Steel sheet and manufacturing method thereof

Abstract

The present invention relates to a steel sheet that can be used for automobile structures and the like, and a method for manufacturing same.

Description

Steel plate and method of manufacturing the same

[0001] The present invention relates to a steel plate that can be used for automobile structures, etc., and a method for manufacturing the same.

[0002] Due to the recent strengthening of safety regulations for automobile passengers and pedestrians, the installation of safety devices has become mandatory, leading to an increase in vehicle body weight that runs counter to the trend of lightweighting aimed at improving fuel efficiency.

[0003] In particular, as consumer interest in eco-friendly and fuel-efficient hybrid and electric vehicles increases, producing such eco-friendly and safe vehicles requires lightweighting of the body structure and ensuring the stability of the body materials.

[0004] However, hybrid vehicles are adding various devices such as electric engines, electric batteries, and secondary fuel storage tanks in addition to the conventional gasoline engine. Furthermore, as driver amenities are continuously added, the weight of the vehicle body is increasing.

[0005] Accordingly, in order to achieve lightweighting of the vehicle body, it is essential to develop materials that are thin yet possess excellent strength, ductility, and bending properties. Therefore, to solve this problem, it is necessary to develop giga-grade steel sheets capable of securing high strength and high ductility with a tensile strength of 980 MPa or higher.

[0006] In addition, in the manufacture of high-strength steel sheets with a tensile strength of 980 MPa or higher, differences in phase transformation may occur due to temperature variations in the width direction of the hot-rolled sheet during ROT cooling and differences in cooling rates between the coil center and edge after coiling. Consequently, the microstructure formed in different parts of the hot-rolled coil varies, which can lead to differences in the material properties of the hot-rolled sheet. Such variations in the width direction of the hot-rolled material cause a decrease in PCM productivity and lead to problems such as scratch defects and meandering in subsequent processes due to FH shape defects. Consequently, issues are being raised, such as cutting off the head / tail portion of the hot-rolled material before feeding it into the PCM.

[0007] To resolve these issues, lowering the coiling temperature to 450℃ or lower reduces material variation, but the increase in martensite fraction can cause cold-rolled sheet fracture due to the cold-rolled roll force load. Therefore, considering this, it is necessary to lower the strength of the hot-rolled steel sheet by applying a heat treatment furnace after hot rolling, but there is a problem that process costs increase significantly when applying a heat treatment furnace.

[0008] As such, variations in yield strength, material properties, and microstructure of the hot-rolled steel sheet in the width direction (Edge, Center) can cause variations in YS material properties and microstructure in different parts of the final steel sheet manufactured after annealing of the cold-rolled material, and such variations can cause formability problems, such as dimensional defects depending on the width direction position, when forming parts using the steel sheet.

[0009] Therefore, research on a method for manufacturing high-strength steel capable of suppressing material variation in the width direction of the aforementioned hot-rolled steel sheet has been ongoing.

[0010] (Patent Document 1) Korean Published Patent Application No. 10-2019-0078259

[0011] (Patent Document 2) Korean Registered Patent Publication No. 10-1561007

[0012] According to one embodiment of the present invention, a steel plate and a method for manufacturing the same may be provided.

[0013] According to another embodiment of the present invention, a high-strength, high-elongation steel plate and a method for manufacturing the same may be provided.

[0014] According to another embodiment of the present invention, a method for manufacturing a low yield ratio, high-strength cold-rolled steel sheet with low material variation of the steel sheet can be provided.

[0015] 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.

[0016] A steel plate according to one embodiment of the present invention is,

[0017] In weight percent, Carbon (C): 0.05–0.1%, Silicon (Si): 0.2–1.0%, Manganese (Mn): 2.2–2.8%, Phosphorus (P): 0.1% or less, Sulfur (S): 0.01% or less, Aluminum (sol.Al): 0.1% or less, Chromium (Cr): 1.0% or less, Molybdenum (Mo): 0.20% or less, Titanium (Ti): 0.04% or less, Niobium (Nb): 0.06% or less, Boron (B): 0.004% or less, Nitrogen (N): 0.01% or less, and the remainder consists of Fe and other unavoidable impurities,

[0018] The microstructure contains 20–50% ferrite, 30–50% martensite, and 20–40% bainite in area %, and

[0019] The difference in ferrite fraction between the surface layer and the center in the thickness direction of the steel plate may be 10% or less.

[0020] The above microstructure may contain 30-50% martensite and 20-40% bainite.

[0021] A steel plate according to another embodiment of the present invention is,

[0022] In weight percent, carbon (C): 0.05–0.1%, silicon (Si): 0.2–1.0%, manganese (Mn): 2.2–2.8%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.20% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.004% or less, nitrogen (N): 0.01% or less, and the remainder being Fe and other unavoidable impurities,

[0023] The microstructure, in area %, consists of ferrite: 20–50%, and the remainder comprises bainite and martensite,

[0024] The above martensite includes a first martensite having five or more laths per unit area with an equivalent diameter of 2 μm, and a second martensite having fewer than five needle-shaped laths per area with an equivalent diameter of 2 μm.

[0025] The share of the first martensite in the total martensite may be 70% or more.

[0026] The above steel plate can satisfy the following [Equation 1] and [Equation 2].

[0027] [Relationship 1]

[0028] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30

[0029] [Relationship 2]

[0030] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50

[0031] In the above equations 1 and 2, each element represents the weight percentage of the corresponding element, and 0 is substituted if it is not added.

[0032] The microstructure may contain 30–50% martensite and 20–40% bainite.

[0033] The share of tempered martensite among the above martensites may be 20% or less.

[0034] The tensile strength of the above steel plate may be 980 MPa or higher.

[0035] The yield ratio of the above steel plate may be 0.75 or less.

[0036] The surface of the above steel plate may include a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.

[0037] A method for manufacturing a steel plate according to another embodiment of the present invention is,

[0038] A step of heating a steel slab comprising, in weight%, carbon (C): 0.05~0.1%, silicon (Si): 0.2~1.0%, manganese (Mn): 2.2~2.8%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.20% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.004% or less, nitrogen (N): 0.01% or less, and the remainder being Fe and other unavoidable impurities;

[0039] 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+50℃ to 950℃;

[0040] A step of cooling the above-mentioned manufactured hot-rolled steel sheet to 450~650℃ and then coiling it;

[0041] A step of obtaining a cold-rolled steel sheet by cold-rolling the above-mentioned coiled hot-rolled steel sheet at a cold reduction rate of 40~80%;

[0042] A step of annealing the above cold-rolled steel sheet at 800~830℃;

[0043] A first cooling step of cooling the annealed steel plate to 630~690℃ at an average cooling rate of 2~14℃ / s;

[0044] A step of cooling the first cooled steel plate to 350~500℃ at an average cooling rate of 10℃ / s or more and maintaining it for 100 seconds or more; and

[0045] The method may include a step of cooling the maintained steel plate to a temperature of 100℃ or lower at an average cooling rate of 1 to 15℃ / s.

[0046] A method for manufacturing a steel plate according to another embodiment of the present invention is,

[0047] A step of heating a steel slab comprising, in weight percent, carbon (C): 0.05~0.1%, silicon (Si): 0.2~1.0%, manganese (Mn): 2.2~2.8%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.20% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.004% or less, nitrogen (N): 0.01% or less, and the remainder being Fe and other unavoidable impurities;

[0048] 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+50℃ to 950℃;

[0049] A step of cooling the above-mentioned manufactured hot-rolled steel sheet to 500~600℃ and then coiling it;

[0050] A step of obtaining a cold-rolled steel sheet by cold-rolling the above-mentioned coiled hot-rolled steel sheet at a cold reduction rate of 40~80%;

[0051] A step of annealing the above cold-rolled steel sheet at 800~830℃;

[0052] A first cooling step of cooling the annealed steel plate to 630~690℃ at an average cooling rate of 2~14℃ / s;

[0053] A step of cooling the first cooled steel plate to 350~500℃ at an average cooling rate of 10℃ / s or more and maintaining it for 100 seconds or more; and

[0054] The method may include a step of cooling the maintained steel plate to a temperature of 100℃ or lower at an average cooling rate of 1 to 15℃ / s.

[0055] The above steel slab may satisfy one or more of the following [Equation 1] and [Equation 2].

[0056] [Relationship 1]

[0057] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30

[0058] [Relationship 2]

[0059] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50

[0060] In the above equations 1 and 2, each element represents the weight percentage of the corresponding element, and 0 is substituted if it is not added.

[0061] The deviation in yield strength measured at the edge and center of the above hot-rolled steel sheet may be 200 MPa or less.

[0062] After the above maintenance, the step of forming a molten zinc plating layer by immersing in a molten zinc plating bath may be further included.

[0063] After forming the above molten zinc plating layer, the step of alloying heat treatment may be further included.

[0064] It may further include a temper rolling step.

[0065] According to the present invention, a steel plate having high strength and high ductility and a method for manufacturing the same can be provided.

[0066] In addition, by minimizing the material variation in the width direction of the hot-rolled material, it is possible to provide a steel plate with minimized material variation in the final steel plate product and a method for manufacturing the same. Furthermore, the steel plate may have high strength and high ductility and an excellent low yield ratio.

[0067] 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.

[0068] Figure 1 is a figure simulating the isothermal phase transformation analysis in the present invention.

[0069] Figure 2 is an SEM image of the edge and center of the hot-rolled material of Invention Example 5 in an embodiment of the present invention.

[0070] Figure 3 is an SEM image of the edge and center of the hot-rolled material of Comparative Example 1 in an embodiment of the present invention.

[0071] Figure 4 is an SEM tissue image of Invention Example 5 in an embodiment of the present invention.

[0072] Figure 5 is an SEM tissue image of Comparative Example 1 in an embodiment of the present invention.

[0073] FIG. 6 is a graph showing the relationship between Equation 1 of Inventive Examples 1-5 and Comparative Examples 1-4 and the YS deviation in the width direction of the hot-rolled steel sheet in the embodiment of the present invention.

[0074] FIG. 7 is a graph showing the relationship between Equation 1 of Inventive Examples 1-5 and Comparative Examples 1-4 and the YS deviation in the width direction of the cold-rolled steel sheet in the embodiment of the present invention.

[0075] Figure 8 is a photograph showing the first martensite and second martensite observed in the steel plate of the present invention.

[0076] Preferred embodiments of the present invention are described below. Reference to the drawings is made as necessary. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0081] In this specification, terms such as 'top', 'upper', 'upper surface', 'lower', 'lower surface', 'lower surface', and 'side surface' are based on the drawings and may actually vary depending on the direction in which the elements or components are arranged.

[0082] Additionally, throughout the specification, when it is said that one part is 'connected' to another part, this includes not only cases where they are 'directly connected,' but also cases where they are 'indirectly connected' with other elements in between.

[0083] 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.

[0084] First, a steel plate, which is an embodiment of the present invention, will be described in detail.

[0085] The above steel plate may contain, in weight percent, carbon (C): 0.05~0.1%, silicon (Si): 0.2~1.0%, manganese (Mn): 2.2~2.8%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.20% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.004% or less, and nitrogen (N): 0.01% or less.

[0086] Carbon (C): 0.05~0.1 wt% (hereinafter, %)

[0087] The above-mentioned carbon (C) is a very important element added for solid solution strengthening. Additionally, C contributes to strength improvement by combining with precipitation elements to form fine carbides. If the content of C is less than 0.05%, it is very difficult to secure the desired strength. On the other hand, if the content of C exceeds 0.1%, the strength increases rapidly due to the excessive formation of martensite during cooling after hot rolling caused by increased hardenability. This can lead to plate fracture under roll force load during cold rolling, and the material properties may be inferior in the final structure due to structural variations caused by the cooling rate in the width direction. Furthermore, weldability is inferior, increasing the likelihood of welding defects occurring during component processing at the customer. Therefore, the content of C may be 0.05 to 0.1%. Preferably, it may be 0.050 to 0.10%. It is more preferable that the lower limit of the C content be 0.06%, and the upper limit be 0.09%.

[0088] Silicon (Si): 0.2~1.0%

[0089] The above-mentioned silicon is one of the five major elements of steel, and a small amount is naturally added during the manufacturing process. This silicon contributes to an increase in strength and suppresses the formation of carbides, thereby preventing carbon from forming carbides during annealing cracking and cooling. Furthermore, by suppressing the formation of pearlite band structures during hot rolling and finely dispersing carbides, the austenite is evenly dispersed during annealing, allowing the martensite to be finely dispersed during final cooling, which is advantageous for securing elongation. If the silicon content is less than 0.2%, it may be difficult to sufficiently secure the aforementioned effects. On the other hand, if the silicon content exceeds 1.0%, it may cause surface scale defects, degrade the plating surface quality, and reduce chemical treatment performance. Therefore, the silicon content may be 0.2% to 1.0%. Preferably, it may be 0.20% to 1.00%. It is more preferable that the lower limit of the silicon content be 0.30%, and it is more preferable that the upper limit be 0.90%.

[0090] Manganese (Mn): 2.2~2.8%

[0091] The above Mn is an element that completely precipitates sulfur in the steel as MnS, thereby preventing hot brittleness caused by the formation of FeS and solid solution strengthening the steel. If the content of the above Mn is less than 2.2%, it is difficult to secure the strength targeted in the present invention. On the other hand, if the content of the above Mn exceeds 2.8%, there is a high possibility that problems such as weldability and hot rolling performance will occur, and at the same time, it may increase hardenability and lead to the excessive formation of martensite, which may result in a decrease in elongation. In addition, there is a problem in that Mn-bands (regions where concentrated Mn exists in a band form) are formed within the microstructure, increasing the risk of processing cracks and plate fracture, and Mn oxides are leached to the surface during annealing, significantly impairing plating properties. Therefore, the content of the above Mn may be 2.2 to 2.8%. Preferably, it may be 2.20 to 2.80%. It is more preferable that the lower limit of the above Mn content is 2.3%, and the upper limit is 2.7%.

[0092] Phosphorus (P): 0.1% or less

[0093] The above-mentioned P is the most advantageous element for securing strength without significantly impairing the formability of steel; however, if added in excess, the possibility of brittle fracture increases significantly, leading to a higher likelihood of plate breakage of the slab during hot rolling. Furthermore, there is a problem in that it acts as an element that impairs the surface characteristics of galvanized steel sheets. Since the aforementioned problems may occur if the P content exceeds 0.1% in the present invention, the above-mentioned P may be included at 0.1% or less, preferably 0.10% or less. Meanwhile, considering the level of P that is inevitably added during the steel manufacturing process, 0% of the content may be excluded.

[0094] Sulfur (S): 0.01% or less

[0095] The above-mentioned S is an impurity inevitably added to steel, and it is effective to manage its content to be as low as possible. In particular, S in steel is highly likely to cause red-hot brittleness. In the present invention, if the content of the above-mentioned S exceeds 0.01%, the aforementioned problems may occur; therefore, the above-mentioned S may be included at 0.01% or less, preferably 0.010% or less. Meanwhile, considering the level of S inevitably added during the steel manufacturing process, 0% of the content may be excluded.

[0096] Aluminum (sol.Al): 0.1% or less

[0097] Aluminum is an element added to steel for grain refinement and deoxidation. In the present invention, if the content of sol.Al exceeds 0.1%, while it is advantageous for increasing the strength of steel due to the grain refinement effect, excessive inclusions are formed during continuous steelmaking operations, increasing the likelihood of surface defects in plated steel sheets. Additionally, there is a concern that economic feasibility may be reduced due to increased manufacturing costs. Therefore, in the present invention, sol.Al may be included in an amount of 0.1% or less, preferably 0.10% or less. According to another embodiment of the present invention, the lower limit of sol.Al may be 0.005% or more, or 0.010% or more.

[0098] Chrome (Cr): 1.0% or less,

[0099] The above Cr is an element that improves hardenability and increases the strength of steel. However, if the content of the above Cr exceeds 1.0%, problems of through-corrosion may occur due to the non-uniform formation of Cr oxides in a saltwater atmosphere, and it is also uneconomical to add Cr. Therefore, in the present invention, the above Cr content may be 1.0% or less. It is more preferable that the content of the above Cr be 0.90% or less, and it is preferable that it be 0.80% or less. Meanwhile, since the effect of improving hardenability and strength can be obtained even with a small amount in the present invention, the lower limit of the above Cr is not specifically limited.

[0100] Molybdenum (Mo): 0.20% or less

[0101] The above Mo is a carbide-forming element that, when combined with carbide-nitride-forming elements such as Ti, Nb, and V, plays a role in improving yield strength and tensile strength by maintaining the size of precipitates finely. Furthermore, the above Mo has the advantage of improving the hardenability of steel, allowing for the fine formation of martensite at grain boundaries, thereby enabling control of the yield ratio. However, since it is an expensive element, there is a disadvantage that manufacturing becomes unfavorable as its content increases; therefore, it is desirable to appropriately control its content. If the content of the above Mo exceeds 0.20%, it leads to a sharp increase in manufacturing costs, resulting in reduced economic feasibility. Moreover, due to excessive grain refinement and solid solution strengthening effects, there is a problem where the ductility of the steel actually decreases. In the present invention, the lower limit of the above Mo content is not limited, but it is preferable that the lower limit of the above Mo content be 0.01%, and more preferable that it be 0.03%. It is more preferable that the upper limit of the above Mo content be 0.18%, and more preferable that it be 0.15%.

[0102] Titanium (Ti): 0.04% or less

[0103] The above Ti contributes to securing yield strength and tensile strength as a fine carbide-forming element. In addition, as a nitride-forming element, Ti precipitates N in the steel as TiN, thereby suppressing AlN precipitation, which has the advantage of reducing the risk of cracking during continuous casting. In the present invention, if the Ti content exceeds 0.04%, coarse carbides precipitate, and a decrease in strength and elongation may occur due to the reduction in dissolved carbon content in the steel, and nozzle clogging may occur during continuous casting. Therefore, it is desirable for the Ti content to be in a range of 0.04% or less. In the present invention, the lower limit of the Ti content is not limited, but it is preferable to set the lower limit to 0.004%, and it is more preferable to set the upper limit to 0.03%.

[0104] Niobium (Nb): 0.06% or less

[0105] The above Nb is an element that segregates at the austenite grain boundaries, suppresses the coarsening of austenite grains during annealing heat treatment, and contributes to an increase in strength by forming fine carbides. In the present invention, if the content of the above Nb exceeds 0.06%, coarse carbides precipitate, and a decrease in strength and elongation may occur due to a reduction in the amount of dissolved carbon in the steel, and there is a problem of increased manufacturing costs. Therefore, it is desirable for the content of the above Nb to have a range of 0.06% or less. The present invention is not limited to a lower limit of the above Nb content, but it is preferable that it be 0.005%, and it is more preferable that the upper limit of the above Nb content be 0.05%.

[0106] Boron (B): 0.004% or less (including 0%)

[0107] The above-mentioned B is an element that contributes significantly to securing the hardenability of steel. However, if the content of the above-mentioned B exceeds 0.004%, boron carbides are formed at the grain boundaries, providing nucleation sites for ferrite, which may actually worsen the hardenability. Therefore, it is desirable for the content of the above-mentioned B to be in a range of 0.004% or less. In the present invention, the lower limit of the above-mentioned B content is not limited but can be 0.0004%, and it is more preferable for the upper limit of the above-mentioned B content to be 0.003%.

[0108] Nitrogen (N): 0.01% or less

[0109] The above N is an impurity that is inevitably added to steel, and it is effective to manage its content to be as low as possible. Since there is a concern that steel refining costs may rise sharply when the content of the above N is reduced to the extreme, the content may be limited to a range where operation is possible, taking this into consideration. As one example, the content of the above N may be limited to 0.01% or less, provided that 0% may be excluded from the content, taking into account the level that is inevitably added during the steel manufacturing process.

[0110] In addition to the aforementioned components, it may contain the remaining Fe and other unavoidable impurities. However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the normal manufacturing process, they cannot be completely excluded. As these impurities are known to anyone with ordinary knowledge in the art, not all details thereof are specifically mentioned in this specification.

[0111] Furthermore, the addition of effective ingredients other than those mentioned above is not entirely excluded.

[0112] The alloy composition of the above steel plate can satisfy the following Equations 1 and 2.

[0113] [Relationship 1]

[0114] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30

[0115] [Relationship 2]

[0116] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50

[0117] In the above equations 1 and 2, each element represents the weight percentage of the corresponding element, and 0 is substituted if it is not added.

[0118] As shown in FIG. 1, the inventors confirmed that it is possible to manufacture a high-strength composite structure steel sheet with a low yield ratio (YR≤0.75) and excellent material variation, by analyzing the isothermal phase transformation of a hot-rolled material and, when the ferrite+pealite fraction generated during isothermal transformation at 600°C defined by the following equation 1 is 30% or less and the bainite fraction generated during isothermal transformation at 450°C defined by the following equation 2 is 50% or more, the YS material variation in the width direction of the hot-rolled material is less than 200 MPa without applying the aforementioned heat treatment furnace, the YS material variation in the width direction of the steel sheet after the annealing process is less than 80 MPa, and the difference in ferrite fraction between the edge portion and the center portion of the coil in the width direction is 15% or less.

[0119] If the ferrite + pearlite fraction generated during the 600°C isothermal transformation defined by the above Equation 1 exceeds 30%, a ferrite + pearlite structure is formed in the coil center and a bainite + martensite structure is formed in the coil edge due to the temperature deviation in the width direction of the hot-rolled sheet during ROT cooling and the difference in cooling rates between the coil center and edge after coiling. Consequently, due to this difference in the width direction structure, the width direction YS material deviation exceeds 200 MPa. This width direction hot-rolled material deviation leads to a decrease in PCM productivity and causes problems such as post-process scratch defects and meandering due to FH shape defects, thereby causing issues such as cutting off the head / tail portion of the hot-rolled material before feeding it into the PCM. In addition, if the yield strength material and microstructure variation in the width direction (Edge, Center) of such hot-rolled steel sheets is large, it causes microstructure variation in the final steel sheet manufactured after annealing of cold-rolled steel, resulting in the YS material variation in the width direction of the product exceeding 100 MPa, and such variation may cause formability problems, such as dimensional defects depending on the width direction position, when forming parts using the steel sheets.

[0120] To resolve these problems, if the coiling temperature is lowered to 450°C or lower, the material variation is reduced. However, if the bainite fraction generated during the 450°C isothermal transformation defined by the above Equation 2 is less than 50%, the martensite fraction increases significantly, causing the strength to rise excessively, which may result in cold-rolled sheet breakage due to the roll force load during cold rolling. Therefore, considering this, it is necessary to apply a heat treatment furnace after hot rolling to lower the strength of the hot-rolled steel sheet, but there is a problem that the process cost increases significantly when applying the heat treatment furnace.

[0121] The microstructure of the above steel plate may include, in area %, ferrite: 20~50%, and the remainder being bainite and martensite. If the ferrite area fraction is less than 20%, the strength is too high to secure the desired elongation, and if it exceeds 50%, there is a problem that the desired strength cannot be secured.

[0122] The above steel plate may have a variation in the ferrite fraction between the surface layer and the center in the thickness direction of the steel plate of 10% or less. The above steel plate may have a variation in the ferrite fraction of 10% or less as a result of observing the microstructure at any two or more points in each region of the surface layer and the center that are collinear in the thickness direction of a cross-section of a steel plate having thickness t. The surface layer may be a region with a thickness (t) of 0.2t to 0.3t, excluding the decarburized extreme surface layer, and the center may be a region with a thickness (t) of 0.3t to 0.7t. Meanwhile, to give statistical significance, it is preferable to calculate the average fraction after measuring at least 20 microstructure images at x5000 magnification in the surface layer and the center that are collinear in the thickness direction, respectively.

[0123] Meanwhile, the above martensite may include a first martensite having five or more laths per unit area with an equivalent diameter of 2 μm, and a second martensite having fewer than five needle-shaped laths per unit area with an equivalent diameter of 2 μm.

[0124] In particular, it may include a first martensite having five or more needle-shaped laths with a width of 0.2 μm or less and an aspect ratio (length / width) of 1.5 or more per unit area with an equivalent diameter of 2 μm, and a second martensite having fewer than five needle-shaped laths per area with an equivalent diameter of 2 μm.

[0125] The above first martensite fraction and second martensite fraction can be confirmed using scanning electron microscope (SEM) images. As a specific example, in a microstructure image at 5,000x magnification observed by SEM after Nital etching of a steel plate, the first martensite, in which there are five or more needle-shaped laths with a width of 0.2 μm or less and an aspect ratio (length / width) of 1.5 or more within a unit area with an equivalent diameter of 2 μm, and the second martensite, in which there are fewer than five needle-shaped laths within an area with an equivalent diameter of 2 μm, can be distinguished and then measured using an image analyzer. Figure 7 is an example showing the distinction between the first martensite and the second martensite in a steel plate SEM image. Meanwhile, it is desirable to measure at least 20 microstructure images to give statistical significance.

[0126] The area share of the first martensite in the above martensite may be 70% or more of the total martensite. If the share of the first martensite is less than 70%, the proportion of the second martensite, which has a high carbon concentration and high strength, increases, resulting in inferior elongation, and the bendability and hole expansion properties become inferior due to an increase in the difference in hardness between phases. More preferably, it is desirable that the share of the first martensite be 75% or more, and most preferably 80% or more.

[0127] The above martensite and bainite may, for example, contain 30-50% martensite and 20-40% bainite in area %. If the martensite area fraction is less than 30%, the strength is too low to secure the desired strength, and if it exceeds 50%, the strength is too high to secure the desired elongation. If the bainite area fraction is less than 20%, the hardness difference between the martensite and ferrite phases is too high, resulting in poor bendability and hole expansion, and if it exceeds 40%, the yield strength increases, exceeding a yield ratio of 0.75.

[0128] In addition, the above martensite may include 20% or less of tempered martensite area.

[0129] The tensile strength of the above steel plate may be 980 MPa or higher, and the yield ratio (YR) may be 0.75 or lower.

[0130] In particular, the steel plate of the present invention has technical significance in that it exhibits small material variation. For example, there is an effect in that the variation in microstructure and physical properties between the edge and the center of the steel plate, which is considered to have the largest material variation, is small. Specifically, the edge may be a point 0 to 10 cm from the edge of the entire width of the steel plate, and the center may refer to the area excluding the edge. The variation in microstructure between the edge and the center may be such that the ferrite fraction is 10% or less, and the variation in physical properties may be a difference of 50 MPa or less based on yield strength.

[0131] The above steel plate may include a plating layer. The type of plating layer is not particularly limited, and any plating that can be performed in the technical field to which the present invention belongs is sufficient. As an example, it may be a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a zinc alloy plating layer containing aluminum and / or magnesium, etc.

[0132] Next, a method for manufacturing a steel plate, which is another embodiment of the present invention, will be described in detail. The above method for manufacturing a steel plate can be performed by passing the steel slab heating, hot rolling, coiling, cold rolling, annealing, and cooling processes to a prepared steel slab. This will be described in detail below.

[0133] Steel slab heating

[0134] The heating process of the steel slab is a process intended to facilitate the hot rolling process described later and to sufficiently obtain the target physical properties of the steel plate. As an example, the steel slab may have the same alloy composition as the steel plate according to one embodiment of the present invention. Additionally, it may satisfy Equations 1 and 2. The description of each alloy element and the description of the compositional relationship equations are replaced by the aforementioned details. In one embodiment of the present invention, it is preferable to heat the steel slab within a temperature range of 1050 to 1300°C. If the heating temperature is below 1050°C, friction between the steel plate and the rolling mill increases, causing a problem where the load applied to the rollers during hot rolling increases rapidly. On the other hand, if the temperature exceeds 1300°C, not only does the energy cost required to raise the temperature increase, but the amount of surface scale also increases, which may lead to material loss. Accordingly, in one embodiment of the present invention, the heating process of the steel slab can be performed in a temperature range of 1050 to 1300°C. According to another embodiment of the present invention, the heating process of the steel slab can be performed at 1100°C or higher, and according to yet another embodiment, it can be performed at 1250°C or lower.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] 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.

[0139] Hot rolling

[0140] The above heated steel slab can be hot-rolled to obtain a hot-rolled steel sheet, i.e., a hot-rolled material.

[0141] In one embodiment of the present invention, a hot-rolled steel sheet can be manufactured by performing a finishing hot rolling in a temperature range of Ar3+50℃ to 950℃ during the hot rolling process. In one embodiment of the present invention, if the finishing hot rolling process is performed at a temperature below Ar3+50℃, a ferrite + austenite two-phase rolling is performed, which may cause material non-uniformity. On the other hand, if the temperature exceeds 950℃, there is a risk of material non-uniformity caused by the formation of abnormal coarse grains due to high-temperature rolling, and as a result, there is a problem of coil warping occurring during subsequent cooling.

[0142] The above Ar3 represents the austenite transformation completion temperature and, as an example, can be calculated as shown below (Equation 1).

[0143] (Equation 1)

[0144] Ac3(℃) = 910 - 203√C + 44.7Si - 30Mn + 11Cr - 15.2Ni + 31.5Mo + 104V - 400Al

[0145] In the above (Equation 1), each element symbol represents the content (weight%) of the element contained in the steel.

[0146] Kwon Chi

[0147] The hot-rolled steel sheet manufactured above can be coiled. In one embodiment of the present invention, the coiling process can be performed in a temperature range of 450 to 650°C. If the coiling temperature is below 450°C, the strength of the hot-rolled steel sheet becomes excessively high, which may cause a rolling load during subsequent cold rolling. In addition, excessive costs and time are required to cool the hot-rolled steel sheet to the coiling temperature, which causes an increase in process costs. On the other hand, if the temperature exceeds 650°C, excessive scale may form on the surface of the hot-rolled steel sheet, which is highly likely to cause surface defects and cause a deterioration in plating properties. The hot-rolled steel sheet coiled as described above may have an edge microstructure containing bainite and martensite, and a core microstructure containing ferrite, bainite, and martensite.

[0148] For example, the edge portion of the hot-rolled material may be a point 0 to 10 cm from the edge of the entire width of the hot-rolled steel sheet, and the center portion of the hot-rolled material may refer to the area excluding the edge portion. The difference in physical properties between the edge portion and the center portion may be a difference of 200 MPa or less based on yield strength.

[0149] Cold rolling

[0150] The above-mentioned coiled hot-rolled steel sheet can be cold-rolled to produce a cold-rolled steel sheet. In one embodiment of the present invention, cold rolling can be performed with a cold reduction rate (total reduction rate) of 40 to 80%. If the cold reduction rate during cold rolling is less than 40%, it becomes difficult to secure the target thickness, and it becomes difficult to correct the shape of the steel sheet. On the other hand, if the cold reduction rate exceeds 80%, there is a high possibility of cracks occurring at the edge of the steel sheet, and there is a problem of generating a load during cold rolling.

[0151] Annealing

[0152] The cold-rolled steel sheet manufactured above can be subjected to annealing treatment, for example, by continuous annealing. According to one embodiment of the present invention, the annealing treatment can be performed in a continuous alloying molten plating furnace. In one embodiment of the present invention, the annealing treatment can be performed in a temperature range of 800 to 830°C. That is, by performing the annealing treatment of the cold-rolled steel sheet in a two-phase region temperature range where ferrite and austenite coexist, it is possible to form ferrite as well as austenite simultaneously with the recrystallization of the structure, while distributing carbon. In the present invention, if the annealing temperature is below 800°C, not only is sufficient recrystallization not achieved, but the austenite phase is also not sufficiently formed, making it impossible to achieve the target microstructure phase composition after the annealing treatment. On the other hand, if the temperature exceeds 830°C, productivity decreases, and due to the excessive formation of the austenite phase, there is a problem in that the fraction of the martensite phase becomes excessive after the subsequent cooling process. In this case, while the yield strength increases, the ductility decreases, making it impossible to secure the intended low yield ratio and high ductility characteristics. Furthermore, the surface concentration of elements such as Si, Mn, and B, which impede the wettability of hot-dip galvanizing in the alloy composition, becomes excessive, which can degrade the plating surface quality.

[0153] Therefore, in the present invention, the annealing treatment can be performed in a temperature range of 800 to 830°C.

[0154] Cooling and maintenance

[0155] Cold-rolled steel sheets treated with annealing can be cooled in the above two-phase temperature range.

[0156] In one embodiment of the present invention, the cooling may be performed in stages, and as one example, the process may involve first cooling the annealed steel plate to a temperature range of 630 to 690°C at a cooling rate of 2 to 14°C / s, and then second cooling the first cooled steel plate to a temperature range of 350 to 450°C at a cooling rate of 10°C / s or more. At this time, the cooling rate during the second cooling may be faster than the cooling rate during the first cooling.

[0157] In this way, the type and fraction of the microstructure formed can be controlled by performing stepwise cooling up to a specific temperature range according to the cooling rate during the cooling of a cold-rolled steel sheet in which a certain fraction of a ferrite phase is formed along with austenite during the annealing process according to one embodiment of the present invention.

[0158] In the present invention, a ferrite phase can be additionally introduced into the steel plate by first cooling the annealed steel plate to a temperature range of 630 to 690°C at a cooling rate of 2 to 14°C / s.

[0159] If the cooling rate during the first cooling is less than 2℃ / s, the austenite phase formed during the annealing process transforms into an excessive fraction of ferrite, making it impossible to adequately secure bainite and martensite phases in the final microstructure. On the other hand, if the cooling rate exceeds 14℃ / s, the additionally introduced ferrite phase is insufficient, and there is a risk that the ductility of the steel sheet will decrease.

[0160] In addition, if the cooling end temperature during the first cooling process is less than 630°C, the ferrite phase is not sufficiently formed, whereas if the temperature exceeds 690°C, there is a problem that the cooling rate must be increased excessively during the subsequent second cooling process, and there is a concern that the fraction of the bainite phase and martensite phase in the final microstructure may not be sufficient.

[0161] According to one embodiment of the present invention, during the first cooling process of an annealed cold-rolled steel sheet, an area fraction of 20 to 30 percent of a ferrite phase can be additionally formed, and together with this, the remaining austenite phase is dispersed.

[0162] Furthermore, in the present invention, after the first cooling of the annealed steel sheet, a second cooling can be performed at a cooling rate of 10°C / s or more up to a temperature range of 350 to 500°C, and a bainite phase can be introduced by maintaining the cold-rolled steel sheet in such a cooled state.

[0163] If the cooling rate during the above secondary cooling is less than 10℃ / s, pearlite may be generated during the cooling process, and the bainite phase may not be sufficiently formed. Meanwhile, the upper limit of the cooling rate during the above secondary cooling is not specifically limited, and a person skilled in the art may select it appropriately by considering the specifications of the cooling equipment. As an example, it may be performed at 100℃ / s or less.

[0164] In addition, if the cooling end temperature during the second cooling is less than 350°C, the martensite phase is excessively formed, and the martensite phase is tempered during the subsequent holding process, making it impossible to secure the intended low yield ratio and high ductility. On the other hand, if the temperature exceeds 500°C, the bainite phase is not sufficiently formed, and thus the effect of reducing the difference in hardness between phases due to the bainite phase cannot be obtained.

[0165] In the present invention, the holding time of the secondary cooled steel plate may be 100 seconds or more. If the holding time is less than 100 seconds, bainite cannot be obtained in a sufficient fraction. The upper limit of the holding time is not specifically limited and may be determined as the time at which the bainite phase is formed in a fraction intended by a person skilled in the art.

[0166] In the present invention, the holding process performed after the second cooling can be performed within the temperature range where the second cooling is completed, so the temperature range is not specifically limited. However, as one example, it can be performed at 400℃ or lower.

[0167] Final cooling

[0168] The steel plate, after undergoing cooling and holding processes, can be finally cooled.

[0169] In one embodiment of the present invention, the final cooling of the steel plate, which has undergone a stepwise cooling and holding process, can be carried out at a cooling rate of 1 to 15°C / s to a temperature of 100°C or lower, and a martensite phase can be introduced during this process.

[0170] If the cooling rate during the final cooling is less than 1℃ / s or the cooling end temperature exceeds 100℃, the martensite phase cannot be secured to the intended level. In one embodiment of the present invention, in terms of forming a certain fraction of the martensite phase, the cooling can be performed at a cooling rate of 15℃ / s or less, for example, and as another example, the final cooling can be performed to room temperature.

[0171] According to one embodiment of the present invention, the austenite remaining after transforming into ferrite and bainite phases during the stepwise cooling and holding process may transform into the martensite phase during the final cooling process. At this time, the austenite phase that transforms into the martensite phase during the final cooling process is uniformly and finely distributed within the steel sheet due to the dispersion effect during the previous stepwise cooling and holding process, so the martensite phase formed during the final cooling process can also be formed with a fine and uniform distribution. In one embodiment of the present invention, a plated steel sheet may be obtained by plating a cold-rolled steel sheet, and as one example, hot-dip galvanizing may be performed on the steel sheet that has undergone the stepwise cooling and holding process prior to the final cooling.

[0172] Hot-dip galvanizing

[0173] A molten zinc-plated steel sheet can be manufactured by immersing a steel sheet according to one embodiment of the present invention, that is, a steel sheet that has undergone a stepwise cooling and holding process, in a molten zinc-plated bath.

[0174] In one embodiment of the present invention, the molten zinc plating process may be performed under normal conditions, but as an example, it may be performed in a temperature range of 430 to 490°C. In addition, the composition of the molten zinc-based plating bath during the molten zinc plating is not particularly limited and may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[0175] Alloying heat treatment

[0176] In addition, if necessary, an alloyed hot-dip galvanized steel sheet can be obtained by alloying a hot-dip galvanized steel sheet produced by hot-dip galvanizing according to one embodiment of the present invention.

[0177] In one embodiment of the present invention, the alloying heat treatment process conditions are not particularly limited and normal conditions are acceptable. As one example, the alloying heat treatment process can be performed in a temperature range of 480 to 600°C.

[0178] Tough rolling

[0179] Furthermore, if necessary, a temper rolling process may be further performed, and the above temper rolling process may be performed not only on the cold-rolled steel sheet that has undergone final cooling, but also on the molten zinc-plated steel sheet or alloyed molten zinc-plated steel sheet that has undergone final cooling.

[0180] According to one embodiment of the present invention, bake hardenability can be further improved by forming a large amount of dislocations within the steel through a temper rolling process, and as an example, it can be performed with a reduction rate of less than 1% (excluding 0%). When the reduction rate during the temper rolling is 1% or more, it is advantageous in terms of dislocation formation, but adverse effects such as plate breakage may occur due to the limitations of equipment capacity.

[0181] 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.

[0182] (Example)

[0183] After heating steel slabs having the alloy composition system of Table 1 below at a temperature of 1050 to 1250°C, each heated slab was finished hot-rolled at Ar3+50°C to 950°C to produce hot-rolled steel sheets. Subsequently, each hot-rolled steel sheet was pickled under normal conditions, coiled in the temperature range of Table 2 below, and then cold-rolled with a cold reduction rate of 40 to 80% to produce cold-rolled steel sheets.

[0184] Afterward, each cold-rolled steel sheet was subjected to continuous annealing treatment under the conditions shown in Table 2 below, followed by stepwise cooling (1st - 2nd cooling), maintained under the conditions of Table 2, cooled to room temperature at the cooling rate of Table 2, and then temper-rolled at a reduction rate of 0.2% to produce the final cold-rolled steel sheet.

[0185] Steel composition (wt%) relationship formula 1 2 Remarks CSM nPS AlCrMoTiNbBN10.06 0.42.30.0110.004 0.0350.850.130.020.050.00250.004459 Invention Steel20.09 0.42.40.0120.0020.030.10.150.020.030.0010.0042589 30.100.22.50.0110.008 0.030.40.100.020.02-0.0052991 40.08 0.72.70.0120.0020.030.10.050.0350.050.00250.003218750. 080.52.60.0150.0040.030.30.050.0250.020.00250.002127060.151.22.40.0140.0070.03--0.020.020.00200.0057758Comparison70.131.02.20.0160.0050.030.70.100.02-0.00100.006514880.141.52.40.0140.0020.03---0.020.00200.004724390.101.12.80.0110.0080.03-----0.0034580

[0186] Steel Type Coiling Temperature (°C) Hot-rolled Material Cold-rolled Properties Annealing Temperature (°C) 1st Cooling 2nd Cooling Holding Time (sec) Final Cooling Rate (°C / s) Remarks Edge Center Deviation End Temperature (°C) Cooling Rate (°C / s) End Temperature (°C) Cooling Rate (°C / s) YS(MPa) YS(MPa) dYS(MPa) 15 20 7 8 8 6 9 19 7 Good 8 10 6 5 0 5 3 6 0 15 3 5 0 2 Invention Example 1 2 5 0 8 5 0 7 0 2 14 8 Good 8 3 0 6 4 0 6 4 4 0 2 3 0 5 Invention Example 2 3 6 0 7 5 1 6 0 11 5 0 Good 8 0 6 8 0 3 4 5 0 3 1 5 0 10 Invention Example 3 4 5 10 7 7 4 6 4 3 13 1 Good 8 2 0 6 3 0 10 4 3 0 2 5 2 0 8 Invention Example 45580761650111 Good 8276704400352503 Invention Example 56580757464293 Inferior 8306503450172005 Comparative Example 17550786512274 Inferior 8106807350203503 Comparative Example 28450840564276 Inferior 800630940030150 8 Comparative Example 396 307 09 466 243 Inferior 8256 705 430 252 50 12 Comparative Example 4 15207 8869 197 Good 760 650 440 3030 12 Comparative Example 5 15207 8869 197 Good 850 640 8420 253 50 10 Comparative Example 6 15207 8869 197 Good 830 650 5300 8020 0 8 Comparative Example 7 15207 8869 197 Good 8207 50 1450 3515 0 5 Comparative Example 8

[0187] In Table 2 above, the yield strength of the hot-rolled steel sheet was tested in the L direction according to JIS standards on specimens taken from the edge and center of the manufactured hot-rolled steel sheet to confirm material uniformity. The edge was measured in the 0~10cm range of the edge of the entire width of the hot-rolled steel sheet, and the center was measured in the area excluding the edge.

[0188] For each steel plate manufactured according to the above, the microstructure was measured and the mechanical properties were evaluated, and the results are shown in Tables 3 to 5.

[0189] First, regarding the microstructure of the steel plate in Table 3, the fractions of ferrite, bainite, and martensite phases were measured using FE-SEM and an image analyzer after Nital etching on specimens taken at the thickness direction t / 4 (t: steel plate thickness (unit mm)). The first martensite fraction was measured using an image analyzer on martensite with five or more acicular laths per 2 μm diameter area in microstructure images observed with SEM at 5000x magnification after Nital etching. To give statistical significance, the average fraction was calculated after measuring on more than 20 microstructure images at 5000x magnification of SEM.

[0190] Meanwhile, the ferrite fraction was measured using FE-SEM and an image analyzer after Nital corrosion at at least two arbitrary points in each region of the surface layer (0.2t–0.3t) and the center (0.3t–0.7t) in the direction of steel plate thickness (t), and the results are shown together in Table 3. To give statistical significance, measurements were taken from at least 20 microstructure images at x5000 magnification of the SEM in the surface layer and center, respectively, on the same line in the thickness direction, and the average fraction was calculated.

[0191] Specimens taken from the center and edge center on the same line (taken at a point 0 to 10 cm from the edge of the full width of the steel plate and at a point 10 cm from the center) were measured, and the results are shown in Table 3.

[0192] In addition, to evaluate the tensile properties of each steel plate in Table 4, tests were conducted on the specimens of the manufactured steel plates in the L direction according to JIS standards. Furthermore, bendability (R / t, R: minimum bending radius, t: specimen thickness) was evaluated using the minimum bending test according to ISO standards.

[0193] Meanwhile, Table 5 shows the results of investigating the material properties and ferrite fraction at the center and edge portions of the steel plate on the same line, where the material variation is expected to be the largest among the steel plates. To evaluate the tensile material properties of each steel plate at the center and edge portions, specimens taken from the edge and center portions of the manufactured cold-rolled steel plates (taken at points 0 to 10 cm from the edge of the entire width of the steel plate and at 10 cm from the center) were subjected to L-direction testing in accordance with JIS standards to confirm material uniformity.

[0194] Steel Microstructure (Area %) 1st M Share (%) Thickness Direction F Fraction and Deviation (%) Remarks FBM 1st M Surface Layer Center Deviation |Surface Layer-Center| 13 139 30 289 330 333 Invention Example 1 22 33 245 33 732 33 18 Invention Example 2 345 20 35 30 86 32 419 Invention Example 3 42 8 25 47 38 81 29 334 Invention Example 4 5 30 32 38 318 22 5 327 Invention Example 5 6 39 22 39 12 31 20 37 17 Comparative Example 1 7 47 1 9341441284516 Comparative Example 284810422150274619 Comparative Example 394515401640284315 Comparative Example 4165102572859645 Comparative Example 5128315747033 Comparative Example 6131861386224273 Comparative Example 7159142793351554 Comparative Example 8

[0195] In Table 3, F represents ferrite, B represents bainite, M represents martensite, and 1 M represents 1 martensite.

[0196] Steel Grade Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) Yield Ratio Bending Properties (R / t) Remarks 17 1 10 3 2 1 10.6 9 0.36 Invention Example 1 2 6 9 7 10 2 4 1 30.6 8 0.71 Invention Example 2 3 6 4 5 10 10 1 30.6 4 1.43 Invention Example 3 4 6 8 4 10 2 4 1 30.6 7 1.07 Invention Example 4 5 7 0 6 10 3 2 1 20.6 80 Invention Example 5 6 6 40 10 0 2 1 2 0.64 2.5 Comparative Example 1 754 89 371 40.58 2.14 Comparative Example 2 853 1986 130.54 2.86 Comparative Example 3 956 1937 150.60 2.5 Comparative Example 4 1490 871 160.56 2.14 Comparative Example 5 1912 1053 90.87 1.43 Comparative Example 6 1859 11058 0.78 1.43 Comparative Example 7 157 1928 150.62 1.79 Comparative Example 8

[0197] Steel Type Steel Sheet Material Steel Sheet Structure Remarks Edge Center Deviation Edge Center Deviation YS(MPa)TS(MPa)El(%)YRYS(MPa)TS(MPa)El(%)YRdYS(MPa)F(%)F(%)dF(%) 17 19 10 48 110.69 71 110 32 110.69 82 73 14 Invention Example 1 27 38 10 36 120.71 69 710 241 30.684120277 Invention Example 236911024130.676451010130.644627358 Invention Example 347151036120.696841024130.673123285 Invention Example 457181047110.697061032120.681225283 Invention Example 567601048100. 736401002120.64120213918 Comparative Example 1 76501018130.64548937140.58102304717 Comparative Example 2 86401033110.62531986130.54109304717 Comparative Example 3 96601027120.64561937150.6099294516 Comparison Example 4 1510880150.58490871160.562061654 Comparative Example 5 1920105990.87912105390.878022 Comparative Example 6 1871110570.79859110580.781227314 Comparative Example 7 1590930140.63571928150.621955594 Comparative Example 8

[0198] As shown in Tables 1 to 5 above, Invention Examples 1 to 5, which satisfy both the alloy composition system and manufacturing conditions according to one embodiment of the present invention, have a microstructure formed as intended, so it can be confirmed that they have a tensile strength of 980 MPa or more, an elongation of 10% or more, and a yield ratio of 0.75 or less. In addition, excellent machinability with an R / t of 2.0 or less could be secured.

[0199] In addition, it can be seen that the material variation between the edge and center of the steel plate in the above Examples 1 to 5 is excellent. In particular, in the case of the examples of the present invention, the first martensite share was 70% or more in all of them. In the present invention, when a bainite + martensite structure is formed during hot rolling, fine carbides and carbon are evenly distributed in the bainite and martensite laths and grain boundaries. The carbides and carbon in these laths and grain boundaries can serve as nucleation sites for austenite generated during two-phase annealing. Compared to the ferrite + pearlite structure of the comparative example, the bainite + martensite structure of the examples of the present invention has more sites where austenite can nucleate, and its distribution is also uniformly dispersed. As a result, in the present invention, there are many austenite nucleation sites, so austenite nucleates and grows rapidly during two-phase annealing. Consequently, the austenite fraction increases at the same annealing temperature, which lowers the carbon concentration within the austenite and also ensures a uniform distribution of the austenite. Therefore, after two-phase annealing, the austenite is rapidly cooled to the bainite region, and then maintained for a certain period of time to form bainite. During the cooling process to room temperature, a significant amount of the remaining austenite is produced as first martensite with a low carbon concentration, thereby allowing for the production of less second martensite.

[0200] On the other hand, Comparative Examples 1 to 8, which do not satisfy at least one of the alloy composition system and manufacturing conditions according to one embodiment of the present invention, did not form a microstructure as intended. In addition, a large amount of secondary martensite is generated, which is believed to be due to the following reason. To explain in detail, when a mixed structure consisting of ferrite and pearlite bands is generated during hot rolling, the carbon concentration in the ferrite is low and plate-shaped carbides are intensively formed in the pearlite band structure; thus, the pearlite band becomes the main nucleation site for austenite generated during two-phase annealing. That is, compared to the mixed structure of bainite and martensite of the present invention, the sites where austenite can be nucleated are reduced, and nucleation sites are concentrated in the pearlite band region. Therefore, since there are fewer austenite nucleation sites, when annealing in the two-phase region, the austenite nucleates and grows locally in the pearlite band region, so the austenite fraction is lower compared to the invention example, and the carbon content in the austenite increases. Therefore, when rapidly cooling to the bainite region after annealing in the two-phase region and then maintaining it for a certain period of time, bainite is formed, and when the remaining austenite is cooled to room temperature, a small amount of first martensite with a high carbon concentration is produced, and a large amount of second martensite with a high carbon concentration is produced.

[0201] Specifically, Comparative Examples 1 to 4 did not satisfy the steel composition presented by the present invention, and it was confirmed that even though the manufacturing process was satisfied, the first martensite share was low. As a result, the required bending characteristics could not be secured, and in some cases, strength could not be secured.

[0202] In addition, as a composite structure consisting of ferrite and pearlite bands is formed during the hot rolling stage, the fraction of the second martensite is high during two-phase annealing due to the reasons mentioned above, so it was not possible to secure a share of 70% of the total martensite occupied by the first martensite.

[0203] Comparisons 5 to 8 satisfy the steel composition presented in the present invention, but fall outside the annealing process temperature and fail to satisfy the first martensite share. In the case of Comparative Example 5, the ferrite fraction was too high to secure sufficient strength, and in the case of Comparative Example 6, the ferrite fraction was too low to secure sufficient elongation.

[0204] In addition, Comparative Examples 5 to 8 secured bainite and martensite structures during the hot rolling stage, but were unable to secure a first martensite share of 70% as they fell outside the range of annealing conditions of the present invention.

[0205] Specifically, Comparative Example 5 has an annealing temperature of 760°C which is too low, so the ferrite fraction exceeds 50%, and the martensite fraction is low, so the carbon concentration in the martensite is high, making it impossible to secure a first martensite share of 70% or more.

[0206] In Comparative Example 6, the annealing temperature was too high at 850°C, resulting in a ferrite fraction that was too low and an austenite fraction that was too high, which lowered the carbon concentration. However, during holding after the second cooling, the bainite fraction increased significantly and was concentrated in the remaining 15% of the austenite, and during the subsequent cooling, the second martensite fraction increased, so it was not possible to secure a first martensite share of 70% or more, and the total fraction of bainite and martensite was too high, resulting in a yield ratio exceeding 0.75.

[0207] In Comparative Example 7, the second cooling end temperature was too low at 300℃ and the cooling rate was too fast at 80℃ / s, so the primary martensite share of 70% could not be secured due to the effect of improving hardenability, and when maintained after the second cooling ended, the martensite was tempered and the yield ratio exceeded 0.75.

[0208] In Comparative Example 8, the first cooling end temperature was too high at 750℃ and the cooling rate was too slow at 1℃ / s, so the amount of ferrite transformation during cooling increased to over 50%, and as a result, the martensite fraction was low and the carbon concentration in the martensite was high, so it was not possible to secure a first martensite share of 70% or more.

[0209] Meanwhile, Figure 2 is an SEM micrograph of the edge and center of the hot-rolled material of Example 5 in an embodiment of the present invention. The microstructure of the edge and center of the steel plate consists of bainite and martensite structures, and shows that the two phases are finely and uniformly distributed.

[0210] Figure 3 is an SEM image of the edge and center of the hot-rolled material of Comparative Example 1 in an embodiment of the present invention. It can be seen that the edge of the steel plate has a bainite and martensite structure, while the center consists of ferrite and coarse pearlite band structures.

[0211] Figure 4 is an SEM image of the microstructure of Example 5 in the embodiment of the present invention. It shows that the ferrite + martensite + bainite structure is uniformly distributed in both the edge portion and the center of the steel plate.

[0212] Figure 5 is an SEM image of Comparative Example 1 in an embodiment of the present invention. It can be seen that the edge portion of the steel plate has a uniformly distributed structure of ferrite + bainite + martensite, but the center portion, although composed of ferrite + bainite + martensite, has a non-uniform structure in which the distribution is not uniform and the martensite is densely gathered in a band form.

[0213] FIG. 6 is a figure showing the relationship between Equation 1 of Inventive Examples 1 to 5 and Comparative Examples 1 to 4 and the YS deviation in the width direction of the hot-rolled steel sheet in the embodiment of the present invention, showing that when Equation 1 is 30 or less, the YS deviation in the width direction of the hot-rolled steel sheet can be secured at 200 MPa or less.

[0214] FIG. 7 is a figure showing the relationship between Equation 1 of Inventive Examples 1 to 5 and Comparative Examples 1 to 4 in the embodiment of the present invention and the YS deviation in the width direction of the cold-rolled steel sheet, and it can be seen that when Equation 1 is 30 or less, the YS deviation in the width direction can be secured at 80 MPa or less.

Claims

1. In wt%, Carbon (C): 0.05–0.1%, Silicon (Si): 0.2–1.0%, Manganese (Mn): 2.2–2.8%, Phosphorus (P): 0.1% or less, Sulfur (S): 0.01% or less, Aluminum (sol.Al): 0.1% or less, Chromium (Cr): 1.0% or less, Molybdenum (Mo): 0.20% or less, Titanium (Ti): 0.04% or less, Niobium (Nb): 0.06% or less, Boron (B): 0.004% or less, Nitrogen (N): 0.01% or less, and the remainder comprises Fe and other unavoidable impurities, The microstructure contains 20–50% ferrite, 30–50% martensite, and 20–40% bainite in area %, and A steel plate having a difference in ferrite fraction between the surface layer and the center in the thickness direction of the steel plate of 10% or less.

2. In wt%, carbon (C): 0.05–0.1%, silicon (Si): 0.2–1.0%, manganese (Mn): 2.2–2.8%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.20% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.004% or less, nitrogen (N): 0.01% or less, and the remainder being Fe and other unavoidable impurities, The microstructure, in area %, consists of ferrite: 20–50%, and the remainder comprises bainite and martensite, The above martensite includes a first martensite having five or more laths per unit area with an equivalent diameter of 2 μm, and a second martensite having fewer than five needle-shaped laths per area with an equivalent diameter of 2 μm. A steel plate in which the share of the first martensite among the total martensite is 70% or more.

3. In paragraph 1 or 2, the steel plate is a steel plate satisfying the following [Equation 1] and [Equation 2]. [Relationship 1] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30 [Relationship 2] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50 In the above equations 1 and 2, each element represents the weight percentage of the corresponding element, and 0 is substituted if it is not added.

4. In Paragraph 2, The above microstructure is a steel plate containing 30-50% martensite and 20-40% bainite.

5. In Paragraph 1 or 2, A steel plate in which the proportion of tempered martensite among the above martensites is 20% or less.

6. In Paragraph 1 or 2, A steel plate having a tensile strength of 980 MPa or higher.

7. In Paragraph 1 or 2, A steel plate having a yield ratio of 0.75 or less.

8. In Paragraph 1 or 2, A steel plate having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the above steel plate.

9. A step of heating a steel slab comprising, in weight%, carbon (C): 0.05~0.1%, silicon (Si): 0.2~1.0%, manganese (Mn): 2.2~2.8%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.20% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.004% or less, nitrogen (N): 0.01% or less, and the remainder being Fe and other unavoidable impurities; 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+50℃ to 950℃; A step of cooling the above-mentioned manufactured hot-rolled steel sheet to 450~650℃ and then coiling it; A step of obtaining a cold-rolled steel sheet by cold-rolling the above-mentioned coiled hot-rolled steel sheet at a cold reduction rate of 40~80%; A step of annealing the above cold-rolled steel sheet at 800~830℃; A first cooling step of cooling the annealed steel plate to 630~690℃ at an average cooling rate of 2~14℃ / s; A step of cooling the first cooled steel plate to 350~500℃ at an average cooling rate of 10℃ / s or more and maintaining it for 100 seconds or more; and A method for manufacturing a steel plate according to claim 1, comprising the step of cooling the maintained steel plate to a temperature of 100℃ or lower at an average cooling rate of 1 to 15℃ / s.

10. A step of heating a steel slab comprising, in wt%, carbon (C): 0.05~0.1%, silicon (Si): 0.2~1.0%, manganese (Mn): 2.2~2.8%, phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (sol.Al): 0.1% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.20% or less, titanium (Ti): 0.04% or less, niobium (Nb): 0.06% or less, boron (B): 0.004% or less, nitrogen (N): 0.01% or less, and the remainder being Fe and other unavoidable impurities; 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+50℃ to 950℃; A step of cooling the above-mentioned manufactured hot-rolled steel sheet to 500~600℃ and then coiling it; A step of obtaining a cold-rolled steel sheet by cold-rolling the above-mentioned coiled hot-rolled steel sheet at a cold reduction rate of 40~80%; A step of annealing the above cold-rolled steel sheet at 800~830℃; A first cooling step of cooling the annealed steel plate to 630~690℃ at an average cooling rate of 2~14℃ / s; A step of cooling the first cooled steel plate to 350~500℃ at an average cooling rate of 10℃ / s or more and maintaining it for 100 seconds or more; and A method for manufacturing a steel plate according to claim 2, comprising the step of cooling the maintained steel plate to a temperature of 100℃ or lower at an average cooling rate of 1 to 15℃ / s.

11. In Paragraph 9 or 10, The above steel slab is a method for manufacturing a steel plate satisfying one or more of the following [Equation 1] and [Equation 2]. [Relationship 1] 459-244*C+21*Si-146*Mn-123*Al-39*Cr-423*Mo+684*Ti+138*Nb-16510*B ≤ 30 [Relationship 2] 192-411*C-10*Si-21*Mn-33*Al-37*Cr-90*Mo+807*Ti+163*Nb-14400*B ≥ 50 In the above equations 1 and 2, each element represents the weight percentage of the corresponding element, and 0 is substituted if it is not added.

12. In Paragraph 9, A method for manufacturing a steel sheet, wherein the deviation in yield strength measured at the edge and center of the hot-rolled steel sheet is 200 MPa or less.

13. In Paragraph 9 or 10, A method for manufacturing a steel plate, further comprising the step of forming a molten zinc plating layer by immersing in a molten zinc plating bath after maintaining the above.

14. In Paragraph 13, A method for manufacturing a steel plate comprising the step of forming the above-mentioned molten zinc plating layer and then performing an alloying heat treatment.

15. In Paragraph 9 or 10, A method for manufacturing a steel plate comprising an additional step of temper rolling.

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