A high-temperature carburized MnCr gear steel with a narrow hardenability band and a manufacturing method thereof

Abstract

The invention discloses a narrow hardenability band high temperature carburized MnCr gear steel, which contains Fe and inevitable impurities, and also contains the following chemical elements in the following mass percentages: C: 0.175-0.195%, Si: 0.17-0.25%, Mn: 1.27-1.35%, S: 0.01-0.025%, Cr: 0.92-1.0%, Mo: 0.08-0.14%, Al: 0.01-0.03%, N: 0.008-0.014%, and Nb: 0.008-0.025%. Correspondingly, the present invention also discloses a method for manufacturing the narrow hardenability bandwidth high temperature carburized MnCr gear steel, which comprises the following steps: (1) smelting; (2) continuous casting; (3) heating: the heating temperature of the ingot is 1130-1250°C, and the holding time is 4-12 hours; (4) rolling: the ingot is processed into the finished product size by a two-fire material forming process, and air-cooled or slowly cooled to room temperature in a holding pit after rolling; (5) straightening. The narrow hardenability bandwidth high temperature carburized MnCr gear steel described in the present invention has good fine grain effect and narrow hardenability bandwidth characteristics, and its chemical composition control range is relatively wide, which is convenient for large-scale commercial production.

Description

Technical Field

[0001] The present invention relates to a steel material and a manufacturing method thereof, and in particular to a gear steel and a manufacturing method thereof. Background Art

[0002] In recent years, with the advancement of manufacturing technology and material production technology in industries such as automobiles and high-speed railways, the market has also put forward higher requirements for steel used in automotive gears.

[0003] At present, the carburized gear steel for automobiles at home and abroad is developed on the basis of carburized structural steel, which mainly includes Cr series, Mn-Cr series, Cr-Mo series, Ni-Cr-Mo series, Ni-Cr series, etc. Among them, Mn-Cr series gear steel has become the main gear steel for domestic cars and light commercial vehicles.

[0004] In the production of gear parts, the consistency of materials will have a serious impact on carburizing deformation, resulting in poor quality of the final gear parts. In order to improve the quality level of automotive gear parts and gearboxes, the quality stability and consistency requirements for gear steel are becoming more and more stringent, especially the hardenability bandwidth requirements, from 12HRC to 6HRC, and then to 4HRC. The hardenability bandwidth requirements for high-performance gear steel are getting narrower and narrower.

[0005] In the prior art, the main way to obtain narrow hardenability of gear steel is to control the chemical element composition of the steel within a very narrow range.

[0006] For example, a Chinese patent document with publication number CN108193029A, publication date June 22, 2018, and titled "A Control Method for Narrow Hardenability Bandwidth of Gear Steel", discloses a control method for narrow hardenability bandwidth of gear steel, which first uses production data to optimize the hardenability calculation formula to obtain calculation parameters suitable for gear steel grades, and then calculates the upper and lower limits of the composition of elements C, Si, Mn, Cr, Ni, Mo, Ti, and B that affect hardenability according to the optimized calculation formula, requiring the hardenability bandwidth at J5, J9, J11, and J13 to be less than 3.5HRC, and finally determines the narrowest control range as the composition control range of gear steel with narrow hardenability bandwidth requirements, achieving a hardenability bandwidth of less than 4HRC. This patented technical solution requires the upper and lower limits of each element to be calculated separately, and the resulting composition design control range is very narrow, which is not manufacturable in large-scale production.

[0007] Another example is the Chinese patent document with publication number CN101289731A, publication date October 22, 2008, and titled “CRMNTI series narrow hardenability belt gear steel and its manufacturing method”, which discloses a CrMnTi series narrow hardenability belt gear steel and its manufacturing method, and its chemical composition is: C: 0.13-0.23%, Si: 0.17-0.30%, Mn: 0.80-1.25%, Ti: 0.04-0.10%, Cr: 0.90-1.35%, Cu≤0.20%, S:≤0.050%, P≤0.030%, Mo≤0.05%, Ni≤0.25%, [O]≤20×10 －6 , [H]≤2.5×10 －6 This patented technical solution achieves the narrow hardenability target by taking measures such as narrow composition steel grade design and smelting process control, fine grain and banded structure control.

[0008] Of course, in addition to controlling the chemical element composition to achieve the narrow hardenability target, in order to improve the performance of the steel, most gear steels also need to be carburized, which requires long-term heat preservation at high temperatures, consumes a lot of energy and has low efficiency. Therefore, currently, the method of increasing the carburizing temperature is mostly used to shorten the carburizing time. Every 50°C increase in the carburizing temperature can reduce the process time by about half, significantly improve production efficiency, and have a very significant energy-saving effect.

[0009] Although high temperature carburizing technology can improve carburizing efficiency and save energy, this technology also places higher requirements on carburized gear steel. During high temperature carburizing, the austenite grains in the carburized gear steel are easy to grow, resulting in coarsening of the structure and affecting the service life of the gear. Therefore, when designing gear steel, how to control the size of austenite grains under high temperature carburizing conditions is particularly important for carburized gear steel.

[0010] In the prior art, most conventional high temperature carburized gear steels are added with Al elements to refine the grains with AlN. 2 O 3 The presence of inclusions can easily lead to the initiation of fatigue cracks, reducing the fatigue performance of gears; moreover, AlN has poor high-temperature stability and is prone to dissolution when the temperature exceeds 950°C, thus losing its function of pinning the grain boundaries.

[0011] At present, the main method for controlling the grain growth of high-temperature carburized austenite is to add microalloying elements Nb and Ti. The technology for controlling the grain growth of high-temperature carburized gear steel at home and abroad generally adopts the solution of adding microalloying elements to form carbide precipitation phase to hinder the grain growth; however, the precipitation of microalloying elements such as Nb and Ti will reduce the content of solid solution C in austenite, thereby reducing the hardenability of steel. Gear steel is a type of steel with strict requirements on hardenability, and steel containing Ti and Nb has a high-temperature brittle zone. During the rolling process, the precipitation of Ti and Nb elements at the grain boundary leads to the occurrence of steel billet fracture accidents due to the weakness of the grain boundary, resulting in a large amount of waste. Therefore, in the field of gear steel, the actual application effect of the technical solution of controlling grain growth by adding excessive Nb and Ti elements is not very ideal.

[0012] Based on this, it is necessary to provide a high-temperature carburizing MnCr gear steel with a narrow hardenability bandwidth and no grain growth under high-temperature carburizing conditions to meet the needs of the automotive gear industry. Summary of the invention

[0013] One of the purposes of the present invention is to provide a narrow hardenability bandwidth high temperature carburized MnCr gear steel, which controls the type and amount of nano-scale precipitates generated in the steel by optimizing the content ratio of Al, N, and Nb elements, thereby achieving the effect of grain refinement, so as to ensure that the austenite grains do not grow significantly under high temperature carburizing conditions of 980-1000°C. In addition, the present invention can obtain a MnCr gear steel with a hardenability bandwidth lower than 4HRC through reasonable chemical element design, by controlling the composition range of alloying elements that affect hardenability and combining with controlling the production process.

[0014] In order to achieve the above object, the present invention provides a narrow hardenability band high temperature carburized MnCr gear steel, which contains Fe and inevitable impurities, and also contains the following chemical elements in the following mass percentages:

[0015] C: 0.175~0.195%, Si: 0.17~0.25%, Mn: 1.27~1.35%, S: 0.01~0.025%, Cr: 0.92 ~1.0%, Mo: 0.08~0.14%, Al: 0.01~0.03%, N: 0.008~0.014%, Nb: 0.008~0.025%.

[0016] Furthermore, in the narrow hardenability band high temperature carburized MnCr gear steel of the present invention, the mass percentage of each chemical element is:

[0017] C: 0.175~0.195%, Si: 0.17~0.25%, Mn: 1.27~1.35%, S: 0.01~0.025%, Cr: 0.92~1.0%, Mo: 0.08~0.14%, Al: 0.01~0.03%, N: 0.008~0.014%, Nb: 0.008~0.025%; the balance is Fe and unavoidable impurities.

[0018] In the above technical scheme of the present invention, the present invention reasonably designs the composition range of alloy elements that affect hardenability, and controls the type and amount of nanoscale precipitation phases generated in the steel by optimizing the content ratio of Al, N, and Nb elements, thereby achieving the effect of grain refinement and ensuring that the austenite grains do not grow significantly under high-temperature carburizing conditions of 980-1000°C.

[0019] In the narrow hardenability band high temperature carburized MnCr gear steel of the present invention, the design principles of each chemical element are as follows:

[0020] C: In the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, C is a necessary component in the steel, and it is also one of the most important elements affecting the hardenability of the steel; when the C content in the steel is too low, the strength of the steel is insufficient, and good hardenability requirements cannot be guaranteed. It should be noted that the C content in the steel should not be too high. When the C content in the steel is too high, it will affect the performance of the steel, causing the steel to be unable to meet the toughness requirements of the gear core. Therefore, in the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, the mass percentage of the C element is controlled between 0.175 and 0.195%.

[0021] Si: In the narrow hardenability bandwidth high temperature carburized MnCr gear steel described in the present invention, Si element can dissolve into ferrite, strengthen ferrite, and improve the strength, hardness, wear resistance, elasticity and elastic limit of the steel. However, at the same time, the Si element content in the steel should not be too high. When the Si element content in the steel is too high, the brittleness of the steel will increase. Therefore, in the narrow hardenability bandwidth high temperature carburized MnCr gear steel described in the present invention, the mass percentage of Si element is controlled between 0.17 and 0.25%.

[0022] Mn: In the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, Mn is one of the main elements that affect the hardenability of steel. Adding an appropriate amount of Mn element to the steel can not only strongly increase the hardenability of the steel, but also effectively reduce the toughness of the steel. In addition, Mn can also dissolve into ferrite, improve the strength and hardness of the steel, and make the steel obtain pearlite with finer layers and higher strength when it is cooled after hot rolling. However, it should be noted that the Mn element content in the steel should not be too high. When the Mn element content in the steel is too high, the thermoplasticity of the steel will deteriorate. Therefore, in the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, the mass percentage of the Mn element is controlled between 1.27 and 1.35%.

[0023] S: In the narrow hardenability bandwidth high temperature carburized MnCr gear steel of the present invention, the S element will significantly reduce the plasticity and toughness of the steel, but a certain content of the S element can form non-metallic inclusions with the Mn element to improve the cutting performance of the steel. Therefore, considering the beneficial effects and adverse effects of the S element, in the narrow hardenability bandwidth high temperature carburized MnCr gear steel of the present invention, the mass percentage of the S element is controlled between 0.01 and 0.025%.

[0024] Cr: In the narrow hardenability bandwidth high temperature carburized MnCr gear steel of the present invention, adding an appropriate amount of Cr element can significantly improve the hardenability, strength and wear resistance of the steel. At the same time, Cr can also prevent decarburization during heating, rolling and heat treatment, but too high a content of Cr element will significantly reduce the toughness of the quenched and tempered steel, forming coarse carbides distributed along the grain boundaries. Therefore, in the narrow hardenability bandwidth high temperature carburized MnCr gear steel of the present invention, the mass percentage of Cr element is controlled between 0.92 and 1.0%.

[0025] Mo: In the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, Mo as a medium-strong carbide forming element can strongly hinder the nucleation and growth of carbides, and at the same time can effectively improve the hardenability of the steel. In the present technical solution, the Mo element can work in conjunction with the Mn element, thereby significantly improving the stability of austenite and improving the hardenability of the steel. In addition, the Mo element can also delay the pearlite transformation and promote the formation of acicular ferrite structure. By adding an appropriate amount of Mo, the formation of banded structure can be effectively inhibited. However, it should be noted that the Mo content in the steel should not be too high, and too high a content of Mo will increase the smelting cost. Therefore, considering the production cost and the beneficial effects of adding the Mo element, in the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, the mass percentage of the Mo element is controlled between 0.08 and 0.14%.

[0026] Al: In the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, Al is a grain refining element, which can be combined with N to form AlN, thereby further refining the grains and improving the strength and toughness of the steel. However, it should be noted that the Al content in the steel should not be too high. When the Al content in the steel is too high, it will not only increase the chance of inclusions in the steel, affecting the casting performance of the steel, but also damage the toughness of the steel. Therefore, in the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, the mass percentage of the Al element is controlled between 0.01 and 0.03%.

[0027] N: In the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, N is an interstitial atom, which can combine with Al and Nb microalloys in the steel to form MN-type precipitates, which can pin the grain boundaries at high temperatures, thereby inhibiting the growth of austenite grains. When the N content in the steel is low, fewer MN-type precipitates are formed, and the pinning effect is not obvious; when the N content in the steel is too high, it will precipitate and grow in the steel prematurely, reducing the effect of inhibiting grain growth and reducing the toughness of the steel. Therefore, in the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, the mass percentage of the N element is controlled between 0.008 and 0.014%.

[0028] Nb: In the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, the Nb element can form a fine precipitate phase in the steel, which can inhibit the recrystallization of the steel and effectively refine the grains. However, it should be noted that the Nb content in the steel should not be too high. When the Nb content in the steel is too high, coarse NbC particles will be formed during the high temperature tempering process, which will reduce the impact toughness of the steel and weaken the grain refinement effect. Therefore, in the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, the mass percentage of the Nb element is controlled between 0.008 and 0.025%.

[0029] Furthermore, in the narrow hardenability band high temperature carburizing MnCr gear steel of the present invention, it satisfies: 2.0≤Al / (N-0.13Nb)≤2.5; wherein Al and N are substituted into the mass percentage of the corresponding elements respectively.

[0030] In the above technical solution of the present invention, the narrow hardenability bandwidth high temperature carburizing MnCr gear steel of the present invention controls the mass percentage of the single chemical elements of Al, Nb, and N, and at the same time specifies the ratio of the three elements, so as to promote the formation of a certain number and proportion of AlN and Nb (CN) precipitation phases in the steel to refine the grains and inhibit the growth of austenite grains during carburizing. Since a certain amount of N element will be consumed in the precipitation process of the microalloying element Nb (CN), in order to make full use of the role of N element in steel in promoting precipitation, in the narrow hardenability bandwidth high temperature carburizing MnCr gear steel of the present invention, the Al, N, and Nb element contents in the steel are controlled to meet the following requirements: 2.0≤Al / (N-0.13Nb)≤2.5;

[0031] In addition, the inventors found that after the Al / (N-0.13Nb) value in the steel exceeds 2, the number of AlN particles that can be formed in the steel is mainly determined by the N content, and the excess Al in the steel will increase the precipitation temperature of the AlN particles. However, when the Al content is too high, the AlN particles will precipitate prematurely during the high-temperature cooling process of the steel, forming large precipitates, which will reduce the effect of AlN in refining grains.

[0032] When the narrow hardenability bandwidth high temperature carburizing MnCr gear steel of the present invention controls the mass percentage of each chemical element and further controls to satisfy: 2.0≤Al / (N-0.13Nb)≤2.5, the precipitation temperature of AlN particles can be reduced. A lower precipitation temperature can make the precipitation of AlN particles finer and more dispersed, and the effect of inhibiting grain growth is better.

[0033] It should be noted that when AlN particles are heated to 980-1000°C for high-temperature carburizing, about 18-25% of the AlN particles dissolve into austenite, and the dissolution of the AlN particles first occurs at the grain boundaries, resulting in poor effect of using AlN particles alone to inhibit austenite grain growth in high-temperature carburized gear steel, which easily leads to coarse grains.

[0034] Similarly, Nb (C, N) particles also have the same problem, so Al, Nb, and N need to be used in combination to achieve the best effect of inhibiting grain growth.

[0035] Therefore, in the narrow hardenability bandwidth high temperature carburizing MnCr gear steel described in the present invention, it is preferably stipulated that the mass percentage content of Nb, Al and N elements satisfies: 2.0≤Al / (N-0.13Nb)≤2.5, so as to ensure that the Al and N elements in the steel have an optimal ratio to control the sufficient precipitation of AlN particles.

[0036] Furthermore, in the narrow hardenability band high temperature carburizing MnCr gear steel of the present invention, it satisfies: 0.02%≤(0.87Nb+N)≤0.035%; wherein Nb and N are substituted into the mass percentage of the corresponding elements respectively.

[0037] In addition, in the above technical solution, the present invention also preferably stipulates that the mass percentage of Nb and N elements satisfies: 0.02%≤(0.87Nb+N)≤0.035% to ensure that there are sufficient Nb (C, N) particles and AlN particles in the steel.

[0038] The inventors found in experiments that when the mass percentage of a single chemical element is controlled and the mass percentage of Al, Nb and N elements in the steel is further controlled to satisfy the following conditions: 2.0≤Al / (N-0.13Nb)≤2.5, 0.02%≤(0.87Nb+N)≤0.035%, the Nb(C, N) particles precipitated in the steel interact with the AlN particles, and can jointly inhibit the growth of austenite grains during high-temperature carburizing, completely offsetting the effect of mixed crystals caused by the dissolution of AlN particles at 980-1000°C, and avoiding the adverse effects of large-sized precipitated particles on the impact and fatigue properties of gear steel.

[0039] After the MnCr gear steel of the present invention adopts the above design and is carburized at 980-1000°C for 4 hours, the austenite grain size is 6-8, and there is no coarse grain or mixed crystal phenomenon.

[0040] Furthermore, in the narrow hardenability bandwidth high temperature carburized MnCr gear steel described in the present invention, among the inevitable impurities, P≤0.015%, B≤0.0008%, O≤0.0015%, H≤0.0002%, Ni≤0.02%, Cu≤0.02%, V≤0.02%, and Ti≤0.01%.

[0041] In the above technical solution, P element, B element, O element, H element, Ni element, Cu element, V element and Ti element are all impurity elements in steel. If technical conditions permit, in order to obtain gear steel with a narrow hardenability bandwidth and stable performance, the content of impurity elements in the material should be reduced as much as possible.

[0042] In the narrow hardenability bandwidth high temperature carburized MnCr gear steel of the present invention, the impurity element B is an element that is relatively sensitive to hardenability. Since the B element is easily segregated, a slight change in the B content will cause a large fluctuation in the hardenability of the steel. Therefore, the B element is not conducive to the narrow bandwidth control of the hardenability bandwidth of the gear steel, and the B element content in the steel must be strictly controlled to control the impurity element B to meet B≤0.0008%.

[0043] Furthermore, in the narrow hardenability bandwidth high temperature carburized MnCr gear steel of the present invention, in order to reduce the adverse effect of composition fluctuation on the hardenability bandwidth, it is also stipulated that in the hot rolled round steel, the C element also satisfies: 0.98≤C x / C 0 ≤1.02, where C x is the mass percentage of C element at each point in the area from 1 / 2R position to the edge of the round steel, C 0 It is the average value of the mass percentage of C element at each point in the area from 1 / 2R position to the edge of the round steel, and R represents the radius of the round steel.

[0044] In the present invention, in addition to the influence of the fluctuation of the main elements in the steel during the smelting process on the hardenability bandwidth, the distribution of the main elements in the finished round steel will also affect the hardenability bandwidth. Among the main segregation-prone elements of the steel of the present invention, the C element has the greatest influence on the hardenability. Therefore, in order to ensure that the hardenability bandwidth of the hot-rolled round steel is lower than 4HRC, in the present invention, the C element of the steel in the hot-rolled round steel state after the rolling process can be further controlled to meet the following conditions: 0.98≤C x / C 0 ≤1.02.

[0045] The MnCr gear steel of the present invention can calculate the target values ​​of the main chemical elements in the MnCr gear steel according to the hardenability target value requirements of gear steels J5 and J9 and the SEP1664 standard. The upper and lower control limits of the main elements are reasonably set according to the hardenability bandwidth lower than 4HRC and the control level of the chemical composition in the on-site production. At the same time, by controlling the continuous casting and rolling processes in the steel manufacturing process, the C content in the hot-rolled round steel after rolling must also meet the following requirements: 0.98≤C x / C 0 ≤1.02. Through the above measures, the MnCr gear steel of the present invention can have both a narrow hardenability bandwidth and a wide composition control range, so as to facilitate large-scale production.

[0046] Furthermore, in the narrow hardenability band high temperature carburized MnCr gear steel of the present invention, after high temperature carburizing at 980-1000°C, the austenite grain size is 6-8.

[0047] Furthermore, in the narrow hardenability bandwidth high-temperature carburized MnCr gear steel described in the present invention, its hardenability bandwidth is lower than 4HRC.

[0048] Furthermore, in the narrow hardenability band high temperature carburized MnCr gear steel of the present invention, the diameter of the prepared round steel is 25-130 mm.

[0049] Correspondingly, another object of the present invention is to provide a method for manufacturing the above-mentioned narrow hardenability bandwidth high-temperature carburized MnCr gear steel. The manufacturing method is simple to produce, and the obtained MnCr gear steel does not grow grains under high-temperature carburizing conditions and has a narrow hardenability bandwidth, the hardenability bandwidth of which is lower than 4HRC, and after high-temperature carburizing at 980-1000°C, the austenite grain size is 6 to 8 levels.

[0050] In order to achieve the above object, the present invention proposes a method for manufacturing the narrow hardenability band high temperature carburized MnCr gear steel, which comprises the steps of:

[0051] (1) Smelting;

[0052] (2) Continuous casting;

[0053] (3) Heating: The ingot heating temperature is 1130-1250°C, and the holding time is 4-12 hours;

[0054] (4) Rolling: The ingot is processed into finished product size by two-fire process, and then air-cooled or slowly cooled to room temperature in a heat preservation pit after rolling;

[0055] (5) Straightening.

[0056] In the above technical solution of the present invention, in order to achieve the control of the C element content in the hot-rolled round steel, the inventors also optimized the manufacturing process. Among them, the two-fire material forming process refers to first rolling the ingot to a specified intermediate billet size, and then reheating and rolling the intermediate billet to the final finished product size.

[0057] In the present invention, the ingot obtained by continuous casting can be input into a heating furnace for heating, and the heating temperature of the ingot is controlled to be 1130-1250°C, and the holding time is 4-12 hours to ensure that AlN, Nb (C, N) and MC type carbides in the steel can be completely dissolved into the steel. Among them, for every 20°C increase in the heating temperature, the corresponding holding time can be reduced by 1-2 hours to prevent the grains from being too coarse and overoxidized.

[0058] The ingot is taken out of the heating furnace to ensure that a certain number of AlN and Nb (C, N) particles have begun to precipitate in the steel when the ingot is rolled. After the steel of the present invention undergoes dynamic recrystallization during rolling, the precipitated AlN and Nb (C, N) particles can inhibit the growth of recrystallized grains and play a role in grain refinement. However, it should be noted that if the rolling temperature is too low or the rolling ratio is too low, uniform dynamic recrystallization is not easy to occur during the rolling process, and uneven deformation of the steel leads to uneven microstructure after rolling, affecting the distribution of C elements in the hot-rolled round steel; at the same time, if the rolling temperature is too high, the grains of the steel will grow rapidly after dynamic recrystallization occurs, which is not conducive to grain refinement.

[0059] After rolling, the round steel can be air-cooled or slowly cooled to room temperature in an insulation pit. Finally, the hot-rolled round steel can be straightened as required, and conventional treatment processes such as post-rolling annealing or peeling can be selected according to specific requirements.

[0060] Furthermore, in the manufacturing method described in the present invention, in step (2), the baking temperature of the tundish during continuous casting is ≥1050°C, and the baking time is ≥4h; during the casting process, the superheat of the molten steel in the tundish is controlled to be 10-25°C, and the pulling speed is controlled to be 0.4-0.8m / min.

[0061] In the above technical solution of the present invention, the molten steel whose chemical element composition meets the design requirements can be cast into continuous casting billets by continuous casting process. During continuous casting, the baking temperature of the tundish can be controlled to be ≥1050℃, and the baking time can be ≥4h; during the casting process, the superheat of the molten steel in the tundish is controlled to be 10-25℃, the pulling speed is controlled to be 0.4-0.8m / min, and the electromagnetic stirring current and frequency of the crystallizer are controlled to be 200A-500A / 4HZ; then the continuous casting billet is hot-sent or slowly-cooled, and the slow-cooling pit temperature is controlled to be ≥600℃, and the slow-cooling time is ≥24h.

[0062] Furthermore, in the manufacturing method described in the present invention, in step (4), the rolling ratio of the continuous casting billet to the intermediate billet is controlled to be ≥2.0; the rolling ratio of the intermediate billet to the finished round steel is controlled to be ≥5.0; the ingot starting rolling temperature is ≥1050°C, and the final rolling temperature is ≥850°C; the heating temperature of the intermediate billet is 1100-1200°C, the holding time is 3-10h, the starting rolling temperature of the intermediate billet is ≥1030°C, and the final rolling temperature is ≥880°C.

[0063] Compared with the prior art, the narrow hardenability band high temperature carburized MnCr gear steel and the manufacturing method thereof described in the present invention have the following advantages and beneficial effects:

[0064] The narrow hardenability bandwidth high temperature carburized MnCr gear steel of the present invention combines the use of the microalloying element Nb on the basis of conventional AlN grain refinement, reasonably configures the content ratio of Al, Nb and N elements, regulates the proportion and quantity of AlN and Nb (C, N) precipitated particles generated in the steel, and allows AlN and Nb (C, N) particles to precipitate at a lower temperature on the basis of using less Al, Nb and N element content, thereby making the precipitated particles smaller and more dispersed, and achieving better effects of refining grains and inhibiting the growth of austenite grains, thereby ensuring that the gear steel obtained will not have coarse grains or mixed crystals after high temperature carburizing at 980-1000°C for 4 hours.

[0065] The narrow hardenability bandwidth high-temperature carburized MnCr gear steel of the present invention can effectively control the distribution of C element content in the finished hot-rolled round steel after rolling by reasonably setting relatively wide upper and lower limit ranges of main elements and combining with optimized steel manufacturing process, so that the steel has a narrow hardenability bandwidth.

[0066] Accordingly, during the manufacturing process, the narrow hardenability bandwidth high-temperature carburized MnCr gear steel described in the present invention can be processed into finished product size by a two-fire material forming process during the rolling process. Its chemical element composition and manufacturing process are reasonably designed, the production process window is wide, and it is very easy to realize mass commercial production. DETAILED DESCRIPTION

[0067] The narrow hardenability band high temperature carburizing MnCr gear steel and its manufacturing method described in the present invention will be further explained and illustrated in conjunction with specific embodiments below. However, such explanation and illustration do not constitute an improper limitation to the technical solution of the present invention.

[0068] Examples 1-6 and Comparative Examples 1-3

[0069] The narrow hardenability band high temperature carburized MnCr gear steels of Examples 1-6 are all prepared by the following steps:

[0070] (1) Smelting is carried out according to the chemical composition shown in Table 1-1 and Table 1-2 below: The smelting can be carried out by electric furnace or converter, and after LF refining and VD or RH vacuum treatment, the steel is produced after the composition meets the design target requirements.

[0071] (2) Continuous casting: Continuous casting is to cast the molten steel that meets the composition design requirements into ingots. During continuous casting, the baking temperature of the tundish is controlled to be ≥1050℃ and the baking time is ≥4h. During the casting process, the superheat of the molten steel in the tundish is controlled to be 10-25℃, the pulling speed is controlled to be 0.4-0.8m / min, and the electromagnetic stirring current and frequency of the crystallizer are controlled to be 200A-500A / 4HZ. After the ingot is off the line, it is hot transported or put into the pit for slow cooling. The slow cooling pit temperature is controlled to be ≥600℃ and the slow cooling time is ≥24h.

[0072] (3) Heating: The ingot is placed in a heating furnace for heating, and the heating temperature of the ingot is controlled to be 1130-1250°C, and the insulation time is 4-12 hours. After leaving the heating furnace, high-pressure water is used to remove phosphorus and oxide scale on the surface of the ingot before rolling.

[0073] (4) Rolling: The ingot is processed into the finished product size by the two-fire material-making process to obtain the finished round steel with a diameter of 25-130 mm, and then air-cooled or slowly cooled to room temperature in a heat-insulating pit after rolling; in the two-fire material-making process, the rolling ratio of the continuous casting billet to the intermediate billet is controlled to be ≥2.0; the rolling ratio of the intermediate billet to the finished round steel is ≥5.0; the ingot starting rolling temperature is ≥1050℃, and the final rolling temperature is ≥850℃; the heating temperature of the intermediate billet is 1100-1200℃, the holding time is 3-10h, the starting rolling temperature of the intermediate billet is ≥1030℃, and the final rolling temperature is ≥880℃.

[0074] (5) Straightening: Straighten the round steel after rolling.

[0075] Of course, in some other embodiments, in the actual process of manufacturing gear steel, the round steel obtained after rolling can also be subjected to conventional treatments such as heat treatment and peeling, which will not be described in detail here.

[0076] In the present invention, the chemical composition design and related processes of the narrow hardenability bandwidth high temperature carburized MnCr gear steels of Examples 1-6 meet the design specification requirements of the present invention.

[0077] Correspondingly, although comparative examples 1-3 are also prepared using the above-mentioned process flow of "smelting, continuous casting, heating, rolling and straightening", their chemical composition design and related processes have parameters that do not meet the design specification requirements of the present invention. Their chemical composition design can be found in the following Tables 1-1 and 1-2.

[0078] Tables 1-1 and 1-2 list the mass percentages of the chemical elements of the narrow hardenability bandwidth high temperature carburized MnCr gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3.

[0079] Table 1-1. (wt.%, the balance is Fe and other inevitable impurities except P, B, O, H, Ni, Cu, V and Ti)

[0080]

[0081] Table 1-2.

[0082] serial number Al / (N-0.13Nb) 0.87Nb+N Example 1 2.33 0.020 Example 2 2.00 0.029 Example 3 2.36 0.023 Example 4 2.20 0.022 Example 5 2.48 0.034 Example 6 2.50 0.026 Comparative Example 1 2.13 <![CDATA[ 0.015 ]]> Comparative Example 2 <![CDATA[ 1.25 ]]> <![CDATA[ 0.012 ]]> Comparative Example 3 2.33 <![CDATA[ 0.015 ]]>

[0083] Note: In the above table, each chemical element in the formulas of "Al / (N-0.13Nb)" and "0.87Nb+N" is substituted with its mass percentage.

[0084] The specific production process of the narrow hardenability band high temperature carburized MnCr gear steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3 is as follows:

[0085] Example 1

[0086] According to the chemical composition shown in Table 1-1 and Table 1-2, electric furnace smelting is carried out, and after LF refining and VD vacuum treatment, continuous casting billets are cast. The cross-sectional size of the continuous casting billet is 320mm×425mm. During continuous casting, the baking temperature of the tundish is controlled to be 1080℃, and the baking time is controlled to be 6h; during pouring, the superheat of the molten steel in the tundish is controlled to be 11~25℃, the pulling speed is 0.65m / min, the electromagnetic stirring current of the crystallizer is 300A / 4Hz, and the slow cooling time of the casting billet is controlled to be 30h. The continuous casting billet is placed in a heating furnace for heating to 1210℃ and kept warm for 4 hours; the billet is removed from the heating furnace and descaled by high-pressure water before rolling, with the start rolling temperature controlled at 1120℃, the final rolling temperature at 960℃, and the rolling ratio at 2.81, to obtain an intermediate billet with a size of 220mm×220mm; the intermediate billet is heated to 1130℃ and kept warm for 6 hours, and then removed from the furnace and descaled by high-pressure water before rolling again, with the start rolling temperature controlled at 1050℃, the final rolling temperature controlled at 920℃, and the rolling ratio controlled at 17.13, to obtain a φ60mm finished bar. After rolling, it is slowly cooled to room temperature in a heat preservation pit and then straightened.

[0087] Example 2

[0088] According to the chemical composition shown in Table 1-1 and Table 1-2, electric furnace smelting was carried out, and after LF refining and VD vacuum treatment, continuous casting billets were cast. The cross-sectional size of the continuous casting billet was 320mm×425mm. During continuous casting, the tundish baking temperature was controlled to be 1100℃, and the baking time was controlled to be 4h; during casting, the superheat of the molten steel in the tundish was controlled to be 12~22℃, the pulling speed was controlled to be 0.75m / min, the electromagnetic stirring current of the crystallizer was 200A / 4Hz, and the slow cooling time of the casting billet was controlled to be 48h. The continuous casting billet is placed in a heating furnace for heating to 1200℃ and kept warm for 6 hours; the billet is removed from the heating furnace and descaled by high-pressure water before rolling, with the start rolling temperature controlled at 1100℃, the final rolling temperature at 880℃, and the rolling ratio at 2.01, to obtain an intermediate billet with a size of 260mm×260mm; the intermediate billet is heated to 1180℃ and kept warm for 4 hours, and then removed from the furnace and descaled by high-pressure water before rolling again, with the start rolling temperature controlled at 1080℃, the final rolling temperature controlled at 950℃, and the rolling ratio controlled at 10.63, to obtain a φ75mm finished bar. After rolling, it is slowly cooled to room temperature in a heat preservation pit and then straightened.

[0089] Example 3

[0090] According to the chemical composition shown in Table 1-1 and Table 1-2, electric furnace smelting was carried out, and after LF refining and RH vacuum treatment, continuous casting billets were cast. The cross-sectional size of the continuous casting billet was 320mm×425mm. During continuous casting, the tundish baking temperature was controlled to be 1150℃, and the baking time was controlled to be 4h; during casting, the superheat of the molten steel in the tundish was controlled to be 12~23℃, the pulling speed was controlled to be 0.45m / min, the electromagnetic stirring current of the crystallizer was 300A / 4Hz, and the slow cooling time of the casting billet was controlled to be 36h. The continuous casting billet is placed in a heating furnace and heated to 1140℃ for 12 hours. The billet is descaled by high-pressure water and then rolled. The starting rolling temperature is controlled to be 1060℃, the final rolling temperature is controlled to be 920℃, and the rolling ratio is controlled to be 2.01 to obtain an intermediate billet with a size of 260mm×260mm. The intermediate billet is heated to 1120℃ and kept for 10 hours. After being descaled by high-pressure water, it is rolled again. The starting rolling temperature is controlled to be 1030℃, the final rolling temperature is controlled to be 930℃, and the rolling ratio is controlled to be 5.10 to obtain a φ130mm finished bar. After rolling, it is air-cooled and then straightened.

[0091] Example 4

[0092] According to the chemical composition shown in Table 1-1 and Table 1-2, electric furnace smelting was carried out, and continuous casting billets were cast after LF refining and RH vacuum treatment. The cross-sectional size of the continuous casting billet is 320mm×425mm. During continuous casting, the tundish baking temperature is controlled to be 1200℃, and the baking time is controlled to be 4.5h; during casting, the superheat of the molten steel in the tundish is controlled to be 15-25℃, the pulling speed is controlled to be 0.5m / min, the electromagnetic stirring current of the crystallizer is 400A / 4Hz, and the slow cooling time of the casting billet is 24h. The continuous casting billet is placed in a heating furnace and heated to 1240℃ for 3 hours. The billet is descaled by high-pressure water and then rolled. The starting rolling temperature is controlled to be 1120℃, the final rolling temperature is controlled to be 900℃, and the rolling ratio is controlled to be 5.31 to obtain an intermediate billet with a size of 160mm×160mm. The intermediate billet is heated to 1150℃ and kept for 8 hours. After being descaled by high-pressure water, it is rolled again. The starting rolling temperature is controlled to be 1050℃, the final rolling temperature is controlled to be 900℃, and the rolling ratio is controlled to be 26.62 to obtain a φ35mm finished bar. After rolling, it is air-cooled and then straightened.

[0093] Example 5

[0094] According to the chemical composition shown in Table 1-1 and Table 1-2, converter smelting is carried out, and after LF refining and RH vacuum treatment, continuous casting billets are cast. The cross-sectional size of the continuous casting billet is 320mm×425mm. During continuous casting, the baking temperature of the tundish is controlled to be 1100℃, and the baking time is 5h; during pouring, the superheat of the molten steel in the tundish is controlled to be 13-25℃, the pulling speed is 0.55m / min, the electromagnetic stirring current of the crystallizer is 300A / 4Hz, and the slow cooling time of the casting billet is 24h. The continuous casting billet is placed in the heating furnace and heated to 1180℃, and the insulation time is 10h; the billet is removed from the heating furnace and descaled by high-pressure water and then rolled, and the starting rolling temperature is controlled to be 1080℃, the final rolling temperature is 950℃, and the rolling ratio is 5.31, so as to obtain an intermediate billet with a size of 160mm×160mm. The intermediate billet was heated to 1150℃ and kept at this temperature for 6 hours, and then removed from the furnace and descaled by high-pressure water and rolled again, with the start rolling temperature controlled at 1080℃, the final rolling temperature controlled at 880℃, and the rolling ratio controlled at 52.18, to obtain a φ25mm finished bar. After rolling, it was air-cooled and then straightened.

[0095] Example 6

[0096] According to the chemical composition shown in Table 1-1 and Table 1-2, the converter is smelted, and after LF refining and RH vacuum treatment, it is cast into a continuous casting billet. The cross-sectional size of the continuous casting billet is 320mm×425mm. During continuous casting, the baking temperature of the tundish is controlled to be 1180℃, and the baking time is 5.5h; during pouring, the superheat of the molten steel in the tundish is controlled to be 10-20℃, the pulling speed is 0.60m / min, the electromagnetic stirring current of the crystallizer is 500A / 4Hz, and the slow cooling time of the casting billet is 40h. The continuous casting billet is placed in the heating furnace and heated to 1160℃, and the insulation time is 8h; the billet is removed from the heating furnace and descaled by high-pressure water and then rolled, and the starting rolling temperature is controlled to be 1100℃, the final rolling temperature is 950℃, and the rolling ratio is 2.81, so as to obtain an intermediate billet with a size of 220mm×220mm. The intermediate billet is heated to 1100℃ and kept at this temperature for 5h, then removed from the furnace and descaled by high-pressure water and rolled again, with the start rolling temperature controlled at 1050℃, the final rolling temperature controlled at 900℃, and the rolling ratio controlled at 24.66, to obtain a φ50mm finished bar. After rolling, it is air-cooled and then straightened.

[0097] Comparative Example 1

[0098] According to the chemical composition shown in Table 1-1 and Table 1-2, converter smelting is carried out, and after LF refining and RH vacuum treatment, continuous casting billets are cast. The cross-sectional size of the continuous casting billet is 320mm×425mm. During continuous casting, the baking temperature of the tundish is controlled to be 1100℃, and the baking time is 3h; during pouring, the superheat of the molten steel in the tundish is controlled to be 15-40℃, the pulling speed is 0.73m / min, the electromagnetic stirring current of the crystallizer is 200A / 4Hz, and the slow cooling time of the casting billet is 12h. The continuous casting billet is placed in the heating furnace and heated to 1150℃, and the insulation time is 3h; the billet is removed from the heating furnace and descaled by high-pressure water and then rolled, and the starting rolling temperature is controlled to be 1100℃, the final rolling temperature is 900℃, and the rolling ratio is 2.81, so as to obtain an intermediate billet with a size of 220mm×220mm. The intermediate billet is heated to 1100℃ and kept at this temperature for 3 hours, then removed from the furnace and descaled by high-pressure water and rolled again, with the start rolling temperature controlled at 1050℃, the final rolling temperature controlled at 880℃, and the rolling ratio controlled at 3.65, to obtain a φ130mm finished bar. After rolling, it is air-cooled and then straightened.

[0099] Comparative Example 2

[0100] According to the chemical composition shown in Table 1-1 and Table 1-2, the converter is smelted, and after LF refining and RH vacuum treatment, it is cast into a continuous casting billet. The cross-sectional size of the continuous casting billet is 320mm×425mm. During continuous casting, the baking temperature of the tundish is controlled to be 1150℃, and the baking time is 3h; during pouring, the superheat of the molten steel in the tundish is controlled to be 20-40℃, the pulling speed is 0.45m / min, the electromagnetic stirring current of the crystallizer is 200A / 4Hz, and the slow cooling time of the casting billet is 12h. The continuous casting billet is placed in a heating furnace and heated to 1100℃, and the insulation time is 5h; the billet is removed from the heating furnace and descaled by high-pressure water and then rolled, and the starting rolling temperature is controlled to be 1050℃, the final rolling temperature is 850℃, and the rolling ratio is 5.31, so as to obtain an intermediate billet with a size of 160mm×160mm. The intermediate billet is heated to 1080℃ and kept at this temperature for 3 hours, then removed from the furnace and descaled by high-pressure water and rolled again, with the start rolling temperature controlled at 1030℃, the final rolling temperature controlled at 850℃, and the rolling ratio controlled at 9.06, to obtain a φ60mm finished bar. After rolling, it is air-cooled and then straightened.

[0101] Comparative Example 3

[0102] According to the chemical composition shown in Table 1-1 and Table 1-2, converter smelting is carried out, and after LF refining and RH vacuum treatment, continuous casting billets are cast. The cross-sectional size of the continuous casting billet is 320mm×425mm. During continuous casting, the baking temperature of the tundish is controlled to be 1200℃, and the baking time is 2.5h; during pouring, the superheat of the molten steel in the tundish is controlled to be 15-30℃, the pulling speed is 0.61m / min, the electromagnetic stirring current of the crystallizer is 200A / 4Hz, and the slow cooling time of the casting billet is 12h. The continuous casting billet is placed in the heating furnace and heated to 1200℃, and the insulation time is 3h; the billet is removed from the heating furnace and descaled by high-pressure water and then rolled, and the starting rolling temperature is controlled to be 1080℃, the final rolling temperature is 870℃, and the rolling ratio is 2.81, so as to obtain an intermediate billet with a size of 220mm×220mm. The intermediate billet is heated to 1120℃ and kept at this temperature for 3h, then removed from the furnace and descaled by high-pressure water and rolled again, with the start rolling temperature controlled at 1060℃, the final rolling temperature controlled at 820℃, and the rolling ratio controlled at 50.33, to obtain a φ35mm finished bar. After rolling, it is air-cooled and then straightened.

[0103] Table 2-1 and Table 2-2 list the specific process parameters of the narrow hardenability bandwidth high temperature carburized MnCr gear steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3 in the above process steps.

[0104] Table 2-1.

[0105]

[0106] Table 2-2.

[0107]

[0108]

[0109] In the above Table 2-1, since the superheat of the molten steel in the tundish changes during the pouring process, what is shown in Table 2-1 is a range of values ​​rather than a point value.

[0110] The obtained finished products of Examples 1-6 with narrow hardenability band and high temperature carburized MnCr gear steel and the comparative steel of Comparative Examples 1-3 were sampled respectively, and the round steel samples of each Example and Comparative Example were used to perform simulated carburizing and quenching tests. In the simulated carburizing and quenching test, different samples were kept at 980°C for 4 hours and 1000°C for 4 hours, taken out and then water quenched, and then the metallographic samples were prepared and corroded to observe the organization of each Example and Comparative Example, and the austenite grain size was evaluated according to the standard GB / T 6394-2017. The test results of the austenite grain size of each Example and Comparative Example are listed in Table 3.

[0111] Table 3 lists the austenite grain size test results of the narrow hardenability bandwidth high temperature carburized MnCr gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3.

[0112] Table 3.

[0113] serial number Grain size (980℃x4h) Grain size (1000℃x4h) Example 1 7.5 6.5 Example 2 7.5 7 Example 3 7 6.5 Example 4 8 7.5 Example 5 8 7.5 Example 6 7.5 7 Comparative Example 1 6.5 <![CDATA[ 5.5 ]]> Comparative Example 2 6 <![CDATA[ 5. Partial mixed crystal ]]> Comparative Example 3 6.5 <![CDATA[ 5 ]]>

[0114] Accordingly, the round steel samples of Examples 1-6 and Comparative Examples 1-3 were further observed and tested. A 5 mm diameter drill was used to take chip samples at the 1 / 2R position and the edge position on the cross section of the round steel samples of Examples 1-6 and Comparative Examples 1-3 according to the cross method. The C content at the 1 / 2R position and the edge position on the cross section of the round steel samples was analyzed using a carbon-sulfur analyzer, and the C content C at each point of the round steel was calculated. x and the average value of the average C content of the sampling points 0 The ratio of C of the round steel in each embodiment and comparative example is x / C 0 The ranges are listed in Table 4.

[0115] Table 4.

[0116] serial number <![CDATA[Round steel C x / C 0 > Example 1 0.989-1.019 Example 2 0.991-1.016 Example 3 0.983-1.015 Example 4 0.99-1.017 Example 5 0.985-1.019 Example 6 0.98-1.01 Comparative Example 1 <![CDATA[ 0.972-1.035 ]]> Comparative Example 2 <![CDATA[ 0.961-1.043 ]]> Comparative Example 3 <![CDATA[ 0.963-1.037 ]]>

[0117] Note: In the above table, the formula “C x / C 0 "C" x It is expressed as: the mass percentage of C element at each point in the area from 1 / 2R position to the edge of the round steel in the hot-rolled round steel state; C 0 It is expressed as: the average value of the mass percentage of C element at each point in the area from 1 / 2R position to the edge of the round steel in the hot-rolled round steel state, and R represents the radius of the round steel.

[0118] After completing the above observation and detection, the round steel samples of Examples 1-6 and Comparative Examples 1-3 can be further taken and prepared according to GB / T225-2006 standard, and the samples are subjected to end quenching test after being kept at 925°C for 1 hour, and the hardness of the position J5 5 mm from the end and the position J9 9 mm from the end of the sample is measured. The end hardenability test results of each embodiment and comparative example are respectively listed in Table 5.

[0119] Table 5 lists the end hardenability test results of the narrow hardenability bandwidth high temperature carburized MnCr gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3.

[0120] Table 5.

[0121]

[0122] It can be seen from Table 3 that the austenite grain size of the narrow hardenability bandwidth high temperature carburized MnCr gear steels of Examples 1-6 of the present invention after simulated carburization at 980-1000°C is 6.5-8. The austenite grain size of the comparative steels of Comparative Examples 1-3 after simulated carburization at 980-1000°C is 5-6.5, and abnormal growth of austenite grains and mixed crystal phenomenon are found in some samples, indicating that the steel of the present invention has a good effect of inhibiting austenite grain growth under high temperature carburizing conditions.

[0123] Accordingly, it can be seen from Table 4 and Table 5 that the C of the round steel of the narrow hardenability band high temperature carburized MnCr gear steel of Examples 1-6 of the present invention is x / C 0 The fluctuation range is lower than that of the comparative steel round steel of Comparative Examples 1-3, and the deviation of the end hardenability test results of each embodiment is also smaller than that of the comparative examples. The end hardenability J5 and J9 bandwidths of the narrow hardenability bandwidth of high-temperature carburized MnCr gear steel of Examples 1-6 are 2.2 and 3.4HRC respectively, while the end hardenability J5 and J9 bandwidths of Comparative Examples 1-3 are 3.9 and 5.2HRC respectively, indicating that the MnCr gear steel of the present invention has a narrow hardenability bandwidth, and the J5 and J9 hardenability bandwidths are both lower than 4HRC.

[0124] From the above, it can be seen that the present invention can obtain a narrow hardenability bandwidth high-temperature carburized MnCr gear steel with excellent performance through reasonable chemical composition design combined with optimized process. The narrow hardenability bandwidth high-temperature carburized MnCr gear steel not only has a narrow hardenability bandwidth, its hardenability bandwidth is lower than 4HRC, and after high-temperature carburizing at 980-1000°C, the austenite grain size is 6 to 8.

[0125] In addition, the production process window of this narrow hardenability bandwidth high temperature carburizing MnCr gear steel is wide, and it is very easy to achieve mass commercial production.

[0126] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0127] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and all should belong to the protection scope of the present invention.

Claims

1. A high temperature carburized MnCr gear steel with a narrow hardenability band, characterized in that: The mass percentage of each chemical element is: C: 0.175-0.195%, Si: 0.17-0.25%, Mn: 1.27-1.35%, S: 0.01-0.025%, Cr: 0.92-1.0%, Mo: 0.08-0.14%, Al: 0.01-0.03%, N: 0.008-0.014%, Nb: 0.008-0.025%; the balance is Fe and unavoidable impurities; The content of each chemical element also satisfies: 2.0≤Al / (N-0.13Nb)≤2.5; 0.02%≤(0.87Nb+N)≤0.035%; When it is in the hot-rolled round steel state, the C element also satisfies: 0.98≤C x / C0≤1.02, where C x is the mass percentage of C element at each point in the area from 1 / 2R position to the edge of the round steel, C0 is the average mass percentage of C element at each point in the area from 1 / 2R position to the edge of the round steel, and R represents the radius of the round steel.

2. The narrow hardenability band high temperature carburized MnCr gear steel according to claim 1, characterized in that: Among the inevitable impurities, P ≤ 0.015%, B ≤ 0.0008%, O ≤ 0.0015%, H ≤ 0.0002%, Ni ≤ 0.02%, Cu ≤ 0.02%, V ≤ 0.02%, and Ti ≤ 0.01%.

3. The narrow hardenability band high temperature carburized MnCr gear steel according to claim 1, characterized in that: After high temperature carburizing at 980-1000℃, the austenite grain size is 6-8.

4. The narrow hardenability band high temperature carburizing MnCr gear steel according to claim 1, characterized in that: Its hardenability bandwidth is lower than 4HRC.

5. The narrow hardenability band high temperature carburizing MnCr gear steel according to claim 1, characterized in that: The diameter of the round steel produced is 25-130 mm.

6. The method for manufacturing a narrow hardenability band high temperature carburized MnCr gear steel according to any one of claims 1 to 5, characterized in that: It includes the steps of: (1) Smelting; (2) Continuous casting; (3) Heating: The ingot heating temperature is 1130-1250°C, and the holding time is 4-12 hours; (4) Rolling: The ingot is processed into finished product size using a two-fire process, and then air-cooled or slowly cooled to room temperature in a heat preservation pit after rolling; (5) Straightening.

7. The manufacturing method according to claim 6, characterized in that: In step (2), during continuous casting, the baking temperature of the tundish is ≥1050°C, and the baking time is ≥4h; during the casting process, the superheat of the molten steel in the tundish is controlled to be 10-25°C, and the casting speed is controlled to be 0.4-0.8m / min.

8. The manufacturing method according to claim 6, characterized in that: In step (4), the rolling ratio of the continuous casting billet to the intermediate billet is controlled to be ≥2.0; the rolling ratio of the intermediate billet to the finished round steel is controlled to be ≥5.0; the ingot starting rolling temperature is ≥1050°C, and the final rolling temperature is ≥850°C; the heating temperature of the intermediate billet is 1100-1200°C, the holding time is 3-10h, the starting rolling temperature of the intermediate billet is ≥1030°C, and the final rolling temperature is ≥880°C.

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