US3908744A

US3908744A - Method of continuously casting wide slabs, in particular slabs wider than 1000 mm

Info

Publication number: US3908744A
Application number: US387371A
Authority: US
Inventors: Thorwald Fastner
Original assignee: Voestalpine AG
Current assignee: Voestalpine AG
Priority date: 1972-08-10
Filing date: 1973-08-10
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30
Also published as: FR2327834B1; JPS537128B2; BR7306109D0; GB1438826A; DE2340291B2; AT323919B; BE803408A; JPS4953531A; CH562071A5; ES417694A1; DE2340291A1; IT990361B; CA1003618A; SE384331B; FR2327834A1

Abstract

A method of continuously casting wide slabs, in particular slabs wider than 1000 mm from steels tending to segregate, by using a vertical, water-cooled mold, into which the steel is allowed to flow and from which the strand, having a liquid core and a solidified skin layer, is withdrawn. The stream of molten steel penetrating into the liquid core is cast at a velocity in dependence upon the length of the liquid core.

Description

United States Patent Fastner Sept. 30, 1975 [54] METHOD OF CONTINUOUSLY CASTING 2,458,236 1/1949 Wolff 164/281 UX W E SLABS, IN PARTICULAR S S 3.465.811 9/1969 De Castelet 164/281 3.536.122 10/1970 Willim et a1. 164/281 X WIDER THAN 1000 MM 4/1974 Babel et a1 164/49 Inventor:

Assignee:

Filed:

Appl. N5;

Thorwald Fastner, Linz, Austria Vereinigte Osterrei chische Eisenund Stahlwerke Alpine Montan Aktiengesellschaft, Linz, Austria Aug. 10, 1973 Foreign Application Priority Data Aug. 10, 1972 Austria 6910/72 References Cited UNITED STATES PATENTS Jun ghans 164/281 Primary E.\'aminerR. Spencer Annear Attorney, Agent, or Firm-Brumbaugh, Graves, Donohue & Raymond 57 ABSTRACT A method of continuously casting wide slabsyin particular slabs wider than 1000 mm from steels tending to segregate, by using a vertical, water-cooled mold, into which the steel is allowed to flow and from which the strand, having a liquid core and a solidified skin layer, is withdrawn. The stream of molten steel penetrating into the liquid core is cast at a velocity in dependence upon the length of the liquid core.

7 Claims, 4 Drawing Figures III 4 U.S. Patent Sept. 30,1975 Sheet2of3 3,908,744

[cm/sec] US. Patent Sept. 30,1975 Sheet 3 of3 3,908,744

METHOD OF CONTINUOUSLY CASTING WIDE SLABS, IN PARTICULAR SLABS WIDER THAN 1000 MM The invention relates to a method of continuously casting wide slabs, in particular slabs wider than 1000 mm, in particular from steels tending to segregate by using a vertical, preferably straight, water-cooled mold, into which the steel is allowed to flow and from which the cast strand having a liquid core and a solidified skin layer is withdrawn.

Segregations in the core of cast strands are known to occur, because various elements, such as C, Mn and S, have a higher solubility in liquid steel than in solidified steel. These elements are therefore being enriched in the remaining melt during solidification. In zones where the steel solidifies last, as, e.g., in the core of continuously cast strands, therefore substantially higher contents of these elements can be found than in the surface zone.

Segre gations cause a deterioration in quality of rolled products made from cast strands; i.e., segregated sheets show lower elongationand notch-impact values. Their ductility is also deteriorated by the segregations. In

.welded constructions, inner cracks may. occur in the welding seams. These disadvantages are observed in particular in C- and Mn-containing steels, from which sheets or plates of great strength are made, which are primarily used in mechanical engineering and structural steel engineering, particularly in pipe-line construction.

Efforts have been made to avoid segregations or to reduce them to a minimum, respectively, and for this purpose, the observance of a steel casting temperature as low as possible, lying slightly above the liquidus, an increased cooling of the cast strand to be withdrawn and a withdrawal speed of the strand as low as possible have been proposed. In large-scale productions, it is, however, not possible to observe these conditions. It is often inevitable to cast steel at a higher casting temperature. Neither is it possible town a continuous casting plant at an extremely low strand withdrawal speed, as the withdrawal speed of the cast strand has to be coordinated with the melt sequence of the steel production plant. Finally, when cooling cast strands, one has to take care in the first place that cracks be avoided during solidification and it is therfore not possible to increase the cooling above a certain scale, as would be desirable for diminishing segregations.

It is an object of the invention to avoid the above mentioned disadvantages and difficulties and to create a continuous casting method which facilitates the production of continuously cast slabs of low segregation from steels which normally tend to segregate strongly. A particular aim of the invention consists in being able to produce steels without cracks on-a high-capacity continuous casting plant, without having to keep the casting temperature within narrow limits and without having to apply a too heavy secondary cooling.

In a method of the above mentioned kind, the invention consists in that the velocity of the casting stream penetrating into the liquid core in vertical direction is kept at a value in dependence on the length of the liquid core, which value is higher than a lower limit value lying within the range of 60 to l cm/sec, represented by curve C of FIG. 2 of the drawings, as subsequently explained. For carrying out the method of the invention, it is suitable to use refractory casting tubes known per se, immersing below the casting level in the continuous casting mold and being open at their lower end, in order to enable the freely emerging casting stream to penetrate as deep as possible into the liquid core of the strand. In the past, the opinion has been held that a deep penetration of the casting stream into the liquid core is detrimental and is to be avoided, as on account of the flow forming in the liquid core the thickness of the solidified skin layer might become irregular, thus causing the danger of crack formation (see, e.g., .RADEX-Rundschau 1971, Heft 5,, page 595/599). It has been believed that a deep penetration is also undesirable, because the non-metallic inclusions contained in the steel are conveyed deeply into the core and from there could. hardly get to the core surface again. For this reason, casting tubes were commonly used in which the liquid metal emerges through lateral or horizontal openings or through openings inclined downwardly or upwardly. The method of the invention may be used very advantageously in the production of cast slabs from a killed steel having the following composition:

balance substantially Fe This steel is commonly killed with Si and Al, the Sicontent lying approx. at 0.3 and the Al-content below 0.1 The P- and S-contents should be as low as possible; they lie at approx. 0.020 In some cases, for achieving particular technological properties, various alloy elements, such as Nb, V, Cr, Ni, Ti and Mo may be added at an amount of up to 2 In the following table, the composition (casting analysis) of various melts is given which have been produced by the method of the invention, wherein no quality diminishing influence in sheets or plates made from these cast strands has been found.

Advantageously, these steels are cast in continuous casting plants having a vertical, straight mold, followed by a vertical, straight guiding path, after which the strand can be gradually bent into the horizontal. When using plants having a curved mold, the strand skin at the lower side of the strand might be too strongly eroded by the vertically penetrating casting stream. As a consequence, cracks and steel-breakthroughs might occur. Principally, it is also possible to allow the steel to flow freely falling into the continuous casting mold. For avoiding the occurrence of non-metallic inclusions,

as a consequence of the effect of the atmosphere upon the casting stream, on the one hand, and for a better guiding of the casting stream, in order to achieve a greater depth penetration, on the other hand, as a rule casting tubes will be used.

It is characteristic of slabs produced by the method of this invention that the hardness determined in the segregated core zone exceeds the hardness of the nonsegregated surface zone by maximumly 3O It is to be observed that the method of this invention is not suited for any strand cross section, but can only be used for casting slabs having a width of at least 1000 mm. This width is necessary in order to achieve a marked compensation flow by the stream penetrating deeply into the liquid core, which would not be possible in narrow slabs or billets. It has been found that the decrease of segregation, achieved by the present invention, does not involve any disadvantages as to quality. In particular, no cracks on the slab surface'and no strand breakthroughs occurred. As to the content in non-metallic inclusions, no difference as against cast strands, produced by conventional methods, has been found. In the advantageously used type of steel, the deoxidation products are present substantially in the form of manganese-silicates, which are liquid during casting, coagulate rapidly and are precipitated to a large degree already in the pouring ladle or in the tundish. Only a small portion gets into the continuous casting mold and, on account of this, the kind and intensity of the flow in the liquid core have no considerable influence as to the inclusion content of the strand.

Further features and details of the invention are explained with reference to the accompanying drawings.

FIG. 1 is a vertical section of the upper part of a continuous casting plant which is particularly advantageous for carrying out the method of the invention.

FIG. 2 illustrates the relationship between the velocity of exit v of the casting stream from the casting tube, respectively the relationship between the velocity of penetration of the casting stream into the liquid core of the strand, and the core length L.

FIG. 3 is a diagram which illustrates the correlation between the increase in hardness as a consequence of the segregations in the strand core and the casting temperature.

FIG. 4 shows a segregated test piece, taken from the center of the strand, in which the increase in hardness in the segregation zone is determined in comparison with the increase in hardness in the non-segregated zone.

In FIG. 1, a tundish is denoted with 1, from which the liquid steel 2 is conveyed vertically in direction of the arrow through a refractory casting tube 3 into a straight, vertical water-cooled mold 4. The velocity of exit v of the liquid steel from the casting tube 3 is regulated by a liftable and lowerable refractory stopper 5. 6 and 7 denote horizontal planes through the casting level in the mold 4, 'and through the lower edge of the casting tube 3, respectively. The casting tube 3 immerses several cm below the casting level covered with casting powder, so that the steel cannot get into touch with the atmosphere. 8 denotes the liquid core of the cast strand having a solidified skin layer 9, the cast strand being continuously supported and guided by the

rollers

10, 11 and 12. The rollers form, together with the straight mold, a vertical, straight strand guiding of 2 to 3 m length, followed by a bending device formed by the rollers 11 and a circular arc-shaped strand guiding device formed by the rollers 12. At the end of the circular arc-shaped strand guiding, a straightening device (not illustrated) and a degree for drawing out the cast strand running out horizontally are provided. The lowest point of the liquid pool 13 may reach as far as the end of the circular arc-shaped strand guiding depending upon the casting velocity and the type of plant. The velocity v of the casting stream 14, freely leaving the casting tube 3 and penetrating into the liquid core 8, respectively measured within the plane 7 is adjusted in such a way that the depth effect becomes greater as the liquid core length L increases. Thereby, a turbulence and a flow are caused in the liquid core 8. Whereas in the strand center a strong downward movement of the steel occurs, the steel streams upward, in particular in the surface zone, i.e., at the narrow sides of the slab. This compensating flow facilitates a deep penetration of the casting stream into the liquid core. For slabs having a width of at least 1000 mm, a sufficiently great compensating flow is present. The upward flow is indicated schematically by the arrows 15. In the range of the strong flow, enrichments of the segregating elements, such as C, Mn and S, are avoided. This means that the liquid steel in the core 8 can be less enriched with these elements, the deeper the casting stream penetrates into the liquid core 8.

FIG. 2 is a diagram, in which, on the abscissa, the core length L in m is plotted and, on the ordinate, the velocity of exit v of the casting stream from the casting tube 3 in cm/sec. With 16 and 17 limiting lines are denoted, within which the invention is applied advantageously. According to the invention, the velocity of exit v is to lie within these limiting

lines

16, 17, above the lower limiting curve C, i.e., in the range B, indicated by hatched lines, whose upper limit is defined approx. by the curved line 18. Therefore with a core length L of 8 m, the velocity of exit v is to amount to at least 60 cm/sec. and should be gradually increased up to at least 1 l0 cm/sec, when the core length increases up to 15 m, as a consequence of a higher withdrawal speed of the strand. According to the illustration in FIG. 2, approximate values for the upper limit of the velocity v to be chosen lie at or cm/sec, respectively. If the velocity of exit v lies below field B in the range A, strong segregations occur in the core of the cast slabs and cause a deterioration in quality.

FIG. 3 illustrates a correlation between casting temperature and segregation. On the abscissa, the casting temperature in C measured in the tundish 1 is plotted and on the ordinate, the increase in hardness in the segregated zone of the cast slab is plotted in of the hardness of the non-segregated zone. Normally, the casting temperature in a type of steel preferred according to the invention lies within the limiting

lines

19, 20, i.e., between 15 10 and 1530C. When this steel is cast under normal conditions i.e., at a velocity v lying in field A of FIG. 2, an increase in hardness in the segregated part of the slab occurs, which lies within a field "A', limited by the lines 21, 22. When applying the method of this invention, i.e., at a velocity v lying within range B of FIG. 2, the increase in hardness lies within field B, limited by the

lines

23, 24. D denotes a limiting line running horizontally, which lies at 30 increase in hardness. It has been found that an increase in hardness of up to 30 does not cause any disadvantages as to the quality of sheets or plates, manufactured from such cast strands. Sheets or plates, manufactured from more strongly segregated continuously cast slabs show lower elongation'and notch-impact values and their ductility is lower. Moreover, at a hardness increase of more than 45 inner cracks in the seams of welded sheets or sheet samples, respectively, are occasionally observed. As can be seen from FIG. 3, the casting temperature strongly influences the segregation intensity, if one works with velocities of exit v of the stream lying in the field A. When observing velocities v, which, according to the invention, lie in the range of field B, the influence of the casting temperature upon the segregation is considerably lower: the

lines

23, 24 run considerably flatter than the lines 21, 22 and the field B lies below the limit line D. Thus, when applying the method of the invention, a cast strand may be produced, even if the range of temperature is relatively large, in which cast strand the segregation has no quality-diminishing influence upon the sheets manufactured from said strand. In a conventional mode of operation, however, the casting temperature would always have to lie below 1515C, so as surely not to exceed an increase in hardness of 30%. In practice, however, this is hardly possible.

In order to determine the increase in hardness, a test piece is cut out of the cast strand, as illustrated in FIG. 4. The test piece is cut from the range of the longitudinal axis 25 of the slab, i.e., at approx, half the length of the strand (the vertical axis through the strand center is denoted with 26). The test piece suitably has a width 27 of 200 mm and a thickness which corresponds to the strand thickness d. Its cross-sectional area 28 is ground and deep-etched with hydrochloric acid, whereby in the core zone 29 segregations 30 become visible. These segregations 30 illustrated slightly exaggerated appear dark and are clearly set off against the structure of the rest of the cross section. The stronger the core zone is enriched with the segregating elements, the darker the segregations 30 appear after the deep etching. In the segregated zones 30 in continuously cast steels which tend to segregate, C-contents in the cast strand are determined which lie up to I00 above the value of the casting analysis. Moreover, Mncontents, higher by up to 30 and S-contents, higher by up to 50 are detected there. The increased C- content in the segregated zone is also to be regarded as the reason for the occurrence of dark spots after the deep-etching. Segregated zones have a higher pearlite content than non-segregated ones, and a structure with a higher pearlite content behaves differently during t etching than a structure with a lower pearlite content.

By the increase of the C- and Mn-content in the core zone 29, the hardness of the steel is greater there than in the remaining casting material. Therefore the hardness increase in the segregated zone of these steels can be regarded as a good indicator for the intensity of the segregation. The hardness is tested through the segregated zones 30 along a'line 31 after abrading the surface 28 at the measuring points 32, which are arranged at distances of lO'mm each. The line 31 is led parallel in relation to line 29, so that the largest partpossible of itslongitudinal extension lies in segregated zones 30. The line 31 may, but need not coincide with the line 29. For testing the hardness, for example, the" Vickers hardness test (HV 5) may be applied, wherein the mean value of all measuring points 32 is compared with EXAMPLE 1 In a continuous casting plant for slabs a steel of the following chemical composition was cast: (casting analysis) 0.20 C 0.025 S 0.30, Si 0.030 Al 1,45 Mn 0.05 Nb 0.020 P balance Fe The size of the mold was 1600 X 225 mm, and the withdrawal speed of the strand was 0.6 m/min. From this, a casting output of 1.55 t/min. results. The liquid core length L may be calculated from the following relationships: I

c-VT...(mm)

and

L a r respectively d denotes the strand thickness in mm, c is the solidification factor, inserted with 27 mm/min, t denotes the solidification time in min, and a stands for the withdrawal speed of the strand in mm/min.

In the present example, the liquid core length L was calculated to measure 10.4 m. The temperature of the steel (casting temperature) measured in the tundish l was 1530C. According to curve C in FIG. 2, a minimum velocity of exit of the steel from the casting tube 3 of 68 cm/sec is necessary in order to avoid disturbing segregations. Therefore a casting tube was used, being open at its lower end and having a lumen of mm. From the relationship casting output (in em /sec) equals the product of cross section of casting tube. (in cm and velocity of exit v (in cm/sec), the velocity of exit v was calculated to be 127 cm/sec in I this example (point 34 in FIG. 2). At half the length of the strand, a test piece was taken out, was deep-etched and the segregation was judged. It stands out only to a minor degree against the segregation-free cross section of the slab, which means that the segregation intensity is low. The value of 210 kplmm determined in the Vickers hardness test, according to the previously described method, was higher by only 25 than the comparative value at the measuring points 33 (point 35 in FIG. 3). The increase in hardness therefore lies within the range B or below the limiting line D, respectively, which means that no deterioration in quality due to segregations could be found in sheets or plates manufactured from such a cast strand. The deep-etched test pieces and the sulphur (Baumann) prints, made for determining the sulphur distribution, moreover showed lines running parallel to the strand surface, which, while they are of no importance for the quality of the strand or the sheets, respectively, indicate that a strong flow was present in the liquid core 8 of the strand.

The results of this melt were compared with another melt, which had been cast at otherwise completely equal conditions by applying the conventional casting technology, i.e., by using a casting tube with two lateral exit openings inclined downwardly at an angle of 45, each of them having a diameter of 42 mm. Whereas thereby an exit velocity of the stream of I30 cm/sec was achieved, a deep penetration of the casting stream into the liquid core 8 was avoided, which is characteristic of the operation with these tubes. A test piece taken from the center of the strand showed a clearly stronger segregation, as had been expected. The increase in hardness compared with the segregation-free material amounted to 45 (point 36 in FIG. 3). The sheets manufactured from this steel were of inferior quality and clearly showed worse technological properties than the sheets rolled from the strand which had been cast according to the invention.

EXAMPLE 2 In the same continuous casting plant another melt of the following chemical composition was cast: (casting analysis) balance Fe The mold had been adjusted to a strand cross section of 1000 X 250 mm, the withdrawal speed of the strand was 0.70 m/min. Therefrom a casting output of 1.26 t/min results. The length of the liquid core was m due to the higher casting output. At this core length, according to the invention and according to curve C of FIG. 2, a lower limit value for the exit velocity of the casting strand amounting to 110 cm/sec has to be exceeded, in order to get a cast slab with low segregation. Therefore a casting tube open at its lower end and having a vertical bore ofa diameter of 55 mm was used, so that the velocity of exit v was 120 cm/sec (point 37 of FIG. 2). The casting temperature measured in the tundish 1 was 1525C. The Vickers hardness of the test piece taken from the center of the strand and examined in the same way amounted to 170 kplmm a value which exceeded the hardness value determined in the segregation-free zone by only (point 38 in FIG. 3).

For comparative purposes, a melt was cast with a casting tube also open at its lower end, the vertical bore of which had however a diameter of 70 mm, whereby the exit velocity v was only 76 cm/sec (point 39 of FIG. 2). As a consequence of this low exit velocity lying in the field A, a considerably stronger segregation was determined in the core zone of this cast slab. Accordingly an increase in hardness by 35 (point 40 in FIG. 3) was determined and the sheets manufactured from this slab had lower elongationand notch-impact valves.

I claim:

1. In a method of continuously casting wide slabs,

particularly slabs having a width of more than 1000 mm, from steels, particularly from steels tending to segregate, in which a stream of molten steel is cast into a vertical, water-cooled mold to form a cast strand and said cast strand having a liquid core of a certain length and a solidified skin layer is withdrawn from the mold, the improvement comprising that the stream of molten steel vertically penetrates into the liquid core and is, in dependence upon the length (L) of the liquid core, cast at a velocity (v) kept at a value higher than a lower limit value that increases with increasing length of the liquid core and that lies within 60 to 110 cm/sec for lengths (L) of from 8 to 15 m, the range of lower limit values being represented by curve C in FIG. 2.

2. A method according to claim 1, wherein a straight mold is used.

3. A slab produced according to the method of claim 1, comprising a segregated core zone and a nonsegregated skin layer zone, the segregated core zone having a hardness exceeding the hardness determined in the non-segregated skin layer zone by maximumly 30 4. In a method of continuously casting wide slabs, particularly slabs having a width of more than 1000 mm,-from steels, particularly from steels tending to segregate, in which a stream of molten steel is cast into a vertical, water-cooled mold to form a cast strand and said cast strand having a liquid core of a certain length and a solidified skin layer is withdrawn from the mold, the improvement comprising that the stream of molten steel vertically penetrates into the liquid core and is, in dependence upon the length (L) of the liquid core, cast as a velocity (v) kept at a value higher than a lower limit value that increases with increasing length of the liquid core and that lies within 60 to 110 cm/sec for lengths (L) of from 8 to 15 m, the range of lower limit values being represented by curve C in FIG. 2 and a killed steel of the following composition being used:

C 0.15 to 0.22 Si 0.20 to 0.40 Mn 1.0 to L5 P max. 0.025 S max. 0.030 Al max. 0.1

5. A slab produced according to the method of claim 4, comprising a segregated core zone and a nonsegregated skin layer zone, the segregated core zone having a hardness exceeding the hardness determined in the non-segregated skin layer zone by maximumly 30 6. A method according to claim 1, wherein the velocoty (v) of the stream of molten steel is kept at a value lower than an upper limit value that increases with increasing length (L) of the liquid core and that lies within to cm/sec for lengths (L) of from 8 to 15m, the range of upper limit values being repesented by curve 18 in FIG. 2.

7. A method according to claim 4, wherein the velocity (v) of the stream of molten steel is kept at a value lower than an upper limit value that increases with increasing length (L) of the liquid core and that lies within 120 to 180 cm/sec for lengths (L) of from 8 to 15 m, the range of upper limit values being represented by curve 18 in FIG. 2.

Claims

1. In a method of continuously casting wide slabs, particularly slabs having a width of more than 1000 mm, from steels, particularly from steels tending to segregate, in which a stream of molten steel is cast into a vertical, water-cooled mold to form a cast strand and said cast strand having a liquid core of a certain length and a solidified skin layer is withdrawn from the mold, the improvement comprising that the stream of molten steel vertically penetrates into the liquid core and is, in dependence upon the length (L) of the liquid core, cast at a velocity (v) kept at a value higher than a lower limit value that increases with increasing length of the liquid core and that lies within 60 to 110 cm/sec for lengths (L) of from 8 to 15 m, the range of lower limit values being represented by curve C in FIG. 2.

2. A method according to claim 1, wherein a straight mold is used.

3. A slab produced according to the method of claim 1, comprising a segregated core zone and a non-segregated skin layer zone, the segregated core zone having a hardness exceeding the hardness determined in the non-segregated skin layer zone by maximumly 30 %.

4. In a method of continuously casting wide slabs, particularly slabs having a width of more than 1000 mm, from steels, particularly from steels tending to segregate, in which a stream of molten steel is cast into a vertical, water-cooled mold to form a cast strand and said cast strand having a liquid core of a certain length and a solidified skin layer is withdrawn from the mold, the improvement comprising that the stream of molten steel vertically penetrates into the liquid core and is, in dependence upon the length (L) of the liquid core, cast as a velocity (v) kept at a value higher than a lower limit value that increases with increasing length of the liquid core and that lies within 60 to 110 cm/sec for lengths (L) of from 8 to 15 m, the range of lower limit values being represented by curve C in FIG. 2 and a killed steel of the following composition being used:

5. A slab produced according to the method of claim 4, comprising a segregated core zone and a non-segregated skin layer zone, the segregated core zone having a hardness exceeding the hardness determined in the non-segregated skin layer zone by maximumly 30 %.

6. A method according to claim 1, wherein the velocoty (v) of the stream of molten steel is kept at a value lower than an upper limit value that increases with increasing length (L) of the liquid core and that lies within 120 to 180 cm/sec for lengths (L) of from 8 to 15m, the range of upper limit values being rEpesented by curve 18 in FIG. 2.