KR101518583B1 - High strength wire rod, steel wire having excellent drawability and manufacturing method thereof - Google Patents

Abstract

According to the present invention, it is possible to provide a high-strength wire rod, steel wire and wire rod excellent in drafting ability which can be twisted more than 60 times without occurrence of very high strength and delamination, And a method for producing the same.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength wire rod, a steel wire, and a method of manufacturing the same,

TECHNICAL FIELD The present invention relates to a high-strength wire rod, a steel wire, and a manufacturing method thereof excellent in drawability.

Tire cords and tire cords used as reinforcements on belts and saw wires for cutting Si ingots are manufactured through wet drawing and require mechanical strength such as clinical strength and torsion. A typical processing method of these is as follows.

The molten steel whose carbon strength is in the range of vacancy-overturning is rolled into a bloom and the billet is homogenized in a heating furnace to homogenize the structure with austenite single phase, down-sizing through rolling of the post- And then cooled at a proper rate at which no pro-eutectoid ferrite or cobalt cementite is formed on the pellet to produce a wire material having fully pearlite structure. After that, it is fresh according to its use and usage by the customer, and it is subjected to an intermediate heat treatment at the middle line between 1-2 mm to give ductility to the material, and is wet-drawn by using 25 or more WC, And the tire cord, which is the final product, is manufactured through the twisting operation in which the same drawing material is twisted.

The required mechanical properties include strength, torsion, and the like. For example, the tire cord is required to have high strength because the rolling resistance is improved due to the high strength of the tire cord, and the final automobile fuel economy is increased. The high strength is achieved by increasing the strength of the wire or heat treatment material, increasing the pearlite interlayer spacing and increasing the amount of drawing process, according to the EF empirical formula proposed in 1960.

Up to now, the high strength of products has been achieved by: 1) increasing the strength of wire or heat treatment material by adding C, Cr, Si or alloys, 2) And the increase in the amount of fresh processing. Each method has advantages and disadvantages. However, when optimization is not performed, only the strength is increased and the torsional characteristic is not good. This is because low-temperature microstructures such as pearlite cementite, martensite, and the like are formed in the structure and coarse secondary ferrite is formed in the microstructure. According to the data reported so far, it is known that the freshness limit or freshness is decreased because more than 1 wt% C is a fragile cementite fraction.

In addition, since the strength of the material itself is high, it is impossible to omit the intermediate heat treatment like the steel cord product of 3200 MPa or less or the MD type tire cord, and a total of two or more heat treatments should be included. That is, the wire material is manufactured by performing the heat treatment step as shown in FIG. 1 (b). The inclusion of the intermediate heat treatment causes a problem that the manufacturing cost increases.

The microstructure of commercially available products to date is pearlite. The reason for this is that, as mentioned above, pearlite has a higher work hardening rate than bainite and martensite. In other words, when the amount of drawing is increased, linear strength increase tendency is shown. However, when the processing amount is 2.1 or more, exponential strength increase is effective in securing strength. Since bainite is contained in low-temperature structure, it is known to have a high strength with a light phase compared with pearlite. However, bainite formed at isothermal holding at 400 to 500 ° C has lower strength and higher ductility than pearlite . As shown in FIG. 2, the bainite shows the characteristics of the section reduction ratio (RA,%) observed in the LP heat treatment. However, since the bend knight has a long transformation end time, it is somewhat difficult to use it in the customer's company because it is not suitable for productivity.

According to the present invention, it is possible to provide a high-strength wire rod, steel wire and wire rod excellent in drafting ability which can be twisted more than 60 times without occurrence of very high strength and delamination, And a method for producing the same.

In one aspect of the present invention, there is provided a high strength wire rod excellent in drawability, comprising 0.92 to 1.12% of C, 0.3 to 0.7% of Si, 0.1 to 0.5% of Cr, 0.07% or less of Mn, S: 0.015% or less, the balance Fe, and other unavoidable impurities, and contains 95% or more of bainite in terms of area fraction%.

Another aspect of the present invention is to provide a method of manufacturing a high-strength wire having excellent workability, comprising: 0.92 to 1.12% of C, 0.3 to 0.7% of Si, 0.1 to 0.5% of Cr, 0.07% 0.015% or less, S: 0.015% or less, the balance Fe and other unavoidable impurities in a temperature range of 1000 to 1100 캜, hot rolling the heated strip at a temperature of 950 캜 or higher, Cooling the wire rod at a cooling rate of 20 ° C / sec or more, and isothermal annealing the cooled wire rod at a temperature of 450 to 500 ° C.

A high strength steel wire excellent in drawability, which is another aspect of the present invention, contains 0.92 to 1.12% of C, 0.3 to 0.7% of Si, 0.1 to 0.5% of Cr, 0.07% or less of Mn, 0.015% or less of P 0.015% or less of S, the balance being Fe and other unavoidable impurities, and having a tensile strength of 4400 MPa or more, containing 95% or more of pearlite in area fraction%.

A method of manufacturing a high strength steel wire excellent in drawability, which is another aspect of the present invention, comprises the steps of: 0.92 to 1.12% of C, 0.3 to 0.7% of Si, 0.1 to 0.5% of Cr, 0.07% 0.015% or less, S: 0.015% or less, the balance Fe and other unavoidable impurities in a temperature range of 1000 to 1100 캜, hot rolling the heated strip at a temperature of 950 캜 or higher, A step of cooling the wire rod at a cooling rate of 20 ° C / sec or more, an isothermal heat treatment of the cooled wire rod at a temperature of 450 to 500 ° C, and a step of drawing the wire rod to produce a steel wire, The step of freshly manufacturing the steel wire includes the steps of a primary drawing step, a heat treatment step and a final drawing step.

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.

In one aspect of the present invention, the microstructure of the wire rod is controlled by bainite excellent in ductility, so that there is an effect that even if the intermediate heat treatment included in the fresh shredding step is omitted twice, the freshness can be obtained. In addition, it is possible not only to produce a 4400 MPa super high strength tire cord but also to manufacture ultra fine wires such as a saw wire for semiconductor cutting, and the manufacturing cost can be reduced because the intermediate heat treatment can be omitted.

Fig. 1 (a) is a schematic view showing an example of a manufacturing method proposed by the present invention, and Fig. 1 (b) is a schematic view showing a conventional manufacturing method.
Fig. 2 is a graph showing the cross-sectional reduction characteristics of bainite formed during LP heat treatment.
3 is a graph showing cooling rate history and conditions for forming bainite structure in Stelmor.
4 (a) is a surface portion, (b) is a 1/4 Q position, and FIG. 4 (c) is a photograph showing a central structure.

The inventors of the present invention have found that the bainite control of the microstructure of the wire rod eliminates the double intermediate heat treatment included in the fresh fiber manufacturing process once and also has the same strength and torsional characteristics as the conventional double heat treatment And the present invention has been devised.

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

In one aspect of the present invention, there is provided a high strength wire rod excellent in drawability, comprising 0.92 to 1.12% of C, 0.3 to 0.7% of Si, 0.1 to 0.5% of Cr, 0.07% or less of Mn, S: 0.015% or less, the balance Fe, and other unavoidable impurities, and contains 95% or more of bainite in percent area percent.

Carbon (C): 0.92 to 1.12 wt%

Carbon is a key element for securing strength. C is present as a carbide in bainite and as cementite in pearlite. Both factors are key factors in increasing strength. When the carbon content is less than 0.92%, it is difficult to secure the target strength to be secured in the present invention. On the other hand, when the content of carbon is more than 1.12% by weight, there is a problem of deterioration of freshness due to formation of center segregation and grain boundary formation of the cornerstone cementite.

Silicon (Si): 0.5 to 0.7 wt%

Si is a ferrite solid solution strengthening element, which is dissolved in ferrite and is segregated even at ferrite / cementite grain boundary when no drawing is applied. It has been reported to increase the yield strength of 14 ~ 16MPa when 0.1% Si is added. Since ferrite strengthening element, ferrite hardness is increased by 5Hv when 0.1% Si is added. Silicon is also known as a cementite decomposition inhibiting element because it is segregated at the ferrite and cementite interface. When the amount of silicon added is less than 0.5% by weight, it is difficult to secure a desired level of strength in the present invention. On the other hand, when the amount exceeds 0.7% by weight, the scale peeling property is not effective.

Cr (Cr): 0.1 to 0.5 wt%

Chromium serves to uniformize and distribute the carbide in the bainite structure to make it finer and to refine the spacing or grain size. Cr can be easily substituted with Fe in the cementite to make the cementite thinner. If the content of chromium is less than 0.1% by weight, the cementite thickness becomes thinner than the critical thickness and the freshness is deteriorated. As the amount of Cr increases, the thickness of the cementite becomes finer, but it is expensive and the thickness of the cementite does not increase proportionally with the increase in the amount of Cr. Therefore, the upper limit is preferably limited to 0.5 wt%.

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

Mn is an austenite stabilizing element and has a deoxidizing effect, and binds with S present in the steel to form MnS, thereby preventing sulfur embrittlement. However, a large amount of Mn is not required since the content of S causing the generation of the red embrittlement can be largely lowered by the development of the steelmaking technique. In addition, since it is an element that causes decomposition of cementite during drawing, it deteriorates the freshness. Therefore, it is preferable to keep Mn at 0.07% by weight or less, because it can be added as an impurity to molten iron even though it is not 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 phosphorus and sulfur are generally referred to as impurities, a brief description thereof is as follows.

P and S: 0.015% or less, respectively

P and S are as small as impurities, but if they are too limited, the cost of removing impurities in the steelmaking process increases. Typically, the content of P and S is 0.015% or less. More preferably, when the amounts of P and S are increased, the ductility of the material is decreased and the drawing workability is lowered. Therefore, it is preferable to limit the amount of P + S to 0.030 wt% or less in the condition that P is included.

The wire rod proposed by the present invention preferably contains 95% or more of bainite in terms of an area fraction%. By securing the bainite, it is possible to omit the subsequent primary LP heat treatment at the time of drawing. Also, the ductility is excellent and the freshness is increased. When the bainite content is less than 95%, there is a problem that cracking occurs during drawing and breakage is caused.

The wire preferably has a section reduction ratio of 1270 MPa or more and 47% or more.

Hereinafter, a high strength steel wire excellent in drawability, which is another aspect of the present invention, will be described in detail.

The steel wire satisfying the above-mentioned composition and microstructure can be drawn and processed to produce a steel wire. The steel wire thus obtained preferably has a torsion number of not less than 60 times without delamination. In this case, the number of twists means the number of twists per 100 x D length (where D is the diameter of the wire).

Further, the steel wire includes a pearlite structure, thereby securing a tensile strength of 4400 MPa or more. That is, the cementite and ferrite are elongated in the longitudinal direction at the time of drawing. When the drawing amount is high, the carbon present in the cementite partially escapes into the ferrite, so that the solid solution strengthening effect is obtained and the tensile strength is increased. This effect is expressed at e above 2.1.

Hereinafter, a method for manufacturing a high strength wire having excellent drawability, which is another aspect of the present invention, will be described in detail.

Another aspect of the present invention is to provide a method of manufacturing a high-strength wire having excellent workability, comprising: 0.92 to 1.12% of C, 0.3 to 0.7% of Si, 0.1 to 0.5% of Cr, 0.07% 0.015% or less, S: 0.015% or less, the balance Fe and other unavoidable impurities in a temperature range of 1000 to 1100 캜, hot rolling the heated strip at a temperature of 950 캜 or higher, Cooling the wire rod at a cooling rate of 20 ° C / sec or more, and isothermal annealing the cooled wire rod at a temperature of 450 to 500 ° C.

Heating step

And the billet satisfying the above-mentioned component system is heated to 1000 to 1100 캜. By heating the steel strip in the temperature range described above, 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. When the heating temperature of the steel strip is less than 1000 캜, it may be difficult to obtain the above effect by heating. On the other hand, when it exceeds 1100 DEG C, the austenite grains become very coarse and it is difficult to secure a high strength and high-strength wire rod. Here, the term "steel" refers to all semi-finished products such as blooms and billets that can be produced by wire rods.

Hot-rolled  step

It is preferable that the hot-rolled steel strip is heated to produce a wire rod. The hot rolling is preferably carried out in a temperature range of 950 캜 or higher. More preferably in a temperature range of 950 to 1050 캜. When the hot rolling is performed at a temperature lower than 950 占 폚, there is a problem that the rolling load becomes large and the roll life becomes short. On the other hand, when it exceeds 1050 DEG C, there is a problem that the ductility of the austenite grains is reduced.

Cooling step

Followed by the step of cooling after the hot rolling. More preferably, cooling is carried out in Stelmor. By cooling in the Stelmor, a homogeneous structure can be obtained. At this time, the cooling is preferably performed at a cooling rate of 20 ° C / second or more. More preferably at a cooling rate of 20 to 30 DEG C / second. By performing the cooling at the above-mentioned speed, the quartzite cementite and the pearlite are not formed. When the cooling rate is less than 20 ° C / second, there is a problem that quartzite cementite and pearlite are formed. When the cooling rate is more than 30 ° C / second, it is impossible on the stellum phase.

Isothermal Heat-treated  step

Followed by an isothermal heat treatment after the cooling step. By performing the isothermal heat treatment process, uniform bainite can be formed. In addition, the isothermal heat treatment is preferably performed in a batch furnace. By performing the isothermal heat treatment in the above arrangement furnace, the above effects can be further manifested.

In addition, the isothermal heat treatment is preferably performed in a temperature range of 450 to 500 ° C. If the isothermal annealing temperature is less than 450 ° C, the lower bainite is formed and the ductility is low, so that there is a problem that disconnection occurs during the drawing. On the other hand, when the temperature exceeds 500 ° C, a pearlite and bainite mixed structure is generated and breakage occurs during drawing. The isothermal heat treatment is preferably maintained for 20 minutes or more. In the case where the isothermal heat treatment holding time is less than 20 minutes, the bainite transformation does not terminate and bainite and pearlite composite structure are formed.

FIG. 4 shows the microstructure of the surface of the wire, 1 / 4Q, and the center of the wire after the wire was manufactured through the above-described manufacturing process. As shown in FIG. 4, bainite exists in all regions.

Hereinafter, a method of manufacturing a high strength steel wire excellent in drawability, which is another aspect of the present invention, will be described in detail.

And a step of drawing the wire rod after the wire rod is manufactured by the above-described method to produce a high strength steel wire having excellent drawability.

As described above, when the wire rod is drawn to produce a steel wire, it is preferable to include a step of primary drawing, a step of heat treatment, and a step of final drawing. That is, as shown in Fig. 1 (a), it is preferable to carry out the intermediate heat treatment one time and finally carry out freshness.

At this time, it is preferable that the wire rod is drawn in the range of 5 to 20% in the cross-sectional reduction rate per pass at the final drawing step. In order to obtain a high strength per pass, it is preferable that the cross-sectional reduction rate per pass is 5% or more. In order to prevent surface deterioration due to material and dice friction, it is desirable to control the cross-sectional reduction rate per pass to 20% or less.

It is preferable that the LP heat treatment is performed in the heat treatment step. More preferably, the heat treatment is preferably performed in a temperature range of 550 to 600 ° C.

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)

A steel satisfying the composition shown in Table 1 below was cast. In case of Inventive Example 1, it was manufactured in accordance with the process flow chart of FIG. 1 (b). First, the wire rod was manufactured by a process of heating at a temperature of 1050 ° C, hot rolling at 1000 ° C, cooling at a cooling rate of 25 ° C / sec, and isothermal heat treatment at a temperature range of 470 ° C. At this time, the diameter of the wire rod is 5.5 mm. 3, cooling was carried out at a cooling rate of 25 DEG C / sec to a temperature of 470 DEG C (within 16 to 19 seconds) at which the bainite can be formed. In the isothermal annealing, Min.

Thereafter, the wire rod produced by the above-described manufacturing method was subjected to primary dry drawing, followed by primary heat treatment and final drawing. In this case, the first heat treatment is austenitizing at 1000 ° C., followed by rapid cooling at a cooling rate of 160 ° C./second, isothermal heat treatment at a temperature of 550 to 600 ° C., and cooling to obtain a first heat treatment material. The texture of the first heat treated material was pearlite, and it was confirmed that the section reduction ratio was 1669 MPa and 36%.

In the case of Comparative Example 1, it was manufactured in accordance with the process flow chart of FIG. 1 (a). The wire of Comparative Example 1 was produced by a conventional method.

Thereafter, the wire rod produced by the above-described manufacturing method was subjected to primary dry-type drying, primary heat-treatment, secondary wet-type drawing, secondary heat treatment, and then final drawing. At this time, the primary and secondary heat treatments were carried out in the same manner as in the primary heat treatment method of Inventive Example 1.

Table 2 shows the strength and the section reduction ratio for each step.

division C (% by weight) Si (% by weight) Mn (% by weight) Cr (wt%) P (% by weight) S (% by weight) Inventory 1 1.02 0.5 0.07 0.2 0.0148 0.0112 Comparative Example 1 1.02 0.5 0.07 0.2 0.0148 0.0112

division Wire rod Primary
Fresh material Primary
Heat treatment material Secondary
Fresh material Secondary
Heat treatment material final
Fresh material TS
(MPa) RA
(%) TS
(MPa) torsion
(time) TS
(MPa) RA
(%) TS
(MPa) torsion
(time) TS
(MPa) RA
(%) TS
(MPa) torsion
(time) Inventory 1 1270 47 1840 38 skip skip skip skip 1669 36 4416 64 Comparative Example 1 1415 28 2535 22 1651 32 2541 24 1674 35 4420 68

As shown in Table 2, even if the first heat treatment is omitted, it can be confirmed that the final steel wire has tensile strength and twist characteristics equal to or higher than those of the final steel wire produced by the second heat treatment.

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 (9)

And the balance Fe and other unavoidable impurities are contained in an amount of 0.92 to 1.12% of C, 0.5 to 0.7% of Si, 0.1 to 0.5% of Cr, 0.07% or less of Mn, 0.015% or less of P, And a high strength wire rod excellent in drawability including bainite of 95% or more at an area fraction%.
The method according to claim 1,
The wire rod has a tensile strength of 1270 MPa or more and a section reduction ratio of 47% or more.
And the balance Fe and other unavoidable impurities are contained in an amount of 0.92 to 1.12% of C, 0.5 to 0.7% of Si, 0.1 to 0.5% of Cr, 0.07% or less of Mn, 0.015% or less of P, Heating the slab containing the steel strip at a temperature in the range of 1000 to 1100 캜;
Hot-rolling the heated billet in a temperature range of 950 占 폚 or higher to produce a wire rod;
Cooling the wire rod at a cooling rate of 20 DEG C / sec or more; And
And a step of subjecting the cooled wire rod to an isothermal heat treatment at a temperature of 450 to 500 占 폚.
The method of claim 3,
Wherein the isothermal heat treatment is maintained for 20 minutes or longer.
And the balance Fe and other unavoidable impurities are contained in an amount of 0.92 to 1.12% of C, 0.3 to 0.7% of Si, 0.1 to 0.5% of Cr, 0.07% or less of Mn, 0.015% or less of P, And having a tensile strength of 4400 MPa or more and containing 95% or more of pearlite and having an area percentage of 10% or more.
6. The method of claim 5,
Wherein the steel wire has a twist characteristic of 60 times or more and is excellent in drawability.
And the balance Fe and other unavoidable impurities are contained in an amount of 0.92 to 1.12% of C, 0.3 to 0.7% of Si, 0.1 to 0.5% of Cr, 0.07% or less of Mn, 0.015% or less of P, Heating the slab containing the steel strip at a temperature in the range of 1000 to 1100 캜;
Hot-rolling the heated billet in a temperature range of 950 占 폚 or higher to produce a wire rod;
Cooling the wire rod at a cooling rate of 20 DEG C / sec or more;
Isothermal annealing the cooled wire at a temperature of 450 to 500 ° C; And
The method of manufacturing a high strength steel wire according to any one of the preceding claims, wherein the step of freshly forming a steel wire comprises a first preliminary step, a heat treatment step and a final fresh step.
8. The method of claim 7,
Wherein the isothermal heat treatment is maintained for 20 minutes or longer.
8. The method of claim 7,
Wherein the heat treatment step is a LP heat treatment and is excellent in drawability.

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