Hot-rolled pickling low-alloy high-strength steel and production method thereof

Abstract

The invention relates to hot-rolled and pickled low-alloy high-strength steel and a production method thereof. The high-strength steel comprises the following chemical elements in percentage by weight: 0.35%-0.55% of C, 0.15%-0.2% of Si, 1.25%-1.35% of Mn, 0.002%-0.004% of P, 0.002%-0.006% of S, 0.05%-0.07% of Cr, 0.05%-0.07% of Ni, 0.18%-0.22% of Mo, 0.08%-0.10% of Al, 0.1%-0.15% of Ti, 0.0030%-0.0060% of N and the balance of Fe and inevitable impurities. The hot-rolled and pickled low-alloy high-strength steel has the advantages of good strength and plasticity, formability and service performance, and meets the use requirements of light weight on complex structural parts of a vehicle body.

Description

Technical Field

[0001] The invention belongs to the field of high-strength steel materials, and in particular relates to a hot-rolled pickled low-alloy high-strength steel and a production method. Background Art

[0002] The main strengthening mechanisms of low alloy high strength steel are grain refinement and precipitation strengthening, which are controlled by different types and precipitates. The dependence of precipitate characteristics on the chemical composition and hot rolling process of low alloy high strength steel, as well as the influence of precipitates on the microstructure and mechanical properties of low alloy high strength steel, have always been a hot topic of research.

[0003] In the prior art, the patent application number is CN202111189125.X, which discloses a method for preparing 800MPa grade cold-rolled low-alloy high-strength steel with excellent hole expansion performance. In the hot rolling process, the coiling temperature is 400-480°C, which effectively inhibits the precipitation of the precipitated phase at this stage, and fully precipitates in the subsequent annealing process to form fine dispersed second phase particles. The present invention uses the design of Nb, Ti, and Mo composite additives, matches reasonable hot rolling and annealing processes, gives full play to the fine grain strengthening and precipitation strengthening effects of the second phase particles, greatly improves the product strength, and obtains high-strength low-alloy steel with excellent surface quality, stable mechanical properties and good forming performance. Its hole expansion performance is significantly improved, and the hole expansion rate can reach more than 55%. This method is a cold-rolled low-alloy high-strength steel, and the production method includes cold rolling and annealing processes, which is not conducive to reducing production costs, and the hole expansion performance is low. The patent application number is CN202010497258.2, which discloses a hot-rolled pickled low-alloy high-strength steel with enhanced hole expansion performance and a production method thereof, which significantly improves the hole expansion rate while ensuring the tensile strength of the hot-rolled pickled strip steel. In this method, the coiled steel coil needs to be sent to a heat preservation pit for slow cooling and heat preservation for 48 hours, which increases the process, management and operation costs and reduces production efficiency. Summary of the invention

[0004] The purpose of the present invention is to provide a hot-rolled pickled low-alloy high-strength steel and a production method, which obtains a single-phase ferrite structure through composition design combined with process design, and has higher strength, enhanced plasticity, formability and service performance, meets the requirements of lightweight for complex structural parts of the car body and personalized use, and reduces production costs.

[0005] To achieve the above object, the present invention is implemented through the following technical solutions:

[0006] A hot-rolled pickled low-alloy high-strength steel, wherein the chemical elements of the high-strength steel include, by weight percentage:

[0007] C 0.35%-0.55%, Si 0.15%-0.2%, Mn 1.25%-1.35%, P 0.002%-0.004%, S0.002%-0.006%, Cr 0.05%-0.07%, Ni 0.05%-0.07%, Mo 0.18%-0.22%, Al 0.08%-0.10%, Ti 0.1%-0.15%, N0.0030%-0.0060%, and the rest are Fe and unavoidable impurities.

[0008] The thickness of the low-alloy high-strength steel is 2.0-5.0 mm.

[0009] The low alloy high strength steel has a yield strength of 650-730 MPa, a tensile strength of 780-860 MPa, an elongation of ≥23%, and a hole expansion rate of ≥65%.

[0010] A method for producing hot-rolled pickled low-alloy high-strength steel comprises the following steps:

[0011] 1) Continuous casting adopts independent pouring, protective pouring throughout the whole process, and the superheat of the tundish is controlled at 10-30℃;

[0012] 2) Slab heating: The slab heating temperature is 1230-1280°C and kept at this temperature for 1.0-2.5h;

[0013] 3) Controlled rolling: finishing rolling start temperature ≥ 1050°C, final rolling temperature 930-980°C;

[0014] 4) Controlled cooling: After finishing rolling, laminar cooling is carried out from groups 4-8, cooling to 680-690°C, cooling rate ≥20°C / s, followed by air cooling;

[0015] 5) Coiling: air cooling to 640-660℃ for coiling;

[0016] 6) Pickling: Pickle the flattened steel coil at a speed of 50-150 m / min, a temperature of 65-85°C, and a rinsing temperature of 30-50°C.

[0017] The invention also includes smelting, wherein the smelting includes the following steps:

[0018] 1) Hot metal pretreatment: Use KR hot metal pretreatment process, add magnesium powder and lime powder in a weight ratio of 1: (3-4), so that the hot metal S is ≤ 0.003%, and the hot metal temperature is ≥ 1300°C;

[0019] 2) Converter smelting: iron ore, limestone and fluorite are added to the molten iron to make slag, the oxygen blowing time is 8 to 10 minutes, the tapping temperature is 1620-1660°C, aluminum particles are added for deoxidation during the tapping process, and molybdenum iron is added for alloying;

[0020] 3) LF+RH refining:

[0021] During the LF process, pre-melted slag and lime are added to make white slag, and aluminum is added for deoxidation. The S in the molten steel is controlled to be ≤0.001% and N ≤0.0030%. Titanium-iron alloying is added, and the soft blowing time is 15-25min. After that, the alloy composition is fine-tuned and the slag is removed. The slag layer thickness is 35-45mm. During the RH process, aluminum wire is added for deoxidation, the cycle time is 10-40min, the carbon content is controlled at 0.35%-0.55%, and the calming time is 10-30min.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. In the chemical composition design of the present invention, a higher content of titanium is added and a lower content of nitrogen is controlled. Stable titanium nitride and titanium carbon sulfide are formed in austenite at the hot temperature of the slab, and nitrogen is completely removed from the austenite. After the slab is heated, a sufficiently high titanium content is retained. Chromium and nickel are added to form fine oxides to improve the microstructure of the ingot.

[0024] 2. During the finishing and final rolling processes, due to the high rolling speed, low temperature and short pass interval, the strain-induced precipitation of titanium carbide is limited, and a relatively high titanium content is still retained in the steel strip after final rolling.

[0025] 3. According to calculation, the temperature of A3 is 875.8°С, and the temperature of A1 is 688.4°C. During the final rolling and coiling process, the formation of titanium carbide will consume a large amount of carbon, forming titanium carbide at the phase interface and further precipitating in the ferrite. The formation temperature of molybdenum carbide and composite iron carbide is within 600-640°C, so the precipitation of cementite, molybdenum carbide and composite iron carbide is almost completely suppressed. Molybdenum exists more in the form of solid solution in ferrite, and a small part forms nano-scale precipitation phase during the cooling process after coiling, further strengthening the matrix.

[0026] 4. The structure is ferrite, mainly composed of equiaxed ferrite. The front cooling is used to make the high-density dislocations in the deformed austenite inherit to the ferrite after phase transformation, strengthening the ferrite structure. The hot-rolled pickled low-alloy high-strength steel has good strength, plasticity, formability and service performance, meeting the requirements for lightweight use of complex structural parts of the vehicle body. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a CCT curve diagram of the present invention.

[0028] Figure 2 It is the metallographic organization diagram of the present invention. DETAILED DESCRIPTION

[0029] The present invention is described in detail below in conjunction with the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0030] A hot-rolled pickled low-alloy high-strength steel comprises, by weight percentage:

[0031] C 0.35%-0.55%, Si 0.15%-0.2%, Mn 1.25%-1.35%, P 0.002%-0.004%, S0.002%-0.006%, Cr 0.05%-0.07%, Ni 0.05%-0.07%, Mo 0.18%-0.22%, Al 0.08%-0.10%, Ti 0.1%-0.15%, N0.0030%-0.0060%, the rest is Fe and unavoidable impurities, see Figure 2 .

[0032] A method for producing hot-rolled pickled low-alloy high-strength steel comprises the following steps:

[0033] 1) Hot metal pretreatment: Use KR hot metal pretreatment process, add magnesium powder and lime powder in a weight ratio of 1:3-4, so that the hot metal S is ≤0.003% and the hot metal temperature is ≥1300℃;

[0034] 2) Converter smelting: iron ore, limestone and fluorite are added to the molten iron to make slag, the oxygen blowing time is 8 to 10 minutes, the tapping temperature is 1620-1660°C, aluminum particles are added for deoxidation during the tapping process, and molybdenum iron is added for alloying;

[0035] 3) LF+RH refining:

[0036] During the LF process, pre-melted slag and lime are added to make white slag, and aluminum is added for deoxidation. The S in the molten steel is controlled to be ≤0.001% and N ≤0.0030%. Titanium-iron alloying is added, and the soft blowing time is 15-25min. After that, the alloy composition is fine-tuned and the slag is removed. The slag layer thickness is 35-45mm. During the RH process, aluminum wire is added for deoxidation, the cycle time is 10-40min, the carbon content is controlled at 0.35%-0.55%, and the calming time is 10-30min.

[0037] 4) Continuous casting adopts independent pouring, protective pouring throughout the whole process, and the superheat of the tundish is controlled at 10-30℃;

[0038] 5) Slab heating: The slab heating temperature is 1230-1280°C and kept at this temperature for 1.0-2.5h;

[0039] 6) Controlled rolling: finishing rolling start temperature ≥ 1050°C, final rolling temperature 930-980°C;

[0040] 7) Controlled cooling: After finishing rolling, laminar cooling is carried out from group 4 to group 8 to increase the cooling intensity, refine the grain size of austenite to ferrite, avoid the transformation of other organizations, cool to 680-690℃, cooling rate ≥20℃ / s, and then air cool; see Figure 1 According to the A1 temperature value, cooling to 680-690℃ is conducive to the transformation of austenite to ferrite.

[0041] 8) Coiling: air cooling to 640-660℃ for coiling;

[0042] 9) Leveling: The steel coil cooled to room temperature after coiling is leveled, and the surface roughness of the strip is 0.6-1.9μm;

[0043] 10) Pickling: Pickle the flattened steel coil at a speed of 50-150 m / min, a temperature of 65-85°C, and a rinsing temperature of 30-50°C.

[0044] Example 1

[0045] A method for producing hot-rolled pickled low-alloy high-strength steel comprises the following steps:

[0046] 1. Hot metal pretreatment: KR hot metal pretreatment process is adopted, magnesium powder and lime powder are added in a ratio of 1:3.2, S in the hot metal is 0.002%, and the hot metal temperature is 1356℃.

[0047] 2. Converter smelting: Add iron ore, limestone and fluorite to the molten iron to make slag, blow oxygen for 9 minutes, measure temperature and take samples, and steel is tapped when the temperature reaches 1644℃. During the tapping process, add aluminum particles for deoxidation and add molybdenum iron for alloying;

[0048] 3. LF+RH refining:

[0049] During the LF process, pre-melted slag and lime are added to make white slag, and aluminum is added for deoxidation. The S content in the molten steel is 0.0008%, and the N content is 0.0022%. Titanium-iron alloying is added, and the soft blowing time is 17 minutes. After that, the alloy composition is fine-tuned to meet the corresponding composition range requirements, and the slag removal operation is performed. The slag layer thickness is controlled at 42mm. During the RH process, aluminum wire is added for deoxidation, the cycle time is 25 minutes, the carbon content is 0.38%, and the calming time is 23 minutes.

[0050] The obtained molten steel has a chemical composition of C 0.38%, Si 0.17%, Mn 1.26%, P 0.003%, S 0.004%, Cr 0.05%, Ni 0.06%, Mo 0.21%, Al 0.083%, Ti 0.13%, N 0.0035%, and the rest is Fe and inevitable impurities.

[0051] 4. Continuous casting adopts independent pouring, protective pouring throughout the whole process, and the superheat of the tundish is 13℃.

[0052] 5. Slab heating: The slab heating temperature is 1248℃ and kept at this temperature for 1.6h;

[0053] 6. Controlled rolling: Finishing rolling F1 start rolling temperature 1055℃, F7 final rolling temperature 946℃;

[0054] 7. Controlled cooling: After finishing rolling, layer cooling is carried out from group 4 to 688°C, with a cooling rate of 22°C / s, followed by air cooling;

[0055] 8. Coiling: Air cool to 642℃ for coiling;

[0056] 9. Leveling: The steel coil cooled to room temperature after coiling is leveled to improve the plate quality. The surface roughness of the strip is 0.9μm;

[0057] 10. Pickling: Pickle the flattened steel coil at a speed of 95 m / min, a temperature of 78°C, and a rinsing temperature of 39°C.

[0058] Example 2

[0059] A method for producing hot-rolled pickled low-alloy high-strength steel comprises the following steps:

[0060] 1. Hot metal pretreatment: KR hot metal pretreatment process is adopted, magnesium powder and lime powder are added in a ratio of 1:3.5, S in the hot metal is 0.001%, and the hot metal temperature is 1388℃.

[0061] 2. Converter smelting: Add iron ore, limestone and fluorite to the molten iron to make slag, blow oxygen for 10 minutes, measure temperature and take samples, and steel is tapped when the temperature reaches 1653℃. During the tapping process, add aluminum particles for deoxidation and add molybdenum iron for alloying;

[0062] 3. LF+RH refining:

[0063] During the LF process, pre-melted slag and lime are added to make white slag, and aluminum is added for deoxidation. The S content in the molten steel is 0.0007%, and the N content is 0.0018%. Titanium-iron alloying is added, and the soft blowing time is 22 minutes. After that, the alloy composition is fine-tuned to meet the corresponding composition range requirements, and the slag removal operation is performed. The slag layer thickness is 41 mm. During the RH process, aluminum wire is added for deoxidation, the cycle time is 33 minutes, the carbon content is 0.42%, and the calming time is 19 minutes.

[0064] The obtained molten steel has a chemical composition of C 0.38%, Si 0.16%, Mn 1.29%, P 0.003%, S 0.004%, Cr 0.06%, Ni 0.05%, Mo 0.20%, Al 0.084%, Ti 0.14%, N 0.0039%, and the rest is Fe and inevitable impurities.

[0065] 4. Continuous casting adopts independent pouring, protective pouring throughout the whole process, and the superheat of the tundish is 22℃.

[0066] 5. Slab heating: The slab heating temperature is 1252℃, and it is kept at this temperature for 2.1h;

[0067] 6. Controlled rolling: Finishing rolling F1 start rolling temperature 1056℃, F7 final rolling temperature 958℃;

[0068] 7. Controlled cooling: After finishing rolling, layer cooling is carried out from group 6 to 685°C, with a cooling rate of ≥24°C / s, followed by air cooling;

[0069] 8. Coiling: Air cool to 658℃ for coiling;

[0070] 9. Leveling: The steel coil cooled to room temperature after coiling is leveled to improve the plate quality. The surface roughness of the strip is 1.2μm;

[0071] 10. Pickling: Pickle the flattened steel coil at a speed of 135 m / min, a temperature of 82°C, and a rinsing temperature of 45°C.

[0072] Example 3

[0073] A method for producing hot-rolled pickled low-alloy high-strength steel comprises the following steps:

[0074] 1. Hot metal pretreatment: KR hot metal pretreatment process is adopted, magnesium powder and lime powder are added in a ratio of 1:3.7, hot metal S0.002%, and hot metal temperature is 1381℃.

[0075] 2. Converter smelting: Add iron ore, limestone and fluorite to the molten iron to make slag, blow oxygen for 9 minutes, measure temperature and take samples, and steel is tapped when the temperature reaches 1630℃. During the tapping process, add aluminum particles for deoxidation and add molybdenum iron for alloying;

[0076] 3. LF+RH refining:

[0077] During the LF process, pre-melted slag and lime are added to make white slag, and aluminum is added for deoxidation. The S and N in the molten steel are controlled at 0.0007% and 0.0020%, and titanium-iron alloying is added. The soft blowing time is 19 minutes. After that, the alloy composition is fine-tuned to meet the corresponding composition range requirements, and the slag removal operation is performed to control the slag layer thickness to 39 mm. During the RH process, aluminum wire is added for deoxidation, the cycle time is 28 minutes, the carbon content is 0.51%, and the calming time is 26 minutes.

[0078] The obtained molten steel has a chemical composition of C 0.51%, Si 0.19%, Mn 1.31%, P 0.003%, S 0.004%, Cr 0.05%, Ni 0.07%, Mo 0.2%, Al 0.081%, Ti 0.12%, N 0.0028%, and the rest is Fe and inevitable impurities.

[0079] 4. Continuous casting adopts independent pouring, protective pouring throughout the whole process, and the superheat of the tundish is 15℃.

[0080] 5. Slab heating: The slab heating temperature is 1253℃, and it is kept at this temperature for 2.2h;

[0081] 6. Controlled rolling: Finishing rolling F1 start rolling temperature 1063℃, F7 final rolling temperature 964℃;

[0082] 7. Controlled cooling: After finishing rolling, layer cooling is carried out from group 8 to 686°C, with a cooling rate of ≥23°C / s, followed by air cooling;

[0083] 8. Coiling: Air cool to 653℃ for coiling;

[0084] 9. Leveling: The steel coil cooled to room temperature after coiling is leveled to improve the plate quality. The surface roughness of the strip is 1.3μm;

[0085] 10. Pickling: Pickle the flattened steel coil at a speed of 140 m / min, a temperature of 82°C, and a rinsing temperature of 45°C.

[0086] The finished steel plate of the above embodiment was subjected to a mechanical property test, the test standard was GB / T228.1-2021, a P14 specimen was used, the specimen direction was longitudinal, and the mechanical property test results were as follows:

[0087]

[0088] In the chemical composition design of the present invention, a higher content of titanium is added and a lower content of nitrogen is controlled. Stable titanium nitride and titanium carbon sulfide are formed in austenite at the hot temperature of the slab, and nitrogen is completely removed from the austenite. After the slab is heated, a sufficiently high titanium content is retained. Chromium and nickel are added to form fine oxides to improve the microstructure of the ingot. During the finishing and final rolling processes, due to the high rolling speed, low temperature drop and short pass interval time, the strain-induced precipitation of titanium carbide is limited, and a relatively high titanium content is still retained in the steel strip after final rolling.

Claims

1. A hot-rolled pickled low-alloy high-strength steel, characterized in that: The chemical elements of the high-strength steel include, by weight percentage: C 0.35%-0.55%, Si 0.15%-0.2%, Mn 1.25%-1.35%, P 0.002%-0.004%, S0.002%-0.006%, Cr 0.05%-0.07%, Ni 0.05%-0.07%, Mo 0.18%-0.22%, Al 0.08%-0.10%, Ti 0.1%-0.15%, N0.0030%-0.0060%, and the rest are Fe and unavoidable impurities.

2. The hot-rolled pickled low-alloy high-strength steel according to claim 1, characterized in that: The thickness of the low-alloy high-strength steel is 2.0-5.0 mm.

3. The hot-rolled pickled low-alloy high-strength steel according to claim 1, characterized in that: The low alloy high strength steel has a yield strength of 650-730 MPa, a tensile strength of 780-860 MPa, an elongation of ≥23%, and a hole expansion rate of ≥65%.

4. A method for producing hot-rolled pickled low-alloy high-strength steel as claimed in any one of claims 1 to 3, characterized in that: The following steps are involved: 1) Continuous casting adopts independent pouring, protective pouring throughout the whole process, and the superheat of the tundish is controlled at 10-30℃; 2) Slab heating: The slab heating temperature is 1230-1280°C and kept at this temperature for 1.0-2.5h; 3) Controlled rolling: finishing rolling start temperature ≥ 1050°C, final rolling temperature 930-980°C; 4) Controlled cooling: After finishing rolling, laminar cooling is carried out from groups 4-8, cooling to 680-690°C, cooling rate ≥20°C / s, followed by air cooling; 5) Coiling: air cooling to 640-660℃ for coiling; 6) Pickling: Pickle the flattened steel coil at a speed of 50-150 m / min, a temperature of 65-85°C, and a rinsing temperature of 30-50°C.

5. The method for producing hot-rolled pickled low-alloy high-strength steel according to claim 4, characterized in that: The process also includes smelting, wherein the smelting includes the following steps: 1) Hot metal pretreatment: Use KR hot metal pretreatment process, add magnesium powder and lime powder in a weight ratio of 1: (3-4), so that the hot metal S is ≤ 0.003%, and the hot metal temperature is ≥ 1300°C; 2) Converter smelting: iron ore, limestone and fluorite are added to the molten iron to make slag, the oxygen blowing time is 8 to 10 minutes, the tapping temperature is 1620-1660°C, aluminum particles are added for deoxidation during the tapping process, and molybdenum iron is added for alloying; 3) LF+RH refining: During the LF process, pre-melted slag and lime are added to make white slag, and aluminum is added for deoxidation. The S in the molten steel is controlled to be ≤0.001% and N ≤0.0030%. Titanium-iron alloying is added, and the soft blowing time is 15-25min. After that, the alloy composition is fine-tuned and the slag is removed. The slag layer thickness is 35-45mm. During the RH process, aluminum wire is added for deoxidation, the cycle time is 10-40min, the carbon content is controlled at 0.35%-0.55%, and the calming time is 10-30min.

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