Titanium is a highly active element, which at room temperature normally forms a stable oxide coating on its surface, which limits further oxidation. At steelmaking temperatures, it forms stable compounds with oxygen, carbon, nitrogen and sulfur. Because of this property, it is often used in steelmaking to fix these elements, so lessening their harmful effects. Titanium also acts as a grain refiner in many steels, and in many respects it has a similar function to the addition of both aluminum and columbium (niobium). Titanium is more expensive than aluminum, so its use as a deoxidizer has diminished considerably.
The reactivity of titanium is such that, like magnesium, thin ribbons such as machine turnings, or powder, can quite easily be set on fire, e.g. by welding sparks or by addition to the surface of a steel melt without plunging. Titanium burns with a very bright white flame, which can be harmful to look at. A titanium powder fire is doubly dangerous, because the convective current caused by the fire can cause the unburned powder to become airborne, with the risk of explosion. Ferrotitanium powder is also flammable, with the finest sizes and highest Ti content being the most hazardous.
As the ninth most abundant element in the earth's crust (0.6%), titanium does not suffer from the availability problems which affect some other alloying constituents. However, the variable demands for titanium alloys from the aerospace industry makes the price of scrap fluctuate widely. Large quantities of titanium ores, principally ilmenite (FeOTiO2) and rutile (TiO2) are mined throughout the world. About 95% of titania feedstocks are used to make titania pigments; the more pure, and relatively scarcer rutile is preferred for metal production. The conversion of rutile into titanium is generally by the Kroll process. Chlorine gas is passed over a mixture of rutile and coke at 800 C (1472 F) to produce chloride:
TiO2 + 2Cl2 +2C = TiCl4 + 2CO
The TiCl4 is then passed over molten magnesium, forming magnesium chloride and titanium sponge, which is then compressed and remelted into ingots under vacuum.
Titanium addition agents generally fall into three general categories: metal scrap, ferroalloys and master alloys. These are considered separately below:
Titanium scrap may be of commercial purity (CP) titanium, or one of the many titanium-rich alloys. The most common of these is 6%Al-4%V (6-4), followed by 6%Al-2%Sn-4%Zr-2%Mo. Scrap is generally available in the form of solid scrap, turnings or sponge. Solid scrap is generally clean, but turnings must be degreased before use. Sponge is often from the Kroll process (above), and may occasionally be contaminated with MgCl2, which reacts with water vapor, making some sponge potentially dangerous to melt. Titanium turnings may be contaminated with lead or bismuth, which are in the brazing alloy that is often used to fix a boss onto a piece of titanium prior to machining it.
It should be noted that the melting point of titanium is higher than that of the bath to which it is generally being added. This means that titanium added as metal generally enters the steel by dissolution, rather than melting. The low density of titanium means that it has to be plunged into the melt; otherwise, oxidation can cause very low recovery into the melt.
Ferrotitanium is available in many grades, but by far the most common contains approximately 70% Ti. The use of 90-6-4 scrap makes a ferroalloy with approximately 4.5% Al and 3% V, which is the basis of many specifications. CP grades of scrap can be used to make alloys with <1% Al, but generally at a significant price premium. Tin is generally unwanted in most steels, so the tin bearing grades of scrap are used in the lower grades of ferrotitanium.
Ferrotitanium is now almost invariably made in an electric induction furnace by melting scrap titanium with iron units. Previously, the alloy was produced by aluminothermic reduction of ilmenite, using a variation of the Thermite process. This produced an alloy with about 40% Ti and 8% Al, balance Fe. This was used in the formulation of many welding rods, and such an alloy remains available today, although it is mostly used for welding and similar operations.
Since the main use of titanium is as a scavenger for carbon, nitrogen and oxygen, the steelmaker naturally prefers to buy ferrotitanium that has the lowest possible content of these elements. This is especially true as the alloy is generally priced on the titanium content, and 1% N can combine with over 3.4% Ti. To minimize the content of carbon, oxygen and nitrogen, the titanium scrap must be carefully selected, and extra measures are taken during the ferrotitanium production process, causing a slight increase in its price.
Other impurity elements in ferrotitanium may be picked up from the titanium scrap that is used in its manufacture. These can include Cr, Ni (from stainless steel which may get mixed into the titanium scrap before it reaches the ferroalloy manufacturer), Zr and Cu.
Proprietary alloys will contain titanium plus additional elements such as aluminum, zirconium, silicon or chromium. They are not commonly used as titanium additions per se, but rather for a combination effect, such as sulfide shape control or increased yield strength. In some cases, they are added simply as a protection mechanism for boron, which would otherwise be lost from the steel melt by its reaction with nitrogen or oxygen (see Boron).
Ferrosilicon-titanium alloys permit the simultaneous addition of silicon and titanium. They usually contain roughly equal amounts of silicon and titanium; 20%-20%, 30%-30% and 45%-45% being the most common grades.
Ferrotitanium alloys are normally supplied in crushed and sieved form, often as 50 mm x down (2 in. by down). Many customers apply a minimum size limit, such as 5 mm (1/4 in.), in order to minimize the amount of fine material, which can give poor recoveries in less well-controlled melting practices. Packing is normally loose in 250 kg or 500 lb. drums, alternatively pre-packed in plastic or paper sacks of up to 20 kg or 50 lb. each. Ferrotitanium is also supplied in supersacks.
There has been a trend towards the use of cored wire for the addition of ferrotitanium at the steelworks. Ferrotitanium (70% Ti) of size 2 mm (0.08 in.) or finer is encapsulated inside a sheath of mild steel, wound onto a coil. By this technique, the ferrotitanium can be added continuously to a bath, launder or tundish of steel. The steel sheath protects the ferroalloy from oxidation. The fine size of the ferroalloy ensures quick and high recovery of titanium into the melt.
An alloy of 70% titanium 30% iron falls at the eutectic point of the system, with a melting point of 1085 C (1985 F), and so is known as eutectic ferrotitanium. As this is well below steelmaking temperatures, the ferroalloy melts, which is far quicker than the dissolution required for metallic titanium to enter the steel. It is for this reason that eutectic ferrotitanium is the preferred analysis for use in steelmaking.
Because titanium is so easily oxidizable, great care must be taken when adding either the metal or its ferroalloys to steel. Failure to do so will drastically reduce alloy recoveries. All efforts should therefore be made to avoid contact between the titanium addition and air or oxidizing slags.
Titanium additions are typically made in the ladle as scrap or lump during tapping, as cored wire into the ladle, or as lump in ladle degassers or CAS-OB processes. The steel should be thoroughly deoxidized first, usually with a prior addition of aluminum. Most steelmakers prefer to add the titanium late in the tap, when the ladle is 1/2 to 3/4 full, and after all other additions, except boron. The object is to leave as little time as possible for reoxidization of the steel to occur, since the titanium will take up any available oxygen, thereby reducing its effectiveness. Contact with oxidizing slags should likewise be avoided, since (a) slag/metal reactions will be detrimental to recovery and (b) the coating action of the slag will impede the otherwise rapid melting and dispersion of the ferroalloy in the melt.
When furnace additions are necessary, titanium should be added only after thorough deoxidation and just before the tap. A thin layer of reducing slag will help prevent reoxidation and will act as a nitrogen barrier as well.
Titanium recoveries are invariably lower than those for most other additives. Recovery can vary between about 50% and 90% depending on the level of care taken by the steelmaker, the steelmaking practice and the type of additive used. Because of their higher density (less likely to float on the melt surface and oxidize) and their lower melting temperature, ferrotitanium alloys generally give higher recoveries than scrap. They are therefore the preferred addition agents for most products.
The processing of titanium-bearing steels on the hot strip mill is fairly straightforward and follows, in general, the pattern established for other controlled rolled products (cf., for example, Columbium). Titanium nitride, formed while the steel is still liquid, will be carried over into the slab: thus, changes in soaking temperature will have little effect unless columbium is present as well. Most of the strengthening imparted by titanium derives from the precipitation of titanium carbonitrides; a lesser amount comes from grain refinement. Thus, titanium steels will have poorer impact resistance than other microalloyed grades unless extra precautions are taken to insure the finest possible grain size in the finished product. Lowering the finishing temperature to about 840 C (1550 F) has a beneficial effect on impact transition temperature. For higher strength products (Y.S. above 550 MPa, 80 ksi), coiling temperatures should be kept below 650 C (1200 F).
One of the more important uses for titanium in steel is the control of sulfide morphology. Without titanium (or zirconium or the rare earths), sulfides tend to become elongated during hot rolling. This leads to poor impact properties and reduced ductility in the through thickness dimension, and can be particularly harmful in welded structures. Titanium hardens the sulfides and allows them to retain a less harmful globular shape throughout the hot rolling process (see Cerium & Rare Earths).
The effects of titanium on heat treatment and microstructure are directly related to the element's reactivity, particularly that with carbon. Thus, pearlitic titanium-bearing steels will contain less cementite (less pearlite) since any titanium not already combined with carbon or nitrogen will form the highly stable carbide, TiC. Carbon scavenged in this manner will be about one quarter the weight percent of the available titanium. Said another way, pearlite will be completely absent in steels containing titanium equal to more than four times the carbon content. There are no double carbides of iron and titanium. Titanium raises the grain coarsening temperature. It is much more effective than aluminum when concentrations of either element exceed 0.035%. However, a steel containing 0.03% Al and 0.005% Ti will have a grain coarsening temperature elevated by about 30 C (50 F).
The effect of titanium on hardenability is complex. As titanium concentration increases, hardenability generally decreases unless austenitizing temperatures are raised. Reason: the formation of TiC (see above) lowers the carbon concentration in austenite. Further, titanium refines the grain size, and this decreases hardenability all the more. If only acid soluble (uncombined) titanium is considered, its hardenability factor is about the same as that for molybdenum.
Titanium steels do exhibit secondary hardening upon tempering due to the precipitation of TiC. The effect is increased when austenitizing temperatures are above 980 C (1800 F).
Titanium is valuable as a carbide stabilizer in stainless steels. Type 321, an austenitic grade, contains titanium equal to at least five times the carbon content in order to prevent the precipitation of chromium carbides on grain boundaries during extended holding at elevated temperatures. Without the presence of Ti, the chromium would be depleted at grain boundaries, leading to intergranular corrosion (see Chromium).
The carbide fixing properties of titanium are also valuable in Type 409 stainless steel, a ferritic grade widely used in the manufacture of automotive catalytic converters and other exhaust components. Low strength, highly formable steels are required for auto body pressings of complex geometry, and for tinplate beverage cans. Vacuum degassed, interstitial free steels are used here. Ti is added at typically ten times the (low) carbon content of the steel, to form TiC and TiN, so freeing the steel of dissolved carbon and nitrogen. The resulting steel can have carbon levels <30 ppm and nitrogen <40 ppm, giving a low yield strength and good formability.
High strength low alloy (HSLA) steels rely on a combination of precipitation of carbides and nitrides, and grain refining for their strengthening mechanism. Columbium (niobium) and vanadium are the principal elements for this, but titanium is also used, especially for the precipitation of TiCN, following controlled rolling and rapid cooling. TiCN is the only micro-alloy carbo-nitride that is stable at the high temperatures attained in the HAZ during welding, where it reduces grain growth and increases toughness. Ti also forms its nitride at very high temperatures and is therefore used to reduce grain growth of austenite during hot rolling of plates.
Titanium is used to protect boron in hardenability steels. The titanium is added before the boron, to tie up any oxygen and nitrogen, so improving the effectiveness of the boron addition (see Boron).
In low alloy steels that have been grain refined with aluminum, AlN can cause intergranular fracture, known as "panel cracking". An addition of Ti causes TiN to be precipitated uniformly in the matrix and increases ductility. A Ti content of 0.05% would be typical for this application.
The precipitation of titanium intermetallic compounds is one of the principal strengthening mechanisms in maraging steels, which may attain yield strengths in excess of 3450 MPa (500 ksi).
Enameling steels require an addition of typically 0.15% Ti to control the surface roughness, to allow a good, even coating with the enamel.
Titanium is occasionally used in tool steels, which it can make less susceptible to quench cracking. Because of their reduced air-hardening tendencies, such steels will develop tougher core structures.
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