US 3900347 A
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United States Patent [1 1 Lorenzetti et al.
[4 1 Aug. 19, 1975 COLD-DRAWN, STRAIGHTENED AND STRESS RELIEVED STEEL WIRE FOR PRESTRESSED CONCRETE AND METHOD FOR PRODUCTION THEREOF  Inventors: James J. Lorenzetti, Franklin; James N. Cordea, Monroe, both of Ohio  Assignee: Armco Steel Corporation,
Middletown, Ohio  Filed: Aug. 27, 1974 21 Appl.No.:500,932
 U.S. Cl. 148/12 B; 148/36  Int. Cl C21d 9/52  Field of Search 148/12 B, 36
 References Cited UNITED STATES PATENTS 3,532,560 10/1970 Tomioka et al. 148/12 B 3,844,848 10/1974 Stacey 148/12 B Primary Examiner-W. Stallard Attorney, Agent, or Firm-Melville, Strasser, Foster & Hoffman [5 7] ABSTRACT Cold-drawn, straightened and stress relieved high carbon steel wire for prestressed concrete, and a method for the production thereof, having reduced load loss due to stress relaxation and good ductility. The steel comprises, by weight percent, from about 0.70 percent to about 0.90 percent carbon, about 0.5 percent to about 110 percent manganese, about 0.025 percent maximum phosphorus, about 0.035 percent maximum sulfur, about 0.15 percent to about 0.35 percent silicon, about 0.010 percent to about 0.020 percent nitrogen, about 0.010 percent to about 0.030 percent columbium, and remainder essentially iron except for incidental impurities. The cold-drawn wire has a pearlitic microstructure wherein the pearlite colony size is reduced by adding columbium to confer good ductility, and sufficient uncombined nitrogen is present to provide improved stress relaxation properties. Processing includes austenitizing a hot-reduced rod stock at about 980C to about 1030C and isothermal treating at about 540 to about 590C, followed by air cooling. The stock is then cold-drawn into wire, straightened, and stress relieved.
7 Claims, N0 Drawings COLD-DRAWN, STRAIGHTENED AND STRESS RELIEVED STEEL WIRE FOR PRESTRESSED CONCRETE AND METHOD FOR PRODUCTION THEREOF BACKGROUND OF THE INVENTION lished Dec. 14, 1971, discloses high carbon steel wire having excellent resistance against relaxation by reason of a total nitrogen content of 0.008 to 0.05 percent by weight (the steel further containing 0.55 to 0.85 percent carbon, 0.30 to 0.90 percent manganese, 0.15 to 0.35 percent silicon and remainder iron except for impurities unavoidable in the steel-making process) wherein aluminum is restricted to a maximum of 0.015 percent by weight. Other nitride formers such as titanium, vanadium, columbium and the like, should also be kept at a low level, according to the disclosure, in order to permit the presence of at least about 0.008 percent by weight uncombined nitrogen, thereby attaining steel wire having reduced load loss due to stress relaxation. Silicon is-used as a deoxidant in order to obtain a fully killed starting material.
United States Pat. No. 3,671,334, issued June 20, 1.972 to J. H. Bucher et a1, discloses a formed and strainaged steel article having a yield strength above 70 ksi, and a process for the production thereof, the steel containing 0.08 to 0.18 percent carbon, 0.3 to 1.0 percent manganese, 0.01 to 0.05 percent columbium, 0.008 to 0.014 percent nitrogen, 0.10 percent maximum silicon, less than a total of about 0.02 percent of aluminum, zirconium, vanadium and titanium, and the balance essentially iron. The steel of this reference is thus a low car- I grain size of the steels. It. is pointed out that columbium is the only one of the conventionally employed grain refining elements, i.e., zirconium, vanadium and titanium, which is not also a strong nitride former; and, consequently, is the only one of these elements which can be used in conjuction with nitrogen to produce a strainaging steel.
An article by T. Gladman et al. in Journal of the Iron and Steel Institute, pages 916-930 (December 1972) discusses the influence of aluminum, titanium, vanadium and niobium (columbium) as grain refiners in steels containing 0.4 to 0.6 percent carbon, 1.4 to 1.5 percent manganese, 0.3 to 0.9 percent silicon, 0.016 to 0.021 percent nitrogen, and balance essentially iron. From experimental data reported therein, it was concluded that less than the expected dispersion strengthening was obtained when useing niobium, and may be attributed to the very restricted solubility of niobium in i a high-carbon austenite. The article was concerned with factors controlling yield and tensile strengths and impact transition temperature in high carbon ferritepearlite steels, and it was found that low uncombined or free nitrogen was necessary in order to obtain low impact transition temperatures. Hence, only aluminum, titanium and vanadium were considered advantageous in steels having a high total nitrogen content.
While the above-mentioned Japanese Patent Publication disclosed high carbon wire having reduced load loss due to stress relaxation, by reason of a relatively high uncombined nitrogen content, this increase in nitrogen would render the steel subject to excessive breakage during wire drawing and cold straightening if heated above about 940C before drawing;
The problem of obtaining adequate ductility in prestressed wire of the type disclosed in this Japanese reference is not believed to be obvious to a person skilled in the art in view of the disclosures of the above-mentioned Bucher et al. patent or the Journal of the Iron and Steel Institute articles. The Japanese reference emphasizes the criticality of maintaining the elements aluminum, titanium, vanadium and columbium at a low level. More specifically, the maximum aluminum content is stated to be 0.015 percent while the minimum total nitrogen content is 0.008 percent.
The disclosure in the Bucher et al patent that columbium is not a strong nitride former would be of no benefit to a person skilled in the art if faced with the problem of obtaining good ductility in a cold drawn wire of the type dislcosed in the Japanese reference. Bucher et al disclose a relatively low carbon steel having a fully ferritic microstructure in the cold rolled and strained condition. The columbium addition in such a steel controls grain size after the austenite to ferrite transformation, a mechanism which is entirely different from that of transformation of a high carbon steel to pearlite.
In ferritic steels particles such as carbides in the grain boundaries restrict the ferritic grain size. A fully pearlitic structure has grain boundaries which are not as mobile as ferrite boundaries. Hence the pearlite colony size, after transfon'nation, is controlled completely by the grain size of the prior austenite. In contrast to this, in a ferritic structure, or in a ferritic-pearlitic structure, ferrite grain size is controlled only to a minor extent by particles in the prior austenite phase and predominantly by particles suchas columbium carbides after transformation to ferrite.
Similar observations apply to the ferrite-pearlitic structures of the 0.4 to 0.6 percent carbon steels of the above-mentioned Journal article, despite the unsupported statement at page 922 thereof that pearlite colony size depends on the prior austenite grain size which in turn can be controlled by grain-refining additions. However, in-the reported data the addition of 0.05 percent columbium results in no refining of the pearlite colony size, apparently due to the cooling cycle followed therein.
Moreover, in the Journal article columbium was added in an amount of 0.05 percent. This columbium level forms a carbide which is stable up to about 1370C in a steel containing 0.8 percent carbon and a nitride which is stable up to about 1130C in a steel containing 0.012 percent nitrogen. Accordingly, the addition of 0.05 percent columbium to a high carbon steel may result in precipitation of large carbides during orimmediately after solidification of the cast ingot, and such carbides cannot be dissolved during subsequent processing since heating to a temperature up to 3 about 1370C cannot be practiced without adverse effect on mechanical properties.
It is therefore evident that the prior art has not disclosed nor suggested the provision of cold-drawn, high carbon steel wire having a fully pearlitic microstructure which combines reduced load loss due to stress relaxation with good ductility.
It is a principal object of the present invention to provide such a product and a method for the production thereof.
SUMMARY The present invention provides cold-drawn, high carbon steel wire for prestressed concrete having a fully pearlitic microstructure, reduced load loss due to stress relaxation and good ductility, consisting essentially of, by weight percent, from about 0.70 percent to about 0.90 percent carbon, about 0.5 to about 1.0 percent manganese, about 0.025 percent maximum phosphorus, about 0.035 percent maximum sulfur, about 0.15 to about 0.35 percent silicon, about 0.010 to about 0.020 percent total nitrogen, about 0.010 to about 0.030 columbium, substantially all the columbium being combined with carbon, and remainder essentially iron except for incidental impurities.
The method of the present invention comprises providing a hot reduced rod stock of the above composition, austenitizing the stock by heating in the range of about 980 to about 1030C, transforming the stock to a fully pearlitic microstructure by isothermal heating at about 540 to about 590C, air cooling, whereby to obtain a pearlite colony size ranging between about 15 and about 30p., and cold-drawing, straightening and stress relieving the rod stock into wire for prestressed concrete.
The austenitizing and isothermal heat treatments of the process of the invention are rapid (comprising a time of about minutes for austenitizing and about 1 to 2 minutes for austenite to pearlite transformation, these times being subject to minor variations depending upon rod diameter), and hence the process provides an efficient and high production rate.
In the present invention the addition of from 0.010 to 0.030 percent columbium to a steel containing from about 0.70 to about 0.90 percent carbon is critical in achieving improved ductility. More specifically, the hot reduced rod stock after heat treatment will exhibit tensile reduction-in-area values ranging from about 25 to about 35 percent. By way of comparison, a steel having a similar composition without columbium addition exhibits tensile reduction-in-area of about 20 to about 22 percent when subjected to the same heat treatment.
Cold-drawn steel wire for prestressed concrete produced in accordance with the invention exhibits a load loss by the 1000 hours stress relaxation test at 20C and 67.5 percent initial stress of not greater than about 3 percent. By comparison, cold-drawn steel wire containing no columbium and having a total nitrogen content not greater than about 0.007 percent exhibits a load loss by the same test of about 6 percent.
DESCRIPTION OF THE PREFERRED EMBODIMENTS While the composition of the steel has been set forth above in broad limits, optimum properties are obtained in a steel in which carbon is from about 0.80 to about I 0.85 percent, manganese about 0.80 to about 0.90 percent, silicon about 0.20 to about 0.30 percent, nitrogen from about 0.011 to about 0.016 percent, columbium from about 0.014 to about 0.020 percent, with the maximum phosphorus and sulfur contents being as set forth above, and remainder essentially iron except for incidental impurities. At least about 0.008 percent uncombined nitrogen is preferably present in the final product.
Although not critical, preferably incidental impurities other than phosphorus and sulfur are maintained within the following limits:
Copper 0.20% maximum Chromium 0.15% maximum Nickel 0.15% maximum Molybdenum 0.05% maximum The carbon, manganese, nitrogen and columbium ranges are critical, and departure therefrom results in loss of one or more of the desirable properties.
A minimum of about 0.70 percent carbon and preferably about 0.80 percent is necessary in order to provide adequate strength and to insure transformation to a fully pearlitic microstructure under the desired heat treatment conditions. More than about 0.90 percent carbon would adversely affect the cold-drawing properties of the steel. Preferably a maximum of about 0.85 percent carbon is observed for optimum properties.
A minimum of about 0.5 percent manganese, preferably about 0.80 percent, is believed to be desired for its effect in holding nitrogen in solution in the steel. However, a maximum of about 1.0 percent manganese and preferably about 0.90 percent, must be observed in order to avoid unduly long austenite to pearlite transformation time during heat treatment.
About 0.010 percent minimum nitrogen is necessary in order to achieve the marked improvement in reduced load loss due to stress relaxation which is one of the principal objects of the present invention. More than about 0.020 percent total nitrogen is undesirable because of its tendency to produce extensive strain aging and consequent brittleness. A preferred maximum of 0.016 percent is observed for this reason.
To obtain homogeneously sound steel it is necessary to add silicon as a deoxidizer during the steel making process in the range of about 0.15 percent to about 0.35 percent silicon. Preferably aluminum is not added since it ties up free nitrogen.
A columbium range of about 0.010 percent to about 0.030 percent, and preferably from about 0.014 percent to about 0.020 percent, is necessary, at the carbon levels involved, in order to produce relatively small columbium carbide particles only after solidification of the molten steel in amounts which will concentrate in the austenite grain boundaries. Columbium in excess of about 0.030 percent results in formation of relatively massive carbides which are stable up to temperatures well above the austentizing heat treatment range of 980 to 1030C of the present process. As indicated previously, fine columbium carbide particle size and distribution in the grain boundaries of the prior austenite grains are essential in the present invention for control of pearlite colony size and resultant improved ductility. v
Heats of similar composition have been prepared with and without columbium additions, hot reduced to rod stock, austenitized at about 980C, transferred to a lead bath at about 555C for controlled transformation to pearlite, and air cooled. Pearlite colony size and ASTM grain size are compared below for two such heats together with analyses of the elements carbon, manganese, silicon, nitrogen and columbium:
ASTM Prior Pearlite Austenite Colony Example Composition Grain Size Size-p.
C Mn Si N Cb A .82 .82 .24 .010 none 484 5 100-70 B .85 .83 .27 .012 .014 8&9 25-18 load loss after the 1000 hour stress relaxation test at 20C and 67.5 percent initial stress was about 2.8 percent for both Examples, indicating no detrimental effect of columbium on load loss characteristics. The reduced load loss and improved ductility of the columbium containing wire of the present invention are therefore significant.
In summary, the composition of the steel, and the heat treatment in the method of the present invention are critical in achiveing the combination of reduced load loss due to stress relaxation, good ductility and high strength.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
l. Cold-drawn, high carbon steel wire for prestressed concrete having a fully pearlitic microstructure, reduced load loss due to stress relaxation, and good ductility, said wire being cold-drawn from hot reduced rod stock having a tensile reduction-in-area value of at leash about 25 percent after austenitizing and transformation to pearlite, said steel consisting essentially of, by weight percent, from about 0.70 to about 0.90 percent carbon, about 0.5 to about 1.0 percent manganese, about 0.025 percent maximum phosphorus, about 0.035 percent maximum sulfur, about 0.15 to about 0.35 percent silicon, about 0.010 to about 0.020percent total nitrogen, about 0.010 to about 0.030 percent columbium, substantially all the columbium being combined with carbon, and remainder essentially iron except for inciden tal impurities.
2. The wire claimed in claim 1, wherein carbon is from about 0.80 to about 0.85 percent, manganese is from about 0.80 to about 0.90 percent, silicon is from about 0.20 to about 0.30 percent, nitrogen is from about 0.011 to about 0.016 percent, and columbium is from about 0.014 to about 0.020 percent.
3. The wire claimed in claim 1, wherein at least about 0.008 percent uncombined nitrogen is present.
4. A method of producing cold-drawn steel wire for prestressed concrete having reduced load loss due to stress relaxation and good ductility, comprising the steps of providing a silicon killed, hot reduced rod stock consisting essentially of, by weight percent, from about 0.70 to about 0.90 percent carbon, about 0.5 to about 1.0 percent manganese, about 0.025 percent maximum phosphorus, about 0.035 maximum sulfur, about 0.15 to about 0.35 percent silicon, about 0.010 to about 0.020 percent total nitrogen, about 0.010 to about 0.030 percent columbium, and remainder iron except for incidental impurities, austenitizing said stock by heating in the range of about 980 to about 1030C, transforming said stock to a fully pearlitic microstructure by isothermal heating to about 540 to "about 590C, and air cooling, whereby to obtain a pearlite colony size ranging between about 15 and 30p, and cold-drawing, straightening and stress relieving said rod stock into wire of desired final diameter.
5. The method claimed in claim 4, wherein said rod stock contains from about 0.80 to about 0.85 percent carbon, about 0.80 to about 0.90 percent manganese, about 0.20 to about 0.30 percent silicon, about 0.01 1
' to about 0.016 percent nitrogen, and about 0.014 persize.