Low hardenability narrow-width mn-cr gear steel and method for manufacturing the same

Abstract

A low hardenability narrow bandwidth Mn-Cr gear round steel and a manufacturing method thereof, the component mass percentage of the gear round steel 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%, the balance contains Fe and other inevitable impurities. The Mn-Cr gear round steel satisfies J5=33-36HRC, J9=25-28HRC, J15=20-24HRC, the hardenability bandwidth of each point before J9≤3HRC, the purpose of reducing heat treatment deformation is achieved, and the performance requirements of gear steel material in application scenarios such as new energy vehicles can be met, and the stability and safety of the performance of the steel for new energy vehicles are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of gear steel manufacturing, in particular to a low hardenability narrow bandwidth Mn-Cr gear round steel and a manufacturing method thereof. BACKGROUND

[0002] Gear steel is one of the key materials with high requirements in the field of automobile, railway, ship and engineering machinery, and is the manufacturing material of the core components to ensure safety. With the development of industrial technology, gear steel is developing towards high performance, long service life, smooth gear operation, low noise, safety, low cost, easy processing and multi-variety.

[0003] Mn-Cr gear steel is a high-standard car gear steel series. The hardenability of the series steel can reach J5mm=36-45HRC, J9mm=34-40HRC, and J15mm=28-34HRC for the most widely used 20MnCr5. The size of the hardenability value reflects the core hardness of gears of different sizes, and the hardenability bandwidth has an important influence on the heat treatment deformation of gears. The hardenability bandwidth of high-quality gear steel abroad can be controlled within 4HRC, while the advanced level in China can reach 4-6HRC, which still has a certain gap with the control level abroad. Low hardenability narrow bandwidth gear steel is easy to forge and has good cutting performance and small heat treatment deformation, which is beneficial to the processing and manufacturing of some parts of users. At present, the demand for low hardenability narrow bandwidth gear steel 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 bandwidth of gear steel mainly depends on the chemical composition and its uniformity. According to the influence of carbon and alloy elements in the steel on the hardness value of each point of hardenability, the internal control composition range of the steel is determined.

[0005] As disclosed in Chinese patent CN108193029A, a control method for gear steel narrow hardenability bandwidth is adopted, which optimizes the hardenability calculation formula with production data, obtains the calculation parameters suitable for gear steel grade, and then calculates the upper and lower limits of elements C, Si, Mn, Cr, Ni, Mo, Ti, B affecting hardenability according to the optimized calculation formula, requires that the hardenability bandwidth at J5, J9, J11, J13 is less than 3.5HRC, and the obtained composition range is compared with ΔC≤±0.01%, ΔSi≤±0.02%, ΔMn≤±0.02%, ΔCr≤±0.01%, ΔNi≤±0.01%, ΔMo≤±0.01%, ΔTi≤±0.01%, ΔAl≤±0.01%, ΔB≤±0.005%, ΔS≤±0.002%, and finally the narrowest control range is determined as the composition control range of gear steel with narrow hardenability bandwidth requirement. The gear steel rod with hardenability bandwidth less than 4HRC is obtained by adopting low superheat pouring of 10-20℃, ensuring the continuous casting billet heating temperature fluctuation ≤±7℃, and the holding time fluctuation ≤±5min. However, the composition control requirement of this patent design is extremely high, which is not easy for industrial mass production, and the hardness value at J5, J9 is still high.

[0006] Chinese patent CN115029618A discloses a narrow hardenability cold forging gear steel and its manufacturing method, which contains, by mass percent: C: 0.15-0.17%, Si: 0.10-0.20%, Mn: 1.0-1.10%, Cr: 0.80-0.90%, Al: 0.02-0.04%. The hardenability of the cold forging gear steel satisfies J5mm=30-34HRC, J9mm=21-25HRC. Although the hardenability value is low, the strength is also decreased, so it cannot meet the requirement of low hardenability steel on strength.

[0007] Chinese patent CN111945070A discloses a narrow hardenability gear steel and a manufacturing method thereof. The narrow hardenability gear steel comprises, by mass percent: 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%, and the balance of Fe and inevitable impurities, wherein Al / N = 3-3.6. The obtained narrow hardenability gear steel not only meets the hardenability requirements of the steel grade, but also reduces the tendency of the steel composition to affect the surface quality, effectively improving the surface quality of the narrow hardenability gear steel. By adjusting the content of Ni and optimizing the Al / N ratio, the steel surface crack sensitivity is reduced, and the end hardenability of the gear steel meets J4.7mm = 36-41HRC and J7.9mm = 28-32HRC. However, the gear steel hardenability value of the invention still cannot meet the current user processing requirements. SUMMARY

[0008] The purpose of the present application is to provide a low hardenability narrow bandwidth Mn-Cr gear round steel and a manufacturing method thereof, which meets J5 = 33-36HRC, J9 = 25-28HRC, J15 = 20-24HRC, and the hardenability bandwidth of each point before J9 is ≤3HRC, achieving the purpose of reducing heat treatment deformation; and the yield strength is ≥800MPa, the tensile strength is ≥1100MPa, the elongation is ≥12%, the reduction of area is ≥50%, the impact energy KU2 is ≥80J, and the grain size is not less than 6 levels. The gear round steel can meet the performance requirements of gear steel materials in new energy vehicle application scenarios, and improve the stability and safety of new energy vehicle steel performance.

[0009] To achieve the above purpose, the technical solution of the present application is:

[0010] A Mn-Cr gear round steel with low hardenability and narrow bandwidth, the chemical composition by mass percentage of which 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%, the balance includes Fe and other unavoidable impurities; and the length b of the full martensite zone obtained at the end of the end quenching specimen made of gear round steel is in the range of: 0.70≤b≤1.16, 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];

[0011] At the same time, control the hardenability coefficient 1.18≤k≤1.65,

[0012] k=0.54*[C]*(-1.12+5.1*[Mn])*(1+0.7*[Si])*(1+2.16*[Cr]).

[0013] Furthermore, the balance is Fe and other inevitable impurities.

[0014] Preferably, among the unavoidable impurities, Ti≤0.003%, B≤0.0004%, and Ca≤0.003%.

[0015] Preferably, 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 amount of harmful elements is ≤0.05%.

[0016] The measured hardenability values ​​of the gear round steel of the present invention satisfy J5=33-36HRC, J9=25-28HRC, J15=20-24HRC, and the hardenability bandwidth before J9 is ≤3HRC.

[0017] The gear round steel 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 energy KU2 of 90 J, and a grain size of not less than grade 6.

[0018] In the composition design of the gear round steel of the present invention:

[0019] C: C is the most important element affecting the hardenability of gear steel, and has a great influence on the end hardenability value. It is also the most basic effective strengthening element in the steel. Since the carburized gear steel needs to ensure the toughness of the gear core and a certain strength, the content of C is controlled at 0.15-0.18% in the application.

[0020] Si: Si has a strong solid solution strengthening effect in steel, replaces Fe atoms in a substitutional manner, thereby hindering dislocation movement, and can significantly improve the yield strength of the steel; at the same time, Si can reduce the critical cooling rate of the steel to a certain extent, increase the hardenability value, and has a great influence on the value after J5mm of the end hardenability. However, it mainly improves the stability of the supercooled austenite in the medium temperature zone, and has little effect on the high temperature zone of 650-700℃. The content of Si is required to be in the range of 0.040-0.10% in the application.

[0021] Mn: Mn has the effects of solid solution strengthening, grain refinement and thus improving the strength of the steel, and can significantly improve the hardenability of the steel, and has a greater influence on the value after J7mm of the end hardenability. However, Mn can reduce the A1 temperature of the steel, promote the growth of austenite grains, and increase the overheating sensitivity of the steel. Therefore, the content of Mn is controlled at 1.10-1.40% in the application.

[0022] Cr: Cr can increase the nucleation work and transformation activation energy of pearlite, reduce the nucleation rate and growth rate of pearlite, increase the stability of supercooled austenite, improve the hardenability of the steel, obtain strengthening effect, and have good effect on impact toughness. Cr has a greater influence on the values at J5, J9 and J15, and is controlled in the range of 1.00-1.20% in the application.

[0023] Ni: Ni can effectively improve the toughness of the core of gear steel, reduce the ductile-brittle transition temperature, improve the low-temperature impact performance, and can refine the structure to obtain strengthening effect. However, too high Ni content will reduce the machinability of the gear steel after hot working. Ni has a certain influence on J5, and has a greater influence on J9 and J15. The content of Ni is required to be in the range of 0.020-0.10% in the application.

[0024] Mo: Reasonable cooperation of Mo and Cr can significantly improve the hardenability and tempering resistance of the steel, and Mo can refine the grains. Too high Mo content will lead to the formation of grain boundary ferrite film, which is not conducive to the hot plasticity of the steel, increases the reheat cracking tendency of the steel, and increases the cost. Mo has little effect on the values before J5, and has a greater influence on the values at positions greater than J7. Therefore, the content of Mo is in the range of 0.006-0.020% in the application.

[0025] The hardenability is the ability of the steel to obtain martensite after austenitizing, and the size is related to the cooling speed of each point of the cross section of the round steel, and if the cooling speed is higher than the critical quenching speed of the steel, full martensite will be obtained. C is the main component affecting the hardenability, and Si, Mn, Cr and Ni will reduce the critical quenching speed of the steel and improve the hardenability of the steel. In order to ensure the hardenability of the gear steel of the application, the length b of the end quenching sample of the round steel of the application to obtain the full martensite zone satisfies: 0.70≤b≤1.16, b=0.22h-0.45,

[0026] 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].

[0027] The end quenching sample is made of the gear round steel of the application according to GB / T 225-2006 "Steel Quenching End Quenching Test Method".

[0028] In order to ensure the hardenability of the gear steel of the application, and at the same time limit the distance from the end face of the round steel after quenching to the half martensite zone, according to the calculation and data analysis, the hardenability coefficient k of the round steel of the application satisfies: 1.18≤k≤1.65;

[0029] k=0.54*[C]*(-1.12+5.1*[Mn])*(1+0.7*[Si])*(1+2.16*[Cr]).

[0030] Wherein, C is the main element affecting the hardenability, the volume and size of the carbide formed by C and the carbide element will affect the hardness at different distances of the end of the end quenching sample, and the pinning effect of the carbide on the austenite grain will affect the austenite grain size, thereby affecting the hardenability; the retarding effect of Mn, Cr and Si on the bainite transformation is much greater than the influence of the pearlite transformation, the retarding effect of Mo on the bainite transformation is much greater than the bainite, and Ni will also retard the pearlite transformation, so Mn, Cr, Si, Mo and Ni can increase the stability of the supercooled austenite and improve the hardenability of the round steel.

[0031] In order to make the gear steel of the application meet the high-temperature carburizing process, Al: 0.020-0.050%, N: 0.007-0.015%, Al / N≥2.0, Nb: 0.004-0.03% are controlled. This is because AlN formed by Al and N can effectively hinder the growth of austenite grains and refine the austenite grains during rolling heating and high-temperature carburizing, but AlN will aggregate and dissolve at about 940℃, and with the increase of temperature, the inhibition of Al element on the austenite grain size plays a harmful role; niobium is a very effective grain refining alloying element, Nb(C, N) can pin the grain boundary and hinder the growth of austenite grains, effectively reducing the carburizing and quenching deformation. However, when the niobium content exceeds a certain value, the Nb element carbonitride precipitated phase in the casting blank cannot dissolve into the gear steel matrix, and with the execution of subsequent hot working or heat treatment process, some precipitated phases will coarsen, and the coarse precipitated phases cannot prevent the growth of austenite grains during high-temperature carburizing. Therefore, the content of Al in the gear steel designed by the application is 0.020-0.050%, and the content of Nb is 0.004-0.03%.

[0032] The manufacturing method of the low-quenching and narrow-bandwidth Mn-Cr gear round steel provided by the application comprises the following steps:

[0033] 1) Smelting and casting

[0034] Smelting and casting into a casting blank according to the above-mentioned composition;

[0035] 2) Heating

[0036] The temperature of the preheating section is 840-880℃, the temperature of the heating section is controlled at 1120-1175℃, the temperature of the soaking section is 1150-1230℃, and the total heating time in the heating furnace is not less than 4h;

[0037] 3) Rolling

[0038] The open rolling temperature is controlled at 1100-1170℃, and the finish rolling temperature is 950-1000℃;

[0039] 4) Quenching + tempering

[0040] The first quenching temperature is 850-910℃, the holding time is 30-60min, and oil cooling or water cooling is performed to room temperature; the second quenching temperature is 800-830℃, the holding time is 30-60min, and oil cooling or water cooling is performed to room temperature; the tempering temperature is 150-200℃, the holding time is 60-150min, and air cooling is performed to room temperature after tempering.

[0041] Preferably, the smelting adopts an electric furnace or a converter, and then LF refining and VD vacuum degassing; in the LF refining process, the refining time is ≥50 min, the FeO content in the slag is less than 1.0%, and the white slag time is ≥20 min; in the VD vacuum degassing process, the vacuum degree is controlled to be ≤66.7 Pa, the high vacuum time is ≥20 min, the quenching time is ≥10 min, and the VD final temperature is 1550-1580℃.

[0042] Preferably, the casting adopts continuous casting, in order to ensure the composition uniformity of the continuous casting billet from the surface to the inside, the superheating degree of the molten steel is controlled, and simultaneously according to the narrow composition requirements of the steel to Cr, Ni and Mo, the narrow hardenability band width requirement, and in combination with the cross-sectional size of the continuous casting billet, the frequency of the mold electromagnetic stirring is controlled to be 2.0-3.0 HZ, and the current is 150-300 A; the frequency of the end electromagnetic stirring is 8.0-8.5 HZ, and the current is 300-350 A; the superheating degree of the molten steel is controlled to be 22-38℃, and the continuous casting withdrawal speed is controlled to be between 0.62-0.68 m / min.

[0043] In the manufacturing method,

[0044] The whole process of the VD vacuum degassing treatment can specifically adopt bottom blowing nitrogen to increase nitrogen, so as to prevent the secondary oxidation of the molten steel and improve the purity of the molten steel.

[0045] In the continuous casting process, the frequency of the mold electromagnetic stirring is controlled to be 2.0-3.0 HZ, and the current is 150-300 A; the frequency of the end electromagnetic stirring is 8.0-8.5 HZ, and the current is 300-350 A. In the continuous casting process, through the electromagnetic stirring, the size of the larger inclusions can be promoted to float up as soon as possible, the purity of the molten steel is improved, the inclusions are prevented from gathering, the size of the inclusions is reduced, and the harm of the inclusions to the performance of the steel is reduced.

[0046] In the heating of the continuous casting billet, the temperature of the preheating section is increased by 40℃ compared with the prior art, so that hot charging into the furnace is required, the heating speed from the preheating section to the heating section is increased, the composition is more homogenized, the dissolution of the precipitated phase is promoted, so as to diffuse and precipitate in the rolling process and the cooling process, and the hardenability of the round steel is stabilized.

[0047] In the rolling process, the opening rolling temperature is controlled to be 1100-1170℃, and the finish rolling temperature is 950-1000℃; air cooling (such as the above cooling bed) is adopted to obtain ferrite+pearlite structure, blowing cold air and water spraying are avoided, and the bainite structure is prevented from being generated, so as to avoid the inhomogeneous structure and the influence on the performance of the round steel after the subsequent heat treatment;

[0048] The heat treatment process is quenching + tempering, wherein the first quenching temperature is 850-910 DEG C, the holding time is 30-60 min, oil cooling or water cooling to room temperature, promoting the formation of more nucleation particles, refining the austenite grain; the second quenching temperature is 800-830 DEG C, the holding time is 30-60 min, oil cooling or water cooling to room temperature; the tempering temperature is 150-200 DEG C, the holding time is 60-150 min, so that the carbide is fully dispersed and precipitated, and after tempering, air cooling to room temperature.

[0049] Compared with the prior art, the advantages and beneficial effects of the present application are:

[0050] The present application is based on the physical metallurgy single factor method, studies the influence of C, Si, Mn, Cr, Mo and Ni elements on hardenability, designs and manufactures a low hardenability narrow bandwidth Mn-Cr series gear round steel, meets J5 = 33-36HRC, J9 = 25-28HRC, J15 = 20-24HRC, J9 front hardenability bandwidth ≤3HRC, can meet the purpose of small heat treatment deformation of users; at the same time, the mechanical properties meet yield strength ≥800MPa, tensile strength ≥1100MPa, elongation ≥12%, reduction of area ≥50%, impact energy KU2 ≥90J.

[0051] Compared with Chinese patents CN115029618A and CN111945070A, the gear round steel reduces the J5, J9 and J15 values, meets the processing and use requirements of low hardenability gear steel, and is more suitable for new energy automobile transmission gear ring and other application scenarios with strict heat treatment deformation requirements.

[0052] Compared with Chinese patent CN115029618A, the gear round steel of the present application puts forward more specific requirements for the content of C, Si, Mn, Cr, Mo and Ni elements, utilizes the interaction between elements, and does not reduce the mechanical properties under the condition of meeting the low hardenability requirement, and the application scenarios are different.

[0053] The gear round steel of the present application can realize the matching of high strength and high toughness by adopting the secondary quenching + tempering process, and meets the use requirements of high strength steel. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 The hardenability curves of examples 1-6 and comparative examples 1-2 of the present application are shown in the following table:

[0055] Figure 2 The microstructure metallographic photos (500x magnification) of example 2 of the present application after quenching + tempering treatment are shown in the following table: DETAILED DESCRIPTION

[0056] The present invention will be further explained and illustrated below in conjunction with the accompanying drawings and specific embodiments. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

[0057] The composition of the embodiment of the low hardenability and narrow bandwidth Mn-Cr gear steel of the present invention is shown in Table 1, with the remainder being Fe and unavoidable impurities.

[0058] The composition coefficients of the round steel for gears in the embodiments are shown in Table 2. It can be seen that the length b of the martensite region of the round steel in each embodiment ranges from 0.70 to 1.16, and the hardenability coefficient k ranges from 1.18 to 1.65.

[0059] The manufacturing method of the embodiment of the present invention is shown in Table 4 and Table 5, which adopts electric furnace smelting-LF refining-VD vacuum degassing-continuous casting-heating-rolling-slow cooling; the obtained round steel is subjected to end quenching test according to GB / T 225-2006, and the hardness at different positions away from the end of the sample is measured, as shown in Table 6 and Figure 1 shown.

[0060] Room temperature tensile and room temperature impact tests were carried out according to GB / T 228.1-2010 and GB / T 229-2007. The mechanical properties are shown in Table 6.

[0061] Since different heats of steel are obtained after smelting in each embodiment and comparative example, multiple batches of bars can be produced from each heat of steel corresponding to each embodiment and comparative example. The hardenability of multiple batches of bars obtained from the same embodiment or comparative example at points J5, J9, and J15 are not exactly the same. Therefore, in Table 4, the hardenability values ​​at points J5, J9, and J15 of Examples 1 to 6 and Comparative Examples 1 and 2 are within a range.

[0062] As can be seen from Table 6, compared with Comparative Examples 1 and 2, the round steel hardenability bandwidth of Examples 1 to 6 is narrower, and can reach ≤3HRC before J9; and the values ​​of J5, J9, and J15 are significantly lower than those of the comparative examples, which have excellent technical effects and meet the requirements of the present invention for low hardenability narrow bandwidth.

[0063] As shown in Table 7, the yield strength of the embodiment is ≥800 MPa, the tensile strength is ≥1100 MPa, the elongation is ≥12%, the cross-sectional shrinkage is ≥50%, and the impact energy KU2 is ≥90 J, which meets the design requirements of the present invention and has excellent comprehensive mechanical properties.

[0064] Depend on Figure 2 It can be seen that the gear round steel according to the embodiment of the present invention obtains a fine tempered martensite structure after quenching and tempering treatment.

[0065] The low quenching through narrow bandwidth gear round steel prepared by the method satisfies J5=33-36HRC, J9=25-28HRC, J15=20-24HRC, and the quenching through bandwidth before J9 is less than or equal to 3HRC; meanwhile, the Al and N contents are controlled, and the Nb element is added, so that the grain size of the hot-rolled round steel after 980 DEG C heat preservation for 4 hours is not less than 6 levels. Therefore, the low quenching through narrow bandwidth gear round steel prepared by the method can satisfy the processing requirements of the high-temperature carburizing gear ring of the new energy vehicle.

[0066] The above examples are to enable those skilled in the art to understand the content of the present application and to implement the same, and the protection scope of the present application is not limited to the above examples. Therefore, any equivalent changes or modifications made according to the principles and design ideas disclosed by the present application are within the protection scope of the present application.

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[0068]

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[0070]

[0071]

Claims

1. A Mn-Cr gear round steel with low hardenability and narrow bandwidth, the chemical composition by mass percentage of which 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%, the balance being Fe and other unavoidable impurities; and the length b of the full martensite zone obtained at the end of the end quenching specimen made of gear round steel is in the range of: 0.70 ≤ b ≤ 1.16, 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, control the hardenability coefficient 1.18≤k≤1.65, k=0.54*[C]*(-1.12+5.1*[Mn])*(1+0.7*[Si])*(1+2.16*[Cr]); The measured hardenability values ​​of the gear round steel satisfy J5=33~36HRC, J9=25~28HRC, J15=20~24HRC, and the hardenability bandwidth before J9 is ≤3HRC; its mechanical properties satisfy yield strength ≥800MPa, tensile strength ≥1100MPa, elongation ≥12%, cross-sectional shrinkage ≥50%, impact energy KU2 ≥90J, and grain size is not less than level 6.

2. The Mn-Cr gear round steel with low hardenability and narrow bandwidth according to claim 1, characterized in that: Among the unavoidable impurities, Ti≤0.003%, B≤0.0004%, and Ca≤0.003%.

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

4. The low hardenability and narrow bandwidth Mn-Cr gear round steel according to claim 1 or 2, characterized in that: The microstructure of the hot-rolled gear round steel is ferrite+pearlite.

5. The low hardenability and narrow bandwidth Mn-Cr gear round steel according to claim 3, characterized in that: The microstructure of the hot-rolled gear round steel is ferrite+pearlite.

6. The method for producing a Mn-Cr gear round steel with low hardenability and narrow bandwidth according to any one of claims 1 to 5, wherein: The steps include: 1) Smelting and casting Smelting and casting the gear round steel into a billet according to the composition of the gear round steel; 2) Heating The temperature of the preheating section is 840-880°C, the temperature of the heating section is controlled at 1120-1175°C, the temperature of the soaking section is 1150-1230°C, and the total heating time in the heating furnace is not less than 4 hours; 3) Rolling Control the starting rolling temperature to 1100-1170℃ and the finishing rolling temperature to 950-1000℃; 4) Quenching + tempering The primary quenching temperature is 850-910°C, the holding time is 30-60 minutes, and the steel is oil-cooled or water-cooled to room temperature. The secondary quenching temperature is 800-830°C, the holding time is 30-60 minutes, and the steel is oil-cooled or water-cooled to room temperature. The tempering temperature is 150-200°C, the holding time is 60-150 minutes, and the steel is air-cooled to room temperature after tempering.

7. The method for manufacturing Mn-Cr gear round steel with low hardenability and narrow bandwidth according to claim 6, wherein: The smelting adopts an electric furnace or a converter, and then undergoes LF refining and VD vacuum degassing; in the LF refining process, the refining time is ≥50 minutes, the FeO content in the slag is less than 1.0%, and the white slag time is ≥20 minutes; in the VD vacuum degassing process, the vacuum degree is controlled to be ≤66.7 Pa, the vacuum time is ≥20 minutes, the calming time is ≥10 minutes, the VD final temperature is 1550°C to 1580°C, and bottom blowing nitrogen is used throughout this step to increase nitrogen.

8. The method for manufacturing Mn-Cr gear round steel with low hardenability and narrow bandwidth according to claim 6, wherein: The casting adopts continuous casting, and the frequency of electromagnetic stirring of the crystallizer is controlled to be 2.0-3.0HZ and the current is 150-300A; the frequency of electromagnetic stirring at the end is 8.0-8.5HZ and the current is 300-350A; the superheat of molten steel is controlled at 22-38°C, and the continuous casting casting speed is controlled between 0.62-0.68m / min.

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