CN113430461B

CN113430461B - Nb and V microalloyed gear steel, preparation method thereof, heat treatment method, carburizing treatment method and carburized gear steel

Info

Publication number: CN113430461B
Application number: CN202110708256.8A
Authority: CN
Inventors: 丁毅; 胡芳忠; 胡乃悦; 汪开忠; 张建; 龚志翔; 杨志强; 陈世杰; 金国忠; 尹德福
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2021-06-24
Filing date: 2021-06-24
Publication date: 2022-05-17
Anticipated expiration: 2041-06-24
Also published as: CN113430461A

Abstract

The invention discloses Nb and V microalloyed gear steel and a preparation method, a heat treatment method, a carburization treatment method and carburized gear steel thereof, wherein Nb and V microalloyed gear steel with ferrite and pearlite structures is obtained by Nb and V microalloying; the method adopts the processes of electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, hot rolling and slow cooling for production; after the gear steel after hot rolling and slow cooling is subjected to heat treatment, Nb and V microalloyed gear steel with the tensile strength of more than or equal to 1465MPa, the yield strength of more than or equal to 1340MPa, the elongation of more than or equal to 14 percent, the reduction of area of more than or equal to 50 percent and the impact energy KV2 of more than or equal to 95J is obtained; the gear steel after hot rolling and slow cooling is carburized to obtain Nb-V microalloying carburized gear steel with high contact fatigue performance, and the rated fatigue life L of the gear steel is L under the condition that the compressive stress is 4.0GPa₁₀≥5×10⁷Median fatigue life L₅₀≥8.2×10⁷。

Description

Nb and V microalloyed gear steel and preparation method, heat treatment method, carburization method and carburized gear steel thereof

Technical Field

The invention belongs to the technical field of gear steel, and relates to Nb and V microalloyed gear steel, a preparation method, a heat treatment method, a carburizing treatment method and carburized gear steel, in particular to Nb and V microalloyed gear steel, a preparation method, a heat treatment method and a carburizing treatment method thereof, and Nb and V microalloyed carburized gear steel with high contact fatigue performance.

Background

Gear steel is a key material with large consumption and high requirement in the field of special steel, and is widely applied to the fields of machinery, traffic, energy and the like. The performance requirements of the gear steel not only influence the technical and economic indexes such as the service life of equipment, but also influence the requirements such as the use safety. The working environment of the gear is complex and severe, and the main failure modes are meshing surface abrasion, pit peeling caused by contact fatigue, crack or fracture caused by tooth root bending fatigue and the like. The material is generally required to have good toughness and wear resistance, so the performance of the material can be reflected by the contact fatigue of the material.

Chinese patent CN 101319294A discloses a fine-grain carburized gear steel and a manufacturing method thereof, and the invention particularly relates to a fine-grain carburized gear steel and a manufacturing method thereof. The steel comprises the following chemical components in percentage by weight: 0.15 to 0.25 percent of C, less than or equal to 0.35 percent of Si, 0.60 to 0.90 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.010 percent of S, 0.80 to 1.20 percent of Cr, 0.15 to 0.35 percent of Mo, 0.02 to 0.08 percent of Nb, 0.0005 to 0.0035 percent of B, 0.02 to 0.06 percent of Al, 0.01 to 0.04 percent of Ti, and [ N ]]≤0.015％，[O]Less than or equal to 0.0015 percent, and the balance of Fe and inevitable impurities. While Ti is required to be not less than 2[ N ]]，B≥([N]-Ti/3.4)/1.4 + 0.001. And a rolling production process with the finishing temperature lower than 900 ℃ is adopted. Steel of the invention and existing carburized gearsCompared with the steel 20CrMoH, the grain size of the steel after carburization and quenching is more than 10 grades, the bending fatigue strength (sigma-1) is improved by more than 15 percent, and the contact fatigue life (L10) is improved by more than 30 percent. But also has a rated contact fatigue of only 2.0X 10⁷On the other hand, with the increasing performance requirements of high-performance gear steels in the industries of high-speed rail, wind power and the like, it has not been able to meet the current requirements of high-contact fatigue performance materials, and therefore, it is necessary to develop carburized gear steels with better performance.

Disclosure of Invention

One of the objects of the present invention is to provide Nb and V microalloyed gear steel, which has a ferrite + pearlite structure and excellent performance by controlling the chemical composition and content of the gear steel.

The invention also aims to provide a preparation method of Nb-V microalloyed gear steel, which can obtain gear steel with ferrite + pearlite structure and excellent performance by controlling various production process parameters.

The invention also aims to provide a heat treatment method of Nb and V microalloyed gear steel, wherein the tensile strength of the Nb and V microalloyed gear steel after heat treatment is more than or equal to 1465MPa, the yield strength is more than or equal to 1340MPa, the elongation is more than or equal to 14%, the reduction of area is more than or equal to 50%, and the impact power KV2 is more than or equal to 95J.

The fourth purpose of the invention is to provide a carburizing treatment method of Nb and V microalloyed gear steel, which can ensure that the gear steel obtains good fatigue performance.

The fifth purpose of the invention is to provide Nb and V microalloyed carburized gear steel with high contact fatigue performance, which is obtained by preparing the Nb and V microalloyed gear steel by the preparation method provided by the invention and then processing the Nb and V microalloyed gear steel by a carburization treatment method, and has high contact fatigue performance.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

the Nb and V microalloyed gear steel comprises the following chemical components in percentage by weight: c: 0.15 to 0.19%, Si: 0.15-0.30%, Mn: 0.60-0.90%, Cr: 1.60-1.80%, Mo: 0.20 to 0.35%, Nb: 0.025 to 0.040%, Ni: 1.50-1.70%, Al: 0.020 to 0.040%, P: less than or equal to 0.010 percent, S: less than or equal to 0.010 percent, and 0.05 to 0.20 percent of V; T.O: less than or equal to 10ppm, [ H ]: 1.0ppm or less, [ N ]: 80-120ppm, Al/[ N ]: 2.5 to 5.0, and the balance of Fe and inevitable impurity elements, wherein Km is C + Mn/6+ (Cr + Mo + V)/5+ (Ni + Cu)/15+10Nb, and Km: 0.97 to 1.37.

The Nb and V microalloyed gear steel preferably comprises the following chemical components in percentage by weight: c: 0.17 to 0.19%, Si: 0.25 to 0.28%, Mn: 0.72-0.75%, Cr: 1.65-1.70%, Mo: 0.25 to 0.30%, Nb: 0.030-0.035%, Ni: 1.60-1.65%, Al: 0.030-0.035%, P: less than or equal to 0.010 percent, S: less than or equal to 0.001 percent, and 0.10 to 0.15 percent of V; T.O: less than or equal to 10ppm, [ H ]: 1.0ppm or less, [ N ]: 95-100ppm, Al/[ N ]: 3.0-3.5, and the balance of Fe and inevitable impurity elements, and Km: 1.10 to 1.20.

The metallographic structure of the Nb-V microalloyed gear steel is ferrite plus pearlite, and the grain size grade is more than 9.0 grade.

The invention provides a preparation method of Nb and V microalloyed gear steel, which comprises the following steps: smelting in an electric arc furnace, LF refining, RH vacuum treatment, continuous casting, hot rolling and slow cooling.

During smelting in an electric arc furnace and RH vacuum treatment, adding the ferrocolumbium before tapping in the electric furnace, and adding V at the later stage of RH vacuum treatment; the Nb is added before the tapping of the electric furnace to ensure the yield of the Nb, and the Nb added earlier is difficult to be burnt due to the higher melting point of the Nb. V has higher yield in the smelting process, and can be added in the early stage of LF under the normal condition to ensure that the formed large-particle V (C, N) inclusion floats upwards, but the content of N in the invention is higher to control, so the invention requires to be added in the later stage of RH, reduces the chance of forming large-particle V (C, N) inclusion, ensures that the V (C, N) inclusion fully floats upwards by prolonging the soft blowing time, and ensures that the V element is homogenized.

In the hot rolling process, round steel rolling is carried out after a continuous casting billet is heated at 1230-1280 ℃ and is kept warm for more than or equal to 5 hours, the temperature uniformity of the casting billet is guaranteed by reasonable temperature and warm keeping time, the economy is considered, the initial rolling temperature is 1120-1180 ℃, the material deformation stability is guaranteed, the final rolling temperature is 930-980 ℃, and the high final rolling temperature can prevent Nb from being unevenly precipitated at the crystal boundary and mixed crystals from occurring.

In the slow cooling process, after rolling, cooling the rolled steel plate to 600-650 ℃ through a cooling bed, and entering a pit for slow cooling; the slow cooling time is more than or equal to 45 hours, on-line annealing is realized by utilizing the residual temperature of steel, the metallographic structure of ferrite and pearlite is ensured to be obtained, and the surface is ensured to have no decarburization and zero defect by polishing and skinning after being taken out of a pit.

Preferably, the round steel rolling is carried out after the continuous casting billet is heated and insulated for 6.0-7.0 h at 1250-1260 ℃, the initial rolling temperature is 1140-1160 ℃, the final rolling temperature is 950-960 ℃, and the round steel is cooled to 630-640 ℃ by a cooling bed and then enters a pit for slow cooling for 45-50 h.

The heat treatment method of the Nb-V microalloyed gear steel provided by the invention comprises the following steps: the gear steel after hot rolling and slow cooling is subjected to quenching at 830 +/-20 ℃ and tempering at 180 +/-30 ℃ for heat treatment.

The tensile strength of the Nb and V microalloyed gear steel after heat treatment is more than or equal to 1465MPa, the yield strength is more than or equal to 1340MPa, the elongation is more than or equal to 14 percent, the reduction of area is more than or equal to 50 percent, and the impact energy KV2 is more than or equal to 95J.

The carburizing treatment method of the Nb-V microalloyed gear steel provided by the invention performs carburizing by a process of twice carburizing and twice diffusing at 930 +/-20 ℃, after carburizing, the sample is cooled to room temperature, then is heated to 880 +/-20 ℃, is subjected to oil quenching, and finally is tempered at 180 +/-30 ℃.

The Nb-V microalloyed carburized gear steel with high contact fatigue performance is obtained by preparing the Nb-V microalloyed gear steel by the preparation method and then processing the Nb-V microalloyed gear steel by the carburization processing method.

The metallographic structure of a carburized layer of the Nb and V microalloying carburized gear steel with high contact fatigue performance is acicular martensite, and the metallographic structure of the core is lath martensite; the grain size of the carburized layer is more than or equal to 10.0 grade, and the grain size is less than or equal to 11.2 mu m; the grain size of the core is more than or equal to 8.5 grade, and the grain size is less than or equal to 18.5 mu m.

The rated fatigue life L of the Nb-V microalloying carburized gear steel with high contact fatigue performance is L under the condition that the compressive stress is 4.0GPa₁₀≥5×10⁷Median fatigue life L₅₀≥8.2×10⁷。

The components of the gear steel provided by the invention are controlled as follows:

c: c is the most basic effective strengthening element in steel and the most effective element influencing hardenability, the content of C cannot be lower than 0.15 percent in order to ensure the sufficient strength and hardenability of the gear steel, and the content of C cannot be higher than 0.19 percent in order to ensure the toughness of a core part because the gear steel is carburized gear steel, so that the content of C is determined to be 0.15-0.19 percent.

Si: si is a deoxidizer, and can improve the hardenability of the gear steel by simultaneously improving the strong hardness of the steel through solid solution strengthening, the content of Si is not less than 0.15 percent, but the excessive silicon increases the activity of C, promotes the decarburization and graphitization tendency of the steel during rolling and heat treatment, makes a carburized layer easy to oxidize, and therefore the content of Si is not more than 0.30 percent. The Si content is controlled to be 0.15-0.30%.

Mn: mn can be dissolved in ferrite, so that the hardness and strength of the ferrite and austenite in the steel are improved, and meanwhile, the stability of an austenite structure can be improved, and the hardenability of the steel is obviously improved. However, excessive Mn lowers the plasticity of the steel, and the toughness of the steel deteriorates during hot rolling. The Mn content is controlled to be 0.60-90%.

Cr: cr can improve the hardenability and strength of steel, Cr can also reduce the activity of C, can reduce the decarburization tendency of the steel surface in the heating, rolling and heat treatment processes, and is beneficial to obtaining high fatigue resistance, so the Cr content cannot be lower than 1.60 percent, excessively high Cr can reduce the toughness of steel, simultaneously a large amount of carbide appears in a carburized layer structure to influence the performance of the carburized layer, and the Cr content cannot be higher than 1.80 percent. The Cr content is controlled to be 1.60-1.80%.

Mo: mo can obviously improve the hardenability of steel and prevent temper brittleness and overheating tendency. In addition, the reasonable matching of the Mo element and the Cr element can obviously improve the hardenability and the tempering resistance, and the Mo can refine grains. And if the Mo content is too low, the effect is limited, if the Mo content is too high, the formation of a grain boundary ferrite film is promoted, the thermoplasticity of the steel is not facilitated, the reheating crack tendency of the steel is increased, and the cost is higher. Therefore, the Mo content is controlled to be 0.20 to 0.35%.

Ni: ni can effectively improve the core toughness of steel, reduce the ductile-brittle transition temperature, improve the low-temperature impact property, and has the effect of improving the fatigue strength of steel materials, and the machinability after hot working can be reduced due to the excessively high Ni content. Therefore, the Ni content is controlled to 1.50 to 1.70%.

Al: al is an effective deoxidizer and can form AlN refined grains, and when the Al content is less than 0.020%, the effect is not obvious, and when the Al content is more than 0.040%, coarse inclusions are easily formed, and the performance of the steel is deteriorated. Therefore, the Al content should be controlled to 0.020-0.040%.

[ N ]: can form compounds with Nb, Al and the like to refine grains, reasonable Al/[ N ] has obvious effect on grain refinement, and excessive [ N ] can form continuous casting defects such as bubbles and the like. Therefore, the [ N ] content should be controlled to 80-120ppm, while controlling Al/[ N ]: 2.0 to 4.0.

P and S: the sulfur is easy to form MnS inclusion with manganese in the steel, so that the steel is hot-brittle; p is an element with strong segregation tendency, increases the cold brittleness of steel, reduces the plasticity and is harmful to the uniformity of the product structure and performance. Controlling P to be less than or equal to 0.010 percent, and S: less than or equal to 0.010 percent.

T.O and [ H ]: forming oxide inclusions in the steel by the T.O, and controlling the T.O to be less than or equal to 10 ppm; [H] white spots are formed in steel, the product performance is seriously influenced, and the [ H ] is controlled to be less than or equal to 1.0 ppm.

Nb: nb is a microalloying element which is very effective in refining grains, and the carbonitride of Nb can pin the grain boundary, prevent austenite grains from growing and effectively reduce carburizing and quenching deformation. When the Nb content is less than 0.025 percent, the carburizing temperature exceeds 980 ℃, the heat preservation time exceeds 10 hours, the grain size requirement cannot be well met, and the effect of excessive Nb is not obviously increased. Therefore, the Nb content is controlled to 0.025 to 0.040%.

V: vanadium is a widely used microalloying element and has the effect of preventing austenite grains from growing when heated. The addition of vanadium can prevent the growth of austenite grains through the precipitation of V (C, N) and the grain boundary pinning effect of undissolved V (C, N) grains; thereby improving the toughness of the steel, but at the same time, reducing the hardenability of the steel. Excessive V addition increases production cost, so the V content should be controlled to be 0.05-0.20% of V.

In the production process of Nb and V microalloyed gear steel, V (C, N) and Nb (C, N) will precipitate, and undissolved carbonitride particles will play a role in pinning grain boundaries and preventing recrystallization when the gear steel is heated to austenitize, thereby refining austenite grains. V (C, N) has the function of preventing austenite grains from growing when austenitizing below 900 ℃, and Nb (C, N) is difficult to dissolve below 1100 ℃ and has the function of inhibiting the growth of the austenite grains. In addition, niobium and vanadium in a solid solution state or a precipitation state can delay austenite recrystallization and play a role in refining austenite grains. By adding a small amount of Nb and V alloy and adopting reasonable proportion, the grain refinement of the gear steel in the carburizing process is ensured, the strength of the carburized gear is improved, the use of alloy elements such as Cr, Ni and Mo can be reduced, and the production cost is reduced. According to the test results, the addition amount of the Nb and V alloy meets the requirement of the component range and also meets the Km: 0.97 to 1.37, wherein Km is C + Mn/6+ (Cr + Mo + V)/5+ (Ni + Cu)/15+10 Nb.

Nb and V exist in steel as substitutional solute atoms, and the Nb and V atoms are larger than iron atoms in size, are easy to be deviated on dislocation lines, generate strong dragging action on dislocation climbing, and inhibit recrystallization nucleation, so that the steel has strong prevention action on recrystallization. During high-temperature carburization, the effect of Nb and V elements on recrystallization is shown as a solute dragging mechanism, so that in the carburization process, crystal boundary extension is influenced, and in the carburization process, a large number of carbon atoms enter a carburization layer, so that the probability of forming MC by interstitial atoms Nb and V is increased, and the dislocation density is also increased. And slowly cooling after carburizing, and heating to the temperature higher than austenitizing temperature, so that the solute in the crystal has enough time to migrate to the newly formed MC on the dislocation and inoculates a new crystal boundary, and further finer crystal grains are obtained when quenching is carried out again.

Compared with the prior art, the invention has the following beneficial effects:

1. the chemical components and the content of the gear steel are controlled, and Nb and V microalloying is adopted to obtain Nb and V microalloyed gear steel with a ferrite plus pearlite structure and crystal grains of more than 9.0 grade;

2. the invention carries out heat treatment on the gear steel after hot rolling and slow cooling through quenching at 830 +/-20 ℃ and tempering at 180 +/-30 ℃ to obtain Nb and V microalloyed gear steel with the tensile strength of more than or equal to 1465MPa, the yield strength of more than or equal to 1340MPa, the elongation of more than or equal to 14 percent, the reduction of area of more than or equal to 50 percent and the impact power KV2 of more than or equal to 95J;

3. the Nb-V microalloyed gear steel is prepared by the preparation method of the invention and then carburized by a process of twice carburization and twice diffusion at 930 +/-20 ℃, the carburization mode can ensure that a carburized surface layer has higher carbon content and the surface hardness after quenching, meanwhile, the sectional diffusion ensures the uniformity of diffusion, ensures the good transition of the carburized layer and the matrix, ensures the performance of the matrix and the carburized layer to be excessively stable, cools to room temperature after carburization, then heating to 880 +/-20 ℃ for oil quenching, cooling and quenching again to ensure grain refinement, meanwhile, the content of residual austenite is reduced, the contact fatigue property of the material is ensured, and finally the Nb-V microalloyed carburized gear steel with high contact fatigue property is obtained after tempering treatment at 180 +/-30 ℃, wherein the rated fatigue life of the carburized gear steel is L under the condition that the compressive stress is 4.0 GPa.₁₀≥5×10⁷Median fatigue life L₅₀≥8.2×10⁷。

Drawings

FIG. 1 is a grain size diagram of a carburized layer of a pinion steel in example 1 after the carburizing treatment;

FIG. 2 is a grain size diagram of a carburized layer of the pinion steel in example 2 after the carburizing treatment;

FIG. 3 is a grain size diagram of a carburized layer of the pinion steel in example 3 after carburization;

FIG. 4 is a grain size diagram of a carburized layer of the pinion steel in comparative example 1 after the carburizing treatment;

FIG. 5 is a grain size diagram of a carburized layer of the pinion steel in comparative example 2 after the carburizing treatment;

FIG. 6 is a grain size diagram of a carburized layer of the pinion steel in comparative example 3 after the carburizing treatment;

FIG. 7 is a grain size diagram of a carburized layer after carburization of the pinion steel in comparative example 4;

FIG. 8 is a grain size diagram of the core of the pinion steel in example 1 after carburizing treatment;

FIG. 9 is a grain size diagram of the core of the pinion steel of example 2 after carburizing treatment;

FIG. 10 is a grain size diagram of the core of the pinion steel of example 3 after carburizing treatment;

FIG. 11 is a grain size diagram of the core portion of the gear steel in comparative example 1 after carburizing treatment;

FIG. 12 is a grain size diagram of the core portion of the gear steel in comparative example 2 after carburizing treatment;

FIG. 13 is a grain size diagram of the core portion of the gear steel in comparative example 3 after carburizing treatment;

FIG. 14 is a grain size diagram of the core portion of the pinion steel in comparative example 4 after carburizing treatment;

FIG. 15 is a microstructure view of hot-rolled round steel of example 3;

FIG. 16 is a microstructure view of a hot-rolled round steel of comparative example 3;

FIG. 17 is a microstructure view of a carburized layer of the gear steel in example 1 after carburization treatment;

FIG. 18 is a microstructure view of a carburized layer of a gear steel in example 2 after the carburizing treatment;

FIG. 19 is a microstructure view of a carburized layer of a gear steel in example 3 after the carburizing treatment;

FIG. 20 is a microstructure diagram of a carburized layer of the gear steel in comparative example 1 after the carburizing treatment;

FIG. 21 is a microstructure diagram of a carburized layer of the gear steel in comparative example 2 after the carburizing treatment;

FIG. 22 is a microstructure diagram of a carburized layer of the gear steel in comparative example 3 after the carburizing treatment;

FIG. 23 is a microstructure diagram of a carburized layer of the pinion steel in comparative example 4 after the carburizing treatment;

FIG. 24 is a microstructure view of a core portion of a gear steel in example 1 after carburizing treatment;

FIG. 25 is a microstructure of a core portion of a gear steel in example 2 after carburizing treatment;

FIG. 26 is a microstructure diagram of a core portion of a gear steel after carburizing treatment in example 3;

FIG. 27 is a microstructure diagram of a core portion of the gear steel in comparative example 1 after carburizing treatment;

FIG. 28 is a microstructure diagram of a core portion of the gear steel in comparative example 2 after carburizing treatment;

FIG. 29 is a microstructure of a core portion of the gear steel in comparative example 3 after carburizing treatment;

FIG. 30 is a microstructure view of a core portion of the gear steel in comparative example 4 after carburizing treatment.

Detailed Description

The Nb and V microalloyed gear steel comprises the following chemical components in percentage by weight: c: 0.15 to 0.19%, Si: 0.15-0.30%, Mn: 0.60-0.90%, Cr: 1.60-1.80%, Mo: 0.20 to 0.35%, Nb: 0.025 to 0.040%, Ni: 1.50-1.70%, Al: 0.020 to 0.040%, P: less than or equal to 0.010 percent, S: less than or equal to 0.010 percent, and 0.05 to 0.20 percent of V; T.O: less than or equal to 10ppm, [ H ]: 1.0ppm or less, [ N ]: 80-120ppm, Al/[ N ]: 2.5-5.0, and the balance of Fe and inevitable impurity elements, wherein Km is C + Mn/6+ (Cr + Mo + V)/5+ (Ni + Cu)/15+10Nb, and Km: 0.97 to 1.37.

The production is carried out by adopting the processes of electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, hot rolling and slow cooling, and the method comprises the following steps:

heating the continuous casting billet at 1230-1280 ℃ for more than or equal to 5 hours, and then rolling round steel at the beginning rolling temperature: 1120-1180 ℃, the finishing temperature of 930-980 ℃, cooling to be less than or equal to 650 ℃ through a cooling bed after rolling, entering a pit for slow cooling, slowly cooling for 48 hours, and polishing and peeling after leaving the pit to ensure that the surface has no decarburization and zero defect.

2 furnace 18CrNiMo7-6 steel (centre line) was produced as a comparison steel according to the requirements in EN 10084 and according to the same production process.

After the gear steel of the test steel subjected to hot rolling and slow cooling is subjected to quenching at 830 +/-20 ℃ and tempering at 180 +/-30 ℃, the mechanical property of the gear steel is tested.

The test steel after hot rolling and slow cooling is processed into a contact fatigue sample, carburization is carried out according to the process of twice carburization and twice diffusion at 930 ℃, the sample is cooled to room temperature after carburization, then is heated to 880 ℃ for oil quenching, finally the sample is tempered at 180 ℃, and then grain size inspection and fatigue performance inspection are carried out. The 18CrNiMo7-6 steel was rolled and processed into a contact fatigue test specimen as a comparative steel, and grain size test and fatigue property test were carried out in the same manner as described above.

The present invention will be described in detail with reference to examples.

TABLE 1 chemical composition of the gear steel in examples and comparative examples (unit: [ N ] is ppm, the remainder is%)

The gear steel with the components shown in the table 1 is produced according to the process of electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, hot rolling and slow cooling, wherein the process parameters of the hot rolling and the slow cooling are shown in the table 2.

TABLE 2 Steel Rolling production Process parameters

	Soaking temperature/. degree.C	Total heating time/h	The initial rolling temperature/. degree.C	Final Rolling temperature/. degree.C	Pit entry temperature/. degree.C	Slow cooling time/h
							Example 1	1255	6.5	1150	952	634	48
Example 2	1255	6.5	1150	955	633	48
							Example 3	1255	6.5	1150	955	639	48
Comparative example 1	1255	6.5	1150	953	638	48
							Comparative example 2	1255	6.5	1150	955	633	48
Comparative example 3	1200	6.5	1100	900	600	30
							Comparative example 4	1255	6.5	1150	955	639	48

Note: example 3 is a gear steel identical to comparative example 4 except that the subsequent carburizing treatment is performed according to different carburizing processes

The mechanical properties of the gear steels obtained after the rolling slow cooling in the above examples and comparative examples after the heat treatment by quenching at 830 ℃ and tempering at 180 ℃ are shown in table 3.

TABLE 3

Carburizing the gear steel obtained after rolling in the examples 1-3 and the comparative examples 1-3 according to a process of twice carburizing and twice diffusing at 930 ℃, cooling the sample to room temperature after carburizing, heating to 880 ℃, performing oil quenching, and finally tempering the sample at 180 ℃; the gear steel obtained after rolling in the comparative example 4 is carburized according to the process of twice carburizing and twice diffusing at 930 ℃, after the carburization, the sample is cooled to 880 ℃ for oil quenching, and finally the sample is tempered at 180 ℃; after heat treatment, grain size inspection and fatigue performance inspection are carried out, the inspection results are respectively shown in tables 4 and 5, and as can be seen from table 4, after the gear steel in the embodiments 1 to 3 of the invention is carburized at high temperature, the grain size of the carburized layer is over 10.0 grade, the grain size is 5.3 to 7.5 μm, the grain size of a comparative example is 9.0 to 9.5 grade, and the grain size is 13.1 to 15.3 μm; the grain size of the center part in the examples is above grade 9.0 and the grain size is 12.6-15.2 μm, while the grain size in the comparative examples is 8.0-8.5 and the grain size is 17.2-21.2 μm.

TABLE 4 grain size of grain size grade after carburization

Table 5 shows a comparison of contact fatigue between examples and comparative examples, and it can be seen from Table 5 that the contact fatigue of the gear steel in the examples is improved by 30% or more as compared with that of the comparative example, and that the rated fatigue life L is obtained under the condition that the compressive stress is 4.0GPa₁₀≥5.2×10⁷Median fatigue life L₅₀≥8.4×10⁷。

TABLE 5 contact fatigue rated fatigue Life and median fatigue Life

	Contact stress/GPa	L₁₀/cycle	L₅₀/cycle
				Example 1	4.0	5.29×10⁷	8.72×10⁷
Example 2	4.0	6.28×10⁷	9.68×10⁷
				Example 3	4.0	5.20×10⁷	8.46×10⁷
Comparative example 1	4.0	3.36×10⁷	5.60×10⁷
				Comparative example 2	4.0	3.48×10⁷	5.62×10⁷
Comparative example 3	4.0	4.33×10⁷	6.58×10⁷
				Comparative example 4	4.0	3.29×10⁷	5.63×10⁷

As can be seen from FIGS. 1-2 and tables 1-5, the steel of the present invention provides a gear steel with high contact fatigue through alloy composition design, reasonable production process control and carburization process, after high temperature carburization at 930 deg.C, the grain size of carburized layer is less than or equal to 8.6 μm, the grain size of core is less than or equal to 15.5 μm, and under the condition of compressive stress of 4.0GPa, the rated fatigue life L is₁₀≥5.2×10⁷Median fatigue life L₅₀≥8.4×10⁷。

The above detailed description of an Nb, V microalloyed gear steel and the method of manufacturing, heat treatment, carburization and carburized gear steel with reference to the examples is illustrative and not restrictive, and several examples may be cited within the scope of the present invention, and it is therefore intended that variations and modifications within the spirit and scope of the present invention be embraced by the present invention.

Claims

1. The Nb and V microalloyed gear steel is characterized by comprising the following chemical components in percentage by weight: c: 0.15 to 0.19%, Si: 0.15-0.30%, Mn: 0.60-0.90%, Cr: 1.60-1.80%, Mo: 0.20 to 0.35%, Nb: 0.025 to 0.040%, Ni: 1.50-1.70%, Al: 0.020 to 0.040%, P: less than or equal to 0.010 percent, S: less than or equal to 0.010 percent, and 0.05 to 0.20 percent of V; T.O: less than or equal to 10ppm, [ H ]: 1.0ppm or less, [ N ]: 80-120ppm, Al/[ N ]: 2.5-5.0, and the balance of Fe and inevitable impurity elements, wherein Km = C + Mn/6+ (Cr + Mo + V)/5+ (Ni + Cu)/15+10Nb, and Km: 0.97 to 1.37;

the preparation method of the Nb and V microalloyed gear steel comprises the following steps: smelting in an electric arc furnace, LF refining, RH vacuum treatment, continuous casting, hot rolling and slow cooling; in the slow cooling process, after rolling, the rolled steel is cooled to 600-650 ℃ by a cooling bed and then enters a pit for slow cooling, and the slow cooling time is more than or equal to 45 hours;

the metallographic structure of a carburized layer of the Nb-V microalloyed gear steel after carburization is acicular martensite, and the metallographic structure of the core is lath martensite; the grain size of the carburized layer is more than or equal to 10.0 grade, and the grain size is less than or equal to 11.2 mu m; the grain size of the core is more than or equal to 8.5 grade, and the grain size is less than or equal to 18.5 mu m.

2. The Nb, V microalloyed gear steel according to claim 1, characterized by comprising the following chemical composition in percentage by weight: c: 0.17 to 0.19%, Si: 0.25 to 0.28%, Mn: 0.72-0.75%, Cr: 1.65-1.70%, Mo: 0.25 to 0.30%, Nb: 0.030-0.035%, Ni: 1.60-1.65%, Al: 0.030-0.035%, P: less than or equal to 0.010 percent, S: less than or equal to 0.001 percent, and 0.10 to 0.15 percent of V; T.O: less than or equal to 10ppm, [ H ]: 1.0ppm or less, [ N ]: 95-100ppm, Al/[ N ]: 3.0-3.5, and the balance of Fe and inevitable impurity elements, and Km: 1.10 to 1.20.

3. The Nb, V microalloyed gear steel according to claim 1 or 2, wherein the metallographic structure of the Nb, V microalloyed gear steel is ferrite + pearlite with a grain size classification of 9.0 or more.

4. A method of producing Nb, V microalloyed gear steel in accordance with any one of claims 1 to 3, the method including the steps of: smelting in an electric arc furnace, LF refining, RH vacuum treatment, continuous casting, hot rolling and slow cooling.

5. The method according to claim 4, wherein the ferrocolumbium is added before tapping of the electric furnace, and the V is added after RH vacuum treatment.

6. The preparation method of claim 4, wherein in the hot rolling process, round steel rolling is performed after the continuous casting billet is heated at 1230-1280 ℃ and kept at the temperature for more than or equal to 5 hours, the initial rolling temperature is 1120-1180 ℃, and the final rolling temperature is 930-980 ℃.

7. The preparation method of claim 4, wherein in the slow cooling process, the rolled steel is cooled to 600-650 ℃ by a cooling bed and then is put into a pit for slow cooling, and the slow cooling time is not less than 45 hours.

8. The method for heat-treating Nb, V microalloyed gear steel according to any one of claims 1 to 3, wherein the gear steel after the hot rolling and slow cooling is heat-treated by quenching at 830. + -. 20 ℃ and tempering at 180. + -. 30 ℃.

9. The heat treatment method of Nb, V microalloyed pinion steel according to claim 8, characterized in that the Nb, V microalloyed pinion steel after heat treatment has tensile strength not less than 1465MPa, yield strength not less than 1340MPa, elongation not less than 14%, reduction of area not less than 50%, and impact energy KV2 not less than 95J.

10. A carburizing treatment method for Nb, V microalloyed gear steel according to any one of claims 1-3, characterized in that carburizing is carried out by a process of twice carburizing and twice diffusing at 930 +/-20 ℃, after carburizing, the sample is cooled to room temperature, then heated to 880 +/-20 ℃, oil quenched and finally tempered at 180 +/-30 ℃.

11. An Nb-V microalloyed carburized gear steel with high contact fatigue performance, characterized in that the Nb-V microalloyed gear steel according to any one of claims 1 to 3 is prepared by the preparation method according to any one of claims 4 to 7 and then is treated by the carburization treatment method according to claim 10.

12. The Nb, V microalloyed carburized gear steel for high contact fatigue performance according to claim 11, wherein a metallurgical structure of a carburized layer of the Nb, V microalloyed carburized gear steel for high contact fatigue performance is needle martensite, and a metallurgical structure of a core is lath martensite; the grain size of the carburized layer is more than or equal to 10.0 grade, and the grain size is less than or equal to 11.2 mu m; the grain size of the core is more than or equal to 8.5 grade, and the grain size is less than or equal to 18.5 mu m.

13. The Nb, V microalloyed carburized gear steel for high contact fatigue performance as in claim 11, wherein said Nb, V microalloyed carburized gear steel for high contact fatigue performance has a rated fatigue life L under a compressive stress of 4.0GPa₁₀≥5×10⁷Median fatigue life L₅₀≥8.2×10⁷。