Low-carbon-footprint if steel and manufacturing method therefor

Abstract

Disclosed in the present invention is low-carbon-footprint IF steel, which comprises Fe and inevitable impurities, and further comprises the following chemical elements in percentages by mass: 0<C≤0.0030%, 0<Si≤0.03%, Mn: 0.05-0.20%, Ti: 0.06-0.09%, Al: 0.02-0.06%, N: 0.005-0.01%; S: 0.01-0.015%; and B: 0.0002-0.0006%, wherein in inevitable impurities, the content of O is controlled to be less than or equal to 0.0040%. Another object of the present invention is to provide a method for manufacturing low-carbon-footprint IF steel, which method comprises the steps of: performing electric furnace smelting, LF, RH and continuous casting by using full scrap steel; hot rolling and coiling; cold rolling; and continuous annealing.

Description

A low-carbon-footprint IF steel and its manufacturing method Technical Field

[0001] This invention relates to steel plates and their manufacturing methods, and more particularly to an IF steel and its manufacturing method. Background Technology

[0002] IF steel, also known as interstitial atom-free steel, possesses excellent deep-drawing properties, a high plastic strain ratio, high elongation, a relatively low yield strength ratio, and excellent non-aging properties. IF steel is characterized by ultra-low carbon, microalloying, and ultra-purity. Its essence lies in the addition of Ti and Nb to the steel to form corresponding nitrides and carbides, thereby obtaining a clean ferritic steel without interstitial atoms.

[0003] The existing IF steel production process mainly adopts the conventional smelting method of blast furnace ironmaking - converter blowing - RH vacuum treatment - continuous casting. This process is relatively mature, and its main features are that the converter reduces carbon and increases oxygen, and the RH process further reduces carbon and removes nitrogen and oxygen through cyclic degassing. With the requirements of technological development, the production of IF steel has further placed demands on low carbon footprint. Since carbon emissions from the smelting process account for the highest proportion of the carbon footprint in the entire product manufacturing process, green and low-carbon requirements have also been put forward for the smelting process of IF steel. Summary of the Invention

[0004] One objective of this invention is to provide a low-carbon footprint IF steel. This IF steel, through specific composition design combined with controlled Ti inclusions of a certain size, achieves a suitable grain structure while ensuring the material's plasticity, bending, and porosity performance. In this document, low carbon footprint refers to a carbon footprint ≤ 1.2 kgCO2eq / kg.

[0005] To achieve the above objectives, the present invention provides a low-carbon footprint IF steel containing Fe and unavoidable impurities, and further containing the following chemical elements in the following mass percentages:

[0006] 0 < C ≤ 0.0030%, 0 < Si ≤ 0.03%, Mn: 0.05-0.20%, Ti: 0.06-0.09%, Al: 0.02-0.06%, N: 0.005-0.01%, S: 0.01-0.015%, B: 0.0002-0.0006%; wherein, in unavoidable impurities, O ≤ 0.0040%.

[0007] Furthermore, in the IF steel described in this invention, the mass percentage content of each chemical element is as follows:

[0008] 0 < C ≤ 0.0030%, 0 < Si ≤ 0.03%, Mn: 0.05-0.20%, Ti: 0.06-0.09%, Al: 0.02-0.06%, N: 0.005-0.01%, S: 0.01-0.015%, B: 0.0002-0.0006%; the balance is Fe and unavoidable impurities; among the unavoidable impurities, O ≤ 0.0040%.

[0009] Furthermore, the design principles of each chemical element in the IF steel described in this invention are as follows:

[0010] C: In the IF steel described in this invention, carbon (C) directly affects the strength, weldability, and formability of the steel sheet / strip. Simultaneously, C is a critical interstitial atom; controlling the C content at a low level allows for good development of the material texture and excellent deep-drawing performance. Therefore, in the IF steel described in this invention, the mass percentage of C is controlled to be 0 < C ≤ 0.0030%. In some embodiments, the mass percentage of C can be controlled to be 0 < C ≤ 0.0020%.

[0011] Si: In the IF steel described in this invention, excessive Si content will affect the surface quality of the steel plate and lead to poor toughness, thus affecting its performance. Therefore, in the IF steel described in this invention, the mass percentage of Si is controlled to be 0 < Si ≤ 0.03%.

[0012] Mn: In the IF steel described in this invention, Mn is generally used as a major solid solution strengthening element and desulfurizer, which is beneficial to improving the strength of the steel. However, when the Mn content is too high, it is easy to cause segregation problems of banded structure. Therefore, in the IF steel described in this invention, the mass percentage of Mn is controlled between 0.05% and 0.20%. In some embodiments, the mass percentage of Mn can be controlled between 0.1% and 0.15%.

[0013] Ti: In the IF steel described in this invention, Ti mainly functions as a key interstitial element for fixing C and N in the steel. Excessive dissolved C and N atoms in the steel will hinder the formation of the {111} texture and cause a sharp decrease in the r-value. Furthermore, Ti needs to combine with S in the steel to form S-oxide inclusions to mitigate the detrimental effects of bulk TiN inclusions on material formability. Therefore, in the IF steel described in this invention, the mass percentage of Ti is controlled between 0.06% and 0.09%. In some embodiments, the mass percentage of Ti can be controlled between 0.07% and 0.08%.

[0014] Al: In the IF steel described in this invention, Al mainly plays a deoxidizing role. However, when the Al content in the steel is too high, it will worsen the continuous casting playability and easily form a large number of oxide inclusions, which is detrimental to the cleanliness of the molten steel. Therefore, in the IF steel described in this invention, the mass percentage of Al is controlled between 0.02% and 0.06%. In some embodiments, the mass percentage of Al can be controlled between 0.02% and 0.05%.

[0015] N: In the IF steel described in this invention, the electric furnace smelting route results in a relatively high nitrogen (N) content. Ideally, N in IF steel should exist as a precipitated solidified form, as excessive free N will affect the deep-drawing performance and R-value of the IF steel. However, an excessively high N content will introduce too many TiN-type inclusions, which may become crack initiation sites during deep deformation, thus affecting the material's formability. Therefore, in the IF steel described in this invention, the mass percentage of N is controlled between 0.005% and 0.01%.

[0016] S: In steel, sulfur (S) generally acts as an impurity element, playing a detrimental role, primarily due to the hot brittleness caused by sulfur compounds reaching a certain level. However, in the IF steel described in this invention, due to its relatively low strength and excellent plasticity, the control requirements for S content are relatively relaxed. Introducing a certain amount of S can induce the precipitation of sulfur compound inclusions of a certain size and suppress the precipitation of fine TiC inclusions, while also modifying TiN-type blocky inclusions to some extent. However, excessive S content will still have a detrimental effect on the material's plasticity. Therefore, in the IF steel described in this invention, the mass percentage of S is controlled between 0.01% and 0.015%.

[0017] B: In the IF steel described in this invention, element B mainly plays a role in improving secondary processing brittleness. Because the C element content in IF steel is extremely low, and high material purity is required, the grain boundaries of IF steel lack the strengthening effect of C-bearing precipitates, easily leading to insufficient grain boundary strength and secondary brittleness. Adding a small amount of element B can strengthen the grain boundaries and, to some extent, inhibit the segregation of harmful elements at the grain boundaries. However, excessively high B content will increase the steel strength, which is not conducive to obtaining good plasticity. Therefore, in the IF steel described in this invention, the mass percentage of element B is controlled between 0.0002% and 0.0006%. In some embodiments, the mass percentage of element B can be controlled between 0.0002% and 0.0004%.

[0018] Furthermore, in this invention, since the electric furnace smelting route leads to a higher N content, introducing a certain amount of S element can precipitate larger-sized TiS and Ti4C2S2 inclusions, and can also suppress the precipitation of nanoscale fine TiC phases, thereby avoiding excessively fine grains due to recrystallization growth of IF steel and affecting the development of texture. At the same time, S-bearing inclusions tend to surround the bulk TiN inclusions to form composite spherical inclusions, thereby reducing the harm of bulk TiN inclusions to the formability of the material.

[0019] Furthermore, the IF steel described in this invention also contains at least one or at least two of Cr, Ni, Cu, Mo, and V. Preferably, when contained, the percentage of each of Cr, Ni, Cu, Mo, and V does not exceed 0.05 wt%.

[0020] In some embodiments, the IF steel of the present invention further contains at least one of the following residual elements:

[0021] Cr ≤ 0.05 wt%;

[0022] Ni ≤ 0.05 wt%;

[0023] Cu ≤ 0.05 wt%;

[0024] Mo ≤ 0.05 wt%;

[0025] V≤0.05wt%.

[0026] In this invention, since all scrap steel is used as the smelting raw material, residual elements such as Cr, Ni, Cu, Mo, and V, which are difficult to remove during steelmaking, may be introduced into the raw material. These elements are all harmful residual elements for IF steel. Because IF steel is an ultra-pure ferritic steel, the introduction of these residual elements into the matrix easily generates excess precipitates, which refines the material grains and affects the development of texture, ultimately adversely affecting the deep-drawing performance and plasticity of IF steel. Therefore, it is necessary to control the Cr, Ni, Cu, Mo, and V elements within the ranges described above.

[0027] Furthermore, in the IF steel described in this invention, 0.02% ≤ Ti eq ≤0.04%, of which Ti eq =Ti-(4C+3.43N+1.5S), where each chemical element is substituted with its mass percentage.

[0028] In this invention, through the above-mentioned composition design, the Ti element in the material can completely fix the interstitial or impurity atoms of C, N, and S, while the Ti element is not so excessive that it affects the stability of the continuous casting process of molten steel.

[0029] Furthermore, in the unavoidable impurities of the IF steel described in this invention, P ≤ 0.015% and Sn ≤ 0.01%.

[0030] In this invention, unavoidable impurities mainly include phosphorus (P) and tin (Sn). The lower the content of these elements, the purer and better the steel's performance. Excessive P content weakens grain boundaries, increases material brittleness, and deteriorates weldability. Therefore, in the IF steel described in this invention, the mass percentage of P can be controlled to P ≤ 0.015%. Sn is also a harmful element in steel; its segregation at grain boundaries negatively impacts the material's toughness and plasticity, and excessive Sn content also deteriorates the material's coating performance. Therefore, in the IF steel described in this invention, the mass percentage of Sn can be controlled to Sn ≤ 0.01%.

[0031] Furthermore, in the IF steel described in this invention, the harmful element factor Hem ≤ 0.04%, where Hem = Sn + N / 2 + 2S, and each chemical element is substituted with its mass percentage content.

[0032] In this invention, by controlling the aforementioned harmful element factors, it is possible to further ensure that the bending and pore-expanding properties of the material do not deteriorate.

[0033] Furthermore, in the IF steel described in this invention, its microstructure includes ferrite, wherein the ferrite grain size is grade 6-9.

[0034] Furthermore, in the IF steel described in this invention, its microstructure also includes titanium nitride inclusions and / or titanium sulfide inclusions.

[0035] Furthermore, in the IF steel described in this invention, the size of the titanium nitride inclusions and the titanium sulfide inclusions are each 3-10 μm.

[0036] Furthermore, in the IF steel described in this invention, the total number density of titanium nitride inclusions and titanium sulfide inclusions is ≤10 inclusions / mm². 2 In some embodiments, the total number density of inclusions in the IF steel of the present invention is 5 to 10 inclusions / mm. 2 .

[0037] Furthermore, in the IF steel described in this invention, its yield strength is 110-220 MPa, tensile strength is 260-370 MPa, elongation at break is ≥40%, R-value is ≥2.1, porosity is ≥130%, and carbon footprint is ≤1.2 kgCO2eq / kg. In some embodiments, the R-value of the IF steel described in this invention is between 2.1 and 3.0. In some embodiments, the porosity of the IF steel described in this invention is between 130% and 180%.

[0038] Another objective of this invention is to provide a method for manufacturing low-carbon footprint IF steel. This method employs an electric arc furnace smelting process using all-scrap steel, resulting in IF steel with green and low-carbon technological advantages. Furthermore, by controlling the scrap steel raw materials and steelmaking technology, impurities and residual elements in the product are kept at low and controllable levels, thus not affecting the material's performance.

[0039] To achieve the above objectives, the present invention provides a method for manufacturing IF steel with a low carbon footprint, comprising the steps of:

[0040] All scrap steel is used for electric furnace smelting, LF, RH and continuous casting; in the LF process, reheating is performed to make the LF outlet temperature ≥1660℃; in the RH process, 1.0-2.5kg / t steel of deep decarburizing agent is added during vacuum treatment, the vacuum treatment time is ≤15min, and soft blowing is performed for 15-25min after breaking the vacuum.

[0041] Hot rolling and coiling;

[0042] Cold rolling;

[0043] Continuous annealing: First, hold at an annealing temperature of 790-840℃ for 60-200s, then cool at a first cooling rate of 3-10℃ / s to the rapid cooling start temperature of 650-690℃, then cool at a second cooling rate of 40-80℃ / s to the rapid cooling end temperature of 370-420℃, then hold for 200-400s for aging treatment, and finally cool at a third cooling rate of 2-10℃ / s to 100℃ or below.

[0044] In the LF process of this invention, after the ladle enters the LF station, because IF steel requires ultra-low carbon control, to prevent carbon reversion due to prolonged processing, no modification, slag formation, or desulfurization is performed in the LF process. Instead, only bottom-blowing argon is used to homogenize the steel composition and molten pool temperature. Simultaneously, to better coordinate with the RH process for decarbonization and degassing, a certain amount of supplemental heating is required. The LF outlet temperature can be controlled to ≥1660℃, generally controlled at 1660~1670℃, while the entire processing time is controlled to ≤20min.

[0045] In the RH process of this invention, after the RH enters the station, a vacuum is drawn to ≤100Pa and maintained for 5 minutes. Then, 1.0-2.5 kg / t of deep decarburizing agent is added to the molten steel. The vacuum treatment time of the steel is controlled to ≤15 minutes, such as 7-15 minutes. After breaking the vacuum, the [O] content of the molten steel is measured and corresponding Al particles are added to achieve deoxidation and alloying. At the same time, Ti wire is fed for alloying. After the above treatment, soft blowing is performed for 15-25 minutes. The soft blowing effect is just enough to not blow away the slag surface. Then, it is hoisted to the continuous casting for casting.

[0046] In this invention, the carbon-oxygen reaction during vacuum circulation extraction can simultaneously remove free [C] and [O] from molten steel. However, the free oxygen from electric furnace tapping alone is insufficient for smelting ultra-low carbon steel; therefore, a deep decarburizing agent needs to be added for further decarburization. In some embodiments, the deep decarburizing agent comprises CaO: 10-30 wt%, Al2O3: 10-30 wt%, CaF2: 0-6 wt%, SiO2 ≤ 5 wt%, with the balance being FeO and unavoidable impurities. In some embodiments, the FeO content in the deep decarburizing agent is ≥ 60 wt%.

[0047] In this invention, vacuum treatment can also reduce the nitrogen (N) content in molten steel, but the reduction is not significant when the vacuum treatment time is too long. Furthermore, prolonged soft blowing after vacuum breaking can remove inclusions (especially ultra-large inclusions ≥10µm in size), allowing them to be adsorbed into the slag layer or undergo plasticization. Therefore, the RH vacuum refining process, employing short vacuum time + deep decarburizing agent treatment and long soft blowing time, can effectively remove carbon (C) and nitrogen (N) and control inclusions in the steel. Since the N content of steel from the scrap electric arc furnace is significantly higher than that of conventional converter processes, the controllable lower limit through RH treatment is at 50ppm. However, the treatment can significantly remove large-sized TiN inclusions in the molten steel. Simultaneously, due to limited sulfur removal during the smelting process, a relatively large number of sulfur compounds are introduced, which also has a certain modifying effect on TiN blocky inclusions.

[0048] In the continuous annealing step of this invention, the recrystallization of the cold-rolled steel sheet is achieved by controlling the annealing holding temperature and time. If the annealing holding temperature is too low, incomplete recrystallization of the steel sheet will occur, while if the temperature is too high, it will easily cause coarse grains and dissolution and decomposition of precipitates, thus affecting the properties of the steel sheet material. Therefore, in this invention, the annealing holding temperature is controlled at 790-840℃.

[0049] Meanwhile, time control is also crucial in this process. Too short an annealing holding time will result in incomplete recrystallization of the steel sheet, severely deteriorating the material's formability due to banded structures; while too long an annealing holding time can easily lead to abnormally coarse grains and abnormal decomposition of precipitates. Therefore, in this invention, the annealing holding time is controlled between 60-200 seconds.

[0050] Furthermore, controlling the initial cooling rate of the heat-insulated steel strip at 3-10℃ / s and slowly cooling it to 650-690℃ allows some of the austenite in the steel sheet to transform into ferrite. This ferrite, being relatively large, is beneficial for the coordinated deformation of the material. Controlling the second cooling rate at 40-80℃ / s and using rapid air cooling to quickly cool the slowly cooled steel strip to 370-420℃, followed by holding at that temperature for 200-400s for aging treatment, is crucial. Too slow a cooling rate could lead to the formation of pearlite, reducing the steel sheet's performance; while too fast a cooling rate increases production difficulties and manufacturing costs. Therefore, controlling the second cooling rate at 40-80℃ / s is essential. After the rapid cooling process, the steel strip is almost entirely transformed into a ferrite structure, and the grain defects and internal stresses within the material are largely eliminated after aging treatment. Finally, the aged steel strip is cooled to 100℃ or below at a third cooling rate of 2-10℃ / s, below which the steel's microstructure essentially remains unchanged.

[0051] Furthermore, in the electric furnace smelting process of the manufacturing method described in this invention, the scrap steel used includes three types: light scrap steel, medium scrap steel, and heavy scrap steel, with a weight ratio of light scrap steel: medium scrap steel: heavy scrap steel = (3-3.5):(1.5-2):1. In this document, light scrap steel, medium scrap steel, and heavy scrap steel have the meanings known in the art. For example, light scrap steel can refer to scrap steel with a thickness of less than 3 mm. Light scrap steel can be waste generated during steel manufacturing, such as flat plates, thin materials, pipes, angle iron, etc., which are characterized by small volume and light weight. Medium scrap steel can refer to scrap steel products with a thickness between 3-6 mm, such as hot-rolled plates, cold-rolled strips, profiles, etc. Heavy scrap steel can refer to scrap steel with a thickness exceeding 6 mm and a large weight, such as steel plates, steel pipes, large structures, etc. In some embodiments, scrap steel containing Cu, Cr, Ni, and Mo elements is avoided.

[0052] In some implementation schemes, when adding scrap steel charge, 1-2 tons of light scrap steel is first laid on the bottom of the electric furnace as a lining to prevent the impact and damage to the furnace lining when a large amount of scrap steel (especially heavy scrap steel) is poured in.

[0053] In some implementations, high-flow-rate oxygen lances are used for oxygen blowing to aid melting and dephosphorization during the smelting process. In some implementations, the final composition of the electric furnace is controlled as follows: C ≤ 0.04 wt%, P ≤ 0.01 wt%, S ≤ 0.01 wt%, while T is also controlled. 出钢 ≥1630℃. After tapping, the steel is moved to the slag removal station to remove slag, and the amount of slag left is controlled to be ≤30mm.

[0054] Furthermore, in the continuous casting process of the manufacturing method described in this invention, the superheat is controlled at 15-30°C, the crystallizer taper is controlled at 1.0-1.3%, and the continuous casting billet pulling speed is controlled at 1.5-2.0 m / min.

[0055] In this invention, a certain degree of superheat is required in the molten steel to ensure smooth continuous casting. When the superheat is below 15°C, it is detrimental to the stability of the continuous casting process; when the superheat is above 30°C, it is unfavorable to the internal quality of the billet center, easily leading to defects such as segregation and shrinkage cavities. Furthermore, to better control the surface quality of the billet and prevent small longitudinal cracks, a chamfered crystallizer can be selected to improve stress distribution, and the crystallizer taper can be controlled between 1.0% and 1.3%. Simultaneously, to balance production efficiency and billet surface quality, the continuous casting speed can be controlled between 1.5 and 2.0 m / min. In addition, dynamic light reduction and electromagnetic stirring are used during the casting process to better improve the degree of segregation in the billet.

[0056] Furthermore, in the hot rolling step of the manufacturing method described in this invention, the slab heating temperature is controlled at 1140-1200℃, and the finishing rolling temperature is controlled at 910-950℃.

[0057] In the hot rolling process of this invention, the slab can be first heated at high temperature in the full austenitic region to soften the material and ensure complete and uniform diffusion of the composition. However, the heating temperature should not be too high to avoid the re-dissolution of large-sized phases precipitated during continuous casting. After rough rolling to the intermediate slab thickness specification, the coarse structure generated in the heating furnace is broken down and refined. After finish rolling, it is further rolled to the required thickness, forming a uniform and fine recrystallized material structure. Controlling the final rolling temperature of the finish rolling at 910-950℃ will facilitate the controllability and stability of the material rolling process and also make it easier to control the subsequent high coiling temperature.

[0058] Furthermore, in the winding step of the manufacturing method described in this invention, the winding temperature is controlled to be 660-700°C.

[0059] In this invention, by controlling the winding temperature between 660-700℃, the size of the precipitated phase during the winding process can be controlled to prevent it from becoming too small. At the same time, a higher winding temperature also results in lower material strength, facilitating subsequent cold rolling with large deformation.

[0060] Furthermore, in the cold rolling step of the manufacturing method described in this invention, the cold rolling reduction rate is controlled to be 70-90%.

[0061] In this invention, by controlling the cold rolling reduction rate to 70-90%, the steel plate can reach the target thickness and accumulate sufficient deformation energy, which is more conducive to subsequent annealing recrystallization and the development of deformation texture.

[0062] The low-carbon-footprint IF steel and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0063] The low-carbon footprint IF steel described in this invention achieves a suitable grain structure through specific composition design combined with control of Ti inclusions of a certain size, while ensuring the material's plasticity, bending, and pore-expanding performance.

[0064] The method for manufacturing low-carbon footprint IF steel described in this invention employs an electric arc furnace smelting process using all scrap steel, resulting in IF steel with green and low-carbon technological advantages. Furthermore, by controlling the scrap steel raw materials and steelmaking technology, the impurities and residual elements in the product are kept at a low and controllable level, thus not affecting the material's performance.

[0065] In some embodiments of the present invention, the low-carbon footprint IF steel of the present invention has a yield strength of 110-220 MPa, a tensile strength of 260-370 MPa, an elongation at break ≥40%, an R-value ≥2.1, and a hole expansion rate ≥130%. The low-carbon footprint IF steel of the present invention has a carbon footprint ≤1.2 kgCO2eq / kg. Attached Figure Description

[0066] Figure 1 shows the microstructure of the low-carbon footprint IF steel of Example 7 of the present invention.

[0067] Figure 2 shows the Ti inclusion morphology of the low-carbon footprint IF steel of Example 7 of the present invention.

[0068] Figure 3 shows the Ti inclusion distribution of the low-carbon footprint IF steel of Example 7 of the present invention. Detailed Implementation

[0069] The following will further explain and illustrate the low-carbon footprint IF steel and its manufacturing method according to the present invention with reference to specific embodiments. However, this explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0070] Examples 1-20 and Comparative Examples 1-3

[0071] The low-carbon footprint IF steels of Examples 1-20 and the comparative steels of Comparative Examples 1-3 of this invention were all prepared using the following steps:

[0072] (1) Electric furnace smelting, LF, RH and continuous casting using all scrap steel:

[0073] In the electric arc furnace (EAF) smelting process, the scrap steel is categorized into three types: light, medium, and heavy. Clean and pure materials are selected, and scrap steel containing Cu, Cr, Ni, and Mo is avoided as much as possible. When adding scrap steel charge, 1-2 tons of light scrap steel are first laid on the furnace bottom as a lining to prevent impact and damage to the furnace lining when large amounts of scrap steel (especially heavy scrap steel) are poured in. High-flow-rate oxygen lances are used for oxygen blowing to aid melting and dephosphorization during the smelting process. The final composition of the EAF is controlled as follows: C ≤ 0.04 wt%, P ≤ 0.01 wt%, S ≤ 0.01 wt%, while T is also controlled. 出钢 ≥1630℃. After tapping, the steel is moved to the slag removal station to remove slag, and the amount of slag left is controlled to be ≤30mm.

[0074] The weight ratio of the three types of scrap steel is light scrap steel: medium scrap steel: heavy scrap steel = (3-3.5):(1.5-2):1. The specific ratios of each embodiment are shown in Table 2-1.

[0075] In the LF process, IF steel requires ultra-low carbon control. To prevent carbon reversion due to prolonged processing, the LF process does not involve modification, slag formation, or desulfurization. Instead, it only involves bottom-blowing argon to homogenize the steel composition and molten pool temperature. At the same time, to better coordinate with the RH process for decarbonization and degassing, a certain amount of supplemental heating is required to ensure that the LF outlet temperature is ≥1660℃. The entire process time is ≤20 minutes.

[0076] In the RH process: After the RH enters the station, a vacuum is drawn to ≤100Pa. After maintaining the vacuum for 5 minutes, a deep decarburizing agent of 1-2.5 kg / t steel is added to the molten steel. The vacuum treatment time is controlled to ≤15 minutes. After breaking the vacuum, the [O] content of the molten steel is measured, and corresponding Al particles are added to achieve deoxidation and alloying. At the same time, Ti wire is fed for alloying. After the above treatment, soft blowing is performed for 15-25 minutes. The soft blowing effect is just enough to not blow away the slag surface. Then, it is hoisted to the continuous casting for casting. The composition of the deep decarburizing agent is CaO: 10-30%, Al2O3: 10-30%, FeO≥60%, CaF2: 0-6%, SiO2≤5%, and the balance is unavoidable impurities.

[0077] In the continuous casting process: the superheat is controlled at 15-30℃, the crystallizer taper is controlled between 1.0-1.3%, and the continuous casting speed is between 1.5-2.0m / min. The chemical composition of the obtained billet is shown in Tables 1-1 and 1-2.

[0078] (2) Hot rolling and coiling: The slab heating temperature is 1140-1200℃, rough rolling is followed by finish rolling, the finish rolling temperature is 910-950℃, and after rolling, it is water cooled to the coiling temperature of 660-700℃.

[0079] (3) Pickling removes iron oxide scale.

[0080] (4) Cold rolling: Control the cold rolling reduction rate to 70-90%.

[0081] (5) Continuous annealing: First, hold at an annealing temperature of 790-840℃ for 60-200s, then cool at a first cooling rate of 3-10℃ / s to the rapid cooling start temperature of 650-690℃, then cool at a second cooling rate of 40-80℃ / s to the rapid cooling end temperature of 370-420℃, then hold for 200-400s for aging treatment, and finally cool at a third cooling rate of 2-10℃ / s to 100℃ and below.

[0082] Tables 1-1 and 1-2 list the chemical composition ratios of the low-carbon footprint IF steels of Examples 1-20 and the comparative steels of Comparative Examples 1-3. Table 1-3 lists the Hem and Ti content of each steel grade. eq .

[0083] Table 1-1. (wt%, balance Fe and other unavoidable impurities besides P and Sn)

[0084] Table 1-2. (wt%, balance Fe and other unavoidable impurities besides P and Sn)

[0085] Table 1-3.

[0086] Note: In Table 3, Hem = Sn + N / 2 + 2S, Ti eq =Ti-(4C+3.43N+1.5S), where each chemical element is substituted with its mass percentage.

[0087] In Tables 1-1, 1-2, and 1-3, steel grade AE ​​represents the steel grade number used in the embodiments of the present invention, and ac represents the steel grade number used in the comparative examples.

[0088] Tables 2-1 and 2-2 list the specific process parameters for the low-carbon footprint IF steels of Examples 1-20 and the comparative steels of Comparative Examples 1-3 in the above process steps.

[0089] Table 2-1.

[0090] Note: The "-" column for the scrap steel weight ratio in Comparative Example 1 indicates that it does not use the electric arc furnace full scrap steel smelting process, but still uses the converter steelmaking process and conventional blast furnace molten iron.

[0091] Table 2-2.

[0092] The samples from Examples 1-20 and Comparative Examples 1-3 were etched with a 4% nitric acid-alcohol solution onto their polished, smooth surfaces free of obvious scratches. Clean cotton balls were used to wipe the surfaces with the etching solution for approximately 15 seconds to observe the microstructure. The microstructure showed that the majority of the area was ferrite; the color differences were related to differences in grain orientation, leading to variations in the etching effect. In addition to the ferrite matrix, a small amount of granular orange or gray material was present, which were Ti nitride or sulfide inclusions. The observation results are listed in Table 3, where the grain size rating was performed according to GB / T 6394-2002 "Evaluation Standard for Grain Size of Metallic Materials".

[0093] Figure 1 shows the microstructure of the low-carbon footprint IF steel of Example 7 of the present invention.

[0094] As shown in Figure 1, the microstructure of this low-carbon footprint IF steel consists of pure ferrite with a small amount of Ti-containing inclusions.

[0095] Figure 2 shows the Ti inclusion morphology of the low-carbon footprint IF steel of Example 7 of the present invention.

[0096] As shown in Figure 2, Example 7 contains a small amount of particulate or strip-shaped Ti nitride or sulfide inclusions.

[0097] Figure 3 shows the Ti inclusion distribution of the low-carbon footprint IF steel of Example 7 of the present invention.

[0098] As shown in Figure 3, the size of the Ti inclusions in Example 7 is in the range of 3-6 μm, and the corresponding number is relatively small.

[0099] In addition, the specimens of Examples 1-20 and Comparative Examples 1-3 were tested using ISO 6892:1998 (Metallic materials, tensile testing at room temperature) and P6 (A) standard. 80 The tensile specimens were used to perform mechanical property tests (transverse properties) and to calculate the carbon footprint. The test results are listed in Table 3. The carbon footprint calculation boundary is from cradle to gate. The specific calculation method and product classification comply with ISO 14067 "Greenhouse gases – Carbon footprint of products – Quantification requirements and guidelines" and China Iron and Steel Association EPD platform PCR 2022:01 "Ordinary steel products and special steel products". The hole expansion rate test method is based on the standard GB / T 15825.4-2008 "Forming properties and test methods of sheet metal – Part 4: Hole expansion test".

[0100] Table 3 lists the test results of the low carbon footprint IF steel of Examples 1-20 and the control steel of Comparative Examples 1-3 described in this invention.

[0101] Table 3.

[0102] Note: In Table 3, (a, b) represents the minimum inclusion size as a μm and the maximum inclusion size as b μm.

[0103] As can be seen from Table 3 above, through reasonable chemical element composition design and optimized process parameters, the embodiments of the present invention 1-20 obtained ideal microstructure characteristics. Their yield strength is between 110-219 MPa, tensile strength is between 261-369 MPa, elongation at break is greater than 40%, R value is greater than 2.1, and the performance of the material is extremely excellent. The porosity meets the requirement of being greater than 130%.

[0104] It can also be seen that the carbon footprint of Examples 1-20 of the present invention is less than or equal to 1.2 kgCO2eq / kg, which is a low level of carbon emissions and meets the technical requirements of green and low-carbon technology. In contrast, the process path of Comparative Example 1 still uses the conventional converter path and blast furnace molten iron, so its carbon footprint is relatively high.

[0105] Unlike the present invention, the comparative examples do not meet the design requirements of the present invention due to their components or process parameters, and therefore cannot achieve the excellent technical effects that the present invention can achieve.

[0106] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0107] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A low-carbon footprint IF steel, containing Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following percentages by mass: 0<C≤0.0030%, 0<Si≤0.03%, Mn: 0.05-0.20%, Ti: 0.06-0.09%, Al: 0.02-0.06%, N: 0.005-0.01%, S: 0.01-0.015%, B: 0.0002-0.0006%; Among the unavoidable impurities, O ≤ 0.0040%.

2. The IF steel as described in claim 1, characterized in that, Its mass percentage content of each chemical element is as follows: 0 < C ≤ 0.0030%, 0 < Si ≤ 0.03%, Mn: 0.05-0.20%, Ti: 0.06-0.09%, Al: 0.02-0.06%, N: 0.005-0.01%, S: 0.01-0.015%, B: 0.0002-0.0006%; the balance is Fe and unavoidable impurities; among the unavoidable impurities, O ≤ 0.0040%.

3. The IF steel as described in claim 1 or 2, characterized in that, It also contains at least one of the following residual elements: Cr ≤ 0.05 wt%; Ni ≤ 0.05 wt%; Cu ≤ 0.05 wt%; Mo ≤ 0.05 wt%; V≤0.05wt%.

4. The IF steel as described in claim 1 or 2, characterized in that, 0.02%≤Ti eq ≤0.04%, of which Ti eq =Ti-(4C+3.43N+1.5S), where each chemical element is substituted with its mass percentage.

5. The IF steel as described in claim 1 or 2, characterized in that, In its unavoidable impurities, P ≤ 0.015% and Sn ≤ 0.01%.

6. The IF steel as described in claim 5, characterized in that, Its harmful element factor Hem≤0.04%, where Hem=Sn+N / 2+2S, and each chemical element is substituted with its mass percentage content.

7. The IF steel as described in claim 1 or 2, characterized in that, Its microstructure includes ferrite, with ferrite grain size ranging from 6 to 9.

8. The IF steel as described in claim 1 or 2, characterized in that, Its microstructure also includes titanium nitride inclusions and / or titanium sulfide inclusions.

9. The IF steel as described in claim 8, characterized in that, The size of the titanium nitride inclusions and titanium sulfide inclusions is 3-10 μm.

10. The IF steel as described in claim 8, characterized in that, The total number density of titanium nitride inclusions and titanium sulfide inclusions is ≤10 inclusions / mm². 2 .

11. The IF steel as described in claim 1 or 2, characterized in that, Its yield strength is 110-220MPa, tensile strength is 260-370MPa, elongation at break is ≥40%, R value is ≥2.1, porosity is ≥130%, and carbon footprint is ≤1.2kgCO2eq / kg.

12. The method for manufacturing low-carbon footprint IF steel according to any one of claims 1-11, characterized in that, It includes the following steps: All scrap steel is used for electric furnace smelting, LF, RH and continuous casting; in the LF process, reheating is performed to make the LF outlet temperature ≥1660℃; in the RH process, 1.0-2.5kg / t steel of deep decarburizing agent is added during vacuum treatment, the vacuum treatment time is ≤15min, and soft blowing is performed for 15-25min after breaking the vacuum. Hot rolling and coiling; Cold rolling; Continuous annealing: First, hold at an annealing temperature of 790-840℃ for 60-200s, then cool at a first cooling rate of 3-10℃ / s to the rapid cooling start temperature of 650-690℃, then cool at a second cooling rate of 40-80℃ / s to the rapid cooling end temperature of 370-420℃, then hold for 200-400s for aging treatment, and finally cool at a third cooling rate of 2-10℃ / s to 100℃ or below.

13. The manufacturing method as described in claim 12, characterized in that, In the electric arc furnace smelting process, the scrap steel used includes three types: light scrap steel, medium scrap steel and heavy scrap steel, and their weight ratio is light scrap steel: medium scrap steel: heavy scrap steel = (3-3.5): (1.5-2):

1.

14. The manufacturing method as described in claim 12, characterized in that, In the continuous casting process, the superheat is controlled at 15-30℃, the crystallizer taper is controlled at 1.0-1.3%, and the continuous casting billet pulling speed is controlled at 1.5-2.0m / min.

15. The manufacturing method as described in claim 12, characterized in that, In the hot rolling process, the slab heating temperature is controlled at 1140-1200℃, and the finishing rolling temperature is controlled at 910-950℃.

16. The manufacturing method as described in claim 12, characterized in that, During the winding process, the winding temperature is controlled at 660-700℃.

17. The manufacturing method as described in claim 12, characterized in that, In the cold rolling process, the cold rolling reduction rate is controlled to be 70-90%.

18. The manufacturing method as described in claim 12, characterized in that, In the vacuum treatment, the deep decarburizing agent is composed of CaO: 10-30wt%, Al2O3: 10-30wt%, CaF2: 0-6wt%, SiO2 ≤ 5wt%, with the balance being FeO and unavoidable impurities; preferably, the FeO content in the deep decarburizing agent is ≥ 60wt%; preferably, the vacuum treatment time is 7-15 min.

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