JP3153618B2 - Manufacturing method of hypereutectoid steel wire - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hypereutectoid steel wire used for producing a small diameter high strength steel wire such as a steel cord or a bead wire.

[0002]

2. Description of the Related Art As a measure for increasing the strength of a high carbon steel wire,
Increasing the C content is the most industrially desirable method because it is inexpensive and has a high effect. However, in the hypereutectoid region, that is, the region where C usually exceeds 0.9%,
In the wire after hot rolling, brittle proeutectoid cementite is formed in a network along the old austenite grain boundaries. For this reason, if the wire rod after rolling is drawn as it is, breakage occurs frequently due to generation of grain boundary cracks along the pro-eutectoid cementite.

Conventionally, a heat treatment method has been developed to improve the drawability of hypereutectoid steel. For example,
No. 8993 discloses a method of changing the structure to a pearlite structure in which granular cementite is dispersed by heat treatment. This is because a hypereutectoid steel wire is austenitized and then oil quenched to obtain a martensite structure, and then 770
To 930 ° C. to rapidly precipitate granular cementite, and immediately after reaching the target heating temperature, 535 to 66 ° C.
This is a method of performing a patenting process at a temperature of 0 ° C. Although this method is excellent as a method for raising the drawing limit, it is difficult to apply it to direct heat treatment after wire rolling because the heat treatment process becomes complicated.

[0004]

As described above, according to the prior art, it was impossible to directly use the hypereutectoid steel wire after hot rolling for wire drawing. An object of the present invention is to provide a wire having a particularly high C content among hypereutectoid steel wires, that is, 1.10.
% Of C is completely prevented from forming intergranular pro-eutectoid cementite in a hypereutectoid steel wire rod, and high elongation wire drawing is applied to the as-hot hypereutectoid steel wire rod. To make things possible.

[0005]

Means and Actions for Solving the Problems That is, the present invention provides: 1) a weight ratio of C: 1.10 to 1.30%, Si: 0.15 to 0.50%, Mn: 0.30 to 0.65%, the balance of steel consisting of Fe and unavoidable impurities is wire-rolled, wound at less than 900 to 750 ° C, and immediately
Production of a hypereutectoid steel wire characterized by being immersed in a molten salt bath maintained at 00 to 650 ° C. and being transformed in the molten salt to obtain a fine pearlite structure free of proeutectoid cementite. Method.

2) C: 1.10 to 1.30%, Si: 0.15 to 0.50%, Mn: 0.30 to 0.60%, Cr: 0.10 to 0.50 by weight ratio %, The balance of steel consisting of Fe and unavoidable impurities is rolled, then wound at less than 900 to 750 ° C.
Production of a hypereutectoid steel wire characterized by being immersed in a molten salt bath maintained at 00 to 650 ° C. and being transformed in the molten salt to obtain a fine pearlite structure free of proeutectoid cementite. Method. That is all.

Hereinafter, the present invention will be described in detail. The present inventors have conducted many experiments in order to prevent the formation of proeutectoid cementite during patenting of a hypereutectoid steel wire. As a result, as shown below, after the wire rod is rolled, it is wound in a predetermined temperature range, the wound wire is quenched in a molten salt maintained at 500 to 650 ° C., and the isothermal transformation is completed in the molten salt. By doing so, new knowledge was obtained that the production of proeutectoid cementite was prevented, and a wire rod having a fine pearlite structure with good wire drawing workability could be produced.

In order to prevent the formation of proeutectoid cementite, it is necessary to increase the cooling rate from the austenite region sufficiently. As in the case of ordinary direct patenting, it is impossible to prevent the production of proeutectoid cementite in a hypereutectoid steel having a C content of 1.1% or more simply by air-cooling the wire after rolling. Thus, the present inventors have found that the production of proeutectoid cementite can be prevented by winding the wire after rolling the wire at a predetermined temperature and immersing the glowing wire in a molten salt bath from the winding temperature.

When the molten salt temperature is lower than 500 ° C., the cooling rate can be increased, but a large amount of bainite is generated on the surface layer of the wire, and martensite is generated in the central segregation portion, thereby lowering the limit of wire drawing. On the other hand, when the temperature exceeds 650 ° C., the cementite constituting pearlite becomes thick, and therefore,
Drawability decreases. Therefore, the molten salt temperature is 50
0 to 650 ° C.

[0010] Regarding the winding temperature after wire rod rolling, if the winding temperature is 900 ° C or higher , pro-eutectoid cementite is formed at austenite grain boundaries during cooling after winding, and wire drawing workability is degraded. On the other hand, when the winding temperature is less than 750 ° C,
Since the layer structure of pearlite is insufficiently developed, the drawing limit is lowered. For the above reasons, the winding temperature is 950
~ 750 ° C.

Next, the reasons for limiting the components of the present invention will be described. C is an effective and economical element for increasing the strength, and is one of the most important elements of the present invention. As the C content increases, the strength after patenting and the amount of work hardening during wire drawing increase. Therefore, in order to obtain a high-strength steel wire by drawing, it is advantageous that the C content is high, and in the present invention, the C content is set to 1.10% or more. On the other hand, if the C content exceeds 1.30%, it becomes impossible to prevent the occurrence of proeutectoid cementite, so the upper limit of the C content is 1.30%.

Si is added in an amount of 0.15% or more as a deoxidizing agent. On the other hand, Si, as an alloy element, forms a solid solution in ferrite and exhibits a remarkable solid solution strengthening action. In addition, S in ferrite
i is an indispensable element in the production of high-strength steel wires because it has the effect of reducing the strength reduction during hot-dip galvanizing after drawing and bluing. However, Si promotes the formation of bainite and lowers the drawability, so that the upper limit is 0.5%.

Mn is also added as a deoxidizing agent in an amount of 0.3% or more. In addition, since Mn has a large effect of improving hardenability, when the wire diameter is large, it is possible to increase the Mn content to improve the uniformity in the cross section, and to improve the ductility of the steel wire after drawing. It is valid. However, if it exceeds 0.60%, martensite is generated in the central segregation part during continuous cooling, and the wire drawing workability deteriorates. Therefore, the upper limit is 0.60%.

Cr is added in an amount of 0.10% or more as necessary to reduce the lamellar spacing of pearlite and to improve the strength and drawability of the steel wire. If it is less than 0.10%, the effect is not sufficient, while if it exceeds 0.50%, the time required for transformation becomes longer, martensite is formed during continuous cooling, and the drawability is significantly reduced. % As the upper limit.

[0015]

[Embodiment] The tensile strength is 140 kgf / mm ² or more.
The production result of a high-strength steel wire having a drawing limit of 90% or more will be described. Diameter 5.5 of the chemical components shown in Table 1
The hot-rolled wire was cooled by molten salt and drawn to the wire drawing limit.

Steel A has not reached the target strength because the C content is less than 1.10%. On the other hand, in the case of steel J, a proeutectoid cementite was formed because a cooling rate corresponding to a high C content was not obtained, and thus the drawability was significantly deteriorated. B-4 steel has a molten salt temperature of less than 500 ° C., and the drawing is high, but the drawing limit is low due to the inclusion of bainite. On the other hand, since the molten salt temperature of H-4 steel exceeded 650 ° C., it became coarse pearlite, and the drawing limit was 80% or less. For the B-2 and H-2 steels, since the winding temperature was 900 ° C. or higher, proeutectoid cementite was formed, and the drawing and drawing limits were significantly reduced.

The winding temperature of B-3 and H-3 steels is 750.
Since the temperature was lower than ℃, the drawability was low despite the relatively high drawing. C having an Mn content of more than 0.6%
In steel and steel F in which the amount of Cr exceeds 0.50%, martensite was formed in the central segregation portion, and both the drawing and the drawing limit were reduced. Further, since the amount of Si exceeds 0.5% in the G steel, the drawing limit has not reached the target value even though the drawing is high. B-5 and H-5 steels were produced by a conventional method, ie, a method in which wire rod rolling was followed by impingement cooling, and since proeutectoid cementite was formed, disconnection occurred in the first die and subsequent drawing was impossible. Met.

On the other hand, all of the wires produced by the method of the present invention sufficiently satisfy the target strength and drawability.

[0019]

As described above, according to the method of the present invention, it becomes possible to produce a hypereutectoid steel wire having higher strength than the conventional method and excellent in drawability.

[0020]

[Table 1] Chemical composition of 5.5mm wire, manufacturing conditions, drawability

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-271329 (JP, A) JP-A-62-142725 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8 / 06,9 / 52 C22C 38 / 00,38 / 04

Claims (2)

(57) [Claims] 1. A weight ratio of C: 1.10 to 1.30%, Si: 0.15 to 0.50%, Mn: 0.30 to 0.60%, the balance being Fe and unavoidable impurities. After rolling the steel rod, wind it at less than 900 to 750 ° C.
Production of a hypereutectoid steel wire characterized by being immersed in a molten salt bath maintained at 00 to 650 ° C. and being transformed in the molten salt to obtain a fine pearlite structure free of proeutectoid cementite. Method. 2. C: 1.10 to 1.30%, Si: 0.15 to 0.50%, Mn: 0.30 to 0.60%, Cr: 0.10 to 0.50 by weight ratio. % After rolling the steel consisting of Fe and unavoidable impurities with a balance of less than 900 to 750 ° C.,
Production of a hypereutectoid steel wire characterized by being immersed in a molten salt bath maintained at 00 to 650 ° C. and being transformed in the molten salt to obtain a fine pearlite structure free of proeutectoid cementite. Method.