A high-strength steel for green low-carbon vehicles and a method for manufacturing the same

Abstract

This invention belongs to the technical field of automotive steel manufacturing, specifically relating to a green, low-carbon high-strength automotive steel and its preparation method. The invention adds nickel-doped modified niobium carbide to the high-strength steel, dispersing it in the iron matrix to effectively improve the strength and toughness of the steel plate and increase its porosity. Simultaneously, the addition of Ca, Mg, and lanthanum and cerium carbonate as composite refining agents to the alloy melt improves casting quality and mechanical properties. Furthermore, precise control of the composition of elements such as C, Mn, Si, Cr, Mo, Cu, and Ti, combined with hot rolling, cold rolling, continuous annealing, or hot-dip galvanizing processes, enables the material to possess both high strength and excellent porosity.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel manufacturing, specifically relating to a green and low-carbon high-strength automotive steel and its preparation method. Background Technology

[0002] In recent years, as the automotive industry has placed increasingly higher demands on the formability of materials, traditional high-strength steel has become insufficient to meet the high drawability requirements of complex stamped parts. Dual-phase steel has thus become a research hotspot in the development of automotive steels. Simultaneously, under the "dual-carbon" background, the steel industry, as a crucial material supplier for automobile manufacturing, urgently needs to develop green and low-carbon automotive steels to meet the multiple demands of lightweighting, high strength, and low carbon emissions in automobiles. Developing green and low-carbon high-strength automotive steel products—namely, multi-purpose alloy designs and short-process, low-cost manufacturing technologies—not only meets the specific needs of the automotive industry, such as multiple parts, diversified user demands, multiple specifications, and small-batch orders, but also represents an effective measure for steel companies to enhance their competitiveness.

[0003] Traditional high-strength steels, while pursuing high strength, often face problems such as decreased plasticity and increased forming difficulty, making it difficult to meet the manufacturing requirements of complex vehicle body structural components. Based on the current research status, solving the problem of poor formability of automotive high-strength steels with short-process, low-cost production has become a research hotspot in the industry. On the one hand, by optimizing composition design, such as adding microalloying elements and using multiphase microstructure control, the strength and plasticity of steel can be improved while reducing carbon emissions during material production. On the other hand, advanced manufacturing processes, such as continuous casting and rolling, controlled rolling and controlled cooling, can optimize the microstructure of the material, further improving its comprehensive mechanical and formability properties. This invention develops a green, low-carbon automotive high-strength steel and its preparation method, which, while meeting the basic performance indicators of the product, can achieve multiple uses from a single steel and shorten the production process on traditional production lines to reduce energy consumption. Summary of the Invention

[0004] The primary objective of this invention is to provide a green and low-carbon high-strength steel for automobiles, wherein the high-strength steel has a tensile strength greater than 1000 MPa, an elongation after fracture of A80 greater than 15%, and a hole expansion rate greater than 30%.

[0005] The second objective of this invention is to provide a method for preparing the above-mentioned green and low-carbon high-strength steel for automobiles.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A green, low-carbon high-strength steel for automobiles, comprising, by weight percentage: C: 0.05-0.20 wt%, Mn: 0.5-1.5 wt%, Si: 0.2-1.5 wt%, Cr: 0.02-0.60 wt%, Mo: 0.02-0.60 wt%, Cu: 0.02-0.60 wt%, Ti: 0.01-0.150 wt%, P≤0.02 wt%, S≤0.02 wt%, N≤0.01 wt%, Nb: 0.1-0.15 wt%, Ni: 0.003-0.021 wt%, Ca: 0.01-0.12 wt%, Mg: 0.01-0.15 wt%, La: 0.02-0.03 wt%, Ce: 0.02-0.03 wt%. wt%, balance is Fe and unavoidable impurities; of which Nb and Ni are derived from raw material modified niobium carbide; La and Ce are derived from raw material composite refining agents; The modified niobium carbide is prepared by the following process: Niobium hydroxide, nickel hydroxide, and carbon black are mixed, sintered, and cooled to obtain modified niobium carbide.

[0007] Further, the molar ratio of niobium hydroxide, nickel hydroxide and carbon black is 1:(0.1-0.3):(1.5-2); the sintering temperature is 1220-1300 ℃ and the time is 10-12 h.

[0008] The proportioning design mechanism of each raw material in the high-strength steel of this invention is as follows: C: Carbon strengthens steel through solid solution, ensuring its strength requirements. Sufficient carbon helps stabilize austenite, thereby improving the steel's formability. Too low a carbon content will not yield the mechanical properties of the steel described in this invention; too high a content will cause the steel to become brittle, leading to delayed fracture.

[0009] Manganese (Mn) is an austenite stabilizing element in steel. It can expand the austenite phase region, reduce the critical quenching rate of steel, and refine grains, thus contributing to solid solution strengthening and improving strength. If the Mn content is too low, the supercooled austenite will be unstable, reducing the processing properties of the steel plate, such as plasticity and toughness. If the Mn content is too high, the weldability of the steel plate will deteriorate, and production costs will increase, which is detrimental to industrial production.

[0010] Silicon (Si): Silicon has a certain solid solution strengthening effect in ferrite, ensuring that the steel has sufficient strength. At the same time, Si can also inhibit the decomposition of retained austenite and the precipitation of carbides, reducing inclusions in the steel. If the Si content is too low, it will not have a strengthening effect; if the Si content is too high, it will reduce the surface quality and weldability of the steel plate.

[0011] Cr: Chromium increases the hardenability of steel to ensure its strength and stabilizes retained austenite, which helps improve the crack resistance and high strength of products, facilitating the design of multi-purpose steel products. Too low a Cr content will affect the hardenability of the steel, while too high a content will increase production costs.

[0012] Mo: Molybdenum is a strengthening element in steel, which helps stabilize residual austenite and has a significant effect on improving the hardenability of steel. When used in combination with Ti, Mo can form a large number of TiMoC precipitates, which helps to disperse diffusible hydrogen in the steel and reduce the accumulation of diffusible hydrogen. Therefore, it can balance high strength and resistance to hydrogen embrittlement.

[0013] Cu: Cu is a solid solution strengthening element that can not only improve the hardenability of steel, but also effectively improve the thermodynamic stability of austenite, which helps to form stable residual austenite at room temperature, thereby improving the plasticity and resistance to delayed fracture of the material; it also helps to improve the strength and toughness of products and is conducive to the design of multi-purpose steel products.

[0014] Ti: The microalloying element Ti improves the overall performance of materials through grain refinement and precipitation strengthening.

[0015] P: P is a harmful element in steel, which seriously reduces the plasticity and deformation properties of steel. The lower its content, the better.

[0016] S: S is a harmful element in steel, which seriously affects the formability of steel. The lower its content, the better.

[0017] Nitrogen (N): Nitrogen is a harmful element in steel, which seriously affects the overall performance of steel. The lower its content, the better.

[0018] Ca: The addition of a small amount of calcium is due to its low price. When added to the master alloy, it can be used as a deoxidizer and inoculant, playing a role in microalloying, which can significantly refine the grains and improve the overall properties of steel, such as plasticity and weldability. At the same time, Ca has a good desulfurization effect, which can change the composition, quantity and morphology of non-metallic inclusions, improve resistance to hydrogen-induced cracking and lamellar tearing, and extend the service life of parts.

[0019] Mg: Magnesium is a good deoxidizer, desulfurizer, and spheroidizing agent in steel. Mg can reduce the number, size, and uniformity of inclusions in steel, and improve their morphology. Trace amounts of magnesium can improve the size and distribution of carbides in DH steel, promoting fine and uniform carbide particles.

[0020] Nb and Ni: These are derived from the added raw material modified niobium carbide. During the smelting process, the addition of modified niobium carbide can act as a reinforcing phase, pinning it at the grain boundaries and hindering grain growth and grain boundary migration. Through dispersion strengthening and grain refinement strengthening mechanisms, it improves the uniformity of the microstructure.

[0021] La and Ce: These are derived from the added raw materials lanthanum and cerium carbonate. During the smelting process, lanthanum and cerium carbonate are added and decomposed and reduced at the melting temperature. The generated gas can adsorb the dissolved gas in the melt and remove impurities such as oxides by floating them to the surface, thus achieving the effect of degassing and slag removal. At the same time, the reduced rare earth elements La and Ce aggregate at the grain boundaries, which can act as pinning agents, hindering the migration of grain boundaries and dislocations and improving the strength of the material.

[0022] Furthermore, the composite refining agent is composed of Ca, Mg and lanthanum and cerium carbonate.

[0023] The above-mentioned method for preparing green and low-carbon high-strength steel for automobiles includes the following steps: (1) The raw materials of the high-strength steel are mixed, smelted, cast, and cooled to obtain a continuous casting billet; (2) The continuously cast billet is rolled, coiled, pickled and cold rolled to obtain cold-rolled steel sheet; (3) High-strength steel is obtained by using continuous annealing process I or continuous annealing process II or continuous hot-dip galvanizing process I or continuous hot-dip galvanizing process II on cold-rolled steel sheets.

[0024] Furthermore, before cold rolling, the hot-rolled steel coil is treated with acid to remove the iron oxide scale on its surface; the cold rolling reduction rate is 45%-80%, otherwise, if the reduction rate is too high, the deformation resistance will be too large, making it difficult to roll to the target thickness; if the reduction rate is too low, the elongation of the cold-rolled steel sheet will decrease.

[0025] Further, the smelting temperature in step (1) is 1600-1700 ℃, and the casting temperature is 1530-1580 ℃; the initial rolling temperature in step (2) is 1000-1150 ℃, and the final rolling temperature is above 800 ℃; the coiling temperature is 350-500 ℃; the pickling uses hydrochloric acid with a mass concentration of 15-20%; and the cold rolling reduction rate is 45-80%. Further, the process parameters of the continuous annealing process I described in step (3) are as follows: belt speed control is 60-240 m / min, furnace temperature in the soaking section is 780-900 ℃, soaking time is 10-600 s, slow cooling outlet temperature is 700-750 ℃, rapid cooling rate is greater than 25 ℃ / s, rapid cooling outlet temperature is 160-300 ℃, finishing rolling force is 1500-3000 kN, and rolling tension is 500-1500 kN.

[0026] Furthermore, by adopting continuous annealing process scheme I, the microstructure of the final product is 20-40% (volume ratio) ferrite and 30-70% (volume ratio) martensite.

[0027] Further, the process parameters of the continuous annealing process II in step (3) are as follows: belt speed controlled at 60-200 m / min, annealing temperature at 760-880 ℃, soaking time at 10-600 s, slow cooling outlet temperature at 700-760 ℃, rapid cooling rate greater than 25 ℃ / s, rapid cooling temperature to 280-350 ℃, aging temperature at 280-440 ℃, aging time at 60-1000 s, and finishing elongation at 0.3%-1.0%.

[0028] Furthermore, by adopting the continuous annealing process II scheme, the microstructure of the final product is 10-30% (volume ratio) ferrite and 50-80% (volume ratio) martensite.

[0029] Further, the process parameters for continuous hot-dip galvanizing I in step (3) are as follows: the strip speed is controlled at 60-180 m / min, the annealing temperature is between 760-880 ℃, the dew point temperature is -20--10 ℃, the annealing time is 30-300 s, the slow cooling outlet temperature is 680-720 ℃, the rapid cooling rate is greater than 20 ℃ / s, and the rapid cooling outlet temperature is 450-470 ℃; the plating bath temperature is 450-470 ℃, after galvanizing, the strip is first cooled to 400-420 ℃ by air knife, and then cooled to 250-300 ℃ by air cooling, the finishing rolling force is 1500-3000kN, and the rolling tension is 500-1500kN.

[0030] Furthermore, by adopting the continuous hot-dip galvanizing scheme I, the microstructure of the final product is 20-40% (volume ratio) ferrite and 30-70% (volume ratio) martensite.

[0031] Further, the process parameters for continuous hot-dip galvanizing II in step (3) are as follows: belt speed control is 60-180 m / min, annealing temperature is 800-900 ℃, dew point temperature is -20--10 ℃, annealing time is 30-300 s, slow cooling outlet temperature is 730-760 ℃, rapid cooling rate is greater than 20 ℃ / s, rapid cooling outlet temperature is 350-420 ℃; plating bath temperature is 450-470 ℃, after galvanizing, the strip steel is first air-cooled to 450-470 ℃, then air-cooled to 250-300 ℃, and the finishing elongation is 0.3%-1.0%.

[0032] Furthermore, by adopting the continuous hot-dip galvanizing II scheme, the microstructure of the final product is 10-30% (volume ratio) ferrite and 50-80% (volume ratio) martensite.

[0033] Furthermore, the plating solution is composed of the following raw materials by mass percentage: Al: 0.16-0.25 wt%, with the balance being Zn and unavoidable impurities.

[0034] The beneficial technical effects of this invention are as follows: 1. This invention discloses a method for preparing 800-1000MPa grade cold-rolled high-strength steel using scrap steel. This method uses scrap steel as raw material and significantly reduces carbon emissions and energy consumption through a short process without adding new equipment, embodying the green and low-carbon design concept. More importantly, through precise composition design and heat treatment process control, this composition system can not only cover the strength levels from 800 to 1000MPa, but also be flexibly applied to various product forms such as cold-rolled sheet (CR) and hot-dip galvanized sheet (GI), achieving a breakthrough effect of "one steel for multiple uses". This greatly simplifies the production process, improves material utilization, and provides an innovative solution for industries such as automobiles that combines low-carbon attributes with high application versatility.

[0035] 2. This invention adds nickel-doped modified niobium carbide to high-strength steel, which, when dispersed in the iron matrix, effectively improves the strength and toughness of the steel plate, while delaying crack propagation and increasing porosity. Nickel-doped modified niobium carbide acts as a reinforcing phase, pinning it at grain boundaries and hindering grain growth and migration. Through dispersion strengthening and grain refinement mechanisms, it improves the uniformity of the microstructure. Compared to in-situ generated niobium carbide, the size, morphology, and amount of directly added niobium carbide particles are controllable, meeting the needs of different addition amounts. Furthermore, it has low dependence on heat treatment conditions and good material formability. Nickel doping modification promotes the formation of intermetallic compounds at the interface between niobium carbide and steel, improving the wettability between them, enhancing the interfacial bonding performance between niobium carbide and iron, and further improving the strength of the steel.

[0036] 3. This invention adds Ca, Mg, and lanthanum and cerium carbonate as composite refining agents to the alloy melt, which can purify the alloy melt and improve the casting quality and mechanical properties of the material. Ca, Mg, and lanthanum and cerium carbonate all have good deoxidation and desulfurization effects. Ca can act as an inoculant, providing nucleation sites during the solidification of molten steel, thereby refining grains and improving the strength of the steel. Mg can also act as a spheroidizing agent, promoting graphite spheroidization and uniform distribution, improving the alloy microstructure and enhancing alloy performance. Lanthanum and cerium carbonate undergo decomposition and reduction at the melting temperature. The generated gas can adsorb dissolved gases in the melt and remove inclusions such as oxides, achieving the effect of degassing and slag removal. At the same time, the reduced rare earth elements La and Ce aggregate at the grain boundaries, which can act as pinning agents, hindering grain boundary and dislocation migration and improving the strength of the material.

[0037] 4. This invention controls the composition of elements such as C, Mn, Si, Cr, Mo, Cu, and Ti, and processes such as hot rolling, cold rolling, continuous annealing, or continuous hot-dip galvanizing to regulate the microstructure of the material, obtaining a metallographic structure containing 20-40% ferrite and 30-70% martensite. This results in a material with high strength and high toughness, a tensile strength ≥800 MPa, an elongation after fracture of A80 ≥15%, and a porosity ≥30%. Attached Figure Description

[0038] Figure 1 A scanning electron microscope image of the modified niobium carbide prepared in Example 1 of this invention; Figure 2 This is a scanning electron microscope image of the high-strength steel prepared by continuous annealing process I in Example 1 of the present invention; Figure 3 This is a scanning electron microscope image of the high-strength steel prepared by continuous annealing process II in Example 1 of the present invention; Figure 4 This is a scanning electron microscope image of the high-strength steel prepared by continuous hot-dip galvanizing in Example 1 of the present invention; Figure 5 This is a scanning electron microscope image of the high-strength steel prepared by continuous hot-dip galvanizing II in Example 1 of the present invention; Figure 6 The mechanical curves of high-strength steel prepared by continuous annealing process I, continuous annealing process II, continuous hot-dip galvanizing process I, and continuous hot-dip galvanizing process II in Example 1 of the present invention are shown. Detailed Implementation

[0039] The following is a further detailed description of the present invention in conjunction with specific preferred embodiments, and it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention. Specific conditions not specified in the embodiments are performed according to conventional conditions or conditions recommended by the manufacturer. Unless otherwise specified, all reagents or instruments used are conventional products obtained through commercial channels.

[0040] Preparation Example Preparation Example 1 A modified niobium carbide is prepared by the following process: The niobium hydroxide, nickel hydroxide, and carbon black were mixed uniformly according to a molar ratio of 1:0.2:1.8, sintered at 1250 °C for 11 h, and then cooled to obtain modified niobium carbide. The scanning electron microscope image of the modified niobium carbide obtained in this preparation example is shown below. Figure 1 As shown.

[0041] Preparation Example 2 A modified niobium carbide is prepared by the following process: The niobium hydroxide, nickel hydroxide, and carbon black were mixed evenly according to a molar ratio of 1:0.1:1.5, sintered at 1220 °C for 12 h, and then cooled to obtain modified niobium carbide.

[0042] Preparation Example 3 A modified niobium carbide is prepared by the following process: The niobium hydroxide, nickel hydroxide, and carbon black were mixed evenly according to a molar ratio of 1:0.3:2, sintered at 1300 °C for 10 h, and then cooled to obtain modified niobium carbide.

[0043] (I) Implementation Examples Examples 1-6 Examples 1-6 provide a green, low-carbon high-strength steel for automobiles, comprising, by weight percentage: C: 0.05-0.20 wt%, Mn: 0.5-1.5 wt%, Si: 0.2-1.5 wt%, Cr: 0.02-0.60 wt%, Mo: 0.02-0.60 wt%, Cu: 0.02-0.60 wt%, Ti: 0.01-0.150 wt%, P≤0.02 wt%, S≤0.02 wt%, N≤0.01 wt%, Nb: 0.1-0.15 wt%, Ni: 0.003-0.021 wt%, Ca: 0.01-0.12 wt%, Mg: 0.01-0.15 wt%, La: 0.02-0.03 wt%, Ce: 0.02-0.03 wt%. wt%, balance being Fe and unavoidable impurities; wherein Nb and Ni are derived from the modified niobium carbide of Preparation Examples 1-3; and La and Ce are derived from the raw material composite refining agent, which is composed of Ca, Mg and lanthanum and cerium carbonate in a certain mass ratio.

[0044] The specific composition of Examples 1-6 is shown in Table 1.

[0045] Table 1. Composition and content (wt%) of high-strength steel raw materials in Examples 1-6 The specific steps of the above-mentioned preparation method for high-strength steel for green and low-carbon automobiles are as follows: (1) Smelting and continuous casting: Weigh the above-mentioned high-strength steel raw materials and mix them evenly. Smelt, cast, and cool to obtain a continuous casting billet; the smelting and casting temperatures of Examples 1-6 are shown in Table 2. (2) Hot rolling and pickling cold rolling: The continuously cast billet is rolled, coiled, pickled and cold rolled to obtain cold rolled steel sheet; the main process parameters of rolling and coiling in Examples 1-6 are shown in Table 2; the mechanical properties of the hot-rolled steel in Examples are shown in Table 3; (3) The green and low-carbon high-strength steel for automobiles can be obtained by using any one of the following processes: continuous annealing process I, continuous annealing process II, continuous hot-dip galvanizing process I, or continuous hot-dip galvanizing process II on cold-rolled steel sheets. The specific parameters of continuous annealing process I used in Examples 1-6 are shown in Table 4; the specific parameters of continuous annealing process II used in Examples 1-6 are shown in Table 5; the specific parameters of continuous hot-dip galvanizing process I used in Examples 1-6 are shown in Table 6; the specific parameters of continuous hot-dip galvanizing process II used in Examples 1-6 are shown in Table 7. In the continuous hot-dip galvanizing process I and continuous hot-dip galvanizing process II, the plating solution is a mixture of zinc and aluminum, wherein the mass percentage of aluminum is 0.16-0.25%.

[0046] Table 2 Process parameters for smelting, casting, rolling, and coiling of high-strength steel in Examples 1-6 Table 3 Mechanical properties of hot-rolled steel in the examples Table 4. Main process parameters of high-strength steels in Examples 1-6 using continuous annealing process I. Table 5. Main process parameters of high-strength steel using continuous annealing process II in Examples 1-6 of this invention. Table 6. Main process parameters for high-strength steel using continuous hot-dip galvanizing I in Examples 1-6 Table 7 Main process parameters for high-strength steel using continuous hot-dip galvanizing II in Examples 1-6 The scanning electron microscope image of the high-strength steel prepared by continuous annealing process I in Example 1 is shown below. Figure 2 As shown; the scanning electron microscope image of the high-strength steel prepared by continuous annealing process II in Example 1 is shown below. Figure 3 As shown; the scanning electron microscope image of the high-strength steel prepared by continuous hot-dip galvanizing in Example 1 is shown below. Figure 4 As shown; the scanning electron microscope image of the high-strength steel prepared by continuous hot-dip galvanizing II in Example 1 is shown below. Figure 5 As shown.

[0047] (ii) Comparative Example Comparative Example 1 Comparative Example 1 is basically the same as Example 1 using the continuous hot-dip galvanizing II process, except that lanthanum and cerium carbonate in the composite refining agent of Example 1 are omitted.

[0048] Comparative Example 2 Comparative Example 2 is basically the same as Example 1 which uses the continuous hot-dip galvanizing II process, except that lanthanum cerium carbonate in the composite refining agent of Example 1 is replaced with cerium carbonate.

[0049] Comparative Example 3 Comparative Example 3 is basically the same as Example 1 which uses the continuous hot-dip galvanizing process II, except that the modified niobium carbide in Example 1 is replaced with niobium.

[0050] Comparative Example 4 Comparative Example 4 is basically the same as Example 1 which uses the continuous hot-dip galvanizing II process, except that the modified niobium carbide in Example 1 is replaced with a mixture of niobium carbide and nickel, wherein the molar ratio of niobium carbide to nickel is 1:0.2.

[0051] (III) Test Examples The high-strength steels prepared in Examples 1-6 and Comparative Examples 1-4 were subjected to performance tests.

[0052] Mechanical property testing: The mechanical properties of the high-strength steels in Examples 1-6 and Comparative Examples 1-4 were tested according to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Room temperature test method". The results are shown in Table 8-11.

[0053] Hole expansion test: The hole expansion rate of the high-strength steels in Examples 1-6 and Comparative Examples 1-4 was tested according to GB / T 15825.4-2008 "Forming properties and test methods of sheet metal - Part 4: Hole expansion test". The results are shown in Table 8-11.

[0054] Table 8. Test results of mechanical properties and porosity of high-strength steel prepared by continuous annealing process I. Table 9. Test results of mechanical properties and porosity of high-strength steel prepared by continuous annealing process II. Table 10 Test results of mechanical properties and porosity of high-strength steel produced by continuous hot-dip galvanizing I. Table 11 Test results of mechanical properties and hole expansion rate of high-strength steel produced by continuous hot-dip galvanizing process II. The mechanical curves of high-strength steel prepared by continuous annealing process I, continuous annealing process II, continuous hot-dip galvanizing process I, and continuous hot-dip galvanizing process II in Example 1 are as follows: Figure 6 As shown.

[0055] As shown in Tables 8-11, the high-strength steels obtained in Examples 1-6 of the present invention have good mechanical properties and excellent porosity.

[0056] As shown in Table 11, compared to Example 1, Comparative Example 1 omits lanthanum and cerium carbonate in the composite refining agent, while Comparative Example 2 replaces lanthanum and cerium carbonate in the composite refining agent with cerium carbonate. This results in a significant decrease in mechanical properties and porosity, indicating that the composite refining agent prepared in this invention can improve mechanical properties and porosity. Specifically, Ca, Mg, and lanthanum and cerium carbonate in this composite refining agent all have good deoxidation and desulfurization effects. Ca can act as an inoculant, providing nucleation sites during steel solidification, thus refining grains and increasing steel strength. Mg can also act as a spheroidizing agent, promoting graphite spheroidization and uniform distribution, improving alloy structure, and enhancing alloy performance. Lanthanum and cerium carbonate undergo decomposition and reduction at the melting temperature, generating gases that adsorb dissolved gases in the melt and remove inclusions such as oxides, achieving degassing and slag removal. Simultaneously, the reduced rare earth elements La and Ce aggregate at grain boundaries, acting as pinning agents, hindering grain boundary and dislocation migration, and improving material strength.

[0057] Compared to Example 1, Comparative Example 3 replaced modified niobium carbide with niobium, and Comparative Example 4 replaced modified niobium carbide with a mixture of niobium carbide and nickel. The mechanical properties and porosity decreased significantly, indicating that the modified niobium carbide prepared in this invention can improve mechanical properties and porosity. This is because nickel-doped modified niobium carbide can act as a reinforcing phase, pinning it at grain boundaries and hindering grain growth and grain boundary migration. Through dispersion strengthening and grain refinement mechanisms, it improves the uniformity of the microstructure. Compared to in-situ generated niobium carbide, the size, morphology, and amount of directly added niobium carbide particles are controllable, meeting the needs of different addition amounts. Furthermore, it has low dependence on heat treatment conditions and good material formability. In addition, nickel doping modification can also promote the formation of intermetallic compounds at the interface between niobium carbide and steel, improving the wettability between them, enhancing the interfacial bonding performance between niobium carbide and iron, and further improving the strength of the steel.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. The basic principles and main features of the present invention have been described above with specific implementation schemes. Based on the present invention, some modifications or substitutions can be made, but these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of protection claimed by the present invention.

Claims

1. A green and low-carbon high-strength steel for automobiles, characterized in that, The high-strength steel, by weight percentage, comprises: C: 0.05-0.20 wt%, Mn: 0.5-1.5 wt%, Si: 0.2-1.5 wt%, Cr: 0.02-0.60 wt%, Mo: 0.02-0.60 wt%, Cu: 0.02-0.60 wt%, Ti: 0.01-0.150 wt%, P≤0.02 wt%, S≤0.02 wt%, N≤0.01 wt%, Nb: 0.1-0.15 wt%, Ni: 0.003-0.021 wt%, Ca: 0.01-0.12 wt%, Mg: 0.01-0.15 wt%, La: 0.02-0.03 wt%, Ce: 0.02-0.03 wt%. wt%, balance is Fe and unavoidable impurities; of which Nb and Ni are derived from raw material modified niobium carbide; La and Ce are derived from raw material composite refining agents; The modified niobium carbide is prepared by the following process: Niobium hydroxide, nickel hydroxide, and carbon black are mixed, sintered, and cooled to obtain modified niobium carbide.

2. The green and low-carbon high-strength steel for automobiles according to claim 1, characterized in that, The molar ratio of niobium hydroxide, nickel hydroxide, and carbon black is 1:(0.1-0.3):(1.5-2); the sintering temperature is 1220-1300 ℃, and the time is 10-12 h.

3. The green and low-carbon high-strength steel for automobiles according to claim 1, characterized in that, The composite refining agent is composed of Ca, Mg and lanthanum and cerium carbonate.

4. A method for preparing green, low-carbon high-strength steel for automobiles according to any one of claims 1-3, characterized in that, Includes the following steps: (1) The raw materials of the high-strength steel are mixed, smelted, cast, and cooled to obtain a continuous casting billet; (2) The continuously cast billet is rolled, coiled, pickled and cold rolled to obtain cold-rolled steel sheet; (3) The cold-rolled steel sheet is processed by continuous annealing process I or continuous annealing process II or continuous hot-dip galvanizing process I or continuous hot-dip galvanizing process II to obtain the high-strength steel.

5. The method for preparing green and low-carbon high-strength steel for automobiles according to claim 4, characterized in that, The smelting temperature in step (1) is 1600-1700 ℃, and the casting temperature is 1530-1580 ℃; the initial rolling temperature in step (2) is 1000-1150 ℃, and the final rolling temperature is above 800 ℃; the coiling temperature is 350-500 ℃; the pickling uses hydrochloric acid with a mass concentration of 15-20%; and the cold rolling reduction rate is 45-80%.

6. The method for preparing green and low-carbon high-strength steel for automobiles according to claim 4, characterized in that, The process parameters of the continuous annealing process I described in step (3) are as follows: belt speed control is 60-240 m / min, furnace temperature in the soaking section is 780-900 ℃, soaking time is 10-600 s, slow cooling outlet temperature is 700-750 ℃, rapid cooling rate is greater than 25 ℃ / s, rapid cooling outlet temperature is 160-300 ℃, finishing rolling force is 1500-3000kN, and rolling tension is 500-1500kN.

7. The method for preparing green and low-carbon high-strength steel for automobiles according to claim 4, characterized in that, The process parameters for the continuous annealing process II described in step (3) are as follows: belt speed controlled at 60-200 m / min, annealing temperature at 760-880 ℃, soaking time at 10-600 s, slow cooling outlet temperature at 700-760 ℃, rapid cooling rate greater than 25 ℃ / s, rapid cooling temperature to 280-350 ℃, aging temperature at 280-440 ℃, aging time at 60-1000 s, and finishing elongation at 0.3%-1.0%.

8. The method for preparing green and low-carbon high-strength steel for automobiles according to claim 4, characterized in that, The process parameters for continuous hot-dip galvanizing I in step (3) are as follows: the strip speed is controlled at 60-180 m / min, the annealing temperature is between 760-880 ℃, the dew point temperature is -20--10 ℃, the annealing time is 30-300 s, the slow cooling outlet temperature is 680-720 ℃, the rapid cooling rate is greater than 20 ℃ / s, and the rapid cooling outlet temperature is 450-470 ℃; the plating bath temperature is 450-470 ℃, after galvanizing, the strip is first cooled to 400-420 ℃ by air knife, and then cooled to 250-300 ℃ by air cooling, the finishing rolling force is 1500-3000kN, and the rolling tension is 500-1500kN.

9. The method for preparing green and low-carbon high-strength steel for automobiles according to claim 4, characterized in that, The process parameters for continuous hot-dip galvanizing II in step (3) are as follows: belt speed control is 60-180 m / min, annealing temperature is 800-900 ℃, dew point temperature is -20--10 ℃, annealing time is 30-300 s, slow cooling outlet temperature is 730-760 ℃, rapid cooling rate is greater than 20 ℃ / s, rapid cooling outlet temperature is 450-470 ℃; plating bath temperature is 450-470 ℃, after galvanizing, the strip steel is first air-cooled to 400-420 ℃, then air-cooled to 250-300 ℃, and the finishing elongation is 0.3%-1.0%.

10. The method for preparing green and low-carbon high-strength steel for automobiles according to claim 8 or 9, characterized in that, The plating solution consists of the following raw materials in the indicated mass percentages. composition: Al: 0.16-0.25 wt%, balance Zn and unavoidable impurities.

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