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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/021—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving particular fabrication steps or treatments of ingots or slabs
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • 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
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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• • C—CHEMISTRY; METALLURGY
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • 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
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  • 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 steel plate and a manufacturing method thereof.
• Patent Document 1 describes a high-strength steel plate containing C: 0.1-0.25%, Si: 0.1-0.5%, Mn: 0.5-2.0%, Cr: 0.1-1.5%, Mo: 0.1-0.5%, Ti: 0.01-0.05%, Nb: 0.01-0.05%, V: 0.01-0.05% and/or B: 0.0001-0.005%, with the balance consisting of iron and unavoidable impurities, and characterized in that the average grain size of prior austenite is 20 ⁇ m or less and the standard deviation ( ⁇ ) of the prior austenite grain size distribution is 5 ⁇ m or less.
• Patent Document 1 also teaches that the above configuration can refine the prior austenite grain size and reduce its variation, thereby realizing a high-strength steel plate with improved bending workability while maintaining a high tensile strength of 980 MPa or more.
• Patent Document 2 describes a high-strength, high-ductility fine martensite structure steel material that contains 0.075-0.3 wt% C, 3-10 wt% Mn, 0-2.5 wt% Si, with the balance being Fe and unavoidable impurities, has a prior ⁇ grain size of 2.0 ⁇ m or less, and has a structure that is equiaxed martensite with a single block.
• Patent Document 2 also teaches that the high-strength, high-ductility fine martensite structure steel material can achieve a tensile strength of 1200 MPa or more and a total elongation of 10% or more.
• Patent Document 3 describes a high-tensile steel material containing, by mass, C: 0.06-0.19%, Si: 0.15-0.60%, Mn: 0.60-1.80%, Cr: 0.05-1.20%, Mo: 0.05-1.00%, and one or more of Nb: 0.005-0.10%, V: 0.005-0.10%, and Ti: 0.005-0.10%, and containing, by volume, 0.01-0.8% carbonitrides of Nb, Ti or V having a particle size of 100 nm or less, and in which the prior ⁇ grains have a grain size number of 7 or more and contain a martensite structure or a mixed structure of martensite and bainite within the prior ⁇ grains.
• Patent Document 3 also teaches that the above configuration makes it possible to provide a high-strength steel material that is excellent in toughness, arrestability, and weldability, has a large uniform elongation characteristic exceeding 10%, and is suitable for mass production.
• the present invention was made in consideration of these circumstances, and its purpose is to provide a steel plate and its manufacturing method that, through a novel configuration, has high strength but also has improved hole expansion properties and work hardening ability.
• the inventors conducted research focusing on the metal structure of steel plate, particularly hot-rolled steel plate.
• the inventors discovered that by configuring the metal structure of a hot-rolled steel plate having a specified chemical composition to be mainly composed of martensite, it is possible to achieve high strength and improved hole expandability, and by limiting the average grain size of prior austenite grains in the metal structure to within a specified range while increasing the variation in the grain size of the prior austenite grains, it is possible to significantly improve the work hardening ability, thus completing the present invention.
• the present invention which has achieved the above object is as follows.
• the chemical composition is, in mass%, Ti: 0.001 to 0.200%, V: 0.001-0.300%, Cu: 0.001-0.40%, Cr: 0.001-0.90%, Mo: 0.001-0.12%, Ni: 0.001 to 0.30%, B: 0.0001 to 0.0030%, Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, Bi: 0.001-0.010%, Zr: 0.001 to 0.050%, Co: 0.001 to 0.010%, Zn: 0.001-0.010%, W: 0.001-0.100%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, and REM: 0.0001 to 0.0100%
• the steel sheet according to the above (1) characterized in that it contains at least one of the following: (3)
• the metal structure further comprises, in area percent: Ferrite: 10.0% or less,
• the steel plate according to the above (1) or (2) characterized in that it contains at least one of bainite: 10.0% or less
• a method for producing a steel plate comprising: a cooling step of starting cooling of the finish-rolled steel plate within 0.5 to 10.0 seconds after completion of the hot rolling step, and then cooling the steel plate to a temperature of 400°C or less within 20.0 seconds from the start of cooling; and a coiling step of coiling the cooled steel plate in a temperature range of 400°C or less.
• the present invention provides a steel sheet, particularly a hot-rolled steel sheet, and a manufacturing method thereof, that has high strength but also has improved hole expansion properties and work hardening ability.
• the steel sheet according to the embodiment of the present invention has a chemical composition, in mass%, C: 0.040-0.200%, Si: 0.30-2.00%, Mn: 1.00-4.00%, sol.
• the metal structure is, in area percent, Martensite: 90.0% or more; and Retained austenite: 3.0% or less;
• the properties such as hole expandability decrease with increasing strength of steel.
• a steel sheet that has excellent hole expandability while maintaining high strength, especially a high strength of tensile strength of 980 MPa or more that enables weight reduction, is required.
• the metal structure of the steel sheet is composed mainly of martensite.
• martensitic steel has a hierarchical structure that includes substructures such as packets, blocks, and laths in the prior austenite grains, and although it has excellent strength, it generally has a problem of low workability.
• the inventors therefore conducted a study focusing on the metal structure of the hot-rolled steel sheet, in addition to making the chemical composition of the steel sheet, particularly the hot-rolled steel sheet, appropriate.
• the inventors discovered that by forming the metal structure of a hot-rolled steel sheet having a predetermined chemical composition with a structure mainly composed of martensite, more specifically, a structure containing martensite: 90.0% or more and retained austenite: 3.0% or less in area percentage, it is possible to achieve high strength, for example, high strength with a tensile strength of 980 MPa or more, while significantly improving the hole expandability of the resulting hot-rolled steel sheet.
• the metal structure by making the metal structure into a more uniform structure with martensite at 90.0% or more in area percentage, it is possible to reduce the hardness difference in the metal structure compared to a case in which other structures softer than martensite, such as ferrite, are relatively contained in large amounts, and it is believed that the hole expandability can be improved due to such a reduction in hardness difference.
• retained austenite can be the starting point of fracture during deformation in press forming, etc., by controlling martensite to 90.0% or more by area, and limiting the retained austenite to 3.0% or less by area, it is possible to more significantly improve hole expansion properties.
• the inventors of the present invention have investigated the improvement of work hardening ability from the viewpoint of optimizing the grain size of the prior austenite grains in a metal structure mainly composed of martensite, because the prior austenite grain boundaries act as a resistance force against the movement of dislocations and are considered to be effective in improving work hardening ability. More specifically, by refining the prior austenite grains, the density of the prior austenite grain boundaries can be increased. Therefore, by refining the prior austenite grains, it is possible to increase the hindrance of dislocations, and therefore to improve the work hardening ability.
• the inventors discovered that by refining the prior austenite grains within a predetermined range, more specifically by controlling the average grain size of the prior austenite grains to 30.0 ⁇ m or less, the work-hardening ability of the entire hot-rolled steel sheet can be improved, while increasing the variation in the grain size of the prior austenite grains, more specifically by controlling the standard deviation in the grain size of the prior austenite grains to 4.0 ⁇ m or more, a high work-hardening rate can be achieved even in a state where a certain amount of strain has been introduced, such as in the later stages of deformation in press forming.
• a mixed grain structure in which coarse grains and fine grains are mixed can be formed, and such a mixed grain structure contributes to a high work hardening rate in the later stage of deformation such as press forming. More specifically, by forming a mixed grain structure in which coarse grains and fine grains are mixed, non-uniform deformation is induced during processing such as press forming, and as a result, sufficient work hardening ability can be maintained even in the later stage of deformation, and therefore a high work hardening rate can be achieved.
• the steel sheet according to the embodiment of the present invention can be stably formed because it maintains high work hardening ability.
• the fact that the work hardening ability of a steel sheet can be improved by increasing the variation in the grain size of prior austenite grains to form a mixed grain structure in which coarse grains and fine grains are mixed in a metal structure mainly composed of martensite was not previously known, and was now revealed for the first time by the present inventors.
• the steel sheet according to the embodiment of the present invention it is possible to significantly improve the hole expansion ability and work hardening ability, despite the high strength, for example, tensile strength of 980 MPa or more. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent workability, and is therefore particularly useful in the automotive field where both properties are required to be achieved.
• C is an element effective in increasing the strength of steel plate.
• C forms carbides and/or carbonitrides with Nb in steel, and refines the structure due to the pinning effect of the precipitates formed.
• the C content is set to 0.040% or more.
• the C content is set to 0.060% or more, 0.080% or more, 0.100% or more, or 0 120% or more.
• the C content is set to 0.200% or less.
• the C content is set to 0.180 % or less, 0.160% or less, 0.150% or less, or 0.140% or less.
• Silicon is an effective element for increasing strength as a solid solution strengthening element.
• the silicon content is set to 0.30% or more.
• the silicon content is set to 0.40% or more. More than 0.50%, 0.51% or more, 0.52% or more, 0.53% or more, 0.54% or more, 0.55% or more, more than 0.55%, 0.60% or more, 0.
• the Si content is set to 2.00% or less.
• the Si content is set to 1.80% or less, 1.60% or less, 1.50% or less, or 1.40% or less. % or less.
• Mn is an element that is effective in increasing strength as an element for hardenability and solid solution strengthening.
• the Mn content is set to 1.00% or more.
• the Mn content is 1.20%.
• the Mn content may be 1.50% or more, 1.80% or more, 2.00% or more, or 2.20% or more.
• the Mn content is set to 4.00% or less.
• the Mn content is set to 3.80% or less, 3.50% or less, 3.20% or less, 3.00% or less, or 2.80% or less. Good too.
• sol. Al is an element that acts as a deoxidizer for molten steel.
• Sol. Al is also an element that suppresses the precipitation of cementite, which is detrimental to hole expandability.
• sol. The sol. Al content is 0.001% or more.
• the sol. Al content is 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, or 0.100% or more.
• the sol. Al content is set to 0.500% or less.
• the content may be 0.400% or less, 0.300% or less, or 0.200% or less.
• Sol. Al means acid-soluble Al, which is present in the steel in a solid solution state. This indicates that.
• P 0.100% or less
• the P content is set to 0.100% or less.
• the P content is set to 0.050% or less, 0.030% or less
• the lower limit of the P content is not particularly limited and may be 0%, but excessive reduction of the P content leads to an increase in costs.
• the content may be 0.0001% or more, 0.001% or more, or 0.005% or more.
• S 0.0300% or less
• S content is set to 0.0300% or less.
• the S content is set to 0.0200% or less.
• the lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs.
• the amount may be 0.0001% or more, 0.0010% or more, or 0.0030% or more.
• N 0.0070% or less
• the N content is set to 0.0050% or less, and 0.
• the lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the N content is It may be 0.0001% or more, or 0.0005% or more.
• O is an element that is mixed in during the manufacturing process. If the O content is excessive, coarse inclusions may form, which may reduce the workability of the steel sheet. Therefore, the O content is set to 0.0100% or less.
• the O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
• the lower limit of the O content is not particularly limited and may be 0%, but 0. To reduce the O content to less than 0.0001%, a long period of time is required for refining, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.
• Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to the refinement of prior austenite grains through a pinning effect, thereby contributing to the high strength of the steel sheet.
• the Nb content is set to 0.001% or more.
• the Nb content is set to 0.005% or more, 0.010% or more, 0.050% or more, 0.100% or more, 0.200% or more, or
• the Nb content is set to 1.
• the Nb content may be 0.800% or less, 0.600% or less, or 0.500% or less.
• the basic chemical composition of the steel plate according to the embodiment of the present invention is as described above. Furthermore, the steel plate may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.
• Cr is an element that enhances the hardenability of steel and contributes to improving strength and/or corrosion resistance.
• the Cr content may be 0%, but in order to obtain these effects, the Cr content is The content of Cr is preferably 0.001% or more, and may be 0.01% or more, 0.05% or more, or 0.10% or more.
• the Cr content is preferably 0.90% or less, more preferably 0.70% or less, 0.50% or less, 0.40% or less, or 0.30% or less. may be also possible.
• Ti, V, Cu, Mo, Ni, B, Ca, Mg, Bi, Zr, Co, Zn, W, Sn, As, and REM may be contained in the steel sheet as optional elements, or may be added to the tramp.
• These elements may be present in the steel sheet as elements.
• the contents of these elements are as follows: Ti: 0 to 0.200% or 0.100%, V: 0 to 0.300% or 0.200%, Cu: 0 Up to 0.40% or 0.20%, Mo: 0-0.12%, 0.09%, 0.08%, 0.06% or 0.04%, Ni: 0-0.30% or 0 .
• the lower limit of these elements may be, for example, Ti, V, Cu, Mo, Ni, Bi, Zr, Co, Zn, W, Sn, and The As content may be 0.001% or more, 0.005% or more, or 0.008% or more, respectively.
• the B, Ca, Mg and REM contents may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• the remainder other than the above elements consists of Fe and impurities.
• Impurities are, for example, components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel plate is industrially manufactured. It is permissible for them to be included within a range that does not affect the effects of the present invention.
• the chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method.
• the chemical composition of the steel plate may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES).
• C and S may be measured using the combustion-infrared absorption method
• N may be measured using the inert gas fusion-thermal conductivity method
• O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
• the metal structure of the steel plate according to the embodiment of the present invention includes, in terms of area%, martensite: 90.0% or more, and retained austenite: 3.0% or less.
• the hard martensite by controlling the hard martensite to within a range of 90.0% or more in terms of area% to make a more uniform structure, not only can it contribute to high strength, but it can also reduce the hardness difference in the metal structure, and the hole expandability can be improved due to such a reduction in hardness difference.
• the area ratio of martensite is less than 90.0%, it is not possible to achieve the desired strength and hole expandability. From the viewpoint of further increasing strength and improving hole expandability, the higher the area ratio of martensite, the more preferable it is, and it may be, for example, 92.0% or more, 94.0% or more, 96.0% or more, or 98.0% or more.
• the upper limit of the area ratio of martensite is not particularly limited and may be 100.0%, for example, 99.0% or less.
• the retained austenite can be a starting point of fracture during deformation such as press forming, in addition to controlling the martensite to 90.0% or more in area%, the retained austenite can be restricted to 3.0% or less in area%, thereby making it possible to improve the hole expandability more significantly. If the area ratio of the retained austenite exceeds 3.0%, it becomes a starting point of fracture during deformation, and the hole expandability decreases.
• the lower the area ratio of the retained austenite the more preferable it is, and may be, for example, 2.5% or less, 2.0% or less, 1.5% or less, or 1.0% or less.
• the lower limit of the area ratio of the retained austenite is not particularly limited and may be 0%, for example, 0.5% or more.
• the remaining structure other than martensite and retained austenite may be 0% in terms of area percent, but when the remaining structure exists, the remaining structure may include at least one of ferrite: 10.0% or less, bainite: 10.0% or less, and pearlite: 10.0% or less. If the area ratio of at least one of ferrite, bainite, and pearlite exceeds 10.0% in total, the area ratio of martensite will be less than 90.0%, and as a result, the desired strength and hole expandability cannot be achieved.
• the lower limits of ferrite, bainite, and pearlite may each be 0%, and may be, for example, 0.1% or more, 0.5% or more, 1.0% or more, 2.0% or more, or 3.0% or more, respectively.
• the upper limits of ferrite, bainite, and pearlite may each be 8.0% or less, 6.0% or less, 5.0% or less, or 4.0% or less, respectively.
• Identification of the metal structure in the steel sheet and calculation of the area ratio are performed by optical microscope observation after corrosion using a Nital reagent or Lepera solution and X-ray diffraction method.
• the microstructure observation by optical microscope is performed on the plate thickness cross section in the direction perpendicular to the plate surface.
• the plate thickness cross section is preferably parallel to the rolling direction. Specifically, first, a sample is taken from the steel sheet, and the observation surface of the sample is etched with Nital.
• image analysis is performed on a microstructure photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope, thereby calculating the total area ratio of martensite and bainite, and each area ratio of ferrite and pearlite.
• image analysis is performed on a microstructure photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope, thereby calculating the total area ratio of martensite and retained austenite.
• the volume fraction of retained austenite is calculated by X-ray diffraction measurement. Since the volume fraction of retained austenite is equivalent to the area fraction, this is taken as the area fraction of retained austenite.
• the area fraction of martensite is calculated by subtracting the obtained area fraction of retained austenite from the total area fraction of martensite and retained austenite previously calculated.
• the area fraction of bainite is calculated by subtracting the obtained area fraction of martensite from the total area fraction of martensite and bainite previously calculated in the same manner.
• the average grain size of the prior austenite grains is 30.0 ⁇ m or less.
• the prior austenite grain boundaries act as a resistance force against the movement of dislocations, and are considered to be effective in improving the work hardening ability.
• the density of the prior austenite grain boundaries can be increased by refining the prior austenite grains. Therefore, by refining the prior austenite grains to 30.0 ⁇ m or less, it is possible to increase the hindrance of dislocations, and therefore it is possible to improve the work hardening ability of the resulting steel plate.
• the average grain size of the prior austenite grains may be, for example, 4.0 ⁇ m or more, 4.1 ⁇ m or more, 4.2 ⁇ m or more, 4.5 ⁇ m or more, 4.7 ⁇ m or more, 5.0 ⁇ m or more, 8.0 ⁇ m or more, 10.0 ⁇ m or more, or 12.0 ⁇ m or more.
• the standard deviation in the grain size of the prior austenite grains is 4.0 ⁇ m or more.
• the upper limit is not particularly specified, since the average grain size of the prior austenite grains is 30.0 ⁇ m or less, the upper limit of the standard deviation is naturally limited and cannot take any value. Although the upper limit is not particularly limited, the standard deviation in the grain size of the prior austenite grains may be, for example, 20.0 ⁇ m or less, 15.0 ⁇ m or less, 12.0 ⁇ m or less, 10.0 ⁇ m or less, or 8.0 ⁇ m or less.
• the grain size of the prior austenite grains in the metal structure is 30.0 ⁇ m or less while controlling the standard deviation in the grain size of the prior austenite grains to 4.0 ⁇ m or more. This is because if either one of the characteristics is not satisfied, at least one of the effects of improving the work hardening ability due to the refinement of the prior austenite grains and the effect of improving the work hardening ability due to the mixed grain structure of coarse grains and fine grains will be insufficient.
• the grain size of the prior austenite grains is generally relatively uniform, that is, the standard deviation in the grain size is relatively small.
• the inventors have discovered for the first time that by carrying out the continuous casting process, hot rolling process, and cooling process of the slab under appropriate conditions, it is possible to form a metal structure in which coarse grains and fine grains are mixed while refining the prior austenite grains, and further, to improve the work hardening ability based on such a metal structure. Therefore, according to the steel sheet according to the embodiment of the present invention, it is possible to achieve high strength and improved hole expansion property by using a metal structure that is mainly composed of martensite, has refined prior austenite grains, and is a mixture of coarse grains and fine grains, while significantly improving the work hardening ability.
• the average aspect ratio of the prior austenite grains is not particularly limited, but may be, for example, 3.0 or less, 2.5 or less, 2.0 or less, 1.8 or less, 1.6 or less, or 1.4 or less.
• the lower limit is not particularly limited, but, for example, the average aspect ratio of the prior austenite grains may be 0.6 or more, 0.7 or more, or 0.8 or more.
• the present invention aims to provide a steel sheet having high strength but improved hole expandability and work hardening ability, and achieves the above object by forming the metal structure of a steel sheet having a predetermined chemical composition from a structure mainly composed of martensite, and limiting the average grain size of the prior austenite grains in the metal structure to a predetermined range while increasing the variation in the grain size of the prior austenite grains. Therefore, it is clear that the average aspect ratio of the prior austenite grains is not an essential technical feature for achieving the object of the present invention.
• the average grain size of the prior austenite grains, the standard deviation in the grain size of prior austenite grains, and the average aspect ratio of prior austenite grains are determined as follows. First, a sample is cut out from an arbitrary position 50 mm or more away from the end face of the steel plate (if a sample cannot be taken from this position, a position avoiding the end) so that the plate thickness cross section perpendicular to the plate surface can be observed.
• the plate thickness cross section is preferably parallel to the rolling direction.
• the size of the sample depends on the measuring device, but is set to a size that allows observation of about 10 mm in the direction perpendicular to the plate thickness direction.
• the cross section of the above sample is polished using silicon carbide paper of #600 to #1500, and then finished to a mirror surface using a dilution liquid such as alcohol or a liquid in which diamond powder with a grain size of 1 to 6 ⁇ m is dispersed in pure water. Next, the observation surface is finished by electrolytic polishing.
• an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used, for example, an EBSD analyzer consisting of a JSM-7001F manufactured by JEOL and a DVC5 type detector manufactured by TSL.
• the degree of vacuum in the EBSD analyzer may be 9.6 ⁇ 10 ⁇ 5 Pa or less
• the acceleration voltage may be 15 kV
• the irradiation current level may be 13.
• the crystal orientation of the prior austenite grains is calculated from the crystal orientation relationship between general prior austenite grains and crystal grains having a body-centered structure after transformation. The following method is used to calculate the crystal orientation of the prior austenite grains. First, a crystal orientation map of the prior austenite grains is created by the method described in Acta Materialia, 58 (2010), 6393-6403.
• the average value of the shortest diameter and the longest diameter is calculated, and this average value is taken as the grain size of that prior austenite grain.
• the above operation is performed for all prior austenite grains, excluding prior austenite grains whose entire crystal grains are not included in the observation field, such as the ends of the observation field, to determine the grain sizes of all prior austenite grains in the observation field.
• the average grain size and standard deviation are calculated from the grain sizes of all the prior austenite grains obtained, thereby determining the average grain size and standard deviation of the grain size of the prior austenite grains.
• the ratio of the diameter in the plate thickness direction to the diameter in the rolling direction is calculated, and this value is regarded as the aspect ratio of the prior austenite grain. If the rolling direction is unknown, the cross section is observed at 0°, 45°, 90°, and 135° to an arbitrary direction, and the cross section with the highest aspect ratio among them is regarded as the cross section parallel to the rolling direction, and the ratio of the diameter in the plate thickness direction to the diameter in the rolling direction (rolling direction diameter/plate thickness direction diameter) is calculated.
• the above operation is performed for all prior austenite grains, except for prior austenite grains whose entire crystal grains are not included in the photographed field, such as the ends of the photographed field, to determine the aspect ratio of all prior austenite grains in the photographed field.
• the average aspect ratio of the prior austenite grains is determined by arithmetically averaging the aspect ratios of all the prior austenite grains obtained.
• the steel sheet according to the embodiment of the present invention generally has a sheet thickness of 1.0 to 8.0 mm, although it is not particularly limited thereto.
• the sheet thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 7.0 mm or less, 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.4 mm or less, 4.2 mm or less, or 4.0 mm or less.
• an automotive part particularly an automobile suspension part
• examples of automobile suspension parts include lower arms and trailing arms.
• These automotive parts, particularly automobile suspension parts only need to include the steel plate according to the embodiment of the present invention in at least a part of these parts, and therefore at least a part of these parts will satisfy the above-mentioned chemical composition and structure characteristics.
• the characteristics of the steel plate do not change particularly before and after forming.
• a part of the steel plate that has been processed relatively less is determined by characteristics such as a smooth shape that has not been subjected to deformation such as bending, and a small rate of increase or decrease in plate thickness.
• the upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel sheet may be 1780 MPa or less, 1700 MPa or less, or 1600 MPa or less.
• the tensile strength is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011. For example, when it is difficult to take a JIS No. 5 test piece due to dimensional constraints, other test pieces described in JIS Z 2241:2011 can be used.
• the lower limit is set to 0.5 mm in order to perform an appropriate evaluation.
• a micro-Vickers test in accordance with JIS Z 2244-1:2020 can be performed, and the hardness (HV) converted into tensile strength can be used.
• the sample to be subjected to the micro-Vickers test can be prepared in the same manner as the sample to evaluate the average grain size and aspect ratio of the prior austenite grains.
• the micro-Vickers test can be performed by measuring 30 points at 1/4 of the plate thickness with a load of 500 gf, and the average value can be used.
• the conversion can be performed by the following formula.
• Tensile strength [MPa] 3.12 x Vickers hardness [HV] + 16
• the hole expansion ratio may be preferably 50% or more, more preferably 60% or more or 70% or more.
• the upper limit of the hole expansion ratio is not particularly limited, but for example, the hole expansion ratio may be 150% or less, 120% or less, or 100% or less.
• the hole expansion ratio is determined as follows.
• the initial hole is expanded with a conical punch with an apex angle of 60° until a crack penetrating the plate thickness occurs, and the hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio ⁇ (%) of each test piece is calculated by the following formula.
• the method for producing a steel sheet according to an embodiment of the present invention includes: A continuous casting process for casting a slab having the chemical composition described above in relation to the steel plate, the average cooling rate at 600-900°C being controlled to be 10-50°C/min and the average cooling rate gradient being 40°C/ min2 or less; A heating step of heating the cast slab and holding it at a temperature of 1100° C. or higher for 6000 seconds or more; a hot rolling step including finish rolling the slab, the finish rolling satisfying the following conditions (a) to (c); (a) the rolling reduction in each rolling pass of the rolling pass one stage before the final stage and the final stage is 20 to 50%; (b) a total rolling reduction of 90% or more, and (c) a final rolling temperature of 960 to 1100°C.
• the method is characterized by including a cooling step in which cooling of the finish-rolled steel sheet is started within 0.5 to 10.0 seconds after the completion of the hot rolling step, and then cooling the steel sheet to a temperature of 400°C or less within 20.0 seconds from the start of cooling, and a coiling step in which the cooled steel sheet is coiled in a temperature range of 400°C or less.
• the temperatures described for the slab and the steel sheet refer to the surface temperature of the slab and the surface temperature of the steel sheet, respectively.
• a slab having the chemical composition described above in relation to the steel plate is cast in a continuous casting process, and the temperature history during solidification is appropriately controlled, more specifically, the average cooling rate at 600-900°C is controlled to be 10-50°C/min and the average cooling rate gradient is controlled to be 40°C/min2 or less.
• the average cooling rate at 600-900°C is 10-50°C/min and the average cooling rate gradient is 40°C/ min2 or less, it becomes possible to achieve the desired average grain size and standard deviation of the grain size of prior austenite grains in the metal structure of the finally obtained steel plate.
• the average cooling rate at 600 to 900°C is less than 10°C/min, the crystal grains that transform into a body-centered cubic structure (bcc structure) during solidification will become coarse due to the slow average cooling rate, and the average grain size of the prior austenite grains in the final metal structure will be larger than 30.0 ⁇ m. In this case, sufficient work hardening capacity will not be achieved in the resulting steel plate.
• the average cooling rate at 600 to 900°C exceeds 50°C/min, the average cooling rate is too fast, so the crystal grains become fine and uniform during the transformation process of the solidification structure, and although the average grain size of the prior austenite grains in the final metal structure will be small, the variation in grain size cannot be increased. In other words, the standard deviation in the grain size of the prior austenite grains will be smaller than 4.0 ⁇ m, and similarly sufficient work hardening capacity will not be achieved.
• the average cooling rate gradient at 600 to 900 ° C. refers to the average rate of change of the cooling rate per unit time at 600 to 900 ° C.
• the average cooling rate gradient in this manufacturing method is 40 ° C./min 2
• the average cooling rate gradient in this manufacturing method is 40 ° C./min 2.
• the average cooling rate gradient from 600 to 900° C. is preferably 30° C./min 2 or less. Although there is no particular lower limit, the average cooling rate gradient from 600 to 900° C. may be 2° C./min 2 or more, or 3° C./min 2 or more.
• the cast slab is heated in the next heating step and held at a temperature range of 1100 ° C or higher for 6000 seconds or more.
• holding at a temperature range of 1100 ° C or higher includes not only the case where the slab temperature is held at a constant temperature of 1100 ° C or higher, but also the case where the slab temperature is held fluctuatingly in a temperature range of 1100 ° C or higher.
• the coarse carbides present in the structure can be completely dissolved, and the starting point of cracks can be eliminated. If the holding temperature is less than 1100 ° C or the holding time is less than 6000 seconds, the solid solution of the coarse carbides is incomplete.
• the upper limit of the heating temperature of the slab is preferably 1300° C. or less or 1200° C. or less.
• the upper limit of the holding time in the temperature range of 1100° C. or more is preferably 10,000 seconds or less.
• the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
• the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
• recrystallization can be promoted to refine the metal structure, and the average aspect ratio of the prior austenite grains can also be reduced. If the reduction ratio in each rolling pass from the final stage to the stage before the final stage and/or the final stage is less than 20%, recrystallization may not be completed or may not be sufficiently promoted, and the desired average grain size of the prior austenite grains may not be achieved in the metal structure of the finally obtained steel sheet, and/or the average aspect ratio of the prior austenite grains may become relatively large. If the desired average grain size of prior austenite grains cannot be achieved, sufficient work hardening ability cannot be obtained.
• the reduction ratio in each rolling pass from the last stage to the stage before and/or the last stage is set to 50% or less.
• the reduction ratio in each rolling pass from the last stage to the stage before and the last stage is set to 45% or less.
• the total reduction in the finish rolling is controlled to 90% or more. Since Mn contained in the steel is an element that reduces the fracture energy of the grain boundary, if there is a region where Mn is locally concentrated, crack generation during plastic deformation in press forming or the like may be promoted. Therefore, from the viewpoint of further improving the hole expandability, it is effective to suppress or reduce the local concentration of Mn.
• Mn can be diffused into the steel, and in connection with this, it is possible to suppress or reduce the variation in the Mn concentration in the steel, that is, to suppress or reduce the local concentration of Mn.
• Total rolling reduction (%) (thickness before finish rolling - thickness after finish rolling) / thickness before finish rolling x 100
• the final rolling temperature (the end temperature of the finish rolling) is also very important in controlling the metal structure of the steel sheet. If the final rolling temperature is less than 960°C, recrystallization may not be completed or may not be sufficiently promoted, and the desired average grain size of the prior austenite grains may not be achieved in the metal structure of the finally obtained steel sheet, and/or the average aspect ratio of the prior austenite grains may become relatively large. If the desired average grain size of the prior austenite grains cannot be achieved, sufficient work hardening ability cannot be obtained.
• the final rolling temperature is more than 1100°C
• the prior austenite grains become coarse overall, and the desired average grain size of the prior austenite grains and/or the standard deviation in the grain size of the prior austenite grains may not be achieved. In this case, it is natural that sufficient work hardening ability cannot be obtained.
• the cooled steel sheet is coiled in a temperature range of 400° C. or less to produce a steel sheet. If the coiling temperature exceeds 400° C., the area ratio of martensite becomes less than 90.0%, as in the case of the cooling process, and as a result, the desired strength and/or hole expandability cannot be obtained.
• the steel sheet manufactured by the above manufacturing method has a more uniform metal structure containing, by area percentage, 90.0% or more of martensite and 3.0% or less of retained austenite, and thus can achieve high strength, for example, tensile strength of 980 MPa or more, while significantly improving hole expandability due to reduced hardness difference. Furthermore, by controlling the average grain size of the prior austenite grains in the metal structure to 30.0 ⁇ m or less, the work hardening ability of the steel sheet as a whole can be improved, while controlling the standard deviation in the grain size of the prior austenite grains to 4.0 ⁇ m or more, it is possible to achieve a high work hardening rate even in a state where a certain degree of strain is introduced, such as in the later stage of deformation in press forming. Therefore, the steel sheet manufactured by the above manufacturing method can reliably achieve the contradictory properties of high strength and excellent workability at the same time, and is particularly useful in the automotive field where both properties are required to be achieved.
• steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the tensile strength (TS), hole expansion ratio ( ⁇ ), and work hardening ratio (WHR) of the resulting steel sheets were investigated.
• TS tensile strength
• ⁇ hole expansion ratio
• WHR work hardening ratio
• molten steel was cast by continuous casting under the conditions shown in Table 3 to form slabs having various chemical compositions shown in Tables 1 and 2. These slabs were heated to a temperature of 1100-1200°C and held for the times shown in Table 3, and then hot-rolled. Hot rolling was performed by rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all examples and comparative examples, and the finish rolling was performed under the conditions shown in Table 3 using a tandem rolling mill consisting of five rolling stands. Finally, the finish-rolled steel plates were cooled and coiled under the conditions shown in Table 3 to obtain steel plates having the plate thicknesses shown in Table 4.
• the properties of the resulting steel plates were measured and evaluated using the following methods.
• TS Tensile strength
• the burr was placed on the die side, and the initial hole was pushed open with a conical punch having an apex angle of 60 ° until a crack penetrating the plate thickness occurred, and the hole diameter d1 mm at the time of the crack occurrence was measured, and the hole expansion ratio ⁇ (%) of each test piece was calculated using the following formula.
• Comparative Example 4 the average cooling rate at 600 to 900°C in the continuous casting process was slow, which is believed to have caused the crystal grains to become coarse. As a result, the average grain size of the prior austenite grains in the final metal structure became large, and the work-hardening ability of the steel plate decreased. In Comparative Example 5, the average cooling rate at 600 to 900°C in the continuous casting process was fast, which is believed to have caused the crystal grains to become fine and uniform during the transformation process of the solidification structure. As a result, the standard deviation in the grain size of the prior austenite grains in the final metal structure became small, and the work-hardening ability of the steel plate decreased.
• Comparative Example 11 since the final rolling temperature in the finish rolling was high, it is considered that the prior austenite grains became coarse overall. As a result, the average grain size and standard deviation of the grain size of the prior austenite grains in the final metal structure became large, and the work hardening ability of the steel sheet decreased. In Comparative Example 12, it is considered that the grain growth did not proceed sufficiently because the time from the completion of the hot rolling process to the start of the cooling process was short. As a result, the desired standard deviation of the grain size of the prior austenite grains could not be obtained, and the work hardening ability of the steel sheet decreased. In Comparative Example 13, it is considered that the grain growth proceeded too much overall because the time from the completion of the hot rolling process to the start of the cooling process was long.
• Comparative Examples 36 and 38 TS decreased due to the low C and Si contents.
• Comparative Examples 37 and 39 C and Si contents were high, respectively, so that a relatively large amount of retained austenite was generated, and ⁇ decreased.
• Comparative Example 40 the hardenability decreased due to the low Mn content, and as a result, the area ratio of martensite decreased, and TS and ⁇ decreased.
• Comparative Example 41 ⁇ decreased due to the high Mn content.
• Comparative Example 42 it is considered that the precipitation of cementite could not be sufficiently suppressed due to the low sol. Al content. As a result, ⁇ decreased.
• Comparative Example 43 it is considered that the refinement of prior austenite grains due to the pinning effect could not be sufficiently promoted due to the low Nb content. As a result, the average grain size of prior austenite grains in the finally obtained metal structure became large, and the work hardening ability of the steel sheet decreased. In Comparative Example 44, it is considered that coarse carbides, etc. were generated in the steel due to the high Nb content. As a result, ⁇ decreased.
• the metal structure contains, by area percentage, 90.0% or more of martensite and 3.0% or less of retained austenite, the average grain size of prior austenite grains is 30.0 ⁇ m or less, and the standard deviation of the grain size of prior austenite grains is 4.0 ⁇ m or more. Furthermore, as a result, it was possible to significantly improve the hole expansion ability and work hardening ability, despite the high strength of the tensile strength being 980 MPa or more.

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