Galvanized steel sheet with excellent surface quality and electric resistance spot weldability, and its manufacturing method

Abstract

Provided are high-strength hot-dip galvanized steel sheets with excellent surface quality and spot weldability, and methods for manufacturing the same. [Solution] 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, and an inhibitor layer made of an Fe-Al alloy is formed between the base steel sheet and the hot-dip galvanized layer at an area ratio of 60% or more.

Description

[Technical Field]

[0001] The present 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. Regarding the method. [Background technology]

[0002] Due to issues such as environmental pollution, regulations on automobile emissions and fuel efficiency are becoming stricter every day. Therefore, there is a growing demand for reducing fuel consumption through weight reduction of automotive steel sheets. Various types of high-strength steel plates with high strength per unit thickness have been developed and released. are.

[0003] High strength steel usually means steel with a strength of 490 MPa or more, but this does not necessarily mean Without being limited thereto, transformation induced plasticity Plasticity;TRIP) steel, Twin Induced P lasticity;TWIP) steel, dual phase (DP) steel, composite Complex Phase (CP) steels and the like can also be included.

[0004] On the other hand, automotive steel is made of plated steel sheets with a surface that is plated to ensure corrosion resistance. Among them, zinc-coated steel sheet (GI), highly corrosion-resistant coated steel sheet (ZM) or Galvannealed steel sheets (GA) have high corrosion resistance due to the sacrificial corrosion protection properties of zinc. Therefore, it is widely used as an automotive material.

[0005] However, when the surface of high-strength steel plate is zinc-plated, the spot weldability becomes weak. In other words, in the case of high strength steel, the yield strength is high as well as the tensile strength. In addition, the tensile stress generated during welding is difficult to eliminate through plastic deformation, resulting in micro-cracks on the surface. When welding is performed on high-strength galvanized steel sheets, the zinc with a low melting point is likely to cause cracks. Lead can penetrate into microcracks in the steel, resulting in liquid metal embrittlement. A phenomenon called LME occurs, and fatigue This can lead to problems such as the steel plate breaking at high strength. This becomes a major obstacle. Summary of the Invention [Problem to be solved by the invention]

[0006] According to one aspect of the present invention, a high-strength hot-dip galvanized steel sheet having excellent surface quality and spot weldability is provided. A plate and method for manufacturing the same are provided.

[0007] The object of the present invention is not limited to the above. Those skilled in the art will be able to understand the further object of the present invention from the overall content of the specification of the present invention. There is no difficulty in understanding it. [Means for solving the problem]

[0008] The hot-dip galvanized steel sheet according to one aspect of the present invention comprises a base steel sheet and a galvanized layer formed on the surface of the base steel sheet. and between the base steel sheet and the hot-dip galvanized layer, there is provided an Fe-A The suppression layer made of the alloy may be formed at an area ratio of 60% or more.

[0009] According to another aspect of the present invention, a method for manufacturing a hot-dip galvanized steel sheet includes the steps of providing a steel slab. reheating the slab at a temperature of 950 to 1300°C; Finish rolling start temperature 900~1,150℃, finish rolling temperature 850~1,050℃ a step of hot rolling to obtain a steel sheet; and a step of coiling the steel sheet at a temperature in the range of 590 to 750°C; pickling the steel sheet at a threading speed of 180 to 250 mpm; The soaking zone temperature and dew point temperature are 650-900°C and -1 The above cooling conditions were carried out under the conditions of 0 to +30°C and the atmospheric gas containing humidified nitrogen containing 5 to 10% by volume of H2. The step of heating the cold-rolled steel sheet for recrystallization annealing; the steel sheet is drawn in at a temperature of 420 to 50°C. The step of hot-dipping the plated material may include immersing the plated material in a hot-dip plating bath at a temperature in the range of 0°C. [Effects of the Invention]

[0010] As described above, the present invention provides a method for uniformly and widely forming an inhibitor layer between a hot-dip galvanized layer and a base steel sheet. By forming it over a small area, the plating layer melted during welding can be formed into micro-clumps without any additional means. This has the effect of suppressing penetration into the welded block and improving weldability. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an electron microscope photograph of the surface of the inhibitor layer of a hot-dip galvanized steel sheet obtained in one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] The terminology used herein refers only to particular embodiments and does not limit the invention. The singular forms used herein are not intended to limit the scope of the term unless the context clearly indicates otherwise. Unless otherwise specified, plural forms are included.

[0013] As used in the specification, "comprises" means any specific properties, regions, integers, steps, acts, elements and and / or components, and other specific properties, regions, integers, steps, operations, elements, components, and The presence or addition of / or groups is not excluded.

[0014] All terms, including technical and scientific terms, used herein unless otherwise defined The meanings are the same as those generally understood by a person having ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries have the same meaning as those in the relevant technical literature and currently disclosed. Unless further construed and defined, the term "ideal" or "unclear" shall have a meaning consistent with the content of the term. is not to be interpreted in a very formal sense.

[0015] The following describes a high strength plating method with excellent plating quality according to one aspect of the present invention, which has been completed through the research of the present inventors. In the present invention, when the content of each element is indicated, It should be noted that unless otherwise specified, the percentages are by weight. The percentage of fabric is based on area unless otherwise specified, and the gas content is based on area unless otherwise specified. Unless otherwise specified, the volume is used.

[0016] The inventors of the present invention have discovered that liquid metal embrittlement (LME) that occurs during welding occurs in the molten coating layer. The idea is that the cracks penetrate into the microcracks in the base steel sheet, and even if the base steel sheet is Even if small cracks exist, they do not prevent the plating solution from coming into contact with the base steel sheet in a molten state. If the metal is broken or blocked but still cannot penetrate into the microcracks, liquid metal embrittlement occurs. The present invention was made based on the discovery that the above phenomenon can be significantly suppressed.

[0017] That is, one aspect of the present invention is an Fe-Al alloy formed of a hot-dip galvanized steel sheet of high strength steel. By controlling the morphology and distribution of the interface alloy layer (also called the inhibition layer), liquid metal embrittlement can be suppressed. The Fe-Al interfacial alloy layer has a higher melting point than the hot-dip galvanized layer, and the When the hot-dip galvanized steel sheet is heated, the coating layer is further alloyed, and the coating layer By increasing the melting point and viscosity of the plating solution, the opportunity for the plating solution to come into contact with the base steel sheet in a molten state is eliminated. If they do come into contact, they will be trapped in microcracks due to their significantly reduced fluidity (high viscosity). This can prevent the penetration of the coating liquid.

[0018] Normally, during hot-dip galvanizing, a problem occurs between the heated base steel sheet and the hot-dip galvanizing bath. The diffusion reaction creates a certain degree of inhibition layer (IL). However, the high Mn and / or Si content required for high strength steels can be Not only does the content make it difficult for diffusion to occur on the surface, but the properties of the inhibiting layer that is formed also make it difficult for diffusion to occur on the surface. However, it is not easy to form the suppression layer uniformly over the entire interface.

[0019] In view of this, the inventors of the present invention have developed a method for forming a suppressing layer between the hot-dip galvanized layer and the base steel sheet. The shape of the suppression layer is appropriately controlled as shown in Figure 1, and the distribution of the areas where the suppression layer is not formed is controlled. By limiting the amount of welding, the present invention aims to minimize the occurrence of liquid metal embrittlement during welding. The preferable conditions for the suppression layer will be explained in detail below.

[0020] Inhibition layer ratio: 60% or more If the area fraction of the suppression layer is less than 60%, the hot-dip coating layer will not come into contact with the base steel sheet. Therefore, in one embodiment of the present invention, the ratio of the suppression layer The amount of the liquid metal embrittlement caused by such an inhibitor layer must be limited to 60% or more. In order to enhance the suppression effect, the proportion of the suppression layer in one embodiment of the present invention is set to 70% or more. In another embodiment of the present invention, the ratio of the suppression layer can be set to 80% or more. The larger the ratio of the suppression layer, the more advantageous it is, so there is no particular upper limit to the ratio. Considering the form of the suppression layer described later, the upper limit of the suppression layer ratio is determined to be 95% or 90%. It can be done.

[0021] Inhibition layer morphology The suppression layer in the present invention is not simply distributed over a wide area, but is formed in a manner similar to that shown in FIG. In this case, the particles can be completely separated from each other. It is difficult to determine the size of the particles because they are sometimes difficult to see. When a straight line is drawn in one direction, the length of the line that overlaps with the particle can be determined as the particle width. In the present invention, the average width of such particles is limited to 4 μm or less. By forming fine particles, the particles are closely connected even with the same area fraction of the suppression layer. This can prevent the sintered and molten coating layer from coming into contact with the base steel sheet. Such particles may not be formed of only a single layer, but may be formed of multiple layers, At this time, the smaller the average particle width, the narrower and more complex the path that the molten coating takes to reach the base steel. Since the particles need to flow toward the sheet, it is difficult for them to come into contact with the base steel sheet. Since the smaller the average value, the more advantageous it is, it is not necessary to determine the lower limit. According to the results of the investigation, the thickness can be set to about 0.5 μm. The lower limit of the average particle width may be 1.0 μm. When the thickness is small, the interface roughness between the alloying suppression layer and the plating layer is low, and sufficient mechanical bonding strength is generated. This may result in peeling.

[0022] Shape of the area where the inhibition layer is not formed The inhibitor layer does not completely cover the surface of the base steel sheet, so the inhibitor layer is not formed. However, even if the occupancy rate of the area where the suppression layer is not formed is the same, However, the manner in which the liquid metal embrittlement phenomenon occurs can change depending on the form of dispersion. If the unformed portions of the suppression layer are finely dispersed, they will melt due to the surrounding suppression layer. However, the unformed part of the inhibitor layer is large in size. When a small number of such electrodes are formed, it is difficult to block the inflow of the molten plating layer. In one embodiment, the average circular diameter of the portion where the suppression layer is not formed (the portion where the suppression layer is not formed) The average size of the unformed part of the inhibition layer is limited to 15 μm or less. In another embodiment of the present invention, the inflow of molten metal can be effectively blocked. The average size (circle equivalent diameter) of the unformed portion of the suppression layer can be limited to 10 μm or less. In another embodiment, the average equivalent circle diameter of the unformed portion of the suppression layer is limited to 7 μm or less. In addition, simply limiting the average size of the unformed portions of the suppression layer can prevent large unformed portions from forming. Since it is difficult to completely restrict the formation of the circular portion, in one embodiment of the present invention, The number of unformed portions of the inhibition layer having a diameter of 20 μm or more can be limited to one or less. In this state, the number of unformed portions of the inhibition layer having a circle equivalent diameter of 15 μm or more can be limited to one or less. In another embodiment, the number of unformed portions of the suppression layer having a circle equivalent diameter of 10 μm or more is set to two. If there are many large unformed portions of the suppression layer, the suppression layer The mechanical bond between the coating and the zinc coating is insufficient, which can cause peeling of the coating. During spot welding, the area where liquid zinc can come into contact with the base steel increases, causing liquid phase embrittlement cracks to develop. It is possible.

[0023] In the present invention, the inhibiting layer is analyzed in the following manner. First, the inhibiting layer of the hot-dip galvanized steel sheet is analyzed. The surface is not alloyed with dichromate solution (CrO3 + HNO3 + ZnSO4 + H2O) A thin plating layer (usually consisting of Zn on eta, but not necessarily limited to this) The resulting surface is then neutralized and washed to expose only the inhibitory layer on the surface. After this, in order to analyze the occupancy rate (area ratio) of the suppression layer, The photographs were taken at 500x magnification to analyze the Al distribution, and the maximum concentration of Al (measured by CPS) was The area with a concentration (CPS) of 10% or more of the Al inhibition layer is considered to be the area where the Al inhibition layer exists. The area ratio can be used as the occupancy rate of the suppression layer. The shape of the minute can also be determined in the same manner.

[0024] In addition, when a line is drawn on an SEM photograph at the same magnification, the width of the inhibitory layer particles is determined by the overlap of the line and the inhibitory layer. The intervals between the two points can be regarded as one width of the suppression layer, and the average of these widths can be calculated. do.

[0025] In one embodiment of the present invention, the shape and occupancy of the suppression layer and the shape of the unformed part of the suppression layer, etc. The value obtained at the center of the steel plate in the width direction can be used. However, it is not necessarily the case that the value is obtained at this position. However, this is not limited to the above, and for example, when spot weldability of the edge in the width direction is an issue, Since there may be more values, the values ​​obtained at the edge portions in the width direction can also be used. The edge portion in the width direction means both end points of the cross section of the steel plate cut in the width direction. However, if there is a problem with the integrity of the test piece, such as contamination occurring at the above points, It can mean a point 1 mm inward in the width direction.

[0026] The steel plate targeted by this invention is a high-strength steel plate with a strength of 780 MPa or more. However, although not necessarily limited to this, the steel sheet targeted by the present invention is , by weight percentage, C: 0.05 to 1.5%, Si: 2.0% or less, Mn: 1.0 to 10%, 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, It may have a composition including Sb+Sn+Bi: 0.1% or less, and N: 0.01% or less. The remaining components are iron and other impurities, and other elements not listed above that are present in steel. This does not exclude materials that further contain elements that may be contained in the present invention in a total range of 1.0% or less. The content of each component element in the description is expressed on a weight basis unless otherwise specified. The composition refers to the bulk composition of the steel plate, that is, the composition at the 1 / 4 point of the steel plate thickness (hereinafter, identical).

[0027] However, in some embodiments of the present invention, the high-strength steel plate may be a TRIP steel, a DP steel, and CP steel, etc. Each steel may have the following composition:

[0028] Steel composition 1: C: 0.05 to 0.30% (preferably 0.10 to 0.25%), Si: 0 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 preferably 0.004% or less), Nb: 0.1% or less (preferably 0.05% or less), Ti : 0.1% or less (preferably 0.001 to 0.05%), Sb + Sn + Bi: 0.05% The following contains N: 0.01% or less, the balance Fe and unavoidable impurities. Although not listed, elements that may be contained in steel may also be included up to a total of 1.0% or less. This can be done.

[0029] Although not necessarily limited to this, the steel of the above steel composition 1 may be a TRIP steel or XF steel, each of which can have a tensile strength of 900 MPa or more.

[0030] Steel composition 2: C: 0.05-0.30% (preferably 0.10-0.2%), Si: 0. 5% or less (preferably 0.3% or less), Mn: 4.0 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 0.35%), B: 0.005% or less (preferably 0.004% or less), Nb: 0.1% or less below (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, balance Fe and unavoidable Contains impurities. In some cases, it may contain elements not listed above that may be present in steel. It may further contain up to a total of 1.0% or less.

[0031] Although not necessarily limited thereto, the steel of the above steel composition 2 includes TRIP steel and XF, etc., which can have a tensile strength of 1000 MPa or more.

[0032] According to one embodiment of the present invention, the surface of the steel sheet includes one or more plating layers. The plating layer is GI (Galvanized) or ZM (Zinc-Magnetic) In the present invention, the zinc-based plating layer may contain, for example, zinc-based ions such as ethoxylated ions. Since the oxygen concentration in the surface layer is properly controlled, even if a zinc-based plating layer is formed on the surface of the steel sheet, This can suppress the problem of liquid metal embrittlement that occurs during spot welding.

[0033] The coating weight of the zinc-based plating layer is 30 to 70 g / m 2 It can be said that: If the coating weight is too small, it is difficult to obtain sufficient corrosion resistance. If it is too large, there is a possibility that the manufacturing cost will increase and problems of liquid metal embrittlement will occur. The coating weight is more preferably controlled within the above range. 2 and It is possible.

[0034] Hereinafter, one embodiment of the method for producing the steel sheet of the present invention will be described. It is not necessarily required to manufacture the steel sheet according to the following examples, and the following examples are not intended to be limiting. It should be noted that this is one preferred method.

[0035] First, a steel slab having the above-mentioned composition is reheated and hot-rolled through rough rolling and finish rolling. After that, it is cooled on the ROT (Run Out Table) and then coiled into hot rolled steel. There are no particular restrictions on the hot rolling conditions, such as ROT cooling. In one embodiment of the invention, the slab reheating temperature, the finish rolling start and finish temperatures, and the coiling temperature The degree can be limited as follows:

[0036] Slab reheating temperature: 950 to 1,300°C Slab reheating is performed to heat the material before hot rolling to ensure rollability. During heating, the surface of the slab combines with the oxygen in the furnace to form scale, which is an oxide. If the temperature is sufficiently high, the composition of the surface and inner oxides of the steel sheet is changed by the interaction with the process described later. However, if the heating temperature is too high, The grains may grow excessively, which may cause deterioration of the steel sheet's properties. Reheat the bread.

[0037] Finish rolling starting temperature: 900 to 1,150°C If the finish rolling start temperature is too high, the surface hot rolling scale will develop excessively, resulting in the surface roughness of the final product. Since there is a possibility that the number of surface defects caused by scale may increase, the upper limit is set at 1,150°C. In addition, if the finish rolling start temperature is less than 900°C, the stiffness of the bar will decrease due to the temperature reduction. Therefore, the finish rolling process should be carried out within the above range. The starting temperature can be limited.

[0038] Finishing temperature: 850-1,050℃ If the finishing temperature exceeds 1,050°C, the material will be descaled during finishing. The scale that was removed re-forms excessively on the surface, increasing the number of surface defects and preventing the end of finish rolling. If the temperature is less than 850°C, the hot rolling property is deteriorated, so the finish rolling temperature is set to the above-mentioned The range can be limited to a certain range.

[0039] Winding temperature: 590 to 750°C The hot-rolled steel sheets are then wound into coils and stored. The sheet undergoes a slow cooling process. This process removes the oxidizing elements contained in the surface layer of the steel sheet. However, if the coiling temperature of the hot-rolled steel sheet is too low, the amount of these elements will decrease. The coil is cooled slowly at a temperature lower than that required for oxidation removal, making it difficult to achieve sufficient results. In addition, if the coiling temperature is too high, it becomes difficult to ensure the tensile strength and other properties of the steel sheet. There is a possibility that excessive internal oxidation during hot rolling will cause a decline in plating quality, such as plating adhesion. Sometimes I do.

[0040] Heating of hot rolled coil edge: 600-800℃ for 5-24 hours In one embodiment of the present invention, the average value of Mn / Si in the oxides in the surface layer of the edge portion is increased, and the steel In order to reduce the average Mn / Si value of the internal oxides at a depth of 100 nm or more inside the plate, The edge of the hot rolled coil can also be heated. This means that both ends in the width direction of the sheet, i.e., the edge portions, are heated. By this heating, the edge portion is preferentially heated to a temperature suitable for oxidation. The inside of the coil remains hot, but the edges cool relatively quickly, which Therefore, the time that the temperature is maintained at a level suitable for internal oxidation is shorter than that at the edge. The removal of oxidizing elements is less active at the edge than at the center of the workpiece. This can be used as a method for removing oxidizing elements from the edge portion.

[0041] In other words, when heating the edge portion, the edge portion takes priority, as opposed to the case of cooling after winding. The edge temperature is maintained at a suitable temperature for internal oxidation. As a result, the thickness of the internal oxide layer at the edge portion increases. The heat temperature must be 600°C or higher (based on the temperature of the steel plate edge). If the temperature is too high, the tensile strength of the steel sheet will decrease and excessive scaling will occur at the edges during heating. or a porous high-oxide scale (hematite) is formed on the surface after pickling. The edge temperature may be set to 800°C or less, as this may cause deterioration. A more preferable edge heating temperature is 600 to 750°C. The edge heating can be carried out in a heat treatment furnace.

[0042] In addition, the Mn / S oxide in the surface layer between the edge and center in the width direction generated during winding The average value of the Mn / Si ratio and the average value of the internal oxide at a depth of 100 nm or more inside the steel sheet are uneven. To eliminate this problem, the edge heating time must be 5 hours or more. If the edge heating time is too long, the tensile strength of the steel sheet will decrease and excessive scale will form. Or, on the contrary, the average value of Mn / Si value of oxides in the surface layer of the steel sheet at the edge becomes excessively high. The average Mn / Si value of the oxide inside the steel sheet may become too low. The heating time can be up to 24 hours.

[0043] According to one embodiment of the present invention, the edge portion heating is performed by a combustion heating method using an air-fuel ratio adjustment. In other words, the oxygen fraction in the atmosphere can be changed by adjusting the air-fuel ratio. However, when the oxygen partial pressure is high, the highly oxidizing Hematite (Fe2O3) and the interface oxide Fayalite (Fe2SiO4), which is a metal oxide, is generated excessively, which deteriorates the quality of the plating surface. If the oxygen partial pressure is low, it may be difficult to ensure the appropriate heating temperature. Although not limited thereto, in one embodiment of the present invention, oxygen is adjusted to 1 to 2 by adjusting the air-fuel ratio. % nitrogen atmosphere. Anyone with experience in this field will be able to control the oxygen fraction through adjustment of the air-fuel ratio without any particular difficulty. Therefore, this will not be discussed separately.

[0044] Pickling treatment: carried out at a threading speed of 180 to 250 mpm The hot-rolled steel sheet that has undergone the above process is placed in a hydrochloric acid bath to remove hot-rolling scale. Pickling treatment is carried out. During pickling, the concentration of hydrochloric acid in the hydrochloric acid bath is in the range of 10 to 30% by volume. The pickling speed is 180-250 mpm. If the pickling speed exceeds 250 mpm, The surface scale of the steel sheet may not be completely removed, and the pickling speed is lower than 180mpm. If this is not done, the surface of the base steel will be corroded by hydrochloric acid, so the pressure should be 180 mpm or more.

[0045] Cold rolling: reduction rate 35-60% After pickling, cold rolling is carried out. The cold rolling reduction is in the range of 35 to 60%. If the cold rolling reduction is less than 35%, there is no particular problem, but the driving force for recrystallization during annealing is insufficient. In addition, it can be difficult to adequately control the microstructure. When the oxide is annealed, the average Mn / Si value of the appropriate surface oxide and the depth of 100 mm inside the steel sheet are It is difficult to have an average Mn / Si value of m or more in the inner oxide.

[0046] The above-mentioned cold rolling process may be followed by a process of annealing the steel sheet. Even during the annealing process, the average Mn / Si value of the oxides in the surface layer of the steel sheet and the depth of 100 nm or more in the steel sheet Since the average value of Mn / Si in the inner oxide is significantly different, in one embodiment of the present invention, The average Mn / Si value of the oxide and the Mn / Si value of the internal oxide at a depth of 100 nm or more inside the steel sheet The annealing process can be controlled under conditions that appropriately control the average value of the annealing temperature. The dew point in the annealing furnace can be controlled by the following conditions.

[0047] Sheet threading speed during annealing: 40 to 130 mpm To ensure sufficient productivity, the threading speed of the cold-rolled steel sheet must be 40 mpm or more. However, if the threading speed is too fast, it may be disadvantageous in terms of securing the quality of the material. Therefore, in one embodiment of the present invention, the upper limit of the threading speed can be set to 130 mpm. Cut.

[0048] Annealing furnace soaking zone temperature and dew point control: Controlled from 650-900°C to -10-30°C In order to control the Mn / Si ratio in the oxides in the interior and surface layers within an appropriate range, the annealing furnace is soaked. It is advantageous to control the dew point in the zone. If the dew point is too low, the surface oxidation will occur instead of the internal oxidation. Surface oxidation may occur, resulting in the formation of oxides of Si, Mn, etc. on the surface. Therefore, the dew point must be controlled to -10°C or higher. If the dew point is too high, oxidation of Fe may occur, so the dew point should be kept below 30°C. In this way, the temperature for dew point control must be controlled so that a sufficient internal oxidation effect is produced. However, if the temperature is too high, the silicon and other The formation of surface oxides not only hinders oxygen from diffusing into the interior, but also prevents the uniform heating of the During heating, excessive austenite is generated, which reduces the carbon diffusion rate and thus the internal oxidation level. The size of the austenite in the soaking zone may grow excessively, causing material softening. It also generates a load on the annealing furnace, shortening the equipment life and increasing the process cost. Therefore, the temperature for controlling the dew point should be 900°C or less. It is possible.

[0049] At this time, the dew point can be determined by adding humidified nitrogen containing water vapor (N2 + H2O) to the annealing furnace. It can be adjusted.

[0050] Hydrogen concentration in the annealing furnace (soaking zone): 5 to 10% by volume The atmosphere in the annealing furnace is maintained as a reducing atmosphere by adding 5 to 10% by volume of hydrogen to nitrogen gas. If the hydrogen concentration in the annealing furnace is less than 5% by volume, the reduction ability decreases and the surface oxide becomes excessive. This leads to a problem of poor surface quality and plating adhesion, and low resistance to LME. When the hydrogen concentration is high, no particular problems occur, but as the amount of hydrogen gas used increases, The hydrogen concentration is limited due to the associated increase in cost and the risk of explosion inside the furnace due to the increased hydrogen concentration.

[0051] Steel sheet drawing temperature in the plating bath: 420~500℃ If the temperature at which the steel sheet is drawn into the plating bath is low, the wettability at the contact interface between the steel sheet and the liquid zinc decreases. If the temperature is too high, the steel sheet and The reaction with the liquid zinc occurs excessively, and the Zetta phase, an Fe-Zn alloy phase, forms at the interface. This causes the adhesion of the coating layer to deteriorate, and the amount of iron element eluted from the steel sheet in the coating bath becomes excessive. There is a problem of dross being generated in the plating bath.

[0052] The Al concentration in the plating bath must be at an appropriate level to ensure the wettability of the plating layer and the fluidity of the plating bath. GI should be maintained at 0.2-0.25 wt% and ZM at 0.7% Al. ~13.2 wt%, Mg 0.7~5.7 wt% to maintain appropriate levels. Therefore, dross formation in the plating bath is maintained at an appropriate level, and the plating surface quality is improved. In the present invention, the coating weight is not particularly limited, and It can be adjusted appropriately to suit the quality required. [Example]

[0053] The present invention will be described in more detail below through examples. However, the following examples are merely examples of the present invention. The present invention is not intended to limit the scope of the present invention. It should be noted that the scope of the present invention is limited to the matters described in the claims and This is because the matter is determined by the matter and matters that can be reasonably inferred from it.

[0054] (Example) Steel slabs having the composition shown in Table 1 below (the remaining components not shown in the table are Fe and In addition, B and N in the table are shown in ppm units, and the remaining components are shown in weight percent units. The temperature at the start of finish rolling and the temperature at the end of finish rolling are then reheated to 1,213°C. The hot rolled coils were then heated to 1,117°C and 972°C, respectively, and the edge Heating was carried out for 8 hours in a nitrogen atmosphere with an oxygen concentration of 1.5% by volume or less. The sheet was passed through a 15% by volume hydrochloric acid bath at 180 mpm, pickled, and cold-rolled. The sheet was annealed in an annealing furnace in a soaking zone in an atmosphere of 8% hydrogen by volume and the remainder nitrogen gas. I is a plating bath containing 0.24 wt% Al, ZM is a plating bath containing 1.75 wt% Al and 1.55 wt% Al. % Mg in a zinc-based plating bath to perform hot-dip galvanizing, and then an air knife The coating weight was adjusted through a pressure of 1000 kJ / min, and the hot-dip galvanized steel sheet was obtained by cooling.

[0055] In all examples, the drawing temperature of the steel sheet into the hot dip galvanizing bath was 475°C. Other conditions for each example are as shown in Table 2.

[0056] [Table 1]

[0057] [Table 2]

[0058] The properties of the hot-dip galvanized steel sheet manufactured by the above process were measured, and the liquid The results are shown in Table 3. Unless otherwise specified, all test results are taken at a point 1 mm away from the edge of the steel plate. When the test piece is large, the edge The test piece was measured with the specified dimensions starting from a point 1 mm away from the edge. Specifically, the steel plate was cut widthwise and spot welded along each cut edge. After applying the spot welding current twice, a hold time of 1 cycle is maintained. Spot welding was performed on three different materials. The 4t materials were stacked in order and spot welded. When spot welding, a new electrode was placed on the soft material. After 15 weldings, the electrode was worn and then used as the target material for spot welding. Measure the upper limit current at which ON occurs. After measuring the upper limit current, set it to 0.5 and Spot welding was performed eight times at a current 1.0 kA lower than the standard current. After precision machining by electrochemical machining, the specimen was epoxy mounted and polished, and the crack length was measured using an optical microscope. The magnification of the optical microscope was set to 100 times, and cracks were found at that magnification. If no cracks are found, it is determined that liquid metal embrittlement has not occurred. The length was measured using image analysis software. PE cracks are 100 μm or less, and C-type cracks are judged to be in good condition when not observed. Ta.

[0059] The lower inhibition layer (IL) of the coating layer (mainly consisting of Zn-eta phase) is applied to the surface of the steel sheet. The ratio of the plating solution to the total weight of the plated metal is 100%. After dissolving the layer, the surface was neutralized and washed to clean it, and then EPMA magnification 500x In photographs where Al distribution was analyzed at three or more points, the highest concentration (CPS standard) was 10% or more. The area with the above Al concentration is considered to be the area where the inhibition layer exists, and the proportion of this area is analyzed by image analysis. The software calculates the average area percentage, and if this value is 60 area percent or more, it is considered good. The size of the unformed portion of the suppression layer was also classified in the same manner. In all examples, there was no portion where the suppression layer having a size of 15 μm was not formed. It was confirmed that the average width of the particles that make up the layer falls within the range of 0.5 to 4 μm.

[0060] The tensile strength was measured by a tensile test using a sample prepared in the C direction of the JIS-5 standard. The plating weight was measured by a wet dissolution method using a hydrochloric acid solution.

[0061] The sealer adhesion is measured by adhering the D-type automotive structural adhesive to the plated surface and then applying it to the steel. The plate was bent 90 degrees to check whether the plating would come off.

[0062] Visually check for defects such as unplated steel sheets, check the surface quality, and If defects were found during visual inspection, the product was judged to be defective.

[0063] [Table 3]

[0064] Inventive Examples 1, 2, 3, 4, 5, 6, 7, and 8, the steel composition satisfies the range proposed by the present invention. The manufacturing method also satisfies the scope of the present invention, and the tensile strength, plating quality, plating adhesion amount and spot The weld LME crack length was also good.

[0065] In Comparative Examples 1 and 8, although the manufacturing method satisfies the ranges proposed by the present invention, the steel composition does not satisfy the ranges proposed by the present invention. In Comparative Examples 1 and 8, Mn and Si, respectively, exceeded the ranges proposed by the present invention. The area ratio of the inhibitor layer applied to the steel sheet does not meet the standard when the dew point of the annealing furnace rises. The excessive Si and Mn contents also caused excessive formation of surface oxides during annealing. As a result, a sufficient decarburized layer cannot be formed, and the alloying suppression layer cannot be formed sufficiently at the interface between the coating layer and the base steel. The LME cracks did not meet the standards, and the surface was left unplated, resulting in a deterioration in surface quality. However, SBT peeling occurred and plating adhesion was poor.

[0066] In Comparative Examples 6 and 11, the coiling temperature during the hot rolling process did not satisfy the range proposed by the present invention. In Comparative Example 6, the hot rolling coiling temperature was lower than the range suggested by the present invention, and the internal The amount of oxidation is insufficient, so the area ratio of the inhibitor layer applied to the steel sheet is less than 60%. Comparative Example 11 is a hot rolled coiler according to the present invention. The amount of internal oxidation that occurs during the hot rolling process is sufficient, so the LME characteristics are not obtained. However, excessive hot rolling scale was generated and the scale was not completely removed during pickling. , the surface quality is poor due to the occurrence of uncoated parts, and the hot rolling temperature is too high, Softening occurred and did not recover even after annealing, resulting in poor quality of the material.

[0067] In Comparative Examples 13 and 14, the heat treatment temperature during the heat treatment process of the edge portion was within the range proposed by the present invention. Comparative Example 13 is a case where the heating temperature in the heat treatment was outside the range proposed by the present invention. Excessive interfacial scale and highly oxidized scale are formed on the edge, resulting in scale deterioration after pickling. The surface quality is poor and the edge is rough because the silicon oxide remains in the form of silicon oxide. The part was heated too much, causing softening and failing to meet the tensile strength requirement. The surface reaction was not active in a high dew point atmosphere due to the material, so the LME resistance did not meet the standard. In Comparative Example 14, the heating temperature of the edge portion was lower than the range proposed by the present invention, and the inner The partial oxide layer could not be formed sufficiently, and a sufficient inhibition layer could not be formed during the plating process. The surface quality and sealer adhesion were poor and the LME resistance was not met.

[0068] In Comparative Example 4, the dew point in the furnace during annealing was lower than the range suggested by the present invention. Even if a sufficient internal oxide layer is generated over the entire width during the rolling and heating process, the dew point during the annealing process after cold rolling is not high enough, the internal oxide layer is not formed sufficiently, and the area of ​​the inhibitor layer applied to the steel sheet The area ratio is less than 60%, which means that a sufficient decarburization level cannot be achieved. The GI material has a low dew point, which prevents sufficient internal oxidation, and the surface oxidation There was excessive buildup and poor surface quality.

[0069] In Comparative Example 2, the internal dew point range of the annealing furnace exceeded the range suggested by the present invention. The internal oxidation was sufficient and LME was satisfied, but the material deteriorated due to excessive internal oxidation. and did not meet the standards.

[0070] Comparative Examples 3 and 7 were produced under conditions where the speed of the steel sheet during annealing was higher than the range suggested by the present invention. This is the case when the steel sheet is manufactured in such a way that there is sufficient time for the decarburization reaction to occur between the steam in the annealing furnace and the steel sheet. As a result, internal oxidation of the surface layer of the steel sheet after annealing is not sufficiently formed, and the steel sheet is deformed by the applied suppression film. The area ratio of the layer is less than 60% and exceeds the standard for spot welding LME crack evaluation. It was poorly done.

[0071] In Comparative Example 12, the temperature of the soaking zone in the annealing furnace exceeded the range suggested by the present invention. The amount of external oxidation increases, and the amount of internal oxidation is not sufficient, so the steel plate is coated. The area ratio of the suppression layer was less than 60% and the LME crack did not meet the criteria. In addition, excessive austenite formation and growth occurred in the soaking zone. As a result, the material did not meet the standards and was defective.

[0072] Comparative Example 10 is a case where the temperature of the soaking zone in the annealing furnace was lower than the range proposed in the present invention. The annealing temperature is low, so the oxidation reaction between the steam and the steel sheet is insufficient, and internal oxidation is insufficient. The area ratio of the inhibitor layer applied to the steel sheet is less than 60% by area, and the LME The rack did not meet the standard, resulting in poor spot weldability. Because crystallization was insufficient, the target microstructure was not formed and the material did not meet the standards. It was therefore defective.

[0073] Comparative Example 5 is a case where the speed of the steel sheet passing through the annealing chamber was lower than the range suggested by the present invention. Although sufficient reaction time with the steam in the annealing furnace was ensured to form a sufficient internal oxide layer, excessive The grain growth caused the material to not meet the standards.

[0074] Comparative Example 9 was produced with a cold rolling reduction rate exceeding the standard set forth in the present invention. The internal oxide layer that is formed by cold rolling is thinned due to excessive cold rolling, and the amount of internal oxidation is insufficient. The area ratio of the applied inhibitor layer is less than 60% and the LME crack does not meet the standard. It was therefore defective.

[0075] Therefore, the advantageous effects of the present invention were confirmed.

Claims

1. Base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, Between the base steel sheet and the hot-dip galvanized layer, there is provided an inhibition layer made of an Fe-Al alloy. Hot-dip galvanized steel sheet, which is formed at a ratio of 60% or more in total.

2. 2. The hot-dip galvanizing method according to claim 1, wherein the suppression layer is formed with an area ratio of 70% or more. steel plate.

3. 2. The hot-dip galvanizing method according to claim 1, wherein the average width of the particles constituting the inhibiting layer is 4 μm or less. Plated steel. Here, the average width of the particles is determined by drawing a straight line in one direction in a photograph of the suppression layer viewed from above. This means that the length of the line segment that overlaps with the particle is taken as the width of the particle, and the average is calculated.

4. The molten zinc oxide according to claim 1, wherein the average width of the particles constituting the suppression layer is 0.5 μm or more. Lead-plated steel sheet.

5. 2. The portion of the sheet on which the suppression layer is not formed has an average equivalent circle diameter of 15 μm or less. The hot-dip galvanized steel sheet according to claim 1.

6. The base steel sheet contains C: 0.05 to 1.5%, Si: 2.0% or less, Mn: 1.0 to 10 %, S-Al (acid soluble aluminum): 3% or less, Cr: 2.5% or less, Mo: 1% or more Bottom, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, V: 0.2% or less and a composition containing Sb + Sn + Bi: 0.1% or less and N: 0.01% or less.

2. The hot-dip galvanized steel sheet according to claim 1.

7. providing a steel slab; reheating the slab at a temperature of 950-1300°C; The reheated slab is subjected to finish rolling at a starting temperature of 900 to 1,150°C. hot rolling at a temperature of 850 to 1,050°C to obtain a steel plate; coiling the steel sheet at a temperature in the range of 590 to 750°C; pickling the steel sheet at a threading speed of 180 to 250 mpm; cold rolling the steel sheet at a reduction ratio of 35 to 60%; The soaking zone temperature and dew point temperature are 650 to 900°C and -10 to +30°C, respectively. The cold-rolled steel sheet is heated under humidified nitrogen conditions containing 5 to 10% by volume of H2 in the ambient gas. and then recrystallization annealing; The steel sheet is immersed in a hot-dip plating bath at a drawing temperature of 420 to 500°C. a step of:

8. The hot-dip galvanizing bath according to claim 7, wherein the hot-dip galvanizing bath contains 0.2 to 0.25 wt % Al. Manufacturing method for coated steel sheets.

9. The hot-dip plating bath contains 0.7 to 13.2 wt. % Al and 0.7 to 5.7 wt. % Mg. The method for producing a hot-dip galvanized steel sheet according to claim 7, comprising:

10. 10. Any one of claims 7 to 9, wherein the sheet threading speed during the recrystallization annealing is 40 to 130 mpm. Item 1. A method for producing a hot-dip galvanized steel sheet according to item 1.

11. The coiled steel sheet is edge-heated at 600 to 800°C for 5 to 24 hours. The method for producing a hot-dip galvanized steel sheet according to any one of claims 7 to 9.

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