High-strength low-manganese light steel and method of making

Abstract

The application discloses a preparation method of high-strength low-manganese light steel, and belongs to the technical field of steel production. The preparation method comprises the following steps: annealing a hot-rolled plate to obtain an annealed plate; after the annealed plate is subjected to multiple warm rolling, water cooling is performed to room temperature to obtain a warm-rolled plate; the warm-rolled plate is rapidly heated to a set temperature and kept for a set time, and then oil cooling is performed to room temperature to obtain the high-strength low-manganese light steel, wherein the rapid heating rate is between 50 DEG C / s and 150 DEG C / s, and the components of the hot-rolled plate are as follows: carbon: 0.28-0.32wt%; manganese: 2.5-3.0wt%; aluminum: 3.5-4.0wt%; chromium: 0.4-0.6wt%; vanadium: 0.18-0.22wt%; niobium: 0.03-0.09wt%; molybdenum: 0.18-0.22wt%; and the balance is Fe. The application further discloses a high-strength low-manganese light steel. The application ensures sufficient ductility, improves the tensile strength of the low-manganese light steel, has a simple process route, a short heat treatment cycle, and realizes low-cost and high-efficiency production.

Description

Technical Field

[0001] This invention relates to the field of advanced high-strength steel preparation technology, and in particular to a method for preparing high-strength low-manganese lightweight steel and a high-strength low-manganese lightweight steel. Background Technology

[0002] At present, exploring the dual lightweight material design concept of "high strength and thinning" and "low density" has become an inevitable trend in its development. As a new type of steel material, low-manganese lightweight steel shows great application potential in the automotive field due to its low density, excellent strength and plasticity, and high productivity.

[0003] Low-manganese lightweight steels typically achieve density reduction by adding a significant amount of the alloying element Al. Each 1% addition of Al reduces density by approximately 1.5%. However, excessive Al addition leads to the formation of the δ-ferrite phase, which cannot be completely eliminated during subsequent rolling and phase transformation heat treatment. Because δ-ferrite is relatively soft and has coarse grains, it exhibits high stress-strain inhomogeneity with other matrix phases, resulting in poor overall strength and ductility.

[0004] The lightweight steels produced by hot rolling, cold rolling, and continuous annealing processes, such as the "780MPa grade high-strength and high-toughness lightweight steel with good cold working performance and its manufacturing method" disclosed in Chinese invention patent CN 106498307 A, the "a high-strength medium-carbon chromium-containing low-manganese lightweight steel and its preparation method" disclosed in Chinese invention patent CN 106086658 A, and the "800MPa grade high-ductility low-density steel and its manufacturing method" disclosed in Chinese invention patent CN 104928569 A, have not achieved a tensile strength exceeding 1GPa, and the processes are complex and cumbersome.

[0005] Therefore, while ensuring sufficient ductility, low-manganese lightweight steel still has significant room for improvement in strength. At the same time, it is necessary to explore simpler processing methods to reduce costs and energy consumption. Summary of the Invention

[0006] The present invention aims to solve one of the technical problems in the related technologies to a certain extent, and provides a method for preparing high-strength low-manganese lightweight steel and a high-strength low-manganese lightweight steel.

[0007] As a first aspect of the present invention, a method for preparing high-strength, low-manganese lightweight steel is provided, comprising:

[0008] The hot-rolled sheet is annealed to obtain annealed sheet material;

[0009] After the annealed sheet is subjected to multiple warm rolling processes, it is cooled to room temperature with water to obtain a warm-rolled sheet.

[0010] The hot-rolled plate is rapidly heated to a set temperature and held at that temperature for a set time, then oil-cooled to room temperature to obtain the high-strength, low-manganese lightweight steel. The rapid heating rate is between 50°C / s and 150°C / s. The composition of the hot-rolled plate is as follows:

[0011]

[0012] Furthermore, in the step of performing multiple warm rolling operations on the annealed sheet, the number of warm rolling passes is between 5 and 7, and the reduction rate per pass is between 18% and 25%.

[0013] Furthermore, in the step of performing multiple warm rolling processes on the annealed sheet, the total reduction rate is between 75% and 85%.

[0014] Furthermore, the set time is between 1 second and 6 seconds, the oil cooling rate is between 30°C / s and 50°C / s, and the set temperature satisfies the following formula:

[0015] T0 = ​​Ac1 + (150~200℃);

[0016] Where T0 is the set temperature;

[0017] Ac1 is the starting temperature of the transformation from pearlite to austenite during heating.

[0018] Furthermore, prior to annealing the hot-rolled plate, the preparation method further includes:

[0019] The steel billet is hot-rolled to obtain the hot-rolled plate, wherein the final rolling temperature of the hot rolling is between 950°C and 980°C, and the total reduction rate of the hot rolling is between 90% and 92%.

[0020] Furthermore, the annealing time of the hot-rolled plate is between 0.5 h and 1.5 h, and the annealing temperature of the hot-rolled plate satisfies the following formula:

[0021] T = Ac1 + (0~150℃);

[0022] Where T is the annealing temperature of the hot-rolled plate;

[0023] Ac1 is the starting temperature of the transformation from pearlite to austenite during heating.

[0024] As a second aspect of the present invention, a high-strength, low-manganese lightweight steel is also provided, wherein the high-strength, low-manganese lightweight steel is prepared by the above-described preparation method.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] (1) The use of multiple warm rolling processes can not only induce a large amount of carbide precipitation and dislocation density of microalloyed Nb, V and Mo by strain, but also allow the interaction between the soft and hard phases of ferrite and martensite during the warm rolling process to fully deform them. Combined with the pinning and refining of microalloyed materials, the grains can be refined to the maximum extent, with a refinement degree higher than that of hot rolling or cold rolling.

[0027] (2) Rapid heating treatment significantly delays grain recovery and recrystallization, preserving the advantages of dislocation density and grain refinement, and inducing explosive nucleation of intercritical austenite, thereby obtaining a mixed microstructure of ultrafine-grained ferrite and austenite. Small-sized retained austenite near α-ferrite and martensite, together with larger-sized retained austenite near δ-ferrite, form a heterogeneous morphology, providing more heterogeneous strengthening and alleviating stress concentration. At the same time, the soft phase δ-ferrite is significantly reduced and refined, while the increased hard phase martensite provides more strength. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0029] Figure 1 This is a flowchart of one embodiment of the method for preparing high-strength, low-manganese lightweight steel provided by the present invention;

[0030] Figure 2 This is the electron backscatter diffraction (EBSD) phase diagram of Embodiment 1 of the present invention;

[0031] Figure 3 This is a statistical analysis of the average grain size at room temperature in Example 1 of the present invention;

[0032] Figure 4 This is the engineering stress-strain curve of Embodiment 1 of the present invention. Detailed Implementation

[0033] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described are intended to explain the present invention and should not be construed as limiting the invention.

[0034] The terms "an embodiment," "example," or "trademark" used in this specification refer to a particular feature, structure, or characteristic described in connection with the embodiment itself that may be included in at least one embodiment disclosed in this invention. The phrase "in an embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment.

[0035] like Figure 1 As shown in the figure, this invention provides a method for preparing high-strength, low-manganese lightweight steel, wherein the preparation method includes:

[0036] In step S110, the hot-rolled plate is annealed to obtain an annealed plate.

[0037] In step S120, the annealed sheet is subjected to multiple warm rolling processes and then cooled to room temperature with water to obtain a warm-rolled sheet.

[0038] In step S130, the warm-rolled plate is rapidly heated to a set temperature and held at that temperature for a set time, then oil-cooled to room temperature to obtain high-strength, low-manganese lightweight steel.

[0039] The rapid heating rate is between 50℃ / s and 150℃ / s, and the composition of the hot-rolled plate is as follows:

[0040]

[0041] In step S120, multiple warm rolling processes are used, resulting in a large total deformation rate. This not only induces a large amount of carbide precipitation in the microalloyed Nb, V, and Mo and significantly increases the dislocation density, but also allows the interaction between the soft and hard phases of ferrite and martensite during the warm rolling process to fully deform the microalloyed grains. Combined with the pinning and refining of the microalloyed grains, the grains can be refined to the maximum extent, with a refinement degree higher than that of hot rolling or cold rolling.

[0042] In step S130, rapid heating treatment is employed, which significantly delays grain recovery and recrystallization, preserving the advantages of dislocation density and grain refinement, and inducing explosive nucleation of intercritical austenite, thereby obtaining a mixed microstructure of ultrafine-grained ferrite and austenite. Small-sized retained austenite near α-ferrite and martensite, together with larger-sized retained austenite near δ-ferrite, constitute a heterogeneous morphology, providing more heterogeneous strengthening and alleviating stress concentration. Simultaneously, the soft δ-ferrite phase is significantly reduced and refined, while the increased hard martensite phase provides greater strength.

[0043] In the process of repeatedly warm rolling the annealed sheet, the number of warm rolling passes is between 5 and 7, and the reduction rate per pass is between 18% and 25%. In this invention, there are no special limitations on the number of warm rolling passes and the reduction rate per pass, as long as the total reduction rate is sufficient to achieve deformation strengthening.

[0044] In the process of hot rolling the annealed sheet multiple times, the total reduction rate is between 75% and 85%. This range of reduction rate allows the hot-rolled sheet to undergo sufficient deformation to achieve strengthening.

[0045] As an optional implementation, the set time is between 1 second and 6 seconds, the oil cooling rate is between 30°C / s and 50°C / s, and the set temperature satisfies the following formula:

[0046] T0 = ​​Ac1 + (150~200℃);

[0047] Where T0 is the set temperature;

[0048] Ac1 is the starting temperature of the transformation from pearlite to austenite during heating. In this invention, T0 is not specifically limited, the set time is not specifically limited, and the oil cooling rate has no special requirements. As long as the set temperature is within the two-phase region, the set time is within the time during which the grain structure will not undergo significant recovery and recrystallization that would increase the grain size, and the cooling rate is only required to ensure that the warm-rolled plate does not produce heat treatment defects, including conventional defects such as deformation and cracking, a suitable oil cooling rate can be set.

[0049] In this embodiment of the invention, no special limitation is made on how the hot-rolled plate is obtained. For example, the hot-rolled plate can be obtained by purchasing it externally. Another example is that the hot-rolled plate can be obtained directly by rolling steel billets. Specifically, before annealing the hot-rolled plate, the preparation method further includes:

[0050] The steel billet is hot-rolled to obtain a hot-rolled plate. The final rolling temperature of the hot rolling is between 950℃ and 980℃, and the total reduction rate of the hot rolling is between 90% and 92%.

[0051] The annealing time of the hot-rolled plate is between 0.5 h and 1.5 h, and the annealing temperature of the hot-rolled plate satisfies the following formula:

[0052] T = Ac1 + (0~150℃);

[0053] Wherein, T is the annealing temperature of the hot-rolled plate;

[0054] Ac1 is the starting temperature of the transformation from pearlite to austenite during heating.

[0055] This invention does not impose special limitations on the processing technology of hot-rolled plates. T is satisfied within the range of Ac1+ (0~150℃). The annealing time is not specially limited. It ensures that the hot-rolled plates achieve grain refinement, uniform microstructure, elimination of internal stress and processing defects, and meets the annealing time of conventional processes.

[0056] The process of this invention is simple and the heat treatment cycle is greatly reduced, which is conducive to achieving low-cost and high-efficiency production.

[0057] The high-strength, low-manganese lightweight steel in this invention is obtained by the above-described preparation method.

[0058] The present invention will be further illustrated below through specific embodiments and comparative examples.

[0059] Example

[0060] In the following embodiments of the present invention, the composition of the steel billet used is as follows:

[0061]

[0062] Example 1

[0063] A method for preparing high-strength, low-manganese lightweight steel, comprising:

[0064] The steel billet is hot-rolled to obtain a hot-rolled plate. The hot rolling process involves holding at 1200℃ for 2 hours, hot rolling in 5 passes, with a total reduction of 90%, a final rolling temperature of 950℃, and water cooling to room temperature to obtain the hot-rolled plate.

[0065] The austenite transformation start temperature (Ac1) of the hot-rolled plate was measured to be 700℃ using a TA instrument DIL805 thermal expansion phase transformation meter. The hot-rolled plate was then annealed at a temperature of 800℃ (Ac1+100℃) for 0.5h.

[0066] The annealed sheet was subjected to 5 passes of warm rolling, with a reduction rate of 23% in each pass and a total reduction rate of 75%. The sheet was then water-cooled to room temperature to obtain the warm-rolled sheet.

[0067] The warm-rolled plate was rapidly heated to the set temperature of 860℃ at a heating rate of 150℃ / s, and the holding time was set to 6s. It was then oil-cooled to room temperature at a cooling rate of approximately 40℃ / s to obtain sample 1.

[0068] Example 2

[0069] High-strength, low-manganese lightweight steel was prepared using the method provided in Example 1. The difference was that in Example 2, the reduction rate of each pass in the warm rolling process was controlled at 25%, the total reduction rate after 6 passes was 82%, the heating rate was 100℃ / s, the temperature was heated to a set temperature of 900℃, and the holding time was set to 2s. The specific process parameters are shown in Table 1, and sample 2 was obtained.

[0070] Example 3

[0071] High-strength, low-manganese lightweight steel was prepared using the method provided in Example 1. The difference was that in Example 3, the reduction rate of each pass in the warm rolling process was controlled at 24%, the total reduction rate after 7 passes was 85%, the heating rate was 50℃ / s, the temperature was heated to the set temperature of 880℃, and the holding time was set to 4s. The specific process parameters are shown in Table 1, and sample 3 was obtained.

[0072] Comparative Example 1

[0073] High-strength, low-manganese lightweight steel was prepared using the method provided in Example 1. The difference was that the warm-rolled plate of Comparative Example 1 was heated to a set temperature of 800°C at a heating rate of 150°C / s. The specific process parameters are shown in Table 1, and sample 4 was obtained.

[0074] Comparative Example 2

[0075] High-strength, low-manganese lightweight steel was prepared using the method provided in Example 1. The difference was that the reduction rate of each pass in the warm rolling process of Comparative Example 2 was controlled at 10%, and the total reduction rate after 6 passes was 47%. The specific process parameters are shown in Table 1, and Sample 5 was obtained.

[0076] Comparative Example 3

[0077] High-strength, low-manganese lightweight steel was prepared using the method provided in Example 1. The difference was that the annealing of Comparative Example 3 was carried out in a muffle furnace, the warm rolled plate was cold-charged into the furnace, the heating rate was much lower than 50℃ / s, and the holding time was set to 300s. The specific process parameters are shown in Table 1, and Sample 6 was obtained.

[0078] Test case

[0079] Microstructure images were obtained using electron backscatter diffraction Symmetry S2, and grain size was measured using Nano Measure software. The final microstructure of sample 1 is shown below. Figure 2 As shown, the microstructure of sample 1 consists of δ-ferrite, ultrafine-grained α-ferrite, martensite, and retained austenite. The grain size statistics of sample 1 are as follows: Figure 3 As shown, the horizontal axis represents the grain size (equivalent circle diameter) in μm, and the vertical axis represents the number of grains (count). There are a total of 731 grains, with an average grain size of approximately 600 nm, a minimum grain size of 0.3 μm, and a maximum grain size of 4.6 μm. After measurement and calculation, it was found that the volume fraction of retained austenite is greater than 15%, of which 55% have a grain size of less than 250 nm.

[0080] Sample 2 improved the total reduction rate during warm rolling, and the grains were further refined. The microstructure was the same as that of Sample 1, consisting of δ ferrite, ultrafine α ferrite, martensite and retained austenite.

[0081] Sample 3 further improved the total reduction rate of warm rolling, and the grains were further refined. The microstructure was the same as that of Sample 1, consisting of δ ferrite, ultrafine α ferrite, martensite and retained austenite.

[0082] Sample 4 was heated to a lower temperature, resulting in insufficient superheat, and the microstructure consisted only of (δ+α) ferrite and cementite.

[0083] Sample 5 reduced the total reduction rate, resulting in insufficient deformation and a decrease in dislocation accumulation inside, leading to a reduction in austenite nucleation sites and a decrease in the content of retained austenite. The microstructure consisted of (δ+α) ferrite, martensite, and a small amount of retained austenite.

[0084] When the heating rate of sample 6 was slowed down, the microstructure underwent a great degree of recovery and recrystallization, and the grains grew. The microstructure consisted of (δ+α)ferrite, a small amount of martensite, and retained austenite.

[0085] Mechanical properties were tested using a universal testing machine (MTS E45.305), such as... Figure 4 As shown, the horizontal axis represents engineering strain (%), and the vertical axis represents engineering stress (MPa). The sample name 860℃ indicates that the rapid heating setting temperature of sample 1 is 860℃. The yield strength (YS) of sample 1 is approximately 475MPa, the ultratensile strength (UTS) is approximately 1423MPa, and the tensile elongation (TE) is approximately 23%. The remaining mechanical properties of the sample are shown in Table 2.

[0086] After increasing the warm rolling reduction rate, the total reduction rate of sample 2 was 82%, the grains were further refined, and the overall performance was further improved. The yield strength of sample 2 was 552 MPa, the tensile strength was 1512 MPa, and the elongation was 20%.

[0087] Sample 3 further improved the warm rolling reduction rate, with a total reduction rate of 85%, and the grains were sufficiently refined, resulting in further improvement in overall performance. Sample 3 had a yield strength of 528 MPa, a tensile strength of 1487 MPa, and an elongation of 24%.

[0088] During the rapid heating process, the set heating temperature of sample 4 was reduced. Although the heating temperature reached the two-phase region, the short holding time resulted in low superheat. Only cementite was precipitated, and no reverse austenite was formed. The yield strength of sample 4 was 1078 MPa, the tensile strength was 1127 MPa, and the elongation was 13%.

[0089] Sample 5 has a lower reduction rate, less dislocation accumulation, a corresponding decrease in austenite nucleation sites, and a lower residual austenite content. Sample 5 has a yield strength of 503 MPa, a tensile strength of 1168 MPa, and an elongation of 10%.

[0090] During rapid heating, the temperature rise rate of sample 6 was relatively slow, and the microstructure underwent a great deal of recovery and recrystallization, resulting in grain growth and a significant reduction in the advantages of fine grain strengthening and dislocation strengthening. Sample 6 had a yield strength of 706 MPa, a tensile strength of 1215 MPa, and an elongation of 17%.

[0091] Table 1. Process parameters of embodiments and comparative examples of the present invention.

[0092]

[0093] Table 2 Mechanical properties and microstructure of embodiments and comparative examples of the present invention

[0094]

[0095] As can be seen from Tables 1 and 2, the total reduction rate of warm rolling in Example 1 is smaller than that in Examples 2 and 3, and the grain size is larger than that in Examples 2 and 3. Example 1 did not receive sufficient strengthening, and its tensile strength is lower than that in Examples 2 and 3.

[0096] The set temperature for rapid heating in Comparative Example 1 was lower than that in Example 1, and no reverse austenite was generated, resulting in a tensile strength of only 1127 MPa and an elongation of 13%, both of which were poor in terms of overall performance.

[0097] The total reduction rate of warm rolling in Comparative Example 2 was lower than that in Example 1, resulting in less dislocation accumulation, a corresponding decrease in austenite nucleation sites, and a reduction in the content of residual austenite. Consequently, its tensile strength only reached 1168 MPa, and its elongation was 10%, indicating poor overall performance.

[0098] The heating rate of Comparative Example 3 was much lower than that of Example 1. The microstructure was sufficiently restored and recrystallized, and the grains grew. The advantages of fine grain strengthening and dislocation strengthening were significantly reduced, resulting in a tensile strength of only 1215 MPa and an elongation of 17%, with poor overall performance.

[0099] Therefore, by using the components and process of the present invention, low-manganese lightweight steel with better comprehensive performance in terms of tensile strength and elongation can be obtained. The product of Example 2 has a yield strength of up to 552 MPa, a tensile strength of up to 1512 MPa or more, and an elongation of 20%.

[0100] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing high-strength, low-manganese lightweight steel, characterized in that, include: The hot-rolled sheet is annealed to obtain an annealed sheet. The annealing time of the hot-rolled sheet is between 0.5 h and 1.5 h, and the annealing temperature of the hot-rolled sheet satisfies the following formula: T = Ac1 + (0~150℃), where T is the annealing temperature of the hot-rolled plate and Ac1 is the starting temperature of the transformation of pearlite to austenite during heating; After the annealed sheet is subjected to multiple warm rolling processes, it is cooled to room temperature with water to obtain a warm-rolled sheet. The warm-rolled plate is rapidly heated to a set temperature and held at that temperature for a set time, then oil-cooled to room temperature to obtain the high-strength, low-manganese lightweight steel. The rapid heating rate is between 50°C / s and 150°C / s, the set time is between 1s and 6s, and the oil cooling rate is between 30°C / s and 50°C / s. The set temperature satisfies the following formula: T0 = ​​Ac1 + (150~200℃), where T0 is the set temperature, Ac1 is the starting temperature of the transformation from pearlite to austenite during heating, and the composition of the hot-rolled plate is: Carbon 0.28~0.32wt%; Manganese 2.5~3.0 wt%; Aluminum 3.5~4.0 wt%; Chromium 0.4~0.6wt%; Vanadium 0.18~0.22wt%; Niobium 0.03~0.09wt%; Molybdenum 0.18~0.22wt%; The balance is Fe.

2. The preparation method according to claim 1, characterized in that, In the step of performing multiple warm rolling on the annealed sheet, the number of warm rolling passes is between 5 and 7, and the reduction rate per pass is between 18% and 25%.

3. The preparation method according to claim 2, characterized in that, In the step of performing multiple warm rolling processes on the annealed sheet, the total reduction rate is between 75% and 85%.

4. The preparation method according to any one of claims 1 to 3, characterized in that, Before annealing the hot-rolled sheet, the preparation method further includes: The steel billet is hot-rolled to obtain the hot-rolled plate, wherein the final rolling temperature of the hot rolling is between 950°C and 980°C, and the total reduction rate of the hot rolling is between 90% and 92%.

5. A high-strength, low-manganese lightweight steel, characterized in that, The high-strength, low-manganese lightweight steel is obtained by the preparation method described in any one of claims 1 to 4.

Images