Title of Invention

"A HIGH-TOUGHNESS OIL-TEMPERED WIRE"

Abstract

A high-toughness oil-tempered wire for springs which is less likely to suffer a permanent set and high in strength and toughness. The wire is made of a steel containing predetermined amounts of C, Si, Mn, Al and Ti, to which are selectively added predetermined amounts of V, Mo, W and Nb. After quenching and tempering, the content of residual I is 1-5 vol. %, and/or the density of carbides 0.05 µm2 or more in particle diameter is 5 pieces per µm2 or less as viewed on a structure observation photo.

Full Text

The present invention relates to a high-toughness oil-tempered wire. The present invention relates to an oil-tempered wire, and more specifically an oil-tempered wire having sufficient toughness as a material for high-strength springs used valve springs for automotive engines. Valve springs for automotive engines are used in extremely harsh conditions in which they are subjected to high stress and high revolving speed. In particular, valve springs used in recent car engines, which are small in size and consume less fuel, are used in still severer environments. It is therefore desirable to increase the strength of material for such valve springs still further. Valve springs are formed from an oil-tempered wire of chrome-vanadium steel' for valve springs or an oil-tempered wire of silicon-chrome steel for valve springs. Efforts are being made to increase the strength of these wire materials. But a wire having increased strength tends to be low in toughness and ductility, so that it is liable to be broken while being formed into springs. In order to solve this problem, Examined Japanese Publication 3-6981 proposes to control the content of vanadium and the quenching conditions so that the crystal grain size will be 10 or more, thereby keeping high toughness of the wire. For the same purpose. Unexamined Japanese Patent Publication 3-162550 proposes an'oil-tempered wire having a tempered martensite, that is, a matrix after tempering, in which is present a residual austenite phase by 5-20%. But in the former, it is impossible to markedly increase the strength and toughness if the crytal grain size is 10 or more. In the latter, if the residual austenite phase is present in a large amount, it may transform into a martensite phase while the wire is used as springs. If this happens, it may suffer a permanent set due to increased volume. That is, such a wire is less resistant to permanent setting. An object of the present invention is to provide an oil-tempered wire for springs which is less likely to —suffer a permanent set and high in strength and toughness. As a result of our efforts, we have discovered that it is possible to increase toughness while keeping high resistance to permanent setting by finely dispersing a residual austenite phase in a tempered martensite at a volume rate of 1% to 5% and by controlling the density of unresolved carbides having particle diameters of 0.05 µl or more to 5 pieces per µm2 or less as observed on a structure observation photograph. SUMMARY OF THE INVENTION According to this invention, there is provided a high-toughness oil-tempered wire for springs made of a steel containing in weight percent 0.5-0.8% C, 1.2-2.5% Si, 0.4-0.8% Mn, 0.7-1.0% Cr, 0.005% or less Al and 0.005% or less Ti, the steel containing, after quenching and tempering, 1% to 5% by volume of residual . The steel may further contain 0.05-0.15% by weight of vanadium, or further at least one of 0.05-0.5% by weight of Mo, 0.05-0.5% by weight of W and 0.05-0.15% by weight of Nb. In another arrangement, the density of unresolved carbides having particle diameters not less than 0.05 Mm is 2 5 pieces per µm2 or less as observed on a structure observation photo, instead of restricting the content of residual . In still another arrangement, both the density of carbides and the content of residual γ restricted. The present invention also provides a method of manufacturing oil-tempered wires as described above under specific quenching and tempering conditions. Now we will explain why the steel composition has been restricted. 1) C: 0.5-0.8 wt.% C is essential to increase the strength of the steel wire. If its content is less than 0.5%, the strength of the wire will be insufficient. On the other hand, a steel wire containing more than 0.8% carbon is low in toughness. Such a wire is not reliable enough because it is more liable to get marred. 2 ) Si: 1.2-2.5 wt.% Si helps increase the strength of ferrite and thus improve the resistance to permanent set. If its content is less than 1.2%, this effect cannot be achieved sufficiently. If over 2.5%, hot and cold machinability will drop. Also, such a large amount will promote decarbonization during heat treatment. 3) Mn: 0.4-0.8 wt.% Mn improves the hardening properties of the steel and prevents any harmful effect caused by suffer in the steel by fixing it. If its content is less than 0.4%, this effect cannot be achieved sufficiently. If over 0.8%, the toughness will drop. 4) Cr: 0.7-1.0 wt.% Like Mn, Cr improves the hardening properties of the steel. It also serves to increase the toughness of the wire by patenting after hot rolling and to increase the resistance to softening during tempering after quenching and thus the strength of the wire. If its content is less than 0.7%, this effect cannot be achieved sufficiently. If over 1.0%, Cr will hinder carbides from turning into solid solution, thus lowering the strength of the wire. Also, such a large amount will cause excessive tempering action, leading to reduced toughness. 5) V: 0.05-0.15 wt.% Vanadium helps the formation of carbides during tempering, thus increasing the"resistance to softening of the wire. If its content is less than 0.05%, this effect will be insufficient. If over 0.15%, a large amount of carbides will be formed during heating for quenching, which will lower the toughness of the wire. 6) Mo: 0.05-0.15 wt.% Mo helps the formation of carbides during tempering, thus increasing the resistance to softening of the wire. If its content is less than 0.05%, this effect will be insufficient. If over 0.5%, wire drawing will become difficult. 7) W: 0.05-0.15 wt.% Tungsten helps the formation of carbides during tempering, thus increasing the resistance to softening of the wire. If its content is less than 0.05%, this effect will be insufficient. If over 0.15%, too large an amount of carbides will be formed during heating for quenching so that the toughness of the wire will drop. 8) Nb: 0.05-0.15 wt.% Nb helps the formation of carbides during tempering, thus increasing the resistance to softening of the wire. If its content is less than 0.05%, this effect will be insufficient. If over 0.15%, too large an amount of carbides will be formed during heating for quenching, so that the toughness of the wire will drop. 9) Al, Ti: 0.005 wt.% or less They form Al2O3 and TiO which are high-melting point, non-metallic inclusions. These inclusions are hard and can markedly lower the fatigue strength if present near the steel wire surface. Thus, though they are unavoidable impurities, their contents have to be 0.005 wt.% or less. For this purpose, a raw material containing lesser impurities should be selected. 10) Reason why the content of residual is restricted to 1-5% by volume A residual phase present in the tempered martensite improves the toughness of the steel wire. If its content is less than 1%, the effect will be insufficient. But if its content is more than 5%, the resistance to permanent set will decrease due to martensitic transformation while the wire is used a spring. 11) Reason why the number of unresolved carbides (0.05 Mm or more in particle diameter) is restricted to 5 pieces per 2 p.m as observed on a structure observation photo. Unresolved carbides having particle diameters of 0.05 µm or more can be starting points of destruction while . forming springs. Thus, if the number of such carbides 2 exceeds 5 pieces per µm 2 as observed on a structure observation photo, the toughness of the wire will drop markedly. The content of residual and the density of carbides can be adjusted to the abovementioned values by subjecting the wire to the following heat treatment. The heating time for quenching in the quenching/tempering step before the cooling step is started, should be within 15 seconds. Otherwise, crystal grains will grow too large, lowering the toughness of the wire. If the heating rate is 150 °C/sec or lower, it is impossible to resolve carbides sufficiently within the 15- second interval before the cooling step begins. If the heating temperature is 1100°C or higher, crystal grains will grow too large, thus lowering the toughness or causing decarbonization. If T ( °C ) is equal to 500 + 750.C + 500V or less (wherein C is the content of carbon in weight % and V is the content of vanadium in weight %), carbides will not be resolved sufficiently. Tempering during the quenching/tempering step has to be finished within 15 seconds before the cooling step is started, while keeping the heating rate at 150 °C/sec or higher. Otherwise, the residual austenite phase will decrease to less than 1% by volume. DETAILED DESCRIPTION OF THE EXAMPLES 4.0-mm-diameter wires were formed by melting, rolling, heat-treating and drawing specimens having the chemical compositions shown in Table 1. After quenching and tempering these wires under predetermined conditions, the amount of residual Y~ phase was measured using X-rays, and the amount of carbides was measured by observing the wire structure. Also, they were subjected to a tensile test to measure the toughness in terms of reduction of area. (Example 1) After quenching and tempering Specimens A-I under the conditions shown in Table 2, measurement of residual and a tensile test were carried out. The results for Specimens A, B, C and I are shown in Table 3. The amounts of residual in the specimens manufactured by the method of the present invention were 1-5 vol. %. It is thus apparent that their toughness is sufficiently high. (Example 2) After quenching and tempering Specimens A-I under the conditions shown in Table 4, the amount of carbides (0.05 urn or more) in each specimen was measured, and then the specimens were subjected to a tensile test. The results for Specimens A, B, D and H are shown in Table 5. From Table 5, it is apparent that the specimens according to Example 2, having 5 or less carbides per square micrometer, are sufficiently tough. As described above, the oil-tempered wire for springs according to the present invention is highly resistant to permanent set and highly strong and tough. TABLE 1 (Table Removed) TABLE 2 Quenching / tempering conditions (Table Removed) I • II : Examples III • IV . V • VI : Comparative examples * Heating time is the time from start of heating to start of cooling. TABLE 3 Residual 7 content and reduction of area (Table Removed) Residual 7 content (vol£) Reduction of area {% TABLE 4 Quenching / tempering conditions (Table Removed) I : Example II • III • IV • V • VI : Comparative examples * Heating time is the time from start of heating to start of cooling. TABLE 5 Density of carbides and reduction of area (Table Removed) We Claim: 1. A high-toughness oil-tempered wire for springs made of a steel containing in weight percent 0.5-0.8% C, 1.2- 2.5% Si, 0.4-0.8% Mn, 0.7-1.0% Cr, 0.005% or less Al and 0.005% or less Ti, characterized in that said steel containing, after quenching and tempering, 1% to 5% by volume of residual Y and the density of carbides having a particle diameter not less than 0.05 µm is 5 pieces per µm2 or less after quenching and tempering as viewed on a structure observation photo. 2. A high-toughness oil-tempered wire as claimed in any of the preceding claims, wherein said steel contains 0.05-0.15% by weight of V. 3. A high-toughness oil-tempered wire as claimed in any of the preceding claims wherein said steel contains at least one of 0.05-0.5% by weight of Mo, 0.05-0.15% by weight of W and 0.05-0.15% by weight of Nb. 4. A high-toughness oil-tempered wire substantially as herein described with reference to the foregoing examples.

Documents:

1701-del-2004-abstract.pdf

1701-del-2004-claims.pdf

1701-del-2004-complete specification (granted).pdf

1701-del-2004-correspondence-others.pdf

1701-del-2004-correspondence-po.pdf

1701-del-2004-description (complete).pdf

1701-DEL-2004-Form-1.pdf

1701-del-2004-form-18.pdf

1701-del-2004-form-2.pdf

1701-del-2004-form-3.pdf

1701-del-2004-form-5.pdf

1701-del-2004-gpa.pdf

1701-del-2004-petition-137.pdf