Rare earth composite microalloyed 550MPa-grade extra-thick maritime work steel and preparation method thereof

Abstract

The invention discloses rare earth composite microalloyed 550MPa-grade extra-thick maritime work steel and a preparation method thereof. The steel comprises the following chemical components in percentage by weight: 0.06%-0.09% of C, 0.15%-0.30% of Si, 1.20%-1.50% of Mn, less than or equal to 0.012% of P, less than or equal to 0.002% of S, 0.50%-1.00% of Ni, 0.020%-0.050% of Alt, 0.20%-0.5% of Cr, 0.008%-0.020% of Ti, 0.10%-0.25% of Mo, 0.10%-0.30% of W, 0.03%-0.06% of V, 0.02%-0.05% of Nb and 0.0015%-0.0035% of rare earth Ce / La mixture RE. The method comprises the key process steps that RE-Fe alloy wires are fed under the low-oxygen condition at the later stage of RH vacuum treatment for rare earth denaturation treatment; electromagnetic stirring and dynamic soft reduction are adopted for continuous casting; in the rolling stage, the large reduction rate larger than or equal to 60% is implemented in a non-recrystallization area; and carrying out final quenching and tempering heat treatment. And the thickness specification of the produced super-thick steel plate is 120-180mm.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of advanced steel material manufacturing and heat treatment technology, and relates to a 550MPa grade extra-thick steel plate suitable for marine engineering structures and having excellent large heat input weldability and lamellar tearing resistance and a preparation method thereof. BACKGROUND

[0002] With the development of large-scale marine platforms and jacket structures, 120mm or more thick steel plates are required for key joint parts. These parts require a large amount of welding work, and the steel plate is required to be able to withstand a heat input of 50kJ / cm or more to improve construction efficiency. However, in order to ensure strength, the traditional 550MPa grade marine steel usually adopts a certain carbon equivalent design, which leads to grain coarsening in the heat-affected zone (HAZ) and a sharp decrease in toughness during large heat input welding. At the same time, the addition of the valuable alloy element Mo increases the cost. In addition, the traditional process produces a serious center segregation in the thick plate, and the sulfide inclusions are distributed in bands, resulting in poor Z-direction (thickness direction) performance and insufficient lamellar tearing resistance.

[0003] Chinese patent CN 202211610421.7 discloses "a low yield ratio ultra-low temperature toughness extra-thick marine steel plate and a manufacturing method thereof", which is produced by a twice-heating-rolling process for the billet, the second heating temperature is 1000-1150℃, and the heating time is 8-10 hours. The original structure of the billet is refined by the twice-rolling and low-temperature heating process to obtain good toughness.

[0004] CN 202310077258.0 discloses "a 550MPa grade extra-thick steel plate and a manufacturing method thereof", which is a high toughness and high strength steel plate manufacturing method applied to large hydroelectric generator units and large steel structures in high-cold regions, and does not involve weldability and lamellar tearing resistance. CN 202110131561.5 discloses "a production method of a 550MPa grade extra-thick anti-seismic weathering steel", which develops an anti-seismic weathering steel with a thickness of 180-230mm through water-cooled mold casting, heating, rolling, and one-step quenching + intercritical quenching + tempering heat treatment processes.

[0005] The above-mentioned patents mainly improve the strength and toughness and corrosion resistance through composition and process design, do not involve the weldability and lamellar tearing resistance of the steel plate, and have high alloy content, complex process and high production cost. SUMMARY

[0006] The present application aims to overcome the above-mentioned deficiencies of the prior art, and provides a 550MPa-grade extra-thick marine engineering steel with low cost and excellent large heat input welding adaptability, which has a maximum of 80kJ / cm and a lamellar tearing resistance, and a Z-direction section shrinkage rate of ≥65%; another object of the present application is to provide a preparation method of the above-mentioned 550MPa-grade extra-thick marine engineering steel, which has a stable production process and is easy to implement in industry.

[0007] The technical scheme of the present application: A rare earth composite micro-alloyed 550MPa-grade extra-thick marine steel, characterized in that the chemical composition of the steel contains C=0.06%-0.09%, Si=0.15%-0.30%, Mn=1.20%-1.50%, P≤0.012%, S≤0.002%, Ni=0.50%-1.00%, Alt=0.020%-0.050%, Cr=0.20%-0.5%, Ti=0.008%-0.020%, Mo=0.10%-0.25%, W=0.10%-0.30%, V=0.03%-0.06%, Nb=0.02%-0.05%, RE (rare earth Ce / La mixture) =0.0015%-0.0035%, the balance being Fe and inevitable impurities, and satisfying W / (Mo+0.5W) =0.35-0.63 and RE / S≥1.2; the production thickness specification is 120-180mm, the yield strength of the steel is ≥550MPa, the tensile strength is ≥620MPa, the elongation is ≥17%, the Charpy impact energy at-40℃ is ≥160J, the Z-direction section shrinkage rate is ≥65%, and the maximum welding heat input is 80kJ / cm.

[0008] Further, the original austenite grain size in the microstructure of the steel is ≥8 levels, and the thickness direction grain size difference is ≤1.5 levels.

[0009] A preparation method of a rare earth composite micro-alloyed 550MPa-grade extra-thick marine steel, the production process route of the steel is hot metal pretreatment-converter smelting-LF refining-RH vacuum treatment-rare earth treatment-continuous casting-slab heating-rolling-accelerated cooling-heat treatment-finishing-performance testing-flaw detection; the key process steps include: LF refining: a high basicity slag system (CaO / SiO2≥8) is used, white slag holding time is ≥30 minutes, and deep desulfurization is performed to [S]≤0.0015%.

[0010] (1) RH vacuum treatment: vacuum degree ≤0.5mbar, pure degassing time ≥18 minutes, [H]≤1.5ppm, [N]≤40ppm.

[0011] (2) Rare earth treatment: in the late RH treatment, when [O]≤10ppm, RE-Fe alloy wire is fed at a speed of 3-5 m / s, and the soft argon blowing time after feeding is ≥15 min.

[0012] (3) Continuous casting: the tundish superheat is 15-25 ℃, the section size is 450 mm×2050 mm, and electromagnetic stirring and dynamic soft reduction (total reduction 4-8 mm) are matched.

[0013] (4) Heat treatment: quenching at 900-920 ℃, the holding time is the steel plate thickness mm×(1.2-1.5) min / mm; tempering at 620-650 ℃, the holding time is the steel plate thickness mm×(2.0-2.5) min / mm; and water mist cooling after tempering.

[0014] A preparation method of a rare earth composite micro-alloyed 550 MPa grade super-thick marine steel, characterized in that step (4) RH vacuum treatment: after RH vacuum treatment, when [O]≤10ppm, RE-Fe alloy wire is fed at a speed of 3-5 m / s; the continuous casting superheat is 15-25 ℃, the dynamic soft reduction is 4-8 mm; the cumulative reduction rate in the unrecrystallization zone is ≥60%; and quenching at 900-920 ℃ + tempering at 620-650 ℃.

[0015] The innovation principle and advantages of the present application are as follows: 1) Innovative W-Mo economic alloy design: by introducing W element to partially replace the precious Mo, the more stable characteristics of W carbide are utilized, the alloy cost is significantly reduced while the strength and hardenability of the steel plate are ensured. The synergistic effect of W and Mo enhances the high-temperature tempering stability of the steel plate.

[0016] 2) Rare earth precise modification technology: by controlling the RE / S ratio, the harmful inclusions such as long strip-shaped and sharp corner-shaped MnS and Al2O3 are successfully converted into fine spherical rare earth oxysulfides such as RE2O2S, which completely eliminates the stress concentration source causing lamellar tearing, and greatly improves the Z-direction performance.

[0017] 3) Excellent welding adaptability: the combined effect of ultra-low sulfur content S≤0.002% and rare earth modification makes the spherical inclusions in the coarse grain zone of the HAZ become the nucleation core of acicular ferrite when the steel plate is subjected to large heat input welding, thereby refining the HAZ structure and ensuring the toughness of the welded joint. The steel plate of the present application can adapt to a welding heat input of up to 80 kJ / cm.

[0018] 4) Clean and homogenization control throughout the whole process: from hot metal pretreatment to dynamic soft reduction in continuous casting, the purity of molten steel and solidification structure are controlled throughout the whole process, ensuring the uniformity of the core and surface of the 120-180 mm super-thick plate, and the performance deviation is less than 15%.

[0019] The beneficial effects of the present application: through reasonable component design and inclusion modification treatment, match the appropriate production process, finally obtain the thick ocean engineering steel with good weldability and resistance to layered tearing performance mainly with tempered bainite. Thickness specification 120~180mm, yield strength ≥550MPa, tensile strength ≥620MPa, elongation ≥17%, -40℃ charpy impact energy ≥160J, Z direction section shrinkage rate ≥65%, the maximum welding heat input reaches 80kJ / cm. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 Typical inclusion diagram of the comparative steel.

[0021] Figure 2 Typical inclusion diagram of the rare earth composite microalloyed steel of the embodiment 1 of the present application. DETAILED DESCRIPTION

[0022] The content of the present application is further illustrated below in combination with the embodiments.

[0023] Embodiment 1: Production of 120mm thick steel plate The chemical composition of the steel is C=0.06%, Si=0.17%, Mn=1.22%, P=0.012%, S=0.0011%, Ni=0.51%, Alt=0.020%, Cr=0.21%, Ti=0.008%, Mo=0.11%, W=0.10%, V=0.03%, Nb=0.05%, RE (rare earth Ce / La mixture) =0.0015%, the balance being Fe and unavoidable impurities, W / (Mo+0.5W) =0.625, RE / S=1.36, in terms of weight percentage. The key process steps include: (1) LF refining: high basicity slag system (CaO / SiO2≥8) is used, white slag holding time is 32 minutes, deep desulfurization to [S]≤0.001%.

[0024] (2) RH vacuum treatment: vacuum degree ≤0.5mbar, pure degassing time 18 minutes, [H]=1.4ppm, [N]=40ppm.

[0025] (3) Rare earth treatment: at the end of RH treatment, when [O]=10ppm, RE-Fe alloy wire is fed at a speed of 3~5m / s, and soft blowing argon time is 16min after feeding the wire.

[0026] (4) Continuous casting: tundish superheat 25℃, cross section size 450mm×2050mm, combined with electromagnetic stirring and dynamic soft reduction (total reduction 4mm).

[0027] (5) Heat treatment: quenching at 920°C, holding time is steel plate thickness mm x 1.2 min / mm; tempering at 650°C, holding time is steel plate thickness mm x 2.0 min / mm; water mist cooling after tempering.

[0028] Example 2: Production of 150 mm thick steel plate The chemical composition of the steel is, in weight percent, C = 0.07%, Si = 0.25%, Mn = 1.35%, P = 0.010%, S = 0.0015%, Ni = 0.75%, Alt = 0.035%, Cr = 0.35%, Ti = 0.015%, Mo = 0.20%, W = 0.13%, V = 0.045%, Nb = 0.035%, RE (rare earth Ce / La mixture) = 0.0023%, the balance being Fe and unavoidable impurities, W / (Mo+0.5W) = 0.49, RE / S = 1.53. Key process steps include: (1) LF refining: high basicity slag system (CaO / SiO2≥8) is used, white slag holding time ≥ 35 minutes, deep desulphurization to [S]≤0.0013%.

[0029] (2) RH vacuum treatment: vacuum degree ≤ 0.5 mbar, pure degassing time 20 minutes, [H]=1.2 ppm, [N]=35 ppm.

[0030] (3) Rare earth treatment: at the end of RH treatment, when [O]=8 ppm, RE-Fe alloy wire is fed at a speed of 3~5 m / s, soft argon blowing time after wire feeding is 18 min.

[0031] (4) Continuous casting: tundish superheat 20°C, cross section size 450 mm x 2050 mm, with electromagnetic stirring and dynamic soft reduction (total reduction 6 mm).

[0032] (5) Heat treatment: quenching at 920°C, holding time is steel plate thickness mm x 1.2 min / mm; tempering at 650°C, holding time is steel plate thickness mm x 2.0 min / mm; water mist cooling after tempering.

[0033] Example 3: Production of 180 mm thick steel plate The chemical composition of the steel, by weight percentage, is: C=0.09%, Si=0.30%, Mn=1.50%, P=0.008%, S=0.0012%, Ni=1.00%, Alt=0.050%, Cr=0.5%, Ti=0.020%, Mo=0.25%, W=0.20%, V=0.06%, Nb=0.02%, RE (rare earth Ce / La mixture) =0.0035%, with the balance being Fe and unavoidable impurities. W / (Mo+0.5W) =0.57, RE / S =2.9. Key process steps include: (1) LF refining: High basicity slag system (CaO / SiO2≥8) is adopted, white slag is held for 38 minutes, and deep desulfurization is carried out to [S]=0.0013%.

[0034] (2) RH vacuum treatment: vacuum degree ≤ 0.5 mbar, pure degassing time 22 minutes, [H] = 1.0 ppm, [N] = 30 ppm.

[0035] (3) Rare earth treatment: In the later stage of RH treatment, when [O]=7ppm, RE-Fe alloy wire is fed in at a speed of 3~5m / s, and the soft blowing time of argon after feeding the wire is 20min.

[0036] (4) Continuous casting: The tundish is superheated at 15°C and has a cross-sectional size of 450mm×2050mm. It is combined with electromagnetic stirring and dynamic light reduction (total reduction of 8mm).

[0037] (5) Heat treatment: Quenching at 900℃, holding time is 1.5 min / mm of steel plate thickness (mm); Tempering at 620℃, holding time is 2.5 min / mm of steel plate thickness (mm); Water mist cooling after tempering.

[0038] Table 1 shows a comparison of the chemical composition and performance of each embodiment with that of the conventional process.

[0039] Table 1 Mechanical properties of different compositions .

Claims

1. A rare earth composite microalloyed 550MPa grade extra-thick marine steel, characterized in that: The chemical composition of the steel is as follows: C = 0.06%~0.09%, Si = 0.15%~0.30%, Mn = 1.20%~1.50%, P ≤ 0.012%, S ≤ 0.002%, Ni = 0.50%~1.00%, Alt = 0.020%~0.050%, Cr = 0.20%~0.5%, Ti = 0.008%~0.020%, Mo = 0.10%~0.25%, W = 0.10%~0.30%, V = 0.03%~0.06%, Nb = 0.02%~0.05%, RE (rare earth Ce / La mixture) = 0.0015%~0.0035%, with the balance being Fe and unavoidable impurities, satisfying W / (Mo+0.5W) = 0.35~0.63, RE / S≥1.2; production thickness specifications 120~180mm, steel yield strength ≥550MPa, tensile strength ≥620MPa, elongation ≥17%, Charpy impact energy at -40℃ ≥160J, Z-direction reduction of area ≥65%, maximum welding heat input is 80kJ / cm.

2. The rare earth composite microalloyed 550MPa grade extra-thick marine steel as described in claim 1, characterized in that: The microstructure of the steel has a pre-austenite grain size ≥ grade 8 and a grain size difference in the thickness direction ≤ grade 1.

5.

3. A method for preparing rare earth composite microalloyed 550MPa grade extra-thick marine engineering steel, wherein the steel production process route is: hot metal pretreatment - converter smelting - LF refining - RH vacuum treatment - rare earth treatment - continuous casting - slab heating - rolling - accelerated cooling - heat treatment - finishing - performance testing - flaw detection, characterized in that... Key process steps include: (1) LF refining: using a high basicity slag system with CaO / SiO2≥8, white slag retention time≥30 minutes, and deep desulfurization to [S]≤0.0015%; (2) RH vacuum treatment: vacuum degree ≤ 0.5 mbar, pure degassing time ≥ 18 minutes, [H] ≤ 1.5 ppm, [N] ≤ 40 ppm; (3) Rare earth treatment: In the later stage of RH treatment, when [O] ≤ 10 ppm, RE-Fe alloy wire is fed in at a speed of 3~5 m / s, and the soft blowing argon time after feeding the wire is ≥ 15 min; (4) Continuous casting: The tundish is superheated to 15~25℃, with a cross-sectional size of 450mm×2050mm. It is combined with electromagnetic stirring and dynamic light reduction, with a total reduction of 4~8mm. (5) Heat treatment: Quenching at 900~920℃, holding time is the thickness of the steel plate mm×(1.2~1.5)min / mm; Tempering at 620~650℃, holding time is the thickness of the steel plate mm×(2.0~2.5)min / mm; Water mist cooling after tempering.

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