Microstructures developed during thermomechanical treatment of HSLA steels
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THE ultimate purpose of this study is to optimize controlled rolling schedules for carbon and microalloyed steel plate so as to produce the finest and most uniform ferrite grain structures for maximum strength and toughness. One prerequisite for producing fine ferrite is closely spaced austenite boundaries at the transformation-start temperature. To achieve this structure requires control of microstructural changes in three areas of the thermomechanical treatment (TMT) of austenite: grain coarsening during reheat, recrystallized grain size during deformation in the complete-recrystallization range, and grain-shape changes during deformation below the recrystallization range. In addition; the fineness of the ferrite structures that result from the refined austenite depends on control of the transformation conditions. This report considers only those microstructural changes induced by hot deformation of austenite. The effects of compositional and processing variables on these microstructural changes suggest several guidelines for controlled rolling.
examined (0.007 and 0.015 wt pct). The major part of the work was done on the 0.05 Nb-N steel; the others were used in on!y one or two parts of the program. The steels were laboratory heats of 45 or 135 kgm (100 or 300 lb) which were hot-rolled to 25-mm (1 in) plate at 1250 to 850 ~ Specimens cut from these plates were reheated to 1300 o C to redissolve the microalloy additions and then cooled to room temperature at a rate, approximating the cooling of 175-mm (7 in) continuous cast slabs. This pretreatment which was performed on all specimens for subsequent studies, produced ferritepearlite structures similar to those observed in actual slabs. For grain-coarsening studies, specimens were reheated to the temperature of interest (900 to 1300 ~ at a rate similar to that for the reheating of slabs for rolling, held 30 min, and brine-quenched. Preliminary work had demonstrated that holds up to
Table I. Steel Compositions, Wt Pct Steel
EXPERIMENTAL PROCEDURE The base steel for these studies was a nominal 0.07 C, 1.4 Mn, 0.25 Si steel. To this were added Nb, V, and A1 as shown in Table I. In addition, two levels of N were L. J. C U D D Y is A s s o c i a t e R e s e a r c h C o n s u l t a n t , U . S . Steel C o r p . , R e s e a r c h L a b o r a t o r y , M o n r o e v i l l e , P A 15146. M a n u s c r i p t s u b m i t t e d M a r c h 13, 1980.
METALLURGICAL
TRANSACTIONS
A
9
C
Mn
Si
N
V
Nb
C-Mn 0.07AI 0.06 N b 0.10V
0.065 0.065 0.077 0.078
1.37 1.43 1.45 1.38
0.27 0.26 0.28 0.27
0.007 0.005 0.007 0.006
---0.098
--0.057 --
-0.068 ---
0.05 N b - N 0.12 N b - N Nb-V-N 0.11 V - N 0.25 V - N
0.064 0.057 0.081 0.078 0.010
1.43 1.44 1.38 1.38 1.42
0.29 0.24 0.26 0.24 0.23
0.017 0.015 0.012 0.013 0.016
--0.069 0.11 0.251
0.048 0.118 0.025 ---
--0.020 0.018
ISSN 0360-2133/81/0713-1313500.75/0 1981 A M E R I C A N S O C I E T Y F O R M E T A L S A N D THE METALLURGICAL SOCIETY OF AIME
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12A, JULY 1981--1313
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