High-strength non-quenched and tempered steel for automobile front axle and production method thereof

Abstract

The application belongs to the technical field of steel metallurgy, and particularly relates to a high-strength non-quenched and tempered steel for automobile front axle and a production method thereof. The high-strength non-quenched and tempered steel for automobile front axle has the following chemical components and mass percentages: C: 0.23-0.28%, Si: 0.6-0.8%, Mn: 1.8-2.1%, Cr: 0.80-1.20%, Nb: 0.02-0.03%, Ti: 0.02-0.03%, Al: 0.010-0.030%, B: 0.004-0.006%, P: <=0.025%, S: 0.045-0.065%, lanthanide: 0.0003-0.0005%, N: 0.009-0.0130%, and the rest is Fe and inevitable impurities. The high-strength non-quenched and tempered steel for automobile front axle has low alloy cost of components, has cost advantages, and is beneficial to large-scale popularization and application.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and specifically relates to a high-strength non-quenched and tempered steel for automobile front axles and its production method. Background Technology

[0002] The front axle is one of the most important load-bearing components in the front axle assembly of an automobile. Its inherent quality and material properties directly affect the safety and reliability of the vehicle's steering system and the vehicle's service life. The front axle simultaneously bears alternating stresses such as bending fatigue and impact loads within the front axle assembly, operating in an extremely harsh environment. Therefore, the materials used in the front axle must possess high resistance to bending fatigue, while also exhibiting a good balance between strength and toughness.

[0003] 25Mn2CrVS is a relatively mature non-quenched and tempered steel currently used in automotive front axles. The chemical composition of this steel is as follows (by mass percentage): C: 0.23-0.28%, Si: 0.30-0.40%, Mn: 1.8-2.0%, Cr: 0.50-0.60%, V: 0.12-0.15%, P: ≤0.025%, S: 0.045-0.065%, with the remainder being Fe and unavoidable impurities. However, this steel grade faces the following two main constraints in engineering applications: (1) The hardenability of this material is slightly insufficient, and forced cooling is required when making large cross-section workpieces, which increases the difficulty of the process; (2) This material contains 0.12 to 0.15% V, and the cost of V alloy reaches 240-300 yuan / ton of steel, which makes its cost disadvantageous. In addition, V microalloying is usually used to improve the strength of the material by precipitating vanadium carbides or carbonitrides in the temperature range of 600-800℃. However, the residence time in this temperature range during the continuous cooling process of 25Mn2CrVS workpiece after forging is short, and the effect of V microalloying precipitation strengthening is not significant, which wastes valuable vanadium resources.

[0004] Chinese Patent CN 202210801542.3 discloses a high-strength non-quenched and tempered steel front axle for heavy-duty vehicles and its manufacturing process, belonging to the field of automobile manufacturing. The chemical composition (mass percentage) is as follows: C 0.10-0.15%, Si 0.80-1.40%, Mn 2.40-3.00%, P≤0.015%, S 0.020-0.050%, Cr 0.40-0.80%, V 0.05-0.10%, Nb 0.040-0.060%, Al 0.015-0.035%, Ti 0.010-0.030%, B 0.0020-0.0040%, O≤0.0015%, N0.014-0.020%, H≤0.0002%, with the balance being Fe and unavoidable impurities. The product has a yield strength ≥900MPa, good toughness, low-temperature impact energy ≥50J, and grain size ≥9.0 grade, which can meet the performance requirements of heavy-duty vehicle front axles. However, the V and Nb content in the chemical composition of the technology described in this patent is still relatively high, with the sum of the mass percentages of V and Nb being 0.09-0.16%, resulting in a relatively high alloy cost.

[0005] In summary, the existing technology has the following three main problems: (1) In the existing technology, the sum of the mass percentages of V and Nb in the steel composition of automobile front axles is generally greater than 0.10%, but the precipitation of V and Nb carbonitrides within the bainitic cooling rate range is insufficient. A large portion of V and Nb exists in the steel in the form of solid solution, and the precipitation strengthening effect is not obvious, resulting in a waste of V and Nb alloy. However, high-strength non-quenched and tempered steel for automobile front axles needs to improve strength through bainitic phase transformation. The precipitation of V and Nb elements and bainitic phase transformation are contradictory, and the two cannot undergo sufficient precipitation or phase transformation at the same time. (2) The existing technology has unreasonable design of the hardenability of steel. There is no clear description of the appropriate range for hardenability setting. Too low or too high hardenability may cause pearlitic or martensitic structures to appear in automobile front axles, causing the mechanical properties of automobile front axles to deviate from the normal use requirements. (3) Fatigue resistance is a very important indicator of automobile front axles, but the existing technology does not address how to improve the fatigue resistance of automobile front axles. Summary of the Invention

[0006] To address the above-mentioned problems, this invention provides a high-strength non-quenched and tempered steel for automotive front axles and its production method. The chemical composition and mass percentage of the high-strength non-quenched and tempered steel for automotive front axles involved in this invention are as follows: C: 0.23-0.28%, Si: 0.6-0.8%, Mn: 1.8-2.1%, Cr: 0.80-1.20%, Nb: 0.02-0.03%, Ti: 0.02-0.03%, Al: 0.010-0.030%, B: 0.004-0.006%, P: ≤0.025%, S: 0.045-0.065%, lanthanide rare earth elements: 0.0003-0.0005%, N: 0.009-0.0130%, with the remainder being Fe and unavoidable impurities.

[0007] Furthermore, based on the effects of chemical composition on the steel in the high-strength non-quenched and tempered steel for automotive front axles described in the following section, the inventors summarized the contribution of chemical composition to hardenability in the high-strength non-quenched and tempered steel for front axles, as expressed by the following formula ①:

[0008] H 淬透性 =93*(ω C )+5*(ω Si )+8.8*(ω Mn )+13*(ω Cr )+780*(ω B )+210*(ω Nb )+93*(ω Ti )+6380*(ω 镧系稀土 )①;

[0009] Furthermore, based on long-term research and development and production practices of special steel grades, the inventors summarized that, in addition to meeting the required composition range, the contribution of the composition to hardenability of high-strength non-quenched and tempered steel for automotive front axles must also meet Equation ②, in order to achieve the goal of lower bainite as the phase transformation structure within the cooling rate range of 0.5~2.5℃ / S:

[0010] 72≤H 淬透性 ≤75②;

[0011] Since the cooling rate of the forged front axle made of high-strength non-quenched and tempered steel is generally in the range of 0.5 to 2.5℃ / s during continuous cooling, the H calculated from the steel composition... 淬透性 If equation ② is satisfied, it can be guaranteed that the microstructure of the entire cross section of the high-strength non-quenched and tempered steel for the front axle after forging is lower bainite.

[0012] Furthermore, in addition to meeting the composition requirements, the Nb, Ti, and N content in high-strength non-quenched and tempered steel for automotive front axles must also meet the following formula ③, so as to achieve the purpose of fully fixing N element with Nb and Ti elements in the steel to generate NbN and TiN.

[0013] 3.5*(ω N )≤0.516*(ω Nb )+(ω Ti )≤4.5*(ω N )③;

[0014] In equations ① to ③, ω C ω Si ω Mn ω Cr ω B ω Nb ω Ti ω 镧系稀土 ω N These represent the mass percentage (%) of C, Si, Mn, Cr, B, Nb, Ti, lanthanide rare earth elements, and N in the steel composition; H 淬透性 This represents the contribution of chemical composition to hardenability in high-strength non-quenched and tempered steel for front axles, as summarized by the inventor based on production practice.

[0015] The effects of chemical composition on high-strength non-quenched and tempered steel used in automotive front axles are as follows:

[0016] C: C can play a solid solution strengthening role in steel, increasing the yield strength of steel, but it will deteriorate the toughness and plasticity of steel. In addition, according to the Fe-C phase diagram, in hypoeutectoid steel, the temperature at which austenite undergoes ferrite transformation decreases after the carbon content increases. Therefore, C can improve the stability of austenite and improve hardenability. Taking all factors into consideration, the carbon content in the steel is set in the range of 0.23% to 0.28% in this invention.

[0017] Si: Si can strengthen steel through solid solution treatment, increasing its yield strength, but it reduces its ductility. Furthermore, Si is a non-carbide-forming element, almost insoluble in carbides. Therefore, before carbide nucleation, Si must first migrate from near the interface into austenite or ferrite. Since Si is a substitutional atom, its diffusion rate is quite slow at low temperatures, thus slowing down carbide nucleation and growth. Consequently, the formation rate of Fe3C slows down, resulting in more carbon in bainite and increased bainite formation. Si can also slow down the growth rate of bainitic ferrite lamellars, leading to a longer bainite transformation incubation period and causing the transformation to occur at lower temperatures, forming fine acicular lower bainite. This allows for the formation of lower bainite over a wider temperature range. Considering all these factors, this invention sets the silicon content in the steel to be in the range of 0.6% to 0.8%.

[0018] Mn: Mn can play a solid solution strengthening role in steel and improve the yield strength of steel. However, excessive manganese content will cause defects such as segregation, porosity and shrinkage cavities in the billet. In addition, manganese forms a substitutional solid solution with iron. Since the affinity between manganese and carbon is stronger than that between manganese and iron, Mn can prevent carbon from diffusing and precipitating from the solid solution, thus playing a role in stabilizing and expanding the austenite region and improving hardenability. Taking all factors into consideration, the present invention sets the manganese content in steel in the range of 1.8% to 2.1%.

[0019] Cr: Cr plays a similar role in steel as Mn, and can also strengthen steel through solid solution. However, while strengthening steel, Cr also reduces plasticity and increases temper brittleness. Furthermore, because the affinity between Cr and C atoms is stronger than that of Mn, Cr has a stronger ability to prevent C from diffusing from the solid solution. As a result, Cr has a greater ability to improve the hardenability of steel than Mn. Taking all factors into consideration, this invention sets the Cr content in steel in the range of 0.8% to 1.2%.

[0020] Nitrogen (Nb): As a microalloying element, Nb can play a role in precipitation strengthening and grain refinement strengthening of steel. However, under the cooling rate conditions of bainitic steel, its precipitation rate is not high, and some Nb exists in solid solution. Therefore, if the Nb content is too high, Nb resources may be wasted. Furthermore, since the atomic radius of Nb atoms is larger than that of Fe atoms, in order to reduce the distortion energy of Nb in the Fe matrix, the solid-solution Nb atoms tend to interact with crystal defects such as dislocations and grain boundaries to form grain boundary segregation layers. Nb atoms segregated on the grain boundaries have a tendency to form carbides with C atoms. Nb atoms have a strong affinity for C atoms. Therefore, Nb atoms can hinder carbon atoms from passing through grain boundaries and hinder the diffusion and transport of C atoms, thereby improving the stability of austenite and improving hardenability. Taking all factors into consideration, this invention sets the Nb content in the steel in the range of 0.02% to 0.03%.

[0021] Ti: As a microalloying element, Ti can play a role in precipitation strengthening and grain refinement strengthening of steel. However, excessive Ti content will impair the toughness of steel. Ti can improve the hardenability of steel, and its principle of improving the hardenability of steel is similar to that of Nb. However, since the radius difference between Ti atoms and iron atoms is smaller than that between Nb atoms and iron atoms, Ti's ability to improve the hardenability of steel is weaker than that of Nb. In this invention, Ti mainly pins grain boundaries in the form of TiN, hindering the growth of austenite grains during the heating process of the workpiece. Taking all factors into consideration, this invention sets the Ti content in steel in the range of 0.02% to 0.03%.

[0022] Al: Al can perform deep deoxidation on molten steel, which is beneficial to reduce oxide inclusions. However, the reducing slag produced by deep deoxidation will also remove sulfur from the molten steel. Since the sulfur content in the composition provided by this invention is in the range of 0.045% to 0.065%, it is not beneficial to control the sulfur content within a high range if too much Al is added. Taking all factors into consideration, this invention sets the Al content in the steel in the range of 0.01% to 0.03%.

[0023] B: Since the atomic radius of B atoms is much smaller than that of Fe atoms, in order to reduce the distortion energy of B in the Fe matrix, the B atoms in solid solution tend to form a grain boundary segregation layer. The B atoms segregated on the grain boundary have a strong ability to form carbides with C atoms. Therefore, B atoms can also prevent carbon atoms from passing through the grain boundary, thereby improving the stability of austenite and improving hardenability. However, if the B content is too high, it will cause grain boundary embrittlement. Taking all factors into consideration, the present invention sets the B content in the steel in the range of 0.004% to 0.006%.

[0024] P: P is harmful to the toughness of steel and will increase the cold brittleness of steel. However, if the P content is too low, the dephosphorization cost of the primary smelting furnace will increase. Taking all factors into consideration, the present invention sets the P content in steel to no more than 0.025%.

[0025] S: S increases the hot brittleness of steel and easily leads to cracks in the cast billet. However, appropriately increasing the S content can improve the machinability of steel and enhance its processing capability. Taking all factors into consideration, this invention sets the S content in steel to be in the range of 0.045% to 0.065%.

[0026] Lanthanide rare earth elements: Because the atomic radius of rare earth atoms is much larger than that of Fe atoms, in order to reduce the distortion energy of rare earths in the Fe matrix, the dissolved rare earth atoms tend to form a grain boundary segregation layer. Rare earth atoms segregated on the grain boundaries have a strong ability to form carbides with C atoms. Therefore, rare earth atoms can also hinder carbon atoms from passing through the grain boundaries, thereby improving austenite stability and hardenability. Since the radius difference between lanthanide rare earth atoms and iron atoms is greater than that between Nb atoms and iron atoms, lanthanide rare earth elements have a stronger ability to improve the hardenability of steel than Nb. Furthermore, rare earth elements can modify inclusions such as Al2O3. The composite inclusions formed after modification have a thermal expansion coefficient close to that of the iron matrix, which can reduce the gap between the iron matrix and inclusions when the workpiece undergoes temperature changes, thus improving the fatigue resistance of the workpiece. However, rare earth elements are relatively expensive. Considering all factors, this invention sets the lanthanide rare earth content in the steel to be in the range of 0.0003% to 0.0005%.

[0027] N: The interaction of N with Ti and Nb can generate TiN and NbN, which can refine the grains. However, excessive nitrogen content will damage the toughness of steel. Taking all factors into consideration, the present invention sets the N content in steel in the range of 0.009 to 0.013%.

[0028] The production method of high-strength non-quenched and tempered steel for automobile front axles is as follows:

[0029] The steelmaking process is as follows: converter → LF furnace refining + RH vacuum degassing treatment → continuous casting (electromagnetic stirring) → slow cooling / hot delivery → rolling → air cooling → finishing → warehousing.

[0030] The converter smelting process is as follows: molten iron temperature entering the converter ≥1300℃, sulfur content in the molten iron controlled below 0.040%, final slag basicity controlled between 2.8 and 3.4, final C content ≥0.10%, P ≤0.012%; tapping temperature 1600-1680℃. During ladle alloying, alloys are added according to the lower limit of the composition for adjustment, and slag-forming material is added at approximately 6-15 kg / t. The amount of slag discharged from the converter is less than 0.5 kg / ton of steel.

[0031] The LF furnace refining process involves controlling the basicity of the refining slag to be greater than 3.0 and the refining cycle to be greater than 40 minutes. Before taking a sample, aluminum wire is fed in as needed, and a sample is taken under the white slag to test the composition. Based on the analysis results of the first sample, the contents of C, Si, Mn, Cr, B, Nb, Ti, lanthanide rare earth elements, and Al are adjusted according to the target composition requirements, so that while meeting the composition range, H is also balanced. 淬透性 The requirements of formula ② must be met. The lanthanide rare earth elements are added using the following process: 1 m of rare earth cored wire is fed per ton of steel at a feeding speed of 3 m / s. The core powder weight of the rare earth cored wire is 190 g / m, and the core powder composition is: Si: 38-45%, N: 24-30%, lanthanide rare earth elements: 12-15%, S≤0.015%, P≤0.015%, with the remainder being unavoidable impurities. After feeding, the molten steel is stirred at a high flow rate through the bottom blower for 1-2 minutes to homogenize the steel composition.

[0032] RH vacuum refining process: Nitrogen blowing and enrichment are carried out throughout the vacuum treatment process. The pure vacuum treatment time is 15-40 minutes. After RH treatment, manganese nitride wire is fed in according to the nitrogen content target. Soft blowing time is ≥15 minutes. RH outlet temperature control: First continuous casting furnace: liquidus temperature increased by 80-105℃; from the second furnace onwards, liquidus temperature increased by 45-80℃.

[0033] Continuous casting process: The liquid level in the tundish should not be lower than 750mm, the liquid level fluctuation in the crystallizer should be ≤±3mm, the superheat in the tundish should be controlled at 20~30℃, the casting speed for a Φ500mm casting section should be controlled at 0.4-0.5m / min, the electric stirring parameters in the crystallizer are: 150A, 1.5Hz, and the electric stirring parameters at the end of solidification are: 200A, 7.0Hz. The temperature of the billet entering the straightening machine should be ≥900℃. Slow cooling requirements for billets: They cannot stay on the cooling bed and must be put into the cooling pit for slow cooling in a timely manner. Slow cooling regime: The temperature entering the cooling pit should be greater than 500℃, and the temperature exiting the cooling pit should be less than 300℃, of which the slow cooling time for Φ500mm continuous casting round billets should not be less than 36 hours;

[0034] The rolling process is as follows: billet → furnace loading → heating → rolling → sawing → air cooling → finishing → flaw detection → warehousing. The heating regime in the billet heating furnace is as follows: preheating section temperature 500~1000℃, heating section temperature 1050~1270℃, soaking section temperature 1170~1230℃, and the total time the billet is in the furnace ≥5 hours. The initial rolling temperature of the billet is 1120℃~1180℃, and the final rolling temperature is 920℃~1000℃. Cooling requirements for the steel cooling bed and lower cooling bed: Insulation covers are strictly prohibited on the cooling bed; steel should be stored with two steps of clearance between each piece.

[0035] Each steel piece was subjected to magnetic flux leakage and ultrasonic testing. Magnetic flux leakage testing was performed according to Level 3 in Table 1 of GB / T32547; ultrasonic testing was performed according to Level B in GB / T4162.

[0036] Advantages of this invention:

[0037] 1. This invention is based on the understanding that the precipitation of V and Nb elements and the bainitic transformation cannot occur simultaneously and fully. It abandons the current technical approach of using microalloying precipitation strengthening + bainitic transformation strengthening to improve the strength of high-strength non-quenched and tempered steel for automotive front axles. Instead, it adopts the technical approach of microalloying grain refinement strengthening + bainitic transformation strengthening to improve the strength of high-strength non-quenched and tempered steel for automotive front axles. The grain refinement strengthening effect of TiN and NbN occurs during the workpiece heating and forging stages, which are at high temperatures above 800°C. This stage is unrelated to the bainitic transformation. Therefore, the microalloying grain refinement strengthening of Ti and Nb and the bainitic transformation strengthening do not contradict each other. The microalloying grain refinement strengthening + bainitic transformation strengthening can occur fully, and there is no waste of expensive microalloying elements such as Ti and Nb.

[0038] 2. Based on long-term production practice, this invention summarizes the contribution rate of various alloying elements in steel to hardenability, and summarizes the range within which the total contribution of various alloying elements in steel to hardenability can ensure that the entire cross-sectional structure of the forged high-strength non-quenched and tempered steel workpiece for automobile front axles is of lower bainite structure during continuous cooling (cooling rate, 0.5~2.5℃ / S), thus clarifying the requirements for the hardenability range of steel.

[0039] 3. This invention adds rare earth elements to high-strength non-quenched and tempered steel for automobile front axles. The strong hardenability improvement ability of rare earth elements is conducive to obtaining lower bainite structure, and the ability of rare earth elements to modify inclusions is conducive to improving the fatigue resistance of steel.

[0040] 4. The high-strength non-quenched and tempered steel for automotive front axles provided by this invention contains only 0.02-0.03% Nb and 0.0003-0.0005% lanthanide rare earth elements. Based on current alloy prices, the cost of 0.02-0.03% Nb is 40-60 yuan / ton of steel, and the cost of 0.0003-0.0005% lanthanide rare earth elements is 6-10 yuan / ton of steel, totaling only 46-70 yuan / ton of steel. This is lower than the alloy cost of existing publicly available high-strength non-quenched and tempered steel for automotive front axles, giving it a cost advantage and facilitating large-scale application. Attached Figure Description

[0041] Figure 1 The metallographic image of the high-strength non-quenched and tempered steel for automobile front axle obtained using the technology of the present invention in Example 1 is magnified by 1000 times.

[0042] Figure 2 For comparison, the metallographic structure of high-strength non-quenched and tempered steel for automobile front axles obtained using existing publicly available technology in Example 1 is magnified by 1000 times.

[0043] Figure 3 For comparison, the metallographic structure of high-strength non-quenched and tempered steel for automotive front axles obtained by the technology described in Example 2 is magnified by 1000 times.

[0044] Figure 4 The metallographic image of high-strength non-quenched and tempered steel for automotive front axles obtained using the technique described in Comparative Example 3 is magnified by 1000 times. Detailed Implementation

[0045] The following is accompanied by the instruction manual. Figure 1-4 A high-strength non-quenched and tempered steel for automobile front axles and its manufacturing method are further described by way of specific embodiments.

[0046] Example 1:

[0047] A factory produces high-strength non-quenched and tempered steel for automotive front axles using the following process:

[0048] The chemical composition and mass percentage of high-strength non-quenched and tempered steel for automotive front axles are as follows: C: 0.23–0.28%, Si: 0.6–0.8%, Mn: 1.8–2.1%, Cr: 0.80–1.20%, Nb: 0.02–0.03%, Ti: 0.02–0.03%, Al: 0.010–0.030%, B: 0.004–0.006%, P: ≤0.025%, S: 0.045–0.065%, lanthanide rare earth elements: 0.0003–0.0005%, N: 0.009–0.0130%, with the remainder being Fe and unavoidable impurities. In addition to meeting the composition requirements, the element content of high-strength non-quenched and tempered steel for automotive front axles must also satisfy the following formulas ①, ②, and ③:

[0049] H 淬透性 =93*(ω C )+5*(ω Si )+8.8*(ω Mn )+13*(ω Cr )+780*(ω B )+210*

[0050] (ω Nb )+93*(ω Ti )+6380*(ω 镧系稀土 )①

[0051] 72≤H 淬透性 ≤75 ②

[0052] 3*(ω N )≤0.516*(ω Nb )+(ω Ti )≤4*(ω N ) ③

[0053] In equations ① to ③, ω C ω Si ω Mn ω Cr ω B ω Nb ω Ti ω 镧系稀土 ω N These represent the mass percentage (%) of C, Si, Mn, Cr, B, Nb, Ti, lanthanide rare earth elements, and N in the steel composition; H 淬透性 This represents the contribution of chemical composition to hardenability in high-strength non-quenched and tempered steel for front axles, as summarized by the inventor based on production practice.

[0054] The steelmaking process is as follows: converter → LF furnace refining + RH vacuum degassing treatment → continuous casting (electromagnetic stirring) → slow cooling / hot delivery → rolling → air cooling → finishing → warehousing.

[0055] The converter smelting process is as follows: molten iron inlet temperature is 1380℃, sulfur content is 0.023%, final slag basicity is controlled at 3.1, final carbon content is 0.12%, phosphorus content is 0.011%, and tapping temperature is 1654℃. During ladle alloying, alloys are added according to the lower limit of the composition for adjustment, and slag-forming material is added at approximately 12 kg / t. The amount of slag discharged from the converter is less than 0.5 kg / ton of steel.

[0056] The LF furnace refining process is as follows: the basicity of the refining slag is controlled at 6.3, and the refining cycle is 48 minutes. Before taking the first sample, aluminum wire is fed in as needed, and a sample is taken under the white slag for composition analysis. Based on the analysis results of the first sample, the contents of C, Si, Mn, Cr, B, Nb, Ti, lanthanide rare earths, Al, and S are adjusted according to the target composition requirements to ensure that the composition is within the specified range while also allowing H to be present. 淬透性 The requirements of formula ② must be met. The lanthanide rare earth elements are added using the following process: 1 m of rare earth cored wire per ton of steel is fed in during the later stages of LF refining at a feeding speed of 3 m / s. The core powder weight of the rare earth cored wire is 190 g / m, and the core powder composition is: Si: 40%, N: 27%, lanthanide rare earth elements: 14%, S≤0.015%, P≤0.015%, with the remainder being unavoidable impurities. After feeding, the molten steel is stirred at a high flow rate at the bottom of the ladle for 1.5 min to homogenize the steel composition.

[0057] The composition of the molten steel was determined by sampling at the refining endpoint: C: 0.26%, Si: 0.68%, Mn: 2.03%, Cr: 0.98%, Nb: 0.028%, Ti: 0.032%, Al: 0.016%, B: 0.005%, P: 0.013%, S: 0.056%, lanthanide rare earth elements: 0.00045%, N: 0.0042%.

[0058] Calculated:

[0059] H 淬透性 = 93*0.26+5*0.68+8.8*2.03+13*0.98+780*0.005+210*0.028+93*0.032+6380*

[0060] 0.00045 = 73.81;

[0061] H 淬透性 It satisfies the requirements of equation ②.

[0062] RH vacuum refining process: The entire vacuum treatment process utilizes bottom-blowing nitrogen from the ladle for nitrogen enrichment. The pure vacuum treatment time is 22 minutes. After RH vacuum breaking, manganese nitride wire (0.5 m / ton of steel) is fed in according to the nitrogen content target, with a soft blowing time of 18 minutes. After RH treatment, samples are taken to test the nitrogen content, which is 0.0115% in the steel.

[0063] Calculated:

[0064] 0.516*(ω Nb )+(ω Ti )=4.04*(ω N );

[0065] The relationship between Nb, Ti and N content satisfies the requirements of equation ③.

[0066] Controlling the RH outlet temperature: For the first continuous casting furnace: add 80-105℃ to the liquidus temperature; for the second furnace onwards: add 45-80℃ to the liquidus temperature.

[0067] Continuous casting process: The liquid level in the tundish should not be lower than 750mm, the liquid level fluctuation in the crystallizer should be ≤±3mm, the superheat in the tundish should be controlled at 20~30℃, the casting speed for a Φ500mm casting section should be controlled at 0.4-0.5m / min, the electric stirring parameters in the crystallizer are: 150A, 1.5Hz, and the electric stirring parameters at the end of solidification are: 200A, 7.0Hz. The temperature of the billet entering the straightening machine should be ≥900℃. Slow cooling requirements for billets: They cannot stay on the cooling bed and must be put into the cooling pit for slow cooling in a timely manner. Slow cooling regime: The temperature entering the cooling pit should be greater than 500℃, and the temperature exiting the cooling pit should be less than 300℃, of which the slow cooling time for Φ500mm continuous casting round billets should not be less than 36 hours;

[0068] The rolling process is as follows: billet → furnace loading → heating → rolling → sawing → air cooling → finishing → flaw detection → warehousing. The heating regime in the billet heating furnace is as follows: preheating section temperature 500–1000℃, heating section temperature 1050–1270℃, soaking section temperature 1170–1230℃, and the total time the billet is in the furnace is 6 hours. The initial rolling temperature is 1160℃, and the final rolling temperature is 930℃. Cooling requirements for the steel cooling bed and lower cooling bed: Insulation covers are strictly prohibited on the cooling bed; steel should be stored with two steps of clearance between each piece.

[0069] Each steel piece was subjected to magnetic flux leakage and ultrasonic testing. Magnetic flux leakage testing was performed according to Level 3 in Table 1 of GB / T32547; ultrasonic testing was performed according to Level B in GB / T4162.

[0070] Automobile front axle manufacturers use the aforementioned high-strength non-quenched and tempered steel as raw material, and obtain the aforementioned high-strength non-quenched and tempered steel front axle for heavy-duty vehicles after sawing, induction heating, forging, edge trimming, correction, controlled cooling, machining, and inspection.

[0071] The process parameters are as follows: the high-strength non-quenched and tempered steel is heated to a temperature of 1250-1280℃ after induction heating, and the forging start temperature is...

[0072] The forging temperature is 1200-1230℃, the forging end temperature is 1030-1060℃, the trimming end temperature is 980-1020℃, and the straightening end temperature is 930-960℃. The workpiece is then suspended in a rapid cooling chamber with an inlet temperature of 900-950℃ and an outlet temperature of 400-450℃, for a time of 20-30 minutes.

[0073] The finished workpieces were subjected to yield strength, tensile strength, elongation, reduction of area, -40℃ impact energy (U-notch), and fatigue life tests. The test results are shown in Table 1 below, and the metallographic images are shown in […]. Figure 1 .

[0074] Comparative Example 1

[0075] A steel mill uses existing publicly available technology to produce high-strength non-quenched and tempered steel. The technical approach employed is vanadium microalloying precipitation strengthening combined with bainitic phase transformation strengthening to enhance the strength of high-strength non-quenched and tempered steel for automotive front axles. Because the atomic radius of vanadium is smaller than that of Nb and Ti, and the radius difference between vanadium and iron atoms is smaller than that between Nb, Ti, and iron atoms, vanadium's ability to improve the hardenability of steel is weaker than that of Nb and Ti. Based on production experience, the contribution of vanadium to the hardenability of steel is expressed by the following formula:

[0076] H 淬透性_V =140*(ω v );

[0077] This steel mill does not feed rare earth cored wire into the LF refining process during the later stages of producing high-strength non-quenched and tempered steel. The final steel composition at the LF refining endpoint is: C: 0.26%, Si: 0.34%, Mn: 1.86%, Cr: 0.53%, V: 0.14%, P: 0.016%, S: 0.053%, N: 0.014%.

[0078] The entire RH vacuum treatment process uses bottom-blowing argon gas from a steel ladle.

[0079] Everything else is the same as in Example 1.

[0080] The contribution of chemical composition to hardenability in high-strength non-quenched and tempered steel for front axles is calculated as follows:

[0081] H 淬透性 =93*0.26+5*0.34+8.8*1.86+13*0.53+110*0.14=64.5;

[0082] The finished workpiece was subjected to yield strength, tensile strength, elongation, reduction of area, -40℃ impact energy (U-notch), and fatigue life tests. The test results are shown in Table 1 below, and the metallographic images are shown in […]. Figure 2 .

[0083] Comparative Example 2

[0084] A steel plant uses the composition range provided by this invention to produce high-strength non-quenched and tempered steel. However, because some alloy components are controlled at the lower limit, the total contribution of chemical components to hardenability in high-strength non-quenched and tempered steel does not meet the requirements of Equation ②.

[0085] The composition of the molten steel was determined by sampling at the refining endpoint: C: 0.28%, Si: 0.63%, Mn: 2.05%, Cr: 0.93%, Nb: 0.022%, Ti: 0.023%, Al: 0.015%, B: 0.0045%, P: 0.011%, S: 0.042%, lanthanide rare earth elements: 0.00035%, N: 0.0040%.

[0086] Calculated:

[0087] H 淬透性 = 93*0.28 + 5*0.63 + 8.8*2.05 + 13*0.93 + 780*0.0045 + 210*0.022 + 93*0.023 + 6380

[0088] *0.00035 = 70.9;

[0089] H 淬透性 The requirements of equation ② are not met.

[0090] RH vacuum refining process: Nitrogen is blown throughout the vacuum treatment process, with a pure vacuum treatment time of 20 minutes. After the RH vacuum is broken, manganese nitride wire (0.2 m / ton of steel) is fed in according to the nitrogen content target, with a soft blowing time of 17 minutes. After RH treatment, samples are taken to test the nitrogen content, which is 0.0093% in the steel.

[0091] Calculated:

[0092] 0.516*(ω Nb )+(ω Ti )=3.7*(ω N );

[0093] The relationship between Nb, Ti and N content satisfies the requirements of equation ③.

[0094] Everything else is the same as in Example 1.

[0095] The finished workpiece was subjected to yield strength, tensile strength, elongation, reduction of area, -40℃ impact energy (U-notch), and fatigue life tests. The test results are shown in Table 1 below, and the metallographic images are shown in […]. Figure 3 .

[0096] Comparative Example 3

[0097] A steel plant uses the composition range provided by this invention to produce high-strength non-quenched and tempered steel. However, because the Nb and Ti in the alloy composition are controlled at the lower limit, the relationship between the chemical composition Nb, Ti and N content in the high-strength non-quenched and tempered steel does not meet the requirements of Equation ③.

[0098] The composition of the molten steel was determined by sampling at the refining endpoint: C: 0.26%, Si: 0.74%, Mn: 2.04%, Cr: 1.11%, Nb: 0.020%, Ti: 0.020%, Al: 0.016%, B: 0.0056%, P: 0.014%, S: 0.048%, lanthanide rare earth elements: 0.00046%, N: 0.0040%.

[0099] Calculated:

[0100] H 淬透性 = 93*0.26 + 5*0.74 + 8.8*2.04 + 13*1.11 + 780*0.0056 + 210*0.020 + 93*0.020 + 6380

[0101] *0.00046 = 72.8;

[0102] H 淬透性 It satisfies the requirements of equation ②.

[0103] RH vacuum refining process: Nitrogen is blown throughout the vacuum treatment process, with a pure vacuum treatment time of 20 minutes. After the RH vacuum is broken, manganese nitride wire (0.2 m / ton of steel) is fed in according to the nitrogen content target, with a soft blowing time of 17 minutes. After RH treatment, samples are taken to test the nitrogen content, which is 0.0095% in the steel.

[0104] Calculated:

[0105] 0.516*(ω Nb )+(ω Ti )=3.2*(ω N );

[0106] The relationship between Nb, Ti and N content does not meet the requirements of equation ③.

[0107] Everything else is the same as in Example 1.

[0108] The finished workpiece was subjected to yield strength, tensile strength, elongation, reduction of area, -40℃ impact energy (U-notch), and fatigue life tests. The test results are shown in Table 1 below, and the metallographic images are shown in […]. Figure 4 .

[0109] Table 1. Mechanical properties and fatigue life test results of the workpieces in Example 1 and Comparative Examples 1-3.

[0110]

[0111] Comparing Example 1 and Comparative Example 1, Table 1 shows that the yield strength and tensile strength of Example 1 are both approximately 250 MPa higher than those of Comparative Example 1. This is because the total contribution of the alloying elements in Example 1 to hardenability satisfies the requirements of Equation ②, resulting in suitable hardenability. Therefore, the microstructure is a fine, acicular lower bainitic structure with excellent strength and toughness, as shown in the attached specification. Figure 1 In contrast, the total contribution of the alloying elements in Example 1 to hardenability does not meet the requirements of Equation ②, and the hardenability is lower than the lower limit requirement. Therefore, the microstructure is ferrite + granular bainite, as shown in the appendix to the specification. Figure 2 The contribution of bainitic phase transformation to strength is relatively small; the fatigue life of Example 1 is much higher than that of Comparative Example 1, for two reasons. First, the microstructure of Example 1 is lower bainitic, which has good toughness, while the microstructure of Comparative Example 1 is mainly granular bainitic, which has poor toughness. The toughness of steel contributes positively to its resistance to fatigue. Second, rare earth elements were added in Example 1 to modify inclusions. The coefficient of thermal expansion between the composite inclusions generated after modification and the iron matrix is ​​close, which can reduce the gap between the iron matrix and inclusions when the workpiece undergoes temperature changes, thus improving the fatigue resistance of the workpiece. Based on current alloy prices, and considering the reduction in steel consumption (the "weight of added alloy" ultimately translates to the "increase in billet production"), the alloy cost of Example 1 is 95 yuan / ton lower than that of Comparative Example 1. Furthermore, the alloy cost of Example 1 is lower, while its mechanical properties are higher. One reason for this is the higher V (0.14%) added in Comparative Example 1. This portion of vanadium cannot be fully precipitated within the bainitic phase transformation cooling rate range, resulting in waste of vanadium alloy. Example 1, however, abandons the current method of using V microalloying precipitation strengthening + bainitic phase transformation strengthening to improve the strength of high-strength non-quenched and tempered steel for automotive front axles. Instead of the traditional approach, this paper adopts a technical approach that combines Nb and Ti microalloying grain refinement strengthening with bainitic transformation strengthening to enhance the strength of high-strength non-quenched and tempered steel for automotive front axles. The grain refinement strengthening effect of TiN and NbN occurs during the workpiece heating and forging stages, which are at high temperatures above 800℃. This stage is unrelated to bainitic transformation, so there is no contradiction between Ti and Nb microalloying grain refinement strengthening and bainitic transformation strengthening. The grain refinement strengthening and bainitic transformation strengthening can occur simultaneously and fully, without wasting expensive microalloying elements such as Ti and Nb, and the utilization efficiency of Ti and Nb alloying elements is high.

[0112] Comparing Example 1 and Comparative Example 2, Table 1 shows that the yield strength and tensile strength of Example 1 are both approximately 120 MPa higher than those of Comparative Example 2. Based on current alloy prices, and considering the reduction in steel consumption (the "weight of added alloy" ultimately translates to the "increase in billet production"), the alloy cost of Example 1 is approximately 30 yuan / ton higher than that of Comparative Example 2. This is because the total contribution of the alloying elements in Example 1 to hardenability satisfies the requirements of Equation ②, resulting in suitable hardenability. Therefore, the microstructure is a fine, acicular lower bainitic structure with excellent strength and toughness, as shown in the appendix to the instruction manual. Figure 1 However, the total contribution of the alloying elements in Comparative Example 2 to hardenability does not meet the requirements of Equation ②, and the hardenability is slightly lower than the lower limit requirement. Therefore, the microstructure is a small amount of ferrite + lower bainite microstructure, as shown in the appendix to the specification. Figure 3 The contribution of phase transformation to strength decreases, and the ferrite that precipitates first cannot provide dislocation strengthening caused by carbon supersaturation, thus resulting in a significant reduction in strength.

[0113] Comparing Example 1 and Comparative Example 3, Table 1 shows that the yield strength and tensile strength of Example 1 are both about 30 MPa higher than those of Comparative Example 2. Based on current alloy prices, and considering the reduction in steel consumption (the "weight of added alloy" ultimately translates to the "increase in billet production"), the alloy cost of Example 1 is about 10 yuan / ton lower than that of Comparative Example 3. Comparative Example 3 has a higher alloy cost but lower strength. This is because the relationship between Nb, Ti, and N content in the alloy elements of Example 1 satisfies the requirements of Equation ③. The interaction of N with Ti and Nb elements can generate TiN and NbN, which refine grain size. Furthermore, the higher content of Nb and Ti elements allows for the pinning of grain boundaries during workpiece heating and forging, preventing grain growth. Figure 1 , Figure 4 It can be seen that the grain size of the microstructure in Example 1 is 1.5 grades higher than that in Comparative Example 2. However, the relationship between the Nb, Ti and N contents in the alloying elements in Comparative Example 3 does not meet the requirements of Equation ③. The amount of TiN and NbN generated by the interaction of N with Ti and Nb elements, which play a role in refining the grains, is small, so the grain refining effect is worse, as shown in the appendix to the specification. Figure 4 The contribution rate of fine grain strengthening decreases.

[0114] The upper and lower limits of the process parameters (such as temperature, time, etc.) and the range values ​​of the present invention can all achieve the method, and examples are not listed here.

[0115] All aspects not described in detail in this invention can be covered using conventional technical knowledge in the field.

[0116] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A high-strength non-quenched and tempered steel for automobile front axles, characterized in that, The chemical composition and mass percentage of the high-strength non-quenched and tempered steel for automotive front axles are as follows: C: 0.23–0.28%, Si: 0.6–0.8%, Mn: 1.8–2.1%, Cr: 0.80–1.20%, Nb: 0.02–0.03%, Ti: 0.02–0.03%, Al: 0.010–0.030%, B: 0.004–0.006%, P: ≤0.025%, S: 0.045–0.065%, lanthanide rare earth elements: 0.0003–0.0005%, N: 0.009–0.0130%, with the remainder being Fe and unavoidable impurities. The production method of the high-strength non-quenched and tempered steel for automobile front axles includes the following steps: Converter smelting → LF furnace refining + RH vacuum degassing treatment → continuous casting → slow cooling / hot delivery → rolling → air cooling → finishing → warehousing; LF furnace refining is as follows: Control the basicity of the refining slag to be greater than 3.0, and the refining cycle to be greater than 40 minutes. Before taking the first sample, feed in aluminum wire as needed, and take a sample under the white slag to test the composition. Based on the analysis results of the first sample, adjust the content of C, Si, Mn, Cr, B, Nb, Ti, lanthanide rare earth elements, and Al according to the target composition requirements, so that it meets the composition range while also achieving the desired effect. Satisfy the requirements of equation ②; in, =93* +5* +8.8* +13* +780* +210* +93* +6380* ①; 72≤ ≤75 ②； 3.5* ≤0.516* + ≤4.5* ③； Mode ~③ , , , , , , , , These represent the mass percentages of C, Si, Mn, Cr, B, Nb, Ti, lanthanide rare earth elements, and N in the chemical composition of steel, respectively. The contribution of chemical composition to hardenability in high-strength non-quenched and tempered steel for automotive front axles; RH vacuum degassing treatment is as follows: The entire vacuum treatment process involves nitrogen blowing and nitrogen enrichment. The pure vacuum treatment time is 15-40 minutes. After RH treatment, manganese nitride wires are fed in according to the nitrogen content target. The soft blowing time is ≥15 minutes. The RH outlet temperature is controlled as follows: for the first continuous casting furnace, the liquidus temperature is increased by 80-105℃, and for the second furnace onwards, the liquidus temperature is increased by 45-80℃. The rolling process is as follows: The preheating zone temperature is 500~1000℃, the heating zone temperature is 1050~1270℃, the soaking zone temperature is 1170~1230℃, and the total time the billet is in the furnace is ≥5h; the billet initial rolling temperature is 1120℃~1180℃, and the final rolling temperature is 920℃~1000℃.

2. The high-strength non-quenched and tempered steel for automotive front axles according to claim 1, characterized in that, Converter smelting is as follows: The temperature of molten iron entering the converter should be ≥1300℃, the sulfur content of the molten iron should be controlled below 0.040%, the basicity of the final slag in the converter should be controlled between 2.8 and 3.4, and the final carbon content of the converter smelting should be ≥0.10% and P ≤0.012%; the tapping temperature should be 1600-1680℃; when alloying the ladle, alloys should be added according to the lower limit of the composition, and slag-forming materials should be added at a rate of 6-15 kg / t; the amount of slag discharged from the converter should be less than 0.5 kg / ton of steel.

3. The high-strength non-quenched and tempered steel for automotive front axles according to claim 1, characterized in that, In the LF furnace refining process, lanthanide rare earth elements are added using the following process: 1 m of rare earth cored wire is fed per ton of steel at a feeding speed of 3 m / s. The core powder weight of the rare earth cored wire is 190 g / m, and the core powder composition is: Si: 38-45%, N: 24-30%, lanthanide rare earth elements: 12-15%, S≤0.015%, P≤0.015%, with the remainder being unavoidable impurities. After feeding, the steel ladle is bottom-blown with a high flow rate for 1-2 minutes to homogenize the steel composition.

4. The high-strength non-quenched and tempered steel for automotive front axles according to claim 1, characterized in that, The continuous casting process is as follows: The liquid level in the tundish should not be lower than 750 mm, the liquid level fluctuation in the crystallizer should be ±3 mm, the superheat in the tundish should be controlled at 20-30℃, and the temperature of the billet entering the straightening machine should be ≥900℃.

5. The high-strength non-quenched and tempered steel for automotive front axles according to claim 1, characterized in that, Each steel piece is subjected to magnetic flux leakage and ultrasonic testing; magnetic flux leakage testing is performed according to Level 3 in Table 1 of GB / T32547; ultrasonic testing is performed according to Level B in GB / T4162.

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