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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/021—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving particular fabrication steps or treatments of ingots or slabs
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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  • C22C—ALLOYS
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • 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
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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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
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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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
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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
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • 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
  • C21D2211/005—Ferrite
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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 and a method of production of the same.
• PTL 1 describes a high strength steel sheet containing C: 0.1 to 0.25%, Si: 0.1 to 0.5%, Mn: 0.5 to 2.0%, Cr: 0.1 to 1.5%, Mo: 0.1 to 0.5%, Ti: 0.01 to 0.05%, and Nb: 0.01 to 0.05%, additionally containing V: 0.01 to 0.05% and/or B: 0.0001 to 0.005%, and having a balance of iron and unavoidable impurities, having a mean particle size of prior austenite of 20 ⁇ m or less, and having a standard deviation ( ⁇ ) of prior austenite grains size distribution of 5 ⁇ m or less.
• PTL 1 teaches that, according to the above-mentioned constitution, it is possible to refine the prior austenite particle size and reduce fluctuation in the same and possible to realize a high strength steel sheet maintaining a high strength of a tensile strength of 980 MPa or more while being improved in bendability.
• PTL 2 describes high strength/high ductility fine-martensite structure steel material containing C: 0.075 to 0.3 wt%, Mn: 3 to 10 wt%, and Si: 0 to 2.5 wt% and having a balance of Fe and unavoidable impurities, having a prior ⁇ particle size of 2.0 ⁇ m or less, and having a structure of equiaxed martensite having single blocks. Further, PTL 2 teaches that according to this high strength/high ductility fine martensite structure steel material, a tensile strength of 1200 MPa or more and a total elongation of 10% or more can be achieved.
• PTL 3 describes a high strength steel material with prior ⁇ -grains of a spherical shape containing, by mass%, 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 also containing one or more of Nb: 0.005 to 0.10%, V: 0.005 to 0.10%, and Ti: 0.005 to 0.10%, having particle size 100 nm or less carbonitrides of Nb, Ti, or V in a volume ratio of 0.01 to 0.8%, and having prior ⁇ -grains of a particle size number 7 or more and having, inside the prior ⁇ -grains, a martensite structure or a mixed structure of martensite and bainite.
• PTL 3 teaches that according to the above-mentioned constitution, it becomes possible to provide a high strength steel material excellent in toughness, arrestability, and weldability, having a large uniform elongation of over 10%, and having good mass producibility.
• 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, and a method of production of the same.
• 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 increasing the variation in particle size of the prior austenite grains, 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, and a method of production of same.
• 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 making the variation in particle size of the prior austenite grains greater, more specifically by controlling the standard deviation in the particle size of prior austenite grains to 4.0 ⁇ m or more, it is possible to achieve a high work hardening rate even in a state where a certain extent of strain is introduced such as at the latter period of press-forming.
• the steel sheet according to an embodiment of the present invention for example, despite the tensile strength being a high 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 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.
• 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.
• the Si content is 0.30% or more.
• the Si content may also be 0.40% or more, more than 0.50%, 0.51% or more, 0.52% or more, 0.53% or more, 0.54% or more, 0.55% or more, more than 0.55%, 0.60% or more, 0.70% or more, 0.85% or more, 1.00% or more, or 1.20% or more.
• 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 prior 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%, 0.09%, 0.08%, 0.06%, or 0.04%, 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 0.05
• 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 structure besides the martensite and retained austenite may be an area% of 0%, but if there is a balance structure present, the balance 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 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 may be 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 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, 4.0 ⁇ m or more, 4.1 ⁇ m or more, 4.2 ⁇ m or more, 4.5 ⁇ m or more, 4.7 ⁇ m or more, 5.0 ⁇ m or more, 8.0 ⁇ m or more, 10.0 ⁇ m or more, or 12.0 ⁇ m or more.
• the standard deviation in particle size of prior austenite grains is 4.0 ⁇ m or more.
• the mean particle size of the prior austenite grains 30.0 ⁇ m or less while making the standard deviation in particle size of the prior austenite grains 4.0 ⁇ m or more, i.e., by increasing the variation in particle size of the prior austenite grains, it is possible to form a mixed structure of coarse grains and fine grains mixed together. It is believed that by forming such a mixed grain structure, uneven deformation is induced during press-forming and other working. As a result, sufficient work hardening ability can be maintained even in a state where a certain extent of strain is introduced such as the latter period of deformation in press-forming.
• it may be 4.5 ⁇ m or more, 5.0 ⁇ m or more, more than 5.0 ⁇ m, 5.1 ⁇ m or more, 5.2 ⁇ m or more, 5.3 ⁇ m or more, 5.4 ⁇ m or more, 5.5 ⁇ m or more, 6.0 ⁇ m or more, 8.0 ⁇ m or more, or 10.0 ⁇ m or more.
• the upper limit is not particularly prescribed, but the mean particle size of the prior austenite grains is 30.0 ⁇ m or less, and therefore the upper limit of the standard deviation is self set and cannot become any value.
• the upper limit is not particularly prescribed, but the standard deviation in particle size of prior austenite grains may be, for example, 20.0 ⁇ m or less, 15.0 ⁇ m or less, 12.0 ⁇ m or less, 10.0 ⁇ m or less, or 8.0 ⁇ m or less.
• limiting the mean particle size of the prior austenite grains in the microstructure to 30.0 ⁇ m or less while controlling the standard deviation in particle size of prior austenite grains to 4.0 ⁇ m or more is extremely important. That is to say, this is because if either of the features is not satisfied, at least one of the effect of improvement of the work hardening ability due to refinement of the prior austenite grains and the effect of improvement of the work hardening ability due to the mixed grain structure of coarse grains and fine grains becomes insufficient.
• the particle size of the prior austenite grains becomes relatively uniform, i.e., the standard deviation in the particle size becomes a relatively small value.
• the average aspect ratio of the prior austenite grains is not particularly limited, but, for example, it may be 3.0 or less, 2.5 or less, 2.0 or less, 1.8 or less, 1.6 or less, or 1.4 or less.
• the lower limit is not particularly prescribed, but, for example, the average aspect ratio of the prior austenite grains may be 0.6 or more, 0.7 or more, or 0.8 or more.
• the present invention as explained above, has as its object the provision of sheet sheet which is high strength, yet despite this, is improved in hole expandability and work hardening ability.
• the above-mentioned object is achieved by forming the microstructure of steel sheet having a predetermined chemical composition by a structure mainly comprised of martensite and by limiting the mean particle size of the prior austenite grains in the microstructure to within a predetermined range while increasing the variation in particle size of the prior austenite grains. Therefore, it is clear that the average aspect ratio of the 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, standard deviation in particle size of prior austenite grains, and average aspect ratio of 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. By calculating the mean particle size and standard deviation from the particle sizes of all of the prior austenite grains obtained, the mean particle size and standard deviation of the particle size of the prior austenite grains are determined.
• 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. By arithmetically averaging the aspect ratios of all of the prior austenite grains obtained, the average aspect ratio of the prior austenite grains is determined.
• 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 may be 150% or less, 120% or less, or 100% or less.
• the temperatures described for the slab and steel sheet respectively mean the surface temperature of the slab and thee surface temperature of the steel sheet.
• a slab having the chemical composition explained in relation to the steel sheet is cast in the continuous casting step.
• the temperature history at the time of solidification is suitably controlled, more specifically is controlled so that the average cooling speed at 600 to 900°C becomes 10 to 50°C/min and the average cooling speed gradient becomes 40°C/min 2 or less.
• the average cooling speed at 600 to 900°C is less than 10°C/min, since the average cooling speed is slow, the crystal grains formed by transformation to body centered cubic structures (bcc structures) at the time of solidification become coarser and the mean particle size of the prior austenite grains in the finally obtained microstructure becomes greater than 30.0 ⁇ m. In this case, it becomes no longer possible to achieve a sufficient work hardening ability in the obtained steel sheet.
• the average cooling speed at 600 to 900°C is more than 50°C/min, since the average cooling speed is fast, the crystal grains become fine and uniform in the process of transformation of the solidified structure.
• the mean particle size of the prior austenite grains in the finally obtained microstructure becomes smaller, but it is not possible to increase the variation in the particle size. That is, the standard deviation in particle size of the prior austenite grains becomes smaller than 4.0 ⁇ m and similarly sufficient work hardening ability can no longer be achieved.
• the "average cooling speed gradient at 600 to 900°C” means the average of the rate of change of the cooling speed per unit time in 600 to 900°C.
• the average cooling speed gradient in the present method of production becomes 40°C/min 2 .
• the average cooling speed gradient in the present method of production becomes 40°C/min 2 . If the average cooling speed gradient at 600 to 900°C becomes more than 40°C/min 2 , the fluctuation of the cooling speed becomes too great, and therefore uneven cooling occurs.
• the average cooling speed gradient at 600 to 900°C is preferably 30°C/min 2 or less.
• the lower limit is not particularly prescribed, but the average cooling speed gradient at 600 to 900°C may also be 2°C/min 2 or more or 3°C/min 2 or more.
• the cast slab is heated at the next heating step and is held in the temperature region of 1100°C or more for 6000 seconds or more.
• "holding at 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 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 i.e., one stage before the last stage and the last stage, is controlled to 20 to 50%.
• the rolling reduction at each rolling pass at one stage before the last stage and the last stage by rolling by such a relatively high rolling reduction, recrystallization is promoted and the microstructure can be made finer and in addition the average aspect ratio of the prior austenite grains can be reduced.
• the rolling reduction at each rolling pass at one stage before the last stage and the last stage is less than 20%, recrystallization either is not completed or is not sufficiently promoted and in the microstructure of the finally obtained steel sheet, the desired mean particle size of the prior austenite grains sometimes cannot be reached and/or the average aspect ratio of the prior austenite grains sometimes becomes a relatively large value. If the desired mean particle size of the prior austenite grains cannot be achieved, sufficient work hardening ability no longer can be obtained.
• 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.
• Total rolling reduction (%) (sheet thickness before finish rolling-sheet thickness after finish rolling)/sheet thickness before finish rolling ⁇ 100
• the final rolling temperature (end temperature of finish rolling) is also extremely important in controlling the microstructure of the steel sheet. If the final rolling temperature is less than 960°C, recrystallization either is not completed or is not sufficiently promoted and in the microstructure of the finally obtained steel sheet, the desired mean particle size of the prior austenite grains sometimes cannot be reached and/or the average aspect ratio of the prior austenite grains sometimes becomes a relatively large value. If not possible to achieve the desired mean particle size of the prior austenite grains, it becomes no longer possible to obtain sufficient work hardening ability.
• the prior austenite grains become coarser overall and sometimes it is not possible to achieve the desired mean particle size of the prior austenite grains and/or standard deviation in particle size of the prior austenite grains. In this case as well, only naturally, it becomes no longer possible to obtain a sufficient work hardening ability.
• 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 cooling time from the start of cooling down to 400°C or less is more than 20.0 seconds or if the cooling stop temperature is more than 400°C, 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 cooled steel sheet is coiled up at a temperature region of 400°C or less whereby the steel sheet is produced. If the coiling temperature is more than 400°C, in the same way as the case of the cooling step, 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 steel sheet produced by above-mentioned method of production by configuring the microstructure by a more uniform structure containing, by area%, 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 under the conditions shown in Table 3 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. Finally, the finish rolled steel sheets were cooled and coiled under the conditions shown in Table 3 to obtain steel sheets having the sheet thicknesses shown in Table 4.
• 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 Example 4 the average cooling speed at 600 to 900°C in the continuous casting step was slow, and therefore it is believed the crystal grains became coarser. 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 5, the average cooling speed at 600 to 900°C in the continuous casting step was fast, and therefore it is believed that crystal grains became fine and uniform in the process of transformation of the solidified structure. As a result, the standard deviation in the particle size of the prior austenite grains in the finally obtained microstructure became smaller and the work hardening ability of the steel sheet fell.
• Comparative Example 11 the final rolling temperature in the finish rolling was high, and therefore it is believed the prior austenite grains became coarser overall. As a result, the mean particle size and particle size of the prior austenite grains in the finally obtained microstructure became greater and the work hardening ability of the steel sheet fell.
• Comparative Example 12 the time from after completion of the hot rolling step to the start of the cooling step was short, and therefore it is believed grain growth did not sufficiently proceed. As a result, the desired standard deviation in the particle size of the prior austenite grains could not be obtained and the work hardening ability of the steel sheet fell. In Comparative Example 13, the time from after completion of the hot rolling step to the start of the cooling step was long, and therefore it is believed that overall grain growth proceeded too much.
• Comparative Example 43 the Nb content was low, and therefore it is believed refinement of the prior austenite grains by 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 44, the Nb content was high, and therefore it is believed coarse carbides, etc., were formed in the steel. As a result, ⁇ 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, by area%, 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 a standard deviation in particle size of prior austenite grains of 4.0 ⁇ m or more. 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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