Effect of cold rolling on texture evolution during primary recrystallization of an IF steel

Effect of cold rolling on texture evolution during primary recrystallization of an IF steel

M. Ramoul¹, A. Ayad^1,2, N. Rouag¹

1Laboratoire de microstructures et défauts dans les matériaux, Université Frères Mentouri Constantine 1, Alegria.

2Département de pharmacie, Faculté de Médecine, Université Constantine 3, Nouvelle ville Ali Mendjeli, BP. 67A Constantine, Algeria.

ramoul_meriem@outlook.fr, abdelhak_ayad@yahoo.com, nadjetrouag@yahoo.fr

Abstract— In the present study, we present the effect of cold rolling on texture evolution during primary recrystallization of an IF steel that is destined to the automotive industry. From the hot rolled sheet, three deformation rates were performed by cold rolling: 25, 50 and 75 % followed by several heat treatments in order to obtain the fully recrystallized states. Microhardness Vichers was used to estimate the recrystallized fraction and XRD (X-ray diffraction) for the texture characterization. The results show that texture of the hot rolled sample is near random and its evolution is very low for the 25% reduced sample. However, texture become more important for 50% and 75% reduces states, with the strengthening of α-fiber and the appearance of γ-fiber. The evolution of texture during primary recrystallization is quite different. Recrystallization of the 25% and 50% cold rolled states shows a maximum of the ODF on {223}<110> component and texture tends generally to be random. The recrystallization texture of the 75% reduced sample is characterized by a γ-fiber more intense than α- fiber and a maximum of the ODF around the component {111}<110>. It was observed also that γ-fiber starts to develop during the primary recrystallization of the 50% rolled state, and becomes more pronounced during recrystallization of the 75% cold rolled sample.

Keywords—IF steel; cold rolling; primary recrystallization; texture; XRD; α-fiber; γ-fiber.

I. INTRODUCTION

Interstitial free (IF) steel is an important class of deep drawing quality (DDQ) steels [1-3]. In IF steel, carbon is mostly arrested by suitable interstitial elements (e.g. Ti, Nb).

IF steels have been designed to obtain an excellent compromise between formability and strength with a specific metallurgy without interstitial elements, this behavior is related to a good elongation to failure and excellent coefficients of hardening and anisotropy. As in previous studies [1], textures of IF steels are discussed mainly considering the α (RD//<110>) and γ (ND//<111 >) fiber components. At the same time, plastic deformation also leads to a subdivision within the original grains, which may occur on both macroscopic and microscopic length scales [4].

Considerable interest exists in understanding how these two processes are linked, as together they determine the spatial distribution of the texture components in the deformed microstructure, which in turn is important for determining the recrystallization behavior.

Significant industrial interest exists in the control and prediction of recrystallization in IF steels, as this process is used to develop a strong, ideally uniform, γ-fiber texture, to minimize material losses during deep drawing [5]. This improved formability is primarily due to the strong γ-fiber recrystallization texture [6-8 ]. Global texture measurements (e.g. XRD) were often used to characterize and to understand the phenomenon [9].

Recent developments in local orientation measurements, especially for OIM (orientation imaging microscopy) [10], may provide link(s) between the more general global texture

developments and the actual changes in the microstructure.

Typically, the development of γ recrystallization texture is attributed to preferred nucleation and/or selective growth of γ grains, presumably from γ-oriented deformed regions [1,11].

Considerable scientific efforts have been directed to the discussion and clarification of these two mechanisms. In general, although the existence of preferred nucleation is widely accepted [12], questions still exist on the relative contribution of selective growth [11,12]. Rather than doing in- depth research to resolve (i.e. if possible) the ''dispute'' between preferred nucleation and/or selective growth, it is possibly far more important, at least technologically, to identify the factors responsible for the developments in γ recrystallization texture and to relate them to some physical characteristics of the deformed microstructure/texture. A recent study [12,13] on Ti-bearing IF steel has shown that recrystallized grains of α- (RD// <110>) and γ-fiber nucleate from the deformed bands of these respective orientations.

This paper is a contribution to understand the microstructural/textural changes during deformation and recrystallization. It aims to study the effect of cold rolling on γ fiber development during primary recrystallization of an IF steel that is destined to the automotive industry.

II. EXPERIMENTAL

The chemical composition of the IF steel under this study is given in table 1.

TABLE 1. Chemical composition of the studied IF steel.

Element C Mn P S N Si Cu Ni Cr Al Ti

10^-3% mass 8 196 4 10 3.1 4 7 18 14 41 97

(a) recrystallized state (b) 25% rolled state

(c) 50% rolled state (d) 75% rolled state Fig. 1. Microstructures evolution during cold rolling.

The hot-rolled material, with (2.72±0.01) mm of thickness, was cold-rolled to 25, 50 and 75% reductions. After cold rolling, IF steel samples were recrystallized at 670°C (for 25%

rolled samples), 640°C (for 50% rolled samples) and 610°C (for 75% rolled samples) for different times, in order to obtain different recrystallized fractions

The ODFs (Orientation Distribution Functions) were calculated from poles figure measurements that are obtained from X-Rays Diffraction (XRD) using the MTEX software [14]. Three experimental poles figures (110), (200) and (112) were performed using a texture goniometer of an empyrean diffractometer of PANALYTICAL.

III. RESULTS AND DISCUSSION A. Morphological analysis

Fig. 1 shows the optical micrographs of the deformed and recrystallized microstructures.

The recrystallized sample shows a homogeneous equi-axed microstructure, it present clusters of small and large grains (Fig. 1a); which may leads to a difference in response to the same external solicitation that may causes local thinning or micro-cracks.

Indeed, it is the control of the manufacturing conditions, which gives the material the appropriate structural features for a subsequent forming. During cold rolling, the grains become increasingly elongated in the direction of rolling and microstructural changes become more noticeable from the50%

cold rolled state.

B. Deformation texture

Development of deformation texture in cold-rolled IF-steel was investigated using XRD. During cold-rolling, both the α (RD//<110>) and the γ (ND//<111>) fibers were observed.

Fig. 2 shows the evolution of texture during deformation.

It presents the φ2= 45° sections of the ODF of the hot rolled sample and the 25%, 50% and 75% cold rolled states.

According to these figures, the hot rolled sample is characterized by a very low texture, because the distribution of orientations is almost random, it showed a slight maximum, around the Goss component, i.e. {110}<001> component which characterized by (φ1=90°, Φ=90°, φ2=45°) Euler angles. The maximum of the texture function is f(g)=2.36 (Fig. 2a). This texture is a characteristic of the hot-rolled steel sheets in particular the presence Goss component that resulted from the dynamic recrystallization during the hot rolling of steel sheets.

The hot rolled state results from a low texture transformation; this initial texture is very important for the development of the texture after cold rolling and can affect the final texture after primary recrystallization.

For the 25% cold rolled samples (Fig. 2b), texture changes little and leads to the appearance of the {223}<011>

component (φ1=0°, Φ=40°, φ2=45°), of the α-fiber, at the expense of {110}<011> and {110}<001> (Goss) components.

The 50% cold rolled sample shows a significant change in texture with the strengthening of the α-fiber, especially the {233}<011> component and the appearance of the γ-fiber (Fig. 2c).

Texture of the 75% cold rolled sample is characterized also by the presence and the strengthening of both α and γ fibers (Fig. 2d) with α-fiber more intense than γ-fiber. The maximum of the ODF is always on the α-fiber around the same component, i.e. {223}<110> component and f (g) = 10.37.

The texture behavior during cold rolling can be justified by:

 The random grains of the hot-rolled sheet tend to move towards more stable orientations, therefore, the orientation {223}<110> was identified as the final stable orientation of the polycristallin material rolled texture (CC) [15]

 During cold rolling, grains of α-fiber are more stable against rotation relatively to those of γ-fiber. This stability depends on the accumulation of stored energy (ie, intragranular disorientation) [15].

C. Recrystallization texture

Recrystallization is a thermally activated process ; the recrystallization temperature decreases with the amount of deformation increases, and the stored energy which provides the motive force in recrystallization is proportional to the rate of hardening.

(a) hot-rolled state (b) 25% rolled state (c) 50% rolled state (d) 75% rolled state Fig. 2. Texture (φ2=45° ODF sections) evolution during cold rolling.

(a) 25% rolled state (b) 50% rolled state

(c) 75% rolled state

Fig. 3. Recrystallized fraction evolution with annealing time.

(a) 25% rolled state (670°C,60min) (b) 50% rolled state (640°C,60min)

(c) 75% rolled state (610°C,45min)

Fig. 4. φ2=45° ODF sections of the fully recrystallized states.

In this study, microhardness Vickers is used as the main indicator of the metallurgical state of the IF steel, particularly the recrystallized state. Noting that at the end of primary recrystallization, the microhardness overstates the recrystallized fraction [16]. A law is used to deduce the recrystallized fraction (Xv), that is given by the following equation [16]:

max mes

v max min

HV HV (%) 100x

X HV HV

 

 ⁽¹⁾

HVmes: average value of the measured hardness.

HVmin: recrystallized sample hardness.

HVmax: deformed sample hardness.

After characterizing the deformed states, recrystallized fraction evolutions with annealing time for the three reductions were estimated from microhardness Vickers measurements using (Eq. 1) and plotted on Fig. 3. Therefore, annealing conditions at the end of primary recrystallization (Xv~100%) of the 25, 50 and 75% cold rolled samples are identified as (670°C for 60min), (640°C for 60min) and (610°C for 45min) respectively.

Fig. 4 groups the φ2=45 ° ODF sections of the fully recrystallized states of the 25%, 50% and 75% reduction rates.

The sample 25% rolled has a reinforcement around the component {223<011> of the α fiber with a value of the

texture function equal to 1.91, for samples completely recrystallized (Fig. 4a).

The sample 50% rolled also has a reinforcement on the same component {223}<011> with a value of the crystal orientation distribution function (FDOC) equal to 3.95 (Fig.4b).

For both 25% and 50% rolling rates, the recrystallization texture tends to randomise. This tendency toward isotropy can be justified by a strong germination can occur at high temperatures recrystallization [17]. In our case, the temperatures are maintained relatively high (670°C and 640°C) for 25% and 50% rolled samples, respectively. The heating speed is relatively high and can lead to a considerable rate of germination. This increases the number of nuclei which can causes this trend to isotropy.

After recrystallization, texture of the 75% rolled sample is characterized by a γ-fiber more intense than the α-fiber, with a

maximum of FDOC around the component {111}<110>, where f (g) = 12.89 for fully recrystallized sample (Fig. 4c).

According to these ODFs, the γ-fiber recrystallization texture starts to develop after recrystallization of the 50% cold rolled sample (Fig. 4b) and becomes more pronounced after recrystallization of 75% rolling reduction rate. However, the recrystallization of the 25% cold rolled samples did not allow the development of this fiber. It is important to note that the increase in γ-fibre intensity during recrystallization is often attributed to preferred/selective nucleation [18,19] and/or growth [20,21] of the γ grains. It should be noted that the presence of the γ-fiber in the final product enhances considerably its drawability[5].

IV. CONCLUSION

In this paper, the effect of deformation on recrystallization texture development was studied. The main results are:

 The hot-rolled sheet has a heterogeneous distribution of grains size; it has a bimodal distribution of grain sizes.

The overall texture, obtained by XRD, is almost random with a slight maximum on Goss component.

 During the cold rolling, the microhardness increases by the hardening of the steel and the loss of its ductility.

The grains become increasingly elongated in the rolling direction. Concerning texture, cold rolling led to the strengthening of α and γ fibers which appears from 50% reduction rate, with α-fiber stronger than γ-fiber.

The maximum of the ODF is found on the α-fiber for the {223}<110> component for the 75% cold rolled sample.

 Evolution of recrystallization textures for 25% and 50% rolling rates, weakened during annealing and tends to randomise. However, for the 75% reduction rate, texture is characterized by intensification of γ- fiber at the expense of α-fiber, the ODF maximum is observed to be on γ-fiber around the {111}<110>

component, where f(g)=12.89. The presence of the γ- fiber in the industrial sheets gives a good aptitude for forming by deep drawing.

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