Non-magnetic rare earth corrosion-resistant reinforcing steel

Abstract

This invention relates to the field of iron and steel smelting technology, and specifically provides a rare earth non-magnetic corrosion-resistant steel bar and its manufacturing method. The chemical composition of the rare earth non-magnetic corrosion-resistant steel bar includes: carbon (C) ≤0.20-0.55%, silicon (Si): 0.40%-0.60%, manganese (Mn): 18%-25%, phosphorus (P) ≤0.035%, sulfur (S) ≤0.020%, chromium (Cr): 3%-6%, copper (Cu): 0.10%-0.20%, rare earth elements (La+Ce): 0.013%-0.018%, with the balance being... The production process of rare earth corrosion-resistant steel bars, which contains iron (Fe) and unavoidable impurities, includes: converter steelmaking → LF refining → billet continuous casting. This invention significantly improves the corrosion resistance of steel bars by designing the composition of rare earth corrosion-resistant steel bars, optimizing the refining process, and adding continuous casting wire feeding process. This meets the modern building's demand for high quality and green environmental protection. The enhanced corrosion resistance not only improves the safety of building structures but also extends their service life. In addition, the addition of rare earth elements helps promote the rational development of my country's rare earth resources and drives the steel industry towards green and low-carbon development.

Description

Technical Field

[0001] This invention belongs to the technical field of hot-rolled steel bars for reinforced concrete and their production process, specifically relating to a high-quality non-magnetic steel bar and its production process. Background Technology

[0002] Reinforced concrete structures face the dual threats of chloride ion corrosion and low-temperature brittle fracture in harsh environments such as marine engineering, polar facilities, and liquefied natural gas (LNG) storage tanks. Traditional steel reinforcement has significant drawbacks in these conditions:

[0003] 1. Insufficient corrosion resistance: High-manganese austenitic non-magnetic steel bars (such as 40Mn18Cr3) have low chromium content (3-4%) and high carbon content (>0.4%), making the passivation film easily ruptured in a chloride ion environment, resulting in a higher pitting corrosion rate than ordinary carbon steel; Stainless steel bars (such as 316L) have excellent corrosion resistance, but the high nickel (12-14%) and molybdenum (2-3%) content leads to a cost increase of more than 300%, making it difficult to use on a large scale.

[0004] 2. Insufficient low-temperature toughness: The ductile-brittle transition temperature (DBTT) of 40Mn18Cr3 is approximately -60℃, and its impact energy at -40℃ is only 30-100J (depending on the process), which is insufficient for ultra-low temperature conditions below -60℃; carbides (Cr) are easily precipitated in the heat-affected zone after cold working or welding. 23 C6) further deteriorates low-temperature toughness, leading to the risk of brittle fracture.

[0005] Rare earth elements (REs, such as Ce, La, Y) can simultaneously enhance corrosion resistance and toughen at low temperatures in steel:

[0006] Corrosion resistance mechanism:

[0007] Inclusion modification: Rare earth combines with sulfur and oxygen to form spherical rare earth sulfur oxides (such as Ce2O2S), which replace the long strip-shaped MnS inclusions and reduce pitting corrosion initiation points.

[0008] Rust layer densification: Rare earth elements are enriched at the rust layer / matrix interface, forming a nanocrystalline CeO2 / La2O3 barrier layer, which inhibits C1 - Permeation (diffusion rate reduced by more than 50%);

[0009] Low-temperature toughening mechanism:

[0010] Grain boundary purification: Rare earth elements accumulate at grain boundaries, adsorbing harmful impurities (P, Sn) and eliminating the tendency for grain boundary embrittlement.

[0011] Microstructure refinement: Rare earth elements inhibit austenite grain growth, improve grain size by 1-2 grades (ASTM 8-9 grade), and increase grain boundary area, significantly hindering crack propagation;

[0012] Carbide control: Rare earth elements reduce carbon activity and inhibit Cr. 23 C6 precipitates at grain boundaries to prevent a sharp drop in low-temperature impact energy.

[0013] However, existing rare earth steel reinforcement technology still faces bottlenecks:

[0014] Conflict between non-magnetism and toughness: High-manganese non-magnetic steel requires the addition of >17% Mn to stabilize austenite, but high manganese leads to a reduction in stacking fault energy, which easily induces ε-martensite during cold deformation and impairs low-temperature toughness;

[0015] The rare earth addition process is unstable: the traditional addition method has a yield of less than 30%, and component segregation leads to performance fluctuations. Summary of the Invention

[0016] I. Technical Issues

[0017] This invention addresses three major defects of existing non-magnetic steel bars:

[0018] Poor corrosion resistance: High-manganese non-magnetic steel (such as 40Mn18Cr3) has a high pitting corrosion rate in chloride ion environment due to its low chromium (≤4%) and high carbon (>0.4%).

[0019] Insufficient low-temperature toughness: Impact energy at -60℃ ≤25J, toughness of the welded area drops sharply by more than 50%;

[0020] Excessive permeability during cold working: Martensite is induced after deformation, and the permeability μ>1.005.

[0021] II. Technical Solution

[0022] This patent proposes a rare-earth-chromium synergistic microalloyed non-magnetic steel bar, with the following core composition design and process:

[0023] 1. Chemical composition [mass percentage %]

[0024] C: 0.32-0.38, carbon control inhibits carbide precipitation and ensures low-temperature toughness.

[0025] Mn: 17.5-18.5, stable austenitic matrix, Mn / C ratio ≥46 [suppresses strain martensite].

[0026] Cr: 0.80-0.94, which works synergistically with rare earth elements to improve the stability of the passivation film and balance cost and corrosion resistance.

[0027] RE: 0.015-0.025, rare earth [Ce / La / Y mixed rare earth], purifies grain boundaries, refines grains, and modifies inclusions.

[0028] N: 0.08-0.12, stabilizes austenite, and replaces part of the cost of Ni.

[0029] S: ≤0.008, ultra-low sulfur reduces MnS inclusions, RE forms spherical Ce2O2S.

[0030] P: ≤0.015, low phosphorus avoids grain boundary embrittlement.

[0031] Balance: Fe and unavoidable impurities

[0032] Innovation Point 1: Through RE-Cr-N ternary synergy (0.015% RE + 0.9% Cr + 0.1% N), corrosion resistance and austenitic stability are simultaneously improved at low cost.

[0033] 2. Key Processes

[0034] Rare earth addition process:

[0035] Rare earth alloy wire (Ce:La = 2:1) is fed through a crystallizer using a wire feeding method, achieving a yield of ≥80% (compared to <30% in traditional processes). The wire feeding tube has a diameter of 22mm; the diameter of the arc at the front end of the wire feeding tube is 1200mm, and the arc length is 1134mm; the straight tube length at the rear end of the wire feeding tube is 760mm.

[0036] Controlled rolling and controlled cooling process:

[0037] Austenitic rolling: rough rolling at 1050-1100℃, final rolling temperature 850-880℃;

[0038] Ultra-rapid cooling: Cooling to 500-550℃ at ≥30℃ / s to suppress carbide precipitation;

[0039] Relaxation treatment: Hold at 550℃ for 20 minutes to enrich rare earth elements at the grain boundaries.

[0040] Solution treatment:

[0041] After rolling, the steel bars are water quenched at 1080±10℃ to obtain a uniform single-phase austenitic structure (grain size ≥ ASTM grade 8).

[0042] Innovation Point 2: Post-rolling relaxation + solution heat treatment promotes rare earth agglomeration at grain boundaries, improving low-temperature toughness by more than 20%.

[0043] III. Performance Indicators

[0044] performance This invention Comparative Example (40Mn18Cr3) Test Standards magnetic permeability μ ≤1.003 (50% cold deformation) 1.008-1.015 GB / T 3656-2022 -60℃ impact energy ≥45J 18-25J GB / T 229-2020 Yield strength 550-580MPa 450-500MPa GB / T 228.1-2021 pitting potential ≥0.35V.SCE 0.05-0.10V.SCE ASTM G61-86(2024)

[0045] IV. Beneficial Effects

[0046] 1. Breakthrough in low-temperature toughness: RE purification of grain boundaries and refinement of grains increase the impact energy at -60℃ by 80% [from 25J to 45J], meeting the requirements of ultra-low temperature conditions such as the -165℃ insulation layer support structure of LNG storage tanks;

[0047] 2. Increased corrosion resistance: The RE-modified rust layer, synergistic with 0.9% Cr, achieves a pitting potential of 0.35V SCE (250% higher than 40Mn18Cr3), while the corrosion rate of ordinary peripheral immersion decreases (according to YB / T4368-2014, for φ12 specimens, the corrosion rate of ordinary specimens is 6.5 g / (m²)). 2 ×h) Based on this, rare earth elements are added, and the corrosion rate is 3.0 g / (m 2 ×h).

[0048] 3. Cost Advantage: With N-type Ni and low Cr design, the cost is only 38% of that of 316L stainless steel, while the strength is increased by 15%. To improve the corrosion resistance of steel bars used in construction, this invention provides a high-quality corrosion-resistant steel bar and its production process.

[0049] 4. This invention utilizes rare earth wire wrapping protection technology to prevent low-melting-point active lanthanum and cerium elements from entering the slag layer, effectively avoiding flocculation and nodule formation at the continuous casting nozzle, forming a stable and efficient rare earth addition technology, and realizing smooth continuous casting production. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of the wire feeding tube;

[0051] Figure 2 The corrosion rate diagram of the immersion sample is shown.

[0052] Figure 3 This is a schematic diagram of an impact test specimen. Detailed Implementation

[0053] The technical solution of the present invention will be further described below with reference to the embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0054] Example 1

[0055] Production process flow

[0056] Converter steelmaking

[0057] Steel materials: 250t molten iron + high-quality scrap steel (total loading)

[0058] Bottom-blown argon: Flow rate 28 Nm 3 / h, pressure 1.2 bar

[0059] Endpoint control: C = 0.08%, P ≤ 0.018%, S ≤ 0.012%, tapping temperature 1650℃

[0060] Alloying: High-carbon ferromanganese (Mn 78%) is added to the target manganese content during steelmaking, and chromium nitride (Cr 60%, N 8%) is added to the target chromium and nitrogen content.

[0061] LF Refining

[0062] Refining time: 25 minutes

[0063] Argon gas stirring: flow rate 280 L / min (controlling the exposed diameter of molten steel ≈ 400 mm)

[0064] Temperature compensation: 1555℃ constant temperature

[0065] Rare earth recovery rate: 92%

[0066] Continuous casting of billets

[0067] Rare earth element addition: Ce-La alloy wire (Φ13mm, Ce:La = 2:1) is injected using a wire feeder at a speed of 2.0 m / min. Protective casting: long nozzle + argon sealing.

[0068] Superheat: 22℃

[0069] Casting speed: 2.5 m / min (170 mm × 170 mm square billet)

[0070] Crystallizer cooling water flow rate: 188m³ 3 / h

[0071] Controlled rolling and controlled cooling

[0072] 1. Billet heating: The heating temperature is 1100℃ and the duration is 2 hours.

[0073] 2. Rough rolling: The initial rolling temperature is 1080℃, and there are 4 passes.

[0074] 3. Finish rolling: The final rolling temperature is 870℃, consisting of 6 passes.

[0075] 4. Ultra-fast cooling: The water cooling rate is 35℃ / s, cooling to 520℃.

[0076] 5. Relaxation treatment: Performed at 520℃ for 25 minutes.

[0077] Solution treatment

[0078] Heating: 1080℃ x 45 minutes

[0079] Cooling: High-pressure water quenching to room temperature (cooling rate > 50℃ / s)

[0080] Example 2

[0081] Core process adjustment

[0082] LF Refining

[0083] Rare earth wire feeding speed: 1.8 m / min (yield 94%)

[0084] After refining, soft argon blowing: 15 minutes (flow rate 100L / min)

[0085] Controlled rolling and controlled cooling

[0086] Final rolling temperature: 860℃

[0087] Relaxation treatment: 540℃ × 20 minutes (to promote RE grain boundary segregation)

[0088] Solution treatment

[0089] Heating: 1075℃ × 50 minutes

[0090] Example 3

[0091] Core process adjustment

[0092] Continuous casting

[0093] Superheat: 18℃

[0094] Controlled rolling and controlled cooling

[0095] Cooling rate: 30℃ / s

[0096] Relaxation temperature: 500℃ × 30 minutes

[0097] Solution treatment

[0098] Heating: 1090℃ x 40 minutes

[0099] Comparative example (conventional 40Mn18Cr3 non-magnetic steel bar)

[0100] Process Flow

[0101] Converter endpoint: C = 0.10%, tapping temperature 1630℃

[0102] LF Refining: No rare earth elements added, refining time 20 minutes.

[0103] Continuous casting: superheat 30℃, casting speed 2.8m / min

[0104] Rolling process: 1050℃ final rolling → air cooling (without controlled cooling and solution treatment)

[0105] The smelting compositions of Examples 1, 2, 3 and the comparative examples are shown in the table.

[0106] Chemical composition (wt%)

[0107] Classification C Mn Cr Ce+La N S P Fe Example 1 0.35 18.0 0.85 0.020 0.10 0.005 0.012 margin Example 2 0.33 17.8 0.90 0.018 0.11 0.006 0.010 margin Example 3 0.38 18.5 0.80 0.025 0.08 0.008 0.015 margin Comparative Example 0.42 18.2 3.5 - 0.005 0.020 0.025 margin

[0108] Performance results:

[0109]

[0110]

[0111] Process Comparison

[0112] Key processes Embodiments of the present invention Comparative Example (40Mn18Cr3) Rare earth addition Silk feeding method (yield ≥ 90%) No additives Final rolling temperature 850-880℃ (austenitic region) 1050℃ (coarse-grained region) Post-rolling cooling ≥30℃ / s water cooling + relaxation treatment air cooling Solution treatment Water quenching at 1080℃ (single-phase austenitic) Untreated (carbide precipitation)

[0113] The rare earth corrosion-resistant non-magnetic steel bar of this invention achieves three major technological breakthroughs through synergistic innovation in "composition-process-structure":

[0114] Innovation in Rare Earth Microalloying Mechanism:

[0115] Adding 0.015-0.025% RE (Ce / La / Y) through the triple action of "inclusion spheroidization ← grain boundary purification ← carbide suppression" increases the impact energy at -60℃ to ≥45J (104% higher than traditional high-manganese non-magnetic steel), breaking through the bottleneck of low-temperature brittleness.

[0116] Corrosion resistance - non-magnetic synergistic design:

[0117] The innovative use of "0.8-0.94%Cr + 0.08-0.12%N" to replace the high-cost Ni-Mo alloy, combined with rare earth modified rust layer, results in a pitting potential of 0.35-0.38V.SCE (an increase of 337%), and the magnetic permeability μ≤1.003 after 50% cold deformation, meeting the stringent requirements of strong magnetic field environment;

[0118] Process-performance binding effect:

[0119] The process chain of rare earth addition by wire feeding (yield ≥90%) → controlled rolling relaxation treatment (520-550℃×20-30min) → solution water quenching (1080℃) ensures single-phase austenite (grain size ≥8) in the whole cross section, solving the stubborn problem of carbide precipitation in high manganese steel.

[0120] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A rare earth corrosion-resistant non-magnetic steel bar, characterized in that, The following components are included by weight percentage: C: 0.32-0.38%, Mn: 17.5-18.5%, Cr: 0.80-0.94%, RE: 0.015-0.025% [RE is a mixture of at least two rare earth elements from Ce, La, and Y], N: 0.08-0.12%, S≤0.008%, P≤0.015%, balance being Fe and unavoidable impurities; After controlled rolling, controlled cooling and solution treatment, the steel bars have a single-phase austenite microstructure with a magnetic permeability μ≤1.003 and a Charpy V-notch impact energy ≥45J at -60℃.

2. The rare earth corrosion-resistant non-magnetic steel bar according to claim 1, characterized in that: The mass ratio of Ce to La in the RE is (6-7):(3-4), and the Y content is ≤10% of the total RE.

3. The rare earth corrosion-resistant non-magnetic steel bar according to claim 1, characterized in that: The mass ratio of Mn / C is ≥46, and the mass ratio of N to Cr satisfies: N / Cr=0.10-0.

14.

4. The rare earth corrosion-resistant non-magnetic steel bar according to claim 1, characterized in that: The amount of RE added is 0.018-0.022%, and the grain size is ≥ ASTM grade 8.

5. The rare earth corrosion-resistant non-magnetic steel bar according to claim 1, characterized in that: The pitting potential measured in 3.5 wt% NaCl solution is ≥0.35 V·SCE.

6. The rare earth corrosion-resistant non-magnetic steel bar according to claim 1, characterized in that: Includes the following steps: (a) Smelting: Smelting is carried out in an electric arc furnace or converter. Ferromanganese and ferrochromium are added during the tapping process for alloying. The molten steel is then refined in an LF furnace. (b) Rare earth addition: After refining, rare earth alloy wire (Ce-La system) is fed into the crystallizer through a wire feeder at a feeding speed of 1.8-2.2 m / min, and the rare earth recovery rate is ≥80%. (c) Continuous casting: Protective casting, superheat control at 15-25℃, and slow cooling of the billet to room temperature; (d) Controlled rolling and controlled cooling: Heat the billet to 1080-1120℃ and hold for 1.5-2 hours; The initial rolling temperature for roughing is 1050-1080℃, and the final rolling temperature for finishing is 850-880℃. After rolling, the water is cooled to 500-550℃ at a cooling rate of ≥30℃ / s, followed by relaxation treatment for 20-30 minutes; (e) Solution treatment: Heat the steel bars to 1080±10℃ and hold for 40-60 minutes, then quench them in water to room temperature.

7. The rare earth corrosion-resistant non-magnetic steel bar according to claim 6, characterized in that: The relaxation treatment temperature is 520-550℃, during which the concentration of rare earth elements segregating towards the grain boundaries increases to 3-5 times that of the matrix.

8. The rare earth corrosion-resistant non-magnetic steel bar according to claim 1, characterized in that: After 50% cold deformation, the magnetic permeability μ is still ≤1.003 and the yield strength is ≥650MPa.

9. The rare earth corrosion-resistant non-magnetic steel bar according to claim 1, characterized in that: Suitable for liquefied natural gas storage tanks, load-bearing components of offshore platforms, or foundations for medical equipment with strong magnetic fields, operating at temperatures ≥-196℃.

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