Method for improving stability of retained austenite in hot-rolled martensitic steel

Abstract

The application discloses a method for improving the stability of residual austenite in hot-rolled martensitic steel, and belongs to the technical field of hot-rolled high-strength steel. The method comprises the following steps: smelting, refining, continuous casting or casting to obtain a billet or an ingot by using an electric furnace or a converter; carrying out two-stage rolling of rough rolling and finish rolling after reheating the slab; carrying out laminar cooling; and carrying out stacking and slow cooling after coiling the steel coil in a coiling process to obtain residual austenite plasticized hot-rolled martensitic steel. The microstructure of the residual austenite plasticized hot-rolled martensitic steel is ferrite, martensite and residual austenite, wherein the volume fraction of the residual austenite is 4% to 8%. The yield strength of the above-mentioned residual austenite plasticized hot-rolled martensitic steel is 1100 to 1200 MPa, the tensile strength is 1200 to 1300 MPa, the elongation is greater than or equal to 14%, and the fracture of the tensile sample is not layered. The application can effectively solve the problem that the prior art cannot realize or meet the performance requirements of hot-rolled high-strength steel through a conventional hot-rolling production process.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled high-strength steel technology, and relates to a production method of hot-rolled high-strength steel, specifically a method for improving the stability of residual austenite in hot-rolled martensitic steel. Background Technology

[0002] Martensitic steels contain high-density dislocations or twins, which significantly enhances their strength. However, their slightly insufficient toughness limits their application in hot-rolled high-strength steels. Generally, martensitic steels require tempering heat treatment to promote the re-dissolution of some carbon elements in the martensite, softening the martensite structure and improving its toughness and plasticity. In recent years, the development of cold-rolled TRIP steel and plasticized dual-phase steel has provided new ideas for the development of hot-rolled high-strength steels: introducing some retained austenite into hot-rolled martensitic steel to improve its toughness and plasticity. However, how to improve the stability of retained austenite through conventional hot-rolling processes remains a pressing issue.

[0003] A search revealed that CN114908245B discloses a method for controlling the forming of retained austenite in bearings with high fatigue durability and dimensional stability. The method includes the following steps: S1, pre-controlling the retained austenite microstructure through a cold rolling process; S2, performing multiple divisions of the retained austenite microstructure through a graded quenching process; S3, controlling the retained austenite content through a cold treatment process; and S4, increasing the retained austenite carbon content and improving the retained austenite stability through a low-temperature tempering process. This invention also provides a bearing obtained using the above method.

[0004] CN112063931B discloses a low-carbon, medium-manganese, high-residual-austenitic, high-strength and high-toughness steel and its heat treatment method. The steel's chemical composition by mass percentage is: C: 0.10-0.25%, Mn: 4.0-8.0%, Al: 1.0-2.5%, with the balance being Fe and other unavoidable impurities. The method involves smelting, continuous casting, and hot rolling of the steel, followed by the following heat treatment on the hot-rolled steel plate: low-temperature tempering (300-500℃, 1h) - cold rolling (reduction > 70%) - rapid heating (10-50℃ / s) - very short-time isothermal treatment in the two-phase region (740℃-800℃, 0.5-2s) - rapid cooling (> 20℃ / s). The final high-strength steel has an average residual austenite grain size of less than 200 nm and a volume fraction of 15%-40%. The matrix is ​​a heterogeneous structure of equiaxed recrystallized ferrite and strip-shaped unrecrystallized ferrite. The yield strength is in the range of 800 MPa-1200 MPa, the tensile strength is in the range of 1200 MPa-1500 MPa, and the uniform elongation can reach 10%-20%.

[0005] CN114107794B discloses a 980MPa grade ultra-low carbon martensitic steel with residual austenitic structure and its manufacturing method. The chemical composition by weight percentage is: C 0.03%–0.06%, Si 0.8%–2.0%, Mn 1.0%–2.0%, P≤0.02%, S≤0.003%, Al 0.02%–0.08%, N≤0.004%, Mo 0.1%–0.5%, Ti 0.01%–0.05%, O≤0.0030%, with the remainder being Fe and other unavoidable impurities. The hot rolling process of the high-expansion steel of this invention is as follows: 1) Reheating of the cast billet. Heating temperature 1100-1200℃, holding time 1–2 hours; 2) Hot rolling. The initial rolling temperature is 950-1100℃, with 3-5 passes of heavy rolling at 950℃ and a cumulative deformation of ≥50%; then the intermediate billet is heated to 920-950℃, followed by 3-5 final rolling passes with a cumulative deformation of ≥70%; the final rolling temperature is 800-920℃; 3) Cooling. After rolling, the strip is first air-cooled for 0-10s, then water-cooled at a rate of ≥50℃ / s until it is below the martensitic transformation start point Ms before coiling, and then cooled to room temperature; 4) Pickling. The pickling speed of the strip is adjusted within the range of 30-100m / min, the pickling temperature is controlled between 75-85℃, the tensile leveling rate is controlled at ≤2%, then rinsing, drying the strip surface, and oiling. The final high-expansion steel has a yield strength ≥800MPa, tensile strength ≥980MPa, elongation (transverse A50 ≥10%), cold bending performance (d≤4a, 180°), and expansion rate ≥80%.

[0006] As can be seen from the above, existing technologies mostly employ heat treatment to obtain stable retained austenite. Specifically, patent CN114908245B uses a multi-stage quenching followed by low-temperature tempering, while CN112063931B uses a two-phase annealing process. Although CN114107794B produces retained austenite using a hot-rolled steel production process, the resulting high-expansion steel has low strength, failing to meet the strength requirements or application needs of high-strength steel. Therefore, there is an urgent need to research a new method to improve the stability of retained austenite in hot-rolled high-strength steel through a conventional hot-rolling production process. Summary of the Invention

[0007] The technical problem to be solved by this invention is that existing methods for obtaining stable residual austenite cannot be achieved through conventional hot rolling production processes or cannot meet the performance requirements of hot-rolled high-strength steel.

[0008] The technical solution adopted by this invention to solve its technical problem is:

[0009] In a first aspect, the present invention provides a method for improving the stability of retained austenite in hot-rolled martensitic steel, comprising the following steps:

[0010] S1 uses electric furnace or converter to smelt, refine, continuously cast or cast steel billets or ingots to obtain steel billets or ingots with the target composition design range;

[0011] S2 involves reheating the slab and then rolling it in two stages: roughing and finishing. The deformation temperature in the roughing stage is higher than the critical temperature for austenite recrystallization of the target steel composition, while the deformation temperature in the finishing stage is lower than the critical temperature for austenite recrystallization of the target steel composition. The final finishing temperature is higher than the ferrite phase transformation initiation temperature.

[0012] S3 is subjected to laminar flow cooling, and the final cooling temperature is controlled within 20°C below the martensitic transformation initiation temperature of the target composition steel.

[0013] The S4 coiling process involves coiling the steel coil and stacking it for slow cooling to obtain hot-rolled martensitic steel with residual austenite plasticization.

[0014] In step S1 above, the chemical composition of the hot-rolled martensitic steel, by weight percentage, includes: C 0.18%–0.25%, Si 1.40%–1.80%, Mn 1.80%–2.20%, Cr 0.50%–0.70%, B 0.0010%–0.0040%, N 0–0.0030%, with the remainder being Fe and unavoidable impurity elements.

[0015] In step S2 above, the reheating temperature of the slab is 1160℃~1180℃, and the holding time is 3h~4h.

[0016] In step S2 above, the rough rolling temperature is 1050℃~1150℃, the final rolling temperature is 1000℃~1050℃, and the deformation is ≥60%.

[0017] In step S2 above, the initial rolling temperature is 950℃~1050℃, the final rolling temperature is 860℃~920℃, and the deformation is ≥70%.

[0018] In step S3 above, the cooling rate of laminar flow cooling is 5 to 15 °C / s.

[0019] In step S3 above, the final cooling temperature of laminar flow cooling is 380℃~400℃.

[0020] In step S4 above, the steel coils are stacked and slowly cooled for 5 to 10 hours within a temperature range of 300℃ to 350℃, and then naturally air-cooled when the temperature is below 300℃.

[0021] Secondly, the present invention provides a residual austenite-plasticized hot-rolled martensitic steel prepared by the above-described method for improving the stability of residual austenite in hot-rolled martensitic steel.

[0022] The microstructure of the above-mentioned hot-rolled martensitic steel with retained austenite plasticization consists of ferrite, martensite, and retained austenite, with the volume fraction of retained austenite being 4% to 8%.

[0023] The aforementioned hot-rolled martensitic steel with residual austenite plasticization has a yield strength of 1100MPa to 1200MPa, a tensile strength of 1200MPa to 1300MPa, an elongation of ≥14%, and the tensile specimen fracture surface does not delaminate.

[0024] The beneficial effects of this invention are as follows: This invention provides a production method for obtaining residual austenite-plasticized hot-rolled martensitic steel through hot continuous rolling and laminar flow cooling. By optimizing the production process of conventional hot-rolled high-strength steel, the residual austenite stability in hot-rolled high-strength steel is improved. Compared with the cold rolling-continuous annealing process, it has the advantages of low production cost and simple operation. The hot-rolled martensitic steel provided by this invention has low alloy cost, with only inexpensive microalloying elements such as Si, Cr, and B added. The steel provided by this invention has high strength and good plasticity, and the tensile test specimens do not delaminate, indicating that the steel is not prone to delamination defects during forming and punching. The residual austenite-plasticized hot-rolled martensitic steel of this invention has excellent performance and can meet the application requirements of hot-rolled high-strength steel. Attached Figure Description

[0025] Figure 1 The microstructure (F+M) of the steel in Example 1;

[0026] Figure 2 The EBSD results are for the steel in Example 1;

[0027] Figure 3 The microstructure of steel in Comparative Example 1;

[0028] Figure 4 The EBSD results are for Comparative Example 1 steel. Detailed Implementation

[0029] The technical solution of the present invention can be implemented in the following manner.

[0030] The hot-rolled martensitic steel described in this invention possesses both high strength (1100MPa~1200MPa) and elongation (≥14%), and the tensile specimen fracture surface does not delaminate. This is because the retained austenite is a soft phase structure, which can absorb energy during deformation to undergo deformation-induced phase transformation, thereby improving plasticity. Simultaneously, since the main mode of plastic deformation in martensitic steel is dislocation slip along the slip plane, the dislocation density increases with the degree of deformation, increasing the steel's high strength and hardness. Furthermore, the large-angle grain boundaries and the "Cottrell atmospheres" formed by the segregation of C atoms on edge dislocations hinder dislocation movement, enabling the martensitic steel to resist deformation during tensile testing and suppressing the formation of tensile delamination.

[0031] Next, the reasons for the limitation on the alloy element content of the steel described in this invention will be explained.

[0032] C and Mn are important austenite stabilizing elements in steel. Higher C and Mn contents can improve the hardenability of steel and promote the martensitic transformation process. However, excessively high C and Mn contents can easily cause segregation in the cast billet, affecting the material's formability. This invention limits the C and Mn contents to 0.18%–0.25% and 1.80%–2.20%, respectively.

[0033] Si can increase the activity of C and promote the enrichment of C into austenite, thereby improving the stability of austenite. In this invention, the Si content is limited to 1.40%–1.80%.

[0034] Cr and B are important hardenability elements that can promote the martensitic phase transformation process. Therefore, this invention requires the addition of 0.50–0.70% Cr and 0.0010%–0.0040% B. Meanwhile, B readily combines with N to form BN inclusions, which segregate at grain boundaries, significantly affecting the material's plasticity. Therefore, this invention requires limiting the N content to below 0.0030%.

[0035] Furthermore, in conjunction with the requirement of improving the stability of residual austenite in hot-rolled martensite as described in this invention, the reasons for the limitations of the production process are explained.

[0036] High-Si steel is prone to producing FeSi2O4 when the slab is heated. This phase has a low melting point of ~1180℃. In order to avoid the melting range of this phase and prevent the iron oxide scale on the surface from sticking to the steel base and being difficult to remove later, this invention requires the slab heating temperature to be lower than the melting point of this phase to prevent the phase from melting. Specifically, the temperature is required to be controlled between 1160℃ and 1180℃.

[0037] The rolling process employs a two-stage process: roughing and finishing. During roughing, austenite recrystallization occurs, requiring a larger reduction and a higher deformation temperature (above the critical austenite recrystallization temperature) to promote recrystallization. Therefore, this invention limits the initial roughing temperature to 1050℃–1150℃ and the final roughing temperature to 1000℃–1050℃, with a deformation amount limited to ≥60%. The finishing process occurs in the non-recrystallization region of austenite, resulting in austenite flattening and refinement, providing more nucleation sites for subsequent phase transformations and promoting grain refinement. Therefore, this invention requires a lower finishing temperature and a larger finishing deformation amount, with the initial finishing temperature limited to 950℃–1050℃ and the finishing deformation amount limited to ≥70%.

[0038] Phase transformation occurs during the laminar flow cooling process. Firstly, the initial cooling temperature, also known as the finishing rolling temperature, must be higher than the ferrite initiation transformation temperature A. e3 The steel phase transformation temperature A of the chemical composition described in this invention e3The temperature range is 825℃ to 845℃, therefore, this invention requires the initial cooling temperature, i.e., the finishing rolling temperature, to be controlled within the range of 860℃ to 920℃. Secondly, the cooling rate is crucial. The cooling rate must be higher than the critical martensite transformation rate. The critical transformation rate of the steel with the chemical composition described in this invention is ~5℃, therefore, this invention requires a cooling rate higher than 5℃ / s. Simultaneously, the cooling rate should not be too high, otherwise it will reduce the stability of the retained austenite; therefore, this invention requires a cooling rate lower than 15℃ / s. Finally, the final cooling temperature is required to be lower than the martensite initiation temperature M. s The steel M with the chemical composition described in this invention s The point temperature should be around 400℃, and the final cooling temperature should not be too low, preferably controlled at M. s The temperature is set within 20°C below the final cooling temperature to allow for the formation of some retained austenite during the martensitic transformation. Therefore, this invention requires the final cooling temperature to be controlled within the range of 380°C to 400°C.

[0039] The coiling process requires stacking and slowly cooling the steel coils for 5 to 10 hours within a temperature range of 300℃ to 350℃. This is mainly to promote the distribution of carbon elements from martensite to retained austenite through isothermal tempering at medium and low temperatures, thereby improving the stability of retained austenite.

[0040] The technical solution and effects of the present invention will be further explained below through practical examples.

[0041] Example

[0042] The process flow of this invention is as follows: steel billets or ingots within the target composition design range shown in Table 1 are obtained by smelting in a converter or electric furnace → vacuum refining → continuous casting → billet (ingot) heating → rough rolling + finish rolling → laminar flow cooling → coiling → finished steel; wherein, in Examples 1-2 and Comparative Example 1, the steel is stacked and slowly cooled after coiling, while in Comparative Example 2, the steel is directly cooled after coiling. Specific production process parameters for the examples and comparative examples are shown in Table 2.

[0043] Table 1 Chemical composition (by mass percentage, %)

[0044]

[0045] Table 2 Production Process Parameters

[0046]

[0047] Taking Example 1 and Comparative Example 1 as examples, the microstructure of the finished hot-rolled steel obtained therefrom was examined, such as... Figures 1-4 As shown. (Attached) Figure 1 The microstructure of the steel plate in Example 1 of this invention shows that it mainly consists of ferrite and martensite; (See attached image) Figure 2The EBSD test results for the steel in Example 1 show that it contains 6.8% retained austenite. (See attached image.) Figure 3 The microstructure of the steel in Comparative Example 1 of this invention shows that the microstructure is mainly composed of martensite; (See attached image) Figure 4 The EBSD test results for Comparative Example 1 steel show that it does not contain retained austenite.

[0048] The finished hot-rolled steels obtained in Examples 1-2 and Comparative Examples 1-2 were tested, and the mechanical properties test results are shown in Table 3.

[0049] Table 3 Mechanical Properties

[0050] Performance indicators Yield strength / MPa Tensile strength / MPa Elongation / % Tensile fracture morphology Example 1 1123 1211 15.5％ No layering Example 2 1176 1255 15.0％ No layering Comparative Example 1 1189 1268 11.5％ No layering Comparative Example 2 1153 1237 11.0％ No layering

[0051] As shown in Table 3, the steel prepared using the composition and process provided by this invention has a yield strength in the range of 1100 MPa to 1200 MPa, a tensile strength in the range of 1200 MPa to 1300 MPa, an elongation of ≥14%, and no delamination during tensile testing. The yield strength and tensile strength of Comparative Example 1 and Comparative Example 2 steels meet the requirements, but their elongation is low. This is mainly because the laminar cooling rate of Comparative Example 1 steel is too high, resulting in the inability to form stable retained austenite. The Si content in Comparative Example 2 steel is low, and the enrichment of C into austenite is low, leading to reduced austenite stability. Furthermore, the steel coils in Comparative Example 2 were not stacked for slow cooling after coiling, and the coil insulation temperature was too low (153℃), preventing C from diffusing between martensite and austenite. Therefore, retained austenite was also not formed in Comparative Example 2 steel.

Claims

1. A method for improving the stability of retained austenite in hot-rolled martensitic steel, characterized in that, Includes the following steps: S1 steel billets or ingots are obtained by smelting, refining, continuous casting or casting in an electric furnace or converter to obtain steel billets or ingots with the target composition design range; The chemical composition of hot-rolled martensitic steel, by weight percentage, includes: C 0.18%~0.25%, Si 1.40%~1.80%, Mn 1.80%~2.20%, Cr 0.50%~0.70%, B 0.0010%~0.0040%, N 0~0.0030%, with the remainder being Fe and unavoidable impurity elements; S2 involves reheating the slab and then performing two-stage rolling: roughing and finishing. The deformation temperature in the roughing stage is higher than the critical temperature for austenite recrystallization of the target composition steel, and the initial rolling deformation is ≥60%. The deformation temperature in the finishing stage is lower than the critical temperature for austenite recrystallization of the target composition steel, and the final finishing rolling temperature is higher than the ferrite transformation initiation temperature, with a finishing rolling deformation of ≥70%. S3 undergoes laminar flow cooling at a rate of 5~15℃ / s, with a final cooling temperature of 380℃~400℃. The S4 coiling process involves coiling the steel coil and stacking it for slow cooling to obtain hot-rolled martensitic steel with residual austenite plasticization.

2. The method for improving the stability of retained austenite in hot-rolled martensitic steel according to claim 1, characterized in that: In step S2, the slab is reheated at a temperature of 1160℃~1180℃ and held for 3h~4h.

3. The method for improving the stability of retained austenite in hot-rolled martensitic steel according to claim 1, characterized in that: In step S2, the roughing rolling temperature is 1050℃~1150℃, and the final rolling temperature is 1000℃~1050℃.

4. The method for improving the stability of retained austenite in hot-rolled martensitic steel according to claim 1, characterized in that: In step S2, the initial rolling temperature of the finishing mill is 950℃~1050℃, and the final rolling temperature is 860℃~920℃.

5. The method for improving the stability of retained austenite in hot-rolled martensitic steel according to claim 1, characterized in that: In step S4, the steel coils are stacked and slowly cooled for 5 to 10 hours within a temperature range of 300℃ to 350℃, and then naturally air-cooled when the temperature is below 300℃.

6. The method for improving the stability of retained austenite in hot-rolled martensitic steel according to claim 1, characterized in that: In step S4, the microstructure of the retained austenite-plasticized hot-rolled martensitic steel consists of ferrite, martensite, and retained austenite, wherein the volume fraction of retained austenite is 4% to 8%.

7. The method for improving the stability of retained austenite in hot-rolled martensitic steel according to claim 6, characterized in that: The retained austenite-plasticized hot-rolled martensitic steel has a yield strength of 1100MPa~1200MPa, a tensile strength of 1200MPa~1300MPa, an elongation of ≥14%, and the tensile specimen fracture surface does not delaminate.

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