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Microstructure-mechanical property relationships in high strength low alloy steels for automotive applications

Chapter 1

Introduction

The production of steel is an ancient process which has evolved over time. Where and when Steel was first created is unknown and a topic of much debate, however most historians believe earliest production of steel originates from China from as early as 202BC. A later form of steel named Wootz Steel was later developed in India, which used wind power to fuel a furnace producing nearly pure steel. In the 11th century China developed steel further was the first country to mass produce steel. Two methods were developed. A "berganesque" method which produced inhomogeneous steel, and a process which that relied on partial decarbonisation through repeated forging under a cold blast, this was seen as the superior method, and one which lead on to the Bessemer process [1].The Bessemer process involved using a blast furnace to extract iron from its ore and is the basis of modern steel extraction.

Figure 1 Bessemer converter, Kelham Island Museum, Sheffield, England (2002). [1].

Steel is produced firstly by extracting iron from its ore. Iron extraction differs slightly from other metals as it can only be found naturally in oxide form. This means that a smelting process is required. This involves a reduction reaction followed by alloying with additional elements like carbon to stabilise and strengthen the steel. Iron smelting requires a high temperature which produces a ferrous material made of a combination of iron and steel.

The addition of alloying elements such as carbon affect the materials properties greatly.

Changing the temperature at which the iron is smelted affects the phase of the resultant steel, giving rise to the possibility of producing steels with varying properties which are suitable for a range of applications.

In the automotive industry, body frames were originally made of hardwood. This was replaced in 1923 when the American Rolling Company developed steel sheet production.

The wooden frames were inferior in energy absorption which was a big safety issue. Steel was also much easier to form than wood and did not warp over time. As the automobile has evolved over time, there has been an increasing public awareness of the environmental impact of the car. This has forced manufacturers to produce lighter cars which are more economical. This brought about the development of thin, highly formable sheet steel.

The main competitor to steel in the automotive industry is Aluminium, which offers a much better strength to weight ratio and also a better resistance to corrosion. However steel is still the most commonly used material mainly due to lower production cost. Increasing competition from aluminium is forcing the development of modern steels. Steel naturally has a higher formability and elongation than aluminium which is one of the reasons it is used so extensively in the automotive sector. This can be seen in Figure 1.1:

Figure 1.1- Yield strength vs total elongation of aluminium alloys and automotive steels [3]

Ultra low carbon (ULC) steels are used commonly in the production of automobiles. Their, highy formability and suitability for hot dip galvanising make them very attractive to automobile producers[4]. Pressure is being put on the manufacturers to produce lightweight cars that minimise emissions without compromising safety. Metallic properties required to achieve this consist of a high tensile strength, high r- value, good ductility and also the ability to be made resistant to corrosion (either naturally or through the use of chemical surface treatment). Various high performance steels have been developed to meet these requirements, of these, one of the most important being HSLA steels.

High strength low alloy steels provide a much better strength to weight ratio than conventional low carbon steels allowing for thinner grades to be used, saving weight. HSLA steels have a manganese content of up to 1.5%, as well as microalloying elements such as vanadium and titanium.

HSLA steels are increasingly replacing traditional low carbon steels for many automotive parts. This is due to their ability to reduce weight without compromising strength and dent resistance. Typical applications include door-intrusion beams, chassis members, reinforcing and mounting brackets, steering and suspension parts, bumpers, and wheels [5].

High strength low alloy steel properties are determined by the way in which they are processed. High deep drawability, can be achieved through precipitation of elements by annealing to produce a strong {111} recrystallisation texture [7], producing highly formable steels which are very desirable for automotive applications.

In this study, two grades of IFHS strips are studied.

A titanium only stabilised steel grade and a titanium-vanadium stabilised steel grade. These have been treated using a Viking tube furnace and studied using a scanning electron microscope, Photoshop and Optilab Software. Both steel grades have been studied using carefully selected thermo mechanical heat treatment cycles.

The heating variables are expected to cause varying effects to the mechanical properties and microstructure of the two materials. The addition of vanadium in one of the steel grades is also expected to influence the mechanical properties.

With the data obtained from my experiments I hope to determine the optimum processing route for similar HSLA steels.


Chapter 2

Aims


Chapter 3

Literature Review

3.1 AUTOMOTIVE STEELS

Automotive manufacturers make use of many different metals in the production of cars, of which the most predominant being steel. This is for several reasons, steel is relatively easy to recycle in comparison with polymers and aluminium, and this is an issue which is growing in importance as the public are becoming more and more environmentally aware. Steel is also a very good material in terms of its practicality, as it is easily welded, has good formability, elongation and ductility. As the environmental impact of cars is becoming more and more important, stringent regulations regarding emissions are being forced upon manufacturers. One of the ways that manufacturers have chosen to meet these requirements is to make the cars lighter by switching from mild steel to high strength steel grades which enables components to have a thinner cross section, saving weight.

The three main types of steels used in automobiles today are;

These steel types can be seen below on figure 3.1 comparing their elongation and strength.

Figure 3.1: Classification of automotive steels [8].

3.1.1 Mild Steels

Mild steels are normally found in two different forms for automotive purposes. Drawn Quality and Aluminium killed. These are both cheap to manufacture are used for high volume parts. They are usually of a ferrite microstructure. [8]

3.1.2 Interstitial Free Steels

IF steels are used for car body panels extensively due largely to their deep drawability. The high elongation achieved in comparison with other steel grades can be seen in figure 3.1.The main characteristic of IF steel is a low carbon and nitrogen content. These elements are removed from solution by adding specific elements for alloys. Commonly used elements for this microalloying process include Manganese, Sulphur, Titanium and Niobium. As well as a deep drawability, IF steel have low yield strength but a poor dent resistance which is undesirable for certain automotive applications [6]

Bake Hardening Steels

BH steels keep carbon in solution either during processing before it is precipitated or during the paint baking state [8]. This strengthens the steel through solid solution strengthening, resulting in steel with both high formability and high strength.

3.1.4 Carbon-Manganese Steels

Carbon-manganese steels are solid solution strengthened and are used in strip form on automobile bodies, although they are becoming replaced by lighter steel grades. They offer high drawability and are relatively cheap to produce. [9] D.T.Llewellyn: 'Steel: Metallurgy and Applications', Butterworth-Heinemann Ltd, Great Britain, 1992.

3.1.5 High-Strength Low-Alloy (HSLA) Steels

HSLA steels are strengthened through the addition of microallying elements. These react with the carbon and nitrogen within the steel to form carbides and nitrides. Common elements include Nb, V and Ti. The resultant steel has both high strength and a high formability due to very fine grain sizes [10]

Dual-Phase (DP) Steels

Dual-phase steels contain two phases within their microstructure. These are ferrite and martensite. This two phase structure is produced through a complex series of contolled heating and cooling. Martensite regions are produced by heating and rapidly cooling. It is the marteniste regions tha give the hardness to the material where as the ferrite regions are much softer. The structure of DP steels takes advantages of the properties of each of the phases, where the hard maternsite regions are surrounded by softer ferrite which reduces brittleness, shown in figure 3.2. DP steel has good ductility, low yield strength but high work hardening rate [8].

Figure 3.2: Microstructure of DP steel [8].

3.1.7 Transformation-Induced Plasticity (TRIP) Steels

TRIP steels consist of a mainly ferrite microstructure with a low austenite content within the matrix. An isothermal hold during production at an intermediate temperature is used to produce bainite [8]. Strength is increased by transformationing of austenite regions to harder martensite regions. TRIP steels have a good work hardening rate and good strength. Work hardening in TRIP steels continues at higher strain levels than those of DP steels so TRIP steels is a superior material from this aspect. Figure 3.3 shows the multi phase microstructure of TRIP steel.

Figure 3.3: Microstructure of TRIP steel [8].

Martensitic (MS) Steel

MS steels are mainly of a martensitic microstructure but contain small amounts of ferrite and bainite. During heat treatment the steel is rapidly cooled transforming austenite into martensite. This gives a very high tensile strength since martensite produces a very hard material, but the drawback is this also gives a low formability. In order to overcome this low formability further processing such as heat treatments must be undertaken. [11]

3.1.9 High Strength Interstitial Free (HS-IF) Steels

HSIF steels are strengthened through the addition of microalloying elements. Commonly used alloying elements include P, B, Si, Mn, Ti, N. The combinations in which the microalloying elements are used have an effect on the properties of resultant steel allowing a range of requirements to be met. HSIF steels can produce nearly twice the potential yield strength as conventional IF steels, although there is a reduction in formability.

3.2 Microalloying Elements

3.2.1 Carbon

Carbon is one of the most important interstitial elements within steel, giving very different mechanical properties as its percentage content is altered and therefore must be studied in depth. Carbon is an element commonly found in automotive steels due to its high strength properties. Although adding carbon increases strength, it also affects the formability, i.e. its deep drawability.

A set of experiments were carried out to determine the effect of carbon content within steel.

When analysing the tensile test results it was noted that the ultimate tensile strength, the proof stress and the yield stress all increased as the amount of carbon increased in the steel. The plastic region as well as the general elongation of the steel under tensile stress decreased as the carbon content increased. These are significant changes in the mechanical properties. Hardness and Tensile strength increase as carbon content approaches 0.85% C as shown in figure 3.4. The elongation percentage decreases as the carbon content increases. This suggests that the more carbon present in the material, the stronger and less ductile it becomes.

Figure 3.4: Affect of Carbon content in Steel

Yield Strength

Carbon content influences the yield strength of steel because carbon molecules fit into the interstitial crystal lattice sites of the body-centred cubic arrangement of the iron molecules. The interstitial carbons make it more difficult for any dislocation to occur as it reduces mobility. This has a hardening effect on the metal.

Phase diagram

Using the phase diagram one can understand why the properties of steels change with differing carbon content.

Figure 3.5: Phase Diagram

The gamma phase, relates to an Austenite range which has a Face Centred Cubic (FCC) structure. The alpha phase relates to a ferritic Body Centered Cubic crystal structure. Ferrite is found extensively in automotive steels, its BCC structure is much less dense than the FCC of austenite which makes it easily formable and therefore relatively cheap to manufacture. Fe3C refers to cementite and the mixture of alpha (ferrite) + cementite is called pearlite. On the phase diagram steels only apply up to about 1.4% carbon.

The eutectoid point is at 723 degrees and is where there are three phases in equilibrium. The eutectoid composition is Fe-0.83%C. The reaction that happens at the eutectoid point is:

    austenite > ferrite + cementite

    gamma > alpha + Fe3C

High carbon content means a greater precense of austenite, whereas low carbon content will give less austenite and a more ferritic microstructure. The affect of these differing microstructures is reflected in their mechanical properties. This is because Ferrite is soft and ductile and Cementite is hard and brittle. It can be seen by looking at figure 3.5 that as the carbon content is increased, strength increases. This relationship occurs up to the eutectoid point after which it starts to reduce. This where cementite grain-boundaries are created.

The figure below shows how the varying content of carbon in steel affects its properties and suitability for different applications.

Figure 3.6 Carbon Steel Applications

Lever rule

The lever rule can be used to calculate expected proportions of the phases present in each of the tested carbon steel specimens. These values can then be compared to the values obtained through testing.

Figure 3.7 Lever Rule

Calculations:

a = Ferrite a + Fe3C = Pearlite

0.1wt%C Normalised Steel Tensile Specimen:

% Ferrite = (0.8- 0.1)    = 0.897
                 (0.8-0.02)

% Pearlite= (0.1- 0.02)   = 0.103
                 (0.8- 0.02)

0.4wt%C Normalised Steel Tensile Specimen:

% Ferrite = (0.8- 0.4)    = 0.513
                 (0.8-0.02)

% Pearlite= (0.4- 0.02)   = 0.487
                 (0.8- 0.02)

0.8wt%C Normalised Steel Tensile Specimen:

% Ferrite = (0.8- 0.8)   = 0
                  (0.8-0.02)

% Pearlite= (0.8- 0.02)  = 1
                  (0.8- 0.02)

These results suggest that as the carbon content increases the pearlite to ferrite ratio also increases. So the ratio of Pearlite to ferrite increases as carbon content is increased the material is made harder, stronger and more brittle but less ductile. These results obtained using the lever rule support the results obtained from the tensile test, showing the steel with the highest carbon content to be the least ductile and most brittle. The results are also supported by the findings from the hardness test which shows the steel with the highest carbon content to be the hardest.

3.2.2 Titanium

The addition of Titanium to IFHS steels is particularly useful in the manufacturing of strip steels where good drawability is a requirement. The addition of Ti or Nb results in a lower Yield Strength/Tensile Strength ratio giving an increased formability. This can be seen in figure 3.8. When Titanium reacts with Carbon and Nitrogen it forms TiC and TiN, these precipitates work to delay recrystallisation of austenite, thus refining the grains to a favourable smaller size [12].

Figure 3.8: The effect of Titanium and Niobium on Yield Srength/UTS ratio [12]

Titanium precipitates exist within steels and these affect the mechanical properties. TiN precipitates help to promote recrystallisation and encourage the {111} texture.

TiS precipitates are commonly found in the austenite region as well as Ti4C2S2, Ti4C2S2 is formed by reacting with Carbon and in the highest regions of the austenite range there is little to no Carbon. These conditions are created at very high temperatures similar to those during hot rolling processes. This leaves the steel highly formable and suitable for deep drawability application such as car body panels. It is very difficult however to form Ti4C2S2 as it is less stable than TiS, although it can be encouraged through specific heat treatment processes. [13]

3.2.3 Vanadium

Titanium is commonly added with Niobium to steels to increase formability through precipitation. However these additions can result in a retardation of recrystallisation meaning a higher temperature or longer soaking time is required for recyrstallisation to occur. Vanadium offers a replacement to Niobium in the form of carbides and nitrides, VC and VN, which does not cause such a drastic retardation of recyrstallisation. This is attractive to manufacturers as lower temperatures and shorter processing time during annealing are more cost effective. The effectiveness of Vandium in essentially lowering the recrystallisation temperature is shown in Figure 3.9.

Figure 3.9: The effect of Ti + Nb, Ti + V and V stabilised steels on the Temperature for Complete Recrystallisation in 30 Seconds [44].

Figure 3.9 shows that the V only stabilised steel recrystallises at a lower temperature than the TiV and TiNb steels.


3.2.4 Sulphur

Sulphur is found in all steels including Interstitial Free High Strength Steels. It acts as an interstitial elements and other elements to form precipitates such as TiS, MnS and Ti4C2S2. These precipitates have different effects on the mechanical properties of the material. In particular the precipitation of carbosulphides is beneficial to the steel as this causes the steel to form in the austenite range and helps to reduce the TiC formation which could occur during heat treatment processing and cause the material to become less likely to form the {111} texture.[13] Promoting Ti4C2S2 therefore encourages the formation of the favourable {111} texture, increasing the formability of the material. In order for Ti4C2S2 to develop, Sulphur, Carbon and Titanium must all be present, and processed in such a way as to form a reaction, which can difficult.

3.2.5 Niobium

Niobium if found extensively in IFHS Steels reacting with carbon to form carbides such as NbC. Solute Niobium can be used to segregate austenite and ferrite grain boundaries and increase the strength of the austenite region [14]. As Niobium content increases the r-value decreases as well as the ductility. Generally Nb content is minimised as much as possible as the positive effect it has on strength in the austenite region is relatively small and is outweighed by the negative effect it has on ductility. Boron can be used instead of Niobium as it has a much greater effect on strength than Niobium. This can be seen in figure 3.9

Figure 3.9: Average Flow Stress vs. Temperature for B, C, and Nb and Mo solutes in steel [15].

3.2.6 Phosphorus

Phosphorus, P, is a common alloy of IFHS steel, offering increases in strength through solid solution hardening. Adding Phosphorus can also have a direct effect on the grains within a structure by increasing the "Hall-Petch slope" (described below). Adding P however can have a negative effect on the brittleness of the material. This can be particularly problematic during the cold working process where brittle fracture is a distinct possibility.

The Hall-Petch relationship says that as the grain size decreases the yield strength of a material increases. This is due to the dislocations piling up at grain boundaries, which act as barriers to dislocation movement at low temperatures. If the grain size is large, then a high number of dislocations will "pile up" at the edge of the slip plane. When the stress exceeds a critical value the dislocations cross the boundary. So the larger the grain size, the lower the applied stress required to reach this critical stress at the grain boundary, meaning the larger the grain size, the lower the yield stress due to easier dislocation movement. This is true down to a grain size of 100nm. Below this size the yield strength remains constant or starts to decrease. This is effect is called the reverse Hall-Petch effect.

Phosphorus along with Silicon and Manganese are added via solid solution strengthening to strengthen steel allowing for a thinner sheet of metal to be used for car body panels, and thus reducing the weight. Phosphorus is the most effective out of the three elements in terms of cost and strengthening effect. This can be seen below in figure 3.11 where the effects of P and S additions are compared.

Figure 3.11: Comparison of Stress vs. Temperature between Phosphorus and Silicon microalloyed Steels [16].

Phosphorus is also found in the form of FeTiP precipitates. These precipitates have a negative affect on strength and drawability. The effects of these precipitates are greater in batch annealed steels than in continuous steels. This is due to the long soaking times required in batch annealing which provides optimum conditions and sufficient time for these precipitates to form [17].

3.2.7 Manganese

Manganese is added through solid solution strengthening to IFHS steels in a low concentration in order to react with the Sulphur to produce MnS precipitates. These MnS precipitates act to refine grain structure during processing when there is a transformation in phase between austenite and ferrite.

Mn is to strengthen steels through solid solution strengthening. The effect of Mn is relatively small in the austenite range but compared to the ferrite range. This is due to a difference in Mn solubility between the austenite and ferrite ranges. Where Mn in ferrite is 10wt% higher than in austenite [18]

Mn acts to stabilize the austenite region and slows down the rate of austenite transformation and also the temperature at which the transformation takes place. This lowering of transformation temperature between austenite and ferrite promotes finer grains through grain refinement.

Mn can be found in oxide and sulphide forms as well as combinations of the two, oxysulphides. These oxides and sulphides act to deoxidise and desulphurise the steel. When in sulphide form, MnS helps to reduce embrittlement of steel without reducing hardness. When mixed with common impurities such as Al2O3, SiO2, MnO, CaO, CaS and FeS an increase in hardness and strength occurs [19]. When in the oxide form, MnO at the surface acts a barrier layer to prevent surface oxidisation and corrosion.

3.2.8 Silicon

Silicon is a useful element and is used to increase the strength through solid solution strengthening, although there is a compromise as increasing Silicon content decreases ductility.

Silicon is also found in oxide form, as silicon dioxide. Silicon dioxide is found with Manganese Oxide or as Silicomanganese to give a strong oxygen stabilisation and prevent corrosion of steel. [20].

3.2.9 Aluminium

Aluminium is used to deoxidise steel by reacting with oxygen within the steel to form Al2O3. These Aluminium Oxides are later removed leaving an oxygen free steel. However the low density of Aluminium means that oxidisation could occur at the steel interface resulting in corrosion.

Aluminium content can have a negative effect on formability. This is due to the precipitation of AlN during recrystallisation preventing the {111} development and thus preventing the formation of finer grains. So minimising the amount of AlN in solid solution results in higher formability. A more stable alternative to AlN which is commonly used in IFHS steels is TiN.

3.3 Hardening and processing

There are many different compositions of steel which offer various advantageous properties. The main reason for altering composition or alloying is to strengthen the material. This can be done in several ways;

3.3.1 Precipitation strengthening

This process uses heat treatment to raise the yield strength of a material. As temperature changes during heat treatment processing, fine particles are produced due to differing melting points of impurities. These fine particles impede dislocation movement. This in turn reduces the ductility and plasticity of the material and increases its hardness.

3.2.2 Solid solution strengthening

Solid solution strengthening is a form of alloying. It is a commonly used technique to improve the strength of a material. Atoms of the alloying element are added to the crystal lattice of the base metal via diffusion.

There are two ways in which this can occur, depending on the size of the alloying alloying element. These are via substitutional solid solution, and interstitial solid solution.

Substitutional solid solution

This takes place when the sizes of the alloying atoms are equal in size to the base atoms, (Differing in size by no more than 15% according to the "Hume-Rothery rules") The alloying atoms replace the solvent atoms and assume their lattice positions. The solute atoms can produce a slight distortion of the crystal lattice, due to the size variation. The amount of distortion increases with the size of the solute atom. This distortion has an effect on microstructural properties. The formation of slip planes is altered making dislocation movement more difficult, meaning a higher stress is required to move the dislocations. This gives the material a higher strength. A generalisation associated with substitution is that large substitutional atoms put the structure under compressive stress, and small substitutional atoms give tensile stress.

Interstitial solid solution

This occurs when the alloying atoms are much smaller than the base atoms. The alloying atoms fit into spaces within the crystal lattice. This is the case with carbon in steel, where carbon is a solute in the iron solvent lattice. The carbon atoms are less than half the size of the iron atoms so an interstitial solid solution forms.

3.3.3 Processing

The final properties of steel are greatly affected by the manner in which it is first made and then processed. Typical processes include steel making, casting, hot and cold rolling and annealing. Each individual process has a distinct affect on the properties of the steel.

To make the steel free from interstitial elements, Ti and Nb are often added to react with interstitials after a process called vacuum degassing. Vacuum degassing is the name given to the process where a metal is melted within a vacuum and the gasses are evaporated out.

Hot and cold rolling

Hot rolling is the first process to take place after steel making. After steel has been cast into uniform slabs or billets it is the rolled under a high temperature to reduce its cross sectional thickness. The hot rolling process is undertaken at a temperature above that at which recrystallisation occurs. Hot rolling reduces allows recrysallisation to occur during processing (dynamic recrystallisation) and the material is left stress free due the new grain nucleation and equiaxed grains.

Effect of hot working on microstructure:

Hot working occurs at high temperatures, this means that there is often enough thermal energy present for recrsytallisation to occur during deformation. This is called dynamic recrystallisation and it occurs with most metals, apart from aluminium. Recrystallisation occurs during the working process and also as the metal is cooling.

Dynamic recrystallisation occurs by new grains nucleating at existing grain boundaries. The amount of recyrstallisation depends on several factors. It depends on the strain rate, temperature and amount of strain on the metal. Generally, as strain within the metal increases, so does the amount of recrystallisation.

Cold working is when steel is plastically deformed below its recrystallisation temperature. This process increases the yield strength due to the plastic deformation causing slight defects within the microstructure of the metal. These defects make it difficult for slip planes to move. The grain size of the metal is also reduced, making the material harder through a process called Hall petch hardening.

Hall Petch hardening, also known as grain boundary strengthening, increases materials strength by altering the grain size. This is because grain boundaries act as barriers to dislocation movement. So altering the grain size, through hot and cold rolling at various temperatures and rates will have an effect on dislocation movement and yield strength.

Cold working will increase the strength of the metal by making it increasingly difficult for slip to occur. However as more and more of the larger grains split to form smaller grains the ductility is greatly reduced as the material hardens. Eventually fracture would occur. To avoid this, the material is annealed.

Cold working occurs at a temperature below 0.4 of the metals melting point. Some of the energy created by the process is expelled as heat but some energy is stored within the structure putting it into a high energy state. The energy is stored within the grain boundaries of the deformed crystals and within the stress fields of the dislocations created through the plastic deformation. The structure is highly stressed after cold working and would prefer to return to its former low energy state. It is however frozen in this high energy state and can only return when heated above 0.5 of its melting temperature. The following process which occurs is called recrystallisation. This is where new stress free grains nucleate from within the old deformed structure; these grains are low energy and equiaxed. The nucleation spreads through grain boundary movement and eventually the entire structure is stress free and only new grains exist. This recrystallisation process is a means of controlling the grain size of a material by altering the conditions under which recrystallisation occurs. The more cold work done on a metal, the higher the nucleation density. This is because more cold work will put the structure into a higher energy state. So more nucleation will occur, and the higher the nucleation density the smaller the grain size. There are however other factors which influence grain size. The grain size increases as the heating time increases as grain growth occurs at high temperatures.

Annealing

Annealing is a process where the material is reheated. This softens the metal allowing grain growth to occur, and allowing the metal to become ductile once again. This process is often followed by further cold working. The heating of material provides the structure with a high enough energy level for nucleation to occur, replacing deformed grains with stress free equiaxed grains.

The annealing process consists of 3 stages; recovery, recrystallization and grain growth.

During recovery the material contains a high number of dislocations arranged randomly throughout the structure. When heat is applied the dislocations can move to a lower energy configuration via diffusion. The mechanical properties are unaltered at this stage, although thermal and electrical conductivity is reduced.

The mechanical properties change dramatically during recyrstallisation. After recovery there is still the same number of dislocations. The structure is under a high enough stain to allow stress free grains with a low dislocation density to nucleate. The areas of highest energy are the grain boundaries so these act as the initial nucleation sites. New equiaxed grains continue to grow converting the entire structure, until there are now deformed grains left.

All previously deformed grains have been converted into strain free grains. Continued annealing promotes further grain growth. This decreases grain boundary area, and lowers the general reducing the energy within the structure.

3.3.4 Grain refinement

When a metal is cast, as it is commonly for automotive applications, a nucleation and growth process takes place. The grain size within the structure is governed by the number of nuclei, where the more nuclei that exist the smaller the grain size. The number of nuclei depends on a number of variables. The lower the casting temperature before solidification begins, the higher the nucleation. Within the casting mould, there are areas of different temperature, with the mould walls being the coolest. This is where there will be a significant temperature drop as soon as the molten metal is poured. This will give a very small grain size concentrated around the edge of the casting compared to larger grain sizes in the more central areas of the mould. It is by using this principal that grain sizes gain be controlled in casting by artificially including chilling particles within a casting. In most alloys nucleation first occurs at mould walls due to them being the first area to solidify. This is the basis for introducing solid nucleating particles into the mould in order to reduce grain size. This is called grain refinement and different grain refinement.


Chapter 4

Experimental Procedures

Two cold rolled experimental HSLA steel grades were used in the study, one based on a Ti only micro alloy addition and one based on combined Ti-V micro alloy additions. The samples were prepared using pure raw materials to minimize unwanted alloying additions. The composition of the experimental HSLA steel grades is given in the table 4.1.

Table 4.1: Chemical Composition of Experimental HS-LA steels (wt %)

Element /  Steel code

Steel 60 (Ti)

Steel 58 (Ti-V)

C

0.0025

0.0033

Si

0.0160

0.0140

Mn

0.1600

0.1700

S

0.0030

0.0030

P

0.0120

0.0120

Ti

0.0260

0.0200

Nb

0.0030

0.0040

V

0.0010

0.0810

Al

0.0200

0.0360

N

0.0038

0.0029

Fe

Balance

Balance

4.1 Heat treatment Processing

A Viking tube furnace (VF1246) was used during heat treatment of samples during this project. The furnace uses an inert gas and creates a vacuum in which the steel is contained. The Viking Furnace was programmed with the desired heat treatment data. Two grades (60 Ti only and 58 TiV) of strip steel were treated under the following conditions.

The steel was heated from room temperature to 475oC at a rate of 90oC/hr, then heated further to the peak annealing temperature at a rate of 10oC/hr where it was held for 20hrs. Then steel was then cooled to room temperature at a rate of 30oC/hr.

This process was repeated with peak annealing temperatures of 675oC, 700oC and 725oC.

The heat treatment process is shown graphically in figure 4.1

Figure 4.1: Heat treatment details

4.2 Preparation

Following heat treatment the strip steels were collected and cut into sample shape using a guillotine. The samples were cut into batches of two sizes, one batch for hardness testing and microstructural examination, and the other for tensile testing.

Samples for hardness testing and microstructural examination were mounted into castable resins which consisted of MetPrep Kleer-Set Type FF resin and MetPrep Kleer-Set Hardener (shown in figure 4.2). These samples were subjected to grinding, starting with 240-, 320-, 600-, and finishing with 1200 grit silicon carbide paper. This was followed by polishing using 6m and 1m diamond paste impregnated cloths, using paraffin as lubricant leaving a smooth scratch free surface.

Figure 4.2: Specimen mounted in resin

4.3 Hardness Test

In order to assess the mechanical properties of samples, each sample was subject to a hardness test using a Vickers indenter, under a load of 10N for 10 seconds. Each sample was tested 15 times in order to get an accurate value by averaging out anomalies.

After hardness tests had been carried out, each sample was re grinded and polished in preparation for scanning electron microscopy.

4.4 Tensile Test

Samples which had been previously cut to tensile test dimensions were used. Each specimen was subjected to a tensile test in order to assess the strength of the material.

A Hounsfield Tensile Tester was used. The dimensions of the tensile specimen were measured and noted. The machine was switched on, as well as the Hounsfield data logger on the corresponding computer. The cross head of the machine was moved close to the specimen and put into the grips securely. The dimensions of the specimen as well as the material identification code, the cross head speed (5mm/min) and the sampling rate (250 milliseconds = 4 readings/second) were added to the screen on the Hounsfield logger. The load and position were zeroed and the test was started. When the sample fractured the test was stopped and the file was saved. The fractured sample was removed and the cross head was reposition for the next test.

4.5 Microstructural Examination

After hardness testing the samples were re-grinded, polished and etched. Etching involved submerging the surface of each sample in 2% Nital Solution for 10 second intervals monitoring the surface closely. Clouding of the surface indicated a fully etched sample. Once etched, the samples were dried to prevent corrosion and the exposed microstructure was ready to be studied using the scanning electron microscope.

4.5.1 Scanning Electron Microscopy

The Scanning electron microscope creates an image of the surface of a material by firing electrons at the surface in a high energy beam. The interaction of the electrons and the elements on the surface of the sample produces a signal which is converted into an image of the surface.

A Philips XL 30 SEM machine was used in the study to determine the microstructure of the various samples. An accelerated voltage of 20kV with a working distance of 10mm was used on all samples.

An image of each sample was captured at magnifications of 500x, 750x and 1000x.

4.6 Grain Size Measurement

Using the 1000x magnitude images of each sample previously captured using the SEM, further analysis was undertaken through Photoshop and Optilab software to determine average grain sizes and obtain data to calculate elongation. Measuring the actual grain sizes of each sample is an accurate way of determining the grain elongation, and therefore quantifying the effect of annealing temperature on grain evolution.


Chapter 5

5.1 Microstructural Examination

5.1.1 As received Condition

Figure 5.1 shows the microstructures of both grades (Ti and TiV) of steel in as received, un annealed condition. The microstructures of both grades show pure ferrite structure which are both highly plastically deformed and elongated due to previous rolling in their manufacture into strip steel form.

Figure 5.1: Scanning electron Micrograph of (a) Ti only Steel in the as received condition (b) Ti-V steel in the as received condition.

5.1.2 Heat Treated condition

Figure 5.2 shows the SEM microstructure images for the Ti only steel grade when annealed at 675oC, 700oC and 725oC, showing the grain evolution as a result of annealing temperature.

Figure 5.2: Scanning electron micrographs recorded in the Ti only steel employing variable annealing temperatures (a) 675oC, (b) 700oC and (c) 725oC.

In figure 5.3 a set of micrographs for the Ti-V steels are presented, employing variable finishing temperatures of 675oC, 700oC or 725oC

5.2 Hardness

Effect of Annealing Temperature

Table 5.1: Hardness evolution for the Ti only steel in relation to annealing temperature.

Annealing Temperature(°C)

Vickers Hardness Value

As received

118.5

675

74.9

700

73.6

725

70.9

Table 5.2: Hardness evolution for the Ti-V steel in relation to annealing temperature.

Heat treatment Temp

Vickers Hardness Value (average)

As received

175.4

675°C

75.0

700°C

70.3

725°C

69.2

5.3 Grain Size measurements

Grain size measurements were undertaken using first Photoshop software followed by further analysis using Optilab software. Each microstructure was examined at 1000x magnification and the results shown in tables 5.3 and 5.4 were obtained.

Table 5.3 Grain size measurements for Ti only Steel.

Ti only

Annealing Temp(°C)

area(µm²)

Elongation

675

101.00

3.09

700

130.00

2.69

725

155.37

2.59

Table 5.4 Grain size measurements for TiV Steel.

TiV

Annealing Temp(°C)

area(µm²)

Elongation

675

91.10

3.12

700

129.00

2.82

725

185.00

2.44


Chapter 6

Discussion

6.1 Grain Size Evolution

In this study three annealing temperatures were tested, 675C, 700C and 725C. The variation of annealing temperature had a noticeable effect on both the Ti and TiV steel grades in terms of microstructure and mechanical properties.

Data obtained through image analysis of obtained microstructures can be used to compare grain size directly to annealing temperature. Figure 6.1 shows the relationship between grain size and annealing temperature of both steel grades.

Figure 6.1: Graph showing relationship between Annealing Temperature and Grain Size

Figure 6.1 shows clearly that as the annealing temperature is increased the grain size increases. This is true for both steel grades. This increase in grain size can be explained by studying the annealing process in depth.

The annealing process which was undertaken in this project involved a heat treatment cycle which heated the steel to its peak annealing temperature which was below the austenising temperature and in the ferrite range, which can be seen in the phase diagram in figure 6.2. The steel was held at this annealing temperature for 20 hours, which allowed any stresses in the material to be relieved due to recovery and recrystallisation.

Figure 6.2: Phase diagram [http://en.wikipedia.org/wiki/File:Steel_pd.svg]

During the annealing process recovery and recrystallisation occur, which greatly alter the mechanical properties and can account for the increase in grain size seen in figure 6.1. These stages involve the replacement of previously deformed grains by equiaxed ferrite grains.

During the heating of the steel, heat energy is supplied which allows dislocations within the steel, created by rolling into strip form, to rearrange via diffusion into a lower energy configuration. This is known as polygonisation. This recovery process does not alter the mechanical properties greatly, it transforms the material from a high internal energy state to a lower energy state without decreasing the number of dislocations.

The next stage of the annealing process occurs when the steel is held at its peak annealing temperature for a sufficient amount of time for recrystallisation to occur. After recovery dislocations still exist and are under a strain which is sufficiently high for the initiation and growth of new equiaxed grains of a low dislocation density to occur. These new grains form at the grain boundaries as these are the areas of highest energy. This process continues until all deformed grains have been replaced by new stress free grains.

Since the reaarangement of dislocations which occurs in annealing is controlled by diffusion the rate at which this occurs is determined by time and temperature. In this particular study the annealing time was fixed, leaving only the temperature as a variable. For fixed times it would be expected that increases the temperature would result in an increased rate of annealing, meaning recrystallisation occurs at a faster rate, and therefore larger recyrstallised ferrite grains. This is as increasing the temperature provides higher energy making the breaking of bonds during the rearrangement of dislocations easier. This is supported in my results where as the annealing temperature is increased from 675C to 700C and 725C the average grain size measured for both steel grades shows an increase.

6.2 Hardness

The temperature at which steel is annealed greatly affects the grain size, as has been previously discussed and highlighted in Figure 6.1. Annealing at a higher temperature provides more energy allowing for dislocations to move to lower energy configurations and bonds to be broken more easily. As a result, the steels annealed at higher temperatures undergo higher recrystallisation followed by grain growth and therefore exhibit larger grains. The alteration in dislocations allows metals to deform more easily which increases their ductility and reduces hardness. The hardness is reduced with increasing grain size as large grains pack together poorly with large gaps between grains which allow dislocation movement to occur, decreasing the strength and hardness of the material. Results from hardness testing and grain size analysis undertaken during this study are presented in a comparison graph shown in figure 6.3a. Figure 6.3b shows the direct relationship obtained between hardness and annealing temperature.

Figure 6.3a: Hardness Vs Grain size Graph

Figure 6.3b: Comparison of hardness evolution for Ti only and TiV steel in relation to annealing temperature.


The general trend that can be seen in the results of this experiment is a smaller grain size relates to a harder steel. This can be explained by grain boundary hardening and the hall petch relationship.

The smaller the grain boundary size, the smaller the possibility of dislocation pile up. This pile up refers to the build up of grains unable to diffuse accross a grain boundary in a stress material. As there is little build up of dislocations at the boundary, an increased level of stress in required to allow dislocations to cross the grain boundaries. Therefore the smaller the grain boundary size the higher the yield strength. This relationship is defined in equation 6.1 by the Hall Petch equation.

Equation 6.1: Hall Petch Equation [Schuh, Christopher; Nieh, T.G. (2003). "Hardness and Abrasion Resistance of Nanocrystalline Nickel Alloys Near the Hall-Petch Breakdown Regime". Mat. Res. Soc. Symp. Proc. 740.]

Where;

    ky is the strengthening coefficient,

    so is the stress required to initiate dislocation movement,

    d is the grain diameter,

    sy is the yield stress.

6.3 Effect of alloying elements

The addition of alloying elements to HSLA steels has a great effect on mechanical properties and theses can be seen in the results obtained during this study. The variations in composition between the two steel grades studied are displayed in table 4.1. The element with the largest wt % difference between the two steels grades is Vanadium with a wt% content of 0.0810 in the 58 grade compared to a wt% content of only 0.0010 in 60 grade steel. This variation in V content greatly affects the microstructure and mechanical properties of the steels and this can be seen in the results of this project. Figure 6.1 compares both steel grades , in terms of their relationship between grain size and annealing temperature. From figure 6.1 it is possible to draw several conclusions regarding the addition of Vanadium since this is this only significant variable between the two grades.

Figure 6.1 shows that the Ti only grade notably increases in grain size evolution at 700C. This suggests that this is the temperature at which the material fully recrystallises and grain growth starts. In comparison the TiV shows a linear relationship between annealing temperature and grain size, suggesting that recrystallisation has already occurred at a lower temperature, and that between 675C and 725C the material is in a grain growth phase were the grains are increasing in size at a constant rate.

This suggests that the addition of Vanadium acts to lower the recrystallisation temperature of the steel as ferrite grains nucleate faster when vanadium additions are present.

In hot rolled products Vanadium is used extensively in combination with other micro alloying elements such as Nb and Ti. Vanadium is used to strengthen steel through precipitation strengthening, and is also used for grain refinement (due to austenite conditioning during hot rolling)

In order that microalloying elements such as Vanadium are most effective, they must be dissolved during processing and precipitated at a lower temperature. This means that the solubility of microalloying elements plays an important role and is affected by temperature and chemical composition.

Vanadium has a high solubility, and the solubility of Vanadium Carbide is much greater that other common precipitates such as TiC and NbC. As VC is soluble at austenite rolling temperatures, it possible to hot roll steel at low reheating temperatures, and this is the reason it is used extensively for hot rolled high strength low alloy supplications.

In cold rolled high strength low alloy steels, Vanadium is a relatively new addition and its affects are still be studied. The strengthening effect of Vanadium is not as great in cold rolled products as it is in hot rolled products, this is due to the precipitates which form.

The most important precipitate as far a strengthening is concerned is Vanadium Carbide, VC. During heat treatment processing, these VC precipitates act to coarsen the microstructure due to a high solubility. Coarsening increases size but reduces the strengthening contribution of the precipitates. So strength is reduced during annealing, due to microalloy coarsening.[21]


Chapter 7

Conclusions


Chapter 8

Suggestions for Further Work

The effectiveness of vanadium as a strengthener in cold-rolled HSLA products should be examined further in steels where sufficient nitrogen is available to enhance the coarsening resistance of the microalloy precipitates.


Chapter 9

References

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