EP0120153B1 - Verfahren zum Elektromagnetischem Rühren von geschmolzenem Stahl beim Stranggiessen - Google Patents

Verfahren zum Elektromagnetischem Rühren von geschmolzenem Stahl beim Stranggiessen Download PDF

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Publication number
EP0120153B1
EP0120153B1 EP83301618A EP83301618A EP0120153B1 EP 0120153 B1 EP0120153 B1 EP 0120153B1 EP 83301618 A EP83301618 A EP 83301618A EP 83301618 A EP83301618 A EP 83301618A EP 0120153 B1 EP0120153 B1 EP 0120153B1
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EP
European Patent Office
Prior art keywords
thickness
molten steel
bloom
unsolidified
cast slab
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Expired
Application number
EP83301618A
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English (en)
French (fr)
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EP0120153A1 (de
Inventor
Minoru Kitamura
Kenta Yoshii
Shozo Kawasaki
Katsuyoshi Matsuo
Mitsuo Tomonaga
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Kobe Steel Ltd
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Kobe Steel Ltd
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Application filed by Kobe Steel Ltd filed Critical Kobe Steel Ltd
Priority to AT83301618T priority Critical patent/ATE25015T1/de
Priority to EP83301618A priority patent/EP0120153B1/de
Priority to DE8383301618T priority patent/DE3369258D1/de
Publication of EP0120153A1 publication Critical patent/EP0120153A1/de
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/12—Accessories for subsequent treating or working cast stock in situ
    • B22D11/122—Accessories for subsequent treating or working cast stock in situ using magnetic fields

Definitions

  • the present invention relates to a method of electromagnetic stirring intended to provide a satisfactory solidified structure in continuous casting, according to the preamble of claim 1.
  • molten steel contains various alloying elements and impurity elements, and the solidification of molten steel is sometimes attended by appreciable segregation of segregative elements, such as C, P, and S, into the final solidifying portion of the steel ingot or cast slab or bloom.
  • Products made of a material having such a segregated portion are inferior in their characteristics, due to non-uniformity of their mechanical properties, and experience case trouble during welding.
  • noticeable segregation develops in a direction at right angles to the cast slab or bloom drawing direction.
  • past examination of various operating conditions has not been successful in improving the mechanical properties of the cast slab or bloom.
  • DE-A-2 424 610 forming the first part of claim 1, discloses a method of electromagnetically stirring molten steel in continuous casting, wherein in producing a cast slab or bloom an electromagnetic stirring force is applied to the unsolidified molten steel in the cast slab or bloom being drawn by an electromagnetic stirrer installed, for example, at a drawing position where the unsolidified thickness at viewed in the direction of the thickness is 30% of the thickness of the cast slab or bloom.
  • the present invention is intended to establish electromagnetic stirring conditions for enhancing the effect of breaking columnar crystals to reduce negative segregation and avoid the formation of white bands.
  • a method of the type disclosed in DE-A-2 424 610 characterised in that said electromagnetic stirrer or a plurality of such stirrers is or are installed within a region defined between drawing positions where said unsolidified thickness in 45% and 15%, respectively, of the thickness as viewed in the direction of the thickness of the cast slab or bloom and in that stirring in the casting direction is applied to the unsolidified molten steel in such a manner that the product of the magnetic flux density at the interface between the unsolidified and solidified portions and the stirring time, expressed as the ratio of the length in m.
  • the region of the cast slab to which electromagnetic stirring is applied to the casting speed in m/min is 0.16 Tesla-min (1,600 gauss-min) or more per m 3 of the total volume of unsolidified molten steel present in a region extending to the drawing side of the electromagnetic stirrer or stirrers.
  • the electromagnetic stirrer is installed between drawing positions where the unsolidified thickness is 35% and 20%, respectively, of the thickness as viewed in the direction of the thickness of the cast slab or bloom.
  • the interface between the unsolidified and solidified portions is hereinafter referred to as the solidification interface.
  • the solid-liquid coexistence condition is established in several regions along the cast slab or bloom drawing direction and these regions flow in block but the flow of these regions takes place successively with some time lag, with the lower side flowing first. Therefore, between adjacent regions, the dendrites separate from each other in accordance with the flow time lag, so that cavities with some periodicity are formed.
  • Such a cavity has a temperature gradient in a direction at right angles to the cast slab or bloom drawing direction and a flow of molten steel is formed between the dendrites, so that the aforesaid suction effect becomes greater toward the centre of the cast slab or bloom. Under these influences, the aforesaid cavities assume a V-shape inclined toward the centre axis, and it seems that the surrounding segregated liquid present between the dendrites flows into the V-shaped cavities, resulting in V segregation.
  • the present invention aims to reduce segregation in the central portion of the cast slab or bloom by adjusting the electromagnetic stirring force so as to change the aforesaid solidification mechanism.
  • the region where V segregation takes place is, after all, a region with little temperature gradient.
  • the factors which determine the size of this region are supposed to include the molten steel composition (particularly the carbon concentration) and super-heating of molten steel, but a statistical examination of regions where V segregation is formed has revealed that even the maximum value does not exceed 45% of the thickness as viewed in the direction of the thickness of the cast slab or bloom.
  • Fig. 1 is a graph showing the relationship between the carbon concentration in molten steel and percentage of equiaxed crystal zone on the upper curve side.
  • the percentage of equiaxed crystal zone on the upper curve side is low in the low and high carbon ranges but very high in the medium carbon range. It is thought that this is because the solidification of single phases 5 and y, in the low and high carbon ranges results in the formation of fewer equiaxed crystals, whereas in the medium carbon range the two-phase solidification, liquid + 5 phase - y phase, takes place, so that a long time is expended in the course of this transformation, resulting in the survival of more of the nuclei for equiaxed crystals.
  • the percentage of equiaxed crystal zone corresponds to the distance from the centre axis of the cast slab or bloom to the portion where V segregation takes place, expressed in terms of its ratio to the thickness as viewed in the thickness of the cast slab or bloom, and the results of continuous casting under the conditions shown in the figure (v is the cast slab or bloom drawing speed and At is the superheating of molten steel) have led the present inventors to the conclusion that the region where V segregation takes place extends from the centre axis up to 45%, preferably 35% of the thickness as viewed in the direction of the thickness as viewed in the direction of the thickness of the cast slab or bloom.
  • the present inventors have thought that to eliminate said V segregation by electromagnetic stirring, it is necessary to stir said region and reached the conclusion that it is suitable to locate an electromagnetic stirrer at a position nearer to the drawing side than a position were the unsolidifed thickness is 45%, preferably 35%, of the thickness as viewed in the direction of the thickness of the cast slab or bloom.
  • the upper limit of the proportion of the unsolidified thickness to the thickness of the cast slab or bloom is 45%, preferably 35%
  • the lower limit must be 15%, perferably 20%. The reason for this is that the amount of unsolidified molten steel remaining in the cast slab or bloom in the region where the proportion is below said lower limit is relatively small and its temperature has dropped so that the viscosity of the molten steel itself is high, which means that stirring is difficult and that the improvement effect on the quality of the cast slab or bloom is lessened.
  • Fig. 2 is a schematic view for explaining a V segregation reducing mechanism according to the present invention, wherein A refers to an instance applying no electromagnetic stirring, B refers to an instance using a conventional electromagnetic stirring technique, and C refers to the present invention; in each case, the cast slab or bloom moves vertically downward.
  • An examination of the macro-structure in the case of A has revealed that columnar crystals extend as far as the centre of the cast slab or bloom thickness, forming centre porosities at their junction, and in the case of B equiaxed crystals are multiplied by the breakage of columnar crystals, and the solidified structure in the centre part is reduced greatly but not to the extent of eliminating V segregation and micro-porosities.
  • the V-shaped segregation angle is changed to an extremely sharp angle; in other words, the end edges are successfully turned parallel with the surface of the cast slab or bloom or orientated in the cast slab or bloom drawing direction.
  • the electromagnetic stirring according to the invention causes the flow of the V segregation forming region in the casting direction to diffuse rather than gathering toward the centre, and more particularly it causes said flow due to the contracting force exerted in the last stage of solidification to be artificially diffused in a direction perpendicular to the cast slab or bloom drawing direction by forming a temperature gradient in said perpendicular direction.
  • the segregated liquid formed in the last stage of solidification is circumferentially diffused and solidified without being allowed to produce V-shaped segregation.
  • such an artificial flow could be produced in the direction opposite to the cast slab or bloom drawing direction, but this is economically disadvantageous, for example as regards the power source capacity.
  • it should be produced in the cast slab or bloom drawing direction.
  • Figs. 3-7 are schematic views showing how the present invention is embodied.
  • One or more electromagnetic stirrers 2 are installed at a position nearer to the drawing side than is the position which satisfies said conditions.
  • the product (B T) of the magnetic flux density (B gauss) at the solidification interface and the stirring time (T.min.) should be 0.16 Tesla-min (1,600 gauss-min.) or more per m 3 of the volume of the unsolidified molten steel.
  • Table 1 shows conditions where, in the continuous casting of cast bloom having a cross-section of 380 mm x 550 mm, an electromagnetic stirrer having a stirring effective length / of 1,300 mm is installed at a position 13 m (Test No. 1-8) or 17 m (Test No. 9-12) apart from the meniscus [which position satisfies the aforesaid installation condition (45% or less)] and the output is changed.
  • the mm notation in the solidified portion indicates the thickness.
  • the solidification percentage when the casting speed is 0.45 m/min. is calculated as follows.
  • the unsolidified volume from the stirrer is calculated as follows, on the assumption that this portion is pyramidal.
  • the gauss values used in the calculations are those shown in Table 2.
  • Table 1 the values of BT are in gauss-min. (i.e. Tesla-min x 10 4 ).
  • the magnetic flux density B at the solidification interface is given by the following equation.
  • Fig. 8 shows the values of Table 1 plotted in a graph, the vertical axis indicating the stirring force (B.T) and the horizontal axis the unsolidified molten steel volume (mm 3 ).
  • the mark 0 refers to cases where the central V segregation was reduced and the mark • refers to cases where there was no such effect.
  • the longitudinal/horizontal axis ratio iunit:gauss-min/m 3 ) for each plot is also shown in the graph.
  • We have concluded from Fig. 8 that the V segregation reducing effect is remarkable if the value of B. T/m 3 is 1,600 ot more.
  • Fig. 9 shows an example in which a cast slab or bloom with a superheating of molten steel AT of 15-40°C and a cross-section of 380 x 550 (mm) was continuously cast at a casting speed of 0.6 m/min.
  • the mark • refers to a comparative example using no electromagnetic stirring and the mark ⁇ refers to an example of the present invention wherein an electromagnetic stirrer is installed at a position where the unsolidified thickness is 40%.
  • the comparative example exhibited extremely noticeable C segregation
  • the example according to the invention yielded a cast slab or bloom having little C segregation. Further, is did not develop negative segregation, either, nor did it form a white band.
  • the use of the present invention makes it possible to prevent formation not only of V-shaped segregation in the central portioh of the cast slab or bloom but also of negative segregation, thereby improving the mechanical properties of continuously cast products.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Continuous Casting (AREA)
  • Treatment Of Steel In Its Molten State (AREA)

Claims (2)

1. Verfahren zum elektromagnetischen Rühren von geschmolzenem Stahl beim Strangguß, wobei bei der Herstellung eines gegossenen Stranges oder Blockes auf den nicht erstarrten, geschmolzenen Stahl, der in dem gegossenen Strang oder Block, der gezogen wird, vorhanden ist, durch einen elektromagnetischen Rührer, der beispielsweise bei einer Ziehlage angebracht ist, wo die in der Richtung der Dicke betrachtete Dikke des nicht erstarrten Teils 30% der Dicke des gegossenen Stanges oder Blockes beträgt, eine elektromagnetische Rührkraft ausgeübt wird, dadurch gekennzeichnet, daß der erwähnte elektromagnetische Rührer oder mehr als ein solcher Rührer innerhalb eines Bereichs, der zwischen Ziehlagen festgelegt ist, wo die erwähnte Dicke des nicht erstarrten Teils 45% bzw. 15% der in der Richtung der Dicke betrachteten Dicke des gegossenen Stranges oder Blockes beträgt, angebracht ist oder sind und daß der nicht erstarrte, geschmolzene Stahl in der Weise in der Gießrichtung gerührt wird, daß das Produkt der magnetischen Flußdichte an der Grenzfläche zwischen dem nicht erstarrten und dem erstarrten Teil und der Rührdauer, die als das Verhältnis der Länge (in m) desjenigen Bereichs des gegossenen Stranges, auf den das elektromagnetische Rühren angewandt wird, zu der Gießgeschwindigkeit (in m/min) ansgedrückt wird, 0,16 Tesla.min (1600 Gauss-min) oder mehr je m3 des Gesamtvolumens des nicht erstarrten, geschmolzenen Stahls beträgt, der in einem Bereich vorhanden ist, der sich bis zu der Ziehseite' des elektromagnetischen Rührers oder der elektromagnetischen Rührer erstreckt.
2. Verfahren nach Anspruch 1, bei dem sich der oder jeder elektromagnetische Rührer zwischen Ziehlagen befindet, wo die Dicke des nicht erstarrten Teils 35% bzw. 20% der in der Richtung der Dicke betrachteten Dicke des gegossenen Stranges oder Blockes beträgt.
EP83301618A 1983-03-23 1983-03-23 Verfahren zum Elektromagnetischem Rühren von geschmolzenem Stahl beim Stranggiessen Expired EP0120153B1 (de)

Priority Applications (3)

Application Number Priority Date Filing Date Title
AT83301618T ATE25015T1 (de) 1983-03-23 1983-03-23 Verfahren zum elektromagnetischem ruehren von geschmolzenem stahl beim stranggiessen.
EP83301618A EP0120153B1 (de) 1983-03-23 1983-03-23 Verfahren zum Elektromagnetischem Rühren von geschmolzenem Stahl beim Stranggiessen
DE8383301618T DE3369258D1 (en) 1983-03-23 1983-03-23 Method of electromagnetically stirring molten steel in continuous casting

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
EP83301618A EP0120153B1 (de) 1983-03-23 1983-03-23 Verfahren zum Elektromagnetischem Rühren von geschmolzenem Stahl beim Stranggiessen

Publications (2)

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EP0120153A1 EP0120153A1 (de) 1984-10-03
EP0120153B1 true EP0120153B1 (de) 1987-01-21

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EP83301618A Expired EP0120153B1 (de) 1983-03-23 1983-03-23 Verfahren zum Elektromagnetischem Rühren von geschmolzenem Stahl beim Stranggiessen

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DE (1) DE3369258D1 (de)

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* Cited by examiner, † Cited by third party
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CN111913452B (zh) * 2020-08-07 2021-12-17 武汉钢铁有限公司 基于生产节奏控制的中包自动烘烤系统及方法

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2236584B1 (de) * 1973-05-21 1976-05-28 Siderurgie Fse Inst Rech
FR2358222A1 (fr) * 1976-07-13 1978-02-10 Siderurgie Fse Inst Rech Nouveaux procede et dispositif pour le brassage electromagnetique de produits metalliques coules en continu
IT1168118B (it) * 1980-04-02 1987-05-20 Kobe Steel Ltd Processo per la colata in continuo di acciaio

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DE3369258D1 (en) 1987-02-26
EP0120153A1 (de) 1984-10-03
ATE25015T1 (de) 1987-02-15

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