Manufacturing method of spring steel

Abstract

The present invention comprises the steps of: manufacturing a billet composed, in weight percent, of C: 0.51~0.59%, Si: 1.20~1.60%, Mn: 0.60~0.80%, P: 0.020% or less (excluding 0%), S: 0.020% or less (excluding 0%), Cr: 0.60~0.80%, and the remainder being Fe and other unavoidable impurities; making the billet into a wire rod by reheating and then rough rolling and finish rolling; manufacturing a wire rod coil by winding the wire rod; and cooling the wound wire rod coil using a conveyor process; wherein the cooling step comprises: a first cooling step of cooling at a rate of 6~10℃ / s in a temperature range of 910~960℃; and a second cooling step of cooling at a rate of 2~4℃ / s in a temperature range of 730~910℃. A method for manufacturing spring steel is provided, comprising: a step of air cooling to room temperature after the second cooling step.

Description

Technology Field

[0001] The present invention relates to a method for manufacturing spring steel, and more specifically, to a method for manufacturing spring steel with excellent peelability. Background Technology

[0002] Generally, spring steel is used in automotive suspension systems and is utilized in suspension springs, valve springs, stabilizers, torsion bars, seat springs, etc. Since these springs are easily exposed to the external environment and subjected to continuous stress within the components, strict surface quality is required.

[0003] Generally, the wire rod production process involves heating a billet to approximately 1000 to 1200°C in a heating furnace, then cooling the hot-rolled wire rod—which has undergone rough rolling, intermediate rough rolling, intermediate finishing rolling, and finishing rolling—on a water cooling line, and finally producing a wire rod coil through a winding process. In a typical wire rod coil production facility, as schematically illustrated in FIG. 2, a wire rod coil (12) wound from a laying head (11) is stacked in a ring shape on a conveying roller (10) and conveyed. The conveyed wire rod coil (12) is cooled by blowing air (D) from a blowing unit (40) installed below the conveying roller (10). Then, the wound wire rod coil (12) is dropped onto an accumulator (14) and accumulated. The spring steel produced in this way is manufactured into a final product (processed product or part) at a wire drawing machine.

[0004] If a low-temperature structure is present within the steel, there is a risk of wire breakage during the subsequent drawing process; therefore, such a structure must be eliminated. Additionally, during the manufacturing of spring steel, a ferrite decarburization layer forms on the surface of the material during the reheating, hot rolling, and cooling processes. Since this ferrite decarburization layer reduces the fatigue life of the material, the wire drawing company must remove the surface scale generated during wire rod manufacturing before performing the processing or drawing process. Two methods are used to remove this scale: chemical methods involving pickling with chemicals, and physical methods such as shot blasting and belt grinders. In the case of spring steel, due to its high strength, even small surface defects can lead to product failure; therefore, physical methods are preferred over chemical methods for scale removal. The problem to be solved

[0005] The present invention aims to solve such conventional problems by manufacturing spring steel with improved ability to physically remove scale within a range that avoids cooling conditions in which ferrite decarburization and low-temperature microstructure occur. However, this objective is exemplary and does not limit the scope of the present invention. means of solving the problem

[0006] According to one aspect of the present invention, a method for manufacturing spring steel is provided.

[0007] The method for manufacturing the above spring steel comprises, in weight percent, a step of manufacturing a billet consisting of C: 0.51~0.59%, Si: 1.20~1.60%, Mn: 0.60~0.80%, P: 0.020% or less (excluding 0%), S: 0.020% or less (excluding 0%), Cr: 0.60~0.80%, and the remainder being Fe and other unavoidable impurities; a step of forming the billet into a wire rod through rough rolling and finish rolling after reheating; a step of manufacturing a wire rod coil by winding the wire rod; and a step of cooling the wound wire rod coil using a conveyor process; wherein the cooling step comprises a first cooling step of cooling at a rate of 6~10℃ / s in a temperature range of 910~960℃; and a second cooling step of cooling at a rate of 2~4℃ / s in a temperature range of 730~910℃. and a step of air cooling to room temperature after the second cooling step;

[0008] According to one embodiment, the first cooling step may be characterized as a temperature range in which the FeO fraction among the scale formed on the wire coil increases.

[0009] According to one embodiment, the cooling speed may be characterized by being controlled by the airflow rate or conveyor speed.

[0010] According to one embodiment, the cooling step may include the step of measuring the temperature of the wire coil moving along the conveyor.

[0011] According to one embodiment, the wire coil that has undergone the cooling step may be characterized by having a scale with a thickness of 10 μm or less, and the ratio of FeO in the scale being 2 to 3 volume%. Effects of the invention

[0012] According to an embodiment of the present invention as described above, by controlling the cooling conditions during the rolling process, spring steel with excellent peelability compared to conventional steel can be manufactured, and there is an effect of reducing the amount of scale remaining on the surface after descaling.

[0013] Of course, the scope of the present invention is not limited by these effects. Brief explanation of the drawing

[0014] Figure 1 is a graph illustrating a cooling method in a method for manufacturing spring steel according to one embodiment of the present invention. FIG. 2 is a schematic diagram illustrating an apparatus in which a method for manufacturing a spring steel material according to one embodiment of the present invention is implemented. FIG. 3 is a diagram showing the cross-sectional structure of scale formed on a wire coil implemented according to a method for manufacturing spring steel according to one embodiment of the present invention. Specific details for implementing the invention

[0015] Hereinafter, various embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the spirit of the present invention to those skilled in the art. In addition, the thickness or size of each layer in the drawings is exaggerated for convenience and clarity of explanation.

[0016] Hereinafter, a method for manufacturing a spring steel according to an embodiment of the present invention will be described.

[0017] The method for manufacturing the above spring steel comprises, in weight percent, a step of manufacturing a billet consisting of C: 0.51~0.59%, Si: 1.20~1.60%, Mn: 0.60~0.80%, P: 0.020% or less (excluding 0%), S: 0.020% or less (excluding 0%), Cr: 0.60~0.80%, and the remainder being Fe and other unavoidable impurities; a step of forming the billet into a wire rod through rough rolling and finish rolling after reheating; a step of manufacturing a wire rod coil by winding the wire rod; and a step of cooling the wound wire rod coil using a conveyor process; wherein the cooling step comprises a first cooling step of cooling at a rate of 6~10℃ / s in a temperature range of 910~960℃; and a second cooling step of cooling at a rate of 2~4℃ / s in a temperature range of 730~910℃. and a step of air cooling to room temperature after the second cooling step;

[0018] The above billet is composed of, in weight percent, C: 0.51~0.59%, Si: 1.20~1.60%, Mn: 0.60~0.80%, P: 0.020% or less (excluding 0%), S: 0.020% or less (excluding 0%), Cr: 0.60~0.80%, and the remainder being Fe and other unavoidable impurities.

[0019] After reheating the billet formed as described above to 900~1,050℃, rough rolling and finish rolling are performed.

[0020] FIG. 1 is a graph exemplarily illustrating a cooling method in a method for manufacturing spring steel according to one embodiment of the present invention. Referring to FIG. 1, section (a) represents the process of hot rolling a billet, which corresponds to the section where magnetite / hematite occurs. During the rolling process, mechanical descaling occurs due to the pressure of the rolls.

[0021] After rolling as described above, the wound wire coil moves through the conveyor's transfer rollers. When the wire coil, extracted by being continuously wound into a coil shape from the laying head, drops onto the conveyor for transport, the wire coil can be cooled to a low temperature by blowing air from a blower unit positioned at the bottom of the conveyor.

[0022] At this time, the wound wire coil is cooled in the following way.

[0023] The first cooling step is a step of cooling the wire coil at a rate of 6 to 10°C / s in a temperature range of 910 to 960°C. This corresponds to section (b) of FIG. 1. 960°C may be the temperature at the point when the wire coil extracted from the laying head falls onto the conveyor, and the temperature after cooling at a rate of 6 to 10°C / s may be 910°C.

[0024] Ferrite decarburization is prevented during the first cooling step, which can suppress the formation of a ferrite decarburization layer on the material surface. This is highly related to the microstructure of the scale formed in the corresponding temperature range.

[0025] Generally, the scale layer is a compound of Fe and O, largely composed of three layers: wüstite (FeO), magnetite (Fe3O4), and hematite (Fe2O3). At high temperatures above 570°C, Fe 2+ , O 2- It is formed through ion exchange, and wüstite is dominant at high temperatures, while magnetite is dominant at room temperature. Among scale structures, the wüstite (FeO) structure, which has low hardness and good peelability, is known to be advantageous for the descaling process.

[0026] In an embodiment of the present invention, the FeO fraction in the scale formed on the wire coil can be increased by cooling at a cooling rate of 6 to 10°C / s in a first cooling step, which is a temperature range (910 to 960°C) where wüstite (FeO) is easily formed. That is, the reason for cooling at a cooling rate of 6 to 10°C / s in the first cooling step is to prevent the formation of ferrite and to promote the formation of FeO. If the cooling rate is less than 6°C / second, it is difficult to suppress the formation of ferrite, and if it exceeds 10°C / second, it is difficult to form FeO sufficiently.

[0027] Next, the second cooling step is a step of cooling at a rate of 2 to 4°C / s in a temperature range of 730 to 910°C. This corresponds to section (c) of Fig. 1. If the cooling rate is less than 2°C / sec, there is a possibility that ferrite decarburization will occur during the initial cooling stage, and if it exceeds 4°C / sec, it is difficult to sufficiently form FeO.

[0028] The above cooling speed can be controlled by the airflow rate or conveyor speed. Specifically, a method for controlling the cooling speed will be explained with reference to FIG. 2.

[0029] FIG. 2 is a schematic diagram illustrating an apparatus in which a method for manufacturing a spring steel material according to one embodiment of the present invention is implemented.

[0030] As illustrated in FIG. 2, the control unit (120, 130) can control the amount of air blown from the blower unit (40) or the speed of the conveyor roller (10). Through this, the cooling speed can be controlled, and specifically, by controlling the cooling speed of the second cooling stage to be slower than the cooling speed of the first cooling stage, the occurrence of low-temperature tissue in the wire coil (12) can be suppressed.

[0031] Optionally, the temperature measuring unit (110) may be configured to measure the temperature of the wire coil (12) moving along the transfer roller (10), and the temperature measuring unit (110) may be placed at multiple locations along the transfer roller (10). For example, it may include a first measuring unit (111) and a second measuring unit (112). The first measuring unit (111) and the second measuring unit (112) may be installed along the longitudinal direction of the transfer roller (10) above the wire coil (12) to measure the temperature of the wire coil (12). By measuring the temperature at which the wire coil (12) is cooled through the temperature measuring unit (110), the structure of the scale formed on the wire coil (12) can be controlled.

[0032] After the second cooling step above, air cooling to room temperature is performed as shown in (d) of FIG. 1. This effectively suppresses the formation of a low-temperature structure in the wire coil.

[0033] The wire coil that has undergone the cooling step as described above has a scale with a thickness of 10 μm or less, and the FeO ratio in the scale may correspond to 2 to 3 volume%.

[0034] Hereinafter, the specific details of the present invention will be explained in detail through the following examples, but this is for illustrative purposes only and is not intended to limit the scope of protection defined by the appended claims.

[0035] <Example>

[0036] A billet was prepared composed of, in weight percent, C: 0.53%, Si: 1.40%, Mn: 0.70%, P: 0.010%, S: 0.010%, Cr: 0.70%, and the remainder being Fe and other unavoidable impurities. Subsequently, it was reheated to 1,050°C, hot-rolled, and coiled. The wire coil was cooled under cooling conditions as shown in Table 1. The first cooling rate is a cooling rate in the temperature range of 910 to 960°C, and the second cooling rate is a cooling rate in the temperature range of 730 to 910°C.

[0037] condition First cooling rate Second cooling rate Analysis Results (Percentage, %) FeO Fe3O4 Fe2O3 Comparative Example 1 16.5 ℃ / sec 2.87 ℃ / sec 0.9 14.0 85.1 Comparative Example 2 11 ℃ / sec 2.87 ℃ / sec 0.8 31.0 68.3 Example 1 6.0 ℃ / sec 2.87 ℃ / sec 2.3 28.2 69.5 Example 2 8.0 ℃ / sec 2.87 ℃ / sec 2.6 38.7 58.8

[0038] Referring to Table 1, when the first cooling rate exceeds 10 ℃ / sec, the phase analysis results of the scale show a low fraction of FeO, indicating that FeO is not sufficiently formed.

[0039] FIG. 3 is a diagram showing the cross-sectional structure of scale formed on a wire coil implemented according to the method of manufacturing spring steel according to (a) Comparative Example 2 and (b) Example 2 above.

[0040] Referring to FIG. 3, it can be seen that a scale with a thickness of 8.2 μm is formed on the surface of the wire coil according to (b) Example 2, and a scale with a thickness of 10.7 μm is formed on the surface of the wire coil according to (a) Comparative Example 2. Therefore, it can be seen that the thickness of the scale is determined according to the first cooling rate, and that a scale with a thickness of 10 μm or less can be obtained when the first cooling rate is adjusted to the range according to the embodiment of the present invention.

[0041] According to an embodiment of the present invention as described above, by controlling the cooling conditions during the rolling process, spring steel with excellent peelability compared to conventional steel can be manufactured, and there is an effect of reducing the amount of scale remaining on the surface after descaling.

[0042] The present invention has been described with reference to the embodiments illustrated in the drawings, but this is merely illustrative, and those skilled in the art will understand that various modifications and equivalent alternative embodiments are possible therefrom. Accordingly, the true technical scope of protection of the present invention should be determined by the technical spirit of the appended claims.

Claims

Claim 1 The method comprises the steps of: manufacturing a billet consisting of, in weight percent, C: 0.51~0.59%, Si: 1.20~1.40%, Mn: 0.60~0.80%, P: 0.020% or less (excluding 0%), S: 0.020% or less (excluding 0%), Cr: 0.60~0.80%, and the remainder being Fe and other unavoidable impurities; making the billet into a wire rod by reheating and then rough rolling and finish rolling; manufacturing a wire rod coil by winding the wire rod; and cooling the wound wire rod coil using a conveyor process; wherein the cooling step comprises: a first cooling step of cooling at a cooling rate of 6~10℃ / s in a temperature range of 910~960℃; and a second cooling step of cooling at a cooling rate of 2~4℃ / s in a temperature range of 730~910℃. A method for manufacturing spring steel, comprising: a step of air cooling to room temperature after the second cooling step; wherein the wire coil that has undergone the cooling step has a scale with a thickness of 10 μm or less, and the ratio of FeO in the scale is 2 to 3 volume%. Claim 2 A method for manufacturing spring steel according to claim 1, wherein the first cooling step is a temperature range in which the FeO fraction of the scale formed on the wire coil increases. Claim 3 A method for manufacturing spring steel according to claim 1, characterized in that the cooling speed of the first cooling step and the second cooling step is controlled by the airflow rate or conveyor speed. Claim 4 A method for manufacturing spring steel according to claim 1, wherein the cooling step comprises the step of measuring the temperature of the wire coil moving along a conveyor. Claim 5 delete

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