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

Abstract

A round steel bar for gears with low hardenability and narrow band width, and a method for manufacturing the same, wherein the mass percentage of its components is 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.

Description

Technical Field

[0001] The present invention relates to the field of gear steel manufacturing, and particularly to round steel for Mn-Cr series gears with low hardenability and narrow band width, and a manufacturing method thereof.

Background Art

[0002] Gear steel is one of the important materials with high requirements among special alloy steels used in the fields of automobiles, railways, ships, and construction machinery, and is a manufacturing material for core components to ensure safety. With the development of industrial technology, gear steel is developing in the directions of high performance, long life, stability of gear operation, low noise, safety, low cost, easy processing, and multiple varieties.

[0003] Mn-Cr series gear steel is a series of high-standard steel for passenger car gears. For this series of steel grades, the hardenability is good. Taking 20MnCr5, which is the most widely applied, as an example, its hardenability can reach J5mm = 36 - 45HRC, J9mm = 34 - 40HRC, and J15mm = 28 - 34HRC. The magnitude of the hardenability value reflects the core hardness of gears of different sizes, and the hardenability band width has an important influence on the heat treatment deformation of gears. The hardenability band width of high-quality gear steel overseas can be controlled within 4HRC, while at the domestic advanced level, it can reach 4 - 6HRC, still having a certain gap compared with the overseas control level. Gear steel with low hardenability and narrow band width is easy to forge, has good cutting performance, and small heat treatment deformation, so it is advantageous for the processing and manufacturing of some parts by users. Currently, the demand for gear steel with low hardenability and narrow band width has also begun to increase, and the high hardenability of 20MnCr5 can no longer meet the special requirements of users.

[0004] The control of the hardenability and hardenability band width of gear steel mainly depends on chemical composition and its uniformity. Based on the influence of carbon elements and alloy elements in the steel on the hardness values at each point of hardenability, the internal control component range of the steel is determined.

[0005] 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 to obtain 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 range is △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. By casting at a low superheat of 10-20°C and ensuring that the heating temperature fluctuation of the continuously cast billet is ±7°C or less and the holding time fluctuation is ±5 min or less, gear steel bars with a hardenability band width of less than 4HRC are 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 are still high.

[0006] Chinese patent application CN115029618A discloses a narrowly hardenable cold-forged gear steel containing 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% by mass percentage, and a method for manufacturing the same. The hardenability of this cold-forged gear steel satisfies J5mm = 30~34HRC and J9mm = 21~25HRC, but if the hardenability value is low, the strength decreases and it cannot meet the strength requirements of low hardenability steel.

[0007] 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 composition 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 and achieves 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 users. [Overview of the project]

[0008] The object of the present invention is to provide a Mn-Cr round steel for gears with low hardenability and narrow band width, and a method for manufacturing the same. This 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, thereby achieving the objective of reducing heat treatment deformation. Furthermore, it has a yield strength of 800MPa or more, a tensile strength of 1100MPa or more, an elongation of 12% or more, a cross-sectional shrinkage of 50% or more, an impact work KU2 of 80J or more, and a grain size of grade 6 or higher. The round steel for gears of the present invention can meet the performance requirements of gear steel materials in application scenes such as new energy vehicles, and can improve the stability and safety of the performance of steel for new energy vehicles.

[0009] To achieve the above objective, the technical solution of the present invention is as follows. 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 length b of the total martensite region 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 The compound is +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]).

[0010] Furthermore, in the chemical composition of the round steel for gears of the present invention, the remainder consists of Fe and other unavoidable impurities.

[0011] Preferably, the unavoidable impurities contain Ti ≤ 0.003%, B ≤ 0.0004%, and Ca ≤ 0.003%. Preferably, 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 total amount of harmful elements ≤ 0.05%.

[0012] The measured hardenability values ​​of the round steel for gears of the present invention satisfy J5=33~36HRC, J9=25~28HRC, J15=20~24HRC, and the hardenability band width before J9 ≤ 3HRC. In some embodiments, 1 HRC ≤ hardenability band width before J9 ≤ 3 HRC.

[0013] The round steel for gears of the present invention has a yield strength of 800 MPa or more, a tensile strength of 1100 MPa or more, an elongation of 12% or more, a cross-sectional shrinkage of 50% or more, an impact work KU2 of 90 J or more, and a grain size of grade 6 or higher.

[0014] In some embodiments, the yield strength of the round steel for gears of the present invention is 800 to 820 MPa. In some embodiments, the tensile strength of the round steel for gears of the present invention is 1100 to 1150 MPa.

[0015] In some embodiments, the elongation of the round steel for gears of the present invention is 12% to 19%. In some embodiments, the cross-sectional shrinkage rate of the round steel for gears of the present invention is 50% to 65%.

[0016] In some embodiments, the impact work KU2 of the round steel for gears of the present invention is 90 to 100 J. In some embodiments, the grain size of the round steel for gears of the present invention is 7.0 to 9.0.

[0017] In the design of the components of round steel for gears of the present invention, Carbon (C) is the most important element affecting the hardenability of gear steel, significantly influencing the end hardenability value, and is also the most fundamental effective strengthening element in steel. Since carburized gear steel needs to guarantee toughness and a certain level of strength in the gear core, the present invention controls the carbon content to 0.15-0.18%.

[0018] Si: Si has a strong solid solution strengthening effect in steel, replacing Fe atoms through substitution and inhibiting dislocation motion, thereby significantly improving the yield strength of the steel. At the same time, Si reduces the critical cooling rate of steel to a certain extent, increasing the hardenability value and greatly affecting the hardenability value beyond J5mm. However, it mainly improves the stability in the medium temperature range of supercooled austenite, and its effect on the high temperature range of 650°C to 700°C is small, and in some cases, it may even decrease its stability. In this invention, the Si content range is set to 0.040 to 0.10%.

[0019] Mn: Mn has the effect of improving the strength of steel through solid solution strengthening and grain refinement, and can significantly improve the hardenability of steel, further having a greater impact on the hardenability value beyond J7mm. However, Mn lowers the A1 temperature of steel, promotes the growth of austenite grains, and increases the overheating sensitivity of the steel. Therefore, in this invention, the Mn content is controlled to 1.10 to 1.40%.

[0020] Cr:Cr improves the nucleation work and transformation activation energy of pearlite, reduces the nucleation rate and growth rate of pearlite, increases the stability of supercooled austenite, improves the hardenability of steel, provides a strengthening effect, and also has a positive effect on impact toughness. Cr has a significant impact on the values ​​of J5, J9, and J15, and in this invention, Cr is controlled within the range of 1.00 to 1.20%.

[0021] Ni:Ni effectively improves the core toughness of gear steel, lowers the toughness-brittle transition temperature, improves low-temperature impact performance, and simultaneously refines the microstructure, resulting in a strengthening effect. However, excessive Ni content reduces the machinability of gear steel after thermal processing. Ni has a certain effect on J5 and a significant effect on J9 and J15, and in this invention, the Ni content range is set to 0.020~0.10%.

[0022] The appropriate blend of Mo and Cr significantly improves the hardenability and tempering resistance of steel, and Mo can refine the crystal grains. Excessive Mo content leads to the formation of grain boundary ferrite films, which is detrimental to the thermoplasticity of steel, increases the tendency of steel to reheat and crack, and raises costs. The effect of Mo on values ​​before J5 is not clear, but it has a significant effect on values ​​above J7. Therefore, in this invention, the Mo content range is set to 0.006 to 0.020%.

[0023] In this invention, the P content is set to ≤0.02%. In some embodiments, the P content range is 0.005 to 0.02%, for example, 0.007% and 0.010%.

[0024] In the present invention, the [O] content is set to be ≤ 0.0015%. In some embodiments, the [O] content range is 0.0010 - 0.0015%, for example, 0.0012%.

[0025] Hardening ability refers to the ability of steel to obtain martensite during quenching after austenitization. Its magnitude is related to the cooling rate at each point of the round steel cross-section. When the cooling rate exceeds the critical hardening rate of the steel, all martensite can be obtained. C is the main component that affects hardening ability and hardness. Si, Mn, Cr, and Ni decrease the critical hardening rate of the steel and improve the hardening ability of the steel. To ensure the hardening ability of the gear steel of the present invention, it is required that the length b of the fully martensite region obtained at the end of the end-face hardening test piece made from the round steel of the present invention satisfies 0.70 ≤ b ≤ 1.16 and 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].

[0026] In some embodiments, b satisfies 0.80 ≤ b ≤ 1.15. The end-face hardening test piece is a standard end-face hardening test piece made from the round steel for gears of the present invention in accordance with GB / T 225 - 2006 "Method of End-Quench Test for Hardenability of Steel".

[0027] To ensure the hardening hardenability of the gear steel of the present invention and at the same time limit the distance from the end face of the round steel after quenching to the semi-martensite region, based on calculations and data analysis, it is required that the hardening coefficient k of the round steel of the present invention satisfies 1.18 ≤ k ≤ 1.65, k = 0.54 * [C] * (-1.12 + 5.1 * [Mn]) (1 + 0.7 * [Si]) * (1 + 2.16 * [Cr]).

[0028] In some embodiments, k satisfies 1.25 ≤ k ≤ 1.55. Of these, carbon (C) is the main element that affects hardenability. The volume and size of the carbides formed by C and carbide elements affect the hardness at different distances from the end of the end-face hardened test specimen. At the same time, the pinning effect of the carbides on the austenite grains affects the austenite grain size, which in turn affects hardenability. The delaying effect of Mn, Cr, and Si on bainite transformation is much greater than that of pearlite transformation, and the delaying effect of Mo on pearlite transformation is much greater than that of bainite. Ni also delays pearlite transformation. Therefore, Mn, Cr, Si, Mo, and Ni improve the hardenability of round steel by increasing the stability of supercooled austenite.

[0029] To ensure the gear steel of this invention satisfies the high-temperature carburizing process, the Al: 0.020~0.050%, N: 0.007~0.015%, Al / N ≥ 2.0, and Nb: 0.004~0.03% are controlled. This is because AlN, formed by Al and N, effectively inhibits the growth of austenite grains during rolling and high-temperature carburizing, thereby refining the austenite grains. However, AlN undergoes aggregation and melting around 940°C, and as the temperature rises, the Al element exhibits a detrimental effect on suppressing the austenite grain size. Niobium is a very effective grain-refining alloying element, and Nb(C,N) pins the grain boundaries, inhibiting the growth of austenite grains and effectively reducing carburizing and quenching deformation. However, if the niobium content exceeds a certain value, the carbonitride precipitate phase of Nb in the ingot does not redissolve in the gear steel base material. As a result of subsequent heat processing or heat treatment processes, some of the precipitate phase coarses, and the coarse precipitate phase cannot inhibit the growth of austenite crystal grains in the high-temperature carburizing process. Therefore, the Al content in the gear steel designed in this invention is 0.020 to 0.050%, and the Nb content is 0.004 to 0.03%. In this invention, Al / N ≥ 2.0. In some embodiments, 2.0 ≤ Al / N ≤ 4.0.

[0030] The present invention provides a method for manufacturing Mn-Cr round steel for gears with low hardenability and narrow band width, comprising the following steps: 1) Smelting and casting The above components are smelted and cast 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) Hardening + Tempering The primary quenching temperature is 850-910°C, the holding time is 30-60 mins, and the material is cooled to room temperature by oil or water cooling. The secondary quenching temperature is 800-830°C, the holding time is 30-60 mins, and the material is cooled to room temperature by oil or water cooling. The tempering temperature is 150-200°C, the holding time is 60-150 mins, and the material is cooled to room temperature by air cooling after tempering.

[0031] In some embodiments, the total heating time in the heating furnace is 4 to 6 hours, for example, 4 to 5.5 hours. Preferably, the smelting is carried out using an electric furnace or converter, and further includes LF (Ladle Furnace) smelting and VD (Vacuum Degassing) vacuum degassing. In the LF smelting process, the smelting 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 is 20 min or more, the standing time is 10 min or more, and the VD completion temperature is controlled to 1550°C to 1580°C. In some embodiments, the smelting time is 50 to 65 minutes. In some embodiments, the white slag time is 20 to 30 minutes. In some embodiments, the high vacuum time is 20 to 35 minutes. In some embodiments, the standing time is 10 to 20 minutes.

[0032] Preferably, the casting employs continuous casting, and in order to ensure uniformity of the composition from front to back of the continuously cast billet, it is combined with molten steel superheat control, and at the same time, based on the narrow composition range of Cr, Ni, and Mo required by the steel of the present invention, the requirement for a narrow hardenability band width, and the ingot cross-sectional size, the frequency of crystallizer electromagnetic stirring is controlled to 2.0~3.0 Hz and the current to 150~300 A. The frequency of terminal electromagnetic stirring is controlled to 8.0~8.5 Hz and the current to 300~350 A. The molten steel superheat is controlled to 22~38°C and the continuous casting draw speed is controlled to between 0.62~0.68 m / min.

[0033] In the manufacturing method of the present invention, In the entire VD vacuum degassing process, nitrogen enrichment using bottom-blowing nitrogen gas can be specifically employed to prevent secondary oxidation of molten steel and improve its purity.

[0034] In the continuous casting process, the frequency of crystallizer electromagnetic stirring is controlled to 2.0-3.0 Hz and the current to 150-300 A, while the frequency of terminal electromagnetic stirring is controlled to 8.0-8.5 Hz and the current to 300-350 A. In the continuous casting process, electromagnetic stirring promotes the rapid budding of larger inclusions, improves the purity of the molten steel, prevents inclusion aggregation, reduces the size of inclusions, and minimizes the harm that inclusions pose to the steel's performance.

[0035] In continuous casting billet heating, the preheating temperature is 40°C higher compared to existing technologies, requiring hot charging of the heating furnace. This increases the heating rate from the preheating section to the heating section, leading to greater uniformity of the components, promotion of the dissolution of the precipitated phase, dispersion and precipitation during the rolling and cooling processes, and stabilization of the hardenability of the round steel.

[0036] In the rolling process, the rolling start temperature is controlled to 1100-1170°C, and the rolling end temperature to 950-1000°C. A ferrite-pearlite structure is obtained by air cooling (e.g., using a cooling bed). To prevent the formation of a bainite structure, which would cause structural inhomogeneity and affect the performance of the round steel after subsequent heat treatment, cold air blowing and water spraying are avoided.

[0037] The heat treatment process consists of quenching and tempering. The primary quenching temperature is 850-910°C, with a holding time of 30-60 minutes. The material is then cooled to room temperature by oil or water quenching to form more nucleation sites and refine the austenite grains. The secondary quenching temperature is 800-830°C, with a holding time of 30-60 minutes. The material is then cooled to room temperature by oil or water quenching. The tempering temperature is 150-200°C, with a holding time of 60-150 minutes to allow sufficient dispersion and precipitation of carbides. After tempering, the material is cooled to room temperature by air quenching.

[0038] Compared to existing technologies, the advantages and beneficial effects of the present invention are as follows: This invention, based on the physical metallurgy single-factor method, studies the influence of elements C, Si, Mn, Cr, Mo, and Ni on hardenability, and designs and manufactures low-hardenability, narrow-bandwidth Mn-Cr round steel for gears that satisfies J5=33~36HRC, J9=25~28HRC, J15=20~24HRC, and hardenability band width before J9 ≤3HRC, achieving the objective of reducing heat treatment deformation for the user. At the same time, the mechanical performance satisfies yield strength ≥800MPa, tensile strength ≥1100MPa, elongation ≥12%, cross-sectional shrinkage ≥50%, and impact work KU2 ≥90J.

[0039] Compared to Chinese patent applications CN115029618A and CN111945070A, the round steel for gears of the present invention reduces the J5, J9, and J15 values, meeting the processing and usage requirements for low hardenability gear steel, and is more suitable for applications with stringent requirements for heat treatment deformation, such as new energy vehicle transmission gearing.

[0040] Compared to Chinese patent application CN115029618A, the present invention proposes clearer requirements regarding the relationship between the elemental content of C, Si, Mn, Cr, Mo, and Ni, and utilizes the interaction between elements to satisfy the requirement for low hardenability without degrading mechanical performance, and has different application scenarios.

[0041] The round steel for gears of the present invention achieves both high strength and high toughness by employing a secondary quenching + tempering process, thereby meeting the requirements for using high-strength steel. [Brief explanation of the drawing]

[0042] [Figure 1] The hardenability curves for Examples 1-6 and Comparative Examples 1-2 of the present invention are shown. [Figure 2] This shows a metallurgical micrograph (magnification 500x) of the microstructure after quenching and tempering treatment of Example 2 of the present invention. [Modes for carrying out the invention]

[0043] The present invention will be further interpreted and described below in conjunction with the drawings and specific embodiments, but this interpretation and description will not unduly limit the technical solutions of the present invention.

[0044] Tables 1 and 2 show the component composition of the low-hardenability, narrow-bandwidth Mn-Cr gear steel of the present invention, with the remainder being Fe and unavoidable impurities.

[0045] Table 3 shows the component coefficients in the round steel gears of the examples. It can be seen that the length b of the martensite region of the round steel in each example is between 0.70 and 1.16, and at the same time, the hardenability coefficient k is between 1.18 and 1.65.

[0046] The process parameters in the manufacturing method of the embodiment of the present invention are shown in Tables 4-1, 4-2, and 5, employing electric furnace smelting - LF refining - VD vacuum degassing - continuous casting - heating - rolling - slow cooling, and including the following steps: 1) Smelting and casting Ingots were formed by smelting and casting according to the components shown in Tables 1 and 2. An electric furnace was used for smelting, and further LF refining and VD vacuum degassing were performed. In the LF refining process, the refining time was 50 min or more, the FeO content in the slag was less than 1.0 wt%, and the white slag time was 20 min or more. In the VD vacuum degassing process, the vacuum level was 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 to 1550°C to 1580°C. Nitrogenization using bottom-blowing nitrogen gas was employed throughout the entire VD vacuum degassing process. Continuous casting was used for casting, and the frequency of crystallizer electromagnetic stirring was controlled to 2.0 to 3.0 Hz and the current to 150 to 300 A. The frequency of terminal electromagnetic stirring was controlled to 8.0 to 8.5 Hz and the current to 300 to 350 A. The molten steel superheating temperature was controlled to 22-38°C, and the continuous casting and drawing speed was controlled to be between 0.62 and 0.68 m / min.

[0047] 2) Heating The preheating section temperature was controlled to 840-880°C, the heating section temperature to 1120-1175°C, and the soaking section temperature to 1150-1230°C. The total heating time in the furnace was 4 hours or more.

[0048] 3) Rolling The rolling start temperature was controlled to 1100-1170°C, and the rolling end temperature was controlled to 950-1000°C.

[0049] 4) Hardening + Tempering The primary quenching temperature was 850-910°C, the holding time was 30-60 mins, and the material was cooled to room temperature by oil or water cooling. The secondary quenching temperature was 800-830°C, the holding time was 30-60 mins, and the material was cooled to room temperature by oil or water cooling. The tempering temperature was 150-200°C, the holding time was 60-150 mins, and the material was cooled to room temperature by air cooling after tempering.

[0050] The microstructure of the round steel for gears obtained in Step 3) in the hot-rolled state was observed using metallographic methods, and the results showed that the microstructure of the round steel for gears in Examples 1 to 6 in the hot-rolled state was all ferrite + pearlite.

[0051] The obtained round steel pieces were subjected to end-face hardening experiments according to GB / T 225-2006, and the hardness was measured at different positions from the end of the specimen (hardness test standards: GB / T 230.1, GB / T 230.2, GB / T 230.3). The results are shown in Table 6 and Figure 1.

[0052] Room temperature tensile and impact tests were conducted according to GB / T 228.1-2010 and GB / T 229-2007, and the measured mechanical performance values ​​are shown in Table 7.

[0053] Each example and comparative example yielded different batches of steel after smelting. Multiple batches of bar stock could be manufactured from the steel of each corresponding batch for each example and comparative example. Since the hardenability at points J5, J9, and J15 was not exactly the same for multiple batches of bar stock obtained from the same example or comparative example, Table 6 shows the hardenability values ​​at points J5, J9, and J15 for Examples 1-6 and Comparative Examples 1 and 2 within a single range.

[0054] As can be seen from Table 6, compared to Comparative Examples 1 and 2, the round steels of Examples 1 to 6 have a narrower hardenable band width, can reach 3 HRC or less before J9, and the values ​​of J5, J9, and J15 are clearly lower than those of the comparative examples, demonstrating superior technical effects and satisfying the requirements of the present invention for low hardenability and narrow band width.

[0055] Table 7 shows that the examples met the design requirements of the present invention and possessed excellent overall mechanical performance, with a yield strength of ≥800 MPa, tensile strength of ≥1100 MPa, elongation of ≥12%, cross-sectional shrinkage of ≥50%, and impact work KU2 of ≥90 J. KU2 represents the impact absorption energy of the U-notch specimen in the Charpy pendulum impact test.

[0056] As can be seen from Figure 2, the round steel for gears in the embodiment of the present invention obtained a fine tempered martensite structure after quenching and tempering treatment.

[0057] After the hot-rolled round steel samples from Examples 1 to 6 were kept at 980°C for 4 hours, their grain size was tested according to the standard GB / T 6394-2017 "Method for Measuring Average Grain Size of Metals," and the test results are shown in Table 8.

[0058] The low-hardenability, narrow-bandwidth round steel for gears manufactured by the method of the present invention satisfies the requirements of J5=33~36HRC, J9=25~28HRC, J15=20~24HRC, and hardenability band width ≤3HRC before J9. Simultaneously, by controlling the Al and N content and adding Nb, the grain size of the hot-rolled round steel after being heated at 980°C for 4 hours was grade 6 or higher. Therefore, the low-hardenability, narrow-bandwidth round steel for gears manufactured by the method of the present invention was able to meet the processing requirements for high-temperature carburized gearing in new energy vehicles.

[0059] The above embodiments are provided to enable those skilled in the art to understand and implement the present invention, and the scope of protection of the present invention is not limited to these embodiments. Accordingly, any equivalent changes or modifications made based on the principles and design concepts disclosed in the present invention are included within the scope of protection of the present invention.

[0060] [Table 1]

[0061] [Table 2]

[0062] [Table 3]

[0063] [Table 4-1]

[0064] [Table 4-2]

[0065] Table 5

[0066] Table 6

[0067] Table 7

[0068] Table 8

Claims

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]であり、 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]) is a Mn-Cr type round steel for gears with low hardenability and narrow band width.

2. The low hardenability, narrow band width Mn-Cr round steel for gears according to claim 1, characterized in that the remainder of its chemical composition is Fe and other unavoidable impurities.

3. The low hardenability and narrow band width 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 low-hardenability, narrow-bandwidth Mn-Cr round steel for gears according to claim 1, 2, or 3, characterized in that the harmful elements in the composition are As ≤ 0.02%, Sn ≤ 0.02%, Pb ≤ 0.012%, Sb ≤ 0.012%, Bi ≤ 0.012%, and the total sum of harmful elements ≤ 0.05%.

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=33-36HRC, J9=25-28HRC, J15=20-24HRC, and the hardenability band width before J9 is ≤3HRC, characterized in that the low hardenability and narrow band width Mn-Cr round steel for gears according to any one of claims 1 to 5.

7. The aforementioned round steel for gears is characterized by having a yield strength of ≥800 MPa, a tensile strength of ≥1100 MPa, an elongation of ≥12%, a cross-sectional shrinkage of ≥50%, an impact work of KU2 of ≥90 J, and a grain size of 6 or higher, as described in any one of claims 1 to 6, which is a low-hardenability, narrow-bandwidth Mn-Cr round steel for gears.

8. The low hardenability and narrow bandwidth Mn-Cr round steel for gears according to claim 6, characterized in that the hardenability band width before J9 satisfies 1 HRC ≤ hardenability band width before J9 ≤ 3 HRC.

9. The round steel for gears described in claim 7 is characterized in that the yield strength of the round steel for gears is 800 to 820 MPa, the tensile strength is 1100 to 1150 MPa, the elongation is 12% to 19%, the cross-sectional shrinkage is 50% to 65%, the impact work KU2 is 90 to 100 J, and the grain size is 7.0 to 9.

0.

10. The low hardenability, narrow bandwidth round steel for Mn-Cr 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 to form an ingot according to the components described in claim 1, 2, 3, or 4 (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) Hardening + Tempering The primary quenching temperature is 850-910°C, the holding time is 30-60 mins, and the material is cooled to room temperature by oil or water cooling. The secondary quenching temperature is 800-830°C, the holding time is 30-60 mins, and the material is cooled to room temperature by oil or water cooling. The tempering temperature is 150-200°C, the holding time is 60-150 mins, and the material is cooled to room temperature by air cooling after tempering. A manufacturing method characterized by including the following step.

12. The method for manufacturing Mn-Cr round steel for gears with low hardenability and narrow band width, as described in 11, is characterized in that the smelting process employs an electric furnace or converter, and further involves LF refining and VD vacuum degassing, in which the LF refining process is characterized by a refining time of 50 min or more, a FeO content in the slag of less than 1.0 wt%, and a white slag time of 20 min or more, in which the VD vacuum degassing process is characterized by controlling the vacuum level 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 to 1550°C to 1580°C, and employing nitrogen enhancement by bottom-blowing nitrogen gas in the entire VD vacuum degassing process.

13. The method for producing low hardenability and narrow band width Mn-Cr round steel for gears according to claim 11, characterized in that the casting employs continuous casting, the frequency of crystallizer electromagnetic stirring is controlled to 2.0 to 3.0 Hz and the current to 150 to 300 A, the frequency of terminal electromagnetic stirring is controlled to 8.0 to 8.5 Hz and the current to 300 to 350 A, the molten steel superheating is controlled to 22 to 38°C, and the continuous casting and drawing speed is controlled to between 0.62 and 0.68 m / min.

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