Method for producing a ferritic rolled ultra-low carbon bake hardening steel with an ultra-short process and a steel strip

Abstract

The application provides a method for preparing a ferrite-rolled ultra-low-carbon bake-hardening steel in an ultra-short process and a steel strip, and belongs to the technical field of metallurgy. The method completely omits the cold rolling and continuous annealing process, and comprises the following steps: heating a slab with a predetermined chemical composition; rough rolling the slab in an austenite zone to obtain an intermediate slab; finish rolling the intermediate slab in a ferrite zone, wherein the cumulative equivalent strain of the finish rolling is greater than or equal to 2.3, and a steel strip is obtained; coiling the steel strip, wherein the coiling temperature is 680-720 DEG C, and the coiling is kept for 30-60 min, and then the steel strip is cooled to room temperature to obtain a target steel strip. The method for preparing a ferrite-rolled ultra-low-carbon bake-hardening steel in an ultra-short process can directly obtain an ULCBH steel with high deep-drawing performance and high bake-hardening performance in a hot-rolled state.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, and more specifically, relates to a method for ultra-short process ferritic rolling of ultra-low carbon bake-hardening steel and the strip steel. Background Technology

[0002] Ultra-low carbon bake-hardening steel (ULC-BH) is a key material for lightweighting automobile bodies and improving dent resistance. It is widely used in deep-drawn structural parts such as doors, hoods, and fenders.

[0003] Traditional ULC-BH steel production employs a long process of "continuous casting → hot rolling → pickling → cold rolling → continuous annealing → leveling," which has the following prominent problems: 1) The process is long, energy consumption is high, and carbon emissions are large. The continuous annealing furnace accounts for a high proportion of energy consumption, and the cost of thick-gauge products remains high. 2) The production process involves many stages and a long cycle, making it difficult to control product consistency; 3) The {111} / / ND texture can only be formed by cold rolling and annealing, and high r value (plastic strain ratio) deep drawing performance cannot be obtained directly in the hot rolled state.

[0004] Existing technologies struggle to achieve a synergistic effect of high strength, high deep-drawing properties, and high bake-hardening properties while maintaining low carbon footprint, short process, and low cost. Therefore, developing a green and low-carbon ULC-BH steel preparation technology that relies solely on short-process hot rolling has significant engineering value and scientific implications. Summary of the Invention

[0005] The purpose of this invention is to provide a method and strip for ultra-short-process ferritic rolling of ultra-low carbon bake-hardening steel. By completely omitting the cold rolling and continuous annealing processes, and relying solely on ultra-short-process rolling with large strain final rolling and high-temperature coiling in the ferrite region, high deep-drawing performance and high bake-hardening performance of ULC can be directly obtained in the hot-rolled state. BH steel.

[0006] To achieve the above objectives, the technical solution adopted by this invention is: to provide a method for ultra-short process ferritic rolling of ultra-low carbon bake-hardening steel, completely omitting the cold rolling and continuous annealing processes, including the following steps: (1) Heating a slab with a predetermined chemical composition; (2) The slab is rough rolled in the austenitic region to obtain an intermediate slab; (3) The intermediate billet is rolled in the ferrite region, and the cumulative equivalent strain of the final rolling is ≥2.3, to obtain steel strip; (4) The steel strip is wound up at a temperature of 680-720°C and kept at that temperature for 30-60 minutes. Then it is cooled to room temperature to obtain the target steel strip.

[0007] In one possible implementation, the predetermined chemical composition of the slab, by mass percentage, is: C: 0.0015~0.0030%; Mn: 0.10~0.30%; Si≤0.02%; Als: 0.020~0.040%; N≤0.0030%; The balance is Fe and unavoidable impurities.

[0008] In one possible implementation, in step (1), the heating temperature of the slab is 1120 to 1180°C.

[0009] In one possible implementation, in step (2), the rough rolling temperature of the slab is 1020 to 1080°C, and the total reduction rate is 80% to 85%.

[0010] In one possible implementation, in step (3), the final rolling temperature of the intermediate billet is 760–800°C.

[0011] In one possible implementation, in step (3), the final rolling includes multiple rolling passes, and the reduction rate of the first rolling pass is not less than 50%.

[0012] The beneficial effects of the ultra-short process ferritic rolling method for ultra-low carbon bake-hardening steel provided by this invention are as follows: Compared with the prior art, the ultra-short process ferritic rolling method for ultra-low carbon bake-hardening steel of this invention adopts an ultra-short process design of multi-pass large strain final rolling and high-temperature heat preservation coiling in the ferrite region, which greatly shortens the production process, effectively reduces production energy consumption and carbon emissions, and meets the needs of green and low-carbon industrial development; the method of this invention can directly form a strong {111} / / ND texture in the hot-rolled state, so that the strip steel has both high deep drawing performance and high bake-hardening performance, while possessing excellent strength-plasticity matching characteristics and anti-aging properties.

[0013] The present invention also provides a strip steel, which is obtained by the above-described method of ultra-short process ferritic rolling ultra-low carbon bake hardening steel.

[0014] The present invention provides a strip steel that, compared with the prior art, uses the above-mentioned rolling method, which is a direct hot-rolled forming process without the need for subsequent cold rolling, continuous annealing or other additional processing. It has excellent surface quality and can be directly applied to automotive stamping. The strip steel forms a strong {111} / / ND texture inside, which has both high deep drawing performance and high bake hardening performance. It also has excellent strength-plasticity matching characteristics and anti-aging properties, resulting in excellent overall performance. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is the ODF diagram of the hot-rolled ULC-BH steel strip obtained in an embodiment of the present invention. Detailed Implementation

[0017] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0018] The present invention will now describe a method for producing ultra-short-process ferritic rolled ultra-low-carbon bake-hardening steel. This method completely omits the cold rolling and continuous annealing processes and specifically includes the following steps: (1) Heating a slab with a predetermined chemical composition.

[0019] In step (1), the predetermined chemical composition of the slab by mass percentage is: C: 0.0015~0.0030%; Mn: 0.10~0.30%; Si≤0.02%; Als: 0.020~0.040%; N≤0.0030%; the balance is Fe and unavoidable impurities.

[0020] In this step, the chemical composition ratio of the slab is based on the performance requirements of ULC-BH steel and its compatibility with the ultra-short process rolling technology, achieving deep synergy between "composition-process-performance". The mechanism of action and design basis of each chemical element are as follows: 1. C element: 0.0015~0.0030% Carbon (C) is the core element affecting the bake-hardening performance of ULC-BH steel. Ultra-low C content prevents carbide precipitation during rolling and cooling, ensuring sufficient dissolved C atoms in the steel. During subsequent baking, these dissolved C atoms undergo dislocation pinning, achieving bake-hardening. If the C content is below 0.0015%, insufficient dissolved C atoms lead to a significant decrease in the BH value, failing to meet the bake-hardening requirements for automotive steel. If the C content is above 0.0030%, fine carbide precipitates easily form, not only reducing bake-hardening performance but also hindering the nucleation and growth of the {111} / / ND texture during high-strain rolling in the ferrite region, resulting in a decrease in the r-value and deep-drawing performance. Simultaneously, ultra-low C content provides a good microstructure foundation for high-strain rolling in the ferrite region, avoiding excessively high steel strength and decreased plasticity caused by high C content, ensuring sufficient strain is introduced during rolling.

[0021] 2. Mn element: 0.10~0.30% Mn, as a solid solution strengthening element, can improve the strength of steel when added in appropriate amounts, while lowering the austenite transformation temperature and creating conditions for rolling in the ferrite region. A low Mn content design can avoid Mn segregation at grain boundaries, preventing grain boundary embrittlement and ensuring the plasticity and deep-drawing performance of the steel. If the Mn content is below 0.10%, the strengthening effect is insufficient, and the steel strength cannot meet the basic requirements for automotive steel. If the Mn content is above 0.30%, it will lead to excessive work hardening, making cracks more likely to occur during ferrite region rolling, while also inhibiting the formation of the {111} / / ND texture and reducing the deep-drawing r-value.

[0022] 3. Si element: ≤0.02% While silicon (Si) can increase the strength of steel, it significantly reduces the surface quality, leading to problems such as poor coating adhesion and blistering during subsequent painting processes. Simultaneously, Si promotes the formation of the {100} texture and inhibits the development of the {111} / / ND texture, drastically reducing deep-drawing performance. Therefore, this invention strictly controls the Si content to ≤0.02%, minimizing the negative impact of Si on surface quality and deep-drawing performance while ensuring the basic strength of the steel. This allows the product to meet automotive painting and stamping requirements without subsequent pickling and leveling processes.

[0023] 4. Al element: 0.020~0.040% The core function of Als is to fix N atoms in steel, forming fine AlN precipitates. This prevents free N atoms from pinning dislocations during room temperature storage, thus improving the steel's resistance to aging. Simultaneously, the AlN precipitates can refine grains during rolling, further enhancing the strength-ductility balance of the steel. If the Als content is below 0.020%, the N atoms in the steel cannot be completely fixed, and free N atoms will cause natural aging of the steel, leading to increased yield strength and decreased ductility. If the Als content is above 0.040%, excessive AlN precipitates will form, increasing smelting costs and hindering dislocation movement during rolling in the ferrite region, reducing nucleation sites for the {111} / / ND texture, and decreasing deep-drawing performance.

[0024] 5. N element: ≤0.0030% Nitrogen (N) is a key harmful element causing age hardening in ULC-BH steel. Free N atoms slowly diffuse and pin dislocations at room temperature, leading to increased yield strength, decreased elongation, and impaired deep-drawing properties. This invention effectively eliminates free N atoms in the steel by controlling the N content to ≤0.0030% through a low-nitrogen control process during smelting, combined with the nitrogen-fixing effect of Als.

[0025] Compared with existing technologies, the chemical composition design of this invention achieves a balance of elements, providing a chemical basis for the high strength, high bake hardening, and high deep drawing performance of ULC-BH steel. It is also highly compatible with the process of large strain rolling + high temperature coiling in the ferrite region, which is a prerequisite for the direct formation of strong {111} / / ND texture in the hot-rolled state.

[0026] In step (1), the slab is heated in a furnace at a temperature of 1120–1180°C and held for a certain period to ensure sufficient austenitization and uniform internal and external temperatures. The holding time is based on the slab thickness. When the slab thickness is 200–250 mm, the holding time is 2–3 hours. The specific adjustment logic is as follows: the holding time is extended as the slab thickness increases and appropriately shortened as the slab thickness decreases, to ensure that slabs of different thicknesses can obtain a uniform and stable heating effect.

[0027] The purpose of slab heating is to achieve homogenization of the internal structure, eliminate stress in the as-cast structure, ensure sufficient and uniform austenitization of the austenite grains, and maintain the slab's plasticity to avoid defects such as splitting and cracking during subsequent rough rolling. When the heating temperature is below 1120℃, the slab's austenitization is insufficient, resulting in poor microstructure uniformity and uneven deformation during rough rolling. When the heating temperature is above 1180℃, it leads to abnormal growth of austenite grains, forming coarse austenite grains that cannot be effectively refined during subsequent rolling, ultimately affecting the strength-ductility balance of the steel. Holding the slab at this temperature for a period of time ensures uniform temperature from the surface to the core, preventing differences in rolling deformation caused by uneven temperature.

[0028] (2) The slab is rough rolled in the austenitic region to obtain an intermediate slab.

[0029] In step (2), the rough rolling process is carried out in the austenitic region, the rolling temperature is controlled at 1020 to 1080°C, and 6 to 7 rolling passes are performed. The total reduction rate is 80% to 85%, and the slab is rolled into an intermediate slab with a thickness of 35 to 45 mm.

[0030] The purpose of this step is to break up the coarse austenite grains through high reduction rate deformation, achieving preliminary austenite grain refinement, while introducing a certain amount of deformation energy storage to provide driving force for subsequent ferrite phase transformation and grain nucleation. Controlling the rolling temperature within the austenite region ensures the plasticity of the billet and enables high reduction rate deformation. A total reduction rate of 80%–85% is crucial for achieving austenite grain refinement. If the total reduction rate is below 80%, the austenite grain refinement effect is poor, and the deformation energy storage is insufficient. If the total reduction rate is above 85%, the billet temperature will drop too quickly during rolling, prematurely entering the ferrite region and resulting in a mixed-grain structure.

[0031] (3) The intermediate billet is rolled in the ferrite region and the cumulative equivalent strain of the final rolling is ≥2.3 to obtain the target steel strip.

[0032] Step (3) is one of the core processes of the present invention. In practice, the intermediate billet after rough rolling is rolled in the ferrite region and the final rolling temperature is controlled at 760-800℃ (ferrite stable region). This process includes multiple rolling passes, with a cumulative equivalent strain ≥2.3 and the reduction rate of the first rolling pass is not less than 50%.

[0033] In this step, the final rolling temperature is 760–800℃. Within this temperature range, the steel microstructure is predominantly ferrite (ferrite volume fraction ≥90%), allowing for direct large-strain deformation of the ferrite, rather than the indirect method of "austenite deformation → phase transformation" in traditional processes. This ensures that strain acts directly on the ferrite, introducing high-density dislocations and shear bands. These high-density dislocations and shear bands provide sufficient nucleation sites and driving forces for the {111} / / ND texture, breaking the technical bottleneck of traditional processes that rely on cold rolling deformation to introduce sufficient strain. If the final rolling temperature is below 760℃, the plasticity of the ferrite decreases, and rolling cracks are prone to occur during large-strain rolling. If the temperature is above 800℃, some austenite still exists in the steel, and the deformation is mainly concentrated in the austenite, failing to introduce sufficient dislocations and shear bands into the ferrite.

[0034] In this step, the cumulative equivalent strain is ≥2.3. Through multi-pass rolling and strain superposition, the dislocation density within the ferrite is greatly increased, providing sufficient energy and sites for the nucleation of the {111} texture. If the cumulative equivalent strain is <2.3, there are insufficient nucleation sites, making it impossible to induce a large number of nucleations of the {111} / / ND texture, ultimately resulting in a low deep-drawing r-value. The core function of the traditional cold rolling process is to introduce large strain, while this invention achieves strain introduction "using heat instead of cold" through multi-pass final rolling in the ferrite region. Simultaneously, in this step, the reduction rate of the first rolling pass must not be less than 50%. A large reduction rate in the first pass can promote the formation of shear bands, further increasing the nucleation sites for the {111} texture and ensuring effective strain accumulation.

[0035] (4) The steel strip is wound up at a temperature of 680-720°C and kept at that temperature for 30-60 minutes. Then it is cooled to room temperature to obtain the target steel strip.

[0036] This step is another core process of the method of this invention. After final rolling, the strip is rapidly cooled to 680-720°C and coiled, while holding the coil at this temperature for 30-60 minutes. The purpose of rapid cooling is to prevent grain growth in the strip during the cooling process, maintain the fine-grained state of ferrite, and freeze the high-density dislocations and shear bands introduced by large-strain rolling within the ferrite. The coiling temperature of 680-720°C is within the recovery and recrystallization temperature range of ferrite. Holding the coil at this temperature allows for the recovery and online recrystallization of ferrite using the residual heat during the coiling process, eliminating the need for an additional continuous annealing furnace to provide heat.

[0037] In this step, setting the coiling temperature to 680–720℃ and the holding time to 30–60 minutes ensures sufficient recrystallization of ferrite, allowing the high-density dislocations introduced by large-strain rolling to slip and climb, forming new dislocation-free recrystallized grains. Simultaneously, during recrystallization, the nuclei of the {111} / / ND texture preferentially grow, gradually forming a strong {111} / / ND texture, while the {100} and {110} textures, which are detrimental to deep-drawing performance, are suppressed. If the coiling temperature is below 680℃, the recrystallization rate of ferrite is too slow, and sufficient recrystallization cannot be completed within the holding time, dislocations cannot be effectively eliminated, the internal stress of the steel is too high, and plasticity decreases. If the temperature is above 720℃, the recrystallized ferrite grains will grow abnormally, leading to a decrease in steel strength. If the holding time is less than 30 minutes, recrystallization is insufficient, and the texture strengthening effect is poor. If it is longer than 60 minutes, it will increase the production cycle, reduce production efficiency, and easily lead to grain growth.

[0038] This step utilizes the residual heat from coiling to achieve online recrystallization of ferrite, completely replacing the continuous annealing process in traditional technology and saving energy consumption in the annealing furnace; at the same time, it achieves preferential growth of the {111} / / ND texture, providing the steel with high r-value deep drawing performance.

[0039] In this step, after high-temperature coiling, the steel coil is naturally air-cooled to room temperature without the need for additional cooling equipment or heat treatment processes, directly yielding usable hot-rolled ULC-BH steel. During air cooling, the steel temperature decreases slowly, internal stress is further released, and the dissolved carbon atoms remain in a stable solid solution state without precipitation, ensuring the steel's bake-hardening properties.

[0040] This invention provides a method for ultra-short process ferritic rolling of ultra-low carbon bake-hardening steel. Compared with the prior art, the method adopts an ultra-short process design of multi-pass large strain final rolling in the ferrite region and high-temperature heat preservation coiling, which significantly shortens the production process, effectively reduces production energy consumption and carbon emissions, and meets the needs of green and low-carbon industrial development. The method of this invention can directly form a strong {111} / / ND texture in the hot-rolled state, so that the strip steel has both high deep drawing performance and high bake-hardening performance, while also possessing excellent strength-plasticity matching characteristics and anti-aging properties.

[0041] Example 1. Slab chemical composition ratio: by mass fraction, C: 0.0022%, Mn: 0.18%, Si: 0.01%, Als: 0.030%, N: 0.0025%, with the balance being Fe and unavoidable impurities. Slab thickness: 220±2mm.

[0042] 2. Slab rolling process steps: (1) Slab heating: heating temperature 1150℃, holding time 2.5h; (2) Austenitic region rough rolling: The rolling temperature is 1050℃, and after 6 passes of rolling, the total reduction rate is 82%, and the intermediate billet with a thickness of 35±1mm is rolled. (3) Large strain final rolling in ferrite region: The final rolling temperature is 800℃. After 7 passes of rolling, the reduction rate of the first pass is 50%, and the reduction rate of the remaining passes is about 23%, with a cumulative equivalent strain of 2.3. (4) High-temperature winding: After final rolling, the temperature is rapidly cooled to 720℃ and then wound at that temperature for 45 minutes; (5) Air cooling to room temperature: The steel coil is naturally air cooled to room temperature to obtain hot-rolled ULC-BH steel strip with a thickness of 3.5±0.1mm.

[0043] 3. Performance test results: Yield strength 215MPa, tensile strength 335MPa, elongation 43%, deep drawing r value 2.1, bake hardening BH value 52MPa. Figure 1 This is the ODF plot of the steel strip. The texture poles in the plot are clearly clustered, indicating that a strong {111} / / ND texture has formed inside the steel body, with a texture volume fraction ≥70%.

[0044] The strip steel prepared in the above embodiments has excellent deep drawing performance and bake hardening performance, meeting the requirements for automotive steel, thus proving the stability and reliability of the technical solution of the present invention.

[0045] The present invention also provides a strip steel, which is obtained by the above-described method of ultra-short process ferritic rolling ultra-low carbon bake hardening steel.

[0046] The present invention provides a strip steel that, compared with the prior art, is directly formed in a hot-rolled state without the need for subsequent cold rolling, continuous annealing or other additional processing. It has excellent surface quality and can be directly applied to automotive stamping. The strip steel forms a strong {111} / / ND texture inside, which has both high deep drawing performance and high bake hardening performance. It also has excellent strength-plasticity matching characteristics and anti-aging properties, and has excellent comprehensive performance.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for ultra-short process ferritic rolling of ultra-low carbon bake-hardening steel, characterized in that, Includes the following steps: (1) Heating a slab with a predetermined chemical composition; (2) The slab is rough rolled in the austenitic region to obtain an intermediate slab; (3) The intermediate billet is rolled in the ferrite region, and the cumulative equivalent strain of the final rolling is ≥2.3, to obtain steel strip; (4) The steel strip is wound up at a temperature of 680-720°C and kept at that temperature for 30-60 minutes. Then it is cooled to room temperature to obtain the target steel strip. The method completely omits the cold rolling and continuous annealing processes.

2. The method for ultra-short process ferritic rolling of ultra-low carbon bake-hardening steel as described in claim 1, characterized in that, The predetermined chemical composition of the slab, by mass percentage, is: C：0.0015～0.0030%； Mn: 0.10~0.30%; Si≤0.02%; Als: 0.020~0.040%; N≤0.0030%； The balance is Fe and unavoidable impurities.

3. The method for ultra-short process ferritic rolling of ultra-low carbon bake-hardening steel as described in claim 1, characterized in that, In step (1), the heating temperature of the slab is 1120 to 1180°C.

4. The method for ultra-short process ferritic rolling of ultra-low carbon bake-hardening steel as described in claim 1, characterized in that, In step (2), the rough rolling temperature of the slab is 1020-1080℃, and the total reduction rate is 80%-85%.

5. The method for ultra-short process ferritic rolling of ultra-low carbon bake-hardening steel as described in claim 1, characterized in that, In step (3), the final rolling temperature of the intermediate billet is 760-800℃.

6. The method for ultra-short process ferritic rolling of ultra-low carbon bake-hardening steel as described in claim 1, characterized in that, In step (3), the final rolling includes multiple rolling passes, and the reduction rate of the first rolling pass is not less than 50%.

7. A strip steel, characterized in that, Prepared by the method described in any one of claims 1-6.

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