Effects of thermomechanical treatment on the occurrence of coincident site lattice boundaries in high strength low alloy steel

Effects of thermomechanical treatment on the occurrence of coincident site lattice boundaries in high strength low alloy steel

Renato Soares de Castro^{I, II}; Ricardo Artur Sanguinetti Ferreira^I,*; Igor Rafael Vilarouco Pedrosa^{I, II}; Yogendra Prasad Yadava^I

^IDepartment of Mechanical Engineering, Federal University of Pernambuco – UFPE, Recife, PE, Brasil

^IIDepartament of Mechanical, Federal Institute of Education, Science and Technology of Pernambuco – IFPE, Recife, PE, Brasil

ABSTRACT

Many studies propose thermomechanical routes to improve HSLA (high strength low alloy) steels by microstructural modification but ignore the study of occurrence of CSL boundaries. The occurrence of these special boundaries is influenced by thermomechanical treatment in which the modification of misorientation profile is related with the increase of Σ3 and Σ11 boundaries and inhibition of Σ5 Σ7 and Σ9 ones. The formation of Σ3 boundaries, beneficial to improve strength, is highly stimulated by this treatment in which deformation plays an important role in the development of CSL boundaries.

Keywords: HSLA steel, thermomechanical treatment, acicular ferrite, coincident site lattice

1. Introduction

In recent years studies have led attention to CSL (Coincident Site Lattice) boundaries, mainly to investigate the influence of special boundaries in metallic material properties^1-3. The development of special boundaries has been frequently achieved by annealing treatments. Many of these studies are concerned to stainless steel³, where low Σ boundaries should be beneficial to minimize crack corrosion mainly due to the low energy of those boundaries. Otherwise low attention has been taken to the occurrence of CSL boundaries in HSLA (High Strength Low Alloy Steel) steel. Many authors have investigated the relationship between thermomechanical treatment and improvement of mechanical properties^4-6 relating this improvement to the presence of particular microstructure, such as acicular ferrite. HSLA steels with acicular microstructure achieve good properties combination, such as high strength, excellent toughness and superior fatigue behavior^7,5. Many studies propose thermomechanical routes to improve microstructural changes in HSLA steel but ignore the occurrence of CSL boundaries. This work investigates the misorientetion boundary profile and occurrence of CSL boundaries in HSLA X80 steel submitted to thermomechanical treatment for increasing toughness by microstructural modification.

2. Experimental Procedure

The material used in this experiment was an HSLA steel composed of (wt. %) C 0.07 - S 0.004 - Al 0.036 - Si 0.27 - P 0.016 - Ti 0.018 - V 0.022 - Cr 0.017 - Mn 1.55 - Ni 0.01 - Cu 0.01 - Nb 0.069 - Mo 0.19. Samples were produced in the form of strips with dimensions of 100 × 9.5 × 6.0 mm. The faces to be rolled were finished with 6.0 mm thickness and tolerance of ± 0.2 mm.

Samples were heated in electrical furnace until 950 ºC, maintained for 900 s and then hot rolled at non recristalization temperature of austenite by a duo rolling cylinder at a strain rate of 1,8 s ^–1. The rate in applied strain varied from 10% to 35%.

SEM images were obtained from cross sections of samples submitted to standard metallographic preparation and finally etched with nital at 3%. EBSD analysis were performed in a JSM-5900LV JEOL microscope with Channel 5 software (HKL Company) for data acquisition and data analysis. Samples for EBSD were sanded in 1500 MESH sandpaper and polished with diamond paste. To eliminate possible surface deformations produced by grinding and mechanical polishing, samples were finally polished in colloidal silica of 1 µm for 7.2 × 10³ s. Micro hardness Vickers test was carried out with load of 0.1 kg for 15s. For each sample five measurements were performed on the central axis of the specimen.

3. Results and Discussions

As-received steel had a microstructure with well-defined polygonal ferrite grains with average grain size of 3.6 ± 0.3 µm as shown in Figure 1a. After thermomechanical treatment, the predominance of polygonal ferrite in microstructure was eliminated. The sample deformed at 10% presented the morphology of a chaotic structure with lamellar and acicular aspects (Figure 1b), which literature has reported as acicular ferrite^8-10. In samples deformed at 25% and 35% (Figures 1c, d) predominant presence of acicular ferrite was more evident, characterized by bundles of ferritic laths interwoven in a chaotic distribution.

The change from polygonal to acicular microstructure increases strength and toughness in steel¹¹ due to finer grain size, higher density of dislocations and presence of subunits in acicular ferrite⁷. Results of Vickers hardness test in Table 1 confirm the trend of increasing yield strength due to increasing hardness in treated samples when compared to the as-received material.

Predominance of acicular ferrite, in specimens more intensely deformed, can be attributed to plastic deformation of austenite below non recristalization temperature⁴ which for this steel is around 1100 ºC. The high density of dislocations in deformed austenite seems to be beneficial to the formation of acicular ferrite to the detriment of the nucleation of bainite. Plastic deformation of austenite suppresses the formation of the bainitic structure from the grain boundaries and favors the formation of acicular structure stimulated by intragranular nucleation⁴.

It can be observed in the grain boundaries map (Figure 2) the evolution from a regular and well-designed grain shape microstructure in as-received steel (Figure 2a) to a finer and less regular one in the sample submitted to thermomechanical treatment (Figure 2b). It can be seen, from histograms in Figure 3a, b, that as-received steel has a misorientation frequency close to normal distribution, while for sample deformed at 10% an increase of frequency at low angles (2 º to 15 º) as well as at high angle (50 º to 60 º) is observed (Figure 3b). This tendency is maintained in other treatment conditions, with the occurrence of the highest frequency of low angle in the sample deformed at 25%. This result, reinforced by SEM images, shows that change in misorientation profile is highly influenced by plastic deformation, which also encourages austenite nucleation and formation of subunits in acicular ferrite⁶.

The increase of frequency in low angle indicates the refinement of the structure¹² with the presence of crystallographic packages of acicular ferrite with misorientation degree between them of about 5º, and subunits with misorientation angle^5,12 from 1º to 2º. It was also identified changes in hardness, crescent with the level of deformation (Table 1) that, in steels with acicular microstructure, can be attributed to the high presence of misorientation angle¹² up to 3º.

Thermomechanical treatment increased the occurrence of CSL boundaries, particularly Σ3 type, in all conditions of treatment, as detected in (Figure 3d) when compared to the as-received material (Figure 3c). The Σ3/total CSL rate of 28.7 % in the as-received steel rises to 43.3% in the sample subjected to 10% strain (Table 1) and remains at high level for greater deformations, indicating that the mechanism of deformation is an important factor in shaping Σ3 boundaries.

It was detected some relationship between the profile of disorientation and the occurrence of CSL boundaries. Considering that in CCC lattices the Σ3 boundaries are generated by the rotation angle of 60º of the plane <111>, it is expected that, due to Σ3 increase, there should be an equivalent increment of frequency in misorientation profile close to 60º. In the as-received steel, misorientation frequency between 58-60º corresponds only to 5.6% of total CSL boundaries while in the sample deformed at 10% (where there is high increase of Σ3 boundaries) this frequency rises to 11.4%, computing a variation of 5,8% (Table 2). Samples subjected to greater deformation also presented increases of Σ3 boundaries and misorientations around 60 º when compared to non treated samples.

As for Σ3 boundaries, modification of quantity of other Σ boundaries can be detected by changes of misorientation profile. Table 2 shows the variation of occurrence of CSL boundaries and respective modification of misorientation profile around specific angle for each Σ-symmetry. The increase of frequency of Σ3 and Σ11 boundaries, as well as the reduction of Σ5, Σ7 and Σ9 boundaries detected in 10% deformed samples, compared to not treated one, have respective variations on misorientation profile (Table 2). This modification was also detected for 25% and 35% deformation conditions.

Variation in misorientation profile can be clearly detected with the comparison between histograms of Figure 3a and Figure 3b. The reduction of Σ5, Σ7 and Σ9 boundaries is verified by means of decrease in the frequency of misorienteation degree from 33.5º to 41.5º in misorientation profile, while the increase of boundaries of Σ11 type is revealed with the increasing frequency misorientation around 50º (Figure 3b).

According to Arafin³ and Gadzer et al.¹³, the increasing fraction of special boundaries has significant influence on the properties of the material. Low angle boundaries, as well as CSL's with low Σ value are inactive and more resistant to intragranular fracture than high energy random boundaries¹⁴. Thus it is expected that the sample subjected to thermomechanical treatment with deformation of 25% shows the highest resistance to intragranular fracture since it combines a high fraction of Σ3 boundaries (Table 1) with low occurrence of contours with Σ > 29. Suzuky et al.¹ found not only the importance of low Σ boundaries in metallic materials, but also measured their limits of resistance by means of a method to evaluate the resistance of CSL boundaries. They compared the values of grain boundary fracture strength, in a Cr-Ni alloy, for different types of CSL boundaries and achieved that Σ3 type presented the greatest value of strength, while Σ5, Σ9, Σ7 and Σ11 had decreasing values in this order.

Considering the results of Suzuky¹, the significant increase from 3.5% to 7.1% of Σ3 boundaries, in sample deformed at 10% (Table 1), should increase the strength of the material. However, the decrease of the amounts of Σ5, Σ7, Σ9 boundaries, weaker than Σ3, can decrease this benefit which was not totally evaluated by that author.

The finding of high fracture strength in Σ3 boundaries for Cr-Ni alloys¹ reinforces the positive influence of the thermomechanical treatment on HSLA steels. Once applied, this treatment caused significant increase in Σ3 boundaries in all samples, when compared to the as-received material. It was also observed, in intragranular regions of treated samples, the presence of twin events (Figure 4). This phenomenon is a favorable factor to the formation of special type of boundaries² and separates regions of symmetric misorientations, named as twins.

Some studies related to the occurrence of CSL boundaries show the effect of annealing treatments to stimulate or inhibit the formation of special boundaries^15,16. However no studies were found reporting the effect of thermomechanical treatment on the formation of CSLs in HSLA steels. Randle¹⁶, when investigated the mechanism of formation of CSL boundaries stimulated by annealing, presented a model called "Σ3 regeneration" which shows that the mobility of CSL boundaries, causes interactions among these types of boundaries which, depending on its nature, results in predominant Σ3 boundaries type. According to this model, a Σ3 boundary when meeting a Σ9, also highly presented in cubic lattices, originates a new Σ3 boundary, just as meeting a Σ9 and Σ27 also originate a Σ3. This phenomenon would explain the fact that Σ3 boundaries are highly present in treated samples.

Despite that the treatment applied here does not involve annealing process to allow high mobility of boundaries by diffusion, it is observed that plastic deformation in thermomechanical treatment has an important role in forming CSL boundaries. The movement of preferred slip planes and the interaction between dislocations during strain or shear transformations favor the mobility of boundaries and their interaction in forming new CSL boundaries.

4. Conclusions

By thermomechanical treatment, at non-recristalization temperature of austenite, it is possible to obtain dominant acicular ferrite microstructure in HSLA steel. Higher levels of deformation result in higher resistance level revealed by the increase in hardness of treated samples. This treatment modifies the misorientation profile, increasing low and high angle frequencies, tending to a bimodal distribution.

The Formation of CSL boundaries is influenced by thermomechanical treatment and it is possible to relate the modification of misorientation profile with the increase of Σ3 and Σ11 boundaries and inhibition of the formation of Σ5 Σ7 and Σ9 boundaries. The formation of Σ3 boundaries, beneficial to improve strength, is highly stimulated by this treatment where the intensity of deformation plays an important role in the development of special CSL boundaries. However further studies are needed to clarify this mechanism. Mapping of Σ3, Σ9 Σ27 boundaries in different processing conditions associated with texture study by EBSD analysis are possible paths for further investigations.

Acknowledgements

The authors would like to thank FACEPE for financial support and the LME/LNNano for technical support during EBSD analysis.

Received: November 28, 2012

Revised: July 6, 2013

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Publication Dates

• Publication in this collection
  13 Aug 2013
• Date of issue
  Dec 2013

History

• Received
  28 Nov 2012
• Accepted
  06 July 2013