AISTECH 2011, 2-5 May 2011, Indianapolis, Indiana ...

AISTECH 2011, 2‐5 May 2011, Indianapolis, Indiana, USA

Development of Grain Refiner Alloys for Steels Casper van der Eijk SINTEF Alfred Getz vei 2 7465 Trondheim, Norway Phone: +47 98283989 E-mail: [email protected] Fredrik Haakonsen SINTEF Alfred Getz vei 2 7465 Trondheim, Norway Phone: +47 95239469 E-mail: [email protected] Ole Svein Klevan Elkem Alfred Getz vei 2 7465 Trondheim, Norway Phone: +47 73590713 E-mail: [email protected] Øystein Grong NTNU Alfred Getz vei 2 7491 Trondheim, Norway Phone: +47 73594896 E-mail: [email protected] Key words: Steel, Grain Refinement, Cerium, Inclusions

INTRODUCTION The demand for higher performance materials with optimum combinations of properties is steadily increasing. For steels, the microstructure controls the resulting mechanical properties and hence, the desired property profile requires the development of a properly adjusted microstructure. The traditional way of producing a fine-grained microstructure yielding the optimum combination of strength and toughness is through thermomechanical processing. However, some non-metallic inclusions can have a profound influence on the microstructure of steel 1,2). THEORY In 1990, the term “Oxide Metallurgy” was first used to describe the concept that oxides with a size from submicron to several microns are used as nucleation points during the austenite to ferrite phase transformation 3). Later it appeared that not only oxides but also sulfide, nitride and carbide inclusions can have an influence on the microstructure so the more correct term “Particle Metallurgy” has been coined to describe this phenomenon.


Non-metallic inclusions in steel can influence the microstructure evolution in three different ways: 1) By acting as nucleating agents for the solidifying steel resulting in a refinement of the solidification structure. The presence of these nucleating agents to the steel contributes to better castability, giving reduced porosity, reduced hot cracking and reduced segregation of alloying elements. It is generally accepted that effective inclusions have a low planar lattice disregistry with respect to the nucleus in order to serve as potent nucleation sites during solidification. It is also important that the inclusion is thermodynamically stable in the liquid steel 4,5). 2) By contributing to pinning of the grain boundaries, thereby inhibiting austenitic grain growth. To function optimally, the pinning inclusions should have a size below 100 nm and be finely distributed. TiN and MgO inclusions are often utilized for this purpose in TMCP-rolled steel plates 6). 3) By promoting nucleation of intragranular acicular ferrite during the austenite to ferrite transformation. The effective inclusions are often oxides with surface layers of MnS and TiN. The mechanism behind these nucleation events is not yet fully understood. However, both the crystal structure, as well as, the formation of a manganese-depleted zone adjacent to the inclusions seem to play a role 7-9). In addition, if cerium is used as a grain refining element a refinement of the solidification substructure through reduction in the dendrite arm spacing is also frequently observed 10). Oxides, nitrides and sulfides of Ti and Ce are most commonly used as dispersoids in steels. Some of these compounds have a low planar lattice disregistry with respect to ferrite 11), as shown in Figure 1. This makes these compounds well suited as nucleating agents in liquid steel.

Figure 1. Relationship between planar lattice disregistry and undercooling for different nucleants in steel11). It has been reported in the literature that TiN can promote grain refinement during solidification of ferritic steels4, 12-15). A fine distribution of TiN will also retard austenite grain growth through Zener pinning5, 16 ). Moreover, Ti-oxides along with Ti-nitrides are known to enhance the formation of intragranular acicular ferrite in low alloyed steels8,9,17). Ce is a very strong sulfide and oxide former when added to steel. Ce additions to steel are known to grain refine a cast structure which solidifies in a ferritic manner5, 18-20). The most probable cause for this is the low planar lattice disregistry between ferrite and the Ce compounds. When the distribution of Ce-oxides is fine enough then the growth of austenite grains is impeded 21,22). Ce-sulfides are also reported to enhance the formation of intragranular acicular ferrite 23,24). A strict control of the balance between the deoxidizing/desulfurizing agent on the one hand and oxygen, sulfur and nitrogen on the other hand is necessary to tailor the right type of inclusion that can improve the microstructure. Therefore, and for the sake of inclusion size control, a number of special “grain refiner alloys” for steels is now under development.


GRAIN REFINER ALLOYS In order to utilize inclusions for grain refinement purposes, it is essential to have control over their size distribution. An average size of about 1 µm is desirable. This is a compromise between two conflicting requirements. On the one hand, a submicron particle size implies that the dispersoids start to lose their potency because a curved interface increases the associated energy barrier against nucleation. On the other hand, if the particles are large (>10 µm), the dispersoids may become detrimental to toughness. At the same time the particle number density drops rapidly, which makes grain nucleation at such sites less likely1). So far, the new steel developments have been hampered by the fact that the nucleating dispersoids used to control the microstructure evolution must be created within the system as a result of deoxidation or desulfurization reactions. The problem is the uncontrolled coarsening of the inclusions, with subsequent loss of toughness. A decade ago, a Ce containing ferroalloy was developed 25) that can be added as a cored wire at a late stage during steelmaking reducing the time for coarsening. The advantages of using such a ferroalloy to add Ce contra the addition of pure Ce metal are a higher Ce yield and no oxidation of such a ferroalloy in air. This is a so-called first generation grain refiner alloy. Such a ferroalloy has become commercially available and is referred to as the Elkem Grain Refiner (EGR). EXAMPLES OF THE USE OF THE FIRST GENERATION GRAIN REFINERS Grain refinement of highly alloyed austenitic steels is important because these steels maintain their solidification microstructure during cooling due to the absence of a phase transformation in the solid state. Reproducible refinement of the microstructure with the use of a Fe-Si-Ce grain refiner alloy has been obtained for steels that solidify austenitic and maintain this phase at room temperature. Examples are austenitic stainless steels like 254 SMO10) and a modified Hadfield steel which containing 1.3 % C, 0.3 % Si and 18 % Mn 26). In these cases, the growth of the columnar grains from the side of the casting block is suppressed thanks to the nucleation and growth of new equiaxed grains ahead of the advancing solid/liquid interface caused by the Ce-compounds. This is a positive attribute being valuable for the steel producer, since castings with a large columnar zone are prone to hot cracking. Except for the smaller columnar zone, a reduction of the primary and secondary dendritic arm spacing is also observed after addition of grain refiner, as shown in Figure 2.

(b)

(a)

0.5 mm

0.5 mm

Figure 2. 254 SMO stainless steel treated with mischmetal (a) and EGR (b)10). This refinement of the solidification structure has a profound influence on the distribution of alloying elements. During solidification, alloying and impurity elements will segregate extensively to the center parts of the intercellular and interdendritic spaces, leading to large compositional variations on a microscopic scale within the as-cast material, as illustrated in Figure 3. Therefore, within each grain, the composition will vary. Homogenizing (i.e. high temperature solution heat treatment) of as-cast material will lead to equalization of the microsegregations by diffusion. As a result, a more uniform microstructure and thus improved mechanical properties should be expected after solution heat treatment, as shown in Figure 4. For example, treatment of 17% manganese steel with EGR will have a profound influence on the microsegregation pattern, leading to reduced Mn concentration gradients within the


grain interiors following homogenizing, as shown in Figure 5. The dendrite arm spacing λ is a measure of the distance between the segregations peaks within the grains of as-cast materials. Hence, when a grain refiner is used, refinement of the solidification substructure leading to a corresponding reduction λ will make it easier to achieve equalization of the microsegregations during homogenizing. This is illustrated in Figure 6. Concentration

As cast condition

Solute diffusion Liq.

Co (mean concentration)

sol Micro segregation As homogenized condition Position within a grain

Figure 4. The effect of homogenization on element distribution.

22

22

21

21 Mn‐content (Wt%)

Mn‐content (Wt%)

Figure 3: Illustration of segregation between dendrites.

20 19 18 17 16 15

EGR‐treated

20 19 18 17 16 15

0

20 40 60 80 Relative distance through grain (%)

100

0 20 40 60 80 100 Relative distance through grain (%) (a) (b) Figure 5. Fluctuations in Mn concentration in a 17% Mn steel without (a) and with (b) EGR treatment following homogenizing at 1050°C for 6 hours.

Figure 6. Schematic illustration of the element distribution in steels being treated with Ce compared to the situation where no Ce treatment is used.


EFFECT OF A REDUCED DENDRITE ARM SPACING ON THE SEGREGATION PATTERN The starting point is Fick’s 1st law of diffusion, where the flux of substance J (in mole per area and second) diffusing down a concentration gradient dC/dx is given by:

1 In this case D is the element diffusivity (with dimension m2/s), defined as:

exp

2

where Qd is activation energy for diffusion (with dimension J/mol), T is the temperature and R is the gas constant. For the purpose of simplicity linear concentration profiles are assumed, where the situation at time t=0 and after and arbitrary time t at a given temperature T is as shown in Figure 7.

Figure 7. Schematic representation of the concentration profiles before and during homogenization. By considering the shaded area A in the graph, an expression for the flux of matter diffusing down the concentration gradient can be developed, based on Fick’s first law of diffusion:

1 2

⁄4

4

3

This equation can be transformed into a first order separable differential equation:

32

4


which following integration between the limits C=Cm at t= 0 and C=C(t) at an arbitrary time t reads:

exp

32

5

EFFECT OF GRAIN REFINEMENT ON HOMOGINEZATION OF AS-CAST MATERIALS In order to illustrate the effect of grain refinement on the homogenization, an example calculation can be made. The reduction in the microsegregation level can be expressed as:

100%

1

6)

This means that the time required to obtain a certain reduction in the microsegregation level is given as:

ln

7

32 For homogenization of austenitic steels containing manganese segregations, the diffusion coefficient, D, is 27

/

17.8 exp

264000

8

Figure 8 shows how the homogenizing time, th, required to achieve 20% reduction in the Mn segregation level during heat treatment varies with temperature and the dendrite arm spacing within the ingot. This figure clearly demonstrates the potential in energy saving that can be achieved during such heat treatment if the liquid steel has been properly treated with a Ce-containing grain refiner prior to the casting operation.

Homogenization time, hours

60 50 40 30

1050°C

20

1100°C 1150°C

10 0 0

50

100

150

200

Dendritic arm distance, λ, μm Figure 8. Calculated homogenizing time, th, required to achieve 20% reduction in the Mn segregation level during high temperature annealing as a function of the dendrite arm spacing within the ingot.


FUTURE DEVELOPMENTS OF GRAIN REFINERS FOR STEELS The commercial application of the grain refiner alloys are (up to now) limited to cast ingots. Addition of the grain refiner alloy during continuous casting of steel is challenging. Addition in the ladle or tundish results in an even distribution of the alloying elements but the long time until casting can lead to inclusion coarsening and clustering. Moreover nozzle clogging can be a problem when Ce is added to steel. Addition in the casting mould requires a very rapid dissolution of the wire fed Fe-Si-Ce alloy. Therefore, before EGR can be applied to continuously cast steels more research is needed to solve these challenges. Second Generation Grain Refiner Alloys So far, the new steel developments have been hampered by the fact that the nucleating dispersoids used to control the microstructure evolution must be created within the system as a result of deoxidation or desulfurization reactions. The problem is the uncontrolled coarsening of the inclusions, with subsequent loss of toughness. This barrier may be overcome by the use of specially designed grain refiners (in the following designated second generation grain refiner alloys) containing a fine distribution of the nucleating dispersoids, analogous to that done in grain refinement of aluminium alloys. Provided that the resulting particle number density and volume fraction are of the correct order of magnitude, these master alloys can be added late in the process, either in the tundish or the casting mould, and thus enable full-scale production of new steel grades without changing the steelmaking process itself 1). There are two different ways the master alloys can be produced. The melting & quenching route means that the alloy components first are mixed and melted in an induction furnace and then superheated to make sure that all elements, including oxygen and sulfur, are in solution. This superheated melt is then rapidly quenched to achieve the desired distribution of the dispersoids in the grain refiner. Alternatively, a powder metallurgy route can be employed. This method involves mixing of iron oxide powder (optionally iron powder) with other metals or oxides. The pellets made from these blends are subsequently reduced in a controlled atmosphere at high temperatures to remove excess oxygen from the master alloy, leaving behind a fine dispersion of stable oxides in the iron matrix 28). The second generation of grain refiners is still in an early stage of development, and their effects in steels are currently being examined in the laboratory 29 . REFERENCES

1

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13

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