An ultrahigh-strength narrow strip steel and a method for producing the same

Abstract

The application discloses a kind of ultrahigh strength narrow strip steel and production method thereof, belong to metal material processing technical field, the method is obtained by smelting the chemical composition is according to mass percentage C:0.15~0.50%, Si:0.60~0.90%, Mn:1.50~3.50%, Cr:1.50~3.50%, Ti:0.02~0.06%, Nb:0.02~0.06%, Ni≤0.50%, Cu≤0.20%, Al:0.01~0.05%, O≤0.0008%, P≤0.015%, S≤0.010%, the balance is Fe and unavoidable impurity element molten steel, continuous casting into rectangular billet, hot rolling after heating to 1180~1230 DEG C, final rolling temperature 800~900 DEG C, immediately after rolling with the cooling rate ≥10 DEG C / s ultrafast cooling to ≤300 DEG C and directly coiling, the method adopts electric furnace+LF+VD+continuous casting short process smelting process, improves cleanliness, directly hot rolling to 1.4~15mm specification, avoid cold rolling and offline heat treatment, after rolling rapid cooling, directly obtain martensite structure, produce tensile strength high, good quality stable narrow strip steel.

Description

Technical Field

[0001] This invention relates to a type of steel and its production method, specifically to an ultra-high strength narrow strip steel and its production method, belonging to the field of metal material processing technology. Background Technology

[0002] Ultra-high strength steel has wide applications in automotive lightweighting, construction machinery, rail transportation, aerospace, and other fields. Particularly in automotive safety structural components (such as A-pillars, B-pillars, and crash beams), ultra-high strength steel can significantly improve collision safety performance and reduce vehicle weight. Ultra-high strength narrow strip steel is one type of ultra-high strength steel. Currently, ultra-high strength automotive steel mainly adopts the following two technical approaches: (1) Hot-formed steel: The steel plate is heated to the austenitic region to re-austenitize the material (usually above 900°C), and then quickly transferred to a mold with a cooling system for stamping and rapid quenching to obtain a full martensitic structure. Its disadvantages are: high energy consumption: the steel plate needs to be heated to above 900°C and kept at that temperature, resulting in significant energy consumption; slow production pace: the heating, transfer, press quenching and other processes have long cycles, and the production pace of a single piece is slow; high mold cost: specially designed molds with cooling channels are required, and the manufacturing and maintenance costs are expensive; subsequent laser trimming is required: due to the extremely high hardness of the material after hot forming (above HRC50), traditional punching cannot be processed, and laser cutting must be used for trimming and punching, which further increases the manufacturing cost and process cycle.

[0003] (2) Cold-rolled + quenched and tempered martensitic steel: Cold-rolled strip steel is continuously quenched and tempered to obtain tempered martensitic structure; its disadvantages are: it requires a special heat treatment line: the continuous quenching and tempering production line has a large investment and a large area, and most steel companies do not have it; narrow process window: parameters such as quenching temperature, cooling rate, and tempering temperature need to be precisely controlled, and slight deviations will lead to unqualified performance or poor plate shape; quenching deformation needs to be straightened: the strip steel deforms due to thermal stress and structural stress during the quenching process, and a straightening machine needs to be configured, which increases the complexity of the equipment and the difficulty of process control; poor uniformity of structure in the thickness direction: for steel plates with a thickness of more than 2mm, due to the gradient of cooling rate along the thickness direction, the core structure may not be able to obtain complete martensite, affecting the overall mechanical properties.

[0004] To address the above issues, patent CN115491593B currently... The hot-rolled thin strip steel with a tensile strength ≥1800MPa produced by the TSR production line and its manufacturing method have a tensile strength ≥1800MPa and a fully martensitic structure. However, its TSR belongs to the near-net-shape manufacturing technology route, which is fundamentally different from the technical path of this invention. Patent CN114086071B low-cost 1200MPa grade cold-rolled high-strength martensitic steel and its manufacturing method mainly adopt the addition of trace amounts of low-valence boron element to reduce the addition of Si, Mn, Cr, and Nb elements, thereby reducing alloy costs. At the same time, it adopts a lower continuous annealing heating temperature in the production process to avoid high energy consumption caused by high temperature, reduce equipment wear, and reduce production costs. However, its tensile strength needs to be improved, and it discloses coiling at 560-660℃ and points out the shortcomings of the final cooling temperature below 560℃. It cannot perform thermal history control of ultra-fast cooling (≥10℃ / s) and extremely low final cooling temperature (≤300℃), which makes its ultra-high strength not high.

[0005] Therefore, developing a method for producing ultra-high strength narrow strip steel with a width ≤450mm and a thickness of 1.4-15mm directly through an integrated "smelting-continuous casting-hot rolling-rapid cooling" route, without the need for subsequent hot forming or tempering, and obtaining a fully martensitic structure, has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] The technical problem to be solved by this invention is to address the shortcomings of the existing technology by proposing an ultra-high strength narrow strip steel and its production method. This method adopts a short-process smelting process of electric furnace + LF + VD + continuous casting to improve cleanliness, directly hot-roll to 1.4-15mm specifications, avoid cold rolling and offline heat treatment, and rapidly cool after rolling to directly obtain martensitic structure, producing narrow strip steel with high tensile strength, good elongation and stable quality.

[0007] The technical solution of this invention to solve the above technical problems is: An ultra-high strength narrow strip steel has the following chemical composition by mass percentage: C: 0.15-0.50%, Si: 0.60-0.90%, Mn: 1.50-3.50%, Cr: 1.50-3.50%, Ti: 0.02-0.06%, Nb: 0.02-0.06%, Ni≤0.50%, Cu≤0.20%, Al: 0.01-0.05%, O≤0.0008%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurity elements. The sum of the mass percentages of the above components is 100%.

[0008] This invention also designs a method for producing ultra-high strength narrow strip steel, specifically including the following steps: (1) Electric furnace smelting The process involves electric arc furnace smelting, LF refining, and VD vacuum degassing, with the tapping temperature of the electric arc furnace controlled at ≥1620℃ and the final carbon content ≥0.08% to reduce over-oxidation. LF refining adjusts composition and temperature, white residue retention time ≥40min; VD vacuum degree ≤1mbar, pressure holding time ≥12min, to achieve degassing and deoxygenation; (2) Continuous casting The molten steel obtained from smelting is continuously cast into billets. During continuous casting, the existing crystallizer is used for electromagnetic stirring and secondary cooling, and the casting speed is controlled at 0.7 to 1.2 m / min. (3) Hot rolling The billet is hot-rolled, with the heating temperature controlled at 1180~1230℃; Rolling temperature: 1080~1120℃; Final rolling temperature: 800~900℃; (4) Cooling after rolling The hot-rolled strip is immediately subjected to ultra-rapid cooling at a rate of ≥10℃ / s (from the final rolling temperature to ≤300℃). Final cooling temperature control: ≤300℃; Cooling medium: High-pressure water + mist mixture cooling, preferably with water pressure controlled at 0.5-1.5MPa; (5) Winding The strip is directly coiled at the final cooling temperature, with a coiling temperature ≤300℃. After coiling, the strip relies on its own residual heat for low-temperature self-tempering or does not require tempering to obtain ultra-high strength narrow strip steel.

[0009] The technical solution further defined in this invention is: Furthermore, in the aforementioned production method of ultra-high strength narrow strip steel, in step (2) continuous casting, molten steel is continuously cast into a rectangular billet with a continuous casting cross section of 250mm×300mm.

[0010] Furthermore, in the aforementioned production method of ultra-high strength narrow strip steel, the low magnification structure of the billet in step (2) has a center segregation of ≤1.0 grade and a center porosity of ≤1.0 grade.

[0011] Furthermore, in the aforementioned production method of ultra-high strength narrow strip steel, in step (3) hot rolling, the existing multi-stand hot continuous rolling is adopted. Preferably, the existing mill thickness ACG control system is used for high-precision automatic thickness control during hot rolling. By monitoring the strip thickness in real time, the rolling force and tension are automatically adjusted to achieve automatic closed-loop stable control of thickness.

[0012] Furthermore, in the aforementioned production method of ultra-high strength narrow strip steel, the finished ultra-high strength narrow strip steel has a thickness of 1.4 to 15 mm and a width of 150 to 450 mm.

[0013] Furthermore, in the aforementioned production method of ultra-high strength narrow strip steel, the ultra-high strength narrow strip steel obtained in step (5) has a martensitic microstructure, a grain size ≥11, a tensile strength ≥1800 MPa, a yield strength ≥1400 MPa, and an elongation after fracture ≥7%.

[0014] The beneficial effects of this invention are: (1) Integrated short process: The full martensitic structure is obtained directly through the route of "smelting → continuous casting → hot rolling → ultra-fast cooling". There is no need for hot forming mold quenching, cold rolling and special heat treatment line, which significantly reduces energy consumption and equipment investment and improves production efficiency.

[0015] (2) High strength: Current patent CN114086071A suggests that a final cooling temperature below 560℃ will lead to insufficient strength and the formation of bainite. However, this invention uses ultra-fast cooling (≥10℃ / s) combined with a high Cr and high Mn composition design to actively control the final cooling temperature at ≤300℃, obtaining a fully martensitic structure, and the tensile strength reaches over 1800MPa, breaking through the technical bias of final cooling temperature. Specifically: During the cooling process, fully austenitic steel first begins to form martensite at the Ms point (the temperature at which martensite begins to transform) and completes the transformation at the Mf point (the temperature at which martensite transformation ends). The primary factor determining the Ms and Mf points of a steel grade is the carbon content. For high-carbon steel, the Mf point may be lower than room temperature. Considering the combined effects of C (0.15-0.50%) and Mn / Cr / Ti / Nb ratios, the Mf point is below 300℃. Controlling the final cooling temperature to ≤300℃ is the optimal threshold to ensure that the Mf point has been passed before coiling (or to ensure a high martensite volume fraction in the Ms-Mf range), ensuring a fully martensitic microstructure and thus achieving ultra-high strength. Furthermore, under ultra-rapid cooling conditions, a lower final cooling temperature means a greater degree of supercooling, which helps to refine the martensite lath bundles. This is also an important guarantee for the product grain size to be ≥11.

[0016] Ultra-high strength steel is more sensitive to hydrogen embrittlement. This invention employs ultra-rapid cooling, which can suppress the precipitation of harmful carbides at grain boundaries and improve resistance to HIDF (hydrogen-induced delayed cracking). The dispersed carbides formed by Ti and Nb microalloying can act as irreversible hydrogen traps, effectively reducing the hydrogen diffusion rate. Some austenite may be retained at extremely low temperatures, which can reduce the material's hardness. The composition design and cooling rate of ≥10℃ / s in this invention significantly suppress the formation of retained austenite through a large degree of supercooling.

[0017] (3) Excellent mechanical properties: The finished product has a tensile strength ≥1800MPa, a yield strength ≥1400MPa, an elongation after fracture ≥7%, and a grain size ≥11, achieving a balance between ultra-high strength and good plasticity.

[0018] (4) Narrow specification adaptability: This invention is particularly suitable for narrow strip steel products with a width of 150-450mm and a thickness of 1.4-15mm, filling the gap in the field of ultra-high strength structural steel in the narrow strip steel field in the prior art. It can be promoted on conventional hot continuous rolling production lines and has strong adaptability.

[0019] (5) Good uniformity of structure: Due to the ultra-fast cooling to ≤300℃ after direct hot rolling, the problem of uneven structure in the thickness direction in the traditional cold rolling + quenching and tempering process is avoided, and the entire thickness is fine lath martensite. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this application clearer, a further detailed description is provided below. The described embodiments should not be considered as limitations on this application. All other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application. Unless otherwise defined, all technical and scientific terms used in the embodiments of this application have the same meaning as commonly understood by those skilled in the art. The terminology used in the embodiments of this application is for the purpose of describing the embodiments of this application only and is not intended to limit this application.

[0021] To address the problems of high energy consumption, slow production pace, high cost, narrow process window, and poor microstructure uniformity in the current production of ultra-high strength automotive steel, this paper proposes an ultra-high strength narrow strip steel and its production method. This method employs a short-process smelting process of electric furnace + LF + VD + continuous casting to improve cleanliness, directly hot-rolling to 1.4-15mm specifications, avoiding cold rolling and offline heat treatment. Rapid cooling after rolling directly yields a martensitic microstructure, with tensile strength ≥1800MPa, yield strength ≥1400MPa, and elongation ≥7%, replacing hot-formed steel and quenched and tempered martensitic steel. The finished product prepared by this method has stable overall quality. Example 1

[0022] This embodiment provides a method for producing ultra-high strength narrow strip steel, which adopts an integrated process route of "smelting-continuous casting-hot rolling-rapid cooling" to produce narrow strip steel and obtain the finished product that meets the technical requirements. The specific steps are as follows: (1) Electric furnace smelting: Electric furnace smelting, LF refining, and VD vacuum degassing are carried out in sequence. The tapping temperature of the electric furnace is controlled at 1630℃, and the final [C] is 0.09% to reduce over-oxidation. The composition of the LF refining is adjusted to meet the requirements, and the temperature is adjusted so that the white slag is held for 42 minutes. The VD vacuum degree is 0.8mbar and the holding time is 12 minutes to achieve degassing and deoxidation. (2) Continuous casting: The molten steel obtained from smelting is continuously cast into a rectangular billet with a cross section of 250mm×300mm. During continuous casting, the existing crystallizer is used for electromagnetic stirring and secondary cooling, and the casting speed is controlled at 0.8m / min. Low-magnification microstructure requirements for cast billets: center segregation grade 0.5, center porosity grade 0.5; (3) Hot rolling: The billet is heated to 1190°C, the initial rolling temperature is 1080°C, and the final rolling temperature is 820°C; Finished product thickness: 1.5mm, width: 330mm. It adopts the existing multi-stand hot continuous rolling mill and is equipped with the existing mill thickness ACG control system for high-precision automatic thickness control. By monitoring the strip thickness in real time, it automatically adjusts the rolling force and tension to achieve automatic closed-loop stable control of thickness. (4) Post-rolling cooling: After hot rolling, the strip immediately enters ultra-fast cooling and is cooled by water cooling + air mist mixed cooling. Water sprayed from the existing high-pressure nozzle and air mist sprayed from the existing air mist fan are used for cooling. The cooling rate is 12℃ / s from the final rolling temperature to 280℃. The cooling medium is 0.5MPa high-pressure water + air mist mixed cooling. (5) Coiling: Coiling is carried out directly at the final cooling temperature. The coiling temperature is 280℃. After coiling, the strip steel relies on its own residual heat to undergo low-temperature self-tempering to obtain ultra-high strength narrow strip steel. According to the test, its structure is full martensite with a grain size of 12, a tensile strength of 1910MPa, a yield strength of 1650MPa, and an elongation after fracture of 8%.

[0023] The chemical composition of the above narrow strip steel, by mass percentage, is as follows: C: 0.30%, Si: 0.70%, Mn: 2.1%, Cr: 2.6%, Ti: 0.03%, Nb: 0.03%, Ni: 0.20%, Cu: 0.05%, Al: 0.04%, O: 0.0006%, P: 0.007%, S: 0.002%, with the balance being Fe and unavoidable impurity elements. The sum of the mass percentages of the above components is 100%. Example 2

[0024] This embodiment provides a method for producing ultra-high strength narrow strip steel, which adopts an integrated process route of "smelting-continuous casting-hot rolling-rapid cooling" to produce narrow strip steel and obtain the finished product that meets the technical requirements. The specific steps are as follows: (1) Electric furnace smelting: Electric furnace smelting, LF refining, and VD vacuum degassing are carried out in sequence. The tapping temperature of the electric furnace is controlled at 1625℃, and the final [C] is 0.08% to reduce over-oxidation. The composition of the LF refining is adjusted to meet the requirements, and the temperature is adjusted so that the white slag is held for 40 minutes. The VD vacuum degree is 1 mbar and the holding time is 12 minutes to achieve degassing and deoxidation. (2) Continuous casting: The molten steel obtained from smelting is continuously cast into a rectangular billet with a cross section of 250mm×300mm. During continuous casting, the existing crystallizer is used for electromagnetic stirring and secondary cooling, and the casting speed is controlled at 0.8m / min. Low-magnification microstructure requirements for cast billets: center segregation grade 0.5, center porosity grade 0.5; (3) Hot rolling: The billet is heated to 1200℃, the initial rolling temperature is 1090℃, and the final rolling temperature is 810℃; Finished product thickness: 1.4mm, width: 320mm. It adopts the existing multi-stand hot continuous rolling mill and is equipped with the existing mill thickness ACG control system for high-precision automatic thickness control. By monitoring the strip thickness in real time, it automatically adjusts the rolling force and tension to achieve automatic closed-loop stable control of thickness. (4) Post-rolling cooling: After hot rolling, the strip immediately enters the ultra-fast cooling of water cooling + air mist mixed cooling. Water is sprayed out through existing high-pressure nozzles, and air mist is sprayed out through existing air mist blowers. The cooling rate is 11℃ / s from the final rolling temperature to 290℃. The cooling medium is 1.5MPa high-pressure water + air mist mixed cooling. (5) Coiling: Coiling is carried out directly at the final cooling temperature. The coiling temperature is 290℃. After coiling, the strip steel relies on its own residual heat to undergo low-temperature self-tempering to obtain ultra-high strength narrow strip steel. According to the test, its microstructure is full martensite with a grain size of grade 12, tensile strength of 1880MPa, yield strength of 1640MPa, and elongation after fracture of 9%.

[0025] The chemical composition of the above narrow strip steel, by mass percentage, is as follows: C: 0.29%, Si: 0.71%, Mn: 2.0%, Cr: 2.5%, Ti: 0.03%, Nb: 0.03%, Ni: 0.20%, Cu: 0.04%, Al: 0.04%, O: 0.0005%, P: 0.007%, S: 0.002%, with the balance being Fe and unavoidable impurity elements. The sum of the mass percentages of the above components is 100%.

[0026] This invention employs a short-process smelting technology of electric furnace + LF + VD + continuous casting, improving cleanliness and directly hot-rolling to 1.4-15mm specifications, avoiding cold rolling and offline heat treatment. Rapid cooling after rolling directly yields a martensitic structure, resulting in high-performance steel with tensile strength ≥1800MPa, yield strength ≥1400MPa, and elongation ≥7%. This steel replaces hot-formed steel and quenched-tempered martensitic steel, eliminating heating and mold quenching processes in hot forming, simplifying the parts manufacturing process, and reducing energy consumption. Narrow strip steel (width ≤450mm) is suitable for high-frequency welded pipes, cold-formed steel, and stamped parts, especially suitable for lightweight automotive design. Compared to hot-formed steel, this invention can reduce the overall cost of parts manufacturing by 20-30%, mainly saving on heating energy consumption, mold amortization, and subsequent trimming processes. Compared to quenched-tempered martensitic steel, it eliminates the offline heat treatment process, reducing material costs by approximately 15% and shortening the delivery cycle.

[0027] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A type of ultra-high strength narrow strip steel, characterized in that, The chemical composition of this narrow strip steel, by mass percentage, is as follows: C: 0.15–0.50%, Si: 0.60–0.90%, Mn: 1.50–3.50%, Cr: 1.50–3.50%, Ti: 0.02–0.06%, Nb: 0.02–0.06%, Ni≤0.50%, Cu≤0.20%, Al: 0.01–0.05%, O≤0.0008%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurity elements. The sum of the mass percentages of the above components is 100%.

2. The ultra-high strength narrow strip steel according to claim 1, characterized in that, The chemical composition of the narrow strip steel, by mass percentage, is as follows: C: 0.30%, Si: 0.70%, Mn: 2.1%, Cr: 2.6%, Ti: 0.03%, Nb: 0.03%, Ni: 0.20%, Cu: 0.05%, Al: 0.04%, O: 0.0006%, P: 0.007%, S: 0.002%, with the balance being Fe and unavoidable impurity elements. The sum of the mass percentages of the above components is 100%.

3. The ultra-high strength narrow strip steel according to claim 1, characterized in that, The chemical composition of the narrow strip steel, by mass percentage, is as follows: C: 0.29%, Si: 0.71%, Mn: 2.0%, Cr: 2.5%, Ti: 0.03%, Nb: 0.03%, Ni: 0.20%, Cu: 0.04%, Al: 0.04%, O: 0.0005%, P: 0.007%, S: 0.002%, with the balance being Fe and unavoidable impurity elements. The sum of the mass percentages of the above components is 100%.

4. A method for producing ultra-high strength narrow strip steel as described in any one of claims 1-3, characterized in that, Specifically, the following steps are included: (1) Electric furnace smelting The process involves electric arc furnace smelting, LF refining, and VD vacuum degassing, with the electric arc furnace tapping temperature controlled at ≥1620℃ and the final carbon content at ≥0.08%. The white slag holding time during LF refining is ≥40 min, and the VD vacuum degree is ≤1 mbar with a holding time of ≥12 min. (2) Continuous casting The molten steel obtained from smelting is continuously cast into billets. During continuous casting, electromagnetic stirring in the crystallizer and weak cooling in the secondary cooling system are used, and the casting speed is controlled at 0.7 to 1.2 m / min. (3) Hot rolling The billet is heated to 1180-1230℃, the initial rolling temperature is 1080-1120℃, and the final rolling temperature is 800-900℃; (4) Cooling after rolling The strip steel is immediately subjected to ultra-fast cooling after hot rolling, cooling from the final rolling temperature to ≤300℃ at a cooling rate of ≥10℃ / s, with the cooling medium being a mixture of high-pressure water and air mist. (5) Winding The strip is directly coiled at the final cooling temperature, with a coiling temperature ≤300℃. After coiling, the strip relies on its own residual heat for low-temperature self-tempering or does not require tempering to obtain ultra-high strength narrow strip steel.

5. The method for producing ultra-high strength narrow strip steel according to claim 4, characterized in that: In step (2), the molten steel is continuously cast into a rectangular billet with a cross section of 250mm×300mm.

6. The method for producing ultra-high strength narrow strip steel according to claim 4, characterized in that: In step (2), the low-magnification microstructure of the continuously cast billet has a center segregation of ≤1.0 grade and a center porosity of ≤1.0 grade.

7. The method for producing ultra-high strength narrow strip steel according to claim 4, characterized in that: In step (3), multi-stand hot rolling is used during hot rolling.

8. The method for producing ultra-high strength narrow strip steel according to claim 4, characterized in that: The resulting ultra-high strength narrow strip steel has a thickness of 1.4–15 mm and a width of 150–450 mm.

9. The method for producing ultra-high strength narrow strip steel according to claim 4, characterized in that: The ultra-high strength narrow strip steel obtained in step (5) has a martensitic microstructure, a grain size ≥11, a tensile strength ≥1800 MPa, a yield strength ≥1400 MPa, and an elongation after fracture ≥7%.