Study on ultra fast cooling process and product properties of ultra wide X70M steel plate

In order to meet the requirements of an ultra wide specification high-grade steel plate export project, by optimizing the composition design and smelting rolling process, an ultra wide X70M steel plate with thickness of 19.7 ~ 22.2mm and width of 4376mm was developed by UFC ultra fast cooling process, and stable production was realized. The mechanical properties of mass-produced steel plates are tested. The test results show that the average yield strength of steel plates is 552mpa, the average tensile strength is 646mpa, the average yield strength ratio is 0.85%, the average impact energy at – 30 ℃ is more than 300j, and the average shear area of DWTT at – 15 ℃ is 92%. The research shows that the developed 4376mm ultra wide X70M steel plate meets the requirements of pipe making.

In recent years, with the increasing demand for oil and gas resources in the world, oil and gas transmission pipelines are also developing towards large-scale in order to obtain higher benefits. Taking China as an example, the diameter of the main pipeline of the second west east gas pipeline is 1219mm, while the diameter of the China Russia east pipeline under construction is 1422mm [1-3]. The requirement for medium thickness steel plate is to increase the width of the plate, which also needs to be solved [6-6]. At present, few domestic enterprises have the equipment conditions to produce ultra wide pipeline steel with 1422mm pipe diameter and can realize stable production. Therefore, the development of ultra wide specification high-grade pipeline steel plays a positive role in improving the technical capacity and market competitiveness of enterprises.
Minmetals has an advanced 5m wide and thick plate mill. In recent years, with the launch of extra thick slab caster, pre straightener and UFC ultrafast cooling system, it has laid a foundation for the development of ultra wide pipeline steel. On this condition, combined with the demand of export projects, the ultra wide X70M pipeline steel plate was developed by optimizing the composition design, smelting and rolling process and cooperating with UFC ultra fast cooling process, and stable production was realized.

Technical requirements and analysis of steel plate

The ordering specification of steel plate is 19.7 ~ 22.2mm thick and 4376mm wide. It is required that the yield strength difference and tensile strength difference of the same batch of steel plate are ≤ 60MPa, the yield ratio is ≤ 0.90, the single value of impact energy at – 30 ℃ at 1/2 thickness position is ≥ 120J, and the average value is ≥ 150j- The single value of 15 ℃ DWTT shear area ≥ 70%, the average value ≥ 85%, and the average value of – 15 ℃ DWTT batch contract ≥ 90%. Meanwhile, the width tolerance of steel plate is required to be ± 10mm, unevenness ≤ 10mm/2m, sickle bend ≤ 8mm/12m. See Table 1 for specific requirements of export order.
Compared with the technical requirements of common X70M hot rolled steel plate in China, the export order requires higher yield strength and lower yield ratio; For toughness, it is required to take 1/2 of the thickness of the steel plate as the impact sample, and the average shear area of – 15 ℃ DWTT is required to be more than 90%. The requirements on the overall dimension of the steel plate are also very strict, which makes it difficult to produce in batch.

Table.1 mechanical property requirements of X70M export order

Composition design and smelting control

In order to meet the mechanical property requirements of the order, the alloy system of low carbon + high manganese and Nb + Cr + Mo is still used in the composition design, and some elements are optimized. Among them, in order to reduce the impact of ultra-low carbon tapping on converter lining and improve batch smelting capacity, element C floats 0.01% higher than the conventional range. At the same time, in order to keep cepcm not rising and stabilize the toughness of steel plate, the content of Cr is greatly reduced and a small amount of Mo is added. The addition of Mo plays a significant role in inhibiting ferrite phase transformation and obtaining acicular ferrite structure [7]. The chemical composition of ultra wide X70M steel plate is shown in Table 2.

Table.2 chemical composition of ultra wide X70M steel plate%

The smelting process adopts the process flow of converter lf RH continuous casting, adopts the preferred molten iron to ensure that the molten iron w (P) ≤ 0.110%, and adopts Kr pretreatment desulfurization to ensure that the molten iron w (s) ≤ 0.010%. The converter adopts double slag smelting to control w (c) ≤ 0.050%, w (P) ≤ 0.010% and w (s) ≤ 0.007% in the tapping composition. LF white slag desulfurization, high-quality white ash addition ≥ 12kg/T, white slag holding time ≥ 15min. The high vacuum (vacuum degree ≤ 67pa) cycle time of RH furnace is ≥ 15min, the outgoing calcium feeding line is > 100m, and the static argon blowing time after line feeding is ≥ 10min. The above measures can promote the deformation of inclusions and the effective removal of large particle inclusions in steel. 250mm is used in continuous casting process × 2400mm section, whole process protective pouring, nitrogen increase ≤ 0.0003%, avoiding secondary oxidation of molten steel, pouring with low superheat, actual superheat ≤ 25 ℃, maintaining constant drawing speed, and liquid level fluctuation controlled within ± 3mm. Through the above measures, the inclusion exceeding the standard caused by process fluctuation can be avoided and the internal quality of billet can meet the requirements.

More than 640 heats of X70M pipeline steel are batch smelted by the above process, and the microstructure of cold acid corrosion samples is randomly inspected and evaluated according to mannesman standard. The evaluation results show that the proportion of samples with tissue center segregation ≤ grade 2 is 100%. Through furnace by furnace inspection of inclusions in pipeline steel, it is found that spherical class D inclusions are mainly found, and a few samples have class A and class B inclusions. See Fig. 1 for the specific detection proportion of various inclusions.

Fig.1 inclusion type and detection ratio

Rolling and UFC ultrafast cooling process

The rolling process of ultra wide X70M steel plate should comprehensively consider the mechanical properties and shape requirements. Because the steel plate widening ratio is greater than 1.8, there are many rough rolling widening passes, and the reduction of passes is difficult to increase, the reduction rate of longitudinal rolling passes after widening can only be guaranteed to be 15% ~ 17%. Compared with conventional steel plate, the refining effect of recrystallized austenite is weakened, The size of recrystallized austenite has an obvious effect on the DWTT properties of pipeline steel [8-9]. Therefore, in order to refine the austenite grain as much as possible, the heating temperature of the billet should not be too high, and should be controlled at 1160 ~ 1220 ℃. At the same time, in order to ensure the full solid solution of C and N compounds and avoid skip passes in the rough rolling stage caused by insufficient burning and thorough burning of the billet, the time of cold charged billet in the furnace should be controlled at least 280min. In the finish rolling stage, in order to ensure the cumulative reduction effect in the non recrystallized zone, provide more nucleation points for subsequent phase transformation and refine the microstructure after phase transformation, the thickness of intermediate billet is designed to be ≥ 3.5 times the thickness of finished product, the start rolling temperature of finish rolling is ≤ 900 ℃, and the final rolling temperature is controlled at 790 ~ 840 ℃. In the finish rolling stage, in addition to strictly controlling the temperature, the reduction rate should also be controlled to avoid excessive reduction rate in the last pass, The original wave shape of the steel plate is serious, and the reduction rate distribution of rolling passes is shown in Figure 2.

Fig.2 rolling pass reduction rate distribution
After rolling, the steel plate quickly enters the cooling system through the pre straightener. The cooling system is divided into two sections a and B, each with 15 groups of nozzles, of which the first four groups of section a are slot nozzles and the rest are high-density nozzles. Under the condition of water supply from the high-level water tank, the cooling water pressure is 0.15Mpa. After starting the booster pump, the cooling water pressure of section a can be adjusted to 0.5MPa to realize the ultra fast cooling function. The first rolling test of ultra wide X70M is compared with ultra fast cooling and ACC mode respectively, and the differences of steel plate properties and shape under the condition of consistent rolling and controlled cooling temperature requirements are compared. The specific properties and organization are shown in Table 3 and figure 3.

Table. 3 Comparison of mechanical properties between ultra wide X70M ultra fast cooling steel grade and ACC mode steel plate

Fig.3 Comparison of metallographic structure between ultra wide X70M ultra fast cooling steel plate and ACC mode steel plate
From the comparison results, there is no obvious difference in the shape of the test steel plate after water outlet and straightening. Compared with ACC mode steel plate, the yield strength of ultra fast cooling mode steel plate is increased by 27mpa, the tensile strength is increased by 21MPa, the yield ratio is increased by 0.02, and the impact properties and DWTT properties are also significantly improved. The metallographic structure of steel plate produced by ACC mode is quasi polygonal ferrite + granular bainite + a small amount of pearlite, while the quasi polygonal ferrite and pearlite of steel plate produced by ultrafast cooling mode are significantly reduced, and the structure is transformed into typical acicular ferrite. It can be seen that the larger cooling rate after the ultrafast cooling process inhibits the ferrite transformation. After the final cooling, the acicular ferrite transformation occurs in the steel plate under a larger undercooling degree, the microstructure is more uniform and refined, and the comprehensive properties of the steel plate are better.
Therefore, in order to obtain better mechanical properties, the ultra wide X70M steel plate produced in batch is cooled by ultra fast cooling mode, so as to improve the starting cooling temperature as much as possible and avoid the precipitation of more eutectoid ferrite before entering the water. The water flow in section a of ultra fast cooling is large, the water supply and drainage ratio is > 1.5, the final cooling temperature is controlled at 470 ~ 520 ℃, the edge cavity coefficient is controlled at 0.35 ~ 0.45, and the head and tail shielding function is put into operation, Avoid uneven performance and steel plate bending caused by uneven cooling of steel plate.

Batch production results

The X70M steel plate with thickness of 19.7 ~ 22.2mm and width of 4376mm produced by this process has accumulated more than 65000 tons, and the inspection batch is 2176, and the performance qualification rate is 99.4%. See Table 4 for the performance inspection results.

Table.4 test results of mechanical properties of ultra wide X70M ultra fast cooling steel plate

It can be seen from the performance test results in Table 4 that the mechanical properties of the steel plate are excellent. Among them, the average yield strength is 552mpa, the average tensile strength is 646MPa, and the average yield ratio is 0.85, which can meet the requirements of strength change after X70M steel pipe making; In terms of toughness, the average impact energy at – 30 ℃ is more than 300j, and the average shear area of DWTT at – 15 ℃ is 92%. A series of temperature tests were carried out on the impact performance and DWTT performance of steel plate, and the results are shown in Figure 4. It can be seen from Figure 4 that the impact energy of steel plate at – 60 ℃ is still greater than 200J, and the dwtt50% fat temperature is lower than – 40 ℃, indicating that the toughness allowance of steel plate is sufficient.

Fig.4 impact toughness and DWTT series temperature test results of ultra wide X70M steel plate
The steel plates produced in batch shall be subject to flaw detection according to grade b4e3 of bs5996 specification for ultrasonic flaw detection, and the qualified rate of steel plate flaw detection is 99.6%; The average width tolerance of the measured steel plate is 6mm, the sickle bending and unevenness are accepted according to the standards of ≤ 8mm/12m and ≤ 10mm/2m respectively, and the qualified rate reaches 100%.

Conclusion

• (1) 4376mm ultra wide X70M pipeline steel plate was developed by optimizing composition design, improving smelting and rolling process and cooperating with UFC ultra fast cooling mode, and stable production was realized.
• (2) The tensile properties of 4376mm ultra wide X70M steel plate are controlled evenly, the average yield strength is 552mpa, the average tensile strength is 646mpa, the average yield strength ratio is 0.85, – the average impact energy at – 30 ℃ is more than 300j, and the average shear area of DWTT at – 15 ℃ is 92%, which meets the requirements of X70M steel pipe making.
• (3) The rolling process of ultra wide X70M steel plate comprehensively considers the mechanical properties and shape requirements. The billet heating temperature is 1160 ~ 1220 ℃, the intermediate billet thickness is ≥ 3.5 times the finished product thickness, the finish rolling start temperature is ≤ 900 ℃, and the final rolling temperature is controlled between 790 ~ 840 ℃, so as to avoid excessive reduction rate in the last few passes.
• (4) Compared with the conventional ACC rolling process, the larger cooling speed of the ultrafast cooling process inhibits the occurrence of ferrite phase transformation. After final cooling, the steel plate undergoes acicular ferrite phase transformation under a larger undercooling degree, the microstructure is more uniform and refined, and the comprehensive properties of the steel plate are more excellent.

Author: QU Zhiguo，LIU Zhenhua，YANG Haifeng

Source: Steel Plate Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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