Production method of low-cost high-toughness rare earth-containing 700MPa-grade high-strength steel

Abstract

The invention discloses a production method of low-cost high-toughness rare-earth-containing 700MPa-grade high-strength steel, which comprises the following smelting production process steps: a steelmaking process: blast furnace ironmaking, molten iron pretreatment, converter top and bottom combined blowing smelting, LF (ladle furnace) external refining, RH (relative humidity) refining and slab continuous casting; heating and rolling; main relevant parameters are limited; meanwhile, the specific components and content are limited. The invention aims to provide the production method of the low-cost high-toughness rare-earth-containing 700MPa-grade high-strength steel, and develops the 700MPa-grade high-strength steel which does not need precious metal, utilizes composite rare earth to cooperate with microalloying, has low cost and high strength and toughness, and has important industrial application value.

Description

Technical Field

[0001] This invention belongs to the field of research and development and production technology of low alloy high strength steel, and particularly relates to a low-cost, high-toughness, rare earth-containing 700MPa grade high strength steel production method. Background Technology

[0002] With the development of lightweight equipment and large-scale engineering, the demand for 700MPa high-strength steel continues to grow due to its ability to significantly reduce structural weight and improve load-bearing efficiency. However, existing technologies have three major drawbacks: First, some solutions rely on precious metals such as Ni and Mo to improve hardenability, resulting in high alloy costs; second, when adding a single rare earth element, the effect of inclusion modification is limited, and the strength-toughness match is poor; and third, the production process often requires additional tempering treatment, which is energy-intensive and inefficient.

[0003] The existing literature and patent research are as follows:

[0004] 1. The patent "CN202210711785.8 - A high-weldability 700MPa grade rare earth high-strength structural steel and its production method" contains precious metals such as Ni (0.15-0.20%) and Mo (0.2-0.3%). Although the welding performance is good, the alloy cost is high, and the use of a single Ce rare earth element leaves room for improvement in the inclusion refinement effect.

[0005] 2. According to the patent "CN201610953280.7-700MPa grade steel plate for engineering machinery and its preparation method", the 700MPa grade steel for engineering machinery is strengthened by adding Mo, but the tensile strength is less than 800MPa. Moreover, there is no rare earth modification design, and the adverse effect of inclusions on toughness is not improved.

[0006] 3. A search of the literature "The Influence of Rare Earth Cerium Treatment on the Microstructure and Properties of Low Alloy High Strength Steel" (by She Renjie, Zhang Bo, et al.) shows that the addition of Ce alone can refine inclusions, but the improvement in corrosion resistance is limited. Summary of the Invention

[0007] The purpose of this invention is to provide a low-cost, high-toughness, rare-earth-containing 700MPa-grade high-strength steel production method, and to develop a 700MPa-grade high-strength steel that does not require precious metals, utilizes composite rare earth synergistic microalloying, and balances low cost and high strength and toughness, which has important industrial application value.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0009] This invention discloses a low-cost, high-toughness, rare-earth-containing 700MPa grade high-strength steel production method, comprising:

[0010] 1) Chemical composition design and principles

[0011] Low-carbon design C: 0.06~0.08%;

[0012] Major element optimization: Mn 1.6-1.8%, to improve strength through solid solution strengthening; Si 0.15-0.23%, to improve deoxidation effect;

[0013] Composite microalloying: Nb 0.06~0.10%, refines austenite grains; V 0.025~0.055%, enhances strength through precipitation strengthening; Ti 0.07~0.09%, fixes N element, forms fine TiN particles, and inhibits grain growth;

[0014] Ce-La composite rare earth 0.0005~0.010%: Ce and La work synergistically to deeply desulfurize and deoxidize, transforming long strip-shaped MnS inclusions into spherical Ce-La-OS composite inclusions, reducing the size from 3.4μm to below 2.0μm; at the same time, it induces intracrystalline ferrite nucleation and refines the microstructure;

[0015] Strict impurity control: P≤0.010%, S≤0.005%, to reduce the adverse effects of impurities on toughness and improve the purity of molten steel;

[0016] 2) Smelting production process steps

[0017] Steelmaking process: Blast furnace ironmaking—hot metal pretreatment—converter top and bottom blowing smelting—LF ladle refining—RH refining—slab continuous casting;

[0018] 2.1) Smelting conditions

[0019] (1) The requirements for molten iron supplied for steelmaking shall meet the following table; molten iron temperature ≥ 1350℃;

[0020] Iron composition unit: %

[0021] element Si P S Element ≤0.60 ≤0.12 ≤0.050

[0022] (2) The molten iron must undergo KR desulfurization treatment, and the sulfur content of the molten iron entering the converter must be ≤0.005%;

[0023] (3) Converter

[0024] The target composition range for tapped steel is shown in the table below; the converter endpoint should be hit once or the number of additional blowing cycles should not exceed one, and the tapping temperature should be ≥1620℃;

[0025] Converter for LF refining target composition range (unit: %)

[0026]

[0027] (4) LF Refining

[0028] The refining temperature is ≥1560℃, and ferroniobium, ferrovanadium and ferrotitanium are added during the LF furnace refining process to adjust the composition.

[0029] (5) RH refining

[0030] ① Temperature measurement, sampling and fine-tuning of composition are carried out during RH vacuum treatment, and rare earth lanthanum iron and cerium iron alloy are added 5 minutes before the end;

[0031] ②After the RH vacuum treatment is completed, adjust the argon flow rate to put the molten steel in a soft blowing state, feed in Si-Ca wire for calcium treatment, and ensure that the soft blowing time is ≥10min after feeding the wire.

[0032] (6) Continuous casting

[0033] The main process parameters for continuous casting are shown in the table below, with a casting speed of 0.9–1.1 m / min;

[0034] Continuous casting process parameters

[0035]

[0036] 3) Heating and rolling process

[0037] (1) Slab heating

[0038] The slab thickness is 230mm, the heating time is greater than or equal to 240min, the soaking time is 40-60min, and the furnace exit temperature is controlled at 1220-1260℃.

[0039] (2) Rolling process

[0040] The rolling process adopts a two-stage controlled rolling. The roughing mode is a 3+3 mode, with a roughing rolling start temperature of 1140~1180℃, a finishing rolling temperature ≥1020℃, and a cumulative reduction rate of 68~85%. The finishing rolling start temperature is 860~910℃, the finishing rolling temperature is 850~870℃, the single-pass reduction rate is ≥10%, the cooling water flow rate between stands is ≥30%, and the roll gap lubrication flow rate is 100~300ml / min.

[0041] (3) Laminar flow cooling: Laminar flow cooling is performed immediately after rolling, and the final cooling coiling temperature is 540-580℃ to obtain a fine-grained ferrite + bainite multiphase structure.

[0042] (4) Leveling treatment: Leveling elongation rate of 0.5-0.8% to improve plate shape and surface uniformity, and surface roughness Ra controlled at 0.8-2.1μm.

[0043] Furthermore, its mechanical properties meet the following requirements: yield strength ≥700MPa, tensile strength 820~900MPa, elongation ≥18%, and low-temperature impact energy ≥120J at -20℃.

[0044] Furthermore, the chemical composition of the high-strength steel by mass percentage is as follows: C: 0.075%; Mn: 1.8%; Si: 0.22%; Nb: 0.08%; V: 0.04%; Ti: 0.09%; Ce+La composite rare earth: 0.0025%; P: 0.009%; S: 0.005%; with the remainder being Fe and unavoidable impurities.

[0045] Furthermore, the chemical composition of the high-strength steel by mass percentage is as follows: C: 0.065%; Mn: 1.7%; Si: 0.18%; Nb: 0.07%; V: 0.03%; Ti: 0.08%; Ce+La composite rare earth: 0.0023%; P: 0.010%; S: 0.002%; with the remainder being Fe and unavoidable impurities.

[0046] Furthermore, the chemical composition of the high-strength steel by mass percentage is as follows: C: 0.075%; Mn: 1.6%; Si: 0.14%; Nb: 0.06%; V: 0.03%; Ti: 0.07%; Ce+La composite rare earth: 0.0019%; P: 0.008%; S: 0.004%; with the remainder being Fe and unavoidable impurities.

[0047] Furthermore, the furnace exit temperature is 1253-1259℃.

[0048] Furthermore, the roughing rolling temperature is 1160-1179℃; the coiling temperature is 555-566℃.

[0049] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0050] The optimal combination of strength and toughness: yield strength ≥700MPa, tensile strength 820~900MPa, elongation ≥18%, and low-temperature impact energy at -20℃ ≥120J, which is 5~10% higher than the existing technology, and solves the problem of "strength improvement accompanied by toughness decrease" in high-strength steel.

[0051] Significant cost reduction: The addition of precious metals such as Ni and Mo is eliminated, and the amount of rare earth is controlled at (0.0005-0.010%), reducing the cost of alloy steel per ton by more than 80 yuan, and increasing the utilization rate of cast billets to more than 98%.

[0052] Simplified production process: No additional heat treatment process is required; it can be achieved using a conventional hot continuous rolling production line. The cooling rate control is less difficult than that of the ultra-fast cooling process, resulting in high production stability. Attached Figure Description

[0053] The present invention will be further described below with reference to the accompanying drawings.

[0054] Figure 1 The microstructure is shown in Example 1.

[0055] Figure 2 The tissue under a scanning electron microscope is shown in Example 1. Detailed Implementation

[0056] A method for producing low-cost, high-toughness, rare-earth-containing 700MPa grade high-strength steel:

[0057] ①Chemical composition

[0058] Chemical composition control is shown in Table 1.

[0059] Table 1 Actual Chemical Composition Control

[0060]

[0061]

[0062] ② Heating furnace process

[0063] The heating furnace process is shown in Table 2.

[0064] Table 2 Heating Process

[0065] Example Heating time / min Soaking time / min Furnace temperature / ℃ Example 1 245 45 1253 Example 2 255 47 1259 Example 3 256 49 1257

[0066] ③ Rolling process

[0067] Table 3 Rolling Process

[0068]

[0069] In addition, in Examples 1-3: the finishing mill has a total of 7 stands, with F1 reduction rate of 48%, F2 reduction rate of 42%, F3 reduction rate of 37%, F4 reduction rate of 30%, F5 reduction rate of 24%, F6 reduction rate of 17%, and F7 reduction rate of 10.5%. The flow rate of each mill is 35% (opening), and the lubrication of each mill roll gap is 200ml / min.

[0070] ④ Leveling machine

[0071] The elongation of the leveling machine and the surface roughness of the strip are shown in Table 4.

[0072] Table 4. Strip surface roughness and leveling machine elongation

[0073]

[0074]

[0075] ⑤ Product mechanical properties

[0076] The mechanical properties of the product are shown in Table 5.

[0077] Table 5 Product Mechanical Properties

[0078]

[0079] ⑥ Metallographic structure and scanned structure

[0080] Figure 1 The microstructure is shown in Example 1.

[0081] Figure 2 The tissue under a scanning electron microscope is shown in Example 1.

[0082] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for producing low-cost, high-toughness, rare-earth-containing 700MPa grade high-strength steel, characterized in that: include: 1) Design and principle of chemical composition based on mass percentage Low-carbon design C: 0.06~0.08%; Major element optimization: Mn 1.6-1.8%, to improve strength through solid solution strengthening; Si 0.15-0.23%, to improve deoxidation effect; Composite microalloying: Nb 0.06~0.10%, refines austenite grains; V 0.025~0.055%, enhances strength through precipitation strengthening; Ti 0.07~0.09%, fixes N element, forms fine TiN particles, and inhibits grain growth; Ce-La composite rare earth 0.0005~0.010%: Ce and La work synergistically to deeply desulfurize and deoxidize, transforming long strip-shaped MnS inclusions into spherical Ce-La-OS composite inclusions, reducing the size from 3.4μm to below 2.0μm; at the same time, it induces intracrystalline ferrite nucleation and refines the microstructure; Strict impurity control: P≤0.010%, S≤0.005%, to reduce the adverse effects of impurities on toughness and improve the purity of molten steel; 2) Smelting production process steps Steelmaking process: Blast furnace ironmaking—hot metal pretreatment—converter top and bottom blowing smelting—LF ladle refining—RH refining—slab continuous casting; 2.1) Smelting conditions (1) The requirements for molten iron supplied for steelmaking shall meet the following table; molten iron temperature ≥ 1350℃; Iron composition unit: % (2) The molten iron must undergo KR desulfurization treatment, and the sulfur content of the molten iron entering the converter must be ≤0.005%; (3) Converter The target composition range for tapped steel is shown in the table below; the converter endpoint should be hit once or the number of additional blowing cycles should not exceed one, and the tapping temperature should be ≥1620℃; Converter for LF refining target composition range (unit: %) (4) LF Refining The refining temperature is ≥1560℃, and ferroniobium, ferrovanadium and ferrotitanium are added during the LF furnace refining process to adjust the composition. (5) RH refining ① Temperature measurement, sampling and fine-tuning of composition are carried out during RH vacuum treatment, and rare earth lanthanum iron and cerium iron alloy are added 5 minutes before the end; ②After the RH vacuum treatment is completed, adjust the argon flow rate to put the molten steel in a soft blowing state, feed in Si-Ca wire for calcium treatment, and ensure that the soft blowing time is ≥10min after feeding the wire. (6) Continuous casting The main process parameters for continuous casting are shown in the table below, with a casting speed of 0.9–1.1 m / min; Continuous casting process parameters 3) Heating and rolling process (1) Slab heating The slab thickness is 230mm, the heating time is greater than or equal to 240min, the soaking time is 40-60min, and the furnace exit temperature is controlled at 1220-1260℃. (2) Rolling process The rolling process adopts a two-stage controlled rolling. The roughing mode is a 3+3 mode, with a roughing rolling start temperature of 1140~1180℃, a finishing rolling temperature ≥1020℃, and a cumulative reduction rate of 68~85%. The finishing rolling start temperature is 860~910℃, the finishing rolling temperature is 850~870℃, the single-pass reduction rate is ≥10%, the cooling water flow rate between stands is ≥30%, and the roll gap lubrication flow rate is 100~300ml / min. (3) Laminar flow cooling: Laminar flow cooling is performed immediately after rolling, and the final cooling coiling temperature is 540-580℃ to obtain a fine-grained ferrite + bainite multiphase structure. (4) Leveling treatment: Leveling elongation rate of 0.5-0.8% to improve plate shape and surface uniformity, and surface roughness Ra controlled at 0.8-2.1μm.

2. The method for producing low-cost, high-toughness, rare-earth-containing 700MPa grade high-strength steel according to claim 1, characterized in that: Its mechanical properties meet the following requirements: yield strength ≥700MPa, tensile strength 820~900MPa, elongation ≥18%, and low-temperature impact energy ≥120J at -20℃.

3. The method for producing low-cost, high-toughness, rare-earth-containing 700MPa grade high-strength steel according to claim 1, characterized in that: The chemical composition of the high-strength steel by mass percentage is as follows: C: 0.075%; Mn: 1.8%; Si: 0.22%; Nb: 0.08%; V: 0.04%; Ti: 0.09%; Ce+La composite rare earth: 0.0025%; P: 0.009%; S: 0.005%; with the remainder being Fe and unavoidable impurities.

4. The method for producing low-cost, high-toughness, rare-earth-containing 700MPa grade high-strength steel according to claim 1, characterized in that: The chemical composition of the high-strength steel by mass percentage is as follows: C: 0.065%; Mn: 1.7%; Si: 0.18%; Nb: 0.07%; V 0.03%; Ti 0.08%; Ce+La composite rare earth 0.0023%; P: 0.010%, S: 0.002%, the remainder being Fe and unavoidable impurities.

5. The method for producing low-cost, high-toughness, rare-earth-containing 700MPa grade high-strength steel according to claim 1, characterized in that: The chemical composition of the high-strength steel by mass percentage is as follows: C: 0.075%; Mn: 1.6%; Si: 0.14%; Nb: 0.06%; V 0.03%; Ti 0.07%; Ce+La composite rare earth 0.0019%; P: 0.008%, S: 0.004%, the remainder being Fe and unavoidable impurities.

6. The method for producing low-cost, high-toughness, rare-earth-containing 700MPa grade high-strength steel according to claim 1, characterized in that: The furnace exit temperature is 1253-1259℃.

7. The method for producing low-cost, high-toughness, rare-earth-containing 700MPa grade high-strength steel according to claim 1, characterized in that: Roughing temperature: 1160-1179℃; Coiling temperature: 555-566℃.

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