**3.3 Crystallographic texture**

ODFs were constructed from the XRD and EBSD data and plotted in colored intensities diagrams using Bunge notation in Euler space, as depicted in **Figure 5**. The crystallographic representation for rolled steels is composed of a plane {hkl} which lies parallel to the normal plane and a direction <uvw>, which is parallel to the rolling direction [38] as schematically shown in **Figure 5e**. The reference system is based on RD, TD and ND, where their related planes are rolling, transversal and normal planes which lies perpendicular to their respective directions. The development of texture produced during hot-rolling at full austenitic region causes

**Figure 2.** *SEM micrographs of (a)–(c) AIR-steel and (d)–(f) ACC-steel.*

*Effect of Textures and Microstructures on the Occurrence of Delamination… DOI: http://dx.doi.org/10.5772/intechopen.88001*

#### **Figure 3.**

*EDS analysis of MnS elongated particles in the AIR-steel.*

**Figure 4.**

*Through-thickness hardness profiles in the rolling and transverse planes of the steel of ACC-steel and AIR-steel.*

strong alpha-fiber development, which consists of {001} <110> to {110} <110> in ϕ2 = 45°, ϕ1 = 0°, ϕ = 0–90° [38]. It is a rotation on the {110} direction axis, being a fiber parallel to RD. As temperature decreases and cold-work increases, there is an increase of gamma-fiber, which consists of {111} <110> to {111} <112> in ϕ2 = 45°, ϕ1 = 0–90°, ϕ = 55° [38]. It is a rotation around the {111} plane, i.e., around the perpendicular direction of the plane, which is [111], being a parallel fiber to ND. In **Figure 5a**–**d** are retrieved the ODFs colored intensities, showing the presence of alpha-fiber and gamma-fiber. Both steels presented low gamma-fiber intensity compared to alpha-fiber. Comparing crystallographic textures in both steels, AIR-steel presented higher alpha-fiber and higher crystallographic intensity in XRD and EBSD measurements than ACC-steel as depicted in **Figure 5a**–**f**. Another component that plays an important role is the cube side lattice {100} since it is the cleavage plane on body-centered cubic (BCC) steels. The family plane {100} is likely to have a relationship to the occurrence of delaminations, as is described further in the present work.

#### **Figure 5.**

*ODF plots at ϕ2 = 45° highlighting alpha-fiber presence, location and high intensity of (001)[110]: (a, b) XRD ODF analysis: (a) ACC-steel, (b) AIR-steel; (c, d) EBSD ODF analysis: (c) ACC-steel, (d) AIR-steel;(e) ODF map [39], (f) quantification of alpha-fiber intensity by XRD, showing higher values for AIR-steel, and (g) schematic representation of crystallographic (001) planes perpendicular to ND and [110] direction parallel to RD.*

The crystallographic orientation distribution from EBSD analyses was performed to compare and validate that of XRD. The EBSD analyses were carried on rolling plane of both investigated samples out at areas of 1500 × 1300 μm2 with a step size of 2.5 μm containing about 1500 grains. Their related ODFs were calculated using the statistical method of kernel density estimation and presented in **Figure 5**. The results obtained by EBSD are similar to previous ODF results calculated by XRD. A certain difference between these methods could be attributed with a high statistical symmetry at each pole figure (XRD) and local grain orientation (EBSD). Overall texture intensity of AIR-steel was higher rather than ACC-steel due to the formation of {110}//RD grains accompanied by banded microstructure. Suikkanen et al. [40] characterized a {110} plane of the carbide-free bainitic ferrite similar to the martensitic transformation originated from close-packed {111} austenite plane because of the accommodation of transformation strains, enhancing the bainitic transformation. The formation of (211)[011] component could be explained by Shackleton and Kelly works which reported that the habit plane of cementite in lower bainitic ferrite is corresponding to {**001**}//{**211**} [41]. In other words, the proeutectoid cementite habit induced large shear deformations because of stereological effects to form a displacive bainite structure.

In order to investigate the role of local crystal orientations and boundary types, detailed EBSD measurements were carried out in rolling transversal planes of both AIR and ACC steels. Orientation image (OI) map, Kernel average misorientation (KAM) map, grain boundary (GB) map, Taylor factor (TF) maps, and normal direction pole figure (IPF) maps of each measurement were presented in **Figure 6** for AIR-steel and **Figure 7** for ACC-steel.

The orientation data collected with EBSD displayed by OIM or Euler colored maps, provides a basic presentation of the measured orientation. Grain boundaries are considered as crystal lattice defects, which can be identified by the point-topoint misorientation between neighboring data points, **Figures 6a** and **7a**. Grain

*Effect of Textures and Microstructures on the Occurrence of Delamination… DOI: http://dx.doi.org/10.5772/intechopen.88001*

#### **Figure 6.**

*(a) OI map, (b) KAM map, (c) GB map, (d) TF maps, and (e) normal direction IPF maps of the rolling plane of AIR-steel.*

boundary energy is attributed by the interfacial region geometry between two adjacent crystals with different orientations. For example, the high angle boundaries (HABs), greater than 15°, are determined by point-to-point misorientation analysis [42, 43]. Low angle boundaries (LABs) and dislocation tangles were characterized from their misorientation angle (5–15° and 2–5°, respectively), **Figures 6c** and **7c**. Also, coincident site lattice (CSL) boundaries with HABs with low stored energy due to good atomic fit between neighboring crystals can act as crack arrester.

#### **Figure 7.**

*(a) OI map, (b) KAM map, (c) GB map, (d) TF maps, and (e) normal direction IPF maps of the rolling plane of ACC-steel.*

KAM map indicates the local plastic strain variations measured between the central point and its nearest neighbors when the misorientation between them exceeds 5° in eliminating the effect of grain boundaries [44, 45]. The high KAM angles appear cumulative build-up of misorientation inside the grain or the presence of sub-grain boundaries, **Figures 6b** and **7b**.

Pencil glide occurs on {110}, {112}, {123} slip planes along the slip direction <111>, in BCC materials such as steel. The external stress imposes during deformation leads to activate the potential activated slip systems for each grain individually according to the crystal rotation axis method based on their Schmid factors [46–48]. Polycrystalline deformation is determined through all possible combinations of the potential slip systems compared with the imposed macroscopic stress state. Then, the yield response of individual grain is predicted based on favorably and unfavorably oriented for easier slip activation according to the geometrical lattice rotation axes. For instance, soft grains with low Taylor factor value (in blue color) have the least resistance to slip, while red-colored grains represent hard grains with highest Taylor factor, i.e., highest resistance to slip [46, 47], **Figures 6d** and **7d**. Also, the corresponding rolling direction IPF obtained from each EBSD data is shown in **Figures 6e** and **7e**, which can quantify the volume fraction of the crystallographic orientation of the grains.

#### **3.4 Tensile tests**

Elongation measured at tensile tests superior to 10% and yield strength above 400 MPa for all the evaluated conditions were found. Yield strength (YS) and ultimate tensile strength (UTS) are summarized in **Figure 8**, according to the tested temperature. As shown in **Figure 8a** and considering the results at room temperature, the ACC-steel fulfills the API standard requirement for the minimum yield strength (555 MPa) [28], in the rolling and transverse planes, to be classified as X80. The AIR-steel reaches the X80 grade specification requirement in the transverse plane, but not in the rolling plane, in which it reaches only the X70 specification requirement. Considering the ultimate tensile strength at room temperature (25°C), as shown in **Figure 8b**, both plates of steel fit into the X80 grade requirement.

Regarding the results presented in **Figure 8**, it is possible to state that anisotropy was found in both plates of steel, being more intense in the AIR-steel than in the ACC-steel, which presented a smaller difference in tensile results between the rolling and transverse planes. In addition, the AIR-steel tensile test results varied considerably between temperatures due to the heterogeneity of the microstructure.

AIR-steel fails the API X80 criterion for yield stress. The ACC-steel plates were machined from a pipe, a finished product, while AIR-steel plates came directly from TMCP plate production. For AIR-steel, a further U-O-E process would be necessary to transform plates into tubes, which would promote enough work-hardening to reach the yield stress of X80-grade specification [24]. Also, the YS/UTS ratio was not the predominant factor affecting the fracture toughness, where usually high YS/ UTS ratios lead to low toughness, however, in this study high YS/UTS lead to high toughness.

#### **3.5 Fracture toughness tests**

Fracture toughness results, measured by the CTOD parameter in mm, are summarized in **Figure 9**. For 7-mm-thick samples, AIR-steel presented lower average CTOD than ACC-steel, as shown in **Figure 9a**. AIR-steel presented difference of CTOD value between L-T and T-L geometries: L-T presented higher values than T-L. ACCsteel presented statistically equal results for both L-T and T-L. Therefore, ACC-steel presented higher toughness and less crystallographic texture effect than AIR-steel.

#### *Effect of Textures and Microstructures on the Occurrence of Delamination… DOI: http://dx.doi.org/10.5772/intechopen.88001*

For 15-mm-thick samples, both steels presented a CTOD decreasing trend lowering temperatures, as depicted in **Figure 9b**. The ACC-steel depicted a better toughness behavior at 0 and −20°C than AIR-steel; however, at −40°C, both steels showed similar CTOD. CTOD results obtained at 0°C were similar to those from 7-mm-thick samples at 25°C. Below 0°C, the drop in temperature caused a partial shift of biaxial tension state towards triaxial tension state, increasing the triaxiality state at the crack-tip and reducing the material ductility [23].

To analyze the fracture surfaces, samples were submerged in liquid nitrogen and then broken by impact to separate samples into two halves. Thus, fracture surfaces of 7-mm-thick samples in **Figure 10** and 15-mm-thick samples in **Figure 11** can be observed. Fatigue pre-cracking region, crack propagation region during the CTOD test and final fracture caused by impact are also shown. The crack opening displacement (COD) and the applied force figure has been added to show the behavior of the material during the CTOD test.

For 15-mm thick samples, the AIR-steel and ACC-steel specimens were tested in L-T and T-L geometries, respectively, due to the best CTOD results of 7-mm thick samples. For AIR-steel CTOD samples, all cases tested at −20 and −40°C presented delamination during CTOD test, totalizing 6 cases. Samples tested at 0°C presented no delamination occurrence. Most of the occurred delaminations manifested as pop-ins in the CTOD test curves and four cases the delamination were considered

**Figure 8.**

*Summary of the tensile tests results of the AIR-steel and ACC-steel: (a) yield strength (YS) and (b) ultimate strength (UTS). Tests were conducted in the transverse and parallel directions relative to the rolling direction.*

#### **Figure 9.**

*Fracture toughness using CTOD parameter for ACC-steel and AIR-steel: (a) 7-mm-thick samples tested at 25°C; (b) 15-mm-thick samples tested at 0, −20 and −40°C. The L-T and T-L correspond to the ASTM E1823 [30] notches nomenclature. Tests were performed in the L-T and T-L direction in SE(B) samples. Results from ACC-steel with 15-mm-thick were published at [31].*

#### **Figure 10.**

*Fractured surfaces of the toughness of 7-mm thick samples tested at room temperature. AIR-steel: (a) L-T geometry, plastic behavior and occurrence of delamination during sample break; (b) T-L geometry, plastic behavior on CTOD zone and flat and brittle like behavior during sample break; (c) T-L geometry, occurrence of delamination during sample break; (d) force vs. COD experimental graph during CTOD test of the sample shown in (c), displaying red line at maximum achieved force. ACC-steel: (e) L-T geometry, plastic behavior and occurrence of delamination during sample break; (f) T-L geometry, plastic behavior on CTOD zone and flat and brittle like behavior during sample break; (g) T-L geometry, plastic behavior during CTOD and final fracture, with occurrence of delamination during sample break; (h) force vs. COD experimental graph during CTOD test of the sample shown in (g), displaying red line at maximum achieved force. COD: crack open displacement at the crack mouth.*

significant according to the ASTM 1820 standard [29], two at −20°C and the other two at −40°C, out of a total of 6 cases of delaminations. Notice that a pop-in event does not mean the start of brittle crack propagation, it is just a disparity of the stable crack propagation and is recommended to calculate CTOD values when they appear; however, the crack propagation continues stably until the end of the test.

Moreover, when the crack front suffers delamination, it is divided into several fronts with specific stress state in each one. Each condition is not considered in the equations to calculate CTOD by the ASTM 1820 standard [29]. This observation was previously reported by [24].

For 7-mm-thick samples, all L-T samples presented a plastic fracture aspect, with lateral deformation and a large crack propagation region during CTOD, with no delaminations, but rather after the CTOD test, during sample break, as shown in **Figure 10a**, **e**. For T-L configuration, the surface fracture presented less plastic deformation aspect in comparison to L-T, with a flatter surface. The CTOD tests did not present any abrupt drop in force, but due to high plasticity, only a gradual drop of force was presented on CTOD graphic, as depicted in **Figure 10d**, **h**.

*Effect of Textures and Microstructures on the Occurrence of Delamination… DOI: http://dx.doi.org/10.5772/intechopen.88001*

#### **Figure 11.**

*Fractured surface of API X80 15-mm thick samples tested at different temperatures. AIR-steel in L-T geometry: (a) 0°C, plastic behavior only in CTOD zone, and no occurrence of delamination during CTOD; (b) −20°C, low plastic behavior in CTOD zone and next to delaminations. Delaminations occurred during CTOD test, causing pop-in; (c) brittle behavior with the occurrence of delamination during CTOD test; (d) force vs. COD experimental graph during CTOD test of the sample shown in (c), displaying early delaminations but not significant to end the test, and a red line at maximum achieved force. ACC-steel in T-L geometry: (e) 0°C, plastic behavior in CTOD zone; (f) low plastic behavior in CTOD zone and occurrence of delamination during CTOD test; (g) −40°C, brittle behavior during CTOD and final fracture, with occurrence of delamination during CTOD; (h) force vs. COD experimental graph during CTOD test of the sample shown in (g), displaying red line at occurred delamination, ending the test.*

For ACC-steel, 4 cases tested at −20 and −40°C presented delamination during CTOD test. Samples tested at 0°C presented no delamination occurrence. All the occurred delaminations in ACC-steel were considered significant according to the ASTM 1820 standard, two at −20°C and two at −40°C.

Some fracture surfaces of 15-mm samples are presented in **Figure 11**. In general, all delaminations occurred in AIR-steel that was not considered to be significant, presented a high zone of plastic deformation near it, as in **Figure 11b**. Strain hardening is followed by pop-ins in CTOD curve, presenting a drop on CTOD force with subsequent increase of force.

#### **4. Discussion**

This study presents two TMCP X80 plates of steel with different characteristics to assess fracture of toughness and mechanisms of crack propagation and delamination of the steels. Microstructural features, such as precipitates, phase boundaries, and grain boundaries, play an important role in determining the crack propagation by offering a weak path ahead of the crack tip. However, phase or grain boundaries can act as strong crack arrester when a crack propagates across grain boundary. Crack propagation rate inside a single-crystal grain is slower compared with the crack propagation rate along a crystal grain boundary. **Figure 12** shows the frequency of dislocation tangle, LABs, HABs and CSL for both steels in order to evaluate the capacity to offer resistance to a crack. The fraction of HABs in all planes of AIR-steel sample is greater than ACC-steel. This behavior could be attributed to the banded microstructure, secondary phases, and segregation, which produces more grain boundaries by the presence of more phases and constituents. The presence of sub-grain boundaries or LABs has indicated the arrangement of dislocation inside a single-crystal grain, retarding the crack growth. The portion of sub-grain boundaries is higher in ACC-steel, decreasing the crack propagation rate by retarding the movement of dislocation. A higher fraction of dislocation tangles of ACCsteel could be related to the more lattice distortion due to bainitic transformation without diffusion and insufficient dynamic recovery, which also decreases crack propagation rate inside a single-crystal grain. Morales-Rivas et al. reported that {**211**} habit plane identified in bainitic structure act as a barrier to the dislocation motion inside bainitic ferrite, consequently postponing the crack initiation process [49]. Although low energy CSL boundaries can effectively increase the fracture resistance and block crack propagation, no significant variation of its fraction was observed in investigated samples.

The frequency distribution of main crystallographic orientations such as {001}, {101}, and {111} with about 15° deviation from ideal planes were calculated from EBSD data of AIR and ACC-steel samples and presented in **Figure 13**. The {111} and {101} planes were predominant in the transversal and rolling planes, respectively, of the AIR-steel sample with banded ferritic microstructure with the presence of iron carbide of its structure. The {001} family of planes were found in similar proportion among the rolling and normal planes. The {001}//ND textural components were dominant on a normal plane, and {111}//ND and {101}//ND were characterized by transversal and rolling planes, respectively, in ACC-steel with bainitic structure. Such orientation-dependent mechanical properties are largely attributed to texture and crystallographic orientation, causing anisotropy of mechanical properties. It is well understood that the formation of grains lying parallel to compact planes in BCC-ferritic steel with low carbon content such as {110} and {112} enhance ductile fracture by enhancing dislocation movements by adequate activated slip system, while cleavage occurs in non-compact planes such as {001} [40, 42, 43, 50]. It is

**Figure 12.** *Distribution of boundary types of both steels: (a) AIR and (b) ACC steel.*

*Effect of Textures and Microstructures on the Occurrence of Delamination… DOI: http://dx.doi.org/10.5772/intechopen.88001*

**Figure 13.**

*Frequency distribution of main crystallographic orientations of (a) AIR-steel and (b) ACC-steel.*

suggested that cleavage fracture in AIR-steel found to be influenced by banded ferrite-pearlite microstructure and formation of segregation zone. The rapid cooling at the surface allowed the formation of finer bainite orientated towards {001}// ND. Blondé et al. have been discovered that low carbon content austenite grains transform first to martensite/bainite under accelerating cooling. Das Bakshi et al. [5] investigated the Charpy impact toughness of bainitic structure of microalloyed API X70 steels. They also characterized a dominance of a large fraction of {111}// ND grains accompanied by the presence of {001} and {101} because of the less of recrystallized austenite before the bainitic transformation of austenitic grains. However, this bainitic structure has less distortion rather than martensite structure, thereby, exhibited less risk of cleavage fracture.

KAM values indirectly measure the dislocation density and the local plastic strain developed in the steel during rolling processing and were presented in **Figures 6** and **7**. The presence of banded ferrite—pearlite microstructure with the dispersion of secondary particles induced local stress sites concentration that would make AIR-steel sample prone to crack initiation due to the local high elastic energy stored. Inhomogeneous dislocation accumulation at interfaces traps carbon atoms, resulting in brittle carbide precipitates containing high dislocation density, leading to early fracture. It is included that the dislocation piles up concentrated at ferrite grain boundaries increase the risk of microcrack nucleation and crack propagation as well. However, the bainitic packet containing strain localization inside the soft ferrite lath phase decreases the strain gradient between the inside and at boundary region [51], resulting in higher strain hardenability and more fracture resistance.

Full constraint Taylor approach was used to evaluate the potential activated slip systems dependent on the grain orientations (**Figures 6** and **7**). It is found that the AIR-steels had a more distribution of high Taylor factor in comparison with ACC-steel, indicating the higher capability of storage of energy due to the accumulation of larger dislocation densities or dislocation piles-up, due to more active slip systems in a certain direction. ACC-steel with low Taylor Factor bainitic structure limits the deformation ability by not offering adequate slip systems. These assessments agree with the tensile results obtained in **Figure 8** where AIR-steel presented a higher UTS/YS relation in comparison to ACC-steel.

AIR-steel presented higher intensity on {001} <110> to {111} <110> alpha-fiber, and {111} <110> to {111} <112> gamma-fiber than ACC-steel because of a lower finishing rolling temperature. AIR-steel presented stronger {100} <011>, known as rotated

cube texture, than ACC-steel, which possess a low Taylor factor, i.e., high strain stored energy, or low further deformation ability. This result indicates intense cold deformation, low finish rolling temperature and low inter-pass rolling time [52, 53].

This non-random distribution of crystal orientations influenced toughness properties, mainly by triggering delamination phenomena. The occurrence of delamination for 7-mm thick samples only occurred out of CTOD zone, after test, during sample break. For 15-mm thick sample, delaminations occurred during CTOD tests, causing pop-in and instant drop on the force.

Crystallographic orientation has been reported to be the major role causing impact toughness anisotropy of the steels [5, 8, 10, 17, 23, 54]. Some atomic planes are more important, as {110}, {112} and {123}, which are the major slip system for BCC steels [5, 8], and the {100}, which is the cleavage plane, the cube side lattice, for BCC steels [5, 8, 10, 17, 23, 54]. Also, it is important to bear in mind that the effect of these planes upon mechanical properties will depend on the volume, distribution and mainly on the position relative to the applied forces and planes presented during crack propagation at the crack-tip. For instance, for cleavage to occur, plane {100} must be presented perpendicular to the applied force, i.e., parallel to the fracture plane. To trigger the {112} slip system, the plane must be placed parallel to the applied force, i.e., perpendicular to the fracture plane.

The alpha-fiber is a parallel fiber to the RD and presents some of the planes mentioned above as {100}, {112} and {113}. It was reported that the sharpening the {112} <110> and {113} <110> components results in improvement of impact toughness [5, 8, 25, 54]. As shown in **Figure 5f**, AIR-steel has a peak intensity at {001} <110>, followed by a dip and then peak at ϕ = 20–30°, region of {112} and {113} components. ACC-steel presents a plateau between ϕ = 10–35°. Values of intensity are higher for AIR-steel than for ACC-steel. Nonetheless, ACC-steel presented higher CTOD toughness with better isotropy, while AIR-steel presented lower values and anisotropy according to the geometry sample. These results are consistent with another study [8] in which was reported a higher fraction of {112} <110> at L-T than T-L and yet much higher CTOD results were obtained with T-L than L-T, showing no specific correlation of increasing toughness by increasing {112}.

The obtained results show slight crystallographic differences between L-T and T-L configurations, and, for ACC-steel, these differences did not seem to affect toughness. However, it is suggested that the anisotropy displayed by AIR-steel between L-T and T-L geometries relies not only crystallographic orientation discrepancies but the microstructure anisotropy, as also reported [27]. 7-mm AIR-steel presented a higher fracture toughness for L-T configuration than T-L, in which the crack travels on a parallel plane to the rolling plane. Comparing L-T to T-L configurations of AIR-steel, nothing between is different regarding the chemical composition of phases and constituents, but only regarding the distribution and configuration of the microstructure.

For AIR-steel, there is anisotropy of grain morphology and segregation configuration, as depicted schematically in **Figure 14**. Segregation is present on both geometries, but its configuration is different in each case, as for L-T geometry, segregation is transverse to the rolling plane while for T-L, segregation is coincident to the rolling plane as shown in **Figure 14**. Banded regions and the presence of aligned microphases and non-metallic inclusions, such as MnS, create an easy path for crack propagation, influencing toughness according to their volume, morphology and distribution [5, 15]. Ferrite resists the crack propagation better than bands composed of secondary phases and constituents, which act as brittle sites for crack initiation or as a concentrated stress spot, an easy path for the crack propagation [22]. For AIR-steel 7-mm L-T geometry, the fine equiaxed grain is encountered, while for T-L geometry, elongated rolled grains are found. Grain morphology of

#### *Effect of Textures and Microstructures on the Occurrence of Delamination… DOI: http://dx.doi.org/10.5772/intechopen.88001*

7-mm ACC-steel was similar in the transverse and rolling planes, the tensile and toughness values were also similar in both directions.

For 15-mm thick samples, a general decrease is observed with a reduction of temperature. An approximate DBTT is −20°C, once, at this temperature, the values of toughness resulted from CTOD tests were close to a mean value of 0°C with −40°C condition tests. Overall, AIR-steel presented lower toughness than ACC-steel. As the temperature was lowered, the fracture started to present a more brittle-like behavior — this induced delamination occurrence, as well as a dropping tendency on toughness. Many 15-mm thick samples presented delaminations, and all delaminations were of divider type, as shown in **Figure 15**.

For all delaminations not considered significant, posterior plastic deformation and strain-hardening are achieved, increasing the CTOD force during test. These delaminations happen because the divider delamination branches the crack into two or more crack fronts, causing a relaxation of the triaxial tension towards a state of biaxial tension resulting in the decrease of the overall material constraint, promoting strain-hardening [22, 23]. In the cases of significant delamination, little or no

#### **Figure 14.**

*Schematic illustration of the used AIR-steel. CTOD specimens in L-T and T-L configuration shown to better visualize the relationship between the microstructure on the CTOD crack propagation path and global plate microstructure.*

**Figure 15.** *Delamination morphology: divide or arrester.*

#### **Figure 16.**

*CTOD fracture surface of API X80 AIR-steel samples tested at different temperatures. Condition tested at (a) 25°C, showing a more plastic extension of CTOD zone, and plastic deformation on delamination edges; (b) −40°C, still exhibiting a plastic character in CTOD zone, but for a shorter extension. Yellow pointed line representing CTOD zone delimitation.*

strain-hardening ability was promoted. **Figure 16** shows a fracture surface with non-significant and significant delamination, showing an aspect more plastic in the former case.

Many works studied the cause of delamination, assigning it to the presence of {100} cleavage plane [5, 8, 10, 23], but without further explanations. Some authors believe that delaminations are consequence mostly of the microstructure [26, 27]. In fact, microstructure exerts an effect on delamination occurrence, since AIR-steel presented more cases of delamination than ACC-steel, but not playing a major role. ACC-steel presented delamination on CTOD for 15-mm and on sample breaking for 7-mm, and yet presented a homogeneous microstructure with no detected inclusions or oxides, and not presenting mid-thickness segregation as AIR-steel.

Delamination phenomena are mainly governed by the crystallographic orientation presented next to the crack propagation path. It is necessary to fulfill embrittlement factors to satisfy the delamination criterion, i.e., many related parameters as temperature, crystallographic planes and orientations and presence of brittle phases. Regarding crystallographic orientation, the many previous works cited above correlated all delamination cases to the presence of {001}.

The X-ray diffraction results showed low presence of {001} parallel to fracture plane in L-T and T-L geometries, and intense {001} at 45° to fracture plane. **Figure 17** shows exactly the display of alpha-fiber according to L-T configuration. The delamination takes place traveling the {001} cubes face, macroscopically display at 45° to the fracture plane.

**Figure 18** illustrates the CTOD sample and the path of crack propagation, containing grains with random crystallographic orientations. The red-colored cube representing the {100} <011>, and the presence a possible clustering of {100} <011>. If two adjacent grains possess the same orientation, then they belong to the same grain. Therefore, it is assumed the existence of slight misorientation between schematic cubes. During the CTOD test, an external force produces internal stresses, causing transmission of forces in each atom lattice. This force transmission assuming the cubic lattice reaches a maximum shear component exactly when the vertex of the cube is pointing out to the applied force. In other words, a maximum shear in the lattice is achieved when the plane <011> is parallel to the applied force.

*Effect of Textures and Microstructures on the Occurrence of Delamination… DOI: http://dx.doi.org/10.5772/intechopen.88001*

#### **Figure 17.**

*Schematic illustration containing half of SE(B) CTOD samples and crystallographic representation on fracture plane with alpha-fiber for L-T configuration.*

#### **Figure 18.**

*Schematic illustration of CTOD sample and the crystallographic orientations contained by the fracture plane. To a given a textured sample with high alpha-fiber, the first component is (001)[110] of the system {100} <011>, will present high intensity, existing the possibility of very similar oriented grains to be close to each other, forming a cluster.*

That is exactly the role of the {100} <011> in delaminations occurrence. Applying a 45° rotation, the face of the cube coincides the imaginary plane of fracture. In this configuration, when an external force is applied, there is no shear component but only uniaxial tension directly on the cleavage plane {100}. Thus, plastic deformation is suppressed, lowering any mechanical property measured.

The literature agrees with the proposed model of **Figure 19**. Mouriño et al. reported the lowest impact toughness at 45° from the RD. This result can be attributed to the highest volume fraction of {001} cleavage planes parallel to the 45° to the RD macroscopic fracture plane [54].

Yang et al. presented three different rolled and heat-treated conditions for the same chemical composition and no macrosegregation. The final difference was grain morphology (elongated for the cold-worked) and texture. Results reported showed the lowest impact toughness and presence of delamination for the elongated grain morphology and the highest fraction of {100} on the fracture plane [25]. Joo et al. reported similar results, the lowest toughness at 45° with a high fraction of {100} planes [10]. In this study, samples were machined from the same rolled and treated plate, excluding any microstructure influence. It

**Figure 19.**

*Schematic illustration of CTOD sample in L-T, T-L configurations, and tilted 45° from RD. Explaining how delamination occurs, and why the 45° tilted sample presents the worst toughness according to the literature.*

was concluded that the dominant aspect of anisotropy was the crystallographic texture. Furthermore, the anisotropy was enhanced when a near DBTT temperature was used [10]. These cited results are very similar to the obtained result in the present paper.

The microstructure may have affected the delamination occurrence, once AIR-steel presented more delaminations than ACC-steel and showed a macrosegregation in the mid-thickness of the plate and elongated grains. It is also important to highlight that the observed delaminations occurred not only next to the midthickness of the samples, showing that microstructure banding is not the major or unique factor to trigger delamination. As expected, testing samples in RD or TD showed little difference for a homogeneous microstructure. Changing from RD to TD results only in slight crystallographic difference for a cubic system. AIR-steel presented a decrease of toughness mainly due to the change of polygonal to elongated microstructure.

Toughness value is mainly controlled by crystallographic feature, and delaminations have a straight relation to the presence of {100} specifically on the plane of fracture [5, 8–10, 17, 23, 25, 54]. The alpha-fiber plays an important role because it aligns the [110] direction to RD, as shown in **Figure 17**, and, usually, the rolling process produces strong alpha-fiber, therefore, strong (100)[011]. This condition leads to a strong (100)[010] at 45° from RD, resulting in the lowest toughness as reported [10, 25, 54], and it is in agreement with the present work and proposed model in **Figure 19**. However, the 45° to RD configuration is not the cause of low toughness, but the presence of {100} on the fracture plane. Bakshi et al. obtained high toughness at 45° and low toughness at RD and ND (0 and 90° to RD) due to low {100} at 45° to RD [5].

*Effect of Textures and Microstructures on the Occurrence of Delamination… DOI: http://dx.doi.org/10.5772/intechopen.88001*
