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WO2023018081A1 - Tôle d'acier haute résistance laminée à chaud présentant une excellente aptitude au formage, et procédé de fabrication associé - Google Patents

Tôle d'acier haute résistance laminée à chaud présentant une excellente aptitude au formage, et procédé de fabrication associé Download PDF

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Publication number
WO2023018081A1
WO2023018081A1 PCT/KR2022/011282 KR2022011282W WO2023018081A1 WO 2023018081 A1 WO2023018081 A1 WO 2023018081A1 KR 2022011282 W KR2022011282 W KR 2022011282W WO 2023018081 A1 WO2023018081 A1 WO 2023018081A1
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steel sheet
hot
rolled steel
cooling
temperature
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Korean (ko)
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송태진
허원
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Posco Holdings Inc
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Posco Co Ltd
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Priority to US18/293,772 priority Critical patent/US20240254585A1/en
Priority to EP22856080.1A priority patent/EP4386103A4/fr
Priority to JP2024507911A priority patent/JP7751074B2/ja
Priority to CN202280054897.9A priority patent/CN117795118A/zh
Publication of WO2023018081A1 publication Critical patent/WO2023018081A1/fr
Anticipated expiration legal-status Critical
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    • 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
    • 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
    • C21D1/18—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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties 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
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • 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
    • C22C38/38—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
    • 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
    • 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
    • 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

Definitions

  • the present invention relates to a hot-rolled steel sheet that can be suitably applied to a chassis structural member of an automobile, and more particularly, to a high-strength hot-rolled steel sheet with excellent formability and a manufacturing method thereof.
  • the type of parts constituting the vehicle is also changed, and the weight of the vehicle is also changed.
  • the weight of the electric vehicle increases by approximately the weight of the battery compared to the internal combustion engine vehicle.
  • chassis parts of automobiles serve to support a vehicle body and are important parts for securing ride comfort and driving stability by absorbing vibrations and shocks of a road surface during driving.
  • the fatigue load applied to the chassis components increases. Therefore, steel materials applied to chassis components of electric vehicles and the like are required to have excellent fatigue strength.
  • chassis parts are manufactured by press forming, it is required to improve tensile strength and yield strength to improve fatigue strength, as well as secure formability such as elongation and hole expandability suitable for press forming.
  • Patent Document 1 discloses a method of forming a steel microstructure of 90% or more of bainitic ferrite and controlling the fractions of martensite and bainite to 5% or less, respectively, to improve hole expandability.
  • the tensile strength of the hot-rolled steel sheet is 980 MPa or more and the hole expandability is 70% or more, but the elongation improvement required for press forming is not disclosed.
  • chassis parts such as eco-friendly vehicles such as electric vehicles
  • tensile strength and yield strength are high, and fatigue life is excellent, as well as moldability such as elongation and hole expandability to facilitate press molding.
  • moldability such as elongation and hole expandability to facilitate press molding.
  • Patent Document 1 Japanese Unexamined Patent Publication No. 2008-255484
  • One aspect of the present invention is to provide a hot-rolled steel sheet having high strength, excellent fatigue performance, and excellent formability and a manufacturing method thereof.
  • the subject of the present invention is not limited to the above.
  • the subject of the present invention will be understood from the entire contents of this specification, and those skilled in the art will have no difficulty in understanding the additional subject of the present invention.
  • carbon (C) 0.05 ⁇ 0.17%
  • silicon (Si) 0.01 ⁇ 1.5%
  • manganese (Mn) 1.5 ⁇ 3.0%
  • aluminum (Al) 0.01 ⁇ 0.1%
  • Chromium (Cr) 2.0% or less (including 0%)
  • Molybdenum (Mo) 2.0% or less (including 0%)
  • Titanium (Ti) 0.01 to 0.15%
  • Phosphorus (P) 0.001 to 0.05%
  • nitrogen (N) 0.0001 to 0.02%
  • the balance including Fe and other unavoidable impurities and satisfying the following relational expression 1,
  • a high-strength hot-rolled steel sheet with excellent formability consisting of a microstructure of a base structure of 70-90% area fraction of acyclic ferrite or bainitic ferrite and at least one second phase among low-temperature bainite, tempered martensite, and MA phase.
  • each element means a weight content.
  • Another aspect of the present invention comprises the steps of reheating a steel slab satisfying the above-described alloy composition and relational expression 1 in a temperature range of 1100 to 1350 ° C; manufacturing a hot-rolled steel sheet by hot-rolling the reheated steel slab; firstly cooling the hot-rolled steel sheet to a temperature of Bs or less at a cooling rate of 70° C./s or more; Secondary cooling at a cooling rate of 20° C./s or less to a temperature of (Bs+Ms)/2 or higher after the primary cooling; Thirdly cooling at a cooling rate of 30°C/s or more to a temperature range of Ms -20°C to 500°C after the secondary cooling; And a step of winding in the tertiary cooled temperature range,
  • finish hot rolling is performed within a temperature range of 750 to 1150 ° C. to satisfy the following relational expression 2, and the total reduction of the final two passes is 10 to 40%.
  • a method for manufacturing a steel sheet is provided.
  • Du is an index representing the effective grain size of austenite immediately before primary cooling after hot rolling
  • Du FDT + (7.35 ⁇ [C]) - (24.7 ⁇ [Si]) - (4.7 ⁇ [Mn] ) - (3.9 ⁇ [Cr]) - (5.2 ⁇ [Mo]) - (560 ⁇ [Ti]) - (1110 ⁇ [Nb])
  • FDT means the rolling end temperature (°C)
  • each element means weight content.
  • the steel material of the present invention has an effect suitable for a chassis structural member of an automobile.
  • 1 is a graph showing the classification of phase types of the second phase according to the size of the second phase of a hot-rolled steel sheet according to an embodiment of the present invention.
  • Figure 2 shows microstructure observation photographs of Inventive Example (a) and Comparative Example (b) according to an embodiment of the present invention.
  • High-strength hot-rolled steel sheets applied to chassis parts in the prior art add a large amount of carbonitride-forming elements, such as Ti, Nb, and V, and are wound at a high temperature around 600°C to induce precipitation of fine carbonitrides in the ferrite base structure, thereby yielding Precipitation hardened steel with excellent strength, elongation, and hole expandability has been widely applied.
  • carbonitride-forming elements such as Ti, Nb, and V
  • the inventors of the present invention conducted in-depth research to develop a composite structure hot-rolled steel sheet having high strength even in a winding process at a low temperature by utilizing a low-temperature transformation structure.
  • the low-temperature transformation structure is a term referring to a microstructure generated by shear transformation (Displacive Phase Transformation), and representative phases include bainite and martensite.
  • Bainite can be defined as a composite structure composed of bainitic ferrite produced by shear transformation without diffusion and secondary products produced by diffusion of interstitial alloying elements such as carbon.
  • a large amount of dislocations are generated in the tissue to accommodate the amount of shear strain generated during the shear transformation of bainitic ferrite.
  • ) phenomenon causes the density to gradually decrease.
  • the speed of the recovery phenomenon is greatly affected by the temperature, the dislocation density existing inside the bainitic ferrite is different depending on the formation and maintenance temperature of bainitic ferrite. Therefore, when the phase fraction and internal dislocation density are adjusted by adjusting the formation temperature and transformation time of bainite while the matrix structure is bainitic ferrite, the elongation and yield strength of the hot rolled steel can be controlled.
  • carbon (C) 0.05 to 0.17%
  • silicon (Si) 0.01 to 1.5%
  • manganese (Mn) 1.5 to 3.0%
  • aluminum (Al) 0.01 to 0.1%
  • Chromium (Cr) 2.0% or less (including 0%)
  • Molybdenum (Mo) 2.0% or less (including 0%)
  • Titanium (Ti) 0.01 to 0.15%
  • Phosphorus (P ) 0.001 to 0.05%
  • sulfur (S) 0.0001 to 0.05%
  • nitrogen (N) 0.0001 to 0.02%.
  • the reason for limiting the alloy composition of the hot-rolled steel sheet provided in the present invention as described above will be described in detail. Meanwhile, in the present invention, unless otherwise specified, the content of each element is based on weight, and the ratio of tissue is based on area.
  • Carbon (C) is the most economical and effective element for strengthening steel, and as the content of C increases, the generation of ferrite during cooling is suppressed.
  • the C diffuses into austenite during bainite transformation to stabilize austenite, and is transformed into the second phase, low-temperature bainite, tempered martensite, martensite-austenite composite phase (MA phase) in the subsequent cooling process, It is effective in improving the tensile strength and yield strength of steel.
  • the C may be included in 0.05 to 0.17%, and more advantageously may be included in 0.06% or more and 0.15% or less.
  • Silicon (Si) is an element that improves the hardenability of steel, and serves to improve strength through a solid solution strengthening effect.
  • the second phase is formed of low-temperature bainite, tempered martensite, and MA phase, thereby improving strength.
  • the Si content is less than 0.01%, carbides are formed and the MA phase fraction is relatively low, making it difficult to secure tensile strength.
  • the content exceeds 1.5%, Fe-Si composite oxide is formed on the surface of the slab during reheating, resulting in poor surface quality and poor weldability.
  • the Si may be included in an amount of 0.01 to 1.5%, more preferably 0.1% or more, and even more advantageously 0.3% or more. In addition, it will be effective to include the Si at 1.3% or less.
  • Manganese (Mn) is an element that improves the hardenability of steel, and facilitates the formation of a low-temperature transformation structure by preventing the formation of ferrite during cooling after finish rolling.
  • the Mn content is less than 1.5%, there is a problem in that the ferrite fraction excessively increases due to insufficient hardenability.
  • the content exceeds 3.0%, the hardenability is greatly increased, and the bainite transformation does not occur smoothly in the cooling zone, so the holding time for sufficiently forming acecular ferrite or bainitic ferrite to be obtained as the base structure in the present invention increases excessively, and the elongation decreases.
  • the Mn may be included in 1.5 to 3.0%, more advantageously, 1.8% or more and 2.4% or less.
  • Aluminum (Al) is an element added for deoxidation of molten steel, and partially exists in the steel after deoxidation.
  • Al content exceeds 0.1%, oxide and nitride-based inclusions increase in the steel, deteriorating the formability of the steel sheet.
  • the Al content is excessively reduced to less than 0.01%, it is economically unfavorable to cause an unnecessary increase in refining cost.
  • the Al may be included in 0.01 to 0.1%.
  • Chromium is an element that improves the hardenability of steel and suppresses the formation of ferrite during cooling after finish rolling.
  • chromium has an excellent affinity with carbon to slow down the diffusion rate of carbon to prevent carbon enrichment into untransformed austenite after coiling, thereby suppressing the production of pearlite and inducing the second phase to become a low-temperature transformation phase, thereby increasing the yield strength and Contribute to improvement of tensile strength.
  • the Cr may be included in an amount of 2.0% or less, and more advantageously, 1.5% or less.
  • Molybdenum is an element that improves hardenability of steel, serves to improve strength through solid solution strengthening effect, and suppresses formation of ferrite during cooling after finish rolling. In addition, Mo slows down the carbon diffusion rate to prevent carbon overconcentration into untransformed austenite after coiling, thereby suppressing the production of pearlite and enabling the second phase to become a low-temperature transformation phase to improve yield strength and tensile strength.
  • the Mo may include 2.0% or less, more advantageously 1.0% or less, and even more advantageously 0.5% or less.
  • 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. added to obtain a preventive effect.
  • the content exceeds 0.15%, the fraction of the MA phase constituting the second phase becomes excessive, resulting in poor hole expandability.
  • Ti may be included in an amount of 0.01 to 0.15%, more advantageously, 0.05% or more and 0.10% or less.
  • Phosphorus (P) is an impurity unavoidably contained in steel, and is an element that is a major cause of impairing the workability of steel due to segregation. Therefore, it is desirable to control the content as low as possible.
  • the P content it is advantageous to limit the P content to 0%, but excessive manufacturing costs are required to control the P content to less than 0.001%, so the lower limit may be set to 0.001%. However, if the content exceeds 0.05%, there is a concern that processability may deteriorate, so the upper limit of P may be limited to 0.05%.
  • S Sulfur
  • the upper limit of S may be limited to 0.05%.
  • Nitrogen (N) is an impurity that is inevitably contained in steel, and there is a problem of inhibiting the workability of steel by forming nitrides by combining with Al and the like. Therefore, it is desirable to control the content as low as possible.
  • the N content it is advantageous to limit the N content to 0%, but excessive manufacturing costs are required to control the N content to less than 0.0001%, so the lower limit may be set to 0.0001%. However, if the content exceeds 0.02%, there is a concern that workability may deteriorate, so the upper limit of N may be limited to 0.02%.
  • the hot-rolled steel sheet of the present invention may further include at least one of niobium (Nb) and boron (B) in addition to the above-described alloy composition.
  • niobium (Nb) has an effect of preventing the generation of pearlite by slowing down the diffusion rate of carbon.
  • recrystallization is delayed during hot rolling compared to Ti, the effect of refining austenite grains is large, and when the content exceeds 0.1%, the second phase MA phase is excessively formed, resulting in poor hole expandability. .
  • B Boron
  • B is an element that greatly improves the hardenability of steel by segregating at austenite grain boundaries and delaying the nucleation of ferrite.
  • the addition of B has an excellent effect of suppressing the formation of ferrite during cooling after hot rolling.
  • the present inventors have found that the transformation rate of bainite is also delayed when the B is added. That is, since the addition of B affects the fraction of acyclic ferrite or bainitic ferrite generated during cooling after hot rolling (preferably during secondary cooling), in the present invention, by adding B, the secondary cooling condition can be easily adjusted.
  • the remaining component of the present invention is iron (Fe).
  • Fe iron
  • the content relationship of specific elements satisfies the following relational expression 1.
  • each element means a weight content.
  • the low-temperature transformation phase should be formed as intended, which should suppress the generation of pearlite after winding the hot-rolled sheet. Since the driving force for formation of pearlite increases as the content of carbon enriched in untransformed austenite increases, it is necessary to prevent over-enrichment of carbon by adding an element that slows down the diffusion rate of carbon.
  • Cr or Mo is added along with Ti as an element that slows down the diffusion rate of carbon, and it is confirmed that pearlite formation is delayed by preventing hyperenrichment of untransformed austenite.
  • Si has a low solubility in iron carbides constituting pearlite, it serves to prevent the formation of carbides, and consequently prevents the formation of pearlite similarly to the above-mentioned elements.
  • the second phase is lowered while preventing the formation of pearlite By securing it in a transformed state, it is possible to improve the yield strength and tensile strength.
  • the hot-rolled steel sheet of the present invention which satisfies the above-described alloy composition and component relational expression (Relational Expression 1), may have a microstructure including acyclic ferrite or bainitic ferrite as a base structure, and the base structure has an area fraction of 70 to 90%. It is preferable to include
  • the steel of the present invention is cooled to a temperature below Bs (bainite transformation start temperature) by avoiding ferrite phase transformation during primary cooling after hot rolling, and then bainite transformation proceeds by slow cooling during subsequent secondary cooling. Since the bainite transformation at this time occurs in the high-temperature bainite transformation zone, the formation of bainitic ferrite and the diffusion of carbon into untransformed austenite occur, and there is a characteristic that carbides are not generated inside the bainitic ferrite. On the other hand, although a large amount of dislocations exist inside the bainitic ferrite generated by shear transformation, the dislocation density is reduced to an appropriate level by the recovery phenomenon after the secondary cooling and subsequent winding, so that the elongation of the steel sheet is improved.
  • Bs bainite transformation start temperature
  • the present invention manages the total fraction of the bainitic ferrite and acecular ferrite.
  • acyclic ferrite or bainitic ferrite If the total fraction of the base structure, acyclic ferrite or bainitic ferrite, is less than 70%, it is difficult to secure elongation, whereas if the fraction exceeds 90%, securing a low-temperature transformation structure that serves to improve strength. There is a problem that makes it difficult.
  • the hot-rolled steel sheet of the present invention includes a second phase in addition to the above-described base structure, and the second phase is a low-temperature transformation structure, preferably one or more of low-temperature bainite, tempered martensite, and MA phase, and has an area fraction of 10 to 30 % can be included.
  • the second phase is a low-temperature transformation structure, preferably one or more of low-temperature bainite, tempered martensite, and MA phase, and has an area fraction of 10 to 30 % can be included.
  • diffusion of carbon into untransformed austenite proceeds with the formation of bainitic ferrite during secondary cooling during the cooling performed after hot rolling. In the cooling process), it transforms into the second phase, low-temperature bainite, tempered martensite and MA phase.
  • untransformed austenite distributed in the structure has a different size for each position, and thus the type of the second phase also varies.
  • Relatively large untransformed austenite has a low carbon content and therefore can transform to low temperature bainite during cooling to the subsequent coiling temperature, while smaller austenite transforms to martensite at lower temperatures. Since the martensite is transformed at a relatively high temperature, a tempering phenomenon occurs after the martensite transformation, so that the final structure becomes tempered martensite.
  • the low-temperature bainite and tempered martensite commonly contain iron carbide at the grain boundary and in the grain in the lath structure, it is revealed that the total fraction is managed.
  • small-sized austenite since small-sized austenite has the highest carbon concentration during secondary cooling, it does not transform into low-temperature bainite or martensite immediately after winding, and does not transform into martensite or martensite in the final cooling step. Otherwise, it may remain as austenite.
  • martensite with a high carbon content is characterized by having a plate type martensite rather than a lath shape, and since the internal twin structure is not clearly observed during nital etching, the low temperature bay It can be clearly distinguished from knight and tempered martensite.
  • This MA phase is effective in improving the yield strength and tensile strength, but has a high hardness difference between the phase (pgase) and bainitic ferrite (or acyclic ferrite), which is a base structure, and deteriorates hole expandability.
  • the present invention it is preferable to include the area fraction of 10% or more of the second phase in terms of securing yield strength and tensile strength, and it is preferable to limit it to 30% or less to secure elongation at the same time.
  • the present invention controls the ratio of the MA phase in the second phase for the purpose of improving the hole expandability of the steel sheet, and preferably includes the MA phase at a ratio of less than 30% of the total area fraction of the second phase.
  • the hot-rolled steel sheet of the present invention may include one or more of ferrite and carbide as other structures in addition to the above-described base structure and second phase, but it is preferable that the area fraction of these is controlled to less than 5%.
  • ferrite means granular ferrite.
  • Ferrite produced during cooling after hot rolling is typically produced by diffusion transformation, and thus has low strength.
  • ferrite when ferrite is formed at less than 5%, ferrite formed previously is subjected to shear strain in order to accommodate grain strain generated when residual austenite is transformed into bainite and martensite after ferrite is formed. It was confirmed that the internal dislocation density was maintained at a high level and the strength of the steel was not greatly reduced. However, when the fraction is 5% or more, the strength of the steel is lowered, which is not preferable.
  • iron deoxide may be generated with carbon diffusion into austenite. Since the present invention seeks to improve the strength by utilizing the low-temperature transformation structure as the second phase, the generation of iron carbide may cause a decrease in the fraction of the second phase. That is, excessive production of iron carbide hinders the strengthening effect targeted by the present invention.
  • alloy carbonitrides may be formed, and in this case, an additional strengthening effect by grain refinement can be expected.
  • coarse carbides impair the toughness of the steel, they are present in the hot-rolled steel sheet of the present invention. It is preferable that the amount of carbides used is less than 5%.
  • the hot-rolled steel sheet of the present invention having the above-described alloy composition and microstructure has a yield strength of 750 MPa or more and a tensile strength of 980 MPa or more, high strength, elongation of 9% or more, and hole expansion rate of 30% or more, which is characterized by excellent formability.
  • the hot-rolled steel sheet according to the present invention can be manufactured by performing a series of processes of [reheating - hot rolling - cooling - winding] a steel slab satisfying the alloy composition and relational expression 1 proposed in the present invention.
  • the reheated steel slab can be hot-rolled to produce a hot-rolled steel sheet.
  • the hot-rolling is performed in a temperature range of 750 to 1150 ° C, and the total reduction of the final 2 passes is controlled to 10 to 40%. It is desirable to do
  • performing multi-stage rolling during hot rolling is to reduce the rolling load and precisely control the thickness.
  • the total reduction ratio of the final 2 passes exceeds 40%, the rolling load of the final 2 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 is rapidly lowered, resulting in shape defects.
  • the crystal grain size of austenite after hot rolling is affected by alloy components, rolling end temperature, and rolling reduction, which affect ferrite and bainite formation behavior and final microstructure in the subsequent cooling process. Further, in the present invention, the proportion of the MA phase in the second phase, which is a major constituent phase, is greatly influenced by austenite grains after hot rolling.
  • the size (grain size) of this second phase is affected by the nucleation behavior in bainite transformation, it is difficult to control the size of the second phase because the size of the second phase cannot be larger than the size of austenite before transformation due to the nature of shear transformation. For this purpose, it is advantageous to control the grain size of austenite after hot rolling.
  • the effective grain size of austenite after hot rolling is derived as a relationship between the rolling end temperature (FDT) and a specific alloy composition, and is specifically defined by the following relational expression 2. If the value of Du according to the following relational expression 2 is 800 or more, the MA phase is properly formed and the hole expansion rate can be secured at 30% or more. On the other hand, if the value exceeds 1106, the austenite grain size is excessively coarsened, resulting in bainite transformation. There is a problem that the elongation rate is inferior according to the delay.
  • Du is an index representing the effective grain size of austenite immediately before primary cooling after hot rolling
  • Du FDT + (7.35 ⁇ [C]) - (24.7 ⁇ [Si]) - (4.7 ⁇ [Mn] ) - (3.9 ⁇ [Cr]) - (5.2 ⁇ [Mo]) - (560 ⁇ [Ti]) - (1110 ⁇ [Nb])
  • FDT means the rolling end temperature (°C)
  • each element means weight content.
  • the hot-rolled steel sheet manufactured according to the above is cooled, and at this time, it is preferable to perform it in stages according to the cooling temperature.
  • the hot-rolled steel sheet is firstly cooled at a cooling rate of 70°C/s or more to a temperature of Bs or less, and then secondarily cooled at a cooling rate of 20°C/s or less to a temperature of (Bs+Ms)/2 or more , It is preferable to perform tertiary cooling at a cooling rate of 30 ° C / s or more to a temperature range of Ms -20 ° C to 500 ° C.
  • the hot-rolled steel sheet manufactured according to the above is quickly cooled below the temperature at which bainite starts to be formed (Bs) to suppress the formation of ferrite (granular ferrite), and then the bainite initiation temperature (Bs) and the martensite initiation temperature (Ms) ) By slowly cooling to an intermediate temperature or higher temperature, it is possible to secure acyclic ferrite or bainitic ferrite as a base structure.
  • the primary cooling is performed at a temperature of Bs or lower after completion of the hot rolling, if the cooling rate is less than 70° C./s, there is a problem in that a ferrite phase is excessively formed during cooling.
  • the upper limit of the primary cooling rate is not particularly limited, but may be limited to 200° C./s or less because there is a concern that the plate shape may be distorted when the steel plate is excessively cooled.
  • the lower limit of the cooling end temperature during the primary cooling is not particularly limited, but if it is excessively low, there is a concern that the cooling time during the subsequent secondary cooling may not be sufficient, so it is revealed that it can be limited to Bs-100 ° C. put
  • the hard cooling is terminated, and secondary cooling can be performed at a temperature of (Bs+Ms)/2 or higher at a cooling rate of 20 ° C./s or less.
  • k(T) is an index representing the growth rate of bainitic ferrite, and is affected by the alloy components of the steel as well as the phase transformation temperature and the grain size after hot rolling. Accordingly, when the value of relational expression 3, that is, the relationship between k(T) and holding time (exp(-k(T) ⁇ (ts) 2 )) is less than 0.1, the fraction of the base tissue becomes excessive and the elongation is excellent, but the target level strength cannot be obtained. On the other hand, when the value exceeds 0.3, there is a problem in that elongation is deteriorated.
  • T1 is the end temperature of the first cooling (°C)
  • T2 is the end temperature of the second cooling (°C).
  • the temperature of the steel sheet may rise due to transformation exotherm caused by bainite phase transformation during secondary cooling according to the above conditions.
  • the cooling rate during the secondary cooling may be controlled to 20° C./s or less in order to minimize the temperature rise of the steel sheet due to transformation heat generation.
  • the cooling rate exceeds 20° C./s, there is a risk that the plate shape may be distorted.
  • the secondary cooling in the present invention also includes an air cooling process.
  • the end temperature of the tertiary cooling that is, the winding temperature can be applied lower than Ms, and if the fraction of bainitic ferrite is formed to 70% or more, cooling can be performed up to Ms -20 ° C.
  • Transformation of low-temperature bainite proceeds during the tertiary cooling, and depending on the carbon content in austenite, some may be transformed into martensite even after winding. Therefore, by setting the cooling rate to 30° C./s or more during the tertiary cooling, it is possible to avoid the formation of additional high-temperature bainite during cooling.
  • the upper limit of the cooling rate is not particularly limited, but may be 100° C./s or less in order to prevent distortion of the plate shape.
  • Bs and Ms can be derived by the formula below, and each element means a weight content.
  • Ms (°C) 550 - (330 ⁇ [C]) - (41 ⁇ [Mn]) - (20 ⁇ [Si]) - (20 ⁇ [Cr]) - (10 ⁇ [Mo]) + (30 ⁇ [Al])
  • final cooling may be performed to obtain a target hot-rolled steel sheet.
  • final cooling can be completed by performing air cooling to room temperature.
  • the hot-rolled steel sheet of the present invention obtained by completing the final cooling may be additionally pickled and oiled.
  • a hot-dip galvanizing process may be performed by heating the pickled and lubricated hot-rolled steel sheet in a temperature range of 450 to 740 ° C.
  • the hot-dip galvanizing process may use a zinc-based plating bath, and the alloy composition in the zinc-based plating bath is not particularly limited.
  • Each of the prepared steel slabs was reheated at 1200° C., and then subjected to hot rolling, cooling, winding, and final cooling (air cooling) processes under the conditions shown in Table 2 below to prepare a hot-rolled steel sheet having a thickness of 2.5 mm.
  • the total reduction ratio of the final 2 passes was applied at 25%, and the cooling rate during the 3rd cooling was uniformly applied at 35°C/s.
  • yield strength, tensile strength and elongation were measured at room temperature using a universal tensile tester after taking a JIS-5 standard test piece in a direction perpendicular to the rolling direction. At this time, yield strength, tensile strength, and elongation were expressed as 0.2% off-set yield strength, maximum tensile strength, and breaking elongation, respectively.
  • the hole expandability was measured according to the ISO TS16630 standard method for the same specimen as in the tensile test.
  • each hot-rolled steel sheet was observed at 10,000 magnification using a scanning electron microscope and an image analyzer after etching the same specimen as in the tensile test with a Nital etching method, and Fractions were calculated. At this time, the microstructure was observed for a cross section of the specimen, that is, a cross section perpendicular to the rolling direction.
  • Inventive Examples 1 to 13 satisfying both the alloy composition and manufacturing conditions proposed in the present invention have sufficiently formed acyclic ferrite or bainitic ferrite as a base structure, and a low temperature as a second phase. As the transformed structure (LB+TM+MA) was appropriately formed, the target strength and moldability could be secured.
  • Comparative Examples 1 to 3 which are unsatisfactory for the alloy component system (relational expression 1) proposed in the present invention, it was impossible to secure the target strength as a large amount of pearlite was formed in the microstructure, and the elongation rate was relatively high due to such low strength. showed
  • 1 is a graph showing the classification of phase types of the second phase according to the size of the second phase of each hot-rolled steel sheet.
  • Figure 2 shows a photograph of the microstructures of Inventive Example 4 and Comparative Example 3 observed with a scanning microscope.
  • inventive example 4 (a) has a microstructure in which the base structure and the second phase to be implemented in the present invention are appropriately formed, whereas in comparative example 3 (b), pearlite, which is not intended in the present invention, is excessively formed It can be seen that it has been created.

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Abstract

La présente invention concerne une tôle d'acier laminée à chaud, qui peut être appliquée de manière appropriée à un élément de structure de châssis d'automobile, et analogue, et, plus particulièrement, une tôle d'acier haute résistance laminée à chaud présentant une excellente aptitude au formage, et un procédé de fabrication associé.
PCT/KR2022/011282 2021-08-09 2022-08-01 Tôle d'acier haute résistance laminée à chaud présentant une excellente aptitude au formage, et procédé de fabrication associé Ceased WO2023018081A1 (fr)

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US18/293,772 US20240254585A1 (en) 2021-08-09 2022-08-01 High strength hot-rolled steel sheet having excellent formability, and manufacturing method therefor
EP22856080.1A EP4386103A4 (fr) 2021-08-09 2022-08-01 Tôle d'acier haute résistance laminée à chaud présentant une excellente aptitude au formage, et procédé de fabrication associé
JP2024507911A JP7751074B2 (ja) 2021-08-09 2022-08-01 成形性に優れた高強度熱延鋼板及びその製造方法
CN202280054897.9A CN117795118A (zh) 2021-08-09 2022-08-01 成型性优异的高强度热轧钢板及其制造方法

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