Steel sheet and manufacturing method thereof

Abstract

One aspect of the present invention provides a steel sheet having excellent bendability and a manufacturing method thereof.

Description

Steel plate and method for manufacturing the same

[0001] The present invention relates to a steel plate and a method for manufacturing the same.

[0002] In recent years, automotive safety regulations have been strengthened to ensure safety in the event of a collision. Consequently, attempts have been made to increase the strength and / or thickness of automotive steel plates. However, there is a growing demand for lighter vehicle bodies to address environmental concerns and improve fuel efficiency.

[0003] Therefore, to simultaneously ensure crashworthiness and lightweight vehicles, high-strength steel plates are essential. Common methods for strengthening steel include solid solution strengthening, precipitation strengthening, grain refinement strengthening, and transformation strengthening.

[0004] Among these, high-strength steel using precipitation strengthening is a technology that secures strength by strengthening the steel sheet by precipitating carbon and nitride by adding carbon and nitride forming elements such as Cu, Nb, Ti, and V, or by refining the crystal grains through suppression of crystal grain growth by fine precipitates.

[0005] This precipitation strengthening technology can easily achieve high strength at low manufacturing costs, but it has the disadvantage that the recrystallization temperature rises rapidly due to fine precipitates, so high-temperature annealing is required to secure ductility through sufficient recrystallization. In addition, precipitation strengthening steel, which strengthens the ferrite matrix by precipitating carbon and nitrides, has the problem of increased cost due to the excessive addition of precipitated elements to obtain high-strength steels of the 600 MPa class or higher.

[0006] Meanwhile, various types of steel have been developed using transformation strengthening, such as ferrite-martensite dual phase steel containing hard martensite in a ferrite matrix structure, TRIP (Transformation Induced Plasticity) steel utilizing transformation-induced plasticity of retained austenite, and CP (Complexed Phase) steel composed of ferrite and hard bainite (or martensite) structures.

[0007] As a prior art for such high-strength steel plates, Patent Document 1 discloses a cold-rolled steel plate having a composite structure including ferrite, bainite, martensite, and retained austenite. However, Patent Document 1 has limitations in that, due to the excessive addition of Si, dents may occur in the furnace during continuous annealing, and liquid metal embrittlement may occur during spot welding.

[0008] (Patent Document 1) Korean Patent Publication No. 10-2019-0076258

[0009] One aspect of the present invention is to provide a steel plate having excellent bendability and a method for manufacturing the same.

[0010] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0011] One aspect of the present invention is to provide a steel plate. The above steel plate contains, in wt%, carbon (C): 0.05 to 0.18%, silicon (Si): 0.1% or less, manganese (Mn): 1.0 to 2.3%, aluminum (sol.Al): 1.0% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the remainder iron (Fe) and other unavoidable impurities, and has a T value defined in the following relational expression 1 of 1380 to 1648, and the microstructure may include, in area%, 65 to 85% ferrite, 10% or less bainite, 6 to 20% fresh martensite and 5% or less retained austenite.

[0012] [Relationship 1]

[0013] T = 279×[C] + 711×[Mn] + 474×[Nb] + 177×[Ti] - 75×[Cr]

[0014] (In the above relational expression 1, [C], [Mn], [Nb], [Ti] and [Cr] represent the content (weight %) of each element.)

[0015] In addition, in the steel plate described above, the steel plate may have an RT value defined in the following relational expression 2 of 0.01 or more.

[0016] [Relationship 2]

[0017] RT = [Si] + [Nb] + [Ti]

[0018] (In the above equation 2, [Si], [Nb], and [Ti] represent the content (weight%) of each element.)

[0019] In addition, in one of the aforementioned steel plates, the steel plate may have a tensile strength of 590 MPa or more and an elongation of 20% or more.

[0020] In addition, in one of the aforementioned steel plates, the steel plate can satisfy the following relationship 3.

[0021] [Relationship 3]

[0022] A / TH ≥ 60 (° / mm)

[0023] (In the above relational expression 3, A represents the bending angle (°) at which no crack occurs in the bending section during a 180° bending test, and TH represents the thickness of the steel plate (mm).)

[0024] Additionally, in one of the aforementioned steel plates, the steel plate may further include a zinc-plated layer or an alloyed zinc-plated layer on the surface.

[0025] Another aspect of the present invention provides a method for manufacturing a steel plate. The method comprises the steps of: reheating a steel slab containing, in wt%, carbon (C): 0.05 to 0.18%, silicon (Si): 0.1% or less, manganese (Mn): 1.0 to 2.3%, aluminum (sol.Al): 1.0% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the remainder iron (Fe) and other unavoidable impurities, and having a T value defined in the following equation 1 of 1380 to 1648; hot-rolling the reheated steel slab to obtain a hot-rolled steel plate; coiling the hot-rolled steel plate; A method for producing a cold-rolled steel sheet, comprising: a step of cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; a step of heating the cold-rolled steel sheet to an annealing temperature of 800 to 850°C, cooling it to a cooling end temperature of 450 to 600°C, and then maintaining the temperature at a constant temperature to continuously anneale it; and a step of cooling the continuously annealed steel sheet to room temperature. The R value defined in the following equation 4 may be 1,543 to 1,590.

[0026] [Relationship 1]

[0027] T = 279×[C] + 711×[Mn] + 474×[Nb] + 177×[Ti] - 75×[Cr]

[0028] (In the above relational expression 1, [C], [Mn], [Nb], [Ti] and [Cr] represent the weight% of each element.)

[0029] [Relationship 4]

[0030] R = 174×[C] + 680×[Mn] + 370×[Nb] + 177×[Ti] - 86×[Cr] + 0.33×[T1] - 0.05×[T2]

[0031] (In the above relational expression 4, [C], [Mn], [Nb], [Ti] and [Cr] represent the content (weight %) of each element, and [T1] and [T2] represent the annealing temperature (℃) and the cooling end temperature (℃) in the continuous annealing step, respectively.)

[0032] In addition, in the method described above, in the reheating step, the steel slab may have an RT value defined in the following relational expression 2 of 0.01 or more.

[0033] [Relationship 2]

[0034] RT = [Si] + [Nb] + [Ti]

[0035] (In the above equation 2, [Si], [Nb], and [Ti] represent the content (weight%) of each element.)

[0036] Additionally, in one of the aforementioned methods, the reheating step may be performed at 1,100 to 1,300°C, and the step of obtaining the hot-rolled steel sheet may be performed at a finishing rolling temperature of 800 to 950°C.

[0037] In addition, in one of the above-described methods, in the coiling step, the hot-rolled steel sheet can be coiled at 400 to 700°C and then cooled to room temperature at an average cooling rate of 0.1°C / s or less.

[0038] Additionally, in one of the aforementioned methods, the step of obtaining the cold rolled steel sheet can be performed at a reduction ratio of 40 to 70%.

[0039] In addition, in one of the above-described methods, in the continuous annealing step, cooling can be performed at an average cooling rate of 20°C / s or less to the cooling end temperature.

[0040] In addition, in one of the above-described methods, in the continuous annealing step, the cold-rolled steel sheet can be maintained at a constant temperature for 50 seconds or more at the cooling end temperature.

[0041] In addition, in one of the above-described methods, after the continuous annealing step, a step of zinc plating the continuously annealed steel sheet at 430 to 490°C may be further included.

[0042] In addition, in one of the above-described methods, after the zinc plating step, a step of alloying the plated steel sheet at 460 to 530°C may be further included.

[0043] According to the present invention, a steel plate having excellent bendability and a method for manufacturing the same can be provided.

[0044] In addition, according to the present invention, the steel plate has improved processability, can be used in complex shapes during press forming, and can be appropriately utilized for automobile structural members.

[0045] 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 course of explaining specific embodiments of the present invention.

[0046] In order to more fully understand the drawings cited in the detailed description of the present invention, a brief description of each drawing is provided.

[0047] Figure 1 is a photograph of the microstructure of Invention Example 13 according to one embodiment of the present invention observed using an electron microscope.

[0048] Figure 2 is a photograph of the microstructure of Comparative Example 6 according to one embodiment of the present invention observed using an electron microscope.

[0049] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified in various other forms, and the terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include plural forms, unless the relevant definition clearly indicates a contrary meaning.

[0050] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0051] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0052] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0053] First, the alloy composition of the steel plate according to one embodiment of the present invention will be described. The content of the alloy composition refers to weight percent unless otherwise specified.

[0054] Carbon (C): 0.05 to 0.18%

[0055] Carbon (C) is an element added to strengthen the transformation structure, promote high strength, and promote the formation of martensite in composite phase steel. When the C content is less than 0.05%, it may be difficult to secure the desired level of strength. For example, as the C content increases, the fraction of martensite in the steel may increase. When the C content exceeds 0.18%, the strength of martensite increases, but the strength difference with ferrite, which has a low carbon concentration, may increase. In this case, due to the strength difference, fracture easily occurs at the interphase interface when stress is applied, which may reduce bendability. In addition, when the C content exceeds 0.18%, weldability is poor, and welding defects may occur during component processing. That is, the C content may be 0.05 to 0.18%, more specifically, 0.06 to 0.17%, and even more specifically, 0.07 to 0.15%.

[0056] Silicon (Si): 0.1% or less

[0057] Silicon (Si) is a ferrite stabilizing element that can contribute to the formation of martensite by promoting ferrite transformation and encouraging C enrichment into untransformed austenite. In addition, since Si has excellent solid solution strengthening ability, it can increase the strength of ferrite, thereby reducing the hardness difference between phases, and can play a role in securing strength without reducing the ductility of the steel sheet. If the Si content exceeds 0.1%, it may cause surface scale defects, which may deteriorate the plating surface quality, or cause liquid metal embrittlement during welding of the plating material. That is, the Si content may be 0.1% or less, specifically 0.10% or less, more specifically 0.08% or less, and even more specifically 0.05% or less. In addition, there is no need to specifically limit the lower limit of the Si content, but as a non-limiting example, the Si content may be greater than 0%, may be 0.001% or more, or may be 0.002% or more.

[0058] Manganese (Mn): 1.0 to 2.3%

[0059] Manganese (Mn) refines grains without reducing ductility, prevents hot embrittlement caused by the formation of FeS by precipitating S in steel as MnS, and strengthens the steel. In addition, Mn can play a role in forming martensite more easily by reducing the critical cooling rate at which martensite is obtained in composite phase steel. If the Mn content is less than 1.0%, it may be difficult to secure the target strength. If the Mn content exceeds 2.3%, there may be problems such as reduced weldability and hot rollability, excessive formation of martensite, deterioration of the material (e.g., bendability and elongation), or increased risk of processing cracks and plate fracture due to the formation of Mn-Bands (bands of Mn oxide) within the structure. In addition, if the Mn content exceeds 2.3%, there may be a problem that Mn oxide is dissolved on the surface during annealing, which significantly reduces the plating property. That is, the content of Mn may be 1.0 to 2.3%, more specifically 1.3 to 2.2%, and even more specifically 1.6 to 2.0%.

[0060] Aluminum (sol.Al): 1.0% or less

[0061] Aluminum (Al) is added to refine grain size and deoxidize steel, and as a ferrite stabilizing element, it can distribute C in ferrite into austenite to improve martensitic hardenability. In addition, Al can improve the ductility of the steel sheet by suppressing the precipitation of carbides in bainite when maintaining a constant temperature in the bainite transformation region. When the Al content exceeds 1.0%, it is advantageous for increasing the strength through the grain refinement effect, but it may cause problems such as excessive formation of inclusions during steelmaking and continuous casting, which may cause surface defects in the plated steel sheet or increase the manufacturing cost. That is, the Al content may be 1.0% or less, more specifically, 0.8% or less, and even more specifically, 0.5% or less. In addition, there is no need to specifically limit the lower limit of the Al content, but as a non-limiting example, the Al content may exceed 0%, may be 0.01% or more, or may be 0.02% or more.

[0062] Chromium (Cr): 0.1 to 1.0%

[0063] Chromium (Cr) can be included to improve the hardenability of steel and secure high strength. In addition, Cr is an element that plays an important role in the formation of martensite, and can be advantageous in the production of composite phase steel with high ductility by minimizing the decrease in elongation compared to the increase in strength. Specifically, during the hot rolling process, Cr 23 Cr-based carbides, such as C6, are formed. Some of these carbides dissolve during the annealing process, while others remain undissolved. Accordingly, Cr can control the content of dissolved C in martensite after cooling to an appropriate level, thereby suppressing the occurrence of yield point elongation (YP-El), which can be advantageous for the production of composite phase steels with low yield ratios.

[0064] When the Cr content is less than 0.1%, the above-described effect cannot be sufficiently obtained. When the Cr content exceeds 1.0%, not only the above-described effect is saturated, but there may be a problem of deterioration in cold rolling properties due to excessive increase in hot rolling strength. In addition, when the Cr content exceeds 1.0%, the fraction of Cr-based carbides increases and coarsens, which may coarsen the martensite size after annealing, resulting in a decrease in elongation. That is, the Cr content may be 0.1 to 1.0%, more specifically 0.1 to 0.9%, and even more specifically 0.2 to 0.8%.

[0065] Niobium (Nb): 0.05% or less

[0066] Niobium (Nb) can segregate at austenite grain boundaries, suppress coarsening of austenite grains during annealing heat treatment, and contribute to increasing strength by forming fine carbides. When the Nb content exceeds 0.05%, coarse carbides may precipitate, and the strength and elongation may decrease due to a decrease in the carbon content in the steel, and the manufacturing cost may increase. That is, the Nb content may be 0.05% or less, specifically 0.050% or less, more specifically 0.045%, and even more specifically 0.040% or less. In addition, there is no need to specifically limit the lower limit of the Nb content, but as a non-limiting example, the Nb content may exceed 0%, may be 0.001% or more, or may be 0.002% or more.

[0067] Titanium (Ti): 0.05% or less

[0068] Titanium (Ti) is an element that forms fine carbides, which can contribute to securing yield strength and tensile strength. In addition, Ti is an element that forms nitrides, which can suppress the precipitation of AlN by precipitating N in the steel as TiN, thereby reducing the risk of cracks occurring during casting. If the Ti content exceeds 0.05%, coarse carbides may be precipitated, and the strength and elongation may decrease due to a decrease in the carbon content in the steel, and nozzle clogging may occur during casting. That is, the Ti content may be 0.05% or less, more specifically, 0.04% or less, and even more specifically, 0.03% or less. In addition, there is no need to specifically limit the lower limit of the Ti content, but as a non-limiting example, the Ti content may be more than 0%, may be 0.001% or more, or may be 0.002% or more.

[0069] Phosphorus (P): 0.1% or less

[0070] Phosphorus (P) is a substitutional element with a solid solution strengthening effect, which can improve in-plane anisotropy and secure strength without significantly reducing formability. When the P content exceeds 0.1%, the possibility of brittle fracture increases excessively, which can cause slab breakage during hot rolling or deteriorate the surface properties of the plating. That is, the P content may be 0.1% or less, specifically 0.10% or less, more specifically 0.08% or less, and even more specifically 0.05% or less. In addition, there is no need to specifically limit the lower limit of the P content, but as a non-limiting example, in consideration of the case where it is inevitably added, the P content may exceed 0%, and may be 0.001% or more.

[0071] Sulfur (S): 0.01% or less

[0072] Sulfur (S) is an impurity element that is inevitably added to steel, and can lower ductility and weldability. Therefore, lowering the S content can be effective in securing the properties of the steel sheet. If the S content exceeds 0.01%, ductility and weldability may be lowered, or in particular, the possibility of hot embrittlement may increase. That is, the S content may be 0.01% or less, specifically 0.010% or less, more specifically 0.008% or less, and even more specifically 0.005% or less. In addition, there is no need to specifically limit the lower limit of the S content, but as a non-limiting example, in consideration of the case where it is inevitably added, the S content may exceed 0%, or may be 0.001% or more.

[0073] Nitrogen (N): 0.01% or less

[0074] Nitrogen (N) can play an effective role in stabilizing austenite. When the N content exceeds 0.01%, there may be a problem that the refining cost of steel increases rapidly. In addition, when the N content exceeds 0.01%, the risk of cracks occurring due to AlN formation during casting may increase. That is, the N content may be 0.01% or less, specifically 0.010% or less, more specifically 0.008% or less, and even more specifically 0.005% or less. In addition, there is no need to specifically limit the lower limit of the N content, but as a non-limiting example, in consideration of the case where it is inevitably added, the N content may exceed 0%, and may be 0.001% or more.

[0075] A steel sheet according to one embodiment of the present invention may include the above components, as well as iron (Fe) as the remaining component. Furthermore, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during a typical manufacturing process, this cannot be ruled out. For example, the steel may further include elements that can be included in the steel in a total amount of up to 1.0%. Since these impurities are readily apparent to anyone skilled in the art, their full content is not specifically discussed herein.

[0076] According to one embodiment of the present invention, the steel plate may have a T value defined in the following relational expression 1 of 1380 or more and less than 1648.

[0077] [Relationship 1]

[0078] T = 279×[C] + 711×[Mn] + 474×[Nb] + 177×[Ti] - 75×[Cr]

[0079] (In the above relational expression 1, [C], [Mn], [Nb], [Ti] and [Cr] represent the content (weight %) of each element.)

[0080] The above equation 1 is a quantitative expression of how the alloying components contained in the steel plate can contribute to the strength and bendability of the steel plate. The contents of C, Mn, Nb, Ti, and Cr among the alloying components contained in the steel plate can be adjusted to satisfy the following equation 1. By adjusting the contents of the alloying components in this way, the strength and bendability of the steel plate can be secured at the target levels.

[0081] Specifically, C and Mn can increase the strength of steel plates through the effect of reinforcing the steel sheet. At this time, the contribution of each element to the strength of the steel plate is different, and the constant value multiplied by each component in the above relational expression 1 can relatively represent the degree to which each element contributes to the strength.

[0082] Nb and Ti can contribute to strength enhancement due to their precipitation strengthening effect. In addition, Nb and Ti precipitate into the ferrite matrix in DP steel, thereby strengthening the ferrite, and can reduce the interphase hardness difference between ferrite and martensite, thereby increasing the bendability of the steel sheet. Therefore, in the above relational expression 1, the constant value multiplied by the content of Nb and Ti can be expressed as a positive value.

[0083] Among the above elements, Cr has the least effect on strengthening and can significantly increase hardenability. However, if added in excessive amounts, a large amount of martensite may be generated, which may reduce bending properties. Therefore, the value of the constant multiplied by the content of Cr in the above equation 1 may be expressed as a negative value.

[0084] If the T value defined in the above relational expression 1 is less than 1380, there may be a problem that fresh martensite is excessively formed in the continuous annealing process, and the target strength and bendability characteristics of the steel sheet cannot be secured. There is no need to specifically limit the upper limit of the T value, but if the T value is 1648 or more, the alloying component is included in an excessive amount, and there may be a problem that the strength increases excessively or the bendability or elongation cannot reach the target level. That is, the T value may be 1380 or more and less than 1648, more specifically, may be 1380 to 1600, and even more specifically, may be 1380 to 1500.

[0085] A steel plate according to one embodiment of the present invention may have an RT value defined in the following relational expression 2 of 0.01 or more.

[0086] [Relationship 2]

[0087] RT = [Si] + [Nb] + [Ti]

[0088] (In the above equation 2, [Si], [Nb], and [Ti] represent the content (weight%) of each element.)

[0089] In general, when Si is added in large amounts, there may be problems such as dent defects in steel sheets in annealing furnaces, phosphate treatability of cold-rolled steel sheets, and poor liquid metal embrittlement and plating properties of plated steel sheets. The steel sheet according to one embodiment of the present invention has the characteristic that the above problems can be solved by minimizing the Si content. Since there is a possibility that mechanical properties may be deteriorated when the Si content is reduced, the contents of Nb and Ti are controlled in consideration of this, thereby preventing deterioration of mechanical properties through carbide precipitation. That is, by appropriately adjusting the RT value defined by the above relationship 2, the problems caused by excessive Si can be minimized, and the mechanical properties can be improved.

[0090] If the RT value defined in the above relational expression 2 is less than 0.01, the desired level of physical properties cannot be secured. That is, the RT value may be 0.01 or more, more specifically 0.02 or more, and even more specifically 0.03 or more. In addition, there is no need to specifically limit the upper limit of the RT value, but the upper limit of the RT value may be 0.2, which is the same as the maximum addition within the limited range of each alloying component.

[0091] Hereinafter, the microstructure of a steel sheet according to one embodiment of the present invention will be described. The microstructure fraction refers to area % unless otherwise specified. In addition, the microstructure fraction can be measured by analyzing the matrix structure at a point 1 / 4 the thickness of the steel sheet, and specifically, the microstructure fraction can be measured using FE-SEM, an image analyzer, and XRD.

[0092] For example, the microstructure of the steel plate may include 65 to 85% ferrite, 10% or less bainite, 6 to 20% fresh martensite, and 5% or less retained austenite.

[0093] Ferrite is a soft structure that can contribute to the ductility of steel sheets. If the area fraction of ferrite in the total microstructure of the steel sheet is less than 65%, it may be difficult to secure the target bendability. If the area fraction of ferrite exceeds 85%, it may be difficult to secure the target level of strength. In other words, the area fraction of ferrite may be 65 to 85%, more specifically 68 to 83%, and even more specifically 73 to 83%.

[0094] Bainite is a structure having an intermediate hardness between ferrite and martensite, and by including bainite in an area fraction within the above range, the ductility and hardness of the steel sheet can be adjusted to a target level. If the area fraction of bainite exceeds 10% of the entire microstructure included in the steel sheet, the strength may be reduced. That is, the area fraction of bainite may be 10% or less, more specifically 9% or less, and even more specifically 8% or less. In addition, there is no need to specifically limit the lower limit of the area fraction of bainite, but as a non-limiting example, the area fraction of bainite may be greater than 0%, may be 1% or more, or may be 2% or more.

[0095] Fresh martensite can contribute to increasing the strength of the steel sheet. If the area fraction of fresh martensite is less than 6% of the total microstructure contained in the steel sheet, the target strength cannot be secured. Furthermore, if the area fraction of fresh martensite exceeds 20%, the area fraction of bainite is relatively reduced, which may cause a problem of poor bendability. In other words, the area fraction of fresh martensite can be 6 to 20%, more specifically 8 to 18%, and even more specifically 9 to 17%.

[0096] Retained austenite may be generated in small amounts during the final cooling process. If the area fraction of retained austenite exceeds 5% of the total microstructure contained in the steel plate, the steel plate may be susceptible to liquid metal embrittlement during spot welding. That is, the area fraction of retained austenite may be 5% or less, more specifically 4% or less, and even more specifically 3% or less. In addition, there is no need to specifically limit the lower limit of the area fraction of retained austenite, but as a non-limiting example, the area fraction of retained austenite may be greater than 0%, may be 0.1% or more, or may be 0.3% or more.

[0097] For example, the steel plate may further include a zinc-plated layer or an alloyed zinc-plated layer on the surface. The zinc-plated layer and / or the alloyed zinc-plated layer may be provided on at least one surface of the steel plate to improve the corrosion resistance of the steel plate.

[0098] The zinc plating layer may be composed of any one selected from zinc and a zinc-based alloy. For example, when composed of a zinc-based alloy, the plating layer may include at least one selected from aluminum (Al), magnesium (Mg), nickel (Ni), and iron (Fe), with the remainder being zinc (Zn). However, the composition of the zinc plating layer is not limited thereto, and may have a composition commonly used in the relevant technical field.

[0099] The zinc plating layer may be formed by hot-dip galvanizing at least one surface of the steel plate. However, the plating method is not limited thereto, and methods commonly used in the relevant technical field, such as electrolytic plating, vacuum deposition plating, and cladding, may be applied.

[0100] The alloyed zinc plating layer is obtained by heat diffusion into the zinc plating layer and can be provided on one or both sides of the steel sheet. Specifically, the alloyed zinc plating layer can be obtained by heat treating the zinc plating layer so that some of the alloy components of the zinc plating layer and the steel sheet are alloyed.

[0101] According to one embodiment of the present invention, a steel sheet can secure sufficient strength and bendability of the steel sheet by controlling the components so that the T value defined in relational expression 1 is 1380 or more and less than 1648 by controlling the content of the alloy components and the RT value defined in relational expression 2 is 0.01 or more. In addition, according to one embodiment of the present invention, by controlling the type and area fraction of the microstructure, the ductility and hardness of the steel sheet can be secured at a target level, and the bendability of the steel sheet can be improved.

[0102] For example, the steel plate may have a tensile strength (TS) of 590 MPa or greater and an elongation (El) of 20% or greater. Additionally / alternatively, the steel plate may satisfy the following relationship 3. That is, the steel plate can secure excellent strength and bendability properties by controlling the alloy composition and microstructure. In addition, the steel plate with excellent strength and bendability has improved workability, can be used in complex shapes during press forming, and can be appropriately utilized for automotive structural members.

[0103] [Relationship 3]

[0104] A / TH ≥ 60 (° / mm)

[0105] (In the above relational expression 3, A represents the bending angle (°) at which no crack occurs in the bending section during a 180° bending test, and TH represents the thickness of the steel plate (mm).)

[0106] Hereinafter, a method for manufacturing a steel plate according to one embodiment of the present invention will be described in detail. However, this does not necessarily mean that the steel plate according to the present invention must be manufactured using the following manufacturing method.

[0107] [Slab reheating stage]

[0108] A steel slab having the aforementioned composition can be reheated. Reheating facilitates the subsequent rolling process and ensures sufficient physical properties of the steel plate. At this time, the composition of the steel slab is identical to that of the aforementioned steel plate, and the reasons for adding and limiting the content of each component in the slab are equally applicable to the explanations for the aforementioned steel plate.

[0109] For example, the reheating step may be performed at 1,100 to 1,300°C. When the reheating temperature is lower than 1,100°C, the re-solution of precipitated elements such as Nb and Ti may decrease, thereby reducing the addition effect of the elements. In addition, when the reheating temperature exceeds 1,300°C, the process cost may increase, or a large amount of hot-rolled oxide may be generated, which may deteriorate the surface quality of the steel sheet. That is, the reheating temperature may be 1,100 to 1,300°C, more specifically 1,120 to 1,250°C, and even more specifically 1,150 to 1,240°C. However, the reheating temperature is not limited thereto, and typical reheating conditions may be applied within a range that does not impair the effects of the present invention.

[0110] [Hot-rolled steel sheet production stage]

[0111] The slab reheated in the aforementioned step can be hot-rolled at a conventional hot-rolling temperature to obtain a hot-rolled steel sheet. Hot rolling produces a hot-rolled steel sheet with finely dispersed carbides, which can serve as austenite nucleation sites. By evenly dispersing the fine carbides during the hot rolling process, the austenite produced during the annealing process, when the carbides are dissolved, can be finely dispersed. Consequently, the martensite produced during cooling after annealing can be finely and uniformly dispersed, contributing to improved strength and elongation of the final steel sheet.

[0112] For example, the step of obtaining a hot-rolled steel sheet may be performed at a finishing rolling temperature of 800 to 950°C. If the finishing rolling temperature is less than 800°C, there may be a problem that the hot rolling temperature is low and the hot rolling load increases. In addition, if the finishing rolling temperature exceeds 950°C, the grains become coarser, which reduces the strength of the steel sheet, and hot-rolled oxides increase on the surface layer, which may cause the surface quality of the steel sheet to deteriorate. That is, the finishing rolling temperature may be 800 to 950°C, more specifically 830 to 940°C, and even more specifically 850 to 920°C.

[0113] [Winding stage]

[0114] The hot-rolled steel sheet obtained in the above-described step can be coiled. For example, the coiling temperature may be 400 to 700°C. If the coiling temperature is lower than 400°C, a large amount of low-temperature transformation structures such as martensite or bainite are generated, which excessively increases the strength of the hot-rolled steel sheet, which may cause a problem of rolling load occurring in the cold rolling process. In addition, if the coiling temperature exceeds 700°C, the microstructure becomes coarse, which reduces the strength of the final annealed steel sheet, and the surface quality and plating properties of the steel sheet may deteriorate due to an increase in oxides on the surface of the steel sheet. That is, the coiling temperature may be 400 to 700°C, more specifically 450 to 680°C, and even more specifically 500 to 650°C.

[0115] For example, in the coiling step, after coiling the hot-rolled steel sheet, it can be cooled to room temperature at an average cooling rate of 0.1°C / s or less. If the average cooling rate exceeds 0.1°C / s, excessive formation of low-temperature transformation structures may cause rolling loads in the cold rolling process. In addition, if the average cooling rate exceeds 0.1°C / s, the shape of the hot-rolled steel sheet may be deformed due to the rapid cooling rate, and plate breakage may occur in the cold rolling process. That is, the average cooling rate may be 0.1°C / s or less, specifically 0.10°C / s or less, more specifically 0.08°C / s or less, and even more specifically 0.07°C / s or less. Additionally, there is no need to specifically limit the lower limit of the average cooling rate, but as a non-limiting example, the average cooling rate may be greater than 0°C / s, may be greater than or equal to 0.01°C / s, or may be greater than or equal to 0.02°C / s.

[0116] [Cold-rolled steel sheet production stage]

[0117] The hot rolled steel sheet coiled in the above-described step can be cold rolled to obtain a cold rolled steel sheet. For example, cold rolling can be performed at a reduction ratio of 40 to 70%. If the reduction ratio is less than 40%, it is difficult to secure the target thickness, and shape correction of the steel sheet may be difficult. In addition, if the reduction ratio exceeds 70%, the possibility of cracks occurring at the edge of the steel sheet increases, or there may be a problem involving a rolling load during the cold rolling process. That is, the reduction ratio can be 40 to 70%, more specifically 45 to 65%, and even more specifically 45 to 60%.

[0118] For example, prior to cold rolling, a pickling process may further include a pickling process for the coiled hot-rolled steel sheet. Pickling can remove the oxide layer formed on the surface of the steel sheet. The pickling process is not particularly limited, and any method commonly used in the art (e.g., immersing the steel sheet in a hydrochloric acid bath) can be applied.

[0119] [Continuous annealing stage]

[0120] The cold-rolled steel sheet obtained in the aforementioned step can be heated to an annealing temperature (T1) within a predetermined range, cooled to a cooling end temperature (T2) within a predetermined range, and then continuously annealed while maintaining a constant temperature. Continuous annealing can be performed to form ferrite and austenite and distribute carbon simultaneously with recrystallization.

[0121] In the continuous annealing step, the annealing temperature (T1) may be 800 to 850°C. By controlling the annealing temperature (T1) within the above range, the fractions of ideal austenite and ferrite in the steel sheet are determined, and the strength of the annealed steel sheet can be differently controlled according to the determined fractions. In general, as the fraction of ideal austenite increases, the strength of the annealed steel sheet can increase; however, since the subsequent process also affects the microstructure, the properties of the final manufactured steel sheet may vary.

[0122] When the annealing temperature (T1) is below 800℃, not only is sufficient recrystallization not achieved, but it is also difficult to form sufficient ideal austenite, making it difficult to secure the desired martensite and bainite fractions after annealing. Furthermore, when the annealing temperature (T1) is below 800℃, a high fraction of ferrite is formed, making it difficult to secure the desired mechanical properties (e.g., strength, elongation, bendability, etc.).

[0123] When the annealing temperature (T1) exceeds 850℃, productivity may decrease, or the yield strength may increase and ductility may decrease because excessive austenite is formed, which greatly increases the fraction of bainite and martensite after cooling. In addition, when the annealing temperature (T1) exceeds 850℃, the surface thickening due to elements that reduce the wettability of the hot-dip galvanizing (e.g., Si, Mn, or B) may become severe, which may deteriorate the plating surface quality. That is, the annealing temperature (T1) during continuous annealing may be 800 to 850℃, more specifically 810 to 840℃, and even more specifically 810 to 830℃.

[0124] In the continuous annealing step, the cooling end temperature (T2) may be 450 to 600°C. Through cooling of the heated steel sheet, the austenite in the ideal region within the steel sheet may be transformed into ferrite. At this time, by controlling the cooling end temperature (T2) within the above range, the fraction of ferrite transformed from the austenite in the ideal region may be controlled differently.

[0125] During continuous annealing, if the cooling end temperature (T2) is less than 450°C, the fraction of bainite may increase excessively and the formation of martensite may decrease during the constant temperature holding process, which may cause a problem of reduced strength. In addition, if the cooling end temperature (T2) exceeds 600°C, a large amount of ferrite transformation may occur, which may cause a problem of reduced strength. That is, the cooling end temperature (T2) may be 450 to 600°C, more specifically 470 to 580°C, and even more specifically 480 to 570°C.

[0126] In the continuous annealing step, cooling can be performed at an average cooling rate of 20°C / s or less to the cooling end temperature (T2). If the average cooling rate exceeds 20°C / s, the material deviation within the steel sheet may increase due to cooling unevenness, and the problem of the sheet shape becoming inferior may occur. That is, the average cooling rate may be 20°C / s or less, more specifically, 18°C / s or less, and even more specifically, 16°C / s or less. In addition, there is no need to specifically limit the lower limit of the average cooling rate, but as a non-limiting example, the average cooling rate may exceed 0°C / s, may be 1°C / s or more, or may be 3°C / s or more.

[0127] In the continuous annealing step, after cooling to the cooling end temperature (T2), the cold-rolled steel sheet may be maintained at a constant temperature for 50 seconds or longer at the cooling end temperature (T2). If the constant temperature holding time is less than 50 seconds, martensite may be excessively generated, resulting in reduced bendability. That is, the constant temperature holding time may be 50 seconds or longer, more specifically, 60 seconds or longer, and even more specifically, 70 seconds or longer. In addition, there is no need to specifically limit the upper limit of the constant temperature holding time, but as a non-limiting example, the upper limit of the constant temperature holding time may be 500 seconds or 400 seconds.

[0128] In the continuous annealing step, the R value defined in the following relational expression 4 may be 1,543 to 1,590.

[0129] [Relationship 4]

[0130] R = 174×[C] + 680×[Mn] + 370×[Nb] + 177×[Ti] - 86×[Cr] + 0.33×[T1] - 0.05×[T2]

[0131] (In the above relational expression 4, [C], [Mn], [Nb], [Ti] and [Cr] represent the content (weight %) of each element, and [T1] and [T2] represent the annealing temperature (℃) and the cooling end temperature (℃) in the continuous annealing step, respectively.)

[0132] The above equation 4 is a quantitative expression that simultaneously secures strength and bendability by controlling the content of alloy components of the steel slab and the annealing conditions in the continuous annealing step. Specifically, by optimizing the content of C, Mn, Nb, Ti, and Cr components and the annealing temperature (T1) and cooling end temperature (T2) conditions in the continuous annealing step, the strength and bendability can be secured at the target levels.

[0133] Specifically, in Equation 4, T1 represents the annealing temperature in the continuous annealing step. The fractions of ideal austenite and ferrite in the steel sheet are determined by T1, and the strength of the annealed steel sheet can be controlled differently depending on the determined fraction. In general, as the fraction of ideal austenite increases, the strength of the annealed steel sheet can increase; however, since the subsequent process also affects the microstructure, the properties of the final manufactured steel sheet may vary.

[0134] In Equation 4, T2 represents the cooling end temperature during the continuous annealing step. In the subsequent cooling process, the austenite in the ideal region can undergo further transformation into ferrite depending on T2. ​​Therefore, the ferrite fraction can be controlled differently, thereby affecting the physical properties of the steel sheet. Furthermore, the fractions of bainite, retained austenite, and martensite, which are the microstructures of the annealed steel sheet, can be controlled differently depending on T2.

[0135] For example, if T2 is higher than the bainite transformation start temperature or lower than the martensite transformation start temperature, bainite cannot be introduced into the structure of the steel plate. In other words, T2 must be set to a temperature between the bainite transformation start temperature and the martensite transformation start temperature.

[0136] T1 and T2, along with the alloying components of the steel plate, can affect the microstructure of the annealed steel plate, ultimately influencing its physical properties. Therefore, to secure the desired properties, the optimized relationship (Equation 4) must be satisfied. This allows for the production of high-strength steel plates with excellent bending properties while minimizing the Si content within the steel slab and still achieving the desired strength.

[0137] If the R value defined in the above relational expression 4 is less than 1,543, it is difficult to secure the microstructure at the target level, such as excessive formation of ferrite or excessive formation of martensite, and the strength of the steel plate may be reduced. In addition, if the R value exceeds 1,590, it is difficult to secure the microstructure at the target level, such as trace formation of ferrite or excessive formation of bainite, and it may be difficult to secure the bendability of the steel plate. That is, the R value may be 1,543 to 1,590, more specifically, 1,543 to 1,580, and even more specifically, 1,543 to 1,578.

[0138] For example, after the continuous annealing step, the continuously annealed steel sheet can be galvanized. Through the galvanizing step, a galvanized layer can be formed on the surface of the steel sheet. For example, the galvanizing step can be performed by immersing the continuously annealed steel sheet in a molten zinc plating bath. However, this is not a limitation, and the galvanizing can be performed under typical conditions applicable in the relevant technical field.

[0139] The zinc plating step may be performed at a temperature of 430 to 490°C. If the plating temperature is lower than 430°C, sufficient wettability may not be secured at the contact interface between the steel sheet and the zinc. In addition, if the plating temperature exceeds 490°C, excessive reaction between the steel sheet and the zinc may occur, forming an Fe-Zn alloy phase at the interface, thereby reducing the adhesion of the plating. That is, the plating temperature may be 430 to 490°C, more specifically 440 to 480°C, and even more specifically 450 to 470°C.

[0140] For example, after the zinc plating step, a step of alloying heat treating the plated steel sheet may be further included. Through the alloying heat treating step, some of the alloy components of the steel sheet are diffused into the zinc plating layer, thereby forming an alloyed zinc plating layer on the surface of the steel sheet.

[0141] The alloying heat treatment step may be performed at 460 to 530°C. If the alloying heat treatment temperature is lower than 460°C, the diffusion amount of Fe is small, resulting in insufficient alloying, and thus deterioration of the plating properties. In addition, if the alloying heat treatment temperature exceeds 530°C, excessive alloying may cause powdering problems. That is, the alloying temperature may be 460 to 530°C, more specifically 470 to 520°C, and even more specifically 480 to 520°C.

[0142] [Cooling stage]

[0143] In the aforementioned step, the continuously annealed steel sheet can be cooled to room temperature. Additionally / alternatively, if a zinc plating step is additionally performed, the zinc-plated steel sheet can be cooled to room temperature. Additionally / alternatively, if an alloying heat treatment step is additionally performed, the alloying heat-treated steel sheet can be cooled to room temperature. In the cooling step, the cooling conditions are not particularly limited, and methods commonly used in the art (e.g., air cooling) can be applied.

[0144] A method for manufacturing a steel sheet according to one embodiment of the present invention can appropriately control the type and area fraction of the steel sheet's microstructure by adjusting the content of the alloying components and the conditions during the continuous annealing step. Accordingly, the ductility and hardness of the steel sheet can be secured at target levels, and the bendability of the steel sheet can be improved.

[0145] For example, a steel sheet manufactured using the aforementioned manufacturing method may have a tensile strength (TS) of 590 MPa or more and an elongation (El) of 20% or more. Additionally / alternatively, the steel sheet may satisfy the following relationship 3. That is, the steel sheet may secure excellent strength and bendability by controlling the alloy composition and annealing conditions. In addition, a steel sheet having excellent strength and bendability has improved workability, can be used in complex shapes during press forming, and can be appropriately utilized for automotive structural members.

[0146] [Relationship 3]

[0147] A / TH ≥ 60 (° / mm)

[0148] (In the above relational expression 3, A represents the bending angle (°) at which no crack occurs in the bending section during a 180° bending test, and TH represents the thickness of the steel plate (mm).)

[0149] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0150] (Example)

[0151] After manufacturing a steel slab having the composition (remaining components of the alloy composition: Fe and unavoidable impurities) described in Table 1 below, it was reheated under the conditions described in Table 2 below, and finish hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was coiled under the conditions described in Table 2 below, cooled to room temperature, pickled, and cold-rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet having a thickness of 1.2 mm. Thereafter, the cold-rolled steel sheet was heated to an annealing temperature (T1) as described in Table 2 below, cooled to a cooling end temperature (T2), and then maintained at a constant temperature for 50 seconds or more to obtain a steel sheet. Finally, the steel sheet was hot-dip galvanized at a temperature of 460°C (the composition of the plating bath was Al: 0.2%, Mg: 1%, and Zn: the balance) and cooled to room temperature.

[0152] Steel alloy composition (weight %) Relationship 1 Relationship 2 CSiMnSol.AlNbTiCrPSNA0.130.011.90.040.0200.20.0080.0020.00413820.03B0.070.0220.040.0200.50.0070.0020.00414140.04C0.1501.90.04000.20.0060.0020.00413780D0.150.031.90.0400.020 .20.0080.0020.00413810.05E0.110.031.90.040.0400.20.0080.0020.00413860.07F0.1301.90.040.040.020.20.0070.0020.00413950.06G0.150.082.250.040.040.020.20.0070.0020.00316490.14[Relationship 1]T = 279×[C] + 711×[Mn] + 474×[Nb] + 177×[Ti] - 75×[Cr](In the above equation 1, [C], [Mn], [Nb], [Ti] and [Cr] represent the content (weight %) of each element.)[Equation 2]RT = [Si] + [Nb] + [Ti](In the above equation 2, [Si], [Nb] and [Ti] represent the content (weight %) of each element.)

[0153]

[0154] Classification Steel grade Heating temperature (℃) Finish rolling temperature (℃) Coiling continuous annealing relationship 4 Temperature (℃) Cooling speed (℃ / s) Annealing temperature (T1, ℃) Cooling end temperature (T2, ℃) Comparative example 1A12109056100.057705201533 Comparative example 2A12109056100.057905201540 Invention example 1A12109056100.058105201546 Invention example 2A12109056100.058305201553 Comparative example 3B11988996050.037705201565 Invention example 3B11988996050.038005601573 Invention Example 4B11988996050.038105601576 Invention Example 5B11988996050.038305601582 Comparative Example 4B11988996050.038504501595 Comparative Example 5C12018876080.048105601540 Comparative Example 6C12018876080.048305601547 Invention Example 6D12018876080.048105201546 Invention Example 7D12018876080.048305201552 Invention Example 8D12018876080.048105601544 Invention example 9D12018876080.048305601550 Invention example 10E11889115990.038105201550 Invention example 11E11889115990.038305201557 Comparative example 7F12028886150.047705601542 Comparative example 8F12028886150.047905601548 Invention example 12F12028886150.048105601555 Invention example 13F12028886150.048305601562 Comparative example 9F12028886150.048304001570Comparative Example 10F12028886150.048705601575Comparative Example 11F12028886150.048306301558Comparative Example 12G12088906050.048305601803[Relationship 4]R = 174×[C] + 680×[Mn] + 370×[Nb] + 177×[Ti] - 86×[Cr] + 0.33×[T1] - 0.05×[T2] (In the above relational expression 4, [C], [Mn], [Nb], [Ti] and [Cr] represent the content (weight %) of each element, and [T1] and [T2] represent the annealing temperature (℃) and the cooling end temperature (℃) in the continuous annealing step, respectively.).

[0155]

[0156] For each steel plate, the fraction of microstructure and mechanical properties (yield strength, tensile strength, elongation, and bendability) were measured, and the results are shown in Table 3.

[0157] For the fraction of microstructure, the matrix was analyzed at the 1 / 4 point of the sheet thickness of each steel plate, and the results were used. Specifically, the fractions of ferrite (F), bainite (B), fresh martensite (FM), and retained austenite (A) were measured using FE-SEM, an image analyzer, and XRD.

[0158] Among the mechanical properties, yield strength, tensile strength, and elongation were evaluated for tensile properties at room temperature in the L direction using ASTM standards for each test piece.

[0159] Among the mechanical properties, for bendability, a 180° bending test was performed on each test piece, and the bending angle (°) at which no cracks occur in the bent portion was measured and evaluated by dividing the value by the thickness (mm) of the test piece. Here, the bend portion can refer to the portion of the steel plate where the bending angle is applied, and can generally refer to the portion where bending is applied.

[0160] ClassificationMicrostructure(Area%)Mechanical PropertiesFBFMAYield Strength(MPa)Tensile Strength(MPa)Elongation(%)Bendability(° / mm)Comparative Example 19009140171116.846Comparative Example 288011138972817.045Invention Example 180513235761425.871Invention Example 282314136860429.072Comparative Example 39117140975617.764Invention Example 383015236659925.385Invention Example 483016137062627.480Invention Example 578516136762829.381Comparative Example 4642511040068022.455 Comparative example 572621136969921.656 Comparative example 672522137970222.457 Invention example 681710234462826.166 Invention example 780513233861827.672 Invention example 882314136560429.271 Invention example 982215136259228.374 Invention example 1081512236063529.566 Invention example 1178516136363327.671 Comparative example 79109039272118.851 Comparative example 888012028268216.661 Invention example 1282512135363726.272 Invention example 1377715134663126.071 Comparative example 976185138356719.881 Comparative example 10781210046670118.655 Comparative example 1179137140171117.858 Comparative example 1270921043282217.559

[0161]

[0162] As shown in Table 3, in the case of invention examples 1 to 13 that satisfy the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the physical properties targeted by the present invention were also secured.

[0163] As a representative example of the invention, a photograph of the microstructure of Invention Example 13 according to one embodiment of the present invention observed using an electron microscope is shown in Fig. 1.

[0164] Referring to Fig. 1, the relatively dark region represents ferrite, and the relatively bright region represents fresh martensite. That is, it was found that Inventive Example 13 satisfied the alloy composition and manufacturing conditions of the present invention, and as the T value defined in Relationship 1 was adjusted to be 1380 or more and less than 1648, ferrite (F) and fresh martensite (FM) were formed at an area fraction within the example range.

[0165] Meanwhile, during continuous annealing, Comparative Examples 1 and 2, which had low annealing temperatures (T1) and low values ​​for equation 4, did not achieve the desired elongation and bendability. This is because the annealing temperature (T1) and the values ​​for equation 4 were low, which prevented sufficient recrystallization, and thus prevented sufficient austenite from being formed during the continuous annealing process.

[0166] Although the relational expression 4 proposed in the present invention is satisfied, comparative example 3, which had a low annealing temperature (T1), measured a high fraction of ferrite and had poor elongation.

[0167] Although the annealing temperature (T1) and cooling end temperature (T2) conditions proposed in the present invention are satisfied, Comparative Example 4, which has a high value in relational expression 4, was measured to have a high area fraction of bainite and a low area fraction of ferrite. Accordingly, Comparative Example 4 failed to secure the bending properties at the desired level.

[0168] Comparative examples 5 and 6, which do not satisfy the relationship 1 and 2 proposed in the present invention, were measured to have a high area fraction of fresh martensite, and thus had poor bending properties.

[0169] Comparative examples 7 and 8, which had low annealing temperatures (T1), showed insufficient recrystallization of ferrite and a high area fraction of ferrite, resulting in a decrease in elongation.

[0170] Comparative Example 9, which had a low cooling end temperature (T2), had a high measured area fraction of bainite and a low measured area fraction of martensite, and therefore failed to secure the strength at the target level.

[0171] Comparative Example 10, which had a high annealing temperature (T1), and Comparative Example 11, which had a high cooling end temperature (T2), had an excessive fraction of bainite, resulting in reduced elongation and bendability.

[0172] Comparative Example 12, which does not satisfy the relational expression 1 proposed in the present invention, had an excessive fresh martensite fraction, resulting in poor elongation and bendability.

[0173] As a representative comparative example, a photograph of the microstructure of Comparative Example 6 according to one embodiment of the present invention observed using an electron microscope is shown in Fig. 2.

[0174] Referring to Figure 2, the relatively dark areas represent ferrite, and the relatively bright areas represent fresh martensite. That is, since Comparative Example 6 did not satisfy the relationships 1 and 2 proposed in the present invention, it can be confirmed that martensite was excessively formed.

[0175] The above embodiments are merely examples and the present invention is not limited thereto. Anything that has substantially the same configuration and achieves the same operational effects as the technical concepts described in the claims of the present invention is included within the technical scope of the present invention.

Claims

1. Contains, in wt%, carbon (C): 0.05 to 0.18%, silicon (Si): 0.1% or less, manganese (Mn): 1.0 to 2.3%, aluminum (sol.Al): 1.0% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the remainder iron (Fe) and other unavoidable impurities. The T value defined in the following relational expression 1 is greater than or equal to 1380 and less than or equal to 1648, Steel sheet having a microstructure of, in area %, 65 to 85% ferrite, 10% or less bainite, 6 to 20% fresh martensite and 5% or less retained austenite. [Relationship 1] T = 279×[C] + 711×[Mn] + 474×[Nb] + 177×[Ti] - 75×[Cr] (In the above relational expression 1, [C], [Mn], [Nb], [Ti], and [Cr] represent the content (weight %) of each element.) 2. In paragraph 1, The above steel plate is a steel plate having an RT value of 0.01 or greater as defined in the following relational expression 2. [Relationship 2] RT = [Si] + [Nb] + [Ti] (In the above equation 2, [Si], [Nb], and [Ti] represent the content (weight%) of each element.) 3. In paragraph 1, The above steel plate is a steel plate having a tensile strength of 590 MPa or more and an elongation of 20% or more.

4. In paragraph 1, The above steel plate is a steel plate that satisfies the following relationship 3. [Relationship 3] A / TH ≥ 60 (° / mm) (In the above relational expression 3, A represents the bending angle (°) at which no crack occurs in the bending section during a 180° bending test, and TH represents the thickness of the steel plate (mm).) 5. In paragraph 1, The above steel plate is a steel plate further including a zinc-plated layer or an alloyed zinc-plated layer on the surface.

6. A step of reheating a steel slab containing, by weight%, carbon (C): 0.05 to 0.18%, silicon (Si): 0.1% or less, manganese (Mn): 1.0 to 2.3%, aluminum (sol.Al): 1.0% or less, chromium (Cr): 0.1 to 1.0%, niobium (Nb): 0.05% or less, titanium (Ti): 0.05% or less, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, nitrogen (N): 0.01% or less, the remainder iron (Fe) and other unavoidable impurities, and having a T value defined in the following Relationship 1 of 1380 to 1648; A step of hot rolling the above reheated steel slab to obtain a hot rolled steel sheet; A step of coiling the above hot-rolled steel plate; A step of cold rolling the above-mentioned coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; A step of heating the above cold rolled steel sheet to an annealing temperature of 800 to 850°C, cooling it to a cooling end temperature of 450 to 600°C, and then continuously annealing it while maintaining the temperature; and Comprising a step of cooling the continuously annealed steel plate to room temperature, A method for manufacturing a steel plate having an R value of 1,543 to 1,590 as defined in the following relational expression 4. [Relationship 1] T = 279×[C] + 711×[Mn] + 474×[Nb] + 177×[Ti] - 75×[Cr] (In the above equation 1, [C], [Mn], [Nb], [Ti], and [Cr] represent the weight% of each element.) [Relationship 4] R = 174×[C] + 680×[Mn] + 370×[Nb] + 177×[Ti] - 86×[Cr] + 0.33×[T1] - 0.05×[T2] (In the above relational expression 4, [C], [Mn], [Nb], [Ti], and [Cr] represent the content (weight %) of each element, and [T1] and [T2] represent the annealing temperature (℃) and the cooling end temperature (℃) in the continuous annealing step, respectively.) 7. In paragraph 6, A method for manufacturing a steel plate in which, in the above reheating step, the steel slab has an RT value of 0.01 or more as defined in the following relational expression 2. [Relationship 2] RT = [Si] + [Nb] + [Ti] (In the above equation 2, [Si], [Nb], and [Ti] represent the content (weight%) of each element.) 8. In paragraph 6, A method for manufacturing a steel sheet, wherein the reheating step is performed at 1,100 to 1,300°C, and the step of obtaining the hot-rolled steel sheet is performed at a finishing rolling temperature of 800 to 950°C.

9. In paragraph 6, A method for manufacturing a steel plate, wherein in the above-mentioned coiling step, the hot-rolled steel plate is coiled at 400 to 700°C and then cooled to room temperature at an average cooling rate of 0.1°C / s or less.

10. In paragraph 6, A method for manufacturing a steel plate, wherein the step of obtaining the above cold rolled steel plate is performed at a reduction ratio of 40 to 70%.

11. In paragraph 6, A method for manufacturing a steel sheet, wherein, in the above continuous annealing step, cooling is performed at an average cooling rate of 20°C / s or less to the cooling end temperature.

12. In paragraph 6, A method for manufacturing a steel sheet, wherein, in the above continuous annealing step, the cold rolled steel sheet is kept at a constant temperature for 50 seconds or longer at the above cooling end temperature.

13. In paragraph 6, A method for manufacturing a steel sheet, further comprising, after the above-mentioned continuous annealing step, a step of zinc-plating the above-mentioned continuously annealed steel sheet at 430 to 490°C.

14. In paragraph 13, A method for manufacturing a steel sheet, further comprising, after the zinc plating step, a step of alloying the plated steel sheet at 460 to 530°C.

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