WO2020111639A1 - Ultra-high strength hot rolled steel sheet having excellent shape quality and bendability, and method for manufacturing same - Google Patents

Ultra-high strength hot rolled steel sheet having excellent shape quality and bendability, and method for manufacturing same Download PDF

Info

Publication number
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
Authority
WO
WIPO (PCT)
Prior art keywords
steel sheet
rolled steel
ultra
rolling
bendability
Prior art date
Application number
PCT/KR2019/015934
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
Publication of WO2020111639A1 publication Critical patent/WO2020111639A1/en

Links

Images

Classifications

    • 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.

Landscapes

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

Abstract

The present invention pertains to an ultra-high strength hot rolled steel sheet having excellent shape quality and bendability, and a method for manufacturing same. An embodiment of the present invention provides: an ultra-high strength hot rolled steel sheet having excellent shape quality and bendability, and a method for manufacturing same, the ultra-high strength hot rolled steel sheet containing, in weight %, 0.16-0.26% of C, 0.75-1.50% of Mn, 0.0009-0.0050% of B, 0.009-0.070% of Ti, and 0.001-0.010% of N, with the remainder comprising Fe and miscellaneous inevitable impurities, wherein relational expressions 1 to 3 below are satisfied, and the sum of martensite and tempered martensite constitutes at least 93% and ferrite constitutes 7% or less (including 0%) of the microstructure by area percentage. [relational expression 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53 [relational expression 2] 130 ≤ (C+0.2Mn)/B ≤ 450 [relational expression 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45 (in relational expressions 1 to 3, the contents of C, Mn, B, and Ti are based on wt%)

Description

형상 품질 및 굽힘성이 우수한 초고강도 열연강판 및 그 제조방법Ultra-high-strength hot-rolled steel sheet excellent in shape quality and bendability and its manufacturing method
본 발명은 형상 품질 및 굽힘성이 우수한 초고강도 열연강판 및 그 제조방법에 관한 것이다.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.
국제 환경 규제 강화 및 자동차 연비규제 강화 추세에 따라 차체 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 강도와 동시에 롤 포밍(Roll Forming) 성형하기 때문에 우수한 굽힘 가공성 및 형상 품질이 요구된다.According to 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 body, and development of ultra-high-strength steel sheets having a level of 1.0 GPa or more is actively being progressed. The high-strength hot-rolled steel sheet used in most automobile body reinforcement materials such as bumper reinforcement and door impact beam is required to have excellent bending processability and shape quality because of high strength and roll forming.
이러한 물성을 만족시키기 위하여 자동차 구조부재용 강판은 기본적으로 페라이트(Ferrite), 베이나이트(Baintie), 마르텐사이트(Martensite), 및 템퍼드 마르텐사이트(Tempered Martensite) 상의 조합으로 구성되며, 이들 상의 구성 비율에 따라, DP(Dual Phase)강, TRIP(Transformation Induced Plasticity)강, 복합조직(Complex Phase)강, MART강 등으로 분류되어 적용되고 있다In order to satisfy these properties, 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.
이 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 초고강도에 따른 연신율의 감소는 피할 수 없어 열간압연 후 냉연 및 연속 소둔 열처리(CAL, Continuous Annealing Line)를 거치거나 또는 열간압연 후 급속냉각 및 가공을 수행하는 HPF(Hot Press Forming)와 같은 공정을 거쳐야 하기 때문에 제조비가 상승하는 단점이 있다. 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.
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 representative examples thereof are Patent Documents 1 to 5.
특허문헌 1은 중량%로 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 제조시 불가피하게 함유되는 원소를 포함한 알루미늄킬드강을 1050~1300℃에서 균질화 처리 후 Ar3 변태점 직상인 850~950℃에서 마무리 열간압연한 다음 550~650℃에서 열연권취하는 단계; 상기 강판을 30~80%의 냉간압하율로 냉간압연한 다음 A3 온도 이상에서 연속소둔하는 단계; 및 상기 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.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 a method for manufacturing a tensile strength 1.2GPa grade ultra-high strength cold rolled steel sheet for an automobile bumper reinforcement comprising the steps of:
특허문헌 2는 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 방법을 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 개시하고 있다.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. In addition, after quenching the steel sheet at a high temperature using a continuous annealing heat treatment facility, by over-aging in a temperature range of 150 to 200°C, plate shape defects due to tempering treatment after normal water quenching (width of the steel sheet) It is also disclosed that the direction deformation) can be improved.
특허문헌 3은 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 단축 평균 길이가 1㎛ 이하이며, 평균 축비(장축/단축)가 5 이상인 내수소취화 특성을 갖는 인장강도 1470MPa급 초고강도 냉연강판 제조 방법에 관하여 소개하고 있다.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℃, cooling at a cooling rate of 3℃/s or higher, and maintaining a constant temperature in the range of (Ms-100℃) to Bs (vanite start temperature), the phase fraction of retained austenite before processing is 10% or more. In addition, 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 µm or less and an average axial ratio (long axis/short axis) of 5 or more is introduced.
특허문헌 4는 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.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℃/s [(Ac3-90 ℃)~(Ac3±15℃)], then heated to a temperature range of 500~750℃ at a cooling rate of 1~3℃/s, and a cooling rate of 3~50℃/s Cold rolling with a tensile strength of 1.5 GPa through a continuous annealing step of secondary cooling to the temperature range of [(Ms-120)~460℃] and then constant temperature transformation for 6~500sec or slow cooling at a cooling rate of 1℃/s or less It introduces how to manufacture 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 있다.However, according to 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.
또한, 특허문헌 5는 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 냉연강판을 열간 프레스 성형을 하여 인장강도가 1.8GPa 이상인 강판 부재의 제조방법에 대해 개시하고 있다.In addition, 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.
그러나, 특허문헌 5에 따를 경우 1.8GPa의 초고강도는 확보할 수 있으나, 냉연강판을 열간 프레스 성형(Hot Press Forming)해야 하기 때문에 제조 단가가 더욱 높아지는 문제점이 있다.However, according to 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.
따라서, 기존의 초고강도 냉연강판 및 열간성형강을 대체 가능할 뿐만 아니라, 보다 우수한 인장강도, 굽힘특성 및 형상 품질을 확보할 수 있고 획기적으로 제조 단가를 낮출 수 있는 초고강도 열연강판 및 그 제조방법에 대한 개발이 요구되고 있는 실정이다.Therefore, it is possible to replace the existing ultra-high strength cold rolled steel sheet and hot-formed steel, as well as to obtain superior tensile strength, bending characteristics and shape quality, and to significantly reduce the manufacturing cost of the ultra-high strength hot rolled steel sheet and its manufacturing method. Korea is in need of development.
(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) 국제 공개특허공보 제2007-129676호(Patent Document 5) International Publication No. 2007-129676
본 발명의 일 측면은 연속주조-압연 직결 공정에서 연연속압연 모드를 이용하여 열연 공정만으로도 형상 품질 및 굽힘성이 우수한 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 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.
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: 0.16~0.26%, Mn: 0.75~1.50%, B: 0.0009~0.0050%, Ti: 0.009~0.070%, N: 0.001~0.010%, 잔부 Fe 및 기타 불가피한 불순물을 포함하고, 하기 관계식 1 내지 3을 만족하며, 면적분율로 마르텐사이트와 템퍼드 마르텐사이트의 합이 93%이상이고, 페라이트가 7%이하(0%를 포함)인 미세조직을 포함하는 형상 품질 및 굽힘성이 우수한 초고강도 열연강판을 제공한다.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%). Provided is an ultra-high strength hot rolled steel sheet having excellent shape quality and bendability.
[관계식 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53[Relationship 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53
[관계식 2] 130 ≤ (C+0.2Mn)/B ≤ 450[Relationship 2] 130 ≤ (C+0.2Mn)/B ≤ 450
[관계식 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45[Relationship 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45
(단, 상기 관계식 1 내지 3의 C, Mn, B, Ti의 함량은 중량%임.)(However, the content of C, Mn, B, Ti of the above formula 1 to 3 is weight%.)
본 발명의 다른 실시형태는 중량%로, C: 0.16~0.26%, Mn: 0.75~1.50%, B: 0.0009~0.0050%, Ti: 0.009~0.070%, 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 바의 표면에 1m²당 5~45ℓ/min의 윤활유를 분사하는 단계; 상기 윤활유가 분사된 바를 Ar3+10℃~Ar3+60℃의 온도에서 압연기의 페어 크로스(pair cross) 각도를 제어하면서 마무리 압연하여 열연강판을 얻는 단계; 상기 열연강판을 30~400℃/s의 냉각속도로 냉각하는 단계; 상기 냉각된 열연강판을 Mf-200℃~Mf-50℃에서 권취하는 단계를 포함하며, 상기 각 단계는 연속주조-압연 직결공정을 통해 연속적으로 행해지는 것을 특징으로 하는 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법을 제공한다.Other embodiments 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 Continuously casting molten steel containing inevitable impurities and satisfying the following relations 1 to 3 to obtain a thin slab; Rough rolling the thin slab to obtain a bar; Injecting a lubricant of 5 ~ 45ℓ / min per 1m² on the surface of the bar; Obtaining a hot rolled steel sheet by finishing rolling the bar where the lubricant is injected while controlling a pair cross angle of a rolling machine at a temperature of Ar3+10°C to Ar3+60°C; Cooling the hot rolled steel sheet at a cooling rate of 30 to 400°C/s; And winding the cooled hot-rolled steel sheet at Mf-200°C to Mf-50°C, each of which is continuously performed through a continuous casting-rolling direct connection process, and is excellent in shape quality and bendability. Provides a method for manufacturing ultra-high strength hot rolled steel sheet.
[관계식 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53[Relationship 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53
[관계식 2] 130 ≤ (C+0.2Mn)/B ≤ 450[Relationship 2] 130 ≤ (C+0.2Mn)/B ≤ 450
[관계식 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45[Relationship 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45
(단, 상기 관계식 1 내지 3의 C, Mn, B, Ti의 함량은 중량%임.)(However, the content of C, Mn, B, Ti of the above formula 1 to 3 is weight%.)
본 발명의 일측면에 따르면, 합금조성과 제조조건을 적절히 제어하고, 연속주조-압연 직결 공정에서 연연속압연 모드를 이용하여 열연 공정만으로도 형상 품질 및 굽힘성이 우수한 초고강도 열연강판을 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, it is possible to provide an ultra-high strength hot-rolled steel sheet having excellent shape quality and bending property by using a continuous rolling mode in a continuous casting-rolling direct connection process, appropriately controlling alloy composition and manufacturing conditions. have. 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, it is possible to use steel that melts scraps of scrap metal or the like in an electric furnace through the thin slab playing method, thereby increasing the recyclability of resources.
도 1은 본 발명의 열연강판 제조에 적용 가능한 연속주조-압연 직결공정을 위한 설비의 모식도이다. 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.
도 2는 본 발명의 열연강판 제조에 적용 가능한 연속주조-압연 직결공정을 위한 설비의 또 다른 모식도이다.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.
도 3은 본 발명의 일 실시예에 따른 발명예 9를 EBSD로 관찰한 미세조직 사진이다.FIG. 3 is a microstructure photograph of Inventive Example 9 according to an embodiment of the present invention observed with EBSD.
도 4는 본 발명의 일 실시예에 따른 발명예 9를 주사전자현미경(SEM)으로 관찰한 미세조직 사진이다.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).
도 5는 본 발명의 일 실시예에 따른 발명예 9를 투과전자현미경(TEM)으로 관찰한 미세조직 사진이다.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).
도 6은 본 발명의 일 실시예에 따른 비교예 11을 주사전자현미경(SEM)으로 관찰한 미세조직 사진이다.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).
도 7은 본 발명의 일 실시예에 따른 비교예 15를 주사전자현미경(SEM)으로 관찰한 미세조직 사진이다.7 is a microstructure photograph of Comparative Example 15 according to an embodiment of the present invention observed with a scanning electron microscope (SEM).
이하, 본 발명의 일 실시형태에 따른 형상 품질 및 굽힘성이 우수한 초고강도 열연강판에 대하여 설명한다. 먼저, 본 발명의 합금조성을 설명한다. 하기 설명되는 합금조성의 함량은 별도의 언급이 없는 한 중량%를 의미한다.Hereinafter, an ultra-high strength hot rolled steel sheet excellent in shape quality and bendability 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 means weight% unless otherwise specified.
C: 0.16~0.26%C: 0.16 to 0.26%
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 0.26% 초과인 경우에는 용접성 및 과도한 강도 상승으로 굽힘 가공성이 저하되는 문제점이 있다. 따라서, C 함량은 0.16~0.26%인 것이 바람직하다. 상기 C 함량의 하한은 0.17%인 것이 보다 바람직하고, 0.18%인 것이 보다 더 바람직하다. 상기 C 함량의 상한은 0.25%인 것이 보다 바람직하고, 0.24%인 것이 보다 더 바람직하다.Carbon (C) is a very important element that increases strength by making microstructure martensite when quenching 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 0.26%, 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%.
Mn: 0.75~1.50%Mn: 0.75~1.50%
망간(Mn)은 페라이트 형성을 억제하며, 오스테나이트 안정성을 높여 저온 변태상의 형성을 용이하게 함으로써 강도를 증가시킨다. Mn 함량이 0.75% 미만인 경우에는 본 발명에서 목표로 하는 강도 확보가 어려울 수 있다. 반면에 Mn 함량이 1.50% 초과인 경우에는 슬라브 및 열연강판의 내부 또는 외부나 이들 모두에 편석대를 형성시켜 크랙의 발생과 전파를 유발해 강판의 최종 품질을 저하시키고, 용접성 및 굽힘 가공성를 열위하게 할 수 있다. 따라서, Mn 함량은 0.75~1.50%인 것이 바람직하다. 상기 Mn 함량의 하한은 0.80%인 것이 보다 바람직하고, 0.85%인 것이 보다 더 바람직하다. 상기 Mn 함량의 상한은 1.40%인 것이 보다 바람직하고, 1.30%인 것이 보다 더 바람직하다. Manganese (Mn) inhibits ferrite formation and increases austenite stability to increase the strength by facilitating the formation of a low-temperature transformation phase. When the Mn content is less than 0.75%, it may be difficult to secure the target strength in the present invention. On the other hand, when the 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: 0.0009~0.0050%B: 0.0009~0.0050%
보론(B)은 강의 경화능을 증가시키는 원소이다. B 함량이 0.0009% 미만인 경우 상기 효과를 얻을 수 없으며, 0.0050%를 초과하게 되면 오스테나이트 재결정 온도를 상승시키며 용접성을 저하시킨다. 따라서, B의 함량은 0.0009~0.0050%인 것이 바람직하다. 상기 B 함량의 하한은 0.0010%인 것이 보다 바람직하고, 0.0015%인 것이 보다 더 바람직하다. 상기 B 함량의 상한은 0.0045%인 것이 보다 바람직하고, 0.0040%인 것이 보다 더 바람직하다. Boron (B) is an element that increases the hardenability of steel. When the B content is less than 0.0009%, the above effect cannot be obtained, and when it exceeds 0.0050%, the austenite recrystallization temperature increases and the weldability decreases. Therefore, 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%.
Ti: 0.009~0.070%Ti: 0.009~0.070%
티타늄(Ti)은 석출물 및 질화물 형성원소로서, 압연 중에 TiC, TiN, Ti(C,N)으로 석출되어 오스테나이트 결정립 사이즈를 감소시킴으로써 석출강화 및 결정립 미세화를 통해 강의 강도 및 굽힘 특성을 향상시키는 원소이다. 또한 Ti은 응고온도 근처에서 TiN의 형성을 통해 고용 N를 제거하여 AlN 등의 석출물 형성을 감소시킴으로써 고온연성 저하를 방지하여 에지(Edge) 크랙 발생 민감성을 감소시키는 원소이다. Ti 함량이 0.009% 미만인 경우에는 미세한 AlN 또는 BN 석출물의 과다 석출에 의해 슬라브의 연성 감소를 초래하여 슬라브 품질을 저하시킬 수 있고, TiN, TiC, Ti(C,N) 석출물의 형성이 적어 오스테나이트 결정립 사이즈 미세화에 한계가 있어 굽힘 특성이 열위해질 수 있다. 반면에 Ti 함량이 0.070% 초과인 경우에는 조대한 TiN, TiC, Ti(C,N) 석출물의 형성에 따른 결정립 미세화 효과를 기대하기 어려울 뿐만 아니라 제조 비용이 상승한다. 따라서 Ti 함량은 0.009~0.070%인 것이 바람직하다. 상기 Ti 함량의 하한은 0.012%인 것이 보다 바람직하고, 0.015%인 것이 보다 더 바람직하다. 상기 Ti 함량의 상한은 0.060%인 것이 보다 바람직하고, 0.050%인 것이 보다 더 바람직하다. 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. In addition, 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. When the Ti content is less than 0.009%, excessive precipitation of fine AlN or BN precipitates may lead to a decrease in the ductility of the slab and deteriorate the slab quality, and there is little formation of TiN, TiC, Ti(C,N) precipitates, resulting in less austenite. Since there is a limit to the refinement of the grain size, the bending characteristics may be inferior. On the other hand, when the Ti content is more than 0.070%, it is difficult to expect a grain refinement effect due to the formation of coarse TiN, TiC, and Ti(C,N) precipitates, and the manufacturing cost increases. Therefore, 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: 0.001~0.010%N: 0.001 to 0.010%
질소(N)는 오스테나이트 안정화에 기여하고 질화물을 형성하여 결정립 미세화를 통해 강의 강도 및 굽힘 특성을 향상시키는 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 0.002%인 것이 보다 바람직하고, 0.003%인 것이 보다 더 바람직하다. 상기 N 함량의 상한은 0.008%인 것이 보다 바람직하고, 0.006%인 것이 보다 더 바람직하다. Nitrogen (N) is an element that contributes to the stabilization of austenite and forms nitrides to improve the strength and bending properties of steel through grain refinement. 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, it is preferable that the N content is 0.001 to 0.010%. 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%.
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, 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.
한편, 본 발명의 열연강판은 전술한 합금성분 중 C, Mn, B 및 Ti가 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 C, Mn, B, Ti의 함량은 중량%이다.On the other hand, in the hot-rolled steel sheet of the present invention, it is preferable that 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. However, the contents of C, Mn, B, and Ti described in relations 1 to 3 below are weight%.
[관계식 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53[Relationship 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53
상기 관계식 1은 본 발명이 얻고자 하는 강도를 확보하기 위한 성분관계식이다. 상기 관계식 1의 값이 0.32 미만인 경우에는 본 발명이 목표로 하는 강도를 확보하기 곤란하고, 0.53을 초과할 경우에는 연신율이 낮아져 가공시 크랙이 발생할 수 있다. 따라서, 상기 관계식 1의 값은 0.32~0.53의 범위를 갖는 것이 바람직하다. 상기 관계식 1의 값의 하한은 0.33인 것이 보다 바람직하고, 0.35인 것이 보다 더 바람직하다. 상기 관계식 1의 값의 상한은 0.52인 것이 보다 바람직하고, 0.50인 것이 보다 더 바람직하다. The relational expression 1 is a component relational expression for securing the strength desired by the present invention. When 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.
[관계식 2] 130 ≤ (C+0.2Mn)/B ≤ 450[Relationship 2] 130 ≤ (C+0.2Mn)/B ≤ 450
상기 관계식 2는 본 발명이 목표로 하는 강도를 확보하기 위한 성분관계식으로 오스테나이트 촉진 원소인 C, Mn과 경화능이 우수한 B 사이의 관계식이다. 상기 관계식 2의 값이 130 미만인 경우에는 본 발명이 목표로 하는 강도를 확보하기 곤란하고, 450을 초과할 경우에는 연신율이 낮아져 가공시 크랙이 발생할 수 있다. 따라서, 상기 관계식 2의 값은 130~450의 범위를 갖는 것이 바람직하다. 상기 관계식 2의 값의 하한은 140인 것이 보다 바람직하고, 150인 것이 보다 더 바람직하다. 상기 관계식 2의 값의 상한은 400인 것이 보다 바람직하고, 350인 것이 보다 더 바람직하다. 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. When 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.
[관계식 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45[Relationship 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45
상기 관계식 3은 본 발명이 목표로 하는 강도 및 우수한 굽힘 특성을 확보하기 위한 성분관계식이다. 상기 관계식 3의 값이 5 미만인 경우에는 Ti이 과다 첨가된 경우로 조대한 TiN, TiC, Ti(C,N) 석출물의 형성에 따른 결정립 미세화 효과를 기대하기 어려울 뿐만 아니라 제조 비용이 상승할 수 있으며, 45를 초과하는 경우에는 C, Mn이 과다 첨가하거나, Ti이 적게 첨가되는 경우로서 연신율 확보가 어렵고, TiN, TiC, Ti(C,N) 석출물의 형성이 적어 오스테나이트 결정립 사이즈 미세화에 한계가 있어 굽힘 특성이 열위해질 수 있다. 따라서, 상기 관계식 3의 값은 5~45의 범위를 갖는 것이 바람직하다. 상기 관계식 3의 값의 하한은 10인 것이 보다 바람직하고, 15인 것이 보다 더 바람직하다. 상기 관계식 3의 값의 상한은 40인 것이 보다 바람직하고, 35인 것이 보다 더 바람직하다. The relational expression 3 is a component relational expression for securing the strength and excellent bending characteristics targeted by the present invention. When 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. , When 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.
한편, 본 발명의 열연강판은 트램프 원소로서 P, S, Si, Nb, V, 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 슬래그(slag), 합금철 및 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 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.
본 발명의 열연강판은 면적분율로 마르텐사이트와 템퍼드 마르텐사이트의 합이 93% 이상이고, 페라이트가 7%이하(0%를 포함)인 미세조직을 포함하는 것이 바람직하다. 상기 마르텐사이트와 템퍼드 마르텐사이트 조직은 본 발명이 목포로 하는 강도를 얻기 위한 필수적인 조직으로서, 그 분율이 93% 미만일 경우에는 강도 확보가 곤란하다. 본 발명에서는 연성 확보를 위하여 페라이트를 7% 이하의 범위로 포함할 수 있으며, 다만, 그 분율이 7%를 초과하는 경우에는 연성은 증가하나 강도 확보에 어려움이 있을 수 있다. 한편, 상기 마르텐사이트와 템퍼드 마르텐사이트의 합의 분율은 94% 이상인 것이 보다 바람직하고, 95% 이상인 것이 보다 더 바람직하다.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%. In the present invention, 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. On the other hand, the fraction of the sum of martensite and tempered martensite is more preferably 94% or more, and even more preferably 95% or more.
본 발명의 열연강판은 구 오스테나이트 결정립 평균 크기(Prior austenite grain boundy size)가 4~20㎛인 것이 바람직하다. 상기 구 오스테나이트 결정립 평균 크기는 강도 뿐만 아니라 특히, 굽힘 특성에 영향을 미칠 수 있다. 상기 구 오스테나이트 결정립 평균 크기가 20㎛를 초과하는 경우에는 목표로 하는 굽힘 특성을 확보하기 어려울 수 있으며, 구 오스테나이트 결정립 평균 크기는 작으면 작을수록 굽힘 특성 및 강도 확보에 유리하지만, 구 오스테나이트 결정립 평균 크기를 4㎛미만으로 미세화하기 위해서는 고가의 합금원소인 Nb, Mo, V 등을 첨가해야 하기 때문에 제조원가가 상승하는 문제가 발생할 수 있다. 따라서, 상기 구 오스테나이트 결정립 사이즈는 4~20㎛인 것이 바람직하다. 상기 구 오스테나이트 결정립 평균 크기의 하한은 6㎛인 것이 보다 바람직하고, 8㎛인 것이 보다 더 바람직하며, 상한은 18㎛인 것이 보다 바람직하고, 16㎛인 것이 보다 더 바람직하다.The hot rolled steel sheet of the present invention preferably has an average austenite grain boundy size of 4 to 20 µm. 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 μm, 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 µm, 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 µm. The lower limit of the average austenite grain average size is more preferably 6 μm, even more preferably 8 μm, the upper limit is more preferably 18 μm, and even more preferably 16 μm.
상기와 같이 제공되는 본 발명의 열연강판은 항복강도가 1080~1600MPa, 인장강도가 1380~1800MPa, 연신율이 4.0%이상일 수 있다. 상기 항복강도는 1100~1550MPa인 것이 보다 바람직하고, 1150~1500MPa인 것이 보다 더 바람직하다. 상기 인장강도는 1400~1780MPa인 것이 보다 바람직하고, 1450~1750MPa인 것이 보다 더 바람직하다. 상기 연신율은 4.5%이상인 것이 보다 바람직하고, 5.0%이상인 것이 보다 더 바람직하다. 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.
또한, 본 발명의 열연강판은 폭 방향 두께 편차가 10~60㎛일 수 있다. 상기 폭 방향 두께 편차(Crown)는 폭 방향으로 두께가 가장 두꺼운 부분과 얇은 부분의 차이를 의미한다. 상기 폭 방향 두께 편차는 15~55㎛인 것이 보다 바람직하고, 20~50㎛인 것이 보다 더 바람직하다. 한편, 상기 폭 방향 두께 편차는 작으면 작을수록 좋으나, 이를 위해서는 추가적인 설비가 필요할 수 있어 제조원가가 상승할 수 있다.In addition, the hot-rolled steel sheet of the present invention may have a thickness variation in the width direction of 10 to 60 μm. 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 μm, and even more preferably 20 to 50 μm. On the other hand, the smaller the thickness deviation in the width direction, the better, but for this, additional equipment may be required, which may increase manufacturing cost.
더하여, 본 발명의 열연강판은 굽힘반경(R)을 강판 두께(t)로 나눈 값이 2.9 일 때 90° 굽힘가공 시험시 크랙이 발생하지 않아 우수한 굽힘특성을 확보할 수 있다. 통상적으로 굽힘반경(R)을 강판 두께(t)로 나눈 값이 약 3일 때 90° 굽힘가공 시험하여 크랙이 발생하지 않으면 우수한 굽힘 특성을 갖는 것으로 평가된다.In addition, 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. Typically, when 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.
본 발명의 열연강판의 두께는 0.6~1.4mm일 수 있으며, 보다 바람직하게는 0.7~1.3mm일 수 있고, 보다 더 바람직하게는 0.8~1.2mm일 수 있다. 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.
전술한 바와 같이, 본 발명의 열연강판은 우수한 강도, 굽힘 특성 및 형상 품질을 가짐으로써 초고강도 냉연강판 및 열간성형강을 효과적으로 대체할 수 있다.As described above, 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.
이하, 본 발명의 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법의 일 실시형태에 대하여 설명한다. Hereinafter, one embodiment of a method for manufacturing an ultra-high strength hot rolled steel sheet having excellent shape quality and bendability according to the present invention will be described.
도 1은 본 발명의 열연강판 제조에 적용 가능한 연속주조-압연 직결공정을 위한 설비의 모식도이다. 본 발명의 일 실시형태에 따른 형상 품질 및 굽힘성이 우수한 초고강도 열연강판은 도 1과 같은 연속주조-압연 직결 설비를 적용하여 생산될 수 있다. 연속주조-압연 직결 설비는 크게 연속주조기(100), 조압연기(400), 마무리 압연기(600)로 구성된다. 상기 연속주조-압연 직결 설비는 제1두께를 갖는 박 슬라브(Slab)(a)를 생산하는 고속 연속주조기(100)와, 상기 박 슬라브를 상기 제1두께보다 얇은 제2두께를 갖는 바(Bar)(b)로 압연시키는 조압연기(400), 상기 제2두께를 갖는 바를 제3두께를 갖는 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 열연강판을 산세시 표면품질이 우수한 PO(Pickled & Oiled)강판의 생산이 가능하다. 또한, 연속주조-압연 직결공정으로 등온등속압연이 가능하여 강판 폭, 길이 방향 온도 편차가 현저히 낮아 ROT[Run Out Table(700)](이하 "런아웃 테이블")에서 정밀 냉각제어가 가능하여 재질이 균일한 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 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. In addition, 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. In addition, it is possible to perform isothermal constant-speed rolling through a continuous casting-rolling direct connection process, so that the temperature variation in the width and length of the steel sheet is remarkably low, allowing precise cooling control in the ROT [Run Out Table (700)] (hereinafter referred to as "run-out table"). Uniform ultra-high strength hot rolled steel sheet can be produced. The strip, which has been rolled and cooled, is cut by the high-speed shearing machine 800 and wound up by the winding machine 900 to be produced as a product. Meanwhile, in front of the finish rolling scale breaker 500, a heater 200 for additionally heating the bar may be provided.
도 2는 본 발명의 열연강판 제조에 적용 가능한 연속주조-압연 직결공정을 위한 설비의 또 다른 모식도이다. 도 2에 개시된 연속주조-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.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. As it is easy to secure the slab edge temperature, the occurrence of edge defects is low, which is advantageous for securing the surface quality. In addition, since the space of at least one slab is secured before the rough rolling machine, batch type rolling is possible.
본 발명의 형상 품질 및 굽힘성이 우수한 초고강도 열연강판은 도 1 및 2에 개시된 연속주조-압연 직결 설비에서 모두 생산이 가능하다.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.
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 4.0~7.5mpm(m/min)의 주조속도로 행하는 것이 바람직하다. 주조속도를 4.0mpm 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 굽힘 특성 확보가 어려울 뿐만 아니라, 폭 방향 또는 길이 방향으로의 재질편차가 발생할 위험성이 커지게 된다. 만약 7.5mpm을 초과하는 경우에는 용강 탕면 불안정에 의해 조업 성공율이 저감될 수 있으므로, 상기 주조속도는 4.0~7.5mpm의 범위를 갖는 것이 바람직하다. 상기 주조속도의 하한은 4.5mpm인 것이 보다 바람직하고, 5.0mpm인 것이 보다 더 바람직하다. 상기 주조속도의 상한은 7.0mpm인 것이 보다 바람직하고, 6.5mpm인 것이 보다 더 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 4.0 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. 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 bending characteristics, and the risk of material deviation in the width direction or the length direction increases. If it exceeds 7.5mpm, 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 4.0 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.
한편, 상기 박 슬라브는 두께는 75~125mm인 것이 바람직하다. 상기 박 슬라브의 두께가 125mm를 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 75~125mm로 제어하는 것이 바람직하다. 상기 박 슬라브의 두께의 하한은 80mm인 것이 보다 바람직하고, 85mm인 것이 보다 더 바람직하다. 상기 박 슬라브의 두께의 상한은 120mm인 것이 보다 바람직하고, 115mm인 것이 보다 더 바람직하며, 110mm인 것이 가장 바람직하다.On the other hand, the thickness of the thin slab is preferably 75 ~ 125mm. When 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. In order to solve this, it is possible to additionally install a heating facility, but this is a factor that improves the production cost, so it is desirable to exclude as much as possible. Therefore, it is preferable to control the thickness of the thin slab to 75 to 125 mm. 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.
이후, 상기 박 슬라브를 조압연하여 바(Bar)를 얻는다. 상기 조압연 단계는 연속주조된 박 슬라브를 2~5개의 압연기로 구성된 조압연기에서 조압연함으로써 수행될 수 있다. Thereafter, the thin slab is rough-rolled to obtain a bar. 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.
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 exit side is preferably 850 to 1000°C. When 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. On the other hand, when 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. In the rough rolling, 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. In the rough rolling, 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.
이후, 상기 바의 표면에 1m²당 5~45ℓ/min의 윤활유를 분사한다. 상기 윤활유 분사는 상기 바와 마무리압연기 중 압연하중이 아주 큰 첫 번째 압연기의 롤 간의 마찰계수를 감소시켜 압연 하중을 감소시키기 위한 것이다. 따라서, 상기 윤활유 분사는 상기 큰 첫 번째 압연기의 입측에서 이루어지는 것이 바람직하다. 상기 윤활유 분사량이 바의 표면 1m²당 5ℓ/min 미만일 경우에는 마찰계수 감소를 통한 압연 하중 감소 효과가 충분하지 않아 폭 방향 두께 편차(크라운, Crown)를 감소 시키기 어렵다는 단점이 있다. 반면, 상기 윤활유 분사량이 바의 표면 1m²당 45ℓ/min를 초과하는 경우에는 윤활유의 사용이 과도하게 많아 제조원가가 상승할 수 있다. 상기 윤활유 분사량의 상한은 바의 표면 1m²당 40ℓ/min인 것이 보다 바람직하고, 바의 표면 1m²당 35ℓ/min인 것이 보다 더 바람직하며, 하한은 바의 표면 1m²당 10ℓ/min인 것이 보다 바람직하고, 바의 표면 1m²당 15ℓ/min인 것이 보다 더 바람직하다.Thereafter, 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. When 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. On the other hand, if 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.
이후, 상기 윤활유가 분사된 바를 Ar3+10℃~Ar3+60℃의 온도에서 압연기의 페어 크로스(pair cross) 각도를 제어하면서 마무리 압연하여 열연강판을 얻는다. 상기 Ar3+10℃~Ar3+60℃의 온도 범위는 마무리 압연시 출측온도인 것이 바람직하다. 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 다량 형성될 수 있어 냉각 후 충분한 마르텐사이트 분율을 얻을 수 없다. 반면, 상기 마무리 압연 출측온도가 Ar3+60℃를 초과하는 경우에는 결정립이 조대해져 목표로 하는 강도 또는 굽힘 특성을 확보할 수 없으며, 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+15℃인 것이 보다 바람직하고, Ar3+25℃인 것이 보다 더 바람직하다. 상기 마무리 압연 출측온도의 상한은 Ar3+55℃인 것이 보다 바람직하고, Ar3+50℃인 것이 보다 더 바람직하다.Thereafter, 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. When 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. On the other hand, when the finish rolling out temperature exceeds Ar3+60°C, the grains become coarse and cannot secure the targeted strength or bending properties, and the disadvantage of having to cool faster is to obtain a sufficient martensite fraction. have. 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+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.
한편, 상기 마무리 압연(Finish rolling Mill, FM)은 3~6개의 압연기로 이루어진 마무리압연기에서 행할 수 있으며, 상기 압연기는 5~6개로 이루어지는 것이 보다 바람직하다. 본 발명에서는 상기 마무리 압연시 마무리압연기의 페어 크로스(Pair Cross) 각도를 정밀히 제어하여 폭 방향 두께 편차를 최소화할 수 있다. 특히, 압연 하중이 아주 큰 첫 번째 압연기와 압연 하중이 비교적 작은 마지막 압연기는 페어 크로스의 각도를 제어하더라도 폭 방향 두께 편차 제어에 큰 영향을 미치지 않으므로, 본 발명에서는 상기 첫 번째 압연기와 마지막 압연기 사이에 구비되는 압연기들을 제어하는 것이 바람직하다. 한편, 상기 페어 크로스 각도란 압연기의 상·하측 롤이 비틀어짐으로써 형성되는 각도를 의미한다.On the other hand, 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. In the present invention, the thickness of the width direction can be minimized by precisely controlling the pair cross angle of the finishing mill during the finishing rolling. In particular, 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. In the present invention, between the first rolling mill and the last rolling mill, It is desirable to control the rolling mills provided. Meanwhile, the pair cross angle means an angle formed by twisting the upper and lower rolls of the rolling mill.
예를 들어, 5개의 스탠드로 구성되는 마무리 압연기에서 마무리 압연을 수행하는 경우, 두 번째 압연기(FM2)의 페어 크로스(pair cross) 각도는 0.30~0.80°인 것이 바람직하고, 세 번째 압연기(FM3)의 페어 크로스(pair cross) 각도는 0.30~0.80°인 것이 바람직하며, 네 번째 압연기(FM4)의 페어 크로스(pair cross) 각도는 0.20~0.70°인 것이 바람직하다. 본 발명에서는 상기와 같이 압연기의 페어 크로스 각도를 제어함으로써 폭 방향 두께 편차를 최소화할 수 있다. 상기 두 번째 압연기의 페어 크로스(pair cross) 각도는 0.35~0.75°인 것이 보다 바람직하고, 0.40~0.70°인 것이 보다 더 바람직하다. 상기 세 번째 압연기의 페어 크로스(pair cross) 각도는 0.35~0.75°인 것이 보다 바람직하고, 0.40~0.70°인 것이 보다 더 바람직하다. 상기 네 번째 압연기의 페어 크로스(pair cross) 각도는 0.25~0.65°인 것이 보다 바람직하고, 0.30~0.60°인 것이 보다 더 바람직하다. For example, when finishing rolling is performed on a finishing mill composed of 5 stands, 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°, and the pair cross angle of the fourth mill FM4 is preferably 0.20 to 0.70°. In the present invention, 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°.
이후, 상기 열연강판을 30~400℃/s의 냉각속도로 냉각하고, 상기 냉각된 열연강판을 Mf-200℃~Mf-50℃에서 권취한다. 상기 냉각속도가 30℃/s 미만이거나 권취온도가 Mf-50℃를 초과하는 경우에는 페라이트 및 베이나이트가 형성될 수 있어 충분한 마르텐사이트 조직을 확보하기가 어렵다. 상기 냉각속도가 400℃/s를 초과하거나 권취온도가 Mf-200℃미만인 경우에는 권취 후 제품 형상이 불량해질 수 있으며, 코일 내부에 잔류하는 냉각수가 상당히 많이 존재하게 되고, 이러한 상태에서 정정(Skin Pass)을 실시할 경우 잔류 냉각수와 스케일이 서로 밀착되어 있어 열연강판 표면에 치명적인 압입흠이 발생할 수 있다. 상기 냉각속도의 하한은 50℃/s인 것이 보다 바람직하고, 70℃/s인 것이 보다 더 바람직하며, 상한은 350℃/s인 것이 보다 바람직하고, 300℃/s인 것이 보다 더 바람직하다. 권취온도의 하한은 Mf-180℃인 것이 보다 바람직하고, Mf-160℃인 것이 보다 더 바람직하며, 상한은 Mf-60℃인 것이 보다 바람직하고, Mf-70℃인 것이 보다 더 바람직하다.Thereafter, 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. When 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. If the cooling rate exceeds 400°C/s or the winding temperature is less than Mf-200°C, the product shape may become poor after winding, and there is a considerable amount of cooling water remaining inside the coil, and correction in this state (Skin When Pass) is performed, the residual coolant and the scale are in close contact with each other, so that a fatal indentation may occur on the surface of the hot rolled steel sheet. 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.
한편, 상기 권취하는 단계 후에는 권취된 열연강판을 산세 처리하는 단계를 추가로 포함할 수 있고, 상기 산체 처리하는 단계 후에는 산체 처리된 열연강판을 도금하는 단계를 추가로 포함할 수 있다. 상기 산세 및 도금 처리를 통해 PO(Pickled & Oiled)재 및 도금재를 얻을 수 있다. 본 발명에서는 박 슬라브 및 바 스케일 제거 단계에서 스케일을 충분히 제거할 수 있으므로, 일반적인 산세 처리 및 도금처리로도 표면품질이 우수한 PO재 및 도금재를 얻을 수 있다. 따라서 본 발명에서는 열연산세공정 및 도금공정에서 일반적으로 사용되는 방법이라면 모두 적용 가능하므로 산세 처리 및 도금 방법에 대하여 특별히 제한하지 않는다.Meanwhile, after the winding step, 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. Through the pickling and plating treatment, a PO (Pickled & Oiled) material and a plating material can be obtained. 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 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.
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 matters described in the claims and reasonably inferred therefrom.
(실시예 1)(Example 1)
하기 표 1의 합금조성을 갖는 용강을 준비한 뒤, 연주-압연 직결 공정을 적용하여 하기 표 2에 기재된 제조조건으로 1.2mm 두께의 열연강판으로 제조하였다. 이 열연강판을 산세 처리하여 PO재를 얻은 뒤, 미세조직, 인장특성, 굽힘 특성 및 두께 편차를 측정한 후, 그 결과를 하기 표 3에 나타내었다. After the molten steel having the alloy composition of Table 1 was prepared, 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.
한편, 하기 표 2에서의 Ar3(오스테나이트 변태온도) 및 Mf(마르텐사이트 변태종료온도)는 상용 열역학 소프트웨어인 JmatPro V-8를 이용하여 계산한 값이다.Meanwhile, 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.
미세조직은 주사전자현미경(SEM) 및 투과전자현미경(TEM)으로 관찰하여 마르텐사이트(M), 템퍼드 마르텐사이트(T.M) 및 페라이트(F)의 면적 분율을 측정하였다. 구 오스테나이트 결정립 평균 크기(Prior austenite grain boundy size, AGS)는 후방산란전자회절(Electron BackScatter Diffraction, EBSD) 장비를 이용하여 측정하였다.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). The former austenite grain boundy size (AGS) was measured using an Electron BackScatter Diffraction (EBSD) instrument.
인장특성(항복강도(YS), 인장강도(TS) 및 연신율(EL))은 JIS 5호 규격을 스트립의 전폭[일정한 간격(7군데)]에 대해 압연방향(L방향)으로 인장 시편을 채취하여 측정한 평균값으로 나타내었다. Tensile properties (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.
굽힘 특성은 굽힘반경(R)을 강판 두께(t)로 나눈 값이 2.9 및 3.3이 되도록 하여 90° 굽힘가공 시험을 행한 후 크랙이 발생하는지 여부로 평가하였다.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.
폭 방향 두께 편차(Crown)는 폭 방향으로 두께가 가장 두꺼운 부분의 두께와 얇은 부분의 두께를 측정한 뒤, 그 차이값으로 나타내었다.The thickness variation in the width direction (Crown) 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.
Figure PCTKR2019015934-appb-img-000001
Figure PCTKR2019015934-appb-img-000001
Figure PCTKR2019015934-appb-img-000002
Figure PCTKR2019015934-appb-img-000002
Figure PCTKR2019015934-appb-img-000003
Figure PCTKR2019015934-appb-img-000003
상기 표 1 내지 3을 통해 알 수 있듯이, 본 발명이 제안하는 합금조성, 관계식 1 내지 3과 제조조건을 모두 만족하는 발명예 1 내지 6의 경우에는 본 발명의 목표로 하는 미세조직을 확보하고 있으며, 인장특성 및 굽힘특성이 우수한 수준임을 알 수 있다.As can be seen from Tables 1 to 3, in the case of Inventive Examples 1 to 6 satisfying all of the alloy composition, relations 1 to 3, and manufacturing conditions proposed by the present invention, the microstructure targeted by the present invention is secured. , It can be seen that the tensile properties and bending properties are excellent.
반면, 본 발명이 제안하는 제조조건은 만족하나, 합금조성이나 관계식 1 내지 3을 만족하지 않는 비교예 1 내지 9의 경우에는 본 발명이 목표로 하는 미세조직을 확보하지 못하거나, 인장특성 및 굽힘특성이 낮은 수준임을 알 수 있다.On the other hand, in the case of Comparative Examples 1 to 9, which satisfies the manufacturing conditions proposed by the present invention, but does not satisfy the alloy composition or relations 1 to 3, the microstructure targeted by the present invention is not secured, or the tensile properties and bending It can be seen that the characteristics are low.
(실시예 2)(Example 2)
상기 표 1에 기재된 강종 1 및 2의 합금조성을 갖는 용강을 준비한 뒤, 연주-압연 직결 공정을 적용하여 하기 표 4에 기재된 제조조건으로 1.2mm 두께의 열연강판으로 제조하였다. 이 열연강판을 산세 처리하여 PO재를 얻은 뒤, 미세조직, 인장특성, 굽힘 특성 및 두께 편차를 측정한 후, 그 결과를 하기 표 5에 나타내었다. 상기 미세조직, 인장특성, 굽힘 특성 및 두께 편차의 측정은 실시예 1과 동일하게 진행하였다.After preparing the molten steel having the alloy composition of Steel Types 1 and 2 described in Table 1, 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. After pickling the hot-rolled steel sheet to obtain a PO material, 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.
Figure PCTKR2019015934-appb-img-000004
Figure PCTKR2019015934-appb-img-000004
Figure PCTKR2019015934-appb-img-000005
Figure PCTKR2019015934-appb-img-000005
상기 표 4 및 5를 통해 알 수 있듯이, 본 발명이 제안하는 합금조성, 관계식 1 내지 3과 제조조건을 모두 만족하는 발명예 7 내지 9의 경우에는 본 발명의 목표로 하는 미세조직을 확보하고 있으며, 인장특성 및 굽힘특성이 우수한 수준임을 알 수 있다.As can be seen from Tables 4 and 5, in the case of Inventive Examples 7 to 9 satisfying all of the alloy composition, relations 1 to 3, and manufacturing conditions proposed by the present invention, the microstructure targeting the present invention is secured. , It can be seen that the tensile properties and bending properties are excellent.
본 발명이 제안하는 합금조성과 관계식 1 내지 3은 만족하나 제조조건 중 마무리 압연온도가 낮아 본 발명의 조건을 만족하지 않는 비교예 10 및 11의 경우에는 충분한 마르텐사이트+템퍼드 마르텐사이트 조직을 확보하지 못하고, 이에 따라 강도가 낮은 수준임을 알 수 있다.In the case of Comparative Examples 10 and 11 in which the alloy composition proposed by the present invention and relational formulas 1 to 3 are satisfied, but the finish rolling temperature among the manufacturing conditions is not satisfied with the conditions of the present invention, sufficient martensite + tempered martensite structures are secured. However, it can be seen that the strength is low.
본 발명이 제안하는 합금조성과 관계식 1 내지 3은 만족하나 제조조건 중 마무리 압연온도가 높아 본 발명의 조건을 만족하지 않는 비교예 12 및 13의 경우에는 본 발명이 제안하는 구 오스테나이트 결정립 평균 크기를 초과함에 따라 양호한 굽힘 특성을 확보하지 못하고 있음을 알 수 있다. In the case of Comparative Examples 12 and 13 in which the alloy composition and relational formulas 1 to 3 proposed by the present invention are satisfied, but the finish rolling temperature is high among the manufacturing conditions and does not satisfy the conditions of the present invention, the average size of the old austenite grains proposed by the present invention It can be seen that as it exceeds, it is not possible to secure good bending characteristics.
본 발명이 제안하는 합금조성과 관계식 1 내지 3은 만족하나 제조조건 중 권취온도가 높아 본 발명의 조건을 만족하지 않는 비교예 14의 경우에는 충분한 마르텐사이트+템퍼드 마르텐사이트 조직을 확보하지 못하고, 이에 따라 강도가 낮은 수준임을 알 수 있다.In the case of 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.
본 발명이 제안하는 합금조성과 관계식 1 내지 3은 만족하나 제조조건 중 본 발명이 제안하는 권취온도를 만족하지 않는 비교예 15 및 16의 경우에는 충분한 마르텐사이트+템퍼드 마르텐사이트 조직을 확보하지 못하고, 이에 따라 강도가 낮은 수준임을 알 수 있다.In the case of Comparative Examples 15 and 16 that satisfy the alloy composition and relations 1 to 3 proposed by the present invention, but do not satisfy the coiling temperature suggested by the present invention among the manufacturing conditions, it is impossible to secure sufficient martensite + tempered martensite structures. , Accordingly, it can be seen that the strength is low.
도 3은 본 발명의 일 실시예에 따른 발명예 9를 EBSD로 관찰한 미세조직 사진이다. 도 3을 통해 알 수 있듯이, 발명예 9는 구 오스테나이트 결정립 평균 크기가 11㎛로서 미세함을 확인할 수 있다.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㎛ fine.
도 4는 본 발명의 일 실시예에 따른 발명예 9를 주사전자현미경(SEM)으로 관찰한 미세조직 사진이다. 도 4를 통해 알 수 있듯이, 발명예 9는 일부 페라이트 조직이 형성되어 있으나, 주로 마르텐사이트+템퍼드 마르텐사이트 조직을 가지고 있음을 알 수 있다. 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.
도 5는 본 발명의 일 실시예에 따른 발명예 9를 투과전자현미경(TEM)으로 관찰한 미세조직 사진이다. 도 5를 통해 알 수 있듯이, 발명예 9는 래스(lath) 사이에 탄화물이 존재하는 것으로부터 템퍼드 마르텐사이트가 존재함을 확인할 수 있다.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.
도 6은 본 발명의 일 실시예에 따른 비교예 11을 주사전자현미경(SEM)으로 관찰한 미세조직 사진이고, 도 7은 본 발명의 일 실시예에 따른 비교예 15를 주사전자현미경(SEM)으로 관찰한 미세조직 사진이다. 도 6 및 7을 통해 알 수 있듯이, 비교예 11 및 15의 경우에는 페라이트 조직이 다량 존재함을 확인할 수 있다.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.
(실시예 3)(Example 3)
상기 표 1에 기재된 강종 1의 합금조성을 갖는 용강을 준비한 뒤, 연주-압연 직결 공정을 적용하여 하기 표 6에 기재된 제조조건으로 1.2mm 두께의 열연강판으로 제조하였다. 이 열연강판을 산세 처리하여 PO재를 얻은 뒤, 두께 편차를 측정한 후, 그 결과를 하기 표 6에 나타내었다. 상기 두께 편차의 측정은 실시예 1과 동일하게 진행하였다.After preparing the molten steel having the alloy composition of Steel Type 1 listed in Table 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 direct-linking process. After pickling the hot-rolled steel sheet to obtain a PO material, thickness variation was measured, and the results are shown in Table 6 below. The thickness deviation was measured in the same manner as in Example 1.
Figure PCTKR2019015934-appb-img-000006
Figure PCTKR2019015934-appb-img-000006
상기 표 6에서 알 수 있듯이, 본 발명이 제안하는 합금조성, 관계식 1 내지 3과 제조조건을 모두 만족하는 발명예 10의 경우에는 본 발명의 목표로 하는 두께 편차를 낮아 형상 품질이 우수한 수준임을 알 수 있다.As can be seen from Table 6, in the case of Inventive Example 10 that satisfies all of the alloy composition, relations 1 to 3, and manufacturing conditions proposed by the present invention, it is understood that the shape quality is excellent by lowering the thickness deviation targeted by the present invention. Can be.
본 발명이 제안하는 합금조성과 관계식 1 내지 3은 만족하나 제조조건 중 윤활유 분사량이 본 발명의 조건을 만족하지 않는 비교예 17 내지 19의 경우에는 두께 편차가 커 형상 품질이 낮은 수준임을 알 수 있다.In the case of Comparative Examples 17 to 19 in which the alloy composition and relational formulas 1 to 3 proposed by the present invention are satisfied, but the injection amount of lubricant among the manufacturing conditions does not satisfy the conditions of the present invention, it can be seen that the thickness quality is large and the shape quality is low. .
본 발명이 제안하는 합금조성과 관계식 1 내지 3은 만족하나 제조조건 중 마무리압연기의 페어 크로스 각도가 본 발명의 조건을 만족하지 않는 비교예 20 내지 25의 경우에는 두께 편차가 커 형상 품질이 낮은 수준임을 알 수 있다.In the case of Comparative Examples 20 to 25 in which the alloy composition proposed by the present invention and the relational expressions 1 to 3 satisfy the conditions of the present invention, but the fair rolling angle of the finish rolling mill does not satisfy the conditions of the present invention, the shape quality is low and the shape quality is low. Can be seen.
[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 (15)

  1. 중량%로, C: 0.16~0.26%, Mn: 0.75~1.50%, B: 0.0009~0.0050%, Ti: 0.009~0.070%, N: 0.001~0.010%, 잔부 Fe 및 기타 불가피한 불순물을 포함하고, 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 other inevitable impurities,
    하기 관계식 1 내지 3을 만족하며,The following relational expressions 1 to 3 are satisfied,
    면적분율로 마르텐사이트와 템퍼드 마르텐사이트의 합이 93%이상이고, 페라이트가 7%이하(0%를 포함)인 미세조직을 포함하는 형상 품질 및 굽힘성이 우수한 초고강도 열연강판.An ultra-high-strength hot-rolled steel sheet with excellent shape quality and bendability, including microstructures with an area fraction of at least 93% of martensite and tempered martensite and less than 7% of ferrite (including 0%).
    [관계식 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53[Relationship 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53
    [관계식 2] 130 ≤ (C+0.2Mn)/B ≤ 450[Relationship 2] 130 ≤ (C+0.2Mn)/B ≤ 450
    [관계식 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45[Relationship 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45
    (단, 상기 관계식 1 내지 3의 C, Mn, B, Ti의 함량은 중량%임.)(However, the content of C, Mn, B, Ti of the above formula 1 to 3 is weight%.)
  2. 청구항 1에 있어서,The method according to claim 1,
    상기 열연강판은 트램프 원소로서 P, S, Si, Nb, V, 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 형상 품질 및 굽힘성이 우수한 초고강도 열연강판.The hot-rolled steel sheet is one or more 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. An ultra-high-strength hot-rolled steel sheet excellent in shape quality and bendability, including a total of 0.1% by weight or less.
  3. 청구항 1에 있어서,The method according to claim 1,
    상기 열연강판은 구 오스테나이트 결정립 평균 크기가 4~20㎛인 형상 품질 및 굽힘성이 우수한 초고강도 열연강판.The hot-rolled steel sheet is an ultra-high-strength hot-rolled steel sheet excellent in shape quality and bendability with an average austenite grain average size of 4 to 20 µm.
  4. 청구항 1에 있어서,The method according to claim 1,
    상기 열연강판은 항복강도: 1080~1600MPa, 인장강도: 1380~1800MPa, 연신율: 4.0%이상 및 폭 방향 두께 편차: 10~60㎛이고, 굽힘반경(R)을 강판 두께(t)로 나눈 값이 2.9일 때 90° 굽힘가공 시험시 크랙이 발생하지 않는 형상 품질 및 굽힘성이 우수한 초고강도 열연강판.The hot-rolled steel sheet has a yield strength: 1080 to 1600 MPa, a tensile strength: 1380 to 1800 MPa, an elongation: 4.0% or more, and a thickness deviation in the width direction: 10 to 60 μm, and a value obtained by dividing the bending radius (R) by the steel sheet thickness (t) Ultra-high strength hot rolled steel sheet with excellent shape quality and bendability that does not crack during 90° bending test when 2.9.
  5. 청구항 1에 있어서,The method according to claim 1,
    상기 열연강판은 두께가 0.6~1.4mm인 형상 품질 및 굽힘성이 우수한 초고강도 열연강판.The hot-rolled steel sheet is an ultra-high-strength hot-rolled steel sheet excellent in shape quality and bendability with a thickness of 0.6 to 1.4 mm.
  6. 중량%로, C: 0.16~0.26%, Mn: 0.75~1.50%, B: 0.0009~0.0050%, Ti: 0.009~0.070%, 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 0.26%, 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 other inevitable impurities, and Continuously casting molten steel satisfying the 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 바의 표면에 1m²당 5~45ℓ/min의 윤활유를 분사하는 단계; Injecting a lubricant of 5 ~ 45ℓ / min per 1m² on the surface of the bar;
    상기 윤활유가 분사된 바를 Ar3+10℃~Ar3+60℃의 온도에서 압연기의 페어 크로스(pair cross) 각도를 제어하면서 마무리 압연하여 열연강판을 얻는 단계; Obtaining a hot rolled steel sheet by finishing rolling the bar where the lubricant is injected while controlling a pair cross angle of a rolling machine at a temperature of Ar3+10°C to Ar3+60°C;
    상기 열연강판을 30~400℃/s의 냉각속도로 냉각하는 단계; Cooling the hot rolled steel sheet at a cooling rate of 30 to 400°C/s;
    상기 냉각된 열연강판을 Mf-200℃~Mf-50℃에서 권취하는 단계를 포함하며,Winding the cooled hot-rolled steel sheet at Mf-200 ℃ ~ Mf-50 ℃,
    상기 각 단계는 연속주조-압연 직결공정을 통해 연속적으로 행해지는 것을 특징으로 하는 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법.Each step is a continuous casting-rolling method of manufacturing ultra-high strength hot-rolled steel sheet having excellent shape quality and bendability, characterized in that it is continuously performed through a direct connection process.
    [관계식 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53[Relationship 1] 0.32 ≤ (C+0.2Mn) ≤ 0.53
    [관계식 2] 130 ≤ (C+0.2Mn)/B ≤ 450[Relationship 2] 130 ≤ (C+0.2Mn)/B ≤ 450
    [관계식 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45[Relationship 3] 5 ≤ (C+0.2Mn)/Ti ≤ 45
    (단, 상기 관계식 1 내지 3의 C, Mn, B, Ti의 함량은 중량%임.)(However, the content of C, Mn, B, Ti of the above formula 1 to 3 is weight%.)
  7. 청구항 6에 있어서,The method according to claim 6,
    상기 용강은 트램프 원소로서 P, S, Si, Nb, V, 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 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법.The molten steel is a sum of one or more 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. A method of manufacturing a super-high strength hot rolled steel sheet having excellent shape quality and bendability in a range of 0.1% by weight or less.
  8. 청구항 6에 있어서,The method according to claim 6,
    상기 연속 주조시 주조속도는 4.0~7.5mpm(m/min)인 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법.The method of manufacturing the ultra-high-strength hot-rolled steel sheet having excellent shape quality and bendability with a casting speed of 4.0 to 7.5mpm (m/min) during continuous casting.
  9. 청구항 6에 있어서,The method according to claim 6,
    상기 박 슬라브는 두께가 75~125mm인 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법.The thin slab is a method of manufacturing a super-high-strength hot-rolled steel sheet excellent in shape quality and bendability with a thickness of 75 to 125 mm.
  10. 청구항 6에 있어서,The method according to claim 6,
    상기 조압연시 조압연 출측에서의 바 에지부 온도는 850~1000℃인 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법.The method of manufacturing a super-high strength hot rolled steel sheet having excellent shape quality and bendability at a bar edge portion temperature of 850 to 1000°C during rough rolling during the rough rolling.
  11. 청구항 6에 있어서,The method according to claim 6,
    상기 마무리 압연시 두 번째 압연기의 페어 크로스(pair cross) 각도는 0.30~0.80°인 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법.The method of manufacturing the ultra-high strength hot rolled steel sheet having excellent shape quality and bending property in which the pair cross angle of the second rolling mill is 0.30 to 0.80° during the finish rolling.
  12. 청구항 6에 있어서,The method according to claim 6,
    상기 마무리 압연시 세 번째 압연기의 페어 크로스(pair cross) 각도는 0.30~0.80°인 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법.The method of manufacturing the ultra-high strength hot rolled steel sheet excellent in shape quality and bending property in which the pair cross angle of the third rolling mill is 0.30 to 0.80° during the finish rolling.
  13. 청구항 6에 있어서,The method according to claim 6,
    상기 마무리 압연시 네 번째 압연기의 페어 크로스(pair cross) 각도는 0.20~0.70°인 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법.The method of manufacturing the ultra-high strength hot rolled steel sheet having excellent shape quality and bending property in which the pair cross angle of the fourth rolling mill is 0.20 to 0.70° during the finish rolling.
  14. 청구항 6에 있어서,The method according to claim 6,
    상기 권취하는 단계 후, 권취된 열연강판을 산세 처리하여 PO(Pickled & Oiled)재를 얻는 단계를 추가로 포함하는 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법.After the step of winding, the method of manufacturing the ultra-high strength hot rolled steel sheet excellent in shape quality and bendability further comprising the step of pickling the wound hot rolled steel sheet to obtain a PO (Pickled & Oiled) material.
  15. 청구항 14에 있어서,The method according to claim 14,
    상기 산세 처리하는 단계 후, 산세 처리된 열연강판을 도금하여 도금재를 얻는 단계를 추가로 포함하는 형상 품질 및 굽힘성이 우수한 초고강도 열연강판의 제조방법.After the step of the pickling treatment, the method of manufacturing an ultra-high strength hot rolled steel sheet excellent in shape quality and bendability further comprising the step of obtaining a plating material by plating the pickled hot rolled steel sheet.
PCT/KR2019/015934 2018-11-26 2019-11-20 Ultra-high strength hot rolled steel sheet having excellent shape quality and bendability, and method for manufacturing same WO2020111639A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
KR1020180146880A KR102164108B1 (en) 2018-11-26 2018-11-26 Ultra high strength hot rolled steel sheet having excellent shape and bendability properties and method of manufacturing the same
KR10-2018-0146880 2018-11-26

Publications (1)

Publication Number Publication Date
WO2020111639A1 true WO2020111639A1 (en) 2020-06-04

Family

ID=70851816

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/KR2019/015934 WO2020111639A1 (en) 2018-11-26 2019-11-20 Ultra-high strength hot rolled steel sheet having excellent shape quality and bendability, and method for manufacturing same

Country Status (2)

Country Link
KR (1) KR102164108B1 (en)
WO (1) WO2020111639A1 (en)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114798762A (en) * 2022-05-18 2022-07-29 首钢长治钢铁有限公司 Rolled piece temperature control method and device

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN113042563A (en) * 2021-03-17 2021-06-29 包头钢铁（集团）有限责任公司 Manufacturing method of SPHE hot-rolled pickled plate for stamping

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR20010060791A (en) * 1999-12-28 2001-07-07 이구택 Estimation Method of the strip flatness at the hot rolling mill
KR100627487B1 (en) * 2005-05-16 2006-09-25 주식회사 포스코 Method for controlling thickness of strip
JP4412418B2 (en) * 2007-02-02 2010-02-10 住友金属工業株式会社 Method for producing hot-rolled steel sheet having fine ferrite structure, and hot-rolled steel sheet
JP2011102434A (en) * 2004-11-30 2011-05-26 Jfe Steel Corp Method for manufacturing high-strength hot-rolled steel sheet
EP2489748B1 (en) * 2011-02-18 2017-12-13 ThyssenKrupp Steel Europe AG Hot-rolled steel surface product produced from a complex phase steel and method for the manufacture

Family Cites Families (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR100946066B1 (en) 2002-12-26 2010-03-10 주식회사 포스코 Method for Manufacturing Ultra High Strength Cold-rolled Steel Sheets for Automotive Bumper Reinforcements
JP5234876B2 (en) 2005-09-30 2013-07-10 Ｊｆｅスチール株式会社 Manufacturing method of high-tensile cold-rolled steel sheet
CN101351570B (en) 2005-12-28 2013-01-30 株式会社神户制钢所 Ultrahigh-strength thin steel sheet
TWI435970B (en) 2006-09-29 2014-05-01 Inventio Ag Flat-belt-like supporting and drive means with tensile carriers
JP5704721B2 (en) * 2011-08-10 2015-04-22 株式会社神戸製鋼所 High strength steel plate with excellent seam weldability
KR101299896B1 (en) 2013-05-30 2013-08-23 주식회사 포스코 METHOD FOR MANUFACTURING TENSILE STRENGTH 1.5GPa CLASS STEEL SHEET
KR101797387B1 (en) * 2016-08-31 2017-11-14 주식회사 포스코 Ultra high strength thin hot-rolled steel sheet having excellent formability and method for manufacturing same

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR20010060791A (en) * 1999-12-28 2001-07-07 이구택 Estimation Method of the strip flatness at the hot rolling mill
JP2011102434A (en) * 2004-11-30 2011-05-26 Jfe Steel Corp Method for manufacturing high-strength hot-rolled steel sheet
KR100627487B1 (en) * 2005-05-16 2006-09-25 주식회사 포스코 Method for controlling thickness of strip
JP4412418B2 (en) * 2007-02-02 2010-02-10 住友金属工業株式会社 Method for producing hot-rolled steel sheet having fine ferrite structure, and hot-rolled steel sheet
EP2489748B1 (en) * 2011-02-18 2017-12-13 ThyssenKrupp Steel Europe AG Hot-rolled steel surface product produced from a complex phase steel and method for the manufacture

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114798762A (en) * 2022-05-18 2022-07-29 首钢长治钢铁有限公司 Rolled piece temperature control method and device
CN114798762B (en) * 2022-05-18 2024-02-27 首钢长治钢铁有限公司 Method and device for controlling temperature of rolled piece

Also Published As

Publication number Publication date
KR102164108B1 (en) 2020-10-12
KR20200061513A (en) 2020-06-03

Similar Documents

Publication Publication Date Title
WO2018117552A1 (en) Ultra-high strength hot-rolled steel sheet having excellent bending processability and method for manufacturing same
WO2017222189A1 (en) Ultrahigh-strength high-ductility steel sheet having excellent yield strength, and manufacturing method therefor
WO2018117501A1 (en) Ultra-high strength steel sheet having excellent bendability and manufacturing method therefor
WO2016104881A1 (en) Hot press-formed (hpf) member having excellent bending characteristics and method for manufacturing same
WO2012064129A2 (en) Method for manufacturing high-strength cold-rolled/hot-rolled trip steel having a tensile strength of 590 mpa grade, superior workability, and low mechanical-property deviation
WO2017171366A1 (en) High-strength cold rolled steel sheet with excellent yield strength and ductility, coated steel plate, and method for manufacturing same
WO2019132426A1 (en) Non-oriented and thin electrical steel sheet having excellent magnetic and shape properties and method for manufacturing same
WO2020111640A1 (en) Non-oriented electrical steel sheet having low iron loss and excellent surface quality, and manufacturing method therefor
WO2017105025A1 (en) Ultra-high strength steel sheet having excellent chemical conversion treatability and bending processability and method for manufacturing same
WO2020022778A1 (en) High-strength steel sheet having excellent impact resistant property and method for manufacturing thereof
WO2020111639A1 (en) Ultra-high strength hot rolled steel sheet having excellent shape quality and bendability, and method for manufacturing same
WO2018117470A1 (en) High-strength steel plate having excellent burring workability in low temperature range and manufacturing method therefor
WO2019132420A1 (en) Non-oriented and thin electrical steel sheet having excellent magnetic properties and shapes and method for manufacturing same
WO2017111322A1 (en) Super strength hot-rolled steel sheet excellent ductility and manufacturing therefor
WO2019132317A1 (en) Non-oriented electrical steel sheet excellent in magnetic properties and having small thickness variation, and method for manufacturing same
WO2022139191A1 (en) Highly thick steel material having excellent low-temperature impact toughness and manufacturing method therefor
WO2020226301A1 (en) Ultra-high strength steel sheet having excellent shear workability and method for manufacturing same
WO2019132408A1 (en) Ultra-thin hot rolled steel sheet having excellent isotropy and manufacturing method thereof
WO2019132315A1 (en) Non-oriented electrical steel sheet having excellent shape property, and manufacturing method therefor
WO2013154254A1 (en) High carbon hot rolled steel sheet having excellent uniformity and method for manufacturing same
WO2019132400A1 (en) High-strength cold-rolled steel sheet with low variation in mechanical property and excellent stretch flangeability and recovery, and manufacturing method therefor
WO2020071654A1 (en) Ultra high strength hot rolled steel sheet having excellent surface qualities and low mechanical properties deviation and method of manufacturing the same
WO2022065797A1 (en) High-strength thick hot-rolled steel sheet and method for manufacturing same
WO2021112488A1 (en) Thick composite-phase steel having excellent durability and manufacturing method therefor
WO2018106016A1 (en) Wire rod for springs with excellent corrosion fatigue resistance, steel wire, and manufacturing method thereof

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 19889950

Country of ref document: EP

Kind code of ref document: A1

NENP Non-entry into the national phase

Ref country code: DE

122 Ep: pct application non-entry in european phase

Ref document number: 19889950

Country of ref document: EP

Kind code of ref document: A1