A method for producing a high-impact hull steel

Abstract

The application relates to a preparation method of a high-impact-resistance hull steel, and the chemical components of the steel are as follows in percentage by mass: C: 0.03-0.05%, Si: 0.25-0.35%, Mn: 1.30%-1.40%, P<=0.015%, S<=0.005%, Nb: 0.03-0.04%, Ti: 0.008-0.02%, Al: 0.015%-0.05%, Cr: 0.20-0.30%, Ni: 0.30-0.40%, and the rest is Fe and inevitable impurities; key preparation steps include smelting, continuous casting, heating, rolling, cooling and heat treatment. The hull structure steel has not only good strength and toughness, with yield strength being greater than or equal to 315 Mpa, tensile strength being 450-570 Mpa and impact energy at-40 DEG C being greater than or equal to 200 J, but also elongation being greater than or equal to 45%, and has very good impact resistance, and can better meet the anti-collision requirements of military and civilian ships.

Description

Technical Field

[0001] This invention belongs to the field of steel material preparation and relates to a method for preparing high-impact-resistant ship hull steel. Technical Background

[0002] In recent years, with the rapid development of the global economy and maritime trade, maritime accidents have also increased year by year. Statistics show that over 51% of maritime accidents are caused by collisions, contact, or grounding. As a countermeasure, some oil tankers employ double-hull structures and spaced areas on the sides of the hull for oil tanks to ensure ship safety. However, these measures increase the difficulty and cost of ship construction due to reduced propulsion efficiency and smaller loading capacity, hindering widespread application. Therefore, in recent years, some classification societies and shipping associations have attempted to use high-ductility steel plates to improve the collision resistance of ships.

[0003] Traditional medium-thick plate materials are processed and strengthened using two-stage rolling to achieve high strength and toughness, but the ductility limit decreases as strength increases. Therefore, the elongation of typical high-strength ship plates rarely exceeds 35%, making these hull steel plates highly susceptible to fracture during collisions, potentially leading to maritime accidents. Currently, there are few patents related to improving the collision resistance of steel plates, and most focus on hot-rolled or cold-rolled thin plates used in automobiles. Methods using rolling followed by normalizing heat treatment to improve the ductility of medium-thick plates have not been reported. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing high-impact-resistant ship hull steel with a strength grade of 32 kg and a particularly high elongation of ≥45%, exhibiting high impact resistance.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for preparing high-collision-resistant ship hull steel, wherein the chemical composition and weight percentage of the steel are: C = 0.03–0.05%, Si = 0.25–0.35%, Mn = 1.30%–1.40%, P ≤ 0.015%, S ≤ 0.005%, Nb = 0.03–0.04%, Ti = 0.008–0.02%, Al = 0.015%–0.05%, Cr = 0.20–0.30%, Ni = 0.30–0.40%, with the balance being Fe and unavoidable impurity elements; the key preparation steps include: smelting the steel in a converter according to the above composition and casting it into steel billets; then heating the steel billets and performing two-stage rolling, controlling the heating temperature at 1150–1000°C. The roughing rolling temperature is ≥1000℃, and the ratio of slab thickness to final product thickness when the finishing rolling temperature is lower than Tnr℃ is ≥4:1. The finishing rolling temperature is 10-30℃ above Ar3. After rolling, the slab is water-cooled to below 200℃. Then, it undergoes normalizing heat treatment at 880±10℃ for 2.0-2.5 min / mm×slab thickness min. This yields a high-impact-resistant hull steel with a thickness of 10-50mm, yield strength ≥315MPa, tensile strength 450-570MPa, impact energy at -40℃ ≥200J, and tensile elongation after fracture ≥45%. The metallographic structure is ferrite + a small amount of dispersed pearlite with a grain size of 10-12.

[0007] Tnr is the recrystallization temperature, and the commonly used professional calculation formula is:

[0008] Tnr=887+464×C+890×Ti+363×Al-357×Si+6445×(Nb×0.75)

[0009] -644×[Sqrt(Nb×0.75)]+[732×V-230×(Sqrt(V)];

[0010] Ar3 is the phase transformation initiation temperature during steel plate cooling, and the commonly used professional calculation formula is:

[0011] 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, where the element symbol represents the mass percentage of this element in %; H represents the thickness of the finished steel plate in mm.

[0012] The principle of this invention:

[0013] By designing an ultra-low carbon and high manganese composition, the hard phase pearlite structure in the steel is reduced; niobium-titanium microalloying expands the non-recrystallized austenite region during rolling, and the original austenite grains are flattened by rolling with a large compression ratio in the non-recrystallized region, while the final rolling temperature and final cooling temperature are precisely controlled to retain the slender austenite grain boundaries in the rolled state; finally, during the normalizing process, austenite grains preferentially form at the rolled grain boundaries, and through the composite addition of alloying elements chromium and nickel and the inheritance of the slender austenite grain boundary structure in the rolled state, the carbon fixation, solid solution strengthening, and austenite grain refinement effects during the normalizing process are ensured, achieving uniform refinement of ferrite grains after air cooling phase transformation, and dispersed distribution of carbides, thereby improving the strength, toughness, and ductility of the steel plate.

[0014] The beneficial effects of the present invention are as follows: The high impact-resistant steel plate of the present invention has an elongation at break of more than 20% higher than that of conventional steel plates. When the hull collides or runs aground, the steel plate has excellent ductility and has a higher ability to resist hull fracture and cracking. It can suppress the outflow of cargo and fuel, prevent marine pollution and hull sinking, and achieve safer and more reliable maritime transportation. Attached Figure Description

[0015] Figure 1 Metallographic image of the steel plate in Example 1 of this invention;

[0016] Figure 2 Metallographic image of the steel plate in Comparative Example 1 of this invention. Detailed Implementation

[0017] According to the present invention, a method for preparing high-collision-resistance ship hull steel involves smelting in a 120t converter, refining in an LF furnace, vacuum treatment in an RH furnace, and casting into continuously cast billets of 220-350mm. These billets are then rolled to the final thickness on a five-meter double-stand medium-thick plate production line, and finally subjected to normalizing heat treatment in a radiant tube heated normalizing furnace. The invention is further illustrated below with examples and comparative examples. The key process parameters for the steel plates in the examples and comparative examples are as follows.

[0018] Example 1

[0019] The preparation of a high-collision-resistance ship hull steel involves a steel plate thickness of 30 mm and a continuously cast slab thickness of 300 mm. Chemical composition calculations show that Tnr is 881℃ and Ar3 is 772℃. After heating, a two-stage rolling process is performed at a heating temperature of 1182℃. After exiting the furnace, the steel is directly rolled. The roughing rolling finish temperature is 1026℃, the intermediate slab thickness is 130 mm, and the finishing rolling start temperature is 830℃. During finishing rolling, the ratio of slab thickness to final product thickness at a rolling temperature below 881℃ is 4.33:1, and the final rolling temperature is 786℃. After rolling, the steel is water-cooled to below 200℃, followed by normalizing heat treatment at 880℃ for 60 minutes.

[0020] Comparative Example 1

[0021] This comparative example uses EH32 marine engineering steel processed by normalizing. The steel plate thickness is 30mm, and the continuous casting billet thickness is 300mm. The calculated Tnr is 923℃, and Ar3 is 756℃. After heating, it undergoes two-stage rolling at a heating temperature of 1180℃. After exiting the furnace, it is directly rolled. The roughing rolling finishing temperature is 1032℃, the intermediate billet thickness is 130mm, and the finishing rolling starting temperature is 820℃. During finishing rolling, the ratio of billet thickness to final product thickness when the rolling temperature is below 923℃ is 4.33:1, and the finishing rolling temperature is 774℃. After rolling, it is water-cooled to below 200℃. Then, it undergoes normalizing heat treatment at a normalizing temperature of 880℃ for 60 minutes.

[0022] Example 2

[0023] The preparation of a high-collision-resistance ship hull steel involves a steel plate thickness of 10 mm and a continuously cast slab thickness of 220 mm. Chemical composition calculations show Tnr at 907℃ and Ar3 at 765℃. After heating, a two-stage rolling process is performed at a heating temperature of 1195℃. After exiting the furnace, the steel is rolled directly. The roughing rolling finish temperature is 1059℃, the intermediate slab thickness is 80 mm, and the finishing rolling start temperature is 920℃. Nine finishing rolling passes are performed. After the first pass, the slab temperature is 901℃ and the slab thickness is 67.8 mm. The ratio of slab thickness to final product thickness when the finishing rolling temperature is below 907℃ is 6.78:1. The remaining eight passes are rolled, with a final rolling temperature of 778℃. After rolling, the steel is water-cooled to below 200℃. Then, a normalizing heat treatment is performed at 880℃ for 25 minutes.

[0024] Comparative Example 2

[0025] This comparative example uses a common ship hull steel with a plate thickness of 10mm and a continuously cast slab thickness of 220mm. The chemical composition shows a Tnr of 907℃ and an Ar3 of 765℃. After heating, a two-stage rolling process is performed at a heating temperature of 1178℃. After exiting the furnace, the steel is rolled directly. The roughing rolling finish temperature is 1062℃, resulting in an intermediate slab thickness of 80mm. The finishing rolling start temperature is 920℃, and nine finishing rolling passes are performed. After the first pass, the slab temperature is 903℃, and the slab thickness is 67.6mm. The ratio of slab thickness to the final finished product thickness when the finishing rolling temperature is below 907℃ is 6.76:1. The remaining eight passes are rolled, with a final rolling temperature of 780℃. After rolling, the steel is air-cooled to room temperature without subsequent normalizing heat treatment.

[0026] Example 3

[0027] The preparation of a high-collision-resistance ship hull steel involves a steel plate thickness of 50 mm and a continuously cast slab thickness of 350 mm. Chemical composition calculations show that Tnr is 879 °C and Ar3 is 789 °C. After heating, a two-stage rolling process is performed at a heating temperature of 1189 °C. After exiting the furnace, the steel is directly rolled. The roughing rolling temperature is 1023 °C, the intermediate slab thickness is 200 mm, and the finishing rolling temperature is 820 °C. During finishing rolling, the ratio of slab thickness to final product thickness at a rolling temperature below 879 °C is 4:1, and the final rolling temperature is 796 °C. After rolling, the steel is water-cooled to below 200 °C. Then, a normalizing heat treatment is performed at 880 °C for 100 min.

[0028] Comparative Example 3

[0029] This comparative example uses TMCP process ship plate EH32, with a plate thickness of 50mm and a continuous casting billet thickness of 350mm. After heating, it undergoes two-stage rolling. The heating temperature is 1186℃, and it is rolled directly after exiting the furnace. The roughing rolling temperature is 1011℃, the intermediate billet is 120mm, the finishing rolling temperature is 840℃, and the finishing rolling temperature is 826℃. After rolling, it is air-cooled to room temperature and no normalizing heat treatment is performed.

[0030] The chemical composition of the steel plates in each embodiment and comparative example is shown in Table 1.

[0031] The comprehensive mechanical properties of the steel plates in each embodiment and comparative example are shown in Table 2.

[0032] Table 1. Chemical composition (by weight, %) of the steel plates used in the examples and comparative examples.

[0033]

[0034] Table 2. Comprehensive mechanical properties of the steel plates from the examples and comparative examples.

[0035]

[0036] The steel plates of Examples 1, 2, and 3 of this invention meet the requirements for comprehensive mechanical properties, especially the elongation >45%, exhibiting high impact resistance. Metallographic observation of the steel plates from these examples shows a microstructure composed of ferrite and a small amount of dispersed pearlite, with a grain size of 10–12. Figure 1 As shown. The elongation of the comparative steel plates was all <35%, and their impact resistance was inferior to that of the examples. Comparative Example 1 was ordinary normalized marine steel, with a composition design significantly different from that of this application. Although produced according to the rolling and heat treatment processes of this application, its elongation was only 34.6%. From a metallographic perspective, the grain size was coarser and the pearlite content was higher than that of the examples. Figure 2As shown, Comparative Example 2 has the same composition and rolling process as Example 2, but without post-rolling water cooling and heat treatment. Compared with Example 2, the microstructure of Comparative Example 2 did not undergo carbide redistribution and homogenization treatment after normalizing, and its ductility did not meet the requirements. Comparative Example 3 steel is TMCP high-strength ship plate EH32, produced using conventional controlled rolling technology. Its strength and low-temperature toughness meet the requirements, but its elongation is only 28.4%, resulting in poor impact resistance. Therefore, using conventional composition design, rolling, and normalizing processes, it is difficult to achieve the high impact resistance performance requirement of elongation ≥35% in industrial production. This fully demonstrates the ingenuity, comprehensiveness, and uniqueness of the present invention in its composition and process design.

Claims

1. A method for preparing high-collision-resistance ship hull steel, characterized in that: The chemical composition and weight percentage of the steel are: C = 0.03–0.05%, Si = 0.25–0.35%, Mn = 1.30%–1.40%, P ≤ 0.015%, S ≤ 0.005%, Nb = 0.03–0.04%, Ti = 0.008–0.02%, Al = 0.015%–0.05%, Cr = 0.20–0.30%, Ni = 0.30–0.40%, with the balance being Fe and unavoidable impurity elements; The key preparation steps include: smelting the above-mentioned components in a converter and casting them into steel billets; then heating the steel billets and performing two-stage rolling, controlling the heating temperature to be 1150~1200℃, the roughing rolling temperature to be ≥1000℃, the ratio of the billet thickness to the final product thickness when the rolling temperature during finishing rolling is lower than Tnr℃ to be ≥4:1, and the finishing rolling temperature to be 10~30℃ above Ar3; water cooling to below 200℃ after rolling; then normalizing heat treatment, the normalizing temperature is 880±10℃, the normalizing time is 2.0~2.5min / mm×plate thickness min, to obtain high-impact hull steel with a thickness of 10~50mm, yield strength ≥315Mpa, tensile strength 450-570Mpa, impact energy at -40℃ ≥200J, tensile elongation after fracture ≥45%, and a metallographic structure of ferrite + a small amount of dispersed pearlite with a grain size of 10~12.

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