New energy automobile high-hardenability gear steel bar and manufacturing method thereof

Abstract

The invention relates to a high-hardenability gear steel bar for a new energy automobile and a manufacturing method of the high-hardenability gear steel bar. The high-hardenability gear steel bar comprises the following chemical components in percentage by mass: 0.17-0.25% of C, less than or equal to 0.37% of Si, 0.50-0.90% of Mn, 0.85-1.25% of Cr, less than or equal to 0.025% of P, less than or equal to 0.030% of S, less than or equal to 0.30% of Cu, 0.02-0.06% of Ti, 0.02-0.05% of Al, less than or equal to 0.30% of Ni, 0.15-0.45% of Mo, 0.0005-0.0020% of B, 0.020-0.080% of Nb, less than or equal to 0.0030% of Ca, 0.0090-0.0150% of N, less than or equal to The production and manufacturing process comprises the steps of converter treatment, external refining, vacuum degassing, continuous casting, continuous rolling, shearing or saw cutting, stack cooling and finishing flaw detection. According to the invention, fine Nb (C, N) and (Nb, Ti) (C, N) particles are precipitated by microalloying Nb and Ti, the growth of crystal grains is effectively inhibited, and meanwhile, the hardenability of the material is improved by adding a proper amount of B element. According to the invention, Nb, Ti and B are adopted for composite microalloying at the same time, so that the fine-grain and high-hardenability gear steel material is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of special steel smelting, and particularly relates to a high hardenability gear steel bar for new energy vehicles and a manufacturing method thereof. Background Art

[0002] In recent years, new energy vehicles in China have developed rapidly. In 2023, the sales volume of new energy vehicles in China reached 9.5 million, far leading the world. With the rapid development of new energy vehicles, the demand for gear steel used in the speed reducers of new energy vehicles is increasing. The technical requirements for its materials are to be able to withstand higher impact loads and have higher fatigue strength, which requires the gear steel materials to have the characteristics of high strength and toughness.

[0003] By adopting Nb and Ti microalloying, Nb and Ti can form carbides, nitrides and carbonitrides with C and N, thereby forming grain refinement and precipitation strengthening. Fine grain strengthening can not only improve the strength of the material, but also improve the plasticity and toughness of the material. After the precipitation of carbonitrides, they pin the grain boundaries, increase the austenite recrystallization temperature, and inhibit grain growth.

[0004] By adopting Ti / B microalloying, the hardenability of the gear material is improved. Boron element in steel can form nitride BN with nitrogen element and cannot improve the hardenability of the material. Titanium element in steel can also combine with nitrogen element to form nitride. Among them, titanium has a stronger binding force with nitrogen element than boron element. Therefore, by adding titanium, nitrogen can be fixed to protect boron, thereby forming effective boron and improving the hardenability of steel.

[0005] One of the energy-saving and environmental protection methods for gear manufacturing at home and abroad is to adopt high-temperature carburizing technology. When the carburizing temperature is increased from the conventional 930 °C to 950 °C, the carburizing cycle can be reduced by about 30%. When the carburizing temperature is increased to 980 °C, the carburizing cycle can be reduced by about 55%, and the production efficiency is significantly improved, and the energy-saving and emission-reduction effect is extremely remarkable. Since the carburizing temperature of ordinary gear steel materials reaches 950 °C and above, it is extremely easy to obtain coarse-grained and mixed-grained structures, which greatly reduces the fatigue life of gears. Therefore, it is of great value to develop gear steel suitable for high-temperature vacuum carburizing. The present invention proposes a production and manufacturing control method for high-temperature carburizing gear steel with niobium-titanium composite control. Nb and Ti microalloying elements are added to the steel at the same time, which can meet the requirement of raising the carburizing temperature of gears to 980 °C. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a Nb, Ti, B microalloyed high hardenability carburizing gear steel for new energy vehicles according to the above-mentioned prior art, and solve the key technologies of high hardenability gears and high-temperature carburizing green gear manufacturing.

[0007] The technical solution adopted by the present invention to solve the above problems is as follows: A high hardenability gear steel bar for new energy vehicles, the chemical composition is designed by mass percentage as C: 0.17 - 0.25%, Si: ≤0.37%, Mn: 0.50 - 0.90%, Cr: 0.85 - 1.25%, P: ≤0.025%, S: ≤0.030%, Cu: ≤0.30%, Ti: 0.02 - 0.06%, Al: 0.02 - 0.05%, Ni: ≤0.30%, Mo: 0.15 - 0.45%, B: 0.0005 - 0.0020%, Nb: 0.020 - 0.080%, Ca ≤0.0030%, N: 0.0090 - 0.0150%, [O] ≤0.0015%, and the balance is Fe and unavoidable impurity elements. The design principle of the steel composition of the present invention:

[0008] 1. 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 at the core after carburizing heat treatment of gears, an appropriate carbon content is required in the steel. The carbon content range of the present invention is determined to be 0.17 - 0.25%.

[0009] 2. Si can be used as a deoxidizing element and is also a basic solid solution strengthening element to improve hardenability; Si is an easily oxidized element and is prone to intergranular oxidation of gear steel during carburizing. The silicon content range of the present invention is determined to be ≤0.37%.

[0010] 3. Mn, as an element acting as a deoxidizer, increases the strength of 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 and replaces a part of the iron atoms in the cementite. In steel, Mn plays a role in refining pearlite by reducing the critical transformation temperature, and also indirectly plays a role in increasing 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 manganese content range of the present invention is determined to be 0.50 - 0.90%.

[0011] 4. Cr can increase the hardenability of steel and has a secondary hardening effect, can play a solid solution strengthening role on the steel, and improve the hardenability of the steel. In addition, Cr reduces the activity of C, can reduce the overheating tendency and surface decarburization rate of the steel. The chromium content range of the present invention is determined to be 0.85 - 1.25%.

[0012] 5. P is an unavoidable impurity, which is prone to forming segregation, inclusions and other defects. Especially, P will reduce the cold deformation performance of the material, resulting in easy brittle fracture of the steel during cold processing, that is, the so-called "cold brittleness" phenomenon. The phosphorus content range of the present invention is determined to be ≤0.025%.

[0013] 6. Sulfur will cause hot brittleness in steel, reducing the plasticity and toughness of steel. In the present invention, the sulfur content range is determined to be ≤0.030%.

[0014] 7. The binding force between Ti element and N is very strong. Ti belongs to a strong nitrogen-fixing element. The stoichiometric ratio of Ti / N is 3.42. By forming TiN and Ti(CN) through binding with N, adding an appropriate amount of Ti element in steel can form particles for refining grains, and at the same time can fix nitrogen and protect boron, improving the hardenability of steel. Therefore, in the present invention, the Ti content is required to be controlled within 0.02 - 0.06%.

[0015] 8. As an effective deoxidizing element, Al is mainly used for deoxidation and grain refinement. In the present invention, the aluminum content range is determined to be Al: 0.02 - 0.05%.

[0016] 9. Nb: In steel, the Nb element combines with C and N to form NbCN precipitates, which can significantly improve the stability of high-temperature austenite in steel and prevent the growth of high-temperature austenite grains. Therefore, in the present invention, the Nb content is required to be controlled within 0.020 - 0.080%.

[0017] 10. The N element precipitates in the form of nitrides with Nb, Ti, Al, etc., which can significantly improve the stability of high-temperature austenite grains in gear steel and prevent the growth of high-temperature austenite grains. In the present invention, the nitrogen content range is determined to be N: 0.0090 - 0.0150%.

[0018] 11. O mainly exists in the form of oxide inclusions such as Al2O3, MnO, CaO, SiO2, etc. in steel. After a large number of studies, a large amount of oxide inclusions in steel significantly deteriorate the fatigue life of the steel. In the present invention, the oxygen content range is determined to be O: ≤0.0015%.

[0019] 12. When the B element exists in a free state in steel, it has the effect of inhibiting the precipitation of ferrite (in steel, Ti needs to be added first to form TiN to form free B, otherwise the B element in steel will exist in the form of combined BN). It can improve the hardenability of steel. In the present invention, the boron content range is determined to be B: 0.0005 - 0.0020%.

[0020] 13. Ni strengthens ferrite and refines pearlite in steel. The overall effect is to increase strength, and it has no significant impact on the plasticity and toughness of steel. In addition, Ni also has a certain effect on improving the hardenability of steel. Since Ni is a precious metal, in the present invention, the Ni content ≤0.30%.

[0021] 14. Mo in steel can improve hardenability and hot strength, prevent temper brittleness, and can also reduce the tendency of carbides to form a continuous network at grain boundaries in the carburized layer, reduce the retained austenite in the carburized layer, and relatively increase the wear resistance of the surface layer. The range of molybdenum content in the present invention is determined to be Mo: 0.15 - 0.45%.

[0022] For the application of the above Nb, Ti, B microalloyed new energy vehicle high hardenability carburized gear steel, the hardenability of this gear steel is J9≥44HRC, J15≥37HRC, high temperature vacuum carburizing is carried out at 980°C, the carburizing time is 5h, and the grain size of the steel remains above grade 5.0.

[0023] The growth of austenite grains is closely related to the undissolved precipitated phases in steel. The microalloying elements Ti and Nb are easy to combine with C and N in steel to form precipitated phases such as TiN, NbC, (Ti,Nb)(C,N), etc. These precipitated phases can pin the austenite grain boundaries during carburizing, hinder the migration of grain boundaries, and thus control grain growth. For the grain growth trends of different microalloyed gear steels, the following tests are carried out: The main chemical compositions (wt%) of the test steels Steels C Si Mn P S Cr Al Ti Nb [N] a 0.19 0.18 0.85 0.011 0.003 1.12 0.035 - - 0.0100 b 0.22 0.20 0.84 0.017 0.011 1.08 0.033 0.054 - 0.0100 c 0.21 0.25 0.87 0.015 0.006 1.13 0.031 - 0.065 0.0022 d 0.20 0.23 0.86 0.018 0.005 1.11 0.034 0.038 0.048 0.0018 As Figure 1 shown, the grain growth trends of the 4 steels during austenitization at temperatures above 930°C: Steels a, b, and c have a relatively small grain growth trend during austenitization below 960°C, and when the austenitization temperature continues to rise, their grain growth trends are all relatively large; Steel d has a significantly smaller grain growth trend during austenitization below 1050°C; the grain sizes of the 4 test steels increase with the increase of the austenitization temperature. Comparison result: Through the grain growth trend test, it can be seen that by using Nb and Ti composite microalloying, the smallest grain growth trend of austenite and the most stable austenite grains can be obtained.

[0024] The production and manufacturing process of the present invention: converter - secondary refining - vacuum degassing - continuous casting - continuous rolling - shearing or sawing - stacking cooling - finishing and flaw detection. The process flow mainly includes: Smelt molten steel that meets the chemical composition of the steel, cast it into continuous casting billets. High-performance refining synthetic slag is used in steelmaking to ensure that inclusions have good deformability. The ladle maintains a long inclusion removal process, enabling non-metallic inclusions to fully float up, ensuring the high purity of the steel. Select special refining slag and tundish covering slag to better adsorb inclusions. Advanced equipment such as mold electromagnetic stirring, final electromagnetic stirring, and tundish induction heating is used during the continuous casting process to reduce the segregation of materials. During the molten steel refining process, ferrotitanium wire is added first. Since the affinity between titanium and nitrogen elements in the molten steel is much greater than the affinity between titanium and carbon, the titanium added to the steel first combines with nitrogen (TiN), and then the excess titanium combines with carbon (TiC). When Ti / N ≥ 3.43, all the nitrogen elements in the molten steel will combine with titanium to form TiN, and the subsequent added boron element will all exist in the molten steel in a free state.

[0025] Before rolling, heat the continuous casting billet to 1200 - 1280 °C, hold it for 4 hours or more and then take it out of the furnace. By fully increasing the heating temperature, elements such as Nb and Ti can be fully and evenly dissolved in the austenite phase. The preferred heating temperature is above 1200 °C. In addition, if the heating temperature is too high, the austenite phase has a tendency to coarsen and overheat. Therefore, the heating temperature is preferably below 1280 °C. Rolling start temperature: 1100 - 1150 °C, final rolling temperature 950 - 980 °C. After rolling, it is cooled to 200 - 500 °C by a cooling bed and taken offline. The stacking cooling method is adopted, and the cooling rate ≤ 25 °C / min.

[0026] Since the microalloying element Ti preferentially forms nitride precipitation phases with N in the steel over the B element, in order to ensure that this product fully obtains effective boron (preventing the formation of boron nitride), therefore, in this invention, Ti is added to the steel first and then B is added to improve the hardenability of this product and ensure that the final hardenability J9 ≥ 44HRC and J15 ≥ 37HRC.

[0027] The 980 °C high-temperature carburized gear steel uses composite microalloying means to control grain growth. The fine and dispersed undissolved carbonitride compounds in the steel can play a role in refining grains during high-temperature carburization. Nb, Ti, and V are the most commonly used microalloying elements, and their pinning effects on grain boundaries decrease in turn. This invention uses the addition of microalloying elements Nb and Ti. Through the precipitation of Nb(C,N) and (Nb,Ti)(C,N) particles in the steel, the growth trend of high-temperature austenite grains in the steel is effectively controlled. This invention focuses on studying the test and analysis of the grain size corresponding to different austenitizing temperatures and times of this material by adding microalloying elements Nb and Ti.

[0028] Compared with the prior art, the advantages of this invention are as follows: The present invention adopts the Nb and Ti complex microalloying technology to prevent the growth of austenite grains during the high-temperature carburizing process of gear steel, and avoid the abnormal growth and mixed crystal structure of austenite grains during the high-temperature vacuum carburizing process of gear steel at temperatures above 930°C, enabling the gear to meet the condition of grain size above grade 5 under the high-temperature carburizing condition of 980°C, and greatly shortening the carburizing heat treatment process cycle.

[0029] The present invention adopts the microalloying technology of titanium and boron elements to improve the hardenability of gear materials. Boron elements in steel form nitrides BN with nitrogen elements and cannot improve the hardenability of materials. Titanium elements in steel can also combine with nitrogen elements to form nitrides. Among them, titanium has a stronger binding force with nitrogen elements compared with boron elements. Therefore, by adding titanium, nitrogen can be fixed to protect boron, thereby forming effective boron and improving the hardenability of steel.

[0030] In the composition design of the steel of the present invention, the microalloying technology of titanium and boron elements is adopted to improve the hardenability of gear materials. Boron elements in steel can form nitrides BN with nitrogen elements and cannot improve the hardenability of materials. Titanium elements in steel can also combine with nitrogen elements to form nitrides. Among them, titanium has a stronger binding force with nitrogen elements compared with boron elements. Therefore, by adding titanium, nitrogen can be fixed to protect boron, thereby forming effective boron and improving the hardenability of steel. Its hardenability J9≥44HRC, J15≥37HRC. Brief Description of the Drawings

[0031] Figure 1 It is a diagram showing the grain growth trend of different microalloyed gear steels in the present invention.

[0032] Figure 2 It is a diagram showing the sampling position and specimen size of the end-quenched specimen in the embodiment of the present invention.

[0033] Figures 3 - 5 It is a grain diagram (100 times, rated according to ASTM E112) of the steel of the present invention and conventional steel under the austenitizing temperature test of 930°C×5h.

[0034] Figures 6 - 10 It is a grain diagram (100 times, rated according to ASTM E112) of the steel of the present invention and conventional steel under the austenitizing temperature test of 980°C×5h; Detailed Description of the Invention

[0035] The technical solution of the present invention will be described in more detail in combination with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred embodiments of the present invention and cannot impose any limitation on the scope of the present invention.

[0036] The present invention uses gear steel with specific components. A total of three furnaces of the steel of the present invention are produced, and converter smelting - LF refining - RH vacuum treatment - continuous casting - rolling - finishing are adopted. The continuous casting billet is taken out of the furnace after being kept at 1200 - 1280°C for 4 hours or more, and round steel rolling is carried out. The starting rolling temperature is 1100 - 1150°C, and the final rolling temperature is 950 - 980°C. After rolling, it is cooled to 200 - 500°C by a cooling bed and taken off the production line. The round steel is rolled into round steel with a diameter of φ45mm, and round steel samples are taken for hardenability test and grain size test respectively.

[0037] The chemical compositions of three groups of the invented steel and one group of conventional steel are shown in Table 1 below: Table 1: Chemical Compositions of Three Groups of the Invented Steel and One Group of Ordinary Steel

[0038] The heat treatment system of the end-quenched specimen shall be implemented according to the provisions of Table 2 below.

[0039] Table 2: Heat Treatment System of End-Quenched Specimen

[0040] The comparison of end-quench tests of three groups of the invented steel and one group of conventional steel is shown in Table 3.

[0041] Table 3: Test Results of End-Quenched Specimens

[0042] It can be seen from this that, compared with ordinary steel, the hardenability performance of the steel of the present invention is significantly improved. For the invented steel, its hardenability J9 ≥ 44HRC and J15 ≥ 37HRC. Grain size test:

[0043] The samples are subjected to simulated carburizing and quenching, and the process is as follows: the carburizing temperatures are 930°C and 980°C respectively, and after holding for 5h, quenching is carried out.

[0044] Three groups of the invented steel and one group of conventional steel are quenched and cooled after austenitization at 930°C and 980°C for 5 hours. After the samples are corroded by supersaturated picric acid, the grain size of the samples is observed under a microscope. The test results are shown in the appendix Figures 2 - 9 . It can be seen from the test results that the austenite grains of ordinary steel are fine grains under the condition of 930°C × 5h, and the austenite grains are in a coarse grain state under the condition of 980°C × 5h. However, under the high-temperature simulated carburizing conditions of holding for 5 hours at 930°C - 980°C, the austenite grains of the three groups of the invented steel all remain above grade 5, and no grain growth is found.

[0045] Therefore, after the steel of the present invention is treated by a simulated carburizing process at 980°C, the austenite grain size of its entire cross-section can still be maintained above grade 5, and no mixed grains or coarse grain structures are found, which can meet the technical requirements of the anti-grain coarsening ability required for high-temperature carburizing, can effectively shorten the carburizing time, save energy consumption, reduce carbon emissions, and at the same time reduce production costs.

[0046] Although the preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various changes and modifications can be made to the present invention by those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A high hardenability gear steel bar for new energy vehicles, characterized in that: The chemical composition of the bar is designed by mass percentage as follows: C: 0.17 - 0.25%, Si: ≤0.37%, Mn: 0.50 - 0.90%, Cr: 0.85 - 1.25%, P: ≤0.025%, S: ≤0.030%, Cu: ≤0.30%, Ti: 0.02 - 0.06%, Al: 0.02 - 0.05%, Ni: ≤0.30%, Mo: 0.15 - 0.45%, B: 0.0005 - 0.0020%, Nb: 0.020 - 0.080%, Ca ≤0.0030%, N: 0.0090 - 0.0150%, [O] ≤0.0015%, and the balance is Fe and inevitable impurity elements.

2. The high hardenability gear steel bar for new energy vehicles according to claim 1, characterized in that: The bar adopts the microalloying technology of titanium and boron elements to improve the hardenability of the gear material.

3. The high hardenability gear steel bar for new energy vehicles according to claim 2, wherein: The hardenability of the bar meets J9≥44HRC and J15≥37HRC.

4. A high hardenability gear steel bar for new energy vehicles according to claim 1, characterized in that: The bar adopts the composite microalloying technology of Nb and Ti to prevent the growth of austenite grains during the high-temperature carburizing process of the gear steel.

5. A high hardenability gear steel bar for new energy vehicles according to claim 4, characterized in that: The bar is subjected to high-temperature carburizing at 980°C × 5h, and the grain size reaches above grade 5.

6. A manufacturing method of a high hardenability gear steel bar for new energy vehicles as described in claim 1, characterized in that: The method mainly includes the following steps: 1) Smelt molten steel that meets the chemical composition of the steel and cast it into a continuous casting billet; 2) Before rolling, heat the continuous casting billet to 1200 - 1280°C, hold for 4 hours or more and then take it out of the furnace. The starting rolling temperature is 1100 - 1150°C, the final rolling temperature is 950 - 980°C, and after rolling, it is cooled to 200 - 500°C by a cooling bed and then taken off the production line.

7. The manufacturing method of a high hardenability gear steel bar for new energy vehicles according to claim 1, characterized in that: In step 1), during the molten steel refining process, first add ferrotitanium wire, and when Ti / N≥3.43, add boron element.

8. The manufacturing method of a high hardenability gear steel bar for new energy vehicles according to claim 1, characterized in that: In step 1), the continuous casting process adopts mold electromagnetic stirring, final electromagnetic stirring, and tundish induction heating.

9. The manufacturing method of a high hardenability gear steel bar for new energy vehicles according to claim 1, characterized in that: In step 2), the bar is taken off the production line by stacking cooling after rolling, and the cooling rate ≤25°C / min.

Images