High Strength Linepipe Steels Research Articles

Effects of granular bainite and polygonal ferrite on yield strength anisotropy in API X65 linepipe steel

Linepipe steels fabricated using thermo-mechanically controlled processes exhibit microstructural inhomogeneity and a characteristic texture; therefore, they often lead to anisotropic yield strength properties. Although yield strength anisotropy is considered as a major guaranteed property, particularly in pipe forming processes, systematic verification of the dominant microstructural component affecting the yield strength is challenging owing to complicated microstructures and inhomogeneous distributions. In this study, the microstructures were controlled by varying the rolling reduction ratios and start rolling temperatures, specifically for polygonal ferrite (PF) and granular bainite (GB); the Taylor factor (TF) and orientation distribution function (ODF) of each microstructure were quantitatively analyzed by electron backscatter diffraction to study the effects of microstructural characteristics on the yield strength. The deviation of TF with tensile direction in GB was larger than that in PF; the intensity of {113}<110> components in ODF maps was greater in GB than in PF. The results indicated that GB induced more yield strength anisotropy than PF; thus, steels with a greater GB fraction exhibited greater yield strength anisotropy. This study can be used for lowering the yield strength anisotropy in linepipe steel plate design, with promising prospects for wider industrialization of high-strength linepipe steels.

Effect of prestrain on ductility and toughness in a high-strength line pipe steel

Fracture properties of a mother plate for API grade X100 line pipe after pre-straining up to 6% are investigated using tensile notched bars and CT pre-cracked specimens. The material has an anisotropic plastic and damage behavior due to the thermo-mechanical control rolling process. Experiments evidence a decrease in both ductility and toughness for both rolling and long transverse direction with increasing prestrains. This effect is however more pronounced at low prestrain levels ( $$0\rightarrow 2\%$$ ) than at higher levels ( $$2\rightarrow 4\rightarrow 6\%$$ ). The modified GTN model proposed by Shinohara et al. (Int J Fract 197:127–145, 2016) is used to represent the database. A good agreement is obtained provided some damage model parameters are modified so as to obtain a slightly higher damage rate for the prestrained materials. This represents the fact that void growth tends to be faster for materials with a lower work hardening rate as evidenced by unit cell calculations. In addition, stress/strain distributions in test specimens are modified for reduced hardening so that stress triaxiality is increased at failure initiation points. This further lowers measured mechanical properties.

The Impact of Isothermal Treatment on the Microstructural Evolution and the Precipitation Behavior in High Strength Linepipe Steel.

Isothermal treatment affects the microstructural evolution and the precipitation behavior of high-strength low alloy (HSLA) steels. In this regard, thermal simulation of different isothermal treatment temperatures was adopted by using a thermomechanical simulator. The results showed that hardness reached the maximum value at 600 °C holding temperature, which was related to a finer grain structure and granular bainite. The strengthening effect of precipitates was remarkable due to the combination of small particle size and small interparticle spacing. It is presumed that the precipitation started after 600 s at 600 °C. Precipitation strengthening continued to exist, even though coarsening of ferrite grains led to softening phenomena when the specimen was isothermally held at 750 °C, which led to relatively high hardness. The precipitates were fcc (Ti, Nb) (N, C) particles, and belonged to MX-type precipitates. Average size of precipitates increased from 3.14 to 4.83 nm when the specimens were isothermally held between 600 °C and 800 °C. Interparticle spacing of precipitates also increased with increasing isothermal treatment temperatures. These led to a reduction in precipitation strengthening. At the same time the polygonal ferrite content increased and ferrite grain size got larger, such that the hardness decreased continuously.

Critical Crack Tip Opening Displacement for X70 and X80 Steels - A Parametric Study

High strength line-pipe steels are widely used in long distance gas pipelines. Fracture toughness is one major parameter in the performance evaluation of these line-pipe steels. For high strength line-pipe steels, critical crack tip opening displacement (CTOD) is one typical quantity for fracture toughness. In this paper, a series of experimental studies were conducted to investigate the influences of steel property and specimen thickness on critical CTOD by three points bending tests for X70 and X80 line-pipe steel. Results showed that the critical CTOD is mainly depended on the plastic crack mouth opening displacement of the specimen. For the same size specimens, the critical CTOD of X80 steel was much less than X70 steel. The specimen thickness had a significant influence on the plastic crack mouth opening displacement. With the decrease of the specimen thickness, the critical CTOD increased.

Hydrogen Induced Cracking and Sulphide Stress Corrosion Resistance of High Strength Line Pipe Steels for Sour Gas Application

Hydrogen Induced Cracking and Sulphide Stress Corrosion Resistance of High Strength Line Pipe Steels for Sour Gas Application

Anisotropic ductile failure of a high-strength line pipe steel

Fracture properties of a mother plate for API grade X100 line pipe were investigated using tensile notched bars, CT and SENB pre-cracked specimens. The material had an anisotropic plastic behaviour due to the thermo-mechanical control rolling process. In addition, anisotropic rupture properties were also observed. Specimens tested along the rolling direction were more ductile and more crack growth resistant than those tested along the long transverse direction. Unit cell calculations were used to show that this fracture behaviour is not related to plastic anisotropy. Assuming that fracture is controlled by internal necking between anisotropically spaced voids, a model combining GTN and Thomason models is proposed which enables describing rupture anisotropy. A modified phenomenological model is also proposed so as to reduce the computational cost.

Austenite–ferrite transformation in enhanced niobium, low carbon steel

The austenite to ferrite transformation characteristics of a commercial high strength line pipe steel containing 0·05 wt-% carbon and 0·095 wt-% niobium have been rigorously studied by continuous cooling experiments in the range between 960 and 1260°C. A significant delay in the austenite to allotriomorphic ferrite transformation has been demonstrated to occur under practically relevant thermal processing conditions. The effects of prior austenite grain size and soluble niobium have been carefully evaluated and isolated and it has been concluded that the amount of niobium in solution in the austenite is primarily responsible for the retardation. Alternative hypotheses to explain the mechanism whereby niobium exerts this effect on the hardenability of steel are discussed in detail. Soluble niobium reducing the austenite grain boundary energy is argued to be the most convincing explanation of the phenomenon and a reduction of grain boundary energy of 0·286 J m−2 per wt-% of soluble niobium content has been proposed.

Influence of prior austenite grain size on martensite–austenite constituent and toughness in the heat affected zone of 700 MPa high strength linepipe steel

Structure–mechanical property relationship studies were carried out on Gleeble simulated intercritically reheated coarse-grained heat affected zone (ICCGHAZ) of 700MPa linepipe steel microalloyed with Nb. The design of experiments was aimed at varying reheat temperature in the first pass to obtain different coarse grain size in the HAZ. This enabled the study of the effect of prior austenite grain size on martensite–austenite (M–A) constituent during the second pass reheating and its consequent influence on impact toughness. We elucidate here the role of phase transformation and the fraction, size, shape, distribution, and carbon content of M–A constituent on impact toughness. The data suggests that the fraction of M–A constituent is not influenced by grain size, but the size of M–A constituent is influenced by the prior austenite grain size, which consequently governs toughness. Coarse austenite grain size increases the size of M–A constituent and lowers the HAZ toughness. Coarse austenite grain associated with coarse M–A constituent along grain boundary is the dominant factor in promoting brittle fracture. The combination of fine prior austenite grain size and smaller M–A constituent is favorable in obtaining high toughness. Good toughness is obtained on refining the prior austenite grain size in the CGHAZ during first pass and hence ICCGHAZ in the second pass.

Effect of Reheat Conditions on Microstructure Evolution and Precipitation Behavior in High Strength Linepipe Steel

The effects of slab reheat temperature and soaking time are studied to characterize austenite grain growth, microstructure homogeneity and dissolution of precipitates in linepipe X80 grade steel. It is shown that the uniformity of austenite microstructure strongly depends on the slab reheat temperature and soaking time. With increasing reheat temperature an abnormal growth of individual grains is observed that stems from gradual dissolution of microalloy carbonitrides. As the result, individual grain boundaries become unpinned and mobile thus "nucleating" secondary recrystallization. The highest reheat temperature at which the dissolution kinetics of precipitates is still slow enough to prevent the onset of secondary recrystallization within long soaking times is 1160°C. The as reheated austenite microstructure and the character of austenite grain size distribution are inherited throughout the entire roughing rolling sequence and even further downstream to the finishing rolling entry. The effects of reheat soaking time on shear fracture area and impact toughness are also described.

Mathematical Modelling of Hot Rolling: A Practical Tool to Improve Rolling Schedules and Steel Properties

Most of the commercial metallic materials undergo at least one hot deformation stage during fabrication. Hot deformation processing leads to the production of plates, strips, rods, pipes and other shapes at lower overall cost when compared to the cold deformation/annealing route. Comprehensive study of the metallurgical phenomena during hot deformation has enormous potential application in the control of industrial rolling processes. Understanding of the microstructural and mean flow stress evolution lead to sound steel developments and innovative rolling schedules. The models predict parameters such as grain size, fractional softening (static and dynamic) and strain induced precipitation which are useful to improve rolling schedules. Effects such as incomplete softening and strain accumulation can be easily detected as well as their consequences on the final grain size and mechanical properties. In this regard, special attention must be given to steels, the most important metallic material in terms of history, present and future. In this paper, three hot rolling routes will be analyzed in order to produce high strength linepipe steels. Examples were selected on how the use of modelling during development stage can help to meet mechanical properties, mainly toughness and drop weight tear test. Firstly, it is presented a brief overview on mathematical models applied to hot rolling. Thin slab casting/direct rolling, hot strip mill and plate mill are exemplified in the present work. The development of new steel grades can greatly accelerated with the aid of modelling, which is an useful, low-cost technique.

Latest developments in mechanical properties and metallurgical features of high strength line pipe steels

In response to the increasing demand to improve both transportation efficiency and performance, the steel pipe industry has conducted extensive efforts to develop line pipe steel grades with superior metallurgical and mechanical (strength, toughness and ductility) properties in order to allow exploitation in hostile environments. This paper aims to give an overview of recent developments of high strength pipe steel grades as API 5L X70 and beyond, providing a detailed understanding of the continuous improvements with respect to a strain-based design context. Information regarding the metallurgy and processing, such as chemical composition, microstructural design, thermo-mechanical controlled process (TMCP) and accelerated cooling process (AcC), to achieve the target strength, ductility and toughness properties are discussed.

Effect of Reheating on Grain Size Evolution in High Strength Linepipe Steel

The effects of slab reheat temperature and soaking time are studied to characterize austenite grain growth, microstructure homogeneity and dissolution of precipitates in linepipe X80 grade steel. It is shown that the uniformity of austenite microstructure strongly depends on the slab reheat temperature and soaking time. With increasing reheat temperature an abnormal growth of individual grains is observed that stems from gradual dissolution of microalloy carbonitrides. As the result, individual grain boundaries become unpinned and mobile thus "nucleating" secondary recrystallization. The highest reheat temperature at which the dissolution kinetics of precipitates is still slow enough to prevent the onset of secondary recrystallization within long soaking times is 1160°C. The as reheated austenite microstructure and the character of austenite grain size distribution are inherited throughout the entire roughing rolling sequence and even further downstream to the finishing rolling entry.

Role of Ti and N in line pipe steel welds

High strength line pipe steels exhibit a combination of excellent toughness and high strength achieved through microalloy additions and thermomechanical controlled processing. During welding, severe thermal cycles experienced by the heat affected zone (HAZ) result in precipitate coarsening/dissolution and subsequent grain growth. This significantly reduces toughness in this region. It is well known that small Ti additions are utilised to control grain growth in the HAZ through grain boundary pinning action of TiN precipitates. Because of a lack of systematic and controlled study, it has been difficult to quantify the effect of TiN in the variety of steels. Hence, the optimum levels proposed in the literature are inconsistent and even contradict each other when compared. This paper mainly reviews the effect of different levels of Ti, N and Ti/N ratios on steels and pipes manufactured using different processes, with particular focus on the HAZ toughness.

Effects of Distribution and the Formation Process of MA on Deformation and Toughness of High Strength Linepipe Steel

Effects of Distribution and the Formation Process of MA on Deformation and Toughness of High Strength Linepipe Steel

Modification of the Stress-Strain Curve for High-Strength Line Pipe Steel

Modification of the Stress-Strain Curve for High-Strength Line Pipe Steel

Effects of Processing Parameters on Microstructure and Properties of Ultra High Strength Linepipe Steel

To examine the effect of processing parameters on microstructural evolution and to obtain the excellent combination of strength and toughness, simulation of thermo-mechanical processing was conducted using the Gleeble machine. Trial production was then conducted under the conditions obtained by Gleeble tests. Based on the results of microstructure analysis and mechanical property evaluation, the relationship between microstructural features and mechanical properties was elucidated. The result shows that the volume fraction of constituted phases can be controlled through adjusting the cooling rate and finish cooling temperature in order to get different strength levels. As cooling rate increases, the volume fraction of upper bainite increases, which leads to the increase of strength. The upper shelf energy (USE) increases with increasing volume fraction of acicular ferrite in bainite base because of the small effective acicular ferrite grain size. Ductile-brittle transition temperature (DBTT) decreases with increasing acicular ferrite volume fraction. High reduction in the rough stage has great influence on grain refinement.

Understanding mechanical property anisotropy in high strength niobium-microalloyed linepipe steels

Thermo-mechanical processing of linepipe steels may result in anisotropy in mechanical properties, notably, yield strength and toughness, depending on the chemical composition and microstructure. These relationships are exceptionally important in spiral-welded pipe. Given the current interest in the development of high strength linepipe steels, we have examined in detail the mechanical property anisotropy phenomena using a combination of electron microscopy and crystallographic texture analysis in API L485 (X70) and API L555 (X80) steels. In this presentation, we describe the results of a study that has enhanced our understanding of the relationship between the microstructure and texture with anisotropy in mechanical properties.The microstructure of both X70 and X80 microalloyed linepipe steels as imaged via electron microscopy was similar and did not exhibit any significant anisotropy in the plane of the sheet. The microstructural constituents, polygonal ferrite and acicular ferrite (bainitic ferrite: ∼5–10% in X70 and ∼15–20% in X80) were also distributed uniformly throughout the volume of the specimens (0°, 45°, and 90° with respect to the rolling direction).The anisotropy in yield strength and Charpy impact toughness of X70-Nb and X80-Nb–Mo microalloyed linepipe steels was examined by using orientation distribution function analysis. The texture fibers of X70 and X80 microalloyed steels were similar but with significant differences in the intensity. Deformation textures mainly consisted of α-fiber (〈110〉//RD), γ-fiber (〈111〉//RD), and ε-fiber (〈110〉//TD). The major components of texture observed were {112}〈110〉, {332}〈113〉, {110}〈001〉, and {001}〈110〉 orientations. The observations suggested that the RD fiber centered at {112}〈110〉, {113}〈110〉, and {223}〈110〉 was responsible for the anisotropy in yield strength. The intensity of these texture components was higher in both X70 and X80 microalloyed steels in the rolling directions (RD) as compared to the 45° to the RD.

Microstructural evolution in friction stir welding of high-strength linepipe steel

Microstructural evolution during friction stir welding (FSW) of a high-strength linepipe steel was studied. The various grain structures developed through a complex process including the rearrangement of low-angle boundaries, continuous dynamic recrystallization and phase transformation. In most parts of the stir zone (SZ), acicular-shaped bainitic ferrites were formed by the phase transformation during the FSW process. A fine-grained microstructure developed mainly in the thermomechanically affected zone (TMAZ), where continuous dynamic recrystallization occurs. The shear texture in the SZ became considerably weak due to the phase transformation during the FSW process. The hardness of the SZ was significantly higher than that of the other FSWed regions due to the bainitic ferrites.

Exploiting a Simple Spectrophotometric Method for the Determination of Manganese in High Strength Line Pipe Steels

The use of high strength line pipe steels is beneficial for the reduction the cost of gas transmission pipelines by enabling high pressure transmission of large volumes of gas. The high strength line pipe steels will become the preferred materials for modern natural gas transmission pipeline. It was well known that manganese was an important element in the high strength line pipe steels. In this paper, a simple spectrophotometric method was described for determination of manganese in high strength line pipe steels. The method was based on the oxidation-reduction reaction between ammonium persulfate and manganese(II) producing manganese(VII) in the presence of silver nitrate as a catalyst. The characteristic wavelength of maximum absorption of manganese(VII) was obtained locating at 530 nm. Under the optimum reaction conditions the absorption value was proportional to the concentration of manganese in the range of 0.18%~2.0% (R2 = 0.9997), and the relative standard deviation was less than 3.0% (n=5). The proposed method was applied successfully to determine manganese in API grade X80 line pipe steel and API grade X70 line pipe steel samples.

OS14F117 Effect of Microstructures on Ductile Tearing Resistance of High Strength Linepipe Steel

This paper presents effect of microstructures on ductile tearing resistance of high strength linepipe steel with low sulfur content. Especially, it is focused on the investigation of the ductile tearing behavior obtain from single edge notched tension (SENT) specimens. SENT tests are conducted after applying a simulated arc welding heat cycle. The peak heating temperature is 1400 ℃ simulated Coarse grained HAZ. Ductile tearing resistance curves between the base metal and the simulated CGHAZ is good agreement. However, ductile tearing behaviors between the base metal and the simulated CGHAZ were entirely different. In the base metal, a ductile crack is initiated from the deepest point of the notch tip and propagates in thickness direction straight. In the case of CGHAZ specimen, a ductile crack is initiated from the deepest point of the notch tip, and propagates toward the 45° inclined direction to the thickness with a zigzag manner. The ductile tearing curve of tested material such as high strength steel with low sulfur content is little affected by welding heat cycle. On the other hand, the difference of ductile tearing behavior between base metal and CGHAZ is associated with micro voids nucleation place near crack front, which can be associated with the difference of the grain size of base metal and CGHAZ.