A bainite non-quenched and tempered steel and a method for manufacturing the same

Abstract

This invention belongs to the technical field of automotive steel and relates to a bainitic non-quenched and tempered steel and its preparation method. The composition of this bainitic non-quenched and tempered steel, by chemical mass percentage, includes: C: 0.22-0.29%, Si: 0.25-0.40%, Mn: 1.80-2.00%, Cr: 0.40-0.60%, V: 0.01-0.08%, P: ≤0.015%, S: 0.020-0.050%, Ti: 0.020-0.050%, Zr: 0.020-0.040%, Al: ≥0.010%, N: 0.010-0.020%, [O] ≤15ppm, [H] ≤2ppm, with the remainder being Fe and unavoidable impurities. In the production process, through reasonable composition design, controlled rolling and cooling processes, and slow cooling temperature design upon entering the pit, this invention enables the steel to achieve excellent strength and toughness. The design employs low carbon, high manganese, and trace amounts of V, Ti, and Zr to fully leverage the effects of solid solution strengthening, precipitation strengthening, and grain refinement strengthening. Zr effectively improves the morphology and distribution of MnS, fully utilizing the metallurgical properties of MnS. Through multi-tank cooling technology and slow cooling temperature design, the strength-toughness matching, uniformity, hardness, and microstructure of the steel are well controlled.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel technology, specifically relating to a bainitic non-quenched and tempered steel and its preparation method. Background Technology

[0002] Non-quenched and tempered steel is made by adding vanadium, titanium, and niobium microalloying elements to medium carbon steel. These elements dissolve in austenite during heating. Through temperature-controlled rolling and cooling, the microalloying elements vanadium, titanium, and niobium precipitate as fine carbides and nitrides in the ferrite and pearlite that have already precipitated. These precipitates maintain a coherent relationship with the parent phase, which strengthens the steel. After rolling (forging), it can achieve the same mechanical properties as quenched and tempered alloy structural steel or carbon structural steel without quenching and tempering. This simplifies the production process, reduces energy consumption, and lowers the manufacturing cost by 25% to 38% compared to quenched and tempered steel.

[0003] Currently, major automotive structural component manufacturers are striving to develop low-cost non-quenched and tempered steels that can replace quenched and tempered steels, especially bainitic non-quenched and tempered steels with good comprehensive properties. For this reason, there is an urgent need to provide a method for preparing bainitic non-quenched and tempered steels. Summary of the Invention

[0004] This invention provides a bainitic non-quenched and tempered steel for automotive structural components. By rationally designing the chemical composition and controlling the smelting, rolling, and cooling processes, the cleanliness of the steel is improved, the morphology and distribution of non-metallic inclusions are controlled, and the homogeneity of the material's microstructure and properties is optimized, enabling the material to obtain excellent mechanical properties, microstructure, and fatigue life, thus giving the material good comprehensive performance.

[0005] The technical solution of the present invention is as follows:

[0006] This invention provides a bainitic non-quenched and tempered steel for use in automotive structural parts. The composition of the bainitic non-quenched and tempered steel, by mass percentage of chemical composition, includes: C: 0.22-0.29%, Si: 0.25-0.40%, Mn: 1.80-2.00%, Cr: 0.40-0.60%, V: 0.01-0.08%, P: ≤0.015%, S: 0.020-0.050%, Ti: 0.020-0.050%, Zr: 0.020-0.040%, Al: ≥0.010%, N: 0.010-0.020%, [O] ≤15ppm, [H] ≤2ppm, with the remainder being Fe and unavoidable impurities.

[0007] Preferably, the bainitic non-quenched and tempered steel is used for automotive structural parts. The composition of the bainitic non-quenched and tempered steel, by mass percentage of chemical composition, includes: C: 0.26-0.28%, Si: 0.25-0.40%, Mn: 1.85-1.95%, Cr: 0.50-0.60%, V: 0.05-0.07%, P: ≤0.015%, S: 0.025-0.040%, Ti: 0.030-0.045%, Zr: 0.025-0.035%, Al: 0.015-0.040%, N: 0.013-0.017%, [O] ≤15ppm, [H] ≤2ppm, with the remainder being Fe and unavoidable impurities.

[0008] This invention provides a method for preparing bainitic non-quenched and tempered steel for use in automotive structural parts. The preparation method includes: electric furnace smelting, LF refining, VD vacuum degassing, continuous casting, rolling, water cooling in a water tank, cooling on a cooling bed, slow cooling in a pit, and obtaining round steel.

[0009] Step 1: Electric Furnace Smelting: Electric furnace smelting is used, with molten iron and scrap steel as the main raw materials, and the proportion of molten iron ≥ 40%. The final carbon content in the electric furnace is ≥ 0.10%, and phosphorus content is ≤ 0.010%. The tapping temperature is 1640–1680℃. Before tapping, 1.3–1.6 kg / t of pure aluminum blocks are added, and 9–12 kg / t of slag-forming material is added. During the tapping process, alloying is performed by adding 19–21 kg / t of metallic manganese and 3.5–4.5 kg / t of ferrosilicon, instead of using manganese ferrosilicon, high-manganese, or medium-manganese alloys. The purity of metallic manganese is ≥ 98%, and the silicon content of ferrosilicon is ≥ 78%, which improves the purity of the steel, reduces the content of impurity elements, and stabilizes the performance of the steel.

[0010] Step 2: LF Refining: With LF in place, Al is controlled at 0.045–0.060 wt%. Rapid slag adjustment is performed. The slag composition is: Al₂O₃ = 28–36%, CaO = 44–50%, SiO₂ = 8–10%, MgO = 7–9%, with basicity controlled at 4–6. Add 0.8–1.2 kg / t of ferrozirconium according to the composition requirements, with a zirconium content of 35–40%. This forms zirconium oxide, providing heterogeneous nucleation sites for MnS inclusions, resulting in fine, dispersed spindle-shaped MnS particles that pin the austenite grains, refining the grain size. After composition adjustment, argon is blown and stirred at a flow rate of 600–1000 NL / min. Aluminum wire is added as needed, with aluminum controlled at 0.040–0.055%. The refining tapping temperature is 1655–1675℃.

[0011] Step 3: VD Vacuum: Before entering the VD furnace, remove slag, maintain a vacuum of less than 67 Pa for ≥12 minutes, and use argon flow rate of 50-70 NL / min; after breaking the vacuum, first feed in 3.0-5.0 m / t manganese nitride wire, then feed in sulfur wire, 1.0-1.5 m / t steel, to improve N and S recovery rate, use argon flow rate of 10-20 NL / min, soft blowing argon time not less than 15 minutes, and control the steel temperature of normal furnace cycles to 1553-1578℃.

[0012] Step 4: Continuous Casting: A double-layer tundish covering agent is used in continuous casting. The lower layer uses an alkaline tundish covering agent, and the upper layer uses a hollow granular tundish covering agent. The tundish superheat is 15–30℃. Electromagnetic stirring is employed. The electromagnetic stirring current in the crystallizer is 200–300A, with a frequency of 3–4 Hz. The electromagnetic stirring current at the end is 200–300A, with a frequency of 12–14 Hz. The specific water content is 0.25–0.27 L / kg. The casting speed for a 260mm × 300mm square billet is 0.5–0.55 m / min. The straightening temperature is ≥920℃, and the billet is allowed to cool slowly on the ground for ≥24 hours.

[0013] Step 5: Rolling: Cold billet is put into the furnace. During heating, the temperature of the high-temperature section is controlled at 1200-1250℃, the heat soaking temperature is 1180-1230℃, the heating time is ≥4h, the initial rolling temperature is 1010-1060℃, and the final rolling temperature is 900-950℃.

[0014] Step 6: Water Tank Cooling: All roller conveyor insulation covers are opened, and a six-tank cooling system is used. After water cooling and reheating, the temperature on the cooling bed is 580–630℃. Specifically, the temperatures exiting water tank #1 are 850–900℃, #2 790–850℃, #3 730–780℃, #4 680–720℃, #5 600–670℃, and #6 530–600℃. The water tanks are 5–8 meters long. The rolling speed is 1.5–1.8 m / s.

[0015] Step 7: Cooling the cooling bed: Open the insulation cover of the cooling bed, control the production rhythm, and allow the cooling bed to air cool normally. The temperature of the cooling bed for the next multiple length of steel is 380-430℃.

[0016] Step 8: Slow cooling in the pit: After the steel is collected, it is quickly placed into the pit for slow cooling. The temperature in the pit is 270-330℃, the slow cooling time is ≥48h, and the temperature when it comes out of the pit is ≤100℃.

[0017] For any process conditions not specified in the above preparation methods, please refer to conventional techniques in this field.

[0018] Compared with the prior art, the advantages of the present invention are:

[0019] 1) In the production process of this invention, through reasonable composition design, controlled rolling and cooling process, and slow cooling temperature design upon entering the pit, the steel achieves excellent strength and toughness. The design employs low carbon, high manganese, and trace amounts of V, Ti, and Zr to fully utilize the effects of solid solution strengthening, precipitation strengthening, and grain refinement strengthening. Zr effectively improves the morphology and distribution of MnS, fully leveraging the metallurgical properties of MnS. Simultaneously, through multi-tank cooling technology and slow cooling temperature design, the strength-toughness matching degree, uniformity, hardness, and microstructure of the steel are well controlled. The hot-rolled mechanical properties reach or even exceed the level of 40Cr quenched and tempered steel, reducing the production cost of automotive structural parts by approximately 500 yuan / ton.

[0020] 2) Through reasonable design of the smelting process, the amount and timing of aluminum addition, and measures such as adding MnN line first and then sulfur line after VD cavitation, the nitrogen and sulfur recovery rate is guaranteed, while the cleanliness of the steel is improved, the non-metallic inclusions in the steel are well controlled, and the fatigue life reaches more than 1 million cycles. Detailed Implementation

[0021] The present invention will be further described below with reference to the embodiments.

[0022] Examples 1-3:

[0023] This invention provides a bainitic non-quenched and tempered steel. It is smelted using a UHP ultra-high power electric furnace, LF refining, and VD vacuum degassing process, followed by continuous casting of the billet and controlled rolling and cooling. Examples 1-3 illustrate how this invention is implemented using the production process of Φ80mm steel.

[0024] The composition of steel by mass percentage is as follows:

[0025] C: 0.26-0.28%, Si: 0.25-0.40%, Mn: 1.90-2.00%, Cr: 0.50-0.60%, V: 0.05-0.07%, P: ≤0.015%, S: 0.025-0.035%, Ti: 0.030-0.045%, Zr: 0.025-0.035%, Al: 0.015-0.030%, N: 0.013-0.017%, [O] ≤15ppm, [H] ≤2ppm, the remainder being Fe and unavoidable impurities.

[0026] The production process is as follows:

[0027] Smelting: Electric furnace smelting is adopted, with molten iron and scrap steel as the main raw materials, and the proportion of molten iron being 50-70%. The final carbon content (C) of the electric furnace is 0.12-0.15%, and phosphorus (P) is ≤0.010%. The tapping temperature is 1640-1680℃. Before tapping, 1.3-1.6 kg / t of pure aluminum blocks are added, and 9-12 kg / t of slag-forming material is added. During the tapping process, alloying is achieved by adding 19-21 kg / t of metallic manganese and 3.5-4.5 kg / t of ferrosilicon. The purity of metallic manganese is ≥98%, and the silicon content of the ferrosilicon is ≥78%. The Al content in the LF feed wire is controlled at 0.050–0.060%, and the slag composition is controlled as follows: Al₂O₃ = 30–36%, CaO = 45–50%, SiO₂ = 8–10%, MgO = 7–9%, with a basicity of 5–6. Add 0.8–1.2 kg / t of ferrozirconium, with a zirconium content of 35–40%. After adjusting the composition, stir with argon gas at a flow rate of 600–1000 NL / min. Based on the actual aluminum wire feed, control the aluminum content at 0.045–0.055%, and refine the steel at a tapping temperature of 1655–1675℃. Before entering the VD furnace, remove slag, maintain a vacuum of less than 67 Pa for 12-15 minutes, and use an argon flow rate of 50-70 NL / min. After breaking the vacuum, first feed in 3.5-4.5 m / t manganese nitride wire, then feed in sulfur wire, 1.0-1.5 m / t steel, use an argon flow rate of 10-20 NL / min, and use soft blowing argon for 20-25 minutes. Control the steel temperature for normal furnace runs to 1553-1570℃.

[0028] Continuous casting: A double-layer tundish covering agent is used in continuous casting. The lower layer uses an alkaline tundish covering agent, and the upper layer uses a hollow granular tundish covering agent. The tundish superheat is 15-30℃. Electromagnetic stirring is used. The electromagnetic stirring current in the crystallizer is 200-300A, and the frequency is 3-4Hz. The electromagnetic stirring current at the end is 200-300A, and the frequency is 12-14Hz. The specific water content is 0.25-0.27L / kg. The casting speed for a 260mm×300mm square billet is 0.5-0.55m / min. The straightening temperature is ≥920℃, and the billet is allowed to cool slowly on the ground for ≥24 hours.

[0029] Controlled rolling and cooling: Cold billets are fed into the furnace. During heating, the temperature of the high-temperature section is controlled at 1200–1250℃, the soaking temperature at 1180–1230℃, and the heating time at least 4 hours. The initial rolling temperature is 1010–1060℃, and the final rolling temperature is 900–950℃. After rolling, all the insulation covers on the roller table are opened, and six water tanks are used for cooling. After water cooling and reheating, the temperature on the cooling bed is 580–630℃. Specifically, the temperatures exiting water tank #1 are 850–900℃, #2 790–850℃, #3 730–780℃, #4 680–720℃, #5 600–670℃, and #6 530–600℃. The insulation cover on the cooling bed is opened, and air cooling is used. The temperature on the cooling bed for the next multiple length of steel is 380–430℃. After collection, the steel is quickly placed into a pit for slow cooling. The temperature inside the pit is 270-330℃, the slow cooling time is ≥48h, and the temperature outside the pit is ≤100℃.

[0030] The specific process parameters are shown in Tables 1 and 2. Table 1 shows the chemical composition of the bainitic non-quenched and tempered steel in Examples 1-3, and Table 2 shows the process parameters for continuous casting and rolling.

[0031] Table 1 Chemical composition (wt, %) of examples of bainitic non-quenched and tempered steel

[0032]

[0033] Table 2. Process parameters for continuous casting and rolling.

[0034]

[0035] The performance test results of each batch of steel products in Examples 1-3 are shown in Tables 3 and 4.

[0036] Table 3. Results of Low-Magnification Inspection of Non-Metallic Inclusions

[0037]

[0038] Table 4. Test results of mechanical properties, hardness, and grain size.

[0039]

[0040] As can be seen from the above embodiments, this invention, through reasonable composition design, smelting and controlled rolling and cooling process design, and slow cooling process design, enables the steel to obtain excellent mechanical properties, with good strength and toughness matching, yield strength ≥780MPa, tensile strength ≥980MPa, elongation after fracture ≥14%, reduction of area ≥47%, impact energy ≥62%, and uniform mechanical properties of steel from different sections of the same furnace. The test results show that the round steel has excellent strength and plasticity, with fine and uniform grains. When this product is used to manufacture automotive structural parts, bench tests show that fatigue resistance exceeds 1 million cycles, demonstrating excellent comprehensive performance.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A bainitic non-quenched and tempered steel, characterized in that, The composition of the bainitic non-quenched and tempered steel, by chemical mass percentage, includes: C: 0.22-0.29%, Si: 0.25-0.40%, Mn: 1.80-2.00%, Cr: 0.40-0.60%, V: 0.01-0.08%, P: ≤0.015%, S: 0.020-0.050%, Ti: 0.020-0.050%, Zr: 0.020-0.040%, Al: ≥0.010%, N: 0.010-0.020%, [O] ≤15ppm, [H] ≤2ppm, with the remainder being Fe and unavoidable impurities; The preparation method of the bainitic non-quenched and tempered steel includes the following steps: Step 1: Electric furnace smelting: tapping temperature 1640~1680℃. Before tapping, aluminum blocks are added. During the tapping process, alloying is carried out by adding metallic manganese and ferrosilicon. Step 2: LF refining: LF is brought in with Al controlled at 0.045-0.060 wt%, slag is adjusted, zirconium iron is added, the composition is adjusted, argon is blown and stirred, and aluminum wire is fed in; Step 3: VD Vacuum: After breaking the vacuum, first feed in manganese nitride wire, then feed in sulfur wire; Step 4: Continuous casting; Step 5: Rolling: Cold billet enters the furnace, initial rolling temperature 1010~1060℃, final rolling temperature 900~950℃; Step 6: Water Tank Cooling: The roller conveyor insulation cover is opened for cooling. After water cooling and reheating, the temperature on the cooling bed is 580-630℃. A six-water-tank cooling system is used, with the following temperatures exiting each tank: 850-900℃ from tank #1, 790-850℃ from tank #2, 730-780℃ from tank #3, 680-720℃ from tank #4, 600-670℃ from tank #5, and 530-600℃ from tank #6. The length of each water tank is 5-8 meters. The rolling speed is 1.5-1.8 m / s. Step 7: Cooling the cooling bed: Open the insulation cover of the cooling bed, and the temperature of the cooling bed of the lower multiple length of steel should be 380-430℃; Step 8: Slow cooling in the pit: After the steel is collected, it is placed in the pit for slow cooling; the temperature in the pit is 270~330℃, the slow cooling time is ≥48h, and the temperature when it comes out of the pit is ≤100℃.

2. The bainitic non-quenched and tempered steel according to claim 1, characterized in that, The composition of the bainitic non-quenched and tempered steel, by mass percentage of chemical composition, includes: C: 0.26-0.28%, Si: 0.25-0.40%, Mn: 1.85-1.95%, Cr: 0.50-0.60%, V: 0.05-0.07%, P: ≤0.015%, S: 0.025-0.040%, Ti: 0.030-0.045%, Zr: 0.025-0.035%, Al: 0.015-0.040%, N: 0.013-0.017%, [O] ≤15ppm, [H] ≤2ppm, with the remainder being Fe and unavoidable impurities.

3. The bainitic non-quenched and tempered steel according to claim 1, characterized in that, In step 1, electric furnace smelting: electric furnace smelting is adopted, and the main raw materials are molten iron and scrap steel, with the proportion of molten iron ≥40%; the final C of the electric furnace is ≥0.10%, P ≤0.010%; before tapping, 1.3~1.6kg / t of aluminum blocks are added to the steel, and slag-forming material is added at (9~12)kg / t; during the tapping process, alloying is carried out by adding 19~21kg / t of metallic manganese and 3.5~4.5kg / t of ferrosilicon.

4. The bainitic non-quenched and tempered steel according to claim 1, characterized in that, In step 2, the slag composition is: Al2O3=28~36%, CaO=44~50%, SiO2=8~10%, MgO=7~9%, basicity controlled at 4~6, 0.8~1.2kg / t zirconium iron is added, and argon is blown and stirred at an argon flow rate of 600~1000NL / min; aluminum wire is fed in, and aluminum is controlled at 0.040~0.055%, and the refining and tapping temperature is 1655~1675℃.

5. The bainitic non-quenched and tempered steel according to claim 1, characterized in that, In step 3, before entering the VD furnace, slag is removed, the vacuum degree is less than 67 Pa, the holding time is ≥12 minutes, and the argon flow rate is 50-70 NL / min. After breaking the vacuum, the amount of manganese nitride wire fed is 3.0-5.0 m / t steel, the amount of sulfur wire fed is 1.0-1.5 m / t steel, the argon flow rate is 10-20 NL / min, the soft blowing argon time is not less than 15 minutes, and the normal furnace temperature is controlled at 1553-1578℃.

6. The bainitic non-quenched and tempered steel according to claim 1, characterized in that, In step 4, a double-layer tundish covering agent is used in continuous casting. The lower layer uses an alkaline tundish covering agent, and the upper layer uses a hollow granular tundish covering agent. The tundish superheat is 15-30℃. Electromagnetic stirring is used. The electromagnetic stirring current in the crystallizer is 200-300A, and the frequency is 3-4Hz. The electromagnetic stirring current at the end is 200-300A, and the frequency is 12-14Hz. The specific water content is 0.25-0.27 L / kg. The casting speed of a 260mm×300mm square billet is 0.5-0.55 m / min. The straightening temperature is ≥920℃, and the billet is slowly cooled to ground for ≥24 hours.

7. The bainitic non-quenched and tempered steel according to claim 1, characterized in that, In step 5, the cold billet is put into the furnace. During heating, the temperature of the high-temperature section is controlled at 1200-1250℃, the soaking temperature is 1180-1230℃, and the heating time is ≥4h.

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