WO2020111639A1 - Tôle d'acier laminée à chaud à ultra-haute résistance ayant une excellente qualité de forme et une excellente aptitude au pliage, et son procédé de fabrication - Google Patents

Tôle d'acier laminée à chaud à ultra-haute résistance ayant une excellente qualité de forme et une excellente aptitude au pliage, et son procédé de fabrication Download PDF

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WO2020111639A1
WO2020111639A1 PCT/KR2019/015934 KR2019015934W WO2020111639A1 WO 2020111639 A1 WO2020111639 A1 WO 2020111639A1 KR 2019015934 W KR2019015934 W KR 2019015934W WO 2020111639 A1 WO2020111639 A1 WO 2020111639A1
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steel sheet
rolled steel
ultra
rolling
bendability
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PCT/KR2019/015934
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English (en)
Korean (ko)
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공종판
고영주
박경미
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주식회사 포스코
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    • 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
    • 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
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B37/00—Control devices or methods specially adapted for metal-rolling mills or the work produced thereby
    • B21B37/74—Temperature control, e.g. by cooling or heating the rolls or the product
    • 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
    • C22C38/14—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
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to an ultra-high strength hot rolled steel sheet excellent in shape quality and bendability and a method for manufacturing the same.
  • the steel plate for automobile structural members is basically composed of a combination of Ferrite, Baintie, Martensite, and Tempered Martensite phases, and the proportion Accordingly, 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
  • MART MART steel
  • These steels are mainly applied to parts that require high energy absorption in case of 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 cannot avoid the reduction in elongation due to ultra-high strength, so they undergo cold rolling and continuous annealing (CAL) after hot rolling, or HPF (Hot Press Forming) that performs rapid cooling and processing after hot rolling. There is a disadvantage that the manufacturing cost increases because it has to go through the same process.
  • CAL cold rolling and continuous annealing
  • HPF Hot Press Forming
  • Patent Literature 1 is weight% 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%, Nb 0.03 to 0.06%, S 0.02% or less, P 0.02% or less, N 0.005% or less is added, and when the steel is manufactured, the aluminum kiln steel containing an element that is inevitably contained is homogenized at 1050 to 1300°C, then hot rolled at 850 to 950°C, which is directly above the Ar3 transformation point, and then 550 to Hot-rolling at 650°C; 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 performing the first slow cooling of the steel plate to 600 to 700°C, and secondarily rapidly cooling to 350 to 300°C at a cooling rate of -10 to -50°C/sec, and then maintaining at least 1 minute while cooling slowly between 350 and 250°C. Disclosed is
  • Patent Document 2 is 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, and Ni in steels containing impurities and a small amount of alloying elements. : A method of manufacturing a cold rolled steel sheet having a good shape having a strength of 1180 to 1400 MPa and a bending/twisting of a steel sheet of 10 mm or less by adding one or two or more of 0.3% or less is disclosed.
  • Patent Document 3 is a cold rolled steel sheet 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 %. 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 retained austenite before processing is 10% or more.
  • a method for manufacturing a tensile strength 1470 MPa grade ultra-high strength cold rolled steel sheet having a hydrogenation embrittlement property of an austenite grain having a short axis average length of 1 ⁇ m or less and an average axial ratio (long axis/short axis) of 5 or more is introduced.
  • Patent Document 4 is 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 residual Fe and other unavoidable impurities at a heating rate of 1 ⁇ 5°C/s [(Ac3-90 °C) ⁇ (Ac3 ⁇ 15°C)], then heated to a temperature range of 500 ⁇ 750°C at a cooling rate of 1 ⁇ 3°C/s, and a cooling rate of 3 ⁇ 50°C/s Cold rolling with a tensile strength of 1.5 GPa through a continuous annealing step
  • Patent Documents 1 to 4 there is a disadvantage in that the manufacturing cost rises rapidly because it has to undergo a cold rolling and annealing heat treatment (CAL) process after hot rolling.
  • CAL cold rolling and annealing heat treatment
  • Patent Document 5 is 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 in weight percent, and further Si : 0.5% or less, Ni: 2% or less, Cu: 1% or less, V: 1% or less, Al: 1% or less, 1 or 2 or more types, B: 0.01% or less in some cases, Nb: 1.0% or less , Mo: 1.0% or less, Ca: 0.001 to 0.005% of cold rolled steel sheets containing one or two or more types of hot-rolled steel sheets having a tensile strength of 1.8 GPa or more is disclosed.
  • Patent Document 5 Although the ultra-high strength of 1.8GPa can be secured, there is a problem in that the manufacturing cost is higher because the cold rolled steel sheet must be hot pressed.
  • 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 International Publication No. 2007-129676
  • One aspect of the present invention is to provide an ultra-high strength hot rolled steel sheet excellent in shape quality and bendability using only a hot rolling process using a continuous rolling mode in a continuous casting-rolling direct connection process and a method for manufacturing the same.
  • One embodiment of the present invention in weight percent, C: 0.16 to 0.26%, Mn: 0.75 to 1.50%, B: 0.0009 to 0.0050%, Ti: 0.009 to 0.070%, N: 0.001 to 0.010%, balance Fe and others Containing unavoidable impurities, satisfying the following relations 1 to 3, comprising a microstructure in which the sum of martensite and tempered martensite in an area fraction is 93% or more and ferrite is 7% or less (including 0%).
  • an ultra-high strength hot rolled steel sheet having excellent shape quality and bendability.
  • 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.
  • FIG. 1 is a schematic diagram of a facility for a continuous casting-rolling direct connection process applicable to the production of hot rolled steel sheet of the present invention.
  • FIG. 2 is another schematic view of a facility for a continuous casting-rolling direct connection process applicable to the production of a hot rolled steel sheet of the present invention.
  • FIG. 3 is a microstructure photograph of Inventive Example 9 according to an embodiment of the present invention observed with EBSD.
  • FIG. 4 is a microstructure photograph of Inventive Example 9 according to an embodiment of the present invention observed with a scanning electron microscope (SEM).
  • FIG. 5 is a microstructure photograph of Inventive Example 9 according to an embodiment of the present invention observed with a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • FIG. 6 is a microstructure photograph of Comparative Example 11 according to an embodiment of the present invention observed with a scanning electron microscope (SEM).
  • Comparative Example 15 is a microstructure photograph of Comparative Example 15 according to an embodiment of the present invention observed with a scanning electron microscope (SEM).
  • Carbon (C) is a very important element that increases strength by making microstructure martensite when quenching 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 0.26%, 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 0.26%.
  • the lower limit of the C content is more preferably 0.17%, and even more preferably 0.18%.
  • the upper limit of the C content is more preferably 0.25%, and even more 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.
  • Mn content is less than 0.75%, it may be difficult to secure the target strength in the present invention.
  • Mn content is more than 1.50%, segregation zones are formed on the inside or outside of the slab and hot-rolled steel sheet to cause crack generation and propagation, deteriorating the final quality of the steel sheet, and deteriorating weldability and bending workability. can do. Therefore, the Mn content is preferably 0.75 to 1.50%.
  • the lower limit of the Mn content is more preferably 0.80%, and even more preferably 0.85%.
  • the upper limit of the Mn content is more preferably 1.40%, and even more preferably 1.30%.
  • B Boron
  • B is an element that increases the hardenability of steel.
  • the content of B is preferably 0.0009 to 0.0050%.
  • the lower limit of the B content is more preferably 0.0010%, and even more preferably 0.0015%.
  • the upper limit of the B content is more preferably 0.0045%, and even more preferably 0.0040%.
  • Titanium (Ti) is a precipitate and nitride forming element, which is precipitated with TiC, TiN, and Ti(C,N) during rolling to reduce the austenite grain size, thereby improving the strength and bending properties of steel through precipitation strengthening and grain refinement. to be.
  • Ti is an element that reduces high-temperature ductility deterioration by reducing the formation of precipitates such as AlN by removing solid solution N through the formation of TiN near the solidification temperature, thereby reducing the sensitivity of edge crack generation.
  • the Ti content is preferably 0.009 to 0.070%.
  • the lower limit of the Ti content is more preferably 0.012%, and even more preferably 0.015%.
  • the upper limit of the Ti content is more preferably 0.060%, and even more preferably 0.050%.
  • N Nitrogen
  • the lower limit of the N content is more preferably 0.002%, and even more preferably 0.003%.
  • the upper limit of the N content is more preferably 0.008%, and even more preferably 0.006%.
  • the remaining component of the invention is iron (Fe).
  • impurities that are not intended from the raw material or the surrounding environment may be inevitably mixed, and therefore cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, they are not specifically mentioned in this specification.
  • C, Mn, B, and Ti among the above-described alloy components respectively satisfy the following relations 1 to 3, through which the mechanical properties and excellent surface quality targeted by the present invention are achieved. Can be secured.
  • the contents of C, Mn, B, and Ti described in relations 1 to 3 below are weight%.
  • the relational expression 1 is a component relational expression for securing the strength desired by the present invention.
  • the value of the relational expression 1 is less than 0.32, it is difficult to secure the target strength of the present invention, and when it exceeds 0.53, 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 0.32 to 0.53.
  • the lower limit of the value of the relational expression 1 is more preferably 0.33, and even more preferably 0.35.
  • the upper limit of the value of the relational expression 1 is more preferably 0.52, and even more preferably 0.50.
  • the relational expression 2 is a component relational expression for securing the strength targeted by the present invention, and is a relational expression between C, Mn, which is an austenite-promoting element, and B having excellent curing ability.
  • the value of the relational expression 2 is less than 130, it is difficult to secure the target strength of the present invention, and when it exceeds 450, the elongation is lowered and cracking may occur during processing. Therefore, it is preferable that the value of the relational expression 2 has a range of 130 to 450.
  • the lower limit of the value of the relational expression 2 is more preferably 140, and even more preferably 150.
  • the upper limit of the value of the relational expression 2 is more preferably 400, and even more preferably 350.
  • the relational expression 3 is a component relational expression for securing the strength and excellent bending characteristics targeted by the present invention.
  • the value of the relational expression 3 is less than 5, Ti is excessively added, and it is difficult to expect a grain refinement effect due to the formation of coarse TiN, TiC, and Ti(C,N) precipitates, and manufacturing cost may increase.
  • it exceeds 45 it is difficult to secure elongation as C, Mn is added excessively or when Ti is added less, and the formation of TiN, TiC, and Ti(C,N) precipitates is small, thereby limiting the refinement of austenite grain size. Therefore, the bending characteristics may be inferior. Therefore, it is preferable that the value of the relational expression 3 has a range of 5 to 45.
  • the lower limit of the value of the relational expression 3 is more preferably 10, and even more preferably 15.
  • the upper limit of the value of the relational expression 3 is more preferably 40, and even more preferably 35.
  • the hot-rolled steel sheet of the present invention is selected from the group consisting of P, S, Si, Nb, V, Mo, Cu, Cr, Ni, Zn, Se, Sb, Zr, W, Ga, Ge and Mg as tram elements 1
  • the total amount of the species may include 0.1% by weight or less.
  • the tramp element is an impurity element derived from slag, alloy iron and scrap used as a raw material in a steelmaking process, ladle and tundish refractory material, etc., and when the total exceeds 0.1%, the thin slab By cracking on the surface of the can be reduced the surface quality of the hot-rolled steel sheet.
  • the hot-rolled steel sheet of the present invention preferably contains a microstructure in which the sum of martensite and tempered martensite in an area fraction is 93% or more and ferrite is 7% or less (including 0%).
  • the martensite and 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 93%.
  • ferrite may be included in a range of 7% or less in order to secure ductility. However, if the fraction exceeds 7%, ductility increases, but strength may be difficult to secure.
  • the fraction of the sum of martensite and tempered martensite is more preferably 94% or more, and even more preferably 95% or more.
  • the hot rolled steel sheet of the present invention preferably has an average austenite grain boundy size of 4 to 20 ⁇ m.
  • the average size of the old austenite grains can affect not only the strength, but in particular the bending properties. If the average size of the old austenite grains exceeds 20 ⁇ m, it may be difficult to secure a target bending property. The smaller the average austenite grain size, the better the bending property and strength, but the older the austenite grains, the better. In order to refine the average grain size to less than 4 ⁇ m, expensive alloying elements such as Nb, Mo, and V must be added, which may cause a problem that manufacturing cost increases. Therefore, it is preferable that the old austenite grain size is 4 to 20 ⁇ m.
  • the lower limit of the average austenite grain average size is more preferably 6 ⁇ m, even more preferably 8 ⁇ m, the upper limit is more preferably 18 ⁇ m, and even more preferably 16 ⁇ m.
  • the hot-rolled steel sheet of the present invention provided as described above may have a yield strength of 1080 to 1600 MPa, a tensile strength of 1380 to 1800 MPa, and an elongation of 4.0% or more.
  • the yield strength is more preferably from 1100 to 1550 MPa, and even more preferably from 1150 to 1500 MPa.
  • the tensile strength is more preferably from 1400 to 1780 MPa, and even more preferably from 1450 to 1750 MPa.
  • the elongation is more preferably 4.5% or more, and even more preferably 5.0% or more.
  • the hot-rolled steel sheet of the present invention may have a thickness variation in the width direction of 10 to 60 ⁇ m.
  • the width variation in the width direction (Crown) means the difference between the thickest portion and the thinnest portion in the width direction.
  • the thickness variation in the width direction is more preferably 15 to 55 ⁇ m, and even more preferably 20 to 50 ⁇ m.
  • the smaller the thickness deviation in the width direction the better, but for this, additional equipment may be required, which may increase manufacturing cost.
  • the hot-rolled steel sheet of the present invention can secure excellent bending characteristics because no crack occurs during a 90° bending process test when the bending radius (R) divided by the steel plate thickness (t) is 2.9.
  • the bending radius (R) divided by the steel plate thickness (t) is about 3, it is evaluated to have excellent bending properties if cracking does not occur by testing 90° bending.
  • the thickness of the hot-rolled steel sheet of the present invention may be 0.6 to 1.4 mm, more preferably 0.7 to 1.3 mm, and even more preferably 0.8 to 1.2 mm.
  • 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 strength, bending characteristics, and shape quality.
  • FIG. 1 is a schematic diagram of a facility for a continuous casting-rolling direct connection process applicable to the production of hot rolled steel sheet of the present invention.
  • the ultra-high-strength hot-rolled steel sheet excellent in shape quality and bendability according to an embodiment of the present invention may be produced by applying a continuous casting-rolling direct connection facility as shown in FIG. 1.
  • the continuous casting-rolling direct connection facility is largely composed of a continuous casting machine 100, a rough rolling machine 400, and a finishing rolling machine 600.
  • the continuous casting-rolling direct connection facility is a high-speed continuous casting machine 100 producing a thin slab (a) having a first thickness, and a bar having a second thickness thinner than the first thickness (Bar) ) (b) includes a rough rolling mill (400), a finishing mill for rolling the bar having the second thickness into a strip (c) having a third thickness, and a winding machine (900) for winding the strip. can do.
  • the rough rolling mill breaker 300 in front of the rough rolling mill 400 (Roughing Mill Scale Breaker,'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, making it 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 view of a facility for a continuous casting-rolling direct connection process applicable to the production of a hot rolled steel sheet of the present invention.
  • the continuous casting-rolling direct connection facility disclosed in FIG. 2 is mostly the same in configuration as the facility disclosed in FIG. 1, but is equipped with a heater 200' that additionally heats the slab in front of the rough rolling mill 400 and the rough rolling scale breaker 300.
  • a heater 200' that additionally heats the slab in front of the rough rolling mill 400 and the rough rolling scale breaker 300.
  • the slab edge temperature the occurrence of edge defects is low, which is advantageous for securing the surface quality.
  • batch type rolling is possible.
  • the ultra-high-strength hot-rolled steel sheet excellent in shape quality and bendability of the present invention can be produced in both the continuous casting-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 4.0 to 7.5mpm (m/min).
  • the reason why the casting speed is 4.0MPm or more is that 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 bending characteristics, and the risk of material deviation in the width direction or the length direction increases.
  • the casting speed is preferably in the range of 4.0 to 7.5mpm.
  • the lower limit of the casting speed is more preferably 4.5mpm, and even more preferably 5.0mpm.
  • the upper limit of the casting speed is more preferably 7.0mpm, and even more preferably 6.5mpm.
  • the thickness of the thin slab is preferably 75 ⁇ 125mm.
  • the thickness of the thin slab exceeds 125mm, not only high-speed casting is difficult, but also the rolling load increases during rough rolling, and when it is less than 80mm, the temperature drop of the cast slab occurs rapidly and it is difficult to form a uniform structure.
  • the lower limit of the thickness of the thin slab is more preferably 80 mm, and even more preferably 85 mm.
  • the upper limit of the thickness of the thin slab is more preferably 120 mm, even more preferably 115 mm, and most preferably 110 mm.
  • the rough rolling step may be performed by rough rolling a continuously cast thin slab in a rough rolling mill composed of 2 to 5 rolling mills.
  • the bar edge temperature at the rough rolling exit side is preferably 850 to 1000°C.
  • the temperature of the bar edge is less than 850°C, a large amount of AlN precipitates and the like is generated, and as the high temperature ductility decreases, the susceptibility to edge cracking is very high.
  • the temperature of the bar edge portion is higher than 1000°C, the surface quality after pickling may deteriorate 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 at the side of the rough rolling during the rough rolling is 850 to 1000°C.
  • the lower limit of the temperature of the bar edge at the rough rolling side is more preferably 860°C, even more preferably 870°C, and most preferably 880°C.
  • the upper limit of the bar edge temperature at the rough rolling exit side is more preferably 990°C, more preferably 980°C, and most preferably 970°C.
  • a lubricant of 5 to 45 l/min per 1 m2 is sprayed onto the surface of the bar.
  • the lubricating oil injection is intended to reduce the rolling load by reducing the friction coefficient between the rolls of the first rolling mill having a very large rolling load among the above bars and the finishing mill. Therefore, it is preferable that the lubricating oil injection is made at the mouth of the large first rolling mill.
  • the amount of lubricant injection is less than 5 l/min per 1 m2 of the surface of the bar, there is a disadvantage in that it is difficult to reduce the thickness deviation (crown, crown) in the width direction because the effect of reducing the rolling load through reducing the friction coefficient is insufficient.
  • the injection amount of the lubricating oil exceeds 45 l/min per 1 m2 of the surface of the bar, the use of the lubricating oil is excessive and manufacturing cost may increase.
  • the upper limit of the injection amount of the lubricating oil is more preferably 40 l/min per 1 m2 of the surface of the bar, more preferably 35 l/min per 1 m2 of the surface of the bar, and the lower limit is more preferably 10 l/min per 1 m2 of the surface of the bar. , It is more preferable that it is 15 L/min per 1 m2 of the bar.
  • the bar in which the lubricant is injected is finish-rolled while controlling a pair cross angle of a rolling machine at a temperature of Ar3+10°C to Ar3+60°C to obtain a hot rolled steel sheet.
  • the temperature range of Ar3+10°C to Ar3+60°C is preferably the exit temperature during finish rolling.
  • 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 large amount of cornerstone ferrite can be formed, and after cooling, a sufficient martensite fraction cannot be obtained.
  • 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+15°C, and even more preferably Ar3+25°C.
  • the upper limit of the finish rolling exit temperature is more preferably Ar3+55°C, and even more preferably Ar3+50°C.
  • the finishing rolling (Finish rolling Mill, FM) may be performed in a finishing mill consisting of 3 to 6 rolling mills, and the rolling mill is more preferably made of 5 to 6 rolling mills.
  • the thickness of the width direction can be minimized by precisely controlling the pair cross angle of the finishing mill during the finishing rolling.
  • the first rolling mill having a very large rolling load and the last rolling mill having a relatively small rolling load do not significantly affect the thickness deviation control in the width direction even if the angle of the pair cross is controlled.
  • the pair cross angle means an angle formed by twisting the upper and lower rolls of the rolling mill.
  • the pair cross angle of the second rolling mill FM2 is preferably 0.30 to 0.80°, and the third rolling mill FM3
  • the pair cross angle of the pair is preferably 0.30 to 0.80°
  • the pair cross angle of the fourth mill FM4 is preferably 0.20 to 0.70°.
  • the thickness variation in the width direction can be minimized by controlling the fair cross angle of the rolling mill as described above.
  • the pair cross angle of the second rolling mill is more preferably 0.35 to 0.75°, and even more preferably 0.40 to 0.70°.
  • the pair cross angle of the third rolling mill is more preferably 0.35 to 0.75°, and even more preferably 0.40 to 0.70°.
  • the pair cross angle of the fourth rolling mill is more preferably 0.25 to 0.65°, and even more preferably 0.30 to 0.60°.
  • the hot-rolled steel sheet is cooled at a cooling rate of 30 to 400°C/s, and the cooled hot-rolled steel sheet is wound at Mf-200°C to Mf-50°C.
  • the cooling rate is less than 30°C/s or the coiling temperature exceeds Mf-50°C, ferrite and bainite may be formed, making it difficult to secure a sufficient martensite structure.
  • the lower limit of the cooling rate is more preferably 50°C/s, even more preferably 70°C/s, the upper limit is more preferably 350°C/s, and even more preferably 300°C/s.
  • the lower limit of the coiling temperature is more preferably Mf-180°C, even more preferably Mf-160°C, the upper limit is more preferably Mf-60°C, and even more preferably Mf-70°C.
  • the step of pickling the wound hot-rolled steel sheet may be further included, and after the step of the acid treatment, the step of plating the acid-treated hot rolled steel sheet may be further included.
  • a PO (Pickled & Oiled) material and a plating material can be obtained.
  • 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 and a plating material having excellent surface quality even with a general pickling treatment and plating treatment. Therefore, in the present invention, any method that is generally used in the hot acid pickling process and the plating process is applicable, so the pickling treatment and plating method are not particularly limited.
  • a rolling-rolled steel sheet having a thickness of 1.2 mm was manufactured under the manufacturing conditions shown in Table 2 by applying a direct rolling process. After pickling the hot-rolled steel sheet to obtain a PO material, after measuring the microstructure, tensile properties, bending properties and thickness deviation, the results are shown in Table 3 below.
  • Ar3 (austenitic transformation temperature) and Mf (martensitic transformation termination temperature) in Table 2 below are values calculated using a commercial thermodynamic software, JmatPro V-8.
  • the microstructure was observed with a scanning electron microscope (SEM) and a transmission electron microscope (TEM) to measure the area fractions of martensite (M), tempered martensite (T.M) and ferrite (F).
  • SEM scanning electron microscope
  • TEM transmission electron microscope
  • ALS former austenite grain boundy size
  • Yield strength (YS), tensile strength (TS), and elongation (EL)) are obtained according to JIS No. 5 standards in the rolling direction (L direction) for the entire width of the strip (at regular intervals (7 locations)) It was represented by the average value measured.
  • the bending properties were evaluated by whether or not cracks occurred after performing a 90° bending process test so that the values obtained by dividing the bending radius (R) by the steel plate thickness (t) were 2.9 and 3.3.
  • the thickness variation in the width direction was measured by measuring the thickness of the thickest portion and the thickness of the thinnest portion in the width direction, and expressed as the difference value.
  • the performance-rolling direct connection process was applied to prepare a hot-rolled steel sheet having a thickness of 1.2 mm under the manufacturing conditions shown in Table 4 below.
  • the microstructure, tensile properties, bending properties and thickness deviations were measured, and the results are shown in Table 5 below. Measurement of the microstructure, tensile properties, bending properties and thickness deviation was performed in the same manner as in Example 1.
  • Comparative Example 14 which satisfies the alloy composition and the relational formulas 1 to 3 proposed by the present invention, but does not satisfy the conditions of the present invention due to the high coiling temperature among the manufacturing conditions, a sufficient martensite + tempered martensite structure is not secured. Accordingly, it can be seen that the strength is low.
  • FIG. 3 is a microstructure photograph of Inventive Example 9 according to an embodiment of the present invention observed with EBSD. As can be seen through Figure 3, Inventive Example 9 can be confirmed that the old austenite grain average size is 11 ⁇ m fine.
  • FIG. 4 is a microstructure photograph of Inventive Example 9 according to an embodiment of the present invention observed with a scanning electron microscope (SEM). As can be seen through FIG. 4, it can be seen that in Example 9, some ferrite structures are formed, but mainly have martensite + tempered martensite structures.
  • FIG. 5 is a microstructure photograph of Inventive Example 9 according to an embodiment of the present invention observed with a transmission electron microscope (TEM). As can be seen through FIG. 5, Inventive Example 9 confirms the presence of tempered martensite from the presence of carbides between the laths.
  • TEM transmission electron microscope
  • FIG. 6 is a microstructure photograph observed in Comparative Example 11 according to an embodiment of the present invention with a scanning electron microscope (SEM), and FIG. 7 is a scanning electron microscope (SEM) in Comparative Example 15 according to an embodiment of the present invention. It is a microstructure photograph observed with. As can be seen through Figures 6 and 7, in Comparative Examples 11 and 15, it can be confirmed that a large amount of ferrite structure.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
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  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

La présente invention concerne une tôle d'acier laminée à chaud à ultra-haute résistance ayant une excellente qualité de forme et une excellente aptitude au pliage, et son procédé de fabrication. Un mode de réalisation de la présente invention concerne : une tôle d'acier laminée à chaud à ultra-haute résistance ayant une excellente qualité de forme et une excellente aptitude au pliage, et son procédé de fabrication, la tôle d'acier laminée à chaud à ultra-haute résistance contenant, en % en poids, 0,16-0,26 % de C, 0,75-1,50 % de Mn, 0,0009-0,0050 % de B, 0,009-0,070 % de Ti, et 0,001-0,010 % de N, le reste comprenant Fe et des impuretés inévitables diverses, les expressions relationnelles 1 à 3 ci-dessous étant satisfaites, et la somme de martensite et de martensite revenue constituant au moins 93 % et la ferrite représentant 7 % ou moins (y compris 0 %) de la microstructure par pourcentage de surface. [expression relationnelle 1] 0,32 ≤ (C+0,2Mn) ≤ 0,53 [expression relationnelle 2] 130 ≤ (C+0,2Mn)/B ≤ 450 [expression relationnelle 3] 5 ≤ (C+0,2Mn)/Ti ≤ 45 (dans les expressions relationnelles 1 à 3, les teneurs en C, Mn, B et Ti sont basées sur des % en poids)
PCT/KR2019/015934 2018-11-26 2019-11-20 Tôle d'acier laminée à chaud à ultra-haute résistance ayant une excellente qualité de forme et une excellente aptitude au pliage, et son procédé de fabrication WO2020111639A1 (fr)

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