High-strength hot-dip galvanized steel sheet that achieves excellent spot weldability uniformly along the width direction and its manufacturing method

Abstract

According to one aspect of the present invention, it is possible to provide a high-strength hot-dip galvanized steel sheet in which excellent spot weldability is uniformly achieved along the width direction, and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a high-strength hot-dip galvanized steel sheet that exhibits excellent spot weldability uniformly across the width, and a method for manufacturing the same. [Background technology]

[0002] Due to issues such as environmental pollution, regulations on automobile exhaust gases and fuel efficiency are becoming stricter day by day. This has led to increased demand for lighter automotive steel sheets to reduce fuel consumption. As a result, various types of high-strength steel sheets with high strength per unit thickness have been developed and released.

[0003] High-strength steel generally refers to steel having a strength of 490 MPa or more, but is not necessarily limited to this, and may also include transformation induced plasticity (TRIP) steel, twin induced plasticity (TWIP) steel, dual phase (DP) steel, complex phase (CP) steel, etc.

[0004] On the other hand, automotive steel is supplied in the form of plated steel sheets, with the surface plated to ensure corrosion resistance. Among these, galvanized steel sheets (GI steel sheets), highly corrosion-resistant plated steel sheets (ZM) and galvannealed steel sheets (GA) are widely used as automotive materials because they have high corrosion resistance due to the sacrificial corrosion protection properties of zinc.

[0005] However, when high-strength steel sheets are surface-plated with zinc, spot weldability becomes weak. High-strength steel sheets have high tensile strength and yield strength, making it difficult to relieve the tensile stress generated during welding through plastic deformation, leading to the likelihood of microcracks forming on the surface. When high-strength galvanized steel sheets are welded, zinc, which has a low melting point, penetrates into the microcracks in the steel sheet. This can lead to a phenomenon known as liquid metal embrittlement (LME), which can lead to the steel sheet breaking under fatigue conditions. This problem poses a major obstacle to increasing the strength of steel sheets. Summary of the Invention [Problem to be solved by the invention]

[0006] According to one aspect of the present invention, it is possible to provide a high-strength hot-dip galvanized steel sheet in which excellent spot weldability is uniformly achieved along the width direction, and a manufacturing method thereof.

[0007] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the overall content of this specification. [Means for solving the problem]

[0008] A galvanized steel sheet according to one aspect of the present invention includes a base steel sheet and a zinc-based coating layer formed on a surface of the base steel sheet, wherein an average depth (a) of an internal oxidation layer formed on the base steel sheet is 2 μm or more, and a difference (bc) between an average depth (b) of the internal oxidation layer on an edge portion side in a width direction of the galvanized steel sheet and an average depth (c) of the internal oxidation layer on a center portion in a width direction of the galvanized steel sheet may exceed 0.

[0009] The average internal oxide layer depth (b) on the edge side may be the average value of the internal oxide layer depths measured at a point 0.5 cm away from the edge of the plated steel sheet along the width direction toward the center of the plated steel sheet and a point 1.0 cm away from the edge of the plated steel sheet along the width direction toward the center of the plated steel sheet. The average internal oxide layer depth (c) at the center may be the average value of the internal oxide layer depths measured at a point 15 cm away from the edge of the plated steel sheet along the width direction toward the center of the plated steel sheet, a point 30 cm away from the edge of the plated steel sheet along the width direction toward the center of the plated steel sheet, and the center of the plated steel sheet in the width direction. The average internal oxide layer depth (a) formed on the base steel sheet may be the average value of the average internal oxide layer depth (b) on the edge side and the average internal oxide layer depth (c) at the center.

[0010] The coating weight of the zinc-based coating layer is 30 to 70 g / m 2 It can be said that:

[0011] The base steel sheet contains, in weight percent, C: 0.05 to 1.5%, Si: 2.5% or less, Mn: 1.5 to 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, the balance being Fe and unavoidable impurities.

[0012] The tensile strength of the galvanized steel sheet can be 900 MPa or more.

[0013] The thickness of the base steel sheet can be 1.0 to 2.0 mm.

[0014] A method for producing a galvanized steel sheet according to one aspect of the present invention includes the steps of: reheating a steel slab in 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; coiling the hot rolled steel sheet in a temperature range of 590 to 750°C; heating both edges of the coiled hot rolled coil to a temperature range of 600 to 800°C at a heating rate of 10°C / s or more for 5 to 24 hours; The method may include annealing the hot-rolled steel sheet in an atmosphere gas and a soaking zone at a temperature range of 650 to 900°C; slowly cooling the annealed hot-rolled steel sheet in a slow cooling zone at a temperature range of 550 to 700°C; quenching the slowly cooled hot-rolled steel sheet in a quenching zone at a temperature range of 270 to 550°C; reheating the quenched hot-rolled steel sheet and then immersing it in a zinc-based coating bath at a drawing temperature of 420 to 550°C to form a zinc-based coating layer; and optionally, alloying the steel sheet on which the zinc-based coating layer has been formed by heating it at a temperature range of 480 to 560°C.

[0015] The sheet passing speed during the annealing may be set to 40 to 130 mpm.

[0016] The steel slab contains, by weight, C: 0.05 to 0.30%, Si: 2.5% or less, Mn: 1.5 to 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, the balance being Fe and unavoidable impurities.

[0017] The above solution to the problem does not list all the features of the present invention, and 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 examples. [Effects of the Invention]

[0018] According to one aspect of the present invention, an internal oxide layer of a uniform thickness is formed on the surface of the base steel directly below the galvanized layer, and the internal oxide layer has a uniform thickness along the width direction of the steel sheet. This makes it possible to provide excellent crack resistance uniformly along the width direction of the steel sheet even when tensile stress is applied during spot welding. As a result, liquid metal embrittlement (LME), which occurs when the hot-dip galvanized layer penetrates along cracks, can be uniformly suppressed across the width direction of the steel sheet.

[0019] The effects of the present invention are not limited to the above-mentioned matters, but can be construed to include technical effects that a person skilled in the art can infer from the matters described below. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention relates to a high-strength hot-dip galvanized steel sheet that exhibits excellent spot weldability uniformly across the width direction, and a manufacturing method thereof. Preferred embodiments of the present invention will be described below. The present invention may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are intended to provide the present invention in more detail to those skilled in the art to which the present invention pertains.

[0021] Hereinafter, the galvanized steel sheet of the present invention will be described through several embodiments.

[0022] It should be noted that the term "galvanized steel sheet" as used herein refers not only to galvanized steel sheet (GI steel sheet) but also to alloyed galvanized steel sheet (GA), and encompasses all coated steel sheets having a zinc-based coating layer containing zinc as the main component. The term "mainly containing zinc" means that zinc accounts for the highest proportion of the elements contained in the coating layer. For example, this term may include highly corrosion-resistant coated steel sheet (ZM). However, alloyed galvanized steel sheets may contain a higher proportion of iron than zinc, and the scope of the present invention may encompass steel sheets having the highest proportion of zinc among the remaining components excluding iron.

[0023] The inventors of the present invention discovered that liquid metal embrittlement (LME) that occurs during welding is caused by microcracks that develop on the surface of the steel sheet, and studied means for suppressing surface microcracks. They discovered that in order to suppress these cracks, it is necessary to specifically control the microstructure of the steel sheet surface, which led to the present invention.

[0024] High-strength steels typically contain large amounts of elements such as carbon (C), manganese (Mn), and silicon (Si) to ensure the steel's hardenability and austenite stability. However, these elements increase the steel's susceptibility to cracking. Therefore, steels containing large amounts of these elements are prone to microcracks, ultimately causing liquid metal embrittlement during welding. Our research has shown that the behavior of microcracks is closely related to carbon concentration. A lower carbon concentration in the surface layer of a steel sheet leads to the formation of a softened ferrite layer in the surface layer, which prevents cracks from occurring due to tensile stress generated during spot welding. Plastic deformation relieves stress, preventing cracks and reducing cracking in spot welds. Because the fraction of soft ferrite formation is affected by the depth of internal oxidation in the surface layer, the level of LME crack improvement in spot welds is proportional to the thickness of the internal oxidation layer formed in the surface layer.

[0025] Furthermore, if an internal oxide layer is locally formed in a certain region across the entire width of the steel sheet, it will be impossible to provide uniform LME crack resistance. Therefore, it is important that the internal oxide layer formed to a certain depth or more is uniformly formed across the entire width of the steel sheet.

[0026] According to one embodiment of the present invention, there is provided a galvanized steel sheet including a base steel sheet and a zinc-based coating layer formed on a surface of the base steel sheet, wherein an average depth (a) of an internal oxide layer formed on the base steel sheet is 2 μm or more, and a difference (bc) between an average depth (b) of the internal oxide layer on an edge portion side in a width direction of the galvanized steel sheet and an average depth (c) of the internal oxide layer on a center portion in a width direction of the galvanized steel sheet may exceed 0. A preferable difference (bc) in the depth of the internal oxide layer may be greater than 0 and not greater than 1.5.

[0027] The average internal oxide layer depth (b) on the edge side may be the average value of the internal oxide layer depths measured at a point 0.5 cm away from the edge of the plated steel sheet along the width direction toward the center of the plated steel sheet and a point 1.0 cm away from the edge of the plated steel sheet along the width direction toward the center of the plated steel sheet. The average internal oxide layer depth (c) at the center may be the average value of the internal oxide layer depths measured at a point 15 cm away from the edge of the plated steel sheet along the width direction toward the center of the plated steel sheet, a point 30 cm away from the edge of the plated steel sheet along the width direction toward the center of the plated steel sheet, and the center of the plated steel sheet in the width direction. The average internal oxide layer depth (a) formed in the base steel sheet may be the average value of the average internal oxide layer depth (b) on the edge side and the average internal oxide layer depth (c) at the center. A person skilled in the art can measure the average depth (a) of the internal oxide layer formed on the base steel sheet, the average depth (b) of the internal oxide layer on the edge portion, and the average depth (c) of the internal oxide layer in the center portion without any particular technical difficulty by utilizing known measurement methods.

[0028] According to one embodiment of the present invention, the average depth (a) of the internal oxide layer formed in the base steel sheet is controlled to a level of 2 μm or more, so that the soft surface layer can be formed with a sufficient thickness. Therefore, plastic deformation occurs in the soft surface layer during spot welding, and the tensile stress generated during spot welding is consumed, thereby effectively suppressing the crack susceptibility of the steel sheet.

[0029] Meanwhile, when cold-rolled galvanized steel sheets are manufactured under conventional process conditions, the internal oxidation layer formed in the center in the width direction is deeper than the internal oxidation layer formed in the edge in the width direction. Manufacturing cold-rolled steel sheets requires a process of winding the hot-rolled steel sheet into a hot-rolled coil at a certain temperature range. The center of the hot-rolled coil, which is wound above a certain temperature range, is maintained at a relatively high temperature for a long time compared to the edge of the hot-rolled coil, resulting in more active internal oxidation occurring in the center of the hot-rolled coil than in the edge of the hot-rolled coil. This tendency for internal oxidation persists in the final cold-rolled galvanized steel sheet, resulting in variations in LME resistance along the width direction of the final steel sheet.

[0030] Meanwhile, a galvanized steel sheet according to an embodiment of the present invention not only has an internal oxide layer with an average depth of 2 μm or more in the surface layer portion of the base steel sheet, but also controls the internal oxide layer formed near the center of the galvanized steel sheet to have a greater thickness than the internal oxide layer formed near the edge portion of the galvanized steel sheet, thereby achieving excellent LME resistance uniformly across the width direction of the steel sheet.

[0031] 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 targeted by the present invention may contain, by weight, C: 0.05 to 1.5%, Si: 2.5% or less, Mn: 1.5 to 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, and the balance being Fe and unavoidable impurities. In some cases, elements that may be contained in the steel other than those listed above may also be contained up to a total of 1.0 wt.% or less. The content of each component element in the present invention is expressed on a weight basis unless otherwise specified. The above-mentioned composition means the bulk composition of the steel sheet, that is, the composition at the 1 / 4 point of the steel sheet thickness (the same applies hereinafter).

[0032] In some embodiments of the present invention, the high-strength base steel sheet may be a TRIP steel, a DP steel, a CP steel, etc. These steels may have the following compositions when classified in detail.

[0033] 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, balance Fe and unavoidable impurities. In some cases, elements not listed above but that may be contained in steel may also be included up to a total content of 1.0% or less.

[0034] Steel composition 2: C: 0.05 to 0.30% (preferably 0.10 to 0.2%), Si: 0.5% or less (preferably 0.3% or less), Mn: 4.0 to 10.0% (preferably 5.0 to 9.0%), S-Al: 0.05% or less (preferably 0.001 to 0.04%), Cr: 2.0% or less (preferably 1.0% or less), Mo: 0.5% or less (preferably 0.1 to 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 to 0.1%), Sb+Sn+Bi: 0.05% or less, N: 0.01% or less, the balance being Fe and unavoidable impurities. In some cases, elements not listed above that may be contained in steel may further be contained up to a total content of 1.0% or less.

[0035] Furthermore, when the lower limit of the content of each of the above-mentioned component elements is not specified, it means that these elements may be regarded as optional elements, and the content thereof may be 0%.

[0036] Although not necessarily limited to this, the thickness of the base steel sheet according to one embodiment of the present invention may be 1.0 to 2.0 mm.

[0037] According to one embodiment of the present invention, the surface of the steel sheet may include one or more plating layers, and the plating layer may be a zinc-based plating layer including a GI (Galvanized), GA (Galva-annealed), or ZM (Zinc-Magnesium-Aluminum) layer. In the present invention, the ferrite fraction and average grain size of the surface layer are controlled within appropriate ranges as described above, so that 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.

[0038] According to one embodiment of the present invention, when the zinc-based coating layer is a GA layer, the alloying degree (meaning the Fe content in the coating layer) can be controlled to 8 to 13 wt %, preferably 10 to 12 wt %. If the alloying degree is insufficient, zinc in the zinc-based coating layer may penetrate into microcracks, causing problems such as liquid metal embrittlement. On the other hand, if the alloying degree is too high, problems such as powdering may occur.

[0039] The coating weight of the zinc-based plating layer is 30 to 70 g / m 2 If the coating weight is too small, it is difficult to obtain sufficient corrosion resistance. On the other hand, if the coating weight is too large, problems such as increased production costs and liquid metal embrittlement may occur, so it is controlled within the above-mentioned range. A more preferable range of the coating weight is 40 to 60 g / m 2 The above coating weight refers to the amount of the coating layer attached to the final product. When the coating layer is GA, the coating weight increases with alloying, so its weight before alloying may decrease slightly, depending on the degree of alloying. Therefore, although not necessarily limited to this, the coating weight before alloying (i.e., the amount of coating attached from the plating bath) may be a value that is approximately 10% less than the coating weight.

[0040] Hereinafter, one embodiment for producing the steel sheet of the present invention will be described, however, it should be noted that the steel sheet of the present invention does not necessarily have to be produced according to the following embodiment, and that the following embodiment is one preferred method for producing the steel sheet of the present invention.

[0041] First, a hot-rolled steel sheet can be produced by reheating a steel slab having the above-described composition, hot-rolling it through rough rolling and finish rolling, and then ROT (Run Out Table) cooling and coiling. The produced steel sheet can then be pickled and cold-rolled, and the resulting cold-rolled steel sheet can be annealed and coated. While there are no particular limitations on the hot-rolling conditions, such as ROT cooling, in one embodiment of the present invention, the slab heating temperature, finish-rolling start and end temperatures, coiling temperature, pickling conditions, cold-rolling conditions, annealing conditions, and coating conditions can be limited as follows.

[0042] Slab heating temperature: 950~1300℃ Slab heating is performed to heat the material before hot rolling and ensure rollability. During slab reheating, the slab surface combines with oxygen in the furnace to form an oxide scale. As the scale forms, it also reacts with the carbon in the steel, causing a decarburization reaction that forms carbon monoxide gas, and the amount of decarburization increases the higher the slab reheating temperature. If the slab reheating temperature is too high, an excessive decarburization layer will form, causing the problem of softening the material of the final product. If the temperature 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 LME improvement.

[0043] Finish rolling starting temperature: 900~1150℃ If the finish rolling start temperature is too high, the surface hot rolling scale may grow excessively, increasing the number of surface defects caused by the scale in the final product, so the upper limit is set to 1150° C. Also, if the finish rolling start temperature is less than 900° C., the temperature decrease may increase the rigidity of the bar, significantly reducing the hot rollability, so the finish rolling start temperature can be limited within the above range.

[0044] Finishing temperature: 850-1050℃ If the finish rolling end temperature exceeds 1,050°C, the scale removed by descaling during finish rolling will be excessively formed again on the surface, increasing the number of surface defects. If the finish rolling end temperature is less than 850°C, the hot rollability will be reduced. Therefore, the finish rolling end temperature can be limited to the above-mentioned range.

[0045] Winding temperature: 590 to 750°C The hot-rolled steel sheet is then wound into a coil and stored, and the wound steel sheet undergoes a slow cooling process. This process removes hardening elements contained in the surface layer of the steel sheet. However, if the coiling temperature of the hot-rolled steel sheet is too low, the coil is slowly cooled at a temperature lower than the temperature required for oxidizing and removing these elements, making it difficult to obtain a sufficient effect.

[0046] Heating the edge of the hot rolled coil: Heat to a temperature range of 600-800°C at a heating rate of 10°C / s or more and heat for 5-24 hours In one embodiment of the present invention, the edge portion of a hot-rolled coil can be heated to reduce the difference in depth of the internal oxidation layer and the difference in LME resistance between the edge portion and the inner region of the edge portion in the width direction. Edge heating of a hot-rolled coil refers to heating both ends of the coil in the width direction, i.e., the edge portion, and edge heating preferentially heats the edge portion to a temperature suitable for oxidation. That is, while the coiled coil maintains a high temperature inside, the edge portion cools relatively quickly, so the time maintained at a temperature suitable for internal oxidation is shorter than that of the edge portion. Therefore, removal of oxidizing elements is less active in the edge portion than in the center portion in the width direction. Edge heating can be used as one method for removing oxidizing elements from the edge portion.

[0047] That is, when edge heating is performed, the edge is preferentially heated, as opposed to cooling after coiling. This maintains the temperature of the edge in the width direction at a level suitable for internal oxidation, resulting in an increase in the thickness of the internal oxide layer at the edge. To achieve this, the edge heating temperature must be 600°C or higher (based on the temperature of the steel sheet edge). However, if the temperature is too high, excessive scale may form at the edge during heating, or porous highly oxidized scale (hematite) may form, resulting in poor surface condition after pickling. Therefore, the edge temperature can be set to 800°C or lower. A more preferred edge heating temperature is 600 to 750°C.

[0048] Furthermore, in order to eliminate variations in the depth of the internal oxide layer of the steel sheet between the edge and center in the width direction that occur during coiling, the edge heating time needs to be 5 hours or more. However, if the edge heating time is too long, excessive scale may be formed or the grain boundary embrittlement of the internal oxide layer at the edge may increase. Therefore, the edge heating time can be set to 24 hours or less.

[0049] In addition, the heating rate during edge heating of the hot-rolled coil is preferably 10°C / s or higher. A heating rate below 10°C / s can result in excessive generation of Si-based oxides, such as Fe2SiO4, in the low-temperature region, suppressing the formation of internal oxides in the final steel sheet. Since the excessive Fe2SiO4 formed in the low-temperature region remains in the steel sheet in the form of SiO2 even after pickling, even if the dew point temperature during annealing is increased, this can inhibit the penetration and diffusion of oxygen into the surface layer of the steel sheet, suppressing internal oxidation and potentially reducing LME resistance. Furthermore, Si-based oxides remaining on the steel sheet surface can grow during annealing, potentially reducing the wettability and physical properties of the steel sheet to molten zinc.

[0050] According to one embodiment of the present invention, the edge heating can be performed by a combustion heating method using air-fuel ratio adjustment. That is, the oxygen fraction in the atmosphere can be changed by adjusting the air-fuel ratio, and as the oxygen partial pressure increases, the oxygen concentration in contact with the surface layer of the steel sheet increases, which can increase decarburization and internal oxidation. Although not necessarily limited thereto, in one embodiment of the present invention, a nitrogen atmosphere containing 1 to 2% oxygen can be controlled by adjusting the air-fuel ratio. Since a person skilled in the art to which the present invention pertains can control the oxygen fraction by adjusting the air-fuel ratio without any particular difficulty, a separate description of this method will not be provided.

[0051] Pickling treatment: carried out at a threading speed of 180 to 250 mpm After undergoing the above process, the hot-rolled steel sheet is subjected to pickling treatment by being placed in a hydrochloric acid bath to remove hot-rolling scale. During pickling, the hydrochloric acid concentration in the hydrochloric acid bath is set to 10-30%, and the pickling speed is set to 180-250 mpm. If the pickling speed exceeds 250 mpm, the surface scale of the hot-rolled steel sheet may not be completely removed. If the pickling speed is lower than 180 mpm, the surface layer of the base steel may be corroded by the hydrochloric acid, so the pickling speed should be set to 180 mpm or higher.

[0052] Cold rolling: reduction rate 35-60% After pickling, the steel is cold-rolled. The cold reduction during cold rolling is in the range of 35 to 60%. If the cold reduction is less than 35%, there are no particular problems, but the driving force for recrystallization during annealing may be insufficient, making it difficult to adequately control the microstructure. If the cold reduction exceeds 60%, the thickness of the soft layer secured during hot rolling becomes thin, making it difficult to sufficiently reduce the hardness within a region 20 μm deep on the steel sheet surface after annealing.

[0053] The above-described cold rolling process may be followed by an annealing process of the steel sheet. The average grain size and fraction of ferrite in the surface region of the steel sheet may also vary significantly during the annealing process. Therefore, in one embodiment of the present invention, the annealing process may be controlled under conditions that appropriately control the average grain size and fraction of ferrite in the region within 50 μm from the surface of the steel sheet.

[0054] Threading speed: 40~130mpm In order to ensure sufficient productivity, the threading speed of the cold-rolled steel sheet needs to be 40 mpm or more. However, if the threading speed is too fast, it may be disadvantageous in terms of ensuring the quality of the material. Therefore, in one embodiment of the present invention, the upper limit of the threading speed can be set to 130 mpm.

[0055] Heating rate in the heating zone: 1.3 to 4.3°C / s To ensure an appropriate range of 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 silicon 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 dissociating into oxygen upon contact with the steel sheet surface. The oxide film inhibits the reaction between carbon and oxygen on the surface, preventing sufficient decarburization and potentially reducing LME resistance. Furthermore, the formation of an oxide film on the surface can reduce plating wettability and degrade 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.

[0056] On the other hand, if the heating rate in the heating zone is high, recrystallization during the heating process and austenite phase transformation in the temperature range above the two-phase region may not be smooth. In TRIP steel, during the simultaneous formation of ferrite and austenite in the two-phase region, carbon in cementite dissociates, and partitioning occurs in austenite with high carbon solubility. This increases the amount of carbon dissolved, stabilizing hard low-temperature phases such as martensite. On the other hand, if the heating rate is high, the austenite fraction decreases, and the low-temperature phase may not be fully formed due to reduced carbon partitioning, resulting in reduced strength. Therefore, in one embodiment of the present invention, the upper limit of the heating rate in the heating zone may be set to 4.3°C / s.

[0057] Dew point control in annealing furnace: Controlled from 650 to 900°C to -10 to +30°C To obtain an appropriate range of surface ferrite fraction and average grain size, it is advantageous to control the dew point in the annealing furnace. If the dew point is too low, surface oxidation may occur instead of internal oxidation, resulting in the formation of oxides of silicon and manganese on the surface. These oxides adversely affect plating. Therefore, the dew point must be controlled to -10°C or higher. On the other hand, if the dew point is too high, oxidation of iron may occur, so the dew point must be controlled to 30°C or lower. The temperature for dew point control can be 650°C or higher, which is the temperature at which sufficient internal oxidation is achieved. However, if the temperature is too high, not only can the formation of surface oxides such as silicon hinder the internal diffusion of oxygen, but excessive austenite can be generated during soaking zone heating, reducing the carbon diffusion rate and thereby reducing the level of internal oxidation. This can lead to excessive growth of austenite in the soaking zone, resulting in material softening. Furthermore, this can increase the load on the annealing furnace, shortening equipment life, and increasing process costs. Therefore, the temperature for controlling the dew point can be 900°C or lower.

[0058] At this time, the dew point can be adjusted by introducing moist nitrogen containing water vapor (N2+H2O) into the annealing furnace.

[0059] Hydrogen concentration in the annealing furnace: 5 to 10 Vol% The atmosphere inside the annealing furnace is maintained as a reducing atmosphere by adding 5-10 vol% hydrogen to nitrogen gas. If the hydrogen concentration inside the annealing furnace is less than 5 vol%, the reducing ability decreases, causing excessive surface oxide formation, which reduces surface quality and coating adhesion. The surface oxide also inhibits the reaction between oxygen and carbon in the steel, reducing the amount of decarburization and lowering the level of LME improvement. While a high hydrogen concentration does not cause any particular problems, it is limited due to the increased cost associated with increased hydrogen gas consumption and the risk of explosion inside the furnace caused by increased hydrogen concentration.

[0060] The steel sheet annealed by the above-mentioned process may be cooled through slow cooling and rapid cooling steps.

[0061] Slow cooling zone temperature: 550-700℃ The slow cooling zone is a zone where the cooling rate is 3 to 5°C / s. If the slow cooling zone temperature exceeds 700°C, excessive soft ferrite may be formed during slow cooling, resulting in a decrease in tensile strength. On the other hand, if the slow cooling zone temperature is less than 550°C, excessive bainite or martensite may be formed, resulting in an excessive increase in tensile strength and a decrease in elongation. Therefore, the slow cooling zone temperature can be limited to the above range.

[0062] Quenching zone temperature during quenching: 270-550℃ The quenching zone is a section where the cooling rate is 12 to 20°C / s, and if the quenching zone temperature exceeds 550°C, martensite is formed at an appropriate level or less during quenching, which may result in insufficient tensile strength.On the other hand, if the quenching zone temperature is less than 270°C, excessive martensite is formed, which may result in insufficient elongation.

[0063] The steel sheet annealed by this process is immediately immersed in a galvanizing bath to be hot-dip galvanized. If the steel sheet is cooled, a step of heating the steel sheet may be further included. The heating temperature must be higher than the drawing temperature of the steel sheet, which will be described later, and may be higher than the temperature of the galvanizing bath in some cases.

[0064] Steel sheet drawing temperature in the plating bath: 420~500℃ If the temperature at which the steel sheet is drawn into the coating bath is too low, sufficient wettability at the contact interface between the steel sheet and the liquid zinc is not ensured, so it must be maintained at 420°C or higher. If the temperature is too high, excessive reaction occurs between the steel sheet and the liquid zinc, resulting in the formation of a Zetta phase, an Fe-Zn alloy phase, at the interface, reducing the adhesion of the coating layer and causing excessive elution of Fe elements from the steel sheet in the coating bath, resulting in the formation of dross in the coating bath. Therefore, the temperature at which the steel sheet is drawn into the coating bath can be limited to 500°C or lower.

[0065] Al concentration in plating bath: 0.10 to 13.0% The Al concentration in the plating bath must be maintained at an appropriate level to ensure the wettability of the plating layer and the fluidity of the plating bath. By controlling the Al concentration to 0.10-0.15% for GA, 0.2-0.25% for GI, and 0.7-13.0% for ZM, dross formation in the plating bath can be maintained at an appropriate level, ensuring the quality and performance of the plating surface.

[0066] The hot-dip galvanized steel sheet coated by the above-mentioned process may then be subjected to an alloying heat treatment process, if necessary. Preferred conditions for the alloying heat treatment are as follows.

[0067] Alloying (GA) temperature: 480~560℃ If the temperature is below 480°C, the amount of Fe diffusion is small and the degree of alloying is insufficient, resulting in poor plating properties. If the temperature exceeds 560°C, powdering problems may occur due to excessive alloying, and the material may deteriorate due to the ferrite transformation of retained austenite. Therefore, the alloying temperature is determined to be within the above range. [Example]

[0068] The present invention will be described in more detail with reference to the following examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0069] (Example) A steel slab having the composition shown in Table 1 below (the remaining components not shown in the table are Fe and inevitable impurities. In addition, B in the table is shown in ppm, and the remaining components are shown in weight %) was heated to 1230°C and hot-rolled with finish rolling start and end temperatures of 1015°C and 950°C, respectively. Then, coiling and edge heating of the hot-rolled coil were carried out under the conditions shown in Table 2. After edge heating, the steel was pickled with a 19.2% by volume hydrochloric acid solution and then cold-rolled. The obtained 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 resulting steel sheets were then heated and immersed in a hot-dip galvanizing bath containing 0.13% Al for GA, 0.24% by weight for GI, and 1.75% Al and 1.55% Mg for ZM. The resulting hot-dip galvanized steel sheets were then subjected to a galvannealing (GA) heat treatment at 520°C as needed to finally obtain galvannealed steel sheets.

[0070] In all examples, the drawing temperature of the steel sheet into the hot-dip galvanizing bath was 475° C. Other conditions for each example are as shown in Table 2, and the process conditions not specifically mentioned above were carried out so as to satisfy the process conditions of the present invention described above.

[0071] [Table 1]

[0072] [Table 2]

[0073] The properties of the hot-dip galvanized steel sheets manufactured using the above process were measured to determine whether liquid metal embrittlement (LME) occurred during spot welding. The results are shown in Table 3. Spot welding was performed along each cut edge of the steel sheet, which was cut widthwise. The spot welding current was applied twice, followed by a one-cycle hold time. Spot welding was performed on three dissimilar materials. Test material - GA 980DP 1.4t (composition: 0.12 wt% C, 0.1 wt% Si, 2.2 wt% Mn) was stacked in this order. Spot welding was performed on a new electrode 15 times on a soft material. After the electrode was worn, the upper limit current at which expulsion occurred was measured. After measuring the upper limit current, spot welding was performed eight times at currents 0.5 and 1.0 kA lower than the upper limit current. The cross-section of the spot weld was precisely machined using electrical discharge machining, epoxy-mounted, and polished. The crack length was measured using an optical microscope. Crack lengths were measured at points 0.5 cm, 1.0 cm, 15 cm, and 30 cm away from the edge of the plate toward the center of the plate in the width direction, as well as at the center of the plate in the width direction. The optical microscope magnification was set to 100x. If no cracks were found at that magnification, it was determined that liquid metal embrittlement had not occurred. If a crack was found, its length was measured using image analysis software. The maximum crack length measured at each point was evaluated. Type B cracks occurring at the shoulder of the spot weld were 100 μm or less, and Type C cracks were considered good when not observed. The Type B and Type C crack lengths listed in Table 3 refer to the maximum crack lengths observed.

[0074] To measure the depth of the internal oxide layer, the cross section of the steel sheet was observed using a scanning electron microscope (SEM). Specifically, SEM observation was performed on the cross section of the steel sheet at points 0.5 cm, 1.0 cm, 15 cm, and 30 cm away from the edge of the steel sheet toward the center in the width direction of the steel sheet, as well as at the center in the width direction of the plated steel sheet, and the internal oxide depth was measured using image analysis software.

[0075] Tensile strength was measured by peeling off tensile strength tests on samples prepared in the C direction according to JIS No. 5. Plating adhesion was measured using a wet dissolution method using a hydrochloric acid solution. Sealer adhesion was measured by adhering automotive structural adhesive D-type to the plated surface and then bending the steel sheet 90 degrees to check for any loss of plating. Powdering was measured by bending the plated material 90 degrees, applying tape to the bent area, then peeling it off to check for the number of millimeters of plating layer that had peeled off from the tape. A defect was determined if the length of the plating layer that peeled off from the tape exceeded 10 mm. Flaking was measured by bending the steel sheet into a U-shape and checking for any loss of plating layer at the bent area. GI and ZM steel sheets were subjected to a sealer bending test (SBT) to check for any loss of plating layer at the bent area after applying automotive structural adhesive to the surface. The steel sheets were visually inspected for defects such as unplated areas, and the surface quality was checked. If defects such as unplated areas were found during visual inspection, the sheets were judged to be defective.

[0076] [Table 3]

[0077] It was confirmed that the test specimens that satisfied all the conditions of the present invention had good coating quality and spot weld LME crack length, while the test specimens that did not satisfy any one of the conditions of the present invention showed deterioration in at least one of tensile strength, coating quality, and spot weld LME crack length.

[0078] Although the present invention has been described in detail with reference to the above embodiments, other embodiments are possible, and the spirit and scope of the following claims are not limited to the embodiments.

Claims

1. A galvanized steel sheet including a base steel sheet and a zinc-based plating layer provided on a surface of the base steel sheet, The average depth (a) of the internal oxide layer formed on the base steel sheet is 2 μm or more, a difference (b-c) between an average internal oxide layer depth (b) on the edge portion side in the width direction of the plated steel sheet and an average internal oxide layer depth (c) on the center portion in the width direction of the plated steel sheet exceeds 0; The base steel sheet is a galvanized steel sheet containing, by mass%, C: 0.05 to 1.5%, Si: 2.5% or less, Mn: 1.5 to 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, and N: 0.01% or less, with the remainder being unavoidable impurities.

2. the average depth (b) of the internal oxide layer on the edge portion side is an average value of the depths of the internal oxide layer measured at a point 0.5 cm away from the edge in the width direction of the plated steel sheet toward the center of the plated steel sheet along the width direction of the plated steel sheet and at a point 1.0 cm away from the edge in the width direction of the plated steel sheet toward the center of the plated steel sheet along the width direction of the plated steel sheet, the average internal oxide layer depth (c) at the center is an average value of the depths of the internal oxide layer measured at a point 15 cm away from the edge in the width direction of the plated steel sheet toward the center of the plated steel sheet along the width direction of the plated steel sheet, a point 30 cm away from the edge in the width direction of the plated steel sheet toward the center of the plated steel sheet along the width direction of the plated steel sheet, and the center in the width direction of the plated steel sheet, 2. The galvanized steel sheet according to claim 1, wherein the average depth (a) of the internal oxidation layer formed in the base steel sheet is an average value of the average depth (b) of the internal oxidation layer on the edge portion side and the average depth (c) of the internal oxidation layer in the center portion.

3. The coating weight of the zinc-based plating layer is 30 to 70 g / m ２ The galvanized steel sheet according to claim 1, wherein

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

6. reheating the steel slab to a temperature range of 950-1300°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; coiling the hot-rolled steel sheet at a temperature in the range of 590 to 750°C; heating both edges of the coiled hot-rolled coil to a temperature range of 600 to 800°C at a heating rate of 10°C / s or more for 5 to 24 hours; pickling the heated hot-rolled steel sheet at a sheet passing speed of 180 to 250 mpm, and then cold-rolling the same at a rolling reduction of 35 to 60% to obtain a cold-rolled steel sheet; Dew point temperature -10 to +30°C, N ２ -5 to 10% H ２ annealing the cold-rolled steel sheet in an atmosphere of the above gas and in a soaking zone at a temperature in the range of 650 to 900°C; Slowly cooling the annealed cold-rolled steel sheet in a slow cooling zone at a temperature range of 550 to 700°C; quenching the slowly cooled cold-rolled steel sheet in a quenching zone at a temperature range of 270 to 550°C; Reheating the quenched cold-rolled steel sheet and then immersing it in a zinc-based coating bath at a drawing temperature of 420 to 550°C to form a zinc-based coating layer; and 2. The method for manufacturing a galvanized steel sheet according to claim 1, further comprising the step of: selectively heating the steel sheet on which the zinc-based coating layer is formed at a temperature in the range of 480 to 560°C to alloy it.

7. The method for producing a galvanized steel sheet according to claim 6, wherein the sheet threading speed during the annealing is 40 to 130 mpm.

8. 7. The method for producing a galvanized steel sheet according to claim 6, wherein the steel slab contains, in mass%, C: 0.05 to 0.30%, Si: 2.5% or less, Mn: 1.5 to 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, and N: 0.01% or less, with the balance consisting of Fe and inevitable impurities.

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