Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
  • B21C47/02—Winding-up or coiling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
  • C21D1/60—Aqueous agents
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/62—Quenching devices
  • C21D1/667—Quenching devices for spray quenching
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
  • C23G1/08—Iron or steel
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present disclosure relates to a hot-rolled steel sheet and a method for manufacturing the same.
• a wear-resistant hot-rolled steel sheet having high hardness has high strength and hardness by mainly using a martensite-series microstructure, and such high hardness leads to high wear resistance, so the wear-resistant hot-rolled steel sheet has been used as a component requiring wear resistance.
• it has the disadvantage of being used with only minimal processing applied because processing is limited due to poor bending workability due to the high strength. This also acts as a limiting factor similarly in a hot-rolled steel material having high strength using martensite as a main phase, and various technologies have been proposed to overcome this.
• a high-strength steel sheet was comprised of tempered martensite in a central portion and ferrite and pearlite in a surface layer portion as main phases in a thickness direction of the steel sheet, so that it was intended to improve bendability.
• Patent Document 1 discloses a technology which requires high-temperature rolling and requires control of a texture of steel, which is difficult to control, and thus has difficulties in the manufacturing process.
• Patent Document 1 has the disadvantage that it is difficult to be used as a wear-resistant steel since it is difficult to secure uniform hardness as a bainite phase is included in the central portion, as a main phase in addition to martensite that can secure high hardness.
• Patent Document 2 as the surface layer portion is comprised of ferrite and pearlite, which are excessively soft structures, compared to the central portion, the surface layer portion has low hardness, it has low hardness, and at the same time, deformation is concentrated in the surface layer portion during bending, so that Patent Document 2 has the disadvantage that it is unsuitable to be used as a wear-resistant steel.
• alloying components such as Si, Mn, Mo, Cr, Cu, Ni, or the like which are mainly used to manufacture the steels having high hardness described above, are effective in improving hardness and formability, but if a large amount of alloying components are added to improve properties, segregation of alloying components and unevenness of microstructure occur, resulting in poor bending workability.
• a steel with high hardenability is sensitive to a change in microstructures when cooled, so a low-temperature transformation structure is formed unevenly, making it difficult to obtain higher bending workability.
• An aspect of the present disclosure is to provide a hot-rolled steel sheet and a method for manufacturing the same.
• a preferred aspect of the present disclosure is to provide a hot-rolled steel sheet having excellent bending workability, strength, and hardness and a method for manufacturing the same.
• a hot-rolled steel sheet including by weight%: 0.17 to 0.26% of C, 0.01 to 0.5% of Si, 0.3 to 2.0% of Mn, 0.005 to 0.5% of Cr, 0.005 to 0.55% of Mo, 0.005 to 0.05% of Nb, 0.005 to 0.08% of Ti, 0.005 to 0.2% of V, 0.01 to 0.5% of Al, 0.003 to 0.05% of P, 0.001 to 0.01% of S, 0.001 to 0.01% of N, 0.0005 to 0.005% of B, with a remainder of Fe and other inevitable impurities, and satisfying the following Relational Expressions 1 and 2, wherein a microstructure has a central portion including by area%, 90% or more of the sum of martensite and auto-tempered martensite and 10% or less of at least one of pearlite and bainite, and a surface layer portion including by area%, 90% or more of bainite and 10% or less of at least one of pearlite and bainite, and a surface layer portion including by area%
• Ti * Ti ⁇ 3.42 N ⁇ 1.5 S in the Relational Expression 1, Ti* is represented by the [Equation 1], and a content of each alloy composition in the Relational Expressions 1 and 2 and Equation 1 refers to % by weight.
• the central portion may have Rockwell hardness of 44 to 50 HrC.
• the surface layer portion may have Rockwell hardness of 38 to 46 HrC.
• the hot-rolled steel sheet may have a value of [bending workability (R/t) ⁇ (tensile strength - 1000)] ⁇ 1000 of 5 or less.
• a method of manufacturing a hot-rolled steel sheet including: heating a slab including by weight%, 0.17 to 0.26% of C, 0.01 to 0.5% of Si, 0.3 to 2.0% of Mn, 0.005 to 0.5% of Cr, 0.005 to 0.55% of Mo, 0.005 to 0.05% of Nb, 0.005 to 0.08% of Ti, 0.005 to 0.2% of V, 0.01 to 0.5% of Al, 0.003 to 0.05% of P, 0.001 to 0.01% of S, 0.001 to 0.01% of N, 0.0005 to 0.005% of B, with a remainder of Fe and other inevitable impurities, and satisfying the following Relational Expressions 1 and 2, to a temperature within a range of 1150 to 1350°C; completing rough rolling of the heated slab at a rough rolling temperature (RDT) of 880 to SCT+170°C based on 1/2t, where t is a thickness of a steel material, to obtain a bar;
• RDT rough rolling temperature
• Ti* is represented by the [Equation 1]
• a content of each alloy composition in the Relational Expressions 1 and 2 and Equations 1 to 3 refers to % by weight.
• the surface temperature of the bar and the surface temperature of the hot-rolled steel sheet may be controlled by a water spraying device.
• pickling and oiling the coiled hot-rolled steel sheet may be further included.
• a hot-rolled steel sheet and a method for manufacturing the same may be provided.
• a hot-rolled steel sheet having excellent bending formability and hardness and a method for manufacturing the same may be provided.
• a content of an alloy composition described below refers to % by weight, unless otherwise specified.
• Carbon (C) is an element which is the most economical and effective for strengthening steel, and has a significant influence on a hardness value. As a content of added C increases, hardenability increases, making it easier to form hard phases such as bainite, martensite, or the like, in a microstructure, which increases a tensile strength. In addition, C forms fine precipitates together with Ti and Nb, which have a high affinity with C, and both a yield strength and tensile strength increase through precipitation strengthening. However, when the content of C exceeds 0.26%, there is a problem in that hardness of martensite itself increases excessively, resulting in excessive increase in strength and reduced bending workability, and it may be difficult to secure sufficient weldability.
• the content of C when the content of C is less than 0.17%, it is difficult to obtain a sufficient strengthening effect. Therefore, it is preferable that the content of C be within the range of 0.17 to 0.26%.
• a lower limit of the content of C is more preferably 0.175%, even more preferably 0.18%, and most preferably 0.185%.
• An upper limit of the content of C is more preferably 0.25%, even more preferably 0.24%, and most preferably 0.23%.
• Silicon (Si) is an element which is advantageous in deoxidizing molten steel, exhibiting a solid solution strengthening effect, and improving formability by delaying the formation of coarse carbides.
• the content of Si is less than 0.01%, the effects of solid solution strengthening and the formation of bainite and martensite may not be sufficiently obtained.
• the content of Si exceeds 0.5%, it is not easy to remove red scales formed on a surface of the steel sheet during hot rolling, and as a result thereof, the surface quality of the steel sheet may become very poor.
• the ductility and weldability are also reduced. Therefore, it is preferable that the content of Si be within a range of 0.01 to 0.5%.
• a lower limit of the content of Si is more preferably 0.012%, even more preferably 0.015%, and most preferably 0.02%.
• An upper limit of the content of Si is more preferably 0.4%, even more preferably 0.35%, and most preferably 0.3%.
• Manganese (Mn), like Si, is an element which is effective in strengthening steel by solid solution, and increases the hardenability of steel, facilitates the formation of bainite and martensite, which are hard phases, during cooling after hot rolling.
• Mn manganese
• the content of Mn is less than 0.3%, the effects of solid solution strengthening and the formation of bainite and martensite may not be sufficiently obtained.
• the content of Mn exceeds 2.0%, grain boundaries becomes weak, which causes a problem such as low-temperature cracking, or the like.
• the strength may increase excessively, making it difficult to secure sufficient formability, and in a continuous casting process, a segregation zone is greatly developed in a central portion of the thickness during slab casting, and when cooling is performed after hot rolling, a microstructure is formed unevenly in the thickness direction, resulting in poor bending workability.
• a lower limit of the content of Mn is more preferably 0.35%, even more preferably 0.4%, and most preferably 0.45%.
• An upper limit of the content of Mn is more preferably 1.9%, even more preferably 1.85%, and most preferably 1.8%.
• Chromium (Cr) strengthens steel by solid solution and delays ferrite phase transformation during cooling, thereby serving to help the formation of martensite and bainite.
• the content of Cr is less than 0.005%, the effects of solid solution strengthening and the formation of martensite and bainite may not be sufficiently obtained.
• the content of Cr exceeds 0.5%, similarly to Mn, a segregation zone in a central portion of the thickness is greatly developed and a microstructure in the thickness direction is formed unevenly, which reduces the bending workability. Therefore, it is preferable that the content of Cr be within the range of 0.005 to 0.5%.
• a lower limit of the content of Cr is more preferably 0.007%, even more preferably 0.008%, and most preferably 0.01%.
• An upper limit of the content of Cr is more preferably 0.4%, even more preferably 0.35%, and most preferably 0.3%.
• Molybdenum (Mo) increases the hardenability of steel to facilitate the formation of martensite and bainite.
• the content of Mo is less than 0.005%, the above-described effect may not be sufficiently obtained.
• the content of Mo exceeds 0.55%, martensite is formed in the surface layer portion due to an excessive increase in hardenability, which significantly deteriorates bending workability, is economically disadvantageous, and it may be difficult to secure sufficient weldability. Therefore, it is preferable that the content of Mo be within the range of 0.005 to 0.55%.
• a lower limit of the content of Mo is more preferably 0.01%, even more preferably 0.02%, and most preferably 0.03%.
• An upper limit of the content of Mo is more preferably 0.52%, even more preferably 0.5%, and most preferably 0.45%.
• Niobium (Nb) is a representative precipitation strengthening element, together with Ti and V, and is precipitated as a precipitate during hot rolling and exerts a grain refinement effect by delaying recrystallization, thereby effectively improving the strength and impact toughness of steel.
• the content of Nb is less than 0.005%, the above-described effect may not be sufficiently obtained.
• the content of Nb exceeds 0.05%, coarse composite precipitates are formed during hot rolling, which deteriorates bending workability. Therefore, it is preferable that the content of Nb be within the range of 0.005 to 0.05%.
• a lower limit of the content of Nb is more preferably 0.007%, even more preferably 0.008%, and most preferably 0.01%.
• An upper limit of the content of Nb is more preferably 0.04%, even more preferably 0.03%, and most preferably 0.02%.
• Titanium (Ti) is a representative precipitation strengthening element together with Nb and V, and forms coarse TiN through strong affinity with nitrogen.
• the TiN has the effect of suppressing grain growth during a heating process for hot rolling.
• Ti remaining after reacting Ni is dissolved in steel, and is combined with carbon to form TiC precipitates, which is a useful component for improving the strength of steel.
• the content of Ti is less than 0.005%, the effects of suppressing grain growth and improving strength may not be sufficiently obtained.
• the content of Ti exceeds 0.08%, coarse TiN occurs and the precipitates become coarse, which deteriorates the bending workability during forming. Therefore, it is preferable that the content of Ti be within the range of 0.005 to 0.08%.
• a lower limit of the content of Ti is more preferably 0.01%, even more preferably 0.015%, and most preferably 0.02%.
• An upper limit of the content of Ti is more preferably 0.07%, even more preferably 0.06%, and most preferably 0.045%.
• V Vanadium (V): 0.005 to 0.2%
• Vanadium (V) is a representative precipitation strengthening element together with Nb and Ti, and V hardly precipitates during hot rolling, but V forms precipitates after high-temperature coiling, cooling, or tempering, thereby improving the strength of steel. Therefore, V is effective in improving the additional strength without increasing deformation resistance and a rolling load due to recrystallization delay during hot rolling.
• the content of V is less than 0.005%, the effect of improving strength may not be sufficiently obtained.
• the content of V exceeds 0.2%, coarse precipitates are formed, which deteriorates bending workability and is also economically disadvantageous. Therefore, it is preferable that the content of V be within the range of 0.005 to 0.2%.
• a lower limit of the content of V is more preferably 0.006%, even more preferably 0.008%, and most preferably 0.01%.
• An upper limit of the content of V is more preferably 0.2%, even more preferably 0.1%, and most preferably 0.05%.
• Aluminum (Al) is an element added mainly for deoxidation.
• the content of Al is less than 0.01%, the deoxidation effect may not be sufficiently obtained.
• the content of Al exceeds 0.5%, Al combines with nitrogen to form excessive AlN, which makes it easy for corner cracks to occur in a slab during continuous casting, and defects due to the formation of inclusions are likely to occur. Therefore, it is preferable that the content of Al be within the range of 0.01 to 0.5%.
• a lower limit of the content of Al is more preferably 0.015%, and even more preferably 0.02%.
• An upper limit of the content of Al is more preferably 0.1%, and even more preferably 0.08%, and even more preferably 0.05%.
• Phosphorus (P), like Si, has the effects of both solid solution strengthening and ferrite transformation promotion.
• the content of P in order to control the content of P to be less than 0.003%, it requires a lot of manufacturing costs, which is economically disadvantageous and is insufficient to obtain the strength.
• the content of P exceeds 0.05%, brittleness due to grain boundary segregation may occur, microcracks are likely to occur during bending, and ductility and impact resistance properties are significantly reduced. Therefore, it is preferable that the content of P be within the range of 0.003 to 0.05%.
• a lower limit of the content of P is more preferably 0.005%, even more preferably 0.007%, and most preferably 0.01%.
• An upper limit of the content of P is more preferably 0.03%.
• Sulfur (S) is an impurity present in steel, and when the content of S exceeds 0.01%, S combines with Mn, or the like to form non-metallic inclusions, and accordingly, S has the problem in that microcracks are likely to occur during bending of steel and impact resistance is significantly reduced.
• a lower limit of the content of S is not particularly limited, but in order to control a lower limit of the content of S to be less than 0.001%, it takes a lot of time during steelmaking, which reduces productivity, so considering the same, the lower limit of the content of S may be limited to 0.001%. Therefore, it is preferable that the content of S be within the range of 0.001 to 0.01%.
• the lower limit of the content of S is more preferably 0.002%.
• An upper limit of the content of S is more preferably 0.008%, even more preferably 0.006%, and most preferably 0.005%.
• Nitrogen (N) is a representative solid solution strengthening element together with C, and forms coarse precipitates together with Ti, Al, or the like.
• N is a representative solid solution strengthening element together with C, and forms coarse precipitates together with Ti, Al, or the like.
• the content of N is less than 0.001%, it is difficult to sufficiently obtain the effects of solid solution strengthening and precipitate formation, and in order to control the content of N to be less than 0.001%, it takes a lot of time during steelmaking, which reduces productivity.
• the solid solution strengthening effect of N is generally superior to that of carbon, there is a problem in that the toughness is significantly reduced when the content of N exceeds 0.01%. Therefore, it is preferable that the content of N be within the range of 0.001 to 0.01%.
• a lower limit of the content of N is more preferably 0.002%, and even more preferably 0.003%.
• An upper limit of the content of N is more preferably 0.008%, and even more preferably 0.007%, and even more preferably 0.006%.
• B When boron (B) is present in steel, in a solid-solution state, B is mainly segregated at grain boundaries and has the effect of improving the brittleness of steel by stabilizing grain boundaries, and plays a role in suppressing the formation of coarse AlN nitrides by stabilizing solid solution N. In addition, B delays ferrite phase transformation and is effective in the formation of bainite and martensite, which are hard phases. When the content of B is less than 0.0005%, the effects of improving brittleness, suppressing the formation of coarse AlN nitrides, and forming bainite and martensite may not be sufficiently obtained.
• the content of B when the content of B exceeds 0.005%, the above-described effects no longer increase, and there is a disadvantage in that the ductility decreases and the formability deteriorates. Therefore, it is preferable that the content of B be within the range of 0.0005 to 0.005%.
• a lower limit of the content of B is more preferably 0.0006%, even more preferably 0.0008%, and most preferably 0.001%.
• An upper limit of the content of B is more preferably 0.004%, and even more preferably 0.003%.
• the remaining component of the present disclosure is iron (Fe).
• Fe iron
• the component since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.
• the hot-rolled steel sheet of the present disclosure satisfies the alloy composition described above, and satisfies the following Relational Expressions 1 and 2.
• the Relational Expression 1 is intended to balance a precipitation phenomenon and hardenability to be advantageous for bendability.
• the X value is less than 0.1, grain growth is facilitated during reheating, and recrystallization becomes uneven during hot rolling, so coarse grains are formed locally, and solid solution C and solid solution N become excessively high, which tends to increase a hardness value of the hard phase, which may ultimately result in poor bending workability.
• the X value exceeds 0.6, the formation of precipitates increases excessively, and also, when the hot-rolled steel sheet is cooled, the solid solution C and solid solution N atoms in an untransformed phase are insufficient, making it difficult to stably form a hard phase and grain boundaries become weak, resulting in poor bending workability.
• the X value be within the range of 0.1 to 0.6.
• a lower limit of the X value is more preferably 0.13, even more preferably 0.15, and most preferably 0.18.
• An upper limit of the X value is more preferably 0.58, even more preferably 0.56, and most preferably 0.55.
• 1.5 ⁇ T Mn+2.8Mo+1.5Cr+500B ⁇ 5.0
• the Relational Expression 2 is factorization of a combination of alloying elements that can maintain the formation of bainite and martensite, which are hard phases, in the microstructure of the steel of the present disclosure, at an appropriate level.
• the T value is less than 1.5, it is difficult to obtain the desired hardness value because the hard phase is not sufficiently secured.
• the larger the T value the more the formation of hard phases such as bainite, martensite, and MA phase increases, and the hardness value of each of the hard phases also increases. Therefore, the larger the T value, the more advantageous it is for securing strength and hardness.
• the T value exceeds 5.0 there is a problem that the bending workability deteriorates and the material deviation increases in the overall length and overall width of the hot-rolled steel sheet.
• the T value be within the range of 1.5 to 5.0.
• a lower limit of the T value is more preferably 1.7, even more preferably 2.0, and most preferably 2.5.
• An upper limit of the T value is more preferably 4.9, even more preferably 4.7, and most preferably 4.5.
• the microstructure of the hot-rolled steel sheet of the present disclosure includes a central portion including by area %, 90% or more of the sum of martensite and auto-tempered martensite and 10% or less of at least one of pearlite and bainite, and a surface layer portion including by area %, 90% or more of bainite and 10% or less of at least one of ferrite, martensite, and pearlite.
• a total fraction of martensite and auto-tempered martensite in the microstructure of the central portion is less than 90% or a total fraction of at least one of pearlite and bainite exceeds 10%, there is a disadvantage in that hardness of the central portion becomes excessively low, making it difficult to obtain the desired high hardness.
• the total fraction of martensite and auto-tempered martensite in the microstructure of the central portion is more preferably 92% or more, even more preferably 94% or more, and most preferably 95% or more.
• the total fraction of at least one of ferrite, martensite, and pearlite in the microstructure of the central portion is more preferably 8% or less, even more preferably 6% or less, and most preferably 5% or less.
• the fraction of bainite in the microstructure of the surface layer portion is more preferably 92% or more, even more preferably 94% or more, and most preferably 95% or less.
• the fraction of at least one of ferrite, martensite, and pearlite in the microstructure of the surface layer portion is more preferably 8% or less, even more preferably 6% or less, and most preferably 5% or less.
• An average thickness of the surface layer portion is preferably 30 to 200 ⁇ m. When the average thickness of the surface layer portion is less than 30 ⁇ m, a soft surface layer portion may not be sufficiently secured, resulting in poor bending workability.
• an upper limit of the average thickness of the surface layer portion is not particularly limited, but it is not easy to exceed 200 ⁇ m in the manufacturing process.
• a lower limit of the average thickness of the surface layer portion is more preferably 32 ⁇ m, even more preferably 35 ⁇ m, and most preferably 40 ⁇ m.
• the upper limit of the average thickness of the surface layer portion is more preferably 150 ⁇ m, even more preferably 120 ⁇ m, and most preferably 100 ⁇ m.
• the surface layer portion means a region [(from a surface of the steel sheet to a point 30 ⁇ m deep in a thickness direction) to (from a surface of the steel sheet to a point 200 ⁇ m deep in the thickness direction)], and the central portion means a region outside the surface layer portion.
• the hot-rolled steel sheet of the present disclosure may have an oxide layer formed on the surface, the surface layer portion does not include the oxide layer, and therefore, the surface layer portion may be a region from directly beneath the oxide layer of the steel sheet to 30 to 200 ⁇ m in the thickness direction.
• the present inventors have confirmed that an average dislocation density, hereinafter referred to as 'Geometrical Necessary Dislocation (GND)', of the surface layer portion is an important factor in the balance between the strength and bending workability of a steel material. More specifically, the average potential density of the surface layer is preferably 1.7 ⁇ 10 14 to 3.0 ⁇ 10 14 m -2 . When the average dislocation density of the surface layer portion is less than 1.7 ⁇ 10 14 m -2 , deformation is concentrated in the surface layer portion and the continuity is reduced, resulting in poor bending workability. When the average dislocation density of the surface layer portion exceeds 3.0 ⁇ 10 14 m -2 , the surface layer portion is not softened and becomes vulnerable to bending deformation.
• GTD Global Necessary Dislocation
• the average potential density of the surface layer be within the range of 1.7 ⁇ 10 14 to 3.0 ⁇ 10 14 m -2 .
• a lower limit of the average dislocation density of the surface layer portion is more preferably 1.8 ⁇ 10 14 m -2 , even more preferably 1.9 ⁇ 10 14 m -2 , and most preferably 2.0 ⁇ 10 14 m -2 .
• An upper limit of the average dislocation density of the surface layer portion is more preferably 2.9 ⁇ 10 14 m -2 , and most preferably 2.8 ⁇ 10 14 m -2 .
• the average dislocation density may be calculated using kernel average misorientation (KAM) data measured by EBSD as expressed in the following Equation 1.
• such calculation may be performed using software such as OIM analysis TM (EDAX), which analyzes the EBSD measurement results.
• EDAX OIM analysis TM
• the EBSD measurement may be performed based on a cross-section of the steel sheet, parallel to the rolling direction at 1/4 a thickness of the steel sheet.
• GND m ⁇ 2 2 ⁇ / ub in the Expression 4
• ⁇ is average misorientation (KAM values)
• u is a unit length (step size in the EBSD measurement)
• b is a burgers vector.
• the central portion has an aspect ratio of old austenite of 5 or more.
• the aspect ratio of old austenite of the central portion is more preferably 6 or more, even more preferably 7 or more, and most preferably 8 or more.
• the upper limit thereof is not particularly limited. However, it is difficult for the aspect ratio of old austenite of the central portion to exceed 30 in the manufacturing process.
• a hot-rolled steel sheet of the present disclosure provided may have Rockwell hardness of 44 to 50 HrC of the central portion, and have Rockwell hardness of 38 to 46 HrC of the surface layer portion.
• the hot-rolled steel sheet may have a value of [bending workability (R/t) ⁇ (tensile strength - 1000)] ⁇ 1000 of 5 or less.
• the Rockwell hardness may be measured by using a Rockwell hardness tester (C scale) to measure the surface at five points in accordance with ASTM-E18-22 and calculate an average value thereof.
• a slab satisfying the above-described alloy composition and Relational Expressions 1 and 2 is heated to a temperature within a range of 1150 to 1350°C.
• the slab heating temperature is lower than 1150°C, precipitates are not sufficiently redissolved, so the formation of precipitates is reduced in a process after hot rolling, coarse TiN remains, and the slab is not sufficiently heated, making it difficult to control a temperature of a steel sheet at a constant level during hot rolling.
• the slab heating temperature exceeds 1350°C, the strength is reduced due to abnormal grain growth of austenite crystal grains. Therefore, it is preferable that the slab heating temperature be within the range of 1150 to 1350°C.
• a lower limit of the slab heating temperature is more preferably 1155°C, and even more preferably 1160°C.
• An upper limit of the slab heating temperature is more preferably 1340°C, even more preferably 1330°C, and most preferably 1320°C.
• the heated slab is subjected to completing rough rolling at a rough rolling temperature (RDT) of 880 to SCT+170°C based on 1/2t, where t is a thickness of a steel material, to obtain a bar.
• RDT rough rolling temperature
• the rough rolling temperature is lower than 880°C, a problem may occur in equipment operation due to an excessive high rolling load, and when the rough rolling temperature exceeds SCT+170°C, it is difficult to sufficiently elongate old austenite, making it difficult to improve bending workability. Therefore, it is preferable that the rough rolling temperature be within the range of 880 to SCT+150°C.
• a lower limit of the rough rolling temperature is more preferably 890°C, even more preferably 900°C, and most preferably 910°C.
• An upper limit of the rolling temperature is more preferably SCT+165°C, even more preferably SCT+160°C, and most preferably SCT+155°C.
• the SCT may be obtained by Equation 2, which is represented as follows.
• the 1/2t which is mentioned above, refers to a point of 1/2 in the thickness direction from a surface of the steel sheet.
• SCT(°C) 741+134C-137Si+75.4Mn- 21.4Cr+24.8Mo-1391Nb-13Ti+19330B
• a surface temperature (RST) of a bar at the end of the rough rolling is 750 to RDT-40°C.
• RST surface temperature
• the surface temperature of the bar at the end of the rough rolling is lower than 750°C, a problem may occur in equipment operation due to an excessively large rolling load.
• the surface temperature of the bar exceeds RDT-40°C, there is a disadvantage in that bainite may not be sufficiently formed in the surface layer portion.
• the surface temperature of the bar at the end of the rough rolling be within the range of 750 to RDT-40°C.
• a lower limit of the surface temperature of the bar at the end of the rough rolling is more preferably 765°C, even more preferably 780°C, and most preferably 800°C.
• An upper limit of the surface temperature of the bar at the end of the rough rolling is more preferably RDT-42°C, even more preferably RDT-45°C, and most preferably RDT-50°C.
• a method for controlling the surface temperature of the bar at the end of the rough rolling is not particularly limited, and for example, a water spraying device such as a descaler, or the like may be used.
• the bar is subjected to completing finishing rolling at a finishing rolling temperature (FDT) of 780 to SCT+30°C based on 1/2t, where t is a thickness of a steel material, to obtain a hot-rolled steel sheet.
• FDT finishing rolling temperature
• the finishing rolling temperature is lower than 780°C, a problem may occur in equipment operation due to an excessively large rolling load, and when the finishing rolling temperature exceeds SCT+30°C, it is difficult to sufficiently elongate old austenite, which has the disadvantage of making it difficult to improve bending workability. Therefore, it is preferable that the finishing rolling temperature be within the range of 780 to SCT+30°C.
• a lower limit of the finishing rolling temperature is more preferably 790°C, even more preferably 800°C, and most preferably 810°C.
• An upper limit of the finishing rolling temperature is more preferably SCT+25°C, even more preferably SCT+22°C, and most preferably SCT+20°C.
• a surface temperature (FST) of the hot-rolled steel sheet at the end of the finishing rolling is 700 to FDT-40°C.
• FST surface temperature
• the surface temperature of the bar at the end of the finishing rolling is lower than 700°C, a problem may occur in equipment operation due to an excessively large rolling load, and when the surface temperature of the bar exceeds FDT-40°C, there is a disadvantage in that bainite may not be sufficiently formed in the surface layer portion. Therefore, it is preferable that the surface temperature of the bar at the end of the finishing rolling be within the range of 700 to FDT-40°C.
• a lower limit of the surface temperature of the bar at the end of the finishing rolling is more preferably 710°C, even more preferably 730°C, and most preferably 750°C.
• An upper limit of the surface temperature of the bar at the end of the finishing rolling is more preferably FDT-42°C, even more preferably FDT-43°C, and most preferably FDT-45°C.
• a method for controlling the surface temperature of the bar at the end of the finishing rolling is not particularly limited, and for example, a water spraying device such as a descaler, or the like may be used.
• cooling start temperature 700°C to SCT+10°C and primarily cooled to a primary cooling stop temperature of Ms to Ms+50°C at a primary average cooling rate of 50 to 100°C/sec.
• WCT cooling start temperature
• a high-temperature phase such as ferrite or pearlite is formed instead of a low-temperature phase such as bainite or martensite, which has the disadvantage of significantly reducing strength and hardness.
• the primary cooling start temperature exceeds SCT+10°C, the driving force for forming martensite rather than bainite in the surface layer portion is increased, so that bainite is not sufficiently formed in the surface layer portion or a thickness of the surface layer portion is insufficient. Therefore, it is preferable that the primary cooling start temperature be within the range of 700°C to SCT+10°C.
• a lower limit of the primary cooling start temperature is more preferably 710°C, even more preferably 730°C, and most preferably 750°C.
• An upper limit of the primary cooling start temperature is more preferably SCT+5°C, even more preferably SCT+2°C, and most preferably SCT°C.
• the primary cooling stop temperature When the primary cooling stop temperature is lower than Ms, a high-temperature phase such as ferrite or pearlite is formed before a low-temperature phase such as martensite or bainite is formed, which has the disadvantage of significantly reducing the strength and hardness of the steel sheet or making the material uneven.
• the primary cooling stop temperature exceeds Ms+50°C, martensite is not sufficiently formed in the central portion, so the hardness is decreased. Therefore, it is preferable that the primary cooling stop temperature be within the range of Ms to Ms+50°C.
• a lower limit of the primary cooling stop temperature is more preferably Ms+5°C, even more preferably Ms+8°C, and more preferably Ms+10°C.
• An upper limit of the primary cooling stop temperature is more preferably Ms+45°C, even more preferably Ms+40°C, and most preferably Ms+30°C.
• the primary average cooling rate is less than 50°C/sec, the formation of martensite and bainite may become uneven, and the driving force for forming the low-temperature phase may be low, making it difficult to obtain sufficient strength and hardness.
• the primary average cooling rate exceeds 100°C/sec, the risk of accidents such as plate entrapment within the equipment may increase due to a rapid volume change in the steel sheet caused by excessively rapid phase transformation. Therefore, it is preferable that the primary average cooling rate be within the range of 50 to 100°C/sec.
• a lower limit of the primary average cooling rate is more preferably 52°C/sec, even more preferably 55°C/sec, and most preferably 60°C/sec.
• An upper limit of the primary average cooling rate is more preferably 97°C/sec, even more preferably 93°C/sec, and most preferably 90°C/sec.
• the primarily-cooled hot-rolled steel sheet is secondarily cooled at a second average cooling rate of 1 to 40°C/sec to a coiling temperature (CT) of 70°C to Ms-50°C, and then coiled.
• CT coiling temperature
• the secondary average cooling rate has a range of 1 to 40°C/sec.
• a lower limit of the secondary average cooling rate is more preferably 2°C/sec, even more preferably 3°C/sec, and most preferably 5°C/sec.
• An upper limit of the secondary average cooling rate is more preferably 39°C/sec, even more preferably 37°C/sec, and most preferably 35°C/sec.
• a wear-resistant steel with high hardness is generally manufactured by performing a heat treatment to make the microstructure tempered martensite in order to overcome poor workability.
• a heat treatment is performed, the surface layer portion is decarburized and the surface hardness is excessively lowered to the level of ferrite, which has the disadvantage of greatly reducing the function as a wear-resistant steel material. To prevent this, it is preferable to omit the heat treatment in the present disclosure.
• a coiling temperature to 70°C to Ms-50°C in order to add an auto-tempering (self-tempering) effect to the bainite of the surface layer portion and the martensite of the central portion.
• the coiling temperature is lower than 70°C, the auto-tempering effect is not significant, there is a disadvantage in that it is difficult to obtain the effect of improving bending workability.
• the coiling temperature exceeds Ms-50°C, a stress formed by phase transformation inside a low-temperature phase may not be sufficient, and thus high hardness may not be obtained.
• the coiling temperature be within the range of 70°C to Ms-50°C.
• a lower limit of the coiling temperature is more preferably 80°C, even more preferably 90°C, and most preferably 100°C.
• An upper limit of the coiling temperature is more preferably Ms-60°C, even more preferably Ms-80°C, and most preferably Ms-100°C.
• pickling and oiling the coiled hot-rolled steel sheet may be further included.
• the pickling and oiling processes are not specifically limited, and all methods commonly used in the relevant technical field may be used.
• a slab having the alloy composition illustrated in Tables 1 and 2 below was prepared and then a hot-rolled steel sheet was manufactured under the conditions illustrated in Tables 3 and 4 below. Meanwhile, a temperature of a bar at the end of rough rolling and a temperature of the hot-rolled steel sheet at the end of finishing rolling illustrated in Tables 3 and 4 below were based on 1/2t, where t is a thickness of a steel material, and a slab heating temperature, a cooling start temperature, and a coiling temperature were based on 1/2t, where t is a thickness of a steel material.
• a type and fraction of a microstructure, an aspect ratio of old austenite of a central portion, an average thickness of a surface layer portion were measured using an electron microscope after collecting specimens from the surface layer portion and the central portion of a hot-rolled steel sheet, at 1/4 a thickness of the steel sheet.
• the aspect ratio of old austenite of the central portion was measured in a transverse direction (TD), which is a side surface of the steel sheet, and a horizontal direction in the image was set to be a rolling direction (RD), and a vertical direction in the image was set to be a normal direction (ND).
• GMD average potential density
• EDAX OIM analysis TM
• EBSD electron back scattered diffraction
• Hardness was measured by measuring a surface and a central portion, at 1/4 the thickness of the steel sheet, of a hot-rolled steel sheet at 5 points on the surface using a Rockwell hardness tester (C scale) in accordance with ASTM-E18-22, and an average value thereof was calculated.
• Bending workability refers to a ratio of a bending radius (R) and a thickness (t) of a steel sheet, and is represented based on a minimum bending radius at which no cracks occur on the surface after a 90-degree bending test of the steel sheet.
• a bending specimen was processed to be long in a direction perpendicular to a rolling direction, and a bending test was performed so that a bending line of the bending specimen was parallel to the rolling direction.
• Comparative Example 1 showed that an X value of Relational Expression 1 was excessively high, so that a driving force for forming precipitates was excessive, and accordingly, carbon and nitrogen did not sufficiently play a role in improving strength within martensite and bainite, resulting in insufficient hardness.
• GND falls short of the scope of the present disclosure.
• Comparative Example 2 showed that a T value of Relational Expression 1 was excessively high, which failed to properly form bainite or martensite, so that an entire steel sheet was comprised of almost martensite without a surface layer portion, resulting in high surface hardness and poor bending workability. In addition, it was confirmed that GND exceeded the scope of the present disclosure.
• Comparative Example 3 showed that an X value of Relational Expression 1 was excessively low, so there was almost no driving force for forming precipitates, and accordingly, the role of carbon and nitrogen in improving strength within martensite and bainite was excessive, resulting in poor bending workability. In addition, it was confirmed that GND exceeded the scope of the present disclosure.
• Comparative Example 4 showed that a T value of Relational Expression 1 was excessively low, which failed to properly form bainite or martensite, resulting in excessive low hardness. In addition, it can be seen that the GND falls short of the range of the present disclosure.
• Comparative Example 5 showed that the alloy composition of the present disclosure is satisfied but a cooling start temperature is high, a surface layer portion was cooled rapidly and a driving force to form martensite than bainite was strong so that a thickness of the surface layer portion was insufficient. As a result, it was shown that the bending workability was poor.
• Comparative Example 6 showed that the alloy composition of the present disclosure is satisfied, but a finishing rolling temperature was high, so an aspect ratio of old austenite of martensite of the central portion is as small as 3, and as a result, it can be seen that the bending workability is inferior to the strength.
• Comparative Examples 7 and 8 showed that the alloy composition of the present disclosure is satisfied, but a difference in temperatures between RDT and RST or FDT and FST was not sufficiently obtained, so a driving force to form bainite in the surface layer portion is low, and a surface layer having a sufficient thickness could not be obtained. As a result, it can be seen that the bending workability was inferior.
• Comparative Examples 9 and 10 showed that the manufacturing conditions of the present disclosure are all satisfied due to excessive high content of carbon, but it can seen that both the surface layer portion and the central portion have excessively high hardness, and as a result, the bending workability is inferior. In addition, it was confirmed that GND exceeds the range of the present disclosure.
• Comparative Example 11 showed that the alloy composition of the present disclosure is satisfied, but a coiling temperature is excessively low, so the auto-tempering effect is not obtained, and thus it can be seen that bending workability is inferior to the strength.
• Comparative Example 12 showed that the alloy composition of the present disclosure is satisfied, but has excessively high coiling temperature so has a weak driving force to form martensite and bainite to have low hardness and strength. In addition, it can be seen that GND falls short of the range of the present disclosure.
• FIG. 1 is a graph illustrating a relationship between bending workability (R/t) according to tensile strength of Inventive Examples 1 to 10 and Comparative Examples 1 to 12. As shown in FIG. 1 , in the case of Comparative Examples 1 to 12, it can be seen that the bending workability (R/t) tends to deteriorate overall as the tensile strength increases. In the case of Inventive Examples 1 to 10, the bending workability is relatively good compared to the tensile strength, so that a value of [bending workability (R/t) ⁇ (tensile strength - 1000)] ⁇ 1000 is 5 or less, and therefore, it can be confirmed that the bending workability is excellent.
• FIG. 2 is a photograph of Inventive Example 1 observed using an electron microscope.
• FIG. 3 is a photograph of a surface layer portion of Inventive Example 1 observed using an electron microscope.
• FIG. 4 is a photograph of a central portion of Inventive Example 1 observed using an electron microscope. As can be seen from FIGS. 2 to 4 , in the case of Inventive Example 1, it could be confirmed that the microstructures of the surface layer portion and the central portion that the present disclosure intends to be obtained are formed.

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