KR20110047383A

KR20110047383A - High strength wire rod for drawing having with superior drawability and manufacturing method the same

Info

Publication number: KR20110047383A
Application number: KR1020090103984A
Authority: KR
Inventors: 김현진
Original assignee: 주식회사 포스코
Priority date: 2009-10-30
Filing date: 2009-10-30
Publication date: 2011-05-09

Abstract

PURPOSE: A high strength wire rod with excellent wire drawability and a manufacturing method thereof are provided to improve wire drawability and strength by preventing bainite generation. CONSTITUTION: A high strength wire rod with excellent wire drawability comprises carbon 0.6~1.0%, silicon 0.2~1.0%, manganese 0.3~1.5%, chrome 0.6~1.0%, oxygen less than 0.0015%, phosphorous less than 0.02%, sulfur less than 0.02%, nitrogen less than 0.02%, and residual Fe and inevitable impurities. The wire rod comprises pearlite more than 95% and bainite less than 5%. The interlamellar spacing in pearlite is less than 100nm.

Description

High strength wire rod for drawing having with superior drawability and manufacturing method the same}

The present invention relates to a wire rod used as a wire rod for wire drawing and a manufacturing method thereof, and more particularly, to a wire rod for wire drawing excellent in drawability and strength and a manufacturing method thereof.

The high carbon steel wire is characterized by having the highest porosity among steel materials, including the over-vacuum component and harsh drawing, and using a pearlite structure to withstand the drawing and show excellent work hardening rate. In particular, research has been conducted to improve the strength, freshness and work hardening rate by minimizing the interlamellar spacing of pearlite.

It is known that the lamella spacing of pearlite is controlled by the supercooling degree from the vacancy temperature to the transformation temperature at which the pearlite transformation occurs. The higher the subcooling temperature, that is, the lower the transformation temperature, the finer the laminar spacing of the pearlite can be. When securing microstructure through continuous cooling, the temperature range is wide from the high temperature at which perlite transformation starts to the low temperature at which transformation is completed, and thus, microstructures having a wide range of pearlite layer spacings can be obtained, thus improving fresh workability and strength. it's difficult.

Therefore, the pearlite transformation of the high strength wire rod is usually a constant temperature transformation using a salt bath or molten lead, and this is called a patterning heat treatment. As mentioned above, lowering the constant temperature transformation (patterning) temperature can make the perlite layer spacing fine, but if the transformation is made too low, the upper bainite transformation starts before the pearlite transformation is completed. There is a problem in that a mixed structure of silver pearlite and bainite is formed, and tensile strength and freshness are deteriorated due to mixing of upper bainite.

An object of the present invention is to provide a wire rod for producing a fine pearlite structure or a structure including a pearlite and a small amount of upper bainite and improving the drawability and strength.

In one embodiment, the present invention provides, in weight percent, carbon (C): 0.6-1.0%, silicon (Si): 0.2-1.0%, manganese (Mn): 0.3-1.5%, chromium (Cr): 0.6-1.0 %, Oxygen (O): 0.0015% or less, Phosphorus (P): 0.02% or less, Sulfur (S): 0.02% or less, Nitrogen (N): 0.02% or less, for high-strength drawing containing residual Fe and other unavoidable impurities Provide wire rod.

The wire rod preferably contains 95% or more of pearlite and 5% or less of upper bainite.

The interlamellar spacing of the pearlite is preferably 100 nm or less.

As another embodiment of the present invention, in weight percent, carbon (C): 0.6-1.0%, silicon (Si): 0.2-1.0%, manganese (Mn): 0.3-1.5%, chromium (Cr): 0.6-1.0 %, Oxygen (O): 0.0015% or less, phosphorus (P): 0.02% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, wires containing residual Fe and other unavoidable impurities It provides a method for producing a high-strength drawing wire rod comprising the step of austenitizing at or above ℃ and a constant temperature heat treatment at a nose temperature of -40 ℃ or more in the TTT curve below the nose temperature of the TTT curve.

Through the present invention it is possible to reduce the layer spacing of the pearlite, it is possible to provide a wire for the wire to prevent the generation of bainite to improve the drawability and strength.

According to the present invention, a pearlite transformation curve and a bainite transformation curve are separated on a TTT curve (time-temperature-transformation curve) using Cr, and the wire rod is incubated at a temperature below the nose temperature of the TTT curve. It is possible to form a pearlite with a narrow layer spacing, and to minimize the formation of upper bainite.

Hereinafter, the component system of the wire rod of the present invention will be described.

Carbon (C): 0.6-1.0 wt%

Carbon is an essential element included to secure the strength of the base metal. In order to secure fresh permeability and work hardening rate, the carbon content is preferably 0.6% by weight or more in order to secure a complete pearlite structure containing no cornerstone ferrite. However, if the carbon content is too much, the cornerstone cementite is produced, which may also drastically lower the fresh workability. Therefore, the upper limit is preferably limited to 1.0% by weight.

Silicon (Si): 0.2-1.0 wt%

Silicon is dissolved in ferrite in pearlite and serves to enhance the strength of the base material. When the content of the silicon is less than 0.2% by weight, the effect of strengthening the base material strength is not sufficient because silicon is dissolved in the ferrite. On the other hand, when the content of silicon exceeds 1.0% by weight, the workability of the ferrite is lowered, the fresh workability is lowered, and there is a problem of increasing the activity of carbon during patterning heat treatment to promote surface decarburization. . Therefore, the content of silicon is preferably limited to 0.2 to 1.0% by weight.

Manganese (Mn): 0.3-1.5 wt%

Manganese is an element added to control the transformation rate from austenite to pearlite, and the overall transformation rate is slowed by adding manganese. When the content of manganese is less than 0.3% by weight, transformation occurs so quickly that even in the case of patterning of large diameter wire rods with thick wire diameters, even when patterning (patenting), transformation is initiated before reaching the transformation temperature, and a heterogeneous microstructure is easily obtained. On the contrary, if the content exceeds 1.5% by weight, the metamorphosis may be too slow, and the metamorphic tissue may generate martensite during patterning. Therefore, the content of manganese is preferably limited to 0.3 to 1.5% by weight.

Chromium (Cr): 0.6-1.0 wt%

Chromium (Cr) is an element that plays an important role in the present invention because it has an effect of separating the pearlite transformation curve and the bainite transformation curve during constant temperature transformation from austenite to pearlite. The addition of more than 0.6% by weight separates the bainite transformation curve from the pearlite transformation curve to a lower temperature, thereby enabling pearlite transformation without incorporation of bainite at temperatures lower than the nose temperature on the TTT curve. However, chromium is a strong carbide-forming element, and when its content exceeds 1.0% by weight, it forms carbides other than cementite, thereby deteriorating fresh workability. Therefore, the amount of chromium added is preferably limited to 0.6 to 1.0% by weight.

Oxygen (O): 0.0015 wt% or less

Oxygen is an element that is inevitably contained during forcing, and when oxygen is added, oxide-based nonmetallic inclusions are coarsened and fresh workability is drastically reduced, so it is desirable to control them as low as possible, and in theory, limit the oxygen content to 0%. Advantageously, but inevitably added in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the content of oxygen is preferably limited to 0.0015% by weight.

Phosphorus (P): 0.02 wt% or less

Phosphorus is an element that is inevitably contained during forcing, and when phosphorus is added, it is segregated at the grain boundary and degrades fresh workability. Therefore, it is preferable to control it as low as possible. It can only be added. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the content of phosphorus is preferably limited to 0.02% by weight.

Sulfur (S): 0.02 wt% or less

Sulfur is an element that is inevitably contained during forcing, and it is desirable to control it as low as possible because it is a low melting point element, which adversely affects fresh workability by forming grain boundary segregation and emulsion, and it is advantageous to limit sulfur content to 0% in theory. Inevitably, it is inevitably added in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the content of sulfur is preferably limited to 0.02% by weight.

Nitrogen (N): 0.02 wt% or less

Nitrogen is an element that is inevitably contained during forcing, and it is preferable to control it as low as possible because it causes deformation aging during fresh processing, which lowers the freshness. In theory, it is advantageous to limit the nitrogen content to 0%, but in the manufacturing process It is inevitably added. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the content of nitrogen is preferably limited to 0.02% by weight. However, it is more preferable to limit the content to 0.001 to 0.02% by weight.

The manufacturing method of the wire rod of this invention is demonstrated.

The manufacturing method of the present invention includes the step of austenitizing a wire rod that satisfies the above component system at 900 ° C or higher, and carrying out constant temperature transformation treatment at a nose temperature of -40 ° C or higher on a TTT curve or lower than a nose temperature on a TTT curve. do. It is preferable to austenitize the wire rod at 900 ° C. or higher, and the upper limit of the temperature range is not necessarily limited, but may be limited in consideration of process or economic aspects. In addition, the cooling method between the austenitization and the constant temperature transformation heat treatment process is not necessarily limited, and may be subjected to general air cooling and cooling.

As the transformation temperature to pearlite is lower, the layer spacing of pearlite can be finely controlled. However, if transformation to pearlite is initiated at too low a temperature, upper bainite may be produced before the pearlite transformation is complete.

Therefore, in the present invention, the transformation curve of bainite is separated from the pearlite transformation curve at low temperature using chromium. By separating the perlite curve from the pearlite and the bainite transformation curve, the formation of upper bainite can be suppressed even when constant temperature heat treatment is performed at a temperature below the nose temperature on the TTT curve, and the perlite transformation occurs at a low temperature. The stratified spacing becomes narrower. However, since the upper bainite may be generated when the constant temperature transformation is performed at too low temperature, the lower limit of the constant temperature transformation heat treatment temperature is preferably limited to the nose temperature of 40 ° C on the TTT curve.

In addition, in the case of constant temperature transformation heat treatment in the above temperature range, the layered spacing of pearlite may be controlled to 100 nm or less. In addition, the microstructure of the wire rod produced by the above production method is made of a single phase of pearlite or includes 95% or more of pearlite and 5% or less of upper bainite. However, the microstructure without other cementite cementite or martensite is the most preferable microstructure. As a result, the present invention can provide a wire rod with excellent freshness and strength by securing a fine structure of pearlite having a low mixing of upper bainite and a narrow layer spacing.

Hereinafter, the present invention will be described through examples.

(Example)

The wire rods of the invention examples and examples satisfying the component systems shown in Table 1 were austenized at 1000 ° C., and then subjected to constant temperature transformation at 520 ° C., 540 ° C. and 560 ° C. to confirm the temperature at which bainite was incorporated.

division C (% by weight) Si (% by weight) Mn (% by weight) Cr (% by weight) P (wt%) S (% by weight) N (% by weight) O (% by weight) Inventive Example 0.98 0.60 0.50 0.6 0.008 0.009 0.049 0.015 Comparative example 0.98 0.60 0.50 - 0.009 0.012 0.050 0.016

In addition, TTT curves were derived by performing dilatometry experiments on the invention and comparative examples, which are shown in FIG. 1. Referring to FIG. 1, in the comparative example, the bainite transformation curve and the pearlite transformation curve were not separated, but in the case of the inventive example, the separated bainite transformation curve can be clearly identified.

In addition, the invention and the comparative example, the constant temperature heat treatment at 520 ℃, 540 ℃ and 560 ℃ by observing each microstructure using an optical microscope is shown in FIG. Figure 2 (a) is a constant temperature heat treatment of the invention example at 560 ℃, it can be seen that does not contain the bainite structure. Figure 2 (b) is a constant temperature heat treatment of the invention example at 540 ℃, it can be seen that the bainite tissue begins to be mixed. Figure 2 (c) is a constant temperature heat treatment of the invention example at 520 ℃, it can be seen that the bainite structure is significantly mixed. In addition, Figure 2 (d) is a constant-temperature transformation heat treatment of the comparative example at 560 ℃, it can be confirmed that does not contain the bainite structure. FIG. 2 (e) shows that the comparative example is subjected to induction transformation heat treatment at 540 ° C., and the bainite structure starts to be mixed. Figure 2 (f) is a constant temperature heat treatment of the comparative example at 520 ℃, it can be seen that the bainite structure is significantly mixed. Through this, it can be seen that the invention examples and comparative examples start to incorporate the bainite at constant temperature transformation heat treatment at 540 ℃.

Constant temperature and hardness were measured, and the comparative example is shown in FIG. 3, and the invention example is shown in FIG. In the comparative example, the nose temperature was measured at 580 ° C, and in the invention example, the nose temperature was measured at 620 ° C. It can be seen that both the comparative examples and the inventive examples rapidly decrease the hardness at around 540 ° C., because the upper bainite starts to be mixed. In the case of the invention, the bainite is not mixed when the constant temperature transformation heat treatment is performed between the nose temperature of 620 ° C and the temperature at which bainite starts to be mixed (540 ° C). On the other hand, in the comparative example, bainite is not mixed when the constant temperature heat treatment is performed between the nose temperature of 580 ° C and the temperature at which bainite starts to be mixed (540 ° C). That is, according to the present invention is to ensure a wider temperature range during constant temperature transformation heat treatment.

Inventive examples and comparative examples were subjected to constant temperature transformation heat treatment at 580 ° C. to measure the layer spacing of pearlite, and are shown in FIG. 5, and the microstructures of the microstructures were shown in FIGS. 6 and 7, respectively. Referring to FIG. 5, it can be seen that in the comparative example, even when constant temperature transformation heat treatment is performed at 580 ° C. as the nose temperature, the lamellar spacing of the pearlite is measured larger than that of the invention example. 6 shows fine layer spacing as the microstructure of the invention example. On the other hand, Figure 7 is a microstructure of the comparative example, it can be seen that the layer spacing is not fine.

In addition, the inventive examples and the comparative examples were subjected to constant temperature transformation at 580 ° C., and then measured in tensile strength and cross-sectional reduction rate.

division Tensile Strength (MPa) Reduction of Area (%) Inventive Example 1391 26.3 Comparative example 1174 17.4

It can be seen that the invention examples satisfying the conditions of the present invention are superior in tensile strength and cross-sectional reduction rate than the comparative examples.

1 is a graph showing the TTT curve of the comparative example and the invention example.

Figure 2 is a photograph of the microstructure observed by the optical microscope after induction transformation heat treatment according to the temperature of the comparative example and the invention example.

3 is a graph showing a correlation between hardness and transformation temperature for a comparative example.

4 is a graph showing a correlation between hardness and transformation temperature for the invention example.

5 is a graph showing the pearlite layer spacing measured after the constant temperature transformation heat treatment at 580 ℃ Comparative Example and Example.

Figure 6 is a photograph of the microstructure of the invention after the transformation at constant temperature transformation at 580 ℃ optical microscope.

Figure 7 is a photograph of the microstructure of the microstructure after the induction transformation heat treatment at 580 ° C Comparative Example.

Claims

By weight%, carbon (C): 0.6-1.0%, silicon (Si): 0.2-1.0%, manganese (Mn): 0.3-1.5%, chromium (Cr): 0.6-1.0%, oxygen (O): 0.0015 High-strength wire rod containing up to%, phosphorus (P): 0.02% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, residual Fe and other unavoidable impurities.

The wire rod of claim 1, wherein the wire includes 95% or more of pearlite and 5% or less of upper bainite.

The wire rod for high strength wire according to claim 2, wherein the pearlite interlamellar spacing is 100 nm or less.

By weight%, carbon (C): 0.6-1.0%, silicon (Si): 0.2-1.0%, manganese (Mn): 0.3-1.5%, chromium (Cr): 0.6-1.0%, oxygen (O): 0.0015 Austenitizing a wire containing 900% or less, phosphorus (P): 0.02% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, balance Fe and other unavoidable impurities And a constant temperature heat treatment at a nose temperature of 40 ° C. or higher on a TTT curve or lower than a nose temperature on a TTT curve.