Melting, Casting, and Hot Processing

Melting, Casting, and Hot Processing

Summary

THE PRIMARY PRODUCTION PROC- ESSES of melting, casting, and hot processing are invisible to the end user. The vast majority of stainless steel is made by arc furnace melting fol- lowed by argon oxygen decarburization (AOD) refining and continuous casting. It is not normal, and it is seldom beneficial for the end user to specify processing paths. The end user should, however, be knowledgeable and require the pro- ducer to document the process and the producer’s control of it.

Introduction

The manner in which stainless steel is made at the producing mill can have a great impact on its final properties. These production methods have undergone a major evolution over the last 50 years and are mainly responsible for stain- less steels becoming the practical, widespread engineering materials they are today. Traditional carbon and alloy steel-making methods are not suitable for stainless steels. The fundamental difference is that the basic decarburization step, which is common to all steel making, is thermo- dynamically very difficult in stainless steel be- cause the essential element, chromium, reacts more strongly with the purifying agent, oxygen, than does carbon. Thus, early stainless steel mak- ing, done in an arc furnace, was a lengthy process that necessarily involved high chromium losses to the slag as carbon was removed. This process was not only very expensive, the carbon levels that could be achieved were not much below 0.10%, making most of today’s stainless steels, whose carbon levels range from 0.010% in sta- bilized ferritic alloys to about 0.07% in normal

austenitic alloys, impossible to produce. The advent of AOD, continuous casting, ladle metal- lurgy, and powerful hot rolling mills has led to stainless steels of much higher quality produced at lower cost. Ironically, the low processing cost of stainless steel has spurred demand and made some of its ingredients, such as molybdenum and nickel, which are relatively scarce and ex- pensive commodities, even more costly, forcing the cost of many alloys to spike even higher than in earlier years.

Melting and Refining

The arc furnace is nearly universally used for the first step in the production of stainless steel. The arc furnace is quite flexible in the types of charge materials it can accept. Since the charge materials for stainless steel are typically carbon steel and stainless steel scrap, this flexibility al- lows scrap of all types to be used. The necessary chromium is added as ferrochromium, whose cost is inversely related to its carbon content. The carbon content of the heat of steel is roughly 1.5 to 2.5% when it is melted and ready to charge into the separate refining vessel.

It is this carbon whose removal is the primary focus of refining. In the 1960s, Union Carbide engineers perfected a method, the previously mentioned AOD process, of removing nearly all the carbon from molten stainless steel without significant loss of chromium. This process is based on the following chemical reaction: Cr₃O₄(Solid) + yC = yCO (gas) + Cr (Eq 1)

The equilibrium for this reaction is:

(Eq 2) Ln (K G T ) 4575 = −Δ

where K is the equilibrium constant, and G is the Gibbs free energy.

Working through the thermodynamics yields the relationship that summarizes the important relationship among carbon, chromium, and CO (Ref 1):

(Eq 3) Thus, increasing the temperature works to in- crease the elimination of carbon as CO, which evolves from the melt. This is similar in principle to the basic oxygen furnace (BOF) process for carbon steel in which oxygen is injected into molten steel to remove carbon by oxidizing it. The key to the AOD process, though, is the in- jection of oxygen and argon into the bath to keep the partial pressure of CO (p_CO) very low. This is done at a temperature consistent with economic refractory life. The injection is done through tubes called tuyeres in the bottom of the barrel- shaped vessel. The injection and the reaction cause extremely thorough mixing, which would never happen in the flat, stagnant, arc furnace bath. This mixing not only allows the CO-pro- ducing reaction to reach equilibrium, but also the mixing of the slag and metal also permits desulfurization. By increasing the ratio of argon to oxygen in the injected gas as the refining pro- ceeds, the carbon is selectively oxidized with- out concurrent chromium oxidation. A typical starting ratio is 3 to 1 oxygen to argon/nitrogen by volume. The ending ratio can be as low as 1 to 9, oxygen to argon/nitrogen. The choice of which inert gas to use, argon or nitrogen, is based on cost and final nitrogen content desired. Stabi- lized stainless steels require low carbon and ni- trogen levels, for instance, so the more expensive argon must be used.

It is possible to use a vacuum system to keep the partial pressure of CO low when refining with injected oxygen. This is the vacuum oxy- gen decarburization (VOD) process. The VOD process can achieve slightly lower carbon levels but does not achieve cleaner steel as some believe.

In both processes, after final carbon content has been achieved ferrosilicon is added to reduce the chromium in the slag and have it return to the molten steel. The excellent mixing of the slag and metal in the AOD permits this to be done efficiently. The silicon plus the manganese in the

steel combine to reduce the oxygen content of the steel to around 100 ppm. This could be fur- ther reduced by aluminum, but aluminum-based inclusions are generally undesirable. The ther- modynamic activity of aluminum is consider- ably reduced in iron as chromium levels in- crease, so its role as a deoxidizer is less valuable in stainless steels. Titanium, on the other hand, is enhanced as a deoxidizer in chromium-iron al- loys, and consequently small amounts of it are sometimes used as a supplementary deoxidant in alloys even though an alloy specification may not call for any. Titanium is believed to reduce hot working defects. More active deoxidants, such as calcium and magnesium, can be used when required. Also note that even if no inten- tional addition of metallic calcium is made, strong deoxidation with aluminum or titanium can reduce small amounts of calcium from the CaO in the slag, producing measurable calcium content in the metal.

Besides carbon and oxygen, other impurities can be removed from the molten stainless. Once the steel has been deoxidized, sulfur can be readily removed by contact with a basic slag. Sulfur can be reduced to less than 0.001% in the AOD, and this excellent purity level is com- mercially furnished without additional price premium. Sulfur, although a harmful impurity from a corrosion standpoint, is often deliber- ately kept at moderate levels (0.008 to 0.015%) for tungsten inert gas (TIG) welding penetration (see Chapter 17) and at high levels (0.15%+) for machinability (see Chapter 15). These trade-offs, which are beneficial to processors, should be viewed with skepticism by end users, whose product integrity is compromised. There are processing methods for which higher levels of sulfur are not necessary that are preferable to the end user while not compromising welding or machining costs. For example, machinability can be improved by calcium additions that pro- duce malleable oxides to replace the deleterious sulfides (see Chapter 15), and welding methods, such as laser welding, can be used in many cases to eliminate the need for the weld penetra- tion enhancement of sulfur while increasing welding speeds.

Phosphorus is an impurity for which no prac- tical removal technology exists in stainless steel. Any known process to remove it first re- moves chromium. Thus, it exists in almost all stainless steel at levels close to its normal speci- fication limit, about 0.030% in austenitic alloys and 0.020% or less in ferritic alloys, which are

Log Cr %C 13,800 CO =% = − + . − . log T 8 76 0 925 p

made from a higher percentage of low-phospho- rus carbon steel scrap. The deleterious effects of phosphorus on corrosion are not avoided unless much lower levels are achieved. Consequently, its presence is tolerated since it has no differen- tial effect over the range in which it is found.

Heavy metals are eliminated by high-temper- ature AOD blowing, as is hydrogen. Care must be taken not to reintroduce such impurities after refining, which is a risk when using damp or contaminated scrap for coolant.

Alloy adjustment can be done in the AOD or preferably in a treatment-and-transfer ladle. The tapped molten steel generally has excess heat from the highly exothermic refining process. This allows the composition to be measured and adjusted before it must be cast. This can be done very precisely by wire feeding of alloying ele- ments through the slag into the heat, which can be stirred by argon bubbling via porous plugs. This technique is very effective for the fine- tuning of reactive elements such as titanium.

The refining treatments used for carbon steel and stainless steel are very similar, but there are subtle differences because of the difference in the thermodynamics of dilute solutions like car- bon steel and highly alloyed, nondilute solutions like stainless steel. Table 1 shows the factors by which additions of various elements to stainless steel (j) alter the thermodynamic activity of other alloying elements (i).

Equation 4 is used to calculate the activity of elements in steel. The activity coefficient γ varies with the concentration of alloying ele- ment x by:

(Eq 4)

This calculation is best left to computer pro- grams such as Thermo-Calc that have been per- fected for these lengthy procedures. It should be noted that chromium, which is always pres- ent in nondilute quantities, has a powerful effect

on interstitial solubility. The higher solubility of carbon, nitrogen, and oxygen in stainless steels is significant. A manganese/silicon deox- idized stainless steel will still have about 100 ppm of dissolved oxygen at the freezing tem- perature as opposed to the less than 10 ppm of oxygen found in aluminum-killed carbon steel. This oxygen precipitates as oxides in the solid state.

Vacuum induction melting (VIM) is another method of melting stainless steels. This is a nearly slag-free process, and little refining is possible. Melt purity is largely controlled by the purity of the starting material, and use of AOD master melt stock for VIM remelting is com- mon. Limited decarburization is possible via in- jection of oxides such as Fe₃O₄or SiO₂to create CO evolution inside the vessel. Using this tech- nique, very low carbon levels (less than 50 ppm) are achievable commercially. Use of VIM is generally limited to high-value, high-purity, or low-tonnage melts.

Remelting

Some stainless steels and related alloys are remelted to refine composition or ingot struc- ture. There are two principal remelt processes: vacuum arc remelting (VAR) and electroslag remelting (ESR).

In VAR, the material to be remelted is cast into a cylindrical electrode and placed inside a cylindrical water-cooled vacuum chamber. A high-current direct current (dc) arc is estab- lished between the electrode and a starter plate at the bottom of the chamber. The end of the electrode is melted, and the molten drops fall through the intervening vacuum. Volatile con- stituents escape from the molten drops, and the purified drops collect to form a molten pool on top of the starter plate. VAR parameters are ad- justed to maintain a shallow pool, which solidi- fies in a bottom-up fashion. The shallowness of the molten pool produces a refined grain

RT RT RT x i i j n n i j lnγ lnγ δ γ δ = + =

∑

Table 1 Influence of alloying elements on the thermodynamic activity of carbon, nitrogen, sulfur, and oxygen

J i Al C Cr Mn Mo N Ni O S Si Ti W O .04 .14 –.02 –.01 –.01 .11 .01 –.34 .05 .08 . . . –.005 N –.03 .13 –.05 –.02 –.01 0.0 .01 .05 .01 .05 –.53 –.001 S . . . .11 –.01 –.03 .003 .01 0.0 –.27 –.03 .06 –.07 .01 0 –.39 –.45 –.04 –.02 .003 .06 .006 –.20 –.13 –.13 –.6 –.01

structure with less solidification segregation than found in typical cast product.

In ESR, the material to be remelted is cast into an electrode of similar shape, but slightly smaller than the water-cooled mold. A gap be- tween the electrode and a starter plate at the bot- tom of the mold is filled with a prepared slag. Typically, this slag is calcium fluoride-based with high lime (CaO) content. Additional ingre- dients control the basicity, fluidity, oxidizing potential, and other properties of the slag. A high current is used to melt the slag, which in turn melts the end of the electrode, and the molten drops fall through the slag. Reaction of the molten drops with the slag removes sulfur and some other impurities, and the purified drops collect to form a molten pool on top of the starter plate. ESR melting typically is done at a higher rate than VAR, and the molten pool is deeper. This deeper pool produces a grain struc- ture between that of VAR and typical cast prod- uct, with commensurate intermediate segrega- tion patterns.

Casting

Continuous slab, billet, and bloom casting have become the standard methods of making stainless steel primary products, replacing the obsolete ingot method. There are some alloys that cannot be continuously cast, but these repre- sent a miniscule percentage of stainless produc- tion. Continuous casting produces slabs directly, thus removing the costly soaking and slab- rolling processes. In a well-executed continuous casting operation, slabs are of sufficient quality that they require no surface conditioning before being hot rolled. Slabs range in thickness from 13 to 63 cm (5 to 15 in.). The segregation in con- tinuous casters is less than in ingots because of the smaller section size. It is not eliminated, however, and certain alloying elements concen- trate at the centerline, where they defy homoge- nization. Carbon and molybdenum are examples of alloying elements with this tendency.

In properly executed continuous casting, the ladle feeds by a slide gate, or preferably a stop- per rod gate, into a ceramic tube into the large tundish situated over the caster mold. The metal in the tundish is covered with a protective slag cover, and flow patterns within the tundish are designed to minimize dead spots and encourage removal of inclusions by impingement with the slag cover. The metal feeds through another

ceramic tube, called the submerged entry

nozzle, into the mold, which is covered with a

consumable protective and lubricating slag cover, called a mold powder. The mold powder, which melts in the mold as it is added, contains ceramics, fluxes, and carbon. The level of the molten metal should be carefully controlled by ultrasonic measurement, or other methods, to prevent fluctuations in level that may entrap slag in the slab surface. The entire water-cooled, copper alloy mold oscillates in a precise pattern as the solidifying strand of steel is withdrawn from the mold bottom by pinch rolls and sprayed with water to cool it. The pinch rolls apply enough pressure to slightly deform the slab. This deformation has a crucial, seldom-rec- ognized effect. It causes a beneficial recrystal- lization that improves hot working characteris- tics of austenitic and duplex alloys. In ferritic alloys, it can cause excessive grain growth, which detracts from hot workability. The initial portion of slab cast in a sequence is seldom of adequate quality to be used because of exogenous inclusions, entrapped mold powder, and non-steady-state solidification structure. The defective portion must be identified and scrapped or diverted to low-quality requirement end uses.

The strand is bent from an initial slightly curved shape to flat and cut into slabs. More than one heat of steel may be cast sequentially without restarting the process. This is ideal eco- nomically and for quality reasons since initial and final segments of a casting can contain more inclusions and aberrant structure. Some end users stipulate that no first slabs be applied to their orders. Producers generally apply first slabs to less-critical uses or discard suspect sec- tions of them. If casting conditions are not opti- mal, the result can be slabs with poor surface quality that must be surface ground.

Slabs are sometimes quenched to avoid pre- cipitation of phases; however, they may be held at high enough temperatures prior to hot rolling to stay above the temperature range in which embrittlement can occur or to stay above the temperature at which an embrittled slab can fracture. Ferritic and martensitic alloys are es- pecially prone to these problems.

There has been great interest for decades in producing stainless steel coils directly from the melt in so-called strip casters. Elimination of hot rolling could be quite valuable in stain- less steel, whose hot rolling from slab can be both expensive and problematic. There are a

number of such machines in pilot or limited production. They have not had sufficient com- mercial or technical success to have become a factor in the industry. Since their development is only being undertaken by those large stain- less steel producers who already have the hot rolling assets that strip casting would replace, it seems unlikely that strip casting will soon become a major factor even if it is perfected technically.

Another method of shortcutting the casting/ ingot step has been perfected: the powder met- allurgy approach. In powder metallurgy, the re- fined molten metal is atomized by gas or liquid and made to freeze into small particles. These particles, having been quenched extremely rapidly, are quite homogeneous. Powder tech- nology methods allow for the design of alloys that would otherwise freeze with too much segregation and too coarse a structure with conventional production methods. Traditional powder metallurgy production methods are used to make small near-net shape compo- nents, avoiding most of the costly machining steps. More impressively, powder technology is also used to produce massive components. For example, very high carbon/vanadium stainless tool steel components can be made by encapsulating powder in an evacuated canister in which it can be sintered and hot worked to 100% density and virtually complete homo- geneity. Chapter 9 on martensitic alloys dis- cusses these materials.

Hot Rolling

Hot rolling remains an essential process for the vast majority of stainless steel used. Hot rolling characteristics of stainless steels vary greatly. Ferritic stainless steels are extremely easy to hot roll since they have a soft, single- phase structure at hot rolling temperatures. Martensitic stainless steels roll like their carbon and alloy steel counterparts since their mi- crostructure during hot rolling is a moderately alloyed austenite similar to alloy steels. The mi- crostructure during hot rolling is the crucial fac- tor. Austenitic stainless steels have high strength at hot rolling temperatures. Furthermore, the low diffusion rates in austenite slow recrystal- lization so that the steel does not always soften between stands in tandem mills. This increases mill loads, and lower reductions must be taken than for alloy steels. Powerful hot strip tandem

mills that routinely roll carbon steel to 1.5 mm (0.06 in.) can struggle to attain 4.5-mm (0.18-in.) thickness for 316 stainless.