US3718024A

US3718024A - Apparatus including a fluidized bed for cooling steel rod through transformation

Info

Publication number: US3718024A
Authority: US
Inventors: V Vitelli
Original assignee: Morgan Construction Co
Current assignee: Siemens Industry Inc
Priority date: 1971-02-12
Filing date: 1971-02-12
Publication date: 1973-02-27
Anticipated expiration: 1990-02-27

Abstract

Apparatus and method are disclosed for the controlled cooling of steel rod, in a system which includes at least two cooling sections, at least one of which has a fluidized bed cooling means, each having means to control the rate of cooling therein. Several such control means are disclosed; among them are means to vary the fluidizing gas flow rate, means to recycle the fluidizing gas, means to preheat the fluidizing gas, means for converting a section from fluidized bed cooling to forced- or still-air cooling, and heat exchange means in contact with the cooling medium. The employment in such a system of plural cooling sections and such control means obtains a controlled-cooling method having a great latitude of cooling rates and of combinations thereof, and provides a system capable of handling a wide range of rod types, both plain carbon and alloy steels, and of producing a wide variety of end products.

Description

United States Patent [191 Vitelli [111 3,718,024 51 Feb. 27, 1973 APPARATUS INCLUDING A FLUIDIZED BED FOR COOLING STEEL ROD THROUGH TRANSFORMATION [75] Inventor: Vito J. Vitelli, Shrewsbury, Mass.

[73] Assignee: Morgan Construction Company,

Worcester, Mass.

[22] Filed: Feb. 12, 1971 211 App]. No.: 114,999

[52] US. Cl ..72/201, 140/2, 266/3 R [51] Int. Cl. ..C21d 9/56, B21c 1/00 [58] Field of Search ..72/201, 200, 202, 342, 364;

3,615,083 10/1971 Feinman et a1. ..266/3 R Primary Examiner--Richard J. Herbst Assistant Examiner-E. M. Combs AttorneyRussell & Nields 57] ABSTRACT Apparatus and method are disclosed for the controlled cooling of steel rod, in a system which includes at least two cooling sections, at least one of which has a fluidized bed cooling means, each having means to control the rate of cooling therein. Several such control means are disclosed; among them are means to vary the fluidizing gas flow rate, means to recycle the fluidizing gas, means to preheat the fluidizing gas, means for converting a section from fluidized bed cooling to forcedor still-air cooling, and heat exchange means in contact with the cooling medium. The employment in such a system of plural cooling sections and such control means obtains a controlledcooling method having a great latitude of cooling rates and of combinations thereof, and provides a system capable of handling a wide range of rod types, both plain carbon and alloy steels, and of producing a wide variety of end products.

17 Claims, 19 Drawing Figures 'GAS FLOW PARTlCLE FLOW COOLING MEDIUM FLOW PATENTED 2 7 1375 saw 1 BF 5 INVENTOR. VITO J. VITELLI ATTORNEYS PATENTEUFEBN 197a

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APPARATUS INCLUDING A FLUIDIZED BED FOR COOLING STEEL ROD THROUGH TRANSFORMATION BACKGROUND OF THE INVENTION Apparatus and procedures have been devised for cooling steel rod through transformation to obtain a product having uniform properties throughout the length of rod. This procedure has been referred to as a controlled cooling, in that, besides the result of product uniformity, the process is adaptable to cooling a variety of steel compositions in a controlled manner.

In general terms, the process of controlled cooling includes forming steel rod immediately after hot rolling (and, if desired, after a preliminary water-cooling) into overlapping non-concentric rings upon a moving conveyor, and conveying the laid rod through a cooling zone in contact with ambient air forced at a mass flow rate most desirable for the particular steel grade being processed. At the end of the conveyor, after the rod has been transformed, the rod is formed into coils and collected. This process has been described in detail in many publications; see, e.g., Rolling and Controlled Cooling of Wire Rod, H. Beck et al., Wire Journal, Dec. 1969, pp. 54-59; and D. W. McLean et al., U.S. Pat. Nos. 3,231,432; 3,320,101; and 3,390,432.

Controlled cooling by means of forced, ambient air is limited at its upper end by the maximum rate of heat exchange available with such a medium. In the general case, this limitation presents no problems, for the cooling rate is adequate to provide an excellent product suitable immediately for wire drawing or other cold working. In certain instances, however, for example with higher carbon steels or where exceptionally high tensile strengths, in combination with maximum ductility, are desired, it would be beneficial to extend the cooling rate beyond that attainable with Forced, ambient air. Furthermore, an increased controlled cooling rate could avoid in certain instances the need of submitting the rod to a lead patenting step, if such a further treatment would otherwise be needed.

Fluidized beds have been suggested as the cooling medium in controlled cooling of steel rod in order to achieve cooling rates in excess of those attainable with air as the medium. See H. Geipel et al., U.S. Pat. Nos. 3,492,740 and 3,506,468; and H. L. F. Bond, U.S. Pat. No. 3,445,100.

The use of a fluidized bed as the cooling medium has achieved the desired objective of increasing the cooling rate. There is a concomitant problem, however, in that the resulting cooling rate is for most purposes too high, generally well in excess of 30 F. per second (on the average through transformation). Since the comparable rates for controlled cooling with forced air are less than about F. per second, it is evident that there remains a significant gap between forced air and fluidized bed with respect to the cooling rates now available for controlled cooling.

It is, therefore, a major objective of this invention to provide apparatus and a process for the controlled cooling in that region between the cooling rates of forced air and fluidized beds, namely in the cooling rate region of about 20 to 30 F. per second. It is a further objective to provide a cooling system capable of rates far wider than heretofore observed in a single system, overlapping the above region as well as the forced air or fluidized bed ranges, or both. One of the results achieved by such a system of wide latitude is, of course, the capability of treating a wide range of steel rod compositions for a wide diversity of end uses. It is, accordingly, a further objective to provide a controlled cooling system capable of cooling steel rod, throughout a wide range of composition, to make the rod suitable for immediate cold working without the need of further patenting or similar treatment steps.

BRIEF DESCRIPTION OF THE INVENTION In the present apparatus a controlled-cooling system is provided having at least two cooling sections, each having variable means to control the rate of cooling. At least one such section is provided with a fluidized bed and variable means to control the cooling rate thereof.

The apparatus includes means for forming hot-rolled steel rod into overlapping non-concentric rings upon a conveyor. The conveyor includes means to convey such rings through the controlled-cooling system, and is preferably a single, endless conveyor extending in direct sequence through both cooling sections.

Each of the cooling sections has variable means of controlling the heat transfer rate therein and consequently the rate of cooling the rod rings. In a fluidized bed, the cooling rate may be varied by adjusting the flow rate of the fluidizing gas, e.g. air, nitrogen, reducing gas or the like. The cooling rate may also be varied by means of a variable-flow heat exchange medium circulating through the cooling medium, as described more fully hereafter, or by using a heat fluidizing gas, preferably with at least a part recycled to the same or a different cooling section. Another means is to convert temporarily the section from a fluidized bed cooling medium to a forced or still gas cooling medium. It is also possible to alter the residence time in a section by changing the conveyor speed, although'this approach is not preferred, since normally operation is improved by maintaining residence time as constant as possible.

In the present apparatus the above means of varying the cooling rate are preferably coupled with control means including, for example, means to determine, directly or indirectly, rod temperature at one or more selected points throughout the cooling sections, or to determine the point at which transformation is complete.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view in side elevation of a controlled cooling apparatus, in cross-section, which diagrammatically shows an embodiment having two cooling sections, each with a fluidized bed and means for varying the cooling rate.

FIG. 2 is an end-elevation view in cross-section, of a cooling section of an embodiment of the present controlled cooling apparatus, viewed in the direction of conveyor travel, as shown along lines 2-2 of FIGS. 3 and 4.

FIG. 3 is a side-elevation view in cross-section of the apparatus shown in FIG. 2, taken along lines 3--3 of FIGS. 2 and 4.

FIG. 4 is a plan view in cross-section of the apparatus shown in FIGS. 2 and 3, taken along lines 4-4 thereof.

FIG. 5 is a curve showing the temperature of the fluidized bed particles, at an arbitrary scale, across line 5-5 shown in FIG. 2.

FIGS. 6 and 7 are an end-elevation view in cross-section of an alternative embodiment employing means to change from fluid-bed cooling to forcedor still-air cooling, FIG. 6 showing the apparatus during fluid-bed cooling and FIG. 7, during air cooling.

FIGS. 8a-e are diagrammatic views of several embodiments of two-section cooling systems, showing five different combinations of means of cooling rate varia tion in the sections.

FIGS. 9a-e show the temperature-time curves, on arbitrary scale, for the respective cooling systems of FIGS. Sa-e.

FIG. 10 is a chart showing the curves of FIGS. 9a-e superimposed on a single graph.

FIG. 11 is a diagrammatic view in side elevation of a controlled cooling apparatus having two cooling sections and means for controlling or varying the cooling rate in each section, including a control circuit and means for sensing rod temperature in each section.

DETAILED DESCRIPTION OF THE INVENTION The present apparatus and method incorporate into a system for the controlled-cooling of steel rod novel means of cooling rate control, whereby greater latitude in cooling rates is possible than in prior systems. In order to appreciate the need for increased control over the rate of cooling steel rod through transformation, and to appreciate the variable means suitable in the present systems, these subjects will be discussed briefly here.

As is well known, steel assumes different allotropic forms, depending upon its temperature and composition, and undergoes transformation from one form to another where it is heated or cooled through critical temperature ranges. A critical temperature is that temperature at which a change in crystal or physical structure occurs. Above about I300-l670 F., depending upon the carbon content of the steeL-y-iron or austenite is stable. those of major concern for steel rod purposes when the austenite is cooled from a stable temperature to a temperature where it becomes unstable (again depending upon carbon content), when or ferrite, having a different atomic lattice and being essentially free of carbon, is formed while the residue austenite becomes richer in carbon. As cooling continues ferrite is formed until the carbon content of residual austenite reaches its point of saturation, about 0.8 percent, where both ferrite and a third component, iron carbide or cementite, an iron-carbon compound (Fe' C), are formed concurrently in a lamellar structure ofalternating ferrite and cementite known as pearlite. Thus the final structure consists theoretically of pearlite colonies embedded in ferrite at the grain boundaries. This process occurs only under very slow cooling, since the carbon atoms must migrate for the formation of ferrite and cementite. Diffusion of carbon is necessary because the carbon in that portion of the autenite which transforms to ferrite must migrate to the remaining austenite. Furthermore, when the remaining austenite is transformed to pearlite, the carbon must again migrate from the ferrite lamellae to adjacent cementite lamellae.

In practice the idealized conditions are not met. As cooling becomes more rapid, more of the grain-boundary ferrite becomes trapped within the pearlite colonies, the pearlite lamallae become finer, and the colonies, smaller. To an extent this is desirable, because the cooled product obtains improved properties, such as increased tensile strength and ductility. Generally speaking, the finer the pearlite lamallae and the less the grain-boundary ferrite, the better. But as the cooling becomes very rapid a temperature is reached at which carbon diffusion is prevented, and in effect, the carbon atoms are trapped in the wrong places. First, a different structure called bainite may form, and then, as cooling is made even faster, martensite is ultimately formed. These constituents are strong but too hard and brittle, and hence undesirable for wire drawing. Thus a common objective of heat treating steel rod is to cool through transformation as rapidly as possible but without formation of bainite or martensite.

Several methods of heat treating are employed for steel rod. Patenting is one method, which has been extensively employed for many years. In general, patenting involves re-heating the rod above the transformation temperature and cooling it, either in air or in a molten lead bath, through transformation. Air patenting employs a slower cooling rate than lead patenting, and the product of the latter is generally recognized as superior for most purposes. On the other hand, lead patenting is more cumbersome and expensive. Traditionally, depending on end use and the amount of cold reduction by wire drawing, certain steel rod is untreated, some is air patented and some is lead patented.

More recently the process known as controlled cooling has been employed to provide steel rod superior to air patented rod, and unique in several respects. In the controlled cooling method the steel rod is cooled through transformation immediately after being rolled. One decided advantage of such a single cooling is that the austenite grains of the just-rolled rod are substantially finer than those of the reheated rod for patenting, which results in a finer-grained structure after transformation and significantly greater uniformity along the rod length and across its width.

Attempts have been made to explain the process of transformation and the phenomena occuring in patenting, controlled cooling or similar heat treating methods. This effort has led to the preparation of transformation diagrams, which show graphically the points in time and temperature at which transformation starts and ends, for small steel samples under laboratory conditions, for both isothermal and continuous cooling. These diagrams also define those areas during transformation where the various constituents, i.e., ferrite, pearlite, bainite, martensite, are formed. See, e.g., Atlas to Heat Treatment of Steels, Stahleisen (Dusseldorf, 1961); and I-T Diagrams, United States Steel (Pittsburgh, 1963). It has been recognized, however, that the isothermal diagrams are not numerically valid to predict accurately optimum cooling rates for continuous cooling.

First, there exist countless steel and alloy grades and compositions, each exhibiting slightly or widely different transformation characteristics. Further, even with a single steel or alloy, the prior history of the rod, as evidenced for one example by the austenite grain size, significantly affects the transformation process. Finally, the end use of the rod is of great importance: an ideal cooling procedure for one use may be unsatisfactory for another, and the diagrams convey little information about product mechanical properties.

In any event, the present invention has for its main objective the provision of a system capable of a wide range of cooling rates, in a method of controlled cooling, and capable of operating at a preselected rate of cooling within a wide range available.

The present invention achieves its objectives by combining into apparatus and process several elements in a new combination capable of controlled cooling steel rod over a wider range of cooling rate and with more precise control than heretofore available. This result is due in part to an extensive control over those parameters which affect cooling. Cooling of steel rod can be represented by the following equation:

q=AhAT;

wherein q is the cooling rate; A is the area of heat exchange surface (theoretically the rod surface area); h is the heat transfer coefficient between the cooling medium and the rod; and A T is the mean temperature difference between the rod surface and the cooling medium.

The desire cooling rate, q, will of course depend upon the starting temperature and size of the rod, as well as upon the desired manner of cooling through transformation. Essentially, its optimum value is fixed by the given rod and by the desired physical properties. One advantage with the present system is that by means of two cooling sections the cooling rate can be different for each. Thus, the rate can be high in the first section to cool rapidly before transformation is initiated, and very low in a second section to approximate isothermal transformation and to avoid undercooling. On the other hand, for certain purposes, it may be preferable to have slow cooling in a first section, followed by very rapid cooling through transformation in a second section. In the present invention, the cooling rate can be preselected within a wide range, and controlled accurately in each of the two sections.

The heat exchange area, A, ideally is the rod surface area. This ideal can be approached in a controlled cooling by laying the rod on a suitable conveyor in the form of overlapping, non-concentric rings, as is now well known. In such a configuration only a small fraction of the rod surface is lost by contact with adjacent rod or conveyor surfaces, and it has been found that such losses do not substantially affect the uniformity of cooling to the extent of measurably affecting product properties. It is of course possible to increase even further the effective heat exchange area, for example by conveying the rod as an open helix, if one were willing to assume the added burden of more complex conveying means. In any case, variation of the rod configuration has little effect upon cooling rate, and the present invention is not restricted to any single rod configuration during cooling, although the form of overlapping, nonconcentric rings is preferred. Some variation of available area is possible by control of the spacing between adjacent, overlapping rings the greater the spacing the more exchange area open but this means is limited. The effective area, however, can be varied over a substantial range by changing the residence time in a cooling section, i.e., the conveyor speed.

The heat exchange coefficient, h, can be varied a number of ways. For fluidized bed cooling the coefficient is in the order of -140 BTU/ft -hr- F. as currently employed; for forced air cooling, about 20-30 BTU/ft -hr- F; and for free air, even lower. Employing certain methods, such as hereafter described, the coefficient for fluidized beds can be lowered to about 50, or even less. One method of varying the coefficient, in either a fluid-bed or forced-air system, is to change the air flow rate. Particularly as the fluidized particle size becomes larger than about 0.0015 ft., this means can achieve substantial variation of the coefficient in a fluid bed. The coefficient can also be varied by changing the particle size, density or, to a lesser extent, shape, but this alternative is less desirable because it would involve having at hand two or more particulate materials and means for substituting one for the other. Another method of variation is, of course, to change from a fluidized bed to a forcedor free-air cooling medium. Variation of the co-efficient by changing the fluidizing gas flow rate is preferred, however, particularly since this means gives a continuous variation.

The temperature differenceA T, between the rod surface and the cooling means, i.e., the fluidized bed or the forced or free air can be varied widely. This latitude of control means is, of course, in part due to the fact that the term is a difference between two temperatures. There is, however, a number of ways to control the two temperatures to vary the term. While it is possible to vary the temperature of the rod surface, for example by rapid precooling with water, ordinarily this temperature is an independent variable and given. The temperature of the cooling medium can be varied over a wide range by several means. With a fluidized-bed cooling medium either of the two constituents, the fluidizing gas or the particles, can be heated or cooled, although it is impractical to cool below ambient temperature. A preferred method of raising the temperature is to heat the fluidizing gas, preferably while recycling part or all of the gas removed from one or more of the cooling sections. The particles can be removed from the system and heated or cooled. Another preferred means is to circulate a heatexchange medium through the fluid bed in cooling pipes or coils. This heat-exchange medium can be used either to raise the temperature of the cooling medium, e.g., with steam or a molten metal such as lead, or to lower it, e.g., with cooling water. Furthermore, such a circulating heat-exchange medium permits ready control of the cooling medium temperature by adjusting its rate of flow.

The present apparatus and method are directed to a controlled-cooling system incorporating a wide latitude of cooling rates for steel rod. It is, of course, not necessary to adopt all or even most of the above mentioned means of control to obtain the advantages of the present system. In any case, however, the present system comprises at least two cooling sections capable of providing two different cooling rates, each section containing means to control the cooling rate therein. A preferred means of cooling rate control includes heating the cooling gas in at least one section, and varying the cooling gas flow rate, either together or alone.

Another preferred means is to circulate a heatexchange medium of variable flow through the cooling medium of at least one section either a cold fluid to induce rapid cooling or a heated fluid to retard the rate of cooling. Another preferred means is to vary the conveyor speed to control residence time in the cooling sections. A further preferred means employs apparatus for converting the cooling medium of a section from fluidized-bed to forced-air cooling, or the reverse.

With reference to the drawings, there is shown in FIG. 1 a preferred embodiment of the present invention incorporating many of the available control means in a single controlled-cooling system. Steel rod immediately after rolling, and at a temperature above transformation, is delivered to laying head 1 and formed into non-concentric, overlapping rings 2 upon conveyor 3. The rings 2 are conveyed through first cooling section 4 and then immediately through second cooling section 5, and finally to collector 6 where the rod is formed into a coil, The details of a suitable laying head 1 and collector 6 are well known to those in the art and are not shown here.

Particulate material 7, such as sand, to be fluidized is fed to first cooling section 4 through inlet chute 8 and inlet zone 9, as shown in FIG. 1 by open arrows. In section 4 the particles 10 are fluidized and come in contact with the rod rings 2 on the conveyor 3. Particles overflow from section 4 to section 5 through section divider 11, where they are also fluidized, and finally leave the system through exit zone 12 and chute 13. By means not shown the particles leaving the system may be treated to cool them and to remove fines before being recycled to inlet chute 8. The desirability and extent of such treatment would depend on the flow rate of the particles. Where particle flow is high, much of the system heat may be removed from the system without requiring external heat exchange to cool the particles, and it may be necessary to remove the tines in order to avoid particle elutriation in the fluidized bed. On the other hand, where the residence time of the particles is very high, and only a very small proportion removed, it may prove unnecessary or uneconomical to recycle the sand, particularly if that portion removed is predominately fines or other material to be discarded. Part of the particulate material is removed with the fluidizing gas, as described below. The particle flow through

cooling sections

4 and 5 has been shown as concurrent with conveyor 3; it is of course possible to have countercurrent flow as well, or some other equivalent process of feeding and removing particulate material.

Air or other suitable fluidizing gas is supplied through inlet 14 to blower 15 and through

lines

16 and 17 to cooling

sections

4 and 5, respectively. Upon entering the

cooling sections

4 and 5, the air is dispersed into air zones 18 and 19, and through grates 20 and 21 to the fluidized beds 10 in each section. If desired, air may be provided to outlet zone 12, to fluidize or assist the movement of the particles therein, via line 22 and valve 23. The air is provided at such a rate, controlled by

valves

24 and 25, to fluidize the particles in the two cooling

sections

4 and 5. Adjustment of the air flow rate to the cooling sections, as described above, is one means of varying the cooling rate of the fluidized bed therein. Furthermore, the air may also be preheated, by

suitable means not shown before it enters the cooling sections.

In the system shown in FIG. 1, several means of controlling the cooling rate in the two cooling sections are employed. As mentioned above, the entering fluidizing gas flow may be controlled by

valves

24 or 25, in order to vary the heat exchange coefficient in

section

4 or 5. It is also possible to raise the temperature of the fluidized bed and hence reduce the factor A T. This can be accomplished by heating the air delivered to line 14 by means not shown. If such preheating is desired, as an alternative or supplemental procedure, at least part of the fluidizing gas removed from the system through

lines

26, 27 or 28, via cyclone separators 29, 30 and 31, is recycled to the gas inlets. This is accomplished in the illustrated system by closing, partially or completely,

valve

32 or 33, opening

bypass valve

34 or 35, and operating recycle pump 36 to deliver recycle gas to line 16 or 17 via line 37. If desired, the recycle gas may be heated before delivery to thecooling sections by heater 38. It is, of course, possible to change the configuration or operation of the above lines, pumps, valves and heater in order that the recycle be changed, for example to recycle to only a single cooling section or from one section to another, or to heat the recycle gas to one section only, or the like. Such modifications are not shown here for the sake of simplicity in presentation.

Further control over the temperature of the fluidized bed may be achieved in the system shown byproviding heat-exchange means 39 or 40 to the cooling sections, controlled by valves 41 or 42. In the ordinary case the bed temperature can be reduced by circulating cooling water or the like through the heat exchange means. But it is also possible to raise the bed temperature, for example to approach an isothermal transformation of the rod, by the use of a heat-exchange medium at high temperature, such as superheated steam or, preferably, a molten metal like lead. The heat exchange means 39 and 40 are shown diagramatically as vertically oriented tubes, but other means of the countless available may also be used, provided that entrainment thereon of the fluidized particles and entanglement of the rod are avoided.

FIGS. 2-4 show in greater detail a preferred embodiment of a part of the present invention, namely a portion of a cooling section. As in the system shown in FIG. 1, over-lapping,non-concentric rod rings 2 are conveyed through the cooling section portion 50 along conveyor 3, in the direction shown by arrows in FIGS. 3 and 4. Fluidizing gas, also shown by arrows in FIGS. 2 and 3, is supplied through inlet 51 into distribution chamber 52 and through grate 59 up into the cooling section 56 in sufficient flow rate to suspend the particulate material as a fluidized bed 53, ultimately to leave the section by outlet 54. Heat-exchange medium, shown by dashed arrows in FIG. 3, is provided in

means

55 and 56, consisting of vertically oriented tubes 57 having exchange fins 58.

The particular orientation of heat-exchange means 55 and 56 shown in FIGS. 2-4 is preferred for several reasons. The vertical orientation of tubes 57 and fins 58 avoids entrainment of the fluidized particles and minimizes any possible entanglement with the rod rings 2.

Another advantage of this orientation along the conveyor edges is illustrated in FIG. 5, which is a diagram of the temperature gradient of the fluidized bed taken along line -5 in FIG. 2. As can be seen, the bed temperature, T decreases as the heat exchange means are approached, and increases to a maximum at the center of the section, i.e., at the position of the center of the rings 2 on conveyor 3, see FIG. 2. This is due to the principle that, since heat flows from the rod rings 2 to the heat exchange means 55 and 56 beyond the ring edges and since under fundamental thermodynamics heat can flow only from a point of higher temperature to a point of lower temperature, the bed temperature about the overlapping rings must be highest at or near the center between means 55 and 56 and lowest at the ring edges, as shown in FIG. 5. Hence it follows that the temperature difference A T between the bed (T,,) and the rod surface (T assumed to be constant) is less at the center of the rings (A T than at the edges (A T as shown in FIG. 5.

This aspect of the invention provides a very important benefit. Due to the configuration of the overlapping rod rings the rod mass (and hence heat content) is greater at the edge of the conveyor than at the center, as best shown in FIG. 2. This inevitable imbalance of rod mass across the conveyor requiring for uniformity a greater removal of heat at the edges, is compensated for in the preferred embodiment shown, since the A T is highest at the edge where a higher cooling rate, or q, is desired. The extent of such an advantage will depend upon how much of the rod heat is removed through the heat exchange means at the rod edges, in comparison to the amount removed by the fluidizing gas and particles. The higher the proportion removed by the former means, the greater the above advantage. This factor then will depend upon a number of other factors, and for this reason the scale of FIG. 5 is arbitrary, and in certain cases the advantage due to differences between A T, and A T, may be too small to be significant, as could be the case where T,, assumes the shape shown by the dotted line.

FIGS. 6 and 7 illustrate in part a preferred embodiment of the present system whereby the cooling section can be converted from fluid-bed cooling to forced or free-air cooling as required. With fluid-bed cooling shown in FIG. 6, as in the above systems, fluidizing gas is provided through valve 60 and inlet line 61 to distribution zone 62, and through grate 63 to fluidize the particles 64 in the cooling section 65. As above, the overlapping rod rings 2 are advanced through the cooling section 65 on conveyor 3. When it is desired to convert the section to forced or free-air cooling, valve 60 may be closed and the particles will form a compact, still bed 66, as shown in FIG. 7. For forced-air cooling, fan 67 is turned on to blow air through valve 68 and nozzles 69 into cooling section 65. To convert back to fluid-bed cooling, the above steps are reversed, once again fluidizing the particles. Other means of converting a cooling section from fluid-bed to forcedor free air cooling and the reverse are of course possible.

FIGS. 8a-e show diagrammatically five controlled cooling systems, each embodying the present invention but employing different combinations of cooling sections for the different cooling rate results and objectives in each case. In each system rod rings 2 in overlapping, non-concentric relationship are transported by conveyor 3 through a first cooling section 4 and immediately thereafter through a second cooling section 5, and upon leaving the cooling systems collected at 6 in bundles.

In the first cooling sections 4 of the systems 8a, b and d, the cooling rates are substantially the same for identical rods, since the sections contain the same cooling means, a fluidized bed of sand (s) fluidized by air (a) from blower l5, and cooled by circulating cooling water (cw) through heat exchange means not shown. In the first cooling section of system 8c the cooling rate is somewhat slower, and the means include a fluidized bed of sand without the cooling water heat exchange means. In the first cooling section of system 8e the cooling rate is slower still, since a fluidized bed is not employed but the cooling means is forced air driven by blower 67.

The second cooling section of system 8a obtains a relatively moderate cooling rate by the use of a fluid sand bed (the sand being carried over from the first section) fluidized by recycled and heated air, driven by blower 36 and heated by heater 38. In system 8b the cooling in the second section is substantially the same as the first sections of systems 8a, b and d, namely a bed of sand (carried over from the first section) fluidized by ambient air and cooled with cooling water, except, of course, that the rod enters the second section of 8b, other things being equal, at a lower temperature than that which enters the first section of 8a, b or d. The second cooling section of system is similar to the first section of 8e and has no fluidized bed but forced air cooling only, the sand in the first section of 8c being removed at its end by outlet '70. The second cooling section of system 8d achieves little or no actual cooling of the rod but provides substantially isothermal treatment. A fluidized bed is employed, and recycled, heated air is used (as in 8a), and in addition molten lead (Pb) is circulated through a heat-exchange means not shown, in order to maintain the bed temperature as close as possible to the temperature of isothermal transformation desired. In the second cooling section of 8e a rapid cooling by means of a fluid bed with ambient air and water cooling, similar to the means in the first section of 8a, b and d and in the second section of 8b is employed. The sand is introduced at inlet '71, since none is employed in the first section of system 8e.

FIGS. 9a-e are curves on an arbitrary but identical scale showing the theoretical rod temperature as the rod passes through the cooling sections of systems 8a-e, respectively. The juncture of the two sections are shown at the dashed vertical line on the time scale, it being assumed for simplicity that the conveyor rate is identical for each of the five examples. It should be noted that on these curves the effect of heat of transformation is not shown, which effect can cause a momentary increase in rod temperature or at least some retardation in cooling rate as the rod passes through transformation. The reasons this effect is not shown are that, first, the point of its occurence and its degree are not constant, depending upon the steel or alloy, austenite grain size, cooling rate and the like; second, that the effect if shown would only complicate the disclosure; and third, it is not necessary to show it in order to present the principles intended to be shown in FIGS. 9a-e.

FIG. 9a illustrates the rapid initial cooling and less rapid subsequent cooling in system 8a. FIG. 9b illustrates the rapid cooling throughout system 8b. FIG. 90

illustrates the faster initial cooling and relatively slow subsequent cooling in system 8c. FIG. 9d shows the rapid initial cooling and substantially isothermal subsequent treatment of system 8d. And FIG. 92 shows the rather slow cooling initially, followed by rapid cooling in the second section of system 8e.

FIG. 10 shows the curves of FIGS. 9a-e superimposed in order to better illustrate the cooling rate differences between systems Ba-e. It can be seen readily that curve b has the highest overall cooling rate, while curve c has the lowest average cooling rate. The distinctive characteristics of curves a, d and e are also apparent.

It should be apparent that the present invention is suitable for further combinations of cooling means and means to control cooling rates than are shown in the five examples above. It should not be inferred by the number of examples given, which is believed to be reasonable for the purpose of this disclosure, that the invention does not extend to other examples embodying its substance. Further, it is not intended that the systems shown are necessarily the best, although one or more may be the best for one or more particular purpose, since for certain rod characteristics or rod properties even a system not shown may prove best. But to illustrate systems adequate to cover all of the best for all possible purposes would be to require an unreasonable disclosure of countless examples.

FIG. 11 shows a control circuit for the operation of a controlled cooling system embodying the principles of the present invention in two, tandem cooling sections, the cooling rates of which are continuously monitored for the purpose of controlling the cooling. In the system shown, rod rings 2 are transported on conveyor 3 through first cooling section 4 and second cooling section 5, each containing a fluidized bed cooling means. Sand for the fluidized beds employed is introduced at inlet 8 and removed from outlet 13. Ambient air is used to fluidize the cooling beds, in the case where maximum cooling is desired, and is introduced by blower 15 through

inlets

16 and 17 leaving the system via

lines

27 and 28. Cooling water or the like may be used as a heat-exchange medium by means not shown, through

inletvalves

80 and 81. The internal cooling and conveying means are not shown, for simplicity.

Several means for controlling the cooling rate of the system of FIG. 11 are employed. The fluidizing air flow rate to the two sections may be controlled by

valves

82 and 83. The cooling water flow rate may be controlled by

valves

80 and 81. Partor all of the air leaving the second section through line 28 may be recycled, thereby heating the inlet air to the section, by closing control valve 84 in line 28 and opening control valve 85 in recycle line 86, while correspondingly adjusting control valve 83 to lower incoming ambient air, and the recycle air may be heated by controlled heater 87. Another control means is variable drive motor '88, which can change the speed of conveyor 3.

The cooling rates of both cooling

sections

4 and 5 are detected indirectly by temperature sensing means 89 and 90, each located so as to detect the rod temperature, directly or indirectly, at the end of the section.

These means may be thermocouples, coming in contact with the rod top surfaces in sequence, photoelectric devices to sense the visible or infra-red light given off by the rods, or any other suitable devices. The sensing means 89 and 90 convey information to temperature determining means 91 and 92, which in turn is conveyed to control

computers

93 and 94. The

computers

93 and 94 are programmed to maintain the temperatures at points 89 and 90 at any level within the capacity of the cooling system. This is done by

lines

95 and 96, which control

valves

82 and 83;

lines

97 and 98, which control valves and 81;

lines

99 and 100, which control valves 84 and

lines

101 and 102, which control motor 88; and line 103 which controls variable heater 87. The computers are programmed so that should the temperature in either section depart from the desired level, one or more of the control meansi.e.,

valves

80 or 82 in section 4;

valves

81, 83, 84 and 85 and heater 87 in section 5; and motor 88 are employed to raise or lower the rod temperature to the desired level.

The control system shown in FIG. 11 is, of course, simplified and can be modified by those skilled in the art to increase or reduce its effectiveness or versatility. Among the possibilities: recycle air may be employed in section 4; the inlet air could pass through a control heater; a heating heat-exchange medium may be employed in one or both sections; or the sand flow rate may be controlled.

I claim:

1. Apparatus for the controlled cooling through transformation of steel rod comprising:

conveying means for receiving steel rod immediately after hot rolling at a temperature above transfor' mation and for conveying said rod in a form with a major portion of its surface exposed through said apparatus;

at least two cooling sections in direct sequence for cooling said rod through transformation while upon said conveying means, each cooling section having control means associated therewith for controlling therein the rate of cooling of said rod upon said conveying mean, and at least oneof said cooling sections having converting means to provide a cooling medium of a fluidized bed in intimate contact with said rod therein or to provide a cooling medium of forced air, wherein said converting means includes means to convert the mode of said section between a fluidized-bed and a forced-air cooling medium; and

collecting means for receiving said rod from said conveying means after cooling through transformation.

2. The controlled cooling apparatus of claim 1, wherein said control means of at least one of said cooling sections comprises means for providing a heated gas as a cooling medium.

3. The controlled cooling apparatus of claim 2, wherein said heated gas is provided at least in part by recycling at least a portion of gas after contact with said rod from at least one of said cooling sections.

4. The controlled cooling apparatus of claim 3, wherein said recycled portion of the forced gas is removed from one cooling section and recycled to another cooling section.

5. The controlled cooling apparatus of claim 1, wherein said control means of at least one of said cooling sections comprises means to vary the fluidizing gas flow rate in said fluidized-bed cooling medium.

6. The controlled cooling apparatus of claim 1, wherein at least one cooling section employs a forced gas as a cooling medium in contact with said rod, and wherein said fluidizing gas is preheated by recycling at least a portion of the forced gas or fluidizing gas.

7. The controlled cooling apparatus of claim 1, wherein a heat exchange medium is circulated through heat exchange means along the edges of said conveying means, whereby when said heat exchange medium cools said cooling medium a lower temperature difference between the rod and the cooling medium occurs at the center of the conveying means remote from said heat exchange means than at the edges adjacent same.

8. The controlled cooling apparatus of claim 1, wherein said control means includes means for varying the speed of said conveying means.

9. Apparatus for the controlled cooling through transformation of steel rod immediately after hot rolling, comprising rod coiling means for receiving steel rod immediately after hot rolling and for laying the rod upon a surface in the form of rings before transformation is initiated;

conveying means for receiving the rod coils from said coiling means and conveying it through means for cooling the rod through transformation, which cooling means includes at least two cooling sections employing a forced-gas cooling medium, each having control means associated therewith for controlling therein the rate of cooling said rod rings comprising means to vary the rate of flow of said forced gas, and at least one of said cooling sections having means for converting the mode thereof between a forced-air cooling medium and a fluidized-bed cooling medium in intimate contact with said rod therein; and

10. The controlled cooling apparatus of claim 9, wherein said control means of at least one of said cooling sections comprises means for providing a heated gas as a cooling medium.

11. The controlled cooling apparatus of claim 10, wherein said heated gas is provided at least in part by recycling at least a portion of the forced gas after contact with said rod from at least one of said cooling sections.

12. The controlled cooling apparatus of claim 9, wherein a heat exchange medium is circulated through heat exchange means along the edges of said conveying means, whereby when said heat exchange medium cools said cooling medium a lower temperature difference between the rod and the cooling medium occurs at the center of the conveying means remote from said heat exchange means than at the edges adjacent same.

13. Apparatus for the controlled cooling through transformation of steel rod immediately after hot rollin ,c mprising ro coiling means for receiving steel rod immediately after hot rolling and for laying the rod upon a surface in the form of rings before transformation is initiated;

conveying means for receiving said rings from said coiling means and conveying it through means for cooling the rod through transformation, which cooling means includes at least two cooling stations employing a forced gas cooling medium, at least one section having a fluidized-bed cooling medium in intimate contact with the rod therein and means within said fluidized-bed cooling medium for circulating a heat exchange medium substantially along the edges of said conveying means and immediately adjacent said rings and solely outside said conveying means and said rings; and

14. The controlled cooling apparatus of claim 13, wherein at least one cooling section employs a forced gas as a cooling medium in contact with said rod, and wherein said heated gas is provided at least in part by recycling at least a portion of the forced gas after contact with said rod from at least one of said cooling sections.

15. The controlled cooling apparatus of claim 14, wherein said recycled portion of the forced gas is removed from one cooling section and recycled to another cooling section.

16. The controlled cooling apparatus of claim 13, wherein at least one other cooling section employs a forced gas as a cooling medium in contact with said rod, and wherein said fluidizing gas is preheated by recycling at least a portion of the forced gas or fluidizing gas.

17. The controlled cooling apparatus of claim 13, wherein at least one cooling section includes means for converting the mode of at least one cooling section between forced-air cooling and fluidized bed cooling.

Claims

1. Apparatus for the controlled cooling through transformation of steel rod comprising: conveying means for receiving steel rod immediately after hot rolling at a temperature above transformation and for conveying said rod in a form with a major portion of its surface exposed through said apparatus; at least two cooling sections in direct sequence for cooling said rod through transformation while upon said conveying means, each cooling section having control means associated therewith for controlling therein the rate of cooling of said rod upon said conveying mean, and at least one of said cooling sections having converting means to provide a cooling medium of a fluidized bed in intimate contact with said rod therein or to provide a cooling medium of forced air, wherein said converting means includes means to convert the mode of said section between a fluidized-bed and a forced-air cooling medium; and collecting means for receiving said rod from said conveying means after cooling through transformation.

9. Apparatus for the controlled cooling through transformation of steel rod immediately after hot rolling, comprising rod coiling means for receiving steel rod immediately after hot rolling and for laying the rod upon a surface in the form of rings before transformation is initiated; conveying means for receiving the rod coils from said coiling means aNd conveying it through means for cooling the rod through transformation, which cooling means includes at least two cooling sections employing a forced-gas cooling medium, each having control means associated therewith for controlling therein the rate of cooling said rod rings comprising means to vary the rate of flow of said forced gas, and at least one of said cooling sections having means for converting the mode thereof between a forced-air cooling medium and a fluidized-bed cooling medium in intimate contact with said rod therein; and collecting means for receiving said rod from said conveying means after cooling through transformation.

13. Apparatus for the controlled cooling through transformation of steel rod immediately after hot rolling, comprising rod coiling means for receiving steel rod immediately after hot rolling and for laying the rod upon a surface in the form of rings before transformation is initiated; conveying means for receiving said rings from said coiling means and conveying it through means for cooling the rod through transformation, which cooling means includes at least two cooling stations employing a forced gas cooling medium, at least one section having a fluidized-bed cooling medium in intimate contact with the rod therein and means within said fluidized-bed cooling medium for circulating a heat exchange medium substantially along the edges of said conveying means and immediately adjacent said rings and solely outside said conveying means and said rings; and collecting means for receiving said rod from said conveying means after cooling through transformation.