Steel plate and method for manufacturing the same

Abstract

The present invention relates to a steel sheet that can be used for automobiles and the like, and relates to a steel sheet capable of ensuring improved plating characteristics and 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 relates to a steel sheet capable of ensuring improved plating characteristics and a method for manufacturing the same.

Background Art

[0002] Recently, issues such as energy conservation, exhaust gas reduction, collision stability, and durability improvement have become important issues that automobile manufacturers need to solve. Emphasis is placed on reducing the weight of automobiles for energy conservation and exhaust gas reduction. On the other hand, at the same time as reducing the weight of automobiles, it is necessary to increase the strength of steel materials for improving collision stability and durability, and excellent formability is also required.

[0003] In order to improve the strength of steel materials, various alloying elements can be included to enhance the hardenability. However, in this case, there is a problem that the workability becomes poor, such as the springback phenomenon.

[0004] In order to solve such problems, a hot forming method (hot press forming method) has been proposed. The hot press forming method is a method of processing steel materials at a high temperature (800 ° C or higher) at which the steel materials are easily processed and then rapidly cooling them to a low temperature to form a low-temperature structure such as martensite in the steel materials to increase the strength of the final product. Such a hot press forming method can minimize the workability problem when manufacturing a member having high strength.

[0005] For hot press forming steel materials, various alloying elements are added to improve the hardenability so that martensite can be easily generated during cooling. In particular, elements having a high oxidation tendency with respect to Fe such as Mn, Si, Al, Cr, and B can be added.

[0006] On the other hand, in order to prevent decarburization and oxidation of the steel sheet during hot press forming or to ensure the corrosion resistance of parts, various types of plating may be performed on 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 aluminum plating is often used.

[0007] Although it is common to perform annealing before plating in the above-mentioned hot dip plating, there is a problem that the plating property deteriorates due to the surface oxides formed during annealing caused by alloying elements such as Mn, Si, Al, Cr, and B contained in the above-mentioned steel material. That is, during the annealing process, the above alloying elements diffuse to the surface side of the steel material and react with trace amounts of oxygen or water vapor present in the annealing furnace to form single or composite oxides of the above elements. These oxides may interfere with the plating wettability during the above plating, causing unplated areas, or may induce plating peeling, deteriorating the plating quality.

[0008] In order to improve the plating quality, various techniques have been proposed. Among them, Patent Document 1 controls the air-fuel ratio of air and fuel to 0.80 to 0.95 during the annealing process, oxidizes the steel plate in a direct flame furnace with an oxidizing atmosphere, forms iron oxides containing single or composite oxides of Si, Mn, or Al to a certain depth inside the steel plate, and then performs reduction annealing of the iron oxides in a reducing atmosphere, and then performs hot dip zinc plating to provide a hot dip zinc plating or an alloyed hot dip zinc plating steel plate with excellent plating quality.

[0009] When using the method of reduction after oxidation in the annealing process as in Patent Document 1, components with a large affinity for oxygen such as Si, Mn, and Al are internally oxidized at a certain depth from the surface layer of the steel plate, suppressing the diffusion to the surface layer. Therefore, in the surface layer, single or composite oxides of Si, Mn, or Al relatively decrease, improving the wettability with zinc and reducing unplated areas. However, in the case of steel grades added with Si, Si concentrates directly under the iron oxide during the reduction process to form strip-shaped Si oxides, resulting in peeling in the surface layer including the plating layer, that is, peeling occurs at the interface between the reduced iron and the underlying base iron, making it difficult to ensure the adhesion of the plating layer.

[0010] On the other hand, as yet another method for improving the plating properties of hot-dip galvanized steel sheets, Patent Document 2 discloses a method of maintaining a high dew point in an annealing furnace to cause internal oxidation of alloy components such as Mn, Si, and Al, which are easily oxidized, inside the steel, thereby reducing the amount of oxides externally oxidized on the surface of the steel sheet after annealing and improving the plating properties. However, as a method according to Patent Document 2, although the problem of plating properties due to external oxidation of Si, which is easily internally oxidized, can be solved, when a large amount of Mn, which is relatively difficult to internally oxidize, is added, there is a problem that the effect is only slight.

[0011] Further, even if the plating properties are improved by internal oxidation, linear unplated areas may occur due to surface oxides formed unevenly on the surface, or when producing an alloyed hot-dip galvanized steel sheet (GA steel sheet) through alloying heat treatment after plating, problems such as linear defects due to uneven alloying may occur on the surface of the alloyed hot-dip galvanized steel sheet.

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0013] One aspect of the present invention is to provide a steel sheet having excellent plating properties by suppressing unplated areas or peeling of the plating layer during plating, and a method for manufacturing the same.

[0014] The problems of the present invention are not limited to the above-described content. Those having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding further problems of the present invention from the overall matters of the specification of the present invention.

Means for Solving the Problem

[0015] An example of the present invention is a steel plate containing one or two of Mn and Si in steel, In the GDS profile in which one or two components of Mn and Si are observed in the depth direction from the surface of the steel plate, there are two or more valleys where the content of one or two of Mn and Si is 60% or less of the base material content up to 1 μm from the surface of the steel plate. This is for the steel plate.

[0016] Another example of the present invention is a step of preparing a base steel plate containing one or two of Mn and Si in steel; Fe pre-plating is performed on the surface of the base steel plate at 0.5 g / m 2 excess to 3 g / m 2 or less to form an Fe plating layer; and A method for manufacturing a steel plate including a step of annealing a base steel plate on which the Fe plating layer is formed at a temperature of 600 to 950 °C in an atmosphere having a dew point temperature of 10 °C or less.

Advantages of the Invention

[0017] The present invention can provide a steel plate having excellent plating characteristics by suppressing the formation of oxides such as Mn and Si on the surface of the steel material and suppressing unplated or peeled plating during molten plating by using a method different from the conventional method.

[0018] The various and significant advantages and effects of the present invention are not limited to the above-described content, and can be more easily understood in the process of explaining the specific embodiments of the present invention.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0020] The terms used in this specification are for the purpose of explaining the present invention and are not intended to limit the present invention. Also, the singular forms used in this specification include plural forms as well, unless the relevant definition clearly indicates a different meaning.

[0021] The meaning of "including" used in this specification does not concretize the composition and does not exclude the existence or addition of other compositions.

[0022] Unless otherwise defined, all terms including technical terms and scientific terms used in this specification have the same meaning as generally understood by those having ordinary knowledge in the technical field to which the present invention belongs. Terms defined in a dictionary are interpreted to have a meaning consistent with the related technical literature and the currently disclosed content.

[0023] Hereinafter, the present invention will be specifically described in detail.

[0024] First, the steel sheet, which is one aspect of the present invention, will be described in detail.

[0025] The above steel sheet contains oxide-forming elements such as Mn and Si in the steel. The above Mn, Si, etc. can form oxides on the surface of the steel sheet during the annealing process before plating, thereby reducing the plating properties.

[0026] The above steel sheet can have the following characteristics in the GDS concentration profiles of oxidizing elements such as Mn and Si that form oxides on the surface and reduce plating properties. The above GDS concentration profile means the concentration and concentration profile measured using a glow discharge optical emission spectrometer.

[0027] FIG. 1 and FIG. 2 show the concentration profiles of Mn and Si measured in the depth direction from the surface of the steel sheets of Comparative Example 1 and Invention Example 6 in the examples described later. It can be seen that the concentration profiles measured from the surface show valleys where the content decreases and peaks where the content increases.

[0028] In the GDS profile in which one or two of Mn and Si are observed in the depth direction from the surface of the steel sheet of the present invention, there can be two or more valleys where the content of one or two of Mn and Si is 60% or less of the base material content within 1 μm from the surface of the above steel sheet.

[0029] Here, the base material content can be taken as the average content of the component in the steel sheet manufactured by adjustment in the steelmaking process, and typically, the one measured at the 1 / 4 point (1 / 4*t) of the thickness (t, unit: mm) of the steel sheet can be used.

[0030] When the number of the above valleys is less than two, surface enrichment of Mn and Si elements may occur, deteriorating the plating property. However, when the number of the above valleys is two or more, a depletion layer of the element is formed, suppressing surface enrichment, so that the plating property can be improved.

[0031] On the one hand, for the above steel plate, it is effective that the content of one or both of Mn and Si contained in the intragranular or grain boundary of grains with a grain size within 1 μm from the surface in the thickness direction is 40% or more of the component content measured at the 1 / 4 point (1 / 4*t) of the thickness (t, unit: mm) of the steel plate. Generally, the diffusion of elements in steel is a function of concentration and time. In order to minimize the diffusion of elements forming surface oxides to the surface, it is preferable to anneal and pass the steel plate at the fastest possible speed. However, in order to satisfy the mechanical properties, there are target temperatures and times for different sections of the annealing furnace. Considering the industrial production environment of the steel plate continuously passing through various facilities such as the annealing furnace and the plating bath, when the above component content is 40% or more, it can have an optimal effect in terms of plating properties and manufacturing efficiency.

[0032] Regarding the alloy composition of the steel plate containing Mn, Si, etc. among the above steels, the present invention is not particularly limited, and it is sufficient if it can be recognized by an ordinary technician that it can be used in the technical field to which the present invention belongs.

[0033] As an example, the above steel plate can contain, by weight%, C: 0.02 to 0.6%, Si: 0.001 to 2%, Al: 0.001 to 1%, Mn: 0.1 to 4%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, N: 0.02% or less, Ti: 0 to 0.1%, B: 0.0001 to 0.01%, Cu: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.01%, Nb: 0 to 0.1%, Sn: 0 to 1%, W: 0 to 1%, Sb: 0 to 1%, Mg: 0 to 0.1%, Co: 0 to 1%, As: 0 to 1%, Zr: 0 to 1%, Bi: 0 to 1%, REM: 0 to 0.3%, and the remaining Fe and unavoidable impurities.

[0034] On the one hand, the steel sheet can contain an oxide composed of one or more of Mn, Si, Al, Cr, and B in the grain boundaries from the surface to a depth of 10 μm in the depth direction. During annealing, the pro-oxidizing elements in the steel such as the above Mn, Si, Al, Cr, B, etc. mainly diffuse to the matrix iron grain boundaries and move to the surface layer. At the same time, the oxygen contained in the Fe plating layer can diffuse into the interior of the steel and combine with the pro-oxidizing elements to form an oxide.

[0035] The depth at which the above oxide is formed can be proportional to the absolute amount of oxygen contained in the Fe plating layer. As will be described later, the oxygen in the above Fe plating layer can be contained in an amount of 5 to 50% by weight. As a result, the thickness of the oxide after annealing is 1 μm or more and 10 μm or less. If it is less than 1 μm, the surface concentration suppression level is insufficient and the plating property is not improved. If it exceeds 10 μm, an additional electroplating cell must be provided, which causes a decrease in the production speed and is not preferable from an economic point of view.

[0036] The surface of the above steel sheet can include a plating layer such as a hot-dip zinc plating layer or a hot-dip aluminum plating layer. The type and method of the above plating layer are not particularly limited and can include all types and methods that can be carried out in the technical field to which the present invention belongs.

[0037] Next, a method for manufacturing a steel sheet, which is another aspect of the present invention, will be described in detail.

[0038] First, a base steel sheet containing one or two of Mn and Si in the steel is prepared.

[0039] In the present invention, as long as it is a base steel plate having the above-described alloy composition, it can be applied without limitation as the base steel plate of the plating steel plate or the hot-dip plating steel plate according to the present invention. Therefore, the method for manufacturing the base steel plate cannot be specifically limited. As an example, the above steel plate contains, by weight%, C: 0.02 to 0.6%, Si: 0.001 to 2%, Al: 0.001 to 1%, Mn: 0.1 to 4%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, N: 0.02% or less, Ti: 0 to 0.1%, B: 0.0001 to 0.01%, Cu: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.01%, Nb: 0 to 0.1%, Sn: 0 to 1%, W: 0 to 1%, Sb: 0 to 1%, Mg: 0 to 0.1%, Co: 0 to 1%, As: 0 to 1%, Zr: 0 to 1%, Bi: 0 to 1%, REM: 0 to 0.3%, and the balance Fe and inevitable impurities.

[0040] A specific example of the method for manufacturing the above base steel plate will be described in detail.

[0041] The base steel plate according to an embodiment of the present invention can be manufactured by reheating, hot rolling, coiling, and cold rolling a steel slab that satisfies the above-described alloy composition.

[0042] Reheating The steel slab that satisfies the alloy composition of the present invention can be reheated in a temperature range of 1200°C or higher. In order to mostly redissolve the precipitates present in the steel, it can be reheated to a temperature of 1200°C or higher. As an example of the present invention, the reheating temperature can be 1250°C or higher.

[0043] Hot rolling The above reheated steel slab can be hot rolled to a finish rolling temperature of Ar3 to 1000°C. During hot rolling, when the finish rolling temperature is lower than the Ar3 (γ→α transformation temperature during cooling) temperature, two-phase region rolling is likely to occur and a mixed grain structure is generated on the surface layer, which may cause difficulties in controlling the shape of the hot rolled steel plate. On the other hand, when the finish rolling temperature exceeds 1000°C, uniform hot rolling cannot be performed over the entire thickness, and there is a problem that grain refinement is insufficient.

[0044] Winding The hot-rolled steel sheet can be coiled at a temperature in the range of above Ms (martensite phase transformation start temperature) to 750°C. If the coiling temperature is below Ms, the strength of the hot-rolled steel sheet will be too high, reducing its cold rolling properties. If the coiling temperature exceeds 750°C, an increase in the thickness of the oxide layer and oxidation of the surface grain boundaries will occur, resulting in poor pickling properties and possibly the detachment of the surface grain boundaries during annealing in a continuous annealing furnace.

[0045] In the present invention, the cooling conditions to the coiling temperature after hot rolling are not particularly limited, and cooling can be performed under normal conditions applied in the same technical field. In one embodiment of the present invention, air cooling can be performed.

[0046] Pickling The hot-rolled steel sheet that has undergone the above-described process can be subjected to pickling treatment by immersing it in a hydrochloric acid bath to remove hot-rolling scale. The hydrochloric acid concentration of the hydrochloric acid bath during pickling is in the range of 10 to 30%, and the pickling speed is 100 to 250 mpm. If the pickling speed exceeds 250 mpm, the surface scale of the hot-rolled steel sheet may not be completely removed, and if the pickling speed is lower than 100 mpm, the surface layer of the base steel may be corroded by the hydrochloric acid. Therefore, the pickling speed should be 180 mpm or higher.

[0047] cold rolling The coiled steel sheet can be cold-rolled at a cumulative reduction of 30-90% to obtain a cold-rolled steel sheet. If the reduction is less than 30%, the roll and tension control may be inaccurate, which may cause distortion of the sheet, and recrystallization during annealing may be insufficient, resulting in increased anisotropy of the material even after hot forming. On the other hand, if the reduction exceeds 90%, the load on the cold roll during rolling may make it impossible to produce the product.

[0048] The surface of the base steel sheet is subjected to Fe pre-plating before annealing to form an Fe-plated layer.

[0049] The method for forming the Fe plating layer is not particularly limited, and an electroplating method or the like can be used.

[0050] The plating deposition amount of the above Fe pre-plating is 0.5 g / m 2 exceeding ~ 3 g / m 2 It is effective that it is as follows.

[0051] In order to ensure the quality of the hot-dip plating of a steel sheet containing Mn and Si, the plating amount of the Fe plating layer is 0.5 g / m based on the iron concentration 2 exceeding ~ 3.0 g / m 2 It is preferably processed as follows. The upper limit of the Fe plating amount is not particularly limited, but when it exceeds 3.0 g / m in the continuous plating process, a plurality of plating cells are required or the production speed decreases, which is not economical. Not only that, when the Fe plating amount is large, the Fe electroplating solution rapidly denatures in the continuous process and the pH decreases, resulting in a large decrease in plating efficiency and difficulty in solution management. On the other hand, when the Fe plating amount is 0.5 g / m 2 or less, the oxygen contained in the Fe plating layer is rapidly reduced and removed, so that it becomes impossible to effectively suppress the diffusion of Mn and Si from the base iron to form surface oxides, resulting in a problem of deterioration of the hot-dip plating quality. The above Fe plating amount has a thickness of about 0.05 to 0.4 μm when the iron concentration contained in the plating layer is completely reduced during annealing of the Fe plating layer. 2 In one embodiment of the present invention, the Fe plating layer can be formed on the surface of the base iron through an electroplating method, and the oxygen concentration of the Fe plating layer formed by appropriately controlling the conditions of the electroplating solution and the plating conditions can be controlled.

[0052] That is, in the present invention, in order to form an Fe plating layer, an electroplating solution containing ferrous ions including ferrous ions and ferric ions; a complexing agent; and inevitable impurities can be used, and the concentration of ferric ions in the above ferrous ions is 5 to 60% by weight.

[0053]

[0054] ​According to an embodiment of the present invention, the electroplating solution contains a first iron ion and a second iron ion. In order to obtain a high plating efficiency, it may be advantageous to contain only the first iron ion. However, when only the first iron ion is contained, the solution deteriorates and the plating efficiency drops sharply, which may induce quality variations in the continuous electroplating process. Therefore, the second iron ion can be further contained. At this time, the concentration of the second iron ion is preferably 5 to 60% by weight, more preferably 5 to 40% by weight, based on the total of the first iron and the second iron ions. If it is less than 5%, the rate at which the second iron is reduced to the first iron at the cathode is smaller than the rate at which the first iron is oxidized to the second iron at the anode, and the concentration of the second iron increases sharply while the pH drops sharply and the plating efficiency continuously decreases. On the other hand, when the concentration of the second iron ion exceeds 60%, the amount of the reaction in which the second iron is reduced to the first iron at the cathode increases significantly more than the amount of the reaction in which the first iron is reduced and deposited as metallic iron. Therefore, the plating efficiency drops significantly and the plating quality deteriorates. Therefore, considering equipment and process characteristics such as the plating amount, working current density, solution replenishment amount, the amount of solution adhering to and lost from the strip, and the rate of concentration change due to evaporation, it is preferable to set the concentration of the second iron ion in the iron ions to 5 to 60% by weight.

[0055] The concentration of the iron ions is preferably 1 to 80 g per liter of the electroplating solution, more preferably 10 to 50 g per liter. If it is less than 1 g / L, there is a problem that the plating efficiency and plating quality drop sharply. On the other hand, if it exceeds 80 g / L, it may exceed the solubility and precipitation may occur, increasing the raw material loss due to solution loss in the continuous plating process, which is not economical.

[0056] The electroplating solution of the present invention contains a complexing agent. In order to maintain a high plating efficiency without generating sludge while containing a large amount of the second iron, it is preferable to use an amino acid or an amino acid polymer as the complexing agent.

[0057] An amino acid is an organic molecule in which a carboxyl group (-COOH) and an amine group (-NH2) are bonded, and an amino acid polymer is an organic molecule formed by polymerizing two or more amino acids, and the amino acid polymer exhibits complexing properties similar to those of amino acids. Therefore, in the following description, amino acids and amino acid polymers will be collectively referred to as amino acids.

[0058] When amino acids dissolve in neutral water, the amines combine with hydrogen ions to acquire a positive charge, while the carboxyl groups acquire a negative charge due to the dissociation of hydrogen ions, so the amino acid molecules maintain a neutral charge. On the other hand, when the solution becomes acidic, the carboxyl groups recombine with hydrogen ions to become charge-neutral, and the amines acquire a positive charge, so the amino acid molecules form cations. In other words, amino acids become charge-neutral or form cations in a weakly acidic aqueous solution.

[0059] When an amino acid is added to an acidic electrolyte containing iron ions, it forms a complex with ferrous and ferric ions. However, the iron ions complexed with the amino acid remain cationic even when complexed. Therefore, it exhibits electrical properties opposite to those of a typical complexing agent with multiple carboxyl groups, which becomes negatively charged in a weakly acidic aqueous solution.

[0060] Although amino acids form fewer bonds with iron ions and weaker bonds than complexing agents containing multiple carboxyl groups, such as citric acid and EDTA, their binding to ferric ions, which cause sludge, is strong enough to prevent precipitation by ferric ions. Furthermore, because amino acids maintain their cation status even when complexed with ferric ions, they are easily transported to the cathode and reduced to ferrous ions, allowing them to participate in the plating reaction. However, their migration to the anode is inhibited, slowing the rate of ferric ion generation. This allows the ferric ion concentration to remain constant even during long-term continuous plating, maintaining consistent plating efficiency and eliminating the need for electrolyte replacement.

[0061] Meanwhile, in the continuous electroplating process, when iron ions in the solution are depleted through plating, the solution becomes acidic. However, even if the same amount of iron ions are deposited, a solution containing ferric ions also exhibits less pH change than a solution containing only ferrous ions. As the pH increases, some ferric ions combine with hydroxide ions, and as the pH decreases, the hydroxide ions separate and are neutralized. Therefore, a solution containing ferric ions acts as a pH buffer, slowing down pH change even without a separate pH buffer, and allowing electroplating efficiency to be maintained consistently during the continuous electroplating process.

[0062] Therefore, by using amino acids as a complexing agent, sludge can be prevented, and not only ferrous ions but also ferric ions can be used as plating raw materials. When ferrous ions and ferric ions are used in combination, the pH change of the solution can be slowed down and the accumulation of ferric ions can be easily prevented, so that electroplating efficiency and plating quality can be maintained constant in a continuous electroplating process.

[0063] The complexing agent is preferably added in an amount such that the molar concentration ratio of iron ions to complexing agent is 1:0.05-2.0, more preferably 1:0.5-1.0. If the molar ratio is less than 0.05, the excess ferric ions cannot be prevented from combining with hydroxide ions or oxygen to form sludge, resulting in a significant decrease in plating efficiency even without ferric ions, and furthermore, burning can occur, deteriorating plating quality. On the other hand, if the molar ratio exceeds 2.0, the sludge-inhibiting effect and plating quality are maintained, but the overvoltage increases, reducing plating efficiency. Furthermore, the use of an unnecessarily excessive amount of amino acids, which are relatively expensive compared to raw materials containing iron ions such as ferrous sulfate, increases raw material costs, making this uneconomical.

[0064] The complexing agent is preferably one or more selected from amino acids or amino acid polymers, and may be, for example, one or more selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid, and glycylglycine.

[0065] Using the above amino acid as a complexing agent, while maintaining the solution temperature at 80 °C or lower and the pH at 2.0 to 5.0, electroplating is carried out at a current density of 3 to 120 A / dm 2 Then, high plating efficiency can be achieved, and an Fe plating layer with a high oxygen concentration can be obtained.

[0066] The temperature of the Fe electroplating solution does not significantly affect the quality of the Fe plating layer. However, when it exceeds 80 °C, the evaporation of the solution becomes severe, the concentration of the solution continuously changes, and it becomes difficult to perform uniform electroplating.

[0067] When the pH of the Fe electroplating solution is less than 2.0, the electroplating efficiency decreases and it is not suitable for the continuous plating process. When the pH exceeds 5.0, the plating efficiency increases, but sludge in which iron hydroxide precipitates is generated during continuous electroplating, causing problems such as pipe blockage, roll and equipment contamination.

[0068] When the current density is less than 3 A / dm 2 it is not suitable for the continuous plating process because the plating overvoltage of the cathode decreases and the Fe electroplating efficiency decreases. When it exceeds 120 A / dm 2 burning occurs on the plating surface, the electroplating layer becomes non-uniform, and there is a problem that the Fe plating layer easily peels off.

[0069] As described above, in the present invention, it is preferable that the Fe plating layer contains 5 to 50% by weight of oxygen. The reason for the oxygen to be mixed into the Fe plating layer is as follows. During the process of iron being deposited on the surface of the steel plate to which the negative electrode is applied, at the same time, hydrogen ions are reduced to hydrogen gas and the pH rises. Therefore, both ferrous and ferric ions may temporarily combine with OH - ions and be mixed in together when the Fe plating layer is formed. If an anionic complexing agent such as acetic acid, lactic acid, citric acid, or EDTA is used, the complexing agent combines with OH -When the iron ions combined with ions are negatively charged on average and a cathode is applied for electroplating, an electrical repulsive force is generated, suppressing the contamination into the Fe plating layer. On the other hand, amino acids are electrically neutral at pH 2.0 to 5.0, and become positively charged in strong acids with a pH less than 2.0. However, even if 1 to 2 OH - are combined with the iron ions bound to amino acids and they become positively charged, an electrical attractive force is generated with the cathode for electroplating, resulting in a large amount of oxygen being mixed in. Therefore, when amino acids are used as a complexing agent so that the molar concentration ratio of iron ions to amino acids is 1:0.05 to 1:2.0 and Fe electroplating is carried out while maintaining pH 2.0 to 5.0, the plating efficiency is high, and an Fe plating layer containing 5 to 50% by weight of oxygen can be obtained while suppressing sludge generation.

[0070] Next, the base steel plate on which the above Fe plating layer is formed is annealed.

[0071] The above annealing can be carried out by maintaining at 600 to 950°C for 1 to 1000 seconds with a dew point temperature of +10°C or lower.

[0072] When the dew point temperature exceeds +10°C during the above annealing heat treatment, there is a risk that the base steel plate itself will oxidize. However, if the dew point temperature is excessively low, there is a problem that the plating performance deteriorates. Therefore, considering this, the lower limit of the dew point temperature can be set to -50°C, preferably -10°C.

[0073] The above annealing can be heat-treated in the temperature range of 600 to 950°C. When the heat treatment temperature is less than 600°C, it is difficult for the rolling structure generated by cold rolling to recover and recrystallize, and it is difficult to ensure sufficient tensile physical properties. On the other hand, when it exceeds 950°C, decarburization occurs excessively, the fatigue characteristics deteriorate, a large amount of B oxide is formed, which may reduce the plating adhesion during plating, and may cause the annealing equipment to deteriorate, resulting in an increase in process costs due to frequent replacement of equipment, etc.

[0074] Furthermore, the annealing time can be 1 to 1000 seconds. When the annealing time is less than 1 second, it is difficult to ensure the annealing effect, and when it exceeds 1000 seconds, the production line may decline.

[0075] On the other hand, when heating for the annealing heat treatment, humid nitrogen can be introduced when the temperature is raised in the heating section, preferably to 700 °C or higher. This is to induce internal oxidation of the oxidizing elements, and the humid nitrogen can be introduced at a flow rate of 50 to 200 Nm 3 / h (on the other hand, the moisture content within the range of 50 to 200 Nm of humid nitrogen can be calculated as 5 to 40 L / h). When the amount of the above-mentioned humid nitrogen is less than 50 Nm 3 / h, the effect of raising the dew point is insufficient and the formation of the internal oxidation layer is weak. When it exceeds 200 Nm 3 / h, the dew point exceeds +10 °C, becoming excessively high, and there is a problem that the base iron itself is oxidized. 3 After the annealing stage, the annealed steel sheet can be cooled. Since the cooling conditions in the cooling stage after the annealing stage do not have a great influence on the surface quality of the final product, that is, the plating quality, there is no need to particularly limit the cooling conditions in the present invention. However, in order to prevent oxidation of the iron component during the cooling process, a reducing atmosphere can be applied at least for iron.

[0076] The above-mentioned manufactured steel sheet can be further plated. The above plating is not particularly limited. As an example, the annealed steel sheet can be immersed in a molten zinc plating bath or a molten aluminum plating bath to form a molten plating layer or an alloyed molten plating layer. The above molten plating layer can be a molten aluminum plating layer, a molten Al-Si plating layer, a molten Al-Si-Mg plating layer, a molten zinc plating layer, a molten Zn-Mg plating layer, etc., and the above alloyed molten plating layer can be an alloyed molten aluminum plating layer, an alloyed molten Al-Si plating layer, an alloyed molten Al-Si-Mg plating layer, an alloyed molten zinc plating layer, an alloyed molten Zn-Mg plating layer, etc.

[0077]

[0078] ​ The plating layer may contain Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, Ca, Sr, Mg, etc. The adhesion amount of the plating layer is not particularly limited, and for example, it may be an adhesion amount within a general range. Similar to the steel sheet, a plating layer or an alloyed plating layer may be provided on the steel member after heat treatment.

Examples

[0079] Hereinafter, examples of the present invention will be described. It goes without saying that those having ordinary knowledge in the technical field to which the present invention pertains can make various modifications without departing from the scope of the present invention. The following examples are for understanding the present invention, and the scope of rights of the present invention should not be limited to the following examples, but should be determined not only by the scope of claims described later but also by equivalents thereof.

[0080] (Example) A cold-rolled steel sheet containing, 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%, with the balance being Fe and inevitable impurities, was prepared. After forming an Fe plating layer with the Fe adhesion amount shown in Table 1 below, annealing was performed under the dew point and temperature conditions shown in Table 1 below. On the other hand, after annealing, the cold-rolled steel sheet was immersed in an Al plating bath containing 9 wt.% Si to perform aluminum plating.

[0081] The above Fe plating layer was formed by immersing the cold-rolled steel sheet in the above Fe plating solution and applying a current density of 20 A / dm 2 to perform Fe electroplating, and controlling the plating time to control the adhesion amount (at this time, the temperature of the solution was maintained at 50°C). The plating time according to the target adhesion amount was calculated based on measuring the dissolution of the Fe plating layer and the total amount of Fe with a 5 - 10 wt.% hydrochloric acid solution after performing Fe electroplating on a copper sheet in the same solution beforehand, calculating the electroplating adhesion amount and plating efficiency.

[0082] For the steel sheet manufactured as described above, glow discharge optical emission spectrometer (GDS) analysis was performed to measure the content profiles of Mn and Si in the depth direction from the surface of the steel sheet. The GDS profiles of Comparative Example 1 and Inventive Example 6 are shown in FIGS. 1 and 2 below.

[0083] The GDS analysis of the present invention was performed using the GDS850A equipment of LECO Corporation and measured at intervals of 0.01 - 0.03 μm from the surface layer to a depth of 2 μm in the RF mode.

[0084] From the GDS analysis results of each of the above steel sheets, the number of valleys where the content of each element is 60% or less of the base material content from the surface to a thickness of 1 μm is defined as A, and the content of the deepest valley is defined as B (unit: wt.%), and shown in Table 1 below. The above valleys do not mean the points of the lowest values in GDS, but mean that they are represented by the trends observed on the profile. In FIG. 1 above, the number of valleys of Comparative Example 1 for Mn is 1, and the number of valleys of Inventive Example 6 is observed to be 2. In FIG. 2, although the number of valleys of Comparative Example 1 for Si was not observed, one was observed in Inventive Example 6.

[0085] On the other hand, the plating properties of each steel sheet were evaluated, and the results are also shown in Table 1 below. The above plating properties were evaluated by plating adhesion. This was evaluated using the structural adhesive SA - 1607E of Hosokawa Corporation. First, on a plated steel sheet with a size of 30 × 80 mm 2 a Teflon jig was used, and the horizontal × vertical × height was 10 × 50 × 10 mm respectively 3After applying an adhesive to a rectangular parallelepiped, baking was performed at a firing temperature of 170 °C for 20 minutes. After baking was completed, it was stored at room temperature for one day, and then the adhesive and the plated steel sheet were fixed. After that, the steel sheet was bent at 90° to forcibly separate the adhesive and the plated steel sheet. At this time, if peeling occurred inside the adhesive, it was judged as normal, and if separation occurred between the plating layer and the adhesive, it was judged as peeling. Specifically, if a peeled area with a diameter of 3 mm or more occurred, it was judged as peeling. If peeling occurred but in a very small area with a diameter of 3 mm or less, re-judgment was performed through re-experimentation. Samples in which peeling occurred 3 times or more after being implemented 5 times were marked as "×", 2 times or more and 1 time or more as "△", and no peeling as "○" grade.

[0086]

Table 1

[0087] In Invention Examples 1 to 4, the Fe plating layer was adhered at 1000 to 3000 mg / m 2 or less. After that, as a result of GDS analysis of the annealed and plated material, two minimum points of Mn were formed, which means that a section lacking Mn was formed in the outermost layer. Therefore, since the surface layer Mn oxide was reduced, the plating adhesion was judged to be good. Invention Examples 5 to 8 adhered the Fe plating layer to the steel sheet at 1000 to 3000 mg / m 2 or less, and after annealing while maintaining the dew point in the annealing furnace at 3 to 7 °C, hot-dip zinc plating was performed. The internal oxidation effect due to the oxygen contained in the Fe plating layer and the moisture in the annealing furnace was aggravated, and the plating adhesion could be improved epoch-makingly.

[0088] On the other hand, in Comparative Example 1, the base iron without the Fe plating layer was annealed and hot-dip aluminum plated under the same conditions as described above. From the GDS measurement results, since the surface Mn and Si concentrations were high, it was judged that the plating property deteriorated due to the formation of the surface layer oxide. In the case of Comparative Example 2, although the Fe plating layer was adhered at 500 mg / m 2 it was insufficient to stably ensure the plating adhesion.

[0089] In Comparative Example 3, by raising the dew point of the annealing furnace to 5°C, an internal oxide was formed with parent oxidizing elements to suppress surface enrichment, but the effect was insufficient and the plating adhesion was poor. In Comparative Example 4, 500 mg / m 2 of an Fe plating layer was adhered before annealing at a dew point of 4°C, but the plating adhesion only improved slightly, which is still judged to be due to the insufficient surface suppression level of Mn and Si.

[0090] In Comparative Examples 5 and 6, an Fe plating layer of 3000 mg / m 2 or more was formed on the steel sheet. However, during the observation of the sample after electroplating, peeling occurred within the plating layer and dust was generated, making it impossible to proceed with subsequent annealing and plating processes.

Claims

1. A steel plate containing one or both of Mn and Si in steel, in the GDS profile observing one or both of the components of Mn and Si in the depth direction from the surface of the steel plate, there are two or more valleys where the content of one or both of the components of Mn and Si is 60% or less of the base metal content up to 1 μm from the surface of the steel plate. Steel plate.

2. In the steel plate, the content of one or both of Mn and Si contained in the intragranular or grain boundary of the crystal grains (grain) within 1 μm in the depth direction from the surface is 40% or more of the content of each component in the base metal. The steel plate according to claim 1.

3. The steel plate according to claim 1, wherein the grain boundaries from the surface to a depth of 10 μm contain oxides composed of one or more of Mn, Si, Al, Cr, and B.

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

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

6. The step of preparing a base steel plate containing one or both of Mn and Si in steel; On the surface of the base steel sheet, electroplating with Fe is carried out at a thickness of 0.5 g / m ２ exceeding to 3 g / m ２ or less to form an Fe plating layer; and The method for manufacturing a steel plate includes annealing the base steel plate with the Fe plating layer formed at a temperature of 600 to 950 °C in an atmosphere with 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 annealing time is 1 to 1000 seconds. The method for manufacturing a steel plate according to claim 6.

9. Inject moist nitrogen at a flow rate of 50 to 200 Nm ３ / h during heating for the annealing, the method for manufacturing a steel sheet according to claim 6.

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

11. The step of reheating the steel slab in a temperature range of 1200°C or higher for the base steel sheet; The step of hot rolling at a finish rolling temperature of Ar3 to 1000°C after the reheating; The step of coiling in a temperature range exceeding Ms (martensite phase transformation start temperature) and 750°C or lower; and The method for manufacturing the steel sheet according to claim 6, which includes the step of cold rolling with a cumulative reduction ratio of 30 to 90%.

12. The method for manufacturing the steel sheet according to any one of claims 6 to 11, further including the step of forming a plating layer by dipping in a molten zinc plating bath or a molten aluminum plating bath after the annealing.

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