US20130174940A1 - Grain oriented steel strip with high magnetic characteristics, and manufacturing process of the same - Google Patents

Grain oriented steel strip with high magnetic characteristics, and manufacturing process of the same Download PDF

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US20130174940A1
US20130174940A1 US13/635,743 US201113635743A US2013174940A1 US 20130174940 A1 US20130174940 A1 US 20130174940A1 US 201113635743 A US201113635743 A US 201113635743A US 2013174940 A1 US2013174940 A1 US 2013174940A1
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strip
temperature
rolling
hot
amount
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Stefano Cicale
Giuseppe Abbruzzese
Marco Antonio Da Cunha
Helcio De Araujo Quintao
Ronaldo Claret Ribeiro Silva
Angelo Jose De Faria Fonseca
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Aperam SA
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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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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 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
    • C21D8/0278—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 a particular surface treatment 
    • C21D8/0284—Application of a separating or insulating coating
    • 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 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
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1216—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the working steps
    • C21D8/1222—Hot rolling
    • 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
    • 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
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/04—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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1216—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the working steps
    • C21D8/1233—Cold rolling
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1244—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the heat treatment
    • C21D8/1255—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the heat treatment 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1244—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the heat treatment
    • C21D8/1261—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the heat treatment following hot rolling
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1244—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the heat treatment
    • C21D8/1272—Final recrystallisation annealing

Definitions

  • the present invention deals with a process for the production of silicon-iron grain oriented electrical steel strips, generally used in the manufacturing of the cores of electric transformers.
  • these products have ferritic (BCC) grains with a size ranging from some millimetres to some centimetres, with the ⁇ 100> crystallographic direction aligned to the rolling direction and the ⁇ 110 ⁇ crystallographic plane almost parallel to the rolling plane.
  • BCC ferritic
  • This high “orientation” of the grains is obtained by exploiting a metallurgical phenomenon called “secondary recrystallization”: during batch annealing, performed at high temperature after primary recrystallization annealing, some of the grains of the microstructure, namely the ones with better orientation, abnormally grow consuming the other grains of the microstructure.
  • This phenomenon is influenced by the delicate balance among size and size distribution of primary recrystallized grains, their texture and fine dispersion of second phases (typically sulphides and/or selenides and/or nitrides), which inhibits grain growth during the secondary recrystallization annealing, allowing the selection mechanisms to operate and to produce a proper oriented final grains structure.
  • second phases typically sulphides and/or selenides and/or nitrides
  • Primary recrystallized microstructure and second phases distribution are in their turn influenced by the upstream process and the attainment of the best metallurgical results is influenced in a complex manner by parameters distributed along the entire production process.
  • Proper precipitation of second phases is obtained by the presence in the alloy of controlled amounts of elements capable of forming them (S and/or Se and/or N together with Mn, Cu, Al, etc.), the heating of the slab before the hot-roiling up to very high temperatures (>1350° C.), so as to dissolve a significant amount of the second phases, precipitated during casting in a coarse form, incapable of controlling the secondary recrystallization. After dissolution they can re-precipitate during the hot-rolling and the subsequent processes, in a form capable of controlling the secondary recrystallization.
  • the fine precipitation of sulphides is obtained during hot rolling, choosing the starting rolling temperature so that MnS, dissolved during slab reheating, are in strongly over-saturated solution; the fine second phase particles distribution is obtained thanks to the high density of dislocations generated by deformation, acting as nucleation site.
  • the same mechanism cannot be adopted for the precipitation of nitrides, in fact when precipitation of nitrides happens during hot rolling, it is in coarse form, not suitable to control secondary recrystallization. This is due to the lower precipitation temperature of aluminium nitrides at the concentration used in this technology, if compared to the one of MnS, and to specific difference in the chemical-physical constants which controls the precipitation (diffusion coefficients, interface energy . . . ) between AlN and MnS.
  • nitrides precipitation during hot rolling is to be avoided and the precipitation in fine form of nitrides is obtained in the following annealing and quenching of the hot rolled strip, which in this technology is necessary.
  • the strip needs to be rapidly cooled after hot rolling in order to prevent at maximum extent the precipitation of nitrides also during coiling; minor unavoidable precipitation during cooling is acceptable if the coiling temperature is below 600° C. In fact, coiling below the mentioned temperature guarantees the precipitation of residual nitrogen present in solid solution as silicon nitrides, preventing in this way the coarsening of aluminium nitrides during coiled strip cooling.
  • a first drawback is related to the control of secondary recrystallization phenomenon, which is quite delicate. Quite often in the industrial practice, due to fluctuations in process parameters it can became unstable, producing as a result finished product which contains fine grains with magnetically unfavourable orientation and determining as a consequence poor magnetic quality.
  • One of the most critical point is the dissolution and re-precipitation of second phases (sulphides/selenides/nitrides) for the control of secondary recrystallization. It depends, besides the adopted temperature, on the chemical activities and consequently on concentration of constitutive elements (Al, N, Mn, S, Se . . . ).
  • Another drawback is related to the abnormal growth of the grains in the slab microstructure.
  • some phenomena of selective secondary grain growth happens in the slab microstructure, producing strong microstructure heterogeneities, with some grains abnormally grown in comparison with others.
  • These micro-structural heterogeneities are not completely overcome during conventional hot rolling and, without the adoption of adequate countermeasures, their inheritance remains down to finished product: fine grains of unfavourable orientation appear together with properly secondary recrystallized grains.
  • fine grains are grouped in colonies elongated along rolling direction a few centimetres in width and a few decimetres in length, correspondent to the big grains grown in the slab microstructure (so called “streaks defect”).
  • a further drawback is related to brittleness of the hot rolled strip due to presence of silicon in concentration above 3%.
  • the heterogeneities in the hot rolled strip microstructure favour the micro-cracks propagation enhancing such brittleness phenomena.
  • This brittleness of the alloy negatively generates ruptures of the strip during the processing down to the finished product, especially during cold rolling, decreasing cold rolling productivity and yield.
  • a first object of the invention is a method for the production of hot-rolled steel strip comprising the following steps:
  • a steel slab comprising, in weight percentages: Si: 2.5 to 3.5%, C: 0.05 to 0.1%, Mn: 0.05 to 0.1%, Als: 0.015 to 0.026%, N: 0.0050 to 0.0100%, and further comprising S and/or Se so that S+ (32/79) Se is in an amount of 0.018 to 0.030%, and optionally comprising one or more elements chosen among Sb in an amount of 0.015 to 0.035%, Cu in an amount of 0.08% to 0.25%, Sn in an amount of 0.06% to 0.15%, P in an amount of 0.005% to 0.015%, the balance being iron and unavoidable impurities,
  • finishing hot-rolling of said blank to produce a hot rolled strip in three rolling passes or more, the temperature of said blank during the first pass being above 1150° C. and at least one rolling pass being performed with a reduction of 40% or more and being immediately followed by a holding of more than 20 sec, the average interpass holding temperature Tav, as defined by eq. 2 below, being settled between 1000 and 1200° C., the total finishing hot rolling time t (TFRT) being controlled so that the value of Tav for any portion of said strip further respects the under mentioned equation:
  • the roughing hot rolling and finishing hot rolling are preferably performed using a reversible hot rolling mill.
  • the end hot rolling temperature T end is preferably higher than [T 1 + ⁇ 1 (t ⁇ 78)] ⁇ 60° C.
  • the reheating of the slab is performed at a temperature between 1350° C. and 1430° C.
  • Another object of the invention is a hot-rolled strip, obtainable by the method according to anyone of the modes above, comprising, in weight percentages: Si: 2.5 to 3.5%, C: 0.05 to 0.1%, Mn: 0.05 to 0.1%, Als: 0.015 to 0.026%, N: 0.0050 to 0.0100%, and further comprising S and/or Se so that S+ (32/79) Se is in an amount of 0.018 to 0.030%, and optionally comprising one or more elements chosen among Sb in an amount of 0.015 to 0.035%, Cu in an amount of 0.08% to 0.25%, Sn in an amount of 0.06% to 0.15%, P in an amount of 0.005% to 0.015%, the balance being iron and unavoidable impurities, comprising less than 0.0025% of nitrogen linked to aluminium under the form of AlN and presenting five different layers across the strip thickness composed of grains populations with different characteristics.
  • the hot-rolled strip comprises preferably outer layers 1 and 5, the zones extending from the surface to 1 ⁇ 6 of strip thickness having a ferritic microstructure composed of more than 80% of equi-axial recrystallized grains with an average grain size d ⁇ o lower than 50 ⁇ m and preferably lower than 30 ⁇ m, and optionally, atop of at least one of the outer surface of said outer layers, coarse decarburised grains with an average grain size of at least 2 d ⁇ o .
  • the hot-rolled strip comprises intermediate layers 2 and 4, the zones extending from 1 ⁇ 6 of the strip thickness to 2/6 of strip thickness, as measured from the surface, having a ferritic microstructure composed of more than 80% of recrystallized grains with an average grain size d ⁇ i lower than 80 ⁇ m and preferably lower than 50 ⁇ m, d ⁇ i being such that: d ⁇ i >d ⁇ o .
  • the hot-rolled strip comprises preferably a central layer, the central zone of the strip equal to 1 ⁇ 3 of strip thickness, having an ⁇ -fiber texture with less than 60%, and preferably less than 50% of the area percentage of the layer with grains having a disorientation less than 20° from the ⁇ -fiber ( ⁇ 110> crystallographic direction parallel to rolling direction).
  • Another object of the invention is a method for the production of cold-rolled grain oriented magnetic strip comprising the following steps:
  • the reduction ratio of the last cold rolling down to final thickness being comprised between 80% and 95%
  • the first annealing of the strip is performed in two steps, the strip being first held at a temperature between 1050° C. and 1170° C. during 10 to 60 s, and then cooled and held at a temperature between 800° C. and 950° C. during 40 to 240 sec.
  • the cold rolling of the annealed strip is preferably performed in three passes or more and the strip is held at a temperature between 170° C. and 300° C. after the first cold rolling pass.
  • the strip under rolling is held at a temperature between 170° C. and 300° C. in at least one interpass step after the first cold rolling pass.
  • the primary recrystallization annealing of the strip comprises an holding at a temperature between 780° C. and 900° C., during 60 to 300 sec, in an atmosphere consisting of N 2 , H 2 and H 2 O, the ratio between partial pressure of H 2 O and partial pressure of H 2 being between 0.40 and 0.70.
  • the heating rate of the strip to reach said holding temperature between 780 and 900° C. is preferably at least 150° C./sec in the range between 200° C. and 700° C.
  • the secondary recrystallization annealing comprises a heating of the strip to a temperature between 1000° C. and 1250° C. with a heating rate between 5° C./h and 40° C./h in an atmosphere consisting of N 2 and H 2 , and then a holding of said strip during 5 to 30 h at this temperature in an atmosphere consisting of H 2 .
  • FIGS. 1 to 3 illustrate the influence of TFRT and average holding temperature Tav on the magnetic induction at 800 A/m (B800)
  • FIGS. 1 and 2 are related to two different values of soluble aluminium.
  • FIG. 4 a is an example of the through-thickness microstructure of a hot rolled strip according to the invention
  • FIG. 4 b is a schematic presentation of the different layers of FIG. 4 a )
  • FIG. 5 illustrates the influence of the parameter [T 1 + ⁇ 1 (t ⁇ 78)] ⁇ T end ] on the magnetic induction B800.
  • the present inventors found out that the combination of the specific composition of the steel and the particular hot rolling procedure of the invention allows better control of the secondary recrystallization, decreasing its sensitivity to second phase distribution fluctuations and overcoming the occurrence of “streaks defect”, without introducing complex pre-rolling procedures, and reducing the risk of breakage during cold rolling.
  • Such finishing hot rolling practice can for example be performed using a reversible hot rolling mill, also called Steckel mill, consisting of a single cage hot rolling mill with two coilers (each at any side of the cage) inside coil boxes at high temperature, where hot rolling is performed reversibly.
  • a reversible hot rolling mill also called Steckel mill, consisting of a single cage hot rolling mill with two coilers (each at any side of the cage) inside coil boxes at high temperature, where hot rolling is performed reversibly.
  • composition of the steel is an essential part of the invention and will now be detailed, all percentages being weight percentages.
  • the amount of silicon is between 2.5 and 3.5%, and preferably between 2.90 and 3.3%.
  • the addition of this element to the steel is essential and allows increasing the electric resistivity, decreasing in such way the iron losses. Silicon addition below the specified minimum limit does not produce suitable effect on iron losses reduction, while silicon contents above such mentioned maximum limit induces brittleness phenomena which make the hot rolling and the following transformation down to finished product quite difficult to be performed.
  • the soluble aluminium amount (Als) is between 0.0150 and 0.0260%.
  • Als is the aluminium soluble in acid, and is understood as being total aluminium minus aluminium bound as oxide.
  • the addition of this essential element is necessary for the formation of proper amount of aluminium nitrides (AlN) suitable to control the secondary recrystallization.
  • Aluminium amount below the minimum specified limit decreases the volume fraction of AlN so that secondary recrystallization becomes unstable.
  • Its addition above the mentioned maximum limit provokes a coarse precipitation of AlN so that they are ineffective to control the grain growth during the secondary recrystallization annealing.
  • aluminium content above the mentioned limit increases the precipitation temperature of AlN, increasing in such way their driving force for precipitation during hot rolling. This makes impossible to maintain them in solution during hot rolling phase, producing as a consequence coarse precipitation of AlN.
  • the amount of nitrogen is between 0.0050 and 0.0100%.
  • the addition of this essential element to the steel below the proposed minimum limit produces low volume fraction of nitrides, while nitrogen content above the mentioned upper limit is difficult to be obtained in conventional steelmaking operations, due its low solubility in steel, and may create a specific defect on the finished product occurrence, the so called “blister defect”.
  • the amount of carbon in the hot-rolled steel is between 0.05 and 0.1%, but preferably under 0.08%. Its presence in the alloy has a positive effect on the magnetic characteristics. Carbon generates hard phases and fine carbides during the quenching process, thus increasing the strain hardening rate during the cold-rolling, in this way it improves the texture of cold rolled sheet, increasing the orientation of the Goss grains nuclei, which will produce better oriented Goss grains after secondary recrystallization. A carbon level below the mentioned lower limit does not produce the mentioned beneficial effects while carbon contents above the mentioned upper limit do not produce additional positive effects and increases the decarburization time unduly.
  • the amount of manganese is between 0.05 and 0.1%. Its addition below the mentioned limit produces insufficient volume fraction of sulphides and or selenides (MnS and MnSe) necessary for the proper control of secondary recrystallization, while manganese content above the mentioned upper limit increases the solubility product of the mentioned second phases and consequently provokes the coarse precipitation of the MnS/MnSe in a way ineffective to control the secondary recrystallization.
  • the amounts of sulphur and selenium must be so that S+(32/79)Se is between 0.0180 and 0.0300%.
  • S+(32/79)Se is between 0.0180 and 0.0300%.
  • the volume fraction of sulphides and/or selenides becomes insufficient to produce secondary recrystallization, while contents above the upper limit produce brittleness phenomena due to sulphur and/or selenium segregation in the central part of the slab.
  • the steel according to the invention may also contain optionally one or more elements chosen among Sb in an amount of 0.015 to 0.035%, Cu in an amount of 0.08% to 0.25%, Sn in an amount of 0.06% to 0.15%, P in an amount of 0.005% to 0.015%.
  • Antimony can be used as a segregating element which can help in controlling grain growth during secondary recrystallization, in addition to AlN, MnS and/or MnSe.
  • Copper is able to form sulphides, alone or in combination with manganese. Its presence in the alloy contribute to have a distribution of sulphide second phase more suitable to control grain growth during secondary recrystallization annealing, improving the magnetic characteristics of the finished product. An addition below the minimum limit does not produce the mentioned positive effect, while above the maximum limit no further improvement of the magnetic characteristics are observed and cost of the alloys is unduly increased.
  • the method of production of a hot rolled steel strip according to the invention starts by reheating a slab having the above composition.
  • a slab can be obtained by usual steelmaking methods such as continuous casting.
  • the slab In order to carry out the hot rolling method, the slab has to be reheated at a high temperature, above 1300° C.
  • the reheating temperature has to be above the level necessary to almost completely dissolve any sulphides and/or nitrides present in the steel, allowing having enough free nitrogen and free sulphur to generate second phases precipitates necessary to control secondary recrystallization.
  • Reheating temperatures above the upper limit of 1430° C. do not produce additional advantages, increasing the energy consumption and decreasing the yield of reheating process due to the oxidation of the slab.
  • the hot rolling is divided in two phases: “roughing hot rolling” until the blank thickness is below 50 mm and “finishing hot rolling” from this thickness to the final thickness, which is usually between 1 and 3 mm.
  • finishing hot rolling After the roughing hot rolling, a blank thickness above the upper maximum limit makes difficult to fulfil the proposed conditions for the following finishing hot rolling because the total finishing reduction rate became too high and consequently the total rolling time is increased above the total maximum limit.
  • the roughing rolling can be performed in any roughing stand but is preferably performed in a reversible rolling mill.
  • finishing hot rolling which is also preferably performed in a reversible hot rolling mill.
  • the finishing hot rolling is performed in three rolling passes or more. It is preferred to have three or five rolling passes, a most preferred embodiment being to have three rolling passes.
  • the total reduction ratio depends of the thickness of the slab.
  • the start temperature of finishing hot rolling (i.e. temperature of the blank measured on the run-in table just before the first finishing hot rolling pass) must be above 1150° C. in order to avoid the beginning of any second phase precipitation before the hot rolling starts, which would cause a coarsening of sulphides and/or selenides.
  • a starting finishing rolling temperature above 1180° C.; it helps to control fluctuations in distribution of precipitating elements, produced by fluctuation in distribution of solubility product through the blank, due to segregation of precipitating elements.
  • T i (x) being the holding temperature of the part of the strip of abscissa x when it is located in the interpass i between two rolling passes
  • ⁇ i (x) being the holding time of the part of the strip of abscissa x when it is located in the interpass between two rolling passes.
  • the way how to measure the interpass holding temperature depends on the specific furnace used to perform the holding. There also is the possibility that the holding temperature during the time interval ⁇ i (x) is not constant, being so a function of time t beside of the abscissa x: T i (t; x). In this case the interpass holding temperature T i (x) will be determined averaging this variable temperature according to the following equation.
  • the inventors while not willing to be bound by any specific measuring method, point out that in the examples 1, 2 and 3 and as well as in the experiments used to determine the graphs in FIGS. 1 , 2 and 3 , where a reversible hot rolling mill has been used, the above eq. 3 has been estimated by averaging the entry and exit Temperature of the strip in the coiling furnaces, and by assuming a linear variation of the temperature of the strip T i (t; x) during holding time. This linear approximation is justified by the small difference in the measured entry and exit temperature: less than 20° C. for most length of the strip. Only for the head part of the strip, the difference between entry and exit temperature is about 50° C.
  • the end hot rolling temperature T end (i.e. the temperature measured on the run-out table just after the last hot rolling pass), is an important parameter related to the magnetic quality of the finished product.
  • T end is higher than [T 1 + ⁇ 1 (t ⁇ 78)] ⁇ 60° C. (with T 1 , ⁇ 1 , t, as defined above), or what is equivalent, if [T 1 + ⁇ 1 (t ⁇ 78)] ⁇ T end is lower than 60° C., the magnetic quality of the finished product has superior characteristics with B800 being superior or equal to 1920 mT, or 1.92 T.
  • the inventors also observed that an high average interpass holding temperatures towards the higher limit of the range 1000° C.-1200° C. decrease the driving force for precipitation, allowing longer rolling time without promoting precipitation of nitrides, as expected according to Eq. 1.
  • the driving force for precipitation is so high that it happens irrespectively of the chosen holding time.
  • average interpass holding temperatures above 1200° C. are difficult to reach and it does not provide additional advantages.
  • T end in particular the difference between the minimum average holding temperature as defined above and T end ([T 1 + ⁇ 1 (t ⁇ 78)] ⁇ T end ), is an important parameter in order to guarantee the finished product has a superior magnetic quality, has not been completely understood, the inventors have nevertheless observed that, provided that the previous specifications for Tav and TFRT are fulfilled, if the difference ([T 1 + ⁇ 1 (t ⁇ 78)] ⁇ T end ) is higher than 60° C., then a minor amount of precipitation of AlN starts in any case on the run-out table and the magnetic quality of the finished product, although still having a B800 ⁇ 1900 mT, starts to deteriorate. If instead the said difference is lower than 60° C., no precipitation on the runout table is observed and the magnetic quality of the finished product is superior with B800 ⁇ 1920 mT
  • equation 1 is drawn on FIGS. 1 and 2 , for two particular Al s values, together with the experimental data which allowed to determine it.
  • FIG. 1 the experimental data related to different steel samples containing 0.0250% ⁇ 0.0010% Als.
  • the B800 of each single sample is represented as a function of average interpass holding temperature Tav and total finishing rolling time t with the following symbols:
  • FIG. 3 illustrate the magnetic properties of samples that have been extracted in different positions along the length of a strip of chemical composition fulfilling the teaching of the present invention (Als measured on the strip: 0.0247% ⁇ 0.0010%), finishing hot rolled by a reversible hot rolling mill, having as a consequence local variations in TFRT and T av along the strip length, represented by the losange marks and continuous line in the graph.
  • the magnetic properties remain high and quasi-constant.
  • B800 drops very sharply.
  • the length of the part hot rolled strip which can fall outside the invention limit, at hot rolled strip tail, during a normal rolling operation at conventional reversible hot rolling mill, is usually very short and it is usually cut away in the normal scrapping operation performed for other reasons during the production process.
  • the cooling of the hot-rolled strip from the end rolling temperature to a temperature below 600° C. has to be done in less than 10 sec to avoid early precipitation of nitrides.
  • the end hot rolling temperature T end is the temperature measured on the run-out table just after last rolling pass.
  • the coiling is performed next at a maximum temperature of 600° C. There again, coiling above the mentioned maximum limit produces a hot rolled strip with a coarse precipitation of AlN unsuitable to control secondary recrystallization.
  • the coiling at a temperature according to the invention produces a hot rolled strip where the nitrogen is captured by precipitation as silicon nitrides, which can be re-dissociated in the subsequent annealing of the hot rolled strip and nitrogen can be so re-precipitated as fine aluminium nitrides.
  • the hot rolled band obtained through this method shows specific characteristics as can be seen on FIG. 4 a ) showing a micrograph of sample n° 15 of the under examples and on FIG. 4 b ) schematizing the layers structure across the thickness.
  • “RD” and “ND” on FIG. 4 a ) stand respectively for the Rolling Direction and the Normal Direction of the band.
  • This fully ferritic microstructure presents a higher degree of recrystallization compared to what is known in the state of art when the hot rolling is conducted without holding the strip at high temperature in the interpass between consecutive hot rolling passes.
  • This microstructure shows specific features:
  • FIGS. 4 a - b illustrate an example wherein these surface grains evidenced as sub-layers 6 and 7 are present on both sides of the sheet, which are probably due to a superficial decarburization occurring incidentally during the interpass holding at high temperature. When these surface coarse grains are present, they are not considered in the grain size calculation.
  • the inventors characterised the microstructure with “Orientation Imaging Microscopy” (OIM) technique. Grain size was determined based on diameter of circle of equivalent area respect the grains; the grains area was determined by 5° tolerance angle criterion (see “Electron Backscattered Diffraction in Materials Science” Kluwer Academic/Plenum Publishers, New York, 2000, ISBN 0-306-46487-X for details on the used techniques). Drawing in FIG. 4 a was obtained with such technique. The average grain sizes in the outer layers 1 and 5 are not necessarily equal to each other and for that reason d ⁇ o is measured as the average grain size of both layers all together. The same criterion applies to the measurement of the average grain size of the intermediate layers 2 and 4, d ⁇ l , for the same reason.
  • OFIM Orientation Imaging Microscopy
  • the present inventors retain it is linked to the recrystallization of the microstructure which happens during holding at a high temperature after a reduction greater than 40% one or more time between consecutive rolling passes, the average interpass holding temperature being settled between 1000° C. and 1200° C.
  • the present inventors also retain the strong recrystallization of the microstructure obtained with this method is also responsible for the avoidance of the streaks defect without introducing the pre-rolling procedure and for the improved stability of the magnetic quality.
  • the hot-rolled band contains MnS and/or MnSe precipitated in fine form suitable to control the secondary recrystallization.
  • Nitrogen is precipitated in SiN or in other low temperature stable compounds; in minor part it is also precipitated as AlN but less than 0.0025% and preferably less than 0.0015% of nitrogen is precipitated as AlN in the hot rolled strip.
  • the above described hot rolled strip can be considered a product itself, which can be used as starting product for the production of cold rolled grain oriented electrical steel following the teaching, well known by the man skilled in the art, for the production of Grain Oriented electrical steel based on MnS and AlN inhibitors completely dissolved during hot rolling and re-precipitated during the process.
  • the strip can be optionally cold-rolled and then annealed during 40 to 300 seconds in one or more steps, the first step being performed at 1050-1170° C. and optional further steps being performed at lower temperatures.
  • Annealing directly after hot rolling or after this first cold rolling has different purposes. It is first necessary to dissolve the silicon nitrides, precipitated during last part of cooling and coiling of the hot rolled strip, and to re-precipitate them in form of AlN. Such precipitation of AlN happens both during the heating stage of this annealing, simultaneously with silicon nitrides dissolution and during cooling before quenching.
  • Annealing temperatures and times as well as starting quenching temperatures outside the mentioned ranges do not provide the proper fine distribution of carbides and nitrides so that the magnetic quality of the finished product is worsened.
  • First holding annealing temperature below the mentioned lower limits does not guarantee the proper dissolution of nitrides, as well as the formation of the proper quantity of gamma phase necessary to form martensite during quenching.
  • First holding annealing temperature, as well as time, above the maximum limit produces coarsening of sulphides by Oswald ripening, which makes their size distribution less suitable to control the secondary recrystallization, producing a worsening of the magnetic quality.
  • the annealing of the strip after hot rolling or after the first cold rolling is carried out in a double stage holding temperature, with a first holding at a temperature comprised in the range 1050° C.-1170° C., during 10-60 seconds, followed by a cooling down to a second holding temperature in the range 800° C.-950° C. during 40-240 seconds, followed by cooling down to a starting quenching temperature in the range 750° C.-940° C.
  • This double stage annealing allows a better control of the distribution of nitrides and hard phases and also an easier control of the starting quenching, less dependent on the possible fluctuations of the annealing line speed.
  • the presence of the lower temperature holding, during which the precipitation of the main part AlN is performed, guarantees a better control of the size distribution of AlN.
  • the strip is cooled down to 750-940° C. and further quenched at a temperature below 300° C.
  • This quenching of the strip is necessary to generate fine carbides and hard phases useful to increase the work hardening during cold rolling.
  • the quenching time is less than 30 s.
  • Next step is cold rolling of the annealed strip, which is necessary, besides the thickness reduction of the strip, to cold harden the strip in order to provide a suitable microstructure and texture after recrystallization.
  • the reduction ratio of the last cold rolling, down to final thickness must be between 80 and 95%. Below, the minimum value of 80%, the obtained magnetic properties after secondary recrystallization are poor. Above the maximum value of 95%, unstable secondary recrystallization is obtained and too fine grains appear in the finished product with consequently poor magnetic quality. The latter is probably due to the very fine grain size produced in the decarburized strip which corresponds to a very high driving force to growth during following secondary recrystallization annealing. The produced distribution of fine second phases is not able to control such high driving force to growth, producing an unstable secondary recrystallization.
  • this “cold-rolling” describes a step performed on a cold-rolling mill.
  • the cold rolling operation is performed in three passes or more and the strip is held at a temperature comprised between 170 and 300° C., in at least one interpass step, after the first cold rolling pass.
  • the function of this holding within the proposed temperature interval is to favour the migration of carbon in solid solution onto the dislocations generated by the rolling process, thereby favouring the generation of new dislocations. This is reflected on the magnetic quality of the final product, showing a more homogeneous and better-oriented grain. Holding temperatures lower than the minimum value does not allow the phenomenon of carbon migration onto the dislocations to occur in a sufficiently effective manner. Temperatures higher than the maximum limit yield no significant improvements and entail phenomena of rapid degradation of the lubricant utilised, making it difficult to industrialise the method.
  • a primary recrystallization is carried out simultaneously with a decarburization so as to reach a carbon amount below 0.0030% and preferably below 0.0025%.
  • this recrystallization is performed at a temperature comprised between 780° C. and 900° C. during 60 to 300 sec, in an atmosphere consisting of N 2 , H 2 and H 2 O, the ratio between partial pressure of H 2 O and partial pressure of H 2 being between 0.40 and 0.70.
  • Temperatures and time lower than the minimum limits cause a non-optimal recrystallization of the sheet that worsens the magnetic characteristics, whereas temperatures higher than the maximum limits, as well as ratios between water and hydrogen partial pressure higher than the maximum value indicated, cause an excessive oxidation of the sheet surface, worsening the magnetic characteristics, as well as the surface quality of the final product. Ratios between water and hydrogen partial pressure below the minimum value indicated produce insufficient decarburization of the strip.
  • this recrystallization is carried out with a heating rate of at least 150° C./sec in the temperature range comprised between 200° C. and 700° C.
  • an annealing separator usually made of MgO is applied to the strip surface and a secondary recrystallization is performed.
  • the secondary recrystallization annealing is carried out by first heating the strip to a temperature between 1000° C. and 1250° C. with a heating rate between 5° C./h and 40° C./h and in an atmosphere consisting of N 2 and H 2 , and then holding the strip during 5 to 30 h at this temperature in an atmosphere consisting of H 2 .
  • Heating rates higher than the maximum indicated cause a too rapid evolution of the distribution of second phases formed during the hot-rolling, required for controlling the secondary recrystallization, so that the latter is not adequately controlled and the result is a worsening of the magnetic characteristics of the final product. Heating rates lower than the minimum one proposed does not produce special advantage and unnecessarily lengthen of the annealing times.
  • the produced slabs were reheated at 1420° C. during 30 min, roughed and finished using a reversible hot rolling mill.
  • the finishing hot rolling was performed in 3 passes.
  • the strip was held, at least once, for a time variable along the strip length, but in any case greater than 20 s, at a temperature in the range 1000-1200° C. depending on coil and on the position along the coil length.
  • the coils were cooled to a temperature less than 600° C. in less than 10 seconds and coiled. From some of the hot rolled strips, samples were taken changing the sampling position along the coil length. The Al s analysis was repeated on the strip samples; for all the samples the measured value was in agreement with table 1 values within ⁇ 0.0010%.
  • Hot rolled sheets manufactured according to the invention display a five layer disposal as described above. Less than 0.0015% of nitrogen bound to aluminium under the form of AlN was measured in the hot rolled samples manufactured according to the invention.
  • the different samples were then transformed into finished product performing a double stage annealing of the hot rolled strip, with a first holding at 1100° C. for 15 sec, followed by a cooling down to 900° C. in 15 sec, holding at 900° C. for 60 sec, cooling down to 800° C. and final quenching to room temperature with water.
  • the annealed strips were cold rolled to 0.30 mm in five passes, at the intermediate thicknesses of 1.0 mm, 0.7 mm and 0.5 mm, the temperature of the strip has being maintained at 250° C. for 10 nm.
  • Decarburization was performed at 840° C. for 200 sec in an H 2 /N 2 /H 2 O atmosphere with a ratio between, partial pressure of H 2 O and partial pressure of H 2 of 0.55, reaching a carbon amount below 0.0025%.
  • the heating rate in the temperature range between 200 and 700° C. was 40° C./sec.
  • the strips were coated with an annealing separator constituted mainly of MgO and then subjected to a secondary recrystallization annealing with the following cycle:
  • a steel of chemical composition shown in table 4 in weight percentages was industrially cast by continuous casting, the balance being iron and unavoidable impurities.
  • the strip was held at least once for a time variable along the strip length, but in any case greater than 20 sec at a temperature in the range 1000-1200° C. depending on coil and on the position along the coil length.
  • the minimum value to be respected by Tav is 1100° C., meaning that these samples are according to the invention.
  • the strip was held at least once for a time variable along the strip length, but in any case greater than 20 sec at a temperature in the range 1000-1200° C. depending on coil and on the position along the coil length.
  • the minimum value to be respected by Tav is 1093° C., meaning that this sample is according to the invention.
  • the samples were annealed with a single step annealing at 1105° C. during 90 sec, a cooling down to 920° C. and a water quenching down to a temperature of 40° C.
  • Heats 5 and 6 have an Als amount around 250 ppm
  • heats 7 and 8 have an Als amount around 190 ppm.
  • the produced slabs were reheated at 1415° C. during 30 min, roughed and finished using a Steckel reversible hot rolling mill.
  • the finishing hot rolling was performed in 3 passes.
  • the strip was held at least once for a time variable along the strip length, but in any case greater than 20 sec at a temperature in the range 1000-1200° C. depending on coil and on the position along the coil length.
  • the coils were cooled to a temperature less than 600° C. in less than 10 seconds and coiled. From some of the hot rolled strips, samples were taken changing the sampling position along the coil length.
  • the end hot rolling temperature T end is reported in the table.
  • the difference between EQ1 and T end i.e. (EQ1 ⁇ T end ) is also reported in the table.
  • the different samples were then transformed into finished product performing a double stage annealing of the hot rolled strip, with a first holding at 1100° C. for 15 sec, followed by a cooling down to 900° C. in 15 sec, holding at 900° C. for 60 sec, cooling down to 800° C. and final quenching to room temperature with water.
  • the annealed strips were cold rolled to 0.30 mm in five passes, at the intermediate thicknesses of 1.0 mm, 0.7 mm and 0.5 mm, the temperature of the strip has been maintained at 250° C. for 10 min.
  • Decarburization was performed at 840° C. for 200 sec in an H 2 /N 2 /H 2 O atmosphere with a ratio between partial pressure of H 2 O and partial pressure of H 2 of 0.55, reaching a carbon amount below 0.0025%.
  • the heating rate in the temperature range between 200 and 700° C. was 40° C./sec.
  • the strips were coated with an annealing separator constituted mainly of MgO and then subjected to a secondary recrystallization annealing with the following cycle:

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US20170271061A1 (en) * 2014-11-27 2017-09-21 Posco Grain-oriented electrical steel sheet and manufacturing method therefor
WO2019131853A1 (fr) * 2017-12-28 2019-07-04 Ｊｆｅスチール株式会社 Tôle d'acier électrique à grains orientés et à faible perte de fer et son procédé de fabrication
JP2022514794A (ja) * 2018-12-19 2022-02-15 ポスコ 方向性電磁鋼板およびその製造方法
JPWO2022186299A1 (fr) * 2021-03-04 2022-09-09
WO2022186300A1 (fr) * 2021-03-04 2022-09-09 Ｊｆｅスチール株式会社 Procédé de production d'une feuille d'acier électrique à grains orientés
WO2024043294A1 (fr) * 2022-08-24 2024-02-29 Ｊｆｅスチール株式会社 Procédé de production d'une tôle d'acier électromagnétique à grains orientés, et tôle laminée à chaud pour tôle d'acier électromagnétique à grains orientés
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US20170271061A1 (en) * 2014-11-27 2017-09-21 Posco Grain-oriented electrical steel sheet and manufacturing method therefor
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WO2022186299A1 (fr) * 2021-03-04 2022-09-09 Ｊｆｅスチール株式会社 Procédé de fabrication d'une tôle d'acier électromagnétique directionnelle et tôle d'acier laminée à chaud pour tôle d'acier électromagnétique directionnelle
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US12626844B2 (en) 2021-03-04 2026-05-12 Jfe Steel Corporation Method of manufacturing grain-oriented electrical steel sheet
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JPWO2024043294A1 (fr) * 2022-08-24 2024-02-29
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EP2547799A2 (fr) 2013-01-23
WO2011114227A3 (fr) 2012-11-22

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