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JP-2026514472-A - Low hardenability and narrow band width Mn-Cr round steel for gears and method for manufacturing the same

JP2026514472AJP 2026514472 AJP2026514472 AJP 2026514472AJP-2026514472-A

Abstract

A round steel bar for Mn-Cr gears with low hardenability and narrow band width, and a method for manufacturing the same, wherein the mass percentages of the components are: C 0.15-0.18%, Si 0.040-0.10%, Mn 1.10-1.40%, P≦0.02%, S 0.015-0.030%, Cr 1.00-1.20%, Mo 0.006-0.02%, Ni 0.020-0.10%, Al 0.020-0.050%, N 0.007-0.015%, Al/N≧2.0, Nb 0.004-0.03%, [O]≦0.0015%, with the remainder being Fe and other unavoidable impurities. The aforementioned Mn-Cr round steel for gears satisfies the requirements of J5=33-36HRC, J9=25-28HRC, J15=20-24HRC, and hardenability band width before J9 ≤3HRC, achieving the objective of reducing heat treatment deformation, meeting the performance requirements for gear steel materials in application scenarios such as new energy vehicles, and improving the stability and safety of steel performance for new energy vehicles.

Inventors

  • 王 強
  • 趙 四 新
  • 高 加 強
  • 余 大 江
  • 王 成 全
  • 章 軍
  • 王 維

Assignees

  • 宝山鋼鉄股▲分▼有限公司

Dates

Publication Date
20260511
Application Date
20240422
Priority Date
20230423

Claims (13)

  1. A round steel alloy for gears with low hardenability and narrow band width, whose chemical composition by mass percentage is as follows: C: 0.15-0.18%, Si: 0.040-0.10%, Mn: 1.10-1.40%, P ≤ 0.02%, S: 0.015-0.030%, Cr: 1.00-1.20%, Mo: 0.006-0.02%, Ni: 0.020-0.10%, Al: 0.02% The composition is 0–0.050%, N: 0.007–0.015%, Al/N≧2.0, Nb: 0.004–0.03%, [O]≦0.0015%, with the remainder being Fe and other unavoidable impurities. Furthermore, the total martensite region length b obtained at the end of a face-hardened test specimen made from round steel for gears is in the range of 0.70≦b≦1.16, where b=0.22h-0.45. h=(6.9[Mn] 2 +3.2[Si]+22.6[Cr] 2 +23.1[Mo]+(13.0[Cr]+2.5[Mo]+9.7)[Ni]+2.5)[C], At the same time, the hardenability coefficient k satisfies 1.18 ≤ k ≤ 1.65. k = 0.54 * [C] * (-1.12 + 5.1 * [Mn])(1 + 0.7 * [Si]) * (1 + 2.16 * [Cr]), a round steel for Mn-Cr gears with low hardenability and narrow band width.
  2. The low hardenability, narrow bandwidth Mn-Cr round steel for gears according to claim 1, characterized in that the remainder of its chemical composition consists of Fe and other unavoidable impurities.
  3. The low hardenability and narrow bandwidth Mn-Cr round steel for gears according to claim 1 or 2, characterized in that the aforementioned unavoidable impurities contain Ti ≤ 0.003%, B ≤ 0.0004%, and Ca ≤ 0.003%.
  4. A round steel bar for Mn-Cr gears with low hardenability and narrow band width, characterized in that the harmful elements in the composition are As ≤ 0.02%, Sn ≤ 0.02%, Pb ≤ 0.012%, Sb ≤ 0.012%, and Bi ≤ 0.012%, and the sum of the harmful elements is ≤ 0.05%, as described in claim 1, 2, or 3.
  5. The low hardenability and narrow band width Mn-Cr round steel for gears according to any one of claims 1 to 4, characterized in that the microstructure of the round steel for gears in the hot-rolled state is ferrite + pearlite.
  6. The measured hardenability values of the round steel for gears are J5 = 33 to 36 HRC, J9 = 25 to 28 HRC, J15 = 20 to 24 HRC, and the hardenability band width before J9 is ≤ 3 HRC, characterized in that it is a low-hardenability, narrow-bandwidth Mn-Cr round steel for gears according to any one of claims 1 to 5.
  7. The mechanical properties of the round steel for gears described above are: yield strength ≥ 800 MPa, tensile strength ≥ 1100 MPa, elongation ≥ 12%, cross-sectional shrinkage ≥ 50%, impact work KU2 ≥ 90 J, and grain size of grade 6 or higher, as described in any one of claims 1 to 6, characterized by low hardenability and narrow band width Mn-Cr round steel for gears.
  8. The low-hardenability, narrow-bandwidth Mn-Cr round steel for gears according to claim 6, characterized in that the hardenability band width before J9 satisfies 1 HRC ≤ J9 ≤ 3 HRC.
  9. The aforementioned round steel for gears has a yield strength of 800 to 820 MPa, a tensile strength of 1100 to 1150 MPa, an elongation of 12% to 19%, a cross-sectional shrinkage of 50% to 65%, an impact work KU2 of 90 to 100 J, and a grain size of 7.0 to 9.0, as described in claim 7, characterized by low hardenability and narrow band width Mn-Cr round steel for gears.
  10. The low-hardenability, narrow-bandwidth Mn-Cr round steel for gears according to claim 1, characterized in that b satisfies 0.80 ≤ b ≤ 1.15, and/or k satisfies 1.25 ≤ k ≤ 1.55.
  11. A method for manufacturing Mn-Cr round steel for gears with low hardenability and narrow band width according to any one of claims 1 to 10, (1) Smelting and casting: Smelting and casting according to the components of claim 1, 2, 3, or 4 to form an ingot. (2) Heating: The preheating section temperature is controlled to 840-880°C, the heating section temperature to 1120-1175°C, the soaking section temperature to 1150-1230°C, and the total heating time in the heating furnace is 4 hours or more. (3) Rolling The rolling start temperature is controlled to 1100-1170°C, and the rolling end temperature is controlled to 950-1000°C. (4) Quenching + Tempering A manufacturing method characterized by including the steps of: primary quenching at a temperature of 850 to 910°C, holding time of 30 to 60 min, and cooling to room temperature by oil or water cooling; secondary quenching at a temperature of 800 to 830°C, holding time of 30 to 60 min, and cooling to room temperature by oil or water cooling; and tempering at a temperature of 150 to 200°C, holding time of 60 to 150 min, and cooling to room temperature by air cooling after tempering.
  12. The smelting process employs an electric furnace or converter, further involving LF refining and VD vacuum degassing. In the LF refining process, the refining time is 50 min or more, the FeO content in the slag is less than 1.0 wt%, and the white slag time is 20 min or more. In the VD vacuum degassing process, the vacuum level is controlled to 66.7 Pa or less, the high vacuum time to 20 min or more, the standing time to 10 min or more, and the VD completion temperature is controlled to 1550°C to 1580°C. Nitrogen enhancement using bottom-blowing nitrogen gas is employed throughout the entire VD vacuum degassing process, as described in claim 11.
  13. The method for producing low-hardenability, narrow-bandwidth Mn-Cr round steel for gears, as described in claim 11, is characterized by employing continuous casting, controlling the frequency of crystallizer electromagnetic stirring to 2.0–3.0 Hz and the current to 150–300 A, controlling the frequency of terminal electromagnetic stirring to 8.0–8.5 Hz and the current to 300–350 A, controlling the molten steel superheating to 22–38°C, and controlling the continuous casting and drawing speed to between 0.62 and 0.68 m/min.

Description

This invention relates to the field of gear steel manufacturing, and more particularly to Mn-Cr round steel for gears with low hardenability and narrow band width, and a method for manufacturing the same. Gear steel is one of the most in-demand and important materials among special alloy steels used in the automotive, railway, marine, and construction machinery sectors, and is a core material for manufacturing safety-enhancing components. With the advancement of industrial technology, gear steel is evolving in directions such as high performance, long service life, stable gear operation, low noise, safety, low cost, ease of processing, and versatility. Mn-Cr gear steel is a high-standard series of steels for passenger car gears. The steel grades in this series have good hardenability; taking the most widely used 20MnCr5 as an example, its hardenability can reach J5mm = 36-45 HRC, J9mm = 34-40 HRC, and J15mm = 28-34 HRC. The magnitude of the hardenability value reflects the core hardness of gears of different sizes, and the hardenability band width significantly affects the heat treatment deformation of the gears. While the hardenability band width of high-quality gear steels overseas can be controlled within 4 HRC, advanced domestic standards can reach 4-6 HRC, indicating a certain gap compared to overseas control levels. Gear steel with low hardenability and narrow band width is easy to forge, has good machinability, and exhibits minimal heat treatment deformation, making it advantageous for the processing and manufacturing of specific parts for users. Currently, demand for gear steel with low hardenability and narrow band width is increasing, and the high hardenability of 20MnCr5 can no longer meet the specific requirements of users. The control of hardenability and hardenability band width in gear steel primarily depends on its chemical composition and uniformity. The internal control range of the steel is determined based on the influence of carbon and alloying elements on the hardenability point hardness values. For example, Chinese patent application CN108193029A discloses a method for controlling the narrow hardenability band width of gear steel. It optimizes the hardenability calculation formula using production data, obtains calculation parameters suitable for the gear steel grade, and then calculates the upper and lower limits of the elements C, Si, Mn, Cr, Ni, Mo, Ti, and B that affect hardenability based on the optimized formula. It requires that the hardenability band width for J5, J9, J11, and J13 be less than 3.5 HRC, and the resulting component ranges are defined as △C ≤ ±0.01% and △Si ≤ ±0.02%. By comparing the following ranges: %, △Mn≦±0.02%, △Cr≦±0.01%, △Ni≦±0.01%, △Mo≦±0.01%, △Ti≦±0.01%, △Al≦±0.01%, △B≦±0.005%, and △S≦±0.002%, the narrowest control range was ultimately determined as the component control range for gear steel with a narrow hardenability band width requirement. Casting was performed at a low superheat of 10-20°C, and by ensuring that the heating temperature fluctuation of the continuously cast billet was ±7°C or less and the holding time fluctuation was ±5 min or less, gear steel bars with a hardenability band width of less than 4HRC were obtained. However, the component control requirements designed in this patent application are extremely high and unsuitable for industrial mass production, and the hardness values at J5 and J9 remain high. Chinese patent application CN115029618A discloses a narrowly hardenable cold-forged gear steel containing, by mass percentage, C: 0.15–0.17%, Si: 0.10–0.20%, Mn: 1.0–1.10%, Cr: 0.80–0.90%, and Al: 0.02–0.04%, and a method for manufacturing the same. While this cold-forged gear steel meets the hardenability requirements of J5mm = 30–34 HRC and J9mm = 21–25 HRC, a lower hardenability value results in reduced strength, making it impossible to meet the strength requirements of low-hardenability steel. Chinese patent application CN111945070A discloses narrow hardenable gear steel and a method for manufacturing the same. The narrow hardenable gear steel contains, by mass percentage, C: 0.19–0.21%, Si: 0.22–0.28%, Mn: 0.82–0.87%, Cr: 0.52–0.58%, Ni: 0.50–0.55%, Mo: 0.19–0.21%, Al: 0.025–0.035%, N: 0.0080–0.0120%, as well as the remaining Fe and unavoidable impurities, with Al/N = 3–3.6. The resulting narrow hardenable gear steel not only satisfies the hardenability requirements of the steel grade but also reduces the tendency for the steel grade components to have an influence on surface quality, effectively improving the surface quality of the narrow hardenable gear steel. This patent application describes how adjusting the Ni content and optimizing the Al/N ratio reduces the crack susceptibility of the steel surface, thereby achieving end hardenability of gear steels to J4.7mm = 36-41 HRC and J7.9mm = 28-32 HRC. However, the hardenability values of the gear steels of this invention still do not meet the processing requirements of current use