KR102443412B1 - High strength wire rod with excellent fatigue properties and manufacturing method thereof - Google Patents

Abstract

Disclosed is a high-strength wire rod having excellent fatigue properties. High-strength wire rod having excellent fatigue properties according to an embodiment of the present invention, by weight, C: 0.60 to 0.92%, Si: 0.1 to 0.5%, Mn: 0.4 to 1.0%, P: 0.03% or less, S: 0.03 % or less, Al: 0.01 to 0.05%, N: 0.01% or less, the remaining Fe and other impurities, and the hardness ratio of the surface layer portion and the center portion, which is a region from the surface toward the center to the diameter × 0.04, is 1.23 to 1.25:1.

Description

High-strength wire rod with excellent fatigue properties and manufacturing method thereof

The present invention relates to a high-strength wire having excellent fatigue properties and a method for manufacturing the same, and more particularly, to a high-strength wire having excellent fatigue properties by controlling the microstructure of the surface layer, and a method for manufacturing the same.

In general, bead wire is a reinforcing material used for fixing tires and rims, and improves driving stability and tire lifespan.

Since the increase in carbon content means high strength of the product, there is a trend to use high-carbon bead wire with a carbon content of 0.6% or more for vehicles that require tire stability.

Such a high carbon bead wire can increase the fatigue strength or fatigue by suppressing the formation of cracks on the surface by increasing the hardness of the surface through nitriding or carburizing treatment. However, there is a problem in that a separate process is added to the nitriding or carburizing treatment, thereby increasing the manufacturing cost.

In addition, the hardness of the surface can be increased by controlling the microstructure without nitriding or carburizing treatment, but when a martensite phase is formed on the surface, the ductility is insufficient, and thus machining disconnection may occur during wire drawing.

An object of the present invention is to provide a high-strength wire rod exhibiting excellent fatigue characteristics by controlling the microstructure of the surface layer and a method for manufacturing the same.

As a means for achieving the above object, the present specification is, in wt%, C: 0.60 to 0.92%, Si: 0.1 to 0.5%, Mn: 0.4 to 1.0%, P: 0.03% or less, S: 0.03% or less, Al : 0.01 to 0.05%, N: 0.01% or less, the remaining Fe and other impurities, and the hardness ratio of the surface layer and the center, which is the area from the surface toward the center to the diameter × 0.04, is 1.23 to 1.25: 1. Disclosed is a high-strength wire rod.

In addition, the hardness of the surface layer may be 400 to 500 Hv, and the hardness of the central portion may be 300 to 380 Hv.

In addition, the tensile strength may be 2,000 MPa or more, and fatigue may be 1,040 MPa or more.

As a means for achieving the above object, the present specification is, in wt%, C: 0.60 to 0.92%, Si: 0.1 to 0.5%, Mn: 0.4 to 1.0%, P: 0.03% or less, S: 0.03% or less, Al : 0.01 to 0.05%, N: 0.01% or less, the remaining Fe and other impurities are heated at 1,000 to 1,200° C. for 90 to 150 minutes to proceed to finishing rolling, and the surface temperature of the wire rod progressed to finishing rolling is 200 to Cooling down to Ms or less at 300°C/s, final rolling the surface-cooled wire rod and winding it at 800 to 850°C, first cooling the wound wire rod to A1-50°C at 20-25°C/s, and then Disclosed is a method for manufacturing a high-strength wire rod having excellent fatigue characteristics by secondary cooling at 5 to 10°C/s.

In addition, the winding may be performed using recuperation due to processing heat generated in the final rolling.

According to an embodiment of the present invention, it is possible to improve the strength and fatigue characteristics of the bead wire by controlling the microstructure of the surface layer portion.

According to an embodiment of the present invention, the surface layer hardness of the high carbon steel is 1.23 to 1.25 times higher than that of the central part, thereby suppressing cracks occurring on the surface, thereby satisfying high strength and increasing fatigue of the wire rod.

BRIEF DESCRIPTION OF THE DRAWINGS It is the cross-sectional photograph which observed the hardened surface layer of Invention Example 2 of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes preferred embodiments of the present invention. However, the embodiments of the present invention may be modified in various other forms, and the technical spirit of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.

The terms used in this application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression unless the context clearly requires it to be singular. In addition, terms such as "comprises" or "comprises" used in the present application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, and other features It should be noted that it is not intended to be used to preliminarily exclude the existence of elements, steps, functions, components, or combinations thereof.

On the other hand, unless otherwise defined, all terms used herein should be regarded as having the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Accordingly, unless explicitly defined herein, certain terms should not be construed in an unduly idealistic or formal sense. For example, a singular expression herein includes a plural expression unless the context clearly dictates otherwise.

In addition, in this specification, "about", "substantially", etc. are used in or close to the numerical value when manufacturing and material tolerances inherent in the stated meaning are presented, and are used in a precise sense to aid the understanding of the present invention. or absolute figures are used to prevent unreasonable use by unconscionable infringers of the mentioned disclosure.

The high-strength wire rod having excellent fatigue properties according to the present invention is, by weight, C: 0.60 to 0.92%, Si: 0.1 to 0.5%, Mn: 0.4 to 1.0%, P: 0.03% or less, S: 0.03% or less, Al: 0.01 to 0.05%, N: 0.01% or less, and the remainder of Fe and other impurities.

Hereinafter, the reason for limiting the alloy composition will be described in detail. All of the following component compositions refer to weight % unless otherwise specified.

C (carbon): 0.60 to 0.92%

C is the element that can increase the strength of the material most effectively, and when 0.1% C is increased, there is an effect of increasing the strength of 100 MPa. When less than 0.60% is added, it is less than the lower strength of the product regulation, and when it is added more than 0.92%, it is preferable to maintain it at 0.60 to 0.92% because the product exceeds the upper limit and the risk of wire breakage during processing is high.

Si (silicon): 0.1 to 0.5%

Si is a ferrite solid solution strengthening element, and when 0.1% Si is added, the strength of 14 to 16 MPa is improved. Since the bead wire is Si-killed steel, when it is maintained at less than 0.1%, there is a problem that the oxide formed in the steel causes disconnection during wire drawing, because it is not deoxidized when it is maintained at less than 0.1%. It is desirable to control it below that level because there is a difficulty in it.

Mn (manganese): 0.4 to 1.0%

Mn is a solid solution strengthening element, and when it is increased by 0.1%, it increases the strength by 20 MPa or gives sufficient hardenability during heat treatment. When it is less than 0.4%, it is difficult to secure the target tensile strength, and when it exceeds 1.0%, central segregation is severe and causes disconnection during wire drawing, so it is desirable to control it below that level.

P (phosphorus) and S (sulfur): 0.03% or less

P and S are impurities, and although content is not specifically prescribed|regulated, it is preferable to set it as 0.03 % or less, respectively, from a viewpoint of ensuring ductility similarly to the conventional steel wire.

Al (aluminum): 0.01 to 0.05%

Al combines with N present in steel to form AlN, and is an element that inhibits grain growth because it exists at the austenite grain boundary. When Al is less than 0.01%, the effect of grain refinement is small, and when Al exceeds 0.05%, it is highly likely that non-uniform crystal grains due to coarse AlN formation, etc. are induced, so it is preferable to add less than that.

N (nitrogen): 0.01% or less

N is an element that greatly increases strength like C, and is an effective element in terms of strength that increases strength by 100 MPa when 0.1% is added. However, since N forms a Cottrell atmosphere to fix dislocations and reduces ductility, it is preferable to control it to 0.01% or less.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. All of the impurities are not specifically mentioned in the present specification, since any person skilled in the art can know the impurities.

In the high-strength wire rod having excellent fatigue properties according to an embodiment of the present invention, a hardened surface layer having a microstructure including tempered martensite is formed in the surface layer, which is an area up to a diameter (D)×0.04 from the surface to the center. Here, the center means the center of the cross-section in the thickness direction of the wire rod.

In addition, the hardness of the surface layer may be 400 to 500 Hv, the hardness of the center may be made of 300 to 380 Hv, the hardness ratio of the surface layer to the center may be made of 1.23 to 1.25: 1. When the hardness of the surface layer is 1.23 to 1.25 times greater than that of the center, the high strength of the wire rod is satisfied, and the surface cracks are suppressed to increase fatigue fatigue, and disconnection can be prevented during wire drawing.

In addition, the tensile strength may be 2,000Mpa or more, and fatigue may be made of 1,040Mpa or more.

Hereinafter, a method for manufacturing a high-strength wire rod having excellent fatigue properties according to the present invention will be described.

The method for manufacturing a high-strength wire rod having excellent fatigue properties according to an embodiment of the present invention includes the steps of heating a billet satisfying the above-described component system to finish rolling, cooling the surface of the wire rod that has progressed to finishing rolling, and the surface is cooled. It includes the steps of winding the wire rod by final rolling (RSM, reducing sizing mill) and cooling the wound wire rod.

In the step of heating the billet to finish rolling, it is maintained in a temperature range of 1,000 to 1,200° C. for 90 to 150 minutes in a wire heating furnace to form a uniform austenite, and then rough rolling and finishing rolling are performed.

Heating in the temperature range of 1,000 to 1,200 ℃ is maintained in the austenite single phase is a condition in which coarsening of the austenite grains does not occur. If it is less than 1,000 ℃, it takes more than 150 minutes to form uniform austenite grains, and if it exceeds 1,200 ℃, the productivity is reduced due to the rapid growth of austenite grains and increased loss due to high-temperature scale formation. It is preferable to keep it below. Similarly, if the heating time exceeds 150 minutes, the loss due to the rapid growth of austenite grains and high-temperature scale formation may occur, so it is preferable to maintain the heating time at 90 to 150 minutes.

The step of cooling the surface of the wire rod progressed to the finishing rolling is carried out between the front end of the final rolling (RSM), that is, between the four rolling and the final rolling (RSM), and the finishing rolling is performed to secure the tempered martens targeted in the present invention. The surface temperature of the wire rod progressed up to 200 to 300 °C/s is cooled to below Ms (martensite start). Cooling may be performed in a water cooling zone.

When the cooling rate is less than 200℃/s, since the wire rolling speed has to be significantly lowered, product competitiveness is lowered due to decreased productivity and increased manufacturing cost. Since this is formed, disconnection may occur during processing of the drawn yarn, so it is desirable to control it below that level.

In the step of final rolling and winding the surface-cooled wire rod, the surface-cooled wire rod is finally rolled and wound at 800 to 850°C using recuperative heat generated by processing heat generated in the final rolling.

In consideration of the alloy composition, the Acm is 780 ° C. In order to avoid the formation of proeutectoid cementite, it is desirable to control the coiling temperature to 800 ° C or higher, and it is not allowed to exceed 850 ° C in consideration of the loss due to the increase in scale thickness.

In the step of cooling the wound wire rod, the wound wire rod is first cooled to A1-50° C. at 20 to 25° C./s, followed by secondary cooling at 5 to 10° C./s. Cooling may be performed in an air cooling zone.

Since it is advantageous in terms of wire-drawn yarn processing to control the proeutectoid cementite formed at the grain boundary at the center of the cross-section of the wire to 2% or less, the maximum cooling rate is given up to A1 - 50°C, and at a temperature below that, multi-stage cooling is performed at 5 to 10°C/s. Induces tempered martensitic transformation of low-temperature tissue formed in the surface layer.

The core hardness of the wire rod manufactured in this way has a hardness of 300 to 400 Hv, and the hardness of the surface layer has a hardness of 450 to 550 Hv. In addition, the hardness of the surface layer/hardness of the center portion has a value of 1.23 to 1.25. If the value of surface layer hardness/core hardness exceeds 1.25, disconnection occurs during wire drawing, and if it is less than 1.23, it is not effective to increase tensile strength and fatigue, so it is desirable to keep it higher than that.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are provided to illustrate the present invention in more detail, and the technical spirit of the present invention is not limited to the following examples.

(Example)

In the present invention, a 160 × 160 mm ² billet was manufactured through steelmaking/casting and slab rolling of high-carbon steel as shown in Table 1, and after heating at 1,120° C. for 100 minutes with a wire heating furnace, a conventional rolling process was performed. In the RSM shear water cooling zone, changes were made as shown in Table 1, and it was wound at a coiling temperature of 830 °C.

division Alloy composition (wt.%) RSM Shear Water Cooling Zone C Si Mn P S Al N cooling rate
(℃/s) Invention Example 1 0.62 0.2 0.6 0.02 0.009 0.03 0.006 250 Invention Example 2 0.72 0.2 0.6 0.02 0.009 0.03 0.006 250 Invention example 3 0.82 0.2 0.6 0.02 0.009 0.03 0.006 250 Invention Example 4 0.92 0.2 0.6 0.02 0.009 0.03 0.006 250 Comparative Example 1 0.72 0.2 0.6 0.02 0.009 0.03 0.006 150 Comparative Example 2 0.72 0.2 0.6 0.02 0.009 0.03 0.006 380 Comparative Example 3 0.52 0.2 0.6 0.02 0.009 0.03 0.006 250 Comparative Example 4 0.98 0.2 0.6 0.02 0.009 0.03 0.006 250 Comparative Example 5 0.72 0.2 0.6 0.02 0.009 0.03 0.006 normal conditions

The wound wire rod was cooled at 23°C/s up to 700°C in the cooling zone, and finished at 10°C/s until the next 200°C. For the cooled wire rod, the hardened surface layer was observed, and the hardness of the surface layer portion on which the hardened surface layer was formed and the hardness of the central portion were measured. After that, the wire rod was descaled through pickling by the customer, and the steel wire was obtained through 93.4% drawing application, and the tensile strength and fatigue limit were measured. Each result is shown in Table 2 below.

division wire rod steel wire hardened surface layer Surface hardness (Hv) Core hardness (Hv) superficial/central
hardness ratio fresh
machinability The tensile strength
(MPa) tired
(MPa) Invention Example 1 Within D×0.04 405 330 1.23 Good 2001 1041 Invention Example 2 Within D×0.04 428 349 1.23 Good 2110 1097 Invention example 3 Within D×0.04 449 360 1.25 Good 2200 1144 Invention Example 4 Within D×0.04 470 378 1.24 Good 2315 1204 Comparative Example 1 Within D×0.04 384 350 1.10 Good 1915 996 Comparative Example 2 More than D×0.04 524 348 1.51 monorail not measurable not measurable Comparative Example 3 Within D×0.04 370 301 1.23 Good 1905 972 Comparative Example 4 Within D×0.04 498 399 1.25 monorail not measurable not measurable Comparative Example 5 doesn't exist 345 349 0.99 Good 1897 941

Looking at Tables 1 and 2 together, Inventive Examples 1 to 4 are cases where the C content is 0.62 to 0.92%, and the cooling rate is cooled to 250° C./s in the water cooling zone preceding the RSM, and the surface layer is finely cooled by multi-stage cooling in the cooling zone. A hardened surface layer was formed within D×0.04 through tissue control, and the tensile strength of the product was 2,000 MPa or more, and the fatigue resistance was 1,040 MPa or more, confirming excellent fatigue properties.

Comparative Example 1 has the same alloy components as Inventive Example 2, but the cooling rate in the RSM shear water cooling zone is less than 200°C/s, the surface hardness is less than 400Hv, and the surface layer/center hardness ratio is less than 1.23, so the tensile strength and fatigue are lower. All were confirmed to appear low.

Comparative Example 2 has the same alloy components as Inventive Example 2, but it was confirmed that the cooling rate in the RSM shear water cooling zone exceeded 300° C./s, so that the hardened surface layer exceeded the D×0.04 region, and disconnection occurred during wire drawing.

In Comparative Examples 3 and 4, the cooling rate in the RSM shear water cooling zone was included in the scope of the present invention, but the carbon content was different from the component range of the present invention, so that the tensile strength and fatigue did not reach the target values, or disconnection occurred. can be checked

Comparative Example 5 is a wire manufactured by winding and cooling after rough rolling, sand rolling, and final rolling as in the general method of manufacturing a wire rod. did.

In addition, it can be seen that the hardness ratio between the surface layer and the center was less than 1.23 due to the non-formation of the hardened surface layer, and fatigue was less than 1,040 MPa.

1 is a photograph of observing a cross section of Inventive Example 2 of the present invention. Referring to FIG. 1, it can be confirmed that the wire rod of Inventive Example 2 secured after the transformation is completed through the water cooling zone and the cooling zone has a surface hardened layer formed in the surface layer, which is the area from the surface toward the center to the diameter (D) × 0.04. can

In the above description, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art will not depart from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.

Claims (5)

By weight%, C: 0.60 to 0.92%, Si: 0.1 to 0.5%, Mn: 0.4 to 1.0%, P: 0.03% or less, S: 0.03% or less, Al: 0.01 to 0.05%, N: 0.01% or less, containing the remaining Fe and other impurities,
A high-strength wire with excellent fatigue properties with a hardness ratio of 1.23 to 1.25:1 between the surface layer and the center, which is the area from the surface to the center with a diameter of 0.04. According to claim 1,
The hardness of the surface layer is 400 to 500 Hv, and the hardness of the center is 300 to 380 Hv. According to claim 1,
High-strength wire rod with excellent fatigue properties with a tensile strength of 2,000 MPa or more and a fatigue resistance of 1,040 MPa or more. By weight%, C: 0.60 to 0.92%, Si: 0.1 to 0.5%, Mn: 0.4 to 1.0%, P: 0.03% or less, S: 0.03% or less, Al: 0.01 to 0.05%, N: 0.01% or less, The billet containing the remaining Fe and other impurities is heated at 1,000 to 1,200° C. for 90 to 150 minutes to proceed to finishing rolling,
Cooling the surface temperature of the wire rod progressed to finishing rolling at 200 to 300°C/s to Ms or less,
The wire rod with the surface cooled is finally rolled and wound at 800 to 850° C. using recuperative heat generated by the processing heat generated in the final rolling,
A method of manufacturing a high-strength wire rod with excellent fatigue properties, in which the wound wire rod is first cooled to A1-50° C. at 20 to 25° C./s, and then is cooled secondarily at 5 to 10° C./s. delete

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