High-strength new energy automobile gear steel and production method thereof

Abstract

The invention relates to high-strength new energy automobile gear steel and a production method thereof, and belongs to the field of iron-based alloys. Nonmetallic inclusions of the steel are inspected according to a DIN 50602 standard K method, and the condition that K4 (O + S) is smaller than or equal to 5 is met; sulfides are graded according to SEP 1572, and the sulfides are less than or equal to grade 3; a macrostructure is inspected according to ASTM E381, C, R and S do not exceed 1.0 grade, shrinkage cavities, cracks and subcutaneous bubbles do not exist, the tensile strength Rm is larger than or equal to 1600 MPa, the yield strength Rel is larger than or equal to 1300 MPa, and the gear steel is pure and high in strength. Continuous casting and rolling forming are adopted in steel production, and compared with electroslag remelting, the method has the advantages of being good in production continuity and high in yield.

Description

Technical Field

[0001] The invention belongs to the field of iron-based alloys in metallurgy, and more specifically relates to steel for automobile gears and a production method thereof. Background Art

[0002] With the rapid development of low-energy, low-pollution new energy vehicle technology, the sales volume and market share of new energy vehicles have hit new highs. As the core component for transmitting power in the automobile transmission system, gears have very demanding market requirements for their uniformity, strength and impact toughness. Compared with traditional fuel vehicles, new energy vehicles start quickly, and the transmission system is instantly subjected to huge alternating loads and impacts, so new energy vehicles have higher strength requirements for gears in the transmission system.

[0003] Gear steel should have the following properties: high uniformity of structure, high strength, long life, etc. At present, high-performance gear steel is mainly produced by electroslag remelting process, which can obtain billets with higher purity and more uniform structure, but electroslag remelting has obvious production disadvantages such as low production efficiency, high energy consumption, low yield rate, poor production discontinuity, and poor stability of batch product quality. The continuous casting process has the advantages of high yield rate, high production efficiency, and low energy consumption, but the uniformity and purity of the billet structure are not as good as those of the electroslag remelting process. Therefore, the development of continuous casting production process for high-performance gear steel has great development prospects and helps to reduce the production cost of high-performance gear steel. Summary of the invention

[0004] The purpose of the present invention is to provide a method for producing high-strength new energy vehicle gear steel. By redesigning the chemical composition of the steel, further reducing harmful elements in the smelting process, optimizing the inclusion composition, reducing the inclusion size, and improving the purity of the molten steel, a non-uniform temperature rolling process is used during steel forming and rolling to improve the uniformity of the structure, and finally obtaining a method for producing high-strength new energy vehicle gear steel.

[0005] The technical solution adopted by the present invention to solve the above problems is: a high-strength new energy vehicle gear steel, the chemical composition of which is C: 0.19-0.24%, Si≤0.12%, S: 0.015-0.030%, Mn: 1.2-1.5%, Cr: 1.0-1.3%, Al: 0.02-0.05%, B: 0.002-0.004%, V: 0.003-0.005%, Nb: 0.005-0.009%, P≤0.020%, Cu≤0.20%, Ni≤0.20%, Mo≤0.10%, O≤0.0008%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, Ti≤0.001%, and the balance is Fe and unavoidable impurities.

[0006] The setting basis of the elemental composition is as follows:

[0007] 1) Determination of the C content

[0008] C is the most basic element in steel and also the most economical strengthening element. Among all elements, carbon has the greatest ability to increase the strength of steel. The strengthening effect of carbon on quenched and tempered steel is approximately 9 times that of chromium and 18 times that of manganese. Therefore, to ensure good strength and toughness in the core of the gear after carburizing heat treatment, an appropriate carbon content is required in the steel. The range of the C content in the present invention is determined to be 0.19 - 0.24%.

[0009] 2) Determination of the Si content

[0010] Si is dissolved in the ferrite phase and has a strong solid-solution strengthening effect, which can improve strength, elastic limit and hardenability. However, Si increases the overheating sensitivity, crack and decarburization tendency of the steel, and is prone to cause grain boundary oxidation during carburizing heat treatment, which is not conducive to tissue stability. The range of the Si content in the steel of the present invention is determined to be ≤0.12%.

[0011] 3) Determination of the Mn content

[0012] Mn, as an element acting as a deoxidizer, increases the strength of the steel through solid-solution strengthening. In addition, Mn fixes the S in the steel in the form of MnS, which can eliminate or reduce the hot brittleness caused by sulfur, thereby improving the hot working performance of the steel. Mn is a carbide-forming element, which enters the cementite and replaces a part of the iron atoms. In the steel, Mn plays a role in refining pearlite by reducing the critical transformation temperature, and also indirectly improves the strength of pearlite. The ability of Mn to stabilize the austenite structure is second only to Ni, and it also strongly increases the hardenability of the steel. The range of the Mn content in the present invention is determined to be 1.2 - 1.5%.

[0013] 4) Determination of the Cr content

[0014] Cr is a carbide-forming element, which can improve the hardenability, wear resistance and corrosion resistance of the steel. A part of the Cr in the steel replaces iron to form alloy cementite, which improves the tempering stability of the steel; a part is dissolved in the ferrite to produce solid-solution strengthening and improve the strength and hardness of the ferrite. In addition, Cr can also reduce the overheating tendency and surface decarburization rate of the steel. However, if the Cr content is too high, it will combine with the carbon in the steel and easily form large carbide, which will reduce the toughness of the steel and the gear life. Therefore, the range of the Cr content in the present invention is determined to be 1.0 - 1.3%.

[0015] 5) Determination of the Ni, Cu, Mo contents

[0016] Elements such as Ni, Cu, and Mo can improve the hardenability, strength, wear resistance, and corrosion resistance of steel to a certain extent, but Ni, Cu, and Mo all belong to precious metals. In this invention, the Ni content is determined to be ≤0.20%, the Cu content is determined to be ≤0.20%, and the Mo content is determined to be ≤0.10%.

[0017] 6) Determination of B content

[0018] Element B in steel has the effect of inhibiting the precipitation of ferrite and at the same time improving the hardenability of steel. The range of B content in this invention is determined to be 0.002 - 0.004%.

[0019] 7) Determination of Nb and V contents

[0020] By adding Nb and V into steel in combination, the CCT curve of steel can be shifted downward and to the right, which can increase the stability of austenite. In steel, they mostly exist in the form of precipitates such as carbides, nitrides, and carbonitrides. These precipitates usually have nanoscale sizes, and these dispersed small-sized precipitates can pin the austenite grain boundaries, hinder the growth of austenite grains, and refine the austenite grains of steel. In this invention, the Nb content is designed to be 0.005 - 0.009%, and the V content is designed to be 0.003 - 0.005%.

[0021] 8) Determination of Al content

[0022] Al is added as a deoxidizing element in steel. In addition to reducing the dissolved oxygen in the molten steel, Al can form dispersed and fine aluminum nitride with N to refine the grains. However, when the Al content is too high, large particles of Al2O3 and other brittle inclusions are easily formed during the melting process of molten steel, reducing the purity of molten steel and affecting the service life of steel. The range of Al content in this invention is determined to be 0.02 - 0.05%.

[0023] 9) Determination of Ti content

[0024] Ti is extremely easy to combine with N or C elements during the solidification process of molten steel to form non-metallic inclusions such as TiN or Ti(C, N). In terms of morphology, they are usually blocky inclusions with "edges and corners". Such inclusions have a relatively high hardness, seriously affecting the tissue uniformity, and are prone to stress concentration at the edges and corners during operation, severely reducing the fatigue life of the bearing. The range of Ti content in this invention is determined to be ≤0.001%.

[0025] 10) Determination of O content

[0026] Oxygen enters the steel naturally during the steelmaking process. The oxygen remaining in the steel in the later stage mainly exists in the form of non-metallic inclusions such as SiO2 and Al2O3 in the steel. In particular, Al2O3 inclusions seriously affect the fatigue life of bearings. A large number of fatigue life tests have shown that reducing the oxygen content is significantly beneficial to improving the purity of steel, especially reducing the content and size of oxide brittle inclusions. The range of O content in the present invention is determined to be ≤0.0008%.

[0027] 11) Determination of P and S contents

[0028] P causes serious segregation during solidification in steel. P dissolves in ferrite, causing the grains to distort and coarsen, and increasing cold brittleness. The range of P content in the present invention is determined to be ≤0.020%; S causes hot brittleness in steel, reducing the ductility and toughness of the steel, but adding an appropriate amount of S to the steel can improve the machinability. The range of S content in the present invention is determined to be 0.015 - 0.030%.

[0029] 12) Determination of As, Sn, Sb, Pb contents

[0030] Trace elements such as As, Sn, Sb, and Pb are all non-ferrous metals with low melting points. Their presence in steel causes soft spots and uneven hardness on the surface of parts. Therefore, they are regarded as harmful elements in steel. The range of the contents of these elements in the present invention is determined to be As ≤0.04%, Sn ≤0.03%, Sb ≤0.005%, and Pb ≤0.002%.

[0031] The production method of the high-strength new energy vehicle gear steel of the present invention has the following process flow: (KR) hot metal pretreatment → (converter or electric furnace) primary melting → (ladle refining furnace) refining → (RH furnace or VD furnace) vacuum degassing → continuous casting CCM → heating and blooming → slow cooling → heating and rolling → stacking cooling → finishing → surface and internal flaw detection → packaging.

[0032] The main steps are as follows:

[0033] (1) Steel melting:[[]]END]]

[0034] KR (Hot Metal Pretreatment), BOF or EAF Primary Melting: For hot metal pretreatment in KR, the insertion depth of the KR stirrer is 160 - 200 mm, and the rotation speed is 150 - 180 rpm. At the bottom of the hot metal ladle, a CaO-FeO-CaF2 composite powder is injected with the help of nitrogen for dephosphorization and desiliconization treatment. Under the action of mechanical stirring, chemical reactions for dephosphorization (P ≤ 0.050%) and desiliconization (Si ≤ 0.20%) are carried out, and then the harmful slag obtained from the reaction is removed by a robotic arm to ensure the cleanliness of the hot metal. Secondly, for primary melting in a BOF or EAF, oxygen is blown from the top of the furnace mouth for oxidation reaction, and argon is blown from the bottom for stirring. 20 - 30 cubic meters of oxygen and 1 - 2 cubic meters of argon are blown per ton of steel, and active composite lime CaO-MnO-CaF2 is added. Under the combined action of oxygen and argon, harmful element phosphorus (P ≤ 0.020%) is removed. The end point carbon at the time of tapping from the primary melting furnace is 0.06% - 0.10%, the tapping temperature ≥ 1660 °C. Slag blocking is adopted during tapping, and some alloys (preliminary composition adjustment) are added during tapping. After tapping, it is quickly lifted and transported to the refining LF furnace for smelting.

[0035] Ladle Furnace Refining: During LF refining, precipitation deoxidation is carried out in the molten steel by adding Al grains, and a CaO-Al2O3-MgO high-performance composite slag-making agent is added on the surface of the molten steel for diffusion deoxidation and adsorption to remove harmful non-metallic inclusions. Preferably: During the smelting process, first add Al grains (100 - 150 Kg) to the molten steel, then add a CaO-Al2O3-MgO high-performance composite slag-making agent (350 - 400 Kg), cover the ladle with a protective cover, connect argon at the bottom of the ladle, and then insert the electrode into the slag for submerged arc power-on. Power is cut off every 10 minutes to measure the temperature and take samples for analysis of the molten steel. The required main elements (B, V, Nb, etc.) are added according to the target requirements. The number of temperature measurement and sampling times in the refining furnace is controlled at 3 - 4 times until the composition meets the product requirements. No alloys are allowed to be added within 10 minutes before the end of refining. Finally, power is cut off, and the argon flow rate at the bottom of the ladle is adjusted to ensure that the molten steel is not exposed to the air. The refining time is controlled to be more than 40 minutes, and the soft blowing time of the molten steel is 15 - 20 minutes to allow harmful inclusions to float up and be removed.

[0036] Vacuum Degassing: During RH or VD vacuum degassing, it is preferably set that the maximum vacuum degree in the vacuum furnace ≤ 0.7 mbar, and the molten steel vacuum circulation treatment time is maintained for 10 - 15 minutes to ensure effective removal of harmful gases in the steel. After the vacuum treatment is completed, argon is blown at the bottom of the ladle, and the argon flow rate is controlled to ensure that the molten steel is not exposed to the air. The soft blowing time of the molten steel is preferably ≤ 15 minutes to further remove harmful gases and non-metallic inclusions in the steel grade.

[0037] (2)Continuous casting: Argon gas protection casting is adopted throughout the process to prevent secondary pollution and oxidation of molten steel; low superheat casting is adopted (superheat △T ≤ 20°C); the molten steel volume in the tundish is controlled at 10 - 20 tons, and a fixed casting speed of 1.0 m / min is adopted; an appropriate steel flow ratio of water volume (1.2 - 1.4 L / kg) is used, and electromagnetic stirring (frequency = 3 HZ, current = 800 A) is used in the tundish to break the bridging in the columnar crystal zone during the solidification of the continuous casting billet; through the above various control technologies, the center segregation of the steel is further improved, and the microstructure uniformity of the steel is enhanced.

[0038] (3)Heating and blooming: The continuous casting billet is hot-transported to a reheating furnace with a neutral or weakly oxidizing atmosphere and heated to 1200°C - 1240°C, and the heating and holding time ≥ 5 h to fully austenitize the microstructure of the steel. After the billet is taken out of the furnace, it is first descaled by high-pressure water before rolling to remove the scale on the surface of the continuous casting billet, and then multi-pass large reduction is adopted. Preferably, 4-pass rolling with large reduction is used, and the reduction ratios of the four passes are 25% - 30%, 20% - 25%, 25% - 30%, and 10% - 15% respectively, so that the microstructure of the core of the billet undergoes deformation and recrystallization preferentially during the deformation process, thereby making the center microstructure of the steel more uniform and dense under the action of large reduction. Finally, it is rolled and bloomed into an intermediate billet of 200 mm × 200 mm - 250 mm × 250 mm, and the intermediate billet is taken offline at ≥ 550°C and sent to a slow cooling pit for slow cooling, and the slow cooling time ≥ 48 hours.

[0039] (4)Heating and rolling: The intermediate billet is cold-charged into a reheating furnace with a neutral or weakly oxidizing atmosphere, heated and then rolled into round bar steel. The specific rolling process is as follows: The intermediate billet enters the walking beam reheating furnace through the conveying roller table, and the steel is heated to 1150°C - 1200°C in the reheating furnace, with a holding time of 1.5 - 2 h, the starting rolling temperature is 950°C - 1000°C, and the final rolling temperature is controlled at 850°C - 900°C. The steel is then alternately rolled by 6 horizontal-vertical rolling mills and finally rolled into round bar steel with a diameter of φ20 mm - φ60 mm, and is taken offline for stacking and cooling.

[0040] (5)Finishing: It includes finishing processes such as straightening and chamfering to ensure that indicators such as dimensions and camber meet the requirements.

[0041] (6)100% non-destructive testing is carried out on the surface and inside, and only the qualified ones can become qualified products.

[0042] Compared with the prior art, the advantages of the present invention are as follows: The present invention is a new type of new energy vehicle gear steel with a new chemical composition. Compared with the domestic electroslag remelting process, the "vacuum degassing + continuous casting" short process adopted by the present invention has the advantages of high production efficiency, low energy consumption, good continuity, and high quality stability. The steel of the present invention not only has high stable and reliable purity and microstructure uniformity, but also has high strength that the traditional new energy vehicle gear steel does not have.

[0043] The steel has the following technical specifications:

[0044] The macrostructure of the steel is inspected and rated according to ASTM E381, meeting the requirements that C, R, and S do not exceed grade 1.0, and no shrinkage cavities, cracks, or subcutaneous air bubbles appear.

[0045] The grain size of the steel is inspected according to ISO 643 at 960 ± 10 °C for 4 hours, water quenched, and the austenite grain size of the steel ≥ 7.5 grades.

[0046] The non-metallic inclusions in the steel are inspected according to the K method of DIN 50602 standard, meeting K4 (O + S) ≤ 5; the sulfides are rated according to SEP1572, meeting ≤ grade 3.

[0047] The end hardenability of the steel is inspected according to GB / T225. The heat treatment system is: normalizing at 920 ± 10 °C for 1 hour, air cooling; end quenching at 900 ± 5 °C, water cooling, meeting J5: 46 - 50 HRC, J9: 42 - 46 HRC, J15: 38 - 42 HRC.

[0048] The mechanical properties of the steel are inspected according to GB / T228, as shown in Table 1.

[0049] Table 1

[0050]

[0051] The steel of the present invention is subjected to a rolling contact fatigue test according to YB / T5345 - 2014. Under the conditions of room temperature (20 °C ± 5 °C) and a cyclic stress of 2.4 GPa, the number of rolling contact fatigue cycles of the steel is required to be ≥ 2.5 × 10 7 times. Detailed implementation manners

[0052] The present invention will be further described in detail below in conjunction with embodiments. The embodiments are exemplary and are intended to explain the present invention, and should not be construed as limiting the present invention.

[0053] The chemical compositions (wt%) of the new energy vehicle gear steel in each embodiment are shown in Table 2 and Table 3.

[0054] Table 2

[0055] C Si Mn P S Cr Cu Ni V Al Example 1 of the present invention 0.21 0.10 1.39 0.007 0.020 1.24 0.02 0.02 0.0040 0.033 Example 2 of the present invention 0.22 0.09 1.40 0.008 0.021 1.25 0.03 0.02 0.0039 0.034 Example 3 of the present invention 0.21 0.10 1.41 0.007 0.022 1.24 0.02 0.03 0.0040 0.032

[0056] Table 3

[0057] Mo As Sn Sb Pb Ti B Nb O Example 1 of the present invention 0.02 0.0055 0.0022 0.0011 0.001 0.0008 0.0031 0.0070 0.00063 Example 2 of the present invention 0.02 0.0057 0.0023 0.0013 0.001 0.0007 0.0032 0.0069 0.00067 Example 3 of the present invention 0.02 0.0054 0.0024 0.0015 0.001 0.0008 0.0031 0.0069 0.00065

[0058] Table 4 Non-metallic inclusions of the steel in each embodiment

[0059] K4(O + S) Sulfide Example 1 of the present invention Φ60mm 0 Grade 1.5 Example 2 of the present invention Φ60mm 0 Grade 1.5 Example 3 of the present invention Φ60mm 0 Grade 2.0

[0060] Table 5 Macrostructure data of steel materials for each example

[0061] C R S Crack Shrinkage cavity Subcutaneous air bubble Example 1 of the present invention Φ60mm 0.5 0.5 0 None None None Example 2 of the present invention Φ60mm 0.5 0.5 0 None None None Example 3 of the present invention Φ60mm 0.5 0.5 0 None None None

[0062] Table 6 Grain size data of steel materials for each example

[0063] Grain size / grade Example 1 of the present invention Φ60mm 8.5 Example 2 of the present invention Φ60mm 8.5 Example 3 of the present invention Φ60mm 8.5

[0064] Table 7 Mechanical property data of steel materials for each example

[0065] <![CDATA[Tensile strength R m MPa]]> Yield strength Rel MPa Elongation A % Reduction of area Z % Example 1 of the present invention Φ60mm 1666 1356 28 82 Example 2 of the present invention Φ60mm 1672 1386 29 80 Example 3 of the present invention Φ60mm 1678 1374 27 79

[0066] Table 8 End-quench hardenability data of steel materials for each example

[0067]

[0068] Table 9 Rolling contact fatigue life data of steel materials for each example

[0069] Test temperature / °C Test cyclic stress / GPa Life / times Example 1 of the present invention Φ60mm 22 2.4 <![CDATA[2.75×10 7 > Example 2 of the present invention Φ60mm 22 2.4 <![CDATA[2.92×10 7 > Example 3 of the present invention Φ60mm 22 2.4 <![CDATA[2.86×10 7 >

[0070] Production method of gear steel, process: KR (hot metal pretreatment) → primary melting in converter or electric furnace → refining in ladle furnace (LF furnace) → vacuum degassing in RH furnace or VD furnace → continuous casting CCM (large cross-section) → heating and blooming → slow cooling → hot rolling → piling cooling → finishing → surface and internal flaw detection → packaging.

[0071] Specifically, hot metal pretreatment is carried out in KR. The insertion depth of the KR stirrer is 160 - 200 mm, and the rotation speed is 150 - 180 revolutions per minute. 2 - 3 tons of CaO - FeO - CaF2 composite powder is injected at the bottom of the hot metal ladle with the help of nitrogen for dephosphorization and desiliconization treatment. Under the action of mechanical stirring, chemical reactions for dephosphorization (P: 0.040% - 0.050%) and desiliconization (Si: 0.15% - 0.20%) are carried out, and then the harmful slag obtained from the reaction is removed by a robotic arm to ensure the cleanliness of the hot metal. Secondly, primary melting is carried out in a converter or an electric furnace. Oxygen is blown from the top of the furnace mouth for oxidation reaction, and argon is blown from the bottom for stirring. 20 - 30 cubic meters of oxygen and 1 - 2 cubic meters of argon are blown per ton of steel, and 1 - 2 tons of active composite lime CaO - MnO - CaF2 are added. Under the combined action of oxygen and argon, the end - point carbon at the time of tapping from the primary melting furnace is 0.06% - 0.10%, the end - point P ≤ 0.020%, the tapping temperature is 1660°C - 1670°C, slag - stopping is adopted during tapping, and some alloys (pre - adjusted composition) are added during tapping. After tapping, it is quickly lifted and transported to the refining LF furnace. At the LF refining furnace, first, Al pellets (100 - 150 Kg) are added to the molten steel, then a high - performance composite slag - forming agent of CaO - Al2O3 - MgO (350 - 400 Kg) is added again, the ladle cover is covered, argon is connected to the bottom of the ladle, and then the electrode is inserted into the slag for submerged - arc power - on. The power is cut off every 10 minutes to measure the temperature and take samples of the molten steel for analysis, and the required main elements (B, V, Nb, etc.) are added according to the target requirements. The number of times of temperature measurement and sampling in the refining furnace is controlled at 3 - 4 times until the composition meets the product requirements.It is not allowed to add alloys within 10 minutes before the end of refining. Finally, cut off the power supply, and adjust the argon gas flow rate at the bottom of the ladle to ensure that the molten steel is not exposed to the air. The refining time should be controlled above 40 minutes, and the soft blowing time of the molten steel is 15 - 20 minutes. During RH or VD vacuum degassing, the highest vacuum degree in the vacuum furnace should be ≤ 0.7 mbar, and the vacuum circulation treatment time of the molten steel should be maintained for 10 - 15 minutes to ensure effective removal of harmful gases in the steel. After the vacuum treatment, argon gas is blown into the bottom of the ladle, and the argon gas flow rate is controlled to ensure that the molten steel is not exposed to the air. The soft blowing time of the molten steel should be ≤ 15 minutes. Continuous casting adopts argon protection pouring throughout the process to prevent the molten steel from being contaminated by secondary oxidation. Low superheat pouring is adopted (superheat △T ≤ 20 °C). The molten steel volume in the tundish is controlled at 10 - 20 tons, and a fixed casting speed of 1.0 m / min is adopted. An appropriate steel flow ratio to water volume (1.2 - 1.4 L / kg) is used, and electromagnetic stirring (frequency = 3 HZ, current = 800 A) is used in the tundish. The continuous casting billet is hot-transported to a heating furnace with a neutral or weakly oxidizing atmosphere and heated to 1200 °C - 1240 °C. The heating and holding time should be ≥ 5 hours. After the continuous casting billet is taken out of the furnace, it is first descaled by high-pressure water before rolling to remove the scale on the surface of the continuous casting billet, and then the 4-pass large reduction technology is adopted. The reduction amounts in the four passes are 25% - 30%, 20% - 25%, 25% - 30%, and 10% - 15% respectively. Finally, it is rolled into an intermediate billet of 200 mm × 200 mm - 250 mm × 250 mm. The intermediate billet (temperature ≥ 550 °C) is taken offline and put into a slow cooling pit for slow cooling for more than 48 hours. The intermediate billet is cold-charged to a walking beam heating furnace with a neutral or weakly oxidizing atmosphere through a conveying roller table. The steel is heated to 1150 °C - 1200 °C in the heating furnace, with a holding time of 1.5 - 2 hours. The starting rolling temperature is 950 °C - 1000 °C, and the final rolling temperature is controlled at 850 °C - 900 °C. The steel is then alternately rolled by 6 horizontal and vertical rolling mills and finally rolled into round bars with a diameter of φ20 mm - φ60 mm. After being taken out of the furnace, it is stacked and cooled to room temperature, and then subsequent flaw detection and finishing are carried out on the bars.

[0072] As can be seen from Tables 2, 3, 4, 5, 6, 7, 8, and 9, for the high tensile strength and high yield strength new energy vehicle gear steel in each embodiment of the present invention, in terms of chemical composition, non-metallic inclusions, macrostructure, mechanical properties, end hardenability, grain size of the structure, and rolling contact fatigue life results, all the indexes of the present invention meet the requirements of high tensile strength and high yield strength new energy vehicle gear steel.

[0073] In summary, for a kind of high tensile strength and high yield strength new energy vehicle gear steel in each embodiment of the present invention, through unique composition design and process innovation, a short process production route of "vacuum degassing + continuous casting + continuous rolling" with continuous, stable, reliable quality, high production efficiency, and low cost is adopted, thereby obtaining a new steel with high purity, high and stable hardenability, high tensile strength and high yield strength, and high rolling contact fatigue life.

[0074] In addition to the above embodiments, the present invention also includes other implementation manners. Any technical solutions formed by equivalent transformation or equivalent substitution shall fall within the protection scope of the claims of the present invention.

Claims

1. A high-strength gear steel for new energy vehicles, characterized in that: The element composition by mass percentage is C: 0.19 - 0.24%, Si ≤ 0.12%, S: 0.015 - 0.030%, Mn: 1.2 - 1.5%, Cr: 1.0 - 1.3%, Al: 0.02 - 0.05%, B: 0.002 - 0.004%, V: 0.003 - 0.005%, Nb: 0.005 - 0.009%, P ≤ 0.020%, Cu ≤ 0.20%, Ni ≤ 0.20%, Mo ≤ 0.10%, O ≤ 0.0008%, As ≤ 0.04%, Sn ≤ 0.03%, Sb ≤ 0.005%, Pb ≤ 0.002%, Ti ≤ 0.001%, and the balance is Fe and unavoidable impurities.

2. The steel according to claim 1, characterized in that: The steel grain size is in accordance with ISO 643, held at 960 ± 10 °C for 4 hours, water quenched, and the austenite grain size ≥ 7.5 grades; the end hardenability is inspected according to GB / T225, heat treatment: normalizing temperature: 920 ± 10 °C, held for 1 hour, air cooled; end quenching temperature: 900 ± 5 °C, water cooled, meeting J5: 46 - 50HRC, J9: 42 - 46HRC, J15: 38 - 42HRC; non-metallic inclusions are inspected according to the K method of DIN 50602 standard, meeting K4 (O + S) ≤ 5; sulfides are rated according to SEP 1572, sulfides ≤ grade 3; the macrostructure is inspected according to ASTM E381, meeting that C, R, and S do not exceed grade 1.0, and there are no shrinkage cavities, cracks, and subcutaneous bubbles.

3. The steel according to claim 1, wherein: The mechanical properties are inspected in accordance with GB / T 228, meeting the requirements of tensile strength Rm≥1600 MPa, yield strength Rel≥1300 MPa, elongation A≥25%, and reduction of area Z≥75%; the rolling contact fatigue test is carried out in accordance with YB / T 5345-2014. Under the conditions of room temperature 20℃±5℃ and cyclic stress of 2.4 GPa, it meets the requirement of the number of rolling contact fatigue cycles≥2.5×10 7 times.

4. A method for producing the steel according to claim 1, characterized in that: Including, Step 1, molten steel smelting: involving hot metal pretreatment, primary melting, refining, and vacuum degassing. Hot metal pretreatment: the insertion depth of the KR stirrer is 160 - 200 mm, the rotation speed: 150 - 180 revolutions per minute. At the bottom of the hot metal ladle, CaO-FeO-CaF2 composite powder is injected with nitrogen for dephosphorization and desiliconization treatment. Under the action of mechanical stirring, phosphorus is reduced to P ≤ 0.050% and silicon is reduced to Si ≤ 0.20% through chemical reactions, and then the reaction slag is removed by a robotic arm; primary melting: oxygen is blown from the top for oxidation reaction, and argon is blown from the bottom for stirring. 20 - 30 cubic meters of oxygen and 1 - 2 cubic meters of argon are blown per ton of steel, and active composite lime CaO-MnO-CaF2 is added. Under the combined action of oxygen and argon, phosphorus is further dephosphorized to P ≤ 0.020%. The end point carbon of the molten steel during tapping is 0.06% - 0.10%, the tapping temperature ≥ 1660 °C, slag blocking is carried out during tapping and part of the alloy is added to initially adjust the composition; refining: Al pellets are added to the molten steel for precipitation deoxidation, and CaO-Al2O3-MgO composite slag-making agent is added on the surface of the molten steel for diffusion deoxidation and adsorption of non-metallic inclusions. Argon is connected to the bottom of the ladle for stirring to promote the floating of inclusions; vacuum degassing: remove the light gas elements in the molten steel. After vacuum degassing, argon is blown into the bottom of the ladle to promote the floating of gases and inclusions; Step 2, casting: cast the molten steel into a billet; Step 3. Heating and blooming: The cast billet is reheated to fully austenitize the structure, and then hot-rolled with multiple heavy drafts. The reduction per pass is 10% - 30%. The structure of the cast billet undergoes deformation and recrystallization, and the obtained intermediate billet is slowly cooled. Step 4. Rolling into shape: The intermediate billet is reheated to 1150°C - 1200°C and held for 1.5 - 2 h. Control the starting rolling temperature at 950°C - 1000°C and the final rolling temperature at 850°C - 900°C. Alternately roll through a horizontal-vertical rolling mill, and finally roll into round steel, which is taken off the production line and stacked for air cooling.

5. The production method according to claim 4, characterized in that: Step 1. During the refining process, first add 100 - 150 kg of Al pellets into the molten steel, then add 350 - 400 kg of CaO - Al2O3 - MgO composite slag-making agent. Cover the ladle with a protective cover, connect argon gas at the bottom of the ladle, insert the electrode into the slag for submerged arc power supply. Stop power supply every 10 minutes to measure the temperature and take samples of the molten steel for analysis, and add alloying elements according to the target requirements. The number of times of temperature measurement and sampling is controlled at 3 - 4 times until the composition meets the target requirements. No alloy is added within 10 minutes before the end of refining. Finally, stop power supply and adjust the argon gas flow at the bottom of the ladle to ensure that the molten steel is not exposed to the air. The refining time is controlled to be more than 40 minutes, and the soft blowing time of the molten steel is 15 - 20 minutes to make harmful inclusions float up and be removed.

6. The production method according to claim 4, characterized in that: Step 1. The maximum vacuum degree in the vacuum degassing furnace is ≤0.7 mbar, and the vacuum circulation treatment time is 10 - 15 minutes; after vacuum degassing treatment, the argon gas flow rate is based on ensuring that the molten steel is not exposed, and the soft blowing time of the molten steel is ≤15 minutes.

7. The production method according to claim 4, characterized in that: Step 2. The molten steel is cast into billets by continuous casting. During the casting process, argon gas is used for protection to isolate the air. The casting superheat △T ≤ 20°C, the molten steel volume in the tundish is controlled at 10 - 20 tons, the molten steel in the tundish is subjected to electromagnetic stirring, the fixed casting speed is 1.0 m / min, and the specific water flow rate of the steel stream is controlled at 1.2 - 1.4 L / kg.

8. The production method according to claim 4, characterized in that: Step 3. The cast billet is hot-transported to a heating furnace with a neutral or weakly oxidizing atmosphere and heated to 1200°C - 1240°C. The heating and holding time is ≥5 h. After leaving the furnace, first remove the surface oxide layer by high-pressure water descaling, and then hot-roll with 4 heavy drafts. The reduction in the four passes is 25% - 30%, 20% - 25%, 25% - 30%, and 10% - 15% respectively. The obtained intermediate billet is taken off the production line at ≥550°C and enters a slow-cooling pit for slow cooling.

Images