Bainite wire rod and wire for drawing and methods of producing t

Details Bainite wire rod and wire for drawing and methods of producing t Bainite wire rod and wire for drawing and methods of producing t

1995-10-05

1997-09-02

Yee, Deborah

Metal treatment

Stock

Ferrous

148333, 148598, 148599, C21D 806, C22C 3818, C22C 3800

Patent

active

056627475

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to bainite wire rod and wire for drawing and methods of producing the same.
In this invention, "wire rod," when termed as a product, means wire rod processed for drawing by subjecting it to direct heat treatment immediately after rolling from a steel slab, while, "wire," when termed as a product, means wire subjected to heat treatment in preparation for drawing before drawing or after hot rolling and wire subjected to heat treatment for secondary drawing after being subjected to primary drawing by cold working following hot rolling.


BACKGROUND ART

Wire rod and wire are ordinarily drawn into final products matched to the purpose of use. Before conducting the drawing process, however, it is necessary to put the wire rod or wire in a condition for drawing.
In the case of high-carbon steel wire rod or wire, the prior art requires that a mixed texture of uniform, fine pearlite and a small amount of pro-eutectoid ferrite be established before drawing, and, therefore, a special wire rod or wire heat treatment called "parenting" is conducted. This treatment heats the wire rod or wire to the austenite formation temperature and then cools it at an appropriate cooling rate to complete pearlite trans formation, thereby establishing a mixed texture of fine pearlite and a small amount of pro-eutectoid ferrite.
In the wire rod production method of Japanese Patent Publication No. Sho 60-56215, a heat treatment is conducted for obtaining a mixed texture of fine pearlite and a small amount of pro-eutectoid ferrite by immersing the wire rod heated to the austenite formation temperature in molten salt and then cooling it from 800.degree.-600.degree. C. at a cooling rate of 15.degree.-100.degree. C./sec.
However, pearlite texture involves the problems of ductility degradation during drawing at a high reduction of area and-of cracking in twist test (hereinafter referred to as "delamination").
The object of this invention is to provide bainite wire rod or wire excellent in ductility and not giving rise to the foregoing problems during drawing, and to provide methods of producing the same.


DISCLOSURE OF THE INVENTION

For achieving this object, the present invention provides bainite-texture wire rod or wire having a chemical composition containing C, Mn, Si, and, if required, further containing Cr in an amount specified by the invention, the upper limit value of P and S content being restricted, and further having prescribed tensile strength and reduction of area.
For achieving this object, the present invention also provides bainite wire rod or wire by increasing the cooling rate up to the nose position in the TTT diagram during cooling of wire rod after hot rolling or during heat treatment of wire after heat treatment at austenite formation temperature, thereby preventing formation of pearlite texture, and then isothermally holding the wire rod or wire at 350.degree.-500.degree. C. In other words, following rolling of the wire rod or heating of the steel wire it is cooled from the temperature range of 1100.degree.-755.degree.C. to the temperature range of 350.degree.-500.degree. C. at a cooling rate of 60.degree.-300.degree. C./sec and maintained at this temperature for at least a specified period to suppress formation of micromartensite texture and thus provide bainite-texture wire rod or wire excellent in drawability, whereby there is obtained wire rod or wire excellent in drawability even at a high reduction of area.
Specifically, the gist of the invention is as set out below.
(1) Bainite wire rod or wire for drawing characterized in that equations (1) and (2),
(2) Bainite wire rod or wire for drawing according to paragraph 1 above characterized in that it further contains Cr : 0.10-0.30% as an alloying component.
(3) Bainite wire rod or wire for drawing according to paragraph 1 or 2 above characterized in that it has a microstructure of not less than 80% upper bainite texture in terms of area ratio and an Hv of not more than 450.
(4) A method of producing bainite wire rod for dra

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