A method for manufacturing a three-dimensional heterogeneous steel sheet using high-temperature twisting and forging

Abstract

The application belongs to the field of material preparation and specifically relates to a method for preparing three-dimensional heterogeneous steel plates by high-temperature twisting and forging. The method comprises the following steps: (1) surface pretreatment: the surfaces of low-carbon steel rods, interstitial-free steel rods and austenitic stainless steel rods are pretreated; (2) mixed arrangement; (3) high-temperature knobbing at 800-1200 DEG C; (4) high-temperature forging: the steel blank is subjected to high-temperature free forging, and the direction of forging is perpendicular to the radial direction; (5) hot rolling: hot rolling is carried out along the radial direction; (6) low-temperature rolling: cold rolling is carried out along the radial direction; (7) heat treatment, and a heterogeneous steel material composed of austenite, ferrite and martensite is obtained. The process flow of the application is simple, the combination of various steels is realized through high-temperature twisting and forging, and the three-dimensional heterogeneous steel material with ferrite, martensite and austenite alternately distributed is formed through subsequent "high-temperature hot rolling-low-temperature rolling-heat treatment".

Description

Technical Field

[0001] This invention belongs to the field of materials preparation, specifically relating to a method for preparing three-dimensional heterogeneous steel plates using high-temperature torsion and forging. Background Technology

[0002] In the development of modern industry, steel, as the most commonly used structural material, has been widely applied in transportation construction, facilities and equipment, automobiles, and other fields. However, with technological advancements, new materials are emerging in an endless stream, and the service performance requirements for materials are constantly increasing. Steel materials are also continuously improving, transforming towards functionalization and lightweighting. Increasing carbon content is the most common strengthening method for steel materials, but while increasing carbon content enhances the strength and hardness of steel, it reduces plasticity and toughness. Therefore, effectively improving the comprehensive performance of steel is of great significance for promoting the application of steel materials.

[0003] Different microstructures of steel can impart different properties to it. Ferrite and austenite exhibit better plasticity, while martensite effectively enhances the material's strength. Therefore, by adjusting the microstructure of steel to create a multiphase heterogeneous structure, steel can simultaneously possess both plasticity and strength, resulting in superior performance. Common methods for controlling microstructure include plastic deformation and heat treatment.

[0004] A search revealed that He et al. published a paper entitled "Improving ductility by increasing fraction of interfacial zone in low Csteel / 304SS laminates" in Mater. Sci. Eng. A 726(2018)288-297. This paper proposed a method to combine low carbon steel and 304 austenitic stainless steel through hot rolling, followed by subsequent cold rolling and heat treatment to obtain a laminated structure composed of martensite and austenite, achieving good mechanical properties. The advantages of this technique are its relatively simple process and the high strength provided by martensite and the elongation provided by austenite in the resulting three-layer material. However, the disadvantages of this technique are: (1) it is difficult to ensure good interfacial bonding during hot rolling, which affects the final mechanical properties; (2) it is relatively difficult to prepare multilayer structures.

[0005] Further research revealed that invention patent CN 202111370521.2 discloses a method for preparing austenitic / ferritic / martensitic multiphase heterogeneous steel materials. Specifically, it utilizes a rotary friction welding method to alternately weld two steel materials, and then performs high-temperature forging, rolling, and heat treatment on the resulting cylindrical laminated sample to prepare a multiphase heterogeneous steel material composed of ultrafine austenite, ferrite, and martensite. The advantage of this technology is that it obtains multiphase heterogeneous steel plates with good bonding between the heterogeneous material interfaces formed by friction welding. However, this method has the following disadvantages: (1) the prepared plates only have heterogeneous interfaces in the cross-section and do not achieve three-dimensional heterogeneity; (2) both ferrite and martensite structures are provided by low-carbon steel, limiting the adjustment of the multiphase ratio; (3) it requires multiple friction welding and wire cutting processes, resulting in low efficiency. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing three-dimensional heterogeneous steel plates using high-temperature torsion and forging.

[0007] The technical solution to achieve the objective of this invention is: a method for preparing three-dimensional heterogeneous steel plates using high-temperature torsion and forging, comprising the following steps:

[0008] Step (1): Surface pretreatment: Pretreatment is performed on the surfaces of low carbon steel bars, interstitial steel bars and austenitic stainless steel bars;

[0009] Step (2): Mixed arrangement: The three different types of steel bars pretreated in step (1) are distributed at intervals and mixed outward in a spiral shape, and fixed by welding;

[0010] Step (3): High-temperature knob: Turn the knob at 800-1200℃;

[0011] Step (4): High-temperature forging: The steel billet obtained in step (3) is subjected to high-temperature free forging, and the forging direction is perpendicular to the radial direction;

[0012] Step (5): Hot rolling: Hot rolling is performed in the radial direction to improve the interfacial bonding between steel grades;

[0013] Step (6): Low-temperature rolling: Cold rolling is performed in the radial direction to refine the microstructure and cause stress-induced martensitic transformation in the austenite region;

[0014] Step (7): Heat treatment: After vacuum heat treatment, the steel plate is quenched to obtain a heterogeneous steel material composed of austenite, ferrite and martensite.

[0015] Furthermore, the low-carbon steel bar, interstitial steel bar, and austenitic stainless steel bar in step (1) have a diameter of 0.5-10 mm and a length of 50-200 mm.

[0016] Furthermore, the carbon content of low-carbon steel is 0.08-0.25 wt.%, the carbon content of interstitial atomless steel is less than 0.01 wt.%, and austenitic stainless steel is 3-series austenitic stainless steel.

[0017] Furthermore, the surface pretreatment in step (1) specifically involves: using sandpaper to polish the surface of the steel rod and then pickling it to remove surface oil and impurities.

[0018] Furthermore, the number of layers in the mixed arrangement in step (2) is 2-20.

[0019] Furthermore, in step (3), the high-temperature knob is operated using a torsion forming device. One end of the steel billet obtained in step (2) is fixed, and the other end is rotated through a chuck knob. The rotation speed of the chuck is 10-30 revolutions per minute, and the number of turns of the knob is 1-10 turns.

[0020] Furthermore, in step (4), the forging temperature of high-temperature free forging is 900-1200℃. Before each forging, the billet is heated to 1100-1200℃ and held for 10-60 minutes. The deformation of the forging is 10%-50%.

[0021] Furthermore, step (5) hot rolling specifically involves: rolling temperature of 900-1000℃, rolling amount of 5%-10% per rolling pass, and cumulative rolling amount of 10%-50%;

[0022] Step (6) Low-temperature rolling specifically involves rolling at a temperature of 25℃-400℃, with each rolling depth being 1%-5%, and the cumulative rolling depth being 50%-95%.

[0023] Furthermore, step (7) heat treatment specifically involves: performing vacuum heat treatment at 700℃-950℃ on the steel plate after low-temperature rolling, holding for 1-60 minutes, and then quenching the steel plate.

[0024] A three-dimensional heterogeneous steel plate is prepared using the method described above.

[0025] A method for preparing three-dimensional heterogeneous steel plates using high-temperature torsion and forging involves first selecting several low-carbon steel, ultra-low-carbon steel, and austenitic stainless steel wires or bars of a certain length and diameter; then, mixing and arranging these wires or bars in a specific pattern, and fixing them by welding; subsequently, using a torsion device, the mixed and arranged wires / bars are subjected to high-temperature torsion, causing the dissimilar wires / bars to bond together, forming a spiral steel billet; then, the spiral steel billet is forged into a plate shape through high-temperature forging, resulting in an alternating distribution of the three materials in all directions of the plate; finally, the forged plate billet is subjected to rolling processing, including hot rolling and low-temperature rolling. The rolling process involves two parts: hot rolling further enhances the bonding between dissimilar materials, while low-temperature rolling refines the microstructure and induces stress-induced martensitic transformation in austenitic steel. Finally, the rolled plate undergoes vacuum annealing and water quenching. Through heat treatment, the low-carbon steel region transforms into martensite or ferrite-martensite dual-phase structure, the ultra-low-carbon steel region recrystallizes to form fine ferrite, and the austenitic region undergoes reverse martensitic transformation to form fine austenite grains, resulting in a heterogeneous plate with alternating distributions of martensite, austenite, and ferrite in three-dimensional space. The excellent plasticity of ferrite and austenite, along with the high strength of martensite, work synergistically to improve the overall mechanical properties of the material.

[0026] Compared with the prior art, the significant advantages of this invention are:

[0027] (1) This invention uses high-temperature twisting and forging of steel wire / bar to form a whole billet of low carbon low alloy steel, ultra low carbon steel, austenitic stainless steel, etc., realizing the combination of multiple steel materials. Through subsequent "high temperature hot rolling-low temperature rolling-heat treatment", the low carbon low alloy steel region forms a high-strength hard phase region mainly composed of martensite, and the ultra low carbon steel and austenitic stainless steel regions form a soft phase region with ultra-fine ferrite and austenitic grains, forming a multi-phase synergistic toughening heterogeneous steel material with both strength and plasticity.

[0028] (2) The shape and layer thickness of the billet can be flexibly controlled by high-temperature twisting and forging. After heat treatment, the characteristics of dissimilar materials are distributed alternately in all directions, resulting in a steel plate with a three-dimensional heterogeneous structure. Compared with traditional materials with only a layered heterogeneous cross section, the comprehensive mechanical properties of three-dimensional heterogeneous materials are better.

[0029] (3) This invention utilizes high-temperature torsion to arrange the bars sequentially along the spiral direction, and obtains plates with alternating distribution of steels of different compositions through subsequent high-temperature forging. The number of heterogeneous interfaces in the three-dimensional space of the heterogeneous plate can be effectively increased by increasing the number of high-temperature torsion turns. In contrast, traditional methods for preparing multiphase heterogeneous materials can only increase the number of heterogeneous interfaces by increasing the number of plates or bars arranged. Therefore, this patent can save materials while more flexibly controlling the number of heterogeneous interface layers in multiphase heterogeneous plates. Thus, the microstructure characteristics of each type of steel can be flexibly controlled by controlling the rolling and heat treatment processes.

[0030] (4) The present invention employs a variety of high-temperature plastic processing methods, such as high-temperature twisting, high-temperature free forging, and high-temperature rolling, which can improve the interface bonding quality of heterogeneous materials. The process flow is simple and the production equipment is simple. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the preparation method of the present invention.

[0032] Figure 2 Figure 1 shows the structural diagrams of the steel plate before and after heat treatment; where a is a schematic diagram of the overall distribution of different materials in the steel plate before heat treatment; b is a schematic diagram of the microstructure corresponding to the upper surface of figure a after heat treatment; c is a schematic diagram of the microstructure corresponding to the front surface of figure a after heat treatment; c is a schematic diagram of the microstructure corresponding to the cross section of figure a after heat treatment.

[0033] 1-Austenitic steel bar, 2-IF steel bar, 3-Low carbon steel bar, 4-Cylindrical steel billet, 5-Torsion device, 6-Torsioned spiral specimen, 7-Heating furnace, 8-Forging machine whetstone, 9-Heterogeneous plate, 10-Roll. Detailed Implementation

[0034] The present invention will now be described in further detail with reference to the accompanying drawings.

[0035] like Figure 1 As shown, a method for preparing three-dimensional heterogeneous steel plates using high-temperature torsion and forging mainly includes the following steps:

[0036] The first step is surface pretreatment: Prepare a certain quantity and size of low-carbon steel, interstitial steel and austenitic stainless steel wire / bar. Use sandpaper to grind the surface of the wire / bar and pickle it to remove surface oil and impurities. The diameter of the wire / bar is 0.5-10mm, the carbon content of the low-carbon steel is 0.08-0.25wt.%, the carbon content of the ultra-low carbon steel is less than 0.01wt.%, and the austenitic stainless steel is 3 series austenitic stainless steel.

[0037] The second step is mixed arrangement: three types of steel wires / rods of the same length are obtained by electrical discharge wire cutting, with a length range of 50-200mm. The three different types of steel wires / rods are distributed alternately and mixed outward in a spiral shape. The number of layers from the inside to the outside is 2-20. The steel wires / rods are fixed by welding.

[0038] The third step is high-temperature twisting and joining: using a twisting forming device, one end of the steel billet is fixed to the left side of the device, and the other end can be twisted through a chuck to join dissimilar materials and form a spiral-shaped steel billet. The shape and internal structure of the spiral steel billet can be controlled by adjusting the number of previously arranged welding turns and the number of twisting turns. The rotation speed of the chuck of the twisting device is 10-30 revolutions per minute, and the number of twisting turns is 1-10 turns. The twisting and joining is carried out at a high temperature of 800-1200℃. An external induction heating device is used to heat the steel billet. After heating to the specified temperature and holding for 10-60 minutes, the twisting is performed.

[0039] The fourth step is high-temperature forging: the twisted steel billet is subjected to high-temperature free forging. Through multiple forging passes, the interfacial bonding of dissimilar materials is further improved, and the twisted sample is forged into a cubic plate. The forging temperature of high-temperature free forging is 900-1200℃. Before each forging, the spiral steel billet is heated to 1100-1200℃ and held for 10-60 minutes. The total thickness of the cubic laminated sample obtained by free forging is 10-25mm. To facilitate subsequent low-temperature rolling, the width of the final cubic plate is greater than the total thickness.

[0040] The fifth step is rolling: Rolling is divided into two steps: hot rolling and cold rolling. Hot rolling further improves the interfacial bonding between different steel grades, while cold rolling refines the microstructure and causes stress-induced martensitic transformation in the austenite region. The hot rolling temperature is 900-1000℃, with each rolling increment being 5%-10% and the cumulative rolling increment being 10%-50%. The low-temperature rolling temperature is 25℃-400℃, with each rolling increment being 1%-5% and the cumulative rolling increment being 50%-95%. Before hot rolling and low-temperature rolling, the samples are placed in a muffle furnace and heated and held for 5-30 minutes.

[0041] Step 6, heat treatment: The rolled steel plate is subjected to vacuum heat treatment at 700℃-950℃. During the heating process, inert gas argon is introduced as a protective atmosphere and the holding time is 1-60 minutes. After that, the steel plate is quenched to obtain a heterogeneous steel plate composed of martensite, ferrite and austenite.

[0042] Example 1

[0043] The three types of steel bars selected are 304 austenitic stainless steel, Q235 low carbon steel, and ultra-low carbon steel with a composition of Fe-0.003C-0.005Si-0.15Mn-0.054Al-0.042Ti (wt.%).

[0044] (1) Surface pretreatment: Select two 304 austenitic stainless steel bars with a diameter of 8mm and a length of 100mm, three Q235 low carbon steel bars and two ultra-low carbon steel bars. Use sandpaper to polish the surface of the steel bars and pickle them to remove surface oil and impurities.

[0045] (2) Mixed arrangement: The three different types of steel bars are distributed at intervals and arranged in a spiral outward. There are two layers from the inside to the outside. The center is Q235 low carbon steel, and the six steel bars in the outer layer are arranged in the order of low carbon steel, ultra-low carbon steel and austenitic steel. The steel bars are fixed by welding.

[0046] (3) High-temperature twisting and joining: Using a twisting and forming device, one end of the mixed and arranged steel billet is fixed on the left side of the device, and the other end can be twisted by rotating the chuck to combine dissimilar materials and form a spiral-shaped steel billet. Before twisting, the steel billet is heated to 1000℃ by an external induction heating device and kept at that temperature for 20 minutes. After that, the heating device is removed and the billet is twisted. The rotation speed of the chuck is 30 revolutions / minute, and the number of twisting turns is 5 turns.

[0047] (4) High-temperature forging: The steel billet after twisting is subjected to high-temperature free forging. After multiple forging passes, the interface bonding of dissimilar materials is further improved and the twisted sample is forged into a cubic plate. The forging temperature range of high-temperature free forging is 900-1100℃. Before each forging, the spiral steel billet is heated to 1100℃ and held for 10 minutes. The total thickness of the cubic steel billet obtained by multiple forgings is 15mm.

[0048] (5) Rolling process: Rolling is divided into two steps: hot rolling and cold rolling. Hot rolling further improves the interfacial bonding between different steel grades, while cold rolling refines the microstructure and causes stress-induced martensitic transformation in the austenite region. The billet is first hot rolled at 1100℃, and the final rolling temperature is 850℃. It is then air-cooled to room temperature to reduce the plate thickness to 8mm. Subsequently, the steel plate is rolled at room temperature with a reduction of 0.3mm each time, and the final plate thickness is 2mm.

[0049] (6) Heat treatment: The rolled steel plate is heat-treated at 850℃ for 2 minutes, followed by quenching. The structural diagrams of the steel billet before and after heat treatment are shown in the figure. Figure 2 As shown in the figure, the above steps yielded a three-dimensional heterogeneous steel material composed of austenite, ferrite, and martensite.

[0050] Example 2

[0051] The three types of steel bars selected are 304 austenitic stainless steel, Q235 low carbon steel, and ultra-low carbon steel with a composition of Fe-0.003C-0.005Si-0.15Mn-0.054Al-0.042Ti (wt.%).

[0052] (1) Surface pretreatment: Select 6 304 austenitic stainless steel bars with a diameter of 8mm and a length of 100mm, 6 Q235 low carbon steel bars and 7 ultra-low carbon steel bars. Use sandpaper to polish the surface of the steel bars and pickle them to remove surface oil and impurities.

[0053] (2) Mixed arrangement: The three different types of steel bars are distributed at intervals and arranged in a spiral outwards. There are 3 layers from the inside to the outside. The center is made of ultra-low carbon steel. The outermost layer of 6 steel bars are arranged in the order of ultra-low carbon steel, low carbon steel and austenitic steel. The outermost layer of 12 steel bars are also arranged in the order of ultra-low carbon steel, low carbon steel and austenitic steel. The steel bars are fixed by welding.

[0054] (3) High-temperature twisting and joining: Using a twisting and forming device, one end of the mixed and arranged steel billet is fixed on the left side of the device, and the other end can be twisted by rotating the chuck to combine dissimilar materials and form a spiral-shaped steel billet. Before twisting, the steel billet is heated to 1000℃ by an external induction heating device and kept at that temperature for 20 minutes. After that, the heating device is removed and the billet is twisted. The rotation speed of the chuck is 30 revolutions / minute, and the number of twisting turns is 6.

[0055] (4) High-temperature forging: The steel billet after twisting is subjected to high-temperature free forging. After multiple forging passes, the interface bonding of dissimilar materials is further improved and the twisted sample is forged into a cubic plate. The forging temperature range of high-temperature free forging is 900-1100℃. Before each forging, the spiral steel billet is heated to 1100℃ and held for 10 minutes. The total thickness of the cubic steel billet obtained by multiple forgings is 20mm.

[0056] (5) Rolling process: Rolling is divided into two steps: hot rolling and cold rolling. Hot rolling further improves the interfacial bonding between different steel grades, while cold rolling refines the microstructure and causes stress-induced martensitic transformation in the austenite region. The billet is first hot rolled at 1100℃, and the final rolling temperature is 850℃. It is then air-cooled to room temperature to reduce the plate thickness to 8mm. Subsequently, the steel plate is rolled at room temperature with a reduction of 0.3mm each time, and the final plate thickness is 2mm.

[0057] (6) Heat treatment: The rolled steel plate is heat treated at 850℃ for 2 minutes, and then the steel plate is quenched to obtain a heterogeneous steel material composed of austenite, ferrite and martensite.

[0058] Example 3

[0059] The three types of steel bars selected are 304 austenitic stainless steel, Q235 low carbon steel, and ultra-low carbon steel with a composition of Fe-0.003C-0.005Si-0.15Mn-0.054Al-0.042Ti (wt.%).

[0060] (1) Surface pretreatment: Select 13 304 austenitic stainless steel bars with a diameter of 8mm and a length of 100mm, 12 Q235 low carbon steel bars and 12 ultra-low carbon steel bars. Use sandpaper to polish the surface of the steel bars and pickle them to remove surface oil and impurities.

[0061] (2) Mixed arrangement: The three different types of steel bars are distributed at intervals and arranged in a spiral outwards. The number of layers from the inside to the outside is 4. The center position is austenitic steel, and the outer layers are arranged in the order of austenitic steel, low carbon steel and ultra-low carbon steel until the end. The steel bars are fixed by welding.

[0062] (3) High-temperature twisting and joining: Using a twisting and forming device, one end of the mixed and arranged steel billet is fixed on the left side of the device, and the other end can be twisted by rotating the chuck to combine dissimilar materials and form a spiral-shaped steel billet. Before twisting, the steel billet is heated to 1000℃ by an external induction heating device and kept at that temperature for 20 minutes. After that, the heating device is removed and the billet is twisted. The rotation speed of the chuck is 30 revolutions / minute, and the number of twisting turns is 7.

[0063] (4) High-temperature forging: The steel billet after twisting is subjected to high-temperature free forging. After multiple forging passes, the interface bonding of dissimilar materials is further improved and the twisted sample is forged into a cubic plate. The forging temperature range of high-temperature free forging is 900-1100℃. Before each forging, the spiral steel billet is heated to 1100℃ and held for 10 minutes. The total thickness of the cubic steel billet obtained by multiple forgings is 18mm.

[0064] (5) Rolling process: Rolling is divided into two steps: hot rolling and cold rolling. Hot rolling further improves the interfacial bonding between different steel grades, while cold rolling refines the microstructure and causes stress-induced martensitic transformation in the austenite region. The billet is first hot rolled at 1100℃, and the final rolling temperature is 850℃. It is then air-cooled to room temperature to reduce the plate thickness to 8mm. Subsequently, the steel plate is rolled at room temperature with a reduction of 0.3mm each time, and the final plate thickness is 1.8mm.

[0065] (6) Heat treatment: The rolled steel plate is heat treated at 850℃ for 2 minutes, and then the steel plate is quenched to obtain a heterogeneous steel material composed of austenite, ferrite and martensite.

Claims

1. A method for preparing three-dimensional heterogeneous steel plates using high-temperature torsion and forging, characterized in that, Includes the following steps: Step (1): Surface pretreatment: Pretreatment is performed on the surfaces of low carbon steel bars, interstitial steel bars and austenitic stainless steel bars; Step (2): Mixed arrangement: The three different types of steel bars pretreated in step (1) are distributed at intervals and mixed outward in a spiral shape, and fixed by welding; Step (3): High-temperature rotation: Rotate the knob at 800-1200 ℃; Step (4): High-temperature forging: The steel billet obtained in step (3) is subjected to high-temperature free forging, with the forging direction perpendicular to the radial direction; the forging temperature of the high-temperature free forging in step (4) is 900-1200 ℃, and the steel billet is heated before each forging at a temperature of 1100-1200 ℃ for a holding time of 10-60 min; the deformation amount of forging is 10%-50%; Step (5): Hot rolling: Hot rolling is carried out in the radial direction to improve the interfacial bonding between steel grades; Specifically, hot rolling in step (5) is as follows: rolling temperature 900-1000 ℃, rolling amount per roll is 5%-10%, and cumulative rolling amount is 10%-50%; Step (6): Low-temperature rolling: Cold rolling is performed in the radial direction to refine the microstructure and cause stress-induced martensitic transformation in the austenite region; Specifically, the low-temperature rolling in step (6) is as follows: the rolling temperature is 25 ºC-400 ºC, the rolling amount per rolling pass is 1%-5%, and the cumulative rolling amount is 50%-95%; Step (7): Heat treatment: After vacuum heat treatment, the steel plate is quenched to obtain a heterogeneous steel material composed of austenite, ferrite and martensite.

2. The method according to claim 1, characterized in that, The low-carbon steel bar, interstitial steel bar, and austenitic stainless steel bar in step (1) have a diameter of 0.5-10 mm and a length of 50-200 mm.

3. The method according to claim 2, characterized in that, Low-carbon steel has a carbon content of 0.08-0.25 wt.%, interstitial atomless steel has a carbon content of less than 0.01 wt.%, and austenitic stainless steel is a series 3 austenitic stainless steel.

4. The method according to claim 3, characterized in that, The surface pretreatment in step (1) is as follows: the surface of the steel bar is polished with sandpaper and acid-washed to remove surface oil and impurities.

5. The method according to claim 4, characterized in that, In step (2), the number of mixed layers is 2-20.

6. The method according to claim 5, characterized in that, Step (3) High-temperature twisting is performed using a torsion forming device. One end of the steel billet obtained in step (2) is fixed, and the other end is twisted through a chuck. The rotation speed of the chuck is 10-30 revolutions / minute, and the number of twists is 1-10.

7. The method according to claim 6, characterized in that, Step (7) heat treatment specifically involves: performing vacuum heat treatment at 700 ºC-950 ºC on the steel plate after low-temperature rolling, holding for 1-60 min, and then quenching the steel plate.

8. A three-dimensional heterogeneous steel plate, characterized in that, Prepared using the method described in any one of claims 1-7.

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