Steel sheet with excellent dent resistance and manufacturing method thereof

Abstract

The present invention relates to a steel sheet used in automobiles and the like, and to a steel sheet having excellent dent resistance and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet used in automobiles and the like, and to a steel sheet having excellent dent resistance and a method for manufacturing the same. [Background technology]

[0002] Recently, technological development of automotive steel has focused on ensuring safety through increased strength and reducing weight through reduced thickness. Technologies for increasing the strength of steel include solid solution strengthening and precipitation strengthening through the addition of alloying elements, as well as AHSS (Advanced High Strength Steel) steel, which utilizes phase transformation. As a result, active development is being carried out on steel that simultaneously increases tensile strength and elongation.

[0003] To increase the strength of steel, it is essential to add alloying elements such as Mn, Si, Cr, and B. These alloying elements have a high tendency to oxidize, so they bind with oxygen during annealing and diffuse to the surface. The surface oxides formed during the annealing process deteriorate the surface reactivity, significantly reducing the plating quality, chemical conversion treatability, etc.

[0004] Such surface oxides not only deteriorate surface quality, but also adhere to and grow on the surface of hearth rolls in an annealing furnace, potentially causing dent problems in the annealing furnace, which damage the surface of the steel sheet during threading. In particular, Mn build-up dents caused by Mn surface oxides reduce continuous productivity, and if the problem persists, it may even lead to production stoppages for repairs, causing a significant decrease in productivity.

[0005] Various techniques have been proposed to improve the above-mentioned dents. One method is to change the material of the hearth roll surface through thermal spray coating to suppress chemical reactions at high temperatures after Mn surface oxides adhere to the hearth roll surface (Patent Document 1). The ceramic content of the hearth roll coating material can be increased through thermal spray coating, or 100% ceramic can be used to suppress chemical reactions. However, changing the thermal spray coating material to ceramic increases the cost of the coating and reduces wear resistance, shortening the coating life.

[0006] Other methods for reducing the formation of Mn surface oxides include controlling the oxygen partial pressure, dew point temperature, etc. in the annealing furnace, but these methods have limitations in that they cannot completely suppress the formation of Mn surface oxides. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 5,466,208 Summary of the Invention [Problem to be solved by the invention]

[0008] SUMMARY OF THE INVENTION One aspect of the present invention is to provide a steel sheet having excellent dent resistance and a method for manufacturing the same.

[0009] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is technically described in the general content of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the further object of the present invention from the content described in the specification of the present invention. [Means for solving the problem]

[0010] One aspect of the present invention contains, by weight, Mn: 0.1 to 8.0%, Si: 0.05 to 3.0%, C: 0.001 to 0.6%, Sol.Al: 0.005 to 3%, P: 0.1% or less (0% excluded), S: 0.02% or less (0% excluded), Cr: 1.5% or less (0% included), B: 0.005% or less (0% included), the balance being Fe and unavoidable impurities, It contains a fine grain layer consisting of crystal grains with a minor axis length of 0.5 μm or less and a major axis length of 3 μm or less within a maximum depth of 1 μm from the surface, The fine grain layer has a length occupancy of 5% or more in the transverse direction of the cross section of the steel sheet.

[0011] The steel sheet may further contain one or more of Ti, Mo and Nb in an amount of 1.2% or less.

[0012] The steel sheet may include a hot-dip coating layer, which is any one of a hot-dip galvanized (GI) layer, a galvannealed (GA) layer, a ternary zinc alloy coating (Zn-Al-Mg) layer, and a hot-dip aluminum alloy coating layer.

[0013] The steel sheet may include any one of a Ni plating layer and a Zn plating layer.

[0014] The steel sheet may include an electrogalvanized (EG) layer.

[0015] Another aspect of the present invention is a method for manufacturing a steel sheet, the method comprising the steps of preparing a steel sheet containing, by weight, Mn: 0.1 to 8.0%, Si: 0.05 to 3.0%, C: 0.001 to 0.6%, Sol.Al: 0.005 to 3%, P: 0.1% or less (excluding 0%), S: 0.02% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), the balance being Fe and unavoidable impurities; 0.5 to 3.0 g / m on the surface of the above base steel sheet 2 forming an Fe plating layer with a coating weight of Annealing the base steel sheet on which the Fe-plated layer is formed at a temperature of 600 to 900°C in a nitrogen (N2) gas atmosphere containing 1 to 80 vol.% of hydrogen (H2) at a dew point temperature of -60 to 30°C; and cooling the annealed base steel sheet.

[0016] The manufacturing method may further include a step of performing hot-dip coating before the completion of the cooling, and the hot-dip coating may be any one of hot-dip galvanizing (GI), galvannealed hot-dip coating (GA), ternary zinc alloy coating (Zn-Al-Mg), and aluminum alloy coating.

[0017] The method further includes a step of pickling after the cooling, and the pickling can be carried out in a 5 to 18 wt % acid solution at 50 to 80°C.

[0018] In the above manufacturing method, after the above pickling, 2 The method further includes the step of forming a metal plating layer by electroplating with a deposition amount of Ni or Zn.

[0019] The manufacturing method may further include a step of performing electrogalvanizing (EG) after the metal plating.

[0020] The step of preparing the base steel sheet includes: heating the steel slab to 1100-1300°C; hot rolling the heated steel slab to produce a hot-rolled steel sheet; a step of cooling the hot-rolled steel sheet and then coiling it at 800°C or less; and pickling and cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet. [Effects of the Invention]

[0021] According to the present invention, a fine crystal grain layer having a certain depth is formed on the surface of a steel sheet, thereby suppressing the formation of surface oxides during annealing, preventing the formed surface oxides from falling off, and reducing adhesion to the hearth roll surface of an annealing furnace, thereby providing a steel sheet with excellent dent resistance, which has the advantage of improving productivity.

[0022] The various advantageous and beneficial effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0023] [Figure 1] 1(a) is a cross-sectional conceptual diagram of an example of a conventional steel plate, and FIG. 1(b) is a cross-sectional conceptual diagram of an example of a steel plate of the present invention. [Figure 2] 1 is a cross-sectional photograph of Example 4 in the examples of the present invention, and is a photograph for deriving the length occupancy ratio of a fine crystal grain layer. [Figure 3] Graphs (a) and (b) show the Fe coating weight and the Mn integral values ​​at −40° C. and −20° C. for steel types A and B in the examples. [Figure 4] 1(a) is a surface photograph of Comparative Example 1 and Invention Example 4 in the examples of the present invention, and FIG. 1(b) is a surface photograph of Comparative Example 4 and Invention Example 16. [Figure 5] 1(a) is a cross-sectional photograph of Comparative Example 1 and Invention Example 4 in the examples of the present invention, and FIG. 1(b) is a cross-sectional photograph of Comparative Example 4 and Invention Example 16. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0024] The terminology used herein is for the purpose of describing the invention and is not intended to limit the invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the relevant definition clearly indicates otherwise.

[0025] The meaning of "comprises" as used in the specification is to specify features and does not exclude the presence or addition of other features.

[0026] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content.

[0027] The inventors of the present invention have made extensive research into steel sheets with excellent dent resistance and have arrived at the present invention.

[0028] First, a steel sheet according to one embodiment of the present invention will be described in detail. It should be noted that the content of each element in the present invention is expressed in weight percent unless otherwise specified. Furthermore, the ratio of crystals and structures is based on area unless otherwise specified. Furthermore, the content of gas is based on volume unless otherwise specified.

[0029] The composition of the above steel sheet includes, in weight percent, Mn: 0.1 to 8.0%, Si: 0.05 to 3.0%, C: 0.001 to 0.6%, Sol.Al: 0.005 to 3%, P: 0.1% or less (excluding 0%), S: 0.02% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), and optionally further includes 1.2% or less (including 0%) of one or more of Ti, Mo, and Nb, with the balance being Fe and unavoidable impurities.

[0030] Manganese (Mn): 0.1 to 8.0% Mn is an essential element in transformable microstructure steel because it forms and stabilizes retained austenite and inhibits ferrite transformation during cooling. Furthermore, to ensure sufficient austenite and ensure strength and ductility, Mn can be included at 0.1% or more. However, if the Mn content exceeds 8.0%, excessive band formation due to segregation induced during the slab and hot rolling processes can occur, impairing physical properties. Therefore, the upper limit of the Mn content can be limited to 8.0% or less.

[0031] Silicon (Si): 0.05 to 3.0% The above-mentioned Si inhibits the precipitation of carbides in ferrite and promotes the diffusion of carbon from ferrite into austenite, thereby contributing to the stabilization of retained austenite. To achieve these effects, Si must be added in an amount of 0.05% or more. However, excessive addition can reduce surface reactivity and cause problems with plating and phosphate treatment, so the upper limit of Si content can be limited to 3.0%.

[0032] Carbon (C): 0.001~0.6% The C content is an important element added to stabilize the retained austenite, and for this reason, it is preferable to add 0.001% or more. On the other hand, if the C content exceeds 0.6%, the problem of deteriorated weldability may occur. Therefore, the C content in the present invention can be limited to 0.001 to 0.6%.

[0033] Solid solution aluminum (Sol.Al): 0.005 to 3% Al is an element that contributes to the stabilization of retained austenite by inhibiting the formation of carbides in ferrite, and can be added in an amount of 0.005% or more to achieve this effect. However, if the Al content exceeds 3%, it may be difficult to produce a sound slab due to a reaction with mold flux during casting, and surface oxides may form, hindering hot-dip galvanization. Therefore, the upper limit of the Al content can be limited to 3% or less.

[0034] Phosphorus (P): 0.1% or less (0% excluded) The above-mentioned P is a solid solution strengthening element, but if its content exceeds 0.1%, weldability decreases and there is a high risk of the steel becoming brittle, so the upper limit can be set to 0.1%.

[0035] Sulfur (S): 0.02% or less (0% excluded) The above-mentioned S is an impurity element that impairs the ductility and weldability of a steel sheet. Therefore, if the S content is high, there is a high possibility that the ductility and weldability of the steel sheet will be impaired. Taking this into consideration, the upper limit of S content is set to 0.02%.

[0036] Chromium (Cr): 1.5% or less (including 0%) Cr is a hardening-enhancing element that suppresses the formation of ferrite. Therefore, a small amount of Cr can be added as needed to ensure 5 to 30% retained austenite. However, if the content is too high, the amount of ferroalloy added may be excessive, which may increase costs. Therefore, the upper limit of the Cr content can be limited to 1.5%.

[0037] Boron (B): 0.005% or less (including 0%) The above-mentioned B is an element that can be selectively added to ensure strength. If the B content exceeds 0.005%, it will concentrate on the surface of the annealed material and significantly deteriorate the surface quality. Therefore, the B content is preferably 0.005% or less.

[0038] In addition to the alloy composition described above, one or more of titanium (Ti), molybdenum (Mo) and niobium (Nb) may be contained in an amount of 1.2% or less (including 0%).

[0039] Mo can contribute to improving strength. In particular, it can ensure strength without reducing the wettability of molten metals such as zinc. Ti can form nitrides to reduce the N concentration in steel. However, excessive Ti content can lead to the precipitation of carbides, which can reduce the carbon concentration and strength of martensite. Nb segregates in the form of carbides at austenite grain boundaries and can suppress coarsening of austenite grains during annealing heat treatment, thereby increasing strength. However, excessive Nb content can increase costs. In consideration of this, one or more of Ti, Mo, and Nb can be contained in an amount of 1.2% or less.

[0040] In addition to the above-mentioned steel composition, the remainder may contain Fe and unavoidable impurities. Unavoidable impurities are those that may be unintentionally mixed in during a typical steel manufacturing process, and they cannot be completely eliminated. This meaning is easily understood by engineers in the field of typical steel manufacturing. Furthermore, the present invention does not completely exclude the addition of other components than the above-mentioned steel composition.

[0041] The steel sheet includes a fine grain layer composed of crystal grains with a minor axis length of 0.5 μm or less and a major axis length of 3 μm or less within a maximum depth of 1 μm from the surface. Figure 1(a) shows a conventional steel sheet without a fine grain layer on the surface, while Figure 1(b) is a schematic diagram showing the steel sheet of the present invention with a fine grain layer on the surface. As shown in Figure 1, in the conventional steel sheet, large amounts of coarse annealing oxides formed on the surface of the steel sheet easily shed from the surface, causing dent problems. In contrast, as shown in Figure 1(b), the present invention has a fine grain layer, resulting in less annealing oxides on the surface. Furthermore, the oxides that do form are small in size and mainly form at the grain boundaries of fine grains exposed on the surface, making them less likely to shed. Since the amount of shed oxides is extremely small, excellent dent resistance can be ensured.

[0042] On the other hand, it is effective that the fine crystal grain layer occupies 5% or more of the length in the transverse direction of the cross section of the steel sheet.

[0043] Here, the measurement of the length occupancy ratio refers to the ratio of the length of the fine crystal grain layer present in the horizontal direction to the reference length in the cross section of the steel sheet of the present invention. More detailed explanation will be given with reference to FIG. 2. FIG. 2 below shows the cross section of Example 4 in the Examples described later. Looking at the cross section of the steel sheet, there are areas where the fine crystal grain layer is formed and areas where it is not formed. The length occupancy ratio can be calculated by measuring the length of the area where the fine crystal grain layer is formed and the length of the area where it is not formed per unit length in the horizontal direction, which is the observation direction. For example, it is preferable that the length of the area where the fine crystal grain layer is formed is 1 μm or more within 20 μm in the horizontal direction, which is the observation direction of the cross section of the steel sheet.

[0044] If the length occupancy of the fine grain layer is less than 5%, the effect of the fine grain layer in preventing the annealing oxides from falling off is insufficient, and there is no significant advantage in preventing dents. Therefore, if the length occupancy of the fine grain layer is at least 5%, the effect of reducing the surface annealing oxides and the effect of preventing dents from falling off are improved.

[0045] The steel sheet may include a plating layer.

[0046] The plating layer may be a hot-dip plating layer, and the hot-dip plating layer may be a zinc-based alloy plating layer or an aluminum-based alloy plating layer. For example, the zinc-based alloy plating layer may be a hot-dip galvannealed (GI) layer, a galvannealed (GA) layer, a ternary zinc alloy plating (Zn-Al-Mg) layer, or a hot-dip aluminum-based alloy plating layer.

[0047] Meanwhile, the plating layer may be a metal plating layer, for example, a Ni plating layer or a Zn plating layer, which is distinguished from the hot-dip plating layer in that it is manufactured by electroplating, unlike the hot-dip plating layer.

[0048] Meanwhile, the steel sheet may further include an electrogalvanized (EG) layer.

[0049] Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described in detail. The manufacturing method includes the steps of preparing a base steel sheet, forming an Fe-plated layer on the surface of the base steel sheet, annealing the base steel sheet on which the Fe-plated layer has been formed, and cooling the annealed steel sheet. Each step will be described in detail below.

[0050] The base steel sheet is not particularly limited in type, such as a hot-rolled steel sheet or a cold-rolled steel sheet, and can be any steel sheet that can be applied in the technical field to which the present invention pertains. Therefore, the method for manufacturing the base steel sheet does not need to be specifically limited.

[0051] As a specific example of a method for producing the above-mentioned base steel sheet, a steel slab containing, in weight percentages as described above, Mn: 0.1 to 8.0%, Si: 0.05 to 3.0%, C: 0.001 to 0.6%, Sol. Al: 0.005 to 3%, P: 0.1% or less (excluding 0%), S: 0.02% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), and optionally one or more of Ti, Mo, and Nb containing 1.2% or less (including 0%), the balance being Fe and unavoidable impurities, is prepared, and the steel slab is heated in a temperature range of 1100 to 1300°C.

[0052] The heated steel slab can be hot-rolled to obtain a hot-rolled steel sheet.

[0053] After cooling, the hot-rolled steel sheet is coiled at a temperature of 800° C. or less. If necessary, the hot-rolled steel sheet can be pickled and cold-rolled to obtain a cold-rolled steel sheet.

[0054] An Fe plating layer is formed on the surface of the base steel sheet prepared as above.

[0055] The Fe plating layer has a thickness of 0.5 to 3.0 g / m 2 It is effective to perform Fe plating with a coating weight of

[0056] The method for forming the Fe plating layer is not particularly limited, but as an example, the Fe plating layer can be formed by electroplating. As a specific example, an electroplating solution containing iron ions including ferrous ions and ferric ions, a complexing agent, and inevitable impurities, in which the concentration of ferric ions among the iron ions is 5 to 60 wt %, can be used. On the other hand, the concentration of the iron ions can be 1 to 80 g per 1 L of the electroplating solution. The electroplating solution can be electroplated at 80°C or less, with a current density of 3 to 120 A / dm 2 Electroplating can be carried out at

[0057] The base steel sheet on which the Fe plating layer is formed can be subjected to annealing heat treatment at a temperature of 600 to 900°C in a nitrogen (N2) gas atmosphere containing 1 to 80 vol.% hydrogen (H2) with a dew point temperature of -60 to 30°C.

[0058] During annealing, keeping the dew point temperature below -60°C requires the injection of very dry gas, which can make it very difficult to maintain the airtightness of the equipment and can be inefficient. On the other hand, a dew point above +30°C is in the range where Fe oxidizes, adversely affecting surface quality. On the other hand, when annealing is performed at a dew point temperature of -20 to 30°C, internal oxidation can occur, in which alloying elements such as Si and Mn form oxides at the internal grain boundaries of the steel sheet. This internal oxidation can have benefits for surface quality, such as suppressing decarburization and Si surface segregation. Even in the dew point temperature range of -20 to 30°C, where internal oxidation occurs, the present invention achieves the targeted effects of reducing the fine grain layer and surface annealing oxides.

[0059] Meanwhile, the annealing atmosphere gas contains nitrogen (N2) and a certain amount of hydrogen (H2). If the hydrogen concentration is less than 1 vol.%, the Fe reducing power is insufficient and Fe oxidation may occur. If the hydrogen concentration is more than 80 vol.%, there is a risk of explosion when the gas leaks, and the cost increases when high-hydrogen operation is performed to rapidly cool the steel sheet.

[0060] If the annealing temperature is less than 600°C, the recrystallization of the cold-rolled steel sheet may not be sufficient, and if it exceeds 900°C, problems of equipment damage and increased costs may occur. Therefore, the annealing temperature may be set to 600 to 900°C.

[0061] The annealed steel sheet can be cooled. For example, the cooling is performed by slow cooling to the annealing temperature of 650°C, followed by rapid cooling depending on the desired properties. If necessary, the rapidly cooled steel sheet is reheated to a certain temperature for tempering and then cooled to room temperature.

[0062] In the present invention, the cooling conditions may vary depending on the conditions for achieving the target material. Furthermore, since the formation of surface annealing oxides mostly occurs in a relatively high temperature region, the cooling conditions do not need to be particularly limited in the present invention. However, to prevent oxidation of iron components during the cooling process, a reducing atmosphere may be applied at a minimum.

[0063] After the annealing heat treatment, hot-dip galvanization can be performed before the end of cooling. The hot-dip galvanization may be zinc-based alloy plating, aluminum-based alloy plating, etc. Specifically, the hot-dip galvanization may be hot-dip galvanizing (GI), galvannealed hot-dip galvanizing (GA), ternary zinc alloy plating (Zn-Al-Mg), hot-dip aluminum-based alloy plating, etc.

[0064] The steel sheet annealed during the zinc-based alloy plating can be hot-dip plated after cooling to a temperature range of 400 to 500°C, and the steel sheet annealed during the aluminum-based alloy plating can be hot-dip plated after cooling to a temperature range of 600 to 700°C. Meanwhile, the alloying temperature in the galvannealed hot-dip plating (GA) can be 480 to 580°C.

[0065] The cooled steel sheet can be pickled. The pickling can be carried out with a 5 to 18 wt % acid solution at 50 to 80°C. As a specific example, pickling can be carried out with 5 wt % hydrochloric acid at 50 to 60°C, and in the case of steel types that require some strong pickling, pickling can be carried out with 18 wt % hydrochloric acid at 80°C.

[0066] To improve surface reactivity after pickling, 5 to 100 mg / m 2 In this case, the plating may be Ni plating, Zn plating, etc., and the type is not particularly limited and can be applied depending on the purpose.

[0067] Meanwhile, after forming the metal plating layer, electrogalvanizing (EG) can be further carried out. [Example]

[0068] Next, an embodiment of the present invention will be described.

[0069] It goes without saying that various modifications of the following examples are possible within the scope of the present invention, as long as they do not deviate from the scope of the present invention, by those skilled in the art. The following examples are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following examples, but should be defined by the claims below as well as equivalents thereof.

[0070] (Example) Two types of cold-rolled steel sheets, A and B, having the compositions shown in Table 1 below were prepared. An Fe-plated layer was formed on each of the cold-rolled steel sheets at the Fe coating weights shown in Table 2. The Fe plating was performed by electroplating.

[0071] [Table 1]

[0072] The steel sheets having the Fe-plated layers formed thereon were subjected to annealing heat treatment and cooling. Specifically, the annealing furnace atmosphere was a reducing atmosphere of nitrogen gas containing 5% hydrogen, and the dew point temperatures were −40° C., −20° C., and +5° C. as shown in Table 2 below.

[0073] The specific annealing conditions for steel type A were as follows: heating to 840°C at a heating rate of 3.1°C / sec, maintaining this temperature for 65 seconds, primary cooling to 650°C at a cooling rate of 2.7°C / sec, followed by rapid cooling to 450°C at a cooling rate of 9°C / sec (secondary cooling), followed by slow cooling to 360°C at a cooling rate of 0.2°C / sec (tertiary cooling), and final cooling to room temperature at a cooling rate of 10°C / sec (quaternary cooling).

[0074] Steel type B was heated to 800°C at a heating rate of 3.2°C / sec, maintained at that temperature for 61 seconds, and then cooled to 650°C at 2.3°C / sec (primary cooling), followed by rapid cooling to 450°C at a cooling rate of 9.5°C / sec (secondary cooling).It was then slowly cooled to 400°C at a cooling rate of 0.1°C / sec (tertiary cooling), and finally cooled to room temperature at a cooling rate of 10°C / sec (quaternary cooling).

[0075] The cross-sectional lengthwise occupancy rate of the fine grain layer of the steel sheets manufactured as above was measured and the results are shown in Table 2.

[0076] Furthermore, the integral values ​​of Mn wt % from the surface of the steel sheet to 0.03 μm through the GDS profile were calculated and are shown in Table 2.

[0077] [Table 2] TIFF2026500350000004.tif125165

[0078] The method for measuring the occupancy rate of the fine crystal grain layer is as follows. - Cross-section processing using FIB (Focused Ion Beam) and observation using STEM (Scanning Transmission Electron Microscopy) at a minimum magnification of 20,000 (20,000 times or more) - FIB-STEM for length occupancy analysis can be performed by sampling at random positions and analyzing 20 μm or more in the lateral direction of the total cross section, or by connecting 20 μm or more in one position and analyzing.

[0079] The above Mn integral value was estimated by assuming the Mn profile up to a depth of 0.03 μm in the GDS data as the annealed surface oxide, and the Mn concentration at a depth of 0.03 μm was assumed to be solute Mn. After subtracting the solute Mn concentration from the Mn concentration at a depth of 0 to 0.03 μm, the integral value was entered in Table 2.

[0080] It can be estimated that the larger the Mn integral value after removing dissolved Mn up to 0.03 μm, the higher the dent sensitivity due to Mn buildup on the hearth roll surface inside the annealing furnace. On the other hand, the smaller the Mn integral value, the less Mn buildup on the hearth roll surface there is, and the lower the dent sensitivity.

[0081] Figure 3 below shows the Mn integral at all dew-point temperatures as the Fe coating weight increases before annealing. As shown in Figure 3, it can be seen that the Mn integral at all dew-point temperatures decreases as the Fe coating weight increases before annealing.

[0082] Figure 4 is a photograph of the surfaces of the steel sheets manufactured in Examples 4 and 16 and Comparative Examples 1 and 4. Fig. 4 shows the presence or absence of fine grains and the location of surface annealing oxides, depending on whether or not Fe plating was applied. That is, Fe plating results in the formation of fine grains, and the annealing oxides formed on the surface become smaller and are located at the grain boundaries of the fine grains exposed on the steel sheet surface.

[0083] Meanwhile, Fig. 5 is a photograph of the cross section of Examples 4 and 16 and Comparative Examples 1 and 4. The presence or absence of a fine grain layer can be confirmed depending on whether or not Fe plating is applied. When Fe plating is applied, a fine grain layer is formed on the surface of the steel sheet, whereas no fine grain layer appears on the surface of the steel sheet that is not Fe plated.

Claims

1. In weight percent, Mn: 0.1 to 8.0%, Si: 0.05 to 3.0%, C: 0.001 to 0.6%, Sol. Al: 0.005 to 3%, P: 0.1% or less (0% excluded), S: 0.02% or less (0% excluded), Cr: 1.5% or less (0% included), B: 0.005% or less (0% included), the balance being Fe and unavoidable impurities. A fine crystal grain layer is included within a maximum depth of 1 μm from the surface, the fine crystal grain layer being composed of crystal grains having a minor axis length of 0.5 μm or less and a major axis length of 3 μm or less, The fine grain layer has a length occupancy of 5% or more in the lateral direction of the cross section.

2. The steel plate according to claim 1 , further comprising at most 1.2% of one or more of Ti, Mo, and Nb.

3. 2. The steel sheet according to claim 1, further comprising a hot-dip coating layer selected from the group consisting of a hot-dip galvanized (GI) layer, a galvannealed (GA) layer, a ternary zinc alloy coating (Zn—Al—Mg) layer, and a hot-dip aluminum alloy coating layer.

4. The steel sheet according to claim 1 , further comprising a metal plating layer selected from the group consisting of a Ni plating layer and a Zn plating layer.

5. The steel sheet according to claim 1 , wherein the steel sheet further comprises an electrogalvanized (EG) layer.

6. preparing a base steel sheet containing, by weight, Mn: 0.1 to 8.0%, Si: 0.05 to 3.0%, C: 0.001 to 0.6%, Sol. Al: 0.005 to 3%, P: 0.1% or less (0% excluded), S: 0.02% or less (0% excluded), Cr: 1.5% or less (0% included), B: 0.005% or less (0% included), the balance being Fe and unavoidable impurities; 0.5 to 3.0 g / m on the surface of the base steel sheet ２ forming an Fe plating layer with a deposition amount of The base steel sheet on which the Fe plating layer was formed was subjected to a dew point temperature of -60 to 30°C in an atmosphere of 1 to 80 vol. % hydrogen (H ２ ) containing nitrogen (N ２ ) annealing the alloy at a temperature of 600 to 900°C in a gas atmosphere; cooling the annealed base steel sheet; A method for manufacturing a steel plate, comprising:

7. The step of performing hot dip coating before the completion of the cooling may further be included. The method for manufacturing a steel sheet according to claim 6, wherein the hot-dip coating is any one of hot-dip galvanizing (GI), galvannealed hot-dip galvanizing (GA), ternary zinc alloy coating (Zn-Al-Mg), and aluminum alloy coating.

8. The method for manufacturing a steel sheet according to claim 6, further comprising the step of pickling after the cooling, wherein the pickling is performed in an acid solution of 5 to 18 wt % at 50 to 80°C.

9. 5 to 100 mg / m after the pickling ２ 9. The method of claim 8, further comprising the step of forming a metal plating layer by electroplating with a coating weight of 0.1 to 0.5 wt %, wherein the metal plating is one of Ni plating and Zn plating.

10. The method for manufacturing a steel sheet according to claim 9, further comprising the step of performing electrogalvanizing (EG) after the metal plating.

11. The step of preparing the base steel sheet includes: heating the steel slab to 1100-1300°C; hot rolling the heated steel slab to produce a hot-rolled steel sheet; and winding the hot-rolled steel sheet at 800°C or less after cooling. pickling and cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; The method for manufacturing a steel sheet according to claim 6, comprising:

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