Method for improving impact toughness of 5.5Ni steel plate

Abstract

The present application relates to a kind of methods for improving the impact toughness of plate 5.5Ni steel, the chemical composition of the plate 5.5Ni steel ingot is 0.05~0.10C, 0.60~1.00Mn, 5.2~5.8Ni, 0.06~0.12Si, 0.05~0.10Cr, 0.15~0.20Mo, ≤0.005P, ≤0.003S, the balance Fe.The present application adopts a kind of process based on TMCP-QLT, and the initial organization is controlled from rolling process, is treated by QLT process, avoid local enrichment in organization Alloy element, promote its uniform diffusion, produce more metastable structure, after quenching, change into rich alloy element lath martensite.Using the method of the present application, the impact toughness of steel plate can be greatly improved under the condition of ensuring that the strength and plasticity are basically unchanged, and has good application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ultra-low temperature energy storage material manufacturing, and relates to a method for improving impact toughness of 5.5Ni steel plate. BACKGROUND

[0002] Liquefied natural gas (LNG) is a green clean energy and has been widely used in industrial production and city gas and other fields. Due to the flammable, explosive and phase change of LNG, it is usually stored in a low-temperature liquefied manner. In order to promote the rapid development of the natural gas industry, low-temperature storage materials have become the focus of current domestic and foreign research.

[0003] Among many low-temperature storage materials, Ni-based low-temperature steel is widely used in LNG storage tanks and pipelines as low-temperature structural materials due to its excellent toughness and strength in a low-temperature environment. At present, the commonly used LNG storage tank is 9Ni material. However, due to the scarcity of Ni-based resources in China and the high international nickel price, reducing the cost of Ni-based low-temperature steel has become the current research focus. In addition, with the reduction of Ni content, the strength, plasticity and toughness will inevitably be reduced. Therefore, how to reduce the Ni content while maintaining the strength, plasticity and toughness has become a key problem restricting the research and development of Ni-based low-temperature steel. SUMMARY

[0004] In view of the above problems of the prior art, the application provides a method for improving the impact toughness of 5.5Ni steel plate. The strength, plasticity and toughness in the structure are ensured while the Ni content is reduced. Finally, the mechanical properties of the 5.5Ni steel plate are comparable to those of the traditional 9Ni steel.

[0005] To achieve the above object, the technical scheme of the application is as follows:

[0006] A method for improving the impact toughness of 5.5Ni steel plate, the chemical composition of the 5.5Ni steel ingot is 0.05-0.10C, 0.60-1.00Mn, 5.2-5.8Ni, 0.06-0.12Si, 0.05-0.10Cr, 0.15-0.20Mo, ≤0.005P, ≤0.003S, and the balance is Fe, a 60mm-thick steel plate is obtained through TMCP process, and the impact toughness thereof is improved through QLT process; the method comprises the following process steps:

[0007] (1) TMCP process

[0008] ①Place the ingot with a size of 140mm*180mm*300mm vertically in a heating furnace at 1200℃ and keep it for 2h to perform microstructure homogenization treatment;

[0009] ②Take out the ingot, air cool to 1150~1100℃, and carry out one-stage rough rolling; select 180mm as the thickness direction, pass through four passes of rolling, rough roll to 90mm, the reduction is 50%, and the rolling schedule is 180→155→130→110→90;

[0010] ③Air cool to 900~850℃, and carry out two-stage finish rolling; pass through five passes of rolling, finish roll to 60mm, the reduction is 33%, and the rolling schedule is 90→80→72→66→62→60;

[0011] ④When the finish rolling temperature is 770~820℃, air cool to room temperature;

[0012] (2) QLT process

[0013] ①Q temperature stage: the heating furnace is heated to 950~1000℃ at a heating speed of 10℃ / min, and then the plate is held for 120~144min; A C3 ~A C3 + 20℃ temperature interval, and then the 5.5Ni steel plate is water cooled to room temperature after homogenization treatment, so that strip martensite rich in multiple alloy elements is obtained;

[0014] ②L temperature stage: the 5.5Ni steel plate is transferred to a temperature interval of 350~380℃; A C1 ~A C1 +20℃, and then water cooled to room temperature after holding for 120~144min; 10~20% soft phase reversed austenite and critical ferrite are introduced into the structure, so that the plasticity and toughness of the 5.5Ni steel plate are ensured;

[0015] ③T temperature stage: the 5.5Ni steel plate is transferred to 570~600℃, held for 60~72min, and then air cooled to room temperature, high-temperature tempering is carried out to remove residual stress in the structure, so that tempered martensite structure + reversed austenite + critical ferrite are obtained.

[0016] The TMCP process is used in the application, the rolling structure state is refined, and more internal stress of the structure is preserved to room temperature. In the subsequent QLT process, because of the internal stress in the structure, a large phase change driving force is generated in the process of holding and heating, and then the required temperature and energy consumption of the Q / L temperature stage are reduced. Finally, the 5.5Ni steel plate with the room temperature yield strength of 580~620 MPa, the tensile strength of 710-730 Mpa, the elongation of 26%~29%, and the impact energy at-196℃ of 220~250 J is obtained.

[0017] The two-stage rolling temperature in step (1) is that one-stage rough rolling is performed in an austenite recrystallization zone, and two-stage finish rolling is performed in an austenite non-recrystallization zone.

[0018] The austenite phase transition temperature in step (2) A C3 and A C1 Determined according to the chemical composition content of the plate 5.5Ni steel using Thermo-Calc phase transition simulation. The calculation method of the holding time is that according to the thickness of the plate, the holding time in the Q temperature stage and the L temperature stage = plate thickness mm x (2~2.4) min / mm, and the holding time in the T temperature stage = plate thickness mm x (1~1.2) min / mm.

[0019] Compared with the prior art, the innovation and beneficial effects of the present application are that: by means of the TMCP-QLT process, the rolling organization state of the plate 5.5Ni steel is regulated and controlled, and a larger internal stress is introduced to room temperature. Subsequently, the QLT process, because of the existence of internal stress in the organization, generates a larger phase transition driving force in the process of holding and heating, thereby reducing the required temperature and energy consumption in the Q / L temperature stage. Through the Q temperature stage, the initial organization is refined, and a strip martensite rich in multiple alloy elements is obtained, so as to ensure the strength of the plate 5.5Ni steel. Subsequently, through the L temperature stage, soft phase reversed austenite and critical ferrite are introduced into the organization, so as to ensure the plasticity and toughness of the plate 5.5Ni steel. Finally, through the T temperature stage, high-temperature tempering is carried out to remove the residual stress in the organization, and a tempered martensite organization + reversed austenite + critical ferrite is obtained, so as to further improve the impact toughness. The properties of the plate prepared by the TMCP-QLT process of the present application are relatively uniform at different thickness positions. Under the mutual coordination deformation of the soft and hard phase organizations, the strength, plasticity and toughness of the plate 5.5Ni steel are equivalent to those of the 9Ni steel, and the improvement of the impact toughness is more significant. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is the process flow chart of the present application.

[0021] Figure 2 is the organization morphology diagram of the plate prepared by the TMCP process at different thicknesses.

[0022] Figure 3 is the organization morphology diagram of the plate prepared by the TMCP-QLT process at different thicknesses.

[0023] Figure 4 is the mechanical property test result diagram of Example 1 of the present application.

[0024] Figure 5is a graph of the mechanical property test results of embodiment 2 of the present application. DETAILED DESCRIPTION

[0025] The specific embodiment is described in detail below in combination with the drawings and examples.

[0026] The tensile specimen in the embodiment of the present application is prepared according to the ASTM E8 standard using an NSC-M3 wire cutting machine, and a tensile test is performed at room temperature at a tensile rate of 2 mm / min using a 50KN electronic universal testing machine AGX-V. The yield strength, tensile strength, and elongation after fracture at different thickness positions of the medium plate 5.5Ni steel are obtained by test calculation. According to the Chinese standard GB / T229-207, Charpy impact tests are performed at -196℃ on the medium plate 5.5Ni steel at different thickness positions. The size of the Charpy impact toughness sample is 10mm ×10mm ×55 mm, and has a V-shaped notch. The TMCP processes of embodiment 1 and embodiment 2 are the same, only the QLT process is different. The specific process steps are as follows:

[0027] TMCP process

[0028] ①Place the ingot with a size of 140mm ×180mm ×300mm vertically in a heating furnace at 1200℃ for 2h for homogenization treatment;

[0029] ②Take out the ingot and air cool to 1145℃, then perform one-stage rough rolling. Select 180mm as the thickness direction, and roll through four passes to 90mm with a reduction of 50%, and the rolling schedule is as follows:

[0030] 180→155→130→110→90;

[0031] ③Air cool to 865℃, then perform two-stage finish rolling. Roll through five passes to 60mm with a reduction of 33%, and the rolling schedule is as follows: 90→80→72→66→62→60;

[0032] ④Air cool to room temperature at a final rolling temperature of 816℃.

[0033] After the TMCP process, the surface oxide is removed by sanding, cleaned with alcohol, and dried after rust removal and oil removal to make the surface smooth and avoid uneven heating during heat treatment. The steps are as follows: Embodiment 1

[0034] (2) QLT process

[0035] ① Q temperature stage: the heating furnace is heated to a temperature range of 820°C at a heating rate of 10°C / min, and the plate 5.5Ni steel is homogenized again for 144 min to promote the uniform diffusion of alloying elements, and then water-cooled to room temperature to refine the initial structure and obtain a lath martensite rich in multiple alloying elements, thereby ensuring the strength of the plate 5.5Ni steel;

[0036] ② L temperature stage: the plate 5.5Ni steel is transferred to a temperature range of 670°C, and then water-cooled to room temperature after being kept for 144 min, so as to introduce soft reverse austenite and critical ferrite into the structure, thereby ensuring the plasticity and toughness of the plate 5.5Ni steel;

[0037] ③ T temperature stage: the plate 5.5Ni steel is transferred to 590°C, and then air-cooled to room temperature after being kept for 72 min, so as to remove the residual stress in the structure by high-temperature tempering, thereby obtaining a tempered martensite structure + reverse austenite + critical ferrite and further improving the impact toughness.

[0038] Finally, the plate 5.5Ni steel has an average yield strength of 597.55 MPa, an average tensile strength of 716.97 MPa, an average elongation of 27.71%, and an average impact energy of 244.22 J at -196°C at different thickness positions at room temperature. Example 2

[0039] (2) QLT process

[0040] ① Q temperature stage: the heating furnace is heated to a temperature range of 820°C at a heating rate of 10°C / min, and the plate 5.5Ni steel is homogenized again for 144 min to promote the uniform diffusion of alloying elements, and then water-cooled to room temperature to refine the initial structure and obtain a lath martensite rich in multiple alloying elements, thereby ensuring the strength of the plate 5.5Ni steel;

[0041] ② L temperature stage: the plate 5.5Ni steel is transferred to a temperature range of 690°C, and then water-cooled to room temperature after being kept for 144 min, so as to introduce soft reverse austenite and critical ferrite into the structure, thereby ensuring the plasticity and toughness of the plate 5.5Ni steel;

[0042] ③ T temperature stage: the plate 5.5Ni steel is transferred to 590°C, and then air-cooled to room temperature after being kept for 72 min, so as to remove the residual stress in the structure by high-temperature tempering, thereby obtaining a tempered martensite structure + reverse austenite + critical ferrite and further improving the impact toughness.

[0043] Finally, the medium plate 5.5Ni steel with the average yield strength of 587.99 MPa, the average tensile strength of 700.34 MPa, the average elongation of 27.54%, and the average impact energy of 211.82 J at -196℃ at the different thickness positions at room temperature is obtained.

Claims

1. A method for improving the impact toughness of a plate 5.5Ni steel, characterized in that: The chemical composition of the medium plate 5.5Ni steel ingot is 0.05-0.10C, 0.60-1.00Mn, 5.2-5.8Ni, 0.06-0.12Si, 0.05-0.10Cr, 0.15-0.20Mo, ≤0.005P, ≤0.003S, and the balance of Fe; a 60mm thick steel plate is obtained through a TMCP process, and the impact toughness thereof is improved through a QLT process; The method comprises the following process steps: (1) TMCP process ①Place the ingot with a size of 140mm×180mm×300mm vertically in a heating furnace at 1200℃ for 2h to perform microstructure homogenization treatment; ②Take out the ingot and air cool it to 1150-1100℃, and then perform one-stage rough rolling; select 180mm as the thickness direction, and perform four-pass rolling to rough roll it to 90mm, with a reduction of 50%, and the rolling schedule is 180→155→130→110→90; ③Air cool it to 900-850℃, and then perform two-stage finish rolling; perform five-pass rolling to finish roll it to 60mm, with a reduction of 33%, and the rolling schedule is 90→80→72→66→62→60; ④When the finish rolling temperature is 770-820℃, air cool it to room temperature; (2) QLT process ①Q temperature stage: The heating furnace heats up to 10℃ / min. A C3 ~A C3 + Within a temperature range of 20℃, with a holding time of 120~144min, the medium-thick 5.5Ni steel plate was homogenized again and then water-cooled to room temperature to obtain lath martensite rich in multiple alloying elements. (ii) L temperature stage: transferring the plate 5.5Ni steel to A C1 ~A C1 +20℃ temperature interval, holding for 120~144 min, and then water cooling to room temperature; introducing 10%~20% soft phase reversed austenite and critical ferrite in the structure, ensuring the plasticity and toughness of the plate 5.5Ni steel; ③T temperature stage: transfer the medium plate 5.5Ni steel to 570-600℃, and after holding for 60-72min, air cool it to room temperature to perform high-temperature tempering to remove residual stress in the microstructure, and obtain tempered martensite microstructure+reversed austenite+critical ferrite.

2. The method for improving impact toughness of the medium plate 5.5Ni steel according to claim 1, characterized in that: The calculation method of the holding time in the step (2) QLT process is that the holding time in the Q temperature stage and the L temperature stage=steel plate thickness mm×(2-2.4)min / mm, and the holding time in the T temperature stage=steel plate thickness mm×(1-1.2)min / mm.

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