Hot-rolled steel sheet having excellent multi-stage press formability and method for manufacturing same

Abstract

The present invention relates to a hot-rolled steel sheet having excellent multi-stage press formability and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet that can be used for a wheel disc of an automobile or the like. More specifically, the present invention relates to a high-strength hot-rolled steel sheet having excellent multi-stage stamping formability and a method for manufacturing the same. Background Art

[0002] In recent years, in order to mitigate global warming, the whole society has an increasing demand for producing environmentally friendly products. In the automotive industry, in the past, many efforts have been made in the development of technologies for reducing exhaust gas generated during the driving of internal combustion engine vehicles. However, with the recent accelerating transformation to electric vehicles, carbon reduction throughout the entire life cycle, including the driving of vehicles as well as the production and recycling processes of vehicles, is being considered. To this end, attempts are being made to regulate carbon emissions during the production and recycling of raw materials for manufacturing vehicles.

[0003] In the past, in order to reduce carbon emissions during the driving of internal combustion engine vehicles, the application of lightweight materials such as aluminum has been expanded. However, in recent years, the application ratio of aluminum bodies with high carbon emissions during the manufacturing process has tended to decrease, and the application ratio of steel materials with relatively low carbon emissions for vehicle bodies has tended to increase again.

[0004] Among the components constituting an automobile, the wheel is located on the path for transmitting ground impact force to the suspension and is a safety-critical component that requires high fatigue durability. In addition, the wheel is a component exposed to the outside of the vehicle and is a component that requires aesthetics including design to stimulate the purchasing desire of vehicle buyers. The passenger car wheel component is manufactured by casting an aluminum alloy material and can be realized in various shapes, and the required aesthetics can be ensured. The fatigue durability can be ensured by thick casting of the durable and vulnerable parts.

[0005] In addition, the hot-rolled steel sheet used for the existing wheel component has excellent strength but insufficient formability and cannot be formed into a wheel disc with a complex shape by stamping. Therefore, passenger car wheels are mainly manufactured in the form of cast aluminum alloy products. However, from the perspective of the recent entire life cycle, in order to reduce carbon emissions, automobile manufacturers also require wheel components to be made of steel. Therefore, it is necessary to develop a hot-rolled steel sheet with more excellent formability compared to existing steel products.

[0006] When producing a wheel disc using a steel plate, for maximizing productivity, it is usually produced at high speed through multi-stage stamping in 7 to 10 consecutively arranged presses. Additionally, in order to improve the design of the wheel disc and smoothly perform air cooling of the wheel disc, a large number of holes need to be machined on the wheel disc. As the area of the holes increases, when the same level of fatigue load can be supported, the thickness of the steel plate needs to be increased. Therefore, in order to meet the required durability life of the customer, a steel plate with a tensile strength of 590 MPa grade usually has to use a material with a thickness of 3.5 mm or more. When continuously stamping and forming thick materials at high speed, in the initial stage of forming, it is formed at normal temperature, but as the forming accumulates, due to processing heat generation, the temperature of the material rises, and the cooling of the thick material is not smooth. Therefore, during the forming process, the temperature of the material may rise to 70 - 90 °C. Considering the formability of the steel plate for wheels, it is necessary to consider both the formability at normal temperature in the initial stage of forming and the warm-intermediate high-speed formability considering the processing heat generation during the forming process.

[0007] As a method of improving the formability of materials in steel materials, a scheme using the plastic-induced phase transformation phenomenon of retained austenite is widely applied. Patent Document 1 proposes a manufacturing method of a hot-rolled steel plate containing ferrite with a volume fraction of 50% or more and austenite with a volume fraction of 3% or more in steel to simultaneously achieve a high level of tensile strength and elongation. However, only the formability at normal temperature state is considered in the above Patent Document 1, and the warm-intermediate high-speed formability is not mentioned.

[0008] Patent Document 2 proposes a manufacturing method that ensures the strength of a cold-rolled steel plate with ferrite and / or bainite as the main phase and containing 3 - 50% by volume of retained austenite under high-speed deformation. However, only the strength is considered from the perspective of the collision performance during high-speed forming in the above Patent Document 2, and the formability is not mentioned.

[0009] Therefore, in order to manufacture environmentally friendly wheel components, it is necessary to develop a steel material with excellent strength and excellent formability in the multi-stage stamping process at various forming temperatures.

[0010] [Prior Art Documents]

[0011] [Patent Documents]

[0012] (Patent Document 1) Japanese Patent Publication Gazette No. 2002 - 030385

[0013] (Patent Document 2) Japanese Patent Publication Gazette No. 1999 - 193439 Summary of the Invention

[0014] (I) Technical Problems to be Solved

[0015] An object of the present invention is to provide a hot-rolled steel sheet having excellent strength, excellent room-temperature formability, and excellent warm-forming property, and a method for manufacturing the same.

[0016] The technical problems of the present invention are not limited to the above. Those skilled in the art to which the present invention pertains can easily understand the additional technical problems of the present invention from the overall content of this specification.

[0017] (II) Technical solution

[0018] One aspect of the present invention provides a hot-rolled steel sheet which, by weight %, comprises: carbon (C): 0.06 - 0.18%, silicon (Si): 1.2 - 2.5%, manganese (Mn): 0.80 - 2.50%, aluminum (Al): 0.001 - 0.100%, phosphorus (P): 0.0001 - 0.0500%, sulfur (S): 0.0001 - 0.0500%, nitrogen (N): 0.0001 - 0.0200%, the balance being Fe and other inevitable impurities, and the average carbon content in the retained austenite contained in the surface layer part is 1.10 - 1.40% by weight %.

[0019] In addition, another aspect of the present invention provides a method for manufacturing a hot-rolled steel sheet, the manufacturing method comprising the following steps: reheating a steel slab at 1050 - 1300°C, and by weight %, the steel slab comprises: carbon (C): 0.06 - 0.18%, silicon (Si): 1.2 - 2.5%, manganese (Mn): 0.80 - 2.50%, aluminum (Al): 0.001 - 0.100%, phosphorus (P): 0.0001 - 0.0500%, sulfur (S): 0.0001 - 0.0500%, nitrogen (N): 0.0001 - 0.0200%, the balance being Fe and other inevitable impurities; hot finish rolling the reheated steel slab at 800 - 1150°C as the final rolling temperature (FDT); performing primary cooling on the hot-rolled steel sheet after the hot finish rolling at an average cooling rate of 50 - 150°C / second to a temperature T1 of 550 - 750°C; after the primary cooling, isothermally holding for a time ts at a temperature T2 of 550 - 750°C, or performing secondary cooling for a time ts at a cooling rate of 20°C / second or less (except 0°C / second) to a temperature T2 of 550 - 750°C lower than the T1; after the isothermal holding or secondary cooling, performing tertiary cooling at a cooling rate of 150°C / second or more to a temperature T3 below the temperature at which martensite starts to form; after the tertiary cooling, air-cooling for 2 seconds or more and equalizing the temperature in the plate thickness direction to T4; and after coiling the hot-rolled steel sheet after the air-cooling, performing quaternary cooling to room temperature.

[0020] (III) Advantageous effects

[0021] According to one aspect of the present invention, there can be provided a hot-rolled steel sheet having excellent strength, excellent room-temperature formability, and excellent warm-forming formability, and a method for manufacturing the same.

[0022] 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 specific embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram for measuring the thickness of the deep part of the hot-rolled steel sheet. Figure 1 (a) thereof is a schematic diagram for measuring the line intersection length of the size distribution of austenite grains at a specific thickness position. Figure 1 (b) thereof is a schematic diagram of the fine structure to be achieved in this steel grade. Figure 1 (c) thereof is a schematic diagram showing the average value and standard deviation of the austenite grain size measured by the line intersection length at a specific thickness position.

[0024] Figure 2 It is a photograph obtained by observing the fine structure of the steel sheet obtained from Invention Example 2 of the present invention by backscattered electron method after installing it in a scanning electron microscope (SEM). Figure 2 (a) thereof shows the fine structure at 100 μm of the surface layer part of Invention Example 2, Figure 2 and (b) thereof shows the deep-layer fine structure of Invention Example 2. The white areas in each tissue photograph represent austenite. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] Preferred embodiments of the present invention will be described below. However, the embodiments of the present invention can be deformed into various other forms, and it should not be construed that the scope of the present invention is limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely describe the present invention to those skilled in the art.

[0026] In addition, the terms used in this specification are used to describe 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 the plural forms. In addition, the meanings of "comprising" and "including" used in the specification are used to specify the constitution and do not exclude the existence or addition of other constitutions.

[0027] The present inventors recognized that in existing high-strength hot-rolled steel sheets, the plastic-induced phase transformation (TRIP) phenomenon of retained austenite can be utilized to manufacture steel materials having excellent elongation measured at room temperature, but the formability under warm-forming conditions has not been considered. To solve this problem, in-depth research has been carried out.

[0028] The principle of the plastic-induced phase transformation phenomenon is that when a material is deformed under external stress, by transforming retained austenite into martensite, the work hardening ability of the steel is increased, local deformation concentration is prevented, and thus formability is improved. To optimize formability, the transformation of austenite to martensite must continue as the material deforms. However, when the stability of austenite is too low, the phase transformation is completed at the initial stage of deformation, and no improvement in elongation can be expected. When the stability of austenite is too high, no phase transformation occurs, so no improvement in elongation can be expected. Therefore, when designing TRIP steel, the fraction and stability of retained austenite must be considered simultaneously to ensure the required formability.

[0029] In addition, it is known that the stability of austenite is greatly affected by the internal carbon content and is sensitive to the deformation temperature and strain rate. Generally, when the deformation temperature is high, excellent formability is achieved only when the carbon content at the deformation temperature is lower than that at room temperature. It is known that the strain rate affects the stability of austenite less sensitively than temperature. Therefore, to ensure formability at high temperatures, a low carbon content in austenite is advantageous, but considering formability at room temperature, a high carbon content in austenite is advantageous.

[0030] In order to make the carbon content of austenite distributed in the steel sheet in an ideal ratio, the present inventors adjusted the structure to be non-uniform in the thickness direction. Thus, in the stamping process of applying initial drawing, since the maximum forming amount is applied to the surface layer portion, a method is sought in which austenite with a high carbon content is formed to have excellent room temperature formability, and austenite with a low carbon content can exist inside the steel material to ensure warm forming property in subsequent continuous stamping processes.

[0031] The internal carbon content of austenite is affected by the size of austenite. When the size of austenite grains is small, the confirmation of carbon is easy, so there is austenite with a high carbon content. When the size of austenite grains is large, the diffusion of carbon inside austenite is difficult, and the enrichment of carbon progresses slowly, so there is austenite with a low average carbon content. Therefore, by changing the size of austenite grains in the surface layer portion and the deep layer portion, austenite with different carbon contents can be formed at each position.

[0032] To finely disperse the retained austenite in TRIP steel, the Quenching&Partitioning (Q&P) process is widely used. This process cools the steel sheet to a temperature below Ms and then raises the temperature of the steel sheet again, thereby improving strength, elongation, and bendability. However, in order to raise the temperature of the steel sheet cooled below Ms again, a separate heating device is required, so there is a problem that it is difficult to apply to the hot rolling process of cooling and coiling in sequence during the rolling process.

[0033] In the manufacturing process of hot-rolled steel sheets, after hot finish rolling, the hot-rolled steel sheets are cooled by cooling water injected from the top and bottom. At this time, the surface layer of the sheet transfers heat to the cooling water and is cooled, while the interior of the sheet is cooled by heat conduction. Generally, the rate of heat transfer occurring in the surface layer is faster than the rate of heat conduction in the interior of the sheet, so a temperature gradient is formed in the interior of the sheet along the thickness direction. However, the present inventors have found the following phenomenon: the surface layer of the sheet is cooled below Ms, and on the other hand, when the deep layer of the sheet is maintained at a temperature above Ms and the cooling water is removed at an appropriate time point, due to heat transfer in the interior of the sheet, the surface temperature of the steel sheet rises again.

[0034] That is, it is recognized that even without a separate heating device, the surface layer of the steel sheet is cooled below Ms and then heated again to a temperature above Ms, so there is fine dispersion of retained austenite and austenite with a high carbon content, and coarse austenite with a low carbon content exists in the deep layer, so that a steel sheet capable of ensuring formability under both normal temperature and warm temperature conditions can be obtained, and thus the present invention is completed.

[0035] First, the alloy composition of the hot-rolled steel sheet of the present invention will be described. The high-strength hot-rolled steel sheet having excellent formability at normal temperature and warm temperature according to the invention contains, by weight%: carbon (C): 0.06 - 0.18%, silicon (Si): 1.2 - 2.5%, manganese (Mn): 0.80 - 2.50%, aluminum (Al): 0.001 - 0.100%, phosphorus (P): 0.0001 - 0.0500%, sulfur (S): 0.0001 - 0.0500%, nitrogen (N): 0.0001 - 0.0200%, and the balance of Fe and other inevitable impurities.

[0036] Hereinafter, first, the alloy composition components of the high-strength hot-rolled steel sheet having excellent bendability and elongation of the present invention and the reasons for limiting their contents will be described in detail. At this time, unless otherwise specifically stated, the content of each element represents weight%.

[0037] Carbon (C): 0.06 - 0.18%

[0038] Carbon (C) is an important element that diffuses and moves to austenite during bainite phase transformation to stabilize austenite, thereby forming retained austenite. As the content of this C increases, the fraction of retained austenite increases, thereby simultaneously improving elongation and tensile strength. When the content of the C is less than 0.06%, the fraction of retained austenite is low, so elongation and tensile strength cannot be ensured. In addition, when the content of C exceeds 0.18%, the Ms temperature decreases excessively, the tensile strength increases excessively, and there are problems of poor formability and weldability. Therefore, in the present invention, the content of the C is preferably 0.08 - 0.18%. More preferably, the content of the C can be 0.08 - 0.15%.

[0039] Silicon (Si): 1.2 - 2.5%

[0040] Silicon (Si) is an important element that delays the formation of carbides during the bainite phase 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 1.2%, carbides are formed, the fraction of retained austenite is low, and it is difficult to ensure elongation. On the other hand, when the content of the silicon (Si) exceeds 2.5%, Fe - Si composite oxides are formed on the slab surface during reheating, the surface quality of the steel plate may deteriorate, and there is also a problem that the weldability will be reduced. Therefore, in the present invention, the content of the Si is preferably 1.2 - 2.5%. Additionally, in terms of further improving the above effects, the lower limit of the content of the Si can be 1.8%, or the upper limit of the content of the Si can be 2.2%.

[0041] Manganese (Mn): 0.80 - 2.50%

[0042] Manganese (Mn) is an element that improves the hardenability of steel, preventing excessive formation of granular ferrite during the cooling process after finish rolling, thus facilitating the formation of bainite and retained austenite.

[0043] When the content of the Mn is less than 0.80%, the hardenability is insufficient, and the fraction of granular ferrite increases rapidly during the cooling process, there is a problem that it is difficult to control the full cooling time at T2 temperature. On the other hand, when the content of the Mn exceeds 2.50%, the growth rate of ferrite is too slow, there is a problem that the time required in the full cooling interval exceeds the time that the equipment can control. Therefore, in the present invention, the content of the Mn is preferably 0.80 - 2.50%, and more preferably, the content of the Mn can be 1.00 - 2.00%.

[0044] Aluminum (Al): 0.001 - 0.100%

[0045] Aluminum (Al) is an element added for deoxidation, and a part remains in the steel after deoxidation. When the content of this Al exceeds 0.100%, it causes an increase in oxide and nitride - based inclusions in the steel, resulting in poor formability of the steel plate. Additionally, when the content of the Al is excessively reduced to less than 0.001%, it causes an unnecessary increase in refining costs. Therefore, in the present invention, the content of the Al is preferably 0.001 - 0.100%.

[0046] Phosphorus (P): 0.0001 - 0.0500%

[0047] Phosphorus (P) is an inevitably contained impurity 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. Theoretically, it is advantageous to limit the content of phosphorus to 0%, but in order to make the content of the P less than 0.0001%, the manufacturing cost increases excessively. Therefore, in the present invention, the content of the P is preferably 0.0001 - 0.0500%.

[0048] Sulfur (S): 0.0001 - 0.0500%

[0049] Sulfur (S) is an inevitably contained impurity and forms non-metallic inclusions in combination with Mn, etc., and is thus an element that is the main cause of reducing the workability of steel. Therefore, it is preferable to control the content of the sulfur (S) as low as possible. Theoretically, it is advantageous to limit the content of S to 0%, but in order to make the content of the S less than 0.0001%, the manufacturing cost increases excessively. Therefore, in the present invention, the content of the S is preferably 0.0001 - 0.0500%.

[0050] Nitrogen (N): 0.0001 - 0.0200%

[0051] Nitrogen is an inevitably contained impurity and is an element that reacts with aluminum to precipitate fine nitrides, thereby reducing the workability of steel. Therefore, it is preferable to control the content of the nitrogen as low as possible. Theoretically, it is advantageous to limit the content of N to 0%, but in order to make the content of the N less than 0.0001%, the manufacturing cost increases excessively. Therefore, in the present invention, the content of the N is preferably 0.0001 - 0.0200%.

[0052] In the present invention, in addition to the above components, one or more of 0.01 - 2.00% of chromium (Cr), 0.01 - 2.00% of molybdenum (Mo), 0.01 - 0.20% of titanium (Ti), and 0.01 - 0.10% of niobium (Nb) can be selectively further contained. Hereinafter, as selectively added elements, the content of each component and the reason for limiting the content will be specifically described.

[0053] Chromium (Cr): 0.01 - 2.00%

[0054] Chromium (Cr) is an element that improves the hardenability of steel and slows down the formation of ferrite during the cooling process after finish rolling, thereby facilitating the formation of austenite. When the content of the Cr is less than 0.01%, the addition effect may not be obtained sufficiently. On the other hand, when the content of chromium (Cr) exceeds 2.00%, there is a problem that the phosphate treatability of the steel plate deteriorates. Therefore, in the present invention, the content of the Cr is preferably 0.01 - 2.00%, and the content of the Cr is more preferably 0.10 - 1.50%.

[0055] Molybdenum (Mo): 0.01 - 2.00%

[0056] Molybdenum (Mo) is an element that improves the hardenability of steel and plays a role in increasing strength through solid solution strengthening. When the content of Mo is less than 0.01%, the addition effect of suppressing the formation of ferrite during the cooling process after finish rolling cannot be fully obtained. On the other hand, when the content of molybdenum (Mo) exceeds 2.00%, the weldability deteriorates, and there is a problem of excessive cost increase. Therefore, in the present invention, the content of Mo is preferably 0.01 - 2.00%, and more preferably 0.05 - 1.00%.

[0057] Titanium (Ti): 0.01 - 0.20%

[0058] Titanium (Ti) is an element that forms carbonitrides. During hot rolling, it delays recrystallization, refines the grains of austenite, promotes ferrite phase transformation, and refines the grains of ferrite, thus playing a role in increasing strength. When the content of Ti is less than 0.01%, the addition effect cannot be fully obtained. In addition, when the content of Ti exceeds 0.20%, coarse carbonitrides are formed, so the toughness of the steel plate decreases. Therefore, in the present invention, in order to obtain the above-mentioned addition effect of Ti and further improve the physical properties, the content of Ti can be set to 0.01 - 0.20%. In addition, in terms of further improving the above effect, the lower limit of the content of Ti can be 0.02%, or the upper limit of the content of Ti can be 0.10%.

[0059] Niobium (Nb): 0.01 - 0.10%

[0060] Niobium (Nb) is similar to Ti and is an element that forms carbonitrides. When niobium is added, through the delay of recrystallization during hot rolling, it refines the grains of austenite, promotes ferrite phase transformation, and refines the grains of ferrite, thus playing a role in increasing strength.

[0061] When the content of Nb is less than 0.01%, the addition effect cannot be fully obtained, but when the content of Nb exceeds 0.10%, coarse carbonitrides are formed, the toughness of the steel plate decreases, and the rolling load increases during rolling, so the workability deteriorates. Therefore, in the present invention, the content of Nb is preferably 0.01 - 0.10%. In addition, in terms of further improving the above effect, the content of Nb can be 0.01 - 0.05%.

[0062] The remaining component of the present invention is iron (Fe). However, in the usual manufacturing process, inevitable impurities may inadvertently be mixed in from raw materials or the surrounding environment, and thus these impurities cannot be excluded. These impurities are well known to those skilled in the field of ordinary steel manufacturing, and thus all of their details are not specifically described in this specification.

[0063] Although not particularly limited, according to one aspect of the present invention, in the hot-rolled steel sheet, the microstructure of the surface layer portion may contain, by area %: the sum of ferrite and bainite: 85.0 - 96.5%, retained austenite: 3.5 - 15.0%, and martensite: 3.0% or less (including 0%).

[0064] According to one aspect of the present invention, in terms of fraction, the sum of ferrite and bainite in the microstructure of the surface layer portion is in the range of 85.0 - 96.5%. The specific gravity of aluminum alloy is lower than that of steel, but its strength is also lower than that of steel. Therefore, when manufacturing wheels using steel sheets with a tensile strength of 590 MPa or more, it is possible to ensure a component weight similar to that of aluminum alloy wheels. Thus, if the elongation rate can be maximized under the condition that the tensile strength meets the level of 590 MPa or more, it is possible to manufacture environmentally friendly and low-cost wheel components with a weight and design similar to those of aluminum alloy wheels. In the present invention, the improvement in formability is achieved by controlling the phase stability and fraction of retained austenite and controlling the fraction of ferrite and bainite as the matrix microstructure. The ferrite phase transformation is achieved in the step of slow cooling or isothermal holding in the temperature range of 550 - 750 °C after hot rolling and primary cooling, and the matrix microstructure is formed. At this time, carbon diffuses into austenite, and thus as ferrite grows, the Ms temperature, which is the martensite formation temperature, gradually decreases. In the present invention, the coiling temperature is set to the temperature at which the surface layer portion and the deep layer portion that are supercooled below Ms after the third cooling reach thermal equilibrium. Therefore, if the fraction of ferrite is too low and the Ms temperature is too high, carbides are formed in the microstructure, and thus there is a problem of a decrease in the fraction of retained austenite. Therefore, the fraction of ferrite is preferably 70% or more. Additionally, when there is too much ferrite, the Ms temperature is too low, so the carbon diffusion of the bainite phase transformation is not smooth, and thus the fraction of austenite decreases, and martensite can be transformed in the final cooling step. Therefore, the fraction of ferrite formed during the primary cooling process is preferably 90% or less.

[0065] As described above, nucleation and growth of martensite occur after the surface layer is cooled below Ms. However, after cooling, the temperature of the steel sheet rises above Ms again by heat transfer, so when the martensite phase transformation is not completed, it enters the bainite phase transformation temperature range again. Therefore, the martensite formed after cooling exists in the form of tempered martensite through tempering, and bainite ferrite grows inside the austenite that has not undergone phase transformation below Ms. The tempered martensite and bainite ferrite present in the surface layer have a common lath shape, and the microstructure contains a large number of dislocations. Therefore, it is difficult to distinguish tempered martensite and bainite ferrite from the fine microstructure. Since their effects on physical properties are also similar, they are not distinguished separately and are collectively referred to as bainite and controlled. The fraction of bainite formed after coiling is determined by the fraction of austenite after three-stage cooling and the maximum carbon content soluble in austenite determined by the coiling temperature. Compared with ferrite and retained austenite, its effect on the physical properties of the steel sheet is very small. Therefore, it is effective to control the sum of the fractions of ferrite and bainite. When the sum of ferrite and bainite is less than 85.0%, the carbon content that must be added to the steel to ensure stable austenite is too high, which may hinder the weldability of the steel. When the sum of ferrite and bainite exceeds 96.5%, the fraction of retained austenite cannot be ensured sufficiently, so there is a problem of poor formability.

[0066] In addition, according to one aspect of the present invention, in terms of area%, the fine microstructure in the surface layer of the present invention may contain 3.5 - 15% of retained austenite. Retained austenite plays an important role in improving the formability of steel. When the fraction of retained austenite is less than 3.5%, there is a problem of poor elongation of the steel. On the other hand, in order to make the fraction of retained austenite exceed 15%, an excessive amount of C must be added. Therefore, the strength of the steel sheet increases excessively, and there is a problem of poor weldability.

[0067] At this time, in terms of weight%, the average carbon content in the retained austenite contained in the surface layer preferably satisfies the range of 1.10 - 1.40%. When the average carbon content in the retained austenite contained in the surface layer is less than 1.10%, plastic-induced phase transformation to martensite occurs at the initial stage of deformation at room temperature, so an improvement in formability can be expected. On the other hand, when the average carbon content in the retained austenite contained in the surface layer exceeds 1.40%, due to excessive stability, even if sufficient deformation is formed, plastic-induced phase transformation does not occur, and an improvement in formability cannot be expected. Therefore, there is a problem of poor formability at room temperature.

[0068] In addition, in the present invention, the above-mentioned surface layer refers to the region located in the surface layer along the thickness direction from the surface of the hot-rolled steel sheet.

[0069] According to one aspect of the present invention, in the hot-rolled steel sheet, the surface layer portion and the deep layer portion can be distinguished by the dimensional change of retained austenite. The method for distinguishing the surface layer portion and the deep layer portion is not particularly limited, but for example, it can be distinguished by the following method. Specifically, first, the austenite structure throughout the thickness of the steel sheet is distinguished by the reference etching method, and then the deep layer portion and the surface layer portion can be distinguished using a micrograph measured at a magnification of 1000 under an optical microscope.

[0070] That is, according to one aspect of the present invention, the surface layer portion and the deep layer portion can be distinguished by the difference in the size (e.g., equivalent circle diameter) of retained austenite. At this time, since the equivalent circle diameter of retained austenite at each thickness position of a specific hot-rolled steel sheet is very important, the average value and standard deviation of the equivalent circle diameter of retained austenite at each thickness position are measured by the method shown in (a) of Figure 1 . To measure the average value and standard deviation of the sizes (e.g., equivalent circle diameters) of retained austenite labeled A to D with different sizes at each position passing through the dotted line marking a specific thickness position, the lengths of the intersections of the dotted line with each retained austenite are measured respectively, and the arithmetic average value and standard deviation are calculated using the number of retained austenite at the intersections.

[0071] In addition, as shown in (b) of Figure 1 , when calculating the average value and standard deviation at each position at equal intervals starting from a specific position on the surface, as shown in (c) of Figure 1 , the average value and standard deviation of the retained austenite grain size can be expressed according to the distance from the surface layer. Based on such measurement results, the surface layer portion of the hot-rolled steel sheet according to the present invention has the characteristics of fine and uniformly dispersed retained austenite size, and thus exhibits the characteristics of low average value and standard deviation. In contrast, in the deep layer portion of the hot-rolled steel sheet according to the present invention, due to the presence of coarse retained austenite, the average value and standard deviation increase rapidly. The position until the average value and standard deviation of the retained austenite size (e.g., equivalent circle diameter) increase rapidly is defined as the surface layer portion. However, the above line intersection method can easily measure the size (e.g., equivalent circle diameter), so it is suitable as a method for measuring the depth of the surface layer portion.

[0072] However, due to the limited measurement accuracy of the fraction and size (e.g., equivalent circle diameter) of the structure, the fine structure of the surface layer and the deep layer, as well as the average carbon content in the retained austenite, etc. can be defined using the following method. At this time, the measurement method is not particularly limited. As an example, for the fine structure of the surface layer, at a position 50 μm from the surface, using the reference etching method, the fraction of ferrite and bainite can be distinguished through the result of analysis at a magnification of 1000 using an optical microscope and an image analyzer. By the same method, the fraction of retained austenite and the equivalent circle diameter (μm) of austenite are measured. In addition, the average carbon content (wt%) and Cγ in the retained austenite can be calculated using the following formula 1 through X-Ray diffraction analysis. In addition, the deep layer can also be measured by the same method as the above-mentioned surface layer, and the measurement method is not particularly limited. As an example, in the case of the deep layer, the fraction of the fine structure, the equivalent circle diameter (μm) of the retained austenite, and the average carbon content (wt%) in the retained austenite at the center position (1 / 2t) of the steel plate thickness are analyzed by the same method as the surface layer.

[0073] [Formula 1]

[0074] aγ = 3.578 + 0.033×[Cγ] + 0.0095×[Mn] - 0.00124×[Si]

[0075] (In the above formula 1, aγ is the lattice constant (angstrom) of austenite calculated by X-ray diffraction analysis, and [Mn] and [Si] are weight contents.)

[0076] Although there is no particular limitation, according to an embodiment of the present invention, the average equivalent circle diameter of the retained austenite contained in the surface layer may be 0.2 - 2.0 μm, more preferably may be 0.5 μm or more, or may be 1.8 μm or less. When the average equivalent circle diameter of the retained austenite contained in the surface layer is less than 0.2 μm, the phase stability increases rapidly, and the internal carbon content is generally high. Therefore, even if sufficient deformation occurs, plastic-induced phase transformation does not occur, and thus it may be difficult to expect an improvement in formability. On the other hand, when the average equivalent circle diameter of the retained austenite in the surface layer exceeds 2.0 μm, the diffusion distance of carbon increases, and it may be difficult to ensure that the average carbon content in the retained austenite is 1.10% or more.

[0077] In addition, the average equivalent circle diameter of the retained austenite contained in the deep layer may be greater than the average equivalent circle diameter of the retained austenite contained in the above-mentioned surface layer.

[0078] In addition, according to one aspect of the present invention, in terms of area %, the microstructure of the deep layer portion may include: the sum of ferrite and bainite: 85.0 - 96.5%, retained austenite: 3.5 - 15.0%, and martensite: 5.0% or less (including 0%). Ferrite is formed in the secondary cooling step with uniform temperature in the thickness direction, so the ferrite has the same fraction as the surface layer portion. Bainite is also formed after coiling with uniform temperature in the thickness direction, so the bainite has a fraction similar to that of the surface layer portion.

[0079] In addition, according to one aspect of the present invention, the average carbon content in the retained austenite contained in the deep layer portion may be less than the average carbon content in the retained austenite contained in the surface layer portion. Specifically, the average carbon content in the retained austenite in the deep layer portion may be 0.80% or more and less than 1.10%. After the temperature of the surface layer portion of the hot-rolled steel sheet according to the present invention is instantaneously cooled below Ms and then heated again, the fine-grained austenite is distributed and carbon enrichment proceeds smoothly. In contrast, after the deep layer portion is coiled while maintaining the temperature above the bainite transformation temperature of Ms, there is coarse austenite, and the time required for carbon diffusion increases, so that the carbon content inside the austenite after final cooling is lower than that of the surface layer portion. At this time, when the average carbon content in the retained austenite in the deep layer portion is less than 0.8%, plastic-induced phase transformation into martensite occurs at the initial stage of deformation during warm forming, and thus improvement in formability cannot be expected. On the other hand, when the average carbon content in the retained austenite in the deep layer portion is 1.1% or more, the stability is too high, and even if sufficient deformation is achieved, plastic-induced phase transformation does not occur, so improvement in formability cannot be expected, and thus the warm formability may deteriorate.

[0080] In addition, according to one aspect of the present invention, the average equivalent circle diameter of the retained austenite contained in the deep layer portion may be more than 2.0 μm and 5.0 μm or less, more preferably 2.2 μm or more, or 3.0 μm or less. When the average equivalent circle diameter of the retained austenite contained in the deep layer portion is 2.0 μm or less, carbon is excessively enriched inside the austenite, and there may be a problem that the average carbon content in the retained austenite in the deep layer portion becomes 1.10% or more. On the other hand, when the average equivalent circle diameter of the retained austenite contained in the deep layer portion exceeds 5.0 μm, as the distance required for carbon diffusion increases, it may be difficult to ensure that the internal carbon content of the retained austenite contained in the deep layer portion is 0.80% or more.

[0081] According to one embodiment of the present invention, the average thickness (t) of the hot-rolled steel sheet may be 1.5 - 12.0 mm. When the average thickness of the hot-rolled steel sheet is less than 1.5 mm, heat exchange in the thickness direction easily occurs, and it is difficult to ensure the two-component structure of the deep layer portion and the surface layer portion. When the average thickness of the hot-rolled steel sheet exceeds 12.0 mm, it may be difficult to be used for the purpose of a wheel component.

[0082] In addition, although there is no particular limitation, according to an embodiment of the present invention, the average thickness of the surface layer portion may be 100 μm or more, and may vary depending on the average thickness of the hot-rolled steel sheet, but the average thickness of the surface layer portion may be 30% or less of the average thickness of the hot-rolled steel sheet. When the average thickness of the surface layer portion is less than 100 μm, the effect of improving formability at normal temperature may be small. In addition, when the average thickness of the surface layer portion exceeds 30% of the total average thickness of the hot-rolled steel sheet, after heat transfer occurs in the steel sheet, it is difficult for the temperature of the surface layer portion to recover to a temperature above Ms. Therefore, coiling at a temperature below the Ms temperature results in shape defects, and the formability in the warm forming step with a high deformation amount may deteriorate. In addition, in terms of further improving the above effects, the upper limit of the average thickness of the surface layer portion may be 25%, or the lower limit of the average thickness of the surface layer portion may be 5%. At this time, the surface layer portions may be provided on both surfaces of the hot-rolled steel sheet respectively. In this case, the above-mentioned average thickness of the surface layer portion refers to the sum of the average thicknesses of the respective surface layer portions measured from the two surfaces in the thickness direction of the steel sheet.

[0083] In addition, according to one aspect of the present invention, when the coiling temperature is too low, the diffusion of carbon is insufficient, and martensite may transform in the step of finally cooling to normal temperature in some regions. This martensite plays a role in increasing strength. However, when an excessive amount of martensite is formed, the result is a decrease in the fraction of retained austenite, so there is a problem of deteriorated formability. In the present invention, there is no need to limit the lower limit of the martensite fraction in the deep layer portion, but when the martensite fraction exceeds 5%, the elongation may deteriorate, so the martensite fraction is preferably controlled below 5%.

[0084] According to one aspect of the present invention, the present invention can provide a high-strength hot-rolled steel sheet having the above alloy composition and fine structure and having excellent normal-temperature formability and warm-forming formability, wherein the tensile strength of the hot-rolled steel sheet is 590 MPa or more, the draw ratio at normal temperature satisfies 2.0 or more, and the elongation measured in the range of 70 - 90 °C is 30% or more.

[0085] In addition, according to another aspect of the present invention, a high-strength hot-rolled steel sheet having excellent normal-temperature formability and warm-forming formability can be provided, wherein the tensile strength of the hot-rolled steel sheet is 590 MPa or more, the draw ratio at normal temperature satisfies 2.0, and the elongation in the warm tensile test at 80 °C is 30% or more.

[0086] Next, a manufacturing method of a high-strength hot-rolled steel sheet having excellent bendability and elongation according to another aspect of the present invention will be described in detail.

[0087] Reheating of the steel billet

[0088] In the present invention, before hot rolling, the steel billet is subjected to a reheating and homogenization process. At this time, the reheating process is preferably carried out at 1050 - 1300 °C. When the reheating temperature is lower than 1050 °C, there is a problem of insufficient homogenization of alloying elements. On the other hand, when the reheating temperature exceeds 1300 °C, excessive oxides are formed on the slab surface, reducing the surface quality of the steel plate, so it is not preferred.

[0089] Hot rolling

[0090] Next, the reheated steel billet is hot rolled to produce a hot rolled steel plate. At this time, the finish hot rolling temperature (FDT) of the hot rolled plate after finish hot rolling is controlled between 800 - 1150 °C.

[0091] When the hot rolling is carried out at a temperature where FDT is higher than 1150 °C, excessive oxides are formed on the surface of the rolled steel plate and cannot be effectively removed even after pickling, so the surface quality deteriorates. In addition, when hot rolling is carried out at a temperature where FDT is lower than 800 °C, the rolling load increases excessively, so the workability deteriorates.

[0092] At this time, it is preferable to control the sum of the reduction ratios in the last two passes of hot rolling within the range of 10 - 40%. The main reason for performing multi-stage conventional hot rolling is to reduce the rolling load and precisely control the thickness. When the sum of the reduction ratios in the last two passes exceeds 40%, the rolling load in the last two passes increases excessively, so there is a problem of deteriorated workability. On the other hand, when the sum of the reduction ratios in the last two passes is less than 10%, the temperature of the steel plate drops rapidly, and the workability may be poor.

[0093] Cooling step

[0094] The steel plate after the finish hot rolling is cooled once at an average cooling rate between 50 - 150 °C to a temperature T1 of 550 - 750 °C.

[0095] After the first cooling, it is isothermally held for ts [unit: second (sec)] at a temperature T2 [unit: °C] of 550 - 750 °C, or cooled for a time ts [unit: second] at an average cooling rate of 20 °C / second or less (except 0 °C / second) to a temperature T2 [unit: °C] of 550 - 750 °C lower than the T1.

[0096] During the isothermal holding or secondary cooling process, ferrite is formed and the matrix structure is formed, and carbon is enriched in austenite. Therefore, the Ms temperature of the steel gradually decreases. At this time, in terms of area%, the fraction of ferrite formed during secondary cooling is preferably 70-90%, so that the Ms temperature of the steel is between 250-450 °C. For this purpose, it is preferable to control the temperature and time of isothermal holding or secondary cooling according to the following relational expression 1.

[0097] [Relational expression 1]

[0098] 70≤Vα≤90

[0099] (In the relational expression 1, the Vα represents the fraction [area%] of ferrite formed during the isothermal holding or secondary cooling.)

[0100] At this time, the Vα can be defined by the following relational expression 2.

[0101] [Relational expression 2]

[0102] Vα=100×(1-exp(-k(T)×(ts) 1.5 ))

[0103] (In the relational expression 2, k(T) is an index representing the growth rate of ferrite and is defined by the following relational expression 3.)

[0104] [Relational expression 3]

[0105]

[0106] (In the relational expression 3, P is defined by the following relational expression 4, Du is an index representing the effective grain size of austenite before the first cooling after hot rolling, and is defined by the following relational expression 5.)

[0107] [Relational expression 4]

[0108] P=633-529×[C]-(29-32×[C])×[Mn]+(70-86×[C])×[Si]-(10-[C])×[Cr]-(15+[C])×[Mo]

[0109] (In the relational expression 4, the [C], [Si], [Mn], [Cr], and [Mo] respectively represent the weight% contents of the elements in the parentheses.)

[0110] [Relational expression 5]

[0111] 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

[0112] (In the relation 5, [C], [Si], [Mn], [Cr], [Mo], [Ti] and [Nb] respectively represent the weight % content of the elements in the brackets, and FDT represents the hot finish rolling temperature (°C).)

[0113] The finishing cooling termination temperature is set using the ferrite fraction formed during the secondary cooling. Therefore, in order to perform accurate calculations, ferrite phase transformation needs to be avoided during the primary cooling step. Thus, the average cooling rate of the primary cooling is preferably 50 °C / second or more. Additionally, when the average cooling rate during primary cooling is too fast, a temperature deviation may occur between the surface layer and the deep layer. Therefore, the average cooling rate of the primary cooling is preferably 150 °C / second or less.

[0114] According to one aspect of the present invention, in order to achieve the desired fine microstructure of the present invention, it is important that the finishing cooling performed after the secondary cooling is rapidly cooled at an average cooling rate of 150 °C / second or more (alternatively, 150 - 250 °C / second) so that the temperature of the surface layer portion satisfies the following relation 6.

[0115] [Relation 6]

[0116] Ms - 100 ≤ T3 ≤ Ms - 30

[0117] (In the relation 6, T3 is the temperature [°C] of the steel plate measured at the surface after the finishing cooling, Ms is the temperature [°C] at which austenite present in the steel plate starts to form martensite after the secondary cooling, and the Ms is defined by the following relation 7.)

[0118] [Relation 7]

[0119] Ms (°C) = 550 - (330×[C']) - (41×[Mn]) - (20×[Si]) - (20×[Cr]) - (10×[Mo]) + (30×[Al])

[0120] (In the relation 7, [C'] represents a value considering the enrichment of carbon diffused in ferrite, and the [C'] is defined by the following relation 8. Additionally, [Mn], [Si], [Cr], [Mo] and [Al] respectively represent the weight % content of the elements in the brackets.)

[0121] [Relation 8]

[0122] [C'] = 0.9×([C] - Vα×0.02) / (1 - Vα)

[0123] (In the said relation 8, the [C] represents the weight % content of the element within the brackets, and the Vα is as described above.)

[0124] According to one aspect of the present invention, the average cooling rate of the three - stage cooling can be 150°C / second or more (alternatively, between 150 - 250°C / second). When the average cooling rate of the three - stage cooling is less than 150°C / second, it is impossible to ensure a sufficient surface layer thickness, so the room - temperature formability may deteriorate. On the other hand, when the average cooling rate of the three - stage cooling exceeds 250°C / second, the thickness of the surface layer is too large, so it may be difficult to ensure the warm - forming property.

[0125] Homogenization step

[0126] According to one aspect of the present invention, after the three - stage cooling, air - cool for 2 seconds or more (the upper limit is not particularly limited), and make the temperature in the plate thickness direction uniform to T4. The T4 can be in the range of 200 - 400°C. Through the air - cooling in the above - mentioned homogenization step, the temperature in the thickness direction can be made uniform by heat transfer in the steel plate. The surface layer cooled to a temperature below Ms is reheated to a temperature above Ms by the heat transferred from the deep layer, and carbon easily diffuses from the martensite in the state of excessive carbon inclusion to austenite, and austenite can be stabilized.

[0127] Coiling and final cooling step

[0128] Subsequently, in the present invention, the hot - rolled steel plate after the said air - cooling can be coiled and then subjected to four - stage cooling to be cooled to room temperature. That is, the hot - rolled steel plate homogenized at the temperature of T4 after the three - stage cooling is coiled to make a coiled sheet, and then cooled to room temperature.

[0129] Next, in the present invention, optionally, a pickled and oiled (PO) steel plate can be manufactured by further pickling and oiling the hot - rolled steel plate that has completed the said final cooling (four - stage cooling).

[0130] Or, optionally, the hot - rolled steel plate after the said final cooling is pickled and then heated to a temperature range of 400 - 750°C, so that hot - dip galvanizing can be carried out. Detailed implementation mode

[0131] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are only used to illustrate the present invention for more detailed description and do not 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.

[0132] (Example)

[0133] Manufacture a steel billet having the alloy composition shown in Table 1 below. Then, using the manufacturing conditions shown in Table 2 below, manufacture a hot-rolled steel sheet with a thickness of 4 mm using the manufactured steel billet. At this time, the reheating temperature of the steel billet should be 1150°C, the sum of the reduction ratios of the final two passes of finish rolling should be 25%, and the average cooling rate of primary cooling should be 60°C / second.

[0134] For the third cooling, apply the same water injection amount and the moving speed of the steel sheet, then change the water injection time and change the cooling rate of the steel sheet. After the water injection is completed, represent the temperature measuring the surface layer temperature of the steel sheet as T3, and represent the temperature (homogenization temperature) before coiling after flowing for more than 2 seconds from this as T4.

[0135] For each steel sheet manufactured as described above, observe the microstructure and show the results in Table 3 below. At this time, in Table 3 below, F represents ferrite, B represents bainite, P represents pearlite, and A represents austenite.

[0136] In addition, measure the thickness and proportion of the surface layer part and show them in Table 3 below. For the thickness of the surface layer part, arbitrarily select 10 points, measure the thickness of the surface layer part, and then show the average value. For the proportion of the surface layer part, calculate the proportion using the average value of the thickness of the surface layer part and show it.

[0137] In addition, for each steel sheet manufactured as described above, process it into circular plates with diameters of 200 mm and 160 mm, perform cup forming using a punch with a diameter of 100 mm, and verify the drawability at room temperature. When the circular plate with a diameter of 160 mm is formed into a cup shape and no cracks occur, it is judged that the drawing ratio (DR) satisfies 1.6. When the circular plate with a diameter of 200 mm does not crack during cup forming, it is judged that the drawing ratio satisfies 2.0, and mark the results as O and × in Table 4.

[0138] To verify the warm forming property, make a specimen with a measuring width of 20 mm and a measuring length of 50 mm along the direction parallel to the rolling direction, keep it in a furnace at 80°C for 1 hour to make the temperature uniform, and then perform a tensile test at a measuring speed with a strain rate of 50 mm / minute in the furnace, and measure the yield strength (YS), tensile strength (TS), uniform elongation (U-El), and elongation (El), and show the results in Table 4. Specifically, the yield strength and tensile strength represent the lower yield point and the maximum tensile strength respectively, and the elongation represents the fracture elongation.

[0139] When the drawing ratio of room temperature forming satisfies 2.0, and the tensile strength of the warm tensile test is 590 MPa or more and the elongation is 30% or more, it is judged as a good level.

[0140] [Table 1]

[0141]

[0142] [Table 2]

[0143]

[0144] T1 : Completion temperature of primary cooling [°C]

[0145] T2 : Isothermal holding temperature [°C] or completion temperature of secondary cooling [°C]

[0146] T3 : Completion temperature of tertiary cooling [°C]

[0147] T4 : Homogenization temperature [°C]

[0148] [Table 3]

[0149]

[0150] Phase A: Retained austenite

[0151] [Table 4]

[0152]

[0153] As shown in Tables 1 to 4 above, it can be seen that in Invention Examples 1 to 7 that all satisfy the alloy composition and manufacturing conditions proposed by the present invention, it includes a surface layer fine structure with an average thickness of 100 μm or more and 30% or less of the total thickness from its steel surface.

[0154] In addition, it can be confirmed that, by area%, the fine structure present in the surface layer portion includes: a total of 85.0 - 96.5% of ferrite and bainite, 3.5 - 15.0% of retained austenite, and martensite within 3.0%. By weight%, the average carbon content in the retained austenite satisfies 1.10 - 1.40%, and excellent formability can be ensured at normal temperature. In addition, by area%, it can be confirmed that the deep layer portion includes a total of 85.0 - 96.5% of ferrite and bainite, and by weight%, includes 3.5 - 15.0% of retained austenite, and the average carbon content in the retained austenite is 0.80 - 1.10%, and it also has excellent warm forming properties.

[0155] In contrast, in Comparative Example 1, since the content of C is 0.06% or less, sufficient retained austenite cannot be ensured. Although the drawability at normal temperature is good, the strength of 590 MPa or more and the elongation of 30% or more cannot be ensured during warm forming.

[0156] In Comparative Example 2, when Si is less than 1.2%, pearlite is formed during coiling, so that a sufficient amount of austenite cannot be ensured. As a result, drawability at normal temperature and strength of 590 MPa or more and elongation of 30% or more during warm forming cannot be ensured.

[0157] In Comparative Example 3, since the ferrite fraction during secondary cooling is too high and the coiling temperature is low, bainite transformation does not proceed smoothly after coiling, and most of the austenite is transformed into martensite. As a result, the drawability is poor, and an elongation of 30% cannot be ensured during warm forming.

[0158] In addition, in the cases of Comparative Examples 1 to 3 described above, the retained austenite size in the surface layer portion and the deep layer portion is too small and less than 0.2 μm, and the size is also fine, so that the equivalent circle diameter of the retained austenite cannot be reliably measured.

[0159] In Comparative Example 4, since the cooling rate during tertiary cooling is too fast, as a result, the drawability at normal temperature is excellent, but a sufficient elongation cannot be ensured during warm tensile testing.

[0160] In Comparative Example 5, the cooling rate during tertiary cooling is slow. As a result, the warm formability is excellent, but sufficient formability cannot be ensured during cold forming.

[0161] In addition, Figure 1 Fig. (a) is a photograph obtained by observing, through backscattered electrons, the microstructure of Invention Example 2 after mounting the microstructure on a scanning electron microscope.

[0162] Figure 1 The microstructure at a depth of 50 μm in the depth direction from the surface of the surface layer portion of Invention Example 2 is shown in (a). The average equivalent circle diameter of the retained austenite is measured to be 1.4 μm. In addition, the average carbon content in the retained austenite contained in the surface layer portion, which is measured by measuring the austenite lattice constant of X-ray diffraction and calculating, is 1.23 wt%.

[0163] Figure 1 Fig. (b) shows the deep layer microstructure, which is the microstructure of the deep layer portion (in the present invention, the thickness center portion corresponds to (1 / 2t)) of Invention Example 2. The average value of the average equivalent circle diameter of the retained austenite contained in the deep layer portion is 2.5 μm, and the average carbon content in the retained austenite contained in the deep layer portion measured by X-ray diffraction is 1.02 wt%. It can be confirmed that the surface layer portion is cooled below Ms, and due to the finely distributed austenite, the carbon content of the surface layer portion is high. On the other hand, in the deep layer portion maintained at a temperature above Ms, the retained austenite is coarsely distributed, and the carbon content distributed in the deep layer portion is low.

[0164] As described above, the preferred embodiments of the present invention have been described in the detailed description of the present invention, but those skilled in the art can make various modifications without departing from the scope of the present invention. Therefore, the scope of the rights of the present invention is not limited to the described embodiments and should be determined by the claims and their equivalents.

Claims

1. A hot-rolled steel sheet, by weight %, the hot-rolled steel sheet contains: carbon (C): 0.06 - 0.18%, silicon (Si): 1.2 - 2.5%, manganese (Mn): 0.80 - 2.50%, aluminum (Al): 0.001 - 0.100%, phosphorus (P): 0.0001 - 0.0500%, sulfur (S): 0.0001 - 0.0500%, nitrogen (N): 0.0001 - 0.0200%, the balance of Fe and other inevitable impurities, By weight %, the average carbon content in the retained austenite contained in the surface layer is 1.10 - 1.40%.

2. The hot-rolled steel sheet according to claim 1, wherein, By area %, the microstructure of the surface layer contains: the sum of ferrite and bainite: 85.0 - 96.5%, retained austenite: 3.5 - 15.0% and martensite: 3.0% or less and including 0%.

3. The hot-rolled steel sheet according to claim 1, wherein, By weight %, the hot-rolled steel sheet further contains one or more selected from 0.01 - 2.00% of chromium (Cr), 0.01 - 2.00% of molybdenum (Mo), 0.01 - 0.20% of titanium (Ti) and 0.01 - 0.10% of niobium (Nb).

4. The hot-rolled steel sheet according to claim 1, wherein, The average carbon content in the retained austenite contained in the deep layer is less than the average carbon content in the retained austenite contained in the surface layer.

5. The hot-rolled steel sheet according to claim 1, wherein, By weight %, the average carbon content in the retained austenite contained in the deep layer is 0.8% or more and less than 1.10%.

6. The hot-rolled steel sheet according to claim 1, wherein, By area %, the microstructure of the deep layer contains: the sum of ferrite and bainite: 85.0 - 96.5%, retained austenite: 3.5 - 15.0% and martensite: 5.0% or less and including 0%.

7. The hot-rolled steel sheet according to claim 1, wherein, The thickness of the surface layer is 30% or less of the thickness of the hot-rolled steel sheet.

8. The hot-rolled steel sheet according to claim 1, wherein, The average equivalent circle diameter of the retained austenite contained in the deep layer is greater than the average equivalent circle diameter of the retained austenite contained in the surface layer.

9. The hot-rolled steel sheet according to claim 1, wherein, The average equivalent circle diameter of the retained austenite contained in the surface layer is 0.2 - 2.0 μm.

10. The hot-rolled steel sheet according to claim 1, wherein, The average equivalent circle diameter of the retained austenite contained in the deep layer is more than 2.0 μm and 5.0 μm or less.

11. A method for manufacturing a hot-rolled steel sheet, which includes the following steps: Reheat the steel billet at 1050 - 1300 °C. By weight %, the steel billet contains: carbon (C): 0.06 - 0.18%, silicon (Si): 1.2 - 2.5%, manganese (Mn): 0.80 - 2.50%, aluminum (Al): 0.001 - 0.100%, phosphorus (P): 0.0001 - 0.0500%, sulfur (S): 0.0001 - 0.0500%, nitrogen (N): 0.0001 - 0.0200%, the balance of Fe and other inevitable impurities; Hot finish roll the reheated steel billet at 800 - 1150 °C as the final rolling temperature FDT; Cool the hot finish rolled steel sheet at an average cooling rate of 50 - 150 °C / second for the first cooling to a temperature T1 of 550 - 750 °C; After the first cooling, it is isothermally held for a time ts at a temperature T2 of 550 - 750°C, or secondarily cooled for a time ts at a cooling rate of 20°C / second or less (excluding 0°C / second), and cooled to a temperature T2 of 550 - 750°C lower than the T1; After the isothermal holding or secondary cooling, it is tertiary cooled at a cooling rate of 150°C / second or more, and cooled to a temperature T3 below the martensite start temperature Ms; After the tertiary cooling, it is air-cooled for 2 seconds or more, and the temperature in the plate thickness direction is equalized to T4; and After coiling the hot-rolled steel plate that has been air-cooled, it is quaternarily cooled to room temperature.

12. The manufacturing method of the hot-rolled steel sheet according to claim 11, wherein, The isothermal holding or secondary cooling step satisfies the following relational expression 1, [Relational expression 1] 70≤Vα≤90 In the relational expression 1, the Vα represents the fraction of ferrite formed during isothermal holding or secondary cooling.

13. The manufacturing method of the hot-rolled steel sheet according to claim 12, wherein, The Vα is defined by the following relational expression 2, [Relational expression 2] Vα = 100×(1 - exp(-k(T)×(ts) 1.5 )) In the relational expression 2, k(T) is an index representing the growth rate of ferrite, and is defined by the following relational expression 3, [Relational expression 3] In the relational expression 3, P is defined by the following relational expression 4, Du is an index representing the effective grain size of austenite before the first cooling after hot rolling, and is defined by the following relational expression 5, [Relational expression 4] P =633 - 529×[C] - (29 - 32×[C])×[Mn] + (70 - 86×[C])×[Si] - (10 - [C])×[Cr] - (15 + [C])×[Mo] In the relational expression 4, the [C], [Si], [Mn], [Cr], and [Mo] respectively represent the weight % contents of the elements in the brackets, [Relational expression 5] 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 5, the [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] respectively represent the weight % contents of the elements in the brackets, and FDT represents the rolling termination temperature (°C).

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