Ti-v-b composite microalloyed large-size low-cost bainite non-quenched and tempered steel, manufacturing process and application thereof

Abstract

The application provides a Ti-V-B composite microalloyed large-size low-cost bainite non-quenched and tempered steel and a manufacturing process and application thereof, C: 0.23-0.30%, Si: 0.50-1.00%, Mn: 1.20-1.60%, Cr: 0.20-0.40%, Al: 0.030-0.050%, P: <=0.010%, S: 0.020-0.050%, Ti: 0.030-0.070%, V: 0.10-0.20%, B: 10-35ppm, T.O: 8-12ppm, [N]: 30-60ppm, and the rest is Fe and inevitable impurity elements, the application does not need air cooling treatment in the forging process through suitable element proportioning, greatly improves the consistency of part performance and the process window, accelerates the market promotion of the non-quenched and tempered steel, and the manufactured product has excellent performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of non-quenched and tempered steel, and particularly relates to a Ti-V-B composite micro-alloyed large-size low-cost bainite non-quenched and tempered steel, a manufacturing process and application thereof, and is used for automobile manufacturing. BACKGROUND

[0002] Non-quenched and tempered steel has become the focus of green manufacturing due to its characteristics of canceling quenching and tempering treatment and thus reducing carbon emissions. According to statistics, about 0.19 KWh of electricity is consumed and about 0.1976 kg of CO2 is emitted for every kilogram of quenched and tempered steel. Taking a truck as an example, the steel consumption of each truck is about 10 tons, and the CO2 emission can be reduced by 1.98 tons for each truck manufactured, which accounts for about 60% of the carbon emissions in the vehicle manufacturing process. At the same time, the quenching and tempering process produces prominent pollution problems such as oil pollution and water pollution, and the quenching deformation and oxidation decarburization of parts during the quenching process also lead to low part yield, which results in low production cost of quenched and tempered steel parts and high environmental pressure. Therefore, using non-quenched and tempered steel to replace quenched and tempered steel to manufacture automobile parts is one of the key materials for future automobile part manufacturing, and is also conducive to improving the comprehensive competitiveness of enterprises.

[0003] Traditional non-quenched and tempered steel is ferrite-pearlite type non-quenched and tempered steel, which obtains high mechanical properties by adding a large amount of micro-alloying elements (Nb, V), but its tensile strength is about 900 MPa. The high content of alloying elements in the martensitic non-quenched and tempered steel and the narrow process window lead to poor matching in large-scale process production, making it difficult to be mass-produced and applied. Bainite type non-quenched and tempered steel has become the focus of future automobile and engineering machinery part non-quenched and tempered steel due to its good strength and toughness and low alloy cost.

[0004] Traditional bainite non-quenched and tempered steel is usually strengthened by micro-alloying elements such as Nb, Ti and V to improve its strength and toughness. On May 8, 2020, the patent with the publication number CN111118403A discloses a Ti micro-alloyed high strength and toughness bainite non-quenched and tempered steel and its controlled forging and controlled cooling process and production process, the tensile strength is ≥1000 MPa, the yield strength is ≥700 MPa, the elongation after fracture is ≥18%, and the room temperature impact energy KU2 is ≥65 J; on July 16, 2021, the patent with the publication number CN113122776A discloses a high strength and toughness medium and large size non-quenched and tempered steel for direct cutting and its production process, the tensile strength is 983-1036 MPa, the yield strength is 721-812 MPa, the elongation is 17-20%, the surface reduction is 39-53%, and the impact energy is 56-82 J. However, the tensile strength of the above-mentioned patents is low, which cannot meet the lightweight demand of parts.

[0005] Furthermore, controlled forging and controlled cooling processes are another crucial factor determining the properties of non-quenched and tempered steels. Traditional bainitic non-quenched and tempered steels employ rapid cooling after forging to control material microstructure and properties. Chinese patent CN111118403A discloses a Ti microalloyed high-strength and high-toughness bainitic non-quenched and tempered steel, which undergoes forced-air cooling treatment after forging at a cooling rate of 0.8℃ / s to 2.4℃ / s; Chinese patent CN 113122776A describes a controlled cooling process for forging. During the production of these parts, due to the complexity of the part structure and gas flow, forced-air cooling cannot guarantee the consistency of part performance and the stability of mass production.

[0006] A production method for non-quenched and tempered steel SY740, disclosed in CN 111378891A published on June 15, 2021, involves the following steps: After rolling, round bars are placed in a close-packed, insulated hood for slow cooling at a rate ≤0.13℃ / s; the hood temperature is 730~750℃, and the insulation time is ≥35min. After exiting the hood, the bars are rapidly transferred to a cooling pit for slow cooling. For diameters φ50~φ100mm, a cooling bed-to-insulated-hood process is used for slow cooling in the pit at a temperature ≥300℃, with a covering cooling time of at least 24h. However, the yield strength is only 627-718MPa, and the tensile strength is only 888-953MPa, indicating poor performance. This method cannot meet the production needs of large-diameter (≥100mm) parts.

[0007] Traditional bainitic non-quenched and tempered steel faces three main challenges: First, it is difficult to achieve a balance between alloy cost and performance, as the high amount of microalloying elements results in a cost that is not significantly different from that of quenched and tempered steel. Second, the performance consistency of parts is poor, with narrow forging process windows and harsh post-forging cooling conditions leading to large fluctuations in part performance after batch production. Third, for steel used in large-size parts, existing material compositions and manufacturing methods cannot meet the performance requirements. Summary of the Invention

[0008] The purpose of this invention is to provide a Ti-VB composite microalloyed large-size, low-cost bainitic non-quenched and tempered steel and its manufacturing process. In the composition design of this invention, based on the Si-Mn-Cr alloy system, the comprehensive use of Si, Ti, and V elements suppresses bainite formation, increases bainite content, and simultaneously refines the M / A islands, resulting in a refined material microstructure and improved toughness. Furthermore, through the combined effects of Ti, S, V, and TO in the steel, Ti₂O₃ serves as the nucleation core for the softening phase (retained austenite and ferrite), thereby enhancing the material's toughness. Additionally, through appropriate elemental ratios, this invention eliminates the need for special air cooling during forging, significantly improving the consistency of part performance and the process window, accelerating the market promotion of non-quenched and tempered steel, and producing products with excellent performance.

[0009] Another objective of this invention is to provide an application of Ti-VB composite microalloyed large-size, low-cost bainitic non-quenched and tempered steel for the manufacture of automotive parts. The Ti-VB composite microalloyed large-size, low-cost bainitic non-quenched and tempered steel has a tensile strength of 1100MPa–1200MPa, a yield strength of 850–950MPa, an elongation after fracture ≥16%, a reduction of area ≥47%, a yield strength ratio ≥0.76, a room temperature impact energy (KU2) ≥70J, and a bending fatigue strength ≥500MPa, meeting the performance requirements for automotive parts manufacturing.

[0010] The specific technical solution of this invention is as follows:

[0011] This invention provides a Ti-VB composite microalloyed large-size, low-cost bainitic non-quenched and tempered steel, comprising the following components by mass percentage:

[0012] C: 0.23-0.30%, Si: 0.50-1.00%, Mn: 1.20-1.60%, Cr: 0.20-0.40%, Al: 0.030-0.050%, P: ≤0.010%, S: 0.020-0.050%, Ti: 0.030-0.070%, V: 0.10-0.20%, B: 10-35ppm, TO: 8-12ppm, [N]: 30-60ppm, with the remainder being Fe and unavoidable impurity elements.

[0013] The composition of the Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel satisfies: X = 30×C + 10×Si + 30×Mn + 15×S + 20×Cr + 10×Al + 40×Ti + 30×V + 2×B + 35×TO, X ≤ 500. Where C, Si, Mn, S, Cr, Al, Ti, and V are in wt%, and TO and B are in ppm.

[0014] The microstructure of the Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel is bainite + M / A islands, with M / A island size ≤30μm and bainite area ratio content ≥90%.

[0015] The Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel has a tensile strength of 1100MPa~1200MPa, a yield strength of 850~950MPa, an elongation after fracture ≥16%, a reduction of area ≥47%, a yield strength ratio ≥0.76, a room temperature impact energy (KU2) ≥70J, and a bending fatigue strength ≥500MPa.

[0016] The present invention provides a manufacturing process for large-size, low-cost Ti-VB composite microalloyed bainitic non-quenched and tempered steel, comprising the following process flow: smelting - refining - RH vacuum treatment - round billet / square billet continuous casting - rolling - forging - air cooling;

[0017] The smelting process involves using an electric furnace / converter to reduce the phosphorus content in the steel.

[0018] The RH vacuum treatment utilizes precise control of the blowing volume during the RH vacuum degassing process to adjust the oxygen content to the target value. By controlling the oxygen content during vacuum degassing, the strength and toughness of the gear steel are improved.

[0019] The rolling process involves heating the round steel at a temperature ≥1230℃ for a heating time ≥300min; starting rolling at a temperature ≥1180℃; and finishing rolling at a temperature of 700~950℃. After rolling, the steel is either covered with an insulation cover or placed in a slow cooling pit for slow cooling.

[0020] The forging process is carried out at a forging heating temperature of 1150–1250°C, an initial forging temperature of 1050–1200°C, and a final forging temperature of 900–1000°C.

[0021] The air cooling process involves air cooling the forged material to 200–400°C, followed by stack cooling to room temperature.

[0022] The design concept of this invention is as follows:

[0023] Carbon (C): Carbon is the most basic element in steel, essential for achieving high strength and hardness. While a high C content is beneficial for increasing steel strength (therefore, the C content should be ≥0.23%), a high carbon content also leads to decreased toughness and increases the likelihood of a decarburized layer, resulting in a decreased strength-to-yield ratio and fatigue performance. Therefore, a C content ≤0.30% is recommended, thus the C content range is determined to be 0.23%–0.30%.

[0024] Si (Si): Si is a strong oxidizing element in steel, which can increase the activity of carbon (C) and effectively inhibit the coarsening of carbides in steel, thus improving the yield strength and yield ratio. Furthermore, Si can improve the hardenability of steel and lower the bainitic transformation initiation temperature, thereby increasing the forging process window of parts and improving the consistency of part performance. Therefore, the Si content should be ≥0.50%; however, a higher Si content can easily lower the bainitic transformation termination temperature of steel, deteriorating the impact energy and toughness. Therefore, the Si content should be ≤1.00%. Thus, the Si content range is determined to be 0.50–1.00%.

[0025] Mn: Mn is a bainite-forming element, which can expand the bainite transformation region. Mn is also an austenite-forming element, which is beneficial for improving the strength and toughness of steel. Therefore, the Mn content should be ≥1.20%. However, Mn is a segregating element, and higher Mn levels can easily lead to the formation of coarse, blocky austenite in the steel. This not only hinders the consistency of part performance but also reduces the yield strength ratio and worsens the fatigue performance of the steel. Therefore, the Mn content should be ≤1.60%. Thus, the Mn content range is determined to be 1.20–1.60%.

[0026] Cr: Cr can improve the hardenability and strength of steel and introduce an appropriate amount of ferrite, thereby improving the toughness of the material. In addition, Cr combines with carbon in steel to form fine carbides, which improves the yield strength ratio and fatigue performance of the material. Therefore, the Cr content is ≥0.20%. However, higher Cr content tends to increase the decarburization tendency of the steel surface, which is not conducive to obtaining high fatigue strength. Therefore, the Cr content is ≤0.40%. Thus, the Cr content range is determined to be 0.20~0.40%.

[0027] Ti readily combines with C and N in steel to form Ti(C,N) precipitates. On one hand, during forging, the residual heat and stress conditions of forging facilitate precipitation, increasing the tensile and yield strength of the steel. On the other hand, Ti combines with C / N in the steel to form nanoscale Ti(C,N) particles. These particles cannot act as nucleation sites for ferrite, inhibiting ferrite precipitation and promoting bainite formation, thus refining the bainite and MA island microstructure. Therefore, the Ti content should be ≥0.030%. However, a higher Ti content can easily lead to the formation of harmful inclusions like TiN, reducing the fatigue strength of the steel. Therefore, the N content should be ≤0.070%. Thus, the Ti content range is determined to be 0.030–0.070%.

[0028] V (V): V combines with C and N in steel to form V(C,N) precipitates, inhibiting austenite grain growth, thus refining the austenite grains and improving the material's toughness. Furthermore, V precipitates at grain boundaries, leading to a decrease in carbon content around the grain boundaries. This improves the material's toughness and inhibits ferrite formation around the grain boundaries, further increasing the bainite content. Therefore, V precipitates at grain boundaries, resulting in a V content ≥ 0.10%. However, excessive V can lead to coarsening of the precipitates and higher alloy costs. Therefore, the V content range is determined to be 0.10–0.20%.

[0029] Botanicals (B): Botanicals (B) combine with nitrogen (N) in steel to form BN, which precipitates at the austenite grain boundaries, improving the material's toughness. In this invention, the combination of B and N releases more Ti, resulting in more Ti₂O₃, which is beneficial for improving the material's strength and toughness. Furthermore, for large-size round steel bars, the core and surface microstructures and properties differ significantly. Botanicals can increase the bainitic transformation temperature, improve the uniformity of properties in large-size round steel bars, and reduce the performance differences between the surface and core. Therefore, the B content is ≥10 ppm. However, higher B content tends to segregate, leading to poorer material uniformity. Therefore, the B content is ≤35 ppm. Thus, the B content range is determined to be 10–35 ppm.

[0030] Al: Al is an effective deoxidizer and combines with nitrogen in steel to form AlN precipitates, refining the austenite grains and simultaneously improving both strength and toughness. When the Al content is below 0.030%, its effect is not significant; above 0.050%, it easily forms coarse inclusions, deteriorating the steel's properties. Therefore, the timing of Al addition during steelmaking should be adjusted to ensure the Al content is controlled between 0.030% and 0.050%.

[0031] Sulfur (S): Sulfur readily combines with manganese in steel to form MnS inclusions, causing hot brittleness. However, adding a small amount of S, without affecting product performance, allows the appropriate amount of MnS inclusions to liquefy during cutting, resulting in fragmented cutting and preventing iron filings from scratching the tool surface, thus improving the material's machinability. Furthermore, S can combine with Mn and Ti in steel to form precipitates, further enhancing the material's strength and toughness. Therefore, the S content should be ≥0.020%. However, higher S content can easily lead to the deterioration of sulfide morphology in steel, which in turn worsens the steel's machinability and fatigue performance. Therefore, the S content should be ≤0.050%. Thus, the S content range is determined to be 0.020–0.050%.

[0032] P: P is an element with a strong tendency to segregate, increasing the cold brittleness of steel, reducing plasticity, and being detrimental to the uniformity of product microstructure and properties. P should be controlled to ≤0.010%.

[0033] [N]: [N] can form compounds with Ti, Al, etc., refining the grain size. A reasonable Al / [N] ratio has a significant effect on grain refinement, while excessive [N] can lead to defects such as bubbles and TiN inclusions. Therefore, the [N] content should be controlled between 30-60 ppm.

[0034] TO (Oxygen) is the main source of inclusions in steel; therefore, controlling oxygen levels is crucial for determining the properties of non-quenched and tempered steel. For ordinary steel grades, lower oxygen levels are preferable. However, in this invention, precise control of oxygen content during the RH smelting process maintains an appropriate oxygen content in the non-quenched and tempered steel. This promotes the combination of Ti and O in the steel to form Ti₂O₃. MnS can nucleate around Ti₂O₃, refining sulfide sizes, improving inclusion ratings and machinability; and forming Mn-depleted zones around Ti₂O₃ inclusions, which is superior to improving material toughness. Therefore, the TO content is ≥8 ppm. However, higher oxygen levels tend to increase the number of oxide inclusions in the steel and worsen Class D inclusions; therefore, TO ≤12 ppm. Thus, the TO content is controlled between 8 and 12 ppm.

[0035] This invention controls the X value to increase the bainite content and refine the M / A island size, thereby improving the material's strength and toughness and broadening its forging process window. This invention refines the bainite and M / A island size through the inhibitory effect of Si on carbon diffusion. Simultaneously, the combined effects of elements such as C, Si, Mn, S, Cr, Al, Ti, and TO in the steel, along with the precipitation of second phases such as Ti(C, N) and Ti2O3, enhance the steel's strength and toughness. Furthermore, controlling the forging process window is also a key factor limiting the application of non-quenched and tempered steel. Conventional non-quenched and tempered steel uses air cooling for microstructure refinement and strength improvement, but for complex parts, uneven cooling on the outer surface and insufficient cooling conditions on the back side often result in "uneven cooling on one side and one side on the other," hindering material application. Therefore, to achieve the performance described in this patent, alloy composition matching is necessary, and the elements should satisfy the following formula:

[0036] X = 30×C + 10×Si + 30×Mn + 15×S + 20×Cr + 10×Al + 40×Ti + 30×V + 2×B + 35×TO, X ≤ 500. Where C, Mn, Si, S, Cr, Al, Ti, and V are in wt%, and TO and B are in ppm.

[0037] Compared with existing technologies, this invention, based on the Si-Mn-Cr alloy system, utilizes the combined effects of Si, Ti, and V elements to suppress bainite formation, increase bainite content, and simultaneously refine M / A islands, thereby refining the material microstructure and improving toughness. Furthermore, through the combined effects of Ti, S, V, and TO in the steel, Ti₂O₃ serves as the nucleation core for softening phases (retained austenite and ferrite), thus enhancing the material's toughness. Additionally, this invention, with its appropriate elemental ratios, eliminates the need for special air-cooling during forging, significantly improving the consistency of part performance and the process window, accelerating the market promotion of non-quenched and tempered steel, and resulting in products with excellent performance. The products exhibit tensile strength of 1100MPa–1200MPa, yield strength of 850–950MPa, elongation after fracture ≥16%, reduction of area ≥47%, yield strength ratio ≥0.76, room temperature impact energy (KU₂) ≥70J, and bending fatigue strength ≥500MPa. Attached Figure Description

[0038] Figure 1 This is a tissue diagram of Example 1;

[0039] Figure 2 This is an organizational diagram of Example 2;

[0040] Figure 3 This is a tissue diagram of Example 3;

[0041] Figure 4 This is a tissue diagram of Comparative Example 1;

[0042] Figure 5 This is the tissue diagram for Comparative Example 2. Detailed Implementation

[0043] This invention provides a Ti-VB composite microalloyed large-size, low-cost bainitic non-quenched and tempered steel, comprising the following components by mass percentage:

[0044] C: 0.23-0.30%, Si: 0.50-1.00%, Mn: 1.20-1.60%, Cr: 0.20-0.40%, Al: 0.030-0.050%, P: ≤0.010%, S: 0.020-0.050%, Ti: 0.030-0.070%, V: 0.10-0.20%, B: 10-35ppm, TO: 8-12ppm, [N]: 30-60ppm, with the remainder being Fe and unavoidable impurity elements.

[0045] The composition of the Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel satisfies: X = 30×C + 10×Si + 30×Mn + 15×S + 20×Cr + 10×Al + 40×Ti + 30×V + 2×B + 35×TO, X ≤ 500. Where C, Si, Mn, S, Cr, Al, Ti, and V are in wt%, and TO and B are in ppm.

[0046] The manufacturing process of the above-mentioned Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel includes the following process flow: smelting - refining - RH vacuum treatment - round billet / square billet continuous casting - rolling - forging - air cooling.

[0047] Non-quenched and tempered steel production method: electric arc / converter smelting - LF refining - RH vacuum treatment - continuous casting - rolling (finishing) into finished products.

[0048] The following requirements should be met during the continuous casting billet smelting and manufacturing process:

[0049] The smelting process involves using an electric furnace / converter to reduce the phosphorus content in the steel.

[0050] The RH vacuum treatment utilizes precise control of the blowing volume during the RH vacuum degassing process to adjust the oxygen content to the target value. By controlling the oxygen content during vacuum degassing, the strength and toughness of the gear steel are improved.

[0051] The rolling process involves heating the round steel at a temperature ≥1230℃ for a heating time ≥300min; starting rolling at a temperature ≥1180℃; and finishing rolling at a temperature of 700~950℃. After rolling, the steel is either covered with an insulation cover or placed in a slow cooling pit for slow cooling.

[0052] The forging process is carried out at a forging heating temperature of 1150–1250°C, an initial forging temperature of 1050–1200°C, and a final forging temperature of 900–1000°C.

[0053] The air cooling process involves air cooling the forged material to 200–400°C, followed by stack cooling to room temperature.

[0054] The microstructure of the Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel manufactured above is bainite + M / A islands. Its tensile strength is 1100MPa~1200MPa, yield strength is 850~950MPa, elongation after fracture is ≥16%, reduction of area is ≥47%, yield ratio is ≥0.76, room temperature impact energy (KU2) is ≥70J, and bending fatigue strength is ≥500MPa.

[0055] The present invention will be further described in detail below with reference to the embodiments.

[0056] Examples 1-3

[0057] A Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel comprises the following mass percentage composition as shown in Table 1. The balance not shown in Table 1 is Fe and unavoidable impurities.

[0058] Table 1 Chemical composition of embodiments of the present invention (unit: TO, [N] and B are ppm, others are wt%)

[0059]

[0060]

[0061] The manufacturing processes of the non-quenched and tempered steels in the above embodiments and comparative examples include the following process flow: electric arc / converter smelting - LF refining - RH vacuum treatment - continuous casting - rolling (finishing) into finished products - forging - air cooling.

[0062] Specifically, the process is as follows: electric arc / converter smelting - LF refining - RH vacuum treatment - continuous casting - rolling (finishing) into finished products.

[0063] The following requirements should be met during the continuous casting billet smelting and manufacturing process:

[0064] The smelting process involves using an electric furnace / converter to reduce the phosphorus content in the steel.

[0065] The RH vacuum treatment utilizes precise control of the blowing volume during the RH vacuum degassing process to adjust the oxygen content to the target value. By controlling the oxygen content during vacuum degassing, the strength and toughness of the gear steel are improved.

[0066] The rolling process involves heating the round steel at a temperature ≥1230℃ for a heating time ≥300min; starting rolling at a temperature ≥1180℃; and finishing rolling at a temperature of 700~950℃. After rolling, the steel is either covered with an insulation cover or placed in a slow cooling pit for slow cooling.

[0067] The forging process is carried out at a forging heating temperature of 1150–1250°C, an initial forging temperature of 1050–1200°C, and a final forging temperature of 900–1000°C.

[0068] The air cooling process involves air cooling the forged material to 200–400°C, followed by stack cooling to room temperature.

[0069] in,

[0070] Example 1: Rolling, the round steel is heated to 1245℃ for 310 minutes; the initial rolling temperature is 1180℃ and the final rolling temperature is 750℃. After rolling, it is either covered with a heat preservation cover or placed in a slow cooling pit for slow cooling.

[0071] The forging process involves a forging heating temperature of 1180℃, an initial forging temperature of 1100℃, and a final forging temperature of 952℃. After forging, the temperature is air-cooled to 255℃ and then stacked to room temperature.

[0072] Example 2: Rolling process, the round steel is heated to 1235℃ for 300 minutes; the initial rolling temperature is 1200℃ and the final rolling temperature is 850℃. After rolling, it is cooled by a heat preservation cover or placed in a slow cooling pit.

[0073] The forging process involves a forging heating temperature of 1150°C, an initial forging temperature of 1080°C, and a final forging temperature of 909°C. After forging, the material is air-cooled to 300°C and then stacked to room temperature.

[0074] Example 3: Rolling: Round steel is heated to 1250℃ for 320 minutes; initial rolling temperature is 1185℃, final rolling temperature is 835℃; after rolling, it is cooled under an insulation cover or in a slow cooling pit. Forging: Forging heating temperature is 1200℃, initial forging temperature is 1120℃, final forging temperature is 950℃; after forging, it is air-cooled to 340℃, and then stacked to room temperature.

[0075] In Comparative Example 1, the round steel was heated to 1230℃ for 315 minutes; the initial rolling temperature was 1170℃, and the final rolling temperature was 850℃. After rolling, it was cooled under an insulation cover or in a slow cooling pit. In the forging process, the heating temperature was 1200℃, the initial forging temperature was 1110℃, and the final forging temperature was 950℃. After forging... Air cooling It is heated to 120°C and then cooled to room temperature.

[0076] Comparative Example 2: Rolling: Round steel heated to 1230℃ for 300 minutes; initial rolling temperature 1200℃, final rolling temperature 850℃; after rolling, it was cooled under an insulation cover or in a slow cooling pit. Forging: Forging heating temperature 1200℃, initial forging temperature 1150℃, final forging temperature 1000℃; after forging... Air cooling It is heated to 140°C and then cooled to room temperature.

[0077] Table 2 shows the forging process of the samples, and Table 3 shows the test results of mechanical properties and rotational bending fatigue properties of the sampled parts of the examples and comparative examples. The cross-sectional dimensions of the parts are φ≥100mm. Comparative Example 1 uses 25Mn2CrVS with conventional heating and smelting processes; Comparative Example 2 uses the same composition and smelting method as Example 1, but does not use the forging process of the present invention.

[0078] Table 2 Mechanical properties, bending fatigue strength, and yield strength ratio of the examples and comparative examples.

[0079]

[0080]

[0081] Examples 1-3 are non-quenched steels produced using the specific composition and smelting process of this invention. Comparative Example 1 is 25Mn2CrVS produced according to the requirements of GB / T 15712 standard and using conventional smelting and rolling processes. The smelting and rolling processes for the examples and comparative examples are the same.

[0082] As shown in Tables 1 to 3, this invention proposes a non-quenched and tempered steel that meets the performance requirements of large-scale automotive and engineering machinery parts through alloy design and production process control.

[0083] This invention, based on the Si-Mn-Cr alloy system, utilizes the combined effects of Si, Ti, and V elements to suppress bainite formation, increase bainite content, and simultaneously refine the M / A islands, thereby refining the material microstructure and improving toughness. Furthermore, through the combined effects of Ti, S, V, and TO in the steel, Ti₂O₃ serves as the nucleation core for the softening phases (retained austenite and ferrite), thus enhancing the material's toughness. Additionally, this invention, with its appropriate elemental ratios, eliminates the need for special air-cooling during forging, significantly improving the consistency of part performance and extending the process window, thus accelerating the market promotion of non-quenched and tempered steel.

[0084] Furthermore, the alloy design and performance control concepts involved in this invention are also applicable to other non-adjustable steels.

Claims

1. A Ti-VB composite microalloyed large-size, low-cost bainitic non-quenched and tempered steel, characterized in that, The Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel comprises the following mass percentage composition: C: 0.23-0.30%, Si: 0.50-1.00%, Mn: 1.20-1.60%, Cr: 0.20-0.40%, Al: 0.030-0.050%, P: ≤0.010%, S: 0.020-0.050%, Ti: 0.030-0.070%, V: 0.10-0.20%, B: 10-35ppm, TO: 8-12ppm, [N]: 30-60ppm, with the remainder being Fe and unavoidable impurity elements; The composition of the Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel satisfies: X=30×C+10×Si+30×Mn+15×S+20×Cr+10×Al+40×Ti+30×V+2×B+35×TO, X≤500; where C, Si, Mn, S, Cr, Al, Ti and V are in wt%, TO and B are in ppm; The microstructure of the Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel is bainite + M / A islands, with M / A island size ≤30μm and bainite area ratio content ≥90%.

2. The Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel according to claim 1, characterized in that, The Ti-VB composite microalloyed large-size low-cost bainitic non-quenched and tempered steel has a tensile strength of 1100MPa~1200MPa, a yield strength of 850~950MPa, an elongation after fracture ≥16%, a reduction of area ≥47%, a yield strength ratio ≥0.76, a room temperature impact energy (KU2) ≥70J, and a bending fatigue strength ≥500MPa.

3. A manufacturing process for the Ti-VB composite microalloyed large-size, low-cost bainitic non-quenched and tempered steel as described in claim 1 or 2, characterized in that, The manufacturing process includes the following steps: smelting, refining, RH vacuum treatment, continuous casting of round / square billets, rolling, forging, and air cooling.

4. The manufacturing process according to claim 3, characterized in that, The rolling process involves heating the round steel at a temperature ≥1230℃ for a duration ≥300 min; starting rolling at a temperature ≥1180℃; and finishing rolling at a temperature of 700~950℃. After rolling, the steel is either covered with an insulation cover or placed in a slow cooling pit for slow cooling.

5. The manufacturing process according to claim 3, characterized in that, The forging process is carried out at a forging heating temperature of 1150~1250℃, an initial forging temperature of 1050~1200℃, and a final forging temperature of 900~1000℃.

6. The manufacturing process according to claim 3, characterized in that, The air cooling process involves air cooling the forged material to 200-400°C, followed by stack cooling to room temperature.

7. An application of the Ti-VB composite microalloyed large-size, low-cost bainitic non-quenched and tempered steel as described in claim 1 or 2, characterized in that, Used in automobile manufacturing.

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