Galvanized steel sheet with excellent fatigue strength of electrical resistance spot welded joints, and method for manufacturing the same.

Abstract

To provide a zinc plated steel sheet having excellent fatigue strength of electrical resistance spot welds, and a manufacturing method thereof.SOLUTION: According to an aspect of the present disclosure, a zinc plated steel sheet includes a base steel sheet and a zinc-based plating layer formed on a surface of the base steel sheet. Concentration profiles of oxygen and one or two of silicon and manganese measured in a depth direction from the surface of the base steel sheet have forms in which the maximum points appear in the depth direction from the surface. An absolute value of a difference between a depth at which the maximum point of the concentration profile of oxygen is formed and a depth at which the maximum point of the concentration profile of one of silicon and manganese is formed is 0.5 μm or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a galvanized steel sheet having excellent fatigue strength at an electric resistance spot weld and a method for producing the same.

Background Art

[0002] Due to problems such as environmental pollution, regulations on automobile exhaust gas and fuel consumption are being strengthened day by day. As a result, the demand for reducing fuel consumption by reducing the weight of automobile steel sheets has increased, and accordingly, various types of high-strength steel sheets with high strength per unit thickness have been developed and put on the market.

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

[0004] On the other hand, automobile steel materials are supplied in the form of galvanized steel sheets with plating on the surface to ensure corrosion resistance. Among them, galvanized steel sheets (GI steel sheets) or alloyed galvanized steel sheets (GA) have high corrosion resistance by using the sacrificial corrosion protection characteristics of zinc, and are therefore widely used as automobile materials. <00​​​​​​​​​​​Tensile stress generated during welding is difficult to relieve through plastic deformation, leading to the formation of microcracks on the surface. It is highly likely that when welding is performed on high-strength galvanized steel sheets, the zinc, which has a low melting point, will melt into the steel sheet. It can penetrate into microcracks. As a result, liquid-phase metal embrittlement occurs. A phenomenon called al emrittlement (LME) occurs, causing the steel plate to break. Not only does this occur, but there is also a possibility that fatigue strength will decrease, which is a problem for steel plates. This poses a major obstacle to increasing strength. [Overview of the project] [Problems that the invention aims to solve]

[0006] According to one aspect of the present invention, a galvanized steel sheet with excellent fatigue strength in the electrical resistance spot welded joint. A method for manufacturing the same is provided.

[0007] The problems that the present invention will address are not limited to those described above. Anyone with knowledge of this subject can understand the overall content of the specification of this invention and further problems of the present invention. It is not difficult to understand. [Means for solving the problem]

[0008] According to one aspect of the present invention, a galvanized steel sheet is formed on a base steel sheet and on the surface of the base steel sheet. A galvanized steel sheet containing a zinc-based plating layer, wherein the depth measurement from the surface of the base steel sheet is The defined concentration profiles of oxygen and one or two of silicon and manganese are on the surface. It has a form in which a maximum point appears in the depth direction, and the maximum point of the oxygen concentration profile is formed. The depth at which the maximum concentration profile of one of the silicon and manganese described above is formed is determined by the depth at which the maximum concentration of one of the silicon and manganese is formed. The galvanized steel sheet may be one in which the difference in depth of the coating is 0.5 μm or less in absolute value.

[0009] Also, a method for manufacturing a galvanized steel sheet according to one aspect of the present invention includes heating a steel slab at a temperature of 950 to 135 0°C, hot-rolling the steel slab to obtain a steel sheet, winding the steel sheet at a temperature of 5 90 to 750°C to obtain a hot-rolled steel sheet, pickling the hot-rolled steel sheet at a through-feed speed of 180 to 250 mpm, cold-rolling the hot-rolled steel sheet with a reduction rate of 35 to 60% to obtain a cold-rolled steel sheet, recrystallization annealing the cold-rolled steel sheet in an atmosphere with a dew point of -10 to 30°C at 650 to 900°C, and hot-dip galvanizing the annealed cold-rolled steel sheet. This is good. [[ID=^13]]

Advantages of the Invention

[0010] As described above, the present invention can achieve softening of the base steel sheet near the surface of the base steel sheet, that is, near the interface between the plating layer and the base steel sheet, by appropriately controlling the concentration profiles of O, Si, and Mn formed inside the base steel sheet on which the plating layer is formed. By forming a soft layer on the surface, the tensile stress generated during spot welding is eliminated by the plastic deformation of the soft layer, resulting in a reduction in the number and length of cracks. As a result, a high-strength galvanized steel sheet with excellent fatigue strength at the spot weld can be manufactured.

Brief Description of the Drawings

[0011] [Figure 1] It is a graph showing the GDOES profiles of oxygen (O) and silicon (Si) or manganese (Mn) for measuring the difference in maximum point depth according to an embodiment of the present invention.

Modes for Carrying Out the Invention

[0012] Hereinafter, the present invention will be described in detail with reference to several embodiments. It should be noted that there seems to be an error in the original text where "135" in line 3 is likely incomplete. Also, the translation of "

発明の効果

Advantages of the Invention

Effects of the Invention

[0013] In the present invention, the galvanized steel sheet includes not only galvanized steel sheets (GI steel sheets), but also alloy galvanized steel sheets (GA), and it should be noted that it is a concept including all plated steel sheets on which a zinc-based plating layer mainly containing zinc is formed. When it is said that zinc is mainly contained, it means that the proportion of zinc among the elements contained in the plating layer is the highest. However, in the case of alloy galvanized steel sheets, the proportion of iron may be higher than that of zinc, and steel sheets in which the proportion of zinc among the remaining components excluding iron is the highest can be included in the scope of the present invention.

[0014] The inventors of the present invention focused on the fact that liquid metal embrittlement (LME) occurring during welding is caused by microcracks generated from the surface of the steel sheet, which may lead to fatigue fracture, and conducted research on means for suppressing surface microcracks. As a result, they found that it is necessary to soften the surface of the steel sheet, and thus the present invention was achieved.

[0015] Generally, in the case of high-strength steel, in order to ensure the hardenability and austenite stability of the steel, a large amount of elements such as carbon (C), manganese (Mn), and silicon (Si) can be contained. However, these elements play a role in increasing the sensitivity of the steel to cracks. Therefore, steel containing a large amount of these elements is likely to generate microcracks, which ultimately cause liquid metal embrittlement during welding.

[0016] According to the research results of the present inventors, the oxygen (O) concentration profile formed in the depth direction from the surface of the base steel sheet, and the concentration profiles of silicon (Si) and / or manganese (Mn) ​​​​​​​​​​When the relationship between the two is properly controlled, the resistance of the spot weld to LME increases, and This can increase the fatigue strength of the spot weld.

[0017] In other words, as illustrated in Figure 1, the depth at which a maximum point appears in the oxygen (O) concentration profile (meaning the depth from the surface of the base steel sheet) and silicon (Si) and / or manganese ( The difference in depth at which the maximum point appears in the concentration profile of Mn must be controlled to within 0.5 μm. There is.

[0018] Thus, maximum concentration profiles for oxygen, silicon, and / or manganese appear. When matching depths, silicon and / or manganese are present in the form of oxides inside the base steel sheet. This means that it is fixed to a certain depth, thereby preventing silicon and / or manganese from forming on the surface. The concentration can be reduced.

[0019] Typically, during the annealing process, silicon and / or manganese diffuse to the surface, forming a surface oxide. In addition, the activity of the silicon and / or manganese on the surface is also The activity of silicon and / or manganese on the surface (proportional to the content) increases. A higher level means not only that the content of these elements increases, but also the table This means reducing the oxygen activity on the surface, making it difficult to remove carbon present on the surface. Thus, when the silicon and manganese content on the surface increases and decarburization does not occur This increases the surface's susceptibility to cracks, making it more prone to cracking, and as a result The likelihood of an increase in LMEs is high.

[0020] By the way, as in the present invention, the concentration profiles of oxygen and silicon and / or manganese When the relationship is controlled, it is not just a matter of reducing the concentration of silicon and / or manganese on the surface. Furthermore, decarburization can occur smoothly, allowing a soft layer to form on the surface. In this case, even if tensile stress is applied during welding, the stress is absorbed by plastic deformation. This allows for effective suppression of crack formation.

[0021] Therefore, the present invention relates to the depth at which the maximum point of the oxygen (O) concentration profile occurs (hereinafter (Also simply called "maximum point depth") and the concentration of silicon (Si) and / or manganese (Mn) By controlling the depth difference at which the maximum point of the degree profile occurs to within 0.5 μm, the surface layer The goal is to soften the material and suppress LME generation. In one embodiment of the present invention, the difference in depth is 2 This can be interpreted as the absolute value of the difference between the depths at which the maximum values ​​of the two profiles are formed. According to one embodiment of the invention, the above depth difference can be limited to within 0.3 μm, and other In one embodiment, the value can be limited to within 0.2 μm or within 0.1 μm. Since the maximum points may be formed at the same location, no lower limit is specifically set for the difference in their values, 0 μm. This may also be the case. Furthermore, even if one or more maximum points are formed in the concentration profile for each element, However, in this invention, the term "maximum point" refers to the maximum point closest to the surface.

[0022] In one embodiment of the present invention, the maximum depth of the oxygen is the silicon and / or manganese This can be smaller than the maximum point depth of the element. This more effectively suppresses the diffusion of silicon and / or manganese onto the surface. can.

[0023] According to one embodiment of the present invention, the maximum point depth of oxygen and one of silicon and manganese Alternatively, if the difference between the maximum depths of the two elements satisfies the above-mentioned range, the advantageous effects of the present invention can be obtained. This is possible. However, in order to more reliably obtain the effects of the present invention, the maximum depth of the oxygen is important. The difference between this and the maximum depth of silicon and manganese can fall within the range described above.

[0024] The concentration profiles of oxygen, silicon, and manganese are known in various fields. Since it can be measured by law, it is not necessarily limited. However, in one embodiment of the present invention, GDOES(Glow Discharge Optical Emission Sp Using ectrometry, the profile measured from the surface to the interior of the raw steel sheet is utilized. It can be used.

[0025] In one embodiment of the present invention, in order to more reliably form a soft layer, the oxygen concentration at the maximum point is A degree of 0.3% by weight or higher is even more advantageous. The higher the oxygen maximum point appears, the better the steel. This securely fixes the silicon and manganese present inside the plate and prevents them from migrating to the surface. This is because it is possible to do so. Furthermore, according to one embodiment of the present invention, the oxygen concentration at the above maximum point is It may be 0.4% by weight or more, and in some cases 0.5% by weight or more. There is no need to specifically define an upper limit for oxygen concentration at a point, but typically the upper limit for oxygen concentration at a maximum point is 1 It can be defined as 0.0% by weight or less, 0.8% by weight or less, or 0.7% by weight or less.

[0026] Therefore, in one embodiment of the present invention, the overall composition of the steel is high alloy steel for high strength. The composition is made to be high, but a soft layer is formed in the surface layer where cracks occur. At the same time, by controlling the distribution of internal oxides, resistance to LME during welding is improved. This can improve the fatigue strength of the welded joint.

[0027] In this invention, the maximum points of each element and the concentrations at those maximum points can be determined as follows. First, as shown in Figure 1, the GDOES profile is obtained. At this time, the above GDOE The S profile can be obtained at depth intervals of 10 to 30 nm, according to the present invention. In one embodiment, data obtained at a depth interval of 20 nm was used. The first data obtained As shown in Figure 1, it has a shape with a near-maximal point, but determining its exact position This can sometimes be a little difficult. In this case, the oxygen concentration at each point is measured at that point and 2% before and after it. When calculated using the 5-point average obtained by averaging the data values ​​of each location, it shows a relatively smooth shape. It is possible.

[0028] From the oxygen concentration profile obtained through this process, the maximum point and the corresponding point can be identified. The oxygen concentration can be determined. The minimum point is in the smoothed oxygen concentration profile. The point where the lowest value is observed is the local maximum, and the point where the highest value is observed after the local minimum is the local maximum. Meaning: In one embodiment of the present invention, the maximum point of the oxygen concentration profile is the steel plate It may appear at a depth of 4 μm from the surface. This occurs when the maximum point appears at a depth too close to the surface. Furthermore, since a soft layer may not easily form on the surface, the above maximum point is located at a certain depth or more from the surface. It is advantageous for it to be formed at a certain point. Conversely, if the maximum point is very far from the surface... However, since the depletion effect of silicon, manganese, etc. may not be sufficient, one embodiment of the present invention Depending on the morphology, the above maximum point may appear at a depth of 15 μm or less from the surface of the steel plate. According to another embodiment of the invention, the maximum point of the oxygen concentration profile is 10 μm from the surface. It appears at a depth of less than 10 μm from the surface, and more preferably at a depth of less than 10 μm from the surface.

[0029] In one embodiment of the present invention, the oxygen concentration profile of the GDOES in the depth direction is that of steel Measurements taken at the center of the plate in the width direction can be used. However, generally speaking, steel Because the values ​​are often higher at the edges in the width direction compared to the center in the width direction of the board, To more effectively improve pot weldability, use the profile measured at the edge. It is also possible to do so. In this case, the edge refers to both ends of the steel plate, but at the above point If contamination occurs or there are other problems with the integrity of the test specimen, check within 1 mm in the width direction from the edge. It can also mean a point on the side.

[0030] The steel plates covered by this invention are high-strength steel plates with a strength of 490 MPa or higher, and their types are as follows: Not limited. However, the steel plates covered by this invention are not necessarily limited to these. The weight ratios are: C: 0.05~1.5%, Si: 2.0% or less, Mn: 1.0~30%. S-Al (acid-soluble aluminum): 3% or less, Cr: 2.5% or less, Mo: 1% or less , B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, V: 0.2% or less It can have a composition containing Sb+Sn+Bi: 0.1% or less, and N: 0.01% or less. The remaining components are iron and other impurities, as well as others not listed above. This does not rule out the possibility of further inclusion of elements that may be present in steel, up to a total of 1.0% or less. In this invention, the content of each component element is expressed by weight unless otherwise specified. The composition described above refers to the bulk composition of the steel plate, that is, the composition at a point 1 / 4 of the steel plate thickness. (The same applies hereafter.)

[0031] In some embodiments of the present invention, TRIP steel and the like are used as the high-strength steel plate. These steels can be further classified into the following compositions:

[0032] Steel composition 1:C:0.05~0.30% (preferably 0.10~0.25%), Si:0 0.5-2.5% (preferably 1.0-1.8%), Mn: 1.5-4.0% (preferably 2.0-3.0%), S-Al: 1.0% or less (preferably 0.05% or less), Cr: 2 0% or less (preferably 1.0% or less), Mo: 0.2% or less (preferably 0.1% or less) ), B: 0.005% or less (preferably 0.004% or less), Nb: 0.1% or less (preferably (or 0.05% or less), Ti: 0.1% or less (preferably 0.001-0.05%) Sb+Sn+Bi: 0.05% or less, N: 0.01% or less, remainder Fe and unavoidable impurities. Includes. In some cases, elements not listed above that may be present in the steel may be included in total. It can also include values ​​below 0%.

[0033] Steel composition 2: C: 0.05~0.30% (preferably 0.10~0.2%), Si: 0. 5% or less (preferably 0.3% or less), Mn: 4.0~10.0% (preferably 5.0~ 9.0%), S-Al: 0.05% or less (preferably 0.001-0.04%), Cr: 2.0% or less (preferably 1.0% or less), Mo: 0.5% or less (preferably 0.1-0 0.35%), B: 0.005% or less (preferably 0.004% or less), Nb: 0.1% or less Lower (preferably 0.05% or less), Ti: 0.15% or less (preferably 0.001-0. 1%), Sb+Sn+Bi: 0.05% or less, N: 0.01% or less, remainder Fe and unavoidable Contains impurities. In some cases, elements that may be present in the steel, although not listed above, may also be present. It can further include up to a total of 1.0% or less.

[0034] Furthermore, if the lower limit of the content of each of the above-mentioned component elements is not specified, these will be left to the discretion of the user. It means that it can be considered an element, and its content can be 0%.

[0035] According to one embodiment of the present invention, the surface of the steel sheet has one or more plating layers. The above plating layer is GI (Galvanized) or GA (Galvanized). It may be a zinc-based plating layer containing (nealed), etc. In the present invention, as described above, The difference between the maximum depth of oxygen and the maximum depth of silicon and / or manganese was appropriately controlled. Therefore, even if a zinc-based plating layer is formed on the surface of the steel plate, liquid phase metal generated during spot welding may still be present. This can mitigate the problem of embrittlement (LME).

[0036] According to one embodiment of the present invention, if the zinc-based plating layer is a GA layer, the degree of alloying ( The Fe content (meaning the Fe content within the layer) is controlled to 8-13% by weight, preferably 10-12% by weight. It can be controlled. If the degree of alloying is insufficient, the zinc in the zinc-based plating layer will be minutely crystalline. There remains a possibility that it may penetrate the rack and cause problems with liquid-phase metal embrittlement, or conversely, alloying. If the concentration is too high, problems such as powdering may occur.

[0037] Furthermore, the amount of zinc-based plating layer deposited is 30-70 g / m². 2That's fine. If the amount of plating is too small, it will be difficult to obtain sufficient corrosion resistance, while if the amount of plating is too large... If this is the case, there is a possibility of increased manufacturing costs and problems with liquid phase metal embrittlement, therefore, as mentioned above. It is controlled within the specified range. A more preferable range for the amount of plating deposited is 40-60 g / m². 2 and Good. The amount of plating adhesion refers to the amount of plating layer attached to the final product, and the plating layer If the layer is a GA layer, the amount of plating adhesion increases due to alloying, therefore, before alloying, The weight may decrease slightly. This varies depending on the degree of alloying, so it is not necessarily limited to this. It is not the case that the amount of plating deposited before alloying (i.e., the amount of plating deposited from the plating bath) It is acceptable for the value to be approximately 10% lower than that.

[0038] The following describes one embodiment of how to manufacture the steel sheet of the present invention. However, the steel sheet of the present invention is It is not necessarily required to be manufactured according to the embodiments described below; the embodiments described below are steel plates of the present invention. It should be noted that this is a preferred method for manufacturing [the product].

[0039] First, a steel slab with the above composition is provided, and then hot-rolled and wound. Steel plates can be manufactured. While there are no particular restrictions on conditions such as hot rolling, this invention... In one embodiment, the slab heating temperature and winding temperature can be limited as follows.

[0040] Slab heating: 950~1350℃ To sufficiently dissolve the solid solution elements and reduce rolling resistance, the slab is heated at a temperature of 950°C or higher. Heating is required. In the case of the present invention, a large amount of alloying elements may be present, therefore the above The heating temperature is 1000°C or higher, preferably 1100°C or higher, and more preferably 1 The temperature should be 150°C or higher. However, if the slab heating temperature is too high, oxidation of the solid solution elements may occur. Problems can occur, and the size of the austenite grains may become coarse. It is also not advantageous in terms of energy. Therefore, the upper limit of the above heating temperature is 1,350°C, preferably The temperature can be 1,300°C, more preferably 1,280°C or lower.

[0041] Winding temperature: 590~750℃ The hot-rolled steel sheets are then wound into coils for storage, but the wound-up The steel sheet undergoes a slow cooling process. Through this process, the oxidation contained in the surface layer of the steel sheet is removed. Although the elemental properties are removed, if the winding temperature of the hot-rolled steel sheet is too low, these Because the coil cools slowly at a temperature lower than the temperature required for the oxidation and removal of elements, sufficient effect can be obtained. That's difficult.

[0042] Pickling treatment: Performed at a feed speed of 180-250 mpm. To remove the hot-rolled scale from the hot-rolled steel sheet that has undergone the process described above, it is placed in a hydrochloric acid bath. The pickling process is then carried out. The hydrochloric acid concentration of the hydrochloric acid bath during pickling is in the range of 10 to 30 volume percent. The sheet metal feeding speed should be 180-250 mpm. If the pickling speed exceeds 250 mpm, In some cases, the scale on the surface of the hot-rolled steel sheet may not be completely removed, and the pickling rate is 18 If the pressure is lower than 0 mpm, the surface layer of the base iron may corrode due to hydrochloric acid, therefore 18 Perform this at 0 mpm or higher.

[0043] Cold rolling: Reduction ratio 35-60% After pickling, cold rolling is performed. The cold reduction ratio during cold rolling is in the range of 35-60%. If the cold reduction ratio is less than 35%, there are no particular problems, but the recrystallization driving force during annealing will be insufficient. This can result in situations where it is difficult to adequately control the microstructure. The cold reduction ratio exceeds 60%. As a result, the thickness of the internal oxide layer secured during hot rolling becomes thinner, and sufficient depth of internal oxidation is not achieved after annealing. Furthermore, it is difficult to obtain a maximum value for oxygen concentration.

[0044] Following the cold rolling process described above, a recrystallization annealing process of the steel sheet may be carried out. Even during the annealing process, the oxygen, silicon, and manganese concentration profiles of the GDOES in the surface layer are large. They may sound different. Therefore, in one embodiment of the present invention, each element of the GDOES in the surface layer The annealing process can be controlled under conditions that appropriately control the concentration profile in the depth direction of the element. Of these, the plate speed and the dew point inside the annealing furnace can be controlled under the following conditions.

[0045] Threading speed: 40~130mpm To ensure sufficient productivity, the feed speed of the cold-rolled steel sheets must be 40 mpm or higher. However, if the plate passing speed is excessively fast, it is disadvantageous in terms of material availability, therefore In one embodiment of the invention, the upper limit of the plate passing speed can be set to 130 mpm.

[0046] Dew point control in the annealing furnace: Controlled from 650-900°C to a range of -10-30°C. To obtain an appropriate range of surface decarburization rates, it is advantageous to control the dew point inside the annealing furnace. Yes, if the dew point is too low, surface oxidation occurs instead of internal oxidation, and Si and Mn are formed on the surface. These oxides may form, which can adversely affect the plating process. Therefore, the dew point needs to be controlled to be above -10°C. On the other hand, if the dew point is too high... Therefore, the dew point needs to be controlled to 30°C or below, as this may cause oxidation of Fe. Thus, the temperature for dew point control is 650°C, which is the temperature at which sufficient internal oxidation effect is achieved. The above may be sufficient. In one embodiment of the present invention, the temperature and dew point inside the annealing furnace described above are It can be determined based on the temperature and dew point of the uniform tropical zone. However, if the temperature is too high, S Surface oxides such as i are formed, which not only prevent oxygen from diffusing into the interior, but also contribute to the sonication of the humid climate. During heating, excessive austenite formation occurs, reducing the carbon diffusion rate and leading to internal oxidation. The level may decrease, and the size of the austenite in the uniform zone may grow excessively, resulting in softer material. This causes chemical reactions. Furthermore, it puts a load on the annealing furnace, shortening the equipment's lifespan and increasing process costs. Because this can cause problems, the temperature used to control the dew point should be below 900°C. That's fine.

[0047] At this time, the dew point is determined by introducing humid nitrogen (N2 + H2O) gas containing water vapor into the annealing furnace. This can be adjusted. According to one embodiment of the present invention, the nitrogen gas is 5-10% It can contain hydrogen (H2), which allows the dew point to be controlled within an appropriate range. Cut.

[0048] The steel sheets annealed through this process are immediately immersed in a plating bath and hot-dip galvanized. After this, the hot-dip galvanized steel sheet undergoes an alloying heat treatment process as needed. This can be done. The preferred conditions for plating and alloying heat treatment are as follows.

[0049] Draw-in temperature for plating bath steel sheets: 420~500℃ If the drawing temperature of the steel sheet in the plating bath is low, the wettability at the contact interface between the steel sheet and liquid phase zinc will be low. Because sufficient temperature cannot be secured, it is necessary to maintain a temperature of 420 degrees or higher. The temperature is excessively high. In this case, the reaction between the steel plate and liquid-phase zinc occurs too much, and the zeta phase, which is an Fe-Zn alloy phase, forms at the interface. A Zetta phase is formed, reducing the adhesion of the plating layer and causing the Fe element in the steel plate to dissolve in the plating bath. There is a problem where the output is excessive, causing dross to form in the plating bath.

[0050] Al concentration in the plating bath: 0.10~0.25% The Al concentration in the plating bath is appropriate to ensure the wettability of the plating layer and the fluidity of the plating bath. It is necessary to maintain a positive concentration. For this purpose, in this invention, the Al concentration in the plating bath is set to 0.1 It is controlled within the range of 0-0.25%. Also, depending on whether or not alloying treatment is performed, GA (alloyed molten sub Lead-plated (Galvannealed) steel sheets and GI (Hot-dip galvanized, Galvannealed) steel sheets. Although it is divided into (zed) steel plates, in one embodiment of the present invention, the dros in the plating bath (dros s) In order to maintain the formation at an appropriate level and ensure the quality and performance of the plated surface, the GA steel sheet In the case of GI steel plates, the Al content can be 0.10 to 0.15%, and in the case of GI steel plates, the Al content The amount can be controlled to between 0.2% and 0.25%.

[0051] Alloying (GA) temperature: 480~560℃ Below 480℃, the amount of Fe diffusion is low and the degree of alloying is insufficient, resulting in poor plating properties. In some cases, if the temperature exceeds 560°C, excessive alloying can cause powdering. A ring-like problem occurs, and the material deteriorates due to the ferrite transformation of retained austenite. For this reason, the alloying temperature is set within the range described above.

[0052] In this way, the galvanized steel sheet of the present invention can be obtained. However, the present invention In one embodiment, in order to further improve the weldability of the edge portion, the heating process of the edge portion is It can also include more.

[0053] Heating of the edges of hot-rolled coils: Performed at 600-800°C for 5-24 hours. In one embodiment of the present invention, the oxygen depth profile of the GDOES at the edge is polar To further increase the difference between the minimum and maximum values, the edges of the hot-rolled coil are heated. It can also be done. Heating the edges of a hot-rolled coil means heating both ends in the width direction of the wound coil. In other words, it means heating the edge, and heating the edge causes the acid to... It is first heated to a temperature suitable for heating. In other words, the inside of the wound coil is kept at a high temperature. However, the edges cool relatively quickly, which brings them to a temperature suitable for internal oxidation. The time it is maintained is shorter. Therefore, the acid is more concentrated at the edges compared to the center in the width direction. The removal of oxidizing elements is not actively performed. Heating the edge removes oxidizing elements at the edge. It can be used as one method for that purpose.

[0054] In other words, when heating the edge, the edge is heated in the opposite way to when cooling after winding. First, it is heated, and in conjunction with this, the temperature of the edges in the width direction is maintained to suit internal oxidation. As a result, the thickness of the internal oxide layer at the edge increases. The heating temperature of the edge must be 600°C or higher (based on the temperature of the steel plate edge). Yes, it is possible. However, if the temperature is too high, excessive scale may form on the edges during heating. Alternatively, a porous, highly oxidized scale (hematite) may form, resulting in poor surface condition after pickling. Because this can happen, the temperature of the edge portion may be 800°C or less. The heating temperature of the lid is 600-750°C.

[0055] Furthermore, the GDOES in the surface layer between the widthwise edge and the center that occurs during winding To eliminate the non-uniformity of the difference between the minimum and maximum values ​​in the oxygen depth profile, The heating time for the edge portion mentioned above must be 5 hours or more. However, if the heating time for the edge portion is long If too much is applied, excessive scaling may occur, or conversely, GDOES may form on the surface of the edge. In the oxygen depth profile, the difference between the minimum and maximum values ​​can sometimes be too high. Therefore, the heating time for the edge can be 24 hours or less.

[0056] According to one embodiment of the present invention, heating of the edge portion is achieved through combustion heating via adjustment of the air-fuel ratio. This can be done by adjusting the air-fuel ratio, which changes the oxygen fraction in the atmosphere. Although this can change, the higher the partial pressure of oxygen, the greater the oxygen concentration in contact with the surface layer of the steel plate, leading to decarburization. Internal oxidation may increase. However, this invention is not necessarily limited to this. In this embodiment, the atmosphere can be controlled to contain 1-2% oxygen by adjusting the air-fuel ratio. It is possible. Anyone with ordinary skill in the art to which this invention belongs can easily obtain the air-fuel ratio without any particular difficulty. Since the oxygen fraction can be controlled through the adjustment of [a specific mechanism], this will not be explained separately. [Examples]

[0057] The present invention will be described more specifically below with reference to examples. However, the following examples are based on the present invention. This is merely illustrative and for the purpose of providing a more detailed explanation, and is not intended to limit the scope of the present invention. It is important to note that this is not the case. The scope of the rights of this invention is as described in the claims. This is because it is determined by the factors that were determined and by factors that can be reasonably inferred from them.

[0058] (Examples) Steel slabs having the composition listed in Table 1 below (the remaining components not listed in the table are F e and unavoidable impurities. Note that B and N in the table are expressed in ppm units, and the remaining components are The amounts are expressed in weight percentages, and any components not listed in the table are considered to have a weight percentage of 0%. After heating the material to 1,230°C and hot-rolling it, the edges of the hot-rolled coil are heated. Subsequently, the steel plates were passed through a 100mm long pickling line at a speed of 210mpm for 19 minutes. After pickling with a 2 vol% hydrochloric acid solution and then cold rolling, the resulting cold-rolled steel sheet is annealed in an annealing furnace. Immediately, GA was placed in a plating bath containing 0.13% Al, and GI was placed in a 4% Al bath containing 0.24% by weight. The material was immersed in a zinc-based plating bath at 56°C to perform hot-dip galvanizing. The resulting hot-dip galvanized steel The sheet is subjected to alloying (GA) heat treatment as needed, and finally an alloyed hot-dip galvanized steel sheet is obtained. Ta.

[0059] In all examples, the temperature at which the steel sheet is drawn into the molten zinc plating bath is set to 475°C. The other conditions for each embodiment are as shown in Table 2.

[0060] [Table 1]

[0061] [Table 2]

[0062] The properties of the hot-dip galvanized steel sheet manufactured by the above process were measured, and the properties during spot welding were measured. Table 3 shows the results of observations regarding whether or not liquid phase metal embrittlement (LME) occurred during spot welding. This was done after cutting the steel plate in the width direction, and then along each of the cut peripheral edges. After applying welding current twice and energizing, the holding time for one cycle is set. The time (of welding time) was maintained. Spot welding was performed on three layers of different materials. Evaluation materials - Evaluation materials - GA 980DP 1.4t(C 0.12wt%, Si 0.1wt%, Mn 2.2 The materials (having a composition of %) were stacked in order and spot-welded. After welding the electrode to the soft material 15 times, wear down the electrode and then apply it to the material to be spot welded. Measure the upper limit current at which explosion occurs. After measuring the upper limit current, Spot welding was performed eight times at currents 0.5 and 1.0 kA lower than the current, and the spot welding was performed eight times for each welding current. After precisely machining the cross-section of the welded area using electrical discharge machining, it is epoxy mounted and polished. The crack length was measured using a microscope. The magnification for observation with the optical microscope was set to 100x. If no cracks were found at that magnification, it was assumed that liquid-phase metal embrittlement did not occur. After making a judgment, if a crack was found, its length was measured using image analysis software. B-type cracks occurring at the shoulder of a pot weld are 100 μm or less, C-type cracks Rack was judged to be in good condition before observation.

[0063] Maximum points in the GDOES concentration profiles of oxygen, silicon, and manganese surface layers. The depth and concentration at that time were determined by averaging the data obtained from the GDOES profile over 5 points. The calculation was performed using the concentration values ​​for each depth. Specifically, the oxygen, silicon and Obtain a manganese concentration profile and find the depth at which the maximum points for each element appear in the above profile. After determining the depth, the smaller of the differences in the maximum depths of oxygen, silicon, and manganese is used as the depth difference. That's what I decided.

[0064] Tensile strength was measured by tensile testing using a sample prepared in the C direction according to JIS-5 standard. The degree of alloying and the amount of plating adhesion were measured using a wet dissolution method with hydrochloric acid solution.

[0065] Powdering and flaking tests were performed on the GA steel sheets. After bending the plated material to 90 degrees, tape is applied to the bent part and then peeled off, and the tape is then applied to the bent part. We checked how many millimeters of detached material from the plating layer had adhered. The length of the plating layer peeled off the tape was 10 If the result exceeded mm, it was confirmed to be defective. In the Flaking test, after processing into an inverted "U" shape... We checked whether the plating layer would peel off in the processing area.

[0066] For GI steel plates, an automotive structural adhesive is applied to the surface, and the steel plate is bent at a 90-degree angle. Sometimes, check the sealer bending to see if the plating layer has peeled off and adhered to the surface where the sealer has come off. A sealing test (SBT) was performed on the unsealed steel plate. The surface quality is checked visually to see if there are any defects such as scratches, and any defects such as unplated areas are found during visual inspection. If this was visible, it was judged to be defective.

[0067] [Table 3]

[0068] In Table 3 above, 1) is the oxygen concentration profile and silicon and / or manganese profile. The smaller of the differences in the depth of the file's local maximums is: 2) powdering length (mm), 3) The length (μm) of the B-type LME crack generated during electrical resistance spot welding is 4) This refers to the length (μm) of the C-type LME crack that occurred during electrical resistance spot welding. In the table, ND means Not Detected.

[0069] Examples 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 of the invention are steel compositions as presented in the present invention. The scope is met, and the manufacturing method also meets the scope of the present invention, including tensile strength, plating quality, and plating. The adhesion amount and the LME crack length of the spot welds were also good.

[0070] Comparative Examples 3 and 9 represent cases where the hydrogen concentration inside the annealing furnace was very low. As a result, the surface oxidation of the surface layer is excessive, and the oxygen and Si of the GDOES in the surface layer The difference between the maximum depth of Mn was not large. As a result, a sufficient decarburized layer could not be formed. Failure to adequately form an alloying-suppressing layer at the interface between the scratches, the plating layer, and the base iron, resulting in LME cracks. Failure to meet the standards results in unplated areas on the surface, degrading surface quality, and SBT peeling. The adhesion has deteriorated.

[0071] Comparative Example 1 and Reference example In case 6, the winding temperature during the hot rolling process did not meet the range presented in the present invention. In comparative example 1, the hot rolling winding temperature was lower than the range presented in the present invention, and the amount of internal oxidation generated during hot rolling was insufficient. As a result, the difference between the depth of the maximum oxygen concentration in the surface layer and the depth of the maximum Si or Mn concentration, as measured by GDOES, exceeded 0.5 μm, and consequently, the LME crack did not meet the criteria. Reference example Material 6 was manufactured at a temperature exceeding the hot rolling and winding temperature specified in the present invention. Although the amount of internal oxidation generated during the hot rolling process was sufficient and the LME characteristics were good, excessive hot rolling scale was generated, and the scale was not completely removed during pickling, resulting in unplated areas and poor surface quality. Plating peeling occurred during flaking evaluation, and the excessively high hot rolling and winding temperature caused softening of the hot-rolled material, which could not be recovered even after annealing, resulting in material degradation.

[0072] Comparative Example 12 is a case where the dew point in the furnace during annealing is controlled to be lower than the range presented in the present invention. Even if a sufficient internal oxide layer is generated across the entire width during the hot rolling heating process, during the annealing process after cold rolling... The dew point is not high enough, so internal oxidation does not occur sufficiently, and the oxygen concentration of the surface layer according to GDOES is The smaller of the difference between the maximum point depth and the maximum point depth of Si or Mn concentration is 0.5 μm. The above is the case, and the degree of surface decarburization is insufficient, resulting in poor LME crack length in the spot welds. there were.

[0073] Reference example Case 8 is when the dew point inside the annealing furnace exceeds the range specified in this invention. Although internal oxidation occurred sufficiently due to the excessively high dew point, resulting in a good LME crack length, the material deteriorated due to excessive internal oxidation and failed to meet the tensile strength standard. Furthermore, the excessive dew point resulted in a large amount of surface oxide generation, leading to SBT and, as a result, plating peeling.

[0074] Comparative Example 10 is the case where the steel sheet passing speed in the annealing furnace was higher than the range presented in the present invention. Yes, it happens. The annealing process is not completed because sufficient time is not given for the decarburization reaction that occurs when the steam in the annealing furnace reacts with the steel sheet. Insufficient internal oxidation was formed in the subsequent surface layer of the steel plate, resulting in a maximum oxygen concentration depth according to GDOES. The difference between the depth of the maximum Si or Mn concentration and the spot welding is 0.5 μm or more. It was found to be defective, exceeding the criteria for LME crack evaluation.

[0075] Comparative Example 2 is the case where the sheet speed of the steel plate during annealing is lower than the range presented in the present invention. The decarburization reaction time in the furnace, where steam reacts with the steel plate, is excessively long, causing the internal oxides of Si to... It was formed deep within the base iron. The depth of the oxygen concentration maximum point in the surface layer by GDOES and Si The difference between the depth of the Mn concentration maximum point and the LME spot welding point is 0.5 μm or more. Although the rack met the evaluation criteria, the material quality was unsatisfactory due to excessive decarburization.

[0076] In Comparative Example 5, the uniform temperature inside the annealing furnace exceeded the range presented in the present invention. When the temperature becomes excessively low, the amount of external oxidation increases, and sufficient internal oxidation does not occur, resulting in GDO in the surface layer. The difference between the maximum oxygen concentration depth and the maximum Si or Mn concentration depth in ES is 0.5 μm. The pressure exceeded m, resulting in LME cracks failing to meet the standards and poor spot weldability. Furthermore, in homogeneous tropical regions, austenite may form and grow excessively, affecting material properties such as tensile strength. I couldn't satisfy it.

[0077] In Comparative Example 11, the uniform temperature inside the annealing furnace was controlled to be lower than the range presented in the present invention. It is a combination. The annealing temperature is low, and the oxidation reaction between the steam and the steel sheet is insufficient, resulting in internal oxidation. This process was not carried out sufficiently, resulting in the maximum depth of oxygen and Si or Mn in the surface GDOES. The difference between the two values ​​exceeds 0.5 μm, and therefore the LME crack does not meet the criteria, spoilage The weldability was poor. Furthermore, recrystallization did not occur sufficiently during annealing, resulting in the target material not being met. The necessary microstructure is not formed, and the material does not meet the standards for tensile strength, etc., resulting in a defect. The evaluation revealed that delamination occurred.

[0078] Comparative Example 7 is a case where the reduction ratio during cold rolling exceeds the standard presented in the present invention. The internal oxide layer formed inside becomes thinner closer to the surface due to excessive cold rolling. The maximum point is formed at a deep depth, and the difference between the maximum points is 0.5 μm or more, indicating an LME crack. It failed to meet the standards and was deemed defective.

[0079] Reference example Case 4 is when the hydrogen concentration in the annealing furnace is less than 5% by volume and the reducing atmosphere composition in the annealing furnace is insufficient. Due to the rise in the dew point, internal oxidation and decarburization are sufficiently formed, and the difference in the depth of the elemental maximums satisfies the criteria of the present invention, and the LME crack length meets the criteria, but the formation of excessive surface oxides results in unplated areas and deterioration of surface quality, and SBT plating peeling occurs.

[0080] From the above, the advantageous effects of the present invention have been confirmed.

Claims

1. Raw steel sheet, and zinc-based plating layer formed on the surface of the base steel sheet A galvanized steel sheet containing, The concentration profiles of one or two of the oxygen, silicon, and manganese measured in the depth direction from the surface of the base steel sheet have a form in which maximum points appear in the depth direction from the surface. A galvanized steel sheet in which the difference in absolute value between the depth at which the maximum point of the oxygen concentration profile is formed and the depth at which the maximum point of one of the silicon and manganese concentration profiles is formed is 0.5 μm or less.

2. The galvanized steel sheet according to claim 1, wherein the depth at which the maximum point of the oxygen concentration profile is formed is 4 to 15 μm.

3. The zinc-plated steel sheet according to claim 2, wherein the oxygen concentration at the maximum point is 0.3% by mass or more.

4. The galvanized steel sheet according to claim 1, wherein the depth at which the maximum point of the oxygen concentration profile is formed is smaller than the depth at which the maximum points of one or both of the silicon and manganese concentration profiles are formed.

5. The zinc-plated steel sheet according to claim 1, wherein the concentration profile is measured by GDOES.

6. The plating amount of the aforementioned zinc-based plating layer is 30 to 70 g / m². ２ The galvanized steel sheet according to claim 1.

7. The zinc-plated steel sheet according to any one of claims 1 to 6, wherein the steel sheet has a composition comprising, by mass%, C: 0.05 to 1.5%, Si: 2.0% or less, Mn: 1.0 to 30%, S-Al (acid-soluble aluminum): 3% or less, Cr: 2.5% or less, Mo: 1% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, V: 0.2% or less, Sb + Sn + Bi: 0.1% or less, and N: 0.01% or less.

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