Rack steel plate with thickness up to 160 mm for offshore wind power installation platform and manufacturing method therefor

Abstract

The present invention relates to a rack steel plate up to 160 mm in thickness for offshore wind power installation platform and manufacturing method thereof, wherein the elemental composition, in mass percentages, is: C: 0.30-0.40%, Si: 0.15-0.35%, Mn: 0.85-1.20%, P: ≤0.010%, S: ≤0.002%, Cr: 0.45-0.75%, Mo: 0.45-0.65%, Ni: 1.3-1.8%, Al: 0.06-0.10%, V: 0.02-0.05%, Nb: 0.01-0.04%, Ti: ≤0.006%, N: ≤0.007%, B: 0.001-0.002%, and the balance being iron and unavoidable impurity elements. The steel plate has a production thickness of ≤160 mm, a yield strength of ≥1000 MPa, a tensile strength of ≥1100 MPa, an elongation of ≥12%, a transverse Charpy impact energy of ≥46 J at -40°C at a quarter thickness position, and a transverse Charpy impact energy of ≥46 J at -30°C at a half-thickness position. The near-surface region of the steel plate has a microstructure of tempered troostite, the quarter thickness position of the steel plate has a microstructure of tempered troostite + tempered lower bainite, and the half-thickness position of the steel plate has a microstructure of tempered troostite + tempered lower bainite + a small amount of tempered granular bainite. It has high strength, high hardness and hardness uniformity, good plasticity, high low-temperature toughness and good weldability,

Description

TECHNICAL FIELD

[0001] The present invention belongs to the technical field of iron-based alloys, and in particular relates to a rack steel plate and a manufacturing method thereof.BACKGROUND

[0002] Self-elevating offshore wind power installation platforms are mainly of two types, one type being hydraulic pin-and-hole, and the other type being rack-and-pinion. The leg racks of the latter are often manufactured by using a 690 MPa grade rack steel plate having a yield strength of ≥690 MPa, a tensile strength of 770-940 MPa, and a Brinell hardness of 240-290 HBW at the quarter thickness position. This material originates from the manufacturing of leg racks for self-elevating drilling platforms used in offshore oil and gas extraction, and can meet the service requirements of offshore wind power installation platforms to a certain extent. However, the weight of to-be-installed wind power equipment is continuously increasing, and the lifting and lowering of the installation platform are more frequent than those of offshore oil and gas extraction platforms, thus, the platform is becoming ever larger, the wear of the leg racks is becoming increasingly severe, the service life of the platform is severely shortened, and the weight of to-be-installed wind power equipment is greatly limited. In order to solve these problems, manufacturers of offshore wind power installation platforms wish to use extra-thick steel plates having higher strength, higher hardness and better hardness uniformity along the thickness cross-section to manufacture leg racks. In addition, this new material still needs to satisfy the high toughness and the good welding and processing performance that the existing 690 MPa grade rack steel plates possess. That is to say, it is necessary to develop a novel rack steel plate having a thickness of up to 160 mm, a yield strength of ≥1000 MPa, a tensile strength of ≥1100 MPa, an elongation of ≥12%, a transverse Charpy impact energy at low temperature (no matter whether at -40°C at a quarter thickness position or at -30°C at a half-thickness position) of ≥46 J, a Brinell hardness of ≥320 HBW across the full thickness of the steel plate, and good weldability, to meet the construction requirements of offshore installation platforms with greater load-bearing capacity.CONTENT OF THE INVENTION

[0003] The object of the present invention is to develop a rack steel plate having a thickness of up to 160 mm for offshore wind power installation platforms and a manufacturing method thereof. The steel plate has a target yield strength of ≥1000 MPa, the tensile strength is ≥1100 MPa, the elongation is ≥12%, and the transverse Charpy impact energy of the steel plate at the quarter thickness position (at -40°C) and that at the half-thickness position (at -30°C) should both be ≥46 J. The Brinell hardness across the full thickness of the steel plate is ≥320 HBW, and the steel plate has good weldability.

[0004] The technical solution of the present invention is: a rack steel plate, in particular a rack steel plate having a thickness of up to 160 mm that can be used for offshore wind power installation platforms, wherein the chemical composition of the steel plate, in mass percentages, is: C: 0.30-0.40%, Si: 0.15-0.35%, Mn: 0.85-1.20%, P: ≤0.010%, S: ≤0.002%, Cr: 0.45-0.75%, Mo: 0.45-0.65%, Ni: 1.3-1.8%, Al: 0.06-0.10%, V: 0.02-0.05%, Nb: 0.01-0.04%, Ti: ≤0.006%, N: ≤0.007%, B: 0.001-0.002%, and the balance being iron and unavoidable impurity elements.

[0005] The rack steel plate is produced with a thickness of up to 160 mm. The yield strength is ≥1000 MPa, the tensile strength is ≥1100 MPa, the elongation is ≥12%, the transverse Charpy impact energy of the steel plate at the quarter thickness position (at -40°C) and that at the half-thickness position (at -30°C) are both ≥46 J, the Brinell hardness across the full thickness of the steel plate is ≥320 HBW, and the steel plate has good weldability.

[0006] The settings of the elemental composition and elemental contents of the rack steel plate are based on the following:

[0007] C: is an element necessary for ensuring the strength and hardness of the steel plate. Increasing the carbon content in the steel increases the martensite transformation capability and the tetragonality of the martensite, and thus improves the strength and hardness of the steel. However, an excessively high C content is detrimental to the ductility, toughness and weldability of the steel. In the present invention, the content of C is controlled to be 0.30-0.40%.

[0008] Si: is a deoxidizing element in the steel, and improves the strength and hardness of the steel in the form of solid solution strengthening. When the Si content is lower than 0.10%, the deoxidation effect is relatively poor. When the Si content is relatively high, the toughness is reduced. In the present invention, the content of Si is controlled to be 0.15-0.35%.

[0009] Mn: is an element that improves the hardenability of the steel, and plays a role of solid solution strengthening, so as to compensate for the loss of strength and hardness caused by the reduction of the C content in the steel. When the Mn content in the steel is lower than 0.8%, the effect of ensuring the strength and hardness cannot be fully brought into play; however, when the Mn content is excessively high, it tends to cause temper embrittlement and also impairs the weldability of the material. Therefore, the content of Mn in the present invention is controlled to be 0.85-1.20%.

[0010] Ni: is an element that improves the hardenability of the steel plate and can significantly improve the low-temperature toughness thereof, and has a good influence on the impact toughness and the ductile-to-brittle transition temperature. However, when the Ni content is excessively high, iron oxide scale having high viscosity is easily formed on the surface of the steel slab, which is difficult to remove and adversely affects the surface quality of the steel plate. In addition, Ni is also a costly metal, and an excessively high content thereof increases the cost. Therefore, in the present invention, the content of Ni is controlled to be 1.3-1.8%, which is conducive to achieving an optimal cost-performance ratio.

[0011] Cr: is an element that improves the hardenability of the steel, and thus contributes to the improvement of the strength and hardness. The addition of an appropriate amount of Cr can ensure that the steel plate achieves the required strength and hardness; however, an excessive addition of Cr will reduce the toughness of the material and also impair the welding performance of the material. In the present invention, the content of Cr is controlled to be 0.45-0.75%.

[0012] Mo: can significantly improve the hardenability of the steel, and thus improve the strength and hardness thereof. The addition of a small amount of Mo in the steel can also reduce or eliminate temper embrittlement, and thus improve the heat treatment performance of the steel; however, Mo is a costly metal, and an excessively high content thereof increases the cost and also reduces the welding performance of the material. In the present invention, the content of Mo is controlled to be 0.45-0.65%.

[0013] V: is an element that causes V(C, N) to precipitate, which can significantly improve the strength and hardness of the steel in the form of dispersed precipitation. However, an excessively high addition amount of V will reduce the toughness and welding performance of the steel plate. In the present invention, the content of V is controlled to be 0.02-0.05%.

[0014] Nb: is an element that plays a significant role in grain refinement during the rolling and heat treatment processes. During the rolling and heat treatment stages of the steel plate, Nb hinders the growth of austenite grains, and thus refines the grains, and provides a basis to achieve a large-thickness steel plate with a fine microstructure after quenching and tempering treatment, which is conducive to improving the strength, hardness and toughness of the steel plate. However, an excessively high amount of Nb cannot be fully taken into solid solution, and therefore fails to play its intended role while increasing the cost. Moreover, an excessively high amount of Nb adversely affects the welding performance. In the present invention, the content of Nb is controlled to be 0.01-0.04%.

[0015] Al: mainly plays the roles of nitrogen fixation and deoxidation. The AlN formed by the combination of Al and N can effectively refine the grains, and can also protect the addition of element B in the steel. However, if the content of Al is excessively high, the toughness of the steel will be damaged. In the present invention, the content thereof (Alt) is controlled to be 0.06-0.10%.

[0016] B: is the most significant element for improving the hardenability of the steel. An excessively low or excessively high content thereof is detrimental to the hardenability. An excessively high content of B increases the welding crack sensitivity, and thus reduces the welding performance of the steel plate. In the present invention, the content of B is controlled to be 0.0010-0.0020%.

[0017] S and P: are harmful impurity elements in the steel, and tend to form defects such as segregation and inclusions. As impurity elements, they bring adverse effects to the toughness of the steel plate (in particular the toughness of the core portion of the steel plate) and the toughness near the weld, so the contents thereof should be minimized. In the present invention, P is controlled to be ≤0.010% and S is controlled to be ≤0.002%; and during the smelting of the molten steel, calcium treatment is used to spheroidize the inclusions and homogenize the distribution of inclusions uniform, so as to reduce their adverse effect on plasticity and toughness.

[0018] The present application further provides a method for manufacturing the above rack steel plate, comprising the following steps:

[0019] Steel smelting: Raw materials for smelting are prepared according to the elemental composition by using high-quality pig iron and steel plate scrap, the raw materials are sequentially subjected to electric furnace smelting, LF refining, VD refining and mold casting, so as to produce molten steel having S ≤ 0.002%, P ≤ 0.010%, H ≤ 0.00015%, O ≤ 0.0015%, and N ≤ 0.007%; preferably, after the vacuum in the VD refining process is broken, a calcium treatment is performed by feeding a silicon-calcium wire, so as to modify inclusions.

[0020] Casting: Preferably, the molten steel is cast into a flat steel ingot by mold casting (ingot casting). Preferably, a steel ingot mold and a bottom plate are preheated to 60-130°C so as to be thoroughly dried. During pouring, it is preferable to adopt a low superheat of 30-40 °C, with argon protection throughout the entire pouring process. After hot top stripping, the steel ingot is placed into a pit with the mold for slow cooling; after slow cooling for about 48 hours, the steel ingot is stripped from the mold.

[0021] Cogging: The ingot (e.g., after stripping from the mold) is hot-charged into a soaking furnace, heated to 1220-1260°C and sufficiently soaked; for a relatively thick ingot, the soaking time can be 10-20 hours; after the ingot is discharged from the furnace, it is subjected to high-pressure water descaling, and then cogging is performed to obtain an intermediate slab. The intermediate slab is air-cooled on a cooling bed. After controlled cooling to 300-400°C, the intermediate slab is taken off the cooling bed, and then a hydrogen diffusion heat treatment is performed. Preferably, the intermediate slab is charged into a furnace and heated to within 650±20°C, soaked for 48 hours, and then furnace-cooled; after the slab is cooled to about 300°C, it is discharged from the furnace and air-cooled. The hydrogen diffusion heat treatment is performed to sufficiently reduce the H content in the steel slab, so as to ensure that the core performance of the finished steel plate meets the requirements.

[0022] Rolling: A hot rolling deformation process is employed. At first, the intermediate slab is heated. Preferably, the intermediate slab is first preheated so that a surface temperature is ≥150°C, and then the intermediate slab is charged into a continuous heating furnace and heated to 1220-1260°C, and soaked for 2.5-4 hours, so as to allow full solid solution of alloying elements in the steel and to ensure the uniformity of the composition and performance of the final product. After the slab is discharged from the furnace, the slab is subjected to high-pressure water descaling treatment, and then heavy-reduction rolling is performed at a temperature of ≥1000°C. The highest single-pass reduction ratio is 22.5%. The rolled steel plate is sent to a cooling bed for air cooling.

[0023] After the steel plate is air-cooled on the cooling bed to 500-600°C, the steel plate is removed from the cooling bed and covered for slow cooling. After the surface temperature of the steel plate drops to 100-200°C by slow cooling, the cover is removed and the steel plate is air-cooled to room temperature.

[0024] Quenching and tempering treatment: The slowly-cooled steel plate is subjected to quenching and tempering treatment, so as to obtain a finished rack steel plate. Preferably, the quenching heating in the quenching and tempering stage is performed by using a continuous furnace. For quenching, the heating temperature is 850-900°C, the soaking time of the steel plate in the furnace is 1.8-2.2 min / mm, and the steel plate is water-quenched to ≤100°C by using a roller-type quenching machine, followed by air cooling to room temperature. The tempering treatment is also performed by using a continuous furnace. For tempering, the heating temperature is 580-620°C, the soaking time of the steel plate in the furnace is 3.5-5.5 min / mm. After the steel plate is discharged from the furnace, it is air-cooled to room temperature.

[0025] The present invention, aiming to satisfy the demands in the offshore equipment manufacturing industry for rack steel plates used in offshore wind power installation platforms, uses an optimized composition design, molten steel having high purity, and mold-cast flat steel ingots produced by low-superheat casting / pouring with full-process argon protection, and uses subsequent cogging, rolling and quenching and tempering heat treatment, as well as sufficiently reducing the H content throughout the entire production process, so as to produce rack steel plates having high strength, high hardness and hardness uniformity, good plasticity, high low-temperature toughness and good weldability, which can be used for manufacturing the leg racks of offshore wind power installation platforms. The thickness of the steel plate manufactured by the present invention can reach 160 mm.

[0026] The rack steel plate not only has excellent tensile properties (yield strength ≥1000 MPa, tensile strength ≥1100 MPa, elongation ≥12%), but also has a Brinell hardness of ≥320 HBW across the full thickness of the steel plate , a transverse Charpy impact energy of ≥46 J at low temperature (no matter whether at -40°C at a quarter thickness position or at -30°C at a half-thickness position), and good weldability.DESCRIPTION OF DRAWINGS

[0027] Fig. 1 shows the microstructures under optical metallographic microscopy and scanning transmission electron microscopy (STEM), at the near-surface position (a) and (b), at the quarter thickness position (c) and (d), and at the half-thickness position (e) and (f) of the steel plate; Fig. 2 shows the hardness variation along the thickness direction of the steel plate; Fig. 3 shows curves of cold cracking susceptibility as a function of temperature, which are measured on samples taken at the quarter thickness position and at the half-thickness position of the steel plate, respectively. DETAILED EMBODIMENTS

[0028] The present invention will be further described in detail below in conjunction with the examples. The examples described below are exemplary and intended to explain the present invention, but should not be understood as limitations to the present invention.Example 1

[0029] The rack steel plate involved in this example has a thickness of 160 mm, and the components contained therein and the mass percentages thereof are: C: 0.35%, Si: 0.23%, Mn: 0.93%, P: 0.005%, S: 0.001%, Cr: 0.57%, Mo: 0.58%, Ni: 1.48%, Al: 0.072%, V: 0.03%, Nb: 0.02%, Ti: 0.002%, N: 0.005%, B: 0.0013%, the balance being iron and unavoidable impurity elements.

[0030] The production process of the rack steel plate is as follows:

[0031] Raw materials for smelting are prepared according to the chemical composition of the above rack steel plate, and sequentially subjected to: electric furnace smelting - LF refining - VD refining - mold casting - slow cooling in a slow cooling pit - steel ingot heating - cogging - hydrogen diffusion treatment - (intermediate slab) steel slab heating - high-pressure water descaling - rolling - slow cooling under a cover- quenching and tempering treatment.

[0032] Further, the processes of the above manufacturing steps involving cogging, steel slab rolling, hydrogen diffusion treatment, slow cooling under a cover and cooling are specifically as follows: the steel ingot after being stripped from the mold is directly hot-charged into a soaking furnace, heated to 1240-1260°C and soaked for 18 hours, then discharged from the furnace, subjected to high-pressure water descaling, and then cogging is performed to obtain a steel slab having a thickness of 450 mm. The steel slab is air-cooled on a cooling bed to about 350°C, taken off the cooling bed, and subjected to hydrogen diffusion annealing (heated to 650°C, soaked for 48 hours, and then furnace-cooled). After the steel slab is furnace-cooled to about 300°C, it is discharged from the furnace and air-cooled, and the steel slab is subjected to warm cleaning (at 150-300°C).

[0033] The cogged steel slab is preheated until a surface temperature reaches about 350°C, then the steel slab is charged into a continuous heating furnace, heated to 1230°C and soaked for 3 hours, and then discharged from the furnace. After high-pressure water descaling treatment, a heavy-reduction rolling is performed on the steel slab at a temperature of ≥1030°C. The highest single-pass reduction ratio of the rolling is 22.5%. The rolled steel plate is sent to a cooling bed for air cooling. The final plate has a thickness of 160 mm. After the steel plate is air-cooled on the cooling bed to about 520°C, the steel plate is removed from the cooling bed and covered for slow cooling. After the surface temperature of the steel plate drops to about 180°C by slow cooling, the cover is removed and the steel plate is air-cooled to room temperature.

[0034] The slowly-cooled steel plate enters a continuous furnace for quenching heating. For quenching, the heating temperature is 870°C, the soaking time of the steel plate in the furnace is 2.0 min / mm, and the steel plate is water-quenched to a surface temperature of about 70°C by using a roller-type quenching machine, and then sent to a cooling bed and air-cooled to room temperature. The quenched steel plate is subjected to tempering treatment by using a continuous furnace. For tempering, the heating temperature is 600°C, the soaking time of the steel plate in the furnace is 4.5 min / mm. After the steel plate is discharged from the furnace, it is air-cooled to room temperature.

[0035] The microstructures of the finished steel plate manufactured by the above process at different positions in the thickness direction of the steel plate are shown in Fig. 1. The near-surface region of the steel plate has a microstructure of tempered troostite, the quarter thickness position of the steel plate has a microstructure of tempered troostite + tempered lower bainite, and the core portion of the steel plate, i.e., the half-thickness position of the steel plate, has a microstructure of tempered troostite + tempered lower bainite + a small amount of tempered granular bainite. Across the entire thickness cross-section of the steel plate, the precipitated phases are relatively fine. As a result, the finished steel plate not only has high strength, good plasticity and high low-temperature toughness, but also has high hardness and hardness uniformity, and the maximum variation in hardness along the thickness direction is <30 HBW. The finished steel plate has excellent comprehensive properties, and the mechanical properties thereof are shown in Table 1 and Fig. 2.

[0036] The weldability of the finished steel plate manufactured with the above composition and process is evaluated by means of an oblique Y-groove cold cracking susceptibility test. The test is performed in accordance with the national standard GB / T 32260.2-2015 "Destructive tests on welds in metallic materials - Cold cracking tests for weldments - Arc welding processes", and the results are shown in Fig. 3. When calculated on the basis of a critical value of 20% cracking, the preheating temperature at the half-thickness position of the plate is about 120°C, and the preheating temperature at the quarter thickness position is about 135°C; when calculated on the basis of 0% cracking, the preheating temperature at the half-thickness position is 190°C, and that at the quarter thickness position is 175°C. It can be seen that, even when the most stringent evaluation method is adopted (i.e., on the basis of 0% cracking), the steel plate manufactured by the present invention still has good weldability. Table 1. Mechanical properties of the large-thickness rack steel plate manufactured in the exampleExampl eSampling positionYield strength (MPa)Tensile strength (MPa)Elongation (%)Charpy impact propertiesTemperature (°C)Impact energy (J, transverse)1Quarter thickness1078113413.5-40645960Half-thickness1037113314.5-30596256

[0037] In addition to the above examples, the present invention may include other embodiments. Any technical solutions formed by equivalent transformation or equivalent substitution shall fall within the protection scope of the claims of the present invention.

Claims

1. A rack steel plate, characterized in that the elemental composition, in mass percentages, is: C: 0.30-0.40%, Si: 0.15-0.35%, Mn: 0.85-1.20%, P: ≤0.010%, S: ≤0.002%, Cr: 0.45-0.75%, Mo: 0.45-0.65%, Ni: 1.3-1.8%, Al: 0.06-0.10%, V: 0.02-0.05%, Nb: 0.01-0.04%, Ti: ≤0.006%, N: ≤0.007%, B: 0.001-0.002%, and the balance being iron and unavoidable impurity elements.

2. The rack steel plate according to claim 1, characterized in that the steel plate has a production thickness of ≤160 mm, a yield strength of ≥1000 MPa, a tensile strength of ≥1100 MPa, an elongation of ≥12%, a transverse Charpy impact energy of ≥46 J at -40°C at a quarter thickness position, and a transverse Charpy impact energy of ≥46 J at -30°C at a half-thickness position.

3. The rack steel plate according to claim 1, characterized in that a near-surface region of the steel plate has a microstructure of tempered troostite, the quarter thickness position of the steel plate has a microstructure of tempered troostite + tempered lower bainite, and the half-thickness position of the steel plate has a microstructure of tempered troostite + tempered lower bainite + a small amount of tempered granular bainite.

4. A method for manufacturing the rack steel plate according to claim 1, <b>characterized in comprising the following steps: Step 1, steel smelting: smelting molten steel according to the designed composition; Step 2, casting and slow cooling: casting the molten steel into a steel ingot or slab, and slowly cooling the steel ingot or slab; Step 3, cogging: reheating the steel ingot or slab to 1220-1260°C and soaking, so as to allow solid solution of elements and to allow homogenization of microstructure, then performing cogging to obtain an intermediate slab, and slowly cooling the intermediate slab to 300-400°C; Step 4, hydrogen diffusion treatment: reheating the intermediate slab to induce hydrogen diffusion; Step 5, rolling: reheating the hydrogen-diffusion-treated intermediate slab to 1220-1260°C and soaking, so as to allow solid solution of elements and to allow homogenization of microstructure; after the slab is discharged from the furnace, descaling the slab; starting rolling at a temperature above 1000°C by employing heavy-reduction rolling wherein the highest reduction ratio of a single pass is ≥20%, to obtain a steel plate of a target thickness; after rolling, placing the steel plate on a cooling bed for air cooling; after air cooling to 500-600°C, removing the steel plate from the cooling bed and covering the steel plate for slow cooling; after a surface temperature of the steel plate drops to 100-200°C by slow cooling, uncovering the steel plate and air cooling the steel plate to room temperature; Step 6, quenching and tempering treatment: subjecting the steel plate to a quenching and tempering treatment.

5. The method according to claim 4, characterized in that in Step 1, raw materials for smelting are prepared according to the elemental composition by using high-quality pig iron and steel plate scrap, the raw materials are sequentially subjected to electric furnace smelting, LF refining, and VD refining; after the vacuum in the VD refining process is broken, a calcium treatment is performed, so as to produce molten steel having S ≤ 0.002%, P ≤ 0.010%, H ≤ 0.00015%, O ≤ 0.0015%, and N ≤ 0.007%.

6. The method according to claim 4, characterized in that in Step 2, the molten steel is cast into a flat steel ingot by mold casting, wherein a superheat during pouring is 30-40°C and a pouring process is protected with argon gas so as to be isolated from air; after hot top stripping, the steel ingot is placed into a pit with the mold for slow cooling; after slow cooling, the steel ingot is stripped from the mold.

7. The method according to claim 6, characterized in that before pouring, a steel ingot mold and a bottom plate are preheated to 60-130°C so as to be thoroughly dried.

8. The method according to claim 4, characterized in that in Step 4, the intermediate slab is reheated to 650±(0-20)°C and soaked, and then furnace-cooled to approximately 300°C, and then air-cooled.

9. The method according to claim 4, characterized in that in Step 6, for quenching a heating temperature is 850-900°C, a soaking time in furnace is 1.8-2.2 min / mm, and water quenching to ≤100°C, and then air cooling; and for tempering a heating temperature is 580-620°C, a soaking time in furnace is 3.5-5.5 min / mm, and air cooling to room temperature after the steel plate is discharged from the furnace.

Images