Hot-rolled steel sheet and method for manufacturing same

Abstract

The present invention relates to a material that can be used for automobile chassis, frame parts, and the like, and relates to a high-strength hot-rolled steel sheet having excellent formability and a method for manufacturing same.

Description

Hot-rolled steel sheet and method of manufacturing the same

[0001] The present invention relates to a material that can be used for automobile chassis, frame parts, etc., and to a high-strength hot-rolled steel sheet with excellent formability and a method for manufacturing the same.

[0002] Conventional high-strength hot-rolled steel sheets used for automobile chassis and frames are becoming thinner and stronger for weight reduction. At the same time, excellent formability is required considering the shape of the parts, and the demand for yield strength is increasing to maximize the durability of the parts. Previously, excellent hole expansion was required for steel with a tensile strength of 590 MPa or less, but recently, excellent hole expansion is also being required for ultra-high-strength hot-rolled steel with a tensile strength of 700 MPa or more. However, since hole expansion is a characteristic that deteriorates as the strength of the steel increases, there is a need for technology that compensates for this by providing excellent hole expansion along with high strength.

[0003] Patent Document 1 describes a technology related to a composite microstructure steel having a matrix structure of bainitic ferrite and granular bainitic ferrite, which are ferrite phases formed in the low-temperature region. However, since Cu must be utilized to secure additional strength, surface defects and high-temperature brittleness may occur during hot rolling, and there is a disadvantage that Ni must be added to prevent this.

[0004] Patent Document 2 relates to a burring-resistant high-strength steel sheet with a tensile strength of 540 MPa or higher and a method for manufacturing the same, and expects a precipitation strengthening effect by adding Nb and Ti. However, in order to secure good burring and processability, it aims for a single ferrite phase and allows for the inclusion of some bainite as necessary.

[0005] (Patent Document 1) Korean Registered Patent No. 10-1114672

[0006] (Patent Document 2) Korean Registered Patent No. 10-0962745

[0007] According to one embodiment of the present invention, the aim is to provide a hot-rolled steel sheet capable of securing ultra-high strength and excellent hole expansion properties, and a method for manufacturing the same.

[0008] 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.

[0009] A hot-rolled steel sheet according to one embodiment of the present invention comprises, in weight percent, C: 0.05~0.10%, Si: 0.01~1.0%, Mn: 1.4~2.1%, Al: 0.01~0.1%, Cr: 0.005~0.4%, Mo: 0.005~0.3%, P: 0.001~0.05%, S: 0.001~0.01%, N: 0.001~0.01%, Nb: 0.005~0.05%, Ti: 0.005~0.13%, and the remainder being Fe and unavoidable impurities.

[0010] The microstructure, in terms of area fraction, contains 92% or more of the sum of ferrite and bainite and 8% or less of the sum of martensite, MA, and pearlite, and

[0011] The following relationship 1 can be satisfied.

[0012] [Relationship 1]

[0013] A > 1

[0014] A: Number of (Nb, Ti)-based precipitates with a maximum major axis length of 50 nm or less / Number of (Nb, Ti)-based precipitates with a maximum major axis length exceeding 50 nm

[0015] The above hot-rolled steel sheet may further contain 1.5% or less of the sum of one or more of V, Ni, and B.

[0016] The above hot-rolled steel sheet may have a tensile strength of 730 MPa or more and a 50% hole expansion (HER) value.

[0017] The above hot-rolled steel sheet can satisfy the following relationship 2.

[0018] [Relationship 2]

[0019] 82 ≤ B ≤ 98

[0020] B = YR + YP_El

[0021] YR: (Yield Strength / Tensile Strength) * 100

[0022] YP_El: Discontinuous yield elongation value occurring in the yield zone during uniaxial tensile testing

[0023] A method for manufacturing a hot-rolled steel sheet according to another embodiment of the present invention comprises the step of heating a steel slab comprising, in weight percent, C: 0.05~0.10%, Si: 0.01~1.0%, Mn: 1.4~2.1%, Al: 0.01~0.1%, Cr: 0.005~0.4%, Mo: 0.005~0.3%, P: 0.001~0.05%, S: 0.001~0.01%, N: 0.001~0.01%, Nb: 0.005~0.05%, Ti: 0.005~0.13%, and the remainder being Fe and unavoidable impurities;

[0024] A step of manufacturing a hot-rolled steel plate by hot-rolling the above steel slab;

[0025] A step of first cooling the above hot-rolled steel plate to a temperature range of 530 to 650℃ at an average cooling rate of 50 to 100℃ / sec;

[0026] A step of maintaining for 6 to 11 seconds after the above first cooling;

[0027] A step of secondary cooling to a temperature range of 250 to 500℃ after the above maintenance at an average cooling rate of 5 to 50℃ / sec; and

[0028] After the above second cooling, a third cooling step may be included to a temperature range of room temperature to 200℃ at an average cooling rate of 0.1 to 25℃ / hour.

[0029] The above steel slab may further contain 1.5% or less of the sum of one or more of V, Ni, and B.

[0030] The heating of the above steel slab can be performed at a temperature of 1100 to 1350°C.

[0031] The above hot rolling can be performed in a temperature range of 850 to 1150°C.

[0032] The step of winding after the above secondary cooling may be further included.

[0033] According to one embodiment of the present invention, a composite structure hot-rolled steel sheet can be provided that has excellent hole expansion and excellent strength, and can be used in parts used in automobile chassis parts, such as members, lower arms, reinforcing materials, connecting materials, and frames.

[0034] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0035] Figure 1 is a photograph of the (Nb, Ti)-based precipitate of specimen 1 in the example observed through a TEM Replica sample.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0041] 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.

[0042] The inventors of the present invention have researched various components and microstructures to expand the applicability of hot-rolled steel sheets to parts such as automobile chassis, and by utilizing methods for precise temperature and time control in the hot-rolling manufacturing process, they have been able to derive a technology capable of securing ultra-high strength while simultaneously ensuring excellent hole expansion capabilities.

[0043] First, a hot-rolled steel sheet, which is an embodiment of the present invention, will be described in detail.

[0044] The hot-rolled steel sheet of the present invention comprises, in weight%, C: 0.05~0.10%, Si: 0.01~1.0%, Mn: 1.4~2.1%, Al: 0.01~0.1%, Cr: 0.005~0.4%, Mo: 0.005~0.3%, P: 0.001~0.05%, S: 0.001~0.01%, N: 0.001~0.01%, Nb: 0.005~0.05%, Ti: 0.005~0.13%, and the remainder being Fe and unavoidable impurities, additionally containing the sum of one or more of V, Ni, and B in an amount of 1.5% or less.

[0045] Carbon (C): 0.05–0.10% (hereinafter indicated simply as '%')

[0046] The above-mentioned carbon is the most economical and effective element for strengthening steel, and increasing the amount added increases the precipitation strengthening effect or the low-temperature phase fraction, thereby increasing tensile strength. If the content is less than 0.05%, it is difficult to achieve sufficient precipitation strengthening and low-temperature phase formation, making it difficult to secure the target strength and bake hardenability. On the other hand, if it exceeds 0.10%, excessive low-temperature phases and carbides are formed, resulting in inferior formability. Additionally, increasing the carbon equivalent generally leads to inferior weldability. Furthermore, depending on the characteristics of the microstructure generated when excessive carbon content is added, there is a possibility that the tensile strength may significantly decrease after heat treatment due to the deterioration of the low-temperature phase and the formation of additional excess carbides during additional heat treatment following hot rolling. Therefore, the content of the above-mentioned carbon may be included in an amount of 0.05 to 0.10%, and preferably 0.065 to 0.095%.

[0047] Silicon (Si): 0.01~1.0%

[0048] The above Si deoxidizes the molten steel and has a solid solution strengthening effect, and is advantageous for improving formability by delaying the formation of coarse carbides. In addition, it has the effect of suppressing carbide formation during heat treatment in the 300 to 600°C range. However, if the content is less than 0.01%, the effect of delaying carbide formation is small, making it difficult to improve formability, and there may be almost no effect on improving strength. If it exceeds 1.0%, red scale caused by Si forms on the surface of the steel sheet during hot rolling, which not only significantly degrades the surface quality of the steel sheet but also causes problems such as reduced ductility and weldability; therefore, it is desirable to limit the content to within 1.0%. Meanwhile, it may be desirable to include the above Si in an amount of 0.1 to 0.9%.

[0049] Manganese (Mn): 1.4~2.1%

[0050] The above-mentioned Mn, like Si, is an effective element for solid solution strengthening of steel and additionally improves the hardenability of steel, thereby delaying ferrite transformation at the same cooling rate and facilitating the formation of low-temperature phases such as bainite and martensite. However, if the content is less than 1.4%, the effects of solid solution strengthening and hardenability improvement are small, so the desired increase in strength cannot be achieved. On the other hand, if it exceeds 2.1%, hardenability increases significantly, causing the martensite phase fraction to exceed the intended purpose. Consequently, during slab casting in the continuous casting process, segregation develops significantly at the center of the thickness, resulting in inferior formability and potentially causing poor welding quality. Therefore, the content of the above-mentioned Mn can be included in the range of 1.4 to 2.1%. Preferably, it can be included in the range of 1.5 to 2.0%.

[0051] Aluminum (Al): 0.01–0.1%,

[0052] The above Al typically refers to the content of Sol.Al. The above Al is a component added primarily for deoxidation; if its content is less than 0.01%, the deoxidation effect is insufficient, and if it exceeds 0.1%, it combines with nitrogen to form AlN, which makes corner cracks prone to occur in the slab during continuous casting and casting, and also makes defects caused by the formation of inclusions prone to occur. Therefore, it is desirable to set the content to 0.01~0.1%.

[0053] Chromium (Cr): 0.005–0.4%,

[0054] The above Cr strengthens the steel through solid solution and can play a role in assisting in the formation of bainite by delaying the ferrite phase transformation upon cooling. However, if the content is less than 0.005%, it is difficult to obtain the above effects from the addition, and if it exceeds 0.4%, the ferrite transformation is excessively delayed, which may result in inferior elongation due to the formation of a martensite phase. Furthermore, similar to Mn, segregation zones develop significantly at the center of the thickness, causing non-uniformity in the microstructure along the thickness direction and potentially leading to inferior elongation flangeability. Additionally, if the Cr content is excessively high, it may impair the corrosion resistance of the material. Therefore, the above Cr may be included in an amount of 0.005 to 0.4%. Preferably, it may be 0.1 to 0.4%.

[0055] Molybdenum (Mo): 0.005~0.30%

[0056] Mo can increase the hardenability of steel and facilitate the formation of a bainite structure. However, if the amount is less than 0.005%, the above effect from the addition cannot be obtained, and if it exceeds 0.30%, the excessive increase in hardenability may lead to the formation of a martensite phase, which can drastically degrade formability. Furthermore, it is economically disadvantageous and detrimental to weldability. Therefore, the Mo may be included in an amount of 0.005 to 0.3%. Preferably, it may be 0.01 to 0.20%.

[0057] Phosphorus (P): 0.001–0.05%

[0058] Like Si, the above-mentioned P simultaneously possesses solid solution strengthening and ferrite transformation promoting effects. However, manufacturing with a P content of less than 0.001% is economically disadvantageous due to high manufacturing costs and may be insufficient to obtain sufficient strength. On the other hand, if the content exceeds 0.05%, brittleness due to grain boundary segregation occurs, fine cracks are prone to form during forming, and ductility and impact resistance properties may be significantly degraded. Therefore, it is desirable to include the above-mentioned P in an amount of 0.001 to 0.05%.

[0059] Sulfur (S): 0.001–0.01%

[0060] The above-mentioned S is an impurity present in steel, and manufacturing it with a content of less than 0.001% requires a long time during steelmaking operations, which may lead to a decrease in productivity. On the other hand, if it exceeds 0.01%, it combines with Mn, etc., to form non-metallic inclusions, and consequently, fine cracks are prone to occur during the cutting process of steel. Therefore, it is desirable to include a content of 0.001 to 0.01%.

[0061] Nitrogen (N): 0.001–0.01%

[0062] The above N is a representative solid solution strengthening element along with C and forms coarse precipitates together with Ti, Al, etc. Generally, the solid solution strengthening effect of N is superior to that of carbon, but there is a problem in that toughness decreases significantly as the amount of N in steel increases. In addition, manufacturing with a content of less than 0.001% requires a long time during steelmaking operations, resulting in reduced productivity. Therefore, in the present invention, it is preferable to include a content of 0.001 to 0.01%.

[0063] Niobium (Nb): 0.005–0.05%

[0064] The above Nb is a representative precipitation strengthening element along with Ti and V, and it is effective in improving the strength and impact toughness of steel through the grain refinement effect caused by delayed recrystallization during hot rolling. Therefore, if the Nb content is less than 0.005%, it is difficult to obtain the above effect, and if the Nb content exceeds 0.05%, there is a problem that formability is compromised due to the formation of elongated grains and coarse composite precipitates caused by excessive delayed recrystallization during hot rolling. Therefore, it is desirable to include a content of 0.005 to 0.05%.

[0065] Titanium (Ti): 0.005–0.13%

[0066] The above-mentioned Ti is a representative precipitation-strengthening element along with Nb and V, and forms coarse TiN in steel due to its strong affinity for N. TiN has the effect of inhibiting grain growth during the heating process for hot rolling. In addition, the Ti remaining after reacting with nitrogen is dissolved in the steel and combines with carbon to form TiC precipitates, making it a useful component for improving the strength of the steel. Therefore, if the Ti content is less than 0.005%, the above effect cannot be obtained, and if the Ti content exceeds 0.13%, there is a problem of inferior formability due to the occurrence of coarse TiN and the coarsening of TiC precipitates. Therefore, it is desirable to include a content of 0.005 to 0.13%.

[0067] In addition, the present invention may include one or more components among vanadium (V), nickel (Ni), and boron (B) depending on the case, and the total content thereof is within 1.5%. V is an element that forms carbides in steel, while Ni and B have the effect of improving the hardenability of steel and increasing material strength. However, if V is added in excessive amounts, while the precipitation strengthening effect may be improved, it may be excessively distributed within the steel, which may degrade the quality of the shear surface; and if Ni and B are added in excessive amounts, the strength may exceed the target. Therefore, it is desirable to control the total content of V, Ni, and B to within 1.5% to ensure strength.

[0068] In addition to the components described above, the product may contain the remaining Fe and other unavoidable impurities. Since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during normal manufacturing processes, 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. Furthermore, the addition of effective components other than those mentioned above is not entirely excluded.

[0069] The microstructure of the hot-rolled steel sheet of the present invention will be described in detail below.

[0070] The hot-rolled steel sheet of the present invention is composed of a total of 92% or more of ferrite and bainite phases in terms of area fraction, and may also contain 8% or less of one or more of pearlite, martensite, and MA (martensite-austenite constituents) phases in total. It is important in the present invention that the total of the main phases, ferrite and bainite, is 92% or more. This is because, in the present invention, the ferrite is precipitation-strengthened ferrite with finely distributed fine precipitates, and there is no significant difference in strength compared to bainite. Therefore, to be more precise, precipitation-strengthened ferrite and bainite are the main phases of the microstructure. If the pearlite, martensite, and MA phases other than the two aforementioned phases exceed 8%, it may be difficult to secure the desired strength and formability because the mechanical properties are very different.

[0071] The hot-rolled steel sheet of the present invention satisfies the conditions of the following relationship 1.

[0072] [Relationship 1]

[0073] A > 1

[0074] A: Number of (Nb, Ti)-based precipitates with a maximum major axis length of 50 nm or less / Number of (Nb, Ti)-based precipitates with a maximum major axis length exceeding 50 nm

[0075] The strength contribution effect of Ni and Ti-based precipitates varies depending on their size; the finer the size, the greater the strength contribution effect, and as the size becomes coarser, the strength contribution effect becomes negligible. In the present invention, when the size of Ni and Ti-based precipitates is 50 nm or less, they contribute significantly to increasing strength. Therefore, in the present invention, it is preferable that the ratio of the precipitates of 50 nm or less to relatively larger precipitates is greater than 1. It is more effective for A to be greater than 1.3. It is more effective for it to be greater than 1.5.

[0076] The hot-rolled steel sheet of the present invention has a tensile strength of 730 MPa or more and a 50% hole expansion (HER) value.

[0077] In addition, the above hot-rolled steel sheet satisfies the following relationship 2.

[0078] [Relationship 2]

[0079] 82 ≤ B ≤ 98

[0080] B = YR + YP_El

[0081] YR: (Yield Strength / Tensile Strength) * 100

[0082] YP_El: Discontinuous yield elongation value occurring in the yield zone during uniaxial tensile testing

[0083] When the composition and microstructure characteristics of the present invention are satisfied, the strength of the material, particularly the yield strength (YS), can be significantly increased, and consequently, the YR of the material can be increased. In addition, when a large number of fine precipitates are distributed within the material, discontinuous yielding (YP_El) behavior is exhibited during uniaxial tension. If the sum of YR and YP_El in Equation 2 is less than 82 or exceeds 98, it is difficult to secure the desired combination of strength and formability.

[0084]

[0085] Next, a method for manufacturing a hot-rolled steel sheet, which is another embodiment of the present invention, will be described in detail. It can be manufactured by heating, hot-rolling, and cooling a steel slab satisfying the alloy composition described above.

[0086] Steel slab heating

[0087] A steel slab satisfying the above alloy composition range is heated at a temperature of 1100 to 1350°C. At this time, if the heating temperature is below 1100°C, the re-solidification rate of precipitates including Ti, Nb, Mo, and V decreases, and the formation of fine precipitates in the process after hot rolling is reduced. If the temperature exceeds 1350°C, the strength decreases due to austenite grain coarsening, so it is preferable to perform the heating at a temperature of 1100 to 1350°C.

[0088] 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.

[0089] 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.

[0090] 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 this invention may be included in the molten steel within permissible limits.

[0091] 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.

[0092] Hot rolling

[0093] When hot rolling the above heated steel slab, the process is carried out at a temperature in the range of 850 to 1150°C. If hot rolling is initiated at a temperature higher than 1150°C, the temperature of the hot-rolled steel sheet increases, causing the grain size to become coarser and the surface quality of the hot-rolled steel sheet to deteriorate. Furthermore, if hot rolling is terminated at a temperature lower than 850°C, excessive recrystallization delay leads to the development of elongated grains, resulting in severe anisotropy and deterioration in formability.

[0094] cooling

[0095] Next, the hot-rolled steel sheet is first cooled to a temperature in the range of 530 to 650°C at an average cooling rate of 50 to 100°C / sec. More preferably, the first cooling is performed to a temperature in the range of 560 to 600°C. At this time, if the first cooling is performed below 530°C, the amount of precipitates formed within the ferrite during cooling when forming the final microstructure is reduced, which may result in a decrease in strength. In addition, if the first cooling is performed above 650°C, some pearlite transformation may occur when forming the final microstructure, which may result in poor formability. It is preferable to set the cooling rate during the first cooling to an average of 50 to 100°C / sec. If the cooling rate is less than 50°C / sec, the ferrite phase fraction may be formed excessively, which is disadvantageous for securing strength, and if the cooling rate exceeds 100°C / sec, the ferrite phase fraction decreases significantly in the region where the first cooling end temperature is low, resulting in insufficient elongation.

[0096] Next, the first cooled steel plate is held for 6 to 11 seconds. After the first cooling, the cooling is stopped to maximize the amount of precipitation within the steel plate. At this time, if the cooling stop time is less than 6 seconds, the effects of ferrite phase transformation and precipitation are negligible, and if it exceeds 11 seconds, the fraction of the ferrite phase in the microstructure increases significantly and the hard phases of bainite and bainitic ferrite decrease, so the desired microstructure cannot be obtained.

[0097] Next, the steel plate is subjected to secondary cooling. During secondary cooling, the final temperature is preferably in the range of 250 to 500°C, and more preferably in the range of 320 to 450°C. If the final temperature of secondary cooling is excessively high, bainite may not be sufficiently formed, making it difficult to secure strength; conversely, if it is excessively low, bainite, martensite, and MA phases may be formed in excessive amounts, resulting in inferior ductility and hole expansion of the steel. During secondary cooling, it is preferable to set the cooling rate to an average of 5 to 50°C / sec. If the cooling rate is excessively high, the MA phase is prone to forming, and an excessive amount of bainite is formed, leading to a decrease in elongation. Although there is no specific limit on the lower limit of the cooling rate, controlling the cooling rate to be slower than 5°C / sec requires separate cooling and heat retention equipment, which may be economically disadvantageous; therefore, considering this, the lower limit may be restricted to 5°C / sec.

[0098] After the above secondary cooling, winding can be performed to manufacture a coil.

[0099] After the above coiling, the third cooling was not controlled separately, and natural cooling to room temperature was performed to manufacture hot-rolled steel sheets.

[0100] After the above secondary cooling, the steel is cooled to a temperature in the range of room temperature to 200°C at an average cooling rate of 0.1 to 25°C / hour. If the cooling rate exceeds 25°C / hour, the MA phase is prone to forming in the steel, which degrades the hole expandability of the steel. Furthermore, controlling the cooling rate to less than 0.1°C / hour requires separate heating equipment, which is economically disadvantageous. Preferably, it is better to cool at a rate of 1 to 10°C / hour.

[0101] 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.

[0102] (Example)

[0103] After manufacturing a steel slab having the composition of Table 1 below (unit weight %, the remainder being Fe and unavoidable impurities), the steel slab was heated to a temperature of 1200°C and hot-rolled at 1050°C to produce a hot-rolled steel sheet with a thickness of 3.0 mm.

[0104] After the above hot rolling, cooling was performed and maintained until the first cooling end temperature (T1) of Table 2. The holding time (HT) at this time is shown in Table 2. Afterwards, second cooling was performed until the second cooling end temperature (T2), and coiling was performed.

[0105] Classification CsiMnAlPSNTiNbCrMo Steel Grade 10.1570.311.760.0350.0080.0020.0040.0950.0110.0080.001 Steel Grade 20.0810.340.110.0360.0130.0020.0040.0850.0310.0420.022 Steel Grade 30.0751.921.680.030.0090.0020.0030.0880.0260.150.028 Steel Grade 40.0710.422.740.0320.0090.0020.0040.0740.0210.250.121 Steel Grade 50.0590.711.850.0330.0110.0020.0030.0420.0190.950.098 Steel Type 60.0880.321.650.0320.0090.0020.0040.1060.0360.240.131 Steel Type 70.0760.421.790.0310.0120.0020.0030.0980.0260.310.087 Steel Type 80.0910.531.590.030.0090.0020.0030.0110.0310.190.121 Steel Type 90.0770.531.780.0220.0120.0030.0040.0890.0190.340.116Gangjong 100.0680.611.690.0250.0110.0020.0030.0840.0250.270.109

[0106] The microstructure and physical properties of the manufactured steel plates were measured and are shown in Tables 2 and 3 below.

[0107] The microstructure fraction was observed at a point 1 / 4 of the way from the surface of the manufactured steel plate in the direction of the center of thickness, and was measured from the results of analysis using SEM at magnifications of x4000 and x8000. After Nital and Lepera etching, analysis was performed at magnification of x1000 using an optical microscope and an image analyzer.

[0108] In addition, the results for yield strength (YS), tensile strength (TS), elongation (El), and hole expansion (HER) of the manufactured steel plates are presented. Tensile performance was evaluated by taking specimens of JIS standard test pieces perpendicular to the rolling direction, and the tensile evaluation was measured at room temperature.

[0109] The hole expandability (HER) test was conducted with a punching clearance (cl) of 12%, and the results were presented based on the JFST 1001-1996 standard. The evaluation values ​​for HER1 and HER2 were expressed as the average values ​​after three trials.

[0110] Classification T1 HTT2 Microstructure (Area %) Remarks FBM MAP Steel Type 159 56.23 69 323 527 60 Specimen Type 1 261 17.14 529 440 11 Specimen Type 2 360 57.94 129 240 22 Specimen Type 3 457 58.240 239 29 31 10 Specimen Type 4 559 99.530 968 11 1920 Specimen Type 5 660 38.24 263 460 240 Specimen Type 6 759 59.94 324 1580 10 Specimen Type 7 858 58.840 137 564 30 Specimen Type 8 955 610.23 89 425 2330 Specimen Type 9 106057.73574653010 Specimen 10 Steel Type 87718.34458110009 Specimen 11 Steel Type 860815.6475926002 Specimen 12 Steel Type 94217.834242451210 Specimen 13 Steel Type 95596.1674654810 Specimen 14 Steel Type 106489.8624909010 Specimen 15T1: 1st cooling end temperature (°C)HT: Holding time after 1st cooling (sec)T2: 2nd cooling end temperature (°C) (= coiling temperature)F: Ferrite phase, B: Bainite phase, M: Martensite phase, MA: MA phase, P: Pearlite phase

[0111] Classification Mechanical Properties Remarks TSYRTP_ElHER Formula 2 Formula 1st Class 196575031751.32 Psalms 1st Class 2613811.14282.11.75 Psalms 2nd Class 3684832.54785.51.65 Psalms 3rd Class 499868027680.87 Psalms 4th Class 5887781.93179.91.72 Psalms 5th Class 6795902.36592.31.88 Psalms 6th Class 7801850.17185.12.11 Psalms 7th Class 88118707071.45 Psalms 8th Class 9776882.68690.61.97 Psalms 9th Class 10820863.16289.11.68 Psalm, Class 10 8675812.23883.20.84 Psalm, Class 11 8688954.96099.90.49 Psalm, Class 12 9845721.33073.30.77 Psalm, Class 13 997068029681.45 Psalm, Class 14 10659883.86591.80.92 Psalm, Class 15 Formula 1: Number of (Nb, Ti) precipitates with a final major axis length of 50 nm or less / Number of (Nb, Ti) precipitates with a maximum major axis length exceeding 50 nm Formula 2: YR+YP_El

[0112] As shown in Table 3 above, in the case of a specimen satisfying the alloy composition and manufacturing conditions of the present invention, the microstructural characteristics proposed in the present invention were satisfied, and the desired physical properties were also secured. Meanwhile, Figure 1 is a photograph of the precipitates in specimen 6 observed through a TEM replica sample, and it can be confirmed that numerous fine precipitates of 50 nm or less were formed.

[0113] In contrast, it was confirmed that it is difficult to secure sufficient strength and / or hole expansion properties when the alloy composition or manufacturing conditions proposed in the present invention are not satisfied.

[0114] Although the present invention has been described in detail through embodiments above, other forms of embodiments are also possible. Therefore, the technical concept and scope of the claims described below are not limited to the embodiments.

Claims

1. In wt%, C: 0.05~0.10%, Si: 0.01~1.0%, Mn: 1.4~2.1%, Al: 0.01~0.1%, Cr: 0.005~0.4%, Mo: 0.005~0.3%, P: 0.001~0.05%, S: 0.001~0.01%, N: 0.001~0.01%, Nb: 0.005~0.05%, Ti: 0.005~0.13%, the remainder being Fe and unavoidable impurities, and The microstructure, in terms of area fraction, contains 92% or more of the sum of ferrite and bainite and 8% or less of the sum of martensite, MA, and pearlite, and Hot-rolled steel sheet satisfying the following relationship 1. [Relationship 1] A > 1 A: Number of (Nb, Ti)-based precipitates with a maximum major axis length of 50 nm or less / Number of (Nb, Ti)-based precipitates with a maximum major axis length exceeding 50 nm 2. In Paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet further containing 1.5% or less of the sum of one or more of V, Ni, and B.

3. In Paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having a tensile strength of 730 MPa or more and a 50% hole expansion (HER) value.

4. In Paragraph 1, The above hot-rolled steel sheet is a hot-rolled steel sheet satisfying the following relationship 2. [Relationship 2] 82 ≤ B ≤ 98 B = YR + YP_El YR: (Yield Strength / Tensile Strength) * 100 YP_El: Discontinuous yield elongation value occurring in the yield zone during uniaxial tensile testing 5. A step of heating a steel slab comprising, in wt%, C: 0.05~0.10%, Si: 0.01~1.0%, Mn: 1.4~2.1%, Al: 0.01~0.1%, Cr: 0.005~0.4%, Mo: 0.005~0.3%, P: 0.001~0.05%, S: 0.001~0.01%, N: 0.001~0.01%, Nb: 0.005~0.05%, Ti: 0.005~0.13%, and the remainder being Fe and unavoidable impurities; A step of manufacturing a hot-rolled steel plate by hot-rolling the above steel slab; A step of first cooling the above hot-rolled steel plate to a temperature range of 530 to 650℃ at an average cooling rate of 50 to 100℃ / sec; A step of maintaining for 6 to 11 seconds after the above first cooling; A step of secondary cooling to a temperature range of 250 to 500℃ after the above maintenance at an average cooling rate of 5 to 50℃ / sec; and A step of tertiary cooling after the above secondary cooling to a temperature range of room temperature to 200℃ at an average cooling rate of 0.1 to 25℃ / hour. A method for manufacturing hot-rolled steel sheets including 6. In Paragraph 5, A method for manufacturing a hot-rolled steel sheet in which the above steel slab further contains 1.5% or less of the sum of one or more of V, Ni, and B.

7. In Paragraph 5, A method for manufacturing hot-rolled steel sheets in which the above steel slab heating is performed at a temperature of 1100 to 1350℃.

8. In Paragraph 5, The above hot rolling is a method for manufacturing hot-rolled steel sheets performed in a temperature range of 850 to 1150℃.

9. In Paragraph 5, A method for manufacturing a hot-rolled steel sheet, further comprising the step of coiling after the second cooling above.

Images