2024 Volume 64 Issue 7 Pages 1172-1177
A new type of online accelerated cooling equipment for seamless steel pipe and its application experiments were introduced in the present work. The equipment can simultaneously cool the inner and outer surface of seamless steel pipes. By using online accelerated cooling process, the mechanical properties of the test steel pipe made of Q345B composition meet the requirements of Q460E steel grade index, with the yield strength of 460–475 MPa, the tensile strength of 601–603 MPa, the elongation of 21.5–26.5%, and the −40°C impact energy of 136J.
In the past 40 years, many research, development and production practices on steel have proved that the online accelerated cooling process is an important means to improve the mechanical properties of hot-rolled steels.1,2,3,4,5,6,7,8) Online accelerated cooling equipments were used widely in the production of heavy plate, strip, bars, wires, H-beams and rail, which made the application of microalloying technology and TMCP possible.9,10,11,12,13,14,15,16,17) Unlike the above products, the seamless steel pipes are hollow structures, making it difficult to develop equipment that can simultaneously cool the inner and outer surfaces of steel pipe. Only by cooling the outer surface of the steel pipe, it is difficult to obtain a large temperature drop and improve the mechanical properties.
A new type of online accelerated cooling equipment for seamless steel pipe and its application experiments were introduced in the present work. The equipment was installed on Baotou Φ460 mm seamless steel pipe plant, which can simultaneously cool the inner and outer surface of seamless steel pipes. The designer named it Accelerated Cooling Inner & Outer System or ACIOS for short.
Figure 1 shows the installation location of the accelerated cooling equipment in the workshop. It was installed on the runout table between the hot sizing mill and the cooling bed. The total length of runout table is 110 meters, which was divided into three groups. The No. 1 group is adjacent to the outlet rollers of the hot sizing mill, with a length of 40 meters. The No. 2 group is fully equipped with the accelerated cooling device, and the length is 32 meters. The No. 3 group is adjacent to the entrance rollers of the cooling bed, with a length of 40 meters. Infrared thermometers were installed at the entrance of ACIOS and the cooling bed respectively to measure the temperature of the steel pipe.
Figure 2 shows the structure diagram and physical photos of the cooling unit. The accelerated cooling equipment for pipe steel consists of 16 identical cooling units. The cooling unit was designed with a flipped structure. When the equipment is not in use, the cooling units are turned up to prevent baking deformation caused by high temperature steel pipes, as shown in Fig. 2(a). When the equipment is use, the cooling units are turned over to cool the steel pipe, as shown in Fig. 2(b).
All original runout table rollers were remodeled. On the premise of ensuring that the original roller table elevation remains unchanged, a horizontal turntable was added to each roller table base, and the original roller table body, coupling and gear motor were installed on this turntable. As mentioned earlier, the runout table rollers were divided into three groups, and the rollers in each group were connected to each other by pull rods, and the horizontal rotation and rotation angle α were synchronized by a fixed stroke hydraulic cylinder, as in Fig. 3 shown. When the central axis of the roller body is not perpendicular to the steel pipe, the static friction force F between the roller body and the steel pipe will produce a component F·sinα along the circumference of the steel pipe and tangent to the outer surface of the steel pipe. It makes the steel pipe spiral forward on the roller table. Spiral forward is a necessary condition to ensure that no bending deformation occurs during the accelerated cooling process of steel pipe.
In the case of normal use of the equipment, when the steel pipe is disengaged from the hot sizing mill and fully enters the No. 1 group runout table, the three sets of hydraulic cylinders simultaneously control the rotation of the turntable in their respective group, that is, α rotates from 0° to 15°. The steel pipe does not need to stop when the roller table rotates. When the accelerated cooling process is completed and the steel pipe fully enters the No. 3 group runout table, the three sets of hydraulic cylinders simultaneously control the rotation of the turntable in each group, that is, α rotates from 15° to 0°, in preparation for cooling the next steel pipe.
2.4. Equipment Operating Conditions, Control Policies and Cooling PrincipleThe cooling medium is turbid ring water, which is taken from the original water system of Baotou Φ460 mm seamless steel pipe plant. The cooling water is stored in the intermediate water tank and pressurized by the water pump to ensure that the working pressure of the cooling water is between 0.3–0.4 MPa. The working water temperature should not exceed 36°C. During normal production, the maximum instantaneous flow is 500 M3/h.
The roller motors are controlled by the frequency converter to control the running speed of the steel pipe and the cooling time in the cooling equipment. The working pressure of the cooling water is controlled by the pump motor controlled by the frequency converter. By adjusting the cooling time and cooling water working pressure, the temperature drop degree of the steel pipe can be controlled.
Figure 4 shows the principle diagram of simultaneous cooling of the inner and outer surfaces of the steel pipe. Some Fan nozzles were installed on the flippable spray beam. Some fan nozzles were installed between adjacent rollers. The outer surface of the steel pipe was cooled by water spray from the fan nozzles on the flippable spray beam. The inner surface of the steel pipe was cooled by forced cold and wet air convection from the fan nozzles between adjacent rollers, which can not only ensure the cooling uniformity in the longitudinal direction of the steel pipe, but also improve the effectiveness of water spray cooling on the outer surface.
Table 1 shows the actual composition of the test steel billet. The test steels were Q345B (GB/T8163-2018, CHN) continuous casting billet smelted by Baotou Φ460 mm seamless steel pipe plant. The billet making processes are, hot metal pretreatment, top and bottom double blowing converter smelting, LF furnace refining, VD vacuum treatment, round billet continuous casting, and tube billet fixed-length cutting. The furnace number of the tube blank was B4124010290A, and its size was Φ350 mm (diameter) × 3300 mm (length).
| C | Si | Mn | Nb | V | Ti | S | P | Als |
|---|---|---|---|---|---|---|---|---|
| 0.17 | 0.26 | 1.57 | 0.001 | 0.009 | 0.01 | 0.007 | 0.015 | 0.0075 |
AlS: acid solution aluminum
The tube billet was reheated at 1230°C, pierced by the Bacterial Piercing, rolled by the Premium Quality Finishing. The temperature of the steel pipe after hot sizing was 950°C, and the measured temperature before entering the cooling equipment was 910°C. The outer diameter of the product was 273 mm, the wall thickness was 15 mm, and the length was about 28 meters.
Table 2 shows the experimental process parameters. No. 1 test steel pipe was produced by using the conventional process without accelerated cooling. No. 2, No. 3, No. 4 and No. 5 test steel pipes were produced by using four accelerated cooling processes. After accelerated cooling, the pipes were cooled to room temperature on the cooling bed. The maximum temperature of the steel pipe measured on the cooling bed with a handheld infrared thermometer, was considered the final cooling temperature or FCT for short.
| Test No. | Runout table roller speed, cm/s | Temperature at the entrance of cooling bed, °C | Final cooling temperature (Max. temperature on cooling bed), °C | |
|---|---|---|---|---|
| Minimum | Maximum | |||
| 1 | 180 | Using conventional process without accelerated cooling | ||
| 2 | 140 | 648 | 668 | 665 |
| 3 | 130 | 625 | 650 | 640 |
| 4 | 120 | 600 | 630 | 616 |
| 5 | 110 | 585 | 610 | 602 |
According to the sampling method specified in GB/T2975-2018, tensile samples were taken in the longitudinal direction of the head and tail of each steel pipe, which is 500 mm away from the end of the pipe. The width of the sample was 20 mm, and the proportional gauge distance was taken. According to the GB/T228.1-2021 test method, the tensile test was carried out on the model 1500HDX-04-G70 tensile testing machine. According to the sampling method specified in GB/T2975-2018, a group of 3 impact samples were taken in the longitudinal direction of the head of each steel pipe. The sample size was 10 mm × 10 mm × 55 mm. The notch depth was 2 mm, and the hammer edge was 2 mm, that is, KV2. According to the GB/T229-2022 test method, the impact test was carried out on the model JBNW-300 impact testing machine. Metallographic samples taken at each test pipe head were etched in 2% nital. The microstructure photos were taken with ZEISS metallographic microscope and image analyzer. The shooting positions were 1/8 thickness, 1/4 thickness, 1/2 thickness, 3/4 thickness and 7/8 thickness from the outer surface.
Figure 5 shows the microstructures at different positions in the thickness direction of the test steels under the optical microscope, followed by 1/8 thickness, 1/4 thickness, 1/2 thickness, 3/4 thickness and 7/8 thickness from the outer surface.
No. 1 test steel was air-cooled, and its microstructures were polygonal ferrite and pearlite. The average grain diameter of polygonal ferrite was about 20 μm.
The final cooling temperature of No. 2 test steel was 665°C, and its microstructures were polygonal ferrite and pearlite. The average grain diameter of polygonal ferrite at 1/8 thickness was about 10 μm, and the average grain diameter of polygonal ferrite from 1/4 thickness to 7/8 thickness was about 15 μm.
The final cooling temperature of No. 3 test steel was 640°C. The microstructure at 1/8 thickness was quasi-polygonal ferrite. The microstructures at 1/4 thickness were quasi-polygonal ferrite, polygonal ferrite and pearlite. The microstructures from 1/2 thickness to 7/8 thickness were polygonal ferrite and pearlite, and the average grain diameter of polygonal ferrite was about 15 μm.
The final cooling temperature of No. 4 test steel was 616°C. The microstructure at 1/8 thickness was granular bainite. The microstructures at 1/4 thickness were quasi-polygonal ferrite and a small amount of granular bainite. The microstructures at 1/2 thickness were quasi-polygonal ferrite, polygonal ferrite and pearlite. The microstructures from 3/4 thickness to 7/8 thickness were polygonal ferrite and pearlite. The average grain diameter of polygonal ferrite was about 15 μm.
The final cooling temperature of No. 5 test steel was 602°C. The microstructures from 1/8 thickness to 1/4 thickness were granular bainite and a small amount of acicular ferrite. The microstructures at 1/2 thickness were granular bainite and quasi-polygonal ferrite. The microstructure from 3/4 thickness to 7/8 thickness was quasi-polygonal ferrite.
4.2. Mechanical PropertiesTable 3 shows the tensile and impact test mechanical properties of the test steels. It can be seen from the tensile test results that the mechanical properties of the head and tail of the test steels were not much different, which proves that the cooling device did not cause a significant difference in the mechanical properties of the tube length direction. Obviously, both the yield strength and the tensile strength had increased significantly, the yield strength had increased from 368–376 MPa to 460–475 MPa, and the tensile strength had increased from 548–552 MPa to more than 600 MPa. Normally, as the strength of the experimental steel increased, the elongation decreased, but it still meet the Q460E standard requirements. The impact test results show that the impact toughness of the test steels after accelerated cooling were significantly improved at all test temperatures. The test results show that the accelerated cooling process improved both the strength and toughness of the test steels. When the final cooling temperature drop to 602°C, the highest comprehensive properties of strength and toughness was obtained, which meet the requirements of Q460E in GB/T8163-2018.
| Test No. | Tensile test | Impact test, KV2, J | ||||||
|---|---|---|---|---|---|---|---|---|
| Position | Rel MPa | Rm MPa | A % | 24°C | 0°C | −20°C | −40°C | |
| 1 | Head | 376 | 548 | 29.0 | 148 152 148 | 168 146 156 | 102 95 95 | 65 68 71 |
| Tail | 368 | 552 | 30.5 | 149 | 157 | 97 | 68 | |
| 2 | Head | 401 | 558 | 29.5 | 148 151 161 | 146 144 156 | 111 97 99 | 98 48 62 |
| Tail | 405 | 560 | 24.5 | 153 | 149 | 99 | 69 | |
| 3 | Head | 415 | 562 | 26.5 | 155 161 180 | 164 153 187 | 149 144 156 | 108 108 93 |
| Tail | 420 | 587 | 25.5 | 165 | 168 | 150 | 103 | |
| 4 | Head | 433 | 601 | 21.5 | 201 209 209 | 181 183 182 | 170 200 164 | 129 128 120 |
| Tail | 447 | 600 | 21.5 | 206 | 182 | 178 | 126 | |
| 5 | Head | 460 | 603 | 26.5 | 215 203 217 | 214 208 174 | 225 151 99 | 152 92 163 |
| Tail | 475 | 601 | 21.5 | 212 | 199 | 158 | 136 | |
| GB/T8163-2018 Q460E | Rel≥460 | 720≥Rm≥550 | A≥17 | −40°C, KV2≥27J | ||||
The outer surface of the test steel pipes was cooled by water spraying. In order to obtain a gradually decreasing final cooling temperature, the cooling intensity of the outer surface of the steel pipe must also be gradually increased. In the thickness direction, the cooling rate obtained by the part near the outer surface will also gradually increase. It was manifested in the change of microstructure at 1/8 thickness of each sample, from grain-refined polygonal ferrite to quasi-polygonal ferrite, granular bainite and acicular ferrite that appear in sequence.
The difference in microstructure at different thicknesses of the same sample was also related to the difference in cooling rate at different thicknesses. In the thickness direction of the steel pipe, the closer to the outer surface, the faster the cooling rate. This difference in cooling rate causes the microstructures change in the thickness direction of the steel pipe. When accelerated cooling was not used, the difference in cooling rate between the inside and outside of the steel pipe was small. The microstructures of entire thickness direction were polygonal ferrite and pearlite, and the grain size was uniform, such as No. 1. When accelerated cooling was used, along the thickness direction from the outside to the inside, the cooling rate gradually decreased. Accordingly, a variety of microstructures appeared in the thickness direction. In fact, the coexistence of various microstructures in the thickness direction is a common phenomenon, and the percentage of various microstructures need to control.
With the decrease of the final cooling temperature, the microstructure at the 7/8 thickness of the experimental steel also changed significantly. This change is caused by the increase of the cooling intensity of the inner surface of the steel pipe. Only relying on the cooling of the outer surface of the steel pipe cannot cause the change of the microstructure near the inner surface of the steel pipe, or even at the center of the thickness of the steel pipe.
5.2. Final Cold Temperature and Mechanical PropertiesFigure 6 shows the relationship between the mechanical properties of the test steel and the final cooling temperature, (a) tensile test, and (b) impact test. With the decrease of the final cooling temperature, the microstructure in the thickness direction of the steel pipe, from the polygonal ferrite with finer grains to the gradually increasing quasi-polygonal ferrite, granular bainite, acicular ferrite, etc., were the reasons for the gradual improvement of strength and toughness. The strengthening and toughening mechanism of quasi-polygonal ferrite, granular bainite, and acicular ferrite had been widely researched in the development of X70 pipeline steel.18,19,20,21,22) Compared with polygonal ferrite, quasi-polygonal ferrite, granular bainite and acicular ferrite all have higher dislocation density and substructure, which can not only improve the yield strength, but also effectively hinder the propagation of cracks.
The austenite transformation products were changed by the on-line accelerated cooling process, and polygonal ferrite, quasi-polygonal ferrite, granular bainite and acicular ferrite were produced in the final microstructure of the experimental steel. In addition, accelerated cooling can also be used to promote the general precipitation of microalloying elements carbonitride in ferrite grains, such as V (CN), to improve steel strength. It is worth considering the use of precipitation strengthening mechanisms to develop non-quenched and tempered high-strength pipeline steels, such as APIX65/X70.
(1) The new online accelerated cooling equipment installed at Baotou Φ460 mm seamless steel pipe plant provides an effective method for simultaneous cooling of the inner and outer surfaces of steel pipes.
(2) In this experiment, as the final cooling temperature decreases, the proportion of phase with high density dislocation and substructure in the microstructure of the experimental steel gradually increases along the thickness direction from the outer surface to the inner surface, such as quasi-polygonal ferrite, granular bainite and acicular ferrite.
(3) By using online accelerated cooling process, the mechanical properties of the test steel pipe made of Q345B composition meet the requirements of Q460E steel grade index, with the yield strength of 460–475 MPa, the tensile strength of 601–603 MPa, the elongation of 21.5–26.5%, and the −40°Cimpact energy of 136J.
This work was supported by Inner Mongolia BaoTou Steel Union Co., Ltd China and Shanghai Ningzhen Machinery Engineer Technology Co., Ltd China.
