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

Abstract

The invention discloses a method for improving the stability of retained 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, continuously casting or casting by adopting an electric furnace or a converter to obtain steel billets or steel ingots; the plate blank is reheated and then subjected to two-stage rolling of rough rolling and finish rolling; carrying out laminar cooling; and in the coiling process, the steel coil is coiled, stacked and slowly cooled, and the retained austenite plasticizing type hot-rolled martensitic steel is obtained. The microstructure of the retained austenite plasticized hot-rolled martensitic steel is ferrite, martensite and retained austenite, wherein the volume fraction of the retained austenite is 4-8%. The yield strength of the retained austenite plasticized hot-rolled martensitic steel is 1100-1200 MPa, the tensile strength is 1200-1300 MPa, the ductility is larger than or equal to 14%, and a tensile sample fracture is not layered. The problem that in the prior art, the performance requirement of the hot-rolled high-strength steel cannot be achieved or cannot be met through a conventional hot rolling production process can be effectively solved.

Description

Technical Field

[0001] The invention belongs to the technical field of hot-rolled high-strength steel, relates to a production method of hot-rolled high-strength steel, and specifically relates to a method for improving the stability of retained austenite in hot-rolled martensitic steel. Background Art

[0002] The martensitic structure contains a high density of dislocations or twin structures, which can significantly improve the strength of steel. However, the toughness of martensitic steel is slightly insufficient, which limits the application of martensitic structure in hot-rolled high-strength steel. Generally, martensitic steel needs to be tempered to promote the dissolution of part of the carbon elements in the martensite to soften the martensitic structure and improve the toughness and plasticity of martensitic steel. In recent years, with the technological development of cold-rolled TRIP steel and plasticized dual-phase steel, a new idea has been provided for the development of hot-rolled high-strength steel, that is, introducing some residual austenite structure into hot-rolled martensitic steel to improve the toughness and plasticity of the steel. However, how to improve the stability of residual austenite through conventional hot rolling production processes has become an urgent problem to be solved.

[0003] After searching, CN114908245B discloses a method for forming and manufacturing austenite of a bearing with high fatigue durability and dimensional stability, which includes the following steps: S1, pre-regulating the residual austenite structure through a cold rolling forming process; S2, dividing the residual austenite structure multiple times through a graded quenching process; S3, controlling the residual austenite content of the residual austenite structure through a cold treatment process; S4, increasing the residual austenite carbon content of the residual austenite structure through a low-temperature tempering process to improve the residual austenite stability of the residual austenite structure. The invention also provides a bearing obtained by the above method.

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

[0005] CN114107794B discloses a 980MPa grade ultra-low carbon martensite and residual Austrian type ultra-high hole expansion steel and its manufacturing method, the chemical composition 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%, the rest is Fe and other inevitable impurities. The hot rolling process of the high hole expansion steel of the present invention is: 1) reheating of the casting. The heating temperature is 1100-1200℃, and the holding time is 1-2 hours; 2) hot rolling. The starting rolling temperature is 950-1100℃, and the strip is rolled down for 3-5 passes above 950℃ with a cumulative deformation of ≥50%; the intermediate billet is then heated to 920-950℃, and then the last 3-5 passes are rolled with a cumulative deformation of ≥70%; the final rolling temperature is 800-920℃; 3) Cooling. After rolling, air cooling is first performed for 0-10s, and then the strip is water-cooled at a cooling rate of ≥50℃ / s to below the martensitic phase transformation starting point Ms for coiling, and then cooled to room temperature after coiling; 4) Pickling. The pickling speed of the strip is adjusted in the range of 30-100m / min, the pickling temperature is controlled between 75-85℃, the straightening rate is controlled at ≤2%, and then rinsed, the strip surface is dried, and oiled. The high hole expansion steel finally obtained has a yield strength of ≥800MPa, a tensile strength of ≥980MPa, an elongation (transverse A50 ≥10%), cold bending performance (d≤4a, 180°), and a hole expansion rate of ≥80%.

[0006] As can be seen from the above, in order to obtain stable retained austenite in the prior art, heat treatment is mostly used. Among them, the CN114908245B patent adopts a multi-stage quenching and low-temperature tempering process, and the CN112063931B adopts a two-phase zone annealing process. Although CN114107794B prepares para-Austenite steel through a hot-rolled steel production process, the high-expansion steel obtained has low strength and cannot meet the strength requirements or application needs of high-strength steel. Therefore, it is urgent to study a new method to improve the stability of para-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 the present invention is that the existing method for obtaining stable retained austenite cannot be achieved through a conventional hot rolling production process or cannot meet the performance requirements of hot-rolled high-strength steel.

[0008] The technical solution adopted by the present 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 smelting, refining, continuous casting or casting to obtain steel billets or ingots within the target composition design range;

[0011] S2 reheats the slab and then performs rough rolling and finish rolling in two stages, wherein the deformation temperature in the rough rolling stage is higher than the austenite recrystallization critical temperature of the target steel component, the deformation temperature in the finish rolling stage is lower than the austenite recrystallization critical temperature of the target steel component, and the final rolling temperature of the finish rolling is higher than the ferrite phase transformation starting temperature;

[0012] S3 performs laminar cooling, and the final cooling temperature is controlled within a range of 20°C below the martensitic transformation starting temperature of the target composition steel;

[0013] In the S4 coiling process, the steel coils are coiled and stacked for slow cooling to obtain retained austenite plasticized hot-rolled martensitic steel.

[0014] In the above step S1, the chemical composition of the hot-rolled martensitic steel includes, by weight percentage: 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%, and the rest is Fe and unavoidable impurity elements.

[0015] In the above step S2, the heating temperature of the slab reheating is 1160°C to 1180°C, and the insulation time is 3h to 4h.

[0016] In the above step S2, the starting temperature of the rough rolling is 1050°C to 1150°C, the finishing temperature is 1000°C to 1050°C, and the deformation amount is ≥60%.

[0017] In the above step S2, the start rolling temperature of the finishing rolling is 950°C to 1050°C, the final rolling temperature is 860°C to 920°C, and the deformation amount is ≥70%.

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

[0019] In the above step S3, the final cooling temperature of the laminar cooling is 380°C to 400°C.

[0020] In the above step S4, the steel coil stack is slowly cooled for 5 h to 10 h in the temperature range of 300° C. to 350° C., and is naturally air-cooled when the temperature is below 300° C.

[0021] In a second aspect, the present invention provides a retained austenite plasticized hot-rolled martensitic steel produced by the above method for improving the stability of retained austenite in hot-rolled martensitic steel.

[0022] The microstructure of the retained austenite plasticized hot-rolled martensitic steel is ferrite, martensite and retained austenite, wherein the volume fraction of the retained austenite is 4% to 8%.

[0023] The yield strength of the retained austenite plasticized hot-rolled martensitic steel is 1100MPa-1200MPa, the tensile strength is 1200MPa-1300MPa, the elongation is ≥14%, and the fracture of the tensile specimen is not delaminated.

[0024] The beneficial effects of the present invention are as follows: the present invention provides a production method for obtaining retained austenite plasticized hot-rolled martensitic steel by hot rolling-laminar cooling, and improves the residual austenite stability in hot-rolled high-strength steel by optimizing the production process of conventional hot-rolled high-strength steel. Compared with the process method of cold rolling-continuous annealing, the present invention has the advantages of low production cost and simple operation method; the hot-rolled martensitic steel alloy provided by the present invention has low cost, and only inexpensive microalloying elements such as Si, Cr, and B are added; the steel provided by the present invention has good plasticity while having high strength, and the fracture of the tensile specimen is not stratified, indicating that the steel is not prone to stratification defects during forming and punching. The retained austenite plasticized hot-rolled martensitic steel of the present invention has excellent performance and can meet the application requirements of hot-rolled high-strength steel. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0026] Figure 2 EBSD results of the steel in Example 1;

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

[0028] Figure 4 This is the EBSD result of the steel in comparative example 1. DETAILED DESCRIPTION

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

[0030] The hot-rolled martensitic steel of the present invention has both high strength (1100MPa-1200MPa) and elongation (≥14%), and the fracture of the tensile specimen is not delaminated. This is because the residual austenite belongs to a soft phase structure, which can absorb energy during the deformation process to induce deformation phase transformation, thereby improving plasticity. At the same time, since the main mode of plastic deformation of martensitic steel is the slip of dislocations along the slip plane, as the deformation degree increases, the dislocation density increases, increasing the high strength and hardness of the steel. At the same time, the "Cottrell gas mass" formed by the segregation of large-angle grain boundaries and C atoms on edge dislocations hinders the movement of dislocations, allowing the martensitic steel to resist deformation during the tensile process and inhibit the formation of tensile delamination.

[0031] Next, the reasons for limiting the alloy element content of the steel of the present invention are explained.

[0032] C and Mn are important austenite stabilizing elements in steel. Using higher C and Mn contents can also improve the hardenability of steel and promote the martensitic phase transformation process. However, the C and Mn elements should not be too high, otherwise it will easily cause segregation of the casting and affect the forming performance of the material. The present invention limits the C and Mn contents to 0.18% to 0.25% and 1.80% to 2.20% respectively.

[0033] The Si element can increase the activity of C and promote the enrichment of C element into austenite, thereby improving the stability of austenite. The present invention limits the Si content to 1.40% to 1.80%.

[0034] Cr and B are important hardenability elements that can promote the martensitic transformation process. Therefore, the present invention requires the addition of 0.50-0.70% Cr and 0.0010%-0.0040% B. At the same time, the B element is easy to combine with the N element to form BN inclusions, and segregate and distribute at the grain boundaries, greatly affecting the plasticity of the material, so the present invention requires that the N content be limited to less than 0.0030%.

[0035] Combined with the requirement of improving the stability of retained austenite in the hot-rolled martensite of the present invention, the reasons for the limitation of the production process are explained.

[0036] Steel with high Si content is prone to produce FeSi2O4 when the slab is heated. The melting point of this phase is relatively low, ~1180°C. In order to avoid the melting range of this phase and prevent the surface oxide scale from sticking to the steel base and being difficult to remove later, the present invention requires that the slab heating temperature be lower than the melting point of this phase to prevent the phase from melting, and specifically requires that it be controlled at 1160°C ~ 1180°C.

[0037] The rolling process adopts two-stage rolling of rough rolling and finishing rolling, wherein austenite recrystallization occurs during the rough rolling process, requiring a larger reduction and a higher deformation temperature (higher than the critical temperature of austenite recrystallization) to promote austenite recrystallization. Therefore, the present invention limits the rough rolling start temperature to 1050℃~1150℃, the rough rolling final rolling temperature to 1000℃~1050℃, and the deformation amount to more than 60%. The finishing rolling process is in the austenite non-recrystallization zone, and flattening and refinement of austenite occur, providing more nucleation particles for subsequent phase transformation and promoting grain refinement. Therefore, the present invention requires the use of a lower finishing rolling temperature and a larger finishing rolling deformation, wherein the finishing rolling start temperature is limited to 950℃~1050℃, and the finishing rolling deformation amount is limited to ≥70%.

[0038] Phase change occurs in the laminar cooling process. First, the cooling temperature, that is, the final rolling temperature, is required to be higher than the ferrite transformation temperature A. e3 , the steel phase transition temperature A of the chemical composition of the present invention e3In the range of 825℃~845℃, the present invention requires that the start cooling temperature, that is, the finishing rolling temperature, be controlled in the range of 860℃~920℃. The second is the cooling rate. The cooling rate must be higher than the critical transformation rate of martensite. The critical transformation rate of the steel with the chemical composition described in the present invention is 5℃~5℃, so the present invention requires the cooling rate to be higher than 5℃ / s. At the same time, the cooling rate should not be too high, otherwise it will reduce the stability of the residual austenite, so the present invention requires the cooling rate to be lower than 15℃ / s. Finally, the final cooling temperature. It is required to be lower than the martensite start transformation temperature M s , the chemical composition of the steel M of the present invention s The final cooling temperature should not be too low and should be controlled at M s The temperature is controlled within 20°C below the final cooling point in order to form a portion of residual austenite during the martensitic transformation. Therefore, the present invention requires that the final cooling temperature be controlled within the range of 380°C to 400°C.

[0039] The coiling process requires that the steel coil stack be slowly cooled for 5h to 10h in the temperature range of 300℃ to 350℃, mainly to promote the distribution of carbon elements in martensite to retained austenite through isothermal tempering at medium and low temperatures, so as to improve the stability of retained austenite.

[0040] The technical solution and effects of the present invention are further illustrated below through practical examples.

[0041] Example

[0042] The process flow of the embodiment of the present invention is: using a converter or electric furnace to smelt to obtain a steel billet or ingot with a target composition design range as described in Table 1 → vacuum refining → continuous casting → heating of the steel billet (ingot) → rough rolling + finishing rolling → laminar cooling → coiling → finished steel; wherein, embodiments 1 to 2 and comparative example 1 are stacked and slowly cooled after coiling, and comparative example 2 is directly cooled after coiling. The specific production process parameters of the embodiment and comparative example 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 microstructures of the hot-rolled steel products obtained therefrom were detected. Figures 1 to 4 Attached Figure 1 The microstructure of the steel plate in Example 1 of the present invention shows that the microstructure is mainly composed of ferrite and martensite. Figure 2EBSD test results of the steel in Example 1 show that the steel in Example 1 contains 6.8% retained austenite. Figure 3 The microstructure of the steel of comparative example 1 of the present invention can be seen that the microstructure is mainly composed of martensite; Figure 4 This is the EBSD test result of the steel in comparative example 1. It can be seen that the steel in comparative example 1 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 property 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 stratification Example 2 1176 1255 15.0％ No stratification Comparative Example 1 1189 1268 11.5％ No stratification Comparative Example 2 1153 1237 11.0％ No stratification

[0051] As can be seen from Table 3, the yield strength of the steel prepared by the composition and process provided by the present invention is in the range of 1100MPa to 1200MPa, the tensile strength is in the range of 1200MPa to 1300MPa, the elongation is ≥14%, and there is no delamination in stretching. The yield strength and tensile strength of the steels of Comparative Examples 1 and 2 meet the requirements, but the elongation is low, mainly because the laminar cooling rate of the steel of Comparative Example 1 is too high, resulting in the inability to form stable residual austenite in the steel; the Si content in the steel of Comparative Example 2 is low, and the degree of C enrichment in austenite is low, resulting in reduced stability of austenite. At the same time, the steel coil of Comparative Example 2 is not stacked and slowly cooled after coiling, and the temperature of the steel coil insulation is low (153°C), and C cannot be distributed between martensite and austenite by diffusion, so residual austenite is not formed in the steel of Comparative Example 2.

Claims

1. A method for improving the stability of retained austenite in hot-rolled martensitic steel, characterized in that: The steps include: S1 uses electric furnace or converter smelting, refining, continuous casting or casting to obtain steel billets or ingots within the target composition design range; S2 reheats the slab and then performs rough rolling and finish rolling in two stages, wherein the deformation temperature in the rough rolling stage is higher than the austenite recrystallization critical temperature of the target steel component, the deformation temperature in the finish rolling stage is lower than the austenite recrystallization critical temperature of the target steel component, and the final rolling temperature of the finish rolling is higher than the ferrite phase transformation starting temperature; S3 performs laminar cooling, and the final cooling temperature is controlled within a range of 20°C below the martensitic transformation starting temperature of the target composition steel; In the S4 coiling process, the steel coils are coiled and stacked for slow cooling to obtain retained austenite plasticized hot-rolled martensitic steel.

2. The method for improving the stability of retained austenite in hot-rolled martensitic steel according to claim 1, characterized in that: In step S1, the chemical composition of the hot-rolled martensitic steel includes, by weight percentage, 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%, and the rest is Fe and unavoidable impurity elements.

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 heating temperature of the slab reheating is 1160°C to 1180°C, and the insulation time is 3h to 4h.

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 starting temperature of the rough rolling is 1050°C to 1150°C, the final rolling temperature is 1000°C to 1050°C, and the deformation amount is ≥60%.

5. 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 start rolling temperature of the finishing rolling is 950°C to 1050°C, the final rolling temperature is 860°C to 920°C, and the deformation amount is ≥70%.

6. The method for improving the stability of retained austenite in hot-rolled martensitic steel according to claim 1, characterized in that: In step S3, the cooling rate of laminar cooling is 5-15°C / s.

7. The method for improving the stability of retained austenite in hot-rolled martensitic steel according to claim 1, characterized in that: In step S3, the final cooling temperature of the laminar cooling is 380°C to 400°C.

8. 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 coil stack is slowly cooled within a temperature range of 300° C. to 350° C. for 5 h to 10 h, and is naturally air-cooled when the temperature is below 300° C.

9. 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 is ferrite, martensite and retained austenite, wherein the volume fraction of the retained austenite is 4% to 8%.

10. The method for improving the stability of retained austenite in hot-rolled martensitic steel according to claim 9, characterized in that: The yield strength of the retained austenite plasticized hot-rolled martensitic steel is 1100MPa-1200MPa, the tensile strength is 1200MPa-1300MPa, the elongation is ≥14%, and the fracture of the tensile specimen is not delaminated.

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