Cold-bending of linepipe steel plate to pipe, detrimental or beneficial?
Introduction
Being an integral part of economic transmission of crude oil and natural gas, linepipe steels have been a topic of significant research interest since decades. An excellent combination of strength, ductility, and impact toughness are the essential requirement in steel plates used for linepipe applications. As per the American Petroleum Institute (API), typical grades of the linepipe steels are designated considering their minimum yield strength requirement in ksi, e.g. X65, X70, X80, X100, X120 etc. [1], [2], [3]. Detailed discussion on the composition, microstructure, and properties of linepipe steels are available in different review articles [1], [2], [3]. The requirement of minimum yield strength of linepipe steels has more than doubled in past four decades, starting from X42 and X52 to X100 and X120 [2], [4]. The grades mostly used at present (X70, X80 etc.) are based on the thermomechanical processing of high-strength low-alloy (HSLA) steel plates, containing microalloying elements such as Nb, Ti, and V [5], [6], [7], [8]. Ferrite / martensite dual-phase microstructure has been found to possess superior combination of strength and toughness [5], however, crystallographic texture can play a vital role resulting in property anisotropy [6]. It is suggested that an acicular ferrite / bainite microstructure with a minor amount of pearlite is best suitable for pipeline applications [7], [9], [10]. For achieving the desired microstructures in the steel plates, extensive studies have been carried out to standardize the optimum processing parameters such as soaking temperature, controlled rolling, accelerated cooling strategies, and coiling practices [9], [10], [11], [12]. Grain refinement is a well-practised method to enhance strength and toughness simultaneously, which are mutually exclusive [11], [13]. It is reported that the effective grain size (EGS) reduces significantly with the decrease in finish rolling temperature, but does not depend much on the cooling rate or coiling temperature [12]. Several investigations relating the nature, size, and fractions of different microstructural constituents (e.g. polygonal- and acicular-ferrite, pearlite, upper- and lower-bainite, and martensite-austenite (MA) constituents) to the mechanical properties (e.g. tensile properties and impact toughness) helped to identify the desired microstructures for the linepipe steel plates [13], [14], [15], [16]. Austenite reversion treatment is considered as a key factor which affects both strength and toughness during thermo-mechanical processing [14], [15]. Bainite has a detrimental effect on low-temperature toughness which can be optimized by developing a microstructure with acicular ferrite + polygonal ferrite as a second phase [16], [17]. Besides microstructure, control over the crystallographic texture, microalloy precipitates (carbides/nitrides of Nb, Ti, and V) and non-metallic inclusions (Al2O3 or MnS) are also necessary to satisfy the requirements of linepipe steel plates [17], [18], [19], [20]. Yang et al. suggested that rolling in the non-crystallization region of austenite resulted in less formation of detrimental texture component i.e. rotated cube {001}<110> component [18]. All damage stages are known to be anisotropic, including void initiation and crack propagation, where porosity and void aspect ratio play a vital role [19]. Formation of MC type carbides within ferritic matrix in microalloyed (specially Nb) steels results in precipitation strengthening and an overall better strength-toughness combination [20].
Although significant research attention has been paid on the processing, microstructure, and properties of linepipe steel, there is a dearth of research on the effect of plate-to-pipe forming process on the final properties of the pipe, which is crucial from the application point of view. Cold-bending of steel plate imparts a strain distribution within the microstructure which is expected to influence the tensile and impact properties of the pipe [21], [22], [23]. Different states of residual stress develop during the manufacturing stages of plate-to-pipe formation and the Bauschinger effect plays a significant role in the magnitude of the resulting plastic strains and residual stresses [24]. As a result, continuous yielding and low yield ratios are anticipated in the inner side, whereas, discontinuous yielding and high yield ratios are expected in the outer side [21]. It is reported that the yield strength is affected more during strain-reversal tests in the presence of more amount of M-A constituents and this phenomenon can be explained by the difference in the density of mobile dislocations and by the competing mechanism between Bauschinger effect and strain hardening [22]. The UOE pipe-forming process is practised industrially where the steel plate is first subjected to ‘U’‐shaped bending, then ‘O’‐shape forming, and finally an ‘E’- expanding [25]. Objective of the present study is to understand the effect of cold-bending which is the primary step in UOE pipe forming process i.e., U-formation. As of now, the entire UOE process is not simulated in laboratory scale and authors aim to investigate in future the effect of pipe forming on mechanical properties inclusive of all the steps that are followed industrially.
Section snippets
Material and Methods
A 24 mm thick industrially controlled rolled plate of API X80 grade linepipe steel was used in this study as the starting material. Details of the chemical composition and the pipe-forming condition are given in Table 1, Table 2, respectively. The plate was bent by hydraulic pressure at ambient temperature and at a loading rate of ~1.1 t-s−1 in such a way that it resembles a curved section taken out from a pipe where the rolling direction remained parallel to the major axis of the pipe, Fig. 1.
Microstructural observation
Composition of the investigated steel is leaner with C, S and P content in comparison to the standard specified by API, Table 1. Hence, the chances to find detrimental inclusions (especially MnS) within the microstructure are less and the investigated steel is expected to show a smaller amount of directional property [3]. SEM micrographs taken from different planes shown in Fig. 1 and the corresponding fractions of different microstructural constituents listed in Table 3 indicate that the
Discussion
The as-received plate exhibited a gradient in hardness from the surfaces to the centre. This is understandable as the grains are finer at the surface regions compared to the mid-thickness portion due to the following reasons: (i) Surface shear effect during rolling and (ii) higher cooling rate and lesser available time for the grain growth. Since, the material was processed by controlled-rolling and accelerated-cooling, it developed both {100}< 011 >α and {332}< 113 >α components at RT. These
Conclusions
Major findings of the present study are as follows:
- i)
Cold-bending increased local strain by the generation of dislocations, especially near the interphase boundaries. The dislocation density observed in outer diameter (OD) side of cold-bent plate was as high as ~ 3.0 × 1014 m−2 and it was ~ 2.0 × 1014 m−2 at inner diameter (ID) side.
- ii)
Cold-bending promoted rotated goss {011}< 110 > and close to {011} texture component, respectively in ID and OD, especially at the expense of rotated cube texture
Acknowledgement
Authors gratefully concede the financial support received from Tata Steel Ltd., Jamshedpur, India and experimental facilities of Central Research Facility and Dept. of MME, IIT Kharagpur (including Institute SGDRI-2015 grant). Authors would also like to acknowledge Dr. Saurabh Kundu (Chief, Product Research) and Dr. A. N. Bhagat (Head, Research Application - Product) from R&D, Tata Steel Ltd. for their support to continue this collaborative project.
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