Ti microalloyed low-cost air-cooled bainite non-quenched and tempered steel, manufacturing method and application thereof

Abstract

The application provides a Ti micro-alloying low-cost air-cooled bainite non-quenched and tempered steel and a manufacturing method and application thereof, and the composition is as follows: C: 0.24-0.30%, Si: 0.58-1.00%, Mn: 1.90-2.20%, Cr: 0.20-0.40%, Al: 0.030-0.050%, P: <=0.010%, S: 0.020-0.050%, Ti: 0.038-0.070%, T.O: 8-12ppm, [N]: 30-60ppm, and the rest is Fe and inevitable impurity elements. Compared with the prior art, the application greatly improves the consistency of the performance of the part and the process window by suitable element proportioning, and the material does not need to be specially air-cooled during forging, and the market popularization of the non-quenched and tempered steel is accelerated. Moreover, the product has excellent performance.

Description

Technical Field

[0001] This invention belongs to the field of non-quenched and tempered steel, specifically a low-cost air-cooled bainitic non-quenched and tempered Ti microalloyed steel, its manufacturing method and application, which is suitable for manufacturing high-quality automotive parts. Background Technology

[0002] Non-quenched and tempered steel has become a focus of green manufacturing due to its ability to reduce carbon emissions by eliminating the quenching and tempering process. Statistics show that processing one kilogram of quenched and tempered steel requires approximately 0.19 kWh of electricity and emits about 0.1976 kilograms of CO2. Taking a truck as an example, each truck uses about 10 tons of steel; manufacturing one truck can reduce CO2 emissions by 1.98 tons, accounting for about 60% of the carbon emissions from vehicle manufacturing. However, the quenching and tempering process generates significant pollution problems such as oil and water contamination. Furthermore, quenching deformation and decarburization during the quenching process can lead to low yield rates, resulting in low production costs for quenched and tempered steel parts while placing a significant environmental burden. Therefore, using non-quenched and tempered steel to replace quenched and tempered steel in the manufacture of automotive parts is one of the key materials for future automotive parts manufacturing and will also help improve the overall competitiveness of enterprises.

[0003] Traditional non-quenched and tempered steels are ferritic-pearlitic type, achieving high mechanical properties through the addition of large amounts of microalloying elements (Nb, V), but their tensile strength is approximately 900 MPa. Martensitic non-quenched and tempered steels have high alloying element content and narrow processing windows, resulting in poor compatibility with large-scale production processes and hindering their mass production applications. In contrast, bainitic non-quenched and tempered steels, with their good strength and toughness and low alloy cost, have become a key focus for future automotive and construction machinery parts.

[0004] Traditional bainitic non-quenched and tempered steels are typically strengthened with microalloying elements such as Nb, Ti, and V to improve their strength and toughness. Chinese patent CN111118403A discloses a Ti-microalloyed high-strength and high-toughness bainitic non-quenched and tempered steel with a tensile strength ≥1000MPa, yield strength ≥700MPa, elongation at break ≥18%, and room temperature impact energy KU2 ≥65J. Chinese patent CN115747639A discloses a bainitic non-quenched and tempered steel for steering knuckles and its manufacturing method, with a tensile strength of 900MPa~1050MPa, a non-proportional elongation strength of 700MPa~800MPa, an elongation at break of 13%~18%, and a reduction of area of ​​38%~50%. However, the tensile strength of these patents is relatively low and cannot meet the requirements for lightweight parts.

[0005] Furthermore, controlled forging and controlled cooling processes are another crucial factor determining the properties of non-quenched and tempered steel. 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 115747639A discloses a bainitic non-quenched and tempered steel for steering knuckles and its manufacturing method, with the cooling rate controlled at 1.0℃ / s to 1.5℃ / s. During the production of these parts, due to the complexity of the part structure and gas flow, forced-air treatment cannot guarantee the consistency of part performance and the stability of mass production.

[0006] Traditional bainitic non-quenched and tempered steel faces two 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 issues such as a narrow forging process window and harsh post-forging cooling conditions leading to large fluctuations in part performance after batch production. Summary of the Invention

[0007] The purpose of this invention is to provide a low-cost, air-cooled, non-quenched and tempered Ti microalloyed bainitic steel and its manufacturing method. Based on the Si-Mn-Cr alloy system, the combined effect of Si and Ti elements suppresses bainite formation, increases bainite content, and simultaneously refines the M / A islands, resulting in a refined material microstructure and improved toughness. Furthermore, by introducing an appropriate amount of Ti₂O₃ particles as nucleation sites for softening phases (retained austenite and ferrite), the toughness of the material is further enhanced. Additionally, this patent, through appropriate elemental proportions, eliminates the need for special air-cooling treatment during forging, significantly improving the consistency of part performance and the process window, thus accelerating the market promotion of non-quenched and tempered steel. Moreover, the non-quenched and tempered steel produced by this invention exhibits excellent performance, with a tensile strength of 1000-1100 MPa, a yield strength of 750-850 MPa, elongation after fracture ≥18%, reduction of area ≥48%, yield strength ratio ≥0.70, room temperature impact energy (KU₂) ≥70 J, and bending fatigue strength ≥500 MPa.

[0008] Another objective of this invention is to provide an application of Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel for automobile manufacturing.

[0009] The specific technical solution of this invention is as follows:

[0010] A low-cost, air-cooled, non-quenched and tempered Ti microalloyed bainitic steel comprises the following mass percentage composition: C: 0.24-0.30%, Si: 0.58-1.00%, Mn: 1.90-2.20%, Cr: 0.20-0.40%, Al: 0.030-0.050%, P: ≤0.010%, S: 0.020-0.050%, Ti: 0.038-0.070%, TO: 8-12ppm, [N]: 30-60ppm, with the remainder being Fe and unavoidable impurity elements.

[0011] The composition of the Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel meets the following requirements: X = 30×C + 10×Si + 30×Mn + 15×S + 20×Cr + 10×Al + 40×Ti + 30×V + 35×TO, X ≤ 500. Where C, Si, Mn, S, Cr, Al, Ti, and V are in wt%, and TO is in ppm.

[0012] The microstructure of the Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel is bainite + M / A islands, with M / A island size ≤ 15 μm and bainite area ratio content ≥ 90%.

[0013] The Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel has a tensile strength of 1000MPa~1100MPa, a yield strength of 750~850MPa, an elongation after fracture ≥18%, a reduction of area ≥48%, a yield strength ratio ≥0.70, a room temperature impact energy (KU2) ≥70J, and a bending fatigue strength ≥500MPa.

[0014] This invention provides a manufacturing process for low-cost, air-cooled bainitic non-quenched and tempered Ti microalloyed steel, comprising the following process flow:

[0015] Smelting - Refining - RH vacuum treatment - Continuous casting of round / square billets - Rolling - Forging - Air cooling.

[0016] The smelting process, using an electric furnace / converter, reduces the phosphorus content in the steel to ≤0.005%.

[0017] The RH vacuum degassing process utilizes appropriate control of the blowing volume during RH vacuum degassing to adjust the oxygen content to the target value; by controlling the oxygen content during vacuum degassing, the material strength and toughness of the gear steel are improved.

[0018] The rolling process involves heating the round steel at a temperature ≥1200℃ for a duration ≥200min; starting rolling at a temperature ≥1150℃; finishing rolling at a temperature 600~900℃; and then using an insulation cover or a slow cooling pit for slow cooling after rolling.

[0019] The forging heating temperature is 1180~1250℃, the initial forging temperature is 1100~1200℃, and the final forging temperature is 900~1000℃;

[0020] The air cooling process involves air cooling to 200–400°C after forging, followed by stack cooling to room temperature.

[0021] This invention provides an application of Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel for automobile manufacturing.

[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 greater than or equal to 0.24%), 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 defining the C content range as 0.24%–0.30%.

[0024] Si: Si is a strong oxidizing element in steel, which can increase the activity of carbon and effectively inhibit the coarsening of carbides in steel, thus improving the yield strength and yield strength ratio. Furthermore, on the one hand, Si increases the residual austenite content by inhibiting carbon diffusion, thereby improving the toughness of the material; on the other hand, Si can refine the bainite structure and M / A island size, resulting in a better strength-toughness balance. Therefore, the Si content should be ≥0.70%; however, a higher Si content tends to result in a lower bainitic transformation termination temperature, worsening the impact energy and toughness of the steel. Therefore, the Si content should be ≤1.00%. Thus, the Si content range is determined to be 0.70–1.00%.

[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 ≥1.90%. However, Mn is a segregating element, and higher Mn content 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 ≤2.20%. Thus, the Mn content range is determined to be 1.90–2.20%.

[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: Ti readily combines with C and N in steel to form Ti(C,N) precipitates. On one hand, this precipitation strengthens the steel, increasing its yield strength. On the other hand, the solid-solution Ti effectively increases the bainite content, refining the bainite and MA island structure. Therefore, the Ti content should be ≥0.040%. However, a high Ti content can easily lead to the formation of harmful inclusions like TiN, reducing the steel's fatigue strength. Therefore, the N content should be ≤0.070%. Thus, the Ti content range is determined to be 0.038–0.070%.

[0028] 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%.

[0029] Sulfur (S) readily combines with manganese (MnS) 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. 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. Therefore, the S content should be ≤0.050%. Thus, the S content range is determined to be 0.020–0.050%.

[0030] 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%.

[0031] [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.

[0032] TO (Oxygen) is the main source of inclusions in steel; therefore, controlling oxygen levels is crucial for the performance of non-quenched and tempered steel. For ordinary steel grades, lower oxygen levels are preferable. However, in this patent, 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. Furthermore, it can form Mn-depleted regions around Ti₂O₃ inclusions, serving as limited nucleation areas for ferrite. A small amount of ferrite in the steel can coordinate material deformation, thereby improving the steel's toughness. Therefore, the TO content is ≥8 ppm. However, higher oxygen levels can lead to an increase in the number of oxide inclusions and worsen Class D inclusions; therefore, TO ≤12 ppm. Thus, the TO content is controlled between 8 and 12 ppm.

[0033] This invention provides a low-cost, non-quenched and tempered steel with Ti microalloying, which can increase the bainite content and refine the M / A island size, thereby improving the material's strength and toughness and broadening the forging process window. This invention utilizes the inhibitory effect of Si on carbon diffusion to refine the bainite and M / A island sizes. Simultaneously, the combined effects of elements such as C, Mn, Al, Ti, S, and O in the steel, along with the precipitation of second phases such as Ti(C, N) and Ti₂O₃, enhance the steel's strength and toughness. In summary, to achieve the optimal alloying effect, the elements should satisfy the following formula:

[0034] X = 30×C + 10×Si + 30×Mn + 15×S + 20×Cr + 10×Al + 40×Ti + 30×V + 35×TO, X ≤ 500. Where C, Si, Mn, S, Cr, Al, Ti, and V are in wt%, and TO is in ppm.

[0035] Compared with existing technologies, this invention, based on the Si-Mn-Cr alloy system, utilizes the combined effects of Si and Ti elements to suppress bainite formation, increase bainite content, and simultaneously refine M / A islands, thereby refining the material microstructure and improving toughness. Furthermore, by introducing an appropriate amount of Ti2O3 particles as nucleation sites for softening phases (retained austenite and ferrite), the material's toughness is further enhanced. Additionally, this patent, through appropriate elemental ratios, eliminates the need for special air-cooling treatment during forging, significantly improving the consistency of part performance and the process window, accelerating the market promotion of non-quenched and tempered steel. Moreover, the non-quenched and tempered steel produced by this invention exhibits excellent performance, with tensile strength of 1000-1100 MPa, yield strength of 750-850 MPa, elongation after fracture ≥18%, reduction of area ≥48%, yield strength ratio ≥0.70, room temperature impact energy (KU2) ≥70 J, and bending fatigue strength ≥500 MPa. Attached Figure Description

[0036] Figure 1 This is a tissue diagram of Example 1;

[0037] Figure 2 This is an organizational diagram of Example 2;

[0038] Figure 3 This is a tissue diagram of Example 3;

[0039] Figure 4 This is a tissue diagram of Comparative Example 1;

[0040] Figure 5 This is the tissue diagram for Comparative Example 2. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to the embodiments.

[0042] Examples 1-3

[0043] A low-cost, air-cooled, non-quenched and tempered Ti microalloyed bainitic steel comprises the following mass percentage composition as shown in Table 1, where the balance not shown in Table 1 is Fe and unavoidable impurities.

[0044] Comparative Example 1 - Comparative Example 2

[0045] A bainitic non-quenched and tempered steel comprises the following mass percentage composition as shown in Table 1, where the balance not shown in Table 1 is Fe and unavoidable impurities.

[0046] Table 1 Chemical composition of embodiments of the present invention (unit: TO, [N] are ppm, others are wt%)

[0047] Example 1 0.24 0.58 1.93 0.007 0.032 0.22 0.032 0.038 / 8.2 35 364.6 Example 2 0.26 0.75 1.92 0.007 0.043 0.35 0.041 0.053 / 10.3 42 443.6 Example 3 0.29 0.93 1.98 0.006 0.048 0.37 0.047 0.068 / 11.3 53 484.2 Comparative Example 1 0.27 0.33 1.82 0.009 0.030 0.53 0.031 0 0.15 13.4 125 552.6 Comparative Example 2 0.24 0.58 1.93 0.007 0.032 0.22 0.032 0.038 / 8.2 35 364.6

[0048] The manufacturing processes for the non-quenched and tempered steels in the above embodiments and comparative examples include the following process flow:

[0049] Smelting - Refining - RH vacuum treatment - Continuous casting of round / square billets - Rolling - Forging - Air cooling.

[0050] The smelting process involves using an electric furnace / converter to reduce the phosphorus content in the steel.

[0051] The RH vacuum degassing process utilizes appropriate control of the blowing volume during RH vacuum degassing to adjust the oxygen content to the target value; by controlling the oxygen content during vacuum degassing, the material strength and toughness of the gear steel are improved.

[0052] The rolling process involves heating the round steel at a temperature ≥1200℃ for a duration ≥200min; starting rolling at a temperature ≥1150℃; finishing rolling at a temperature 600~900℃; and then using an insulation cover or a slow cooling pit for slow cooling after rolling.

[0053] The forging heating temperature is 1180~1250℃, the initial forging temperature is 1100~1200℃, and the final forging temperature is 900~1000℃;

[0054] The air cooling process involves air cooling to 200–400°C after forging, followed by stack cooling to room temperature.

[0055] 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 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. Table 2 shows the forging process parameters for the samples.

[0056] Table 2 Forging process parameters

[0057]

[0058]

[0059] Mechanical property test specimens were sampled in accordance with GB / T 2975 standard, and tensile (room temperature) and impact (room temperature) properties were tested in accordance with GB / T 228.1 and GB / T229 standards. Rotational bending fatigue properties were tested in accordance with GB / T 4337 standard.

[0060] The performance of the products produced in the above embodiments and comparative examples was tested. Table 3 shows the results of mechanical properties and rotational bending fatigue tests of the materials of the embodiments and comparative examples after forging treatment.

[0061] Table 3 Mechanical properties, bending fatigue strength, and yield strength ratio of the examples and comparative examples.

[0062]

[0063] Comparative Example 1 uses 25Mn2CrVS with conventional heating and smelting processes; Comparative Example 2 uses the components and smelting method of this patent, but not the forging process of this patent. The performance of both Comparative Examples 1 and 2 fails to meet the requirements of this invention.

Claims

1. A low-cost, air-cooled, non-quenched and tempered Ti microalloyed bainitic steel, characterized in that, The Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel comprises the following mass percentage components. C: 0.24-0.30%, Si: 0.58-1.00%, Mn: 1.90-2.20%, Cr: 0.20-0.40%, Al: 0.030-0.050%, P: ≤0.010%, S: 0.020-0.050%, Ti: 0.038-0.070%, TO: 8-12ppm, [N]: 30-60ppm, with the remainder being Fe and unavoidable impurity elements; The composition of the Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel meets the following requirements: X = 30×C + 10×Si + 30×Mn + 15×S + 20×Cr + 10×Al + 40×Ti + 30×V + 35×TO, X ≤ 500; where C, Si, Mn, S, Cr, Al, Ti and V are in wt%, and TO is in ppm; The manufacturing process of the aforementioned Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel includes the following process flow: Smelting - Refining - RH vacuum treatment - Continuous casting of round / square billets - Rolling - Forging - Air cooling; The air cooling process involves air cooling to 200-400°C after forging, followed by stack cooling to room temperature. The microstructure of the Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel is bainite + M / A islands, with M / A island size ≤ 15 μm and bainite area ratio content ≥ 90%.

2. The Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel according to claim 1, characterized in that, The Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel has a tensile strength of 1000MPa~1100MPa, a yield strength of 750~850MPa, an elongation after fracture ≥18%, a reduction of area ≥48%, a yield strength ratio ≥0.70, a room temperature impact energy (KU2) ≥70J, and a bending fatigue strength ≥500MPa.

3. A manufacturing process for the Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel according to claim 1 or 2, characterized in that, The manufacturing process includes the following process flow: Smelting - Refining - RH vacuum treatment - Continuous casting of round / square billets - Rolling - Forging - Air cooling; The air cooling process involves air cooling to 200-400°C after forging, followed by stack cooling to room temperature.

4. The manufacturing process according to claim 3, characterized in that, The rolling process involves heating the round steel at a temperature ≥1200℃ for a duration ≥200min; starting rolling at a temperature ≥1150℃; finishing rolling at a temperature 600~900℃; and then using an insulation cover or a slow cooling pit for slow cooling after rolling.

5. The manufacturing process according to claim 3, characterized in that, The forging heating temperature is 1180~1250℃, the initial forging temperature is 1100~1200℃, and the final forging temperature is 900~1000℃.

6. An application of the Ti microalloyed low-cost air-cooled bainitic non-quenched and tempered steel as described in claim 1 or 2, characterized in that, Used in automobile manufacturing.

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