Variable strength duplex steel sheet having a normalized composition and its flexible manufacturing method
A duplex steel with a normalized composition and flexible manufacturing process addresses the challenge of achieving high elongation and hole expansion in high-strength steels, ensuring consistent mechanical properties and formability across varying strength levels.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- BAOSHAN IRON & STEEL CO LTD
- Filing Date
- 2024-05-30
- Publication Date
- 2026-06-02
Smart Images
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Abstract
Description
Technical Field
[0001] The present invention relates to a two-phase steel manufacturing technology, and particularly to a variable-strength two-phase steel sheet having a normalized composition and a softening manufacturing method thereof.
Background Art
[0002] With the intensification of the global energy crisis and environmental problems, energy conservation and safety have become the main development directions of the automobile manufacturing industry. The lightweighting of vehicles is one of the measures for energy conservation and emission reduction. Ultra-high-strength steel has excellent mechanical properties and service performance, is suitable for the manufacture of automobile structural parts, and can effectively lightweight the vehicle by widespread practical application. In recent years, the demand for ultra-high-strength steel for automobiles has been increasing steadily.
[0003] There are many types in the ultra-high-strength steel family for automobiles. Two-phase steel has excellent strength and plasticity, but the hole expansion rate (about 20 - 35%) is much lower than that of conventional soft steel for automobiles. On the other hand, although bainite steel and duplex steel have a high hole expansion rate, their elongation rate is too low. Therefore, on the premise of not falling below the elongation rate of two-phase steel, if a product with an improved hole expansion rate is developed, a wide range of application scenarios can be expected.
[0004] The white body requires the application of high-strength steel with various mechanical properties, and conventionally, it has been necessary to adopt different composition designs and manufacturing processes.
[0005] Chinese patent application CN103215516A discloses high-strength hot-rolled Q&P steel and a method for producing the same, the chemical composition of which contains C: 0.15%~0.40%, Si: 1.0%~2.0%, Mn: 1.5%~3.0%, P≦0.015%, S≦0.005%, Al: 0.3%~1.0%, N≦0.006%, Ti: 0.005%~0.015%, with the remainder being Fe; the yield strength is ≥700 MPa, the tensile strength is ≥1300 MPa, and the elongation is >10%. This invention, based on a normal C-Mn steel composition, suppresses cementite precipitation by increasing the Si content, refines austenite grains by adding trace amounts of Ti, and accelerates the dynamics of austenite transformation during the air-cooling process by increasing the Al content. At the same time, by employing a combination of a hot continuous rolling process and a stepwise cooling process, it obtains a structure containing protereminate ferrite + martensite + retained austenite while significantly reducing alloy costs.
[0006] Chinese patent application CN104451436A discloses a bainite-martensite-austenite biphase wear-resistant steel sheet and a method for producing the same. The chemical composition of the steel sheet is, by weight percentage, C: 0.20~0.40; Mn: 0.30~1.50; Si: 0.80~1.20; Cr: 0.60~1.00; Ni: 0.20~0.60; Mo: 0.20~0.40; Cu: 0.20~0.50; B: 0.0005~0.003; S ≤ 0.010, P ≤ 0.015, with the remainder being Fe and unavoidable impurity elements. From the rolled material, a bainite-martensite-retaining austenite multiphase structure can be obtained, with a retaining austenite volume fraction of 5-15%, a yield strength exceeding 1000 MPa, a tensile strength exceeding 1300 MPa, an elongation exceeding 15%, a hardness HB of 420-500, and machining and welding performance meeting the requirements of equipment manufacturing; the wear resistance against abrasive grain abrasion is more than 1.3 times that of Hardox 450, and more than 1.5 times that of Hardox 450 under weakly acidic working environment conditions.
[0007] Chinese patent application CN102776438A discloses a niobium-lanthanum trace alloyed Mn-B ultra-high-strength steel sheet and its heat treatment process, which belongs to the technical field of heat treatment of iron materials. The chemical composition and content (by weight percentage) of the steel sheet are C 0.14%~0.35%, Mn 1.5%~2.0%, Si 0.6%~1.0%, P≦0.015%, S≦0.002%, Nb 0.01%~0.06%, B 0.0005%~0.0040%, La 0.001%~0.5%, with the remainder being Fe and unavoidable impurities. The heat treatment process involves an austenitizing temperature of 880~940°C, a holding time of 0.5~5 hours followed by water granulation, and a tempering temperature of 190~250°C with a holding time of 1~15 hours. Steel sheets produced by this heat treatment process possess excellent mechanical properties, with a tensile strength of 1200-1400 MPa, a yield strength of 1000-1300 MPa, and an elongation of 6-15%. They also have low production costs and enable the industrial production of steel sheets with standard thicknesses of 5-25 mm.
[0008] Chinese patent application CN102321841A discloses a steel for track boots with a tensile strength of 1300 MPa, the chemical composition of which, by weight percentage, is C: 0.20-0.30%, Mn: 0.80-1.40%, Si: 0.15-0.35%, P: 0-0.015%, S: 0-0.016%, Cr: 0-0.30%, Ni: 0-0.25%, Cu: 0-0.30%, Ti: 0.01-0.02%, Al: 0.02-0.06%, B: 0.0005-0.0035%, with the remainder being Fe and unavoidable impurity elements. The steel used for these track boots has a tensile strength of 1340 MPa or higher, a fracture elongation of less than 12%, and a U-notch impact absorption energy of over 72 J for the structural steel. It boasts high strength, minimal quenching and internal cracking, and a long service life. As can be seen from the above patent applications, conventional high-strength steels all consist of a single compositional design and a corresponding single manufacturing process design.
[0009] The technical solutions described in Chinese patent applications CN103215516A and CN104451436A obtain sufficient retained austenite by adding high levels of Si·Al, and achieve high elongation through the TRIP effect of the retained austenite; hole expansion performance is not considered.
[0010] The ultra-high-strength steels described in Chinese patent applications CN102776438A and CN102321841A achieve good mechanical performance through the addition of trace alloys such as niobium, lanthanum, nickel, cadmium, and copper, respectively, but fail to achieve high elongation and high hole-expanding performance. [Overview of the Initiative] [Problems that the invention aims to solve]
[0011] The object of the present invention is to provide a variable-strength duplex steel sheet having a normalized composition and a method for manufacturing the same, thereby enabling the production of various steel grades with different strength levels required for white bodies with a single chemical composition, enabling tensile strength at the 450-1310 MPa level, resulting in good formability, a strength-to-elongation product exceeding 11000, and a hole expansion ratio exceeding 35%; the microstructure of the duplex steel includes ferrite + martensite + retained austenite + nanoscale precipitates, wherein martensite accounts for more than 30% by volume fraction, and it contains a sufficient amount of retained austenite (more than 3%) + a sufficient amount of nanoscale precipitates (the mass fraction ratio of nanoscale precipitates with a size of less than 50 nm to the total precipitates is more than 8%). [Means for solving the problem]
[0012] To achieve the above objective, the technical solution of the present invention is: It is a duplex steel, with a chemical composition by mass percentage of C:0.06~0.08%, Si:0.4~0.6%, Mn:2.4~2.6%, Al:0.01~0.05%, Ti:0.02~0.04%, B:0.0015~0.0025%, P≦0.015%, S≦0.003%, N≦0.005%, with the remainder being Fe and unavoidable impurities; The microstructure of the duplex steel according to the present invention comprises ferrite + martensite + retained austenite + nanoscale precipitates, wherein martensite accounts for 30% or more by volume; The duplex steel according to the present invention has a tensile strength of 450 to 1310 MPa, a strength-to-elongation product exceeding 11000%, and a hole expansion ratio exceeding 35%.
[0013] Furthermore, the remainder consists of Fe and unavoidable impurities.
[0014] In some embodiments, the duplex steel according to the present invention is a variable-strength duplex steel having a normalized composition.
[0015] In some embodiments, the nanoscale precipitates in the duplex steel according to the present invention are ε-carbides.
[0016] In some embodiments, the volume fraction of retained austenite in the duplex steel according to the present invention is greater than 3%.
[0017] In some embodiments, the mass fraction ratio of nanoscale precipitates with a size of less than 50 nm in the duplex steel according to the present invention is greater than 8% of the total precipitates.
[0018] In some embodiments, the yield strength of the duplex steel according to the present invention is 310 to 1200 MPa.
[0019] In some embodiments, the hole expansion ratio of the duplex steel according to the present invention is 36-55%.
[0020] In some embodiments, the duplex steel according to the present invention has a tensile strength of 450 to 590 MPa, preferably 470 to 590 MPa, a yield strength of 310 to 370 MPa, a strength-to-elongation product of 12000 to 14500%, and a hole expansion ratio of 36 to 40%.
[0021] In some embodiments, the dual-phase steel according to the present invention has a tensile strength of 591 - 980 MPa, preferably 650 - 980 MPa, a yield strength of 360 - 580 MPa, a strength-elongation product of 12500 - 14700%, and a hole expansion rate of 45 - 55%.
[0022] In some embodiments, the dual-phase steel according to the present invention has a tensile strength of 981 - 1310 MPa, preferably 1000 - 1310 MPa, a yield strength of 600 - 1200 MPa, a strength-elongation product of 11500 - 16000%, and a hole expansion rate of 40 - 50%.
[0023] In some embodiments, in the microstructure of the dual-phase steel according to the present invention, the volume fraction of martensite is 32 - 91%, the volume fraction of retained austenite is 4 - 15%, and the volume fraction of ferrite is 2 - 65%.
[0024] In some embodiments, the mass fraction ratio of nano-scale precipitates with a size of less than 50 nm in the dual-phase steel according to the present invention to all precipitates is 9 - 30%. In some embodiments, the mass fraction ratio of nano-scale precipitates with a size of less than 50 nm in the dual-phase steel according to the present invention to all precipitates is 9 - 28%.
[0025] In the composition design of the variable-strength dual-phase steel according to the present invention: C: By adding C element, the strength of the steel is improved and the occurrence of martensite phase transformation is ensured. When the mass percentage of C is less than 0.06%, sufficient martensite formation cannot be ensured in the annealing process, which affects the strength of the steel plate. However, when the mass percentage of C exceeds 0.08%, a peritectic reaction occurs in the continuous casting process, which is not at all suitable for continuous casting production at a high drawing speed. Therefore, in the present invention, the content of C is controlled to be 0.06% - 0.08%.
[0026] Si: Si exhibits a solid solution strengthening effect, enhances the tempering resistance of martensite, and can suppress the precipitation and growth of Fe3C. Especially when the tempering temperature is high, it can form ε-carbide to suppress the precipitation of Fe3C, but it is disadvantageous to the steel plate surface. When Si is less than 0.4%, it is disadvantageous to suppress the precipitation and growth of Fe3C during tempering. However, when it exceeds 0.6%, it is likely to affect the surface quality. Therefore, the range of Si is controlled to be 0.4 - 0.6%.
[0027] Mn: The addition of Mn element contributes to the improvement of the hardenability of steel and effectively increases the strength of the steel plate. In the hot coil heat preservation cover annealing according to the present invention, a large amount of carbides are generated, resulting in insufficient carbon equivalent in the matrix structure. When the mass percentage of Mn is less than 2.4%, due to the insufficient carbon equivalent, the hardenability is insufficient, and a sufficient amount of martensite cannot be generated during the annealing process, resulting in insufficient strength of the steel plate. However, when the mass percentage of Mn exceeds 2.6%, the carbon equivalent increases significantly, which has an adverse effect on the welding performance and resistance to delayed cracking. Therefore, in the present invention, the content of Mn is controlled to be 2.4 - 2.6%.
[0028] Al: The addition of aluminum has a deoxidation effect and a grain refinement effect. Therefore, in the present invention, the content of Al is controlled to be 0.01 - 0.05%.
[0029] Ti: Ti is added at 0.02 - 0.04% for the following reasons: Although Ti is the main compounding element of precipitates, it shows the effect of strongly suppressing the growth of austenite grain size and refining the grain size even at high temperatures. However, in low-carbon steel, if there are too many carbide-forming elements such as Nb and Ti, it will affect the subsequent phase transformation. Therefore, it is necessary to control the upper limit of the content of alloying elements.
[0030] B: Boron is an element that significantly improves hardenability, and the addition of boron promotes the formation of martensite, thereby ensuring the strength of martensitic steel. However, if boron is added further after grain boundary defects have been completely filled, the plasticity decreases due to the precipitation of a "boron phase" at the grain boundaries. If B is below 0.0015%, the effect is insufficient, but if it exceeds 0.0025%, it is unfavorable for the plasticity of the steel. Therefore, in this invention, 0.0015 to 0.0025% of B is added.
[0031] In the technical solution of the present invention, the impurity elements include P, N, and S. The lower the impurity content is controlled, the better the implementation effect, so the P content is controlled to P ≤ 0.015%; MnS formed by S severely affects molding performance, so the S content is controlled to S ≤ 0.003%; N tends to cause cracks and bubbles on the slab surface, so in the present invention, the N content is controlled to ≤ 0.005%.
[0032] The method for manufacturing duplex steel according to the present invention includes the following steps:
[0033] 1) Smelting and casting The mixture is smelted according to the aforementioned composition and cast into a billet;
[0034] 2) Hot rolling When manufacturing duplex steel with a tensile strength of 450-590 MPa, the cast billet is heated to 1100-1250°C and kept warm for 0.5 hours or more; the rolling end temperature is controlled to 860-900°C, and the winding temperature is controlled to 231-260°C;
[0035] When manufacturing duplex steel with a tensile strength of 591-980 MPa, the cast billet is heated to 1100-1250°C and kept warm for 0.5 hours or more; the rolling end temperature is controlled to 860-900°C, and the winding temperature is controlled to 201-230°C;
[0036] When manufacturing duplex steel with a tensile strength of 981-1310 MPa, the cast billet is heated to 1100-1250°C and kept warm for 0.5 hours or more; the rolling end temperature is controlled to 860-900°C, and the winding temperature is controlled to 160-200°C;
[0037] 3) Hot coil insulation cover annealing When manufacturing duplex steel with a tensile strength of 450-590 MPa, after winding, insulating cover annealing is performed, with an annealing time of 0.5-2 hours, utilizing the internal heat of the steel coil with the insulating cover, and the temperature decrease rate per hour is less than 6°C / s.
[0038] When manufacturing duplex steel with a tensile strength of 591 to 980 MPa, after winding, insulating cover annealing is performed, with an annealing time of 2.1 to 4 hours, utilizing the internal heat of the steel coil with the insulating cover, and the temperature decrease rate per hour is less than 6°C / s.
[0039] When manufacturing duplex steel with a tensile strength of 981 to 1310 MPa, after winding, insulating cover annealing is performed, with an annealing time of 4.1 to 6 hours, utilizing the internal heat of the steel coil with the insulating cover, and the temperature decrease rate per hour is less than 6°C / s.
[0040] 4) Cold rolling: The cold rolling reduction ratio is controlled to 0-50%;
[0041] 5) Continuous annealing When manufacturing duplex steel with a tensile strength of 450-590 MPa, the annealing temperature is set to 760-820°C, with a holding time of 50-100 seconds; then, it is cooled at a rate of 3-10°C / s to the rapid cooling start temperature of 660-760°C, and then cooled to room temperature at a rate of 50-150°C / s;
[0042] When manufacturing duplex steel with a tensile strength of 591-980 MPa, the annealing temperature is set to 760-820°C, with a holding time of 50-100 seconds; then, it is cooled at a rate of 3-10°C / s to the rapid cooling start temperature of 660-760°C, and then cooled to room temperature at a rate of 151-350°C / s;
[0043] When manufacturing duplex steel with a tensile strength of 981-1310 MPa, the annealing temperature is set to 760-820°C, with a holding time of 50-100 seconds; then, it is cooled at a rate of 3-10°C / s to a rapid cooling start temperature of 660-760°C, and then cooled to room temperature at a rate of 351-600°C / s;
[0044] 6) Tempering When manufacturing duplex steel with a tensile strength of 450-590 MPa, the tempering temperature should be 160-260°C and the tempering time 0.5-3 hours.
[0045] When manufacturing duplex steel with a tensile strength of 591 to 980 MPa, the tempering temperature should be 160 to 260°C and the tempering time 2 to 6 hours.
[0046] When manufacturing duplex steel with a tensile strength of 981 to 1310 MPa, the tempering temperature should be 160 to 260°C and the tempering time 2 to 4 hours.
[0047] 7) Level up at a leveling rate of 0-0.3%.
[0048] Preferably, in step 1), casting is performed using a thin slab continuous casting process.
[0049] In this text, a cold rolling reduction ratio of 0% means that the cold rolling process is not performed.
[0050] In some embodiments, when manufacturing duplex steel with a tensile strength of 450 to 590 MPa, the heat retention time in the hot rolling process is set to 0.5 to 3.5 hours. When manufacturing duplex steel with a tensile strength of 450 to 590 MPa, the coiling temperature in the hot rolling process is set to 232 to 260°C.
[0051] In some embodiments, when manufacturing duplex steel with a tensile strength of 591 to 980 MPa, the heat retention time in the hot rolling process is set to 0.5 to 2.5 hours. When manufacturing duplex steel with a tensile strength of 591 to 980 MPa, the coiling temperature in the hot rolling process is set to 203 to 228°C.
[0052] In some embodiments, when manufacturing duplex steel with a tensile strength of 981 to 1310 MPa, the heat retention time in the hot rolling process is set to 1.2 to 3.2 hours.
[0053] In some embodiments, in the hot coil insulation cover annealing process, when manufacturing duplex steel with a tensile strength of 450 to 590 MPa, the annealing time is set to 0.5 to 2 hours; when manufacturing duplex steel with a tensile strength of 591 to 980 MPa, the annealing time is set to 2.2 to 4 hours; and when manufacturing duplex steel with a tensile strength of 981 to 1310 MPa, the annealing time is set to 4.3 to 6 hours.
[0054] In some embodiments, during the continuous annealing process, when producing duplex steel with a tensile strength of 450 to 590 MPa, the material is cooled to room temperature at a rate of 50 to 150°C / s; when producing duplex steel with a tensile strength of 591 to 980 MPa, the material is cooled to room temperature at a rate of 160 to 350°C / s; and when producing duplex steel with a tensile strength of 981 to 1310 MPa, the material is cooled to room temperature at a rate of 360 to 600°C / s.
[0055] In the manufacturing method according to the present invention: This invention employs a flexible manufacturing process and obtains desired mechanical performance by adjusting the process control differently according to the different compositional designs of duplex steels with different strengths.
[0056] The present invention varies mainly as follows, depending on the flexibility process of different strengths: Adjustment of hot rolling coil temperature parameters: For steel grades with higher strength, a lower hot rolling coil temperature, i.e., a lower insulation cover annealing temperature, is required. This allows for better precipitation strengthening without promoting precipitation growth.
[0057] Adjustment of insulation cover annealing parameters: For higher strength steel grades, a longer insulation time is required to obtain more precipitates.
[0058] Adjusting continuous annealing parameters: For higher strength steel grades, a faster cooling rate is required to achieve higher strength. By controlling the cooling rate differently during the softening process, the amount of martensite obtained can be controlled, ultimately resulting in different strength levels in the final product.
[0059] Adjustment of tempering hold time parameters: In the tempering process, longer hold times are required for higher strength steel grades to obtain more retained austenite and nanoscale precipitates. However, if the hold time is too long, the strength of the continuously annealed product will decrease. Therefore, it is necessary to determine a reasonable hold time process interval according to different strength ranges, thereby ensuring both strength and a sufficient amount of retained austenite and nanoscale precipitates.
[0060] In the softening process, the thermal insulation cover annealing process parameters, annealing process parameters, and tempering process parameters are controlled. The amount and size of the obtained retained austenite and finely dispersed precipitates are controlled to ensure that the retained austenite is greater than 3%, and that the mass fraction ratio of nanoscale precipitates with a size of less than 50 nm in the final structure to the total precipitates is greater than 8%, thereby obtaining different strength levels and overall performance depending on the product. The duplex steel will have a tensile strength of 450 to 1310 MPa, a strength-to-elongation product exceeding 11000%, and a hole expansion ratio exceeding 35%. [Effects of the Invention]
[0061] Compared to conventional technology, the beneficial effects of the present invention are: This invention employs a normalized composition design, combined with flexible manufacturing technology, and provides different hot coil insulation cover annealing, insulation cover annealing, continuous annealing, and tempering processes to obtain duplex steel with a tensile strength of 450 to 1310 MPa.
[0062] Developing and commercializing large quantities of high-performance steels with different compositions and mechanical properties makes white bodies safer and lighter. However, such multi-steel solutions create numerous challenges, including resistance spot welding of dissimilar steels, process optimization, and recycling. This invention solves these problems by producing steel grades with diverse strength levels required for white bodies from a single chemical composition.
[0063] By promptly applying an insulating cover annealing mask after winding the hot coil, and combining this with a rational design of the composition and process, finely dispersed ε-carbides are generated by insulating and annealing at a relatively low temperature for a long period of time. Furthermore, the rational process design ensures that these finely dispersed ε-carbides are transferred to the final continuously annealed product. The finely dispersed precipitates in the annealed product improve overall strength, reduce strength differences between phases, reduce strength differences at grain boundaries and within the crystal, and strengthen grain boundaries during the deformation process, resulting in a dual effect of improved strength and hole expansion. By controlling the hot coil winding temperature and insulating time during softening, the amount and size of the obtained finely dispersed precipitates are controlled, ensuring that nanoscale precipitates with a size of less than 50 nm account for more than 8% of the total precipitates by mass fraction, allowing for different strength levels and overall performance depending on the product.
[0064] Another feature of this invention is the application of discontinuous ultra-low temperature long-time tempering, which aims to generate finely dispersed ε-carbides again without reducing the strength of the martensitic structure due to excessive tempering, while also localizing carbon in the untransformed austenite. This ultimately results in obtaining a sufficient amount of retained austenite (over 3%) + a sufficient amount of nanoscale precipitates (ensuring that nanoscale precipitates with a size of less than 50 nm in the final structure account for more than 8% of the total precipitates by mass). This contributes significantly to both strength and elongation. By controlling the tempering temperature and tempering time during softening, the amount and size of finely dispersed precipitates can be obtained, resulting in different strength levels and overall performance depending on the product.
[0065] In contrast, the prior art described in Chinese patent applications CN103215516A and CN104451436A obtains sufficient retained austenite by adding high levels of Si·Al and then obtains a high elongation rate through the TRIP effect of the retained austenite, but hole expansion performance is not considered and is different from the concept of the present invention.
[0066] The ultra-high-strength steels described in Chinese patent applications CN102776438A and CN102321841A achieve good mechanical performance through the addition of trace alloys such as niobium, lanthanum, nickel, cadmium, and copper, respectively, but they cannot achieve the high elongation and high hole-expanding performance indicators of the present invention. [Modes for carrying out the invention]
[0067] The present invention will be further interpreted and described below based on specific embodiments, but this interpretation and description will not unduly limit the technical solutions of the present invention.
[0068] The composition of the steel embodiment according to the present invention is shown in Table 1, and the remainder of the composition consists of Fe and P, other unavoidable impurities besides S and N. The manufacturing process parameters of the steel sheet according to the embodiment of the present invention are shown in Table 2. The microstructure ratio of the steel sheet according to the embodiment of the present invention is shown in Table 3. The relevant performance parameters of the steel sheet according to the embodiment of the present invention are shown in Table 4.
[0069] The flexibility processes were as follows:
[0070] Softening Process 1: When the tensile strength of the duplex steel is 450-590 MPa, the hot rolling and coiling temperature is set to 231-260°C; the insulation cover annealing time is set to 0.5-2 hours; the continuous annealing is cooled to room temperature at a rate of 50-150°C / s; and the tempering time is set to 0.5-3 hours.
[0071] Softening Process 2: When the tensile strength of the duplex steel is 591-980 MPa, the hot rolling and coiling temperature is set to 201-230°C; the insulation cover annealing time is set to 2.1-4 hours; the continuous annealing is cooled to room temperature at a rate of 151-350°C / s; and the tempering time is set to 2-6 hours.
[0072] Softening Process 3: When the tensile strength of the duplex steel is 981-1310 MPa, the hot rolling and coiling temperature is set to 160-200°C; the insulation cover annealing time is set to 4.1-6 hours; the material is cooled to room temperature at a rate of 351-600°C / s during continuous annealing; and the tempering time is set to 2-6 hours.
[0073] As can be seen from Table 3, the microstructure of the duplex steel according to the present invention includes ferrite + martensite + retained austenite + nanoscale precipitates, where martensite accounts for more than 30% by volume, retained austenite accounts for more than 3% by volume, and nanoscale precipitates with a size of less than 50 nm account for more than 8% by mass of the total precipitates.
[0074] As can be seen from Table 4, by adopting the same composition design, combining it with a flexible manufacturing method, and adjusting parameters such as continuous annealing and tempering processes, the present invention can obtain martensitic steel with a tensile strength of 450 to 1310 MPa. The resulting steel sheets exhibit good formability and delayed crack resistance, a strength-to-elongation product exceeding 11,000, and a hole expansion ratio exceeding 35%.
[0075] [Table 1]
[0076] [Table 2]
[0077] [Table 3]
[0078] Table 4
Claims
1. The duplex steel has a chemical composition by mass percentage of C: 0.06–0.08%, Si: 0.4–0.6%, Mn: 2.4–2.6%, Al: 0.01–0.05%, Ti: 0.02–0.04%, B: 0.0015–0.0025%, P ≤ 0.015%, S ≤ 0.003%, N ≤ 0.005%, with the remainder being Fe and other unavoidable impurities; The microstructure of the aforementioned duplex steel includes ferrite + martensite + retained austenite + nanoscale precipitates, wherein martensite accounts for more than 30% by volume, retained austenite accounts for more than 3% by volume, and nanoscale precipitates with a size of less than 50 nm account for more than 8% by mass of the total precipitates; The aforementioned duplex steel has a tensile strength of 450 to 1310 MPa, a strength-to-elongation product exceeding 11000%, and a hole expansion ratio exceeding 35%. A duplex steel characterized by the following features.
2. The duplex steel according to claim 1, characterized in that the remainder is Fe and other unavoidable impurities.
3. The duplex steel according to claim 1 or 2, characterized in that the nanoscale precipitate is ε-carbide.
4. The duplex steel according to claim 1 or 2, characterized in that the volume fraction of martensite is 32 to 91%, the volume fraction of retained austenite is 4 to 15%, the volume fraction of ferrite is 2 to 65%, and / or the mass fraction ratio of nanoscale precipitates with a size of less than 50 nm to the total precipitate is 9 to 30%, for example, 9 to 28%.
5. The duplex steel according to any one of claims 1 to 4, characterized in that the yield strength of the duplex steel is 310 to 1200 MPa.
6. The duplex steel according to any one of claims 1 to 5, characterized in that the hole expansion ratio of the duplex steel according to the present invention is 36 to 55%.
7. The duplex steel has a tensile strength of 450 to 590 MPa, preferably 470 to 590 MPa, a yield strength of 310 to 370 MPa, a strength-to-elongation product of 12000 to 14500%, and a hole expansion ratio of 36 to 40%; or, the duplex steel has a tensile strength of 591 to 980 MPa, preferably 650 to 980 MPa, a yield strength of 360 to 580 MPa, and a strength-to-elongation product of 12000 to 14500%. The duplex steel according to any one of claims 1 to 4, characterized in that the product of magnitude is 12,500 to 14,700% and the hole expansion ratio is 45 to 55%; or the duplex steel is characterized in that the tensile strength is 981 to 1,310 MPa, preferably 1,000 to 1,310 MPa, the yield strength is 600 to 1,200 MPa, the product of strength and elongation is 11,500 to 16,000% and the hole expansion ratio is 40 to 50%.
8. A method for producing duplex steel according to any one of claims 1 to 7, characterized by including the following steps: 1) Smelting and casting Smelt according to the composition of claim 1 or 2 and cast into a billet; 2) Hot rolling When manufacturing duplex steel with a tensile strength of 450 to 590 MPa, the cast billet is heated to 1100 to 1250°C and kept warm for 0.5 hours or more; the rolling end temperature is controlled to 860 to 900°C, and the winding temperature is controlled to 231 to 260°C; When manufacturing duplex steel with a tensile strength of 591 to 980 MPa, the cast billet is heated to 1100 to 1250°C and kept warm for 0.5 hours or more; the rolling end temperature is controlled to 860 to 900°C, and the winding temperature is controlled to 201 to 230°C; When manufacturing duplex steel with a tensile strength of 981 to 1310 MPa, the cast billet is heated to 1100 to 1250°C and kept warm for 0.5 hours or more; the rolling end temperature is controlled to 860 to 900°C, and the winding temperature is controlled to 160 to 200°C; 3) Hot coil insulation cover annealing When manufacturing duplex steel with a tensile strength of 450 to 590 MPa, after winding, insulating cover annealing is performed, with an annealing time of 0.5 to 2 hours, utilizing the internal heat of the steel coil with the insulating cover, and the temperature decrease rate per hour is less than 6°C / s; When manufacturing duplex steel with a tensile strength of 591 to 980 MPa, after winding, insulating cover annealing is performed, with an annealing time of 2.1 to 4 hours, utilizing the internal heat of the steel coil with the insulating cover, and the temperature decrease rate per hour is less than 6°C / s; When manufacturing duplex steel with a tensile strength of 981 to 1310 MPa, after winding, insulating cover annealing is performed, with an annealing time of 4.1 to 6 hours, utilizing the internal heat of the steel coil with the insulating cover, and the temperature decrease rate per hour is less than 6°C / s; 4) Cold rolling: The cold rolling reduction ratio is controlled to 0-50%; 5) Continuous annealing When manufacturing duplex steel with a tensile strength of 450 to 590 MPa, the annealing temperature is set to 760 to 820°C, and the holding time is set to 50 to 100 s; thereafter, it is cooled at a rate of 3 to 10°C / s to the rapid cooling start temperature of 660 to 760°C, and then cooled to room temperature at a rate of 50 to 150°C / s; When manufacturing duplex steel with a tensile strength of 591 to 980 MPa, the annealing temperature is set to 760 to 820°C, and the holding time is set to 50 to 100 s; thereafter, it is cooled at a rate of 3 to 10°C / s to a rapid cooling start temperature of 660 to 760°C, and then cooled to room temperature at a rate of 151 to 350°C / s; When manufacturing duplex steel with a tensile strength of 981 to 1310 MPa, the annealing temperature is set to 760 to 820°C, and the holding time is set to 50 to 100 s; thereafter, it is cooled at a rate of 3 to 10°C / s to a rapid cooling start temperature of 660 to 760°C, and then cooled to room temperature at a rate of 351 to 600°C / s; 6) Tempering When manufacturing duplex steel with a tensile strength of 450 to 590 MPa, the tempering temperature should be 160 to 260°C and the tempering time 0.5 to 3 hours. When manufacturing duplex steel with a tensile strength of 591 to 980 MPa, the tempering temperature should be 160 to 260°C and the tempering time 2 to 6 hours. When manufacturing duplex steel with a tensile strength of 981 to 1310 MPa, the tempering temperature should be 160 to 260°C and the tempering time 2 to 6 hours. 7) Level at a leveling rate of 0-0.3%.
9. A method for producing duplex steel according to claim 8, characterized in that in step 1), casting is performed using a thin slab continuous casting process.
10. The method for manufacturing duplex steel according to claim 8, characterized in that, when manufacturing duplex steel with a tensile strength of 450 to 590 MPa, the heat retention time in the hot rolling process is 0.5 to 3.5 hours; and when manufacturing duplex steel with a tensile strength of 450 to 590 MPa, the winding temperature in the hot rolling process is 232 to 260°C.
11. The method for manufacturing duplex steel according to claim 8, characterized in that, when manufacturing duplex steel having a tensile strength of 591 to 980 MPa, the heat retention time in the hot rolling process is 0.5 to 2.5 hours; and when manufacturing duplex steel having a tensile strength of 591 to 980 MPa, the coiling temperature in the hot rolling process is 203 to 228°C.
12. The method for manufacturing duplex steel according to claim 8, characterized in that, when manufacturing duplex steel having a tensile strength of 981 to 1310 MPa, the heat retention time in the hot rolling process is 1.2 to 3.2 hours.
13. The method for manufacturing duplex steel according to claim 8, characterized in that, in the hot coil insulation cover annealing process, when manufacturing duplex steel with a tensile strength of 450 to 590 MPa, the annealing time is 0.5 to 2 hours; when manufacturing duplex steel with a tensile strength of 591 to 980 MPa, the annealing time is 2.2 to 4 hours; and when manufacturing duplex steel with a tensile strength of 981 to 1310 MPa, the annealing time is 4.3 to 6 hours.
14. The method for manufacturing duplex steel according to claim 8, characterized in that, in the continuous annealing process, when manufacturing duplex steel with a tensile strength of 450 to 590 MPa, the material is cooled to room temperature at a rate of 50 to 150°C / s; when manufacturing duplex steel with a tensile strength of 591 to 980 MPa, the material is cooled to room temperature at a rate of 160 to 350°C / s; and when manufacturing duplex steel with a tensile strength of 981 to 1310 MPa, the material is cooled to room temperature at a rate of 360 to 600°C / s.