Nb-ti microalloyed low-cost bainite non-quenched and tempered steel, manufacturing process and application thereof

Abstract

This invention provides a low-cost Nb-Ti microalloyed bainitic non-quenched and tempered steel, its manufacturing process, and its applications. The composition of this invention is: C: 0.23–0.30%, Si: 0.50–1.00%, Mn: 1.60–2.20%, Cr: 0.20–0.40%, Al: 0.030–0.050%, P: ≤0.010%, S: 0.020–0.050%, Ti: 0.030–0.070%, Nb: 0.03–0.05%, TO: 8–12 ppm, [N]: 30–60 ppm, with the remainder being Fe and unavoidable impurity elements. Compared with existing technologies, this invention, through compositional control, increases the bainite content and refines the M / A island size, thereby improving the material's strength and toughness and broadening the forging process window. In production, after forging, the material is air-cooled to room temperature; the process is simple, stable, and controllable.

Description

Technical Field

[0001] This invention belongs to the field of non-quenched and tempered steel, specifically to non-quenched and tempered steel for automotive forgings. It relates to a low-cost Nb-Ti microalloyed bainitic non-quenched and tempered steel, its manufacturing process, and its application, which is suitable for manufacturing steel for large-size, 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 usually strengthened by microalloying elements such as Nb, Ti, and V to improve their strength and toughness. Chinese patent CN111118403A, published on May 8, 2020, discloses a Ti microalloyed high-strength and high-toughness bainitic non-quenched and tempered steel, its controlled forging and controlled cooling process, and its production process. The composition is: C 0.20-0.30%, Si 0.20-0.40%, Mn 1.90-2.10%, P≤0.010%, S 0.030-0.050%, Cr 0.40-0.60%, V≤0.010%, Ti 0.030-0.050%, Ni≤0.20%, Mo≤0.20%, Al 0.020-0.045%, N 40-60ppm, with the remainder being Fe and unavoidable impurity elements. Furthermore, Ti-3.43*N≥0.017% and Ti×N≤0.00016%. Its tensile strength is ≥1000MPa, and its yield strength is...

[0005] The tensile strength is ≥700MPa, elongation at break ≥18%, and room temperature impact energy KU2 ≥65J. Chinese patent CN 115747639A, published on March 7, 2023, discloses a bainitic non-quenched and tempered steel material, a steering knuckle, and a preparation method. It contains the following components by weight percentage: C: 0.33%–0.37%, Si: 0.15%–0.35%, Mn: 1.9%–2.05%, P≤0.03%, S: 0.04%–0.07%, Cr: 0.20%–0.50%, V: 0.20%–0.30%. Its tensile strength is 900MPa–1050MPa, non-proportional elongation strength is 700MPa–800MPa, elongation at break is 13%–18%, and reduction of area is 38%–50%. However, the tensile strength of the above patent is relatively low and cannot meet the requirements for lightweight parts.

[0006] 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 steel employs rapid cooling after forging to control its microstructure and properties. The aforementioned Chinese patent CN111118403A discloses a post-forging forced-air cooling treatment with a cooling rate of 0.8℃ / s to 2.4℃ / s; the technical solution disclosed in Chinese patent CN 115747639A controls the cooling rate 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.

[0007] 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.

[0008] In summary, traditional bainitic non-quenched and tempered steel has low tensile strength, and its technical characteristics make it difficult to promote and apply. Furthermore, the post-forging cooling conditions are harsh, resulting in large performance fluctuations. Summary of the Invention

[0009] The purpose of this invention is to provide a low-cost Nb-Ti microalloyed bainitic non-quenched and tempered steel and its manufacturing process. Through compositional design, the cost is low. Combined with the production method, the product exhibits a tensile strength of 1200-1300 MPa, a yield strength of 900-990 MPa, elongation after fracture ≥14%, reduction of area ≥47%, yield strength ratio ≥0.75, room temperature impact energy (KU2) ≥80 J, and bending fatigue strength ≥550 MPa. The production method involves air cooling to room temperature after forging; the process is simple, stable, and controllable, and the performance of parts remains stable after batch production.

[0010] Another objective of this invention is to provide an application of Nb-Ti microalloyed low-cost bainitic non-quenched and tempered steel for automobile manufacturing, especially for manufacturing large-size, high-quality automotive parts.

[0011] The specific technical solution of this invention is as follows:

[0012] A low-cost Nb-Ti microalloyed bainitic non-quenched and tempered steel comprises the following mass percentage composition: C: 0.23–0.30%, Si: 0.50–1.00%, Mn: 1.60–2.20%, Cr: 0.20–0.40%, Al: 0.030–0.050%, P: ≤0.010%, S: 0.020–0.050%, Ti: 0.030–0.070%, Nb: 0.03–0.05%, TO: 8–12 ppm, [N]: 30–60 ppm, with the remainder being Fe and unavoidable impurity elements.

[0013] The composition of the Nb-Ti microalloyed low-cost bainitic non-quenched and tempered steel also satisfies: X value

[0014] = 30×C+10×Si+30×Mn+15×S+20×Cr+10×Al+40×Ti+30×V+20×Nb+35×TO, X value ≤500.

[0015] When calculating the X value, the units for C, Si, Mn, S, Cr, Al, Ti, V, and Nb are wt%, and the unit for TO is ppm; please directly substitute the values ​​before the above units into the formula for calculation.

[0016] The microstructure of the Nb-Ti microalloyed low-cost bainitic non-quenched and tempered steel is lath bainite + M / A islands, with M / A island size ≤10μm and bainite area ratio content ≥90%.

[0017] The Nb-Ti microalloyed low-cost bainitic non-quenched and tempered steel has a tensile strength of 1200MPa~1300MPa, a yield strength of 900~990MPa, an elongation after fracture ≥14%, a reduction of area ≥47%, a yield strength ratio ≥0.75, a room temperature impact energy (KU2) ≥80J, and a bending fatigue strength ≥550MPa.

[0018] The present invention provides a manufacturing process for Nb-Ti microalloyed low-cost bainitic non-quenched and tempered steel, including forging and post-forging air cooling.

[0019] The forging process involves a heating temperature of 1100–1250°C, an initial forging temperature of 1000–1200°C, and a final forging temperature of 800–1000°C.

[0020] Preferably, the forging is followed by air cooling to 200–400°C, and then stack cooling to room temperature.

[0021] Furthermore, the manufacturing process also includes smelting, refining, RH vacuum treatment, continuous casting, and rolling;

[0022] The smelting process, conducted via electric furnace / converter, reduces the phosphorus content in the steel to ≤0.010%.

[0023] The RH vacuum treatment utilizes appropriate control of the blowing volume during the RH vacuum degassing process to adjust the oxygen content to a target value of 8-12 ppm. By controlling the oxygen content during the vacuum degassing process, the strength and toughness of the gear steel are improved.

[0024] The rolling process involves heating the round steel at a temperature ≥1230℃ for a heating time ≥200min; starting rolling at a temperature ≥1180℃; finishing rolling at a temperature 700~950℃; and then using an insulation cover or slow cooling pit for slow cooling after rolling.

[0025] This invention provides an application of Nb-Ti microalloyed low-cost bainitic non-quenched and tempered steel for automobile manufacturing, especially for manufacturing large-size, high-quality automotive parts.

[0026] The design concept of this invention is as follows:

[0027] 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, thus requiring a C content of ≥0.22%, a high carbon content also leads to decreased toughness and increases the likelihood of a decarburized layer, resulting in a lower strength-to-yield ratio and reduced fatigue performance. Therefore, a C content ≤0.30% is recommended, resulting in a C content range of 0.23–0.30%.

[0028] 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 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 ≥0.50%; 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 0.50–1.00%.

[0029] 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 ≥1.60%. 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 1.60–2.20%.

[0030] 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%.

[0031] 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%.

[0032] Nb combines with C and N in steel to form Nb(C,N) precipitates, inhibiting austenite grain growth, thus refining the austenite grains and improving the material's toughness. Furthermore, Nb precipitation at grain boundaries leads to a decrease in carbon content around the grain boundaries, which improves toughness and inhibits ferrite formation, further increasing bainite content. Therefore, Nb content ≥ 0.03% is preferred for grain boundary precipitation. However, excessive Nb can lead to coarsening of the precipitates and higher alloy costs. Unmelted Nb can also become a fatigue source, reducing the material's fatigue strength. Therefore, Nb content ≤ 0.05% is preferred. Thus, the Nb content range is determined to be 0.03%–0.05%.

[0033] 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%.

[0034] 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%.

[0035] 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%.

[0036] [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.

[0037] 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 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 enhances the material'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.

[0038] This invention controls the X value to increase the bainite content and refine the M / A island size, thereby improving the strength and toughness of the material and broadening the forging process window. This invention refines the bainite and M / A island size by inhibiting carbon diffusion through Si. Simultaneously, the combined effects of elements such as C, Si, Mn, S, Cr, Al, Ti, V, Nb, and TO in the steel, along with the precipitation of second phases such as Ti(C,N), Nb(C,N), and Ti₂O₃, enhance the strength and toughness of the steel. Furthermore, the addition of a small amount of Nb to the steel, through the pinning effect of Nb precipitates on grain boundaries, refines the austenite grains, providing favorable sites for bainite nucleation, thus refining the bainite microstructure of the material. Furthermore, the control of 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 to refine the microstructure and improve strength. However, for complex parts, uneven cooling on the outer surface of the forging and insufficient cooling conditions on the back side often result in "yin-yang" issues, which are detrimental to the application of the material. Therefore, to achieve the performance described in this patent, the alloy composition needs to be matched, and the elements should satisfy the following formula:

[0039] X value

[0040] = 30×C+10×Si+30×Mn+15×S+20×Cr+10×Al+40×Ti+30×V+20×Nb+35×TO, X value ≤500.

[0041] Compared with existing technologies, this invention controls the matching of multiple elements through component design to achieve the desired X value.

[0042] =30×C+10×Si+30×Mn+15×S+20×Cr+10×Al+40×Ti+30×V+20×Nb+35×TO, X value ≤500, 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. After forging, the material of this invention is air-cooled to 200-400℃, and then stack-cooled to room temperature. The process is simple, stable, and controllable. After batch production, the parts exhibit stable performance, with a tensile strength of 1200MPa-1300MPa, a yield strength of 900-990MPa, an elongation after fracture ≥14%, and a reduction of area.

[0043] ≥47%, yield strength ratio ≥0.75, room temperature impact energy (KU2) ≥80J, flexural fatigue strength ≥550MPa. Attached Figure Description

[0044] Figure 1 This is a tissue diagram of Example 1;

[0045] Figure 2 This is an organizational diagram of Example 2;

[0046] Figure 3 This is a tissue diagram of Example 3;

[0047] Figure 4 This is a tissue diagram of Comparative Example 1;

[0048] Figure 5 This is the tissue diagram for Comparative Example 2. Detailed Implementation

[0049] The present invention provides a low-cost Nb-Ti microalloyed bainitic non-quenched and tempered steel, comprising the following mass percentage composition: C: 0.23-0.30%, Si: 0.50-1.00%, Mn: 1.60-2.20%, Cr: 0.20-0.40%, Al: 0.030-0.050%, P: ≤0.010%, S: 0.020-0.050%, Ti: 0.030-0.070%, Nb: 0.03-0.05%, TO: 8-12ppm, [N]: 30-60ppm, with the remainder being Fe and unavoidable impurity elements.

[0050] The composition of the Nb-Ti microalloyed low-cost bainitic non-quenched and tempered steel also satisfies: X value = 30×C + 10×Si + 30×Mn + 15×S + 20×Cr + 10×Al + 40×Ti + 30×V + 20×Nb + 35×TO, X value ≤ 500.

[0051] The manufacturing process of the above-mentioned Nb-Ti microalloyed low-cost bainitic non-quenched and tempered steel includes the following process flow: electric arc / converter smelting - LF refining - RH vacuum treatment - continuous casting - rolling (finishing) into finished products - forging - air cooling.

[0052] In the manufacturing process, the phosphorus (P) content in the steel is first reduced to ≤0.010% through electric furnace / converter smelting. Then, the oxygen content is adjusted to the target value of 8–12 ppm by appropriately controlling the amount of air blown during RH vacuum degassing. The bar rolling process should meet the following requirements: round steel heating temperature ≥1230℃, heating time ≥200 min; initial rolling temperature…

[0053] The rolling temperature is ≥1180℃, and the final rolling temperature is 700~950℃. After rolling, the material is cooled by using an insulation cover or a slow cooling pit. The forging heating temperature is controlled at 1100~1250℃, the initial forging temperature is 1000~1200℃, and the final forging temperature is 800~1000℃. After forging, the material is air-cooled to 200~400℃ and then stacked to cool to room temperature.

[0054] The present invention will be further described in detail below with reference to the embodiments.

[0055] Examples 1-3

[0056] A low-cost Nb-Ti microalloyed 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.

[0057] Comparative Example 1

[0058] Comparative Example 1 is 25Mn2CrVS non-quenched and tempered steel produced according to GB / T 8713 standard, which includes the following mass percentage composition as shown in Table 1. The balance not shown in Table 1 is Fe and unavoidable impurities.

[0059] Table 1 Chemical composition of embodiments of the present invention (unit: TO, [N] are ppm, others are wt%)

[0060] Example C Si Mn P S Cr Al Ti V Nb TO [N] X value Example 1 0.24 0.56 1.65 0.008 0.035 0.23 0.035 0.041 / 0.032 9.4 36 399.1 Example 2 0.25 0.73 1.92 0.006 0.041 0.32 0.042 0.053 / 0.043 10.3 43 443.3 Example 3 0.28 0.96 2.16 0.005 0.048 0.38 0.049 0.068 / 0.048 11.2 48 487.3 Comparative Example 1 0.24 0.30 1.80 0.008 0.032 0.53 0.032 / 0.13 / 13.5 120 552 Comparative Example 2 0.24 0.56 1.65 0.008 0.035 0.23 0.035 0.041 / 0.032 9.4 36 399.1

[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] In the manufacturing process, the phosphorus (P) content in the steel is first reduced by smelting in an electric furnace / converter. The oxygen content is then adjusted to the target value by appropriately controlling the amount of air blown during the RH vacuum degassing process. The bar rolling process should meet the following requirements: round steel heating temperature ≥1230℃, heating time ≥200min; initial rolling temperature ≥1180℃, final rolling temperature 700~950℃, followed by slow cooling under an insulation cover or in a slow cooling pit. The forging heating temperature is controlled at 1100~1250℃, initial forging temperature at 1000~1200℃, and final forging temperature at 800~1000℃; after forging, the steel is air-cooled to 200~400℃, and then pile-cooled to room temperature. Table 2 shows the forging process parameters for each embodiment and comparative sample. Table 3 shows the mechanical properties and rotational bending fatigue properties of the materials after forging treatment in the embodiments and comparative samples.

[0063] Comparative Example 1 uses 25Mn2CrVS with conventional heating and smelting processes; Comparative Example 2 uses the composition and smelting method of Example 1 of this invention, but does not use the forging process of this invention.

[0064] Table 2 Forging process parameters

[0065]

[0066] Mechanical property test specimens were sampled according to GB / T 2975 standard, and tensile (room temperature) and impact (room temperature) properties were tested according to GB / T 228.1 and GB / T 229 standards. Rotational bending fatigue properties were tested according to GB / T 4337 standard. The size of the M / A islands was measured and statistically analyzed by scanning electron microscopy, and the bainite content was measured by XRD and metallography.

[0067] Table 3 Mechanical properties, bending fatigue strength, and yield strength ratio of the examples and comparative examples.

[0068]

[0069] 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.

[0070] The data underlined above do not meet the requirements of this invention.

[0071] As shown in Tables 1-3, this invention proposes a non-quenched and tempered steel suitable for automotive and engineering machinery applications through alloy design and production process control. Based on the Si-Mn-Cr alloy system, this invention utilizes the combined effects of Si, Ti, and Nb elements to suppress bainite formation, increase bainite content, and simultaneously refine 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, 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, thus accelerating the market promotion of non-quenched and tempered steel. Moreover, the alloy design and performance control concepts involved in this invention are also applicable to other non-quenched and tempered steels. Comparative Example 1 did not meet the requirements of this invention in terms of composition and X value; air cooling after forging during production resulted in reduced product strength. Comparative Example 2, although meeting the composition requirements of this invention, also experienced reduced product strength due to air cooling after forging.

Claims

1. A Nb-Ti microalloyed low cost bainitic non-quenched and tempered steel, characterized in that, The Nb-Ti microalloyed low-cost bainite non-quenched and tempered steel comprises the following mass percentage components: C: 0.23-0.30%, Si: 0.50-1.00%, Mn: 1.60-2.20%, Cr: 0.20-0.40%, Al: 0.030-0.050%, P: ≤0.010%, S: 0.020-0.050%, Ti: 0.030-0.070%, Nb: 0.03-0.05%, T.O: 8-12 ppm, [N]: 30-60 ppm, and the rest is Fe and inevitable impurity elements; The components of the Nb-Ti microalloyed low-cost bainite non-quenched and tempered steel also satisfy: X value = 30xC+10xSi+30xMn+15xS+20xCr+10xAl+40xTi+30xV+20xNb+35xT.O, X value ≤500; wherein C, Si, Mn, S, Cr, Al, Ti, V and Nb are in wt%, and T.O is in ppm when calculating the X value; The microstructure of the Nb-Ti microalloyed low-cost bainite non-quenched and tempered steel is lath bainite+M / A island, the size of the M / A island is ≤10 μm, and the bainite area ratio content is ≥90%; The Nb-Ti microalloyed low-cost bainite non-quenched and tempered steel has a tensile strength of 1200-1300 MPa, a yield strength of 900-990 MPa, an elongation after fracture of ≥14%, a reduction of area of ≥47%, a yield strength ratio of ≥0.75, a room temperature impact energy (KU2) of ≥80 J, and a rotary bending fatigue strength of ≥550 MPa.

2. A process for the production of the Nb-Ti microalloyed low cost bainitic non- quenched and tempered steel according to claim 1, characterized in that, The manufacturing process comprises forging and post-forging air cooling.

3. The manufacturing process of claim 2, wherein, The forging has a heating temperature of 1100-1250 ℃, an initial forging temperature of 1000-1200 ℃, and a final forging temperature of 800-1000 ℃.

4. The manufacturing process according to claim 2 or 3, characterized in that, The post-forging air cooling is to 200-400 ℃, and then to room temperature by stack cooling.

5. The manufacturing process according to claim 2 or 3, characterized in that, The manufacturing process comprises rolling, and the rolling has a round steel heating temperature of ≥1230 ℃ and a heating time of ≥200 min; The open rolling temperature is ≥1180 ℃, the final rolling temperature is 700-950 ℃, and the rolling is followed by air cooling with a heat preservation cover or entering a slow cooling pit for slow cooling.

6. Use of the Nb-Ti microalloyed low cost bainitic non-quenched and tempered steel according to claim 1, characterized in that, The steel is used for automobile manufacturing.

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