2500MPa-grade high-carbon Ti composite reinforced hot-formed steel and preparation method thereof

Abstract

The invention discloses 2500MPa-grade high-carbon Ti composite reinforced hot-formed steel and a preparation method thereof, and relates to the field of metal materials, and the 2500MPa-grade high-carbon Ti composite reinforced hot-formed steel comprises the following chemical components in percentage by mass: 0.63%-0.68% of C, 1.4%-1.7% of Si, 1.8%-2.0% of Mn, less than or equal to 0.008% of P, less than or equal to 0.005% of S, 1.1%-1.3% of Cr, 0.12%-0.14% of Mo, 0.09%-0.12% of V, 0.02%-0.04% of Nb and the balance of Fe. According to the 2500MPa-grade high-carbon Ti composite reinforced hot-formed steel and the preparation method thereof, through collaborative innovation of a high-carbon-high-titanium-precise vanadium / niobium component system and a tin-bismuth alloy cooling process, breakthrough balance of multiple technical effects is achieved.

Description

Technical Field

[0001] This invention relates to metal materials technology, specifically to a 2500MPa grade high-carbon Ti composite reinforced hot-formed steel and its preparation method. Background Technology

[0002] The automotive industry's pursuit of lightweighting and enhanced safety has placed higher demands on the strength of hot-formed steel, exceeding 2000 MPa. Existing technologies primarily employ two approaches to achieve this high strength:

[0003] Adding precious alloying elements: significantly increasing the content of elements such as Mo, V, and Nb to improve hardenability and precipitation strengthening effect, but this leads to a surge in material costs (30%-40% higher than conventional steel grades), and excessive addition may impair toughness and weldability.

[0004] The high-carbon design combined with traditional cooling strengthens the martensite by increasing the carbon content, but when the carbon content exceeds 0.6%, the material's toughness, ductility, and resistance to cold welding cracks deteriorate sharply. At the same time, relying on traditional cooling processes such as water quenching (uneven cooling, high stress), oil quenching (environmental issues), or lead-based alloy quenching (toxicity) cannot meet the environmental and safety standards of modern production while ensuring performance.

[0005] In addition, the addition of microalloying elements such as Ti, V, and Nb in existing technologies is mostly based on experience, lacking systematic optimization of the synergistic effects between them and between them and high-carbon matrices and specific cooling processes. This makes it difficult to stably generate uniformly dispersed nanoscale composite carbide structures that can support strengths of 2500 MPa or higher.

[0006] Therefore, there is an urgent need in this field for an innovative material and preparation method that can simultaneously solve a series of mutually restrictive problems such as ultra-high strength, high toughness, excellent weldability, low cost, and environmentally friendly production. Summary of the Invention

[0007] The present invention aims to overcome the above-mentioned defects of the prior art and provide a 2500MPa grade high carbon Ti composite reinforced hot-formed steel and its preparation method. This solution does not rely on the simple stacking of precious alloys, but achieves the optimal balance of strength, toughness, cost and environmental protection through precise component ratio and innovative cooling process.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a 2500MPa grade high-carbon Ti composite reinforced hot-formed steel, the chemical composition of which, by mass percentage, is as follows:

[0009] C: 0.63%~0.68%, Si: 1.4%~1.7%, Mn: 1.8%~2.0%, P: ≤0.008%, S: ≤0.005%, Cr: 1.1%~1.3%, Mo: 0.12%~0.14%, V: 0.09%~0.12%, Nb: 0.02%~0.04%, Ti: 0.28%~0.32%, Ni: 0.25%~0.30%, Al: 0.08%~0.15%, Total impurities: ≤0.08%, Balance: Fe;

[0010] The microstructure of the hot-formed steel contains TiC, VC and NbC composite carbides, and the size of the composite carbides is 3nm to 8nm.

[0011] Furthermore, the total content of the composite carbide is ≥0.52%.

[0012] Furthermore, the mechanical properties of the hot-formed steel meet the following requirements: tensile strength of 2500MPa~2650MPa, yield strength ≥2300MPa, impact energy at -20℃ ≥25J, and welding cold cracking rate ≤2%.

[0013] Furthermore, its chemical composition, by mass percentage, is preferably as follows:

[0014] C: 0.65%~0.67%, Si: 1.5%~1.6%, Mn: 1.85%~1.95%, Cr: 1.2%~1.3%, Mo: 0.13%~0.14%, V: 0.10%~0.11%, Nb: 0.03%~0.04%, Ti: 0.29%~0.31%, Ni: 0.27%~0.29%, Al: 0.10%~0.13%.

[0015] A method for preparing the 2500MPa grade high-carbon Ti composite reinforced hot-formed steel includes the following steps:

[0016] S1. Austenitizing treatment: Heat the steel billet to 1060℃~1080℃ and hold for 4min~5min;

[0017] S2. Rapid quenching: The steel billet treated by S1 is quenched in a tin-bismuth alloy cooling medium at a temperature of 150℃~180℃ within a transfer time of ≤2s. The tin-bismuth alloy cooling medium is composed of 58% Bi and 42% Sn by mass percentage. The medium is stirred during the quenching process so that the steel billet obtains an average cooling rate of 90℃ / s~110℃ / s.

[0018] S3. Tempering treatment: Hold the steel billet treated with S2 at 200℃ for 8 to 10 minutes.

[0019] Furthermore, in step S2, the stirring rate of the tin-bismuth alloy cooling medium is ≥1.5 m / s.

[0020] Furthermore, in step S1, austenitization is performed by induction heating at a heating rate of 20°C / min to 30°C / min.

[0021] Furthermore, in step S2, the tin-bismuth alloy cooling medium is filtered through a filter with a pore size of no more than 50 μm before use.

[0022] Furthermore, the effective thickness of the steel billet is ≤8mm.

[0023] Furthermore, after quenching in step S2, the core temperature of the steel billet is ≤200℃.

[0024] Compared with existing technologies, this invention provides a 2500MPa-grade high-carbon Ti composite reinforced hot-formed steel. Through the synergistic innovation of a high-carbon, high-titanium, precise vanadium / niobium composition system and a tin-bismuth alloy cooling process, it achieves a breakthrough balance of multiple technical effects. Its core lies in using low-cost titanium (0.28-0.32%) as the main component, in conjunction with trace amounts of vanadium and niobium, to form 3-8nm ultrafine composite carbides under specific processes. This provides core dispersion reinforcement, enabling the material to achieve a stable tensile strength of 2500-2650MPa, a yield strength ≥2300MPa, and a strength fluctuation ≤±30MPa. While achieving ultra-high strength, due to the toughening effect and refined microstructure of nickel, the material maintains excellent comprehensive performance: impact energy ≥25J at -20℃, elongation after fracture 6-7%, and welding cold cracking rate ≤2%, effectively overcoming the inherent brittleness and welding difficulties of high-carbon steel. In terms of cost, replacing expensive vanadium and niobium with high-titanium alloys, combined with a long-life tin-bismuth alloy medium, reduces the total material cost by 10-15% compared to similar high-alloy solutions. The environmental and safety benefits are significant. The use of lead-free and non-toxic tin-bismuth alloys completely replaces traditional lead quenching or oil quenching, resulting in high cooling uniformity, low residual stress, and a substantial reduction in environmental risks and workpiece cracking rates. The entire process parameters are compatible with existing production lines, requiring no major equipment modifications. Low oxide scale and medium losses demonstrate excellent mass production feasibility, providing a complete material solution for lightweight automotive safety components that combines ultra-high strength, high toughness, good economy, and environmental friendliness. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0026] Figure 1 This is a schematic diagram illustrating the component design and enhancement mechanism in an embodiment of the present invention;

[0027] Figure 2 This is a flowchart illustrating the entire process of the preparation method described in this invention. Detailed Implementation

[0028] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0029] Raw materials and equipment:

[0030] The raw materials used include industrial pure iron, high-carbon ferrochrome (Cr≥50%), ferromolybdenum (Mo≥60%), ferrovanadium (V≥50%), ferroniobium (Nb≥65%), ferrotitanium (Ti≥30%), ferrosilicon (Si≥75%), silicon-manganese alloy, nickel plate (Ni≥99.5%), and aluminum ingot (Al≥99.7%).

[0031] The melting was carried out using a 50kg medium-frequency induction melting furnace, equipped with an LF refining and VD vacuum degassing device (vacuum degree ≤10Pa).

[0032] The heat treatment equipment includes: a medium-frequency induction heating furnace (heating rate adjustable from 0-50℃ / min), a custom tin-bismuth alloy quenching tank (500L volume, with stirring and temperature control), a comparative water quenching tank, a comparative lead-based alloy quenching tank, and a box-type tempering furnace.

[0033] The testing equipment includes: a direct-reading spectrometer (ARL 4460), a universal tensile testing machine (WAW-1000), a cryogenic impact testing machine (JBW-500Z), a field emission scanning electron microscope (Zeiss Sigma 300), a transmission electron microscope (FEI Talos F200X), and an X-ray stress analyzer (μ-X360s).

[0034] General testing standards:

[0035] Tensile properties were tested according to GB / T 228.1-2010, with the sampling direction perpendicular to the rolling direction.

[0036] Impact toughness at -20℃ is tested according to GB / T 229-2020.

[0037] The welding cold crack rate was evaluated according to the "Test Method for Welding Cracks in Oblique Y-groove". The welding parameters were: current 120A, voltage 22V, speed 10cm / min, and the test was conducted 48 hours after welding.

[0038] The residual stress was determined using X-ray diffraction.

[0039] The size and morphology of the carbides were observed using TEM bright-field images, and the average size was statistically analyzed using image analysis software.

[0040] As attached Figure 1 and attached Figure 2 As shown:

[0041] Example 1:

[0042] This embodiment provides a 2500MPa grade high-carbon Ti composite reinforced hot-formed steel and its preparation method;

[0043] Chemical composition (mass percentage): C: 0.63%, Si: 1.40%, Mn: 1.80%, P: 0.003%, S: 0.002%, Cr: 1.10%, Mo: 0.12%, V: 0.09%, Nb: 0.02%, Ti: 0.28%, Ni: 0.25%, Al: 0.08%, [O]: 0.0015%, [N]: 0.0030%, with the remainder being Fe and unavoidable trace elements. The total impurities are 0.06%.

[0044] Preparation method:

[0045] Melting and casting: After melting, refining and vacuum degassing according to the above composition, the steel is cast into ingots and then forged into slabs with a thickness of 8mm.

[0046] Austenitizing: Place the slab in an induction heating furnace and heat it to 1070°C at a rate of 25°C / min, and hold it for 4.5 minutes.

[0047] Quenching: After the holding period, the slab was transferred to a 160°C tin-bismuth alloy cooling medium (Bi: 58 wt.%, Sn: 42 wt.%) within 1.5 seconds. The stirrer was turned on and the stirring linear velocity was maintained at 1.8 m / s. The measured average cooling rate of the slab core was 100°C / s.

[0048] Tempering: Immediately place the quenched slab into a tempering furnace at 200℃, hold for 9 minutes, and then air cool.

[0049] Properties and microstructure: The obtained steel plate has a tensile strength of 2520 MPa, a yield strength of 2310 MPa, an elongation after fracture of 6.1%, an impact energy of 26 J at -20℃, a weld cold cracking rate of 1.2%, and a surface residual stress of 230 MPa. TEM observation shows that the microstructure contains dispersed composite carbides with a size of 4-7 nm, and energy dispersive spectroscopy analysis indicates that they are mainly (Ti, V, Nb)C.

[0050] Example 2:

[0051] The difference between this embodiment and Example 1 is that the chemical composition (mass percentage) is as follows: C: 0.68%, Si: 1.70%, Mn: 2.00%, P: 0.004%, S: 0.003%, Cr: 1.30%, Mo: 0.14%, V: 0.12%, Nb: 0.04%, Ti: 0.32%, Ni: 0.30%, Al: 0.15%, total impurities are 0.07%, and the balance is Fe.

[0052] Preparation method: The austenitizing temperature was adjusted to 1080℃, and the tin-bismuth alloy medium temperature was adjusted to 170℃. The measured average cooling rate was 105℃ / s. The remaining steps were the same as in Example 1.

[0053] Properties and microstructure: The obtained steel plate has a tensile strength of 2630 MPa, a yield strength of 2440 MPa, an elongation after fracture of 6.3%, an impact energy of 29 J at -20℃, and a weld cold cracking rate of 1.0%. The carbide size is 3-6 nm.

[0054] Example 3:

[0055] The difference between this embodiment and Example 1 is that the chemical composition (mass percentage) is as follows: C: 0.66%, Si: 1.55%, Mn: 1.90%, P: 0.003%, S: 0.002%, Cr: 1.25%, Mo: 0.135%, V: 0.105%, Nb: 0.035%, Ti: 0.30%, Ni: 0.28%, Al: 0.11%, total impurities are 0.05%, and the balance is Fe.

[0056] Preparation method: The austenitizing temperature was 1075℃, and the tin-bismuth alloy medium temperature was 165℃. The measured average cooling rate was 102℃ / s. The remaining steps were the same as in Example 1.

[0057] Properties and microstructure: The obtained steel plate has a tensile strength of 2620 MPa, a yield strength of 2420 MPa, an elongation after fracture of 6.5%, an impact energy of 30 J at -20℃, a weld cold cracking rate of 0.8%, and a surface residual stress of 220 MPa. The carbide size is more uniform, ranging from 3 to 5 nm.

[0058] Comparative Example 1:

[0059] This comparative example aims to illustrate that, with the same composition, the traditional water quenching process cannot achieve the effect of the present invention.

[0060] Chemical composition (mass percentage): exactly the same as in Example 3.

[0061] Preparation method changed: the quenching medium was changed to room temperature circulating water, and forced stirring was removed. The measured average cooling rate was approximately 80℃ / s, and the cooling uniformity was poor (fluctuation > ±8℃ / s). The austenitizing and tempering processes were the same as in Example 3.

[0062] Results: The tensile strength of the obtained steel plate was only 2380 MPa, the yield strength was 2200 MPa, the impact energy at -20℃ dropped to 22 J, the welding cold cracking rate was as high as 3.5%, and the residual stress reached 380 MPa. A small amount of pearlite appeared in the microstructure, and the carbide size was uneven (5-10 nm).

[0063] Comparative Example 2:

[0064] This comparative example aims to illustrate that, even with alloy quenching, the overall benefits of the existing conventional strengthening approach of "high V, Nb, low Ti" are still inferior to those of this invention.

[0065] Chemical composition changes: Based on Example 3, the Ti content was reduced to 0.15%, while the V content was increased to 0.15% and the Nb content was increased to 0.06%. The contents of other elements remained unchanged.

[0066] Preparation method changed: A lead-based alloy (Bi: 56 wt.%, Pb: 44 wt.%) was used as the quenching medium, the temperature was 165℃, the stirring speed was 1.8 m / s, and the measured average cooling rate was approximately 95℃ / s. The remaining processes were the same as in Example 3.

[0067] Results: The obtained steel plate had a tensile strength of 2450 MPa, a yield strength of 2280 MPa, an impact energy of 24 J at -20℃, and a weld cold cracking rate of 2.8%. TEM analysis showed that the carbides were mainly (V,Nb)C, with large sizes ranging from 6 to 12 nm. The material cost per ton of steel was approximately 11.5% higher than that of Example 3.

[0068] Effect Analysis:

[0069] The data from the above embodiments and comparative examples are summarized in the table below:

[0070] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Tensile strength (MPa) 2520 2630 2620 2380 2450 Impact energy at -20℃ (J) 26 29 30 22 24 Welding cold cracking rate (%) 1.2 1.0 0.8 3.5 2.8 Residual stress (MPa) 230 240 220 380 280 Carbide size (nm) 4-7 3-6 3-5 5-10 (uneven) 6-12 Main carbide types (Ti, V, Nb)C (Ti, V, Nb)C (Ti, V, Nb)C (Ti, V, Nb)C (V, Nb)C is the main component Environmental protection Lead-free and non-toxic Lead-free and non-toxic Lead-free and non-toxic Environmentally friendly (poor uniformity) Lead is toxic

[0071] It can be seen from the above table:

[0072] All embodiments of the present invention (using specific components + tin-bismuth alloy cooling) have stably achieved ultra-high strength of ≥2500MPa, while also possessing high toughness (KV2≥26J), low welding cold cracking rate (≤1.2%) and low residual stress (≤250MPa), and small carbide size (3-7nm).

[0073] Comparative Example 1 demonstrates that even with identical composition, simply changing the cooling process to traditional water quenching leads to a comprehensive deterioration in strength, toughness, and weldability due to insufficient cooling rate and poor uniformity. This verifies that the tin-bismuth alloy cooling process of this invention is an essential technical feature for achieving the desired effect.

[0074] Comparative Example 2 demonstrates that if the "high Ti-dominant, V / Nb fine-tuning" composition design principle of this invention is abandoned, and instead the conventional approach of increasing precious V and Nb elements is adopted, even with alloy quenching, its core properties such as strength and toughness are still lower than those of the preferred embodiment of this invention, and the cost increases significantly, while also introducing environmental hazards. This highlights the non-obviousness of the synergistic composition design of this invention and its dual advantages in terms of technology and economy.

[0075] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A 2500MPa grade high-carbon Ti composite reinforced hot-formed steel, characterized in that, Its chemical composition, by mass percentage, is as follows: C: 0.63%~0.68%, Si: 1.4%~1.7%, Mn: 1.8%~2.0%, P: ≤0.008%, S: ≤0.005%, Cr: 1.1%~1.3%, Mo: 0.12%~0.14%, V: 0.09%~0.12%, Nb: 0.02%~0.04%, Ti: 0.28%~0.32%, Ni: 0.25%~0.30%, Al: 0.08%~0.15%, Total impurities: ≤0.08%, Balance: Fe; The microstructure of the hot-formed steel contains TiC, VC and NbC composite carbides, and the size of the composite carbides is 3nm to 8nm.

2. The 2500MPa grade high-carbon Ti composite reinforced hot-formed steel according to claim 1, characterized in that, The total content of the composite carbides is ≥0.52%.

3. The 2500MPa grade high-carbon Ti composite reinforced hot-formed steel according to claim 1, characterized in that, The mechanical properties of the hot-formed steel meet the following requirements: tensile strength of 2500MPa~2650MPa, yield strength ≥2300MPa, impact energy at -20℃ ≥25J, and welding cold cracking rate ≤2%.

4. The 2500MPa grade high-carbon Ti composite reinforced hot-formed steel according to claim 1, characterized in that, Its chemical composition, by mass percentage, is preferably as follows: C: 0.65%~0.67%, Si: 1.5%~1.6%, Mn: 1.85%~1.95%, Cr: 1.2%~1.3%, Mo: 0.13%~0.14%, V: 0.10%~0.11%, Nb: 0.03%~0.04%, Ti: 0.29%~0.31%, Ni: 0.27%~0.29%, Al: 0.10%~0.13%.

5. A method for preparing 2500MPa grade high-carbon Ti composite reinforced hot-formed steel as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Austenitizing treatment: Heat the steel billet to 1060℃~1080℃ and hold for 4min~5min; S2. Rapid quenching: The steel billet treated by S1 is quenched in a tin-bismuth alloy cooling medium at a temperature of 150℃~180℃ within a transfer time of ≤2s. The tin-bismuth alloy cooling medium is composed of 58% Bi and 42% Sn by mass percentage. The medium is stirred during the quenching process so that the steel billet obtains an average cooling rate of 90℃ / s~110℃ / s. S3. Tempering treatment: Hold the steel billet treated with S2 at 200℃ for 8 to 10 minutes.

6. The method for preparing a 2500MPa grade high-carbon Ti composite reinforced hot-formed steel according to claim 5, characterized in that, In step S2, the stirring rate of the tin-bismuth alloy cooling medium is ≥1.5 m / s.

7. The method for preparing a 2500MPa grade high-carbon Ti composite reinforced hot-formed steel according to claim 5, characterized in that, In step S1, austenitization is performed by induction heating at a rate of 20°C / min to 30°C / min.

8. The method for preparing a 2500MPa grade high-carbon Ti composite reinforced hot-formed steel according to claim 5, characterized in that, In step S2, the tin-bismuth alloy cooling medium is filtered through a filter with a pore size of no more than 50 μm before use.

9. The method for preparing a 2500MPa grade high-carbon Ti composite reinforced hot-formed steel according to claim 5, characterized in that, The effective thickness of the steel billet is ≤8mm.

10. The method for preparing a 2500MPa grade high-carbon Ti composite reinforced hot-formed steel according to claim 5, characterized in that, After quenching in step S2, the core temperature of the steel billet is ≤200℃.

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