Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-10-09
2003-11-11
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S332000, C148S333000, C148S598000, C148S599000
Reexamination Certificate
active
06645319
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wire rod for drawing and a method for production thereof, said wire rod exhibiting good twisting characteristics when drawn into steel cord, wire saw, and steel wire for PC wire rope.
2. Description of the Related Arts
Hard drawn steel wire for steel cord and steel ropes is usually produced from wire rods by cold drawing which follows patenting. Drawing imparts strength to the steel wire. However, the resulting steel wire is subject to longitudinal cracking if it acquires an excessively high strength at the time of drawing. Therefore, wire rods for drawing are basically required to have good drawing characteristics.
The above-mentioned hard steel wire is examined for quality according to “Twisting Test” provided in Japanese Industrial Standards. It should meet requirements for the number of twists, the state of fracture, the uniformity of twist, and so forth, stipulated in the standard. (These requirements are referred to as “twisting characteristics” hereinafter.) One of the characteristics required of hard steel wire is resistance to longitudinal cracking called delamination.
It is common practice to use pearlite transformation in production of the above-mentioned steel wire. According to this method, wire rods undergo heating at a temperature (say, 900-1100° C.) above the A
3
transformation point for austenitic transformation (or &ggr;-transformation), quenching and isothermal transformation (patenting) at 550-600° C. to give the pearlite structure, and cold drawing.
Wire rods to be drawn into steel wire are required to have good drawing characteristics as well as good twisting characteristics.
Japanese Patent Laid-open No. 302120/1993 intended to meet this requirement discloses a method of minimizing the network-like or thick cementite structure on the assumption that such cementite structure affects drawability. According to this disclosure, the object is achieved by subjecting the steel wire to austenitic transformation and performing patenting on the steel wire being drawn before or during transformation at a temperature below the A
1
transformation point.
This disclosure suggests that the resulting steel wire is free from delamination in the twisting test. In actual, however, the resulting steel wire is not so improved in twisting characteristics because the disclosed method is basically intended to improve drawability.
Also, Japanese Patent Laid-open No. 199978/1999 discloses a wire rod for drawing which is produced from the standpoint of improving twisting characteristics such that eutectoid steel or hyper-eutectoid steel has a ferrite particle diameter of 4.0 &mgr;m on average. The disclosed technology, however, does not meet the recent requirements for drawability and twisting characteristics.
OBJECT AND SUMMARY OF THE INVENTION
The present invention was completed in view of the foregoing. It is an object of the present invention to provide a wire rod for drawing superior in twisting characteristics as well as drawability and a method for production thereof.
The present invention is directed to a wire rod for drawing superior in twisting characteristics characterized in that the raw material thereof is a eutectoid steel or hyper-eutectoid steel containing 0.1-2.0 mass % Si and 0.2-2.0 mass % Mn and the pearlite structure therein accounts for no less than 80 area % of microstructure and the maximum length of ferrite as the second phase therein is no larger than 10 &mgr;m. The wire rod in a preferred embodiment is characterized in that the pearlite nodule therein is no larger than 20 &mgr;m in size.
The wire rod of the present invention is made of a eutectoid steel or hyper-eutectoid steel (with 0.65-1.2 mass % C) containing Si and Mn in specific amounts. It may optionally contain any one or more species selected from the following.
(a) less than 0.1 mass % Cu (0 mass % exclusive)
(b) no more than 0.8 mass % Cr (0 mass % exclusive)
(c) no more than 1 mass % Ni (0 mass % exclusive)
(d) 0.0003-0.005 mass % B (no less than 0.0003 mass % B in solid solution)
(e) no more than 0.1 mass % V (0 mass % exclusive), no more than 0.1 mass % Ti (0 mass % exclusive), no more than 0.1 mass % Nb (0 mass % exclusive), and no more than 0.1 mass % Mo (0 mass % exclusive).
The wire rod is improved differently in characteristic properties depending on the elements added.
The wire rod for drawing as specified in the present invention is produced by drawing with a true strain of 1.5 or above and patenting at a temperature (T° C.) defined by the equation (1) below.
354[C]+5.15[Cr]+1000[B]+600≦T≦354[C]+5.15[Cr]+1000[B]+620 (1)
where, [C], [Cr], and [B] denote respectively the content (mass %) of C, Cr, and B.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to realize the wire rod for drawing which meets the above-mentioned requirements, the present inventors carried out extensive studies. As the result, it was found that the object is achieved if the wire rod is made of a eutectoid steel or hyper-eutectoid steel containing Si and Mn in specific amounts in which the pearlite structure accounts for no less than 80 area % of microstructure and the maximum length of ferrite as the second phase is no larger than 10 &mgr;m. This finding led to the present invention.
The present inventors continued their researches assuming that the twisting characteristics is affected by the maximum length of ferrite as the second phase. As the result, it was found that the major factor that controls the maximum length of ferrite is the grain size of austenite and the content of undissolved carbide resulting from insufficient heating in patenting. The undissolved carbide functions as ferrite nucleating sites and also prevents the growth of austenite crystal grains.
It is desirable to eliminate undissolved carbide almost completely from the view point of eliminating ferrite nucleating sites; however, a small amount of undissolved carbide is necessary to control the grain size of austenite. According to the present invention, the condition of patenting is established so as to control the grain size of austenite and the amount of undissolved carbide, thereby specifying the maximum length of ferrite. Thus it became possible to realize the wire rod for drawing which is superior in twisting characteristics.
Incidentally, it is suggested in Japanese Patent Laid-open No. 199978/1999 that the maximum grain size (or the length of major axis) of ferrite should desirably be no larger than 12 &mgr;m for prevention of longitudinal cracking. However, no concrete means is disclosed to achieve the object. What is intended for is to improve twisting characteristics by specifying the average grain size of ferrite instead of controlling and detecting the actual grain size of ferrite with difficulties. By contrast, the present invention makes it possible to keep the maximum length of ferrite no larger than 10 &mgr;m by specifying the composition and heat treatment conditions.
The maximum length of ferrite as the second phase implies the length of the major axis of the crystal grain of ferrite not having the pearlite structure. According to the present invention, the wire rod exhibits good twisting characteristics when the maximum length of ferrite is no larger than 10 &mgr;m. If the maximum length of ferrite exceeds 10 &mgr;m, the wire rod is poor in twisting characteristics and liable to longitudinal cracking called delamination.
The wired rod of the present invention contains the main phase of pearlite structure formed by patenting. The content of pearlite structure in the wire rod should be no less than 80 area %. Otherwise, the wire rod is poor in drawability due to increase in bainite structure. In addition, as is apparent from the foregoing, the wire rod of the present invention should desirably contain no ferrite. However, the effect of ferrite can be minimized by adequately controlling the grain siz
Ibaraki Nobuhiko
Minamida Takaaki
Nagao Mamoru
Ochiai Kenji
Yaguchi Hiroshi
(Kobe Steel, Ltd.)
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yee Deborah
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