Stainless steel bar for bearing pipe penetration and pipe penetration preparation process thereof

Abstract

This invention discloses a stainless steel bar for bearing tube insertion and its tube insertion preparation process. The bar has a carbon content of 0.55%~0.65%, and the content of harmful elements is strictly controlled by optimizing the Cr, Mo, Ni ratio and adding trace amounts of Nb and V. The diameter is 40~110mm. The preparation process includes smelting and casting billets, grinding and inspection, billet rolling, annealing and flaw detection, length cutting, hot piercing, steel tube cooling and annealing, and final heat treatment. During hot piercing, the heating temperature is controlled at 1140~1180℃, and the reheating temperature T before final rolling and the cross-sectional machining degree R satisfy T>44.4×ln(R)+821. This invention improves the high-temperature plasticity of the material, reduces deformation resistance, inhibits δ-ferrite and phase transformation cracks, widens the hot working window, and achieves a tube insertion yield of over 95%. The hardness of the steel tube after heat treatment is HRC55~60, meeting the requirements for bearing rings, and is suitable for the large-scale production of seamless steel tubes for bearings.

Description

Technical Field

[0001] This invention relates to the field of stainless steel technology, and in particular to a stainless steel bar for bearing tube insertion and its tube insertion preparation process. Background Technology

[0002] Martensitic stainless steel, with its high hardness, excellent wear resistance and corrosion resistance, has become the core material for key wear-resistant parts such as bearing rings, rolling elements, and precision cutting tools. Among them, G65Cr14Mo, as a typical high-carbon martensitic stainless steel with a carbon content of about 0.65%, is a commonly used base material for the preparation of bearing parts. Seamless steel tubes for bearings are the core blanks for bearing rings. The mainstream manufacturing process involves hot piercing stainless steel bars with a diameter of 40-110mm. However, due to its inherent compositional characteristics, high-carbon martensitic stainless steel faces numerous technical bottlenecks in the entire hot piercing tube manufacturing process, severely restricting its industrial application in the bearing steel tube field. To ensure hardness and wear resistance, high-carbon martensitic stainless steel typically has a carbon content >0.60%, and traditional grades such as G65Cr14Mo have a molybdenum content as high as 0.50%-0.80%. This combination of high carbon and high molybdenum composition means that the material maintains high strength in the high-temperature range of hot piercing, making it difficult to achieve uniform plastic deformation. Problems such as motor overload and irregular forming are prone to occur during the piercing process, increasing the difficulty of tube manufacturing process control. Furthermore, it has a significant tendency for air-cooling hardening, which easily leads to phase transformation cracks: the critical cooling rate for martensitic phase transformation of high-carbon martensitic stainless steel is relatively low. During the air cooling process after hot piercing, the material is prone to rapid martensitic phase transformation, resulting in microstructures... Stress can cause microcracks on the surface and inside of steel pipes, significantly reducing the surface quality and internal integrity of the pipes, becoming a fatal defect in subsequent processing. Furthermore, the chromium content of this type of stainless steel is generally above 13%. If the heating temperature before hot piercing is not properly controlled, brittle phases such as δ-ferrite are easily precipitated at high temperatures. The presence of this phase will significantly reduce the high-temperature plasticity and toughness of the material, making the bar stock prone to cracking during the piercing deformation process, further aggravating the quality risks in the pipe manufacturing process. In production and processing, bearing components have strict standards for dimensional accuracy, surface quality, and internal metallurgical quality. However, due to the above-mentioned process defects, existing high-carbon martensitic stainless steel is prone to cracks, internal wall tearing, and excessive dimensional deviations after hot piercing, resulting in a low yield. At the same time, the existing process lacks quantitative matching control of heating temperature and deformation amount, and there is no full-process quality control system, making it difficult to stably produce seamless steel pipes that meet the requirements of bearing pipes, thus limiting the large-scale application of high-carbon martensitic stainless steel in the field of bearing steel pipes. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a stainless steel bar for bearing tube insertion and its tube insertion preparation process. It provides a method that, while maintaining a carbon content of approximately 0.6%, significantly improves high-temperature plasticity, reduces deformation resistance, and inhibits the formation of δ-ferrite by optimizing alloy composition design and cooperating with full-process process control from billet to bar to steel pipe.

[0004] To achieve the above objectives, the present invention provides a stainless steel bar for bearing tube insertion, wherein the chemical composition of the bar, by mass percentage, is: C: 0.55%~0.65%; Cr: 12.5%~13.5%; Mo: 0.10%~0.20%; Ni: 0.30%~0.50%; Mn: 0.30%~0.60%; Si: 0.20%~0.40%; Nb: 0.02%~0.08%; V: 0.05%~0.15%; P: ≤0.020%; S: ≤0.005%; N: ≤0.030%; the balance being Fe and unavoidable impurities; the diameter of the bar is 40~110mm.

[0005] The beneficial effects of this design are as follows: By precisely defining the chemical composition, mass percentage, and diameter specifications of the stainless steel bars used for bearing tube insertion, synergistic optimization of the alloy system is achieved. While retaining a high carbon content of 0.55%~0.65% to ensure the core wear resistance and high hardness of the bearing material, the design specifically addresses the processing pain points of traditional high-carbon martensitic stainless steel. The Mo content is significantly reduced to 0.10%~0.20%, effectively reducing the material's resistance to high-temperature deformation and solving the problem of uneven deformation during hot piercing. The Cr content is controlled at 12.5%~13.5%, avoiding the formation of brittle δ-ferrite phases at high temperatures and ensuring a single, stable austenitic structure during hot working. The Ni content is increased to 0.30%~0.50%, with the addition of trace amounts of Nb and V, which both inhibits δ-ferrite formation and widens the temperature window of the single-phase austenite region, and significantly improves high-temperature plasticity and toughness by pinning grain boundaries and promoting dynamic recrystallization through fine carbonitrides. Strict control of harmful elements such as P, S, and N reduces the risk of material cracking and inclusions. With diameters ranging from 40 to 110 mm, the product precisely matches the mainstream billet requirements for hot piercing of seamless steel tubes for bearings. It optimizes the hot working process performance at the base material level, providing a high-quality and suitable billet foundation for subsequent tube forming, and solving the core problem of poor tube forming compatibility of traditional stainless steel bars.

[0006] Further, the preferred range of the chemical composition by mass percentage is: C: 0.58%~0.62%; Cr: 12.8%~13.2%; Ni: 0.35%~0.45%.

[0007] The beneficial effects of this configuration are as follows: Precise control of C (0.58%~0.62%), Cr (12.8%~13.2%), and Ni (0.35%~0.45%) optimizes the synergistic effect of each alloying element. Narrow-range C content control ensures high hardness after final heat treatment while minimizing high-temperature deformation resistance and air-cooling crack sensitivity, further optimizing hot working performance. Precise Cr content limitation completely avoids the risk of δ-ferrite formation, ensuring the stability of the austenite structure within the hot piercing temperature range. The optimized Ni content range further expands the temperature window of the single-phase austenite region, improving the tolerance of hot working temperature control and reducing processing defects caused by temperature fluctuations in industrial production. This optimized composition further enhances the high-temperature plasticity, crack resistance, and hot working adaptability of the bar stock, resulting in higher forming stability during tube insertion and effectively reducing defects such as surface microcracks and internal wall tensile cracks. This allows for a more precise match between the bar stock's processing performance and the requirements of bearing tube insertion, laying a superior material foundation for the production of high-quality bearing steel tubes.

[0008] Furthermore, the present invention also discloses a process for preparing stainless steel bars for bearing tube insertion, comprising the following steps: Step 1, Smelting and casting: The continuous casting billet is prepared by electric arc furnace + AOD / VOD refining + continuous casting process. The superheat of molten steel is controlled to be ≤30℃. After the continuous casting billet is taken off the line, it is slowly cooled or annealed at 650~720℃ in time. The rolling ratio of the continuous casting billet is greater than 10. Step 2, Billet Grinding and Inspection: The surface of the continuously cast billet is ground to a depth of ≥1.5mm to remove surface defects, and ultrasonic testing is used to detect the internal quality of the billet. Step 3, billet rolling: Heat the qualified continuously cast billet to 1150~1180℃, and hold it for 1.0~1.5min / mm. Then, hot roll it into round bars with a diameter of 40~110mm through multiple passes. Control the final rolling temperature to ≥950℃. After rolling, the bars are cooled by stacking or slow cooling at a rate of ≤0.5℃ / s. Step 4, Bar Annealing and Inspection: Heat the rolled bar to 800~860℃, hold for 3~5 hours, furnace cool to below 600℃ and air cool. After annealing, the hardness of the bar is ≤HB240. Surface flaw detection and ultrasonic flaw detection are performed on the annealed bar. Step 5, Bar Cutting and Length Setting: Cut the qualified bars into lengths of 800~1200mm, chamfer both ends of the bars and remove burrs; Step 6, Bar Piercing: Heat the standard-length bars to 1140~1180℃. During the heating process, heat slowly below 900℃ and rapidly above 900℃, controlling the temperature difference between the inside and outside of the bar within ±20℃. Use hot piercing technology to form the bars into seamless steel pipes. The piercing ratio is controlled at 0.3~0.7, and the reheating temperature T before final rolling and the final rolling section processing degree R are controlled to satisfy the following relationship: T>44.4×ln(R)+821; where T is in ℃, R is the final rolling section processing degree, calculated according to the formula R=(A0-A1) / A0×100%, A0 is the cross-sectional area of ​​the bar before piercing, and A1 is the cross-sectional area of ​​the steel pipe after piercing. Step 7, steel pipe cooling: Immediately cool the pierced steel pipe in a pile or slow cooling pit at a rate of ≤0.5℃ / s until the steel pipe temperature drops below 200℃; Step 8, steel pipe annealing: Heat the cooled steel pipe to 800~860℃, hold for 3~5 hours, furnace cool to below 600℃ and air cool. After annealing, the hardness of the steel pipe is ≤HB240. Step 9, final heat treatment: The annealed steel pipe is quenched and tempered. The quenching temperature is 1000~1040℃, and the cooling method is oil cooling or air cooling. The tempering temperature is 180~220℃ or 500~550℃.

[0009] The beneficial effect of this setup is that the nine steps from smelting the billet to the final heat treatment form a closed-loop process control, and the technical bottlenecks of traditional processes are broken one by one to address the material characteristics of high-carbon martensitic stainless steel. In the billet smelting stage, controlling the superheat of molten steel and implementing slow cooling / annealing eliminates internal stress in the billet, prevents cracking, and ensures the metallurgical quality of the billet. Billet grinding and flaw detection remove defects at the source, preventing them from amplifying in subsequent processing. Temperature, holding, and cooling control during billet rolling ensures the quality of bar forming and prevents post-rolling cracks. The bar annealing process softens the microstructure, eliminates rolling stress, and controls hardness to a easily machinable range; flaw detection further ensures billet quality. Length cutting and chamfering / deburring adapt to the requirements of the piercing mill, reducing clamping and forming errors. Segmented heating, temperature control, piercing ratio, and quantitative matching of temperature and machinability in hot piercing achieve uniform plastic deformation of the bar, avoiding excessive deformation resistance and grain coarsening. Slow cooling and annealing of the steel pipe after piercing effectively suppresses martensitic phase transformation cracks and further softens the microstructure to suit subsequent processing. Finally, heat treatment achieves a precise match between hardness and toughness, meeting the requirements of bearing rings. The combination of end-to-end process technology and optimized composition significantly improves the yield of hot piercing. The seamless steel pipes produced meet the stringent standards for bearing components in terms of dimensional accuracy, surface quality, and internal metallurgical quality, solving the industry problems of low yield and unstable product quality caused by traditional processes.

[0010] Furthermore, the heating equipment for the fixed-length bars mentioned in step six is ​​an induction furnace or a ring furnace.

[0011] The beneficial effects of this setup are as follows: the induction furnace utilizes electromagnetic induction to simultaneously heat the inside and outside of the bar, offering controllable heating rates and high heating efficiency. It can precisely match the process requirements of rapid heating above 900℃ and easily control the temperature difference between the inside and outside of the bar. The ring furnace enables continuous heating of the bar, adapting to the needs of large-scale industrial production. During the heating process, the bar is heated evenly, effectively ensuring temperature consistency during slow heating below 900℃. Both heating devices can precisely control the temperature difference between the inside and outside of the bar within ±20℃, avoiding uneven heating that leads to differences in local deformation resistance and irregular forming. Furthermore, it can be combined with segmented heating processes to reduce the high-temperature residence time of the bar, preventing excessive grain growth and surface decarburization. This ensures the uniformity of the bar's microstructure and surface quality before hot piercing, providing crucial support for uniform plastic deformation during hot piercing and effectively reducing forming defects such as inner wall cracking and surface cracks in the steel pipe. Detailed Implementation

[0012] An embodiment of the stainless steel bar for bearing tube insertion and its tube insertion preparation process of the present invention is described. The stainless steel of the present invention is smelted according to the following composition (mass %): C 0.60%, Cr 13.1%, Mo 0.15%, Ni 0.40%, Mn 0.45%, Si 0.30%, Nb 0.05%, V 0.10%, P 0.015%, S 0.002%, N 0.020%, balance Fe. The tube insertion preparation process of the stainless steel bar for bearing tube insertion includes the following steps: The smelting process adopts electric arc furnace + AOD refining + continuous casting. The superheat of molten steel is controlled at 25℃ throughout the process. The continuous casting forms 200mm×200mm square billets. After the continuous casting billets come off the line, they are immediately sent to a slow cooling furnace for slow cooling treatment at 680℃ to eliminate the internal stress of the billets. The subsequent rolling ratio of the continuous casting billets is designed to be 16, which meets the requirement of a rolling ratio greater than 10.

[0013] After slow cooling, the surface of the continuously cast billet is fully ground to a depth of 2.0 mm to completely remove defects such as cracks, inclusions, and pits from the billet surface. After grinding, ultrasonic testing is used to inspect the internal quality of the billet to ensure that there are no excessive internal defects such as central porosity and shrinkage cavities. After passing the test, it is ready for use.

[0014] The qualified billet is sent to the heating furnace and heated to 1160℃. It is held at a holding time of 1.2 min / mm to ensure uniform temperature inside and outside the billet. After multiple hot rolling passes, it is rolled into round bars with a diameter of 50 mm. The final rolling temperature is controlled at 980℃ during the rolling process to meet the process requirement of ≥950℃. The rolled bars are cooled by stacking cooling, and the cooling rate is controlled at 0.4℃ / s to avoid post-rolling cracks.

[0015] The cooled bars were sent to an annealing furnace, heated to 820°C and held for 4 hours, then furnace cooled to 580°C and air cooled. The hardness of the bars after annealing was tested and found to be HB228, which meets the requirement of ≤HB240. The annealed bars were subjected to eddy current surface testing and ultrasonic internal testing in sequence. Bars with surface defects or internal cracks were rejected, and qualified bars were retained.

[0016] According to the requirements of the hot piercing machine, the qualified bars are cut into fixed lengths of 1000mm, and the two ends of the bars are chamfered at 45° and the burrs on the end faces are completely removed to prevent stress concentration during the piercing process.

[0017] The induction furnace is used to heat the fixed-length bars. The heating process is carried out in two stages: slow heating at 5℃ / min is used below 900℃ to make the temperature inside and outside of the bar uniform; rapid heating at 20℃ / min is used above 900℃ until the bar temperature reaches 1170℃. The temperature difference between the inside and outside of the bar is controlled at 15℃ throughout the process to reduce the high-temperature dwell time and prevent excessive grain growth and surface decarburization. Hot piercing is used to form heated bars into seamless steel tubes with an outer diameter of 50 mm and a wall thickness of 8 mm. The piercing ratio is controlled at 0.53, and the mandrel extension is adjusted to 35 mm. Calculate the final rolling section machining degree under this process: R=(A0-A1) / A0×100%=(π×252-π×(25-8)×8×2) / π×252×100%=46.3%; Verification using the temperature-processing degree relationship: T>44.4×ln(46.3)+821=44.4×3.835+821=170.3+821=991.3℃. The actual heating temperature of 1170℃ is much higher than the theoretical minimum temperature, which meets the process matching requirements.

[0018] After piercing, the steel pipe is immediately sent to a slow cooling pit for slow cooling treatment. The cooling rate is controlled at 0.3℃ / s and continues to cool until the temperature of the steel pipe drops below 150℃. Air cooling or rapid cooling by spraying water is avoided throughout the process to prevent the formation of martensitic phase transformation cracks.

[0019] The cooled steel pipe is sent into an annealing furnace and annealed using the same process as the bar stock: heated to 820℃ and held for 4 hours, then furnace cooled to 550℃ and air cooled. After annealing, the hardness of the steel pipe is tested to be HB222, which meets the requirement of ≤HB240, making it convenient for subsequent cold working or turning.

[0020] According to the performance requirements of the bearing rings, the steel pipe is subjected to quenching and low-temperature tempering treatment: the steel pipe is heated to 1020℃, held at the temperature and then quenched by oil cooling; after quenching, it is heated to 200℃ for tempering treatment, held at the temperature and then air cooled.

[0021] Finally, the performance of the heat-treated steel pipe was tested. The steel pipe had no cracks on the surface and no tensile cracks on the inner wall. The metallurgical structure was uniform and the hardness reached HRC58, which meets the requirements of bearing rings for high hardness and high wear resistance. The overall yield rate of this embodiment reached 97%, which is far higher than the level of traditional processes.

[0022] Comparative Example 1 This comparative example uses conventional G65Cr14Mo high-carbon martensitic stainless steel, whose chemical composition by mass percentage is: C 0.67%, Cr 13.4%, Mo 0.64%, Ni 0.06%, with the balance being Fe and conventional impurities. It is rolled into bars with a diameter of 50 mm and hot-pierced to form seamless steel pipes with an outer diameter of 50 mm and a wall thickness of 8 mm using the same preparation and tube-piercing process as in Example 1.

[0023] The comparative example exhibited significant technical problems during the piercing process: the material's resistance to high-temperature deformation increased significantly, leading to excessive load on the piercing machine motor and disrupting the forming process; after air cooling, the pierced steel pipes showed multiple micro-cracks on their surface, and after heat cooling, flaw detection revealed multiple surface cracks with lengths of 2-5mm. Ultimately, the yield of qualified steel pipes was less than 60%, and no qualified steel pipes met the quality requirements for bearing rings.

[0024] Comparative Example 2 This comparative example uses the exact same chemical composition as Example 1 to prepare stainless steel bars with a diameter of 50 mm. The only difference is the heating temperature in the hot piercing process, which is adjusted to 1050°C. All other preparation and tube-piercing process parameters are the same as in Example 1, and the same seamless steel pipe of the same specification is formed by hot piercing.

[0025] Calculations showed that the final rolled section processing degree was still 46.3%, and the theoretical minimum heating temperature was 991.3℃. Although the actual heating temperature of 1050℃ was higher than the theoretical value, it did not reach the recommended temperature range of 1140~1180℃ of this invention. In this comparative example, the deformation resistance increased during the piercing process, and the inner wall of the formed steel pipe showed slight tensile cracking defects. After flaw detection, the unqualified products were removed, and the final yield dropped to 82%. Although it was better than the comparative example 1, it was far lower than the yield of example 1. Moreover, the inner wall defects would greatly increase the risk of product scrapping in subsequent processing.

[0026] A comparison of Example 1 with Comparative Examples 1 and 2 clearly shows that: This invention effectively solves the technical problems of traditional high-carbon martensitic stainless steel, such as high resistance to high-temperature deformation, easy formation of brittle phases, and strong tendency to crack during air quenching, by optimizing the chemical composition, reducing the Mo content, increasing the Ni content, and adding trace amounts of Nb and V. It significantly improves the high-temperature plasticity and crack resistance of the material. The hot piercing heating temperature range of 1140~1180℃ set by this invention, and the quantitative matching relationship between the reheating temperature T before final rolling and the processing degree R of the final rolling section, provide a precise quantitative control basis for the piercing process and avoid forming defects caused by the mismatch between temperature and deformation. This invention achieves precise control of process parameters throughout the entire process, from smelting and casting billets to final heat treatment, establishing a complete quality control chain. This effectively ensures the surface quality, internal quality, and performance stability of seamless steel pipes for bearings, increasing the pipe-making yield to over 95%, and ensuring that the final product meets the hardness requirements of bearing rings at HRC55~60.

[0027] The above examples are merely one preferred embodiment of the present invention. Ordinary variations and substitutions made by those skilled in the art within the scope of the technical solution of the present invention are all included within the protection scope of the present invention.

Claims

1. A stainless steel bar for bearing tube insertion, characterized in that: The chemical composition of the rod, by mass percentage, is: C: 0.55%~0.65%; Cr: 12.5%~13.5%; Mo: 0.10%~0.20%; Ni: 0.30%~0.50%; Mn: 0.30%~0.60%; Si: 0.20%~0.40%; Nb: 0.02%~0.08%; V: 0.05%~0.15%; P：≤0.020%； S: ≤0.005%; N: ≤0.030%; balance is Fe and unavoidable impurities; the diameter of the rod is 40~110mm.

2. The stainless steel bar for bearing tube insertion according to claim 1, characterized in that: The preferred range of the chemical composition by mass percentage is: C: 0.58%~0.62%; Cr: 12.8%~13.2%; Ni: 0.35%~0.45%.

3. A process for preparing stainless steel rods for bearing tube insertion according to claim 1, characterized in that, Includes the following steps: Step 1, Smelting and casting: The continuous casting billet is prepared by electric arc furnace + AOD / VOD refining + continuous casting process. The superheat of molten steel is controlled to be ≤30℃. After the continuous casting billet is taken off the line, it is slowly cooled or annealed at 650~720℃ in time. The rolling ratio of the continuous casting billet is greater than 10. Step 2, Billet Grinding and Inspection: The surface of the continuously cast billet is ground to a depth of ≥1.5mm to remove surface defects, and ultrasonic testing is used to detect the internal quality of the billet. Step 3, billet rolling: Heat the qualified continuously cast billet to 1150~1180℃, and hold it for 1.0~1.5min / mm. Then, hot roll it into round bars with a diameter of 40~110mm through multiple passes. Control the final rolling temperature to ≥950℃. After rolling, the bars are cooled by stacking or slow cooling at a rate of ≤0.5℃ / s. Step 4, Bar Annealing and Inspection: Heat the rolled bar to 800~860℃, hold for 3~5 hours, furnace cool to below 600℃ and air cool. After annealing, the hardness of the bar is ≤HB240. Surface flaw detection and ultrasonic flaw detection are performed on the annealed bar. Step 5, Bar Cutting and Length Setting: Cut the qualified bars into lengths of 800~1200mm, chamfer both ends of the bars and remove burrs; Step 6, Bar Piercing: Heat the standard-length bars to 1140~1180℃. During the heating process, heat slowly below 900℃ and rapidly above 900℃, controlling the temperature difference between the inside and outside of the bar within ±20℃. Use hot piercing technology to form the bars into seamless steel pipes. The piercing ratio is controlled at 0.3~0.7, and the reheating temperature T before final rolling and the final rolling section processing degree R are controlled to satisfy the following relationship: T>44.4×ln(R)+821; where T is in ℃, R is the final rolling section processing degree, calculated according to the formula R=(A0-A1) / A0×100%, A0 is the cross-sectional area of ​​the bar before piercing, and A1 is the cross-sectional area of ​​the steel pipe after piercing. Step 7, steel pipe cooling: Immediately cool the pierced steel pipe in a pile or slow cooling pit at a rate of ≤0.5℃ / s until the steel pipe temperature drops below 200℃; Step 8, steel pipe annealing: Heat the cooled steel pipe to 800~860℃, hold for 3~5 hours, furnace cool to below 600℃ and air cool. After annealing, the hardness of the steel pipe is ≤HB240. Step 9, final heat treatment: The annealed steel pipe is quenched and tempered. The quenching temperature is 1000~1040℃, and the cooling method is oil cooling or air cooling. The tempering temperature is 180~220℃ or 500~550℃.

4. The tube-passing preparation process of the stainless steel bar for bearing tube passing according to claim 3, characterized in that: The heating equipment for the fixed-length bars mentioned in step six is ​​an induction furnace or a ring furnace.