WO2020071654A1 - Ultra high strength hot rolled steel sheet having excellent surface qualities and low mechanical properties deviation and method of manufacturing the same - Google Patents

Ultra high strength hot rolled steel sheet having excellent surface qualities and low mechanical properties deviation and method of manufacturing the same

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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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WO
WIPO (PCT)
Prior art keywords
steel sheet
rolled steel
less
surface quality
manufacturing
Prior art date
Application number
PCT/KR2019/011700
Other languages
French (fr)
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 JP2021517743A priority Critical patent/JP7186291B2/en
Priority to CN201980064021.0A priority patent/CN112996939B/en
Publication of WO2020071654A1 publication Critical patent/WO2020071654A1/en

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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

One embodiment of the present invention provides a super high strength hot rolled steel sheet with excellent surface quality and low material deviation, and a manufacturing method thereof. The super high strength hot rolled steel sheet comprises: by weight%, 0.16-0.27% of C; 0.8-2.6% of Mn; 0.05-0.3% of Si; 0.05% or less of Al; 0.01-0.08% of Ti; 0.001-0.005% of B; 0.001%-0.005% of Ca; 0.001%-0.010% of N; and the balance being Fe and other unavoidable impurities, a microstructure satisfying the following relations 1 to 3, and in which, by area fraction, the sum of martensite and auto tempered martensite is 95% or more and ferrite is 5% or less (including 0%), and M(X) (M = Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X = C, N) composite precipitates with an average size of 40 nm or less. [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 (wherein the content of the alloying components described in relations 1 to 3 is in weight%)

Description

표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판 및 그 제조방법Super high-strength hot-rolled steel sheet with excellent surface quality and little material deviation, and its manufacturing method
본 발명은 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판 및 그 제조방법에 관한 것이다.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.
국제 환경 규제 강화 및 자동차 연비규제 강화 추세에 따라 차체의 초고강도화 및 초경량화의 실현이 필요하게 되어, 1.0GPa급 이상의 초고강도 강판의 개발이 활발히 진행되고 있다. 대부분의 자동차 차체 보강재인 범퍼 보강재 및 도어 임팩트빔 등에 사용되는 초고강도 열연강판은 높은 강도가 요구됨은 물론, 롤 포밍(Roll Forming) 성형하기 때문에 우수한 굽힘 가공성과 적은 재질편차가 요구된다. 이러한 물성을 만족시키기 위하여 자동차 구조부재용 강판은 기본적으로 페라이트(Ferrite), 베이나이트(Baintie), 마르텐사이트(Martensite) 및 템퍼드 마르텐사이트(Tempered Martensite) 상(Phase)의 조합으로 구성되며, 이들 상의 구성 비율에 따라, DP(Dual Phase)강, TRIP(Transformation Induced Plasticity)강, 복합조직(Complex Phase)강, MART강 등으로 분류되어 적용되고 있다.In accordance with the trend of strengthening international environmental regulations and strengthening automobile fuel economy regulations, it is necessary to realize ultra-high strength and ultra-light weight of the vehicle body, and development of ultra-high-strength steel sheets having a level of 1.0 GPa or more is actively being progressed. The ultra-high strength hot rolled steel sheet used in most automobile body reinforcement materials such as bumper reinforcement and door impact beam requires high strength as well as excellent bending processability and less material deviation due to roll forming. In order to satisfy these physical properties, 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.
이러한 강들은 멤버류, 필라류, 범퍼보강재, 실사이드 등 차량 충돌 시 높은 에너지 흡수능이 요구되는 부품에 주로 적용되며 롤 포밍을 이용해 가공하기 때문에 1.0GPa 이상의 인장강도와 더불어 높은 연신율을 가져야 한다. 그러나 이러한 강들은 초고강도 확보에 따른 연신율의 감소를 피하기 어려워 열간압연 후 냉연 및 소둔 열처리(CAL, Continuous Annealing Line) 또는 열연 후 급속냉각을 이용한 가공 공정인 HPF(Hot Press Forming)와 같은 신규공정을 거쳐야 하기 때문에 제조비가 상승하는 단점이 있다. 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.
한편, 자동차 차체 보강재 부품으로 사용되는 인장강도 1.2GPa급 이상의 초고강도강을 제공하기 위하여 많은 연구 및 개발이 진행되었으며, 그 대표적인 예로는 특허문헌 1 내지 5가 있다.Meanwhile, many researches and developments have been conducted to provide ultra-high strength steel having a tensile strength of 1.2 GPa or higher, which is used as an automobile body reinforcement component, and typical examples thereof are Patent Documents 1 to 5.
특허문헌 1에서는, 화학성분 중량비로 C: 0.15~0.20%, Si: 0.3~0.8%, Mn: 1.8~2.5%, Al: 0.02~0.06%, Mo: 0.1~0.4%, Nb: 0.03~0.06%, S: 0.02% 이하, P: 0.02% 이하, N: 0.005% 이하를 첨가하고 강의 제조 시 불가피하게 함유되는 원소를 포함한 알루미늄 킬드강을 1050~1300℃에서 균질화 처리 후 Ar3 변태점 직상인 850~950℃에서 마무리 열간압연한 다음 550~650℃에서 열연권취하는 단계; 이 강판을 30~80%의 냉간압하율로 냉간압연한 다음 A3 온도 이상에서 연속소둔하는 단계; 및 이 강판을 600~700℃까지 1차 서냉을 실시하고, 2차로 350~300℃까지 -10~-50℃/sec의 냉각속도로 급냉한 다음 350~250℃ 사이에서 서냉하면서 1분 이상 유지하는 단계를 포함하는 자동차 범퍼 보강재용 인장강도 1.2GPa급 초고강도 냉연강판 제조방법에 대하여 개시하고 있다. In 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. , S: 0.02% or less, P: 0.02% or less, N: 0.005% or less added, and homogenized aluminum chelated steel containing elements that are inevitably contained in steel production at 1050 to 1300 ° C, then 850 to 950, which is directly at the Ar3 transformation point Finishing hot rolling at ℃ ℃ and then hot-rolling at 550 ~ 650 ℃; Cold-rolling the steel sheet at a cold rolling reduction rate of 30 to 80%, followed by continuous annealing at a temperature above A3; And the first slow cooling of the steel sheet to 600 to 700 ° C, and secondarily rapid cooling to 350 to 300 ° C at a cooling rate of -10 to -50 ° C / sec, and then cooling at 350 to 250 ° C for 1 minute or more. Disclosed is a method for manufacturing an ultra-high strength cold rolled steel sheet having a tensile strength of 1.2 GPa for a bumper reinforcement of an automobile including a step of
특허문헌 2에서는, 중량%로 C: 0.05 ~ 0.20%, Si: 2.5% 이하, Mn: 3.0% 이하 및 불순물과 소량의 합금 원소를 함유한 강에 Cr: 0.3% 이하, Mo: 0.3% 이하, Ni: 0.3% 이하 중 1 또는 2종 이상을 첨가하여 1180~1400MPa 강도를 갖고 강판의 휘어짐/뒤틀림이 10mm 이하인 양호한 형상을 갖는 냉연강판을 제조하는 방법에 대하여 개시하고 있다. 또한, 연속소둔 열처리 설비를 이용하여 강판을 고온에서 급냉한 후, 150~200℃ 온도 범위에서 과시효 처리함에 의해 통상의 수냉(quenching) 후 뜨임(tempering) 처리에 의한 판 형상 불량(강판의 폭 방향 변형)을 개선할 수 있음도 개시되어 있다. In 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. In addition, after quenching the steel sheet at a high temperature using a continuous annealing heat treatment facility, by over-aging treatment in a temperature range of 150 to 200 ° C, plate shape defects due to tempering treatment after normal water quenching (the width of the steel sheet) It is also disclosed that it is possible to improve the direction deformation).
특허문헌 3에서는, 중량%로 C:0.1~0.6%, Si: 1.0~3.0%, Mn: 1.0~3.5%, Al: 1.5% 이하 및 Cr: 0.003~2.0%를 함유하는 냉연 강판을 Ac3~Ac3+50℃ 온도로 가열한 후 3℃/s 이상의 냉각속도로 냉각하고, (Ms-100℃)~Bs(베이나이트 개시 온도) 범위에서 항온 유지함에 의해 가공 전 잔류 오스테나이트의 상분율이 10% 이상이고, 오스테나이트 결정립의 길이가 단축으로 1 마이크론 이상이며, 평균 축비(장축/단축)가 5 이상인 내 수소취하 특성을 갖는 인장강도 1470MPa급 초고강도 냉연강판 제조 방법에 관하여 소개하고 있다.In Patent Document 3, 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. After heating to a temperature of + 50 ° C, cooling at a cooling rate of 3 ° C / s or higher, and maintaining a constant temperature in the range of (Ms-100 ° C) to Bs (vanite start temperature), the phase fraction of residual austenite before processing is 10% As described above, 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.
특허문헌 4에서는, 중량% C: 0.10~0.27%, Si: 0.001~1.0%, Mn: 2.3~ 3.5%, Al: 1.0% 이하(0% 제외), Cr: 2% 이하(0% 제외), P: 0.02% 이하(0% 제외), S: 0.01% 이하(0% 제외), N: 0.01% 이하(0% 제외), B: 0.005% 이하(0% 제외), Ti: 0.004 ~ 0.03%, Mo: 0.2% 이하(0% 제외), Nb: 0.05% 이하(0% 제외), 잔부 Fe 및 기타 불가피한 불순물이 함유한 냉간 압연된 스트립을 1~5℃/s 가열속도로 [(Ac3-90℃)~(Ac3±15℃)]의 온도 범위까지 가열한 다음, 1~3℃/s의 냉각속도로 500~750℃의 온도 범위까지 1차 냉각하고, 3~50℃/s의 냉각속도로 [(Ms-120)~460℃]의 온도 범위까지 2차 냉각한 다음 6~500sec 동안에 항온변태 유지하거나 1℃/s 이하의 냉각속도로 서냉하는 연속소둔 단계를 걸쳐 인장강도 1.5GPa의 냉연강판 제조 방법에 관하여 소개하고 있다. In Patent Document 4, weight% C: 0.10 to 0.27%, Si: 0.001 to 1.0%, Mn: 2.3 to 3.5%, Al: 1.0% or less (excluding 0%), Cr: 2% or less (excluding 0%), P: 0.02% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), Ti: 0.004 to 0.03% , Mo: 0.2% or less (excluding 0%), Nb: 0.05% or less (excluding 0%), cold rolled strips containing the remaining Fe and other unavoidable impurities at a heating rate of 1 ~ 5 ℃ / s [(Ac3- 90 ℃) ~ (Ac3 ± 15 ℃)], then heated to a temperature range of 500 ~ 750 ℃ at a cooling rate of 1 ~ 3 ℃ / s, and cooled to 3 ~ 50 ℃ / s Secondary cooling to the temperature range of [(Ms-120) ~ 460 ℃] at a rate and then maintaining constant temperature transformation for 6 ~ 500sec or slow cooling at a cooling rate of 1 ℃ / s or less, the tensile strength of 1.5GPa It introduces how to manufacture cold rolled steel sheet.
그러나 특허문헌 1 내지 4에 따를 경우, 열간압연 후 냉연 및 소둔 열처리(CAL, Continuous Annealing Line) 공정을 거쳐야 하기 때문에 제조비가 급격히 상승하는 단점이 있을 뿐만 아니라, 현재 상업적으로 사용되고 있는 자동차용 범퍼 또는 보강재에 적용하기에는 상대적으로 인장강도가 낮은 문제점이 있다. However, according to 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.
또한, 특허문헌 5에서는, 중량%로 C：0.26∼0.45％, Mn＋Cr：0.5∼3.0％, Nb：0.02∼1.0％, 3.42N＋0.001≤Ti≤3.42N＋0.5를 만족하는 양의 Ti, 나아가 Si：0.5％ 이하, Ni：2％ 이하, Cu：1％ 이하, V：1％ 이하 및 Al: 1% 이하의 1종 또는 2종 이상, 경우에 따라 B: 0.01% 이하, Nb : 1.0% 이하, Mo : 1.0% 이하, Ca : 0.001∼0.005%의 1종 또는 2종 이상을 함유한 냉간 압연강판을 열간 프레스 성형을 통해 인장강도가 1.8GPa인 초고강도 제조 방법에 대해 개시하고 있다. In 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.
특허문헌 5에 따를 경우 1.8GPa의 초고강도는 확보할 수 있으나, 냉간 압연된 강판을 다시 열간 프레스 성형 단계(Hot Press Forming)를 거쳐야 하기 때문에 제조 단가가 더욱 높아지는 문제점이 있다. According to 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.
따라서, 기존의 초고강도 냉연강판 및 열간성형강을 대체 가능할 뿐만 아니라, 보다 높은 인장강도를 확보할 수 있고 획기적으로 제조 단가를 낮출 수 있는 초고강도 열연강판 및 그 제조방법에 대한 개발이 요구되고 있는 실정이다.Therefore, not only is it possible to replace the existing ultra-high strength cold rolled steel sheet and hot-formed steel, but also to develop a super high-strength hot-rolled steel sheet capable of securing higher tensile strength and significantly lowering manufacturing cost, and a method for manufacturing the same This is true.
[선행기술문헌][Advanced technical literature]
(특허문헌 1) 한국 공개특허공보 제2004-0057777호(Patent Document 1) Korean Patent Publication No. 2004-0057777
(특허문헌 2) 일본 공개특허공보 제2007-100114호(Patent Document 2) Japanese Patent Publication No. 2007-100114
(특허문헌 3) 한국 공개특허공보 제2008-0073763호(Patent Document 3) Korean Patent Publication No. 2008-0073763
(특허문헌 4) 한국 공개특허공보 제2013-0069699호(Patent Document 4) Korean Patent Publication No. 2013-0069699
(특허문헌 5) 한국 공개특허공보 제2008-0111549호(Patent Document 5) Korean Patent Publication No. 2008-0111549
본 발명의 일 측면은 연주~압연 직결 공정에서 연연속압연 모드를 이용하여 열연 공정만으로도 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판 및 그 제조방법을 제공하고자 하는 것이다.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.
한편, 본 발명의 과제는 상술한 내용에 한정하지 않는다. 본 발명의 과제는 본 명세서의 내용 전반으로부터 이해될 수 있을 것이며, 본 발명이 속하는 기술분야에서 통상의 지식을 가지는 자라면 본 발명의 부가적인 과제를 이해하는데 아무런 어려움이 없을 것이다.In addition, the subject of this invention is not limited to the above-mentioned content. The subject matter of the present invention will be understood from the entire contents of the present specification, and those skilled in the art to which the present invention pertains will have no difficulty in understanding the additional subject matter of the present invention.
본 발명의 일 실시형태는 중량%로, C: 0.16~0.27%, Mn: 0.8~2.6%, Si: 0.05~0.3%, Al: 0.05% 이하, Ti: 0.01~0.08%, B: 0.001~0.005%, Ca: 0.001~0.005%, N: 0.001~0.010%, 잔부 Fe 및 기타 불가피한 불순물을 포함하고, 하기 관계식 1 내지 3을 만족하며, 면적분율로 마르텐사이트와 오토 템퍼드(Auto Tempered) 마르텐사이트의 합이 95%이상이고, 페라이트가 5%이하(0%를 포함)인 미세조직을 포함하고, 평균 크기가 40nm 이하인 M(X) (M=Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X=C, N) 복합 석출물을 포함하는 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판을 제공한다.One embodiment of the present invention in weight percent, C: 0.16 to 0.27%, Mn: 0.8 to 2.6%, Si: 0.05 to 0.3%, Al: 0.05% or less, Ti: 0.01 to 0.08%, B: 0.001 to 0.005 %, Ca: 0.001 to 0.005%, N: 0.001 to 0.010%, including residual Fe and other inevitable impurities, satisfying the following relations 1 to 3, and martensite and auto tempered martensite in area fraction M (X) (M = Ti, Nb, Si, Al, B, Mg, which has a microstructure with a sum of 95% or more, a ferrite of 5% or less (including 0%), and an average size of 40 nm or less. Cr, Ca, P, X = C, N) Provides an ultra-high-strength hot-rolled steel sheet having excellent surface quality including a composite precipitate and low material deviation.
[관계식 1] 16 ≤ 100(C+Mn/100+B/10) ≤ 28 [Relationship 1] 16 ≤ 100 (C + Mn / 100 + B / 10) ≤ 28
[관계식 2] 1 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)] ≤ 14[Relationship 2] 1 ≤ [(Al / 27) × (N / 14)] / [(Ti / 48) × (B / 11)] ≤ 14
[관계식 3] 0.05 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/100(C+Mn/100+B/10) ≤ 0.66[Relationship 3] 0.05 ≤ [(Al / 27) × (N / 14)] / [(Ti / 48) × (B / 11)] / 100 (C + Mn / 100 + B / 10) ≤ 0.66
(단, 상기 관계식 1 내지 3에 기재된 합금성분의 함량은 중량%임.)(However, the content of the alloy component described in the above formula 1 to 3 is weight%.)
본 발명의 다른 실시형태는 중량%로, C: 0.16~0.27%, Mn: 0.8~2.6%, Si: 0.05~0.3%, Al: 0.05% 이하, Ti: 0.01~0.08%, B: 0.001~0.005%, Ca: 0.001~0.005%, N: 0.001~0.010%, 잔부 Fe 및 기타 불가피한 불순물을 포함하고, 하기 관계식 1 내지 3을 만족하는 용강을 연속 주조하여 박 슬라브를 얻는 단계; 상기 박 슬라브를 조압연하여 바(Bar)를 얻는 단계; 상기 바를 마무리 압연 출측온도가 Ar3+10℃~Ar3+60℃가 되도록 마무리 압연하여 열연강판을 얻는 단계; 및 상기 열연강판을 Ar3 직상에서 200℃/sec 이상으로 냉각하여 Mf-50℃ 이하에서 권취하는 단계를 포함하며, 상기 각 단계는 연속적으로 행해지는 것을 특징으로 하는 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법을 제공한다.Other embodiments of the present invention in weight percent, C: 0.16 to 0.27%, Mn: 0.8 to 2.6%, Si: 0.05 to 0.3%, Al: 0.05% or less, Ti: 0.01 to 0.08%, B: 0.001 to 0.005 %, Ca: 0.001 ~ 0.005%, N: 0.001 ~ 0.010%, including the remaining Fe and other unavoidable impurities, continuously casting molten steel satisfying the following relations 1 to 3 to obtain a thin slab; Rough rolling the thin slab to obtain a bar; Finish rolling the bar so that the exit temperature of the finish rolling is Ar3 + 10 ° C to Ar3 + 60 ° C to obtain a hot rolled steel sheet; And cooling the hot-rolled steel sheet to 200 ° C / sec or more at Ar3 and winding it up to Mf-50 ° C or less, wherein each step is performed continuously, and has excellent material quality and material deviation. It provides a method for manufacturing a very high strength hot rolled steel sheet.
[관계식 1] 16 ≤ 100(C+Mn/100+B/10) ≤ 28 [Relationship 1] 16 ≤ 100 (C + Mn / 100 + B / 10) ≤ 28
[관계식 2] 1 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)] ≤ 14[Relationship 2] 1 ≤ [(Al / 27) × (N / 14)] / [(Ti / 48) × (B / 11)] ≤ 14
[관계식 3] 0.05 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/100(C+Mn/100+B/10) ≤ 0.66[Relationship 3] 0.05 ≤ [(Al / 27) × (N / 14)] / [(Ti / 48) × (B / 11)] / 100 (C + Mn / 100 + B / 10) ≤ 0.66
(단, 상기 관계식 1 내지 3에 기재된 합금성분의 함량은 중량%임.)(However, the content of the alloy component described in the above formula 1 to 3 is weight%.)
본 발명의 일측면에 따르면, 합금조성과 제조조건을 적절히 제어함으로써 연주~압연 직결 공정에서 연연속압연 모드를 이용하여 열연 공정만으로도 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판 및 그 제조방법을 제공할 수 있다. 또한, 본 발명의 열연강판은 보다 높은 인장강도를 확보할 수 있어 초고강도 냉연강판 및 열간성형강을 대체할 수 있을 뿐만 아니라, 획기적으로 제조 단가를 낮출 수 있는 효과가 있다. 아울러, 박 슬라브 연주법을 통해 전기로에서 고철 등의 스크랩을 용해한 강을 사용할 수 있어 자원의 재활용성을 높일 수 있다.According to one aspect of the present invention, by controlling the alloy composition and manufacturing conditions appropriately, 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. In addition, 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. In addition, 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.
도 1은 본 발명의 열연강판 제조에 적용 가능한 연주~압연 직결공정을 위한 설비의 모식도이다. 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.
도 2는 본 발명의 열연강판 제조에 적용 가능한 연주~압연 직결공정을 위한 설비의 또 다른 모식도이다.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.
도 3은 본 발명의 일 실시예에 따른 발명예 1 내지 15와 비교예 1 내지 13에 대한 관계식 1 및 2의 값을 나타낸 그래프이다.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.
도 4는 본 발명의 일 실시예에 따른 발명예 1을 주사전자현미경(SEM)으로 관찰한 미세조직 사진이다.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).
도 5 (a) 및 (b)는 본 발명의 일 실시예에 따른 발명예 1을 투과전자현미경(TEM)으로 관찰한 미세조직 사진이다.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).
도 6은 본 발명의 일 실시예에 따른 발명예 1의 마르텐사이트 및 오토 템퍼드 마르텐사이트 래스 폭에 대한 분포를 나타낸 그래프이다.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.
도 7은 본 발명의 일 실시예에 따른 발명예 1의 석출물을 투과전자현미경(TEM)으로 관찰한 사진이다. 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).
도 8은 본 발명의 일 실시예에 따른 비교예 8의 석출물을 투과전자현미경(TEM)으로 관찰한 사진이다.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).
이하, 본 발명의 일 실시형태에 따른 열연강판에 대하여 설명한다. 먼저, 본 발명의 합금조성을 설명한다. 하기 설명되는 합금조성의 함량은 별도의 언급이 없는 한 중량%를 의미한다.Hereinafter, a hot rolled steel sheet according to an embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. The content of the alloy composition described below refers to weight percent unless otherwise specified.
C: 0.16~0.27%C: 0.16 to 0.27%
탄소(C)는 열간압연후 급냉시 미세조직을 마르텐사이트로 만들어 강도를 증가시키는 매우 중요한 원소이다. C 함량이 0.16% 미만인 경우에는 마르텐사이트 자체 강도가 낮아 본 발명에서 목표로 하는 강도 확보가 어려울 수 있다. 반면에 C 함량이 0.27% 초과인 경우에는 용접성 및 과도한 강도 상승으로 굽힘 가공성이 저하되는 문제점이 있다. 따라서, C 함량은 0.16~0.27%인 것이 바람직하다. 상기 C 함량의 하한은 0.17%인 것이 보다 바람직하고, 0.18%인 것이 보다 더 바람직하며, 0.19%인 것이 가장 바람직하다. 상기 C 함량의 상한은 0.26%인 것이 보다 바람직하고, 0.25%인 것이 보다 더 바람직하며, 0.24%인 것이 가장 바람직하다Carbon (C) is a very important element that increases strength by making microstructure martensite upon rapid cooling after hot rolling. When 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. On the other hand, when 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%.
Mn: 0.8~2.6%Mn: 0.8-2.6%
망간(Mn)은 페라이트 형성을 억제하며, 오스테나이트 안정성을 높여 저온 변태상의 형성을 용이하게 함으로써 강도를 증가시킨다. Mn 함량이 0.8% 미만인 경우에는 본 발명에서 목표로 하는 강도 확보가 어려울 수 있다. 반면에 Mn 함량이 2.6% 초과인 경우에는 연주 슬라브 및 열연강판의 내부 또는 외부 혹은 이들 모두에 편석대를 형성시켜 크랙의 발생과 전파를 유발해 강판의 최종품질을 저하시키고 용접성 및 굽힘 가공성를 열위하게 할 수 있다. 따라서, Mn 함량은 0.8~2.6%인 것이 바람직하다. 상기 Mn 함량의 하한은 0.85%인 것이 보다 바람직하고, 0.90%인 것이 보다 더 바람직하며, 0.95%인 것이 가장 바람직하다. 상기 Mn 함량의 상한은 2.5%인 것이 보다 바람직하고, 2.4%인 것이 보다 더 바람직하며, 2.3%인 것이 가장 바람직하다.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%.
Si: 0.05~0.3%Si: 0.05 ~ 0.3%
규소(Si)는 강판의 연성을 저하시키지 않으면서 강도를 확보할 수 있는 유용한 원소이다. 또한, 페라이트 형성을 촉진하고 미변태 오스테나이트로의 C 농축을 조장함으로써 마르텐사이트 형성을 촉진하는 원소이다. Si 함량이 0.05% 미만인 경우에는 상술한 효과를 충분히 확보하기 어렵다. 반면에, Si 함량이 0.3% 초과인 경우에는 강판 표면에 적 스케일이 생성되어 산세 후 강판 표면에 흔적이 잔류하여 표면 품질이 저하될 수 있다. 따라서, Si 함량은 0.05~0.3%인 것이 바람직하다. 상기 Si 함량의 하한은 0.06%인 것이 보다 바람직하고,0.07%인 것이 보다 더 바람직하며, 0.08%인 것이 가장 바람직하다. 상기 Si 함량의 상한은 0.28%인 것이 보다 바람직하고, 0.26%인 것이 보다 더 바람직하며, 0.24%인 것이 가장 바람직하다.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. When the Si content is less than 0.05%, it is difficult to sufficiently secure the above-described effect. On the other hand, when 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%.
Al: 0.05% 이하Al: 0.05% or less
알루미늄(Al)은 강판의 표면에 농화되어 도금성을 나쁘게 할 수 있는 반면 탄화물 형성을 억제하여 강의 연성을 증가시킨다. 한편, 강중 알루미늄(Al)은 질소(N)와 반응하여 AlN 석출이 되는데, 박 슬라브 제조시 이들 석출물이 석출하는 주편 냉각 조건에서 슬라브 크랙을 유발하여 주편 또는 열연강판의 품질을 저하시킬 수 있다. 따라서 그 함량을 가능한 낮게 제어하여야 하며, 0.05% 이하로 제어하는 것이 바람직하다. 상기 Al 함량은 0.048% 이하인 것이 보다 바람직하고, 0.046% 이하인 것이 보다 더 바람직하며, 0.045% 이하인 것이 가장 바람직하다.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. On the other hand, aluminum (Al) in the steel reacts with nitrogen (N) to precipitate AlN. When manufacturing thin slabs, slab cracks can be caused in the cooling conditions of the castings of these precipitates, thereby deteriorating the quality of the cast or hot rolled steel sheet. Therefore, 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.
Ti: 0.01~0.08%Ti: 0.01 ~ 0.08%
티타늄(Ti)은 석출물 및 질화물 형성원소로서 강의 강도를 증가시키는 원소이다. 또한 Ti은 응고온도 근처에서 TiN의 형성을 통해 고용 N를 제거하여 AlN 등의 석출물량을 감소시킴으로써 고온연성 저하를 방지하여 에지(Edge) 크랙 발생 민감성을 감소시키는 원소이다. Ti 함량이 0.01% 미만인 경우에는 미세한 AlN 또는 BN 석출물의 과다 석출에 따른 주조 슬라브의 연성 감소를 초래하여 슬라브 품질을 저하시킨다. 반면에 Ti 함량이 0.08% 초과인 경우에는 조대한 TiN 석출물의 형성에 따른 결정립 미세화 효과를 기대하기 어려울 뿐만 아니라 제조 비용이 상승한다. 따라서 Ti 함량은 0.01~0.08%인 것이 바람직하다. 상기 Ti 함량의 하한은 0.012%인 것이 보다 바람직하고, 0.014%인 것이 보다 더 바람직하며, 0.016%인 것이 가장 바람직하다. 상기 Ti 함량의 상한은 0.07%인 것이 보다 바람직하고, 0.06%인 것이 보다 더 바람직하며, 0.05%인 것이 가장 바람직하다.Titanium (Ti) is an element that increases the strength of steel as a precipitate and nitride forming element. In addition, 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. When 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. On the other hand, when 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. Therefore, 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%.
B: 0.001~0.005%B: 0.001 to 0.005%
보론(B)은 강의 경화능을 증가시키는 원소이다. 그 함량이 0.001% 미만인 경우 상기 효과를 얻을 수 없으며, 0.005%를 초과하게 되면 오스테나이트 재결정 온도를 상승시키며 용접성을 나쁘게 한다. 따라서, B의 함량은 0.001~0.005%로 제한하는 것이 바람직하다. 상기 B 함량의 하한은 0.0012%인 것이 보다 바람직하고, 0.0014%인 것이 보다 더 바람직하며, 0.0016%인 것이 가장 바람직하다. 상기 B 함량의 상한은 0.0045%인 것이 보다 바람직하고, 0.0040%인 것이 보다 더 바람직하며, 0.0035%인 것이 가장 바람직하다.Boron (B) is an element that increases the hardenability of steel. When 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%.
Ca: 0.001~0.005%Ca: 0.001 ~ 0.005%
칼슘(Ca)은 용강 중의 Al, O와 반응하여 저융점인 구상의 개재물(12CaO·17Al 2O 3)을 형성하여 노즐막힘 방지와 개재물 분리부상을 용이하게 하는 원소이다. Ca 함량이 0.001% 미만인 경우 상기 효과를 확보하기 어렵다. 반면에, Ca 함량이 0.005% 초과인 경우에는 고융점 개재물이 형성되어 노즐막힘을 조장하여 주조중단이 발생할 수 있고, 대형 개재물(>50㎛)이 형성되어 강판의 가공성을 열위하게 할 수 있다. 따라서, Ca의 함량은 0.001~0.005%로 제어하는 것이 바람직하다. 상기 Ca 함량의 하한은 0.0012%인 것이 보다 바람직하고, 0.0014%인 것이 보다 더 바람직하며, 0.0016%인 것이 가장 바람직하다. 상기 Ca 함량의 상한은 0.0045%인 것이 보다 바람직하고, 0.0040%인 것이 보다 더 바람직하며, 0.0035%인 것이 가장 바람직하다.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. When the Ca content is less than 0.001%, it is difficult to secure the above effect. On the other hand, when 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 µm) 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: 0.001~0.010%N: 0.001 to 0.010%
질소(N)는 오스테나이트 안정화 및 질화물 형성 원소이다. N 함량이 0.001% 미만인 경우에는 상술한 효과가 불충분하다. 반면에 N 함량이 0.010% 초과인 경우에는 석출물 형성 원소와 반응하여 석출 강화 효과를 증가시키지만, 연성의 급격한 하락을 초래할 수 있다. 따라서 N 함량은 0.001~0.010%인 것이 바람직하다. 상기 N 함량의 하한은 0.0012%인 것이 보다 바람직하고, 0.0014%인 것이 보다 더 바람직하며, 0.0016%인 것이 가장 바람직하다. 상기 N 함량의 상한은 0.009%인 것이 보다 바람직하고, 0.008%인 것이 보다 더 바람직하며, 0.007%인 것이 가장 바람직하다.Nitrogen (N) is an austenite stabilizing and nitride forming element. When the N content is less than 0.001%, the above-described effect is insufficient. On the other hand, when the N content is more than 0.010%, it increases the precipitation strengthening effect by reacting with the precipitate-forming element, but may cause a sharp drop in ductility. Therefore, 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%.
본 발명의 나머지 성분은 철(Fe)이다. 다만, 통상의 제조과정에서는 원료 또는 주위 환경으로부터 의도되지 않는 불순물들이 불가피하게 혼입될 수 있으므로, 이를 배제할 수는 없다. 이들 불순물들은 통상의 제조과정의 기술자라면 누구라도 알 수 있는 것이기 때문에 그 모든 내용을 특별히 본 명세서에서 언급하지는 않는다.The remaining component of the invention is iron (Fe). However, in the normal manufacturing process, 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.
한편, 본 발명의 열연강판은 전술한 합금성분 중 C, Mn, B, Al, Ti 및 N이 하기 관계식 1 내지 3을 각각 만족하는 것이 바람직하며, 이를 통해, 본 발명이 목표로 하는 기계적 물성과 우수한 표면 품질을 확보할 수 있다. 단, 하기 관계식 1 내지 3에 기재된 합금성분의 함량은 중량%이다.On the other hand, in 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. However, the content of the alloy component described in the following relations 1 to 3 is weight%.
[관계식 1] 16 ≤ 100(C+Mn/100+B/10) ≤ 28 [Relationship 1] 16 ≤ 100 (C + Mn / 100 + B / 10) ≤ 28
상기 관계식 1은 본 발명이 얻고자 하는 기계적 물성을 확보하기 위한 성분관계식이다. 상기 관계식 1의 값이 16 미만인 경우에는 본 발명이 목표로 하는 강도를 확보하기 곤란하고, 28을 초과할 경우에는 연신율이 낮아져 가공시 크랙이 발생할 수 있다. 따라서, 상기 관계식 1의 값은 16~28의 범위를 갖는 것이 바람직하다. 상기 관계식 1의 값의 하한은 17인 것이 보다 바람직하고, 18인 것이 보다 더 바람직하며, 19인 것이 가장 바람직하다. 상기 관계식 1의 값의 상한은 27인 것이 보다 바람직하고, 26인 것이 보다 더 바람직하며, 25인 것이 가장 바람직하다.The relational expression 1 is a component relational expression for securing the mechanical properties desired by the present invention. When 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.
*[관계식 2] 1 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)] ≤ 14* [Relationship 2] 1 ≤ [(Al / 27) × (N / 14)] / [(Ti / 48) × (B / 11)] ≤ 14
상기 관계식 2는 슬라브 또는 바의 에지(Edge) 품질을 확보하여 최종적으로 얻어지는 열연강판의 표면 품질을 향상시키기 위한 성분관계식이다. 상기 관계식 2의 값이 1 미만인 경우에는 Ti 또는 B 함량이 높거나 Al 또는 N 함량이 낮은 경우로서 조대한 Ti(C,N) 및 B(C,N) 석출물의 과다 석출에 따른 고온연성 감소를 초래하여 슬라브 또는 바의 에지에 크랙이 발생할 수 있고, 14를 초과하는 경우에는 Ti 또는 B 함량이 낮거나 Al 또는 N 함량이 높은 경우로서 AlN이 과다 석출되어 고온연성이 저하됨에 따라 슬라브 또는 바의 에지 품질이 저하될 수 있다. 따라서, 상기 관계식 2의 값은 1~14의 범위를 갖는 것이 바람직하다. 상기 관계식 2의 값의 하한은 1.1인 것이 보다 바람직하고, 1.2인 것이 보다 더 바람직하며, 1.3인 것이 가장 바람직하다. 상기 관계식 2의 값의 상한은 13인 것이 보다 바람직하고, 12인 것이 보다 더 바람직하며, 11인 것이 가장 바람직하다.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. When 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.
[관계식 3] 0.05 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/100(C+Mn/100+B/10) ≤ 0.66[Relationship 3] 0.05 ≤ [(Al / 27) × (N / 14)] / [(Ti / 48) × (B / 11)] / 100 (C + Mn / 100 + B / 10) ≤ 0.66
상기 관계식 3은 본 발명이 목표로 하는 기계적 물성과 우수한 표면 품질을 확보하기 위한 성분관계식이다. 상기 관계식 3의 값이 0.05 미만인 경우에는 목표로 하는 강도를 확보하기 곤란할 수 있고, 0.66을 초과하는 경우에는 석출물의 과다 석출에 따른 고온연성 감소를 초래하여 슬라브 또는 바의 에지에 크랙이 발생하는 문제가 발생할 수 있다. 따라서, 상기 관계식 3의 값은 0.05~0.66의 범위를 갖는 것이 바람직하다. 상기 관계식 3의 값의 하한은 0.06인 것이 보다 바람직하고, 0.08인 것이 보다 더 바람직하며, 0.10인 것이 가장 바람직하다. 상기 관계식 3의 값의 상한은 0.62인 것이 보다 바람직하고, 0.58인 것이 보다 더 바람직하며, 0.56인 것이 가장 바람직하다.The relational expression 3 is a component relational expression for securing the mechanical properties and excellent surface quality targeted by the present invention. When 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.
한편, 본 발명의 열연강판은 트램프 원소로서 Nb, V, Ti, Mo, Cu, Cr, Ni, Zn, Se, Sb, Zr, W, Ga, Ge 및 Mg로 이루어지는 그룹으로부터 선택된 1종 이상을 그 합계가 0.1중량% 이하의 범위로 포함할 수 있다. 상기 트램프 원소는 제강공정에서 원료로 사용하는 합금철 또는 스크랩이나, 래들(Ladle) 및 턴디쉬(Tundish) 내화물 등에서 비롯된 불순물 원소로서, 그 합계가 0.1% 초과하는 경우에는 박 슬라브의 표면에 크랙을 발생시켜 열연강판의 표면 품질을 저하시킬 수 있다.On the other hand, 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.
본 발명의 열연강판은 면적분율로 마르텐사이트와 오토 템퍼드(Auto Tempered) 마르텐사이트의 합이 95%이상이고, 페라이트가 5%이하(0%를 포함)인 미세조직을 포함하는 것이 바람직하다. 상기 마르텐사이트와 오토 템퍼드 마르텐사이트 조직은 본 발명이 목포로 하는 강도를 얻기 위한 필수적인 조직으로서, 그 분율이 95% 미만일 경우에는 강도 확보가 곤란하다. 본 발명에서는 연성 확보를 위하여 페라이트를 5% 이하의 범위로 포함할 수 있으며, 다만, 그 분율이 5%를 초과하는 경우에는 연성은 증가하나 강도 확보에 어려움이 있을 수 있다. 한편, 상기 마르텐사이트와 오토 템퍼드 마르텐사이트의 합의 분율은 96% 이상인 것이 보다 바람직하고, 97% 이상인 것이 보다 더 바람직하며, 98% 이상인 것이 가장 바람직하다.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%. In the present invention, 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. On the other hand, 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.
본 발명의 주요 미세조직은 마르텐사이트 오토 템퍼드 마르텐사이트이며, 이 때, 상기 마르텐사이트 및 오토 템퍼드 마르텐사이트의 래스 평균 폭은 강도 및 가공성에 영향을 미칠 수 있다. 따라서, 상기 마르텐사이트 및 오토 템퍼드 마르텐사이트의 래스 평균 폭은 단축을 기준으로 1㎛ 이하인 것이 바람직하다. 상기 마르텐사이트 및 오토 템퍼드 마르텐사이트의 래스 평균 폭이 1㎛를 초과하는 경우에는 목표로 하는 강도 및 가공성을 확보하기 어려울 수 있다. 상기 마르텐사이트 및 오토 템퍼드 마르텐사이트의 래스 평균 폭은 좁으면 좁을수록 강도 확보에 유리하나, 통상적인 냉각 조건에서 0.1㎛ 미만으로 제어하기에는 어려움이 있을 수 있다. 상기 마르텐사이트 및 오토 템퍼드 마르텐사이트의 래스 평균 폭의 하한은 0.12㎛인 것이 보다 바람직하고, 0.14㎛인 것이 보다 더 바람직하며, 0.16㎛인 것이 가장 바람직하다. 상기 마르텐사이트 및 오토 템퍼드 마르텐사이트의 평균 폭의 상한은 0.9㎛인 것이 보다 바람직하고, 0.8㎛인 것이 보다 더 바람직하며, 0.7㎛인 것이 가장 바람직하다.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 µm or less based on the shortening. When the average width of lath of the martensite and auto-tempered martensite exceeds 1 μm, 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 μm under normal cooling conditions. The lower limit of the average lath width of the martensite and auto-tempered martensite is more preferably 0.12 μm, even more preferably 0.14 μm, and most preferably 0.16 μm. The upper limit of the average width of the martensite and auto-tempered martensite is more preferably 0.9 µm, even more preferably 0.8 µm, and most preferably 0.7 µm.
본 발명의 열연강판은 평균 크기가 40nm 이하인 M(X) (M=Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X=C, N) 복합 석출물을 포함하는 것이 바람직하다. 상기 복합 석출물의 평균 크기가 40nm를 초과하는 경우에는 효과적으로 강도를 확보하기 어려울 수 있고, 에지 크랙이 발생하여 에지 품질이 저하될 수 있다. 상기 복합 석출물의 평균 크기는 작으면 작을수록 강도 확보에 유리하나 본 발명의 제조조건에서는 5nm 미만으로 제어하기가 어려울 수 있다. 상기 복합 석출물의 평균 크기는 38nm 이하인 것이 보다 바람직하고, 34nm 이하인 것이 보다 더 바람직하며, 30nm 이하인 것이 가장 바람직하다.The hot-rolled steel sheet of the present invention preferably includes M (X) (M = Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X = C, N) composite precipitates having an average size of 40 nm or less. . When the average size of the composite precipitate exceeds 40 nm, it may be difficult to effectively secure strength, and edge cracking may occur, thereby deteriorating edge quality. The smaller the average size of the composite precipitate is, the more advantageous it is to secure strength, but under the manufacturing conditions of the present invention, it may be difficult to control to less than 5 nm. 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.
본 발명이 제공하는 열연강판은 항복강도가 1060~1400MPa이며, 인장강도가 1470~1800MPa이고, 연신율이 5% 이상이며, 비커스 경도가 420~550Hv(0.5kgf)이고, 스트립의 폭 방향 인장강도 편차가 100MPa이하이며, 스트립의 폭 방향 비커스 경도 편차가 50Hv(0.5kgf)이하일 수 있다. 또한, 본 발명의 열연강판은 두께가 1.6mm 이하일 수 있으며, 보다 바람직하게는 1.4mm 이하일 수 있고, 보다 더 바람직하게는 1.3mm 이하일 수 있으며, 가장 바람직하게는 1.2mm 이하일 수 있다. 본 발명의 열연강판은 상술한 바와 같이 우수한 기계적 물성 및 표면 품질과 낮은 재질편차를 가짐으로써 초고강도 냉연강판 및 열간성형강을 효과적으로 대체할 수 있다.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. In addition, 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.
이하, 본 발명의 열연강판 제조방법의 일 실시형태에 대하여 설명한다. Hereinafter, one embodiment of the method for manufacturing a hot-rolled steel sheet of the present invention will be described.
도 1은 본 발명의 열연강판 제조에 적용 가능한 연주~압연 직결공정을 위한 설비의 모식도이다. 본 발명의 일 실시형태에 따른 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판은 도 1과 같은 연주~압연 직결 설비를 적용하여 생산될 수 있다. 연주~압연 직결 설비는 크게 연속주조기(100), 조압연기(400), 마무리 압연기(600)로 구성된다. 상기 연주~압연 직결 설비는 제1두께의 박 슬라브(Slab)(a)를 생산하는 고속 연속주조기(100)와, 상기 슬라브를 상기 제1두께보다 얇은 제2두께의 바(Bar)(b)로 압연시키는 조압연기(400), 상기 제2두께의 바를 제3두께의 스트립(c)으로 압연시키는 마무리 압연기(600), 상기 스트립을 권취하는 권취기(900)를 포함할 수 있다. 추가로, 상기 조압연기(400) 앞에 조압연 스케일 브레이커(300)(Roughing Mill Scale Breaker, 이하 'RSB')와 마무리 압연기(600) 앞에 마무리 압연 스케일 브레이커(500)(Fishing Mill Scale Breaker, 이하 'FSB')를 추가로 포함할 수 있으며, 표면 스케일 제거가 용이하여 후공정에서 열연 강판을 산세시 표면품질이 우수한 PO(Pickled & Oiled)강판의 생산이 가능하다. 또한, 연주~압연 직결공정으로 등온등속압연이 가능하여 강판 폭, 길이 방향 온도 편차가 현저히 낮아 ROT[Run Out Table(700)](이하 "런아웃 테이블")에서 정밀 냉각제어가 가능하여 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 생산이 가능하다. 이렇게 압연 및 냉각이 완료된 스트립은 고속전단기(800)에 의해 절단되고, 권취기(900)에 의해 권취되어 제품으로 생산될 수 있다. 한편, 마무리 압연 스케일 브레이커(500) 앞에는 바를 추가로 가열하는 가열기(200)가 구비될 수 있다.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. It may include a rough rolling mill 400 to be rolled, a finishing mill 600 to roll the bar of the second thickness into a strip (c) of the third thickness, and a winder 900 to wind the strip. In addition, 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. In addition, it is possible to perform isothermal constant-speed rolling in a direct-to-rolling process, so that the temperature variation in the width and length of the steel sheet is remarkably low. It is possible to produce ultra-high-strength hot-rolled steel sheet with excellent material and little material deviation. The strip, which has been rolled and cooled, is cut by a high-speed shearing machine 800 and wound up by a winding machine 900 to be produced as a product. On the other hand, a heater 200 for additionally heating the bar may be provided in front of the finish rolling scale breaker 500.
도 2는 본 발명의 열연강판 제조에 적용 가능한 연주~압연 직결공정을 위한 설비의 또 다른 모식도이다. 도 2에 개시된 연주~압연 직결 설비는 도 1에 개시된 설비와 구성이 대부분 동일하나, 조압연기(400) 및 조압연 스케일 브레이커(300) 앞에 슬라브를 추가로 가열하는 가열기(200')가 구비되어, 슬라브 에지 온도 확보가 용이하여 에지 결함 발생을 낮게 되어 표면 품질 확보에 유리하다. 또한 조압연기 이전에 슬라브 1매 이상의 길이만큼의 공간을 확보하고 있어, 배치(Batch)식 압연도 가능하다.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. 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. , It is easy to secure the slab edge temperature, which lowers the occurrence of edge defects, which is advantageous for securing the surface quality. In addition, since the space of at least one slab is secured before the rough rolling, batch type rolling is possible.
본 발명의 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판은 도 1 및 2에 개시된 연주~압연 직결 설비에서 모두 생산이 가능하다.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.
이하, 본 발명의 열연강판 제조방법의 일 실시형태에 대하여 상세히 설명한다. Hereinafter, one embodiment of the method for manufacturing a hot-rolled steel sheet of the present invention will be described in detail.
우선, 전술한 합금조성을 갖는 용강을 연속주조하여 박 슬라브를 얻는다. 이때, 상기 연속주조는 4~8mpm(m/min)의 주조속도로 행하는 것이 바람직하다. 주조속도를 4mpm 이상으로 하는 이유는 고속주조와 압연과정이 연결되어 이루어져, 목표 압연 온도를 확보하기 위해서는 일정 이상의 주조 속도가 요구되기 때문이다. 다만, 주조속도가 느릴 경우 주편에서부터 편석이 발생할 위험이 있으며, 이러한 편석이 발생하면 강도 및 가공성 확보가 어려울 뿐만 아니라, 폭 방향 또는 길이 방향으로의 재질편차가 발생할 위험성이 커지게 된다. 만약 8mpm을 초과하는 경우에는 용강 탕면 불안정에 의해 조업 성공율이 저감될 수 있으므로, 상기 주조속도는 4~8mpm의 범위를 갖는 것이 바람직하다. 상기 주조속도의 하한은 4.2mpm인 것이 보다 바람직하고, 4.4mpm인 것이 보다 더 바람직하며, 4.6mpm인 것이 가장 바람직하다. 상기 주조속도의 상한은 7.5mpm인 것이 보다 바람직하고, 7.0mpm인 것이 보다 더 바람직하며, 6.5mpm인 것이 가장 바람직하다.First, a molten slab having the above-described alloy composition is continuously cast to obtain a thin slab. At this time, 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. However, if 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. If it exceeds 8mpm, the operation success rate may be reduced due to the instability of the molten steel bath surface, so 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.
한편, 상기 박 슬라브는 두께는 80~120mm인 것이 바람직하다. 상기 박 슬라브의 두께가 120mm를 초과하는 경우에는 고속주조가 어려울 뿐만 아니라, 조압연 시 압연 부하가 증가하게 되고, 80mm 미만인 경우에는 주편의 온도 하락이 급격하게 일어나 균일한 조직을 형성하기 어렵다. 이를 해결하기 위해서는 부가적으로 가열 설비를 설치할 수 있으나, 이는 생산 원가를 향상시키는 요인이 되므로, 가능한 배제하는 것이 바람직하다. 따라서, 박 슬라브의 두께는 80~120mm로 제어하는 것이 바람직하다. 상기 박 슬라브의 두께의 하한은 82mm인 것이 보다 바람직하고, 84mm인 것이 보다 더 바람직하며, 86mm인 것이 가장 바람직하다. 상기 박 슬라브의 두께의 상한은 116mm인 것이 보다 바람직하고, 114mm인 것이 보다 더 바람직하며, 110mm인 것이 가장 바람직하다.On the other hand, the thickness of the thin slab is preferably 80 ~ 120mm. When 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. In order to solve this, 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.
또한, 상기 연속주조시 몰드 플럭스의 염기도는 0.8~1.5인 것이 바람직하다. 여기서 염기도는 CaO(%)/SiO2(%) 비를 나타낸다. 본 발명강의 경우 높은 강도를 확보하기 위해 C, Mn 및 B 등 첨가되는 합금원소가 많아 선형크랙 민감성이 아주 높다. 따라서, 염기도가 0.8 미만인 몰드 플럭스를 사용하게 되면, 전열량이 높아 슬라브 표면이 강냉됨에 따라 선형크랙이 발생할 수 있다. 반면, 염기도가 1.5 초과인 몰드 플럭스를 사용하는 경우에는 전열량이 너무 낮아, 건전한 응고셀을 얻기가 어려울 수 있다. 따라서, 상기 연속주조시 몰드 플럭스의 염기도는 0.8~1.5인 것이 바람직하다. 상기 몰드 플럭스의 염기도의 하한은 0.85인 것이 보다 바람직하고, 0.90인 것이 보다 더 바람직하며, 0.95인 것이 가장 바람직하다. 상기 몰드 플럭스의 염기도의 상한은 1.45인 것이 보다 바람직하고, 1.40인 것이 보다 더 바람직하며, 1.35인 것이 가장 바람직하다.In addition, the basicity of the mold flux during the continuous casting is preferably 0.8 to 1.5. Here, the basicity represents the CaO (%) / SiO2 (%) ratio. In the case of the steel of the present invention, there are many 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. On the other hand, when using a mold flux having a basicity of more than 1.5, the heat transfer amount is too low, and it may be difficult to obtain a healthy solidification cell. Therefore, it is preferable that 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.
또한, 상기 연속주조시 2차 냉각 비수량은 1.5~2.5ℓ/㎏인 것이 바람직하다. 상기 연속주조시 2차 냉각 비수량이 2.5ℓ/㎏을 초과하게 되면 선형크랙이 발생하여 슬라브 품질이 떨어질 위험이 있고, 슬라브 또는 바의 에지 온도가 낮아져 에지 크랙 발생 위험성이 높다. 반면, 상기 연속주조시 2차 냉각 비수량이 1.5ℓ/㎏ 미만인 경우 연주 출측에서 슬라브 미응고에 따른 용강 유출 등의 문제가 야기 될 수 있고, 세그먼트(Segment) 롤이 열화될 수 있어 설비 이상의 문제가 발생할 수 있다. 따라서, 상기 연속주조시 2차 냉각 비수량은 1.5~2.5ℓ/㎏인 것이 바람직하다. 상기 연속주조시 2차 냉각 비수량의 하한은 1.55ℓ/㎏인 것이 보다 바람직하고, 1.60ℓ/㎏인 것이 보다 더 바람직하며, 1.65ℓ/㎏인 것이 가장 바람직하다. 상기 연속주조시 2차 냉각 비수량의 상한은 2.45ℓ/㎏인 것이 보다 바람직하고, 2.40ℓ/㎏인 것이 보다 더 바람직하며, 2.35ℓ/㎏인 것이 가장 바람직하다.In addition, the secondary cooling specific water amount during the continuous casting is preferably 1.5 ~ 2.5ℓ / ㎏. In the case of the continuous casting, when 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. On the other hand, in the case of the continuous casting, when 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. Therefore, 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.
이후, 상기 박 슬라브를 조압연하여 바(Bar)를 얻는다. 상기 조압연 단계는 연속주조된 박 슬라브를 2~5개의 스탠드로 구성된 조압연기에서 조압연함으로써 수행될 수 있다. Then, the thin slab is rough rolled to obtain a bar. 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.
상기 조압연시 조압연 출측에서의 바 에지부 온도는 850~1000℃인 것이 바람직하다. 상기 바 에지부 온도가 850℃ 미만인 경우에는 AlN 석출물 등이 다량 생성되어 고온연성이 저하됨에 따라 에지 크랙 발생 민감성이 아주 높아지는 문제점이 있다. 반면, 상기 바 에지부 온도가 1000℃ 초과인 경우에는 바의 에지부 뿐만 아니라 중심부 온도 또한 높아 스케일이 다량 발생함에 따라 산세 후 표면 품질이 떨어질 수 있다. 따라서, 상기 조압연시 조압연 출측에서의 바 에지부 온도는 850~1000℃인 것이 바람직하다. 상기 조압연시 조압연 출측에서의 바 에지부 온도의 하한은 860℃인 것이 보다 바람직하고, 870℃인 것이 보다 더 바람직하며, 880℃인 것이 가장 바람직하다. 상기 조압연시 조압연 출측에서의 바 에지부 온도의 상한은 990℃인 것이 보다 바람직하고, 980℃인 것이 보다 더 바람직하며, 970℃인 것이 가장 바람직하다.In the rough rolling, the bar edge temperature at the rough rolling side is preferably 850 to 1000 ° C. When 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. On the other hand, when 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. In the rough rolling, 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. In the rough rolling, 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.
상기 바를 얻는 단계 후에는 상기 바에 냉각수를 분사하여 스케일을 제거하는 단계를 추가로 포함할 수 있다. 예를 들어, 바를 마무리 압연하기 전에 마무리압연 스케일 브레이커(Finishing Mill Scale Breaker, 이하 'FSB'라 함) 노즐에서 냉각수를 200~300bar 압력으로 분사하여 표면 스케일을 30㎛ 이하의 두께로 제거할 수 있다. 상기 냉각수 분사 압력이 200bar 미만인 경우에는 스케일이 제거가 불충분하여 마무리 압연 후 강판 표면에 방추형, 비늘형 스케일이 다량 생성되어 산세 후 표면 품질이 열위하게 된다. 반면에 상기 냉각수 분사 압력이 300bar를 초과하는 경우에는 마무리압연 출측온도가 너무 낮아지게 되어 효과적인 오스테나이트 분율을 확보하기 곤란하여 목표로 하는 인장강도를 확보하기 곤란할 수 있다. 따라서, 상기 냉각수 분사 압력은 200~300bar인 것이 바람직하다. 상기 냉각수 분사 압력의 하한은 210bar인 것이 보다 바람직하고, 220bar인 것이 보다 더 바람직하며, 230bar인 것이 가장 바람직하다. 상기 냉각수 분사 압력의 상한은 290bar인 것이 보다 바람직하고, 280bar인 것이 보다 더 바람직하며, 270bar인 것이 가장 바람직하다.After the step of obtaining the bar, a step of removing the scale by spraying coolant on the bar may be further included. For example, the surface scale can be removed to a thickness of 30 µm 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. . When 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. On the other hand, when the cooling water injection pressure exceeds 300 bar, the finish rolling exit temperature becomes too low, so it is difficult to secure an effective austenite fraction, and thus it may be difficult to secure a target tensile strength. Therefore, 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.
또한, 상기 냉각수 분사시, 냉각수의 겹침(overlap) 면적율은 5~25%인 것이 바람직하다. 상기 냉각수의 겹침 면적율이 5% 미만인 경우 냉각수 겹침 면적이 너무 작아 바의 온도가 국부적으로 상승하게 되어 폭 방향으로 온도가 불균일해질 수 있고, 이로 인해 스케일이 완전히 제거되지 못하여 표면품질이 저하될 수 있으며, 최종적으로 얻어지는 열연강판의 폭 방향 인장강도 편차를 100MPa 이하로 제어하는 것이 곤란할 수 있다. 또한, 냉각수 분사 겹침 면적율이 25%를 초과할 경우 국부적으로 냉각이 많이 되어 폭 방향으로 온도 편차가 발생하여 최종적으로 얻어지는 열연강판의 재질편차가 심해질 수 있다. 따라서, 상기 냉각수 분사시, 냉각수의 겹침(overlap) 면적율은 5~25%인 것이 바람직하다. 상기 냉각수의 겹침 면적율의 하한은 6%인 것이 보다 바람직하고, 7%인 것이 보다 더 바람직하며, 8%인 것이 가장 바람직하다. 상기 냉각수의 겹침 면적율의 상한은 24%인 것이 보다 바람직하고, 23%인 것이 보다 더 바람직하며, 22%인 것이 가장 바람직하다.In addition, when spraying the cooling water, the overlap area ratio of the cooling water is preferably 5 to 25%. When 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. , It may be difficult to control the tensile strength deviation in the width direction of the finally obtained hot rolled steel sheet to 100 MPa or less. In addition, when 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. Therefore, when spraying the cooling water, 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%.
이후, 상기 바를 마무리 압연 출측온도가 Ar3+10℃~Ar3+60℃가 되도록 마무리 압연하여 열연강판을 얻는다. 상기 마무리 압연 단계는 조압연기에서 만들어진 바를 3~7개의 스탠드로 이루어진 마무리 압연기에서 마무리 압연함으로써 수행될 수 있다. 상기 마무리 압연 출측온도가 Ar3+10℃ 미만인 경우에는 열간압연시 롤의 부하가 크게 증가함에 따라 에너지 소비가 증가하고, 작업속도가 늦어지며, 폭 방향 온도 편차 발생시 열연강판의 온도가 국부적으로 Ar3 이하로 내려감에 따라 초석 페라이트가 발생할 수 있어 냉각 후 충분한 마르텐사이트 분율을 얻을 수 없다. 반면, 상기 마무리 압연 출측온도가 Ar3+60℃를 초과하는 경우에는 결정립이 조대해져 높은 강도를 얻을 수 없고, 충분한 마르텐사이트 분율을 얻기 위해서는 냉각속도를 더욱 빨리해야 하는 단점이 있다. 따라서, 상기 마무리 압연 출측온도는 Ar3+10℃~Ar3+60℃인 것이 바람직하다. 상기 마무리 압연 출측온도의 하한은 Ar3+12℃인 것이 보다 바람직하고, Ar3+14℃인 것이 보다 더 바람직하며, Ar3+16℃인 것이 가장 바람직하다. 상기 마무리 압연 출측온도의 상한은 Ar3+58℃인 것이 보다 바람직하고, Ar3+56℃인 것이 보다 더 바람직하며, Ar3+52℃인 것이 가장 바람직하다.Thereafter, the bar is subjected to finish rolling so that the exit temperature of the finish rolling is Ar3 + 10 ° C to Ar3 + 60 ° C to obtain a hot rolled steel sheet. 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. In the case where 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. On the other hand, when the exit temperature of the finish rolling exceeds Ar3 + 60 ° C, crystal grains become coarse and high strength cannot be obtained, and in order to obtain a sufficient martensite fraction, there is a disadvantage in that the cooling rate must be faster. Therefore, 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.
상기 마무리 압연시 압연 속도 편차는 50mpm 이하인 것이 바람직하다. 본 발명에서 목적으로 하는 초고강도강은 변태조직의 형성을 강화기구로 이용하고 있기 때문에 마무리 압연시 변형속도에 따라 재질특성이 변할 가능성이 매우 높다. 즉, 다수 개의 스탠드로 이루어진 마무리 압연기 내에서 압연속도의 차이가 50mpm를 초과하게 되면 후속하는 런 아웃 테이블(ROT, Run Out Table)에서 균일한 냉각속도 및 목표 권취온도를 확보하기 어려워 스트립(Srtip)의 폭 또는 길이방향의 재질편차를 크게 발생시키는 원인이 될 수 있다. 따라서, 상기 마무리 압연시 압연 속도 편차는 50mpm 이하인 것이 바람직하다. 상기 마무리 압연시 압연 속도 편차는 48mpm 이하인 것이 보다 바람직하고, 46mpm 이하인 것이 보다 더 바람직하며, 42mpm 이하인 것이 가장 바람직하다. 한편, 본 발명에서는 상기 마무리 압연시 압연 속도 편차가 낮으면 낮을수록 유리하므로, 그 하한에 대해서는 특별히 한정하지 않는다.It is preferable that 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. In the finish rolling, the rolling speed deviation is more preferably 48mpm or less, more preferably 46mpm or less, and most preferably 42mpm or less. On the other hand, in the present invention, the lower the rolling speed variation in the finish rolling, the lower the advantage, so the lower limit is not particularly limited.
상기 마무리 압연시 열연강판의 폭 방향 온도 편차는 50℃ 이하인 것이 바람직하다. 상기 마무리 압연시 열연강판의 폭 방향 온도 편차가 50℃를 초과할 경우 국부적으로 오스테나이트 분율 및 결정립 사이즈의 차이가 발생하여 재질편차가 심해질 수 있다. 따라서, 상기 마무리 압연시 열연강판의 폭 방향 온도 편차는 50℃ 이하인 것이 바람직하다. 상기 마무리 압연시 열연강판의 폭 방향 온도 편차는 48℃ 이하인 것이 보다 바람직하고, 46℃ 이하인 것이 보다 더 바람직하며, 42℃ 이하인 것이 가장 바람직하다. 한편, 본 발명에서는 상기 마무리 압연시 열연강판의 폭 방향 온도 편차가 낮으면 낮을수록 유리하므로, 그 하한에 대해서는 특별히 한정하지 않는다.In the finish rolling, the temperature range in the width direction of the hot-rolled steel sheet is preferably 50 ° C or less. When the temperature range in the width direction of the hot-rolled steel sheet exceeds 50 ° C during the finish rolling, a difference in austenite fraction and grain size may occur locally, resulting in severe material deviation. Therefore, it is preferable that 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. On the other hand, in the present invention, 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.
상기 마무리 압연시 압연 속도는 200~600mpm인 것이 바람직하다. 상기 마무리 압연시 압연 속도가 600mpm를 초과할 경우 판파단과 같은 조업 사고가 일어날 수 있으며, 등온등속 압연이 어려워 균일한 온도가 확보되지 않아 재질편차가 발생할 수 있다. 반면, 상기 마무리 압연시 압연 속도가 200mpm 미만이 되면 마무리 압연 속도가 너무 느려 본 발명이 목표로 하는 마무리 압연 온도를 확보하기가 어려울 수 있다. 따라서, 상기 마무리 압연시 압연 속도는 200~600mpm인 것이 바람직하다. 상기 마무리 압연시 압연 속도의 하한은 220mpm인 것이 보다 바람직하고, 250mpm인 것이 보다 더 바람직하며, 280mpm인 것이 가장 바람직하다. 상기 마무리 압연시 압연 속도의 상한은 580mpm인 것이 보다 바람직하고, 550mpm인 것이 보다 더 바람직하며, 500mpm인 것이 가장 바람직하다.When the finish rolling, it is preferable that the rolling speed is 200 to 600 mpm. When 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. On the other hand, when 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.
이후, 상기 열연강판을 Ar3 직상에서 200℃/sec 이상으로 냉각하여 Mf(90)-50℃ 이하에서 권취한다. 상기 냉각속도가 200℃/sec 미만인 경우에는 페라이트 및 베이나이트가 형성될 수 있어 충분한 마르텐사이트 조직을 확보하기가 어렵다. 따라서, 상기 냉각속도는 200℃/sec 이상인 것이 바람직하다. 상기 냉각속도는 220℃/sec 이상인 것이 보다 바람직하고, 240℃/sec 이상인 것이 보다 더 바람직하며, 260℃/sec 이상인 것이 가장 바람직하다. 또한, 상기 권취 온도가 Mf(90)-50℃를 초과하는 경우에는 마르텐사이트 조직을 얻기가 어려울 뿐만 아니라 냉각에 의해 얻어진 마르텐사이트 조직이 지나치게 오토 템퍼링(Auto Tempering)되어 본 발명이 목표로 하는 인장강도를 얻기가 어려워질 수 있다. 따라서, 상기 권취온도는 Mf-50℃ 이하인 것이 바람직하다. 상기 권취온도는 Mf-60℃ 이하인 것이 보다 바람직하고, Mf-70℃ 이하인 것이 보다 더 바람직하며 Mf-80℃ 이하인 것이 가장 바람직하다. 한편, 상기 Mf는 오스테나이트 조직이 마르텐사이트로 변태가 100% 완료되는 온도를 의미한다.Thereafter, 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. When the cooling rate is less than 200 ° C / sec, ferrite and bainite may be formed, making it difficult to secure a sufficient martensite structure. Therefore, 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. In addition, when the coiling temperature exceeds Mf (90) -50 ° C, it is difficult to obtain a martensitic structure, and the martensitic structure obtained by cooling is too auto-tempering, which is the target of the present invention. It can be difficult to obtain strength. Therefore, 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. On the other hand, Mf means the temperature at which the austenite structure is 100% transformed into martensite.
한편, 상기 냉각시 냉각 노즐의 간격은 150~400mm인 것이 바람직하다. 상기 냉각 노즐의 간격이 400mm를 초과할 경우에는 열연강판의 온도가 국부적으로 상승하게 되어 재질편차가 심해질 수 있고, 150mm 미만인 경우에는 열연강판의 온도가 국부적으로 낮아지게 되어 재질편차가 심해질 수 있다. 따라서, 상기 냉각시 냉각 노즐의 간격은 150~400mm인 것이 바람직하다. 상기 냉각시 냉각 노즐의 간격의 하한은 160mm인 것이 보다 바람직하고, 170mm인 것이 보다 더 바람직하며, 180mm인 것이 가장 바람직하다. 상기 냉각시 냉각 노즐의 간격의 상한은 380mm인 것이 보다 바람직하고, 360mm인 것이 보다 더 바람직하며, 340mm인 것이 가장 바람직하다.On the other hand, the interval of the cooling nozzle during the cooling is preferably 150 ~ 400mm. When 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. When cooling, 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. When cooling, the upper limit of the interval of the cooling nozzle is more preferably 380 mm, more preferably 360 mm, and most preferably 340 mm.
상기 권취하는 단계 후에는 권취된 열연강판을 산세 처리하는 단계를 추가로 포함할 수 있으며, 상기 산세 처리를 통해 PO(Pickled & Oiled)재를 얻을 수 있다. 본 발명에서는 박 슬라브 및 바 스케일 제거 단계에서 스케일을 충분히 제거할 수 있으므로, 일반적인 산세처리로도 표면품질이 우수한 PO재를 얻을 수 있다. 따라서 본 발명에서는 열연산세공정에서 일반적으로 사용되는 방법이라면 모두 적용 가능하므로 산세 처리 방법에 대하여 특별히 제한하지 않는다.After the winding step, 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. In the present invention, since 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.
이하, 실시예를 통해 본 발명을 보다 상세히 설명한다. 다만, 하기의 실시예는 본 발명을 예시하여 보다 상세하게 설명하기 위한 것일 뿐, 본 발명의 권리범위를 한정하기 위한 것이 아니라는 점에 유의할 필요가 있다. 본 발명의 권리범위는 특허청구범위에 기재된 사항과 이로부터 합리적으로 유추되는 사항에 의해 결정되는 것이기 때문이다.Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the items described in the claims and the items reasonably inferred therefrom.
(실시예 1)(Example 1)
하기 표 1의 합금조성을 갖는 용강을 준비한 뒤, 연주-압연 직결 공정을 적용하여 하기 표 2 및 3에 기재된 제조조건으로 1.2mm 두께의 열연강판으로 제조하였다. 이 열연강판을 산세 처리한 뒤, 노즐막힘유무를 관찰하고, 미세조직 및 석출물을 측정한 후, 그 결과를 하기 표 4에 나타내었으며, 항복강도(YS), 인장강도(TS), 연신율(El), 비커스 경도(Hv(0.5kgf)), 인장강도 편차(△TS) 및 비커스 경도 편차(△Hv(0.5kgf))와 크랙 발생 유무를 측정한 뒤, 그 결과를 하기 표 5에 나타내었다. 한편, 하기 표 3에서의 Ar3 및 Mf 온도는 상용 열역학 소프트웨인 JmatPro V-8를 이용하여 계산한 값이다.After preparing the molten steel having the alloy composition of Table 1, by applying a direct-rolling direct-linking process, it was manufactured as a hot-rolled steel sheet having a thickness of 1.2 mm under the manufacturing conditions shown in Tables 2 and 3 below. After pickling the hot-rolled steel sheet, the presence or absence of nozzle clogging was observed, the microstructure and precipitates were measured, and the results are shown in Table 4 below. Yield strength (YS), tensile strength (TS), and elongation (El) ), Vickers hardness (Hv (0.5kgf)), tensile strength deviation (ΔTS) and Vickers hardness deviation (ΔHv (0.5kgf)), and whether cracks were generated, and the results are shown in Table 5 below. Meanwhile, the Ar3 and Mf temperatures in Table 3 are values calculated using a commercial thermodynamic software JmatPro V-8.
미세조직 및 석출물은 주사전자현미경(SEM)과 투과전자현미경(TEM)을 이용하여 관찰하였다.Microstructure and precipitates were observed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM).
항복강도, 인장강도 및 연신율은 스트립의 전폭[일정한 간격(7군데)]에 대하여 압연방향으로 채취한 JIS 5호 규격의 시편들을 측정한 뒤 평균값을 기재하였다. 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.
경도는 비커스 경도기를 이용하여 하중 0.5kgf로 10회 측정한 뒤 평균값을 기재하였다. The hardness was measured 10 times with a load of 0.5 kgf using a Vickers hardness tester and the average value was described.
인장강도 편차(△TS) 및 비커스 경도 편차(△Hv(0.5kgf))는 전폭에서 측정된 값 중 최대값과 최소값의 차이를 나타낸다.The tensile strength deviation (ΔTS) and Vickers hardness deviation (ΔHv (0.5kgf)) represent the difference between the maximum value and the minimum value among the values measured at the full width.
크랙 발생 유무는 슬라브, 바 및 스트립에서 육안으로 1차 확인하고, 표면 결함 디텍터(Detector)인 SDD(Surface Defect Detector) 장치를 이용하여 2차 확인하였다. The presence or absence of cracks was first checked visually on slabs, bars, and strips, and secondly confirmed using a surface defect detector (SDD) device, which is a surface defect detector.
Figure PCTKR2019011700-appb-img-000001
Figure PCTKR2019011700-appb-img-000001
Figure PCTKR2019011700-appb-img-000002
Figure PCTKR2019011700-appb-img-000002
Figure PCTKR2019011700-appb-img-000003
Figure PCTKR2019011700-appb-img-000003
Figure PCTKR2019011700-appb-img-000004
Figure PCTKR2019011700-appb-img-000004
Figure PCTKR2019011700-appb-img-000005
Figure PCTKR2019011700-appb-img-000005
상기 표 1 내지 5에서 알 수 있는 바와 같이, 본 발명이 제안하는 합금조성, 관계식 1 내지 3과 제조조건을 모두 만족하는 발명예 1 내지 15의 경우에는 본 발명의 미세조직 및 석출물 조건을 만족하고 있음을 알 수 있다. 또한, 선형크랙 및 에지크랙이 발생하지 않아 양호한 표면품질을 확보하고 있음을 확인할 수 있다. 아울러, 본 발명이 목표로 하는 항복강도, 인장강도, 연신율, 비커스 경도, 스트립의 폭 방향 인장강도 편차 및 스트립의 폭 방향 비커스 경도 편차를 확보하고 있음을 확인할 수 있다. As can be seen from Tables 1 to 5, in the case of Inventive Examples 1 to 15 satisfying all of the alloy composition, relations 1 to 3 and manufacturing conditions proposed by the present invention, the microstructure and precipitate conditions of the present invention are satisfied, You can see that there is. In addition, it can be confirmed that the linear crack and the edge crack are not generated, thereby ensuring good surface quality. In addition, it can be seen that the target yield strength, tensile strength, elongation, Vickers hardness, the width direction tensile strength deviation of the strip, and the width direction Vickers hardness deviation of the strip are secured.
그러나, 본 발명이 제안하는 합금조성, 관계식 1 내지 3, 및 제조조건(마무리 압연 출측온도) 중 하나 이상을 만족하지 않는 비교예 1 내지 12의 경우에는 에지 크랙이 발생하거나, 본 발명이 목표로 하는 기계적 물성과 재질편차 조건을 확보하고 있지 않음을 확인할 수 있다.However, in the case of Comparative Examples 1 to 12 that do not satisfy one or more of the alloy composition, relations 1 to 3, and manufacturing conditions (finish rolled exit temperature) proposed by the present invention, edge cracking occurs, or the present invention aims It can be confirmed that the mechanical properties and material deviation conditions are not secured.
비교예 13는 본 발명이 제안하는 합금조성 중 Ca 함량 범위를 만족하지 않는 경우로서, 노즐막힘으로 인하여 주조중단이 발생하였음을 확인할 수 있다.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.
비교예 14 및 15는 본 발명이 제안하는 합금조성, 관계식 1 내지 3은 만족하나, 제조조건(마무리 압연 출측온도)을 만족하지 않는 경우로서, 본 발명이 제안하는 미세조직을 확보하지 못함에 따라 본 발명이 목표로 하는 기계적 물성과 재질편차 조건을 확보하고 있지 않음을 확인할 수 있다.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.
도 3은 발명예 1 내지 15와 비교예 1 내지 13에 대한 관계식 1 및 2의 값을 나타낸 그래프이다. 발명 영역의 경우, 본 발명의 관계식 3을 만족하는 범위로서, 발명예 1 내지 15의 경우 상기 발명 영역에 포함되는 것을 확인할 수 있는 반면, 비교예 1 내지 12의 경우에는 상기 발명 영역을 벗어나고 있음을 확인할 수 있다. 비교예 13는 상기 발명 영역에 포함되어 있으나 본 발명의 Ca 함량 범위를 만족하지 않는 경우이다.3 is a graph showing the values of relations 1 and 2 for Inventive Examples 1 to 15 and Comparative Examples 1 to 13. In the case of the invention region, it can be confirmed that 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.
도 4는 발명예 1을 주사전자현미경(SEM)으로 관찰한 미세조직 사진이다. 도 4를 통해 알 수 있듯이, 발명예 1은 마르텐사이트 및 오토 템퍼드 마르텐사이트가 주조직이며, 일부 페라이트가 형성되어 있음을 확인할 수 있다.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.
도 5 (a) 및 (b)는 발명예 1을 투과전자현미경(TEM)으로 관찰한 미세조직 사진이다. 도 5(a) 및 (b)를 통해 알 수 있듯이, 발명예 1은 마르텐사이트 래스가 미세하게 잘 발달되어 있을 뿐만 아니라, 마르텐사이트 래스 내에는 미세한 탄화물이 존재하여 오토 템퍼드 마르텐사이트 조직이 함께 존재하고 있음을 확인할 수 있다.5 (a) and (b) are microstructure photographs of Inventive Example 1 observed with a transmission electron microscope (TEM). As can be seen through Figure 5 (a) and (b), Inventive Example 1, as well as finely developed martensitic lath, there is a fine carbide in the martensitic lath, the auto-tempered martensitic tissue together You can confirm that it exists.
도 6은 발명예 1의 마르텐사이트 및 오토 템퍼드 마르텐사이트 래스 폭에 대한 분포를 나타낸 그래프이다. 도 6을 통해 알 수 있듯이, 발명예 1의 경우 마르텐사이트 및 오토 템퍼드 마르텐사이트 래스는 0.05~1.0㎛ 범위에 존재하며, 0.3㎛의 폭을 갖는 마르텐사이트 및 오토 템퍼드 마르텐사이트 래스가 많이 존재하고 있음을 확인할 수 있다.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 μm, and martensitic and auto-tempered martensitic laths having a width of 0.3 μm are present. You can confirm that
도 7 및 8은 각각 발명예 1과 비교예 8의 석출물을 투과전자현미경(TEM)으로 관찰한 사진이다. 이 때, TEM 시편은 카본 레프리카 방법으로 샘플을 제작하였다. 도 7 및 8을 통해 알 수 있듯이, 발명예 1의 경우에는 40nm 이하의 미세한 복합 석출물(M(X) (M=Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X=C, N))이 분포하고 있는 반면, 비교예 8의 경우에는 복합 석출물이 40nm를 초과하여 상당히 조대한 것을 확인할 수 있다.7 and 8 are photographs of the precipitates of Inventive Example 1 and Comparative Example 8, respectively, observed with a transmission electron microscope (TEM). At this time, the TEM specimen was prepared by a carbon replica method. 7 and 8, in the case of Inventive Example 1, fine complex precipitates of 40 nm or less (M (X) (M = Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X = While C, N)) is distributed, it can be confirmed that in the case of Comparative Example 8, the complex precipitate exceeded 40 nm and was significantly coarse.
(실시예 2)(Example 2)
발명강 1의 합금조성을 갖는 용강을 준비한 뒤, 연주-압연 직결 공정을 적용하여 하기 표 6에 기재된 제조조건으로 1.2mm 두께의 열연강판으로 제조하였다. 하기 표 6에 기재된 제조조건 외 스케일 제거, 마무리 압연 및 냉각 조건은 상기 표 2의 발명예 1의 조건과 동일하게 수행하였다. 상기 제조된 열연강판을 산세 처리한 뒤, 선형크랙 및 에지크랙의 발생 정도를 측정한 뒤, 그 결과를 하기 표 6에 나타내었다. After preparing the molten steel having the alloy composition of Inventive Steel 1, it was manufactured into a hot-rolled steel sheet having a thickness of 1.2 mm under the manufacturing conditions shown in Table 6 below by applying a direct rolling process. In addition to the manufacturing conditions described in Table 6, the scale removal, finish rolling, and cooling conditions were performed in the same manner as in Example 1 of Table 2. After the hot-rolled steel sheet prepared above was pickled, the degree of occurrence of linear cracks and edge cracks was measured, and the results are shown in Table 6 below.
Figure PCTKR2019011700-appb-img-000006
Figure PCTKR2019011700-appb-img-000006
상기 표 6에서 알 수 있듯이, 본 발명이 제안하는 합금조성 및 제조조건을 만족하는 발명예 16 내지 18은 선행크랙 및 에지크랙이 발생하지 않았음을 확인할 수 있다.As can be seen from Table 6, it can be seen that the inventive examples 16 to 18 satisfying the alloy composition and manufacturing conditions proposed by the present invention did not generate any preceding cracks and edge cracks.
반면, 비교예 16 내지 19는 본 발명이 제안하는 합금조성은 만족하나, 제조조건 중 몰드 플럭스 염기도, 2차 냉각 비수량 및 조압연 출측 바 에지부 온도 조건 중 하나를 만족하지 않아 선행크랙 및 에지크랙이 발생하였음을 확인할 수 있다.On the other hand, 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.
(실시예 3)(Example 3)
발명강 5의 합금조성을 갖는 용강을 준비한 뒤, 연주-압연 직결 공정을 적용하여 하기 표 7 및 8에 기재된 제조조건으로 1.2mm 두께의 열연강판으로 제조하였다. 하기 표 7 및 8에 기재된 제조조건 외 연속주조 및 조압연 조건은 상기 표 2의 발명예 5의 조건과 동일하게 수행하였다. 상기 제조된 열연강판을 산세 처리한 뒤, 항복강도(YS), 인장강도(TS), 연신율(El), 비커스 경도(Hv(0.5kgf)), 인장강도 편차(△TS) 및 비커스 경도 편차(△Hv(0.5kgf))를 측정한 뒤, 그 결과를 하기 표 9에 나타내었다. 하기 표 7에서의 Ar3 및 Mf 온도는 상용 열역학 소프트웨인 JmatPro V-8를 이용하여 계산한 값이다.After preparing the molten steel having the alloy composition of Inventive Steel 5, it was manufactured as a hot-rolled steel sheet having a thickness of 1.2 mm under the manufacturing conditions shown in Tables 7 and 8 by applying a direct-rolling direct connection process. Continuous casting and rough rolling conditions in addition to the manufacturing conditions described in Tables 7 and 8 were performed in the same manner as in Example 5 of Table 2. After the prepared hot-rolled steel sheet is pickled, yield strength (YS), tensile strength (TS), elongation (El), Vickers hardness (Hv (0.5kgf)), tensile strength deviation (△ TS) and Vickers hardness deviation ( After measuring ΔHv (0.5kgf), the results are shown in Table 9 below. Ar3 and Mf temperatures in Table 7 are values calculated using a commercial thermodynamic software JmatPro V-8.
Figure PCTKR2019011700-appb-img-000007
Figure PCTKR2019011700-appb-img-000007
Figure PCTKR2019011700-appb-img-000008
Figure PCTKR2019011700-appb-img-000008
Figure PCTKR2019011700-appb-img-000009
Figure PCTKR2019011700-appb-img-000009
상기 표 7 내지 9에서 알 수 있듯이, 본 발명이 제안하는 합금조성 및 제조조건을 발명예 19 내지 21은 본 발명이 목표로 하는 항복강도, 인장강도, 연신율, 비커스 경도, 스트립의 폭 방향 인장강도 편차 및 스트립의 폭 방향 비커스 경도 편차를 확보하고 있음을 확인할 수 있다. As can be seen from Tables 7 to 9, the alloy composition and the manufacturing conditions proposed by the present invention, Examples 19 to 21, 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.
반면, 비교예 20 내지 24는 본 발명이 제안하는 합금조성은 만족하나, 제조조건 중 냉각수 분사 겹침 면적율, 마무리 압연시 압연 속도 편차, 마무리 압연시 열연강판의 폭 방향 온도 편차, 및 냉각시 냉각 노즐의 간격 중 하나를 만족하지 않아 선행크랙 및 에지크랙이 발생하였음을 확인할 수 있다.On the other hand, 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.
[부호의 설명][Description of codes]
a: 슬라브 b: 바a: slab b: bar
c: 스트립c: strip
100: 연속주조기 200, 200': 가열기 100: continuous casting machine 200, 200 ': heater
300: RSB(Roughing Mill Scale Breaker, 조압연 스케일 브레이커)300: RSB (Roughing Mill Scale Breaker)
400: 조압연기400: rough rolling mill
500: FSB(Fishing Mill Scale Breaker, 마무리 압연 스케일 브레이커) 500: FSB (Fishing Mill Scale Breaker)
600: 마무리 압연기 700: 런아웃 테이블 600: finishing mill 700: runout table
800: 고속전단기 900: 권취기800: High-speed shearing machine 900: Winding machine

Claims (18)

  1. 중량%로, C: 0.16~0.27%, Mn: 0.8~2.6%, Si: 0.05~0.3%, Al: 0.05% 이하, Ti: 0.01~0.08%, B: 0.001~0.005%, Ca: 0.001~0.005%, N: 0.001~0.010%, 잔부 Fe 및 기타 불가피한 불순물을 포함하고, In weight percent, C: 0.16 to 0.27%, Mn: 0.8 to 2.6%, Si: 0.05 to 0.3%, Al: 0.05% or less, Ti: 0.01 to 0.08%, B: 0.001 to 0.005%, Ca: 0.001 to 0.005 %, N: 0.001 ~ 0.010%, contains the balance Fe and other inevitable impurities,
    하기 관계식 1 내지 3을 만족하며,The following relational expressions 1 to 3 are satisfied,
    면적분율로 마르텐사이트와 오토 템퍼드(Auto Tempered) 마르텐사이트의 합이 95%이상이고, 페라이트가 5%이하(0%를 포함)인 미세조직을 포함하고,The area fraction includes microstructures in which the sum of martensite and auto tempered martensite is 95% or more and ferrite is 5% or less (including 0%),
    평균 크기가 40nm 이하인 M(X) (M=Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X=C, N) 복합 석출물을 포함하는 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판.Excellent surface quality including M (X) (M = Ti, Nb, Si, Al, B, Mg, Cr, Ca, P, X = C, N) composite precipitates having an average size of 40 nm or less, and material deviation Less ultra-high strength hot rolled steel sheet.
    [관계식 1] 16 ≤ 100(C+Mn/100+B/10) ≤ 28 [Relationship 1] 16 ≤ 100 (C + Mn / 100 + B / 10) ≤ 28
    [관계식 2] 1 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)] ≤ 14[Relationship 2] 1 ≤ [(Al / 27) × (N / 14)] / [(Ti / 48) × (B / 11)] ≤ 14
    [관계식 3] 0.05 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/100(C+Mn/100+B/10) ≤ 0.66[Relationship 3] 0.05 ≤ [(Al / 27) × (N / 14)] / [(Ti / 48) × (B / 11)] / 100 (C + Mn / 100 + B / 10) ≤ 0.66
    (단, 상기 관계식 1 내지 3에 기재된 합금성분의 함량은 중량%임.)(However, the content of the alloy component described in the above formula 1 to 3 is weight%.)
  2. 청구항 1에 있어서,The method according to claim 1,
    상기 열연강판은 트램프 원소로서 Nb, V, Ti, Mo, Cu, Cr, Ni, Zn, Se, Sb, Zr, W, Ga, Ge 및 Mg로 이루어지는 그룹으로부터 선택된 1종 이상을 그 합계가 0.1중량% 이하의 범위로 포함하는 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판.The hot-rolled steel sheet has a total weight of 0.1 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 tram element. Super high strength hot rolled steel sheet with excellent surface quality and less material deviation, which is included in the range below%.
  3. 청구항 1에 있어서,The method according to claim 1,
    상기 열연강판은 상기 마르텐사이트 래스(lath)의 평균 폭은 1㎛ 이하인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판.The hot-rolled steel sheet is an ultra-high-strength hot-rolled steel sheet having excellent surface quality with an average width of 1 µm or less of the martensite lath and less material deviation.
  4. 청구항 1에 있어서,The method according to claim 1,
    상기 열연강판은 항복강도가 1060~1400MPa이며, 인장강도가 1470~1800MPa이고, 연신율이 5% 이상이며, 비커스 경도가 420~550Hv(0.5kgf)이고, 스트립의 폭 방향 인장강도 편차가 100MPa이하이며, 스트립의 폭 방향 비커스 경도 편차가 50Hv(0.5kgf)이하인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판.The hot-rolled steel sheet 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 deviation in tensile strength of the strip in the width direction of 100 MPa or less. , Super high-strength hot-rolled steel sheet with excellent surface quality with less than 50Hv (0.5kgf) variation in Vickers hardness in the width direction of the strip.
  5. 청구항 1에 있어서,The method according to claim 1,
    상기 열연강판은 두께가 1.6mm 이하인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판.The hot-rolled steel sheet is an ultra-high-strength hot-rolled steel sheet having excellent surface quality with a thickness of 1.6 mm or less and less material deviation.
  6. 중량%로, C: 0.16~0.27%, Mn: 0.8~2.6%, Si: 0.05~0.3%, Al: 0.05% 이하, Ti: 0.01~0.08%, B: 0.001~0.005%, Ca: 0.001~0.005%, N: 0.001~0.010%, 잔부 Fe 및 기타 불가피한 불순물을 포함하고, 하기 관계식 1 내지 3을 만족하는 용강을 연속 주조하여 박 슬라브를 얻는 단계; In weight percent, C: 0.16 to 0.27%, Mn: 0.8 to 2.6%, Si: 0.05 to 0.3%, Al: 0.05% or less, Ti: 0.01 to 0.08%, B: 0.001 to 0.005%, Ca: 0.001 to 0.005 %, N: 0.001 ~ 0.010%, including the residual Fe and other inevitable impurities, and continuously casting molten steel satisfying the following relations 1 to 3 to obtain a thin slab;
    상기 박 슬라브를 조압연하여 바(Bar)를 얻는 단계; Rough rolling the thin slab to obtain a bar;
    상기 바를 마무리 압연 출측온도가 Ar3+10℃~Ar3+60℃가 되도록 마무리 압연하여 열연강판을 얻는 단계; 및Finish rolling the bar so that the exit temperature of the finish rolling is Ar3 + 10 ° C to Ar3 + 60 ° C to obtain a hot rolled steel sheet; And
    상기 열연강판을 Ar3 직상에서 200℃/sec 이상으로 냉각하여 Mf-50℃ 이하에서 권취하는 단계를 포함하며,Cooling the hot-rolled steel sheet above Ar3 to 200 ° C / sec or more and winding it up to Mf-50 ° C or less,
    상기 각 단계는 연속적으로 행해지는 것을 특징으로 하는 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.Each step is a method of manufacturing a super high strength hot-rolled steel sheet having excellent surface quality and less material deviation characterized in that it is continuously performed.
    [관계식 1] 16 ≤ 100(C+Mn/100+B/10) ≤ 28 [Relationship 1] 16 ≤ 100 (C + Mn / 100 + B / 10) ≤ 28
    [관계식 2] 1 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)] ≤ 14[Relationship 2] 1 ≤ [(Al / 27) × (N / 14)] / [(Ti / 48) × (B / 11)] ≤ 14
    [관계식 3] 0.05 ≤ [(Al/27)×(N/14)]/[(Ti/48)×(B/11)]/100(C+Mn/100+B/10) ≤ 0.66[Relationship 3] 0.05 ≤ [(Al / 27) × (N / 14)] / [(Ti / 48) × (B / 11)] / 100 (C + Mn / 100 + B / 10) ≤ 0.66
    (단, 상기 관계식 1 내지 3에 기재된 합금성분의 함량은 중량%임.)(However, the content of the alloy component described in the above formula 1 to 3 is weight%.)
  7. 청구항 6에 있어서,The method according to claim 6,
    상기 연속주조시 주조속도는 4~8mpm(m/min)인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.The method of manufacturing a super high strength hot rolled steel sheet having excellent surface quality with a casting speed of 4 to 8mpm (m / min) and less material deviation during the continuous casting.
  8. 청구항 6에 있어서,The method according to claim 6,
    상기 박 슬라브는 두께가 80~120mm인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.The thin slab is a method of manufacturing a super high strength hot rolled steel sheet having excellent surface quality with a thickness of 80 to 120 mm and less material deviation.
  9. 청구항 6에 있어서,The method according to claim 6,
    상기 연속주조시 몰드 플럭스의 염기도는 0.8~1.5인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.The method of manufacturing the ultra-high strength hot rolled steel sheet having excellent surface quality with a basicity of 0.8 to 1.5 and a small material deviation during the continuous casting.
  10. 청구항 6에 있어서,The method according to claim 6,
    상기 연속주조시 2차 냉각 비수량은 1.5~2.5ℓ/㎏인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.The method of manufacturing a super-high strength hot rolled steel sheet having excellent surface quality of 1.5 to 2.5 L / kg and a small material deviation in the secondary cooling specific water amount during the continuous casting.
  11. 청구항 6에 있어서,The method according to claim 6,
    상기 조압연시 조압연 출측에서의 바 에지부 온도는 850~1000℃인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.The method of manufacturing the ultra-high strength hot rolled steel sheet having excellent surface quality of 850 to 1000 ° C. and a material deviation with a bar edge temperature at the side of the rough rolling during rough rolling.
  12. 청구항 6에 있어서,The method according to claim 6,
    상기 바를 얻는 단계 후, 상기 바에 200~300bar의 압력으로 냉각수를 분사하는 단계를 추가로 포함하는 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.After the step of obtaining the bar, the method of manufacturing a super high strength hot-rolled steel sheet having excellent surface quality and less material deviation, further comprising spraying cooling water at a pressure of 200 to 300 bar to the bar.
  13. 청구항 12에 있어서,The method according to claim 12,
    상기 냉각수 분사시, 냉각수의 겹침(overlap) 면적율은 5~25%인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.When the cooling water is sprayed, the overlap area ratio of the cooling water is 5 to 25%, the surface quality is excellent, and the method for manufacturing the ultra-high strength hot rolled steel sheet with little material deviation.
  14. 청구항 6에 있어서,The method according to claim 6,
    상기 마무리 압연시 압연 속도 편차는 50mpm 이하인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.The method of manufacturing the ultra-high strength hot rolled steel sheet having excellent surface quality of less than 50mpm and less material deviation in the rolling speed variation during the finish rolling.
  15. 청구항 6에 있어서,The method according to claim 6,
    상기 마무리 압연시 열연강판의 폭 방향 온도 편차는 50℃ 이하인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.The method for manufacturing a hot rolled steel sheet having a high surface quality of 50 ° C. or less and a material deviation with a temperature variation in the width direction of the hot rolled steel sheet during the finish rolling.
  16. 청구항 6에 있어서,The method according to claim 6,
    상기 마무리 압연시 압연 속도는 200~600mpm인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.When the finish rolling, the rolling speed is excellent in the surface quality of 200 ~ 600mpm, the manufacturing method of the ultra-high strength hot rolled steel sheet with less material deviation.
  17. 청구항 6에 있어서,The method according to claim 6,
    상기 냉각시 냉각 노즐의 간격은 150~400mm인 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.When cooling, the interval between the cooling nozzles is 150 to 400 mm, the surface quality is excellent, and the manufacturing method of the ultra-high strength hot rolled steel sheet with little material deviation.
  18. 청구항 6에 있어서,The method according to claim 6,
    상기 권취하는 단계 후, 권취된 열연강판을 산세 처리하는 단계를 추가로 포함하는 표면 품질이 우수하고, 재질편차가 적은 초고강도 열연강판의 제조방법.After the step of winding, the method of manufacturing an ultra-high strength hot rolled steel sheet having excellent surface quality and less material deviation, further comprising the step of pickling the wound hot rolled steel sheet.
PCT/KR2019/011700 2018-10-01 2019-09-10 Ultra high strength hot rolled steel sheet having excellent surface qualities and low mechanical properties deviation and method of manufacturing the same WO2020071654A1 (en)

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