US3656536A

US3656536A - Method for cooling the cast strand in curved-guide continuous casting plants

Info

Publication number: US3656536A
Application number: US880101A
Authority: US
Inventors: Piero Colombo
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-11-28
Filing date: 1969-11-26
Publication date: 1972-04-18
Anticipated expiration: 1989-04-18
Also published as: DE1957758A1; JPS4910896B1; DE1957758B2

Abstract

In the continuous casting of molten steel into a mold having a vertical casting space, from which the cast metal is passed as a molten core surrounded by a peripheral crust on into a curved guide path section, the improvement comprising cooling, while passing through the straight as well as the curved guide path, that side of the metal which is to form the side of greater radius of curvature more intensively than the opposing side to form a crust of greater thickness, while maintaining an intensity of cooling in the region of the straight guide path section approximately 3 to 7 times greater than the intensity of cooling in the curved guide path section, thereby strengthening the cast metal, and thereafter, after deflection of the cast metal into the curved guide path and while the core is still molten, reversing the differential cooling so as to apply a cooling of greater intensity on the side of the lesser radius of curvature, whereby the solidified crusts along all sides of the cast metal assume substantially the same thickness and the cast metal assumes a symmetrical shape at a point prior to complete solidification.

Description

15] 3,656,536 [45] Apr. 18, 1972 FOREIGN PATENTS OR APPLICATIONS ,066 5/1954 Germany.................................164/89 ABSTRACT In the continuous casting of molten steel into a mold having a vertical casting space, from which the cast metal is passed as a Primary Exarhiner--R. Spencer Annear Attorney-Imirie and Smiley molten core surrounded by a peripheral crust on into a curved guide path section, the improvement comprising cooling, while passing through the straight as well as the curved guide path, that side of the metal which is to form the side of greater United States Patent Colombo STRAND IN CURVED-GUIDE CONTINUOUS CASTING PLANTS [72] Inventor: Piero Colombo, 27 Via Leopardi, Udine,

Italy [22] Filed: Nov. 26, 1969 [21] Appl. No.: 880,101

[30] Foreign Application Priority Data Nov. 28, 1968 Italy.................;.................7495 A/68 [54] METHOD FOR COOLING THE CAST "u u "n 4 S m "H" 5 mm T n u. g

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- sum 26F 2 J06 581 E S E 408 ,y v4 5 n H65 PRIOR ART INVENToR PiERo COLOMBO BY 1/ MJIAW BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for the continuous casting of metals, particularly of steel, in which molten metal is continuously cast in a casting mould having a straight vertical casting space, and passes as a molten core surrounded by a marginal crust of solidified metal into and through a straight vertical guide path section and a succeeding curved guide path section.

The curved guide path section may be of arcuate form struck about a center of curvature, or it may be of progressively increasing curvature, for example parabolic or hyperbolic.

2. Description of the Prior Art In the known processes the cast metal, as it passes through the straight vertical path section is cooled equally along sides thereof which are to form the sides of greater and lesser curvature as the cast metal passes through the curved guide path section so that the solidified marginal crusts of the cast section possess practically the same thickness along the outer as well as inner curvatures of the cast section.

In all curved continuous casting plants of the said known kind, the marginal crust along the outer curvature of the cast section is stressed in tension in the region of the curved path section, and the marginal crust along the inner curvature is stressed in compression. Whereas the compressive stress on the marginal crust along the inner curvature does not raise any difficulties as a rule, the tensile stress on the marginal crust along the outer curvature can lead to the forming of cracks in the transition area (dendrites area) between the molten core of the cast section and the solidified marginal crust, especially in the case of some grades of steel, and even in the solidified marginal crust, if the elongations caused by the tensile strains are too great for the. strength of the material of the cast section.

It is the object of the invention to eliminate these shortcomings and to develop a process by means of which the deleterious effect of bending of the cast section described is eliminated in practice, that is the forming of cracks in the part of the cast section along the outer, that is the greater curvature, and stresses in tension are prevented or at least reduced to a degree which is insignificant in respect of the quality of the sections cast.

SUMMARY According to the invention, the problem is resolved by the improvement which consists of, while the cast metal is passing through the straight vertical guide path section, cooling that side of the cast metal which is to form the side of greater curvature while passing through the curved guide path section more intensively than the side which is to form the side of lesser curvature thereby to provide the side of greater curvature with a marginal crust of greater thickness than that of the marginal crust of the side of lesser curvature.

As a result of the greater thickness of the marginal crust along the side of greater curvature, the zero-line of the distribution of tensile and compressive stress is displaced in the cross-section of the cast section towards the side of greater curvature of the cast section during bending of the cast section. The maximum specific elongations of the fiber of the material in the part of the cross-section of the cast section corresponding to the greater curvature and proportional to the distance from this zero-line, are reduced correlatively and cracks which may be caused by these elongations in the solidified marginal crust along the side of greater curvature in the area of the dendrites between the solidified marginal crust and the molten core of the section are prevented.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a vertical section through a curved continuous casting plant for steel,

FIG. 6 is a cross-section through a cast section cooled by the process according to the invention, in the region of the line II-II of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The'curved continuous casting plant for steel illustrated diagrammatically in FIG. 1 comprises a water-cooled continuous casting mold 2 having a straight vertical casting space which leads into a

guide path

3, 4 situated below the mold 2. The guide path consists of a straight vertical path section 3 situated immediately below and co-axial with the mold 2, and a subsequent arcuate path section 4 that is an arcuate path struck from a center of curvature. The

path sections

3 and 4 are defined by rotatably arranged guide rollers 5 which are arranged omnilaterally around the cast steel section S in the region of the straight vertical path section 3, whereas in the region of the subsequent arcuate guide path section 4, the rollers 5 are situated at least along the outer and inner curvatures E and A respectively of the cast steel section. Water-spraying

norzles

6, 106, 206 are arranged to direct water inwards against the cast steel section S and are situated on all sides around the cast steel section. The noules are located between the guide rollers S in both

guide path sections

3, 4.

The straight vertical guide path section 3 may be of optional height H but preferably is between one and two times the height H1 of the mould. The arcuate guide path section 4 may correspond to sector angle C having a maximum included angle of about The molten steel is cast from a teeming or distributing vessel 7 into the continuous casting mold 2 in which a solidified layer 8 of steel, which represents the so-called marginal crust of the cast section S, is fonned on all sides along the cooled walls of the mould 2. This solidified marginal crust 8 is initially in contact with the walls of the mould, that is in the upper portion L1 of the mold 2, and as it slides down the walls of the mold the thickness of the crust progressively increases. In the lower portion L2 of the mold 2, in which the thickness of the solidified marginal crust 8 is sufficient to withstand the ferrostatic pressure of the molten core 9 of the cast section, the marginal crust 8 is separated from the walls of the mold due to shrinkage. The cast section S emerging from the bottom of the casting mold 2 thus consists of a marginal crust 8 solidified all around and of a molten core 9.

After emerging from the mold 2, the cast section S initially traverses the straight vertical guide path section 3 and is postcooled on all sides by the spraying

nozzles

6, 106. The path for the cast section S, defined by the guide rollers 5 of the straight guide path section 3, corresponds to the actual cross-section of the cast section at the egress end of the continuous casting mold 2, within tolerance limits. In one example, the casting space of the continuous casting mold 2 has a square cross-section of which each side has a width of 101 mm. 0.4 to 0.0). A shrinkage of the cast section is assumed, amounting to 1.3 to 1.5 percent of the width of the sides of the casting mold space. The open square passage opening for the cast section formed between the guide rollers 5 of the straight guide path section 3 is thus adjusted for a side-width of 99.5 mm. In the region of transition between the straight vertical guide path section 3 and the subsequent arcuate guide path section 4, that is in the region of line 11-1] of FIG. 1, the section S is deflected, the bending moment being applied by the final guide roller sets of the guide path section 3 and the first guide roller sets of the guide path section 4. The cast section S, which is then curved, subsequently traverses the arcuate guide path section and is post-cooled by the water-spraying

noutles

6, 206 until its complete solidification, that is until the molten core 9 of the section has solidified. The average intensity of the water cooling action on the cast section S, related to the unit of area of the surface of the cast section, is approximately 3 to 7 times and preferably 4 to 6 times greater in the region of the straight guide section 3 than the cooling action effect in the region of the arcuate guide path section 4. Consequently, the thickness of the marginal crust 8 is so increased in the straight guide path section, that the cast section S can withstand mechanical bending upon entering the arcuate guide path section 4, without incurring damage.

In the known curved continuous casting plants of this kind, the cast section S1, FIG. 5, is cooled with the same intensity along the side which forms the outer curvature E, FIG. 1, as along the side which forms the inner curvature A of the section at all points along the cast section, below the mould 2. The solidified marginal crust 8 of a cast section Sl obtained by this known cooling method consequently has practically the same thickness, as indicated diagrammatically in FIG. 5, especially in the terminal portion of the straight vertical guide path section 3 along the sides E and A of the cast section. In the region of bending of this cast section S1, between the straight vertical guide path section 3 and the subsequent arcuate guide path section 4, the zero-line Nl-Nl separating the tensile portion corresponding to the outer curvature from the compressive portion corresponding to the inner curvature is approximately in the center of the cross-section of the cast section, that is at the same distance 11 from the outer curvature E of the cast section and from the inner curvature A thereof. In these circumstances the elongations engendered by the tensile stresses during the bending of the cast section in the portion of the cast section corresponding to the outer curvature can reach so great a value, especially in the case of some grades of steel, that they cause undesirable harmful cracks in the area of transition D (dendrite area) between the molten core 9 of the cast section and the solidified marginal crust 80 along the outer curvature, and even in the marginal crust 80 along the outer curvature.

According to the invention, each part of the cast section is cooled much more intensively along the side thereof which is to form the outer curvature E in the region of the straight vertical guide path section 3 than it is along the other sides of the cast section and especially more than along the side of the cast section which is to form the inner curvature A of the section, that is in such manner that along the side which is to form the outer curvature there is formed a marginal crust 108 which has a substantially greater thickness than the marginal crusts along the other sides of the section and which is especially greater than the marginal crust 208 along the side which is to form the inner curvature, as shown in FIGS. 2 and 6. In the region of the bend of the cast section S between the straight vertical guide path section 3 and the subsequent arcuate guide path section 4, the zero-line N-N, FIG. 6, which separates the tensile portion corresponding to the outer curvature from the compressive portion of the cross-section of the cast section corresponding to the inner curvature is accordingly offset from the center of the cast section. The zero-line N--N is displaced considerably towards the outer curvature E of the cast section as compared with that of the cast sections S1 according to FIG. which are obtained by the known cooling methods and are cooled with equal intensity along the outer and inner curvatures of the cast section. As can be seen from FIG. 6, the distance hl of the zero-line N-N from the outer curvature E of the cast section is considerably less than the distance M of the zero-line NN from the inner curvature A of the cast section.

The maximum specific expansions and compressions of the metal during bending of the cast section S are directly proportional to the distance hl of the zero-line N-N from the tensionally stressed outer curvature E to the distance h2 of the zero-line N-N from the compressively stressed inner curvature A of the cast section. Consequently, the maximum specific elongations in the solidified tensionally stressed portion of outer curvature of the cast section are so reduced due to the lesser distance hl from the zero-line N-N, compared to that of the zero-line N1-N1 of the cast sections S1 according to FIG. 5 which are cooled by conventional methods, that practically no cracks are formed in the solidified marginal crust 108 along the outer curvature and in the transition area D (dendritic area) between this marginal crust 108 and the molten core 9 of the section, and the number of cracks are reduced to an extent which is certainly acceptable qualitatively. At the same time, the stresses and the maximum specific strains created in the solidified compressively stressed internal curvature part of the cast section may well be increased according to the greater distance h2 from the zero-line N--N as compared to those of the cast sections S1 according to FIG. 5, which are cooled by conventional methods, and the higher tensile and compressive strains can be withstood without damage to the cast section S and do not cause cracks in the marginal crust 208 along the internal curvature in the dendritic area between the marginal crust 208 and the molten core 9 of the cast section. The inner curvature side A of the cast section and the two lateral sides of the cast section are preferably cooled with equal intensity in such manner that the marginal crust 208 along the inner curvature and the two lateral marginal crusts of the cast section have approximately the same thickness as the corresponding marginal crusts of the cast sections S1 obtained by the conventional methods, as can be seen by comparison of FIGS. 5 and 6.

In addition to the provision of the step of unequal cooling of the outer curvature E and inner curvature A regions of the cast section in the region of the straight vertical guide path section 3, the positioning of the guide path section 3 is of importance. The straight guide path section 3 serves not only the purpose of supporting the cast section which still largely consists of molten steel, but also serves to prevent automatic bending of the cast section S due to the differential cooling action on the outer and inner curvatures E and A of the section. This prevents not only the intervention of harmful strains and deformations in the solidified marginal crust 8 of the cast section S, but also precludes disturbance in the dendrite formation in the area of transition between the solidified marginal crust 8 and the molten core 9 of the cast section. As a result, the thickness of the solidified marginal crust 8 of the cast section S can increase rapidly and without disturbance during traversal of the straight guide path section 3 until the ratio is established which is desirable for the subsequent guided bending of the cast section S between the thicknesses of the thicker marginal crust 108 along the outer curvature and the thinner marginal crust 208 along the inner curvature.

The more intensive cooling action on the side E of the cast section along the outer curvature, in the region of the straight guide path section 3, may be accomplished in practice by means of optional structural devices or measures. For example, it is possible to arrange a greater number of more closely spaced spraying nozzles 106 at the side E of the section along the outer curvature than are arranged at the other sides of the cast section, as illustrated in FIG. 2. It is also possible to arrange the sarne number of spraying nozzles at each side of the section and to employ noules having a higher spraying performance at the side E of the section along the outer curvature. The increase in the intensity of the cooling action on the cast section along the outer curvature in comparison with the cooling action on the other sides of the cast section, especially as compared with the cooling action on the inner curvature of the section, depends on the dimensions and the shape of the cross-section of the section cast, as well as on the properties of the steel and on the permissible maximum limits of elongation in the tensionally stressed part of the cross-section of the cast section, for example, the cooling action on the outer curvature side E of the cast section may be 20 to percent more intensive than the cooling action on any other side of the cast section and particularly on the inner curvature side A of the cast section.

After deflection of the cast section S from the straight vertical guide path section 3 into the subsequent arcuate guide difference between the higher intensity of cooling of the outer curvature side E of the cast section and the lower intensity of the cooling of the other sides of the cast section, especially of the inner curvature side A, to be reduced or nullified suddenly or progressively in the longitudinal direction of the cast section, or even reversed, that is in such manner that the solidified marginal crusts on all sides of the section, and in particular the

marginal crusts

108, 208 along the outer and inner curvature sides E and A of the cast section, assume the same thickness before the complete solidification throughout the cross-section of the cast section. To this end, it is possible to reduce the higher cooling intensity on the outer curvature side A and/or to increase the lower intensity of the cooling of the other sides of the cast section, especially of the inner curvature side A of the cast section. The zero-line N-N of the cross-section of the cast section displaced from the center in the region on the bend of the cast section S, that is displaced in the direction towards the outer curvature side E of the section, consequently creeps towards the inner curvature side A of the section and stops in the center of the cross-section of the cast section. The core 9 of the cast section, which is still molten and offset from the center in the region of the bend accordingly assumes a central position and the cross-section of the cast section again assumes a symmetrical shape prior to complete solidification of the cast section S, as shown in FIG. 4. FIG. 3 shows the cross-section of the cast section in a transitional stage between FIG. 2 and FIG. 4. It will be seen from FIGS. 3 and 4 that in the region of the arcuate guide path section 4, the number of spraying nozzles 206 along the outer curvature side E of the section is equal to the number of spraying nozzles 6 at each of the other sides of the section, that is in the example illustrated, the unification of the thickness of the marginal crusts on all sides of the cast section behind the region of the bend in the cast section is accomplished by reducing the intensity of the cooling action on the outer curvature side E of the cast section, which had initially been increased. In conjunction with the unification of the thickness of the marginal crusts, the arcuate guide path section 4 serves not only the purpose of supporting the cast section S, but also to hold the bent cast section along the scheduled arcuate path, that is against the warping tendencies of the cast section engendered by the differential cooling of the sides of the section and by the differential increase in the thickness of the marginal crusts.

The invention may also be applied in such curved continuous casting plants in which the straight vertical guide path section 3 is followed by a curved guide path section which is not arcuate, that is struck about a center of curvature, but has a progressively increasing curvature, e.g. parabolic, hyperbolic or the like, the cast section traversing this guide path section accordingly being bent continually. In this case, it may be sufficient to cool the outer curvature side E of the cast section more intensively only in the region of the straight vertical guide path region 3 than the other sides of the cast section, and especially the inner curvature side A. Although the cast section running from the straight vertical guide path section 3 into the curved subsequent guide path section has a marginal crust 108 along the outer curvature of greater thickness than that of the marginal crusts at the other sides of the section, and especially along the inner curvature side A, the zero-line N N between the tensile and compressive portions of the crosssection of the cast section retains its position offset from the center without change, as reached at the end of the straight guide path, during the subsequent and progressively increasing bending action on the cast section S in the curved guide path, that is it does not creep towards the outer curvature side E of the section, as required for an increase inthe curvature of the section. On the other hand, in the case of guide path sections for cast sections comprising a progressively increasing curvature, the more intensive cooling action applied on the outer curvature side E of the section in the region of the straight vertical guide path section 3 may also be continued in the region of the subsequent curved guide path section and may be increased progressively in the longitudinal direction of the cast section according to the increasing curvature of the cast section. Consequently, the difference between the thicknesses of the

marginal crusts

108 and 208 along the outer and inner curvatures of the cast section also increases progressively in the region of the curved guide path section. Accordingly, the zero-line N-N between the tensile and compressive portions of the cross-section of the cast section travels progressively towards the outer curvature side E of the cast section corresponding to the increase in the curvature of the cast section in the curved guide path section. In both cases, the difference in the intensity of the cooling action between the more intensive one on the outer curvature of the cast section and the less intensive one on the other sides of the cast section may, for unification of the thicknesses of the marginal crusts of all sides of the cast section, be suddenly or gradually reduced, cancelled or even reversed, in the terminal region of the curved guide .path section at an appropriate point prior to the complete solidification of the cast section.

I claim:

1. In a process for the continuous casting of molten metals,

particularly of steel, which process includes the steps of continuously casting the metal in a casting mould having a straight vertical casting space, and passing the metal so cast as a molten core surrounded by a peripheral crust of solidified metal into and through a straight vertical guide path section and a succeeding curved guide path section, the improvement which comprises:

a. commencing while the cat metal is passing through the straight vertical guide path section, cooling while passing through the straight as well as the curved guide path section that side of the cast metal which is to form the side of greater radius of curvature more intensively than the side which is to form the side of lesser radius of curvature to provide the side of greater radius of curvature with a crust of greater thickness than that of the side of lesser radius of curvature, constraining the difierentially cooled metal casting along said curved guide path section,

. maintaining an intensity of cooling in the region of said straight guide path section approximately 3 to 7 times greater than the intensity of cooling in said curved guide path to so increase the thickness of the peripheral crust in said straight guide path section that the cast metal can withstand entry into said curved guide path section without mechanical damage, and

c. after deflection of the cast metal into said guide path and while the core is still molten reversing said differential cooling so as to apply a cooling of greater intensity to the side of lesser radius of curvature of the cast metal in the terminal region of the curved guide path section whereby the solidified crusts along all sides of said cast metal assume substantially the same thickness and the solidified metal assumes a symmetrical shape at a point prior to the complete solidification of said cast metal.

Claims

1. In a process for the continuous casting of molten metals, particularly of steel, which process includes the steps of continuously casting the metal in a casting mould having a straight vertical casting space, and passing the metal so cast as a molten core surrounded by a peripheral crust of solidified metal into and through a straight vertical guide path section and a succeeding curved guide path section, the improvement which comprises: a. commencing while the cast metal is passing through the straight vertical guide path section, cooling while passing through the straight as well as the curved guide path section that side of the cast metal which is to form the side of greater radius of curvature more intensively than the side which is to form the side of lesser radius of curvature to provide the side of greater radius of curvature with a crust of greater thickness than that of the side of lesser radius of curvature, constraining the differentially cooled metal casting along said curved guide path section, b. maintaining an intensity of cooling in the region of said straight guide path section approximately 3 to 7 times greater than the intensity of cooling in said curved guide path to so increase the thickness of the peripheral crust in said straight guide path section that the cast metal can withstand entry into said curved guide path section without mechanical damage, and c. after deflection of the cast metal into said guide path and while the core is still molten reversing said differential cooling so as to apply a cooling of greater intensity to the side of lesser radius of curvature of the cast metal in the terminal region of the curved guide path section whereby the solidified crusts along all sides of said cast metal assume substantially the same thickness and the solidified metal assumes a symmetrical shape at a point prior to the complete solidification of said cast metal.