US3410734A - Quench system - Google Patents

Description

H. L. TAYLOR 3,410,734

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NOV. 12, 1968 H, L, TAYLOR 3,410,734 v QUENCH SYSTEM 1965 5 Sheets-Sheet Nov. 12, 1968 Filed Jan. 18, 1965 H. L. TAYLOR QUENCH SYSTEM 5 Sheets-Sheet 5 United States Patent O 3,410,734 QUENCH SYSTEM Harold L. Taylor, Hammond, Ind., assignor to Inland Steel Company, Chicago, Ill., a corporation of Delaware Continuation-impart of application Ser. No. 153,834,

Nov. 21, 1961. This application Jan. 18, 1965. Ser.

11 Claims. (Cl. 148-143) ABSTRACT OF THE DISCLOSURE A method and apparatus for drastic quench-ing of metal strip (eg. low carbon steel strip to obtain martensite) by passing the heated strip downwardly through a restricted quench channel having high velocity quench liquid flowing through the channel, either countercurrently or concurrently, and with submerged streams of quench liquid, preferably sheet-like sprays, being directed perpendicularly against the opposite surfaces of the strip adjacent to but below the liquid surface at the upper end of the channel. Sufficient turbulence is created at the strip surfaces to avoid vapor accumulation while the liquid surface at the upper end of the `channel remains smooth and non-turbulent at the location where the -strip initially contacts the quench liquid. Uniform quenching of the strip is obtained without excessive distortion of the quenched strip. Pressure rolls may be provided in the channel to assist in maintaining strip atness.

This invention relates to a novel method and apparatus for continuously quenching metal strip.

The quench system of the present invention is applicable to the continuous quenching of metal strip generally, but the invention is particularly advantageous and useful in t-he quenching of steel strip to obtain a microstructure which is at least partially martensitic and preferably fully martensitic.

The principal constituents of steel which determine its properties are ferrite and cementite. At a relatively high temperature, which is dependent upon the carbon content, steel exists in the form known as austenite which is a solid solution of carbon or cementite in ferrite. When steel is cooled slowly from a high temperature at which austenite is stable, the ferrite and cementite precipitate together in a characteristic lamellar structure known as pearlite. However, dependent upon the rate of quenching and other factors, the transformation from austenite to pearlite proceeds through a series of different microstructures. The low temperature decomposition product in the transformation of austenite upon cooling is martensite which is a body-centered tetragonal structure in which the carbon atoms are thoroughly dispersed. Martensitic steels are characterized by high tensile and yield strengths.

Plain carbon steels of relatively high carbon content and certain alloy steels, particularly those containing hardenability agents such as boron, are more easily quenched to a martensitic structure, but the plain carbon steels of relatively lowcarbon content are considerably more diicult to quench to martensite. As will be understood from the customary isothermal transformation diagrams, plain carbon steels of low carbon content (.03- .25 wt. require extremely rapid quenching in order to achieve a substantially fully martenstic structure. In general, it may be stated that low carbon steel strip must be quenched from the austenitizing temperature to below the temperature for the start of martensite formation in from about .l to about .4 second. In addition, the quenching must be accomplished uniformly so as to obtain a uniform microstructure and so as to avoid excessive warpage 3,410,734 Patented Nov. 12, 1968 or distortion of the strip. Heretofore, no satisfactory means has been known to obtain this result.

Accordingly, the broad object of the present invention is to provide novel and improved means for rapidly and continuously quenching a metal strip.

A more particular object of the invention is to provide novel continuous means for rapidly and uniformly quenching low carbon steel strip so as to obtain a martensitic microstructure and acceptable strip tlatness.

A further object of the invention is. to provide a novel and improved method for achieving the aforementioned results.

Another object of the invention is to provide a novel and improved apparatus for achieving the aforementioned results.

Other objects and advantages of the invention will become apparent from the subsequent detailed description taken in conjunction with the accompanying drawing, wherein:

FIG. l is a schematic diagram of a continuous heat treating and quenching line for the production of martensitic steel strip;

FIG. 2 is an enlarged vertical `sectional view of the quench apparatus of the line shown in FIG. l;

FIG. 3 is a vertical sectional view taken along the line 3-3 of FIG. 2;

FIG. 4 is a horizontal sectional view taken along the line 4-4 of FIG. 3;

FIG. 5 is an enlarged fragmentary :internal view taken along the line 5-5 of FIG. 4;

FIG. 6 is an enlarged cross-sectional view taken along the line 6-6 of FIG. 5;

FIGS. 7 and 8 are fragmentary views similar to FIG. 2 but showing modifications of the apparatus;

FIG. 9 is a schematic view showing another embodiment of the quench apparatus of the invention;

FIG. l0 is a schematic view showing still another embodiment of the quench apparatus; and

FIG. 11 is a fragmentary view similar to FIG. l0 but showing a further modication.

The quench system of the present invention is illustrated in FIG. l as embodied in a continuous heat treating and quenching line for making martensitic steel strip. As more fully described in the copending application by William H. McFarland, Serial No. 398,626 tiled Sept. 23, 1964, the resulting martensitic steel strip may be tin plated, galvanized, or aluminum coated, whereby a coated steel product of exceptionally high tensile strength is obtained because of the microstructure and Without the development of poor ductility and a high degree of anisotropy which are characteristic of severely cold worked products.

The steel employed as the starting material is plain carbon steel having the following composition range (wt. percent: carbon .O3-.25, manganese .20-.60, phosphorus .05 max., sulfur .03 max., and the balance iron with residual elements in the usual amounts. Preferably, the steel strip starting material is in work hardened or as-cold reduced condition, and although the gauge of the strip will usually and preferably be within the range of from about .002 to about .050 inch, the invention in its broadest aspect is also applicable to steel strip having a thickness as low as about .0002 inch andl as high as about .100 inch.

As seen in FIG. l, steel strip 10 is fed from a payoff reel 11 through a bridle 12 and a looper 13 to a conventional cleaning and rinsing step 14 in which the residual rolling oil is removed. For example, an alkaline cleaning medium may be used either with or without electrolytic means.

The cleaned strip then passes through the usual roll system and downwardly through a furnace 15 where the steel strip is heated to a uniform temperature above the A1 critical point so that the steel is at least partially austenitized. This temperature may range from about 1330 F. to as high as about 2l00 F., dependent upon the carbon content, but from a practical standpoint effective results may be obtained within the range of from about 1330 F. to about 1750o F. In order to obtain a fully martensitic product the steel strip must be heated above the A3 critical point, i.e., to a temperature within the range of from about 1525 F. to about 2100o F. and particularly within the range of from about 1525 F. to about 1750 F.

Immediately upon leaving the furnace 15 the heated strip passes into a quench system 16 (comprising the subject matter of the present invention and more fully described below) where the strip is rapidly quenched to ambient or room temperature so as to obtain at least a partially martensitic microstructure. Preferably, the strip is quenched at a rate in excess of the critical cooling rate so that substantially all of the austenite is transformed to martensite. The oxide scale formed during quenching is removed from the surface of the strip by pickling in an acid dip 17, and after passage through another looper 18 and bridle 19 the martensitic steel strip is recoiled on a take-up reel Z0.

As shown in FIGS. 2 to 4, the quench system 16 comprises a tank 30 provided with a drain 30 and a strip exit chute 31 and containing a sinker roll 32 journaled at the sides of the tank 30 by means of water seaed bearings 32. Water, or other quench liquid, is supplied continuously to the tank 30 through an inlet 33. Extending upwardly from the tank 30 is an elongated conduit section 34 of rectangular cross-section which provides a restricted quench channel 35. Quench water flows upwardly through the conduit 34 and spills over the upper edge into a trough 36 having a rectangular cross-section and surrounding the conduit 34. An upright rectangular baffle or weir 37 extends upwardly from the bottom of the trough 36 surrounding the conduit 34 and terminating below the upper edge of the conduit 34. Extending downwardly from the outlet end of the furnace 15 is a tubular connecting or seal section 33 the lower end of which extends into the trough 36 below the upper edge of the weir 37 and disposed between the conduit 34- and the weir 37. Effluent water fiows over the upper edge of the weir 37 and is discharged at one end of the trough 36 through drain lines 39 extending from the bottom thereof.

Since the water level in the space between the weir 37 and the conduit 34 is determined by the height of the weir 37, it will be recognized that the lower end of the connecting or seal section 38 is sealed by the water confined in the rectangular weir 37 so as to prevent infiltration of air into the furnace 15. If desired, a reducing or other non-oxidizing gas may be supplied to the section 38 (by means not shown) for passage upwardly through the furnace 15, thereby preventing oxidation of the strip. A plurality of View ports 46 and 40 are provided in the tubular section 38 and the conduit 34, respectively, to permit observation of the quench action.

Closely adjacent the upper end of the conduit 34, a plurality of submerged jet or spray units 41 (described more fully below) are provided in the opposite walls of the conduit 34 for directing streams of liquid toward opposite sides of the strip 10 across the entire width thereof. In the 4embodiment shown in FIGS. 2 to 4, two superimposed units 41 are mounted at each side of the strip 10, but any desired number of such units may be used. Each unit 41 is supplied with water or other quench liquid by means of a supply conduit 42 having a control valve 43 and communicating with a main header 44.

As best seen in FIGS. and 6, each spray unit 41 comprises a pair of elongated angle members (FIG. 6) welded together to provide an elongated box-like structure having a front wall 46, a rear wall 47, an upper wall 48, and

a bottom wall 49. End closure plates 51 (FIG. 5) are secured to the opposite ends of the box-like structure. The quench-liquid supply conduit 42 extends into an opening in the rear wall 47, and an elongated discharge slot 52 is provided in the front wall 46 extending substantially the entire length of the unit 41 and across the entire width of the strip 10.

The interior of the enclosure is provided with a pair of elongated baffles 52 and 53 extending between the end plates 51 in transversely spaced relation between the front wall 46 and the rear wall 47. As seen in FIG. 6, the baffle 52 extends from the upper wall 48 and is spaced slightly from the bottom wall 49, as at 52, whereas the bafiie 53 extends upwardly from the bottom wall 49 and is spaced slightly from the upper wall 48, as at 53.

A pair of elongated adjustable orifice strips 54 are mounted on the outside of the front wall 46 by a means of a plurality of screws 56 extending through transverse slots 57 in the strips 54 into threaded openings in the front wall 46. By means of the slot and screw arrangement the innermost edges of the strips 54 are spaced apart to any desired extent to provide an elongated rectangular or slit orifice 58 overlying the discharge slot 52. A pair of end blocks 59 (FIG. 5) are also secured to the front wall 46 in fixed relation at the opposite ends of the adjustable orifice strips 54. The liquid coolant in passing from the inlet 42 to the orifice 58 follows a tortuous path, as indicated by the arrows in FIG. 6, thereby insuring a uniform distribution of liquid across the entire length of the orifice opening 58. Moreover, the restricted spacing between the free edges of the baffles 52'53 and the walls 49-48 serves to trap any large particles of foreign material and thereby prevent obstruction of the orifice 58. If desired, the strips 54 may be adjusted so that the baffle clearances 52" and 53 are more restricted than the orifice 58, thereby insuring that no foreign particles will clog the orifice 58.

As will be evident particularly from FIGS. 1 and 4, the submerged spray units 41 are mounted in suitable openings in the walls of the conduit 34 with the orifice strips 54 projecting slightly into the channel 35 so that sheets or curtains of water are directed from the slit orifices 58 substantially perpendicularly toward Opposite sides of the strip 10. In fiuid kinematics terminology, the sheet or curtain of water `from an elongated rectangular orifice, such as 58, may be characterized as having two-dimensional flow, i.e. the flow is identical in parallel planes so as to extend uniformly across the width of the strip 10.

Reference is made to the copending application of Harold L. Taylor and John M. Marshall, Serial No. 484,616 filed September 2, 1965, now Patent No. 3,360,202, wherein the spray unit 41 per se is described and claimed.

As the heated steel strip 10 moves downwardly from the furnace 15 it passes through the seal section 38 and enters the upper end of the water filled quench channel 35 where it is immediately immersed in the upwardly flowing stream of wate-r. In addition, the submerged spray units 41 direct water streams against the strip in a direction generally normal to the path of movement of the strip, thereby creating a high degree of turbulence in the uppermost portion or strip entry end of the quench channel. As the strip 1t] leaves the lower end of the quench section 34 it enters the tank 30, passes bene-ath the roll 32, and emerges fro-m the exit chute 31. As will be noted particularly from FIG. 4, the dimensions of the conduit 34 are as restricted as possible so as to provide a relatively high velocity of water ow through the quench channel 35 while at the same time allowing suflicient clearance to permit passage of thestrip 10 Without scraping the walls of the conduit.

As previously mentioned, uniformity of quenching is essential not only for the sake of obtaining a strip having uniform microstructure and uniform physical properties but also to avoid warpage and distortion of the strip. Irregular vaporization of the water or other quenching medium in contract with the strip can result in substantial differentials in heat transfer rates between portions of the strip surface in contact with liquid water and other portions in contact with Water Vapor. These differentials cause different rates of contraction in the steel strip and result in quenching stresses and deformation. However, in the quench system of the present invention, desired uniformity of quenching is realized as a result of several cooperating factors. The provision of the restricted quench channel 35 results in a water velocity relative to the strip which, by Way of example, may be on the order of 1 to 10 ft./sec. in a direction generally parallel to the strip 10. Furthermore, the submerged sprays 41 are designed so as to provide Water streams in a direction generally normal to the strip 10 at relatively low pressures and relatively high flow rates so as to create substantial turbulence within the channel adjacent the strip entry end thereof. For example, the water pressure in the sprays 41 may be on the order of to 30 p.s.i. at the inlet 42 and on the order of 5 to l0 p.s.i. at the discharge slot 52. Although the submerged jets 41 are designed to create internal turbulence in the quench channel, nevertheless, the surface of the liquid in the channel where the strip 10 first contacts the quench liquid is maintained substantially smooth, non-splashing, and non-turbulent so that every point across the width of the strip makes initial Contact with the quench liquid at substantially the same time The low pressure of the high flow rate submerged jets 41 makes it possible to provide the desired surface smoothness while at the same time providing the required internal turbulence below the liquid surface. Thus, a quenched strip of at least partially martensitic structure is obtained which is either flat enough for its intended use or can easily be rolled to atness.

Within the broadest scope of the invention any suitable quenching liquid may be used including Water, brine or other aqueous salt solution, oil, liquid nitrogen, etc. However, for quenching a heated steel strip to convert austenite to martensite, the preferred quenching media are water and aqueous brine or other aqueous salt solutions. For the latter purpose, the volumel rate of fiow of the quech liquid must be high enough to achieve the cooling rate required to transform the austenite to martensite, and the turbulence of the quench liquid relative to the strip, particularly at the strip entry end of the quench channel, must be great enough to prevent the accumulation of vapor film which would lead to non-uniformity of quenching and consequent distortion of the strip.

Typical line speeds may range from about 100 ft./min. to about 2000 ft./min. dependent upon the gauge of the strip and the carbon content. The water introduced to the quench system may be at the ordinary available temperature, e.g. from about 35 F. to about 65 F., and the steel strip will normally be cooled from its austenitizing temperature range of 13302l00 F. (15252l00 F. in the case of a fully martensitic product) to approximately the water temperature before leaving the quench tank. If desired, the water or other quench medium may be recirculated through a heat exchanger for temperature control.

Assuming that the quenching operation has been carried out under optimum conditions, as discussed above, so as to achieve uniformity of quenching across the full width of the strip, the final quenched product will have accepta-ble atness for many end uses, as mentioned above. However, the quenched strip can readily be rolled, as on a temper mill, to provide adequate commercial flatness for any desired end use. For example, successful flattening is usually obtained by a single pass through a twin stand four high temper mill, each stand having two work rolls and two back-up rolls. Because of the unusual hardness of a fully martensitic strip, the work rolls may have a high degree of roughness without impairing the surface of the strip, thereby providing adequate flattening in a single pass.

Although quench systems have been proposed heretofore in which a continuous metal strand is passed through a restricted quench channel, such a system is not adequate when extremely rapid and uniform quenching is necessary As previously explained, the quenching of low carbon steel strip to obtain a martensitic microstructure while at the same time retaining acceptable iiatness in the strip is a particularly difcult problem. In accordance with the present invention, however, it has been found that exceptionally uniform quenching can be achieved if a high degree of turbulence is maintained in the restricted quench channel at the strip entry end of the channel, optimum results being obtained when the liquid surface in the quench channel where the heated strip first contacts the quench liquid is maintained in a substantially smooth non-splashing con-dition.

Thus, the most important feature of the invention is the provision of the submerged spray units 41 or other equivalent means for inducing turbulence in the quench channel 35 adjacent the strip entry end thereof by directing low pressure-high volume streams of quench liquid generally perpendicularly against the strip and uniformly across the width of the strip. The submerged spray units 41 having the rectangular or slit orifices 58 are highly effective for this purpose because of their ability to direct against the strip a plurality of sheets or curtains of quench liquid which have a `relatively high velocity for the flow rates involved and which are essentially uniform across the width of the strip. The volume o'w rate and velocity of the Water or other coolant streams Idischarged from the orifices 58 are controlled by means of the valves 43 and adjustment of the spacing between the orifice strips S4.

Other means may also be employed for creating the necessary turbulence in the quench channel 35 by causing high velocity transverse flow of coolant in addition to longitudinal flow of coolant through the channel. For eX- ample, the quench channel may be provided with transverse baiiles extending inwardly from the outer walls of the channel toward the strip so as to direct a portion of the quench medium from its generally longitudinal path parallel with the strip to a transverse path generally normal to the strip.

The currents or streams of water directed transversely or generally perpendicularly against the strip by the submerged sprays 41, in cooperation with the high relative velocity of the main flow of water through the restricted quench channel 35 parallel to the strip, cause sufficient turbulence in the quench medium to eliminate steam pockets which would otherwise tend to form or accumulate at the strip surface. Such vapor formation causes a vapor barrier which results in non-uniform quenching, non-uniform transformation to martensite, and consequent warping and distortion of the strip. Experiments have shown that when the quench apparatus illustrated in FIGS. 2-6 is operated without the use of the submerged spray units 41, the uniformity of quenching is inadequate to obtain a fully martensitic steel strip of acceptable flatness. Moreover, observation through the view ports 40' reveals that steam bubbles formed at the strip surface are dragged downwardly into the quench channel 35 and red hot spots can be seen on the strip indicating non-uniform quench action. However, when the spray units 41 are employed, a high degree of strip flatness is obtained, and no steam bubbles or hot spots can be observed through the ports 40. Thus, turbulence at the strip entry end of the quench channel is effective in eliminating the adverse effects of vapor formation and accumulation at the strip surface.

The foregoing description of the invention relates particularly to the quenching of steel strip which has been heated to an austenitizing temperature so as to obtain a product which is at least partially martensitic. However, in its broadest aspect the invention has particular utility in the quenching of any metal strip which has been heated 7 to a temperature which would normally cause film boiling of the quench liquid as distinguished from nucleate boiling. As is well known, under conditions of film boiling the rate of heat transfer is diminished drastically and it also becomes increasingly diiiicult to obtain uniform quenching. In accordance with the present invention, iilm boiling is avoided by the creation of suflicient turbulence to avoid vapor accumulation even though the temperature of the metal surface is such that film boiling would normally be expected. In the case of aqueous quench liquids, including water, brine, and other aqueous salt solutions, the invention is useful in quenching metal strip which has been heated to a minimum temperature of about 800 F.

While the effective suppression of vapor formation and accumulationy which is characteristic of the present quench system is of critical functional importance when water is used as the quench medium, there are also additional economic advantages if other more expensive quench media are used. For example, when liquid nitrogen is used as a quench medium or coolant, the elimination of vapor loss from the quench system prevents excessive and costly consumption of nitrogen.

FIG. 7 shows a modification of the quench system which is similar to the arrangement of FIGS. 2 to 6 eX- cept that an additional pair of spray units 61 are provided in the seal section 38. These spray units 61 are identical in construction with the units 41 but are spaced above the surface of the water in the conduit 34 instead of being submerged in the quench channel. The spray units 61 are oriented downwardly at an angle so as to direct curtain jets of water against opposite sides of the strip closely adjacent the point where the strip 10 lirst contacts the surface of the water in the channel 35. Thus, the jetS or sprays from the units 61 provide additional initial quenching, contribute further to the desired turbulent condition at the entry end of the quench channel, assist in maintaining surface smoothness of the water, and also serve to seal off the surface of quench bath in the channel 35 so as to further inhibit vapor evolution.

FIG. 8 shows another modification similar to FIG. 2 but in this instance a plurality of pairs of pinch rolls 62 are mounted in the quench channel 35 for engaging opposite sides of the strip 10. As shown, the pinch rolls 62 are preferably disposed intermediate the submerged sprays 41 so that the rolls do not interfere with the water streams from the sprays. By the mechanical action of the rolls 62 the strip 10 is held fiat, and, in addition, the presence of the rolls 62 in the channel 35 increases the turbulence in the quench liquid thereby further insuring uniformity of quench action -for the reasons already discussed.

FIG. 9 illustrates a slightly different embodiment of the invention wherein an enlarged hood or enclosure 63 is provided around the strip 10 as it emerges from the furnace. The bottom of the enclosure 63 has a central upwardly projecting portion 64 which communicates with a quench conduit 66 similar to the conduit 34 in FIGS. 2-6. Submerged spray units 67, which are the same as the units 41 in FIGS. 2-6, are mounted in the opposite walls of the conduit 66 for the same purpose previously described. The lower end of the conduit 66 (not shown) is connected to a tank 30 just as in the previously described embodiments. A pair of large diameter pinch rolls 68 are housed in the enclosure 63 and engage the opposite sides of the strip 10 before the latter enters the conduit 66. The rolls 68 are also engaged by smaller diameter back-up or squeegee rolls 69. A pair of upright weirs 71 extend from the bottom of the enclosure 63 outwardly of the rolls 68 and high enough to enclose the lower portions of the rolls, and the water flowing upwardly through the quench channel in the conduit 66 overflows the upper edges of the weirs 71 and is discharged through outlets 72, as indicated by the arrows.

The use of pinch rolls 68 in the location shown in FIG. 9 insures a llat strip at the point where the strip first contacts the quench medium. The pinch rolls 68 also serve to seal olf the upwardly flowing quench medium from the conduit 66 so as to insure that there is noexposed liquid surface and that there is uniform initial contact between the quench liquid and the strip across the width of the latter. Thus, a high upward velocity of liquid in the conduit 66 and a high degree of induced turbulence by the sprays 67 can be maintained. The pinch rolls`68 are maintained at the temperature of the quench medium by being partially submerged in the body of quench medium coniined by the weirs 71. The high degree of strip flatness as the strip enters the quench channel from the pinch rolls 68 also permits the useof a more'rstricted conduit 66 without danger of the strip'scrapir'ig the conduit walls thereby providing higher liquid Velocities and higher heat transfer rates for a given quench liquid flow rate.

Te embodiment of the invention illustrated in FIG. 10 differs from the form of FIGS. 2 to 6 primarily by the use of a concurrent ow relationship between the quench medium and the strip instead of the countercurrentlioyv arrangement of the previous embodiments. Thus, in FIG. l() the strip 10 as it emerges from the furnace enters a central opening 73 in the top wall 74 of a quench tankvr76 containing a supply 77 of water or other quench liquid. At each side of the opening 73 a generally Ulshaped partition depends from the top wall 74 to provide spaced outer and inner partition walls 78 and 79 interconnected by bottom portions 81. A pair of depending baies 82 also extend from the top 4wall 74 between the respective pairs of partition walls 78-79 and terminate above the botton portions 81.

As shown in FIG. 10, the quench medium is pumped upwardly through conduits 83 by pumps 84 from the main supply 77 in the tank 76 and is discharged through coolers or heat exchangers 85 into the spaces between the partition walls 78 and 79. As indicated by the arrows in FIG. l0, the quench liquid flows downwardly and thence-upwardly around the baffles 82 and overows the upper edges of the walls 79 into a restricted quench channel 86 defined between the walls 79. As the strip 10 passes downwardly through the restricted channel 86 it is immersed in the quench medium which has a higherl level in the channel 86 than in the main portion of the tank 76, the difference in level being exaggerated inthe drawing for the sake of illustration. The quench medium flows downwardly through the channel 86 tothe main quench supply 77 and` is recirculated by the pumps 84. The Tstrip 10 passes downwardly around a sinker roll' 87V and then passes angularly out of the tank 76 through a chute 88. For the reasons already explained, the walls 79 of the quench channel 86 are provided with submerged spray units 89 identical with the units 41- heretofore described Instead of a recirculating system having the coolers `or heat exchangers 85, separate inlet and outlet water lines could be provided as in the previous embodiments. t

In FIG. 11, the structure is the same as FIG. 10 except that a plurality of pinch Vrolls 91 are mounted in the quench channel 86 between the sprays 89. Thus, the additional benetits of pinch rolls in the quench channel are obtained as described abovein connection with FIG. 8.

Although the invention has-been described with reference to particular structural embodiments `thereof, it should be understood that Various modifications and equivalent structures may be used withoutdepartingafrom the scope of the invention as defined invthe appended claims.

Iclaim: V,

1. A method -of quenching a heated metal ystrip with a quench liquid wherein the temperature ofthev strip #is such as to normally cause iilrn boiling ofthe quenchl'iqui'd with consequent non-uniform quenching fi and excessive distortion of the quenched strip; said method comprising the steps of passing' the strip continuously in a4 gen' "ally downward path through an` elongated re'strictedquench channel, passing the quench liquid continuously through said channel with t-he strip immersed in the quench liquid and with the quench liquid flowing at a high velocity relative to the strip and generally parallel to the strip, said strip as it enters said channel being at an elevated temperature such as to normally cause lm boiling of said quench liquid, and also directing quench liquid substantially perpendicularly against the Opposite surfaces of the strip and uniformly across the `width thereof at a submerged location within said channel adjacent to but spaced from the liquid surface at the strip entry end of said channel, whereby to create substantial turbulence within said channel at said strip surfaces so as to prevent vapor accumulation at said strip surfaces while maintaining said liquid surface substantially smooth and non-turbulent at the location where the strip initially contacts the quench liquid and thereby obtaining substantially uniform quenching and avoiding excessive distortion of the quenched strip.

2. The method of claim 1 further characterized in that said strip is passed downwardly through said channel and said quench liquid is passed upwardly through said channel in countercurrent relation to said strip.

3. The method of claim 2 further characterized in that said strip is passed between a pair of opposed pressure rolls disposed immediately above said channel, the lower portions of said rolls being immersed in the quench liquid owing upwardly from said channel.

4. The method of claim 1 further characterized in that said strip and said quench liquid are passed downwardly through said channel in concurrent relation.

5. The method of claim 1 further characterized in that said strip is also subjected to pressure between opposed rolls within said channel to assist in maintaining strip flatness.

6. The method of claim 1 further characterized in that said strip as it enters said channel has a temperature of at least about 800 F. and said quench liquid comprises an laqueous liquid at a temperature of from about 35 F. to about 65 F.

7. The method of claim 1 further characterized in that quench liquid is introduced into said channel at one end thereof and additional quench liquid is introduced into said channel at said submerged location and is directed substantially perpendicularly against the opposite surfaces of said strip.

8. The method of claim 7 further characterized in that said additional quench liquid is directed against the strip surfaces in the form of liquid sheets extending uniformly across the width of the strip.

9. A continuous method of making martensitic steel strip which comprises the steps of passing plain carbon steel strip having a carbon content of from about .03 wt. percent to about .25 wt. percent continuously through a heating zone fand heating the strip to a temperature above the A1 critical point so as to at least partially austenitize the steel, immediately thereafter passing the strip continuously in a generally downward path through an elongated restricted quench channel, passing a quench liquid continuously through said channel lwith the strip immersed in the quench liquid and with the quench liquid flowing at a high velocity relative to the strip and generally parallel to the strip, and also directing quench liquid substantially perpendicularly against the opposite surfaces of the strip and uniformly across the width thereof at a submerged location within said channel adjacent to but spaced from the liquid surface at the strip entry end of said channel, whereby to create substantial turbulence within said channel at said strip surfaces so as to prevent vapor accumulation at said strip surfaces while maintaining said liquid surface substantially smooth and non-turbulent at the location where the strip initially contacts the quench liquid and thereby obtaining substantially uniform quenching and avoiding excessive distortion of the quenched strip.

10. The method of Claim 9 further characterized in that the step of directing quench liquid substantially perpendicularly against the opposite surfaces of the strip is accomplished by introducing yadditional quench liquid into said channel in liquid sheets directed against opposite surfaces of the strip, said liquid sheets extending uniformly across the width of the strip at said submerged location.

11. The method of claim 9 further characterized in that said strip is heated in said heating zone to a temperature above the A3 critical point.

References Cited UNITED STATES PATENTS 2,348,232 5/1944 Trautman et al. 148-153 X 2,776,230 1/1957 Scott 148-156 X 3,027,308 3/ 1962 Zulkoski 266-3 X 3,148,093 9/1964 Williams et al. 148-156 X 3,240,480 3/1966` Cary 266-4 FOREIGN PATENTS 931,153 7/1963 Great Britain.

OTHER REFERENCES Metals Handbook, 1948 Ed., Published by A. S. M., pp. 615-622.

CHARLES N. LOVELL, Primal;7 Examiner.

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