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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
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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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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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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
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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
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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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
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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
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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 sheet.
• PTL 1 describes a hot rolled steel sheet having a chemical composition containing, by mass%, C: 0.020 to 0.070%, Si: 0.10 to 1.70%, Mn: 0.60 to 2.50%, Al: 0.01 to 1.00%, Ti: 0.015 to 0.170%, Nb: 0.005 to 0.050%, etc., limiting P: 0.05% or less, S: 0.010% or less, and N: 0.0060% or less, and having a balance of Fe and impurities, having a structure containing an area ratio of 5 to 60% of ferrite and 30 to 95% of bainite, in which structure, a boundary with an orientation difference of 15° or more being deemed a grain boundary, when defining a region surrounded by the grain boundaries and with a circle equivalent diameter of 0.3 ⁇ m or more as a "crystal grain", having a ratio of crystal grains with an orientation difference in the grains of 5 to 14° of an area% of 20 to 100%.
• PTL 2 describes a high strength steel material with prior ⁇ grains of a spherical shape containing, by mass ratio, C: 0.06 to 0.19%, Si: 0.15 to 0.60%, Mn: 0.60 to 1.80%, Cr: 0.05 to 1.20%, and Mo: 0.05 to 1.00% and containing one or more of Nb: 0.005 to 0.10%, V: 0.005 to 0.10%, and Ti: 0.005 to 0.10%, which steel material containing carbonitrides of Nb, Ti, or V with a particle size of 100 nm or less in a volume ratio of 0.01 to 0.8%, having prior ⁇ grains of a particle size no.
• PTL 2 teaches that according to the above configuration, it becomes possible to provide high strength steel material excellent in toughness, arrestability, and weldability and having a large uniform elongation property of more than 10% and a good mass producibility.
• PTL 3 describes a high carbon steel sheet member containing C: 0.80 mass% to 1.10 mass%, Si: 0.05 mass% to 0.40 mass%, Mn: 0.05 mass% to 0.50 mass%, Cr: 0.01 mass% to 0.35 mass%, P: 0.03 mass% or less, and S: 0.03 mass% or less and having a balance of Fe and unavoidable impurities, in which high carbon steel sheet member, a circle equivalent diameter of martensite blocks defined by an orientation difference of 15° or more being 3.2 ⁇ m or less, carbides of a circle equivalent diameter of 0.2 ⁇ m or more being present in 0.6 to 5.0 area%, an average particle size of the carbides being 0.3 ⁇ m or more and 2.0 ⁇ m or less, a carbon occupancy on the prior austenite grain boundaries being 0.35% or less, and an average value of KAM measured by EBSD being 0.690 to 0.710. Further, PTL 3 teaches that according to this configuration, it is possible to obtain a high carbon steel sheet member with high levels of both hard
• press-forming is performed divided into several steps, and therefore there are a relatively large number of locations for example receiving primary deformation and accumulating strain inside the steel sheet and, in that state, again receiving separate deformation (stretch flange deformation, etc.)
• strain is introduced into a steel sheet, work hardening occurs and the strength becomes higher, and therefore the workability in the later steps generally falls.
• a steel sheet is being asked to exhibit high formability by for example having excellent work hardening ability (performance of continuing to become harder) even in a state where a certain degree of strain is introduced.
• the present invention was made in consideration of this actual situation and has as its object to provide, by a novel constitution, a steel sheet which, despite being high strength, is improved in hole expandability and work hardening ability.
• the inventors engaged in studies focusing on the microstructure of a steel sheet, in particular a hot rolled steel sheet, to achieve the above object.
• the inventors discovered that by making the microstructure of a hot rolled steel sheet having a predetermined chemical composition a structure mainly comprised of martensite, it is possible to achieve higher strength and improved hole expandability and that by limiting the mean particle size of prior austenite grains in the microstructure to within a predetermined range while suitably controlling the ratio of crystal grains having a predetermined grain orientation difference in the martensite structure, it is possible to remarkably improve the work hardening ability, and thereby completed the present invention.
• the present invention able to achieve the above object is as follows:
• a steel sheet in particular a hot rolled steel sheet, which, despite being high strength, is improved in hole expandability and work hardening ability.
• the steel sheet according to an embodiment of the present invention in particular the hot rolled steel sheet, has a chemical composition comprising, by mass%,
• the hole expandability and other properties fall.
• a steel sheet securing a high strength, in particular high strength of a tensile strength of 980 MPa or more enabling reduction of weight, while having excellent hole expandability is being sought.
• the microstructure of the steel sheet is preferably made a structure mainly comprised of martensite.
• martensitic steel has a layered structure including packets, blocks, laths, and other substructures in the prior austenite grains.
• the inventors engaged in studies focusing in particular on the microstructure of the hot rolled steel sheet in addition to prescribing a suitable chemical composition of the steel sheet, in particular the hot rolled steel sheet.
• the inventors discovered that by making the microstructure of hot rolled steel sheet having a predetermined chemical composition a structure mainly comprised of martensite, more specifically, a structure containing, by area%, martensite: 90.0% or more and retained austenite: 3.0% or less, it is possible to achieve high strength, for example, high strength of a tensile strength of 980 MPa or more, while remarkably improving the hole expandability of the hot rolled steel sheet.
• the hole expandability can be improved.
• retained austenite can become starting points for fracture during deformation in press-forming, etc., and therefore by limiting the retained austenite to an area% of 3.0% or less in addition to controlling the martensite to an area% of 90.0% or more, the hole expandability can be improved more remarkably.
• prior austenite grain boundaries act as resistance against motion of dislocations and would be effective for improving work hardening ability
• the inventors studied improvement of the work hardening ability from the viewpoint of making the particle size of the prior austenite grains in a microstructure mainly comprised of martensite a suitable one. More specifically, by making the prior austenite grains finer, it is possible to increase the density of prior austenite grain boundaries. For this reason, it is possible to increase the obstacles to dislocation by making the prior austenite grains finer and therefore it becomes possible to raise the work hardening ability.
• the inventors discovered that by making the prior austenite grains finer within a predetermined range, more specifically controlling the mean particle size of prior austenite grains to 30.0 ⁇ m or less, the work hardening ability of the hot rolled steel sheet as a whole is improved while by suitably controlling the ratio of specific crystal grains in the martensite structure, more specifically, when defining a region surrounded by grain boundaries with an orientation difference of 15° or more in the martensite as a "crystal grain", by controlling a ratio of crystal grains with an orientation difference in the grains of 4° or more to a range of, by area%, 45.0 to 70.0%, it is possible to achieve a high work hardening rate even in a state where a certain extent of strain is introduced such as at the latter period of press-forming.
• crystal grains raised in orientation difference in the grains in the martensite in a suitable ratio and establishing the presence of a certain amount of a structure with dislocations unevenly dispersed, uneven deformation occurs during press-forming and other working. As a result, even in the latter period of deformation, a sufficient work hardening ability can be maintained and therefore it is believed a high work hardening rate can be achieved.
• crystal grains with an orientation difference in the grains of 4° or more are crystal grains sufficient for causing uneven deformation. By controlling such crystal grains to within a range of, by area%, 45.0 to 70.0%, it becomes possible to achieve the desired work hardening rate.
• the steel sheet according to an embodiment of the present invention for example, despite being a high strength of a tensile strength of 980 MPa or more, the hole expandability and work hardening ability can be remarkably improved. Therefore, the steel sheet according to an embodiment of the present invention can reliably achieve the contradictory properties of high strength and excellent workability, and therefore is particularly useful in use in the automotive field where realization of both of these properties is sought.
• C is an element effective for raising the strength of steel sheet. Further, C forms carbides and/or carbonitrides with Nb in the steel and contributes to refinement of the structure by the pinning effect of the precipitates formed. To sufficiently obtain these effects, the C content is 0.040% or more. The C content may also be 0.060% or more, 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, if excessively containing C, sometimes the workability falls. Therefore, the C content is 0.200% or less. The C content may also be 0.180% or less, 0.160% or less, 0.150% or less, or 0.140% or less.
• the Si is an element effective for raising the strength as a solution strengthening element. To sufficiently obtain such an effect, the Si content is 0.30% or more. The Si content may also be 0.40% or more, 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. On the other hand, if excessively containing Si, the chemical convertability and workability fall and during hot rolling, slab cracking sometimes occurs. Therefore, the Si content is 2.00% or less. The Si content may also be 1.80% or less, 1.60% or less, 1.50% or less, or 1.40% or less.
• Mn is an element effective for raising the hardenability and the strength as a solution strengthening element. To sufficiently obtain these effects, the Mn content is 1.00% or more. The Mn content may also be 1.20% or more, 1.50% or more, 1.80% or more, 2.00% or more, or 2.20% or more. On the other hand, if excessively containing Mn, the workability sometimes falls. Therefore, the Mn content is 4.00% or less. The Mn content may also be 3.80% or less, 3.50% or less, 3.20% or less, 3.00% or less, or 2.80% or less.
• sol. Al is an element acting as a deoxidizer of molten steel. Further, sol. Al is an element suppressing the precipitation of the cementite so harmful to hole expandability. To obtain these effects, the sol. Al content is 0.001% or more. The sol. Al content may also be 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, or 0.100% or more. On the other hand, even if excessively containing sol. Al, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the sol. Al content is 0.500% or less. The sol. Al content may also be 0.400% or less, 0.300% or less, or 0.200% or less. "sol. Al” means acid soluble Al and indicates solid solution Al present in the steel in a solid solution state.
• the P content is 0.100% or less.
• the P content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less.
• the lower limit of the P content is not particularly prescribed and may also be 0%, but excessive reduction would invite a rise in costs. Therefore, the P content may also be 0.0001% or more, 0.001% or more, or 0.005% or more.
• the S content is 0.0300% or less.
• the S content may also be 0.0200% or less, 0.0100% or less, or 0.0050% or less.
• the lower limit of the S content is not particularly prescribed and may also be 0%, but excessive reduction would invite a rise in costs. Therefore, the S content may also be 0.0001% or more, 0.0010% or more, or 0.0030% or more.
• the N content is 0.0070% or less.
• the N content may also be 0.0050% or less, 0.0040% or less, or 0.0030% or less.
• the lower limit of the N content is not particularly prescribed and may also be 0%, but excessive reduction would invite a rise in costs. Therefore, the N content may also be 0.0001% or more or 0.0005% or more.
• the O content is an element entering in the production process. If excessively containing O, coarse inclusions are formed and the workability of the steel sheet is liable to fall. Therefore, the O content is 0.0100% or less.
• the O content may also be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
• the lower limit of the O content is not particularly prescribed and may also be 0%, but reduction to less than 0.0001% would require time for refining and invite a drop in productivity. Therefore, the O content may also be 0.0001% or more or 0.0005% or more.
• Nb is an element forming carbides, nitrides, and/or carbonitrides in the steel and contributes to refinement of the structure of the retained austenite grains and in turn higher strength of the steel sheet by the pinning effect.
• the Nb content is 0.001% or more.
• the Nb content may also be 0.005% or more, 0.010% or more, 0.050% or more, 0.100% or more, 0.200% or more, or 0.300% or more.
• the Nb content is 1.000% or less.
• the Nb content may also be 0.800% or less, 0.600% or less, or 0.500% or less.
• the basic chemical composition of the steel sheet according to an embodiment of the present invention is as explained above. Furthermore, the steel sheet may, according to need, further contain at least one of the following elements in place of part of the balance of Fe.
• Cr is an element raising the hardenability of steel and contributing to improvement of the strength and/or corrosion resistance.
• the Cr content may also be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more and may also be 0.01% or more, 0.05% or more, or 0.10% or more.
• the Cr content is preferably 0.90% or less and may also be 0.70% or less, 0.50% or less, 0.40% or less, or 0.30% or less.
• 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 sometimes are present in the steel sheet as trump elements.
• the contents of these elements may also be Ti: 0 to 0.200%, or 0.100%, V: 0 to 0.300%, or 0.200%, Cu: 0 to 0.40%, or 0.20%, Mo: 0 to 0.12% or 0.08%, Ni: 0 to 0.30%, or 0.15%, B: 0 to 0.0030%, or 0.0015%, Ca: 0 to 0.0010%, or 0.0008%, Mg: 0 to 0.0010%, or 0.0008%, Bi: 0 to 0.010%, Zr: 0 to 0.050%, or 0.030%, Co: 0 to 0.010%, Zn: 0 to 0.010%, W: 0 to 0.100%, or 0.050%, Sn: 0 to 0.040%, or
• the Ti, V, Cu, Mo, Ni, Bi, Zr, Co, Zn, W, Sn, and As contents may also be 0.001% or more, 0.005% or more, or 0.008% or more.
• the B, Ca, Mg and REM content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• the balance besides the above-mentioned elements is comprised of Fe and impurities.
• the "impurities" are constituents, etc., entering from ore, scrap, and other such starting materials due to various factors in the production process when, for example, industrially producing the steel sheet. They are allowed to be included in a range not affecting the effect of the present invention.
• the chemical composition of the steel sheet according to an embodiment of the present invention may be measured by a general analysis method.
• the chemical composition of the steel sheet may be measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES).
• C and S can be measured using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-nondispersive type infrared absorption method.
• the microstructure of the steel sheet according to an embodiment of the present invention includes, by area%, martensite: 90.0% or more and retained austenite: 3.0% or less.
• martensite 90.0% or more
• retained austenite 3.0% or less.
• the area ratio of the martensite is less than 90.0%, the desired strength and hole expandability cannot be achieved. From the viewpoint of further higher strength and improved hole expandability, the higher the area ratio of martensite, the more preferable. For example, it may be 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 prescribed and may also be 100.0%. For example, it may be 99.0% or less.
• retained austenite can form starting points for fracture during deformation in press-forming, etc., and therefore by controlling the martensite to an area% of 90.0% or more plus controlling the retained austenite to an area% of 3.0% or less, it becomes possible to more remarkably improve the hole expandability.
• the area ratio of the retained austenite is more than 3.0%, the grains form starting points for fracture during deformation and the hole expandability falls.
• the lower the area ratio of the retained austenite the more preferable. For example, it may be 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, it may be 0.5% or more.
• the balance structures besides the martensite and retained austenite may be an area% of 0%, but if there are balance structures present, the balance structures 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 the at least one of ferrite, bainite, and pearlite is a total of more than 10.0%, the area ratio of martensite becomes less than 90.0%, and therefore as a result the desired strength and hole expandability can no longer be achieved.
• the lower limits of ferrite, bainite, and pearlite may respectively be 0%. For example, they are respectively 0.1% or more, 0.5% or more, 1.0% or more, 2.0% or more, or 3.0% or more. Similarly, the upper limits of ferrite, bainite, and pearlite may be respectively 8.0% or less, 6.0% or less, 5.0% or less, or 4.0% or less.
• the microstructure in steel sheet is identified and the area ratios are calculated by examination under an optical microscope and X-ray diffraction after corrosion using a Nital reagent or LePera solution.
• the structure is examined under an optical microscope at a sheet thickness cross-section in a direction vertical to the sheet surface. Note that the sheet thickness cross-section is preferably parallel to the rolling direction. Specifically, first, a sample is taken from the steel sheet and examined surface of the sample is etched by Nital. Next, an optical microscope is used to photograph a 300 ⁇ m ⁇ 300 ⁇ m field at the 1/4 depth position of sheet thickness. The obtained structural photograph is analyzed to calculate the total area of the martensite and bainite and the individual area ratios of ferrite and pearlite.
• the sample with the examined surface corroded by the LePera solution is used and an optical microscope is similarly used to photograph a 300 ⁇ m ⁇ 300 ⁇ m field at the 1/4 depth position of sheet thickness.
• the obtained structural photograph is analyzed to calculate the total area ratio of martensite and retained austenite.
• a sample ground at its surface down to 1/4 depth of sheet thickness from the logarithmic direction of the rolled surface is used to calculate the volume ratio of the retained austenite by X-ray diffraction measurement.
• the volume ratio of retained austenite is equal to the area ratio, and therefore this is deemed the area ratio of the retained austenite.
• the obtained area ratio of retained austenite is subtracted from the total area ratio of martensite and retained austenite similarly calculated previously to calculate the area ratio of martensite. Finally, the obtained area ratio of martensite is subtracted from the total area ratio of martensite and bainite similarly calculated in in advance to thereby calculate the area ratio of the bainite.
• the mean particle size of the prior austenite grains is 30.0 ⁇ m or less.
• prior austenite grain boundaries act as resistance to motion of dislocations and are believed effective for improvement of the work hardening ability.
• refining the prior austenite grains it is possible to increase the density of the prior austenite grain boundaries.
• refining the prior austenite grains it is possible to increase the obstacles to dislocation and therefore possible to raise the work hardening ability of the obtained steel sheet. From the viewpoint of further raising the work hardening ability of the steel sheet, the smaller the mean particle size of the prior austenite grains, the more preferable.
• the mean particle 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 with an aspect ratio of 2.0 or less in all prior austenite grains may be an area% of for example 90.0% or more, 92.0% or more, 94.0% or more, or 96.0% or more.
• the upper limit is not particularly prescribed, but for example the ratio of prior austenite grains with an aspect ratio of 2.0 or less in all prior austenite grains may be 100.0% and may also be 99.0% or less or 98.0% or less.
• the present invention has as its object the provision of steel sheet which, while high strength, is improved in hole expandability and work hardening ability. It achieves the above object by making the microstructure of steel sheet having a predetermined chemical composition a structure mainly comprised of martensite and limiting the mean particle size of prior austenite grains in the microstructure to within a predetermined range while suitably controlling the ratio of crystal grains having a predetermined grain orientation difference in the martensite structure. Therefore, it is clear that the ratio of prior austenite grains with an aspect ratio of 2.0 or less in all prior austenite grains is not a technical feature essential in achieving the object of the present invention.
• the mean particle size of prior austenite grains and the ratio of prior austenite grains with an aspect ratio of 2.0 or less in all prior austenite grains are determined in the following way.
• a sample is cut out from any position 50 mm or more away from the end faces of the steel sheet (if not possible to take a sample from that position, a position avoiding the end parts) so that a vertical sheet thickness cross-section can be examined.
• the sheet thickness cross-section is preferably parallel to the rolling direction.
• the size of the sample while depending on the measurement device, is made a size enabling examination of about 10 mm in the direction vertical to the sheet thickness direction.
• the cross-section of the sample is polished using #600 to #1500 silicon carbide paper, then is finished to a mirror surface using particle size 1 to 6 ⁇ m diamond powder made to disperse in alcohol or other diluent or pure water.
• electrolytic polishing is used to finish the examined surface.
• a length 50 ⁇ m and sheet thickness direction 50 ⁇ m region is measured by electron backscatter diffraction at 0.1 ⁇ m measurement intervals to obtain crystal orientation information.
• an EBSD analysis apparatus comprised of a thermal field emission type scan electron microscope and EBSD detector may be used.
• an EBSD analysis apparatus comprised of a JSM-7001F made by JEOL and a DVC5 type detector made by TSL may be used.
• the vacuum degree inside the EBSD analysis apparatus may be 9.6 ⁇ 10 - 5 Pa or less
• the acceleration voltage may be 15 kV
• the probe current level may be 13.
• the obtained crystal orientation information is used to calculate the crystal orientation of the prior austenite grains from the crystal orientation relationship of general prior austenite grains and crystal grains having a body centered structure after transformation.
• the method of calculating the crystal orientation of prior austenite grains the following method is used. First, the method described in Acta Materialia, 58(2010), 6393-6403 is used to prepare a crystal orientation map of the prior austenite grains.
• the average value of the shortest diameter and the longest diameter is calculated.
• the average value is made the particle size of the prior austenite grains.
• the above operation is performed for all of the prior austenite grains except for the prior austenite grains not contained in the photographed field in the entireties of the crystal grains such as at the end parts of the photographed field.
• the particle size of all of the prior austenite grains in the photographed field is sought.
• the mean particle size is determined by calculating the mean particle size from the particle sizes of all of the prior austenite grains obtained.
• the ratio of the diameter in the sheet thickness direction and diameter in the rolling direction is calculated and that value is used as the aspect ratio of the prior austenite grains. If the rolling direction is unclear, the cross-section is examined at a direction of 0°, 45°, 90°, and 135° with respect to any direction, the cross-section with the highest aspect ratio among them is deemed the cross-section parallel to the rolling direction, and the ratio of the diameter in the sheet thickness direction and diameter in the rolling direction (rolling direction diameter/sheet thickness direction diameter) is calculated.
• the above operation is performed for all of the prior austenite grains except for the prior austenite grains not contained in the photographed field in the entireties of the crystal grains such as at the end parts of the photographed field.
• the aspect ratio of all of the prior austenite grains in the photographed field is sought.
• the total number of the prior austenite grains with an aspect ratio of 2.0 or less divided by the number of all prior austenite grains, the ratio of prior austenite grains with an aspect ratio of 2.0 or less in all prior austenite grains is determined.
• crystal grain In the martensite structure of the steel sheet according to an embodiment of the present invention, if a region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a "crystal grain", the ratio of crystal grains with an orientation difference in the grains of 4° or more is controlled to a range of, by area%, 45.0 to 70.0%. As explained earlier, the more unevenly dislocations are dispersed, generally the greater the orientation difference in the grains becomes. Crystal grains with an orientation difference in the grains of 4° or more are crystal grains having an orientation difference sufficient for forming a structure with dislocations unevenly dispersed and causing spread of uneven deformation.
• the ratio of crystal grains with an orientation difference in the grains in the martensite structure of 4° or more may be for example an area% of 48.0% or more, 50.0% or more, or 55.0% or more.
• the ratio of crystal grains with an orientation difference in the grains in the martensite structure of 4° or more may also be for example an area% of 65.0% or less, 62.0% or less, or 60.0% or less.
• the ratio of crystal grains with an orientation difference in the grains of 4° or more in the martensite is measured by electron backscatter diffraction (EBSD). More specifically, first, a sample is taken from the steel sheet so that a sheet thickness cross-section in a direction vertical to the sheet surface can be examined. At the 1/4 depth position of sheet thickness from the steel sheet surface, a region of 200 ⁇ m in the direction vertical to the sheet thickness direction and 100 ⁇ m region in the sheet thickness direction is measured by EBSD at 0.2 ⁇ m measurement intervals to obtain crystal orientation information.
• EBSD electron backscatter diffraction
• an EBSD analysis apparatus comprised of a thermal field emission type scan electron microscope (JSM-7001F made by JEOL) and EBSD detector (HIKARI detector made by TSL) is used for analysis at a speed of 50 to 300 points/s.
• the vacuum degree inside the EBSD analysis apparatus may be 9.6 ⁇ 10 -5 Pa or less
• the acceleration voltage may be 15 kV
• the probe current level may be 13.
• the obtained crystal orientation information is used to identify the martensite structure using a "Phase Map" function installed in the "OIM Analysis ® " software attached to the EBSD analysis apparatus.
• the average orientation difference in the grains of the crystal grains is calculated and the ratio of crystal grains with an orientation difference in the grains of 4° or more is found.
• the crystal grains defined as explained above and the average orientation difference in the grains can similarly be calculated using the "OIM Analysis ® " software attached to the EBSD analysis apparatus.
• the "orientation difference in the grains” expresses the dispersion of the orientations in a crystal grain called the "grain orientation spread (GOS)".
• the value of the orientation difference in the grains is found as the average value of misorientation of all measurement points from the crystal orientation used as a reference in the same crystal grain.
• the crystal orientation used as the reference is the orientation obtained by averaging all measurement points in the same crystal grain.
• the value of GOS can be calculated using the "OIM Analysis ® Version 7.0.1" software attached to the EBSD analysis apparatus.
• the steel sheet according to an embodiment of the present invention is not particularly limited, but in general it has a 1.0 to 8.0 mm sheet thickness.
• the sheet thickness may also be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more and/or may also be 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.
• the steel sheet according to an embodiment of the present invention can reliably realize the contradictory properties of high strength and excellent workability and is useful for use for parts in technical fields in which achievement of both of these properties is sought, etc. In particular, it is useful for use for parts in the automotive field, etc.
• an auto part including steel sheet according to an embodiment of the present invention in particular, a transmission of an automobile, is provided.
• transmission parts of automobiles a lower arm, trailing arm, etc.
• These auto parts, in particular transmission parts of automobiles need only contain the steel sheet according to an embodiment of the present invention in at least portions of these parts. For this reason, at least portions of these parts satisfy the above features of the chemical composition and structure.
• the features of the steel sheet do not particularly change before and after forming. Portions of the steel sheet with relatively low degrees of working are judged by being flat in shape without being bent or otherwise deformed, by being small in rate of change of sheet thickness, and other features.
• the steel sheet having the above-mentioned chemical composition and microstructure in particular hot rolled steel sheet, it is possible to achieve a high tensile strength, specifically a tensile strength of 980 MPa or more.
• the tensile strength is preferably 1000 MPa or more, 1080 MPa or more, or 1180 MPa or more.
• the steel sheet according to an embodiment of the present invention despite having such an extremely high tensile strength, it is possible to realize excellent hole expandability and work hardening ability by a specific combination of the chemical composition and microstructure explained above.
• the upper limit of the tensile strength is not particularly prescribed, but, for example, the tensile strength of the steel sheet is 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 an orientation (C direction) where the longitudinal direction of the test piece becomes parallel to the rolling perpendicular direction of the steel sheet and performing a tensile test based on JIS Z 2241: 2011. For example, if it is difficult to obtain a JIS No. 5 test piece due to dimensional restrictions, it is possible to use another test piece described in JIS Z 2241: 2011. However, if the sheet thickness is less than 0.5 mm, 0.5 mm is made the lower limit for performing suitable evaluation.
• JIS Z 2244-1 2020
• the sample used for the micro Vickers test can be prepared by the same method as the sample for evaluation of the mean particle size and aspect ratio of the prior austenite grains.
• the micro Vickers test may be performed by measuring 30 points at the sheet thickness 1/4 position by a load of 500 gf and using the average value.
• the hole expansion rate may be preferably 50% or more, more preferably 60% or more or 70% or more.
• the upper limit of the hole expansion rate is not particularly prescribed, but, for example, the hole expansion rate is 150% or less, 120% or less, or 100% or less.
• a slab having a chemical composition explained above in relation to the steel sheet is heated and is held in a temperature region of 1100°C or more for 6000 seconds or more.
• a slab obtained by continuous casting is preferable from the viewpoint of productivity, but it is also possible to use a slab obtained by casting and blooming. In accordance with need, these which are hot worked or cold worked may also be used.
• "holding in the temperature region of 1100°C or more” includes not only the case of holding the temperature of the slab at a 1100°C or more fixed temperature but encompasses the case of holding the temperature of the slab while fluctuating in the temperature region of 1100°C or more.
• the slab By holding the slab at the temperature region of 1100°C or more for 6000 seconds or more, it is possible to make the coarse carbides present in the structure completely dissolve and possible to eliminate starting points of cracking. If the holding temperature is less than 1100°C or the holding time is less than 6000 seconds, the coarse carbides become incompletely dissolved. If the coarse carbides are incompletely dissolved, in the cooling step explained later, due to the occurrence of ferrite or bainite transformation starting from such carbides, the area ratio of martensite becomes less than 90.0% and as a result it becomes no longer possible to obtain the desired strength and/or hole expandability.
• 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 at the temperature region of 1100°C or more is preferably 10000 seconds or less.
• the heated slab may be rough rolled before the finish rolling so as to adjust the sheet thickness, etc.
• the rough rolling need only be able to secure the desired sheet bar dimensions.
• the conditions are not particularly limited.
• the heated slab or the slab additionally rough rolled in accordance with need is next finish rolled.
• the finish rolling is performed using a tandem rolling mill comprised of several rolling stands, for example, five or more rolling stands.
• the rolling reduction at each rolling pass at the last two stages has to be suitably controlled. Specifically, the rolling reduction at the rolling pass one stage before the last stage is controlled to 30 to 50% while the rolling reduction at the rolling pass at the last stage is controlled to 20 to 50%.
• the rolling reduction at each rolling pass of one stage before the last stage and/or the last stage is too high, the rolling load becomes excessive and the burden of the rolling mill and other facilities becomes higher. For this reason, the rolling reduction at each rolling pass of one stage before the last stage and the last stage is 50% or less. Preferably the rolling reduction at each rolling pass of one stage before the last stage and the last stage is 45% or less.
• the total rolling reduction in the final rolling is controlled to 90% or more.
• the Mn contained in the steel is an element causing a drop in the fracture energy of the grain boundaries, and therefore if there are regions where Mn is locally concentrated, sometimes occurrence of cracking is promoted at the time of plastic deformation in the press-forming, etc. There, from the viewpoint of further improving the hole expandability, suppressing or reducing local concentration of Mn would be effective.
• By controlling the total rolling reduction in finish rolling to 90% or more it is possible to make the Mn disperse in the steel and in turn suppress or reduce the variation in Mn concentration in the steel, i.e., suppress or reduce the local concentration of Mn.
• the total rolling reduction in the finish rolling is less than 90%, the variation in Mn concentration becomes relatively high and, sometimes, locally Mn concentrates and growth of regions with reduced fracture energy cannot be sufficiently suppressed.
• the total rolling reduction in finish rolling is less than 90%, the cumulative strain during the rolling becomes insufficient, and therefore sometimes the recrystallization will not be completed or not be sufficiently promoted and the desired mean particle size of prior austenite grains in the microstructure of the finally obtained steel sheet will not be able to be achieved and/or the ratio of the prior austenite grains with an aspect ratio of 2.0 or less will become a relatively small value.
• the upper limit of the total rolling reduction in the finish rolling is, for example, 99% or less or 98% or less.
• the rolling temperature at the rolling pass of one stage before the last stage (entry side temperature of rolling pass of one stage before the last stage) and the final rolling temperature (end temperature of finish rolling) are also extremely important in control of the microstructure of the steel sheet. If the rolling temperature at the rolling pass of one stage before the last stage is less than 970°C and/or the final rolling temperature is less than 960°C, sometimes the recrystallization is not completed or is not sufficiently promoted and the desired mean particle size of the prior austenite grains in the microstructure of the finally obtained steel sheet cannot be achieved and/or sometimes the ratio of prior austenite grains with an aspect ratio of 2.0 or less becomes a relatively small value.
• the rolling temperature of the one stage before the last stage is more than 1100°C and/or the final rolling temperature is more than 1050°C, the prior austenite grains become coarser and sometimes the desired mean particle size of the prior austenite grains cannot be achieved. In this case as well, only naturally, sufficient work hardening ability can no longer be obtained.
• the finish rolled steel sheet starts to be cooled in the next cooling step within 0.5 to 10.0 seconds after the completion of the hot rolling step, then is cooled down to a temperature of 400°C or less within 20.0 seconds from the start of cooling.
• the time from the completion of the hot rolling step to the start of cooling is less than 0.5 second, sometimes the recrystallization will not be completed or not be sufficiently promoted and the desired mean particle size of prior austenite grains in the microstructure of the finally obtained steel sheet will not be able to be achieved and/or the ratio of the prior austenite grains with an aspect ratio of 2.0 or less will not be able to be obtained. If the time from the completion of the hot rolling step to the start of cooling is more than 10.0 seconds, grain growth will proceed too much and the desired mean particle size of the prior austenite grains will no longer be able to be obtained. As a result, in either case, it will become no longer possible to achieve a sufficient work hardening ability in the steel sheet.
• the cooling time from the start of cooling down to 400°C or less is more than 20.0 seconds or the cooling stop temperature is more than 400°C, ferrite, bainite, and/or pearlite will be produced during the cooling and the area ratio of martensite will become less than 90.0%. As a result, the desired strength and/or hole expansion ability will no longer be able to be obtained.
• the cooled steel sheet is coiled up at a temperature region of 400°C or less. If the coiling temperature is more than 400°C, in the same way as the case of the cooling step, ferrite, bainite and/or pearlite is formed, the area ratio of the martensite becomes less than 90.0%, and as a result it becomes no longer possible to obtain the desired strength and/or hole expandability.
• the obtained steel sheet was given strain becoming an absolute value of 0.16 to 1.40% at a sheet thickness 1/4 position of the steel sheet three times or more while repeatedly switching between positive and negative to thereby produce the steel sheet.
• strain By repeatedly imparting such strain to the steel sheet, it is possible to unevenly introduce dislocations inside the crystal grains of the martensite structure and as a result control the ratio of crystal grains with an orientation difference in the grains of 4° or more to a range of, by area%, 45.0 to 70.0%.
• the absolute value of the strain imparted is less than 0.16% or the number of times of imparting strain is less than three times, the uneven introduction of dislocations in the crystal grains becomes insufficient, and therefore the ratio of crystal grains with an orientation difference in the grains of 4° or more becomes an area% of less than 45.0% and it becomes no longer possible to achieve the desired work hardening rate.
• the absolute value of the strain impart becomes more than 1.40%, the accumulation of dislocations at the crystal grains becomes excessive whereby the ratio of crystal grains with an orientation difference in the grains of 4° or more becomes more than an area% of 70.0% and similarly the desired work hardening rate can no longer be achieved.
• the upper limit of the number of times of imparting strain is not particularly prescribed, but from the viewpoint of productivity is preferably 10 times or less, 7 times or less, or 5 times or less.
• the martensite structure inherits the dislocations in the austenite grains before transformation, and therefore in general the crystal grains of the martensite structure formed from the nonrecrystallized austenite grains become high in value of orientation difference in the grains while the crystal grains of the martensite structure formed from the recrystallized austenite grains become low in value of orientation difference in the grains.
• recrystallization is promoted in the hot rolling step to make the structure finer whereby the mean particle size of the prior austenite grains is controlled to 30.0 ⁇ m or less.
• martensite is formed from the recrystallized austenite grains, and therefore the crystal grains of the martensite structure become low in value of the orientation difference in the grains and the ratio of crystal grains with an orientation difference in the grains of 4° or more can no longer be made an area% of 45.0 to 70.0%.
• the prior austenite structure by suitably controlling the conditions of the hot rolling step to make the prior austenite structure a structure with recrystallization substantially or completely completed, it is possible to refine the structure and improve the work hardening ability of the steel sheet as a whole while by unevenly introducing dislocations in some of the crystal grains of the martensite structure in the strain imparting step to suitably form the orientation difference, it becomes possible to achieve a high work hardening rate even in a state with a certain degree of strain introduced such as in the latter period of deformation of press-forming.
• the strain imparting step can be performed by any suitable method known to persons skilled in the art. While not particularly limited to this, as such a method, for example bending-unbending deformation by a tension leveler, etc., may be mentioned. In this case, by changing the size of the rolls or the relative positional relationship of the rolls with each other in the tension leveler and further the sheet thickness of the steel sheet, etc., it is possible to easily change the absolute value of the strain imparted. Therefore, by suitably controlling these parameters, it is possible to easily impart the desired strain to the sheet thickness 1/4 position of the steel sheet.
• the steel sheet produced by above-mentioned method of production by configuring the microstructure by a more uniform structure containing an area% of martensite: 90.0% or more and retained austenite: 3.0% or less, it is possible to achieve a high strength, for example, a high strength of a tensile strength of 980 MPa or more, while remarkably improving the hole expandability due to the reduction of the hardness difference, etc.
• the steel sheet produced according to the above-mentioned method of production can reliably achieve both the contradictory properties of high strength and excellent workability, and therefore is particularly useful in use in the automotive field where realization of both of these properties is sought.
• steel sheets according to an embodiment of the present invention in particular hot rolled steel sheets, were produced under various conditions and investigated for the tensile strength (TS), hole expansion rate ( ⁇ ), and work hardening rate (WHR) of the obtained steel sheets.
• TS tensile strength
• ⁇ hole expansion rate
• WHR work hardening rate
• molten steels were cast by the continuous casting method to form slabs having the various chemical compositions shown in Tables 1 and 2. These slabs were heated to 1100 to 1200°C in temperature and held over the time periods shown in Table 3, then were hot rolled.
• the hot rolling was performed by rough rolling and finish rolling. More specifically, the rough rolling was performed under the same conditions in all of the examples and comparative examples while the finish rolling was performed under the conditions shown in Table 3 using a tandem rolling mill comprised of five rolling stands.
• the finish rolled steel sheets were cooled and coiled under the conditions shown in Table 3.
• strain was imparted to give an absolute value shown in Table 3 at a sheet thickness 1/4 position of the steel sheet a predetermined number of times while repeatedly switching between positive and negative.
• the tensile strength (TS) was measured by taking a JIS No. 5 test piece from an orientation(C direction) where the longitudinal direction of the test piece became parallel with a rolling perpendicular direction of each steel sheet and performing a tensile test based on JIS Z 2241: 2011.
• Comparative Examples 4 and 7 the respective rolling temperature and final rolling temperature in one stage before the last stage in the finish rolling were low, and therefore it is believed recrystallization was not completed or was not sufficiently promoted. As a result, the mean particle size of the prior austenite grains in the finally obtained microstructure became larger and the work hardening ability of the steel sheet fell. In Comparative Examples 5 and 8, the respective rolling temperature and final rolling temperature in one stage before the last stage in the finish rolling were high, and therefore it is believed the prior austenite grains became coarser overall. As a result, the mean particle size of the prior austenite grains in the finally obtained microstructure became larger and the work hardening ability of the steel sheet fell.
• Comparative Example 9 the time from the completion of the hot rolling step to the start of the cooling step was short, and therefore it is believed the recrystallization was not completed or was not sufficiently promoted. As a result, the mean particle size of the prior austenite grains in the finally obtained microstructure became larger and the work hardening ability of the steel sheet fell. In Comparative Example 10, the time from the completion of the hot rolling step to the start of the cooling step was long, and therefore it is believed the grain growth proceeded too much overall. As a result, it was not possible to obtain the desired mean particle size of the prior austenite grains and the work hardening ability of the steel sheet fell.
• Comparative Example 11 the time from the start of cooling to when becoming 400°C or less in the cooling step was long, and therefore the area ratio of martensite became less than 90.0% and the TS and ⁇ fell.
• Comparative Example 12 the coiling temperature was high, and therefore similarly the area ratio of martensite became less than 90.0% and the TS and ⁇ fell.
• Comparative Example 13 the absolute value of strain imparted in the strain imparting step was small, and therefore it is believed dislocations were not sufficiently introduced in the crystal grains. As a result, in the martensite, the ratio of crystal grains with an orientation difference in the grains of 4° or more became smaller and the work hardening ability of the steel sheet fell.
• Comparative Example 14 the absolute value of strain imparted in the strain imparting step was large, and therefore it is believed dislocations were excessively accumulated in the crystal grains. As a result, in the martensite, the ratio of crystal grains with an orientation difference in the grains of 4° or more became larger and the work hardening ability of the steel sheet fell. In Comparative Example 15, the number of times of imparting strain in the strain imparting step was small, and therefore it is believed that dislocations were not sufficiently unevenly introduced inside of the crystal grains. As a result, in the martensite, the ratio of crystal grains with an orientation difference in the grains of 4° or more became smaller and the work hardening ability of the steel sheet fell.
• Comparative Examples 37 and 39 the respective C and Si contents were low, and therefore the TS fell.
• Comparative Example 38 and in Comparative Example 40 the respective C and Si contents were high, and therefore the retained austenite was formed in a relatively large amount and the ⁇ fell.
• the Mn content was low, and therefore the hardenability fell and as a result the area ratio of martensite became lower and the TS fell.
• Comparative Example 42 the Mn content was high, and therefore the ⁇ fell.
• the sol. Al content was low, and therefore it is believed precipitation of cementite could not be sufficiently suppressed. As a result, the ⁇ fell.
• Comparative Example 44 the Nb content was low, and therefore it is believed refinement of the prior austenite grains due to the pinning effect could not be sufficiently promoted. As a result, the mean particle size of the prior austenite grains in the finally obtained microstructure became larger and the work hardening ability of the steel sheet fell. In Comparative Example 45, the Nb content was high, and therefore it is believed coarse carbides, etc., formed in the steel. As a result, the ⁇ fell.
• steel sheet according to all of the invention examples by having a predetermined chemical composition and, furthermore, by suitably controlling the conditions in the method of production, it was possible to obtain steel sheet having a microstructure containing an area% of martensite: 90.0% or more and retained austenite: 3.0% or less, having a mean particle size of prior austenite grains of 30.0 ⁇ m or less, and having in the martensite a ratio of crystal grains with an orientation difference in the grains of 4° or more of an area% of 45.0 to 70.0%. Further, as a result, regardless of being a high strength of a tensile strength of 980 MPa or more, it was possible to remarkably improve the hole expandability and work hardening ability.

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