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

Abstract

The present invention relates to a high-strength hot-dip galvanized steel sheet having excellent surface quality and spot weldability, and a method for producing the same. A hot-dip galvanized steel sheet according to one aspect of the present invention includes a base steel sheet and a hot-dip galvanized layer formed on a surface of the base steel sheet, and the difference between the average Mn / Si value of surface layer oxides present in a surface layer region from the interface between the hot-dip galvanized layer and the base steel sheet to a depth of 15 nm and the average Mn / Si value of internal oxides present at a depth of 50 to 100 nm from the interface can be 0.5 or more. Here, Mn and Si in each oxide refer to the content (wt%) of Mn and Si components in the oxide measured by EDS, and the average Mn / Si value refers to the average of the Mn / Si values ​​measured for each oxide.

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 producing the same.

Background Art

[0002] Due to problems such as environmental pollution, regulations on automotive exhaust gases and fuel consumption are being strengthened day by day. Therefore, there is a strong demand for reducing fuel consumption by lightweighting automotive 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 fall under this category.

[0004] On the other hand, automotive steel materials are supplied in the form of plated steel sheets plated on the surface to ensure corrosion resistance. Among them, galvanized steel sheets (GI), high-corrosion-resistant plated steel sheets (ZM), or alloyed galvanized steel sheets (GA) are widely used as automotive materials 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. [Overview of the project] [Problems that the invention aims to solve]

[0006] 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 are provided.

[0007] The problems addressed by the present invention are not limited to those described above. Anyone with ordinary skill in the art to which the present invention pertains should have no difficulty understanding further problems addressed by the present invention from the overall content of the specification. [Means for solving the problem]

[0008] A hot-dip galvanized steel sheet according to one aspect of the present invention includes a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, wherein the value obtained by subtracting the average Mn / Si value of the internal oxide located at a depth of 50 to 100 nm from the interface between the hot-dip galvanized layer and the base steel sheet from the average Mn / Si value of the surface oxide located in the surface region up to a depth of 15 nm toward the base steel sheet can be 0.5 or more.

[0009] Here, Mn and Si for each oxide refer to the content (weight %) of Mn and Si components in the oxide as measured by EDS, and the average Mn / Si value refers to the average of the Mn / Si values ​​measured for each oxide.

[0010] A method for manufacturing a hot-dip galvanized steel sheet according to another aspect of the present invention comprises the steps of: providing a steel slab; reheating the slab to a temperature 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 obtain a steel sheet; winding the steel sheet in a temperature range of 590 to 750°C; pickling the steel sheet at a feed speed of 180 to 250 mpm; and the steel The process may include the steps of: cold rolling a sheet with a reduction ratio of 35-60%; recrystallizing annealing the cold-rolled steel sheet under conditions of humid nitrogen containing 5-10 volume% H2 in an atmospheric gas, with a uniform temperature of 650-900°C and a dew point of -10-+30°C, respectively, and cooling in a rapid cooling zone at a cooling rate of 5-30°C / s; and immersing the steel sheet in a molten plating bath at an entry temperature in the range of 420-500°C for molten plating. [Effects of the Invention]

[0011] As described above, the present invention has the effect of suppressing the occurrence of microcracks in the surface layer by controlling the difference between the Mn / Si values ​​of the surface oxide and the Mn / Si values ​​of the internal oxide of the base steel sheet to be large, thereby greatly improving spot weldability. [Modes for carrying out the invention]

[0012] The technical terms used herein are for the sole purpose of referring to specific embodiments and are not intended to limit the invention. The singular forms used herein also include plural forms unless the phrase clearly indicates the opposite meaning.

[0013] As used in this specification, "includes" embodies a particular characteristic, area, integer, stage, operation, element and / or component, and does not exclude the presence or addition of other particular characteristics, areas, integers, stages, operations, elements, components and / or groups.

[0014] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as that generally understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries are further interpreted to have the meaning consistent with the relevant technical literature and the content currently disclosed, and are not interpreted in their ideal or highly formal sense unless otherwise defined.

[0015] The following describes in detail a high-strength hot-dip galvanized steel sheet with excellent plating quality, which is one aspect of the present invention completed through the inventor's research. It should be noted that, unless otherwise specified, the weight percentages of each element in this invention refer to the weight percentage. Furthermore, unless otherwise specified, the ratio of crystals and structures is based on area, and unless otherwise specified, the gas content is based on volume.

[0016] The inventors of this invention focused on the fact that liquid metal embrittlement (LME) that occurs during welding is caused by microcracks that originate from the surface of steel sheets. They researched means of suppressing these microcracks on the surface and found that it is necessary to appropriately control the oxide composition, which led to the present invention.

[0017] Normally, high-strength steel can contain large amounts of elements such as C, Mn, Si, Cr, Mo, and V to ensure the hardening ability and austenite stability of the steel. However, these elements play a role in increasing the steel's susceptibility to cracking. Therefore, steel containing large amounts of these elements is prone to the development of microcracks, which ultimately leads to liquid metal embrittlement during welding. According to our research, the larger the difference between the average Mn / Si value of oxides present in the surface layer (surface oxides) up to a depth of 15 nm from the surface of the base steel sheet (in the case of hot-dip galvanized steel sheets) and the average Mn / Si value of oxides present in the region from a depth of 50 nm to 100 nm (internal oxides) (i.e., the value obtained by subtracting the average Mn / Si value of internal oxides from the average Mn / Si value of surface oxides), the less likely microcracks are to occur. Here, Mn and Si for each oxide refer to the content (weight %) of Mn and Si components in the oxide as measured by EDS, and the average Mn / Si value refers to the average of the Mn / Si values ​​measured for each oxide.

[0018] Therefore, in one embodiment of the present invention, the (average value of Mn / Si values ​​of the surface oxide - average value of Mn / Si values ​​of the internal oxide) (hereinafter also referred to as the "Mn / Si difference") is limited to 0.5 or more. This means that the average value of Mn / Si values ​​of the surface oxide is at least 0.5 greater than the average value of Mn / Si values ​​of the internal oxide, and that the Mn content is higher than the Si content in the surface oxide. By controlling the oxide composition in this way, the hardness of the surface layer can be controlled to be soft, and it is possible to prevent the development of microcracks even when stress is applied during plastic processing.

[0019] In another implementation of the present invention, the Mn / Si difference can be 0.8 or greater, and in yet another implementation, the Mn / Si difference can be 0.9 or greater or 1.2 or greater. Since a larger Mn / Si difference is advantageous, there is no need to specifically set an upper limit for the Mn / Si difference. However, considering the values ​​that are usually formed, the Mn / Si difference can also be 1.5 or less.

[0020] The above-mentioned Mn / Si difference can be achieved by increasing the average value of the Mn / Si value in the surface layer. In one implementation of the present invention, the average value of the Mn / Si value in the surface layer oxide can be limited to 1.5 or more. In another implementation, the average value of the Mn / Si value in the surface layer can be set to 1.7 or more. In yet another implementation, the average value of the Mn / Si value in the surface layer can be set to 1.9 or more. Since a larger Mn / Si value in the surface layer oxide is advantageous, there is no particular upper limit, but it can be set to a value of 2.2 or less.

[0021] Furthermore, as another method to increase the above-mentioned Mn / Si difference, a method can be used to keep the average value of the Mn / Si value of the internal oxide low. In one implementation of the present invention, the average value of the Mn / Si value of the internal oxide may be 1.0 or less, in another implementation, the average value of the Mn / Si value of the internal oxide may be 0.9 or less, and in yet another implementation, the average value of the Mn / Si value of the internal oxide may be 0.8 or less or 0.7 or less. Since a lower average value of the Mn / Si value of the internal oxide is advantageous, there is no particular lower limit, but it can be set to a value of 0.4 or higher.

[0022] In one embodiment of the present invention, the above Mn / Si difference can be a value obtained at the center of the steel plate in the width direction. However, it is not necessarily limited to this position, and for example, since spot weldability at the edge portion in the width direction may be a problem in many other cases, a value obtained at the edge portion in the width direction can also be used. Here, the edge portion in the width direction refers to both endpoints of the cross-section obtained by cutting the steel plate in the width direction, but if there is a problem with the integrity of the test piece, such as contamination occurring at the above point, it can refer to a point 1 mm inward in the width direction from the endpoint.

[0023] In the present invention, the steel plate targeted is not limited in type as long as it is a high-strength steel plate with a strength of 780 MPa or more. However, although it is not necessarily limited to this, the steel plate targeted in the present invention can have a composition containing, by weight ratio, C: 0.05 to 1.5%, Si: 2.0% or less, Mn: 1.0 to 20%, 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 although not listed above, it is not excluded to further contain elements that can be contained in steel within a total range of 1.0% or less. In the present invention, the content of each component element is expressed based on weight unless otherwise specified. The above-described 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).

[0024] However, in some embodiments of the present invention, as the above high-strength steel plate, TRIP steel, DP steel, CP steel, etc. can be targeted. Each steel can have the following composition.

[0025] Steel composition 1: C: 0.05 to 0.30% (preferably 0.10 to 0.25%), Si: 0.5 to 2.5% (preferably 1.0 to 1.8%), Mn: 1.5 to 4.0% (preferably 2.0 to 3.0%), S-Al: 1.0% 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 from 0.001 to 0.05%), Sb + Sn + Bi: 0.05% or less, N: 0.01% or less, and the balance Fe and unavoidable impurities. In some cases, although not listed above, it can further contain elements that can be contained in steel within a total range of 1.0% or less.

[0026] Although not necessarily limited thereto, examples of the steel of the above steel composition 1 include TRIP steel or XF steel, and each can have a tensile strength of 900 MPa or more.

[0027] 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, although not listed above, elements that can be contained in the steel can be further contained up to a total of 1.0% or less.

[0028] Although not necessarily limited thereto, examples of the steel of the above steel composition 2 include TRIP steel and XF, etc., and can have a tensile strength of 1000 MPa or more.

[0029] According to one embodiment of the present invention, the surface of the above steel plate may contain one or more plating layers, and the above plating layer may be a zinc - based plating layer including GI (Galvanized), ZM (Zinc - Magnesium), or GA (Galva - annealed), etc. In the present invention, as described above, since the oxygen concentration of the surface layer portion is appropriately controlled, even when a zinc - based plating layer is formed on the surface of the steel plate, the problem of liquid metal embrittlement generated during spot welding can be suppressed.

[0030] When the above zinc-based plating layer is a GA layer, the degree of alloying can be controlled to 8-13%, preferably 10-12%. If the degree of alloying is insufficient, zinc in the zinc 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.

[0031] Furthermore, the plating amount of the zinc-based plating layer mentioned above is 30-70 g / m². 2 This is also acceptable. If the amount of plating is excessively low, it will be difficult to obtain sufficient corrosion resistance, while if the amount of plating is excessively high, it may lead to increased manufacturing costs and problems with liquid metal embrittlement. Therefore, it should be controlled within the range described above. A more preferable range for the amount of plating is 40-60 g / m². 2 It is possible.

[0032] 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.

[0033] First, a hot-rolled steel sheet can be manufactured by reheating a steel slab of the above composition, performing rough rolling and finish rolling, followed by hot rolling, and then winding it after ROT (Run Out Table) cooling. There are no particular restrictions on the hot rolling conditions such as ROT cooling, but in one embodiment of the present invention, the slab reheating temperature, the start and end temperatures of finish rolling, and the winding temperature can be limited as follows.

[0034] Slab reheating temperature: 950~1,300℃ Slab reheating is performed before hot rolling to ensure rollability by heating the material. During slab reheating, the surface layer of the slab combines with oxygen in the furnace to form scale, which is an oxide. If the heating temperature is sufficiently high, the composition of the surface layer and internal oxides of the steel sheet can be controlled to an appropriate range through interaction with the process described later. However, if the heating temperature is too high, the crystal grains may grow excessively, potentially degrading the material properties of the steel sheet; therefore, the slab is reheated within the temperature range described above.

[0035] Finish rolling start temperature: 900~1,150℃ 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 1,150°C. Also, if the finish rolling start temperature is below 900°C, the stiffness of the bar may increase as the temperature decreases, significantly reducing its hot-rollability. Therefore, the finish rolling start temperature can be restricted within the range described above.

[0036] Finishing rolling completion temperature: 850~1,050℃ 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 range described above.

[0037] Winding temperature: 590~750℃ Hot-rolled steel sheets are then wound into coils for storage, but the wound sheets undergo a slow cooling process. This process removes oxidizing 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 at a temperature lower than necessary to remove these elements, making it difficult to achieve sufficient effect. Conversely, if the winding temperature is too high, it may be difficult to ensure the same material properties as the tensile strength of the steel sheet, potentially leading to a deterioration in plating quality.

[0038] Heating of the edges of hot-rolled coils: Performed at 600-800°C for 5-24 hours. In one embodiment of the present invention, the edges of a hot-rolled coil can be heated to increase the average Mn / Si value of the surface oxide at the edge and decrease the average Mn / Si value of the internal oxide at a depth of 100 nm or more inside the steel sheet. Heating the edges of a hot-rolled coil means heating both ends in the width direction of the wound coil, i.e., the edges, and heating the edges preferentially heats them to a temperature suitable for oxidation. That is, the inside of the wound coil is kept at a high temperature, but the edges cool relatively quickly, and as a result, the time spent at a temperature suitable for internal oxidation is shorter at the edges than at the center. Therefore, the removal of oxidizing elements is not as active at the edges compared to the center in the width direction. Heating the edges can be used as one method for removing oxidizing elements at the edges.

[0039] In other words, when heating the edges, unlike when cooling after winding, the edges are preferentially heated, which allows the temperature of the edges in the width direction to be suitably maintained for internal oxidation, resulting in an increase in the thickness of the internal oxidation layer at the edges. For this to occur, the heating temperature of the edges must be 600°C or higher (based on the temperature of the steel sheet edges). However, if the temperature is too high, the tensile strength of the steel sheet decreases, excessive scale may form at the edges during heating, or a porous, highly oxidized scale (hematite) may form, potentially resulting in a poor surface condition after pickling. Therefore, the heating temperature of the edges can be 800°C or lower. Also, the Mn / Si ratio may increase excessively in both the surface and interior, and the difference may not meet the value specified in this invention. A more preferable heating temperature for the edges is 600-750°C. According to one embodiment of this invention, the heating of the edges can be performed in a heat treatment furnace.

[0040] Furthermore, in order to eliminate the non-uniformity between the average Mn / Si value of the surface oxide between the edge and the center in the width direction that occurs during winding, and the average Mn / Si value of the internal oxide at a depth of 100 nm or more inside the steel sheet, the heating time for the edge portion must be 5 hours or more. However, if the heating time for the edge portion is too long, the tensile strength of the steel sheet may decrease, excessive scale may form, or conversely, the average Mn / Si value of the surface and internal oxide of the steel sheet at the edge portion may become excessively high. Therefore, the heating time for the edge portion can be 24 hours or less.

[0041] According to one embodiment of the present invention, the heating of the edge portion 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 the higher the oxygen partial pressure, the higher the Mn / Si ratio in the surface layer of the steel plate can be increased. Although not necessarily limited to this, in one embodiment of the present invention, the atmosphere can be controlled to contain 1-2% oxygen in a nitrogen atmosphere by adjusting the air-fuel ratio. Since the oxygen fraction can be controlled by adjusting the air-fuel ratio without any particular difficulty by a person with ordinary skill in the art to which the present invention belongs, this will not be explained separately.

[0042] Pickling treatment: Performed at a feed speed of 180-250 mpm. The hot-rolled steel sheets that have undergone the process described above 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 hydrochloric acid bath is in the range of 10 to 30 volume percent, and the pickling speed is 180 to 250 mpm. If the pickling speed exceeds 250 mpm, the surface scale of the hot-rolled steel sheet 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, so it is necessary to perform the pickling at 180 mpm or higher.

[0043] Cold rolling: Reduction ratio 35-60% After pickling, cold rolling is performed. 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%, it becomes difficult to obtain an appropriate average Mn / Si value for the surface oxide and the internal oxide at a depth of 100 nm or more in the steel sheet after annealing.

[0044] Following the cold rolling process described above, a steel sheet annealing process may follow. During the annealing process, the average Mn / Si value of the surface oxides and the average Mn / Si value of the internal oxides at a depth of 100 nm or more can differ significantly. Therefore, in one embodiment of the present invention, the annealing process can be controlled under conditions that appropriately control the average Mn / Si value of the surface oxides and the average Mn / Si value of the internal oxides at a depth of 100 nm or more. Of these, the sheet feeding speed and the dew point in the annealing furnace can be controlled under the following conditions.

[0045] Plate feeding speed during annealing: 40-130 mpm To ensure sufficient productivity, the feed speed of the cold-rolled steel sheet must be 40 mpm or higher. If the feed speed is too slow, the grain size may grow excessively, potentially leading to a decrease in strength. Conversely, if the feed speed is too fast, the time the sheet is held at high temperatures will be shortened, potentially reducing the amount of austenite and the resulting cooling phases of martensite and bainite. Therefore, since this can be disadvantageous from the standpoint of ensuring material quality, in one embodiment of the present invention, the upper limit of the feed speed can be set to 130 mpm.

[0046] Control of the uniform temperature and dew point of the annealing furnace: Controlled within the range of -10 to 30°C at 650 to 900°C. To control the Mn / Si ratio in the internal and surface oxide layers to an appropriate range, it is advantageous to control the dew point within the uniform zone of 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 form 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 dew point control may be 650°C or higher, which is the temperature at which a sufficient internal oxidation effect is achieved. However, if the temperature is too high, surface oxides such as Si are formed, not only hindering oxygen diffusion into the interior, but excessive austenite formation during heating in the uniform zone reduces the carbon diffusion rate, thereby decreasing the internal oxidation level. This can lead to excessive growth of the austenite size in the uniform zone, causing material softening. Furthermore, it can 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.

[0047] In this case, the dew point can be adjusted by introducing humid nitrogen (N2 + H2O) containing water vapor into the annealing furnace.

[0048] Hydrogen concentration in the annealing furnace (uniform zone): 5-10% by volume The atmosphere inside the sonic zone of the annealing furnace is maintained by adding 5-10 volume percent hydrogen to nitrogen gas to create a reducing atmosphere. If the hydrogen concentration in the annealing furnace is less than 5 volume percent, the reduction capacity decreases, leading to excessive surface oxide formation, which reduces surface quality and plating adhesion, and lowers resistance to LME. While no particular problems occur at high hydrogen concentrations, the hydrogen concentration is limited due to the increased cost associated with increased hydrogen gas usage and the risk of furnace explosion due to the high hydrogen concentration.

[0049] Hydrogen concentration in the rapidly cooled zone: 25-80% by volume In annealing furnaces, hydrogen and nitrogen are typically used as cooling refrigerants in the rapid cooling zone. To ensure an appropriate cooling rate and suppress surface oxidation by the refrigerant during cooling, an appropriate hydrogen concentration must be maintained. If the hydrogen concentration is less than 25%, the reducing hydrogen gas concentration is insufficient, and oxidation of oxidizing elements such as Si occurs on the surface of the steel plate during cooling, degrading the plating wettability and potentially lowering the Mn / Si ratio in the surface layer. Furthermore, if the hydrogen concentration is low, the cooling capacity decreases, making it difficult to secure an appropriate phase fraction of the cooled secondary phase, and problems arise in securing material quality. If the hydrogen concentration exceeds 80%, the cost of hydrogen production may increase, and the risk of hydrogen explosion during high-hydrogen operations increases, so the upper limit is restricted to 80%. In one implementation of the present invention, the cooling rate of the rapid cooling zone can be set to 5-30°C / s, and in another implementation, the cooling rate of the rapid cooling zone can be set to 10-30°C / s.

[0050] The steel sheets annealed through this process are immediately immersed in a plating bath and hot-dip galvanized.

[0051] 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.

[0052] Draw-in temperature for plating bath steel sheets: 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, an Fe-Zn alloy phase, at the interface, which reduces the adhesion of the plating layer. This also leads to excessive leaching of Fe element from the steel sheet in the plating bath, resulting in the formation of dross in the plating bath.

[0053] 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 Al concentration to 0.10-0.15% for GA, 0.2-0.25% for GI, and 0.7-5.7% by weight of Al and 0.7-5.7% by weight of Mg 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 plating surface. In this invention, the amount of plating deposited is not particularly limited and can be adjusted as appropriate according to the quality required by the customer.

[0054] 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.

[0055] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are for illustrative purposes only and do not 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.

[0056] Skin pass reduction ratio: 0~0.35 After hot-dip galvanizing or alloying, skin pass rolling (SPM) is performed to adjust the yield strength and surface roughness of the steel sheet. When skin pass rolling is performed, the yield strength increases in proportion to the reduction ratio due to work hardening by cold rolling, and the surface roughness is increased as the roughness of the skin pass rolling roll surface is transferred to the steel sheet. Therefore, skin pass rolling has the advantage of ensuring appropriate yield strength and surface roughness by adjusting the reduction ratio, ensuring stability of tensile material, and ensuring sealer adhesion by increasing surface roughness. If adjustment of yield strength and surface roughness after hot-dip galvanizing is not necessary, skin pass rolling may not be performed. Furthermore, if the reduction ratio exceeds 0.35, the yield strength may increase excessively, potentially exceeding the target material, and the surface roughness may increase excessively, potentially leading to poor degreasing due to capillary action caused by the roughness after oiling.

[0057] 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, and the remaining components are expressed in weight percent) is reheated to 1,210°C, and hot-rolled with finish rolling start and end temperatures of 945°C and 870°C, respectively. After that, the edges of the hot-rolled coil are heated in a nitrogen atmosphere containing 1.5 volume% or less of oxygen, and then hydrochloric acid is added to a concentration of 12. The sheets were pickled in a 3% hydrochloric acid bath, cold-rolled to a reduction ratio of 53%, heated in the homogenized zone of an annealing furnace, and then rapidly cooled in a hydrogen atmosphere of 60 vol% - remaining nitrogen (cooling rate: 15°C / s). Immediately afterward, the GA sheets were immersed in a plating bath containing 0.13% Al, the GI sheets in a plating bath containing 0.24 wt% Al, and the ZM sheets in a zinc-based plating bath containing 1.75 wt% Al and 1.55 wt% Mg. The amount of coating was then adjusted using an air knife, and the sheets were cooled to perform hot-dip galvanizing. In the case of GA, the obtained hot-dip galvanized steel sheets were subjected to alloying (GA) heat treatment as needed within a preferred range of 480-560°C to finally obtain alloyed hot-dip galvanized steel sheets. The obtained hot-dip galvanized steel sheets or alloyed hot-dip galvanized steel sheets were subjected to skin-pass rolling at a reduction ratio of 8%.

[0058] 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.

[0059] [Table 1]

[0060] [Table 2]

[0061] 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. Unless otherwise specified, all test results in Table 3 were measured using samples taken at a point 1 mm away from the edge of the steel sheet. When the sample size was large, a sample of a predetermined size was measured starting from a point 1 mm away from the edge. Specifically, spot welding was performed by cutting the steel sheet in the width direction and welding along the cut ends. After applying the spot welding current twice, a hold time of one cycle was maintained. Spot welding was performed in a triple layer of different materials. Evaluation material - Evaluation material - GA 980DP Spot welding was performed on stacked 1.4t materials. 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 spot welding target material was measured. After measuring the upper limit current, spot welding was performed 8 times at currents 0.5 and 1.0kA lower than the upper limit current, and the cross-section of the spot weld was precisely machined by electrical discharge machining, then epoxy mounted and polished, and the length of the crack was measured with an optical microscope. The magnification for observation with the optical microscope was specified as 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. If B-type cracks occurring at the shoulder of the spot weld were 100μm or less, and if C-type cracks were not observed, it was determined that the work was good.

[0062] The average weight ratio of Mn / Si in the surface oxides and internal oxides at a depth of 100 nm or more within the steel sheet was determined by processing the steel sheet with a Focused Ion Beam (FIB), and then performing spot analysis at each location more than 10 times using TEM EDS on the surface oxides and the internal oxides at a depth of 50-100 nm. The average value of the weight percentages of Mn and Si at each location, measured as a weight ratio, was calculated as Mn / Si and then used to determine the average value of the results.

[0063] Tensile strength was measured by tensile testing using C-direction samples prepared according to JIS-5 standard. The degree of alloying and plating adhesion were measured using a wet dissolution method with hydrochloric acid solution.

[0064] To test the sealer adhesion, a D-type structural adhesive for automotive use was applied to the plated surface, and then the steel sheet was bent at a 90-degree angle to check whether the plating peeled off. GI and ZM steel sheets underwent sealer adhesion testing.

[0065] For the powdering test, the plated material was bent at a 90-degree angle, tape was applied to the bent section, and then the tape was removed. The length of the peeled-off plating layer onto the tape was then checked. If the length of the peeled-off plating layer exceeded 10 mm, it was considered defective.

[0066] For the Flaking process, after processing the material into an inverted "U" shape, it was checked whether the plating layer peeled off the processed area. Surface quality was checked by visually inspecting the steel plate for defects such as unplated areas, and if defects such as unplated areas were found during the visual inspection, the material was judged to be defective.

[0067] [Table 3]

[0068] Examples of Inventions 1, 2, 3, 4, 5, 6, 7, 8 , and 9 The steel composition met the range presented in this invention, and the manufacturing method also met the range of this invention. The tensile strength, plating quality, plating adhesion, and spot weld LME crack length were also good.

[0069] In Comparative Examples 9, 16, and 17, even though the manufacturing method met the scope presented in the present invention, the steel composition exceeded the scope presented in the present invention. In Comparative Examples 5, 13, and 20, Mn and Si exceeded the scope presented in the present invention, respectively. Even when the dew point of the annealing furnace was increased, the amount of surface oxidation in the surface layer became excessive, the average value of the Mn / Si value of the surface oxide of the steel sheet fell outside the standard, and the average value of the weight ratio of Mn / Si value to the internal oxide also fell outside the standard, resulting in the LME crack length not meeting the standard. Furthermore, due to excessive surface oxidation, an alloying suppression layer was not sufficiently formed at the interface between the plating layer and the base iron, resulting in unplated areas on the surface, poor surface quality, and SBT peeling, resulting in poor plating adhesion.

[0070] In Comparative Examples 2 and 7, the coiling temperature during the hot rolling process did not meet the range presented in the present invention. In Comparative Example 2, the hot rolling coiling temperature was lower than the range presented in the present invention. As a result, the average value of the Mn / Si value of the surface oxide of the steel sheet fell outside the standard, and the average value of the weight ratio of the Mn / Si value to the internal oxide also fell outside the standard, resulting in LME cracks not meeting the standard. In Comparative Example 7, the hot rolling coiling temperature exceeded the range presented in the present invention. Although the LME properties were good, excessive hot rolling scale was generated, and the scale was not completely removed during pickling, resulting in unplated areas and poor surface quality. As a result, plating peeling occurred during flaking evaluation, and powdering properties were poor due to non-uniform alloying. In addition, the hot rolling coiling temperature was excessively high, causing softening of the hot-rolled material, which did not recover even after annealing, resulting in insufficient tensile strength.

[0071] In Comparative Example 15, the heating temperature of the heat treatment furnace exceeded the range presented in the present invention, causing peroxidation at the edges during the heat treatment process, resulting in the formation of red hematite on the surface scale, which became excessively thick. After hot rolling, the edges were excessively pickled during the pickling process, leading to increased surface roughness. After plating, the surface shape became uneven, resulting in color unevenness defects where the surface color differed from the center, and the powdering properties were poor due to uneven alloying. Furthermore, the average weight ratio of the Mn / Si value increased in both the surface and interior, and the difference between the two values ​​was less than 0.5, thus failing to meet the LME standard.

[0072] Comparative Example 8 is a case where the heating temperature of the heat treatment furnace for heating the edge portion is lower than the range specified in the present invention. As a result, the Mn / Si ratio of the surface layer and the internal oxide could not be controlled, and consequently, it was deemed defective as it did not meet the criteria for spot welding LME crack evaluation.

[0073] Comparative Example 13 is a case where the heating temperature for the heat treatment of the edge portion met the range of the present invention, but the heating time exceeded it. As a result, peroxidation occurred at the edge portion during the heat treatment process, and the surface scale formed red hematite, which became excessively deep. After hot rolling, the edge portion was excessively pickled during the pickling process, resulting in high surface roughness. After plating, the surface shape became uneven, color unevenness defects occurred where the surface color differed from the central part, and powdering properties were poor due to uneven alloying. In addition, the Mn / Si ratio increased in both the surface and interior, and the difference became less than 0.5, resulting in poor weldability.

[0074] Comparative Example 5 is a case where the heating temperature of the heat treatment furnace meets the range of the present invention, but the heating time of the heat treatment furnace is short. As a result, the oxide composition could not be controlled within the range specified by the present invention, and the criteria for spot welding LME crack evaluation could not be met.

[0075] Comparative Examples 1 and 14 did not meet the pickling rate range presented in the present invention. In Comparative Example 1, the pickling rate was lower than the standard, and the excessively long pickling time caused the surface layer to dissolve and be removed deeply. As a result, the average Mn / Si value of the surface oxide of the steel sheet deviated from the standard, and the average weight ratio of Mn / Si value to internal oxide also deviated from the standard, causing LME cracks. The grain boundaries of the hot-rolled internal oxidation were corroded by the acid solution, degrading the grain boundary integrity and causing delamination during the flaking test. In Comparative Example 14, the pickling rate was higher than the standard, and the hot-rolled scale was not completely removed from the surface of the steel sheet, remaining and degrading the surface quality. The GA alloying was also uneven, resulting in reduced powdering properties.

[0076] In Comparative Example 10, the uniform temperature in the annealing furnace exceeded the range presented by the present invention. The annealing temperature was excessive, increasing the amount of external oxidation and preventing the formation of sufficient internal oxidation. As a result, the average value of the Mn / Si ratio of the surface oxides of the steel sheet fell outside the standard, and the average value of the weight ratio of Mn / Si to internal oxides also fell outside the standard, resulting in LME cracks not meeting the standard and poor spot weldability. Furthermore, excessive austenite formation and growth occurred in the uniform temperature zone, causing the material to fail to meet the standard and be of poor quality.

[0077] Comparative Example 12 is a case where the uniform temperature in the annealing furnace is lower than the range presented in the present invention. Due to the low annealing temperature, the oxidation reaction between the steam and the steel sheet was insufficient, the average value of the Mn / Si value of the surface oxide of the steel sheet fell outside the standard, and the average value of the weight ratio of the Mn / Si value to the internal oxide also fell outside the standard, resulting in LME cracks not meeting the standard and poor spot weldability. In addition, recrystallization did not occur sufficiently during annealing, and the target microstructure was not formed, so the material did not meet the standard and was of poor quality.

[0078] Comparative Example 6 was manufactured in which the furnace dew point during annealing was lower than the range presented in the present invention. Even though a sufficient internal oxide layer was generated across the entire width during the hot rolling heating process, the dew point during the annealing process after cold rolling was not sufficiently high. As a result, the average value of the Mn / Si value of the surface oxide of the steel sheet fell outside the standard, and the average value of the weight ratio of the Mn / Si value to the internal oxide also fell outside the standard, causing the length of the spot weld LME crack to not meet the standard. In the case of GI material, the dew point was low, and sufficient internal oxidation could not be generated, resulting in excessive surface oxide generation, poor surface quality, and SBT delamination.

[0079] In Comparative Example 3, the hydrogen concentration in the annealing furnace was less than 5 Vol%, indicating an insufficient composition of the reducing atmosphere within the furnace. Excessive surface oxide formation resulted in unplated areas, degrading surface quality, SBT plating peeling, and an inability to control the oxide composition, leading to LME cracks not meeting the standards.

[0080] Comparative Examples 4 and 11 show that the sheet feed speed in the annealing furnace falls outside the range specified by the present invention. In Comparative Example 4, the sheet feed speed was higher than the range specified by the present invention, and the oxide composition was not controlled within the range defined by the present invention. As a result, LME cracks did not meet the criteria, and sufficient recrystallization time could not be secured in the annealing furnace, resulting in poor material quality. In Comparative Example 11, the sheet feed speed was lower than the range specified by the present invention. Although spot weldability was good due to meeting the oxide composition conditions, there was an excess of oxide on the surface, resulting in insufficient surface quality of the plated steel sheet and the occurrence of powdering.

[0081] Therefore, we were able to confirm the advantageous effects of the present invention.

Claims

1. The raw material consists of steel sheets with the following composition in weight percent: C: 0.05-1.5%, Si: 2.0% or less, Mn: 1.0-20%, 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, N: 0.01% or less, the remainder being Fe, and unavoidable impurities. The above includes a hot-dip galvanized layer formed on the surface of the base steel sheet, The value obtained by subtracting the average Mn / Si value of the internal oxide located at a depth of 50 to 100 nm from the interface from the average Mn / Si value of the surface oxide located in the surface region, which is the region from the interface of the hot-dip galvanized layer to a depth of 15 nm, is 0.5 or greater. The aforementioned base steel sheet is a hot-dip galvanized steel sheet further containing elements that may be present in steel but are not listed above, in a total amount of 1.0% by weight or less. Here, Mn and Si for each oxide refer to the content (weight %) of Mn and Si components in the oxide as measured by EDS, and the average Mn / Si value refers to the average of the Mn / Si values ​​measured for each oxide.

2. The hot-dip galvanized steel sheet according to claim 1, wherein the difference in the above-mentioned average values ​​is 0.8 or more.

3. The hot-dip galvanized steel sheet according to claim 1, wherein the average value of the Mn / Si value of the surface oxide is 1.5 or more.

4. The hot-dip galvanized steel sheet according to claim 1, wherein the average value of the Mn / Si value of the surface oxide is 1.7 or more.

5. The hot-dip galvanized steel sheet according to claim 1, wherein the average value of the Mn / Si value of the internal oxide is 1.0 or less.

6. The hot-dip galvanized steel sheet according to claim 1, wherein the average value of the Mn / Si value of the internal oxide is 0.9 or less.

7. A method for producing a hot-dip galvanized steel sheet according to claim 1, The process involves providing a steel slab composed of, by weight percent, C: 0.05-1.5%, Si: 2.0% or less, Mn: 1.0-20%, 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, N: 0.01% or less, with the remainder being Fe and unavoidable impurities. The steps include reheating the steel slab to a temperature of 950 to 1300°C, The process involves hot-rolling the reheated steel slab at a finish rolling start temperature of 900 to 1,150°C and a finish rolling end temperature of 850 to 1,050°C to obtain a steel sheet, The steps include winding the steel plate at a temperature range of 590 to 750°C, The process involves pickling the steel plate at a feed speed of 180 to 250 mph, The steps include cold rolling the steel sheet at a reduction ratio of 35-60%, The uniform tropical temperature and dew point temperature are 650 to 900°C and -10 to +30°C, respectively, and the atmospheric gas is 5 to 10 volume percent H ２ The process involves heating the cold-rolled steel sheet under conditions of humid nitrogen containing and then recrystallizing it by cooling it in a rapid cooling zone at a cooling rate of 5 to 30°C / s, The process involves drawing in the steel plate and immersing it in a molten plating bath at a temperature in the range of 420 to 500°C to perform molten plating, Includes, During the aforementioned recrystallization annealing, the sheet passing speed is 40 to 130 mph. During the aforementioned recrystallization annealing, the hydrogen concentration in the rapidly cooled zone is 25 to 80% by volume. The edges of the rolled steel plate are heated at 600 to 800°C for 5 to 24 hours. The method wherein the steel slab further contains elements that may be present in the steel but are not listed above, in a total amount of 1.0% by weight or less.

8. The method according to claim 7, further comprising the step of alloying the molten-plated steel sheet at a temperature of 480 to 560°C.

9. The method according to claim 8, wherein the molten plating bath contains 0.10 to 0.15% by weight of Al.

10. The method according to claim 7, wherein the molten plating bath contains 0.2 to 0.25% by weight of Al.

11. The method according to claim 7, wherein the molten plating bath contains 0.7 to 5.7% by weight of Al and 0.7 to 5.7% by weight of Mg.

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