FH40-HD50 high-ductility ship plate steel having multiphase structure, and preparation method therefor

Abstract

An FH40-HD50 high-ductility ship plate steel having a multiphase structure, and a preparation method therefor, belonging to the field of ship plate steel manufacturing. The chemical composition of the ship plate steel is as follows: C, Mn, Si, Ni, Nb, Alt, S, and P, the rest being Fe and unavoidable impurity elements; the FH40-HD50 high-ductility ship plate steel has a composition design with low-C-and-Nb microalloying as a core, and does not contain metal elements such as Cu, V, and Ti. The preparation method mainly comprises procedures such as smelting, continuous casting, hot rolling, and cooling; on-line optimization of a TMCP process involving three stages of cooling, i.e. "relaxation + ultrafast cooling + air cooling", enables control of a ferrite + pearlite + bainite multiphase structure, produces FH40-HD50 high-ductility ship plate steel having far higher plasticity and toughness than traditional high-strength ship plate steel, and improves the collision resistance of large ships without changing the structural design of the ships.

Description

A multiphase structured FH40-HD50 high ductility ship plate steel and its preparation method Technical Field

[0001] The invention belongs to the field of ship plate steel manufacturing and relates to a FH40-HD50 high-ductility ship plate steel with a multiphase structure and a preparation method thereof. Background Art

[0002] Safety is an enduring theme in the transportation industry. With the aging of ships, the resulting safety hazards pose a serious threat to human life, property, and the marine ecosystem. The combined effects of shipping digitalization and decarbonization strategies are significantly impacting the healthy development of the shipping industry. Therefore, new high-grade ship plate steels and alternative fuels will play an increasingly important role. With the implementation of carbon intensity ratings in 2023, maritime traffic safety accidents are becoming increasingly complex and uncertain, especially those primarily involving collisions and mechanical damage. Therefore, ship plate steel in critical areas such as ship hulls, cargo holds, and fuel tanks must possess high strength, excellent ductility, and sufficient collision resistance to ensure safe navigation.

[0003] Currently, patent publication number CN 113957343A proposes a "TMCP state EH830 ultra-high strength marine steel plate and its manufacturing method." The steel involved contains microalloying elements such as Nb, Ti, Ni, Cu, and Mo, resulting in high production costs. While the strength can reach EH830 levels, its drawbacks include poor ductility and toughness, with an elongation of only approximately 14%.

[0004] Patent publication number CN 108517462A proposes "a high-ductility EH40 grade ship plate steel and its preparation method". The invention adopts a three-stage cooling method of water cooling + air cooling + water cooling. The steel produced has a dual-phase structure of ferrite + bainite, a strength of EH40 grade, an elongation of more than 30%, and a low-temperature impact energy of more than 230J at -40℃. However, its disadvantages are: this type of structure and high yield strength ratio are not conducive to the production of thick and high-ductility ship plate steel; the above invention cases mainly produce single-phase or dual-phase steel. At present, the use of multiphase steel for high-ductility ship plate steel is not widely used. There is little research on microstructure manipulation to improve strength-plasticity matching; multiphase microstructure manipulation can enable dislocation engineering to break through the limits of strength-plasticity balance. During plastic deformation, the soft and hard phases can coordinate with each other. Steels with a multiphase microstructure containing soft and hard phases usually exhibit better strain hardening behavior; by optimizing the volume fraction and size of the multiphase structure and controlling the morphology and distribution of the hard phase, a better strength-plasticity matching can be achieved; multiphase microstructure manipulation will become an important development direction for simultaneously improving strength and ductility in the research and development of high-ductility ship plate steel.

[0005] Summary of the Invention

[0006] Purpose of the invention: The purpose of the present invention is to provide a FH40-HD50 high-ductility ship plate steel with a multiphase structure and a preparation method thereof. Based on a low-cost composition design with low C and Nb microalloying and no metal elements such as Cu, V, and Ti, the TMCP process of "relaxation + ultra-fast cooling + air cooling" is optimized online to achieve multiphase structure control with a strong plasticity matching gradient. The structure type is ferrite + pearlite + bainite, and the FH40-HD50 high-ductility ship plate steel with a plastic toughness far higher than that of traditional high-strength ship plate steel is obtained.

[0007] Technical solution: The FH40-HD50 high-ductility ship plate steel with a multiphase structure described in the present invention has the following chemical composition by weight percentage: C: 0.05% to 0.14%, Mn: 0.8% to 1.6%, Si: 0.12% to 0.28%, Ni: 0.05% to 0.15%, Nb: 0.01% to 0.04%, Alt: 0.02% to 0.05%, S≤0.01%, P≤0.001%, and the rest are Fe and unavoidable impurity elements.

[0008] Furthermore, its chemical composition is as follows by weight percentage: C: 0.05%, Mn: 0.8%, Si: 0.12%, Ni: 0.05%, Nb: 0.01%, Alt: 0.02%, S: 0.005%, P: 0.0005%, and the rest are Fe and inevitable impurity elements.

[0009] Furthermore, its chemical composition is as follows by weight percentage: C: 0.14%, Mn: 1.6%, Si: 0.28%, Ni: 0.15%, Nb: 0.04%, Alt: 0.05%, S: 0.01%, P: 0.001%, and the rest are Fe and inevitable impurity elements.

[0010] Furthermore, its chemical composition is as follows by weight percentage: C: 0.10%, Mn: 1.2%, Si: 0.20%, Ni: 0.10%, Nb: 0.25%, Alt: 0.35%, S: 0.007%, P: 0.0008%, and the rest are Fe and inevitable impurity elements.

[0011] Furthermore, a method for preparing FH40-HD50 high ductility ship plate steel with a multiphase structure specifically comprises the following steps:

[0012] Step 1: According to the design of each chemical composition, the steel is smelted and cast into a steel ingot using a vacuum induction melting furnace, and the riser is removed and forged into a steel billet with a thickness of 80 to 320 mm;

[0013] Step 2: The steel billet is kept at 1130-1220°C for 1.2-2h, and then subjected to three passes of rough rolling. The rough rolling stage is in the austenite recrystallization zone, the cumulative reduction rate is ≥60%, the rough rolling finishing temperature is 1000-1060°C, and the intermediate billet thickness is 40-160mm; after the rough rolling, two passes of finishing rolling are carried out. The finishing rolling stage is in the austenite non-recrystallization zone, the cumulative reduction rate is ≥35%, and the finishing temperature is 830-920°C;

[0014] Step 3: The hot rolled steel plate is cooled in three stages: relaxation + ultra-fast cooling + air cooling. First, the hot rolled steel plate is relaxed to a temperature close to A. r3 Temperature, the relaxation stage is controlled as much as possible in the austenite single phase region, the measured steel plate A r3 The temperature is 740-800℃; then, the ultra-fast cooling system based on the hot rolling production line is used to cool at a cooling rate of 50-100℃ / s, and the final cooling temperature is close to A r1 Temperature, measured steel plate A r1 The temperature is 600-630℃; finally, the steel plate is air-cooled to room temperature to complete the preparation of FH40-HD50 high ductility ship plate steel, and the thickness of the finished steel plate is 16-50mm.

[0015] Furthermore, the prepared ship plate steel has a multiphase structure of ferrite + pearlite + bainite, in which the volume fraction of soft phase ferrite is 60-75%, the volume fraction of hard phase pearlite is 15-25%, the volume fraction of hard phase bainite is 10-15%, and the average grain size of ferrite is 8.5-11.5 μm, and the bainite is mainly granular bainite, with a small amount of lamellar bainite.

[0016] Furthermore, the present invention relates to a production standard and performance testing method for a multiphase structured FH40-HD50 high-ductility ship plate steel, with reference to the "MATERIAL REQUIREMENTS FOR HIGHER-DUCTILITY HULL SRUCTURAL STEEL PLATES AND SECTIONS" guide of the American ABS classification society, and a full-thickness A5 proportion specimen is used as the tensile specimen.

[0017] Furthermore, the prepared ship plate steel has a yield strength of 410-500 MPa, a tensile strength of 520-645 MPa, an elongation after fracture of 32%-38%, and an impact energy of more than 280 J at -60°C.

[0018] Beneficial effects: Compared with the prior art, the present invention has the following characteristics: 1. The FH40-HD50 high ductility ship plate steel of the present invention adds 0.01% to 0.04% of Nb element only on the basis of conventional C and Mn components, which greatly reduces the production cost; 2. Based on actual industrial production conditions, the present invention optimizes the TMCP process online, and controls 60-75% of ferrite, 15-25% of pearlite, and 10-15% of bainite (mainly granular bainite) through a three-stage cooling method of "relaxation + ultra-fast cooling + air cooling". The yield strength of the produced FH40-HD50 high ductility ship plate steel is 41 0~500MPa, tensile strength is 520~645MPa, elongation after fracture is 32%~38%, and impact energy at -60℃ is greater than 280J, which is much higher than the performance indicators of traditional high-strength ship plate steel and high-ductility ship plate steel specified by classification societies, greatly improving the collision resistance of ships; 3. The FH40-HD50 high ductility ship plate steel described in the present invention has both high strength and excellent plasticity and toughness. On the one hand, it is due to the precipitation strengthening and fine grain strengthening effects brought by the Nb element. Fine grains are also conducive to the improvement of plasticity and toughness. On the other hand, it makes full use of phase transformation strengthening and the coordinated deformation ability of soft and hard phases, which is of great significance to the development of high ductility ship plate steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG1 is a SEM microstructure diagram of a FH40-HD50 high ductility ship plate steel with a multiphase structure in Example 1 of the present invention;

[0020] FIG2 is an engineering stress-strain curve diagram of a FH40-HD50 high ductility ship plate steel with a multiphase structure in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] The chemical composition and mass percentage of FH40-HD50 high ductility ship plate steel with a multiphase structure in each embodiment of the present invention are shown in Table 1, wherein the balance is Fe and unavoidable impurities.

[0023] Table 1 Chemical composition of FH40-HD50 high ductility ship plate steel in various examples (mass fraction / wt%)

[0024] Example 1

[0025] The chemical composition and mass percentage of a multiphase structured FH40-HD50 high ductility ship plate steel are shown in Table 1. The preparation method thereof specifically comprises the following steps:

[0026] Step 1: According to the composition design in Table 1, a vacuum induction melting furnace is used to smelt and cast a steel ingot, which is then forged into a 150 mm thick billet after the riser is removed;

[0027] Step 2: The steel billet is kept at 1150℃ for 1.5h, and then subjected to three passes of rough rolling. The rough rolling stage is in the austenite recrystallization zone, the cumulative reduction rate is 76%, the rough rolling finishing temperature is 1030℃, and the intermediate billet thickness is 36mm; after the rough rolling, two passes of finishing rolling are carried out. The finishing rolling stage is in the austenite non-recrystallization zone, the cumulative reduction rate is 50%, and the finishing temperature is 850℃;

[0028] Step 3: The hot rolled steel plate is cooled in three stages: relaxation + ultra-fast cooling + air cooling. First, the hot rolled steel plate is relaxed to a temperature close to A. r3 Temperature, the relaxation stage is controlled as much as possible in the austenite single phase region, the measured steel plate A r3 The temperature is 760℃; then, the ultra-fast cooling system based on the hot rolling production line is used to cool at a cooling rate of 80℃ / s, and the final cooling temperature is close to A r1 Temperature, measured steel plate A r1 The temperature is 614℃; finally, the steel plate is air-cooled to room temperature to complete the preparation of FH40-HD50 high ductility ship plate steel, and the finished steel plate thickness is 18mm.

[0029] The ship plate steel produced in this example exhibits a multiphase structure of ferrite, pearlite, and bainite, with a volume fraction of 72% soft ferrite, 16% hard pearlite, and 12% hard bainite. The average ferrite grain size is 11.2 μm, and the bainite is primarily granular, with a small amount of lath-like bainite. Full-thickness tensile testing and low-temperature impact testing were performed on the ship plate steel produced in this example. The engineering stress-strain curves are shown in Figure 2, and specific mechanical properties are shown in Table 2.

[0030] Example 2

[0031] The chemical composition and mass percentage of a multiphase structured FH40-HD50 high ductility ship plate steel are shown in Table 1. The preparation method thereof specifically comprises the following steps:

[0032] Step 1: According to the composition design in Table 1, the steel is smelted and cast into a steel ingot using a vacuum induction melting furnace. The riser is removed and the steel is forged into a 300 mm thick billet.

[0033] Step 2: The steel billet is kept at 1180℃ for 1.3h, and then subjected to three passes of rough rolling. The rough rolling stage is in the austenite recrystallization zone, the cumulative reduction rate is 68%, the rough rolling finishing temperature is 1050℃, and the intermediate billet thickness is 96mm; after the rough rolling, two passes of finishing rolling are carried out. The finishing rolling stage is in the austenite non-recrystallization zone, the cumulative reduction rate is 56%, and the finishing temperature is 870℃;

[0034] Step 3: The hot rolled steel plate is cooled in three stages: relaxation + ultra-fast cooling + air cooling. First, the hot rolled steel plate is relaxed to a temperature close to A. r3 Temperature, the relaxation stage is controlled as much as possible in the austenite single phase region, the measured steel plate A r3 The temperature is 780℃; then, the ultra-fast cooling system based on the hot rolling production line is used to cool at a cooling rate of 60℃ / s, and the final cooling temperature is close to A r1 Temperature, measured steel plate A r1 The temperature is 622℃; finally, the steel plate is air-cooled to room temperature to complete the preparation of FH40-HD50 high ductility ship plate steel, and the finished steel plate thickness is 42mm.

[0035] The ship plate steel produced in this example exhibits a multiphase structure of ferrite, pearlite, and bainite, with a volume fraction of 65% soft ferrite, 21% hard pearlite, and 14% hard bainite. The average ferrite grain size is 9.7 μm, and the bainite is primarily granular, with a small amount of lath-like bainite. Full-thickness tensile testing and low-temperature impact testing were performed on the ship plate steel produced in this example. Specific mechanical properties are shown in Table 2.

[0036] Example 3

[0037] The chemical composition and mass percentage of a multiphase structured FH40-HD50 high ductility ship plate steel are shown in Table 1. The preparation method thereof specifically comprises the following steps:

[0038] Step 1: According to the composition design in Table 1, a vacuum induction melting furnace is used to smelt and cast a steel ingot, which is then forged into a 320 mm thick billet after removing the riser.

[0039] Step 2: The steel billet is kept at 1210℃ for 1.2h, and then subjected to three passes of rough rolling. The rough rolling stage is in the austenite recrystallization zone, the cumulative reduction rate is 64%, the rough rolling finishing temperature is 1055℃, and the intermediate billet thickness is 115mm; after the rough rolling, two passes of finishing rolling are carried out. The finishing rolling stage is in the austenite non-recrystallization zone, the cumulative reduction rate is 44%, and the finishing temperature is 840℃;

[0040] Step 3: The hot rolled steel plate is cooled in three stages: relaxation + ultra-fast cooling + air cooling. First, the hot rolled steel plate is relaxed to a temperature close to A. r3 Temperature, the relaxation stage is controlled as much as possible in the austenite single phase region, the measured steel plate A r3 The temperature is 750℃; then, the ultra-fast cooling system based on the hot rolling production line is used to cool at a cooling rate of 90℃ / s, and the final cooling temperature is close to A r1 Temperature, measured steel plate A r1The temperature is 608℃; finally, the steel plate is air-cooled to room temperature to complete the preparation of FH40-HD50 high ductility ship plate steel, and the finished steel plate thickness is 64mm.

[0041] The ship plate steel produced in this example exhibits a multiphase structure of ferrite, pearlite, and bainite, with a volume fraction of 67% soft ferrite, 20% hard pearlite, and 13% hard bainite. The average ferrite grain size is 8.9 μm, and the bainite is primarily granular, with a small amount of lath-like bainite. Full-thickness tensile testing and low-temperature impact testing were performed on the ship plate steel produced in this example. Specific mechanical properties are shown in Table 2.

[0042] Example 4

[0043] The chemical composition and mass percentage of a multiphase structured FH40-HD50 high ductility ship plate steel are shown in Table 1. The preparation method thereof specifically comprises the following steps:

[0044] Step 1: According to the composition design in Table 1, a vacuum induction melting furnace is used to smelt and cast a steel ingot, which is then forged into a 280 mm thick billet after removing the riser.

[0045] Step 2: The steel billet is kept at 1170℃ for 1.8h, and then subjected to three passes of rough rolling. The rough rolling stage is in the austenite recrystallization zone, the cumulative reduction rate is 74%, the rough rolling finishing temperature is 1020℃, and the intermediate billet thickness is 72mm; after the rough rolling, two passes of finishing rolling are carried out. The finishing rolling stage is in the austenite non-recrystallization zone, the cumulative reduction rate is 53%, and the finishing temperature is 900℃;

[0046] Step 3: The hot rolled steel plate is cooled in three stages: relaxation + ultra-fast cooling + air cooling. First, the hot rolled steel plate is relaxed to a temperature close to A. r3 Temperature, the relaxation stage is controlled as much as possible in the austenite single phase region, the measured steel plate A r3 The temperature is 785℃; then, the ultra-fast cooling system based on the hot rolling production line is used to cool at a cooling rate of 75℃ / s, and the final cooling temperature is close to A r1 Temperature, measured steel plate A r1 The temperature is 617℃; finally, the steel plate is air-cooled to room temperature to complete the preparation of FH40-HD50 high ductility ship plate steel, and the finished steel plate thickness is 34mm.

[0047] The ship plate steel produced in this example exhibits a multiphase structure of ferrite, pearlite, and bainite, with a volume fraction of 62% soft ferrite, 23% hard pearlite, and 15% hard bainite. The average ferrite grain size is 10.3 μm, and the bainite is primarily granular, with a small amount of lath-like bainite. Full-thickness tensile testing and low-temperature impact testing were performed on the ship plate steel produced in this example. Specific mechanical properties are shown in Table 2.

[0048] Table 2 Mechanical properties of FH40-HD50 high ductility ship plate steel in various examples

Claims

1. A FH40-HD50 high ductility ship plate steel with multiphase structure, characterized in that: Its chemical composition by weight percentage is: C: 0.05% ~ 0.14%, Mn: 0.8% ~ 1.6%, Si: 0.12% ~ 0.28%, Ni: 0.05% ~ 0.15%, Nb: 0.01% ~ 0.04%, Alt: 0.02 ~ 0.05%, S ≤ 0.01%, P ≤ 0.001%, and the rest are Fe and unavoidable impurity elements.

2. The FH40-HD50 high ductility ship plate steel with multiphase structure according to claim 1, characterized in that: Its chemical composition by weight percentage is: C: 0.05%, Mn: 0.8%, Si: 0.12%, Ni: 0.05%, Nb: 0.01%, Alt: 0.02%, S: 0.005%, P: 0.0005%, and the rest is Fe and unavoidable impurity elements.

3. The FH40-HD50 high ductility ship plate steel with multiphase structure according to claim 1, characterized in that: Its chemical composition by weight percentage is: C: 0.14%, Mn: 1.6%, Si: 0.28%, Ni: 0.15%, Nb: 0.04%, Alt: 0.05%, S: 0.01%, P: 0.001%, and the rest is Fe and unavoidable impurity elements.

4. The FH40-HD50 high ductility ship plate steel with multiphase structure according to claim 1, characterized in that: Its chemical composition by weight percentage is: C: 0.10%, Mn: 1.2%, Si: 0.20%, Ni: 0.10%, Nb: 0.25%, Alt: 0.35%, S: 0.007%, P: 0.0008%, and the rest is Fe and unavoidable impurity elements.

5. The method for preparing the FH40-HD50 high ductility ship plate steel with multiphase structure according to any one of claims 1 to 4, characterized in that: The preparation steps are as follows: (1): Prepare various compounds in proportion, smelt and cast into steel ingots using a vacuum induction melting furnace, and forge into steel billets after cutting off the risers; (2): The steel billet is kept at 1130-1220°C for 1.2-2h, and then subjected to three passes of rough rolling; (3): The hot-rolled steel plate is subjected to a three-stage cooling process of "relaxation + ultra-fast cooling + air cooling".

6. The method for preparing FH40-HD50 high ductility ship plate steel with multiphase structure according to claim 5, characterized in that: In step (1), the thickness of the steel billet is 80 to 320 mm.

7. The method for preparing FH40-HD50 high ductility ship plate steel with multiphase structure according to claim 5, characterized in that: In step (2), the three rough rolling passes are as follows: the rough rolling stage is in the austenite recrystallization zone, the cumulative reduction rate is ≥60%, the rough rolling final rolling temperature is 1000-1060°C, and the intermediate billet thickness is 40-160mm; after the rough rolling is completed, two finishing rolling passes are performed, the finishing rolling stage is in the austenite non-recrystallization zone, the cumulative reduction rate is ≥35%, and the final rolling temperature is 830-920°C.

8. The method for preparing FH40-HD50 high ductility ship plate steel with multiphase structure according to claim 5, characterized in that: In step (3), the three-stage cooling process is as follows: first, the hot-rolled steel plate is relaxed to a temperature close to A. r3 Temperature, the relaxation stage is controlled in the austenite single phase region, the measured steel plate A r3 The temperature is 740-800℃; then, the ultra-fast cooling system based on the hot rolling production line is used to cool at a cooling rate of 50-100℃ / s, and the final cooling temperature is close to A r1 Temperature, measured steel plate A r1 The temperature is 600-630°C; finally, the steel plate is air-cooled to room temperature to complete the preparation of FH40-HD50 high ductility ship plate steel, and the thickness of the finished steel plate is 16-50 mm.

9. The method for preparing FH40-HD50 high ductility ship plate steel with multiphase structure according to claim 5, characterized in that: The prepared ship plate steel has a multiphase structure of ferrite + pearlite + bainite; The volume fraction of soft-phase ferrite is 60-75%, the volume fraction of hard-phase pearlite is 15-25%, the volume fraction of hard-phase bainite is 10-15%, and the average grain size of ferrite is 8.5-11.5 μm.

10. The method for preparing FH40-HD50 high ductility ship plate steel with multiphase structure according to claim 5, characterized in that: The yield strength of the prepared ship plate steel is 410-500MPa, the tensile strength is 520-645MPa, the elongation after fracture is 32%-38%, and the impact energy at -60°C is greater than 280J.

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