Carbon - Effect of Important Alloying Elements on Dual Phase Steels

2. LITERATURE REVIEW

2.4. Effect of Important Alloying Elements on Dual Phase Steels

2.4.1. Carbon

In order to control the amount of carbon in ferrite and martensite, it is usually wanted for carbon content to be not higher than 0,30% weight percent. Carbon is the main alloying element that determines martensite hardness. In order to avoid high contrast of hardness between ferrite and martensite phases, low carbon content is desired.

Increasing annealing temperature, carbon content in austenite (martensite, after quenching) reduces and the amount of martensite increases while carbon content in

ferrite remains relatively the same. Main effect of carbon in austenite is the control of hardenability [13], and at high temperatures, transformation of austenite to martensite would require higher cooling rates.

Effect of C content on the mechanical properties of dual phase steels arises from the amount of martensite. In many studies, it is shown that increasing C content increases yield and tensile strength of the steel.

Effect of C content of martensite to martensite hardness and yield strength in 1.45Mn containing steels that are annealed at 760ºC for 30 min then water quenched are shown in Figure 2.15 and Figure 2.16. Increasing the carbon content, hardness of the martensite and yield strength of the samples increase almost linearly. In Figure 2.17, C content of the steel versus fraction of austenite formed for two different starting microstructures is given [46]. It can be concluded that, C content both increases the martensite amount and hardness of the martensite phase.

Figure 2.15. Microhardness of martensite depending on the C content [55].

Figure 2.16. Yield strength of DP steels depending on C content [55]

Figure 2.17. C content vs austenite fraction formed in 1.50Mn steel with two different starting microstructures [46].

21 2.4.2. Manganese

The effect of manganese content is mainly to the grain refinement of austenite [13].

Calcagnotto et al has reported that addition of manganese has 4 main effects on the austenite. These are, decrease of A1 temperature and so required annealing temperatures, expanding ferrite + cementite + austenite phase region, in where austenite growth is very limited, refinement of cementite, which in turn causes a great pinning effect on the grain boundaries, and reduction of grain boundary movement due to solute drag effect [48]. These effects cause an increase in strength while keeping the elongation similar in the steel.

In Figure 2.18, change of the A1 and A3 temperatures with changing Mn content in 0.16C steel is shown. Increasing Mn content reduces both A1 and A3 temperatures.

Figure 2.18. Change of the intercritical temperatures with Mn content in 0.16C steel [56].

Mn content in austenite for production of DP steels are of great importance as Mn is an austenite stabilizing element. Due to diffusion of Mn from ferrite to austenite grain being slow, a Mn rich zone at the grain boundary between ferrite and austenite is expected [57] [58]. Their work on a steel with 0.1C-6Mn steel has shown that at the ferrite side of the grain boundary, a Mn depleted zone occurs and with increasing time, homogenization of the Mn in ferrite slowly proceeds (Figure 2.19).

Figure 2.19. Mn profile in ferrite in a 0.1C-6Mn steel at 650ºC [57].

Figure 2.20. Mn profile in austenite in a 0.1C-6Mn steel at 650ºC [57].

At the austenite side of the grain boundary however, Mn rich zone is formed and as time passes by, homogenization of Mn in austenite proceeds (Figure 2.20).

As dashed lines from Figure 2.19 and Figure 2.20 correspond to the same annealing times, it is seen that Mn diffusion in ferrite is faster than Mn diffusion in austenite.

For 1 minute holding between 750ºC and 850ºC, diffusion distance of Mn in ferrite is around 0.2μm and in austenite is lower than 0.01μm [59]. In his work with 0.06C-1.5Mn steels, Speich et. al. has found that the Mn diffusion in ferrite is almost third order of the Mn diffusion in austenite [60]. In Figure 2.21, a similar result to Moor’s work has been obtained at different temperature by Speich. Note that 1 hour annealing at intercritical temperature (740ºC) is not sufficient for complete Mn diffusion and homogenization for 0.1C-1.5Mn steel.

Figure 2.21. Mn concentration of ferrite and austenite phases in 0.06C-1.5Mn steel after 1h annealing at 740ºC [60].

As previously mentioned, Mn is a austenite stabilizing element and delays pearlite or ferrite formation to a degree. If the steel is cooled before the Mn homogenization, Mn rich zone at the austenite side of the grain boundary will show different transformation kinetics than middle of the austenite grain (Figure 2.22). If diffusion kinetics are not enough, it is very possible for martensite formation at the edge of the austenite grain, while inner sections of austenite transforming to pearlite, bainite or ferrite, schematically shown in Figure 2.23 [60].

Figure 2.22. Mn partitioning by diffusion during annealing [60].

Figure 2.23. Microstructure of 0.06C-1.5Mn steel after annealing at 740ºC followed with slow cooling [60].

Speich was able to plot Figure 2.24 using his experimental data, which is very important and useful for production of DP steels containing low carbon and high Mn.

It can be seen that at low intercritical temperatures dissolution of pearlite takes long time, which may cause, for short annealing times, presence of pearlite in the final microstructure. After this region ends, diffusion on Mn starts which could take up to 270 hours. His theoretical calculations showed no Mn diffusion zone above 840ºC.

Figure 2.24. Austenite formation diagram for 0.12C-1.5Mn steel [60].

2.4.3. Silicon

Silicon is usually the main alloying element in the dual phase steel alongside with manganese. As Si is a ferrite forming element, it increases A1 temperature, causing ferrite grains to be larger during heating, reduces the amount of Fe3C precipitates.

Reduction of Si content causes formation of brittle carbides along ferrite / austenite grain boundaries.

Nouri et al has reported that at higher Si contents, Mn partitioning between ferrite and austenite is higher [61]. Presence of Si in ferrite phase, promote higher flux of carbon migration to austenite [62]. It both increases the yield and tensile strength of the steel by solid solution strengthening and enhances mechanical properties by improving the elongation-tensile strength balance [63] [64] (Figure 2.25).

Figure 2.25. Effect of Si content on the tensile strength and elongation in dual phase steels [64].

2.4.4. Aluminum

Aluminum is also a ferrite-forming element similar to Si. Addition of Al, as well as Si, has an indirect effect on formation of new ferrite, which is crucial for controlling carbon content in austenite and stability of austenite [13]. Addition of Si increases A1 and A3 temperatures, making formation of ferrite faster [65] In their experiments, Girina et al has found that increasing Al content, formation of new ferrite is accelerated and carbon distribution between ferrite and austenite is improved, which leads to delay of bainitic transformation and increase in martensite amount [66]. Al also has a grain refinement effect on ferrite and may bind with Nitrogen to form AlN, preventing Nitrogen to bind with other precious alloying elements such as Titanium.

Effect of Al content on the tensile properties of dual phase steels are mainly due to increase of A3 temperature, hence reduced amount of austenite formed, and to refinement of grains. Girina and Fonstein reported that increase in Aluminum content could balance tensile strength and elongation relationship in dual phase steel by refinement of ferrite grains [66]

Figure 2.26. Tensile strength-elongation relationship with different amount of Al content in a dual phase steel [66].

2.4.5. Molybdenum

Molybdenum promotes martensitic transformation by improving hardenability of austenite. It delays pearlitic and ferritic transformation reaction, and reduces bainite start temperature. At low cooling rates, Mo is the best austenite stabilizer among other elements such as Mn and Cr, as indicated by Irie et al [67]. For galvanizing process, where galvanizing bath is between 450ºC-500ºC, addition of Mo extends the required time for bainitic reaction to start and reduces bainitic reaction start temperature, avoiding formation of bainite in the final microstructure for dual phase steels. Hara el

al has reported that, addition of Mo to low carbon steels causes Mo-C precipitate to form, leaving more precious alloying elements such as B or Nb to carbon-free and in the solid solution [68].

2.4.6. Chromium

Chromium increases hardenability of austenite and retards austenitic transformations, enabling, at relatively slow cooling rates, formation of martensite [69]. At intercritical annealing temperatures, due to high carbon content of austenite, effect of Cr is more significant. If added extensively, steel’s performance during deep drawing is negatively affected.

31 CHAPTER 3

3. EXPERIMENTAL PROCEDURE

3.1. Steel Used for the Experiments

In this study, a 20 ton, 800 meters long, 1142 mm wide and 2,8 mm thick steel strip coil is used. Coil was purchased by Borçelik, a joint venture of ArcelorMittal and Borusan Holding, as hot rolled condition. Chemical analysis of the steel is given in Table 3.3.1. The steel coil in the hot rolled state is shown in Figure 3.1.

Table 3.3.1. Chemical analysis of the steel used in this study.

Element C Mn Si Cr+V Ti+Nb Ni+Al Ceq

(%) 0.14 1.98 0.21 <0.40 <0.05 <0.1 0.56

3.2. Sample Taking

The coil (also called as the mother coil) is first pickled oiled in Borçelik Continuous Pickling Line (CPL) to remove the rust on the coil surface. The first and last 25 meters are cut and discarded in order to avoid inhomogeneities which can occur after hot rolling and coiling due to the differences in chemical composition and cooling rates.

After, the steel is cut into 3 equal parts (also called as baby coils) of 250 meters each.

At the end of each baby coil, 3 30 cm flat samples are taken for initial microstructural analysis. The places where PO (pickled oiled) samples are taken are shown in Figure 3.2. Some production lines are equipped with high technology monitoring systems to adjust the cooling rate just before the coiling so that no homogeneity occurs

throughout the coil. First cut part is called Part3, second cut part is called Part2, third cut part is called Part1 in this study.

Figure 3.1. Steel strip coil in hot rolled state.

Figure 3.2. Temperature heterogeneity in hot rolled steel during cooling, and places where PO samples are taken [70].

After taking of the samples, each baby coil (parts) is cold rolled into different thicknesses in Reversible Cold Rolling Mill in Borçelik (Figure 3.3). Reduction ratios are chosen so that effect of annealing in the intercritical region can be seen in low, middle and high cold rolling ratios. Part1, Part2 and Part3 coils are cold rolled to their final thicknesses in 3, 5 and 7 passes, respectively (Table 3.2).

After the rolling of each coil in Borçelik Reversible Cold Rolling Mill line, around 20 meters of strip is cut to the scrap from the coil head. Then 10 samples from each baby coil, each with a length of 30 cm, are taken (Figure 3.4).

Table 3.2. Reduction ratios of each baby coils (Parts).

Part1 Part2 Part3

Starting Thickness (mm) 2,80 2,80 2,80

Reduction Ratio (%CR) 60% 45% 30%

Final Thickness (mm) 1,12 1,54 1,96

Number of Passes 7 5 3

Figure 3.3. Borçelik cold rolling mill line.

Figure 3.4. Flat samples taken after cold rolling.

Figure 3.5. Mechanical tensile test specimen press machine.

The tensile test specimens were prepared parallel in the direction of hot rolling and cold rolling. Specimens were punched from the sheets directly using a die and hydraulic press, as shown in Figure 3.5.

3.3. Heat Treatment

Heat treatments were done in muffle furnace (Figure 3.6). The furnace has a maximum temperature capacity of 1200ºC. The samples were heated to the required temperatures at a rate of 1^oC/sec. Thermocouple is placed inside the furnace, very close to the samples each time. Thermocouple is connected to DATAPAQ data logger, which has a capability of logging data every second for up to 6 hours (Figure 3.7).

In annealing, the samples were hold in a temperature range of 680C-800C for either 5 min or 10 min. Following each heat treatment, samples were quenched in water.

Figure 3.6. Atmospheric heat treatment furnace.

Figure 3.7. DATAPAQ data logger.

The heat treatments were planned such that the effect of the amount of cold reduction, the soaking time and annealing temperature on the microstructure can be observed.

For comparison purposes, a specimen cut from the as-received (NR HT (Not rolled, heat treated)) is also heat treated. Samples were designated as

*temperature*-*%CR*-*annealing time*. For example 740-60-10 means that 60% cold rolled sample annealed at 740ºC for 10 minutes.

All tests were repeated 3 times in order to increase accuracy and minimize error.

3.4. Microstructural Analysis

After heat treatment, the samples were sectioned parallel to the rolling direction and mounted in Bakalite. After grinding and polishing, they were etched (in 2% Nital etchant) in order to analyze the microstructure.

In this study, optical microscopy (Huvitz) and Field Emission Scanning Electron Microscopy (FESEM) (FEI NOVA NANOSEM 430) was used to analyze the microstructure of the samples. Optical microscopy was used in order to find the volume fractions of ferrite and martensite, and also for general microstructural examination. FESEM is used especially to analyze the carbides, to determine the austenite nucleation regions and to see the effect of cold reduction on the Ae1 temperature by closely analyzing martensite formation.

3.5. Quantitative Analysis

Volume ratio and grain sizes of the phases were calculated by an image analyzer software, MIPAR. Different recipes for all samples in order to identify ferrite and to emphasize grain boundaries were created for analyses to be accurate (Figure 3.8).

Figure 3.8. Ferrite fraction detection using MIPAR.

3.6. Tensile Tests

Tensile tests of the heat treated specimens were performed in Borçelik. The tensile test machine is INSTRON, has a maximum capacity of 25 tons (Figure 3.9). Tensile tests are done at a speed of 32 mm/minute.

Figure 3.9. INSTRON tensile test machine.

39 CHAPTER 4

4. RESULTS

The aim of this work is to see the effect of rolling ratio, annealing temperature and annealing time on the microstructure and tensile properties of dual phase steels.

Pickled-oiled (not cold rolled), 30% cold rolled, 45% cold rolled, 60% cold rolled steel specimens were annealed at the pre-determined temperatures for 5 minutes and 10 minutes. After annealing, specimens were quenched in water.

4.1. Microstructures and Mechanical Properties of As-Pickled Samples

The microstructures of each batch are shown in Figure 4.1, Figure 4.2 and Figure 4.3 and SEM image is given in Figure 4.4. Starting microstructure is ferrite-pearlite. In order to measure the amount of the primary ferrite grains, the microstructure is analyzed quantitatively. Quantitative analysis results of PO samples are given in Figure 4.5. A slight reduction of ferrite fraction from Part3 to Part1 can be seen.

During coiling of the hot rolled strip, cooling rate is different throughout the length due to overlapping the strip onto the cold recoiler [70]. Also, edges of the strip may cool faster due to being exposed directly to air. Therefore, slight differences in ferrite fraction and grain sizes in the parts are expected.

Figure 4.1. Microstructure of Part3. Etched with %2 Nital. Dark regions are pearlite, white regions are ferrite.

Figure 4.2. Microstructure of Part2. Etched with %2 Nital. Dark regions are pearlite, white regions are ferrite.

Figure 4.3. Microstructure of Part1. Etched with %2 Nital. Dark regions are pearlite, white regions are ferrite.

Figure 4.4. SEM image of Part2. F represents ferrite, P represents pearlite.

Figure 4.5. Ferrite fraction and grain size of each part prior to cold rolling.

Mechanical properties of PO specimens taken from Part1, Part2 and Part3 regions of the mother coil is given in Figure 4.6. There are slight differences between the mechanical properties, Part1 has the highest tensile strength and Part3 has the lowest.

As mentioned above, due to different procedures on cooling rates during coiling, change of ferrite fraction and grain size seems to affect the mechanical properties of the PO samples slightly.

Figure 4.6. Mechanical properties of each part of the PO.

4.2. Microstructures and Mechanical Properties of Cold Rolled Samples

The effect of 30%, 45% and 60% cold rolling on the microstructures can be seen in Figure 4.7, Figure 4.8 and Figure 4.9 respectively. As rolling ratio increases, the aspect ratio of ferrite grains increase. Another effect of cold rolling is fragmentation of pearlite. For 60% CR specimen, broken cementite particles seems to be scattered inside ferrite grains, while this fragmentation is less pronounced in 30% and 60% CR specimens

Figure 4.7. 30% CR specimen before heat treatment.

Figure 4.8. 45% CR specimen before heat treatment.

Figure 4.9. 60% CR specimen before heat treatment.

Mechanical properties of cold rolled samples are given in Figure 4.10 and Figure 4.11.

As seen, an increase in the amount of cold work causes an increase in yield and tensile strength values but a decrease in % elongation as expected. The 60% cold rolled specimen did not form neck due to excessive amount of cold rolling and failed in a brittle manner. For this reason, yield strength and elongation of 60% cold rolled specimen could not be measured as seen in Figure 4.10. The abrupt increase in hardness of 60% cold rolled sample is seen in Figure 4.11.

Figure 4.10. Yield strength, tensile strength and elongation of samples after cold rolling.

Figure 4.11. Hardness Vickers versus cold reduction ratio.

4.3. The Microstructure and Mechanical Properties of Heat Treated Samples

In order to see the effect of annealing temperature on the mechanical properties and microstructure of the cold rolled specimens, several samples were annealed below and above the intercritical temperature. The heat treatment details are given in Table 4.1.

Table 4.1. Summary of the heat treatments.

NR HT 30% CR 45% CR 60% CR

5 Min 5 Min 10 Min 5 Min 10 Min 5 Min 10 Min

680ºC x x x x x x

700ºC x x x

710ºC x x x

720ºC x x x x x x x

730ºC x x x

740ºC x x x x x x x

760ºC x x x x x x x

780ºC x x x

800ºC x x x x x x x

Critical transformation temperatures, Ac1 and Ac3, of the samples are calculated using Eq.2 and Eq.3 are as follows:

Ac1: 711,2ºC Ac3: 802,2ºC

Where Ac1 is the temperature where austenite is first starts to nucleate upon heating and Ac3 is the lowest temperature for the composition where the microstructure consists of only austenite. All the specimens are quenched in water after annealing.

4.3.1. Microstructures of Heat Treated Samples

In Figure 4.12 and Figure 4.13, SEM micrographs of 700-30-5 are given. Even at well below Ae1 temperature, nucleation of austenite phase (which most probably transformed to martensite upon quenching) can be seen at grain boundaries, especially at locations adjacent to prior pearlite or close to carbides.

Thermodynamically, formation of austenite at this temperature is not expected under equilibrium conditions. However, due to composition gradients and short annealing times, austenite formation is favored at carbon rich grain boundaries.

A higher magnification of a prior pearlite region, martensite formation and cementite globularization is given in Figure 4.13. Due to cold deformation, fragmented

globular cementite particles are present in the microstructures, which formed as a result of breakdown of several pearlite colonies into smaller, spherical cementite particles. The presence of martensite islands indicate that upon heating at 700ºC and waiting for only 5 minutes is enough for formation of austenite islands which transform to martensite upon quenching to room temperature.. With an increase in the amount of cold work the amount of fragmented cementite particles increase such that in 60% CR sample only fragmented cementite particles and martensite is seen besides ferrite grains (Figure 4.14, Figure 4.15). There is no pearlite morphology is observed.

Figure 4.12. Microstructure of 700-30-5. F represents ferrite, PP represents prior pearlite.

Figure 4.13. High magnification image of 700-30-5. M represents martensite, PP represents prior pearlite, GC represents globular cementite.

Figure 4.14. SEM micrograph of 700-45-5. M represents martensite, FC represents fragmented cementite.

Figure 4.15. SEM micrograph of 700-60-5. M represents martensite, FC represents fragmented cementite.

In Figure 4.16, four different regions where austenite nucleation starts are designated as “1” for ferrite-ferrite grain boundary, “2” for ferrite/pearlite phase boundary, “3”

for triple grain boundary and “4” for inside the ferrite grain, in 710-45-5 sample. As discussed before, austenite requires carbon for nucleation, therefore point “2” and

“3” are ideal locations for austenite nucleation as diffusion of carbon into austenite is

In document EFFECT OF COLD ROLLING AND ANNEALING PARAMETERS ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF DUAL PHASE STEELS (Page 36-0)