A hot-rolled pickled low-alloy high-strength steel and its production method

Abstract

This invention relates to a hot-rolled pickled low-alloy high-strength steel and its production method. The chemical elements of this high-strength steel, by weight percentage, include: C 0.35%-0.55%, Si 0.15%-0.2%, Mn 1.25%-1.35%, P 0.002%-0.004%, S 0.002%-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%, N 0.0030%-0.0060%, with the remainder being Fe and unavoidable impurities. The advantages are: this hot-rolled pickled low-alloy high-strength steel possesses excellent strength, plasticity, formability, and service performance, meeting the requirements for lightweighting complex structural components in vehicle bodies.

Description

Technical Field

[0001] This invention belongs to the field of high-strength steel materials, and particularly relates to a hot-rolled pickled low-alloy high-strength steel and its production method. Background Technology

[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 research hotspot.

[0003] In the prior art, patent application number CN202111189125.X discloses a method for preparing 800MPa grade cold-rolled low-alloy high-strength steel with excellent hole-expanding performance. In the hot rolling process, the coiling temperature is 400-480℃, effectively suppressing the precipitation of precipitates at this stage, and allowing them to fully precipitate during the subsequent annealing process, forming fine and dispersed second-phase particles. This invention, through the design of Nb, Ti, and Mo composite additives and matching with reasonable hot rolling and annealing processes, fully utilizes the grain-refining and precipitation-strengthening effects of the second-phase particles, significantly improving product strength and obtaining high-strength low-alloy steel with excellent surface quality, stable mechanical properties, and good formability. Its hole-expanding performance is significantly improved, with a hole-expanding rate of over 55%. However, this method, which produces cold-rolled low-alloy high-strength steel, includes cold rolling and annealing processes, which is not conducive to reducing production costs, and also results in relatively low hole-expanding performance. Patent application CN202010497258.2 discloses a hot-rolled pickled low-alloy high-strength steel with enhanced hole-expansion performance and its production method, which significantly improves the hole expansion rate while ensuring the tensile strength of the hot-rolled pickled strip. However, this method requires sending the coiled steel coil to an insulated pit for slow cooling and holding at that temperature for 48 hours, increasing process and management costs and reducing production efficiency. Summary of the Invention

[0004] The purpose of this invention is to provide a hot-rolled pickled low-alloy high-strength steel and its production method. By combining composition design with process design, a single-phase ferrite structure is obtained, which has high strength, enhanced plasticity, formability and service performance, to meet the requirements of lightweighting for complex structural parts of the car body and personalized use, and to reduce production costs.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

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

[0007] C 0.35%-0.55%, Si 0.15%-0.2%, Mn 1.25%-1.35%, P 0.002%-0.004%, S 0.002%-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%, N 0.0030%-0.0060%, with the remainder being 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 650-730MPa, a tensile strength of 780-860MPa, an elongation of ≥23%, and a porosity of ≥65%.

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

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

[0012] 2) Slab heating: The slab is heated to 1230-1280℃ and held at this temperature for 1.0-2.5 hours;

[0013] 3) Controlled rolling: Finishing rolling start temperature ≥1050℃, finishing rolling temperature 930-980℃;

[0014] 4) Controlled cooling: After exiting the finishing mill, laminar flow cooling is carried out from groups 4-8, cooling to 680-690℃, with a cooling rate ≥20℃ / s, followed by air cooling;

[0015] 5) Winding: Air cool to 640-660℃ and then wind up;

[0016] 6) Pickling: Pickling is performed on the flattened steel coils at a speed of 50-150 m / min and a temperature of 65-85℃. The rinsing temperature is 30-50℃.

[0017] It also includes smelting, which includes the following steps:

[0018] 1) Hot metal pretreatment: The KR hot metal pretreatment process is adopted, and magnesium powder and lime powder are added at a weight ratio of 1:(3-4) to make the S in the hot metal ≤0.003% and the hot metal temperature ≥1300℃;

[0019] 2) Converter smelting: Iron ore, limestone and fluorite are added to the molten iron to form slag, oxygen blowing time is 8 to 10 minutes, and the tapping temperature is 1620-1660℃. During the tapping process, aluminum particles are added for deoxidation and ferromolybdenum 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 content in the molten steel is controlled to be ≤0.001% and N to be ≤0.0030%. Titanium-iron alloying is added, and the soft blowing time is 15-25 min. After that, the alloy composition is finely adjusted, and slag is removed. The slag layer thickness is 35-45 mm. During the RH process, aluminum wire is added for deoxidation, the circulation time is 10-40 min, the carbon content is controlled to be 0.35%-0.55%, and the killing time is 10-30 min.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] 1. The chemical composition design of this invention adds a high content of titanium and controls a low nitrogen content, so that stable titanium nitride and titanium carbide are formed in the austenite at the hot temperature of the slab, and nitrogen is completely removed from the austenite. The slab retains a sufficiently high titanium content after heating, and the addition of chromium and nickel forms fine oxides to improve the microstructure of the slab.

[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 the steel strip still retains a high titanium content after final rolling.

[0025] 3. Based on calculations, the temperature of A3 is 875.8°C, 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 ferrite. The formation temperature of molybdenum carbide and composite iron carbide is within 600-640°C. Therefore, the precipitation of cementite, molybdenum carbide, and composite iron carbide is almost completely suppressed. Molybdenum exists more in solid solution form in ferrite, and a small portion forms nanoscale precipitates during the cooling process after coiling, further strengthening the matrix.

[0026] 4. The microstructure is ferrite, mainly composed of equiaxed ferrite. Pre-cooling is employed to transfer high-density dislocations from the deformed austenite to the transformed ferrite, strengthening the ferrite microstructure. This hot-rolled, pickled, low-alloy high-strength steel exhibits excellent strength, plasticity, formability, and service performance, meeting the requirements for lightweighting complex structural components in vehicle bodies. Attached Figure Description

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

[0028] Figure 2 This is a metallographic diagram of the present invention. Detailed Implementation

[0029] The present invention will now be described in detail with reference to 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%, S 0.002%-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%, N 0.0030%-0.0060%, with the remainder being Fe and unavoidable impurities, see [see source]. Figure 2 .

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

[0033] 1) Hot metal pretreatment: The KR hot metal pretreatment process is adopted, and magnesium powder and lime powder are added at a weight ratio of 1:3-4 to make the S in the hot metal ≤0.003% and the hot metal temperature ≥1300℃;

[0034] 2) Converter smelting: Iron ore, limestone and fluorite are added to the molten iron to form slag, oxygen blowing time is 8 to 10 minutes, and the tapping temperature is 1620-1660℃. During the tapping process, aluminum particles are added for deoxidation and ferromolybdenum 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 content in the molten steel is controlled to be ≤0.001% and N to be ≤0.0030%. Titanium-iron alloying is added, and the soft blowing time is 15-25 min. After that, the alloy composition is finely adjusted, and slag is removed. The slag layer thickness is 35-45 mm. During the RH process, aluminum wire is added for deoxidation, the circulation time is 10-40 min, the carbon content is controlled to be 0.35%-0.55%, and the killing time is 10-30 min.

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

[0038] 5) Slab heating: The slab is heated to 1230-1280℃ and held at this temperature for 1.0-2.5 hours;

[0039] 6) Controlled rolling: Finishing rolling start temperature ≥1050℃, finishing rolling temperature 930-980℃;

[0040] 7) Controlled Cooling: After finishing rolling, laminar flow cooling begins from groups 4-8 to increase cooling intensity, refine the grain size for austenite to ferrite transformation, and prevent other microstructure transformations. Cool to 680-690℃ at a cooling rate ≥20℃ / s, followed by air cooling; see [link to relevant documentation]. Figure 1 Cooling to 680-690℃ based on the A1 temperature value is beneficial for the transformation of austenite to ferrite.

[0041] 8) Winding: Air cool to 640-660℃ and then wind up;

[0042] 9) Leveling: Level the steel coils after they have been coiled and cooled to room temperature. The surface roughness of the strip should be 0.6-1.9μm.

[0043] 10) Pickling: Pickling is performed on the flattened steel coil. The pickling speed is 50-150m / min, the pickling temperature is 65-85℃, and the rinsing temperature is 30-50℃.

[0044] Example 1

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

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

[0047] 2. Converter smelting: Iron ore, limestone and fluorite are added to the molten iron to form slag. Oxygen is blown for 9 minutes. Temperature is measured and sampled. Steel is tapped when the temperature reaches 1644℃. During the tapping process, aluminum particles are added for deoxidation and ferromolybdenum is added 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 molten steel contains 0.0008% S and 0.0022% N. Titanium-iron alloying is added, and the soft blowing time is 17 minutes. After that, the alloy composition is finely adjusted to meet the corresponding composition range requirements, and slag removal is performed. The slag layer thickness is controlled at 42 mm. During the RH process, aluminum wire is added for deoxidation, the circulation time is 25 minutes, the carbon content is 0.38%, and the killing time is 23 minutes.

[0050] The resulting molten steel had the following chemical composition: 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%, with the remainder being Fe and unavoidable impurities.

[0051] 4. Continuous casting adopts independent pouring, with full-process protective pouring, and the tundish superheat is 13℃.

[0052] 5. Slab heating: The slab is heated to 1248℃ and held at this temperature for 1.6 hours;

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

[0054] 7. Controlled cooling: After exiting the finishing mill, start layer cooling from group 4 to 688℃ at a cooling rate of 22℃ / s, followed by air cooling;

[0055] 8. Winding: Air cool to 642℃ before winding;

[0056] 9. Leveling: Level the steel coils after they have been coiled and cooled to room temperature to improve the strip shape 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℃, and a rinsing temperature of 39℃.

[0058] Example 2

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

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

[0061] 2. Converter smelting: Iron ore, limestone and fluorite are added to the molten iron to form slag. Oxygen is blown for 10 minutes. Temperature is measured and sampled. Steel is tapped when the temperature reaches 1653℃. During the tapping process, aluminum particles are added for deoxidation and ferromolybdenum is added 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 molten steel contains 0.0007% S and 0.0018% N. Titanium-iron alloying is added, and the soft blowing time is 22 minutes. After that, the alloy composition is finely adjusted to meet the corresponding composition range requirements, and slag removal is performed. The slag layer thickness is 41 mm. During the RH process, aluminum wire is added for deoxidation, the circulation time is 33 minutes, the carbon content is 0.42%, and the killing time is 19 minutes.

[0064] The resulting molten steel had the following chemical composition: 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%, with the remainder being Fe and unavoidable impurities.

[0065] 4. Continuous casting adopts independent pouring, with full-process protective pouring, and the tundish superheat is 22℃.

[0066] 5. Slab heating: The slab is heated to 1252℃ and held at this temperature for 2.1 hours;

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

[0068] 7. Controlled cooling: After exiting the finishing mill, start layer cooling from group 6 to 685℃, with a cooling rate ≥24℃ / s, followed by air cooling;

[0069] 8. Winding: Air cool to 658℃ before winding;

[0070] 9. Leveling: Level the steel coils after they have been coiled and cooled to room temperature to improve the strip shape 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℃, and a rinsing temperature of 45℃.

[0072] Example 3

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

[0074] 1. Hot metal pretreatment: The KR hot metal pretreatment process is adopted, with magnesium powder and lime powder added in a ratio of 1:3.7, the sulfur content of the hot metal is 0.002%, and the hot metal temperature is 1381℃.

[0075] 2. Converter smelting: Iron ore, limestone and fluorite are added to the molten iron to form slag. Oxygen is blown for 9 minutes. Temperature is measured and sampled. Steel is tapped when the temperature reaches 1630℃. During the tapping process, aluminum particles are added for deoxidation and ferromolybdenum is added 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 content in the molten steel is controlled at 0.0007% and N at 0.0020%. Titanium-iron alloying is added, and the soft blowing time is 19 minutes. After that, the alloy composition is finely adjusted to meet the corresponding composition range requirements. Slag removal operation is performed to control the slag layer thickness at 39 mm. During the RH process, aluminum wire is added for deoxidation, the circulation time is 28 minutes, the carbon content is 0.51%, and the killing time is 26 minutes.

[0078] The resulting molten steel had the following chemical composition: 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%, with the remainder being Fe and unavoidable impurities.

[0079] 4. Continuous casting adopts independent pouring, with full-process protective pouring, and the tundish superheat is 15℃.

[0080] 5. Slab heating: The slab is heated to 1253℃ and held at this temperature for 2.2 hours;

[0081] 6. Controlled rolling: The starting rolling temperature of finishing mill F1 is 1063℃, and the finishing rolling temperature of F7 is 964℃;

[0082] 7. Controlled cooling: After exiting the finishing mill, start from group 8 for layer cooling to 686℃, with a cooling rate ≥23℃ / s, followed by air cooling;

[0083] 8. Winding: Air cool to 653℃ before winding;

[0084] 9. Leveling: Level the steel coils after they have been coiled and cooled to room temperature to improve the strip shape 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℃, and a rinsing temperature of 45℃.

[0086] The finished steel plates from the above embodiments were subjected to mechanical property tests according to GB / T228.1-2021, using P14 specimens with the specimen orientation longitudinal. The mechanical property test results are as follows:

[0087]

[0088] This invention incorporates a high titanium content and controls a low nitrogen content in its chemical composition design. At the slab's hot temperature, stable titanium nitride and titanium carbide are formed in the austenite, with nitrogen completely removed from the austenite. The slab retains a sufficiently high titanium content after heating. The addition of chromium and nickel forms fine oxides, improving the slab's microstructure. During finishing and final rolling, due to the high rolling speed, low temperature, and short pass intervals, strain-induced precipitation of titanium carbide is limited, resulting in a high titanium content remaining 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, by weight percentage, include: C 0.35%-0.55%, Si 0.15%-0.2%, Mn 1.25%-1.35%, P 0.002%-0.004%, S 0.002%-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%, N 0.0030%-0.0060%, with the remainder being Fe and unavoidable impurities; The method for producing hot-rolled pickled low-alloy high-strength steel includes the following steps: 1) Continuous casting adopts independent pouring, with full-process protective pouring, and the superheat of the tundish is controlled at 10-30℃; 2) Slab heating: The slab is heated to 1230-1280℃ and held at this temperature for 1.0-2.5 hours; 3) Controlled rolling: Finishing rolling start temperature ≥1050℃, finishing rolling temperature 930-980℃; 4) Controlled cooling: After exiting the finishing mill, laminar flow cooling is carried out from groups 4-8, cooling to 680-690℃, with a cooling rate ≥20℃ / s, followed by air cooling; 5) Winding: Air cool to 640-660℃ and then wind up; 6) Pickling: Pickling is performed on the flattened steel coils at a speed of 50-150 m / min and a temperature of 65-85℃. The rinsing temperature is 30-50℃.

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 described in any one of claims 1-3, characterized in that, Includes the following steps: 1) Continuous casting adopts independent pouring, with full-process protective pouring, and the superheat of the tundish is controlled at 10-30℃; 2) Slab heating: The slab is heated to 1230-1280℃ and held at this temperature for 1.0-2.5 hours; 3) Controlled rolling: Finishing rolling start temperature ≥1050℃, finishing rolling temperature 930-980℃; 4) Controlled cooling: After exiting the finishing mill, laminar flow cooling is carried out from groups 4-8, cooling to 680-690℃, with a cooling rate ≥20℃ / s, followed by air cooling; 5) Winding: Air cool to 640-660℃ and then wind up; 6) Pickling: Pickling is performed on the flattened steel coils at a speed of 50-150 m / min and a temperature of 65-85℃. The rinsing temperature is 30-50℃.

5. A method for producing hot-rolled pickled low-alloy high-strength steel according to claim 4, characterized in that, It also includes smelting, which includes the following steps: 1) Hot metal pretreatment: The KR hot metal pretreatment process is adopted, and magnesium powder and lime powder are added at a weight ratio of 1:(3-4) to make the S in the hot metal ≤0.003% and the hot metal temperature ≥1300℃; 2) Converter smelting: Iron ore, limestone and fluorite are added to the molten iron to form slag, oxygen blowing time is 8~10 minutes, and the tapping temperature is 1620-1660℃. During the tapping process, aluminum particles are added for deoxidation and ferromolybdenum 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 content in the molten steel is controlled to be ≤0.001% and N to be ≤0.0030%. Titanium-iron alloying is added, and the soft blowing time is 15-25 min. After that, the alloy composition is finely adjusted, and slag is removed, with a slag layer thickness of 35-45 mm. During the RH process, aluminum wire is added for deoxidation, the circulation time is 10-40 min, the carbon content is controlled to be 0.35%-0.55%, and the killing time is 10-30 min.

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