Rare earth-boron composite microalloyed 490MPa-grade high-strength and high-toughness refractory steel and preparation method thereof

Abstract

The invention discloses rare earth-boron composite microalloyed 490MPa-grade high strength and toughness refractory steel and a preparation method thereof. The rare earth-boron composite microalloyed 490MPa-grade high strength and toughness refractory steel comprises the following chemical components in percentage by mass: 0.025%-0.05% of C, 0.1%-0.3% of Si, 0.5%-1.0% of Mn, less than or equal to 0.0010% of S, less than or equal to 0.008% of P, 0.8%-1.5% of Cr, 0.2%-0.6% of Mo, 0.02%-0.08% of V, 0.01%-0.03% of Ti, 0.01%-0.05% of Al, 0.003%-0.008% of N, 0.001%-0.003% of B, 0.01%-0.05% of RE and the balance of Fe and inevitable impurities. By optimizing alloy components and a multi-stage controlled rolling manufacturing method, the forming and welding capacity of the alloy is improved, grains are refined, the form size of inclusions is improved, and the toughness and the high-temperature strength are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of materials and metallurgy, and more particularly to a rare earth-boron composite microalloyed 490MPa grade high-strength and toughness refractory steel and a preparation method thereof. Background Art

[0002] Traditional structural steel, exposed to high temperatures, can cause building collapse due to a sharp drop in strength. Existing refractory steels often utilize a Cr-Mo alloy system, enhancing high-temperature strength through carbide strengthening. However, these steels suffer from several drawbacks: poor weldability: a high carbon content of 0.2% to 0.3% leads to embrittlement in the weld heat-affected zone; high cost: reliance on large amounts of precious metals such as Mo and Nb; complex processing: normalizing treatment is required for microstructure refinement, resulting in high energy consumption; and short refractory aging: strength retention at 600°C is less than two hours. Research both domestically and internationally has focused on optimizing low-carbon microalloying and controlled rolling processes, but has yet to overcome the technical bottleneck of achieving a synergistic improvement in strength, toughness, and fire resistance.

[0003] Comparing relevant patent documents, CN112779473A discloses a refractory steel with a refractory temperature of up to 600°C and its preparation method: C: 0.08-0.11%, Si: 0.6-1.2%, Mn: 0.1-0.35%, Ti: 0.01-0.025%, Mo: 0.01-0.02%, Cr: 0.15-0.3%, Nb: 0.02-0.04%, Al: 0.01-0.05%, and the remainder is Fe. During heat treatment, the refractory steel billet has a ferrite content of 50-65%, a pearlite content of 10-35%, and a granular bainite content of 15-30%. The steel is smelted in a converter, refined outside the furnace, heat treated, rolled, cooled, and tempered to obtain a refractory steel with a yield strength of ≥600 MPa. Although the alloy cost is relatively low, it requires a heat treatment process, which increases process cost and time, making it difficult for companies to promote large-scale production. While the AlN mentioned in the article does improve strength and corrosion resistance, the large amount of AlN precipitation may affect the cracking of the ingot.

[0004] CN109680215A discloses a fire-resistant steel and its preparation method, as does CN112011736A, which discloses a method for preparing a 460MPa-grade, seismic-resistant, corrosion-resistant, fire-resistant steel for construction containing rare earth elements. Both use rare earth elements to improve inclusion morphology, refine the size of granular bainite, and enhance seismic performance. They also reduce the precipitation rate of second phases in austenite and increase the precipitation rate and amount in ferrite, thereby improving high-temperature performance. However, these methods increase the C and Mn content to reduce alloying elements such as Mo and Cr, which can lead to difficulties in subsequent forming and welding processes. Meanwhile, patent document CN112011736A discloses a method for preparing a 460MPa-grade, seismic-resistant, corrosion-resistant, fire-resistant steel for construction containing rare earth elements. While the cooling rate is not specified, it is likely to be very fast based on the steel's composition and microstructure. The final rolling temperature mentioned in the patent is 950-969°C, which is cooled to room temperature, making large-scale production very challenging. Patent document CN111763881A discloses a high-strength, low-carbon bainitic refractory steel and its preparation method: C: 0.07%-0.1%, Si: 0.7%-0.9%, Mn: 1.0%-1.5%, Cr: 0.7%-0.8%, Ni: 1.0%-1.3%, Cu: 0.3%-0.35%, Mo: 0.6%-0.8%, Nb: 0.025%-0.035%, V: 0.09%-0.15%, Ti: 0.01%-0.015%, (Nb+V+Ti)<0.2%, Alt<0.02%, S<0.003%, P<0.008%, with the remainder being iron and unavoidable impurities. The steel is air-cooled or laminar-cooled to below 370°C after continuous casting or rolling. The production process window is narrow, and temperature control is difficult in actual production. Summary of the Invention

[0005] The purpose of the present invention is to overcome the above-mentioned defects of the prior art and provide a rare earth-boron composite microalloyed 490MPa grade high-strength and toughness refractory steel and its preparation method. By optimizing the alloy composition and using a multi-stage controlled rolling manufacturing method, the traditional Cr-Mo series is mainly used to significantly reduce the C and Mn contents, thereby improving its forming and welding capabilities. By adjusting the process window through the addition of Mo and B elements, and synergistically acting with rare earth (RE) elements, the toughness and high-temperature strength of the steel are improved by refining the grains and improving the morphology and size of inclusions.

[0006] To achieve the above object, the technical solution of the present invention is as follows:

[0007] Disclosed is a rare earth-boron composite microalloyed 490MPa grade high-strength and toughness fire-resistant steel. The chemical composition and mass percentage of the high-strength and toughness fire-resistant steel are as follows: C: 0.025%-0.05%, Si: 0.1%-0.3%, Mn: 0.5%-1.0%, S≤0.0010%, P≤0.008%, Cr: 0.8%-1.5%, Mo: 0.2%-0.6%, V: 0.02%-0.08%, Ti: 0.01%-0.03%, Al: 0.01%-0.05%, N: 0.003%-0.008%, B: 0.001%-0.003%, RE: 0.01%-0.05%, and the remainder is Fe and unavoidable impurities.

[0008] The present invention also discloses a method for preparing the rare earth-boron composite microalloyed 490 MPa grade high-strength and toughness refractory steel, comprising the following steps:

[0009] (1) weighing the raw materials of each component according to the formula ratio, and then smelting to obtain molten steel; the smelting steps include converter smelting, LF refining, and RH refining;

[0010] (2) Casting the molten steel into a continuous casting billet, heating, hot rolling, cooling and coiling the continuous casting billet to obtain high-strength and toughness fire-resistant steel; wherein the hot rolling includes rough rolling and finish rolling, the rough rolling is carried out in the austenite recrystallization zone, and the finish rolling is carried out in the austenite non-recrystallization zone.

[0011] The implementation of the present invention will have the following beneficial effects:

[0012] (1) The composition design of the present invention adopts a low C design, combined with appropriate amounts of Cr and Mo, which not only ensures high-temperature strength and toughness, but also improves welding performance.

[0013] (2) In the composition design of the present invention, alloy elements such as Mo and Cr, and composite carbides formed by chromium and molybdenum (such as Cr-Mo carbides) are more stable at high temperatures and can further improve the high-temperature performance of steel.

[0014] (3) The composition design of the present invention allows the addition of rare earth elements to react with oxygen, sulfur, phosphorus, hydrogen and other elements in the molten steel, thereby effectively purifying the molten steel. Rare earth elements refine the grain size and shape of inclusions in the steel, significantly improving the toughness and high-temperature strength of the steel.

[0015] (4) In the composition design of the present invention, the addition of V element inhibits the transformation of austenite to ferrite and pearlite, and promotes the transformation of bainite. At the same time, V can combine with C and N elements to form a compound with a higher melting point, which is dissolved in the matrix and improves the high-temperature performance of the steel.

[0016] (5) The present invention adopts a layer cooling process to control the cooling rate between 10℃ / s and 20℃ / s, cools to 600℃~650℃, and then air cools to 530℃~580℃ for coiling, thereby adjusting the structure to a large amount of B + a small amount of M, thereby improving the corrosion resistance of the structure.

[0017] (6) The high-strength and toughness fire-resistant steel produced by the present invention has a yield strength of ≥490 MPa at room temperature, a tensile strength of ≥600 MPa, a yield strength ratio ≤0.85, and an elongation ≥20%; an impact energy of ≥100 J at -40°C; a yield strength at 600°C not less than 2 / 3 of the minimum yield strength specified at room temperature; and a relative corrosion rate ≤60%. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the microstructure diagram of the high-strength and toughness fire-resistant steel produced in Example 1 of the present invention. DETAILED DESCRIPTION

[0019] The present invention will be further described below with reference to specific examples, but the present invention is not limited thereto in any way.

[0020] 1. Chemical composition and mechanical properties

[0021] The invention discloses a rare earth-boron composite microalloyed 490MPa grade high-strength and toughness fire-resistant steel. The chemical composition and mass percentage of the high-strength and toughness fire-resistant steel are as follows: C: 0.025%-0.05%, Si: 0.1%-0.3%, Mn: 0.5%-1.0%, S≤0.0010%, P≤0.008%, Cr: 0.8%-1.5%, Mo: 0.2%-0.6%, V: 0.02%-0.08%, Ti: 0.01%-0.03%, Al: 0.01%-0.05%, N: 0.003%-0.008%, B: 0.001%-0.003%, RE: 0.01%-0.05%, and the remainder is Fe and unavoidable impurities.

[0022] Specifically, the mechanism of action of each alloy component in the steel of the present invention is as follows:

[0023] Rare earth elements (RE): Rare earth elements react with oxygen, sulfur, phosphorus, and other elements in molten steel, effectively purifying the steel. They refine the steel's grain size and improve the shape and size of inclusions, significantly enhancing the steel's toughness and high-temperature strength. Therefore, the RE content is controlled between 0.01% and 0.05%.

[0024] Cr: Chromium dissolves in ferrite or austenite, increasing the high-temperature strength of the matrix. It also forms carbides with carbon, providing stability at high temperatures and hindering dislocation movement, thereby improving the creep resistance of the steel. Chromium also improves the corrosion resistance of steel and reduces damage from high-temperature corrosion. Therefore, the Cr content is controlled between 0.8% and 1.5%.

[0025] Mo: Molybdenum inhibits the coarsening and aggregation of carbides, maintaining the structural stability of steel at high temperatures. It also improves the steel's tempering resistance, allowing it to maintain high hardness and strength at high temperatures. Therefore, the Mo content is controlled at 0.2% to 0.6%.

[0026] Boron: At high temperatures, it can form stable borides that pin dislocations and grain boundaries, hindering dislocation movement, thereby improving the high-temperature strength and creep resistance of steel. Therefore, the B content is controlled at 0.001% to 0.003%.

[0027] In a specific embodiment, the yield strength of the high-strength and toughness refractory steel at room temperature is ≥490MPa, the tensile strength is ≥600MPa, the yield strength ratio is ≤0.85, and the elongation is ≥20%; the impact energy at -40°C is ≥100J; the yield strength at 600°C is not less than 2 / 3 of the minimum yield strength specified at room temperature; and the relative corrosion rate is ≤60%.

[0028] 2. Production Technology

[0029] The present invention also discloses a method for preparing the rare earth-boron composite microalloyed 490 MPa grade high-strength and toughness refractory steel, comprising the following steps:

[0030] (1) Weighing each component raw material according to the formula ratio, and then smelting to obtain molten steel; the smelting steps include converter smelting, LF refining, and RH refining.

[0031] (2) Casting the molten steel into a continuous casting billet, heating, hot rolling, cooling and coiling the continuous casting billet to obtain high-strength and toughness refractory steel; wherein the hot rolling includes rough rolling and finish rolling, the rough rolling is carried out in the austenite recrystallization zone, and the finish rolling is carried out in the austenite non-recrystallization zone.

[0032] In a specific embodiment, in step (2), the starting rolling temperature of the rough rolling is 1150° C. to 1180° C., the finishing rolling temperature is 1050° C. to 1080° C., and the cumulative reduction is ≥70%.

[0033] In a specific embodiment, in step (2), the start rolling temperature of the finishing rolling is 950° C. to 980° C., the final rolling temperature is 850° C. to 880° C., and the cumulative reduction is ≥70%.

[0034] In a specific embodiment, in step (2), laminar water cooling is used for cooling, the cooling rate is 10°C / s to 20°C / s, the final cooling temperature is 600°C to 650°C, and then air cooling is performed to 530°C to 580°C for coiling to ensure structural uniformity and performance stability.

[0035] In a specific embodiment, in step (1), the converter smelting, LF refining, and RH refining steps of the present invention can be performed according to conventional preparation methods.

[0036] In a specific embodiment, in step (1), the molten steel includes the following chemical components in mass fraction: C: 0.025% to 0.05%, Si: 0.1% to 0.3%, Mn: 0.5% to 1.0%, S≤0.0010%, P≤0.008%, Cr: 0.8% to 1.5%, Mo: 0.2% to 0.6%, V: 0.04% to 0.1%, Ti: 0.01% to 0.03%, Al: 0.01% to 0.05%, N: 0.003% to 0.008%, B: 0.001% to 0.003%, RE: 0.01% to 0.05%, and the rest is Fe and unavoidable impurities.

[0037] In a specific embodiment, in step (2), the heating temperature is 1200° C. to 1250° C. and the heating time is 3 h to 4 h to ensure that the alloy elements are fully dissolved.

[0038] The following are specific embodiments

[0039] Example 1

[0040] In this embodiment 1, the fire-resistant steel with a product specification of 12.0 mm produced by our company is used as an example to further illustrate the present invention.

[0041] The raw materials of each component are weighed according to the alloying element ratio in Table 1 (the balance is Fe and unavoidable impurities), and then smelted to obtain molten steel; the smelting steps include converter smelting, LF refining, and RH refining. The converter smelting, LF refining, and RH refining steps of this embodiment can be performed according to conventional preparation methods.

[0042] After the molten steel is cast into continuous casting slabs, they are slowly cooled in a slow cooling pit. After 36 hours, the slabs are placed in a heating furnace. The heating temperature is 1238°C for 221 minutes. Rough rolling is carried out in the austenite recrystallization zone, with a start temperature of 1168°C and a finish temperature of 1071°C, and a cumulative reduction of 78%. Finishing rolling is carried out in the austenite non-recrystallization zone, with a start temperature of 961°C and a finish temperature of 868°C, and a cumulative reduction of ≥70%. Cooling is done by laminar water flow at a cooling rate of 10°C / s to a final cooling temperature of 631°C. The slabs are then air-cooled to 540°C for coiling.

[0043] The performance indicators of the steel strip produced according to the steps of Example 1 are shown in Table 2.

[0044] Example 2

[0045] This embodiment 2 uses the fire-resistant steel with a product specification of 12.0 mm produced by our company as an example to further illustrate the present invention.

[0046] The raw materials of each component are weighed according to the alloying element ratio in Table 1 (the balance is Fe and unavoidable impurities), and then smelted to obtain molten steel; the smelting steps include converter smelting, LF refining, and RH refining. The converter smelting, LF refining, and RH refining steps of this embodiment can be performed according to conventional preparation methods.

[0047] After the molten steel is cast into continuous casting slabs, they are slowly cooled in a slow cooling pit. After 36 hours, the slabs are placed in a heating furnace. The heating temperature is 1235°C for 235 minutes. Rough rolling is carried out in the austenite recrystallization zone, with a start temperature of 1170°C and a finish temperature of 1065°C, and a cumulative reduction of 76%. Finishing rolling is carried out in the austenite non-recrystallization zone, with a start temperature of 971°C and a finish temperature of 858°C, with a cumulative reduction of ≥72%. Cooling is done by laminar water flow at a cooling rate of 15°C / s to a final cooling temperature of 604°C. The slabs are then air-cooled to 531°C for coiling.

[0048] The performance indicators of the steel strip produced according to the steps of Example 2 are shown in Table 2.

[0049] Example 3

[0050] This embodiment 3 uses the fire-resistant steel with a product specification of 12.0 mm produced by our company as an example to further illustrate the present invention.

[0051] The raw materials of each component are weighed according to the alloying element ratio in Table 1 (the balance is Fe and unavoidable impurities), and then smelted to obtain molten steel; the smelting steps include converter smelting, LF refining, and RH refining. The converter smelting, LF refining, and RH refining steps of this embodiment can be performed according to conventional preparation methods.

[0052] After the molten steel is cast into continuous casting slabs, they are slowly cooled in a slow cooling pit. After 36 hours, the slabs are placed in a heating furnace. The heating temperature is 1241°C for 232 minutes. Rough rolling is carried out in the austenite recrystallization zone, with a start temperature of 1179°C and a finish temperature of 1071°C, and a cumulative reduction of 75%. Finishing rolling is carried out in the austenite non-recrystallization zone, with a start temperature of 962°C and a finish temperature of 878°C, with a cumulative reduction of ≥74%. Cooling is done by laminar water flow at a cooling rate of 12°C / s to a final cooling temperature of 649°C. The slabs are then air-cooled to 562°C for coiling.

[0053] The performance indicators of the steel strip produced according to step 3 of this embodiment are shown in Table 2.

[0054] Table 1: % of ingredients in Examples 1-3

[0055]

[0056] Table 2 Mechanical properties of Examples 1-3

[0057]

[0058] The above-described embodiments merely represent several implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A rare earth-boron composite microalloyed 490MPa grade high strength and toughness refractory steel, characterized in that: The chemical composition and mass percentage of the high-strength and toughness fire-resistant steel are: C: 0.025% ~ 0.05%, Si: 0.1% ~ 0.3%, Mn: 0.5% ~ 1.0%, S ≤ 0.0010%, P ≤ 0.008%, Cr: 0.8% ~ 1.5%, Mo: 0.2% ~ 0.6%, V: 0.02% ~ 0.08%, Ti: 0.01% ~ 0.03%, Al: 0.01% ~ 0.05%, N: 0.003% ~ 0.008%, B: 0.001% ~ 0.003%, RE: 0.01% ~ 0.05%, the rest are Fe and unavoidable impurities.

2. The rare earth-boron composite microalloyed 490 MPa grade high strength and toughness refractory steel according to claim 1, characterized in that: The high-strength and toughness fire-resistant steel has a yield strength of ≥490MPa at room temperature, a tensile strength of ≥600MPa, a yield strength ratio of ≤0.85, and an elongation of ≥20%; an impact energy of ≥100J at -40°C; a yield strength at 600°C not less than 2 / 3 of the minimum yield strength specified at room temperature; and a relative corrosion rate of ≤60%.

3. A method for preparing the rare earth-boron composite microalloyed 490 MPa grade high strength and toughness refractory steel according to any one of claims 1 to 2, characterized in that: The following steps are involved: (1) weighing the raw materials of each component according to the formula ratio, and then smelting to obtain molten steel; the smelting steps include converter smelting, LF refining, and RH refining; (2) Casting the molten steel into a continuous casting billet, heating, hot rolling, cooling and coiling the continuous casting billet to obtain high-strength and toughness fire-resistant steel; wherein the hot rolling includes rough rolling and finish rolling, the rough rolling is carried out in the austenite recrystallization zone, and the finish rolling is carried out in the austenite non-recrystallization zone.

4. The preparation method according to claim 3, characterized in that In step (2), the starting rolling temperature of the rough rolling is 1150° C. to 1180° C., the finishing rolling temperature is 1050° C. to 1080° C., and the cumulative reduction is ≥70%.

5. The preparation method according to claim 3, characterized in that In step (2), the start rolling temperature of the finishing rolling is 950°C to 980°C, the final rolling temperature is 850°C to 880°C, and the cumulative reduction is ≥70%.

6. The preparation method according to claim 3, characterized in that In step (2), the cooling adopts laminar water cooling, the cooling rate is 10℃ / s~20℃ / s, the final cooling temperature is 600℃~650℃, and then air cooling is carried out to 530℃~580℃ for coiling.

7. The preparation method according to claim 3, characterized in that In step (1), the molten steel includes the following chemical components by mass fraction: C: 0.025% to 0.05%, Si: 0.1% to 0.3%, Mn: 0.5% to 1.0%, S≤0.0010%, P≤0.008%, Cr: 0.8% to 1.5%, Mo: 0.2% to 0.6%, V: 0.04% to 0.1%, Ti: 0.01% to 0.03%, Al: 0.01% to 0.05%, N: 0.003% to 0.008%, B: 0.001% to 0.003%, RE: 0.01% to 0.05%, and the rest is Fe and unavoidable impurities.

8. The preparation method according to claim 3, characterized in that In step (2), the heating temperature is 1200° C. to 1250° C., and the heating time is 3 h to 4 h.

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