WO2019132400A1 - Tôle d'acier laminée à froid à haute résistance présentant une faible variation de propriété mécanique et une excellente aptitude à la formation de bords par étirage et à la récupération et son procédé de fabrication - Google Patents

Tôle d'acier laminée à froid à haute résistance présentant une faible variation de propriété mécanique et une excellente aptitude à la formation de bords par étirage et à la récupération et son procédé de fabrication Download PDF

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WO2019132400A1
WO2019132400A1 PCT/KR2018/016263 KR2018016263W WO2019132400A1 WO 2019132400 A1 WO2019132400 A1 WO 2019132400A1 KR 2018016263 W KR2018016263 W KR 2018016263W WO 2019132400 A1 WO2019132400 A1 WO 2019132400A1
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steel sheet
rolled steel
rolling
cold
less
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PCT/KR2018/016263
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English (en)
Korean (ko)
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공종판
정제숙
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주식회사 포스코
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0268—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment between cold rolling steps
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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 a high-strength cold-rolled steel sheet having a small material variation, excellent stretch flangeability, and a high yield rate, and a method for producing the same.
  • Molding modes of automobile parts are divided into extrusion molding, stretch flange molding, drawing and bending. It is very important to secure proper moldability as well as strength in order to satisfy these requirements.
  • the Hole Expanding Ratio is a mechanical characteristic as important as tensile strength and elongation. It is also known that stretch flangeability is more dependent on microstructure than mechanical properties. The stretch flangeability is defined as the ultimate strain that can withstand a large deformation exceeding the stretch.
  • the AHSS of 780 MPa or more has a problem of increasing the rolling load during cold rolling since the alloy element and the added amount are increased to secure high strength and ductility / formability at the same time, so that the shape of the strip is limited, ⁇ 1.0 mm).
  • this portion since the top portion is inserted in the rolling mill for each coil and the tail portion must exit from the rolling mill, this portion must be cut to open the thickness deviation and scale quality to the top and tail portions It is difficult to manufacture hot rolled steel sheets because of a very low yield and a high risk of plate fracture, so that there are many restrictions on the thickness of the final cold rolled steel sheet.
  • Patent Literatures 1 and 2 disclose technologies related to the manufacture of such AHSS steel, but all of these methods relate to a method of manufacturing in a conventional hot-melt mill, and it is difficult to avoid the problem that a material deviation in a production line is largely generated in a width and a length direction It is true. This is because, in the batch process of conventional hot-rolled mill, the top portion is drawn into the rolling mill for every coil, and the tail portion must exit the rolling mill. Therefore, this portion must be cut in order to open the thickness variation and scale quality to the top and tail portions Since the rate of realization is very low and the risk of sheet breakage is high, it is difficult to manufacture hot rolled steel sheets, and therefore, there are many restrictions (? 1.0 mm) in securing the thickness of the final cold rolled steel sheet.
  • Patent Document 1 United States Patent No. 4285741
  • Patent Document 2 United States Patent Publication No. 4325751
  • One aspect of the present invention relates to a high strength cold rolled steel sheet having a small variation in material, excellent elongation flangeability, and a high yield rate by deriving optimal alloy components and process conditions as well as a continuous continuous rolling mode in a performance- .
  • An embodiment of the present invention is a steel sheet comprising, by weight%, 0.030 to 0.080% of C, 1.5 to 3.0% of Mn, 0.03 to 0.50% of Si, 0.001 to 0.045% of P, 0.0005 to 0.0035% of S, 0.001 to 0.04% N, 0.001 to 0.010% N, 0.001 to 0.010% of B, and the balance of Fe and other unavoidable impurities, 4, and the microstructure has an area fraction of 30 to 70% for ferrite and 30 to 70% for martensite and autotemplated martensite: 30 to 70%, and has excellent stretch flangeability and excellent water- .
  • a steel sheet comprising: 0.030 to 0.080% of C, 1.5 to 3.0% of Mn, 0.03 to 0.50% of Si, 0.001 to 0.045% of P, 0.0005 to 0.0035% of S, 0.001 to 0.04% N, 0.001 to 0.010% N, 0.001 to 0.010% of B, and the balance of Fe and other unavoidable impurities, 4 is continuously cast to obtain a thin slab; Subjecting the thin slab to rough rolling to obtain a bar; Hot rolling the bar at a temperature of Ar3 or higher to obtain a hot-rolled steel sheet; And winding the hot-rolled steel sheet at 400 to 650 ° C, wherein each of the steps is performed continuously, cold-rolling the rolled hot-rolled steel sheet to produce a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet in a temperature range of Ac3-50 DEG C to Ac3 + 30 DEG C; A primary cooling step of cooling the continuously annealed cold rolled
  • a high-strength cold-rolled steel sheet having a small variation in material, excellent elongation flangeability, and a high yield rate by deriving optimum alloy components and process conditions while using continuous continuous rolling mode in a performance- Method can be provided.
  • the steel sheet is manufactured in the continuous continuous rolling mode in the performance-rolling direct rolling process, there is an advantage in that the steel sheet is excellent in the thickness deviation and the surface quality without the top and tail portions, so that the material deviation is small and the slip rate is high.
  • FIG. 1 is a schematic view of a facility for a performance-rolling direct process that can be applied to the present invention.
  • FIG. 2 is another schematic diagram of a facility for a performance-rolling direct process that can be applied to the present invention.
  • FIG 3 is a schematic diagram showing one embodiment of a Finishing Mill Scale Breaker (FSB).
  • FFB Finishing Mill Scale Breaker
  • FIG 4 is an optical microscope tissue photograph of the inventive example 1 according to an embodiment of the present invention.
  • Example 5 is a SEM micrograph of Inventive Example 1 according to an embodiment of the present invention.
  • FIG. 6 is a transmission electron microscope (TEM) micrograph of the inventive example 1 according to an embodiment of the present invention.
  • FIG. 6 is a TEM micrograph of 50,000 times on the left side and 100,000 times magnification of the [X] and [Y] It is an organization photograph.
  • TEM transmission electron microscope
  • Example 7 is a graph showing the ferrite (F) uniaxial crystal grain size distribution of Inventive Example 1 according to an embodiment of the present invention.
  • Example 8 is a graph showing martensite (M) + autothermite martensite (A.M) single axis grain size distribution of Inventive Example 1 according to an embodiment of the present invention.
  • FIG. 9 is a photograph of a precipitate of Inventive Example 1 taken by a transmission electron microscope (TEM) according to an embodiment of the present invention.
  • TEM transmission electron microscope
  • FIG 10 is a graph showing a distribution of size of precipitates of Inventive Example 1 according to an embodiment of the present invention.
  • FIG. 11 is a graph showing the distribution of the distance between precipitates of Inventive Example 1 according to an embodiment of the present invention.
  • Carbon (C) is a very important element for increasing the strength of a steel sheet and securing a composite structure composed of ferrite and martensite.
  • the C content is less than 0.030%, it may be difficult to secure the desired strength in the present invention.
  • the C content is more than 0.080%, an apodization reaction (L + Delta Ferrite ⁇ Austentite) occurs at the time of solidification of molten steel, so that a solidified cell having a non-uniform thickness is formed and molten steel outflow may occur. Therefore, the C content is preferably 0.030 to 0.080%, more preferably 0.035 to 0.075%.
  • Manganese (Mn) is an element that has a very strong effect of solid solution strengthening and at the same time promotes the formation of composite structure composed of ferrite and martensite.
  • Mn content is less than 1.5%, it may be difficult to obtain the intended strength in the present invention.
  • Mn content is more than 3.0%, it is difficult to obtain the desired elongation, and the weldability and hot rolling property may be deteriorated.
  • the Mn content is preferably 1.5 to 3.0%, more preferably 1.8 to 2.8%.
  • Silicon (Si) is a useful element that can secure strength without deteriorating the ductility of the steel sheet. It is also an element promoting the formation of martensite by promoting ferrite formation and promoting C concentration in untransformed austenite. When the Si content is less than 0.03%, it is difficult to sufficiently secure the above effect. On the other hand, when the Si content is 0.50% or more, the scale is generated on the surface of the steel sheet, traces remain on the surface of the steel sheet after pickling, and the surface quality may be deteriorated. Therefore, the Si content is preferably 0.03 to 0.50%, more preferably 0.05 to 0.30%.
  • Phosphorus (P) is an element showing the effect of strengthening the steel sheet.
  • the P content is less than 0.001%, it is difficult to secure the effect.
  • the P content exceeds 0.045%, the grain boundary and / or the intergranular grain boundary may be segregated to cause brittleness, and the mechanical properties of the welded portion may be deteriorated. Therefore, the content of P is preferably 0.001 to 0.045%, more preferably 0.002 to 0.035%.
  • S Sulfur
  • the content should be controlled as low as possible, and it is preferable to control the content to 0.035% or less.
  • the lower limit is preferably 0.0005%. Therefore, the content of S is preferably 0.0005 to 0.0035%, more preferably 0.0008 to 0.0020%.
  • Chromium (Cr) is an element that improves hardenability and increases the strength of steel.
  • the Cr content is preferably 0.5%, more preferably 0.7 to 2.0%.
  • Aluminum (Al) is concentrated on the surface of the steel sheet to deteriorate the plating ability, while suppressing carbide formation, thereby increasing the ductility of the steel.
  • the reheating process in the conventional hot melt mill can be omitted, energy can be saved and the production can be improved, but the temperature of the slab surface or the edge portion may be lowered due to cooling of the surface of the slab.
  • the AlN is excessively precipitated and deterioration of the high temperature ductility may deteriorate the edge quality of the cast steel and / or the bar. Therefore, in the present invention, the Al content should be controlled as low as possible, preferably 0.05% or less, more preferably 0.035% or less.
  • Titanium (Ti) is an element for forming precipitates and nitrides, which increases the strength of steel.
  • Ti is an element that reduces the amount of AlN precipitate by removing solute N through the formation of TiN near the solidification temperature, thereby preventing deterioration of high temperature ductility and reducing sensitivity to edge cracking.
  • the Ti content is less than 0.005%, the above-mentioned effect is insufficient.
  • the Ti content exceeds 0.10%, the manufacturing cost may increase and the ductility of the ferrite may be deteriorated. Therefore, the Ti content is preferably 0.005 to 0.10%, more preferably 0.01 to 0.08%.
  • Niobium (Nb) is an element effective for increasing the strength and grain size of a steel sheet.
  • Nb content is less than 0.005%, the above-mentioned effect is insufficient.
  • the Nb content exceeds 0.10%, the manufacturing cost may increase, the ductility of the ferrite may decrease, and the edge crack of the slab / bar may be caused. Therefore, the content of Nb is preferably 0.005 to 0.10%, more preferably 0.01 to 0.07%.
  • B Boron
  • B is an element that serves to delay the transformation of austenite into pearlite during cooling during annealing.
  • the content of B is preferably 0.0005 to 0.004%, and more preferably 0.0010 to 0.0035%.
  • N Nitrogen
  • the N content is preferably 0.001 to 0.010%, and more preferably 0.002 to 0.008%.
  • the alloy composition of the cold-rolled steel sheet of the present invention preferably satisfies the following relational expressions (1) to (4).
  • [Cp] in the following relational expressions 1 to 4 is 0.09-0.038C-0.013Mn-0.028Si + 0.00015Cr, and [Ceq] is C + Si / 30 + Mn / 20 + 2P + 3S.
  • [Cp] is a formula for obtaining the critical C content at which the apodization reaction occurs.
  • the C content of the low-carbon content exceeds [Cp]
  • an apodization reaction occurs, So that it is possible to cause an accident due to the leakage of molten steel. Therefore, it is preferable that the C content of molten steel is equal to or lower than [Cp] in order to produce a sound thin slab through high-speed casting.
  • the value of [C x Cp] x 1000 is preferably 1.5 to 3.5, more preferably 1.6 to 3.3.
  • [Ceq] x1000 is a component relational expression for securing the weldability of the steel sheet and the mechanical properties of the welded portion.
  • the value of [Ceq] x 1000 is preferably 165 to 235, more preferably 170 to 230.
  • [Ceq] / ([CxCp]) is less than 50 in the above-mentioned relational expression 4, apodization reaction may occur and it may be difficult to manufacture sound thin slabs. If it exceeds 150, the mechanical properties of the welds may be deteriorated have. Accordingly, the value of [Ceq] / ([C x Cp]) is preferably 50 to 150, more preferably 60 to 140.
  • the remainder of the present invention is iron (Fe).
  • impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.
  • the cold-rolled steel sheet of the present invention may contain at least one selected from the group consisting of V, Mo, Cu, Ni, Zn, Se, Sb, Zr, W, Ga, By weight is not more than 0.2% by weight.
  • the tramp element is an impurity element derived from scrap used as a raw material in the steelmaking process, ladle or tundish refractory, etc. When the total amount exceeds 0.2%, the surface / edge crack of the thin slab and the cold- It is possible to reduce the surface quality of the substrate.
  • the cold-rolled steel sheet of the present invention preferably has a microstructure in an area fraction of 30 to 70% of ferrite and 30 to 70% of martensite and autotemplated martensite.
  • the ferrite fraction is more than 70%, it is difficult to secure the desired tensile strength.
  • the ferrite fraction is less than 30%, the fraction of the remaining martensite and autotempered martensite structure becomes too high to secure the elongation and stretch flangeability. .
  • the sum of the martensite and the auto-tempered martensite fraction exceeds 70%, the strength becomes too high to secure workability. If the sum is less than 30%, it may be difficult to secure the desired strength.
  • Auto-tempered martensite refers to a structure having properties similar to those of tempered martensite without any additional tempering treatment.
  • the average short axis size of the ferrite crystal grains is 0.2 to 3.0 mu m.
  • the smaller grain size is advantageous in terms of strength and workability, but it is difficult to control the grain size to 0.2 ⁇ or less in the rolling and annealing conditions of the present invention. If the average short axis grain size of the ferrite exceeds 3.0 ⁇ , it may be difficult to secure the desired strength and workability.
  • the average short axis size of the ferrite crystal grains is preferably in the range of 0.2 to 3.0 mu m, more preferably 0.5 to 2.5 mu m.
  • the average grain size of the martensite and the auto-tempered martensite is preferably 0.2 to 3.0 mu m.
  • the martensite and the auto-tempered martensite structure are important for ensuring the desired strength. The smaller the grain size is, the more advantageous in securing strength and workability.
  • the average short axis size of the crystal grains of the martensite and the auto-tempered martensite is preferably in the range of 0.2 to 3.0 mu m, more preferably 0.5 to 2.5 mu m.
  • the average short axis size of the martensite and the auto-tempered martensite is preferably 100 nm or less.
  • the average short axis size of the laths of the martensite and the auto tempered martensite exceeds 100 nm, it may be difficult to secure the desired strength. Therefore, the average short axis size of the martensite and the auto-tempered martensite is preferably 100 nm or less, more preferably 50 nm or less. In the present invention, the smaller the average shortening size of the lath is, the more advantageous in securing the strength, so that the lower limit is not set.
  • the average size of the M (X) is preferably in the range of 2 to 15 nm.
  • the cold-rolled steel sheet may have a tensile strength of 800 MPa or more, an elongation of 10% or more, a strip length and width tensile strength deviation of 35 MPa or less, and a stretch flange ratio of 35% or more. Further, the cold-rolled steel sheet of the present invention can have a thickness of 1 mm or less, and a high water-rejection ratio can be secured.
  • the cold-rolled steel sheet of the present invention preferably has a ductility ratio (CTS / TSS x 100) of 35% or more of an electrical resistance spot weld.
  • CTS is the Cross Tensile Strength (kN)
  • TSS is the Tensile Shear Strength (kN).
  • the ductility ratio is defined as the ratio of the TSS to the CTS and is used as a comprehensive index to judge the mechanical properties of the weld in the electrical resistance spot welding of the Advanced High Strength Steel (AHSS).
  • the cold-rolled steel sheet of the present invention may have a ductility ratio of 35% or more at an appropriate welding current range capable of ensuring a sound weld.
  • FIG. 1 is a schematic diagram of a facility for a performance-to-rolling direct process that can be applied to the present invention, and is a schematic diagram of a performance-to-rolling direct process facility applicable to the manufacture of hot rolled steel sheets for obtaining cold- rolled steel sheets.
  • a cold-rolled steel sheet having a small material deviation and excellent stretch flangeability and an excellent water-repelling ratio according to an embodiment of the present invention can be manufactured from a hot-rolled steel sheet produced by applying a performance-to-rolling direct connection facility as shown in Fig.
  • the rolling-to-rolling direct-connection facility comprises the steps of: preparing a thin slab a of a first thickness in a continuous casting machine 100 and forming the slab in a roughing mill 400 to produce a bar b of a second thickness thinner than the first thickness And a hot rolled steel sheet (c) having a second thickness and a third thickness is produced in the finishing mill (600).
  • the hot-rolled steel sheet (b) having a thickness of 3.0 mm or less can be continuously rolled because there is no coil box between the rough rolling mill (400) and the finish rolling mill (600) It is possible to produce it stably.
  • finishing mill scale breaker (FSB) 500 in front of the roughing mill scale breaker (RSB) and the finish rolling mill 600 before the roughing mill 400 It is possible to produce cold rolled steel sheets having excellent surface quality during pickling and cold rolling of hot rolled steel sheets in a subsequent process. Further, it is possible to perform isothermal / constant-speed rolling at a rolling speed difference of 10% or less between the top and tail in one strip in the finish rolling step, so that the strip width and the longitudinal direction temperature deviation are remarkably low, Out Table, ROT), it is possible to manufacture steel sheet with excellent material deviation.
  • the thus rolled strip is cut by a high-speed shear machine 800 and wound by a winder 900.
  • the finishing rolling scale breaker 500 may be provided with a heater 200 for further heating the bar.
  • FIG. 2 is another schematic diagram of a facility for a performance-rolling direct process that can be applied to the present invention.
  • the apparatus for direct rolling-to-rolling process disclosed in FIG. 2 is substantially identical in construction to the apparatus disclosed in FIG. 1, but includes a heater 200 'for further heating a slab in front of the rough rolling mill 400, It is possible to lower the occurrence of edge defects and is advantageous in securing the surface quality. In addition, a space of at least one slab length is secured before the roughing mill, and batch rolling is possible.
  • the high-strength cold-rolled steel sheet of which the material deviation of the present invention is small, and excellent in the stretch flangeability and the rate of realization can be produced in all of the performance-rolling direct connection facilities disclosed in Figs.
  • molten steel 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 8.0 mpm (m / min).
  • the reason why the casting speed is set to 4.0 mpm or more is that a high speed casting and rolling process are connected and a casting speed higher than a certain level is required to secure the target rolling temperature.
  • the casting speed is slow, there is a risk of segregation from the cast steel. If such a segregation occurs, it is difficult to secure strength and workability, and the risk of material variation in the width direction or the longitudinal direction is increased.
  • the casting speed is preferably in the range of 4.0 to 8.0 mpm, more preferably 4.5 to 7.5 mpm, more preferably 5.0 to 7.0 mpm, since the operation success rate may be reduced by instability of the molten steel bath surface. mpm is more preferable.
  • the thin slab preferably has a thickness of 60 to 120 mm.
  • the thickness of the thin slab is more than 120 mm, high-speed casting is difficult, and the rolling load during rough rolling is increased.
  • the thickness is less than 60 mm, the temperature of the cast steel is rapidly decreased and uniform texture is hardly formed.
  • the thickness of the thin slab is preferably in the range of 60 to 120 mm, more preferably 70 to 115 mm, and even more preferably 80 to 110 mm.
  • cooling water may be sprayed to the thin slab to remove the scale.
  • cooling water of 50 ° C or less is sprayed at a pressure of 50 to 350 bar from a roughing scale breaker (RSB) nozzle to remove the surface scale of the thin slab to a thickness of 250 ⁇ m or less .
  • RSB roughing scale breaker
  • the cooling water injection pressure in the RSB is preferably 50 to 350 bar, more preferably 100 to 300 bar, and even more preferably 150 to 250 bar.
  • the thin slab is roughly rolled to obtain a bar.
  • a continuously cast thin slab can be rough rolled in a roughing mill consisting of 2 to 5 stands.
  • the surface temperature of the thin slab drawn in the rough rolling is preferably 900 to 1200 ° C. If the surface temperature of the thin slab is less than 900 ° C, there is a possibility that cracks may occur in the edge of the bar during the rough rolling load increase and rough rolling, which may lead to defects at the edge of the hot-rolled steel sheet. If the surface temperature of the thin slab is more than 1200 ° C, problems such as deterioration of the hot-rolled surface due to the remnant of the hot-rolled scale may occur. In addition, the internal temperature of the cast steel may be too high to cause the non-solidification, so that casting may be interrupted due to swelling of the cast steel before rough rolling.
  • the surface temperature of the thin slab drawn in the rough rolling is preferably in the range of 900 to 1200 ° C, more preferably in the range of 920 to 1180 ° C, more preferably in the range of 940 to 1160 ° C desirable.
  • the edge temperature of the bar is preferably 800 to 1100 ° C.
  • the edge temperature of the bar is less than 800 ° C, a large amount of complex precipitates such as NbC, Nb (C, N), (Nb, Ti) (C, N), AlN and BN are produced in the edge portion of the bar, There is a problem that the sensitivity to edge crack generation becomes very high as the surface roughness decreases.
  • the edge portion temperature is higher than 1100 ° C, the temperature of the central portion of the thin slab becomes too high, so that a large number of arithmetic scale may occur and the surface quality after the pickling may become poor. Therefore, the edge temperature of the bar is preferably 800 to 1100 ° C, more preferably 850 to 1050 ° C, and even more preferably 900 to 1000 ° C.
  • FIG. 3 is a schematic diagram showing one embodiment of a Finishing Mill Scale Breaker (FSB).
  • a Finishing Mill Scale Breaker (hereinafter referred to as FSB) 500 includes a cooling water spray nozzle 502.
  • the cooling water 504 is injected from the cooling water injection nozzle 502 to remove the scale formed on the surface of the bar.
  • the cooling water injection pressure in the FSB is preferably 50 to 350 bar, more preferably 60 to 230 bar, and even more preferably 70 to 210 bar.
  • the overlap area ratio of the cooling water 502 injected from the cooling water injection nozzle 502 of the finishing rolling scale breaker 500 preferably satisfies the following relational expression (5).
  • the overlap area ratio of the cooling water 502 means the ratio of the overlap area A of the cooling water to the total area of the cooling water sprayed on the bar surface.
  • the combined area ratio of the cooling water influences the scale removal side, it also influences the temperature in the width direction of the steel sheet, which may cause a material deviation, so that it is preferable to control it appropriately. If (X 3 -85X 2 + 1800X) / 1000 is less than 5 in the following relation (5), a temperature deviation in the width direction may occur and a material deviation may occur severely.
  • X means the overlapping area ratio (%) of the cooling water sprayed to the steel sheet from the finish rolling scale breaker.
  • the above bars are subjected to hot rolling at a temperature of Ar3 or higher to obtain a hot-rolled steel sheet.
  • finishing rolling can be performed in a finishing mill having, for example, 3 to 6 stands.
  • the temperature deviation means a difference between the maximum value and the minimum value of the rolling temperature in the last rolling mill.
  • the finish rolling temperature is lower than Ar3
  • the load of the roll during hot rolling is greatly increased to increase the energy consumption and the operation speed, and a sufficient austenite fraction can not be secured, and the target microstructure and material can not be secured.
  • the temperature of the inlet side of the last rolling mill exceeds 75 ⁇ during the manufacturing of one strip in the last rolling mill, the austenite and ferrite fraction may be significantly deviated in the width direction, and the material deviation may be increased.
  • the speed deviation of the last mill during the production of one strip in the last mill is not more than 60 mpm. If the speed difference of the last rolling mill exceeds 60mpm, the temperature and the rolling load become uneven, resulting in material and thickness variations of the hot rolled steel sheet, and uneven rolling during cold rolling can increase the thickness variation of the final product.
  • the speed deviation is more preferably 55 mpm or less, and more preferably 50 mpm or less.
  • the speed deviation means the difference between the maximum value and the minimum value of the rolling speed in the last rolling mill.
  • the average rolling speed of the final rolling mill during the finish rolling is preferably 100 to 600 mpm. If the average passing speed of the final rolling mill is more than 600 mPm during the finishing rolling, there may occur an accident such as a plate rupture, and a uniform temperature may not be secured due to difficulty in isothermal and constant speed rolling. On the other hand, in the case of less than 100 mpm, the finishing rolling speed is too slow to secure the finishing rolling temperature. Accordingly, the average rolling speed of the final rolling mill during the finish rolling is preferably 100 to 600 mpm, more preferably 150 to 550 mpm, and even more preferably 200 to 500 mpm.
  • finishing rolling can be performed in a finishing mill made up of three to six stands of a bar made in a roughing mill.
  • the hot rolled steel sheet obtained as described above may have a thickness of 3.0 mm or less, more preferably 2.5 mm or less, and still more preferably 2.0 mm or less.
  • the hot-rolled steel sheet is wound at 400 to 650 ° C.
  • the coiling temperature is lower than 400 ⁇ , the martensite transformation is promoted and the strength becomes too high, so that there may be a problem that the rolling property and the shape may be poor when cold rolling. If it exceeds 650 ⁇ , The surface quality may be deteriorated. Therefore, the coiling temperature is preferably controlled to 400 to 650 ⁇ ⁇ , and more preferably 450 to 600 ⁇ ⁇ .
  • the above-described method for producing a hot-rolled steel sheet is characterized in that the above-described respective steps are performed continuously by using the continuous rolling mode in the performance-to-rolling direct connection process.
  • the rolled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
  • the reduction ratio in the cold rolling is preferably in the range of 40 to 70%. If the reduction rate is less than 40%, there is a risk that recrystallization does not occur during annealing. If the reduction rate exceeds 70%, the rolling deformation resistance increases greatly and rolling becomes difficult, so that the reduction rate is preferably in the range of 40 to 70% .
  • the cold rolled steel sheet has a widthwise average steepness of 1.0% or less. If the widthwise average steepness exceeds 1.0%, the shape of the cold-rolled steel sheet is poor and it is difficult to pass through the annealing line, which may make production of the final product difficult.
  • the steepness is an important index for evaluating the shape of the strip by dividing the wave width (wave height) of the cold rolled steel strip by the length (wave length) of the wave and multiplying by 100 (wave height / wavelength ⁇ 100).
  • a step of pickling the hot-rolled steel sheet to remove the oxide layer may be further included.
  • the cold-rolled steel sheet is continuously annealed in a temperature range of Ac3-50 ° C to Ac3 + 30 ° C.
  • the present invention is for producing a high-strength steel sheet excellent in both tensile properties and elongation flangeability.
  • control of the subsequent annealing step is important.
  • the temperature range during final annealing is very important.
  • the annealing temperature is lower than Ac3-50 deg. C, strength decreases due to a large amount of residual ferrite.
  • the temperature of the annealing is preferably in the range of Ac3-50 ° C to Ac3 + 30 ° C, more preferably in the range of Ac3-40 ° C to Ac3 + 20 < 0 > C.
  • the above-mentioned Ac3 means a temperature at which ferrite starts to be transformed into austenite at the time of heating.
  • the continuously annealed cold rolled steel sheet is first cooled to a temperature range of Ar1 to Ar3 at an average cooling rate of 1 to 10 ⁇ ⁇ / s. If the primary cooling end temperature is lower than Ar1, a large amount of ferrite transformation may occur and the strength of the steel may be lowered. On the other hand, if the primary cooling end temperature is higher than Ar3, the austenite fraction is too high, And the final cooling section is too long, so that the quality of the shape of the steel sheet may be deteriorated.
  • the above-mentioned Ar3 means a temperature at which a transformation of austenite into ferrite starts at the time of cooling, and Ar1 means a temperature at which a complete transformation of austenite into ferrite is completed upon cooling.
  • the cooling stop temperature is preferably in the range of Ar 1 + 50 ° C to Ar 3 - 20 ° C, more preferably Ar 1 + 40 ° C to Ar 3 - 30 ° C.
  • the primary cooling rate is preferably in the range of 1 to 10 ⁇ ⁇ / s.
  • the primary cooling rate is less than 1 ⁇ / s, a large amount of microstructures such as ferrite and pearlite are formed, and it may be difficult to secure the desired strength in the present invention.
  • the primary cooling rate exceeds 10 DEG C / s, abrupt bainite transformation or martensitic transformation occurs, which may cause a problem in that the quality of the steel sheet may deteriorate and the target ferrite fraction can not be secured It may be difficult to ensure ductility due to a high hard tissue fraction in the final cooling.
  • the primary cooling rate is preferably in the range of 1 to 10 ° C / s, more preferably in the range of 2 to 8 ° C / s.
  • the primary cooled cold rolled steel sheet is secondarily cooled to a temperature range of Mf-50 ° C to Ms + 50 ° C at an average cooling rate of 5 to 30 ° C / s.
  • the secondary cooling quench temperature is lower than Mf-50 ° C, the supersaturated C in the martensite is hardly precipitated as a carbide, so that the transformation into the martensitic structure having a better ductility than that of the martensitic structure occurs little and the ductility and stretch flangeability are secured
  • the temperature exceeds Ms + 50 ⁇ ⁇ , it is difficult to obtain martensite and the target strength may not be obtained.
  • the secondary cooling stop temperature is in the range of Mf-50 ⁇ ⁇ to Ms + 50 ⁇ ⁇ More preferably Mf-40 ° C to Ms + 40 ° C.
  • Ms is the temperature at which the martensitic transformation begins
  • Mf is the temperature at which the martensitic transformation is completed.
  • the secondary cooling rate is preferably in the range of 5 to 30 DEG C / s.
  • the secondary cooling rate is preferably in the range of 5 to 30 ⁇ ⁇ / s and more preferably in the range of 10 to 25 ⁇ ⁇ / s since the martensite fraction is exceeded and it may be difficult to ensure ductility and stretch flangeability.
  • the molten steel having the alloy composition shown in Tables 1 and 2 was prepared, and then the molten steel was continuously cast at a casting speed of 6 mpm in a continuous continuous rolling mode by applying a performance-rolling direct process, thereby obtaining a thin slab having a thickness of 90 mm, Hot-rolled steel sheets having a thickness of 2.4 mm were produced under the manufacturing conditions shown in Table 3 below.
  • a slab having a thickness of 250 mm was cast from a conventional hot-rolling mill, and then a hot-rolled steel sheet having a thickness of 3.5 mm was produced in the conventional batch process under the manufacturing conditions shown in Table 3 below.
  • each hot-rolled steel sheet thus produced was pickled and cold-rolled under the conditions shown in Table 4 to obtain a cold-rolled steel sheet (Full Hard material, hereinafter referred to as FH material), followed by annealing, primary and secondary cooling, (Cold rolled material, hereinafter referred to as CR material).
  • FH material Fel Hard material
  • CR material Cold rolled material
  • the temperatures Ac3, Ar3, Ar1, Ms, and Mf in Table 3 were calculated using JmatPro-v9.1, a commercial thermodynamic software.
  • the cold-rolled steel sheets thus produced were measured for their microstructure and mechanical properties, and the results are shown in Table 6 below.
  • the area fraction of ferrite (F), martensite (M) and auto-tempered martensite (AM) was 10 times at 500 times and 5,000 times magnification using an optical microscope and a scanning electron microscope (SEM) was randomly photographed, and the area ratio was measured using Image-Plus Pro software, and the result was expressed as an average value.
  • the uniaxial grain size and distribution of ferrite (F), martensite (M) and auto-tempered martensite (AM) were randomly photographed at a magnification of 3,000 times using SEM at 10 locations, And the size of the short axis was measured and expressed as an average value.
  • the tensile strength is measured by taking JIS No. 5 specimens in a direction perpendicular to the rolling direction (C direction) at a width of w / 2.
  • the length deviation of the tensile strength is measured by measuring the tensile strength And the difference in intensity values.
  • the Hole Expanding Ratio was measured by punching the hole with a diameter of 10.8 mm and then pushing it up to the cone to increase the diameter of the expanded hole until the crack was generated in the circumference portion to the initial diameter (10.8 mm) Calculated as a percentage.
  • the ductility ratios were evaluated by welding using the electric resistance spot welding conditions (specified in ISO 18728-2) listed in Table 5. The results are shown in Table 6.
  • the target microstructure characteristics The tensile strength of not less than 800 MPa, the elongation of not less than 10%, the tensile strength in the longitudinal direction of not more than 35 MPa, the elongation flange ratio of not less than 35%, and the ductility ratio of not less than 35% Able to know.
  • Inventive Examples 1 to 7 show that the strip cutting length is lower than that of Conventional Example 1 produced in the batch mode in the conventional hot melt mill, resulting in a high rate of yield and a small deviation in tensile strength in the longitudinal direction.
  • Comparative Examples 1 to 5 do not satisfy at least one of the alloy composition and the component relational expressions 1 to 4 proposed by the present invention, so that casting due to the molten steel outflow has occurred or due to a difference in the microstructure fraction, It can be seen that the strength and ductility ratio are not ensured.
  • FIGS. 4 and 5 are photographs of SEM micrographs and SEM micrographs of Inventive Example 1, respectively.
  • ferrite (F) and auto-tempered martensite (AM) are composed of major phases and some martensite (M) structure is not tempered can confirm.
  • FIG. 6 is a TEM photograph of a transmission electron microscope (TEM) of Inventive Example 1, which is a photograph of a magnification of 100,000 magnification obtained by enlarging the [X] and [Y] portions on the left 50,000 times and the right side on the left photograph.
  • TEM transmission electron microscope
  • FIG. 7 and 8 are graphs showing the ferrite (F) single crystal grain size distribution of Inventive Example 1 and the martensite (M) + autothermite martensite (A.M) single axis grain size distribution, respectively.
  • ferrite (F) and martensite (M) + auto-temperate martensite (AM) structures having a minor axis grain size of 0.2 to 5.0 ⁇ m are distributed in Inventive Steel 1, Is in the range of 0.2 to 3.0 ⁇ ⁇ .
  • FIG. 9 is a photograph of a precipitate of Inventive Example 1 taken by a transmission electron microscope (TEM).
  • the precipitate size was a sample prepared by the carbon repulping method, and 5 sheets of 50,000 times and 10 sheets of 100,000 times were taken by TEM at random, and the precipitate size was measured using Image-Plus Pro software.
  • TEM transmission electron microscope
  • FIGS. 10 and 11 are graphs showing the distribution of the size of the precipitate of Inventive Example 1 and the spacing between the precipitates, respectively.
  • precipitates having a size of 1 to 30 nm exist in Inventive Example 1, and precipitates having a size of 1 to 15 nm are mainly distributed. It can also be seen that the precipitates are present in a large amount in the intervals of 10 to 150 nm.
  • the molten steel having the alloy composition of Inventive Steel 1 of Table 1 and the conventional steel was prepared, the molten steel was continuously cast at a casting speed of 6.8 mpm by applying a performance-rolling direct process to obtain a 90 mm thick thin slab, Rolled steel sheets having a thickness of 16 mm, and then hot-rolled steel sheets were produced under the manufacturing conditions described in the following Table 7, and the cold-rolled steel sheets were produced under the production conditions described in Table 8 below.
  • the thus-prepared cold-rolled steel sheet was measured for microstructure, mechanical properties and ductility ratio, and the results are shown in Table 9 below.
  • the physical properties were measured under the same conditions as in Example 1.
  • the target microstructure characteristics are all satisfied, Mechanical properties.
  • the inventive examples 8 to 10 show that the strip cutting length is low compared to the conventional examples 2 and 3 produced in the batch mode in the hot-melt mill, so that the yield rate is high and the tensile strength deviation in the longitudinal direction is small.
  • the molten steel was continuously cast at a casting speed of 6.8 mpm by applying a performance-rolling direct process, thereby obtaining a 90 mm thick thin slab, Hot rolled steel sheets were produced under the manufacturing conditions described in the following Table 10, and then cold rolled and annealed under the same conditions as in Example 2 of Table 4 to prepare cold rolled steel sheets.
  • the thus obtained hot-rolled steel sheet and cold-rolled steel sheet were measured for tensile strength in the width direction, and the results are shown in Table 11 below.

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Abstract

Un mode de réalisation de la présente invention concerne une tôle d'acier laminée à froid à haute résistance ayant une faible variation de propriété mécanique et une excellente aptitude à la formation de bords par étirage et à la récupération, et son procédé de fabrication. La tôle d'acier laminée à froid à haute résistance comprend C : de 0,030 à 0,080 % en poids, Mn : de 1,5 à 3,0 % en poids, Si : de 0,03 à 0,50 % en poids, P : de 0,001 à 0,045 % en poids, S : de 0,0005 à 0,0035 % en poids, Cr : de 0,5 à 2,5 % en poids, Al : 0,05 % en poids ou moins, Ti : de 0,005 à 0,10 %, Nb : de 0,005 à 0,10 % en poids, B : de 0,0005 à 0,004 % en poids, N : de 0,001 à 0,010 % en poids, le reste étant du Fe et d'autres impuretés inévitables et la tôle satisfaisant aux expressions relationnelles suivantes 1 à 4, la microstructure contenant, en fraction surfacique, de 30 à 70 % de ferrite et de 30 à 70 % de martensite et de martensite auto-revenue additionnées. [Expression relationnelle 1] [C] ≤ [Cp], [Expression relationnelle 2] 1,5 ≤ [C × Cp] × 1000 ≤ 3,5, [Expression relationnelle 3] 165 ≤ [Ceq] × 1000 ≤ 235 et [Expression relationnelle 4] 50 ≤ [Ceq]/ ([C × Cp]) ≤ 150 (dans les expressions relationnelles 1 à 4, [Cp] est égal à 0,09 - 0,038C - 0,013Mn - 0,028i + 0,00015Cr, [Ceq] est égal à C + Si/30 + Mn/20 +2P +3S, la teneur de chacun de C, Mn, Si, Cr, P et S étant représentée e pourcentage en poids).
PCT/KR2018/016263 2017-12-26 2018-12-19 Tôle d'acier laminée à froid à haute résistance présentant une faible variation de propriété mécanique et une excellente aptitude à la formation de bords par étirage et à la récupération et son procédé de fabrication WO2019132400A1 (fr)

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