# US 11,898,231 B2 · Steel for gear and method for manufacturing gear using the same

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- Number: US 11,898,231 B2
- Document ID: US11898231B2
- Status: Granted
- Kind: B2
- Application number: 17/234,428
- Filed: 2021-04-19
- Published: 2022-03-24
- Granted: 2024-02-13
- Earliest claimed: 2020-09-21
- Assignees: Hyundai Motor Company; Kia Corporation; Hyundai Steel Company
- Applicants: Hyundai Motor Company; Kia Corporation; Hyundai Steel Company
- Inventors: In Beom Lee; Min Woo Kang; Min Woo Kang; Dong Hwi Kim; Chang Won Kang; Myung Sik Kim; Sung Min Hong; Gung Seung Nam; Sang Won Lee
- Web page: https://patentgrep.com/patent/US11898231B2

## CPC

- C22C 38/48: with niobium or tantalum
- C21D 7/13: by hot working
- C21D 8/00: Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment (hardening articles or materials formed by forging or rolling with no further heating beyond that required for the formation C21D1/02)
- C21D 9/32: for gear wheels, worm wheels, or the like
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- C22C 38/44: with molybdenum or tungsten
- C22C 38/46: with vanadium
- C23C 8/02: Pretreatment of the material to be coated (C23C8/04 takes precedence)
- C23C 8/22: of ferrous surfaces
- C22C: ALLOYS (treatment of alloys C21D, C22F)
- C21D: MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING (surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by this subclass C23F17/00; unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid materials C30B)
- C23C: COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL (making metal-coated products by extrusion B21C23/22; covering with metal by connecting pre-existing layers to articles, see the relevant places, e.g. B21D39/00, B23K; metallising of glass C03C; metallising mortars, concrete, artificial stone, ceramics or natural stone C04B41/00; enamelling of, or applying a vitreous layer to, metals C23D; treating metal surfaces or coating of metals by electrolysis or electrophoresis C25D; single-crystal film growth C30B; by metallising textiles D06M11/83; decorating textiles by locally metallising D06Q1/04)

## Abstract

A steel for a gear includes, based on a total weight of the steel: C: 0.10-0.30 wt %, Si: 0.60-0.80 wt %, Mn: 0.25-0.75 wt %, Cr: 1.80-2.20 wt %, Ni: 0.50-1.50 wt %, Mo: 0.20-0.40 wt %, Nb: 0.025-0.050 wt %, V: 0.030-0.050 wt %, and a balance of Fe and inevitable impurities, wherein contents of Nb and V satisfy the following <Relationship Formula 1>: 0.055<[Nb]+[V]<0.100 <Relationship Formula 1>, wherein [Nb] represents a content of Nb and [V] represents a content of V.

## Claims

### Claim 1

A method for manufacturing a gear, the method comprising:
a molten metal preparing step of preparing a molten metal consisting of, based on a total weight thereof: C: 0.10-0.30 wt %; Si: 0.60-0.80 wt %; Mn: 0.25-0.75 wt %; Cr: 1.80-2.20 wt %; Ni: 0.50-1.50 wt %; Mo: 0.20-0.40 wt %; Nb: 0.025-0.050 wt %; V: 0.030-0.050 wt %; and a balance of Fe and inevitable impurities, wherein contents of Nb and V satisfy the following <Relationship Formula 1>:
0.055<[Nb]+[V]<0.100 <Relationship Formula 1>,
wherein [Nb] represents a content of Nb and [V] represents a content of V;
a pre-rolling heat treatment step of casting molten metal and then thermally treating the cast molten metal;
a rolled steel rolling step of rolling the thermally treated cast molten metal into a rolled steel;
a forged steel forging step of forging the thermally treated, rolled steel into a planetary gear;
a forged steel heat treatment step of thermally treating the forged steel;
an article processing step of processing the forged steel into a final article; and
a carburizing heat treatment step of carburizing the final article,
wherein the final article after the carburizing heat treatment step has a MX precipitate formed at a fraction of 0.03-0.07% therein, the MX precipitate being 150 nm or less in size and amounting to 50 or more per 100 μm2.

### Claim 2 (depends on 1)

The method of claim 1, wherein the heat treatment temperature in the pre-rolling heat treatment step is maintained at or below a liquidus curve of the cast steel.

### Claim 3 (depends on 2)

The method of claim 2, wherein the heat treatment temperature in the pre-rolling heat treatment step is between 1180° C. and 1430° C.

### Claim 4 (depends on 1)

The method of claim 1, wherein the forged steel heat treatment step is carried out in an ISO heat treatment condition.

### Claim 5 (depends on 1)

The method of claim 1, wherein the carburizing heat treatment step is carried out in the following conditions:
heat treatment temperature of 850° C. and 940° C., carbon potential (C.P) of 0.7-1.0, and heat treatment duration: 100 minutes.

### Claim 6 (depends on 1)

The method of claim 1, wherein the final article after the carburizing heat treatment step has a pitting area of less than 12 mm2 as measured by a fatigue test (SAE J 1619).

### Claim 7 (depends on 1)

The method of claim 1, wherein the final article after the carburizing heat treatment step exhibits 4,000 cycles of tooth bending fatigue testing (SAE J 1619).

### Claim 8 (depends on 1)

The method of claim 1, wherein the MX precipitate is at least one of a Nb-based carbide, a Nb-based nitride, a V-based carbide, a V-based nitride, a Nb+V-based carbide, and a Nb+V-based nitride.