KR101560877B1 - Medium carbon sofr wire rod having excellent impact toughness and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a medium carbon wire rod which can improve the deformability of cementite, improve impact toughness and strain aging by using a wire material not containing silicon, which is a ferrite solid solution strengthening element, and a composite design of alloying elements, .

Description

TECHNICAL FIELD [0001] The present invention relates to a medium carbon wire having excellent impact toughness and a method of manufacturing the same. [0002]

The present invention relates to a medium carbon wire rod excellent in impact toughness and a method of manufacturing the same.

Recent developments in cold heading quality wire (CHQ) technology have been focused on the development of high strength wire rods with excellent function along with process abbreviated wire ropes without heat treatment and machining processes.

For example, boron steel, which is currently in the limelight, has a low boron content (B) added even if it does not contain expensive additive improving elements such as chromium (Cr), nickel (Ni) and molybdenum (Mo) Or more than that of the steel material. Thus, since a large amount of alloying elements are not added, the strength of the material after rolling is low, which is also referred to as a step-abbreviated steel material since the softening step such as spheroidizing heat treatment is not performed.

Generally, in the production of cold-rolled wire by cold-pressing, the lifetime of the die is one of the important factors in terms of securing the competitiveness of cold forging products due to productivity and high manufacturing cost. It is known that it is greatly influenced by aging.

The cold-rolled steel die life or wear of the die tends to decrease as the strength of the steel increases, and as a method of lowering the strength of such steel, softening the steel to a spheroidizing heat treatment to soften the cementite Methods are most commonly used. In addition, the strain aging causes the displacement and propagation of the dislocations to occur at the same time in the material during machining, and the elements such as C and N fix the dislocations to increase the deformation strength, which causes a decrease in the life of the die .

The present invention relates to a medium carbon wire rod which can improve the deformability of cementite, improve impact toughness and strain aging by using a wire material not containing silicon, which is a ferrite solid solution strengthening element, and a composite design of alloying elements, .

In one aspect of the present invention, a medium carbon wire rod excellent in impact toughness comprises 0.2 to 0.4% of C, 0.4 to 1.0% of Mn, 0.2 to 2.0% of Co, 0.005 to 0.015% of Ti, 0.005 to 0.015% of Ti, 0.003%, P: not more than 0.035%, S: not more than 0.04%, O: not more than 0.005%, N: not more than 50 ppm, and the balance Fe and other unavoidable impurities.

Another aspect of the present invention is to provide a method for producing a medium carbon wire rod excellent in impact toughness, which comprises 0.2 to 0.4% of C, 0.4 to 1.0% of Mn, 0.2 to 2.0% of Co, 0.005 to 0.015 % of Ti, The billets containing 0.001 to 0.003% of B, 0.035% or less of P, 0.04% or less of S, 0.005% or less of O and 50 ppm or less of N and the balance Fe and other unavoidable impurities are homogenized in a temperature range of 1000 to 1200 캜 Heat-treating the hot-rolled billet, and hot-rolling the homogenized heat-treated billet.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

According to the present invention, cobalt is added to improve the cementite deformability, and titanium is added to improve the impact toughness without lowering the strength of the wire without adding silicon, which is a ferrite solid solution strengthening element. There is an effect of providing a wire having a remarkably improved lifetime of a cold pressing die by suppressing strain aging caused by movement and diffusion of carbon, nitrogen and the like, and a manufacturing method thereof.

The inventors of the present invention have studied a method for improving the deformation aging due to the cold pressing shrinkage and the problem of deterioration of the die life due to the cold pressing in producing the wire rod for cold rolling, The effect of suppressing deformation aging and improving impact toughness was confirmed through appropriate adjustment between alloying elements such as Ti and B, and reached the present invention.

Hereinafter, a heavy carbon wire rod excellent in impact toughness, which is one aspect of the present invention, will be described in detail.

In one aspect of the present invention, a medium carbon wire rod excellent in impact toughness comprises 0.2 to 0.4% of C, 0.4 to 1.0% of Mn, 0.2 to 2.0% of Co, 0.005 to 0.015% of Ti, 0.005 to 0.015% of Ti, 0.003%, P: not more than 0.035%, S: not more than 0.04%, O: not more than 0.005%, N: not more than 50 ppm, the balance Fe and other unavoidable impurities, do.

Carbon (C): 0.2 to 0.4 wt%

Carbon is an element that is essential for securing strength. When the content of carbon is less than 0.2% by weight, it is difficult to obtain sufficient material strength and it is not easy to secure sufficient ingotability after final QT heat treatment. On the other hand, when the content of carbon is more than 0.4% by weight, it is difficult to exhibit the effect of improving the cold rolling composition to be obtained by the present invention due to high material strength.

Manganese (Mn): 0.4 to 1.0 wt%

Manganese increases the strength of the steel, affects the impact properties, increases the rolling property and reduces the brittleness. In order to exhibit the above-mentioned effect, it is preferable to contain 0.4 wt% or more of manganese. On the other hand, when the content of manganese exceeds 1.0% by weight, there is a problem that the hardening phenomenon is intensified by the increase of the strength.

Cobalt (Co): 0.2 to 2.0 wt%

Cobalt is an effective element for accelerating pearlite transformation and softening cementite to secure ductility. When the content of the cobalt is less than 0.2% by weight, the hardness of the cementite is not sufficiently lowered and it is difficult to expect an improvement in the deformability. On the other hand, when the amount exceeds 2.0% by weight, it is not easy to ensure sufficient ingot qualities after the final QT heat treatment.

Titanium (Ti): 0.005-0.015 wt%

Titanium is added as an element that fixes nitrogen for ensuring free boron content for strengthening the grain boundary through boron or improving the ingotability. In order to achieve the above-described effects, the titanium preferably contains 0.005% by weight or more of titanium. If the content of titanium exceeds 0.015% by weight, TiN is coarsened and impact toughness of the wire is lowered and the grain control effect is lowered.

Boron (B): 0.001 to 0.003 wt%

Boron is an element that increases incombustibility. In the present invention, boron was added in order to compensate for the addition of expensive Si as the solid solution strengthening element. When the content of boron is less than 0.001% by weight, the increase of the boron content for securing strength is not sufficient. On the other hand, if it exceeds 0.003% by weight, the effect becomes saturated and the grain boundary is deteriorated, thereby causing problems such as deterioration of physical properties.

Nitrogen (N): 50ppm or less

Nitrogen is an element that increases the deformation strength by adhering to the dislocation when displacement and propagation occur in the material due to the rolling heat generated during cold pressing. The addition of such nitrogen directly affects the lifetime of the die. In particular, interstitial elements such as nitrogen induce dislocation and propagation in the material due to the forging heat generated during cold forging, and the nitrogen element adheres to the electric potential to increase the deformation strength do. In order to achieve the above-described effect, the nitrogen is preferably added in an amount of 50 ppm or less. When the content of nitrogen exceeds 50 ppm, it is difficult to control free nitrogen even when nitrogen fixing elements such as Al are added.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

However, since oxygen, phosphorus and sulfur are generally referred to as impurities, they will be briefly described as follows.

Oxygen (O): 0.005 wt% or less

Oxygen may form an oxide-based non-metallic inclusion, which may cause fatigue life degradation. Therefore, the upper limit of the content is preferably limited to 0.005% by weight.

Phosphorus (P): 0.035% by weight or less

The phosphorus is inevitably contained and is an element which is segregated at the crystal grain boundaries to decrease the toughness and reduce the delayed fracture resistance. Therefore, it is desirable to control the phosphor as low as possible. Theoretically, it is preferable to limit the phosphorus content to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is preferably limited to 0.035% by weight.

Sulfur (S): 0.04% by weight or less

Sulfur is an inevitably contained impurity, which is segregated at grain boundaries as a low melting point element, and is an element which deteriorates toughness and forms an emulsion so as to adversely affect delayed fracture resistance and stress relaxation characteristics. Therefore, it is desirable to control as low as possible. The theoretical sulfur content is advantageous to be limited to 0% but it is inevitably contained in the manufacturing process normally. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is preferably limited to 0.04% by weight.

In order to minimize an increase in strength due to deformation aging during cold forging of the wire rod, it is preferable to control the content of free nitrogen in the wire to be 10 ppm or less. When the content of free nitrogen in the wire rod exceeds 10 ppm, there is a problem that the strength is increased due to strain aging. Free nitrogen (N) in the present invention means nitrogen in a state in which the boron (B) does not bond with the boron in the steel.

Furthermore, the content of the free boron (Free B) in the wire is preferably controlled to 10 to 20 ppm in order to improve the ingotability of the wire. If the content of the free boron is less than 10 ppm, there is a problem that sufficient incombustibility is not obtained. On the other hand, if it exceeds 20 ppm, there is a problem that the grain boundary is deteriorated and the physical properties are lowered.

The microstructure of the wire is preferably ferrite and pearlite composite structure. By having the composite structure of ferrite and pearlite in the microstructure of the wire, excellent cold-rolling composition can be secured.

In order to further improve the excellent cold-rolling composition expressed by having such a microstructure, it is preferable to have a microstructure of ferrite and remainder pearlite of 10 to 30% in area fraction%. If the percentage area fraction of the ferrite is less than 10%, the strength becomes high and a sufficient cold-rolling composition can not be secured. When the area percentage is more than 30%, it is difficult to secure sufficient ingotability after the final Q coining

The wire preferably has an average grain size of 10 to 100 mu m. If the average grain size of the wire rod is less than 10 mu m, the strength becomes high and a sufficient cold-rolling composition can not be secured. On the other hand, when it exceeds 100 탆, it is not easy to secure sufficient ingotability after the final heat-cooling (QT) heat treatment.

The wire rod preferably has an impact value of 80 J or more at room temperature.

Hereinafter, a method for manufacturing a medium carbon wire having excellent impact toughness, which is another aspect of the present invention, will be described in detail.

Another aspect of the present invention is to provide a method for producing a medium carbon wire having excellent impact toughness, which comprises 0.2 to 0.4% of C, 0.4 to 1.0% of Mn, 0.2 to 2.0% of Co, 0.005 to 0.02% of Ti, The billets containing 0.001 to 0.003% of B, 0.035% or less of P, 0.04% or less of S, 0.005% or less of O and 50 ppm or less of N and the balance Fe and other unavoidable impurities are homogenized in a temperature range of 1000 to 1200 캜 Heat-treating the hot-rolled billet, and hot-rolling the homogenized heat-treated billet.

Homogenization  Heat treatment step

The billet satisfying the above-mentioned component system is subjected to homogenization heat treatment in a temperature range of 1000 to 1200 캜. If the homogenization heat treatment temperature is less than 1000 ° C, the microstructure homogenization is not sufficiently performed. If the homogenization heat treatment temperature is more than 1200 ° C, the TiN precipitates are coarsened and the impact characteristics are deteriorated.

Hot rolling step

The billet subjected to the homogenization heat treatment is hot-rolled in a wire form. At this time, hot rolling can be performed by a conventional wire rolling method.

However, in hot rolling, hot rolling is performed at a reduction ratio of 80% or more. If the reduction ratio of the hot rolling is less than 80%, there is a problem that a coarse casting structure can not be sufficiently formed.

It is preferable to perform the hot rolling and then cool down. The cooling is performed by a conventional wire rolling method.

The wire material produced by the above method is characterized by containing aluminum nitride (AlN) as a wire material and having a content of free nitrogen of 10 ppm or less. When such a wire rod is subjected to cold pressing, the phenomenon such as dislocation fixation due to free N is not generated due to fixing of dislocation-fixing N by bonding with Al, so that the increase of strain strength is suppressed.

Therefore, when the deformation rate of the wire rod is changed from room temperature (RT) to 200 ° C at е = 1 and the deformation rate ε '= 1 / s, the material strength is lower than that of the conventional Si addition steel by 100 MPa or more, The strength has a strength value of 60 MPa or more higher than that of the conventional Si-added steel.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred from them.

(Example)

Each of the steels having the composition shown in Table 1 below was prepared, and a 50 Kg ingot was cast using the steels as a sample. The ingot was homogenized at 1200 ° C for 2 hours and hot-rolled to a thickness of 15 mm. At this time, the hot rolling temperature was set to 900 ° C or more, followed by hot rolling and air cooling. The rolling ratio was set to 80% or more. Thereafter, bolt cold forging simulation was performed using a Griple compression test specimen.

The deformation temperature was measured at room temperature (RT) and maximum 200 ℃, and the deformation rate and deformation rate were е = 1 and ε '= 1 / s, respectively. After the bolt cold stamping simulation as described above, the change in the deformation strength was measured. At this time, the increase (+) or the decrease (-) was measured based on the strength value of the conventional material and is shown in Table 2 below. The microstructure fraction and average grain size of the wire were measured and are shown together in Table 2 below.

division C Si Mn Co Ti B P S O N Conventional example 0.29 0.21 0.71 - - - 0.03 0.03 0.004 0.005 Inventory 1 0.30 - 0.69 0.5 0.005 0.0019 0.03 0.03 0.004 0.005 Inventory 2 0.31 - 0.72 1.9 0.014 0.0017 0.03 0.03 0.004 0.004 Comparative Example 1 0.31 - 0.72 2.3 0.005 0.0018 0.03 0.03 0.004 0.004 Comparative Example 2 0.29 - 0.68 0.1 0.014 0.0018 0.03 0.03 0.004 0.005 Comparative Example 3 0.30 - 0.68 1.2 0.003 0.0017 0.03 0.03 0.004 0.005 Comparative Example 4 0.28 - 0.70 1.2 0.022 0.0005 0.03 0.03 0.004 0.004

(The contents of C, Si, Mn, Co, Ti, B, P, S, O and N are added in weight%)

division Microstructure Ferrite fraction (%) Free nitrogen (ppm) Amount of free boron (ppm) Average grain size (占 퐉) Deformation Resistance Strength (MPa) Intensity (MPa) Impact value at 20 占 폚 (U-notch) Room temperature 200 ℃ Conventional example Ferrite + Pearlite 25 50 0 60 740 764 1151 52 Inventory 1 Ferrite + Pearlite 23 9 10 12 -118 -159 -17 82 Inventory 2 Ferrite + Pearlite 22 4 15 10 -144 -167 -53 87 Comparative Example 1 Ferrite + Pearlite 23 10 15 15 -153 -166 -115 79 Comparative Example 2 Ferrite + Pearlite 24 5 14 13 -57 -98 -15 83 Comparative Example 3 Ferrite + Pearlite 23 17 7 52 -54 -31 -58 54 Comparative Example 4 Ferrite + Pearlite 24 4 4 49 -55 -20 -101 22

In comparison between the conventional example in which Si is contained and the case of Examples 1 and 2 in which Si is not contained and the cases of Comparative Examples 1 to 4 in Comparative Examples 1 and 2 and Comparative Examples 1 to 4 in which Si is not contained, It can be confirmed that the high temperature compressive strength is reduced and the solid solution strengthening effect is reduced.

Examples 1 and 2 demonstrate that the steel satisfies both the composition and the composition range proposed by the present invention and has a low deformation resistance strength and an effect equivalent to or less than that of the conventional example. It is also confirmed that the austenite grains are made finer and the impact toughness is improved.

On the other hand, in Comparative Example 1, when the content of cobalt exceeded the range proposed by the present invention, it was confirmed that although the strength of the deformation resistance was further lowered, the degradation of the ingot property also occurred. In Comparative Example 2, the content of cobalt is below the range proposed by the present invention, and it can be confirmed that the deformation resistance strength is not sufficiently lowered.

Further, in Comparative Example 3, when the content of titanium was below the range proposed by the present invention, nitrogen was not fixed and the amount of free nitrogen was increased. As a result, the amount of free boron was decreased to increase the strength due to strain aging, Can be confirmed. In Comparative Example 4, when the content of boron was below the range proposed by the present invention, the strength of the deformation resistance was not sufficiently lowered, the deformation aging effect was increased, the entrapment property was lowered, and the content of titanium was within the range proposed by the present invention It can be confirmed that the effect of controlling the graininess is lost and the impact toughness is lowered.

The variation of the deformation strength of each of the above steel types is not significantly different when the inventive examples 1 and 2 and the comparative examples 1 to 4 are compared with each other, but it is a very big difference when a cold pressing process of thousands or tens of thousands times is actually carried out , And this difference therefore has a great influence on the cold-pressured die life.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (7)

B: 0.001 to 0.003%; P: not more than 0.035%; S: not more than 0.04%; , O: not more than 0.005%, N: not more than 50 ppm (excluding 0%), the balance Fe and other unavoidable impurities, and a free nitrogen content of 10 ppm or less.
The method according to claim 1,
The wire rod has an area fraction percentage of 10 to 30% of ferrite and the balance pearlite composite structure having excellent impact toughness.
The method according to claim 1,
Wherein the wire rod has an average grain size of 10 to 100 占 퐉 and excellent impact toughness.
The method according to claim 1,
Wherein the content of the free boron (B) in the wire is 10 to 20 ppm.
The method according to claim 1,
Wherein said wire rod has an impact resistance of 80 J or more at room temperature.
B: 0.001 to 0.003%; P: not more than 0.035%; S: not more than 0.04%; , 0.005% or less of O, 50 ppm or less of N (excluding 0%), the balance of Fe and other unavoidable impurities is subjected to a homogenization heat treatment in a temperature range of 1000 to 1200 캜; And
And subjecting the homogenized heat treated billet to hot rolling.
The method according to claim 6,
Wherein the hot rolling is performed by hot rolling at a reduction ratio of 80% or more.