Steel sheet having excellent indentation resistance and method for manufacturing same

Abstract

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

[0002] In recent years, the technical development of automotive steel has focused on ensuring safety through high strength and weight reduction through thickness reduction. Technologies for high-strengthening steel include, in addition to solid solution strengthening and precipitation strengthening by adding alloying elements, advanced high-strength steel (AHSS) that utilizes phase transformation. Thus, the development of steel with simultaneously improved tensile strength and elongation has also been actively carried out.

[0003] For high-strengthening of steel, alloying elements such as Mn, Si, Cr, and B must basically be added. These alloying elements have a high oxidation tendency and thus diffuse to the surface during annealing to combine with oxygen. The surface oxides formed during annealing deteriorate the surface reactivity, thereby significantly reducing plating quality and chemical conversion treatability, etc.

[0004] Such surface oxides not only deteriorate the surface quality but also adhere to the surface of the hearth roll in the annealing furnace and grow, and in the process of passing the steel sheet, they cause a dent problem of crushing the surface of the steel sheet. In particular, the Mn build-up dent caused by Mn surface oxides reduces continuous productivity, and if the problem persists, production even has to be interrupted for repair, which becomes a reason for significantly reducing productivity.

[0005] To improve the dent, various technologies have been proposed. After the Mn surface oxide adheres to the surface of the hearth roll, in order to suppress the chemical reaction at high temperature, there is a method of changing the material by thermal spraying on the surface of the hearth roll (Patent Document 1). There is a reaction of suppressing the chemical reaction by increasing the ceramic content of the coating material of the hearth roll by thermal spraying or applying 100% ceramic. However, when changing the thermal spraying material to ceramic, the coating cost increases and the wear resistance decreases, thus having a problem of shortening the coating life.

[0006] In addition, in order to reduce the formation of Mn surface oxides, there are methods of controlling the oxygen partial pressure and dew point temperature in the annealing furnace, etc., but there are limitations in that the formation of Mn surface oxides cannot be completely suppressed.

[0007] (Patent Document 1) U.S. Authorized Patent US5,466,208 Summary of the Invention

[0008] (1) Technical Problems to be Solved

[0009] An object of one aspect of the present invention is to provide a steel sheet having excellent dent resistance and a method for manufacturing the same.

[0010] The technical problems of the present invention are not limited to the above. Additional technical problems of the present invention are described in the entire content of the specification, and those of ordinary skill in the art to which the present invention pertains can easily understand the additional technical problems of the present invention from the content described in the specification of the present invention.

[0011] (II) Technical solution

[0012] One embodiment of the present invention relates to a steel sheet which, by weight %, comprises: Mn: 0.1 - 8.0%, Si: 0.05 - 3.0%, C: 0.001 - 0.6%, acid-soluble aluminum (Sol.Al): 0.005 - 3%, P: 0.1% or less (except 0%), S: 0.02% or less (except 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), the balance being Fe and unavoidable impurities, and comprising a fine grain layer composed of grains having a minor axis length of 0.5 μm or less and a major axis length of 3 μm or less within 1 μm from the surface to the maximum depth, and the length occupancy in the transverse direction of the cross-section of the fine grain layer being 5% or more.

[0013] The steel sheet may further comprise one or more of Ti, Mo, and Nb at 1.2% or less.

[0014] The steel sheet may include any one of hot-dip galvanized (GI) layer, alloyed hot-dip galvanized (GA) layer, ternary galvanized alloy (Zn - Al - Mg) layer, and hot-dip aluminized alloy layer as a hot-dip coating layer.

[0015] The steel sheet may include any one of Ni coating layer and Zn coating layer as a metal coating layer.

[0016] The steel sheet may include an electro-galvanized (EG) layer.

[0017] Another aspect of the present invention relates to a method for manufacturing a steel sheet, the manufacturing method comprising the following steps: preparing a base steel sheet which, by weight %, comprises: Mn: 0.1 - 8.0%, Si: 0.05 - 3.0%, C: 0.001 - 0.6%, acid-soluble aluminum (Sol.Al): 0.005 - 3%, P: 0.1% or less (except 0%), S: 0.02% or less (except 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), the balance being Fe and unavoidable impurities; on the surface of the base steel sheet at 0.5 - 3.0 g / m 2An Fe coating is formed with an adhesion amount; the base steel plate formed with the Fe coating is subjected to annealing heat treatment at a temperature of 600 - 900 °C in a nitrogen (N2) atmosphere with a dew point temperature of -60 °C to 30 °C and containing 1 - 80 vol.% of hydrogen (H2); and the base steel plate subjected to the annealing heat treatment is cooled.

[0018] The manufacturing method may further include a step of hot-dip coating before the termination of the cooling, and the hot-dip coating may be any one of hot-dip galvanizing (GI), alloyed hot-dip galvanizing (GA), ternary galvanized alloy (Zn - Al - Mg), and aluminized alloy.

[0019] The manufacturing method may further include a step of pickling after the cooling, and the pickling may be carried out with a 5 - 18 wt.% acid solution at 50 - 80 °C.

[0020] The manufacturing method may further include a step of electroplating to form a metal coating with an adhesion amount of 5 - 100 mg / m 2 The metal plating may be any one of Ni plating and Zn plating.

[0021] The manufacturing method may further include a step of electro-galvanizing (EG) after the metal plating.

[0022] The step of preparing the base steel plate may include the following steps: heating a steel billet at 1100 - 1300 °C; hot-rolling the heated steel billet to manufacture a hot-rolled steel plate; cooling the hot-rolled steel plate and then coiling it at a temperature below 800 °C; and pickling and cold-rolling the hot-rolled steel plate to manufacture a cold-rolled steel plate.

[0023] (III) Beneficial effects

[0024] According to the present invention, by forming a surface layer fine grain layer with a specified depth on the steel plate surface, the formation of surface oxides during annealing is suppressed, and the shedding of the formed surface oxides is prevented, thereby reducing the situation of adhering to the surface of the bottom roll of the annealing furnace. Therefore, a steel plate with excellent dent resistance can be provided. Thus, it has the advantage of improving the reduction in productivity.

[0025] The various and beneficial advantages and effects of the present invention are not limited to the above content and can be more easily understood during the description of the specific implementation embodiments of the present invention. Description of the drawings

[0026] Figure 1 (a) is a cross-sectional conceptual diagram of an example of an existing steel plate, Figure 1 (b) is a cross-sectional conceptual diagram of an example of the steel plate of the present invention.

[0027] Figure 2 It is a cross-sectional photograph of Invention Example 4 in the embodiments of the present invention and is a photograph for deriving the length occupancy of the fine grain layer.

[0028] Figure 3 Figures (a) and (b) thereof are graphs showing the Fe plating adhesion amount of Steel Grade A and Steel Grade B in the embodiments and the Mn integral value at -40°C and -20°C.

[0029] Figure 4 Figure (a) thereof is a surface photograph of Comparative Example 1 and Invention Example 4 in the embodiments of the present invention, Figure 4 Figure (b) thereof is a surface photograph of Comparative Example 4 and Invention Example 16.

[0030] Figure 5 Figure (a) thereof is a cross-sectional photograph of Comparative Example 1 and Invention Example 4 in the embodiments of the present invention, Figure 5 Figure (b) thereof is a cross-sectional photograph of Comparative Example 4 and Invention Example 16. Best Mode for Carrying Out the Invention

[0031] The terms used in this specification are for explaining the present invention and are not intended to limit the present invention. In addition, unless the relevant definition indicates a clearly opposite meaning, the singular forms used in this specification also include the plural forms.

[0032] The meaning of "comprising" or "including" used in the specification is to specify the components and does not exclude the existence or addition of other components.

[0033] Unless otherwise defined, all terms, including technical terms and scientific terms, used in this specification have the same meaning as commonly understood by those skilled in the technical field to which the present invention pertains. Terms defined in the dictionary should be interpreted as having a meaning consistent with the relevant technical literature and the currently disclosed content.

[0034] The inventors of the present invention have conducted in-depth research on steel plates having excellent dent resistance, thereby completing the present invention.

[0035] First, a steel plate of a specific embodiment of the present invention will be described in detail. It should be noted that in the present invention, unless otherwise specifically and differently defined, the content of each element is expressed in weight %. In addition, unless otherwise specifically and differently indicated, the proportion of crystals or tissues is based on area. In addition, unless otherwise specifically and differently indicated, the content of gases is based on volume.

[0036] The composition of the steel plate, in weight %, may include: Mn: 0.1 - 8.0%, Si: 0.05 - 3.0%, C: 0.001 - 0.6%, acid-soluble aluminum (Sol.Al): 0.005 - 3%, P: less than 0.1% (except 0%), S: less than 0.02% (except 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%). Optionally, it may further include one or more of Ti, Mo, and Nb at 1.2% or less (including 0%), and may include the balance of Fe and inevitable impurities.

[0037] Manganese (Mn): 0.1 - 8.0%

[0038] The Mn forms and stabilizes retained austenite and inhibits the ferrite phase transformation during cooling, so Mn is an essential element in transformation-induced plasticity steel. In addition, to ensure strength and ductility by sufficiently ensuring austenite, Mn of 0.1% or more can be included. On the other hand, when the content of Mn exceeds 8.0%, excessive segregation in the slab and hot rolling processes leads to the formation of bands, which may cause problems in hindering physical properties. Therefore, the upper limit of the Mn content can be limited to 8.0% or less.

[0039] Silicon (Si): 0.05 - 3.0%

[0040] The Si inhibits the precipitation of carbides in ferrite and promotes the diffusion of carbon in ferrite to austenite, thus contributing to the stabilization of retained austenite. To obtain this effect, Si of 0.05% or more needs to be added. However, when too much Si is added, the surface reactivity is reduced, and problems may occur in terms of plating properties, phosphate treatment properties, etc. Therefore, the upper limit of the Si content can be limited to 3.0%.

[0041] Carbon (C): 0.001 - 0.6%

[0042] The C is an important element added for the stabilization of retained austenite, and for this purpose, C of 0.001% or more is preferably added. On the other hand, when the content of the C exceeds 0.6%, problems such as poor weldability may occur. Therefore, in the present invention, the C content can be limited to 0.001 - 0.6%.

[0043] Acid-soluble aluminum (Sol.Al): 0.005 - 3%

[0044] Al is an element that helps stabilize retained austenite by suppressing the formation of carbides in ferrite. To achieve this effect, more than 0.005% of Al can be added. However, when the content of Al exceeds 3%, it is difficult to manufacture a complete slab during casting due to the reaction with the mold flux, and surface oxides are formed, which may hinder hot-dip coating properties. Therefore, the upper limit of the Al content can be limited to 3% or less.

[0045] Phosphorus (P): 0.1% or less (except 0%)

[0046] The said P is a solid-solution strengthening element, but when the P content exceeds 0.1%, the weldability may be reduced and the risk of steel brittleness increases. Therefore, the upper limit of the P content can be defined as 0.1%.

[0047] Sulfur (S): 0.02% or less (except 0%)

[0048] The said S is an impurity element and an element that hinders the ductility and weldability of the steel plate. Therefore, when the S content increases, the possibility of hindering the ductility and weldability of the steel plate increases. Considering this situation, the upper limit of the S content can be defined as 0.02%.

[0049] Chromium (Cr): 1.5% or less (including 0%)

[0050] The said Cr is an element that increases hardenability and plays a role in suppressing the formation of ferrite. Therefore, to ensure 5 - 30% of retained austenite, a small amount can be added as needed. However, when the content of Cr is too high, the amount of alloy iron added is too much, which may cause an increase in cost. Therefore, the upper limit of the Cr content can be limited to 1.5% or less.

[0051] Boron (B): 0.005% or less (including 0%)

[0052] The said B is an element that can be optionally added to ensure strength. When the content of B exceeds 0.005%, it accumulates on the surface of the annealed material, thus significantly reducing the surface quality. Therefore, the B content is preferably 0.005% or less.

[0053] In addition to the above alloy composition, one or more of titanium (Ti), molybdenum (Mo), and niobium (Nb) can be included at 1.2% or less (including 0%).

[0054] The Mo can help improve the strength. In particular, the strength can be ensured without reducing the wettability of molten metals such as zinc. The Ti forms nitrides, thereby reducing the concentration of N in the steel. On the other hand, when an excessive amount of Ti is contained, a reduction in the carbon concentration and strength of martensite may occur due to the precipitation of carbides. The Nb segregates in the form of carbides at the austenite grain boundaries, suppressing the coarsening of austenite grains during annealing heat treatment, thereby increasing the strength. However, when the addition amount is too large, it may lead to an increase in cost. Considering these situations, one or more of the Ti, Mo, and Nb are contained at 1.2% or less.

[0055] In addition to the above steel composition, the balance may contain Fe and inevitable impurities. Inevitable impurities may be inadvertently mixed in during the normal steelmaking process, so it is impossible to completely exclude inevitable impurities, and those skilled in the art of the general steel manufacturing field can easily understand its meaning. In addition, the present invention does not completely exclude the addition of other components outside the above steel composition.

[0056] The steel plate contains a fine grain layer composed of grains with a minor axis length of 0.5 μm or less and a major axis length of 3 μm or less from the surface to within a maximum depth of 1 μm. Figure 1 (a) of is an existing material, and the surface does not contain a fine grain layer, while Figure 1 (b) of is the steel plate of the present invention, showing a schematic diagram of the surface including a fine grain layer. As Figure 1 shown, in the past, a large amount of coarse annealing oxides were generated on the surface of the steel plate and were easily peeled off from the surface, thus causing a dent problem. In contrast, as Figure 1 (b) of shows, in the present invention, the fine grain layer is formed, so that the generation of annealing oxides on the surface itself is less, and the size of the formed oxides themselves is smaller and mainly formed at the grain boundaries of the fine grains exposed on the surface, making them not easily peeled off. Since the peeled-off oxides become extremely few, excellent dent resistance can be ensured.

[0057] In addition, it is more effective when the length occupancy ratio in the transverse direction of the cross-section of the steel plate with the fine grain layer is 5% or more.

[0058] Among them, the measurement of the length occupancy ratio refers to the ratio of the length of the fine grain layer existing in the transverse direction within a reference length in the cross-section of the steel plate of the present invention. More specifically, it will be described with reference to Figure 2 as follows. The following Figure 2It is a view for observing the cross-section of Invention Example 4 in the following embodiments. It can be seen from the cross-section of the steel plate that there are parts where a fine grain layer is formed and parts where a fine grain layer is not formed. The length of the part where the fine grain layer is formed and the length of the part where the fine grain layer is not formed can be measured in the transverse direction as the observation direction within a unit length to derive. As an example, in the transverse direction as the observation direction of the cross-section of the steel plate, the length of the part where the fine grain layer is formed is preferably 1 μm or more in 20 μm.

[0059] When the length occupancy rate of the fine grain layer is less than 5%, the effect of preventing the annealing oxide from falling off brought by the fine grain layer is insufficient, so there is not much advantage in preventing dents. Therefore, when the length occupancy rate of the fine grain layer is at least 5% or more, the effect of reducing the surface annealing oxide and preventing the dents from falling off will be improved.

[0060] The steel plate may include a coating.

[0061] The coating may be a hot-dip coating, and the hot-dip coating may be a zinc-based alloy coating or an aluminum-based alloy coating. As an example, the zinc-based alloy coating may be a hot-dip galvanized (GI) layer, an alloyed hot-dip galvanized (GA) layer, a ternary galvanized alloy (Zn-Al-Mg) layer, or a hot-dip aluminum-based alloy layer.

[0062] In addition, the coating may be a metal coating. As an example, it may be a Ni coating or a Zn coating. Different from the above hot-dip coating, the difference between the metal coating and the hot-dip coating is that it is manufactured by electroplating.

[0063] In addition, the steel plate may further include an electro-galvanized (EG) layer.

[0064] Next, a manufacturing method of the steel plate according to a specific embodiment of the present invention will be described in detail. The manufacturing method includes the following steps: preparing a base steel plate; forming an Fe coating on the surface of the base steel plate; annealing the base steel plate with the Fe coating formed thereon; and cooling after annealing. Hereinafter, each step will be described in detail.

[0065] The base steel plate may be a hot-rolled steel plate, a cold-rolled steel plate, etc. There is no particular limitation on its type, as long as it can be applied in the technical field to which the present invention belongs, it can be applied without limitation. Therefore, the manufacturing method of the base steel plate is not specifically limited.

[0066] As a specific example of the manufacturing method of the base steel plate, prepare the billet described above, and heat the billet to a temperature range of 1100 - 1300 °C. By weight percentage, the billet contains: Mn: 0.1 - 8.0%, Si: 0.05 - 3.0%, C: 0.001 - 0.6%, acid-soluble aluminum (Sol.Al): 0.005 - 3%, P: 0.1% or less (except 0%), S: 0.02% or less (except 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%). Optionally, it may further contain one or more of Ti, Mo, and Nb at 1.2% or less (including 0%), and contains the balance of Fe and inevitable impurities.

[0067] The heated billet can be hot-rolled to obtain a hot-rolled steel plate.

[0068] Cool the hot-rolled steel plate, and then coil it within a temperature range of 800 °C or lower. Optionally, the hot-rolled steel plate can be pickled and cold-rolled to obtain a cold-rolled steel plate.

[0069] Form an Fe coating on the surface of the prepared base steel plate.

[0070] The Fe coating is more effective when Fe plating is carried out with an adhesion amount of 0.5 - 3.0 g / m 2 of the base steel plate.

[0071] There is no particular limitation on the method for forming the Fe coating. As an example, electroplating can be used to form the Fe coating. As a specific example, the following electroplating solution can be used. The electroplating solution contains: ferrous ions and ferric ions, a complexing agent, and inevitable impurities, and the concentration of ferric ions in the ferrous ions is 5 - 60% by weight. In addition, the concentration of the ferrous ions that can be used is 1 - 80 g of the electroplating solution / 1 L of the electroplating solution. The electroplating solution can be electroplated at a current density of 3 - 120 A / dm 2 at a temperature of 80 °C or lower.

[0072] The base steel plate formed with the Fe coating can be subjected to annealing heat treatment at a temperature of 600 - 900 °C in a nitrogen (N2) atmosphere with a dew point temperature of -60 °C to 30 °C and containing 1 - 80% by volume of hydrogen (H2).

[0073] During annealing, in order to make the dew point temperature lower than -60°C, it is necessary to inject very dry gas, and it is very difficult to manage the airtightness of the equipment, etc., so it may be inefficient. On the other hand, when the dew point temperature exceeds +30°C, since it is the area where Fe is oxidized, it has an adverse effect on the surface quality. In addition, when annealing at a dew point temperature of -20°C to 30°C, internal oxidation may occur where alloy elements such as Si and Mn form oxides at the grain boundaries inside the steel plate. This internal oxidation can be beneficial for decarburization and suppressing surface enrichment of Si, etc., for surface quality. The dew point temperature can also show the effect of the fine grain layer and reducing surface annealing oxides desired by the present invention within the range of -20°C to 30°C where internal oxidation occurs.

[0074] In addition, the annealing atmosphere gas contains a specified amount of hydrogen (H2) in nitrogen (N2). When the hydrogen concentration is less than 1% by volume, the reducing power of Fe is insufficient, and Fe oxidation may occur. When the hydrogen concentration exceeds 80% by volume, there is a risk of explosion when the gas leaks, and the cost during high-hydrogen operations for rapid cooling of the steel plate will increase.

[0075] When the annealing temperature is lower than 600°C, recrystallization of the cold-rolled steel plate may not proceed sufficiently. When the annealing temperature exceeds 900°C, problems such as equipment damage and cost increase may occur. Therefore, the annealing temperature can be set to 600 - 900°C.

[0076] The base steel plate subjected to the annealing heat treatment can be cooled. As an example, the cooling is slow cooling from the annealing temperature to the level of 650°C, and then rapid cooling is performed according to the desired material. If necessary, the steel plate subjected to rapid cooling is reheated to a specified temperature for tempering and then cooled to room temperature.

[0077] In the present invention, the cooling conditions can vary according to the conditions for achieving the desired material. In addition, since the formation of surface annealing oxides mostly occurs in the relatively high-temperature region, 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 cooling, an atmosphere that is at least reducing to iron is applied.

[0078] Hot-dip coating can be performed after the annealing heat treatment and before the cooling is completed. The hot-dip coating includes zinc-based alloy coating, aluminum-based alloy coating, etc. Specifically, the hot-dip coating includes hot-dip galvanizing (GI), alloyed hot-dip galvanizing (GA), ternary galvanized alloy (Zn - Al - Mg), hot-dip aluminum-based alloy, etc.

[0079] When the zinc-based alloy is electroplated, the annealed steel sheet can be cooled to the range of 400 - 500 °C and then hot-dip electroplated. When the aluminum-based alloy is electroplated, the annealed steel sheet can be cooled to the range of 600 - 700 °C and then hot-dip electroplated. Additionally, in the alloyed hot-dip galvanized (GA), the alloying temperature can be 480 - 580 °C.

[0080] The cooled steel sheet can be pickled. The pickling can be carried out with an acid solution of 5 - 18 wt% at 50 - 80 °C. As a specific example, pickling can be carried out with 5 wt% hydrochloric acid at 50 - 60 °C. In the case of some steel grades that require strong pickling, pickling can be carried out with 18 wt% hydrochloric acid at 80 °C.

[0081] After pickling, in order to improve the surface reactivity, electroplating can be carried out with an adhesion amount of 5 - 100 mg / m 2 to form a metal coating. At this time, Ni plating, Zn plating, etc. can be electroplated, and there is no special limitation on their types, and they can be applied according to their uses.

[0082] Additionally, electrogalvanized (EG) can be further carried out after forming the metal coating. Detailed implementation mode

[0083] Next, the embodiments of the present invention will be described.

[0084] Ordinary technicians in the technical field to which the present invention pertains can make various modifications to the following embodiments without departing from the scope of the present invention. The following embodiments are for understanding the present invention, and the scope of rights of the present invention should not be limited to the following embodiments, but should be determined by the claims and their equivalents.

[0085] (Embodiment)

[0086] Two cold-rolled steel sheets A and B with the compositions shown in Table 1 below were prepared. An Fe coating was formed on the prepared cold-rolled steel sheets with the Fe plating adhesion amount disclosed in Table 2. At this time, the Fe plating was carried out by electroplating.

[0087] [Table 1]

[0088]

[0089] The steel sheets with the formed Fe coating were subjected to annealing heat treatment and cooling. Specifically, the annealing furnace atmosphere was nitrogen containing 5% hydrogen, which was a reducing atmosphere, and the dew point temperature was carried out at -40 °C, -20 °C, and +5 °C as shown in Table 2.

[0090] Regarding the specific annealing conditions, for steel grade A, it is heated to 840°C at a heating rate of 3.1°C / second (sec) and held for 65 seconds, then cooled at a cooling rate of 2.7°C / second for the first cooling to 650°C, and then rapidly cooled (second cooling) at a cooling rate of 9°C / second to 450°C. After that, it is slowly cooled (third cooling) at a cooling rate of 0.2°C / second to 360°C, and then finally cooled (fourth cooling) at a cooling rate of 10°C / second to room temperature.

[0091] For steel grade B, it is heated to 800°C at a heating rate of 3.2°C / second and held for 61 seconds, then cooled at a cooling rate of 2.3°C / second for the first cooling to 650°C, and then rapidly cooled (second cooling) at a cooling rate of 9.5°C / second to 450°C. After that, it is slowly cooled (third cooling) at a cooling rate of 0.1°C / second to 400°C, and then finally cooled (fourth cooling) at a cooling rate of 10°C / second to room temperature.

[0092] The occupancy rate in the cross-sectional length direction of the fine-grained layer of the manufactured steel plate is measured and shown in Table 2.

[0093] In addition, the integral value is calculated from the GDS spectrum of the Mn weight percentage from the steel plate surface to 0.03μm and shown in Table 2.

[0094] [Table 2]

[0095]

[0096] The method for measuring the occupancy rate of the fine-grained layer is as follows.

[0097] - Process the cross-section with a Focused Ion Beam (FIB) and observe it with a Scanning Transmission Electron Microscopy (STEM) at a magnification of at least 20,000 times (above 20,000 times)

[0098] - The FIB-STEM for analyzing the length occupancy rate can sample at random positions and analyze more than 20μm in the transverse direction of the total cross-section, or connect and analyze more than 20μm at one position.

[0099] The Mn integral value is obtained by inferring the Mn spectrum with a depth up to 0.03μm of the GDS data as the annealing surface oxide and assuming the Mn concentration at a depth of 0.03μm as the solid-solved Mn. The integral value obtained by subtracting the solid-solved Mn concentration from the Mn concentration with a depth of 0 - 0.03μm is recorded in Table 2.

[0100] The larger the Mn integrated value for removing Mn solids up to 0.03 μm, the greater the dent sensitivity caused by Mn accumulation on the surface of the bottom rolls inside the annealing furnace can be inferred, and the smaller the Mn integrated value, the less Mn accumulates on the surface of the bottom rolls, so the dent sensitivity can be considered to be smaller.

[0101] The following Figure 3 shows the curves of the Mn integrated values at all dew point temperatures as the amount of Fe plating attachment before annealing increases. As Figure 3 shown, it can be confirmed that as the amount of Fe plating attachment before annealing increases, the Mn integrated values at all dew point temperatures decrease.

[0102] Figure 4 are photos of observing the surfaces of the above-mentioned Invention Example 4 and Invention Example 16 and Comparative Example 1 and Comparative Example 4. Looking at the Figure 4 it can be confirmed according to the presence or absence of fine grains of Fe plating and the formation position of the surface annealing oxide. That is, when Fe plating is carried out, fine grains are formed, and the size of the annealing oxide formed on the surface becomes smaller and is located at the grain boundaries of the fine grains exposed on the steel plate surface.

[0103] In addition, Figure 5 are photos of observing the cross-sections of the above-mentioned Invention Example 4 and Invention Example 16 and Comparative Example 1 and Comparative Example 4. It can be confirmed according to the presence or absence of a fine grain layer of Fe plating. When Fe plating is carried out, a fine grain layer is formed on the steel plate surface. On the other hand, no fine grain layer appears on the surface of the steel plate without Fe plating.

Claims

1. A steel plate, by weight %, the steel plate comprises: Mn: 0.1 - 8.0%, Si: 0.05 - 3.0%, C: 0.001 - 0.6%, acid-soluble aluminum (Sol.Al): 0.005 - 3%, P: less than 0.1% and excluding 0%, S: less than 0.02% and excluding 0%, Cr: 1.5% or less and including 0%, B: 0.005% or less and including 0%, the balance of Fe and inevitable impurities, containing, within a maximum depth of 1 μm from the surface, a fine grain layer composed of grains with a minor axis length of 0.5 μm or less and a major axis length of 3 μm or less, the length occupancy rate in the transverse direction of the cross-section of the fine grain layer being 5% or more.

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

3. The steel plate according to claim 1, wherein, The steel plate further includes any one of hot-dip galvanized (GI) layer, alloyed hot-dip galvanized (GA) layer, ternary galvanized alloy (Zn - Al - Mg) layer, and hot-dip aluminized alloy layer as a hot-dip coating layer.

4. The steel plate according to claim 1, wherein, The steel plate further includes any one of Ni coating and Zn coating as a metal coating layer.

5. The steel plate according to claim 1, wherein The steel plate further includes an electro-galvanized (EG) layer.

6. A method for manufacturing a steel plate, which comprises the following steps: Preparing a base steel plate, by weight %, the base steel plate comprises: Mn: 0.1 - 8.0%, Si: 0.05 - 3.0%, C: 0.001 - 0.6%, acid-soluble aluminum (Sol.Al): 0.005 - 3%, P: less than 0.1% and excluding 0%, S: less than 0.02% and excluding 0%, Cr: 1.5% or less and including 0%, B: 0.005% or less and including 0%, the balance of Fe and inevitable impurities; On the surface of the base steel plate, an Fe coating is formed with an adhesion amount of 0.5 - 3.0 g / m 2 ; Annealing and heat-treating the base steel plate formed with the Fe coating in a nitrogen (N2) atmosphere with a dew point temperature of -60°C to 30°C and containing 1 - 80 vol% of hydrogen (H2) at a temperature of 600 - 900°C; And Cooling the base steel plate that has undergone the annealing and heat-treatment.

7. The manufacturing method of the steel plate according to claim 6, wherein, The manufacturing method further includes a step of hot-dip coating before the termination of the cooling, and the hot-dip coating is any one of hot-dip galvanizing (GI), alloyed hot-dip galvanizing (GA), ternary galvanized alloy (Zn - Al - Mg), or aluminized alloy.

8. The manufacturing method of the steel plate according to claim 6, wherein, The manufacturing method further includes a step of pickling after the cooling, and the pickling is carried out with a 5 - 18 wt% acid solution at 50 - 80°C.

9. The manufacturing method of the steel plate according to claim 8, wherein, The manufacturing method further includes a step of electroplating at an adhesion amount of 5-100 mg / m 2 to form a metal coating after the pickling, and the metal plating is any one of Ni plating and Zn plating.

10. The manufacturing method of the steel plate according to claim 9, wherein, The manufacturing method further includes a step of electro-galvanizing (EG) after the metal coating.

11. The manufacturing method of the steel plate according to claim 6, wherein, The step of preparing the base steel plate includes the following steps: Heating a steel billet at 1100 - 1300°C; Hot-rolling the heated steel billet to manufacture a hot-rolled steel plate; Cooling the hot-rolled steel plate and then coiling it at 800°C or below; and Pickling and cold-rolling the hot-rolled steel plate to manufacture a cold-rolled steel plate.

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