Ultra-low carbon hot-rolled steel plate with good formability and manufacturing method of ultra-low carbon hot-rolled steel plate

Abstract

The invention discloses an ultra-low carbon hot-rolled steel plate with good formability and a manufacturing method thereof. The ultra-low carbon hot-rolled steel plate comprises the following chemical components in percentage by weight: 0.0005-0.004% of C, less than or equal to 0.05% of Si, 0.1-1.0% of Mn, less than or equal to 0.035% of P, 0.015-0.050% of S, less than or equal to 0.010% of Al, less than or equal to 0.005% of N, 0.025-0.055% of O, 0.02-0.10% of Nb, 0.01-0.10% of Cu, 0.01-0.10% of Cr, less than or equal to 0.05% of Ni, less than or equal to 0.05% of Mo and the balance of Fe and other inevitable impurities. The steel plate has good punch forming performance and aging resistance, the elongation rate of the steel plate is larger than or equal to 40%, the aging index (AI) of the steel plate is smaller than or equal to 45 MPa, the defects of fish scaling, bubbles, pinholes, black spots and the like generated during electrostatic enameling can be completely overcome, and the machining requirements for punching, flanging, cold rolling and the like during water heater inner container manufacturing, baking tray manufacturing and the like are met.

Description

Technical Field

[0001] The invention belongs to the technical field of enameled steel plate processing, and in particular relates to an ultra-low carbon hot-rolled steel plate with good formability and a manufacturing method thereof. Background Art

[0002] In the past, the wet enameling process was primarily used to manufacture enameled bakeware, flat plates, and water heater liners. Steel plates undergo forming, welding, and surface treatment before undergoing a single enameling and sintering step. While wet enameling provides relatively good adhesion and scale resistance between the enamel and the steel, the resulting layer is thick and uneven, with poor surface quality and low production efficiency. Electrostatic enameling, replacing the traditional wet enameling process, makes it easier to control the thickness of the layer, resulting in a thinner and more uniform layer. This layer is denser, more aesthetically pleasing, and improves production efficiency. Electrostatic enameling is increasingly being used in the production of enameled products such as enameled bakeware, flat plates, and water heater liners.

[0003] The electrostatic enameling process places higher demands on the steel plate's enameling performance. When using ordinary enameled steel plates in the electrostatic enameling process, the adhesion between the enamel powder and the steel plate is not as good as that of wet enameling. Furthermore, the electrostatic enameling layer is dense and has poor air permeability, making it more prone to defects such as scale, bubbles, pinholes, and black spots. These defects can seriously damage the inherent and external quality of the enameled product. Especially when only one glaze is applied, the steel plate is subject to even higher requirements in terms of scale resistance, adhesion, and resistance to defects such as black spots, bubbles, and pinholes.

[0004] Among the existing enameled steels, ultra-low carbon high-oxygen steel is the most widely used enameled steel, but it is mainly cold-rolled plate. The high oxygen content mainly forms oxide inclusions in the steel as hydrogen storage traps. The purpose of controlling ultra-low carbon is to reduce enamel defects such as enamel bubbles and pinholes and improve the plasticity of the steel.

[0005] Chinese patent application No. 200780034772.5 discloses "enamel glazing processing products and enameled processing products". The steel is given a control range of elements such as C, Si, Mn, P, S, O, and precious elements such as Ti and Co are added. The steel involves many types of alloy elements and a wide range of contents. By forming an oxide film formed by the oxides of the steel plate components on the surface of the steel plate, the close adhesion between the enamel layer and the steel plate is improved. The steel plate needs to undergo multiple processes such as hot rolling, cold rolling, annealing and tempering rolling.

[0006] Chinese patent application No. 201810663362.7 discloses "A Low-Cost High-Oxygen Enameled Steel Produced by a CSP Process and Its Manufacturing Method." This is also a high-oxygen steel, with low C and Mn content, eliminating the need for expensive alloying elements such as Nb, Ti, and V. The CSP thin slab continuous casting and rolling process is used, with the finishing rolling temperature controlled within a narrow temperature range of 880±20°C. MnO and MnS inclusions formed in the steel act as hydrogen traps, resulting in a metallographic structure of recrystallized ferrite + pearlite + a small amount of deformed microstructure. The resulting steel exhibits excellent phosphorus explosion resistance and mechanical properties. However, pearlite in the steel is prone to defects such as enamel bubbles and pinholes during enameling, and the steel plate composition does not specify lower limits for key elements such as oxygen and sulfur.

[0007] Chinese patent application No. 201710064842.7 discloses "A hot-rolled enameled steel plate for subway decoration and its manufacturing method". The chemical composition of the steel plate is as follows by mass percentage: Si 0.10-0.20%, Mn 0.65-0.95%, P≤0.012%, S≤0.015%, Ti 0.05-0.10%, Cr 0.10-0.20%, and the rest are iron and unavoidable impurities. Although the yield strength of the steel plate reaches more than 390MPa, due to the high carbon content in the steel, ferrite + a small amount of cementite is formed, and the grain size is above level 10, which is not conducive to improving the steel plate's enameling performance such as resistance to bubbles, pinholes or black spots.

[0008] It is difficult for existing low-carbon or ultra-low-carbon, high-oxygen enameled steels to simultaneously achieve good stamping and forming properties and resistance to aging, while also having good resistance to scale explosion, adhesion, and resistance to black spots, bubbles, pinholes and other properties, so as to meet the requirements that the steel plates will not produce defects such as scale explosion, bubbles, pinholes and black spots during the processing of enameled products such as enameled baking trays, flat plates and water heater tanks using the electrostatic enameling process. Summary of the Invention

[0009] The object of the present invention is to provide an ultra-low carbon hot-rolled steel plate with good formability and a manufacturing method thereof. The obtained steel plate has good stamping forming performance and aging resistance, with an elongation of 40% or more and an aging index (AI) of 45MPa or less. In addition, defects such as scale explosion, bubbles, pinholes and black spots on the steel plate during electrostatic enameling can be completely avoided, and the processing requirements such as stamping, flanging and cold rolling for making water heater inner tanks, baking trays, etc. can be met.

[0010] To achieve the above object, the technical solution of the present invention is:

[0011] An ultra-low carbon hot-rolled steel plate with good formability, wherein the chemical composition by weight is as follows: C: 0.0005-0.004%, Si≤0.05%, Mn: 0.1-1.0%, P≤0.035%, S: 0.015-0.050%, Al≤0.010%, N≤0.005%, O: 0.025-0.055%, Nb: 0.02-0.10%, Cu: 0.01-0.10%, Cr: 0.01-0.10%, Ni≤0.05%, Mo≤0.05%, the balance including Fe and other unavoidable impurities, and the following must also be met:

[0012] Nb≥(93 / 12)×C+(93 / 14)×N,

[0013] Mn / [(55 / 32)×S+(55 / 16)×O]≥0.7.

[0014] Preferably, the Nb content is 0.02-0.085%.

[0015] Furthermore, the chemical composition of the steel plate also includes B: 0.0003-0.005%.

[0016] Furthermore, the balance is Fe and other inevitable impurities.

[0017] The microstructure of the steel plate of the present invention is ferrite, the ferrite grain size is 7-10, manganese oxysulfide and ferrous sulfide composite inclusions are dispersed on the ferrite, and the mass percentage of the inclusions is above 0.13%.

[0018] The steel plate of the present invention has a yield strength of ≤270 MPa, a tensile strength of ≥270 MPa, an elongation of ≥40%, and an aging index of ≤45 MPa.

[0019] In the chemical composition design of the ultra-low carbon hot-rolled steel plate with good formability described in the present invention:

[0020] Carbon: The lower the carbon content in steel, the better the plasticity and formability of the steel plate. Carbon can exist in steel in the form of solid solution, cementite, pearlite, and metallic compound precipitation phases. Both solid solution carbon and carbide increase the strength of the steel while reducing its plasticity and formability. In particular, the higher the solid solution carbon content, the more susceptible the steel is to aging, i.e., the higher the aging index. Therefore, the lower the carbon content, the better the formability of the steel plate. The impact of carbon on the surface quality of enamel is also extremely critical. Whether it is solid solution carbon in steel or carbon in the form of cementite or pearlite, carbon monoxide and other gases are generated during the enameling process. This is the main source of enamel bubbles, pinholes, and black spots. Therefore, the lower the carbon content, the fewer bubbles are formed and the better the anti-pinhole performance. By keeping the carbon content low and adding appropriate amounts of strong carbide-forming elements such as niobium, the carbon in the steel is precipitated as carbides, avoiding the formation of pearlite and massive cementite, and reducing or eliminating dissolved carbon. This improves plasticity, reduces or eliminates aging, and enhances enamel quality. However, due to the limited decarburization capacity of smelting and refining during the steelmaking process, the lower the carbon content, the higher the manufacturing cost. Therefore, the carbon content of the present invention is controlled to 0.0005-0.004%.

[0021] Aluminum: Aluminum is a strong deoxidizing element. If the aluminum content is high, the oxygen content will decrease, and the formed aluminum oxide inclusions will have poor plasticity, which will damage the workability of the steel and also damage the adhesion of the enamel. Therefore, the lower the aluminum content, the better. Therefore, the present invention controls the aluminum content to ≤0.010%.

[0022] Silicon: Silicon is a harmful element. A high silicon content impairs adhesion between the steel plate and the enamel during enameling, impairing the enameling properties. Furthermore, silicon easily forms oxide inclusions with poor ductility, impairing the plasticity of the steel. Therefore, the present invention controls the silicon content to ≤ 0.05%.

[0023] Phosphorus: Steel plates with high phosphorus content are prone to bubbles and black spots during enameling, which damages the surface quality of the enamel. The lower the phosphorus content in steel, the better. Therefore, the present invention controls the phosphorus content to ≤0.035%.

[0024] Nitrogen: Like carbon, nitrogen can exist as a solid solution element and also form compounds with alloying elements such as niobium. Nitrogen compounds can strengthen the matrix and act as hydrogen traps to improve fishscale resistance. However, nitrogen in solid solution is susceptible to aging of the steel sheet. This means that if the steel sheet is left standing for too long, its yield strength will increase and its plasticity will decrease. Excessive nitrogen content also results in a high amount of nitrides, which can also impair the steel's plasticity. Therefore, the present invention controls the nitrogen content to ≤ 0.005%.

[0025] Manganese: Manganese reacts with oxygen to form manganese oxide, and also reacts with sulfur to form manganese sulfide. In the present invention, by properly matching the amounts of oxygen, manganese, and sulfur, composite inclusions such as manganese oxysulfide and ferrous sulfide are primarily formed in the steel. When Mn / [(55 / 32)×S+(55 / 16)×O]≥1, composite inclusions primarily composed of manganese oxysulfide are formed; when 0.7≤Mn / [(55 / 32)×S+(55 / 16)×O]<1, composite inclusions such as manganese oxysulfide and ferrous sulfide are formed. These inclusions are generated in molten steel, and their quantity, morphology, and distribution are relatively fixed in the steel, rarely affected by heating, rolling, cooling, or temperature. They serve as stable hydrogen storage traps, and the presence of these inclusions is beneficial for improving scale resistance. The amount of manganese added is set to Mn / [(55 / 32)×S+(55 / 16)×O]≥0.7. However, a large number of inclusions or large inclusions will severely damage the plasticity and formability of steel. Manganese is an effective element for improving strength. If the manganese content is too high, the adhesion of the enamel will deteriorate after pickling, and defects such as bubbles and black spots will easily occur. Therefore, the manganese content in the present invention is controlled at 0.1-1.0%.

[0026] Sulfur: When an appropriate amount of sulfur is added to the steel, and when the oxygen content is also high, sulfur, oxygen, and manganese are more likely to form composite inclusions called manganese oxysulfide. Sulfur may also form ferrous sulfide inclusions, with MnS, FeS, and MnO primarily existing as composite inclusions. This significantly improves the elongated shape of pure MnS after rolling, reducing damage to the transverse plasticity of the steel plate, while also preventing the formation of single, coarse particle inclusions. However, excessive sulfur content, such as above 0.050%, leads to excessive inclusions in the steel, making it difficult to maintain the plasticity of the steel plate. Therefore, the sulfur content in the present invention is controlled between 0.015% and 0.050%.

[0027] Oxygen: Oxygen is a decarburizing element during the smelting process, and higher oxygen levels facilitate decarburization. Oxygen in steel forms oxide inclusions, which act as beneficial hydrogen traps and improve the steel's fishscale resistance. Oxygen can form manganese oxide inclusions with manganese. At higher sulfur contents, it forms manganese oxide sulfide inclusions or a combination of manganese oxide and ferrous sulfide. The type and quantity of these inclusions depend on the oxygen, manganese, and sulfur contents in the steel and their chemical ratios. When the oxygen content is below 0.025%, the amount of inclusions formed is low, insufficient to ensure fishscale resistance. Conversely, when the oxygen content exceeds 0.055%, not only are the number of inclusions high, but the particle size is also large, severely impairing the steel's plasticity. Therefore, the oxygen content in the present invention is controlled between 0.025% and 0.055%.

[0028] Niobium: Interstitial atoms such as carbon and nitrogen dissolved in steel are the primary cause of aging in steel sheets. Niobium is a strong carbon and nitride-forming element, combining with carbon and nitrogen to form niobium carbide, niobium nitride, or composite niobium carbonitrides. The present invention controls Nb to ≥ (93 / 12) × C + (93 / 14) × N, effectively fixing the dissolved carbon and nitrogen content in the steel. This significantly reduces or eliminates aging in the steel sheet, significantly lowering the aging index. Increasing the niobium content decreases the content of interstitial carbon and nitrogen atoms in the steel, reducing or eliminating aging in the steel sheet and improving its plasticity and workability. Therefore, the present invention controls the niobium content to 0.02-0.10%.

[0029] Boron: Boron can form compounds with carbon and nitrogen and can also be dissolved in steel, both of which act as hydrogen traps. Adding an appropriate amount of boron to hot-rolled steel helps improve its anti-scale performance. However, excessive boron content can easily cause cracks in the ingot during continuous casting. Therefore, the boron content in this invention is controlled within a range of 0.0003-0.005%.

[0030] Copper, chromium, nickel, and molybdenum: Appropriate amounts of copper, chromium, nickel, and molybdenum in steel can accelerate the adhesion reaction during the enameling process. However, these elements also affect the pickling speed of the steel sheet. Excessive levels can inhibit the pickling speed and affect the pickling effect. However, these alloying elements are precious metals, and adding them significantly increases the cost of the alloy. Therefore, in the present invention, the copper content is controlled at 0.01-0.10%, the chromium content is 0.01-0.10%, the nickel content is ≤0.05%, and the molybdenum content is ≤0.05%.

[0031] The present invention also provides a method for manufacturing an ultra-low carbon hot-rolled steel plate with good formability, comprising the following steps:

[0032] 1) Smelting, refining and continuous casting

[0033] According to the above composition, converter smelting and RH or LF refining are carried out, and slabs are continuously cast;

[0034] 2) Hot rolling

[0035] Slab heating temperature: 1100-1250℃, followed by hot rolling, and the final rolling temperature is controlled at 800-960℃;

[0036] 3) Coiling

[0037] The hot-rolled steel plate is cooled by water or air and then coiled at a coiling temperature of 520-820°C.

[0038] In the method for manufacturing the steel plate of the present invention:

[0039] Converter blowing and refining ensure that the basic components of the molten steel meet the design requirements of the present invention, decarburize and remove harmful gases and other harmful elements in the steel, add necessary alloying elements, and adjust the composition. In particular, it is necessary to control elements such as carbon, nitrogen, sulfur, and oxygen, and add appropriate amounts of alloying elements such as manganese, copper, chromium, nickel, molybdenum, niobium, and boron. Since inclusions are generated during the steelmaking process, the formation of coarse inclusions and the aggregation of inclusions must be prevented during the smelting and continuous casting processes. By controlling the sulfur content and oxygen content, mainly complex inclusions are formed in the steel, which is conducive to refining the inclusions and also avoids the harm caused by the formation of independent manganese sulfide plastic inclusions.

[0040] Continuous Casting: The smelted molten steel is continuously cast into slabs. To prevent carbon and nitrogen addition, a carbon-free protective slag is used during the casting process, and vacuum or inert gas protection is used to prevent air from entering and causing nitrogen addition. Hot Rolling: The slabs are heated and hot rolled at a temperature of 1100-1250°C. During the heating process, the slabs are transformed into austenite and fully homogenized.

[0041] In addition, the ultra-low carbon content and the resulting ferrite structure in the steel of the present invention allow hot rolling to be completed in either the austenite region (no higher than 960°C) or the ferrite region (no lower than 800°C). As long as the final rolling temperature is no lower than 800°C, the ferrite can complete recrystallization and grain growth, ultimately achieving a ferrite structure with a grain size controlled between 7 and 10, thereby achieving good plasticity. The final hot rolling temperature adopted by the present invention is 800-960°C, and the coiling temperature is 520-820°C. That is, both the final rolling and coiling are carried out within a relatively wide temperature range, making the process easier to implement and more stably controlled.

[0042] The inclusions in the steel can stably maintain their precipitation form and quantity during the slab heating and rolling process, and are difficult to dissolve even after long-term heating at high temperature before hot rolling. Therefore, the inclusions have stability in form, size and quantity, which well guarantees the hydrogen storage performance and anti-scale explosion performance of the steel.

[0043] Therefore, the ultra-low carbon content ensures that the steel plate's microstructure is composed of ferrite, without forming large or substantial amounts of cementite or pearlite. This is crucial for improving the steel plate's plasticity and suppressing defects such as bubbles, pinholes, and black spots during enameling. The lower-carbon alloy design and simplified manufacturing process result in lower alloy and process costs, faster production, and stable control of the steel's microstructure, inclusions, and precipitates. This ensures a perfect match between the steel plate's mechanical properties and enameling performance, meeting the high quality requirements of enameled products from various enameling processes.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] The present invention adopts ultra-low carbon, adds a very small amount of alloying elements such as copper, chromium, nickel, molybdenum, niobium or boron, and controls the content of elements such as Si, P, Al, and N in a relatively low range as much as possible to meet the plasticity requirements of the steel plate. Nb is added and Nb is controlled to be ≥ (93 / 12)×C+(93 / 14)×N, which effectively fixes the solid-solution carbon and nitrogen content in the steel, can significantly reduce or eliminate the aging of the steel plate, and significantly reduce the aging index of the steel plate, so that the steel plate has good stamping forming performance and aging resistance.

[0046] The compositional design combines the controlled contents of oxygen, sulfur, and manganese to form composite inclusions composed of manganese oxysulfide, MnS, FeS, and MnO. The weight percentage of these inclusions in the steel is maintained at above 0.13%. These inclusions act as beneficial hydrogen traps, and the formation of sufficient inclusions in the steel ensures excellent fishscale resistance. This significantly improves the elongated shape of simple MnS inclusions after rolling, mitigating the damage to the transverse plasticity of the steel plate. It also prevents the formation of large, aggregated inclusions, significantly contributing to the stabilization and improvement of the fishscale resistance of the hot-rolled steel plate. By minimizing the addition of precious elements to the steel plate, alloy costs are also lowered.

[0047] The present invention, based on component design and combined with process control of heating and rolling temperatures, ensures that the microstructure of the steel plate is composed of ferrite, the ferrite grain size is controlled between 7 and 10 levels, and no cementite or pearlite is formed, thereby improving the plasticity and formability of the steel plate and suppressing the occurrence of enamel defects such as bubbles, pinholes and black spots during enameling.

[0048] The manufacturing method of this invention utilizes an ultra-low carbon composition design, allowing hot rolling to be performed in either the austenite or ferrite regions, ensuring ferrite recrystallization and grain growth. Each process is easy to implement and stably control. Conventional hot rolling processes, on the other hand, require rolling to be completed within the austenite temperature range and utilize laminar cooling. Compared to cold rolling, this method offers a shorter process and faster production.

[0049] The steel plate of the present invention has a yield strength of ≤270 MPa, a tensile strength of ≥270 MPa, an elongation of ≥40%, and an aging index of ≤45 MPa, achieving a good match between the steel plate's good stamping forming performance, aging resistance, and enameling performance. The steel plate is not only suitable for wet enameling and electrostatic enameling with one-time enameling and one-time firing, but can also be used for double enameling with two-time firing or two-time spraying and one-time firing. The steel plate has good workability and enameling performance in the production of water heater inner tanks, baking trays, flat plates, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is a microstructure photograph of the steel plate of Example 1 of the present invention. DETAILED DESCRIPTION

[0051] The present invention is further illustrated by the following examples, but this is not intended to limit the present invention. Those skilled in the art may make modifications or improvements based on the basic ideas of the invention. As long as they do not deviate from the basic ideas of the invention, they are all within the scope of the present invention.

[0052] The composition and process parameters of the embodiments of the present invention are shown in Table 1 and Table 2, wherein the remainder of the composition is Fe and other unavoidable impurities.

[0053] The samples obtained in Examples 1-10 of the present invention were cut transversely to form tensile specimens. The parallel portion of the specimens had a width of 20 mm and a gauge length of 80 mm. The mechanical properties were tested according to standard GB / T 228.1. The test results are listed in Table 3.

[0054] Aging index (AI) test method: the pre-stretching deformation rate is 8%, the aging treatment process is 100℃×60min, and the aging index, i.e., the AI ​​value, is calculated based on the load corresponding to the two stretches of the sample before and after aging treatment.

[0055] Enameling Performance: Samples approximately 200 x 200 mm were cut and subjected to enameling testing. The enameling test steps included: acid washing or sandblasting followed by degreasing, followed by water washing, drying, and electrostatic dry powder spraying. The enameling process was then fired at 840°C ± 10°C for 4 minutes, resulting in a ceramic layer thickness of 150 μm ± 30 μm. After the enameling test, the enameled samples were exposed to air for 48 hours and visually inspected for defects such as scale, pinholes, bubbles, and black spots. Adhesion was then tested using a drop weight test according to Annex C of EN 10209:2013, with A1 representing the best adhesion.

[0056] As shown in Table 3, the hot-rolled steel sheet obtained by the present invention has an elongation of over 40% and an AI of less than 45 MPa. Using electrostatic spraying conditions, no scale cracking occurs, excellent adhesion is achieved, and the ceramic layer exhibits excellent resistance to pinholes, bubbles, and black spots when the thickness is 150 μm ± 30 μm.

[0057] like Figure 1 As shown, the microstructure of the hot-rolled steel plate of the present invention is composed of ferrite.

[0058] The hot-rolled steel plate obtained by the present invention has good plasticity and low aging resistance, and can solve the problems of anti-scale explosion, adhesion and anti-pinhole defects that ordinary low-carbon steel plates cannot meet in the electrostatic dry powder enamel process. It is widely applicable and has good application prospects.

[0059] It should be noted that the embodiments listed above are only specific embodiments of the present invention. Within the scope of protection of the present invention, similar changes or modifications made can be directly derived or easily associated with by those skilled in the art from the contents disclosed in the present invention, and should all fall within the scope of protection of the present invention.

[0060]

[0061]

[0062]

Claims

1. An ultra-low carbon hot-rolled steel plate with good formability, the chemical composition weight percentage of which is: C: 0.0005-0.004%, Si≤0.05%, Mn: 0.1-1.0%, P≤0.035%, S: 0.015-0.050%, Al≤0.010%, N≤0.005%, O: 0.025-0.055%, Nb: 0.02-0.10%, Cu: 0.01-0.10%, Cr: 0.01-0.10%, Ni≤0.05%, Mo≤0.05%, the balance includes Fe and other unavoidable impurities; and it is also necessary to meet the following requirements at the same time: Nb≥(93 / 12) × C+(93 / 14) × N, Mn / [(55 / 32) × S+(55 / 16) × O]≥0.7。 2. The ultra-low carbon hot-rolled steel sheet with good formability according to claim 1, characterized in that: The balance is Fe and other inevitable impurities.

3. The ultra-low carbon hot-rolled steel sheet with good formability according to claim 1 or 2, characterized in that: Also includes B: 0.0003~0.005%.

4. The ultra-low carbon hot-rolled steel sheet with good formability according to claim 1, 2 or 3, characterized in that: The Nb content is 0.02-0.085%.

5. The ultra-low carbon hot-rolled steel sheet with good formability as claimed in claim 1, 2, 3 or 4, characterized in that: The microstructure of the steel plate is ferrite, the ferrite grain size level is 7-10, manganese oxysulfide and ferrous sulfide composite inclusions are dispersed on the ferrite, and the mass percentage of the inclusions is above 0.13%.

6. The ultra-low carbon hot-rolled steel sheet with good formability according to any one of claims 1 to 5, characterized in that: The steel plate has a yield strength of ≤270 MPa, a tensile strength of ≥270 MPa, an elongation of ≥40%, and an aging index of ≤45 MPa.

7. The method for manufacturing an ultra-low carbon hot-rolled steel sheet with good formability according to any one of claims 1 to 6, characterized in that: The steps include: 1) Smelting, refining and continuous casting Smelting, RH or LF refining, and continuous casting into slabs according to the composition of claim 1, 2, 3 or 4; 2) Hot rolling The slab is heated to 1100-1250°C and then hot rolled. The rolling process includes rough rolling and finish rolling. The final rolling temperature is 800-960°C. 3) Coiling The hot rolled steel plate is cooled by water cooling or / and air cooling, and then coiled at a coiling temperature of 520-820°C.

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