Hot-rolled steel sheet and method for producing same

Abstract

The present invention relates to a hot-rolled steel sheet suitable for a chassis structural member of an automobile, and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet applicable to chassis structural members of automobiles and the like, and a manufacturing method thereof. Background Art

[0002] In recent years, in order to slow down global warming, internal combustion engine vehicles, which are the mainstream in the automotive market, are rapidly transforming into environmentally friendly vehicles such as electric vehicles.

[0003] As internal combustion engine vehicles transform into electric vehicles and the like, the types of components constituting the vehicle are also changing, and the weight of the vehicle is also changing. For example, when comparing the weights of an internal combustion engine vehicle and an electric vehicle of the same model introduced, it is known that the weight of the electric vehicle is approximately increased by the weight of the battery compared to the internal combustion engine vehicle.

[0004] In addition, the chassis components of a vehicle play a role in supporting the vehicle body and are important components for ensuring riding comfort and driving stability by absorbing vibrations and impacts from the road surface during driving. As the vehicle weight increases, the fatigue load applied to the chassis components also increases. Therefore, the steel used for chassis components of electric vehicles and the like needs to have excellent fatigue strength.

[0005] The fatigue strength of steel is proportional to the tensile strength and the yield strength. Therefore, the steel used for the purpose of chassis components of electric vehicles and the like needs to increase the tensile strength and the yield strength. Thus, the steel sheet used for manufacturing chassis components is gradually moving towards high strength.

[0006] Furthermore, chassis components are mainly manufactured by stamping. Although it is possible to achieve weight reduction of components by applying high-strength steel and reducing the thickness of the steel sheet, the shape of the components cannot be significantly changed. Therefore, ensuring formability suitable for stamping of components in the manufacturing of high-strength steel is more important than ever.

[0007] So far, various techniques for improving the strength and formability of hot-rolled steel sheets have been proposed.

[0008] For example, Patent Document 1 discloses a manufacturing method of a high-strength hot-rolled steel sheet with excellent hole expansion property. Among them, for a steel containing 0.01 - 0.05% carbon by weight%, the fine structure has bainitic ferrite without carbide as the main phase. According to this Patent Document 1, it is possible to manufacture a steel sheet with excellent hole expansion property and a tensile strength of 980 grade or more, but the yield strength is low, the fatigue characteristics are poor, the weight reduction effect of the components is negligible, the elongation is poor, and the formability of the components may be poor.

[0009] Therefore, in order to ensure the driving stability of chassis components for environmentally friendly vehicles such as electric vehicles, it is necessary to develop a steel material that not only has excellent fatigue life due to high tensile strength and yield strength, but also has excellent formability such as elongation and hole expansion properties.

[0010] (Patent Document 1) Japanese Unexamined Patent Publication No. 2008-255484 Summary of the Invention

[0011] (I) Technical Problems to be Solved

[0012] An object of one aspect of the present invention is to provide a hot-rolled steel sheet and a method for manufacturing the same, the hot-rolled steel sheet not only having excellent fatigue performance due to high strength, but also having excellent formability and being suitable for stamping.

[0013] In addition, the technical problems of the present invention are not limited to the above. The technical problems of the present invention can be understood from the overall content of this specification, and those skilled in the technical field to which the present invention pertains can easily understand the additional technical problems of the present invention.

[0014] (II) Technical Solutions

[0015] One embodiment of the present invention relates to a hot-rolled steel sheet which, by weight%, contains: carbon (C): 0.09 - 0.25%, silicon (Si): 0.5 - 2.3%, manganese (Mn): 1.5 - 3.5%, aluminum (Al): 0.001 - 1.0%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 - 0.20%, boron (B): 0.0005 - 0.005%, phosphorus (P): 0.0001 - 0.05%, sulfur (S): 0.0001 - 0.05%, nitrogen (N): 0.0001 - 0.05%, the balance being Fe and unavoidable impurities, and by area%, the fine structure contains 75 - 90% of a composite structure of martensite and austenite, 10 - 25% of bainitic ferrite, and contains 3 - 10% of the austenite.

[0016] The hot-rolled steel sheet may further contain 0.01 - 0.2% of niobium (Nb).

[0017] The average grain size of the bainitic ferrite may be 2.0 μm or more.

[0018] The average spacing of the bainitic ferrite may be 3 μm or more.

[0019] The hot-rolled steel sheet may have a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, an elongation of 9% or more, and may have an expansion rate of 45% or more.

[0020] Another embodiment of the present invention relates to a method for manufacturing a hot-rolled steel sheet, the manufacturing method comprising the following steps: reheating a steel slab in a temperature range of 1100 - 1350 °C, wherein, by weight%, the steel slab comprises: carbon (C): 0.09 - 0.25%, silicon (Si): 0.5 - 2.3%, manganese (Mn): 1.5 - 3.5%, aluminum (Al): 0.001 - 1.0%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 - 0.20%, boron (B): 0.0005 - 0.005%, phosphorus (P): 0.0001 - 0.05%, sulfur (S): 0.0001 - 0.05%, nitrogen (N): 0.0001 - 0.05%, the balance being Fe and other inevitable impurities; hot-rolling the reheated steel slab to manufacture a hot-rolled steel sheet; performing a first cooling on the hot-rolled steel sheet at a cooling rate of 50 °C / second or more, cooling to a temperature below B S ; after the first cooling, performing a second cooling for ts seconds (seconds) at a cooling rate of 25 °C / second or less, cooling to a temperature above (B S +M S ) / 2; after the second cooling, performing a third cooling at a cooling rate of 30 °C / second or less, cooling to a temperature range of (M S -20 °C) to 200 °C; and coiling within the temperature range of the third cooling, wherein, during the hot rolling, hot finish rolling is performed in a temperature range of 750 - 1150 °C such that the value of Du defined by the following relational expression 1 satisfies the range of 2 to 10, and the first cooling and the second cooling satisfy the conditions of the following relational expressions 2 to 4.

[0021] [Relational expression 1]

[0022] Du=(FDT+(7.4×[C])-(24.7×[Si])-(4.7×[Mn])-(3.9×[Cr])-(5.2×[Mo])-(560×[Ti])-(1110×[Nb]))×0.049 - 34.2

[0023] (In the relational expression 1, FDT represents the rolling end temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent the weight% content of each element within the parentheses.)

[0024] [Relational expression 2]

[0025] 5.0×10 6 ≤Du×Bat×2.968×10 10 ≤2.0×10 7

[0026] (In the relation 2, Du is defined in the same way as in relation 1, Bat represents 55.845×[B] / (1080.6 + 45.04×[B]), and [B] represents the weight content (%) of boron (B).)

[0027] [Relation 3]

[0028] 0.75 ≤ exp(-k(T)×(ts) 2 ) ≤ 0.9

[0029] (The k(T) represents the value defined by the following relation 4, and ts represents the secondary cooling time.)

[0030] [Relation 4]

[0031]

[0032] (In the relation 4, Du is defined in the same way as in relation 1, and Bat is defined in the same way as in relation 2. In addition, T1 represents the primary cooling termination temperature [°C], and T2 represents the secondary cooling termination temperature [°C]. In addition, [C], [Si], [Mn], [Cr], and [Mo] represent the weight % content of the respective elements in the brackets.)

[0033] During the hot rolling, the total reduction of the final two passes can be 10 - 40%.

[0034] After the coiling, a step of further cooling to room temperature can be included.

[0035] After the final cooling, a step of pickling and oiling can be further included.

[0036] After the pickling and oiling, a step of hot dip galvanizing can be further included.

[0037] (III) Beneficial effects

[0038] According to the present invention, a steel sheet having a high strength with a tensile strength of 980 MPa or more and excellent formability, and a manufacturing method thereof can be provided. Therefore, it can be applied to chassis structural members of automobiles, etc.

[0039] Various beneficial advantages and effects of the present invention are not limited to the above, and the advantages and effects can be more easily understood during the description of the specific embodiments of the present invention. Description of the drawings

[0040] Figure 1 For the figure showing the relationship between the content of boron and Du that simultaneously satisfies relation 1 and relation 2, the desired fine structure of the present invention can be ensured within the solid line connecting A - B - C - D - E - F.

[0041] Figure 2 (a), (b), and (c) of the present invention show photographs of the microstructures of Invention Example 4, Comparative Example 2, and Comparative Example 3, respectively, observed by a scanning electron microscope in an embodiment of the present invention. Best Mode for Carrying Out the Invention

[0042] The preferred embodiments of the present invention will be described below. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

[0043] In addition, the terms used in this specification are for describing specific embodiments and are not intended to limit the present invention. For example, unless the relevant definition clearly indicates the contrary meaning, the singular forms used in this specification also include plural forms. In addition, the meanings of "comprising" and "including" used in the specification are used to specify the components and do not exclude the existence or addition of other components.

[0044] In order to improve the hole expansion property of high-strength steel with a tensile strength of 980 MPa or more, the existing manufacturing method of hot-rolled steel sheets that sets the fraction of a specific microstructure in the steel's microstructure to 90% or more to prevent deterioration of the hole expansion property due to the interphase hardness difference cannot ensure excellent elongation. In addition, when using retained austenite to ensure excellent elongation, although elongation can be ensured, there is a problem that it is difficult to ensure the hole expansion property at the same time.

[0045] Therefore, the present inventors have confirmed that by using martensite and austenite as the matrix microstructures, ensuring both high strength and elongation, and uniformly dispersing bainitic ferrite as the secondary phase in the microstructure to prevent excessive stress concentration at specific positions during deformation and suppress the generation of microcracks, the hole expansion property can be improved, and thus the present invention has been completed. The present invention will be described in more detail below.

[0046] A steel sheet according to a specific embodiment of the present invention will be described. The steel sheet may contain, by weight%: carbon (C): 0.09 - 0.25%, silicon (Si): 0.5 - 2.3%, manganese (Mn): 1.5 - 3.5%, aluminum (Al): 0.001 - 1.0%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 - 0.20%, boron (B): 0.0005 - 0.005%, phosphorus (P): 0.0001 - 0.05%, sulfur (S): 0.0001 - 0.05%, nitrogen (N): 0.0001 - 0.05%.

[0047] Carbon (C): 0.09 - 0.25%

[0048] Carbon (C) is an important element that stabilizes austenite and forms retained austenite by diffusing into austenite after martensitic transformation. As the content of this C increases, the fraction of retained austenite increases, thereby simultaneously increasing elongation and tensile strength. When the content of the C is less than 0.09%, the fraction of retained austenite is low, and elongation and tensile strength cannot be ensured. In addition, when the content of C exceeds 0.25%, the M S temperature drops excessively, resulting in difficult carbon diffusion and the formation of too much fresh martensite, so there is a problem of poor hole expansion. Therefore, in the present invention, the content of the C is preferably 0.09 - 0.25%. The content of the C is more preferably 0.090 - 0.250%. More preferably, the lower limit of the content of the C can be 0.12%, or the upper limit of the C content can be 0.23%.

[0049] Silicon (Si): 0.5 - 2.3%

[0050] Silicon (Si) is an important element that delays the formation of carbides after martensitic transformation and forms retained austenite. In addition, Si plays a role in increasing strength through solid solution strengthening. When the content of the Si is less than 0.5%, carbides are formed and the fraction of retained austenite is low, making it difficult to ensure elongation. On the other hand, when the content of the Si exceeds 2.3%, Fe - Si composite oxides are formed on the slab surface during re - heating, resulting in poor surface quality of the steel plate and a problem of reduced weldability. Therefore, in the present invention, the content of the Si is preferably 0.5 - 2.3%. The content of the Si is more preferably 0.50 - 2.30%. More preferably, the lower limit of the content of the Si can also be 0.7%, or the upper limit of the Si content can be 2.1%.

[0051] Manganese (Mn): 1.5 - 3.5%

[0052] Manganese (Mn) is an element that improves the hardenability of steel and prevents the formation of ferrite during the cooling process after finish rolling, thereby making it easy to form a low - temperature transformation structure.

[0053] When the content of this Mn is less than 1.5%, there are problems of insufficient hardenability and excessive increase in the fraction of ferrite. On the other hand, when the content of the Mn exceeds 3.5%, the hardenability increases significantly, the holding time required to fully form the bainitic ferrite expected to be obtained in the present invention increases excessively, and the hole expansion decreases.

[0054] Therefore, in the present invention, the content of the Mn can be 1.5 - 3.5%, and the content of the Mn is more preferably 1.50 - 3.50%. More preferably, the lower limit of the Mn content can be 1.6%, or the upper limit of the Mn content can be 3.0%.

[0055] Aluminum (Al): 0.001 - 1.0%

[0056] Aluminum (Al) is an element usually added for deoxidizing molten steel. After deoxidation, part of it remains in the steel. Similar to Si, it delays the formation of carbides after martensitic transformation, thus also playing a role in forming retained austenite.

[0057] When the content of the Al is less than 0.001%, carbides are formed and the fraction of retained austenite is low, making it difficult to ensure elongation. On the other hand, when the content of the Al exceeds 1.0%, it causes an increase in oxide and nitride-based inclusions in the steel, thereby deteriorating the formability of the steel plate. Therefore, in the present invention, the content of the Al is preferably 0.001 - 1.0%. More preferably, the lower limit of the content of the Al can be 0.01%, or the upper limit of the content of the Al can be 0.5%.

[0058] Chromium (Cr): 2.5% or less (including 0%)

[0059] Chromium (Cr) is an element that improves the hardenability of steel and inhibits the formation of ferrite during the cooling process after finish rolling. When the content of this Cr exceeds 2.5%, the hardenability increases significantly, and the bainite transformation in the cooling zone cannot proceed smoothly. The holding time for ensuring the fraction of bainite ferrite increases excessively, resulting in poor hole expansion property. Therefore, in the present invention, the content of the Cr can be 2.5% or less, more preferably 2.50% or less, and more preferably, the content of the Cr can be 1.5% or less.

[0060] In addition, in the present invention, even if the Cr is not included, there is not much difficulty in ensuring the target physical properties, so the case where the Cr content is 0% is included. However, it should be noted that when the Cr is added, it is effective to add at least 0.01%.

[0061] Molybdenum (Mo): 2.0% or less (including 0%)

[0062] Molybdenum (Mo) is an element that improves the hardenability of steel and plays a role in increasing strength through solid solution strengthening effect, and inhibits the formation of ferrite during the cooling process after finish rolling. When the content of the Mo exceeds 2.0%, the hardenability increases significantly, and the bainite transformation in the cooling zone cannot proceed smoothly. Therefore, the holding time for ensuring the fraction of bainite ferrite increases excessively, resulting in a decrease in hole expansion property. Therefore, in the present invention, the content of the Mo can be 2.0% or less, more preferably, the content of the Mo can be 1.0% or less, and more preferably, the content of the Mo can be 0.5% or less.

[0063] In addition, even if the Mo is not included in the present invention, there is not much difficulty in ensuring the target physical properties, so the case where the Mo content is 0% is included. However, it should be noted that when the Mo is intentionally added, adding at least 0.01% is effective.

[0064] Titanium (Ti): 0.01 - 0.20%

[0065] Titanium (Ti) is an element that forms carbonitrides in steel and is widely used as described above for inducing the formation of precipitates to ensure the strength of steel. However, in the present invention, by removing nitrogen (N) in the steel, the formation of BN is suppressed, which plays a role in enriching boron (B) at the austenite grain boundaries and is also used for controlling the grain size of austenite before controlled rolling.

[0066] In order to fully obtain the desired effects of the present invention, the content of the Ti is preferably 0.01% or more. In order to remove nitrogen (N) in the steel, the content of the Ti is preferably 2.9 times or more of the nitrogen (N) content. However, when the content of the Ti exceeds 0.20%, oxides are formed during continuous casting, and problems such as clogging of the casting nozzle may occur.

[0067] Therefore, in the present invention, the content of the Ti can be 0.01 - 0.20%, more preferably, the lower limit of the Ti can be 0.015%, or the upper limit of the Ti content can be 0.12%.

[0068] Boron (B): 0.0005 - 0.005%

[0069] Boron is an element that increases the hardenability of steel by reducing the grain boundary energy by enriching at the austenite grain boundaries. In the present invention, by suppressing the phase transformation of ferrite and upper bainite that nucleate during the diffusion phase transformation at the austenite grain boundaries, it plays a role in ensuring the main phase as a composite structure of martensite and austenite.

[0070] In order to fully obtain the desired effects of the present invention, the concentration of B is preferably 0.0005% or more. However, when the content of the B exceeds 0.005%, the hardenability increases significantly, the holding time required to fully form the bainite ferrite expected to be obtained in the present invention increases excessively, and the hole expansion property decreases.

[0071] Therefore, in the present invention, the content of the B can be 0.0005 - 0.005%, more preferably, the lower limit of the B content can be 0.001%, or the upper limit of the B content can be 0.0025%.

[0072] Phosphorus (P): 0.0001 - 0.05%

[0073] Phosphorus (P) is an impurity inevitably contained in steel and is an element that is the main cause of hindering the workability of steel through segregation. Therefore, it is preferable to control the content of the phosphorus (P) as low as possible.

[0074] Theoretically, it is advantageous to limit the content of the P to 0%, but it requires too high a manufacturing cost to control the content of the P to less than 0.0001%, so the lower limit of the P content can be set to 0.0001%. However, when the content of the P exceeds 0.05%, the workability may decrease, so the upper limit of the P content can be limited to 0.05%. However, more preferably, the lower limit of the P content can be 0.0005%, or the upper limit of the P content can be 0.02%.

[0075] Sulfur (S): 0.0001 - 0.05%

[0076] Sulfur (S) is an impurity inevitably contained in steel and combines with Mn, etc. to form non-metallic inclusions, so there is a problem of reduced workability of the steel. Therefore, it is preferable to control the content of the sulfur (S) as low as possible.

[0077] Theoretically, it is advantageous to limit the content of the S to 0%, but it requires too high a manufacturing cost to control the content of the S to less than 0.0001%, so the lower limit of the S content can be set to 0.0001%. However, when the content of the S exceeds 0.05%, the workability may decrease, so the upper limit of the S can be limited to 0.05%. However, more preferably, the lower limit of the S content can be 0.0005%, or the upper limit of the S content can be 0.005%.

[0078] Nitrogen (N): 0.0001 - 0.05%

[0079] Nitrogen (N) is an impurity inevitably contained in steel and combines with Al, etc. to form nitrides, so there is a problem of hindering the workability of the steel. Therefore, it is preferable to control the content of the nitrogen (N) as low as possible.

[0080] Theoretically, it is advantageous to limit the content of the N to 0%, but it requires too high a manufacturing cost to control the content of the N to less than 0.0001%, so the lower limit of the N content can be set to 0.0001%. However, when the content of the N exceeds 0.05%, the workability may decrease, so the upper limit of the N can be limited to 0.05%. However, more preferably, the lower limit of the N content can be 0.001%, or the upper limit of the N content can be 0.006%.

[0081] In addition to the above alloy composition, the hot-rolled steel sheet of the present invention may further include niobium (Nb).

[0082] Niobium (Nb): 0.01 - 0.2%

[0083] Niobium (Nb) is an element that forms carbonitrides in steel and is widely used as described above for ensuring the strength of steel by inducing the formation of precipitates. However, in the present invention, the niobium (Nb) delays recrystallization during hot rolling and functions to control the grain size of austenite. When the content of Nb is less than 0.01%, the effect of controlling the grain size is low. When the content of Nb exceeds 0.2%, the crystal grains of austenite are too fine, and there is a problem of poor formability. Therefore, in the present invention, the content of Nb can be 0.01 - 0.2%.

[0084] The remaining component of the present invention is iron (Fe). However, in the usual manufacturing process, inevitable impurities may be inadvertently mixed in from raw materials or the surrounding environment, so these impurities cannot be excluded. These impurities are well known to those skilled in the art of ordinary steel manufacturing, so not all of their content will be specifically described in this specification.

[0085] The hot-rolled steel sheet of the present invention has a composite structure of martensite and austenite as the matrix structure, and simultaneously ensures a tensile strength of 980 MPa or more and an elongation of 9% or more. Therefore, according to a specific embodiment of the present invention, it may contain 75 - 90% of the composite structure composed of the martensite and austenite by area%.

[0086] In the present invention, during the three cooling processes after hot rolling, a part of the untransformed austenite transforms into martensite below the M S temperature. After coiling, the steel sheet gradually cools and remains in a nearly isothermal state. At this time, an isothermal phase transformation of austenite to martensite occurs, and thus the fraction of martensite increases. Martensite is generated by shear phase transformation (displacive phase transformation). Therefore, due to the screw dislocations generated in the structure to reduce the shear deformation generated during the phase transformation and the edge dislocations generated to accommodate the volume expansion caused by the phase transformation, a high level of dislocation density exists in the structure. Therefore, it is suitable to improve the yield strength and tensile strength of the steel by the fine carbides present in the structure. In addition, the high level of dislocation density and fine carbides hinder the movement of dislocations within the structure, so it has the characteristic of poor elongation.

[0087] Therefore, it is preferable to improve the elongation of high-strength steel by including austenite in the matrix structure and through the phenomenon of plastic-induced phase transformation. After martensitic transformation, the over-absorbed carbon atoms in martensite diffuse and move into austenite, gradually increasing the carbon concentration inside austenite. Austenite, whose stability is improved due to carbon enrichment, does not undergo phase transformation even when cooled to room temperature but remains in the fine structure, thus playing a role in improving the elongation of the steel plate. Therefore, in terms of ensuring tensile strength and elongation, the area fraction of the martensite and austenite composite structure is preferably 75% or more. Additionally, in order to ensure the following hole expansion property, the area fraction of the martensite and austenite composite structure is preferably limited to 90% or less.

[0088] At this time, the austenite can be 3 - 10% by area%. When the area fraction of austenite is less than 3%, the effect of improving elongation brought about by the plastic-induced phase transformation phenomenon is negligible. On the other hand, in order to make the area fraction of the austenite exceed 10%, it is necessary to increase the carbon (C) content added to the steel, so there is a problem that the weldability of the steel deteriorates. Due to M S The temperature is too low, carbon is not easily diffused, and the amount of newly formed martensite generated in the final cooling step from coiling to room temperature increases excessively, resulting in poor hole expansion property.

[0089] The hot-rolled steel plate of the present invention may have bainitic ferrite as a secondary phase of the fine structure and may contain 10 - 25% of the secondary phase by area%.

[0090] When the present invention performs the first cooling after hot rolling, it is cooled to a temperature below B S (the bainite transformation start temperature) to avoid ferrite transformation, and then slowly cooled in the subsequent secondary cooling process to perform bainite transformation. At this time, the bainite transformation occurs in the high-temperature bainite transformation region. Therefore, the phenomena of generation of bainitic ferrite and diffusion of carbon to untransformed austenite occur, and it is characterized by not generating carbides inside the bainitic ferrite. On the other hand, there are a large number of dislocations inside the bainitic ferrite generated by shear transformation, but due to the recovery phenomenon in the secondary cooling, the dislocation density is reduced to an appropriate level, so it is characterized by having a soft property.

[0091] In addition, since martensitic transformation occurs at M SWithin a relatively wide temperature range from the temperature to the termination temperature of the third cooling, the temperature at which the phase transformation starts varies depending on the position within the steel plate. Since the residual phase transformation stress within the steel plate varies according to the martensite formation temperature, the phase transformation stress is unevenly distributed depending on the position within the steel plate and remains within the steel plate even after cooling to room temperature. When deformation is externally applied during component forming, the deformation concentrates in the areas with higher residual stress within the steel plate, thereby promoting the growth and propagation of cracks. Therefore, there is a problem of poor hole expansion performance.

[0092] In addition, when soft bainitic ferrite is uniformly distributed in appropriate sizes within a high-strength matrix structure with an uneven stress distribution, local concentration of stress can be prevented by uniformly absorbing the deformation during the forming process, thereby having the effect of improving hole expansion performance.

[0093] Therefore, in the present invention, when the fraction of bainitic ferrite as the secondary phase is less than 10%, there is a problem of difficulty in ensuring hole expansion performance. On the other hand, when the fraction of the bainitic ferrite exceeds 25%, there is a problem of difficulty in ensuring the martensite and austenite composite structure that plays a role in enhancing strength.

[0094] In addition, the average particle size of the bainitic ferrite can be 2.0 μm or more. Furthermore, the average spacing of the bainitic ferrite can be 3 μm or more.

[0095] At this time, the average particle size of the bainitic ferrite represents the equivalent circle diameter, and the average spacing of the bainitic ferrite represents the average value of the distances between the 5 closest microstructures for each microstructure.

[0096] When the average particle size of the bainitic ferrite as the soft tissue is less than 2.0 μm, the effect of accommodating deformation is low, and an improvement in hole expansion performance cannot be expected. In addition, when the average spacing of the bainitic ferrite is less than 3.0 μm, the fraction of the soft steel increases excessively, so the yield strength and tensile strength may deteriorate. The upper limits of the average particle size and the average spacing of the soft tissue are not controlled individually, but under the condition that the fraction of the soft tissue satisfies the range of 10 - 25%, the average particle size of the soft tissue is preferably 20 μm or less. Furthermore, the average spacing of the soft tissue can be 20 μm or less.

[0097] In addition to the above-mentioned structure, the hot-rolled steel plate of the present invention may contain carbides and fresh martensite as other structures, but their area fractions are preferably controlled to be less than 5%.

[0098] In the manufacturing process of the hot-rolled steel sheet, carbides may be generated. After the martensitic transformation, a part of the carbon atoms form fine carbides inside the laths, which can play a role in increasing the strength. On the other hand, the present invention aims to use austenite to increase the elongation, so the generation of carbides may lead to a decrease in the austenite fraction. That is, the excessive generation of carbides hinders the improvement of the elongation desired in the present invention. However, when Ti and Nb are present in the phase, alloy carbonitrides may be formed. In this case, an additional strengthening effect due to grain refinement can be expected, but coarse carbides hinder the toughness of the steel. Therefore, the content of carbides present in the hot-rolled steel sheet of the present invention is preferably less than 5%.

[0099] In addition, according to a specific embodiment of the present invention, the hot-rolled steel sheet may contain fresh martensite as a fine microstructure. In the present invention, the martensite that undergoes isothermal transformation immediately after the three-stage cooling process and coiling transforms before the carbon enrichment in the austenite begins, so it has a lath shape within the composition range of the present invention. Additionally, when the coiling temperature is too low, carbon is not easily diffused, so the cooling may be completed before the austenite is sufficiently stable. In this case, if the M S temperature of the austenite with carbon enrichment is above room temperature, it may transform into fresh martensite during the cooling process. The fresh martensite generated during the final cooling process undergoes shear deformation during the phase transformation, forming twins instead of dislocations, so it has a plate (Plate) shape and has the characteristic of observing twins in the fine microstructure, so it can be easily distinguished from the martensite in the present invention. Since the fresh martensite with a high carbon concentration has too high hardness and poor hole expansion performance, the fraction of fresh martensite present in the hot-rolled steel sheet of the present invention is preferably less than 5%.

[0100] The hot-rolled steel sheet of the present invention having the above alloy composition and fine microstructure has high strength, with a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, an elongation of 9% or more, a product of tensile strength and elongation of 13000 MPa·% or more, and a hole expansion rate of 45% or more, and has excellent formability characteristics.

[0101] Next, a manufacturing method of a hot-rolled steel sheet according to another specific embodiment of the present invention will be described in detail. However, the hot-rolled steel sheet of the present invention does not necessarily have to be manufactured by the following manufacturing method.

[0102] The manufacturing method can manufacture a steel billet that satisfies the alloy composition proposed by the present invention through a series of processes of [reheating - hot rolling - cooling - coiling].[[]END]]

[0103] Next, the above process conditions will be described in detail.

[0104] Reheating of the steel billet

[0105] Preferably, before the following rolling process, a process of reheating the steel billet for homogenization treatment is carried out. At this time, the reheating of the steel billet can be carried out in the temperature range of 1100 - 1350 °C.

[0106] When the temperature during reheating of the steel billet is lower than 1100 °C, there is a problem of insufficient homogenization of alloying elements. However, when the temperature during reheating of the steel billet exceeds 1350 °C, oxides are excessively formed on the surface of the steel billet, so the surface quality of the steel plate may be reduced.

[0107] Hot rolling

[0108] The reheated steel billet can be hot rolled to manufacture a hot rolled steel plate. At this time, preferably, the hot rolling is carried out in the temperature range of 750 - 1150 °C, and the total reduction of the final two passes is controlled to be 10 - 40%.

[0109] First, when the hot rolling starts at a temperature exceeding 1150 °C, oxides are excessively formed on the surface of the rolled steel plate, and even through pickling process, it cannot be effectively controlled, so the surface quality deteriorates. On the other hand, when the hot rolling is carried out at a temperature lower than 750 °C, there is a problem that the rolling load increases excessively, resulting in reduced workability, and ferrite is generated during the rolling process, thus having a problem of deteriorated anisotropy.

[0110] Generally, sectional rolling is carried out during hot rolling to reduce the rolling load and precisely control the thickness. In the case of hot rolling by this sectional rolling, when the sum of the reduction ratios of the final two passes (the last two passes) exceeds 40%, the rolling load of the final two passes becomes too large, and there is a problem of deteriorated workability. On the other hand, when the sum of the reduction ratios of the final two passes is less than 10%, there is a problem that the temperature of the steel plate drops rapidly, thus inducing poor shape.

[0111] In addition, the grain size of austenite after hot rolling is affected by alloy composition and rolling termination temperature, which affects the formation behavior of bainite and the final microstructure in the subsequent cooling process. In addition, in the present invention, the fraction and size of bainite ferrite as the main constituent phase are greatly affected by the austenite grains after hot rolling.

[0112] The grains of equiaxed ferrite and pearlite grow through element diffusion during the phase transformation process, so the size of the structure after phase transformation is affected by the phase transformation temperature and holding time. On the other hand, bainite ferrite generated by shear phase transformation such as bainite only grows within the austenite grains, so their size cannot be larger than the austenite size before phase transformation. Therefore, in order to control the size of bainite ferrite, it is beneficial to control the grain size of austenite after hot rolling.

[0113] Therefore, in the present invention, the effective grain size of austenite after hot rolling is obtained through the relationship between the finish rolling temperature (FDT) and a specific alloy composition, specifically defined by the following relational expression 1. That is, during hot rolling, hot finish rolling is performed within the temperature range of 750 - 1150 °C so that the Du value defined by the following relational expression 1 satisfies the range of 2 to 10.

[0114] [Relational expression 1]

[0115] Du = (FDT + (7.4 × [C]) - (24.7 × [Si]) - (4.7 × [Mn]) - (3.9 × [Cr]) - (5.2 × [Mo]) - (560 × [Ti]) - (1110 × [Nb])) × 0.049 - 34.2

[0116] (In the relational expression 1, FDT represents the finish rolling temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent the weight % content of the elements within each pair of brackets.)

[0117] The Du is an index representing the effective grain size of austenite before primary cooling after hot rolling. When the Du value defined by the relational expression 1 is 2 or more, the average grain diameter of bainitic ferrite is 2.0 μm or more, thereby ensuring a reaming rate of 45% or more. On the other hand, when the Du value defined by the relational expression 1 exceeds 10, the grain boundary concentration of boron increases excessively, delaying the phase transformation during secondary cooling and making it impossible to ensure a sufficient fraction of bainitic ferrite. Therefore, there is a problem of poor reaming property. The Du value is more preferably 2.0 to 10.0.

[0118] In addition, according to a specific embodiment of the present invention, boron stabilizes the austenite grain boundary through segregation at the austenite grain boundary to delay the nucleation of ferrite and upper bainite, thereby playing a role in reducing the phase transformation rate. To ensure the area fraction and average spacing of bainitic ferrite desired in the present invention, it is important to control the concentration of boron segregated at the austenite grain boundary. Due to the influence of microsegregation during casting and the grain size of austenite, the concentration of boron segregated at the austenite grain boundary shows different values at different grain boundaries. Among them, at the austenite grain boundary where the concentration of boron element is low, the nucleation of bainitic ferrite selectively occurs during the secondary cooling step. Generally, when the grain size of austenite is small, the concentration of boron segregated at each grain boundary is low, so nucleation is easy. When the grain size of austenite is large, the concentration of boron segregated at the grain boundary is high, so nucleation delay can be predicted. Therefore, as shown in relational expression 2, the concentration of boron present at the grain boundary and the phase transformation behavior of bainitic ferrite during secondary cooling are affected by the content of boron added to the steel and the grain size of austenite.

[0119] Relationship 2 is an index representing the concentration of boron (B) distributed at the austenite grain boundaries before cooling. When the value of Du×Bat×2.968×10 10 is less than 5.0×10 6 , the fraction of bainitic ferrite is too high, making it impossible to ensure the yield strength and tensile strength. On the other hand, when the value of Du×Bat×2.968×10 10 exceeds 2.0×10 7 , the secondary cooling time required to ensure the fraction of bainitic ferrite becomes too long, resulting in a problem of poor hole expansion performance.

[0120] [Relationship 2]

[0121] 5.0×10 6 ≤Du×Bat×2.968×10 10 ≤2.0×10 7

[0122] (In the above Relationship 2, Du is defined the same as in Relationship 1, Bat represents 55.845×[B] / (1080.6 + 45.04×[B]), and [B] represents the weight content (%) of boron (B).)

[0123] Figure 1 For the figure showing the relationship between the boron content and Du that simultaneously satisfies the above Relationship 1 and Relationship 2, the desired microstructure of the present invention can be ensured within the solid line connecting A - B - C - D - E - F.

[0124] Cooling and coiling

[0125] Cool the hot - rolled steel sheet manufactured as described above. At this time, cooling is preferably carried out in stages according to the cooling temperature.

[0126] Specifically, it is preferable to first cool the hot - rolled steel sheet at a cooling rate of 50°C / second or more to a temperature below B S , and then perform secondary cooling for ts seconds (seconds) defined in Relationship 3 at a cooling rate of 25°C / second or less to a temperature above (B S +M S ). Then, perform tertiary cooling at a cooling rate of 30°C / second or more to a temperature range of (M S - 20°C) to 200°C.

[0127] Rapidly cool the hot - rolled steel sheet manufactured as described above to a temperature below the bainite start temperature (B S ) to suppress the formation of ferrite (granular ferrite). Then, slowly cool for ts seconds (seconds) to the bainite start temperature (BS ), and the intermediate temperature between the martensite start temperature (M S ) or a temperature above the intermediate temperature, so that bainite ferrite with an area fraction of 10 - 25% can be ensured.

[0128] When performing a first cooling to a temperature below B after the hot rolling is completed, when the cooling rate is less than 50 °C / sec, there is a problem of forming a ferrite phase during the cooling process. At this time, the upper limit of the first cooling rate is not particularly limited, but when the steel plate is rapidly cooled, the shape of the plate may be distorted. Therefore, the upper limit of the first cooling rate can be limited to 200 °C / sec or less. S It should be noted that the lower limit of the cooling termination temperature during the first cooling is not particularly limited, but when the lower limit of the termination temperature during the first cooling is too low, the cooling time during the subsequent second cooling may be insufficient. Therefore, the lower limit of the termination temperature during the first cooling can be limited to B

[0129] -100 °C. S -100 °C.

[0130] When, through the first cooling, the temperature of the hot rolled steel plate is below B S , the cooling of the steel is terminated, and the second cooling can be performed at a cooling rate of 25 °C / sec or less, cooling to a temperature above (B S +M S ) / 2.

[0131] During the period when the hot rolled steel plate after the first cooling is cooled from the temperature of the first cooling to the target temperature of the second cooling, the growth of bainite ferrite occurs. In particular, in order to obtain the fraction desired in the present invention, it is preferable to maintain the second cooling for a time (ts, seconds (sec)) satisfying the following relational expression 3.

[0132] In relational expression 3, k(T) is an index representing the growth rate of bainite ferrite, which is affected not only by the alloy composition of the steel but also by the phase transformation temperature and the grain size after hot rolling. Therefore, when the value of relational expression 3, that is, the relationship between k(T) and the holding time (exp(-k(T)×(ts) 2 )) is less than 0.75, the fraction of bainite ferrite is too large to ensure the strength at the target level. On the other hand, when the value of relational expression 3 exceeds 0.9, there is a problem of poor hole expansion property.

[0133] [Relational expression 3]

[0134] 0.75 ≤ exp(-k(T)×(ts) 2 ) ≤ 0.9

[0135] (The k(T) represents the value defined by the following relational expression 4.)

[0136] [Relationship 4]

[0137]

[0138] (In the said Relationship 4, Du is the same as defined in Relationship 1, and Bat is the same as defined in Relationship 2. In addition, T1 represents the primary cooling termination temperature [°C], and T2 represents the secondary cooling termination temperature [°C]. In addition, [C], [Si], [Mn], [Cr], and [Mo] represent the weight % content of the elements within each bracket.)

[0139] During the secondary cooling according to the above conditions, due to the phase transformation heat generation based on the bainite phase transformation, the temperature of the steel plate may rise. At this time, due to excessive heat generation, the dislocation density may be excessively reduced. Therefore, in order to minimize the temperature rise of the steel plate caused by the phase transformation heat generation, the cooling rate during the secondary cooling can be controlled below 25 °C / second. When the said cooling rate exceeds 25 °C / second, the shape of the plate may be distorted. It should be noted that the secondary cooling in the present invention also includes an air cooling process.

[0140] Preferably, the hot-rolled steel plate that has completed the secondary cooling as described above is subjected to tertiary cooling at a cooling rate of 30 °C / second or less until it reaches a temperature range of (M S -20 °C) to 200 °C, and then coiled at this temperature. During the said tertiary cooling, martensite phase transformation occurs at a temperature below M S and a part of the untransformed austenite can further grow into martensite under isothermal conditions after coiling.

[0141] When martensite phase transformation occurs, if the cooling rate is too fast, the plate shape will be deformed due to rapid volume expansion, which will trigger cooling imbalance again and lead to uneven material distribution. Therefore, in the present invention, by setting the cooling rate in the tertiary cooling step accompanied by a sharp phase transformation to 30 °C / second or less, shape deformation during cooling and the resulting uneven material deviation within the plate can be prevented. In addition, when the cooling rate is too slow, bainite ferrite will grow during cooling, resulting in an excessive fraction of secondary phases, thus unable to ensure the strength of the steel. Therefore, the cooling rate in the tertiary cooling step of the present invention can be 5 °C / second or more.

[0142] In addition, the maximum carbon concentration that can be dissolved in austenite varies with different carbon enrichment temperatures. Generally, the solubility limit of carbon in austenite increases with decreasing temperature. Therefore, when the coiling temperature for carbon enrichment is too high, the carbon enrichment in austenite is insufficient, and the phase stability required for plastic-induced phase transformation cannot be ensured. Even if it remains at room temperature, it will be lost due to stress-induced phase transformation at the initial stage of deformation, and thus the improvement in elongation cannot be expected. Therefore, in the present invention, in order to ensure sufficient phase stability, the upper limit of the termination temperature of the third cooling is preferably M S -20 °C. In addition, when the temperature for enrichment is too low, carbon diffusion is poor, and the carbon concentration inside austenite cannot reach the solid solution limit. In this case, the stability of austenite is insufficient, and it may transform into fresh martensite during the cooling process, thereby reducing the hole expansion property. Therefore, in the present invention, the lower limit of the termination temperature of the third cooling is preferably 200 °C.

[0143] In the present invention, B S and M S can be obtained by the following formula, where each element represents the weight content.

[0144] B S (°C) = 830 - (320 × [C]) - (90 × [Mn]) - (35 × [Si]) - (70 × [Cr]) - (120 × [Mo])

[0145] M S (°C) = 550 - (330 × [C]) - (41 × [Mn]) - (20 × [Si]) - (20 × [Cr]) - (10 × [Mo]) + (30 × [Al])

[0146] Final cooling

[0147] After completing the cooling and coiling processes as described above, final cooling can be performed to obtain the desired hot-rolled steel sheet. At this time, air cooling to room temperature can be carried out to complete the final cooling.

[0148] In addition, as described above, the hot-rolled steel sheet of the present invention obtained by completing the final cooling can be further pickled and oiled.

[0149] In addition, the pickled and oiled hot-rolled steel sheet can be heated to a temperature range of 420 - 740 °C for a hot-dip galvanizing process.

[0150] The hot-dip galvanizing process can utilize a zinc-based plating bath, and the alloy components in the zinc-based plating bath are not particularly limited. Detailed implementation mode

[0151] The present invention will be described in more detail by way of examples below. However, it should be noted that the following examples are only used to illustrate the present invention for more detailed description and are not used to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the matters recorded in the claims and the matters reasonably deduced therefrom.

[0152] (Example)

[0153] Prepare steel billets having the alloy compositions shown in Table 1 below (in wt%, with the balance being Fe and unavoidable impurities).

[0154] Reheat each of the prepared steel billets at 1200 °C, and then perform a hot rolling process, a cooling process, a coiling process, and a final cooling (air cooling) process under the conditions shown in Table 2 below to manufacture a hot-rolled steel sheet with a thickness of 2.5 mm. The total reduction ratio of the final two passes during hot rolling was uniformly applied at 25%, the cooling rate during primary cooling was uniformly applied at 70 °C / second, and the cooling rate during tertiary cooling was uniformly applied at 20 °C / second.

[0155] Measure the mechanical properties of each hot-rolled steel sheet and observe the microstructure, and show the results in Table 3 and Table 4 below.

[0156] The yield strength, tensile strength, and elongation in the mechanical properties were measured at room temperature using a universal tensile testing machine after cutting a JIS-5 standard test piece in a direction perpendicular to the rolling direction. At this time, the yield strength, tensile strength, and elongation are respectively expressed as the 0.2% offset yield strength, the maximum tensile strength, and the elongation at break.

[0157] In addition, the hole expansion property was measured for the same test piece as in the tensile test according to the ISO TS16630 standard method.

[0158] Furthermore, the microstructure of each hot-rolled steel sheet was etched using the nitric acid ethanol (Nital) etching method for the same test piece as in the tensile test, and then observed at a magnification of 10,000 using a scanning electron microscope and an image analyzer, and the fraction of each phase was calculated. The average size of bainitic ferrite represents the equivalent circle diameter, and the average spacing represents the average of the distances between the five closest microstructures to each bainitic ferrite phase.

[0159] Using a Bruker X-Ray Diffractometer, calculate the austenite fraction by using the integrated intensity of the diffraction peaks of each phase.

[0160] At this time, the microstructure was observed for the cross-section of the test piece, that is, observed at the t / 4 position in the thickness direction of the cross-section perpendicular to the rolling direction.

[0161] [Table 1]

[0162]

[0163] [Table 2]

[0164]

[0165] FDT Finish Rolling Temperature (°C)

[0166] Du = (FDT + (7.4 × [C]) - (24.7 × [Si]) - (4.7 × [Mn]) - (3.9 × [Cr]) - (5.2 × [Mo]) - (560 × [Ti]) - (1110 × [Nb])) × 0.049 - 34.2

[0167] Va = Du × Bat × 2.968 × 10 10

[0168] (Bat represents 55.845 × [B] / (1080.6 + 45.04 × [B]), and [B] represents the weight content (%) of boron (B). [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent the weight % content of the elements within each respective parenthesis.)

[0169] Relationship 3 is exp(-k(T) × (ts) 2 )

[0170] (The k(T) is the value defined as follows, and ts is the secondary cooling time)

[0171]

[0172] (T1 represents the primary cooling termination temperature [°C], and T2 represents the secondary cooling termination temperature [°C]. Additionally, [C], [Si], [Mn], [Cr], and [Mo] represent the weight % content of the elements within each respective parenthesis)

[0173] [Table 3]

[0174]

[0175] M: Martensite

[0176] A: Austenite

[0177] FM: Fresh Martensite

[0178] BF: Bainite Ferrite

[0179] [Table 4]

[0180]

[0181] YS: Yield strength

[0182] TS: Tensile strength

[0183] El: Elongation

[0184] TS El: The product of tensile strength and elongation

[0185] HER: Hole expansion ratio

[0186] As shown in Tables 1 to 4, in Invention Examples 1 to 11 that satisfy all the alloy compositions and manufacturing conditions proposed by the present invention, the matrix structure is composed of a composite structure of martensite and austenite with an area fraction of 75 - 90% and a second phase of bainitic ferrite of 10 - 25%. Therefore, the desired strength and formability can be ensured.

[0187] In addition, in Comparative Example 1 where boron is not added and the alloy composition system proposed by the present invention is not satisfied, excessive bainitic ferrite is generated during secondary cooling. Therefore, the target strength cannot be ensured, and due to this low strength, the elongation shows a relatively high tendency. In Comparative Example 2, titanium is not added, and sufficient bainitic ferrite cannot be obtained. Therefore, it is difficult to ensure the hole expandability required by the present invention.

[0188] In addition, Comparative Examples 3 to 5 are cases where the alloy composition satisfies the scope of the present invention, but the manufacturing conditions are not within the scope of the present invention.

[0189] In Comparative Examples 3 and 4, due to the excessively long secondary cooling time, Relationship 3 is not satisfied, and the fraction of bainitic ferrite as the secondary phase is too high to ensure a tensile strength of 980 MPa or more.

[0190] In Comparative Example 5, due to the too low finish temperature of the tertiary cooling, carbon is not easily diffused and moved, resulting in a lower fraction of austenite, so the elongation cannot be ensured, and due to excessive newly formed martensite, the hole expandability is also poor.

[0191] In Comparative Example 6, Si is not added, and the fraction of austenite cannot be ensured, so the elongation is poor.

[0192] Figure 1 A graph showing the relationship between the content of boron and Du that simultaneously satisfies Relationship 1 and Relationship 2. The desired fine structure of the present invention can be ensured within the solid line connecting A - B - C - D - E - F.

[0193] Figure 2 Photographs showing the fine structures of Invention Example 4, Comparative Example 2, and Comparative Example 3 observed with a scanning electron microscope, respectively. As Figure 2As shown in (a) of [description], in Invention Example 4, the matrix structure and the second phase desired to be achieved by the present invention were appropriately formed as the fine structure. On the other hand, as shown in Figure 2 (b) of [description], it was confirmed that the soft structure as the second phase was not sufficiently generated in Comparative Example 2. In addition, as shown in Figure 2 (c) of [description], an excessive amount of the soft structure as the secondary phase was generated in Comparative Example 3.

Claims

1. A hot-rolled steel sheet, by weight %, the hot-rolled steel sheet contains: carbon (C): 0.09 - 0.25%, silicon (Si): 0.5 - 2.3%, manganese (Mn): 1.5 - 3.5%, aluminum (Al): 0.001 - 1.0%, chromium (Cr): 2.5% or less including 0%, molybdenum (Mo): 2.0% or less including 0%, titanium (Ti): 0.01 - 0.20%, boron (B): 0.0005 - 0.005%, phosphorus (P): 0.0001 - 0.05%, sulfur (S): 0.0001 - 0.05%, nitrogen (N): 0.0001 - 0.05%, the balance being Fe and unavoidable impurities, By area %, the microstructure contains 75 - 90% of a composite structure of martensite and austenite, and 10 - 25% of bainitic ferrite, And contains 3 - 10% of the austenite.

2. The hot-rolled steel sheet according to claim 1, wherein, The hot-rolled steel sheet further contains 0.01 - 0.2% of niobium (Nb).

3. The hot-rolled steel sheet according to claim 1, wherein, The average grain size of the bainitic ferrite is 2.0 μm or more.

4. The hot-rolled steel sheet according to claim 1, wherein, The average spacing of the bainitic ferrite is 3 μm or more.

5. The hot-rolled steel sheet according to claim 1, wherein, The hot-rolled steel sheet has a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, an elongation of 9% or more, and a hole expansion rate of 45% or more.

6. A method for manufacturing a hot-rolled steel sheet, which includes the following steps: Reheat the steel billet in the temperature range of 1100 - 1350 °C. By weight %, the steel billet contains: carbon (C): 0.09 - 0.25%, silicon (Si): 0.5 - 2.3%, manganese (Mn): 1.5 - 3.5%, aluminum (Al): 0.001 - 1.0%, chromium (Cr): 2.5% or less including 0%, molybdenum (Mo): 2.0% or less including 0%, titanium (Ti): 0.01 - 0.20%, boron (B): 0.0005 - 0.005%, phosphorus (P): 0.0001 - 0.05%, sulfur (S): 0.0001 - 0.05%, nitrogen (N): 0.0001 - 0.05%, the balance being Fe and other unavoidable impurities; Hot-roll the reheated steel billet to manufacture a hot-rolled steel sheet; The hot-rolled steel plate is subjected to a primary cooling at a cooling rate of 50 °C / second or more until it is cooled to a temperature below the bainite start temperature (B S ); After the first cooling, secondary cooling is carried out for a time of ts at a cooling rate of 25 °C / second or less, and cooled to a temperature above (the bainite start temperature (B S ) + the martensite start temperature (M S )) / 2. The unit of ts time is seconds; After the secondary cooling, tertiary cooling is performed at a cooling rate of 30 °C / second or less to a temperature range of (martensite start temperature (M S ) - 20 °C) to 200 °C; and Coil up within the temperature range of the third cooling, Wherein, during the hot rolling, perform hot finish rolling in the temperature range of 750 - 1150 °C so that the value of Du defined by the following relational expression 1 satisfies the range of 2 to 10, The first cooling and the second cooling satisfy the conditions of the following relational expressions 2 to relational expression 4, [Relational expression 1] Du = (FDT + (7.4 × [C]) - (24.7 × [Si]) - (4.7 × [Mn]) - (3.9 × [Cr]) - (5.2 × [Mo]) - (560 × [Ti]) - (1110 × [Nb])) × 0.049 - 34.2 In the relational expression 1, FDT represents the rolling end temperature, the unit of the rolling end temperature is °C, and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent the weight % contents of the respective elements within the brackets, [Relationship 2] 5.0×10 6 ≤Du×Bat×2.968×10 10 ≤2.0×10 7 In the said relation 2, Du has the same definition as in relation 1, and Bat represents 55.845×[B] / (1080.6 + 45.04×[B]), where [B] represents the weight percentage content of boron (B). [Relationship 3] 0.75 ≤ exp(-k(T)×(ts) 2 ) ≤ 0.9 The said k(T) represents the value defined by the following relation 4, and ts represents the secondary cooling time. [Relation 4] In the said relation 4, Du has the same definition as in relation 1, Bat has the same definition as in relation 2, T1 represents the termination temperature of primary cooling, T2 represents the termination temperature of secondary cooling, the unit of temperature is °C, and [C], [Si], [Mn], [Cr], and [Mo] represent the weight percentage contents of the respective elements within the brackets.

7. The manufacturing method of the hot-rolled steel sheet according to claim 6, wherein, During the said hot rolling, the total reduction ratio of the final two passes is 10 - 40%.

8. The manufacturing method of the hot-rolled steel sheet according to claim 6, wherein, After the said coiling, it further includes the step of finally cooling to room temperature.

9. The manufacturing method of the hot-rolled steel sheet according to claim 8, wherein, After the said final cooling, it further includes the steps of pickling and oiling.

10. The method for manufacturing a hot-rolled steel sheet according to claim 9, wherein after the said pickling and oiling, it further includes the step of hot-dip galvanizing.

Images