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Microstructure-mechanical Property Relationships

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Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of UK Essays.

Published: Wed, 07 Mar 2018

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.

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In the automotive industry, body frames were originally made of hardwood.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

    • Carry out selective batch annealing heat treatments on two microalloyed High Strength IF strip steels.
    • Measure grain size evolution samples using scanning electron microscopy and quantitave optical microscopy techniques.
    • Measure mechanical properties of obtained samples using hardness and tensile testing techniques
  • Determine the optimum processing characteristics, resulting in optimum mechanical property characteristics.

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;

    • Low strength (IF and mild steels),
    • High strength (Carbon manganese, bake hardening, IFHS and HSLA)
  • Advanced high strength steels (dual-phase, complex phase, transformation induced plasticity and matensitic steels)

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 howeve


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