Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-03-07
2001-11-27
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S595000, C148S598000, C148S599000
Reexamination Certificate
active
06322641
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a carbon steel wire for steel wire cords, wire ropes, etc., a steel product as a raw material for said steel wire, and a process for producing them. This carbon steel wire is finished without heat treatment (such as bluing) after cold rolling.
2. Description of the Related Art
Automotive steel radial tires are reinforced with steel wires such as cord wires and bead wires. Their constituent is a strand composed of twisted high-carbon steel wires, each measuring 0.2 mm in diameter and having a strength greater than 310 kgf/mm
2
.
Each wire constituting the strand is produced by wire drawing from a eutectoid or hyper-eutectoid high-carbon steel. The step of wire drawing is followed by patenting, pickling, brass plating (to ensure good adhesion to rubber). Final drawing gives a thin wire about 0.2 mm in diameter. The patenting treatment transforms the austenitic structure into the fine pearlite structure at about 500-550° C., thereby making the steel tougher. Automotive tires recently call for improved durability and said steel wires also call for higher strength than before. An effective way to increase strength is to increase the carbon content. However, a mere increase in carbon content results in longitudinal cracking when the wire is twisted. Longitudinal cracking can be effectively prevented by incorporation with chromium. Japanese Patent Laid-open No. 194147/1990 discloses incorporation with 0.10-0.30% of chromium. Japanese Patent Laid-open No. 049592/1994 discloses incorporation with chromium in a specific amount determined by the boron content. The latter is intended to promote the growth of cementite in pearlite and to improve ductility and fatigue characteristics.
OBJECT AND SUMMARY OF THE INVENTION
The above-mentioned first technology (incorporation with chromium) are limited in increase in tensile strength (360 kgf/mm
2
at most) and in twist value (25 at most). It is desirable not to add chromium if energy for chromium refinement and recycling of steel products are taken into consideration. The above-mentioned second technology has the disadvantage of requiring chromium as an essential component and being limited in the limiting working ratio of wire drawing (not exceeding the conventional level of 3.6). It does not provide fine steel wires having ultra high strength in excess of 4000 MPa.
The present invention was completed to address the above-mentioned problem. It is an object of the present invention to provide a high-carbon steel wire which exhibits higher strength than before and good resistance to longitudinal cracking even though it is not incorporated with chromium. It is another object of the present invention to provide a steel product for said steel wire. It is further another object of the present invention to provide a process for producing said steel wire and steel product. The invention to achieve these objects is defined in the following.
The first aspect of the present invention resides in a high-carbon steel wire superior in resistance to longitudinal cracking which is characterized in that the essential components are C (0.65-1.2 wt %), Si (0.1-2.0wt %), Mn (0.2-2.0 wt %), and Fe, the main phase is pearlite, and the ferrite area ratio is less than 0.40% in the surface layer up to a depth of 50 &mgr;m from the surface. The expression “the main phase is pearlite” means that the pearlite area ratio is more than 80%. The preferable pearlite area ratio is more than 90%.
The second aspect of the present invention resides in a high-carbon steel wire superior in resistance to longitudinal cracking which is characterized in that the essential components are C (0.65-1.2 wt %), Si (0.1-2.0 wt %), Mn (0.2-2.0 wt %), B (0.0003-0.0050 wt %), Ti (less than 0.030 wt %), N (less than 0.0050 wt %), and Fe, with the amounts of B, Ti, and N satisfying the equation (1),
0.03 ≦B/(Ti/3.43−N)≦5.0 (1)
the main phase is pearlite, and the ferrite area ratio is less than 0.40% in the surface layer up to a depth of 50 &mgr;m from the surface.
The third aspect of the present invention resides in a steel for a high-carbon steel wire which has the same chemical composition as defined above in the second aspect and which is characterized in that the maximum particle diameter of TiN inclusion is smaller than 8.0 &mgr;m. This steel is processed into the above-mentioned high-carbon steel wire by diameter reduction (including working after patenting) and ensuing patenting.
The fourth aspect of the present invention resides in a process for producing a steel for a high-carbon steel wire, said process comprising casting a steel having the same chemical composition as defined above in the second aspect, cooling the cast at a rate greater than 5° C./sec in the period from the start of casting to the completion of solidification, and hot-rolling go the resulting billet.
The fifth aspect of the present invention resides in a high-carbon steel wire which is characterized in that the essential components are C (0.65-1.2 wt %), Si (0.1-2.0 wt %), Mn (0.2-2.0 wt %), B (0.0003-0.0050 wt %, B in solid solution accounting for more than 0.0003 wt %), N (less than 0.0050 wt %), and Fe, the content of Ti is limited to 0-0.005 wt %, the main phase is pearlite, and the ferrite area ratio is less than 0.40% in the surface layer up to a depth of 50 &mgr;m from the surface. What is unique to this aspect is that the content of Ti in the second aspect is limited.
The sixth aspect of the present invention resides in a steel for a high-carbon steel wire superior in resistance to longitudinal cracking, said steel having the same chemical composition as defined in the fifth aspect. This steel is processed into the high-carbon steel wire (defined in the fifth aspect) by diameter reduction (including working after patenting) and ensuing patenting.
The seventh aspect of the present invention resides in a process for producing a wire rod for a high-carbon steel wire, said process comprising casting a steel whose essential components are C (0.65-1.2 wt %), Si (0.1-2.0 wt %), Mn (0.2-2.0 wt %), B (0.0003-0.0050 wt %), N (less than 0.0050 wt %), and Fe, with the content of Ti being limited to 0-0.005 wt %, cooling the cast at a rate greater than 5° C./sec in the period from the start of casting to the completion of solidification, thereby forming a billet, heating the resulting billet and hot-rolling it such that the finishing temperature is 900-1100° C., and cooling the hot-rolled product to 850° C. within 30 seconds. This process gives the steel for high-carbon steel wire which is defined above in the sixth aspect.
REFERENCES:
patent: 3900347 (1975-08-01), Lorenzetti et al.
patent: 5211772 (1993-05-01), Ashida et al.
patent: 5575866 (1996-11-01), Minamida et al.
patent: 5873958 (1999-02-01), Makii et al.
patent: 40-8283867-A (1996-10-01), None
Ibaraki Nobuhiko
Inada Atsushi
Makii Koichi
Minamida Takaaki
Nagao Mamoru
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel Ltd).
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yee Deborah
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