A high fatigue life 20mncr5nbh gear steel and a method of producing the same

Abstract

This invention belongs to the field of metallurgical technology, specifically relating to a high fatigue life 20MnCr5NbH gear steel and its production method. The chemical composition of the gear steel billet by weight percentage is: C: 0.17%~0.22%, Si: 0.15%~0.35%, Mn: 1.10%~1.30%, Cr: 1.00%~1.25%, Nb: 0.020%~0.050%, N≤0.012%, with the balance being Fe and unavoidable impurities; the production method includes the following steps: (1) smelting and continuous casting to obtain a steel billet, (2) heating the continuously cast billet to 1220~1260℃ and holding it for ≥280min; (3) using two-stage controlled rolling; (4) water cooling after rolling. This invention, through the addition of trace amounts of Nb and process synergy, increases the gear contact fatigue limit to over 1700MPa and the bending fatigue limit to over 600MPa.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a high fatigue life 20MnCr5NbH gear steel and its production method. Background Technology

[0002] 20MnCr5H is an internationally used carburizing gear steel with good hardenability and machinability. However, with the increasing demands for gear load-bearing capacity and service life from wind power, new energy vehicles, and high-end engineering machinery, the contact fatigue strength and bending fatigue performance of traditional 20MnCr5H gear steel are gradually showing their limitations.

[0003] In existing technologies, two types of measures are typically adopted to improve the fatigue life of gear steel: one is to increase the content of alloying elements or add expensive elements such as Mo and Ni, which leads to a significant increase in cost; the other is to simply refine the grains through controlled rolling and cooling, but under conventional rolling processes, the austenite grains tend to coarsen significantly, and mixed grains are prone to occur during carburizing, affecting the stability of fatigue performance. Microalloying element Nb has a significant grain refining and precipitation strengthening effect in low-carbon steel, but in medium-low carbon alloy steels such as 20MnCr5H, how to fully utilize the synergistic effect of Nb precipitation strengthening and grain refining strengthening through reasonable composition matching and coordinated control of the entire process, while avoiding surface cracks in the cast billet and excessive rolling load, has not yet formed a mature technical solution. Summary of the Invention

[0004] To solve the above-mentioned technical problems, this invention provides a high fatigue life 20MnCr5NbH gear steel and its production method. Based on the national standard 20MnCr5H, the Nb content is strictly limited to 0.020% to 0.050%, and the C and N content matching is optimized (N≤0.012%) to avoid the precipitation of coarse primary Nb (C,N). At the same time, the trace amount of Nb is used to achieve ultra-fine grains in the carburized layer.

[0005] The technical solution adopted in this invention is as follows: The first aspect of this invention provides a high fatigue life 20MnCr5NbH gear steel. The chemical composition of the steel billet, by weight percentage, is: C: 0.17%–0.22%, Si: 0.15%–0.35%, Mn: 1.10%–1.30%, Cr: 1.00%–1.25%, Nb: 0.020%–0.050%, N ≤0.012%, with the balance being Fe and unavoidable impurities. After carburizing treatment, the austenite grain size of the gear steel is finer than grade 9.0, and the microstructure is a uniform ferrite + pearlite. The contact fatigue limit is increased to over 1700 MPa, and the bending fatigue limit is increased to over 600 MPa.

[0006] Furthermore, the Nb content in the steel billet is 0.030% to 0.045%, and the N content is 0.006% to 0.010%.

[0007] Furthermore, the Nb in the gear steel mainly exists in the form of nano-sized Nb(C,N) particles with a particle size of 10~30nm.

[0008] Secondly, the present invention also provides a method for producing high fatigue life 20MnCr5NbH gear steel, comprising the following steps: Step S1: The converter smelting process is adopted, followed by LF refining and VD vacuum degassing to precisely control the Nb and N composition within the target range and implement dynamic light pressure. Step S2: The continuously cast billet is heated in a walking beam furnace, and the temperature of the soaking zone is controlled at 1220~1260℃, with a total furnace time of ≥280min; Step S3: Two-stage controlled rolling is adopted. The roughing end temperature is ≥1050℃. After roughing, the intermediate billet is air-cooled to 920~980℃ on the roller table and then enters the finishing rolling. The cumulative reduction rate of finishing rolling is ≥70%. After finishing rolling, water cooling is performed, and the final rolling temperature is 820~860℃. Step S4: After rolling, water cooling is applied, with the cooling rate controlled at 15~25℃ / s, and the reddening temperature at 580~620℃, followed by air cooling.

[0009] Furthermore, in step S1, the continuous casting process adopts a weak cooling process, with a secondary cooling water volume of 0.20~0.30L / kg and a straightening temperature ≥950℃.

[0010] Furthermore, in step S3, the finishing rolling passes are ≥5, and the reduction rate per pass is ≥15%.

[0011] This invention is based on the following principles: 1. Adding trace amounts of Nb (0.020%–0.050%) to the basic composition of 20MnCr5H utilizes the dual effects of Nb's solid solution dragging effect in austenite and its combination with C and N to form nanoscale Nb(C,N) precipitates. During the heating stage, undissolved Nb(C,N) pins the austenite grain boundaries, inhibiting the coarsening of the original grains; during the rolling stage, strain-induced precipitation hinders recrystallization; and during the carburizing stage, fine Nb(C,N) particles continuously pin the grain boundaries, ensuring uniform and dense grains in the service state.

[0012] 2. The rolling process of "high temperature homogenization + low temperature multi-pass large reduction" is adopted to fully dissolve Nb and induce its precipitation in the later stage of rolling. At the same time, the ultra-fast cooling after rolling is combined to control the size of the precipitated particles in the range of 10-30nm, so as to achieve the optimal match between precipitation strengthening and fine grain strengthening.

[0013] 3. By controlling the smelting endpoint and continuous casting process, the sensitivity of Nb-containing steel billets to transverse cracks at the corners is suppressed, ensuring surface quality.

[0014] Main innovations of this invention: Innovative composition system: Based on the national standard 20MnCr5H, the Nb content is strictly limited to 0.020%~0.050%, and the C and N content matching is optimized (N≤0.012%) to avoid the precipitation of coarse primary Nb (C,N), while using trace amounts of Nb to achieve ultra-fine grains in the carburized layer.

[0015] Process Collaborative Innovation: A pioneering integrated control strategy of "high-temperature diffusion + low-temperature high reduction + ultra-fast cooling" was implemented. Specifically, the heating temperature of the continuously cast billet was set at 1220~1260℃ to ensure sufficient Nb solution; after rough rolling, the billet was allowed to heat up, and the finishing rolling adopted a "low-temperature multi-pass high reduction" process (single-pass reduction rate ≥15%, final rolling temperature 820~860℃) to induce strain-induced precipitation; after rolling, the billet was water-cooled to 580~620℃ to suppress coarsening of the precipitated phase.

[0016] Slab quality control technology: To address the crack sensitivity of Nb-containing steel, a weak or slow cooling process is adopted, the water content in the secondary cooling zone of continuous casting is controlled at 0.20~0.30 L / kg, and the slab straightening temperature is increased to above 950℃, effectively eliminating corner transverse cracks.

[0017] The beneficial effects of this invention are as follows: It adds 0.020%–0.050% Nb to the traditional 20MnCr5H composition and optimizes the entire process of smelting, continuous casting, heating, controlled rolling, and controlled cooling. Continuous casting employs a weak cooling process to suppress surface cracks; heating temperature of 1220–1260℃ promotes Nb solution dissolution; rolling adopts a "low-temperature multi-pass high-reduction" process, with a single-pass reduction rate of ≥15% in finishing rolling and a final rolling temperature of 820–860℃; post-rolling water cooling is rapidly reduced to 580–620℃. This invention, through the addition of trace amounts of Nb and synergistic process, precipitates fine and dispersed Nb (C,N) particles in the steel, significantly refining the carburized layer grains. The gear contact fatigue limit is increased to over 1700 MPa, and the bending fatigue limit is increased to over 600 MPa. The overall performance is superior to conventional 20MnCr5H steel, with minimal cost increase, making it suitable for high-end gear steel manufacturing. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1Example 1 of the present invention: austenite grain size; Figure 2 The austenite grain size is shown in Comparative Example 1. Detailed Implementation

[0020] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present invention.

[0021] This invention provides a high fatigue life 20MnCr5NbH gear steel. The key improvement on the basic composition of the national standard 20MnCr5H lies in the addition of trace amounts of Nb and control of the N content. The chemical composition of the steel billet, by weight percentage, is: C: 0.17%–0.22%, Si: 0.15%–0.35%, Mn: 1.10%–1.30%, Cr: 1.00%–1.25%, Nb: 0.020%–0.050%, N ≤0.012%, with the balance being Fe and unavoidable impurities. After carburizing treatment, the austenite grain size of the gear steel is finer than grade 9.0, increasing the contact fatigue limit to over 1700 MPa and the bending fatigue limit to over 600 MPa.

[0022] Preferably, the Nb content in the steel billet is 0.030% to 0.045%, and the N content is 0.006% to 0.010%. The Nb in the gear steel mainly exists in the form of nano-sized Nb(C,N) particles with a particle size of 10 to 30 nm.

[0023] This invention also provides a method for producing high fatigue life 20MnCr5NbH gear steel, comprising the following steps: Step S1: The steel is smelted in a converter, refined by LF and vacuum degassed by VD, and the Nb and N composition is precisely controlled within the target range. The continuous casting process adopts a weak cooling process, and the water content in the secondary cooling zone is controlled at 0.20~0.30 L / kg. Dynamic light pressure is implemented. The billet straightening temperature is controlled at ≥950℃ to eliminate the sensitivity of Nb-containing steel to transverse cracks at the corners and obtain a continuous casting billet with good surface quality.

[0024] Step S2: The continuously cast billet is heated in a walking beam furnace, and the temperature of the soaking zone is controlled at 1220~1260℃; the total time in the furnace is ≥280min to ensure that the Nb element is fully dissolved into the austenite, while avoiding excessive grain coarsening.

[0025] Step S3: Two-stage controlled rolling is adopted. The roughing end temperature is ≥1050℃. The number of passes and reduction are set according to the conventional method to break the as-cast structure. After roughing, the intermediate billet is air-cooled to 920~980℃ on the roller table and then enters the finishing rolling. The finishing rolling is carried out at low temperature and multiple passes with large reduction. The cumulative reduction rate is ≥70%, the single pass reduction rate is ≥15%, and the number of passes is ≥5. After finishing rolling, water cooling is carried out. The final rolling temperature is 820~860℃. Within this temperature range, strain-induced Nb(C,N) precipitation is induced.

[0026] Step S4: The rolled steel immediately enters the water cooling system, with the cooling rate controlled at 15~25℃ / s and the reddening temperature at 580~620℃. Then it is air-cooled. The coarsening of the precipitated phase is suppressed by ultra-fast cooling to obtain Nb(C,N) particles with a size of 10~30nm.

[0027] The following examples provide further details.

[0028] Example 1: The target composition of the gear steel in this example, by weight percentage, is: C 0.19%, Si 0.25%, Mn 1.18%, Cr 1.15%, S≤0.015%, P≤0.020%, Nb 0.035%, N 0.008%, with the balance being Fe and unavoidable impurities.

[0029] The production steps are as follows: (1) Smelting and continuous casting: The converter-LF-VD process is adopted. The secondary cooling water ratio is 0.25 L / kg during continuous casting. Under dynamic light pressure, the billet straightening temperature is controlled at 980℃.

[0030] (2) Heating: The continuous casting billet is heated in a walking beam furnace with a soaking temperature of 1240℃ and a total furnace time of 280min to ensure that Nb is fully dissolved.

[0031] (3) Rolling: Two-stage controlled rolling is adopted. The final rolling temperature of roughing is 1080℃ and the thickness of the intermediate billet is 160mm; after the temperature reaches 950℃, it enters the finishing rolling, which is 6 passes with an average reduction rate of 18% per pass and a final rolling temperature of 840℃.

[0032] (4) Cooling: After rolling, the rolls immediately enter the water cooling system, the cooling rate is controlled at 18℃ / s, the red temperature is 600℃, and then air cooling is applied.

[0033] The microstructure of the Φ60mm round steel produced by the above method is uniform ferrite + pearlite, with an austenite grain size of 9.5. Figure 1 As shown.

[0034] Example 2: It is basically the same as Example 1, except that the Nb content is 0.045%, the final rolling temperature is adjusted to 830°C, and the reddening temperature is 590°C.

[0035] Comparative Example 1: The material uses a standard 20MnCr5H composition, is Nb-free, and is rolled using a conventional hot rolling process: initial rolling temperature 1150℃, final rolling temperature 950℃, and air cooling after rolling.

[0036] Application effect: The bars obtained in Examples 1, 2, and Comparative Example 1 were processed into standard carburized gear samples, and carburized and quenched under the same conditions at a carburizing temperature of 930℃ for 5 hours. The test results are shown in Table 1.

[0037] Table 1: Performance test results of gear samples produced by the methods of the examples and comparative examples ; As shown in Table 1, the Nb-containing gear steel produced by the method of this invention exhibits significantly refined and uniform grain size after carburizing, exceeding grade 9.5. This results in an increase of approximately 16%–18% in contact fatigue life and 17%–20% in bending fatigue life. Furthermore, due to the grain-refining effect of Nb, the tendency for surface decarburization during the carburizing process is reduced, leading to better surface quality control.

[0038] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present technical solution are within the protection scope of the present invention.

[0039] To facilitate understanding by those skilled in the art of the improvements of this invention over the prior art, some of the accompanying drawings and descriptions have been simplified, and for clarity, some other elements have been omitted from this application. Those skilled in the art should realize that these omitted elements may also constitute the content of this invention.

Claims

1. A high fatigue life 20MnCr5NbH gear steel, characterized in that, The chemical composition of the steel billet, by weight percentage, is as follows: C: 0.17%–0.22%, Si: 0.15%–0.35%, Mn: 1.10%–1.30%, Cr: 1.00%–1.25%, Nb: 0.020%–0.050%, N ≤0.012%, with the balance being Fe and unavoidable impurities. After carburizing treatment, the austenite grain size of the gear steel is finer than grade 9.0, and the microstructure is a uniform ferrite + pearlite. The contact fatigue limit is increased to over 1700 MPa, and the bending fatigue limit is increased to over 600 MPa.

2. The high fatigue life 20MnCr5NbH gear steel as described in claim 1, characterized in that, The billet contains 0.030% to 0.045% Nb and 0.006% to 0.010% N.

3. The high fatigue life 20MnCr5NbH gear steel as described in claim 2, characterized in that, In the gear steel, Nb mainly exists in the form of nano-sized Nb(C,N) particles with a particle size of 10~30nm.

4. A method for producing gear steel as described in any one of claims 1-3, characterized in that, Includes the following steps: Step S1: The converter smelting process is adopted, followed by LF refining and VD vacuum degassing to precisely control the Nb and N composition within the target range, and dynamic light pressure is implemented. Step S2: The continuously cast billet is heated in a walking beam furnace, and the temperature of the soaking zone is controlled at 1220~1260℃, with a total furnace time of ≥280min; Step S3: Two-stage controlled rolling is adopted. The roughing end temperature is ≥1050℃. After roughing, the intermediate billet is air-cooled to 920~980℃ on the roller table and then enters the finishing rolling. The cumulative reduction rate of finishing rolling is ≥70%. After finishing rolling, water cooling is performed, and the final rolling temperature is 820~860℃. Step S4: After rolling, water cooling is performed, with the cooling rate controlled at 15~25℃ / s and the reddening temperature at 580~620℃, followed by air cooling.

5. The method for producing gear steel as described in claim 4, characterized in that, In step S1, the continuous casting process adopts a weak cooling process, with a secondary cooling water ratio of 0.20~0.30L / kg and a straightening temperature ≥950℃.

6. The method for producing gear steel as described in claim 5, characterized in that, In step S3, the finishing rolling passes are ≥5, and the reduction rate per pass is ≥15%.

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