Steel sheet and manufacturing method therefor

Abstract

The present invention relates to a high-strength cold-rolled steel sheet having excellent hole expandability and bendability while also having strength suitable for use as an automobile material, and a manufacturing method therefor.

Description

Steel plate and method of manufacturing the same

[0001] The present invention relates to a steel plate as a material for automotive structural members such as members, seat rails, and pillars, and a method for manufacturing the same.

[0002] Recently, as interest in carbon dioxide reduction and the associated regulatory standards have grown, automakers are making significant efforts to improve fuel efficiency through vehicle body lightweighting. To efficiently achieve this lightweighting, it is essential to adopt high-strength steel, which reduces body weight by decreasing steel plate thickness while simultaneously ensuring passenger safety.

[0003] Recently, there has been an increasing trend of applying high-strength steel to structural components such as members, seat rails, and A / B / C pillars to improve the impact resistance of vehicle bodies. Automotive structural components possess characteristics that are advantageous for absorbing impact energy when the yield ratio (yield strength / tensile strength)—that is, the ratio of yield strength to tensile strength—is higher. However, since increased strength is accompanied by a decline in formability due to reduced ductility, there is a need to develop materials that simultaneously possess high yield ratio characteristics and improved formability. Accordingly, in response to these technical requirements, it is necessary to develop automotive steel sheets that possess not only strength suitable for automotive applications but also excellent hole expansion and bendability.

[0004] Patent Document 1 discloses a technology for manufacturing a steel sheet having a martensite phase with an area fraction of 80% or more by using water cooling and overaging treatment during continuous annealing. When tempering is performed by immersing in water after cracking in the annealing process, a steel sheet having a tempered martensite structure can be manufactured. At this time, although the yield ratio increases due to the tempering effect of the martensite, there is a risk that workability will be reduced because the shape quality of the coil deteriorates due to temperature variations in the width and length directions of the steel sheet, or cracks occur during forming due to material variation issues.

[0005] Meanwhile, Patent Document 2 discloses a technology for manufacturing composite structure steel sheets with excellent workability by utilizing a retained austenite phase. Although this technology enables the simultaneous securing of strength and ductility required by automobile manufacturers by utilizing transformation-induced plasticity, it has the problem of difficulty in securing steelmaking and continuous casting quality due to the large amount of Si and Al added to create the retained austenite.

[0006] Accordingly, there is a need to develop a steel plate that can simultaneously secure not only strength but also hole expandability and bendability while solving the problems of the aforementioned existing technologies.

[0007] (Patent Document 1) Japanese Published Patent Application No. 1992-289120

[0008] (Patent Document 2) Japanese Published Patent Application No. 2015-113504

[0009] According to one embodiment of the present invention, a high-strength steel plate having excellent hole expansion and bendability and a method for manufacturing the same can be provided as a steel plate suitable for use as an automobile material, particularly as a material for a member requiring high formability.

[0010] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0011] A cold-rolled steel sheet according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.05~0.25%, manganese (Mn): 1.0~3.6%, silicon (Si): 2.0% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and chromium (Cr): 1.0% or less, molybdenum (Mo): 0.15% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, boron (B): 0.004% or less, the remainder being iron (Fe) and unavoidable impurities, and in area%, ferrite: 30% or less, fresh martensite: 30% or less. The steel plate has a microstructure comprising 10% or less of residual austenite, the remainder being bainite and tempered martensite, and includes a decarburization layer in the surface layer of the steel plate, wherein the decarburization layer is defined as a first decarburization region extending to a depth of 20 μm in the thickness direction from the surface of the steel plate, and a second decarburization region extending to a depth of 40 μm in the thickness direction from the first decarburization region, and can satisfy the following relationship 3.

[0012] [Relationship 3]

[0013] (F2 / F1) ×100 ≥ 30%

[0014] [Here, F1 represents the ferrite fraction (area %) in the first decarburization region, and F2 represents the ferrite fraction (area %) in the second decarburization region]

[0015] The above F1 may be 15% or more, and the above F2 may be 5% or more.

[0016] In addition, when the region from the surface of the steel plate to 50㎛ in the thickness direction is called the surface layer and the region other than the surface layer is called the center, the ratio (a / b) of the Ceq(a) value measured in the surface layer and the Ceq(b) value measured in the center can satisfy 0.95 to 0.99 according to the following relationship 2.

[0017] [Relationship 2]

[0018] Ceq = C + Mn / 6 + Si / 20 + Cr / 5 + Mo / 4 (%)

[0019] (Here, each element refers to its weight content.)

[0020] In addition, the following relationship 1 can be satisfied.

[0021] [Relationship 1]

[0022] 4.0 ≤ C + 1.1Si + 1.5Mn + 1.6Cr + 1.8Mo ≤ 5.1

[0023] (Here, each element refers to its weight content.)

[0024] In addition, the above cold-rolled steel sheet may have a yield strength of 600 MPa or more, a tensile strength of 980 MPa or more, a yield ratio of 0.6 or more, a hole expansion of 60% or more, and an elongation of 15% or more.

[0025] In addition, the above cold-rolled steel sheet has a product of tensile strength and elongation of 15,000 MPax% or more, a product of hole expansion and elongation of 950%x% or more, and a three-point bending angle of 135 o Above, and the product of the 3-point bending angle and the elongation is 2100%x o It could be more than that.

[0026] A cold-rolled steel sheet according to another embodiment of the present invention comprises, in weight percent, carbon (C): 0.05~0.25%, manganese (Mn): 1.0~3.6%, silicon (Si): 2.0% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and chromium (Cr): 1.0% or less, molybdenum (Mo): 0.15% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, and boron (B): 0.004% or less, wherein the steel sheet according to one embodiment of the present invention comprises

[0027] In weight percent, carbon (C): 0.05–0.25%, manganese (Mn): 1.0–3.6%, silicon (Si): 2.0% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and one or more selected from chromium (Cr): 1.0% or less, molybdenum (Mo): 0.15% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, boron (B): 0.004% or less, and the remainder comprising iron (Fe) and unavoidable impurities.

[0028] It has a microstructure in area % comprising ferrite: 7–30%, fresh martensite: 30% or less, retained austenite: 10% or less, and the remainder comprising one or more of bainite and tempered martensite, and

[0029] When the region extending from the surface of the steel plate to a thickness of 50 μm in the thickness direction is called the surface layer and the region other than the surface layer is called the center, the surface layer includes a decarburized layer, and when the decarburized layer extending from the surface of the steel plate to a depth of 20 μm in the thickness direction is called the first decarburized region and extending from the first decarburized region to a depth of 40 μm in the thickness direction is called the second decarburized region, the following relationship 3 can be satisfied.

[0030] [Relationship 3]

[0031] 30% ≤ (F2 / F1) × 100 ≤ 44%

[0032] In the above equation 3, F1 represents the ferrite fraction (area %) in the first decarburization region, and F2 represents the ferrite fraction (area %) in the second decarburization region.

[0033] The above steel plate can satisfy the following relationship 1.

[0034] [Relationship 1]

[0035] 4.0 ≤ C + 1.1Si + 1.5Mn + 1.6Cr + 1.8Mo ≤ 5.1

[0036] (Here, each element refers to its weight content.)

[0037] The above steel plate can satisfy a ratio (a / b) of 0.97 to 0.98 between the Ceq(a) value measured at the surface layer and the Ceq(b) measured at the center, according to the following relationship 2.

[0038] [Relationship 2]

[0039] Ceq = C + Mn / 6 + Si / 20 + Cr / 5 + Mo / 4 (%)

[0040] In the above equation 2, each element represents the content (weight%).

[0041] At least one surface of the above steel plate may include a molten zinc plating layer or an alloyed molten zinc layer.

[0042] The ferrite fraction in the first decarburization region above may be 10 to 50%.

[0043] The ferrite fraction in the second decarburization region above may be 5 to 20%.

[0044]

[0045] A method for manufacturing a steel plate according to one embodiment of the present invention is,

[0046] A step of providing a cold-rolled steel sheet comprising, in weight percent, carbon (C): 0.05~0.25%, manganese (Mn): 1.0~3.6%, silicon (Si): 2.0% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and one or more selected from chromium (Cr): 1.0% or less, molybdenum (Mo): 0.15% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, boron (B): 0.004% or less, and the remainder being iron (Fe) and unavoidable impurities;

[0047] A step of annealing the provided cold-rolled steel sheet at a temperature range of 800 to 840°C;

[0048] A step of first cooling the annealed cold-rolled steel sheet to a temperature range of 550 to 700℃ at an average cooling rate of 1 to 10℃ / s;

[0049] A step of secondary cooling to a temperature range of 250 to 390°C after the above primary cooling at an average cooling rate of 10 to 55°C / s; and

[0050] The method may include a step of reheating to a temperature in the range of 250 to 500°C after the above secondary cooling and over-aging treatment for 100 to 700 seconds.

[0051] The above annealing may be performed in an annealing furnace with an atmosphere of a dew point temperature of -10 to 20°C.

[0052] The above cold-rolled steel sheet is,

[0053] A step of heating the above steel slab in a temperature range of 1000 to 1350℃;

[0054] A step of manufacturing a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated steel slab in a temperature range of Ar3 to Ar3+50℃;

[0055] A step of winding the above hot-rolled steel sheet at a temperature range of 400 to 700℃; and

[0056] The above-mentioned coiled hot-rolled steel sheet can be manufactured by including the step of cold-rolling to produce a cold-rolled steel sheet.

[0057] The above cold-rolled steel sheet can satisfy the following relationship 1.

[0058] [Relationship 1]

[0059] 4.0 ≤ C + 1.1Si + 1.5Mn + 1.6Cr + 1.8Mo ≤ 5.1

[0060] Here, each element represents the content (weight%).

[0061] The method may further include the step of forming a hot-dip galvanized layer by immersing the above-mentioned over-aged cold-rolled steel sheet in a hot-dip galvanized bath.

[0062] The method may further include a step of forming an alloyed hot-dip galvanized layer by alloying the cold-rolled steel sheet having the above-mentioned hot-dip galvanized layer formed thereon.

[0063] Temper rolling can be performed on the above cold-rolled steel sheet.

[0064] According to the present invention, a steel plate having high strength and excellent hole expansion and bendability can be provided. In particular, the steel plate of the present invention has excellent formability, so it has the effect of being suitable for application to automotive structural parts and the like that require processing into complex shapes.

[0065] The various and beneficial advantages and effects of the present invention are not limited to those described above, and may be more easily understood in the process of explaining specific embodiments of the present invention.

[0066] Figure 1 is an SEM micrograph of a cross-section of the steel material of Invention Example 1 in an embodiment of the present invention.

[0067] FIG. 2 is an SEM image of the cross-section in the thickness direction of the surface layer of the steel material of Invention Example 1 according to one embodiment of the present invention.

[0068] Preferred embodiments of the present invention are described below. The description will be made with reference to the drawings as necessary. However, embodiments of the present invention may be modified in various different forms, and the scope of the present invention is not limited to the embodiments described below.

[0069] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0070] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0071] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0072] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0073] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0074] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0075] It should be noted that, although not essential, the technical solution according to each aspect of the present invention may be usefully applied to other aspects of the technical solution. Furthermore, the composition and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to obtain advantageous effects.

[0076] The inventors of the present invention have conducted extensive research to provide a steel plate with improved hole expansion and bendability, which possesses strength suitable for automotive materials while also having formability that allows it to be processed into parts requiring complex shapes.

[0077] As a result, by optimizing the alloy composition system and manufacturing conditions of the steel, it is possible to have a structure advantageous for securing target physical properties. It was confirmed that this allows for the provision of a steel plate suitable for use in automotive structural members and the like, which require processing into complex shapes, and thus the present invention was completed.

[0078] A high-strength steel sheet with excellent hole expandability and bendability according to one embodiment of the present invention may comprise, in weight%, carbon (C): 0.05~0.25%, manganese (Mn): 1.0~3.6%, silicon (Si): 2.0% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), chromium (Cr): 1.0% or less, molybdenum (Mo): 0.15% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, boron (B): 0.004% or less, and the remainder may be iron (Fe) and unavoidable impurities.

[0079] Below, the reason for limiting the alloy composition of the steel plate provided in the present invention as described above will be explained in detail.

[0080] Carbon (C): 0.05~0.25%

[0081] Carbon (C) is an element advantageous for securing strength by forming martensite within steel, and is an essential element for manufacturing high-strength steel. Generally, the higher the C content, the easier the formation of martensite, which is favorable for the formation of the complex microstructure required for manufacturing high-strength steel. However, to simultaneously control the intended strength and elongation, it is necessary to control the content to an appropriate level.

[0082] In the present invention, the above C may be added at a level of 0.05% or more to secure the target strength and form an appropriate level of martensite. However, if the content is excessive and exceeds 0.25%, there is a problem that weldability and formability become inferior.

[0083] Accordingly, in the present invention, C may be included in an amount of 0.05 to 0.25%, and more advantageously, in an amount of 0.07% or more and 0.23% or less.

[0084] Manganese (Mn): 1.0~3.6%

[0085] Manganese (Mn) is an element that improves the hardenability of steel and plays a particularly important role in the formation of martensite. Furthermore, Mn contributes to the increase in steel strength through solid solution strengthening effects and precipitates sulfur (S), which is inevitably added to steel, as MnS, thereby suppressing plate fracture and high-temperature brittleness caused by sulfur during hot rolling.

[0086] In order to sufficiently obtain the aforementioned effects, it is desirable to add the above Mn at a content of 1.0% or more. However, if the content exceeds 3.6%, not only is weldability inferior, but excessive martensite is formed, making the material unstable, and band-shaped oxide strips are formed, increasing the risk of processing cracks and plate fracture. In addition, there is a concern that manganese oxide may leach out onto the surface of the steel plate during annealing, thereby impairing plating properties.

[0087] Accordingly, in the present invention, the Mn may be included in an amount of 1.0 to 3.6%, and more advantageously, in an amount of 1.2% or more and 3.4% or less.

[0088] Silicon (Si): 2.0% or less

[0089] Silicon (Si) is a useful element that can secure strength without reducing the ductility of steel sheets. This Si promotes the formation of ferrite and is also advantageous for promoting the formation of martensite by encouraging carbon enrichment into untransformed austenite.

[0090] However, if the Si content exceeds 2.0%, there is a concern that it may cause inferiority in weldability as well as plating properties. Therefore, in the present invention, the Si may be included at 2.0% or less, and 0% may be excluded considering the level that is inevitably added to the steel.

[0091] Aluminum for acidity (Sol.Al): 0.1% or less

[0092] Acid-soluble aluminum (Sol.Al) is an element added to steel for deoxidation and grain size refinement. However, if the content of Sol.Al is excessive and exceeds 0.1%, not only is castability reduced during continuous casting, but there is also a problem that the excessive formation of inclusions increases the likelihood of material defects in annealed materials and surface defects in plated materials.

[0093] Accordingly, the above Sol.Al may be included in an amount of 0.1% or less, provided that 0% may be excluded considering the level inevitably added in the steel. More advantageously, the above Sol.Al may be included in an amount of 0.01% or more.

[0094] Phosphorus (P): 0.05% or less

[0095] Phosphorus (P) is the most advantageous element for securing the strength of steel without significantly impairing formability, but when added in excess, it significantly increases the likelihood of brittle fracture, thereby increasing the possibility of plate breakage of slabs during hot rolling. In addition, P also has the problem of acting as an element that impairs the plating surface characteristics.

[0096] Considering this, the above P may be included in an amount of 0.05% or less, and 0% may be excluded considering the level inevitably added in the steel.

[0097] Sulfur (S): 0.01% or less

[0098] Sulfur (S) is an impurity element that is inevitably added to steel, and it is desirable to keep its content as low as possible. In particular, since S in steel can increase the likelihood of red-hot brittleness, it is desirable to limit its content to 0.01% or less. However, 0% may be excluded considering the level that is inevitably added during the manufacturing process.

[0099] Nitrogen (N): 0.01% or less

[0100] Nitrogen (N) is an impurity element that is inevitably added to steel, so it is important to keep its content as low as possible. However, since this causes a sharp increase in steel refining costs, it is desirable to control it to 0.01% or less, which is within the range feasible for operating conditions. However, 0% may be excluded considering the level that is inevitably added during the manufacturing process.

[0101] Meanwhile, in order to advantageously secure the intended physical properties, the steel sheet of the present invention having the alloy composition described above may further include one or more selected from chromium (Cr): 1.0% or less, molybdenum (Mo): 0.15% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, and boron (B): 0.004% or less.

[0102] Chrome (Cr): 1.0% or less

[0103] Chromium (Cr) is an element added to steel to improve its hardenability and ensure high strength. This Cr is not only effective in forming martensite but also minimizes the decrease in elongation relative to the increase in strength, making it advantageous for manufacturing high-strength steel with high ductility.

[0104] However, if the above Cr content exceeds a certain level, it causes a problem in that it excessively increases the martensite formation rate and raises the fraction of coarse Cr-based carbides, leading to a decrease in elongation. In addition, there is a risk of hydrogen embrittlement and deterioration of weldability.

[0105] Therefore, the addition of the above Cr can be limited to 1.0% or less, and more advantageously, to 0.8% or less.

[0106] Molybdenum (Mo): 0.15% or less

[0107] Molybdenum (Mo) is an element that improves the hardenability of steel and contributes to strength enhancement by not only refining ferrite but also forming fine carbides within the steel by combining with Ti, Nb, etc. Molybdenum (Mo) may be added to obtain the aforementioned effects. However, as the content of molybdenum (Mo), a relatively expensive element, increases, it not only raises manufacturing costs but also reduces ductility due to excessive grain refinement and solid solution strengthening effects; therefore, the content may be limited to 0.15% or less. A more desirable upper limit may be 0.13%.

[0108] Niobium (Nb): 0.1% or less

[0109] Niobium (Nb) is an element that segregates at austenite grain boundaries and not only suppresses the coarsening of austenite grains during annealing but also contributes to the improvement of yield strength and tensile strength by forming fine carbides.

[0110] However, if the above Nb content is excessive, there is a risk that strength and elongation will decrease due to the precipitation of coarse carbides and the reduction of carbon content in the steel, and the manufacturing cost may increase, which may worsen economic feasibility.

[0111] Therefore, the addition of the above Nb can be limited to 0.1% or less, and more advantageously, it can be included at 0.05% or less.

[0112] Titanium (Ti): 0.1% or less

[0113] Titanium (Ti) not only contributes to securing yield strength and tensile strength by forming fine carbides, but can also effectively reduce the risk of crack formation during continuous casting by suppressing the precipitation of AlN through the precipitation of N in the steel as TiN.

[0114] However, if the content is excessive, strength and elongation may decrease due to the precipitation of coarse carbides and the reduction of carbon content in the steel, and nozzle clogging may also occur during continuous casting.

[0115] Therefore, the addition of the above Ti can be limited to 0.1% or less, and more advantageously, to 0.05% or less.

[0116] Boron (B): 0.004% or less

[0117] Boron (B) is effective in delaying the transformation of austenite into pearlite during the cooling process after continuous annealing.

[0118] However, if the content of B is excessive, B may become concentrated on the surface of the steel plate, leading to a deterioration in plating adhesion.

[0119] Therefore, when adding the above B, it can be limited to 0.004% or less.

[0120] The remaining component of the above steel plate is iron (Fe). Furthermore, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0121] In the alloy composition of the present invention described above, the content relationship of C, Mn, Cr, and B can satisfy the following relationship 1.

[0122] [Relationship 1]

[0123] 4.0 ≤ C + 1.1Si + 1.5Mn + 1.6Cr + 1.8Mo ≤ 5.1

[0124] In the above equation 1, each element represents the content (weight%).

[0125] The above Equation 1 is a numerical representation of the content relationship of specific components to secure the basic material of the steel plate intended for the present invention. If the value of Equation 1 is less than 4.0, the hardenability of the steel is reduced, so the phase fraction of low-temperature transformation structures such as martensite or bainite is lowered, making it impossible to secure the strength intended for the present invention. On the other hand, if the value exceeds 5.1, the hardenability of the steel becomes excessively high, making it impossible to obtain the formable material desired for the present invention.

[0126] More preferably, the value of the above relationship 1 may be 4.2 or higher and 4.8 or lower.

[0127] Meanwhile, the high-strength steel sheet of the present invention, which has excellent bendability and hole expansion properties, has a microstructure comprising, in area %, ferrite: 7~30%, fresh martensite: 30% or less, retained austenite: 10% or less, and the remainder being one or more of bainite and tempered martensite.

[0128] If the ferrite fraction exceeds 30%, the desired strength cannot be secured, and conversely, if the ferrite fraction is too low at less than 7%, it is difficult to secure sufficient ductility. If the fresh martensite fraction exceeds 30%, not only is the desired ductility compromised, but the increased difference in interphase hardness may also lead to deterioration in hole expandability and bendability. The lower limit of the fresh martensite is not specifically limited, but it is desirable for it to exceed 0%. Furthermore, if the retained austenite fraction exceeds 10%, localized stress concentration due to transformation into fresh martensite during forming may lead to deterioration in hole expandability and bendability. Meanwhile, the lower limit of the retained austenite is not specifically limited, but it is desirable for it to exceed 0%.

[0129] The above steel plate may include a surface layer extending from the surface of the steel plate to a thickness of 50 μm in the thickness direction and a center in the area other than the surface layer. The surface layer includes a decarburization layer, and the decarburization layer may be divided into a first decarburization region extending to a depth of 20 μm in the thickness direction from the surface of the steel plate and a second decarburization region extending to a depth of 40 μm in the thickness direction from the first decarburization region. The ferrite fraction of the first decarburization region and the second decarburization region may satisfy the following relationship Equation 3.

[0130] [Relationship 3]

[0131] 30% ≤ (F2 / F1) × 100 ≤ 44%

[0132] In the above equation 3, F1 represents the ferrite fraction (area %) in the first decarburization region, and F2 represents the ferrite fraction (area %) in the second decarburization region.

[0133] Meanwhile, the ferrite fraction in the first decarburization region may be 10 to 50%. The ferrite fraction in the second decarburization region may be 5 to 20%. In the present invention, by securing ferrite in the first decarburization region and the second decarburization region, bendability can be secured.

[0134] In addition, the ratio (a / b) of the Ceq(a) value measured by the following relationship 2 in the above surface layer and the Ceq(b) value measured at the center of the steel plate can satisfy 0.97 to 0.98.

[0135] [Relationship 2]

[0136] Ceq = C + Mn / 6 + Si / 20 + Cr / 5 + Mo / 4 (%)

[0137] In the above equation 2, each element represents the content (weight%).

[0138] The cold-rolled steel sheet of the present invention having the alloy composition and microstructure as described above may have a yield strength of 600 MPa or more, a tensile strength of 980 MPa or more, and a yield ratio of 0.6 or more. In addition, the steel sheet can secure excellent strength, as well as hole expansion and bendability simultaneously, by having an elongation of 15% or more.

[0139] The present invention can secure not only ductility but also hole expandability of 45% or more and a three-point bending angle of 135° or more by effectively reducing the difference in hardness between phases through martensitic tempering. Accordingly, the steel sheet of the present invention may have a product of tensile strength and elongation of 14,700 MPa·% or more, a product of hole expandability and elongation of 675%·% or more, and a product of three-point bending and elongation of 2,025%·° or more.

[0140]

[0141] Next, a method for manufacturing a high-strength cold-rolled steel sheet with excellent hole expansion and bendability, which is another embodiment of the present invention, will be described in detail.

[0142] The method for manufacturing a cold-rolled steel sheet according to the present invention may include the steps of: heating a steel slab satisfying the alloy composition and Equation 1 described above; hot-rolling the heated steel slab to produce a hot-rolled steel sheet; coiling the hot-rolled steel sheet; cold-rolling the coiled hot-rolled steel sheet to produce a cold-rolled steel sheet; annealing the produced cold-rolled steel sheet in a dew point annealing furnace at a temperature range of 800 to 870°C; first cooling the annealed cold-rolled steel sheet to a temperature range of 550 to 700°C at an average cooling rate of 1 to 10°C / s; second cooling after the first cooling to a temperature range of 250 to 450°C at an average cooling rate of 10 to 55°C / s; and reheating to a temperature range of 250 to 500°C after the second cooling and over-aging for 100 to 700 seconds.

[0143] The conditions for each step are explained in detail below.

[0144] [Heating Steel Slabs]

[0145] After preparing a steel slab having the specified alloy composition described above, it is heated. For example, the heating temperature may be 1000 to 1350°C. This is because if the heating temperature of the steel slab is less than 1000°C, there is a risk that it will be hot-rolled in a temperature range below the desired finishing hot-rolling temperature range, and if the heating temperature of the steel slab exceeds 1350°C, there is a possibility that it will reach the melting point of the steel and melt.

[0146] The steel slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0147] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0148] In the electric furnace process, molten steel can be obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of including a large amount of scrap in this manner, elements not intended in the present invention may be included in the molten steel within permissible limits.

[0149] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0150] [Hot Rolled]

[0151] According to the above, the heated steel slab can be hot-rolled to produce a hot-rolled steel sheet. During the hot rolling process, the finishing hot rolling can be performed in a temperature range of Ar3 to Ar3+50℃.

[0152] If the temperature during the above-mentioned finishing hot rolling is below Ar3, there is a high possibility that the resistance to hot deformation will increase rapidly; on the other hand, if the temperature exceeds Ar3 + 50℃, not only will an excessively thick oxide scale be generated, but the crystal grains of the hot-rolled steel sheet will also be formed coarsely, raising concerns that this may cause a deterioration in the physical properties of the final steel sheet. Here, the above-mentioned finishing hot rolling temperature may be the temperature at the exit side of the finishing rolling mill.

[0153] The above Ar3 represents the temperature at which the transformation from austenite to ferrite begins upon cooling, and as an example, it can be derived as shown in Equation 1 below.

[0154] (Equation 1)

[0155] Ar3(℃)=910-310C-80Mn-20Cu-15Cr-55Ni+80Si

[0156] In the above Equation 1, each element represents the content (weight%), and if not included, 0 can be substituted.

[0157] [Record]

[0158] The hot-rolled steel sheet manufactured according to the above can be cooled and coiled. For example, the coiling can be performed in a temperature range of 400 to 700°C. If the coiling temperature is below 400°C, excessive martensite or bainite may be generated, which may cause manufacturing problems such as shape defects due to load during subsequent cold rolling. On the other hand, if the temperature exceeds 700°C, there is a problem of deterioration in pickling ability due to an increase in surface scale.

[0159] [Cold Rolled]

[0160] The above-mentioned coiled hot-rolled steel sheet can be cold-rolled to produce a cold-rolled steel sheet. At this time, the conditions of the cold rolling (e.g., reduction rate) are not specifically limited, but can be performed with a reduction rate of 30 to 70%.

[0161] If the reduction rate during the above cold rolling is less than 30%, the recrystallization driving force is weakened, making it difficult to secure good recrystallized grains and making shape correction difficult. On the other hand, if the above reduction rate exceeds 70%, the likelihood of cracks occurring at the edge of the steel sheet increases, and there is a concern that the rolling load will increase rapidly.

[0162] Meanwhile, prior to performing the above cold rolling, a pickling process may be further performed to remove scale formed on the surface of the hot-rolled steel sheet. It should be noted that the above pickling process may be performed under normal conditions, and that such conditions are not specifically limited.

[0163] [Sodun]

[0164] The cold-rolled steel sheet manufactured according to the above can be subjected to annealing treatment. For example, continuous annealing can be performed. Through the above annealing treatment, the basis for the microstructure intended in the present invention can be established.

[0165] Specifically, the annealing can be performed on the cold-rolled steel sheet at a temperature range of 800 to 840°C, and it is preferable to maintain it at that temperature for 30 to 300 seconds.

[0166] If the annealing temperature is below 800°C, the presence of a ferrite fraction may result in a decrease in strength, and the difference in hardness between phases may increase, potentially leading to a deterioration in bendability. Meanwhile, if the annealing temperature exceeds 840°C, there is a risk of annealing oxides forming on the surface of the steel sheet. More preferably, the annealing may be performed at a temperature of 810°C or higher and 830°C or lower.

[0167] When a cold-rolled steel sheet is maintained in the above annealing temperature range, if the time is less than 30 seconds, recrystallization does not occur sufficiently, making it difficult to secure elongation, and increasing the likelihood of material variation in the length / width direction of the steel sheet. On the other hand, if the time exceeds 300 seconds, the annealing effect becomes saturated, and there is a problem of reduced productivity.

[0168] In this invention, the bendability of a steel sheet is improved by introducing a soft ferrite decarburization layer on the surface of the steel sheet. When annealing the cold-rolled steel sheet, the dew point of the annealing furnace can be carried out in the range of -10 to 20°C. When the steel sheet is annealed at the corresponding dew point temperature, a decarburization reaction occurs in which carbon present in the steel is discharged from the surface of the steel sheet into the atmosphere of the annealing furnace. Even if the annealing is carried out at a temperature in the austenite single-phase region, if decarburization occurs, ferrite can be formed as an equilibrium phase. As the decarburization reaction increases on the surface of the steel sheet, the fraction of ferrite increases, and the grains may grow and become coarse at that temperature. However, if the dew point of the annealing furnace is less than -10°C, the decarburization reaction is insufficient, making it difficult to secure the target thickness of the ferrite decarburization layer, and thus the target bendability cannot be achieved. On the other hand, if the dew point temperature exceeds 20℃, an excessive decarburization layer is formed, increasing the fraction of the ferrite decarburization layer and causing a problem of deteriorating the strength of the steel sheet.

[0169] [1st Cooling]

[0170] Next, the cold-rolled steel sheet subjected to continuous annealing treatment is cooled in the first stage. In the present invention, the first stage of cooling is performed as slow cooling compared to the subsequent second stage of cooling, and the plate shape degradation caused by a drop in temperature during the second stage of cooling, which is a relatively rapid cooling section, can be suppressed.

[0171] Specifically, the first cooling is performed at an average cooling rate of 1 to 10°C / s up to a temperature range of 550 to 700°C. If the cooling end temperature is less than 550°C or exceeds 700°C, it deviates from the appropriate temperature gradient range with the subsequent second cooling, making it difficult to secure stable cooling capacity.

[0172] In addition, if the average cooling rate during the first cooling exceeds 10℃ / s, there is a concern that sufficient enrichment of C and Mn may not occur in the austenite. In the present invention, the lower limit of the average cooling rate during the first cooling is not specifically limited, but since cooling is performed at a conventional slow cooling rate, it may be 1℃ / s or more.

[0173] [Secondary Cooling]

[0174] When the above first cooled cold-rolled steel sheet is cooled a second time, the cooling speed and the cooling end temperature are appropriately adjusted according to the width and thickness of the steel sheet to be obtained, thereby securing an optimal sheet shape.

[0175] Specifically, in the present invention, the firstly cooled cold-rolled steel sheet is secondarily cooled to a temperature range of 250 to 390°C at an average cooling rate of 10 to 55°C / s. If the cooling end temperature during the second cooling is less than 250°C, the amount of martensite transformation is too high and the amount of retained austenite is too low, resulting in insufficient elongation and increased yield strength, making forming difficult. On the other hand, if the cooling end temperature exceeds 390°C, sufficient martensite is not generated during cooling, making it difficult to secure the chemical and mechanical stability of the retained austenite. Furthermore, the fraction of fresh martensite ultimately increases, making it difficult to obtain sufficient yield strength and elongation. More advantageously, the cooling end temperature of the second cooling may be 280°C or higher and 360°C or lower.

[0176] In addition, the average cooling rate during the second cooling may vary depending on the temperature difference from the first cooling end temperature, and preferably can be performed at 10 to 55°C / s. More advantageously, the average cooling rate may be 15°C / s or more and 50°C / s or less.

[0177] [Reheating and Overaging]

[0178] The above secondary cooled cold-rolled steel sheet can be subjected to over-aging treatment by reheating it to 250 to 500°C and maintaining it at that temperature range for a period of 100 to 700 seconds. Through this process, the interphase carbon distribution necessary for stabilizing the retained austenite and additional bainite phase transformation are obtained. If the reheating temperature is below 250°C, the redistribution of carbon from the martensite generated during cooling to the surrounding austenite is insufficient, making it difficult to secure the stability of the retained austenite. Furthermore, the tempering of the martensite is insufficient, resulting in a problem where the strength is excessively high and the elongation is inferior. On the other hand, if the reheating temperature exceeds 500°C, the tempering of the martensite is excessive, making it difficult to secure strength. Additionally, precipitation of cementite occurs, reducing the stability of the retained austenite and resulting in a problem where the elongation is inferior.

[0179] If the holding time during over-aging treatment is excessive and exceeds 700 seconds, excessive bainite transformation may occur during the holding process, raising concerns that the fraction of the bainite phase in the final microstructure will increase. This leads to a decrease in the fraction of martensite, making it impossible to effectively secure the desired strength. On the other hand, if the time is less than 100 seconds, the bainite nose is avoided, which may result in the final microstructure not containing the bainite phase, causing a decrease in the ductility of the steel.

[0180] [Hot-dip galvanizing, Alloyed hot-dip galvanizing]

[0181] In the present invention, the over-aged cold-rolled steel sheet may be immersed in a plating bath to perform plating as needed and optionally. At this time, a molten zinc-based plating bath may be used, although not limited thereto, and after such molten zinc plating, an alloying heat treatment may be optionally performed.

[0182] In the present invention, the method of hot-dip galvanizing or alloyed hot-dip galvanizing is not particularly limited, and process conditions for hot-dip galvanizing or alloyed hot-dip galvanizing that are commonly applied may be applied. As a non-limiting example, the hot-dip galvanizing may be performed in a zinc-based plating bath at 430 to 490°C, and the alloying heat treatment may be performed in a temperature range of 480 to 600°C.

[0183] In addition, the composition of the plating bath during the above-mentioned hot-dip galvanizing or alloyed hot-dip galvanizing is not specifically limited, but may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[0184] According to the above, cold-rolled steel sheets or steel sheets that have been hot-dip galvanized, or alloyed heat treatment if necessary, can be cooled to room temperature under normal conditions, and the cooling process is not specifically limited. However, it is obvious that it can be replaced with known cooling methods such as water cooling, oil cooling, or furnace cooling.

[0185] [Tough Rolling]

[0186] In the present invention, temper rolling may be further performed on the plated steel sheet, and at this time, it may be performed with a reduction rate of 0.1 to 1.0%.

[0187] Typically, when temper rolling is performed on steel plates, the effect of increasing yield strength can be obtained without accompanying an increase in tensile strength. However, if the reduction rate of the above temper rolling is less than 0.1%, not only is the effect of increasing yield strength insufficient, but shape control is also difficult. On the other hand, if the reduction rate exceeds 1.0%, there is a risk that workability will be significantly inferior due to high elongation.

[0188] The steel sheet of the present invention, manufactured through the aforementioned series of processes, is characterized by having a yield strength of 600 MPa or more, a tensile strength of 980 MPa or more, and a yield ratio of 0.6 or more, as the intended microstructure is formed. In addition, the steel sheet has an elongation of 15% or more, a product of tensile strength and elongation of 14,700 MPa or more, a hole expansion of 45% or more, a product of hole expansion and elongation of 675% or more, a 3-point bend of 135° or more, and a product of 3-point bend and elongation of 2025% or more, thereby providing excellent strength as well as hole expansion and bendability simultaneously.

[0189] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0190] (Example)

[0191] A steel slab having the alloy composition listed in Table 1 below (the remainder being Fe and unavoidable impurities) was vacuum melted, heated in a temperature range of 1200°C, finished rolling was completed at an exit side temperature of 880–920°C, and coiled at 600°C. Afterward, the surface scale was removed by pickling, and then cold-rolled with a cold reduction rate of 50% to produce a cold-rolled steel sheet. Next, the cold-rolled steel sheet was subjected to continuous annealing, stepwise cooling, and overaging treatment in an annealing furnace under the conditions shown in Table 2 below.

[0192] The cold-rolled steel sheet, which had undergone the above-mentioned over-aging treatment, was temper-rolled at a reduction rate of 0.3% to produce a final steel sheet.

[0193] Steel Grade Alloy Composition (Weight%) Relationship Formula 1CSiMnCrMoBNbTiAlPSNA0.10.42.30.30.010.0020.030.030.030.010.0020.0034.5B0.040.42.30.30.010.0020.030.030.030.010.0020.0034.4C0.260.42.30.30.010.0020.030.030.030.010.0020.0034.7D0.12.22.30.30.010.0020.030.030.030.010.0020.0036.5E0.10.40.90.30. 010.0020.030.030.030.010.0020.0032.4F0.10.43.70.30.010.0020. 030.030.030.010.0020.0036.6G0.10.42.31.10.010.0020.030.030.03 0.010.0020.0035.8H0.10.42.30.30.160.0020.030.030.030.010.002 0.00344.8I0.080.62.50.30.030.0020.030.030.030.010.0020.0035.0

[0194] Steel Grade Continuous Annealing 1st Cooling 2nd Coolant Heating / Overaging Classification Temperature (°C) Holding Time (s) Ending Temperature (°C) Cooling Rate (°C / s) Ending Temperature (°C) Cooling Rate (°C / s) Temperature (°C) Holding Time (s) A790 100600 4.4350 29.1350 450 Comparative Example 1 A820 100600 5.1350 29.1350 450 Inventive Example 1 A850 100600 5.7350 29.1350 450 Comparative Example 2 A820 100600 5.1400 23.3400 450 Comparative Example 3 A820 100600 5.1450 17.4450 450 Comparative Example 4A8201006005.150011.6500450Comparative Example 5A8201006005.15505.8550450Comparative Example 6B8201006005.135029.1350450Comparative Example 7C8201006005.135029.1350450Comparative Example 8D8201006005.135029.1350450Comparative Example 9E8201006005.135029.1350450Comparative Example 10F8201006005.135029.1350450Comparative Example 11G8201006005.135029.1350450Comparative Example 12H8201006005.135029.1350450Comparative Example 13I8201006005.135029.1350450Invention Example 2

[0195] The microstructure of the steel plate manufactured in this way was observed and is shown in Table 3 below.

[0196] Among the microstructures, martensite (M), tempered martensite (TM), and bainite (B) were observed through SEM after nital etching of the polished specimen cross-section, and retained austenite (RA) was measured through XRD analysis.

[0197] In addition, the Ceq value (a) according to the following Equation 2 at a depth of 30 μm in the thickness direction from the surface of the steel plate and the Ceq value (b) at a point 1 / 4t of the thickness of the cold-rolled steel plate were measured, and the ratio (a / b) was shown in Table 3 below.

[0198] [Relationship 2]

[0199] Ceq = C + Mn / 6 + Si / 20 + Cr / 5 + Mo / 4 (%)

[0200] In addition, the ferrite area fraction (F1) at a depth of 5 μm in the thickness direction from the surface of the cold-rolled steel sheet and the ferrite area fraction (F2) at a depth of 30 μm in the thickness direction from the surface of the cold-rolled steel sheet were measured, respectively, and the (F2 / F1)×100(%) value of Equation 3 was calculated and shown in Table 3 below.

[0201] In addition, mechanical properties were measured for each specimen, and the results are described in Table 4 below. Yield strength (YS), tensile strength (TS), and elongation (El) were evaluated through tensile testing, and each mechanical property was measured by evaluating specimens taken according to JIS-5 standard based on a 90° direction relative to the rolling direction.

[0202] Classification Microstructure Ceq(a) Ceq(b) Ceq(a / b) F1(Area %) F2(Area %) (F2 / F1) × 100(%) FB TM FM RA Comparative Example 1 35 20 31 11 30.55 20.55 60.99 39 17 44 Inventive Example 1 16 19 49 12 40.55 20.56 60.98 25 8 32 Comparative Example 2 6 20 57 13 40.55 20.56 60.98 12 4 33 Comparative Example 3 15 37 13 31 40.55 20.56 60.98 23 9 39 Comparative Example 4 16 340 46 40.55 20.56 60.98 22 9 41 Comparative Example 5143205040.5520.5660.98201155Comparative Example 6152505640.5520.5660.98231252Comparative Example 71519491430.4920.5060.97221255Comparative Example 81518491530.7110.7260.98211152Comparative Example 91517511430.6430.6560.98201050Comparative Example 101616521330.3230.3330.97241250Comparative Example 111719481240.7850.7990.98221045Comparative Example 121617501430.6290.7260.87241250 Comparative Example 131519511140.5940.6030.99251248 Invention Example 21718491240.5840.5940.9827933

[0203] In Table 3, F represents ferrite, B represents bainite, TM represents tempered martensite, FM represents fresh martensite, and RA represents retained austenite.

[0204] Classification Mechanical Properties YS(MPa)TS(MPa)El(%)YRYSYEl(MPa·%)TSYEl(MPa·%)HER(%)HERYEl(%·%)3-Point Bending( o )3-point bend ΥEl( o ·%) Comparative Example 163299615.00.63948014940284201081620 Inventive Example 1711103015.70.691116316171619581362135 Comparative Example 2919110111.90.831093613102556551271511 Comparative Example 3702102315.90.691116216266396201101749 Comparative Example 4732106015.30.691120016218304591111698 Comparative Example 5788109414.10.721111115425243381081523Comparative Example 6803112315.80.721268717743223481021612Comparative Example 752292718.60.56970917242346321242306Comparative Example 8912117310.60.78966712434444661151219Comparative Example 9847112218.00.751524620196386841122016Comparative Example 1056393017.30.61974016089406921192059 Comparative Example 11898113910.00.79898011390313101171170 Comparative Example 12933117811.10.791035613076273001181310 Comparative Example 13940118312.00.791128014196293481101320 Invention Example 2673102415.50.661043215872639771392155

[0205] As shown in Tables 1 to 4 above, in the case of Invention Examples 1 and 2, in which alloy composition, component ratio, and manufacturing process conditions satisfy the scope of the present invention, it can be seen that both exhibit excellent welding properties as well as mechanical properties. Specifically, Invention Examples 1 and 2 had a yield strength of 600 MPa or more, a tensile strength of 980 MPa or more, and a yield ratio of 0.6 or more. Furthermore, the cold-rolled steel sheet had an elongation of 15% or more, a product of tensile strength and elongation of 14,700 MPa% or more, a hole expansion of 45% or more, a product of hole expansion and elongation of 675%·% or more, a 3-point bend of 135° or more, and a product of 3-point bend and elongation of 2025%·° or more, confirming that it exhibits excellent strength as well as hole expansion and bendability simultaneously.

[0206] In contrast, Comparative Examples 1-6 represent cases where the gold component system satisfies the proposal of the present invention, but the manufacturing conditions deviate from the present invention. In particular, Comparative Examples 1 and 2 represent cases where the annealing temperature is low or high, and it was found that the ferrite intended in the present invention is excessive or insufficient, resulting in insufficient strength or low elongation.

[0207] Meanwhile, Comparative Examples 3 to 6 are cases where the secondary cooling temperature is excessively high, resulting in the excessive formation of fresh martensite in the microstructure and failure to secure sufficient bending characteristics. In particular, Comparative Example 6 corresponds to a case where the secondary cooling rate is insufficient and the reheating temperature is high, resulting in failure to secure sufficient bending characteristics.

[0208] Meanwhile, Comparative Examples 7 to 13 are cases where the manufacturing conditions satisfy the present invention, but the alloy composition system deviates from the proposal of the present invention. Among these, Comparative Example 7 is a case where the carbon content in the steel is insufficient, and it can be understood that the strength intended in the present invention was not secured as the hardenability of the steel was reduced.

[0209] Comparative Example 8 had inferior ductility and bendability as the martensite phase formed by the excessive addition of C content in the steel became relatively harder, increasing the difference in hardness with the surrounding phases.

[0210] Comparative Example 9 is a case where the Si content in the steel was excessively added, and it can be seen that the value of Equation 1 was 6.5, which exceeded 5.1. In other words, solid solution strengthening and high ductility could be secured, but high hole expansion and bendability at the level of the inventive example could not be secured.

[0211] Comparative Example 10 is a case where the Mn content in the steel is insufficient, and the strength is inferior. In particular, Comparative Example 12 is confirmed to have a value of Equation 1 of 2.4, which is less than 4.0; therefore, it can be understood that Comparative Example 12 does not achieve the strength intended by the present invention as the hardenability of the steel is reduced.

[0212] Comparative Example 11 is a case where the Mn content in the steel is excessive, and it can be seen that the value of Equation 1 is 6.6, which exceeds 5.1. That is, in Comparative Example 13, the hardenability of the steel is excessively high, so the ductility, hole expansion, and bendability targeted by the present invention are not secured.

[0213] Comparative Example 12 is a case where Cr is added excessively, and it can be seen that the value of Equation 1 is 5.8, which exceeds 5.1. That is, in Comparative Example 14, the hardenability of the steel was excessively high, so the ductility, hole expandability, and bendability targeted by the present invention were not secured.

[0214] Comparative Example 13 is a case where Mo is added in excess, and it can be understood that the ductility, hole expansion, and bendability targeted in the present invention are not secured as the hardenability of the steel increases.

[0215] Meanwhile, FIG. 1 is an SEM micrograph of the central cross-section of the steel of Invention Example 1 in an embodiment of the present invention, and FIG. 2 is an SEM micrograph of the surface cross-section of the steel of Invention Example 1 in an embodiment of the present invention.

[0216] Referring to FIG. 2, the surface layer of the steel of Invention Example 1 according to one embodiment of the present invention can be divided into three layers in the thickness direction as follows. The first decarburization region appears to have undergone ferritization to a certain extent and can be confirmed to have a depth of 5 μm in the thickness direction from the surface of the steel plate. The second decarburization region can be confirmed to have a depth of 30 μm in the thickness direction. It can also be confirmed to include a base material that is not affected by decarburization.

Claims

1. In weight percent, carbon (C): 0.05–0.25%, manganese (Mn): 1.0–3.6%, silicon (Si): 2.0% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and one or more selected from chromium (Cr): 1.0% or less, molybdenum (Mo): 0.15% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, boron (B): 0.004% or less, and the remainder comprising iron (Fe) and unavoidable impurities, and It has a microstructure in area % comprising ferrite: 7–30%, fresh martensite: 30% or less, retained austenite: 10% or less, and the remainder comprising one or more of bainite and tempered martensite, and A steel plate satisfying the following equation 3, wherein the region extending from the surface of the steel plate to a thickness of 50 μm in the thickness direction is called the surface layer, and the region other than the surface layer is called the center, the surface layer includes a decarburized layer, wherein the decarburized layer extending from the surface of the steel plate to a depth of 20 μm in the thickness direction is called the first decarburized region, and extending from the first decarburized region to a depth of 40 μm in the thickness direction is called the second decarburized region. [Relationship 3] 30% ≤ (F2 / F1) × 100 ≤ 44% In the above equation 3, F1 represents the ferrite fraction (area %) in the first decarburization region, and F2 represents the ferrite fraction (area %) in the second decarburization region.

2. In Paragraph 1, The above steel plate is a steel plate satisfying the following relationship 1. [Relationship 1] 4.0 ≤ C + 1.1Si + 1.5Mn + 1.6Cr + 1.8Mo ≤ 5.1 (Here, each element refers to its weight content.) 3. In Paragraph 1, The above steel plate is a steel plate satisfying the ratio (a / b) of the Ceq(a) value measured at the surface layer and the Ceq(b) measured at the center layer being 0.97 to 0.98 according to the following relationship 2. [Relationship 2] Ceq = C + Mn / 6 + Si / 20 + Cr / 5 + Mo / 4 (%) In the above equation 2, each element represents the content (weight%).

4. In Paragraph 1, A steel plate having a molten zinc plating layer or an alloyed molten zinc layer formed on at least one surface of the steel plate.

5. In Paragraph 1, A steel plate having a ferrite fraction of 10 to 50% in the first decarburization region.

6. In Paragraph 1, A steel plate having a ferrite fraction of 5 to 20% in the second decarburization region.

7. A step of providing a cold-rolled steel sheet comprising, in weight percent, carbon (C): 0.05~0.25%, manganese (Mn): 1.0~3.6%, silicon (Si): 2.0% or less (excluding 0%), acid-soluble aluminum (Sol.Al): 0.1% or less (excluding 0%), phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), and at least one selected from chromium (Cr): 1.0% or less, molybdenum (Mo): 0.15% or less, niobium (Nb): 0.1% or less, titanium (Ti): 0.1% or less, boron (B): 0.004% or less, and the remainder being iron (Fe) and unavoidable impurities; A step of annealing the provided cold-rolled steel sheet at a temperature range of 800 to 840°C; A step of first cooling the annealed cold-rolled steel sheet to a temperature range of 550 to 700℃ at an average cooling rate of 1 to 10℃ / s; A step of secondary cooling to a temperature range of 250 to 390°C after the above primary cooling at an average cooling rate of 10 to 55°C / s; and A method for manufacturing a steel plate comprising the step of reheating to a temperature in the range of 250 to 500℃ after the above secondary cooling and overaging treatment for 100 to 700 seconds.

8. In Paragraph 7, A method for manufacturing a steel sheet in which the above annealing is performed in an annealing furnace with an atmosphere of a dew point temperature of -10 to 20℃.

9. In Paragraph 7, The above cold-rolled steel sheet is, A step of heating the above steel slab in a temperature range of 1000 to 1350℃; A step of manufacturing a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated steel slab in a temperature range of Ar3 to Ar3+50℃; A step of winding the above hot-rolled steel sheet at a temperature range of 400 to 700℃; and The above-mentioned coiled hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet. A method for manufacturing a steel plate including 10. In Paragraph 7, The above cold-rolled steel sheet is a steel sheet manufacturing method satisfying the following relationship 1. [Relationship 1] 4.0 ≤ C + 1.1Si + 1.5Mn + 1.6Cr + 1.8Mo ≤ 5.1 Here, each element represents the content (weight%).

11. In Paragraph 7, A method for manufacturing a steel sheet, further comprising the step of forming a hot-dip galvanized layer by immersing the above-mentioned over-aged cold-rolled steel sheet in a hot-dip galvanized bath.

12. In Paragraph 11, A method for manufacturing a steel sheet, further comprising the step of forming an alloyed hot-dip galvanized layer by alloying the cold-rolled steel sheet having the above-mentioned hot-dip galvanized layer formed thereon.

13. In Paragraph 11 or 12, A method for manufacturing a steel plate, wherein temper rolling is performed on the above cold-rolled steel plate.

Images