Steel processing tecnology (2)

Moderatore: Moderatore Area Tecnica

Steel processing tecnology (2)

Sostienici

Sponsor
 

Steel processing tecnology (2)

Messaggioda Aldebaran » 05/05/2010, 15:35

Cold Rolling
About half of all rolled steel products are sold in the as-hot-rolled condition. This includes such obvious items as rails and
structural sections (I beams, channels, angles, and so on), and plates and seamless tubes, as well as certain relatively thick
grades of sheet and strip that are employed in the as-rolled condition (for example, for the forming of bumpers and car
wheels, or for the manufacture of pipe). Nevertheless, numerous products require much more reduction in thickness or
cross section and considerably better surface quality than can be produced by hot rolling; this is where cold rolling plays a
role. In the cold rolling of flat products, numerous advances have been made in equipment and processes that have
allowed for improved flatness and consistency of gage along the length of a coil. These advances include hydraulically
inflatable rolls for control of the amount of crown, six-high mills with tapered backup rolls that can be inserted and withdrawn laterally as required, as well as a variety of roll-bending techniques .
Cold rolling increases the hardness and yield strength, reduces the ductility and formability, and also introduces the
specific texture components associated with thickness reduction at a constant width (known as plane-strain deformation)
that can be later enhanced by annealing. Some products are sold and used in such a strengthened condition, particularly
when little further deformation or shaping is involved. However, when appreciable further forming operations are to be
carried out, such as in the manufacture of cans or the deep drawing of auto body parts, cold rolling must be followed by
annealing. Annealing removes the work hardening introduced by rolling and thus restores the formability of the material.
When employed after the appropriate processing of selected grades (for example, aluminum-killed drawing quality, also
known as AKDQ, or IF steels), annealing also brings about an increase in the normal anisotropy (R value), which leads to
significant further increases in the deep drawability.
Annealing
Batch Annealing. The annealing of coils weighing 9 to 27 tonnes (10 to 30 tons) is necessarily a very time-consuming
operation. Typical heating and cooling times are of the order of 2 and 3 days, respectively, so that the rate of temperature
change is about 12 °C/h (22 °F/h) during heating and -8 °C/h (-14 °F/h) during cooling. Consequently, a total process
time of nearly 5 days is involved. Because of the considerable capital, maintenance, and energy costs associated with such
heating operations (Fig. 23), the optimization of heating and cooling schedules to improve the productive capacity of
these units is a subject of great interest at present.
The principal mechanism associated with batch annealing is recrystallization, which eliminates the strain hardening
introduced in the previous rolling operations. Once initiated, at temperatures of 600 to 650 °C (1110 to 1200 °F), it takes
only minutes to spread through the material and replace the grain structure flattened by rolling with equiaxed, strain-free
grains. Although precipitate coarsening and particle solution and reprecipitation also occur, they are not of particular
importance for most grades. For AKDQ steels, on the other hand, the characteristics of precipitation and of particle
coarsening must be carefully controlled so that the grain orientations associated with high formability (that is, the
{111}<110> and to a lesser extent the {111}<112> texture components) are favored during recrystallization and can in
this way replace the other sets of grain orientations inherited from the rolling operation. This is brought about by pinning
the undesirable {100}-oriented grains during heating through their optimum growth temperature range by means of AlN
particles. These form during heating from the supersaturated Al and N held in solution as a result of the relatively low
coiling temperatures (550 °C or 1020 °F) employed for the AKDQ grades. The AlN particles gradually redissolved and
coarsen as the temperature is further increased during annealing, permitting growth of the {111}-oriented grains within
the temperature range that favors this orientation.
Continuous Annealing. With the trend toward more and more continuous processing, batch annealing is gradually
being replaced by continuous annealing . The prime advantage of this process is the considerable increase in
product uniformity along the length of a given coil. This is of increasing importance as tolerances and allowable property
variabilities are reduced as a result of the increasing automatization of forming processes. Because of the much shorter
process times involved (for example, 2 min instead of 5 days), the heating and cooling rates are much higher (15 °C/s, or
27 °F/s). Negligible hold times are required at the maximum temperatures of 700 to 800 °C (1290 to 1470 °F), which are
significantly higher than in the batch annealing process.
For commodity grades, the temperatures and heating and cooling rates are not of critical importance. By contrast, for
drawing quality grades such as the niobium- and titanium-base IF steels, the process parameters must again be carefully
adjusted for compatibility with the particular chemistries employed. For these steels, the favored texture components are
{554}<225> and {111}<112> and, to a lesser extent, {111}<110>. These components differ slightly from those
associated with the AKDQ steels and lead to still higher R values. As for the AKDQ grades, the appearance and
disappearance of precipitates (in this case, Nb(C,N) and Ti4C2S2), the mean sizes and volume fractions of these
precipitates, and the amounts of niobium, titanium, carbon, and nitrogen remaining in solution must be carefully
controlled if optimum product properties are to be produced. By tying up the carbon and nitrogen present in the form of
carbonitrides and eliminating the yield drop in this way, low yield stresses are obtained, which lead to high initial workhardening
rates and n values. These, in association with the high R values that follow from the presence of the texture
components described above, are responsible for the excellent drawability properties of these grades.
References cited in this section
14. W.L. Roberts, Hot Rolling of Steel, Marcel Dekker, 1983
15. Microalloying '75, Proceedings of the International Symposium on High Strength Low Alloy Steels, Union
Carbide, 1977
16. F.B. Pickering, Physical Metallurgy and the Design of Steels, Applied Science, 1978
17. S. Yue, F. Boratto, and J.J. Jonas, Designing an Industrial Controlled Rolling Schedule using Simple
Statistical Process Analysis and Laboratory Modelling, in Proceedings of the Conference on Hot and Cold-
Rolled Sheet Steels. R. Pradhan and G. Ludkovsky, Ed., The Metallurgical Society of the American Institute
of Mining, Metallurgical, and Petroleum Engineers, 1988, p 349-359
18. W.J. Liu and J.J. Jonas, Ti(CN) Precipitation in Microalloyed Austenite During Stress Relaxation, Metall.
Trans. A, Vol 19A, 1988, p 1415-1424; Calculation of the Ti(CyN1-y)-Ti4C2S2-MnS-Austenite Equilibrium
in Ti-Bearing Steels, Metall. Trans. A, Vol 20A, 1989, p 1361-1371
19. S. Yue, R. Barbosa, J.J. Jonas, and P.J. Hunt, Manufacture of Seamless Tubing by Means of Recrystallized
Controlled Rolling and Accelerated Cooling, in 30th Mechanical Working and Steel Processing
Conference, Iron and Steel Society of the American Institute of Mining, Metallurgical and Petroleum
Engineers, 1988, p 37-45
20. W. Roberts, in HSLA Steels: Technology and Applications, American Society for Metals, 1984, p 33, 67
21. F.H. Samuel, S. Yue, J.J. Jonas, and K.R. Barnes, Effect of Dynamic Recrystallization of Microstructural
Evolution During Strip Rolling, I.S.I.J. Int., in press
22. L.N. Pussegoda, S. Yue, and J.J. Jonas, Laboratory Stimulation of Seamless Tube Piercing and Rolling
Using Dynamic Recrystallization Schedules, Metall. Trans., in press
23. J.J. Jonas and T. Sakai, A New Approach to Dynamic Recrystallization, in Deformation, Processing and
Structure, G. Krauss, Ed., American Society for Metals 1984 p 185-243
24. W.L. Roberts, Flat Processing of Steel, Marcel Dekker, 1988
25. F.H. Samuel, S. Yue, B. A. Zbinden, and J. J. Jonas, "Recrystallization Characteristics of a Ti-Containing
Interstitial-Free Steel during Hot Rolling," Paper presented at the AIME Symposium on Metallurgy of
Vacuum-Degassed Carbon Steel Products (Indianapolis, IN), American Institute of Mining, Metallurgical,
and Petroleum Engineers, Oct 1989
References
1. R.I.L. Guthrie, Engineering in Process Metallurgy, Oxford Science Publications, Clarendon Press, 1989
2. E.T. Turkdogan, Physical Chemistry of Oxygen Steelmaking, Thermochemistry and Thermodynamics, in
B.O.F. Steelmaking, Vol 2, Theory, Iron and Steel Society, 1975
3. A. Jackson, Oxygen Steelmaking for Steelmakers, 2nd ed., George Newnes Ltd., 1969
4. G. Savarde and R. Lee, French Patent 1,450,718, 1966
5. J.R. Stubbler, The Original Steelmakers, Iron and Steel Society, 1984
6. Y. Sahai and R.I.L. Guthrie, The Formation and Growth of Thermal Accretions in Bottom/Combination
Blown Steelmaking Operations, Iron Steelmaker, April 1984, p 34-38
7. R.L. Reddy, Use of DRI in Steelmaking, in Direct Reduced Iron--Technology and Economics of
Production and Use, Iron and Steel Society, 1980, p 104-118
8. R.I.L. Guthrie, Addition Kinetics in Steelmaking, in Electric Furnace Proceedings, Vol 35, Iron and Steel
Society, 1977, p 30-41
9. R.I.L. Guthrie, The Use of Fluid Mechanics in Ladle Metallurgy, Iron Steelmaker, Vol 9 (No. 10), 1982 p
41-45
10. G.M. Faulring, J.W. Farell, and D.C. Hilty, Steel Flow Through Nozzles: The Influence of Calcium, in
Continuous Casting, Vol 1, L.J. Heaslip, A. McLean, and I.D. Sommerville, Ed., Iron and Steel Institute,
1983, p 57-66; see also p 23-42 for reoxidation inclusions
11. T. Emi and Y. Lida, Impact of Injection Metallurgy on the Quality of Steel Products, In Scaninject III, Part
1, Proceedings of a joint MEFOS/JERNKONTORET Conference (Lulea, Sweden), 1983, p 1-1 to 1-31
12. S. Joo and R.I.L. Guthrie, Mathematical Models and Sensors as an Aid to Steel Quality Assurance for
Direct Rolling Operations, in Proceedings of the Metal Society of the Canadian Institute of Mining and
Metallurgy Vol 10, Proceedings of an International Symposium on Direct Rolling and Hot Charging of
Strand Cast Billets J.J. Jonas, R.W. Pugh, and S. Yue, Ed., Pergamon Press, 1989, p 193-209
13. H. Ichihashi, Sumitomo Metals Internal Report; see D.H. Nakajima, "On the Detection and Behaviour of
Second Phase Particles in Steel Melts," Ph.D. thesis, McGill University, 1986
14. W.L. Roberts, Hot Rolling of Steel, Marcel Dekker, 1983
15. Microalloying '75, Proceedings of the International Symposium on High Strength Low Alloy Steels,
Union Carbide, 1977
16. F.B. Pickering, Physical Metallurgy and the Design of Steels, Applied Science, 1978
17. S. Yue, F. Boratto, and J.J. Jonas, Designing an Industrial Controlled Rolling Schedule using Simple
Statistical Process Analysis and Laboratory Modelling, in Proceedings of the Conference on Hot and
Cold-Rolled Sheet Steels. R. Pradhan and G. Ludkovsky, Ed., The Metallurgical Society of the American
Institute of Mining, Metallurgical, and Petroleum Engineers, 1988, p 349-359
18. W.J. Liu and J.J. Jonas, Ti(CN) Precipitation in Microalloyed Austenite During Stress Relaxation, Metall.
Trans. A, Vol 19A, 1988, p 1415-1424; Calculation of the Ti(CyN1-y)-Ti4C2S2-MnS-Austenite Equilibrium
in Ti-Bearing Steels, Metall. Trans. A, Vol 20A, 1989, p 1361-1371
19. S. Yue, R. Barbosa, J.J. Jonas, and P.J. Hunt, Manufacture of Seamless Tubing by Means of Recrystallized
Controlled Rolling and Accelerated Cooling, in 30th Mechanical Working and Steel Processing
Conference, Iron and Steel Society of the American Institute of Mining, Metallurgical and Petroleum
Engineers, 1988, p 37-45
20. W. Roberts, in HSLA Steels: Technology and Applications, American Society for Metals, 1984, p 33, 67
21. F.H. Samuel, S. Yue, J.J. Jonas, and K.R. Barnes, Effect of Dynamic Recrystallization of Microstructural
Evolution During Strip Rolling, I.S.I.J. Int., in press
22. L.N. Pussegoda, S. Yue, and J.J. Jonas, Laboratory Stimulation of Seamless Tube Piercing and Rolling
Using Dynamic Recrystallization Schedules, Metall. Trans., in press
23. J.J. Jonas and T. Sakai, A New Approach to Dynamic Recrystallization, in Deformation, Processing and
Structure, G. Krauss, Ed., American Society for Metals 1984 p 185-243
24. W.L. Roberts, Flat Processing of Steel, Marcel Dekker, 1988
25. F.H. Samuel, S. Yue, B. A. Zbinden, and J. J. Jonas, "Recrystallization Characteristics of a Ti-Containing
Interstitial-Free Steel during Hot Rolling," Paper presented at the AIME Symposium on Metallurgy of
Vacuum-Degassed Carbon Steel Products (Indianapolis, IN), American Institute of Mining, Metallurgical,
and Petroleum Engineers, Oct 1989
Avatar utente
Aldebaran
 
Messaggi: 5438
Images: 4951
Iscritto il: 31/03/2010, 19:26
Località: Milano - Chieti

Torna a Iron & Steel

Chi c’è in linea

Visitano il forum: Nessuno e 1 ospite