Heat Transfer in Spray zones below the mold

Below the mold, heat flux from the strand surface varies greatly between each pair of support
rolls according to spray nozzle cooling (based on water flux), hspray; radiation, hrad_spray; natural
convection, hconv; and heat conduction to the rolls, hroll, as shown in Fig. Incorporating these
phenomena enables the model to simulate heat transfer during the entire continuous casting
process. The heat extraction due to water sprays is a function of water flow, of the following
form:
 spray water  h(spray) = A⋅Qwater ⋅(1 − b⋅Tspray)

where Qwater (l/m2s) is water flux in spray zones, Tspray is the temperature of the spray cooling
water. In Nozaki’s empirical correlation, A=0.3925, c=0.55, b=0.0075, which has been used
successfully by other modelers.
Radiation is calculated by:

hspray =σ ⋅ε steel(Tsk +Tambk) (Tsk +T2 spray) 

where TsK and TsprayK are Ts and Tspray expressed in Kelvin. Natural convection is treated as a
constant input for every spray zone. For water-cooling only, it is not very important, so was
simplified to 8.7W/m2K everywhere. Larger values can be input for hconv to reflect the stronger
convection when there is air mist in the cooling zone. Heat extraction into the rolls is calculated
based on the fraction of heat extraction to the rolls, froll, which is calibrated for each spray zone:

hroll = [(hrad_spray+hconv + hspray)*Lspray +(hrad_spray + hconv)*(Lsparay_pitch-Lspray-Lrollcontact)/Lroll contact*(1-froll)]*froll

A typical froll value of 0.05 produces local temperature drops beneath the rolls of about 100oC.
Beyond the spray zones, heat transfer simplifies to radiation and natural convection.

Higher C in Stainless Steel

In pure iron, the A₄ (1394 °C) and A₃ (912 °C) transformations take place at constant temperatures. If an element enters into solid solution in iron — forming in that way a binary alloy — each of these transformations are required by the Phase Rule to occur over a range of temperature.

Some elements, such as chromium, lower the A₄ and raise the A₃ transformation temperatures, restricting the gamma loop (γ loop) in the iron-carbon phase diagram. As the binary iron-chromium phase diagram shows, the presence of chromium restricts the gamma loop (Figure 1).

Notice that above approximately 13 wt. % Cr, the binary Fe-Cr alloys are ferritic over the whole temperature range. A narrow (α + γ) range that exists between approximately 12 wt. % Cr and 13 wt. % Cr is also worth noting.

The addition of carbon to the Fe-Cr binary system widens the (α + γ) field and extends the gamma-loop to higher chromium contents (see, for example, the Fe-Cr-C ternary system at 1000 °C and 1100 °C).

 

FigureFe-Cr phase diagram shows which phases are to be expected at equilibrium for different combinations of chromium content and temperature. The Fe-Cr phase diagram was calculated with Thermo-Calc, coupled with PBIN thermodynamic database. The melting point of iron and chromium at the pressure of 101325 Pa is 1538 °C and 1907 °C, respectively.

The sigma (σ) phase, which is an intermetallic FeCr compound, can sometimes form in Fe-Cr alloys, such as AISI 316 or AISI 310 stainless steels. The harmful effects of the sigma phase on mechanical properties (e.g., ductility) and corrosion resistance are well documented.

Stainless Steel

Adding chromium to low carbon steel, gives it stain resistance.In addition to iron, carbon, and chromium, modern stainless steel may also contain other elements, such as nickel, niobium, molybdenum, and titanium. Nickel, molybdenum, niobium, and chromium enhance the corrosion resistance of stainless steel.It is the addition of a minimum of 12% chromium to the steel that makes it resist rust, or stain 'less' than other types of steel. The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome-containing oxide, called the passive film. The sizes of chromium atoms and their oxides are similar, so they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. If the metal is cut or scratched and the passive film is disrupted, more oxide will quickly form and recover the exposed surface, protecting it from oxidative corrosion. Iron, on the other hand, rusts quickly because atomic iron is much smaller than its oxide, so the oxide forms a loose rather than tightly-packed layer and flakes away.

The passive film requires oxygen to self-repair, so stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments. In seawater, chlorides from the salt will attack and destroy the passive film more quickly than it can be repaired in a low oxygen environment. 

Types of Stainless Steel :


The three main types of stainless steels are austenitic, ferritic, and martensitic. These three types of steels are identified by their microstructure or predominant crystal phase.

Austenitic:
Austenitic steels have austenite as their primary phase (face centered cubic crystal). These are alloys containing chromium and nickel (sometimes manganese and nitrogen), structured around the Type 302 composition of iron, 18-20% chromium, and 8-10% nickel.


Ferritic:
Ferritic steels have ferrite (body centered cubic crystal) as their main phase. These steels contain iron and chromium, based on the Type 430 composition of 17% chromium. Ferritic steel is less ductile than austenitic steel and is not hardenable by heat treatment.

Martensitic:
Martensitic steels are low carbon steels built around the Type 410 composition of iron, 12% chromium, and 0.12% carbon.