KR101482358B1 - High carbon steel wire rod for high strength and method for manufacturing thereof - Google Patents

Abstract

One aspect of the present invention is to manufacture a wire cord for a tire cord, which can be drawn to the final steel wire by a single heat treatment and has excellent torsional characteristics. Further, it is possible to provide a steel wire having excellent characteristics of the wire material for tire cords.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high-strength high-

The present invention relates to a high-strength high-carbon wire and a method of manufacturing the same.

The tire cord refers to a reinforcing material or a member that enters the inside of rubber to enhance the durability, running property and stability of the tire in a vehicle such as an automobile. The tire cord material is mainly composed of a polymer synthetic fiber such as polyester or nylon, In recent years, high strength and excellent heat resistance are required, and steel cords are widely used.

As the weight of the tire cord is decreased, it is advantageous to improve the fuel efficiency of the automobile. Therefore, it is advantageous that the tire cord has a high load, that is, a high load per unit weight.

In addition, since the tire cord is used as a cord or the like through a twisted wire of one steel wire, the twist characteristic is positioned as an especially important characteristic so as not to be broken during the ducting process in order to be used as a tire cord.

The torsional characteristics are evaluated by the number of torsions until a delamination phenomenon in which vertical fracture occurs when a torsional stress is applied. In the case of a tire cord, 40 to 50 times or more torsions are required.

The strength, surface flaw and tensile residual stress of the steel wire are known to be factors affecting such torsional characteristics. Generally, when the strength is high, the ductility is decreased, and the final delamination is also affected, and when the strength is too high, delamination occurs. In addition, the surface flaw formed by the use of the defective die acts as a stress concentration spot at the time of applying the stress, and therefore, there is a problem in that initial fracture can be caused.

As other main means, alloying element control, process control, etc. can be used. However, it is not simple to improve the twist characteristic of the tire cord by using these means. If precise control can not be achieved, a problem may arise in that a martensitic structure is formed which is not good in twisting characteristics at the wire rod stage .

Also, if the texture control of the wire rod is not smooth, the pearlite may be non-uniformly present. Such non-uniformly existing pearlite rotates in the direction or axis of drawing, i.e., in the <001> direction. Although it is influenced by the method of experiment, generally, the rotation is severely issued up to the drawing amount e: 0.5 ~ 1.0, and the rearrangement is completed near 1.0, and the direction of the center of the specimen The size is continuously or exponentially decreased. The pearlite placed parallel to the drawing direction does not cause the cementite segregation phenomenon, but there is a problem that the vertically existing or the cementite having a large inclination is segmented to serve as a cracking point which causes breakage in the drawing.

One aspect of the present invention is to manufacture a wire cord for a tire cord, which can be drawn to the final steel wire by a single heat treatment and has excellent torsional characteristics. Further, it is possible to provide a steel wire having excellent characteristics of the wire material for tire cords.

A high strength high-carbon steel wire rod according to one aspect of the present invention comprises, by weight, 0.8 to 1.1% of C, 0.7 to 1.3% of Si, 0.1 to 0.5% of Cr, 0.07% or less of Mn, 0.015% or less, S: 0.015% or less, and a balance Fe and other unavoidable impurities, and a microstructure having an area fraction of 95% or more of pearlite. To {001} direction of the pearlite.

A method of manufacturing a high strength high-carbon wire according to another aspect of the present invention is a method of manufacturing a high-strength high-carbon wire having a composition of C: 0.8 to 1.1%, Si: 0.7 to 1.3%, Cr: 0.1 to 0.5%, Mn: 0.07% ), P: not more than 0.015%, S: not more than 0.015% The present invention relates to a method of manufacturing a wire rod comprising the steps of heating a steel strip containing Fe and other unavoidable impurities at 1100 to 1200 ° C, To start cooling at a temperature of 900 to 1000 占 폚 and cooling at a cooling rate of 15 to 20 占 폚 / sec.

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, it is possible to provide a tire cord excellent in freshness without largely changing the tire cord alloy component or the structure.

Fig. 1 is a schematic diagram showing the separation intervals of electron back scattered diffraction (EBSD) (D: radius of wire rod).

The inventors of the present invention have found a problem that torsional characteristics are deteriorated due to the occurrence of fracture during drawing or twisting in accordance with the crystal orientation of pearlite, in particular, the crystal orientation of cementite, among microstructures of the wire, The research was conducted. As a result, it has been confirmed that the directionality of the ferrite having a directionality similar to that of cementite can be controlled, thereby preventing occurrence of fracture during drawing or twisting, thereby leading to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a high strength high carbon wire as one aspect of the present invention will be described in detail.

A high strength high-carbon steel wire rod according to one aspect of the present invention comprises, by weight, 0.8 to 1.1% of C, 0.7 to 1.3% of Si, 0.1 to 0.5% of Cr, 0.07% or less of Mn, 0.015% or less, S: 0.015% or less, and a balance Fe and other unavoidable impurities, and a microstructure having an area fraction of 95% or more of pearlite. To {001} direction of the pearlite.

Carbon (C): 0.8 to 1.1 wt%

C is a key element in securing strength. The carbon is present mostly in the form of cementite in high carbon steel wire rods. Cementite forms layered pearlite with ferrite and has higher strength than ferrite. Therefore, the strength of the wire rod increases as the cementite fraction increases or the interval between pearlite layers becomes finer. As the carbon content increases, the cementite fraction increases and the layer spacing becomes finer, which is very effective in increasing the strength of the wire rod. However, when a large amount of carbon is added, a quartzite cementite is formed in the grain boundaries and a difference in free energy between austenite and cementite becomes worse and transformation control becomes difficult in the production of wire rods, so that the upper limit is preferably limited to 1.1 wt% . On the other hand, when the content of carbon is too low, the strength is reduced, so that the lower limit is preferably limited to 0.8 wt%. Accordingly, it is preferable that the carbon is contained in an amount of 0.8 to 1.1% by weight.

Silicon (Si): 0.7 to 1.3 wt%

Silicon is dissolved in a ferrite base to increase the strength by solid solution strengthening. It is present in the ferrite grain, ferrite / cementite grain boundary, and can not substitute with the Fe atom in the general type or special type cementite. Is an extremely low element. In addition, the formation of cementite cementite is suppressed in the quartz. Compared with C, V, and Cr after the intermediate heat treatment, the effect of increasing the strength is small, but it is about 2 to 2.5 times (14 kg / mm ² ) larger than that of Mn when the same amount is added. In addition, it is an element that inhibits decomposition of cementite which occurs during wet or dry drawing. In order to secure the intended strength in the present invention, it is preferable to add 0.7 wt% or more. However, when a large amount of silicon is added, a loss due to surface decarburization layer formation and scale formation occurs, so that the upper limit is preferably limited to 1.3 wt%.

Manganese (Mn): 0.07% by weight or less (excluding 0%)

The manganese is an effective element for strengthening the strength of the steel. The more manganese is present in the steel, the more segregation occurs with C and P. Especially, as the carbon content increases, the segregation increases and the steel properties decrease. In addition, manganese can be located in the general form of cementite, which easily substitutes with Fe and deteriorates the cementite during processing because Mn and C bond strength is weaker than the bond strength between Fe and C. Therefore, in the present invention, it is desirable to reduce the manganese content as much as possible, and therefore, it is preferable to control it to 0.07 wt% or less.

Cr (Cr): 0.1 to 0.5 wt%

Chromium is an effective element for increasing the heat treatment strength, increasing the work hardening rate, and increasing the cutting limit of the fresh end by greatly increasing the strength and fineness of the cementite and stabilizing the cementite. This is because Cr is a substitutional element that can easily be placed in a general site in the cementite, so it is easily substituted with Fe to make the cementite thinner. In addition, since it bonds well with the carbon in the cementite, it inhibits the decomposition of cementite and inhibits the occurrence of room temperature aging. In order to exhibit such an effect in the present invention, it is preferable that 0.1 wt% or more is included. However, when a large amount of Cr is added, the transformation end time is prolonged and the productivity is lowered. Therefore, it is preferable to limit Cr to 0.5% or less.

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 phosphorus and sulfur are generally referred to as impurities, a brief description thereof is as follows.

Phosphorus (P): 0.015% by weight or less

The phosphorus is an impurity inevitably contained. It is segregated mainly in the central portion of the steel strip to decrease the toughness and remarkably deteriorate the weldability. Therefore, it is desirable to control the phosphor as low as possible in order to ensure low-temperature impact toughness in the center portion of the post- Theoretically, it is preferable to limit the content of phosphorus to 0 wt%, 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 0.015 wt%.

Sulfur (S): 0.015 wt% or less

Sulfur is inevitably contained as an impurity and forms a nonmetallic inclusion by bonding with Mn or the like, thereby largely deteriorating the ductility, impact toughness and weldability of steel. Therefore, it is desirable to suppress the content to the maximum. 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 0.015 wt%.

In addition, it is preferable that the microstructure of the wire rod proposed by the present invention contains, as a percentage of the area fraction, at least 95% of pearlite and other residual structure. The other remaining structure refers to a quartz cementite, pro-eutectoid ferrite, and ferrite resulting from decarburization from the surface.

If the area fraction of the pearlite is less than 95%, fracture occurs during drawing and it is difficult to secure strength. In addition, if the remaining portion exceeds 5%, breakage occurs and the strength is not secured.

In addition, when the pearlite is a mixture of ferrite and cementite, it is preferable that the pearlite contains 50% or more ferrite having a {001} direction within the range of -1 / 2D to 1 / 2D when measured by EBSD.

The EBSD measurement is performed by measuring the area fraction of ferrite having the {001} direction based on the division of the distance from the center of the wire in the direction of the surface by 1 / 2D as shown in FIG.

As described above, when 50% or more of ferrite having a {001} direction within the range of -1 / 2D to 1 / 2D when measured by EBSD is included, destruction due to bending of the cementite at the time of drawing does not occur, . In addition, when the area is less than -1 / 2D and the area exceeding 1 / 2D, 10% or less of ferrite is included, respectively, to further improve the torsional property.

At this time, the EBSD measurement is performed by setting the step interval to 0.1 mu m in an error range of 10 DEG.

Hereinafter, a method of manufacturing a high-strength high-carbon wire rod according to another aspect of the present invention will be described in detail.

A method of manufacturing a high strength high-carbon wire according to another aspect of the present invention is a method of manufacturing a high-strength high-carbon wire having a composition of C: 0.8 to 1.1%, Si: 0.7 to 1.3%, Cr: 0.1 to 0.5%, Mn: 0.07% ), P: not more than 0.015%, S: not more than 0.015% The present invention relates to a method of manufacturing a wire rod comprising the steps of heating a steel strip containing Fe and other unavoidable impurities at 1100 to 1200 ° C, To start cooling at a temperature of 900 to 1000 占 폚 and cooling at a cooling rate of 15 to 20 占 폚 / sec.

Heating step

And the billet satisfying the above-mentioned component system is heated to 1100 to 1200 占 폚. By heating the steel strip in this temperature range, it is possible to maintain the austenite single phase, prevent the coarsening of the austenite grains, and effectively dissolve the remaining segregation, carbides and inclusions. In the present invention, it is preferable to heat the steel strip at a temperature of 1100 占 폚 or higher in order to exhibit the above effect. On the other hand, when the heating temperature is too high, the austenite grains become very coarse and it is difficult to obtain a high-strength and high-toughness wire. Therefore, it is preferable to control the upper limit to 1200 ° C. Here, the term "steel" refers to all semifinished products such as blooms and billets, which can be produced from wire rods.

Rolling step

The rolled steel strip can be rolled as described above. By positioning the direction of the ferrite parallel to the drawing direction through the rolling, it is possible to draw the wire without breakage without performing a separate heat treatment in the subsequent drawing process.

More preferably, the steel strip is subjected to rough rolling at a temperature of 900 DEG C or higher, and the rough-rolled steel strip is subjected to hot rolling at a temperature of 1150 DEG C or lower and final rolling (RSM) at a temperature of 1000 DEG C or lower, .

If the temperature of the rough rolling is less than 900 캜, the roll wear becomes worse, and a roll load is applied, which lowers the productivity.

The roughly rolled steel strip is preferably subjected to hot rolling. At this time, when the hot finish rolling temperature exceeds 1150 DEG C, the crystal grains are coarsened and the strength and ductility are lowered.

The hot-rolled steel strip is preferably subjected to final rolling (RSM) in order to ensure excellent freshness. For this, the final rolling temperature is 1000 캜 or less.

Cooling step

Followed by cooling step after the hot rolling step. At this time, the cooling start temperature is preferably 900 to 1000 占 폚. The cooling is preferably performed at a rate of 15 to 20 ° C / second. When the temperature is less than 15 ° C / sec, the base stone cementite is formed actively and it is difficult to obtain the microstructure of the present invention. On the other hand, when the heating temperature is higher than 20 DEG C / second, a hard texture is formed and it is difficult to secure ductility.

The method for manufacturing the steel wire of the present invention is not necessarily limited to this, but it is possible to produce a steel wire by drawing wire material satisfying the composition and the microstructure described above. The steel wire thus obtained has a tensile strength of 2300 MPa or more and excellent torsional characteristics of 60 times or more.

At this time, the step of freshness can be carried out according to a usual method, and therefore, the present invention is not particularly limited.

Also, in the case of manufacturing the steel wire by drawing the wire material produced by the above method, the ductility at the time of drawing is not drastically reduced, and the steel wire having the desired diameter can be produced without performing the LP heat treatment. It is possible to solve the environmental problem that arises.

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 therefrom.

(Example 1)

Steels satisfying the component system described in Table 1 below were cast. Thereafter, it was heated at 1050 占 폚 by a conventional method and rolled to produce a wire rod. The produced wire rod was put into a cooling zone at a temperature of 950 캜 and then cooled at a cooling rate of 15 캜 / sec to have a diameter of 5.5 mm. At this time, the rolling was subjected to final rolling (RSM) at 1000 占 폚 after hot rolling at 950 占 폚, hot rolling at 1150 占 폚.

In Examples 1 to 4 and Comparative Examples 2 and 3 produced under the above-described manufacturing conditions, the cooled wire rod was drawn to a drawing processing amount of 2.6, thereby preparing a steel wire. On the other hand, Comparative Example 1 was subjected to LP heat treatment twice to produce a steel wire.

The tensile strengths of the wire and steel wire thus prepared were measured and are shown in Table 2 below. In addition, the number of twist of the steel wire was measured and shown in Table 2 below.

In Table 3, the <001> direction of the ferrite in the pearlite measured by EBSD is expressed as an area fraction. At this time, the error range was 10 占 and the step interval was set to 0.1 占 퐉.

At this time, the area fraction of the ferrite having the <001> direction was measured based on dividing the interval by 1 / 2D from the center to the surface direction as shown in FIG.

division C (% by weight) Si (% by weight) Mn (% by weight) Cr (wt%) Inventory 1 0.912 0.742 0.069 0.21 Inventory 2 0.923 1.03 0.07 0.189 Inventory 3 0.918 1.239 0.068 0.198 Comparative Example 1 0.921 0.49 0.289 0.201 Comparative Example 2 0.923 0.519 0.069 0.19 Comparative Example 3 0.916 1.511 0.07 0.212

division Tensile Strength of Wire (MPa) Tensile strength of steel wire (MPa) Number of twists (based on 200D) Inventory 1 1374 2537 67 Inventory 2 1397 2585 65 Inventory 3 1421 2617 63 Comparative Example 1 1360 2505 44 Comparative Example 2 1346 2410 48 Comparative Example 3 1422 2665 Not fresh

division Ferrite <001> Area fraction (%) -1 to -1 / 2D - 1 / 2D to 0 0 to + 1 / 2D + 1 / 2D to +1 Inventory 1 7 31 34 9 Inventory 2 7 39 41 8 Inventory 3 9 49 45 10 Comparative Example 1 9 20 22 7 Comparative Example 2 8 19 21 9 Comparative Example 3 7 21 17 8

As shown in Tables 1 to 3, in Examples 1 to 3 which satisfy the range proposed by the present invention, it can be confirmed that a steel wire having excellent strength can be produced without performing LP heat treatment.

On the other hand, since the content of Si is smaller than the range proposed by the present invention and the content of Mn is excessively added, it is confirmed that the LP heat treatment is required twice because of generation of segregation upon deformation.

In Comparative Examples 2 and 3, the Si range does not satisfy the range proposed by the present invention. Accordingly, the ferrite area fraction in the range of -1 / 2D to 1 / 2D is not satisfied and the number of twist turns is small Can be confirmed.

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 (4)

(Excluding 0%), P: not more than 0.015%, S: not more than 0.015%, and the balance Fe (Cr) is not more than 0.07% And other inevitable impurities, and a microstructure having an area fraction of 95% or more of pearlite,
The ferrite in the pearlite in the direction of {001} in the range of -1 / 2D to 1 / 2D is 50% or less in the direction of {001} relative to the radial direction D of the wire, High strength high carbon wire.
(Excluding 0%), P: not more than 0.015%, S: not more than 0.015%, and the balance Fe (Cr) is not more than 0.07% And other unavoidable impurities at a temperature of 1100 to 1200 占 폚;
Rolling the heated billet to produce a wire rod; And
And cooling the produced wire rod at a temperature of 900 to 1000 占 폚 and cooling at a cooling rate of 15 to 20 占 폚 / sec.
3. The method of claim 2,
The rolling may include: rough rolling the billet at a temperature of 900 占 폚 or more;
Subjecting the roughly rolled steel strip to hot finish rolling at a temperature of 1150 占 폚 or lower; And
And finally rolling the hot-rolled and rolled steel strip.
The method of claim 3,
Wherein the final rolling is performed at a temperature of 1000 DEG C or less.

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