WO2020111702A1 - 내구성이 우수한 고강도 강재 및 이의 제조방법 - Google Patents

내구성이 우수한 고강도 강재 및 이의 제조방법 Download PDF

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WO2020111702A1
WO2020111702A1 PCT/KR2019/016299 KR2019016299W WO2020111702A1 WO 2020111702 A1 WO2020111702 A1 WO 2020111702A1 KR 2019016299 W KR2019016299 W KR 2019016299W WO 2020111702 A1 WO2020111702 A1 WO 2020111702A1
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steel
cooling
phase
hot
strength
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PCT/KR2019/016299
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English (en)
French (fr)
Korean (ko)
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김성일
나현택
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주식회사 포스코
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Priority to JP2021529852A priority Critical patent/JP7244723B2/ja
Priority to EP19889604.5A priority patent/EP3889298A4/de
Priority to CN201980078073.3A priority patent/CN113166893B/zh
Priority to US17/294,250 priority patent/US20220010399A1/en
Publication of WO2020111702A1 publication Critical patent/WO2020111702A1/ko

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    • C—CHEMISTRY; METALLURGY
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    • 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 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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    • 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
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • 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
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • 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
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a high strength steel material having excellent durability and a method for manufacturing the same.
  • members of chassis parts and wheel disks of commercial vehicles have used high-strength steel plates having a thickness of 5 mm or more and a yield strength of 450 to 600 MPa in order to secure high rigidity due to vehicle characteristics.
  • a high-strength steel material with a tensile strength of 650 MPa or more.
  • Patent Document 1 the ferrite phase is formed as a matrix structure by winding at a high temperature after a hot rolling of an ordinary austenite station, and fine precipitates are formed.
  • Patent Document 2 proposes a technique for cooling the winding temperature to a temperature at which the bainite phase is formed as a matrix structure so that a coarse pearlite structure is not formed.
  • Patent Document 3 discloses a technique for miniaturizing austenite grains by rolling with a rolling reduction of 40% or more in an unrecrystallized region during hot rolling using titanium (Ti), niobium (Nb), or the like.
  • alloy components such as Si, Mn, Al, Mo, and Cr are mainly used.
  • it is effective in improving the strength of the hot-rolled steel sheet, but when a large amount of alloy components is added, some components are segregated in the steel ( segregation), or causing micro-structure non-uniformity, resulting in poor shear formability, and micro-cracks on the shear surface are easily propagated in the fatigue environment, resulting in component damage.
  • the microstructure non-uniformity between the thickness surface layer portion and the center portion increases, resulting in increased cracking of the shear surface, and a faster propagation rate of cracks in a fatigue environment, resulting in poor durability.
  • Patent Documents 1 to 3 do not consider the fatigue properties of thick steel materials having high strength.
  • Patent Document 1 Japanese Patent Publication No. 2002-322541
  • Patent Document 2 Korean Registered Publication No. 10-1528084
  • Patent Document 3 Japanese Patent Publication No. 1997-143570
  • One aspect of the present invention is to provide a steel material having a certain thickness, and not only has high strength, but also has excellent durability and a method for manufacturing the same.
  • carbon (C) 0.05 to 0.15%
  • silicon (Si) 0.01 to 1.0%
  • manganese (Mn) 1.0 to 2.3%
  • aluminum (Al) 0.01 to 0.1%
  • Chromium (Cr) 0.005 to 1.0%
  • Phosphorus (P) 0.001 to 0.05%
  • Sulfur (S) 0.001 to 0.01%
  • Titanium (Ti) 0.005 ⁇ 0.11%
  • the fraction of ferrite and bainite phases is 90% or more, and the proportion of crystal grains (ratio of short side/long side) in the center (t/4 to t/2 point in the thickness direction) is 0.3 or less.
  • the proportion of crystal grains (ratio of short side/long side) in the center (t/4 to t/2 point in the thickness direction) is 0.3 or less.
  • And provides a high strength steel material having excellent durability at a grain boundary length of 700 mm or more observed in a unit area (1 mm 2 ) at the center.
  • Another aspect of the present invention heating the steel slab having the above-described alloy composition in a temperature range of 1200 ⁇ 1350 °C; Hot-rolling the heated steel slab to produce a hot-rolled steel sheet; Cooling the hot-rolled steel sheet to a temperature range of 400 ⁇ 500 °C and then winding (CT); And after the winding, including the step of air cooling to a temperature range of room temperature ⁇ 200 °C,
  • the hot rolling is finished hot rolling at a temperature (FDT (°C)) satisfying the following [Relational Formula 1], and the cooling is performed by the following primary cooling and secondary cooling, and the primary cooling is [Relational Formula 2].
  • FDT °C
  • the secondary cooling provides a method of making high-strength steel material having excellent durability, characterized in that for performing a cooling rate (CR 2) satisfying the following [Expression 3].
  • Tn 730 + 92 ⁇ [C] + 70 ⁇ [Mn] + 45 ⁇ [Cr] + 650 ⁇ [Nb] + 410 ⁇ [Ti]-80 ⁇ [Si]-1.4 ⁇ (t-5) (where Each element means weight content (%), and t means thickness (mm) of the final hot rolled steel sheet)
  • the steel material of the present invention has an effect that can be suitably applied to members of a chassis component of a vehicle and wheel disk.
  • 1 is a graph showing the ratio of fatigue strength and yield strength according to the thickness of an invention steel and a comparative steel in one embodiment of the present invention.
  • the present inventors have studied in depth to solve the problem of deterioration in durability when molding steel materials for existing automobiles.
  • the present inventors investigated the change in the distribution of cracks and durability in the shear surface after molding according to the components and microstructures of the existing thick steel materials, and confirmed that the durability characteristics were changed according to the shape control of the grains in the center of the thickness of the steel material.
  • the present inventors have confirmed that it is possible to provide a steel material having high strength and a targeted durability due to excellent cross-section quality during molding, and to complete the present invention.
  • the high strength steel material having excellent durability according to an aspect of the present invention is in weight%, carbon (C): 0.05 to 0.15%, silicon (Si): 0.01 to 1.0%, manganese (Mn): 1.0 to 2.3%, aluminum (Al ): 0.01 to 0.1%, chromium (Cr): 0.005 to 1.0%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.001 to 0.01%, nitrogen (N): 0.001 to 0.01%, niobium (Nb) ): 0.005 to 0.07%, titanium (Ti): may include 0.005 to 0.11%.
  • the content of each element is based on weight, and the proportion of tissue is based on area.
  • Carbon (C) is the most economical and effective element for reinforcing steel, and as the amount added increases, the precipitation strengthening effect increases or the fraction of bainite phase increases, thereby improving tensile strength.
  • the thicker the thickness of the hot-rolled steel material the slower the cooling rate in the center of the thickness during cooling after hot rolling, so that when the content of C is large, coarse carbide or pearlite is likely to be formed.
  • the content of C is less than 0.05%, it is difficult to sufficiently obtain the strengthening effect of steel, whereas when it exceeds 0.15%, pearlite phase or coarse carbide is formed in the center of the thickness, resulting in poor shear formability and reduced durability. there is a problem.
  • the C may be included as 0.05 to 0.15%, and more advantageously as 0.06 to 0.12%.
  • Silicon (Si) deoxidizes molten steel and has a solid solution strengthening effect, and is advantageous in improving moldability by delaying the formation of coarse carbide.
  • the Si content is less than 0.01%, the solid solution strengthening effect is small, and the effect of delaying carbide formation is also low, making it difficult to improve formability.
  • the content exceeds 1.0%, a red scale formed by Si is formed on the surface of the steel sheet during hot rolling, and not only the surface quality of the steel sheet is very deteriorated, but also ductility and weldability are deteriorated.
  • the Si may be included in an amount of 0.01 to 1.0%, and more advantageously, in an amount of 0.2 to 0.7%.
  • Manganese (Mn), like Si, is an effective element for solid solution strengthening of steel, and increases the hardenability of steel to facilitate formation of a bainite phase during cooling after hot rolling.
  • the Mn content is less than 1.0%, the above-described effect cannot be sufficiently obtained.
  • the content exceeds 2.3%, the hardenability is greatly increased, so that the transformation of martensite phase is likely to occur, and the segregation part is greatly developed at the center of the thickness during slab casting in the playing process, and when cooled after hot rolling, fineness in the thickness direction The tissue is formed non-uniformly, resulting in poor shear formability and durability.
  • the Mn may be included in an amount of 1.0 to 2.3%, and more advantageously, in an amount of 1.1 to 2.0%.
  • Aluminum (Al) is an element mainly added for deoxidation. If the content is less than 0.01%, the additive effect cannot be sufficiently obtained. On the other hand, when the content exceeds 0.1%, by forming AlN in combination with nitrogen (N) in steel, corner cracks are likely to occur in the slab during continuous casting, and there is a risk of defects due to inclusion formation.
  • Al may be included as 0.01 to 0.1.
  • aluminum means soluble aluminum (Sol.Al).
  • Chromium (Cr) solidifies and strengthens the steel, and helps to form bainite at the coiling temperature by delaying transformation of the ferrite phase when cooled.
  • Cr Chromium
  • the segregation part is greatly developed in the center of the thickness, and the shearing formability and durability are deteriorated by making the microstructure in the thickness direction non-uniform.
  • the Cr may be included at 0.005 to 1.0%, and more advantageously at 0.3 to 0.9%.
  • Phosphorus (P) is an element that simultaneously has the effect of strengthening solid solution and promoting ferrite transformation.
  • the manufacturing cost is excessive, which is economically disadvantageous, and it is difficult to secure a target level of strength.
  • the content of P exceeds 0.05%, brittleness by grain boundary segregation occurs, fine cracking is likely to occur during molding, and shear moldability and durability are greatly deteriorated.
  • the P may be included in 0.001 to 0.05%.
  • S Sulfur
  • S is an impurity present in the steel, and when its content exceeds 0.01%, it forms a non-metallic inclusion by combining with Mn and the like, and thus it is easy to cause micro-cracks when cutting steel, and greatly improves shear formability and durability. There is a problem of deterioration.
  • S Sulfur
  • S may be included in 0.001 to 0.01%.
  • N Nitrogen
  • N is a typical solid solution strengthening element with C, and combines with Ti, Al, etc. to form coarse precipitates.
  • the solid solution strengthening effect of N is superior to that of carbon, but there is a problem that the toughness of steel decreases as the amount of N increases in steel.
  • the N may be included as 0.001 to 0.01%.
  • Niobium (Nb) precipitation strengthening element It is effective in improving the strength and impact toughness of steel due to the grain refinement effect due to recrystallization delay by precipitation during hot rolling. In order to sufficiently obtain the above-described effect, it may be included in an amount of 0.005% or more, while when the content exceeds 0.07%, formability and durability are formed by formation of stretched grains due to excessive recrystallization delay during hot rolling and formation of coarse composite precipitates. You are inferior.
  • the Nb may be included as 0.005 to 0.07%, more advantageously as 0.01 to 0.06%.
  • Titanium (Ti) is a representative precipitation strengthening element together with the Nb, and forms a coarse TiN in the steel with strong affinity with N.
  • the TiN has an effect of inhibiting the growth of crystal grains during the heating process for hot rolling.
  • the Ti remaining after reacting with N is dissolved in the steel and bonds with carbon to form a TiC precipitate, which is useful for improving the strength of the steel.
  • the Ti may be included in an amount of 0.005 to 0.11%, and more advantageously, in an amount of 0.01 to 0.1%.
  • the remaining component of the invention is iron (Fe).
  • impurities that are not intended from the raw material or the surrounding environment may be inevitably mixed, and therefore cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, they are not specifically mentioned in this specification.
  • the steel material of the present invention having the above-described alloy composition may be composed of a microstructure is a composite structure of ferrite and bainite.
  • the sum of the fractions of the ferrite and bainite phases is preferably 90% or more of the area fraction, and among these, the bainite phase may be 50% or more of the area fraction.
  • the fraction of the bainite phase is less than 50% of the area fraction, it is difficult to secure the target strength, and when the coarse ferrite phase increases, it has a non-uniform microstructure, and thus, it is easy to generate fine cracks during shear deformation or punching deformation.
  • the ferrite phase means a polygonal ferrite phase that is a high temperature ferrite phase
  • the bainite phase collectively refers to both a needle-shaped ferrite phase and a bainitic ferrite phase, which are low temperature reverse ferrite phases.
  • Residual tissues other than the composite tissue may include a MA phase (a mixture of martensite and austenite) and a martensite phase.
  • the two phases may be combined to include an area fraction of 1 to 10%, of which the MA phase is preferably less than 3%.
  • the MA phase has an average size of 1/10 of that of the martensite phase, but the tendency to crack at the interface of the phase is similar to that of the martensite phase. It is preferred.
  • the steel material of the present invention contains 3% or less (including 0%) of the pearlite phase in addition to the above-described structure, there is no great difficulty in securing the intended physical properties.
  • the steel material of the present invention is formed of crystal grains having a shape ratio (ratio of short side length (short axis)/long side length (long axis), aspect ratio) of 0.3 or less within a center portion corresponding to a t/2 point in a thickness direction. It is preferable that the fraction is less than 50%, and the length of the grain boundaries observed in the unit area (1 mm 2 ) in the center is 700 mm or more.
  • the proportion of crystal grains having a shape ratio of crystal grains of 0.3 or less in the central portion is 50% or more, the growth of cracks is facilitated when cracks are generated, and durability is inferior.
  • the length of the grain boundary in the central portion is less than 700 mm, the strength of the central portion decreases, and cracks are easily propagated, resulting in poor durability.
  • the method for analyzing the shape ratio of the crystal grains and the length of the grain boundaries is not particularly limited, but for example, it can be analyzed by using Back Scattered Diffraction (EBSD). Specifically, from the EBSD measurement result of the rolled section, the grain size having a large-diameter grain boundary of 15° or more is determined as the length of the grain boundary per unit area (1 mm 2 ), and the shape ratio can be determined by the ratio of the shortening of the grain size and the long axis.
  • EBSD Back Scattered Diffraction
  • the steel material of the present invention having the above-described alloy composition and microstructure is a thick steel material having a thickness of 5 mm or more and a maximum of 12 mm, the tensile strength is 650 MPa or more, and the ratio of fatigue limit and yield strength (fatigue limit/yield strength) is 0.25 or more. As a result, it is possible to secure excellent durability with high strength.
  • the high-strength steel according to the present invention can be produced by performing a series of processes of [heating-hot rolling-winding-cooling] a steel slab satisfying the alloy composition proposed in the present invention.
  • the heating temperature is less than 1200°C, the precipitates are not sufficiently re-used, so that the formation of precipitates is reduced in the process after hot rolling, and there is a problem that coarse TiN remains.
  • the temperature exceeds 1350°C, the strength is lowered due to abnormal grain growth of austenite grains, which is not preferable.
  • the hot rolling When the hot rolling is performed at a temperature higher than 1150°C, the temperature of the hot rolled steel sheet becomes high, the grain size becomes coarse, and the surface quality of the hot rolled steel sheet becomes inferior.
  • the hot rolling when hot rolling is performed at a temperature lower than 800° C., the grains stretched due to excessive recrystallization delay develop, anisotropy increases, moldability deteriorates, and unevenness occurs due to rolling at a temperature below the austenite temperature range. Microstructure develops more severely.
  • the hot rolling process of the present invention when rolling is terminated at a temperature higher than the temperature range suggested in the following relational formula 1 (temperature above Tn), the microstructure of the steel is coarse and non-uniform, and phase transformation is delayed to coarse. Due to the formation of the MA phase and martensite phase, fine cracks are excessively formed during shear forming and knitting forming, resulting in poor durability.
  • the thickness direction t/4 under the surface layer having a relatively low temperature in a thick steel material having a thickness of 5 mm or more.
  • Ferrite phase transformation is promoted at the point, and the phase fraction of fine ferrite increases, but it has an elongated grain shape, which causes cracks to spread rapidly, and uneven microstructure can remain in the center of the thickness, which is disadvantageous in securing durability. do.
  • Tn 730 + 92 ⁇ [C] + 70 ⁇ [Mn] + 45 ⁇ [Cr] + 650 ⁇ [Nb] + 410 ⁇ [Ti]-80 ⁇ [Si]-1.4 ⁇ (t-5) (where Each element means weight content (%), and t means thickness (mm) of the final hot rolled steel sheet)
  • the hot-rolled steel sheet manufactured by performing hot rolling as described above can be cooled to a temperature range of 400 to 500° C., and then a winding process can be performed at that temperature.
  • the cooling is performed by primary cooling and secondary cooling, and the primary cooling is a cooling rate (CR 1 ) satisfying [Relational Formula 2], and a cooling rate (CR) satisfying [Relational Formula 3] for the secondary cooling. It is preferable to perform with 2 ).
  • the primary cooling is preferably terminated in a temperature section in which phase transformation of ferrite occurs during cooling, but the temperature at which phase transformation of ferrite occurs may vary according to the alloy composition proposed in the present invention. More specifically, the primary cooling is preferably performed to a temperature at which transformation of hard phases such as bainite phase, MA phase and martensite phase does not occur. Even more preferably, the primary cooling may be performed until the temperature of the hot-rolled steel sheet obtained by hot rolling reaches 600°C.
  • the cooling rate in the center of the thickness of the rolled sheet is slower than the cooling rate in the area directly below the surface layer ⁇ t/4, so adjust the thickness from the center of thickness.
  • the ferrite phase it may have a non-uniform microstructure.
  • Secondary cooling is performed immediately after the primary cooling is completed under the above-described conditions, and the secondary cooling is preferably terminated at a coiling temperature (CT (° C.)).
  • CT coiling temperature
  • the untransformed phase is transformed into the bainite phase over the entire thickness of the steel material, so that 90% (area fraction) of the matrix structure is formed into the ferrite and bainite phases.
  • CR 2 a specific cooling rate
  • carbides are formed rather than the bainite phase to grow coarse, which mainly exists at the grain boundary of the ferrite phase, and when the cooling rate is slower, a pearlite phase is formed and cracks during shear molding or punching molding It is easy to form, and there is a problem that cracks propagate along grain boundaries even with a small external force.
  • the cooling rate exceeds CR Max the MA phase or the martensite phase, which increases the difference in hardness between phases, is excessively formed, resulting in poor durability.
  • the coiling temperature exceeds 500°C during coiling after completing the above-described cooling process, the pearlite phase is formed and the strength of the steel becomes insufficient.
  • it is less than 400°C, the martensite phase is excessively formed, resulting in shear formability and punching formability. Durability is inferior.
  • the area ratio of the crystal grains having an aspect ratio of 0.3 or less in the center of the thickness of the steel material is less than 50% While securing it, the length of the grain boundary observed within the unit area (1 mm 2 ) can be secured to 700 mm or more.
  • relational expressions 2 and 3 correspond to the cooling conditions for optimizing the microstructure so that the strength and durability of the steel can be improved through a phase transformation process during cooling. That is, since not only the type and fraction of the tissue phase, but also the shape ratio of the crystal grains and the size of the grain boundaries vary depending on the cooling conditions, it will be preferable to perform cooling under the conditions satisfying the above relational expressions 2 and 3.
  • the coil obtained by completing the cooling and coiling process according to the above can be air-cooled to a temperature range of room temperature to 200°C.
  • the air cooling process of the coil has a cooling rate of 0.001 to 10°C/hour, which means cooling in the air.
  • the cooling rate exceeds 10°C/hour, some untransformed phases of the steel are easily transformed into the MA phase, thereby deteriorating the shear formability and punching formability and durability of the steel.
  • a separate heating and heat preservation facility is required, which is economically disadvantageous.
  • the air-cooled steel can be pickled and oiled, and then heated to a temperature range of 450 to 740°C to perform a hot dip galvanizing process.
  • the hot dip galvanizing process may use a zinc-based plating bath, the alloy composition in the zinc-based plating bath is not particularly limited, for example, magnesium (Mg): 0.01 to 30% by weight, aluminum (Al): 0.01 It may be a plating bath containing ⁇ 50% by weight and the balance Zn and unavoidable impurities.
  • a steel slab having an alloy composition of Table 1 below was prepared. At this time, the content of the alloy composition is weight%, and the rest includes Fe and unavoidable impurities.
  • the prepared steel slabs were manufactured according to the manufacturing conditions in Table 2 below. At this time, when cooling after hot rolling, the primary cooling was completed at 600°C, and the secondary cooling was completed at the coiling temperature.
  • FDT is the temperature at the time of finishing hot rolling (the end temperature of hot rolling)
  • CT is the winding temperature
  • the yield strength and elongation each indicate 0.2% off-set yield strength and elongation at break
  • the tensile strength was measured by taking specimens of JIS 5 standard specimens in a direction perpendicular to the rolling direction.
  • the durability was evaluated by performing a high-cycle fatigue test (bending fatigue test) on the test piece having the punched molded part.
  • the test piece for the fatigue test was produced by punching a hole of 10 mm in diameter with a clearance of 12% by punching molding in the center of a bending fatigue test piece having a length of 40 mm and a width of 20 mm, and tested under a stress ratio of -1 and a frequency of 15 Hz.
  • the fatigue strength (S Fatigue ) was expressed as the strength ratio (S Fatigue /YS) compared to the yield strength, from which it was possible to confirm the change in cross-section quality and durability of the punched area.
  • the aspect ratio (AR) which is the ratio of grain length and grain size per unit area (1 mm 2 ) corresponding to the area of grain boundaries, is applied to back-scattered electron diffraction (EBSD) for grains with large-angle grain boundaries of 15° or more. It was measured using.
  • EBSD back-scattered electron diffraction
  • the results analyzed at 1000 magnification using an optical microscope and an image analyzer were shown.
  • the phase fractions of martensite (M), ferrite (F), bainite (B), and pearlite (P) were measured from the results analyzed at 3000 magnification and 5000 magnification using an electron scanning microscope (SEM).
  • F denotes a polygonal ferrite having an equiaxed crystal shape
  • B denotes the sum of all fractions of the ferrite phase observed in a low temperature region such as a bainite phase, acicular ferrite, and bainitic ferrite.
  • AR0.3 represents a ratio (area fraction) of crystal grains having an aspect ratio of 0.3 or less, and shows results obtained by observing at 1000 magnification.
  • the inventive steels 1 to 7 satisfying both the alloy composition and manufacturing conditions proposed in the present invention were formed of a ferrite and bainite composite structure.
  • the fraction of crystal grains having a grain shape ratio of 0.3 or less in the center of the thickness direction of the steel was less than 50% (see FIG. 2), and since the grain boundary lengths were all formed to be 700 mm or more, the intended high strength and excellent durability were secured. .
  • the comparative steel 1 to 11 is satisfactory alloy composition proposed in the present invention, but the manufacturing conditions are out of the present invention, it was not possible to secure the intended physical properties.
  • Comparative steel 1 to 3 is a case where the hot-rolled finish temperature does not satisfy the relational expression 1 proposed in the present invention, Comparative steel 1 has a final steel thickness of 2.9 mm, and a ferrite phase stretched from the center is excessively formed, but fatigue The characteristics showed a result that was not significantly inferior. This is due to the fact that, when hot-rolled to a thickness of 2.9 mm, the amount of rolling in the unrecrystallized temperature range greatly increased, and elongated microstructure developed, but the quality of the cross-section of the punched area was good as the microstructure in the thickness direction was uniform. Was judged.
  • Comparative Steels 2 and 3 are thick steels with a thickness of 10 mm and 7 mm, respectively, and Comparative Steel 2 has micro-cracks formed in the cross section when exposed to a fatigue environment as the MA phase develops in the central microstructure and the grain boundary length is less than 700 mm. It has been shown to grow easily and have poor fatigue properties.
  • Comparative Steel 3 due to hot rolling in a low-temperature region, excessively formed crystal grains stretched in the center of the thickness, and it was judged that fatigue fracture occurred along the weak grain boundaries. That is, this is due to the development of fine cracks in the center of the thickness during punching molding along the drawn ferrite grain boundaries.
  • Comparative steels 4 and 5 have the same components, and the conditions of primary cooling when cooling after hot rolling do not satisfy relational expression 2, comparative steel 4 has a thickness of 3.2 mm, and comparative steel 5 has a thickness of 8 mm. To have. Among them, comparative steel 4 having a thickness of less than 5 mm formed many elongated grains similar to comparative steel 1, but even when the cooling rate was slow during primary cooling, coarse carbides were hardly formed at the grain boundaries, so fatigue characteristics were not significantly inferior. .
  • the comparative steel 5 having a thick thickness has a slow cooling rate during the first cooling, so that pearlite is formed in the center of the thickness, and the fraction of ferrite phase is somewhat excessive, and the MA phase is also observed in the crystal grains, indicating that the fatigue properties are deteriorated. .
  • Comparative steels 6 and 7 have the same components as each other, but have thicknesses of 3.3 mm and 9 mm, respectively, and are not satisfied with both relations 1 and 2.
  • Comparative steel 6 is a thin material, and it is judged that the effect of delaying recrystallization can be secured over the entire thickness even when the hot rolling temperature is high. It was good.
  • thick comparative steel 7 has a large microstructure due to a high rolling temperature and a slow cooling rate during primary cooling, and a grain boundary length of less than 700 mm is formed, and a MA phase and a pearlite phase are also formed, resulting in poor fatigue properties.
  • Comparative steels 8 and 9 are cases in which the finishing temperature at the time of hot pressure is lower than the range suggested by the present invention, and the cooling rate at the time of primary cooling is slow. These also have the same components but different thicknesses.
  • the comparative steel 8 which is a thin material, many fine and elongated ferrite phases are formed over the entire thickness, but the fatigue properties are not inferior, whereas the comparative steel 9, a thick material, is MA at the center of the thickness. The phase and pearlite phase were excessively formed, resulting in poor fatigue properties.
  • Comparative steel 10 is a relational formula 3, that is, when the cooling rate during secondary cooling is out of the present invention, the cooling rate during secondary cooling is too fast, so that the martensite phase is excessively formed in the center of the thickness, resulting in an ambient phase when exposed to a fatigue environment. It was judged that the fracture proceeded easily in the region where the hardness difference with) was large.
  • Comparative steel 11 also does not satisfy the relational expression (3), and the cooling rate is too slow during secondary cooling, resulting in excessive formation of the pearlite phase, resulting in inferior fatigue properties.
  • the comparative steel 12 to 17 is the case where the alloy composition is out of the present invention, all of the relational formulas 1 to 3 are satisfied at the time of manufacture, and they are all manufactured to have the same thickness (8 mm), but the fatigue properties are inferior.
  • the comparative steel 12 is a case where the C content is insufficient, the ferrite phase is excessively formed in the center of the thickness, and the bainite phase is not sufficiently formed. Due to this, the microstructure was coarse and the fatigue strength was low.
  • Comparative steel 13 is a case in which the C content is excessively added, and the pearlite and martensite phases are excessively formed due to the high C concentration in the untransformed phase during the phase transformation process, thereby exhibiting lower fatigue strength than the yield strength.
  • Comparative steel 14 is a case where the Si content is too high, and the MA phase is formed together with the bainite phase, and a lot of stretched microstructures are observed. Due to this, the fatigue properties were inferior, which is thought to be due to the formation of many cracks around the relatively hard phase MA phase.
  • Comparative steel 15 is a case where the Mn content is insufficient, and despite the fact that it was prepared by satisfying the relations 1 to 3 to obtain a recrystallization delay effect and a uniform microstructure, the ferrite phase was excessively formed in the center of the thickness, resulting in strength and fatigue strength All appeared low.
  • Min and Max mean the minimum and maximum values of the aspect ratio (short side length of crystal grains/long side length of crystal grains), and Total Fraction corresponds to the range above the minimum value (Min) and below the maximum value (Max). It means the area fraction of crystal grain.
  • the fraction of crystal grains having a shape ratio (ratio of short side/long side) of crystal grains of 0.3 or less is less than 50% (total fraction 0.5).

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JP2010174343A (ja) * 2009-01-30 2010-08-12 Jfe Steel Corp 低温靭性に優れた厚肉高張力熱延鋼板の製造方法
KR101528084B1 (ko) 2010-09-17 2015-06-10 제이에프이 스틸 가부시키가이샤 타발 가공성이 우수한 고강도 열연 강판 및 그 제조 방법
KR20150074943A (ko) * 2013-12-24 2015-07-02 주식회사 포스코 전단변형부 성형이방성 및 내피로특성이 우수한 열연강판 및 그 제조방법

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