Method for preparing high-mechanical-property medium-manganese steel material and application thereof

Abstract

The application discloses a preparation method of high-mechanical-property medium-manganese steel and application thereof and belongs to the technical field of medium-manganese steel processing. The method is based on conventional hot rolling, cold rolling and annealing treatment, and utilizes secondary circulating quenching heat treatment technology after cold rolling, and the temperature is first increased to 900-910 DEG C and kept for 30-35 min, then the temperature is increased to 900-910 DEG C and kept for 10-12 min after water quenching, and then the temperature is increased to 670-690 DEG C and kept for 1-1.5 h for annealing, and then air cooling is carried out to the ambient temperature, so that the content and stability of residual austenite in the medium-manganese steel are further improved, the mechanical properties of the medium-manganese steel product are remarkably improved, the tensile strength is 838 MPa, the total elongation is 90.8%, and the product of the strength and the elongation is 76.1 GPa%. The method can be widely applied to technical fields requiring the use of high-mechanical-property medium-manganese steel.

Description

Technical Field

[0001] This invention relates to the field of high mechanical property manganese steel processing technology, and particularly to a method for preparing high mechanical property manganese steel. Background Technology

[0002] During power line construction, it is unavoidable to cross various ground facilities such as buildings, highways, existing power transmission lines, railways, and bridges. Crossing frames are typically used for safe construction. Previously, during power transmission line crossings, the objects being crossed required segmented power outages, and corresponding temporary road closures were necessary for that section of highway or railway. Traditional crossing frames primarily use wood, bamboo, or steel pipes, but this method has a long construction period, significantly impacts surrounding production and living facilities, and poses certain safety hazards.

[0003] Medium-manganese steel (Mn content 3-11 wt%) has attracted much attention due to its high metastable retained austenite content and unique dual-phase microstructure (fine γ and α) or three-phase microstructure (fine γ, α, and / or martensite). Since the excellent mechanical properties exhibited by medium-manganese steel are mainly influenced by the content and stability of the retained austenite, controlling the retained austenite content is crucial. Generally, the mechanical stability of retained austenite is affected by the content of C and Mn, grain size, and morphology. Medium-manganese steel is generally produced using the reverse austenite transformation (ART) process to obtain a large amount of metastable retained austenite. These varieties of medium-manganese steel are widely used in automobile manufacturing and other fields due to their excellent strength, elongation, impact resistance, and safety. If a medium-manganese steel with high mechanical properties could be designed for the crossing structures required in power line construction, it would have broad application prospects in related fields. Summary of the Invention

[0004] This invention provides a method for preparing high-performance medium-manganese steel. Utilizing rapid heating and secondary cyclic quenching heat treatment technology, the method further improves the content and stability of retained austenite in the medium-manganese steel, significantly enhancing the mechanical properties of the finished product. This method can be widely applied in technical fields requiring high-performance medium-manganese steel, such as for manufacturing trusses (crossing structures) capable of spanning ground facilities. Specifically, it is achieved through the following techniques.

[0005] A method for preparing high-mechanical-performance manganese steel includes the following steps:

[0006] S1. Take a medium manganese steel forging billet and pass it through hot rolling and cold rolling to obtain a cold-rolled steel plate;

[0007] S2. Heat the cold-rolled steel sheet obtained in step S1 to 900-910℃ and hold for 30-35 minutes, then quench to complete the first cycle of quenching; heat to 900-910℃ again and hold for 10-12 minutes, then quench to complete the second cycle of quenching.

[0008] S3. After annealing the material obtained in step S2 at 670-690℃ for 1-1.5h, cool it to room temperature to obtain a high-mechanical-performance medium-manganese steel product.

[0009] This invention employs the aforementioned method to perform hot rolling, cold rolling, and heat treatment on medium-manganese steel. This effectively refines the original austenite grains, thereby refining the residual austenite grains after quenching and the reverse austenite transformation. It also increases the diffusion flux of Mn atoms in austenite and effectively prolongs the annealing holding time, promoting the distribution and enrichment of Mn into austenite and enhancing the stability of residual austenite. As a result, more residual austenite is obtained at ambient temperature, which provides a strong guarantee for the generation of the TRIP effect during deformation. Ultimately, this results in medium-manganese steel possessing superior comprehensive mechanical properties.

[0010] Preferably, in step S1, the chemical composition of the medium manganese steel forging billet includes C 0.20-0.35wt%, Mn 3.51-4.05wt%, Al 1.1-1.25wt%, Si 0.15-0.30wt%, Nb 0.02-0.04wt%, Mo 0.15-0.25wt%, with the balance being Fe and unavoidable impurities.

[0011] More preferably, in step S1, the chemical composition of the medium manganese steel forging billet includes 0.25wt% C, 3.98wt% Mn, 1.22wt% Al, 0.20wt% Si, 0.03wt% Nb, 0.19wt% Mo, with the balance being Fe and unavoidable impurities.

[0012] Preferably, in step S1, the hot rolling and cold rolling process is as follows: the medium manganese steel forging billet is heated to 1200°C, held for 2 hours, and hot rolled several times to obtain a hot-rolled steel plate. After cooling to room temperature, it is cold-rolled into a cold-rolled steel plate.

[0013] Preferably, in step S2, the cold-rolled steel sheet obtained in step S1 is heated to 900°C and held for 30 minutes, and then quenched to complete the first cycle of quenching.

[0014] More preferably, in step S2, the steel plate that has completed the first cycle of quenching is heated again to 900°C and held for 10 minutes, and then quenched to complete the first cycle of quenching.

[0015] Preferably, in step S3, the annealing temperature is 675°C and the annealing time is 1 hour.

[0016] The high-performance medium-manganese steel provided by this invention can be used in ground construction to prepare crossing frames that span ground facilities (such as houses, highways, power transmission lines, railways, or bridges); or it can be used to manufacture motor vehicle parts, such as the body, engine, or chassis of a motor vehicle. Preferably, the crossing frame comprises a truss made of high-performance medium-manganese steel prepared using the method of this invention.

[0017] Based on the lifting principle of truck cranes and the design principle of umbrellas, a telescopic and foldable truss frame assembly, similar to the ribs of an umbrella, is designed. The truss frame assembly is deployed or folded via a power system and linkage mechanism. Similar to umbrella ribs, as the truss frame assembly deploys, the rope system around the truss tightens, connecting with the truss components in all directions to form a square (or circular) platform. A truck crane serves as the power source, using its telescopic boom to lift the aerial work platform.

[0018] Compared with the prior art, the advantages of the present invention are:

[0019] 1. This invention provides a method for preparing high-performance medium-manganese steel. By utilizing rapid heating and secondary cyclic quenching heat treatment technology, the content and stability of retained austenite in the medium-manganese steel are further improved, significantly enhancing the mechanical properties of the finished medium-manganese steel. The tensile strength is 838 MPa, the total elongation is 90.8%, and the product of strength and elongation is 76.1 GPa%. This method can be widely applied in technical fields requiring high-performance medium-manganese steel.

[0020] 2. The crossing frame made of high-performance manganese steel according to the present invention has high strength, flexible contraction, large unfolded area and small contracted volume; it can realize construction of crossing complex terrain. The entire crossing frame is modularly designed, which facilitates transportation and rapid installation and improves work efficiency. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the preparation process of the medium-manganese steel sample in Example 1;

[0022] Figure 2 The images show SEM and TEM images of the medium-manganese steel samples from Example 1 and Comparative Example 1; where, Figure 2 (a, c) correspond to medium manganese steel in ratio 1. Figure 2 (b, d) correspond to the medium manganese steel in Example 1;

[0023] Figure 3 The XRD patterns of the medium-manganese steel samples from Example 1 and Comparative Example 1 are shown below; Figure 3 (a) is the XRD pattern of the specimen before tensile testing. Figure 3 (b) is the XRD pattern of the specimen after stretching;

[0024] Figure 4 The mechanical properties of the medium manganese steel samples of Example 1 and Comparative Example 1 after heat treatment;

[0025] Figure 5 The C and Mn distribution of the medium manganese steel samples of Example 1 and Comparative Example 1 after heat treatment; wherein, γ refers to austenite, γF refers to elongated film-like retained austenite, and γG refers to granular retained austenite. Figure 5 (a) and (b) are respectively, Figure 5 (b) is, Figure 5 (c) shows the change in residual austenite Mn concentration in the medium manganese steel sample. Detailed Implementation

[0026] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] In the following examples and comparative examples, the raw material used was self-made medium-manganese steel (experimental steel, grade 4Mn-Nb-Mo), which was smelted into 15kg steel ingots in a vacuum medium-frequency induction furnace. The chemical composition was tested to be C 0.25%, Mn 3.98%, Al 1.22%, Si 0.20%, Nb 0.03%, Mo 0.19%, with the balance being Fe and other unavoidable impurities.

[0028] Example 1

[0029] The medium-manganese steel provided in this embodiment is prepared by the following method:

[0030] S1. The ingot is forged into a 100mm×30mm forging billet, and then the forging billet is placed in a high-temperature furnace and heated to 1200℃, held for 2 hours, and then hot rolled in six passes to finally roll into a 3.8mm thick plate, which is then air-cooled to room temperature.

[0031] Then, the cold-rolled sheet was rolled to a thickness of 1.9 mm on a cold rolling mill. The critical zone temperatures Ac1 and Ac3 were measured to be 632℃ and 862℃, respectively, using Thermal-Calc software.

[0032] S2. Heat the cold-rolled steel sheet obtained in step S1 to 900℃ and hold for 30 minutes (austenitization), then water quench to complete the first cycle of quenching; heat it again to 900℃ and hold for 10 minutes (austenitization), then water quench to complete the second cycle of quenching.

[0033] S3. After annealing the material obtained in step S2 at 675°C for 1 hour, cool it to room temperature to obtain a high-mechanical-performance medium-manganese steel product.

[0034] The above preparation process is as follows Figure 1 As shown in the figure, HR represents the sixth hot rolling stage, AC represents the air cooling to room temperature stage, CR represents the cold rolling stage; CQ1 and CQ2 represent the first and second cycle quenching stages, respectively; WC represents the water quenching process; and ART represents the austenitic reverse transformation annealing stage.

[0035] Comparative Example 1

[0036] The medium-manganese steel provided in this comparative example was not subjected to secondary quenching; its preparation method is as follows:

[0037] S1. The ingot is forged into a 100mm×30mm forging billet, and then the forging billet is placed in a high-temperature furnace and heated to 1200℃, held for 2 hours, and then hot rolled in six passes to finally roll into a 3.8mm thick plate, which is then air-cooled to room temperature.

[0038] Then, the cold-rolled sheet was rolled to a thickness of 1.9 mm on a cold rolling mill. The critical zone temperatures Ac1 and Ac3 were measured to be 632℃ and 862℃, respectively, using Thermal-Calc software.

[0039] S2. Heat the cold-rolled steel sheet obtained in step S1 to 900℃ and hold for 30 minutes (austenitization), then quench it in water.

[0040] S3. After annealing the material obtained in step S2 at 675°C for 1 hour, cool it to room temperature to obtain a high-mechanical-performance medium-manganese steel product.

[0041] Application example: Microstructure, bulk structure and mechanical property testing of medium manganese steel.

[0042] The medium-manganese steel materials from Example 1 and Comparative Example 1 were selected and named CQ2-ART and CQ1-ART, respectively.

[0043] 1. Microstructure testing

[0044] The microstructure of the CQ-ART test steel samples was observed using SEM and TEM. The samples observed by scanning electron microscopy were etched with 4% nitric acid alcohol by volume.

[0045] like Figure 2 As shown, Figure 2 Figures (a, b) and (c, d) show the SEM and TEM microstructures of the medium-manganese steel samples, respectively. As shown in Figures (a, b), the microstructure of the samples after CQ-ART treatment mainly consists of retained austenite and ferrite. However, the difference lies in the composition of the retained austenite... Figure 2In CQ1-ART samples (Comparative Example 1) of (a) and (c), the samples were elongated strips, while... Figure 2 The CQ2-ART samples (Example 1) in (b) and (d) exhibit a finer and shorter strip shape. These results demonstrate that the rapid heating cyclic quenching austenite reverse transformation process can effectively refine the original austenite grains, thereby refining the residual austenite grains after quenching and austenite reverse transformation.

[0046] The grain size of retained austenite in the two samples was detected using Digital Micrograph and Image Pro Plus software. It was found that the average grain width of retained austenite in the CQ1-ART sample (Comparative Example 1) was 620 nm, while the average grain width of retained austenite in the CQ2-ART sample (Example 1) decreased to 400 nm.

[0047] The rapid heating and cyclic quenching austenitic reverse transformation heat treatment process involves repeatedly and rapidly heating the sample to near austenitization, holding it at that temperature for a short time, and then rapidly cooling it to refine the size of the retained austenite grains. Analysis suggests that the core of this process lies in two aspects: first, repeatedly and rapidly heating the austenite in the experimental steel to prevent it from growing too large, thus generating finer proto-austenite grains, which then allow for the formation of finer, elongated martensite during subsequent water quenching; second, subjecting the water-quenched sample to austenitic reverse transformation, where the transformed austenite tends to nucleate along the boundaries of the fine, elongated martensite; and finally, the experimental steel can generate metastable retained austenite with a finer grain size at room temperature.

[0048] 2. Bulk structure testing

[0049] Medium-manganese steels from Example 1 and Comparative Example 1, namely CQ2-ART and CQ1-ART, were selected and machined into standard plate-shaped tensile test steel samples with a width of 12.5 mm in the rolling direction. The phase composition of the experimental steel was analyzed by XRD with a scanning angle of 40°-120° and a scanning speed of 1° / min. The content of retained austenite was calculated using the diffraction peak intensities of (200)α, (211)α, (311)γ, (200)γ, and (220)γ.

[0050] Figure 3 The XRD patterns of medium-manganese steel specimens from Example 1 and Comparative Example 1 before and after tensile testing are shown. The specimens before tensile testing are as follows: Figure 3 As shown in (a), both the Example 1 (CQ2-ART) and Example 2 (CQ1-ART) samples contain three FCC diffraction peaks: γ(200), γ(220), and γ(311). However, the γ(200) diffraction peak in the CQ2-ART sample is significantly stronger than that in the CQ1-ART sample. Figure 2This is also one of the reasons why the CQ2-ART sample contains more austenite.

[0051] After the specimen is stretched, Figure 3 As shown in (b), the intensity of the two BCC diffraction peaks α(200) and α(211) in the samples of Example 1 (CQ2-ART) and Example 2 (CQ1-ART) is significantly enhanced, while the intensity of the three FCC diffraction peaks γ(200), γ(220) and γ(311) is significantly reduced.

[0052] Analysis suggests that the retained austenite content in the CQ2-ART sample before stretching was significantly higher than that in the CQ1-ART sample, while the retained austenite content in both samples was comparable after stretching. This indicates that during the stretching process, the retained austenite in both experimental steels underwent a phase transformation-induced martensitic transformation to form a martensitic phase, resulting in a more widespread TRIP effect. Furthermore, the retained austenite content and conversion rate of the CQ2-ART sample in Example 1 were higher than those of the CQ1-ART sample in Example 2.

[0053] 3. Mechanical property testing

[0054] Medium manganese steels, namely CQ2-ART and CQ1-ART, from Example 1 and Comparative Example 1 were selected and processed into standard plate-shaped tensile test steel samples with a width of 12.5 mm in the rolling direction. Figure 4 The mechanical properties of samples after two cycles of quenching and reverse austenitic transformation processes are presented.

[0055] The ultimate tensile strength (UTS) of the sample (CQ2-ART) in Example 1 was 838 MPa, the total elongation (TE) was 90.8%, and the product of tensile strength and elongation (UTS×TE, i.e., PSE) was 76.1 GPa.

[0056] The UTS of the sample in Comparative Example 1 (CQ1-ART) was 784 MPa, the TE was 83.7%, and the PSE was 65.6 GPa.

[0057] The PSE of conventional ART steel is 11.5-29.3 GPa, and the austenite content is 24.2-34.2% (volume fraction).

[0058] Combination Figure 3 and Figure 4 It can be seen that as the number of rapid heating cyclic quenching cycles increased from 1 to 2, the UTS, TE, and PSE of the experimental steel all tended to increase. After tensile fracture, approximately 80% and 90% of the residual austenite in the CQ1-ART and CQ2-ART samples underwent phase transformation-induced martensite transformation to form martensite, respectively. This reflects that the content of residual austenite and the transformation rate during the tensile process directly affect the comprehensive mechanical properties of the samples.

[0059] The above results indicate that the rapid heating cyclic quenching austenite reverse phase transformation process can effectively improve the stability of metastable residual austenite at room temperature, providing a basis for the generation of a wide TRIP effect during tensile testing, thereby obtaining excellent mechanical properties.

[0060] 4. The partitioning behavior of Mn and C

[0061] The partitioning behavior of Mn and C elements during the reverse transformation annealing of austenite will cause differences in the stability of the austenite produced by the reverse transformation, ultimately resulting in differences in the volume fraction of metastable residual austenite at room temperature. The Mn content in the residual austenite of medium-manganese steel samples from Example 1 (CQ2-ART) and Comparative Example 1 (CQ1-ART) at room temperature was quantitatively characterized using TEM-EDS, as shown in Table 1. Figure 5 As shown.

[0062] Table 1. Mn concentration in retained austenite characterized by TEM-EDS

[0063] Mn concentration CQ1-ART CQ2-ART Average, wt% 6.16 7.31 Minimum value, wt% 5.24 6.07 Maximum value, wt% 7.69 7.93

[0064] The average Mn concentration in the retained austenite of the CQ1-ART sample was 6.16 wt%, while that of the CQ2-ART sample was 7.19 wt%. Furthermore, the latter had both higher maximum and minimum Mn concentrations than the former. This Mn enrichment stabilizes the retained austenite grains (e.g., due to Mn distribution from adjacent ferrite grains) because it is distributed from them. Figure 5 (a, b)

[0065] The concentration of residual austenite Mn measured in CQ1-ART and CQ2-ART samples is as follows: Figure 5 As shown in (c), Mn exhibits a clear partitioning behavior between the retained austenite and ferrite grains, and granular retained austenite (γ-) can be observed. G The Mn concentration in the ) is greater than that in the elongated film-like retained austenite (γ) F ).

[0066] Furthermore, the Mn concentration in the retained austenite after two cycles of quenching in Example 1 was significantly higher than the Mn concentration after one cycle of quenching in Comparative Example 1, indicating that the average Mn concentration in the retained austenite significantly increased after multiple rapid heating cyclic quenching austenite reversal transformation processes. Meanwhile, C is also a key element for stabilizing retained austenite, and the C concentration in retained austenite can be obtained using the following equation:

[0067] α γ =3.556 + 0.0453x C +0.00095x Mn +0.0056x Al

[0068] In the formula xC x Mn and x Al The concentrations (wt%) of C, Mn, and Al in austenite are respectively; α γ It is the austenite lattice parameter α is calculated from the following formula γ :

[0069]

[0070] In the formula, λ, θ, and (h, k, l) are the X-ray wavelength, diffraction angle, and lattice parameters, respectively. From the above formula, the C concentrations in the retained austenite of Comparative Example 1 (CQ1-ART) and Example 1 (CQ2-ART) samples are 0.5171 wt% and 0.5139 wt%, respectively. The results show that because the total content of retained austenite in the CQ2-ART sample is higher than that in the CQ1-ART sample, the C concentration in individual retained austenite grains is slightly lower. Therefore, the C element distribution behavior in the retained austenite of the samples after rapid heating and cyclic quenching austenite reverse phase transformation treatment is not obvious, while the Mn element exhibits the opposite behavior.

[0071] Analysis suggests that C atoms act as interstitial solutes, rapidly diffusing and dispersing between ferrite and austenite to form a dynamic equilibrium; however, Mn atoms act as substitutional solutes, with a diffusion coefficient much smaller than that of interstitial solutes, requiring extended holding time to promote effective diffusion of Mn.

[0072] The above detailed embodiments describe the implementation of the present invention; however, the present invention is not limited to the specific details described in the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

Claims

1. A method for preparing high-mechanical-performance manganese steel, characterized in that, Includes the following steps: S1. Take a medium manganese steel forging billet and pass it through hot rolling and cold rolling to obtain a cold-rolled steel plate; S2. Heat the cold-rolled steel sheet obtained in step S1 to 900-910℃ and hold for 30-35 minutes, then quench to complete the first cycle of quenching; heat it again to 900-910℃ and hold for 10-12 minutes, then quench to complete the second cycle of quenching, thereby refining the residual austenite grains and increasing the concentration of Mn in the residual austenite. S3. After annealing the material obtained in step S2 at 670-690℃ for 1-1.5h, cool it to room temperature to obtain a high-mechanical-performance medium-manganese steel product.

2. The method for preparing high-mechanical-performance manganese steel according to claim 1, characterized in that, In step S1, the chemical composition of the medium manganese steel forging billet includes C 0.20-0.35wt%, Mn 3.51-4.05wt%, Al 1.1-1.25wt%, Si 0.15-0.30wt%, Nb 0.02-0.04wt%, Mo 0.15-0.25wt%, with the balance being Fe and unavoidable impurities.

3. The method for preparing high-mechanical-performance manganese steel according to claim 2, characterized in that, In step S1, the chemical composition of the medium manganese steel forging billet includes C 0.25wt%, Mn 3.98wt%, Al 1.22wt%, Si 0.20wt%, Nb 0.03wt%, Mo 0.19wt%, with the balance being Fe and unavoidable impurities.

4. The method for preparing high-mechanical-performance manganese steel according to claim 1, characterized in that, In step S1, the hot rolling and cold rolling processes are as follows: the medium manganese steel forging billet is heated to 1200℃, held for 2 hours, and hot rolled several times to obtain a hot-rolled steel plate. After cooling to room temperature, it is cold-rolled into a cold-rolled steel plate.

5. The method for preparing high-mechanical-performance manganese steel according to claim 1, characterized in that, In step S2, the cold-rolled steel sheet obtained in step S1 is heated to 900℃ and held for 30 minutes, and then quenched to complete the first cycle of quenching.

6. The method for preparing high-mechanical-performance manganese steel according to claim 5, characterized in that, In step S2, the steel plate that has completed the first cycle of quenching is heated again to 900℃ and held for 10 minutes, and then the second cycle of quenching is completed.

7. The method for preparing high-mechanical-performance manganese steel according to claim 1, characterized in that, In step S3, the annealing temperature is 675℃ and the annealing time is 1 hour.

8. A high-mechanical-performance medium-manganese steel prepared by the preparation method according to any one of claims 1-7.

9. An application of the high mechanical properties manganese steel as described in claim 8, characterized in that, During ground construction, it is used to prepare a crossing frame that crosses ground facilities, such as houses, highways, power lines, railways, or bridges; or it is used to manufacture motor vehicle parts.

10. A bridging frame, characterized in that, The invention includes a truss, wherein the truss is made of high-mechanical-performance medium-manganese steel prepared by the preparation method described in any one of claims 1-7.

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