The development of large-sized, heavy-wall 4130M-QT-125K mechanical structure seamless steel tubes

The development of large-sized, heavy-wall 4130M-QT-125K mechanical structure seamless steel tubes

Abstract: This article describes the design of the large-sized, heavy-wall 4130M-QT-125K mechanical structure seamless steel tube, involving chemical composition, and processes of steel-melting/casting, pipe rolling, and heat treatment, etc. Also summarized are the control tips for manufacturing the said tube. Relevant operation practice demonstrates that the manufacturing process as designed for the large-sized, heavy-wall 4130M-QT-125K mechanical structure seamless steel tube is reasonable, and the tube itself possesses such characteristics as high controlled-dimensional tolerance, consistent strength and toughness, yield strength: Rt0.65 under 890~1020 MPa, tensile strength: Rm over 960 MPa, after-fracture elongation: A over 18%, reduction of area: Z over 50%, impact energy absorption at 0°C: over 60 J, shearing area rate: over 80%, and full wall thickness Rockwell hardness: 33~37 HRC. All the property values are up to or even over relevant agreed requirements.

Key words: seamless steel tube; mechanical structure tube; high-grade steel; large-sized; machining; heat treatment; mechanical properties

Mechanical processing tubes have always occupied an important position in the production of seamless steel tubes. Replacing solid short forgings and rolled billets with seamless steel tubes to manufacture various hollow shafts, cylinders, sleeves, and other parts can significantly improve production efficiency and save steel. With the improvement of equipment levels in the steel tube industry, mechanical processing tubes have gradually shifted from small and medium diameter thick-walled high-quality carbon steel seamless tubes to large and medium diameter thick-walled low-alloy steel and alloy steel seamless tubes. In response to the demand in the North American market, Tianjin Pipe (Group) Corporation (referred to as Tianjin Pipe) has successfully developed a 273.05 mm in diameter and 31.75 mm in wall thickness, 4130M-QT-125K grade alloy steel seamless tube (supplied in the quenched and tempered state), with a minimum yield strength requirement of 865 MPa (125 ksi), and the full wall thickness Rockwell hardness required to be between 32~38 HRC, and the hardness variation between the inner surface, middle, and outer surface of the same cross-section is not allowed to exceed 5 HRC. The production process is introduced here.

1 Main technical requirements of the product
When customers order large-sized, heavy-wall 4130M-QT-125K mechanical structure seamless steel tubes, they have requirements for their chemical composition, mechanical properties, dimensional tolerances, etc. The chemical composition requirements of the large-sized, heavy-wall 4130M-QT-125K mechanical structure seamless steel tube are shown in Table 1, and the mechanical property requirements are shown in Table 2; the allowable deviation of the outer diameter (-0.75%~+0.75%)D, the allowable deviation of the wall thickness (-10.0%~+12.5%)S; in addition, grain size inspection should be carried out, and the original austenite grain size level should be Grade 7 or finer.

2 Production process and technical difficulties
2.1 Steelmaking process and technical difficulties
2.1.1 Steelmaking process
The continuous casting billet production process is: raw materials (scrap steel, pig iron, molten iron, etc.) → 100 t EBT-EAF (eccentric bottom pouring – ultra-high power electric arc furnace smelting) → LF furnace refining → VD furnace vacuum treatment →喂Ca line → round billet continuous casting machine continuous casting. During the electric furnace pouring, silicon ferrite, manganese iron, aluminum blocks, and other deoxidation alloying are used, and argon protection pouring is used from the steel package to the intermediate package, and submerged nozzle protection pouring is used from the intermediate package to the crystallizer, six-flow arc-shaped round billet continuous casting. The billet cross-section is 310 mm.

The 4130M-QT-125K grade is an improvement based on the AISI 4130 grade. The customer has put forward the range requirements for the main chemical composition elements. Within the chemical composition range specified by the user, Tianjin Steel Pipe considers the needs of the thick-walled seamless steel tube production process, requiring both good hardenability and good toughness performance, and also avoiding large quenching deformation and cracking. Key chemical composition elements have been internally controlled. The internal control chemical composition requirements for the large-sized, heavy-wall 4130M-QT-125K mechanical structure seamless steel tube are shown in Table 3. Comparing Table 1 and Table 3, it can be seen that the internal control chemical composition has increased the content of strong carbide-forming elements Cr and Mo that are beneficial to the stability of austenite, and the content of elements Si and Mn that strengthen the ferrite. This combination can greatly improve the stability of overcooled austenite and significantly improve the hardenability of the steel; considering the characteristics of high-strength quenching, it is necessary to reduce the quenching organizational stress to prevent deformation and cracking. The quenching cracks of the steel pipe are mainly related to the martensite transformation starting temperature Ms:

Ms = 538 – 317w(C) – 33w(Mn) – 28w(Cr) – 17w(Ni) – 11w(Si) – 11w(Mo) – 11w(W), if the steel contains C and alloy elements that lower the Ms point too much, the organizational stress will increase. On the premise of ensuring strength and toughness, the content of the above elements is considered comprehensively, especially the carbon content is reduced.

Note: The customer requires sampling for each batch of the first batch; within the same quadrant, three points are taken for testing on the inner surface, middle, and outer surface.

Table 1 Chemical composition requirements (mass fraction) of 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube %

Full wall thickness Rockwell hardness HRC
Over 52 Over 75 32~38, variation less than 5 935 865~1035 Over 14 Over 30

0°C transverse impact test
Impact energy absorption AKV2/J Shearing area rate FA/%
Room temperature tensile test
Tensile strength Rm/MPa Yield strength Rt0.65/MPa After-fracture elongation A/% Reduction of area Z/%

Table 2 Mechanical property requirements of 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube

Table 3 Internal control chemical composition requirements (mass fraction) of 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube

Note: It is required that w(As) is less than 0.020%, w(Sn) is less than 0.015%, w(Pb) is less than 0.010%, w(Bi) is less than 0.010%, w(Sb) is less than 0.010%.

%

Cu V Al
Less than 0.35 Less than 0.10 0.010~0.040 0.20~0.30
C P S Ni Cr Mo
Less than 0.020 Less than 0.010 Less than 0.50 0.90~1.10 0.45~0.50
Si
0.20~0.35
Mn
0.85~1.00

2.1.2 Steelmaking technical difficulties
(1) Ensure the uniformity and stability of the composition and the internal quality of the billet. Use high-quality pig iron and scrap steel to ensure the purity of the steel raw materials. Control the content of “five harmful” elements, although usually not high, but it will reduce the toughness, strength, and high-temperature mechanical properties of the steel, and even directly cause crack defects on the surface of the seamless steel tube or billet. Due to the micro-adjustment of components in the refining LF, argon blowing, and electromagnetic stirring of the crystallizer, the chemical composition of 4130M-QT-125K steel is stable and fluctuates little. Especially the component segregation will make the material performance uneven, so ensuring the material composition is uniform is one of the important measures to ensure the stability of the material performance.

(2) Ensure the cleanliness of the steel. The steel is treated by vacuum degassing to ensure low nitrogen, oxygen, and hydrogen gas content in the steel.

(3) Control the content of inclusions in the steel and control the morphology of the inclusions in the steel to reduce the impact of the inclusions on the quality of the steel. To improve the cleanliness of the billet, it is necessary to minimize or avoid the inclusions brought by slag, rolling slag, and the refractory of the intermediate package. The measures taken by Tianjin Steel Pipe include: using a slag detector to control the slag when changing the package; adjusting the pulling speed in time during continuous casting to maintain a stable liquid level in the intermediate package; preventing the crystallizer liquid level from fluctuating too much to form rolling slag; using sealed argon gas protection pouring during the opening process of the intermediate package.

Table 6 Chemical composition inspection results of 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube melting and finished product %

890 50 80 Air cooling
Quenching
Temperature/°C Holding time/min Holding time/min Cooling method
Water
Quenching medium
Tempering
Temperature/°C
590 Allowable deviation of outer diameter/mm Allowable deviation of wall thickness/mm Wall thickness unevenness/%
(-0.65%~+0.65%)D (-10%~+10%)S Less than 14

2.2 Pipe rolling process and technical difficulties
2.2.1 Pipe rolling process
The pipe rolling process adopts the MPM limited mandrel continuous pipe rolling machine rolling process technology, and has formulated a circular furnace heating → new guide disc type two-roll drum-shaped roll piercing machine piercing → 7 stands MPM limited mandrel continuous pipe rolling machine rolling → 14 stands three-roll micro-tension reducer sizing machine sizing → chain type cooling bed cooling process route. Considering the characteristics of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube that the inner and outer surfaces need to be machined, the wall thickness requires high precision, and the tolerance of roundness and wall thickness unevenness is strict; therefore, during production, the outer diameter tolerance and wall thickness tolerance of the product are strictly controlled. The internal control outer diameter and wall thickness allowable deviation of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube are shown in Table 4.

2.2.2 Pipe rolling technical difficulties
The steel has a high alloy content and high deformation resistance, and the piercing and continuous rolling host load is high during pipe rolling, and the inner surface of this type of steel is prone to quality defects, so the cooling water of the piercing and continuous rolling equipment is controlled to increase the temperature of the rough pipe during piercing and rolling, ensuring rolling at high temperatures.

2.3 Heat treatment process and technical difficulties
2.3.1 Heat treatment process
The tempering treatment of the steel pipe mainly adopts the rotating immersion type inner and outer spray quenching method, and has formulated a walking beam type high-temperature furnace (quenching furnace) → high-pressure water scale removal → rotating immersion type inner and outer spray water quenching → walking beam type low-temperature furnace (tempering furnace) → hot straightening → chain type cooling bed cooling process route. The choice of water quenching process route is mainly to ensure that the entire wall thickness of the cross-section can obtain martensitic organization to ensure that the mechanical properties after tempering meet the requirements, especially the control requirement of the full wall thickness hardness variation value.

2.3.2 Heat treatment system
According to the formula for the end of the austenite transformation A c3=910-320w(C) + 14w(Ni) – 12w(Cu) – 10w(Mn) + 5w(Cr) + 14w(Mo) + 18w(Si), the A c3 of 4130M-QT-125K steel is calculated to be about 830°C; considering the influence of Mn, Cr, Mo elements, the actual heat treatment quenching temperature is stipulated to be 890°C. The tempering temperature and tempering time have a significant impact on the mechanical properties and organization of the steel pipe. Considering the tensile properties, impact energy, hardness and other performance indicators of the steel pipe and the influence on the wall thickness size of the steel pipe, the tempering temperature is adopted at 590°C, and the holding time is appropriately extended to 80 minutes. The heat treatment system of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube is shown in Table 5.

2.3.3 Heat treatment technical difficulties
(1) Considering the characteristics of the large diameter and thick wall of the seamless steel pipe, it is necessary to have good cooling capacity for the quenching device. The quenching of the steel pipe adopts a clamping rotation and rapid immersion quenching function of the immersion type steel pipe quenching device. This process has strong cooling capacity and high quenching efficiency, equipped with a steel pipe rotation device, which can make the steel pipe cool evenly in the circumferential direction, with minimal cooling deformation, or even no quenching bending deformation.

(2) The heat treatment process uses water as the quenching medium, which requires ensuring the cooling water temperature and water circulation, and performing high-temperature long-time tempering to ensure uniform performance. To ensure the uniformity of mechanical properties, the tempering time for this grade of steel pipe is extended by 10 minutes.

3 Steel inspection results and analysis
3.1 Chemical composition
The chemical composition inspection results of the melting and finished product of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube are shown in Table 6, which shows that the chemical composition of the melting and finished product has reached the internal control requirements.

Table 4 Internal control outer diameter and wall thickness allowable deviation of 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube

Note: Wall thickness unevenness = 2(Smax-Smin) / (Smax+Smin) × 100%.

Table 5 Heat treatment system of 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube

Figure 3 Microstructure of 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube

Figure 2 Grain size of 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube

(a) Yield strength (b) Tensile strength (c) After-fracture elongation (d) Reduction of area (e) Impact energy absorption (f) Rockwell hardness

Figure 1 Mechanical property statistics of 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube

(a) Yield strength (b) Tensile strength (c) After-fracture elongation (d) Reduction of area

3.2 Mechanical properties
Statistical analysis of 76 heat treatment batches (including individual inspection for the first batch) of the mechanical properties and microstructure of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube. The mechanical property statistics of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube are shown in Figure 1, the grain size is shown in Figure 2, the microstructure is shown in Figure 3, and the non-metallic inclusion detection results are shown in Table 7. The test found that the grain size of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube is 8.5 grade, and the microstructure is tempered sorbite. No tower-shaped hairline defects were found during low magnification observation.

When testing the mechanical properties of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube, GB/T 228.1—2010 “Metal Material Tensile Test Part 1: Room Temperature Test Method” was implemented, using a round bar specimen with d0=12.7 mm and L0=50 mm; the impact test was implemented according to GB/T 229—2007 “Metal Material Charpy Pendulum Impact Test Method”, using a 10 mm×10 mm×55 mm V-notch specimen; the hardness test was implemented according to GB/T 230.1—2009 “Metal Material Rockwell Hardness Test Part 1: Test Method (A, B, C, D, E, F, G, H, K, N, T scales)”, non-metallic inclusions were executed according to GB/T 10561—2005 “Steel Non-metallic Inclusion Content Determination Standard Rating Diagram Microscopic Test Method” A method and JK standard rating diagram for evaluation; grain size was executed according to GB/T 6394—2002 “Metal Average Grain Size Determination Method”; microstructure was executed according to GB/T 13298—1991 “Metal Microstructure Examination Method”; low magnification inspection was executed according to GB/T 226—1991 “Steel Low Magnification Organization and Defect Acid Etch Inspection Method” and GB/T 1979—2001 “Structural Steel Low Magnification Organization Defect Rating Diagram”.

4 Results and discussion
The mechanical properties of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube are shown in Table 8, and the hardness inspection results at different positions of the same cross-section are shown in Figure 4. From Table 8 and Figure 4, it can be seen that the mechanical properties of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube after the final heat treatment meet the design requirements, indicating that the internal control requirements of the chemical composition are effective, and the

heat treatment system is reasonable. No quenching cracks were found during subsequent manual surface inspection and automatic ultrasonic flaw detection, and the full wall thickness hardness value is between 33~37 HRC, and the hardness variation at different positions of the same cross-section is controlled within 3 HRC.

Table 8 Mechanical properties of 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube

Note: The average value of 3 points at different positions (inner surface, middle, outer surface) on the same cross-section.

5 Conclusion
The 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube produced by Tianjin Steel Pipe has been successfully trial-produced and has stable performance, and all performance indicators meet customer requirements. During the production of the 4130M-QT-125K large-sized, heavy-wall mechanical structure seamless steel tube, key technical issues such as material chemical composition, steelmaking, continuous casting, continuous rolling, and heat treatment processes have been resolved.