US3923091A

US3923091A - Method of supervising skin thickness in a solidifying body such as a continuously cast ingot

Info

Publication number: US3923091A
Application number: US456899A
Authority: US
Inventors: Wolfgang Dorr; Hartwig Matzner; Tilman Noska
Original assignee: Mannesmann AG
Current assignee: Vodafone GmbH
Priority date: 1973-04-17
Filing date: 1974-04-01
Publication date: 1975-12-02
Anticipated expiration: 1992-12-02
Also published as: ATA28474A; AT337381B; DE2320277A1; FR2226660B1; DE2320277B2; JPS5010195A; FR2226660A1

Abstract

An early indication of skin separation and expected thinning of the skin in a mold for continuous casting is provided by detecting the ratio of heat flow into the mold in two zones of limited width of the mold wall, one above the other and located where the skin is most likely to begin to separate. The resulting parameter K in conjunction with total heat flow Q into the zone is related to skin thickness s, generally, by the relation

wherein A'', B'' and C'' are constant parameters for the mold when operated at a constant casting speed. The total heat flow Q into the two zones is either determined directly by measuring temperature and coolant flow or indirectly by the relation

wherein A, B and C are additional parameters and To is the measured surface temperature of the ingot at the mold bottom opening. In the preferred form, the heat flow ratio is determined by temperature measurements in one cooling duct, traversing the mold side vertically.

Description

United States Patent [1 1 Dorr et a1.

[ Dec.2, 1975 [75] Inventors: Wolfgang Diirr, Essen; Hartwig Matznor; Tilman Noska, both of Duisburg, all of Germany [73] Assignee: Mannesmann Aktiengesellschaft,

Dusseldorf, Germany [22] Filed: Apr. 1, 1974 [21] Appl'. No.: 456,899

[30] Foreign Application Priority Data Apr 17, 1973 Germany 2320277 [52] U.S. Cl. 164/4; 73/295; 164/154 [51] Int. Cl. B22D 11/12 [58] Field of Search 164/4, 82, 154, 283 M; 73/295 [56] References Cited UNITED STATES PATENTS 3,145,567 8/1964 Bobrowsky 73/295 3,204,460 9/1965 Milnes 164/4 UX 3,478,808 11/1969 Adams. 164/82 X 3,502,133 3/1970 Carson. [64/154 X 3,745,828 7/1973 Howellm, 73/295 3,797,310 3/1974 Babcock et al. 164/154 X FOREIGN PATENTS OR APPLICATIONS 1,029,098 3/1953 France 164/154 Primary Examiner-R. Spencer Annear Attorney, Agent, or FirmRalf H. Siegemund [57] ABSTRACT An early indication of skin separation and expected thinning of the skin in a mold for continuous casting is provided by detecting the ratio of heat flow into the mold in two zones of limited width of the mold wall. one above the other and located where the skin is most likely to begin to separate. The resulting parameter K in conjunction with total heat flow 0 into the zone is related to skin thickness 5, generally, by the relation wherein A, B and C are constant parameters for the mold when operated at a constant casting speed. The total heat flow Q into the two zones is either determined directly by measuring temperature and coolant flow or indirectly by the relation log Q B wherein A, B and C are additional parameters and T is the measured surface temperature of the ingot at the mold bottom opening. In the preferred form. the heat flow ratio is determined by temperature measurements in one cooling duct. traversing the mold side vertically.

12 Claims, 4 Drawing Figures (457m; awzeaz I U.S. Patent Dec.2, 1975 'sh eetzf (4577M; (ax rial METHOD OF SUPERVISING SKIN THICKNESS IN A SOLIDIITYING BODY SUCH AS A CONTINUOUSLY CAST INGOT BACKGROUND OF THE INVENTION The present invention relates to a method and equipment for ascertaining, supervising and controlling the thickness of the skin of a solidifying body. More particularly but not exclusively the present invention relates to the supervision of the growth of the skin of a continuously cast ingot as extracted from a liquid cooled mold which is open at top and bottom.

As is well known, particularly from continuous casting of steel, the ingot as withdrawn from a mold is by no means solidified all the way through. Rather, the ingot has a liquidous core which extends inside of the ingot for a distance much longer than the height of the mold; the skin is still quite thin at the bottom of the mold. A significant aspect of continuous casting is to be seen in the desirability of obtaining a skin of adequate and uniform thickness over the entire circumferences,right at the bottom exit of the mold so that the withdrawn ingot has at least some strength at that point. This particular area is most critical and disturbances are most likely to occur here unless duly prevented.

Adequate skin thickness at the mold bottom is obtained by appropriately proportioning the interrelating casting speed, rate of cooling and mold contour (e.g. conicity). Empirical data have been acquired here over long periods of time devoted to experiments and practice, and as a result one can predetermine these parameters today so as to obtain an adequate skin thickness with sufficient certainty, whereby adequate is to mean a skin thickness that will hold and will not rupture or the like, with a considerable margin of safety.

It has to be observed, however, that even today ones does not know with sufficient certainty all of the details of the phenomena and events that actually transpire inside of a mold. The interior of a mold e.g. for continuous casting of steel is not readily accessible for taking direct measurements. It is believed, therefor, that the presently used withdrawal and casting speeds are low primarily for reasons of operating with a "sure margin of safety, but such caution may well be needlessly excessive in cases. Lack of data and information has, therefor, precluded increasing the through-put of molds.

It is known to measure the skin thickness outside of the mold, just underneath its bottom opening, by transmitting radiation transverse to and through the ingot so as to detect skin thickness (vs. liquidous core thickness) and to control the casting speed in dependence thereof. However, one can ascertain the skin thickness below the mold only i.e., at an instant when it may already be too late.

In accordance with another approach the problem is simulated on the basis of a mathematical model by means of which one can attempt to investigate mathematically the processes and phenomena inside of the mold. In a kind of iteration and/or approximation process one can attempt here to gain information on the inside events using the externally accessible data as boundary conditions. It is possible in this manner to obtain some information on the limitations of the casting process as to speed.

The largest unknown" in that model is, however, the local formation of a gap, limiting the accuracy in the determination of the heat transfer into the mold as a function of distance from the bottom. That heat transfer is not constant over the mold height to begin with, and suddenly occurring local changes in the heat transfer conitions due to local separation of the skin from the mold wall are simply not amenable to calculatory prediction. Presently, one simply does not know with sufficient accuracy when the skin will separate from the mold wall at any particular spot (and not elsewhere and not earlier or later) because the modus operandi of that phenomenon is not sufficiently known even though one does know that certain mold wall regions are more likely than others to develop skin separation. Thus, the separation appears to occur, as far as is known at this time, more or less at random. This aspect is only one of the problems; the heat transfer in and through such a gap is quite a complex phenomenon particularly when compared with the heat transfer above the gap as well as when compared with the heat transfer as it is effective prior to gap formation is a local phenomena which does not occur (or only rarely) simultaneously over the entire circumference of the forming ingot, nor will ,the gap form in the same level in the mold in all cases.

Of course, some measurements can be made with regard to the process. For example, one can measure the heat through-put of the mold, possibly in relation to poured-in molten steel per unit time and for a temperature of the molten material which can likewise be measured, at least on the surface. The thermal energy is specifically measured by determining the amount of water passing through the mold (per unit time), and by determining its temperature differential at input and output. The data gained here become more meaningful if measured and recorded separately for the different sides of the mold. On the basis of data gained here, variations in removed thermal energy can be used for control of the casting process.

However, one can see that the heat flow and transfer measured in that manner yields a more or less summary kind of data, integrated for entire mold sides. It is simply not possible to learn sufficient details as to any local conditions, particularly with regard to portions of the mold which are, for example, specifically endangered or prone to exhibit local (and then progressing) gap formation. Total thermal input and variations thereof as measured for the entire mold, or even for each side separately, is simply not adequate information which permits localization of any trouble spot that may cause (or does cause) skin separation resulting in a significant thinning of the skin (and, possibly, rupture) when leaving the mold.

DESCRIPTION OF THE INVENTION It is an object of the present invention to solve the problem broadly outlined above and to avoid the drawbacks inherent the known methods. Particularly, it is an object of the present invention to provide for definition of a particular parameter which permits the drawing of conclusions about the formation of the skin, its thickness at the mold bottom exit, and particularly about the skin configuration at those mold areas which establish most likely trouble spots as has become known empirically (after the fact). Moreover, this parameter should permit a comparison of the skin thickness in various portions around the periphery. Finally, this parameter should permit utilization as an input for process control of the continuous casting operation.

It is a specific object of the present invention to provide for a method for determining and supervising the skin thickness of a solidifying body, particularly the growth of the skin in a continuously cast ingot, during continuous casting while using a liquid cooled mold which is open at top and bottom, whereby the skin of the ingot is rather thin when withdrawn and only gradually thickens so that the total length of the liquidous core is larger than the mold is high.

It is a further object of the present invention to provide for a supervision of the skin growing process of a continuous cast ingot that yields information on conditions in the mold, even prior to emerging e.g. of a thinner-than-normal skin.

In accordance with the preferred embodiment of the present invention it is suggested to define two different, rather narrow cooling zones in a mold wall, extending over a relatively small width as determined in circumferential direction of the mold; the two zones are vertically separated. The ratio of the heat flows in the two zones, from the mold wall into the coolant, is detected on a running basis as the primary parameter K. The total heat transfer Q into the two zones is additionally determined, either directly or indirectly, and the values for K and Q are used to ascertain the thickness of the skin expected to emerge shortly thereafter from the mold bottom exit. The direct determination of heat transfer involves the measurements of coolant flow, and of the coolant temperature before and after heat exchange with the mold wall in both of the two zones. The indirect determination of Q involves measuring the surface temperature T of the skin where leaving the mold and relying on a functional relation between K, Q and T for determining Q.

The zones of detection are located in one of those portions of the mold wall which are predominantly prone to exhibit (or produce?) separation of the skin from the mold wall. In accordance with another feature of the invention, the supervision and indirect measurement of skin thickness is carried out separately in different areas of the mold, and the resulting skin thickness values are compared to ascertain uniformity or non-uniformity of the casting process. I

The invention is based on the discovery that the particular parameter K as defined is a very valuable quantitive representation and indicator of skin separation. When considering a (hypothetical) horizontally taken slice of the ingot, as it drops down in the mold, such a slice begins to cool in the mold and begins particularly to grow a solid skin adjacent the mold side as it progresses down in the mold; the heat transfer from that slice into the mold wall is not uniform as a function of mold height (i.e., distance from the bottom exit). The growing skin as such, i.e., the increasing thickness of the skin with reducing distance from the mold bottom is one factor for non-uniformity of the heat transfer, but separation when actually occurring is another one, having a rather pronounced effect on local heat transfer and on the subsequent growth of the skin. If one plots heat transfer density over distance (from bottom), there is a definite maximum in the upper mold portion i.e., at a relatively large distance from the bottom exit.

The invention utilizes this phenomenon. By means of simulation in a mathematical (computer) model it was found that the (local) skin thickness at the mold exit depends, on the one hand, on the entire heat flow into the mold portion and taken over the width of that mold section that contributed to the skin growth. The skin thickness depends further on the relative distribution of heat flow, i.e., on the heat flow into the mold wall in the upper mold portion in relation to heat flow into the lower mold portion. This holds true with or without skin separation; the separation results in a material disturbance of these heat flow relations and is reflected in a reduced skin thickness. Hence, when the onset of such disturbance can be detected, the expected reduction in skin thickness can be deduced from such a detection.

Upon supervising the heat transfer in upper and lower mold portions and in a width section most likely to produce separation, the onset ofa change in parameter K will be detected before the effect of the skin separation has resulted in a thinner skin at the mold bottom. The skin portion which will be thinner (because of separation) is still inside of the mold when the parameter K begins to change. Thus, one learns of the separation and of the expected thinner skin rather early.

It was further found that the heat transfer distribution and total heat transfer are directly related to the surface temperature of the ingot as emerging from the bottom of the mold. The specific relationship between these parameters will be explained later, but it can be said presently that the total heat flow Q from mold to coolant is functionally related to the ingot surface temperature so that one can be calculated from the other.

It should be mentioned further that the primary parameter in this method is K, as variations of K (in time) or more precisely the onset of a deviation of K from a constant value is already a indication that the normal heat transfer conditions in the supervised zones is disturbed. The parameter Q will also vary in this instance, but changes in K are more pronounced. Thus, the parameter K can already be used directly for control of the casting process.

The invention uses known mold sidings with vertical cooling channels, an inlet for the coolant in the upper mold portion, usually well above the surface level of molten material in the mold, and an outlet for the coolant adjacent the bottom opening. Such a single, vertical cooling duct, for example, is used to define the two cooling zones together, particularly as to their width and location in one mold side. The temperatures at the upper and lower ends of the duct yield the needed information of total heat transfer in both zones, while the temperature in the center (vertical) of the duct when measured separately permits ascertaining of heat transfer into upper and lower zones individually; the location of this central temperature measurement defines the dividing line between the two zones. However, the scope of the invention includes the provision of separate coolant circulations in upper and lower mold wall portions with inlet and outlet temperatures being determined separately to determine the heat transfer into upper and, lower mold portion. However, for reasons below it is mandatory to determine also the coolant flow in a two-circulation system, whereas no such determination needs to be made in the case of a signal coolant circulation.

DESCRIPTION OF THE DRAWINGS While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:

FIG. 1 shows a mold, somewhat schematically and in an isometric view for demonstrating certains aspects of defining mold wall zones;

FIG. 2a and 2b are two related graphs demonstrating functional relations as discovered and relevant for practicing the inventive method; and

FIG. 3 is a cross-section through a mold siding plus schematic representation of measuring instrumentation and probes for practicing the inventive method.

Proceeding now to the detailed description of the drawings. FIG. 1 shows a mold in which the inventive method can be practiced and which is comprised of four sides 1, 2, 3 and 4. The particular mold serves for casting slab ingots and has wide sides 1 and 2, the narrow sides 3 and 4 accordingly. The hatched wall portions refer to those portions of the mold adjacent to which most likely the skin may separate first. Particularly, a central area 5 on a narrow mold side, and zones 6 at a wide mold side are those where the danger of such separation is most prevalent.

These zones 5 and 6 are not physical entities in the sense of inserts, but are defined by width sections taken over the entire height of the mold. Each such zone is divisible into an upper zone, such as 5a or 6a, and a lower zone such as 5b or 6b with a hypothetical (dashed) dividing line separating the zones.

The particular equipment shown in FIG. 3 can be used in any, some or all of these mold side portions. The mold sides is identified here by reference numeral 7 to indicate that it can be any of the sides of the mold. The mold contains predominantly molten steel 17 which is completely liquidous in the surface level 17a, while a narrowing, liquidous core 17b remains underneath and for a long distance from the mold. Reference numeral 16 denotes the growing solidified skin.

A cooling duct 8 traverses the mold in direction of casting. The mold side area in the immediate vicinity of that vertically running duct can be regarded as one of the zones 5 or 6. The circumferential width of such a zone is in the essence defined by the width of that mold wall or side portion that is effectively cooled by this one duct 8. The width dimension of the zone extends transversely to the plance of the drawing of FIG. 3, while a and b in conjunction with the dotted dividing line are the upper and lower zones to be considered. How the upper zone is actually separated from the lower zone will be described shortly.

A flow meter 9 is disposed in the inflow path for water as flowing into duct 8 (see arrows) to ascertain and to meter the amount of flow of cooling water (e.g. per unit time) into the cooling portion of duct 8. The flow meter is, however, optional. Thermo-

elements

10, 11 and 12 are strategically located in duct 8 projecting into the duct with sensing ends or points 13. Particularly, element 10 has its sensing point disposed at a location where the coolant comes into first contact with wall portions of the duct, closest to the inner mold surface. Thermo-element 11 is located in a central portion of duct 8 and element 12 monitors the water temperature where flowing away from the mold side interior. The duct may contain baffles, or the like to enhance turbulent flow, particularly right at the thermo-elements where sensing the water temperature.

As stated, reference numeral 13 denotes for each thermo-element the end point as exposed most critically to the coolant water and defined therewith the point of measurement where the temperature reading is taken. The thermo-element 11 defines the dividing line between upper and lower cooling and mold zones as governed for cooling action by the water in this particular duct 8.

The thermo-elements furnish electrical output signals representing the initial coolant temperature T the temperature T in about the center (longitudinally) of the cooling duct and exit temperature T,,, respectively. The temperature differential T T is indicative of the amount of heat transferred from the upper mold zone (a) considered into the coolant, while the differential T T is indicative of the amount of heat transferred from the lower mold zone (b) into the coolant. The temperature differential T T is proportional to the total heat transfer in both zones.

The amount of water and its heat capacity are additional parameters for determining these heats Qu, Qe and Q respectively. However, the ratio Qu/Qe does not contain the amount of water flow and its heat capacity E. That ratio is merely equal to (1",, T )/(T T A pyrometer 14 may be disposed under the mold, directly adjacent the withdrawing ingot where emerging from the mold. That instrument provides a measuring value representing the casting ingot surface temperature T at the mold exit.

Pyrometer 14 and flow meter 9 has been listed as optional, but one of them has to be provided for. Both of them may be needed for purposes of preparation for practicing the inventive method on a running basis during supervised. production. The actual withdrawal speed of the ingot is measured by means of a friction contact roll 15 and its outout signal (suitably generated by an tachometer like device or the like), V represents the casting speed.

The purpose of the equipment as described is to provide for certain data which are inter-related in such a manner, that the skin thickness s can be determined.

FIG. 2a is a first graph showing a family of curves wherein a parameter K is plotted against the thickness of s of the skin at the mold exit point. K is a dimensionless quantity and defines the'ratio of amount of heat transferred from the mold wall to the coolant in the upper zone (-a) over the amount of heat transferred from the mold into the coolant as flowing through the lower zone (-b). The variable parameter of these curves is total quanity of heat Q fed into the zones, (-a and b), and removed by the coolant as flowing through duct 8.

FIG. 2bis a second graph wherein the same parameter K is plotted against surface temperature T, of the ingot at the mold exit. The parameter of the plurality of curves is again Q.

The two graphs of FIG. 2a and 2b have been intentionally plotted with aligned abscissas, so that comparable values of K are aligned horizontally. One can see, however, that the order of the curves representing different Qs is reversed.

The relationship of the curves in the two graphs can be demonstrated best by an example. A particular surface temperature T, at the mold exit intersects with a particular value for K in FIG. 2b and at a curve associated with a particular parameter Q Or, to state it differently, if the ratio of upper to lower heat transfer is K for a total heat flow in both zones of 0 then the surface temperature of the ingot will be T Taking the same K and using it in the plot of FIG. 2a, then the in- 7 tersection of the particular curve for Q with the horizontal line identifying K defines the skin thickness s. Skin thickness s, ratio K, and total heat flow Q are related by the following formula:

log Q B (l) log, K A logg log, Q B

wherein A, B, and C are also empirically determined constants. FIG. 2b represents this formula.

The measuring equipment shown in FIG. 3 can be used to determined empirically the constants A, A, B, B, C and C. The redundancy of equation (2) with regard to T, and Q permits readily the determination of the parameters A, B and C, while measuring the skin thickness with known techniques permits the determination of parameters A, B and C. A particular set of these constant parameters has, of course, validity only for a speeds V,,, for a particular material that is being cast. The equations as such and the numerical values for the parameters A, A etc can also be determined by way of mathematical analysis and simulation in conjunction with the measurements.

It is an important aspect that these equations described stationary conditions, while changes (in time) disturb the relations. Therefor, a change of the value K, for example, due to skin separation in the mold and observed at a particular instant does not change the skin thickness as emerging from the mold at the same instant. The skin thickness will change only when the separated skin portion, whose separation caused the change in K, emerges from the bottom of the mold. Thus, a change in K when detected yields information that a change in skin thickness can be expected, and that is precisely what one wants to know.

Equation 2 verifies the general statement made above, namely, once the parameters A, B and C have been determined for a particular mold, not all parameters K, T and Q have to be measured for purposes of running supervision of the casting process. Having measured either K and T or K and Q, the respective third parameter can be calculated. One needs Q and K to obtain the skin thickness by operation of equation 1, (FIG. 2a). Please note, that measuring Q and T is feasible as per equation (2) to obtain K, which then could be used in equation (1). In other words FIG. 2b and equation (2) could be interpreted as obviating the need for measuring K, i.e., for separately determining the upper-to-lower heat flow ratio. That however, is a misleading assumption. These equations describe the stationary conditions, and a measured value of K is needed to predict a deviation from the stationary case. Continued utilization of the equations in a transitory, non-stationary or quasistationary case is merely an approximation, but if one uses a measured K, that approximation suffices to predict a change in s.

The measuring equipment as described has as its principle function, during actual casting, to ascertain the heat flow density into the mold in dependancy upon height, and here particularly in a manner permitting distinction between the heat flow into the upper mold part and into the lower mold part.

Readings can be taken on a continuous basis. The temperature readings T and T are indicative of the heat exchange (0,) between mold and coolant in the upper mold half, the readings T and T,, are indicative of the heat exchange (0,.) between mold and coolant in the lower mold half. Coolant flow determination is would be needed for determining these heats Q,,, Q, individually. However only the ratio Qu/Qe K is actually needed for the relevant determination, and the same amount of coolant tranverses upper and lower zones due to the continuous configuration of duct 8. Thus, the temperature readings taken with the instruments as described permit directly the derivation of parameter K:

This derived parameter can be calculated on a continuous basis if for example the measurements are inputted ina process controller. However, known analog circuit networks can simply be used for generating the two differences, as well as the ratio to determine K directly as an electrical (analog) quantity.

This parameter K is referenced in time e.g. against the surface temperature of the ingot T, as measured by the pyrometer. Upon using the equation (2) above or a digital function table in a computer, T and K are used to calculate, also on a running, on-line, real time basis, the parameter Q. Graphically (FIG. 2b) the intersection of T and K defines a curve having a Q as parameter, which is then used in conjunction with FIG. 2a to ascertain skin thickness s.

A computer may use the thus calculated parameter Q and the equation (I) to calculate the skin thickness or to find that value in a look-up table, stored in the computer as digital representation of the two families of curves of FIGS. 2a and 2b. The value can be outputted for immediate reading by a (human operator) and/or can be used to control the casting process.

One can see, however, that analog circuitry can also be employed, since the logarithmic function can be rather simply approximated by linear circuits such as diodes, and the parameters A, A etc are represented by fixed resistors (having been trimmed to values which are valid for a particular mold and a particular casting speed). Equations (1), (2) and (3) can, therefor, be represented by a network of linear and non-linear resistance, receiving e.g. voltages representing T T,,-, T T, and producing an output voltage that is e.g. proportional to logs, or s directly, if one uses another non-linear resistance for extracting the output from the device.

The skin thickness ascertained in this manner is accurate only for the stationary case. However, in the case of separation of the skin inside of the mold, the parameter K will begin to change before the resulting thinner skin emerges from the bottom of the mold and one can indeed use the detection of the onset of a change in K to prevent possible disaster by increasing the coolant flow and/or lowering the casting speed.

If one uses analog circuitry for representing actual skin thickness s, a reference source providing eg a signal representing desired thickness so may be connected in opposision to the source providing the measured/calculated signal representing s, and an error signal may be produced and used further and/or indicated e.g. when a threshold value for such an error is exceeded.

The particular mode of practicing the invention with a common cooling duct for both zones, is somewhat simpler in that it is not necessary to measure the coolant flow to obtain any other values On, Qe and Q. Rather, Q is determined on the basis of the surface temperature as per equation (2). The operation as such is, of course, valid only for a constant casting speed. However, the measurements and calculated results, either K process, separately for the upper half and the lower half of the mold. The two quantities are respectively denoted Q0 and On and their ratio is again the parameter K. While 00 +Qu Q, and equation (1) yields the skin thickness.

The example above shows a common flow path for the coolant in upper and lower mold portions, and upper and lower heat flows are distinguished on the basis of temperature readings in the different portions of the coolant flow. One can, however, construct the mold to have different coolant circulations in upper and lower mold portions, with separate inflow and discharge paths. However, in that case it is essential to meter flow quantities in both circulators to obtain accurate information on the heat flow quotient K as well as on Q.

Using a single circulation as depicted has the advantage that the amount of water flowing through the system does not have to measured. As was explained, Q per se can be treated as a redundant parameter as far as measurements is concerned, and for exactly equal water flow in upper and lower mold zones (as is the case in FIG. 3), the ratio K does no longer contain the water quantity but is reduced to the quotient of temperature differentials, K (T T )/(T T However separate cooling flows in lower and lower zones may be of advantage for the cooling process as such, since the central mold wall region is cooled with fresh, cool coolant. In this case, one needs to meter the coolant flow as the ratio K will no longer be represented by the ratio of temperature differentials as per equation (3). Rather one has to form K Qu/Qe, with Qu and Qe being separately determined in the two vertically, stacked coolant circulations.

The invention is not limited to the embodiments described above but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.

We claim:

1. In a method for providing for an anticipating indication of weakening of the skin of a continuously cast ingot where emerging from the bottom of a liquid cooled mold, and wherein a location of the mold wall has been determined which is particularly prone to exhibit separation of the ingot skin as formed adjacently thereof during the casting, the improvement comprising the steps of:

measuring the heat transfer taking place in an upper portion of the mold at said location, well above the bottom thereof and of a width considerably smaller than the width of a mold side and providing an electrical signal representative thereof; and

detecting electrically the onset of deviation of the representation from a normal value indicative of absence of skin weakening or separation. 2. In a method as in claim 1, including the step of measuring separately the heat transfer in a portion below said upper portion and of the same width and providing an electrical signal representative thereof, and providing a running indication of the ratio of the two signals as representing the two heat transfers.

3. In a method as in claim 1, and including measuring the heat transfer in a second portion of the mold, also well above the bottom thereof and at a mold location displaced for said first mentioned location and providing an electrical signal representative thereof, and relating the signal representations of the two heat transfers to each other.

4. In a method of ascertaining the skin thickness of a body solidfying in a mold wherein liquidous material is added to the mold and the body is withdrawn from the bottom of the mold, with solidified skin around a still liquidous core, the mold provided with liquid cooling, comprising:

measuring separately the heats transferred by the body as in contact with the mold to the coolant in two different, vertically spaced zones, each zone having width considerably smaller than the total circumference of the mold, and providing a signal representative of the ratio K of the heats as measured, said signal being time variable as said body is being withdrawn in dependance upon any variations of any said heat transfers; providing a signal representation for the total heat transfer Q to the coolant in said two zones; and

indirectly measuring the skin thickness s to be expected at the mold exit by processing said signals on the basis of the functional relation log, K=A log;

wherein A, B and C are empirically ascertained pa- I rameters for the mold.

5. In a method as in claim 4, wherein the heat transfer Q is measured indirectly, by detecting the surface temperature T, of the body when withdrawn from the mold, and by providing the signal representing Q on the basis of the relation tion, the coolant flowing vertically down in the mold wall, the temperature being measured in the coolant flow path.

8. Method as in claim 7, wherein the two zones extend respectively in upper and lower half of the mold, the heat ratio detecting step including measuring the temperature T of the coolant as fed to the mold in an upper wall portion thereof, the temperature T,, of the coolant as withdrawn from the mold in a lower wall portion thereof, and the temperature T of the coolant in a central wall portion of the mold, and providing the signal representing K by forming in a measuring circuit the ratio T T T T on the basis of the separate measuring of said temperatures.

9. Method as in claim 8, wherein the common circulation includes a vertical duct, the temperatures being detected by using thermo-elements projecting into the duct.

10. Method as in claim 4, wherein upper and lower mold wall portions are cooled by separate coolant circulations, the temperature in the central wall portion being measured separately for the two circulations, once in the in-flow for the cooling of the lower mold wall portion and once in the out-flow from cooling the upper mold wall portion.

11. In a method of ascertaining the skin thickness of a body solidifying in a mold wherein liquidous material is added to the mold and the body is withdrawn from the bottom of the mold, with solidified skin around a still liquidous core, the mold provided with liquid cooling, comprising:

monitoring the heat transferred by the ingot to the mold inan upper portion of the mold and for a width portion thereof considerably smaller than the total circumference of the mold, and providing a signal representative thereof;

monitoring the heat transferred by the ingot to the mold in a low portion of the mold and for a width portion similar to said width and providing a signal representative thereof;

monitoring concurrently the total amount of heat Q transferred in both said portions, said monitoring steps including measuring temperature of the cooling liquid at various points in said portions as well as the rate of flow of said coolant as effective in said portions; and

processing said signals and a signal representation of the quantity Q to provide an indication of skin thickness s to be expected on the basis of the functional relation log,K=A'log,s-l-

wherein A, B' and C are empirically ascertained parameters for the mold.

12. In a method of ascertaining the skin thickness of a body solidifying in a mold wherein liquidous material is added to the mold and the body is withdrawn from the bottom of the mold, with solidified skin around a still liquidous core, the mold provided with liquid cooling, comprising:

monitoring the vertical temperature differential of the cooling liquid in an upper portion of the mold and for a width portion thereof considerably smaller than the total circumference of the mold; monitoring the vertical temperature differential of the cooling liquid in a lower portion of the mold and for a width portion also considerably smaller than the total circumference of the mold; providing a signal representation of the ratio K of said temperature differentials, and on a running basis; measuring the surface temperature T of the body as withdrawn from the mold; and providing a signal representation of skin thickness by processing said ratio signal K and a signal representing measured surface temperature, on the basis of the two functional relations log Q B wherein A, A, B, B, C and C are empirically determined constants and Q is an auxiliary quantity.

Claims

1. In a method for providing for an anticipating indication of weakening of the skin of a continuously cast ingot where emerging from the bottom of a liquid cooled mold, and wherein a location of the mold wall has been determined which is particularly prone to exhibit separation of the ingot skin as formed adjacently thereof during the casting, the improvement comprising the steps of: measuring the heat transfer taking place in an upper portion of the mold at said location, well above the bottom thereof and of a width considerably smaller than the width of a mold side and providing an electrical signal representative thereof; and detecting electrically the onset of deviation of the representation from a normal value indicative of absence of skin weakening or separation.

2. In a method as in claim 1, including the step of measuring separately the heat transfer in a portion below said upper portion and of the same width and providing an electrical signal representative thereof, and providing a running indication of the ratio of the two signals as representing the two heat transfers.

4. In a method of ascertaining the skin thickness of a body solidfying in a mold wherein liquidous material is added to the mold and the body is withdrawn from the bottom of the mold, with solidified skin around a still liquidous core, the mold provided with liquid cooling, comprising: measuring separately the heats transferred by the body as in contact with the mold to the coolant in two different, vertically spaced zones, each zone having width considerably smaller than the total circumference of the mold, and providing a signal representative of the ratio K of the heats as measured, said signal being time variable as said body is being withdrawn in dependance upon any variations of any said heat transfers; providing a signal representation for the total heat transfer Q to the coolant in said two zones; and indirectly measuring the skin thickness s to be expected at the mold exit by processing said signals on the basis of the functional relation

5. In a method as in claim 4, wherein the heat transfer Q is measured indirectly, by detecting the surface temperature To of the body when withdrawn from the mold, and by providing the signal representing Q on the basis of the relation

6. Method as in claim 4 characterized by carrying out similar steps in different areas of the mold spaced apart along the circumference, and comparing the resulting skin thickness values with each other.

7. Method as in claim 4, wherein upper and lower mold wall portions are cooled by a common circulation, the coolant flowing vertically down in the mold wall, the temperature being measured in the coolant flow path.

8. Method as in claim 7, wherein the two zones extend respectively in upper and lower half of the mold, the heat ratio detecting step including measuring the temperature TE of the coolant as fed to the mold in an upper wall portion thereof, the temperature TA of the coolant as withdrawn from the mold in a lower wall portion thereof, and the temperature TM of the coolant in a central wall portion of the mold, and providing the signal representing K by forming in a measuring circuit the ratio TM -TE / TA - TM on the basis of the separate measuring of said temperatures.

11. In a method of ascertaining the skin thickness of a body solidifying in a mold wherein liquidous material is added to the mold and the body is withdrawn from the bottom of the mold, with solidified skin around a still liquidous core, the mold provided with liquid cooling, comprising: monitoring the heat transferred by the ingot to the mold inan upper portion of the mold and for a width portion thereof considerably smaller than the total circumference of the mold, and providing a signal representative thereof; monitoring the heat transferred by the ingot to the mold in a low portion of the mold and for a width portion similar to said width and providing a signal representative thereof; monitoring concurrently the total amount of heat Q transferred in both said portions, said monitoring steps including measuring temperature of the cooling liquid at various points in said portions as well as the rate of flow of said coolant as effective in said portions; and processing said signals and a signal representation of the quantity Q to provide an indication of skin thickness s to be expected on the basis of the functional relation

12. In a method of ascertaining the skin thickness of a body solidifying in a mold wherein liquidous material is added to the mold and the body is withdrawn from the bottom of the mold, with solidified skin around a still liquidous core, the mold provided with liquid cooling, comprising: monitoring the vertical temperature differential of the cooling liquid in an upper portion of the mold and for a width portion thereof considerably smaller than the total circumference of the mold; monitoring the vertical temperature differential of the cooling liquid in a lower portion of the mold and for a width portion also considerably smaller than the total circumference of the mold; providing a signal representation of the ratio K of said temperature differentials, and on a running basis; measuring the surface temperature To of the body as withdrawn from the mold; and providing a signal representation of skin thickness by processing said ratio signal K and a signal representing measured surface temperature, on the basis of the two functional relations