TECHNICAL ARTICLE:

Abstract

This paper studied the effects of piercing process and the total reduction ratio on pipe rolled with continuous casting round billet, and gave the corresponding process control parameters to guarantee the production quality.

Key Words: continuous casting (cc) round billet, rolling pipe, piercing, total reduction ratio.

1. INTRODUCTION

Today, continuous casting round billet is widely used in seamless steel pipe production. Its products have sufficient high strength, high collapse resistance, and high resistance to low temperature impact. However, the rolling process with continuous casing round billet is different from the process with rolled billet. We studied the effects of piercing process and total reduction ratio on pipe quality.

2. INFLUENCE OF PIERCING PROCESS

Since two-high cross roll piercing technology was invented last century, this technology has been significantly improved, but the control of hole cavity in cross roll piercing is still an important method to guarantee the inner surface quality of the pipe. Due to technology progress, modifiable parameters are increased from three to nine. The original parameters only include the distance between rolls, the distance between discs and the position of plug. But new ones, besides three original parameters, include the feed speed of roll, the rolling speed, the disc speed, the pre-rotary speed of bar, the feed angle and the toe angle. All nine parameters are on-line adjustable, and certain relations exist between parameters, which make it complicated to control piercing process and surface quality. Here, we tried to analysis piercing parameters to suit rolling continuous casting round billet.

2.1 EQUIPMENT AND PROCESS

• Equipment parameters:
  Type: Vertical Diescher Piercer Mill
  Roller: Ø950 -1200°×950mm
  Disc: Ø2000 - 2200mm
  Motor: 3800°×2 kW DC
• Design process parameters:
  Specification of billet: Ø210mm
  Specification of mother: Ø220×22mm
  Steel grade: N80
  Piercing temperature: 1260 °C
  Elongation: 2.51
  Percentage of the outside diameter variation: 4.8%
  Total reduction ratio: 11%
  Reduction ratio of the front of the plug: 5.5%
  Ovality coefficient of pass: 1.12
  Feed angle: 11°
  Toe angle: 0°
  Piercing velocity: 1.0 m/sec

2.2 EXPERIMENT AND ANALYSIS

Within the range of above process parameters, piercing process is steady with high product precision. However, inner fold defects still happen when rolling continuous casting round billet, most of them is in the front end of the pipe. For solving this problem, we adjusted the design parameters carefully to explore the causes of the inner fold defect in the front end of the pipe and seek methods to minimize it.

Suppose the inner fold ratio is a function of main process parameters of piercer, that is,

Y = F (X1,X2,X3,X4,X5,X6,X7) …… (1)
Where: Y—inner fold ratio (%)
X1—adjusted distant between rolls (mm), “+” is to enlarge rolls distant.
X2—adjusted distant between discs (mm), “+” is to enlarge discs distant.
X3—adjusted position of plug (mm), “+” is in the direction of inlet.
X4—feed angle (degree).
X5—rolls speed (rpm).
X6—roll speed rise after fed (%).
X7—pre-rotary speed of bar (rpm).

Results of the experiment were listed in table 1. The test emphases on the effect of each factor on the inner fold ratio in virtue of the multi-regression analysis. The results of t-tests of the factors are shown in table 2

.

From literature(1), t(0.95, 4)=2.132. Therefore, in seven main modifiable parameters, the adjusted distant between rolls (X1) and the feed angle (X4) are not significant and therefore are not the main factors or are indirect factors highly depend on main factors. Inner fold ratio did not improved obviously by adjusting them. All other five parameters are significant and main factors of influencing inner fold ratio. The sequence of their effect on inner fold ratio is as follow: pre-rotary speed of bar (X7), roll speed rise after fed (X6), adjusted distant between discs (X2), adjusted position of plug (X3) and rolls speed (X5).

In piercing process, the bar rotates with rolling pieces, but before fed, pre-rotary of the bar was set for reducing the effect of self-weight of the bar. Therefore, before feeding, pre-rotary of the bar only affects the piercing process and so does the velocity rise after feeding. For feeding easily, lower roll rotation speeds were chosen. The roll rotation speeds recover to normal immediately after feeding. Hence, in the first part of the rolling process, these speed variations could introduce distortions and produce the inner fold defect in the front end of pipe.
Therefore, for the inner fold defect in front end of pipe when rolling cc round billet, the main causes aren’t inner quality of the billet, but was risen from some piercing variation parameters. Those parameters may be optimized and controlled by manual operation. To guarantee the inner surface quality, we would rather pay more attention to the speed variation than the inner quality of the billet.

3. THE INFLUENCE OF THE TOTAL REDUCTION RATIO

Mechanical properties of the steel pipe depend on the inner microstructure of the pipe in a great degree, and the microstructure depends on the chemical composition in steel and processing procedure. For cc round billet, even with the advanced out-furnace refining technology, total reduction ratio of the pipe rolled decides mechanical properties of steel pipe after confirming the chemical composition in steel. Fig.1 to Fig.3 are regression curves of relationship between the values of three properties and the total reduction ratio when rolled line pipe of grade B with cc round billet according to standard API-5L. Here, when the total reduction ratio changed, elongation percentage almost doesn’t change, but tensile strength and yield strength were changed. The yield strength changed in greater degree (ref 2,3).

According to API-5L, the minimum tensile strength of grade B is 413 MPa, the minimum yield strength is 241 MPa. The minimum elongation changes according to the specification, but the maximum value is 29.5% which is lower than the actual values in rolling cc round billet. Hence, in actual production the tensile strength and elongation always can meet the needs of the standard. However, when total reduction ratio is lower than 5.15, it is possible that the value of yield strength is lower than the standard. Our early results of piercing with horizontal cc billet show that some values of 20 is lower than ones of the standard when total reduction ratio is lower than 5.12 (ref 2,3)

A similar situation happened to the casing pipe rolled according to API-5CT. Fig.4 is the first eligibility ratio for yield strength of casing J55. According to this figure, when total reduction ratio is lower than 10.2, 100% of the first eligibility ratio cannot be guaranteed.

After pipe normally rolled with cc round billet, whether total reduction ratio large or small, structure of casting state can always be eliminated. When the reduction ratio is too small, however, inner grains grow. Actual production shows that grain size is correlated with yield strength and impact work, when grain size is larger, yield strength and impact work are always lower. Vice versa, when grain size is smaller, yield strength and impact work are always higher. General standards require that grain size is grade 5 - 6, but we consider grain size had better to be controlled more than grade 7 for OCTG.

When lacking reduction ratio, it is possible to result in lower yield strength, impact work and grain size, and the pipe definitely cannot be delivered to the user. However, the properties may be improved by normalized process and quenching + tempering process. The results of compare with the properties of steel grade K55 before and after normalized process are listed in Table 3.

4. Summary

Any high quality seamless steel pipe can all be produced with continuous casing round billet, but in rolling we should pay attention to some problems as follow.

4.1 To guarantee the inner surface quality of the pipe, piercing modifiable parameters must be strictly controlled, especially those speeds parameters which affecting inner fold defect in the front end of pipe should be chosen very carefully.

4.2 When rolling pipe with cc round billet, total reduction ratio is still the main factor which effects properties and grain size of the pipe. When the total reduction ratio is small and the properties is lower than requirements the standard and the user, properties of pipe can be improved by heat treat process for insuring user demands.

4.3 When rolling line pipe of grade B, 20 and other general seamless pipe with cc round billet, the total reduction ratio should be higher than 5.2. The total reduction ratio should higher than 10.2 and grain size had better to be controlled more than grade 7 for OCTG .

References

1. Shangzhu Chen. Application Statistics. Beijing: Economics Science Publishing House, 1987.

2. Xiaoyong Yang. The research for the properties suitability under steel pipe standard. Metallurgy Standardized & Quality, 1995, (3): p. 36 - 37.

3. Xiaoyong Yang. Experiment On Application of HCC Billet in f76mm Seamless Rolling Mill. Steel Pipe Technology, 1987,(3):p. 53 - 55.