Ultra-high-strength hot-rolled steel sheet with excellent formability and method for manufacturing the same

Abstract

The present invention relates to a hot-rolled steel sheet suitable for use in automobile chassis structural members, etc., and more particularly to an ultra-high strength hot-rolled steel sheet having excellent formability and uniform material properties within the steel sheet, and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet suitably applicable to automotive chassis structural members and the like, and more specifically, to an ultra-high-strength hot-rolled steel sheet with excellent formability and uniform material distribution within the sheet, and a method for manufacturing the same. [Background technology]

[0002] In recent years, in order to reduce global warming, there has been a rapid shift from internal combustion engine vehicles, which are the mainstream in the automobile market, to environmentally friendly vehicles such as electric vehicles.

[0003] As automobiles transition from internal combustion engine vehicles to electric vehicles, the types of components that make up automobiles are changing, and so is the weight of the automobiles. For example, when comparing the weight of an internal combustion engine vehicle and an electric vehicle of the same model, it is known that the weight of the electric vehicle is approximately equal to the weight of the battery.

[0004] On the other hand, automobile chassis components play a role in supporting the vehicle body and are important parts for ensuring ride comfort and driving stability by absorbing road vibrations and shocks during driving. As the weight of the automobile increases, the fatigue load applied to the above-mentioned chassis components also increases, so the steel materials used for chassis components in electric vehicles and other vehicles are required to have excellent fatigue strength.

[0005] Since the fatigue strength of steel is proportional to its tensile strength and yield strength, steel used in applications such as chassis components for electric vehicles needs to have improved tensile strength and yield strength.

[0006] Furthermore, since chassis components are manufactured by press forming, it is necessary to improve tensile strength and yield strength to enhance fatigue strength, as well as to ensure formability such as elongation and hole expandability suitable for press forming. On the other hand, if there are areas within the steel sheet where the microstructure is non-uniform and locally poor formability exists, processing cracks may occur during press forming, which can reduce productivity. Therefore, it is important to manufacture the steel sheet so that formability such as elongation and hole expandability is uniformly distributed within it. In the case of hot-rolled steel sheets used as chassis components, the temperature of the steel sheet is controlled by cooling water after hot rolling. Therefore, localized temperature deviations can induce shape defects during cooling, and shape defects can again induce temperature deviations, potentially increasing material variability. For this reason, it is important to prevent temperature deviations within the steel sheet during manufacturing. In particular, in the case of ultra-high-strength steel sheets that use large amounts of low-temperature bainite and martensite as microstructure, the possibility of shape defects increases due to rapid cooling and rapid phase transformation rates, so special attention is required.

[0007] Various technologies have been proposed to improve the strength and formability of hot-rolled steel sheets.

[0008] As an example, Patent Document 1 discloses a method in which, after hot rolling, the steel is cooled to a temperature below Ms temperature but above 200°C, and then wound up to form a lower bainite phase and / or tempered martensite with a total area ratio of 90% or more of the steel's microstructure. According to Patent Document 1, the tensile strength of the hot-rolled steel sheet can be 1180 MPa or more, and excellent elongation flange formability and bendability can be ensured. However, because it is cooled to a temperature below Ms by water cooling, there are concerns about shape defects and resulting material variations within the steel sheet due to the rapid phase transformation behavior of martensite.

[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 steel materials that not only have high tensile strength and yield strength and excellent fatigue life, but also excellent formability such as elongation and hole-expandability to facilitate press forming, while simultaneously having a uniform material composition within the steel sheet. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Application No. 2019-553584 [Overview of the project] [Problems that the invention aims to solve]

[0011] According to one aspect of the present invention, the aim is to provide a hot-rolled steel sheet and a method for manufacturing the same that not only has high strength and excellent fatigue performance, but also excellent formability and is suitable for press forming, and has a uniform material distribution within the steel sheet.

[0012] The problems addressed by the present invention are not limited to those described above. Anyone with ordinary skill in the art to which the present invention pertains should have no difficulty understanding the further problems addressed by the present invention from the entirety of the specification. [Means for solving the problem]

[0013] One aspect of the present invention is, In weight percent, the composition is as follows: Carbon (C): 0.06-0.18%, Silicon (Si): 0.01-1.8%, Manganese (Mn): 1.6-3.5%, Aluminum (Al): 0.001-0.1%, Chromium (Cr): 2.5% or less (including 0%), Molybdenum (Mo): 2.0% or less (including 0%), Titanium (Ti): 0.01-0.15%, Boron (B): 0.0005-0.003%, Phosphorus (P): 0.0001-0.05%, Sulfur (S): 0.0001-0.05%, Nitrogen (N): 0.0001-0.02%, with the remainder being Fe and other unavoidable impurities. The present invention provides an ultra-high-strength hot-rolled steel sheet having a microstructure comprising, in terms of area fraction, one or more selected from low-temperature bainite and martensite: 75-90%, one or more selected from acicular ferrite and bainitic ferrite: 10-25%, and other phases: 5% or less (including 0%).

[0014] Another aspect of the present invention is, The process involves reheating a steel slab containing, by weight percent, carbon (C): 0.06~0.18%, silicon (Si): 0.01~1.8%, manganese (Mn): 1.6~3.5%, aluminum (Al): 0.001~0.1%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01~0.15%, boron (B): 0.0005~0.003%, phosphorus (P): 0.0001~0.05%, sulfur (S): 0.0001~0.05%, nitrogen (N): 0.0001~0.02%, with the remainder being Fe and other unavoidable impurities, in a temperature range of 1100~1350°C. The above-mentioned reheated steel slab is hot-rolled to produce a hot-rolled steel sheet, The above hot-rolled steel sheet is subjected to a primary cooling step of a temperature below Bs at an average cooling rate of 50°C / s or more, After the primary cooling described above, a secondary cooling step is performed for ts time (seconds) at an average cooling rate of 25°C / s or less until the temperature reaches (Bs+Ms) / 2 or higher. After the above secondary cooling, a tertiary cooling step is performed to a temperature range of Ms℃ to 500℃ at an average cooling rate of 30℃ / s or more. The step includes winding within the above-mentioned tertiary cooled temperature range, The present invention provides a method for manufacturing ultra-high-strength hot-rolled steel sheets, wherein, during the hot rolling process described above, finish hot rolling is performed within a temperature range of 750 to 1150°C so that the value of Du, defined by the following relational equation 1, satisfies the range of 2 to 10.

[0015] [Relationship 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 above relational equation 1, FDT represents the rolling completion temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent the weight percentage content of the element in parentheses, respectively.) [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a hot-rolled steel sheet having an ultra-high strength with a tensile strength of 1180 MPa or more, excellent formability, and a uniform material in the steel sheet, and a method for manufacturing the same. Thereby, it can be suitably applied to chassis structural members of automobiles and the like.

[0017] The various and beneficial advantages and effects of the present invention are not limited to the above-described contents, and can be more easily understood in the process of explaining the specific embodiments of the present invention.

Brief Description of the Drawings

[0018] [Figure 1] It is a graph showing the relationship between the content of boron and Du that simultaneously satisfy Relational Expression 1 and Relational Expression 2, and a fine structure intended in the present invention can be ensured within the solid line connecting A - B - C - D - E - F. [Figure 2] Photographs showing the microstructure of Invention Example 1(a) and Comparative Example 5(b) observed with a scanning electron microscope (SEM) are shown.

Modes for Carrying Out the Invention

[0019] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those having average knowledge in the technical field.

[0020] On the other hand, the terms used in this specification are for explaining specific examples and are not intended to limit the present invention. For example, the singular forms used in this specification include plural forms as well, unless the related definitions clearly indicate the opposite meaning. Further, the meaning of "including" used in the specification does not exclude the existence or addition of other configurations when embodying a configuration.

[0021] Conventional methods for manufacturing hot-rolled steel sheets to obtain ultra-high strength tensile strength of 1180 MPa or more have widely applied a method in which, after hot rolling, a large amount of cooling water is injected to cool the steel sheet below the Ms temperature, thereby making martensite the dominant phase. However, when martensite is the dominant phase, there is a problem in that elongation, bendability, and hole-expandability deteriorate. Furthermore, due to shape defects caused by rapid lattice expansion during cooling and temperature unevenness within the steel sheet, areas where residual stress from phase transformation concentrate are created within the sheet, leading to the problem of forming cracks occurring during forming for parts processing.

[0022] Therefore, the inventors have found that by utilizing bainite and martensite, it is possible to ensure ultra-high strength, and by uniformly dispersing acicular ferrite or bainitic ferrite within the structure, it is possible to prevent excessive stress concentration at specific locations during deformation and thus ensure formability. Furthermore, by winding at a temperature above Ms to avoid the rapid formation of martensite, it is possible to provide an ultra-high-strength hot-rolled steel sheet with a uniform material structure within the steel sheet, thus completing the present invention. The present invention will be described in more detail below.

[0023] An ultra-high-strength hot-rolled steel sheet, according to one aspect of the present invention, which has excellent formability and uniform material composition within the steel sheet, may contain, by weight percent, carbon (C): 0.06~0.18%, silicon (Si): 0.01~1.8%, manganese (Mn): 1.6~3.5%, aluminum (Al): 0.001~0.1%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01~0.15%, boron (B): 0.0005~0.003%, phosphorus (P): 0.0001~0.05%, sulfur (S): 0.0001~0.05%, and nitrogen (N): 0.0001~0.02%.

[0024] The following will specifically explain the reasons for adding components to the hot-rolled steel sheet in this invention and the reasons for limiting their content. Unless otherwise specified in this invention, the content of each element is based on weight, and the proportion of the microstructure is based on area.

[0025] Carbon (C): 0.06~0.18% Carbon (C) is the most economical and effective element for strengthening steel, and the strength of the steel sheet increases as the C content increases. In this invention, the above C exists as fine carbides or in a solid solution state within the lower bainite and martensite, and plays a role in improving the yield strength and tensile strength of the steel.

[0026] If the C content is less than 0.06%, it becomes difficult to secure a tensile strength of 1180 MPa or more. On the other hand, if the C content exceeds 0.18%, there is a problem of poor weldability. Therefore, in the present invention, the C content can be between 0.06% and 0.18%, and more advantageously, the lower limit of the C content may be 0.07%, and more preferably 0.09%. Furthermore, the upper limit of the C content may be 0.15%.

[0027] Silicon (Si): 0.01~1.8% Silicon (Si) is an element that improves the hardening ability of steel, playing a role in increasing strength through solid solution strengthening. It also improves strength by preventing ferrite formation during cooling and allowing the lower bainite and / or martensite phases to form.

[0028] If the Si content is less than 0.01%, the solid solution strengthening effect and ferrite transformation delay effect are reduced, making it difficult to secure tensile strength. On the other hand, if the Si content exceeds 1.8%, Fe-Si composite oxide is formed on the surface of the slab during reheating, which not only deteriorates the surface quality of the steel sheet but also reduces weldability.

[0029] Therefore, in the present invention, the Si content can be 0.01 to 1.8%, more preferably 0.1% or more, and even more preferably 0.2%. The upper limit of the Si content may be 1.5%, and more preferably 1.3%.

[0030] Manganese (Mn): 1.6~3.5% Manganese (Mn) is an element that improves the hardening ability of steel, preventing ferrite formation during cooling after finish rolling and facilitating the formation of low-temperature transformation structures. If the Mn content is less than 1.6%, there is a problem that the hardening ability is insufficient and the ferrite fraction increases excessively. On the other hand, if the Mn content exceeds 3.5%, the hardening ability increases significantly, excessively increasing the holding time required to sufficiently form the acicular ferrite or bainitic ferrite to be obtained in this invention, and reducing the elongation. Therefore, in this invention, the above Mn can be contained in an amount of 1.6 to 3.5%, and more advantageously, the lower limit of the Mn content may be 1.8%, or the upper limit of the Mn content may be 2.5%.

[0031] Aluminum (Al): 0.001~0.1% Aluminum (Al) is an element added to molten steel for deoxidation, and some of it remains in the steel after deoxidation. If the Al content exceeds 0.1%, it leads to an increase in oxide and nitride inclusions in the steel, degrading the formability of the steel sheet. On the other hand, if the Al content is less than 0.001%, it leads to an unnecessary increase in refining costs due to excessive reduction of Al, which is economically disadvantageous. Therefore, in the present invention, the Al can be contained in an amount of 0.001 to 0.1%, and more advantageously, the lower limit of the Al content may be 0.005% or 0.02%. Alternatively, more advantageously, the upper limit of the Al content may be 0.05% or 0.025%.

[0032] Chromium (Cr): 2.5% or less (including 0%) Chromium (Cr) is an element that improves the hardening ability of steel and suppresses ferrite formation during cooling after finish rolling. If the Cr content exceeds 2.5%, the hardening ability increases significantly, bainite transformation does not occur smoothly in the cooling zone, and the holding time required to ensure the fraction of acicular ferrite or bainite ferrite increases excessively, degrading the elongation. Therefore, in the present invention, the Cr content can be 2.5% or less, and more preferably 1.5% or less. On the other hand, according to one embodiment of the present invention, more preferably, the upper limit of the Cr content may be 0.9%.

[0033] On the other hand, the present invention includes cases where the Cr content is 0%, as it does not significantly impede the securing of the intended physical properties even without containing the above-mentioned Cr. However, it is clarified that when the above-mentioned Cr is intentionally added, adding a minimum of 0.01% is effective.

[0034] Molybdenum (Mo): 2.0% or less (including 0%) Molybdenum (Mo) is an element that improves the hardening ability of steel, playing a role in increasing strength through solid solution strengthening and suppressing ferrite formation during cooling after finish rolling. When the Mo content exceeds 2.0%, the hardening ability increases significantly, and bainite transformation does not occur smoothly in the cooling zone. As a result, the holding time required to ensure the fraction of acicular ferrite or bainite ferrite increases excessively, and the elongation decreases.

[0035] Therefore, in the present invention, the above Mo can be included in an amount of 2.0% or less, more preferably 1.0% or less, and even more preferably 0.5% or less.

[0036] On the other hand, the present invention includes cases where the Mo content is 0%, as it does not significantly impede the securing of the intended physical properties even without containing the above-mentioned Mo. However, it is clarified that when the above-mentioned Mo is added, adding a minimum of 0.01% is effective.

[0037] Titanium (Ti): 0.01~0.15% Titanium (Ti) is an element that forms carbonitrides in steel, and is widely used to ensure the strength of steel by inducing the formation of these precipitates. In this invention, however, by removing nitrogen (N) from the steel and suppressing the formation of BN, it plays a role in allowing boron (B) to concentrate at the austenite grain boundaries, and is also used to control the grain size of austenite before rolling.

[0038] In order to fully obtain the effects intended in this invention, it is preferable that the above-mentioned Ti content be 0.01% or more, and preferably 2.9 times or more the nitrogen (N) content in order to remove nitrogen (N) from the steel. However, if the content exceeds 0.15%, oxides may form during continuous casting, potentially causing problems such as clogging of the casting nozzle.

[0039] Therefore, in the present invention, the above Ti can be contained in an amount of 0.01 to 0.15%, and more advantageously, the lower limit of the above Ti content may be 0.02%, or the upper limit of the Ti content may be 0.12%.

[0040] Boron (B): 0.0005~0.003% Boron is an element that improves the hardening ability of steel by concentrating at austenite grain boundaries and reducing grain boundary energy. In this invention, it plays a role in suppressing the phase transformation of ferrite and upper bainite, which occurs at austenite grain boundaries due to diffusion transformation, and ensuring that lower bainite and martensite are secured as the main phases.

[0041] In order to fully obtain the desired effects in this invention, it is preferable that the concentration of B be 0.0005% or more. However, if the content exceeds 0.003%, the curing ability increases significantly, the holding time required to sufficiently form the acicular ferrite or bainitic ferrite to be obtained in this invention increases excessively, and the elongation decreases.

[0042] Therefore, in the present invention, the above B can be included in an amount of 0.0005 to 0.003%, and more advantageously, the lower limit of the B content may be 0.001%, or the upper limit of the B content may be 0.0025%.

[0043] Phosphorus (P): 0.0001~0.05% Phosphorus (P) is an unavoidable impurity in steel and is the main element that inhibits the workability of steel due to segregation. Therefore, it is preferable to control its content to be as low as possible.

[0044] Theoretically, it is advantageous to limit the content of P to 0%, but controlling the content of P to less than 0.0001% requires excessive manufacturing costs, so the lower limit can be set to 0.0001%. However, if the content exceeds 0.05%, the processability may decrease, so the upper limit of P can be limited to 0.05%. More advantageously, however, the lower limit of the P content may be 0.0005%, or the upper limit of the P content may be 0.013%.

[0045] Sulfur (S): 0.0001~0.05% Sulfur (S) is an unavoidable impurity in steel, and it combines with Mn and other elements to form nonmetallic inclusions, which reduces the workability of the steel. Therefore, it is preferable to control its content to be as low as possible.

[0046] Theoretically, it is advantageous to limit the content of S to 0%, but controlling the content of S to less than 0.0001% requires excessive manufacturing costs, so its lower limit can be set to 0.0001%. However, if its content exceeds 0.05%, the processability may decrease, so the upper limit of S can be limited to 0.05%. More advantageously, however, the lower limit of the S content may be 0.0003%, or the upper limit of the S content may be 0.0012%.

[0047] Nitrogen (N): 0.0001~0.02% Nitrogen (N) is an unavoidable impurity in steel, and it has the problem of inhibiting the workability of steel by combining with aluminum and other elements to form nitrides. Therefore, it is preferable to control its content to be as low as possible.

[0048] Theoretically, it is advantageous to limit the content of N to 0%, but controlling the content of N to less than 0.0001% requires excessive manufacturing costs, so the lower limit can be set to 0.0001%. However, if the content exceeds 0.02%, the processability may decrease, so the upper limit of N can be limited to 0.02%. More advantageously, however, the lower limit of the N content may be 0.001%, or the upper limit of the N content may be 0.006%.

[0049] In addition to the alloy composition described above, the hot-rolled steel sheet of the present invention may further selectively contain niobium (Nb).

[0050] Niobium (Nb): 0.01-0.2% Niobium (Nb) is an element that forms carbonitrides in steel and is widely used to ensure the strength of steel by inducing the formation of these precipitates. In this invention, however, Nb plays a role in controlling the grain size of austenite by delaying recrystallization during hot rolling. If the Nb content is less than 0.01%, the effect of controlling grain size is low, and if the Nb content exceeds 0.2%, the grain size of the austenite crystals may become excessively fine, potentially worsening formability. Therefore, in this invention, the Nb content can be between 0.01% and 0.2%.

[0051] The remaining component of this invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be introduced from the raw materials or the surrounding environment, and therefore cannot be eliminated. Since these impurities are easily recognizable to any technician in the normal manufacturing process, this specification does not specifically mention all of them.

[0052] The hot-rolled steel sheet of the present invention that satisfies the alloy composition described above includes one or more low-temperature transformation structures, namely low-temperature bainite and martensite, as its base structure in order to ensure a yield strength of 900 MPa or more and a tensile strength of 1180 MPa or more. Accordingly, according to one embodiment of the present invention, one or more of the low-temperature bainite and martensite can be included in an area fraction of 75 to 90%.

[0053] In this invention, after tertiary cooling following hot rolling, austenite transforms into low-temperature bainite or martensite. Since low-temperature bainite and martensite are produced by shear transformation (displacive phase transformation), a high level of dislocation density exists within the microstructure due to helical dislocations generated to reduce the shear deformation generated during the transformation and edge dislocations generated to accommodate the volume expansion due to the phase transformation. Therefore, the solid-solution carbon and fine carbides within the microstructure are suitable for improving the yield strength and tensile strength of the steel. On the other hand, the high level of dislocation density and the solid-solution carbon and fine carbides hinder the movement of dislocations within the microstructure, resulting in a characteristic of reduced elongation.

[0054] Therefore, from the viewpoint of ensuring yield strength and tensile strength, the present invention preferably contains one or more selected from the above-mentioned low-temperature transformation structures, low-temperature bainite (LB) and martensite (M), in an area percentage of 75% or more, and preferably is limited to 90% or less in order to simultaneously ensure elongation. On the other hand, according to one embodiment of the present invention, more preferably, the lower limit of the fraction of one or more selected from the above-mentioned low-temperature bainite and martensite may be 76.9%, or the upper limit of the fraction of one or more selected from the above-mentioned low-temperature bainite and martensite may be 86.6%.

[0055] In this invention, since both low-temperature bainite and martensite contain iron carbides at grain boundaries and within grains in their lath structure, their total fraction must be controlled. Therefore, in this invention, the fraction of one or more selected materials from low-temperature bainite and martensite is controlled.

[0056] On the other hand, the hot-rolled steel sheet according to the present invention preferably contains 10 to 25% by area fraction of one or more materials selected from acicular ferrite (AC) and bainitic ferrite (BF).

[0057] The steel of the present invention, after hot rolling, is cooled to a temperature below Bs (bainite transformation initiation temperature) to avoid ferrite phase transformation during primary cooling, and then undergoes bainite transformation by slow cooling during the subsequent secondary cooling. This bainite transformation occurs in the high-temperature bainite transformation region. Therefore, bainitic ferrite is formed and carbon diffuses into untransformed austenite, and no carbides are formed inside the bainitic ferrite. On the other hand, although a large amount of dislocations exist inside the bainitic ferrite formed by shear transformation, the dislocation density is reduced to an appropriate level due to the secondary cooling and the subsequent recovery phenomenon after winding, resulting in an improved elongation rate of the steel sheet.

[0058] Since bainitic ferrite produced below the above-mentioned Bs temperature is similar in shape and properties to acicular ferrite produced during supercooling in ultra-low carbon steel, this invention clarifies that the total fraction of bainitic ferrite and acicular ferrite is controlled. Accordingly, this invention controls one or more fractions selected from acicular ferrite and bainitic ferrite.

[0059] If one or more of the above-mentioned acicular ferrites and bainitic ferrites are present in less than 10%, there is a problem in that it becomes difficult to secure the elongation rate. On the other hand, if one or more of the above-mentioned acicular ferrites and bainitic ferrites are present in more than 25%, there is a problem in that it becomes difficult to secure the low-temperature transformation structure that plays a role in improving strength. On the other hand, according to one embodiment of the present invention, the lower limit of the fraction of one or more of the above-mentioned acicular ferrites and bainitic ferrites may be 12.4%, or the upper limit of the fraction of one or more of the above-mentioned acicular ferrites and bainitic ferrites may be 21.0%.

[0060] In this case, according to one embodiment of the present invention, the average size of one or more selected from acicular ferrite and bainitic ferrite may be 2.0 μm or more.

[0061] On the other hand, according to one embodiment of the present invention, more preferably, the lower limit of the average size of one or more selected from the above-mentioned acicular ferrite and bainitic ferrite may be 3.9 μm, or the upper limit of the average size of one or more selected from the above-mentioned acicular ferrite and bainitic ferrite may be 6.2 μm.

[0062] Furthermore, according to one embodiment of the present invention, the average spacing of one or more selected from the acicular ferrite and bainitic ferrite may be 3 μm or more. According to one embodiment of the present invention, more preferably, the lower limit of the average spacing of one or more selected from the acicular ferrite and bainitic ferrite may be 6.2 μm, or the upper limit of the average spacing of one or more selected from the acicular ferrite and bainitic ferrite may be 13.2 μm.

[0063] In this case, the average size of one or more selected acicular ferrites and bainitic ferrites refers to the diameter equivalent to a circle, and the average spacing of one or more selected acicular ferrites and bainitic ferrites refers to the average of the distances between the five nearest adjacent tissues for each microstructure.

[0064] In low-temperature bainite and martensite formed after tertiary cooling, phase transformation begins first at sites within the austenite where nucleation is easily facilitated. Subsequently, the phase transformation continues, and at the site where the final phase transformation occurs, the internal dislocation density becomes excessively high locally. This leads to stress concentration during the molding process, causing fine cracks to form within the microstructure and worsening hole-expandability. On the other hand, if soft acicular ferrite or bainitic ferrite is uniformly distributed in appropriate sizes within the low-temperature transformation matrix, it helps to uniformly accommodate deformation during molding, preventing localized stress concentration and improving hole-expandability.

[0065] Therefore, if the average size of one or more selected soft structures, such as acicular ferrite and bainitic ferrite, is less than 2.0 μm, the deformation acceptance effect is low, and improvement in hole expansion cannot be expected. Also, if the average spacing of one or more selected soft structures, such as acicular ferrite and bainitic ferrite, is less than 3.0 μm, the proportion of soft steel increases excessively, which may reduce yield strength and tensile strength. Although there are no specific regulations regarding the upper limits of the average size and average spacing of the soft structure, under conditions where the total proportion of the soft structure is in the range of 10-25%, the average size of the soft structure is preferably 20 μm or less, and the average spacing of the soft structure is preferably 20 μm or less.

[0066] In addition to the structure described above, the hot-rolled steel sheet of the present invention may contain other phases such as ferrite, carbide, and retained austenite, but it is preferable that their area fraction be controlled to 5% or less. Here, the ferrite described above refers to granular ferrite.

[0067] In particular, ferrite formed during cooling after hot rolling is typically produced by diffusion transformation and therefore has low strength. By controlling the amount of such ferrite to 5% or less (including 0%), the previously formed ferrite undergoes shear deformation to accommodate the lattice deformation that occurs when retained austenite transforms into bainite and martensite after ferrite formation. Therefore, it was confirmed that the dislocation density inside the ferrite is maintained at a high level, and the strength of the steel does not decrease significantly even if other phases such as ferrite are included at 5% or less. However, if the ferrite fraction exceeds 5%, the strength of the steel decreases, which is undesirable.

[0068] On the other hand, carbides can be generated during the manufacturing process of this product. Since the present invention aims to improve strength by utilizing a low-temperature transformation structure as the second phase, the generation of carbides may cause a decrease in the second phase fraction. In other words, excessive generation of carbides hinders the strengthening effect targeted in the present invention. However, if Ti and Nb are present in the phase, alloy carbonitrides can be formed, and in this case, a further strengthening effect due to grain refinement can be expected. However, since coarse carbides hinder the toughness of steel, it is preferable that the amount of carbides present in the hot-rolled steel sheet of the present invention be less than 5%.

[0069] Furthermore, according to one embodiment of the present invention, the hot-rolled steel sheet may contain only retained austenite as the other phases mentioned above in its microstructure. Therefore, the hot-rolled steel sheet may contain retained austenite in an area percentage of 5% or less (including 0%) as its microstructure. During the secondary cooling of the present invention, when bainitic ferrite is formed, isolated island-like austenite in the growing bainitic ferrite can remain at room temperature without undergoing bainite-martensitic transformation in the subsequent cooling process due to carbon enrichment. If the fraction of retained austenite is low, the effect on the physical properties may be slight, but if the fraction exceeds 5%, hole-expanding properties may deteriorate and hydrogen-delayed fracture may occur. Therefore, the fraction of retained austenite in the present invention is preferably 5% or less, and more preferably controlled to 3% or less. On the other hand, according to one embodiment of the present invention, more preferably, the lower limit of the fraction of retained austenite may be 0.9%, or the upper limit of the fraction of retained austenite may be 2.1%.

[0070] The hot-rolled steel sheet of the present invention, having the alloy composition and microstructure described above, is characterized by high strength with a yield strength (YS) of 900 MPa or more (or 948-1064 MPa) and a tensile strength (TS) of 1180 MPa or more (or 1204-1343 MPa), as well as excellent formability with an elongation rate (El) (i.e., average El) of 7% or more (or 8.4-12.6%), a standard deviation of elongation rate of 2% or less (0.3-1.1%), and a hole expansion rate (HER) of 25% or more (or 27-36%), and also by a uniform material distribution within the steel sheet.

[0071] Next, the following describes in detail another aspect of the present invention: a method for manufacturing an ultra-high-strength steel sheet that is highly formable and has a uniform material composition within the sheet. However, this does not necessarily mean that the ultra-high-strength steel sheet of the present invention should be manufactured by the following method.

[0072] The hot-rolled steel sheet according to the present invention can be manufactured by performing a series of steps [reheating - hot rolling - cooling - coiling] on a steel slab that satisfies the alloy composition proposed in the present invention. The conditions for each of the above steps will be described in detail below.

[0073] [Steel slab reheating] In the present invention, it is preferable to reheat the steel slab and homogenize it before the rolling process, and this can be done in a temperature range of 1100 to 1350°C.

[0074] If the reheating temperature of the steel slab is below 1100°C, there is a problem in that the homogenization of the alloying elements will be insufficient. On the other hand, if the temperature exceeds 1350°C, excessive oxides may form on the surface of the slab, potentially degrading the surface quality of the steel sheet.

[0075] [Hot rolling] The reheated steel slab described above can be hot-rolled to produce a hot-rolled steel sheet. In this case, it is preferable to perform the hot rolling in a temperature range of 750 to 1150°C and to control the total reduction amount in the final two passes to 10 to 40%.

[0076] If hot rolling is started at a temperature exceeding 1150°C, excessive oxides are formed on the surface of the steel sheet after rolling, and even pickling does not effectively control this, resulting in a decrease in surface quality. On the other hand, if hot rolling is performed at a temperature lower than 750°C, the rolling load increases excessively, reducing workability, and ferrite is formed during rolling, leading to a deterioration of anisotropy.

[0077] Normally, the reason for performing multi-stage hot rolling is to reduce the rolling load and precisely control the thickness. When performing hot rolling with such multi-stage rolling, if the sum of the reduction ratios of the last two passes (or the total reduction ratio) exceeds 40%, there is a problem that the rolling load of the last two passes becomes excessive and the workability decreases. On the other hand, if the sum of the reduction ratios of the last two passes is less than 10%, there is a problem that the temperature of the steel sheet drops rapidly, inducing shape defects. On the other hand, according to one embodiment of the present invention, the lower limit of the total reduction ratio of the last two passes during hot rolling may more preferably be 25%, or the upper limit of the total reduction ratio of the last two passes during hot rolling may more preferably be 38%.

[0078] On the other hand, the grain size of austenite after hot rolling is influenced by the alloy composition, the rolling completion temperature, and the amount of reduction, which affects the formation behavior of ferrite and bainite in the subsequent cooling process and the final microstructure. Furthermore, the fraction and size of acicular ferrite or bainite, which are the main constituent phases in this invention, are greatly influenced by the austenite grains after hot rolling.

[0079] In equiaxed ferrites and pearlites, grain growth occurs through elemental diffusion during phase transformation, and therefore the size of the microstructure after phase transformation is influenced by the phase transformation temperature and holding time. On the other hand, acicular ferrites and bainitic ferrites, which are produced by shear transformations such as bainite, grow only within austenite grains, and their size cannot exceed the size of the austenite before transformation. Therefore, to control the size of acicular ferrites and bainitic ferrites, it is advantageous to control the grain size of austenite after hot rolling.

[0080] Therefore, the present invention derives the effective grain size of austenite after hot rolling as a relationship between the rolling completion temperature (FDT) and a specific alloy composition, and specifically defines it by the following relational equation 1. That is, during hot rolling, finish hot rolling is performed within a temperature range of 750 to 1150°C so that the value of Du defined by the following relational equation 1 satisfies the range of 2 to 10.

[0081] [Relationship 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 above relational equation 1, FDT represents the rolling completion temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent the weight percentage content of the element in parentheses, respectively.)

[0082] The above-mentioned Du is an index indicating the effective grain size of austenite immediately before primary cooling after hot rolling. When the value of Du defined in relational equation 1 is 2 or greater, the average grain size of acicular ferrite and / or bainitic ferrite becomes 2.0 μm or greater, and a hole expansion ratio of 25% or greater can be ensured. On the other hand, when the value of Du defined in relational equation 1 exceeds 10, the grain size of the austenite before transformation becomes excessively coarse, reducing impact strength, and the grain boundary concentration of boron increases, delaying phase transformation during secondary cooling, which leads to a problem of reduced elongation. On the other hand, according to one embodiment of the present invention, the lower limit of the value of Du defined in relational equation 1 may more preferably be 4.4, or the upper limit of the value of Du defined in relational equation 1 may more preferably be 7.8.

[0083] On the other hand, according to one embodiment of the present invention, boron segregates at the austenite grain boundaries and stabilizes them, thereby delaying the nucleation of ferrite and upper bainite and reducing the phase transformation rate. In order to ensure the area fraction and average spacing of acicular ferrite or bainite ferrite intended in the present invention, it is important to control the concentration of boron segregated at the austenite grain boundaries. The concentration of boron segregated at the austenite grain boundaries shows different values ​​for each grain boundary due to the influence of minute segregation during casting and the grain size of austenite. Among these, nucleation of acicular ferrite or bainite ferrite selectively occurs at the secondary cooling stage in austenite grain boundaries where the concentration of boron is low. Generally, when the austenite grain size is small, the concentration of boron segregated at each grain boundary is low and nucleation is smooth, while when the grain size is large, the concentration of boron segregated at the grain boundaries is high, so it can be expected that nucleation will be delayed. Therefore, the boron concentration present at the grain boundaries and the phase transformation behavior during secondary cooling are influenced by the boron content added to the steel and the austenite grain size, as shown in relational equation 2.

[0084] Relational equation 2 is an indicator of the concentration of boron (B) distributed at the grain boundaries of austenite immediately before cooling, and is given by Du × Bat × 2.968 × 10⁻¹⁰ using relational equation 2 below. 10 The value of (i.e., Va[=Du×Bat×2.968×10 10 The value of ] is 5.0 × 10 6 If the value is less than , the fraction of acicular ferrite or bainitic ferrite is excessive, and the yield strength and tensile strength cannot be secured. On the other hand, Du × Bat × 2.968 × 10 according to the following relational equation 2. 10 The value is 2.0 × 10 7 If it exceeds this, the secondary cooling time required to ensure the fraction of acicular ferrite or bainitic ferrite becomes excessively long, which leads to a problem of reduced elongation. On the other hand, according to one embodiment of the present invention, the lower limit of the above Va value is more preferably 8.60 × 10 6 It may also be the case that the upper limit of the above Va value is more preferably 1.89 × 10 7 That's fine.

[0085] [Relational Expression 2] 5.0×10 6 ≦Du×Bat×2.968×10 10 ≦2.0×10 7 (In the above relational expression 2, Du is the same as the definition in relational expression 1, and the above Bat represents 55.845×[B] / (1080.6 + 45.04×[B]), where [B] represents the weight % content of boron (B).)

[0086] Figure 1 is a graph showing the relationship between the content of boron and Du that simultaneously satisfies the above-described relational expressions 1 and 2. Within the solid line connecting A - B - C - D - E - F, a microstructure intended in the present invention can be ensured.

[0087] [Cooling and Coiling] It is preferable to cool the hot-rolled steel sheet manufactured as described above stepwise according to the temperature at which it is cooled at this time.

[0088] Specifically, after first cooling the above hot-rolled steel sheet to a temperature below Bs at a cooling rate of 50°C / s or more, then second cooling for ts seconds (seconds) defined by relational expression 3 at a cooling rate of 25°C / s or less to a temperature of (Bs + Ms) / 2 or more, and then preferably performing third cooling at a cooling rate of 30°C / s or more in the temperature range from Ms°C to 500°C.

[0089] The hot-rolled steel sheet manufactured as described above is rapidly cooled to a temperature below the temperature at which bainite starts to form (Bs) to suppress the formation of ferrite (granular ferrite). Next, by gradually cooling for ts seconds (seconds) to an intermediate temperature between the bainite start temperature (Bs) and the martensite start temperature (Ms), or a temperature higher than that, acicular ferrite or bainitic ferrite with an area fraction of 10 - 25% can be ensured. On the other hand, according to an embodiment of the present invention, more preferably, the lower limit of the cooling end temperature (T1) of the above first cooling may be 500°C, or the upper limit of the cooling end temperature (T1) of the above first cooling may be 540°C.

[0090] When performing primary cooling at a temperature below Bs after completing the hot rolling described above, if the cooling rate is less than 50°C / s, there is a problem in that an excessive ferrite phase is formed during cooling. In this case, there is no particular upper limit to the primary cooling rate, but if the steel plate is cooled excessively, the shape of the plate may be distorted, so it can be limited to 200°C / s or less. On the other hand, according to one embodiment of the present invention, the lower limit of the cooling rate during primary cooling may more preferably be 70°C / s, or the upper limit of the cooling rate during primary cooling may more preferably be 100°C / s.

[0091] While there is no particular limit to the lower limit of the cooling completion temperature during the primary cooling described above, if it becomes excessively low, there is a risk that the cooling time during the subsequent secondary cooling will be insufficient. Therefore, it is clarified that it can be limited to Bs-100°C.

[0092] Once the temperature of the hot-rolled steel sheet falls below Bs due to the primary cooling described above, the strong cooling can be terminated, and secondary cooling can be performed at a cooling rate of 25°C / s or less at a temperature of (Bs + Ms) / 2 or higher.

[0093] In the hot-rolled steel sheet that has been cooled in the primary stage, bainitic ferrite growth occurs during the cooling process from the primary cooling temperature to the target temperature for secondary cooling. In particular, in order to obtain the target fraction in this invention, it is preferable to maintain the secondary cooling for a time (ts, seconds (sec)) that satisfies the following relational equation 3.

[0094] In relation 3, k(T) is an index indicating the growth rate of bainitic ferrite, and is influenced not only by the alloy composition of the steel but also by the phase transformation temperature and the grain size after hot rolling. Therefore, the relationship between the value of relation 3, i.e., k(T), and the holding time (exp(-k(T)×(ts))) 2 If the ratio is less than 0.75, the proportion of acicular ferrite or bainitic ferrite becomes excessive, resulting in excellent elongation but failing to achieve the target level of strength. On the other hand, if the value exceeds 0.9, there is a problem of degraded elongation.

[0095] [Relationship Equation 3] 0.75 ≤ exp(-k(T) × (ts)) 2 )≦0.9 (The above k(T) represents the value defined by relational equation 4 below.)

[0096] [Relationship Equation 4]

number

[0097] During secondary cooling according to the above conditions, the temperature of the steel sheet may rise due to transformation heat generated by the bainite phase transformation. In this case, excessive heat generation may cause an excessive decrease in dislocation density. Therefore, in order to minimize the temperature rise of the steel sheet due to transformation heat generation, the cooling rate during secondary cooling can be controlled to 25°C / s or less. If the cooling rate exceeds 25°C / s, the shape of the sheet may be distorted. In this invention, it is made clear that the above secondary cooling also includes an air cooling process. On the other hand, according to one embodiment of the present invention, more preferably, the lower limit of the cooling rate during secondary cooling may be 5.0°C / s, or the upper limit of the cooling rate during secondary cooling may be 20.0°C / s.

[0098] Alternatively, according to one embodiment of the present invention, more preferably, exp(-k(T)×(ts)) by the above relational expression 3 2 The lower limit of the value of ) may be 0.79, or exp(-k(T)×(ts)) according to the above relation 3. 2 The upper limit of the value of ) may be 0.88.

[0099] Alternatively, according to one embodiment of the present invention, the cooling completion temperature (T2) of the secondary cooling may be lower than the cooling completion temperature of the primary cooling. Alternatively, preferably, the cooling completion temperature (T2) of the secondary cooling may be 20°C or more lower than the cooling completion temperature (T1) of the primary cooling. More preferably, the cooling completion temperature (T2) of the secondary cooling may be 20°C lower than the cooling completion temperature (T1) of the primary cooling.

[0100] Alternatively, according to one embodiment of the present invention, more preferably, the lower limit of the cooling completion temperature (T2) of the secondary cooling may be 480°C, or the upper limit of the cooling completion temperature (T2) of the secondary cooling may be 520°C.

[0101] Alternatively, according to one embodiment of the present invention, more preferably, the lower limit of the cooling time for the secondary cooling may be 1.0 second, or the upper limit of the cooling time for the secondary cooling may be 4.0 seconds.

[0102] It is preferable to tertiarily cool the hot-rolled steel sheet, which has completed secondary cooling as described above, to a temperature range of Ms~500°C at a cooling rate of 30°C / s or more, and then wind it at that temperature. During the tertiary cooling described above, the low-temperature bainite transformation proceeds, and some of the untransformed austenite can be transformed into martensite even after winding.

[0103] Therefore, by setting the cooling rate during the tertiary cooling stage to 30°C / s or higher, the formation of further high-temperature bainite during cooling can be avoided. On the other hand, if the cooling rate is too fast, the plate shape will be distorted by a large amount of cooling water, and the shape defects will again induce temperature deviations, increasing the variation in material properties within the plate. Therefore, in the present invention, it is preferable to perform the cooling rate in the tertiary cooling stage, which involves a rapid phase transformation, at 100°C / s or less. On the other hand, according to one embodiment of the present invention, the lower limit of the cooling rate during the tertiary cooling stage may more preferably be 35°C / s, or the upper limit of the cooling rate during the tertiary cooling stage may more preferably be 80°C / s.

[0104] On the other hand, if the cooling termination temperature, i.e., the winding temperature, exceeds 500°C, the dislocation density inside the low-temperature bainite and martensite decreases excessively, and the carbides become coarser, which may reduce the yield strength and tensile strength. Conversely, if the winding temperature is below Ms, martensitic transformation proceeds immediately before winding, resulting in shape defects and temperature deviations, and increasing material variation within the steel sheet.

[0105] In this invention, Bs and Ms can be derived by the following formulas, where each element represents its weight content.

[0106] Bs(℃)=830-320×[C]-90×[Mn]-35×[Si]-70×[Cr]-120×[Mo] Ms(℃)=550-330×[C]-41×[Mn]-20×[Si]-20×[Cr]-10×[Mo]+30×[Al]

[0107] On the other hand, according to one embodiment of the present invention, more preferably, the lower limit of the cooling completion temperature for the tertiary cooling may be 400°C, or the upper limit of the cooling completion temperature for the tertiary cooling may be 430°C.

[0108] [Final cooling] After completing the cooling and winding processes as described above, the target hot-rolled steel sheet can be obtained by performing a final cooling. At this time, the final cooling can be completed by air cooling to room temperature.

[0109] On the other hand, the hot-rolled steel sheet of the present invention obtained after the final cooling is completed as described above can be further pickled and oiled.

[0110] Furthermore, the hot-rolled steel sheet that has been pickled and oiled can be heated to a temperature range of 420 to 740°C to perform a hot-dip galvanizing process.

[0111] The above-described hot-dip galvanizing process can use a zinc-based plating bath, and the alloy composition in the zinc-based plating bath is not particularly limited. [Examples]

[0112] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0113] (Examples) Steel slabs having the alloy compositions shown in Table 1 below were prepared, with the residual components of each steel slab being Fe and unavoidable impurities.

[0114] After reheating each prepared steel slab to 1200°C, hot-rolled steel sheets with a thickness of 2.5 mm were manufactured by hot-rolling, cooling, coiling, and final cooling (air cooling) processes under the conditions shown in Table 2 below. During the hot-rolling process, a total reduction ratio of 25% was applied to the final two passes, a cooling rate of 70°C / s was uniformly applied during the primary cooling, and a cooling rate of 35°C / s was uniformly applied during the tertiary cooling.

[0115] The mechanical properties of each hot-rolled steel sheet were measured, and the microstructure was observed. The results are shown in Tables 3 and 4 below.

[0116] Of the mechanical properties, yield strength, tensile strength, and elongation were measured at room temperature using a universal tensile testing machine after taking samples from a JIS-5 standard test specimen at five locations perpendicular to the rolling direction. At this time, yield strength, tensile strength, and elongation were expressed as yield strength, maximum tensile strength, and fracture elongation, respectively, with a 0.2% offset. The standard deviation of the elongation measurements taken at the five locations is also shown.

[0117] Furthermore, hole expansion properties were measured using the same test specimens as those used in the tensile test, based on the ISO TS16630 standard method.

[0118] Furthermore, the microstructure of each hot-rolled steel sheet was observed at 10,000x magnification using a scanning electron microscope and image analyzer after etching the same specimen used in the tensile test described above with the Nital etching method, and the fraction of each phase was calculated. The average size of one or more selected acicular ferrites and bainitic ferrites is expressed as the equivalent diameter of a circle, and the average spacing of one or more selected acicular ferrites and bainitic ferrites is expressed as the average of the distances between the five most adjacent microstructures for each microstructure.

[0119] At this time, the microstructure was observed in the cross-section of the above-mentioned test specimen, that is, in the cross-section perpendicular to the rolling direction.

[0120] [Table 1]

[0121] [Table 2] FDT*: Finishing rolling temperature [°C] 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 Va* = Du × Bat × 2.968 × 10 10

[0122] [Table 3] AC: Asicular Ferrite BF: Baynitik Ferrite LB: Low Temperature Bainite M: Martensite

[0123] [Table 4] YS: Yield strength TS: Tensile strength El: Growth rate HER: Hole expansion rate

[0124] As shown in Tables 1 to 3 above, Invention Examples 1 to 12, which satisfy all of the alloy compositions and manufacturing conditions proposed in this invention, were configured to satisfy the microstructure proposed in this invention, thereby ensuring the target strength and formability.

[0125] On the other hand, Comparative Example 1, which did not contain boron and therefore did not satisfy the alloy composition system proposed in this invention, showed excessive generation of acicular ferrite and bainitic ferrite during secondary cooling, making it impossible to achieve the target strength. As a result of this low strength, it tended to exhibit a relatively high elongation rate.

[0126] On the other hand, Comparative Examples 2 to 8 are cases where the alloy composition satisfies the present invention, but the manufacturing conditions deviate from the present invention.

[0127] Of these, Comparative Examples 2 and 3 are cases where the boron concentration present at the grain boundaries was inappropriate and deviated from relational equation 2. In Comparative Example 2, the boron concentration present at the grain boundaries was excessive, resulting in a deficiency in the secondary soft tissue fraction, and thus the elongation could not be secured. On the other hand, in Comparative Example 3, the boron concentration present at the grain boundaries was insufficient, and the secondary soft tissue fraction was excessive, resulting in a failure to secure yield strength and tensile strength.

[0128] In Comparative Example 4, the value of Du defined by relational equation 1 did not meet the range of 2 to 10, and it was not possible to ensure that the size of the secondary phase, the soft tissue, was 2 μm or larger. As a result, stress concentration during deformation could not be effectively dispersed, and consequently, the hole-expanding properties decreased.

[0129] Comparative Example 5 had insufficient secondary cooling time and could not satisfy relational equation 3. As a result, the proportion of the soft tissue, which is the secondary phase, was insufficient, and the elongation rate decreased.

[0130] In Comparative Example 6, the austenite grain size after hot rolling was excessively coarse, and the Du value defined by relation 1 did not meet the range of 2 to 10. Consequently, the soft structure, which is the secondary phase, could not be distributed uniformly, the average spacing was less than 3 μm, and the secondary phase was densely concentrated in some areas. This prevented effective dispersion of stress concentration during deformation, resulting in reduced hole-expanding properties.

[0131] Comparative Example 7 not only had insufficient Mn content, but the tertiary cooling completion temperature was too low, causing martensite to form during tertiary cooling, resulting in excessive temperature deviation within the sheet and poor material variation within the steel sheet, with a standard deviation of elongation of 2% or more.

[0132] Figure 1 is a graph showing the relationship between boron content and Du when relational equations 1 and 2 are simultaneously satisfied. The microstructure intended in this invention can be secured within the solid line connecting A, B, C, D, E, and F.

[0133] Figure 2 shows scanning electron microscope (SEM) images of the microstructures of Invention Example 1 and Comparative Example 5. As shown in Figure 2(a), Invention Example 1 shows that the matrix structure and secondary phase, which are the target of the present invention, are appropriately formed as microstructure. On the other hand, as shown in Figure 2(b), it can be seen that the soft tissue, which is the secondary phase, is not sufficiently formed in Comparative Example 5.

Claims

1. In weight percent, it contains carbon (C): 0.06-0.18%, silicon (Si): 0.01-1.8%, manganese (Mn): 1.6-3.5%, aluminum (Al): 0.001-0.1%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01-0.15%, boron (B): 0.0005-0.003%, phosphorus (P): 0.0001-0.05%, sulfur (S): 0.0001-0.05%, and nitrogen (N): 0.0001-0.02%, with the remainder being Fe and other unavoidable impurities. The microstructure consists of, by area fraction, one or more selected from low-temperature bainite and martensite: 75-90%, one or more selected from acicular ferrite and bainitic ferrite: 10-25%, and other phases: 5% or less (including 0%). An ultra-high-strength hot-rolled steel sheet in which the average spacing between one or more types selected from the aforementioned acicular ferrite and bainitic ferrite is 3 μm or more.

2. The hot-rolled steel sheet further comprises 0.01 to 0.2% niobium (Nb), as described in claim 1.

3. The ultra-high-strength hot-rolled steel sheet according to claim 1, wherein the average size of one or more selected from the aforementioned acicular ferrite and bainitic ferrite is 2.0 μm or more.

4. The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has a yield strength of 900 MPa or more, a tensile strength of 1180 MPa or more, an elongation of 7% or more, and a standard deviation of elongation of 2% or less.

5. The ultra-high-strength hot-rolled steel sheet according to claim 1, having a hole expansion ratio of 25% or more.

6. The process involves reheating a steel slab containing, by weight percent, carbon (C): 0.06-0.18%, silicon (Si): 0.01-1.8%, manganese (Mn): 1.6-3.5%, aluminum (Al): 0.001-0.1%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01-0.15%, boron (B): 0.0005-0.003%, phosphorus (P): 0.0001-0.05%, sulfur (S): 0.0001-0.05%, and nitrogen (N): 0.0001-0.02%, with the remainder being Fe and other unavoidable impurities, in a temperature range of 1100-1350°C. The steps include: hot-rolling the reheated steel slab to produce a hot-rolled steel sheet; The steps include: first cooling the hot-rolled steel sheet to a temperature of Bs or less at an average cooling rate of 50°C / s or more; After the primary cooling, a secondary cooling step is performed for ts time (seconds) at an average cooling rate of 25°C / s or less until the temperature reaches (Bs + Ms) / 2 or higher. After the aforementioned secondary cooling, a tertiary cooling step is performed to a temperature range of Ms°C to 500°C at an average cooling rate of 30°C / s or more. The step includes winding the winding within the aforementioned tertiary cooled temperature range, During the aforementioned hot rolling, finish hot rolling is performed within a temperature range of 750 to 1150°C such that the value of Du, as defined by the following relational equation 1, satisfies the range of 2 to 10. During the hot rolling process described above, the following relational equation 2 is further satisfied, A method for manufacturing ultra-high-strength hot-rolled steel sheets that satisfies the following relational equation 3. [Relationship 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 above relational formula 1, FDT represents the rolling completion temperature (°C), and [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] each represent the weight percentage content of the element in parentheses. If the element is not present, 0 is substituted.) [Relationship Equation 2] 5.0×10 ６ ≦Du×Bat×2.968×10 １０ ≦2.0×10 ７ (In relational equation 2 above, Du is the same as defined in relational equation 1, and Bat is 55.845 × [B] / (1080.6 + 45.04 × [B]), where [B] represents the weight % content of boron (B).) [Relationship Equation 3] 0.75≦exp(-k(T)×(ts) ２ )≦0.9 (The k(T) mentioned above represents the value defined by the following relational equation 4.) [Relational Equation 4] [Math 1] (In relational equation 4 above, Du is defined the same as in relational equation 1, and Bat is defined the same as in relational equation 2. Also, T1 indicates the primary cooling completion temperature [°C], and T2 indicates the secondary cooling completion temperature [°C]. Furthermore, [C], [Si], [Mn], [Cr], and [Mo] each represent the weight % content of the element in parentheses.)

7. The method for manufacturing an ultra-high-strength hot-rolled steel sheet according to claim 6, wherein the steel slab further comprises 0.01 to 0.2% niobium (Nb).

8. The method for manufacturing an ultra-high-strength hot-rolled steel sheet according to claim 6, wherein the total reduction amount in the final two passes during the hot rolling process is 10 to 40%.

9. A method for manufacturing an ultra-high-strength hot-rolled steel sheet according to claim 6, further comprising a step of final cooling to room temperature after the winding step.

10. The method for manufacturing an ultra-high-strength hot-rolled steel sheet according to claim 9, further comprising the steps of pickling and oiling after the final cooling.

11. The method for manufacturing an ultra-high-strength hot-rolled steel sheet according to claim 10, further comprising the step of hot-dip galvanizing after the pickling and oiling steps.

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