Production process of controlled rolling and controlled cooling round steel for high-strength crankshaft of commercial vehicle

Abstract

The invention belongs to the technical field of controlled rolling and controlled cooling of alloy structural steel, and relates to a production process of controlled rolling and controlled cooling round steel for a high-strength crankshaft of a commercial vehicle. The round steel comprises the following components: 0.42 to 0.50 percent of C, 0.5 to 0.8 percent of Si, 1.20 to 1.4 percent of Mn, less than or equal to 0.035 percent of P, 0.035 to 0.065 percent of S, 0.15 to 0.30 percent of V, 0.015 to 0.025 percent of Ti, 0.025 to 0.06 percent of Nb, 0.30 to 0.50 percent of Cr, 0.10 to 0.20 percent of Ni, 0 to 0.20 percent of Cu, 3 to 7 ppm of B, less than or equal to 1.5 ppm of H, less than or equal to 15 ppm of O, 130 to 200 ppm of N and the balance of Fe and inevitable impurities. The components and the optimization of a controlled rolling and controlled cooling process are combined, the grain size of the raw materials is refined, micro-alloy elements are fully separated out, the comprehensive performance of the round steel is improved, and the requirement for the high-strength crankshaft of the commercial vehicle is met.

Description

Technical Field

[0001] The present invention belongs to the technical field of controlled rolling and controlled cooling of alloy structural steel, and specifically relates to a production process of controlled rolling and controlled cooling round steel for high-strength crankshafts of commercial vehicles. Background Art

[0002] At present, common grades of steel for hot forging non-quenched and tempered crankshafts include 49MnVS3, 48MnV, C38+N2, 38MnVS6 and other steel grades. The properties of this series of steel grades are close to those of quenched and tempered steel. Generally, the yield strength is <650 MPa and the tensile strength is <900 MPa. With the proposal of the "dual carbon" strategy, the competition in the automotive industry is becoming increasingly fierce. New energy vehicles have risen strongly, and the concept of energy conservation and emission reduction is a common understanding. However, the weight reduction of automotive engines requires higher-strength materials, and the performance of traditional non-quenched and tempered steel materials can no longer meet the needs. There is an urgent need to develop new materials with higher strength to better meet the requirements of high-strength crankshafts of commercial vehicles. Summary of the Invention

[0003] The purpose of the present invention is to overcome the technical defects existing in the prior art. Aiming at the technical problem of poor performance of traditional non-quenched and tempered steel materials, a production process of controlled rolling and controlled cooling round steel for high-strength crankshafts of commercial vehicles is provided. Through the effective combination of composition and process control, the grain size of raw materials is refined, the precipitation of microalloying elements is sufficient, and the refined original structure does not coarsen during the forging heating of the crankshaft (induction heating), improving the comprehensive performance and meeting the requirements of high-strength crankshafts of commercial vehicles.

[0004] In order to achieve the above purpose, the present invention adopts the following technical solutions; First, a controlled rolling and controlled cooling round steel for high-strength crankshafts of commercial vehicles is provided, which is composed of the following components by mass percentage: C: 0.42 - 0.50%, Si: 0.5 - 0.8%, Mn: 1.20 - 1.4%, P ≤0.035%, S: 0.035 - 0.065%, V: 0.15 - 0.30%, Ti: 0.015 - 0.025%, Nb: 0.025 - 0.06%, Cr: 0.30 - 0.50%, Ni: 0.10 - 0.20%, Cu: 0 - 0.20%, B: 3 - 7 ppm, H: ≤1.5 ppm, O: ≤15 ppm, N: 130 - 200 ppm, and the balance is Fe and unavoidable impurities.

[0005] Aiming at the technical problem of poor performance of traditional non-quenched and tempered steel materials, the present invention first optimizes the alloy design composition, which is specifically described as follows: C: It can increase the pearlite ratio, strength and hardness, but too high content will reduce plasticity and toughness, and too low content of C will affect the strength of hot-rolled round steel. Therefore, the present invention controls the C content within 0.42 - 0.50%.

[0006] Cr: A carbide - forming element that increases the strength of the material and inhibits grain growth. However, the addition of Cr should not be too high, otherwise it will lead to a decrease in plasticity and toughness. Therefore, in the present invention, the Cr content is controlled within 0.30 - 0.50%.

[0007] Mn: Lowers the lower critical point of the steel, increases the supercooling degree during austenite cooling, refines the pearlite structure to improve its mechanical properties, can significantly improve the hardenability of the steel, but has an adverse tendency to increase grain coarsening and temper brittleness. Therefore, in the present invention, the Mn content is controlled within 1.20 - 1.40%.

[0008] S: Can improve the machinability of the material, improve the processing efficiency and surface quality of the material. It causes hot - shortness phenomenon, deteriorates the quality of the steel, and a high sulfur content has an adverse effect on weldability. Therefore, the content should not be too high. In the present invention, the S content is controlled within 0.035 - 0.065%.

[0009] V: Dissolved in austenite can improve the hardenability of the steel, and dissolved in ferrite has a strong solution strengthening effect. After the crankshaft is forged, it has the effect of precipitation strengthening and grain refinement to improve the low - temperature impact toughness. Therefore, in the present invention, the V content is controlled within 0.15 - 0.30%.

[0010] Ti: Has the effect of refining austenite grains and has a strong solution strengthening effect. If it is too high, the precipitates coarsen, reducing the fatigue life of the crankshaft. Therefore, in the present invention, the Ti content is controlled within 0.010 - 0.025%.

[0011] Ni: Improves plasticity and toughness (more obvious in improving low - temperature toughness), improves corrosion resistance, and when used in combination with chromium and molybdenum, improves thermal strength, enabling the crankshaft to adapt to a more complex environment. Therefore, in the present invention, the Ni content is controlled within 0.10 - 0.20%.

[0012] B: Trace amounts of boron can improve the hardenability of the steel, but as the carbon content in the steel increases, the improvement of hardenability gradually weakens and even completely disappears. Therefore, in the present invention, the B content is controlled within 3 - 7 ppm.

[0013] N: Combines with micro - alloying elements such as Al, Ti, and V in the steel to precipitate, having the effect of refining grains and improving the strength and toughness of the steel. If it is too high, it increases the brittleness of the steel. Therefore, in the present invention, the N content is controlled within 130 - 200 ppm.

[0014] The present invention also provides a production process for controlled - rolling and controlled - cooling round steel for high - strength crankshafts of commercial vehicles, including the following steps: electric - arc furnace steelmaking → LF refining → VD vacuum treatment → continuous casting (Φ500) → heating → controlled - rolling and controlled - cooling → cooling bed collection → pit cooling → straightening → finishing, flaw detection, and warehousing.

[0015] The process requirements for the controlled - rolling and controlled - cooling round steel for high - strength crankshafts of commercial vehicles are as follows: (1) The specific process requirements for the electric furnace steelmaking stage are as follows: ① The final C ≥ 0.08%, and the target carbon content is 0.10% ≥ C ≥ 0.25%; ② The target P ≤ 0.015%; ③ The target temperature ≥ 1620 °C; ④ For every 100 - 105 t of molten steel, 500 kg of lime, 400 kg of purification promoter, and 100 - 120 kg of tapping aluminum per furnace are used.

[0016] (2) The process requirements for the LF refining stage: The white slag time ≥ 20 min, and the smelting time ≥ 40 min; The auxiliary material consumption for the LF refining stage: 1.50 kg of silica sand (SiO2) is added per ton of molten steel, and the silica sand is added 5 minutes before the LF refining station exits.

[0017] (3) The process requirements for the VD vacuum treatment stage are as follows: ① The VD high vacuum degree ≤ 67 Pa, the high vacuum holding time ≥ 12 min, and the soft blowing time ≥ 15 min; ② The wire feeding sequence: The wire feeding sequence is in the order of manganese nitride wire, ferrotitanium wire, calcium silicate wire, sulfur wire, tellurium-manganese wire (Te-Mn); Among them, the feeding amount of tellurium-manganese wire (Te-Mn) is 75 m per furnace; the feeding interval between the sulfur wire and the calcium silicate wire ≥ 5 min; calcium silicate wire: 60 m per furnace for the first furnace, and 50 m per furnace for the continuous casting furnace.

[0018] (4) The continuous casting process: Two-stage water cooling is adopted in combination with mold electromagnetic stirring, strand stirring, and final electromagnetic stirring; The flow parameters of the two-stage water cooling are as follows: The flow rate of the first cooling water is 4000 L / min, the flow rate of the second cooling water in zone 1 is 75 L / min, the flow rate of the second cooling water in zone 2a is 50 L / min, and the flow rate of the second cooling water in zone 2b is 42 L / min; The current of the mold electromagnetic stirring is 200 A, the frequency is 2 Hz, the current of the strand stirring is 180 A, the frequency is 8 Hz, the current of the final stirring is 1100 A, the frequency is 8 Hz; the casting speed is 0.39 m / min.

[0019] (5) The hot rolling process: The heating temperature and heating time are as follows: The temperature of the preheating section < 900 °C, the time > 1.0 h, the temperature of the heating section I is 900 - 1100 °C, the time ≥ 1.0 h, the temperature of the heating section II is 1180 - 1280 °C, the time ≥ 3 h, the soaking section temperature is 1200 - 1260 °C, the time ≥ 3 h, and the total heating time ≥ 6.5 h.

[0020] Preferably, the target temperature of the preheating section is 860 °C, the target temperature of the heating section I is 1000 °C, the target temperature of the heating section II is 1230 °C, and the target temperature of the soaking section is 1240 °C.

[0021] The rolling process is as follows: The φ500 specification round billet is taken out of the furnace and the intermediate billet is 270mm*280mm after passing through the blooming mill, and the head and tail are sheared hydraulically; through the controlled cooling equipment, the temperature entering the continuous rolling is controlled at 830 - 840 °C, and 5-stage water cooling is used to control the cooling. The set parameters of the 5-stage water flow are: the water inlet flow of the first stage is 190 - 210 m 3 / h, the water inlet flow of the second stage is 150 - 170 m 3 / h, the water inlet flow of the third stage is 120 - 140 m 3 / h, the water inlet flow of the fourth stage is 110 - 130 mm 3 / h, the water inlet flow of the fifth stage is 150 - 190 mm 3 / h.

[0022] Rolling speed: To control cooling and reduce the rolling speed, the rolling speed is controlled within 0.45 m / s.

[0023] Rolling is carried out through the 6-stand rolling mill in a vertical and horizontal 90° alternating rolling mill; among them, the 5-stage water cooling corresponds to 5 water tanks, which are respectively denoted as the 1# water tank, 2# water tank, 3# water tank, 4# water tank, and 5# water tank; thus, the arrangement of the 6-stand rolling mill and the 5 water tanks is in turn: 1 horizontal stand → 2 vertical stands → 3 horizontal stands → 4 vertical stands → 5 horizontal stands → 6 vertical stands → 1# water tank → 2# water tank → 3# water tank → 4# water tank → 5# water tank.

[0024] (6) Cooling bed collection and pit cooling: When the temperature of the cooling bed is > 400 °C, it enters the pit for slow cooling, and it is taken out of the pit after slow cooling to a temperature ≤ 200 °C, and the slow cooling time is ≥ 24 h. Beneficial effects:

[0025] 1. Through composition optimization, the present invention increases the incubation period of ferrite transformation by adding a specific B element, making the ferrite phase transformation occur in a relatively low temperature range, which is beneficial to increasing the nucleation rate of ferrite, making the pearlite lamellar spacing shorter, increasing the strength of pearlite, and ultimately increasing the strength of the round bar after water penetration.

[0026] 2. The present invention controls the temperature entering the continuous rolling at 830 - 840 °C to precipitate Nb(C, N) compounds, prevent austenite recrystallization during rolling, and prevent austenite coarsening; moreover, low-temperature rolling stores more distortion energy in the matrix, creating conditions for ferrite nucleation and refining ferrite grains.

[0027] 3. The present invention uses 5-stage water cooling to control cooling, among which the water inlet flow of the first-stage water cooling is 190 - 210 m 3 / h, and the water inlet flow of the second-stage water cooling is 150 - 170 m 3 / h, increase the water flow rate in the first and second stages, control the post-water-passing redness temperature within a relatively low range, and the precipitation of V(C,N) compounds becomes more dispersed and finer, giving full play to the precipitation strengthening mechanism. On the other hand, adding tellurium-manganese wire (Te-Mn) during the VD vacuum treatment stage can modify the inclusions, making the inclusions disperse and further improving the comprehensive performance of the finished crankshaft.

[0028] 4. Through the effective combination of composition and process control, the present invention refines the grain size of the raw materials, fully precipitates microalloying elements, and the refined original structure does not coarsen during the forging heating (induction heating) of the crankshaft. At the same time, it has excellent properties. The tensile strength of the round steel is ≥1046 MPa, the yield strength is ≥881 MPa, the elongation after fracture is ≥16%, the reduction of area is ≥45%, and the impact energy at room temperature is ≥43.7 J, meeting the requirements of high-strength crankshafts for commercial vehicles. Description of the Drawings

[0029] Figure 1 It is a picture of the austenite grain size of the round steel prepared in Example 1, and the size in the figure is marked as 20 μm.

[0030] Figure 2 It is a metallographic structure diagram of the round steel prepared in Example 1, and the size in the figure is marked as 20 μm. Detailed Embodiments

[0031] The present invention will be described in detail below in conjunction with examples, but the present invention is not limited to these examples. Example 1:

[0032] The hot-rolled round steel contains the following chemical components by mass percentage (unit, wt%): C: 0.43%, Si: 0.65%, Mn: 1.3%, P: 0.01%, S: 0.04%, V: 0.20%, Ti: 0.020%, Nb: 0.045%, Cr: 0.4%, Ni: 0.15%, Cu: 0.02%, B: 5 ppm, H: 0.9 ppm, O: 18 ppm, N: 160 ppm, and the balance is Fe and unavoidable impurities.

[0033] Through the processes of electric furnace steelmaking, LF refining, VD vacuum treatment, continuous casting, heating, rolling, and pit cooling, a φ160 mm hot-rolled round steel is obtained; The specific implementation process is as follows: (1) The specific process requirements in the electric furnace steelmaking stage are: Process requirements: ① The target carbon is 0.10% ≥ C ≥ 0.25%; ② The target P ≤ 0.015%; ③ The target T ≥ 1620 °C; ④ For every 100 - 105t of molten steel, 500kg of lime, 400kg of slag cleaning agent, and 100 - 120kg / heat of tapping aluminum are used.

[0034] (2)Process requirements for the LF refining stage: The white slag time ≥ 20min, and the smelting time ≥ 40min; Auxiliary material consumption: 1.50kg of silica sand (SiO2) per ton of molten steel. Add silica sand 5 minutes before the LF refining station.

[0035] (3)Process requirements for the VD vacuum treatment stage: ① The VD high vacuum degree ≤ 67Pa, the high vacuum holding time ≥ 12min, and the soft blowing time ≥ 15min; ② Wire feeding sequence: The wire feeding sequence is in the order of manganese nitride wire, ferrotitanium wire, calcium silicate wire, sulfur wire, tellurium - manganese wire (Te - Mn); The interval between feeding the sulfur wire and the calcium silicate wire ≥ 5min; ③ Calcium silicate wire: 60m for the first heat, 50m for continuous casting heats.

[0036] (4)The continuous casting (φ500mm) process: The flow rate of the first cooling water is 4000L / min, the second cooling water in zone 1 is 75L / min, in zone 2a is 50L / min, and in zone 2b is 42L / min; The current of the mold electromagnetic stirring is 200A, the frequency is 2Hz, the current of the strand stirring is 180A, the frequency is 8Hz, and the current of the final stirring is 1100A, the frequency is 8Hz; The casting speed is 0.39m / min.

[0037] (5)Hot rolling process: The heating temperature and heating time are as follows: The temperature of the pre - heating section < 900℃, the target temperature is 860℃, and the time > 1.0h; The temperature of the heating I section is 900 - 1100℃, the target temperature is 1000℃, and the time ≥ 1.0h; The temperature of the heating II section is 1180 - 1280℃, the target temperature is 1230℃, and the time ≥ 3h; The temperature of the soaking section is 1200 - 1260℃, the target temperature is 1240℃, and the time ≥ 3h; The total heating time ≥ 6.5h.

[0038] In the said rolling, the rolling process is as follows: The φ500 - sized round billet is taken out of the furnace and the intermediate billet is 270mm * 280mm after passing through the blooming mill, and the head and tail are cut by hydraulic shearing; Through the controlled cooling equipment, the temperature entering the continuous rolling is controlled at 830 - 840℃, and it is rolled by a 6 - stand vertical and horizontal 90° alternating rolling mill; And 5 - stage water cooling is used to control the cooling, and the set parameters of the 5 - stage water flow are shown in Table 1; Among them, the 5 - stage water cooling corresponds to 5 water tanks, which are respectively recorded as 1# water tank, 2# water tank, 3# water tank, 4# water tank, and 5# water tank; The arrangement of the rolling mill and the water tanks is: 1 horizontal stand → 2 vertical stands → 3 horizontal stands → 4 vertical stands → 5 horizontal stands → 6 vertical stands → 1# water tank → 2# water tank → 3# water tank → 4# water tank → 5# water tank.

[0039] Rolling speed: To control cooling, the rolling speed is reduced and controlled within 0.45 m / s.

[0040] Table 1: Water cooling parameters:

[0041] (6) Cooling bed collection and pit cooling: When the temperature of the cooling bed is > 400 °C, it is put into the pit for slow cooling. It is slowly cooled until the temperature ≤ 200 °C and then taken out of the pit. The slow cooling time ≥ 24 h. Example 2:

[0042] The hot-rolled round steel contains the following chemical components by mass percentage (unit, wt%): C: 0.435%, Si: 0.66%, Mn: 1.33%, P: 0.009%, S: 0.041%, V: 0.21%, Ti: 0.019%, Nb: 0.05%, Cr: 0.38%, Ni: 0.16%, Cu: 0.02%, B: 4.5 ppm, H: 1.1 ppm, O: 18 ppm, N: 156 ppm, and the balance is Fe and unavoidable impurities.

[0043] Through the processes of electric furnace steelmaking, LF refining, VD vacuum treatment, continuous casting, heating, rolling, and pit cooling, φ85 mm hot-rolled round steel is obtained; The specific implementation process of Example 2 is the same as that of Example 1: The components are detected according to the GB / T 4336 standard, and the performance of the round steel is detected by the impact test according to the metal material Charpy pendulum impact test process GB / T 229 standard. The results are shown in Table 2; Table 2 Performance of round steel

[0044] Figure 1 and 2 are respectively the austenite grain size picture and the metallographic structure picture of the round steel prepared in Example 1. Among them, the obtained austenite grain size of the round steel is grade 10 austenite grain size.

[0045] In summary, through the effective combination of composition and process control, the present invention refines the grain size of the raw materials, fully precipitates the microalloying elements, and the refined original structure does not coarsen during the forging heating of the crankshaft (induction heating). At the same time, it has excellent performance. The tensile strength of the round steel ≥ 1046 MPa, the yield strength ≥ 881 MPa, the elongation after fracture ≥ 16%, the reduction of area ≥ 45%, and the impact energy at room temperature ≥ 43.7 J, meeting the requirements of high-strength crankshafts for commercial vehicles.

[0046] Note: The above embodiments are only used to illustrate the present invention rather than to limit the technical solutions described in the present invention; therefore, although this specification has described the present invention in detail with reference to the above respective embodiments, those of ordinary skill in the art should understand that the present invention can still be modified or equivalently replaced; and all technical solutions and their improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A production process for controlled rolling and controlled cooling round steel for high-strength crankshafts of commercial vehicles, characterized in that: The controlled rolling and controlled cooling round steel is composed of the following components in percentage by mass: composition: C: 0.42-0.50%, Si: 0.5-0.8%, Mn: 1.20-1.4%, P ≤0.035%, S: 0.035-0.065%, V: 0.15-0.30%, Ti: 0.015-0.025%, Nb: 0.025-0.06%, Cr: 0.30-0.50%, Ni: 0.10-0.20%, Cu: 0-0.20%, B: 3-7ppm, H: ≤1.5ppm, O: ≤15ppm, N: 130-200ppm, the balance is Fe and unavoidable impurities; The production process of controlled rolling and controlled cooling round steel is as follows: steelmaking raw materials are sequentially subjected to electric furnace steelmaking, LF refining, VD vacuum treatment, continuous casting, heating, rolling, and pit cooling processes to obtain controlled rolling and controlled cooling round steel; In the electric furnace steelmaking process: end point C ≥ 0.08%; target P ≤ 0.015%; target temperature ≥ 1620℃; In the LF refining process: white slag time ≥ 20min, smelting time ≥ 40min; In the VD vacuum treatment process: the highest vacuum degree is ≤67pa, the high vacuum holding time is ≥12min, and the soft blowing time is ≥15min; the wire feeding sequence is manganese nitride wire, titanium iron wire, calcium silicon wire, sulfur wire, and tellurium manganese wire; In the continuous casting process: two-stage water cooling is used in combination with electromagnetic stirring of the crystallizer, strand stirring and end electromagnetic stirring; In the heating and rolling process: the preheating section temperature is less than 900℃, the time is greater than 1h; the heating section temperature is 900-1100℃, the time is ≥1h; the heating section temperature is 1180-1280℃, the time is ≥3h; the soaking section temperature is 1200-1260℃, the time is ≥3h; the total heating time is greater than 6.5h; the temperature of the continuous rolling is controlled to be 830-840℃, and 5-stage water cooling is used to control the cooling. The setting parameters of the 5-stage water flow are: the water flow of section 1 is 190~210 m 3 / h, 2-stage water flow rate 150~170m 3 / h, 3-stage water flow rate 120~140m 3 / h, 4-stage water flow rate 110~130mm 3 / h, 5-stage water flow rate 150~190mm 3 / h; rolling speed is controlled within 0.45m / s; Pit cooling process: When the temperature of the cooling bed is greater than 400℃, enter the pit for slow cooling, and slowly cool to the temperature ≤200℃ before exiting the pit. The slow cooling time is ≥24h.

2. The production process of controlled rolling and controlled cooling round steel for high-strength crankshafts of commercial vehicles according to claim 1, characterized in that: In the electric furnace steelmaking process, the end point C is: 0.10%≥C≥0.25%; 500 kg of lime and 400 kg of cleaning agent are added to every 100-105t of molten steel, and 100-10kg of aluminum is produced per furnace.

3. The production process of controlled rolling and controlled cooling round steel for high-strength crankshafts of commercial vehicles according to claim 1, characterized in that: In the LF refining process, the amount of auxiliary materials used in the LF refining stage is: 1.50 kg of silica sand per ton of molten steel, and silica sand is added 5 minutes before the LF refining station.

4. The production process of controlled rolling and controlled cooling round steel for high-strength crankshafts of commercial vehicles according to claim 1, characterized in that: In the VD vacuum treatment process, the feeding amount of the tellurium manganese wire is 75m / furnace; the feeding interval between the sulfur wire and the silicon calcium wire is ≥5min; the silicon calcium wire: 60m / furnace for the first furnace and 50m / furnace for the continuous casting furnace.

5. The production process of controlled rolling and controlled cooling round steel for high-strength crankshafts of commercial vehicles according to claim 1, characterized in that: In the continuous casting process, the flow parameters of the two-stage water cooling are: the flow rate of the first cold water is 4000 L / min; the flow rate of the second cold water zone 1 is 75 L / min, the flow rate of the second cold water zone 2a is 50 L / min, and the flow rate of the second cold water zone 2b is 42 L / min; the current of the crystallizer electromagnetic stirring is 200 A and the frequency is 2 Hz, the current of the casting strand stirring is 180 A and the frequency is 8 Hz, and the current of the end electromagnetic stirring is 1100 A and the frequency is 8 Hz; the pulling speed in the continuous casting process is 0.39 m / min.

6. The production process of controlled rolling and controlled cooling round steel for high-strength crankshafts of commercial vehicles according to claim 1, characterized in that: In the heating rolling process: the target temperature of the preheating section is 860°C, the target temperature of the heating section I is 1000°C, the target temperature of the heating section II is 1230°C, and the target temperature of the soaking section is 1240°C.

7. The production process of controlled rolling and controlled cooling round steel for high-strength crankshafts of commercial vehicles according to claim 1, characterized in that: In the hot rolling process, 6 rolling mill stands are used for vertical and horizontal 90° alternating rolling; 5 sections of water cooling correspond to 5 water tanks, which are respectively recorded as 1# water tank, 2# water tank, 3# water tank, 4# water tank and 5# water tank; the arrangement of 6 rolling mills and 5 water tanks is as follows: 1 horizontal stand → 2 vertical stands → 3 horizontal stands → 4 vertical stands → 5 horizontal stands → 6 vertical stands → 1# water tank → 2# water tank → 3# water tank → 4# water tank → 5# water tank.

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