Organic elements and protein in some macrofungi of south east Anatolia in Turkey

Effect of Rolling Conditions on Ferrite Refinement of Low-carbon Steel Ahmed Elkawas

Sabbah Ataya and Samir Ibrahim

Quality department AL Ezz Dekheila Steel Company (EZDK) Alexandria, Egypt

Department of Metallurgy and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez Canal University, 43721, Suez, Egypt

Abstract— Refinement of the ferrite grains provides a promising approach to simultaneously improving both the strength and the toughness of steels. Among recent techniques to obtain ultrafine ferrite, dynamic strain induced transformation (DSIT) is used. This type of treatment is sensitive to steel composition, deformation temperature, the prior austenite grain size, strain and cooling rate.This work is aiming to improve the mechanical properties of steels using ferrite grain refining through trials on industrial scale using a compact strip production plant (CSP) with hot strip mill (HSM) containing six stands F1 – F6 in EZDK Steel Company. The chemical composition of the trials was C (~0.046), Si (0.073) Mn (0.50) and Ti (0.008).The effect of final deformation temperature of 750, 775, 800 and 815°C on the microstructure and mechanical properties was studied. In addition the effect of strain rate variation at the final two rolling stands was correlated with the ferrite refining mechanisms. The ferrite grain size attained after refining varies from 10 µm to be 6 µm. Yield and tensile strength increased slightly through changing deformation temperature, with the advantage of keeping almost the original high ductility .Toughness of the processed steel increased with ferrite grain refining. Keywords—Ferritic rolling, grain refining, dynamic strain induced transformation.

I.

INTRODUCTION

Several strengthening methods are conventionally applied during materials processing industry in order to improve mainly the mechanical properties. These include solid solution strengthening, strengthening by introducing a second phase, precipitation hardening and grain refinement. Grain refinement is considered one of the most effective, since it improves both the strength and the fracture resistance. Moreover, it hardly has a deteriorating effect on ductility and weldability, despite the achievement of considerable strengthening [1]. Ultrafine grained structure in plain carbon steels is gaining high research interest and considered as means of lowering the cost of steel production and opening up the window of high band mechanical properties of steels [2]. There are several methods to obtain fine grains such as severe plastic deformation, recrystallization, dynamic strain induced dynamic transformation and intercritical rolling [3]. Ferrite grain size refinement has been achieved commercially through dynamic strain-induced transformation (DSIT), in which the austenite-

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to-ferrite transformation is enhanced by the dynamic deformation within an appropriate temperature range. However, DSIT is a complex procedure involving interactions of diffusion of solute atoms, evolution of dislocation, propagation of grain boundaries and phase transformation3. Furthermore, it is important to relate this critical condition to the thermo-mechanical parameters that can be controlled during a process [4]. II.

EXPRIMENTAL WORK

A. Production route The trials for production of fine grained ferritic steel was carried out using aluminium killed low carbon steel with the chemical composition shown in Table I. TABLE I.

C% 0.046 Al % 0.034

Si% 0.073 Ca% 0.002

TRIAL CHEMICAL COMPOSITION

Mn% 0.503 Cr% 0.014

P% 0.006 Cu% 0.017

S% 0.002 Ni% 0.012

Ti% 0.008 N% 0.005

Nb% 0.001 V% 0.0001

The rolling process was performed using a compact strip production plant (CSP) with hot strip mill (HSM) containing six mill stands F1 to F6 followed by early laminar cooling as shown in Figure 1. The produced strip thickness and width were 3 mm and 1220 mm, respectively. Pyrometers were used to measure the temperature after every stand. The temperature after the last mill stand (finishing temperature) was a variable, where it was changed in for different trials to be 750, 775, 800 and 815°C . The coiling temperature through trials was kept around 400°C.

Caster & shear Tunnel furnace

descaler

6-Rolling stands

Shear

Figure 1. General layout of CSP

Laminar cooling

Coiler

RESULTS AND DISCUSSION

A. Measurements and description of the processing conditions To explain the effect of the different processing parameters such as the temperature at different processing steps and strain rates at the various rolling stands, these parameters are recorded. Figure 2 includes the measured temperatures at the different stands using pyrometers fixed after every stand. From the spacing between the stands and the rolling speed the time elapsed between the stands was calculated. In hot working processes, the flow stress for hot rolling is a function of both temperature and strain rate. Figure 3 includes the strain rate which is calculated according the following formula [5]:

ε. =

2υr sin θ υ 2υr sin θ = = h h hf + D (1 − cos θ )

(1)

.

Where ε : The strain rate for hot-rolling with sticking friction, θ: bite angle,

υr = 2π Rn and n is in revolutions per second,

D: roll diameter, hf : final thickness. Also the reduction in sheet height at every stand (Figure 3) was calculated from sheet thickness after (hf) and before (h) the stand. Although the reduction in height is high in the roughing stand F1 (~50%) and in the next stand F2 (45%), the slow deformation has led to decreasing of the strain rate at these beginning stands. The strain rate increases rapidly after the stands F1 and F2 (2.5-6.5 s-1) to reach higher values in the stands F5 and in the finishing stand F6 (34.5-54 s-1). Higher strain rates were achieved in the trials with finishing temperatures of 800 and 815°C than those with finishing temperatures of 750 and 775°C. B. Microstructure evaluation Different finishing hot deformation temperatures are used in order to examine the effect of accumulated strain of the multipasses on the produced microstructure and the total

a) Descaler

o

o

Temperature [ C ]

Tunnel furnace

1000 Ae3 = 850 C

800

6 Rolling stands F1 - F6 Cooling showers

600 400 200

Colier

0

300

600

900

Time [ sec ] 1000

o

III.

1200

Temperature [ C ]

B. Mechanical tests and microstructure investigation Tensile test samples according to the standard EN 10002-1 were used for quasi-static tensile tests (b = 20 mm, Lo= 43.4 mm), which were carried out using the tensile testing machine Model Zwick/Roll (250 KN). Impact test was conducted on small size impact samples (10 mm height, 2.5 mm thick and 2mm V-notch depth) according to the standard EN 100045-1. Charpy impact testing machine Type RKP 300/450 was used. Tensile and impact specimens were cut in the rolling direction discarding the un-cooled and off-gauge parts. Samples for microstructure and grain size measurement were taken from sheets cross section perpendicular to rolling direction. Imaging was done using a digital camera AXIOCAM MRC5 attaching an Olympus optical microscope. Grain size was measured according to ASTM E112.

F1 Roughing stand F2

b) F3

900

F4 F5

o

Ae3 = 850 C

800

700 780

F6

o

815 C o 800 C o 775 C o 750 C

790

Finishing stand

800

810

820

Time [ sec ] Figure 2. (a) Temperature profile against rolling time and (b) closer description of rolling stands F1-F6 which shown in (a) strain is kept almost constant. The measured temperatures after each of the six stands are shown in Figure 2, in addition to the amount of strain and the strain rate at each of the deformation stands. An estimated temperature of Ar3 ≈ 850 °C is used to determine with certain limits the temperature and the amount of strain of the prior austenite deformation, i.e., before the start of transformation. The maximum amount of strain on the austenite phase is expected to vary with the changes in the finishing deformation temperature. The maximum amount of accumulated strain and strain rates (Figure 3) in austenite is obtained at finishing temperatures of 800°C and 815°C. Under such conditions, dynamic recrystallization is quite expected to take place and this has been confirmed by simulation studies for industrial processing to produce a thin plate by rolling [6]. This results in an almost fully dynamic recrystallized microstructure which has an important effect on the next step of transformation. At the lower finishing deformation temperatures of 775°C and 750°C, the deformed austenite may only undergo a partial recrystallization with inhomogeneous prior austenite grain size been the outcome of this stage of deformation. Intergranular defects introduced by the continuous hot-deformation increase by increasing the austenite grain size and the strain will be

(a)

-1

Reduction in height [%] , Strain rate [ sec ]

60 For finishing temp. o 800 & 815 C

F1

F6

50 F2 Average reduction in hieght

40

F5 F3 o

750 & 775 C F4

30

F5 F6

20

(b) Strain rate

10

F2 F3

F1

0 780

790

800

810

820

Time on rolling line [ sec] Figure 3. Average strain rate and reduction in height at the rolling stands for trials with different finishing temperatures

more effective on reducing Ar3. Also, the number of intergranular nucleation in a steel having larger prior austenite grain size is shown to increase and will lead to the formation of a fine ferrite [7]. This is contrary to the accepted view that fine austenite grain size leads to fine ferrite grains. A homogeneous distribution of DSIT ferrite is obtained at the higher finishing deformation temperatures, as shown in Figure 4-a,b compared to a relatively larger ferrite with nonhomogenous size distribution observed after lowering deformation temperatures of 750 and 775°C. Homogeneous ferrite grain size distribution was obtained at 800 and 815°C, an example is shown in Figure 4-c. This difference could be attributed to both the size of prior austenite grain size and the degree of recrystallization and accelerated cooling effects. The effect of roll chilling in conjunction with a large shear strain is resulting from roll friction and also additional factors which facilitate a higher density of intergranular nucleated ferrite grain during hot rolling of austenite Figure 5. It has been generally argued that for low-carbon content steel dynamic recrystallization could have a small retarding effect on DSIT process, mainly at lower austenite deformation temperature. The obtained heterogeneous recrystallized austenite produced under this condition is the results of a varying strain distribution on the deformed structure. Therefore, the rate of ferrite nucleation and subsequent inhomogeneousness of the final ferrite grain size observed could be explained in such base. This negative effect of

(c)

Figure 4. Microstructures after rolling at different finishing temperatures, (a) 750ºC, (b) 775ºC, and (c) 800ºC.

recrystallization on DP-steel has been experimentally confirmed [8]. The larger ferrite grains above 6µm obtained at lower deformation temperature can be attributed to a possible growth of the transformed ferrite into deformed ferrite, i.e., unrecrystallized. The deformed ferrite was transformed from austenite before intercritical rolling, and the transformed ferrite resulted from the austenite deformed during intercritical rolling.

to sheet center Sheet surface Figure 5. Roll chilling of steel sheet produced by rolling at finishing temperature of 775ºC.

C. The effects of the on-line processing It is known that deformation induced ferrite transformation is expected to take place above Ar3, the equilibrium temperature for austenite to ferrite transformation where the austenite is relatively more stable than ferrite in the undeformed state. As indicated above the started austenite for transformation could be either fully or partial recrystallized and the continuous hot deformation between stand 3 and 4 will be carried out at a varying temperatures in the range of 880°C to 850°C, according to the pre-determined finish temperature. The accumulated strain at this stage has exceeded a value of 0.5 and partially stored in the structure. In order to lower the rise in free energy due to deformation the process of DSIT will start once a critical stain is attained. The kinetics of the process will be enhanced by the highly localized stain at grain boundaries. Also, the deformation force can contribute to a reduction in nucleation activation energy, since the deformation causes the TTT diagram to move to the left [9]. In other words, the required strain to start austenite transformation at Ar3 deceases in manner of deformation temperature dependence. Increasing the pre-transformation strain will also enhance the process of ferrite grain refining in addition to increasing the volume fraction of transformed ferrite. For strain ≈ 0.36 nucleation is limited to occur at grain boundaries while at higher strain of ≈ 0.7 additional nucleation can take place at grain interior10. This can be correlated with an increase of effective boundaries area through cell formation, deformation bands and dislocation structure. A total deformed strain in the intercritical zone of 1.1 has been demonstrated to give an average ferrite grain size of 3~5 µm [10]. The present continuous hot-deformation regimes allowed only an accumulated strain after recrystallization to reach a maximum value of ≈ 0.96 strain at the finishing deformation temperature of 815°C and resulted in an average ferrite grain size of ≈ 6 µm, as shown in Figure 5-b. This value decreases to be ~0.69 strain with decreasing the finishing temperature to 750°C and produced ferrite grain size with average diameter of ≈10 µm.

Therefore, it is possible to deduce that a higher strain in one single pass is more effective in grain refinement compared to multi-pass regime. The strain percentage has a determining effect on the grain size distribution throughout the microstructure. Moreover, the effective shearing should be taken into consideration which occurs due to the large difference between the working rolls diameters and the deformed sheet thickness. A highly localized strain is formed in the sheet surface which is gradually reduced in moving towards the sheet center in addition to the rolling friction force effect [6]. Based on a finite element model, a higher strain on the surface is estimated to be 2.8 time the nominal given strain [11]. The inhomogeneous strain distribution across the sheet thickness is responsible for a higher degree of grain refinement on the surface layer as shown in Figure 5. A very limited grain growth after the austenite transformation should be expected with finishing temperature of 775°C to 815°C. A slow-growth can be to be due to both the transformation mechanism and the processing parameters mainly the strain rates of deformation. The calculated strain rate values in the last three deformation stands are 26.4, 43.4 and 54 s-1 which is relatively high and would not allow a long time for austenitic transformation. In the mean time, the continuous carbon injection from ferrite grain to austenite during transformation will enrich the γ/α interface with carbon and, in addition to a limited time for diffusion to the interior of austenite grains, the interface movement will be then almost suppressed. This would lead to a limited observed coarsening and such mechanism has been adopted for single pass DP-steel deformation [12].

The microstructure, mainly at higher deformation temperature showed some of the produced ferrite with the mark of substructure deformation, obviously with low-angle boundaries, as shown in Figure 6-b. This could suggest a possible dynamic recrystallization for the ferrite structure. However, ferrite is known to have a higher stacking fault energy which may suppress such possibility. So, it is possible that the rearrangement of dislocation, i.e., dynamic recovery, can account for such observation. The presence of second phase precipitates is observed with all the produced ferrite microstructure, with one difference in the way they are located in the structure. At high finishing deformation temperature 800°C to 815°C, the precipitates are mainly confined to the trip grain boundaries. A rather random distribution of the second phase precipitate can be attributed to lower temperatures deformation conditions. The carbide formed at grain boundaries is essentially related to the transformation process rather than the dynamic recrystallization of austenitic grains [13]. This is not fully in agreement with the current observation and it is possible to attribute the precipitates distribution to the final deformation condition and its relation to the carbon diffusion kinetics.

420

(a)

σUTS

σY and σUTS [MPa]

(a) 390 360

σY

330 300 740

760

780

800

40

820 12

38

Grain size

10

Y2

36

8 34

εFr

6

32 30 740

760

780

800

Grain size [µm]

(b)

Strain at fracture , ε [%]

(b)

4 820

o

Finishing temperature, [ C ] Figure 7. Tensile test results and grain size of steel produced at different finishing temperatures Figure 6. (a) Partial grain growth at finishing temperature of 775°C and (b) Formation of substructure at finishing temperatures of 800°C.

D. Mechanical properties The tensile strengths and yield stresses of the produced DIFsteel are summarized in Figure 7. Microstructure observations confirm that both grain size homogeneities and refinement are improved when increasing the final deformation temperature. A fine grain size microstructure is expected to increase both elongation and tensile strength in accordance with the HallPetch relation. A small variation of 20-30 MPa in tensile strength values is obtained which could be correlated to the grain size reduction from ~ 10 to 5 µm with rolling temperatures. Therefore, the Hall-Petch relation fails to express the observed relation between the microstructure evolution and mechanical behaviors. A different deformation mechanism could then be attributed to different microstructures.

DIF steel can be considered as a kind of supersaturated ferrite where carbon tends to diffuse to grain boundaries or precipitates. Such complicated effects of carbon distribution in DIF and the second phase could either be due to pearlite precipitates or a fine spherodized cementite in the ferrite matrix. At lower deformation temperature the fine cementite particle is quite dominating as shown in Figure 6-a, where the inhomogeneous grain size distribution is also observed. It has been reported [14] that a smaller key value which is the grain boundary resistance, might be the result of a reduction in yield stress by replacing harder pearlite particles with softer ferrite and spherodized cementite. However, the presence of a very fine grain size, less than average, surrounding the coarse high angle ferrite grain could be responsible for the strengthening in this case. For the higher temperature deformed DIF, the effect of a smaller grains in addition to the presence of the second phase mainly at grain boundaries will produce the relatively high tensile strength and yield stress. Moreover, a decrease in strain hardening is observed, however not at the expense of the strength/ductility balance. This could be attributed to either the presence of second phase particles or even to the dispersed cementite [15].

The absorbed energy in the impact test which carried out on the sub-size specimens (10x2.5 mm2 ) was recalculated in accordance to be comparable with the usual standard impact specimens (10x10 mm2 ). The impact energy (Figure 8) increased with increasing the finishing temperature due mainly to the grain refining effect of the microstructure. The achieved ferrite refinement has makeable increased toughness of the produced steels. IV.

Conclusions

1. A homogeneous distribution of DSIT ferrite is obtained at the higher finishing temperature. Relatively larger ferrite grains with non-homogenous size distribution were observed at lower finishing deformation temperatures (750°C and 775°C). This difference could well be attributed to both the size of prior austenite grain size and the degree of recrystallization and accelerated cooling effects. 2. The microstructure at higher finishing temperature (800°C) showed some ferrite with the mark of deformation substructure, obviously with low-angle boundaries. This could suggest a possible dynamic recovery for the ferrite structure. 3. A little decrease in tensile strength (20-30 MPa) and a clear increase of the absorbed impact energy was obtained with the decrease in the grain size from ~ 10 to 6 µm. A different deformation mechanism instead of Hall- Petch relation could then be attributed to the second phase precipitates inside and at the grain boundaries. Further rolling trials are planned to get higher ferrite refinemnet and better achievemnt of improved tensile proprties.

Absorbed Impact energy [J]

225

220

215

210

205

200 740

760

780

800

820

o

Finishing temperature, [ C ] Figure 8. Absorbed impact energy of the produced steels related to the 10x10 mm2 standard impact sample specimens.

V.

ACKNOWLEDGEMENT

This work was performed with a support from EZDK Company. The authors are pleased to thank EZDK Hot Strip Mill staff especially Eng. Moataz Shafeek for his effort to execute the trials. VI.

REFERENCES

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