Metal treatment – Compositions – Heat treating
Patent
1984-08-06
1986-09-23
Stallard, Wayland
Metal treatment
Compositions
Heat treating
148 123, 148 124, 148320, 148337, C21D 952
Patent
active
046133852
DESCRIPTION:
BRIEF SUMMARY
The present invention is directed to a process for making high-strength, high-ductility, low-carbon steel wires, bars and rods by cold drawing dual-phase steels. Here, the term "dual-phase steels" refers to a class of steels which are processed by continuous annealing, bach annealing, or conventional hot rolling to obtain a ferrite matrix with a dispersed second phase such as martensite, bainite and/or retained austenite. The second phase is controlled to be a strong, tough and deformable phase unlike the hard, non-deformable carbide phase found in pearlitic rods and wires. It must be suitably dispersed and in sufficient volume fraction i.e. greater than 10%, to provide a substantial contribution to the strength in the as-heat-treated condition and to increase the work-hardening rate during wire drawing. Various heat treatment paths can be used to develop the dual-phase microstructure and the morphology depends on the particular heat treatment employed. A preferred heat treatment is the intermediate quench method i.e. austenitize and quench to 100% martensite prior to annealing in the two phase .alpha.+.gamma. field and quenching to a ferrite martensite structure. The invention is further directed to the high-structure, high ductility steel wires, bars and rods produced by the process of the present invention.
Steel wire has many known uses, such as for making cables, chains, and springs. It is also used to make steel belts and bead wire for tires, and steel strands are included in multistrand electrical wire to improve the tensile strength of the wire. In these applications, the diameter requirements range from 0.005 inch to more than 0.25 inch with strength requirements ranging from 250 ksi to as much as 400 ksi in the smaller diameters. In all of these applications, it is important to provide a steel wire having a high tensile strength and good ductility at the required diameter.
The oldest and most common method of producing high strength, high ductility wire is by patenting a near eutectoid composition pearlitic steel. However, this process is complex and expensive. A further disadvantage of the patenting method is an inherent limitation in the maximum wire diameter that can be produced at a given strength level.
There is a need for steel wire and rods having higher tensile strength and higher ductility than steel wire and rods produced by the known methods, as well as a more economical method for producing high strength steel wire and rods. The present invention would replace the conventional method of patenting pearlitic steel to produce wwire with a process whereby an alloy of relatively simple composition is cold drawn into wire or rods in a single multipass operation, i.e., without intermediate annealing or patenting heat treatments. Elimination of the patenting heat treatments in the production of high strength steel wire should lower the cost of producing high strength steel wire, especially in light of the present fuel situation.
The cold drawing process requires a low alloy steel composition with a microstructure and morphology which provides high initial strength, high ductility, rapid work hardening, and good cold formability. The steel should be capable of being cold drawn, without intermediate anneals or patenting heat treatments, to the desired diameter, tensile strength, and ductility.
A specific group of steels with a chemical composition specifically developed to impart higher mechanical property values is known in the art as high-strength, low-alloy (HSLA) steel. These steels contain carbon as a strengthening element in an amount reasonably consistent with weldability and ductility. Various levels and types of alloy carbide formers are added to achieve the mechanical properties which characterize these steels. However, the high tensile strength and high ductility needed in many applications for steel wire and rods do not seem to be attainable using HSLA steels.
The factors governing the properties of low carbon steels are primarily its carbon content and microstructure, and secondari
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Koo et al., "Design of Duplex Fe/X/0.1C Stals for Improved Mechanical Properties", Metallurgical Transactions, vol. 8A, Mar. 1977, pp. 525-528.
Iron Age, "Steel Bars Climb to 400,000 Psi", Feb. 1963, pp. 85-87.
Nakagawa Alvin H.
Thomas Gareth
Regents of the University of California
Stallard Wayland
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