Gigapascal grade cold rolled high strength steel and method of production thereof

Abstract

The application discloses a kind of giga-level cold-rolled high-strength steel and its production method, belongs to high-strength steel production technical field.Its chemical composition is C:0.25~0.35%, Si:0.5~1.1%, Mn:2.2~2.9%, P≤0.015%, S≤0.005%, Al:0.04~0.2%, and Si+Al:0.6~1.2%, Cr:0.35~0.6%, Mo:0.12~0.24%, Cu:0.08~0.2%, B:0.002~0.0035%, Ca≤0.007%, 1~2 kinds of Ti and Nb, Nb:0.02~0.04%, Ti:0.02~0.06%, and Nb+Ti≤0.08%, the balance is Fe and inevitable impurities;Its production method includes smelting and continuous casting, hot rolling, cover retreat, acid rolling, continuous annealing process.The high-strength steel has the advantages of high strength and plasticity.

Description

Technical Field

[0001] This invention belongs to the field of high-strength steel production technology, specifically relating to a gigapascal-grade cold-rolled high-strength steel and its production method. Background Technology

[0002] To meet the requirements of energy conservation, emission reduction, environmental protection, and enhanced safety, developing high-strength and ductile automotive steel has become a crucial means of achieving lightweighting and carbon emission reduction in automobiles. Currently, the most widely used cold-rolled high-strength steels are duplex steels and low-alloy steels, but their strength levels are generally 980 MPa and below. With the continuous increase in strength requirements, the application of quenched and fractional steels, medium-manganese steels, hot-formed steels, and martensitic steels in automobile manufacturing is gradually increasing. However, quenched and fractional steels and medium-manganese steels have high alloy content, narrow process windows, and are difficult to produce; martensitic steels have low ductility and are limited to simple reinforcing parts forming; hot-formed steels require reheating at high temperatures during forming, resulting in high energy consumption, which contradicts the trend of low-carbon emission development.

[0003] Chinese patent CN 106244918 A discloses a 1500MPa grade high-strength and high-ductility automotive steel and its manufacturing method. The chemical element mass percentages are: C: 0.1%~0.3%, Si: 0.1%~2.0%, Mn: 7.5%~12%, Al: 0.01%~2.0%; the balance is iron and other unavoidable impurities. It requires two annealing processes after pickling and rolling, the production process is relatively long, and the high Mn content makes it a medium manganese steel, which is difficult to smelt and roll.

[0004] Chinese patent CN 109321828 A discloses a 1600MPa grade cold-rolled martensitic steel and its production method. The composition of the steel plate, by weight percentage, is as follows: C: 0.23%–0.28%, Si: 0.5%–1.0%, Mn: 1.8%–2.3%, Al: 0.02%–0.07%, P: ≤0.02%, S: ≤0.005%, N: ≤0.005%, with the balance being Fe and unavoidable impurities. It can produce ultra-high strength cold-rolled steel plates with tensile strength greater than 1600MPa and elongation greater than 5%. This patent utilizes a water-quenching process, which easily leads to yellowing of the steel plate surface after water quenching, hindering subsequent coating. Furthermore, the cold-rolled steel prepared by this invention has low plasticity, making it suitable only for simple roll forming.

[0005] Currently, the proportion of publicly available gigapascal-grade cold-rolled high-strength steels used in high-strength automotive body parts remains relatively low. The main reason for this is likely that the forming process requires materials with both high strength and high ductility, necessitating annealing equipment with high cooling capacity and a suitable composition and process design. Therefore, developing a gigapascal-grade cold-rolled high-strength steel that combines high strength and high elongation at a lower cost is particularly urgent. Summary of the Invention

[0006] To solve the above-mentioned technical problems, the present invention provides a gigapascal-grade cold-rolled high-strength steel and its production method, the technical solution of which is as follows:

[0007] A gigapascal-grade cold-rolled high-strength steel has the following chemical composition and mass percentage: C: 0.25–0.35%, Si: 0.5–1.1%, Mn: 2.2–2.9%, P≤0.015%, S≤0.005%, Al: 0.04–0.2%, and Si+Al: 0.6–1.2%, Cr: 0.35–0.60%, Mo: 0.12–0.24%, Cu: 0.08–0.20%, B: 0.002–0.0035%, Ca≤0.007%, one or both of Ti and Nb, Nb: 0.02–0.04%, Ti: 0.02–0.06%, and Nb+Ti≤0.08%, with the balance being Fe and unavoidable impurities.

[0008] Furthermore, the microstructure of the high-strength steel is martensite + bainite + retained austenite, wherein, by volume percentage, martensite is 81-90%, bainite is 6-11%, and austenite is 3-8%.

[0009] Furthermore, the high-strength steel has a thickness of 0.9–2.5 mm, is sampled along the rolling direction, has a yield strength of 1296–1380 MPa, a tensile strength ≥1600 MPa, and an elongation after fracture A. 80 ≥10%. Among them, the strength σ c With σ d They respectively satisfy the following relations:

[0010] σ c =(Nb / 0.01*17+Ti / 0.01*7+B / 0.001*5+Cu / 0.05*13+Mo / 0.1*17+Cr / 0.1*17)*1

[0011] 00, and 199≤σc≤287, where Nb, Ti, B, Cu, Mo, and Cr are the mass percentage contents of the corresponding elements in the high-strength steel;

[0012] σ d =8.3*v+10*ε, and 338≤σ d ≤668; where v is the rapid cooling rate of the continuous annealing process, and ε is the cold rolling reduction rate.

[0013] The rationale for the design of the components in this invention is as follows:

[0014] C: It has solid solution strengthening properties and is also a strengthening element that stabilizes austenite. In this invention, it plays a role in increasing the hardness of martensite, significantly affecting the strength of the steel plate; the strength of the steel plate increases significantly with increasing C content. C also improves the stability of retained austenite. If the C content is too low, the stable existence of retained austenite cannot be achieved, and the martensite strength is low; if the C content is too high, the weldability and formability of the material are poor. Therefore, considering both weldability and the mechanical properties of the material, this invention requires the C content to be between 0.25% and 0.35%.

[0015] Si and Al: Both Si and Al have certain solid solution strengthening effects and can also inhibit carbide formation, avoiding the reduction of material properties and residual austenite stability caused by carbide precipitation. Too low a Si content will not be able to inhibit carbide precipitation, while too high a Si content will make surface red rust difficult to remove and exacerbate material brittleness. Too high an Al content will significantly increase the difficulty of continuous casting. Therefore, the Si content should be between 0.5% and 1.1%, Al content between 0.04% and 0.2%, and the Si+Al content between 0.6% and 1.2%.

[0016] Mn: An element that expands the austenite phase region, improving the hardenability of steel and the stability of retained austenite. Too low a Mn content hinders the acquisition of retained austenite, while too high a Mn content increases the difficulty of smelting and rolling, and reduces the weldability of the material. Therefore, the Mn content is typically between 2.2% and 2.9%.

[0017] Nb and Ti: Nb can increase the recrystallization temperature of austenite, refine grains, and improve material strength. In addition to its similar grain-refining and dispersion-strengthening effects to Nb, Ti combines with N to form TiN for nitrogen fixation, also contributing to grain-refining strengthening. If the content is too low, the effects of Nb or Ti are not significant; if too high, it will increase costs and make the cold rolling process more difficult. Therefore, one or both of Ti and Nb are added, with Nb content: 0.02–0.04%, Ti content: 0.02–0.06%, and Nb+Ti≤0.08%.

[0018] Cr: a solid solution strengthening element, which can improve the hardenability of steel, and its combination with B also helps improve the weldability of the material. It can also delay the pearlite and bainite transformations and promote carbon diffusion into austenite. In this invention, the Cr content is selected to be 0.35–0.60%.

[0019] Mo can significantly delay the ferrite-pearlite phase transformation, improve the hardenability of steel, strengthen grain boundaries, and inhibit the growth of microalloyed carbides. Due to its high cost, this invention uses a Mo content of 0.12–0.24%.

[0020] Cu: This is an important element in this invention. As a non-carbide-forming element, it not only opens the austenite phase region but also provides solid solution strengthening and precipitation strengthening. It can precipitate as a copper-rich phase, thus exerting precipitation strengthening and grain refinement effects. Excessive content can cause surface defects or copper embrittlement; therefore, the Cu content is between 0.08% and 0.20%.

[0021] Boron (B) mainly segregates at the original austenite grain boundaries, inhibiting the formation of proeutectoid ferrite. Adding boron to steel can also greatly improve its hardenability. Excessive boron content can cause "boron embrittlement" in the matrix; therefore, the amount of boron added to steel is 0.002–0.0035%.

[0022] Ca can improve the morphology of sulfides such as MnS, causing them to spheroidize and thus improving the morphology of inclusions. Excessive Ca will worsen the formability of the matrix structure; therefore, in this invention, Ca ≤ 0.007%.

[0023] P and S: P and S are residual harmful elements. P easily causes central segregation of the matrix structure and secondary cold working brittleness, and is also detrimental to the welding of steel plates. S easily forms MnS with Mn, which reduces the cold bending and hole expansion performance of steel plates. Therefore, both should be controlled at the lowest possible level. This invention controls P ≤ 0.015% and S ≤ 0.005%.

[0024] σ c =(Nb / 0.01*17+Ti / 0.01*7+B / 0.001*5+Cu / 0.05*13+Mo / 0.1*17+Cr / 0.1*17)*1

[0025] The purpose of setting σc to 00 and 199≤σc≤287 is to leverage the synergistic effect of Nb, Ti, B, Cu, Mo, and Cr elements, improve the utilization rate of the alloy, and achieve strong plasticity of the material. If σc is too low, the strength requirements of the final material cannot be guaranteed. If σc is too high, it may cause the formation of coarse precipitates and deteriorate the microstructure and properties. Therefore, 199≤σc≤287.

[0026] The production method of the above-mentioned gigapascal grade cold-rolled high-strength steel includes smelting and continuous casting, hot rolling, annealing, pickling, and continuous annealing processes:

[0027] Annealing process: The cooled hot-rolled coil is placed in the annealing furnace for annealing. The heating rate is 0.08 to 0.016℃ / s, and the temperature is raised to 670 to 720℃. After holding at this temperature for 210 to 400 minutes, the coil is slowly cooled to room temperature in the furnace.

[0028] Continuous annealing process: heating to 870-910℃ at 2-5℃ / s, holding for 30-150s; rapid cooling to 190-250℃ at 40-80℃ / s; aging temperature 280-330℃, holding time 150-250s, furnace dew point temperature -25 to -45℃.

[0029] Furthermore, the smelting and continuous casting process adopts a converter + LF + RH for smelting; the continuous casting speed is 0.8-1.2m / min, the superheat is 10-25℃, and a dynamic light reduction of 5-8mm is adopted.

[0030] Furthermore, in the hot rolling process, a hot charging and hot delivery process is adopted, in which the 400-700°C billet is charged into the heating furnace, heated to 1180-1260°C, and held for 120-180 minutes.

[0031] Furthermore, in the hot rolling process, the intermediate billet edge temperature is compensated by 30-50°C, the finishing rolling start temperature is 960-1050°C, and the final rolling temperature is 870-920°C. The laminar flow section adopts a front-end centralized rapid cooling process with a cooling rate controlled at 20-70°C / s. The laminar flow fine-tuning section uses high-pressure air horizontal spraying instead of high-pressure water side spraying, with a high-pressure air pressure of 0.3-0.8 bar and a coiling temperature of 300-500°C. After coiling, the plate is placed in a ventilated area to cool to room temperature, resulting in a hot-rolled plate with a thickness of 2.0-5.0 mm and a microstructure of bainite + ferrite + a small amount of martensite.

[0032] Furthermore, in the pickling and rolling process, the stretching elongation is 0.3-1.0%, two bending and one straightening are adopted, the bending roll insertion is 5-20mm, and the straightening roll insertion is 5-11mm; the free acid concentration of the acid solution is 100-200g / L, the acid solution temperature is 75-90℃, the pickling belt speed is 60-150m / min, the cold rolling reduction is 40-60%, the F1-F4 roll changing cycle is ≤300km / time, the roll roughness is 0.9-1.2μm, and the thickness of the strip after cold rolling is 0.9-2.5mm.

[0033] Furthermore, in the continuous annealing process, a high-hydrogen rapid cooling mode is adopted to rapidly cool to 190-250°C at 40-80°C / s, with H2 content of 15-25% and the remainder being N2.

[0034] Furthermore, in the continuous annealing process, the flattening elongation is 0.3-0.6%.

[0035] The beneficial effects of adopting the above technical solution are as follows:

[0036] (1) The addition of Cu in this invention can improve the strength of steel by precipitation strengthening without consuming carbon in the matrix. At the same time, Mo can inhibit the formation of microalloyed carbides. The two elements work together to retain sufficient free carbon in the retained austenite. Meanwhile, Cu, Cr, Nb, Ti, Mo and other elements work synergistically to achieve a combination of fine grain strengthening, precipitation strengthening and solid solution strengthening, thereby improving the strength and plasticity of the material.

[0037] (2) Low-temperature coiling is beneficial for refining precipitates, especially for the formation of nano-precipitates, and also for improving the fine grain strengthening effect and banded structure of the microstructure. High-temperature hooding with short-time heat preservation promotes the spheroidization of carbides and the formation of equiaxed ferrite, reduces the anisotropy of the microstructure and improves the uniformity of the microstructure properties, while preventing the ripening of precipitates and providing nucleation sites for the subsequent formation of retained austenite. It is also beneficial for the formation of ε-Cu phase and helps to improve the crack arrest ability of the material.

[0038] (3) Continuous annealing adopts a high-temperature homogenization + quenching + low-temperature aging partitioning process, which provides conditions for the formation of martensite and bainite structures and realizes the stable existence of retained austenite. At the same time, the high-hydrogen rapid cooling mode realizes the gas-cooled quenching and low dew point control of the material. In synergy with the annealing time and annealing atmosphere, it can reduce the diffusion of surface elements and inhibit the enrichment and decarburization of surface elements, which helps to improve surface quality and obtain a high-strength and plastic structure.

[0039] The product of this invention has a yield strength of 1296-1380 MPa, a tensile strength of ≥1600 MPa, and an elongation of ≥10%. The matrix structure is martensite + bainite + retained austenite, of which martensite accounts for 81-90%, bainite for 6-11%, and austenite for 3-8%. It has the characteristics of high strength and high elongation, which helps to improve the safety of automobiles and is of great significance to the realization of automobile lightweighting. Attached Figure Description

[0040] Figure 1 This is a microstructure diagram of the cold-rolled high-strength steel obtained in this invention. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to the embodiments.

[0042] The chemical composition and mass percentage of the gigapascal grade cold-rolled high-strength steel of this invention are as follows: C: 0.25-0.35%, Si: 0.5-1.1%, Mn: 2.2-2.9%, P≤0.015%, S≤0.005%, Al: 0.04-0.2%, and Si+Al: 0.6-1.2%, Cr: 0.35-0.60%, Mo: 0.12-0.24%, Cu: 0.08-0.20%, B: 0.002-0.0035%, Ca≤0.007%, one or both of Ti and Nb, Nb: 0.02-0.04%, Ti: 0.02-0.06%, and Nb+Ti≤0.08%, with the balance being Fe and unavoidable impurities.

[0043] The microstructure of the aforementioned gigapascal-grade cold-rolled high-strength steel consists of martensite, bainite, and retained austenite, with a volume percentage of 81-90% martensite, 6-11% bainite, and 3-8% austenite. Its thickness is 0.9-2.5 mm, sampled along the rolling direction. The yield strength is 1296-1380 MPa, the tensile strength is ≥1600 MPa, and the elongation after fracture is A... 80 ≥10%.

[0044] The above-mentioned method for preparing gigapascal-grade cold-rolled high-strength steel includes smelting and continuous casting, hot rolling, annealing, pickling, and continuous annealing processes, comprising the following steps:

[0045] (1) Smelting and continuous casting process: The smelting is carried out by converter + LF + RH to obtain molten steel that meets the following composition requirements by mass percentage, and then it is continuously cast. The chemical element content of each embodiment is shown in Table 1, and the continuous casting parameters are shown in Table 2.

[0046] Table 1. Mass percentage of chemical elements in each embodiment (wt / %)

[0047]

[0048] Table 2. Continuous casting parameters for each embodiment

[0049] Example Pulling speed (m / min) Superheat (°C) Press down lightly (mm) 1 0.8 12 5 2 0.9 11 6 3 1.1 15 7 4 1.2 13 8 5 1.0 10 6 6 1.05 18 5 7 0.97 25 7.5

[0050] (2) Hot rolling process: A hot-charging and hot-feeding process is used for heating, rough rolling, finish rolling, laminar flow cooling, and coiling to obtain 2.0–5.0 mm hot-rolled plates. The hot rolling process parameters for each embodiment are shown in Table 3.

[0051] Table 3. Hot rolling process parameters for each embodiment

[0052]

[0053]

[0054] (3) Annealing process: The cooled hot-rolled coil is placed in the annealing furnace for annealing and then slowly cooled to room temperature in the furnace; (4) Pickling process: The hot-rolled coil is straightened, pickled, and cold-rolled to obtain a cold-hardened sheet of 0.9-2.5mm. The parameters of the annealing and pickling processes in each embodiment are shown in Table 4, and the parameters of the pickling rolls are shown in Table 5.

[0055] Table 4. Parameters of the coating removal and pickling processes in each embodiment

[0056]

[0057] Table 5. Parameters of acid-pressed rolls in each embodiment

[0058]

[0059] (5) Continuous annealing process: The cold-hardened strip is heated, homogenized, slowly cooled, quickly cooled and leveled to obtain the final gigap-grade cold-rolled high-strength steel product.

[0060] The parameters for the continuous annealing process in each embodiment are shown in Table 6.

[0061] Table 6. Parameters of the continuous annealing process in each embodiment

[0062]

[0063] The microstructure and mechanical properties of the cold-rolled high-strength steel obtained in the above embodiments are shown in Table 7. Figure 1 It can be seen that the microstructure of Example 1 is mainly martensite, with a small amount of bainite and Mao islands.

[0064] Table 7. Microstructure and Mechanical Properties of Cold-Rolled High-Strength Steel in Each Example

[0065]

Claims

1. A gigapascal grade cold rolled high strength steel characterized in that, The high-strength steel has the following chemical composition and mass percentage: C: 0.25-0.35%, Si: 0.5-1.1%, Mn: 2.2-2.9%, P≤0.015%, S≤0.005%, Al: 0.04-0.2%, Si+Al: 0.6-1.2%, Cr: 0.35-0.60%, Mo: 0.12-0.24%, Cu: 0.08-0.20%, B: 0.002-0.0035%, Ca≤0.007%, one or both of Ti and Nb, Nb: 0.02-0.04%, Ti: 0.02-0.06%, and Nb+Ti≤0.08%, and the balance of Fe and inevitable impurities; The microstructure of the high-strength steel is martensite+behenite+residual austenite, wherein the martensite is 81-90%, the behenite is 6-11%, and the austenite is 3-8% by volume percentage; The production method of the high-strength steel comprises the following steps: smelting and continuous casting, hot rolling, cover annealing, acid rolling, and continuous annealing; The cover annealing process is as follows: the hot-rolled coil after cooling is placed in a cover annealing furnace for annealing, the heating rate is 0.08-0.016 ℃ / s, the temperature is heated to 670-720 ℃, the temperature is kept for 210-400 min, and then the coil is slowly cooled to room temperature in the furnace; The continuous annealing process is as follows: the temperature is heated to 870-910 ℃ at a rate of 2-5 ℃ / s, the temperature is kept for 30-150 s, the temperature is quickly cooled to 190-250 ℃ at a rate of 40-80 ℃ / s, the aging temperature is 280-330 ℃, the temperature is kept for 150-250 s, and the dew point temperature in the furnace is -25--45 ℃.

2. The gigapascal cold rolled high strength steel of claim 1 wherein, The high-strength steel has a thickness of 0.9-2.5 mm, and the yield strength is 1296-1380 MPa, the tensile strength is greater than or equal to 1600 MPa, and the elongation A after fracture is greater than or equal to 10% 80 ; wherein the strength σ c ; wherein the strength σ d respectively satisfy the following relationships: σ c = (Nb / 0.01*17 + Ti / 0.01*7 + B / 0.001*5 + Cu / 0.05*13 + Mo / 0.1*17 + Cr / 0.1*17)*100, and 199≤σc≤287, wherein Nb, Ti, B, Cu, Mo, Cr are the mass percentage content of corresponding elements in the high-strength steel, respectively; σ d = 8.3*v + 10*ε, and 338 < σ d < 668; where v is the fast cooling rate of the continuous annealing process and ε is the cold rolling reduction.

3. The production method of a gigapascal cold rolled high strength steel according to claim 1 or 2, characterized in that, The production method of the high-strength steel comprises the following steps: smelting and continuous casting, hot rolling, cover annealing, acid rolling, and continuous annealing; The cover annealing process is as follows: the hot-rolled coil after cooling is placed in a cover annealing furnace for annealing, the heating rate is 0.08-0.016 ℃ / s, the temperature is heated to 670-720 ℃, the temperature is kept for 210-400 min, and then the coil is slowly cooled to room temperature in the furnace; The continuous annealing process is as follows: the temperature is heated to 870-910 ℃ at a rate of 2-5 ℃ / s, the temperature is kept for 30-150 s, the temperature is quickly cooled to 190-250 ℃ at a rate of 40-80 ℃ / s, the aging temperature is 280-330 ℃, the temperature is kept for 150-250 s, and the dew point temperature in the furnace is -25--45 ℃.

4. The method of producing a gigapascal cold rolled high strength steel according to claim 3, characterized by, The smelting and continuous casting process is performed by adopting converter+LF+RH for smelting, the continuous casting speed is 0.8-1.2 m / min, the superheat degree is 10-25 ℃, and the dynamic light pressing down process is adopted with a pressing down amount of 5-8 mm.

5. The method of producing a gigapascal cold rolled high strength steel according to claim 3, characterized by, In the hot rolling process, the hot charging and hot feeding process is adopted, the 400-700 ℃ cast blank is charged into a heating furnace, the temperature is heated to 1180-1260 ℃, and the temperature is kept for 120-180 min.

6. A method of production of a gigapascal cold rolled high strength steel according to claim 5, characterized in that, The hot rolling process, the intermediate blank edge temperature compensation is 30-50 DEG C, the finish rolling starting temperature is 960-1050 DEG C, and the finish rolling temperature is 870-920 DEG C; the laminar flow section adopts the front section concentrated fast cooling process, the cooling speed is controlled at 20-70 DEG C / s, the laminar flow fine adjustment section adopts the high pressure air transverse spraying instead of the high pressure water side spraying, the high pressure air pressure is 0.3-0.8 bar, the coiling temperature is 300-500 DEG C, the coiled product is placed in the ventilated place after coiling and cooled to room temperature, the hot rolled plate with the thickness of 2.0-5.0 mm and the structure of bainite + ferrite + a small amount of martensite is obtained.

7. A method of production of a gigapascal cold rolled high strength steel according to claim 3, characterized in that, The pickling process, the straightening extension rate is 0.3-1.0%, two bending and one straightening are adopted, the bending roll insertion amount is 5-20 mm, the straightening roll insertion amount is 5-11 mm; the free acid concentration of the acid liquid is 100-200 g / L, the acid liquid temperature is 75-90 DEG C, the pickling strip speed is 60-150 m / min, the cold rolling reduction is 40-60%, the F1-F4 roll changing period is less than or equal to 300 km / time, the roll roughness is 0.9-1.2 mu m, and the cold rolled strip thickness is 0.9-2.5 mm.

8. A method of production of a gigapascal cold rolled high strength steel according to claim 7, characterized in that, The continuous annealing process adopts the high hydrogen fast cooling mode to fast cool to 190-250 DEG C at 40-80 DEG C / s, the fast cooling section H2 content is 15-25%, and the rest is N2.

9. A method of production of a Giga-Pascal grade cold rolled high strength steel according to any of claims 3-8, characterized in that, The continuous annealing process, the flattening extension rate is 0.3-0.6%.

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