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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
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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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  • C22C—ALLOYS
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  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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  • C22C—ALLOYS
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  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
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  • C22C—ALLOYS
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  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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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
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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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
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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  • C22C—ALLOYS
  • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
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  • C22C—ALLOYS
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • 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
  • C21D2211/002—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
  • 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 steel plates.
• Patent Document 1 describes the chemical composition as containing, by mass%, C: 0.020-0.070%, Si: 0.10-1.70%, Mn: 0.60-2.50%, Al: 0.01-1.00%, Ti: 0.015-0.170%, Nb: 0.005-0.050%, etc., with P limited to 0.05% or less, S: 0.010% or less, N: 0.0060% or less, and the balance being Fe and
• the hot-rolled steel sheet described herein is characterized in that the steel sheet is made of impurities, the structure contains 5-60% ferrite and 30-95% bainite in terms of area ratio, and, when the boundaries in the structure where the orientation difference is 15° or more are defined as grain boundaries and the regions surrounded by the grain boundaries and have a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains, the proportion of the crystal grains where the orientation difference within the grains is 5-14° is 20-100% in terms of area ratio.
• Patent Document 1 also teaches that by setting the proportion of crystal grains
• Patent Document 2 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 is characterized in that it contains, by volume, 0.01-0.8% carbonitrides of Nb, Ti or V having a particle size of 100 nm or less, and in that 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 2 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 large uniform elongation characteristics exceeding 10%, and is suitable for mass production.
• Patent Document 3 describes a high-carbon steel plate member containing C: 0.80% to 1.10% by mass, Si: 0.05% to 0.40% by mass, Mn: 0.05% to 0.50% by mass, Cr: 0.01% to 0.35% by mass, P: 0.03% or less, S: 0.03% or less by mass, with the remainder being Fe and unavoidable impurities, in which the circle equivalent diameter of martensite blocks defined by an orientation difference of 15° or more is 3.2 ⁇ m or less, carbides with a circle equivalent diameter of 0.2 ⁇ m or more are present in 0.6 to 5.0 area %, the average grain size of the carbides is 0.3 ⁇ m or more and 2.0 ⁇ m or less, the carbide occupancy rate on prior austenite grain boundaries is 0.35% or less, and the average KAM value measured by EBSD is 0.690 to 0.710.
• Patent Document 3 also teaches that the above configuration makes it possible to obtain a high-carbon steel plate member that has high levels of both hardness and hardness-toughness
• the steel sheet is required to exhibit high formability, for example, by having excellent work-hardening ability (ability to continue to harden) even in a state where a certain degree of strain is introduced.
• the present invention was made in consideration of these circumstances, and its purpose is to provide a steel sheet with a new structure that 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 sheets, particularly hot-rolled steel sheets.
• the inventors discovered that by configuring the metal structure of a hot-rolled steel sheet having a specified chemical composition to be mainly composed of martensite, it is possible to achieve high strength and improved hole expandability, and by appropriately controlling the proportion of crystal grains having a specified intragranular orientation difference in the martensite structure while limiting the average grain size of prior austenite grains in the metal structure to within a specified range, 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 to 0.010%, W: 0.001-0.100%, Sn: 0.001-0.040%, 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
• the present invention makes it possible to provide a steel sheet, particularly a hot-rolled steel sheet, that has improved hole expansion properties and work hardening capacity despite its high strength.
• 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 prior austenite grain boundaries act as a resistance force against dislocation motion 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 increase 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 appropriately controlling the proportion of specific crystal grains in the martensite structure, more specifically by defining a crystal grain as an area surrounded by grain boundaries with an orientation mismatch of 15° or more in martensite, and controlling the proportion of crystal grains with an intragranular orientation mismatch of 4° or more within the range of 45.0 to 70.0% by area, it is possible to achieve a high work-hardening rate even in a state where a certain degree of strain has been introduced, such as in the later stages of deformation in press forming.
• a martensite structure with non-uniformly dispersed dislocations can be formed, and that such a martensite structure contributes to a high work-hardening rate in the later stages of deformation such as press forming.
• the misorientation in crystal grains represents the distribution state of dislocations, and the more non-uniformly dispersed the dislocations are, the larger the misorientation in grains generally becomes.
• crystal grains with increased intragranular misorientation in martensite to create a certain amount of structure with non-uniformly dispersed dislocations
• non-uniform deformation develops during processing such as press forming.
• crystal grains with an intragranular misorientation of 4° or more are crystal grains with a sufficient misorientation to develop non-uniform deformation, and it is possible to achieve a desired work hardening rate by controlling such crystal grains within the range of 45.0 to 70.0% in terms of area percent.
• the steel sheet according to the embodiment of the present invention maintains high work hardening ability and can be stably formed.
• a metal structure mainly composed of martensite the fact that the work hardening ability of a steel sheet can be improved by controlling the proportion of crystal grains with an increased intragranular misorientation within a predetermined range to form a structure in which dislocations are unevenly distributed has not been known in the past, and has now been revealed for the first time by the present inventors.
• the steel sheet according to the embodiment of the present invention for example, despite the high strength of the tensile strength of 980 MPa or more, it is possible to significantly improve the hole expandability and work hardening ability. 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.
• 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.
• the Si content may be 0.50% or more, 0.60% or more, 0.70% or more, 0.85% or more, 1.00% or more, or 1.20% or more.
• the Si content is set to 2.00% or less.
• the Si content is set to 1.80% or less, and 1.00% or less. .60% or less, 1.50% or less, or 1.40% 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. In order to reduce the O content to less than 0.0001%, refining takes time, 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 steel sheets.
• 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% or 0.08%, Ni: 0-0.30% or 0.15%, B: 0-0.0030% or 0.
• each of these elements may be, for example, 0.050% or less. It may be 0.001% or more, 0.005% or more, or 0.008% or more.
• 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 to the extent that they do 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 and X-ray diffraction method after corrosion using a Nital reagent or Lepera solution.
• the structure 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 structure 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 structure 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 smaller the average grain size of the prior austenite grains the more preferable it is, and it may be, for example, 28.0 ⁇ m or less, 25.0 ⁇ m or less, 22.0 ⁇ m or less, 20.0 ⁇ m or less, 18.0 ⁇ m or less, or 15.0 ⁇ m or less.
• the average grain size of the prior austenite grains may be, for example, 3.0 ⁇ 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 ratio of prior austenite grains having an aspect ratio of 2.0 or less to all prior austenite grains may be, for example, 90.0% or more, 92.0% or more, 94.0% or more, or 96.0% or more in area %, without any particular limitation.
• the ratio of prior austenite grains having an aspect ratio of 2.0 or less to all prior austenite grains may be 100.0%, 99.0% or less, or 98.0% or less.
• the present invention aims to provide a steel sheet having high strength but improved hole expandability and work hardening ability, and achieves the above objective by forming the metal structure of a steel sheet having a predetermined chemical composition with a structure mainly composed of martensite, restricting the average grain size of prior austenite grains in the metal structure to a predetermined range, and appropriately controlling the ratio of crystal grains having a predetermined intragranular orientation difference in the martensite structure. Therefore, it is clear that the ratio of prior austenite grains having an aspect ratio of 2.0 or less to all prior austenite grains is not an essential technical feature for achieving the objective of the present invention.
• the average grain size of the prior austenite grains and the proportion of the prior austenite grains with an aspect ratio of 2.0 or less to all the 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 the 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 liquid in which diamond powder with a grain size of 1 to 6 ⁇ m is dispersed in a diluent such as alcohol or 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 may be used.
• 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 grain is calculated from the crystal orientation relationship between the general prior austenite grain and the crystal grain having a body-centered structure after transformation.
• the following method is used to calculate the crystal orientation of the prior austenite grain.
• 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 the average value is set as the grain size of the prior austenite grain.
• the above operation is performed for all prior austenite grains, except for 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 is determined by calculating the average grain size from the grain sizes of all the prior austenite grains obtained.
• 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 that 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 taken 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 of view, such as the ends of the photographed field of view, to determine the aspect ratios of all prior austenite grains in the photographed field of view.
• the total number of prior austenite grains with an aspect ratio of 2.0 or less is divided by the number of all prior austenite grains to determine the proportion of prior austenite grains with an aspect ratio of 2.0 or less to all prior austenite grains.
• the more non-uniformly the dislocations are dispersed the larger the misorientation within the grain is generally, and crystal grains with a misorientation of 4° or more within the grain are crystal grains with a sufficient misorientation to form a structure with non-uniformly dispersed dislocations and develop non-uniform deformation.
• such crystal grains are present in the martensite within the range of 45.0 to 70.0% by area to provide a certain amount of a structure with non-uniformly dispersed dislocations, which develops non-uniform deformation 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. If the proportion of crystal grains having an intragranular misorientation of 4° or more is less than 45.0% or more than 70.0% by area, non-uniform deformation is unlikely to develop during processing such as press forming, and therefore the desired work hardening rate cannot be achieved.
• the proportion of crystal grains having an intragranular misorientation of 4° or more in the martensite structure may be, for example, 48.0% or more, 50.0% or more, or 55.0% or more by area.
• the proportion of crystal grains having an intragranular misorientation of 4° or more in the martensite structure may be, for example, 65.0% or less, 62.0% or less, or 60.0% or less by area.
• the ratio of crystal grains in which the orientation difference in the martensite grains is 4° or more is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from the steel sheet so that the plate thickness cross section in the direction perpendicular to the plate surface is the observation surface. Next, at a depth position of 1/4 of the plate thickness from the steel sheet surface, an area of 200 ⁇ m in the direction perpendicular to the plate thickness direction and 100 ⁇ m in the plate thickness direction is analyzed by EBSD analysis at a measurement interval of 0.2 ⁇ m to obtain crystal orientation information.
• EBSD electron backscattered diffraction
• the EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an EBSD analysis device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL).
• JSM-7001F thermal field emission scanning electron microscope
• EBSD detector HOKARI 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 martensite structure is specified for the obtained crystal orientation information using the “Phase Map” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer.
• a region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a crystal grain, the average orientation difference within the crystal grain is calculated, and the ratio of crystal grains with an orientation difference within the grain of 4° or more is obtained.
• the crystal grains and the average orientation difference within the grains defined as above can be calculated using the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer.
• the “orientation difference within the grain” refers to “Grain Orientation Spread (GOS)”, which is the orientation dispersion within the crystal grain.
• the value of the misorientation in the grain is calculated as the average value of the misorientation between the reference crystal orientation and all the measurement points in the same grain, as described in "Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD Method and X-ray Diffraction Method", Kimura Hidehiko et al., Transactions of the Japan Society of Mechanical Engineers (Series A), Vol. 71, No. 712, 2005, pp. 1722-1728.
• the reference crystal orientation is the orientation averaged over all the measurement points in the same grain.
• the GOS value can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" provided with the EBSD analyzer.
• 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 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 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 heating step in which a slab having the chemical composition described above in relation to the steel sheet is heated and held at a temperature range of 1100° C. or more 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 the rolling pass one stage before the final stage is 30 to 50%, and the rolling reduction in the rolling pass in the final stage is 20 to 50%, (b) a total rolling reduction of 90% or more; and (c) a rolling temperature in the rolling pass one stage before the final stage is 970 to 1100°C, and a final rolling temperature is 960 to 1050°C.
• the method is characterized by including a winding step of winding the cooled steel sheet in a temperature range of 400°C or less, and a strain imparting step of imparting a strain of 0.16 to 1.40% in absolute value at a 1/4 position of the sheet thickness of the obtained steel sheet by repeating positive and negative strains three or more times.
• 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. Each step will be described in detail below.
• a slab having the chemical composition described above in relation to the steel plate is heated and held at a temperature range of 1100°C or higher for 6000 seconds or more. From the viewpoint of productivity, it is preferable to use a slab obtained by continuous casting, but a slab obtained by casting and blooming can also be used, and a slab obtained by hot working or cold working may be used as necessary.
• holding at a temperature range of 1100°C or higher includes not only the case where the temperature of the slab is held at a constant temperature of 1100°C or higher, but also the case where the temperature of the slab is held while fluctuating in a temperature range of 1100°C or higher.
• the coarse carbides present in the structure can be completely solid-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-dissolution of the coarse carbides is incomplete. If the solid solution of the coarse carbides is incomplete, the area ratio of martensite becomes less than 90.0% due to the occurrence of ferrite or bainite transformation originating from such carbides in the cooling process described below, and as a result, the desired strength and/or hole expandability cannot be obtained.
• 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 in addition, the proportion of prior austenite grains having an aspect ratio of 2.0 or less to all prior austenite grains can be increased. If the reduction rate in the rolling pass one stage before the final stage is less than 30% and/or the reduction rate in the rolling pass of 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 prior austenite grains may not be achieved in the metal structure of the finally obtained steel sheet, and/or the proportion of prior austenite grains having an aspect ratio of 2.0 or less may become relatively small.
• the reduction rate in each rolling pass of the rolling pass one stage before the final stage and/or the final stage is too high, the rolling load becomes excessive, and the load on the equipment such as the rolling mill becomes high. For this reason, the reduction rate in each rolling pass of the rolling pass one stage before the final stage and the final stage is 50% or less. Preferably, the reduction rate in each rolling pass of the rolling pass one stage before the final stage and the final stage is 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.
• the total reduction in the finish rolling is less than 90%, the variation in the Mn concentration becomes relatively high, and it may not be possible to sufficiently suppress the development of a region where Mn is locally concentrated and the fracture energy is reduced.
• the total reduction in the finish rolling is less than 90%, the accumulated strain during rolling becomes insufficient, and recrystallization is not completed or not sufficiently promoted, so that the desired average grain size of prior austenite grains may not be achieved in the metal structure of the finally obtained steel sheet, and/or the proportion of prior austenite grains having an aspect ratio of 2.0 or less may become relatively small.
• the upper limit of the total reduction in the finish rolling may be, for example, 99% or less or 98% or less.
• the rolling temperature in the rolling pass one stage before the last stage is less than 970°C and/or 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 prior austenite grains may not be achieved in the metal structure of the finally obtained steel sheet, and/or the proportion of prior austenite grains having an aspect ratio of 2.0 or less may become relatively small. If the desired average grain size of prior austenite grains cannot be achieved, sufficient work hardening ability cannot be obtained. On the other hand, if the rolling temperature in the next to final rolling stage exceeds 1100° C.
• the prior austenite grains become coarse, and it may not be possible to achieve a desired average grain size of the prior austenite grains. In this case as well, it is naturally impossible to obtain sufficient work hardening ability.
• the time from the completion of the hot rolling process to the start of cooling is less than 0.5 seconds, 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 desired proportion of prior austenite grains having an aspect ratio of 2.0 or less may not be obtained. Also, if the time from the completion of the hot rolling process to the start of cooling exceeds 10.0 seconds, grain growth will proceed too far, and the desired average grain size of the prior austenite grains will not be obtained. As a result, in either case, sufficient work hardening ability will not be achieved in the steel sheet.
• the cooled steel sheet is coiled in a temperature range of 400° C. or less. If the coiling temperature exceeds 400° C., ferrite, bainite and/or pearlite are generated during coiling, as in the cooling process, and the area ratio of martensite becomes less than 90.0%, and as a result, the desired strength and/or hole expandability cannot be obtained.
• the absolute value of the applied strain is less than 0.16% or the number of times that strain is applied is less than three times, the introduction of dislocations non-uniformly into the crystal grains becomes insufficient, and the proportion of crystal grains having an intragranular misorientation of 4° or more becomes less than 45.0% in terms of area percentage, making it impossible to achieve the desired work hardening rate.
• the absolute value of the applied strain exceeds 1.40%, the accumulation of dislocations in the crystal grains becomes excessive, and the ratio of crystal grains having an intragranular misorientation of 4° or more exceeds 70.0% by area, which similarly makes it impossible to achieve the desired work hardening rate.
• martensite structures inherit dislocations within the austenite grains before transformation, generally, the grains of a martensite structure formed from unrecrystallized austenite grains have a high intragranular misorientation value, while the grains of a martensite structure formed from recrystallized austenite grains have a low intragranular misorientation value.
• the structure is refined by promoting recrystallization in the hot rolling process, thereby controlling the average grain size of the prior austenite grains to 30.0 ⁇ m or less.
• the conditions of the hot rolling process are appropriately controlled to produce a prior austenite structure in which recrystallization is almost or completely completed, thereby refining the structure and improving the work hardening capacity of the steel sheet as a whole, while dislocations are introduced unevenly into some of the crystal grains of the martensite structure in the strain imparting process to appropriately create misorientation, making it possible to achieve a high work hardening rate even in a state in which a certain degree of strain has been introduced, such as in the later stages of deformation in press forming.
• the strain imparting process can be carried out by any suitable method known to those skilled in the art. Examples of such methods include, but are not limited to, bending and unbending deformation using a tension leveler or the like.
• the absolute value of the strain to be imparted can be easily changed by changing the size of the rolls in the tension leveler, the relative positional relationship between the rolls, and even the thickness of the steel plate. Therefore, by appropriately controlling these parameters, it is possible to easily impart the desired strain at the 1/4 position of the plate thickness of the steel plate.
• 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.
• the work hardening ability of the steel sheet as a whole can be improved, while controlling the proportion of crystal grains in martensite with an intragranular misorientation of 4° or more to within the range of 45.0 to 70.0% by area percentage, it becomes 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 to form slabs with 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 for all examples and comparative examples, and finish rolling was performed under the conditions shown in Table 3 using a tandem rolling mill consisting of five rolling stands. Next, the finish-rolled steel plate was cooled and coiled under the conditions shown in Table 3, and finally, the absolute value of the strain shown in Table 3 was applied at a position 1/4 of the plate thickness a predetermined number of times while repeatedly changing positive and negative, to obtain a steel plate having a plate thickness shown in Table 4.
• the properties of the resulting steel plates were measured and evaluated using the following methods.
• TS Tensile strength
• the tensile strength (TS) was measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the rolling direction of the steel plate (C direction) and performing a tensile test in accordance with JIS Z 2241:2011.
• 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 9 it is considered that the time from the completion of the hot rolling process to the start of the cooling process was short, so that recrystallization was not completed or was not sufficiently promoted. As a result, the average grain size of the prior austenite grains in the finally obtained metal structure became large, and the work hardening ability of the steel sheet decreased. In Comparative Example 10, it is considered that the time from the completion of the hot rolling process to the start of the cooling process was long, so that the grain growth progressed too much overall. As a result, the desired average grain size of the prior austenite grains could not be obtained, and the work hardening ability of the steel sheet decreased.
• Comparative Example 11 the time from the start of cooling in the cooling process to 400 ° C or less was long, so that the area ratio of martensite was less than 90.0%, and TS and ⁇ decreased.
• Comparative Example 12 the area ratio of martensite was similarly less than 90.0% due to the high coiling temperature, and TS and ⁇ were reduced.
• Comparative Example 13 the absolute value of the strain applied in the strain application process was small, so it is considered that the introduction of dislocations in the crystal grains was insufficient. As a result, the proportion of crystal grains in which the orientation difference in the grains was 4° or more in the martensite was small, and the work hardening rate of the steel sheet was reduced.
• Comparative Example 14 the absolute value of the strain applied in the strain application process was large, so it is considered that the accumulation of dislocations in the crystal grains was excessive. As a result, the proportion of crystal grains in which the orientation difference in the grains was 4° or more in the martensite was large, and the work hardening rate of the steel sheet was reduced. In Comparative Example 15, the number of times strain was applied in the strain application process was small, so it is considered that the introduction of dislocations in the crystal grains insufficient. As a result, the proportion of crystal grains in martensite with an intragranular misorientation of 4° or more decreased, and the work-hardening rate of the steel sheet decreased.
• Comparative Examples 37 and 39 the TS was decreased due to the low C and Si contents.
• Comparative Examples 38 and 40 the C and Si contents were high, respectively, so that a relatively large amount of retained austenite was generated, and ⁇ was decreased.
• Comparative Example 41 the hardenability was decreased due to the low Mn content, and as a result, the area ratio of martensite was decreased and TS was decreased.
• Comparative Example 42 ⁇ was decreased due to the high Mn content.
• Comparative Example 43 it is considered that the precipitation of cementite could not be sufficiently suppressed due to the low sol. Al content. As a result, ⁇ was decreased.
• Comparative Example 44 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 was decreased. In Comparative Example 45, it is considered that coarse carbides, etc. were generated in the steel due to the high Nb content. As a result, ⁇ was decreased.

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