**6. Titanium steels**

Titanium alloys are mainly used in the structural materials in the aerospace and chemical industries due to lower density, high strength, and corrosion resistance. Tensile strength/density ratios of titanium alloys are considerably greater than that of steels and Al alloys at ordinary temperature. For these reasons Ti alloys are mostly used in the aerospace industries. And for this Ti is added to the steels to improve its properties to some extent (**Figure 8**).

#### **Figure 8.**

*Comparison of (a) short-time tensile strength and (b) tensile strength/density ratio for titanium alloys, three classes of steel, and 2024-T86 aluminum alloys included for annealed alloys with less than 10% elongation or heat-treated alloys with less than 5% elongation (reprinted from Ref. [8]).*

When Ti is added to the steels, then it improves its high temperature properties as refractory metals [9]. The chemical behavior of Ti always limits its application at moderate to high temperature. At low temperature Ti passives to acids and minerals, but at elevated temperature, Ti oxidizes very fast. Besides, dissolution of hydrogen and nitrogen causes surface hardening.

Pure Ti undergoes allotropic transformations at about 1158 K. Thus if Ti remains in steels at around 1158 K, the properties may vary, because pure Ti at 1158 K transforms from a closely packed hexagonal structure to a body-centered cubic structure. The high temperature BCC of Ti is called alpha phase. On the other hand,

**Figure 9.**

*Phase transformation of titanium with the weight percentage of (a) Cr and (b) Ni stabilizers (reprinted from Ref. [10]).*

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**Figure 10.**

*Phase diagrams of titanium with selected stabilizers (reprinted from Ref. [10]).*

*Phase Transformation in Micro-Alloyed Steels DOI: http://dx.doi.org/10.5772/intechopen.91468*

diffusionless martensitic transformation.

HCP at low temperature forms beta phase. Beta to alpha transformation happens by

Allotropic transformation temperature depends on few alloying elements. Some alloying elements raise the transformation temperature called alpha stabilizers, whereas few lower the transformation temperatures called beta stabilizers.

**Figure 9** shows that carbon, oxygen, and nitrogen are rapidly absorbed by Ti when the metal is hot. All these elements hardened and the solution hardened the alpha Ti. Al has significant solubility over alpha and beta phases. The reason to explain it is that

Elements that lower the transformation temperature are of two classes. One is elements that undergo eutectoid transformations, for example, iron, Cu, N, Co,

Ti has many fold advantages on the steel acting as an alloying element.

#### *Phase Transformation in Micro-Alloyed Steels DOI: http://dx.doi.org/10.5772/intechopen.91468*

*Engineering Steels and High Entropy-Alloys*

and nitrogen causes surface hardening.

When Ti is added to the steels, then it improves its high temperature properties as refractory metals [9]. The chemical behavior of Ti always limits its application at moderate to high temperature. At low temperature Ti passives to acids and minerals, but at elevated temperature, Ti oxidizes very fast. Besides, dissolution of hydrogen

Pure Ti undergoes allotropic transformations at about 1158 K. Thus if Ti remains

*Phase transformation of titanium with the weight percentage of (a) Cr and (b) Ni stabilizers (reprinted from* 

in steels at around 1158 K, the properties may vary, because pure Ti at 1158 K transforms from a closely packed hexagonal structure to a body-centered cubic structure. The high temperature BCC of Ti is called alpha phase. On the other hand,

**46**

**Figure 9.**

*Ref. [10]).*

HCP at low temperature forms beta phase. Beta to alpha transformation happens by diffusionless martensitic transformation.

Allotropic transformation temperature depends on few alloying elements. Some alloying elements raise the transformation temperature called alpha stabilizers, whereas few lower the transformation temperatures called beta stabilizers.

**Figure 9** shows that carbon, oxygen, and nitrogen are rapidly absorbed by Ti when the metal is hot. All these elements hardened and the solution hardened the alpha Ti. Al has significant solubility over alpha and beta phases. The reason to explain it is that Ti has many fold advantages on the steel acting as an alloying element.

Elements that lower the transformation temperature are of two classes. One is elements that undergo eutectoid transformations, for example, iron, Cu, N, Co,

**Figure 10.** *Phase diagrams of titanium with selected stabilizers (reprinted from Ref. [10]).*

Mn, etc. And the other one is those that are isomorphous with beta phase at high temperature and from alpha + beta equilibrium phase at ordinary temperature.

**Figure 10** expresses that Mo, Ta, V, etc. have limited solubility in alpha phases.

The main role of Ti in steel is grain refinement strengthening and precipitation strengthening. The smelting of Ti-micro-alloyed steel should satisfy that most of the Ti dissolves in the molten steel and precipitates in the form of carbide or carbonitride after the subsequent solidification.

The affinity of Ti toward oxygen is less than that of the aluminum toward the oxygen. Besides, Ti has greater affinity than manganese toward oxygen. Thus if the molten steel during smelting is not deoxidized properly, then there is a large amount of titanium oxide.

The high content of nitrogen forms titanium nitride that forms inclusions in molten steels. On the other hand, titanium oxides and nitrides will obstruct the process of continuous casting. During the refining process by pyrometallurgy of Ti-micro-alloyed steel, it is required to remove sulfur, oxygen, and nitrogen. But emphasis should be given on the relationship between Ti, Al, and Ti, which are refractory elements. Compared with Nb or V, in the case of Ti, it is more difficult to control Ti-micro-alloyed steels which attributes to the more type of the secondary phase and wider temperature range of the precipitate.

During smelting Ti2O3 and TiN particles will precipitate in the liquid steel that improves as cast microstructure. During slab cooling process TiN and Ti4S2C2 tens to hundreds of nanometers precipitate in the solid solution plays an important role in controlling the grain growth of austenite during soaking and recrystallization process (**Figure 11**).

During rolling TiC precipitation of TiC with the size below 10mm could result in the significantly precipitation hardening.

As a kind of micro-alloying element, Ti significantly improves the comprehensive properties of steel. However, when compared with niobium and vanadium micro-alloyed technology, Ti has not been used extensively in industry for a long time. Ti-micro-alloyed steel fluctuates largely and production process is not stable. Ti is very reactive and forms TiO and TiS that are very harmful. Formation of these phases consumes a portion of Ti that reduces the volume fraction of TiC precipitation at low temperature but also significantly changes the chemical free

**49**

rolling.

after slow cooling.

and ductility.

*Phase Transformation in Micro-Alloyed Steels DOI: http://dx.doi.org/10.5772/intechopen.91468*

austenite grains during hot rolling.

of steel.

**7. Nickel steels**

austenite gets stable.

**8. Chromium steels**

**9. Manganese steels**

resistance and high temperature properties.

energy of TiC. Precipitation behavior of TiC changes and strengthening effect is greatly affected. Besides, TiC is sensitive to temperature and affects the properties

Besides, high-strength Ti-micro-alloyed steels are the precipitation-hardened ferritic steels. The ferrite grain refinement and TiC precipitation have a good combination in strengthening that plays an important role in obtaining both high strength and high toughness simultaneously for those steels. The ferrite grain refinement depends on the refinement of austenite grain size and on the control of transformation temperature. The refinement of austenite grain size mainly depends on the control of the austenite grain growth before hot rolling and recrystallized

Nickel is the oldest and one of the fundamental alloying elements. It has unlimited solubility in gamma iron and is highly soluble in ferrite. As a result it gives high strength and toughness. Ni lowers the critical temperature of steels and retards the decomposition of austenite. As a result at low temperature or room temperature,

Ni does not form carbide. Besides, it reduces the carbon content of the eutectoid.

Chromium is less expensive than Ni. Chromium is a carbide former and forms (Cr7C3, Cr4C) or complex carbide [(FeCr)3C]. This carbide has high hardness and wear resistance. It has 13% solubility in austenite and unlimited solubility in ferrite. In alloying steels, chromium containing more than 5% improves the corrosion

Manganese is less expensive and mostly acts as deoxidizers. The presence of manganese in alloy steel reduces the prone to the hot shortness. As a result of which,

Besides, the absence of manganese in the steel may form FeS. FeS has low melting temperature. Ehen the steel sample is how rolled then due to the low melting temperature of the FeS it melted first. Thus the few places in steels containing FeS become slippery, and thus the hot-rolled samples may slip during

Mn and Ni both reduce the critical temperature and lowers the amount of carbon in eutectoid. Alloying steels containing more than 10% Mn become austenitic

Hadfield Mn steel is a special type of steel (12% Mn) and has great abrasion resistance. If it is slow-cooled from 1750F, then a large brittle carbide forms surrounding the austenite grain. Ultimately forms the structure with low strength

As a result of which there is high percentage of pearlite forms compared to the equal composition plain carbon steels. Pearlite forms at the lower temperature thus

become finer and tougher than the pearlite in unalloyed steels.

the alloy steel containing manganese can perform the hot work.

**Figure 11.**

*Ti-bearing precipitation (adapted from Ref. [11]).*

#### *Phase Transformation in Micro-Alloyed Steels DOI: http://dx.doi.org/10.5772/intechopen.91468*

energy of TiC. Precipitation behavior of TiC changes and strengthening effect is greatly affected. Besides, TiC is sensitive to temperature and affects the properties of steel.

Besides, high-strength Ti-micro-alloyed steels are the precipitation-hardened ferritic steels. The ferrite grain refinement and TiC precipitation have a good combination in strengthening that plays an important role in obtaining both high strength and high toughness simultaneously for those steels. The ferrite grain refinement depends on the refinement of austenite grain size and on the control of transformation temperature. The refinement of austenite grain size mainly depends on the control of the austenite grain growth before hot rolling and recrystallized austenite grains during hot rolling.
