Method for manufacturing ferritic lightweight steel material and ferritic lightweight steel material using the same
A low-temperature tempering process for ferritic lightweight steel addresses the challenges of high costs and instability in conventional steels, producing ultra-high-strength, flexible, and low-density steel with improved mechanical properties.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2021-11-11
- Publication Date
- 2026-03-27
AI Technical Summary
Conventional lightweight steels face challenges in achieving desired strength and flexibility while maintaining low density, often resulting in high process costs due to the addition of manganese and aluminum, and suffer from instability of austenite particles at high temperatures.
A low-temperature tempering process is applied after thermomechanical processing, adjusting the tempering pathway to split intermediate carbon atoms into metastable austenite, improving strength and flexibility without losing dislocation density, using 2.0-3.0 wt% Mn, 5.0-6.0 wt% Al, and 0.1-0.3 wt% C.
The process produces ultra-high-strength, flexible, and low-density ferritic lightweight steel with reduced process costs, enhancing mechanical properties and stability by stabilizing austenite particles.
Smart Images
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Abstract
Description
Technical Field
[0001] The present invention relates to a method for manufacturing ferrite-based lightweight steel and ferrite-based lightweight steel using the same. More specifically, the present invention relates to a cost-saving method for manufacturing ferrite-based lightweight steel that maximizes the stability of the structure by performing low-temperature tempering over a short period of time, and improves strength while reducing the amount of manganese added.
Background Art
[0002] Generally, for the lightweight strategy in the industrial field, high Mn steel is added to twinning-induced plasticity (TWIP), or a large amount of Al (aluminum) is added to transformation-induced plasticity (TRIP). When 1 wt% of Al is added, the density of the alloy decreases by up to 1.3% at most. However, for engineering applications, high-content Al lightweight or low-density TWIP or TRIP steels have various drawbacks. When various thermodynamic treatment phenomena of lightweight steel occur, the stability of face-centered cubic (f.c.c.) austenite particles at high temperatures is significantly reduced due to the high density of Al content. This makes the austenite particles heterogeneous at room temperature depending on the particle size and distribution of metastable austenite particles. Eventually, the strain of solid-state martensite during the firing strain of heterogeneous metastable austenite cannot be predicted, leading to the early TRIP effect at small strains during the tensile test.
[0003] Although the lightweight steel that has been used conventionally has excellent elongation, it has been difficult to obtain the desired strength by adding aluminum. To compensate for this, a solution was devised to increase the strength by adding manganese. However, adding a large amount of manganese results in a soaring process cost. This causes a great loss from an economic perspective, and as a result, there is a need to devise another solution.
[0004] To address the aforementioned shortcomings, the present invention involves low-temperature tempering after existing thermomachining processes used in a wide range of industrial applications. Here, it was shown that by adjusting the low-temperature tempering pathway, intermediate carbon atoms are split into two types of metastable austenite, simultaneously improving the strength and flexibility of conventional ferrite-based LIGHT-TRIP (transformation-induced plasticity)-DP (duplex) steel. In particular, such a tempering process was carried out without loss of high dislocation density in the metastable austenite particles. [Overview of the project] [Problems that the invention aims to solve]
[0005] The present invention was devised to solve the above-mentioned problems, and its objective is to provide a ferritic lightweight steel that is ultra-high strength, flexible, and low density.
[0006] Furthermore, an object of the present invention is to provide a ferritic lightweight steel that can reduce process costs by using solid solute elements such as Al and Mn, thereby solving the disadvantages of conventional processes that result in high process costs.
[0007] The technical problems that the present invention aims to solve are not limited to the technical problems described above, and other unmentioned technical problems can be clearly understood by a person with ordinary skill in the art to which the present invention belongs from the following description. [Means for solving the problem]
[0008] The present invention relates to a method for manufacturing ferritic lightweight steel, comprising: a first step of solution-treating an alloy at 1200°C for 90 minutes; a second step of hot-rolling the solution-treated alloy at 900°C to 1100°C; a third step of air-cooling the hot-rolled alloy at room temperature at a cooling rate of 10°C / s; a fourth step of cold-rolling the air-cooled alloy at room temperature until its thickness is reduced by 70%; a fifth step of intercritical annealing the cold-rolled alloy at 850°C to 950°C for 90 seconds; and the intercritical annealing. The process is characterized by comprising: a sixth step of cooling an annealed alloy at a cooling rate of -10°C / s; a seventh step of isothermal annealing the cooled alloy at 430°C for 50 seconds; an eighth step of air-cooling the isothermal annealed alloy; and a ninth step of low-temperature heat treatment (LTP) the air-cooled alloy at 300°C for 10 minutes.
[0009] The aforementioned lightweight steel is characterized by containing 2.0 to 3.0 wt% of Mn, 5.0 to 6.0 wt% of Al, and 0.1 to 0.3 wt% of C. [Effects of the Invention]
[0010] By addressing the above-mentioned problems, the present invention makes it possible to manufacture ultra-high-strength, flexible, and low-density ferritic lightweight steel.
[0011] Furthermore, the present invention makes it possible to manufacture cost-saving ferritic lightweight steel that can reduce process costs by using solid solute elements such as Al and Mn, thereby solving the disadvantages of conventional processes that result in high process costs.
[0012] Furthermore, the present invention makes it possible to produce lightweight steel with improved strength and flexibility simultaneously based on conventional ferritic steel by adjusting the low-temperature heat treatment (LTP) process.
[0013] Furthermore, the present invention can maximize the stability of the structure and improve mechanical properties by performing a low-temperature heat treatment (LTP) process. [Brief explanation of the drawing]
[0014] [Figure 1] This is a schematic diagram showing a temperature graph over time in the method for producing ferritic lightweight steel according to the present invention. [Figure 2] Figure 1, shown in section B, is a photograph illustrating the overall microstructure of the cold-rolled steel material before intercritical annealing (ICA). [Figure 3] This is one embodiment of the present invention, a graph showing the equilibrium phase fractions of ferrite, austenite, and κ-carbide for low-temperature heat-treated (LTP) steel, calculated using Thermo-Calc. [Figure 4] The graph shows the change in the metastable austenite particle fraction during tempering. (A) is the volume fraction of metastable austenite particles as a function of tempering temperature for 10 minutes, and (B) is a scanning electron microscope (SEM) image of LTP steel tempered at 300°C (top) and 400°C (bottom). [Figure 5] This is a transmission electron microscope (TEM) image showing the current microstructure of the steel before the tensile test, demonstrating that no dislocation density is lost during LTP (Long-Term Puncture). [Figure 6] This is a 3D (three-dimensional) reconstructed carbon map, showing the corresponding 1D concentration profiles of C, Mn, and Al read from annealed γ particles of 0.1C-850 steel (left) and 0.3C-850 steel (right), as well as from metastable γ particles of each steel. [Figure 7] Typical electron backscatter diffraction (EBSD) pole diagrams of the fcc-austenite phase, where the transverse direction is perpendicular to the planes of 0.1C-850 (left) and 0.3C-850 steel (right), RD is the rolling direction, and ND is the vertical direction. [Figure 8]It is a graph measuring the area fraction for metastable austenite particles of steel critically annealed at 0.1 and 0.3 wt% C before tempering. [Figure 9] It is a graph of the (220) fcc peak of LTP (red) and non-LTP (blue) steels. [Figure 10] It is a graph showing the room-temperature tensile properties. (a) shows the steel subjected to LTP, and curves b-d show non-LTP steels of 0.1 wt% C (c and d) and 0.3 wt% C (b) treated by isothermal annealing. [Figure 11] It is the microstructure of LTP steel after a tensile test, which is the synchrotron XRD (X-ray diffraction) profile of LTP steel at other strains. [Figure 12] It is the microstructure of LTP steel after a tensile test, which is a microscopic analysis photograph for a micro-region (Small Zone) with a diameter of 0.5 μm or less. [Figure 13] It is the microstructure of LTP steel after a tensile test, which is a microscopic analysis photograph for a coarse region (Coarse Zone) with a diameter of 3.0 μm or less. [Figure 14] It is a photograph showing the change in the microstructure according to the amount of rolling change (ε = 0%, ε = 5.2%, ε = 13.5%, ε = 5.2%) in the micro-region and the coarse region after performing the low-temperature heat treatment process (LTP). RD is the rolling direction, ND is the direction perpendicular to RD, and TD is the observation direction. [Figure 15] It is a graph showing the comparison result of the strength before and after performing LTP. [Figure 16] It is a flowchart showing the manufacturing method of the ferritic lightweight steel of the present invention.
Embodiments for Carrying Out the Invention
[0015] The terms used in this specification are briefly explained, and the present invention is described in detail.
[0016] In this invention, while considering the function of the invention, we have selected as many commonly used terms as possible, although this may vary depending on the intentions of engineers in the field, precedents, and the emergence of new technologies. In certain cases, the applicant may have arbitrarily selected terms, in which case their meaning will be described in detail in the section describing the relevant invention. Therefore, the terms used in this invention are not merely names of terms, but are defined based on the meaning of the terms and the overall content of this invention.
[0017] Throughout the specification, when a part is described as "containing" a certain component, this does not exclude other components unless otherwise specified, and means that it may contain other components as well.
[0018] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, so that they can be easily implemented by a person with ordinary skill in the art to which the present invention pertains. However, the present invention can be embodied in various different forms and is not limited in any way to the embodiments described herein.
[0019] The specific details of the problem to be solved by this invention, the means of solving that problem, and the effects of the invention are incorporated into the embodiments and drawings described below. The advantages and features of this invention, as well as the methods for achieving them, will become even clearer when you refer to the embodiments described in detail below in conjunction with the accompanying drawings.
[0020] The present invention will be described in more detail below with reference to the attached drawings.
[0021] While conventionally used lightweight steel possesses excellent elongation, achieving the desired strength through the addition of aluminum has been challenging. To compensate for this, manganese (Mn) has been added to increase strength. However, adding large amounts of manganese significantly increases process costs. This results in substantial economic losses, necessitating the development of alternative methods. For these reasons, the present invention allows for the reduction of manganese addition while still achieving the desired strength through a low-temperature heat treatment (LTP) process, thereby obtaining a so-called "cost-saving" lightweight steel.
[0022] Generally, the austenite phase of fcc structures in metal alloys and steel is stable at high temperatures. However, when various thermodynamic treatments are applied to LIGHT TRIP-DP (transformation-induced plasticity-dual phase) materials, the stability of austenite particles decreases significantly at high temperatures. This causes the austenite particles to become disordered at room temperature, resulting in heterogeneous particle size and distribution of metastable austenite particles. LIGHT TRIP-DP alloys differ from ordinary TRIP-DP with low Al content (less than 5 wt%) in their metastable austenite particles. Specifically, the former consists of heterogeneous lamellar metastable austenite particles attached to a coarse, low-temperature stable BCC (body-centered cubic) ferrite matrix, while the latter forms a uniform distribution along the particle boundaries. Consequently, the solid-state martensite strain cannot be predicted during the firing strain of the heterogeneous metastable austenite, resulting in premature TRIP at small strains during tensile testing.
[0023] Conventional processing methods involve heating at a high temperature, followed by high-temperature rolling and low-temperature rolling, and then annealing before use. However, as mentioned above, existing methods have the disadvantage of being brittle in terms of strength, and variations in grain size occur, requiring compensation. When the microstructure is observed after the rolling process, it can be confirmed that it is highly heterogeneous. Therefore, heat treatment is performed to recrystallize and induce a phase transformation. After the two processes are completed, a low-temperature heat treatment process (hereinafter referred to as LTP) is performed at a relatively lower temperature than the temperatures of the previous processes, 300°C, for about 600 seconds. In the present invention, the stability of the microstructure can be maximized and the mechanical properties can be improved by performing the low-temperature heat treatment process (LTP).
[0024] The ferritic lightweight steel of the present invention is characterized by containing 2.0 to 3.0 wt% of Mn, 5.0 to 6.0 wt% of Al, and 0.1 to 0.3 wt% of C. Furthermore, the ferrite has a volume fraction of 76.9%, and the austenite has a volume fraction of 23.1%.
[0025] The aforementioned Mn is included in an amount of 2.0 to 3.0 wt% to reduce process costs.
[0026] The aforementioned Al was included at 5.0-6.0 wt% to reduce the mass density of the alloy and suppress harmful Fe-C-based precipitation. Figure 2 shows the overall microstructure of the cold-rolled steel before ICA, which is point B in Figure 1. The rolled steel exhibits a two-phase microstructure along the rolling direction RD, containing ferrite and κ-carbide, a C-Al-rich precipitate.
[0027] The aforementioned carbon (C) was included in a concentration of 0.1 to 0.3 wt%, and all rolled steel sheets containing 0.1 to 0.3 wt% of C were subjected to intercritical annealing (ICA) at 850°C and 950°C, respectively.
[0028] In one embodiment, 0.3 wt% carbon was subjected to ICA (S50) at 850°C (hereinafter, 0.3C-850), followed by isothermal annealing (S70), and then low-temperature heat treatment (LTP) at 300°C for 10 minutes. As shown in Figure 4, the resulting material consisted of double particles of ferrite and austenite, and the austenite fraction decreased as the temperature increased. It was also confirmed that at temperatures exceeding 400°C, α-ferrite and κ-carbide were formed, and the austenite volume fraction was lost.
[0029] In one embodiment, as shown in Figure 5, when a specimen treated with ICA(S50) at 0.3C-850 under LTP was subjected to TEM analysis up to 300°C for 10 minutes, it was confirmed that no dislocation density was lost during LTP, demonstrating that LTP can be efficiently utilized for austenite stabilization.
[0030] The composition of ferritic lightweight steel after LTP is as shown in Table 1 below.
[0031] [Table 1]
[0032] As shown by atom probe tomography (APT) in Figure 6, in this invention, the concentration of C separated into metastable austenite particles was measured after the LTP process. The concentration difference of Mn, another austenite-stabilizing element, between the two steel specimens, LTP and 0.3C-850, was within the detection error range. Al atoms showed approximately the same trend as Mn. As shown in Figure 7, all steel specimens, including the LTP sample, exhibit a lamellar microstructure that forms layers within a thick ferrite matrix, constituting clustered metastable austenite particles.
[0033] In annealed (S70) steel, the heterogeneity of metastable austenite particles exhibits positional disorder and indicates a particle size range of 0.45 to 4.2 μm. This heterogeneity arises from a microstructure that forms layers and clusters along the rolling direction (RD) during the rolled state. As shown in Figure 8, measurements of the area fraction of metastable austenite particles, determined by electron backscatter diffraction (EBSD) and existing X-ray diffraction (XRD), revealed that increasing the carbon content or raising the intercritical annealing temperature generates even more austenite fractions.
[0034] As shown in Figure 9, the LTP process, which can further divide more gap carbon atoms into metastable austenite particles, is (220). fcc It reduces the diffraction angle of the plane. This has been revealed by synchrotron XRD and shows an increase in the face-centered cubic (fcc)-austenite lattice parameter using LTP. (220) fcc The calculated interplanar d-spacings for the planes were 0.12880 and 0.12859 nm, respectively. Based on APT and XRD results, adding 1 at% (atomic percent) of carbon to metastable austenite resulted in (220) fcc The inter-plane spacing increases to 0.00018 nm. This is (111) fcc This means that the d-spacing of the sliding plane is effectively increased by LTP.
[0035] Figure 10 shows the nominal stress-strain curves of steel (0.3C-850-LTP(a), 0.3C-850(b), 0.1C-850(c), and 0.1C-950(d)). It can be seen that the tensile properties at room temperature of the ferritic LIGHT-TRIP-DP alloy were significantly improved by performing LTP.
[0036] The yield strength increased from 610 MPa in case (b) to 798 MPa in the case of LTP steel. The maximum tensile strength increased from 900 MPa to 1108 MPa. The total elongation increased from 42.5% to 47% (absolute level). When the carbon content decreased from 0.3 wt% to 0.1 wt%, the tensile properties decreased. The results of LTP (0.3C-850-LTP(a)) confirm that even greater changes can be accepted while maintaining even higher strength. Furthermore, as shown in Figure 15, when the specific strength (strength / weight) was measured, it was observed that the LTP-treated steel had an even higher specific strength.
[0037] To determine the dislocation density of LPT steel during tensile testing, coupled analysis was performed using synchrotron XRD and stepwise strain until the final sample broke. Diffraction results obtained at each strain level, i.e., strain rate (ε) of 0% (annealed state), 13.5%, 25.2%, and 47.1%, are shown in Figure 11(A). Figure 11(B) shows that the metastable austenite phase of the LPT steel before strain had a density of 3.13 × 10¹⁵ m -2 While edge dislocations are almost entirely present, in a coarse ferrite matrix, helical dislocations are present in a quantity of 4.48 × 10¹⁴ m. -2 This indicates that.
[0038] Figure 14 shows the changes in microstructure in response to the amount of rolling after LTP. In the small zone, there is no significant change despite the increase in the amount of rolling, but in the coarse zone, the size decreases as the amount of rolling increases.
[0039] Next, the particle size dependence of strain-induced phase transformation was confirmed in fine and coarse austenite particles. Nano-indentation tests and TEM analyses were performed at the same locations in the austenite region with indenters for fine particles with a diameter of 0.5 μm or less and coarse particles with a diameter of 3.1 μm or less. The load-displacement curve in Figure 12 shows that austenite fine particles resist martensite strain induced by nano-indentation loads.
[0040] This is also confirmed by TEM images and corresponding Selected Area Diffraction Patterns (SADPs), as shown in Figure 13, being targeted and acquired from beneath the small, recessed austenite. In contrast, the pop-in phenomenon of the loading-unloading curve is clearly observed in the LTP alloy during nano-indentation testing of the rough austenite region. Furthermore, the gradient after the initial pop-in phenomenon increased sharply compared to the gradient before the initial pop-in, even at a constant loading rate. This increase in gradient is attributed to the hardening of the dislocation dendritic forest through the formation of continuous phase strain within the rough metastable austenite. This is supported by TEM images, indicating that the austenite was locally strained into a hard α'-martensite phase. The corresponding SADP indicates the existence of a common Kurdjumov-Sachs (KS) relationship between the newly formed bcc α'-martensite and the parent fcc austenite. <110> bcc The total energy generated by strain-induced phase transformation was minimized by growing the material along the direction of the normal. The pop-in phenomenon occurred when external stress was applied along the normal direction by the nanoindenter tip. That is, it is highly likely that the compression axis of the Bain strain is approximately parallel to the indentation direction.
[0041] The method for manufacturing ferritic lightweight steel according to the present invention is carried out as shown in Figure 16.
[0042] First, in the first step (S10), the alloy is subjected to solution treatment at 1200°C for 90 minutes. The solution treatment involves heating the alloying elements to a temperature above the temperature at which they dissolve in a solid solution and holding it for a sufficient amount of time to soften the material. More specifically, when performing the solution treatment, if the temperature is below 1200°C, there is a risk that the austenite phase fraction will be low, and if the temperature exceeds 1200°C, there is a risk that the grain size of the austenite phase will become excessively large. Therefore, it is preferable to perform the treatment under the above conditions. Furthermore, when performing the solution treatment, if the time is less than 90 minutes, there is a risk that the austenite phase fraction will be low, and if the time exceeds 90 minutes, there is a risk that the grain size of the austenite phase will become excessively large. Therefore, it is preferable to perform the treatment under the above conditions.
[0043] The alloy is characterized by containing manganese (Mn), aluminum (Al), and carbon (C), and the lightweight steel produced by the method for producing ferritic lightweight steel according to the present invention is characterized by containing 2.0 to 3.0 wt% manganese (Mn), 5.0 to 6.0 wt% aluminum (Al), and 0.1 to 0.3 wt% carbon (C).
[0044] Next, in the second step (S20), the solution-treated alloy is hot-rolled at 900°C to 1100°C. In the second step (S20), the thickness is reduced to 55% by hot rolling. More specifically, if hot rolling is performed at a temperature below 900°C in the second step (S20), the temperature gap to the finish rolling temperature is narrow, making it impossible to roll the alloy sufficiently to the desired thickness. If hot rolling is performed at a temperature above 1100°C, there is a risk of causing high-temperature brittleness. Therefore, it is preferable to perform the process under the above conditions.
[0045] Next, in the third step (S30), the hot-rolled alloy is air-cooled at room temperature at a cooling rate of 10°C / s. However, the hot-rolled alloy may be rolled at 650°C for 60 minutes and then air-cooled at room temperature. The hot-rolled alloy is formed in a coil shape.
[0046] Next, in the fourth step (S40), the air-cooled alloy is cold-rolled at room temperature until its thickness is reduced to 70%. As shown in Figure 2, the rolled steel produced in the fourth step (S40) exhibits a two-phase microstructure containing ferrite and κ-carbide (volume fraction: 38.6%), which is a C-Al rich precipitate, along the rolling direction RD.
[0047] The formation of the κ-carbide band structure is mainly due to the solute splitting effect during the casting of the high-aluminum lightweight steel. The fourth step (S40) is carried out at a low temperature, such as room temperature.
[0048] Next, in the fifth step (S50), the cold-rolled alloy is subjected to intercritical annealing (ICA) at 850°C to 950°C for 90 seconds. During the fifth step (S50), the cold-rolled alloy will generate ferrite and austenite. The temperature for the fifth step (S50) is set based on thermodynamic calculations to completely dissolve the κ-carbide phase in the fourth-order Fe-Mn-Al-C system.
[0049] More specifically, if two-phase annealing is performed at a temperature below 850°C in the fifth step (S50), there is a risk of residual austenite remaining. Since the aim is to ensure that the alloying elements are sufficiently dissolved, it is not necessary to perform two-phase annealing above 950°C.
[0050] Next, in the sixth step (S60), the intercritically annealed alloy is cooled at -10°C / s.
[0051] Next, in the seventh step (S70), the alloy that has undergone the cooling treatment is isothermal annealed at 430°C for 50 seconds.
[0052] Next, in the eighth step (S80), the isothermally annealed alloy is air-cooled.
[0053] Next, in the ninth step (S90), the air-cooled alloy is subjected to a low-temperature heat treatment (LTP) process at 300°C for 10 minutes. The isothermal annealed alloy undergoes the ninth step (S90) to form a double microstructure consisting of ferrite and austenite particles. In the ninth step (S90), it is preferable to temper the alloy until precipitates are formed and an equilibrium phase is reached.
[0054] More specifically, if the low-temperature heat treatment (LTP) step (S90) is performed at a temperature below 300°C, there is a risk that precipitates will not precipitate. If the low-temperature heat treatment (LTP) step (LTP) is performed at a temperature above 300°C, there is a risk that the coarsening of the precipitates will lead to a decrease in mechanical properties and an increase in manufacturing costs. Therefore, it is preferable to perform the procedure under the above conditions. Furthermore, if the low-temperature heat treatment (LTP) step (S90) is performed for less than 10 minutes, there is a risk that precipitates will not precipitate. If the low-temperature heat treatment (LTP) step (LTP) is performed for more than 10 minutes, sufficient energy will be obtained, which may lead to grain coarsening. Therefore, it is preferable to perform the procedure under the above conditions.
[0055] In previous studies, the main results were a tensile strength of 1 GPa or higher and an elongation of 7% or higher. However, in this invention, using the LTP process, while the ferrite-based structure is the main component, the yield strength increased from 610 MPa to 798 MPa, and the maximum tensile strength increased from 900 MPa to 1108 MPa. Furthermore, the total elongation increased from 42.5% to 47% (absolute level).
[0056] By addressing the above-mentioned problems, the present invention makes it possible to manufacture ultra-high-strength, flexible, and low-density ferritic lightweight steel.
[0057] Furthermore, the present invention makes it possible to manufacture ferritic lightweight steel that can reduce process costs by using solid solute elements such as Al and Mn, thereby solving the problem of high process costs that plagues conventional processes.
[0058] Furthermore, the present invention makes it possible to produce lightweight steel with improved strength and flexibility simultaneously based on conventional ferritic steel by adjusting the low-temperature heat treatment (LTP) process.
[0059] Furthermore, the present invention can maximize the stability of the structure and improve mechanical properties by using a low-temperature heat treatment (LTP) process.
[0060] Thus, the technical configuration of the present invention described above can be implemented in other specific forms by a person skilled in the art in which the present invention belongs, without changing the technical idea or essential features of the present invention.
[0061] Therefore, the embodiments described above should be understood to be illustrative and not limiting in all respects, and the scope of the present invention is expressed more by the claims described below than by the detailed description above, and the meaning and scope of the claims, as well as any modifications or distortions derived from the concept of equivalents thereof, are included in the scope of the present invention.
Claims
1. A material containing 2.0 to 3.0 wt% Mn, 5.0 to 6.0 wt% Al, and 0.1 to 0.3 wt% C, with the remainder being Fe and unavoidable impurities. The tensile strength is 900 MPa to 1,108 MPa. A ferritic lightweight steel material characterized by the following features.
2. The aforementioned ferritic lightweight steel material consists of ferrite and austenite. The ferritic lightweight steel material according to claim 1.
3. The ferrite has a volume fraction of 76.9%, and the austenite has a volume fraction of 23.1%. The ferritic lightweight steel material according to claim 2.
4. The aforementioned ferritic lightweight steel material is The total growth is between 42.5% and 47%. The ferritic lightweight steel material according to claim 1.
5. The aforementioned ferritic lightweight steel material is The yield strength is 610 MPa to 798 MPa. The ferritic lightweight steel material according to claim 1.
6. The first step is to solution-treat the alloy at 1200°C for 90 minutes, A second step is to hot-roll the solution-treated alloy at 900°C to 1100°C, A third step involves air-cooling the hot-rolled alloy at a cooling rate of 10°C / s, A fourth step involves cold-rolling the air-cooled alloy at room temperature until its thickness is reduced by 70%, A fifth step involves subjecting the cold-rolled alloy to intercritical annealing at 850°C to 950°C for 90 seconds, A sixth step involves cooling the alloy that has undergone intercritical annealing at a cooling rate of 10°C / s, A seventh step is to isothermal anneal the alloy that has undergone the cooling treatment at 430°C for 50 seconds, The eighth step is to air-cool the isothermally annealed alloy, A ninth step involves subjecting the air-cooled alloy to a low-temperature heat treatment (LTP) process at 300°C for 10 minutes, This is done by The ferritic lightweight steel material produced by the ninth step described above is It contains 2.0-3.0 wt% Mn, 5.0-6.0 wt% Al, and 0.1-0.3 wt% C, with the remainder being Fe and unavoidable impurities. The tensile strength is 900 MPa to 1,108 MPa. A method for producing ferritic lightweight steel materials, characterized by the following features.
7. The aforementioned ferritic lightweight steel material is The tensile strength is 900 MPa to 1,108 MPa. A method for producing a ferritic lightweight steel material according to claim 6.
8. The aforementioned ferritic lightweight steel material is The total growth is between 42.5% and 47%. A method for producing a ferritic lightweight steel material according to claim 6.
9. The aforementioned ferritic lightweight steel material is The yield strength is 610 MPa to 798 MPa. A method for producing a ferritic lightweight steel material according to claim 6.
10. In the fourth step described above, cold rolling is performed. Ferrite and κ-carbide are arranged alternately, creating a precipitated band structure along the rolling direction. A method for producing a ferritic lightweight steel material according to claim 6.
11. In the second step described above, the material is hot-rolled to reduce the wall thickness to 55%. A method for producing a ferritic lightweight steel material according to claim 6.
12. In the fourth step described above, the material is cold-rolled to reduce the wall thickness to 70%. A method for producing a ferritic lightweight steel material according to claim 6.