Low-cost steel belt for transmission shaft pipe with tensile strength of 800Mpa and production method of low-cost steel belt

Abstract

The invention discloses a low-cost steel belt for a transmission shaft pipe with the tensile strength of 800 MPa and a production method of the steel belt. The steel belt comprises the following chemical components in percentage by mass: 0.040-0.080% of C, 0.07-0.20% of Si, 1.50-2.0% of Mn, less than or equal to 0.015% of P, less than or equal to 0.0050% of S, 0.02-0.05% of Al, 0.07-0.12% of Ti, 0.045-0.065% of Nb, less than or equal to 50 ppm of N and the balance of Fe and inevitable impurities. The production method comprises the steps of molten iron desulphurization pretreatment, converter smelting, LF refining, RH refining, continuous casting, heating, rough rolling, finish rolling, ultra-fast cooling and laminar cooling and curling. The steel strip provided by the invention has excellent obdurability matching and is low in production cost.

Description

Technical Field

[0001] This invention belongs to the field of steel materials technology, specifically relating to a low-cost steel strip for transmission shaft tubes with a tensile strength of 800MPa and its production method. Technical Background

[0002] To address global warming, developing safer and more fuel-efficient vehicles has become a common challenge for the global engineering machinery and automotive industries. To achieve the goals of low fuel consumption, low emissions, and high safety, lightweight and high-strength design have become major development directions. Driveshafts are crucial components of automotive transmission systems. Due to their complex and diverse working environment and high impact loads, high requirements are placed on the fatigue performance, weldability, and cold forming performance of the steel used for driveshaft tubes. Therefore, hot-rolled strip steel for driveshaft tubes, in addition to meeting high strength requirements, also needs low levels of non-metallic inclusions and banded microstructure, as well as good cold forming performance.

[0003] Patent applications CN115679200A, CN112708829A, and CN109763063A all relate to steel for drive shafts. They employ a medium-to-high carbon content design and rely on high-valence alloying elements such as Cr, Mo, and Ni to control microstructure and properties. Furthermore, the product form is limited to bars, fundamentally different from hot-rolled steel strips. Patent application CN106244926A discloses "A Vanadium-Containing Steel for Automotive Drive Shafts and Its Production Method," with a chemical composition of C≤0.14%, Si≤0.40%, Mn: 0.80~1.40%, P≤0.035%, S≤0.025%, V: 0.010~0.080%, Ti≤0.06%, and Als: 0.015~0.060%. This invention is characterized by the presence of the precious V alloy, resulting in higher costs. Its tensile strength is 420~580MPa, and its yield strength is ≥360MPa, indicating a relatively low strength level. Patent application CN109355563A discloses "A hot-rolled steel strip with a tensile strength of 750MPa and a thickness of 3-8mm for automotive drive shaft tubes and its production method". Its chemical composition is C: 0.06-0.08%, Si: 0.10-0.20%, Mn: 1.60-1.75%, P: ≤0.015%, S: ≤0.010%, Alt: 0.020-0.060%, Nb: 0.035-0.045%, Ti: 0.070-0.090%, Ca: 0.0010-0.0030%, Ce: 0.0005-0.0020%. The patent application added a trace amount of Ce to the composition design to modify inclusions. However, the Ce yield fluctuated greatly, and the casting ability of rare earth steel was poor, often resulting in nozzle blockage. This led to a smaller number of tundish castings and thus higher production costs.

[0004] Patent application CN116904880A discloses "A steel for automotive driveshafts with a tensile strength of 800MPa and its manufacturing method." It uses a composition of 0.04–0.07%C, 1.80–1.90%Mn, ≤0.2%Si, ≤0.003%S, ≤0.015%P, 0.30–0.50%Cr, 0.16–0.20%Mo, 0.09–0.11%V, 0.09–0.11%Ti, 0.02–0.06%Al, and ≤0.0060%N to produce steel with a thickness of 2.5–6.0 mm, a tensile strength ≥800MPa, a lower yield strength ≥750MPa, and an elongation of A. 50 The steel used in automotive drive shafts contains ≥17% Cr and Mo, which increase hardenability. Mo also hinders the precipitation and growth of proeutectoid ferrite, improves the thermal stability of TiC precipitates, and stabilizes precipitate size. Vanadium carbonitride particles precipitate at nanometer levels during the heat treatment process after coiling, increasing precipitation strengthening. Through alloying with Cr, Mo, V, and in coordination with Ti and Mn, the microstructure type, microstructure refinement, and precipitate size are controlled, resulting in good control over the steel's strength and toughness. However, the large addition of expensive Cr, Mo, and V alloys increases alloy costs, adding nearly 800 yuan / ton to the steel cost. Furthermore, the coiled steel requires slow cooling in a cold box for 48 hours, extending production time and reducing production efficiency. Summary of the Invention

[0005] The purpose of this invention is to provide a low-cost steel strip for drive shaft tubes with a tensile strength of 800MPa and its production method. Through collaborative innovation of composition design and process, based on low-carbon manganese-containing + Nb-Ti composite microalloying, it fully utilizes the microstructure refinement effect of Mn+Nb, combined with controlled rolling, enhanced laminar cooling and nano-precipitation strengthening process regulation; at the same time, it adopts a clean steel production route with low sulfur, low phosphorus and low oxygen, eliminates the means of inclusion modification treatment, and ultimately completely abandons the technical route of Cr, Mo, V and Ce precious alloy composition, achieving a triple breakthrough of high strength, high toughness and low cost.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: A low-cost 800MPa grade steel strip for drive shaft tubes has the following chemical composition by mass percentage: C: 0.040–0.080%, Si: 0.07–0.20%, Mn: 1.50–2.0%, P≤0.015%, S≤0.0050%, Al: 0.02–0.05%, Ti: 0.07–0.12%, Nb: 0.045–0.065%, N≤50ppm, with the balance being Fe and unavoidable impurities.

[0007] The composition design basis of the low-cost 800MPa grade transmission shaft steel belt described in this invention is as follows: C: Carbon is the most effective and inexpensive strengthening element. In steel, it plays a role in solid solution strengthening, increases pearlite content, and combines with Nb and Ti to form carbide or carbonitride particles, which pin dislocations and perform precipitation strengthening. Excessive carbon content will worsen the cold working and weldability of steel and increase elemental segregation during solidification. Considering all factors, the C content range is set at 0.04% to 0.08%.

[0008] Si: Si plays a role in substitutional solid solution strengthening in steel, which can improve the strength and fatigue limit of steel, and also promote ferrite precipitation. However, high silicon content can easily lead to the formation of spinel olivine phase at the interface between the steel matrix and the iron oxide scale, increasing the difficulty of dephosphorization and resulting in large-area red iron oxide scale defects on the product surface. Taking all factors into consideration, the Si content range is set at 0.07% to 0.20%.

[0009] Mn: Mn mainly plays a role in solid solution strengthening in steel. It can also lower the starting temperature of the high-temperature austenite to ferrite transformation, increase hardenability, refine the microstructure, and improve strength and toughness. However, if the manganese content is too high, the center segregation of the billet will be severe, and the banded structure of the steel strip will be more difficult to control, thus deteriorating the weldability. Considering all factors, the Mn content is set at 1.50-2.0%.

[0010] P: P is a ferrite-strengthening element, but too high a P content will increase cold brittleness and segregation in the center of the billet; too low a P content will increase steelmaking and alloying costs. Based on the current steelmaking level, the P content is set to ≤0.015%.

[0011] Sulfur (S): Sulfur is a harmful element in steel. Excessive levels will increase the number and size of Class A inclusions in the steel. To control sulfide inclusions, additional inclusion modification treatment is required. Based on existing clean steel production technology, the sulfur content is set at S≤0.0050%.

[0012] Al: Al is a commonly used deep deoxidizer. Al content above 0.02% can reduce the free oxygen content in steel to below 5 ppm. Furthermore, dissolved aluminum in the steel can combine with nitrogen to form AlN particles, which refine the grain structure. However, excessively high aluminum content does not further enhance its deoxidizing effect and instead increases production costs. Considering all factors, the Al content is set at 0.02~0.05%.

[0013] Niobium (Nb): Niobium is a commonly used microalloying element. It exists in steel in two forms: dissolved and niobium carbonitride particles. Dissolved Nb can hinder austenite recrystallization and growth through solute atom dragging, while niobium carbonitride particles can restrict austenite recrystallization and growth by pinning dislocations and hindering dislocation movement. 0.05% Nb can raise the complete austenite recrystallization temperature to over 1000℃. Excessive Nb content increases cost and waste. Considering all factors, the Nb content is set at 0.045–0.065%.

[0014] Ti: Ti is a commonly used microalloying element with low cost. Ti forms TiN, which hinders the coarsening of the original austenite structure in the furnace, while TiC precipitates after ferrite formation, exhibiting fine grain size and uniform distribution, playing a crucial role in precipitation strengthening. If the Ti content is too low, the effective titanium content for TiC formation is insufficient; if the Ti content is too high, large TiN inclusions exceeding 10 μm in size will form, reducing impact toughness and fatigue performance. Considering the existing control levels of S and N content in clean steel and the requirement for sufficient effective Ti content, the Ti content is set at 0.07–0.12%.

[0015] Furthermore, the thickness of the steel strip in this invention is 3.0~8.0 mm.

[0016] Furthermore, the steel strip of the present invention has a yield strength ≥720MPa, tensile strength ≥800MPa, impact energy ≥100J at -20℃, and no cracks when the bending mandrel diameter D=1.5a is tested in a 180° cold bending test.

[0017] Furthermore, the steel strip of the present invention has a banded structure level ≤ 2.0, and the non-metallic inclusions A, B, C, D, and Ds are all ≤ 1.5.

[0018] Furthermore, the steel strip production method of the present invention includes hot metal desulfurization pretreatment, converter smelting, LF refining, RH refining, continuous casting, heating, rough rolling, finish rolling, ultra-fast cooling + laminar flow cooling, and coiling.

[0019] Furthermore, in the hot metal desulfurization pretreatment of the present invention, the S content after treatment is ≤0.0050%; in the converter smelting, the S content of the tapped steel is ≤0.01% and the P content is ≤0.010%; in the LF refining, deep desulfurization using white slag is adopted, and the S content at the outlet is ≤0.0020%; in the RH refining, the duration of vacuum degree ≤150Pa is ≥15 minutes, the pure circulation time is ≥6 minutes, the N content at the outlet is ≤40ppm, and the TO content at the outlet is ≤15ppm; in the continuous casting, the casting speed is 1.0~1.5m / min, the superheat is 15~30℃, and dynamic light reduction with a total reduction of 4.0~7.5mm is adopted to obtain a continuous casting slab with a thickness of 200~230mm and a center segregation grade not exceeding C2.0.

[0020] Furthermore, the heating described in this invention involves a heating temperature of 1240–1270°C and a furnace dwell time of 180–300 min, which ensures uniform billet temperature without overheating or burning. This allows for the full solid solution of niobium carbonitride particles in the steel, leaving some TiN particles behind. This effectively suppresses austenite grain coarsening during billet heating, laying a good microstructure foundation for subsequent microstructure control and the full utilization of the physical metallurgical effects of the second-phase particles.

[0021] Furthermore, in the finishing rolling process described in this invention, the inlet temperature is ≤1040℃, the reduction rate of the last stand is ≥10%, and the final rolling temperature is 860~900℃. The finishing rolling process accumulates dislocation density through rolling in the non-recrystallization zone, refining the austenite grain size before phase transformation to ≤20μm.

[0022] Furthermore, the ultra-fast cooling + laminar flow cooling described in this invention cools the steel plate to below 670°C within 4 seconds at a cooling rate of 40-60°C / s, followed by laminar flow cooling. The initial ultra-fast cooling process promotes the formation of acicular ferrite, resulting in a ferrite grain size of grade 10 or higher in the hot-rolled strip.

[0023] Furthermore, the coiling described in this invention, with a coiling temperature of 600–640℃, synergistically promotes the dispersion and precipitation of 5–20 nm TiC nanoparticles, achieving precipitation strengthening. Through the combination of ferrite microstructure refinement and precipitation strengthening, an excellent strength-toughness balance is achieved, with a lower yield strength ≥720 MPa, tensile strength ≥800 MPa, and impact energy ≥100 J at -20℃.

[0024] The beneficial effects of the technical solution of this invention are as follows: This invention completely eliminates high-valence elements such as Cr, Mo, V, and Ni through a low-carbon design combined with Nb-Ti composite microalloying. It employs a billet heating process of 1240–1270℃ to ensure sufficient solid solution of (Nb,Ti)C while utilizing the high-temperature stability of TiN particles to suppress austenite coarsening. Combined with a finishing rolling inlet temperature ≤1040℃, a final pass reduction rate ≥10%, and a final rolling process of 860–900℃, dislocation density is accumulated. Through ultra-rapid cooling (40–60℃ / s) in the initial stage, the phase transformation temperature is reduced to below 650℃, forming acicular ferrite. Combined with coiling at 600–640℃, 5–20 nm (Ti,Nb)C nano-precipitates are triggered, ultimately achieving a strong and tough synergy with a lower yield strength ≥720MPa, tensile strength ≥800MPa, impact energy ≥100J at -20℃, and no cracking even with a 180° cold bending mandrel diameter D=1.5a.

[0025] This invention breaks through the limitations of traditional high-strength steel relying on expensive alloys and complex post-processing by innovating the entire chain of composition, process, microstructure and performance. It replaces precious metals with Nb-Ti microalloying and works in conjunction with controlled rolling and cooling processes to achieve synergistic optimization of "high strength, high toughness, low cost and high efficiency", providing a raw material solution for lightweight drive shafts. Attached Figure Description

[0026] Figure 1 Metallographic structure of steel strip in Example 1 (×100).

[0027] Figure 2 Rating of non-metallic inclusions in steel strip for Example 1.

[0028] Figure 3 Metallographic structure of steel strip in Example 2 (×100).

[0029] Figure 4 Rating of non-metallic inclusions in steel strip for Example 2.

[0030] Figure 5 Metallographic structure of steel strip in Example 3 (×100).

[0031] Figure 6 Rating of non-metallic inclusions in steel strip for Example 3. Detailed Implementation

[0032] To enable those skilled in the art to better understand the technical solutions in this invention, the technical solutions in the embodiments of this invention will be clearly and completely described. Example

[0033] The chemical composition of the steel strip used for the drive shaft tube in this embodiment is shown in Table 1. Its production method includes hot metal desulfurization pretreatment, converter smelting, LF refining, RH refining, continuous casting, heating, rough rolling, finish rolling, ultra-rapid cooling + laminar flow cooling, and coiling; specifically as follows: After pretreatment, the sulfur content of the molten iron is 0.0032%. The desulfurized molten iron is then smelted in a 210t converter with a scrap steel ratio of 20%. The smelting process utilizes bottom-blown argon gas throughout. At the converter's final stage, the carbon content is 0.038%, the phosphorus content is 0.006%, and the sulfur content is 0.0062%, at a temperature of 1658℃. Slag is blocked by a sliding plate during tapping. During the converter tapping process, metallic aluminum is added for deoxidation, and medium-carbon ferromanganese, metallic manganese, ferrosilicon, and ferroniobium are added for alloying. Simultaneously, 2.5 kg / t of lime and 0.5 kg / t of fluorite are added for slag formation. The LF furnace uses aluminum granules to deoxidize the top slag and refine it into white slag, resulting in a TFe content of 0.53% in the slag. The sulfur content of the molten steel exiting the LF furnace is 0.0015%, and the Al content is 0.04%. The RH refining furnace maintained a vacuum of ≤150Pa for 20 minutes. After the last batch of alloy was added, it was circulated for 6 minutes. The hydrogen content at the outlet was 0.70ppm, the nitrogen content was 0.0032%, and the TO content was 12ppm. The continuous casting machine was a 230mm thick slab continuous casting machine with a casting speed of 1.4m / min, superheat control of 18~25℃, and liquid level fluctuation in the crystallizer within ±3mm. The reduction range under dynamic light pressure was the solidification fraction. f s =0.30~0.95, the total reduction is 6.5mm; the center segregation grade of the resulting billet is C1.0.

[0034] (2) The billet is hot-sent to the heating furnace. The surface temperature of the billet entering the furnace is 400~500℃, the furnace dwell time is 225min, and the furnace exit temperature is 1265℃.

[0035] (3) The target thickness of the finished product is 5.0 mm. The roughing rolling is in 1+5 mode, the intermediate billet is 56 mm, the steel is swayed before finishing rolling, the entry temperature of finishing rolling is controlled at 1035℃, the finishing rolling is a 7-stand continuous rolling mill, the reduction rate of the last pass of F7 is 11%, and the final rolling temperature is 880℃.

[0036] (4) The cooling section consists of a UFC ultra-fast cooling section, a normal section, a dense cooling section, and a fine-tuning section. The cooling process adopts an ultra-fast cooling + laminar flow cooling method with centralized front-end cooling. That is, 50% of the manifolds of the UFC ultra-fast cooling section are open, and 100% of the manifolds of the normal section, dense cooling section, and fine-tuning section are open. The steel plate is cooled to 650°C in 4 seconds at a cooling rate of 51°C / s, and the curling temperature is 600°C.

[0037] The metallographic structure of the steel strip in this embodiment is as follows: Figure 1 As shown, by Figure 1 It can be seen that the microstructure is ferrite + pearlite, with a grain size grade of 12.5. The non-metallic inclusion rating of the steel strip in this embodiment is as follows: Figure 2 As shown, by Figure 2 It can be seen that the inclusions are of type B fine inclusions, grade 1.0, and no other types of inclusions were found. Example

[0038] The chemical composition of the steel strip used for the drive shaft tube in this embodiment is shown in Table 1; its production method includes hot metal desulfurization pretreatment, converter smelting, LF refining, RH refining, continuous casting, heating, rough rolling, finish rolling, ultra-fast cooling + laminar flow cooling, and coiling; specifically as follows: (1) After pretreatment, the sulfur content of the molten iron is 0.0028%. The desulfurized molten iron enters a 210t converter for smelting, with a scrap steel ratio of 23%. The smelting process uses bottom-blown argon gas throughout. The final C content of the converter is 0.041%, P content is 0.008%, and S content is 0.0058%. The temperature is 1655℃, and the tapping process uses a sliding plate to block slag. During the tapping process in the converter, metallic aluminum is added for deoxidation, and medium carbon ferromanganese, metallic manganese, ferrosilicon, and ferroniobium are added for alloying. At the same time, 2.5 kg / t of lime and 0.5 kg / t of fluorite are added for slag formation. The LF furnace uses aluminum particles to deoxidize the top slag and refine it into white slag. The TFe content in the slag is 0.63%, and the sulfur content of the molten steel leaving the LF station is 0.0012%, and the Al content is 0.034%. The RH refining furnace maintained a vacuum of ≤150Pa for 23 minutes. After the last batch of alloy was added, it was circulated for 6 minutes. The hydrogen content at the outlet was 0.90ppm, the nitrogen content was 0.0035%, and the TO content was 11ppm. The continuous casting machine was a 230mm thick slab continuous casting machine with a casting speed of 1.4m / min, superheat control of 19~27℃, and liquid level fluctuation in the crystallizer within ±3mm. The reduction range under dynamic light pressure was the solidification fraction. f s =0.30~0.95, the total reduction is 6.5mm; the center segregation grade of the resulting billet is C1.0.

[0039] (2) The billet is hot-sent to the heating furnace. The surface temperature of the billet entering the furnace is 400~500℃, the furnace dwell time is 218min, and the furnace exit temperature is 1268℃.

[0040] (3) The target thickness of the finished product is 3.0 mm. The roughing rolling is in 3+3 mode, the intermediate billet is 54 mm, the steel is swayed before finishing rolling, the entry temperature of finishing rolling is controlled at 1021℃, the finishing rolling is a 7-stand continuous rolling mill, the reduction rate of the last pass of F7 is 10%, and the final rolling temperature is 890℃.

[0041] (4) The cooling section consists of a UFC ultra-fast cooling section, a normal section, a dense cooling section, and a fine-tuning section. The cooling process adopts an ultra-fast cooling + laminar flow cooling method with centralized front-end cooling. That is, 50% of the manifolds of the UFC ultra-fast cooling section are open, and 100% of the manifolds of the normal section, dense cooling section, and fine-tuning section are open. The steel plate is cooled to 645℃ in 4 seconds at a cooling rate of 55℃ / s, and the curling temperature is 620℃.

[0042] The metallographic structure of the steel strip in this embodiment is as follows: Figure 3 As shown, by Figure 3 It can be seen that the microstructure is ferrite + pearlite, with a grain size grade of 12.5. The non-metallic inclusion rating of the steel strip in this embodiment is as follows: Figure 4 As shown, by Figure 4 It can be seen that the inclusions are Class B fine inclusions, grade 0.5, and no other types of inclusions were found. Example

[0043] The chemical composition of the steel strip used for the drive shaft tube in this embodiment is shown in Table 1; its production method includes hot metal desulfurization pretreatment, converter smelting, LF refining, RH refining, continuous casting, heating, rough rolling, finish rolling, ultra-fast cooling + laminar flow cooling, and coiling; specifically as follows: (1) After pretreatment, the sulfur content of the molten iron is 0.0035%. The desulfurized molten iron enters a 210t converter for smelting with a scrap steel ratio of 18%. The smelting process uses bottom-blown argon gas throughout. The final C content of the converter is 0.037%, P content is 0.009%, and S content is 0.0075%. The temperature is 1650℃, and the tapping process uses a sliding plate to block slag. During the tapping process in the converter, metallic aluminum is added for deoxidation, and medium carbon ferromanganese, metallic manganese, ferrosilicon, and ferroniobium are added for alloying. At the same time, 2.5 kg / t of lime and 0.5 kg / t of fluorite are added for slag formation. The LF furnace uses aluminum particles to deoxidize the top slag and refine it into white slag. The TFe content in the slag is 0.58%, and the sulfur content of the molten steel leaving the LF station is 0.0014%, and the Al content is 0.038%. The RH refining furnace maintained a vacuum of ≤150Pa for 22 minutes. After the last batch of alloy was added, it was circulated for 6 minutes. The hydrogen content at the outlet was 1.1ppm, the nitrogen content was 0.0031%, and the TO content was 13ppm. The continuous casting machine was a 230mm thick slab continuous casting machine with a casting speed of 1.4m / min, superheat control of 22~28℃, and liquid level fluctuation in the crystallizer within ±3mm. The reduction range under dynamic light pressure was the solidification fraction. f s =0.30~0.95, the total reduction is 6.5mm; the center segregation grade of the resulting billet is C2.0.

[0044] (2) The billet is hot-sent to the heating furnace. The surface temperature of the billet entering the furnace is 400~500℃, the furnace dwell time is 298min, and the furnace exit temperature is 1240℃.

[0045] (3) The target thickness of the finished product is 8.0 mm. The roughing rolling is in 3+3 mode, the intermediate billet is 58 mm, the steel is swayed before finishing rolling, the entry temperature of finishing rolling is controlled at 1036℃, the finishing rolling is a 7-stand continuous rolling mill, the reduction rate of the last pass of F7 is 11%, and the final rolling temperature is 860℃.

[0046] (4) The cooling section consists of a UFC ultra-fast cooling section, a normal section, a dense cooling section, and a fine-tuning section. The cooling process adopts an ultra-fast cooling + laminar flow cooling method with centralized front-end cooling. That is, 50% of the manifolds of the UFC ultra-fast cooling section are open, and 100% of the manifolds of the normal section, dense cooling section, and fine-tuning section are open. The steel plate is cooled to 652°C in 4 seconds at a cooling rate of 42°C / s, and the curling temperature is 640°C.

[0047] The metallographic structure of the steel strip in this embodiment is as follows: Figure 5 As shown, by Figure 5 It can be seen that the microstructure is ferrite + pearlite, with a grain size grade of 12.5. The non-metallic inclusion rating of the steel strip in this embodiment is as follows: Figure 6 As shown, the inclusions are classified as Class B fine inclusions, grade 1.0, with no other types of inclusions observed. Example

[0048] The chemical composition of the steel strip used for the drive shaft tube in this embodiment is shown in Table 1; its production method includes hot metal desulfurization pretreatment, converter smelting, LF refining, RH refining, continuous casting, heating, rough rolling, finish rolling, ultra-fast cooling + laminar flow cooling, and coiling; specifically as follows: (1) After pretreatment, the sulfur content of the molten iron is 0.0032%. The desulfurized molten iron enters a 210t converter for smelting, with a scrap steel ratio of 21%. The smelting process uses bottom-blown argon gas throughout. The final C content of the converter is 0.035%, P content is 0.008%, S content is 0.0063%, and the temperature is 1643℃. Slag is blocked by a sliding plate when tapping. During the tapping process in the converter, metallic aluminum is added for deoxidation, and medium carbon ferromanganese, metallic manganese, ferrosilicon, and ferroniobium are added for alloying. At the same time, 2.2 kg / t lime and 0.5 kg / t fluorite are added for slag formation. The LF furnace uses aluminum particles to deoxidize the top slag and refine it into white slag. The TFe content in the slag is 0.67%, and the sulfur content of the molten steel leaving the LF station is 0.0022% and the Al content is 0.036%. The RH refining furnace maintained a vacuum of ≤150Pa for 23 minutes. After the last batch of alloy was added, it was circulated for 8 minutes. The hydrogen content at the outlet was 1.2ppm, the nitrogen content was 32ppm, and the TO content was 12ppm. The continuous casting machine was a 230mm thick slab continuous casting machine with a casting speed of 1.5m / min, superheat control of 20~35℃, and liquid level fluctuation in the crystallizer within ±3mm. The reduction range under dynamic light pressure was the solidification fraction. f s =0.30~0.95, the total reduction is 6.5mm; the center segregation grade of the resulting billet is C1.5.

[0049] (2) The billet is hot-sent to the heating furnace. The surface temperature of the billet entering the furnace is 400~500℃, the furnace dwell time is 210min, and the furnace exit temperature is 1263℃.

[0050] (3) The target thickness of the finished product is 4.0 mm. The roughing is in 3+3 mode, the intermediate billet is 55 mm, the steel is slab before finishing, the entry temperature of finishing is controlled at 1003℃, the finishing is a 7-stand continuous rolling mill, the reduction rate of the last pass of F7 is 10.5%, and the final rolling temperature is 885℃.

[0051] (4) The cooling section consists of a UFC ultra-fast cooling section, a normal section, a dense cooling section, and a fine-tuning section. The cooling process adopts an ultra-fast cooling + laminar flow cooling method with centralized front-end cooling. That is, 50% of the manifolds of the UFC ultra-fast cooling section are open, and 100% of the manifolds of the normal section, dense cooling section, and fine-tuning section are open. The steel plate is cooled to 632°C in 4 seconds at a cooling rate of 47°C / s, and the curling temperature is 623°C.

[0052] The metallographic structure of the steel strip in this embodiment is ferrite + pearlite, with a grain size level of 12.5. The non-metallic inclusions of the steel strip in this embodiment are rated as Class D, 0.5, and no other types of inclusions were observed. Example

[0053] The chemical composition of the steel strip used for the drive shaft tube in this embodiment is shown in Table 1; its production method includes hot metal desulfurization pretreatment, converter smelting, LF refining, RH refining, continuous casting, heating, rough rolling, finish rolling, ultra-fast cooling + laminar flow cooling, and coiling; specifically as follows: (1) After pretreatment, the sulfur content of the molten iron is 0.0030%. The desulfurized molten iron enters a 210t converter for smelting, with a scrap steel ratio of 22%. The smelting process uses bottom-blown argon gas throughout. The final C content of the converter is 0.041%, P content is 0.0084%, S content is 0.009%, and the temperature is 1653℃. Slag is blocked by a sliding plate when tapping. During the tapping process in the converter, metallic aluminum is added for deoxidation, and medium carbon ferromanganese, metallic manganese, ferrosilicon, and ferroniobium are added for alloying. At the same time, 2.2 kg / t lime and 0.52 kg / t fluorite are added for slag formation. The LF furnace uses aluminum particles to deoxidize the top slag and refine it into white slag. The TFe content in the slag is 0.59%, and the sulfur content of the molten steel leaving the LF station is 0.0020%, and the Al content is 0.035%. The RH refining furnace maintained a vacuum of ≤150Pa for 22 minutes. After the last batch of alloy was added, it was circulated for 8 minutes. The hydrogen content at the outlet was 0.9ppm, the nitrogen content was 35ppm, and the TO content was 13ppm. The continuous casting machine was a 230mm thick slab continuous casting machine with a casting speed of 1.5m / min, superheat control of 20~33℃, and liquid level fluctuation in the crystallizer within ±3mm. The reduction range under dynamic light pressure was the solidification fraction. f s =0.30~0.95, the total reduction is 6.5mm; the center segregation grade of the resulting billet is C1.0.

[0054] (2) The billet is hot-sent to the heating furnace. The surface temperature of the billet entering the furnace is 400~500℃, the furnace dwell time is 280min, and the furnace exit temperature is 1268℃.

[0055] (3) The target thickness of the finished product is 6.0 mm. The roughing is in 3+3 mode, the intermediate billet is 55 mm, the steel is swayed before finishing, the entry temperature of finishing is controlled at 996℃, the finishing is a 7-stand continuous rolling mill, the reduction rate of the last pass of F7 is 12%, and the final rolling temperature is 890℃.

[0056] (4) The cooling section consists of a UFC ultra-fast cooling section, a normal section, a dense cooling section, and a fine-tuning section. The cooling process adopts an ultra-fast cooling + laminar flow cooling method with centralized front-end cooling. That is, 50% of the manifolds of the UFC ultra-fast cooling section are open, and 100% of the manifolds of the normal section, dense cooling section, and fine-tuning section are open. The steel plate is cooled to 620°C in 4 seconds at a cooling rate of 51°C / s, and the curling temperature is 605°C.

[0057] The metallographic structure of the steel strip in this embodiment is ferrite + pearlite, with a grain size level of 12.5. The non-metallic inclusions of the steel strip in this embodiment are rated as Class B fine inclusions, level 1.0, and no other types of inclusions were observed. Example

[0058] The chemical composition of the steel strip used for the drive shaft tube in this embodiment is shown in Table 1; its production method includes hot metal desulfurization pretreatment, converter smelting, LF refining, RH refining, continuous casting, heating, rough rolling, finish rolling, ultra-fast cooling + laminar flow cooling, and coiling; specifically as follows: (1) After pretreatment, the sulfur content of the molten iron is 0.0031%. The desulfurized molten iron enters a 210t converter for smelting with a scrap steel ratio of 15%. The smelting process uses bottom-blown argon gas throughout. The final C content of the converter is 0.038%, P content is 0.008%, S content is 0.0069%, and the temperature is 1649℃. Slag is blocked by a sliding plate when tapping. During the tapping process in the converter, aluminum is added for deoxidation, and medium carbon ferromanganese, metallic manganese, ferrosilicon, and ferroniobium are added for alloying. At the same time, 2.2 kg / t lime and 0.5 kg / t fluorite are added for slag formation. The LF furnace uses aluminum particles to deoxidize the top slag and refine it into white slag. The TFe content in the slag is 0.88%, and the sulfur content of the molten steel leaving the LF station is 0.0015% and the Al content is 0.035%. The RH refining furnace maintained a vacuum of ≤150Pa for 23 minutes. After the last batch of alloy was added, it was circulated for 8 minutes. The hydrogen content at the outlet was 0.8ppm, the nitrogen content was 36ppm, and the TO content was 14ppm. The continuous casting machine was a 230mm thick slab continuous casting machine with a casting speed of 1.5m / min, superheat control of 20~29℃, and liquid level fluctuation in the crystallizer within ±3mm. The reduction range under dynamic light pressure was the solidification fraction. f s=0.30~0.95, the total reduction is 6.5mm; the center segregation grade of the resulting billet is C1.5.

[0059] (2) The billet is hot-sent to the heating furnace. The surface temperature of the billet entering the furnace is 400~500℃, the furnace dwell time is 181min, and the furnace exit temperature is 1255℃.

[0060] (3) The target thickness of the finished product is 5.0 mm. The roughing is in 3+3 mode, the intermediate billet is 54 mm, the steel is swayed before finishing, the entry temperature of finishing is controlled at 992℃, the finishing is a 7-stand continuous rolling mill, the reduction rate of the last pass of F7 is 12%, and the final rolling temperature is 892℃.

[0061] (4) The cooling section consists of a UFC ultra-fast cooling section, a normal section, a dense cooling section, and a fine-tuning section. The cooling process adopts an ultra-fast cooling + laminar flow cooling method with centralized front-end cooling. That is, 50% of the manifolds of the UFC ultra-fast cooling section are open, and 100% of the manifolds of the normal section, dense cooling section, and fine-tuning section are open. The steel plate is cooled to 640℃ in 4 seconds at a cooling rate of 50℃ / s, and the curling temperature is 620℃.

[0062] The metallographic structure of the steel strip in this embodiment is ferrite + pearlite, with a grain size level of 12.5. The non-metallic inclusions of the steel strip in this embodiment are rated as Class B fine inclusions, level 1.0, and no other types of inclusions were observed.

[0063] Table 1 Chemical composition of steel strip in each embodiment

[0064] The balance in Table 1 is Fe and unavoidable impurities.

[0065] The properties of the steel strips in each embodiment are shown in Table 2.

[0066] Table 2 Performance of steel strips in various embodiments

[0067] The hot-rolled strip steel from each embodiment was used to produce 800MPa grade drive shaft tubes with an inner diameter of 120mm. The production process was as follows: slitting → small coil packaging → uncoiling → butt welding of two coils → five-roll leveling → underground looper → seven-stand forming → high-frequency welding → deburring (internal and external) → weld flaw detection → sizing → straightening → length setting → end flattening and chamfering → marking and warehousing. The slitting results in good sheet shape. The welded pipe was flattened at 0° and 90° with a reduction of 2 / 3D, and no cracks were found in the weld or the base material. No cracks were observed when the welded pipe underwent 10% hole enlargement.

[0068] In summary, the steel strip provided by this invention has the characteristics of high cleanliness, high strength, and high cold forming performance, and can be used for automotive drive shafts in heavy-duty trucks.

[0069] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A low-cost steel strip for transmission shaft tubes with a tensile strength of 800 MPa, characterized in that, Its chemical composition by mass percentage is as follows: C: 0.040–0.080%, Si: 0.07–0.20%, Mn: 1.50–2.0%, P≤0.015%, S≤0.0050%, Al: 0.02–0.05%, Ti: 0.07–0.12%, Nb: 0.045–0.065%, N≤50ppm, with the balance being Fe and unavoidable impurities.

2. The low-cost steel strip for transmission shaft tubes with a tensile strength of 800MPa as described in claim 1, characterized in that, The thickness of the steel strip is 3.0 to 8.0 mm.

3. The low-cost steel strip for transmission shaft tubes with a tensile strength of 800MPa as described in claim 1, characterized in that, The steel strip has a yield strength ≥720MPa, tensile strength ≥800MPa, impact energy ≥100J at -20℃, and no cracks were observed in the 180° cold bending test mandrel diameter D=1.5a.

4. The low-cost steel strip for transmission shaft tubes with a tensile strength of 800 MPa as described in claim 1, characterized in that, The steel strip has a banded structure grade ≤ 2.0, and the non-metallic inclusions A, B, C, D, and Ds are all grade ≤ 1.

5.

5. A method for producing low-cost steel strips for transmission shaft tubes with a tensile strength of 800 MPa as described in any one of claims 1 to 4, characterized in that, This includes hot metal desulfurization pretreatment, converter smelting, LF refining, RH refining, continuous casting, heating, rough rolling, finish rolling, ultra-fast cooling + laminar flow cooling, and coiling.

6. The method for producing low-cost steel strips for transmission shaft tubes with a tensile strength of 800 MPa as described in claim 5, characterized in that, The hot metal desulfurization pretreatment results in an S content ≤0.0050% after treatment; the converter smelting results in an S content ≤0.01% and a P content ≤0.010% in the tapped steel; the LF refining uses deep desulfurization with white slag production, resulting in an S content ≤0.0020% at the outlet; the RH refining uses a vacuum degree ≤150Pa for a duration ≥15 minutes, a pure circulation time ≥6 minutes, an N content ≤40ppm at the outlet, and a TO content ≤15ppm at the outlet; the continuous casting uses a casting speed of 1.0~1.5m / min, a superheat of 15~30℃, and a dynamic light reduction of 4.0~7.5mm to obtain a continuously cast slab with a thickness of 200~230mm and a center segregation grade not exceeding C2.

0.

7. The method for producing low-cost steel strip for transmission shaft tubes with a tensile strength of 800 MPa as described in claim 5, characterized in that, The heating process involves a heating temperature of 1240–1270°C and a furnace dwell time of 180–300 minutes.

8. The method for producing low-cost steel strips for transmission shaft tubes with a tensile strength of 800 MPa as described in claim 5, characterized in that, The finishing mill has an inlet temperature ≤1040℃, a last stand reduction rate ≥10%, and a final rolling temperature of 860~900℃.

9. The method for producing low-cost steel strip for transmission shaft tubes with a tensile strength of 800 MPa as described in claim 5, characterized in that, The ultra-fast cooling + laminar flow cooling method cools the steel plate to below 650°C within 4 seconds at a cooling rate of 40-60°C / s, followed by laminar flow cooling.

10. The method for producing low-cost steel strip for transmission shaft tubes with a tensile strength of 800 MPa as described in claim 5, characterized in that, The curling is performed at a temperature of 600–640°C.

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