Method for synergistically enhancing strength and toughness of ship plate steel

Abstract

The invention discloses a method for synergistically enhancing the strength and toughness of ship plate steel, which comprises the following process steps: 1) primarily heating a blank to an austenite recrystallization temperature interval, preserving heat, and then carrying out first-stage rolling; (2) the steel plate is air-cooled to an austenite non-recrystallization area, and second-stage rolling is conducted; (3) the steel plate is rapidly reheated and subjected to heat preservation; and (4) third-stage rolling is conducted, the steel plate is cooled to the surface temperature of 400 DEG C through a laminar cooling device, and finally air cooling is conducted to the room temperature. The prepared medium-thickness plate steel with the thickness being 30 mm or above has an ideal microstructure, a two-phase microstructure with polygonal ferrite as a main part and granular bainite as an auxiliary part is formed, the ideal average effective grain size of 3.0 microns is obtained, the room-temperature tensile strength of the steel is larger than or equal to 650 MPa, the total elongation is larger than or equal to 25%, the steel plate core impact energy at the temperature of 80 DEG C below zero is larger than or equal to 200 J, and the steel plate core impact energy at the temperature of 100 DEG C below zero is larger than or equal to 120 J; and the production process is simple in procedure and high in feasibility.

Description

Technical Field

[0001] The invention relates to the field of steel material processing and preparation, and relates to a process method for synergistically enhancing the strength and toughness of ship plate steel. Background Art

[0002] High-strength low-alloy HSLA steel has been widely used in large-scale marine equipment due to its excellent mechanical properties, low cost and excellent formability. However, in the industrial production of steel plates, especially in the manufacture of thick steel plates, technical challenges still exist, such as the problem of reduced mechanical properties due to uneven microstructure along the thickness direction. Therefore, in order to ensure the safe use of large-scale marine equipment, the mechanical properties of HSLA steel need to be further improved. The factors affecting the mechanical properties of steel materials are complex and changeable. For example, related factors such as the proportion of high-angle grain boundaries, dislocation density, and texture strength have a significant impact on the performance of steel plates. In addition, there is a contradiction between the strength and toughness of steel materials that is difficult to reconcile. It is also challenging to improve the strength of thick steel plates while ensuring their good toughness.

[0003] In the steel plate production process, the effectiveness of controlled rolling and related heat treatment processes in optimizing the structure of thick steel plates to improve their strength and toughness has been widely recognized and has been studied by many researchers. Chinese patent CN202110952716.1 discloses a 690 MPa grade ultra-high strength and extra-thick ship plate steel, the structure of which is mainly tempered martensite and the tensile strength is ≥800 MPa. However, its manufacturing process is relatively complicated and mainly includes molten iron pretreatment → converter smelting → LF furnace refining → RH furnace vacuum treatment → slab casting → slow cooling of ingot → heating of ingot → rolling → laminar cooling → air cooling → quenching → tempering → air cooling. After rolling is completed, a multi-step heat treatment process is required, which has a long production cycle, high cost and low controllability.

[0004] Patent CN201010560401.4 discloses NV-F690 ultra-high-strength ship plate steel and its preparation method. The finished ship plate steel is formed primarily through heating, insulation, hot rolling, cooling, and heat treatment. The ship plate steel has a high yield strength exceeding 690 MPa. However, its impact energy at -60°C is only greater than 46 J, and its low-temperature toughness is poor, making it unsuitable for use in polar regions and other complex and harsh environments. Patent CN202210215471.9 discloses a low-crack-susceptibility steel plate with a yield strength of 960 MPa and its manufacturing method, with a maximum thickness of 60 mm. However, this steel plate does not meet the impact toughness requirements at -60°C. Summary of the Invention

[0005] The present invention aims to provide a method for synergistically enhancing the strength and toughness of ship plate steel. This method requires controlling the microstructure and refining the grain size under conditions of low compression ratio and high rolling temperature, thereby obtaining medium and thick plate steel with excellent strength and toughness. The core impact energy is still higher than 120 J at -100°C. The process is simple and highly controllable.

[0006] To achieve the above object, the technical solution of the present invention is: A method for synergistically enhancing the strength and toughness of ship plate steel, for producing medium-thickness plate steel of 30 mm or more, comprising the following process steps: 1) The billet is initially heated to the austenite recrystallization temperature range and kept warm, and then the first stage of rolling is carried out; 2) Air-cool the steel plate to the austenite non-recrystallization zone and carry out the second stage rolling; 3) Rapidly reheat the steel plate and keep it warm; 4) performing a third-stage rolling process with a reduction ratio of 35% to 40%. After rolling, the steel plate is cooled to a surface temperature of 300 to 400°C using a laminar cooling device and finally air-cooled to room temperature, thereby obtaining a shipbuilding steel having a tensile strength ≥ 650 MPa, a total elongation ≥ 24%, and a core impact energy ≥ 200 J at -80°C and a core impact energy ≥ 120 J at -100°C.

[0007] Furthermore, in step (1), the initial heating temperature of the billet is 1150-1200 °C, the holding time is 2-4 h, and the first stage rolling reduction rate of the billet after holding is controlled at 30%-35%; Furthermore, in step (2), after the first stage of rolling is completed, the surface temperature is cooled to 780-830 °C, and the second stage of rolling is carried out at this temperature, and the rolling reduction rate is controlled at 40%-45%; Furthermore, in step (3), after the second stage rolling is completed, the steel plate is heated again to 800-850 °C and kept at this temperature for 20-40 min; Furthermore, the core structure of the steel plate is composed of ferrite and granular bainite; the grains in the core of the steel plate are fine and uniform, with an average effective grain size of 3.0 μm and a standard deviation of 3.1 μm.

[0008] The innovations and beneficial effects of the present invention are as follows: (1) The innovative three-stage rolling coupling interpass heat treatment process has obvious advantages in mass production, greatly improving production efficiency, shortening production processes, and reducing production costs; (2) The core mechanical properties of the ship plate steel prepared by the present invention are relatively excellent. It coordinates toughness while ensuring strength and has a wide range of applications. The room temperature tensile strength is ≥ 650 MPa, the total elongation is ≥ 24%, and the impact energy of the steel plate core is ≥ 200 J at -80 °C and ≥ 120 J at -100 °C. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 Schematic diagram of the process route of the steel plate in the embodiment.

[0010] Figure 2 This is the metallographic structure diagram of the steel plate of the embodiment.

[0011] Figure 3 This is the metallographic structure diagram of the steel plate of Comparative Example 1.

[0012] Figure 4 This is the metallographic structure diagram of the steel plate of comparative example 2.

[0013] Figure 5 Schematic diagram of the process route of the steel plate in comparative example 1.

[0014] Figure 6 Schematic diagram of the process route of the steel plate in comparative example 2. DETAILED DESCRIPTION Example 1

[0015] A method for synergistically enhancing the strength and toughness of ship plate steel, the process route is as follows Figure 1 As shown, the specific process steps include the following: (1) The billet was first heated to 1200 °C and kept at this temperature for 4 h. After the billet was kept at this temperature, the first stage of rolling was carried out. The reduction ratio was 35% after four passes. (2) After the first stage of rolling is completed, the surface temperature is air-cooled to 800-830 °C, and the second stage of rolling is directly carried out at this temperature. The reduction rate is controlled at 44% through four passes; (3) After the second stage of rolling, the steel plate is heated again to 800-830 °C and kept at this temperature for 30 min; (4) The steel plate after insulation is directly subjected to the third stage rolling, and the rolling reduction rate is controlled at 36%. After rolling, the steel plate is cooled to a surface temperature of 400 °C using a laminar cooling device, and finally air-cooled to room temperature.

[0016] The final thickness of the medium and thick plate obtained in this embodiment is 32-35 mm. The core structure of the steel is composed of 84% ferrite and 12.5% ​​bainite. The average grain size is 3.0 μm and the standard deviation is 3.1 μm. The specific metallographic structure is shown in the attached figure. Figure 2 The effective grain size of the core structure is shown in Table 1.

[0017] Further mechanical property tests show that the core of the steel plate of the embodiment of the present invention has good comprehensive mechanical properties, and has excellent low-temperature toughness while ensuring high strength. The specific mechanical property parameters are shown in Table 2. To ensure the reliability of the data, more than three samples are taken for each experiment and repeated experiments are carried out to obtain the average value.

[0018] Comparative Example 1: Comparative Example 1 Compared with the embodiment, after the second stage rolling and reheating, the third stage rolling was not carried out. The process route is as follows Figure 5 As shown, the specific process steps include the following: (1) The billet was first heated to 1200 °C and kept at this temperature for 4 h. After the billet was kept at this temperature, the first stage of rolling was carried out. The reduction ratio was 35% after four passes. (2) After the first stage of rolling is completed, the surface temperature is air-cooled to 800-830 °C, and the second stage of rolling is directly carried out at this temperature. The reduction ratio is controlled at 64% through six passes; (3) After the second stage of rolling, the steel plate is heated again to 800-830 °C and kept at this temperature for 30 min; (4) The insulated steel plate is cooled to a surface temperature of 400 °C using a laminar cooling device, and finally air-cooled to room temperature; Comparative Example 2: Comparative Example 2 adopts a conventional two-stage rolling process, the process route is as follows Figure 6 As shown, the specific process steps include the following: (1) The billet was first heated to 1200 °C and kept at this temperature for 4 h. After the billet was kept at this temperature, the first stage of rolling was carried out. The reduction ratio was 35% after four passes. (2) After the first stage of rolling is completed, the surface temperature is air-cooled to 800-830 °C, and the second stage of rolling is directly carried out at this temperature. The reduction ratio is controlled at 64% through six passes; (3) After the second stage of rolling, the steel was directly cooled to a surface temperature of 400 °C using a laminar cooling device and finally air-cooled to room temperature.

[0019] The core structure of the medium and thick plate steel obtained in Comparative Example 1 is composed of a large amount of coarse ferrite, such as Figure 3 Comparative Example 2 shows that the core structure of the medium and thick plate steel obtained is composed of a large amount of bainite and a small amount of ferrite. Figure 4 The effective grain sizes of the core of the medium and thick plate steel obtained in Comparative Examples 1 and 2 are shown in Table 1, and the mechanical properties of the core of the medium and thick plate steel obtained are shown in Table 2.

[0020] Table 1 Average effective grain size of medium and thick plate steel in Examples and Comparative Examples .

[0021] Table 2 Mechanical properties of the core of the medium and thick plate steel in the examples and comparative examples .

[0022] To reduce experimental errors, the chemical composition mass percentages of the steel plates in the examples and comparative examples are C=0.03%~0.05%, Mn=1.55%~1.95%, Si=0.15%~0.25%, Ni=0.5%~1.95%, Cu=0.10%~0.30%, Nb=0.01%~0.05%, Ti=0.01%~0.05%, S<0.01%, P<0.01%, and the balance is Fe and other unavoidable impurity elements.

[0023] According to the Examples and Comparative Examples 1 and 2, even if relatively similar rolling processes are used, the omission of one or more steps can significantly affect the mechanical properties of the final product. In the Examples, the three-stage rolling coupled with the interpass heat treatment process significantly enhances the toughness of the steel plate, achieving room temperature tensile strength ≥ 650 MPa, total elongation ≥ 25%, and core impact energy ≥ 200 J at -80°C and ≥ 120 J at -100°C. These mechanical properties are significantly superior to those of Comparative Examples 1 and 2, demonstrating the superiority and indispensability of the present invention in rolling processes. The proposed rolling process is closely related to the technical effects ultimately achieved.

[0024] In summary, the medium and thick plate steel prepared in the embodiment of the present invention forms a two-phase structure with polyhedral ferrite as the main phase and granular bainite as the auxiliary phase while ensuring the strength through a three-stage rolling coupled interpass heat treatment process, and refines the final microstructure grain size, thereby achieving the goal of greatly promoting the improvement of low-temperature toughness while ensuring the strength.

Claims

1. A method for synergistically enhancing the strength and toughness of ship plate steel, producing medium-thickness plate steel of 30 mm or more, characterized in that The process steps include: (1) The billet is initially heated to the austenite recrystallization temperature range and kept warm, and then the first stage of rolling is carried out; (2) Air cooling the steel plate to the austenite non-recrystallization zone and performing the second stage rolling; (3) Rapidly reheat the steel plate and keep it warm; (4) The third stage of rolling is carried out, and the reduction rate of the third stage rolling is 35%~40%. After the rolling is completed, the steel plate is cooled to a surface temperature of 300~400 °C using a laminar cooling device, and finally air-cooled to room temperature to obtain a ship plate steel with a tensile strength ≥ 650 MPa, a total elongation ≥ 24%, and a core impact energy ≥ 200 J at -80 °C and a core impact energy ≥ 120 J at -100 °C.

2. The method for synergistically enhancing the strength and toughness of ship plate steel according to claim 1, characterized in that: In step (1), the initial heating temperature of the billet is 1150~1200 °C, the holding time is 2~4 h, and the first stage rolling reduction rate after the billet is held is controlled at 30%~35%.

3. The method for synergistically enhancing the strength and toughness of ship plate steel according to claim 1, characterized in that: In step (2), after the first stage of rolling is completed, the surface temperature is cooled to 780~830 °C, and the second stage of rolling is carried out at this temperature, and the rolling reduction rate is controlled at 40%~45%.

4. The method for synergistically enhancing the strength and toughness of ship plate steel according to claim 1, characterized in that: In step (3), after the second stage of rolling is completed, the steel plate is heated again to 800-850 °C and kept warm for 20-40 min.

5. The method for synergistically enhancing the strength and toughness of ship plate steel according to claim 1, characterized in that: The core structure of the steel plate is composed of ferrite and granular bainite; the grains in the core of the steel plate are fine and uniform, with an average effective grain size of 3.0 μm and a standard deviation of 3.1 μm.

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