Large-thickness F-grade ocean engineering steel plate with tensile strength of 1100 MPa and preparation method of large-thickness F-grade ocean engineering steel plate

Abstract

The invention relates to a large-thickness F-grade steel plate with tensile strength of 1100 MPa for ocean engineering. The large-thickness F-grade steel plate comprises the following chemical components in percentage by weight: 0.08 to 0.15 percent of C, 0.10 to 0.20 percent of Si, 0.6 to 1.1 percent of Mn, less than or equal to 0.005 percent of P, less than or equal to 0.002 percent of S, 0.040 to 0.060 percent of V, 0.040 to 0.060 percent of Al, 0.30 to 0.60 percent of Cr, 2.0 to 2.5 percent of Ni, 0.20 to 0.40 percent of Cu, 0.40 to 0.70 percent of Mo, 0.0010 to 0.0020 percent of B and the balance of Fe and inevitable impurity elements. The carbon equivalent CEV is less than or equal to 0.60%, the microstructure is a mixed structure of tempered martensite, bainite and retained austenite, and the grain size is 6-20 microns. A series of advanced technologies such as a high-purity steel smelting technology, a low-speed high-pressure rolling technology, a differential temperature rolling technology and the like are applied to obtain the steel plate with ultrahigh strength, ultralow-temperature toughness, high ductility, high homogeneity, excellent corrosion resistance and welding performance.

Description

Technical Field

[0001] The invention belongs to the field of iron and steel metallurgy, and specifically relates to a thick F-grade marine engineering steel plate with a tensile strength of 1100 MPa and a preparation method thereof. Background Art

[0002] As the strategic importance of clean energy and green environmental protection continues to increase, the development of deep-sea resources by various maritime countries has continued to increase. With the continuous expansion of the scale of engineering construction such as marine resource exploration and exploitation, the demand for marine engineering steel has continued to rise, providing ample space for the development of the marine engineering steel industry.

[0003] The main grades of ultra-high strength steel widely used in offshore platforms are 460MPa, 500MPa, 550MPa, 620MPa and 690MPa in yield strength. Due to the particularity of the service environment, the cost of underwater repair and maintenance is extremely high in order to withstand severe wind and wave conditions for a long time. The steel plates used are gradually developing towards high strength, high toughness, easy welding, good corrosion resistance, large thickness and large standardization. The world has successfully developed marine engineering steel with a strength level of 980 MPa, so high-performance marine engineering steel still mainly relies on imports.

[0004] After searching, Chinese patent publication number CN104480406A discloses a low-alloy high-strength and high-toughness steel plate and its manufacturing method. It is produced by composite billets, with high production costs and long process flow, and it utilizes the refinement and strengthening effects of micro-alloying elements such as Nb and Ti to ensure the mechanical properties and welding performance of the steel plate. The addition of Nb and Ti micro-alloying elements will also form Nb and Ti (C / N) inclusions at the segregation position in the core of the steel plate, affecting the low-temperature impact toughness of the steel plate. Chinese patent CN114480970B is a high-strength and high-toughness steel with a yield strength of 1100MPa. The microstructure is mainly martensite. Although it has high strength, its low-temperature impact toughness and plasticity are poor, and its coverage thickness range is only 30mm. Chinese patent CN105950997B discloses a high-toughness and high-strength thick steel plate and its production method. By adding alloys such as Mn, Ni, Cr, Mo, and adopting the Q+L+T process, a steel plate with a maximum thickness of 100 mm can be produced. The structure is tempered martensite + acicular ferrite, the yield strength of the steel plate is greater than 1200 MPa, and the tensile strength is ≥1400 MPa. Its carbon equivalent is as high as 0.68%, which seriously affects the welding performance. No corrosion-resistant elements are added in its composition design, so it is not suitable for marine engineering. Summary of the invention

[0005] The purpose of the present invention is to solve the above-mentioned deficiencies in the prior art and to provide a thick F-grade marine engineering steel plate with a tensile strength of 1100 MPa and a preparation method thereof.

[0006] The present invention adopts a low-carbon equivalent component design and, through the application of a series of advanced technologies such as high-purity steel smelting technology, low-speed strong pressure rolling technology, and differential temperature rolling technology, has successfully developed a marine engineering steel plate with a maximum production thickness of 120mm, a tensile strength ≥1100MPa, and good -60°C low-temperature impact toughness. The result is a steel plate with ultra-high strength, ultra-low temperature toughness, high ductility, high homogeneity, excellent corrosion resistance, and welding performance.

[0007] The technical solution adopted by the present invention is: a thick F-grade marine engineering steel plate with a tensile strength of 1100MPa, the chemical composition of which is as follows by weight percentage: C: 0.08-0.15%, Si: 0.10-0.20%, Mn: 0.6-1.1%, P: ≤0.005%, S: ≤0.002%, V: 0.040-0.060%, Al: 0.040-0.060%, Cr: 0.30-0.60%, Ni: 2.0-2.5%, Cu: 0.20-0.40%, Mo: 0.40-0.70%, B: 0.0010-0.0020%, and the balance is Fe and unavoidable impurity elements.

[0008] The technical solution adopted by the present invention to solve the above-mentioned problem is a thick F-grade marine engineering steel plate with a tensile strength of 1100 MPa and a preparation method thereof, wherein the carbon equivalent index CEV is less than 0.60%, wherein the carbon equivalent is calculated using formula (1): CEV=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Preferably, the chemical composition of the steel plate is C: 0.11%, Si: 0.12%, Mn: 0.90%, P: 0.003%, S: 0.0015%, V: 0.050%, Al: 0.055%, Cr: 0.55%, Ni: 2.30%, Cu: 0.25%, Mo: 0.55%, B: 0.0015%, CEV=0.58%, and the remainder is Fe and unavoidable impurity elements.

[0009] The chemical composition design principle of the steel of the present invention: C: It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, too high carbon is not good for the welding performance of steel, and will lead to serious center segregation, increased ductile-brittle transition temperature, and easy cracking during quenching. The present invention stipulates that the C content is 0.08-0.15%.

[0010] Si: has a strong solid solution strengthening effect and can be dissolved in ferrite and austenite to improve the hardness and strength of steel. However, for martensitic steel, it can promote the coarsening of crystals and laths, which is not conducive to low-temperature toughness. The present invention stipulates that the Si content is in the range of 0.10-0.20%.

[0011] Mn: The main function of this patent is to improve hardenability, promote martensitic transformation, and improve the strength of the steel plate. However, Mn is an element that is easily segregated. The formed MnS will affect the low-temperature impact toughness of the steel plate. At the same time, as the thickness of the steel plate increases, the rolling deformation decreases, and the steel plate segregates seriously, which will lead to uneven performance in the thickness direction. Therefore, the present invention adopts a low Mn component design, and the content is controlled at: 0.60~1.10%.

[0012] V: VN and V(C, N) hinder the growth of austenite grains during heating and play a role in grain refinement; on the other hand, these nano-scale fine dispersed precipitates can significantly improve the strength of the steel plate. However, if added in excessive amounts, the toughness of the base material and HAZ will be reduced, so the present invention stipulates that the V content is within the range of 0.040-0.060%.

[0013] Al: The main function of the present invention is to fix nitrogen in steel, thereby protecting the hardenability of B element and refining austenite grains. Since Al and O elements also have a strong affinity, some Al elements will be consumed due to deoxidation. Therefore, a lower Al content has little effect on fixing N, protecting B and refining grains. However, too much Al will produce Al 2 O 3 Inclusions affect the low temperature impact toughness of the steel plate. Therefore, the total Al content should be controlled at 0.040-0.060%.

[0014] Ni: It stabilizes austenite, improves hardenability, significantly improves low-temperature toughness of steel, and effectively prevents copper embrittlement of steel. However, considering the high cost of Ni alloy, the present invention stipulates that the Ni content is between 2.0 and 2.5%.

[0015] Cu: It is a crucial element in the present invention. Cu has excellent seawater corrosion resistance and also significantly improves stress corrosion. Cu also stabilizes austenite and improves the strength and low-temperature toughness of steel. However, excessive addition can easily cause copper brittleness, surface quality of the ingot and internal cracking problems. The present invention stipulates that the Cu content is in the range of 0.20-0.40%.

[0016] Cr: improves the hardenability of steel, can inhibit the formation of polygonal ferrite and pearlite, promote the transformation of martensite, and improve the strength of steel. However, too high a Cr content will affect the low-temperature impact toughness of the steel and reduce the welding performance of the steel plate. Therefore, the Cr content of the present invention is controlled at 0.30-0.60%.

[0017] Mo: hardenability element, which can inhibit the formation of massive ferrite, promote martensite transformation, and refine martensite The maximum thickness of the steel plate of the present invention is 120 mm. When Cr and Mo are added at the same time, the steel plate can have sufficient hardenability and ensure the uniformity of the structure and performance in the thickness direction of the steel plate. The Mo content of the present invention is controlled between 0.40% and 0.70%.

[0018] B: The main function is to improve hardenability. The trace amount of boron dissolved in solid solution is easily adsorbed at the grain boundary, which can improve the stability of austenite. More importantly, it can reduce the hardness gradient of quenched steel and improve the over-quenching of the surface layer. The more the solid solution, the more obvious the effect. However, too high a content of B is easy to segregate and form borides, which seriously deteriorates the toughness of the steel plate. Therefore, the present invention controls the B content to be between 0.0010 and 0.0020%.

[0019] P and S are harmful elements in steel. They are detrimental to plasticity and toughness, and also affect welding performance. In the present invention, P is ≤0.005% and S is ≤0.002%.

[0020] The present invention further provides the above-mentioned F-grade marine engineering steel plate with a tensile strength of 1100 MPa and a large thickness and a preparation method thereof, and the specific steps are as follows: 1. Smelting process The smelting raw materials are sequentially subjected to KR hot metal pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting. In order to ensure the internal quality of the ingot, low superheat pouring is adopted, argon protection pouring is used throughout the process, and a scientific dynamic soft reduction process is matched. The casting superheat is controlled at 10-40℃; the center segregation of the ingot is ≤C0.5 level, the center crack is ≤0.5 level, and the center looseness is ≤0.5 level.

[0021] 2. Slow cooling of billet pile After the billets are segmented, they are stacked and slowly cooled (preferably for more than 36 hours) for hydrogen expansion, and surface defects are cleaned after the slow cooling.

[0022] 3. Rolling The cleaned billet is placed in a walking beam heating furnace, heated to 1200-1280℃, and kept warm for 2-4 hours to fully dissolve the alloy elements in the steel to ensure the uniformity of the composition and performance of the final product. The billet is descaled by high-pressure water after it leaves the furnace. Then two-stage rolling is carried out. The rough rolling adopts a low-speed strong pressure rolling process, and the single-pass reduction is ≥30mm. After rolling into an intermediate billet, finishing rolling is carried out. In the first two passes of finishing rolling, the intermediate billet is forced to be cooled by water spraying by the high-pressure water descaling equipment of the finishing mill, which shortens the waiting time of the intermediate billet, eliminates the oxide scale on the surface of the billet, and creates a temperature difference between the surface and the core of the billet. The water spraying intensity is adjusted to make the temperature difference between the surface and the core of the billet between 70-90℃, and the surface temperature of the billet should also be controlled at the finishing rolling temperature of 780-880℃. The temperature difference helps to increase the deformation of the core and bridge the defects of the core of the billet. The last two passes of finishing rolling are forming passes, with a single pass reduction of ≤5mm to ensure good shape of the steel plate.

[0023] 3. Slow cooling of steel plate pile The rolled steel plate is slowly cooled at a high temperature of ≥200°C for more than 24 hours to perform hydrogen expansion.

[0024] 4. Heat treatment After slow cooling, the steel plate is subjected to quenching and tempering heat treatment.

[0025] Quenching: Roller-type quenching furnace is used, the quenching temperature is 860-930℃, and the time from steel plate entering the furnace to leaving the furnace is 1.6-2.5min / mm. After leaving the furnace, water cool to room temperature.

[0026] Tempering: Use roller hearth tempering furnace, the tempering temperature is 500-650℃, the time from entering the furnace to leaving the furnace is 2.0-5.0min / mm, and air-cool to room temperature after leaving the furnace.

[0027] The matrix structure of the steel plate is a mixed structure of tempered martensite + bainite + retained austenite, and the average grain size is 6-20μm.

[0028] The technical features of the present invention are as follows: 1) The steel plate of the present invention with a maximum thickness of 120 mm is produced by using continuous casting billets. Under the premise of low compression ratio, the key technologies such as high-purity steel smelting, low-speed large-reduction rolling, and differential temperature rolling are applied to solve the problem of poor core performance of the steel plate caused by defects in the continuous casting billets.

[0029] 2) The low carbon equivalent composition design ensures the excellent welding performance of the steel plate. By scientifically matching the Ni and Cu alloy elements, the steel plate has good seawater corrosion resistance while avoiding surface quality defects. The quenching + tempering heat treatment process is matched to improve the plasticity and toughness of the steel plate by controlling the composition of the residual austenite.

[0030] 3) Adopt Cr+Mo+B hardenability element composition system, give full play to the equipment capacity advantage, make the steel plate harden to the core, ensure the uniformity of the structure and performance in the thickness direction of the steel plate, and control the composition of the retained austenite through reasonable tempering process design, so as to improve the plasticity and toughness of the steel plate. The yield strength of the steel plate is ≥900MPa, the tensile strength is ≥1100MPa, the elongation is ≥14%, and the -60℃ V-notch Charpy impact energy is ≥120J. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The metallographic structure at 1 / 4 thickness of the steel plate of Example 2 of the present invention is a mixed structure of tempered martensite + bainite + retained austenite.

[0032] Figure 2 The metallographic structure at 1 / 3 thickness of the steel plate of Example 2 of the present invention is a mixed structure of tempered martensite + bainite + retained austenite.

[0033] Figure 3 The metallographic structure at 1 / 2 thickness of the steel plate of Example 2 of the present invention is a mixed structure of tempered martensite + bainite + retained austenite. DETAILED DESCRIPTION

[0034] The present invention is further described in detail below in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0035] The chemical composition and mass percentage of the corresponding 1100MPa grade ultra-low temperature toughness marine engineering steel plate in the embodiment are: C: 0.11%, Si: 0.12%, Mn: 0.90%, P: 0.003%, S: 0.0015%, V: 0.050%, Al: 0.055%, Cr: 0.55%, Ni: 2.30%, Cu: 0.25%, Mo: 0.55%, B: 0.0015%, and the rest is Fe and unavoidable impurity elements.

[0036] The production steps of steel plates are as follows: (1) Smelting: KR hot metal pretreatment, top and bottom double-blowing converter smelting, LF furnace refining and RH vacuum degassing treatment are used in sequence to obtain high-purity molten steel.

[0037] (2) Continuous casting: The pouring process adopts low superheat protection pouring throughout the process. The casting temperature is controlled at 10-40℃ above the liquidus line.

[0038] (3) Hydrogen expansion treatment of continuous casting billets: After the continuous casting billets are segmented, the billets are slowly cooled and hydrogen expanded for more than 36 hours.

[0039] (4) Rolling: Heat the billet to 1200-1280℃ and keep it at this temperature for 2-4 hours. The alloy elements in the steel are fully dissolved to ensure the uniformity of the composition and performance of the final product. The billet is descaled by high-pressure water after it comes out of the furnace. Then it is rolled in two stages: rough rolling and finishing rolling. The rough rolling adopts a low-speed strong pressure rolling process. After the intermediate billet is rolled, it is finished. In the first two passes of finishing rolling, the intermediate billet is forced to be cooled by spraying water through the high-pressure water descaling equipment of the finishing mill. This shortens the waiting time of the intermediate billet, eliminates the oxide scale on the surface of the billet, and creates a temperature difference between the surface and the core of the billet, thereby increasing the deformation of the core and bridging the defects of the core of the billet. The last two passes of finishing rolling are forming passes, and the reduction of each pass is ≤5mm to ensure that the steel plate has a good shape. After rolling off the line, the steel plate is piled and slowly cooled at a high temperature of ≥200℃ for more than 24 hours.

[0040] (5) Heat treatment: The steel plate obtained in step (4) is sent to a non-oxidizing roller-hearth quenching furnace, the quenching temperature is 860-930°C, and the time from the steel plate entering the furnace to exiting the furnace is 1.6-2.5 min / mm. After exiting the furnace, the steel plate is water-cooled to room temperature, and then sent to a roller-hearth open flame tempering furnace, the tempering temperature is 500-650°C, the time from the steel plate entering the furnace to exiting the furnace is 2.0-5.0 min / mm, and after exiting the furnace, the steel plate is air-cooled to room temperature.

[0041] (6) Conduct transverse tensile and transverse impact performance tests and metallographic structure observation on the steel plates after they come off the production line.

[0042] The rolling process parameters of the two thickness steel plates produced according to the above process flow are shown in Table 1, and the properties are shown in Table 2. The properties of the steel plates in the thickness direction are uniform and stable, among which the tensile properties are: yield strength is between 918-945MPa, tensile strength is between 1118-1136MPa, elongation is ≥14%, yield strength ratio is between 0.82-0.84, and transverse Charpy impact energy at -60℃ is ≥120J.

[0043] The flaw detection results of the entire steel plate meet the requirements of ASTM A578 / A578M Class C. There are no defects such as cracks, wrinkles, welts, delamination, and iron oxide scale indentation on the surface of the steel plate.

[0044] Figure 1-3 The microstructure diagram of the steel plate of Example 2 is given, and the microstructure is a mixed structure of tempered martensite + bainite + residual austenite, and the grain size is between 6-20μm; this structure ensures the excellent low-temperature impact toughness of the steel plate.

[0045] Table 1 Rolling process parameters of Example 1-2 Example Finished steel plate thickness, mm Billet heating temperature, ℃ Thickness at temperature, mm Rough rolling single pass reduction, mm Finish rolling start temperature, ℃ Temperature difference between the surface and core of the intermediate billet, ℃ 1 100 1225 140~180 30-55 780-880 50-100 2 120 1240 140~180 35-60 780-880 50-100 Table 2 Performance parameters of steel plates in Example 1-2

Claims

1. A thick F-grade marine engineering steel plate with a tensile strength of 1100 MPa, characterized by: The chemical composition by weight percentage is: C: 0.08-0.15%, Si: 0.10-0.20%, Mn: 0.6-1.1%, P: ≤0.005%, S: ≤0.002%, V: 0.040-0.060%, Al: 0.040-0.060%, Cr: 0.30-0.60%, Ni: 2.0-2.5%, Cu: 0.20-0.40%, Mo: 0.40-0.70%, B: 0.0010-0.0020%, and the balance is Fe and unavoidable impurity elements.

2. The steel plate according to claim 1, characterized in that: Carbon equivalent CEV≤0.60%, calculated by smelting analysis composition according to the formula CEV=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14, where the element symbols represent the percentage of the corresponding elements.

3. The steel plate according to claim 1, characterized in that: The microstructure is a mixed structure of tempered martensite + bainite + retained austenite, and the grain size is 6 to 20 μm.

4. The steel plate according to claim 1, characterized in that: Yield strength: 918 ~ 945MPa, tensile strength: 1118 ~ 1136MPa, elongation ≥ 14%, yield strength ratio: 0.82 ~ 0.84, -60 ℃ transverse Charpy impact energy ≥ 120J.

5. The steel plate according to claim 1, characterized in that: The chemical composition by weight percentage is C: 0.11%, Si: 0.12%, Mn: 0.90%, P: 0.003%, S: 0.0015%, V: 0.050%, Al: 0.055%, Cr: 0.55%, Ni: 2.30%, Cu: 0.25%, Mo: 0.55%, B: 0.0015%, the balance is Fe and unavoidable impurity elements, CEV=0.58%.

6. A method for manufacturing the steel plate according to claim 1, characterized in that: The steps include, Smelting: smelting molten steel according to the chemical composition design, and casting the molten steel into billets; Billet hydrogen expansion and slow cooling: Billet is segmented and then slowly cooled for hydrogen expansion, and surface defects are cleaned after slow cooling; Rolling: The billet is heated in the furnace to allow the alloy elements to fully dissolve. After it comes out of the furnace, it is descaled by high-pressure water, and then it is rolled in two stages: rough rolling + finishing rolling. The rough rolling adopts a low-speed strong pressure rolling process: the single-pass reduction is ≥30mm, and it is rolled into an intermediate billet. The thickness of the intermediate billet is 1.4-1.8 times the thickness of the finished steel plate; then it is finished rolling. In the first two passes of finishing rolling, the intermediate billet is sprayed with water to cool the billet surface temperature at 780-880℃, and the temperature difference between the surface and the core is between 70-90℃; the last two passes of finishing rolling are forming passes, with a reduction of ≤5mm to control the shape of the steel plate. After rolling off the line, the steel plate is piled and slowly cooled at a high temperature of ≥200℃ for more than 24 hours; Heat treatment: After quenching and tempering, the metallographic structure of tempered martensite + bainite + retained austenite is obtained.

7. The method according to claim 6, characterized in that: In the smelting step, a continuous casting process is adopted to cast molten steel, the overheat is controlled at 10-40°C, the center segregation of the ingot is ≤C0.5 level, the center crack is ≤0.5 level, and the center porosity is ≤0.5 level.

8. The method according to claim 6, characterized in that: In the smelting steps, KR molten iron pretreatment, converter smelting, LF furnace refining and RH vacuum degassing refining are used in sequence to obtain high-purity molten steel.

9. The method according to claim 6, characterized in that: In the heat treatment step, the quenching heating temperature is 860-930°C, the steel plate is heated in a non-oxidizing roller bottom quenching furnace, the time from entering the furnace to exiting the furnace is 1.6-2.5min / mm*steel plate thickness, and it is water-cooled to room temperature after exiting the furnace.

10. The method according to claim 6, characterized in that: In the heat treatment step, the tempering heating temperature is 500-650°C, the steel plate is heated in a roller bottom open flame tempering furnace, the time from entering the furnace to leaving the furnace is 2.0~5.0min / mm*steel plate thickness, and it is air-cooled to room temperature after leaving the furnace.

Images