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

Abstract

The invention relates to a new energy automobile high-rotating-speed gear steel bar and a manufacturing method thereof, and belongs to the field of metallurgy. The gear steel comprises the following chemical components in percentage by mass: 0.14 to 0.20 percent of C, less than or equal to 0.30 percent of Si, 1.00 to 1.50 percent of Mn, 1.00 to 1.40 percent of Cr, less than or equal to 0.025 percent of P, 0.010 to 0.030 percent of S, less than or equal to 0.30 percent of Cu, less than or equal to 0.0040 percent of Ti, 0.020 to 0.050 percent of Al, less than or equal to 0.30 percent of Ni, less than or equal to 0.10 percent of Mo, 0.0010 to 0.0050 percent of B, 0.020 to 0.080 percent of Nb, less than or equal to 0.0030 percent of Ca, 0.0090 to 0.0150 percent of N, less than or equal to 0.0015 percent of [O] and the balance of Fe and inevitable impurity elements. The production process of the steel comprises the steps of converter treatment, external refining, vacuum degassing, continuous casting, continuous rolling, shearing or saw cutting, stack cooling and finishing flaw detection. And a NbAl composite microalloying mode is adopted, grain growth is inhibited, and a good high-temperature grain refining effect is obtained. By adding a proper amount of B element, the bending impact performance of the material is improved, and the high-speed gear manufactured by the material is ensured to have good impact resistance.

Description

Technical Field

[0001] The invention belongs to the field of metallurgy, and in particular relates to a method for manufacturing gear steel for automobiles. Background Art

[0002] In recent years, my country's new energy vehicles have developed rapidly, and the market penetration rate has exceeded 30%, far ahead of other countries in the world. With the rapid development of new energy vehicles, the demand for gear steel for high-speed motor reducers is increasing, and the technical requirements for its materials are: high speed (20,000 rpm), impact resistance, energy saving and environmental protection (high temperature carburizing resistance) and other new special performance requirements.

[0003] The traditional way to improve the impact performance of gear steel is to add precious metal elements such as Ni, which can effectively improve the impact performance of the material. However, this traditional method will greatly increase the alloy cost of the material, and the material cost-effectiveness is low.

[0004] One of the energy-saving and environmentally friendly methods for gear manufacturing at home and abroad is to use high-temperature carburizing technology. When the carburizing temperature is increased from the conventional 930℃ to 950℃, the carburizing cycle can be reduced by about 30%, and when the carburizing temperature is increased to 1000℃, the carburizing cycle can be reduced by about 55%, the production efficiency is significantly improved, and the energy-saving and emission reduction effect is extremely significant. Since the carburizing temperature of ordinary gear steel materials reaches 950℃ and above, it is very easy to obtain coarse grains and mixed grain structures, which greatly reduces the fatigue life of the gear; therefore, it is of great value to develop gear steel suitable for high-temperature vacuum carburizing. Summary of the invention

[0005] The purpose of the present invention is to provide a B, Nb, Al micro-alloyed new energy vehicle high-speed carburizing gear steel, to achieve a green manufacturing technology for high-speed gear impact resistance and high-temperature carburizing resistance.

[0006] The present invention forms BN precipitates by adding boron elements, thereby improving the impact performance of the material. The material processed sample has good bending and punching performance, and the bending and punching test value Fdyn≥55KN.

[0007] The present invention adopts Nb and Al composite microalloying technology to prevent the growth of austenite grains during high-temperature carburizing of gear steel, avoid the abnormal growth of austenite grains and mixed crystal structure of gear steel during high-temperature vacuum carburizing at a temperature higher than 930°C, so that the gear meets the grain size of more than level 5 under high-temperature carburizing conditions of 1000°C, and greatly shortens the carburizing heat treatment process cycle.

[0008] The technical solution adopted by the present invention to solve the above-mentioned problem is: a new energy vehicle high-speed gear steel bar and a manufacturing method thereof, wherein the chemical composition is designed by mass percentage as C: 0.14-0.20%, Si: ≤0.30%, Mn: 1.00-1.50%, Cr: 1.00-1.40%, P: ≤0.025%, S: 0.010-0.030%, Cu: ≤0.30%, Ti: ≤0.0040%, Al: 0.020-0.050%, Ni: ≤0.30%, Mo: ≤0.10%, B: 0.0010-0.0050%, 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.

[0009] The production process of the invention comprises the following steps: converter - refining outside the furnace - vacuum degassing - continuous casting - continuous rolling - shearing or sawing - stack cooling - finishing and flaw detection.

[0010] The present invention adopts NbAl composite microalloying to inhibit grain growth and obtain good high-temperature grain refinement effect. At the same time, the present invention improves the bending and punching performance of the material by adding an appropriate amount of B element, ensuring that the high-speed gear manufactured by the material has good impact resistance.

[0011] In the composition design of the steel of the present invention, it is completely different from the smelting method of adding B and Ti elements to the steel at the same time in the traditional boron-containing alloy steel. An appropriate amount of B microalloying element is added to the steel of the present invention, and the addition of Ti is strictly prohibited. At the same time, the residual Ti element content in the steel is strictly controlled (≤40ppm) to ensure that the B element in the steel forms a chemical state BN with the N element in the steel, thereby improving the impact performance of the material, and the bending and punching test value Fdyn≥55KN.

[0012] In the composition design of the steel of the present invention, appropriate amounts of Nb and Al microalloying elements are added to the steel, and Nb and Al composite microalloying technology is used to control the growth of austenite grains during high-temperature carburizing of gear steel, so that the gear meets the grain size of more than level 5 under high-temperature carburizing conditions of 1000°C.

[0013] The application of the above-mentioned B, Nb, Al micro-alloyed high-speed carburized gear steel for new energy vehicles, the bending test value of the gear steel sample Fdyn ≥ 55KN, high-temperature vacuum carburizing is carried out at 1000°C, the carburizing time is 5h, and the grain size of the steel is maintained above grade 5.0.

[0014] The present invention provides a high-speed gear steel bar for new energy vehicles and a manufacturing method thereof, and the process flow mainly includes: Molten steel that meets the chemical composition of steel is smelted and cast into continuous casting billets. High-performance refined synthetic slag is used in steelmaking to ensure that inclusions have good deformability. The ladle maintains a long inclusion removal process to allow non-metallic inclusions to fully float, ensuring the high purity of the steel. Special refined slag and tundish protective slag are selected to better absorb inclusions. Advanced equipment such as crystallizer electromagnetic stirring, end electromagnetic stirring, and tundish induction heating are used in the continuous casting process to reduce material segregation.

[0015] Before rolling, the continuous casting billet is heated to 1220-1270°C and kept for 4 hours or more before being taken out of the furnace. By fully increasing the heating temperature, elements such as Nb and Al can be fully and evenly dissolved in the austenite phase. The preferred heating temperature is above 1220°C. In addition, if the heating temperature is too high, the austenite phase tends to coarsen and overheat, so the heating temperature is preferably below 1270°C. The starting rolling temperature is 1100-1180°C, the final rolling temperature is 960-1000°C, and after rolling, it is cooled to 300-500°C on a cooling bed before being taken off the line.

[0016] The steel composition design principles of the present invention are: C is the most basic element in steel and the most economical strengthening element. Among all elements, carbon has the greatest ability to improve steel strength. The strengthening effect of carbon on quenched and tempered steel is about 9 times that of chromium and 18 times that of manganese. Therefore, in order to ensure that the core of the gear has good toughness after carburizing heat treatment, the steel needs to have a suitable carbon content. The carbon content range of the present invention is determined to be 0.14-0.20%.

[0017] Si can be used as a deoxidizing element and is also a basic solid solution strengthening element that improves hardenability. Si is an easily oxidized element and is prone to cause intergranular oxidation of gear steel during carburizing. The silicon content range of the present invention is determined to be ≤0.30%.

[0018] Mn, as an element acting as a deoxidizer, improves the strength of steel through solid solution strengthening. Mn is a carbide-forming element, which enters cementite to replace a portion of iron atoms. Mn in steel reduces the critical transition temperature, plays a role in refining pearlite, and also indirectly plays a role in improving the strength of pearlite. The ability of Mn to stabilize austenite structure is second only to Ni, and it also strongly increases the hardenability of steel. The manganese content range of the present invention is determined to be 1.00-1.50%.

[0019] Cr can increase the hardenability of steel and has a secondary hardening effect, which can play a solid solution strengthening role on steel and improve the hardenability of steel. In addition, Cr reduces the activity of C, which can reduce the overheating tendency and surface decarburization rate of steel. The present invention determines the range of chromium content to be 1.00-1.40%.

[0020] P is an inevitable impurity, which is easy to form defects such as segregation and inclusion. In particular, P will reduce the cold deformation performance of the material, causing the steel to be easily brittle during cold working, which is the so-called "cold brittle" phenomenon. The present invention determines the range of phosphorus content to be ≤0.025%.

[0021] S can make steel hot brittle and reduce its plastic toughness. Meanwhile, a proper amount of S in steel can improve the chip processing performance of steel. The present invention determines the range of sulfur content to be 0.010 to 0.030%.

[0022] Ti: Ti element has a very strong bonding force with N, and is prone to form sharp-cornered TiN large-particle brittle inclusions, which will significantly reduce the fatigue life of the finished gear. Therefore, the Ti content of the present invention is required to be controlled at ≤0.0040%.

[0023] Al is an effective deoxidation element and is mainly used for deoxidation and grain refinement. The aluminum content range of the present invention is determined to be Al: 0.020-0.050%.

[0024] Nb: The Nb element in the steel combines with C and N to form NbCN precipitates, which can significantly improve the high-temperature austenite stability of the steel and prevent the high-temperature austenite grains from growing. Therefore, the Nb content of the present invention is required to be controlled within a range of 0.020 to 0.080%.

[0025] The precipitation of N element, Nb and Al in the form of nitride can significantly improve the stability of high-temperature austenite grains in gear steel and prevent the growth of high-temperature austenite grains. The nitrogen content of the present invention is determined to be N: 0.0090-0.0150%.

[0026] O exists in steel mainly in the form of oxide inclusions such as Al2O3, MnO, CaO, SiO2, etc. After a lot of research, it is found that a large amount of oxide inclusions in steel significantly deteriorates the fatigue life of the steel. The range of oxygen content in the present invention is determined to be O: ≤0.0015%.

[0027] The free state of B in steel has the effect of inhibiting the precipitation of ferrite (Ti needs to be added to the steel to form TiN first, so that free B can be formed, otherwise the B in the steel will exist in the form of combined BN), which can improve the hardenability of the steel. The range of the boron content of the present invention is determined to be B: 0.0010-0.0050%, and the B in the steel exists in the form of combined BN, which improves the toughness and impact toughness of the steel and promotes grain refinement in the steel.

[0028] Compared with the prior art, the advantages of the present invention are: since the microalloying element Ti forms a nitride precipitation phase with N in the steel before the B element, in order to ensure that the product fully obtains the chemically combined BN, the present invention requires strictly prohibiting the addition of Ti in the steel and strictly controlling the residual Ti element content in the steel (≤40ppm), so that effective chemically combined BN is formed in the steel, which can improve the impact performance of the product and ensure that the final bending test value Fdyn≥55KN.

[0029] 1000℃ high temperature carburizing gear steel adopts composite microalloying to control grain growth. The undissolved fine dispersed carbon and nitrogen compounds in the steel can refine the grains during high temperature carburizing. Nb, Ti, and V are the most commonly used microalloying elements, and their effects on grain boundary pinning decrease in turn. The present invention adopts the addition of microalloying Nb and Al microalloying elements to effectively control the high temperature austenite grain growth trend of the steel through AlN and NbCN precipitates in the steel. The present invention focuses on the addition of Nb and Al microalloying elements to the material, and tests and analyzes the grain size corresponding to different austenitizing temperatures and times. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is the dimension surface of the bending and punching specimen; Figure 2 It is the test surface of the bending and punching specimen; Figure 3 It is the test position of the impact hammer and bending punch specimen; Figure 4 The cross-sectional grain size statistics of the gear steel in Example 1 under the austenitizing temperature test at 930°C × 5h; Figure 5 The cross-sectional grain size statistics of the gear steel in Example 2 under the austenitizing temperature test at 930°C × 5h; Figure 6 The cross-sectional grain size statistics of the gear steel in Example 3 under the austenitizing temperature test at 930°C × 5h; Figure 7 It is the statistics of cross-section grain size of conventional gear steel under the austenitizing temperature test at 930℃×5h; Figure 8 The cross-sectional grain size statistics of the gear steel in Example 1 under the austenitizing temperature test at 1000°C×5h; Fig. 9 The cross-sectional grain size statistics of the gear steel in Example 2 under the austenitizing temperature test at 1000°C×5h; Fig.10 The cross-sectional grain size statistics of the gear steel in Example 3 under the austenitizing temperature test at 1000°C×5h; Fig.11It is the cross-sectional grain size statistics of conventional gear steel under the austenitizing temperature test at 1000℃×5h. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0032] A gear steel with a specific composition was used to produce 3 batches of gear steel bars (round steel) of the present invention, as Examples 1-3, and a group of conventional steels as comparative examples. Converter smelting-LF refining-RH vacuum treatment-continuous casting-rolling-finishing were used. The continuous casting billet was heated at 1220-1270°C, kept at this temperature for more than 4 hours before being taken out of the furnace, and round steel rolling was performed. The starting rolling temperature was 1100-1180°C, the final rolling temperature was 960-1000°C, and after rolling, it was cooled to 300-500°C on a cooling bed before being taken off the line. The round steel was rolled into a φ70mm round steel, and round steel samples were taken for bending and punching tests and grain size tests.

[0033] The chemical compositions of the three groups of example steels and one group of conventional steels are shown in Table 1. Table 1 Elemental chemical compositions of the three groups of example steels and one group of conventional steels element C Si Mn Cr P S Cu Ni Mo Al Ti Nb N B O Example 1 0.16 0.18 1.29 1.24 0.019 0.020 0.02 0.02 0.01 0.038 0.0021 0.046 0.0133 0.0031 0.0007 Example 2 0.15 0.20 1.27 1.23 0.015 0.017 0.03 0.01 0.02 0.037 0.0018 0.043 0.0139 0.0035 0.0009 Example 3 0.16 0.21 1.30 1.26 0.017 0.021 0.02 0.03 0.02 0.040 0.0017 0.045 0.0135 0.0032 0.0008 Conventional steel 0.15 0.19 1.31 1.24 0.014 0.019 0.02 0.02 0.02 0.039 0.0043 0.001 0.0136 0.0002 0.0010

[0034] Processing of bending and punching specimens Figure 1 shown.

[0035] Test point P1: HRC hardness of the bending and punching specimen surface.

[0036] Note: According to ISO 6508-1, surface hardness compensation (+0.5 HRC) is required.

[0037] Test point P2: The carburized layer depth CHD 610 (mm), grain boundary structure (retained austenite) and intergranular oxidation were tested on grinding slices made from bent and punched tooth parts. The carburized layer depth was tested using the HV1 method specified in ISO 2639.

[0038] Limit value GH=610HV1.

[0039] Test point P3: The core hardness (HBW) is determined in accordance with ISO 6506-1 at a distance of 5 mm from the surface of the bend punch specimen cross section.

[0040] Test point P4: The surface carbon content can be measured on the surface of the broken teeth of the bend and punch, or it can be measured on the surface of a carbon-fixed sample that is heat treated in the same furnace as the bend and punch sample.

[0041] Before measuring the surface carbon content, the surface can be ground off by 0.05 mm.

[0042] Heat treatment temperature and time control of bending and punching specimens Surface hardening is achieved through the following process: a. Heat the sample in a carburizing box (solid carbon powder carburizing) at a temperature of 960°C to 980°C. The carburizing time must be sufficient to ensure that the depth of the carburized layer meets the requirements (reference time: 5 hours). b. Cooling to hardening temperature (cooling speed depends on the size of the heating furnace and container, about 1.5 hours): 870℃±1℃ c. Keep the sample at the hardening temperature: 870℃±10℃ / holding time ≥0.5h, then harden the sample directly in the carburizing box. After the sample is taken out of the carburizing agent, quench it in circulating oil at 30℃±10℃.

[0043] d. After hardening, the sample is heated in a circulating air furnace to: 170℃±10℃ / holding time ≥1.5h. In order to ensure uniform cooling, the sample is then cooled in the furnace.

[0044] The measured values ​​of various properties of the bending and punching specimens after heat treatment shall meet the requirements of Table 2.

[0045] Table 2 Performance range of bending and punching specimens after heat treatment (test points see Figure 2 ) Bending and punching test specimens Measured surface hardness [HRC] 59 - 62 Carburized layer depth CHD 610 [mm] 0.8 - 1.2 Measured core hardness [HBW] 325 Surface carbon content [%] 0.65 - 0.85 Intergranular oxidation [µm] 20 Grain boundary structure Martensite and retained austenite ≤ 30 % The surface hardened bending and punching specimens are impacted by a pendulum under specified conditions. The specimens are fixed on the bending and punching machine at an angle to ensure that the angle of the hammer head when impacting the wide side of the specimen wing is 30° (see Figure 3 ).

[0046] The bending and punching test results of three groups of example steels and one group of conventional steels are shown in Table 3.

[0047] Table 3 Test results of bending and punching specimens It can be seen that the bending and punching performance of the steels of Examples 1-3 is significantly improved compared to conventional steels, and the bending and punching test values ​​Fdyn are all ≥55KN.

[0048] The samples were subjected to simulated carburizing and quenching, and the process was as follows: the carburizing temperatures were 930℃ and 1000℃, respectively, and quenching was performed after keeping the temperature for 5h.

[0049] Three groups of example steels and one group of conventional steels were austenitized at 930℃ and 1000℃ for 5 hours and then quenched. After the samples were corroded by supersaturated picric acid, the grain size of the samples was observed under a microscope. The test results are detailed in the attached Figure 4-11The test results show that the austenite grains of conventional steel are fine-grained under the condition of 930℃×5 hours, and the austenite grains are coarse-grained under the condition of 1000℃×5 hours. However, under the high-temperature simulated carburizing conditions of 930℃-1000℃ for 5 hours, the austenite grains of the three groups of example steels are kept above level 5, and no growth of grains is found.

[0050] Therefore, after the example steel is treated with a simulated carburizing process at 1000°C, the austenite grain size of the entire cross-section can still be maintained above level 5, and no mixed crystals or coarse grains are found. This can meet the technical requirements for resistance to grain coarsening required by high-temperature carburizing, effectively shorten the carburizing time, save energy consumption, reduce carbon emissions, and reduce production costs.

[0051] In addition to the above embodiments, the present invention also includes other implementation modes. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the protection scope of the claims of the present invention.

Claims

1. A high-speed gear steel bar for new energy vehicles, characterized in that: The element composition of steel is calculated by mass percentage as follows: C: 0.14~0.20%, Si: ≤0.30%, Mn: 1.00~1.50%, Cr: 1.00~1.40%, P: ≤0.025%, S: 0.010~0.030%, Cu: ≤0.30%, Ti: ≤0.0040%, Al: 0.020~0.050%, Ni: ≤0.30%, Mo: ≤0.10%, B: 0.0010~0.0050%, 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.

2. The steel according to claim 1, characterized in that: Element composition: B: 0.0010~0.0030%, Nb: 0.020~0.060%.

3. The steel according to claim 1, characterized in that: The bending and punching test value of the sample Fdyn≥55KN, and the processed gear meets the requirements of grain size reaching 5.0 or above under high temperature carburizing conditions of 1000℃.

4. A method for manufacturing the steel according to claim 1, characterized in that: Molten steel that meets the chemical composition of steel is smelted and cast into continuous casting billets. The smelting ladle maintains a long-term inclusion removal process to allow non-metallic inclusions to fully float. Refining slag and tundish protection slag are used to absorb inclusions in the molten steel. The continuous casting process uses crystallizer electromagnetic stirring and end electromagnetic stirring, and tundish induction heating technology to reduce material segregation; Before rolling, the continuous casting billet is heated to 1220-1270℃ and kept warm for more than 4 hours before being taken out of the furnace to allow the alloy including Nb and Al elements to be fully and evenly dissolved in the austenite phase. The starting rolling temperature is controlled at 1100-1180℃ and the final rolling temperature is controlled at 960-1000℃. After rolling, the billet is cooled to 300-500℃ on a cooling bed before being taken off the line.

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