US 3507711 A
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United States Patent Office 3,507,711 Patented Apr. 21, 1970 3,507,711 HIGH-STRENGTH STEEL AND NOVEL WIRE PRODUCT Robert M. Fisher, Wilkins Township, Pa., assignor to United States Steel Corporation, a corporation of Delaware No Drawing. Filed May 29, 1967, Ser. No. 642,134
Int. Cl. C22c 39/08 US. Cl. 148-36 5 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a high-strength steel. More particularly, the invention concerns a steel suitable for patenting and for the manufacture of highstrength wire. Steel in accordance with the invention is characterized by a very fine pearlite spacing in the patented condition.
There is presently produced steel wire having a tensile strength of up to 520,000-540,000 p.s.i. at a size of about 0.007-inch or less. However, there is a demand for wire of significantly higher strength, i.e. 600,000 p.s.i. or more in this size range, or alternatively for wire in the 550,000 p.s.i. range but of significantly larger sizes.
High-strength wire has been drawn from pearlitic steel for many years and it has been known that fine pearlite results in a higher-strength product than coarse pearlite. The fineness of the pearlite is determined in part by the heat treatment to which the steel is subjected. A common heat treatmentfor steel used in wire production is referred to as patenting. Patenting is a heat treatment applied to rods and wire having a carbon content of 0.4% and higher. The object of patenting is to obtain a structure which combines high tensile strength with high ductility. In the case of wire this structure provides the ability to withstand heavy drafting to produce the desired finished sizes with the required high tensile strength and toughness.
Patenting is .always conducted as a continuous process and consists of first heating the material to a point well above the critical temperature and then cooling from the critical temperature at a rapid rate to a predetermined temperature level at which a transformation will yield the desired microstructure and mechanical properties.
There are a number of patenting techniques and in a common one, the work is continuously passed through a molten metal bath of controlled temperature. For a fine pearlite structure, the patenting temperature and the speed of the patenting temperature are adjusted to provide minimum time at temperature. However, if the transformation line is lowered too much, i.e. into the bainite range, coarse upper bainite is formed which results in a somewhat weaker metallurgical structure after working, i.e., drawing. Thus, the finest pearlite structure that can be developed for patenting is determined by the isothermal transformation characteristics of the steel being used.
Most alloying elements increase with pearlite spacing and in the past, no deliberate effort has been made to develop an alloy with optimum characteristics for drawing into high-strength wire by providing a very fine pearlite spacing.
The present invention provides a novel steel composition which is capable of achieving a very fine pearlite spacing upon heat treating, e.g. patenting. According to the invention, there is provided a steel consisting essentially of, in percent by weight, 0.85 to 1.0% carbon, 1.0 to 4.0% cobalt, preferably 2.0 to 4.0% cobalt, up to 0.5 preferably 0.2 to 0.5% or at least one element from the group consisting of molybdenum, chromium and tungsten, up to 0.7% manganese, preferably 0.2 to 0.4%, and the balance iron with normal steel-making residuals. Any steel-making processes capable of producing a clean highquality melt may be employed in making an alloy in accordance with the invention.
A characteristic of the novel steel composition described above is that in the patented condition, it can be made to achieve a pearlite spacing not greater than about 400 angstroms in a metallurgical structure consisting of at least pearlite, and preferably as close to pearlite as practical. As used herein in connection with the invention, the term fine pearlite spacing refers to the aforementioned maximum spacing of about 400 angstroms between carbide platelets in the pearlite.
The following examples will illustrate how steel in accordance with the invention may be processed to produce high-strength steel wire of very fine diameter.
A series of 300 lb. heats of steel having the composition of steels A and B described in Table I were vacuum melted and cast into ingots which were then forged into 4-inch billets. The billets were hot rolled to -inch rod. The hot-rolled rod was drawn in four stages to 0.005- inch wire. Before each drawing operation, the rod or wire was patented in continuous-line processing. The first patenting operation was. performed on the -inch rod and comprised heating at 1800 F. for 360 seconds, prequenching to 550 F. for 37 seconds, and reheating to 900 F. for 27 seconds. The rod was then dry drawn to 0.220-inch wire after which it was patented by heating at 1800 F. for seconds, prequenched to 550 F. for 24 seconds and reheated to 900 F. for 17 seconds. Following this second patenting, the O.22()-inch wire was dry drawn to 0.102-inch wire and patented for the third time. The third patenting step comprised heating at 1800" F. for 75 seconds, prequenching to 550 F. for 12 seconds and reheating to 900 F. for 8 /2 seconds. The 0.102-inch wire was dry drawn to 0.032-inch wire and given a final patenting which comprised heating at 1800 F. for 1 8 seconds, prequenching 2 seconds at 550 F. and reheating to 1000 F. for 2% seconds. The patented 0.032-inch wire was then given a flash coating of copper and wet drawn (with water) to 0.005-inch wire.
TABLE I An evaluation of the wire so produced indicates that in both cases, the pearlite spacing was less than 400 angstroms. The 2.0% cobalt wire had a yield strength of about 550,000 p.s.i. and the 4.0% cobalt had a yield strength of about 600,000 p.s.i. When drawn to 0.003- inch diameter, the wire had a yield strength of 700,000 p.s.i. This is to be contrasted with plain wire of the same diameter treated in the same manner which has a yield strength of about 520,000 p.s.i. and is characterized by a pearlite spacing of greater than 400 angstroms.
It has been found that when at least about 1.0% cobalt is included in the high-carbon steel, a significant reduction in pearlite spacing is achieved. However, if more than about 4.0% cobalt is included, it is impossible to obtain the desired fine pearlite spacing without the formation of undesirable massive carbides. Similarly, the carbon must be carefully controlled to within the range of 0.85 to 1.0% to obtain fine pearlite spacing also while avoiding massive carbide formation. The presence of cobalt accelerates the pearlite transformation and to acommodate this steel to present processing practices, it is desirable to add a small but effective amount of at least one of molybdenum, chromium or tungsten to retard the pearlite transformation reaction sufficiently to compensate for the cobalt without affecting the pearlite spacing. By heat treating an alloy of the composition described above, it is possible to manufacture wire having a yield strength of at least 550,000-600,000 p.s.i. in wire having diameters of 0.007-inch and less and 700,000 p.s.i. at 0.003-inch diameter.
It is apparent from the above that various changes and modifications may be made without departing from the invention. Accordingly, the scope of the invention should be limited only by the appended claims whereby what is claimed is:
l. A steel wire not greater than 0.007-inch diameter having a yield strength of at least 550,000 p.s.i. and a pearlite spacing not greater than about 400 angstroms consisting essentially of, in percent by weight, 0.85 to 1.0% carbon, 1.0 to 4.0% cobalt, up to 0.5% of at least one element from the group consisting of molybdenum, chromium and tungsten, up to 0.7% manganese and the balance iron and normal steel-making residuals.
2. A steel wire in accordance with claim 1 of 0.003- inch diameter having a yield strength of 700,000 p.s.i.
3. A steel wire in accordance with claim 1 having 2.0 to 4.0% cobalt.
4. A steel wire in accordance with claim 1 having 0.2 to0.5% of at least one of molybdenum, chromium and tungsten.
5. A steel wire in accordance with claim 1 having 0.2
A to 0.4% manganese.
References Cited UNITED STATES PATENTS CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,507,711 April 21, 1970 Robert M. Fisher It is certified that error appears in the above identified patent and'that said Letters Patent are hereby corrected as shmm below:
Column 1, line 59, "temperature" should read line line 61, "line" should read temperature Signed and sealed this 22nd day of December 1970.
Edward M. Fletcher, Jr. E. JR.
Attesting Officer Commissioner of Patents