Steel member

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]

[0001] 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. [Background technology]

[0002] Steel members such as gears are often subjected to carburizing and quenching in order to increase the surface hardness while maintaining toughness. Carburizing and quenching is a process in which 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.

[0003] A known method of carburizing and quenching is to use a large heat treatment furnace equipped with an oil quenching tank at the exit side to carburize steel members for a long time, and then perform oil quenching immediately after the carburizing. The reason for using oil as the coolant during quenching is that it allows for relatively gentle cooling compared to water, which aims to suppress distortion. However, even with oil quenching, it is difficult to solve the problem of distortion in steel members that have been carburized and quenched using the above-mentioned conventional method, and for members that require high dimensional accuracy, processes such as cutting, grinding, and polishing are required after carburizing and quenching.

[0004] As a hardening treatment after carburizing, it is possible to apply a high-frequency hardening method that hardens locally instead of hardening the entire part. However, simply applying high-frequency hardening is not enough to suppress distortion. This is due to distortion that occurs during cooling immediately after carburizing and before hardening.

[0005] As a method for solving this problem, Patent Document 1 discloses a method for cooling steel members after heat treatment in which the steel members are heated to an austenitizing temperature or higher, in which reduced pressure cooling is performed in which the atmospheric gas is reduced in pressure below atmospheric pressure for a predetermined period from the start of cooling of the steel members. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2008-45200 A Summary of the Invention [Problem to be solved by the invention]

[0007] In Patent Document 1, a step of reducing pressure and cooling is required, which may increase the process cost. The present invention aims to suppress distortion during quenching by designing the chemical composition of steel members, rather than by using manufacturing processes such as reduced pressure cooling. [Means for solving the problem]

[0008] In order to achieve the above object, a steel member according to the present invention has (1) chemical compositions of 0.20 to 0.30 mass% C, 0.30 to 0.80 mass% Si, 0.10 to 0.50 mass% Mn, and 1.50 to 2.20 mass% Cr, with the remainder being Fe and unavoidable impurities.

[0009] The steel member according to the present invention (2) has chemical compositions of 0.20-0.30 mass% C, 0.30-0.80 mass% Si, 0.10-0.50 mass% Mn, 1.50-2.20 mass% Cr, and 0.40-3.50 mass% Ni, with the remainder being Fe and unavoidable impurities.

[0010] The steel member according to the present invention (3) has chemical compositions of 0.20-0.30 mass% C, 0.30-0.80 mass% Si, 0.10-0.50 mass% Mn, 1.50-2.20 mass% Cr, and 0.15-0.45 mass% Mo, with the balance being Fe and unavoidable impurities.

[0011] The steel member according to the present invention (4) has chemical compositions of 0.20-0.30 mass% C, 0.30-0.80 mass% Si, 0.10-0.50 mass% Mn, 1.50-2.20 mass% Cr, 0.40-3.50 mass% Ni, and 0.15-0.45 mass% Mo, with the remainder being Fe and unavoidable impurities. Effect of the Invention

[0012] According to the present invention, the austenite yield strength of a steel member can be increased by chemical composition design, thereby making it possible to suppress the heat treatment distortion (plastic deformation) that inevitably occurs during carburizing and quenching of the steel member. [Brief description of the drawings]

[0013] [Figure 1] FIG. 2 is a schematic diagram of a test piece (first embodiment). [Diagram 2] FIG. 1 is a schematic diagram of a test piece (second embodiment). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The steel member of this embodiment can be widely used for parts that require toughness, surface hardness, and low distortion. Parts of this type include, for example, gears and shafts as vehicle parts. In vehicles (electric vehicles, hybrid vehicles, plug-in hybrid vehicles) that have a motor as a power unit for driving the vehicle, the use of parts with little distortion that have the steel member of this embodiment as a base material can improve quietness during vehicle driving.

[0015] (First embodiment) The steel member of this embodiment has chemical compositions 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 inevitable impurities. The chemical compositions can be determined by molten steel analysis (JIS G0320) or chemical analysis of the steel material or parts. The chemical compositions of the steel member can be adjusted in a molten steel refining process included in the steel manufacturing process. The content of each chemical component and the reasons for the limitations will be explained below.

[0016] (About C) C is an essential chemical component of the steel member. The C content is 0.20-0.30 mass%, preferably 0.22-0.26 mass%, when the entire steel member is taken as 100 mass%. C is an element that affects the hardness of the steel member, the hardenability in the core, the hot and cold forgeability, and the machinability. By setting the C content to 0.20-0.30 mass%, the hardness of the steel member can be ensured, and the workability such as machinability and forgeability can be suppressed from being impaired. If the C content is less than 0.20 mass%, the core hardness 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 mass%, the hardness of the steel material increases, thereby impairing the workability such as machinability and forgeability.

[0017] (About Si) Si is an essential chemical component of steel members. The Si content is 0.30 to 0.80 mass%, preferably 0.40 to 0.70 mass%, when the entire steel member is taken as 100 mass%. Si is an element necessary for deoxidation, and is also an element that increases the strength of steel materials, inhibits structural changes in steel materials due to fatigue, and contributes to improving fatigue life, as well as contributing to reducing heat treatment distortion that inevitably occurs during carburizing treatment. In order to obtain these effects, the Si content needs to be 0.30 mass% or more. On the other hand, if the Si content is higher than 0.80 mass%, the hardness of the steel material increases, thereby impairing processability such as machinability and forgeability, and carburizing.

[0018] (About Mn) Mn is an essential chemical component of steel members. The Mn content is 0.10 to 0.50 mass%, and preferably 0.20 to 0.40 mass%, when the entire steel member is taken as 100 mass%. Mn is an element necessary for ensuring hardenability, and 0.10 mass% or more is required. On the other hand, if the Mn content is higher than 0.50 mass%, the hardness of the steel material increases, thereby impairing processability such as machinability and forgeability.

[0019] (About Cr) Cr is an essential chemical component of steel members. The Cr content is 1.50 to 2.20 mass%, and preferably 1.70 to 1.90 mass%, when the entire steel member is taken as 100 mass%. Cr needs to be 1.50 mass% or more in order to increase the number of spheroidized carbides in the spheroidized annealed structure and to reduce the heat treatment distortion that inevitably occurs during carburizing. On the other hand, if the Cr content is higher than 2.20 mass%, the hardness of the steel increases, thereby impairing processability such as machinability and forgeability.

[0020] (Fe and inevitable impurities) Fe is a main metal of the steel member. An inevitable impurity is an impurity that is unintentionally mixed in during the steel manufacturing process and cannot be completely removed and remains. C, Si, Mn, and Cr are all essential chemical components of the steel member of this embodiment, and are therefore not inevitable impurities. The inevitable impurities of this embodiment may include Ni and Mo. Needless to say, even if Ni is included as an inevitable impurity, its content does not exceed the lower limit (0.40 mass%) shown in the second embodiment described later. Also, even if Mo is included as an inevitable impurity, its content does not exceed the lower limit (0.15 mass%) shown in the third embodiment described later. Note that P and S may be included as inevitable impurities (the same applies to the second to fourth embodiments). P is an inevitable impurity included from scrap, but it segregates at the austenite grain boundaries and reduces toughness such as impact strength and bending strength. Therefore, it is desirable to limit P to 0.030 mass% or less. S is an element that improves machinability. However, S forms nonmetallic inclusions, MnS, which reduces toughness and fatigue strength, so it is desirable to limit S content to 0.030% or less.

[0021] Second embodiment The steel member of this embodiment has chemical compositions of 0.20-0.30 mass% C, 0.30-0.80 mass% Si, 0.10-0.50 mass% Mn, 1.50-2.20 mass% Cr, 0.40-3.50 mass% Ni, with the balance being Fe and unavoidable impurities. The reasons for limiting the contents of C, Si, Mn, and Cr have been described in detail in the first embodiment, and therefore will not be described again.

[0022] (About Ni) The Ni content is 0.40 to 3.50 mass%, and preferably 0.50 to 2.50 mass%, when the entire steel member is taken as 100 mass%. Ni is an element that improves the hardenability and toughness of the steel member and contributes to reducing heat treatment distortion that inevitably occurs during carburizing. In order to achieve this effect, it is preferable to contain Ni at 0.40 mass% or more. On the other hand, if the Ni content is higher than 3.50 mass%, the hardness of the steel increases, which impairs workability such as machinability and forgeability, and increases costs.

[0023] C, Si, Mn, Cr, and Ni are all essential chemical components of the steel member of this embodiment, and are not unavoidable impurities. The unavoidable impurities of this embodiment may include Mo. Even if Mo is included as an unavoidable impurity, its content does not exceed the lower limit (0.15 mass%) shown in the third embodiment described later.

[0024] Third embodiment The steel member of this embodiment has chemical compositions of 0.20-0.30 mass% C, 0.30-0.80 mass% Si, 0.10-0.50 mass% Mn, 1.50-2.20 mass% Cr, 0.15-0.45 mass% Mo, with the balance being Fe and unavoidable impurities. The reasons for limiting the contents of C, Si, Mn, and Cr have been described in detail in the first embodiment, and therefore will not be described again.

[0025] (About Mo) The Mo content is 0.15 to 0.45 mass%, and preferably 0.25 to 0.40 mass%, when the entire steel member is taken as 100 mass%. Mo is an element that improves the hardenability of the steel member and contributes to reducing heat treatment distortion that inevitably occurs during carburizing. In order to achieve this effect, it is preferable to contain Mo at 0.15 mass% or more. On the other hand, if the Mo content is higher than 0.45 mass%, the hardness of the steel material increases, which impairs workability such as machinability and forgeability, and increases costs.

[0026] C, Si, Mn, Cr, and Mo are all essential chemical components of the steel member of this embodiment, and are not unavoidable impurities. Ni may be included as an unavoidable impurity in this embodiment. Even if Ni is included as an unavoidable impurity, its content does not exceed the lower limit (0.40 mass%) shown in the second embodiment described above.

[0027] (Fourth embodiment) The steel member of this embodiment has the following chemical components: C: 0.20-0.30 mass%, Si: 0.30-0.80 mass%, Mn: 0.10-0.50 mass%, Cr: 1.50-2.20 mass%, Ni: 0.40-3.50 mass%, Mo: 0.15-0.45 mass%, with the balance being Fe and inevitable impurities. The reasons for limiting the contents of C, Si, Mn, Cr, Ni, and Mo have been described in detail in the first to third embodiments, and therefore will not be described in detail here. C, Si, Mn, Cr, Ni, and Mo are all essential chemical components of the steel member of this embodiment, and are therefore not inevitable impurities. EXAMPLES

[0028] The present invention will now be described in detail with reference to examples. [Table 1] (First embodiment) The materials consisting of the chemical components of each sample in Table 1 were melted in a vacuum melting furnace to produce a steel ingot of 100 kg. The melted steel ingot was heated at a heating temperature of 1250 ° C for 10.8 ks, then forged into a steel bar with a diameter of 65 mm and air-cooled. Assuming rolling, a normalizing process was performed by heating at a heating temperature of 925 ° C for 3.6 ks and then air-cooling. A test piece with a key groove shown in Figure 1 was cut out from the center of this normalized material. This test piece was held at 850 ° C for 2 hr, and then quenched in oil at 100 ° C perpendicular to the oil surface. The runout of the center of the test piece was measured with a dial gauge. If the runout amount was more than 0.82 mm, it was evaluated as a comparative example, and if the runout amount was 0.82 mm or less, it was evaluated as an inventive example. In Table 1, the inventive example is indicated as the inventive steel, and the comparative example is indicated as the conventional steel. Additionally, elements contained in each sample as unavoidable impurities are indicated with "-".

[0029] (Second Example) The materials consisting of the chemical components of each sample in Table 1 were melted in a vacuum melting furnace to produce a steel ingot of 100 kg. The melted steel ingot was heated at a heating temperature of 1250 ° C for 10.8 ks, then forged into a steel bar with a diameter of 65 mm, and air-cooled. Assuming rolling, a normalizing treatment was performed by heating at a heating temperature of 925 ° C for 3.6 ks and then air-cooling. A tensile test piece shown in Figure 2 was cut out from the central part of this normalized material. The central part of the test piece was heated to 900 ° C to austenitize it, and a tensile test was performed at a tensile speed of 1 mm / s. The 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 inventive example. In addition, even if a steel component contains elements other than the essential elements in addition to the essential elements of the present invention, it is included in the scope of the present invention as long as the steel component satisfies the condition of "a run-out amount of 0.82 (mm) or less and a gamma yield stress of 134 (MPa) or more."

[0030] A comparison of the inventive steel 1 and the conventional steel 10 shows that the gamma yield stress can be improved and the deflection amount can be reduced by increasing the Si content. A comparison of the inventive steel 1 and the conventional steel 11 shows that the gamma yield stress can be improved and the deflection amount can be reduced by increasing the Cr content. A comparison of the inventive steel 1 and the inventive steel 16 shows that the gamma yield stress can be improved and the deflection amount can be reduced by appropriately adding Ni. A comparison of the inventive steel 1 and the inventive steel 21 shows that the gamma yield stress can be improved and the deflection amount can be reduced by appropriately adding Mo.

Claims

1. The chemical composition is as follows: 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%, Ni: 0.40 to 3.50 mass%, The balance of the steel member is Fe and unavoidable impurities.

2. The chemical composition is as follows: 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%, Ni: 0.40 to 3.50 mass%, Mo: 0.15 to 0.45 mass%, The balance of the steel member is Fe and unavoidable impurities.

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