Hot-rolled steel sheet with excellent multi-stage press formability and its manufacturing method

Abstract

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

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet that can be used for automobile wheel discs and the like, and more particularly to a high-strength hot-rolled steel sheet that is excellent in multi-stage press formability and a method for producing the same. [Background technology]

[0002] Recently, society as a whole has been calling for the production of environmentally friendly products in order to reduce global warming. In the automotive industry, much effort has been put into developing technologies to reduce exhaust gases emitted while internal combustion engine vehicles are running. However, with the recent acceleration of the shift to electric vehicles, carbon reduction is being considered not only during vehicle operation but also throughout the entire life cycle, including the production and recycling processes of automobiles. To this end, there is a movement to regulate carbon emissions during the production and recycling of raw materials used in automobile manufacturing.

[0003] In the past, the use of lightweight materials such as aluminum has expanded to reduce carbon emissions while internal combustion engine vehicles are running. However, the proportion of aluminum used in car bodies, which has high carbon emissions during the manufacturing process, has recently been on the decline, while the proportion of steel used in car bodies, which has relatively low carbon emissions, is once again on the rise.

[0004] Among the components that make up an automobile, wheels are located in the path that transmits ground impact to the suspension, making them an important safety component that requires high fatigue durability. Meanwhile, as a component exposed to the exterior of the vehicle, wheels also require aesthetic appeal, including design, to stimulate the desire of vehicle buyers. Passenger vehicle wheel components are manufactured by casting aluminum alloy material, allowing for various shapes to be realized to ensure the aesthetics desired by customers, and durable vulnerable parts are cast thick to ensure fatigue durability.

[0005] Meanwhile, hot-rolled steel sheets, which have traditionally been used for wheel components, have excellent strength but lack formability, making it impossible to form wheel discs with complex shapes by press forming. As a result, passenger car wheels have primarily been made from cast aluminum alloy products. However, in order to reduce carbon from a lifecycle perspective, automakers are now demanding that wheel components be made from steel, which has necessitated the development of hot-rolled steel sheets that have superior formability compared to conventional steel products.

[0006] To maximize productivity, wheel discs are typically produced at high speeds using multi-stage press forming using a 7- to 10-stage continuous press. To improve wheel disc design and facilitate air cooling, a large number of holes must be drilled into the wheel disc. While still able to support the same level of fatigue load, the greater the hole area, the thicker the steel plate must be. Therefore, to meet the customer's required durability life, a steel plate with a tensile strength of 590 MPa typically requires a thickness of 3.5 mm or more. When continuously forming thick materials at high speed, the initial forming is performed at room temperature. However, as the forming process accumulates, the material temperature rises due to heat generation. Because thick materials cannot be cooled smoothly, the material temperature can rise to 70-90°C during forming. Therefore, when considering the formability of steel plates for wheels, it is necessary to consider not only the initial formability at room temperature but also the warm high-speed formability that takes into account the heat generation during forming.

[0007] In the field of steel materials, methods utilizing the plasticity-induced transformation phenomenon of retained austenite have been widely applied as a method for improving the formability of raw materials. Patent Document 1 presents a method for producing hot-rolled steel sheets containing 50% or more ferrite and 3% or more austenite by volume fraction in the steel in order to achieve both high levels of tensile strength and elongation. However, the above-mentioned Patent Document 1 only considers formability at room temperature and does not mention warm high-speed formability.

[0008] Patent Document 2 presents a manufacturing method for ensuring strength during high-speed deformation of a cold-rolled steel sheet having ferrite and / or bainite as the main phase and containing 3 to 50% by volume of retained austenite. However, Patent Document 2 only considers strength from the perspective of collision performance during high-speed forming, and does not mention formability.

[0009] Therefore, in order to manufacture environmentally friendly wheel parts, it is necessary to develop steel materials that are not only strong but also have excellent formability in multi-stage press processes with a variety of forming temperatures. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-030385 [Patent Document 2] Japanese Patent Application Laid-Open No. 1999-193439 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention aims to provide a hot-rolled steel sheet that is excellent in strength, room temperature formability, and warm formability at the same time, and a method for manufacturing the same.

[0012] The object of the present invention is not limited to the above-mentioned content. Anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the entire content of the specification of the present invention. [Means for solving the problem]

[0013] One aspect of the present invention is In weight percent, carbon (C): 0.06 to 0.18%, silicon (Si): 1.2 to 2.5%, manganese (Mn): 0.80 to 2.50%, aluminum (Al): 0.001 to 0.100%, phosphorus (P): 0.0001 to 0.0500%, sulfur (S): 0.0001 to 0.0500%, nitrogen (N): 0.0001 to 0.0200%, with the balance being Fe and other unavoidable impurities. The hot-rolled steel sheet has an average carbon content in the retained austenite contained in the surface layer portion of 1.10 to 1.40% by weight.

[0014] Furthermore, still another aspect of the present invention is a step of reheating a steel slab containing, by weight, 0.06-0.18% carbon (C), 1.2-2.5% silicon (Si), 0.80-2.50% manganese (Mn), 0.001-0.100% aluminum (Al), 0.0001-0.0500% phosphorus (P), 0.0001-0.0500% sulfur (S), 0.0001-0.0500% nitrogen (N), the balance being Fe and other unavoidable impurities, at 1050-1300°C; Finish hot rolling the reheated steel slab at a rolling finish temperature (FDT) of 800 to 1150°C; a step of primarily cooling the finish hot-rolled steel sheet to a temperature T1 of 550 to 750°C at an average cooling rate of 50 to 150°C / s; After the first cooling, maintaining the temperature at T2 of 550 to 750°C for a time ts, or performing a second cooling at a cooling rate of 20°C / s or less (excluding 0°C / s) to a temperature T2 of 550 to 750°C, which is lower than T1, for a time ts; After the isothermal holding or secondary cooling, a step of tertiary cooling at a cooling rate of 150°C / s or more to a temperature T3 below the temperature at which martensite formation begins; After the tertiary cooling, the temperature is uniformly maintained at T4 in the thickness direction by air cooling for 2 seconds or more; and The method for manufacturing a hot-rolled steel sheet further includes a step of coiling the air-cooled hot-rolled steel sheet and then fourthly cooling it to room temperature. [Effects of the Invention]

[0015] According to one aspect of the present invention, it is possible to provide a hot-rolled steel sheet that is excellent in strength, room temperature formability, and warm formability, and a method for manufacturing the same.

[0016] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0017] [Figure 1] Schematic diagrams for measuring the thickness of a deep layer of a hot-rolled steel sheet, where (a) is a schematic diagram of a line intersection length measurement for measuring the size distribution of austenite grains at a specific thickness position, (b) is a schematic diagram of the microstructure to be embodied in this steel type, and (c) is a schematic diagram showing the average and standard deviation of austenite grain size measured at a line intersection length at a specific thickness position. [Figure 2] Photographs of the microstructure of the steel sheet obtained from Example 2 of the present invention were taken using a scanning electron microscope (SEM) equipped with a backscattered electron microscope. (a) shows the microstructure at a depth of 100 μm from the surface of Example 2, and (b) shows the microstructure at a deeper layer of Example 2. The white areas in each microstructure photograph represent austenite. DETAILED DESCRIPTION OF THE INVENTION

[0018] The following describes preferred embodiments of the present invention. However, the embodiments of the present invention can be modified in various ways, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0019] It should be noted that the terms used in this specification are intended to describe specific embodiments and are not intended to limit the present invention. For example, singular expressions used in this specification include plural expressions unless the related definition clearly indicates otherwise. Furthermore, the meaning of "comprises" used in this specification embodies a configuration and does not exclude the presence or addition of other configurations.

[0020] The present inventors recognized that, in conventional high-strength hot-rolled steel sheets, it is possible to manufacture steel materials with excellent elongation measured at room temperature using the phenomenon of plasticity-induced transformation of retained austenite (TRIP), but formability under warm forming conditions is not taken into consideration. They conducted in-depth research to solve this problem.

[0021] The plasticity-induced transformation phenomenon is the principle behind the transformation of retained austenite to martensite when a material is deformed by external stress. This phase transformation increases the work hardening capacity of the steel, preventing localized deformation and improving formability. To optimize formability, the transformation of austenite to martensite must continue as the material deforms. However, if the stability of austenite is too low, the phase transformation will terminate early in the deformation process, resulting in poor elongation. On the other hand, if the stability of austenite is too high, the phase transformation will not occur, resulting in poor elongation. Therefore, when designing TRIP steel, it is important to consider both the fraction and stability of retained austenite to ensure the required formability.

[0022] Meanwhile, it is known that the stability of austenite is significantly affected by the internal carbon content and is sensitive to the temperature and rate at which deformation occurs. Generally, when the deformation temperature is high, a lower carbon content results in excellent formability, and the deformation rate is less sensitive to the temperature. Therefore, to ensure formability at high temperatures, a low carbon content in austenite is advantageous, but when considering formability at room temperature, a high carbon content in austenite is advantageous.

[0023] In order to manufacture a steel sheet with an ideal austenite carbon content, the inventors engineered a non-uniform structure in the thickness direction. As a result, in the press process where initial drawing is applied, the maximum amount of forming is applied to the surface layer, so high-carbon austenite is generated to ensure excellent room-temperature formability, and austenite with a low carbon content can be present inside the steel to ensure warm formability in the subsequent continuous press process.

[0024] The carbon content inside austenite is affected by the size of the austenite. When the austenite grain size is small, carbon is easily identified and austenite with a high carbon content exists. When the austenite grain size is large, carbon diffusion inside the austenite is difficult, so carbon concentration occurs slowly and austenite with a low average carbon content exists. Therefore, by making the size of the austenite grains in the surface and deep layers different, austenite with different carbon contents can be generated in each location.

[0025] To finely disperse the retained austenite in TRIP steel, the Quenching & Partitioning (Q&P) process is widely used. This involves cooling the steel sheet to a temperature below Ms and then reheating it to improve its strength, elongation, and bendability. However, because a separate heating device is required to reheat the steel sheet cooled below Ms, this process is difficult to apply to hot rolling processes, where rolling, cooling, and coiling are performed in a single process.

[0026] In the manufacturing process of hot-rolled steel sheets, after finish hot rolling, the hot-rolled steel sheet is cooled by cooling water poured into the top and bottom. During this process, the surface layer of the sheet is cooled by heat transfer with the cooling water, while the interior of the sheet is cooled by heat conduction. Generally, the rate of heat transfer in the surface layer is faster than the rate of heat conduction in the interior of the sheet, resulting in a temperature gradient in the thickness direction within the sheet. However, the inventors have discovered a phenomenon in which the surface layer of the sheet is cooled to below Ms, while the deeper layer of the sheet is maintained at a temperature above Ms. If the cooling water is removed at an appropriate time, the surface temperature of the steel sheet rises again due to heat transfer within the sheet.

[0027] That is, the inventors recognized that a steel sheet can be obtained that can ensure formability at both room temperature and warm conditions by cooling the surface layer of the steel sheet to below Ms and then heating it again to a temperature above Ms without providing a separate heating device, whereby austenite with a high carbon content containing finely dispersed retained austenite is present, and coarse austenite with a low carbon content is present in the deeper layer. This led to the completion of the present invention.

[0028] First, the alloy composition of the hot-rolled steel sheet of the present invention will be described. The high-strength hot-rolled steel sheet of the present invention, which is excellent in room-temperature and warm formability, contains, by weight, 0.06 to 0.18% carbon (C), 1.2 to 2.5% silicon (Si), 0.80 to 2.50% manganese (Mn), 0.001 to 0.100% aluminum (Al), 0.0001 to 0.0500% phosphorus (P), 0.0001 to 0.0500% sulfur (S), 0.0001 to 0.0200% nitrogen (N), with the balance being Fe and other unavoidable impurities.

[0029] Hereinafter, the alloy composition of the high-strength hot-rolled steel sheet having excellent bendability and elongation according to the present invention and the reasons for limiting the content thereof will be described in detail. Hereinafter, the content of each element means wt% unless otherwise specified.

[0030] Carbon (C): 0.06~0.18% Carbon (C) is an important element that diffuses into austenite during the bainite phase transformation to stabilize the austenite, thereby forming retained austenite. As the C content increases, the fraction of retained austenite increases, resulting in improved elongation and tensile strength. If the C content is less than 0.06%, the fraction of retained austenite is low, making it difficult to ensure the required elongation and tensile strength. On the other hand, if the C content exceeds 0.18%, the Ms temperature becomes excessively low, the tensile strength becomes excessively high, and formability and weldability become poor. Therefore, in the present invention, the C content is preferably 0.08 to 0.18%. More preferably, it can be contained in a range of 0.08 to 0.15%.

[0031] Silicon (Si): 1.2-2.5% Silicon (Si) is an important element that delays the formation of carbides during bainite transformation and forms retained austenite. Si also plays a role in improving strength through solid solution strengthening. If the Si content 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, if the Si content exceeds 2.5%, Fe-Si composite oxides are formed on the surface of the slab during reheating, which not only deteriorates the surface quality of the steel sheet but also reduces weldability. Therefore, in the present invention, the Si content is preferably 1.2 to 2.5%. To further improve the above-mentioned effects, the lower limit of the Si content may be 1.8%, or the upper limit of the Si content may be 2.2%.

[0032] Manganese (Mn): 0.80-2.50% Manganese (Mn) is an element that improves the hardenability of steel, prevents the excessive formation of granular ferrite during cooling after finish rolling, and facilitates the formation of bainite and retained austenite.

[0033] If the Mn content is less than 0.80%, the hardening ability is insufficient, and the fraction of granular ferrite increases rapidly during cooling, making it difficult to control the slow cooling time at the T2 temperature. On the other hand, if the Mn content exceeds 2.50%, the ferrite growth rate becomes too slow, and the time required for the slow cooling section exceeds the time that can be controlled by the equipment. Therefore, the Mn content in the present invention is preferably 0.80 to 2.50%, and more preferably 1.00 to 2.00%.

[0034] Aluminum (Al): 0.001 to 0.100% Aluminum (Al) is an element added for deoxidation, and some of it remains in the steel after deoxidation. If the Al content exceeds 0.100%, it increases oxide and nitride inclusions in the steel, deteriorating the formability of the steel sheet. On the other hand, if the Al content is excessively reduced to less than 0.001%, it will result in an unnecessary increase in refining costs. Therefore, the Al content in the present invention is preferably 0.001 to 0.100%.

[0035] Phosphorus (P): 0.0001 to 0.0500% Phosphorus (P) is an unavoidable impurity and is the main cause of reduced workability of steel due to segregation, so it is preferable to control its content as low as possible. In theory, it is advantageous to limit the phosphorus content to 0%, but reducing the P content to less than 0.0001% would excessively increase production costs. Therefore, the P content in the present invention is preferably 0.0001 to 0.0500%.

[0036] Sulfur (S): 0.0001 to 0.0500% Sulfur (S) is an unavoidable impurity that combines with Mn and other elements to form nonmetallic inclusions, which are the primary cause of reduced workability in steel. Therefore, it is preferable to control its content as low as possible. In theory, it is advantageous to limit the S content to 0%, but reducing the S content to less than 0.0001% would result in excessive increases in manufacturing costs. Therefore, the S content in the present invention is preferably 0.0001 to 0.0500%.

[0037] Nitrogen (N): 0.0001 to 0.0200% Nitrogen is an unavoidable impurity that reacts with aluminum to precipitate fine nitrides, reducing the workability of steel. Therefore, it is preferable to control its content as low as possible. Theoretically, it is advantageous to limit the N content to 0%, but reducing the N content to less than 0.0001% would excessively increase manufacturing costs. Therefore, the N content in the present invention is preferably 0.0001 to 0.0200%.

[0038] In the present invention, in addition to the above compositional components, one or more of chromium (Cr): 0.01 to 2.00%, molybdenum (Mo): 0.01 to 2.00%, titanium (Ti): 0.01 to 0.20%, and niobium (Nb): 0.01 to 0.10% may be selectively further contained. The content of each component as a selective additional element and the reasons for limiting the content will be specifically explained below.

[0039] Chromium (Cr): 0.01 to 2.00% Chromium (Cr) is an element that improves the hardenability of steel, slowing the formation of ferrite during cooling after finish rolling and facilitating the formation of austenite. If the Cr content is less than 0.01%, the effect of adding Cr cannot be fully achieved. On the other hand, if the Cr content exceeds 2.00%, the phosphate treatability of the steel sheet deteriorates. Therefore, the Cr content in the present invention is preferably 0.01 to 2.00%, and more preferably 0.10 to 1.50%.

[0040] Molybdenum (Mo): 0.01 to 2.00% Molybdenum (Mo) is an element that improves the hardenability of steel and plays a role in improving strength through solid solution strengthening. If the Mo content is less than 0.01%, the additive effect of suppressing ferrite formation during cooling after finish rolling cannot be fully obtained. On the other hand, if the Mo content exceeds 2.00%, problems arise such as deterioration of weldability and excessive increase in cost. Therefore, the Mo content in the present invention is preferably 0.01 to 2.00%, more preferably 0.05 to 1.00%.

[0041] Titanium (Ti): 0.01 to 0.20% Titanium (Ti) is an element that forms carbonitrides. It refines austenite grains by delaying recrystallization during hot rolling, accelerating ferrite transformation, and improving strength by refining ferrite grains. If the Ti content is less than 0.01%, the effect of addition is insufficient. On the other hand, if the Ti content exceeds 0.20%, coarse carbonitrides are formed, reducing the toughness of the steel sheet. Therefore, in the present invention, to further improve physical properties while still obtaining the above-mentioned effects of adding Ti, the Ti content may be set to 0.01 to 0.20%. On the other hand, to further improve the above-mentioned effects, the lower limit of the Ti content may be 0.02%, or the upper limit of the Ti content may be 0.10%.

[0042] Niobium (Nb): 0.01 to 0.10% Niobium (Nb) is an element that forms carbonitrides similar to Ti. When added, it refines austenite grains by delaying recrystallization during hot rolling, promoting ferrite transformation, and refines ferrite grains to improve strength.

[0043] If the Nb content is less than 0.01%, the effect of addition is insufficient, while if the Nb content exceeds 0.10%, coarse carbonitrides are formed, reducing the toughness of the steel sheet and increasing the rolling load during rolling, deteriorating workability. Therefore, in the present invention, the Nb content is preferably 0.01 to 0.10%. On the other hand, to further improve the above-mentioned effects, the Nb content may be 0.01 to 0.05%.

[0044] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, the contents of all of them will not be specifically mentioned in this specification.

[0045] Although not particularly limited, according to one aspect of the present invention, the above-mentioned hot-rolled steel sheet has a microstructure in the surface layer that includes, in area percentages, 85.0 to 96.5% of ferrite and bainite combined, 3.5 to 15.0% of retained austenite, and 3.0% or less (including 0%) of martensite.

[0046] According to one aspect of the present invention, the microstructure of the surface layer may contain a total fraction of ferrite and bainite in the range of 85.0 to 96.5%. Because aluminum alloys have a lower specific gravity but also lower strength than steel, manufacturing wheels using steel plates with a tensile strength of 590 MPa or higher can ensure a part weight similar to that of aluminum alloy wheels. Therefore, if the elongation can be maximized while satisfying a tensile strength of 590 MPa or higher, it is possible to manufacture environmentally friendly wheel parts with similar weight and design to aluminum alloy wheels at low manufacturing costs. In the present invention, formability is improved by controlling the phase stability and fraction of retained austenite and the fractions of ferrite and bainite, which are the matrix structure. Ferrite transformation occurs during slow cooling or isothermal holding at a temperature range of 550 to 750°C after primary cooling after hot rolling, forming the matrix structure. During this process, carbon diffuses into austenite, gradually lowering the Ms temperature, the martensite formation temperature, as ferrite grows. In the present invention, the coiling temperature is set to a temperature at which the surface layer, which has been supercooled to below Ms, and the deeper layer reach thermal equilibrium after the tertiary cooling. Therefore, if the ferrite fraction is too low and the Ms temperature is too high, carbides are formed in the structure, resulting in a decrease in the fraction of retained austenite. Therefore, the ferrite fraction is preferably 70% or more. On the other hand, if the ferrite fraction is too high, the Ms temperature becomes too low, preventing smooth carbon diffusion during bainite transformation, resulting in a decrease in the fraction of austenite, which can then transform into martensite in the final cooling stage. Therefore, the fraction of ferrite formed during the primary cooling is preferably 90% or less.

[0047] As mentioned above, martensite nucleation and growth occur immediately after the surface layer is cooled below Ms. However, after cooling, the temperature rises again above Ms due to heat transfer within the steel sheet, causing the sheet to re-enter the bainite transformation temperature range before the martensitic transformation is complete. Therefore, the martensite formed immediately after cooling is tempered and exists as tempered martensite, while bainitic ferrite grows within the untransformed austenite below Ms. Because tempered martensite and bainitic ferrite present in the surface layer share a lath morphology and numerous potentials within their structures, they are difficult to distinguish microstructurally. Because they also have similar effects on physical properties, they are managed as bainite without being classified separately. The fraction of bainite formed after coiling is determined by the austenite fraction immediately after tertiary cooling and the maximum carbon content that can dissolve in austenite, which is determined by the coiling temperature. Since its effect on the physical properties of steel sheets is smaller than that of ferrite and retained austenite, it is efficient to manage the sum of the fractions of ferrite and bainite. If the total of ferrite and bainite is less than 85.0%, the carbon content that must be added to the steel to ensure stable austenite becomes too high, which may impair the weldability of the steel.If the total of ferrite and bainite exceeds 96.5%, the fraction of retained austenite cannot be sufficiently secured, resulting in poor formability.

[0048] According to one aspect of the present invention, the microstructure of the surface layer of the present invention can contain 3.5 to 15% by area of ​​retained austenite. Retained austenite plays an important role in improving the formability of steel, and if the fraction of retained austenite is less than 3.5%, the elongation of the steel deteriorates. On the other hand, if the fraction of retained austenite exceeds 15%, an excessive amount of C must be added, which results in an excessive increase in the strength of the steel sheet and poor weldability.

[0049] In this case, the average carbon content in the retained austenite contained in the surface layer portion preferably falls within the range of 1.10 to 1.40% by weight. If the average carbon content in the retained austenite contained in the surface layer portion is less than 1.10%, plasticity-induced transformation to martensite occurs in the early stage of deformation at room temperature, and improvement in formability cannot be expected. On the other hand, if the average carbon content in the retained austenite contained in the surface layer portion exceeds 1.40%, the stability becomes too high and plasticity-induced transformation does not occur even when sufficient deformation is performed, so improvement in formability cannot be expected and formability at room temperature deteriorates.

[0050] In the present invention, the surface layer portion refers to a region located on the surface of the hot-rolled steel sheet in the thickness direction.

[0051] According to one aspect of the present invention, the surface layer and the deep layer of the hot-rolled steel sheet can be distinguished based on the change in the size of retained austenite. The method for distinguishing the surface layer and the deep layer is not particularly limited, but can be, for example, distinguished by the following method. Specifically, the austenite structure is first distinguished through Répera etching over the entire thickness of the steel sheet, and then the deep layer and the surface layer can be distinguished using a structural photograph taken under an optical microscope at 1,000 magnifications.

[0052] That is, according to one aspect of the present invention, the surface layer and the deep layer can be distinguished based on the difference in the size (e.g., equivalent circle diameter) of the retained austenite. Since it is important to determine the equivalent circle diameter of the retained austenite at each thickness position of the hot-rolled steel sheet, the average and standard deviation of the equivalent circle diameter of the retained austenite at each thickness position were measured using the method shown in FIG. 1(a). To measure the average and standard deviation of the retained austenite sizes (e.g., equivalent circle diameters) marked A to D, each of which has a different size and crosses a dotted line indicating a specific thickness position, the length at which each retained austenite intersects with the dotted line was measured, and the arithmetic mean and standard deviation were calculated based on the number of intersecting retained austenite.

[0053] Furthermore, by starting from a specific position from the surface as shown in FIG. 1(b) and calculating the average and standard deviation at each position at equal intervals, the average and standard deviation of the retained austenite grain size can be expressed as a function of distance from the surface as shown in FIG. 1(c). From these measurement results, the surface layer of the hot-rolled steel sheet according to the present invention is characterized by a low average and standard deviation due to the fine and uniform dispersion of the retained austenite grain size. In contrast, the average and standard deviation increase sharply in the deeper layer of the hot-rolled steel sheet according to the present invention due to the presence of coarse retained austenite grains. The position where the average and standard deviation of the retained austenite grain size (e.g., equivalent circle diameter) increase sharply was defined as the surface layer. However, the line intersection method described above is suitable for measuring the depth of the surface layer because it can easily measure the size (e.g., equivalent circle diameter).

[0054] However, because there are limitations to the accuracy of measuring the fraction and size (e.g., equivalent circle diameter) of the structure, the microstructure of the surface and deep layers and the average carbon content in the retained austenite can be defined using the following method. The measurement method is not particularly limited. For example, the microstructure of the surface layer can be analyzed at a position 50 μm from the surface using a repelling etching method, and the fractions of ferrite and bainite can be determined from the results of analysis at 1000x magnification using an optical microscope and an image analyzer. The fraction of retained austenite and the equivalent circle diameter (μm) of austenite are measured using the same method. The average carbon content (wt%), Cγ, in the retained austenite was calculated using the following equation (1) by X-ray diffraction analysis. The deep layer can also be measured using the same method as the surface layer, and the measurement method is not particularly limited. As an example, for the deep layer, the microstructure fraction at the center of the steel plate thickness (1 / 2t), the circle equivalent diameter (μm) of the retained austenite, and the average carbon content (wt%) in the retained austenite were analyzed using the same method as for the surface layer. [Formula 1] aγ=3.578+0.033×[Cγ]+0.0095×[Mn]-0.00124×[Si] (In the above formula 1, aγ is the lattice constant (Angstroms) of austenite calculated by X-ray diffraction analysis, and [Mn] and [Si] are the weight contents.)

[0055] Although not particularly limited, according to one embodiment of the present invention, the average equivalent circle diameter of the retained austenite contained in the surface layer portion may be 0.2 to 2.0 μm, more preferably 0.5 μm or more, or 1.8 μm or less. If the average equivalent circle diameter of the retained austenite contained in the surface layer portion is less than 0.2 μm, the phase stability increases sharply and the internal carbon content is often high, so that even if sufficient deformation is performed, plasticity-induced transformation does not occur, and it may be difficult to expect improvement in formability. On the other hand, if the average equivalent circle diameter of the retained austenite in the surface layer portion exceeds 2.0 μm, the carbon diffusion distance increases, making it difficult to ensure an average carbon content of 1.10% or more in the retained austenite.

[0056] The average equivalent circle diameter of the retained austenite contained in the deep layer portion may be larger than the average equivalent circle diameter of the retained austenite contained in the surface layer portion.

[0057] According to one aspect of the present invention, the microstructure of the deep layer portion can include, by area percentage, a total of ferrite and bainite: 85.0 to 96.5%, retained austenite: 3.5 to 15.0%, and martensite: 5.0% or less (including 0%). Ferrite has the same fraction as the surface layer portion because it is formed in the secondary cooling stage where the temperature in the thickness direction is uniform, and bainite can also have a fraction similar to that of the surface layer portion because it is formed after coiling where the temperature in the thickness direction is uniform.

[0058] According to another aspect of the present invention, the average carbon content of the retained austenite in the deep layer may be lower than the average carbon content of the retained austenite in the surface layer. Specifically, the average carbon content of the retained austenite in the deep layer may be 0.80% or more and less than 1.10%. In the hot-rolled steel sheet according to the present invention, the surface layer temperature is instantaneously cooled to or below Ms and then raised again, resulting in a fine distribution of austenite and smooth carbon enrichment. Meanwhile, the deep layer temperature is maintained at a bainite transformation temperature above Ms before coiling. This results in coarse austenite, increasing the time required for carbon diffusion, and resulting in a lower carbon content within the austenite after final cooling compared to the surface layer. In this case, if the average carbon content of the retained austenite in the deep layer is less than 0.8%, plasticity-induced transformation to martensite may occur in the early stages of deformation during warm forming, preventing improvement in formability. On the other hand, if the average carbon content in the retained austenite in the deep layer is 1.1% or more, the stability is too high and plasticity-induced transformation does not occur even if sufficient deformation is performed, so that improvement in formability cannot be expected and warm formability may deteriorate.

[0059] According to one aspect of the present invention, the average equivalent circle diameter of the retained austenite contained in the deep layer may be more than 2.0 μm and not more than 5.0 μm, more preferably not less than 2.2 μm, or not more than 3.0 μm. If the average equivalent circle diameter of the retained austenite contained in the deep layer is 2.0 μm or less, carbon concentration within the austenite may proceed excessively, resulting in a problem of the average carbon content of the retained austenite in the deep layer being 1.10% or more. On the other hand, if the average equivalent circle diameter of the retained austenite contained in the deep layer exceeds 5.0 μm, the distance required for carbon diffusion increases, making it difficult to ensure an internal carbon content of 0.80% or more of the retained austenite contained in the deep layer.

[0060] According to one embodiment of the present invention, the average thickness t of the hot-rolled steel sheet may be 1.5 to 12.0 mm. If the average thickness of the hot-rolled steel sheet is less than 1.5 mm, heat exchange in the thickness direction is easy, making it difficult to ensure a dual structure between the deep layer and the surface layer. If the average thickness of the hot-rolled steel sheet exceeds 12.0 mm, it may be difficult to use the hot-rolled steel sheet for wheel components.

[0061] Meanwhile, although not particularly limited, according to one embodiment of the present invention, the average thickness of the surface layer portion may be 100 μm or more, and may be 30% or less of the average thickness of the hot-rolled steel sheet, although this may vary depending on the average thickness of the hot-rolled steel sheet. If the average thickness of the surface layer portion is less than 100 μm, the effect of improving formability at room temperature is minimal. Furthermore, if the average thickness of the surface layer portion exceeds 30% of the overall average thickness of the hot-rolled steel sheet, after heat transfer occurs within the steel sheet, it is difficult for the surface layer temperature to be restored to a temperature above Ms. As a result, the steel sheet is coiled at a temperature below Ms, resulting in poor shape and poor formability during the warm-forming stage, which involves a large amount of deformation. Meanwhile, to further improve the above effect, 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%. In this case, the surface layer portion may be provided on both surfaces of the hot-rolled steel sheet. In this case, the average thickness of the surface layer portion mentioned above means the sum of the average thicknesses of the surface layer portions measured from both surfaces in the thickness direction of the steel plate.

[0062] Meanwhile, according to one aspect of the present invention, if the coiling temperature is too low, carbon diffusion is insufficient, and some regions may transform into martensite during the final cooling stage to room temperature. While such martensite plays a role in improving strength, excessive formation of martensite results in a decrease in the fraction of retained austenite, resulting in poor formability. In the present invention, there is no need to set a lower limit for the martensite fraction in the deep layer. However, if the fraction exceeds 5%, the elongation may deteriorate, so it is preferable to control it to 5% or less.

[0063] According to one aspect of the present invention, the present invention having the above-described alloy composition and microstructure can provide a high-strength hot-rolled steel sheet having excellent room-temperature and warm-temperature formability, which has a tensile strength of 590 MPa or more, a room-temperature drawing ratio of 2.0 or more, and an elongation measured in the range of 70 to 90°C of 30% or more.

[0064] According to yet another aspect of the present invention, there is provided a high-strength hot-rolled steel sheet having excellent room-temperature and warm formability, which has a tensile strength of 590 MPa or more, a room-temperature drawing ratio of 2.0, and an elongation of 30% or more in a warm tensile test at 80°C.

[0065] Next, a method for producing a high strength hot rolled steel sheet excellent in bendability and elongation, which is yet another aspect of the present invention, will be described in detail.

[0066] steel slab reheating In the present invention, before hot rolling, the steel slab is reheated and homogenized, preferably at a temperature of 1050 to 1300°C. If the reheating temperature is less than 1050°C, the homogenization of the alloying elements is insufficient. On the other hand, if the temperature exceeds 1300°C, excessive oxides are formed on the slab surface, which reduces the surface quality of the steel sheet, which is undesirable.

[0067] hot rolling Next, the reheated steel slab is hot rolled to produce a hot-rolled steel sheet. At this time, the finish hot rolling temperature (FDT), which is the temperature of the hot-rolled sheet immediately after finish hot rolling, is controlled to a range of 800 to 1150°C.

[0068] If the FDT temperature during hot rolling is higher than 1150°C, oxides are formed excessively on the surface of the steel sheet after rolling, and cannot be effectively removed even after pickling, resulting in poor surface quality.On the other hand, if the FDT temperature is lower than 800°C, the rolling load increases excessively, resulting in poor workability.

[0069] In this case, it is preferable to control the total reduction in the last two passes of hot rolling within the range of 10 to 40%. The main reason for performing multi-stage hot rolling is to reduce the rolling load and precisely control the thickness, so if the total reduction in the last two passes exceeds 40%, the rolling load in the last two passes increases excessively, resulting in poor workability. On the other hand, if the total reduction in the last two passes is less than 10%, the temperature of the steel sheet will drop rapidly, which may result in poor workability.

[0070] Cooling Phase The above finish hot-rolled steel sheet is primarily cooled to a temperature T1 of 550 to 750°C at an average cooling rate of 50 to 150°C.

[0071] After the above primary cooling, the material is either isothermally maintained at a temperature T2 (unit: °C) of 550 to 750°C for a time ts (unit: seconds (sec)), or secondary cooling is performed to a temperature T2 (unit: °C) lower than T1 above, of 550 to 750°C, at an average cooling rate of 20°C / s or less (excluding 0°C / s), for a time ts (unit: seconds (sec)).

[0072] During the isothermal holding or secondary cooling, ferrite is generated to form a matrix structure, carbon is concentrated in austenite, and the Ms temperature of the steel gradually decreases. At this time, the area percentage of ferrite generated during the secondary cooling is preferably 70 to 90% so that the Ms temperature of the steel is between 250 and 450°C. For this purpose, it is preferable to control the temperature and time of the isothermal holding or secondary cooling according to the following relational expression 1. [Equation 1] 70≦Vα≦90 (In the above relational expression 1, Vα represents the fraction [area %] of ferrite generated during the above isothermal holding or secondary cooling.)

[0073] In this case, Vα can be defined by the following relational expression 2. [Equation 2] Vα=100×(1-exp(-k(T)×(ts) 1.5 )) (In the above relational expression 2, k(T) is an index showing the growth rate of ferrite and is defined by the following relational expression 3.) [Equation 3]

number

[0074] Since the end temperature of the tertiary cooling is set using the ferrite fraction generated in the secondary cooling, it is necessary to avoid ferrite transformation in the primary cooling stage for accurate calculation. Therefore, the average cooling rate of the primary cooling is preferably 50°C / s or more. On the other hand, if the average cooling rate during the primary cooling is excessive, a temperature difference between the surface and the deep layers may occur, so the average cooling rate of the primary cooling is preferably 150°C / s or less.

[0075] According to one aspect of the present invention, in order to realize the microstructure intended in the present invention, it is important that the tertiary cooling performed after the secondary cooling is rapid cooling at an average cooling rate of 150°C / s or more (or 150 to 250°C / s) so that the temperature of the surface layer satisfies the following Relational Formula 6. [Equation 6] Ms-100≦T3≦Ms-30 (In the above relational expression 6, T3 is the temperature [°C] of the steel sheet measured on the surface after the end of the tertiary cooling, and Ms is the martensite formation start temperature [°C] of the austenite present in the steel sheet after the end of the secondary cooling, and is defined by the following relational expression 7.) [Equation 7] Ms(℃)=550-(330×[C'])-(41×[Mn])-(20×[Si])-(20×[Cr])-(10×[Mo])+(30×[Al]) (In the above relational expression 7, [C'] represents a value that takes into account the concentration of carbon diffused in ferrite, and is defined by the following relational expression 8. Furthermore, the above [Mn], [Si], [Cr], [Mo], and [Al] each represent the weight percent content of the element in parentheses.) [Equation 8] [C']=0.9×([C]-Vα×0.02) / (1-Vα) (In the above relational formula 8, the above [C] represents the weight percent content of the element in parentheses, and the above Vα is as described above.)

[0076] According to one aspect of the present invention, the average cooling rate of the tertiary cooling may be 150°C / s or more (or between 150 and 250°C / s). If the average cooling rate of the tertiary cooling is less than 150°C / s, a sufficient thickness of the surface layer may not be ensured, and room-temperature formability may be deteriorated. Conversely, if the average cooling rate of the tertiary cooling exceeds 250°C / s, the thickness of the surface layer may be excessive, making it difficult to ensure warm formability.

[0077] Homogenization stage According to one aspect of the present invention, after the tertiary cooling, the temperature in the sheet thickness direction can be homogenized to T4 by air cooling for 2 seconds or more (the upper limit is not particularly limited), and T4 can be in the range of 200 to 400°C. Heat transfer within the steel sheet by air cooling in the homogenization step described above can homogenize the temperature in the thickness direction. The surface layer cooled to a temperature below Ms is reheated to a temperature above Ms by heat transferred from the deeper layer, facilitating carbon diffusion from martensite in an over-carbon-encapsulated state to austenite, thereby stabilizing the austenite.

[0078] Winding and final cooling stage Subsequently, in the present invention, the air-cooled hot-rolled steel sheet may be coiled and then fourthly cooled to room temperature. That is, the hot-rolled steel sheet homogenized to a temperature of T4 after the third cooling is coiled to produce a coil, and then cooled to room temperature.

[0079] Subsequently, in the present invention, the hot-rolled steel sheet that has been subjected to the final cooling (fourth cooling) can be selectively pickled and oiled to produce a PO (Pickled and Oiled) steel sheet.

[0080] Alternatively, the hot-rolled steel sheet that has been finally cooled can be pickled and then heated to a temperature range of 400 to 750°C to be hot-dip galvanized. [Example]

[0081] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are merely for illustrative purposes and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0082] (Example) A steel slab having the alloy composition shown in Table 1 below was prepared. Then, a hot-rolled steel sheet having a thickness of 4 mm was manufactured from the prepared steel slab under the manufacturing conditions shown in Table 2 below. In this case, the reheating temperature of the steel slab was 1150°C, the total reduction rate of the final two passes of finish rolling was 25%, and the average cooling rate of the primary cooling was 60°C / s.

[0083] For the tertiary cooling, the same amount of water was poured and the same speed at which the steel plate was moved was applied, and then the cooling speed of the steel plate was changed by changing the time for pouring water. The temperature of the surface of the steel plate measured after pouring water was completed was indicated as T3, and the temperature just before coiling (homogenization temperature) after 2 seconds or more had elapsed was indicated as T4.

[0084] The microstructure of each steel sheet manufactured as described above was observed, and the results are shown in Table 3 below. In Table 3 below, F represents ferrite, B represents bainite, P represents pearlite, and A represents austenite.

[0085] The thickness and proportion of the surface layer were measured and are shown in Table 3 below. The thickness of the surface layer was measured at 10 randomly selected points and the average value was used to measure the thickness of the surface layer. The proportion of the surface layer was calculated using the average thickness of the surface layer.

[0086] Furthermore, each steel plate manufactured as described above was processed into 200 mm and 160 mm diameter disks, which were then cup-formed using a 100 mm diameter punch to verify drawing formability at room temperature. If the 160 mm diameter disk was cup-formed without cracking, it was determined to satisfy a drawing ratio (DR) of 1.6, and if the 200 mm diameter disk was cup-formed without cracking, it was determined to satisfy a drawing ratio of 2.0. The results are shown in Table 4 with ○ and ×.

[0087] To verify warm formability, test pieces with a gauge width of 20 mm and a gauge length of 50 mm were prepared parallel to the rolling direction and held in a furnace maintained at 80°C for 1 hour to uniformly heat the specimen. Tensile tests were then conducted in the furnace at a deformation rate of 50 mm per minute to measure the yield strength (YS), tensile strength (TS), uniform elongation (U-El), and elongation (El), and the results are shown in Table 4. Specifically, the yield strength and tensile strength indicated the lower yield point and maximum tensile strength, respectively, and the elongation indicated the fracture elongation.

[0088] A drawing ratio of 2.0 for room temperature forming, a tensile strength of 590 MPa or more in warm tensile testing, and an elongation rate of 30% or more were judged to be good levels.

[0089] [Table 1]

[0090] [Table 2]

[0091] [Table 3]

[0092] [Table 4]

[0093] As shown in Tables 1 to 4 above, Invention Examples 1 to 7, which satisfy all of the alloy compositions and manufacturing conditions proposed in the present invention, contain a fine structure in the surface layer, whose average thickness from the surface of the steel is 100 μm or more and corresponds to 30% or less of the total thickness.

[0094] The microstructure present in the surface layer contains, in area percent, 85.0 to 96.5% ferrite and bainite combined, 3.5 to 15.0% retained austenite, and 3.0% or less martensite, with the average carbon content of the retained austenite being 1.10 to 1.40% by weight, confirming that excellent formability is ensured at room temperature.On the other hand, the deeper layer contains, in area percent, 85.0 to 96.5% ferrite and bainite combined, with the average carbon content of the retained austenite being 0.80 to 1.10% by weight, confirming that excellent warm formability is also ensured.

[0095] In contrast, in Comparative Example 1, the C content was 0.06% or less, and sufficient retained austenite could not be secured. Although the drawing formability at room temperature was good, a strength of 590 MPa or more and an elongation rate of 30% or more could not be secured during warm forming.

[0096] In Comparative Example 2, the Si content was less than 1.2%, and pearlite was generated during coiling, making it impossible to secure a sufficient amount of austenite. As a result, it was impossible to secure draw formability at room temperature, strength of 590 MPa or more, and elongation of 30% or more during warm forming.

[0097] In Comparative Example 3, the ferrite fraction during secondary cooling was excessively high and the coiling temperature was low, resulting in a smooth bainite transformation after coiling and most of the austenite being transformed into martensite. As a result, drawing deteriorated and the elongation of 30% during warm forming could not be secured.

[0098] On the other hand, in the case of the above-mentioned Comparative Examples 1 to 3, the fraction of retained austenite in the surface layer and deep layer was less than 0.2 μm, which was too small and the size was also very fine, so that the circle equivalent diameter of the retained austenite could not be measured clearly.

[0099] In Comparative Example 4, the cooling rate during the tertiary cooling was excessive, and as a result, although the drawing formability at room temperature was excellent, sufficient elongation could not be ensured during the warm tensile test.

[0100] In Comparative Example 5, the cooling rate during tertiary cooling was slow, and as a result, although the warm formability was excellent, sufficient formability could not be ensured during cold forming.

[0101] On the other hand, FIG. 1 is a photograph of the microstructure of Example 2 of the present invention, taken by observing backscattered electrons attached to a scanning electron microscope.

[0102] 1(a) shows the microstructure of Example 2 at a depth of 50 μm from the surface, where the average equivalent circle diameter of the retained austenite was measured to be 1.4 μm. Meanwhile, the average carbon content of the retained austenite contained in the surface layer, calculated by measuring the austenite lattice constant by X-ray diffraction, was 1.23 wt%.

[0103] Figure 1(b) shows the microstructure of the deep layer (corresponding to the center part of the thickness (1 / 2t) in the present invention) of Example 2. The average mean equivalent circle diameter of the retained austenite in the deep layer was 2.5 µm, and the average carbon content of the retained austenite in the deep layer measured by X-ray diffraction was 1.02 wt%. It can be seen that the surface layer was cooled below Ms and had a fine distribution of austenite with a high carbon content, whereas the deep layer, maintained at a temperature above Ms, had a coarse distribution of retained austenite with a low carbon content.

[0104] As described above, the detailed description of the present invention has been given of the preferred embodiments of the present invention, but it goes without saying that a person skilled in the art to which the present invention pertains can make various modifications without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined by the claims below as well as equivalents thereof.

Claims

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

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

3. 2. The hot-rolled steel sheet according to claim 1, further comprising, in weight percent, one or more selected from the group consisting of chromium (Cr): 0.01 to 2.00%, molybdenum (Mo): 0.01 to 2.00%, titanium (Ti): 0.01 to 0.20%, and niobium (Nb): 0.01 to 0.10%.

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

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

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

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

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

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

10. The hot-rolled steel sheet according to claim 1, wherein the retained austenite contained in the deep layer portion has an average equivalent circle diameter of more than 2.0 μm and not more than 5.0 μm.

11. a step of reheating a steel slab containing, in weight percent, 0.06-0.18% carbon (C), 1.2-2.5% silicon (Si), 0.80-2.50% manganese (Mn), 0.001-0.100% aluminum (Al), 0.0001-0.0500% phosphorus (P), 0.0001-0.0500% sulfur (S), 0.0001-0.0200% nitrogen (N), the balance being Fe and other unavoidable impurities, at 1050-1300°C; Finish hot rolling the reheated steel slab at a rolling finish temperature (FDT) of 800 to 1150°C; a step of primarily cooling the finish hot-rolled steel sheet to a temperature T1 of 550 to 750°C at an average cooling rate of 50 to 150°C / s; After the primary cooling, isothermally maintaining the temperature T2 at 550 to 750°C for a time ts, or secondary cooling to a temperature T2 lower than T1, 550 to 750°C, at a cooling rate of 20°C / s or less (excluding 0°C / s) for a time ts; After the isothermal holding or secondary cooling, tertiary cooling is performed at a cooling rate of 150°C / s or more to a temperature T3 that is equal to or lower than the temperature Ms at which martensite formation begins; After the tertiary cooling, the plate is air-cooled for 2 seconds or more to homogenize the temperature in the thickness direction to T4; and The method for manufacturing a hot-rolled steel sheet further comprises a step of coiling the air-cooled hot-rolled steel sheet and then fourthly cooling it to room temperature.

12. The method for manufacturing a hot-rolled steel sheet according to claim 11, wherein the isothermal holding or secondary cooling step satisfies the following relational expression 1: [Relationship 1] 70≦Vα≦90 (In the above-mentioned relational expression 1, Vα represents the fraction of ferrite generated during isothermal holding or secondary cooling.)

13. The method for producing a hot-rolled steel sheet according to claim 12, wherein Vα is defined by the following relational expression 2: [Relationship 2] Vα=100×(1-exp(-k(T)×(ts) １．５ )) (In the above-mentioned relational expression 2, k(T) is an index showing the growth rate of ferrite and is defined by the following relational expression 3.) [Relationship 3] [Equation 1] (In the above-mentioned relational expression 3, P is defined by the following relational expression 4, and Du is an index showing the effective grain size of austenite immediately before primary cooling after hot rolling, and is defined by the following relational expression 5.) [Relationship 4] P=633-529×[C]-(29-32×[C])×[Mn]+(70-86×[C])×[Si]-(10-[C])×[Cr]-(15+[C])×[Mo] (In the above-mentioned relational expression 4, the [C], [Si], [Mn], [Cr], and [Mo] represent the weight percent contents of the elements in the parentheses.) [Relationship 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 above-mentioned relational expression 5, the [C], [Si], [Mn], [Cr], [Mo], [Ti] and [Nb] represent the weight percent contents of the elements in the parentheses, respectively, and FDT represents the rolling finish temperature (°C).)

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