JPH0790495A - High strength steel wire and its production - Google Patents

Abstract

PURPOSE:To produce a wire with which high strength is easily attained and having practical strength, in a steel wire with a pearlitic structure having a specified compsn. constituted of C, Si, Mn, Cr, V, Nb and iron. CONSTITUTION:In a steel wire with a pearlitic structure having a compsn. contg., by weight, 0.7 to 1.0% C, <=1.0% Si, <=0.7% Mn, <=0.5% Cr and/or 0.02 to 1.0% V and/or Nb, and the balance iron with inevitable impurities, the carbide of V or Nb with <=0.1mum size is precipitated into ferrite by 0.05 to 1.0vol.% to obtain a high strength steel wire with which high strength can be attained even at a low draft. This steel wire is produced by holding the wire having the same componental compsn. at 950 to 1200 deg.C, thereafter executing rapid cooling to 650 to 500 deg.C, holding it for 5sec to 5min, next executing cooling to a room temp., subsequently subjecting it to wire drawing at 60 to 98% reduction rate of area and moreover executing heating treatment at 300 to 500 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel wire used for a PC steel wire, a galvanized wire, a steel wire for springs, etc., and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, there are the following techniques for strengthening high strength steel wire. (1) Method of increasing the amount of pearlite steel C to increase the amount of high strength Fe ₃ C (cementite) (Japanese Patent Laid-Open Nos. 5-171276 and 156370)
Or, a method of strengthening by narrowing the interval between cementite and ferrite. A method for strengthening by adding Nb and V to reduce the γ particle size (Japanese Patent Laid-Open Nos. 5-171268 and 63-179017).

(2) Martensitic steel Generally, a method of tempering martensitic steel to precipitate Nb and V, and strengthen it by secondary hardening.

(3) Work hardening It is known that the strength is increased by work hardening associated with the drawing work based on a steel material having relatively good workability represented by pearlite steel. In this case, generally, the higher the working degree is, the higher the strength is, but the strength is determined by the working limit (working degree at which breakage occurs when further working) and the like. On the other hand, by performing special heat treatment on low carbon steel with reduced impurities, the workability can be made extremely large (99.
99%) There is also a method of obtaining a composite structure to obtain a steel wire having a tensile strength of 500 kgf / mm ² (Journal of Japan Institute of Metals, No. 28).
Vol. 4, No. 4, 1989).

[0005]

As described above, various techniques relating to high strength have been proposed. Although the first material is work hardening associated with wire drawing, high strength of 350 kgf / mm ² can be achieved. The higher the amount of C, the more difficult the heat treatment condition for obtaining the material to be processed becomes, and the mass production condition becomes narrower. Further, although it is necessary to increase the workability to some extent in order to increase the strength by work hardening of the two phases of cementite and ferrite, there is also a problem that if the workability is too high, the toughness decreases.

The following materials can be made to have high strength, but the degree of improvement in strength with respect to the additive-free material is small. Further, the material (1) has excellent fatigue properties, but only a low tensile strength can be obtained, and the further technology has a problem that the workability is extremely large and the manufacturing cost becomes too high. In view of such circumstances, it is an object of the present invention to obtain a wire rod that can achieve high strength even if the workability is small and that has practical strength.

[0007]

In order to achieve this object, the wire of the present invention contains C: 0.7 to 1.0 by weight% and Si:
1.0 or less, Mn: 0.7 or less, Cr: 0.5 or less,
A steel wire having a pearlite structure containing 0.02 to 1.0 of at least one of V and Nb, and the balance consisting of iron and unavoidable impurities, wherein the carbide of V or Nb is 0.05% by volume in ferrite. .About.1.0 is precipitated, and the size of the carbide is 0.1 .mu.m or less.

The manufacturing method is characterized in that the wire having the above composition is subjected to the following treatment. Hold the wire at 950 to 1200 ° C, then rapidly cool it to 650 to 500 ° C and hold it for 5 seconds or more and 5 minutes or less. Heat to secondary cure

[0009]

The reason for limiting the above configuration will be described. [High-strength wire] (1) Component C: If the content is less than 0.7, the strength is insufficient, and if it exceeds 1.0, the toughness is insufficient. This is because solid solution strengthening can be achieved with Si: 1.0 or less. 1.
If it exceeds 0, toughness is insufficient. If Mn exceeds 0.7, the toughness and strength are deteriorated due to the abnormal structure caused by segregation. Therefore, the hardenability is secured to 0.7 or less. This is because if the Cr content is 0.5 or less, the structure can be made finer and the strength can be improved. If it exceeds 0.5, the toughness decreases. If V, Nb: less than 0.02, the strength improving effect is small (strength is improved but secondary hardening is small), and conversely, if it exceeds 1.0, the strength improving effect is small. By setting it in the range of 0.02 to 1.0, secondary curing can be sufficiently performed.

(2) V- and Nb carbides are precipitated in ferrite in an amount of 0.05 to 1.0% by volume, and the size of these carbides is 0.1 μm or less. Strength is improved even if they are precipitated in cementite. The effect is small and such a condition is required. If it is less than 0.05%, the effect of improving the strength is small, and if it exceeds 1.0%, the strength is improved but the toughness is deteriorated. The size of this carbide is 0.1μ
If it is less than m, the workability is impaired.

[Manufacturing Method] (1) Hold the wire rod at 950 to 1200 ° C. By this heating, all the carbides are dissolved in the austenite. If it is less than 950 ° C, solid solution is insufficient, and if it exceeds 1200 ° C, γ grains are coarsened and the strength is lowered. (2) Quenching to 650 to 500 ° C. and holding for 5 seconds or more and 5 minutes or less This is to reduce carbides of Nb and V in ferrite as much as possible to obtain a pearlite structure with good workability. If it exceeds 650 ° C., pro-eutectoid cementite will precipitate, and if it is less than 500 ° C., a structure such as bainite having poor workability will precipitate. Further, if V and Nb carbides are precipitated before wire drawing, wire drawing workability is deteriorated. (3) Area-drawing rate of 60 to 98% is applied. If it is less than 60%, the strength improving effect is small, and if it exceeds 98%, the toughness is reduced. (4) Secondary curing by heating to 300 to 600 ° C. Less than 300 ° C, little effect, and above 500 ° C, 2
The softening of the matrix is larger than that in the second hardening, and the strength is lowered as a whole.

[0012]

EXAMPLES Examples of the present invention will be described below. Sample materials No. 1 to 14 having the composition shown in Table 1 were melted and cast to 6 m
A rolled material of mφ was obtained. This rolled material is used as a test material in each of the following examples. No. 1 is a comparative example.

[0013]

[Table 1]

(Embodiment 1) No. 1, 4 and 7 test materials
After heating at 00 ° C. for 15 minutes, it was cooled to 600 ° C. at a cooling rate of 70 ° C./sec or more, then kept in a lead bath at 600 ° C. for 60 seconds and then cooled with water. And each of these materials is 6m
Wire drawing was performed from mφ to 4,3,2,1 mmφ and the tensile strength of each was measured. The result is shown in FIG. As shown in the figure, in both cases, the strength is improved as the workability is increased. Further, when each of these wire rods was subjected to secondary curing by heating at 400 ° C. for 20 minutes, in each of the examples, 10 kg / mm ^{2 was} improved, whereas in the comparative example, it was decreased by 10 kg / mm ² . From these facts, it was confirmed that both Nos. 4 and 7 were able to achieve higher strength than the No. 1 comparative example.

(Embodiment 2) Next, the same material was added to 700 and 65
When kept in a lead bath at 0,500,450 ℃, 700
For those with a temperature of ℃, pro-eutectoid cementite was precipitated, and for those with a temperature of 450 ° C., bainite was precipitated, and both were broken by wire drawing with an area reduction rate of about 80%. From this, it became clear that it was necessary to hold (perlite transformation) at a temperature of 650 to 500 ° C. The holding time was set to 5 seconds or longer so that the pearlite transformation could be completed. If the transformation could be completed, the productivity would be inferior even if the transformation was maintained for more than 5 minutes.

(Example 3) Furthermore, all test materials No. 1 to 14
Is heated at 1000 ° C for 15 minutes and then rapidly cooled to 600 ° C.
It was kept in a lead bath at 0 ° C for 1 minute to complete the pearlite transformation, and then cooled to room temperature. And these materials are 2mm
The wire was wire-drawn, and it was secondarily cured at 400 ° C. to check the strength. The results are shown in Table 2.

[0017]

[Table 2]

As shown in the table, test materials No. 5, 8, 10,
In No. 12, wire breakage occurred during wire drawing. Specimen No. 1, 2,
Although Nos. 6 and 14 had high strength after wire drawing, no strengthening due to secondary hardening was observed, and the strength was equivalent to that of the conventional Nb, V-added material. It was also found that Nos. 11 and 13 were not suitable for practical use because they had high strength but had extremely low toughness values such as elongation and drawing. On the other hand, all of Nos. 3, 4, 7 and 9 were able to be strengthened by secondary curing. Incidentally, No3
In order to achieve high strength equivalent to 4, 7, 9 Nb, V
It has been found that this can be achieved by adding C and adjusting the C amount to 1.0 to 1.1%. However, in that case, the temperature control of the lead furnace was strict, and it was difficult to manufacture under a wide range of good productivity such as 650 to 500 ° C. as in the present invention.

(Embodiment 4) Using the materials No. 3, 4, 7 and 9 which were capable of wire drawing and had an effect of secondary hardening, conditions for obtaining more optimal secondary hardening were examined. As a test method, a rolled material having a diameter of 6 mm is drawn to a diameter of 2 mm and subjected to a heat treatment (secondary hardening) at a different temperature to measure the tensile strength of each material. For comparison, the tensile strength of the material that was not heat treated was also measured. The result is shown in FIG. As shown in the figure, when the heat treatment temperature is 100 ° C. or higher, the strength is improved, but when it exceeds 500 ° C., the strength is decreased. Especially 300 ° C
The strength was remarkably improved in the temperature range of 500 ° C. or lower.

(Example 5) The structures of the materials used in Example 4 were observed with a scanning electron microscope and a transmission electron microscope,
The amount of V.Nb carbide precipitates at each heat treatment temperature was examined. The result is shown in FIG. Furthermore, regarding these test materials,
The relationship between heat treatment temperature and drawing was also investigated. The result is shown in FIG. As shown in FIG. 3, V / Nb carbide is 0.05 vo
It can be seen that the strength increases at l% or more. But,
If it exceeds 1%, the toughness (drawing) is remarkably deteriorated as in the case of sample material No. 4 at 500 ° C. in FIG. 4, so the upper limit is preferably 1%. The size of the precipitate was 0.1 μm or less.

[0021]

As described above, according to the present invention, it is possible to obtain a steel wire having higher strength than the conventional pearlite steel. In particular, it can be manufactured under a wider range of conditions than the conventional methods for increasing the C content to increase the strength.
Further, since secondary hardening is utilized, the wire drawing property is relatively low during wire drawing and the wire drawability is good.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between wire diameter (workability) and tensile strength.

FIG. 2 is a graph showing the relationship between secondary curing temperature and tensile strength.

FIG. 3 is a graph showing the relationship between the secondary hardening temperature and the precipitation amount of V · Nb carbide.

FIG. 4 is a graph showing a relationship between a secondary curing temperature and a diaphragm.

Claims (2)

[Claims] 1. C: 0.7 to 1.0 by weight%, Si:
1.0 or less, Mn: 0.7 or less, Cr: 0.5 or less,
A steel wire having a pearlite structure containing 0.02 to 1.0 of at least one of V and Nb, and the balance consisting of iron and unavoidable impurities, wherein the carbide of V or Nb is 0.05% by volume in ferrite. A high-strength steel wire, characterized in that it precipitates up to 1.0 and the size of the carbide is 0.1 μm or less. 2. C: 0.7 to 1.0, Si: in weight%.
1.0 or less, Mn: 0.7 or less, Cr: 0.5 or less,
A method for producing a high-strength steel wire, which comprises subjecting a wire rod containing 0.02-1.0 of at least one of V and Nb and the balance being iron and unavoidable impurities to the following treatment. Hold the wire at 950 to 1200 ° C, then rapidly cool it to 650 to 500 ° C and hold it for 5 seconds or more and 5 minutes or less. To heat