STEEL SHEET HAVING EXCELLENT DENT RESISTANCE AND METHOD FOR MANUFACTURING SAME

Abstract

 The present invention relates to a steel sheet used in automobiles, etc. and to a steel sheet with excellent dent-resistance and a method for manufacturing same.

Description

Technical Field

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

Background Art

[0002] Recently, an automotive steel material has been developed with a focus on safety through high strength and weight reduction through thickness reduction. Technologies for high strength steel include Advanced High Strength Steel (AHSS) steel using phase transformation in addition to solid solution strengthening and precipitation strengthening through the addition of alloy elements, through which steel development that simultaneously increases tensile strength and elongation has also been actively carried out.

[0003] In order to increase the strength of a steel material, alloy elements such as Mn, Si, Cr, and B are essential, and since these alloy elements have a high oxidation tendency, they diffuse to a surface to combine with oxygen during annealing. Surface oxides formed during the annealing process worsen surface reactivity, which may significantly reduce plating quality and chemical treatment properties.

[0004] These surface oxides not only worsen surface quality, but also stick to a surface of a hearth roll in an annealing furnace and grow, and may cause dent problems in the annealing furnace that stamps a surface of a steel sheet during strip passing. Specifically, Mn build-up dents due to Mn surface oxides reduce continuous productivity, and when the problem persists, this may lead to production stoppage for repair, which significantly reduces productivity.

[0005] Various technologies have been proposed to improve the dents. In order to suppress chemical reactions at high temperatures after Mn surface oxides adhere to the hearth roll surface, there is a method of changing a material of the hearth roll surface through thermal spray coating (Patent Document 1). There is a reaction of suppressing chemical reactions by increasing a ceramic content of the hearth roll coating material through thermal spray coating or applying 100% ceramic. However, there is a problem that coating costs increase and wear resistance decreases when the thermal spray coating material is changed to ceramic, which may shorten a coating lifespan.

[0006] In addition, there is provided a method of controlling the oxygen partial pressure and a dew point temperature in an annealing furnace to reduce the formation of Mn surface oxides, but there is a limitation that the formation of Mn surface oxides may not be completely suppressed.

[0007] (Patent Document 1) US Patent No. US 5,466,208

Disclosure of Invention

Technical Problem

[0008] An aspect of the present invention is to provide a steel sheet having excellent dent-resistance and a method for manufacturing the same.

[0009] An aspect of the present invention is not limited to the aspects described above. Additional aspects of the present invention are described in the overall contents of the specification, and those skilled in the art to which the present invention belongs may have no difficulty in understanding the additional aspects of the present invention from the contents described in the specification of the present invention.

Solution to Problem

[0010] An embodiment of the present invention relates to a steel sheet comprising, by wt%, 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 a balance of Fe and inevitable impurities,

[0011] wherein the steel sheet comprises a fine grain layer comprised of grains having a minor axis length of 0.5µm or less and a major axis length of 3µm or less within a maximum depth of 1µm from a surface,

[0012] wherein the fine grain layer has a length occupancy rate of 5% or more in a transverse direction of a cross section.

[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 further comprise a hot-dip plating layer, which is one a hot-dip galvanized (GI) layer, an alloyed hot-dip galvanized (GA) layer, a ternary zinc alloy plating (Zn-Al-Mg) layer, and a hot-dip aluminum alloy plating layer.

[0015] The steel sheet may further comprise a metal plating layer of either a Ni plating layer or a Zn plating layer.

[0016] The steel sheet may further comprise an electrogalvanized (EG) layer.

[0017] Another embodiment of the present invention relates to a method for manufacturing a steel sheet, comprising: preparing a base steel sheet comprising, by wt%, 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 a balance of Fe and inevitable impurities;

forming an Fe plating layer on a surface of the base steel sheet with a coating weight of 0.5 to 3.0g/m².

performing an annealing heat-treatment on the base steel sheet on which the Fe plating layer is formed in a nitrogen (N₂) gas atmosphere comprising 1 to 80 vol.% of hydrogen (H₂) at a dew point temperature of -60 to 30°C, at a temperature of 600 to 900°C; and

cooling the annealing heat-treated steel sheet.

[0018] The manufacturing method may further comprise performing hot-dip galvanizing before terminating the cooling, wherein the hot-dip galvanizing may be one of hot-dip galvanizing (GI), alloyed hot-dip galvanizing (GA), ternary zinc alloy plating (Zn-Al-Mg), and aluminum alloy plating.

[0019] The manufacturing method may further comprise pickling after the cooling, wherein the pickling is performed with 5 to 18 wt% of an acid solution at 50 to 80°C.

[0020] The manufacturing method may further comprise performing electroplating with a coating weight of 5 to 100mg/m² after the pickling, and forming a metal plating layer, wherein the metal plating is one of Ni plating and Zn plating.

[0021] The manufacturing method may further comprise performing electrogalvanizing (EG) after the metal plating.

[0022] The preparing a base steel sheet may comprise:

heating a steel slab at 1100 to 1300°C;

hot-rolling the heated steel slab to manufacture a hot-rolled steel sheet;

cooling the hot-rolled steel sheet and coiling at 800°C or lower; and

pickling and cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet.

Advantageous Effects of Invention

[0023] According to the present invention, by forming a microcrystalline layer of a certain depth on the surface of the steel sheet, the formation of surface oxides during annealing can be suppressed, and the detachment of the formed surface oxides from the steel sheet surface can be prevented. As a result, the amount of oxides adhering to the hearth roll surface is reduced, thereby providing a steel sheet with excellent dent-resistance properties. This provides the advantage of improving productivity.

[0024] The various and advantageous advantages and effects of the present invention are not limited to the above-described contents, and may be more easily understood in the process of explaining specific embodiments of the present invention.

Brief Description of Drawings

[0025] 

(a) of FIG. 1 is a cross-sectional conceptual diagram of an example of a conventional steel sheet, and (b) of FIG. 1 is a cross-sectional conceptual diagram of an example of a steel sheet of the present invention.

FIG. 2 is a cross-sectional photograph of Inventive Example 4 in an embodiment of the present invention, and is a photograph for deriving a length occupancy rate of a fine grain layer.

(a) and (b) of FIG. 3 are graphs illustrating a Fe coating weight of steel types A and B in an embodiment and Mn integral values at -40°C and -20°C.

(a) of FIG. 4 is a surface photograph of Comparative Example 1 and Inventive Example 4 in an embodiment of the present invention, and (b) of FIG. 4 is a surface photograph of Comparative Example 4 and Inventive Example 16.

(a) of FIG. 5 is a cross-sectional photograph of Comparative Example 1 and Inventive Example 4 in an embodiment of the present invention, and (b) of FIG. 5 is a cross-sectional photograph of Comparative Example 4 and Inventive Example 16.

Best Mode for the Invention

[0026] The terms used in this specification are intended to describe the present invention and are not intended to limit the present invention. In addition, the singular forms used in this specification include the plural forms as well, unless the relevant definition clearly indicates a meaning contrary thereto.

[0027] The meaning of "comprising" and "including" as used in the specification specifies a component and does not exclude the presence or addition of other components.

[0028] Unless otherwise defined, all terms including technical and scientific terms used in this specification have the same meaning as generally understood by those skilled in the art to which the present invention belongs. Terms defined in the dictionary are interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content.

[0029] The inventors of the present invention have researched a steel sheet having excellent dent resistance in depth and have completed the present invention.

[0030] First, a steel sheet, which is an embodiment of the present invention, will be described in detail. It should be noted that when the content of each element is expressed in the present invention, unless otherwise specified, this means wt%. In addition, a ratio of crystals or structures is based on area unless otherwise specified. Also, the gas content is based on volume unless otherwise specifically stated.

[0031] A composition of the steel sheet may include, by wt%, 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 may further include, if necessary, one or more of Ti, Mo and Nb at 1.2% or less (including 0%), and may include a balance of Fe and inevitable impurities.

Manganese (Mn): 0.1 to 8.0%

[0032] The Mn is an essential element in transformation structure steel because this exerts an effect of suppressing ferrite transformation during cooling as well as forming and stabilizing residual austenite. In addition, in order to secure sufficient austenite and secure strength and ductility, Mn may be included in an amount of 0.1% or more. On the other hand, when the content thereof exceeds 8.0%, there is a problem that the band formation caused by segregation in a slab and hot rolling process becomes excessive and deteriorates the physical properties, so that an upper limit of the Mn content may be limited to 8.0% or less.

Silicon (Si): 0.05 to 3.0%

[0033] The Si is an element suppressing the precipitation of carbides in ferrite and promoting the diffusion of carbon in ferrite into austenite, contributing to the stabilization of residual austenite. In order to obtain this effect, it may be necessary to add Si in an amount of 0.05% or more. However, when Si is added significantly excessively, the surface reactivity may be reduced, which may cause problems with plating properties, phosphate treatment properties, and the like, so that an upper limit of the content may be limited to 3.0%.

Carbon (C): 0.001 to 0.6%

[0034] The C is an important element added to stabilize residual austenite, and for this purpose, it may be preferable to add 0.001% or more. On the other hand, when the C content exceeds 0.6%, the problem of poor weldability may occur. Accordingly, the C content in the present invention may be limited to 0.001 to 0.6%.

Solid Solution Aluminum (Sol.Al): 0.005 to 3%

[0035] The Al is an element contributing to stabilizing residual austenite by suppressing the formation of carbides in ferrite, and may be added in an amount of 0.005% or more to obtain this effect. However, when the content exceeds 3%, it may be difficult to manufacture a sound slab through a reaction with a mold flux during casting, and surface oxides may be formed to inhibit melting plating properties, so that an upper limit of the Al content may be limited to 3% or less.

Phosphorus (P): 0.1% or less (excluding 0%)

[0036] The P is a solid solution strengthening element, but when the content thereof exceeds 0.1%, weldability may deteriorate and the risk of steel brittleness may increase, so that an upper limit thereof may be limited to 0.1%.

Sulfur (S): 0.02% or less (excluding 0%)

[0037] The S is an impurity element inhibiting the ductility and weldability of the steel sheet. Accordingly, as the S content increases, the possibility of inhibiting the ductility and weldability of the steel sheet may increases, so that in consideration thereof, an upper limit thereof is limited to 0.02%.

Chromium (Cr): 1.5% or less (including 0%)

[0038] The Cr is a hardening element inhibiting the formation of ferrite. Accordingly, Cr may be added in small amounts as needed to secure 5 to 30% of residual austenite. However, when the content is excessive, an alloy iron input may be excessive, which may cause an increase in costs, so that an upper limit of the Cr content may be limited to 1.5% or less.

Boron (B): 0.005% or less (including 0%)

[0039] The B is an element that may be optionally added to secure strength. When the content of B exceeds 0.005%, B may be enriched on a surface of an annealed material, which may significantly reduce the surface quality, and thus, the content thereof may preferably be 0.005% or less.

[0040] In addition to the alloy composition described above, one or more of titanium (Ti), molybdenum (Mo) and niobium (Nb) may be included in an amount of 1.2% or less (including 0%).

[0041] The Mo may contribute to improving strength. Specifically, Mo may secure strength without lowering the wettability of molten metals such as zinc. The Ti may form a nitride to reduce the concentration of N in the steel. On the other hand, when Mo is excessively included, the carbon concentration and strength of martensite may decrease due to carbide precipitation. The Nb may be segregated in the form of carbides at the austenite grain boundary, which may increase the strength by suppressing the coarsening of austenite grains during annealing heat treatment, but when an input amount thereof is excessive, this may cause an increase in costs. In consideration thereof, one or more of Ti, Mo and Nb may be included in an amount of 1.2% or less.

[0042] In addition to the steel composition described above, the balance may include Fe and inevitable impurities. The inevitable impurities may be unintentionally incorporated during a normal steel manufacturing process, and the impurities may not be completely excluded, and those skilled in the field of normal steel manufacturing may easily understand the meaning. In addition, the present invention does not completely exclude the addition of other compositions other than the steel composition mentioned above.

[0043] The steel sheet includes a fine grain layer comprised of grains having a minor axis length of 0.5µm or less and a major axis length of 3µm or less within a maximum depth of 1µm from a surface. (a) of FIG. 1 is a conventional material, which does not include a fine grain layer on a surface, but (b) of FIG. 1 is a schematic diagram illustrating a steel sheet of the present invention having a fine grain layer on the surface. As shown in FIG. 1, conventionally, annealed surface oxides were coarse and occurred in large quantities on the surface of the steel sheet, and easily fell off from the surface, which caused a dent problem. In contrast, as shown in (b) of FIG. 1, in the present invention, the fine grain layer may be formed, and thus, an annealed surface oxide itself formed on the surface is less likely to occur, and the formed oxide itself is also small in size and is mainly formed on microcrystal grain boundaries exposed on the surface, making it difficult to detach. Since the amount of oxide detached is extremely small, excellent dent resistance may be secured.

[0044] Meanwhile, it is effective that the fine grain layer has a length occupancy rate of 5% or more in the transverse direction of a cross-section of the steel sheet.

[0045] Here, the measurement of the length occupancy refers to a ratio of the length in the cross-section of the steel sheet of the present invention, from a reference length, of the length in which the fine grain layer exists in the transverse direction. Hereinafter, a more detailed explanation will be given with reference to FIG. 2. The following FIG. 2 is an observation of a cross-section of Inventive Example 4 in the following embodiment, and when viewing the cross-section of the steel sheet, there are a part in which the fine grain layer is formed and a part in which the fine grain layer is not formed, and a length of the part in which the fine grain layer is formed and a length of the part in which the fine grain layer is not formed may be measured and derived in a unit length in the transverse direction, which is the observation direction. For example, preferably, the length of the part in which the fine grain layer is formed is 1µm or more out of 20um in the transverse direction, which is the observation direction of the cross-section of the steel sheet.

[0046] When a length occupancy rate of the fine grain layer is less than 5%, an effect of preventing annealed surface oxide from being detached by the fine grain layer is not sufficient, and thus, there is no significant advantage in preventing dents, so that when the fine grain layer is formed by a length occupancy rate of 5% or more, the effect of reducing annealed surface oxides and preventing annealed surface oxides from being detached are improved.

[0047] The steel sheet may include a plating layer.

[0048] The plating layer may be a hot-dip galvanized layer, and the hot-dip galvanized 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 galvanized (GI) layer, an alloyed hot-dip galvanized (GA) layer, a ternary zinc alloy plating (Zn-Al-Mg) layer, or a hot-dip aluminum-based alloy plating layer.

[0049] Meanwhile, the plating layer may be a metal plating layer, and may be, for example, a Ni plating layer or a Zn plating layer. Unlike the hot-dip galvanized layer described above, the metal plating layer is manufactured by an electroplating method, and is thus distinguished from the hot-dip galvanized layer.

[0050] Meanwhile, the steel sheet may further include an electrogalvanized (EG) layer.

[0051] Next, a method for manufacturing a steel sheet according to an embodiment of the present invention will be described in detail. The manufacturing method includes an operation of preparing a base steel sheet, forming an Fe plating layer on a surface of the base steel sheet, an operation of annealing the base steel sheet on which the Fe plating layer is formed, and an operation of performing cooling after the annealing. Each operation is described in detail below.

[0052] The base steel sheet is not particularly limited in type, such as a hot-rolled steel sheet or a cold-rolled steel sheet, and may be applied without limitation as long as the base steel sheet may be applied in the technical field to which the present invention belongs, and thus, the method for manufacturing the base steel sheet may not be specifically limited.

[0053] In a specific example of a method for manufacturing the steel sheet, a steel slab including, by wt%, 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%), as described above, and including, if necessary, one or more of Ti, Mo and Nb including 1.2% or less (including 0%), and including a balance of Fe and inevitable impurities is prepared, and the steel slab is heated to a temperature within a range of 1100 to 1300°C.

[0054] A hot-rolled steel sheet may be obtained by hot-rolling the heated steel slab.

[0055] The hot-rolled steel sheet is cooled and then coiled at a temperature range of 800°C or less. If necessary, the hot-rolled steel sheet may be pickled and cold-rolled to obtain a cold-rolled steel sheet.

[0056] An Fe plating layer is formed on a surface of the prepared base steel sheet.

[0057] It is effective to perform Fe plating with a coating weight range from 0.5 to 3.0 g/m² in the Fe plating layer.

[0058] There is no particular limitation on the method for forming the Fe plating layer, and for example, the Fe plating layer may be formed using electroplating. As a specific example, an electroplating solution including iron ions including ferrous ions and ferric ions, a complexing agent and inevitable impurities, and having a concentration of the ferric ions of 5 to 60 wt%, among the iron ions, may be used. Meanwhile, the concentration of the iron ions may be 1 to 80 g per 1 L of the electroplating solution. The electroplating solution may perform electroplating at a current density of 3 to 120 A/dm² at 80°C or less.

[0059] A base steel sheet on which the Fe plating layer is formed may be subject to an annealing heat-treatment at a temperature of 600 to 900°C in a nitrogen (N₂) gas atmosphere including 1 to 80 vol.% of hydrogen (H₂) at a dew point temperature of -60 to 30°C.

[0060] During annealing, in order to make the dew point temperature below -60°C, significantly dry gas injection may be required, and it may be significantly difficult to manage, such as maintaining the airtightness of equipment, which may be inefficient. On the other hand, when the dew point temperature exceeds +30°C, this is a region in which Fe is oxidized, which has a negative effect on the surface quality. On the other hand, when the annealing is performed at the dew point temperature of -20 to 30°C, internal oxidation may occur in which alloy elements such as Si and Mn form oxides at the grain boundaries inside the steel sheet, and such internal oxidation may have benefits on the surface quality, such as decarburization and suppression of Si surface enrichment. The dew point temperature exhibits the effects of fine grain layer formation and reduction of annealed surface oxides, which are the objectives of the present invention, even in the range of -20 to 30°C where internal oxidation occurs.

[0061] Meanwhile, the annealing atmosphere gas includes a certain amount of hydrogen (H₂) in nitrogen (N₂). When the hydrogen concentration is less than 1 vol.%, Fe oxidation may occur due to insufficient Fe reduction power, and when the hydrogen concentration exceeds 80 vol.%, there is a risk of explosion when gas leaks, and the costs increases when performing high-hydrogen work for rapid cooling of the steel sheet.

[0062] When the annealing temperature is less than 600°C, the recrystallization of the cold-rolled steel sheet may not occur sufficiently, and when the annealing temperature exceeds 900°C, problems of equipment damage and increased costs may occur, so that the annealing temperature may be set to 600 to 900°C.

[0063] The annealing heat-treated base steel sheet may be cooled. For example, cooling is performed by performing slow cooling from the annealing temperature to 650 C, and then performing rapid cooling depending on the target material. The rapidly cooled steel sheet is reheated to a certain temperature for tempering if necessary and then cooled to room temperature.

[0064] In the present invention, the cooling conditions may vary depending on the conditions for achieving the target material. In addition, since the formation of annealed surface oxides mostly occurs in a relatively high temperature range, there is no need to specifically limit the cooling conditions in the present invention. However, in order to prevent oxidation of iron components during the cooling process, a reducing atmosphere may be applied at least for iron.

[0065] After the annealing heat treatment, a hot dip plating may be performed before terminating the cooling. The hot dip plating includes zinc-based alloy plating, aluminum-based alloy plating, and the like. Specifically, the hot dip plating includes hot dip galvanizing (GI), alloyed hot dip galvanizing (GA), ternary zinc alloy plating (Zn-Al-Mg), hot dip aluminum-based alloy plating, and the like.

[0066] The annealed steel sheet for the zinc-based alloy plating may be cooled to a range of 400 to 500°C and may then be subject to molten galvanization, and the annealed steel sheet during aluminum-based alloy plating may be cooled to a range of 600 to 700°C and may then be subject to molten galvanization. Meanwhile, the alloying temperature in the alloyed hot-dip zinc plating (GA) may be 480 to 580°C.

[0067] The cooled steel sheet may be pickled. The pickling may be performed with 5 to 18 wt% of an acid solution at 50 to 80°C. As a specific example, the pickling may be performed with 5 wt% of hydrochloric acid at 50 to 60°C, and in the case of some steel types requiring strong pickling, the pickling may be performed with 18 wt% of hydrochloric acid at 80°C.

[0068] After the pickling, in order to improve surface reactivity, a metal plating layer may be formed by electroplating with a coating weight-of 5 to 100 mg/m². In this case, plating includes Ni plating, Zn plating, and the like, and the type thereof is not particularly limited and may be applied according to the purpose.

[0069] Meanwhile, after forming the metal plating layer, electrolytic zinc plating (EG) may be additionally performed.

Mode for the Invention

[0070] Next, an embodiment of the present invention will be described.

[0071] The following embodiment may be modified in various ways by those skilled in the art without departing from the scope of the present invention. The following embodiment is for understanding the present invention, and the scope of the rights of the present invention should not be limited to the following embodiment, but should be determined by the claims described below as well as equivalents thereto.

(Example)

[0072] Two types of cold-rolled steel sheets, A and B, having the compositions shown in Table 1 below were prepared. An Fe plating layer was formed on the cold-rolled steel sheets prepared in this manner with the Fe coating weight disclosed in Table 2. In this case, the Fe plating was performed by an electroplating method.

[Table 1]

COMPOSITION (WEIGHT %)	C	Mn	Si	Cr	Al	P	S	FE AND IMPURITIES
STEEL TYPE A	0.05	2.5	0.1	0.9	0.04	0.01	0.002	BALANCE
STEEL TYPE B	0.08	1.7	0.2	0	0.02	0.01	0.005	BALANCE

[0073] The steel sheet on which the Fe plating layer was formed was subjected to the annealing heat treatment and the cooling. Specifically, an annealing atmosphere was a reducing atmosphere of nitrogen gas including 5% hydrogen, and the dew point temperatures were -40°C, -20°C, and +5°C as shown in Table 2 below.

[0074] Regarding the specific annealing conditions, for steel type A, the temperature was increased to 840°C at a heating rate of 3.1°C/sec and maintained for 65 seconds, and then primary cooling was performed at a cooling rate of 2.7°C/sec to 650°C, and then rapid cooling (secondary cooling) was performed at a cooling rate of 9°C/sec to 450°C. Then, slow cooling (third cooling) was performed at a cooling rate of 0.2°C/sec to 360°C, and then final cooling (fourth cooling) was performed at a cooling rate of 10°C/sec to room temperature.

[0075] For steel type B, the temperature was increased to 800°C at a heating rate of 3.2°C /sec and maintained for 61 seconds, primary cooling was performed at to 650°C at 2.3°C/sec, and then rapid cooling (secondary cooling) was performed to 450°C at a cooling rate of 9.5°C/sec. Then, slow cooling (third cooling) was performed at a cooling rate of 0.1°C/sec to 400°C, and then final cooling (fourth cooling) was performed at a cooling rate of 10°C/sec to room temperature.

[0076] A cross-sectional longitudinal occupancy rate of the fine grain layer for the manufactured steel sheet was measured and are shown in Table 2.

[0077] In addition, an integral value through the GDS profile of Mn wt% from the steel sheet surface to 0.03µm was obtained and is shown in Table 2.

[Table 2]

STEEL TYPE	FE COATING WEIGHT	DEW POINT TEMPERATURE (°C)	FINE GRAIN LAYER OCCUPANCY RATE	MN WT% PROFILE INTEGRAL VALUE	DIVISION
A	0	-40	<1	0.199	COMPARATIVEEXAMPLE 1
	0	-20	<1	0.349	COMPARATIVEEXAMPLE 2
	0	+5	<3	0.447	COMPARATIVEEXAMPLE 3
	0.5	-40	5.5	0.102	INVENTIVEEXAMPLE 1
	0.5	-20	5.7	0.310	INVENTIVEEXAMPLE 2
	0.5	+5	6.1	-	INVENTIVE EXAMPLE 3
	1	-40	32	0.070	INVENTIVE EXAMPLE 4
	1	-20	35	0.221	INVENTIVEEXAMPLE 5
	1	+5	128	10.139	INVENTIVE EXAMPLE 6
	1.5	-40	45	0.093	INVENTIVEEXAMPLE 7
	1.5	-20	50	0.262	INVENTIVE EXAMPLE 8
	1.5	+5	41	-	INVENTIVE EXAMPLE 9
	2	-40	65	0.087	INVENTIVEEXAMPLE 10
	2	-20	70	0.195	INVENTIVEEXAMPLE 11
	2	+5	66	-	INVENTIVE EXAMPLE 12
B	0	-40	<1	0.266	COMPARATIVE EXAMPLE 4
	0	-20	<1	0.474	COMPARATIVE EXAMPLE 5
	0	+5	<3	0.515	COMPARATIVE EXAMPLE 6
	0.5	-40	15	0.182	INVENTIVE EXAMPLE 13
	0.5	- 20	21	0.284	INVENTIVE EXAMPLE 14
	0.5	+5	27	-	INVENTIVE EXAMPLE 15
	1	-40	48	0.124	INVENTIVE EXAMPLE 16
	1	-20	38	0.155	INVENTIVE EXAMPLE 17
	1	+5	35	0.040	INVENTIVE EXAMPLE 18
	1.5	-40	78	0.080	INVENTIVE EXAMPLE 19
	1.5	-20	85	0.088	INVENTIVE EXAMPLE 20
	1.5	+5	170	-	INVENTIVE EXAMPLE 21
	2	-40	83	0.035	INVENTIVE EXAMPLE 22
	2	-20	75	0.037	INVENTIVE EXAMPLE 23
	2	+5	82	-	INVENTIVE EXAMPLE 24

[0078] The method of measuring an occupancy rate of the fine grain layer is as follows.

• Cross-section is processed with Focused Ion Beam (FIB) and observed with Scanning Transmission Electron Microscopy (STEM) at a minimum magnification of 20,000x (20,000x or more)
• FIB-STEM for length occupancy rate analysis may sample in random positions and may analyze a length of 20um or more in a transverse direction of the total cross-section or may connect and analyze a length of 20um or more in one position.

[0079] The Mn integral value estimated the Mn profile up to a GDS data depth of 0.03µm as an annealed surface oxide, and an Mn concentration at a depth of 0.03µm was assumed to be solid solution Mn. The integral value is entered in Table 2 after subtracting the solid solution Mn concentration from the Mn concentration at a depth of 0 to 0.03µm.

[0080] It may be estimated that as the Mn integral value with a Mn solution content removed up to 0.03µm increases, the dent sensitivity due to Mn build up on the surface of the hearth roll inside the annealing furnace increases, and as the Mn integral value decreases, the Mn build up on the surface of the hearth roll is reduced, which may have low dent sensitivity.

[0081] The following FIG. 3 shows a graph of the Mn integral value at all dew point temperatures as the amount of Fe coating weight before annealing increases. As shown in FIG. 3, it may be confirmed that the Mn integral value decreases at all dew point temperatures as the Fe coating weight before annealing increases.

[0082] FIG. 4 is a photograph of the surface of the manufactured Inventive Examples 4 and 16 and Comparative Examples 1 and 4. When observing FIG. 4, the presence or absence of fine grain layers and the position of the formation of annealed surface oxides may be confirmed depending on whether Fe plating is applied. That is, when Fe plating is performed, fine grain layers are formed, and the size of the annealed oxide formed on the surface becomes smaller and is disposed at the microcrystal grain boundaries exposed on the steel sheet surface.

[0083] Meanwhile, FIG. 5 is a photograph of the cross-section of Inventive Examples 4 and 16 and Comparative Examples 1 and 4. The presence or absence of a fine grain layer depending on whether Fe plating is applied may be confirmed. When the Fe plating is performed, the fine grain layer is formed on the surface of the steel sheet, whereas the fine grain layer does not appear on the surface of the steel sheet without performing the Fe plating.

Claims

1. A steel sheet, comprising: by wt%, 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 a balance of Fe and inevitable impurities,

wherein the steel sheet comprises a fine grain layer comprised of grains having a minor axis length of 0.5µm or less and a major axis length of 3µm or less within a maximum depth of 1µm from a surface,

wherein the fine grain layer has a length occupancy rate of 5% or more in a transverse direction of a cross section.

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

3. The steel sheet of claim 1, wherein the steel sheet further comprises a hot-dip plating layer, which is one a hot-dip galvanized (GI) layer, an alloyed hot-dip galvanized (GA) layer, a ternary zinc alloy plating (Zn-Al-Mg) layer, and a hot-dip aluminum alloy plating layer.

4. The steel sheet of claim 1, wherein the steel sheet further comprises a metal plating layer of either a Ni plating layer or a Zn plating layer.

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

6. A method for manufacturing a steel sheet, comprising:

preparing a base steel sheet comprising, by wt%, 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 a balance of Fe and inevitable impurities;

forming an Fe plating layer on a surface of the base steel sheet with a coating weight of 0.5 to 3.0 g/m²;

performing an annealing heat-treatment on the base steel sheet on which the Fe plating layer is formed in a nitrogen (N₂) gas atmosphere comprising 1 to 80 vol.% of hydrogen (H₂) at a dew point temperature of -60 to 30°C, at a temperature of 600 to 900°C; and

cooling the annealing heat-treated steel sheet.

7. The method for manufacturing a steel sheet of claim 6, further comprising:

performing hot-dip galvanizing before terminating the cooling,

wherein the hot-dip galvanizing is one of hot-dip galvanizing (GI), alloyed hot-dip galvanizing (GA), ternary zinc alloy plating (Zn-Al-Mg), and aluminum alloy plating.

8. The method for manufacturing a steel sheet of claim 6, further comprising:

pickling after the cooling,

wherein the pickling is performed with 5 to 18 wt% of an acid solution at 50 to 80°C.

9. The method for manufacturing a steel sheet of claim 8, further comprising:

performing electroplating with a coating weight of 5 to 100 mg/m² after the pickling, and forming a metal plating layer,

wherein the metal plating is one of Ni plating and Zn plating.

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

11. The method for manufacturing a steel sheet of claim 6, wherein the preparing a base steel sheet comprises:

heating a steel slab at 1100 to 1300°C;

hot-rolling the heated steel slab to manufacture a hot-rolled steel sheet;

cooling the hot-rolled steel sheet and coiling at 800°C or lower; and

pickling and cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet.

Drawing