1. No category

15. - 17. 5. 2013, Brno, Czech Republic, EU
OPTIMIZATION OF THE TUBE ROLLING PROCESS FOR 25CRMO4 STEEL GRADE
Ľudovít PARILÁKa, c, Milan MOJŽIŠa, Tomáš KVAČKAJb, Lucia DOMOVCOVÁa
a
ŽP Research and Development Centre, Podbrezova, Slovak Republic, EU, [email protected]
b
Železiarne Podbrezová, Podbrezova, Slovak Republic, EU, [email protected]
c
Technical University in Kosice, Presov, Slovak Republic, EU
Abstract
In the process of tube rolling in Železiarne Podbrezová it is crucial to properly maintain the selected
thermomechanical process parameters, namely: heating of the tube stock in a walking beam furnace,
maintaining the uniform plastic deformation in all rolling stands of a stretch-reducing mill and, finally, keeping
the finishing rolling temperature of the tube over the Ar3 temperature. In this paper, optimization results for
25CrMo4 tube rolling are being presented, considering the final tube with a 2,6 mm wall thickness. For this,
the stretch-reduction parameter in all rolling stands as well as the finishing rolling temperature has been
optimized.
Keywords: tube rolling, 25CrMo4 steel grade, plastic deformation, stretch-reducing mill
1.
INTRODUCTION
In Železiarne Podbrezová, steel grade 25CrMo4 is being produced and subsequently hot-rolled in seamless
tube rolling mill, producing the tubes with diameter from 31.8 mm up to 76.1 mm and wall thickness from 2.6
mm up to 5.6 mm. These tubes are frequently used as a semi-finished product for pressure vessels and also
for applications in power industry, thanks to their favorable creep-resistant properties up to 400 °C [1].
The tube rolling represents a sophisticated technology, consisting of several technological sub-steps: billet
heating in a rotary hearth furnace (with zone #1 - 1230°C, zone #2 - 1269°C, zone #3 - 1297°C, zone #4. 1285°C), billet piercing, elongation, rolling on a push-bench and mandrel removal on a detaching mill
(reeler). After this, the tube blank is reheated in a walking-beam furnace (temperatures from 1035 °C to 1050
°C), thus ready for final rolling operation on a 28-stand stretch-reducing mill. For 25CrMo4 steel grade and
wall thickness of 2.6 mm, material failure (rupture) between adjacent rolling stands occurred frequently. This
problem plagued the rolling mill economically, rendering this problem a high priority one to be solved [1].
2.
EXPERIMENT
In the first iteration we focused on all parameters of technological sub-steps, going from rotary hearth
furnace up to the walking beam furnace. However, we have encountered no flaws leading to material failure
in the stretch-reducing mill. In the second iteration we focused on: thermal-deformation processes during
rolling, walking-beam furnace temperature and stretch-reducing mill inlet/outlet temperature. We analyzed
the strain rates given by stretch ratio in adjacent stretch-reducing rolling stands. Results presented were
obtained in a plant experiment, using heat No. 13520 (see Tab. 1). Dilatometric analysis, performed on TU
Košice, reveals threshold temperatures Ar1= 687 °C, Ar3 = 764 °C, Ac1 = 758 °C and Ac3 = 814 °C,
respectively [2]. After tempering, 25CrMo4 grade should exhibit the hypoeutectoid, ferritic-pearlitic structure
(with approx. 30 wt% of ferrite), Fig. 1, 2.
15. - 17. 5. 2013, Brno, Czech Republic, EU
Fig 1. 25CrMo4 microstructure
Fig 2. 25CrMo4 microstructure
After final heat treatment - transversal cut [1]
After final heat treatment - longitudinal cut [1]
Tackling the problem both theoretically as well as experimentally on ruptured tubes we can conclude
following:
Samples taken from neighbourhood of a failure area were characterized by low ductility and high hardness,
exhibiting the ferritic-bainitic structure (see Fig. 5,6). The typical characteristics for all ruptured tubes was the
finishing rolling temperature approaching Ar3 from below. It follows, that the material cooling rate was very
high, hindering the deformability, so to increase the material failure probability.
Based on our analysis we prepared an experimental rolling plan with technological parameters as follows:
1)
The primary target was having the finishing rolling temperature not to fall below Ar3
2)
In order to lower the possibility of a tube rupture we checked the stretch ratios (1 round), lower the
nd
stretch ratios (2 round) and, finally, lower the wall-thickness group.
st
Table 1 Chemical composition of heat No. 13520, mechanical properties of 25CrMo4 creep-resistant steel
according to EN 10216-2+A2
Chemical composition [%]
Grade
C
25CrMo
4
3.
0,220,29
Si
0,40
Mn
0,600,90
Pmax
0,02
5
Mechanical properties
Smax
0,02
0
Cr
0,901,20
Ni
0,30
Mo
0,150,30
Cu
0,30
Re
[MPa
]
Rm
[MPa
]
A
[MPa
]
345
540690
18
KV
min
[J]
27
Max.
Tempe
rature
[°C]
400
RESULTS ANALYSIS
For testing other rolling parameters, two rolling heats with different tube dimensions (33.7 x 2.6 mm and 38.0
x 2.6 mm) were performed. Both heats were supervised.
Basic data:
st
1 rolling
Order No. 2133368, heat No. 13520, Tube dimension: 33.7 x 2.6 mm, initial wall-thickness 3.4 mm
The tube blanks were put into the walking-beam furnace with temperature of 1035 °C. Furnace outlet
temperature was in the interval of 963 °C – 983 °C. Finishing rolling temperature was in the interval of 762
°C – 824 °C. [3]
15. - 17. 5. 2013, Brno, Czech Republic, EU
During rolling, we found out that the first failure appeared in the front part of the tube, dividing the stock into
seven pieces with 2 – 7 m of length. This was happening for every tube rolled, having 1035 °C in walkingbeam furnace and 820 °C in the last rolling stand, respectively. Due to these failures, the production on
stretch-reducing mill was ceased drastically. Subsequently, the furnace temperature was raised to 1040 °C,
followed by finishing rolling temperature of 781 °C. Nevertheless, the failure scenario repeated again.
Following this, the stretches were changed, Fig. 3. Again, the material failure occurred repeatedly. In
conclusion, only 5 % out of 40 rolled tubes were flawless.
nd
2 rolling
Order No. 2133367, heat No. 13520, Tube dimension: 33.7 x 2.6 mm, initial wall-thickness 3.2 mm
The tube blanks were put into the walking-beam furnace with temperature from 1015 °C to 1030 °C. Furnace
outlet temperature was in the interval of 897 °C – 975 °C. Finishing rolling temperature was in the interval of
750 °C – 823 °C. [3]
With furnace temperature of 1015 °C and reducing mill inlet of 900 °C, the first tube rupture occurred on the
tube No. 40, separating a 1m piece from the tube end. The next rupture follows on the tube No. 50,
separating a 2m piece from the tube end. For another tubes the ruptures were sparse. This rolling heat can
be thus considered as a success, having extra credit for maintaining the finishing rolling temperature and
also for given stretches, lowered by lowering the initial wall thickness (from 3.4 mm down to 3.2 mm). In
conclusion, 94 % of rolled tubes were flawless, which saved the day.
In Tab. 2 we can see a comparison of both rolling heats.
Table 2 Technological parameters of both experimental rolling heats [3]
1st Rolling
2nd Rolling
Rolling Gauge
AO
Rolling Gauge
AO
No. of rolling stands
28
No. of rolling stands
26
Tube stock diameter [mm]
139,8
Tube stock diameter [mm]
139,4
Initial wall thickness [mm]
3,4
Initial wall thickness [mm]
3,2
Tube diameter [mm]
33,7
Tube diameter [mm]
38
Wall thickness [mm]
2,6
Wall thickness [mm]
2,6
Initial velocity [m/s]
1,33
Initial velocity [m/s]
1,33
Diameter reduction [%]
75,89
Diameter reduction [%]
72,74
Wall thickness reduction [%]
23,51
Wall thickness reduction [%]
18,73
Average stretch
0,61
Average stretch
0,59
Maximum stretch
0,72
Maximum stretch
0,71
Maximum stretch in stand No.
10,11
Maximum stretch in stand No.
11
Elongation [m]
5,67
Elongation [m]
4,69
15. - 17. 5. 2013, Brno, Czech Republic, EU
st
nd
Fig 3 Stretches in all rolling stands for 33.7 x 2.6 mm tube rolling. 1 alternative in black, 2 alternative in
red
Fig 4 Stretches in all rolling stands for 38 x 2.6 mm tube rolling
st
st
In Fig. 3 we can see stretches for 1 rolling on all 28 rolling stands, changing its value from 0.72 (1
nd
alternative) to 0.68 (2 alternative). Although a mild improvement could be seen, we failed to reach desired
nd
goal. In Fig. 4 we can see stretches for 2 rolling heat with 26 rolling stands with maximum stretch of 0.71.
After these experiments took place we focused on tube microstructure. In Fig. 5, 6 there is the microstructure
st
from 1 rolling. The structure is a heterogeneous, two-phase ferritic-bainitic with scarce regions of ferrite.
Volume ratio of the bainite accounts for 65 %. Yield stress Re = 614 MPa, tensile strength Rm = 989 MPa and
ductility reaches 14,5 %.
15. - 17. 5. 2013, Brno, Czech Republic, EU
st
Fig.5 25CrMo4 microstructure, 1 rolling
st
Fig.6 25CrMo4 microstructure, 1 rolling
nd
The microstructure from 2 rolling heat with smaller initial wall thickness is very similar to the structure
depicted in Fig. 1, 2. It is a homogeneous, ferritic-bainitic structure with bainite fraction of 43 %. The
mechanical properties are as follows: Re = 611 MPa, Rm = 921 MPa and ductility reaches 16,8 %.
4.
CONCLUSIONS
The crucial points in flawless 25CrMo4 tube rolling is maintaining the finishing rolling temperature and
recommended stretches, in particular:
a)
Rolling in austenitic region, i.e. over Ar3 = 756 °C we recommend the finishing rolling temperature
>790 (780) °C
b)
Homogenous distribution of stretches for all rolling stands, paying special attention to step changes
between adjacent rolling stands
c)
Reduction of the stretches can be achieved by lowering the initial wall thickness, too
REFERENCES
[1]
Parilák, Ľ., Weiss, M., Beraxa, P., Mojžiš, M., Ďurčík, R., Domovcová, L., Benchmarking vlastností žiarupevných
ocelí do teplôt 560°C, Položka 2.A.4.4.4 Benchmarking
[2]
Fujda, M., Dilatometrické stanovenie teplôt fázovej premeny ocelí, Júl 2011
[3]
Parilák, Ľ., Weiss, M., Mojžiš, M., Ďurčík, R., Domovcová, L., Sledovanie výroby rúr z akosti 25CrMo4, Protokol
z overovacej skúšky č.1/2011