Classifications

• • 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
• • 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
  • 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
  • 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
  • C21D8/0247—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 heat treatment
  • C21D8/0263—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 heat treatment following 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
  • C22C38/18—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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • 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
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • 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
  • C21D2211/001—Austenite
• • 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
  • C21D2211/002—Bainite
• • 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
  • C21D2211/005—Ferrite
• • 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
• • 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

Definitions

• the present disclosure relates to a hot-rolled steel sheet suitable for use in a structural chassis member of an automobile and the like and a method for manufacturing the same.
• chassis parts of automobiles play a role in supporting the body of the automobile and are important parts in ensuring ride comfort and driving stability by absorbing road vibrations and shocks during driving.
• the fatigue load applied to the chassis parts increases, and thus the steel applied to the chassis parts of electric vehicles and the like is required to have excellent fatigue strength.
• chassis parts are mainly manufactured by press forming.
• the weight of parts may be reduced by reducing the thickness of the steel sheet through the application of high-strength steel, the shape of the parts cannot be significantly changed, and thus securing formability suitable for press forming of parts is becoming more important than anything else in the manufacture of high-strength steel.
• Patent Document 1 discloses a method for manufacturing a high-strength hot-rolled steel sheet with excellent hole expandability in which the microstructure is comprised of bainitic ferrite without carbides as the main phase, using steel containing 0.01 to 0.05% of carbon in weight %. According to patent document 1, it is possible to manufacture a steel plate with excellent hole expandability of tensile strength 980 grade or higher, but the yield strength is low, so the fatigue characteristics are inferior, the effect of weight reduction of the part is minimal, and the elongation is inferior, and thus there is concern that the formability of the part will be inferior.
• Patent document 1 Japanese Patent Laid-Open Publication No. 2008-255484
• An aspect of the present disclosure is to provide a hot-rolled steel sheet having high strength and excellent fatigue property, as well as excellent formability, and suitable for press forming, and a method for manufacturing the same.
• the subject matter of the present disclosure is not limited to the above-described contents.
• the subject matter of the present disclosure may be understood from the entire contents of this specification, and a person having ordinary knowledge in the technical field to which the present disclosure belongs will have no difficulty in understanding the additional task of the present disclosure.
• a hot-rolled steel sheet comprises, in weight %, carbon (C): 0.09 to 0.25%, silicon (Si): 0.5 to 2.3%, manganese (Mn): 1.5 to 3.5%, aluminum (Al): 0.001 to 1.0%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 to 0.20%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.0001 to 0.05%, sulfur (S): 0.0001 to 0.05%, nitrogen (N): 0.0001 to 0.05%, a remainder of Fe and unavoidable impurities,
• the hot-rolled steel sheet may further comprise niobium (Nb): 0.01 to 0.2%.
• An average grain size of the bainitic ferrite may be 2.0 ⁇ m or more.
• An average spacing of the bainitic ferrite may be 3 ⁇ m or more.
• the hot-rolled steel sheet may have a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, an elongation of 9% or more, and a hole expansion ratio of 45% or more.
• a method for manufacturing a hot-rolled steel sheet comprises, an operation of reheating a steel slab at a temperature within a temperature range of 1100 to 1350°C, the steel slab comprising, in wt%, carbon (C): 0.09 to 0.25%, silicon (Si): 0.5 to 2.3%, manganese (Mn): 1.5 to 3.5%, aluminum (Al): 0.001 to 1.0%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 to 0.20%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.0001 to 0.05%, sulfur (S): 0.0001 to 0.05%, nitrogen (N): 0.0001 to 0.05%, a remainder of Fe and other unavoidable impurities;
• FDT represents a finishing delivery temperature (°C)
• [C], [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent weight% contents of elements in parentheses, respectively.
• k T is a value defined by Relationship 4 below, and ts represents a second cooling time.
• k T 20 Du ⁇ exp ⁇ T 1 + 2 ⁇ T 2 / 3 ⁇ 557 + 320 ⁇ C + 35 ⁇ Si + 90 ⁇ Mn + 70 ⁇ Cr + 120 ⁇ Mo + 160000 ⁇ Bat 112 1.92
• a total reduction amount of final 2 passes may be 10 to 40%.
• the method for manufacturing a hot-rolled steel sheet may further comprise an operation of final cooling to room temperature after the coiling.
• the method for manufacturing a hot-rolled steel sheet may further comprise an operation of pickling and oiling after the final cooling.
• the method for manufacturing a hot-rolled steel sheet may further comprise an operation of hot-dip zinc plating after the pickling and oiling.
• a steel sheet having a high strength of 980 MPa or more and excellent formability and a method for manufacturing the same may be provided. Accordingly, it may be suitably applied to structural chassis members of automobiles, and the like.
• the hot-rolled steel sheet manufacturing method of the related art which prevents the deterioration of hole expandability due to the difference in hardness between phases by making the fraction of a specific microstructure of the steel 90% or more, could not secure excellent elongation.
• the hot-rolled steel sheet manufacturing method of the related art which prevents the deterioration of hole expandability due to the difference in hardness between phases by making the fraction of a specific microstructure of the steel 90% or more, could not secure excellent elongation.
• hole expandability may be improved by securing high strength and elongation by using martensite and austenite as a matrix structure, while uniformly dispersing bainitic ferrite as a secondary phase within the structure to prevent excessive stress concentration at a specific location during deformation and suppress the occurrence of microcracks, thereby completing the present disclosure.
• the present disclosure will be described in more detail below.
• the steel sheet may comprise, in wt%, carbon (C): 0.09 to 0.25%, silicon (Si): 0.5 to 2.3%, manganese (Mn): 1.5 to 3.5%, aluminum (Al): 0.001 to 1.0%, chromium (Cr): 2.5% or less (including 0%), molybdenum (Mo): 2.0% or less (including 0%), titanium (Ti): 0.01 to 0.20%, boron (B): 0.0005 to 0.005%, phosphorus (P): 0.0001 to 0.05%, sulfur (S): 0.0001 to 0.05%, and nitrogen (N): 0.0001 to 0.05%.
• Carbon (C) is an important element that forms retained austenite by stabilizing austenite by diffusing into austenite after martensite phase transformation. As the content of C increases, the fraction of retained austenite increases, thereby simultaneously improving elongation and tensile strength. If the content of C is less than 0.09%, the fraction of retained austenite is low, so elongation and tensile strength cannot be secured. On the other hand, if the content exceeds 0.25%, the B S temperature is excessively low, making carbon diffusion difficult, resulting in a problem of excessive fresh martensite generation and poor hole expandability. Therefore, in the present disclosure, the content of C is preferably 0.09 to 0.25%. It is more advantageous that the content of C is 0.090 to 0.250%. More advantageously, the lower limit of the C content may be 0.12%, or the upper limit of the C content may be 0.23%.
• Silicon (Si) is an important element that delays the formation of carbides after martensite transformation and forms retained austenite. In addition, Si plays a role in improving strength through the solid solution strengthening effect. If the content of Si is less than 0.5%, carbides are formed and the fraction of retained austenite is low, making it difficult to secure elongation. On the other hand, if the content exceeds 2.3%, Fe-Si composite oxides are formed on the surface of the slab during reheating, which not only deteriorates the surface quality of the steel sheet, but also deteriorates the weldability. Therefore, in the present disclosure, the content of Si is preferably 0.5 to 2.3%. It is more advantageous that the content of Si is 0.50 to 2.30%. More advantageously, the lower limit of the Si content may be 0.7%, or the upper limit of the Si content may be 2.1%.
• Manganese (Mn) is an element that improves the hardenability of steel, and prevents the formation of ferrite during cooling after finish rolling, thereby facilitating the formation of low-temperature transformation structures.
• the content of Mn is less than 1.5%, there is a problem that the hardenability is insufficient and the fraction of ferrite increases excessively. On the other hand, if the content exceeds 3.5%, the hardenability increases significantly, so that the holding time for sufficiently forming the bainitic ferrite to be obtained in the present disclosure excessively increases, and hole expandability deteriorates.
• the Mn may be included in an amount of 1.5 to 3.5%, and it is more advantageous that the Mn content is 1.50 to 3.50%. More advantageously, the lower limit of the Mn content may be 1.6%, or the upper limit of the Mn content may be 3.0%.
• Aluminum (Al) is an element that is usually added to deoxidize molten steel, and may exist in the steel to some extent after deoxidation. However, similar to Si, it also plays a role in forming retained austenite by delaying the formation of carbides after martensite transformation.
• the content of Al is less than 0.001%, carbides are formed and the fraction of retained austenite is low, making it difficult to secure elongation.
• the content exceeds 1.0% it causes an increase in oxide and nitride inclusions in the steel, which deteriorates the formability of the steel sheet. Therefore, in the present disclosure, it is preferable that the content of Al is 0.001 to 1.0%. More advantageously, the lower limit of the Ai content may be 0.01%, or the upper limit of the Al content may be 0.5%.
• Chromium is an element that improves the hardenability of steel and suppresses the formation of ferrite during cooling after finish rolling. If the content of Cr exceeds 2.5%, the hardenability increases significantly, so that the bainite transformation does not occur smoothly in the cooling zone, and the holding time for securing the fraction of bainitic ferrite increases excessively, thereby deteriorating hole expandability. Therefore, in the present disclosure, the Cr may be included in an amount of 2.5% or less, and more advantageously in 2.50% or less, and more advantageously, may be included in an amount of 1.5% or less.
• the present disclosure includes a case where the Cr content is 0% because there is no significant problem in securing the intended properties even if the Cr is not included.
• the Cr content is 0% because there is no significant problem in securing the intended properties even if the Cr is not included.
• Molybdenum is an element that improves the hardenability of steel, plays a role in improving strength through the solid solution strengthening effect, and suppresses the formation of ferrite during cooling after finish rolling. If the content of Mo exceeds 2.0%, the hardenability increases significantly, and the bainite transformation does not occur smoothly in the cooling zone. As a result, the holding time for securing the fraction of bainitic ferrite increases excessively, and hole expandability decreases. Therefore, in the present disclosure, the Mo may be included in an amount of 2.0% or less, more advantageously at 1.0% or less, and even more advantageously at 0.5% or less.
• the present disclosure includes a case where the Mo content is 0% because there is no significant problem in securing the intended properties even if the Mo is not included.
• the Mo content is 0% because there is no significant problem in securing the intended properties even if the Mo is not included.
• Titanium (Ti) is an element that forms carbonitride in steel, and is widely used to secure the strength of steel by inducing the formation of precipitates in this way. However, in the present disclosure, it is used to control the grain size of austenite before rolling by removing nitrogen (N) in the steel to suppress the formation of BN, thereby concentrating boron (B) in the austenite grain boundary.
• the Ti at 0.01% or more, and it is preferable to contain 2.9 times or more the nitrogen (N) content to remove nitrogen (N) in the steel.
• the content exceeds 0.20%, problems such as clogging of the casting nozzle or the like may occur due to the formation of oxides during continuous casting.
• the Ti may be included in an amount of 0.01 to 0.20%, and more advantageously, the lower limit of the Ti may be 0.015%, or the upper limit of the Ti content may be 0.12%.
• Boron is an element that improves the hardenability of steel by concentrating in austenite grain boundaries and reducing grain boundary energy.
• the phase transformation of ferrite and upper bainite may be suppressed, where the nucleus of phase transformation occurs by diffusion transformation in the austenite grain boundaries, so that a composite structure of martensite and austenite may be secured as the main phase.
• the concentration of B at 0.0005% or more.
• the content exceeds 0.005%, the hardenability increases significantly, so that the holding time for sufficiently forming the bainitic ferrite to be obtained in the present disclosure increases excessively, and hole expandability decreases.
• the B may be included in an amount of 0.0005 to 0.005%, and more advantageously, the lower limit of the B content may be 0.001%, or the upper limit of the B content may be 0.0025%.
• Phosphorus (P) is an impurity that is inevitably contained in steel, and is an element that is the main cause of deteriorating the workability of steel due to segregation. Therefore, it is desirable to control the content as low as possible.
• the lower limit may be set to 0.0001%.
• the upper limit of the P may be limited to 0.05%.
• the lower limit of the content of the P may be 0.0005%, or the upper limit of the content of the P may be 0.02%.
• S Sulfur
• S is an impurity that is inevitably contained in steel, and forms non-metallic inclusions by combining with manganese or the like, which causes a problem of deterioration in the workability of the steel. Therefore, it is desirable to control the content as low as possible.
• the lower limit may be set to 0.0001%.
• the upper limit of the S may be limited to 0.05%.
• the lower limit of the S content may be 0.0005%, or the upper limit of the S content may be 0.005%.
• Nitrogen (N) is an impurity that is inevitably contained in steel, and there is a problem that it forms nitrides by combining with Al or the like, thereby hindering the workability of the steel. Therefore, it is desirable to control the content as low as possible.
• the lower limit may be set to 0.0001%.
• the upper limit of the N may be limited to 0.05%.
• the lower limit of the N content may be 0.001%, or the upper limit of the N content may be 0.006%.
• the hot-rolled steel sheet of the present disclosure may further contain niobium (Nb) in addition to the alloy composition described above.
• Niobium (Nb) is an element that forms carbonitrides in steel, and is widely used to secure the strength of steel by inducing the formation of precipitates, but in the present disclosure, it delays recrystallization during hot rolling and controls the grain size of austenite. If the content of Nb is less than 0.01%, the effect of controlling the grain size is low, and if the content exceeds 0.2%, there is a problem that the grain size of austenite is too fine and formability is poor. Therefore, the content of Nb in the present disclosure may contain 0.01 to 0.2%.
• the remaining component in the present disclosure is iron (Fe).
• Fe iron
• unintended impurities may inevitably be mixed in from raw materials or the surrounding environment during a normal manufacturing process, which cannot be ruled out.
• These impurities are known to anyone skilled in the art of manufacturing processes of the related art, and thus are not specifically mentioned in this specification.
• the hot-rolled steel sheet of the present disclosure includes a composite structure of martensite and austenite as a matrix structure to simultaneously secure a tensile strength of 980 MPa or more and an elongation of 9% or more. Therefore, according to an embodiment of the present disclosure, the composite structure comprised of martensite and austenite may be included in an amount of 75 to 90% in terms of area %.
• a portion of the untransformed austenite is transformed into martensite at the M S temperature or less.
• the steel sheet is gradually cooled and maintained in a state close to isothermal, and at this time, an isothermal phase transformation of austenite into martensite occurs, so that the fraction of martensite increases.
• Martensite is formed by shear transformation (displacive phase transformation), so there is a high level of dislocation density in the structure due to the screw dislocations formed in the structure to reduce the shear strain generated during transformation and the blade dislocations formed to accommodate the volume expansion due to phase transformation, and it is suitable for improving the yield strength and tensile strength of steel due to the fine carbides present in the structure.
• the present disclosure preferably includes the martensite and austenite composite structure at an area fraction of 75% or more. Meanwhile, to secure the hole expandability to be described later, it is preferable to limit the area fraction of the martensite and austenite composite structure to 90% or less.
• the austenite may be 3 to 10% in area %. If the area fraction of austenite is less than 3%, the elongation improvement effect due to the transformation-induced plasticity phenomenon is minimal. On the other hand, in order for the area fraction of the austenite to exceed 10%, the carbon (C) content that should be added to the steel should increase, so there is a problem that the weldability of the steel is inferior, and since the M S temperature is excessively low and carbon diffusion is not easy, the amount of fresh martensite generated in the final cooling stage to room temperature after coiling increases excessively, which causes inferior hole expandability.
• C carbon
• the hot-rolled steel sheet of the present disclosure may comprise bainitic ferrite as a secondary phase of the microstructure, and may contain 10 to 25% of the secondary phase in terms of area %.
• soft bainitic ferrite when soft bainitic ferrite is evenly distributed in an appropriate size within a high-strength matrix structure with an uneven stress distribution, it has the effect of preventing local stress concentration by evenly accommodating deformation during forming, thereby improving hole expandability.
• the fraction of bainitic ferrite, the secondary phase is less than 10%, there is a problem that it is difficult to secure hole expandability, and on the other hand, if the fraction exceeds 25%, there is a problem that it is difficult to secure martensite and austenite composite structures that play a role in improving strength.
• the average grain size of the bainitic ferrite may be 2.0 ⁇ m or more.
• the average spacing of the bainitic ferrite may be 3 ⁇ m or more.
• the average grain size of the bainitic ferrite refers to the equivalent circular diameter
• the average spacing of the bainitic ferrite refers to the average distance between the five most adjacent structures for respective microstructures.
• the average grain size of the soft structure bainitic ferrite
• the effect of strain acceptance is low, and thus improvement in hole expandability cannot be expected.
• the average spacing of the bainitic ferrite is less than 3.0 ⁇ m, the fraction of soft steel increases excessively, so the yield strength and tensile strength may be inferior.
• the upper limit of the average grain size and the upper limit of the average spacing of the soft structure are not separately regulated, but under the condition that the fraction of the soft structure satisfies the range of 10 to 25%, the average grain size of the soft structure may be preferably 20 ⁇ m or less. In addition, the average spacing of the vertical structure may be 20 ⁇ m or less.
• the hot-rolled steel sheet of the present disclosure may include carbides and fresh martensite as other structures in addition to the above-described structures, but it is preferable that these are controlled to an area fraction of less than 5%.
• the hot-rolled steel sheet may generate carbides during the manufacturing process. Immediately after martensite transformation, some carbon atoms may form fine carbides inside the lath, which may play a role in improving strength. On the other hand, since the present disclosure seeks to improve the elongation by utilizing austenite, the formation of carbides may cause a decrease in the austenite fraction. In detail, excessive formation of carbides inhibits the improvement of elongation targeted by the present disclosure.
• alloy carbonitrides may be formed, and in this case, an additional strengthening effect due to grain refinement may be expected, but since coarse carbides inhibit the toughness of the steel, it is preferable that the carbides present in the hot-rolled steel sheet of the present disclosure be less than 5%.
• the hot-rolled steel sheet may include fresh martensite as a microstructure.
• the martensite that is transformed isothermally during the third cooling process and immediately after coiling is transformed before the carbon concentration inside the austenite begins, and therefore has a lath form within the component range of the present disclosure.
• the coiling temperature is excessively low, carbon diffusion is not easy, so cooling may be completed before the austenite is sufficiently stabilized.
• the M S temperature of the austenite in which carbon is concentrated is room temperature or more, it may transform into fresh martensite during cooling.
• the fresh martensite generated during the final cooling process accepts shear deformation as the creation of twins rather than dislocations during phase transformation, so it has a plate shape and has the characteristic of twins being observed within the microstructure, so it may be easily distinguished from the martensite in the present disclosure. Since fresh martensite with a high carbon concentration has excessively high hardness, which makes hole expandability inferior, the fraction of fresh martensite present in the hot-rolled steel sheet of the present disclosure is preferably less than 5%.
• the hot-rolled steel sheet of the present disclosure having the above-described alloy composition and microstructure has high strength with a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, an elongation of 9% or more, a product of tensile strength and elongation being 13000 MPa ⁇ % or more, and a hole expansion ratio of 45% or more, and thus has excellent formability.
• the manufacturing method may be performed by manufacturing a steel slab satisfying the alloy composition proposed in the present disclosure by performing a series of processes of [reheating - hot rolling - cooling - coiling].
• the temperature of the reheating of the steel slab is lower than 1100°C, there is a problem that the homogenization of the alloy elements is not sufficient. On the other hand, if the temperature exceeds 1350°C, excessive oxides are formed on the surface of the slab, which may deteriorate the surface quality of the steel sheet.
• the reheated steel slab may be hot-rolled to manufacture a hot-rolled steel sheet.
• the hot rolling is performed at a temperature within a temperature range of 750 to 1150°C, and it is desirable to control the total reduction amount of the final two passes to 10 to 40%.
• the hot rolling is initiated at a temperature exceeding 1150°C, oxides are excessively formed on the surface of the steel sheet after rolling, and even if the pickling process is performed, they cannot be effectively controlled, resulting in poor surface quality.
• the hot rolling is performed at a temperature lower than 750°C, the rolling load increases excessively, resulting in poor workability, and ferrite is generated during rolling, resulting in poor anisotropy.
• multi-stage rolling is performed during hot rolling to reduce the rolling load and precisely control the thickness.
• the total reduction ratio of the final two passes exceeds 40%, the rolling load of the final two passes becomes excessive, resulting in poor workability.
• the total reduction ratio of the final two passes is less than 10%, the temperature of the steel sheet rapidly decreases, resulting in a problem of poor shape.
• the grain size of austenite after hot rolling is affected by the alloy composition and finishing delivery temperature, which affects the bainite formation behavior and final microstructure in the subsequent cooling process.
• the fraction and size of the main constituent phase, bainitic ferrite, in the present disclosure are greatly affected by the austenite grains after hot rolling.
• bainitic ferrite generated by shear transformation such as bainite grows only within the austenite grains, and its size cannot be larger than the austenite size before transformation. Therefore, to control the size of bainitic ferrite, it is advantageous to control the grain size of austenite after hot rolling.
• the present disclosure derives the effective grain size of austenite after hot rolling as a relationship between the finishing delivery temperature (FDT) and a specific alloy composition, which is specifically defined as Relationship 1 below. That is, during hot rolling, finishing hot rolling is performed so that the value of Du defined by Relationship 1 below satisfies the range of 2 to 10 within a temperature range of 750 to 1150°C.
• Du (FDT+(7.4 ⁇ [C])-(24.7 ⁇ [Si])-(4.7 ⁇ [Mn])- (3.9 ⁇ [Cr])-(5.2 ⁇ [Mo])-(560 ⁇ [Ti])-(1110 ⁇ [Nb])) ⁇ 0.049-34.2
• FDT refers to the finishing delivery temperature (°C)
• [Si], [Mn], [Cr], [Mo], [Ti], and [Nb] represent the weight% contents of the elements in parentheses, respectively.
• the Du is an index indicating the effective grain size of austenite immediately before the first cooling after hot rolling. If the Du value defined by the above Relationship 1 is 2 or more, the average grain size of bainitic ferrite becomes 2.0 ⁇ m or more, so that hole expandability may be secured as 45% or more. On the other hand, if the Du value defined by the above Relationship 1 exceeds 10, the grain boundary concentration of boron element increases excessively, which delays the phase transformation during the second cooling, so that a sufficient fraction of bainitic ferrite cannot be secured, which causes a problem in that hole expandability is inferior. The Du value is more preferably 2.0 to 10.0.
• boron segregates at the grain boundaries of austenite and stabilizes the austenite grain boundaries, thereby delaying the nucleation of ferrite and upper bainite, thereby lowering the phase transformation speed.
• it is important to control the concentration of boron segregated at the grain boundaries of austenite.
• the concentration of boron segregated in the austenite grain boundaries exhibits different values for respective grain boundaries due to the microsegregation during casting and the grain size of the austenite, and thereamong, bainitic ferrite nucleation selectively occurs at the austenite grain boundaries with a low concentration of boron element during the second cooling stage.
• the concentration of boron segregated in each grain boundary is low, making nucleation easy, and when the grain size is large, the concentration of boron segregated in the grain boundary is high, and thus it can be expected that nucleation is delayed. Therefore, the concentration of boron existing in the grain boundaries and the phase transformation behavior of bainitic ferrite during the second cooling are affected by the content of boron added to the steel and the grain size of the austenite, as illustrated in Relationship 2.
• Relationship 2 is an indicator of the concentration of boron (B) distributed at the grain boundaries of austenite immediately before cooling. If the value of Du ⁇ Bat ⁇ 2.968 ⁇ 10 10 according to Relationship 2 below is less than 5.0 ⁇ 10 6 , the fraction of bainitic ferrite is excessive, and the yield strength and tensile strength cannot be secured. On the other hand, if the value of Du ⁇ Bat ⁇ 2.968 ⁇ 10 10 according to Relationship 2 below exceeds 2.0 ⁇ 10 7 , the second cooling time for securing the fraction of bainitic ferrite becomes excessively long, which causes a problem of poor hole expandability. 5.0 ⁇ 10 6 ⁇ Du ⁇ Bat ⁇ 2.968 ⁇ 10 10 ⁇ 2.0 ⁇ 10 7
• FIG. 1 is a graph illustrating the relationship between the boron content and Du, which simultaneously satisfies the above Relationship 1 and Relationship 2, and the microstructure intended in the present disclosure may be secured within the solid line connecting A-B-C-D-E-F.
• the hot-rolled steel sheet manufactured as described above is cooled, which is preferably performed in stages according to the cooling temperature.
• the hot-rolled steel sheet manufactured as described above is rapidly cooled to a temperature (B S ) or less at which bainite begins to form, thereby suppressing the formation of ferrite (granular ferrite). Subsequently, by slowly cooling for a time (seconds) of ts to a temperature intermediate between the bainite initiation temperature (B S ) and the martensite initiation temperature (M S ) or a temperature higher than the temperature, bainitic ferrite having an area fraction of 10 to 25% may be secured.
• the upper limit of the first cooling rate is not particularly limited, but since there is a concern that the plate shape may be distorted if the steel sheet is cooled too rapidly, it may be limited to 200°C/s or less.
• the lower limit of the cooling completion temperature during the first cooling is not particularly limited, but since there is a concern that the cooling time for the subsequent second cooling may not be sufficient if it is lowered excessively, it is stated that it may be limited to B S -100°C.
• the steel cooling is terminated, and second cooling may be performed at a temperature of (B S + M S )/2 or more at a cooling rate of 25°C/s or less.
• k(T) is an indicator of the growth rate of bainitic ferrite, which is affected not only by the alloy composition of the steel but also by the phase transformation temperature and the grain size after hot rolling. Accordingly, if the value of Relationship 3, for example, the relationship (exp(-k(T) ⁇ (ts) 2 )) between k(T) and the holding time, is less than 0.75, the fraction of bainitic ferrite becomes excessive, and thus the target level of strength may not be secured. On the other hand, if the value exceeds 0.9, there is a problem of deterioration of hole expandability. 0.75 ⁇ exp ⁇ k T ⁇ ts 2 ⁇ 0.9
• k T 20 Du ⁇ exp ⁇ T 1 + 2 ⁇ T 2 / 3 ⁇ 557 + 320 ⁇ C + 35 ⁇ Si + 90 ⁇ Mn + 70 ⁇ Cr + 120 ⁇ Mo + 160000 ⁇ Bat 112 1.92
• the temperature of the steel sheet may increase due to transformation heat generation caused by the bainite phase transformation.
• the cooling rate may be controlled to 25°C/s or less during the second cooling to minimize the temperature rise of the steel sheet due to transformation heat generation. If the cooling rate exceeds 25°C/s, there is a concern that the plate shape may be distorted. It is disclosed that the second cooling in the present disclosure also includes the process of air cooling.
• the plate shape When the cooling rate is excessive during the martensitic transformation, the plate shape may be distorted due to rapid volume expansion, which may in turn cause cooling imbalance and result in uneven material distribution. Therefore, in the present disclosure, by setting the cooling rate in the third cooling stage, which is accompanied by rapid phase transformation, to 30°C/s or less, shape distortion during cooling and resulting uneven material deviation within the plate may be prevented. On the other hand, if the cooling rate is excessively slow, bainitic ferrite grows during cooling, and the fraction of the secondary phase increases excessively, and thus it may be difficult to secure the strength of the steel. Therefore, in the present disclosure, the cooling rate in the third cooling stage may be performed at 5°C/s or more.
• the maximum carbon concentration that may be dissolved in austenite exhibits different values depending on the temperature at which carbon enrichment proceeds, and generally, the solubility limit of carbon within austenite increases as the temperature decreases. Therefore, if the coiling temperature at which carbon enrichment occurs is too high, austenite cannot secure phase stability for transformation-induced plasticity because carbon enrichment is not sufficient, and even if it remains at room temperature, it is lost due to stress-induced transformation at the initial stage of deformation, so improvement in elongation cannot be expected. Therefore, in the present disclosure, it is preferable that the upper limit of the third cooling end temperature is M S -20°C to secure sufficient phase stability.
• the lower limit of the third cooling end temperature is 200°C.
• Bs and Ms may be derived by the following equations, and each element represents a weight content.
• B S (°C) 830-(320 ⁇ [C])-(90 ⁇ [Mn])-(35 ⁇ [Si])-(70 ⁇ [Cr])- (120 ⁇ [Mo])
• Ms(°C) 550-(330 ⁇ [C])-(41 ⁇ [Mn])-(20 ⁇ [Si])-(20 ⁇ [Cr])- (10 ⁇ [Mo])+(30 ⁇ [Al])
• the target hot-rolled steel sheet may be obtained by final cooling.
• the final cooling may be completed by performing air cooling to room temperature.
• the hot-rolled steel sheet of the present disclosure obtained by completing the final cooling as described above may be additionally pickled and oiled.
• the hot-rolled steel sheet that has been pickled and oiled may be heated to a temperature within a temperature range of 420 to 740°C in a hot-dip galvanizing process.
• the hot-dip galvanizing process may utilize a zinc-based plating bath, and there is no particular limitation on the alloy composition in the zinc-based plating bath.
• Each of the prepared steel slabs was reheated at 1200°C, and then hot-rolled, cooled, coiled, and finally cooled (air-cooled) under the conditions illustrated in Table 2 below to manufacture hot-rolled steel sheets having a thickness of 2.5 mm.
• the total reduction ratio of the final two passes in the hot rolling was applied equally at 25%, and the cooling rate was uniformly applied at 70°C/s during the first cooling, and uniformly applied at 20°C/s during the third cooling.
• the yield strength, tensile strength, and elongation were measured at room temperature using a universal tensile tester after collecting JIS-5 standard test specimens in a direction perpendicular to the rolling direction. At this time, the yield strength, tensile strength, and elongation were expressed as 0.2% off-set yield strength, maximum tensile strength, and fracture elongation, respectively.
• hole expandability was measured according to the ISO TS16630 standard method for the same specimens as those used in the tensile test.
• each hot-rolled steel sheet was observed at 10,000x magnification using a scanning electron microscope and an image analyzer after etching the same specimen as the above tensile test using the Nital etching method, and the fraction of each phase was calculated.
• the average size of bainitic ferrite was expressed as the equivalent circular diameter, and the average spacing was expressed as the average distance between the five closest structures for respective bainitic ferrite phases.
• the austenite fraction was calculated using the integrated intensity of the diffraction peak of each phase using an X-ray diffractometer by Bruker.
• Relationship 3 is exp(-k(T) ⁇ (ts) 2 ).
• k T 20 Du ⁇ exp ⁇ T 1 + 2 ⁇ T 2 / 3 ⁇ 557 + 320 ⁇ C + 35 ⁇ Si + 90 ⁇ Mn + 70 ⁇ Cr + 120 ⁇ Mo + 160000 ⁇ Bat 112 1.92
• Invention Examples 1 to 11 which satisfy all of the alloy compositions and manufacturing conditions proposed by the present disclosure, were comprised of a composite structure of martensite and austenite with an area fraction of 75 to 90% as a matrix structure and a second phase of bainitic ferrite with an area fraction of 10 to 25%, thereby securing the target strength and formability.
• Comparative Example 1 which does not satisfy the alloy composition system proposed by the present disclosure because boron was not added, was unable to secure the target strength due to excessive generation of bainitic ferrite during the second cooling, and showed a relatively high tendency for elongation due to this low strength. Comparative Example 2 did not obtain sufficient bainitic ferrite because titanium was not added, and thus it was difficult to secure hole expandability required by the present disclosure.
• Comparative Examples 3 and 4 did not satisfy Relationship 3 because the second cooling time was too long, and thus the fraction of the secondary phase, bainitic ferrite, was excessive, and thus the tensile strength of 980 MPa or more could not be secured.
• FIG. 1 is a graph illustrating the relationship between the boron content and Du that simultaneously satisfies Relationship 1 and Relationship 2.
• the microstructure intended by the present disclosure may be secured within the solid line connecting A-B-C-D-E-F.
• FIG. 2 exhibits photographs of the microstructures of Invention Example 4, Comparative Example 2, and Comparative Example 3, respectively, observed using a scanning electron microscope.
• the matrix structure and the secondary phase intended to be implemented in the present disclosure were appropriately formed in the microstructure.
• the secondary phase, the soft structure was not sufficiently formed.
• the secondary phase, the soft structure was excessively formed.

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