IMPINGEMENT COOLING

FIELD OF THE INVENTION

(001) The present invention relates to manufacturing of metal strips and wires, primarily to stainless steel metal strips and wires.

BACKGROUND OF THE INVENTION

(002) Stainless steel is used in a wide range of applications because of its resistance to staining and rusting, attributable to the chromium content, usually from 12 to 20 percent of the stainless steel alloy. There are numerous alloys that are included in the stainless steels family that are used in a large number of applications and industries, including bulk material handling equipment, building exteriors, roofing, automobile components, chemical process plants, pulp and paper manufacturing, refining, water supply, consumer products, sporting goods, shipbuilding, transportation, etc.

(003) Several types of stainless steel exist for various uses depending on the microstructure of the alloy. Austenitic stainless steels exhibit good corrosion resistance and high ductility. Ferritic stainless steels have high resistance to stress corrosion but are difficult to weld. Martensitic stainless steels contain iron with a needle-like structure. Duplex stainless steels provide good resistance to pitting and crevice corrosion as well as resistance to cracking.

(004) The manufacture of stainless steel involves a series of processes. The raw materials to make the desired stainless steel are first melted and then cast into one of several shapes, such as blooms (rectangular), billets (round or square), slabs, rods or tubes. Further forming steps can be carried out and then the stainless steel is heat treated followed by cleaning and polishing. The stainless steel is then ready for shipment to end users.

(005) The forming operations begin with hot rolling wherein the steel is heated and rolled. Blooms and billets are generally processed into bar or wire, while slabs become plate, strip, or sheet. Once the initial forming has been completed, annealing is carried out. Annealing comprises heating and cooling the steel under controlled conditions to reduce internal stresses and to achieve the desired characteristics for higher strength.

(006) There are several known methods for annealing of stainless steel. For example, the stainless steel may be annealed in air followed by pickling, the use of a nitric- hydrofluoric acid bath to remove scaling. A further example is the use of transverse flux induction heating to anneal the stainless steel followed by cooling in air or water.

Pickling is again necessary to remove oxide layers formed during the annealing stage. Another example is bright annealing in Radiant Tube Furnaces (RTF) or Electrically Heated Furnaces (ERH) in a reducing atmosphere, used mainly a finishing stage for thin strips. One known method is the induction heating of stainless steel pipes in a controlled atmosphere (such as hydrogen) following be contact cooling with solid graphite blocks in a hydrogen filled encapsulation (from Oppermann Rohrtechnik GmbH).

(007) An example of an induction heating process is shown in Figure 1, wherein the system 100, includes an induction heating stage 20, a final heating/soaking stage 30, a cooling stage 40 and an optional quench stage 50. In this system a strip 10 of stainless steel to be processed is feed through the system 100. The strip 10 is first heated in the induction heating stage 20 using induction coil heaters 25. The induction heating is carried out in air. The strip 10 then passes through the final heating/soaking stage 30 which may comprise radiant, electrical or induction heating. The strip 10 is then cooled in the cooling stage 40 by air or water. The optional quench stage 50 quenches the strip 10 in water or air. (008) There remains a need for improvements to stainless steel processing.

SUMMARY OF THE PRESENT INVENTION

(009) The present invention provides improved techniques for treating stainless steel. BRIEF DESCRIPTION OF THE DRAWINGS

(010) Figure 1 is a schematic diagram of a prior art stainless steel annealing process.

(011) Figure 2 is a schematic diagram of a stainless steel annealing process according to the invention.

(012) Figure 3 is a schematic diagram of a stainless steel annealing process according to the invention.

(013) Figure 4 is a schematic diagram of a stainless steel annealing process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(014) The invention provides new stainless steel annealing processes. The invention will be described with reference to the drawing figures. Figure 2 shows an annealing process system 200 similar to that shown in Figure 1. In particular, the system 200 includes an induction heating stage 220, a final heating/soaking stage 230, a cooling stage 240 and an optional quench stage 250. In this system a strip 210 of stainless steel to be processed is feed through the system 200. The strip 210 is first heated in the induction heating stage 220 using induction coil heaters 225. A hydrogen atmosphere is maintained in the induction heating stage 220. The strip 210 then passes through the final heating/soaking stage 230 which may comprise radiant, electrical or induction heating also in a hydrogen atmosphere. The strip 210 is then cooled in the cooling stage 240. According to the invention the cooling may be carried out using cooling gas impingement shown by arrows 245. The gas used may be one or more of hydrogen, nitrogen, argon, helium, water vapor, carbon dioxide, carbon monoxide, and hydrocarbons. The cooling gas may be recycled and reused through the use of a recycling means 260, which may be a heat exchanger, cooler and fan, compressor, etc. The strip 210 may be optionally quenched in quench stage 250 with water or air.

(015) Another embodiment of the invention is shown in Figure 3 which shows an annealing process system 300. In particular, the system 300 includes an induction heating stage 320, a final heating/soaking stage 330, a cooling stage 340 and an optional quench stage 350. In this system a strip 310 of stainless steel to be processed is feed through the system 300. The strip 310 is first heated in the induction heating stage 320 using induction coil heaters 325. A hydrogen atmosphere is maintained in the induction heating stage 320. In this embodiment of the invention, the induction heating stage 320 is encapsulated within a gas tight housing 322. The strip 310 then passes through the final heating/soaking stage 330 which may comprise radiant, electrical or induction heating also in a hydrogen atmosphere. The strip 310 is then cooled in the cooling stage 340. According to the invention the cooling may be carried out using cooling gas impingement shown by arrows 345. The gas used may be one or more of hydrogen, nitrogen, argon, helium, water vapor, carbon dioxide, carbon monoxide, and

hydrocarbons. The cooling gas may be recycled and reused through the use of a recycling means 360, which may be a heat exchanger, cooler and fan, compressor, etc. The strip 310 may be optionally quenched in quench stage 350 with water or air.

(016) A further embodiment of the invention is shown in Figure 4 which shows an annealing process system 400 that includes an induction heating stage 420, a cooling stage 440 and an optional quench stage 450. In this system a strip 410 of stainless steel to be processed is feed through the system 400. The strip 410 is first heated in the induction heating stage 420 using induction coil heaters 425. A hydrogen atmosphere is maintained in the induction heating stage 420. In this embodiment of the invention, the induction heating stage 420 is again encapsulated within a gas tight housing 422. The final heating/soaking stage is not used in this system and the strip 410 passes directly to the cooling stage 440. According to the invention the cooling may be carried out using cooling gas impingement shown by arrows 445. The gas used may be one or more of hydrogen, nitrogen, argon, helium, water vapor, carbon dioxide, carbon monoxide, and hydrocarbons. The cooling gas may be recycled and reused through the use of a recycling means 460, which may be a heat exchanger, cooler and fan, compressor, etc. The strip 410 may be optionally quenched in quench stage 450 with water or air.

(017) The invention provides many advantages. By using induction heating for the annealing of the stainless steel, the need for an extensive pickling process can be eliminated. For example, process steps related to pickling following air annealing that can be eliminated by using induction heating are oxidizing, scale braking, neolytic pickling, rinsing and brushing, acid pickling, rinsing and skin pass rolling. Induction heating is also advantageous as compared to bright annealing because the capex and footprint for equipment related to bright annealing are much greater that those needed for induction heating and the annealing capacity for bright annealing is more limited than that for induction heating.

(018) The invention wherein a reduced atmosphere is maintained in the induction heating stage provides advantages over prior art induction heating in air. In particular, by carrying out the heating in a reducing gas atmosphere according to the invention, the need for subsequent oxidizing, scale braking, neolytic pickling, rinsing and brushing, acid pickling and rinsing can be eliminated. Further, by cooling with impingement of reducing gas additional processing stages can be eliminated. (019) The invention has been describe with reference to stainless steel production, but is equally applicable to other metals, such as steel of all types, titanium and titanium alloys, aluminum and aluminum alloys, and copper and copper alloys. The invention can be used with any type of induction heater, e.g. selenoidal, transverse flux, etc., and can be used for processing of all different types of products, including rolled strips, wire, rods, plates. Etc.

(020) It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the light of the foregoing description, and it is intended that such embodiments and variations likewise be included within the scope of the invention as set out in the appended claims.