Superplastic 980mpa quenching and partitioning cold rolled steel sheet for automobiles and method of manufacturing the same

Abstract

The present application relates to a kind of superplastic 980MPa quenching partitioning cold-rolled steel plate for automobile and manufacturing method, and the chemical composition in steel is C:0.20%~0.24%, Si:0.90%~1.80%, Mn:1.85%~2.15%, P≤0.010%, S≤0.005%, Ti:0.015%~0.025%, Cr≤0.15%, Mo≤0.05%, Cu≤0.20%, Nb≤0.03%, V≤0.05%, Ca≤0.005%, B≤0.002%, the balance is Fe and inevitable impurities.Manufacturing method includes smelting, hot rolling, pickling, cold rolling, pre-annealing treatment, continuous annealing and finishing process;The present application significantly improves product performance under the premise of not adding alloy cost, so that the 980MPa steel plate produced reaches superplasticity index, i.e. elongation≥26%, FLC0≥0.21.

Description

Technical Field

[0001] This invention relates to the field of automotive steel manufacturing technology, and in particular to a superplastic 980MPa quenched and partitioned cold-rolled continuous annealed steel sheet for automobiles and its manufacturing method. Background Technology

[0002] Currently, the biggest problem facing the application of high-strength steel for automobiles with a strength of 980MPa and above is its insufficient ductility, making it difficult to manufacture complex and relatively complex body parts (such as B-pillars, lower A-pillars, and C-pillars that require multiple processing steps), which seriously slows down the process of vehicle lightweighting.

[0003] To address these issues, in recent years, many scholars and steel mill R&D institutions have focused on developing high-ductility products, such as DH steel (Dual phase with high ductility, i.e., enhanced ductility dual-phase steel), an upgraded product of dual-phase steel. DH steel increases the plasticity of the original DP980 steel from approximately 12% to over 16%, effectively increasing the application rate of dual-phase steel in automotive body parts. Another example is the development and application of quenched and partitioned steel (Q&P). Q&P steel was proposed by Professor Speer et al. in 2003. It introduces metastable retained austenite into dual-phase steel, relying on the strain-induced phase transformation (TRIP effect) of the retained austenite during deformation to delay necking and improve ductility. Based on this theory, my country's industrialized Q&P products have developed rapidly since 2015, with products such as Q&P980, Q&P1180, and Q&P1470 being successively applied. Among these, the application rate of Q&P980 has increased year by year, and its application scenarios have become increasingly diverse.

[0004] Compared to DP980, Q&P980 exhibits a 18%–20% increase in plasticity, significantly improving the ductility of the steel sheet. However, its application remains limited. Compared to the 25%–30% plasticity of high-strength steels such as DP590 and 420LA, which are currently the most widely used steels in car body manufacturing, the Q&P series products still suffer from significant plasticity deficiencies and cannot be used as product substitutes.

[0005] Around 2017, the Japan National Research Institute for New Materials (NRI) announced the development trend of cold-formed 980MPa steel. Reportedly, their 980MPa steel had reached the 20% plasticity stage, with a future plasticity development target of 35%. Of course, a 35% plasticity target will inevitably be a future development direction integrating alloy design, process design, and equipment design. However, at present, based on the existing equipment foundation, achieving a plasticity target of over 25% in stages is of paramount importance for the development of cold-formed high-strength steel for automobiles in my country.

[0006] Chinese patent application CN202011262486.8 discloses "a cold-rolled 980MPa grade quenched steel and its production method." The main chemical composition of the steel is: C: 0.20%–0.22%, Si: 1.50%–1.70%, Mn: 2.0%–2.2%, P≤0.02%, S≤0.005%, Cr: 0.19%–0.22%, Ti: 0.01%–0.015%, Ca: 0.0015%–0.004%, Alt: 0.025%–0.060%, with the remainder being Fe and unavoidable impurities. It employs a cold-rolling-continuous annealing production process, and the product properties are a yield strength of 600–850 MPa, a tensile strength ≥980 MPa, and an elongation ≥15%. The maximum elongation in the example is 16%, which shows that the product is not a high-ductility Q&P steel product, and the plasticity index is far below 25%.

[0007] Chinese patent application CN201810144307.7 discloses a "980MPa grade cold-rolled high-strength Q&P steel for automobiles and its production method." The main chemical composition of the steel is: C: 0.18%–0.24%, Si: 0.60%–1.30%, Mn: 1.60%–2.40%, P: 0.022%–0.04%, S≤0.005%, Nb: 0.045%–0.070%, Al: 0.50%–1.00%, with the remainder being Fe and unavoidable impurities. The product is a cold-rolled annealed steel sheet with a yield strength ≥550MPa, tensile strength ≥980MPa, and elongation ≥18%. The highest elongation in the examples is 23.5% (Example 11). This patent application aims to promote fine-grain strengthening through high Nb addition. However, the addition of 0.045% to 0.070% Nb significantly increases the difficulty of steel plate production. The main difficulty lies in the fact that after hot rolling and coiling, the high Nb addition has a significant dragging and strain-induced precipitation effect, which greatly increases the hot rolling strength of the steel plate, making it unfavorable or even impossible to complete the cold rolling operation. At the same time, the high Nb addition also significantly increases the alloy cost of the steel plate. Summary of the Invention

[0008] This invention provides a superplastic 980MPa quenched and fractionated cold-rolled continuous annealed steel sheet for automobiles and its manufacturing method. Without adding alloy costs, the product performance is significantly improved only through process improvement, so that the produced 980MPa steel sheet meets the superplasticity index, namely elongation ≥26% and FLC0 ≥0.21.

[0009] To achieve the above objectives, the present invention employs the following technical solution:

[0010] A superplastic 980MPa quenched and fractionated cold-rolled continuous annealed steel sheet for automobiles, wherein the chemical composition of the steel, by mass percentage, is: C: 0.20%–0.24%, Si: 0.90%–1.80%, Mn: 1.85%–2.15%, P≤0.010%, S≤0.005%, Ti: 0.015%–0.025%, Cr≤0.15%, Mo≤0.05%, Cu≤0.20%, Nb≤0.03%, V≤0.05%, Ca≤0.005%, B≤0.002%, with the balance being Fe and unavoidable impurities.

[0011] Furthermore, the microstructure of the steel comprises, by volume percentage: 35%–45% recrystallized ferrite, 35%–40% bainitic ferrite, 12%–15% retained austenite, and 3%–8% martensite.

[0012] Furthermore, the grain size of the recrystallized ferrite is 0.75–0.95 μm; the lath width of the bainitic ferrite is 0.15–0.35 μm; the grain size of the retained austenite is 0.15–0.33 μm; and the grain size of the martensite is 0.35–0.62 μm.

[0013] Furthermore, the mechanical properties of the finished steel plate are: tensile strength ≥980MPa, yield strength 600~680MPa, elongation ≥26%, FLC0 ≥0.21.

[0014] A method for manufacturing superplastic 980MPa quenched and continuously annealed cold-rolled steel sheet for automobiles includes smelting, hot rolling, pickling, cold rolling, pre-annealing, continuous annealing, and finishing processes; wherein the following processes are controlled:

[0015] 1) Hot rolling:

[0016] The heating temperature is 1250~1280℃; the rough rolling temperature is 1200~1250℃; the finish rolling temperature is 1100~1200℃; the coiling temperature is 400~470℃; the coiled steel plate is subjected to a bell-type annealing treatment at a temperature of 450~550℃ for 18~36h.

[0017] 2) Cold rolling:

[0018] The cold rolling reduction rate is controlled between 40% and 60%.

[0019] 3) Pre-annealing treatment:

[0020] The steel plate is heated to 865–900℃, isothermaled for 30–80 seconds, and then slowly cooled to 750–780℃ at a rate of 0.5–5℃ / s. It is then water-quenched to 80–100℃, followed by a second pickling process. After this second pickling, the steel plate is heated to 350–550℃, isothermaled for 100–140 seconds, and then coiled. After pre-annealing, an ultrafine-grained fully tempered martensitic matrix is ​​obtained.

[0021] 4) Continuous annealing:

[0022] The pre-annealed steel plate is heated to 800-830℃, isothermal for 120-240s, then cooled to 660-720℃ at a cooling rate of 0.5-2℃ / s, and then cooled to 360-410℃ at a cooling rate of 20-40℃ / s, isothermal for 340-550s.

[0023] 4) Finishing:

[0024] The finishing elongation rate is controlled at 0.2% to 0.4%.

[0025] Furthermore, the thickness of the hot-rolled steel sheet is 2.8mm to 3.3mm.

[0026] Furthermore, the thickness of cold-rolled steel sheets is 1.4mm to 1.6mm.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] 1) This invention solves the problem that it is currently difficult to achieve a superplasticity index of 980MPa (elongation ≥26%, FLC0 ≥0.21) internationally. The steel plate has high ductility, which greatly broadens its application prospects in the automotive field.

[0029] 2) This invention achieves the goal of green and low-carbon development, and significantly improves product performance by improving the process without increasing alloy costs;

[0030] 3) This invention innovatively proposes an initial configuration control technology, that is, to construct an ultrafine crystalline fully tempered martensite matrix after pre-quenching treatment as the initial configuration for continuous annealing. This technology is applicable to all ultrafine crystalline steel manufacturing processes. Attached Figure Description

[0031] Figure 1 This is a metallographic photograph of the finished steel plate of Embodiment 1 of the present invention (with typical TEM structure). Detailed Implementation

[0032] The present invention discloses a superplastic 980MPa quenched and fractionated cold-rolled continuous annealed steel sheet for automobiles. The chemical composition of the steel, by mass percentage, is as follows: C: 0.20%–0.24%, Si: 0.90%–1.80%, Mn: 1.85%–2.15%, P≤0.010%, S≤0.005%, Ti: 0.015%–0.025%, Cr≤0.15%, Mo≤0.05%, Cu≤0.20%, Nb≤0.03%, V≤0.05%, Ca≤0.005%, B≤0.002%, with the balance being Fe and unavoidable impurities.

[0033] Furthermore, the microstructure of the steel comprises, by volume percentage: 35%–45% recrystallized ferrite, 35%–40% bainitic ferrite, 12%–15% retained austenite, and 3%–8% martensite.

[0034] Furthermore, the grain size of the recrystallized ferrite is 0.75–0.95 μm; the lath width of the bainitic ferrite is 0.15–0.35 μm; the grain size of the retained austenite is 0.15–0.33 μm; and the grain size of the martensite is 0.35–0.62 μm.

[0035] Furthermore, the mechanical properties of the finished steel plate are: tensile strength ≥980MPa, yield strength 600~680MPa, elongation ≥26%, FLC0 ≥0.21.

[0036] The present invention discloses a method for manufacturing a superplastic 980MPa quenched and continuously annealed cold-rolled steel sheet for automobiles, comprising smelting, hot rolling, pickling, cold rolling, pre-annealing, continuous annealing, and finishing processes; wherein the following processes are controlled:

[0037] 1) Hot rolling:

[0038] The heating temperature is 1250~1280℃; the rough rolling temperature is 1200~1250℃; the finish rolling temperature is 1100~1200℃; the coiling temperature is 400~470℃; the coiled steel plate is subjected to a bell-type annealing treatment at a temperature of 450~550℃ for 18~36h.

[0039] 3) Cold rolling:

[0040] The cold rolling reduction rate is controlled between 40% and 60%.

[0041] 5) Pre-annealing treatment:

[0042] The steel plate is heated to 865–900℃, isothermaled for 30–80 seconds, and then slowly cooled to 750–780℃ at a rate of 0.5–5℃ / s. It is then water-quenched to 80–100℃, followed by a second pickling process. After this second pickling, the steel plate is heated to 350–550℃, isothermaled for 100–140 seconds, and then coiled. After pre-annealing, an ultrafine-grained fully tempered martensitic matrix is ​​obtained.

[0043] 4) Continuous annealing:

[0044] The pre-annealed steel plate is heated to 800-830℃, isothermal for 120-240s, then cooled to 660-720℃ at a cooling rate of 0.5-2℃ / s, and then cooled to 360-410℃ at a cooling rate of 20-40℃ / s, isothermal for 340-550s.

[0045] 6) Finishing:

[0046] The finishing elongation rate is controlled at 0.2% to 0.4%.

[0047] Furthermore, the thickness of the hot-rolled steel sheet is 2.8mm to 3.3mm.

[0048] Furthermore, the thickness of cold-rolled steel sheets is 1.4mm to 1.6mm.

[0049] The rationale for the alloy design of the superplastic 980MPa quenched and partitioned cold-rolled continuous annealed steel sheet for automobiles described in this invention is as follows:

[0050] Carbon (C) is an essential alloying element in steel, primarily serving as a solid solution strengthening agent. In this invention, C is mainly used to regulate the degree of austenitization during the annealing stage, adjust the behavior of C in supercooled austenite, promote the formation of bainite and C-rich supercooled austenite during cooling, thereby increasing the content and stability of room-temperature retained austenite. Too low a C addition cannot meet the requirements for retained austenite content and stability in this invention, while excessive C addition will lead to a series of negative problems such as brittleness, hardness, and poor weldability in the steel plate. Therefore, this invention controls the C content to be between 0.20% and 0.24%.

[0051] Si: Si itself promotes ferrite formation and strengthens the ferrite matrix. In this invention, the main role of added Si is to inhibit the decomposition of supercooled austenite during the over-aging stage, thereby ensuring the bainite content and the room-temperature retained austenite content. However, adding too much Si will reduce the surface quality of the steel. Therefore, this invention controls the Si content to be between 0.9% and 1.8%.

[0052] Mn: Mn is an economical element commonly added to steel, possessing strong solid solution strengthening properties. In this invention, Mn plays a role in regulating supercooling transformation behavior, improving the hardenability of the steel plate, and delaying the transformation of austenite to pearlite. The key is to strictly control the Mn content. Excessively high Mn content will inhibit bainite formation, affecting the content of bainite and retained austenite. Insufficient Mn content is insufficient to stabilize a sufficient amount of austenite in the critical region and will reduce the phase stability of retained austenite at room temperature, resulting in poor work hardening ability of the experimental steel. Therefore, this invention controls the Mn content to 1.85%–2.15%.

[0053] Ti: Ti is an economical element commonly added to microalloyed high-strength steel, offering a significant cost advantage compared to other elements. Its primary function is to solidify nitrogen (N), while TiN precipitates during solidification, acting as grain boundary pinning. Ti(C,N) precipitation during hot rolling further pins the original austenite grain boundaries and refines the original austenite grains. In this invention, the effective grain refinement significantly promotes bainite formation, determining the bainite content, as well as the content and stability of retained austenite. Therefore, this invention controls the Ti element content to be between 0.015% and 0.025%.

[0054] Cr and Mo: Both Cr and Mo are elements that enhance hardenability, but they are slightly more expensive than Mn. In this invention, when the addition of C and Mn is close to the upper limit, it will lead to a certain degree of segregation and cause banded structure. Therefore, Cr and Mo are used as supplementary elements to Mn in this invention to compensate for the hardenability of the steel plate when the Mn content is low, delay the formation of pearlite and bainite during the cooling stage, and promote the formation of bainite and martensite. In addition, Cr and Mo can change the type of iron oxide scale during the coiling process, effectively control the oxidation behavior inside the steel plate and the grain boundary oxidation behavior, and improve the surface quality of the steel plate.

[0055] Cu: Cu is typically used in high-cost precipitation steels. In this invention, the addition of a small amount of Cu supplements the phase stability of austenite during the austenitization stage, and the precipitation of elemental Cu in austenite during continuous annealing plays a certain precipitation strengthening role. Furthermore, the addition of Cu in combination with Cr and Mo can improve the corrosion resistance of the steel plate to some extent. This invention controls Cu to ≤0.20%.

[0056] Nb: Nb is a microalloying element with a strong grain-refining effect. It can promote strain-induced precipitation during the hot rolling recrystallization stage, promoting the recrystallization of the original austenite grains and thus refining the grain size. In this invention, Nb is added in conjunction with hot rolling and solution treatment temperatures to compensate for the insufficient grain refinement after adding Ti. This invention controls Nb to be ≤0.03%.

[0057] V: V is a relatively expensive microalloying element with a strong precipitation strengthening effect. In this invention, the role of adding V is to mitigate the effects of excessive cold-hardened strength and excessive rolling load on the steel plate caused by high levels of Nb and Ti. This invention controls V to be ≤0.05%.

[0058] Ca: The morphology of inclusions can be controlled by adding an appropriate amount of Ca, thereby improving the quality of cast steel plates. This invention controls Ca to ≤0.005%.

[0059] B: In this invention, the addition of B can improve the hardenability of the steel plate and ensure the formation of martensite during the rapid cooling stage of continuous annealing galvanizing. However, excessive addition of B will increase the brittleness of the steel plate and deteriorate its processing performance. Therefore, this invention controls B to ≤0.002%.

[0060] P: P is a harmful element in steel, and its content should be as low as possible. Considering cost, this invention controls the P content to P≤0.010%.

[0061] S: S is a harmful element in steel, and its content should be as low as possible. Considering cost, this invention controls the S content to S≤0.005%.

[0062] The present invention discloses a method for manufacturing a superplastic 980MPa quenched and fractionated cold-rolled continuous annealed steel sheet for automobiles, comprising a series of processes including smelting, hot rolling, pickling, cold rolling, pre-annealing, continuous annealing, and finishing. The specific process is as follows:

[0063] 1. Smelting: Smelting is carried out in a converter to obtain alloy composition within a limited range.

[0064] 2. Hot rolling:

[0065] ① The heating temperature is controlled between 1250 and 1280℃ to ensure the precipitation behavior of Ti atoms, which has a good effect on consolidating N in the steel plate and ensures the precipitation of Ti(C,N), which plays a role in pinning the original austenite grain boundaries and refining the original austenite grains. At the same time, at this temperature, Nb atoms can also effectively drag the original austenite grains, resulting in a grain refinement and strengthening effect;

[0066] ② The rough rolling temperature is controlled between 1200 and 1250℃. During this stage, the dynamic recovery behavior of the original austenite grains is promoted, the recrystallization behavior of austenite in the finishing rolling stage is promoted, the dragging effect of Nb atoms on grain boundaries is promoted, and the grain refinement is promoted.

[0067] ③ The finishing rolling temperature is controlled between 1100℃ and 1200℃. During this stage, the dynamic and static recrystallization behavior of austenite is promoted. At the same time, Nb and Ti microalloying elements are precipitated through strain-induced precipitation to form Nb(C,N) or Ti(C,N), or (Nb,Ti)(C,N) precipitates, which play a role in pinning the original austenite grains and refining the grains.

[0068] ④ The winding temperature is controlled between 400℃ and 470℃. At this temperature, surface quality problems caused by abnormal oxidation of the surface at high temperatures can be prevented. The thickness of the hot-rolled plate is between 2.8mm and 3.2mm.

[0069] ⑤Bell annealing: The coiled steel sheet is annealed in a bell at a temperature of 450℃~550℃ for 18~36h.

[0070] 3. Pickling: The steel plate after the annealing process is pickled to remove the iron oxide scale on the surface of the steel plate.

[0071] 4. Cold rolling: The cold rolling reduction rate is controlled at 40% to 60% to promote the fibrous structure in the cold rolling configuration; at the same time, it prevents the problem of excessive deformation resistance due to excessive cold rolling reduction rate, making it difficult to roll to the target thickness.

[0072] 5. Pre-annealing treatment: Heat the steel plate to 865℃~900℃, hold it at that temperature for 30~80s, and then slowly cool it to 750~780℃, with the cooling rate controlled at 0.5~5℃ / s; then water quench it to 80~100℃; after a second pickling, heat the steel plate to 350~550℃, hold it at that temperature for 100~140s, and then roll it up.

[0073] The mechanism is as follows: The superplastic 980MPa grade steel plate developed in this invention is mainly achieved by constructing a relatively ideal initial configuration. During the pre-annealing process, the steel plate is first heated to 865℃~900℃ and isothermally held for 30~80s to obtain a fully austenitic microstructure, ensuring that subsequent processing will yield a fully martensitic microstructure. Secondly, the slow cooling temperature is controlled at 750~780℃, maintaining a relatively high slow cooling temperature to prevent the formation of oriented epiphytic ferrite. Subsequently, it is water-quenched to 80~100℃ to obtain a fully martensitic microstructure. Finally, after a second pickling process to remove residual oxides from the surface, the steel plate undergoes tempering to obtain a tempered martensitic microstructure, maintaining a lath morphology with a lath width of 0.25~0.45μm. Thus, the pre-annealing process constructs an ultrafine-grained fully tempered martensitic matrix as the initial configuration for continuous annealing.

[0074] 6. Continuous annealing treatment: The pre-annealed steel plate with ultrafine crystalline fully tempered martensite matrix is ​​heated to 800-830℃, isothermal for 120-240s, then cooled to 660-720℃ at a cooling rate of 0.5-2℃ / s, and then cooled to 360-410℃ at a cooling rate of 20-40℃ / s, isothermal for 340-550s.

[0075] The mechanism is as follows: First, the steel plate is heated to 800–830℃ and isothermally heated for 120–240 seconds to ensure the degree of austenitization at this stage, thereby ensuring the subsequent bainite and retained austenite. Second, the slow cooling temperature is controlled at 660–720℃ to form a small amount of oriented epiphytic ferrite at this stage, adjusting the C distribution in the supercooled austenite. Subsequently, it is cooled at a rapid cooling rate of 20–40℃ / s to prevent the formation of eutectoid products such as pearlite at low cooling rates. Finally, it is isothermally heated at 360–410℃ for 340–550 seconds to ensure the formation of bainite at this stage, thereby ensuring the content of bainitic ferrite (35%–40%) and retained austenite (12%–15%).

[0076] 7. Finishing: Control the finishing elongation rate to 0.2% to 0.4%, and then cool to room temperature.

[0077] In summary, the core technology of this invention lies in forming an ultrafine-grained, fully tempered martensite matrix using pre-annealing treatment as the initial configuration for continuous annealing. During continuous annealing, the overall phase transformation products based on the ultrafine-grained, fully tempered martensite matrix all exhibit a fine-grained effect, specifically: recrystallized ferrite grain size of 0.75–0.95 μm; bainitic ferrite lath width of 0.15–0.35 μm; retained austenite grain size of 0.15–0.33 μm; and martensite grain size of 0.35–0.62 μm. Furthermore, the ultrafine-grained, fully tempered martensite matrix effectively promotes bainitic phase transformation and retains retained austenite, thereby reducing the martensite content after cooling.

[0078] The finished steel obtained by the manufacturing method described in this invention has a tensile strength of over 980 MPa, a yield strength of 600–680 MPa, an elongation of ≥26%, and an FLC0 of ≥0.21.

[0079] The following embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0080] (Example)

[0081] Table 1 lists the chemical composition of the steel in each embodiment and comparative example (balance is Fe and impurities), Table 2 lists the hot rolling process parameters of the steel in each embodiment and comparative example, Table 3 lists the pre-annealing process parameters of the steel in each embodiment and comparative example, Table 4 lists the continuous annealing process parameters of the steel in each embodiment and comparative example, Table 5 lists the phase ratios of the steel in each embodiment and comparative example, and Table 6 lists the mechanical properties of the finished steel plates in each embodiment and comparative example. Figure 1 This is a typical TEM microstructure of the steel plate from Example 1.

[0082] Table 1 Chemical composition of steel, wt%

[0083] 1 0.202 2.03 0.12 - 1.58 0.016 - - 0.001 0.009 0.003 2 0.215 2.12 - 0.05 1.62 0.018 0.012 0.02 0.002 0.01 0.005 3 0.226 1.96 0.14 0.05 1.75 0.017 0.021 0.03 0.002 0.009 0.003 4 0.205 1.99 0.13 - 1.49 0.021 - - 0.002 0.01 0.004 5 0.203 2.05 - 0.04 1.38 0.023 0.021 0.01 0.001 0.007 0.003 6 0.218 2.11 0.09 0.02 1.26 0.024 0.012 - - 0.008 0.005 7 0.227 2.08 0.05 - 0.98 0.018 - 0.05 - 0.01 0.004 8 0.236 1.89 0.13 0.02 1.28 0.022 0.016 0.01 0.002 0.01 0.005 9 0.206 2.03 012 0.05 1.46 0.02 - - 0.002 0.009 0.003 10 0.208 1.95 0.14 - 1.52 0.021 0.015 0.05 - 0.007 0.005 Comparative Example 0.213 2.02 0.08 0.26 1.65 0.022 0.016 - 0.002 0.005 0.003

[0084] Note: "-" indicates that the element is not added.

[0085] Table 2 Hot rolling process parameters for steel

[0086]

[0087] Table 3 Pre-annealing process parameters for steel

[0088]

[0089] Table 4 Continuous Annealing Process Parameters for Steel

[0090]

[0091] Table 5. Phase Proportions of Steel

[0092]

[0093] Table 6 Mechanical Properties of Finished Steel Plates

[0094]

[0095] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for manufacturing a superplastic 980MPa quenched and fractionated cold-rolled continuous annealed steel sheet for automobiles, characterized in that, The chemical composition of the steel, by mass percentage, is as follows: C: 0.20%–0.24%, Si: 0.90%–1.80%, Mn: 1.85%–2.15%, P≤0.010%, S≤0.005%, Ti: 0.015%–0.025%, Cr≤0.15%, Mo≤0.05%, Cu≤0.20%, Nb≤0.03%, V≤0.05%, Ca≤0.005%, B≤0.002%, with the balance being Fe and unavoidable impurities. The manufacturing method of the superplastic 980MPa quenched and continuously annealed cold-rolled steel sheet for automobiles includes smelting, hot rolling, pickling, cold rolling, pre-annealing, continuous annealing, and finishing processes; wherein the following processes are controlled: 1) Cold rolling: The cold rolling reduction rate is controlled between 40% and 60%. 2) Pre-annealing treatment: The steel plate is heated to 865-900℃, held at that temperature for 30-80 seconds, and then slowly cooled to 750-780℃ at a rate of 0.5-5℃ / s. Then it is water quenched to 80-100℃, and after a second pickling, the steel plate is heated to 350-550℃, held at that temperature for 100-140 seconds, and then coiled. After pre-annealing, an ultrafine crystalline fully tempered martensite matrix was obtained; 3) Continuous annealing: The pre-annealed steel plate is heated to 800-830℃, isothermal for 120-240s, then cooled to 660-720℃ at a cooling rate of 0.5-2℃ / s, and then cooled to 360-410℃ at a cooling rate of 20-40℃ / s, isothermal for 340-550s. 4) Finishing: The finishing elongation rate is controlled at 0.2% to 0.4%.

2. The method for manufacturing a superplastic 980MPa quenched and partitioned cold-rolled continuous annealed steel sheet for automobiles according to claim 1, characterized in that, The microstructure of the steel, by volume percentage, comprises: 35%–45% recrystallized ferrite, 35%–40% bainitic ferrite, 12%–15% retained austenite, and 3%–8% martensite.

3. The method for manufacturing a superplastic 980MPa quenched and partitioned cold-rolled continuous annealed steel sheet for automobiles according to claim 2, characterized in that, The recrystallized ferrite has a grain size of 0.75–0.95 μm; the bainitic ferrite has a lath width of 0.15–0.35 μm; the retained austenite has a grain size of 0.15–0.33 μm; and the martensite has a grain size of 0.35–0.62 μm.

4. The method for manufacturing a superplastic 980MPa quenched and partitioned cold-rolled continuous annealed steel sheet for automobiles according to claim 1, characterized in that, The mechanical properties of the finished steel plate are: tensile strength ≥980MPa, yield strength 600~680MPa, elongation ≥26%, FLC0 ≥0.

21.

5. The manufacturing method of a superplastic 980MPa quenched and partitioned cold-rolled continuous annealed steel sheet for automobiles as described in claim 1, characterized in that, The hot rolling process is controlled as follows: heating temperature is 1250~1280℃; roughing temperature is 1200~1250℃; finishing temperature is 1100~1200℃; coiling temperature is 400~470℃; the coiled steel plate is subjected to annealing in a bell-type process at a temperature of 450~550℃ for 18~36h.

6. The manufacturing method of a superplastic 980MPa quenched and partitioned cold-rolled continuous annealed steel sheet for automobiles as described in claim 1, characterized in that, The thickness of hot-rolled steel plates ranges from 2.8 mm to 3.3 mm.

7. The manufacturing method of a superplastic 980MPa quenched and partitioned cold-rolled continuous annealed steel sheet for automobiles as described in claim 1, characterized in that, The thickness of cold-rolled steel sheets is 1.4mm to 1.6mm.

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