Thermomechanically Rolled Flat Products
Thermomechanically Rolled Flat Products
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Introduction |
It is well known that weight reduction of a construction can be achieved by introducing steels with higher strength. This reduction can be highest when just tensile loading occurs, e.g. in pipelines. However, even for bending or torsion loading weight reduction prospects can still be remarkable (1). One example is the increased transportation capacity in gas pipelines with the introduction of high strength low alloy steels (2).
These economical benefits can be used only, if the construction remains safe. First, the steel needs to be tough at operating temperatures, and second it should exhibit sufficient ductility to withstand any ductile crack propagation. It is known from fracture mechanics considerations that the necessary ductility to prevent ductile crack propagation increases exponentially with increasing yield strength (3). Therefore, higher strength steels also require by the same token improved toughness and ductility, which can be achieved only with low carbon clean steels and by maximizing grain refinement. The implementation of the thermomechanical rolling process (TM) is the most efficient practical way to achieve this goal (4).
Since over the last decades welding has become the most relevant fabrication method and with the tendency towards even more economic welding processes with higher heat input, weldability must be considered as an important issue in the development of such steels.
Table 1: Alloy Design and Properties of Typical Pipeline Steels
|
Year |
Grade |
Process |
% C |
% Mn |
% V |
% Nb |
% Ti |
others |
Pcm |
Re |
DWTT |
Av at -20C |
|
|
|
|
|
|
|
|
|
|
|
|
N/mm2 |
C |
Joule |
|
|
1960 |
X52 |
N |
0.18 |
1.40 |
- |
- |
- |
- |
0.26 |
360 |
- |
- |
|
|
1970 |
X60 |
N |
0.18 |
1.40 |
0.12 |
- |
- |
0.015N |
0.27 |
415 |
- |
- |
|
|
|
X60 |
N |
0.18 |
1.40 |
0.07 |
0.040 |
- |
- |
0.27 |
415 |
- |
- |
|
|
1975 |
X60 |
TM |
0.12 |
1.40 |
0.03 |
0.040 |
- |
- |
0.20 |
415 |
0 |
60 |
|
|
1980 |
X70 |
TM |
0.09 |
1.50 |
0.05 |
0.030 |
- |
- |
0.18 |
480 |
-20 |
80 |
|
|
1990 |
X70 |
TMCP |
0.08 |
1.65 |
- |
0.035 |
- |
- |
0.17 |
480 |
-20 |
80 |
|
|
|
X80 |
TMCP |
0.07 |
1.85 |
- |
0.045 |
0.02 |
0.15Mo |
0.18 |
550 |
-20 |
120 |
|
|
. |
X65 |
TMCP |
0.05 |
1.35 |
0.05 |
0.050 |
- |
- |
0.13 |
450 |
-30 |
150 |
|
| SOURGAS | |||||||||||||
Pcm=C+Si/30 + (Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5xB
BDWTT =>85% shear area in-Batelle drop weight tear test at testing temperature.
TM = Thermomechanically processed.
TMCP = Thermomechanically processed plus accelerated cooled.
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Plate and Strip for Welded Pipes |
Steels for large diameter pipe were for many years the most relevant application of the thermomechanical rolling process and many trends in steel development originated from pipe steel development. The economic transport of liquid or gaseous media in pipelines across remote areas with arctic climates or under water has only become feasible with TM-grades. Table I shows the development since the 1960's where by replacing the normalizing process with thermomechanical rolling, strength, toughness, ductility and weldability have all been improved. Furthermore, the transportation of sour gas containing media has also become possible.
The improved property combination of strength and toughness was made possible by maximizing grain refinement thus substituting for the strengthening effect of carbon. In addition, modern steel making processes allow the mass production of low sulfur steels. Figure I indicates, which improvements in ductility are achieved especially in transverse and through thickness direction (5) when lowering the sulfur content. Since the impact energy value in the transverse direction is the dominating property regarding safety in longitudinally welded pipe and the through thickness properties are very important for resistance welded pipe (ERW), the typical sulfur content is actually below 50 ppm. If resistance against hydrogen induced cracking in pipelines is required, even lower sulfur contents are requested: the mean value is actually below 10 ppm and the residual sulfur has to be modified, e.g. by calcium treatment, so that no elongated manganese sulfides exist.
The majority of large diameter pipe is produced from plate, while skelp is mainly applied for ERW and spiral welded pipe. In the thermomechanical rolling of plate, the procedure for obtaining the desired properties is to introduce a high deformation in the temperature range of non-recrystallization of austenite. Therefore, the finish rolling temperature is in the metastable austenite or even in the two phase region alpha plus gamma. To guarantee such low processing temperatures, two step or three step rolling schedules with periods of delay have to be applied. To avoid a reduced output of the mill, rolling schedules have been developed which allow for the simultaneous processing of more than one slab, figure 2 shows one example (6). Similar solutions are possible with plate mills of one stand (7).
The processing of hot strip material usually occurs at a higher temperature level, thus requiring a greater amount of solute niobium for the equivalent deformation to be executed in the area of non-recrystallization. But very often the thickness range for skelp is below that for pipe plate, which compensates with regard to toughness for the reduced grain refinement. Regarding strength, strip material of equal thickness and finishing conditions exhibits higher values since installations for accelerated cooling are standard and the quasi-isothermal heat treatment during coiling results in an additional strength increase by grain refinement and precipitation hardening.
Figure 3 (8) summarizes the most typical influencing factors in alloy design and processing of pipeline steels. As already indicated in table I optimization of thermomechanical treatment allowed for the most relevant pipe steel X 70 to be actually produced by applying niobium as the only microalloy element.
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Plates for Welded Constructions |
Thermomechanically processed steels exhibit better toughness than normalized steels (9). Furthermore, the required strength is obtained with a leaner composition, which results in lower costs and improved weldability. Figure 4 shows the strength levels that are obtained as a function of chemical composition and processing route. Therefore, normalized steels are utilized only if the steel plate is to be subjected to subsequent hot forming processes or is to be used for elevated temperature applications, where the excellent properties of the TM-processed material will deteriorate. TMCP plates exhibit further improved properties, being similar to those of quenched plus tempered material with comparable weldability. Higher strength steels for welded construction can be produced by direct quenching of plates after thermomechanical treatment.
Table 2 shows some examples of thermomechanically processed plates in comparison with traditional heat treated steels. The application range of the TM and TMCP steels is actually in the region of minimum yield strength 350 to 500 N/mm2. Their most typical application is in shipbuilding including icebreakers, offshore construction, vessels especially for low temperature environments, commercial vehicle construction, cranes and other general steel construction. Since the welding of these steels is often with rather high heat input, an addition of titanium stoichiometric to nitrogen has become standard. Recently, the even more effective grain size control in the heat affected zone by 'TiO' instead of TiN is increasingly applied (10). The steel development for thermomechanically processed plates followed also the discussed trend, i.e. reducing the sulfur and carbon contents and maximizing grain refinement.
Table 2: Examples of 50mm Plates, TM-Processed or Heat Treated and Sheet
|
Re(N/mm2) |
Process |
% C |
% Mn |
% Ni |
% Cu |
% Nb |
% Ti |
% Mo |
CE |
|
|
360 |
N |
0.14 |
1.45 |
0.15 |
0.15 |
0.030 |
0.020 |
- |
0.40 |
|
|
365 |
TM |
0.11 |
1.45 |
- |
- |
0.030 |
0.020 |
- |
0.35 |
|
|
365 |
TMCP |
0.07 |
1.45 |
- |
- |
0.020 |
0.015 |
- |
0.31 |
|
|
435 |
TMCP |
0.08 |
1.45 |
- |
- |
0.030 |
0.015 |
- |
0.32 |
|
|
515 |
TMCP |
0.07 |
1.50 |
0.75 |
0.30 |
0.035 |
0.015 |
- |
0.39 |
|
|
515 |
TMCP |
0.10 |
1.50 |
0.50 |
0.25 |
0.030 |
0.015 |
- |
0.40 |
|
|
540 |
QT |
0.10 |
1.50 |
0.60 |
0.30 |
0.020 |
0.020 |
0.30 |
0.45 |
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HSLA Strip and Sheet |
The application of HSLA strip material for pipes has already been noted but these steels are also widely used in the automotive industry, for welded constructions, etc. Due to their reduced pearlite content together with a low sulfur content (11), these steels offer improved cold formability which is often utilized to produce components like truck frames.
Since the typical thickness of hot strip material is below 10 mm, the toughness is usually not a problem. Therefore, the production via a hot strip mill enables high strength levels to be achieved. Neglecting the toughening effect and even considering only the strength increase, niobium is still the most efficient element as shown in figure 5 and small additions are very effective. Therefore in practice, hot strip material up to a minimum yield strength level of 420 N/mm2 is just microalloyed with niobium and additional microalloys are only considered for higher strength levels. Figure 6 summarizes the metallurgical mechanisms applied to achieve high strength levels in hot strip.
Historically, titanium microalloyed steel grades were widely used, especially when steel products with a low sulfur content were not regularly produced. Unlike niobium. or vanadium, which just form carbides and nitrides, titanium also exhibits a rather high affinity for oxygen and sulfur and had been used to control the sulfide shape thus improving cold formability. Since modern steelmaking guarantees the mass production of steel with about 0.005% S or less, more attention was paid to the negative effects of titanium in microalloyed steels. In particular it is difficult to control the processing conditions to guarantee a reasonable narrow scatter of mechanical properties, necessary for a series production (12). Also the weldability is impaired with higher titanium contents (13) and this has influenced alloy design towards niobium. Today titanium microalloying is applied mainly in addition to niobium microalloying in the same way that vanadium is added to niobium HSLA strip for yield strengths greater than 420 N/mm2.
If surface quality requirements are high and the thickness too small for hot rolling then cold rolling plus subsequent recrystallization annealing is applied. In principle, the same mechanisms discussed before apply to produce HSLA cold rolled sheet. Figure 7 underlines this approach. These steels are often produced without any silicon addition to improve the surface, and also other concepts like strength increase by solid solution hardening with phosphorus up to about 0.080% may be applied. With the more modern annealing possibilities such as continuous annealing lines or high convection bells instead of the traditional batch annealing process, a higher consistency of properties is achieved and the strength can be obtained with leaner alloy additions.
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Conclusion |
Thermomechanically processed steels have gained a substantial share of the steel production of industrialized countries. Their concept is based on the requirements of steel users, who increasingly ask for a simultaneous improvement of strength, toughness, ductility and weldability. The described steels provide these property combinations even at lower cost than applying traditional steels or alternative materials. Therefore, the described alloy design and the processes have been introduced successfully to flat products. Thermomechanically processed steels have allowed many structural applications to become feasible and have guaranteed that steel remains the most relevant material for many applications.
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References |
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