A low carbon steel and a method of making the same

Abstract

This application relates to the field of steel production technology, and more particularly to a low-carbon steel and its preparation method. The chemical composition of the low-carbon steel includes: C, Si, Mn, S, P, Alt, B, N, and Fe; wherein, by mass fraction, the content of Alt is 0.05–0.09%, the content of B is ≤0.005%, and the content of N is ≤0.004%; and satisfies the following relationship: [B]-0.79×[N]≥0.0004%, where [B] represents the mass fraction of B, and [N] represents the mass fraction of N. This application solves the technical problem of poor aging resistance of existing low-carbon steel, with an aging index (AI) value of less than 20 MPa for the steel plate.

Description

Technical Field

[0001] This application relates to the field of steel production technology, and in particular to a low-carbon steel and its preparation method. Background Technology

[0002] Low-carbon steel is the most common type of steel. It has a low carbon content, low strength, and low hardness, but good plasticity and toughness. The annealed microstructure of low-carbon steel consists of ferrite, a small amount of pearlite, and free cementite. It has good cold formability and can be cold-formed by methods such as rolling, bending, and stamping. This type of steel also has good weldability, and therefore it is widely used in various fields such as construction, automobiles, and home appliances, with extremely wide applications.

[0003] Low-carbon steel has a significant tendency to age, a spontaneous phenomenon that occurs when a non-equilibrium state transitions to an equilibrium state. The aging of steel is primarily caused by interstitial carbon and nitrogen atoms. These interstitial atoms generally have a certain diffusion capacity at room temperature, and their solubility decreases with decreasing temperature. During the cooling process after annealing, low-carbon steel experiences a supersaturation of these interstitial atoms, leading to aging. Aging increases the strength and hardness of steel while decreasing its plasticity and toughness, making it prone to fracture during processing or causing "orange peel" defects (tensile strain marks) during deformation. Therefore, strict control is essential. Currently, due to limitations in other technical indicators and smelting capabilities, the control level of the anti-aging properties of low-carbon steel is relatively low. Summary of the Invention

[0004] This application provides a low-carbon steel and its preparation method to solve the technical problem of poor anti-aging properties of existing low-carbon steel.

[0005] In a first aspect, this application provides a low-carbon steel, the chemical composition of which includes:

[0006] C, Si, Mn, S, P, Alt, B, N, and Fe; wherein, by mass fraction,

[0007] The content of Alt is 0.05~0.09%, the content of B is ≤0.005%, and the content of N is ≤0.004%.

[0008] And it satisfies the following relationship: [B]-0.79×[N]≥0.0004%,

[0009] In the formula, [B] represents the mass fraction of B, and [N] represents the mass fraction of N.

[0010] Optionally, the chemical composition of the low-carbon steel, by mass fraction,

[0011] The content of C is 0.01~0.08%, the content of Si is ≤0.05%, the content of Mn is 0.05~0.65%, the content of S is ≤0.05%, and the content of P is ≤0.04%.

[0012] Secondly, this application provides a method for preparing low-carbon steel, used to prepare the low-carbon steel described in any embodiment of the first aspect, the method comprising:

[0013] The slab is heated under a set temperature condition;

[0014] The heated slab is rolled and cooled, then coiled, and the coiling temperature is controlled to obtain a hot-rolled coil.

[0015] The hot-rolled coil is pickled and cold-rolled, and the reduction rate of the cold rolling is controlled to obtain a cold-rolled hard coil;

[0016] The cold-hardened coil is annealed and then leveled to obtain low-carbon steel.

[0017] Optionally, the set temperature is 1100~1180℃.

[0018] Optionally, the heating time is 120~200 min.

[0019] Optionally, the winding temperature is 680~750°C.

[0020] Optionally, the cold rolling reduction rate is 40-70%.

[0021] Optionally, the annealing and subsequent leveling of the cold-rolled coil to obtain low-carbon steel includes:

[0022] The cold-rolled coil is continuously annealed and then leveled to obtain low-carbon steel; wherein, the continuous annealing includes:

[0023] The cooling zones are: a warming zone, a slow cooling zone, a rapid cooling zone, and an aging zone.

[0024] The temperature of the heat spreader is 600~780℃, the temperature of the slow cooling section is 500~720℃, the temperature of the rapid cooling section is 300~380℃, and the temperature of the over-aging section is 380~480℃.

[0025] Optionally, the annealing and subsequent leveling of the cold-rolled coil to obtain low-carbon steel includes:

[0026] The cold-rolled coil is annealed, and then the flattening elongation is set according to the thickness of the cold-rolled coil to obtain a low elongation.

[0027] Carbon steel; among which,

[0028] If the thickness of the cold-rolled coil is ≤1.2mm, then the thickness of the cold-rolled coil and the elongation of the flattened surface satisfy the following:

[0029] Relationship: δ≥2.5 h 100%,

[0030] If the thickness of the cold-rolled coil is 1.2 < h ≤ 2.5 mm, then the thickness of the cold-rolled coil and the elongation of the flattened surface meet the requirements.

[0031] The following relationship applies: 1.5 h 100%≤δ≤1.8 h 100%,

[0032] In the formula, δ represents the elongation of the flat surface, and h represents the thickness of the cold-rolled coil.

[0033] Optionally, the final rolling temperature is 830-900℃.

[0034] The technical solutions provided in this application have the following advantages compared with the prior art:

[0035] The low-carbon steel and its preparation method provided in this application have the following characteristics: In the chemical composition of the low-carbon steel, Alt reacts with N to form AlN, thereby significantly reducing the content of dissolved N in the steel and mitigating aging performance. Boron preferentially combines with N before Al to form BN particles. The bonding force between B and N is stronger than that between B and Al, thus allowing B to absorb more N atoms. After the reaction between B and N, a certain amount of dissolved B remains and accumulates at the grain boundaries, thereby promoting the precipitation of a large amount of supersaturated dissolved C within the grains and mitigating aging. N exists as a residual element in the steel and is an important element in the aging process; the lower the content, the better.

[0036] The chemical composition, combined with the subsequent low-carbon steel preparation process, employs process parameters most conducive to carbide precipitation throughout the entire process from hot rolling to leveling. In particular, the rapid cooling and over-aging sections in the annealing process differ from conventional processes where rapid cooling is followed by cooling to the over-aging section. This application uses reverse heating after the rapid cooling section to enter the over-aging process. Low-temperature rapid cooling is conducive to increasing the driving force for carbide precipitation, while high-temperature over-aging is conducive to the diffusion and precipitation of carbides, thereby promoting the precipitation of the maximum amount of carbides and greatly reducing the aging performance.

[0037] In summary, the aging index of this low-carbon steel is within 20 MPa, indicating excellent resistance to aging. Attached Figure Description

[0038] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0039] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 A schematic flowchart illustrating a method for preparing low-carbon steel provided in this application embodiment;

[0041] Figure 2 Metallographic diagram of a low-carbon steel provided in an embodiment of this application. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0043] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0044] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, terms such as "comprising" and "including" mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0045] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0046] Firstly, this application provides a low-carbon steel, which can be found in [reference needed]. Figure 2 The metallographic structure of the low-carbon steel shown is highly uniform. The chemical composition of the low-carbon steel includes:

[0047] C, Si, Mn, S, P, Alt, B, N, and Fe; wherein, by mass fraction,

[0048] The content of Alt is 0.05~0.09%, the content of B is ≤0.005%, and the content of N is ≤0.004%.

[0049] And it satisfies the following relationship: [B]-0.79×[N]≥0.0004%,

[0050] In the formula, [B] represents the mass fraction of B, and [N] represents the mass fraction of N.

[0051] In the embodiments of this application, the positive effects of controlling the content of Alt to 0.05~0.09% are as follows: Aluminum is added as a deoxidizer in conventional processes. In this invention, during hot rolling and subsequent coiling, Alt reacts with N to generate AlN, thereby significantly reducing the content of dissolved N in the steel, mitigating aging properties. The generated AlN has good stability and will not undergo re-dissolution during subsequent continuous annealing, enabling stable control of aging. Specifically, the content of Alt can be 0.05%, 0.07%, 0.09%, etc.

[0052] The positive effects of controlling the B content to ≤0.005% are as follows: B preferentially combines with N elements before Al to form BN particles. The binding force between B and N is stronger than that between B and Al, thus allowing for the absorption of more N atoms. In the embodiments of this application, B and N satisfy [B] - 0.79 × [N] ≥ 0.0004%. After the reaction between B and N, a certain amount of B in solid solution remains and accumulates at the grain boundaries, thereby promoting the precipitation of a large amount of supersaturated solid solution C in the steel within the grains and mitigating aging. However, the B content should not be too high, as excessively high B content can easily cause cracks in the cast billet. Specifically, the B content can be 0.005%, 0.004%, etc.

[0053] The positive effects of controlling the nitrogen (N) content to ≤0.004%: N exists as a residual element in steel and is an important element inducing aging; the lower the content, the better. However, due to limitations in smelting technology and cost, the N content is controlled within a reasonable range. Specifically, the N content can be 0.004%, 0.003%, etc.

[0054] In some embodiments, the chemical composition of the low-carbon steel, by mass fraction, includes C content of 0.01~0.08%, Si content of ≤0.05%, Mn content of 0.05~0.65%, S content of ≤0.05%, and P content of ≤0.04%.

[0055] In the embodiments of this application, the positive effects of controlling the carbon content to be 0.01~0.08% are as follows: For low-carbon steel, the carbon content is generally above 0.01%, which can ensure a certain strength. However, when the carbon content is too high, the plasticity of the steel deteriorates, and it also leads to an increase in the solid solution carbon content. Therefore, the carbon content is limited to a certain range. Specifically, the carbon content can be 0.01%, 0.05%, 0.08%, etc.

[0056] The positive effects of controlling the Si content to ≤0.05% are as follows: In low-carbon steel, a high Si content increases the steel's strength but affects its plasticity; therefore, the Si element is controlled at a low level. Specifically, the Si content can be 0.05%, 0.04%, 0.03%, etc.

[0057] The positive effects of controlling the Mn content to 0.05–0.65% include: manganese reacts with sulfur to form manganese sulfide, eliminating the brittleness of sulfur; at the same time, Mn can improve the strength of steel; and excessive Mn content can easily cause banded structures. Specifically, the Mn content can be 0.05%, 0.1%, 0.3%, 0.65%, etc.

[0058] The positive effects of controlling the sulfur (S) content to ≤0.05% are as follows: Generally, sulfur is an impurity element in steel and easily forms brittle substances. Specifically, the S content can be 0.05%, 0.04%, etc.

[0059] The positive effects of controlling the phosphorus (P) content to ≤0.04% are as follows: Phosphorus is an impurity element that tends to segregate at grain boundaries, increasing the brittleness of the steel plate and impairing its formability. Specifically, the P content can be 0.04%, 0.03%, etc.

[0060] Secondly, this application provides a method for preparing low-carbon steel, which can be found in [reference needed]. Figure 1 The method for preparing the low-carbon steel according to any embodiment of the first aspect comprises:

[0061] S1. Heating the slab at a set temperature;

[0062] In some embodiments, the set temperature is 1100~1180°C.

[0063] In some embodiments, the heating time is 120-200 minutes. The heating time is also the furnace time.

[0064] In this embodiment of the application, the process before step S1 includes: pre-treating the molten iron, followed by converter smelting, RH / LF / CAS...

[0065] Slabs are obtained through refining and continuous casting. The "set temperature" refers to the heating temperature. Controlling this heating temperature to 1100~1180℃ and the heating time to 120~200min has the following positive effects: it ensures full austenitization of the steel billet, and the BN, AlN, and carbides formed in the billet remain in a large-size precipitate state, reducing solid solution content. Specifically, the heating temperature can be 1100℃, 1150℃, 1180℃, etc., and the heating time can be 120min, 160min, 200min, etc.

[0066] S2. The heated slab is rolled and cooled, then coiled, and the coiling temperature is controlled to obtain a hot-rolled coil.

[0067] In this embodiment, the cooling is laminar flow cooling, which uses concentrated water spray for rapid cooling in the front section and air cooling in the rear section.

[0068] In some embodiments, the winding temperature is 680~750°C.

[0069] In the embodiments of this application, the positive effect of controlling the winding temperature to 680~750°C is that during the slow cooling process after winding, the AlN and solid-solid carbon precipitates are fully released, thereby reducing aging. Specifically, the winding temperature can be 680°C, 730°C, 750°C, etc.

[0070] In some embodiments, the final rolling temperature is 830-900°C.

[0071] In the embodiments of this application, the positive effect of controlling the final rolling temperature to 830-900℃ is to reduce strain-induced precipitation. Specifically, the final rolling temperature can be 830℃, 860℃, 900℃, etc. The initial rolling temperature is 1020~1100℃, and the entire rolling range maintains a suitable temperature range, reducing strain-induced precipitation while avoiding rolling in the two-phase region.

[0072] S3. Pickling and cold rolling are performed on the hot-rolled coil, and the reduction rate of the cold rolling is controlled to obtain a cold-rolled hard coil;

[0073] In some embodiments, the cold rolling reduction rate is 40-70%.

[0074] In the embodiments of this application, the positive effects of controlling the cold rolling reduction rate to 40-70% are: a low cold rolling reduction rate can...

[0075] This reduces the degree of carbide fragmentation after coiling, thereby reducing the amount dissolved during annealing. Specifically, the reduction rate of this cold rolling can be 40%, 50%, 60%, 70%, etc.

[0076] S4. Anneal the cold-hardened coil and then flatten it to obtain low-carbon steel.

[0077] In some embodiments, the annealing and subsequent leveling of the cold-rolled coil to obtain low-carbon steel includes:

[0078] The cold-rolled coil is continuously annealed and then leveled to obtain low-carbon steel; wherein, the continuous annealing includes:

[0079] The cooling zones are: a warming zone, a slow cooling zone, a rapid cooling zone, and an aging zone.

[0080] The temperature of the heat spreader is 600~780℃, the temperature of the slow cooling section is 500~720℃, the temperature of the rapid cooling section is 300~380℃, and the temperature of the over-aging section is 380~480℃.

[0081] In this embodiment, the annealing process employs continuous annealing, controlling the annealing homogenization temperature, slow cooling temperature, rapid cooling temperature, and over-aging temperature. Rapid cooling is followed by heating to enter the over-aging stage. During homogenization, the carbide re-dissolution process should not be too hot. Slow cooling is a crucial step in the continuous annealing process; higher temperatures allow for more complete carbide diffusion and increase the amount of cementite within the ferrite grains. Lower rapid cooling temperatures increase the supersaturation of C and the precipitation driving force, which is more conducive to carbon precipitation. However, from the perspective of C diffusion, higher over-aging temperatures within a certain temperature range are more conducive to carbon diffusion. This application involves a reverse heating process after rapid cooling to maximize the precipitation of dissolved carbon. Specifically, the temperature of the soaking zone can be 600℃, 650℃, 700℃, 780℃, etc., the temperature of the slow cooling zone can be 500℃, 550℃, 600℃, 650℃, 720℃, etc., the temperature of the rapid cooling zone can be 300℃, 350℃, 380℃, etc., and the temperature of the over-aging zone can be 380℃, 440℃, 480℃, etc.

[0082] In some embodiments, the annealing and subsequent leveling of the cold-rolled coil to obtain low-carbon steel includes:

[0083] The cold-rolled coil is annealed, and then the flattening elongation is set according to the thickness of the cold-rolled coil to obtain a low elongation.

[0084] Carbon steel; among which,

[0085] If the thickness of the cold-rolled coil is ≤1.2mm, then the thickness of the cold-rolled coil and the elongation of the flattened surface satisfy the following:

[0086] Relationship: δ≥2.5 h 100%,

[0087] If the thickness of the cold-rolled coil is 1.2 < h ≤ 2.5 mm, then the thickness of the cold-rolled coil and the elongation of the flattened surface meet the requirements.

[0088] The following relationship applies: 1.5 h 100%≤δ≤1.8 h 100%,

[0089] In the formula, δ represents the elongation of the flat surface, and h represents the thickness of the cold-rolled coil.

[0090] In this embodiment of the application, the positive effect of setting the flattening elongation rate based on the thickness of the cold-rolled coil is: [further details needed for accurate translation]

[0091] The post-fire leveling is performed online. The principle of the leveling process of this invention is to use a large leveling elongation rate. By leveling, the dislocation density of the steel plate is increased, and the interstitial atoms remaining in the steel are pinned, thereby eliminating the occurrence of tensile strain marks.

[0092] The low-carbon steel prepared using the above method has better resistance to aging. By manufacturing BN and AlN particles in the steel to reduce the content of dissolved N, and by controlling the precipitation of carbon elements through hot rolling, cold rolling, and annealing processes, the total interstitial atom content in the steel is reduced to a minimum, resulting in excellent resistance to aging.

[0093] The preparation method of this low-carbon steel is based on the chemical composition of the low-carbon steel described above. The specific steps for preparing the chemical composition of the low-carbon steel can be referred to in the above embodiments. Since the preparation method of this low-carbon steel adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0094] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0095] Table 1. Chemical composition (wt%) of low-carbon steel, the remainder being Fe and unavoidable impurities.

[0096]

[0097] This application provides a method for preparing low-carbon steel, the method comprising:

[0098] S1. Heating the slab at a set temperature;

[0099] S2. The heated slab is rolled and cooled, then coiled, and the coiling temperature is controlled to obtain a hot-rolled coil.

[0100] S3. Pickling and cold rolling are performed on the hot-rolled coil, and the reduction rate of the cold rolling is controlled to obtain a cold-rolled hard coil;

[0101] S4. Anneal the cold-hardened coil and then flatten it to obtain low-carbon steel. Specific process parameters can be found in Tables 2-3.

[0102] Table 2 Hot rolling process parameters for low carbon steel

[0103]

[0104] Table 3. Manufacturing process parameters for low-carbon steel: cold rolling and annealing process parameters and measured aging index (AI value).

[0105]

[0106] The method for measuring the aging index AI value in Table 3 is to cut a sample at 1 / 4 of the plate width and conduct a tensile test. The measurement method is to first pre-stretch by 8%, hold at 100℃ for 20 minutes, and then stretch again. The data is then processed to obtain the AI ​​value.

[0107] The low-carbon steel prepared through the embodiments of this application has an aging index (AI) of less than 20 MPa, exhibiting excellent resistance to aging. In contrast, Comparative Examples 1-2, which did not employ the methods described in the embodiments of this application, showed poor resistance to aging.

[0108] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A low-carbon steel, characterized in that, The chemical composition of the low-carbon steel includes: The composition comprises C, Si, Mn, S, P, Alt, B, N, and the balance being Fe; wherein, by mass fraction, the content of C is 0.01~0.08%, the content of Si is ≤0.05%, the content of Mn is 0.05~0.65%, the content of S is ≤0.05%, the content of P is ≤0.04%, the content of Alt is 0.05~0.09%, the content of B is ≤0.005%, and the content of N is ≤0.004%. And it satisfies the following relationship: [B]-0.79×[N]≥0.0004%, In the formula, [B] represents the mass fraction of B, and [N] represents the mass fraction of N; The aging index AI value of the low-carbon steel is less than 20 MPa; The method for preparing the low-carbon steel includes: The slab is heated under a set temperature condition; The heated slab is rolled and cooled, then coiled, and the coiling temperature is controlled at 680~750°C to obtain a hot-rolled coil. The final rolling temperature is 830-900°C. The hot-rolled coil is pickled and cold-rolled, and the reduction rate of the cold rolling is controlled to be 40-70% to obtain a cold-rolled coil; The cold-rolled coil is continuously annealed and then leveled to obtain low-carbon steel; wherein, the continuous annealing ladle... It includes: a soaking zone, a slow cooling zone, a rapid cooling zone, and an over-aging zone; the temperature of the soaking zone is 600~780℃, the temperature of the slow cooling zone is 500~720℃, the temperature of the rapid cooling zone is 300~380℃, and the temperature of the over-aging zone is 380~480℃. The process of continuously annealing the cold-hardened coil and then leveling it to obtain low-carbon steel includes: The cold-rolled coil is continuously annealed, and then the flattening elongation is set according to the thickness of the cold-rolled coil. Low-carbon steel was obtained; among which, If the thickness of the cold-rolled coil is ≤1.2mm, then the thickness of the cold-rolled coil and the elongation of the flattened surface satisfy the following conditions: The following relationship applies: δ ≥ 2.5 multiplied by h multiplied by 100%. If the thickness of the cold-rolled coil is 1.2 < h ≤ 2.5 mm, then the thickness of the cold-rolled coil and the flattened extension... The rate satisfies the following relationship: 1.5 multiplied by h multiplied by 100% ≤ δ ≤ 1.8 multiplied by h multiplied by 100%. In the formula, δ represents the elongation of the flat surface, and h represents the thickness of the cold-rolled coil.

2. A method for preparing low-carbon steel, characterized in that, The method for preparing the low-carbon steel according to claim 1 includes: The slab is heated under a set temperature condition; The heated slab is rolled and cooled, then coiled, and the coiling temperature is controlled at 680~750°C to obtain a hot-rolled coil. The final rolling temperature is 830-900°C. The hot-rolled coil is pickled and cold-rolled, and the reduction rate of the cold rolling is controlled to be 40-70% to obtain a cold-rolled coil; The cold-rolled coil is continuously annealed and then leveled to obtain low-carbon steel; wherein, the continuous annealing ladle... It includes: a soaking zone, a slow cooling zone, a rapid cooling zone, and an over-aging zone; the temperature of the soaking zone is 600~780℃, the temperature of the slow cooling zone is 500~720℃, the temperature of the rapid cooling zone is 300~380℃, and the temperature of the over-aging zone is 380~480℃. The process of continuously annealing the cold-hardened coil and then leveling it to obtain low-carbon steel includes: The cold-rolled coil is continuously annealed, and then the flattening elongation is set according to the thickness of the cold-rolled coil. Low-carbon steel was obtained; among which, If the thickness of the cold-rolled coil is ≤1.2mm, then the thickness of the cold-rolled coil and the elongation of the flattened surface satisfy the following conditions: The following relationship applies: δ ≥ 2.5 multiplied by h multiplied by 100%. If the thickness of the cold-rolled coil is 1.2 < h ≤ 2.5 mm, then the thickness of the cold-rolled coil and the flattened extension... The rate satisfies the following relationship: 1.5 multiplied by h multiplied by 100% ≤ δ ≤ 1.8 multiplied by h multiplied by 100%. In the formula, δ represents the elongation of the flat surface, and h represents the thickness of the cold-rolled coil.

3. The method according to claim 2, characterized in that, The set temperature is 1100~1180℃.

4. The method according to claim 2, characterized in that, The heating time is 120~200 min.

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