1. Killed Steel. The term killed comes from the fact that the steel lies quietly after being poured into the mold. Killed steel is a fully deoxidized steel; that is, oxygen is removed and the associated porosity is thus eliminated. In the deoxidation process, the oxygen dissolved in the molten metal is made to react with elements such as aluminum, silicon, manganese, and vanadium that have been added to the melt. These elements have an affinity for oxygen and form metallic oxides. If aluminum is used, the product is called aluminum-killed steel (see Table 16.4). If they are sufficiently large, the oxide inclusions in the molten bath float out and adhere to, or are dissolved in, the slag. A fully killed steel thus is free of any porosity caused by gases; it also is free of any blowholes (large spherical holes near the surfaces of the ingot). Consequently, the chemical and mechanical properties of a killed-steel ingot are relatively uniform throughout. Because of shrinkage during solidification, however, an ingot of this type develops a pipe at the top (also called a shrinkage cavity); it has the appearance of a funnel-like shape. This pipe can take up a substantial volume of the ingot, and it has to be cut off and scrapped.
2. Semi-killed Steel. Semi-killed steel is a partially deoxidized steel. It contains some porosity (generally in the upper central section of the ingot), but it has little or no pipe. Although the piping in semi-killed steels is less, this advantage is offset by the presence of porosity in that region. Semi-killed steels are economical to produce.
3. Rimmed Steel. In a rimmed steel, which generally has a carbon content of less than 0.15%, the evolved gases are only partially killed (or controlled) by the addition of other elements, such as aluminum. The gases produce blowholes along the outer rim of the ingot—hence the term rimmed. These steels have little or no piping, and they have a ductile skin with good surface finish; however, if not controlled properly, blowholes may break through the skin. Furthermore, impurities and inclusions tend to segregate tow ard the center of the ingot. Products made from this steel may thus be defective, hence thorough inspection is essential.
a) Amorphous alloys exhibit excellent corrosion resistance, good ductility, high strength, and very low magnetic hysteresis (utilized in magnetic steel cores for transformers, generators, motors, lamp ballasts, magnetic amplifiers, and linear accelerators).
b) The most important precious (costly) metals, also called noble metals, are the following: • Gold (Au, from the Latin aurum) is soft and ductile, and has good corrosion resistance at any temperature. Typical applications include jewelry, coinage, reflectors, gold leaf for decorative purposes, dental work, electroplating, and electrical contacts and terminals. • Silver (Ag, from the Latin argentum) is ductile and has the highest electrical and thermal conductivity of any metal (see Table 3.2); however, it develops an oxide film that adversely affects its surface characteristics and appearance. Typical applications for silver include tableware, jewelry, coinage, electroplating, solders, bearing linings, and food and chemical equipment. Sterling silver is an alloy of silver,7.5%copper. • Platinum (Pt) is a soft, ductile, grayish-white metal that has good corrosion resistance, even at elevated temperatures. Platinum alloys are used as electrical contacts; for spark-plug electrodes; as catalysts for automobile pollution control devices; in filaments and nozzles; in dies for extruding glass fibers (Section 18.3.4), in thermocouples; and in jewelry and dental work.
c) Low-melting alloys are so named because of their relatively low melting points. The major metals in this category are lead, zinc, tin, and their alloys.
Lead can be used for radiation shielding, zinc alloys can be used for fuel pumps, Tin alloys can be used in dental applications
d) Major applications of superalloys are in jet engines and gas turbines; other applications are in reciprocating engines, rocket engines, tools and dies for hot working of metals, and in the nuclear, chemical, and petrochemical industries.
There are advantages and disadvantages to each. Rolling at high speed is advantageous in that production rate is increased, but it has disadvantages as well, including:
1. The lubricant film thickness entrained will be larger, which can reduce friction and lead to a slick mill condition where the rolls slip against the workpiece. This can lead to a damaged surface finish on the workpiece.
2. The thicker lubricant film associated with higher speeds can result in significant oil peel, or surface roughening.
3. Because of the higher speed, chatter may occur, compromising the surface quality or process viability.
4. There is a limit to speed associated with the motor and power source that drive the rolls.
Rolling at low speed is advantageous because the surface roughness in the workpiece can match that of the rolls (which can be polished).
However, rolling at too low a speed has consequences such as:
1. Production rate will be low, and thus the cost per unit weight will be higher.
2. Because a thick lubricant film cannot be developed and maintained, there is a danger of transferring material from the workpiece to the roll (pickup), thus compromising surface finish.
3. The workpiece may cool excessively before contacting the rolls. This is because a long billet that is rolled slowly loses some of its heat to the environment and also through conduction through the roller conveyor.
Referring to the tandem rolling operation shown in Fig. 13.12, note that mass continuity has to be maintained during rolling. Thus, if the roll speed is not synchronized with the strip thickness in a particular stand, excessive tensions or slack may develop between the stands; some rolls may slip. Also, if the temperature is not controlled properly, strip thickness will change, thus affecting reduction per pass and, consequently, the roll forces involved. This, in turn, will also affect the actual roll gap and roll deflections. Complex control systems have been developed for monitoring and controlling such operations at high rolling speeds.