KR20210077506A - Steel having enhanced cold formability and method for manufacturing the same - Google Patents

Abstract

Provided are a wire rod having excellent cold workability and a method for manufacturing the same. The wire rod of the present invention includes, by weight, 0.20 to 0.50 % of C, 0.5% or less (excluding 0 %) of Si, 0.4 to 1.4 % of Mn, 0.030 % or less of P, 0.030 % or less of S, and the balance Fe and unavoidable impurities and has a microstructure including, by volume, 15 % or less of retained austenite and the residual bainite, and a solid solution carbon content in the retained austenite satisfies a range of 1.0 to 1.8 %.

Description

Wire rod having excellent cold workability and manufacturing method therefor {STEEL HAVING ENHANCED COLD FORMABILITY AND METHOD FOR MANUFACTURING THE SAME}

An object of the present invention is to provide a wire rod that can be used for automobiles, structural bolts, etc. exposed to various stress and corrosive environments, and a method for manufacturing the same.

Wire rods are made into final products of various shapes such as bolts and balls through a cold working process. In order to minimize the wear of the mold used at this time, a method of lowering the strength of the wire rod through microstructure control through spheroidizing heat treatment is used. Usually, the more difficult the shape of the final product, the higher the spheroidization rate is required, and for this, one or more spheroidization heat treatments are required.

In general, although the spheroidizing heat treatment is slightly different depending on the type of steel, it is always disadvantageous in terms of manufacturing cost due to high cost because the steel made of ferrite and pearlite structure must be maintained in a certain temperature range for a long time.

It is known that the spheroidizing heat treatment time can be greatly reduced if the microstructure in the wire rod state can be made into a fine pearlite structure or a low-temperature structure such as bainite or martensite. However, in the conventional wire rod cooling method, there is a difficulty in that it is not possible to produce the fine or low temperature structure as described above due to the limitation of the cooling rate.

An object of the present invention is to provide a wire rod and a method for manufacturing the same, which can reduce the spheroidizing heat treatment time and secure excellent cold workability by controlling the microstructure through the alloy composition and manufacturing method.

The problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the following description.

The present invention for achieving the above object,

By weight%, C: 0.20 to 0.50%, Si: 0.1 to 0.5%, Mn: 0.4 to 1.4%, P: 0.030% or less, S: 0.030% or less, the remainder contains Fe and unavoidable impurities,

By volume fraction, it has a microstructure comprising 15% or less of retained austenite and residual bainite,

And it relates to a wire rod excellent in cold workability, which satisfies the range of 1.0 to 1.8% of the solid solution carbon in the retained austenite.

The microstructure may include 85 to 95% bainite and 5 to 15% retained austenite.

A lath thickness of the bainite may be 1 μm or less.

Also, the present invention

By weight%, C: 0.20 to 0.50%, Si: 0.1 to 0.5%, Mn: 0.4 to 1.4%, P: 0.030% or less, S: 0.030% or less, the remainder after preparing steel containing Fe and unavoidable impurities , a process of wire-rolling it, and then winding it in a coil shape at 700 to 900°C; and

After immersing the wound steel material in molten salt or molten lead at 300 to 400 ° C., and then performing constant temperature maintenance heat treatment, as a volume fraction, it has a microstructure including 15% or less of retained austenite and residual bainite, and the residual It relates to a method for manufacturing a wire rod excellent in cold workability, including a process of manufacturing a wire rod having a solid solution carbon content in austenite in the range of 1.0 to 1.8%.

It is preferable to set the holding time to 5 minutes or more during the constant temperature maintenance heat treatment.

The microstructure of the wire rod may include 85 to 95% of bainite and 5 to 15% of retained austenite.

The present invention according to the above-described configuration can provide a wire rod excellent in cold workability required for materials or parts for industrial machines and automobiles by controlling the matrix structure of the wire rod with bainite.

Hereinafter, the present invention will be described.

The wire rod having excellent cold workability according to the present invention, by weight, C: 0.20 to 0.50%, Si: 0.5% or less (excluding 0%), Mn: 0.4 to 1.4%, P: 0.030% or less, S: 0.030 % or less, the remainder contains Fe and unavoidable impurities, has a microstructure including 15% or less of retained austenite and residual bainite by volume fraction, and a solid solution carbon content in the retained austenite is in the range of 1.0 to 1.8% is satisfied with

First, the compositional component of the wire rod of the present invention and the reason for limiting the compositional range will be described in detail. Hereinafter, unless otherwise specified, "%" means % by weight.

・Carbon (C): 0.20 to 0.50%

Carbon is an element added to secure product strength. When the carbon content is less than 0.20%, it is difficult to secure a target strength, and when it exceeds 0.50%, it is not preferable because the ferrite and pearlite phase transformations proceed rapidly, which may interfere with the formation of bainite.

Silicon (Si): 0.5% or less (excluding 0%)

Silicon is not only useful for deoxidation of steel, but also effective for securing strength through solid solution strengthening, but is an element that has inferior impact properties. If the content exceeds 0.5%, it is not preferable because there is a concern about deterioration of impact properties and cold workability. In addition, since too little Si content may give difficulty in securing strength through deoxidation and solid solution strengthening of steel, in the present invention, it is more preferable to manage the Si content in the range of 0.1 to 0.5%.

Manganese (Mn): 0.4 to 1.4%

Manganese is an element for improving hardenability, and is a very useful element that forms a substitutional solid solution in the matrix structure to produce a solid solution strengthening effect. When the manganese content is less than 0.4%, it is difficult to secure the target strength because the solid solution strengthening effect and hardenability are not sufficient. When the manganese content exceeds 1.4%, it is not preferable because the hardenability increases and it may take a long time to form bainite. .

Phosphorus (P): 0.030% or less (0% is not included)

Phosphorus is segregated at grain boundaries to decrease toughness and is preferably not included as a major cause of reduced resistance to delayed fracture, so the upper limit thereof is set to 0.030%.

Sulfur (S): 0.030% or less (0% is not included)

Sulfur, like phosphorus, segregates at grain boundaries to reduce toughness, and is an element that inhibits hot rolling by forming low-melting emulsions, so it is preferable not to contain it as much as possible, so the upper limit is set to 0.030%.

In addition to the above composition, the remainder includes Fe and unavoidable impurities. The present invention does not exclude the addition of other alloys other than the above-mentioned alloy composition.

On the other hand, the wire rod of the present invention has a microstructure including 15% or less of retained austenite and residual bainite by volume fraction, by volume fraction. If the retained austenite structure fraction exceeds 15%, the solid solution carbon content is lowered and the retained austenite phase becomes unstable, so there is a problem in that the structure becomes non-uniform due to martensite transformation during wire drawing.

Preferably, the wire rod has a microstructure containing 85 to 95% bainite and 5 to 15% retained austenite.

In addition, in the present invention, the content of solid solution carbon in the retained austenite constituting the microstructure is required to satisfy the range of 1.0 to 1.8%. If the dissolved carbon content is less than 1.0%, the phase stability of retained austenite is lowered, so martensite transformation is easy to occur during drawing, and the structure may become non-uniform. If it exceeds 1.8%, deformation twins during drawing are concentrated. may be formed and brittle.

Furthermore, in the present invention, it is preferable that the lath thickness of the bainite constituting the microstructure is 1 μm or less. When the lattice thickness of the bainite exceeds 1 μm, the spheroidizing heat treatment time becomes long and it becomes disadvantageous in terms of process cost, so it is better to control the conditions such as the alloy composition of the steel and the provision process to manage the rath thickness to be 1 μm or less desirable.

Next, the wire rod manufacturing method of the present invention will be described in detail.

The method for manufacturing a wire rod of the present invention includes a step of preparing a steel material having the alloy composition, rolling the wire rod, and then winding it in a coil shape at 700 to 900°C; and a process of manufacturing a wire rod by immersing the wound steel material in molten salt or molten lead at 300 to 400° C., and then performing constant temperature maintenance heat treatment.

After preparing a steel material having the above-mentioned composition and rolling the wire rod, it is wound in a coil shape at 700 to 900 ° C., and then immersed in molten salt or molten lead at 300 to 400 ° C. for 5 minutes or more to complete the constant temperature transformation, followed by air cooling. includes steps.

First, in the present invention, after preparing a steel material having the above-described composition and rolling the wire rod, it is wound in a coil shape at 700 to 900°C.

If the coiling temperature is less than 700°C, proeutectoid ferrite transformation becomes easy, and a wire rod having a bainite matrix structure cannot be obtained. On the other hand, when the coiling temperature exceeds 900°C, the austenite grains become coarse and the lattice thickness of bainite after phase transformation exceeds 1 μm, and the spheroidization heat treatment time becomes longer, which is disadvantageous in terms of process cost. It is preferable to set it as °C.

Thereafter, in the present invention, the wound steel is quenched in a molten salt or molten lead of 300 to 400° C. exceeding the martensitic transformation starting temperature (Ms), and a constant temperature maintenance heat treatment is performed to maintain it in this temperature range. If the constant temperature maintenance heat treatment temperature is less than 300° C., the bainite transformation time becomes longer, which is disadvantageous in terms of productivity, or martensite is generated, so that the microstructure becomes non-uniform, and the spheroidization characteristic is inferior. On the other hand, if it exceeds 400 ℃, pearlite is generated and the spheroidization time may be long. Therefore, in the present invention, it is preferable to perform the constant temperature maintenance heat treatment temperature in the range of 300 ~ 400 ℃.

On the other hand, in the present invention, it is preferable to set the constant temperature maintenance heat treatment time to 5 minutes or more, and if it is maintained for less than 5 minutes, some untransformed austenite is transformed into martensite during air cooling, and the strength is greatly increased more than necessary. The tissue may become non-uniform and the spheroidization properties may be inferior.

Next, in the present invention, after the constant temperature maintenance heat treatment, the transformation-completed wire rod is air-cooled.

Hereinafter, the present invention will be described in detail with reference to Examples.

(Example)

After preparing a steel material having the composition components shown in Table 1 below, reheating to 1050° C., hot rolling, and then performing winding under the conditions of Table 2 below, performing constant temperature transformation heat treatment in molten salt or molten lead, and then air cooling to manufacture a wire rod did.

For these wire rods, the bainite volume fraction, retained austenite volume fraction, and bainite lattice thickness were measured and shown in Table 2. The volume fraction of bainite was measured using an image analyzer, the volume fraction and carbon content of retained austenite were measured using Cu-Kα X-ray diffraction analyzer and EPMA, respectively, and the thickness of bainite lattice was measured by backscattered electrons. It was obtained using diffraction (EBSD).

In addition, after 30% cold drawing of these steels, spheroidizing heat treatment was performed at 730° C. for a certain period of time, and the critical upset rate was obtained through an upset test to evaluate the cold workability, and are shown in Table 2 below.

Steel grade No. C Si Mn P S One 0.48 0.1 0.7 0.019 0.021 2 0.22 0.4 1.2 0.024 0.014 3 0.29 0.3 0.9 0.020 0.017 4 0.43 0.3 0.7 0.019 0.020 5 0.50 0.4 0.6 0.020 0.012 6 0.23 0.2 1.1 0.018 0.010 7 0.38 0.5 0.9 0.015 0.013 8 0.30 0.3 1.3 0.020 0.015 9 0.31 0.1 1.0 0.021 0.022

* In Table 1, the content of each component is expressed in wt%, and the remainder is Fe and inevitable impurities

Steel grade No. division Coiling temperature (℃) constant temperature heat treatment Residual austenite volume fraction (%) Solid carbon content Bainite volume fraction (%) / Lath thickness (㎛) Nodularization rate (%) Critical upset rate (%) maintain
Temperature (℃) maintain
time
(min) One Invention Example 1 760 385 12 13 1.2 87/0.7 90 68 2 Invention Example 2 880 350 15 9 1.4 91/0.7 90 69 3 Invention example 3 810 365 20 11 One. 89/0.6 91 75 4 Invention Example 4 840 325 9 8 1.6 92/0.5 92 73 5 Comparative Example 1 660 330 15 10 1.5 81/0.5 79 56 6 Comparative Example 2 950 350 20 10 13 89/1.2 85 65 7 Comparative Example 3 850 250 30 7 2.0 79/0.9 80 60 8 Comparative Example 4 780 420 8 2 0.7 10/- 78 57 9 Comparative Example 5 870 360 3 4 1.3 55/0.8 83 65 One Comparative Example 6 860 400 15 15 0.9 85/0.9 83 63 2 Comparative Example 7 850 300 30 5 1.9 95/0.5 82 64 3 Comparative Example 8 830 405 9 17 0.9 83/0.6 77 60

As shown in Table 1-2, the wire rods of Inventive Example 1-4 satisfying the emphasis and manufacturing conditions of the present invention have a microstructure containing 15% or less of retained austenite and residual bainite by volume fraction, it can be seen that there is And it can be seen that the lath thickness of bainite is 1 μm or less in all of Inventive Examples 1-4, so that the spheroidization rate during spheroidization heat treatment is high as 90% or more, and the critical upset rate is about 70% or more, showing excellent cold workability. have.

In comparison, Comparative Example 1 showed that the spheroidization rate was low during the spheroidizing heat treatment, and the critical upset rate was also deteriorated because the coiling temperature was low and the proeutectoid ferrite transformation was facilitated and a sufficient amount of bainite was not formed.

Comparative Example 2, in contrast to Comparative Example 1, exhibits a relatively low spheroidization rate and critical upset rate after spheroidization heat treatment because the bainite lattice thickness is relatively thick due to a high coiling temperature.

Comparative Example 3 is a case in which the holding temperature is low during constant temperature heat treatment, and not only martensite is formed, but also the solid solution carbon content of retained austenite is too high, so the deformation is concentrated and embrittled during cold drawing. The microstructure becomes non-uniform and spheroidized. After heat treatment, the spheroidization rate and critical upset rate were inferior.

Comparative Example 4 is a case where the holding temperature is high during constant temperature heat treatment, fine pearlite is formed instead of bainite in the matrix structure, and the carbon content of retained austenite is too low, so martensite transformation may occur during cold drawing. It can be seen that relatively inferior spheroidization rate and critical upset rate are shown.

In Comparative Example 5, the constant temperature heat treatment holding time was too short, so bainite was not sufficiently formed, and untransformed austenite was transformed into martensite during air cooling. The microstructure became non-uniform, resulting in inferior spheroidization rate and critical upset rate after spheroidization heat treatment It has shown that it is not effective for cold workability improvement.

Comparative Examples 6-7 show a case in which the solid solution carbon content in retained austenite is out of the scope of the present invention, even though the wire rod has a mixed structure of bainite and retained austenite satisfying the scope of the present invention.

Specifically, in Comparative Example 6, which had an insufficient solid solution carbon content, the phase stability of retained austenite was lowered, and martensite transformation was easy to occur during wire drawing, resulting in a non-uniform structure, and in Comparative Example 7 with excessive amount of retained austenite, the phase stability of retained austenite was reduced. It became too high, indicating that the spheroidization rate and critical upset rate after wire drawing deteriorated.

On the other hand, Comparative Example 8 is a case in which the retained austenite fraction is excessive, the solid solution carbon content is lowered, the phase stability of the retained austenite is lowered, and the martensitic transformation is easy to occur during wire drawing and the structure becomes non-uniform showed this inferiority.

The present invention is not limited to the above embodiments and embodiments, but can be manufactured in various different forms, and those of ordinary skill in the art to which the present invention pertains can use other methods without changing the technical spirit or essential features of the present invention. It will be understood that it may be implemented in specific forms. Therefore, it should be understood that the embodiments and embodiments described above are illustrative in all respects and not restrictive.

Claims (6)

By weight%, C: 0.20 to 0.50%, Si: 0.5% or less (excluding 0%), Mn: 0.4 to 1.4%, P: 0.030% or less, S: 0.030% or less, the remainder contains Fe and unavoidable impurities and,
By volume fraction, it has a microstructure comprising 15% or less of retained austenite and residual bainite,
and a wire rod having excellent cold workability in which the solid solution carbon content in the retained austenite satisfies the range of 1.0 to 1.8%.
The wire rod having excellent cold workability according to claim 1, wherein the microstructure comprises 85 to 95% of bainite and 5 to 15% of retained austenite.
The wire rod having excellent cold workability according to claim 1, wherein the lath thickness of the bainite is 1 μm or less.
By weight%, C: 0.20 to 0.50%, Si: 0.5% or less (excluding 0%), Mn: 0.4 to 1.4%, P: 0.030% or less, S: 0.030% or less, the remainder contains Fe and unavoidable impurities After providing a steel material to do, the wire rod rolling, and then, the process of winding in a coil shape at 700 ~ 900 ℃; and
After immersing the wound steel material in molten salt or molten lead of 300 to 400 ° C., by constant temperature maintenance heat treatment, by volume fraction, has a microstructure containing 15% or less of retained austenite and residual bainite, and the residual A method of manufacturing a wire rod having excellent cold workability, comprising a process of manufacturing a wire rod having a dissolved carbon content in austenite in the range of 1.0 to 1.8%.
[Claim 5] The method for manufacturing a wire rod having excellent cold workability according to claim 4, wherein the holding time is set to 5 minutes or more during the constant temperature maintenance heat treatment.
5. The method of claim 4, wherein the wire rod microstructure contains 85 to 95% bainite and 5 to 15% retained austenite.