High-ductility high-strength steel plate eh32 and preparation method and application thereof

Abstract

The application discloses a high-ductility high-strength steel plate EH32 and a preparation method and application thereof, and is produced by adopting the steps of smelting, primary rolling, secondary rolling, cooling and heat treatment. The components and weight percentages are as follows: C 0.07%-0.09%, Si 0.25%-0.35%, Mn 0.80%-0.89%, P≤0.010%, S≤0.003%, Nb 0.02%-0.03%, Ti 0.008%-0.02%, Al 0.015%-0.05%, Cr 0.19%-0.29%, and the balance of Fe and inevitable impurity elements. The high-strength steel plate is applied to ship body manufacturing, has good strength and toughness, yield strength and tensile strength, an impact energy at-40 DEG C is greater than or equal to 200J, the elongation of the steel plate is greater than or equal to 40%, has excellent ductility and anti-collision performance, and can preferably meet the anti-collision requirements of military and civilian ships.

Description

Technical Field

[0001] This invention relates to the field of steel material preparation technology, and more specifically to a high-ductility, high-strength steel plate EH32, its preparation method, and its application. Background Technology

[0002] Most maritime accidents are caused by collisions, contact, or grounding. To ensure ship safety, some oil tankers adopt double hull structures (double hulls) and measures such as creating space for oil tankers on the side of the hull. However, this improved structure reduces the ship's propulsion efficiency and cargo capacity, which in turn increases the difficulty and cost of ship construction and transportation costs, making it unsuitable for widespread application. Therefore, using steel plates with improved performance to enhance the collision resistance of ships is currently a hot research topic.

[0003] Traditional medium-thick plate processing employs a two-stage rolling process for strengthening, resulting in high strength and toughness. However, the elongation limit decreases as strength increases. Therefore, it's difficult for high-strength ship plates to achieve an elongation exceeding 35%, making them highly susceptible to fracture during collisions. Current research on improving the collision resistance of steel plates focuses primarily on automotive steel, lacking relevant research on marine steel plates.

[0004] Therefore, how to provide a marine steel plate with high strength, high toughness and high ductility is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] In view of this, the present invention provides a high-strength steel plate EH32 with high ductility, its preparation method and application, with a strength level of 32 kg, a particularly high elongation of ≥40%, and high ductility.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: First, the present invention also provides a method for preparing the high-ductility, high-strength steel plate EH32 as described in the above technical solution, comprising the following steps: (1) Weigh the raw materials according to the weight percentage, smelt them, and cast them into continuous casting billets; (2) One-time rolling: The continuous casting billet is heated to 1200℃±50℃, the soaking time is 20~30min, and after it is taken out of the furnace, it is immediately cooled to Tnr±20℃, and then rolled back into a two-fire billet with a thickness of 150-200mm. After rolling, it is cooled again to below 200℃. (3) Secondary rolling: The second billet is rapidly heated to 900℃±30℃, and the heating time is 20~30min. After exiting the furnace, it is immediately rolled to the required thickness, and the final rolling temperature is controlled at Ar3~Ar3+30℃. (4) Cooling: After secondary rolling, water cooling at 600~660℃, and then stack cooling to below 200℃ after being removed from the line; (5) Heat treatment: After stacking and unstacking, tempering heat treatment is carried out, and after treatment, natural cooling is performed to obtain high ductility and high strength steel plate EH32.

[0007] Preferably, the high-ductility, high-strength steel plate EH32 comprises, by weight percentage: C 0.07%~0.09%, Si 0.25%~0.35%, Mn 0.80%~0.89%, P≤0.010%, S≤0.003%, Nb0.02%~0.03%, Ti 0.008%~0.02%, Al 0.015%~0.05%, Cr 0.19%~0.29%, with the balance being Fe and unavoidable impurity elements.

[0008] The chemical composition design focuses on the ductility of the steel plate. By designing a low-carbon and low-manganese composition and controlling the phosphorus and sulfur elements, segregation defects such as carbides and sulfides in the billet are reduced, ensuring high strength and good low-temperature toughness while maintaining high ductility.

[0009] Furthermore, the steel plate has a thickness of 20~50mm, a yield strength ≥315Mpa, a tensile strength of 450~570Mpa, an impact energy of -40℃ ≥200J, a tensile elongation after fracture ≥40%, a metallographic structure of ferrite + pearlite, a grain size of 10~12, and a banded structure ≤1.

[0010] Preferably, in step (1), the steel is smelted in a converter, refined in an LF furnace, and vacuum treated in a VD furnace before being cast into a continuous casting billet. The final [P] of the converter is ≤0.008%, and slag is blocked at the tapping point. At the end of the LF refining, a calcium treatment operation is performed with a pure calcium feed rate of 200-300m and a sulfur content ≤0.002% at the outlet. The [H] of the VD furnace is set to ≤2.0ppm before the outlet. The continuous casting process is protected, with a superheat of 5~20℃ and a center segregation of C class ≤1.0 grade.

[0011] The Tnr temperature mentioned in step (2) is the recrystallization temperature, specifically: Tnr=887+464×C+890×Ti+363×Al-357×Si+6445×(Nb×0.75)-644×[Sqrt(Nb×0.75)]+[732×V-230×(Sqrt(V)]; The Ar3 temperature mentioned in step (3) is the phase transformation initiation temperature during the cooling of the steel plate, specifically: Ar3={1670-558×[C+(Mn+Mo)÷3.875+Cu÷15.5+Cr÷20.67+Ni÷5.636]+16×[(H÷25.4)-0.315]-32}×5÷9; Wherein, the element symbol represents the weight percentage of the element, in units of %; and H represents the thickness of the finished steel plate, in units of mm.

[0012] Preferably, the cooling in step (2) is performed by Mulpic oscillating cooling. If the billet is relatively thick, it is cooled for more than 300 seconds using the oscillating mode of Mulpic to the steel plate to below 200°C, thus preserving the original refined austenitic grain structure.

[0013] Preferably, in step (2), the single-pass reduction during the rolling process of the two billets is ≥30mm.

[0014] Preferably, the heating rate in the rapid heating described in step (3) is 5~8 min / cm.

[0015] Preferably, the tempering temperature in step (5) is 600~660℃ and the tempering time is 2.5min / mm×plate thickness mm.

[0016] In the above-mentioned preparation process, the first rolling process, through sufficient heating, homogenizes the composition and microstructure of the billet, weakens the segregation in the center of the billet, and utilizes the rapid cooling mechanism of the ultra-fast cooling system to create a large temperature difference between the surface and the core of the billet. This ensures that the austenite grains in the non-recrystallized zone are flattened while eliminating defects such as porosity and segregation in the core. Rapid cooling then preserves the slender original austenite grain boundaries in the rolled state and prevents carbide diffusion to form cementite, maintaining a uniform distribution of composition. The second rolling process, through rapid low-temperature heating, refines the austenite grains and homogenizes the microstructure. Then, through low-temperature high-reduction rolling and controlled cooling, more nucleation sites are provided for ferrite nucleation, and the growth of ferrite grains is inhibited. Finally, the tempering process eliminates rolling stress and dislocation defects, weakens the influence of banded microstructure and segregation, and improves the ductility of the steel plate.

[0017] Furthermore, based on the above preparation method, the present invention also provides a high-ductility, high-strength steel plate EH32 prepared by the above technical solution.

[0018] This invention also provides the application of the high-ductility, high-strength steel plate EH32 described in the above technical solution and the high-ductility, high-strength steel plate EH32 prepared by the method, namely, its application in the shipbuilding process for hull manufacturing.

[0019] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a high-ductility, high-strength steel plate EH32, its preparation method, and its application, which has the following beneficial effects: The high-ductility steel plate provided by this invention and prepared by the method has an elongation at break of more than 20% higher than the specified value of conventional steel plate. When applied to shipbuilding, the steel plate has a higher resistance to hull fracture and cracking due to its excellent ductility, which can suppress the leakage of cargo and fuel, prevent marine pollution and ship sinking, and achieve safer and more reliable maritime transportation. Attached Figure Description

[0020] 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, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0021] Figure 1 Metallographic images of the steel plate from Example 1; Figure 2 The image shows the metallographic structure of the steel plate in Comparative Example 1. Detailed Implementation

[0022] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] In the preparation method of the following embodiments, the continuously cast billet is smelted in a 120t converter, refined in an LF furnace, and vacuum treated in a VD furnace. The final [P] of the converter is ≤0.008%, and slag is blocked at the tapping point. At the end of the LF refining, a calcium treatment operation is performed with a pure calcium wire feed rate of 200-300m and a sulfur content ≤0.002% at the outlet. The [H] of the VD billet is set to ≤2.0ppm before exiting the outlet. The continuous casting process is protected, with a superheat of 5~20℃ and a center segregation of C class ≤1.0 grade.

[0024] The chemical composition of the steel plates of Examples 1-3 and Comparative Example 1 is shown in Table 1. Except for the components in the table below, the balance is Fe and unavoidable impurities.

[0025] Table 1. Chemical composition (weight percentage) and Ceq (weight, %) of steel plates in the examples and comparative examples.

[0026] The specific preparation method is as follows: Example 1

[0027] The target steel plate has a thickness of 20 mm and a chemical composition as shown in Table 1, Example 1, with Tnr at 866℃ and Ar3 at 816℃. The preparation method is as follows: (1) The smelting was carried out in a converter and the billet was cast into a 300mm continuous casting billet; (2) One-time rolling: The continuous casting billet is heated to 1198℃ and the soaking time is 26min. After exiting the furnace, it is immediately cooled to 878℃ ​​by the Mulpic rapid cooling equipment. Then it is rolled in the finishing mill for 4 passes. The reduction of each pass should be ≥30mm. It is rolled into a 150mm two-fire billet. After rolling, it is cooled to below 200℃ by the Mulpic again. (3) Secondary rolling: The second billet is rapidly heated to 892°C, the heating time is 26 min, and it is rolled immediately after exiting the furnace, with a final rolling temperature of 829°C; (4) Cooling: After secondary rolling, water cooling to 655°C, and then offline stack cooling to below 200°C; (5) Heat treatment: After stacking and unstacking, tempering heat treatment is carried out at a temperature of 630℃ and a tempering time of 50min. After treatment, the plate is naturally cooled to obtain a 20mm thick high ductility and high strength steel plate EH32.

[0028] Example 2 The target steel plate has a thickness of 35 mm and a chemical composition as shown in Table 1, Example 2, with Tnr at 853 °C and Ar3 at 821 °C. The preparation method is as follows: (1) The smelting was carried out in a converter and the billet was cast into a 300mm continuous casting billet; (2) One-time rolling: The continuous casting billet is heated to 1202℃ and the soaking time is 28min. After it comes out of the furnace, it is immediately cooled to 860℃ by the Mulpic rapid cooling equipment. Then it is rolled in the finishing mill for 3 passes. The reduction of each pass should be ≥30mm. It is rolled into a 180mm two-fire billet. After rolling, it is cooled to below 200℃ by the Mulpic again. (3) Secondary rolling: The second billet is rapidly heated to 902°C, the heating time is 22 min, and it is rolled immediately after exiting the furnace, with a final rolling temperature of 836°C; (4) Cooling: After secondary rolling, water cooling to 634°C, and then offline stack cooling to below 200°C; (5) Heat treatment: After stacking and unstacking, tempering heat treatment is carried out at a temperature of 620℃ and a tempering time of 87.5min. After treatment, the plate is naturally cooled to obtain a 35mm thick high ductility and high strength steel plate EH32.

[0029] Example 3 The target steel plate has a thickness of 50 mm and a chemical composition as shown in Table 1, Example 3, with Tnr at 883℃ and Ar3 at 830℃. The preparation method is as follows: (1) The smelting was carried out in a converter and the billet was cast into a 350mm continuous casting billet; (2) One-time rolling: The continuous casting billet is heated to 1196℃ and the soaking time is 25min. After it comes out of the furnace, it is immediately cooled to 880℃ by the Mulpic rapid cooling equipment. Then it is rolled in the finishing mill for 4 passes. The reduction of each pass should be ≥30mm. It is rolled into a 200mm two-fire billet. After rolling, it is cooled to below 200℃ by the Mulpic again. (3) Secondary rolling: The second billet is rapidly heated to 905°C, the heating time is 28 minutes, and it is rolled immediately after exiting the furnace, with a final rolling temperature of 832°C; (4) Cooling: After secondary rolling, water cooling to 608°C, and then offline stack cooling to below 200°C; (5) Heat treatment: After stacking and unstacking, tempering heat treatment is carried out at a temperature of 600℃ and a tempering time of 125min. After treatment, the plate is naturally cooled to obtain a 50mm thick high ductility and high strength steel plate EH32.

[0030] Comparative Example 1 The target steel plate is 20mm thick and has the chemical composition shown in Table 1, Comparative Example 1. The preparation method is as follows: (1) The smelting was carried out in a converter and the billet was cast into a 300mm continuous casting billet; (2) Conventional two-stage rolling: The continuous casting billet is heated to 1190℃ and rolled directly after exiting the furnace. The roughing rolling temperature is 1018℃, the intermediate billet is 100mm, the finishing rolling temperature is 880℃, and the final rolling temperature is 824℃. (3) Cooling: After rolling, the steel plate is water-cooled to 652°C, then stacked and cooled to below 200°C before being unstacked and naturally cooled to obtain a 20mm thick steel plate.

[0031] Test case The steel plates prepared in Examples 1-3 and Comparative Example 1 were subjected to comprehensive mechanical property tests, and the test results are shown in Table 2.

[0032] Table 2. Comprehensive mechanical properties of the steel plates in the examples and comparative examples.

[0033] The above results show that the comprehensive mechanical properties of the steel plates of Examples 1, 2, and 3 of this invention meet the requirements, especially the elongation >40%, indicating high ductility. Metallographic observation of the steel plate of Example 1 revealed a microstructure composed of ferrite and pearlite, with a grain size of 10-12 and a banded structure of grade 0. Figure 1As shown. The elongation of the steel plate in Comparative Example 1 is less than 30%, indicating poor ductility. The composition of Comparative Example 1 is the same as that of Example 1 of this application. In terms of manufacturing process, Comparative Example 1 uses conventional two-stage rolling without two rolling and tempering heat treatments. Its elongation is only 28.5%. From a metallographic perspective, the banded structure is obvious, and hard phase structures exist in the core segregated areas, which significantly affect the ductility of the steel plate. Figure 2 As shown. Therefore, it is difficult to achieve the high ductility performance requirement of elongation ≥40% in industrial production using conventional composition design and rolling methods, thus fully demonstrating the ingenuity, comprehensiveness and uniqueness of the present invention in composition and process design.

[0034] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0035] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. 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 the invention. Therefore, the invention 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 disclosed herein.

Claims

1. A method for preparing high-ductility, high-strength steel plate EH32, characterized in that, Includes the following steps: (1) Weigh the raw materials according to the weight percentage, smelt them, and cast them into continuous casting billets; (2) One-time rolling: The continuous casting billet is heated to 1200℃±50℃, the soaking time is 20~30min, and after it is taken out of the furnace, it is immediately cooled to Tnr±20℃, and then returned to the rolling mill to be rolled into a second-fired billet with a thickness of 150-200mm. After rolling, it is cooled again to below 200℃. (3) Secondary rolling: The second billet is rapidly heated to 900℃±30℃, and the heating time is 20~30min. After exiting the furnace, it is immediately rolled to the required thickness, and the final rolling temperature is controlled at Ar3~Ar3+30℃. (4) Cooling: After secondary rolling, water cooling at 600~660℃, and then stack cooling to below 200℃ after being removed from the line; (5) Heat treatment: After stacking and unstacking, tempering heat treatment is carried out, and after treatment, natural cooling is performed to obtain high ductility and high strength steel plate EH32; The cooling described in step (2) is achieved using the Mulpic oscillating cooling system, a rapid cooling device. The rapid heating in step (3) involves a heating rate of 5~8 min / cm.

2. The method for preparing a high-ductility, high-strength steel plate EH32 according to claim 1, characterized in that, The steel plate comprises, by weight percentage: C 0.07%~0.09%, Si 0.25%~0.35%, Mn 0.80%~0.89%, P≤0.010%, S≤0.003%, Nb 0.02%~0.03%, Ti 0.008%~0.02%, Al 0.015%~0.05%, Cr 0.19%~0.29%, with the balance being Fe and unavoidable impurity elements.

3. The method for preparing a high-ductility, high-strength steel plate EH32 according to claim 1, characterized in that, The steel plate has a thickness of 20~50mm, a yield strength ≥315Mpa, a tensile strength of 450~570Mpa, an impact energy of -40℃ ≥200J, a tensile elongation after fracture ≥40%, a metallographic structure of ferrite + pearlite, a grain size of 10~12, and a banded structure ≤1.

4. The method for preparing a high-ductility, high-strength steel plate EH32 according to claim 1, characterized in that, In step (1), the smelting process is carried out in a converter, the refining process is carried out in an LF furnace, and the vacuum treatment process is carried out in a VD furnace before casting into a continuous casting billet.

5. The method for preparing a high-ductility, high-strength steel plate EH32 according to claim 1, characterized in that, In step (2), during the rolling process of the second billet, the reduction per rolling pass is ≥30mm.

6. The method for preparing a high-ductility, high-strength steel plate EH32 according to claim 1, characterized in that, Step (5) The tempering temperature is 600~660℃ and the tempering time is 2.5min / mm×plate thickness mm.

7. A high-ductility, high-strength steel plate EH32 prepared by the method of any one of claims 1-6.

8. The application of the high-ductility, high-strength steel plate EH32 prepared by the method of any one of claims 1-6 in shipbuilding.

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