High-toughness ultra-heavy steel plate and method for producing the same

Abstract

The present application relates to the technical field of metallurgy, and particularly relates to a high-toughness super-thick steel plate and a production method thereof, which comprises smelting, casting blank slow cooling, heating and rolling, normalizing heat treatment process, and the chemical component composition and mass percentage of the casting blank are as follows: C 0.10%-0.20%, Si 0.02%-0.20%, Mn 1.20%-1.60%, P≤0.010%, S≤0.005%, Alt 0.025-0.050%, Ti 0.05-0.10%, Ni 0.20%-1.50%, Ca 0.0010%-0.0025%, N≤0.005%, O: 0.002%-0.008%, H<2ppm, and the rest is Fe and inevitable impurities. The present application can produce the high-toughness super-thick steel plate with the thickness of 120-260mm, and the high-toughness super-thick steel plate has uniform structure and excellent low-temperature impact toughness.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a high-toughness extra-thick steel plate and its production method. Background Technology

[0002] With the development of the metallurgical industry, the requirements for the comprehensive performance of steel are becoming increasingly stringent. In particular, due to the expansion of application environments, many steel products need to operate for extended periods in low-temperature environments, demanding that the steel not only possess high strength, good formability and weldability, but also excellent low-temperature impact toughness. In steel product classification, extra-thick steel plates refer to steel plates with a thickness ≥ 60mm. Extra-thick steel plates can be used in industries such as construction, machinery, shipbuilding, bridges, boilers, and pressure vessels, and have broad application prospects. Therefore, developing extra-thick steel plates with excellent mechanical properties, especially good low-temperature toughness, is a key research direction in the metallurgical industry.

[0003] To improve the mechanical properties of steel, existing processes often employ the addition of Ti and Nb elements, combined with rolling processes to refine the internal grain structure of the steel, thereby enhancing its strength and toughness. For example, Chinese invention patent CN202211016212.X discloses a high-Ti-content thick steel plate with excellent comprehensive performance and its preparation method. This method utilizes Nb addition and a high-titanium composition design, employing rolling to refine the grain structure and thus improve the steel plate's toughness.

[0004] However, the above process is only applicable to steel plates with a thickness of less than 50mm that are suitable for controlled rolling. For extra-thick steel plates, especially those with a thickness of ≥120mm, the large thickness makes it difficult to achieve penetration during rolling production, and the uniformity of the microstructure along the thickness direction is hard to guarantee. The intention to directly refine the internal grains by adding Nb during rolling is difficult to achieve, and may even worsen the internal toughness of the extra-thick steel plate, thus reducing the performance qualification rate of the extra-thick steel plate.

[0005] Furthermore, Chinese invention patent CN202210531136.X discloses a low-alloy high-strength structural steel of Q420 and its production method. This method employs a composition design incorporating v (V) and a production process that allows v to combine with nitrogen (N) in the steel to form a VN alloy, thus solving the instability problem of nitrogen and achieving high strength in the steel plate. However, the method of adding v to form a VN alloy is costly, and the resulting steel lacks excellent low-temperature impact toughness, resulting in deficiencies in mechanical properties. Summary of the Invention

[0006] To address the problems of difficult rolling penetration, uneven microstructure in the thickness direction, and poor toughness in the production of extra-thick steel plates in existing technologies, this invention provides a high-toughness extra-thick steel plate and its production method. It adopts a composition design with added Ni and high Ti, and uses a production process of high-temperature rolling and normalizing to produce high-toughness extra-thick steel plates with a thickness of 120-260mm. The high-toughness extra-thick steel plate has a uniform microstructure and excellent low-temperature impact toughness.

[0007] In a first aspect, the present invention provides a method for producing high-toughness extra-thick steel plates, including smelting, slow cooling of billets, heating and rolling, and normalizing heat treatment processes. The chemical composition and mass percentage of the billet are as follows: C: 0.10%-0.20%, Si: 0.02%-0.20%, Mn: 1.20%-1.60%, P≤0.010%, S≤0.005%, Alt: 0.025%-0.050%, Ti: 0.05%-0.10%, Ni: 0.20%-1.50%, Ca: 0.0010%-0.0025%, N≤0.005%, O: 0.002%-0.008%, H<2ppm, with the remainder being Fe and unavoidable impurities.

[0008] Furthermore, the production steps for high-toughness extra-thick steel plates include:

[0009] (1) Smelting: Alloying the molten steel smelted in the converter, refining the alloyed molten steel with LF+RH, controlling the vacuum degree below 200Pa and the pure degassing time ≥10min; calcium treatment after RH degassing, and then soft blowing and continuous casting to obtain continuous casting billet;

[0010] (2) Slow cooling of the billet: The continuously cast billet is placed in a slow cooling pit or stacked for slow cooling;

[0011] (3) Billet heating and rolling: The continuously cast billet is heated and then rolled. The heating temperature is 1150-1220℃. The reduction of at least three rolling passes is >15%, and the total compression ratio is ≥2.5. The finishing rolling start temperature is controlled at 830-900℃ and the finishing rolling finish temperature is controlled at 830-880℃. After rolling, weak water cooling is performed. The starting cooling temperature is 800-860℃ and the final cooling temperature is 630-680℃. The cooling rate is controlled at 3-8℃ / s to obtain semi-finished steel plates.

[0012] (4) Normalizing heat treatment: The semi-finished steel plate is heated to 30-50°C above the Ac3 temperature (actual phase transformation temperature) for normalizing. The heating time is controlled according to the thickness of the semi-finished steel plate. The heating time is 1.3-1.8 min / mm. After normalizing, it is air-cooled to below 150°C to obtain a high-toughness extra-thick steel plate.

[0013] Furthermore, in step (1), the soft blowing time is ≥12 min; the continuous casting superheat is 10-25℃; and the continuous casting speed is 0.85-0.95 m / min.

[0014] Furthermore, in step (1), the center segregation of the low-magnification structure of the continuously cast billet is better than that of C1.0.

[0015] Furthermore, the slow cooling time in step (2) is ≥72h.

[0016] Furthermore, in step (3), a trolley or roller hearth furnace is used to heat the continuous casting billet. The heating time is calculated based on the thickness of the continuous casting billet, and the heating time is 0.9-1.4 min / mm.

[0017] Secondly, the present invention provides a high-toughness extra-thick steel plate produced by the above-described production method, wherein the thickness of the high-toughness extra-thick steel plate is 120-260 mm.

[0018] Furthermore, the high-toughness extra-thick steel plate has a yield strength ReH≥325MPa, tensile strength Rm≥470MPa, elongation≥21%, and longitudinal impact toughness KV2≥100J at -40℃.

[0019] The beneficial effects of this invention are as follows:

[0020] The high-toughness extra-thick steel plate production method provided by this invention adopts a Ni-added and high-Ti composition design, and employs high-temperature rolling and normalizing processes in the production process. Combined with parameter control at each stage of the production process, it can reduce the rolling load and waiting time of the rolling mill, avoiding the problems of insufficient thickness penetration and inability to refine the deep microstructure that occur with low-temperature controlled rolling of Nb alloyed thick steel plates, thus improving the microstructure uniformity of the high-toughness extra-thick steel plate. The resulting high-toughness extra-thick steel plate has a thickness of 120-260mm, a yield strength ReH≥325MPa, a tensile strength Rm≥470MPa, an elongation ≥21%, and a longitudinal impact toughness KV2≥100J at -40℃, exhibiting both high strength and excellent impact toughness at -40℃. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a metallographic image of the microstructure at 1 / 4 thickness of the high-toughness extra-thick steel plate in Example 1.

[0023] Figure 2This is a metallographic image of the microstructure at half the thickness of the high-toughness extra-thick steel plate in Example 1. Detailed Implementation

[0024] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0025] Example 1

[0026] A high-toughness, extra-thick steel plate, the chemical composition and mass percentage of the cast billet are shown in Table 1, with the balance being Fe and unavoidable impurities.

[0027] Table 1. Steel smelting composition (%) in Example 1

[0028]

[0029] The production steps include:

[0030] (1) Alloying the molten steel smelted in the converter, refining the alloyed molten steel with LF+RH, controlling the vacuum degree below 200Pa and the pure degassing time to be 12min; after RH degassing, calcium treatment is performed to make the calcium content in the tundish 12ppm, and then soft blowing and continuous casting are performed. The soft blowing time is 13min; the continuous casting superheat is 18℃, and the continuous casting speed is 0.90m / min to obtain the continuous casting billet; the continuous casting billet is 2600mm long, 2200mm wide and 600mm thick, and the center segregation of the low magnification structure of the continuous casting billet is better than C1.0;

[0031] (2) Place the continuously cast billet in a slow cooling pit for slow cooling for 72 hours;

[0032] (3) The continuous casting billet is heated in a roller hearth furnace at a temperature of 1185℃ for 10.5 hours and a tapping temperature of 1164℃. Then it is widened and rolled longitudinally. The reduction of the three longitudinal rolling passes is >15%, and the thickness of the intermediate billet is 240mm. The finishing rolling start temperature is controlled at 850℃ and the finishing rolling finish temperature is controlled at 850℃. After rolling, it is subjected to weak water cooling at a start cooling temperature of 840℃ and a finish cooling temperature of 640℃. The cooling rate is controlled at 5℃ / s to obtain a semi-finished steel plate with a thickness of 200mm and a total compression ratio of 3.

[0033] (4) The semi-finished steel plate is heated to 30°C above the Ac3 temperature for normalizing. The Ac3 temperature of the semi-finished steel plate in Example 1 is 850°C, the normalizing temperature is 880°C, and the normalizing time is 4.5h. After normalizing, it is quickly taken out of the furnace and hung on a cooling rack to be air-cooled to below 150°C to obtain a high-toughness extra-thick steel plate with a thickness of 200mm.

[0034] The metallographic image of the microstructure at 1 / 4 thickness of the 200mm high-toughness extra-thick steel plate prepared in Example 1 is shown below. Figure 1 The metallographic image of the microstructure at the 1 / 2 thickness position is shown below. Figure 2 As shown, the internal structure is uniform at the 1 / 4 and 1 / 2 thickness positions.

[0035] The 200mm high-toughness extra-thick steel plate prepared in Example 1 has a yield strength ReH of 357MPa, a tensile strength Rm of 490MPa, an elongation of 21%, and a longitudinal impact toughness KV2 of 134J at -40℃.

[0036] Example 2

[0037] A high-toughness, extra-thick steel plate, the chemical composition and mass percentage of the cast billet are shown in Table 2, with the balance being Fe and unavoidable impurities.

[0038] Table 2. Steel smelting composition (%) in Example 2

[0039]

[0040] The production steps include:

[0041] (1) Alloy the molten steel obtained by converter smelting, and refine the alloyed molten steel by LF+RH, controlling the vacuum degree below 200Pa and the pure degassing time to be 14min; after RH degassing, calcium treatment is performed to make the calcium content of the tundish 16ppm, and then soft blowing and continuous casting are performed, with a soft blowing time of 15min; the continuous casting superheat is 23℃, and the continuous casting speed is 0.91m / min to obtain a continuous casting billet; the continuous casting billet is 2600mm long, 1800mm wide, and 300mm thick, and the center segregation of the low magnification structure of the continuous casting billet is better than C1.0;

[0042] (2) The continuously cast billets are stacked and cooled slowly for 72 hours;

[0043] (3) The continuous casting billet is heated in a roller hearth furnace with a thickness of 300 mm, a heating temperature of 1170 ℃, a heating time of 4.5 h, and a tapping temperature of 1155 ℃. Then, it is rolled longitudinally with a reduction of >15% in three passes and a thickness of 170 mm in the intermediate billet. The finishing rolling temperature is controlled at 843 ℃ and the finishing rolling temperature is controlled at 840 ℃. After rolling, it is subjected to weak water cooling with a cooling temperature of 830 ℃ and a cooling temperature of 652 ℃. The cooling rate is controlled at 6 ℃ / s to obtain a semi-finished steel plate with a thickness of 120 mm and a total compression ratio of 2.5.

[0044] (4) The semi-finished steel plate is heated to 35°C above the Ac3 temperature for normalizing. The Ac3 temperature of the semi-finished steel plate in Example 2 is 840°C, the normalizing temperature is 875°C, and the normalizing time is 3h. After normalizing, it is quickly taken out of the furnace and hung on a cooling rack to be air-cooled to below 150°C to obtain a high-toughness extra-thick steel plate with a thickness of 120mm.

[0045] The 120mm high-toughness extra-thick steel plate prepared in Example 2 has a uniform internal structure, a yield strength ReH of 365MPa, a tensile strength Rm of 517MPa, an elongation of 23%, and a longitudinal impact toughness KV2 of 156J at -40℃.

[0046] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for producing high-toughness extra-thick steel plates, characterized in that, The process includes smelting, slow cooling of the billet, heating and rolling, and normalizing heat treatment. The chemical composition and mass percentage of the billet are as follows: C: 0.10%-0.20%, Si: 0.02%-0.20%, Mn: 1.20%-1.60%, P≤0.010%, S≤0.005%, Alt: 0.025%-0.050%, Ti: 0.05%-0.10%, Ni: 0.20%-1.50%, Ca: 0.0010%-0.0025%, N≤0.005%, O: 0.002%-0.008%, H<2ppm, with the remainder being Fe and unavoidable impurities. The production steps for high-toughness extra-thick steel plates include: (1) Alloy the molten steel smelted in the converter, refine the alloyed molten steel by LF+RH, control the vacuum degree below 200Pa and the pure degassing time ≥10min; after RH degassing, calcium treatment is performed, and then soft blowing and continuous casting are carried out to obtain continuous casting billet; Among them, the soft blowing time is ≥12min; the continuous casting superheat is 10-25℃; the continuous casting speed is 0.85-0.95m / min; and the center segregation of the low magnification structure of the continuous casting billet is better than C1.

0. (2) Place the continuously cast billet in a slow cooling pit for slow cooling or stack it for slow cooling, and the slow cooling time is ≥72h; (3) After heating the continuously cast billet, it is rolled at a heating temperature of 1150-1220℃. The reduction of at least three rolling passes is >15%, and the total compression ratio is ≥2.

5. The finishing rolling start temperature is controlled at 830-900℃ and the finishing rolling finish temperature is controlled at 830-880℃. After rolling, it is subjected to weak water cooling at a start cooling temperature of 800-860℃ and a finish cooling temperature of 630-680℃. The cooling rate is controlled at 3-8℃ / s to obtain a semi-finished steel plate. (4) Heat the semi-finished steel plate to 30-50°C above Ac3 temperature for normalizing. Control the heating time according to the thickness of the semi-finished steel plate. The heating time is 1.3-1.8 min / cm. After normalizing, air cool to below 150°C to obtain a high-toughness extra-thick steel plate with a thickness of 120-260 mm.

2. The production method as described in claim 1, characterized in that, In step (3), the continuous casting billet is heated using a trolley or roller hearth furnace. The heating time is calculated based on the thickness of the continuous casting billet, and the heating time is 9-14 min / cm.

3. A high-toughness, extra-thick steel plate produced using the production method described in any one of claims 1-2, characterized in that, The yield strength ReH of the high-toughness extra-thick steel plate is ≥325MPa, the tensile strength Rm is ≥470MPa, the elongation is ≥21%, and the longitudinal impact toughness KV2 at -40℃ is ≥100J.

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