KR101611724B1 - Wire rod and steel wire having high strength and method manufacturing of wire rod - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a wire and a steel wire used for a tire cord and the like, and more particularly, to provide a method for manufacturing a high strength wire and a steel wire and wire.
According to an aspect of the present invention, there is provided a steel sheet comprising, by weight, 0.05 to 0.15% of C, 0.5 to 1.5% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.02% of N, A high strength wire made of martensite and residual ferrite composed of 25 to 43% of microstructure in an area fraction, and a method for manufacturing a high strength steel wire and wire using the same, and a balance Fe and other unavoidable impurities.
According to the present invention, it is possible not only to provide a high-strength wire rod through control of an alloy component and a two-step heat treatment, but also to provide a high-strength steel wire having a tensile strength of 4000 MPa or more using the wire rod.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength wire rod,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wire rod and a steel wire used for a tire cord and the like, and more particularly to a method of manufacturing a high strength wire rod and a steel wire and a wire rod.

Tire cord and the like are required to have high strength and excellent torsional characteristics due to their use characteristics. For example, when a steel cord made of a high-strength steel wire is used as a tire cord, fatigue life is excellent, so that disconnection during running is reduced, and safety driving performance is greatly improved.

The required strength of the steel wire for use as a tire cord is at least 2800 MPa and is currently commercialized to 4000 MPa. In order to obtain such excellent strength, fine pearlite should be applied to the texture, and drawing or drawing should be applied to the processing method. This is because, unlike other tissues, the pearlite has a large plastic deformation, and the randomly formed pearlite is elongated in the direction of drawing when the total amount of reduction of 50% or more is applied. It is known that the test results can secure a strength of 5000 MPa or more in wet drawing.

High strength can be achieved by two methods: 1) Alloy composition changes such as C increase and Cr addition in C-Si-Mn alloy system 2) Improvement of cutting speed, increase of cutting speed per pass, . However, when an amount of alloy such as C or Cr is excessively added, there may arise a problem that the transformation completion time becomes long or a cornerstone cementite is formed in the grain boundary. On the other hand, There is a problem that there is a limit to increase.

Further, since the steel wire for tire cord is carbon steel using pearlite, its component system is also simple as C-Si-Mn-Cr. Such a component system does not show a great difference in the world, and therefore, it is required to develop a new product through derivation of a unique component system.

Therefore, studies on materials having a structure of ferrite and martensite or more have progressed actively since the '90s. Martensite is a phase in which C is supersaturated in the BCC structure, and its hardness or strength is lower than that of cementite, but plastic deformation can be facilitated because of a large slip system. This means that the range of application of the amount of processing at the time of freshness is wide. However, since the ferrite phase is only slightly harder than cementite, a ferrite base phase capable of absorbing the strain deformation energy is required. Therefore, researches on materials having a ferrite and martensitic structure have been actively conducted, but they are not used for tire cords and the like because the optimal alloy components and suitable heat treatment conditions are not established.

One aspect of the present invention is to provide a high-strength wire rod by controlling an alloy component and performing two-step annealing to form an abnormal structure composed of ferrite and martensite.

Another aspect of the present invention is to provide a method of manufacturing a high-strength wire rod by controlling an alloy component and performing a two-stage heat treatment to form an ideal structure composed of ferrite and martensite.

Another aspect of the present invention is to provide a high strength steel wire using the high strength wire of the present invention.

According to an aspect of the present invention, there is provided a steel sheet comprising, by weight, 0.05 to 0.15% of C, 0.5 to 1.5% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.02% of N, Strength ferrite composed of martensite and residual ferrite composed of 25 to 43% of microstructure in an area fraction, and the balance ferrite and other unavoidable impurities.

According to another aspect of the present invention, there is provided a ferritic stainless steel comprising 0.05 to 0.15% of C, 0.5 to 1.5% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.02% of N, Obtaining a wire rod by hot rolling the billet consisting of the remainder Fe and other unavoidable impurities;

A first heat treatment step of holding the wire rod at 950 to 1150 캜 and cooling the wire rod to room temperature at a cooling rate of 80 캜 / s or more;

The first heat-treated wire rod is maintained at 50 to 150 ° C for 10 to 60 minutes, then heated to 750 to 850 ° C, held for 3 to 7 minutes, and then cooled to room temperature at a cooling rate of 80 ° C / A method of manufacturing a high-strength wire rod is provided.

According to still another aspect of the present invention, there is provided a ferritic stainless steel comprising 0.05 to 0.15% of C, 0.5 to 1.5% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.02% of N, % Or less, the balance Fe and other unavoidable impurities, and has a tensile strength of 4000 MPa or more.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages and effects of the present invention will become more fully understood with reference to the following specific embodiments.

According to the present invention, it is possible not only to provide a high-strength wire rod through control of an alloy component and a two-step heat treatment, but also to provide a high-strength steel wire having a tensile strength of 4000 MPa or more using the wire rod.

The present inventors have conducted studies to derive high strength wire and steel wire. As a result, they have found that ferrite and martensite are formed to secure high strength, and dynamic aging is caused by addition of N, And the present invention has been made based on the results of these studies.

Hereinafter, the composition of the wire rod and the steel wire of the present invention will be described in detail. At this time, the content of the component elements means all the weight percentages.

C (carbon): 0.05 to 0.15%

C is an element added to secure the material strength and is used for forming martensite. When the C content is less than 0.05%, it is difficult to secure the target strength. When the C content is more than 0.15%, the hardness of martensite is high and it is formed in a massive form. Therefore, cracks are formed in the interface between the martensite and the ferrite during drawing, The content of C is preferably limited to 0.05 to 0.15%, more preferably to 0.07 to 0.10%.

Si (silicon): 0.5 to 1.5%

Si is added as a ferrite stabilizing element. For example, when 0.1% is added, Si is an element which improves the strength at a level of 14 to 16 MPa. In addition, since the ferrite and austenite are present in the grain boundary during the anomalous zone heat treatment, it helps to prevent the austenite shape from being agglomerated. When the Si content is less than 0.5%, the shape of the austenite interface becomes uneven, so that it is difficult to secure the martensite of the needle bed. When the Si content exceeds 1.5%, the Fe2O3 scale layer is formed thickly, To 1.5%.

Mn (manganese): 1.0 to 2.0%

Mn is an austenite stabilizing element and added to improve the ingotability since C is added at a level of 0.05 to 0.15%. When the content is less than 1.0%, the formation of fully martensite is difficult in the case of water-cooling, and when the content is more than 2.0%, the central Mn segregation zone which causes disconnection of the wire is severely formed, %. ≪ / RTI >

N (nitrogen): 0.005 to 0.02%

N is an element which is added to C and dissolved in ferrite to increase the strength effectively. It has a strength increasing effect of 80 MPa when 0.1% is added. N dissolved in the ferrite induces dynamic strain aging in the drawing, and the effect of increasing the strength is great.

When the content is less than 0.005%, it is difficult to effectively exhibit the dynamic aging. When the content is more than 0.02% by weight, the draft workability is affected and the torsional property is also deteriorated. Therefore, the nitrogen content is preferably limited to 0.005 to 0.02%, more preferably to 0.008 to 0.010%.

P (phosphorus) and S (sulfur): not more than 0.03%, respectively

The lower the content of P and S is, the better the impurities are. However, if the limit is too high, the cost of removing impurities in the steelmaking process increases. Further, when the content of P and S increases, ductility of the material decreases. Therefore, it is important to control the upper limit of the content of P and S, and in the present invention, the upper limit of P and S is controlled to 0.030 wt%.

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.

The microstructure of the wire according to one aspect of the present invention is preferably a dual phase composed of ferrite and martensite.

It is preferable that the martensite has an area fraction of 25 to 43%.

When the martensite content is less than 25% by area, it is difficult to secure the desired strength after final drawing. On the other hand, when the martensite exceeds 43% by area, there is a problem that cracks are generated at the interface between ferrite and martensite so that the martensite is not drawn. Therefore, the area fraction of martensite is preferably limited to 25 to 43%, and more preferably to 41 to 43%.

The wire may have a tensile strength of 650 to 670 MPa.

In addition, the cross-sectional reduction ratio (RA) of the wire rod is preferably 63% or more.

As described above, when the cross-sectional reduction rate RA of the steel wire is less than 63% per pass, the cross-sectional reduction rate excessively increases the number of passes required to achieve the total reduction ratio, and the strain is accumulated relatively more on the surface of the wire, And the mechanical properties of the final product are adversely affected.

A method of manufacturing a high strength wire according to another aspect of the present invention will be described in detail.

And the balance Fe and other unavoidable impurities are contained in an amount of 0.05 to 0.15%, C: 0.5 to 1.5%, Mn: 1.0 to 2.0%, N: 0.005 to 0.02%, P: 0.03% Hot-rolling the billet to obtain a wire rod;

A first heat treatment step of holding the wire rod at 950 to 1150 캜 and cooling the wire rod to room temperature at a cooling rate of 80 캜 / s or more;

The first heat-treated wire rod is maintained at 50 to 150 ° C for 10 to 60 minutes, then heated to 750 to 850 ° C, held for 3 to 7 minutes, and then cooled to room temperature at a cooling rate of 80 ° C / .

According to one aspect of the present invention, in order to produce a wire rod, a steel strip satisfying the above-mentioned composition is hot-rolled to obtain a wire rod. At this time, the hot rolling is carried out according to an ordinary method and is not particularly limited.

The wire material thus prepared is subjected to first and second heat treatments.

Primary heat treatment

The hot-rolled wire as described above is subjected to a first heat treatment process. In the primary heat treatment step, the wire rod is maintained at 950 to 1150 占 폚 and then cooled to room temperature at a rate of 80 占 폚 / s or more.

The holding temperature of the wire rod is preferably limited to 950 to 1150 ° C. This is because when the holding temperature is less than 950 DEG C, the austenite grain size is small and it takes a long time to suppress the formation of the retained austenite. When the holding temperature is more than 1150 DEG C, (packet) becomes large and the freshness is deteriorated.

It is preferable to restrict the cooling rate to 80 ° C / s or more when cooling after maintaining at 950 to 1150 ° C.

This is because if the cooling rate is less than 80 ° C / sec, there is a problem that retained austenite is formed. On the other hand, the upper limit is not particularly limited because it is advantageous to suppress the formation of retained austenite as the cooling rate increases.

2nd heat treatment

As described above, the primary heat-treated wire rod is subjected to a secondary heat treatment. In the second heat treatment step, the primary heat-treated wire rod is maintained at 50 to 150 DEG C for 10 to 60 minutes, then heated to 750 to 850 DEG C, held for 3 to 7 minutes, cooled to room temperature at a rate of 80 DEG C / do.

The holding temperature of the primary heat-treated wire rod is preferably limited to 50 to 150 ° C. The reason for this is that when the holding temperature is less than 50 ° C, carbon in martensite is difficult to diffuse, and when the holding temperature is more than 150 ° C, the temperature is high, which is not suitable in terms of energy consumption or process cost .

The holding time at the holding temperature of 50 to 150 DEG C is preferably limited to 10 to 60 minutes.

If the holding time is less than 10 minutes, the carbon does not have sufficient time to diffuse. If the holding time is more than 60 minutes, sufficient carbon diffusion has already occurred. It is not.

It is preferable that the primary heat-treated wire rod is maintained at 50 to 150 ° C and then heated to 750 to 850 ° C. The reason for this is that when the temperature is less than 750 ° C, martensite is not formed, or the martensite fraction is small, so there is a problem in securing the target strength. When the temperature exceeds 850 ° C, the martensite fraction increases, There is a problem of growing into a site.

The rate of temperature rise up to 750 to 850 DEG C is preferably limited to 2 DEG C / s or more. This is because when the temperature is raised at a rate of less than 2 캜 / s, the alloy is transformed into austenite before reaching the target temperature, so that the grain size is not uniform. The upper limit of the temperature raising rate need not be technically limited, but may be suitably limited in terms of economy such as energy consumption or process cost.

The holding time at the temperature of 750 to 850 ° C is preferably limited to 3 to 7 minutes. This is because when the span interval is less than 3 minutes, sufficient transformation time and stabilization time are insufficient and when the span interval is longer than 7 minutes, martensite grows and becomes massive.

It is preferable to restrict the cooling rate to 80 ° C / s or more when cooling after maintaining at the temperature of 750 to 850 ° C. The reason is that if the cooling rate is less than 80 ° C / sec, there is a possibility that the austenite is transformed into pearlite. On the other hand, the higher the cooling rate, the more favorable the transformation to martensite, so the upper limit is not particularly limited.

The high strength steel wire, which is another aspect of the present invention, will be described in detail.

The high strength steel wire of the present invention can be produced by drawing the wire material. The steel wire according to the present invention preferably contains 0.05 to 0.15% of C, 0.5 to 1.5% of Si, 1.0 to 2.0% of Mn, 0.005 to 0.02% of N, 0.03% or less of P, 0.03% or less of S, Fe and other unavoidable impurities, and has a tensile strength of 4000 MPa or more.

The steel wire may have a twisting property of 25 times or more without a delamination in a tensile strength x sectional area x0.08 load application and a 100D (D: steel wire diameter) condition twist test.

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 present 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 50 kg ingot having the composition shown in Table 1 was cast and then subjected to a soaking treatment at 1250 ° C. for 12 hours or more in order to suppress the occurrence of segregation such as Mn and P and then heated at a heating furnace temperature of 1100 ° C. hot rolled material (wire rod) was prepared by subjecting a hot rolled steel sheet to wire rolling under the following conditions: x 1 hour, hot rolling temperature: 1000 ° C laying head temperature: 950 ° C, and cooling rate: 5 ° C / s.

Two heat treatment processes were performed to impart ferrite and martensite abnormal texture to the hot rolled material (wire rod) produced as described above.

In the first heat treatment during the heat treatment process, the temperature was maintained at a holding temperature of 1150C for 10 minutes in a batch furnace, and then the temperature was kept at room temperature.

In the second heat treatment during the heat treatment process, in order to sufficiently impart carbon diffusion in the martensite of the first heat-treated rolled material (wire rod), the temperature was maintained at a holding temperature of 100 ° C for 30 minutes in a baking furnace, The temperature was raised to a holding temperature of 830 ° C at a heating rate and maintained for 5 minutes.

The martensite fraction, tensile strength (TS), and section reduction ratio (RA) of the cross section (-1 / 4D to the center to + 1 / 4D position) were measured for the secondarily heat treated rolled material , And the results are shown in Table 2 below.

Next, a steel wire was produced by dry and wet drawing under the usual conditions under which the hot rolled material (wire rod) produced as described above was subjected to a fresh wire drawing.

The wire material of Table 1 below was reduced by 20% per pass and fresh to the total amount of reduction of the amount of 8.03. The test piece was taken at intervals of 1 in increments of 5.13, and the tensile strength was measured. Torsion characteristics were measured for tensile strength x cross section x0.08 and 100D (D: steel wire diameter) conditions for the steel wire, and the results are shown in Table 2 below.

weight (%) C Si Mn N P S Inventory 1 0.1 1.01 1.52 0.01 0.025 0.024 Inventory 2 0.1 1.00 1.51 0.015 0.021 0.024 Inventory 3 0.12 1.04 1.52 0.02 0.02 0.021 Honorable 4 0.11 1.49 1.51 0.012 0.022 0.025 Comparative Example 1 0.02 1.02 1.51 0.012 0.021 0.021 Comparative Example 2 0.049 1.01 1.52 0.012 0.025 0.024 Comparative Example 3 0.3 1.01 1.5 0.011 0.023 0.022 Comparative Example 4 0.11 1.02 1.53 0.05 0.02 0.025 Comparative Example 5 0.10 1.05 1.5 0.03 0.025 0.024 Comparative Example 6 0.11 1.75 1.5 0.011 0.024 0.024 Comparative Example 7 0.12 1.01 2.25 0.012 0.022 0.024 Comparative Example 8 0.1 1.02 0.95 0.011 0.025 0.026 Comparative Example 9 0.1 1.00 1.51 0.0045 0.020 0.022

Wire rod Tensile Strength of Steel Wire (MPa) Twisted steel wire Vm
(area%) TS
(MPa) RA
(%) 5.13 6.02 7.14 8.03 Dil lamination time Inventory 1 42 665 67 2820 3261 3821 4001 Not occurring 28 Inventory 2 42 664 67 2935 3179 3836 4204 Not occurring 27 Inventory 3 41 660 65 2973 3217 3874 4242 Not occurring 26 Honorable 4 43 670 63 2840 3074 3741 4053 Not occurring 27 Comparative Example 1 41 648 70 2651 2845 3510 3815 Not occurring 29 Comparative Example 2 43 657 65 2695 2890 3589 3875 Not occurring 28 Comparative Example 3 48 684 62 2795 monorail x x x x Comparative Example 4 43 659 64 monorail x x x x x Comparative Example 5 43 668 64 monorail x x x x x Comparative Example 6 44 679 63 monorail x x x x x Comparative Example 7 48 Brittle
Destruction x x x x x x x Comparative Example 8 43 640 68 2668 2890 3489 3678 Not occurring 31 Comparative Example 9 41 664 66 2760 3211 3784 3940 Not occurring 28

As can be seen from Table 2, the inventive materials 1 to 4 had a martensite fraction of 41 to 43% at the end face of the wire rod, with a tensile strength of 664 to 670 MPa or more and a reduction ratio of 63 to 67% . It was confirmed that comparative examples except for Comparative Examples 6 and 7 in which Si and Mn were excessively added in Comparative Examples had similar martensite fractions and mechanical properties.

In Table 2, Comparative Examples 3 to 6 are test pieces in which wire breakage occurred during drawing. These characteristics are that C: 0.3%, N: 0.03 and 0.05%, Si: 1.75% and Mn: 2.25% If the content of the above-mentioned alloying element is excessively added, it means that the second heat treatment temperature, that is, the heat treatment at 830 캜, which is the ideal region heat treatment temperature, does not lead to freshness.

In addition, since N plays a role of increasing the strength by aging hardening which occurs in the drawing, the strength is increased up to 0.02%, but no effect is observed when the N content is further exceeded.

In the meantime, Examples 1 to 4 according to the present invention show that the tensile strength is not less than 4000 MPa, the delamination does not occur when the torsional stress is applied, and the value is from 26 to 28 times. On the other hand, Comparative Examples 8 and 9 show a tensile strength lower than the target tensile strength.

Claims (6)

And the balance Fe and other unavoidable impurities are contained in an amount of 0.05 to 0.15%, C: 0.5 to 1.5%, Mn: 1.0 to 2.0%, N: 0.005 to 0.02%, P: 0.03% Lt; / RTI >
High strength wire consisting of 25 ~ 43% of martensite and residual ferrite in microstructure in area fraction.
The method according to claim 1,
Wherein the wire rod has a tensile strength of 650 MPa or more and a section reduction ratio of 63% or more.
And the balance Fe and other unavoidable impurities are contained in an amount of 0.05 to 0.15%, C: 0.5 to 1.5%, Mn: 1.0 to 2.0%, N: 0.005 to 0.02%, P: 0.03% Hot-rolling the billet to obtain a wire rod;
A first heat treatment step of holding the wire rod at 950 to 1150 캜 and cooling the wire rod to room temperature at a cooling rate of 80 캜 / s or more;
The first heat-treated wire rod is maintained at 50 to 150 ° C for 10 to 60 minutes, then heated to 750 to 850 ° C, held for 3 to 7 minutes, and then cooled to room temperature at a cooling rate of 80 ° C / Wherein the high-strength wire rod is made of a high-strength wire.
The method of claim 3,
Wherein the heating rate in the secondary heat treatment step is not less than 2 DEG C / s.
And the balance Fe and other unavoidable impurities are contained in an amount of 0.05 to 0.15%, C: 0.5 to 1.5%, Mn: 1.0 to 2.0%, N: 0.005 to 0.02%, P: 0.03% Lt; / RTI >
High strength steel wire with tensile strength of 4000 MPa or more.
The method of claim 5,
Tensile strength x High tensile steel wire with twist characteristics of 25 times or more without undergoing a delamination in a torsion test under 100D (D: steel wire diameter) condition with a load of x0.08.