Hot-rolled steel sheet and method of manufacturing the same

Abstract

This invention relates to a hot-rolled steel sheet for use in frames, structures, etc., and to a hot-rolled steel sheet and method thereof that ensures high hardness, strength and excellent processing properties.

Description

Technical Field

[0001] This invention relates to a hot-rolled steel sheet for use in frames, structures, etc., and to a hot-rolled steel sheet and method thereof that ensures high hardness, strength and excellent processing properties. Background Technology

[0002] Traditionally, high-hardness, wear-resistant hot-rolled steel sheets have primarily utilized a martensitic microstructure to achieve high strength and hardness. This high hardness translates to high wear resistance, making them suitable for components requiring wear resistance. However, the poor bending workability resulting from this high strength limits processing, thus limiting their use to roll forming or minimal processing.

[0003] This also acts as a similar limiting factor in high-strength hot-rolled steel sheets with martensite as the main phase, and various technologies have been proposed to overcome this.

[0004] Patent Document 1 proposes that when the hardness of martensite and tempered martensite in the microstructure at positions 200 μm and 100 μm from the surface is reduced compared to the center, excellent bending properties can be ensured in 1 GPa grade high-strength martensitic hot-rolled steel sheets. Furthermore, Patent Document 2 proposes that in 1.2 GPa grade martensitic steel, low-temperature impact toughness is improved through microstructure refinement, excellent flange formability is obtained by controlling precipitates, and high bending properties are obtained by controlling surface roughness.

[0005] However, Patent Document 1 struggles to ensure sufficient strength, as the changes in the central and surface portions of the steel plate are minimal, and it fails to control the microstructure and physical properties of the extremely surface portion below 100 μm. Therefore, the improvement in bendability can be considered minimal. Furthermore, Patent Document 2, which relies solely on surface roughness control to improve bendability relative to strength, has limitations. A certain level of surface roughness can typically be achieved through pickling processes alone, and strict manufacturing conditions are necessary to control precipitates, presenting drawbacks.

[0006] Furthermore, while alloying elements such as Si, Mn, Mo, Cr, Cu, and Ni, primarily used in the manufacture of such high-hardness steel, are effective in improving hardness and formability, the addition of large amounts of these alloying elements to enhance physical properties can lead to segregation of the alloying components and inhomogeneity of the microstructure, thereby worsening bending workability. In particular, steels with high hardenability are sensitive to changes in the microstructure during cooling, and the low-temperature phase transformation structure forms unevenly, making it even more difficult to achieve high bending workability.

[0007] (Patent Document 1) Korean Patent Publication No. 10-2023-0085173 (Patent Document 2) International Patent Publication No. WO2020-026593 Summary of the Invention

[0008] (a) Technical problems to be solved One aspect of the present invention aims to provide a hot-rolled steel sheet with excellent bending workability, strength and hardness, and a method for manufacturing the same.

[0009] The technical problems of this invention are not limited to the matters described above. Additional technical problems of this invention are described throughout the entire specification, and those skilled in the art to which this invention pertains can readily understand these additional technical problems from the content described in this specification.

[0010] (II) Technical Solution One aspect of the present invention relates to a hot-rolled steel sheet, comprising, by weight percent: C: 0.17-0.26%, Si: 0.01-0.5%, Mn: 0.4-2.2%, Cr: 0.005-0.6%, Mo: 0.005-0.3%, Nb: 0.001-0.01%, Ti: 0.005-0.08%, V: 0.005-0.05%, Al: 0.01-0.5%, P: 0.003-0.05%, S: 0.001-0.05%. The hot-rolled steel sheet contains 0.01% N, 0.001-0.01% B, 0.0005-0.003% Fe and other unavoidable impurities, and satisfies the following relationship 1: the surface microstructure from the surface to 10 μm contains 5-40% ferrite and 2-10% twinned martensite by area fraction, and the central microstructure contains more than 80% (inclusive) martensite and self-tempered martensite, and less than 20% (inclusive) pearlite and bainite by area fraction, or more of these two types.

[0011] [Relation 1] 30≤T≤100 T=([C] / 10) 0.5 ) (0.7 [Si]+1) (5 [Mn]+1) (2.5 [Cr]+1) (5 [Mo]+1) 25 In Equation 1, [C], [Si], [Mn], [Cr], and [Mo] represent the content (by weight %) of the corresponding alloying elements. These components are replaced with 0 if they were not intentionally added.

[0012] The aspect ratio of the original austenite in the central microstructure can be from 1 to 10.

[0013] The hardness of the central section of the hot-rolled steel plate can be above 400HV, and the tensile strength in the width direction can be above 1300MPa.

[0014] The bending property (R / t) of the hot-rolled steel sheet can be below 4.0.

[0015] The difference between the original bending and bending properties of the hot-rolled steel plate can be greater than 0.5.

[0016] Another aspect of the present invention relates to a method for manufacturing a hot-rolled steel sheet, comprising the following steps: heating a steel billet to a temperature range of 1150-1350°C, wherein the steel billet, by weight%, comprises: C: 0.17-0.26%, Si: 0.01-0.5%, Mn: 0.4-2.2%, Cr: 0.005-0.6%, Mo: 0.005-0.3%, Nb: 0.001-0.01%, Ti: 0.005-0.08%, V: 0.005-0.05%, Al: 0.01-0.5%, P: 0.003-0.05%, S: 0.001-0.01%, N: 0.001-0.0%. 1%, B: 0.0005-0.003%, balance Fe and other unavoidable impurities, the billet satisfies the following relationship 1; the heated billet is rough rolled at the roughing temperature (RDT) of the following relationship 2 to obtain a strip; the strip is finished rolled at the finishing temperature (FDT) of the following relationship 3 to obtain a hot-rolled steel plate; the hot-rolled steel plate is cooled to a primary cooling termination temperature of 150-350°C at a primary average cooling rate of 60-90°C / second; after the primary cooling, it is cooled to a coiling temperature (CT) of 50-200°C at a secondary average cooling rate of 1-50°C / second; and coiling is performed after the secondary cooling.

[0017] [Relation 1] 30≤T≤100 T=([C] / 10) 0.5 ) (0.7 [Si]+1) (5 [Mn]+1) (2.5 [Cr]+1) (5 [Mo]+1) 25 In Equation 1, [C], [Si], [Mn], [Cr], and [Mo] represent the content (by weight %) of the corresponding alloying elements. These components are replaced with 0 if they were not intentionally added.

[0018] [Relationship 2] RDT (°C) ≤ (FT + 80 + strip thickness) 3.8) FT=907+239[Al]-10.3[Cr]-25.8[Mn]+17.9[Mo]+30.9[Si]-254[C] In Equation 2, [Al], [Cr], [Mn], [Mo], [Si], and [C] represent the content (weight %) of the corresponding alloying elements. If these components are not intentionally added, they are replaced with 0. The thickness of the strip is mm.

[0019] [Relationship 3] FDT(°C)≥FT After obtaining the strip plate, the process may further include descaling and surface cooling of the entire width of the strip plate at a water pressure of 150 bar or more.

[0020] The thickness of the strip plate can satisfy the following relationship 4.

[0021] [Relationship 4] The thickness of the strip plate (mm) is greater than or equal to the thickness of the hot-rolled steel plate (mm). 10 The rough rolling can be carried out in a temperature range of 900-1100℃.

[0022] (III) Beneficial Effects According to one aspect of the present invention, a hot-rolled steel sheet with excellent bending formability and hardness, and a method for manufacturing the same, can be provided.

[0023] The various and beneficial advantages and effects of this invention are not limited to those described above, and can be more easily understood in the process of explaining the specific embodiments of this invention. Attached Figure Description

[0024] Figure 1 This is a graph showing the relationship between the central tensile strength (TS) and bending properties (R / t) of the inventive example and the comparative example in the embodiments. Best practice

[0025] The terminology used in this specification is for illustrative purposes and is not intended to limit the scope of the invention. Furthermore, the singular forms used herein include the plural forms unless the relevant definitions explicitly state otherwise.

[0026] The use of "including" or "contains" in this specification means to specify the constituent elements, without excluding the existence or addition of other constituent elements.

[0027] Unless otherwise defined, all terms used in this specification, including technical and scientific terms, shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms as defined in dictionaries shall be interpreted as having the same meaning as those in relevant technical literature and current disclosure.

[0028] To address the problems of existing technologies, the inventors of this invention conducted in-depth research on the changes in hardness and bendability of the central section and the original bendability after surface milling, based on the composition, manufacturing process, and microstructure characteristics of various steels with different compositions and microstructures. The results confirmed the correlation between the detailed characteristics of each constituent phase in the surface layer (surface portion) and the original bendability and the hardness and bendability of the central section. To achieve this, further research was conducted, exploring the steel composition, microstructure, and manufacturing methods of hot-rolled steel sheets, leading to this invention.

[0029] First, an example of the hot-rolled steel sheet of the present invention will be described in detail. The alloy composition and range of the hot-rolled steel sheet will be described below. Unless otherwise specified, the alloy composition content described below is expressed as a percentage by weight.

[0030] Carbon (C): 0.17-0.26% Carbon (C) is the most economical and effective element for strengthening steel, significantly influencing both strength and hardness. Increased C content enhances hardenability and facilitates the formation of hard phases such as bainite and martensite in the microstructure, thereby increasing tensile strength. Furthermore, since C is more readily enriched in austenite than ferrite, when ferrite forms on the surface, the C enriched in austenite provides a mechanism for transformation into twinned martensite during the martensitic phase transformation. However, when the C content exceeds 0.26%, the hardness of the martensite increases excessively, leading to an excessive increase in strength while simultaneously reducing bending workability and making it difficult to ensure sufficient weldability. Additionally, when the C content is less than 0.17%, sufficient strengthening effect is difficult to achieve. Therefore, the C content is preferably in the range of 0.17-0.26%. The lower limit of the C content is more preferably 0.175%, further more preferably 0.18%, and most preferably 0.185%. The upper limit of the C content is more preferably 0.25%, even more preferably 0.24%, and most preferably 0.23%.

[0031] Silicon (Si): 0.01-0.5% The Si is used to deoxidize the molten steel and exert a solid solution strengthening effect, and delays the formation of coarse carbides, which is beneficial to improving formability. When the Si content is less than 0.01%, the solid solution strengthening effect and the formability improvement effect cannot be fully obtained. On the other hand, when the Si content exceeds 0.5%, the red oxide scale formed on the surface of the steel plate during hot rolling is difficult to remove, which may result in very poor surface quality of the steel plate. In addition, there is also the problem of reduced ductility and weldability. Therefore, the Si content is preferably in the range of 0.01-0.5%. The lower limit of the Si content is more preferably 0.012%, further more preferably 0.015%, and most preferably 0.02%. The upper limit of the Si content is more preferably 0.4%, further more preferably 0.35%, and most preferably 0.3%.

[0032] Manganese (Mn): 0.4-2.2% Mn, like Si, is an effective element for solid solution strengthening of steel, increasing its hardenability and facilitating the formation of hard phases bainite and martensite during cooling after hot rolling. Furthermore, when ferrite forms on the surface, it concentrates within austenite, providing a mechanism for transformation into twinned martensite during the subsequent martensitic transformation. When the Mn content is less than 0.4%, sufficient solid solution strengthening and the formation of bainite and martensite cannot be achieved. On the other hand, when the Mn content exceeds 2.2%, grain boundaries become brittle, leading to problems such as low-temperature cracking. Moreover, excessive strength increases may compromise sufficient formability, and during continuous casting, significant segregation occurs in the center of the billet, resulting in uneven formation of fine microstructure along the thickness direction during cooling after hot rolling, thus worsening bending workability. In particular, it is difficult to uniformly create fine microstructure during cooling across the entire length and width of the hot-rolled steel sheet. Therefore, the Mn content is preferably in the range of 0.4-2.2%. The lower limit of the Mn content is more preferably 0.45%, further more preferably 0.5%, and most preferably 0.55%. The upper limit of the Mn content is more preferably 2.1%, further more preferably 2.05%, and most preferably 2.0%.

[0033] Chromium (Cr): 0.005-0.6% The Cr content provides solid solution strengthening to the steel, delays the ferrite phase transformation during cooling, and promotes the formation of martensite and bainite. When the Cr content is less than 0.005%, the solid solution strengthening and martensite and bainite formation effects cannot be fully achieved. On the other hand, when the Cr content exceeds 0.6%, similar to Mn, the segregation at the center of the thickness develops significantly, resulting in uneven microstructure in the thickness direction and reducing bending workability. Therefore, the Cr content is preferably in the range of 0.005-0.6%. The lower limit of the Cr content is more preferably 0.007%, further more preferably 0.008%, and most preferably 0.01%. The upper limit of the Cr content is more preferably 0.5%, further more preferably 0.45%, and most preferably 0.4%.

[0034] Molybdenum (Mo): 0.005-0.3% The Mo content increases the hardenability of the steel, facilitating the formation of martensite and bainite. When the Mo content is less than 0.005%, the aforementioned effects cannot be fully achieved. On the other hand, when the Mo content exceeds 0.3%, excessive hardenability leads to the formation of martensite on the surface, resulting in a sharp deterioration in bending workability, which is economically unfavorable and makes it difficult to ensure sufficient weldability. Furthermore, due to excessively high hardenability, ferrite cannot form on the surface. Therefore, the Mo content is preferably in the range of 0.005-0.3%. The lower limit of the Mo content is more preferably 0.01%, further more preferably 0.02%, and most preferably 0.03%. The upper limit of the Mo content is more preferably 0.25%, further more preferably 0.2%, and most preferably 0.15%.

[0035] Niobium (Nb): 0.001-0.01% Nb, along with Ti and V, is a representative precipitation strengthening element. During hot rolling, it precipitates as a precipitate, exerting a grain refinement effect through delayed recrystallization, effectively improving the strength and impact toughness of the steel. When the Nb content is less than 0.001%, the above-mentioned effects cannot be fully obtained. On the other hand, when the Nb content exceeds 0.01%, coarse composite precipitates are formed during hot rolling, resulting in poor bending workability. Furthermore, it has the effect of excessively increasing the aspect ratio of the austenite parent phase before martensitic transformation. Therefore, the Nb content is preferably in the range of 0.001-0.01%.

[0036] Titanium (Ti): 0.005-0.08% Ti, along with Nb and V, is a representative precipitation strengthening element, forming coarse TiN through a strong affinity for nitrogen. This TiN effectively inhibits grain growth during heating. Furthermore, the remaining Ti after reacting with nitrogen dissolves in the steel and combines with carbon to form TiC precipitates, which are beneficial for improving the steel's strength. When the Ti content is less than 0.005%, the effects of inhibiting grain growth and improving strength cannot be fully achieved. On the other hand, when the Ti content exceeds 0.08%, coarse TiN is produced, and the coarse precipitates worsen, resulting in poor bending workability during forming. Therefore, the Ti content is preferably in the range of 0.005-0.08%. The lower limit of the Ti content is more preferably 0.01%, further more preferably 0.015%, and most preferably 0.02%. The upper limit of the Ti content is more preferably 0.07%, further more preferably 0.06%, and most preferably 0.045%.

[0037] Vanadium (V): 0.005-0.05% V, along with Nb and Ti, is a representative precipitation strengthening element. It hardly precipitates during hot rolling, but forms precipitates after high-temperature coiling, cooling, or tempering, thus increasing the steel's strength. Therefore, during hot rolling, it does not increase deformation resistance and rolling load due to delayed recrystallization, effectively improving strength. When the V content is less than 0.005%, the strength improvement effect cannot be fully achieved. On the other hand, when the V content exceeds 0.05%, coarse precipitates form, worsening bending workability, and like Nb, it cannot maintain the aspect ratio of the parent austenite at a low level, which is also economically disadvantageous. Therefore, the V content is preferably in the range of 0.005-0.05%. The lower limit of the V content is more preferably 0.006%, further more preferably 0.008%, and most preferably 0.01%. The upper limit of the V content is more preferably 0.04%, further more preferably 0.03%, and most preferably 0.02%.

[0038] Aluminum (Al): 0.01-0.5% The Al is added primarily for deoxidation. Here, Al represents Sol-Al. When the Al content is less than 0.01%, sufficient deoxidation cannot be achieved. On the other hand, when the Al content exceeds 0.5%, it combines with nitrogen to form excessive AlN, which easily leads to corner cracks in the billet during continuous casting and defects due to inclusion formation. Therefore, the Al content is preferably in the range of 0.01-0.5%. The lower limit of the Al content is more preferably 0.015%, and even more preferably 0.02%. The upper limit of the Al content is more preferably 0.1%, even more preferably 0.08%, and most preferably 0.05%.

[0039] Phosphorus (P): 0.003-0.05% Like Si, phosphorus (P) simultaneously possesses solid solution strengthening and ferrite phase transformation promoting effects. However, controlling the P content to less than 0.003% requires significant manufacturing costs, which is economically disadvantageous and insufficient to achieve the required strength. On the other hand, when the P content exceeds 0.05%, brittleness due to grain boundary segregation may occur, making it prone to microcracks during bending and significantly reducing ductility and impact resistance. Therefore, the P content is preferably in the range of 0.003-0.05%. The lower limit of the P content is more preferably 0.005%, even more preferably 0.007%, and most preferably 0.01%. The upper limit of the P content is more preferably 0.03%.

[0040] Sulfur (S): 0.001-0.01% S is an impurity present in steel. When its content exceeds 0.01%, it combines with Mn and other metals to form non-metallic inclusions. This makes the steel prone to microcracks during bending and significantly reduces its impact resistance. While this invention does not specifically limit the lower limit of the S content, controlling it to less than 0.001% requires a significant amount of time in steelmaking, leading to decreased productivity. Therefore, considering this, the lower limit of the S content can be limited to 0.001%. Thus, the S content preferably has a range of 0.001-0.01%. The lower limit of the S content is more preferably 0.002%. The upper limit of the S content is more preferably 0.008%, further more preferably 0.006%, and most preferably 0.005%.

[0041] Nitrogen (N): 0.001-0.01% The nitrogen (N) and carbon (C) are representative solid solution strengthening elements, forming coarse precipitates together with Ti, Al, etc. When the N content is less than 0.001%, it is not only difficult to fully achieve the effects of solid solution strengthening and precipitate formation, but also a significant amount of time is required during steelmaking operations to control the N content below 0.001%, leading to a decrease in productivity. Furthermore, while N generally has a better solid solution strengthening effect than carbon, a significant decrease in toughness occurs when the N content exceeds 0.01%. Therefore, the N content is preferably in the range of 0.001-0.01%. The lower limit of the N content is more preferably 0.002%, and even more preferably 0.003%. The upper limit of the N content is more preferably 0.008%, even more preferably 0.007%, and most preferably 0.006%.

[0042] Boron (B): 0.0005-0.003% When boron (B) exists in steel in a solid solution state, it mainly segregates at grain boundaries, stabilizing them and improving the brittleness of the steel. It also stabilizes dissolved nitrogen (N) and inhibits the formation of coarse AlN nitrides. Furthermore, it delays the ferrite phase transformation and is effective in forming hard phases such as bainite and martensite. When the B content is less than 0.0005%, the effects of improving brittleness, inhibiting the formation of coarse AlN nitrides, and promoting the formation of bainite and martensite cannot be fully achieved. On the other hand, when the B content exceeds 0.003%, the above effects no longer increase, and ductility decreases, leading to reduced formability. Therefore, the B content is preferably in the range of 0.0005-0.003%. The lower limit of the B content is more preferably 0.0006%, further more preferably 0.0008%, and most preferably 0.001%. The upper limit of the B content is more preferably 0.0025%, and further more preferably 0.002%.

[0043] The remaining components are iron (Fe), and may include unintentional and unavoidable impurities introduced during the manufacturing process. These impurities are well known to those skilled in the art of ordinary manufacturing processes, and therefore their contents are not specifically mentioned in this specification.

[0044] The hot-rolled steel plate preferably satisfies the following relationship 1.

[0045] [Relation 1] 30≤T≤100 T=([C] / 10) 0.5 ) (0.7 [Si]+1) (5 [Mn]+1) (2.5 [Cr]+1) (5 [Mo]+1) 25 In Equation 1, [C], [Si], [Mn], [Cr], and [Mo] represent the content (by weight %) of the corresponding alloying elements. These components are replaced with 0 if they were not intentionally added.

[0046] Equation 1 is a parameterized formula that combines alloying elements to maintain appropriate levels of hard phases bainite and lath martensite in the central part of the steel microstructure of the present invention, as well as ferrite, twinned martensite, and bainite in the surface part. The larger the "T" value in Equation 1, the more the hard phases bainite and lath martensite are formed, and the harder the hardness of each phase increases. Therefore, a larger value is more beneficial for ensuring strength and hardness, but excessive value prevents the formation of ferrite and twinned martensite in the surface layer, leading to poor bending workability of the steel and increased material deviations along the entire length and width of the hot-rolled steel sheet. Therefore, it is preferable to manage the value of Equation 1 between 30 and 100.

[0047] The microstructure of the hot-rolled steel sheet, from the surface to a 10 μm layer, can contain 5-40% ferrite and 2-10% twinned martensite by area fraction. While the twinned martensite belongs to the broad category of martensite, it differs from lath martensite with a substructure of several μm width and self-tempered martensite containing ε-carbides as fine carbides. Instead, it is martensite with a fine-thickness (tens of nm) twinned substructure exhibiting crystallographic twinning. The twinned martensite is difficult to distinguish phases on FE-SEM, therefore, it can be interpreted using high-resolution transmission electron microscopy (TEM). Furthermore, TEM measurements can be performed by collecting samples from a cross-section parallel to the rolling direction at a position 10 μm above the surface of the hot-rolled steel sheet for evaluation.

[0048] When there is too little ferrite in the surface layer, the surface layer, which is mostly composed of hard phase, has insufficient deformation resistance during bending, easily forming surface irregularities and ultimately leading to fracture. On the other hand, when there is too much ferrite, the deformation that should be distributed throughout the entire thickness is concentrated in a limited surface layer, which may also form surface irregularities even at large bending radii.

[0049] The twinned martensite in the surface layer acts as a buffer against the abrupt strength change between the surface ferrite and the deeper martensite, preventing excessive deformation concentration only in the surface layer. Therefore, by appropriately coordinating the ratio of ferrite to twinned martensite, an effective improvement in bendability can be achieved. This includes not only the surface ferrite but also more than 2% twinned martensite formed due to the influence of carbon and hardenability elements concentrated in the untransformed austenite, thus preventing excessive deformation concentration and ensuring proper machinability. However, excessive twinned martensite in the surface layer reduces surface resistance to deformation and makes it prone to cracking.

[0050] Furthermore, the remaining portion of the surface layer is preferably a hard microstructure. Specifically, it can be bainite, tempered bainite, martensite, tempered martensite, etc.

[0051] The microstructure of the central portion of the hot-rolled steel sheet is predominantly martensite. Depending on the winding temperature, carbides can be observed in tempered martensite (self-tempered martensite with ε-carbides) formed due to the self-tempering effect. Specifically, it may contain more than 80% (inclusive) of martensite and self-tempered martensite, and less than 20% (inclusive) of pearlite and bainite, in terms of area fraction. Typically, the central microstructure is located at approximately one-quarter of the steel sheet thickness.

[0052] By fully ensuring the presence of martensite and self-tempered martensite, excellent strength and hardness can be guaranteed.

[0053] The hot-rolled steel sheet shows proto-austenite extending along the rolling direction. This can be observed using SEM. The aspect ratio of the proto-austenite can be between 1 and 10. It can be confirmed that when the aspect ratio is in the range of 1 to 10, it improves bending workability. This can be attributed to the fact that, since the bending line during bending is parallel to the rolling direction, the deformation occurs in a form that elongates perpendicular to the rolling direction. The fine microstructure formed along the rolling direction has a very small grain size when observed perpendicular to the rolling direction, thus reducing deformation resistance.

[0054] The hardness of the central section of the hot-rolled steel plate can be above 400HV, and the tensile strength in the width direction can be above 1300MPa.

[0055] Furthermore, the bending property (R / t) is 4 or less, and the difference between the original bending property (r / t) and the bending property is 0.5 or more, thus exhibiting excellent strength and hardness while improving bending property, particularly surface bending property. Additionally, the original bending property (r / t) is preferably 6.0 or less.

[0056] The bending property (R / t) evaluation, for example, involves placing a mold with a 90-degree bending angle and a specific V-shaped bending radius at the bottom, and performing a 90-degree bending process by pressing it with a corresponding mold at the top. It is defined as the ratio of the bending radius (R) that does not produce surface unevenness during processing to the thickness (t) of the steel plate. The original bending property is the bending property measured using the same method after milling the surface by 0.1-0.5 mm.

[0057] Next, a specific embodiment of the hot-rolled steel plate manufacturing method of the present invention will be described in detail.

[0058] First, the manufacturing method may include heating, hot rolling, cooling, and coiling a steel billet that satisfies the above alloy composition and relation 1.

[0059] billet heating The steel billet satisfying the above alloy composition and Equation 1 is heated to a temperature range of 1150-1350°C. When the billet heating temperature is below 1150°C, the precipitates are not sufficiently redissolved, resulting in reduced precipitate formation in subsequent hot-rolling processes, coarse TiN residues, and insufficient homogenization of the billet, making it difficult to maintain a constant temperature for the steel plate during hot rolling. On the other hand, when the billet heating temperature exceeds 1350°C, the strength decreases due to abnormal austenite grain growth. Therefore, the billet heating temperature preferably has a range of 1150-1350°C. The lower limit of the billet heating temperature is more preferably 1155°C, and even more preferably 1160°C. The upper limit of the billet heating temperature is more preferably 1340°C, even more preferably 1330°C, and most preferably 1320°C.

[0060] Hot rolling The heated steel billet is rough-rolled at the roughing temperature (RDT) specified in Equation 2 to obtain a strip. The roughing temperature (RDT) is preferably based on t / 2 (t: the thickness of the steel). The thickness of the steel is preferably the thickness of the strip. The roughing can be performed in a temperature range of 900-1100°C.

[0061] [Relationship 2] RDT (°C) ≤ (FT + 80 + strip thickness) 3.8) FT=907+239[Al]-10.3[Cr]-25.8[Mn]+17.9[Mo]+30.9[Si]-254[C] In Equation 2, [Al], [Cr], [Mn], [Mo], [Si], and [C] represent the content (weight %) of the corresponding alloying elements. If these components are not intentionally added, they are replaced with 0. The thickness of the strip is mm.

[0062] The strip is finished rolled at the finishing temperature (FDT) specified in Equation 3 to obtain a hot-rolled steel sheet. The finishing temperature (FDT) is preferably based on t / 2 (t: the thickness of the steel). The thickness of the steel at this point is preferably the thickness of the hot-rolled steel sheet.

[0063] [Relationship 3] FDT(°C)≥FT To improve bending workability relative to hardness, a fine microstructure composition and hardness difference between the surface and central portions are necessary, and the elongation of the original austenite in the central martensite is limited; ideally, the aforementioned microstructure characteristics should be satisfied. Therefore, the RDT and FDT at the end of the roughing and finishing rolling stages have different temperature limits based on Equations 2 and 3, which are derived from the steel composition and strip thickness. When the RDT is higher than the calculation in Equation 2, the desired microstructure composition cannot be induced in the surface and central portions, resulting in ordinary martensitic high-strength steel, which may lead to poor bending workability. When the FDT is lower than the calculation in Equation 3, the aspect ratio of the central microstructure cannot reach the desired level, which may also adversely affect bending workability.

[0064] After obtaining the strip plate, the process may further include descaling and surface cooling the entire width of the strip plate at a water pressure of 150 bar or more. By performing the descaling, the effect of rapid surface cooling can be increased, and the additional surface cooling can more easily form a soft tissue on the surface.

[0065] The thickness of the strip is preferably satisfied by the following relationship 4. During rolling, the surface portion needs to cool faster than the center portion. If the strip thickness is too thin, heat conduction occurs rapidly to the center portion, making it impossible to generate a temperature difference between the surface portion and the center portion. Therefore, it is preferable to satisfy the condition of relationship 4.

[0066] [Relationship 4] The thickness of the strip plate (mm) is greater than or equal to the thickness of the hot-rolled steel plate (mm). 10 cool down After manufacturing the hot-rolled steel sheet, the hot-rolled steel sheet is cooled to a cooling termination temperature of 150-350°C at an average cooling rate of 60-90°C / second.

[0067] After the first cooling, the temperature is further cooled to a winding temperature (CT) of 50-200°C at a secondary average cooling rate of 1-50°C / second.

[0068] The primary cooling is carried out at an average cooling rate of 50-100°C / second to a temperature range of 150-350°C. This is to ensure the even formation of sufficient central martensite and surface ferrite and twinned martensite. Excessively high cooling rates may degrade the coil shape and are therefore preferred to avoid.

[0069] The steel sheet that has undergone the first cooling is then subjected to a second cooling at an average cooling rate of 1-40°C / second, cooling it to a temperature in the range of 50-200°C. At this point, to form the desired fine microstructure, a cooling rate of 1°C / second or higher is preferred. Excessively high-speed cooling may degrade the shape quality of the sheet and offers no further benefit; therefore, the cooling rate is preferably within the range described above. Furthermore, from a productivity perspective, switching to low-speed cooling is advantageous; therefore, it is preferable to introduce low-speed cooling as much as possible.

[0070] Collect After the secondary cooling, winding is performed within the range of 50-200°C. This winding is to achieve a self-tempering effect. Typically, by holding the coil at a temperature below the martensitic transformation initiation temperature where carbides can form for a long time, a fine microstructure containing ε-carbides, known as transition carbides, can be formed even without further heating or heat treatment. In hot rolling processes, this is achieved by utilizing the phenomenon that when winding is performed at a specific temperature range above room temperature, the cooling time of the coil is extended to more than one hour. Therefore, the winding in this invention is preferably performed between 50°C and 200°C. If the temperature is too low, the self-tempering effect cannot be obtained; if the winding temperature is too high, the self-tempering effect is excessive, which may lead to deterioration of strength and flexural strength.

[0071] Pickling and oiling After coiling, the process may further include pickling and oiling the coiled hot-rolled steel sheet. The pickling and oiling process is not particularly limited in this invention, and all methods commonly used in this technical field may be employed. Detailed Implementation

[0072] The following describes embodiments of the present invention. It is self-evident that those skilled in the art can make various modifications to these embodiments without departing from the scope of the invention. These embodiments are provided for understanding the invention, and the scope of the invention should not be limited to these embodiments, but should be determined not only by the following claims, but also by their equivalents.

[0073] (Example) A steel billet with the composition shown in Table 1 below (in weight %, the remainder being Fe and unavoidable impurities) was prepared. T in Table 1 below is calculated using the following formula 1.

[0074] [Relation 1] 30≤T≤100 T=([C] / 10) 0.5 ) (0.7 [Si]+1) (5 [Mn]+1) (2.5 [Cr]+1) (5 [Mo]+1) 25 In Equation 1, [C], [Si], [Mn], [Cr], and [Mo] represent the content (by weight %) of the corresponding alloying elements. These components are replaced with 0 if they were not intentionally added.

[0075] [Table 1] The steel billet was subjected to billet heating, rough rolling, finish rolling, primary cooling, secondary cooling, and coiling under the conditions shown in Table 2 below. In Table 2, RDT is the rough rolling termination temperature, FDT is the finish rolling termination temperature, and CT is the coiling temperature. It was confirmed that these conditions conformed to the following relationships 2 to 3. The thickness of the manufactured hot-rolled steel sheet was 3 mm.

[0076] [Relationship 2] RDT (°C) ≤ (FT + 80 + strip thickness) 3.8) FT=907+239[Al]-10.3[Cr]-25.8[Mn]+17.9[Mo]+30.9[Si]-254[C] In Equation 2, [Al], [Cr], [Mn], [Mo], [Si], and [C] represent the content (weight %) of the corresponding alloying elements. If these components are not intentionally added, they are replaced with 0. The thickness of the strip is mm.

[0077] [Relationship 3] FDT(°C)≥FT [Table 2] The microstructure of the central and surface portions of the hot-rolled steel sheets manufactured according to Tables 1 and 2 is shown in Table 3 below.

[0078] The aspect ratio was calculated using the major axis / minor axis of the original austenite grains in the central martensite. All microstructure observations were performed using transmission electron microscopy and conventional electron microscopy, measured at 1000x and 3000x magnification respectively, where differentiation was possible.

[0079] [Table 3] The hardness, physical properties, and bending workability of the hot-rolled steel sheets described in Table 3 are evaluated, and the results are shown in Table 4 below. In Table 4, surface hardness and central section hardness are Vickers hardness. Surface hardness is measured on the surface, and central section hardness is measured at half the thickness of the section. r / t is the original bending property, and R / t is the bending property, which is the ratio of the bending radius (R) to the steel sheet thickness (t), expressed as the minimum bending radius that does not produce unevenness or cracks on the surface after a 90-degree bend. In this case, the bending specimen is machined to be perpendicular to the rolling direction, so that the bending line is parallel to the rolling direction. The original bending property is measured after milling the surface by 0.2-0.5 mm. YS and TS are the yield strength and tensile strength measured in MPa during tensile testing in the direction perpendicular to the rolling direction, i.e., the width direction of the coil. The tensile test specifications use JIS 5 specification specimens.

[0080] in addition, Figure 1 The graph below shows the relationship between tensile strength (TS) and flexural properties (R / t) based on the results in Table 4.

[0081] [Table 4] As described in Tables 1 to 4 and Figure 1 As shown, all of the inventive examples 1 to 6 that meet the conditions proposed in this invention ensure both excellent strength and hardness, as well as excellent machinability. It can be confirmed that they possess excellent mechanical and physical properties.

[0082] On the other hand, reference Figure 1 It can be seen that in the comparative example, as the strength increases, R / t increases, and the bending workability deteriorates.

[0083] Specifically, Comparative Examples 1 and 2 satisfy the steel composition and relational formula 1 proposed in this invention, but the winding temperature in the manufacturing process exceeds the range of this invention, and cooling is completed at a high temperature. As a result, a large amount of martensite in the central part of the final microstructure is replaced by bainite, resulting in very low strength, and the ferrite and twinned martensite in the surface part are not formed within the appropriate range. Consequently, as the winding temperature increases, sufficient hardness cannot be ensured.

[0084] Comparative Example 7 satisfies the steel composition and Equation 1 proposed in this invention, but the RDT in the manufacturing process is excessively increased, failing to meet the conditions of Equation 2. As a result, the combination of ferrite and twinned martensite in the surface layer fails to form in the fine microstructure, resulting in very poor flexibility.

[0085] Comparative Example 3 involves hardenability elements such as Mn, Cr, and Mo, and the composition-related formula 1 exceeds the scope of this invention. As a result, no ferrite or twinned martensite is formed on the surface layer, and even if all manufacturing process conditions are met, no improvement in bending performance is achieved.

[0086] Comparative Example 4 differs from Comparative Example 3 in that the alloy composition is formed under conditions where the value of Relation 1 is lower than the scope of this invention. Therefore, even if all the manufacturing process conditions proposed in this invention are met, sufficient martensite cannot be ensured, making it impossible to manufacture high-strength steel. Specifically, bainite structure is mostly formed in the central portion, resulting in lower strength and hardness compared to other examples. While the strength and hardness are reduced, machinability is improved, and an appropriate amount of ferrite and twinned martensite is formed on the surface, this is not suitable for the present invention, which aims to improve the bending properties of high-strength steel.

[0087] Comparative Example 5 involves a case where the Mo content and the T value of Equation 1 exceed the scope of this invention, and where the thickness of the strip plate is too small during the manufacturing process. As a result, during the manufacturing process of the hot-rolled steel sheet, the temperature difference between the surface and the center of the strip plate becomes smaller, thus failing to form ferrite + twinned martensite in the surface layer, failing to ensure sufficient flexibility, and the difference between the original flexibility and the flexibility does not fall within the required range.

[0088] Comparative Example 6 is a case of excessive Nb content. As a result, even if all the manufacturing process conditions proposed in this invention are met, the aspect ratio of the original austenite exceeds the scope of this invention, thus resulting in a deterioration in both bending and original bending properties.

Claims

1. A hot-rolled steel sheet, by weight percent, comprising: C: 0.17-0.26%, Si: 0.01-0.5%, Mn: 0.4-2.2%, Cr: 0.005-0.6%, Mo: 0.005-0.3%, Nb: 0.001-0.01%, Ti: 0.005-0.08%, V: 0.005-0.05%, Al: 0.01-0.5%, P: 0.003-0.05%, S: 0.001-0.01%, N: 0.001-0.01%, B: 0.0005-0.003%, with the balance being Fe and other unavoidable impurities, wherein the hot-rolled steel sheet satisfies the following relationship 1. The microstructure from the surface to the 10μm surface layer contains 5-40% ferrite and 2-10% twinned martensite by area fraction. The central microstructure, by area fraction, contains more than 80% and up to 100% martensite and self-tempered martensite, and less than 20% and up to 0% pearlite and bainite, or one or more of these two types. [Relation 1] 30≤T≤100 T=(([C] / 10) 0.5 ) (0.7 [Si]+1) (5 [Mn]+1) (2.5 [Cr]+1) (5 [Mo]+1) 25 In Equation 1, [C], [Si], [Mn], [Cr], and [Mo] represent the content of the corresponding alloying elements in weight percent, and these components are replaced with 0 if they are not intentionally added.

2. The hot-rolled steel plate according to claim 1, wherein, The aspect ratio of the original austenite in the central microstructure is 1 to 10.

3. The hot-rolled steel plate according to claim 1, wherein, The central section hardness of the hot-rolled steel plate is above 400HV, and the tensile strength in the width direction is above 1300MPa.

4. The hot-rolled steel plate according to claim 1, wherein, The bending resistance R / t of the hot-rolled steel plate is below 4.

0.

5. The hot-rolled steel plate according to claim 1, wherein, The difference between the original bending and bending properties of the hot-rolled steel plate is greater than 0.

5.

6. A method for manufacturing a hot-rolled steel plate, comprising the following steps: The steel billet is heated to a temperature range of 1150-1350℃, and the steel billet, by weight%, contains: C: 0.17-0.26%, Si: 0.01-0.5%, Mn: 0.4-2.2%, Cr: 0.005-0.6%, Mo: 0.005-0.3%, Nb: 0.001-0.01%, Ti: 0.005-0.08%, V: 0.005-0.05%, Al: 0.01-0.5%, P: 0.003-0.05%, S: 0.001-0.01%, N: 0.001-0.01%, B: 0.0005-0.003%, with the balance being Fe and other unavoidable impurities. The steel billet satisfies the following relationship 1. The heated steel billet is rough rolled at the rough rolling temperature (RDT) of the following relationship 2 to obtain a strip plate; The strip is finished rolled at the finishing temperature (FDT) of the following formula 3 to obtain a hot-rolled steel sheet; The hot-rolled steel plate is cooled to a cooling termination temperature of 150-350°C at an average cooling rate of 60-90°C / second. After the initial cooling, the temperature is further cooled to a winding temperature (CT) of 50-200°C at a secondary average cooling rate of 1-50°C / second; and After the second cooling, the coil is wound up. [Relation 1] 30≤T≤100 T=(([C] / 10) 0.5 ) (0.7 [Si]+1) (5 [Mn]+1) (2.5 [Cr]+1) (5 [Mo]+1) 25 In Equation 1, [C], [Si], [Mn], [Cr], and [Mo] represent the content of the corresponding alloying elements in weight percent. These components are replaced with 0 if they were not intentionally added. [Relationship 2] RDT (°C) ≤ (FT + 80 + strip thickness) 3.8) FT=907+239[Al]-10.3[Cr]-25.8[Mn]+17.9[Mo]+30.9[Si]-254[C] In Equation 2, [Al], [Cr], [Mn], [Mo], [Si], and [C] represent the content of the corresponding alloying elements in weight percent (%). These components are substituted into zero if they were not intentionally added. The thickness of the strip plate is mm. [Relationship 3] FDT(°C)≥FT.

7. The method for manufacturing hot-rolled steel plate according to claim 6, wherein, After obtaining the strip plate, the manufacturing method further includes the steps of descaling and surface cooling the entire width of the strip plate with a water pressure of 150 bar or more.

8. The method for manufacturing hot-rolled steel sheet according to claim 6, wherein, The thickness of the strip plate satisfies the following relationship 4. [Relationship 4] The thickness of the strip plate (mm) is greater than or equal to the thickness of the hot-rolled steel plate (mm).

10.

9. The method for manufacturing hot-rolled steel sheet according to claim 6, wherein, The rough rolling is carried out in a temperature range of 900-1100℃.

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