KR20200076476A - Spring steel with improved fatigue properties and manufacturing method thereof - Google Patents

Abstract

The present invention relates to spring steel and a manufacturing method thereof and, more specifically, to spring steel with improved fatigue property and a manufacturing method thereof. According to an embodiment of the present invention, the method for manufacturing the spring steel comprises the steps of: refining molten steel containing 0.5-0.7 wt% of C, 1.2-2 wt% of Si, 0.5-0.8 wt% of Mn, 0.5-0.8 wt% of Cr, 0.1-0.4 wt% of Ni, 0.1-0.4 wt% of Cu, 0.05-0.2 wt% of V, 0.01-0.1 wt% of Ti, 0.1 wt% or less of Nb, 0.001-0.07 wt% of sol. Al, 0.007 wt% or less of N, 0.004 wt% or less of B, 0.03 wt% or less of P, 0.03 wt% or less of S, and the remainder consisting of Fe and unavoidable impurities; and casting the refined molten steel to manufacture steel blooms, wherein the step for refining the molten steel includes composite deoxidization of sol. Al and Si.

Description

Spring steel with improved fatigue properties and manufacturing method thereof

The present invention relates to a spring steel and a manufacturing method thereof, and more particularly, to a spring steel having improved fatigue properties and a manufacturing method thereof.

2. Description of the Related Art Recently, demand for high-stress springs is increasing in order to reduce the weight of automobiles and improve fuel efficiency. To this end, there are attempts to increase the stress of the spring through finer grains and enhanced precipitation during tempering by controlling alloy elements, such as adding alloy components such as Ti, V, and Nb to the spring steel.

However, alloy components such as added Ti, V, and Nb are combined with carbon or nitrogen in the steel to generate a large amount of carbonitride in the center of the steel piece, and a large amount of carbonitride generated in the center remains after spring molding, resulting in final spring fatigue There is a problem of deteriorating properties.

Korea Patent Publication No. 10-2015-0133850 (published date: November 30, 2015)

In order to solve the above-mentioned problems, the present invention is to provide a spring steel and a method of manufacturing the same, which can improve the final spring fatigue properties.

Manufacturing method of spring steel according to an example of the present invention is by weight, C: 0.5 ~ 0.7%, Si: 1.2 ~ 2%, Mn: 0.5 ~ 0.8%, Cr: 0.5 ~ 0.8%, Ni: 0.1 ~ 0.4% , Cu: 0.1-0.4%, V: 0.05-0.2%, Ti: 0.01-0.1%, Nb: 0.1% or less, sol. Al: 0.001 ~ 0.07%, N: 0.007% or less, B: 0.004% or less, P: 0.03% or less, S: 0.03% or less, refining the molten steel containing the residual Fe and unavoidable impurities, and the refined molten steel Casting to prepare a steel piece, and the step of refining the molten steel is sol. Al and Si composite deoxidation.

In addition, the step of refining the molten steel is by weight, sol. Al: 0.04 to 0.07%, and may include complex deoxidation.

In addition, the refined molten steel is weight%, and may include N: 0.004 to 0.007%.

In addition, in the step of casting the refined molten steel to produce a steel piece, TiN is formed, and the initial formation temperature of the TiN may be 1430°C or higher.

Spring steel according to another embodiment of the present invention in weight percent, C: 0.5 to 0.7%, Si: 1.2 to 2%, Mn: 0.5 to 0.8%, Cr: 0.5 to 0.8%, Ni: 0.1 to 0.4%, Cu: 0.1-0.4%, V: 0.05-0.2%, Ti: 0.01-0.1%, Nb: 0.1% or less, sol. Al: 0.04 to 0.07%, N: 0.004 to 0.007%, B: 0.004% or less, P: 0.03% or less, S: 0.03% or less, balance Fe and unavoidable impurities.

The present invention is a step of refining the molten steel, and it is possible to increase the nitrogen content in the molten steel to control the initial formation temperature of TiN to 1430°C or higher, so that a large amount of carbonitride formed in the center of the steel piece can be formed by dispersing it throughout the steel piece, resulting in the final spring. Fatigue properties can be improved.

1 is a schematic view of a conventional spring steel casting.
2 is a schematic view of formation of carbonitride.
3 is a schematic diagram of casting of the spring steel of the present invention.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the technical idea of the present invention is not limited to the embodiments described below. In addition, embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

The terms used in this application are only used to describe specific examples. Thus, for example, a singular expression includes a plural expression, unless the context clearly indicates it. In addition, terms such as “comprise” or “include” used in the present application are used to clearly indicate the existence of features, steps, functions, components, or combinations thereof described in the specification, and other features. It should be noted that it is not used to preliminarily exclude the presence of a field or step, function, component, or combination thereof.

On the other hand, unless otherwise defined, all terms used in this specification should be regarded as having the same meaning as generally understood by a person having ordinary skill in the art to which the present invention pertains. Accordingly, unless explicitly defined herein, certain terms should not be construed in excessively ideal or formal sense. For example, in this specification, a singular expression includes a plural expression unless the context clearly has an exception.

In addition, "about", "substantially" and the like in the present specification are used in the sense of or close to the value when manufacturing and substance tolerances unique to the stated meaning are presented, and are used to help understand the present invention. Or, an absolute value is used to prevent unscrupulous use of the disclosed content by unscrupulous intruders.

In addition, in the present specification, "liquid line" is used in the casting step to refer to the starting temperature of solidification from molten steel to steel, and the temperature is about 1460°C. "Solid ship" is used to mean the completion temperature of solidification from molten steel to steel, and the temperature is about 1380°C.

2. Description of the Related Art Recently, demand for high-stress springs is increasing in order to reduce the weight of automobiles and improve fuel efficiency. To this end, there are attempts to increase the stress of the spring through finer grains and enhanced precipitation during tempering by controlling alloy elements, such as adding alloy components such as Ti, V, and Nb to the spring steel.

However, alloy components such as added Ti, V, and Nb form carbonitrides such as TiN and TiC during solidification of the steel pieces by combining with carbon or nitrogen in the steel, and the initial formation temperature and steel pieces of carbonitrides rather than the total amount of carbonitrides. A large amount of carbonitride is generated in the center of the steel piece due to the difference between the liquidus and solidus. The carbonitride generated in a large amount in the center remains after the spring molding, thereby deteriorating the final spring fatigue characteristics. Hereinafter, a process of forming carbonitride present in a large amount in the center will be described in detail with reference to FIGS. 1 and 2.

Referring to FIG. 1, the higher the temperature toward the center of the steel piece, the higher the molten steel at a relatively high temperature in the center of the steel piece. In addition, since the temperature at the solidification completion point where solidification is completed to all the molten steel at the center of the steel piece is lower than the temperature at the solidification start point at which the molten steel starts to solidify, carbonitride is likely to be formed at the center of the steel piece.

The initial formation temperature of TiN in the carbonitride is approximately 1400° C., which is similar to the temperature in the solidus, and as shown in FIG. 1, TiN is formed in a large amount in the center of the steel piece where a relatively high temperature molten steel remains for a long time. . In addition, referring to FIG. 2, Ti-CN (about 1387°C) having a lower formation temperature is additionally precipitated around the TiN formed during the process of casting a steel piece as a nucleus, and the formed Ti-CN as a nucleus. As a result of additional precipitation of Nb-C (about 1257°C) having a lower formation temperature in the surroundings, a large amount of carbonitride is coarsely formed in the center of the steel piece.

Therefore, it is important to control the Ti, Nb, C, and N components that form the carbonitride, and among these, it is possible to control without affecting the Ti component and the spring stress that form the relatively high initial formation temperature of the carbonitride. It is important to control the N component.

In order to solve the above-described problem, the present invention can be formed by dispersing the carbonitride formed in a large amount in the center of the steel piece to form a high TiN initial formation temperature close to the liquid line, thereby improving the final spring fatigue properties. It is intended to provide a spring steel and a method of manufacturing the same.

According to one example, the method of manufacturing the spring steel of the present invention is by weight, C: 0.5 to 0.7%, Si: 1.2 to 2%, Mn: 0.5 to 0.8%, Cr: 0.5 to 0.8%, Ni: 0.1 to 0.4 %, Cu: 0.1-0.4%, V: 0.05-0.2%, Ti: 0.01-0.1%, Nb: 0.1% or less, sol. Al: 0.001 ~ 0.07%, N: 0.007% or less, B: 0.004% or less, P: 0.03% or less, S: 0.03% or less, refining the molten steel containing the residual Fe and unavoidable impurities, and the refined molten steel Casting to produce a steel piece. Hereinafter, each step will be described in detail.

Steps to refine the molten steel

In the present invention, the step of refining molten steel is a key step in which carbonitride formed in a large amount in the center of the steel piece is dispersed throughout the steel piece by increasing the initial formation temperature of TiN formed during casting of the molten steel.

As a result of repeated research, the present inventors have found that the higher the N content ratio compared to the Ti content, the higher the initial formation temperature of TiN. In addition, the N content can be increased through a separate operation in the steelmaking process, but the operation to have has a large variation in the N content, and has a problem of process load. Therefore, in the present invention, in the step of refining molten steel, the N content is increased through a change in the steelmaking deoxidation method.

The step of refining the molten steel according to an example of the present invention is sol. And deoxidizing the Al and Si composites. sol. When deoxidizing Al and Si composites, it is possible to lower the [O] content in the molten steel and increase the [N] content thermodynamically. On the other hand, sol. When deoxidizing Al or Si alone, the [O] content in the molten steel after refining is high, and the [N] content may be low thermodynamically.

According to an example of the present invention, in the step of refining the molten steel, sol. Al can be made to be 0.04 to 0.07% by weight and can be deoxidized in a complex manner. sol. When the Al content is less than 0.04% by weight, the [O] content in the molten steel after refining is high, and the [N] content may be low thermodynamically. sol. When the Al content exceeds 0.07% by weight, the Al-based inclusions do not float and are separated, and the possibility of remaining in molten steel increases, thereby deteriorating the quality of the final steel.

Normally, the N content in the molten steel is 0.003% by weight, and according to an example of the present invention, the molten steel refined as the above-described refining process is N: 0.004 to 0.007% by weight, and may contain nitrogen in a high content.

In the present invention, the nitrogen content may be increased by the above-described refining step, and the initial formation temperature may be controlled to 1430° C. or higher when TiN is formed in the step of producing steel pieces by casting molten steel to be refined.

Hereinafter, a step of manufacturing a steel piece by casting refined molten steel will be described.

Step of casting refined molten steel into steel pieces

In the present invention, the step of manufacturing molten steel by casting the refined molten steel is sufficient and not particularly limited if the molten steel having the above-described alloy composition is refined by the refining process, and then cast and manufactured into steel pieces. A person skilled in the art can appropriately design change the known casting process.

As the above-described refining step, it is possible to increase the nitrogen content in the molten steel and control the initial formation temperature of TiN to 1430°C or higher, so that a large amount of carbonitride formed in the center of the steel piece can be formed by dispersing it throughout the steel piece, thereby improving the final spring fatigue properties. Can be improved. Hereinafter, this will be described in detail with reference to the accompanying FIG. 3.

Referring to FIG. 3, since the initial TiN formation temperature is controlled to be 1430° C. or higher, the initial TiN formation temperature is closer to the temperature of the liquidus line, so that the TiN generation starting point is formed closer to the solidification starting point compared to FIG. 1. Accordingly, TiN is formed by dispersing relatively whole steel pieces, and Ti-CN generated around TiN as a nucleus and Nb-C generated around Ti-CN as a nucleus are also formed to be dispersed throughout the steel piece. do. Therefore, since carbonitride formed in a large amount in the center of the steel piece can be formed by dispersing the entire steel piece, the final spring fatigue properties can be improved.

Spring steel with improved fatigue properties according to an example of the present invention manufactured in the above-described steps is in weight percent, C: 0.5 to 0.7%, Si: 1.2 to 2%, Mn: 0.5 to 0.8%, Cr: 0.5 to 0.8 %, Ni: 0.1-0.4%, Cu: 0.1-0.4%, V: 0.05-0.2%, Ti: 0.01-0.1%, Nb: 0.1% or less, sol. Al: 0.04 to 0.07%, N: 0.004 to 0.007%, B: 0.004% or less, P: 0.03% or less, S: 0.03% or less, balance Fe and unavoidable impurities.

In addition, the spring steel according to an example of the present invention was subjected to a rotation bending fatigue test (Nakamura Test) by applying a load corresponding to 40% of the tensile strength (TS), and the fatigue life may be 10 ⁶ times or more.

As described above, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and a person having ordinary skill in the art does not depart from the concept and scope of the following claims. It will be understood that various modifications and variations are possible.

Claims (5)

In weight percent, C: 0.5 to 0.7%, Si: 1.2 to 2%, Mn: 0.5 to 0.8%, Cr: 0.5 to 0.8%, Ni: 0.1 to 0.4%, Cu: 0.1 to 0.4%, V: 0.05 to 0.2%, Ti: 0.01 to 0.1%, Nb: 0.1% or less, sol. Al: 0.001 to 0.07%, N: 0.007% or less, B: 0.004% or less, P: 0.03% or less, S: 0.03% or less, refining molten steel containing residual Fe and unavoidable impurities; And
Including; casting the refined molten steel to produce a steel piece;
The step of refining the molten steel is sol. Method for manufacturing spring steel comprising deoxidizing Al and Si composites. According to claim 1,
The step of refining the molten steel,
In weight percent, sol. Al: 0.04 ~ 0.07% method of manufacturing a spring steel comprising a complex deoxidation. According to claim 1,
The refined molten steel,
In weight percent, N: a method of manufacturing spring steel comprising 0.004 to 0.007%. According to claim 1,
In the step of casting the refined molten steel into a steel piece,
TiN is formed, and the initial formation temperature of the TiN is 1430°C or higher. In weight percent, C: 0.5 to 0.7%, Si: 1.2 to 2%, Mn: 0.5 to 0.8%, Cr: 0.5 to 0.8%, Ni: 0.1 to 0.4%, Cu: 0.1 to 0.4%, V: 0.05 to 0.2%, Ti: 0.01 to 0.1%, Nb: 0.1% or less, sol. Al: 0.04 to 0.07%, N: 0.004 to 0.007%, B: 0.004% or less, P: 0.03% or less, S: 0.03% or less, spring steel containing residual Fe and unavoidable impurities.

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