US20190161817A1 - Hot-rolled steel sheet for grain-oriented electrical steel sheet and method of producing same, and method of producing grain-oriented electrical steel sheet - Google Patents

Hot-rolled steel sheet for grain-oriented electrical steel sheet and method of producing same, and method of producing grain-oriented electrical steel sheet Download PDF

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US20190161817A1
US20190161817A1 US16/320,612 US201716320612A US2019161817A1 US 20190161817 A1 US20190161817 A1 US 20190161817A1 US 201716320612 A US201716320612 A US 201716320612A US 2019161817 A1 US2019161817 A1 US 2019161817A1
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steel sheet
hot
grain
rolled steel
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Masanori Takenaka
Minoru Takashima
Yukihiro Matsubara
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JFE Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1216—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the working step(s) being of interest
    • C21D8/1233—Cold rolling
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1216—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the working step(s) being of interest
    • C21D8/1222—Hot rolling
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1244—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the heat treatment(s) being of interest
    • C21D8/1255—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the heat treatment(s) being of interest with diffusion of elements, e.g. decarburising, nitriding
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1244—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the heat treatment(s) being of interest
    • C21D8/1272—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
    • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
    • H01F1/16—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys in the form of sheets
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Definitions

  • This disclosure relates to a hot-rolled steel sheet for a grain-oriented electrical steel sheet which is used to produce a grain-oriented electrical steel sheet, a method of producing the hot-rolled steel sheet for a grain-oriented electrical steel sheet, and a method of producing a grain-oriented electrical steel sheet.
  • a grain-oriented electrical steel sheet is a soft magnetic material mainly used as an iron core material of an electrical device such as a transformer or a generator, and has crystal texture in which the ⁇ 001> orientation which is the easy magnetization axis of iron is highly aligned with the rolling direction of the steel sheet.
  • Such texture is formed through secondary recrystallization of preferentially causing the growth of giant crystal grains in the (110)[001] orientation which is called Goss orientation, when secondary recrystallization annealing is performed in the processes of producing the grain-oriented electrical steel sheet.
  • a slab containing about 3 mass % Si and inhibitor components such as MnS, MnSe, and MN is heated at a temperature exceeding 1300° C. to dissolve the inhibitor components.
  • the slab is then hot rolled, and optionally hot band annealed.
  • the sheet is cold rolled once, or twice or more with intermediate annealing performed therebetween, to obtain a cold-rolled sheet with a final sheet thickness.
  • the cold-rolled sheet is then subjected to primary recrystallization annealing in a wet hydrogen atmosphere, to perform primary recrystallization and decarburization.
  • the sheet is applied with an annealing separator mainly composed of magnesia (MgO), and then subjected to final annealing at 1200° C. for about 5 hours to develop secondary recrystallization and purify the inhibitor components (for example, U.S. Pat. No. 1,965,559 A (PTL 1), JP
  • the conventional processes of producing a grain-oriented electrical steel sheet include containing precipitates (inhibitor components) such as MnS, MnSe, and AlN in a slab, heating the slab at a high temperature exceeding 1300° C. to dissolve the inhibitor components, and finely precipitating the inhibitors component in a subsequent process to develop secondary recrystallization.
  • slab heating at a high temperature exceeding 1300° C. is necessary in the conventional processes of producing a grain-oriented electrical steel sheet, which causes very high production costs.
  • the conventional processes thus have a problem of failing to meet the recent demands to reduce production costs.
  • JP 2782086 B2 proposes a method of containing acid-soluble Al (sol. Al) in an amount of 0.010% to 0.060% and, while limiting slab heating to a low temperature, performing nitriding in an appropriate nitriding atmosphere in a decarburization annealing process so that (Al, Si)N is precipitated and used as an inhibitor in secondary recrystallization.
  • (Al, Si)N disperses finely in the steel and functions as an effective inhibitor, and precipitates (Si 3 N 4 or (Si, Mn)N) mainly composed of silicon nitride only form in the surface layer of the steel sheet after the nitriding treatment of the aforementioned production method.
  • the precipitates mainly composed of silicon nitride change to Al-containing nitride ((Al, Si)N or AlN) which is more thermodynamically stable.
  • JP 4593317 B2 (PTL 5) describes that, by leaving non-recrystallized texture in 1/4 area of the surface layer of a hot-rolled sheet and performing cold rolling without hot band annealing, primary recrystallization preferentially develops in the ⁇ 411 ⁇ 148> orientation, and good magnetic properties can be obtained.
  • JP 2000-129356 A (PTL 6) describes a technique (inhibitorless method) that enables secondary recrystallization without containing any inhibitor component.
  • NPL 1 Y. Ushigami et al. “Precipitation Behaviors of Injected Nitride Inhibitors during Secondary Recrystallization Annealing in Grain Oriented Silicon Steel” Materials Science Forum Vols. 204-206 (1996) pp. 593-598
  • the aforementioned inhibitorless method requires no high-temperature slab heating, and is therefore an excellent method in terms of costs and productivity.
  • a low slab heating temperature always leads to a reduction in rolling temperature in the hot rolling.
  • strains are more likely to accumulate in the surface layer of the steel sheet during the hot rolling, and the surface layer of the hot-rolled sheet is more likely to be recrystallized.
  • the surface layer of the produced hot-rolled sheet is completely recrystallized.
  • the recrystallized grains in the surface layer of such a hot-rolled sheet are very fine, and will cause occurrence of abnormal grain growth during the subsequent hot band annealing process.
  • the uniformity of the recrystallized texture in the sheet thickness direction of the hot-rolled and annealed sheet decreases, and the magnetic properties vary largely in the coil of the steel sheet after final annealing.
  • sheet thickness 1/4 layer the recrystallization ratio in a region from the steel sheet surface (the outermost surface layer or the bottommost surface layer) to a depth of 1/4 the sheet thickness (hereinafter referred to as “sheet thickness 1/4 layer”) is 90% or less
  • sheet thickness 1/4 layer the variations in magnetic properties in the coil of a grain-oriented electrical steel sheet, which is the final product.
  • the “recrystallization ratio” here is obtained by measuring the recrystallization ratio in a region from the outermost surface layer of the steel sheet to a depth of 1/4 the sheet thickness and the recrystallization ratio in a region from the bottommost surface layer of the steel sheet to a depth of 1/4 the sheet thickness, and taking the larger one as the result.
  • Rolling with weak lubrication and high friction coefficient is commonly used in the hot rolling process to prevent wear and roughening of the rollers and to suppress slipping and defective biting.
  • regulating the sheet thickness, tension, and supplying amount of lubricant can regulate the friction coefficient in the final pass, and reducing the friction coefficient to a value lower than a conventional one, specifically to 0.35 or less, can regulate the recrystallization ratio in the region of the sheet thickness 1/4 layer of the hot-rolled steel sheet to 90% or less.
  • FIG. 1 illustrates the relationship between the recrystallization ratio in the region of the sheet thickness 1/4 layer of the hot-rolled steel sheet and the friction coefficient in the final pass of the hot rolling based on the results listed in Table 1.
  • the hot-rolled coils were each hot band annealed at 1050° C. for 60 seconds, then cold rolled to a thickness of 0.27 mm, and then subjected to primary recrystallization annealing at 820° C. for 120 seconds.
  • the heating rate between 500° C. and 700° C. in the primary recrystallization annealing was 200° C./s.
  • the normal grain growth inhibiting capability can be obtained even with slab heating in a low-temperature region of 1300° C. or lower by regulating the contents of sol.Al, S and Se in a predetermined amount. In this way, the magnetic properties can be improved.
  • the recrystallization ratio in the region of the sheet thickness 1/4 layer of the hot-rolled steel sheet can be 90% or less, and the variations in magnetic properties of the steel sheet after final annealing can be significantly reduced.
  • a hot-rolled steel sheet for a grain-oriented electrical steel sheet comprising:
  • a hot-rolled and annealed sheet for a grain-oriented electrical steel sheet comprising:
  • a method of producing a hot-rolled steel sheet for a grain-oriented electrical steel sheet comprising:
  • a method of producing a grain-oriented electrical steel sheet comprising:
  • FIG. 1 illustrates the relationship between the recrystallization ratio in the region of the sheet thickness 1/4 layer of a hot-rolled steel sheet and the friction coefficient in the final pass of hot rolling.
  • the C content is set to a range of 0.002% or more and 0.080% or less.
  • the C content is preferably 0.020% or more and 0.070% or less.
  • Si is a very effective element in increasing electrical resistance of steel and reducing eddy current loss which constitutes a part of iron loss.
  • the electrical resistance monotonically increases until the Si content reaches 11%, yet a Si content more than 8.00% remarkably deteriorates the workability.
  • the Si content is set to 2.00% or more and 8.00% or less.
  • the Si content is preferably 2.50% or more and 4.50% or less.
  • Mn 0.02% or More and 0.50% or Less
  • Mn combines with S or Se to form MnS or MnSe.
  • MnS or MnSe acts with a grain boundary segregation element and inhibits normal grain growth in the heating process of the secondary recrystallization annealing.
  • the Mn content is less than 0.02%, the normal grain growth inhibiting capacity is insufficient.
  • the Mn content exceeds 0.50%, high-temperature slab heating is required in the slab heating process before hot rolling in order to completely dissolve Mn, and moreover, the normal grain growth inhibiting capacity is decreased due to coarse precipitation of MnS or MnSe. Therefore, the Mn content is set to 0.02% or more and 0.50% or less.
  • the Mn content is preferably 0.05% or more and 0.20% or less.
  • S and Se combine with Mn to form an inhibitor.
  • the total content of one or more selected from S and Se is less than 0.001%, the absolute amount of inhibitor is insufficient, and therefore the normal grain growth inhibiting capacity is insufficient.
  • the total content of one or more selected from S and Se exceeds 0.010%, iron loss deterioration occurs because of incomplete desulfurization and deselenization in the secondary recrystallization annealing. Therefore, the total content of one or more selected from S and Se is set in a range of 0.001% or more and 0.010% or less.
  • Acid-Soluble Al 0.003% or More and Less than 0.010%
  • Al forms a dense oxide film on the surface, which increases the difficulty of regulating the nitriding amount during the nitriding and hampers decarburization. Therefore, the Al content is limited to less than 0.010% in terms of acid-soluble Al.
  • Al since Al has a high affinity for oxygen, adding a minute amount of Al during steelmaking is expected to reduce the amount of dissolved oxygen in the steel, thereby, for example, reducing oxide inclusions which cause deterioration in properties. In view of this, containing acid-soluble Al in an amount of 0.003% or more can suppress deterioration in magnetic properties.
  • N content increases the difficulty of secondary recrystallization, as with the case of S and Se.
  • N content is 0.006% or more, secondary recrystallization is unlikely to occur and the magnetic properties are deteriorated. Therefore, the N content is limited to less than 0.006%.
  • Ni, Cu, Sb, Sn, Cr, P, Mo, Ti, Nb, V, B, Bi, Te and Ta are each a useful element in improving magnetic properties.
  • the content is less than the lower limit of the corresponding range as listed above, the effect of improving magnetic properties is insufficient.
  • the content exceeds the upper limit of the corresponding range as listed above secondary recrystallization is unstable and magnetic properties are deteriorated.
  • a steel slab having the aforementioned chemical composition is heated.
  • the slab heating temperature is 1300° C. or lower. Heating at a temperature exceeding 1300° C. requires a special heating furnace, such as an induction heating furnace, rather than a normal gas heating furnace, which is disadvantageous in various respects such as costs, productivity and yield rate.
  • a too low slab heating temperature leads to incomplete uniformization of solute elements, and therefore the slab heating temperature is preferably 1200° C. or higher.
  • Hot rolling is performed after the slab heating.
  • the rolling reduction of the hot rolling is 95% or more, and the sheet thickness after the hot rolling is 1.5 mm to 3.5 mm.
  • the rolling finish temperature is desirably 800° C. or higher.
  • the coiling temperature after the hot rolling is desirably about 500° C. to 700° C.
  • the recrystallization ratio in the region of the sheet thickness 1/4 layer of the hot-rolled steel sheet is 90% or less.
  • Methods of obtaining a recrystallization ratio of 90% or less in the region of the sheet thickness 1/4 layer of the hot-rolled steel sheet include lowering the friction coefficient in the final pass of the hot rolling, specifically to 0.35 or less.
  • Methods of lowering the friction coefficient include increasing the amount of lubricant; reducing the roughness of rollers; and reducing the thickness of the surface-layer scale by, for example, an improved the descaling process or a shorter rolling time.
  • hot band annealing is performed to improve the texture of the hot-rolled sheet.
  • the hot band annealing is preferably performed under conditions where the soaking temperature is 800° C. or higher and 1200° C. or lower, and the soaking time is 2 seconds or more and 300 seconds or less.
  • the soaking temperature in the hot band annealing is lower than 800° C.
  • the improvement in the texture of the hot-rolled sheet is incomplete, and non-recrystallized parts remain.
  • the desired texture may not be obtained.
  • the soaking temperature exceeds 1200° C.
  • dissolution of AlN, MnSe and MnS proceeds, the inhibiting capability of inhibitor in the secondary recrystallization process is weakened, and secondary recrystallization is suspended.
  • the soaking temperature in the hot band annealing is preferably 800° C. or higher and 1200° C. or lower.
  • the soaking time in the hot band annealing is preferably 2 seconds or more and 300 seconds or less.
  • This disclosure has a further feature that, in the hot-rolled and annealed sheet, the average recrystallized grain size in regions from the outermost surface layer and the bottommost surface layer to a depth of 1/4 the sheet thickness is three times or less the average recrystallized grain size in regions from the center of the sheet thickness to a length of less than 1/4 the sheet thickness in directions to the outermost surface layer and the bottommost surface layer.
  • the “average recrystallized grain size in regions from the outermost surface layer and the bottommost surface layer to a depth of 1/4 the sheet thickness” here is obtained by measuring the average recrystallized grain size in a region from the outermost surface layer of the steel sheet to a depth of 1/4 the sheet thickness and the average recrystallized grain size in a region from the bottommost surface layer of the steel sheet to a depth of 1/4 the sheet thickness, and taking the larger one as the result.
  • the method of measuring the average recrystallized grain size of the hot-rolled and annealed sheet was the same as the case of hot-rolled sheet, where samples were cut from the central portion in the width direction of the lead and tail ends of the coil, and measurement was performed by EBSD throughout the thickness of a cross section in the transverse direction (direction orthogonal to the rolling direction) (TD cross section). A region where the grain orientation spread (the average value of misorientation angles between one measurement point and all the other measurement points in the same crystal grain) is within 3° was regarded as a recrystallized grain.
  • the steel sheet is subjected to cold rolling twice or more with intermediate annealing performed therebetween to obtain a steel sheet with a final sheet thickness.
  • the intermediate annealing is preferably performed with a soaking temperature of 800° C. or higher and 1200° C. or lower and a soaking time of 2 seconds or more and 300 second or less, for the same reasons as in the hot band annealing.
  • the sheet By setting the rolling reduction in final cold rolling to 80% or more and 95% or less during the cold rolling, the sheet can obtain better texture after subjection to primary recrystallization annealing. It is also effective to perform the rolling with the rolling temperature increased to 100° C. to 250° C., or perform aging treatment once or more in a range of 100° C. to 250° C. during the cold rolling, in terms of developing Goss texture.
  • primary recrystallization annealing is performed preferably with a soaking temperature of 700° C. or higher and 1000° C. or lower.
  • the primary recrystallization annealing may also serve as decarburization of the steel sheet by being performed in, for example, a wet hydrogen atmosphere.
  • the soaking temperature in the primary recrystallization annealing is lower than 700° C., non-recrystallized parts remain, and the desired texture may not be obtained.
  • the soaking temperature exceeds 1000° C. secondary recrystallization of Goss orientation grains may occur. Therefore, the soaking temperature in the primary recrystallization annealing is preferably set to 700° C. or higher and 1000° C. or lower.
  • the average heating rate in a temperature range of 500° C. to 700° C. is preferably 50° C./s.
  • an annealing separator may be applied to the steel sheet as required.
  • an annealing separator mainly composed of MgO may be applied, so that a forsterite film can be formed while developing secondary recrystallized texture during the subsequent secondary recrystallization annealing which also serves as purification annealing.
  • no annealing separator is applied, or, even when applying an annealing separator, materials such as silica or alumina is used instead of the forsterite film-forming MgO.
  • electrostatic coating which introduces no moisture.
  • a heat-resistant inorganic material sheet (silica, alumina, mica) may be used.
  • Secondary recrystallization annealing is subsequently performed.
  • the secondary recrystallization annealing is preferably performed, for example, at a temperature of 800° C. or higher for at least 20 hours.
  • the annealing conditions of the secondary recrystallization annealing are not particularly limited, and any conventionally known annealing conditions may be applied.
  • the secondary recrystallization annealing can also serve as purification annealing.
  • an insulating coating application process and a flattening annealing process may be performed as required, and the desired grain-oriented electrical steel sheet is obtained.
  • the production conditions of the insulating coating application process and the flattening annealing process are not particularly limited, and any conventional methods may be used.
  • the applied coating is mainly composed of glass, and that is commonly obtained by applying, for example, silicate phosphate with a coater.
  • the flattening annealing process if an annealing separator is applied in the preceding processes, then water washing, brushing, or pickling is performed to remove the remaining annealing separator. It is effective to subsequently perform flattening annealing to correct the shape to reduce iron loss.
  • the soaking temperature in the flattening annealing is preferably about 700° C. to 900° C. from the perspective of shape correction.
  • a grain-oriented electrical steel sheet produced according to the aforementioned conditions has a very high magnetic flux density as well as low iron loss properties after the secondary recrystallization.
  • a high magnetic flux density means that the grains have preferentially grown only in Goss orientation and its vicinity during the secondary recrystallization process. In the Goss orientation and its vicinity, the growth rate of secondary recrystallized grains is higher. Therefore, an increase in magnetic flux density indicates a potential increase in secondary recrystallized grain size. This is advantageous in terms of reducing hysteresis loss, yet disadvantageous in terms of reducing eddy current loss.
  • the magnetic domain refining treatment may be any known heat resistant or non-heat resistant magnetic domain refining treatment.
  • the effect of magnetic domain refining can reach the steel sheet inside in the sheet thickness direction.
  • the other production conditions may comply with typical grain-oriented electrical steel sheet production methods.
  • the hot-rolled sheets were each subjected to hot band annealing at 1000° C. for 60 seconds.
  • the average recrystallized grain size in the region of the sheet thickness 1/4 layer and the region of the sheet thickness central layer (the regions from the center of the sheet thickness to a length of less than 1/4 the sheet thickness in directions to the outermost surface layer and the bottommost surface layer) of the hot-rolled and annealed sheets taken from the lead and tail ends of the hot band annealed coil were measured. The results are listed in Table 2.
  • the steel sheets were each subjected to first cold rolling to obtain an intermediate thickness of 1.8 mm. Subsequently, the steel sheets were each subjected to intermediate annealing at 1040° C.
  • the heating rate from 500° C. to 700° C. in the primary recrystallization annealing was 100 ° C/s.
  • an annealing separator mainly composed of MgO was applied to the surface of each steel sheet, and then the steel sheets were subjected to secondary recrystallization annealing serving also as purification annealing at 1200° C. for 10 hours.
  • a phosphate-based insulating tension coating was applied to and baked on each steel sheet, and flattening annealing was performed for the purpose of flattening the steel strips, to obtain products.
  • the recrystallization ratio in the region of the sheet thickness 1/4 layer of the hot-rolled steel sheet can be 90% or less
  • the average recrystallized grain size in the region of the sheet thickness 1/4 layer of the hot-rolled and annealed sheet can be three times or less the average recrystallized grain size in the region of the sheet thickness central layer, and the variations in magnetic properties of the steel sheet after final annealing can be significantly reduced.

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US20210087650A1 (en) * 2019-09-19 2021-03-25 Nucor Corporation Ultra-high strength weathering steel for hot-stamping applications
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