US2400931A - Continuous production of heattreatable ferrous sections - Google Patents

Description

TIoNs May 28, 1946. P. HUME ETAL CONTINUOUS PRODUCTION oF HEAT-TREATABLE FERRoUs sEc original Filed April 12, 1945 5 Sheets-Sheet 1 FHS- 1.4

MNMRUSQ.

P. H. HUME ETAL 2,400,931

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May 2s, 194s. 2,400,931

ON 0F HEAT-TREATABLE FERROUS SECTIONS Pf H. HUME ETAL CONTINUOUS vPRODUCTI 3 Sheets-Shea?l 3 Original Filed April l2, 1943 i my @MW Z wa om Patented May 28, 1946 r- UNITI-:D STAT coNTxNUoUs Pao 'rnEATABLE ESV-PATENT DUCTIQN F EEA FERBGUS SECTIONS 2,400,931 orner-1 `Patrick H. Home, Lakewood, ohio, William F.

McGan-ity, L. Robinson,

Hollldays Cove. W.

Pittsburgh.' Pa.,

Va., and Edward asslgnors to Carnegie-Illinois Steel Corporation, a corporation of New Jersey Original application A Divided an vbei 12, 1945, Serial N0.

T Y 1 Claim.' This invention relates to th'e adjustment of the physical properties of ferrous bodies, especially After composition is established. the next in importance is the structural constituency of the specific metal whereby its allotropic and ancillary p may be adjusted to ailord optimum properties.

I'he least important (except in non-heat-treatable steels) fromthe standpoint of over-all eil'ect is condition of the strucphysical considerations,

on the ilnal properties,

dealing'with purely mad mental, and is directed toward the of the structural constituency of a. steel body in such a manner as to preclude the necessity for elaborate heat-treating equipment and processing cycles characteristic oi prior art technique.I

It is. accordingly, theprincipal object of the present invention to impart the desired nal properties' to ferrous bodies, through adjustment N109 PlOllerties-usually associates am. ...M

pril 12, 1943, Serial No. this application Dccemrate subsequent heat-treatment in furnaces. or thelike.

It is a further object to process steels in accordance with a new technique herein called "differential reflex tempering. Other objects and advantages, including increased production and lowered costs, will become apparent as the description progresses.

. strips of steel of heat-treatable or normalizing operations. i. e., heat-treatment, to develop the required ilnal properties of the metal, in accordancewith conventional practices. volved great expenditure in time, labor and equipmentwhich has added materially to the cost per lton of steel'so produced.vwhile, at the same time. has curtailed the production thereof. Hot-reduced-material, after reduction to gauge,

and preferentially cooled (quenched), attened,4 and, in some cases. heated and flattened again before the ilnal properties are imparted. Such practice, especially in plate gauge material involving heavy masses, entails a slow cycle for heating, warpage upon quenching, consequent l and many steps whichprolong a production cycle into days.

AThe present invention eliminates the need fcr heat-treating and processing cycles' by utilizing heat residual in the metal from previous processing, e. g., hot-ro to obtain the same results continuously, automatically, and substantially in one operation. As a brief outline of a preferred embodiment of the invention. plates, sheets or composition are rolled to gauge on a hot-mill so. as to finish at austenitizing temperature. 'Such stock is then delivered to a series of specially designed sprays. preferably associated withthe run-out table of a continuous hot-mill, where it is cooled in a preferential manner without warpase; all of the stock being taken through thetransformation point at approximately equilibrium vtemperatureor at a depressed temperature, depending upon the structure sought, or part of the stock being cooled and part being left relativelyv hot for mutual structural adjustment by residual heat transference when stacked or coiled together (diierential reflexive tempering).

I'he invention will be better understood when the following description is considered in con- Junction with the accompanying drawings in which:Y

' Figure 1 is a graph showing the effect of dii'- ferent rates of cooling on the transformation temperature of steel as represented by a typical Here the temperature of transformation (ordinates) is plotted against the transforend of a run-out one` method for selectively cooling plates andv act upon relatively thin-gauge `characteristics of the material have "from within the`austenitic range desired structural constituency to 2 to reveal the allotropic structures obtained upon transformation at different levels; Figure 2 is a diagrammatic plan view of a runout table of a continuous hot-mill indicating the placement of sprays in accordance with one embodiment of thepresent invention;

Figure 3 is a side elevational view drawn to vall enlarged scale of a typical spray unit for application as in Figure 2;

Figure 4 is an end view of the apparatus illustrated in Figure 3 as viewed from the left;

Figure 5 is a schematic representation of the table and a piler illustrative of sheets, according to the present invention; and

. Figures 6 and '7 illustrate the principle, described in connection with to strip. l

I-Ieretofore, the continuous production of plates, sheets andstrip on a tandemhot-mill has been accomplished by heating slabs in furnaces and then advancing them through a series of roll stands (some of which may be reversing) called the roughingtrain, and, thence, on through another series of stands continuously, which reduce the material to gauge. This latter is referred'to as the finishing train, and usually has associated with it a long run-Out table at the exit end of the last stand by which the stock is advanced to the coiling or piling mechanisms. Such tables have been constructed of sumcient length to provide for the adequate cooling of the stock to a temperature below ing between contiguous portions of a stack or coil would occur. In some instances, water sprays have been provided to supplement the air-cooling for the purpose of assuring that the temperature of the stock is reduced a sumcient amount.

The sprays associated with conventional arrangements of run-out tables are of small capacity, since, for the most part, they are designed to stock, and then onlyin an auxiliary capacity to assist inthe cooling thereof. Heavier gauge material, such as plates, could not be effectively treated with the conventional sprays, and even the thinner gauges have not been cooled with sufficient rapidity vto alter the transformation temperature of the steel to any material degree. Taking into consideration that temperatures of recrystallization extend to below the transformation range, and, in point of time, are maintained longer, more recent deof higher the condivelopments have `produced a spray efficiency, whereby an adjustment of Y tion of. the structural constituent of the metal (grain size,L dispersion' of segregates, etc.) has been possible, and to this extent the physical been ad- Figure 5, as appliedfture, will vary as that at which Weldlusted and controlled in a manner usually assoelated with such v deals with the third fundamental discussed above, and is the. subject of a patent to Herman et'al., No. 2,271,372.

In accordance with the present invention, it has been discovered that specially'designed sprays, capable of delivering more water at higher velocity, and capable-ofbeing controlled to a critical degree far beyond anything contemplated heretofore, can be applied in the production of hotreduced plates, sheets and strip so as -to control the transformation temperature upon cooling to impart the the metal.

drawings, Figure 1 sets intended to illustrate the aims of the present invention.

In the accompanying condition of the metal. This .75 structure. Below this zone,

,supplied to Figure complished with zones demarking that may be It will be understood that the S-curves of steels, being indicative of the'timerequired to effect transformation at different levels of temperathe composition varies. thus graphic representation, in terms of specific temperatures and times applicable to all steels, an impossibility.l For this reason no specific temperatures or time intervals have 'been rendering any included in Figure l, since the considerationsY here are applicable to steels of any heat-treatable composition. However, with the principle understood in general, specific values may be l to satisfy the conditions of agiven composition.

By reference to Figure 1, several pertinent observations may be made that will be applicable to any steel under treatment. .Let it be assumed that a steel body is at a temperature within the austenitic range. Under conditions (slow-cool) tending toward theoretical equilibrium between the external energy of heat and the internal energy of the metal associatedwith phaserretention or transformation, the auStenite-to-ferrite transformation would occur somewhere along a substantially straight line marked ,Ae. The more rapidly the cooling is conducted, (i. e., the closer the cooling-rate curve falls to the lefthand side of the graph) the lower the transformation temperature is depressed, as is evidenced by the sloping line l, indicative of the commencement of such phase-change. II the cooling is ac,-

sufcient metal from a temperature above' the Ae tempertemperature-represented by abscissa Tri in t1 seconds, the lowest transformation point in the upper temperature levels. from the standpoint of the least time and temperature, will have been met. This po t (where Tri and t1 intersect in Figure 1), commonly referred to as the nose or elbow of the S-curve. has been designated by the letter N in this ngure, and in non-alloy or low-alloy steels is usually attained in a second or less. The cooling rate which, when plotted, dennes a curve or line passing tangentially to the nose N of the S-curve is termed the critical cooling rate, since -transformation above or below this point occurs in a. greater order of time. Therefore, in order to make available those structures resulting from transformation at lower temperatures, the critical must be invoked as the maximum rate in point of time admissible to effect this end, lest a slower rate initiate transformation at a higher temperature.

The second curve 2 of Figure 1 is representative of the time-temperature variant. at which intersection of curve i, is completed, after the which no further adjustment of structure of the metal (barring conditioning under the third fundamental) is possible by cooling alone, without reheating. In accordance with' this, the curve Vof Figure 1 has been divided into the different structures derived from effecting transformation at dierent timetemperature relationships. If austenite is transformed within zone A, and the transformation is completed at such temperature, a substantially uniform structure of coarse pearlite in varying amounts, depending upon the carbon content, will be obtained. Transformation begun and completed in zone B will afford a medium pearlite structure, while that occurring in zone C will be the smallest. most highly pearlitic characterrapidity to cool the rened pearlitic l cooling ratel is. such as istics give way to acicular structures ranging i'ro the softer bainite in zone D down to the hardest martensite in zone E. y, l l For uniformity, transformation should be com? pleted at approximately'the temperature level at at whichlower-phase structures will-.have formed. f

a mixed or fl1 :i-,eregencous. structurev will "result predominated` by that.; structure derived from t a temperature level `at whichthe metal will 4.have

stood the longest. short or completing die` transf obtained vfrom interzonal transformation.`

f Because. of the drastic rate of cooling entailed in constituent structures adjustment, inadequate Vfacilities, and immurement by heat-treating habits of long standing, hot rolled materiaLas produced in accordance with conventional practices, whether subsequently cold reduced or not, has, the residual heat of rolling a manner as to require any preferred structural constituency of the metal, other than that occurring between the Asand connected by risers H in turn, is connected tothe watersupply |1 WWW- Y correspondingly arranged below the passfline oif the work. between the rollers of the table; is

|2" carrying no les la' disposed in emsitonib .tho'seidf the upper bank. In asmnr mannerteniet already e pipes i2: ereonncted to'a; manifold leftye quantities drives-r. the temperature lor which@ formation. Borderline or mixed structures are@ Vrapid and uniform the Tri temperatures (Figure 1) in accordance with the above, to be acquired in separate heattreating operations. The present invention, 'by utilizing the residual` heat of rolling in conjunction with special apparatus for the impartation of preferential cooling, has achieved the adjustment of the structural `constituency without the need of further heat treatment. The manner vby which this is accomplished, in a preferred -form of the invention,-will be understood by, reference to Figures 2 to 7, inclusive, of the drawings.

Figure Zig a schematic plan view ofa hot-mill run-out table to which spray apparatus, in accordance with the present invention, is applied.` The last stand 5v of'the continuous hot-mill is shownat the left-hand end of this iisure. rela. tive to which the stock moves into the direction ci the arrows. After emerging from the roll stand 5, the stock is a descaler l, and one or more spray units 8, which are designed to give eilect to the present invention, and which will be described. in greater detail hereinbelow. is conducted ble 0 to coilers l0, produced, the stock It strip is being produced, the stock by a continuation ofl the roller taor if plates or sheets are being is conducted beyond 'the coilers l0 by a continuation oi' the roller table 9, through another series oiy sprays B, to a stacker or piler unit li, from which they are removed for storage or further processing.

' Each of the spray umts 0 may comprise anindependently controllable series of sprays arranged above and below the' roller table in a manner similar to that shown in Figures .3 and 4.

suitable structure Vto this end is intended, although the one illustrated has proved to be highly eillcient under conditions of actual operation; In these drawings, the conveyer-tablerollers are indicated at 9', relative to which the path line oi' stock-travel falls tangent to their upper surfaces. A plurality of transverselydisposed spray-pipes are horizontally arranged across the roller-table above the pass-line of the VWork. Each comprises a pipe I2 having-a plurality of nozzles It positioned approximately ten inches from the up- Jer surface of the workpiece. As illustrated. here are ten (10) cross-pipes |2 carrying seveneen (17) nozzles each. The cross-pipes I2 are 75 water. The structure adjusted to a uniform 'temperature to safeguard' from thefheaviest plate section, abstraction of heat will be eil'ected without warping material.4 To this end,

water, under 1000 to 1200 lbsyper. square inch pressure, is handled by each unit at the rate or l 2800 gallons per minute, and the units are soarranged in connection with the main water supply as to be turned on other, as' by valves l0 (shown in Figure 2), to admit of cooling in any pre-selected manner.

Except on such installations where coiling and piling can be accomplished at substantially the same location in reference to a run-out table, it is usual to provide separate groupsof sprays for material produced in continuous lengths, which will be coiled as at l0 in Figure 2. and for that which is cut into sheets or plates, which will be piled as at H in this same iigure. Thus, in Figure `2, one or more oi the spray unitsvto the left ofcoilers i0 may be used in the production of strip, while in i v number or none of these iirst units will be used to conserve the heat of the material for its longer travel down the table, relying upon units to the right of the coilers i0 to make the final temperature adjustment fairly close to the pliers ii.

As a typical example of this invention, armor u plate manufa ured from steels' of S. A. E. iflcatlons ranging from 3120 m to 3325 which include steels containing carbonwithin the range of 0.20 to 0.30%, nickel from 1.25 to 3.50%, and chromium from 1.00 to 1.60%, hasv been produced in nal gauges ranging from to 1 thick. Slabs of such material were preheated so as `to leave the last pass or the finish- 'ing mill (Ein Figure 2) at temperatures ranging from 1650 to 1750, or, in any case, within the l the 'hot plates advanced t they' were sprayed by the left or the coller I0, and again by the units 0 to the right of the coller lil adiacent to the plier, at which point they were at a temperature less than the boiling point of derived was hard martensite, evidencing transformation in the middle to lower region of zone E, Figure l, and the plates condition.

vaustenitic range; A3 down the roller table three of the vunites to erable to reheat the I temper the martensitic structure. stances, this hasbeen accomplished by controlling the sprays to reduce the surface temperature of the' plates to a temperature in theprder of 400, allowing the residual heat in the core oi the stock to draw the surface metal to the required temper. The temperature gradient in the heavier sections is one that steadily declines in the central portion ofthe cross sections to a Y to a manifold il, which,

and oil? independently of each the case of sheets'orplates, a lesser' high ballistic rating, is possessed istic very hard `balnite, martensite, or

depending upon w surface. A sharp decline of temperature at the surface, as a result of direct contact 'with water, increase in temperature as the and thermal equilibrium is restored throughout the mass by redistribution of heat f' m the hotter interior to the cooler exterior. 'Instead of the usual homogeneous sorbitic structure elated with quenched and drawn composite structure of upper bainite and martensite has been observed, which is attributable to the interrupted quench-and-draw as here applied. The steel thus produced, in addition to a l of deep hardenability, good weldabality, and, under heavy shock test, does' not spall, which,-particular1y inthe thinner sections (less than is a characterto obtain. Where softer structures are desired, the cooling may be retarded by controlling the sprays 8, aswell as the speed of travel of the plate, to give progressively softer structures ranging from martensite up to pearlite, and, in the case of carbon steel, the' temperature diderentials from the surface to the core of the section has been such as to produce a fine pearlitic center and a spheroidized surface onv this treatment. These are plates subjected .to examples of dilerential reflex tempering.

Y T he'nal structural constituency is ,determined 1 by the rate A austenlte-to-ferrite transformation point. If the :of cooling and its effect upon the latter is depressed below the nose N of the S- curve, as shown in Figure l, ferrite and carbides, .borderline structures, may be developed at progressively lower temperatures,

' here transformation curve is intersected: The resulting structure may be retained as is, or may be' modified by regular tempering treatment or differential reflex tem- E- The sprays are designed to afford uniform cooling to prevent warpage. They have sumcient force .to wipe away steam envelopes which tend to form insuiatory sheaths about the plates and -c'ausenon-uniform cooling conditions, while the volume of water delivered is such as toabstract the-heat required. Any suitable equipment and `iluid medium vis intended. for this purpose. This practice,

as distinguished from the older practices in which the steel had to-be subjected to several reheatings, including a quench in a bath, is greatly simplified and expedited. As has been said, the eir'ect practices was to induce in turn, required several flattening operations in heavy presses, in the case of plate gauge stock, and roller leveling or cold-passing in the case of strip and sheets. All'of this has been eliminated in the present invent'on, which is outstanding because production schedules for armor plate, previously measured in days, are now measurable in minutes by application of the teachings hereof.

In strips and sheets', too, the vigorous sprayquenching, described above in connection with plate-gauge material, can be applied, though less drastically to conform to the requirements of the lighter sections. Since the masses involved in sheet and strip gauges render controlled abstraction of heat more dimcult, there has been developed a differential reflex tempering method, in accordance with the present invention, which, while equally applicable to plate sections and heavier gauges, is ideally adapted to heat-treat ing strips and sheets in a controlled manner sesevere Warpage, which,-

normally asso-V armor plate, av

of the bath-quench in the prior down-and-up (averaging down) gradient at the alternation, hot-.cold-hot-'cold-hot,

lectively to impart the desired metallographic structure to the metal. This method is illustrated in Figure 5 of the drawings. -Successive sheets or plates are alternately quenched (20C) by the spray, to any required'lower temperature down to room temperature, while the remainder (20H) down the table without quenchas VVto reach the than the quenched sections (20C) preferably near or above the austenitic temperature (around 1400nv or above). The relatively :'cold'fand hot sheets or plates, thus derived, arefstacked in etc.. to provide a pile 2l of mutuallysandwiched hot and cold sheets. It is obvious thatthe -heat in the hot sheets or plates will beA transmitted tc their colder neighbors to effect a tempering-fof the quenched structure existing in thev latter, while the rate of abstraction of heat from the sheets or plates by the relative cold ones .will give an accelerated cooling through the critical temperature'range selectively to afford the desired structure depending upon the initial temperature differential between the twoJsets of sheets. As thermal equilibrium is approached, av soaking temperature uniformly distributed will prevail, allowing the pile slowly to cool to room temperature. Covers; or a holding furnace, may be utilized to cool the pile more slowly and'uniformly.

Different structures are *obtainable through different combinations of mass-temperature relationships, e.l g., two or more' hot sheets may be included for every cold one, or vicel versa, thus providing slower or faster cooling for the hot sheets, and higher or lower drawing temperatures for the cold ones, which, in accordance with the transformation as determined by the S-curve of' the steel of thecompositionfin question (of which Figure 1 is a typical example), will afford dictable and controllablemanner. It is obvious that sheets or plates'of intermediate tempera tures may be interspersed with the extremely hot and cold ones to vary the conditions of transformation and tempering as of piling should be noted in order to permit the repiling of sheets and plates into groups of metallographically similar characteristics.

The same idea is adaptable to strips (Figure 6) by quenching one or more continuous lengths 22C while leaving others hot 22H, superimposing one upon another in any desired relationship, and then coiling together 23 for the attainment of thermal equilibrium. More conveniently, contiguous portions cf a single strip 22 may be alternately cooled (as by spraying) and left hot, respectively, so as to provide a strip having hot and cold areas (H and C, Figure '1) in succession. Upon coiling a strip preparedkin this manner 23, isothermal conditions are approached, thus ad- 'justing the allotropic condition of the metallic structure of the entire strip in one operation. As an assumed example, the cold sections C of 'a strip treated in this manner may have been quenched to insure transformation into martensite; i. e., cooled at. a rate so as to attain a temperature below Tri in tx seconds or less for transformation in zone E, Figure v1. The temperature or the hot sections H upon coiling will be suck g., l400 F.) as to draw the temper of the colder martensitic portions into a structure of say, sorbite, while the heatex'change attendan upon this tempering will effectively quench thi hot sections of the strip down to a temperatur desired. IThe order "and unal equilibrium conditions in l may be expressed by the following equation:

heat has been been attained (in the assumed example, this- Should fall between 500 to 700 F.), will afford a slow cooling to room temperature from which a homogeneous structure throughout the 'entire strip is assured.

The. pre-established temperature differential, and the ratio ,of the masses of the hot and cold portions .provide adjustable variables for the at-` tainment of any desired coil or stack-tempering another way, the mass-temperature differential prevailing between the hotter and colder .portions of a stack, pile, or coil, is such as to eiect selff (reilexive) annealing and tempering.

The relationship between mass-temperature pack or coil in which represents the highest theoretical equilibrium temperature; T, the total heat l(in calories); M, total mass under treatment, and K, a constant to correct for heat-losses. to the air, scale, etc. To simplify. the following illustrations,

reduced to its function, temperature. expressed in degrees Fahrenheit, since the individual masses involved have been regarded as unity.

I Example I be assumed that succarbon steel, finished sheared tothe same size, .are advanced from thelast stand (5, Figure 2) of a continuous hot at a temperature of 1678 F. Alternate sheets are quenched to 78 F.

To illustrate this, let it cessive sheets of eutectoid to the same gauge and (room temperatur.) while the remainder is a1- lowed to proceed to' the piler unquenched. Let lit be assumed further that the temperature loss of the hot sheets in traversing the table is about 100 F. Then, with mass as unity, and disregarding the losses which are constant:

Eabample Il c I'he effect of mass may be by which ature of the quenching medi volved at an appointed temperature, and the selectively varied Y ,let one sheet be quenched, and two sheets unquenched. Then:

In this casejtransformation will be performed close to the Ae temperature to afford coarse pearlite with agglomerated carbides due to the arrestedl cooling at s'uch a. high temperature. The quenched sections will have a strong, ductile, nodular structure of sorbite and spheroidized cementite with exceedingly ne grains. 'l

Examples illustrative of the effects derived from varying the mass-temperature relationships could be multiplied indenitely. The foregoing, however, are sucient to illustrate the principle any 'condition of quench, quench, quench and draw, Austempering (Patent No. 1,924,099), Purnelliz'ing (application illed July 18, 1940, Serial N o. 346,224), and related heat-treatments, may be accomplished. In the foregoing examples, fsheets are intended to apply to any cut lengths irrespective of gauge.' or contiguous sections of strip, or other long lengths, whether stacked or coiled. v y

Apart from the obvious advantages of simplicity,

automatic aspects ofthe invention whereby, once the mass-temperature relationships are established empirically for a steel of given composition,

a given set of physical properties, in terms of structural consistency, is duplicatable indefinitely. The nishing temperature of the stock, the quantity and temperthe masses 'inspeed of stock-travel, all maybe or iixed at will to produce the desired results. The promptness with whichthe quenching is followed by tempering reduces the losses occa- `sioned by delays sometimes encountered in this interval, .thus preventing strain cracks and fallures, vand destruction of the properties of the metal. v

It will be appreciated, as set forth inthe following claim, that it is the simpliiled processing to which the present invention is directed by whatever means attained, and that the continuous hot mill, used herein for purposes of illustration, is anexample without limitation.

It should be understood, therefore. that many modifications and vvariations of the present invention may be made, whichthough departing from the letterof this specification, Willfnot be without the spirit` andv scope of the invention as are more particularly apprehended in Vand by illustrated, by the following in furtherance of Example I:

(A) For each sheet piled at 1578 F., let two sheets be piled at quenched temperature (78").

Then:

the appended claim.

We claimr The method of ing them to a hardness Vrequiring tempering;

PATRICK H. HUME. WnmAM n. MCGARRITY. EDWARDL. RoBmsoN.

interrupted economy and expediency, other benefitsare derived from the treating steel strip which incoiler; quenching certain of thejsections to a temperaturel harden-

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