Ultrathin low-carbon steel and manufacturing method thereof

Abstract

According to the manufacturing method, reasonable smelting, continuous casting, heating, rolling, cooling and coiling processes are adopted, a Z-shaped cooling process is adopted for cooling, the length L1 of a high-temperature area of a hot coiling head is set according to different thicknesses h of strip steel finished products, L1 is equal to (100-120) h, the unit of L1 is m, the unit of h is mm, the coiling temperature CT1 of the high-temperature section of the hot coiling head is equal to CT + T, t is equal to 40-60 DEG C, and the coiling temperatures CT of the middle part and the tail part of the hot coil are equal to 600-650 DEG C, so that the problem that the fluctuation of the mechanical property, especially the plastic strain ratio, of an ultra-thin low-carbon steel through coil produced by a traditional hot continuous rolling process is relatively large can be solved, the uniformity of the mechanical property of the low-carbon steel through coil is improved, and the stamping forming cracking probability of the low-carbon steel through coil is reduced. Equipment investment does not need to be increased, and the cost is low.

Description

Technical Field

[0001] The invention belongs to the field of steel production, and in particular relates to an extremely thin low-carbon steel and a manufacturing method thereof. Background Art

[0002] Low carbon steel is widely used in home appliances, hardware, food packaging and other fields due to its good forming performance and cost-effectiveness. In recent years, with the increasing demand for "hot-heat cooling" of plate and strip products, the demand for ultra-thin specifications (≤2.0mm) low carbon steel has continued to increase. According to the production process, ultra-thin specifications of strip steel can be produced in the thin slab continuous casting and rolling process and the traditional hot rolling process. Considering factors such as the number of inclusions, surface quality, and strength, the traditional hot rolling process is still the mainstream production process. Due to the use of the speed-up rolling process, the belt threading speed is close to the upper limit of the equipment capacity, and the rolling speed of the head of the hot coil is lower than that of the middle and tail, the traditional hot rolling process produces ultra-thin specifications of low carbon steel. The final rolling temperature in the length direction of the hot coil often has the problem of "low head, high middle and tail". The final rolling temperature of the head of the hot coil is 40-60℃ or even higher than that of the middle or tail, which leads to large fluctuations in the mechanical properties of the head, middle and tail of the low carbon steel hot coil, especially the large fluctuation of the plastic strain ratio, which directly affects the stamping performance of the material and affects the use of downstream users.

[0003] Therefore, attention should be paid to the large fluctuations in the mechanical properties of ultra-thin low-carbon steel coils produced by the traditional hot rolling process, especially the plastic strain ratio.

[0004] The patent with publication number CN110614280A published on December 27, 2019, discloses a method for online hot coil temperature homogenization. The hot coils that are coiled and bundled after hot rolling are sent to an online mobile enclosed space for insulation and / or heating within 15 minutes, and are directly transferred to the cold rolling unit through a steel coil transport chain or a mobile trolley. During the above-mentioned hot coil transportation process, the hot coils are treated with online insulation in a closed space; the coiling temperature is controlled at 350-650°C, the online insulation time is at least 1 hour, and the hot coils are directly sent to the cold rolling unit. It is to add an online insulation cover equipment after the coiler to improve the temperature uniformity and performance uniformity of the strip in the length and width direction. This method requires additional equipment investment, and has no obvious effect on improving the performance uniformity of fine-grained + solid solution strengthened low-carbon steel. Summary of the invention

[0005] The purpose of the present invention is to provide an ultra-thin low-carbon steel and a manufacturing method thereof, which can solve the problem of large fluctuations in mechanical properties of ultra-thin low-carbon steel coils produced by traditional hot rolling processes, especially in plastic strain ratio, through reasonable smelting, continuous casting, heating, rolling, cooling and coiling processes, improve the uniformity of mechanical properties of low-carbon steel coils, and reduce the probability of cracking during stamping. It does not require additional equipment investment and is low in cost.

[0006] The specific technical solutions of the present invention are as follows:

[0007] A method for manufacturing extremely thin low-carbon steel comprises converter smelting, LF furnace refining, continuous casting, heating, rolling, cooling and coiling.

[0008] After converter smelting and LF furnace refining, alloys are added to the converter steel, where the alloy auxiliary material type of Si is high-purity ferrosilicon, and the chemical element B content is ≤0.0005%, and manganese silicon alloy or ferrosilicon 75B alloy is not used;

[0009] The continuous casting has a casting thickness of 230 mm;

[0010] The heating is performed such that the temperature of the cast billet out of the furnace is 1150-1200°C, and the temperature at the head 0-2m position in the length direction of the cast billet is 20-40°C higher than the temperature at the tail 0-2m position, and the casting time in the furnace is 150-200min.

[0011] The rolling is carried out by using a 2-stand rough rolling and a 7-stand finishing hot rolling mill set. The rough rolling mill set adopts a 3+3 rolling mode. The third pass rolling speed of the R2 stand is ≥4.0 m / s. The thickness H of the intermediate billet after rough rolling and the thickness h of the finished strip steel must satisfy the relationship H=(20~25)×h; the unit of H is mm, and the unit of h is mm.

[0012] The heat preservation covers between the rolling, rough rolling and finishing rolling mills are all put into use;

[0013] In the rolling, the F2-F6 finishing mills use a roll gap lubrication process, the volume ratio of oil and water in the lubricating liquid is 0.30%-0.60%, the lubricating liquid flow rate is 80-120 ml / min, and the final rolling temperature FDT of the F7 finishing mill is 830-880°C.

[0014] The cooling is performed by laminar cooling after finishing rolling, and a "Z-type" cooling process is adopted. The length L1 of the high-temperature zone at the head of the hot coil is set according to the thickness h of the finished strip steel, L1 = (100-120) × h, the unit of L1 is m, the unit of h is mm, and when calculating the formula, the value before the unit is directly substituted for calculation. The coiling temperature of the high-temperature zone length L1 of the hot coil head is CT1 = CT + △T, △T = 40-60 ° C, the coiling temperature of the middle and tail of the hot coil is CT = 600-650 ° C, and the units of CT1, CT and △T are all ° C; the hot coil is air-cooled to room temperature after coiling, and the tail of the hot coil refers to the 150m within the tail of the hot coil.

[0015] The present invention provides an ultra-thin low-carbon steel, which is manufactured by the above method. The ultra-thin low-carbon steel comprises the following chemical components in weight percentage: C 0.020-0.060%; Si

[0016] 0.010~0.060%; Mn 0.08~0.30%; B≤0.0005%; Als 0.015~0.050%, the rest is Fe and unavoidable inclusions.

[0017] The thickness of the ultra-thin low-carbon steel is ≤2.0mm;

[0018] The ultra-thin low-carbon steel hot coil has an average yield strength of 265-270 MPa at the head, middle and tail, an average tensile strength of 366-375 MPa, an average hardness of 59-61 HRB, and an average plastic strain ratio r=0.99-1.01.

[0019] The yield strength fluctuation range of the head, middle and tail of the ultra-thin low-carbon steel hot coil is ≤10MPa, the tensile strength fluctuation range is ≤10MPa, the hardness fluctuation range is ≤5HRB, and the plastic strain ratio r fluctuation range is ≤0.05. The head refers to the head 6m of the hot-rolled coil, the tail refers to the tail 6m of the hot-rolled coil, and the middle refers to the center length position of the hot coil in the length direction.

[0020] The inventors have found that the existing technology mainly improves the uniformity of the mechanical properties of steel strips in the following ways: (1) Research on how to improve the uniformity of the performance of medium and thick plates; (2) Improving the uniformity of the performance of steel strips in the length direction by optimizing the continuous annealing process; (3) The improvement of the uniformity of the performance is concentrated in the width direction of the steel strip, and the length direction is not involved; (4) Improving the uniformity of the performance of steel strips in the length direction by segmented head and tail shielding or U-shaped cooling process; (5) By adding online heat preservation hood equipment. The above methods are not suitable for improving the uniformity of the mechanical properties of extremely thin low-carbon steel coils. Therefore, the present invention provides a method that can be used to improve the uniformity of the mechanical properties of extremely thin low-carbon steel coils without increasing equipment investment.

[0021] The design ideas of the present invention are as follows: the present invention limits the alloy auxiliary material category of Si, mainly because manganese silicon alloy or ferrosilicon 75B alloy contains a certain amount of residual alloy element B, and the content of B element is relatively high, up to 0.0015%. As a typical grain boundary segregation element, B element will significantly inhibit the nucleation and growth of proeutectoid ferrite recrystallization, refine the grain size of the material, and improve the material strength. At the same time, it will also reduce the content of {111} texture components and reduce the plastic strain ratio of the material, which is not conducive to the deep drawing performance of the material. Therefore, in order to avoid the influence of B on the strength and deep drawing performance of the material, the alloy auxiliary material category of Si adopts high-purity ferrosilicon, and the content of chemical element B is ≤0.0005%.

[0022] In the heating process design, the design of the billet out of the furnace temperature and the time in the furnace is mainly based on the need to control the surface oxide scale of the strip steel and improve its surface quality. In order to reduce the temperature drop of the hot coil head and improve the uniformity of the final rolling temperature and mechanical properties of the hot coil head and middle, the billet heating process is designed with temperature compensation, and the head of the billet in the length direction is 20-40℃ higher than the tail.

[0023] In the rolling process design of the present invention, the rough rolling adopts a high-temperature fast rolling strategy, and the rolling speed of the last pass of R2 is controlled to be ≥4.0m / s; the "3+3" mode is selected for the rolling passes of the R1 and R2 frames of the rough rolling. Compared with the "3+5" mode, the residence time of the intermediate billet in the rough rolling unit can be significantly reduced by 16-20s, and the temperature drop of the intermediate billet can be reduced by 15-20°C. The use of all the insulation covers between the rough rolling and finishing rolling units is conducive to reducing the temperature drop of the intermediate billet and improving the temperature difference between the head and tail of the subsequent hot coil. The thickness H of the intermediate billet and the thickness h of the hot coil finished product must satisfy the relationship: H = (20-25) × h. Properly increasing the thickness of the intermediate billet is conducive to reducing the temperature drop of the intermediate billet and increasing the temperature of the intermediate billet. If the thickness of the intermediate billet is too thin, it is easy to cause the temperature drop of the head and tail of the intermediate billet to be too large, and the temperature difference of the head and tail of the hot coil to be even greater, which is not conducive to improving the uniformity of the hot coil through-coiling performance; if the thickness of the intermediate billet is too thick, the rolling pressure of the finishing rolling unit is too large, which is not conducive to the plate shape control of the hot rolled product. The main purpose of using lubrication rolling process in F2~F6 finishing mills is to reduce the friction coefficient between the roller and the surface of the rolled product, reduce the shear strain caused by friction and the unfavorable {110}(1l0) texture caused by uneven deformation, control the consistency of the texture in the thickness direction of the rolled product, and help to form {111} texture to improve the deep drawing performance of the material. The finishing rolling temperature <830℃ is likely to lead to: on the one hand, the rolling speed of the finishing rolling F7 mill is too slow, which increases the difference in the finishing rolling temperature between the head and tail of the hot coil, which is not conducive to the uniformity control of the hot coil through-coiling performance; on the other hand, the final rolling temperature is too low, which is easy to cause mixed crystal problems, which is not conducive to the reduction of material strength and the improvement of plastic strain ratio.

[0024] The present invention adopts a "Z-type" cooling process in the design of the cooling process. The coiling temperature of the head of the hot coil is 40-60°C higher than that of the middle or tail. The main purpose is to perform temperature compensation. The high coiling temperature allows the ferrite to fully recover or statically recrystallize, and then the ferrite grains can fully grow, which is beneficial to reduce the degree of ferrite grain refinement caused by the low final rolling temperature at the head of the hot coil, thereby reducing the performance difference between the head and the middle or tail of the hot coil, and improving the uniformity of the hot coil through-rolling performance. The length L1 of the high temperature zone of the head of the hot coil is set according to the thickness h of the finished strip steel. The thinner the thickness of the finished strip steel, the longer the length of the high temperature zone of the head of the hot coil. The two need to satisfy the relationship L1 = (100-120)h, where L1 is in m and h is in mm.

[0025] Compared with the prior art, the present invention can solve the problem of large fluctuations in the mechanical properties of ultra-thin low-carbon steel coils produced by the traditional hot rolling process, especially the plastic strain ratio, through reasonable smelting, continuous casting, heating, rolling, cooling and coiling processes, improve the uniformity of the mechanical properties of low-carbon steel coils, and reduce the probability of cracking during stamping. And there is no need to increase equipment investment, and the cost is low. The ultra-thin low-carbon steel produced by the present invention has a hot coil yield strength of 265-270MPa, a tensile strength of 366-375MPa, a hardness of 59-61HRB, and a plastic strain ratio r=0.99-1.01. Compared with conventional processes, the ultra-low carbon steel hot coil produced by the present invention has an average yield strength reduction of 7.31-9.26%, an average tensile strength and hardness are basically equivalent, and an average r increase of 6.42-10.26%. The reduction in material yield strength and the increase in r value are both conducive to improving the deep drawing performance of the material. The ultra-thin low-carbon steel produced by the present invention has a yield strength fluctuation range of ≤10MPa at the head, middle and tail of the hot coil, a tensile strength fluctuation range of ≤10MPa, a hardness fluctuation range of ≤5HRB, and a plastic strain ratio r fluctuation range of ≤0.05. Compared with conventional processes, the uniformity of the hot coil through-coiling strength, hardness and plastic strain ratio is significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the "Z-type" cooling process used in the present invention. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] Example 1-Example 3

[0029] An extremely thin gauge low carbon steel comprises the following chemical compositions in weight percentage: as shown in Table 1, wherein the balance not shown in Table 1 is Fe and unavoidable inclusions.

[0030] Comparative Example 1-Comparative Example 6

[0031] An extremely thin gauge low carbon steel comprises the following chemical compositions in weight percentage: as shown in Table 1, wherein the balance not shown in Table 1 is Fe and unavoidable inclusions.

[0032] Table 1 Chemical composition of the embodiments and comparative examples (mass percentage, wt%)

[0033] serial number C Si Mn Als B Example 1 0.035 0.041 0.10 0.030 0.0002 Example 2 0.025 0.015 0.18 0.018 0.0004 Example 3 0.055 0.036 0.25 0.038 0.0001 Comparative Example 1 0.036 0.028 0.10 0.030 0.0012 Comparative Example 2 0.031 0.019 0.11 0.028 0.0003 Comparative Example 3 0.038 0.028 0.10 0.026 0.0004 Comparative Example 4 0.042 0.025 0.12 0.025 0.0003 Comparative Example 5 0.035 0.020 0.10 0.031 0.0003 Comparative Example 6 0.030 0.026 0.11 0.025 0.0008

[0034] The manufacturing method of the ultra-thin low-carbon steel of the above embodiments and comparative examples includes converter smelting, LF furnace refining, continuous casting, heating, rolling, cooling and coiling.

[0035] According to the composition in Table 1, after converter smelting and LF furnace refining, alloy is added to the converter steel, wherein the alloy auxiliary material type of Si adopts high-purity ferrosilicon, and the chemical element B content thereof is ≤0.0005%, and manganese silicon alloy or ferrosilicon 75B alloy is not used;

[0036] The continuous casting has a casting thickness of 230 mm;

[0037] The heating is performed such that the temperature of the cast billet out of the furnace is 1150-1200°C, and the temperature at the head 0-2m position in the length direction of the cast billet is 20-40°C higher than the temperature at the tail 0-2m position, and the casting time in the furnace is 150-200min.

[0038] The rolling is carried out by using a 2-stand rough rolling and a 7-stand finishing hot rolling mill set. The rough rolling mill set adopts a 3+3 rolling mode, the third pass rolling speed of R2 is ≥4.0m / s, and the intermediate billet thickness H and the finished strip steel thickness h after rough rolling need to satisfy the relationship H=(20-25)×h; the unit of H is mm, and the unit of h is mm.

[0039] The insulation covers between the rolling, rough rolling and finishing rolling mills are all put into use;

[0040] In the rolling, the F2-F6 finishing mills use a roll gap lubrication process, the volume ratio of oil and water in the lubricating liquid (oil-water mixture concentration) is 0.30%-0.60%, the lubricating liquid flow rate is 80-120 ml / min, and the final rolling temperature FDT of the F7 finishing mill is 830-880°C.

[0041] The cooling is performed by laminar cooling after finish rolling, and a "Z-type" cooling process is adopted. The length L1 of the high-temperature zone at the head of the hot coil is set according to the thickness h of the finished strip steel, L1=(100-120)×h, the unit of L1 is m, the unit of h is mm, the coiling temperature CT1 of the high-temperature zone length L1 at the head of the hot coil is CT+△T, △T=40-60°C, the coiling temperatures at the middle and tail of the hot coil are both CT=600-650°C, and the units of CT1, CT and △T are all ℃; the tail of the hot coil refers to the 150m within the tail of the hot coil, and the hot coil is air-cooled to room temperature after coiling.

[0042] The hot rolling temperature parameters of each embodiment and comparative example are shown in Table 2, and the lubrication and cooling processes are shown in Table 3.

[0043] Table 2 Hot rolling temperature parameters of the embodiments and comparative examples

[0044]

[0045]

[0046] Table 3 Lubrication process and cooling process parameters of the embodiments and comparative examples

[0047]

[0048] The mechanical properties and fluctuation ranges of the embodiments and comparative examples are shown in Table 4, and the average mechanical properties of the head, middle and tail of the hot coil are shown in Table 5.

[0049] The evaluation method of mechanical properties and their fluctuation range is as follows: 3 samples are taken at the 1 / 4 plate width position corresponding to the head 6m, middle and tail 6m in the length direction of the hot-rolled strip, and 2 longitudinal tensile samples and 1 Rockwell hardness sample are cut from each sample. The yield strength, tensile strength and plastic strain ratio index of the material are obtained by room temperature tensile test according to GB / T 228.1, and the arithmetic average of the test data of the two tensile samples is used as the yield strength, tensile strength and r value test value. The Rockwell hardness HRB index is obtained according to GB / T230.1. Among them, the fluctuation range of the yield strength of the hot coil = Max (yield strength of the head, middle and tail tensile specimens) - Min (yield strength of the head, middle and tail tensile specimens), the fluctuation range of the tensile strength = Max (tensile strength of the head, middle and tail tensile specimens) - Min (tensile strength of the head, middle and tail tensile specimens), the average yield strength is the arithmetic mean of the yield strengths corresponding to the tensile specimens at the head, middle and tail positions, the average tensile strength is the arithmetic mean of the tensile strengths corresponding to the tensile specimens at the head, middle and tail positions, the average hardness is the arithmetic mean of the hardnesses corresponding to the tensile specimens at the head, middle and tail positions, and the average r is the arithmetic mean of the r corresponding to the tensile specimens at the head, middle and tail positions.

[0050] Table 4 Mechanical properties and fluctuation range of examples and comparative examples

[0051]

[0052] Table 5 Average mechanical properties of the head, middle and tail of the hot coil in the embodiments and comparative examples

[0053]

[0054] Comparative Example 1 is produced according to the process of the present invention, but the content of chemical element B is 0.0012%. B will significantly inhibit the nucleation and growth of proeutectoid ferrite recrystallization, refine its grain size, and lead to increased strength of the material and reduced plastic strain ratio.

[0055] In Comparative Example 2, the temperature difference between the head and tail of the slab is -10°C, and the rough rolling adopts the "3+5" mode, resulting in the final rolling temperature of the head of the hot coil being 37-46°C lower than that of the middle or tail. The ferrite grain size at the head of the hot coil is small and the strength is high, resulting in worse performance uniformity in the length direction of the hot coil.

[0056] In comparative example 3, the intermediate billet thickness is 34 mm, H / h<20. The too thin thickness causes the intermediate billet temperature to drop too quickly, and FDT<830℃, so the hot coil final rolling temperature is low, the material strength is high, and the plastic strain ratio is low.

[0057] In Comparative Example 4, the temperature difference between the head and tail of the slab is -15°C, and the cooling adopts the "conventional mode". The "Z-type cooling" is not adopted, resulting in the final rolling temperature of the head of the hot coil being 34-44°C lower than that of the middle or tail. The ferrite grain size at the head of the hot coil is small and the strength is high, resulting in worse performance uniformity in the length direction of the hot coil.

[0058] The ingredients of Comparative Example 5 meet the requirements of the present invention, but the lubrication process is not adopted, resulting in serious shear strain and uneven deformation caused by friction between the roller and the steel plate surface, and the {110} (1l0) texture orientation density increases, which in turn reduces the plastic strain ratio of the material.

[0059] In Comparative Example 6, the rough rolling adopts the "3+5" mode, which will increase the residence time of the intermediate billet in the rough rolling unit and aggravate the temperature drop of the intermediate billet by 15-20°C relative to the "3+3" mode. In addition, the temperature difference between the head and tail of the slab is 2°C, and the cooling does not adopt the "Z-type" cooling mode, resulting in the overall final rolling temperature of the hot coil being low. The final rolling temperature of the head of the hot coil is still lower than that of the middle or tail, and the problem of large performance fluctuations in the length direction of the hot coil still exists.

[0060] The underlined data above do not meet the requirements of the present invention.

[0061] The description of the above embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A method for manufacturing ultra-thin low-carbon steel, comprising converter smelting, LF furnace refining, continuous casting, heating, rolling, cooling and coiling, characterized in that: For the cooling, the length L1 of the high temperature zone at the head of the hot coil is set according to the thickness h of the finished strip steel, L1 = (100-120) × h, the unit of L1 is m, the unit of h is mm, the coiling temperature CT1 of the high temperature zone L1 at the head of the hot coil is = CT + △T, △T = 40-60°C, the coiling temperatures at the middle and tail of the hot coil are both CT = 600-650°C, and the hot coil is air-cooled to room temperature after coiling.

2. The manufacturing method according to claim 1, characterized in that: After converter smelting and LF furnace refining, alloy is added to the converter steel, among which the alloy auxiliary material type of Si is high-purity ferrosilicon, and the content of chemical element B is ≤0.0005%.

3. The manufacturing method according to claim 1, characterized in that: The heating is performed such that the temperature of the cast billet out of the furnace is 1150-1200°C, and the temperature at the head 0-2m position in the length direction of the cast billet is 20-40°C higher than the temperature at the tail 0-2m position, and the casting time in the furnace is 150-200min.

4. The manufacturing method according to claim 1, characterized in that: For the rolling, the rough rolling unit adopts a 3+3 rolling mode, the third pass rolling speed of R2 is ≥4.0 m / s, and the thickness H of the intermediate billet after rough rolling and the thickness h of the finished strip steel must satisfy the relationship H=(20-25)×h.

5. The manufacturing method according to claim 1 or 4, characterized in that: The heat preservation covers between the rolling, rough rolling and finishing rolling mills are all put into use.

6. The manufacturing method according to claim 1 or 4, characterized in that: In the rolling, the F2-F6 finishing mills use a roll gap lubrication process, the oil-water mixture concentration is 0.30%-0.60%, the lubricating liquid flow rate is 80-120 ml / min, and the final rolling temperature FDT of the F7 finishing mill is 830-880°C.

7. An ultra-thin low-carbon steel manufactured by the manufacturing method according to any one of claims 1 to 6, characterized in that: The ultra-thin low-carbon steel comprises the following chemical components in weight percentage: C 0.020-0.060%; Si 0.010-0.060%; Mn 0.08-0.30%; B≤0.0005%; Als 0.015-0.050%, and the rest is Fe and inevitable inclusions.

8. The ultra-thin low-carbon steel according to claim 7, characterized in that: The thickness of the ultra-thin low-carbon steel is ≤2.0 mm.

9. The ultra-thin low-carbon steel according to claim 7 or 8, characterized in that: The ultra-thin low-carbon steel hot coil has a through-coil yield strength of 265-270 MPa, a tensile strength of 366-375 MPa, a hardness of 59-61 HRB, and a plastic strain ratio r=0.99-1.

01.

10. The ultra-thin low-carbon steel according to any one of claims 7 to 9, characterized in that: The yield strength fluctuation range of the head, middle and tail of the ultra-thin low-carbon steel hot coil is ≤10MPa, the tensile strength fluctuation range is ≤10MPa, the hardness fluctuation range is ≤5HRB, and the plastic strain ratio r fluctuation range is ≤0.05.

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