Production method of microalloy rare earth Ce high-strength steel

Abstract

The invention discloses a production method of microalloy rare earth Ce high-strength steel. The production method comprises the following steps: 1) a smelting process; (2) refining; 3) continuous casting and casting blank treatment; 4) cooling the casting blank; (5) rolling and heat treatment are conducted, specifically, rough rolling is conducted through a 3800 mm four-roller reversible rolling mill, the initial rolling temperature is 1050-1150 DEG C, and the reduction rate is larger than or equal to 25%; finish rolling: a 4100mm four-roller mill is adopted, the finish rolling temperature is 850-900 DEG C, and the thickness tolerance is + / -0.5 mm; cooling control is conducted, laminar flow accelerated cooling is adopted, the cooling rate is 15-25 DEG C / s, the final cooling temperature is 500-600 DEG C, and grain coarsening is inhibited; quenching: performing oil quenching to 150 DEG C or below, wherein the cooling rate is greater than or equal to 20 DEG C / s; and tempering: carrying out heat preservation at 620 DEG C for 30 minutes, and carrying out air cooling. The invention aims to prepare the high-strength steel which not only meets the strength requirement but also has good processability and economic benefits by accurately controlling the addition amount of the rare earth element Ce and production process parameters.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal material processing, and particularly relates to a production method of microalloyed rare earth Ce high-strength steel. Background Art

[0002] With the development of modern industry, the requirements for the mechanical properties of steel are getting higher and higher. Especially in the fields of construction, bridges, machinery, etc., the application of high-strength steel is becoming increasingly widespread. However, the traditional high-strength steel production process often has problems such as high cost, complex process, and limited performance improvement. Rare earth elements, with their unique physical and chemical properties, perform excellently in improving the microstructure of steel and enhancing strength and toughness. Among them, the rare earth element Ce has attracted much attention due to its rich resources and relatively low price. Summary of the Invention

[0003] The purpose of the present invention is to provide a production method of microalloyed rare earth Ce high-strength steel. By precisely controlling the addition amount of rare earth element Ce and the production process parameters, high-strength steel that meets both strength requirements and has good processing performance and economic benefits is prepared.

[0004] To solve the above technical problems, the present invention adopts the following technical solutions:

[0005] A production method of microalloyed rare earth Ce high-strength steel of the present invention includes:

[0006] 1). Smelting process:

[0007] A 210t top-bottom combined blowing converter is used, with the end-point carbon-oxygen product ≤ 0.003 and the temperature ≥ 1650 °C;

[0008] Before the hot metal is tapped, an additive of rare earth element Ce is added to the hot metal according to a predetermined ratio, and finally the concentration of Ce in the hot metal reaches 60 ppm. The Ce element is uniformly distributed in the hot metal by stirring;

[0009] 2). Refining:

[0010] The hot metal containing rare earth element Ce is sent to a refining furnace for further refining to remove impurities and gases and improve the purity of the molten steel;

[0011] LF furnace: Lime and fluorite are added to make slag, the calcium treatment desulfurization ≤ 0.005%, and the soft blowing time ≥ 15 min;

[0012] RH vacuum treatment: The vacuum degree ≤ 67 Pa, the treatment time ≥ 25 min, to remove inclusions and homogenize the composition;

[0013] Rare earth Ce addition: Rare earth is added in the LF or RH stage, and the rare earth addition amount is controlled to be 0.0015% - 0.0025% by mass to generate (CeCa)S inclusions to refine the grains;

[0014] 3) Continuous casting and billet processing:

[0015] For the continuous casting process, a vertical bending caster with dimensions of 300mm×2200mm is used. The superheat is controlled at 15 - 25°C, the casting speed is 0.8 - 1.2m / min, and the basicity of the mold powder is 1.2 - 1.5;

[0016] 4). Billet cooling, adopting the "intensive cooling - slow cooling" process: The specific water volume in the secondary cooling zone is 0.8 - 1.0L / kg, and the billet temperature when it exits the pit is ≥900°C;

[0017] 5). Rolling and heat treatment:

[0018] Rough rolling: A four - high reversible rolling mill with a roll diameter of 3800mm is used. The rolling start temperature is 1050 - 1150°C, and the reduction ratio is ≥25%;

[0019] Finish rolling: A four - high rolling mill with a roll diameter of 4100mm is used. The final rolling temperature is 850 - 900°C, and the thickness tolerance is ±0.5mm;

[0020] Cooling control: Adopt laminar flow accelerated cooling, with a cooling rate of 15 - 25°C / s, and the final cooling temperature is 500 - 600°C to inhibit grain coarsening;

[0021] Heat treatment:

[0022] Quenching: Oil quenched to below 150°C, with a cooling rate of ≥20°C / s;

[0023] Tempering: Hold at 620°C for 30min and air - cool.

[0024] Furthermore, the rare - earth additive is CeCl3·7H2O.

[0025] Furthermore, after quenching and tempering, the 00MPa - grade high - strength steel has a yield strength of ≥700MPa, a tensile strength of ≥750MPa, and an elongation of ≥12%; after adding 0.0018% Ce, the impact toughness of the base metal is significantly improved, and the room - temperature impact energy increases from 31.7J to 52.3J.

[0026] 4. The production method of the micro - alloyed rare - earth Ce high - strength steel according to claim 1, characterized in that after quenching + tempering, the microstructure is tempered sorbite.

[0027] Furthermore, the grain size is refined to 5 - 8μm.

[0028] Furthermore, the microstructure in the heat - affected zone of welding:

[0029] Coarse - grain zone: The increase in the austenite grain size of the Ce - containing steel is 23% less than that of the Ce - free steel.

[0030] Fine grain zone: Ce inhibits grain coarsening by pinning grain boundaries, promotes the nucleation of acicular ferrite, and reduces the proportion of brittle upper bainite.

[0031] Furthermore, after adding 0.0025% Ce, the S content in the steel is significantly reduced, the total amount of non-metallic inclusions is reduced by 30%, and the inclusion rating is ≤ 1.5.

[0032] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0033] 1. Improve the strength of steel: By adding trace rare earth element Ce, the grains of steel are refined and the microstructure is improved, thereby enhancing the strength and toughness of steel.

[0034] 2. Optimize process parameters: The present invention realizes the stable improvement of steel performance and reduces production costs by precisely controlling the addition amount of rare earth element Ce and production process parameters.

[0035] 3. Enhance processing performance: The addition of rare earth element Ce also helps to improve the processing performance of steel, such as improving weldability, machinability, etc., which is beneficial to the subsequent processing and use of steel.

[0036] 4. Significant economic benefits: The high-strength steel produced by the present invention not only has excellent performance but also relatively low production costs, with high market competitiveness and economic benefits. Specific embodiments

[0037] I. Selection of iron ore and beneficiation process

[0038] 1. Ore pretreatment

[0039] Use a jaw crusher (200 - 300 mm) for coarse crushing and a cone crusher (50 - 100 mm) for medium and fine crushing to ensure that the ore particle size is ≤ 50 mm.

[0040] Classify through a vibrating screen (10 - 30 mm) to separate qualified and returned ore.

[0041] 2. Beneficiation methods

[0042] Magnetic separation: Preferentially use strong magnetic separation (magnetic field intensity ≥ 1.2 T) to separate iron minerals from gangue, and the recovery rate can reach over 90%.

[0043] Flotation: For refractory siderite and limonite, use a combination of reagents of "depressant + collector" (such as starch + fatty acid), and the flotation recovery rate is increased to 85%.

[0044] Roasting - magnetic separation: For complex ores such as oolitic hematite, use a rotary kiln for low-temperature roasting (800 - 900 °C) to improve magnetism, and then use weak magnetic separation for purification.

[0045] 3. Requirements for concentrate quality

[0046] The iron concentrate grade is ≥65%, SiO2 ≤8%, S ≤0.03%, and P ≤0.05%.

[0047] II. Smelting and Rare Earth Alloying Process

[0048] 1. Converter Smelting

[0049] A 210t top and bottom combined blown converter is used, with the end point carbon-oxygen product ≤0.003 and the temperature ≥1650°C.

[0050] 2. Refining Treatment

[0051] LF furnace: Adding lime and fluorite to make slag, desulfurizing by calcium treatment (≤0.005%), and the soft blowing time ≥15min.

[0052] RH vacuum treatment: The vacuum degree ≤67Pa, the treatment time ≥25min, removing inclusions and homogenizing the composition.

[0053] 3. Addition of Rare Earth Ce

[0054] Adding CeCl3·7H2O in the LF or RH stage, and the addition amount is controlled at 0.0015%-0.0025% (mass fraction) to generate (CeCa)S inclusions to refine the grains.

[0055] III. Continuous Casting and Slab Treatment

[0056] 1. Continuous Casting Process

[0057] A 300mm×2200mm vertical bending caster is used, with the superheat controlled at 15-25°C, the casting speed 0.8-1.2m / min, and the basicity of the mold powder 1.2-1.5.

[0058] 2. Slab Cooling

[0059] Adopting the "strong cooling - slow cooling" process: The specific water volume in the secondary cooling zone is 0.8-1.0L / kg, and the slab out-of-pit temperature ≥900°C.

[0060] IV. Rolling Process

[0061] 1. Rolling Process

[0062] Rough rolling: A 3800mm four-high reversible rolling mill, the starting rolling temperature is 1050-1150°C, and the reduction ratio ≥25%.

[0063] Finish rolling: A 4100mm four-high rolling mill, the final rolling temperature is 850-900°C, and the thickness tolerance is ±0.5mm.

[0064] 2. Cooling Control

[0065] Laminar accelerated cooling (ACC) is adopted, with a cooling rate of 15 - 25 °C / s and a final cooling temperature of 500 - 600 °C, to inhibit grain coarsening.

[0066] V. Heat treatment process

[0067] 1. Quenching

[0068] Quench in oil to below 150 °C, with a cooling rate ≥ 20 °C / s, to obtain a martensite + bainite structure.

[0069] 2. Tempering

[0070] Insulate at 620 °C for 30 min and air cool. After tempering, the strength ≥ 700 MPa and the elongation ≥ 12%.

[0071] 3. Microstructure optimization

[0072] The Ce element inhibits the grain growth in the heat - affected zone (HAZ) of welding (the grain size increase amplitude ≤ 50%) by pinning grain boundaries, and at the same time delays the formation of upper bainite, improving toughness.

[0073] VI. Key quality control indicators

[0074] Process control parameter reference standards

[0075] The rare earth addition amount is 0.0015 - 0.0025%;

[0076] The central porosity of the continuous casting billet macrostructure is ≤ grade 1.5, and the central segregation is ≤ grade 2.0 (GB / T 222 - 2017);

[0077] The roll imprint of the rolled sheet shape is ≤ 3.0 mm, and the flatness is ≤ 5.0 mm / m (enterprise internal control standard);

[0078] The Charpy impact energy (-40 °C) of the welding performance is ≥ 100 J (GB / T 2650 - 2008).

[0079] Mechanical properties:

[0080] 1. Base metal strength

[0081] After quenching and tempering, the 700 MPa grade high - strength steel has a yield strength ≥ 700 MPa, a tensile strength ≥ 750 MPa, and an elongation ≥ 12%.

[0082] After adding 0.0018% Ce, the impact toughness of the base metal is significantly improved, and the room - temperature impact energy increases from 31.7 J to 52.3 J (a 65% increase).

[0083] 2. Heat - affected zone (HAZ) properties of welding

[0084] Impact toughness: At a heat input of 25 kJ / cm, the impact energy of the HAZ of the Ce-containing steel reaches 110 J, which is 30% higher than that of the steel without Ce (84.8 J); it remains 112 J at 50 kJ / cm, while that of the steel without Ce drops to 24.5 J.

[0085] Strength loss: The addition of Ce reduces the strength loss in the HAZ. For example, in a 800 MPa low-alloy steel, the strength of the CGHAZ of the Ce-containing steel is 15% higher than that of the steel without Ce.

[0086] Microstructure:

[0087] 1. Base metal microstructure

[0088] After quenching + tempering, the microstructure is tempered sorbite, and the grain size is refined to 5 - 8 μm.

[0089] 2. Weld heat-affected zone (HAZ) microstructure

[0090] Coarse grain zone (CGHAZ): The increase in the austenite grain size of the Ce-containing steel is 23% less than that of the steel without Ce (for example, when the heat input is 100 kJ / cm, the grain size of the Ce-containing steel increases by 52.4%, while that of the steel without Ce increases by 75.6%).

[0091] Fine grain zone: Ce inhibits grain coarsening by pinning grain boundaries, promotes the nucleation of acicular ferrite, and reduces the proportion of brittle upper bainite.

[0092] Inclusion characteristics:

[0093] 1. Inclusion modification

[0094] Ce converts the original MgO - Al2O3 spinel inclusions into (CeCa)S composite inclusions, and at the same time spheroidizes the MnS inclusions, with the size decreasing from 10 μm to 2 - 5 μm.

[0095] The inclusions are evenly and dispersedly distributed, without long strip or network structures, reducing the splitting effect on the matrix.

[0096] 2. Improvement in cleanliness

[0097] After adding 0.0025% Ce, the S content in the steel is significantly reduced, the total amount of non-metallic inclusions is reduced by 30%, and the inclusion rating ≤ 1.5.

[0098] Welding performance optimization:

[0099] 1. Grain control

[0100] Ce inhibits the abnormal growth of grains during welding by pinning the original austenite grain boundaries. For example, in an 800 MPa steel, the austenite grain size of the CGHAZ is 20% finer than that of the steel without Ce.

[0101] 2. Toughness improvement mechanism

[0102] Ce inclusions promote the formation of large-angle grain boundaries (the proportion increases by 15%-20%), improve the crack propagation energy. For example, in 700 MPa steel, the impact energy of the HAZ is increased by more than 40%.

[0103] Delay the formation of upper bainite, making the HAZ microstructure mainly martensite + lower bainite, with better toughness.

[0104] Through the inclusion modification and grain refinement of rare earth Ce, the process of the present invention realizes the synergistic improvement of the mechanical properties and welding properties of high-strength steel.

[0105] The above-described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for producing microalloyed rare earth Ce high-strength steel, characterized in that: include: 1). Smelting process: Use 210t top and bottom combined blowing converter, the final carbon oxygen product is ≤0.003, the temperature is ≥1650℃; Before the molten iron is discharged from the furnace, the additive of rare earth element Ce is added to the molten iron in a predetermined proportion, and the concentration of Ce in the molten iron finally reaches 60ppm. The Ce element is evenly distributed in the molten iron by stirring; 2) Refining: The molten iron containing the rare earth element Ce is sent to a refining furnace for further refining to remove impurities and gases and improve the purity of the molten steel; LF furnace: add lime and fluorite to make slag, calcium treatment desulfurization ≤ 0.005%, soft blowing time ≥ 15min; RH vacuum treatment: vacuum degree ≤67Pa, treatment time ≥25min, remove inclusions and make the composition uniform; Rare earth Ce addition: rare earth is added in the LF or RH stage, and the amount of rare earth added is controlled to be 0.0015%-0.0025% by mass, generating (CeCa)S inclusions to refine the grains; 3) Continuous casting and ingot processing: The continuous casting process uses a 300mm×2200mm vertical bending casting machine, with superheat controlled at 15-25℃, casting speed at 0.8-1.2m / min, and protective slag basicity at 1.2-1.5; 4). Billet cooling adopts "strong cooling-slow cooling" process: the water volume in the secondary cooling zone is 0.8-1.0L / kg, and the billet exit temperature is ≥900℃; 5). Rolling and heat treatment: Rough rolling: 3800mm four-roll reversible rolling mill, starting rolling temperature 1050-1150℃, reduction ratio ≥25%; Finishing rolling: 4100mm four-roll mill, final rolling temperature 850-900℃, thickness tolerance ±0.5mm; Cooling control: Use laminar accelerated cooling, cooling rate 15-25℃ / s, final cooling temperature 500-600℃ to inhibit grain coarsening; Heat Treatment: Quenching: Oil quenching to below 150°C, cooling rate ≥ 20°C / s; Tempering: 620℃ for 30min, air cooling.

2. The method for producing microalloyed rare earth Ce high-strength steel according to claim 1, characterized in that: The rare earth additive is CeCl3·7H2O.

3. The method for producing microalloyed rare earth Ce high-strength steel according to claim 1, characterized in that: After quenching and tempering, the yield strength of 00MPa grade high-strength steel is ≥700MPa, the tensile strength is ≥750MPa, and the elongation is ≥12%; after adding 0.0018% Ce, the impact toughness of the base material is significantly improved, and the impact energy at room temperature increases from 31.7J to 52.3J.

4. The method for producing microalloyed rare earth Ce high-strength steel according to claim 1, characterized in that: After quenching + tempering, the structure is tempered troostite.

5. The method for producing microalloyed rare earth Ce high-strength steel according to claim 4, characterized in that: The grain size is refined to 5-8μm.

6. The method for producing microalloyed rare earth Ce high-strength steel according to claim 1, characterized in that: Welding heat affected zone structure: Coarse grain area: The increase in austenite grain size of steel containing Ce is 23% less than that of steel without Ce. Fine grain area: Ce inhibits grain coarsening by pinning grain boundaries, promotes the nucleation of acicular ferrite, and reduces the proportion of brittle upper bainite.

7. The method for producing microalloyed rare earth Ce high-strength steel according to claim 1, characterized in that: After adding 0.0025% Ce, the S content in the steel is significantly reduced, the total amount of non-metallic inclusions is reduced by 30%, and the inclusion rating is ≤1.5.