Spheroidized annealing steel for low-temperature resistant high-strength ball screws and method for producing the same

Abstract

The present invention relates to a spheroidized annealed steel for low-temperature resistant and high-strength ball screws, and the chemical components of the steel, calculated in mass percentage, are: C: 0.40 to 0.70%, Si: 1.20 to 1.80%, Mn: 1.00 to 1.60%, Cr: 0.80 to 1.20%, S: ≦0.025%, P≦0.025%, Ni: 0.10 to 0.60%, Cu: 0.30 to 0.80%, Mo: 0.10 to 0.40%, Al≦0.05%, Ca≦0.0010%, Ti≦0.003%, O≦0.0010%, As≦0.04%, Sn≦0.03%, Sb≦0.005%, Pb≦0.002%, and the balance being Fe and unavoidable impurities. The cementite in the microstructure of the steel material is present in a spheroidized state with a diameter of 0.1 to 0.5 μm, preferably 0.3 to 0.5 μm, with the spheroidization rate reaching 95% or more, and the remaining structure being ferrite.

Description

Technical Field

[0001] The present invention relates to the technical field of alloy steel bars, and particularly to a steel applied to the processing of cryogenic high-strength ball screws and a manufacturing method thereof.

Background Art

[0002] In mechanical equipment, ball screws are indispensable transmission components for power and displacement transmission. Due to different usage environments, some ball screws used in extreme environments not only need to have the high precision and high wear resistance of conventional screws, but also need to meet the usage requirements such as maintaining high toughness under harsh environments such as storms, high waves, and severe cold in both polar regions of the earth.

[0003] The high-carbon chromium bearing steel such as the GCr15 brand is used for conventional ball screws. After quenching + tempering, such materials can only meet the usage requirements of contact rigidity with steel balls, and cannot meet the usage requirements in extreme environments for toughness in low-temperature environments. Moreover, since the heat treatment deformation of high-carbon bearing steel is difficult to control, the axial expansion and contraction of such materials are the main factors that ultimately prevent the grinding accuracy of ball screws from reaching the standard. In addition, since this type of steel has a relatively high carbon content, the grinding workability after quenching is quite poor, and the occurrence rate of processing quality problems such as grinding cracks is high.

Summary of the Invention

[0004] The present invention presents a new steel for cryogenic high-strength ball screws and a production method thereof. The processed ball screw products have ultra-high hardness, strength, and wear resistance on the surface even under extreme low-temperature conditions, and at the same time have extremely high low-temperature toughness, and also have good dimensional stability during the processing and use processes, ensuring the precision of the work in the final screw use process.

[0005] In order to achieve the above object, the mechanical properties of the steel for ball screws of the present application reach the following levels or requirements.

[0006] The requirements for non-metallic inclusions in the steel material are as shown in Table 1.

[0007] JPEG0007704883000001.jpg26159

[0008] The mechanical properties after the quenching and tempering treatment of the steel (for example, oil quenching at 880°C + water cooling at 450°C) are as shown in Table 2.

[0009] JPEG0007704883000002.jpg30159

[0010] Steel hardness: When the hardenability at one end was inspected by adopting JIS G 0561 method, the hardness of J9mm was ≧58HRC (the hardness at a depth of 9mm from the surface was ≧58HRC).

[0011] The specific technical means for realizing the above performance in the present invention are as follows.

[0012] The chemical composition of the spheroidizing annealing steel for low-temperature high-strength ball screws of the present invention, calculated by mass percentage, is: C: 0.40 - 0.70%, Si: 1.20 - 1.80%, Mn: 1.00 - 1.60%, Cr: 0.80 - 1.20%, S: ≦0.025%, P ≦0.025%, Ni: 0.10 - 0.60%, Cu: 0.30 - 0.80%, Mo: 0.10 - 0.40%, Al ≦0.05%, Ca ≦0.0010%, Ti ≦0.003%, O ≦0.0010%, As ≦0.04%, Sn ≦0.03%, Sb ≦0.005%, Pb ≦0.002%, and the balance is Fe and inevitable impurities.

[0013] The basis for setting the above chemical composition is as follows.

[0014] 1) Determination of C content C is an element necessary to guarantee wear resistance. The carbon in the steel increases the hardness and strength by increasing the martensite transformation ability, thereby improving the wear resistance. However, when the C content exceeds 0.77%, the cracking sensitivity increases significantly and the low-temperature toughness decreases. In the present invention, its content is controlled to be 0.40 - 0.70%.

[0015] 2) Determination of Si content Si is a deoxidizer in the steelmaking process and can improve the hardness, strength, elastic limit and yield ratio of steel in the form of solid solution strengthening. Si reduces the diffusion rate of C in ferrite, makes it difficult for carbides precipitated during tempering to aggregate, and improves the resistance of steel to temper softening. In addition, Si reduces the oxidation effect during frictional heat generation and improves the wear resistance of the material by increasing the cold deformation hardening rate of the steel. However, if the Si content is too high, the low-temperature toughness will decrease. In the present invention, the Si content is controlled to be 1.20 - 1.80%.

[0016] 3) Determination of Mn content Mn is an effective element for strengthening steel as a deoxidizing element in the steelmaking process and compensates for the strength loss caused by the decrease in C content in the steel by exerting a solid solution strengthening effect. In addition, Mn can increase the hardenability of steel and improve the hot working performance of steel. Mn can remove the influence of S (sulfur). Mn can form high-melting-point MnS together with S in the steelmaking of iron and steel, thereby weakening and removing the adverse effects of S. When the Mn content exceeds 1.60%, the toughness of the steel decreases significantly. In the present invention, the Mn content is controlled to be 1.00 - 1.60%.

[0017] 4) Determination of Cr content Cr is a carbide-forming element and can improve the hardenability, wear resistance and corrosion resistance of steel. A part of Cr in the steel replaces iron to form alloy cementite, improving the tempering stability of the steel, and a part dissolves in ferrite to cause solid solution strengthening, improving the strength and hardness of ferrite. However, if the Cr content is too high, it is easy to combine with carbon in the steel to form large carbide lumps, and these large carbide lumps reduce the contact fatigue life of the steel. After analyzing the above, in the present invention, the range of Cr content is set to 0.80 - 1.20%.

[0018] 5) Determination of Al content Al is a deoxidizer in the smelting process. It can not only reduce the dissolved oxygen in molten steel, but also form fine aluminum nitride inclusions dispersed by Al and N to refine the crystal grains. However, when the Al content exceeds 0.05%, the fluidity of molten steel will decrease significantly, increasing the difficulty of casting. In the present invention, the range of the Al content is set to ≤0.05%.

[0019] 6) Determination of Ni content Ni exists in a solid solution state in steel. In the composition system of the present invention, Ni can reduce the stacking fault energy and significantly improve the low-temperature impact performance of steel. However, if there is too much Ni, the retained austenite content in steel will become too high, reducing the strength and increasing the cost. In the present invention, the range of the Ni content is set to 0.10 - 0.60%.

[0020] 7) Determination of Cu content The Cu element can form fine precipitates during tempering to improve the strength of steel. At the same time, Cu also helps to improve the corrosion resistance of steel in extreme environments. However, if there is too much Cu, the grain boundaries will become weak and cracks will occur. In the present invention, the range of the Cu content is set to 0.30 - 0.80%.

[0021] 8) Determination of Mo content Mo can refine the crystal grains of steel, enhance hardenability and high-temperature performance, and maintain sufficient strength and creep resistance at high temperatures. It can also suppress the brittleness caused by tempering of alloy steel. However, since molybdenum alloys belong to precious alloys, in order to control the cost and obtain the desired effect, in the present invention, the range of the Mo content is set to 0.10 - 0.40%.

[0022] 9) Determination of Ca content The Ca content increases the amount and size of the punctiform oxides in the steel. However, since the punctiform oxides are hard and have poor plasticity, they do not deform when the steel deforms, and gaps are likely to form at the interface, deteriorating the performance of the steel. At the same time, it is also related to the control of the smelting cost. In the present invention, the range of the Ca content is set to ≤0.001%.

[0023] 10) Determination of the Ti content Ti remains in the steel in the form of titanium nitride and titanium carbonitride inclusions, which has an adverse effect on the steel material. This type of inclusion is hard and angular, so it has a serious impact on the fatigue life of the material. Especially when the purity is very high and the amount of other oxide inclusions is small, the damage caused by the titanium-containing inclusions becomes particularly prominent. At the same time, it is also related to the control of the smelting cost. In the present invention, the range of the Ti content is set to ≤0.003%.

[0024] 11) Determination of the O content The oxygen content represents the total amount of oxide inclusions, and the oxide brittle inclusions limit and affect the service life of the finished product. From many tests, it has been found that the decrease in the oxygen content is significantly beneficial to the improvement of the purity of the steel material, especially the decrease in the content of oxide brittle inclusions in the steel. At the same time, it is also related to the control of the smelting cost. In the present invention, the range of the oxygen content is set to ≤0.0010%.

[0025] 12) Determination of the P and S contents P causes serious segregation during solidification in the steel. When P dissolves in ferrite, it distorts and coarsens the grain boundaries and increases the cold brittleness. At the same time, it is also related to the control of the smelting cost. In the present invention, the range of the P content is set to ≤0.025%. S causes hot brittleness in the steel, reducing the ductility and toughness of the steel. At the same time, it is also related to the control of the smelting cost. In the present invention, the range of the S content is set to ≤0.025%.

[0026] 13) Determination of the As, Sn, Sb, and Pb contents Trace elements such as As, Sn, Sb, and Pb all belong to low-melting non-ferrous metals. When present in steel, they cause soft spots on the surface of parts and uneven hardness, so they are regarded as harmful elements in steel. At the same time, it is also related to the control of smelting costs. In the present invention, the ranges of the contents of these elements are set as As≦0.04%, Sn≦0.03%, Sb≦0.005%, and Pb≦0.002%.

[0027] The manufacturing process of the above ball screw steel is: electric furnace or converter - secondary refining - vacuum degassing - continuous casting - continuous rolling - shearing or saw cutting - stack cooling - spheroidizing annealing - finishing - picking and placing, warehousing.

[0028] The characteristics of the main production processes are as follows.

[0029] 1. Adopt high-quality molten iron, scrap steel and raw material auxiliary materials to reduce the content of harmful elements in the molten steel. Strengthen deoxidation in the refining process, ensure the amount of residual aluminum in the steel, utilize good kinetic conditions in the molten steel to perform intensive early deoxidation and vacuum degassing treatment, make non-metallic inclusions float sufficiently, and control the content of gas elements in the molten steel. Continuously perform weak blowing of argon after vacuum degassing to further float inclusions in the molten steel. In the continuous casting process, oxidation of the molten steel must be prevented and protected.

[0030] 2. In the continuous casting process, combine electromagnetic stirring and soft reduction, and adopt casting at a low superheat degree to effectively improve and reduce the component segregation of the continuous casting billet. Especially when advanced equipment such as electromagnetic stirring and soft reduction at the solidification end is added, the density of the solidification structure of the billet increases, the roughness and shrinkage cavity in the center of the billet are effectively controlled, the secondary dendritic arm spacing is significantly improved, the central equiaxed crystal ratio is significantly improved, the crystal grains are refined, thereby significantly improving the quality of the billet and reducing component segregation.

[0031] 3. The product of the present invention passes the smelting raw materials through primary smelting, refining, and vacuum degassing in sequence to obtain molten steel of the target components, and then adopts a continuous casting process to cast the molten steel into a continuous casting square billet with a specification of 390 mm × 510 mm or more. The continuous casting billet is slowly cooled in a pit to prevent cracking. The slow cooling time is not less than 48 hours. Then, the continuous casting billet is sent into a heating furnace in a neutral or weakly acidic atmosphere for heating. The heating temperature is 1000 - 1250°C, and the heating time is more than 5 hours. Then, it is rolled into an intermediate billet of 200 mm × 200 mm - 300 mm × 300 mm. The rolling temperature range is 1000°C - 1200°C, the finish rolling temperature is ≥800°C, the rolling ratio is greater than 5, and the intermediate billet is slowly cooled in a pit, with the pit temperature ≥500°C and the slow cooling time not exceeding 48 hours.

[0032] Next, the intermediate billet is reheated to further proceed with rolling and rolled to the target specification. The specific heating process is as follows. The temperature of the preheating zone is 650 - 900°C, the temperature of the heating zone is 1000 - 1250°C, the temperature of the soaking zone is 1000 - 1250°C, and in order to ensure that the billet is heated sufficiently uniformly, the total heating time must be 2 hours or more. The starting temperature of rolling is 1000°C - 1200°C, the finish rolling temperature is ≥800°C, and after rolling is completed, it is stack-cooled.

[0033] In order to ensure the dimensional accuracy stability of the steel material when manufacturing the ball screw, it is necessary to perform spheroidizing annealing treatment on the above-mentioned steel material, so the following spheroidizing annealing process is used.

[0034] (1) Keep it at a temperature of 805 ± 10°C for 7 hours to keep the microstructure in the two-phase region of ferrite and austenite. At this time, a part of the cementite dissolves into austenite to form secondary cementite. The matrix is ferrite and secondary cementite, and the microstructure has cementite particles that can be used for subsequent nucleation, and the two types of structures of ferrite and secondary cementite reach dynamic equilibrium and coexist.

[0035] (2) Water spray cooling. Different from hypereutectoid steels such as bearing steel GCr15, the products of the present invention belong to hypoeutectoid steels. Therefore, it is necessary to adopt water spray cooling to enhance the driving force of spheroidized supercooling.

[0036] (3) Further perform a two-stage isothermal spheroidizing process. In the first stage, keep the temperature in the range of 745 ± 10 °C for 5 hours, and in the second stage, keep the temperature in the range of 690 ± 10 °C for 4.5 hours to sufficiently precipitate the secondary cementite described in step (1) in a spherical form. In the two-stage isothermal spheroidizing method, the size of the balls and the spheroidization rate are controlled. The diameter of the cementite after spheroidization is controlled to be (0.1 μm - 0.5 μm), preferably (0.3 μm - 0.5 μm).

[0037] If the isothermal spheroidizing temperature is too high, the size of the balls will become too large. If the isothermal spheroidizing temperature is too low, the spheroidization rate will become too low, which will affect the dimensional stability in the subsequent heat treatment process of the ball screw.

[0038] The products after spheroidizing annealing will become the final products after straightening and flaw detection.

[0039] Compared with the prior art, the advantages of the present invention are as follows.

[0040] 1) Different from the conventional GCr15 bearing steel, the chemical composition is optimized. Therefore, the hardenability, yield strength and temper softening resistance of the steel material are significantly improved, and cracks are less likely to occur.

[0041] 2) The spheroidized cementite of the conventional GCr15 bearing steel is relatively coarse, with a diameter of 1 - 3 μm. In comparison, the size of the spheroidized cementite of the products of the present invention is smaller, with a diameter of (0.1 μm - 0.5 μm), and the spheroidization rate reaches more than 95%. The remaining structure is ferrite. The structural strain energy is small, the heat treatment deformation during the process of processing the screw products is small, and the dimensional accuracy is high. Therefore, the accuracy usage requirements of the ball screw can be met.

[0042] 3) The conventional GCr15 bearing steel has very high low-temperature brittleness, with a Charpy impact value AKU2 < 10 J at -40°C. In comparison, the product of the present invention not only has a higher yield strength (≧1380 MPa) and tensile strength (≧1500 MPa), but also has better low-temperature toughness. The Charpy impact value at -40°C is AKU2 ≧ 27 J.

Brief Description of the Drawings

[0043]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0044] Hereinafter, in conjunction with the examples, a more detailed description of the present invention will be given.

[0045] Examples 1 to 3 list the chemical compositions and manufacturing methods of the steel for ball screws of the present invention, and compare them with commercially available GCr15 bearing steel.

[0046] The chemical compositions (wt%) of each example are as shown in Tables 2 and 3

[0047] JPEG0007704883000003.jpg33158

[0048] JPEG0007704883000004.jpg38158

[0049] The inclusions in the steel materials of each example are as shown in Table 4

[0050] JPEG0007704883000005.jpg46158

[0051] The comparison of the mechanical properties (oil quenching at 880°C + water cooling at 450°C) of each example is as shown in Table 5.

[0052] JPEG0007704883000006.jpg36154

[0053] The hardenability data of one end of the steel materials of each example is as shown in Table 6.

[0054] JPEG0007704883000007.jpg36158

[0055] The microstructures of the steel materials of each example are as shown in Figures 1 to 3. Different from the coarse globular cementite of the conventional GCr15 bearing steel, the cementite in the as-delivered state of the steel materials of the present invention exists in a uniform and finer (generally 0.1 to 0.5 μm) globularized state, and the globularization rate reaches 95% or more, and the remaining structure is ferrite. The structure strain energy is small, the heat treatment deformation during the process of processing screw products is small, and the dimensional accuracy is high, so it can meet the accuracy use requirements of ball screws.

[0056] The manufacturing process of the steel for ball screws of each example is: electric furnace or converter - secondary refining - VD or RH vacuum degassing - continuous casting - forming from continuous casting billet to intermediate billet - heating the intermediate billet and rolling it into material - spheroidizing annealing - finishing - picking and placing, warehousing.

[0057] Specifically, when smelting, high-quality molten iron, scrap steel and raw material auxiliary materials are selected, and high-quality deoxidizers and refractory materials are selected. In the production process of the electric furnace / converter, the C at the tapping end of the three examples is controlled to be 0.05 to 0.25% respectively, and the P at the end point needs to be ≤0.025%, and the continuous casting overheat degree is controlled between 15 and 35°C.

[0058] The billet rolling process using the continuous casting billets of each example is as shown in Table 7.

[0059] JPEG0007704883000008.jpg40159

[0060] The specific rolling process of sending the intermediate billet into the heating furnace and rolling it into the target round bar is as follows. The temperature of the preheating zone is controlled at 650 - 900 °C, the temperature of the heating zone is controlled at 1000 - 1250 °C, and the temperature of the soaking zone is controlled at 1100 - 1200 °C. To ensure that the billet receives heat evenly enough, the total heating time is set to be 2 hours or more. The rolling start temperature is controlled at 900 °C - 1100 °C, the finishing rolling temperature is controlled at 800 °C or higher. After rolling is completed, slow cooling is carried out to precipitate the AlN particles in the steel finely, uniformly, and sufficiently, thereby refining the crystal grains and preventing the occurrence of mixed crystals in the steel. After rolling is completed, stack cooling is performed. Spheroidizing annealing treatment is carried out on the rolled finished bar. The process is as shown in the above three-stage spheroidizing process diagram. The bar products after spheroidizing annealing are further subjected to flaw detection treatment, and finally picked and placed into storage.

[0061] From Tables 2, 3, 4, 5, and 6, it can be seen that for the low-temperature high-strength steel for ball screws in each of the above embodiments of the present invention, the control levels of harmful elements such as oxygen, titanium, and non-metallic inclusions are clearly better compared with the conventional GCr15 bearing steel. Especially in terms of mechanical properties, the yield strength, tensile strength, low-temperature impact, and tempering softening resistance of the present invention after being processed in the same quenching and tempering process are clearly superior to the conventional GCr15 bearing steel. The yield strength is improved by nearly 400 MPa, the tensile strength is improved by 300 MPa, the low-temperature impact performance is improved by nearly 30 J, and the hardness is improved by nearly 10 HRC. The hardenability is also clearly superior to the conventional GCr15 bearing steel.

[0062] As described above in detail for the preferred embodiments of the present invention, those skilled in the art must clearly understand that the present invention can be subject to various modifications and changes. Any changes, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall all be included within the protection scope of the present invention.

Claims

1. In the spheroidized annealed steel for ball screws, when the chemical composition of the steel is calculated by mass percentage, C: 0.40 - 0.70%, Si: 1.20 - 1.80%, Mn: 1.00 - 1.60%, Cr: 0.80 - 1.20%, S: ≤0.025%, P ≤0.025%, Ni: 0.10 - 0.60%, Cu: 0.30 - 0.80%, Mo: 0.10 - 0.40%, Al ≤0.05%, Ca ≤0.0010%, Ti ≤0.003%, O ≤0.0010%, As ≤0.04%, Sn ≤0.03%, Sb ≤0.005%, Pb ≤0.002%, and the balance is Fe and unavoidable impurities. The spheroidized annealed steel for ball screws is characterized by this.

2. The steel that has undergone quenching and tempering treatment has a yield strength ≥1380 MPa, a tensile strength ≥1500 MPa, an elongation ≥9%, a -40°C Charpy impact value AKU2 ≥27 J, and the hardenability at one end inspected using the JIS - G - 0561 method satisfies J9mm hardness ≥58 HRC. The spheroidized annealed steel for ball screws according to Claim 1 is characterized by this.

3. The cementite in the microstructure of the steel exists in a spheroidized state with a diameter of 0.1 - 0.5 μm, and the spheroidization rate reaches 95% or more, and the remaining structure is ferrite. The spheroidized annealed steel for ball screws according to Claim 1 is characterized by this.

4. In the manufacturing method of the spheroidized annealed steel for ball screws according to Claim 1, the manufacturing method is as follows: Step 1: Primary smelting, refining, and vacuum degassing of the smelting raw materials are carried out in an electric furnace or a converter in this order to obtain molten steel, and then the molten steel is continuously cast to continuously cast a continuous casting square billet with a specification of 390×510 mm or more that is consistent with the chemical composition of the finished steel product. Step 2: The continuous casting billet is put into a slow cooling pit and slowly cooled, and the slow cooling time is not less than 48 hours. Then, the continuous casting billet is sent into a heating furnace in a neutral or weakly acidic atmosphere and heated, and then rolled into an intermediate billet of 200 mm×200 mm - 300 mm×300 mm. Step 3: The intermediate billet is reheated and rolled to the target specification. Step 4: Then, spheroidizing annealing is performed on the product after rolling is completed. Step 5: The product after spheroidizing annealing is straightened and flaw - detected to obtain a qualified product. The manufacturing method is characterized by including the above steps.

5. In step 1, molten iron, scrap steel, and raw material auxiliary materials are selected, a deoxidizer and a refractory material are selected. In the primary smelting process in an electric furnace or a converter, the C content at the end of smelting is controlled to be 0.05 - 0.25%, the P content at the end is set to ≤0.025%. In the continuous casting process, electromagnetic stirring and light pressure are carried out for the casting during the solidification process, and the superheat degree of continuous casting is 15 - 35°C. The manufacturing method according to claim 4 is characterized by this.

6. In step 2, the heating temperature for the continuous casting billet is 1000 - 1250°C, the heating time is longer than 5 hours, the rolling start temperature during rolling is set to 1000°C - 1200°C, the finish rolling temperature is ≥800°C, the rolling ratio is greater than 5, the intermediate billet obtained by rolling is slowly cooled in a pit, the temperature when the intermediate billet enters the pit is ≥500°C, and the slow cooling time is ≥48 hours. The manufacturing method according to claim 4 is characterized by this.

7. In step 3, the heating method of the intermediate billet is to control the preheating zone temperature to 650 - 900°C, the heating zone temperature to 1000 - 1250°C, the soaking zone temperature to 1000 - 1250°C, the total heating time to be 2 hours or more, control the rolling start temperature of the intermediate billet rolling to 1000°C - 1200°C, control the finish rolling temperature to 800°C or more, and stack-cool the product after rolling is completed. The manufacturing method according to claim 4 is characterized by this.

8. In step 4, the spheroidizing annealing process is first to hold the product at 805 ± 10°C for 7 hours or more, then cool the product to 745°C ± 10°C by water spray cooling and hold it at this temperature for 5 hours or more. Next, cool the product to 690 ± 10°C in the furnace and hold it at this temperature for 4.5 hours or more. Finally, cool the product to 500 ± 10°C in the furnace and then take it out of the furnace. The manufacturing method according to claim 4 is characterized by this.

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