WO2025034059A1 - Steel sheet and method for manufacturing same - Google Patents

Abstract

The present invention relates to a steel sheet suitable for various uses including use in automobile parts and, more specifically, to a steel sheet and a manufacturing method thereof.

Description

Steel plate and its manufacturing method

The present invention relates to a steel plate suitable for various purposes including automobile parts, and more specifically, to a steel plate and a method for manufacturing the same.

In order to respond to the climate crisis on Earth, automakers are demanding steelmakers to supply automotive steel sheets with reduced _CO2 emissions in stages, with the goal of achieving carbon neutrality by 2050. For this reason, steelmakers are currently developing automotive steel sheets with reduced _CO2 emissions through recycling of scrap iron, using existing blast furnace-converter furnace steelmaking processes or electric arc furnace steelmaking processes.

When using scrap iron as a raw material, it is difficult to remove residual elements (Tramp Elements) such as Cu and Ni contained in scrap iron during refining, so they end up being included in the steel after refining. These residual elements have the problem of lowering the properties of the steel or deteriorating the surface quality, so up to now, in the case of high-grade thin plate products represented by automobile steel plates, molten iron has been used as the main raw material, and the C and N in the steel have been greatly reduced in the general blast furnace-converter process, while the content of residual elements has been extremely controlled.

Meanwhile, a technology for manufacturing automobile steel sheets with excellent processability from electric furnace steel containing residual elements using an electric furnace steelmaking method was proposed as follows.

Patent documents 1 and 2 present a technology for manufacturing a cold rolled steel sheet having excellent workability. Specifically, the steel sheet discloses a cold rolled steel sheet having excellent cold workability even though it contains a large amount of residual elements. However, the technologies do not consider at all the purpose of improving the heat-resistant plating adhesion of the steel sheet and the means for achieving that purpose.

In this way, although a technology for manufacturing automotive steel sheets containing residual elements has been proposed, automotive steel sheets with improved processability and plating adhesion despite containing residual elements have not yet been developed.

(Patent Document 1) Japanese Patent Publication No. 1995-118795

(Patent Document 2) Japanese Patent Publication No. 1998-025541

One aspect of the present invention is to provide a steel sheet having improved processability and plating adhesion while containing residual elements when manufacturing automotive steel sheets by recycling iron scrap, and a method for manufacturing the same.

The subject matter of the present invention is not limited to the above-described contents. Those with ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding additional subjects of the present invention from the general contents of the specification of the present invention.

One aspect of the present invention provides a steel sheet including, in wt%, carbon (C): more than 0% to 0.0700%, silicon (Si): more than 0% to 0.800%, manganese (Mn): more than 0% to 1.000%, aluminum (Al): more than 0% to 0.500%, phosphorus (P): 0% to 0.080%, sulfur (S): 0% to 0.0500%, nitrogen (N): 0% to 0.0300%, copper (Cu): more than 0% to 1.000%, nickel (Ni): more than 0% to 1.000%, chromium (Cr): more than 0% to 1.000%, tin (Sn): more than 0% to 0.5000%, and at least one selected from the following groups (i) to (vi), the remainder being Fe and unavoidable impurities.

(i) Molybdenum (Mo): 0~1.000%

(ii) At least one of titanium (Ti): 0 to 0.500%, niobium (Nb): 0 to 0.500%, vanadium (V): 0 to 0.500%, and boron (B): 0 to 0.0200%.

(iii) Magnesium (Mg): 0~0.050%, Calcium (Ca): 0~0.050%, and Rare Earth Elements (REM) excluding Yttrium (Y): 0~0.050%, at least one of these

(iv) Tungsten (W): 0 to 0.50% and zirconium (Zr): 0 to 0.50%, at least one of these

(v) Antimony (Sb): 0~0.5000% and cobalt (Co): 0~0.5000%, at least one of these

(vi) Yttrium (Y): 0 to 0.200% and Hafnium (Hf): 0 to 0.200%, at least one of these

In one embodiment of the present invention, when the average content (weight %) of (Cu+Cr+Ni+Sn) in the surface layer is A and the average content (weight %) of (Cu+Cr+Ni+Sn) in the center layer is B, a steel plate satisfying A/B of 0.15 to 30.00 can be provided.

In one embodiment of the present invention, a steel sheet having a microstructure of a single phase of ferrite can be provided, and the average crystal grain size of the ferrite can be 6 to 50 ㎛.

In one embodiment of the present invention, a steel sheet can be provided in which a value (I/II) obtained by dividing the average grain size (I) of a cold rolled steel sheet by the average grain size (II) of the hot rolled steel sheet satisfies 0.60 to 0.95.

The steel plate according to one embodiment of the present invention has high strength, excellent processability, and can also have excellent plating adhesion when a subsequent plating process is performed.

In one embodiment of the present invention, the steel plate may have a product of tensile strength (TS) and elongation (El) (TS×El) of 4.0 to 30.0 GPa·%, and a product of Lankford value (r-value) and elongation (r-value×El) of 20 to 200%.

In one embodiment of the present invention, the steel plate may include a plating layer formed on at least one surface, and the plating layer may have a powdering peeling width of less than 7 mm.

Another aspect of the present invention provides a method for manufacturing a steel sheet, comprising the steps of: preparing a steel slab; heating the steel slab in a temperature range of 900 to 1,300°C; finish-rolling the heated steel slab in an austenite region higher than the Ar3 transformation point or in a ferrite region lower than the Ar3 transformation point to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 700°C or higher; cooling the coiled hot-rolled steel sheet to a temperature range of 250 to 350°C at a rate of 10°C/min or lower; cold-rolling the hot-rolled steel sheet after the cooling at a cold reduction ratio of 50% or higher to obtain a cold-rolled steel sheet; and annealing the cold-rolled steel sheet at 600°C or higher for 10 seconds or longer.

In one embodiment of the present invention, the steel slab may have the alloy composition described above.

In one embodiment of the present invention, the steel slab may be obtained by continuously casting molten steel obtained through a steelmaking process in an electric furnace.

In one embodiment of the present invention, the annealing treatment step can be performed by heating the cold rolled steel sheet obtained by cold rolling to the annealing temperature at a heating rate of 2 to 60°C/s.

In one embodiment of the present invention, a step of performing a plating treatment on the cold rolled steel sheet after the annealing treatment may be further included, and the plating treatment may be hot-dip plating or electroplating.

In one embodiment of the present invention, the molten zinc plating may be performed at a temperature range of 440 to 520°C, and an alloying heat treatment may be additionally performed after the molten zinc plating.

According to another aspect of the present invention, a steel sheet manufactured by the above-described manufacturing method can satisfy a value of 0.60 to 0.95, which is obtained by dividing the average grain size (I) of the cold rolled steel sheet after the annealing treatment by the average grain size (II) of the hot rolled steel sheet.

According to the present invention, it is possible to provide an automotive steel sheet having improved plating adhesion as well as processability despite containing residual elements.

The steel plate of the present invention is manufactured by recycling iron scrap, and has the advantage of an excellent _CO2 reduction effect when applied as a material for automobile bodies, etc.

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

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms used herein also include the plural forms unless the phrases clearly indicate the opposite.

The word "comprising" as used in the specification means specifying a particular characteristic, region, integer, step, operation, element, and/or component, but does not exclude the presence or addition of any other particular characteristic, region, integer, step, operation, element, component, and/or group.

Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Terms defined in commonly used dictionaries are additionally interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal sense unless defined.

Hereinafter, a steel plate according to one aspect of the present invention, particularly a steel plate having excellent processability and plating adhesion, will be described in detail. It should be noted that when indicating the content of each element in the present invention, unless otherwise specified, it means weight %. In addition, the ratio of crystals or structures is based on area unless otherwise specified.

Hereinafter, when explaining steel plates, it is stated that cold rolled steel plates are meant unless otherwise specified.

According to one aspect of the present invention, a steel plate may include, in wt%, carbon (C): more than 0% to 0.0700%, silicon (Si): more than 0% to 0.800%, manganese (Mn): more than 0% to 1.000%, aluminum (Al): more than 0% to 0.500%, phosphorus (P): 0 to 0.080%, sulfur (S): 0 to 0.0500%, nitrogen (N): 0 to 0.0300%, copper (Cu): more than 0% to 1.000%, nickel (Ni): more than 0% to 1.000%, chromium (Cr): more than 0% to 1.000%, and tin (Sn): more than 0% to 0.5000%.

Below, the reason for limiting the alloy composition of the steel plate provided in the present invention as described above is explained in detail.

Carbon (C): More than 0% ~ 0.0700%

Carbon (C) is an effective element for securing the strength of steel.

In one embodiment of the present invention, if the content of C exceeds 0.0700%, it is difficult to secure the elongation and r-value value targeted by the present invention. Therefore, the C may be contained at 0.0700% or less. In another embodiment of the present invention, the C may be 0.0650% or less or 0.0600% or less.

Meanwhile, C is an essential element in the production of steel, and its content is greater than 0%. However, in one embodiment of the present invention, if the content is less than 0.0003%, the effect of reducing C is hardly obtained, and rather, it causes an increase in the cost of steel production, which is not preferable. Therefore, the C may be included at 0.0003% or more. In another embodiment of the present invention, the C may be included at 0.0010% or more, or 0.0015% or more.

Silicon (Si): greater than 0% to 0.800%

Silicon (Si) is an element that has the effect of improving strength through solid solution strengthening, and is useful for improving workability by strengthening ferrite and uniformizing the structure. In addition, the Si is an element necessary for deoxidation during steelmaking.

In one embodiment of the present invention, if the content of Si exceeds 0.800%, there is a problem of causing plating defects such as non-plating during plating and hindering the weldability of the steel sheet. Accordingly, the Si may be contained at 0.800% or less. In another embodiment of the present invention, the Si may be 0.700% or less.

Meanwhile, in one embodiment of the present invention, the Si may be included in excess of 0%, and the content may be 0.001% or more in consideration of manufacturing cost.

Manganese (Mn): 0% or more to 1.000%

Manganese (Mn) is a useful element for simultaneously improving the strength and ductility of steel.

In one embodiment of the present invention, if the content of Mn exceeds 1.000%, the processability deteriorates, which is not preferable. In another embodiment of the present invention, the Mn may be 0.900% or less, or 0.400% or less.

Meanwhile, in one embodiment of the present invention, the Mn may be included in excess of 0%, and the content may be 0.001% or more in consideration of manufacturing cost.

Aluminum (Al): 0% to 0.500%

Aluminum (Al) is an element that combines with oxygen (O) in steel and has a deoxidizing effect. Also, similar to Si, it is an element that strengthens ferrite, uniformizes the structure, and improves processability.

In one embodiment of the present invention, if the content of Al exceeds 0.500%, there is a problem of deterioration in processability, so it may be included at 0.500% or less. In another embodiment of the present invention, the Al may be included at 0.400% or less.

Meanwhile, in one embodiment of the present invention, Al may be included in excess of 0%. However, since there is a problem that manufacturing costs increase in order to control the content of Al to less than 0.001%, the content may be limited to 0.001% or more in consideration of this.

P: 0.080% or less

Phosphorus (P) is an element added to improve the strength of steel. In one embodiment of the present invention, if the content of P exceeds 0.080%, there is a problem of inhibiting the workability of steel and causing brittleness. Therefore, the P may be included at 0.080% or less. According to another embodiment of the present invention, the P may be included at 0.070% or less.

Meanwhile, in one embodiment of the present invention, the P may be 0%, but the content may be limited to 0.001% or more in consideration of manufacturing costs.

Sulfur (S): 0.0500% or less

Sulfur (S) is an element that is inevitably added during the steel manufacturing process, and combines with Mn in the steel to form MnS inclusions, thereby inhibiting the ductility of the steel. Therefore, in one embodiment of the present invention, the content of S may be limited to 0.0500% or less. In another embodiment of the present invention, S may be included at 0.0400% or less.

Meanwhile, in one embodiment of the present invention, the S may be 0%, but since it may be inevitably contained in the steel, the content may exceed 0%. In one embodiment of the present invention, there is a problem that the manufacturing cost significantly increases in order to control the S content to less than 0.0010%. Therefore, the S may be contained at 0.0010% or more.

Nitrogen (N): 0.0300% or less

Nitrogen (N) is an element that is inevitably added during the steel manufacturing process, and is an element that causes cracks in slabs by forming nitrides during the continuous casting process. Accordingly, in one embodiment of the present invention, N may be included at 0.0300% or less.

Meanwhile, in one embodiment of the present invention, the N may be 0%, but since it may be inevitably contained in the steel, the content may exceed 0%. In one embodiment of the present invention, there is a problem that the manufacturing cost significantly increases in order to control the N content to less than 0.0010%. Therefore, the N may be contained at 0.0010% or more.

Copper (Cu): 0% to 1.000% and Nickel (Ni): 0% to 1.000%

Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. In addition, Cu and Ni are useful elements for inhibiting hydrogen-delayed fracture by concentrating on the surface of the steel plate and preventing the intrusion of hydrogen moving into the steel.

In one embodiment of the present invention, if the content of Cu and Ni is excessive, the processability becomes inferior, so each may be limited to 1.000% or less. In another embodiment of the present invention, each of Cu and Ni may be included at 0.900% or less.

Meanwhile, in one embodiment of the present invention, Cu and Ni may be included in amounts exceeding 0%, and in order to sufficiently obtain the effect of Cu and Ni, each may be included in amounts of 0.0010% or more.

Chromium (Cr): 0% to 1.000%

Chromium (Cr) is an element that suppresses the decomposition of austenite and stabilizes austenite during alloying treatment of manufactured steel plates.

In one embodiment of the present invention, since excessive Cr content deteriorates processability, the Cr content may be limited to 1.000% or less in consideration of this. According to another embodiment of the present invention, the Cr content may be included at 0.900% or less.

Meanwhile, in one embodiment of the present invention, Cr may be included in excess of 0%, but may be included in an amount of 0.001% or more in order to sufficiently obtain the effect of adding Cr.

Sn: >0%~0.5000%

Tin (Sn) is an element that improves the plating wettability and plating adhesion of steel. In one embodiment of the present invention, if the content of tin in the steel exceeds 0.5000%, there is a problem that the brittleness of the steel increases, causing cracks during hot working or cold working. According to another embodiment of the present invention, the Sn may be included at 0.4000% or less.

Meanwhile, in one embodiment of the present invention, Sn may be included in excess of 0%, and in order to sufficiently obtain the effect due to Sn, it may be included in an amount of 0.0005% or more.

In addition to the alloy composition described above, the steel plate of the present invention may further include one or more elements selected from the groups (i) to (vi) described below. Since all of the elements described below can be added selectively, it is stated that the content thereof is 0%. Meanwhile, the elements of the groups (i) to (vi) may be referred to as residual elements.

(i) Molybdenum (Mo): 0~1.000%

(ii) At least one of titanium (Ti): 0 to 0.500%, niobium (Nb): 0 to 0.500%, vanadium (V): 0 to 0.500%, and boron (B): 0 to 0.0200%.

(iii) Magnesium (Mg): 0~0.050%, Calcium (Ca): 0~0.050%, and Rare Earth Elements (REM) excluding Yttrium (Y): 0~0.050%, at least one of these

(iv) Tungsten (W): 0 to 0.50% and zirconium (Zr): 0 to 0.50%, at least one of these

(v) Antimony (Sb): 0~0.5000% and cobalt (Co): 0~0.5000%, at least one of these

(vi) Yttrium (Y): 0 to 0.200% and Hafnium (Hf): 0 to 0.200%, at least one of these

(i) Molybdenum (Mo): 0~1.000%

Molybdenum (Mo) is an element that suppresses the decomposition of austenite and stabilizes austenite during alloying treatment of steel plates manufactured similarly to the Cr described above. In one embodiment of the present invention, if the content of Mo is excessive, workability deteriorates, and therefore, taking this into consideration, the content may be limited to 1.000% or less. In another embodiment of the present invention, the Mo may be included at 0.900% or less. Meanwhile, when the content of Mo is 0.0010% or more when added, the above-described effect can be obtained, and therefore, the lower limit may be limited to 0.0010%.

(ii) At least one of titanium (Ti): 0 to 0.500%, niobium (Nb): 0 to 0.500%, vanadium (V): 0 to 0.500%, and boron (B): 0 to 0.0200%.

Titanium (Ti), niobium (Nb), and vanadium (V) are elements that form precipitates in steel, and the strength and impact toughness of the steel can be improved by the formation of the precipitates. In particular, the Ti, Nb, and V can prevent deterioration of workability due to the solid solution C and solid solution N in the steel by precipitating the solid solution C and solid solution N as carbides, nitrides, etc. In one embodiment of the present invention, in order to obtain the above-described effect, the elements may be added in an amount of 0.001% or more each. However, if the content of each element exceeds 0.500%, there is a problem that the addition effect is saturated and the manufacturing cost increases, and therefore the upper limit of each element may be limited to 0.500%. According to another embodiment of the present invention, each element may be included in an amount of 0.400% or less.

Boron (B) is an element that improves the hardenability of steel, thereby increasing the strength, and suppresses nucleation at grain boundaries. If the content exceeds 0.0200%, there is a concern that the deep drawability of the steel may deteriorate. Therefore, in one embodiment of the present invention, the content of B may be limited to 0.0200% or less when added. According to another embodiment of the present invention, B may be included in an amount of 0.0100% or less. Meanwhile, in one embodiment of the present invention, the content of B may be 0.0001% or more when added, and according to another embodiment, B may be included in an amount of 0.0005% or more, or 0.0010% or more.

(iii) Magnesium (Mg): 0~0.050%, Calcium (Ca): 0~0.050%, and Rare Earth Elements (REM) excluding Yttrium (Y): 0~0.050%, at least one of these

Rare earth elements (REMs), excluding magnesium (Mg), calcium (Ca), and yttrium (Y), are elements that improve the ductility of steel by spheroidizing sulfides in the steel. In one embodiment of the present invention, if the content of each of the above elements exceeds 0.050%, not only will the above-described effect be saturated, but also the manufacturing cost will increase, so the content may be limited to 0.050% or less, respectively.

In general, rare earth elements (REM) refer to a total of 17 metallic elements, including scandium (Sc), yttrium (Y), lanthanum (La), and cerium (Ce). However, according to one embodiment of the present invention, the content of yttrium (Y) is separately limited below, so it is to be noted that in one embodiment of the present invention, REM refers to 16 elements excluding Y.

(iv) Tungsten (W): 0 to 0.50% and zirconium (Zr): 0 to 0.50%, at least one of these

Tungsten (W) and zirconium (Zr) are elements that improve the hardenability of steel and increase the strength of steel. If these elements are added excessively, not only will the above-mentioned effects be saturated, but the manufacturing cost will also increase. Therefore, when adding at least one of the above W and Zr, the content may be limited to 0.50% or less, respectively.

(v) Antimony (Sb): 0~0.5000% and cobalt (Co): 0~0.5000%, at least one of these

Antimony (Sb) and cobalt (Co) are elements that improve the plating wettability and plating adhesion of steel. In one embodiment of the present invention, if the content of each of the elements in the steel exceeds 0.5000%, the brittleness of the steel increases, which causes a problem of cracking during hot working or cold working, so the content of each element may be limited to 0.5000% or less. According to another embodiment of the present invention, each of these elements may be included at 0.4000% or less. Meanwhile, in order to sufficiently obtain the desired effect by containing these elements, each of them may be included at 0.0005% or more.

(vi) Yttrium (Y): 0 to 0.200% and Hafnium (Hf): 0 to 0.200%, at least one of these

Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel. In one embodiment of the present invention, if the content of each of the elements exceeds 0.200%, there is a concern that the ductility of the steel may deteriorate, so the content of each element may be limited to 0.200% or less.

The steel sheet according to one aspect of the present invention may contain the remaining Fe and other unavoidable impurities in addition to the aforementioned components. However, since unintended impurities may inevitably be mixed in from raw materials or the surrounding environment during a normal manufacturing process, they cannot be completely excluded. Since these impurities are known to anyone with ordinary knowledge in the art, not all of the contents are specifically mentioned in this specification. In addition, the addition of effective components other than the aforementioned components is not completely excluded.

The steel plate of the present invention having the alloy composition as described above can improve the workability of the steel plate by controlling the content of residual elements contained at a specific point in the thickness direction as follows.

Specifically, when the average content (weight %) of (Cu+Cr+Ni+Sn) in the surface layer of the steel plate is A and the average content (weight %) of (Cu+Cr+Ni+Sn) in the center layer is B, it is preferable that A/B satisfies 0.15 to 30.00.

In the present invention, A/B is significant in that it is used as an index indicating the workability of the steel plate. If the value of A/B is less than 0.15 or exceeds 30.00, there is a problem in that specific residual elements contained in the steel are not distributed uniformly.

Here, the center refers to the point t/2 in the thickness direction, and the surface refers to the inflection point, which is the point where the concentration of Cu, Cr, Ni, and Sn changes (or changes rapidly) when measuring the concentration gradient from the surface to the thickness direction.

In one embodiment of the present invention, the inflection point may refer to a point corresponding to 99% of the (Cu+Cr+Ni+Sn) content of the center in the concentration gradient measurement value when the (Cu+Cr+Ni+Sn) content of the polar surface is greater than the (Cu+Cr+Ni+Sn) content of the center, and may refer to a point corresponding to 1% of the (Cu+Cr+Ni+Sn) content of the center in the concentration gradient measurement value when the (Cu+Cr+Ni+Sn) content of the polar surface is smaller than the (Cu+Cr+Ni+Sn) content of the center. As a non-limiting example, the concentration gradients of the above-mentioned elements can be measured using FE-SEM or glow discharge spectroscopy (GDS).

As an example, when measuring the concentration of (Cu+Cr+Ni+Sn) in the thickness direction of the steel plate, the (Cu+Cr+Ni+Sn) content at the polar surface has the highest value, and the value may tend to decrease linearly as it goes from the polar surface to the center.

As another example, when measuring the concentration of (Cu+Cr+Ni+Sn) in the thickness direction of the steel plate, the (Cu+Cr+Ni+Sn) content at the polar surface has the lowest value, and the value may tend to increase linearly from the polar surface to the center.

Meanwhile, it is well known in the technical field of the present invention that a steel plate, i.e. a cold rolled steel plate, is obtained through a series of rolling processes. That is, as will be described in detail later, a cold rolled steel plate according to the present invention is obtained through a cold rolling (and subsequent annealing heat treatment) process on a hot rolled steel plate obtained through a hot rolling and coiling process.

For such hot-rolled steel sheets and cold-rolled steel sheets, in one embodiment of the present invention, it is preferable that the ratio of the average grain size of the hot-rolled steel sheet and the average grain size of the cold-rolled steel sheet obtained by cold-rolling and annealing the hot-rolled steel sheet satisfies the following.

Specifically, in one embodiment of the present invention, the value (I/II) obtained by dividing the average grain size (I) of the cold rolled steel sheet by the average grain size (II) of the hot rolled steel sheet can satisfy 0.60 to 0.95.

In this way, according to the present invention, by controlling the grain size of a hot-rolled steel sheet obtained by hot rolling, the grain size of a cold-rolled steel sheet obtained by subsequent cold rolling and annealing heat treatment can be secured, thereby obtaining excellent workability as well as strength.

If the value of the above I/II is less than 0.60, there is a problem that the rigidity increases significantly due to the grain size of the hot-rolled steel sheet becoming excessively fine, whereas if the value exceeds 0.95, the grain size becomes coarse, making it impossible to secure strength and workability.

In one embodiment of the present invention, the microstructure of the steel plate may be a single phase of ferrite.

That is, if the microstructure of the steel plate includes a structure other than ferrite, such as pearlite, or bainite or martensite formed at low temperatures, the intended workability cannot be secured. However, it is not intended to exclude even extremely small amounts of impurity structures that are inevitably included in the process, and they are naturally included in the scope of the present invention.

In one embodiment of the present invention, the average grain size of the ferrite may be 6 to 50 μm. If the average grain size of the ferrite is less than 6 μm, there is a risk that the steel will become brittle due to increased rigidity, whereas if the size exceeds 50 μm, there is a risk that it will be disadvantageous to secure strength through grain refinement.

Meanwhile, it should be noted that the grain size of the hot-rolled steel sheet and cold-rolled steel sheet mentioned above refers to the grain size of ferrite.

A steel plate according to one embodiment of the present invention not only has high strength but also has excellent processability.

Specifically, according to one embodiment of the present invention, the steel plate can satisfy a product of tensile strength (TS) and elongation (El) (TS×El) of 4.0 to 30.0 GPa·%, and further, a product of Lankford value (r-value) and elongation (r-value×El) of 20 to 200%.

A steel plate according to one embodiment of the present invention may have a plating layer on at least one surface.

In one embodiment of the present invention, the plating layer may be a zinc-based plating layer, and the zinc-based plating layer is a plating layer containing zinc (Zn) as a main component, and a composition of the plating layer commonly applied in the relevant technical field may be applied in the same manner. At this time, the zinc-based plating layer may also include a zinc-based plating layer alloyed by alloying treatment.

In this way, the steel sheet having a zinc-based plating layer on at least one side of the steel sheet is a zinc-based plating steel sheet, and may be a galvanized steel sheet (GI), a galvanealed steel sheet (GA), an electrogalvanized steel sheet (EG), an electrogalvanized steel sheet, or the like.

In one embodiment of the present invention, the steel sheet having a zinc-based plating layer formed on at least one surface may be a steel sheet according to one embodiment of the present invention, and it is to be noted that the above-described description can be equally applied to the steel sheet.

Meanwhile, the steel sheet (cold rolled steel sheet) according to one embodiment of the present invention may have a concentration gradient of specific elements, for example, residual elements Cu, Cr, Ni, and Sn, in the thickness direction from the surface. When a plating layer (for example, a zinc-based plating layer) is formed on at least one surface of such a steel sheet, the residual elements may further diffuse into the plating layer due to the influence of the temperature during plating or the temperature during alloying treatment. Accordingly, the concentration gradient of the residual elements in the plating steel sheet may be realized from the interface between the base material, which is the base steel sheet, and the plating layer in the thickness direction of the base material, but may also be realized from an alloy layer formed at the interface between the base material and the plating layer.

That is, the residual elements may have a concentration gradient, previously represented as A/B, due to the difference in diffusion speed of the residual elements that diffuse toward the surface of the steel sheet during the process of manufacturing the cold rolled steel sheet, and these characteristics of the cold rolled steel sheet are also shown in the coated steel sheet.

A plated steel sheet according to one embodiment of the present invention has a characteristic of excellent powdering properties of a plating layer. Specifically, the plating layer may have a characteristic of having a powdering peeling width of less than 7 mm.

Hereinafter, a method for manufacturing a steel plate (cold rolled steel plate) and a plated steel plate according to another aspect of the present invention will be described in detail.

First, the steel sheet according to the present invention, i.e., the cold rolled steel sheet, can be obtained by going through a series of processes, for example, [steel slab heating - hot rolling - coiling - cooling - cold rolling - annealing]. Each process step is described in detail below, and it is to be noted that the following manufacturing process is an example for manufacturing the cold rolled steel sheet of the present invention and the subsequent plated steel sheet.

In one embodiment of the present invention, the steel slab or ingot can be manufactured through an electric furnace or a new blast furnace-converter process, and the raw material used in the electric furnace or the blast furnace-converter process uses pig iron together with iron scrap. Here, pig iron means molten iron obtained from the blast furnace-converter process, or its cold material (Corrugate) or HBI (Hot Briquette Iron).

In the case of an electric furnace, desulfurization treatment can be performed by ladle refining after the electric furnace, and desulfurization and vacuum degassing treatment as a post-process can be performed. In addition, the steel obtained from an electric furnace can be adjusted to the final desired alloy composition by adding alloy components while performing degassing treatment. Here, the RH method and the DH method are common as vacuum degassing treatment methods, but oxygen injection into the degassing tank can be performed in parallel. Regarding oxygen injection, there is an oxygen injection method using a top blowing lance.

[River Slab Heating]

After preparing the steel slab, it can be heated. At this time, the steel slab may be a steel ingot. In one embodiment of the present invention, the steel slab may have the alloy composition described above, and thus, the alloy composition of the steel slab is replaced with the content described above.

In one embodiment of the present invention, the heating temperature during heating of the prepared steel slab or ingot may be in the range of 900 to 1300°C. If the heating temperature exceeds 1300°C, the steel may reach its melting point and melt, whereas if the temperature is lower than 900°C, there is a concern that the rolling load may increase during subsequent hot rolling, thereby lowering the stability of the hot rolling. In another embodiment of the present invention, the heating may be performed at 1160°C or lower.

[Hot Rolling]

The above heated steel slab or ingot can be hot rolled to obtain a hot rolled steel sheet.

In one embodiment of the present invention, the finishing temperature during hot rolling can be either an austenite temperature above the Ar3 transformation point or a ferrite temperature below the Ar3 transformation point. However, if the finishing temperature is too low, the rolling load during hot rolling increases, so the lower limit temperature can be limited to 750°C or higher.

[Winding]

The hot rolled steel sheet obtained by the above hot rolling can be wound into a coil shape.

In one embodiment of the present invention, the coiling process can be performed at a temperature of 700°C or higher. If the temperature during the coiling is lower than 700°C, the grain size of the hot-rolled steel sheet obtained after the coiling cannot be secured as intended. In another embodiment of the present invention, the coiling process can be performed at a temperature of 720°C or higher.

Meanwhile, there is no particular limitation on the upper limit of the above coiling temperature, but considering that scale may be excessively formed on the surface of the hot-rolled coil, the temperature may be limited to 800°C or lower.

The present invention can achieve the grain relationship of a hot-rolled steel sheet and a subsequent cold-rolled steel sheet, i.e., the range of I/II, by performing coiling at the temperature described above.

[cooling]

The hot rolled steel sheet coiled as described above can be cooled while uncoiling.

In one embodiment of the present invention, cooling after coiling can be performed at a cooling rate of 10.0°C/min or less to a temperature range of 250 to 350°C. By performing cooling at such a relatively slow cooling rate, crystal grains within the microstructure can be uniformly secured.

In one embodiment of the present invention, if the cooling rate exceeds 10.0°C/min, there is a possibility that the crystal grains will become coarser. Meanwhile, there is no particular limitation on the lower limit of the cooling rate during the cooling. However, if the cooling rate is excessively low, a strain may be placed on the cooling equipment, and the crystal grains may also become coarser. Therefore, taking this into consideration, it may be limited to 0.2°C/min or more.

If the cooling end temperature is less than 250℃ when cooling at the above-mentioned cooling rate, there is a concern that the cooling time will be excessive and productivity will decrease. On the other hand, if the cooling end temperature exceeds 350℃, there is a problem of causing a load during subsequent cold rolling.

[Cold rolled]

A cold rolled steel sheet can be obtained by cold rolling the above-mentioned cooled uncoiled hot rolled steel sheet.

In one embodiment of the present invention, cold rolling can be performed at a cold reduction ratio of 50% or more. If the cold reduction ratio is less than 50%, it may be difficult to secure the target thickness and it may be difficult to correct the shape of the steel plate. According to one embodiment of the present invention, there is no particular limitation on the upper limit of the cold reduction ratio during cold rolling, but if it is too excessive, a cold rolling load may be induced, and therefore, taking this into consideration, it may be limited to 95% or less.

[Sondu]

The cold rolled steel sheet obtained as described above can be subjected to annealing heat treatment.

In one embodiment of the present invention, the annealing heat treatment can be performed in a general continuous annealing furnace, and it is preferable to perform the annealing heat treatment for 10 seconds or longer after the cold rolled steel sheet is heated to 600°C or higher. If the temperature is lower than 600°C or the time is shorter than 10 seconds during the annealing heat treatment, recrystallization may not be sufficiently performed, and there is a risk that an unrecrystallized structure may occur. In this case, the workability of the steel sheet may be poor.

According to one embodiment of the present invention, the upper limits of temperature and time for the annealing heat treatment are not particularly limited, but considering the risk of equipment trouble due to high-temperature annealing and the inferiority of powdering properties, the upper limits may be set to 960°C and 15 minutes, respectively.

Meanwhile, when heating the cold rolled steel sheet to a temperature for annealing, heating can be performed at a constant heating rate. As an example, heating can be performed at a heating rate of 2 to 60°C/s. If the heating rate is too slow or too fast during the heating, the crystal grain characteristics intended in the present invention may not be secured.

The cold rolled steel sheet manufactured through the above-described series of processes can have the strength and workability targeted in the present invention.

In particular, the cold rolled steel sheet of the present invention can have a product of tensile strength (TS) and elongation (El) (TS × El) of 4.0 to 30.0 GPa·%, and a product of Lankford value (r-value) and elongation (r-value × El) of 20 to 200%.

Meanwhile, according to one aspect of the present invention, a steel plate, that is, a cold rolled steel plate, may be plated to obtain a plated steel plate. At this time, the cold rolled steel plate may be a cold rolled steel plate manufactured through a series of processes according to one embodiment of the present invention. Accordingly, the plated steel plate may have a plated layer on at least one surface of the cold rolled steel plate described above.

In one embodiment of the present invention, the plating process may be a hot-dip plating process or an electroplating process, and each plating process may be performed under conditions commonly practiced in the relevant technical field.

As an example, the above-described hot-dip galvanizing can be performed by immersing the cold-rolled steel sheet, which has undergone the above-described annealing heat treatment, in a hot-dip galvanizing bath containing zinc as a main component. Since the conditions for the hot-dip galvanizing treatment can be based on normal conditions, the present invention does not specifically limit them. However, as an example of implementation, the GI hot-dip galvanizing can be performed under normal conditions in a temperature range of 440 to 520°C.

In addition, according to one embodiment of the present invention, an alloyed hot-dip galvanized steel sheet (GA) can be obtained by selectively performing alloying heat treatment on a plated steel sheet having a zinc-based plating layer formed thereon. The alloying heat treatment can also be performed under normal conditions, and is not specifically limited thereto. However, as an example, the alloying heat treatment can be performed in a temperature range of 500 to 560°C.

In addition, as another example, the electroplating can be performed by placing a base steel sheet (cold rolled steel sheet) on the cathode of an electroplating simulator of a vertical plating cell type, and then circulating a plating solution containing zinc (Zn) to form a zinc-based plating layer on at least one surface of the base steel sheet (cold rolled steel sheet).

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only intended to explain the present invention through examples and are not intended to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

(Example)

After preparing a steel slab having the alloy composition shown in Table 1 below, a steel plate (cold rolled steel plate) was obtained by subjecting the steel slab to a series of processes according to the conditions shown in Table 2 below. At this time, cooling was performed at a temperature range of 250 to 350°C after coiling.

The above steel slab was obtained through the following process. First, iron scrap mixed with the molten iron of an existing blast furnace was put into an electric furnace to obtain molten steel. Then, the molten steel was transferred to a ladle and vacuum degassing was performed in an RH degassing furnace (0.1 torr), and at the same time, alloying components were added to manufacture molten steel having the intended composition. Thereafter, the molten steel was manufactured into a slab by continuous casting.

The microstructure and mechanical properties of the manufactured steel sheets were measured and evaluated, and all results are shown in Table 3 below. At this time, the phase types and properties of the microstructure were measured/evaluated for the cold rolled steel sheets, and the grain size was measured for both the hot rolled steel sheets and the cold rolled steel sheets. The cold rolled steel sheets at this time correspond to the cold rolled steel sheets after annealing.

The microstructure of each cold-rolled steel sheet was observed by collecting specimens at t/4 in the thickness direction, polishing them, and then nital-etching the cross-section of the polished specimens using a scanning electron microscope (SEM). After the nital-etching, the structure without irregularities on the surface of the specimens was determined to be ferrite, and the structure with a spherical or lamellar structure was determined to be cementite. Then, in order to confirm the grain size characteristics of the ferrite, the average grain size of the ferrite was measured through an optical microscope for each specimen collected at t/4 in the thickness direction of the hot-rolled steel sheet and the cold-rolled steel sheet before cold rolling and the cross-section was nital-etched.

In addition, a tensile test was conducted to evaluate the properties of each steel plate. The tensile test was evaluated using specimens collected according to the JIS No. 5 standard at a 90-degree angle to the rolling direction of the rolled plate, and the product of the tensile strength (TS) and the elongation (El) (TS × El) was calculated. In addition, the Lankford value (r-value) for the same specimen was measured by the three-point method after 15% tensile prestrain, and the average values in the rolling direction (L direction), the direction perpendicular to the rolling direction (C direction), and the direction 45 degrees to the rolling direction (D direction) were obtained by the following calculation formula.

r-value = (r _L + 2r _D + r _C ) / 4

Meanwhile, in order to confirm the content (wt%) of specific elements (Cu, Cr, Ni, and Sn) according to the point in the thickness direction of the cold-rolled steel sheet, the cross-section of each specimen was removed by FIB and then measured by component analysis using FE-SEM. At this time, according to the definition of the surface layer, the content within the surface layer and the point at the center (t/2) in the thickness direction were measured, respectively, and each measurement was repeated 30 times in total. After removing the maximum and minimum values of the analyzed values of each element, the content was calculated as the average value.

In addition, for the evaluation of powdering property, each cold rolled steel sheet was plated in a hot-dip zinc plating bath at 440 to 480°C, and the plated steel sheet was evaluated using a 45-degree V-bend test method. The evaluation surface was placed on the inner side of the bend, and a mold with a curvature radius of 1 mm at the tip was used to perform the bending process at 60 degrees. After attaching tape to the inner side and removing the tape, the powdering property of the plating layer peeled off together with the tape was evaluated with a full score of 5. At this time, the evaluation criteria were set as follows, and a score of 2 or higher was evaluated as passing.

Peeling width less than 2mm: 5 points

Peeling width 2mm or more but less than 3mm: 4 points

Peeling width 3mm or more but less than 5mm: 3 points

Peeling width 5mm or more but less than 7mm: 2 points

Peeling width 7mm or more: 1 point

river
bell Alloy composition (weight%) C Si Mn P S Al N Cu Cr Ni Sn Ti etc A 0.0007 0.003 0.054 0.002 0.0015 0.026 0.0015 0.83 0.68 0.85 0.32 0.012 Hf 0.005 B 0.0015 0.004 0.096 0.005 0.004 0.029 0.0038 0.65 0.84 0.47 0.18 0.034 Nb 0.023
Y 0.004 C 0.0026 0.015 0.162 0.012 0.009 0.034 0.0057 0.47 0.35 0.48 0.09 0.051 W 0.02 D 0.0039 0.023 0.205 0.026 0.014 0.031 0.0062 0.38 0.26 0.33 0.05 0.056 Mg 0.002 E 0.0158 0.028 0.251 0.038 0.019 0.034 0.0085 0.26 0.21 0.22 0.03 0.078 Mo 0.03
Zr 0.01 F 0.0282 0.026 0.289 0.049 0.023 0.032 0.0098 0.14 0.12 0.10 0.01 0.095 B 0.0021
Sb 0.002 G 0.0497 0.032 0.375 0.064 0.035 0.033 0.0163 0.003 0.002 0.002 0.001 0.142 Ca 0.005
REM 0.001 XA 0.0731 0.021 0.234 0.028 0.025 0.027 0.0042 0.19 0.23 0.16 0.021 0.038 - XB 0.0023 0.827 0.251 0.025 0.021 0.025 0.0058 0.22 0.21 0.24 0.025 0.053 - XC 0.0020 0.024 1.075 0.017 0.030 0.034 0.0062 0.20 0.25 0.18 0.020 0.057 - XD 0.0025 0.023 0.216 0.019 0.034 0.539 0.0066 0.25 0.16 0.17 0.017 0.061 - XE 0.0022 0.025 0.220 0.020 0.028 0.030 0.0342 0.18 0.17 0.20 0.019 0.295 - XF 0.0024 0.029 0.204 0.023 0.024 0.036 0.0059 1.14 0.21 0.25 0.024 0.052 - XG 0.0025 0.020 0.312 0.028 0.023 0.031 0.0050 0.26 1.08 0.23 0.023 0.044 - Xh 0.0026 0.027 0.281 0.025 0.026 0.025 0.0045 0.32 0.26 1.21 0.028 0.040 - XI 0.0021 0.022 0.237 0.027 0.020 0.027 0.0042 0.34 0.23 0.17 0.523 0.036 - In Table 1, REM means REM excluding Y.

River species Reheat
temperature
(℃) finish
Rolling temperature
(℃) Winding
temperature
(℃) After winding
cooling
(℃/min) cooling
Pressure ratio
(%) Temperature rise
speed
(℃/s) Annealed
temperature
(℃) Annealed
hour
(s) I/II Psalter
No. A 1160 880 750 0.5 76 45 930 40 0.96 1 B 1160 880 750 0.5 76 42 820 40 0.57 2 A 1270 930 830 0.6 76 47 940 600 0.94 3 B 1060 780 400 0.6 76 39 760 20 0.58 4 A 1160 880 750 0.2 76 58 880 40 0.74 5 B 1160 880 790 0.5 76 35 870 40 0.77 6 B 1320 950 750 0.7 76 28 880 40 0.82 7 B 880 630 450 0.5 76 22 880 40 0.73 8 B 1280 950 820 0.6 76 30 880 40 0.64 9 B 1050 820 680 0.8 76 32 880 40 0.82 10 B 1160 880 750 0.9 48 35 880 40 0.51 11 B 1160 880 750 1.0 76 35 580 40 0.38 12 B 1160 880 750 1.0 76 35 970 40 0.98 13 B 1160 880 700 0.7 76 23 880 5 0.37 14 B 1160 880 700 0.7 76 23 880 1000 0.98 15 B 1160 880 700 0.1 76 18 860 40 0.97 16 B 1160 880 750 10.7 76 20 860 40 0.57 17 B 1160 880 750 0.6 76 1 860 40 0.98 18 B 1160 880 700 0.7 76 62 860 40 0.51 19 C 1050 880 750 0.8 76 26 860 800 0.82 20 D 1050 880 720 0.9 76 14 860 40 0.93 21 E 1200 880 720 1.2 76 8 900 40 0.62 22 F 1250 880 750 2.6 76 5 950 40 0.70 23 G 1160 880 780 5.4 76 2 880 40 0.71 24 XA 1160 880 750 0.8 76 18 880 40 0.42 25 XB 1160 880 750 0.6 76 20 880 40 0.51 26 XC 1160 880 750 0.5 76 20 880 40 0.47 27 XD 1160 880 750 0.9 76 24 880 40 0.49 28 XE 1160 880 750 0.7 76 26 880 40 0.55 29 XF 1160 880 750 0.3 76 29 880 40 0.53 30 XG 1160 880 750 0.4 76 17 880 40 0.50 31 XH 1160 880 750 0.5 76 15 880 40 0.51 32 XI 1160 880 750 0.8 76 22 880 40 0.54 33 In Table 2, I/II refers to the value (I/II) obtained by dividing the average grain size (I) of cold-rolled steel sheets by the average grain size (II) of hot-rolled steel sheets.

Psalm No. A/B cold rolled steel sheet
Average grain size
Size (㎛) TS×El
(GPa%) r-value×El
(%) Powdering properties 1 0.53 46 23 189 4 2 0.57 23 17 93 4 3 30.82 53 34 206 1 4 0.51 5 3.3 19 4 5 0.19 39 29 197 5 6 0.34 34 24 165 5 7 0.38 17 3.5 17 4 8 0.54 9 3.1 14 4 9 32.43 30 20 135 1 10 0.41 14 3.2 18 4 11 0.52 18 5.6 12 3 12 0.14 9 3.4 16 1 13 0.36 35 26 143 3 14 0.12 15 3.7 15 1 15 0.23 33 21 156 3 16 0.45 40 25 174 2 17 0.34 14 3.3 17 4 18 0.48 32 20 139 2 19 0.39 13 3.0 15 4 20 0.82 25 23 124 4 21 2.52 22 16 95 4 22 8.49 19 12 67 4 23 16.2 14 7.2 34 3 24 28.5 8 4.3 22 3 25 16.9 4 2.8 16 2 26 26.4 9 3.3 15 2 27 25.8 7 3.7 18 3 28 20.7 8 2.6 14 2 29 21.2 5 2.5 17 3 30 23.0 9 3.8 15 3 31 26.4 12 3.1 13 2 32 15.7 13 2.4 16 2 33 16.9 15 2.7 18 2 In Table 3, A/B means the value (A/B) obtained by dividing the average content (Cu+Cr+Ni+Sn, weight%, A) of the surface layer by the average content (Cu+Cr+Ni+Sn, weight%, B) of the center layer.

As shown in Tables 1 to 3 above, in the case of specimens satisfying both the alloy composition and manufacturing conditions proposed in the present invention, the microstructure was formed to have the intended crystal grains, and both the strength and the processability were excellent.

On the other hand, in the case of specimens that deviate from one or more of the alloy composition and manufacturing conditions of the present invention, the crystal grains of the microstructure are not formed as intended, or at least one or more physical properties such as strength, processability, and plating adhesion are inferior due to the non-uniform distribution of specific elements.

In this way, the present invention has a technical significance in that it can achieve both excellent processability and plating adhesion by controlling the crystal grain size characteristics and the content ratio of specific elements in the thickness direction.

Claims (17)

Carbon (C): exceeding 0% to 0.0700%, Silicon (Si): exceeding 0% to 0.800%, Manganese (Mn): exceeding 0% to 1.000%, Aluminum (Al): exceeding 0% to 0.500%, Phosphorus (P): 0 to 0.080%, Sulfur (S): 0 to 0.0500%, Nitrogen (N): 0 to 0.0300%, Copper (Cu): exceeding 0% to 1.000%, Nickel (Ni): exceeding 0% to 1.000%, Chromium (Cr): exceeding 0% to 1.000%, Tin (Sn): exceeding 0% to 0.5000%, and at least one selected from the following groups (i) to (vi), the remainder being Fe and unavoidable impurities. (i) Molybdenum (Mo): 0~1.000%, (ii) At least one of titanium (Ti): 0 to 0.500%, niobium (Nb): 0 to 0.500%, vanadium (V): 0 to 0.500%, and boron (B): 0 to 0.0200%; (iii) Magnesium (Mg): 0 to 0.050%, Calcium (Ca): 0 to 0.050%, and Rare Earth Elements (REM) excluding Yttrium (Y): 0 to 0.050%, at least one of these; (iv) Tungsten (W): 0 to 0.50% and zirconium (Zr): 0 to 0.50%, at least one of these; (v) Antimony (Sb): 0~0.5000% and cobalt (Co): 0~0.5000%, at least one of these; (vi) At least one of yttrium (Y): 0 to 0.200% and hafnium (Hf): 0 to 0.200%; When the average content (weight%) of (Cu+Cr+Ni+Sn) in the surface layer is A and the average content (weight%) of (Cu+Cr+Ni+Sn) in the center is B, A/B satisfies 0.15 to 30. The above center refers to a point t/2 in the thickness direction, and the above surface refers to an inflection point where the concentration of Cu, Cr, Ni, and Sn changes when the concentration gradient is measured from the surface in the thickness direction, and the inflection point refers to a point corresponding to 99% of the (Cu+Cr+Ni+Sn) content of the center in the concentration gradient measurement value when the (Cu+Cr+Ni+Sn) content of the polar surface is greater than the (Cu+Cr+Ni+Sn) content of the center, or refers to a point corresponding to 1% of the (Cu+Cr+Ni+Sn) content of the center in the concentration gradient measurement value when the (Cu+Cr+Ni+Sn) content of the polar surface is smaller than the (Cu+Cr+Ni+Sn) content of the center. Steel plate with an average grain size of 6–50 μm. In paragraph 1, The above steel plate is a cold rolled steel plate manufactured by cold rolling and annealing heat treatment of a hot rolled steel plate obtained by hot rolling. A steel sheet characterized in that the value (I/II) obtained by dividing the average grain size (I) of the cold rolled steel sheet by the average grain size (II) of the hot rolled steel sheet satisfies 0.60 to 0.95. In paragraph 1, The above steel plate is a steel plate whose microstructure is a single phase ferrite. In paragraph 1, The above steel plate is a steel plate having a product of tensile strength (TS) and elongation (El) (TS × El) of 4.0 to 30.0 GPa·%. In paragraph 1, The above steel plate is a steel plate having a product of the Lankford value (r-value) and the elongation (r-value × El) of 20 to 200%. In paragraph 1, The above steel plate is a steel plate including a plating layer formed on at least one surface. In paragraph 6, A steel plate having a powdering peeling width of the above plating layer of less than 7 mm. A step for preparing a steel slab including, in wt%, carbon (C): more than 0% to 0.0700%, silicon (Si): more than 0% to 0.800%, manganese (Mn): more than 0% to 1.000%, aluminum (Al): more than 0% to 0.500%, phosphorus (P): 0% to 0.080%, sulfur (S): 0% to 0.0500%, nitrogen (N): 0% to 0.0300%, copper (Cu): more than 0% to 1.000%, nickel (Ni): more than 0% to 1.000%, chromium (Cr): more than 0% to 1.000%, tin (Sn): more than 0% to 0.5000%, and at least one selected from the following groups (i) to (vi), the remainder being Fe and unavoidable impurities; (i) Molybdenum (Mo): 0~1.000%, (ii) At least one of titanium (Ti): 0 to 0.500%, niobium (Nb): 0 to 0.500%, vanadium (V): 0 to 0.500%, and boron (B): 0 to 0.0200%; (iii) Magnesium (Mg): 0 to 0.050%, Calcium (Ca): 0 to 0.050%, and Rare Earth Elements (REM) excluding Yttrium (Y): 0 to 0.050%, at least one of these; (iv) Tungsten (W): 0 to 0.50% and zirconium (Zr): 0 to 0.50%, at least one of these; (v) Antimony (Sb): 0~0.5000% and cobalt (Co): 0~0.5000%, at least one of these; (vi) At least one of yttrium (Y): 0 to 0.200% and hafnium (Hf): 0 to 0.200%; A step of heating the above steel slab in a temperature range of 900 to 1300℃; A step of obtaining a hot rolled steel sheet by finish rolling the above heated steel slab in the austenite range above the Ar3 transformation point or in the ferrite range below the Ar3 transformation point; A step of coiling the above hot-rolled steel plate at 700℃ or higher; A step of cooling the above-mentioned coiled hot-rolled steel sheet to a temperature range of 250 to 350°C at a rate of 10°C/min or less; A step of obtaining a cold rolled steel sheet by cold rolling the hot rolled steel sheet after the cooling at a cold reduction ratio of 50% or more; and A method for manufacturing a steel plate, comprising a step of annealing the cold rolled steel plate at 600°C or higher for 10 seconds or longer. In Article 8, A method for manufacturing a steel sheet, characterized in that the value (I/II) obtained by dividing the average grain size (I) of the cold rolled steel sheet after the annealing treatment by the average grain size (II) of the hot rolled steel sheet after the coiling satisfies 0.60 to 0.95. In Article 8, The above steel slab is a method for manufacturing a steel plate by continuously casting molten steel obtained through a steelmaking process in an electric furnace. In Article 8, The above annealing process step is: A method for manufacturing a steel plate, comprising heating the cold rolled steel plate to an annealing temperature at a heating rate of 2 to 60°C/s. In Article 8, It further includes a step of plating the cold rolled steel sheet after the above annealing treatment. The above plating treatment is a method for manufacturing a steel plate that is a hot-dip plating or electroplating process. In Article 12, The above-mentioned method for manufacturing a steel sheet is a method for manufacturing a steel sheet by performing a molten zinc plating process at a temperature range of 440 to 520°C. In Article 13, A method for manufacturing a steel sheet further comprising a step of performing alloying heat treatment after the above molten zinc plating. In a steel plate manufactured by any one of the manufacturing methods of Articles 8 to 14, A steel sheet having a value of 0.60 to 0.95, obtained by dividing the average grain size (I) of the cold rolled steel sheet after the above annealing treatment by the average grain size (II) of the hot rolled steel sheet. In Article 15, A steel plate comprising a plating layer formed on at least one surface of the steel plate. In Article 16, A steel plate having a powdering peeling width of the above plating layer of less than 7 mm.