High carbon steel wire excellent in wire-drawability and in...

Metal treatment – Stock – Ferrous

Reexamination Certificate

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C148S333000, C148S334000, C148S335000, C148S336000

Reexamination Certificate

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06447622

ABSTRACT:

FIELD OF ART
The present invention relates to a high carbon steel wire for wire drawing excellent in wire-drawability and in fatigue resistance after wire drawing, which steel wire is used for, for example, bridge cables, various wires for aircraft, long rubber belts, cords for steel tires, etc., after wire drawing.
BACKGROUND ART
High carbon steel wire for wire drawing is generally required to withstand high-speed drawing and to have excellent fatigue resistance after wire drawing. Hard oxide type non-metallic inclusions are one:of the factors adversely affecting these properties.
Among oxide type inclusions, inclusions of a single composition such as Al
2
O
3
, SiO
2
, CaO, TiO
2
and MgO, etc. are generally hard and inviscid. It is widely known, therefore, that it is necessary to enhance the cleanliness of molten steel and soften oxide type inclusions to manufacture a high carbon steel wire having excellent wire-drawability.
As methods to enhance the cleanliness of steel and soften inviscid inclusions, Japanese Examined Patent Publication No. S57-22969 discloses a method to manufacture a high carbon steel excellent in wire-drawability, and Japanese Unexamined Patent Publication No. S55-24961 discloses a method to manufacture an ultrafine wire. The basic concept of these technologies, however, is limited to the composition control of oxide type non-metallic inclusions in the ternary compound system of Al
2
O
3
-SiO
2
-MnO.
Meanwhile, Japanese Unexamined Patent Publication No. S50-71507 proposes to improve wire-drawability of a product by having the composition of non-metallic inclusions fall within the range of spessartite in the ternary compound system phase diagram of Al
2
O
3
, SiO
2
and MnO, and Japanese Unexamined Patent Publication No. S50-81907 discloses a method to improve wire-drawability by decreasing harmful inclusions through the control of the Al amount added to molten steel.
Additionally, Japanese Examined Patent Publication No. S57-35243 proposes, in relation to manufacturing steel cords for tires with an inviscid inclusion index of 20 or below, to soften inclusions by injecting alloys containing one or more of Ca, Mg and REM after preliminary deoxidation by injecting a flux containing CaO with a carrier gas (inert gas) into molten steel in a ladle with the complete control of Al.
In above proposed methods, in case of reforming ternary system non-metallic inclusions, stable composition control is not easy while, in case of controlling multi-component non-metallic inclusions, the reduction of the size and number of inclusions and the security of ductility are hard to achieve and, thus, an improvement in wire-drawability and fatigue resistance after wire drawing cannot be expected. In Japanese Examined Patent Publication No. H4-8499, a high carbon steel wire having remarkably excellent wire-drawability and fatigue resistance after wire drawing has been realized by; specifying the total oxygen content within a prescribed range and controlling the amount and composition of inviscid inclusions; obtaining favorable distribution of the amount and size of inviscid inclusions by securing the reduction of the size and number of inviscid inclusions and their ductility; and softening inclusions by reforming the composition of inclusions into oxide type inclusions of multi-component system containing SiO
2
and MnO and selectively containing Al
2
O
3
, MgO, CaO and TiO
2
.
In the invention disclosed in Japanese Examined Patent Publication No. H4-8499, a secondary deoxidizer containing Al and two or more of Mg, Ca, Ba, Ti, V, Zr and Na is added to molten steel, for the purpose of reforming inclusions into oxide type inclusions of multi-component system containing SiO
2
and MnO and selectively containing Al
2
O
3
, MgO, CaO and TiO
2
. However, these alloys for deoxidation are expensive and thus it is desirable, for the reduction of production cost, to reduce the use of these costly alloys.
DISCLOSURE OF THE INVENTION
The object of the present invention is to provide a high carbon steel wire remarkably excellent in wire-drawability and fatigue resistance after wire drawing at a low cost by reducing the use of the above-mentioned expensive alloys.
Namely, the gist of the present invention is as follows:
(1) A high carbon steel wire excellent in wire-drawability and fatigue resistance after wire drawing, characterized in that; the total oxygen content is 15 to 50 ppm; among non-metallic inclusions included therein, the number of inviscid inclusions is 1.5 pieces/mm
2
or less in average under the visual field of an optical microscope; among the inviscid inclusions, the number of those having a composition falling within composition A specified below accounts for more than 20% and the total number of those having a composition falling within composition A or B specified below accounts for 80% or more; and the thickness of the inviscid inclusions having a composition falling within composition A specified below is 40 &mgr;m or less;
composition A: containing over 70% of SiO
2
,
composition B: containing 25 to 70% of SiO
2
, 8 to 30% of MnO, 40% or less of MgO, 35% or less of Al
2
O
3
, 25% or less of CaO and 6% or less of TiO
2
, and at least 5% of one or both of Al
2
O
3
and MgO and, additionally, at least 2% of one or both of CaO and TiO
2
.
(2) A high carbon steel wire excellent in wire-drawability and fatigue resistance after wire drawing according to item (1), characterized in that the inclusions having composition B specified above contain 5% or less of other oxides (one or more of the oxides of V, Ba, Zr and Na and trace amount of other oxides inevitably included, hereunder referred to as “other oxides”).
(3) A high carbon steel wire excellent in wire-drawability and fatigue resistance after wire drawing according to item (1) or (2), characterized in that the number of inviscid inclusions having a composition falling within composition A is 1 piece/mm
2
or less in an observation visual field.
(4) A high carbon steel wire excellent in wire-drawability and fatigue resistance after wire drawing according to any one of items (1) to (3), characterized by containing 0.4 to 1.2% of C, 0.1 to 1.5% of Si and 0.1 to 1.5% of Mn in weight.
(5) A high carbon steel wire-excellent in wire-drawability and fatigue resistance after Wire drawing according to any one of items (1) to (3),:characterized. by containing, in weight, 0.4 to 1.2% of C, 0.1 to 1.5% of Si and 0.1 to 1.5% of Mn, additionally P and S controlled not to exceed 0.02%, respectively, and one or more of 0.05 to 1.0% of Cr, 0.05 to 1.0% of Ni, 0.05 to 1.0% of Cu, 0.001 to 0.01% of B. 0.001 to 0.2% of Ti, 0.001 to 0.2% of V, 0.001 to 0.2% of Nb, 0.05 to 1.0% of Mo and 0.1 to 2% of Co.
Here, inviscid inclusions denote inclusions whose length or thickness is 5 &mgr;m or more and the length (1) and the thickness (d) of each of them satisfy the formula 1/d ≦5, under optical microscope observation of a longitudinal section including the center line of a wire.
It is generally known that, when inclusions contain the oxides of simple composition or specific oxides in high quantity, they are hard and their plasticity is poor. The most significant feature of the present invention is a discovery of the fact that the inclusions having a high content of SiO
2
are softer than those having high content of Al
2
O
3
and MgO and that, even if inclusions having high content of said SiO
2
account for more than 20%, the wire-drawability and fatigue resistance after wire drawing of a steel wire are not adversely affected so long as the thickness (d) of said inclusions is controlled not to exceed 40 &mgr;m.
BEST EMBODIMENT FOR CARRYING OUT THE INVENTION
On specifying Total Oxygen Content in the Range of 15 to 50 ppm
When the total oxygen content of steel is high, blowholes are generated during the solidification of molten steel causing surface defects and the amount of inviscid inclusions increases-in a steel wire having a total oxygen content exceeding 50 ppm. For this reason, the upper limit of the total oxygen content is set at 50 ppm.

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