US2756169A

US2756169A - Method of heat treating hot rolled steel rods

Info

Publication number: US2756169A
Application number: US190954A
Authority: US
Inventors: John H Corson; George A Goetz; Lewis Dartrey
Original assignee: JOHN A ROEBLING S SONS CORP
Current assignee: JOHN A ROEBLING S SONS CORP; JOHN A ROEBLING'S SONS Corp
Priority date: 1950-10-19
Filing date: 1950-10-19
Publication date: 1956-07-24
Anticipated expiration: 1973-07-24

Description

July 24, 1956 J. H. CORSON ET AL METHOD OF HEAT TREATING HOT ROLLED STEEL RODS 2 Sheets-Sheet 1 Filed 001;. 19, 1950 NZS R 0 T l E mm N No M C T W A T Y A 2 m E JGAW July 24, 1956 J. H. CORSON ET AL 2,756,169

METHOD OF HEAT TREATING HOT ROLLED STEEL RODS Filed Oct. 19. 1950 2 sheets-sheet 2 l\ 3?. I400 m E I200 KW TIME IN SECONDS INVENTOR Jomv H CoRsoN y 650m: A Goerz DARTREY LEWIS United States Patent C) METHOD OF HEAT TREATING HOT ROLLED STEEL RODS John H. Corson, Williamsport, and George A. Goetz,

Morrisville, Pa., and Dartrey Lewis, Trenton, N. J., assignors, by mesne assignments, to John A. Roeblings Sons Corporation, Trenton, N. J., a corporation of Delaware Application October 19, 1950, Serial No. 190,954 4 Claims. (Cl. 148-21.55)

This invention has to do with hot rolling and heat treating carbon steel rods to make them suitable for being drawn into wire.

In the production of steel wire it is customary to reduce the steel ingots by hot rolling, first to billets and then to rods of the desired size such as 7 or diameter. The rolled rods are then heat treated as a separate operation to perform what is known as patenting. This is a heat treatment which produces a metallurgical structure in the rod that makes it especially suitable for wire drawing. Such patenting involves passing the rod very slowly through a furnace in which it is reheated to a temperature of l5501850 F. The rod as it emerges from the furnace is allowed either to cool naturally in the air, which is known as air patenting, or is quenched in a bath of molten lead usually within the temperature range 900-l300 F.

Most of the steel rods subjected to this treatment are of a plain carbon steel composition in which the carbon content ranges usually from around 0.20 to 0.95%. Rods of such composition as hot rolled contain a very coarse structure of pearlite which is of irregular formation and makes the rods unsuitable for cold wire drawing, except to a very limited extent. The patenting operation improves this structure by first getting the carbon back into solid solution in the iron and then allowing the carbon to come back out of solution in the form of very fine plates of iron carbide (cementite) closely spaced. In fact the pearlitic structure thus obtained is frequently so fine that it cannot be readily resolved under the microscope even at 1000 magnifications. Such a structure has much higher strength and is suitable for cold drawing through six or more successive dies until the total reduction of area is 90% or more of the original cross-sectional area.

It will be apparent that it would be highly advantageous if the hot rolling and patenting process could be combined to avoid the necessity for reheating the rods and the necessity for all the separate handling operations necessarily involved in the foregoing procedure. This has been extremely difiicult to do, heretofore, because of the high finishing speeds which are used in hot rolling the rods, and the fact that the time-temperature regulation needed in the patenting operation which is carried out at very slow speeds is not adaptable to such high finishing speeds. For example, it is general practice in hot rolling rods to roll them in a continuous mill from a 2" x 2" billet through a succession of reducing rolls to a rod that may be any- 1 where from to /8" in diameter. In such a rolling operation the rod is elongated each time its cross section is reduced, and it is customary for the rods to emerge from the last pair of rolls in such a mill at a speed of from 2000 to 5000 F. R. M. Patenting operations, on the other hand, are usually carried out at much lower speeds, ranging from 10 to 100 F. P. M.

Finishing temperatures in a hot rolling mill are frequently of the order of 1800 F., and in order to develop a metallurgical structure in :this rodsimilar to that pro- "ice duced by patenting, it is necessary to cool it rapidly and uniformly from such a temperature to a temperature within the 9001300 F. range and then hold the rod within that range for a period of 10 seconds or more while the carbon comes out of solution. If the rod is not cooled sufliciently the pearlitic structure will be too coarse, especially in the center of the rod. If the rod is cooled too much, the center of the rod may be of the proper fine pearlitic structure, but the surface will be acicular o1 Bainitic in structure. In fact if the cooling is too great, definite hardening may be produced with the formation of sorbite, rnartensite or similar structures which greatly impair the ability of the rods to be drawn.

An object of this invention is to provide a method and apparatus for quickly and uniformly cooling a rapidly moving rod from a temperature of the order of 1800 F. to a temperature within the range of 9001300 F. without cooling the outer portion of the rod substantially below 900 F.

Another object of the invention is to provide a method and apparatus for accomplishing such rapid cooling and immediately thereafter holding the rod within this temperature range for a sufiicient period of time to permit the carbon to separate from the solid solution in the form of fine pearlite.

Another object of this invention is to provide a method and apparatus for patenting plain carbon steel rods which can be carried out while the rods are moving at very high speeds and which can be readily combined with the hot rolling of such rods on a continuous mill.

These and other objects of the invention will be apparent or explained in connection with the following description of the invention and the accompanying drawings in which- Figures 1 and 1A together illustrate somewhat diagrammatically apparatus for heat treating in accordance with this invention steel rods as they come from a conventional mill for the continuous hot rolling of such rods.

Figure 2 is a vertical longitudinal sectional view showing on a larger scale one of the quenching units appearing in Figures 1 and 1A.

Figure 3 is a vertical longitudinal sectional view showing on a larger scale one of the liquid separating units appearing in Figures 1 and 1A.

Figure 4 is a cooling curve indicating the manner and rate at which the rods are cooled in accordance with this invention.

It has been found that a fine grain pearlite structure can be obtained by heat treating hot rolled rods as they emerge from the rolling mill if the rods are rapidly cooled to a temperature within the range of 9001300 F. without cooling the surface of the rods at any time below a temperature of about 900 F., and are then held within the 900-1300 F. temperature range for 10 seconds or more until the carbon is all out of solution. By rapid cooling is meant cooling in a space of time of the order of .75 to 1.5 seconds. Of course, if the cooling is too rapid it will be virtually impossible to avoid cooling the outer surface of the rods below 900 F. and, on the other hand, if the cooling is not rapid enough the pearlitic structure will be toocoarse.

Preferably, the rapid cooling to the temperature range of 900-1300 F. takes place between the time that the rod emerges from the last pair of rolls in the rolling mill and the time that it is delivered to a reel or otherwise formed into a coil. If the rod is cooled to the proper temperature when it is formed into a coil, the mass of steel in the coil will retain the temperature of the metal at the recalescence point for a number of seconds, or at least long enough to permit all of the carbon to come out of solution in the form of fine pearlite.

It has been found that this rapid reduction of temperature without too drestic coolingof the outer surface of the rod can be effected satisfactorily by subjecting the hot rod, as it comes from the rolling mill and while it is traveling at a .high rate of speed, .to successive alternate stages .of liquid quenching and air cooling. While other liquids might be used for quenching, water is readily available .and serves the purpose very well. It is desirable to. avoid cooling the outer surface of the rod substautially below 9.00 F. and this is accomplished by shortening the time of each water quenching stage as the overall temperature of the rod is reduced. In other words, as the rod moves along at a constant speed, the successive quenching units apply water to the surface of the rod for a shorter time.

.One form of apparatus suitable for carrying out heat treatment of steel rods as they issue from a hot rolling mill isillustrated somewhat diagrammatically in the drawings. Figures 1 and 1A, when arranged in tandem, indicate the complete apparatus for performing this heat treatment. The rod, indicated generally at 10, issues from between the .last pair of rolls 11, 11 of a conventional or other suitable continuous hot rolling mill at a temperature .of the order of 1800 F. This hot rod, traveling at a high rate of speed, immediately passes through a water quenching unit 12 and then in succession through a water separating or draining unit 13, an air cooling tube 14, a second water quenching unit 15, water separating device 16 and air cooling tube 17 to a pair of

pinch rolls

18,18. The

pinch rolls

18,, 18 are driven and serve to keep the red hot rod moving smoothly. through the apparatus without cobbling. If the pathof travel for the rod after it leaves the last pair of rolls of the rolling mill and before it reaches the take-up or coiling reel is not too, long, this intermediate pair of

pinch rolls

18, 18, may be omitted. On the other hand, if the path of travel is unusually long, it may be desirable to use more than one pair of such driven pinch rolls.

After the pinch rolls 18, 1.8, the rod is passed successively through another water quenching unit. 19, a 'Water separating unit 20, an air cooling tube 21, a fourth water quenching unit 22, followed by another water separating unit 23 and an air cooling. tube 24. Tube .24 is preferably curved to deliver the hot rod onto a reel, indicated generally at 25. Such a reel orcoiling device mavbeof any conventional or other design and, if desired, may be positively rotated by a ring gear 26 driven by a pinion 27.

The reel 25, may also be provided with a conventional stripper plate 28which ,is elevated when a coil of rod is completed to a position level with the conveyor table .29. A ram,30 of any. suitable construction may be provided to push the finished coil from the stripper plate 28 onto the table 29, whence it is carried away by the conveyor 31. The finished coil is indicated at 32. in a position which, it occupies while being carried away by the conveyor 31.

Figure 2 illustrates in somewhat more detail one. suitable form for the water quenching devices shown in Figures 1 and 1A at 12, 15, Hand 22. Essentially, such a unit may consist of a central tubular member 33 having a flared forward end 34 and a number ofholes or perforations, indicated generally at 35. The tubular member is surrounded by a jacket 36 which issupplied with water or other suitable quenching fluid under pressure through the pipe 37. The quenching fluid; is thus sprayed under. pressure against the outer surface of. the rapidly traveling rod 10, as, it moves through the'quenching unit. The heat of t the rod converts some of this water or quenching fluid to vapor and the excess liquid spillsout from the ends of the tube 33 into a. trough or. other suitable collecting basin, (not shown).

Because the rod. is moving so rapidly it is particularly. desirable to provide further opportunity for. excess water to drain off its surface. This maybe done in a water separating, unit. such as

shownat

13, 16, 20

and 23. One suitable form for such a unit is shown in Figure 3 and consists essentially of a number of short tubular sections 40 spaced from each other along the path of travel of the rod. Each tubular section preferably has a flared forward end to receive the traveling rod. These tube sections may all be held in proper alinement by means of a metal bracket such as indicated at 42. Any excess water or other quenching fluid carried along by the rod from the preceding quenching unit is allowed to drain 01f before the rod passes into one of the air cooling tubes 14, 17, 21 or 24.

It will be noted that while the water separating units and the air cooling tubes are all substantially alike both in structure and size, the quenching units differ in length. For example, the second quenching unit 15 is much shorter in length than the first quenching unit 12. The third quenching unit 19 is in turn substantially shorter than the second unit 15, and the fourth quenching unit 22, if needed, may be made smaller than the third one or it may be made approximately the same size as the unit 19. The purpose of making these quenching units of successively shorter lengths can best be explained in connection with the following example.

Example..It may be assumed for purposes of this example that the rod to be heat treated is a plain carbon steel rod A in diameter and is issuing at a temperature of 1800 F. from the last pair of rolls on a conventional Morgan continuous rod mill with a finishing speed of 4100 F. P. M. (68.3 F. P. S.). It has been found that, the ideal treatment is to cool this rod to a uniform temperature of about 1100 F. as rapidly as possible without subjecting the skin or any other portion of the rod to. a temperature less than about 900 F. during the cooling. When such a rod is passed through the machine of the type illustrated in Figures 1 and 1A, the cooling eflect on the rod may be that indicated generallyv in the graph of Figure 4.

The temperature is plotted against time so that this graphv shows therate of change of temperature as the rod moves along. The line 45 indicates the temperature of the center of therod as it moves along, while the line 46 indicates the temperature of the outer surface of the rod. As the rod. moves into and through the first quenching unit 12 which, for purposes of illustration, may be 3.8 long, the outer surface of the rod is cooled to approximately. 1000 F. in .056. second. The center of the rod, however, is cooled only slightly, as shown by the first small, dipin the line 45.0f Figure 4, and the rod then passes through the, units 13 and 14 which take up a total of 17.1 of travel. During this, time, which is approximately. 0.25 second, an equalization of temperature between the outer surface and center of the rod takes place, so that the outer surface of the rod rises rapidly to a temperature above 1400 F., while the temperature of the center of the rod is cooled down to a temperature of a little less than 1500" F. Before the temperatures of the outer surface and center of. the rod-have completely equalized however, the rod passes through the second quenching unit 15 which is 1.4 in length and rapidly cools the. exterior of the rodto 1000 F. in .02 second. This is indicated at 46b inFigure 4. The center of the rod again is only, slightly cooled during this interval but during passage through the succeeding air cooling sections 16 and 17, which takes up to a distance of 17.1 of travel, the temperature of the outer surface of the rod risesrapidly to just, below l300 F. while the center of the rod is cooled down to around 1300 F.

At this point the rod passes through the third quenching unit, 19 which is 0.7' in length and in the space of .01 second coolsthe outer surface of the rod down to a little less than 1000? F., as shown bythe section 460 of the curve 46 on the graph. Thereupon. another equalization takes placewhiletherod is passing through the air cooling sections 20 and 2-1 for a travel'of: 17.1, the temperature this time being equalized aroundl1509 F. The

fourth quenching unit 22, which is also .7' long, then cools the outer surface of the rod down to around 900 F. in the space of .01 second as shown at 46a. The final equalization of the rod which takes place during another 17.1 of travel through the

units

23 and 24 results in a rod which is delivered to the reel 25 at a substantially uniform temperature throughout this section, that temperature being 1100 F. or a little less.

This whole operation, therefore, in a total time of 1.096 seconds and during a total travel of 75 has reduced the temperature of the rod from 1800 F. to a little below 1100 F. without any time cooling the outer surface or any part of the rod to a temperature below 900 F. which might produce the formation of the objectionable Bainite or martensite structure.

As the rod is being wound up on the reel 25 and even after the complete coil of rod is being carried away by the conveyor 31, it is aflforded ample time at the proper temperature i. e. within the range of 900-1300" F. for all of the carbon in solution to come out in the form of a very fine structure of pearlite. It will be apparent of course that the particular conditions and dimensions will necessarily be varied in the treatment of hot rolled rods of other sizes or of the same or other sizes which may be traveling at different speeds than the speed in the example given. Also, adjustment of course will have to be made for rods which emerge from the rolling mill at a temperature that may be a little above or somewhat below 1800 F.

The terms and expressions employed are used as terms of description and not of limitation, and it is not intended, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

We claim:

1. A method of heat treating hot rolled high carbon steel rods as the hot rod travels from the rolls to a coiling device at a temperature of the order of 1800 F. and at a speed of the order of 2000 F. P. M. or more which comprises water quenching the rod and immediately removing surplus water from the rod'surface, allowing the temperature of the outer and inner portions of the rod to equalize substantially, then again water quenching the rod and removing surplus water from the rod surface, again allowing the temperature of the portions of the rod to equalize substantially, and repeating said treatments successively until the average temperature of the rod is reduced to a temperature of the order of 1100 F. at the time it is coiled, each of said water quenching treatments being of short enough duration so that the outside surface of the rod is not cooled to a temperature substantially below 900 F. before the rod is coiled.

2. A method of producing high carbon steel rods suitable for wire drawing without further heat treatment which comprises hot rolling billets to rods, quickly cooling the rapidly traveling rod coming from the rolling operation at a temperature of the order of 1800 F. by subjecting the rod as it travels along to a series of alternate water quenchings and air coolings until the average rod temperature is reduced to a temperature substantially below 1300 F. but not below 900 F., and then coiling the rod to retard further cooling while the carbon comes out of solution, each of said water quenchings being of short enough duration to avoid cooling the surface of the rod substantially below 900 F., and each of said air coolings being long enough to permit substantial equalizing of the temperatures of the inner and outer portions of the rod before the next water quenching treatment is applied.

3. In a method of preparing steel rods for wire drawing by heat treating a rod as it leaves a hot rolling mill at a temperature of the order of 1800 F., the step of cooling the rod rapidly and relatively uniformly to a temperature within the range of 900-1300 F. by successively and alternately applying a quenching liquid to the rod and immediately removing surplus liquid to allow the rod to equalize substantially in temperature as it moves along in a longitudinal direction, the succeeding quenching treatments being of shorter duration than the first quenching treatment to minimize overcooling of the surface of the rod.

4. -A method of heat treating hot rolled steel rod as the hot rod travels at high speed in an axial direction from the finishing rolls of a hot rod mill to a coiling device which comprises lowering the surface temperature of the rod to a greater degree than its interior temperature by means of a brief water quench, immediately removing the water from the surface of the rod for a sufficient time for the surface temperature of the rod to rise substantially during equalization of the temperature within the rod,

and repeating said quenching and equalizing treatments in rapid succession until the average temperature of the rod is reduced from around 1800 F. to a temperature substantially below 1300 F. but not below 900 F. at the time it is coiled, each of said water quenching treatments being brief enough to that the outside surface of the rod is not cooled to a temperature substantially below 900 F. before the rod is coiled, the rate of cooling of the rod after it is coiled being reduced so that the carbide separates in the form of fine pearlite suitable for wire drawing directly without further heat treatment.

References Cited in the file of this patent UNITED STATES PATENTS 2,023,285 Otis Dec. 3, 1935 2,240,019 Quarnstrom et al. Apr. 29, 1941 2,516,248 OBrien July 25, 1950 FOREIGN PATENTS 7 297,796 Great Britain Feb. 24, 1930 342,189 Great Britain Jan. 29, 1931

Claims

1. A METHOD OF HEAT TREATING HOT ROLLED HIGH CARBON STEEL RODS AS THE HOT ROD TRAVELS FROM THE ROLLS TO A COILING DEVICE AT A TEMPERATURE OF THE ORDER OF 1800* F. AND AT A SPEED OF THE ORDER OF 2000 F.P.M. OR MORE WHICH COMPRISES WATER QUENCHING THE ROD AND IMMEDIATELY REMOVING SURPLUS WATER FROM THE ROD SURFACE, ALLOWING THE TEMPERATURE OF THE OUTER AND INNER PORTIONS OF THE ROD TO EQUALIZE SUBSTANTIALLY, THEN AGAIN WATER QUENCHING THE ROD AND REMOVING SURPLUS WATER FROM THE ROD SURFACE, AGAIN ALLOWING THE TEMPERATURE OF THE PORTIONS OF THE ROD TO EQUALIZE SUBSTANTIALLY, AND REPEATING SAID TREATMENTS SUCCESSIVELY UNTIL THE AVERAGE TEMPERATURE OF THE ROD IS REDUCED TO A TEMPERATURE OF THE ORDER OF 1100* F. AT THE TIME IT IS COILED, EACH OF SAID WATER QUENCHING TREATMENTS BEING OF SHORT ENOUGH DURATION SO THAT THE OUTSIDE SURFACE OF THE ROD IS NOT COOLED TO A TEMPERATURE SUBSTANTIALLY BELOW 900* F. BEFORE THE ROD IS COILED.