Method for eliminating annealing yield platform of thin-gauge ultra-low-carbon steel and thin-gauge ultra-low-carbon steel

Abstract

This invention discloses a method for eliminating the annealing yield plateau of thin-gauge ultra-low carbon steel and the thin-gauge ultra-low carbon steel. The method includes the following steps: 1) Pure iron raw material and La raw material are sequentially added to a vacuum furnace for melting and casting to obtain an ingot; wherein, the mass of La raw material is 0.055-0.095 wt% of the mass of pure iron raw material; 2) The ingot is held at 1000-1250℃ for 1-3 hours, and then hot-rolled at 950-1200℃, with a reduction rate of 65.5-69%, a reduction amount Δh of 9-11 mm, and a final rolling temperature of 830-860℃; after hot rolling, it is held at 600-650℃ for 10-50 minutes, and then air-cooled to below 50℃ to obtain a hot-rolled plate; 3) After removing the surface iron oxide scale from the obtained hot-rolled plate, it is subjected to multiple cold rolling passes, with a reduction rate of 80-85%, to obtain a cold-rolled sheet with a thickness of 0.5-1.0 mm; 4) The cold-rolled sheet is recrystallized and annealed to obtain ultra-low carbon steel. This method can eliminate the annealing yield plateau of the obtained thin-gauge ultra-low carbon steel.

Description

Technical Field

[0001] This invention relates to a method for eliminating the annealing yield plateau of thin-gauge ultra-low carbon steel and to thin-gauge ultra-low carbon steel. Background Technology

[0002] Ultra-low carbon steel has lower strength and hardness, but better plasticity and toughness, exhibiting excellent deep-drawing and formability, and is widely used in manufacturing industries such as automobiles. With the rapid development of the automotive industry, higher demands are being placed on low-carbon steel sheets with good formability.

[0003] For ductile materials such as low-carbon steel and mild steel, a distinct upper and lower yield point will appear in the tensile test, accompanied by a horizontal segment (i.e., yield plateau) where the stress is almost constant but the strain increases significantly.

[0004] In terms of steel plate thickness, the yield plateau in thin-gauge ultra-low carbon steel plates is difficult to eliminate. This is because solute atoms form Cotillard atmospheres, which interact strongly with dislocations, pinning them. During tensile testing, this pinning effect makes plastic deformation difficult, resulting in a high yield stress. When the external stress increases to a certain level, the dislocations break free and begin to move. As the resistance to dislocation movement decreases, the yield stress drops rapidly. Under stress, the dislocations continue to move until they encounter new obstacles or escape to the surface of the object, thus forming a yield plateau. On the tensile curve, this is manifested as a sudden drop in stress after reaching the upper yield strength, followed by a short region where the stress remains almost constant, and finally, as the deformation increases, the stress gradually rises again.

[0005] This pronounced yielding phenomenon is often accompanied by the appearance of Lüders bands on the sample surface. Lüders bands are a metallurgical concept referring to the strip-like wrinkles that form on the surface of annealed low-carbon steel sheets during stamping due to uneven deformation caused by sudden, localized yielding. The presence of Lüders bands significantly affects aesthetics, fails to meet the surface quality requirements of automotive interior and exterior panels, and reduces their stamping formability. Therefore, it is necessary to avoid them as much as possible during the forming process.

[0006] CN102534186A discloses a method for eliminating the yield plateau phenomenon in low-carbon steel during bell-type annealing. This method involves formulating and adopting a reasonable bell-type annealing process and flattening elongation rate. Specifically, the wider the strip width and the greater the total weight of the coils loaded into each furnace, the longer the annealing time; the thicker the strip, the greater its flattening elongation rate. This effectively controls the formation of the yield plateau in the steel and eliminates the yield plateau phenomenon that occurs during bell-type annealing of low-carbon steel.

[0007] CN104164619A discloses a short-process manufacturing method for low-carbon steel plates without a yield plateau, comprising: firstly, by mass percentage, C 0.002~0.010wt%, Si 0.01~0.2wt%, Mn 0.05~0.15wt%, S 0.002~0.02wt%, P 0.005~0.02wt%, sol-Al 0.002-0.02wt%, with the balance being Fe, is used to smelt low-carbon steel. The molten steel flows into the tundish to form a molten pool. The superheat of the molten steel on the upper surface of the molten pool is controlled at 5-90℃. After passing through the crystallizing roll, it solidifies and is discharged to form a low-carbon steel strip. The low-carbon steel strip is cooled to 900-1200℃ at a cooling rate of 10-80℃ / s and then hot-rolled to obtain a hot-rolled strip. It is then cooled to 500-780℃ and coiled. The strip is then uncoiled and pickled to remove iron oxide scale, resulting in a low-carbon steel plate without a yield plateau.

[0008] CN105506437A discloses a method for producing hard ultra-low carbon steel, specifically including the following process steps: During rolling, the finishing mill inlet temperature is controlled at 1050℃~1075℃; for hot-rolled IF steel with a finished thickness of 2.5mm~3.5mm, the intermediate billet thickness is controlled at 38mm~42mm; during cold rolling, the radiant heating zone temperature RTF of the annealing furnace is controlled within a range of approximately 760℃~790℃, and the non-oxidizing zone temperature NOF is 600℃~660℃. The steel produced by this method can meet the requirement of an elongation of 40%, and the yield strength also stably reaches 240MPa, and no obvious yield plateau appears after baking and processing. This method produces IF steel, which does not exhibit a yield plateau after annealing.

[0009] CN111218617A discloses a cold-rolled low-carbon steel strip SPCC with low yield strength and no yield plateau, and its production method. The chemical composition of the cold-rolled low-carbon steel strip SPCC, by mass percentage, is: C 0.01~0.05wt%; Si≤0.03wt%; Mn 0.20~0.40wt%; P≤0.020wt%; S≤0.015wt%; N≤50ppm, with the remainder being Fe and unavoidable impurities. No defects such as orange peel caused by yield plateau were found during use.

[0010] CN112458361A discloses a cold-rolled low-carbon steel and its production method for reducing surface orange peel defects. The chemical composition and mass percentage of the cold-rolled low-carbon steel are as follows: C: 0.01–0.025 wt%, Si: ≤0.02 wt%, Mn: 0.2–0.24 wt%, P: ≤0.02 wt%, S: ≤0.012 wt%, Als: 0.03–0.06 wt%, N: ≤0.0035 wt%, B: 0.002–0.004 wt%, with the remainder being iron and unavoidable impurities. The mass percentages of Al, N, and B in the steel satisfy the following conditions: N / B = 0.5–1.5, N / Als = 0.03–0.1, and the mass percentage of N is controlled between 0.001 and 0.0035 wt%. The resulting cold-rolled low-carbon steel shows no yield plateau in tensile testing and, after further processing and forming, no orange peel defects are observed. Summary of the Invention

[0011] One object of the present invention is to provide a method for eliminating the yield plateau in the annealing of thin-gauge ultra-low carbon steel, which can eliminate the yield plateau in thin-gauge ultra-low carbon steel and essentially avoid the problems of uneven deformation and surface wrinkling that occur during the cold working of ultra-low carbon steel. Another object of the present invention is to provide a thin-gauge ultra-low carbon steel prepared according to the method described above.

[0012] The above-mentioned technical objectives are achieved through the following technical solutions.

[0013] On one hand, the present invention provides a method for eliminating the annealing yield plateau of thin-gauge ultra-low carbon steel, wherein the ultra-low carbon steel has the following composition: C content less than or equal to 0.02wt%, Si content less than or equal to 0.03wt%, Mn content less than or equal to 0.002wt%, P content less than or equal to 0.001wt%, S content less than or equal to 0.001wt%, La content less than 0.095wt%, and Fe as the balance;

[0014] The method includes the following steps:

[0015] 1) Provide pure iron raw materials and La raw materials; add the pure iron raw materials and La raw materials to a vacuum furnace in sequence for melting and casting to obtain ingots; wherein, the mass of La raw materials is 0.055 to 0.095 wt% of the mass of pure iron raw materials;

[0016] 2) Heat the ingot to 1000-1250℃ and hold it at 1000-1250℃ for 1-3 hours. Then, start hot rolling at 950-1200℃. The hot rolling reduction rate is 65.5-69%, the hot rolling reduction Δh is 9-11 mm, and the final rolling temperature is 830-860℃. After hot rolling, hold it at 600-650℃ for 10-50 minutes, and then air cool it to below 50℃ to obtain the hot-rolled plate.

[0017] 3) After removing the surface iron oxide scale from the obtained hot-rolled plate, it is subjected to multiple cold rolling passes with a reduction rate of 80-85% to obtain a cold-rolled sheet with a thickness of 0.5-1.0 mm.

[0018] 4) Recrystallize and anneal the cold-rolled sheet to obtain ultra-low carbon steel.

[0019] According to the method of the present invention, preferably, in step 1), the mass of the La raw material is 0.06 to 0.09 wt% of the mass of the pure iron raw material; and a vacuum electric arc furnace is used for smelting.

[0020] According to the method of the present invention, preferably, in step 2), hot rolling is started at 1000-1100°C.

[0021] According to the method of the present invention, preferably, in step 2), the hot rolling reduction rate is 66-68%.

[0022] According to the method of the present invention, preferably, in step 2), the hot-rolled product is kept at 600-650°C for 15-40 minutes.

[0023] According to the method of the present invention, preferably, in step 2), the temperature is air-cooled to below 40°C.

[0024] According to the method of the present invention, preferably, in step 3), the thickness of the cold-rolled sheet is 0.6 to 0.95 mm.

[0025] According to the method of the present invention, preferably, in step 4), the recrystallization annealing temperature is 600-650°C and the time is 20-40 min.

[0026] According to the method of the present invention, preferably, the obtained ultra-low carbon steel has an Rp0.2 greater than or equal to 105 MPa.

[0027] On the other hand, the present invention also provides a thin-gauge ultra-low carbon steel prepared according to the method described above.

[0028] The method of this invention can eliminate the annealing yield plateau of thin-gauge ultra-low carbon steel, and basically avoid the problems of uneven deformation and surface wrinkles that occur during the cold working of ultra-low carbon steel, thus meeting the quality requirements of automotive sheet metal. The method of this invention is relatively simple to operate. Detailed Implementation

[0029] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0030] In this invention, the reduction (Δh) refers to the decrease in the thickness of the workpiece before and after rolling, which directly represents the absolute thickness change of the workpiece during the rolling process, and the unit is millimeters (mm).

[0031] In this invention, the reduction rate (also known as relative reduction) refers to the percentage of the reduction amount to the thickness of the rolled piece before rolling, reflecting the relative degree of deformation of the rolled piece, and the unit is percentage (%).

[0032] In this invention, Rp0.2 is a symbol representing the specified limit plastic extension strength in the mechanical properties of metallic materials. It belongs to one of the yield strength indices of materials and is often used to measure the ability of materials to resist minute plastic deformation. Rp0.2 is specifically defined as the stress value corresponding to a plastic elongation of 0.2% in a tensile test, and its unit is MPa.

[0033] In this invention, R is the plastic strain value, reflecting the ability of a thin metal sheet to resist thinning or thickening when subjected to tensile or compressive forces in a plane. It is an important parameter for evaluating the deep-drawing performance of metal sheets. R90 indicates that it was measured in a direction at 90° to the rolling direction. In the methods of this invention, the measurement is taken in a direction parallel to the rolling direction, denoted by R0.

[0034] While existing technologies have reported the addition of rare earth elements such as lanthanum and cerium to obtain rare earth low-carbon steel, the addition of rare earth elements is generally used to improve the corrosion resistance and high-temperature performance of low-carbon steel. To date, there are no reports of using the rare earth element lanthanum to eliminate the annealing yield plateau of thin-gauge ultra-low-carbon steel. The method of this invention is not a conventional approach.

[0035] Methods for eliminating the yield plateau in annealing of thin-gauge ultra-low carbon steel.

[0036] The present invention provides a method for eliminating the annealing yield plateau of thin-gauge ultra-low carbon steel, which may include the following steps: (1) raw material preparation and ingot casting; (2) hot rolling; (3) cold rolling; and (4) recrystallization annealing. These steps are described in detail below.

[0037] Raw material preparation and ingot casting steps

[0038] The ultra-low carbon steel of this invention has the following composition: C content ≤ 0.02 wt%, Si content ≤ 0.03 wt%, Mn content ≤ 0.002 wt%, P content ≤ 0.001 wt%, S content ≤ 0.001 wt%, La content ≤ 0.095 wt%, and Fe as the balance. It may contain unavoidable impurities. The La content is preferably 0.06–0.093 wt%. This invention surprisingly discovers that adding a specific amount of La can help eliminate the annealing yield plateau of the resulting ultra-low carbon steel. If the amount of La added is outside the range of this invention, or if La is replaced with Ce, or if a mixture of La and Ce is added, the technical effects of this invention cannot be achieved.

[0039] This invention requires pure iron and lanthanum (La) as raw materials. Pure iron refers to iron with a purity of 99.9 wt% or higher, for example, 99.99 wt%. La has a purity of 99.9 wt% or higher, for example, 99.99 wt%.

[0040] In this invention, pure iron raw material and La raw material are sequentially added to a vacuum furnace for melting and casting to obtain an ingot; wherein, the mass of La raw material is 0.055-0.095 wt% of the mass of pure iron raw material, preferably 0.06-0.093 wt%, more preferably 0.06-0.091 wt%, and even more preferably 0.065-0.09 wt%, for example, 0.065 wt%, 0.067 wt%, 0.07 wt%, 0.075 wt%, 0.08 wt%, 0.085 wt%, and 0.09 wt%.

[0041] In this invention, the vacuum furnace can be a vacuum electric arc furnace. The specific steps of smelting and casting can be referred to known techniques, and will not be elaborated here.

[0042] According to one specific embodiment of the present invention, the ingot is square in shape. The dimensions of the ingot can be 70-90 mm in length, 10-20 mm in width, and 10-20 mm in height. In a specific embodiment, the dimensions of the ingot are 15 mm × 15 mm × 80 mm.

[0043] Hot rolling process

[0044] The ingot is heated to 1000–1250℃ and held at this temperature for 1–3 hours. Then, hot rolling begins at 950–1200℃, with a reduction rate of 65.5–69% and a reduction amount Δh of 9–11 mm. The final rolling temperature is 830–860℃. After hot rolling, the ingot is held at 600–650℃ for 10–50 minutes, and then air-cooled to below 50℃ to obtain the hot-rolled plate. This process is beneficial for obtaining ultra-low carbon steel with stable performance.

[0045] In this invention, the ingot can be heated to 1000–1250°C, preferably to 1100–1250°C, and more preferably to 1150–1200°C. It is then held at 1000–1250°C for 1–3 hours, preferably 1.5–3 hours, and more preferably 2–2.5 hours. This facilitates complete melting of the ingot, which is beneficial for subsequent hot rolling steps.

[0046] In this invention, the starting temperature of hot rolling can be 950-1200°C, preferably 1000-1150°C, more preferably 1000-1100°C, and even more preferably 1050-1100°C.

[0047] The hot-rolled reduction rate can be 65.5%–69%, preferably 66%–68%, and more preferably 66.5%–67.5%. The hot-rolled reduction Δh can be 9–11 mm, preferably 10–11 mm. The final rolling temperature can be 830–860°C, preferably 840–855°C, and more preferably 845–850°C. This is beneficial for obtaining ultra-low carbon steel with stable properties and for eliminating the yield plateau of the obtained ultra-low carbon steel.

[0048] In this invention, after hot rolling, the steel is held at 600–650°C for 10–50 minutes, and then air-cooled to below 50°C to obtain a hot-rolled plate. The holding temperature after hot rolling can be 600–650°C, preferably 600–630°C, and more preferably 600–620°C. The holding time can be 10–50 minutes, preferably 15–40 minutes, more preferably 20–40 minutes, and even more preferably 30–35 minutes. This is beneficial for obtaining ultra-low carbon steel with stable performance. Air cooling refers to natural cooling in air.

[0049] According to a specific embodiment of the present invention, the thickness of the hot-rolled plate can be 4 to 6 mm, for example, 4 mm, 5 mm, or 6 mm.

[0050] Cold rolling process

[0051] After removing the surface iron oxide scale from the obtained hot-rolled sheet, it undergoes multiple cold rolling passes with a reduction rate of 80-85%, resulting in cold-rolled thin sheets with a thickness of 0.5-1.0 mm. This process is beneficial for obtaining ultra-low carbon steel with stable performance.

[0052] The reduction rate of cold rolling can be 80-85%, preferably 80-83%, and more preferably 80-82%. The thickness of the cold-rolled sheet is preferably 0.5-0.95 mm, more preferably 0.6-0.95 mm, and even more preferably 0.7-0.93 mm.

[0053] Recrystallization annealing steps

[0054] Recrystallization annealing of cold-rolled sheet yields ultra-low carbon steel. This method is beneficial for obtaining ultra-low carbon steel with stable properties.

[0055] In this invention, the recrystallization annealing temperature can be 600–650°C, preferably 600–640°C, and more preferably 600–620°C. The recrystallization annealing time can be 20–40 min, preferably 30–40 min, and more preferably 30–35 min.

[0056] According to a specific embodiment of the present invention, a method for eliminating the annealing yield plateau of thin-gauge ultra-low carbon steel is provided, wherein the ultra-low carbon steel has the following composition: C content less than or equal to 0.02 wt%, Si content less than or equal to 0.03 wt%, Mn content less than or equal to 0.002 wt%, P content less than or equal to 0.001 wt%, S content less than or equal to 0.001 wt%, La content less than 0.095 wt%, and Fe as the balance;

[0057] The method includes the following steps:

[0058] 1) Provide pure iron raw materials and La raw materials; add the pure iron raw materials and La raw materials to a vacuum furnace in sequence for melting, and cast them after complete melting to obtain ingots; wherein, the mass of La raw materials is 0.06 to 0.09 wt% of the mass of pure iron raw materials;

[0059] 2) Heat the ingot to 1000-1250℃ and hold it at 1000-1250℃ for 1-3 hours. Then, start hot rolling at 950-1200℃. The hot rolling reduction rate is 65.5-69%, the hot rolling reduction amount Δh is 9-11 mm, and the final rolling temperature is 830-860℃. After hot rolling, hold it at 600-650℃ for 10-50 minutes, and then air cool it to below 40℃ to obtain the hot-rolled plate.

[0060] 3) After removing the surface iron oxide scale from the obtained hot-rolled plate, it is subjected to multiple cold rolling passes with a reduction rate of 80-85% to obtain a cold-rolled sheet with a thickness of 0.5-0.95 mm.

[0061] 4) Cold-rolled sheet is recrystallized and annealed at 600-650℃ for 30-40 minutes to obtain ultra-low carbon steel.

[0062] <Thin-gauge ultra-low carbon steel>

[0063] This invention also provides a thin-gauge ultra-low carbon steel prepared according to the method described above. The ultra-low carbon steel of this invention has the following composition: C content ≤ 0.02 wt%, Si content ≤ 0.03 wt%, Mn content ≤ 0.002 wt%, P content ≤ 0.001 wt%, S content ≤ 0.001 wt%, La content ≤ 0.095 wt%, and Fe as the balance. It may contain unavoidable impurities. The thickness of the obtained thin-gauge ultra-low carbon steel can be 0.5–1.0 mm, preferably 0.5–0.95 mm, more preferably 0.6–0.95 mm, and even more preferably 0.7–0.93 mm. The obtained ultra-low carbon steel has an Rp0.2 greater than or equal to 105 MPa and an R0 less than 3. Testing demonstrates that the ultra-low carbon steel prepared according to the method of this invention eliminates the annealing yield plateau.

[0064] <Performance Description in Examples and Comparative Examples>

[0065] Yield strength: The strength at the lower yield point on the tensile curve.

[0066] Rp0.2: The stress value corresponding to a material exhibiting 0.2% plastic elongation in a tensile test.

[0067] R0: The ratio of strain in the width direction to strain in the thickness direction.

[0068] Example 1

[0069] 1) Provide pure iron raw material with a purity of 99.99wt% and La raw material with a purity of 99.99wt%; add the pure iron raw material and La raw material to a vacuum furnace in sequence for melting and casting to obtain an ingot; wherein, the mass of La raw material is 0.067wt% of the mass of pure iron raw material;

[0070] 2) Heat the ingot to 1200℃ and hold it at 1200℃ for 2 hours. Then, start hot rolling at 1050℃. The hot rolling reduction rate is 66.7%, the hot rolling reduction amount Δh is 10 mm, and the final rolling temperature is 850℃. After hot rolling, hold it at 600℃ for 30 minutes and then air cool it to room temperature to obtain the hot-rolled plate.

[0071] 3) After removing the surface iron oxide scale from the obtained hot-rolled plate, it is subjected to multiple cold rolling passes with a reduction rate of 80% to obtain a cold-rolled sheet with a thickness of 0.77 mm.

[0072] 4) The cold-rolled sheet was recrystallized and annealed at 600℃ for 30 minutes to obtain ultra-low carbon steel.

[0073] Example 2

[0074] 1) Provide pure iron raw material with a purity of 99.99wt% and La raw material with a purity of 99.99wt%; add the pure iron raw material and La raw material to a vacuum furnace in sequence for melting and casting to obtain an ingot; wherein, the mass of La raw material is 0.08wt% of the mass of pure iron raw material.

[0075] 2) Heat the ingot to 1200℃ and hold it at 1200℃ for 2 hours. Then, start hot rolling at 1050℃. The hot rolling reduction rate is 66.7%, the hot rolling reduction amount Δh is 10 mm, and the final rolling temperature is 850℃. After hot rolling, hold it at 600℃ for 30 minutes and then air cool it to room temperature to obtain the hot-rolled plate.

[0076] 3) After removing the surface iron oxide scale from the obtained hot-rolled plate, it is subjected to multiple cold rolling passes with a reduction rate of 80% to obtain a cold-rolled sheet with a thickness of 0.91 mm.

[0077] 4) The cold-rolled sheet was recrystallized and annealed at 600℃ for 30 minutes to obtain ultra-low carbon steel.

[0078] Comparative Example 1

[0079] In this comparative example, no La raw material is added. The specific steps are as follows:

[0080] 1) Provide pure iron raw material with a purity of 99.99wt%; add the pure iron raw material to a vacuum furnace for melting and casting to obtain ingots.

[0081] 2) Heat the ingot to 1200℃ and hold it at 1200℃ for 2 hours. Then, start hot rolling at 1050℃. The hot rolling reduction rate is 66.7%, the hot rolling reduction amount Δh is 10 mm, and the final rolling temperature is 850℃. After hot rolling, hold it at 600℃ for 30 minutes and then air cool it to room temperature to obtain the hot-rolled plate.

[0082] 3) After removing the surface iron oxide scale from the obtained hot-rolled plate, it is subjected to multiple cold rolling passes with a reduction rate of 80% to obtain a cold-rolled sheet with a thickness of 0.83 mm.

[0083] 4) The cold-rolled sheet was recrystallized and annealed at 600℃ for 30 minutes to obtain ultra-low carbon steel.

[0084] Table 1

[0085]

[0086] Note: In the table, "-" indicates "none".

[0087] As can be seen from the table, the ultra-low carbon steel prepared by the method of the present invention eliminates the annealing yield plateau.

[0088] There are generally two methods for representing yield strength. If the tensile curve has a yield plateau, then the yield strength of the steel is considered to be the lower yield point value Rel on the yield plateau; if the tensile curve does not have a yield plateau, then the yield strength value is Rp0.2. In the embodiments of the present invention, the tensile curves do not have yield plateaus, so the yield point is represented by Rp0.2.

[0089] This invention is not limited to the above-described embodiments. Any modifications, improvements, or substitutions that can be conceived by those skilled in the art without departing from the essential content of this invention fall within the scope of this invention.

Claims

1. A method for eliminating the annealing yield plateau in thin-gauge ultra-low carbon steel, characterized in that, The method includes the following steps: 1) Provide pure iron and La raw materials; add the pure iron and La raw materials sequentially into a vacuum furnace for melting and casting to obtain ingots; wherein the mass of the La raw material is 0.065 to 0.095 wt% of the mass of the pure iron raw material; 2) Heat the ingot to 1000–1250℃ and hold at 1000–1250℃ for 1–3 hours, then begin hot rolling at 950–1200℃. The hot rolling reduction rate is 65.5–69%, and the hot rolling reduction amount is… △h The thickness is 9-11 mm, and the final rolling temperature is 830-860℃; after hot rolling, it is held at 600-650℃ for 10-50 minutes, and then air-cooled to below 50℃ to obtain hot-rolled plate; 3) After removing the surface iron oxide scale from the obtained hot-rolled plate, it is subjected to multiple cold rolling passes with a reduction rate of 80-85% to obtain a cold-rolled sheet with a thickness of 0.5-1.0 mm. 4) Recrystallization annealing of cold-rolled sheet yields ultra-low carbon steel; The ultra-low carbon steel has the following composition: C content less than or equal to 0.02 wt%, Si content less than or equal to 0.03 wt%, Mn content less than or equal to 0.002 wt%, P content less than or equal to 0.001 wt%, S content less than or equal to 0.001 wt%, La content greater than or equal to 0.06 wt% and less than 0.095 wt%, with Fe as the balance.

2. The method according to claim 1, characterized in that, In step 1), the mass of the La raw material is 0.065 to 0.09 wt% of the mass of the pure iron raw material; the smelting is carried out in a vacuum electric arc furnace.

3. The method according to claim 1, characterized in that, In step 2), hot rolling begins at 1000–1100°C.

4. The method according to claim 1, characterized in that, In step 2), the reduction rate of hot rolling is 66-68%.

5. The method according to claim 1, characterized in that, In step 2), after hot rolling, the temperature is maintained at 600-650℃ for 15-40 minutes.

6. The method according to claim 1, characterized in that, In step 2), air cool to below 40°C.

7. The method according to claim 1, characterized in that, In step 3), the thickness of the cold-rolled sheet is 0.6 to 0.95 mm.

8. The method according to claim 1, characterized in that, In step 4), the recrystallization annealing temperature is 600-650℃ and the time is 20-40 min.

9. The method according to any one of claims 1 to 8, characterized in that, The obtained ultra-low carbon steel has an Rp0.2 greater than or equal to 105 MPa.

10. A thin-gauge ultra-low carbon steel prepared by the method according to any one of claims 1 to 9.

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