US3224087A - Method of making high-strength aluminum-coated steel wire - Google Patents

Description

2 Sheets-Shee'fl 1 R. A. NICKOLA ETAL Dec. 21, 1965 METHOD 0E MAKING HIGH-STRENGTH ALUMINUM-COATED STEEL WIRE Filed om. 1o, 1962 1 O90/ By @WM M Attorney Dec. 21, 1965 R. A. NlcKoLA ETAL 3,224,087

METHOD OF MAKING HIGH-STRENGTH ALUMINUM-COATED STEEL WIRE Filed Oct. 10, 1962 2 Sheets-Sheet 2 Q /sal PWS/ww Hw/vaals 37/5 Q w. una www 'AJH/1300 dmv/u w Nou rfa/v0 73 Nasa/3d l/JU/ 0/ -mmm @www fanta. unk

...W mam m mim United States Patet ffice 3,224,087 Patented Dec. 21, 1965 3,224,087 METHD E MAKING HIGH-STRENGTH ALUMINUM-@DATED STEEL WERE Richard A. Nickola, Bedford Heights, and Gordon T.

Spare, Chardon, Uhio, assignors to United States Steel Corporation, a corporation of New Jersey Filed Oct. 10, 1962, Ser. No. 229,583 Claims. (Cl. 29-527) The invention relates to the manufacture of aluminumcoated steel wire and more particularly to the manufacture of hot-dip aluminum coated steel wire having a tensile strength in excess of 200,000 p.s.i.

In the drawings:

FIGURE l is a graph showing the effect of cold work on the tensile strength of steels of various carbon contents;

FIGURE 2 is a graph showing the effect of time and temperature on the ductility as percent elongation of steels of carbon contents as indicated thereon, cold drawn 75, 85 and 90%;

`FIGURE 3 is a graph showing the effect of die entry `angle on the mechanical properties of 0.87% aluminum coated carbon steel cold drawn 83% prior to the coating operation;

FIGURE 4 is a similar graph showing the effect of die entry angle on the mechanical properties of the steel of FIGURE 3 when cold drawn 8.6% prior to aluminum coating; and

FIGURE 5 is a sectional view through a drawing die illustrating die entry angle.

Steel wire is commonly coated with aluminum by passing the wire as a strand through -a molten bath of aluminum or alloy thereof. To effect a satisfactory coating, the steel must be brought to a temperature above the melting point of the bath, e.g. 1100-1250 F. depending on bath composition and maintained there for a short period of timerto effect a slight alloying at the innerface of the steel and the aluminum coating. Excessive alloying is detrimental. Since the rate of alloy formation is relatively insignificant below the melting point of the bath but is extremely rapid above this temperature, the time interval, measuring from the moment the wire reaches bath temperature until it is cooled below this temperature, is of critical importance. It should be noted that a major portion of this interval occurs during the cooling of the wire after it emerges from the bath.

The portion of the above interval chargeable to immersion in the bath can be controlled by selecting the bath length with regard to the gage of the wire and the lineal speed of movement thereof through the bath. In this regard it must be remembered that the bath serves to bring the wire to temperature, thus considerations incident to this purpose will fix a minimum bath length. The heating function involves raising the temperature of the wire about 1000 F. and this can be accomplished in wires up to l1 gage in about 1 second. Thus at a speed of say 180 f.p.m. (3 f.p.s.), a bath length of 3 feet is required. Assuming a bath of this length has been provided and is operated on 1l gage wire moving at 180 fpm., the portion of the alloying interval incident to immersion in the coating bath becomes substantially zero. Although this ideal condition will not exist for all conditions of a commercial operation, it is apparent that in a properly designed unit, the portion of the alloying interval chargeable to the bath itself is reduced to a fraction of a second.

The portion of the alloying interval chargeable to the cooling step however, is not so easily reduced. Cooling of the coated wire is accomplished by passing the wire from the bath into the atmosphere and under the most favorable conditions such air-quenching requires approximately 10 seconds to reduce the temperature below l000 F. More drastic quenching cannot be tolerated since it adversely affects the appearance of the coating.

In view of the alloying considerations discussed above, it is obvious that an aluminum coating operation subjects the steel wire to a thermal treatment of approximately 10-12 seconds at temperatures above 900 F. Such con ditions are known to affect the properties of steel in the cold worked condition; in fact the temperatures involved are essential to those of the treatment commonly termed, subcritical annealing.

High tensile wire is manufactured by subjecting the wire to high degree of cold work. In general, the level of tensile strength depends upon the carbon content of the steel and the amount of cold work, as indica-ted by FIGURE l, wherein the strength versus cold work curves of steels of four different carbon contents are plotted. The curves relate to the steels in the uncoated condition and it is apparent therefrom that, at carbon levels above about 0.45%, tensile strengths in excess of 200,000 p.s.i. are readily obtainable in the uncoated wire. However, as noted above, an aluminum coating operation subjects the wire to annealing temperatures. The effect is to reduce the tensile strength; the amount of the reduction varying with the temperature and the time at temperatures above about 900 F. In conventional aluminum coating operations it has been found that the tensile strength of wire cold reduced is lowered as much as and sometimes more than 100,000 p.s.i. Thus in order to insure production of aluminum coated Wire of a specific strength level, it is necessary to start with a wire cold worked to a considerably higher degree than wire of the same strength level for use in the uncoated condition. Use of this expedient heretofore, however, has not provided a satisfactory answer; for, contrary to accepted metallurgical knowledge which predicts that the ductility of steel increases as its tensile strength decreases, the decrease in tensile strength effected during aluminum coating is accompanied by a decrease of ductility of the coated w1re.

In investigating the latter problem, we have discovered the heretofore unobserved phenomenon that during a relatively short initial period at temperatures in the neighborhood of l050l250 F. the ductility of very heavily cold drawn steels is significantly decreased and the steels embrittled. (When the time at temperature is extended, the steels of course soften in the normal manner assocated with a subcritical anneal.) The effects are illustrated in FIGURE 2 which comprises a series of graphs wherein ductility, as expressed by percent elongation, after heat treatment at (a) 1250 F. and (b) 1050 F. has been plotted against the logarithm of time (seconds) at said temperatures for four steels containing 0.85, 0.60, 0.40 and 0.20% carbon, respectively, each cold drawn to three different strength levels, Thus, each graph comprises three curves, and in each the solid line curve is specific to steel of the carbon content noted above the graph when cold drawn about 75%, the dot-dash curve to the same steel when drawn about 85% and the dotted curve when drawn about 90% The strength level (prior to heat treatment) achieved by the drawing has been noted on each curve. As evident from the graphs, the embrittlement is associated with the percentage of cold work in the steel; it does not develop below about 75% cold work but above this value occurs in steels of practically all carbon ranges and persists over longer times at temperature in steels of higher carbon content. It will be noted that the conditions producing the embrittlement are almost precisely those imposed in the production of high tensile, aluminum coated wire.

It is an object of the present invention to provide a method negating the embrittlement of heavily cold work steel occurring during aluminum coating thereof.

We have found this is accomplished by the simple expedient of drafting the wire through dies of smaller than normal entry angle. The conventionally used entry angle ranges from approximately 18 to 24. We have discovered that at least insofar as steels in the range .8 to .9% carbon are concerned when such are cold drawn between 75-85% through dies having entry angles of between 10 and 15 and then hot-dip aluminum coated in the conventional manner, the ductility of the product as measured by the conventional wrap-ductility test is raised from practically to 60% perfect buttons; 100% perfect buttons representing perfect ductility. The effect of entry die angle on the properties of aluminum coated wire are illustrated in the graph of FIGURE 3. The curves of this figure are typical of the results of our invention as applied to nominal 0.85% carbon steel and are specific to such steel cold drawn about 83%. As shown in the figure, decrease in entry angle of the die has little effect on the tensile strength of the wire but markedly increases ductility as measured by percent elongation in the standard tensile test. However, it will be noted that While percent elongation is a maximum at an entry angle of 5, the wrap-ductility drops precipitously as the die angle is decreased below Thus it is apparent that the performance of the coated wire is not determined solely by the ductility of the steel base.

The efficacy of angles between 10 and 15 is not completely understood. It seems improbable that a change in die angle could prevent the embrittlement of the steel; more likely reducing the angle acts only to offset the effects thereof, possibly by more uniformly working the steel throughout the cross section thereof thus minimizing the presence of stress-raisers at the innerface of steel and coating metal; while the loss of effect noted at angles less than 10 is possibly because of an increase in the tendency to galling, which acts to again increase the incidence of such stress-raisers. However we have not been able to establish this hypothesis to our complete satisfaction and accordingly do not wish to be limited thereto. In any event, the beneficial effect of die angles between 10 and 15 is completely lost if the steels are drawn more than about 85% as clearly evident on comparison of the curves of FIGURES 3 and 4. The latter figure is a graph showing the effect of the entry angle of the drawing dies on the properties of the steel of FIG- URE 3 when drawn 86% prior to the aluminum coating operation. At 86% cold work, while the tensile strength and elongation increase as the entry angle is decreased, the wrap ductility is unaffected.

The wrap ductility test is commonly used to determine the suitability of wire to its conditions of use; it comprises sampling a coil of wire at five places, each sample is then wound about its own diameter to form five complete wraps. In the case of aluminum coated wires cold drawn more than 85%, the wire breaks in the first or second wrap.

The following specific example serves to illustrate the preferred practices of our invention in the production of Percent Reduction in Area Draft Diameter (inches) Per Draft Cumulative Any of the conventional wire drawing compounds can be used and, if desired, the entrance angle of the first die of the above series may be increased to 15 or even as much as 24 to promote distribution of the lubricant without adversely affecting the ultimate product. Following drafting the wire is cleaned and coated by passing through a molten bath of aluminum or aluminum alloy in the conventional manner. The resulting product is characterized by a tensile strength of the order of 226,000 p.s.i. and substantially perfect wrap ductility.

While we have disclosed a specific embodiment of our invention, we do not wish to be limited thereto except as defined in the appended claims.

We claim:

1. In the method of producing high tensile strength steel wire containing 0.8 to 0.9% carbon hot-dip coated with aluminum by passing it through a bath of molten aluminum, the improvement comprising drastically cold drawing the steel wire between 75 and 85% prior to coating by drawing it through dies having an entrance angle between 10 and 15 2. In the method of producing aluminum coated, high tensile strength steel wire containing .8 to .9% carbon having a tensile strength in excess of 200,000 p.s.i., said wire being hot dipped coated with aluminum by passing it through a bath of molten aluminum, the improvement comprising cold drawing the wire prior to coating to reduce its cross-sectional area between 75 and 85 by drawing it through dies having an entrance angle between 10 and 15 3. In the method of producing hot-dip aluminum coated steel wire having a tensile strength in excess of 200,000 p.s.i. and improved wrap ductility comprising cold reducing steel wire having a carbon content in the range of 0.8 t0 0.9% by drawing it through dies having an entrance angle between 10 and 15 to reduce its cross-sectional area in excess of 75 but not more than 85% and thereafter passing said wire through a bath of molten aluminum maintained at a temperature above 1100 F., said die angle offsetting embrittlement of the drastically cold drawn Wire when heated in the range 1050 to l250 F. during passage through the aluminum bath.

4. In the method of producing hot-dip aluminum coated steel wire having a tensile strength in excess of 200,000 p.s.i. and improved wrap ductility comprising cold reducing steel wire having a carbon content in the range of 0.8 to 0.9% by drawing it through a series of dies having an entrance angle between 10 and 15 to reduce its 5 6 cross-sectional area in excess of 75% but not more than References Cited by the Examiner 85% taking substantially equal reductions at each die of UNITED STATES PATENTS sa1d ser1es and thereafter lpassing said wire through a bath 297,551 4/1884 Agnew 205 26 of molten aluminum mamtalned at a temperature above 3 118 223 1/1964 Schull et al 29 196 2 X 1100 F., said die angle offsetting embrittlement of the 5 drastically cold drawn wire when heated in the range 1050 OTHER REFERENCES to l250 F. during passage through the aluminum bath. U.S. Steel, The Making, Shaping and Treating of Steel,

5. The method of claim 4 wherein the first die of said 7th ed, 1957, Pp. 684, 711-712. series is provided with an entrance angle greater than 15.

WHITMORE A. WILTZ, Primary Examiner.

Claims (1)

1. IN THE METHOD OF PRODUCING HIGH TRENSILE STRENGTH STEEL WIRE CONTAINING 0.8 TO 0.9% CARBON HOT-DIP COATED WITH ALUMINUM BY PASSING IT THROUGH A BATH OF MOLTEN ALUMINUM, THE IMPROVEMENT COMPRISING DRASTICALLY COLD DRAWING THE STEEL WIRE BETWEEN 75% AND 85% PRIOR TO COATING BY DRAWING IT THROUGH DIES HAVING AN ENTRANCE ANGLE BETWEEN 10 AND 15*.

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