WO2020071654A1 - Tôle d'acier laminée à chaud à ultra-haute résistance présentant d'excellentes qualités de surface et un faible écart de propriétés mécaniques et procédé pour la fabriquer - Google Patents

Tôle d'acier laminée à chaud à ultra-haute résistance présentant d'excellentes qualités de surface et un faible écart de propriétés mécaniques et procédé pour la fabriquer

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Publication number
WO2020071654A1
WO2020071654A1 PCT/KR2019/011700 KR2019011700W WO2020071654A1 WO 2020071654 A1 WO2020071654 A1 WO 2020071654A1 KR 2019011700 W KR2019011700 W KR 2019011700W WO 2020071654 A1 WO2020071654 A1 WO 2020071654A1
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WIPO (PCT)
Prior art keywords
steel sheet
rolled steel
less
surface quality
manufacturing
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Application number
PCT/KR2019/011700
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English (en)
Korean (ko)
Inventor
공종판
고영주
박경미
Original Assignee
주식회사 포스코
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by 주식회사 포스코 filed Critical 주식회사 포스코
Priority to CN201980064021.0A priority Critical patent/CN112996939B/zh
Priority to JP2021517743A priority patent/JP7186291B2/ja
Publication of WO2020071654A1 publication Critical patent/WO2020071654A1/fr

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/16—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling wire rods, bars, merchant bars, rounds wire or material of like small cross-section
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/46—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/46—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting
    • B21B1/463—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting in a continuous process, i.e. the cast not being cut before rolling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/10—Supplying or treating molten metal
    • B22D11/108—Feeding additives, powders, or the like
    • 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 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
    • 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 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
    • 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 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
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to an ultra-high-strength hot-rolled steel sheet having excellent surface quality and less material deviation, and a method for manufacturing the same.
  • the steel sheet for automobile structural members is basically composed of a combination of Ferrite, Baintie, Martensite and Tempered Martensite phases. According to the composition ratio, it is classified into DP (Dual Phase) steel, Transformation Induced Plasticity (TRIP) steel, Complex Phase steel, and MART steel.
  • DP Dual Phase
  • TRIP Transformation Induced Plasticity
  • These steels are mainly applied to parts that require high energy absorption in the event of a vehicle collision, such as members, pillars, bumper reinforcement, and seal side. Since they are processed using roll forming, they must have a high elongation with tensile strength of 1.0 GPa or more. However, these steels have difficulty in avoiding a reduction in elongation due to securing ultra-high strength, and new processes such as hot rolling and annealing heat treatment (CAL, continuous annealing line) after hot rolling or HPF (Hot Press Forming), which is a process using rapid cooling after hot rolling, are used. There is a disadvantage that the manufacturing cost increases because it has to be passed.
  • CAL hot rolling and annealing heat treatment
  • HPF Hot Press Forming
  • Patent Document 1 C: 0.15 to 0.20%, Si: 0.3 to 0.8%, Mn: 1.8 to 2.5%, Al: 0.02 to 0.06%, Mo: 0.1 to 0.4%, and Nb: 0.03 to 0.06% by weight ratio of chemical components.
  • Patent Document 2 C: 0.05 to 0.20% by weight, Si: 2.5% or less, Mn: 3.0% or less, and Cr: 0.3% or less, Mo: 0.3% or less in steel containing impurities and a small amount of alloying elements, Ni: A method for producing a cold rolled steel sheet having a good shape having a strength of 1180 to 1400 MPa and a bending / twisting of the steel sheet of 10 mm or less by adding one or two or more of 0.3% or less is disclosed.
  • cold rolled steel sheets containing C: 0.1 to 0.6%, Si: 1.0 to 3.0%, Mn: 1.0 to 3.5%, Al: 1.5% or less, and Cr: 0.003 to 2.0% in weight% are Ac3 to Ac3.
  • the phase fraction of residual austenite before processing is 10%
  • a method of manufacturing a tensile strength 1470 MPa grade ultra-high strength cold rolled steel sheet having a hydrogen withdrawal characteristic having an austenite grain size of 1 micron or more due to shortening and an average axial ratio (long axis / short axis) of 5 or more is introduced.
  • Patent Documents 1 to 4 there is a disadvantage in that the manufacturing cost is rapidly increased because it has to undergo a cold rolling and annealing heat treatment (CAL) process after hot rolling, as well as a bumper or reinforcement for automobiles that are currently commercially used. There is a problem in that the tensile strength is relatively low to apply to.
  • CAL cold rolling and annealing heat treatment
  • Patent Document 5 Ti in an amount satisfying C: 0.26 to 0.45%, Mn + Cr: 0.5 to 3.0%, Nb: 0.02 to 1.0%, 3.42N + 0.001 ⁇ Ti ⁇ 3.42N + 0.5 by weight%, and further Si: 0.5% or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less, Al: 1% or less, one or two or more, and sometimes B: 0.01% or less, Nb: 1.0% Or less, Mo: 1.0% or less, Ca: 0.001 to 0.005% of a cold rolled steel sheet containing one or two or more types is disclosed for a method of manufacturing ultra high strength having a tensile strength of 1.8 GPa through hot press forming.
  • Patent Document 5 the ultra-high strength of 1.8 GPa can be secured, but there is a problem that the manufacturing cost is higher because the cold rolled steel sheet must be subjected to a hot press forming step again.
  • Patent Document 1 Korean Patent Publication No. 2004-0057777
  • Patent Document 2 Japanese Patent Publication No. 2007-100114
  • Patent Document 3 Korean Patent Publication No. 2008-0073763
  • Patent Document 4 Korean Patent Publication No. 2013-0069699
  • Patent Document 5 Korean Patent Publication No. 2008-0111549
  • One aspect of the present invention is to provide an ultra-high-strength hot-rolled steel sheet having excellent surface quality and low material deviation using only a hot rolling process using a continuous rolling mode in a performance-direct rolling process.
  • the surface quality is excellent only by the hot rolling process by using the continuous rolling mode in the direct rolling process from the performance to rolling, and the ultra-high strength hot rolled steel sheet with less material deviation and its manufacturing Can provide a method.
  • the hot-rolled steel sheet of the present invention can secure a higher tensile strength, and not only can replace the ultra-high-strength cold-rolled steel sheet and hot-formed steel, but also have the effect of significantly lowering the manufacturing cost.
  • through the slab playing method it is possible to use steel in which scraps such as scrap metal are melted in an electric furnace, thereby increasing the recyclability of resources.
  • FIG. 1 is a schematic diagram of a facility for a direct-to-roll rolling process applicable to the production of hot-rolled steel sheets of the present invention.
  • Figure 2 is another schematic diagram of a facility for the direct-to-rolling direct connection process applicable to the production of hot-rolled steel sheet of the present invention.
  • FIG 3 is a graph showing the values of relations 1 and 2 for Inventive Examples 1 to 15 and Comparative Examples 1 to 13 according to an embodiment of the present invention.
  • FIG. 4 is a microstructure photograph of Inventive Example 1 according to an embodiment of the present invention observed with a scanning electron microscope (SEM).
  • FIG. 5 (a) and (b) are microstructure photographs of Inventive Example 1 according to an embodiment of the present invention observed with a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • Example 6 is a graph showing the distribution of martensitic and auto-tempered martensitic lath widths of Inventive Example 1 according to an embodiment of the present invention.
  • FIG. 7 is a photograph of the precipitate of Inventive Example 1 according to an embodiment of the present invention observed with a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • Comparative Example 8 is a photograph of the precipitate of Comparative Example 8 according to an embodiment of the present invention observed with a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • Carbon (C) is a very important element that increases strength by making microstructure martensite upon rapid cooling after hot rolling.
  • the C content is less than 0.16%, the strength of martensite itself is low, and thus it may be difficult to secure the strength targeted in the present invention.
  • the C content is more than 0.27%, there is a problem in that bending workability is deteriorated due to increase in weldability and excessive strength. Therefore, the C content is preferably 0.16 to 0.27%.
  • the lower limit of the C content is more preferably 0.17%, even more preferably 0.18%, and most preferably 0.19%.
  • the upper limit of the C content is more preferably 0.26%, more preferably 0.25%, and most preferably 0.24%.
  • Manganese (Mn) inhibits ferrite formation and increases austenite stability to increase the strength by facilitating the formation of a low-temperature transformation phase. If the Mn content is less than 0.8%, it may be difficult to secure the target strength in the present invention. On the other hand, when the Mn content is more than 2.6%, segregation zones are formed on the inside or outside of the performance slab and hot-rolled steel sheet to cause crack generation and propagation, which degrades the final quality of the steel sheet and degrades weldability and bending workability. can do. Therefore, the Mn content is preferably 0.8 to 2.6%.
  • the lower limit of the Mn content is more preferably 0.85%, even more preferably 0.90%, and most preferably 0.95%.
  • the upper limit of the Mn content is more preferably 2.5%, even more preferably 2.4%, and most preferably 2.3%.
  • Silicon (Si) is a useful element that can secure strength without lowering the ductility of the steel sheet. It is also an element that promotes the formation of martensite by promoting ferrite formation and promoting C concentration into unmodified austenite.
  • the Si content is less than 0.05%, it is difficult to sufficiently secure the above-described effect.
  • the Si content is more than 0.3%, red scale is generated on the surface of the steel sheet, and after pickling, traces may remain on the surface of the steel sheet, thereby deteriorating the surface quality. Therefore, the Si content is preferably 0.05 to 0.3%.
  • the lower limit of the Si content is more preferably 0.06%, even more preferably 0.07%, and most preferably 0.08%.
  • the upper limit of the Si content is more preferably 0.28%, even more preferably 0.26%, and most preferably 0.24%.
  • Aluminum (Al) may thicken on the surface of the steel sheet, thereby deteriorating plating properties, while suppressing carbide formation to increase the ductility of the steel.
  • aluminum (Al) in the steel reacts with nitrogen (N) to precipitate AlN.
  • N nitrogen
  • the content should be controlled as low as possible, and preferably controlled to 0.05% or less.
  • the Al content is more preferably 0.048% or less, even more preferably 0.046% or less, and most preferably 0.045% or less.
  • Titanium (Ti) is an element that increases the strength of steel as a precipitate and nitride forming element.
  • Ti is an element that decreases the susceptibility to edge cracking by preventing high temperature ductility deterioration by reducing the amount of precipitates such as AlN by removing solid solution N through the formation of TiN near the solidification temperature.
  • the Ti content is less than 0.01%, the slab quality is deteriorated by reducing the ductility of the cast slab due to excessive precipitation of fine AlN or BN precipitates.
  • the Ti content is more than 0.08%, it is difficult to expect a grain refinement effect due to the formation of coarse TiN precipitates, and the manufacturing cost increases.
  • the Ti content is preferably 0.01 to 0.08%.
  • the lower limit of the Ti content is more preferably 0.012%, even more preferably 0.014%, and most preferably 0.016%.
  • the upper limit of the Ti content is more preferably 0.07%, even more preferably 0.06%, and most preferably 0.05%.
  • Boron (B) is an element that increases the hardenability of steel.
  • the content is less than 0.001%, the above effect cannot be obtained, and when it exceeds 0.005%, the austenite recrystallization temperature increases and the weldability is deteriorated. Therefore, it is preferable to limit the content of B to 0.001 to 0.005%.
  • the lower limit of the B content is more preferably 0.0012%, even more preferably 0.0014%, and most preferably 0.0016%.
  • the upper limit of the B content is more preferably 0.0045%, even more preferably 0.0040%, and most preferably 0.0035%.
  • Calcium (Ca) is an element that reacts with Al and O in molten steel to form a spherical inclusion (12CaO ⁇ 17Al 2 O 3 ) with a low melting point to prevent nozzle clogging and facilitate inclusion separation.
  • the Ca content is less than 0.001%, it is difficult to secure the above effect.
  • the Ca content is more than 0.005%, a high melting point inclusion is formed to promote nozzle clogging, which may cause casting interruption, and a large inclusion (> 50 ⁇ m) can be formed to degrade the workability of the steel sheet. Therefore, the content of Ca is preferably controlled to 0.001 to 0.005%.
  • the lower limit of the Ca content is more preferably 0.0012%, even more preferably 0.0014%, and most preferably 0.0016%.
  • the upper limit of the Ca content is more preferably 0.0045%, even more preferably 0.0040%, and most preferably 0.0035%.
  • N Nitrogen
  • the N content is preferably 0.001 to 0.010%.
  • the lower limit of the N content is more preferably 0.0012%, even more preferably 0.0014%, and most preferably 0.0016%.
  • the upper limit of the N content is more preferably 0.009%, even more preferably 0.008%, and most preferably 0.007%.
  • the remaining component of the invention is iron (Fe).
  • Fe iron
  • unintended impurities from the raw material or the surrounding environment may inevitably be 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 hot-rolled steel sheet of the present invention it is preferable that C, Mn, B, Al, Ti and N among the above-described alloy components satisfy the following relations 1 to 3, respectively, and through this, the mechanical properties targeted by the present invention Excellent surface quality can be ensured.
  • the content of the alloy component described in the following relations 1 to 3 is weight%.
  • the relational expression 1 is a component relational expression for securing the mechanical properties desired by the present invention.
  • the value of the relational expression 1 is less than 16, it is difficult to secure the strength targeted by the present invention, and when it exceeds 28, the elongation is lowered and cracking may occur during processing. Therefore, it is preferable that the value of the relational expression 1 has a range of 16 to 28.
  • the lower limit of the value of the relational expression 1 is more preferably 17, even more preferably 18, and most preferably 19.
  • the upper limit of the value of the relational expression 1 is more preferably 27, even more preferably 26, and most preferably 25.
  • the relational expression 2 is a component relational expression for improving the surface quality of the hot-rolled steel sheet finally obtained by securing the edge quality of the slab or bar.
  • the value of the relational expression 2 is less than 1, when the Ti or B content is high or the Al or N content is low, the decrease in high temperature ductility due to excessive precipitation of coarse Ti (C, N) and B (C, N) precipitates is reduced. As a result, cracks may occur at the edge of the slab or bar, and when it exceeds 14, the Ti or B content is low or the Al or N content is high. Edge quality may degrade. Therefore, it is preferable that the value of the relational expression 2 has a range of 1 to 14.
  • the lower limit of the value of the relational expression 2 is more preferably 1.1, even more preferably 1.2, and most preferably 1.3.
  • the upper limit of the value of the relational expression 2 is more preferably 13, even more preferably 12, and most preferably 11.
  • the relational expression 3 is a component relational expression for securing the mechanical properties and excellent surface quality targeted by the present invention.
  • the value of the relational expression 3 is less than 0.05, it may be difficult to secure a target strength, and when it exceeds 0.66, a high temperature ductility decrease due to excessive precipitation of precipitates, resulting in cracks at the edge of the slab or bar Can occur. Therefore, it is preferable that the value of the relational expression 3 has a range of 0.05 to 0.66.
  • the lower limit of the value of the relational expression 3 is more preferably 0.06, even more preferably 0.08, and most preferably 0.10.
  • the upper limit of the value of the relational expression 3 is more preferably 0.62, even more preferably 0.58, and most preferably 0.56.
  • the hot-rolled steel sheet of the present invention is one or more selected from the group consisting of Nb, V, Ti, Mo, Cu, Cr, Ni, Zn, Se, Sb, Zr, W, Ga, Ge and Mg as a tramp element.
  • the total may be included in a range of 0.1% by weight or less.
  • the tramp element is an impurity element originating from alloy iron or scrap used as a raw material in the steelmaking process, ladle and tundish refractory material, and when the sum exceeds 0.1%, cracks on the surface of the thin slab It can cause the surface quality of the hot rolled steel sheet to decrease.
  • the hot-rolled steel sheet of the present invention preferably includes a microstructure in which the sum of martensite and auto tempered martensite in an area fraction is 95% or more, and ferrite is 5% or less (including 0%).
  • the martensite and auto-tempered martensite structures are essential structures for obtaining the strength of the present invention as a mokpo, and it is difficult to secure strength when the fraction is less than 95%.
  • ferrite may be included in a range of 5% or less in order to secure ductility. However, when the fraction exceeds 5%, ductility increases but strength may be difficult to secure.
  • the fraction of the sum of the martensite and auto-tempered martensite is more preferably 96% or more, even more preferably 97% or more, and most preferably 98% or more.
  • the main microstructure of the present invention is martensite auto-tempered martensite, wherein the average width of the lathes of martensite and auto-tempered martensite can affect strength and workability. Therefore, the average width of the lath of the martensite and auto-tempered martensite is preferably 1 ⁇ m or less based on the shortening. When the average width of lath of the martensite and auto-tempered martensite exceeds 1 ⁇ m, it may be difficult to secure target strength and workability. The narrower the average lath width of the martensite and auto tempered martensite is, the more advantageous it is to secure strength, but it may be difficult to control to less than 0.1 ⁇ m under normal cooling conditions.
  • the lower limit of the average lath width of the martensite and auto-tempered martensite is more preferably 0.12 ⁇ m, even more preferably 0.14 ⁇ m, and most preferably 0.16 ⁇ m.
  • the upper limit of the average width of the martensite and auto-tempered martensite is more preferably 0.9 ⁇ m, even more preferably 0.8 ⁇ m, and most preferably 0.7 ⁇ m.
  • the average size of the composite precipitate is more preferably 38 nm or less, more preferably 34 nm or less, and most preferably 30 nm or less.
  • the hot-rolled steel sheet provided by the present invention has a yield strength of 1060 to 1400 MPa, a tensile strength of 1470 to 1800 MPa, an elongation of 5% or more, a Vickers hardness of 420 to 550 Hv (0.5 kgf), and a variation in tensile strength in the width direction of the strip. Is less than 100MPa, and the Vickers hardness deviation in the width direction of the strip may be 50Hv (0.5kgf) or less.
  • the hot-rolled steel sheet of the present invention may have a thickness of 1.6 mm or less, more preferably 1.4 mm or less, even more preferably 1.3 mm or less, and most preferably 1.2 mm or less.
  • the hot-rolled steel sheet of the present invention can effectively replace the ultra-high-strength cold-rolled steel sheet and hot-formed steel by having excellent mechanical properties, surface quality, and low material deviation as described above.
  • FIG. 1 is a schematic diagram of a facility for a direct-to-roll rolling process applicable to the production of hot-rolled steel sheets of the present invention.
  • An ultra-high strength hot rolled steel sheet having excellent surface quality and low material deviation according to an embodiment of the present invention can be produced by applying a performance-rolling direct connection facility as shown in FIG. 1.
  • the direct-rolling direct-linking facility is largely composed of a continuous casting machine 100, a rough rolling machine 400, and a finishing rolling machine 600.
  • the performance-rolling direct connection facility includes a high-speed continuous casting machine 100 that produces a thin slab (a) of a first thickness, and a second thickness bar (b) of the slab thinner than the first thickness.
  • the rough rolling mill breaker 300 in front of the rough rolling mill 400 (Roughing Mill Scale Breaker, hereinafter 'RSB') and the finishing rolling scale breaker 500 in front of the finishing mill 600 (Fishing Mill Scale Breaker, hereinafter) FSB ') can be additionally included, and it is easy to remove the surface scale, so it is possible to produce PO (Picked & Oiled) steel sheets with excellent surface quality when pickling hot rolled steel in a later process.
  • FIG. 2 is another schematic diagram of a facility for the direct-to-rolling direct connection process applicable to the production of hot-rolled steel sheet of the present invention.
  • the performance-rolling direct connection facility disclosed in FIG. 2 is mostly the same as the facility disclosed in FIG. 1, but the heater 200 ′ for additionally heating the slab in front of the rough rolling mill 400 and the rough rolling scale breaker 300 is provided.
  • the heater 200 ′ for additionally heating the slab in front of the rough rolling mill 400 and the rough rolling scale breaker 300 is provided.
  • the space of at least one slab is secured before the rough rolling, batch type rolling is possible.
  • the super-high-strength hot-rolled steel sheet having excellent surface quality and less material deviation of the present invention can be produced in both the performance and rolling direct connection facilities disclosed in FIGS. 1 and 2.
  • a molten slab having the above-described alloy composition is continuously cast to obtain a thin slab.
  • the continuous casting is preferably performed at a casting speed of 4 ⁇ 8mpm (m / min).
  • the reason why the casting speed is 4mpm or more is because a high-speed casting and a rolling process are connected, and a casting speed of a predetermined or higher is required to secure a target rolling temperature.
  • the casting speed is slow, there is a risk of segregation from the cast iron. When such segregation occurs, it is difficult to secure strength and processability, and the risk of material deviation in the width direction or the length direction increases.
  • the casting speed is preferably in the range of 4 to 8mpm.
  • the lower limit of the casting speed is more preferably 4.2mpm, even more preferably 4.4mpm, and most preferably 4.6mpm.
  • the upper limit of the casting speed is more preferably 7.5mpm, even more preferably 7.0mpm, and most preferably 6.5mpm.
  • the thickness of the thin slab is preferably 80 ⁇ 120mm.
  • the thickness of the thin slab exceeds 120 mm, not only is high-speed casting difficult, but the rolling load increases during rough rolling, and when it is less than 80 mm, the temperature drop of the cast slab rapidly occurs, making it difficult to form a uniform structure.
  • an additional heating facility may be installed, but this is a factor that improves production cost, and thus it is desirable to exclude it. Therefore, it is preferable to control the thickness of the thin slab to 80 to 120 mm.
  • the lower limit of the thickness of the thin slab is more preferably 82 mm, even more preferably 84 mm, and most preferably 86 mm.
  • the upper limit of the thickness of the thin slab is more preferably 116 mm, even more preferably 114 mm, and most preferably 110 mm.
  • the basicity of the mold flux during the continuous casting is preferably 0.8 to 1.5.
  • the basicity represents the CaO (%) / SiO2 (%) ratio.
  • alloying elements added such as C, Mn and B in order to secure high strength, and thus the linear crack sensitivity is very high. Therefore, when a mold flux having a basicity of less than 0.8 is used, linear cracking may occur as the slab surface is strongly cooled due to high heat transfer.
  • the basicity of the mold flux during the continuous casting is 0.8 to 1.5.
  • the lower limit of the basicity of the mold flux is more preferably 0.85, even more preferably 0.90, and most preferably 0.95.
  • the upper limit of the basicity of the mold flux is more preferably 1.45, more preferably 1.40, and most preferably 1.35.
  • the secondary cooling specific water amount during the continuous casting is preferably 1.5 ⁇ 2.5l / kg.
  • the secondary cooling specific water amount exceeds 2.5 l / kg, there is a risk of deteriorating the slab quality due to the occurrence of a linear crack and a high risk of edge cracking due to a low edge temperature of the slab or bar.
  • the secondary cooling specific water amount is less than 1.5l / kg, problems such as leakage of molten steel due to non-solidification of the slab may occur at the performance exit, and segment rolls may be deteriorated, resulting in problems of equipment failure. Can occur.
  • the secondary cooling specific water amount during the continuous casting is preferably 1.5 to 2.5 l / kg.
  • the lower limit of the secondary cooling specific quantity during the continuous casting is more preferably 1.55 L / kg, more preferably 1.60 L / kg, and most preferably 1.65 L / kg.
  • the upper limit of the secondary cooling specific water amount during the continuous casting is more preferably 2.45 l / kg, more preferably 2.40 l / kg, and most preferably 2.35 l / kg.
  • the rough rolling step may be performed by rough rolling a continuous cast thin slab in a rough rolling mill composed of 2 to 5 stands.
  • the bar edge temperature at the rough rolling side is preferably 850 to 1000 ° C.
  • the bar edge portion temperature is less than 850 ° C, a large amount of AlN precipitates and the like is generated, and as a result, high temperature ductility decreases, there is a problem in that the susceptibility to edge crack generation is very high.
  • the temperature of the bar edge portion exceeds 1000 ° C, the surface quality may be deteriorated after pickling as the scale of the bar is high as well as the central temperature is high. Therefore, it is preferable that the temperature of the bar edge portion on the side of the rough rolling during the rough rolling is 850 to 1000 ° C.
  • the lower limit of the bar edge portion temperature at the rough rolling exit side is more preferably 860 ° C, even more preferably 870 ° C, and most preferably 880 ° C.
  • the upper limit of the bar edge portion temperature at the rough rolling exit side is more preferably 990 ° C, more preferably 980 ° C, and most preferably 970 ° C.
  • a step of removing the scale by spraying coolant on the bar may be further included.
  • the surface scale can be removed to a thickness of 30 ⁇ m or less by spraying cooling water at a pressure of 200 to 300 bar from a nozzle of a Finishing Mill Scale Breaker (hereinafter referred to as 'FSB') before finishing rolling the bar.
  • 'FSB' Finishing Mill Scale Breaker
  • the cooling water injection pressure is less than 200 bar, the scale is insufficiently removed, resulting in a large amount of spindle- and scale-like scales on the surface of the steel sheet after finish rolling, resulting in poor surface quality after pickling.
  • the cooling water injection pressure is preferably 200 ⁇ 300bar.
  • the lower limit of the cooling water injection pressure is more preferably 210 bar, more preferably 220 bar, and most preferably 230 bar.
  • the upper limit of the cooling water injection pressure is more preferably 290 bar, more preferably 280 bar, and most preferably 270 bar.
  • the overlap area ratio of the cooling water is preferably 5 to 25%.
  • the overlapping area ratio of the cooling water is less than 5%, the overlapping area of the cooling water is too small, so that the temperature of the bar rises locally, and the temperature may become non-uniform in the width direction. As a result, the scale may not be completely removed and the surface quality may deteriorate.
  • the overlapping area ratio of the injection of the cooling water exceeds 25%, the cooling is locally localized, and a temperature deviation occurs in the width direction, so that the material deviation of the finally obtained hot rolled steel sheet may be severe.
  • the overlap area ratio of the cooling water is preferably 5 to 25%.
  • the lower limit of the overlapping area ratio of the cooling water is more preferably 6%, even more preferably 7%, and most preferably 8%.
  • the upper limit of the overlapping area ratio of the cooling water is more preferably 24%, more preferably 23%, and most preferably 22%.
  • the finishing rolling step may be performed by finishing rolling on a bar made in a rough rolling mill in a finishing mill consisting of 3 to 7 stands.
  • the finish rolling exit temperature is less than Ar3 + 10 ° C
  • energy consumption increases as the load of the roll increases significantly during hot rolling, the working speed becomes slower, and the temperature of the hot-rolled steel sheet is locally lower than Ar3 when a temperature deviation occurs in the width direction. As it goes down, a cornerstone ferrite may be generated, so that a sufficient martensite fraction cannot be obtained after cooling.
  • the finishing rolling exit temperature is preferably Ar3 + 10 ° C to Ar3 + 60 ° C.
  • the lower limit of the finish rolling exit temperature is more preferably Ar3 + 12 ° C, even more preferably Ar3 + 14 ° C, and most preferably Ar3 + 16 ° C.
  • the upper limit of the finish rolling exit temperature is more preferably Ar3 + 58 ° C, even more preferably Ar3 + 56 ° C, and most preferably Ar3 + 52 ° C.
  • the rolling speed variation during the finish rolling is 50 mpm or less. Since the ultra-high-strength steel targeted in the present invention uses the formation of a metamorphic structure as a reinforcing mechanism, it is very likely that the material properties change depending on the deformation rate during finish rolling. That is, if the difference in rolling speed in the finishing mill made of multiple stands exceeds 50 mpm, it is difficult to secure a uniform cooling rate and target winding temperature in a subsequent run out table (ROT) strip (Srtip) It may cause a large material deviation in the width or length direction of the. Therefore, it is preferable that the rolling speed variation during the finish rolling is 50 mpm or less.
  • ROT run out table
  • the rolling speed deviation is more preferably 48mpm or less, more preferably 46mpm or less, and most preferably 42mpm or less.
  • the lower the rolling speed variation in the finish rolling the lower the advantage, so the lower limit is not particularly limited.
  • the temperature range in the width direction of the hot-rolled steel sheet is preferably 50 ° C or less.
  • the temperature range in the width direction of the hot-rolled steel sheet during the finish rolling is 50 ° C or less.
  • the temperature range in the width direction of the hot-rolled steel sheet during the finish rolling is more preferably 48 ° C or less, even more preferably 46 ° C or less, and most preferably 42 ° C or less.
  • the lower the temperature deviation in the width direction of the hot-rolled steel sheet during the finish rolling the lower the advantage, and thus the lower limit is not particularly limited.
  • the rolling speed is 200 to 600 mpm.
  • the rolling speed exceeds 600 mpm during the finish rolling, an operation accident such as plate breakage may occur, and uniform temperature may not be secured due to difficulty in isothermal constant velocity rolling, which may cause material deviation.
  • the rolling speed during the finish rolling is less than 200mpm, the finish rolling speed may be too slow to secure the finish rolling temperature targeted by the present invention. Therefore, it is preferable that the rolling speed in the finish rolling is 200 to 600 mpm.
  • the lower limit of the rolling speed during the finish rolling is more preferably 220mpm, even more preferably 250mpm, and most preferably 280mpm.
  • the upper limit of the rolling speed during the finish rolling is more preferably 580mpm, even more preferably 550mpm, and most preferably 500mpm.
  • the hot-rolled steel sheet is cooled to 200 ° C./sec or more on the top of Ar3 and wound up below Mf (90) -50 ° C.
  • the cooling rate is preferably 200 ° C / sec or more.
  • the cooling rate is more preferably 220 ° C./sec or more, more preferably 240 ° C./sec or more, and most preferably 260 ° C./sec or more.
  • the coiling temperature is preferably Mf-50 ° C or lower.
  • the coiling temperature is more preferably Mf-60 ° C or less, more preferably Mf-70 ° C or less, and most preferably Mf-80 ° C or less.
  • Mf means the temperature at which the austenite structure is 100% transformed into martensite.
  • the interval of the cooling nozzle during the cooling is preferably 150 ⁇ 400mm.
  • the interval of the cooling nozzle exceeds 400mm, the temperature of the hot-rolled steel sheet is locally increased, which may cause a material deviation, and if it is less than 150mm, the temperature of the hot-rolled steel sheet may be locally lowered, resulting in a severe material deviation. Therefore, it is preferable that the interval of the cooling nozzle during the cooling is 150 to 400 mm.
  • the lower limit of the interval of the cooling nozzle is more preferably 160 mm, even more preferably 170 mm, and most preferably 180 mm.
  • the upper limit of the interval of the cooling nozzle is more preferably 380 mm, more preferably 360 mm, and most preferably 340 mm.
  • a step of pickling the wound hot-rolled steel sheet may be further included, and a pickled & oiled (PO) material may be obtained through the pickling process.
  • a pickled & oiled (PO) material may be obtained through the pickling process.
  • the scale can be sufficiently removed in the step of removing the thin slab and bar scale, it is possible to obtain a PO material having excellent surface quality even with a general pickling treatment. Therefore, in the present invention, any method that is generally used in the hot acid pickling process is applicable, so the pickling treatment method is not particularly limited.
  • Yield strength, tensile strength and elongation were measured by measuring specimens of JIS No. 5 standard collected in the rolling direction with respect to the entire width of the strip (constant intervals (seven locations)), and the average value was recorded.
  • the hardness was measured 10 times with a load of 0.5 kgf using a Vickers hardness tester and the average value was described.
  • ⁇ TS tensile strength deviation
  • ⁇ Hv Vickers hardness deviation
  • SDD surface defect detector
  • Comparative Example 13 is a case that does not satisfy the Ca content range in the alloy composition proposed by the present invention, it can be confirmed that the casting interruption occurred due to nozzle clogging.
  • Comparative Examples 14 and 15 are alloy compositions proposed by the present invention, relations 1 to 3 are satisfactory, but manufacturing conditions (finish rolling exit temperature) are not satisfied, as the present invention does not secure the microstructure proposed. It can be confirmed that the mechanical properties and material deviation conditions targeted by the present invention are not secured.
  • FIG. 3 is a graph showing the values of relations 1 and 2 for Inventive Examples 1 to 15 and Comparative Examples 1 to 13.
  • the invention examples 1 to 15 are included in the invention region as a range that satisfies relational expression 3 of the present invention, whereas in the case of comparative examples 1 to 12, it is out of the invention region Can be confirmed.
  • Comparative Example 13 is included in the invention area, but does not satisfy the Ca content range of the present invention.
  • FIG. 4 is a microstructure photograph of Inventive Example 1 observed with a scanning electron microscope (SEM). As can be seen through Figure 4, Inventive Example 1, it can be confirmed that martensite and auto-tempered martensite are the main structures, and some ferrite is formed.
  • FIG. 5 (a) and (b) are microstructure photographs of Inventive Example 1 observed with a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • FIG. 6 is a graph showing the distribution of martensitic and auto-tempered martensitic lath widths of Inventive Example 1; As can be seen through Figure 6, in the case of Inventive Example 1, martensitic and auto-tempered martensitic laths are present in the range of 0.05 to 1.0 ⁇ m, and martensitic and auto-tempered martensitic laths having a width of 0.3 ⁇ m are present. You can confirm that
  • Comparative Examples 16 to 19 the alloy composition proposed by the present invention is satisfactory, but it does not satisfy one of the mold flux basicity, secondary cooling specific quantity, and rough rolling exit bar edge temperature conditions among the manufacturing conditions. It can be confirmed that cracks have occurred.
  • the alloy composition and the manufacturing conditions proposed by the present invention are the yield strength, tensile strength, elongation, Vickers hardness, and tensile strength in the width direction of the strip, which the present invention aims for. It can be seen that the deviation and the Vickers hardness deviation in the width direction of the strip are secured.
  • Comparative Examples 20 to 24 the alloy composition proposed by the present invention is satisfactory, but the cooling water spray overlap area ratio, the rolling speed deviation during finishing rolling, the temperature deviation in the width direction of the hot rolled steel sheet during finishing rolling, and the cooling nozzle during cooling It can be confirmed that the preceding crack and the edge crack occurred because one of the intervals of was not satisfied.

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Abstract

Un mode de réalisation de la présente invention concerne une tôle d'acier laminée à chaud à très haute résistance présentant une excellente qualité de surface et un faible écart de matériau, et un procédé de fabrication associé. La tôle d'acier laminée à chaud à très haute résistance comprend : en % en poids, de 0,16 à 0,27 % de C ; de 0,8 à 2,6 % de Mn ; de 0,05 à 0,3 % de Si ; de 0,05 % ou moins d'Al ; de 0,01 à 0,08 % de Ti ; 0,001 à 0,005 % de B ; 0,001 % à 0,005 % de Ca ; 0,001 % à 0,010 % de N ; et le reste étant constitué de Fe et d'autres impuretés inévitables, une microstructure satisfaisant les relations suivantes 1 à 3, et dans laquelle, par fraction de surface, la somme de martensite et de martensite autotrempée est de 95 % ou plus et la ferrite est de 5 % ou moins (0 % inclus), et les précipités composites M (X) (M = Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X = C, N) avec une taille moyenne de 40 nm ou moins. [Relation 1] 16 ≤ 100(C+Mn/100+B/10) ≤ 28 [Relation 2] 1 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)] ≤ 14 [Relation 3] 0.05 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/100(C+Mn/100+B/10) ≤ 0,66 (la teneur des composants d'alliage décrits dans les relations 1 à 3 étant en % en poids)
PCT/KR2019/011700 2018-10-01 2019-09-10 Tôle d'acier laminée à chaud à ultra-haute résistance présentant d'excellentes qualités de surface et un faible écart de propriétés mécaniques et procédé pour la fabriquer WO2020071654A1 (fr)

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JP2022503938A (ja) 2022-01-12
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