Continuous Heat Treating of Sheet Steel
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CONTINUOUS HEAT TREATING OF SHEET STEEL N.A. LAZARIDIS, A.G. PREBAN AND P.D. SOUTHWICK Inland Steel Research Laboratories, 3001 East Columbus Drive, East Chicago, IN 46312 USA INTRODUCTION The continuous heat treatment of steel has been a widely used practice throughout the world for a considerable time. However, it is only in the past decade that the enormous potential of continuous annealing was recognized so that it is now possible to produce steels with a much wider variety of properties than is possible by batch annealing. These possibilities have been revealed not only by advances in controlling and configuring continuous annealing lines, but also in up-stream and downstream technology. These advances have made consistency the word which is synonymous with continuously annealed steel. This paper will review the fundamentals of continuous annealing and associated metallurgy, and the effect of processing variables on the product properties. The acceptable variation in processing to give an uniform product will be described. ADVANTAGES OF CONTINUOUS ANNEALING While the advantages of a continuous process over a batch process are well known, the specific advantages of continuous annealing over batch annealing are worth enumerating. 1. Quality - more uniform properties within a coil and between coils - improved steel surface cleanliness 2. Productivity - shorter annealing and overall processing time 3. Products - strength levels from 140 MPa to 1540 MPa yield strength are -possible
TYPES OF CONTINUOUS ANNEALING LINES (CAL) The concept of continuous annealing is not new, but the two types of continuous annealing lines now in use were developed in Japan over the last 10 years[1,2]. The major difference between the various lines is in the rate of cooling from the annealing temperature. All modern CALs embody the latest technology in computer process control and operating functions. The Inland line is of the water quench type, the metallurgical principles of which are as follows. 1.
Anneal in the intercritical region of the phase diagram to produce the required grain size and microstructure (typically 760'C-930°C).
2.
Cool down to a predetermined temperature at a predetermined rate to retain as much or as little of the high temperature phases as are required.
3.
Reheat the steel to a specified temperature to remove carbon from solid solution in the correct form to either increase or decrease strength and increase ductility (typically 150°C-4000C).
As a result of the greater cooling rates, the water quench leaves a considerable amount of carbon in supersaturated solid solution[i]. Consequently, upon reheating after the quench, the precipitation rate is great and therefore the overaging time is short. Also lean alloys are capable of producing dual phase steels (ferrite + martensite) by water Mat. Res. Soc.
Syrup.
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quenching which keeps the alloy cost low. The disadvantages of the water cooling process are two-fold. Firstly, increased energy has t
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