High-strength hot-dip galvanized steel sheet having excellent surface quality and 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 manufacturing the same. According to one aspect of the present invention, the hot-dip galvanized steel sheet includes a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, and the difference between the average of Mn / Si values of oxides present in a surface layer portion of a region having a depth of 15 nm from the interface of the hot-dip galvanized layer and the base steel sheet and the average of Mn / Si values of internal oxides present at a position having a depth of 50-100 nm from the interface can be 0.5 or more. Herein, Mn and Si of each oxide refer to the content (wt%) of Mn and Si components in the oxides measured by EDS, and the average of Mn / Si values refers to measuring the Mn / Si values of each oxide and taking the average.

Description

Technical Field

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

[0002] Due to environmental pollution and other issues, regulations on vehicle emissions and fuel efficiency are becoming increasingly stringent. Therefore, the demand for reducing fuel consumption through lightweight automotive steel sheets is growing, leading to the development and commercialization of various high-strength steel sheets with high strength per unit thickness.

[0003] High-strength steel generally refers to steel with a strength of 490 MPa or higher, but it is not necessarily limited to this. Transformation-induced plasticity (TRIP) steel, twin-induced plasticity (TWIP) steel, dual-phase (DP) steel, and complex-phase (CP) steel can all be classified as high-strength steel.

[0004] In addition, to ensure corrosion resistance, automotive steel is supplied in the form of coated steel sheets with a coating on the surface. Among them, galvanized steel sheet (GI), high corrosion resistant coated steel sheet (ZM), or alloyed galvanized steel sheet (GA) have high corrosion resistance by utilizing the sacrificial corrosion protection properties of zinc, and are therefore widely used as automotive materials.

[0005] However, galvanizing high-strength steel sheets leads to poor weldability. Specifically, high-strength steel exhibits both high tensile strength and high yield strength, making it difficult to eliminate tensile stress generated during welding through plastic deformation. This increases the likelihood of microcracks forming on the surface. When welding high-strength galvanized steel sheets, the low-melting-point zinc penetrates into these microcracks, resulting in a phenomenon known as liquid metal embrittlement (LME). This leads to steel sheet failure in a fatigue environment, significantly hindering the achievement of higher strength levels. Summary of the Invention

[0006] Technical problems to be solved

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

[0008] The technical problems addressed by this invention are not limited to those described above. Those skilled in the art can readily understand the additional technical problems of this invention based on the full text of this specification.

[0009] Technical solution

[0010] According to one aspect of the present invention, a hot-dip galvanized steel sheet includes a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet. The average value of Mn / Si values ​​of the surface oxides present in the surface portion of the region from the interface between the hot-dip galvanized layer and the base steel sheet to a depth of 15 nm toward the base steel sheet, minus the average value of Mn / Si values ​​of the internal oxides present from the interface to a depth of 50-100 nm, can be 0.5 or more.

[0011] Wherein, Mn and Si in each oxide refer to the content (wt%) of Mn and Si in the oxide measured by EDS, and the average value of Mn / Si refers to the average value of the measured Mn / Si values ​​of each oxide.

[0012] A method for manufacturing hot-dip galvanized steel sheet according to another aspect of the present invention comprises the following steps: providing a steel billet; reheating the billet to a temperature of 950-1300°C; hot-rolling the reheated billet at a finishing rolling start temperature of 900-1150°C and a finishing rolling finish temperature of 850-1050°C to obtain a steel sheet; coiling the steel sheet at a temperature range of 590-750°C; and pickling the steel sheet at a plate throughput speed of 180-250 m / min (mpm). The steel sheet is cold-rolled at a reduction rate of 35-60%; the cold-rolled steel sheet is heated under wet nitrogen conditions with a soaking zone temperature of 650-900℃ and a dew point temperature of -10℃ to +30℃, and the atmosphere gas contains 5-10% by volume of H2, and then recrystallized and annealed by cooling at a cooling rate of 5-30℃ / second in a rapid cooling zone; the steel sheet is then immersed in a molten plating bath for hot-dip plating within an introduction temperature range of 420-500℃.

[0013] Beneficial effects

[0014] As described above, by controlling the difference between the Mn / Si values ​​of the oxides on the surface of the base steel plate and the Mn / Si values ​​of the oxides inside the plate to a large level, the present invention can suppress the generation of microcracks in the surface layer, thus significantly improving the spot weldability.

[0015] Best practice

[0016] The technical terms used herein are for describing particular embodiments only and are not intended to limit the invention. Unless the context clearly indicates otherwise, the singular forms used herein also include the plural forms.

[0017] The terms "comprising" or "including" as used in this specification are used to specifically describe a particular characteristic, region, integer, step, operation, element, and / or ingredient, and do not exclude the presence or addition of other particular characteristics, regions, integers, steps, operations, elements, ingredients, and / or groups.

[0018] Unless otherwise defined, all terms used herein, including technical and scientific terms, shall have the same meaning as commonly understood by those skilled in the art. Terms as defined in commonly used dictionaries shall be further interpreted as meaning consistent with the relevant technical literature and currently disclosed content, and shall not be construed as having an ideal or highly formal meaning unless otherwise defined.

[0019] The following is a detailed description of the high-strength hot-dip galvanized steel sheet with excellent coating quality according to one aspect of the present invention, which was developed by the inventors through research. In this invention, it should be noted that, unless otherwise specified, the content of each element is expressed as weight percent. Furthermore, unless otherwise specified, the proportion of crystals or structures is based on area, and unless otherwise specified, the gas content is based on volume.

[0020] The inventors of this invention noticed that the cause of liquid metal embrittlement (LME) during welding is microcracks generated on the surface of the steel plate. They studied methods to suppress microcracks on the surface and found that it was necessary to properly control the composition of the oxides, thus completing this invention.

[0021] Typically, in the case of high-strength steel, to ensure hardenability or austenite stability, large amounts of elements such as C, Mn, SiCr, Mo, and V are included. These elements increase the steel's susceptibility to cracking. Therefore, steels containing large amounts of these elements are prone to microcracks, which can ultimately lead to embrittlement of the liquid metal during welding. According to the inventors' research, the greater the difference between the average Mn / Si value of the oxides present in the surface layer (the interface between the coating and the base steel sheet in hot-dip galvanized steel) to a depth of 15 nm and the average Mn / Si value of the oxides present in the region between 50-100 nm (i.e., the average Mn / Si value of the surface oxides minus the average Mn / Si value of the internal oxides), the less likely microcracks are to occur. Wherein, Mn and Si in each oxide refer to the content (wt%) of Mn and Si in the oxide measured by EDS, and the average value of Mn / Si refers to the average of the measured Mn / Si values ​​of each oxide.

[0022] Therefore, in one specific embodiment of the present invention, the average Mn / Si value of the oxide in the surface layer minus the average Mn / Si value of the oxide in the interior layer (hereinafter also referred to as "difference between Mn / Si") is limited to 0.5 or more. This means that the average Mn / Si value of the oxide in the surface layer is at least 0.5 greater than the average Mn / Si value of the oxide in the interior layer, and that the content of Mn relative to Si in the oxide in the surface layer is increased. When the composition of the oxide is controlled as described above, the hardness of the surface layer can be controlled to be soft, thus preventing the development of microcracks even when stress is applied during plastic processing.

[0023] In another specific embodiment of the present invention, the difference between Mn and Si can be set to 0.8 or higher. In another specific embodiment, the difference between Mn and Si can also be set to 0.9 or higher or 1.2 or higher. A larger difference between Mn and Si is more advantageous, therefore there is no need to specifically set an upper limit for the difference between Mn and Si. However, considering the commonly formed values, the difference between Mn and Si can also be set to 1.5 or lower.

[0024] When the average Mn / Si value of the surface layer is high, the aforementioned Mn / Si difference can be achieved. However, in one specific embodiment of the present invention, the average Mn / Si value of the oxide in the surface layer can be limited to 1.5 or higher. In another specific embodiment, the average Mn / Si value of the surface layer can be 1.7 or higher. In yet another specific embodiment, the average Mn / Si value of the surface layer can be 1.9 or higher. A higher Mn / Si value of the oxide in the surface layer is more advantageous; therefore, there is no particular upper limit, but it can be determined to be a value of 2.2 or lower.

[0025] Furthermore, as a method to increase the Mn / Si difference, one approach is to maintain the average Mn / Si value of the internal oxide at a low level. In one specific embodiment of the invention, the average Mn / Si value of the internal oxide may be below 1.0; in another specific embodiment, the average Mn / Si value of the internal oxide may be below 0.9; and in yet another specific embodiment, the average Mn / Si value of the internal oxide may also be below 0.8 or 0.7. A lower average Mn / Si value of the internal oxide is more advantageous; therefore, there is no particular limitation on its lower limit, but it can be determined to be a value above 0.4.

[0026] In one specific embodiment of the invention, the difference between Mn and Si can be obtained using a value obtained at the center of the steel plate in the width direction. However, it is not necessarily limited to this location. For example, spot weldability is often problematic at the edges in the width direction, so the value obtained at the edges in the width direction can also be used. Here, the edge in the width direction refers to the two ends of the cross-section of the steel plate cut along the width direction, but when the integrity of the specimen is problematic, such as contamination occurring at the above-mentioned location, it can refer to the location 1 mm inward along the width direction from the end point.

[0027] The type of steel plate targeted in this invention is not limited as long as it is a high-strength steel plate with a strength of 780 MPa or higher. However, the steel plate targeted in this invention may have a composition comprising, by weight, 0.05-1.5% C, 2.0% or less Si, 1.0-20% Mn, 3% or less acid-soluble aluminum (S-Al), 2.5% or less Cr, 1% or less Mo, 0.005% or less B, 0.2% or less Nb, 0.2% or less Ti, 0.2% or less V, 0.2% or less Sb+Sn+Bi, 0.1% or less Sb+Sn+Bi, and 0.01% or less N, but is not necessarily limited to this. The remaining components are iron and other impurities. In addition, it is not excluded that elements not listed above but which may be included in steel may be further included in the range of 1.0% or less in total. Unless otherwise specified, the content of each component element in this invention is expressed on a weight basis. The above composition refers to the bulk composition of the steel plate, that is, the composition at 1 / 4 of the thickness of the steel plate (hereinafter the same).

[0028] However, in some specific embodiments of the present invention, TRIP steel, DP steel, and CP steel can be used as the high-strength steel plate. Each steel can have the following composition.

[0029] Steel composition 1: comprising 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: less than 1.0%, Cr: less than 2.0% (preferably less than 1.0%), Mo: less than 0.2% (preferably less than 0.1%), B: less than 0.005% (preferably less than 0.004%), Nb: less than 0.1% (preferably less than 0.05%), Ti: less than 0.1% (preferably 0.001-0.05%), Sb+Sn+Bi: less than 0.05%, N: less than 0.01%, balance Fe, and unavoidable impurities. Depending on the circumstances, elements not listed above but permitted in the steel may be further included in the range of less than 1.0% in total.

[0030] While not necessarily limited to this, the steel used as component 1 may include TRIP steel or XF steel, and may each have a tensile strength of 900 MPa or more.

[0031] Steel composition 2: comprising C: 0.05-0.30% (preferably 0.10-0.2%), Si: less than 0.5% (preferably less than 0.3%), Mn: 4.0-10.0% (preferably 5.0-9.0%), S-Al: less than 0.05% (preferably 0.001-0.04%), Cr: less than 2.0% (preferably less than 1.0%), Mo: less than 0.5% (preferably 0.1-0.35%), B: less than 0.005% (preferably less than 0.004%), Nb: less than 0.1% (preferably less than 0.05%), Ti: less than 0.15% (preferably 0.001-0.1%), Sb+Sn+Bi: less than 0.05%, N: less than 0.01%, balance Fe, and unavoidable impurities. Depending on the circumstances, elements not listed above but permitted in the steel may be further included in the range of less than 1.0% in total.

[0032] While not necessarily limited to this, examples of steels used in steel composition 2 include TRIP steel and XF steel, and they can have tensile strengths of 1000 MPa or more.

[0033] According to one embodiment of the present invention, the surface of the steel plate may include one or more coatings, which may be zinc-based coatings including galvanized (GI), zinc-magnesium (ZM), or galvanized-annealed (GA). In the present invention, by appropriately controlling the oxygen concentration of the surface layer as described above, the problem of liquid metal embrittlement during spot welding can be suppressed even when formed on the surface of a zinc-based coated steel plate.

[0034] When the zinc-based coating is a GA layer, the degree of alloying can be controlled at 8-13%, preferably at 10-12%. When the degree of alloying is insufficient, zinc in the zinc coating may penetrate into the microcracks, which may cause embrittlement of the liquid metal. However, when the degree of alloying is too high, problems such as pulverization may occur.

[0035] Furthermore, the coating thickness of the zinc-based coating can be 30-70 g / m². 2 When the coating thickness is too low, sufficient corrosion resistance is difficult to achieve. On the other hand, when the coating thickness is too high, it may lead to increased manufacturing costs and embrittlement of the liquid metal. Therefore, the coating thickness is controlled within the aforementioned range. A more preferred coating thickness range is 40-60 g / m². 2 .

[0036] The following describes a specific embodiment of manufacturing the steel plate of the present invention. However, it should be noted that the steel plate of the present invention is not necessarily manufactured through the specific embodiment described below; the specific embodiment described below is a preferred method for manufacturing the steel plate of the present invention.

[0037] First, hot-rolled steel sheets can be manufactured through the following process: a steel billet having the above composition is reheated, hot-rolled by roughing and finishing rolling, and then cooled by the run-out table (ROT) before being coiled. No particular restrictions are placed on the hot rolling conditions such as ROT cooling; however, in one specific embodiment of the invention, the slab reheating temperature, finishing rolling start temperature, finishing rolling end temperature, and coiling temperature can be limited as follows.

[0038] Slab reheating temperature: 950-1300℃

[0039] Reheating of the slab is performed to ensure rollability by heating the material before hot rolling. During the reheating process, the surface layer of the slab combines with oxygen in the furnace to form an oxide scale. When the heating temperature is sufficiently high, the composition of the oxides on the surface and inside of the steel sheet can be controlled within an appropriate range due to the interaction with the processes described below. However, when the heating temperature is too high, excessive grain growth may occur, and the material quality of the steel sheet may deteriorate. Therefore, the slab is reheated to the aforementioned temperature range.

[0040] Finishing rolling starting temperature: 900-1150℃

[0041] When the finishing rolling starting temperature is too high, the surface hot-rolled oxide scale becomes excessively developed, which may increase the amount of surface defects caused by the oxide scale in the final product. Therefore, the upper limit of the finishing rolling starting temperature is limited to 1150°C. In addition, when the finishing rolling starting temperature is below 900°C, the rigidity of the bar increases due to the decrease in temperature, which may significantly reduce the hot rollability. Therefore, the finishing rolling starting temperature can be limited to the above range.

[0042] Finishing rolling temperature: 850-1050℃

[0043] When the finishing rolling termination temperature exceeds 1050℃, the oxide scale removed during the finishing rolling process will re-form excessively on the surface, leading to an increase in the amount of surface defects. When the finishing rolling termination temperature is below 850℃, the hot rollability decreases. Therefore, the finishing rolling termination temperature can be limited to the above range.

[0044] Winding temperature: 590-750℃

[0045] Hot-rolled steel sheets are stored in coils, which undergo a slow cooling process. This process removes oxidizing elements from the surface of the steel sheet. However, if the coiling temperature is too low, the coil is slowly cooled below the required temperature for removing these oxidizing elements, making it difficult to achieve sufficient results. Furthermore, if the coiling temperature is too high, it may be difficult to ensure the tensile strength and other material properties of the steel sheet, and the coating quality may deteriorate.

[0046] Heating of the edges of hot-rolled coils: at 600-800℃ for 5-24 hours.

[0047] In one specific embodiment of the present invention, in order to increase the average Mn / Si value of the surface oxides at the edge and decrease the average Mn / Si value of the internal oxides at a depth of 100 nm or more in the steel sheet, the edge of the hot-rolled coil can be heated. Heating the edge of the hot-rolled coil refers to heating both ends of the coil in the width direction, i.e., the edge portion. By heating the edge portion, it is first heated to a temperature suitable for oxidation. That is, the interior of the coil remains at a high temperature, but the edge portion is cooled relatively quickly, thus the time the edge portion remains at a temperature suitable for internal oxidation is relatively short. Therefore, compared to the center portion in the width direction, the removal of oxidizing elements in the edge portion is less active. Heating the edge portion can be used as a method for removing oxidizing elements from the edge portion.

[0048] That is, when heating the edge portion, unlike the cooling process after coiling, the edge portion is heated first, thereby maintaining the temperature of the edge portion in the width direction at a temperature suitable for internal oxidation, resulting in an increase in the thickness of the internal oxide layer at the edge portion. Therefore, the heating temperature of the edge portion needs to be above 600°C (based on the temperature of the steel plate edge). However, when the temperature is too high, the tensile strength of the steel plate decreases, excessive oxide scale is formed at the edge portion during heating, or a porous, highly oxidized oxide scale (ferric oxide) is formed, which may worsen the surface condition after pickling. Therefore, the heating temperature of the edge portion can be below 800°C. Furthermore, when the Mn / Si ratio is excessively increased in both the surface and interior portions, the difference in the Mn / Si ratio may not meet the value specified in this invention. A more preferred heating temperature for the edge portion is 600-750°C. According to a specific embodiment of the invention, the heating of the edge portion can be carried out in a heat treatment furnace.

[0049] Furthermore, to eliminate the unevenness in the average Mn / Si value of the surface oxide between the edge and center portions in the width direction that occurs during winding, and the unevenness in the average Mn / Si value of the internal oxide at a depth of 100 nm or more within the steel sheet, the heating time for the edge portion needs to be 5 hours or more. However, if the heating time for the edge portion is too long, the tensile strength of the steel sheet decreases, excessive oxide scale forms, or conversely, the average Mn / Si value of the surface and internal oxides at the edge portion may become too high. Therefore, the heating time for the edge portion can be 24 hours or less.

[0050] According to one specific embodiment of the present invention, the heating of the edge portion can be achieved by adjusting the air-fuel ratio for combustion heating. That is, the oxygen fraction in the atmosphere can be changed by adjusting the air-fuel ratio, and as the oxygen partial pressure increases, the Mn / Si ratio of the surface layer of the steel plate can be increased. In one specific embodiment of the present invention, the air-fuel ratio can be adjusted to control a nitrogen atmosphere containing 1-2% oxygen, but this is not necessarily the case. Those skilled in the art to which this invention pertains can easily control the oxygen fraction by adjusting the air-fuel ratio, and therefore this will not be described separately.

[0051] Pickling: Performed at a plate throughput speed of 180-250 m / min.

[0052] The hot-rolled steel sheet, after the above process, is added to a hydrochloric acid bath for pickling to remove the hot-rolled oxide scale. The hydrochloric acid concentration in the bath during pickling is in the range of 10-30% by volume, and the plate throughput speed is 180-250 m / min. When the pickling speed exceeds 250 m / min, the oxide scale on the surface of the hot-rolled steel sheet may not be completely removed. When the pickling speed is below 180 m / min, the surface layer of the base iron may be corroded by the hydrochloric acid. Therefore, the pickling is carried out at a plate throughput speed of 180 m / min or higher.

[0053] Cold rolling: reduction rate 35-60%

[0054] After pickling, cold rolling is performed. The cold rolling reduction rate is in the range of 35-60%. When the cold rolling reduction rate is less than 35%, although there are no particular problems, it may be difficult to fully control the microstructure due to insufficient recrystallization driving force during annealing. When the cold rolling reduction rate exceeds 60%, it is difficult to achieve an appropriate average Mn / Si value for the surface oxides after annealing and an average Mn / Si value for the internal oxides of the steel sheet with a depth of more than 100 nm.

[0055] Following the aforementioned cold rolling process, an annealing process can be performed on the steel sheet. During the annealing process, the average Mn / Si value of the oxides on the surface of the steel sheet and the average Mn / Si value of the oxides in the interior of the steel sheet with a depth of 100 nm or more may change significantly. Therefore, in a specific embodiment of the present invention, the annealing process can be controlled under the condition of appropriately controlling the average Mn / Si value of the oxides on the surface and the average Mn / Si value of the oxides in the interior of the steel sheet with a depth of 100 nm or more. The sheet throughput speed and the dew point in the annealing furnace can be controlled under the following conditions.

[0056] Through speed: 40-130 m / min

[0057] To ensure sufficient productivity, the cold-rolled steel sheet needs to be passed through at a speed of 40 meters per minute or more. When the sheet passes through at a slow speed, excessive grain growth may occur, potentially reducing strength. Furthermore, when the sheet passes through at too high a speed, the time spent at high temperatures is shortened, potentially reducing the amount of austenite and the fraction of martensite and bainite, which are the cooling phases based on austenite. Therefore, this may be disadvantageous in terms of material properties; hence, in one specific embodiment of the invention, the upper limit of the sheet passing through speed can be set to 130 meters per minute.

[0058] Temperature and dew point control in the soaking zone of the annealing furnace: maintained within the range of -10℃ to 30℃ at 650-900℃.

[0059] To control the Mn / Si ratio in the oxides of both the internal and surface layers within an appropriate range, it is advantageous to control the dew point in the soaking zone of the annealing furnace. When the dew point is too low, surface oxidation occurs instead of internal oxidation, and oxides such as Si or Mn may form on the surface. These oxides have an adverse effect on the plating. Therefore, it is necessary to control the dew point to above -10°C. On the other hand, when the dew point is too high, Fe oxidation may occur, so it is necessary to control the dew point to below 30°C. As mentioned above, the temperature used to control the dew point can be above 650°C to achieve a sufficient internal oxidation effect. However, when the temperature is too high, surface oxides such as Si form, which not only hinder oxygen diffusion to the interior but also generate excessive austenite during heating in the soaking zone, reducing the carbon diffusion rate. Therefore, the level of internal oxidation may decrease, and excessive austenite growth in the soaking zone may lead to material softening. Furthermore, it may increase the load on the annealing furnace, causing problems such as shortened equipment life and increased process costs. Therefore, the temperature for controlling the dew point can be below 900°C.

[0060] At this point, the dew point can be adjusted by adding wet nitrogen gas (N2+H2O) containing water vapor into the annealing furnace.

[0061] Hydrogen concentration in the annealing furnace (soaking zone): 5-10% by volume

[0062] The atmosphere in the soaking zone of the annealing furnace is maintained as a reducing atmosphere by adding 5-10% by volume of hydrogen to the nitrogen. When the hydrogen concentration in the annealing furnace is less than 5% by volume, excessive surface oxides are formed due to the reduced reducing power, resulting in poor surface quality and coating adhesion, and reduced resistance to LME (Liquid Metal Electrode Regulator). While no particular problems arise at higher hydrogen concentrations, the increased cost due to increased hydrogen usage and the risk of furnace explosion caused by higher hydrogen concentrations necessitate limiting the hydrogen concentration.

[0063] Hydrogen concentration in the rapid cooling zone: 25-80% by volume

[0064] Hydrogen and nitrogen are typically used as the refrigerant in the rapid cooling zone of an annealing furnace. To ensure an appropriate cooling rate and suppress surface oxidation caused by the refrigerant during cooling, a suitable hydrogen concentration must be maintained. When the hydrogen concentration is less than 25%, the concentration of reducing hydrogen is insufficient, leading to oxidation of easily oxidizable elements such as Si on the steel plate surface during cooling. This results in poor plating wettability, and the Mn / Si ratio in the surface layer may decrease. Furthermore, low hydrogen concentrations reduce cooling capacity, making it difficult to ensure an appropriate level of cooling secondary phase fraction, thus causing problems with material quality. When the hydrogen concentration exceeds 80%, the cost of hydrogen production may increase, and the risk of hydrogen explosion during high-hydrogen operation increases; therefore, the upper limit is limited to 80%. In one specific embodiment of the invention, the cooling rate of the rapid cooling zone is set to 5-30°C / second; in another specific embodiment, the cooling rate of the rapid cooling zone can be set to 10-30°C / second.

[0065] The steel sheet annealed according to the process described above is directly immersed in the plating bath for hot-dip galvanizing.

[0066] Depending on the requirements, the hot-dip galvanized steel sheet coated by the above process can subsequently undergo an alloying heat treatment process. The preferred conditions for alloying heat treatment are as follows.

[0067] Introducing temperature of the steel sheet for plating bath: 420-500℃

[0068] When the temperature of the steel sheet introduced into the plating bath is too low, the wettability at the interface between the steel sheet and the liquid zinc cannot be adequately ensured; therefore, it should be maintained above 420℃. When the temperature of the steel sheet introduced into the plating bath is too high, the steel sheet reacts excessively with the liquid zinc, generating a ζ phase at the interface, which is an Fe-Zn alloy phase. This leads to reduced coating adhesion and excessive leaching of Fe from the steel sheet, resulting in scum formation within the plating bath.

[0069] To ensure the wettability of the coating and the fluidity of the plating bath, the Al concentration in the plating bath should be maintained at an appropriate level. For GA, it should be controlled at 0.10-0.15%; for GI, at 0.2-0.25%; and for ZM, Al should be controlled at 0.7-5.7 wt%, and Mg at 0.7-5.7 wt%. This will maintain the formation of dross in the plating bath at an appropriate level and ensure the quality and performance of the coated surface. There are no particular limitations on the coating adhesion amount in this invention, but it can be appropriately adjusted according to the quality requirements of the customer.

[0070] Alloying (GA) temperature: 480-560℃

[0071] When the alloying temperature is below 480℃, the degree of alloying is insufficient due to the low diffusion of Fe, and therefore the physical properties of the coating may be poor. When the alloying temperature exceeds 560℃, powdering may occur due to excessive alloying, and the material may deteriorate due to the transformation of residual austenite into ferrite. Therefore, the alloying temperature is set within the above range.

[0072] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the contents of the claims and the contents reasonably deduced therefrom.

[0073] Leveling reduction ratio: 0 to 0.35

[0074] After hot-dip galvanizing or alloying, surface finishing (SPM) is performed to adjust the yield strength and surface roughness of the steel sheet. During SPM, due to work hardening caused by cold rolling, the yield strength increases proportionally with the reduction rate, and the surface roughness of the SPM rolls is transferred to the steel sheet, thus increasing the surface roughness. Therefore, by adjusting the reduction rate during SPM, appropriate yield strength and surface roughness are ensured, ensuring the stability of the tensile material and increasing surface roughness. This has the advantage of ensuring sealant adhesion. If no adjustment of yield strength and surface roughness is required after hot-dip galvanizing, SPM can be omitted. However, when the reduction rate exceeds 0.35, the yield strength increases excessively, potentially exceeding the target material, and the surface roughness increases excessively. Therefore, capillary action caused by roughness after oiling may result in poor degreasing properties. Detailed Implementation

[0075] A steel billet with the composition listed in Table 1 (the remaining components not listed in the table are Fe and unavoidably contained impurities. Furthermore, B and N in the table are expressed in ppm, and the remaining components are expressed in % by weight) is reheated to 1210°C, hot-rolled with the finishing rolling start and finish temperatures set to 945°C and 870°C respectively, then the hot-rolled coil is heated at the edges in a nitrogen atmosphere containing less than 1.5% by volume of oxygen, and then pickled with a 12.3% hydrochloric acid bath, and finished with 53%... The steel sheet is cold-rolled at a reduction rate, then heated in the soaking zone of an annealing furnace, and then rapidly cooled in an atmosphere of 60% by volume hydrogen minus the remainder nitrogen (cooling rate: 15°C / s). Immediately afterward, GA is immersed in a plating bath containing 0.13% Al, GI in a plating bath containing 0.24% by weight Al, and ZM in a zinc-based plating bath containing 1.75% by weight Al and 1.55% by weight Mg. Hot-dip galvanizing is then performed by adjusting the adhesion amount with an air knife and cooling. In the case of GA, the obtained hot-dip galvanized steel sheet is subjected to alloying (GA) heat treatment within a preferred range of 480-560°C, as needed, to finally obtain alloyed hot-dip galvanized steel sheet. The obtained hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet is then leveled and rolled at a reduction rate of 8%.

[0076] In all embodiments, the introduction temperature of the steel sheet into the hot-dip galvanizing bath is set to 475°C. Other conditions for each embodiment are described in Table 2.

[0077] [Table 1]

[0078]

[0079] [Table 2]

[0080]

[0081]

[0082] The properties of hot-dip galvanized steel sheets manufactured through the above process were measured, and the occurrence of liquid metal embrittlement (LME) during spot welding was observed. The results are shown in Table 3. Unless otherwise specified, the test results shown in Table 3 were measured using samples collected 1 mm from the edge of the steel sheet. When the sample size was large, samples of a specified size were measured starting from 1 mm from the edge. Specifically, spot welding was performed by cutting the steel sheet along its width and along the edges of each cut. Two spot welding currents were applied, followed by a hold time of one cycle. Spot welding was performed in two three-layer configurations. The evaluation material was stacked and spot welded in the order of evaluation material-evaluation material-GA 980DP 1.4t. During spot welding, a new electrode was welded to a soft material 15 times, after which the electrode was worn down, and the upper limit current at which spatter occurred was measured using the material to be spot welded. After measuring the upper limit current, eight spot welds were performed at currents 0.5 kA and 1.0 kA lower than the upper limit current, at each welding current. The cross-section of the spot weld was then precision machined using electrical discharge machining, fixed with epoxy resin (epoxy mounting), and ground. Crack lengths were measured using an optical microscope. When observing with the optical microscope, the magnification was set to 100x. If no cracks were found at this magnification, it was determined that liquid metal embrittlement had not occurred. If cracks were found, their lengths were measured using image analysis software. A good result was considered achieved when B-type cracks at the shoulder of the spot weld were less than 100 μm and no C-type cracks were observed.

[0083] For the average weight ratio of Mn / Si in the oxides on the surface of the steel plate and the internal oxides with a depth of more than 100 nm, the steel plate was processed with a focused ion beam (FIB), and point analysis was performed more than 10 times at each location of the oxides present in the surface of the steel plate and the oxides with a depth of 50-100 nm in the interior of the steel plate using TEM EDS. The weight percentage values ​​of Mn and Si at each location measured by weight ratio were averaged as the result of the Mn / Si calculation.

[0084] Tensile strength was measured by manufacturing a sample in the C direction according to JIS-5 standard and performing a tensile test. The degree of alloying and the amount of plating were measured using a wet dissolution method with hydrochloric acid solution.

[0085] For sealant adhesion, the steel sheet was bent 90 degrees after the automotive structure was bonded to the coated surface with D-type adhesive, and the coating was checked for peeling. GI and ZM steel sheets were tested for sealant adhesion.

[0086] Powdering involves bending the coated material at a 90-degree angle, applying tape to the bent section, and then peeling it off. The length of the coating that has detached from the tape (in millimeters) is then recorded. If the length of the coating detached from the tape exceeds 10mm, it is considered a defect.

[0087] Flaking is a process of processing. After shaping, check whether the coating has peeled off. Visually inspect for defects such as uncoated steel plates to confirm surface quality. If defects such as uncoated plates are observed visually, the plate is deemed defective.

[0088] [Table 3]

[0089]

[0090]

[0091]

[0092] The steel composition of Examples 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 of the invention meets the scope proposed in this invention, and the manufacturing method also meets the scope of this invention. Therefore, they have good tensile strength, plating quality, plating adhesion and spot weld LME crack length.

[0093] In Comparative Examples 9, 16, and 17, the manufacturing methods met the scope of the present invention, but the steel composition was not within the scope of the present invention. In Comparative Examples 5, 13, and 20, the Mn and Si were not within the scope of the present invention. When the dew point of the annealing furnace was increased, the surface oxide content of the surface layer was excessive. Therefore, the average Mn / Si value of the oxides on the surface of the steel plate deviated from the reference, and the average weight ratio of the Mn / Si values ​​of the internal oxides also deviated from the reference. Therefore, the LME crack length did not meet the reference. Furthermore, due to the excessive surface oxides, the alloying inhibition layer at the interface between the coating and the base iron could not be sufficiently formed, resulting in uncoated surfaces, poor surface quality, and SBT peeling. Therefore, the coating adhesion was not excellent.

[0094] In Comparative Examples 2 and 7, the coiling temperature during the hot rolling process did not meet the range proposed in this invention. Comparative Example 2 involved a hot rolling coiling temperature lower than the range proposed in this invention. Therefore, the average Mn / Si value of the oxides on the surface of the steel sheet deviated from the reference, and the average weight ratio of the Mn / Si values ​​of the internal oxides also deviated from the reference, thus failing to meet the LME crack criterion. Comparative Example 7 involved a hot rolling coiling temperature not proposed in this invention. Although the LME characteristics were good, excessive hot-rolled oxide scale was generated, which could not be completely removed during pickling, resulting in uncoated areas and poor surface quality. Consequently, plating peeled off during the peeling evaluation, and due to uneven alloying, pulverization was poor. Furthermore, due to the excessively high hot rolling coiling temperature, the hot-rolled material softened and could not recover after annealing, resulting in insufficient tensile strength.

[0095] The heating temperature of the heat treatment furnace in Comparative Example 15 was outside the range proposed in this invention. During the heat treatment process, over-oxidation occurred at the edges, resulting in the formation of a red ferric oxide scale on the surface, and its thickness became excessive. During the pickling process after hot rolling, the edges were over-pickled, increasing the surface roughness and resulting in an uneven surface shape after plating. Furthermore, defects occurred where the surface color differed from the center, leading to poor powdering properties due to uneven alloying. In addition, the average weight ratio of Mn / Si increased in both the surface and interior layers, with the difference between the two values ​​being less than 0.5, thus failing to meet the LME standard.

[0096] Comparative Example 8 illustrates a case where the heating temperature of the heat treatment furnace used to heat the edge portion is below the range specified in this invention. Therefore, the Mn / Si ratio of the surface and internal oxides cannot be controlled, resulting in the spot weld LME crack evaluation failing to meet the criteria and thus being deemed defective.

[0097] Comparative Example 13 illustrates a case where the heating temperature of the heat treatment furnace used to heat the edge portion meets the range of the present invention but exceeds the heating time. As a result, over-oxidation occurs in the edge portion during heat treatment, forming a reddish ferric oxide scale on the surface, which becomes excessively thick. During the pickling process after hot rolling, the edge portion is over-pickled, increasing surface roughness. Consequently, the surface shape after plating is uneven, resulting in color inconsistencies where the surface color differs from the center. Furthermore, due to uneven alloying, powdering properties are poor. Additionally, the Mn / Si ratio increases in both the surface and interior layers, with a difference of less than 0.5, leading to poor weldability.

[0098] Comparative Example 5 illustrates 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 composition of the oxides cannot be controlled within the range specified in the present invention, and therefore the benchmark is not met when evaluating LME cracks in spot welds.

[0099] In Comparative Examples 1 and 14, the pickling speed did not meet the range proposed in this invention. Comparative Example 1 was manufactured at a pickling speed lower than the benchmark. As the pickling speed became excessively long, the surface layer was deeply dissolved and removed, causing the average Mn / Si value of the oxides on the surface of the steel sheet to deviate from the benchmark, and the average weight ratio of the Mn / Si values ​​of the internal oxides also deviated from the benchmark, resulting in LME cracking. Because the grain boundary integrity deteriorated due to acid corrosion of the internal oxide grain boundaries in the hot-rolled sheet, flaking occurred during the flaking test. Comparative Example 14 was manufactured at a pickling speed higher than the benchmark. The hot-rolled oxide scale was not completely removed and remained on the steel sheet surface, resulting in poor surface quality and poor pulverization due to the uneven GA alloying degree.

[0100] In Comparative Example 10, the temperature in the soaking zone of the annealing furnace exceeded the range proposed in this invention. Because the annealing temperature became excessively high, the external oxidation increased, but insufficient internal oxidation was not formed. The average Mn / Si value of the oxides on the surface of the steel plate deviated from the reference standard, and the average weight ratio of the Mn / Si values ​​of the internal oxides also deviated from the reference standard. This resulted in LME cracks not meeting the reference standard, leading to poor spot weldability. Furthermore, excessive austenite formation and growth occurred in the soaking zone, causing the material to fail to meet the reference standard and resulting in poor weldability.

[0101] Comparative Example 12 illustrates a case where the soaking zone temperature in the annealing furnace is lower than the range proposed in this invention. Due to the low annealing temperature, the oxidation reaction between water vapor and the steel plate is insufficient. The average Mn / Si value of the oxides on the surface of the steel plate deviates from the reference standard, and the average weight ratio of the Mn / Si value of the internal oxides also deviates from the reference standard. Therefore, the LME crack does not meet the reference standard, resulting in poor spot weldability. Furthermore, insufficient recrystallization occurs during annealing, preventing the formation of the target fine microstructure; thus, the material does not meet the reference standard and is defective.

[0102] In Comparative Example 6, the dew point in the furnace during the annealing process was lower than the range proposed in this invention. Even if a sufficient internal oxide layer is formed across the entire width during the hot rolling heating process, the dew point during the annealing process after cold rolling is not high enough. The average Mn / Si value of the oxide on the surface of the steel sheet deviates from the reference, and the average weight ratio of the Mn / Si value of the internal oxide also deviates from the reference. Therefore, the LME crack length in the spot weld does not meet the reference. In the case of GI material, due to the low dew point, sufficient internal oxidation cannot occur, resulting in excessive surface oxides, thus leading to poor surface quality and flaking during SBT.

[0103] Comparative Example 18 is a case where the dew point range in the annealing furnace exceeds the range proposed in this invention. While the spot weldability is excellent due to the excessively high dew point, the material deteriorates due to excessive internal oxidation, failing to meet the benchmark. Furthermore, the amount of surface oxides generated also increases due to the excessively high dew point, resulting in coating peeling in the SBT results.

[0104] In Comparative Example 3, the hydrogen concentration in the annealing furnace was less than 5% by volume, resulting in insufficient reducing atmosphere composition. Excessive surface oxide formation led to incomplete plating, resulting in poor surface quality. Furthermore, plating peeled off during SBT, and the oxide composition could not be controlled, thus failing to meet the LME crack criterion.

[0105] In Comparative Examples 4 and 11, the plate throughput speed in the annealing furnace was outside the range proposed in this invention. Comparative Example 4 involved a plate throughput speed exceeding the range proposed in this invention. Because the oxide composition was not controlled within the range specified in this invention, the LME cracks did not meet the benchmark, and sufficient recrystallization time could not be ensured in the annealing furnace, resulting in poor material quality. Comparative Example 11 involved a plate throughput speed below the range proposed in this invention. While the oxide composition conditions were met, the spot weldability was good, but excessive oxide on the surface resulted in insufficient surface quality of the coated steel sheet, leading to powdering.

[0106] Therefore, the advantageous effects of the present invention can be confirmed.

Claims

1. A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, wherein the average Mn / Si value of the surface oxides present in the surface portion of the region from the interface between the hot-dip galvanized layer and the base steel sheet to a depth of 15 nm, minus the average Mn / Si value of the internal oxides present in the region from the interface to a depth of 50-100 nm, is 0.5 or more. Mn and Si of each oxide refers to the content of Mn and Si components in the oxide measured by EDS, wherein, The content is expressed in weight %, and the average Mn / Si value refers to the average value of the measured Mn / Si values ​​of each oxide. The base steel plate has the following composition: C: 0.05-1.5%, Si: less than 2.0%, Mn: 1.0-20%, acid-soluble aluminum (S-Al): less than 3%, Cr: less than 2.5%, Mo: less than 1%, B: less than 0.005%, Nb: less than 0.2%, Ti: less than 0.2%, V: less than 0.2%, Sb+Sn+Bi: less than 0.1%, N: less than 0.01%, and the balance being Fe and unavoidable impurities.

2. The hot-dip galvanized steel sheet according to claim 1, wherein, The value of the average Mn / Si value of the surface oxides present in the region from the interface between the hot-dip galvanized layer and the base steel plate to a depth of 15 nm, minus the average Mn / Si value of the internal oxides present in the region from the interface to a depth of 50-100 nm, is 0.8 or higher.

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

4. The hot-dip galvanized steel sheet according to claim 3, wherein, The average Mn / Si value of the oxide in the surface layer is 1.7 or higher.

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

0.

6. The hot-dip galvanized steel sheet according to claim 5, wherein, The average Mn / Si value of the internal oxide is below 0.

9.

7. A method for manufacturing a hot-dip galvanized steel sheet according to any one of claims 1-6, comprising the following steps: Provide steel billets; The steel billet is then reheated to a temperature of 950-1300℃; The reheated steel billet is hot-rolled at a finishing rolling start temperature of 900-1150°C and a finishing rolling end temperature of 850-1050°C to obtain a steel plate. The steel plate is coiled within a temperature range of 590-750℃; The coiled steel sheet is heated at 600-800℃ for 5-24 hours at its edges. The steel plate is pickled at a throughput speed of 180-250 meters per minute; The steel plate is cold-rolled with a reduction rate of 35-60%; The cold-rolled steel sheet is heated under wet nitrogen conditions with a soaking zone temperature of 650-900°C and a dew point temperature of -10°C to +30°C, and the atmosphere gas contains 5-10% by volume H2, and then recrystallized annealed by cooling at a cooling rate of 10-30°C / second in a rapid cooling zone; and The steel plate is immersed in a molten plating bath for hot-dip plating within an introduction temperature range of 420-500℃. The hydrogen concentration in the rapid cooling zone during recrystallization annealing is 25-80% by volume.

8. The method for manufacturing hot-dip galvanized steel sheet according to claim 7, wherein, The method further includes the step of alloying the hot-dip galvanized steel sheet at a temperature of 480-560°C.

9. The method for manufacturing hot-dip galvanized steel sheet according to claim 8, wherein, The molten plating bath contains 0.10-0.15% by weight of Al.

10. The method for manufacturing hot-dip galvanized steel sheet according to claim 7, wherein, The molten plating bath contains 0.2-0.25% by weight of Al.

11. The method for manufacturing hot-dip galvanized steel sheet according to claim 7, wherein, The molten plating bath contains 0.7-5.7% by weight Al and 0.7-5.7% by weight Mg.

12. The method for manufacturing hot-dip galvanized steel sheet according to any one of claims 7 to 11, wherein, The plate passing speed during recrystallization annealing is 40-130 meters per minute.

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