Steel sheet and method for manufacturing same

Abstract

The present invention relates to a steel sheet which can be used in automobiles and the like, and to a steel sheet which can ensure improved plating characteristics, and to a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a steel sheet that can be used for automobiles and the like, and to a steel sheet that can ensure improved plating characteristics and a method for producing the same. Background Art

[0002] In recent years, energy conservation, emission reduction, collision stability, and durability improvement have become important issues that automobile manufacturers must address. In order to save energy and reduce emission, emphasis is placed on lightweighting of automobiles. In addition, in order to improve collision stability and durability while reducing the weight of automobiles, steel materials need to be made stronger and have excellent formability.

[0003] In order to increase the strength of steel materials, various alloy elements may be included to improve hardenability. However, in this case, there is a problem that workability deteriorates, such as a springback phenomenon.

[0004] In order to solve this problem, hot forming (hot press forming) was proposed. Hot press forming is a method of processing steel at a high temperature (above 800°C) that is convenient for processing, and then rapidly cooling it to a low temperature to form a low-temperature structure such as martensite in the steel, thereby improving the strength of the final product. This hot press forming method can minimize the processability problem when manufacturing parts with high strength.

[0005] In order to improve the hardenability and to facilitate the formation of martensite during cooling, various alloy elements are added to the steel for hot press forming, in particular, elements with a higher oxidation tendency than Fe, such as Mn, Si, Al, Cr, B, etc., can be added.

[0006] In addition, in order to prevent decarburization or oxidation of the steel sheet during hot press forming or to ensure corrosion resistance of the parts, various plating methods are sometimes applied to the surface of the steel material. Among them, a method of plating the surface of the steel material by hot dip plating, such as hot dip galvanizing or hot dip aluminizing, is widely used.

[0007] The hot dip coating is generally annealed before plating, but due to the surface oxides formed by the alloy elements such as Mn, Si, Al, Cr, B contained in the steel during the annealing process, there is a problem of poor plating properties. That is, during the annealing process, the alloy elements can diffuse to the surface side of the steel and react with trace amounts of oxygen or water vapor present in the annealing furnace to form single oxides or composite oxides of the elements. These oxides hinder the plating wettability during the plating, resulting in non-plating or causing the coating to peel off, so that the plating quality may be reduced.

[0008] In order to improve the coating quality, various technologies have been proposed. Among them, Patent Document 1 proposes a technology in which the air-fuel ratio of air and fuel is controlled to 0.80 to 0.95 during the annealing process, and the steel sheet is oxidized in a direct flame furnace in an oxidizing atmosphere to form iron oxides containing single oxides or composite oxides of Si, Mn or Al at a specified depth inside the steel sheet, and then the iron oxide is reduction annealed in a reducing atmosphere and then hot-dip galvanized, thereby providing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet with excellent coating quality.

[0009] As in Patent Document 1, when a method of oxidation followed by reduction in an annealing process is used, components with high affinity for oxygen, such as Si, Mn, and Al, are internally oxidized from the surface layer of the steel sheet to a predetermined depth and diffusion to the surface layer is suppressed, so that the single oxide or composite oxide of Si, Mn, or Al on the surface layer is relatively reduced, thereby improving wettability with zinc, and thus it is possible to reduce unplated coating. However, in the case of a steel type to which Si is added, Si is enriched in the reduction process directly below the iron oxide to form a band-shaped Si oxide, so that peeling occurs in the surface layer portion including the plated layer, that is, peeling occurs at the interface between the reduced iron and the base iron below it, and thus there is a problem that it is difficult to ensure the adhesion of the plated layer.

[0010] In addition, as another method for improving the coating property of hot-dip plated steel sheets, Patent Document 2 proposes a method in which alloy components such as Mn, Si, and Al that are easily oxidized are internally oxidized inside the steel by maintaining a high dew point in the annealing furnace, and the oxides that are externally oxidized are reduced on the surface of the steel sheet after annealing, thereby improving the coating property. However, although the method according to Patent Document 2 can solve the coating property problem caused by the external oxidation of Si that is easily internally oxidized, there is a problem that the effect is minimal when a large amount of Mn that is relatively difficult to internally oxidize is added.

[0011] In addition, even if the plating property is improved by internal oxidation, linear non-plating may occur due to surface oxides unevenly formed on the surface, or when an alloyed hot-dip galvanized steel sheet (GA steel sheet) is manufactured by alloying heat treatment after plating, linear defects caused by uneven alloying may occur on the surface of the alloyed hot-dip galvanized steel sheet.

[0012] (Patent Document 1) Korean Patent Publication No. 2010-0030627

[0013] (Patent Document 2) Korean Patent Publication No. 2009-0006881 Summary of the invention

[0014] 1. Technical issues to be resolved

[0015] An object of one aspect of the present invention is to provide a steel sheet having excellent plating characteristics by suppressing non-plating or plating peeling during plating, and a method for producing the same.

[0016] The technical problems of the present invention are not limited to the above contents. Those skilled in the art can easily understand the additional technical problems of the present invention from the overall contents of the present invention specification.

[0017] (II) Technical solution

[0018] One example of the present invention relates to a steel plate, wherein the steel plate contains one or both of Mn and Si in the steel, and in a GDS curve of the components of one or both of Mn and Si observed along the depth direction from the surface of the steel plate, there are two or more valleys in which the content of the one or both of Mn and Si from the surface of the steel plate to 1 μm is less than 60% of the content of the base material.

[0019] Another embodiment of the present invention relates to a method for manufacturing a steel plate, the method comprising the steps of: preparing a base steel plate containing one or both of Mn and Si in steel; 2 Up to 3g / m 2 The base steel sheet having the Fe coating layer is pre-plated with Fe to a thickness of 100° C. or less to form an Fe coating layer; and the base steel sheet having the Fe coating layer is annealed at a temperature of 600-950° C. in an atmosphere having a dew point temperature of 10° C. or less.

[0020] (III) Beneficial effects

[0021] The present invention suppresses the formation of oxides such as Mn and Si on the surface of steel materials by using a method different from the prior art, thereby suppressing unplated coating or coating peeling during hot-dip coating, thereby providing a steel sheet having excellent plating properties.

[0022] The multiple beneficial advantages and effects of the present invention are not limited to the above contents, and can be more easily understood in the process of describing the specific embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a graph of a glow discharge optical emission spectrometer (GDS) curve of the Mn content measured along the depth direction on the surface of the steel materials of Comparative Example 1 and Inventive Example 6 according to an embodiment of the present invention.

[0024] Figure 21 is a graph of a glow discharge optical emission spectrometer (GDS) curve of Si content measured along the length direction on the surface of steel materials of Comparative Example 1 and Inventive Example 6 according to an embodiment of the present invention. Best Mode for Carrying Out the Invention

[0025] The terms used in this specification are used to illustrate the present invention and are not intended to limit the present invention. In addition, unless the relevant definition indicates an obviously opposite meaning, the singular form used in this specification also includes the plural form.

[0026] The words “include” and “comprising” used in the specification are intended to specify the components and do not exclude the existence or addition of other components.

[0027] Unless otherwise defined, all terms used in this specification, including technical terms and scientific terms, have the same meanings as those generally understood by those skilled in the art. Terms defined in dictionaries are interpreted as having meanings consistent with relevant technical documents and currently disclosed contents.

[0028] Hereinafter, the present invention will be described specifically and in detail.

[0029] First, a steel sheet according to an embodiment of the present invention will be described in detail.

[0030] The steel sheet contains oxide-forming elements such as Mn and Si in the steel. The Mn, Si, etc. form oxides on the surface of the steel sheet during annealing before plating, thereby reducing plating properties.

[0031] The GDS concentration curve of oxidizing elements such as Mn and Si, which form oxides on the surface of the steel sheet to reduce plating characteristics, may have the following characteristics: The GDS concentration curve refers to a concentration and concentration curve measured using a glow discharge emission spectrometer.

[0032] Figure 1 and Figure 2 The concentration curves of Mn and Si measured from the surface of the steel plate along the depth direction of Comparative Example 1 and Inventive Example 6 in the following embodiment are shown. It can be seen that the concentration curve measured from the surface has a valley where the content decreases and a peak where the content increases.

[0033] In the GDS curve of one or both of the components of Mn and Si observed from the surface along the depth direction of the steel plate of the present invention, there may be two or more valleys where the content of one or both of the components of Mn and Si from the surface of the steel plate to 1 μm is less than 60% of the content of the base material.

[0034] The parent material content refers to the average content of the corresponding components in the steel plate manufactured by adjusting during the steelmaking process. It can be typically used at the 1 / 4 position of the thickness (t, in mm) of the steel plate. The content measured at t).

[0035] When the number of the valleys is less than 2, surface enrichment of Mn and Si occurs, which may deteriorate the plating property. However, when the number of the valleys is more than 2, a depleted layer of the corresponding elements is formed, which can suppress surface enrichment and thus improve the plating property.

[0036] In addition, the content of one or both of Mn and Si contained in the grains or grain boundaries within 1 μm from the surface of the steel plate in the thickness direction is 1 / 4 of the thickness (t, in mm) of the steel plate. t) is effective. Generally, the diffusion of elements in steel is a function of concentration and time. In order to minimize the diffusion of elements that form surface oxides to the surface, it is preferred to anneal the steel sheet and pass the sheet as quickly as possible. However, in order to meet the mechanical and physical properties, there are target temperatures and times for each section of the annealing furnace. Considering the industrial production environment of steel sheets that continuously pass through various equipment such as annealing furnaces and plating baths, when the above content is 40% or more of the content of the component, it can have the best effect in terms of plating characteristics and manufacturing efficiency.

[0037] The present invention does not particularly limit the alloy composition of the steel plate containing Mn, Si, etc., as long as a person skilled in the art can recognize that it can be used in the technical field to which the present invention belongs.

[0038] As an example, the steel plate may include, in weight %, C: 0.02-0.6%, Si: 0.001-2%, Al: 0.001-1%, Mn: 0.1-4%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, N: 0.02% or less, Ti: 0-0.1%, B: 0.0001-0.01%, Cu: 0-1.00%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.01%, Nb: 0-0.1%, Sn: 0-1%, W: 0-1%, Sb: 0-1%, Mg: 0-0.1%, Co: 0-1%, As: 0-1%, Zr: 0-1%, Bi: 0-1%, REM: 0-0.3%, and the balance of Fe and inevitable impurities.

[0039] In addition, the steel sheet may contain an oxide composed of one or more of Mn, Si, Al, Cr and B at the grain boundary from the surface to 10 μm in the depth direction. During annealing, the oxygen-loving elements in the steel such as Mn, Si, Al, Cr, B, etc. diffuse mainly to the grain boundary of the base material iron and move to the surface layer, while the oxygen contained in the Fe plating layer may diffuse into the interior of the steel and combine with the corresponding oxygen-loving elements to form oxides.

[0040] The depth of forming the oxide may be proportional to the absolute amount of oxygen contained in the Fe coating. As described below, the oxygen content in the Fe coating may be 5-50% by weight. Thus, the thickness of the oxide after annealing is from 1 μm to 10 μm. When the thickness of the oxide is less than 1 μm, the level of suppressing surface enrichment is insufficient and the plating property cannot be improved. When the thickness of the oxide exceeds 10 μm, it is necessary to further set up an electroplating unit, resulting in a reduction in production speed, so it is not preferred in terms of economy.

[0041] The surface of the steel plate may include a hot-dip galvanizing layer, a hot-dip aluminum coating layer, etc. The type or method of the coating is not particularly limited, and may include all types or methods that can be performed in the technical field to which the present invention belongs.

[0042] Next, a method for producing a steel sheet according to another embodiment of the present invention will be described in detail.

[0043] First, a base steel plate containing one or both of Mn and Si in steel is prepared.

[0044] In the present invention, any base steel sheet having the above alloy composition can be used as the base steel sheet for the plated steel sheet or the hot-dip plated steel sheet according to the present invention without limitation, and therefore the method for manufacturing the base steel sheet is not particularly limited. As an example, the steel plate may include, in weight %, C: 0.02-0.6%, Si: 0.001-2%, Al: 0.001-1%, Mn: 0.1-4%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, N: 0.02% or less, Ti: 0-0.1%, B: 0.0001-0.01%, Cu: 0-1.00%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.01%, Nb: 0-0.1%, Sn: 0-1%, W: 0-1%, Sb: 0-1%, Mg: 0-0.1%, Co: 0-1%, As: 0-1%, Zr: 0-1%, Bi: 0-1%, REM: 0-0.3%, and the balance of Fe and inevitable impurities.

[0045] An example of a method for producing the base steel plate will be described in detail.

[0046] The base steel plate according to one embodiment of the present invention may be manufactured by reheating, hot rolling, coiling, and cold rolling a steel billet satisfying the above-mentioned alloy composition.

[0047] Reheating

[0048] The steel slab satisfying the alloy composition of the present invention may be reheated to a temperature range of 1200° C. or higher. In order to re-solubilize most of the precipitates present in the steel, the steel slab may be reheated to a temperature of 1200° C. or higher. As one embodiment of the present invention, the reheating temperature may be 1250° C. or higher.

[0049] Hot Rolling

[0050] The reheated steel slab may be hot rolled at a finishing temperature of Ar3 to 1000°C. During hot rolling, when the finishing temperature is lower than Ar3 (γ→α phase transformation temperature during cooling), two-phase rolling is likely to occur, a mixed grain structure is generated in the surface layer, and it may be difficult to control the shape of the hot-rolled steel sheet. On the other hand, when the finishing temperature exceeds 1000°C, uniform hot rolling cannot be achieved throughout the thickness, resulting in insufficient grain refinement.

[0051] Rolling

[0052] The hot-rolled steel sheet may be coiled in a temperature range exceeding Ms (martensitic phase transformation start temperature) and below 750°C. When the coiling temperature is below Ms, the strength of the hot-rolled steel sheet becomes too high, thereby reducing the cold rolling property. When the coiling temperature exceeds 750°C, the thickness of the oxide layer increases and the surface grain boundary is oxidized, so the pickling property deteriorates and the surface grain boundary may fall off during annealing in a continuous annealing furnace.

[0053] In the present invention, the cooling conditions after hot rolling to the coiling temperature are not particularly limited, and cooling can be performed under conventional conditions used in the same technical field. As one embodiment of the present invention, air cooling can be performed.

[0054] Pickling

[0055] The hot rolled steel sheet after the above process can be put into a hydrochloric acid bath for pickling to remove the hot rolled scale. The hydrochloric acid concentration of the hydrochloric acid bath during pickling is in the range of 10-30%, and the pickled sheet passes at a speed of 100-250 meters per minute (mpm). When the pickling speed exceeds 250 mpm, the scale on the surface of the hot rolled steel sheet may not be completely removed. When the pickling speed is lower than 100 mpm, the surface layer of the substrate iron may be corroded by hydrochloric acid, so the pickled sheet passes at a speed of more than 180 mpm.

[0056] Cold Rolling

[0057] The coiled steel sheet is cold rolled at a total reduction rate of 30-90% to obtain a cold rolled steel sheet. When the reduction rate is less than 30%, the roll and tension control are inaccurate, the sheet may be distorted, and recrystallization cannot be fully achieved during annealing, so the anisotropy of the material after hot forming may also increase. On the other hand, when the reduction rate exceeds 90%, the product may not be produced due to the load of the cold rolling roll during rolling.

[0058] The surface of the base steel sheet is pre-plated with Fe before annealing to form an Fe plating layer.

[0059] The method of forming the Fe plating layer is not particularly limited, but an electroplating method or the like may be used.

[0060] The coating weight of the pre-plated Fe is more than 0.5 g / m 2 Up to 3g / m 2 The following is valid.

[0061] In order to ensure the quality of hot-dip plating of steel sheets containing Mn and Si, it is preferred that the concentration of iron be greater than 0.5 g / m 2 Up to 3.0g / m 2 The upper limit of the Fe coating amount is not particularly limited, but in the continuous plating process, when the Fe coating amount exceeds 3.0 g / m 2 When the Fe plating amount is large, multiple plating tanks are required or the production speed is reduced, which is not economical. In addition, when the Fe plating amount is large, the Fe plating solution quickly denatures in the continuous process, the pH decreases, and the plating efficiency is greatly reduced, so there is a problem of difficulty in controlling the solution. On the other hand, when the Fe plating amount is 0.5 g / m 2 When the Fe coating is less than 0.5 μm, the oxygen contained in the Fe coating is quickly reduced and removed, so it is not possible to effectively suppress the diffusion of Mn and Si from the base iron and the formation of surface oxides, so there is a problem of reduced hot dip plating quality. The Fe coating amount is the concentration of iron contained in the coating, and the Fe coating has a thickness of about 0.05-0.4 μm when it is completely reduced during the annealing process.

[0062] In a specific embodiment of the present invention, the Fe plating layer can be formed on the surface of the substrate iron by electroplating. The oxygen concentration of the formed Fe plating layer can be controlled by appropriately controlling the conditions of the electroplating solution and the plating conditions.

[0063] That is, in the present invention, in order to form the Fe plating layer, the following electroplating solution can be used, which contains: iron ions including ferrous ions and ferric ions; a complexing agent; and inevitable impurities, and the concentration of ferric ions in the iron ions is 5-60% by weight.

[0064] According to a specific embodiment of the present invention, the electroplating solution includes ferrous ions and ferric ions. In order to obtain high plating efficiency, it is more advantageous to only include ferrous ions, but when only including ferrous ions, the solution deteriorates, so that the plating efficiency is sharply reduced, so that quality deviation may be caused in the continuous electroplating process, so the ferric ions may be further included. At this time, the concentration of the ferric ions is preferably 5-60% by weight of the total amount of ferrous ions and ferric ions, and more preferably 5-40% by weight. When the concentration of the ferric ions is less than 5%, the speed at which ferrous iron is reduced to ferrous iron in the cathode is less than the speed at which ferrous iron is oxidized to ferric iron in the anode, so the concentration of ferric iron rises sharply, and the plating efficiency continues to decrease while pH is sharply reduced. On the other hand, when the concentration of ferric iron ions exceeds 60%, the reaction amount of ferrous iron being reduced to ferrous iron in the cathode is greatly increased compared with the reaction amount of metallic iron being reduced to ferrous iron, so the plating efficiency is greatly reduced, and the plating quality can be reduced. Therefore, considering the equipment and process characteristics such as the plating amount, operating current density, solution supply amount, the amount of solution lost by adhering to the belt, and the concentration change rate caused by evaporation, the concentration of trivalent iron ions in the iron ions is preferably 5-60% by weight.

[0065] The concentration of the iron ions is preferably 1-80 g, more preferably 10-50 g per 1 L of the electroplating solution. When the concentration of the iron ions is less than 1 g / L, there is a problem of a sharp decrease in plating efficiency and plating quality. On the other hand, when the concentration of the iron ions exceeds 80 g / L, precipitation may occur due to the excess solubility, and the loss of raw materials due to solution loss increases in a continuous plating process, which is uneconomical.

[0066] The electroplating solution of the present invention contains a complexing agent. In order to contain a large amount of trivalent iron without generating sludge and to maintain a high plating efficiency, it is preferred to use an amino acid or an amino acid polymer as the complexing agent.

[0067] Amino acids refer to organic molecules formed by the bonding of carboxyl groups (-COOH) and amine groups (-NH2), and amino acid polymers refer to organic molecules formed by the polymerization of two or more amino acids. Amino acid polymers exhibit complexing agent properties similar to those of amino acids. Therefore, in the following description, amino acids and amino acid polymers are collectively referred to as amino acids.

[0068] When amino acids are dissolved in neutral water, amines combine with hydrogen ions to become positively charged, and carboxyl groups dissociate hydrogen ions to become negatively charged, so the amino acid molecules remain electrically neutral. In addition, when the solution is acidified, the carboxyl groups combine with hydrogen ions again to become electrically neutral, and the amines have a positive charge, so the amino acid molecules form cations. That is, amino acids form electrically neutral or cations in weakly acidic aqueous solutions.

[0069] When amino acids are added to an acidic electrolyte containing iron ions, they will complex with ferrous ions and ferric ions, and the iron ions complexed with amino acids will also remain in a cationic state in the complexed state. Therefore, compared with the conventional complexing agents with multiple carboxyl groups that carry a negative charge in a weakly acidic aqueous solution, they exhibit opposite electrical properties.

[0070] In addition, compared with complexing agents containing multiple carboxyl groups such as citric acid and EDTA, amino acids form fewer bonds with iron ions and have weaker bonding strength, but the bonding strength with trivalent iron ions that produce sludge is strong enough, so precipitation caused by trivalent iron ions can be prevented. In addition, even if trivalent iron ions are complexed, cations can be maintained. Trivalent iron ions are easily transferred to the cathode and reduced to ferrous ions, so they can participate in the plating reaction, but on the other hand, the movement to the anode is suppressed, and the formation rate of trivalent iron ions slows down. Therefore, even if long-term continuous plating is performed, the concentration of trivalent iron ions is maintained at a specified level, and the specified plating efficiency is maintained, and there is no need to replace the electrolyte.

[0071] In addition, in the continuous electroplating process, when the iron ions in the solution are depleted due to plating, the solution is acidified, and even if the same amount of iron ions are precipitated, the pH change of the solution containing trivalent ferrous ions is reduced compared with the solution containing only ferrous ions. When the pH increases, a part of the trivalent ferrous ions combines with the hydroxide ions, and when the pH decreases, the hydroxide ions are separated and neutralized. Therefore, the solution containing trivalent ferrous ions can slow down the pH change even without a separate pH buffer, thereby playing the role of a pH buffer, so that the specified plating efficiency can be maintained in the continuous electroplating process.

[0072] Therefore, sludge can be prevented by using amino acids as chelating agents, not only ferrous ions but also ferric ions can be used as plating raw materials, and when ferrous ions and ferric ions are mixed, the pH change of the solution is slowed down and the accumulation of ferric ions is easily prevented, so the specified plating efficiency and plating quality can be maintained in the continuous electroplating process.

[0073] In addition, the complexing agent is preferably added in an amount of 1:0.05 to 2.0 with a molar concentration ratio of the iron ion and the complexing agent, more preferably in an amount of 1:0.5 to 1.0. When the addition of the complexing agent is less than 0.05, it is impossible to suppress the excessively contained trivalent iron ions to combine with hydroxide ions or oxygen to form sludge, even if trivalent iron is not included, the plating efficiency is greatly reduced, and then combustion is caused, resulting in the deterioration of the plating quality. On the other hand, even if the addition of the complexing agent exceeds 2.0, although the effect and plating quality of the sludge are kept suppressed, due to the rise in overvoltage, the plating efficiency is reduced, and the amino acid with a relatively high price compared to the raw material containing iron ions such as iron sulfate is unnecessarily excessively included, so the raw material cost increases and is uneconomical.

[0074] The complexing agent is preferably one or more selected from amino acids or amino acid polymers, for example, one or more selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid and glycylglycine.

[0075] The amino acid is used as a complexing agent, the solution temperature is below 80°C, the pH is maintained at 2.0 to 5.0, and the current density is 3-120A / dm 2 When electroplating is carried out under the conditions of , an Fe plating layer with high plating efficiency and high oxygen concentration can be obtained.

[0076] Although the temperature of the Fe electroplating solution has little effect on the quality of the Fe coating, when the temperature of the Fe electroplating solution exceeds 80°C, the solution evaporates violently and the concentration of the solution changes continuously, making it difficult to achieve uniform electroplating.

[0077] When the pH of the Fe electroplating solution is less than 2.0, the electroplating efficiency is reduced and is not suitable for a continuous plating process. When the pH of the Fe electroplating solution exceeds 5.0, the plating efficiency increases, but sludge of iron hydroxide precipitation occurs during the continuous electroplating process, resulting in conduit blockage, roller and equipment contamination.

[0078] When the current density is less than 3A / dm 2 When the current density exceeds 120A / dm, the Fe electroplating efficiency decreases due to the decrease of the cathode plating overvoltage, so it is not suitable for continuous plating process. 2 When the Fe plating layer is not uniform, the Fe plating layer is easily peeled off.

[0079] In the present invention, as described above, the Fe coating preferably contains 5-50 wt% of oxygen. The reason why oxygen is mixed into the Fe coating is as follows. In the process of iron precipitation on the surface of the steel sheet to which the cathode is applied, hydrogen ions are simultaneously reduced to hydrogen gas, thereby increasing the pH. Therefore, both ferrous ions and ferric ions are temporarily combined with OH. - Ion combination, and may be mixed in when forming the Fe coating. When using anionic complexing agents such as acetic acid, lactic acid, citric acid, EDTA, etc., due to the complexing agent, with OH - The ion-bound iron ions have an average negative charge, and when a cathode is applied for electroplating, an electric repulsion force is generated to suppress the mixing of Fe plating. On the other hand, amino acids are electrically neutral at pH 2.0 to 5.0, and are cationic in strong acids with a pH of less than 2.0. Even if the iron ions bound to amino acids are combined with 1-2 OH -Combined with cations, it generates an electrical attraction with the cathode for electroplating and mixes a large amount of oxygen. Therefore, by using amino acids as complexing agents to make the molar concentration ratio of iron ions to amino acids 1:0.05 to 1:2.0 and maintaining the pH at 2.0 to 5.0 for Fe electroplating, it is possible to obtain an Fe coating containing 5-50% by weight of oxygen while suppressing the generation of sludge with high plating efficiency.

[0080] Next, the base steel sheet on which the Fe plating layer is formed is annealed.

[0081] In the annealing, the dew point temperature may be below +10° C., and the annealing may be performed by maintaining the temperature at 600-950° C. for 1-1000 seconds.

[0082] When the dew point temperature during the annealing heat treatment exceeds +10°C, the base steel sheet itself may be oxidized. However, when the dew point temperature is too low, the coating performance may be deteriorated. Therefore, in consideration of this, the lower limit of the dew point temperature may be set to -50°C, preferably -10°C.

[0083] The annealing may be heat treated in a temperature range of 600-950° C. When the heat treatment temperature is lower than 600° C., it is difficult to recover and recrystallize the rolled structure generated by cold rolling, and it is difficult to ensure sufficient tensile physical properties. On the other hand, when the heat treatment temperature exceeds 950° C., excessive decarburization occurs, fatigue properties may deteriorate, and a large amount of B oxide is formed, so that plating adhesion may be reduced during plating, and the annealing equipment may be deteriorated and the equipment needs to be frequently replaced, which may become a factor leading to an increase in process costs.

[0084] In addition, the annealing time may be 1 to 1000 seconds. When the annealing time is less than 1 second, it is difficult to ensure the annealing effect, and when the annealing time exceeds 1000 seconds, productivity may decrease.

[0085] In addition, when heating for the annealing heat treatment, wet nitrogen can be added when the heating interval is preferably raised to above 700°C. This is to induce internal oxidation of the oxidizing elements. The nitrogen can be added at a rate of 50-200 Nm 3 / hour flow rate to add wet nitrogen (in addition, wet nitrogen 50-200Nm 3 / hour interval can be calculated as 5-40L / hour). When the amount of wet nitrogen is less than 50Nm 3 / hour, the dew point rise effect is insufficient, so the formation of the internal oxide layer is weak. When the amount of wet nitrogen exceeds 200Nm 3 / hour, the dew point exceeds +10℃. Since the dew point is too high, there is a problem that the base iron itself is oxidized.

[0086] After the annealing step, the annealed steel sheet may be cooled. The cooling conditions in the cooling step after the annealing step do not have a great influence on the surface quality of the final product, that is, the coating quality, and therefore the cooling conditions do not need to be particularly limited in the present invention. However, in order to prevent oxidation of the iron component during the cooling process, an atmosphere that is at least reducing to iron may be applied.

[0087] The obtained steel sheet may be further plated. The plating is not particularly limited. As an example, the annealed steel sheet may be immersed in a hot-dip galvanizing bath or a hot-dip aluminum bath to form a hot-dip coating layer or an alloyed hot-dip coating layer. The hot-dip coating layer may be a hot-dip aluminum coating layer, a hot-dip Al-Si coating layer, a hot-dip Al-Si-Mg coating layer, a hot-dip galvanizing coating layer, a hot-dip Zn-Mg coating layer, etc., and the alloyed hot-dip coating layer may be an alloyed hot-dip aluminum coating layer, an alloyed hot-dip Al-Si coating layer, an alloyed hot-dip Al-Si-Mg coating layer, an alloyed hot-dip galvanizing coating layer, an alloyed hot-dip Zn-Mg coating layer, etc.

[0088] The coating may also contain Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, Ca, Sr, Mg, etc. The coating amount is not particularly limited, and may be set to a conventional amount. Like a steel plate, a coating or alloy coating may be provided on a heat-treated steel part. DETAILED DESCRIPTION

[0089] The following are examples of the present invention. Those skilled in the art may make various modifications to the following examples without departing from the scope of the present invention. The following examples are provided for understanding the present invention, and the scope of the present invention is not limited to the following examples, but shall be determined by the claims and their equivalents.

[0090] (Example)

[0091] A cold-rolled steel sheet was prepared, which contained, by weight %, C: 0.22%, Si: 0.28%, Al: 0.036%, Mn: 1.2%, P: 0.009%, S: 0.0006%, N: 0.0039%, B: 0.0031%, Ti: 0.03%, Cr: 0.21%, and the balance of Fe and inevitable impurities, and then an Fe plating layer was formed with an Fe deposition amount shown in Table 1 below, and then annealing was performed under the dew point and temperature conditions shown in Table 1 below. In addition, after annealing, the cold-rolled steel sheet was immersed in an Al plating bath containing 9 weight % of Si for aluminum plating.

[0092] The Fe coating is prepared by immersing the cold rolled steel sheet in the Fe plating solution and applying 20A / dm 2The Fe plating was performed with a current density of 1000 μm and the adhesion amount was controlled by controlling the plating time. (At this time, the temperature of the solution was maintained at 50°C.) A copper plate was used in advance and Fe plating was performed with the same solution. After that, the Fe plating layer was dissolved with a 5-10 wt% hydrochloric acid solution and the total amount of Fe was measured to calculate the electroplating adhesion amount and plating efficiency, and based on this, the plating time according to the target adhesion amount was calculated.

[0093] The steel sheet prepared as described above was subjected to glow discharge spectrometry (GDS) analysis to measure the content profiles of Mn and Si in the depth direction from the surface of the steel sheet. Figure 1 and Figure 2 The GDS curves of Comparative Example 1 and Inventive Example 6 are shown in FIG.

[0094] The GDS analysis of the present invention uses a GDS850A device manufactured by LECO Corporation, and is measured in RF mode at intervals of 0.01 to 0.03 μm from the surface layer to 2 μm in the depth direction.

[0095] In the GDS analysis results of each of the steel plates, the number of valleys from the surface along the thickness direction to 1 μm where the content of each element is less than 60% of the parent material content is classified as A, and the content of the deepest valley is classified as B (unit, weight %), and is shown in the following Table 1. The valley does not refer to the position of the lowest value in the GDS, but means the trend observed on the curve. Figure 1 In the case of Mn, the number of valleys observed in Comparative Example 1 was 1, and the number of valleys observed in Inventive Example 6 was 2. Figure 2 In the graph, regarding Si, no valleys were observed in Comparative Example 1, but one valley was observed in Inventive Example 6.

[0096] In addition, the galvanizing properties of each steel sheet were evaluated, and the results are shown in Table 1 below. The galvanizing properties were evaluated based on the plating adhesion. This was evaluated using the structural adhesive SA-1607E from Bogwang. First, on a 30×80 mm 2 On the plated steel plate, a Teflon fixture with a width × length × height of 10 × 50 × 10 mm was used. 3After the adhesive is applied to the rectangular parallelepiped form, it is baked at a calcination temperature of 170°C for 20 minutes. After the baking is completed, the adhesive and the plated steel plate are fixed after being stored at room temperature for one day, and then the steel plate is bent 90° to forcibly separate the adhesive and the plated steel plate. At this time, if peeling occurs inside the adhesive, it is judged to be normal, and if separation occurs between the plating layer and the adhesive, it is judged to be peeling. Specifically, if the peeling area has a width of 3mm or more in diameter, it is judged to be peeling. If peeling occurs, but the peeling area is a very small area with a diameter of less than 3mm, it is re-judged by re-testing. Samples that were tested 5 times and peeling occurred more than 3 times were marked as "X" grade, samples that peeled less than 2 times and more than 1 time were marked as "△" grade, and samples that did not peel were marked as "○" grade.

[0097] [Table 1]

[0098]

[0099] Inventive Examples 1 to 4, 1000-3000 mg / m 2 The Fe coating is then attached. Afterwards, the GDS analysis of the annealed and plated material shows that two Mn minimum points are formed, which means that a Mn-depleted zone is formed in the outermost layer. Therefore, the surface Mn oxides are reduced, so it is judged that the coating adhesion is good. In Example 5 to Example 8, 1000-3000 mg / m 2 After the Fe plating layer is attached, annealing is performed while the dew point in the annealing furnace is maintained at 3-7°C, and then hot-dip galvanizing is performed. The plating adhesion can be significantly improved due to the intensification of the internal oxidation effect caused by the oxygen contained in the Fe plating layer and the moisture in the annealing furnace.

[0100] In contrast, in Comparative Example 1, the substrate iron without Fe coating was subjected to annealing and hot-dip aluminum plating under the same conditions as above. The GDS measurement results showed that the concentrations of Mn and Si on the surface were high, which was judged to be due to the formation of surface oxides and poor plating properties. In Comparative Example 2, 500 mg / m 2 The Fe plating layer adhered, but was not sufficient to stably ensure plating adhesion.

[0101] In Comparative Example 3, an attempt was made to suppress surface layer enrichment by increasing the dew point of the annealing furnace to 5°C to form internal oxides of the oxophilic elements, but the effect was insufficient, resulting in poor plating adhesion. In Comparative Example 4, although 500 mg / m 2 The Fe coating was obtained, but the plating adhesion was only slightly improved, which was judged to be because the surface suppression level of Mn and Si was still insufficient.

[0102] In Comparative Examples 5 and 6, 3000 mg / m 2 The above Fe plating layer, but when the sample was observed after electroplating, the plating layer peeled off and dust was generated, making it impossible to carry out the subsequent annealing and plating process.

Claims

1. A steel plate, comprising one or both of Mn and Si in steel, wherein in a GDS curve of the components of one or both of Mn and Si observed along a depth direction from the surface of the steel plate, there are two or more valleys in which the content of the one or both of Mn and Si at 1 μm from the surface of the steel plate is 60% or less of the content of the base material.

2. The steel plate according to claim 1, wherein: The content of one or both of Mn and Si contained in grains or grain boundaries within 1 μm in the depth direction from the surface of the steel sheet is 40% or more of the content of each component in the base material.

3. The steel plate according to claim 1, wherein: The steel sheet includes an oxide composed of one or more of Mn, Si, Al, Cr, and B at a grain boundary up to 10 μm in a depth direction from the surface.

4. The steel plate according to claim 1, wherein: The steel plate includes, by weight%, C: 0.02-0.6%, Si: 0.001-2%, Al: 0.001-1%, Mn: 0.1-4%, P: less than 0.05%, S: less than 0.02%, Cr: less than 1%, N: less than 0.02%, Ti: 0-0.1%, B: 0.0001-0.01%, Cu: 0-1.00%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.01%, Nb: 0-0.1%, Sn: 0-1%, W: 0-1%, Sb: 0-1%, Mg: 0-0.1%, Co: 0-1%, As: 0-1%, Zr: 0-1%, Bi: 0-1%, REM: 0-0.3%, and the balance of Fe and inevitable impurities.

5. The steel plate according to any one of claims 1 to 4, wherein: The steel plate includes a hot-dip galvanized layer or a hot-dip aluminum layer formed on the surface.

6. A method for manufacturing a steel plate, comprising the following steps: Prepare a base steel plate containing one or both of Mn and Si in steel; On the surface of the base steel plate, more than 0.5 g / m 2 Up to 3g / m 2 Pre-plating Fe with a thickness of the following to form an Fe plating layer; and The base steel sheet on which the Fe plating layer is formed is annealed at a temperature of 600 to 950° C. in an atmosphere having a dew point temperature of 10° C. or less.

7. The method for manufacturing a steel plate according to claim 6, wherein: The Fe pre-plating is performed by electroplating.

8. The method for manufacturing a steel plate according to claim 6, wherein: The annealing time is 1-1000 seconds.

9. The method for manufacturing a steel plate according to claim 6, wherein: The heating for annealing is 50-200 Nm 3 Wet nitrogen was added at a flow rate of / hour.

10. The method for manufacturing a steel plate according to claim 6, wherein: The base steel plate comprises, in weight%, C: 0.02-0.6%, Si: 0.001-2%, Al: 0.001-1%, Mn: 0.1-4%, P: less than 0.05%, S: less than 0.02%, Cr: less than 1%, N: less than 0.02%, Ti: 0-0.1%, B: 0.0001-0.01%, Cu: 0-1.00%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0-1.00%, V: 0-1.00%, Ca: 0-0.01%, Nb: 0-0.1%, Sn: 0-1%, W: 0-1%, Sb: 0-1%, Mg: 0-0.1%, Co: 0-1%, As: 0-1%, Zr: 0-1%, Bi: 0-1%, REM: 0-0.3%, and the balance of Fe and inevitable impurities.

11. The method for manufacturing a steel plate according to claim 6, wherein: The base steel plate is manufactured by a method comprising the following steps: Reheating the steel billet to a temperature range above 1200°C; After the reheating, hot rolling is performed at a finishing temperature of Ar3 to 1000°C; Coiling is performed within a temperature range exceeding the martensitic transformation start temperature Ms and below 750°C; and Cold rolling is carried out at a total reduction of 30-90%.

12. The method for manufacturing a steel plate according to any one of claims 6 to 11, wherein: The manufacturing method further comprises the step of immersing the coating in a hot-dip galvanizing bath or a hot-dip aluminum coating bath to form a coating after the annealing.

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