**3. Effect of chemical composition**

High-strength steel requires tensile properties as main requirement, whereas the requirements such as weldability and ductility are also of chief importance. Therefore, carbon which is the chief source of strength should not exceed very high values, and hence high-strength steel requires addition of alloying elements.

The addition of microalloying elements can be divided into two categories [6]:


## **3.1 Addition of niobium, titanium, and vanadium**

Microalloying elements such as niobium, titanium, and vanadium are principally carbide-forming elements. Although the addition of these elements in steel raises its Ar3 temperature, they retard austenite transformation to ferrite by restricting carbon diffusion. Strengthening by addition of one or all of niobium, vanadium, or titanium has shown a remarkable increase in strength of steel. The strengthening phenomenon is caused by fine precipitation of nitrides, carbides, or carbonitrides which are coherent with ferrite matrix but induce strengthening by impeding dislocation movement.

recrystallization will be the first phenomenon. As a basic principle of controlled rolling demands that precipitation should occur during finish rolling, it has been recommended to have higher roughing temperatures along with short rougher

The gamma grain is refined by repeated static recrystallization caused during roughing mill action. Increasing rolling strain has marked effects on facilitating static recrystallization caused by higher dislocation density and increased nucleation sites caused by fine size of austenite which in turn leads to softening of

As has been elucidated above, no-recrystallization temperature (TNR) is important in design of controlled rolling process. This temperature determines where strain is multiplied for austenite grains, leading to strain-induced precipitation of carbonitrides as well as enhanced sites for a fine-grain size ferrite to be nucleated at the sites. Hot rolling being a dynamic process, no-recrystallization temperature depends upon deformation parameters. The influencing factors for TNR are composition of the steel, strain values applied in each pass, the strain rate, and the rolling

During finish rolling the value of TNR tends to dynamically lower down as the strain value or the reduction increases. This phenomenon is attributable on account of static recrystallization caused by increased recrystallization sites owing to finer

The strain rate value is also a determining factor for the onset of dynamic recovery and facilitates static recrystallization which eventually decreases the TNR. During controlled rolling, the interpass time during each rolling reduction also plays an important role as the prime requirement is to roll below TNR temperatures. The precipitation kinetics are accelerated due to strains induced when rolling below TNR. A lower interpass time is preferable as higher interpass will lead to coarsening of precipitate sizes as well as increased tendency of recrystallization detrimental to

The runout table and the coiler in general act like post heat treatment unit which makes possible to achieve phase transformation through control of cooling to gen-

Accelerated cooling after hot rolling leads to further refinement of grains and

The final mechanical properties after accelerated cooling are majorly influenced

phase control, leading to enhancement of properties. The phenomenon for strengthening of microstructure is phase transformations in terms of microstructures avoiding pearlitic transformations, precipitation strengthening through carbides, and nitride precipitates which along with controlled cooling rates lead to the refinement of grain size in the resulting microstructure. The accelerated cooling may be classified into two techniques—continuous accelerated cooling and

grains and higher dislocation density induced during each rolling pass.

erate coils with varied properties and microstructures.

by the alloying content and hot rolling parameters.

material. However, there is a limiting value of grain refinement.

*Grain Boundary Effects on Mechanical Properties: Design Approaches in Steel*

*DOI: http://dx.doi.org/10.5772/intechopen.88564*

**4.3 Finish rolling at no-recrystallization temperatures**

interpass intervals.

interpass time [7, 8].

final strength value of steel.

interrupted accelerated cooling.

**229**

**4.4 Accelerated cooling**

One of the most significant effect of adding individually or simultaneous addition of V, Nb, and Ti is to decrease recrystallization temperature. Which contributes in generating a finer size of gamma (austenite) grains during finish rolling.

The two principal mechanisms that inhibit recrystallization and eventually grain growth are particle pinning and solute drag.

The grain boundary movement can be accounted on strain-induced precipitation of micro carbides on gamma grain boundaries that limit the gamma grain size. Addition of titanium or niobium helps in suppressing gamma grain growth by means of nitride or carbonitride precipitates which are majorly present at grain boundaries and inhibit their movement.

In the case of vanadium addition, addition of nitrogen can be helpful in increasing the strength and toughness. The vanadium nitride precipitates are useful in imparting strength to the steel. The addition of nitrogen however attributes to poor weldability. Likewise, the strengthening may be achieved by adding niobium, but a higher niobium content is bound to give poor weldability. Hence the conventional methods require simultaneous addition of V and Nb.

#### **3.2 Manganese-based strengthening**

The improvement of toughness can be achieved through addition of manganese that leads to decrease in Ar3 temperature. Due to decrease in Ar3 coupled with low coiling temperatures, the alpha (ferrite) grains are refined, thus increasing the strength. Additionally, the fine precipitate size is contributed by niobium carbonitrides and vanadium nitrides.

### **4. Effect of controlled hot rolling parameters**

#### **4.1 Reheating temperatures at reheating furnace**

In general, austenitic grains starts to recrystallize at temperatures above 1050°C. Since an initial finer gamma grain size is helpful in creating a final finer size of alpha, lower reheating temperatures are effective. Also, the microalloying elements also add to refinement of the austenitic grain size by means of undissolved carbides and nitrides that restrict initial austenitic grain size.

Eventually, a lower slab reheating temperature by contributing fine austenitic grain size and lower temperature rolling at roughing mill will induce even finer grain size by reduction at lower temperatures.

#### **4.2 Repeated recrystallization in roughing mill**

Due to repeated reduction in roughing mill, both recrystallization and precipitation are competing phenomena. However, at higher temperatures

recrystallization will be the first phenomenon. As a basic principle of controlled rolling demands that precipitation should occur during finish rolling, it has been recommended to have higher roughing temperatures along with short rougher interpass intervals.

The gamma grain is refined by repeated static recrystallization caused during roughing mill action. Increasing rolling strain has marked effects on facilitating static recrystallization caused by higher dislocation density and increased nucleation sites caused by fine size of austenite which in turn leads to softening of material. However, there is a limiting value of grain refinement.
