Steel sheet and manufacturing method therefor

Abstract

The present invention relates to a steel sheet and a manufacturing method therefor, the steel sheet being mainly used for members and lower arms of vehicle chassis parts, reinforcing materials, connecting materials, and the like.

Description

Steel plate and method of manufacturing the same

[0001] The present invention relates to a steel plate mainly used for members, lower arms, reinforcing materials, connecting materials, etc. of automobile chassis parts, and a method for manufacturing the same.

[0002] In the past, high-strength hot-rolled steel sheets were widely used for applications such as chassis parts. In particular, high-strength hot-rolled steel sheets with a tensile strength of 590 to 780 MPa and high burring properties were widely used for lower arms and upper arms, which have the highest forming volume. The above steel sheets are often composite structural steels such as single ferritic steel, ferritic-bainite steel, or ferritic-bainite-martensite steel. In particular, to improve formability, a technology has been proposed to manufacture two-phase composite structural steel with a mixed ferritic-bainite structure as the basic matrix to improve elongation flangeability, or to manufacture high-strength, high-burring steel with a ferritic phase or a bainite phase as the basic matrix.

[0003] Patent documents 1 and 2 are ferrite single-phase structural steels that secure strength with fine precipitates and have excellent burring properties, but because high-temperature coiling is required, the ferrite grain size is coarse and segregation is easily formed, which causes problems with inferior shear formability and impact resistance, and there are also limitations in securing high strength.

[0004] The technology related to Patent Documents 3 to 5 relates to ferrite-bainite composite structure steel, which is a method of forming a polygonal ferrite and bainite composite structure by controlling the cooling process after hot rolling to a temperature of around 700°C, thereby simultaneously securing elongation flangeability and strength. Although these technologies also utilize the bainite phase, there is a problem in that it is difficult to secure high strength, and if alloying elements such as Si and Cu are excessively added, there is a concern about scale formation and high-temperature brittleness, and the shape quality may deteriorate due to local differences in cooling rates during the control of the cooling process.

[0005] To simultaneously secure high strength, shear formability, impact resistance, and elongation flangeability, it is necessary to utilize the martensite phase and the lamellar martensite (MA) phase, which have the highest medium hardness in the steel composition; however, if hard phases are included, there is a problem in that it is difficult to secure high elongation flangeability and durability due to the large difference in hardness between phases.

[0006] Although alloying elements such as Si, Mn, Mo, Cr, Cu, and Ni, which are primarily used to manufacture high-strength steels, are effective in improving the strength and workability of the aforementioned hot-rolled steel sheets, adding large amounts of these alloying elements to enhance these properties leads to segregation of the alloying elements and microstructural non-uniformity, resulting in inferior workability. In particular, steels with high hardenability undergo sensitive microstructural changes upon cooling, causing non-uniform formation of low-temperature transformation phases. Consequently, variations in the fraction of constituent phases occur, exhibiting a large distribution of elongation flangeability or hole expansion values, making it difficult to ensure stable quality.

[0007] (Patent Document 1) Japanese Published Patent Application No. 2002-322543

[0008] (Patent Document 2) Japanese Published Patent Application No. 2007-262487

[0009] (Patent Document 3) Japanese Published Patent Application No. 1994-293910

[0010] (Patent Document 4) Korean Registered Patent Publication No. 10-1114672

[0011] (Patent Document 5) Korean Registered Patent Publication No. 10-1528084

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

[0013] According to another embodiment of the present invention, a steel plate having excellent strength and elongation, excellent balance of strength and elongation, and excellent hole expansion properties, and a method for manufacturing the same can be provided.

[0014] 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.

[0015] A steel sheet according to one embodiment of the present invention comprises, in weight percent, C: 0.03~0.08%, Si: 0.001~0.14%, Mn: 1.0~2.0%, Al: 0.01~0.8%, P: 0.0001~0.05%, S: 0.0001~0.01%, N: 0.0001~0.01%, and the remainder consists of Fe and unavoidable impurities.

[0016] The microstructure contains 75 area% or more of bainitic ferrite and 25 area% or less of ferrite, and

[0017] GOS(i) of [Relation 1] below ferrite It can satisfy 0.5 to 0.65.

[0018] [Relationship 1]

[0019]

[0020] j(i): Total number of pixels for grain i,

[0021] wij: Angle of discrepancy between the orientation of pixel j and the average orientation of grain i

[0022] a: A constant value for deriving the range having the GOS(i) threshold corresponding to the inflection point in the entire GOS(i) integral graph.

[0023] The above steel plate may contain one or more of chromium (Cr): 0.01~0.8%, molybdenum (Mo): 0.01~0.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, and vanadium (V): 0.001~0.25%.

[0024] The above steel plate may contain one or more of the following: antimony (Sb): 0.005~0.1%, tin (Sn): 0.005~0.1%, arsenic (As): 0.005~0.1%, copper (Cu): 0.01~0.3%, and boron (B): 0.005% or less.

[0025] The above microstructure may include at least one of pearlite, fresh martensite, and MA phase in an area of ​​less than 5%.

[0026] The average grain size of the ferrite above may be 12㎛ or less.

[0027] The aspect ratio (Aspect Ratio, AR) of the above ferrite may be 0.35 to 0.8.

[0028] The above steel plate may have a yield ratio of 0.85 or higher, a hole expansion (HER) of 85% or higher, and a tensile strength of 590 MPa or higher.

[0029] A method for manufacturing a steel plate according to another embodiment of the present invention comprises the step of heating a steel slab comprising, in weight percent, C: 0.03~0.08%, Si: 0.001~0.14%, Mn: 1.0~2.0%, Al: 0.01~0.8%, P: 0.0001~0.05%, S: 0.001~0.01%, N: 0.001~0.01%, and the remainder comprising Fe and unavoidable impurities;

[0030] A step of rough rolling the above steel slab;

[0031] A step of obtaining a hot-rolled steel sheet by finishing rolling a rough-rolled steel slab;

[0032] A step of cooling the above hot-rolled steel sheet to a coiling temperature of 400 to 500°C; and

[0033] It may include a step of winding under conditions satisfying that the AE of [Relationship 2] below is 0.1 to 0.3.

[0034] [Relationship 2]

[0035] AE = A ×(C+Mn / 6)×exp(-Q / RT)

[0036] Here, Q is the activation enthalpy, and the experimental measurement data for solid solution C in steel with a BCC grain structure is 84.1 / KJ mol -1 A is a constant expressed as the activation energy associated with C diffusion and Fe3C growth in the corresponding CT region where solid solution C is not precipitated during hot rolling, R is the gas constant (8.314 J / (mol·K)), T is the coiling temperature (K), and each element symbol represents the alloy composition content (weight%) of the steel.

[0037] The above steel slab may contain one or more of chromium (Cr): 0.01~0.8%, molybdenum (Mo): 0.01~0.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, and vanadium (V): 0.001~0.25%.

[0038] The above steel slab may contain one or more of the following: antimony (Sb): 0.005~0.1%, tin (Sn): 0.005~0.1%, arsenic (As): 0.005~0.1%, copper (Cu): 0.01~0.3%, and boron (B): 0.005% or less.

[0039] The above steel slab can be heated to a temperature range of 1100 to 1350°C.

[0040] The above hot rolling can be performed by rough rolling at a temperature of 1050°C or higher (RDT), and by finishing rolling at a finishing rolling temperature (FDT) of 890°C or higher (FDT).

[0041] After heating the above steel slab, it can be cooled at a cooling rate of 20℃ / s or less until rough rolling.

[0042] After the above rough rolling, cooling can be performed at a cooling rate of 50℃ / s or more until coiling.

[0043] According to one embodiment of the present invention, a steel plate having excellent strength and elongation and excellent hole expansion properties can be provided. The steel plate of one embodiment of the present invention can be applied to lower arms, upper arms, link parts, wheel discs, etc., which require high formability of the flange portion and burring properties.

[0044] 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.

[0045] Preferred embodiments of the present invention are described below. 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] In order to manufacture lower arms, upper arms, link parts, and wheel discs using high-strength steel with a tensile strength of 590 MPa or higher, high formability of the flange and burring properties are required. To achieve this, shearing of the steel sheet before forming, and the occurrence of initial porosity or cracks in the holes and edges of the punched formed parts must be prevented to ensure high-quality formed products during subsequent forming. To this end, manufacturers require a hole expansion ratio (HER), and it is necessary to satisfy a value of at least 85%, and preferably 90% or more.

[0051] The present invention aims to provide a method for securing a tensile strength of 590 MPa or higher while simultaneously ensuring the formability described above. In particular, conventionally, it has been difficult to clearly determine the correlation between the shear formability, burring properties, and flange formability of a steel sheet based solely on the combination of types and fractions of microstructures. Accordingly, the present invention proposes a method for securing excellent strength and hole expansion properties by identifying the dislocation density within the grains and the characteristics of the microstructure at the grain level.

[0052] A steel plate, which is an embodiment of the present invention, will be described in detail. First, the alloy composition of the steel will be described in detail. Regarding the alloy composition of the steel, % means weight % unless otherwise specified.

[0053] The above steel plate contains, in weight%, C: 0.03~0.08%, Si: 0.01~0.14%, Mn: 1.0~2.0%, Al: 0.01~0.8%, P: 0.0001~0.05%, S: 0.0001~0.01%, and N: 0.0001~0.01%.

[0054] Carbon (C): 0.03 to 0.08%

[0055] Carbon (C) is the most economical and effective element for strengthening steel. As the content of C increases, the formation of polygonal ferrite during cooling is suppressed, which can increase the fraction of bainitic ferrite structure. At the same time, it lowers the bainite transformation temperature, thereby maintaining a high dislocation density within the bainitic ferrite. Additionally, C diffuses into austenite during the bainitic ferrite transformation and transforms into low-temperature bainite and / or martensite, which are secondary phases, during the final cooling process, thereby contributing to an improvement in tensile strength. If the content of C is less than 0.03%, hardenability is insufficient, resulting in a low fraction of bainitic ferrite and secondary phases, making it difficult to secure sufficient tensile strength and / or yield strength. On the other hand, if the content of C exceeds 0.08%, the fraction of secondary phases increases excessively, making it difficult to secure elongation and potentially reducing weldability. That is, the content of C may be 0.03 to 0.08%, specifically 0.04 to 0.07%, and more specifically 0.05 to 0.06%.

[0056] Silicon (Si): 0.001 to 0.14%

[0057] Silicon (Si) is an element that improves the hardenability of steel and can improve strength through solid solution strengthening effects. In addition, the Si can improve yield strength and / or tensile strength by delaying the formation of carbides and preventing the formation of pearlite, thereby causing transformation into secondary phases such as low-temperature bainite and / or martensite. If the Si content is less than 0.001%, it may be difficult to secure the effect of strength improvement through solid solution strengthening. On the other hand, if the Si content exceeds 0.14%, Fe-Si complex oxides are formed on the slab surface upon reheating, which not only degrades the surface quality of the steel plate but also causes problems with reduced weldability. The Si content may be 0.001 to 0.14%, and preferably 0.01 to 0.12%.

[0058] Manganese (Mn): 1.0 to 2.0%

[0059] Manganese (Mn) is an element that improves the hardenability of steel and can facilitate the formation of a low-temperature transformation structure by preventing the formation of polygonal ferrite during cooling after finish rolling. If the Mn content is less than 1.0%, it may be difficult to secure sufficient strength due to insufficient fractions of bainitic ferrite, which is a low-temperature transformation structure, and secondary phases. If the Mn content exceeds 2.0%, the hardenability increases significantly, and the delayed transformation of bainitic ferrite in the cooling zone causes untransformed austenante to transform into final martensite, which may reduce elongation. The Mn content may be 1.0 to 2.0%, and specifically 1.2 to 1.8%.

[0060] Aluminum (Al): 0.01 to 0.8%

[0061] Aluminum (Al) is an element added for deoxidation, but it can also improve strength through solid solution strengthening. The above Al may be acid-soluble Al (Sol-Al). If the content of the above Al is less than 0.01%, it may be difficult to sufficiently secure the effect of strength improvement through solid solution strengthening. In addition, if the content of the above Al exceeds 0.8%, it may lead to an increase in oxide and / or nitride inclusions in the steel, thereby reducing the formability of the steel sheet. That is, the content of the above Al may be 0.01 to 0.8%, specifically 0.02 to 0.7%, and more specifically 0.03 to 0.6%.

[0062] Phosphorus (P): 0.0001 to 0.05%

[0063] Phosphorus (P) is an impurity inevitably contained in steel and can be a major cause of reduced workability of steel due to segregation; therefore, the lower its content, the more effective it can be for the workability of steel sheets. The lower limit of the above P content may be 0%, but considering limitations in the manufacturing process or excessive increases in manufacturing costs, the above P content may be 0.0001% or more, and more specifically, 0.0002% or more. In addition, since the workability of steel sheets may be reduced if the above P content exceeds 0.05%, the above P content may be 0.050% or less, more specifically 0.045% or less, and even more specifically 0.040% or less.

[0064] Sulfur (S): 0.0001 to 0.01%

[0065] Sulfur (S) is an impurity inevitably contained in steel and can be a major cause of reduced workability in steel by forming non-metallic inclusions through combination with Mn, etc. Therefore, the lower the content, the more effective it can be for the workability of steel sheets. The lower limit of the S content may be 0%, but considering limitations in the manufacturing process or excessive increases in manufacturing costs, the S content may be 0.0001% or more, and more specifically, 0.0002% or more. In addition, since the workability of steel sheets may be reduced if the S content exceeds 0.01%, the S content may be 0.01% or less, and more specifically, 0.005% or less.

[0066] Nitrogen (N): 0.0001~0.01%

[0067] Nitrogen (N) is an impurity inevitably contained in steel and can reduce the workability of steel by reacting with Al and others to precipitate nitrides; therefore, the lower the content, the more effective it can be for the workability of steel sheets. The lower limit of the N content may be 0%, but considering limitations in the manufacturing process or an excessive increase in manufacturing costs, the N content may be 0.0001% or more, and more specifically, 0.0002% or more. In addition, since the workability of steel sheets may be reduced if the N content exceeds 0.01%, the N content may be 0.01% or less, specifically 0.018% or less, and more specifically 0.008% or less.

[0068] Meanwhile, in addition to the alloy composition described above, it may further include one or more of chromium (Cr): 0.01~0.8%, molybdenum (Mo): 0.01~0.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, and vanadium (V): 0.001~0.25%.

[0069] Chromium (Cr): 0.01 to 0.8%

[0070] Chromium (Cr) is an element that improves the hardenability of steel and can facilitate the formation of a low-temperature transformation structure by suppressing the formation of ferrite during primary cooling after finish rolling. If the content of Cr is less than 0.01%, the above-described effect cannot be sufficiently obtained. Furthermore, if the content of Cr exceeds 0.8%, the hardenability increases excessively, and since bainite transformation does not occur smoothly in the cooling zone, the time required to secure the fraction of bainitic ferrite, which is the matrix structure, increases excessively, which may reduce the elongation. That is, the content of Cr may be 0.01 to 0.8%, specifically 0.01 to 0.7%, and more specifically 0.05 to 0.6%.

[0071] Molybdenum (Mo): 0.01 to 0.5%

[0072] Molybdenum (Mo) is an element that significantly improves the hardenability of steel and can improve strength through solid solution strengthening effects. In addition, the Mo can suppress the formation of ferrite during cooling after finish rolling, thereby facilitating the formation of a low-temperature transformation structure. If the content of the Mo is less than 0.01%, the above-described effects cannot be sufficiently obtained. Furthermore, if the content of the Mo exceeds 0.5%, there is a problem where the alloy cost increases significantly, resulting in poor economic efficiency. Additionally, because the hardenability increases excessively, the bainite transformation does not occur smoothly in the cooling zone, which can lead to an excessive increase in the time required to secure the fraction of bainitic ferrite, the matrix structure, thereby reducing the elongation. That is, the content of the Mo may be 0.01 to 0.5%, specifically 0.01 to 0.4%, more specifically 0.02 to 0.3%, and even more specifically 0.05 to 0.2%.

[0073] Titanium (Ti): 0.001 to 0.25%

[0074] Titanium (Ti) is an element that forms carbonitrides, and by delaying the recrystallization of steel during hot rolling, it refines the grain size of austenite, promotes the transformation into bainite in the cooling zone, and refines the grain size of martensite in the microstructure, thereby improving the strength of the steel. If the content of Ti is less than 0.001%, the above-described effect cannot be sufficiently obtained. Furthermore, if the content of Ti exceeds 0.25%, the coarse carbonitrides generated during the casting stage become excessively stable, and there is a problem in that they are not sufficiently dissolved during the slab reheating stage, thereby degrading the formability of the steel sheet. That is, the content of Ti may be 0.001 to 0.25%, specifically 0.001 to 0.20%, more specifically 0.01 to 0.15%, and even more specifically 0.03 to 0.13%.

[0075] Niobium (Nb): 0.001 to 0.25%

[0076] Niobium (Nb) delays the recrystallization behavior of steel during hot rolling, thereby allowing control of the austenite grain size. If the content of Nb is less than 0.001%, the above-described effect cannot be sufficiently obtained. Furthermore, if the content of Nb exceeds 0.25%, the austenite grain size becomes excessively fine, which makes it difficult to precisely control the bainitic ferrite fraction. That is, the content of Nb may be 0.001 to 0.25%, specifically 0.001 to 0.20%, more specifically 0.01 to 0.15%, and even more specifically 0.02 to 0.08%.

[0077] Vanadium (V): 0.001 to 0.25%

[0078] Vanadium (V) delays the recrystallization behavior of steel during hot rolling, thereby allowing control of the austenite grain size. If the content of V is less than 0.001%, the above-described effect cannot be sufficiently obtained. Furthermore, if the content of V exceeds 0.25%, the austenite grain size becomes excessively fine, which makes it difficult to precisely control the bainitic ferrite fraction. The content of V may be 0.001 to 0.25%, specifically 0.001 to 0.20%, more specifically 0.01 to 0.15%, and even more specifically 0.02 to 0.08%.

[0079] Meanwhile, in addition to the alloy composition described above, it may include one or more of antimony (Sb): 0.005~0.1%, tin (Sn): 0.005~0.1%, arsenic (As): 0.005~0.1%, copper (Cu): 0.01~0.3%, and boron (B): 0.005% or less.

[0080] Antimony (Sb): 0.005~0.1%

[0081] Antimony (Sb) may inevitably be present in steel, and when an appropriate amount exists in a solid solution state, an increase in strength due to solid solution strengthening can be obtained. However, if the content is less than 0.005%, the solid solution effect described above does not occur, and if it exceeds 0.1%, although there is an effect of suppressing the formation of red scale (Fayalite, Fe2SiO4), it acts locally, leading to a higher occurrence of uneven scale defects and causing surface defects due to the localized concentration of Sb on the steel surface. Therefore, more specifically, it is desirable to set 0.08% as the upper limit.

[0082] Note (Sn): 0.005~0.1%

[0083] The above tin (Sn) may inevitably be contained in the steel and, like Sb, has a solid solution strengthening effect. However, if it is contained in excessive amounts like Sb, Sn becomes concentrated on the surface, increasing the occurrence of surface defects. The content of the above Sn may be 0.005 to 0.1%, and more specifically, it is preferable to make it 0.08%.

[0084] Arsenic (As): 0.005~0.1%,

[0085] The above arsenic (As) may inevitably be contained in steel and has the characteristic of exhibiting effects similar to Sb and Sn. However, since strength effects due to solid solution strengthening can be obtained when an appropriate level of content exists in a solid solution state, the As content may be 0.005% or more. As an element that easily segregates at surfaces or grain boundaries, similar to Sb and Sn, an excessive content not only worsens the ductility of the steel but may also cause surface defects. The above As content may be 0.005 to 0.1%.

[0086] Copper (Cu): 0.01~0.3%

[0087] Copper (Cu) can be contained in an amount of 0.01% or more as a hardenable element of steel. However, if the Cu content is excessive, there is a high possibility of slab grain boundary cracking occurring, so the upper limit may be restricted to 0.3%.

[0088] Boron (B): 0.005% or less

[0089] Boron (B) is a hardenable element that delays ferrite transformation in trace amounts, allowing for grain boundary strengthening and solid solution strengthening effects. However, if the B content exceeds 0.005%, Fe, a B-based precipitate, 23 Since (B,C)6 can cause brittle fracture during hot rolling as it forms at the austenite grain boundaries, it is desirable to limit its upper limit to 0.005%.

[0090] In addition to the above composition, the remainder is Fe. However, since unintended and unavoidable impurities from raw materials or the surrounding environment may be incorporated during the normal manufacturing process, they cannot be excluded. As these impurities are known to anyone with ordinary knowledge in the art, all details thereof are not specifically mentioned in this specification. Meanwhile, the addition of effective components other than the above composition is not excluded.

[0091] Next, the microstructure of the above steel plate is described.

[0092] The microstructure of the above steel plate comprises 75 area% or more of bainitic ferrite and 25 area% or less of ferrite. Meanwhile, some structures may be unintentionally formed in the above microstructure. For example, pearlite, fresh martensite, MA phase, etc. may be formed, and these may be less than 5 area%.

[0093] The above-mentioned structure composed of 75% or more of bainitic ferrite and 25% or less of ferrite is intended to secure the elongation and HER required for the application of parts requiring shear and burring properties. However, if the area fraction of bainitic ferrite exceeds 90%, the burring properties may be improved by reducing the elongation due to the required 590MPa-grade tensile strength, but this may result in defects when forming parts subjected to curvature or tensile deformation. Therefore, to achieve the invention, control of the cooling rate and coiling temperature during hot rolling is essential. In order to simultaneously secure high burring properties and elongation while obtaining the corresponding tensile strength, it is preferable that the bainitic ferrite does not exceed 90% of the area and is composed of ferrite, and more preferably does not exceed 85% of the area.

[0094] The above ferrite is GOS(i) of [Equation 1] below ferrite It satisfies 0.5 to 0.65.

[0095] [Relationship 1]

[0096]

[0097] j(i): Total number of pixels for grain i,

[0098] w ij : Angle of discrepancy between the orientation of pixel j and the average orientation of grain i

[0099] a: A constant value for deriving the range having the GOS(i) threshold corresponding to the inflection point in the entire GOS(i) integral graph.

[0100] The above GOS(i) ferrite To derive the value, the electron backscatter diffraction (EBSD) method, capable of quantitatively analyzing microstructural characteristics, was applied. Grains are defined using cross-sectional information obtained from EBSD measurements; as an example in this invention, if the crystal orientations corresponding to critical pixels have a rotation angle of 10° or less, they are defined as identical grains. GOS(i) ferrite Prior to definition, once the grain is defined, the number of pixels J(i) constituting the grain is determined, and the angle of mismatch Wij between the crystal orientations of pixels j constituting the grain is calculated. Ultimately, since GOS(i) is the angle of mismatch between the crystal orientations of pixels within the grain defined with respect to the measurement area, a large GOS value suggests the presence of low-angle grain boundaries within the grain, that is, the existence of crystallographic orientation differences within the grain. Consequently, GOS(i) ferrtie It consists of the product of the maximum value of GOS(i) and a constant (a). The constant (a) can be defined as a constant value for deriving a range having a GOS(i) threshold value corresponding to an inflection point in the overall GOS(i) integral graph, and in the present invention, the a corresponds to a range of 0.5 to 0.6.

[0101] The above GOS(i) ferrite When θ is small, it means that the mismatch angle within the ferrite grains is low, and the grains are composed of high-angle grain boundaries. GOS(i)ferrite When the value is low, coarse ferrite is easily formed by high-temperature hot rolling, making it difficult to secure bainitic ferrite, and thus it is difficult to achieve tensile strength, and GOS(i) ferrite When the set value is large, the inconsistency angle within the grain is high, that is, the fraction of low-angle grain boundaries within the grain increases. Therefore, if it falls outside this range, there is a risk that bainitic ferrite may be included as ferrite, and it becomes difficult to perform quantitative microstructure analysis according to hot rolling conditions.

[0102] It is preferable that the average grain size of the ferrite is 12 μm or less. The average grain size may be the equivalent diameter. The lower limit of the average grain size is not particularly limited, but as an example, it is preferable that it be 2 μm or more. If the average grain size exceeds 12 μm, the area fraction of coarse ferrite increases and the grain boundaries become brittle, which can easily cause fracture along the grain boundaries during forming, in addition to failing to secure the strength of the invention.

[0103] It is preferable that the aspect ratio (AR) of the ferrite be 0.35 to 0.8. If the ferrite aspect ratio is less than 0.35, the area fraction of acicular ferrite (AF) formed at low temperatures increases, which may make it difficult to secure tensile strength due to a decrease in the area fraction of precipitation-strengthened ferrite capable of securing strength. If the ferrite aspect ratio exceeds 0.8, the area fraction of coarse ferrite may increase at high-temperature hot rolling temperatures in relation to grain size.

[0104] The above steel plate may have a yield ratio of 0.85 or higher, a hole expansion (HER) of 85% or higher, and a tensile strength of 590 MPa or higher.

[0105]

[0106] Next, a method for manufacturing a steel plate, which is another embodiment of the present invention, will be described in detail. A steel plate can be manufactured by performing a series of processes including heating, hot rolling, cooling, and coiling a slab having the alloy composition described above. The conditions for each of these processes will be described in detail below.

[0107] Slab heating

[0108] Before performing hot rolling, a homogenization treatment can be performed by heating a slab satisfying the aforementioned alloy composition. The heating temperature in the heating step may be 1100 to 1350°C. If the heating temperature is less than 1100°C, the homogenization of the alloy elements may not be sufficiently performed. In addition, if the heating temperature exceeds 1350°C, an excessive amount of oxide may be formed on the surface of the slab, which may degrade the surface quality of the galvanized steel sheet. That is, the heating temperature may be 1100 to 1350°C, more specifically 1150 to 1300°C, and even more specifically 1180 to 1250°C.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] Hot rolling

[0114] The above heated steel slab is hot-rolled at a temperature in the range of 850 to 1150°C. A hot-rolled steel sheet can be obtained. After hot rolling, the grain size of the austenite is influenced by the alloy composition, the rolling end temperature, and / or the amount of reduction, which can affect the formation behavior of polygonal ferrite and bainite in the subsequent cooling process and the composition of the final microstructure.

[0115] The above hot rolling includes performing rough rolling and finishing rolling, and the above rough rolling can be performed at a temperature of 1050°C or higher (RDT), and the finishing rolling temperature (FDT) can be 890°C or higher.

[0116] In addition, the cooling rate from heating the steel slab to rough rolling may be 20℃ / s or less.

[0117] cooling

[0118] The above hot-rolled steel sheet is cooled to a coiling temperature of 400 to 500°C. More specifically, after the rough rolling temperature, it is cooled to a coiling temperature at a cooling rate of 50°C / s or more.

[0119] Kwon Chi

[0120] After the above cooling, winding is performed. At this time, it is preferable to perform the winding temperature (CT) such that the AE of the following [Equation 2] is 0.1 to 0.3.

[0121] [Relationship 2]

[0122] AE = A ×(C+Mn / 6)×exp(-Q / RT)

[0123] Here, Q is the activation enthalpy, and the experimental measurement data for solid solution C in steel with a BCC grain structure is 84.1 / KJ mol -1 used

[0124] Here, A is a constant expressed as the activation energy associated with C diffusion and Fe3C growth in the corresponding CT region where solid solution C does not precipitate during hot rolling. In this invention, A is a constant 5x10 5 ~9.5x10 5 R is the value of . The above R is the gas constant (8.314 J / (mol·K)), T is the coiling temperature (K), and each element symbol represents the alloy composition content (weight%) of the steel.

[0125] The AE in Equation 3 above refers to the thermodynamic activation energy devised to suppress C diffusion and Fe3C growth in the coiling temperature range without precipitating solid solution C during reheating and coiling. If the AE is less than 0.1, solid solution C exists in the low-temperature phase and may form martensite after coiling. If it exceeds 0.3, the coiling temperature increases, causing non-uniformity in precipitation behavior at grain boundaries and within grains, which may lead to a deterioration in shear plane quality and HER properties.

[0126] After completing the cooling and coiling processes according to the above, the target hot-rolled steel sheet can be obtained by performing final cooling. At this time, the final cooling can be completed by performing air cooling.

[0127] The steel plate after the final cooling is completed can be further pickled and oiled. In addition, after pickling, it can be heated to a temperature range of 400 to 750°C to apply a hot-dip galvanizing process.

[0128] The above-mentioned molten plating process may use zinc (Zn)-based, aluminum (Al)-based, and zinc (Zn)-magnesium (Mg)-aluminum (Al) ternary plating baths, and the alloy composition within the plating bath is not specifically limited.

[0129] 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.

[0130] (Example)

[0131] A steel slab having the alloy composition of Table 1 below (unit weight%, the remaining components being Fe and unavoidable impurities) was prepared, and the steel slab was heated and rough rolled (RDT) and finished rolled (FDT) according to the manufacturing conditions of Table 2, and then cooled and coiled to produce a hot-rolled steel sheet. After hot rolling, the thickness of the steel sheet was set to 2.5 mm, and after coiling, it was air-cooled.

[0132]

[0133]

[0134] For the steel plate manufactured above, the microstructure fraction, average equivalent diameter of ferrite, and aspect ratio (AR) were determined and listed in Table 3 below.

[0135] The fraction of the microstructure and the average equivalent diameter of the ferrite were measured by analyzing the specimen at a magnification of 5,000x using a scanning electron microscope and an image analyzer after etching the specimen using the Nital etching method.

[0136] Meanwhile, for ferrite, GOS(i) of [Equation 1] explained earlier ferrite We derived and presented the results together.

[0137] [Relationship 1]

[0138]

[0139]

[0140] Meanwhile, for each of the above steel plate specimens, yield strength (YS), tensile strength (TS), elongation (El), and hole expansion (HER) were measured, and the results are shown in Table 4. The yield strength, tensile strength, and elongation were obtained by taking specimens of the JIS-5 standard specimen in a direction perpendicular to the rolling direction. In this case, the yield strength and elongation represent the 0.2% off-set yield strength and fracture elongation, respectively.

[0141] Hole expandability (HER) was measured by punching a hole with an initial diameter (d0) of 10 mm and expanding the hole by raising a conical punch at 60°. At this time, the punch was stopped rising the moment the crack penetrated the thickness of the steel plate, and the diameter (d) of the punched hole after the crack penetrated was measured. Subsequently, HER(%) was calculated as = ((d - d0) / d0) × 100. The average value was calculated after conducting three tests for each steel grade.

[0142]

[0143] As shown in Tables 1 to 4 above, when the compositional components and manufacturing process conditions of the steel satisfy the scope of the present invention, the optimal structural area fraction pursued in the present invention, as in Invention Examples 1 to 3, can be secured, and it can be confirmed that it has excellent mechanical properties.

[0144] As in Comparative Examples 1 to 8, when one or more of the ranges of GOS value, aspect ratio, and grain size are not satisfied, the AE mentioned in the present invention was high, and as a result, low tensile strength, yield ratio, or HER value was obtained.

[0145] As can be seen in Comparative Example 9, when the bainitic ferrite area fraction was composed of nearly 100%, the CT was low and the AE value was satisfied, but since the low-temperature transformation phase was composed of the main matrix structure, the tensile strength was relatively high compared to the inventive example and other comparative examples, and it was not possible to secure an elongation of 20% or more.

[0146] Although the present invention has been described with reference to the above embodiments, those skilled in the art will understand that this is intended to explain the invention more specifically and is not intended to limit the scope of the invention.

Claims

1. In wt%, it contains C: 0.03–0.08%, Si: 0.001–0.14%, Mn: 1.0–2.0%, Al: 0.01–0.8%, P: 0.0001–0.05%, S: 0.0001–0.01%, N: 0.0001–0.01%, and the remainder consists of Fe and unavoidable impurities, The microstructure contains 75 area% or more of bainitic ferrite and 25 area% or less of ferrite, and GOS(i) of [Relation 1] below ferrite A steel plate satisfying 0.5 to 0.

65. [Relationship 1] j(i): Total number of pixels for grain i, wij: Angle of discrepancy between the orientation of pixel j and the average orientation of grain i a: A constant value for deriving the range having the GOS(i) threshold corresponding to the inflection point in the entire GOS(i) integral graph.

2. In Paragraph 1, The above steel plate is a steel plate containing one or more of chromium (Cr): 0.01~0.8%, molybdenum (Mo): 0.01~0.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, and vanadium (V): 0.001~0.25%.

3. In Paragraph 1, The above steel plate is a steel plate containing one or more of the following: antimony (Sb): 0.005~0.1%, tin (Sn): 0.005~0.1%, arsenic (As): 0.005~0.1%, copper (Cu): 0.01~0.3%, and boron (B): 0.005% or less.

4. In Paragraph 1, A steel sheet in which the above microstructure contains at least one of pearlite, fresh martensite, and MA phase in an area of ​​less than 5%.

5. In Paragraph 1, A steel plate having an average grain size of 12㎛ or less of the ferrite.

6. In Paragraph 1, A steel plate having an aspect ratio (Aspect Ratio, AR) of the above ferrite of 0.35 to 0.

8.

7. In Paragraph 1, The above steel plate is a steel plate having a yield ratio of 0.85 or higher, a hole expansion (HER) of 85% or higher, and a tensile strength of 590 MPa or higher.

8. A step of heating a steel slab comprising, in wt%, C: 0.03~0.08%, Si: 0.001~0.14%, Mn: 1.0~2.0%, Al: 0.01~0.8%, P: 0.0001~0.05%, S: 0.0001~0.01%, N: 0.0001~0.01%, and the remainder being Fe and unavoidable impurities; A step of rough rolling the above steel slab; A step of obtaining a hot-rolled steel sheet by finishing rolling a rough-rolled steel slab; A step of cooling the above hot-rolled steel sheet to a coiling temperature of 400 to 500°C; and A winding step satisfying the condition that the AE in [Relationship 2] below is 0.1 to 0.3 A method for manufacturing a steel plate including [Relationship 2] AE = A ×(C+Mn / 6)×exp(-Q / RT) Here, Q is the activation enthalpy, and the experimental measurement data for solid solution C in steel with a BCC grain structure is 84.1 / KJ mol -1 A is a constant expressed as the activation energy associated with C diffusion and Fe3C growth in the corresponding CT region where solid solution C is not precipitated during hot rolling, R is the gas constant (8.314 J / (mol·K)), T is the coiling temperature (K), and each element symbol represents the alloy composition content (weight%) of the steel.

9. In Paragraph 8, A method for manufacturing a steel plate in which the above steel slab comprises one or more of chromium (Cr): 0.01~0.8%, molybdenum (Mo): 0.01~0.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, and vanadium (V): 0.001~0.25%.

10. In Paragraph 8, A method for manufacturing a steel plate in which the above steel slab contains one or more of the following: antimony (Sb): 0.005~0.1%, tin (Sn): 0.005~0.1%, arsenic (As): 0.005~0.1%, copper (Cu): 0.01~0.3%, and boron (B): 0.005% or less.

11. In Paragraph 8, A method for manufacturing a steel plate in which the above steel slab is heated to a temperature range of 1100 to 1350℃.

12. In Paragraph 8, A method for manufacturing a steel sheet in which the above hot rolling is performed by rough rolling at a temperature of 1050°C or higher (RDT), and finish rolling is performed at a finish rolling temperature (FDT) of 890°C or higher (FDT).

13. In Paragraph 8, A method for manufacturing a steel plate by cooling the above-mentioned steel slab at a cooling rate of 20℃ / s or less from heating to rough rolling.

14. In Paragraph 8, A method for manufacturing a steel plate by cooling at a cooling rate of 50℃ / s or more from rough rolling to coiling.

Images