Method for producing a coarse-grained ferritic strip steel and coarse-grained ferritic strip steel

Abstract

The application provides a preparation method of a coarse-grained ferrite strip steel and the coarse-grained ferrite strip steel and belongs to the technical field of iron and steel metallurgy, and comprises the following steps: preparing an ultra-low-carbon steel casting blank, rough rolling after heating of the ultra-low-carbon steel casting blank, completion of the rough rolling in the full austenite zone; rapid cooling to the ferrite single-phase zone, fine rolling in the ferrite single-phase zone, a final rolling temperature of 780-820 DEG C; a high-temperature coiling temperature of 680-720 DEG C; cold rolling and annealing after online pickling of the hot-rolled plate to obtain the coarse-grained ferrite strip steel. The preparation method of the coarse-grained ferrite strip steel provided by the application combines the four processes of final rolling in the low-temperature ferrite zone, high-temperature coiling, cold rolling and accurate annealing to obtain a coarse-grained low-strength product, and the problem of easy breaking and serious die wear of the ultra-low-carbon steel during drawing is solved.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and more specifically, it relates to a method for preparing coarse-grained ferritic strip steel and the coarse-grained ferritic strip steel. Background Technology

[0002] Ultra-low carbon ferritic steels are widely used in automobile manufacturing, electronic devices, and pressure vessels due to their excellent plasticity, toughness, and weldability. However, existing ultra-low carbon ferritic cold-rolled annealed strip steels and wires generally suffer from high yield strength (typically ≥220MPa) and high elastic modulus, which makes them prone to fracture during subsequent drawing processes and accelerates die wear (average life is only 1 / 3-1 / 2 of conventional materials), severely restricting their application in high-deformation processing fields.

[0003] From the perspective of materials science principles, the yield strength of metallic materials is closely related to grain size, conforming to the Hall-Petch relationship. This relationship indicates that the smaller the grain size, the higher the yield strength. In conventional production processes, ultra-low carbon steel often undergoes austenitic final rolling (final rolling temperature 860-950℃), resulting in fine ferrite grains (typically ≤20μm) formed through austenite-to-ferrite phase transformation after rolling. This leads to a relatively high yield strength, making it difficult to meet the processing requirements of low strength and high plasticity. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing coarse-grained ferritic strip steel and the coarse-grained ferritic strip steel, which aims to reduce the yield strength of ultra-low carbon ferritic steel to solve the problems of high strength and high elastic modulus of existing ultra-low carbon ferritic cold-rolled annealed steel plates and wires, which lead to easy breakage during drawing and severe wear of dies.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is: to provide a method for preparing coarse-grained ferritic strip steel, comprising the following steps:

[0006] S1. Preparation of ultra-low carbon steel billets;

[0007] S2. Hot rolling treatment of ultra-low carbon steel billets, including:

[0008] S21. Heat the ultra-low carbon steel billet to 1150-1250℃ and hold it at that temperature for 2-4 hours;

[0009] S22. Rough rolling is performed on the ultra-low carbon steel billet when it is in the full austenitic region to obtain rough rolled strip. The rough rolling start temperature is 1000-1100℃.

[0010] S23. Rapid cooling treatment is applied to the rough-rolled strip to allow it to enter the single-phase ferrite region.

[0011] S24. The rough-rolled strip is finished rolled to obtain a finished strip. The finishing rolling temperature is 780-820℃.

[0012] S25, fine-rolled strip steel coiling, coiling temperature is 680-720℃;

[0013] S3. The precision-rolled strip is cold-rolled to obtain cold-rolled strip, with a total deformation of 60-90%.

[0014] S4. Anneal the cold-rolled strip to obtain coarse-grained ferritic strip.

[0015] In one possible implementation, the chemical composition of the ultra-low carbon steel billet, by mass percentage, includes: C < 0.02%, Si < 6%, Mn < 0.5%, P < 0.01%, S < 0.01%, with the balance being Fe.

[0016] In one possible implementation, in step S1, the ultra-low carbon steel billet is obtained by sequentially passing through blast furnace ironmaking, converter steelmaking, LF refining, RH / VD refining, and continuous casting.

[0017] In one possible implementation, in step S21, the heating rate of the ultra-low carbon steel billet is 10-15℃ / s.

[0018] In one possible implementation, in step S23, the rough-rolled strip is rapidly cooled using laminar flow cooling or high-pressure water jet cooling at a rate of 5-10°C / s.

[0019] In one possible implementation, in step S24, the rough-rolled strip is subjected to 4-6 passes of finishing rolling, with the reduction rate decreasing in each pass and the total reduction rate being 50-70%.

[0020] In one possible implementation, the finished strip is subjected to centralized cooling at the rear end before step S25.

[0021] In one possible implementation, in step S4, a continuous annealing furnace is used for annealing. The operating parameters include: belt speed 100-120m / min, heating rate 5-10℃ / s, soaking zone temperature 800-850℃, soaking zone holding time 50-100s, and after cooling in the furnace to 400-500℃, the belt is wound up and then air-cooled after exiting the furnace.

[0022] In one possible implementation, in step S4, a bell-type annealing furnace is used for annealing treatment. The operating parameters include: heating rate of 1-2℃ / s, soaking temperature of 700-750℃, holding time of 10-15h, and after cooling in the furnace to 400-500℃, the furnace is removed and air-cooled.

[0023] The beneficial effects of the method for preparing coarse-grained ferritic strip steel provided by this invention are as follows: Compared with the prior art, the method for preparing coarse-grained ferritic strip steel of this invention firstly provides a "heritable coarse-grained rudimentary form" for subsequent processes through low-temperature ferrite zone final rolling, and avoids fine-grained phase transformation in the austenite zone final rolling by phase transformation control, directly obtaining ferrite deformation structure and laying the foundation for coarse grains; then, the coarse-grained characteristics of the hot-rolled state are amplified by high-temperature coiling, and the "coarse-grained rudimentary form" is developed into a stable coarse ferrite group through recrystallization and grain growth during high-temperature coiling. The process involves four steps: First, hot rolling directly reduces the strength of the hot-rolled material, providing a suitable "genetic matrix" for cold rolling. Next, cold rolling preserves this genetic information, using the original coarse grain boundaries as the deformation core to prevent microstructure refinement. This "writes" the coarse-grained characteristics of hot rolling into the cold-rolled structure, while simultaneously accumulating the deformation energy required for directional nucleation. Finally, precise annealing activates and amplifies the genetic effect, utilizing the deformation energy stored in cold rolling to ensure that annealing recrystallization preferentially occurs along the original coarse grain boundaries. Ultimately, the coarse-grained characteristics of hot rolling are "replicated" and strengthened to the annealed state, achieving a balance between low yield strength and high ductility and toughness. By combining these four processes, a coarse-grained, low-strength product is obtained, solving the problems of easy breakage during drawing and severe die wear in ultra-low carbon steel.

[0024] The present invention also provides a coarse-grained ferritic strip steel, which is prepared by the above-described method for preparing coarse-grained ferritic strip steel.

[0025] The beneficial effects of the coarse-grained ferritic strip steel provided by the present invention are as follows: compared with the prior art, the yield strength is significantly reduced, the elongation is more than 45%, and it has both low strength and high plasticity. It can avoid drawing fracture, improve the forming limit during deep drawing, and reduce defects such as wrinkling. Attached Figure Description

[0026] 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.

[0027] Figure 1 A flowchart illustrating a method for preparing coarse-grained ferritic strip steel according to an embodiment of the present invention;

[0028] Figure 2 Metallographic diagram of the hot-rolled plate sample provided in Embodiment 1 of the present invention;

[0029] Figure 3 Metallographic diagram of the hot-rolled plate sample provided in Embodiment 2 of the present invention;

[0030] Figure 4 The metallographic structure of the hot-rolled plate sample provided in Comparative Example 1 of the present invention is shown in the figure.

[0031] Figure 5 The metallographic structure of the annealed plate sample provided in Embodiment 1 of the present invention is shown in the figure.

[0032] Figure 6 The metallographic structure of the annealed plate sample provided in Embodiment 2 of the present invention is shown in the figure.

[0033] Figure 7 The image shows the metallographic structure of the annealed plate sample provided in Comparative Example 1 of the present invention. Detailed Implementation

[0034] 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.

[0035] Please see Figure 1 The present invention will now describe a method for preparing coarse-grained ferritic strip steel. The method for preparing coarse-grained ferritic strip steel includes the following steps:

[0036] S1. Preparation of ultra-low carbon steel billets.

[0037] In this step, the chemical composition of the ultra-low carbon steel billet is designed as follows: C < 0.02%, Si < 6%, Mn < 0.5%, P < 0.01%, S < 0.01%, with the balance being Fe. The ultra-low carbon composition is fundamental to ensuring the steel has extremely low yield strength and excellent plasticity. Excessive carbon content increases the strength of the steel; a carbon content ≤ 0.02% minimizes the solid solution strengthening effect of carbon in ferrite. Controlling the content of elements such as silicon and manganese at low levels reduces their solid solution strengthening effect while ensuring sufficient hardenability to achieve a full ferrite phase transformation. Phosphorus and sulfur are harmful elements and must be strictly controlled to avoid grain boundary embrittlement.

[0038] In this step, the production process of ultra-low carbon steel billets includes blast furnace ironmaking → converter steelmaking → LF refining → RH / VD refining → continuous casting. The above-mentioned production process of ultra-low carbon steel billets is a conventional technical means for preparing ultra-low carbon steel, and will not be described in detail here.

[0039] S2. Hot rolling treatment of ultra-low carbon steel billets, including:

[0040] S21. Heat the ultra-low carbon steel billet to 1150-1250℃ and hold it for 2-4 hours. In this step, an electric heating furnace is used to heat the ultra-low carbon steel billet at a heating rate of 10-15℃ / s, and the temperature difference between different areas in the furnace is ≤±20℃. In this step, when the ultra-low carbon steel billet is heated to a temperature range of 1150-1250℃, it enters the fully austenitic region, and its microstructure becomes completely austenitic. Too low a heating temperature will result in incomplete austenitization, with undissolved carbides remaining, affecting subsequent phase transformations. Too high a temperature may lead to excessive grain growth, increasing rolling difficulty. The holding time needs to be adjusted between 2-4 hours based on the billet thickness. Holding ensures uniform austenitic grain growth and the accumulation of deformation energy, laying a good foundation for subsequent rolling deformation and phase transformation. In this step, a heating rate that is too slow will result in coarse and uneven grains, while a rate that is too fast will easily cause thermal stress cracks.

[0041] S22. Rough rolling is performed on the ultra-low carbon steel billet while it is in the fully austenitic region to obtain rough-rolled strip. The initial rolling temperature is 1000-1100℃. In this step, 3-5 rolling passes can be used, with a total reduction rate of 70%-80%, ultimately yielding a rough-rolled strip with a thickness of 20-30mm. The main purpose of rough rolling is to reduce the billet thickness through plastic deformation and simultaneously break up the casting structure, providing suitable billet size and microstructure for subsequent finish rolling. Rough rolling in the fully austenitic region can utilize the good plasticity of austenite to reduce rolling force and avoid crack formation.

[0042] S23. Rapid cooling treatment is applied to the rough-rolled strip to bring it into the single-phase ferrite region. In this step, laminar flow cooling or high-pressure water jet cooling can be used for rapid cooling of the rough-rolled strip at a cooling rate of 5-10℃ / s, enabling the strip to enter the single-phase ferrite region quickly and achieve the austenite-to-ferrite phase transformation. In practical applications, the temperature at which ultra-low carbon steel billets with different chemical compositions enter the single-phase ferrite region varies slightly. In this step, based on the ultra-low carbon steel billet provided by this invention, when the rough-rolled strip is cooled to 760-830℃, it is in the single-phase ferrite region. Rapid cooling of the rough-rolled strip in this step can suppress the precipitation of harmful phases such as pearlite, promote the isothermal transformation of austenite to ferrite, and ensure that the rough-rolled strip before finishing rolling is a fully ferrite structure. In this step, insufficient cooling or a slow cooling rate can lead to some austenite residue, affecting the microstructure control during the finishing rolling stage.

[0043] S24. The rough-rolled strip is finished rolled to obtain a finished strip. The finishing rolling temperature is 780-820℃. In this step, the rough-rolled strip is finished rolled in 4-6 passes, with the reduction rate decreasing in each pass. Optionally, the reduction rate in the first pass is 25%-30%, the reduction rate in the last pass is 10%-15%, and the total reduction rate is 50-70%. Ferrite has significantly lower low-temperature deformation resistance than austenite. Finishing in the ferrite region can significantly reduce rolling stress, reduce energy consumption, and protect equipment. Controlling the finishing rolling temperature in the high-temperature range of the ferrite region (780-820℃) can avoid a sudden increase in deformation resistance of ferrite due to excessively low temperature and can also conserve energy for subsequent grain growth.

[0044] S25. Coiling of the finished strip at a temperature of 680-720℃. In this step, the finished strip is coiled at a high temperature of 680-720℃, significantly higher than the conventional coiling temperature for ultra-low carbon steel (550-650℃). High-temperature coiling provides sufficient diffusion energy to the ferrite, allowing dislocations generated during the finishing rolling process to annihilate (recrystallize). Simultaneously, it promotes the merging and growth of adjacent ferrite grains through grain boundary migration, further reducing the yield strength of the finished strip. Compared to conventional coiling temperatures, the significantly increased coiling temperature in this step provides sufficient growth driving force for the ferrite grains, ensuring a coarse and uniform microstructure.

[0045] In practice, the temperature of the finished strip after finishing rolling is 780-820℃ (finishing temperature in the ferrite region), while the target temperature for high-temperature coiling is 680-720℃. To obtain a suitable coiling temperature, the finished strip undergoes centralized cooling at the rear end before this step. This centralized cooling refers to cooling the end of the finished strip closest to the coiling equipment. Through rapid and uniform cooling methods (such as centralized water spraying or laminar flow cooling), the temperature of the finished strip can be accurately reduced from the finishing rolling temperature to the preset coiling temperature range in a short time, enabling immediate coiling after cooling. This ensures that the strip is in a high-temperature window that promotes ferrite recrystallization and grain growth during coiling.

[0046] S3. The finished strip is cold-rolled to obtain cold-rolled strip with a total deformation of 60-90%. Before this step, the finished strip needs to be pickled online on a continuous pickling line to remove surface iron oxide scale before cold rolling. This avoids surface defects caused by oxide scale indentation during cold rolling. In this step, the purpose of cold rolling the finished strip is to obtain the required thickness through plastic deformation while retaining the genetic characteristics of the coarse-grained structure of hot rolling. Specifically, the coarse ferrite grain boundaries of the finished strip become the core carrier of deformation energy storage during the cold rolling stage, and dislocations accumulate at the grain boundaries to form a non-uniformly distributed deformation energy storage. By controlling the deformation amount within the range of 60-90%, the original grain boundary network can be prevented from being completely destroyed, the genetic characteristics of the "coarse grain framework" can be preserved, and the coarse original grain boundaries can be ensured to act as "deformation energy reservoirs" during plastic deformation, providing nucleation sites for subsequent annealing and recrystallization. This ensures that the new grains formed by subsequent annealing preferentially nucleate along the original grain boundaries rather than randomly, thus ensuring the continuity of the microstructure.

[0047] S4. Anneal the cold-rolled strip to obtain coarse-grained ferritic strip. In this step, a continuous annealing furnace or a bell-type annealing furnace can be used for the annealing process.

[0048] When using a continuous annealing furnace for annealing, the annealing parameters can be designed as follows: belt speed 100-120m / min, heating rate 5-10℃ / s, soaking zone temperature 800-850℃, soaking zone holding time 50-100s, and after cooling in the furnace to 400-500℃, the belt is wound up and then air-cooled after exiting the furnace.

[0049] When using a bell-type annealing furnace for annealing, the annealing parameters can be designed as follows: heating rate 1-2℃ / s, soaking temperature 700-750℃, holding time 10-15h, cooling in the furnace to 400-500℃, and then air cooling after exiting the furnace.

[0050] The purpose of this step is to abandon the traditional annealing goal of grain refinement and instead, by precisely controlling the heating rate and homogenization parameters, utilize the deformation energy stored at the original grain boundaries during the cold rolling stage to guide new grains to preferentially nucleate along the original coarse grain boundaries. By employing slow heating (heating rate 1-2℃) in bell-type annealing or precise homogenization (homogenization temperature 700-750℃) in continuous annealing, a time window is provided for grains to merge and grow across the original grain boundaries, ultimately forming uniform and coarse equiaxed ferrite grains.

[0051] By fully recrystallizing and annealing, not only is the work hardening caused by cold rolling eliminated, but also the favorable {111} surface texture is developed, improving the deep drawing performance of the material. At the same time, the coarse grains further reduce the yield strength through the Hall-Petch relationship.

[0052] The present invention provides a method for preparing coarse-grained ferritic strip steel. Compared with the prior art, it constructs a performance closed loop of "coarse-low strength-high plasticity" by combining "low-temperature ferritic zone final rolling, high-temperature coiling, cold rolling and precision annealing". In this method, the above four processes are not simply superimposed, but achieve synergistic effect through "organic genetic chain".

[0053] First, low-temperature ferrite final rolling provides a "heritable coarse-grained prototype" for subsequent processes. Phase transformation control avoids the fine-grained phase transformation of the austenite region during final rolling, directly obtaining a ferrite deformed structure and laying the foundation for coarse grains. Then, high-temperature coiling amplifies the coarse-grained characteristics of the hot-rolled state. During high-temperature coiling, recrystallization and grain growth develop the "coarse-grained prototype" into a stable coarse ferrite structure, directly reducing the strength of the hot-rolled material and providing a qualified "genetic matrix" for cold rolling. Next, cold rolling preserves the genetic information, using the original coarse grain boundaries as the deformation core to avoid microstructure refinement. The hot-rolled coarse-grained characteristics are "written" into the cold-rolled structure, while accumulating the deformation energy required for directional nucleation. Finally, precise annealing activates and amplifies the genetic effect. Utilizing the deformation energy stored in cold rolling, annealing recrystallization preferentially proceeds along the original coarse grain boundaries, ultimately "replicating" and strengthening the hot-rolled coarse-grained characteristics to the annealed state, achieving a balance between low yield strength and high ductility and toughness. By combining the above four processes, a coarse-grained, low-strength product is finally obtained, solving the problems of easy breakage during drawing of ultra-low carbon steel and severe mold wear.

[0054] Example 1

[0055] A method for preparing coarse-grained ferritic strip steel, comprising the following steps:

[0056] 1. Preparation of ultra-low carbon steel billets: The composition of the ultra-low carbon steel billets is: C content 0.01%, Si content 0.5%, Mn content 0.5%, P and S content less than 0.01%, and the remaining element is Fe. The production process of ultra-low carbon steel billets is: blast furnace ironmaking → converter steelmaking → LF refining → RH refining → continuous casting.

[0057] 2. Heating and rough rolling: The ultra-low carbon steel billet is heated to 1250℃ and held for 4 hours. The rough rolling temperature is 1100℃, and it is rolled to 10mm in 5 passes to obtain rough rolled strip steel.

[0058] 3. Rapid cooling: Utilizing an ultra-fast cooling (laminar water) system between the finishing mill and the roughing mill, the temperature of the rough-rolled strip is rapidly reduced to 815±5℃ before entering the finishing mill, so that the temperature of the rough-rolled strip is in the single-phase ferrite phase region, achieving a complete γ→α phase transformation, with a rapid cooling rate of 8℃ / s.

[0059] 4. Finishing rolling: Finishing rolling is carried out in the ferrite region, and is performed in 5 passes with reduction rates of 28%, 25%, 20%, 15%, and 12% respectively, for a total reduction rate of 65%. The finishing rolling temperature is 800℃ to obtain a finished strip with a thickness of 3.5mm.

[0060] 5. Coiling: Between the end of the finishing rolling and the coiling, the finished strip is cooled using a centralized cooling mode at the rear end, and coiled at 700℃.

[0061] 6. Cold rolling: After pickling, the finished strip steel is cold rolled to 0.5mm with a total deformation of 86% to obtain cold-rolled strip steel.

[0062] 7. Continuous annealing: The cold-rolled strip is sent to a continuous annealing furnace for annealing treatment. The design parameters are: strip speed 100m / min, heating rate 7℃ / s, soaking zone temperature 810℃, soaking zone holding time 70s, and then the strip is cooled to 400℃ in the furnace and coiled. It is then air-cooled after exiting the furnace to obtain coarse-grained ferritic strip.

[0063] Example 2

[0064] A method for preparing coarse-grained ferritic strip steel, comprising the following steps:

[0065] 1. Preparation of ultra-low carbon steel billets: The composition of the ultra-low carbon steel billets is: C content 0.008%, Si content 0.3%, Mn content 0.4%, P and S content less than 0.01%, and the remaining element is Fe. The production process of ultra-low carbon steel billets is: blast furnace ironmaking → converter steelmaking → LF refining → RH refining → continuous casting.

[0066] 2. Heating and rough rolling: The ultra-low carbon steel billet is heated to 1100℃ and held for 1 hour. The rough rolling temperature is 1000℃, and it is rolled to 6.5mm in 4 passes to obtain rough rolled strip steel.

[0067] 3. Rapid cooling: Utilizing an ultra-fast cooling (laminar water) system between the finishing mill and the roughing mill, the temperature of the rough-rolled strip is rapidly reduced to 795±5℃ before entering the finishing mill, so that the temperature of the rough-rolled strip is in the single-phase ferrite phase region, achieving a complete γ→α phase transformation, with a rapid cooling rate of 8℃ / s.

[0068] 4. Finishing rolling: Finishing rolling is carried out in the ferrite region and is performed in 4 passes with reduction rates of 30%, 25%, 18%, and 10% respectively, for a total reduction rate of 60%. The finishing rolling temperature is 780℃ to obtain a finished strip with a thickness of 2.5mm.

[0069] 5. Coiling: Between the end of finishing rolling and coiling, the finished strip is cooled using a centralized cooling mode at the rear end, and coiled at 680℃.

[0070] 6. Cold rolling: After pickling, the finished strip steel is cold rolled to 0.5mm with a total deformation of 80% to obtain cold-rolled strip steel.

[0071] 7. Continuous annealing: The cold-rolled strip steel is sent to a bell-type annealing furnace for annealing treatment. The design parameters are: heating rate 1.5℃ / s, soaking temperature 730℃, holding time 13h, and then air cooling after cooling to 420℃ in the furnace to obtain coarse-grained ferritic strip steel.

[0072] Comparative Example 1

[0073] The following steps are used to prepare ultra-low carbon ferritic steel using a conventional austenitic zone final rolling process:

[0074] 1. The ingredients are the same as in Example 1.

[0075] 2. Hot rolling: The billet is heated to 1200℃ and held for 3 hours; both roughing and finishing rolling are carried out in the austenitic region, with a final rolling temperature of 900℃; the coiling temperature is 600℃ to obtain a hot-rolled plate with a thickness of 4mm.

[0076] 3. The cold rolling and annealing processes are the same as in Example 1.

[0077] Samples were taken from Examples 1, 2, and Comparative Example 1 after hot rolling to obtain corresponding hot-rolled plate samples; samples were taken from Examples 1, 2, and Comparative Example 1 after annealing to obtain corresponding annealed plate samples. The hot-rolled and annealed plate samples from Examples 1, 2, and Comparative Example 1 were etched using 4% nitric acid alcohol, and the ferrite grain morphology of the samples was observed using an optical microscope. Figures 2-7 As shown in the metallographic images of the hot-rolled plate sample and the annealed plate sample, the grain size of Example 1 and Example 2 is significantly larger than that of Comparative Example 1.

[0078] Tensile tests were conducted on the coarse-grained ferritic strip steel samples obtained in Examples 1 and 2, as well as the strip steel sample obtained in Comparative Example 1, using a universal testing machine. The tensile strength, yield strength, and elongation were obtained, and the test results are shown in Table 1.

[0079] Table 1

[0080]

[0081] As can be seen from the comparison, the coarse-grained ferritic strip steels of Examples 1 and 2 of the present invention are superior to those of Comparative Example 1 in terms of yield strength, tensile strength and elongation, which fully demonstrates the superiority of the technology of the present invention.

[0082] The present invention also provides a coarse-grained ferritic strip steel, which is prepared by the above-described method for preparing coarse-grained ferritic strip steel.

[0083] The beneficial effects of the coarse-grained ferritic strip steel provided by the present invention are as follows: compared with the prior art, the yield strength is significantly reduced, the elongation is more than 45%, and it has both low strength and high plasticity. It can avoid drawing fracture, improve the forming limit during deep drawing, and reduce defects such as wrinkling.

[0084] 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 of producing a coarse ferrite strip steel, characterized by, The method comprises the following steps: S1, preparing an ultra-low carbon steel casting blank, the chemical composition of the ultra-low carbon steel casting blank comprises, in mass percentage, C < 0.02%, Si < 6%, Mn < 0.5%, P < 0.01%, S < 0.01%, and the balance being Fe; S2, performing hot rolling treatment on the ultra-low carbon steel casting blank, comprising: S21, heating the ultra-low carbon steel casting blank to 1150-1250 ℃ and maintaining for 2-4 h, the heating rate of the ultra-low carbon steel casting blank being 10-15 ℃ / s; S22, performing rough rolling on the ultra-low carbon steel casting blank when the ultra-low carbon steel casting blank is in the full austenite region, to obtain a rough rolling strip, the rough rolling temperature being 1000-1100 ℃; S23, performing fast cooling treatment on the rough rolling strip to make the rough rolling strip enter the single-phase ferrite region, the fast cooling treatment being performed on the rough rolling strip by using laminar flow cooling or high-pressure water jet cooling, the cooling rate being 5-10 ℃ / s; S24, performing finish rolling on the rough rolling strip to obtain a finish rolling strip, the finish rolling final rolling temperature being 780-820 ℃; S25, high-temperature coiling the finish rolling strip, the coiling temperature being 680-720 ℃; S3, performing cold rolling treatment on the finish rolling strip to obtain a cold rolling strip, the total deformation of the cold rolling strip being 60-90%; S4, performing annealing treatment on the cold rolling strip to obtain a coarse-grained ferrite strip; the annealing treatment is performed by using a continuous annealing furnace or a cover annealing furnace, when the annealing treatment is performed by using the continuous annealing furnace, the operation parameters thereof comprise: a strip speed of 100-120 m / min, a heating rate of 5-10 ℃ / s, a soaking section temperature of 800-850 ℃, a soaking section holding time of 50-100 s, coiling after furnace cooling to 400-500 ℃, and air cooling after discharging; when the annealing treatment is performed by using the cover annealing furnace, the operation parameters thereof comprise: a heating rate of 1-2 ℃ / s, a soaking temperature of 700-750 ℃, a holding time of 10-15 h, and air cooling after discharging after furnace cooling to 400-500 ℃.

2. A method of producing a coarse ferrite steel strip as claimed in claim 1, characterized in that, In step S1, the ultra-low carbon steel casting blank is prepared by sequentially performing blast furnace ironmaking, converter steelmaking, LF refining, RH / VD refining, and continuous casting.

3. A method of producing a coarse ferrite steel strip as claimed in claim 1, characterized in that, In step S24, the rough rolling strip is subjected to 4-6 passes of finish rolling, the reduction rate of each pass of finish rolling being decreased, and the total reduction rate being 50-70%.

4. The method of producing a coarse ferrite steel strip according to claim 1, wherein The finish rolling strip is subjected to end concentrated cooling before step S25.

5. A coarse-grained ferritic steel strip, characterized in that The coarse-grained ferrite strip is prepared by using the method according to any one of claims 1-4.

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