A thick plate for ultra-low yield ratio and high toughness marine structure and heat treatment method thereof

Abstract

The present invention discloses a thick plate for ultra-low yield ratio high-toughness marine structure and a heat treatment method thereof, wherein the chemical composition and mass percentage of the steel plate are: C: 0.11-0.14%, Si: 0.15-0.50%, Mn: 1.55-1.65%, P≤0.012%, S≤0.005%, Ni 0.60-0.70%, Nb 0.025-0.040%, Al≥0.020%, and the rest are Fe and unavoidable impurity elements; the steel plate is normalized and heat treated by an open flame heating normalizing furnace, and the normalizing furnace is divided into 3 zones at equal distances from the furnace door to the furnace tail, and a water cooling device is matched after the furnace. The steel plate provided by the present invention is a ferrite + carbon-rich Maho Island two-phase structure, a grain size of 10-11 level, a yield ratio of ≤0.65, and an ultra-low yield ratio is achieved.

Description

Technical Field

[0001] The invention relates to a thick plate for an ultra-low yield ratio and high toughness marine structure and a heat treatment method thereof. Background Art

[0002] The working environment of marine structural steel is special. In addition to bearing gravity loads, it is also necessary to consider the influence of wind loads, current loads, ice loads, earthquake loads, etc. Secondly, with the extension of marine mining to deep-sea areas, higher requirements are also put forward for the safety and reliability of marine structures, which are reflected in the steel plates used to build marine structures. First, the thickness is getting larger and larger; second, under the premise of meeting certain strength and good weldability, it is also necessary to have higher plasticity, low-temperature toughness, yield strength ratio, thickness uniformity and other higher-level requirements; especially the yield strength ratio is used as a measure of the size of the gap between yield strength and tensile strength. The higher the yield strength ratio, the material will break soon after deformation, and the worse the safety performance of the material. By studying the evolution of the organization during the heat treatment process and regulating the microphase morphology and composition, it is a key technology to obtain high-level and high-performance marine structural steel. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a thick plate with ultra-low yield ratio and high toughness for marine structure and a heat treatment method thereof.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0005] A thick plate with ultra-low yield ratio and high toughness for marine structures. The chemical composition and mass percentage of the steel plate are: C: 0.11-0.14%, Si: 0.15-0.50%, Mn: 1.55-1.65%, P≤0.012%, S≤0.005%, Ni0.60-0.70%, Nb 0.025-0.040%, Al≥0.020%, and the rest are Fe and unavoidable impurity elements.

[0006] The ultra-low yield ratio and high toughness thick plate for marine structure of the present invention has a thickness of 70-120 mm.

[0007] The heat treatment method for thick plates with ultra-low yield ratio and high toughness for marine structures of the present invention is specifically as follows: a normalizing furnace is heated by an open flame to perform normalizing heat treatment on the steel plate, wherein the normalizing furnace is divided into three zones with equal distances from the furnace door to the furnace tail, and a water cooling device is matched behind the furnace.

[0008] In the heat treatment method of the present invention, when the steel plate is normalized, the temperatures of the three zones of the normalizing furnace are set as follows: Zone 1 is set at 900-910°C, Zone 2 is set at 880-890°C, and Zone 3 is set at 860-870°C; the heating time of the steel plate in each zone is (0.9-1.1) min / mm*T, where T is the thickness of the steel plate in mm.

[0009] In the heat treatment method of the present invention, the steel plate is cooled in water after being taken out of the furnace, and the cooling rate is controlled in stages, firstly cooled to 640-660°C at a rate of 3-5°C / s, then cooled to 450-470°C at a rate of 8-10°C / s, and finally cooled to room temperature in air after being taken out of the water.

[0010] After the steel plate of the present invention is subjected to normalizing heat treatment, the transverse impact energy at -40°C at the surface, 1 / 4 thickness and 1 / 2 thickness of the steel plate can reach more than 120J; the room temperature tensile strength: the yield strength is 325-360MPa, the tensile strength is 550MPa-600MPa, and the elongation after fracture is ≥25%; in particular, an ultra-low yield strength ratio is achieved, satisfying ≤0.65.

[0011] The steel plate of the present invention has a ferrite + carbon-rich Mao Island two-phase structure after normalizing heat treatment, the carbon-rich Mao Island two-phase structure accounts for 20%-30%, and the grain size is 10-11.

[0012] The performance standard of the ultra-low yield ratio and high toughness thick plate for marine structure described in the present invention refers to EN10225, and the detection method refers to standards EN ISO6892-1 and EN ISO148-1.

[0013] The beneficial effects of adopting the above technical solution are:

[0014] 1. The present invention adopts a low-carbon and high-manganese component system, and then adds a certain amount of Nb and Ni alloy elements. Low carbon is the basis for ensuring high plasticity and toughness; high manganese, firstly, manganese can solid-solution strengthen ferrite to ensure the strength of the steel plate, and secondly, manganese can strongly reduce the martensitic transformation point of steel, laying a good foundation for the subsequent two-phase organization; nickel alloy can not only stabilize the austenite organization, but also significantly improve the toughness of the matrix, which is the guarantee of high toughness; trace niobium alloy can play a role in refining the grains.

[0015] 2. During the heat treatment process, the temperature field in the normalizing furnace is controlled. The temperature of the first zone of the furnace is innovatively set at 900-910℃, and high-temperature rapid heating is adopted to increase the austenite nucleation rate; the temperature setting from zone 2 to zone 3 is gradually reduced to ensure that the austenite is fully homogenized and prevent the austenite from growing.

[0016] 3. Cooling in stages after leaving the furnace. Under the component system of the present invention, combined with the formation conditions of ferrite, bainite and martensite during the cooling process of the steel plate, firstly slow cooling is performed to obtain a part of fine and uniform ferrite structure, then rapid cooling is performed to avoid the bainite area, and stay to the beginning transformation point of martensite, and finally slow cooling is performed in the air so that the carbon-rich austenite is retained, and finally a two-phase structure of ferrite + carbon-rich martensite is obtained.

[0017] 4. A certain proportion of carbon-rich Mao Island structure is the key technical factor to obtain low yield strength ratio and high toughness. After the formation of ferrite, the carbon in the steel is concentrated in the Mao Island structure, which makes it have a higher solid solution strength, and the contribution of solid solution strengthening to tensile strength is significantly greater than yield strength; secondly, the stable residual austenite also has a significant effect on improving the plasticity and toughness of the steel plate.

[0018] 5. The method of the present invention has the characteristics of stable product quality and can realize batch production. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the metallographic structure diagram of the thick plate for ultra-low yield ratio and high toughness marine structure after heat treatment in Example 1 of the present invention.

[0020] Figure 2 This is the metallographic structure diagram of the thick plate for ultra-low yield ratio and high toughness marine structure after heat treatment according to Example 2 of the present invention.

[0021] Figure 3 This is the metallographic structure diagram of the thick plate for ultra-low yield ratio and high toughness marine structure after heat treatment according to Example 3 of the present invention.

[0022] Figure 4 This is the metallographic structure diagram of the thick plate for ultra-low yield ratio and high toughness marine structure after heat treatment according to Example 4 of the present invention.

[0023] Figure 5 This is the metallographic structure diagram of the thick plate for ultra-low yield ratio and high toughness marine structure after heat treatment in comparative example 5 of the present invention. DETAILED DESCRIPTION

[0024] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] Example 1

[0026] The ultra-low yield ratio and high toughness thick plate for marine structure in this embodiment has the following chemical components and mass percentages: C: 0.11%, Si: 0.23%, Mn: 1.60%, P: 0.012%, S: 0.003%, Ni: 0.66%, Nb: 0.031%, Al: 0.020%, and the rest are Fe and unavoidable impurity elements; the thickness of the steel plate is 110 mm.

[0027] The heat treatment method of the thick plate with ultra-low yield ratio and high toughness for marine structure in this embodiment is specifically as follows: the steel plate is subjected to normalizing heat treatment by using an open flame heating normalizing furnace, the furnace is divided into three zones with equal distance from the furnace door to the furnace tail, and a water cooling device is matched behind the furnace; the details are as follows:

[0028] (1) When the steel plate is normalized, zone 1 is set at 904°C, zone 2 is set at 890°C, and zone 3 is set at 865°C, forming a decreasing temperature field. The heating time of the steel plate in each zone is 0.94 min / mm*T, where T is the thickness of the steel plate in mm.

[0029] (2) After the steel plate is taken out of the furnace, it is cooled in water, and the cooling rate is controlled in stages. First, it is cooled to 644°C at a rate of 4.1°C / s, then cooled to 459°C at a rate of 8°C / s, and finally cooled to room temperature in air.

[0030] The mechanical properties of the ultra-low yield ratio high toughness marine structure thick plate after heat treatment in this embodiment are shown in Table 1; the microstructure of the steel plate after heat treatment is shown in Table 1. Figure 1 (500X) is a two-phase structure of ferrite + carbon-rich Mao Island, the carbon-rich Mao Island two-phase structure accounts for 23%, and the grain size is 10.5.

[0031] Table 1 Mechanical properties of steel plate in Example 1

[0032]

[0033] Example 2

[0034] The ultra-low yield ratio and high toughness thick plate for marine structure in this embodiment has the following chemical composition and mass percentage: C: 0.13%, Si: 0.15%, Mn: 1.58%, P: 0.010%, S: 0.002%, Ni: 0.64%, Nb: 0.025%, Al: 0.037%, and the rest are Fe and unavoidable impurity elements; the thickness of the steel plate is 96 mm.

[0035] The heat treatment method of the thick plate with ultra-low yield ratio and high toughness for marine structure in this embodiment is specifically as follows: the steel plate is subjected to normalizing heat treatment by using an open flame heating normalizing furnace, the furnace is divided into three zones with equal distance from the furnace door to the furnace tail, and a water cooling device is matched behind the furnace; the details are as follows:

[0036] (1) When the steel plate is normalized, zone 1 is set at 910°C, zone 2 is set at 887°C, and zone 3 is set at 860°C to form a decreasing temperature field. The heating time of the steel plate in each zone is 0.90 min / mm*T, where T is the thickness of the steel plate in mm.

[0037] (2) After the steel plate is taken out of the furnace, it is cooled in water, and the cooling rate is controlled in stages. First, it is cooled to 660°C at a rate of 3°C / s, then cooled to 470°C at a rate of 8.8°C / s, and finally cooled to room temperature in air.

[0038] The mechanical properties of the ultra-low yield ratio high toughness marine structure thick plate after heat treatment in this embodiment are shown in Table 2; the microstructure of the steel plate after heat treatment is shown in Table 2. Figure 2 (500X) is a two-phase structure of ferrite + carbon-rich Mao Island, the carbon-rich Mao Island two-phase structure accounts for 25%, and the grain size is level 10.

[0039] Table 2 Mechanical properties of steel plate in Example 2

[0040]

[0041] Example 3

[0042] The ultra-low yield ratio and high toughness thick plate for marine structure in this embodiment has the following chemical components and mass percentages: C: 0.12%, Si: 0.50%, Mn: 1.55%, P: 0.008%, S: 0.001%, Ni: 0.60%, Nb: 0.028%, Al: 0.058%, and the rest are Fe and unavoidable impurity elements; the thickness of the steel plate is 88 mm.

[0043] The heat treatment method of the thick plate with ultra-low yield ratio and high toughness for marine structure in this embodiment is specifically as follows: the steel plate is subjected to normalizing heat treatment by using an open flame heating normalizing furnace, the furnace is divided into three zones with equal distance from the furnace door to the furnace tail, and a water cooling device is matched behind the furnace; the details are as follows:

[0044] (1) When the steel plate is normalized, zone 1 is set at 902°C, zone 2 is set at 885°C, and zone 3 is set at 870°C, forming a decreasing temperature field. The heating time of the steel plate in each zone is 1.03 min / mm*T, where T is the thickness of the steel plate in mm.

[0045] (2) After the steel plate is taken out of the furnace, it is cooled in water, and the cooling rate is controlled in stages. First, it is cooled to 652°C at a rate of 3.6°C / s, then cooled to 450°C at a rate of 9.2°C / s, and finally cooled to room temperature in air.

[0046] The mechanical properties of the ultra-low yield ratio high toughness marine structure thick plate after heat treatment in this embodiment are shown in Table 3; the microstructure of the steel plate after heat treatment is shown in Table 3. Figure 3 (500X) is a two-phase structure of ferrite + carbon-rich Mao Island, the carbon-rich Mao Island two-phase structure accounts for 30%, and the grain size is level 10.

[0047] Table 3 Mechanical properties of steel plate in Example 3

[0048]

[0049] Example 4

[0050] The ultra-low yield ratio and high toughness thick plate for marine structure in this embodiment has the following chemical components and mass percentages: C: 0.14%, Si: 0.44%, Mn: 1.62%, P: 0.006%, S: 0.005%, Ni: 0.70%, Nb: 0.036%, Al: 0.061%, and the rest are Fe and unavoidable impurity elements; the thickness of the steel plate is 70 mm.

[0051] The heat treatment method of the thick plate with ultra-low yield ratio and high toughness for marine structure in this embodiment is specifically as follows: the steel plate is subjected to normalizing heat treatment by using an open flame heating normalizing furnace, the furnace is divided into three zones with equal distance from the furnace door to the furnace tail, and a water cooling device is matched behind the furnace; the details are as follows:

[0052] (1) When the steel plate is normalized, zone 1 is set at 900°C, zone 2 is set at 880°C, and zone 3 is set at 868°C to form a decreasing temperature field. The heating time of the steel plate in each zone is 1.1 min / mm*T, where T is the thickness of the steel plate in mm.

[0053] (2) After the steel plate is taken out of the furnace, it is cooled in water, and the cooling rate is controlled in stages. First, it is cooled to 658°C at a rate of 4.7°C / s, then cooled to 453°C at a rate of 10°C / s, and finally cooled to room temperature in air.

[0054] The mechanical properties of the ultra-low yield ratio high toughness marine structure thick plate after heat treatment in this embodiment are shown in Table 4; the microstructure of the steel plate after heat treatment is shown in Table 4. Figure 4 (500X) is a ferrite + carbon-rich Mao Island two-phase structure, the carbon-rich Mao Island two-phase structure accounts for 20%, and the grain size is 10.5.

[0055] Table 4 Mechanical properties of steel plate in Example 4

[0056]

[0057] Example 5

[0058] The ultra-low yield ratio and high toughness thick plate for marine structure in this embodiment has the following chemical composition and mass percentage: C: 0.13%, Si: 0.36%, Mn: 1.65%, P: 0.004%, S: 0.004%, Ni: 0.69%, Nb: 0.040%, Al: 0.044%, and the rest are Fe and unavoidable impurity elements; the thickness of the steel plate is 120 mm.

[0059] The heat treatment method of the thick plate with ultra-low yield ratio and high toughness for marine structure in this embodiment is specifically as follows: the steel plate is subjected to normalizing heat treatment by using an open flame heating normalizing furnace, the furnace is divided into three zones with equal distance from the furnace door to the furnace tail, and a water cooling device is matched behind the furnace; the details are as follows:

[0060] (1) When the steel plate is normalized, zone 1 is set at 907°C, zone 2 is set at 883°C, and zone 3 is set at 862°C to form a decreasing temperature field. The heating time of the steel plate in each zone is 1.0 min / mm*T, where T is the thickness of the steel plate in mm.

[0061] (2) After the steel plate is taken out of the furnace, it is cooled in water and the cooling rate is controlled in stages. First, it is cooled to 640°C at a rate of 5°C / s, then cooled to 466°C at a rate of 9.7°C / s, and finally cooled to room temperature in air.

[0062] The mechanical properties of the ultra-low yield ratio high toughness marine structure thick plate after heat treatment in this embodiment are shown in Table 5; the microstructure of the steel plate after heat treatment is shown in Table 5. Figure 5 (500X) is a ferrite + carbon-rich Mao Island two-phase structure, the carbon-rich Mao Island two-phase structure accounts for 28%, and the grain size is 11 levels.

[0063] Table 5 Mechanical properties of steel plate in Example 5

[0064]

[0065] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A heat treatment method for a thick plate with ultra-low yield ratio and high toughness for marine structures, wherein the chemical composition and mass percentage of the thick plate are: C: 0.11-0.14%, Si: 0.15-0.50%, Mn: 1.55-1.65%, P≤0.012%, S≤0.005%, Ni0.60-0.70%, Nb 0.025-0.040%, Al≥0.020%, and the rest are Fe and unavoidable impurity elements; characterized in that, The steel plate is subjected to normalizing heat treatment by adopting an open flame heating normalizing furnace, the normalizing furnace is divided into three zones with equal distance from the furnace door to the furnace tail, and a water cooling device is matched behind the furnace; when the steel plate is normalizing, the temperatures of the three zones of the normalizing furnace are set as follows: zone 1 is set at 900-910°C, zone 2 is set at 880-890°C, and zone 3 is set at 860-870°C; the heating time of the steel plate in each zone is (0.9-1.1) min / mm*T, T is the thickness of the steel plate, unit is mm; the steel plate is cooled in water after being taken out of the furnace, and the cooling rate is controlled in stages, first cooled to 640-660°C at a rate of 3-5°C / s, then cooled to 450-470°C at a rate of 8-10°C / s, and finally cooled to room temperature by air out of water.

2. The heat treatment method for a thick plate with ultra-low yield ratio and high toughness for marine structure according to claim 1, characterized in that: The thickness of the steel plate is 70-120 mm.

3. The heat treatment method for thick plates for ultra-low yield ratio and high toughness marine structures according to claim 1 or 2, characterized in that: After the steel plate is normalized and heat treated, the transverse impact energy at -40°C on the surface, 1 / 4 thickness, and 1 / 2 thickness of the steel plate can reach more than 120J; Room temperature tensile strength: yield strength 325-360MPa, tensile strength 550MPa-600MPa, elongation after fracture ≥25%; yield strength ratio ≤0.

65.

4. The heat treatment method for thick plates for ultra-low yield ratio and high toughness marine structures according to claim 1 or 2, characterized in that: The steel plate after normalizing heat treatment has a ferrite + carbon-rich Mao Island two-phase structure, the carbon-rich Mao Island two-phase structure accounts for 20%-30%, and the grain size is 10-11.

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