High-strength galvanized steel sheet with excellent electric resistance spot weldability and its manufacturing method

Abstract

The present invention relates to a galvanized steel sheet having excellent spot weldability and a manufacturing method thereof. The galvanized steel sheet according to one aspect of the present invention is a galvanized steel sheet including a base steel sheet and a zinc-based coating layer formed on the upper part of the base steel sheet, and the decarburization rate of the surface layer portion of the base steel sheet, which is represented by the following formula 1, may be 30% or more. [Formula 1] Decarbonization rate of the surface layer (%) = (1 - average carbon concentration of the surface layer / bulk carbon concentration) * 100 Here, the surface layer refers to the region extending from the surface of the base steel sheet to a depth of 35 μm.

Description

[Technical field]

[0001] The present invention relates to a high-strength galvanized steel sheet having excellent electric resistance spot weldability and a method for producing the same. [Background technology]

[0002] Due to problems such as environmental pollution, regulations on automobile exhaust gas and fuel efficiency are becoming increasingly strict. This has led to a strong demand for lighter steel sheets for automobiles to reduce fuel consumption. In response to this demand, various high-strength steel sheets with high strength per unit thickness have been developed and are now commercially available.

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

[0004] Meanwhile, steel materials for automobiles are supplied in the form of plated steel sheets with a plating applied to the surface to ensure corrosion resistance. Among these, zinc-plated steel sheets (GI steel sheets) or 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 the surface of a high-strength steel sheet is plated with zinc, there is a problem that the spot weldability is reduced. That is, since high-strength steel has high tensile strength and high yield strength, it is difficult to eliminate the tensile stress generated during welding by baking deformation, and there is a high possibility that microcracks will occur on the surface. When high-strength zinc-plated steel sheet is welded, zinc, which has a low melting point, penetrates into the microcracks of the steel sheet, which can cause a phenomenon called liquid metal embrittlement (LME), leading to the destruction of the steel sheet. This acts as a major obstacle to increasing the strength of steel sheet.

[0006] Furthermore, in the case of high-strength steel sheets, alloy components such as Mn, Al, and Si contained in the steel sheet react with the atmosphere to form oxides on the surface of the steel sheet. In this case, there is a risk of problems occurring in the surface quality of the plated steel sheet, such as a significant decrease in plating adhesion. Summary of the Invention [Problem to be solved by the invention]

[0007] According to one aspect of the present invention, there is provided a galvanized steel sheet having excellent spot weldability and a manufacturing method thereof.

[0008] The object of the present invention is not limited to the above. A person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding additional objects of the present invention from the contents described throughout the specification of the present invention. [Means for solving the problem]

[0009] A galvanized steel sheet according to one aspect of the present invention is a galvanized steel sheet including a base steel sheet and a zinc-based plating layer formed on an upper portion of the base steel sheet, and the decarburization rate of a surface layer portion of the base steel sheet, represented by the following formula 1, may be 30% or more.

[0010] [Formula 1] Decarbonization rate of surface layer (%) = (1-average carbon concentration of surface layer / bulk carbon concentration)*100

[0011] Here, the surface layer refers to a region from the surface of the base steel sheet to a depth of 35 μm.

[0012] A method for producing a galvanized steel sheet according to another aspect of the present invention includes the steps of hot rolling a steel slab to obtain a hot rolled steel sheet, coiling the hot rolled steel sheet at a temperature of 590 to 750°C to obtain a hot rolled steel sheet, heating an edge portion of the coiled hot rolled steel sheet at 600 to 800°C for 5 to 24 hours in a nitrogen atmosphere containing 0.5 to 2% oxygen, cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet, annealing the cold rolled steel sheet at 650 to 900°C in an atmosphere with a dew point of -10 to 30°C while passing the cold rolled steel sheet at a passing speed of 40 to 130 mpm, and hot-dip galvanizing the annealed cold rolled steel sheet. Effect of the Invention

[0013] As described above, the present invention makes it possible to produce a galvanized steel sheet that has excellent surface quality and excellent LME resistance during resistance spot welding by forming a decarburized layer in the surface layer of the base steel immediately below the galvanized layer. [Brief description of the drawings]

[0014] [Figure 1] 1 is a graph showing an interval for performing integration on a carbon concentration profile in order to measure the degree of decarbonization. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention will be described in detail below.

[0016] In the present invention, it should be noted that the concept of galvanized steel sheet includes not only galvanized steel sheet (GI steel sheet) but also galvannealed steel sheet (GA) and any plated steel sheet containing zinc as a main component. The term "containing zinc as a main component" means that the proportion of zinc is the highest among the elements contained in the plating layer. However, in the case of galvannealed steel sheet, the proportion of iron may be higher than that of zinc, and may include the case where the proportion of zinc is the highest among the remaining components excluding iron.

[0017] The inventors of the present invention noticed that liquid metal embrittlement (LME) that occurs during welding is caused by microcracks that develop on the surface of the steel plate, and as a result of researching means for suppressing surface microcracks, they discovered that in order to do so, it is necessary to soften the surface of the steel plate, which led to the creation of the present invention.

[0018] Generally, high strength steels contain large amounts of elements such as C, Mn, Si, Cr, Mo, and V to ensure the hardenability and austenite stability of the steel, and these elements play a role in increasing the sensitivity of the steel to cracks. Therefore, steels containing large amounts of these elements are prone to microcracks, which can cause liquid metal embrittlement during welding. According to the research results of the present inventors, the behavior of such microcracks is closely related to the carbon concentration, and since cracks occur from the surface and propagate to the inside, the possibility of microcracks occurring increases when the carbon concentration of the surface is high.

[0019] Therefore, in one embodiment of the present invention, the overall composition of the steel has a high carbon concentration for high strength, and the carbon concentration in the surface layer, where cracks will occur, is low to provide resistance to cracks, i.e., a high decarburization rate.

[0020] In the present invention, the surface layer refers to a point within 35 μm in the depth direction from the surface of the steel plate, and the decarburization rate of the surface layer can be expressed by the following formula [1].

[0021] [Formula 1] Decarbonization rate of surface layer (%) = (1-average carbon concentration of surface layer / bulk carbon concentration)*100

[0022] In the above formula 1, the carbon concentration in the surface layer means the average carbon concentration in the surface layer, and can be the value obtained by integrating the value from the surface layer to a depth of 35 μm of the base steel sheet in a carbon GDS (a concept that includes any similar device such as GDOES) profile as shown in Figure 1, and dividing it by the depth (35 μm). Furthermore, the bulk carbon concentration means the carbon concentration at which there is no further change in carbon concentration even when moving in the depth direction on the GDS profile, and usually means the carbon concentration at a depth of 1 / 4 of the steel sheet thickness. In this case, the carbon concentration at a depth of 1 / 4 of the steel sheet thickness can be obtained by removing a part of the steel sheet in the depth direction and then performing a GDS analysis.

[0023] In one embodiment of the present invention, the decarburization rate of the surface layer may be based on a value measured at the center in the width direction. However, since the edge in the width direction of a steel sheet usually has a higher decarburization rate value than the center in the width direction, the spot weldability can be improved when the decarburization rate at the edge satisfies the value specified in the present invention. Here, the edge in the width direction means both end points of the cross section of the steel sheet cut in the width direction, but when there is a problem with the soundness of the test piece, such as contamination occurring at the above-mentioned points, it may mean a point 1 mm inward in the width direction from the end point.

[0024] As described above, in one embodiment of the present invention, in order to ensure sufficient spot weldability by preventing the occurrence of LME, the decarburization rate may be 30% or more. According to another embodiment of the present invention, the decarburization rate may be 40% or more, and in yet another embodiment, the decarburization rate may be 50% or more.

[0025] Since a higher decarburization rate is more advantageous, the upper limit of the decarburization rate does not need to be particularly limited. However, according to one embodiment of the present invention, the upper limit of the decarburization rate may be 90%, in another embodiment, the upper limit of the decarburization rate may be 80%, and in yet another embodiment, the upper limit of the decarburization rate may be 70%.

[0026] According to an embodiment of the present invention, an inner oxide may be present in the surface layer of the base steel sheet. The inner oxide may contain at least one of Si, Mn, Al, and Fe, and may further contain additional elements derived from the composition of the base steel sheet.

[0027] The steel sheet targeted by the present invention is not limited to any particular type as long as it is a high-strength steel sheet having a strength of 490 MPa or more. However, the steel sheet targeted by the present invention may have a composition including, by weight, C: 0.05-1.5%, Si: 2.0% or less, Mn: 1.0-30%, S-Al (acid-soluble aluminum): 3% or less, Cr: 2.5% or less, Mo: 1% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, V: 0.2% or less, Sb+Sn+Bi: 0.1% or less, and N: 0.01% or less. The remaining components are iron and other impurities, and further elements not listed above and which may be contained in the steel may be included in the range of 1.0% or less in total. In the present invention, the content of each component element is expressed by weight unless otherwise specified. The above composition means the bulk composition of the steel plate, that is, the composition at the 1 / 4 point of the thickness of the steel plate (hereinafter the same).

[0028] However, in some embodiments of the present invention, the high strength steel plate may be a TRIP steel, etc. Each steel may have the following composition.

[0029] 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 balance being Fe and unavoidable impurities. In some cases, elements not listed above and that may be contained in the steel may be further included up to a total of 1.0% or less.

[0030] 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 inevitable 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.

[0031] Furthermore, when the lower limit of the content of each of the above-mentioned component elements is not specified, it means that the element may be any element and the content may be 0%.

[0032] According to an 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 GI (Galvanized) or GA (Galva-annealed), etc. In the present invention, since the ratio of Ceq is appropriately controlled as described above, even if a zinc-based plating layer is formed on the surface of the steel sheet, the problem of liquid metal embrittlement occurring during spot welding can be suppressed.

[0033] When the zinc-based plating layer is a GA layer, the degree of alloying (meaning the content of Fe in the plating layer) can be controlled to 8 to 13 wt%, preferably 10 to 12 wt%. If the degree of alloying is insufficient, the zinc in the zinc-based plating layer may penetrate into microcracks, causing liquid metal embrittlement problems, while if the degree of alloying is too high, problems such as powdering may occur.

[0034] The coating weight of the zinc-based coating layer is 30 to 70 g / m 2 If the coating weight is too low, it is difficult to obtain sufficient corrosion resistance, whereas if the coating weight is too high, there is a risk of an increase in production costs and liquid metal embrittlement, so it is controlled within the above range. A more preferable coating weight range is 40 to 60 g / m 2 This plating weight refers to the amount of plating layer attached to the final product, and when the plating layer is a GA layer, the plating weight increases due to alloying, so its weight may be slightly reduced before alloying. Since it varies depending on the degree of alloying, it is not necessarily limited to this, but the adhesion weight before alloying (i.e., the amount of plating attached from the plating bath) can be a value that is about 10% less than that.

[0035] Hereinafter, one embodiment of the method 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 embodiment described below, and that the embodiment described below is one preferred method for producing the steel sheet of the present invention.

[0036] First, a steel slab having the above-mentioned composition is hot-rolled and then coiled to produce a hot-rolled steel sheet. The conditions for heating the slab (temperature control in the case of direct rolling) and hot rolling are not particularly limited, but in one embodiment of the present invention, the coiling temperature can be limited as follows.

[0037] Winding temperature: 590~750℃ The hot-rolled steel sheet is later wound up in the form of a coil and stored, and the wound steel sheet undergoes a slow cooling process. This process removes oxidizing elements contained in the surface layer of the steel sheet, but if the coiling temperature of the slab is too low, the coil is slowly cooled at a temperature lower than the temperature required to remove the oxidation of these elements, making it difficult to achieve a sufficient effect. Conversely, if the coiling temperature is too high, the temperature deviation between the center and edge parts in the width direction becomes large, which increases the material deviation. In this case, the cold rolling property deteriorates, and not only the strength of the final product decreases, but also the formability may deteriorate. In terms of surface oxidation, if the coiling temperature is too high, re-oxidation of the scale occurs, resulting in the formation of Fe. 2 O 3 However, in this case, the surface quality may be deteriorated. Therefore, in one embodiment of the present invention, the upper limit of the winding temperature may be set to 750°C.

[0038] Heating the edges of hot rolled coils: 600-800℃ for 5-24 hours In one embodiment of the present invention, the edge portion of a hot rolled coil is heated to increase the decarburization rate of the edge portion. Heating the edge portion of a hot rolled coil means heating both end portions in the width direction of a coiled coil, i.e., the edge portion, and the edge portion is heated first to a temperature suitable for oxidation by heating the edge portion. That is, the inside of the coiled coil is maintained at a high temperature, but the edge portion is cooled relatively quickly, and thus the time that the edge portion is maintained at a temperature suitable for internal oxidation is shorter. Therefore, the removal of oxidizing elements is less active in the edge portion than in the center portion in the width direction. Heating the edge portion can be used as one method for removing oxidizing elements from the edge portion.

[0039] That is, when the edge portion is heated, the edge portion is heated first, in contrast to the cooling after coiling, and the temperature of the edge portion in the width direction is maintained at a temperature suitable for internal oxidation, resulting in an increase in the thickness of the internal oxidation layer of the edge portion. For this purpose, the heating temperature of the edge portion needs to be 600°C or higher (based on the temperature of the edge portion of the steel sheet). However, if the temperature is too high, there is a risk that excessive scale is formed on the edge portion during heating or porous highly oxidized scale (hematite) is formed, which may deteriorate the surface condition after pickling, so the temperature of the edge portion can be 800°C or lower. A more preferable heating temperature of the edge portion is 600 to 750°C.

[0040] In order to eliminate the unevenness of the surface layer Ceq value between the edge and center in the width direction that occurs during winding, the heating time of the edge must be 5 hours or more. However, if the heating time of the edge is too long, there is a risk that excessive scale will be formed or the decarburization rate value of the surface layer of the edge will be excessively high. Therefore, the heating time of the edge can be 24 hours or less.

[0041] According to one embodiment of the present invention, the edge portion may be heated by a combustion heating method by adjusting the air-fuel ratio. That is, the oxygen fraction in the atmosphere may be changed by adjusting the air-fuel ratio, and the higher the oxygen partial pressure, the higher the oxygen concentration in contact with the surface layer of the steel sheet may be, which may increase decarburization and internal oxidation. Although not necessarily limited thereto, in one embodiment of the present invention, the air-fuel ratio may be adjusted to control the nitrogen atmosphere containing 0.5 to 2% by volume of oxygen. A person having ordinary knowledge in the technical field to which the present invention pertains may control the oxygen fraction by adjusting the air-fuel ratio without any particular difficulty, and therefore, a separate description will not be given here.

[0042] The hot-rolled steel sheet that has undergone the above-mentioned process may be pickled as necessary and then cold-rolled. The above-mentioned cold rolling process may be followed by a process of annealing the steel sheet. Since the decarburization rate of the surface layer may vary significantly during the annealing process of the steel sheet, in one embodiment of the present invention, the annealing process may be controlled under conditions that appropriately control the decarburization rate of the surface layer, and among these, the sheet passing speed and the dew point in the annealing furnace may be controlled to the following conditions.

[0043] 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 is disadvantageous in terms of ensuring the quality of the material, so in one embodiment of the present invention, the upper limit of the threading speed can be set to 130 mpm. Furthermore, the threading speed affects the thickness of the internal oxide layer, and the slower the threading speed, the thicker the internal oxide tends to be and the lower the carbon content tends to be. Therefore, a threading speed that is too fast may be a factor in the internal oxide layer not being formed to a sufficient thickness. In one example, a threading speed exceeding 130 mpm may cause the internal oxide layer to be formed to a thickness of 3 μm or less.

[0044] Annealing conditions: 650 to 900°C and -10 to 30°C dew point atmosphere. In the present invention, the temperature at which annealing is performed may be 650°C or higher, which is a temperature at which a sufficient internal oxidation effect is achieved. However, if the temperature is too high, surface oxides such as Si are formed, which not only prevents oxygen from diffusing into the interior, but also austenite is generated excessively during heating in the soaking zone, which may reduce the carbon diffusion rate and thus reduce the degree of decarburization. In addition, there is a possibility that problems such as a load on the annealing furnace, shortening the equipment life, and increasing the process cost may occur, so the temperature at which the dew point is controlled may be 900°C or lower. In the present invention, the temperature at which annealing is performed means the temperature in the soaking zone.

[0045] In this case, it is advantageous to control the dew point of the atmosphere in the annealing furnace in order to form a sufficient and uniform internal oxide layer. If the dew point is too low, surface oxidation will occur instead of internal oxidation, and oxides of Si, Mn, etc. may be generated on the surface. Therefore, the dew point must be controlled to -10°C or higher. Conversely, if the dew point is too high, oxidation of Fe may occur, so the dew point must be controlled to 30°C or lower.

[0046] In this case, the dew point is measured by measuring the dew point of wet nitrogen (N 2 +H 2 O) can be added to the annealing furnace.

[0047] The steel sheet annealed by this process is immediately immersed in a plating bath to perform hot-dip galvanizing.

[0048] The steel sheet annealed by this process is reheated to a temperature equal to or higher than the coating bath temperature (460 to 500°C) and then immersed in the coating bath to perform hot-dip galvanization. According to one embodiment of the present invention, the thickness of the annealed steel sheet immersed in the coating bath may be adjusted to 1.0 to 2.0 mm. According to one embodiment of the present invention, the coating bath is a zinc-based coating bath and may contain 50 wt% or more of Zn.

[0049] Alloying (GA) temperature: 480~560℃ If the temperature is less than 480°C, the amount of Fe diffusion is small and the degree of alloying is insufficient, which may result in poor plating properties. If the temperature exceeds 560°C, problems with powdering may occur due to excessive alloying, and the material may deteriorate due to the ferrite transformation of retained austenite. Therefore, the alloying temperature is set to the above-mentioned range.

[0050] In one embodiment of the present invention, in order to ensure the sufficient degree of alloying, the alloying heat treatment time may be 1 second or more, but if the alloying heat treatment time is too long, the degree of alloying may exceed the range specified in the present invention, so the upper limit of the alloying heat treatment time may be 5 seconds. EXAMPLES

[0051] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are merely intended to illustrate and embody the present invention, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0052] (Example) A steel slab having the composition shown in Table 1 below (the remaining components not shown in the table are Fe and unavoidably contained impurities. In the table, B and N are shown in ppm units, and the remaining components are shown in weight %) was hot-rolled, and then the hot-rolled coil was heated at the edge in a nitrogen atmosphere containing oxygen, and then pickled. The obtained cold-rolled steel sheet was annealed in an annealing furnace, and then the steel sheet was reheated to 480°C and immersed in a zinc-based plating bath containing 0.13 wt% Al for hot-dip galvanization. The obtained hot-dip galvanized steel sheet was subjected to alloying (GA) heat treatment for 4 seconds as necessary to finally obtain an alloyed hot-dip galvanized steel sheet.

[0053] In addition, when simply obtaining a hot-dip galvanized steel sheet without alloying, the above-mentioned cold-rolled steel sheet was annealed and reheated in an annealing furnace, and then immersed in a zinc-based plating bath containing 0.24 wt. % Al for plating, and then cooled after an air knife to finally obtain a hot-dip galvanized (GI) steel sheet.

[0054] In all examples, the reduction ratio during cold rolling was 52%, the soaking zone temperature during annealing was 830°C, and the ratio of hydrogen contained in the wet nitrogen in the annealing furnace was 5.0% by volume. Other conditions for each example are as shown in Table 2 (in the table, B and N are shown in ppm units, and the remaining components are shown in weight %).

[0055] [Table 1]

[0056] [Table 2]

[0057] The properties of the galvannealed (GA) steel sheets manufactured by the above process were measured, and the results of observing whether liquid metal embrittlement (LME) occurred during spot welding are shown in Table 3. The steel sheets were cut in the width direction and spot welding was performed along each cut edge. After applying spot welding current twice, one cycle of hold time was maintained. Spot welding was performed in triplicate for two types. The test material-test material-GA 980DP 1.4t material were stacked and spot welded. During spot welding, a new electrode was welded to the soft material 15 times to wear the electrode, and the upper limit current at which expulsion occurred in the target material for spot welding was measured. After measuring the upper limit current, spot welding was performed 8 times for each welding current at currents 0.5 and 1.0 kA lower than the upper limit current, and the cross section of the spot weld was processed with high precision by electric discharge machining, epoxy mounted and polished, and the crack length was measured using an optical microscope. When observing with an optical microscope, the magnification was set to 100x, and if no cracks were found at that magnification, it was determined that liquid metal embrittlement had not occurred, and if a crack was found, its length was measured using image analysis software. It was determined that the specimen was good when B-type cracks occurring at the shoulder of the spot weld were 100μm or less, and when no C-type cracks were observed.

[0058] The decarburization rate was calculated by GDOES using the concentration value for each depth by the component. The decarburization rate of the surface layer was calculated by formula 1 as described above. The tensile strength was measured by preparing a JIS-5 standard C-direction sample and performing a tensile test. The alloying degree and plating adhesion were measured by a wet dissolution method using a hydrochloric acid solution. The sealer adhesion was measured by adhering D-type automotive structural adhesive to the plating surface, bending the steel sheet 90 degrees to check whether the plating fell off. Powdering was measured by bending the plated material 90 degrees, adhering tape to the bent area and then peeling it off, and checking how many millimeters of the plated layer fell off the tape. Flaking was measured by processing the material into a U-shape and checking whether the plated layer fell off at the processed area.

[0059] [Table 3]

[0060] In Examples 1, 2, 3, 4, 5, 6, 7, 8, and 9, the steel composition satisfied the range set forth in the present invention, and the manufacturing method also satisfied the range of the present invention, and the tensile strength, plating quality, plating adhesion weight, and LME crack length during spot welding were also good.

[0061] In Comparative Example 1, the alloying temperature during the GA alloying process was controlled to be lower than the range proposed in the present invention, resulting in a lower alloying degree than the standard, an excessively bright surface, poor surface quality, and flaking, resulting in poor plating surface quality.

[0062] In Comparative Example 2, the heating time during the heat treatment of the edge portion exceeded the range specified in the present invention, and overoxidation occurred at the edge portion during the heat treatment process, forming red hematite-based scale on the surface, and the thickness of the scale became excessively thick. In addition, the edge portion was excessively pickled during the pickling process after hot rolling, resulting in high surface roughness, and after plating, the surface shape was non-uniform and the surface color was different from that of the center, resulting in non-uniform color defects.

[0063] Comparative Example 3 is a case where the coiling temperature during the hot rolling process was higher than the range proposed in the present invention. Therefore, sufficient decarburization occurred during the hot rolling process, and the decarburization rate of the surface layer was 40% or more due to the high dew point during annealing, and although the coating surface quality and LME resistance were excellent, material deviation occurred in the width direction, and the cold rolling property and tensile strength were poor.

[0064] In Comparative Example 4, the heating furnace temperature of the heat treatment furnace exceeded the range proposed by the present invention, and overoxidation occurred at the edge during the heat treatment process, red hematite-based scale was formed on the surface, and the thickness of the scale became excessively thick. In addition, the edge was excessively pickled in the pickling process after hot rolling, resulting in high surface roughness, and after plating, the surface shape was non-uniform and the surface color was different from the center, resulting in non-uniform color defects.

[0065] In Comparative Example 5, the coiling temperature during the hot rolling process was controlled to be lower than the range proposed by the present invention, and therefore decarburization during the hot rolling process did not occur sufficiently, resulting in poor LME properties.

[0066] In Comparative Examples 6 and 13, the dew point in the furnace during annealing was lower than the range proposed by the present invention. Even if a sufficient decarburized layer was generated over the entire width during the hot rolling and heating processes in the heat treatment furnace, the dew point during the annealing process after cold rolling was not high enough, so carbon was homogenized, a sufficient level of decarburization was not formed, and the length of the LME crack during spot welding was poor over the entire width. The GI material had a low dew point and did not generate sufficient internal oxidation, so excessive surface oxide was generated, resulting in poor coating adhesion.

[0067] In Comparative Example 7, the steel sheet passing speed in the annealing furnace was lower than the range proposed in the present invention. The time given to the decarburization reaction in which the water vapor in the annealing furnace reacts with the steel sheet was sufficient, and the criteria were met when evaluating LME cracks during spot welding. However, the manufacturing time was long, resulting in poor productivity, and the manufacturing conditions were therefore not suitable.

[0068] In Comparative Examples 8, 11, and 15, the dew point in the furnace during annealing was controlled to be higher than the range proposed in the present invention. Sufficient decarburization occurred during annealing as well as during hot rolling, and the LME resistance and coating surface quality were good, but the excessive generation of Mn-based oxides caused the oxides to physically adhere to the surface of the hearth roll in the annealing furnace, inducing dents, which are depression defects, in the steel sheet, resulting in poor operability.

[0069] In Comparative Example 9, the heating temperature of the heat treatment furnace during the heat treatment of the edge portion was lower than the range of the present invention. A sufficient decarburized layer was not formed during hot rolling, and the evaluation of LME cracks during spot welding did not meet the criteria and was poor.

[0070] In Comparative Example 10, the heating temperature and time of the heat treatment furnace were within the ranges proposed by the present invention, but the oxygen fraction was outside the range. During the heat treatment process, overoxidation occurred at the edges, forming red hematite-based scale on the surface, and the thickness of the scale was excessively thick. In addition, during the pickling process after hot rolling, the edges were excessively pickled, resulting in high surface roughness, and after plating, the surface shape was non-uniform and the surface color was different from the center, resulting in non-uniform color defects.

[0071] In Comparative Example 12, the steel sheet passing speed during annealing heat treatment was controlled to be higher than the range proposed in the present invention. Since the time given to the decarburization reaction in which the steel sheet reacts with the water vapor in the annealing furnace was insufficient, the decarburization rate of the surface layer of the steel sheet after annealing was insufficient, and LME cracks occurred during spot welding in an amount exceeding the standard.

[0072] In Comparative Example 14, the heating temperature and time in the heat treatment furnace were within the ranges set forth in the present invention, but the oxygen fraction was lower than the ranges. The hot-rolled decarburized layer was not sufficiently formed, and the evaluation of LME cracks during spot welding did not meet the criteria, resulting in a poor result.

[0073] In Comparative Example 16, the alloying temperature in the GA alloying process exceeded the range of the present invention. The Fe alloying degree was high, the color appeared dark, and the surface quality was poor. When evaluating GA powdering, excessive powdering occurred.

[0074] In Comparative Example 17, the heating temperature in the heat treatment furnace was within the range of the present invention, but the heating time in the heat treatment furnace was shorter than the range proposed in the present invention. A sufficient hot-rolled decarburized layer was not formed, and the evaluation of LME cracks during spot welding did not meet the criteria, resulting in a poor result.

Claims

1. A galvanized steel sheet including a base steel sheet and a zinc-based plating layer formed on an upper portion of the base steel sheet, The decarburization rate of the surface layer of the base steel sheet, which is represented by the following formula 1 at points 1 mm inward in the width direction from both end points of a cross section cut in the width direction of the steel sheet, is 30% or more, The base steel sheet is C: 0.05 to 0.30%, Si: 0.5 to 2.5%, Mn: 2.0 to 3.0%, S-Al: 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, or 1. A high-strength galvanized steel sheet comprising: C: 0.05 to 0.30%, Si: 0.5% or less, Mn: 4.0 to 10.0%, S-Al: 0.05% or less, Cr: 2.0% or less, Mo: 0.5% or less, B: 0.005% or less, Nb: 0.1% or less, Ti: 0.15% or less, Sb+Sn+Bi: 0.05% or less, N: 0.01% or less, the balance being Fe and unavoidable impurities. [Formula 1] Decarbonization rate of surface layer (%) = (1 - average carbon concentration of surface layer / bulk carbon concentration) * 100 (Note that the surface layer refers to the region from the surface of the base steel sheet to a depth of 35 μm.)

2. The high-strength galvanized steel sheet according to claim 1 , wherein a decarburization rate of the surface layer is 40% or more.

3. The high-strength galvanized steel sheet according to claim 1, wherein the base steel sheet has a tensile strength of 490 MPa or more.

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

5. The high-strength galvanized steel sheet according to any one of claims 1 to 4, wherein the zinc-based plating layer is a galvannealed (GA) layer having a degree of alloying, which means an Fe content in the plating layer, of 8 to 13 wt%.

6. hot rolling the steel slab to obtain a hot rolled steel sheet; coiling the hot-rolled steel sheet at a temperature of 590 to 750° C. to obtain a hot-rolled steel sheet; heating the edge portion of the coiled hot-rolled steel sheet at 600 to 800° C. for 5 to 24 hours in a nitrogen atmosphere containing 0.5 to 2% oxygen; cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet; Annealing the cold-rolled steel sheet in an atmosphere having a dew point of −10 to 30° C. at 650 to 900° C. while passing the cold-rolled steel sheet at a passing speed of 40 to 130 mpm; hot-dip galvanizing the annealed cold-rolled steel sheet; A method for producing a galvanized steel sheet having excellent spot weldability, comprising:

7. The method for producing a galvanized steel sheet having excellent spot weldability according to claim 6, further comprising the step of subjecting the hot-dip galvanized cold rolled steel sheet to an alloying heat treatment.

8. The method for producing a galvanized steel sheet having excellent spot weldability according to claim 7, wherein the alloying heat treatment is carried out at a temperature of 480 to 560°C.

9. 9. The method for producing a galvanized steel sheet having excellent spot weldability according to any one of claims 6 to 8, wherein the steel slab has a composition including C: 0.05 to 1.5%, Si: 2.0% or less, Mn: 1.0 to 30%, S-Al (acid-soluble aluminum): 3% or less, Cr: 2.5% or less, Mo: 1% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, V: 0.2% or less, Sb + Sn + Bi: 0.1% or less, and N: 0.01% or less.

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