Austenitic high manganese steel for brake discs

Abstract

To provide an austenitic high manganese steel for disc brakes having an excellent friction coefficient. [Solution] The present invention relates to a high manganese steel for disc brakes, and discloses an austenitic high manganese steel for disc brakes having an excellent friction coefficient. According to one embodiment of the disclosed austenitic high manganese steel, the steel contains, by weight, 0.2-1.8% C, 8-30% Mn, and the balance being Fe and other unavoidable impurities, and the microstructure contains 90% or more austenite by area fraction, and the length of the austenite crystal grains is 100 μm. 2 The diameter may be 1 μm or more.

Description

[Technical Field]

[0001] The present invention relates to a high manganese steel for disc brakes, and more particularly to an austenitic high manganese steel for disc brakes having an excellent coefficient of friction. [Background technology]

[0002] In order to solve the increasingly serious problem of global warming, regulations on carbon dioxide emissions are being strengthened. In the automotive field, fuel efficiency regulations are being tightened, and efforts are underway to reduce the weight of vehicles in order to improve fuel efficiency.

[0003] Attempts to reduce weight are being made from various angles, and the trend is to reduce the thickness of any part as much as possible while maintaining the same performance. In the case of disc brakes, reducing the thickness is also beneficial for reducing the weight of the vehicle as long as it does not affect braking performance or vehicle safety.

[0004] However, since disc brakes are vehicle components that perform braking through contact friction with brake pads, their thickness can be reduced through repeated friction, and therefore they must have wear resistance to prevent braking problems within their service life. Furthermore, disc brakes are subject to frictional heat due to their characteristics, and the surface that comes into contact with the brake pads can be locally heated to high temperatures, which can reduce their strength. In other words, if the friction coefficient is insufficient, the reduction in thickness can cause fatal safety issues. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Korean Patent Publication No. 10-2008-0058440 Summary of the Invention [Problem to be solved by the invention]

[0006] In order to solve the above-mentioned problems, the present invention aims to provide an austenitic high manganese steel for disc brakes having an excellent friction coefficient.

[0007] However, the problems to be solved by the present application are not limited to those mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below. [Means for solving the problem]

[0008] As a means for achieving the above-mentioned object, an austenitic high manganese steel according to one embodiment of the present invention contains, by weight %, 0.2 to 1.8% C, 8 to 30% Mn, and the balance being Fe and other unavoidable impurities, and the microstructure contains austenite in an area fraction of 90% or more, and the length of the austenite crystal grains is 100 μm. 2 It may be 1 μm or more per particle.

[0009] The austenitic high manganese steel may further contain one or more of Cr, Mo, and W.

[0010] The austenitic high manganese steel may further contain, by weight percent, Cr, Mo, and W in a total amount of 8% or less.

[0011] The austenitic high manganese steel may also contain precipitates formed at the interfaces of the austenite crystal grains in an area fraction of 5% or more.

[0012] In addition, the austenitic high manganese steel has a length of austenite crystal grains of 100 μm per unit area. 2 The thickness may be 30 to 120 μm.

[0013] The austenitic high manganese steel may have a friction coefficient of 0.4 or more. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide an austenitic high manganese steel for disc brakes that has an excellent friction coefficient by controlling the length of austenite crystal grains per unit area. DETAILED DESCRIPTION OF THE INVENTION

[0015] Preferred embodiments of the present invention will be described below. However, the embodiments of the present invention may be modified into various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0016] The terms used in this application are only used to describe specific examples. For example, singular expressions include plural expressions unless the context clearly dictates otherwise. In addition, it should be noted that the terms "comprise" or "comprises" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

[0017] Meanwhile, unless otherwise defined, all terms used herein should be considered to have the same meaning as that commonly understood by a person of ordinary skill in the art to which the present invention pertains. Therefore, unless clearly defined herein, specific terms should not be construed as having an overly ideal or formal meaning. For example, in this specification, the singular expression includes the plural expression unless there is a clear exception in the context.

[0018] Furthermore, in this specification, the terms "about," "substantially," and the like are used in the sense of a numerical value or a value close to the numerical value when tolerances for manufacturing and materials inherent in the meanings mentioned are presented, and are used to prevent unscrupulous infringers from unfairly utilizing the disclosure content in which precise or absolute numerical values ​​are mentioned to aid in the understanding of the present invention.

[0019] The inventors of the present invention conducted research into steels suitable for use in components requiring high-temperature strength, such as disc brakes. As a result, they discovered that austenitic steels possess high strength at high temperatures and are therefore suitable as disc brake materials. Conventional steels with pearlite, ferrite, or high-strength martensite microstructures can experience reduced braking performance due to phase transformation induced at the disc surface during repeated braking. On the other hand, steels with austenitic microstructures exhibit high strength at room and high temperatures because they do not undergo phase transformation at high temperatures. However, at the high temperatures at which disc brakes are heated, simply converting the primary structure to austenite is not sufficient; precipitates must be formed within the steel with an appropriate shape and distribution. Conventional austenitic steels typically limit the amount of precipitates present at grain boundaries to prevent a decrease in softness and impact toughness. However, the present invention improves the wear resistance of steels at high temperatures by forming precipitates at grain boundaries and, at the grain interfaces, preferably to prevent the precipitates from connecting with each other.

[0020] That is, the steel material of the present invention is an austenitic steel material having a large amount of precipitates formed at grain boundaries. Meanwhile, the microstructure of the austenitic steel material of the present invention includes austenite and precipitates. The microstructure contains austenite as the main structure in an area fraction of 90% or more, preferably 99% or more, and 5% or more precipitates. It may additionally contain α'-martensite and ε-martensite as impurities that may be unavoidably formed during the manufacturing process. Furthermore, the high manganese steel of the present invention has the advantages of not only excellent work hardening ability but also wear resistance based on the austenitic structure. Therefore, when an austenite area fraction of 90% or more is secured in the present invention, it can be determined that an austenitic steel type has been formed.

[0021] In the austenitic high manganese steel according to the present invention, the length of the austenite grains is 100 μm per unit area. 2The more grain boundaries there are, which are sites where precipitates (carbides) can form, the more likely it is that precipitates will be easily formed in the carbide formation temperature range. Taking this into consideration, in order to ensure a high friction coefficient, the length of the austenite grains is set to be 1 μm or more per unit area, preferably 30 μm or more. 2 The length of the grain boundary per unit area is preferably 1 μm or more, more preferably 30 μm or more. That is, the smaller the size of the crystal grains and the greater the number of the crystal grains, the greater the length of the crystal grain per unit area. Therefore, the longer the length of the crystal grains per unit area, the more preferable it is. However, taking into consideration the practical limitations of the manufacturing process, in one example, the upper limit of the length of the austenite crystal grains is set to 100 μm per unit area. 2 The thickness may be 120 μm.

[0022] The higher the coefficient of friction of the austenitic high manganese steel according to one embodiment of the present invention, the better. For example, the coefficient of friction may be 0.4 to 0.7, preferably 0.5 to 0.8. When the coefficient of friction is 0.4 or greater, the austenitic steel can be used in disc brakes. Specifically, a low coefficient of friction can result in a long braking distance, whereas an excessively high coefficient of friction can cause problems such as excessive stress on other devices and excessive disc pad wear due to sudden braking.

[0023] An austenitic high manganese steel according to one example of the present invention can contain, by weight, 0.2 to 1.8% C, 8 to 30% Mn, and the remainder being Fe and other unavoidable impurities.

[0024] The reasons for limiting the chemical composition of the austenitic high manganese steel will be specifically explained below.

[0025] The carbon (C) content is 0.2 to 1.8% by weight, and preferably 0.3 to 1.5% by weight.

[0026] C is an element that stabilizes austenite, enabling the formation of an austenite structure at room temperature, thereby increasing the strength of the steel. In particular, C dissolves in austenite to increase work hardening, precipitates at grain boundaries to ensure high wear resistance, and stabilizes the austenite phase to ensure non-magnetism.

[0027] In conventional high-manganese austenitic products that utilize carbon, excessive carbon and the resulting carbide precipitation at grain boundaries reduce the toughness of the material and cause embrittlement of the steel. Therefore, the carbon content is limited to suppress carbide precipitation. However, the present invention utilizes carbides at grain boundaries to improve high-temperature strength and high-temperature wear resistance. Specifically, in the present invention, carbon forms grain boundary carbides, and the temperature range in which these carbides exist stably at grain boundaries is approximately 400 to 900°C. This temperature range corresponds to the temperature range during braking in products such as disc brakes, which require high-temperature strength and high-temperature wear resistance. The carbides present in this temperature range increase the high-temperature strength and wear resistance of the material, significantly improving the performance of products such as disc brakes.

[0028] Considering the above effects, in the present invention, C is added in an amount of 0.2 wt% or more. However, if the C content is too high, an excessive amount of carbides may be precipitated in the steel, which may reduce productivity. Therefore, in the present invention, the upper limit of the C content may be limited to 1.8 wt%.

[0029] The manganese (Mn) content is 8 to 30% by weight, and preferably 10 to 25% by weight.

[0030] Mn is an element that stabilizes austenite, and in order to ensure the austenite phase as the main structure in the present invention, 8 wt% or more of Mn may be added. However, an excessive Mn content can cause problems such as reduced corrosion resistance, difficulty in the manufacturing process, increased manufacturing costs, reduced tensile strength, and reduced work hardening, so the upper limit of the Mn content in the present invention can be limited to 30 wt%.

[0031] Furthermore, the high manganese steel according to the present invention may further contain other alloy elements selectively in addition to the alloy composition described above, and preferably may further contain one or more of Cr, Mo and W.

[0032] An example high manganese steel may further include, by weight percent, Cr, Mo, and W in a total amount of 8% or less.

[0033] The high manganese steel according to the present invention may further contain one or more of chromium (Cr), molybdenum (Mo), and tungsten (W), the total amount of which may be 8 wt% or less. Cr, Mo, and W are elements that improve corrosion resistance and have a solid solution strengthening effect. In particular, Cr, Mo, and W are elements that form carbides, which precipitate at grain boundaries and increase high-temperature strength. However, if the total amount of these elements is excessive, manufacturing costs may increase and ferrite may be formed, preventing the formation of austenite as the main structure.

[0034] The remaining component of the present invention is iron (Fe). However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to exclude them. These impurities are known to anyone skilled in normal manufacturing processes, and therefore, not all of them will be specifically mentioned in this specification.

[0035] However, it is preferable that the austenitic high manganese steel for disc brakes according to the present invention does not contain nickel (Ni). Ni is a very expensive metal and is therefore not suitable for use in brakes, which are consumables. Therefore, the austenitic high manganese steel for disc brakes according to the present invention has the advantage of being able to provide a low-cost brake material by achieving a high friction coefficient without containing nickel.

[0036] A method for manufacturing an austenitic high manganese steel according to the present invention will now be described in detail. According to the present invention, the manufacturing conditions are not particularly limited as long as they are appropriately controlled to satisfy the above-described alloy composition and microstructure. An austenitic high manganese steel according to an example of the present invention can be manufactured by reheating a slab, hot-rolling it through rough rolling and finish rolling, cooling it, and then heat-treating it at about 400 to 900°C to precipitate carbides at the austenite grain boundaries, but the manufacturing conditions are not limited thereto. For example, when the high manganese steel according to the present invention is applied to a disc brake, it can be heated to a temperature range of about 400 to 900°C during brake operation without a separate heat treatment, and carbides can be formed during this process.

[0037] The following examples of the present invention will be described. It should be noted that the following examples are merely provided to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters set forth in the claims and matters that can be reasonably inferred therefrom.

[0038] <Example>

[0039] The chemical compositions of the steels of the examples of the present invention and the comparative examples are shown in Table 1. All components other than the alloy components shown in Table 1 are the balance iron.

[0040] [Table 1]

[0041] Table 2 below shows the area fraction of room temperature austenite, the length of grain boundaries per unit area, the coefficient of friction, and the amount of wear measured for the inventive material and the comparative material.

[0042] Specifically, the length of the grain boundary is 2 The length (μm) of the austenite grain boundary per unit area was measured.

[0043] The coefficient of friction was measured by testing at a temperature of 450° C. for 24 hours through a pin-on-disk test.

[0044] The amount of wear was measured to confirm high temperature strength and abrasion resistance, and was measured by comparing the test pieces before and after the friction coefficient experiment.

[0045] [Table 2]

[0046] As shown in Tables 1 and 2, in the case of Inventive Materials 1 to 4, which satisfy the alloy composition of the present invention, the length of the grain boundary per unit area is 32 μm or more, and a relatively large number of austenite grain boundaries are formed, which makes it easy for carbide precipitation to occur at high temperatures. As a result, it was confirmed that the friction coefficient at 450°C was high at 0.5 or more, and the amount of wear was also low at 0.38 g or less, so it was confirmed that the high-temperature strength and wear resistance were also excellent.

[0047] In the case of Comparative Material 1, a steel commonly used as an existing brake material, no austenite phase was formed, but ferrite and pearlite structures were formed, and the friction coefficient at 450°C was low at 0.3. It was also confirmed that the wear amount was 5.7g, which was inferior to the inventive material in terms of high-temperature strength and wear resistance. In other words, the inventive material is superior to Comparative Material 1, a conventional brake steel, and therefore can reduce manufacturing thickness, making the inventive material more economical and superior.

[0048] In the case of Comparative Material 2, the carbon and manganese contents were too low, which caused deformation of the material, making it impossible to test the friction coefficient and measure the amount of wear.

[0049] While exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and it should be understood that various changes and modifications can be made by those skilled in the art without departing from the concept and scope of the claims set forth below.

Claims

1. In weight percent, it contains C: 0.2 to 1.8%, Mn: 8 to 30%, and the balance is Fe and other unavoidable impurities, The microstructure contains 90% or more austenite by area fraction, and the length of the austenite grain boundary is 100 μm. ２ Austenitic high manganese steel characterized in that the grain size is 1 μm or more per grain.

2. The austenitic high manganese steel according to claim 1, further comprising at least one of Cr, Mo and W.

3. 3. The austenitic high manganese steel according to claim 2, further comprising, by weight percent, Cr, Mo and W in a total amount of up to 8%.

4. 2. The austenitic high manganese steel according to claim 1, wherein the steel contains precipitates formed at the interfaces of the austenite grain boundaries in an area fraction of 5% or more.

5. The length of the austenite grain boundary is 100 μm ２ 2. The austenitic high manganese steel according to claim 1, wherein the grain size is 30 to 120 μm per grain.

6. 2. The austenitic high manganese steel according to claim 1, wherein the austenitic high manganese steel has a friction coefficient of 0.4 or more in a pin-on-disk test at 450°C for 24 hours.

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