JP-2026075483-A - Case-hardened steel with excellent crack resistance

Abstract

[Problem] To provide a case-hardening steel that ensures hardenability while also exhibiting excellent cold workability, particularly crack resistance. [Solution] A case-hardening steel comprising, by mass%, C: 0.15-0.30%, Si: 0.05-1.00%, Mn: less than 0.10-0.50%, Cr: 1.3-2.5%, the remainder being Fe and unavoidable impurities, wherein the P content of the unavoidable impurities is 0.030% or less, S content is 0.025% or less, and the ferrite band rating of the cross-sectional structure parallel to the hot-rolling direction is 1 to 3. [Selection Diagram] Figure 2

Inventors

• 栗田 俊
• 美谷 章生

Assignees

• 山陽特殊製鋼株式会社

Dates

Publication Date

20260508

Application Date

20241022

Claims (5)

1. In mass percent, C: 0.15-0.30%, Si: 0.05-1.00%, Mn: 0.10 to less than 0.50% Cr: 1.3-2.5%, The remainder consists of Fe and unavoidable impurities. The unavoidable impurities are P: 0.030% or less, S: 0.025% or less. Case-hardening steel with a ferrite band rating of 1 to 3 in the cross-sectional microstructure parallel to the hot-rolling direction.
2. In mass percent, It contains the following essential components: C: 0.15-0.30%, Si: 0.05-1.00%, Mn: less than 0.10-0.50%, Cr: 1.3-2.5%. Furthermore, as additional components, it contains at least one or more of the following: Ni: 0.5% or less, Mo: 0.5% or less, Al: 0.020-0.050%, Ti: 0.20% or less, Nb: 0.10% or less, B: 0.0050% or less, N: 0.0040-0.0300%. The remainder consists of Fe and unavoidable impurities. The unavoidable impurities are P: 0.030% or less, S: 0.025% or less. Case-hardening steel with a ferrite band rating of 1 to 3 in the cross-sectional microstructure parallel to the hot-rolling direction.
3. The case-hardening steel according to claim 1 or 2, characterized in that the ratio A4 /A2 of the ferrite + pearlite fraction A4 (area ratio) at position D/4 (middle circumference) to the ferrite + pearlite fraction A2 (area ratio) at position D/ 2 (center) is 0.80 to 1.10.
4. A case-hardening steel according to claim 1 or claim 2, satisfying formula 1 (0.6 ≤ 0.59Cr + 0.26Si - 0.81Mn ≤ 1.4).
5. The ratio A4/A2 of the ferrite + pearlite fraction A4 (area ratio) at position D/4 (mid-circumferential region) to the ferrite + pearlite fraction A2 (area ratio) at position D / 2 (center) is 0.80 to 1.10. Furthermore, it must satisfy equation 1 (0.6 ≤ 0.59Cr + 0.26Si - 0.81Mn ≤ 1.4), The surface hardening steel according to claim 1 or claim 2, characterized by the above.

Description

This invention relates to a case-hardening steel with excellent cold workability and particularly excellent crack resistance, suitable for use as a material for mechanical parts such as gears and shafts in transportation equipment such as automobiles and industrial machinery such as construction machinery. Conventionally, as a case-hardening steel with excellent cold workability, the composition by mass% was as follows: C: 0.1-0.3%, Si: 0.01-0.15%, Mn: 0.2-0.65%, S: 0.005-0.15%, Cr: 0.4-1.25%, B: 0.0005-0.005%, Al: 0.015-0.05%, Nb: 0.002-0.035%, N: less than 0.005 (including 0%), Ti contained in a range of 0.015-3.4N + 0.02% depending on the N content, P: 0.025 or less (including 0%), O: 0.0025 or less (including 0%). A case-hardening steel with excellent cold workability and low carburizing strain properties has been proposed, characterized by the following features: the remainder consisting of iron and unavoidable impurities; the amount of AlN precipitates limited to 0.005% or less; the matrix containing at least 20 Nb precipitates, Ti precipitates, or precipitates with a composite composition of Nb and Ti, each with a diameter of 0.2 μm or less; the bainite microstructure fraction limited to 15% or less; and a ferrite band score of 1 to 5 in the microstructure of a cross-section parallel to the hot-rolling direction (see Patent Document 1). Furthermore, a steel for cold-working machine structures has been proposed, characterized by containing C: 0.3-0.6%, Si: 0.05-0.5%, Mn: 0.2-1.7%, P: greater than 0-0.03%, S: 0.001-0.05%, Al: 0.01-0.1%, and N: 0-0.015%, with the remainder being iron and unavoidable impurities; the steel's microstructure having proterite and pearlite, with a total area ratio of proterite and pearlite to the entire microstructure being 90% or more, a proterite area ratio to the entire microstructure being (1 - 1.25 × [C%]) × 80% or more, and an average block size of the pearlite being 25 μm or less (see Patent Document 2). Japanese Patent Application Publication No. 11-335777Japanese Patent Publication No. 2018-003106 Figure 1 is an explanatory diagram of a micrograph of a metal structure showing a quantitative evaluation of the degree of banding in the ferrite bands.Figure 2 is an optical microscope image of development steel 5, which has a ferrite band rating of 1.Figure 3 is an optical microscope image of comparative steel 3, which has a ferrite band rating of 6.Figure 4 is a schematic diagram illustrating the position of the D/4 and D/2 cross-sections parallel to the rolling direction, using the shape of a steel bar as an example. Prior to describing embodiments of the present invention, the reasons for defining each component of the case-hardening steel of the present invention, the evaluation value of the ferrite band, the ratio of the area ratio of A4 / A2 , and the reasons for defining Formula 1 will be explained. The remainder of the components of the case-hardening steel consists of Fe and unavoidable impurities. Also, the percentages in the components refer to mass percentages. C: 0.15-0.30% C is a component that increases the hardness of the material, and if the C content is less than 0.15%, the strength after cold forging will be insufficient. On the other hand, since increased material hardness reduces cold workability, the C content should be 0.30% or less, preferably 0.25% or less. Si: 0.05-1.00% Si is useful for deoxidation and also increases the hardness of the material. If the Si content is less than 0.05%, deoxidation may be insufficient. Therefore, the Si content should be 0.05% or more. Preferably, the Si content is 0.30% or more. On the other hand, since Si increases the hardness of the material, it reduces cold workability, so the Si content should be 1.00% or less. Preferably, the Si content is 0.80% or less. Mn: 0.10 to less than 0.50% Since Mn is a component that forms ferrite bands, if it is less than 0.10%, the hardenability will be insufficient. Therefore, the Mn content should be 0.10% or more. Preferably, the Mn content is 0.15% or more. On the other hand, if the Mn content is excessive, the cold workability will decrease, so the Mn content should be less than 0.50%. Preferably, the Mn content is 0.40% or less. Cr: 1.3-2.5% Cr is a component that increases the hardness of the material. If the amount of Cr is too low, the hardenability and strength will decrease. Therefore, the amount of Cr should be 1.3% or more. Preferably, the amount of Cr should be 1.5% or more. On the other hand, if the hardness of the material increases too much, the cold workability will decrease, so the amount of Cr should be 2.5% or less. Preferably, the amount of Cr should be 2.2% or less. P: 0.030% or less. P is an unavoidable impurity. Excessive P increases deformation resistance during cold working, thus reducing cold workability. Therefore, P should be kept below 0.030%. S: 0.030% or less. S is an unavoidable impurity. Excessive S forms MnS, which degrades the composition when