JP7680863B2 - steel parts - Google Patents

Description

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

Steel components such as gears are often carburized and quenched to increase the surface hardness while maintaining toughness. Carburized quenching is a process in which the steel component is heated to above the austenitizing temperature 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.

A known method of carburizing and quenching is to use a large heat treatment furnace equipped with an oil quenching tank at the outlet side to carburize steel members for a long period of time, followed immediately by oil quenching. 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 eliminate 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.

As a hardening process after carburizing, it is possible to apply high-frequency hardening, which hardens the part locally rather than hardening the entire part. However, simply applying high-frequency hardening is not enough to sufficiently suppress the occurrence of distortion. This is due to distortion that occurs during cooling immediately after carburizing and before hardening.

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 the 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.

JP 2008-45200 A

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.

To solve the above problems, the steel member according to the present invention has the following chemical compositions: (1) 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 according to the present invention (2) has the following chemical compositions: 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%, with the balance being Fe and unavoidable impurities.

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

The steel member according to the present invention (4) has the following chemical compositions: 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 unavoidable impurities.

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

FIG. 2 is a schematic diagram of a test piece (first embodiment). FIG. 4 is a schematic diagram of a test piece (second embodiment);

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, vehicle parts such as gears and shafts. In vehicles that have a motor as the power unit for driving the vehicle (electric vehicles, hybrid vehicles, plug-in hybrid vehicles), parts with low distortion that use the steel member of this embodiment as a base material can be used to improve quietness while the vehicle is running.

First Embodiment
The steel member of this embodiment has chemical components 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 balance being Fe and inevitable impurities. The chemical components can be determined by molten steel analysis (JIS G0320) or chemical analysis of the steel material or parts. The chemical components 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 reasons for the limitations will be described below.

(Regarding 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.

(Regarding 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 material, suppresses structural changes in steel material due to fatigue, contributes to improving fatigue life, and contributes 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 inhibiting processability such as machinability and forgeability, and carburizing.

(Regarding Mn)
Mn is an essential chemical component of steel members. The Mn content is 0.10 to 0.50 mass%, 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.

(Regarding Cr)
Cr is an essential chemical component of steel members. The Cr content is 1.50 to 2.20 mass%, preferably 1.70 to 1.90 mass%, when the entire steel member is taken as 100 mass%. Cr is required 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.

(Regarding Fe and inevitable impurities)
Fe is the 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 therefore are 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 to 0.030% or less.

Second 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%, Ni: 0.40-3.50 mass%, and the balance is 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.

(Regarding Ni)
The Ni content is 0.40 to 3.50 mass%, 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 the 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 material increases, which impairs workability such as machinability and forgeability, and increases costs.

C, Si, Mn, Cr, and Ni are all essential chemical components of the steel member of this embodiment, and are therefore not unavoidable impurities. In this embodiment, the unavoidable impurities 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.

Third Embodiment
The steel member of this embodiment has chemical compositions of 0.20 to 0.30 mass% C, 0.30 to 0.80 mass% Si, 0.10 to 0.50 mass% Mn, 1.50 to 2.20 mass% Cr, 0.15 to 0.45 mass% Mo, and the balance being Fe and inevitable 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.

(About Mo)
The content of Mo is 0.15 to 0.45 mass%, 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 the 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 increases, which impairs workability such as machinability and forgeability, and increases costs.

C, Si, Mn, Cr, and Mo are all essential chemical components of the steel member of this embodiment, and are therefore not unavoidable impurities. In this embodiment, the unavoidable impurities may include Ni. 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.

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, so a detailed description will be omitted. 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.

The present invention will now be described in detail with reference to examples.
(First embodiment)
A material consisting of the chemical components of each sample in Table 1 was 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 in which the material was heated at a heating temperature of 925 ° C. for 3.6 ks and then air-cooled. A test piece with a key groove as shown in FIG. 1 was cut out from the middle periphery of this normalized material. This test piece was held at 850 ° C. for 2 hrs, and then quenched in oil at 100 ° C. while perpendicular to the oil surface. The runout in the middle of the test piece was measured with a dial gauge. When the runout amount was more than 0.82 mm, it was evaluated as a comparative example, and when the runout amount was 0.82 mm or less, it was evaluated as an inventive example. In Table 1, the inventive example was represented as an inventive steel, and the comparative example was represented as a conventional steel. Additionally, elements contained as unavoidable impurities in each sample are indicated with "-".

Second Example
A material consisting of the chemical components of each sample in Table 1 was 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 in which the material was heated at a heating temperature of 925 (°C) for 3.6 (ks) and then air-cooled. 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 and austenitized, and then 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 member 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 member satisfies the condition of "a run-out amount of 0.82 (mm) or less and a γ yield stress of 134 (MPa) or more."

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 (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%,
The balance of the steel member is Fe and unavoidable impurities. 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.