration in the bath should be in the range of 10-30%, and the pickling speed should be 180-250 mpm. If the pickling speed exceeds 250 mpm, the surface scale of the hot-rolled steel sheets may not be completely removed, and if the pickling speed is lower than 180 mpm, the surface layer of the base iron may be corroded by the hydrochloric acid; therefore, the pickling speed should be 180 mpm or higher.

[0056] Cold rolling: Reduction ratio 35-60% Cold rolling is performed after pickling. The cold reduction ratio during cold rolling should be in the range of 35-60%. If the cold reduction ratio is less than 35%, there is no particular problem, but there is a risk that the recrystallization driving force will be insufficient during annealing, making it difficult to adequately control the microstructure. If the cold reduction ratio exceeds 60%, the thickness of the soft layer secured during hot rolling will be reduced, making it difficult to sufficiently lower the hardness in the region within 20 μm of the steel sheet surface after annealing.

[0057] The cold rolling process described above can be followed by an annealing process for the steel sheet. During the annealing process, the average ferrite grain size and fraction on the surface of the steel sheet can change significantly. Therefore, in one embodiment of the present invention, the annealing process can be controlled under conditions that appropriately control the average ferrite grain size and fraction in the region within 50 μm from the surface of the steel sheet.

[0058] Threading speed: 40~130mpm To ensure sufficient productivity, the feeding speed of the cold-rolled steel sheet must be 40 mpm or higher. However, if the feeding speed is excessively high, it may be disadvantageous in terms of material quality. Therefore, in one embodiment of the present invention, the upper limit of the feeding speed can be set to 130 mpm.

[0059] Heating rate of the heating zone: 1.3~4.3℃ / s To ensure an appropriate ferrite fraction and average grain size in the surface layer, it is advantageous to control the heating rate in the heating zone. If the heating rate in the heating zone is low, the amount of Si oxidation increases in the region above 650°C, forming a continuous film-like oxide film on the surface. This significantly reduces the amount of water vapor that dissociates into oxygen upon contact with the steel plate surface, and the oxide film suppresses the reaction between carbon and oxygen on the surface, resulting in insufficient decarburization and potentially poor LME resistance. Furthermore, the formation of an oxide film on the surface can lead to poor plating wettability and inferior plating surface quality. Therefore, in one embodiment of the present invention, the lower limit of the heating rate in the heating zone can be set to 1.3°C / s.

[0060] Furthermore, if the heating rate of the heating zone is high, recrystallization and austenite phase transformation in the two-phase or higher temperature range may not occur smoothly during the heating process. In TRIP steel, during the process of simultaneously forming ferrite and austenite in the two-phase temperature range, carbon composed of cementite dissociates, and as partitioning progresses into austenite with high carbon solid solubility, the amount of solid-solved carbon increases, and hard, low-temperature phases such as martensite become stable. On the other hand, if the heating rate is high, the austenite fraction decreases, and the low-temperature phase may not be sufficiently formed due to reduced carbon partitioning, which may result in a decrease in strength. Therefore, in one embodiment of the present invention, the upper limit of the heating rate of the heating zone can be set to 4.3°C / s.

[0061] Control of the dew point in the annealing furnace: Controlled within a range of -10 to +30°C at 650-900°C. To obtain an appropriate range of ferrite fraction and average grain size in the surface layer, it is advantageous to control the dew point in the annealing furnace. If the dew point is too low, surface oxidation occurs instead of internal oxidation, and oxides such as Si and Mn may be formed on the surface. These oxides adversely affect the plating. Therefore, the dew point needs to be controlled to -10°C or higher. Conversely, if the dew point is too high, oxidation of Fe may occur, so the dew point needs to be controlled to 30°C or lower. Thus, the temperature for controlling the dew point may be 650°C or higher, which is the temperature at which a sufficient internal oxidation effect appears. However, if the temperature is too high, surface oxides such as Si are formed, which not only hinders the diffusion of oxygen into the interior, but excessive austenite may be generated during heating of the uniform layer, reducing the diffusion rate of carbon, thereby decreasing the internal oxidation level, and the austenite size of the uniform layer may grow excessively, causing material softening. Furthermore, it may lead to problems such as increased load on the annealing furnace, shortening the equipment life and increasing process costs. Therefore, the temperature for controlling the dew point can be 900°C or lower.

[0062] In this case, the dew point can be adjusted by introducing humid nitrogen (N2 + H2O) containing water vapor into the annealing furnace.

[0063] Hydrogen concentration inside the annealing furnace: 5-10 Vol% The atmosphere inside the annealing furnace is maintained by adding 5-10 Vol% hydrogen to nitrogen gas to create a reducing atmosphere. If the hydrogen concentration inside the annealing furnace is less than 5 Vol%, the reducing capacity decreases, leading to excessive surface oxide formation, poor surface quality and plating adhesion, and the surface oxide inhibits the reaction between oxygen and carbon in the steel, reducing the amount of decarburization and lowering the level of LME improvement. While no particular problems occur at high hydrogen concentrations, the hydrogen concentration is limited due to increased costs associated with increased hydrogen gas usage and the risk of furnace explosion due to the high hydrogen concentration.

[0064] The steel plate annealed by the process described above can be cooled through slow cooling and rapid cooling stages.

[0065] Slow cooling zone temperature: 550~750℃ The slow cooling zone refers to the range where the cooling rate is 3-5°C / s. If the slow cooling zone temperature exceeds 750°C, excessive soft ferrite is formed during slow cooling, reducing tensile strength. Conversely, if the slow cooling zone temperature is below 550°C, excessive bainite or martensite may be formed, leading to an excessive increase in tensile strength and a decrease in elongation. Therefore, the slow cooling zone temperature can be limited to the range described above.

[0066] Rapid cooling temperature range: 270~550℃ The rapid cooling zone refers to the range where the cooling rate is 12-20°C / s. If the rapid cooling zone temperature exceeds 550°C, martensite below the appropriate level may form during rapid cooling, resulting in insufficient tensile strength. Conversely, if the rapid cooling zone temperature is below 270°C, excessive martensite formation may occur, leading to insufficient elongation.

[0067] The annealed steel sheet is immediately immersed in a plating bath for hot-dip galvanizing. If the steel sheet is to be cooled, a step of heating the steel sheet may be included. The heating temperature must be higher than the steel sheet drawing temperature described later, and in some cases, it may be higher than the temperature of the plating bath.

[0068] Intake temperature of steel sheets in the plating bath: 420~500℃ If the drawing temperature of the steel sheet in the plating bath is too low, sufficient wettability at the contact interface between the steel sheet and liquid zinc cannot be ensured, so the temperature must be maintained at 420°C or higher. If the temperature is excessively high, the reaction between the steel sheet and liquid zinc becomes excessive, causing the formation of a zetta phase, which is an Fe-Zn alloy phase, at the interface, reducing the adhesion of the plating layer, and leading to excessive dissolution of Fe element from the steel sheet in the plating bath, resulting in the formation of dross in the plating bath. Therefore, the drawing temperature of the steel sheet may be limited to 500°C or lower.

[0069] Al concentration in the plating bath: 0.10~13.0% The Al concentration in the plating bath must be maintained at an appropriate level to ensure wettability of the plating layer and fluidity of the plating bath. Only by controlling the concentration to 0.10-0.15% for GA, 0.2-0.25% for GI, and 0.7-13.0% for ZM can the formation of dross in the plating bath be kept at an appropriate level, thereby ensuring the quality and performance of the plated surface.

[0070] The hot-dip galvanized steel sheet plated by the process described above can then undergo an alloying heat treatment process as needed. The preferred conditions for the alloying heat treatment are as follows:

[0071] Alloying (GA) temperature: 480~560℃ Below 480°C, the amount of Fe diffusion is low and the degree of alloying is insufficient, which may result in poor plating properties. Above 560°C, excessive alloying can lead to powdering problems, and the material may deteriorate due to the ferrite transformation of retained austenite. Therefore, the alloying temperature should be within the range described above. [Examples]

[0072] The present invention will be described in more detail below with reference to examples. However, it should be noted that the examples described below are for illustrative purposes to further illustrate the present invention and are not intended to limit the scope of the rights of the present invention.

[0073] (Examples) A steel slab having the composition listed in Table 1 below (the remaining components not listed in the table are Fe and unavoidable impurities; in the table, B and N are expressed in ppm units, and the remaining components are expressed in weight percent units) was heated to 1230°C, and hot-rolled with finish rolling start and end temperatures of 1015°C and 950°C, respectively, and then coiled at 630°C. After pickling with a 19.2 vol% hydrochloric acid solution, it was cold-rolled, and the resulting cold-rolled steel sheet was annealed in an annealing furnace, slowly cooled at 4.2°C / s in a slow-cooling zone at 620°C, and rapidly cooled at 17°C / s in a rapid-cooling zone at 315°C to obtain an annealed steel sheet. The atmospheric gas used in the homogenized zone was N2-6%H2. Subsequently, the obtained steel sheets were heated and immersed in a plating bath containing 0.13% Al for GA, 0.24 wt% Al for GI, and 1.75% Al and 1.55% Mg for ZM to perform hot-dip galvanizing. The obtained hot-dip galvanized steel sheets were subjected to alloying (GA) heat treatment at 520°C as needed, and finally alloyed hot-dip galvanized steel sheets were obtained.

[0074] In all examples, the temperature at which the steel sheet was drawn into the molten zinc plating bath was set to 475°C. Other conditions specific to each example are as shown in Table 2.

[0075] [Table 1]

[0076] [Table 2]

[0077] Table 3 shows the results of measuring the properties of hot-dip galvanized steel sheets manufactured by the process described above and observing whether or not liquid metal embrittlement (LME) occurred during spot welding. Spot welding was performed along each cut end after cutting the steel sheet in the width direction. After applying the spot welding current twice, a hold time of one cycle was maintained. Spot welding was performed in a triple layer of dissimilar materials. Spot welding was performed by laminating the materials in the following order: evaluation material - evaluation material - GA 980DP 1.4t material (composition having C: 0.12 wt%, Si: 0.1 wt%, Mn: 2.2 wt%). During spot welding, a new electrode was welded to the soft material 15 times, and after the electrode was worn down, the upper limit current at which explosion occurred in the target material of the spot weld was measured. After measuring the upper limit current, spot welding was performed 8 times at currents 0.5 and 1.0 kA lower than the upper limit current for each welding current. After precisely machining the cross-section of the spot weld area by electrical discharge machining, it was epoxy mounted and polished, and the length of the crack was measured with an optical microscope. The magnification for observation using an optical microscope was set to 100x. If no cracks were found at this magnification, it was determined that liquid metal embrittlement had not occurred. If cracks were found, their length was measured using image analysis software. For B-type cracks occurring at the shoulder of the spot weld, a length of 100 μm or less was considered good, and for C-type cracks, the absence of observation was considered good.

[0078] The microstructure fraction was measured using EBSD (Electron Back-Scattered Diffraction) phase maps of the cross-sections of each specimen. In addition, after nital etching of the cross-sections of each specimen, scanning electron microscopy (SEM) analysis was performed, and the average grain size of ferrite was measured using three or more photographs of each specimen.

[0079] The Vickers hardness of the cross-section of each specimen was measured using a nanointentionation Vickers hardness tester under a 5g load condition. The average hardness of the first surface region is the average of the Vickers hardness measured at points 5μm, 10μm, 15μm, and 20μm away from the interface; the average hardness of the second surface region is the average of the Vickers hardness measured at points 30μm, 35μm, 40μm, and 45μm away from the interface; and the average hardness of the center is the average of the Vickers hardness measured at the 1 / 2t point and the 1 / 2t ± 5μm point, respectively.

[0080] Tensile strength was measured by tensile testing using C-direction samples prepared according to JIS-5 standard. Plating adhesion was measured using a wet dissolution method with hydrochloric acid solution. For sealer adhesion, after bonding D-type structural adhesive for automobiles to the plated surface, the steel plate was bent at a 90-degree angle to check if the plating peeled off. For powdering, after bending the plated material at a 90-degree angle, tape was bonded to the bent area and then peeled off, and the length of the peeled-off plating layer onto the tape was checked. If the length of the plating layer peeled off from the tape exceeded 10 mm, it was considered defective. For flaking, after processing into a "U" shape, it was checked whether the plating layer peeled off at the processed area. For GI and ZM steel plates, a sealer bending test (SBT) was performed to check whether the plating layer peeled off and adhered to the sealer-peeling surface when the steel plate was bent at a 90-degree angle with automotive structural adhesive applied to the surface. The surface quality of the steel plates was checked by visual inspection to determine whether there were any defects such as unplated surfaces. If defects such as unplated surfaces were found during the visual inspection, the plates were judged to be defective.

[0081] [Table 3]

[0082] [Table 4]

[0083] Test specimens 2, 4, 5, 6, 7, 9, 10, 11, 14, and 15 satisfy the alloy composition and process conditions of the present invention, and it can be confirmed that their tensile strength, plating quality, and spot weld LME crack length are good. On the other hand, test specimens 1, 3, 8, 12, 13, 16, 17, and 18 do not satisfy any one of the alloy composition and process conditions of the present invention, and it can be confirmed that they are inferior to one or more of the tensile strength, plating quality, and spot weld LME cracks.

[0084] As described above, the present invention has been explained in detail through the examples, but other forms of examples are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. A galvanized steel sheet comprising a base steel sheet and a zinc-based plating layer provided on the surface of the base steel sheet, The aforementioned base steel sheet is In weight percent, C: 0.05-1.5%, Si: 2.5% or less (excluding 0), Mn: 1.5-20.0%, S-Al (acid-soluble aluminum): 3.0% or less (excluding 0), Cr: 2.5% or less, Mo: 1.0% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, Sb+Sn+Bi: 0.1% or less, N: 0.01% or less, the remainder being Fe and unavoidable impurities. A first surface region is a region corresponding to a depth of 25 μm in the thickness direction of the base steel sheet, from the interface between the base steel sheet and the zinc-based plating layer, It includes a second surface region adjacent to the first surface region, which corresponds to a depth of 25 μm to 50 μm in the thickness direction of the base steel sheet, The ferrite fraction of the first surface region is 55 area% or more, and the average crystal grain size of the ferrite contained in the first surface region is 2 to 10 μm. The ferrite fraction of the second surface region is 30 area % or more, and the average crystal grain size of the ferrite contained in the second surface region is 1.35 to 7 μm. A galvanized steel sheet in which the average crystal grain size of the ferrite in the first surface region is even larger than the average crystal grain size of the ferrite in the second surface region.

2. The ratio of the average hardness of the first surface layer region to the average hardness of the center of the base steel sheet is 90% or less. The galvanized steel sheet according to claim 1, wherein the ratio of the average hardness of the second surface layer region to the average hardness of the center of the base steel sheet is 95% or less.

3. The plating amount of the aforementioned zinc-based plating layer is 30 to 70 g / m². ２ The galvanized steel sheet according to claim 1.

4. The galvanized steel sheet according to claim 1, wherein the tensile strength of the galvanized steel sheet is 900 MPa or more.

5. The zinc-plated steel sheet according to claim 1, wherein the surface layer of the base steel sheet contains an oxide containing at least one of Si, Mn, Al, and Fe.

6. The galvanized steel sheet according to any one of claims 1 to 3, wherein the thickness of the base steel sheet is 1.0 to 2.0 mm.

7. A step of reheating a steel slab containing, by weight %, C: 0.05 to 1.5%, Si: 2.5% or less (excluding 0), Mn: 1.5 to 20.0%, S-Al (acid-soluble aluminum): 3.0% or less (excluding 0), Cr: 2.5% or less, Mo: 1.0% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, Sb + Sn + Bi: 0.1% or less, N: 0.01% or less, the remainder being Fe and unavoidable impurities, to a temperature range of 950 to 1300°C, The steps include: providing a hot-rolled steel sheet by hot-rolling the reheated slab at a finish-rolling start temperature of 900 to 1150°C and a finish-rolling end temperature of 850 to 1050°C; The process involves winding the hot-rolled steel sheet at a temperature range of 590 to 750°C, The steps include pickling the wound hot-rolled steel sheet and then cold-rolling it at a reduction ratio of 35-60% to obtain a cold-rolled steel sheet, 1. A step of heating the cold-rolled steel sheet in a heating zone at a heating rate of 1.3 to 4.3°C / s, Dew point temperature of -10 to +30°C, N ２ -5 to 10%H ２ The steps include annealing the cold-rolled steel sheet in an atmospheric gas and a homogenized zone with a temperature range of 650 to 900°C, A step of slowly cooling the annealed cold-rolled steel sheet in a slow-cooling zone with a temperature range of 550 to 700°C, A step of rapidly cooling the slowly cooled cold-rolled steel sheet in a rapid cooling zone with a temperature range of 270 to 550°C, The steps include: reheating the rapidly cooled cold-rolled steel sheet, then immersing it in a zinc-based plating bath at an entry temperature of 420 to 550°C to form a zinc-based plating layer; A method for manufacturing a zinc-plated steel sheet, comprising the step of heating a steel sheet on which the zinc-based plating layer is selectively formed to a temperature range of 480 to 560°C to alloy it.

8. The method for manufacturing a galvanized steel sheet according to claim 7, wherein the sheet feeding speed during annealing is 40 to 130 mpm.

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