**5. Niobium-micro-alloyed steels**

Niobium is a soft gray ductile and transition element. The main commercial source of niobium is mineral pyrochlore. Around 80% of the niobium produced is used in automotive industry, for oil and gas pipelines, and in construction. Adding niobium to steels causes the formation of niobium carbide and niobium nitride which improve grain refinement and retardation of recrystallization. Besides, it enhances precipitation hardening which increases toughness, strength, formability, and weldability of micro-alloyed steel [4].

Large-diameter pipes are used in transportation of oil and gas. It is manufactured by thermomechanical controlled processing (TMCP) [5]. Its performance can be enhanced by inducing its strength and toughness through grain refinements. Grain refinement can be done by controlling austenite parameters by the addition of niobium.

In austenitic-ferritic stainless steel, usually solidification starts at 1450°C with the formation of ferrite (α) which acts as an origin to start the formation of austenite near 1300°C. σ forms at the interphase of austenite and ferrite at 600–950°C, and the toughness of the steel gets reduced.

**Figures 1**–**6** show the microstructural characteristics of the austenitic-ferritic stainless steel with or without niobium, after heat treatment.

**Figure 5(a)** shows the heat-treated steels without niobium with elongated austenitic grains in ferrite matrix. When the annealed sample is aged at 850°C/15 min then it is observed that the beginning of sigma phase forms.

When steel is modified with 0.2% niobium, then a little amount of sigma phase is observed than steel without niobium after being annealed and aged at 850°C/15 min. Besides no Laves phase is seen (**Figure 7**).

When steel is modified with 0.5% niobium after being annealed and aged at 850°C/min, then the Laves phase appears as needles associated with sigma phase.

In the aggressive environments, the preferential attack prone to the reduction of Cr and Mo near and alongside of the sigma phases. That is the reason for the reduction on pitting corrosion resistance in the steels.

The addition of niobium in supermartensitic stainless steel after tempering at 600°C for 2 h improves the mechanical resistance properties with lower degree of sensitization. Besides, given such properties, it never compromises its elongation and pitting corrosion resistance compared to the reference steels.

#### **Figure 4.**

*Range of austenite in chromium steels (reprinted from Ref. [6] with permission).*

#### **Figure 5.**

*Annealed steel without niobium (BSE after Behara etching) with (a) Austenite grains in ferrite matrix and (b) beginning of the sigma phase (adapted from Ref. [7], p. 802).*

**45**

**Figure 8.**

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

**6. Titanium steels**

**Figure 7.**

improve its properties to some extent (**Figure 8**).

*Steel modified with 0.5% niobium (adapted from Ref. [7], p. 803).*

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

*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]).*

**Figure 6.** *Steels modified with 0.2% niobium (adapted from Ref. [7], p. 803).*

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

*Engineering Steels and High Entropy-Alloys*

*Range of austenite in chromium steels (reprinted from Ref. [6] with permission).*

*Annealed steel without niobium (BSE after Behara etching) with (a) Austenite grains in ferrite matrix and* 

**44**

**Figure 6.**

**Figure 4.**

**Figure 5.**

*Steels modified with 0.2% niobium (adapted from Ref. [7], p. 803).*

*(b) beginning of the sigma phase (adapted from Ref. [7], p. 802).*

**Figure 7.** *Steel modified with 0.5% niobium (adapted from Ref. [7], p. 803).*
