JP2022148091A - steel member - Google Patents

Abstract

To inhibit strain during quenching by the chemical compositional design of a steel member, without using a production process such as vacuum-cooling.SOLUTION: A steel member comprises chemical components of C: 0.20-0.30 mass%, Si: 0.30-0.80 mass%, Mn: 0.10-0.50 mass%, Cr: 1.50-2.20 mass%, with the balance being Fe and unavoidable impurities. The steel member also may comprise Ni: 0.40-3.50 mass% and/or Mo: 0.15-0.45 mass%.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a technique for suppressing heat treatment distortion that inevitably occurs during carburizing and quenching of steel members, particularly steel members.

Steel members such as gears are often carburized and quenched to increase surface hardness while maintaining toughness. In the carburizing and quenching process, the steel member is heated to the austenitizing temperature or higher, and then carburized to increase the carbon concentration on the surface, followed by quenching to ensure the toughness of the core and increase the surface hardness. processing.

As a carburizing and quenching treatment, a method is known in which a steel member is carburized for a long period of time in a large heat treatment furnace equipped with an oil quenching tank on the exit side, and then immediately after the oil quenching. The reason why oil is used as a coolant during quenching is to suppress distortion due to relatively gentle cooling compared to water. However, even if oil quenching is performed, it is difficult to solve the problem of distortion in steel members that have been carburized and quenched by the above-mentioned conventional method. Processes such as cutting, grinding, and polishing were required later.

As the hardening treatment after the carburizing treatment, it is conceivable to apply an induction hardening method in which hardening is performed locally instead of hardening the entire part. However, simply applying induction hardening cannot sufficiently suppress the occurrence of distortion. This is due to strain that occurs during cooling immediately after carburizing and before quenching.

As a method for solving this problem, Patent Document 1 discloses a method of cooling a steel member after performing a heat treatment for raising the temperature of the steel member to a temperature equal to or higher than the austenitizing temperature. , discloses a method of performing decompression cooling in which atmospheric gas is cooled in a state of being decompressed below atmospheric pressure.

Japanese Patent Application Laid-Open No. 2008-45200

In Patent Document 1, a process for cooling under reduced pressure is required, which may increase the process cost.
An object of the present invention is to suppress distortion during quenching by designing the chemical composition of a steel member rather than by a manufacturing process such as a decompression cooling method.

In order to solve the above problems, the steel member according to the present invention has (1) C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.10% by mass. 50% by mass, Cr: 1.50 to 2.20% by mass, with the balance being Fe and unavoidable impurities.

The steel member according to the present invention has (2) C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.50% by mass, Cr: 1.0% by mass. It has a chemical composition of 50 to 2.20 mass %, Ni: 0.40 to 3.50 mass %, and the balance consists of Fe and unavoidable impurities.

The steel member according to the present invention has (3) C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.50% by mass, Cr: 1.0% by mass. It has a chemical composition of 50 to 2.20% by mass, Mo: 0.15 to 0.45% by mass, and the balance consists of Fe and unavoidable impurities.

The steel member according to the present invention has (4) C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.50% by mass, Cr: 1.0% by mass. It has chemical components of 50 to 2.20% by mass, Ni: 0.40 to 3.50% by mass, Mo: 0.15 to 0.45% by mass, and the balance is Fe and unavoidable impurities.

According to the present invention, the austenite yield strength of steel members can be increased by chemical composition design. As a result, it is possible to suppress the heat treatment strain (plastic deformation) that inevitably occurs during carburizing and quenching of the steel member.

It is a schematic diagram of a test piece (first example). It is the schematic of a test piece (2nd Example).

The steel member of this embodiment can be widely used for parts that require toughness, surface hardness, and low distortion. Such parts include, for example, gears and shafts as vehicle parts. In a vehicle (electric vehicle, hybrid vehicle, plug-in hybrid vehicle) having a motor as a power unit for running the vehicle, by using parts with less distortion using the steel member of the present embodiment as a base material, Quietness can be enhanced.

(First embodiment)
The steel member of this embodiment has C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.50% by mass, Cr: 1.50 to 2. It has a chemical composition of 20% by mass, and the balance consists of Fe and unavoidable impurities. The chemical composition can be determined by molten steel analysis (JIS G0320) or chemical analysis of steel materials and parts. The chemical composition of the steel member can be adjusted in the molten steel refining process included in the steel manufacturing process. The content of each chemical component and the reason for limitation will be described below.

(About C)
C is an essential chemical component of steel members. The content of C is 0.20 to 0.30% by mass, preferably 0.22 to 0.26% by mass, when the entire steel member is 100% by mass. C is an element that affects the hardness of the steel member, the hardenability of the core, hot and cold forgeability, and machinability. By setting the C content to 0.20 to 0.30% by mass, it is possible to ensure the hardness of the steel member and to suppress deterioration of workability such as machinability and forgeability. If the C content is less than 0.20% by mass, the hardness of the core of the steel member decreases after carburizing treatment, resulting in insufficient strength. On the other hand, if the C content is higher than 0.30% by mass, the hardness of the steel increases, impairing workability such as machinability and forgeability.

(About Si)
Si is an essential chemical component of steel members. The content of Si is 0.30 to 0.80% by mass, preferably 0.40 to 0.70% by mass, when the entire steel member is 100% by mass. Si is an element necessary for deoxidation, and is an element that increases the strength of steel materials, suppresses structural changes in steel materials due to fatigue, and contributes to improving fatigue life. It is an element that contributes to the reduction of heat treatment strain that occurs. In order to obtain these effects, the Si content must be 0.30% by mass or more. On the other hand, when the Si content is higher than 0.80% by mass, the hardness of the steel material increases, impeding workability such as machinability and forgeability, as well as carburization.

(About Mn)
Mn is an essential chemical component of steel members. The Mn content is 0.10 to 0.50% by mass, preferably 0.20 to 0.40% by mass, when the entire steel member is 100% by mass. Mn is an element necessary for ensuring hardenability, and its content is required to be 0.10% by mass or more. On the other hand, when the Mn content is higher than 0.50% by mass, the hardness of the steel increases, impairing workability such as machinability and forgeability.

(About Cr)
Cr is an essential chemical component of steel members. The Cr content is 1.50 to 2.20% by mass, preferably 1.70 to 1.90% by mass, when the entire steel member is 100% by mass. Cr is required to be 1.50% by mass or more in order to increase spheroidized carbides in the spheroidized annealed structure and to reduce heat treatment strain that inevitably occurs during carburizing treatment. On the other hand, if the Cr content is higher than 2.20% by mass, the hardness of the steel increases, impairing workability such as machinability and forgeability.

(Regarding Fe and inevitable impurities)
Fe is the main metal in steel members. Unavoidable impurities are impurities that are unintentionally mixed in during the steel manufacturing process and remain unremoved. C, Si, Mn, and Cr are all essential chemical components of the steel member of the present embodiment, and are not inevitable impurities. The unavoidable impurities of this embodiment may include Ni and Mo. Needless to say, even when Ni is contained as an unavoidable impurity, the content does not exceed the lower limit (0.40% by mass) shown in the second embodiment described later. Moreover, even when Mo is contained as an unavoidable impurity, the content does not exceed the lower limit (0.15% by mass) shown in the third embodiment described later. Incidentally, P and S may be contained as unavoidable impurities (the same applies to the second to fourth embodiments). P, which is an unavoidable impurity contained in scrap, segregates at austenite grain boundaries and lowers toughness such as impact strength and bending strength. Therefore, it is desirable to limit P to 0.030% by mass or less. S is an element that improves machinability. However, S forms non-metallic inclusions MnS to reduce toughness and fatigue strength. Therefore, it is desirable to limit S to 0.030% or less.

(Second embodiment)
The steel member of this embodiment has C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.50% by mass, Cr: 1.50 to 2.20% by mass, Ni: 0.40 to 3.50% by mass, and the balance being Fe and unavoidable impurities. The reason for limiting the contents of C, Si, Mn, and Cr has been explained in detail in the first embodiment, so the explanation will not be repeated.

(About Ni)
The content of Ni is 0.40 to 3.50% by mass, preferably 0.50 to 2.50% by mass, when the entire steel member is 100% by mass. Ni is an element that improves the hardenability and toughness of steel members and contributes to reducing heat treatment strain that inevitably occurs during carburizing. In order to exhibit this effect, it is preferable to contain 0.40% by mass or more of Ni. On the other hand, when the Ni content is higher than 3.50% by mass, the hardness of the steel material increases, impeding workability such as machinability and forgeability, and increasing the cost.

C, Si, Mn, Cr, and Ni are all essential chemical components of the steel member of the present embodiment, and are not inevitable impurities. The unavoidable impurities of this embodiment may contain Mo. Even when Mo is contained as an unavoidable impurity, the content does not exceed the lower limit (0.15% by mass) shown in the third embodiment described later.

(Third embodiment)
The steel member of this embodiment has C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.50% by mass, Cr: 1.50 to 2.20% by mass, Mo: 0.15 to 0.45% by mass, with the balance being Fe and unavoidable impurities. The reason for limiting the contents of C, Si, Mn, and Cr has been explained in detail in the first embodiment, so the explanation will not be repeated.

(About Mo)
The content of Mo is 0.15 to 0.45% by mass, preferably 0.25 to 0.40% by mass, when the entire steel member is 100% by mass. Mo is an element that improves the hardenability of steel members and contributes to reducing heat treatment strain that inevitably occurs during carburizing. In order to express this effect, it is preferable to contain 0.15% by mass or more of Mo. On the other hand, when the Mo content is higher than 0.45% by mass, the hardness of the steel material increases, impeding workability such as machinability and forgeability, and increasing the cost.

C, Si, Mn, Cr, and Mo are all essential chemical components of the steel member of the present embodiment, and are not inevitable impurities. The unavoidable impurities of the present embodiment may contain Ni. Even when Ni is contained as an unavoidable impurity, the content does not exceed the lower limit (0.40% by mass) shown in the second embodiment.

(Fourth embodiment)
The steel member of this embodiment has C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.50% by mass, Cr: 1.50 to 2.20% by mass, Ni: 0.40 to 3.50% by mass, Mo: 0.15 to 0.45% by mass, and the balance consists of Fe and unavoidable impurities. The reasons for limiting the contents of C, Si, Mn, Cr, Ni, and Mo have been explained in detail in the first to third embodiments, so a detailed explanation will be omitted. C, Si, Mn, Cr, Ni, and Mo are all essential chemical components of the steel member of the present embodiment, and are not inevitable impurities.

（第１実施例）
表１の各試料の化学成分からなる素材を真空溶解炉にて溶製し、１００（ｋｇ）の鋼塊を作製した。溶製した鋼塊を１２５０（℃）の加熱温度で１０．８（ｋｓ）加熱した後に直径６５（ｍｍ）の棒鋼に鍛伸し、空冷した。圧延を想定して、９２５（℃）の加熱温度で３．６（ｋｓ）加熱した後に空冷する焼ならし処理を実施した。この焼ならし材の中周部より図１に示すキー溝付き試験片を切り出した。この試験片を８５０（℃）で２（ｈｒ）保定後、油面に対して垂直な状態で１００（℃）の油で焼入れを行った。試験片中部の振れをダイヤルゲージにて測定した。振れ量が０．８２（ｍｍ）超の場合には比較例、振れ量が０．８２（ｍｍ）以下の場合には発明例と評価した。なお、表１では発明例を発明鋼、比較例を従来鋼と表記した。また、各試料に不可避不純物として含まれる元素は、「－」と表記した。 The present invention will be specifically described with reference to examples.

(First embodiment)
A raw material having the chemical composition of each sample shown in Table 1 was melted in a vacuum melting furnace to produce a steel ingot of 100 (kg). After heating the melted steel ingot for 10.8 (ks) at a heating temperature of 1250 (°C), it was forged into a steel bar with a diameter of 65 (mm) and air-cooled. Assuming rolling, a normalizing treatment was performed by heating for 3.6 (ks) at a heating temperature of 925 (°C) and then air cooling. A test piece with a keyway shown in FIG. 1 was cut out from the middle circumference of this normalized material. After holding this test piece at 850° C. for 2 hours, it was quenched in oil at 100° C. in a state perpendicular to the oil surface. The run-out of the middle portion of the test piece was measured with a dial gauge. When the deflection amount exceeded 0.82 (mm), it was evaluated as a comparative example, and when the deflection amount was 0.82 (mm) or less, it was evaluated as an invention example. In Table 1, invention examples are indicated as invention steel, and comparative examples are indicated as conventional steel. An element contained as an unavoidable impurity in each sample is indicated by "-".

(Second embodiment)
A raw material having the chemical composition of each sample shown in Table 1 was melted in a vacuum melting furnace to produce a steel ingot of 100 (kg). After heating the melted steel ingot for 10.8 (ks) at a heating temperature of 1250 (°C), it was forged into a steel bar with a diameter of 65 (mm) and air-cooled. Assuming rolling, a normalizing treatment was performed by heating for 3.6 (ks) at a heating temperature of 925 (°C) and then air cooling. A tensile test piece shown in FIG. 2 was cut out from the middle peripheral portion of this normalized material. After heating the central portion of the test piece to 900° C. to austenitize it, a tensile test was performed at a tensile speed of 1 (mm/s). A yield stress (γ yield stress) was obtained from this stress-strain curve. When the γ yield stress was less than 134 (MPa), it was evaluated as a comparative example, and when the γ yield stress was 134 (MPa) or more, it was evaluated as an invention example. In addition to the essential elements of the present invention, even for steel members containing elements other than essential elements, the condition that "the amount of deflection is 0.82 (mm) or less and the γ yield stress is 134 (MPa) or more" is satisfied. Any satisfactory steel member is within the scope of the present invention.

By comparing invention steel 1 and conventional steel 10, it was found that an increase in the γ yield stress and a decrease in deflection can be achieved by increasing the Si content. By comparing invention steel 1 and conventional steel 11, it was found that an increase in the γ yield stress and a decrease in deflection can be achieved by increasing the Cr content. By comparing invention steel 1 and invention steel 16, it was found that the appropriate addition of Ni can improve the γ yield stress and reduce the amount of deflection. By comparing invention steel 1 and invention steel 21, it was found that by appropriately adding Mo, an improvement in the γ yield stress and a reduction in the amount of deflection can be realized.

Claims (4)

C: 0.20 to 0.30 mass%, Si: 0.30 to 0.80 mass%, Mn: 0.10 to 0.50 mass%, Cr: 1.50 to 2.20 mass% chemical components has
A steel member whose balance consists of Fe and unavoidable impurities. C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.50% by mass, Cr: 1.50 to 2.20% by mass, Ni: Having a chemical composition of 0.40 to 3.50% by mass,
A steel member whose balance is Fe and unavoidable impurities. C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.50% by mass, Cr: 1.50 to 2.20% by mass, Mo: having a chemical composition of 0.15 to 0.45% by mass,
A steel member whose balance consists of Fe and unavoidable impurities. C: 0.20 to 0.30% by mass, Si: 0.30 to 0.80% by mass, Mn: 0.10 to 0.50% by mass, Cr: 1.50 to 2.20% by mass, Ni: 0.40 to 3.50% by mass, Mo: having a chemical composition of 0.15 to 0.45% by mass,
A steel member whose balance is Fe and unavoidable impurities.

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