Transformation behavior and microstructural characteristics of acicular ferrite in linepipe steels

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Abstract

Transformation behavior and morphological characteristics of acicular ferrite in linepipe steels were investigated through the use of a dilatometer and EBSD. The results show that the volume fraction of acicular ferrite increased with an increase in the amount of hot-deformation in the austenite non-recrystallization region because acicular ferrite is formed at nucleation sites such as dislocations within austenite grains. This study found that acicular ferrite is formed at approximately 600 °C, where the transformation behavior can be characterized as a two-stage reaction: (i) nucleation of acicular ferrite and (ii) formation of polygonal ferrite between acicular ferrite grains. EBSD analyses show that an acicular ferrite grain consists of several sub-units misoriented by 1–2° and that a set of adjacent acicular ferrite grains with crystallographic misorientation below 15° makes up the crystallographic packet.

Introduction

In recent years, there have been increasing demands for high-performance linepipe steels which feature not only high strength but also good low-temperature toughness. The steels should also have a low-yield ratio, good weldability and resistance to a corrosion environment. As these properties can generally be improved by controlling the chemical compositions and process parameters, many researchers have studied the relationship between microstructural features and mechanical properties in linepipe steels [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11].

Acicular ferrite, first described by Smith et al. in the early 1970s [12], has been known as the optimum microstructure with an excellent combination of high strength and good low-temperature toughness in steel welds. These are mainly due to the relatively high density of dislocations and the fine-grained nature of the acicular ferrite structure, which improves the toughness of the welds [13], [14], [15], [16], [17]. Acicular ferrite structure in the weld region consists of a fairly chaotic arrangement of ferrite plates facing in many different directions within any given austenite grain. Thus far, most works concerning acicular ferrite have been carried out on welds in which the high density of the inclusions present ensures a high density of nucleation sites [18], resulting in the formation of acicular ferrite instead of bainite.

In contrast, a microstructure with similar morphology, which many researchers have also termed acicular ferrite believing it to have similar properties to those of welds, has been observed in linepipe steels [5], [6], [7], [8], [9], [10], [11]. Despite their morphological similarity, however, the environment for acicular ferrite formation in linepipe steels is quite different from that of welds, as nonmetallic inclusions for nucleation sites are extremely restricted in linepipe steels. This can result in a different formation mechanism of acicular ferrite from that occurring in welds. Although Araki et al. [8] and Xiao et al. [9] agreed that the acicular ferrite in linepipe steels is different from that in welding metals, the transformation behavior and morphological features of such a microstructure in linepipe steels are still controversial and remained unresolved despite its excellent balanced mechanical properties. As the properties of materials in general depend on the microstructural features, a precise comprehension of acicular ferrite should come first.

The present study is aimed at investigating the microstructural characteristics of acicular ferrite formed in high strength linepipe steels. For this relation, the transformation behavior of acicular ferrite during continuous cooling and isothermal holding was examined by dilatometry analysis in addition to an orientation imaging microscopy (OIM) technique based on electron back-scattered diffraction (EBSD) as well as electron microscopy, which was used to obtain some quantitative crystallographic information.

Section snippets

Experimental procedure

In this study, steels with different additions of alloying elements were manufactured by vacuum induction melting. Their chemical compositions are listed in Table 1. The A steel has the chemical composition of commercial API X70-80 grade linepipe steel, and the B steel is a conventional TRIP steel with a higher amount of Si, Ni and Cu. In order to examine the characteristics of acicular ferrite for these steels, the effect of a number of processing parameters on the formation of acicular

Effect of deformation in austenite non-recrystallization region

Acicular ferrite is well known as a microstructure with a fairly chaotic arrangement of ferrite plates facing in many different directions within any given austenite grain [19]. It mainly nucleates on nonmetallic inclusions in weld metals. Although in general nonmetallic inclusions promoting the formation of acicular ferrite are extremely restricted in linepipe steels, this microstructure can be easily observed. This indicates that acicular ferrite nucleates at other sites. As a heavy

Transformation behaviors of acicular ferrite

According to previous reports, acicular ferrite and bainitic ferrite are considered to be formed in the same range of temperatures (approximately 400–600 °C) by the same type of transformation mechanism [13], [20], [21]. In the case of bainitic ferrite, the ferrite nucleates at the austenite grain boundaries, forming sheaves of parallel plates with the same crystallographic orientation, whereas acicular ferrite nucleates intragranularly at nonmetallic inclusions in the welds. The acicular

Conclusions

This study analyzed the formation mechanism and morphological characteristics of acicular ferrite in high-strength linepipe steels. The results can be summarized as follows:

  • (1)

    As the amount of hot-deformation in the austenite non-recrystallization region increases, the heterogeneous nucleation sites within the austenite grain increase, promoting the formation of acicular ferrite. However, an increase in the amount of the austenite stabilizing elements suppresses the formation of acicular ferrite.

  • (2)

Acknowledgement

This work was supported by the National Research Laboratory Program of the Ministry of Science.

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