Martin Liebeherr

Influence of Rolling Temperature and Cooling Rate on Microstructure and Properties of Pipeline Steel Grades

Thermo-mechanical control processing (TMCP) is a powerful tool for development of high strength low alloy (HSLA) steels. The understanding of the effect of process parameters on the microstructure of these steels is a key aspect for the optimization of their mechanical properties. The influence of the rolling temperatures and the cooling conditions on the texture, strength and toughness of HSLA steel was investigated. Two stage controlled rolling (roughing and finishing) was carried out on a laboratory rolling mill. Four different compositions were rolled by maintaining same roughing conditions but varying the process parameters of the finish rolling, namely, start finish rolling temperature (SFRT), finish rolling and cooling temperatures. Subsequent to rolling, two different cooling routes were used, namely, air-cooling (AC) and accelerated water cooling (ACC). For the ACC route, the plates were subsequently heat treated to simulate coiling. The microstructure and texture obtained before and after each TMCP schedule were characterized quantifying the phases, grain size and texture by means of EBSD and XRD and associated with the mechanical properties. It was found that SFRT has a strong impact on both strength and toughness if the material was air-cooled. Plates rolled at lower temperature showed better strength and toughness than ones rolled at high temperature in both air-cooling and ACC due to grain refinement. However, for the material that was processed through ACC and coiling simulation, the strength increased without any substantial effect on the toughness. These results provide an interesting insight on the industrial processing of HSLA steels.

Control of the Austenite Recrystallization in Niobium Microalloyed Steels

The use of heavy gauge steel sheets for structural applications very often requires a combination of high yield strength and adequate toughness. The most cost effective way to realize a high yield strength and a high ductility in a low alloyed steel is grain refinement. In industrial practice, this refinement is realized by controlled processing. This process consists of controlling the slab reheating temperature, applying a large amount of hot deformation below the non-recrystallization temperature (Tnr) and accelerated cooling. A better knowledge of Tnr could optimize the process and the best mechanical properties could be reached against the lowest cost. Tnr can be raised by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at low temperatures either by solute drag or by precipitation pinning. In this study, the recrystallization behavior of five Nb-microalloyed model alloys with various Nb contents, was evaluated by double hit compression tests. Further, the precipitation state of the materials was investigated experimentally by Inductively Couples Mass Spectroscopy and X-ray Diffraction. The construction of recrystallization-time-temperature diagrams and precipitation-time-temperature diagrams showed that both mechanisms, i.e. recrystallization and precipitation, strongly influence each other.

Investigations Into the Microstructure–Toughness Relation in High Frequency Induction Welded Pipes

In the present paper, investigations performed on ArcelorMittal X65 linepipe steel in order to understand the effects of high frequency induction (HFI) welding process and in-line post-weld heat treatment on weld properties are described. The factors that potentially can affect weld toughness, such as microstructure, grain size, precipitates, hardness, inclusions, and texture, are evaluated and discussed systematically in order to correlate weld microstructure with toughness of the HFI welded pipes.Copyright

Mechanical Anisotropy of Hot Rolled Line Pipe Steel Coil

The mechanical anisotropy of hot rolled coils for linepipe grades in the range between X52 and X80 have been investigated in terms of tensile strength and Charpy impact toughness. Samples were taken in different orientations with respect to the strip rolling direction: 0, 30, 45, 60 and 90 degrees. Tensile tests were performed on round tensile specimens to avoid the need for strip levelling. Results from this investigation reveal that the hot rolled coils display different degrees of anisotropy varying as function of strength level and thickness. The material anisotropy is reflected in variations in yield, tensile strength and ovality of the gauge section after fracture. At the same time, variations of the CVN absorbed energy in the upper shelf energy and in the ductile to brittle transition temperature were also observed. Finally, detailed texture studies revealed a relation between mechanical anisotropy and crystallographic texture.Copyright

HIPERC: a novel, high performance, economic steel concept for linepipe and general structural use

The HIPERC project has examined the effects of alloying elements and processing conditions in low carbon, < 0.09 wt %., niobium containing, 0.05 - 0.12 wt.%, steels. Laboratory-scale heats and pilot rolling trials simulating air and water-cooled plate production as well as hot-rolled strip production have been made. The effects of C, Mn, Ni, Cu, Cr, Mo, Nb, Ti and B, on transformation characteristics and temperatures of recrystallisation have been determined along with regression equations for characterisation of microstructure, tensile and impact properties, and for the weldability of these steels. The properties of products processed commercially to plate and coil-plate and made into pipe and to plate for structural use were determined and these compared well with the values predicted from the regression equations. The project has shown that excellent combinations of strength, toughness and weldability can be obtained using this steel type. Additional experiences have been gained in the processing of these steels through three commercial rolling mills and benefits were seen with this steel type due to higher production rates and lower amounts of surface dressing compared with steels currently being used to satisfy equivalent property specifications. Recommendations on the limits for niobium in Euronorms have been proposed; concerns relating to weldability have been addressed by proposing varying limits based on the carbon and manganese contents of the steel. This report makes the output of this project available to CEN working groups to support the revision of Euronorms based on the gathered data.

Development of X80M Line Pipe Steel for Spiral Welded Pipes With Low Temperature Toughness and Excellent Weldability

The unique combination of high strength and low temperature toughness on heavy wall thickness coils allows higher operating pressures in large diameter spiral welded pipes and could represent a 10% reduction in life cycle cost on long distance gas pipe lines. One of the current processing routes for these high thickness grades is the thermo-mechanical controlled processing (TMCP) route, which critically depends on the austenite conditioning during hot forming at specific temperature in relation to the aimed metallurgical mechanisms (recrystallization, strain accumulation, phase transformation). Detailed mechanical and microstructural characterization on selected coils and pipes corresponding to the X80M grade in 24 mm thickness reveals that effective grain size and distribution together with the through thickness gradient are key parameters to control in order to ensure the adequate toughness of the material. Studies on the softening behavior revealed that the grain coarsening in the mid-thickness is related to a decrease of strain accumulation during hot rolling. It was also observed a toughness detrimental effect with the increment of the volume fraction of M/A (martensite/retained austenite) in the middle thickness of the coils, related to the cooling practice. Finally, submerged arc weldability for spiral welded pipe manufacturing was evaluated on coil skelp in 24 mm thickness. The investigations revealed the suitability of the material for spiral welded pipe production, preserving the tensile properties and maintaining acceptable toughness values in the heat-affected zone.The present study revealed that the adequate chemical alloying selection and processing control provide enhanced low temperature toughness on pipes with excellent weldability formed from hot rolled coils X80 grade in 24 mm thickness produced at ArcelorMittal Bremen.Copyright

Measurement of Mechanical Properties on Line Pipe: Comparison of Different Methodologies

Line pipe manufacturers always have to verify the mechanical properties on pipe to make sure that the pipe meets the requirements specified by the standard and/or customer. This involves measurement of mechanical properties along the hoop direction. The most accurate way to do so is by performing a ring expansion test, which, however, requires dedicated tools. The two other methodologies consist of standard tensile tests on either non-flattened round bar samples or so called ‘flattened tensile samples’. Round bar samples have the disadvantage that only part of the pipe’s wall thickness is considered. Furthermore they can only be used in case of larger OD/t ratios. Tests on flattened samples, on the other hand, require a flattening operation, which induces additional plastic deformation. However, this flattening operation is not standardized. Moreover, it was observed that the mechanical properties — especially the yield strength — resulting from tensile tests on flattened samples largely depend on test parameters such as residual deflection, extensometer position, flattening procedure, etc. Most manufacturers prefer to test flattened samples, because sample preparation is straightforward and cheap. Moreover it only requires a standard tensile bench.An extensive FEA (Finite Element Analysis) study was launched to investigate the influence of those parameters on the measured yield strength. The applied FEA methodology consists of three steps. First the complete pipe forming process is modeled (in a simplified way). Next a pipe sample is flattened. Finally a tensile sample is cut from the flattened pipe sample and loaded in tension. The mechanical material behaviour is described by a combined kinematic-isotropic hardening model, which allows taking into account the Bauschinger effect. The results are also compared to simulations of ring expansion tests and tests on round bar samples.Next a dedicated experimental test campaign was performed to verify the results of FEA. Results of ring expansion tests are compared to results obtained on round bar samples and flattened tensile samples.The results of this study have shown that the applied methodology significantly affects the measured yield strength. Moreover tests on insufficiently flattened samples could considerably underestimate the actual yield strength on pipe. Finally some guidelines are provided to improve the reproducibility of the measured yield strength when using flattened samples.Copyright

Characterization of X80 Grade Linepipe Steel Coil With 24 mm Thickness

High strength and high toughness at low temperatures on heavy wall thickness skelp is required to build high pressure gas transportation pipelines. Detailed mechanical and microstructural characterization was carried on 24mm thick ArcelorMittal X80 coils in order to identify the microstructure control required to reach high toughness as determined by the shear fracture appearance after DWTT testing. Detailed microstructural characterization through thickness reveals that the microstructure gradient described by a systematic increase of the average grain size between surface and middle thickness of the strip and the increment of the volume fraction of M/A (martensite/ retained austenite) are the key microstructural parameters to control in order to ensure the adequate toughness of the material. The obtained high toughness of the coils indicates that the microstructure, controlled by an optimized rolling and cooling practice, is homogeneous through thickness of heavy wall linepipe grades.Copyright

Interaction Deformation / Precipitation / Phase Transformation in Nb Micro-Alloyed Steels

Characterization of the phase transformation of Nb-micro-alloyed steels has to be performed taking into account the effect of deformation and precipitation. In the present investigation, the austenite to ferrite phase transformation is characterized in continuous cooling experiments after deformation at high temperature. The resulting phase transformation kinetics and microstructure showed an influence of the soaking temperature. Detailed investigations of the possible causes of the change of mechanism of phase transformation indicate that the amount of niobium in solution correlates with the slowing down in phase transformation kinetics.

Microstructure–Property Relationship in 22mm Thick X80 Coil Skelp

The progress in the development of heavy gauge X80 linepipe steel on coil at ArcelorMittal was recently rewarded with a 6000 ton commercial order for the production of 21.6mm wall thickness spiral welded pipe. The further product development is concentrating on the improvement of the impact toughness at low temperatures. Research is currently focussing on the relationship between the mechanical properties and the microstructure of the steels. In the present study, two industrially hot rolled X80 steels with thickness 21.6mm were investigated. The steels had the same chemical composition but were processed with different parameter sets in the hot strip mill. The two resulting low-carbon bainitic microstructures were composed predominantly of quasi-polygonal ferrite and globular bainitic ferrite / bainitic ferrite, respectively. Emphasis of the microstructure and property characterisation was laid on through-thickness gradients of grain size, hardness, texture, impact toughness and tensile properties. Accordingly, the materials were characterised at different positions in the thickness. Grain size and texture were determined by means of Electron Backscatter Diffraction (EBSD). Sub-size Charpy as well as sub-thickness tensile test specimens were taken at different positions in the cross section. The results show that the link between microstructure and properties is not at all obvious. The influence of mean grain size, grain size distribution and texture is discussed in detail.Copyright