Hot-rolled steel sheet and method for manufacturing same

Abstract

The present invention relates to a hot-rolled steel sheet used in frames, structures, etc. and, to a hot-rolled steel sheet that ensures high hardness, strength, and excellent processing characteristics, and a method for manufacturing same.

Description

Hot-rolled steel sheet and its manufacturing method

[0001] The present invention relates to a hot-rolled steel sheet used for frames, structures, etc., and relates to a hot-rolled steel sheet that secures high hardness, strength, and excellent processing characteristics, and a method for manufacturing the same.

[0002] Conventional wear-resistant hot-rolled steel sheets with high hardness primarily utilize a martensitic microstructure to achieve both high strength and hardness. This high hardness, in turn, leads to high wear resistance, making them ideal for use in components requiring wear resistance. However, the inherently poor bending workability associated with high strength limits their processing, leading to the use of roll-forming techniques or minimal machining.

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

[0004] Patent Document 1 suggests that excellent bendability can be secured in a 1 GPa high-strength martensitic hot-rolled steel sheet when the hard phases of martensite and tempered martensite in the microstructure at positions 200 μm and 100 μm from the surface are reduced compared to the center. In addition, Patent Document 2 suggests that low-temperature impact toughness can be improved through microstructural refinement in a 1.2 GPa martensitic steel, excellent flange formability can be obtained by controlling precipitates, and high bendability can be obtained by controlling surface roughness.

[0005] However, Patent Document 1 may be considered to have a small effect on improving bendability because it is difficult to secure sufficient strength, there is not much change between the center and surface of the steel plate, and there is no control over the microstructure and properties of the extreme surface layer of 100㎛ or less. On the other hand, Patent Document 2 has the disadvantage that there is a limit to improving bendability in relation to strength through surface roughness control alone, a certain level of surface roughness can usually be obtained through the pickling process alone, and manufacturing conditions must be strictly limited to control precipitates.

[0006] In addition, alloying elements such as Si, Mn, Mo, Cr, Cu, and Ni, which are mainly used to manufacture the high-hardness steels described above, are effective in improving hardness and formability, but if a large amount of alloying elements is added to improve properties, segregation of alloying elements and unevenness of microstructure occur, resulting in poor bending workability. In particular, steels with high hardenability are sensitive to changes in microstructure during cooling, so there is a problem that it is difficult to obtain even higher bending workability because the low-temperature transformation structure is formed unevenly.

[0007] (Patent Document 1) Korean Patent Publication No. 10-2023-0085173

[0008] (Patent Document 2) International Patent Publication No. WO 2020-026593

[0009] One aspect of the present invention is to provide a hot-rolled steel sheet having excellent bending workability, strength and hardness, and a method for manufacturing the same.

[0010] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0011] One embodiment of the present invention comprises, in wt%, C: 0.17 to 0.26%, Si: 0.01 to 0.5%, Mn: 0.4 to 2.2%, Cr: 0.005 to 0.6%, Mo: 0.005 to 0.3%, Nb: 0.001 to 0.01%, Ti: 0.005 to 0.08%, V: 0.005 to 0.05%, Al: 0.01 to 0.5%, P: 0.003 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.0005 to 0.003%, the remainder being Fe and other unavoidable impurities, and satisfying the following relational expression 1:

[0012] The surface microstructure from the surface to 10㎛ contains, in area fraction, 5-40% ferrite and 2-10% twinned martensite.

[0013] The core microstructure relates to a hot-rolled steel sheet containing, in terms of area fraction, 80% or more (including 100%) of martensite and auto-tempered martensite, and 20% or less (including 0%) of at least one type of pearlite and bainite.

[0014] [Relationship 1]

[0015] 30 ≤ T ≤ 100

[0016] T =(([C] / 10) 0.5 )*(0.7*[Si]+1)*(5*[Mn]+1)*(2.5*[Cr]+1)*(5*[Mo]+1)*25

[0017] [C], [Si], [Mn], [Cr], and [Mo] in the above relational expression 1 represent the content (weight %) of the corresponding alloy elements, and if these components are not intentionally added, 0 is substituted.

[0018] The above central microstructure may have an aspect ratio of 1 to 10.

[0019] The above hot-rolled steel sheet may have a central cross-sectional hardness of 400 HV or more and a widthwise tensile strength of 1300 MPa or more.

[0020] The above hot-rolled steel sheet may have a bendability (R / t) of 4.0 or less.

[0021] The above hot-rolled steel sheet may have a difference between the original bendability and the bendability of greater than 0.5.

[0022]

[0023] Another aspect of the present invention comprises a step of heating a steel slab, which comprises, in wt%, C: 0.17 to 0.26%, Si: 0.01 to 0.5%, Mn: 0.4 to 2.2%, Cr: 0.005 to 0.6%, Mo: 0.005 to 0.3%, Nb: 0.001 to 0.01%, Ti: 0.005 to 0.08%, V: 0.005 to 0.05%, Al: 0.01 to 0.5%, P: 0.003 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.0005 to 0.003%, the remainder being Fe and other unavoidable impurities, and satisfying the following relational expression 1, in a temperature range of 1150 to 1350°C;

[0024] A step of performing rough rolling on the above-mentioned heated steel slab at a rough rolling temperature (RDT) of the following relational expression 2 to obtain a bar plate;

[0025] A step for obtaining a hot rolled steel sheet by performing finish rolling on the above bar plate at the finish rolling temperature (FDT) of the following relational expression 3;

[0026] A step of first cooling the hot-rolled steel sheet to a first cooling stop temperature of 150 to 350°C at a first average cooling rate of 60 to 90°C / sec;

[0027] A step of secondary cooling to a coiling temperature (CT) of 50 to 200°C at a secondary average cooling rate of 1 to 50°C / sec after the primary cooling; and

[0028] It relates to a method for manufacturing a hot-rolled steel sheet, including a step of coiling after the above-mentioned secondary cooling.

[0029] [Relationship 1]

[0030] 30 ≤ T ≤ 100

[0031] T = (([C] / 10) 0.5 )*(0.7*[Si]+1)*(5*[Mn]+1)*(2.5*[Cr]+1)*(5*[Mo]+1)*25

[0032] [C], [Si], [Mn], [Cr], and [Mo] in the above relational expression 1 represent the content (weight %) of the corresponding alloy elements, and if these components are not intentionally added, 0 is substituted.

[0033] [Relationship 2]

[0034] RDT (℃) ≤ (FT + 80 + bar plate thickness * 3.8)

[0035] FT = 907 + 239[Al] - 10.3[Cr] - 25.8[Mn] + 17.9[Mo] + 30.9[Si] - 254[C]

[0036] [Al], [Cr], [Mn], [Mo], [Si], and [C] in the above relational expression 2 represent the content (weight %) of the corresponding alloy elements. If these components are not intentionally added, 0 is substituted, and the bar plate thickness is mm.

[0037] [Relationship 3]

[0038] FDT (℃) ≥ FT

[0039] After obtaining the above bar plate, a step of descaling and surface cooling the entire width of the bar plate with a water pressure of 150 bar or more may be further included.

[0040] The above bar thickness can satisfy the following relationship 4.

[0041] [Relationship 4]

[0042] Bar plate thickness (mm) ≥ thickness of hot rolled steel plate (mm) * 10

[0043] The above rolling can be performed at a temperature range of 900 to 1100°C.

[0044] According to one aspect of the present invention, a hot-rolled steel sheet having excellent bending formability and hardness and a method for manufacturing the same can be provided.

[0045] The various advantageous and beneficial advantages and effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0046] Figure 1 is a graph showing the relationship between the central tensile strength (TS) and bendability (R / t) of an inventive example and a comparative example in an embodiment.

[0047] The terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include the plural forms, unless the context clearly dictates otherwise.

[0048] The meaning of "comprising" as used in the specification is to specify a configuration and not to exclude the presence or addition of other configurations.

[0049] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0050]

[0051] In order to solve the problems of the prior art, the inventors of the present invention have conducted in-depth research on the changes in the core cross-sectional hardness and bendability and the original bendability after surface milling according to the characteristics of the components, manufacturing process, and microstructure of various steels with different components and microstructures. As a result, they confirmed that the detailed characteristics of each composition of the surface layer (superficial part) and the core part (center part) and the original bendability are correlated with the core cross-sectional hardness and bending characteristics, and as a result of in-depth research on methods for realizing this, they explored the steel composition and microstructure of hot-rolled steel sheets and methods for manufacturing the same, which led to the derivation of the present invention.

[0052]

[0053] First, an example of the hot-rolled steel sheet of the present invention will be described in detail. Below, the alloy composition and range of the hot-rolled steel sheet will be described. Unless otherwise specified, the alloy composition content described below refers to weight percent.

[0054] Carbon (C): 0.17~0.26%

[0055] The above-mentioned C is the most economical and effective element for strengthening steel, and has a great influence on strength and hardness. As its addition amount increases, the hardenability increases, making it easier to form hard phases such as bainite and martensite in the microstructure, thereby increasing the tensile strength. In addition, since C has a tendency to accumulate in austenite rather than ferrite, ferrite is formed in the surface layer, and when the C accumulated in austenite transforms into martensite, it provides a mechanism for changing into twinned martensite. However, when the content of the above-mentioned C exceeds 0.26%, there is a problem that the hardness of martensite increases excessively, resulting in an excessive increase in strength and a decrease in bending workability, and it may be difficult to secure sufficient weldability. On the other hand, when the content of the above-mentioned C is less than 0.17%, it is difficult to obtain a sufficient strengthening effect. Therefore, the content of the above-mentioned C is preferably in the range of 0.17 to 0.26%. The lower limit of the C content is more preferably 0.175%, more preferably 0.18%, and most preferably 0.185%. The upper limit of the C content is more preferably 0.25%, more preferably 0.24%, and most preferably 0.23%.

[0056] Silicon (Si): 0.01~0.5%

[0057] The above-mentioned Si is an element that is advantageous in improving formability by deoxidizing molten steel, exerting a solid solution strengthening effect, and delaying the formation of coarse carbides. When the content of the Si is less than 0.01%, the solid solution strengthening effect and the formability improvement effect cannot be sufficiently obtained. On the other hand, when the content of the Si exceeds 0.5%, it is not easy to remove the red scale formed on the surface of the steel sheet during hot rolling, and this may significantly deteriorate the surface quality of the steel sheet. In addition, there is a problem that ductility and weldability are also reduced. Therefore, the content of the Si is preferably in the range of 0.01 to 0.5%. The lower limit of the Si content is more preferably 0.012%, more preferably 0.015%, and most preferably 0.02%. The upper limit of the Si content is more preferably 0.4%, more preferably 0.35%, and most preferably 0.3%.

[0058] Manganese (Mn): 0.4~2.2%

[0059] The above manganese, like silicon, is an effective element for strengthening steel through solid solution, and increases the hardenability of steel, facilitating the formation of hard phases, bainite and martensite, during cooling after hot rolling. In addition, when ferrite is formed in the surface layer, it is concentrated in austenite, and when it subsequently transforms into martensite, it provides a mechanism for changing into twinned martensite. When the content of manganese is less than 0.4%, the effects of solid solution strengthening and the formation of bainite and martensite cannot be sufficiently obtained. On the other hand, when the content of manganese exceeds 2.2%, the grain boundaries become weak, causing problems such as low-temperature cracking. In addition, the strength may increase excessively, making it difficult to secure sufficient formability, and when slab casting in the casting process, segregation zones develop significantly in the center of the thickness, and when cooling after hot rolling, the microstructure is formed unevenly in the thickness direction, resulting in inferior bending workability. In particular, it makes it difficult to manufacture a uniform microstructure during cooling across the full length and width of the hot-rolled steel sheet. Therefore, it is preferable that the content of Mn be in the range of 0.4 to 2.2%. The lower limit of the Mn content is more preferably 0.45%, more preferably 0.5%, and most preferably 0.55%. The upper limit of the Mn content is more preferably 2.1%, more preferably 2.05%, and most preferably 2.0%.

[0060] Chromium (Cr): 0.005~0.6%

[0061] The Cr content strengthens the steel and delays the ferrite phase transformation during cooling, thereby assisting in the formation of martensite and bainite. If the Cr content is less than 0.005%, the effects of solid solution strengthening and the formation of martensite and bainite cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 0.6%, similar to Mn, the segregation zone in the center of the thickness is greatly developed, and the microstructure in the thickness direction becomes non-uniform, thereby reducing the bending workability. Therefore, the Cr content is preferably in the range of 0.005 to 0.6%. The lower limit of the Cr content is more preferably 0.007%, more preferably 0.008%, and most preferably 0.01%. The upper limit of the Cr content is more preferably 0.5%, more preferably 0.45%, and most preferably 0.4%.

[0062] Molybdenum (Mo): 0.005~0.3%

[0063] The above Mo increases the hardenability of the steel and facilitates the formation of martensite and bainite. When the content of Mo is less than 0.005%, the above-mentioned effect cannot be sufficiently obtained. On the other hand, when the content of Mo exceeds 0.3%, martensite is formed in the surface layer due to excessive increase in hardenability, which rapidly deteriorates bending workability, is economically disadvantageous, and may make it difficult to secure sufficient weldability. In addition, excessively high hardenability makes it impossible to form a ferrite structure in the surface layer. Therefore, the content of Mo is preferably in the range of 0.005 to 0.3%. The lower limit of the Mo content is more preferably 0.01%, more preferably 0.02%, and most preferably 0.03%. The upper limit of the Mo content is more preferably 0.25%, more preferably 0.2%, and most preferably 0.15%.

[0064] Niobium (Nb): 0.001~0.01%

[0065] The above-mentioned Nb, together with Ti and V, is a representative precipitation strengthening element, and is effective in improving the strength and impact toughness of steel by precipitating as a precipitate during hot rolling and exerting the effect of refining grains by delaying recrystallization. If the content of the above-mentioned Nb is less than 0.001%, the above-mentioned effect cannot be sufficiently obtained. On the other hand, if the content of the above-mentioned Nb exceeds 0.01%, coarse composite precipitates are formed during hot rolling, which deteriorates bending workability. In addition, there is an effect of making the aspect ratio of the parent austenite excessively large before the martensite phase transformation. Therefore, the content of the above-mentioned Nb is preferably in the range of 0.001 to 0.01%.

[0066] Titanium (Ti): 0.005~0.08%

[0067] The above Ti, along with Nb and V, is a representative precipitation strengthening element, and forms coarse TiN through a strong affinity with nitrogen. The TiN has the effect of inhibiting grain growth during the heating process for hot rolling. In addition, the Ti remaining after reacting with nitrogen is dissolved in the steel and combines with carbon to form TiC precipitates, which is a useful component for improving the strength of the steel. When the content of the Ti is less than 0.005%, the effects of inhibiting grain growth and improving strength cannot be sufficiently obtained. On the other hand, when the content of the Ti exceeds 0.08%, coarse TiN is generated, and the precipitates become coarser, which deteriorates the bending workability during forming. Therefore, the content of the Ti is preferably in the range of 0.005 to 0.08%. The lower limit of the Ti content is more preferably 0.01%, still more preferably 0.015%, and most preferably 0.02%. The upper limit of the above Ti content is more preferably 0.07%, more preferably 0.06%, and most preferably 0.045%.

[0068] Vanadium (V): 0.005–0.05%

[0069] The above-mentioned V, together with Nb and Ti, is a representative precipitation strengthening element. It hardly precipitates during hot rolling, but it forms precipitates after high-temperature coiling, cooling, or tempering, thereby improving the strength of steel. Therefore, it is effective in additionally improving the strength without increasing the deformation resistance and rolling load due to the delay in recrystallization during hot rolling. If the content of the above-mentioned V is less than 0.005%, the strength improvement effect cannot be sufficiently obtained. On the other hand, if the content of the above-mentioned V exceeds 0.05%, coarse precipitates are formed, which deteriorates the bending workability, and like Nb, it is difficult to maintain a low aspect ratio of the parent austenite, which is also economically disadvantageous. Therefore, the content of the above-mentioned V is preferably in the range of 0.005 to 0.05%. The lower limit of the above-mentioned V content is more preferably 0.006%, still more preferably 0.008%, and most preferably 0.01%. The upper limit of the above V content is more preferably 0.04%, more preferably 0.03%, and most preferably 0.02%.

[0070] Aluminum (Al): 0.01~0.5%

[0071] The above Al is an element mainly added for deoxidation. Here, Al means Sol-Al. When the content of the Al is less than 0.01%, the deoxidation effect cannot be sufficiently obtained. On the other hand, when the content of the Al exceeds 0.5%, it combines with nitrogen to form excessive AlN, which easily causes corner cracks to occur in the slab during continuous casting and easily causes defects due to the formation of inclusions. Therefore, the content of the Al is preferably in the range of 0.01 to 0.5%. The lower limit of the Al content is more preferably 0.015%, and further preferably 0.02%. The upper limit of the Al content is more preferably 0.1%, further preferably 0.08%, and most preferably 0.05%.

[0072] Phosphorus (P): 0.003~0.05%

[0073] The above P, like Si, has both the effect of strengthening the solid solution and promoting ferrite transformation. However, controlling the P content to less than 0.003% requires a lot of manufacturing cost, which is economically disadvantageous and insufficient for obtaining strength. On the other hand, if the P content exceeds 0.05%, embrittlement due to grain boundary segregation may occur, microcracks are likely to occur during bending, and ductility and impact resistance properties are significantly reduced. Therefore, the P content is preferably in the range of 0.003 to 0.05%. The lower limit of the P content is more preferably 0.005%, more preferably 0.007%, and most preferably 0.01%. The upper limit of the P content is more preferably 0.03%.

[0074] Sulfur (S): 0.001~0.01%

[0075] The above S is an impurity present in steel, and if its content exceeds 0.01%, it combines with Mn, etc. to form non-metallic inclusions, which easily causes fine cracks to occur during bending of the steel and significantly reduces impact resistance. In the present invention, the lower limit of the S content is not particularly limited, but in order to control it to less than 0.001%, a lot of time is required during steelmaking, which reduces productivity. Therefore, considering this, the lower limit of the S content may be limited to 0.001%. Therefore, the S content is preferably in the range of 0.001 to 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%, still more preferably 0.006%, and most preferably 0.005%.

[0076] Nitrogen (N): 0.001~0.01%

[0077] The above-mentioned N, together with C, is a representative solid solution strengthening element, and forms coarse precipitates together with Ti, Al, etc. When the content of the above-mentioned N is less than 0.001%, it is difficult to sufficiently obtain the solid solution strengthening and precipitate formation effects, and in order to control the content of the above-mentioned N to be less than 0.001%, it takes a lot of time during the steelmaking operation, which reduces productivity. Meanwhile, although the solid solution strengthening effect of the above-mentioned N is generally superior to that of carbon, there is a problem in that the toughness is greatly reduced when the content of the above-mentioned N exceeds 0.01%. Therefore, the content of the above-mentioned N is preferably in the range of 0.001 to 0.01%. The lower limit of the above-mentioned N content is more preferably 0.002%, and the upper limit of the above-mentioned N content is more preferably 0.008%, and the upper limit of the above-mentioned N content is more preferably 0.007%, and the upper limit of the above-mentioned N content is most preferably 0.006%.

[0078] Boron (B): 0.0005~0.003%

[0079] The above B, when present in a solid solution state in steel, mainly segregates at grain boundaries and has the effect of improving the brittleness of steel by stabilizing grain boundaries and also plays a role in suppressing the formation of coarse AlN nitrides by stabilizing solid solution N. In addition, it is effective in delaying ferrite phase transformation and forming hard phases such as bainite and martensite. When the content of the above B is less than 0.0005%, the effects of improving brittleness, suppressing the formation of coarse AlN nitrides, and forming bainite and martensite cannot be sufficiently obtained. On the other hand, when the content of the above B exceeds 0.003%, the above-mentioned effect does not increase any further, and there is a disadvantage in that the ductility decreases and the formability deteriorates. Therefore, the content of the above B is preferably in the range of 0.0005 to 0.003%. The lower limit of the above B content is more preferably 0.0006%, still more preferably 0.0008%, and most preferably 0.001%. The upper limit of the above B content is more preferably 0.0025%, and even more preferably 0.002%.

[0080] The remaining components are iron (Fe), and may contain some unintended and unavoidable impurities introduced during the manufacturing process. Since these impurities are readily apparent to anyone skilled in the art of manufacturing, their full details are not specifically mentioned in this specification.

[0081] It is preferable that the above hot-rolled steel sheet satisfies the following relationship 1.

[0082] [Relationship 1]

[0083] 30 ≤ T ≤ 100

[0084] T =(([C] / 10) 0.5 )*(0.7*[Si]+1)*(5*[Mn]+1)*(2.5*[Cr]+1)*(5*[Mo]+1)*25

[0085] [C], [Si], [Mn], [Cr], and [Mo] in the above relational expression 1 represent the content (weight %) of the corresponding alloy elements, and if these components are not intentionally added, 0 is substituted.

[0086] The above relational expression 1 is a combination of alloy elements that can form hard phases, bainite and lath martensite, in the center of the steel microstructure of the present invention, and maintain ferrite, twinned martensite, and bainite at appropriate levels in the surface layer. As the "T" value of the relational expression 1 increases, the formation of hard phases, bainite and lath martensite, increases, and the hardness value of each hard phase also increases. Therefore, the larger this value is, the more advantageous it is for securing strength and hardness. However, if it is excessive, ferrite and twinned martensite cannot be formed in the surface layer, which deteriorates the bending workability of the steel and increases the material deviation in the overall length and overall width of the hot-rolled steel sheet. To this end, it is preferable to manage the value of the above relational expression 1 to be 30 or more and 100 or less.

[0087] The surface microstructure of the hot-rolled steel sheet up to 10㎛ may include, in terms of area fraction, 5 to 40% ferrite and 2 to 10% twinned martensite. The twinned martensite is broadly classified as martensite, but is distinguished from lath martensite, which has a plate-like substructure of several ㎛ in width, and autotempered martensite, which includes epsilon carbide as a micro-carbide, and is martensite with a twin substructure of a fine thickness (several tens of nm) that is crystallographically in a twin relationship. Since the twinned martensite is difficult to distinguish in FE-SEM, it can be interpreted using a high-resolution transmission electron microscope (TEM). Meanwhile, TEM measurement can be evaluated by collecting a specimen based on a cross-section parallel to the rolling direction at a location of 10㎛ from the surface of the hot-rolled steel sheet.

[0088] If the ferrite content of the surface layer is too low, the surface layer, which is mostly composed of a hard phase, has insufficient deformation resistance during bending, easily forming surface irregularities and ultimately leading to fracture. On the other hand, if the ferrite content is too high, the deformation that should be distributed throughout the entire thickness is concentrated only in a limited surface layer, which can form surface irregularities despite a large bending radius.

[0089] The twinned martensite in the above-mentioned surface layer serves to buffer the rapid strength change between the ferrite in the surface layer and the martensite in the deep layer, and serves to prevent excessive strain concentration only in the surface layer. Therefore, an effective bendability improvement effect can be obtained by harmonizing the appropriate ratio of the ferrite and twinned martensite, and the twinned martensite formed by the influence of carbon and hardenable elements concentrated in not only the ferrite in the surface layer but also the austenite that has not yet been transformed can be included in an amount of 2% or more to prevent excessive strain concentration and secure appropriate workability. However, if there is too much twinned martensite in the surface layer, there is a problem that the surface becomes vulnerable to deformation, causing cracks to easily occur.

[0090] Meanwhile, the remainder of the surface layer is preferably a hard structure. Specifically, it can be bainite, tempered bainite, martensite, tempered martensite, etc.

[0091] The central microstructure of the above hot-rolled steel sheet is mainly composed of martensite, and depending on the control of the coiling temperature, carbides such as tempered martensite (auto-tempered martensite in which epsilon carbide is observed) due to the self-tempering effect may be observed. Specifically, the hot-rolled steel sheet may include, in terms of area fraction, 80% or more (including 100%) of martensite and auto-tempered martensite, and 20% or less (including 0%) of at least one of pearlite and bainite. Typically, the central microstructure targets a point at 1 / 4 of the thickness of the steel sheet.

[0092] By sufficiently securing the above martensite and auto-tempered martensite, excellent strength and hardness can be secured.

[0093] In the above hot-rolled steel sheet, old austenite that is elongated in the rolling direction can be observed. This can be observed using SEM. At this time, the aspect ratio of the old austenite can be 1 to 10. It can be confirmed that the effect of improving bending workability is exerted when the aspect ratio is in the range of 1 to 10. This is because the bending line during bending is placed parallel to the rolling direction, so the deformation is in the form of elongation perpendicular to the rolling direction, and at this time, the microstructure formed long in the rolling direction has a very small grain size when viewed perpendicular to the rolling direction, which can be judged to be due to the lowering of deformation resistance.

[0094] The above hot-rolled steel plate may have a central cross-sectional hardness of 400 HV or more and a widthwise tensile strength of 1300 MPa or more.

[0095] In addition, since the bendability (R / t) is 4 or less and the difference between the original bendability (r / t) and the bendability is 0.5 or more, it can have excellent strength and hardness while improving the bendability, especially the bendability of the surface. Meanwhile, it is preferable that the original bendability (r / t) is 6.0 or less.

[0096] The above bendability (R / t) evaluation is, for example, performed by placing a mold having a 90-degree bending angle at the bottom and a specific V-shaped bending radius, and pressing with a mold of the same corresponding shape at the top, and performing a 90-degree bending process, and is defined as the ratio of the steel plate thickness (t) to the bending radius (R) at which no surface unevenness occurs during processing, and the above original bendability is measured by the same method after milling the surface by 0.1 to 0.5 mm.

[0097]

[0098] Next, an embodiment of the method for manufacturing a hot-rolled steel sheet of the present invention will be described in detail.

[0099] First, the above manufacturing method may include heating, hot rolling, cooling, and coiling a steel slab satisfying the aforementioned alloy composition and relational expression 1.

[0100] steel slab heating

[0101] A steel slab satisfying the aforementioned alloy composition and equation 1 is heated to a temperature range of 1150 to 1350°C. If the steel slab heating temperature is less than 1150°C, the precipitates are not sufficiently re-dissolved, which reduces the formation of precipitates in the process after hot rolling, coarse TiN remains, and the slab is not sufficiently aged, making it difficult to control the temperature of the steel sheet at a constant level during hot rolling. On the other hand, if the slab heating temperature exceeds 1350°C, the strength is reduced due to abnormal grain growth of austenite grains. Therefore, the slab heating temperature is preferably in the range of 1150 to 1350°C. The lower limit of the slab heating temperature is more preferably 1155°C, and even more preferably 1160°C. The upper limit of the above slab heating temperature is more preferably 1340°C, more preferably 1330°C, and most preferably 1320°C.

[0102] hot rolling

[0103] The above-mentioned heated steel slab is subjected to rough rolling at the rough rolling temperature (RDT) of the following relational expression 2 to obtain a bar plate. The above-mentioned rough rolling temperature (RDT) is preferably performed based on t / 2 (t: thickness of the steel material). At this time, the thickness of the steel material is preferably the thickness of the bar plate. The above-mentioned rough rolling can be performed within a temperature range of 900 to 1100°C.

[0104] [Relationship 2]

[0105] RDT (℃) ≤ (FT + 80 + bar plate thickness * 3.8)

[0106] FT = 907 + 239[Al] - 10.3[Cr] - 25.8[Mn] + 17.9[Mo] + 30.9[Si] - 254[C]

[0107] [Al], [Cr], [Mn], [Mo], [Si], and [C] in the above relational expression 1 represent the content (weight %) of the corresponding alloy elements. If these components are not intentionally added, 0 is substituted, and the bar plate thickness is mm.

[0108] The above bar plate is subjected to finish rolling at the finish rolling temperature (FDT) of the following relational expression 3 to obtain a hot-rolled steel sheet. The above finish rolling temperature (FDT) is preferably performed based on t / 2 (t: thickness of the steel). At this time, the thickness of the steel is preferably the thickness of the hot-rolled steel sheet.

[0109] [Relationship 3]

[0110] FDT (℃) ≥ FT

[0111] In order to improve the bending workability in relation to the hardness, the difference in the microstructure composition and hardness between the surface and the center is essential, and it is desirable that the degree of elongation of the old austenite of the center martensite is limited so as to satisfy the characteristics of the microstructure described above. To this end, the temperatures RDT and FDT at the end of the rough rolling and finish rolling have different temperature limits according to the steel components and bar plate thicknesses, as shown in Equations 2 and 3. If the RDT is higher than the calculated formula by Equation 2, the microstructure formation of the surface and the center cannot be induced to the desired composition, and the steel becomes a typical martensitic high-strength steel, which may result in poor bending workability. If the FDT is lower than the calculated formula by Equation 3, the aspect ratio of the center structure cannot be as desired, which may also adversely affect the bending workability.

[0112] After obtaining the above bar plate, further descaling and surface cooling can be performed across the entire width of the bar plate at a hydraulic pressure of 150 bar or more. The descaling can further enhance the effect of rapidly cooling the surface, and the additional surface cooling can facilitate the formation of soft tissue on the surface.

[0113] It is preferable that the above bar plate thickness satisfies the following relational expression 4. It is necessary that the surface layer cools faster than the center during rolling. At this time, if the bar plate thickness is too thin, heat conduction occurs quickly to the center, preventing a temperature difference from occurring between the surface layer and the center. Therefore, it is preferable to satisfy the condition of the above relational expression 4.

[0114] [Relationship 4]

[0115] Bar plate thickness (mm) ≥ thickness of hot rolled steel plate (mm) * 10

[0116] cooling

[0117] After the above hot-rolled steel sheet is manufactured, the hot-rolled steel sheet is first cooled to a first cooling stop temperature of 150 to 350°C at a first average cooling rate of 60 to 90°C / sec.

[0118] After the above first cooling, the second cooling is performed at an average second cooling rate of 1 to 50°C / sec to a coiling temperature (CT) of 50 to 200°C.

[0119] The above primary cooling is performed at an average cooling rate of 50 to 100°C / sec to a temperature range of 150 to 350°C. This is to ensure the balanced formation of sufficient central martensite, surface ferrite, and twinned martensite. At this time, excessively high cooling rates should be avoided as they can deteriorate the coil shape.

[0120] The above-mentioned primary cooled steel plate is then subjected to secondary cooling at an average cooling rate of 1 to 40°C / sec to a temperature in the range of 50 to 200°C. At this time, the cooling rate is preferably 1°C / sec or higher to form the intended microstructure. Excessively high-speed cooling may deteriorate the shape quality of the plate and provide no further benefits, so it is preferable to maintain the cooling rate within the above range. Meanwhile, from a productivity perspective, switching to low-speed cooling is advantageous, so it is preferable to introduce low-speed cooling as much as possible.

[0121] Winding

[0122] After the secondary cooling, coiling is performed in the range of 50 to 200°C. The coiling is to obtain an auto-tempering effect, and when the time is long at a temperature where carbides can be formed but below the temperature at which martensite phase transformation starts, a microstructure including epsilon carbide, called transition carbide, can be formed without a separate heat treatment process. A method for implementing this in the hot rolling process is to utilize the phenomenon that the coil cooling time becomes longer than 1 hour when coiling is performed in a specific temperature range higher than room temperature. For this purpose, in the present invention, the coiling is preferably performed at a temperature of 50°C or higher and 200°C or lower. If the temperature is too low, the auto-tempering effect cannot be obtained, and if the coiling temperature is too high, the auto-tempering effect may be excessive, resulting in deterioration of strength and bendability.

[0123] Pickling and oiling

[0124] After the above coiling, a step of pickling and oiling the coiled hot-rolled steel sheet may be additionally included. The present invention does not specifically limit the pickling and oiling process, and any method commonly used in the relevant technical field may be used.

[0125] Hereinafter, embodiments of the present invention will be described. It should be apparent to those skilled in the art that various modifications to the following embodiments may be made without departing from the scope of the present invention. The following embodiments are intended to facilitate understanding of the present invention, and the scope of the present invention should not be limited to the following embodiments, but should be determined not only by the claims set forth below but also by their equivalents.

[0126] (Example)

[0127] A steel slab having the composition (unit, weight %, the remainder being Fe and unavoidable impurities) shown in Table 1 below was prepared. In Table 1 below, T was calculated using the following equation 1.

[0128] [Relationship 1]

[0129] 30 ≤ T ≤ 100

[0130] T =(([C] / 10) 0.5 )*(0.7*[Si]+1)*(5*[Mn]+1)*(2.5*[Cr]+1)*(5*[Mo]+1)*25

[0131] [C], [Si], [Mn], [Cr], and [Mo] in the above relational expression 1 represent the content (weight %) of the corresponding alloy elements, and if these components are not intentionally added, 0 is substituted.

[0132] Classification CSiMnCrMoNbTiVAlPSNBT Steel grade 10.210.11.20.30.010.0010.020.010.030.0050.0020.0040.001550 Steel grade 20.180.21.10.40.010.0030.020.020.030.010.0020.0040.00252 Steel grade 30.220.050.80.40.20.0050.020.020.030.010.0020.0050.00277 Steel grade 40.230.110.50.10.0070.040.010.030.010.0050.0060.00282 Steel grade 50.20.020.80.30.020.0070.030.020.020.0030.0040.00235 Steel grade 60.250.010.70.30.050.0070.030.010.020.0050.0030.0030.00239 Steel grade 70.190.21.50.20.10.0050.020.010.020.010.0030.0040.00275 Steel grade 80.230.21.50.20.10.0050.030.010.040.0150.0050.0030.00383 steel grade 90.180.022.30.650.350.0050.040.010.030.0150.0050.0060.002307 steel grade 100.170.050.90.0050.050.010.030.010.040.010.0040.0060.00123 steel grade 110.210.051.20.10.50.010.0250.010.040.0150.0030.0040.0025115 Steel grade 120.220.11.10.30.10.070.070.010.030.0050.0020.0040.001568 Steel grade 130.20.021.70.40.020.0050.020.020.040.0150.0020.0040.00175

[0133] The above steel slab was subjected to heating, rough rolling, finish rolling, primary cooling, secondary cooling, and coiling under the conditions shown in Table 2 below. In Table 2 below, RDT is the rough rolling end temperature, FDT is the finish rolling end temperature, and is the point t / 2 (t: thickness of steel). CT is the coiling temperature. It was confirmed that these are in accordance with the following equations 2 and 3. The thickness of the hot-rolled steel sheet manufactured above is 3 mm.

[0134] [Relationship 2]

[0135] RDT (℃) ≤ (FT + 80 + bar plate thickness * 3.8)

[0136] FT = 907 + 239[Al] - 10.3[Cr] - 25.8[Mn] + 17.9[Mo] + 30.9[Si] - 254[C]

[0137] [Al], [Cr], [Mn], [Mo], [Si], and [C] in the above relational expression 2 represent the content (weight %) of the corresponding alloy elements. If these components are not intentionally added, 0 is substituted, and the bar plate thickness is mm.

[0138] [Relationship 3]

[0139] FDT (℃) ≥ FT

[0140] Classification Steel grade Slab heating temperature RDT (℃) FDT (℃) Bar thickness (mm) FT (℃) Primary cooling rate (℃ / s) Primary cooling stop temperature (℃) Secondary cooling rate (℃ / s) CT (℃) Invention example 1 Steel grade 11242101484334830632282180 Invention example 2 Steel grade 212371026856338428222723126 Invention example 3 Steel grade 31241102685137839722222988 Invention example 4 Steel grade 412361015844348306422519133 Invention example 5 Steel grade 51248101984635838682261667 Invention example 6 Steel grade 612211016844358288123725104Comparative Example 1 Steel Type 712351023849338315524027416Comparative Example 2 Steel Type 812211018845398255223521300Comparative Example 3 Steel Type 912061005828348096322831134Comparative Example 4 Steel Type 101231101083836853612272071Comparative Example 5 Steel Type 111231101985228842852322485Comparative Example 6 Steel Type 1212401029853348327223820137Comparative Example 7 Steel Type 1312531037834328196222720141

[0141] The microstructures of the central and surface layers of the hot-rolled steel sheets manufactured according to Tables 1 and 2 above were observed and are shown in Table 3 below.

[0142] Aspect Ratio is the aspect ratio calculated as the major axis / minor axis of the old austenite grain size of the central martensite. The above microstructural observations were all performed using a transmission electron microscope and a general electron microscope at magnifications of 1000x and 3000x, respectively, and were measured when distinction was possible.

[0143] Classification Steel grade Austenite aspect ratio (major axis / minor axis) Central bainite fraction (%) Central martensite fraction (%) Surface bootwind martensite fraction (%) Surface ferrite fraction (%) Invention example 1 steel type 1301001220 Invention example 2 steel type 245951225 Invention example 3 steel type 3401001125 Invention example 4 steel type 45.401001220 Invention example 5 steel type 545951325 Invention example 6 steel type 63.4595925 Comparative example 1 steel type 767525065 Comparative example 2 steel type 888515055 Comparative example 3 steel type 96010000 Comparative example 4 steel type 103.42575830Comparative example 5 steel type 116010000Comparative example 6 steel type 121401001225Comparative example 7 steel type 132.5010000

[0144] For the hot-rolled steel sheets in Table 3 above, hardness, physical properties, and bending workability were evaluated, and the results are shown in Table 4 below. In Table 4 below, the surface hardness and the central cross-section hardness are Vickers hardness. The surface hardness was measured at the surface, and the central cross-section hardness was measured at a cross-section with a thickness of half the cross-section. r / t is the circular bendability, and R / t is the bendability, which is the ratio of the bending radius (R) to the steel sheet thickness (t), and is expressed based on the minimum bending radius at which no unevenness or cracks occur on the surface even after 90 degrees of bending. At this time, the bent specimen was processed to be long in the direction perpendicular to the rolling direction, and the bending was performed so that the bend line was parallel to the rolling direction. The circular bendability was measured after milling the surface by 0.2 to 0.5 mm. YS and TS are the yield strength and tensile strength measured in the width direction of the coil, which is perpendicular to the rolling direction, during a tensile test. The unit is MPa, and the tensile test standard used was a JIS No. 5 standard test specimen.

[0145] Meanwhile, Fig. 1 is a graph showing the relationship between tensile strength (TS) and bendability (R / t) according to the results in Table 4 below.

[0146] ClassificationSteel gradeSurface hardness(Hv)Center cross-sectional hardness(Hv)Circular bendability(r / t)Bendability(R / t)YS(MPa)TS(MPa)Invention example 1Steel grade 14394743.72.712411489Invention example 2Steel grade 24044383.32.711251378Invention example 3Steel grade 34464803.02.312601507Invention example 4Steel grade 44564924.03.013011546Invention example 5Steel grade 54494844.03.012731517Invention example 6Steel grade 64635044.33.713401581Comparative example 1Steel grade 72552762.33.0636868Comparative example 2 steel type 82602803.03.3651882Comparative example 3 steel type 94184575.35.311851437Comparative example 4 steel type 103694004.74.310081258Comparative example 5 steel type 114334684.74.712211471Comparative example 6 steel type 124254604.74.311951445Comparative example 7 steel type 134404755.75.712441493

[0147] As shown in Tables 1 to 4 and Fig. 1, all of the invention examples 1 to 6, which meet the conditions set forth in the present invention, were able to secure excellent strength and hardness, as well as excellent processability. It can be confirmed that they possess excellent mechanical properties.

[0148] On the other hand, referring to Fig. 1, it can be seen that as the strength increases, the bending workability of the comparative examples deteriorates as R / t increases.

[0149] Specifically, Comparative Examples 1 and 2 satisfied the steel composition and equation 1 presented in the present invention, but the coiling temperature during the manufacturing process exceeded the range of the present invention, and thus cooling was completed at a high temperature. As a result, in the final microstructure, a large amount of martensite in the center was replaced by bainite, resulting in very low strength, and ferrite and twinned martensite were not formed within an appropriate range in the surface layer. Consequently, sufficient hardness could not be secured as the coiling temperature increased.

[0150] Comparative Example 7 satisfied the steel composition and equation 1 presented in the present invention, but did not satisfy the condition of equation 2 due to excessively high RDT among the manufacturing process conditions. As a result, a combination of ferrite and twinned martensite in the surface layer was not formed in the microstructure, and the bendability was very poor.

[0151] Comparative Example 3 is a case where the relationship 1 related to the hardenable elements and components such as Mn, Cr, and Mo exceeds the scope of the present invention. As a result, ferrite and twinned martensite were not formed at all in the surface layer, and even when all manufacturing process conditions were satisfied, no improvement in bendability was observed.

[0152] Comparative Example 4, unlike Example 3, was composed of alloy components under conditions where the value of Equation 1 fell short of the scope of the present invention. Accordingly, it was confirmed that even if all the manufacturing process conditions suggested in the present invention were satisfied, sufficient martensite could not be secured, making it impossible to produce high-strength steel. Specifically, most of the central portion formed a bainite structure, and accordingly, the strength and hardness were lower than those of other examples. Of course, the strength and hardness were lowered, but the workability was improved, and an appropriate amount of ferrite and twinned martensite were formed in the surface layer, but it is not suitable for the present invention, which aims to improve bendability in high-strength steel.

[0153] Comparative Example 5 is a case where the Mo content and the T value of the relational expression 1 are outside the range of the present invention, and the thickness of the bar plate is too small during the manufacturing process. As a result, during the hot-rolled steel sheet manufacturing process, the temperature difference between the surface and the center of the bar plate is small, and as a result, ferrite + twin martensite cannot be formed in the surface layer, and sufficient bendability is not secured, and the difference between the original bendability and the bendability does not fall within the required range.

[0154] Comparative Example 6 is a case where the Nb content is excessively high. Therefore, even if all the manufacturing process conditions presented in the present invention are satisfied, the aspect ratio of the old austenite falls outside the range of the present invention, resulting in poor bendability and original bendability.

Claims

1. In weight%, 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%, the remainder including Fe and other unavoidable impurities, satisfying the following relationship 1, The microstructure of the surface layer up to 10 μm from the surface contains, in area fraction, 5 to 40% ferrite and 2 to 10% twinned martensite. A hot-rolled steel sheet having a core microstructure comprising, in area fraction, 80% or more (including 100%) of martensite and auto-tempered martensite, and 20% or less (including 0%) of at least one of pearlite and bainite. [Relationship 1] 30 ≤ T ≤ 100 T =(([C] / 10) 0.5 )*(0.7*[Si]+1)*(5*[Mn]+1)*(2.5*[Cr]+1)*(5*[Mo]+1)*25 [C], [Si], [Mn], [Cr], and [Mo] in the above relational expression 1 represent the content (weight %) of the corresponding alloy elements, and if these components are not intentionally added, 0 is substituted.

2. In claim 1, The above central microstructure is a hot-rolled steel sheet having an aspect ratio of old austenite of 1 to 10.

3. In claim 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having a central cross-sectional hardness of 400 HV or more and a widthwise tensile strength of 1300 MPa or more.

4. In claim 1, The above hot-rolled steel sheet is a hot-rolled steel sheet having a bendability (R / t) of 4.0 or less.

5. In claim 1, The above hot-rolled steel sheet is a hot-rolled steel sheet in which the difference between the original bendability and the bendability is greater than 0.

5.

6. A step of heating a steel slab containing, by weight%, C: 0.17 to 0.26%, Si: 0.01 to 0.5%, Mn: 0.4 to 2.2%, Cr: 0.005 to 0.6%, Mo: 0.005 to 0.3%, Nb: 0.001 to 0.01%, Ti: 0.005 to 0.08%, V: 0.005 to 0.05%, Al: 0.01 to 0.5%, P: 0.003 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.0005 to 0.003%, the remainder being Fe and other unavoidable impurities, and satisfying the following relationship 1, in a temperature range of 1150 to 1350℃; A step of performing rough rolling on the above heated steel slab at a rough rolling temperature (RDT) of the following relational expression 2 to obtain a bar plate; A step for obtaining a hot rolled steel sheet by performing finish rolling on the above bar plate at the finish rolling temperature (FDT) of the following relational expression 3; A step of first cooling the hot-rolled steel sheet to a first cooling stop temperature of 150 to 350°C at a first average cooling rate of 60 to 90°C / sec; A step of secondary cooling to a coiling temperature (CT) of 50 to 200°C at a secondary average cooling rate of 1 to 50°C / sec after the primary cooling; and Step of winding after the above second cooling A method for manufacturing a hot rolled steel sheet comprising: [Relationship 1] 30 ≤ T ≤ 100 T = (([C] / 10) 0.5 )*(0.7*[Si]+1)*(5*[Mn]+1)*(2.5*[Cr]+1)*(5*[Mo]+1)*25 [C], [Si], [Mn], [Cr], and [Mo] in the above relational expression 1 represent the content (weight %) of the corresponding alloy elements, and if these components are not intentionally added, 0 is substituted. [Relationship 2] RDT (℃) ≤ (FT + 80 + bar plate thickness * 3.8) FT = 907 + 239[Al] - 10.3[Cr] - 25.8[Mn] + 17.9[Mo] + 30.9[Si] - 254[C] [Al], [Cr], [Mn], [Mo], [Si], and [C] in the above relational expression 2 represent the content (weight %) of the corresponding alloy elements. If these components are not intentionally added, 0 is substituted, and the thickness of the bar plate is mm. [Relationship 3] FDT (℃) ≥ FT 7. In claim 6, A method for manufacturing a hot-rolled steel sheet, further comprising the step of descaling and surface cooling the entire width of the bar plate at a hydraulic pressure of 150 bar or more after obtaining the above bar plate.

8. In claim 6, The above bar thickness is a method for manufacturing a hot-rolled steel plate satisfying the following relationship 4. [Relationship 4] Bar plate thickness (mm) ≥ Thickness of hot rolled steel plate (mm) * 10 9. In claim 6, The above rough rolling is a method for manufacturing hot rolled steel sheets, which is performed at a temperature range of 900 to 1100°C.

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