Análisis de los principales destinos de Exportación (2001 – 2010)

suggested that in niobium steels undissolved particles at the austeniti- zing temperature exerted a direct refining effect on ferrite grain size, independent of any effect they may have on the austenite grain size. A similar effect of additional ferrite grain refinement due to niobium

and vanadium additions even at identical austenite grain size has been observed by Sekine and Maruyama^"^^. Eozasu et a l ^ ^ determined the ratio of the linear-intercept grain diameter of ferrite to that of austenite,which was 2.3> and suggested that this is in agreement with the results of Karuyama et al^*^.

It has been noted^^ that deformation refines the ferrite grain size although the austenite grain boundary area per unit volume does not change. High deformation leads to the formation of deformation bands, and their density increases with increasing deformation. Deformation bands are also known to nucleate ferrite, therefore Kozasu et a l ^ ^

included deformation bands in their calculation of the effective austenite interface area (S^), which was ultimately related to the ferrite grain size. They found that for a fixed S^, deformation below the recrystalli­ zation temperature refined the ferrite more* effectively. Roberts and Ahlblcm^2a\ however, suggested that the‘relationship between and

nucleation frequency for ferrite is not linear when S becomes very large, and therefore it would not be possible to reduce the austenite grain size indefinitely and expect to obtain ever finer ferrite.

CHAPTER 10

PRECIPITATION IN MICROALLOYED STEELS

10.1 — Niobium-Steels--- Niobium is added to C-Mn steels mainly to refine the ferrite grain size and also to impart some precipitation strengthening due to formation of semi-coherent Nb(C.N) precipitates in ferrite. Niobium is known^*^ to form an isomorphous series of solid solutions between its carbide and its nitride, and Nb(C,N) has a f.c.c. structure with a lattice parameter intermediate^between that of FdN(0.442 nra) and NbC(0.447 nm).

Gauthier and LeBon^*^ suggested that Nb(c,N) is carbon rich and very (172)

close to the carbide NbC, However, Hoogendoom et al' 7 observed an increase of nitrogen in Nb(C,N) with increasing precipitation temperatu­ re. Also the composition of Nb(C,N) depends on the composition of the

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Precipitation of Nb(C,N) occurs in austenite, during the transformation of austenite to ferrite, and in the ferrite. The preferential nucleation sites for Nb(C,N) are dislocations^2-^, existing n u c l e i ^ sub- grain and grain boundaries

10.1.1 Precipitation in austenite

It is well known^^*^-^ that precipitation of Nb(C,N) in austenite occurs as coarse particles which do not contribute to strengthening and will reduce the subsequent precipitation strengthening which can other­ wise be achieved.

Due to the small number of lattice defects in undeformed austenite, precipitation of Nb(C,N) occurs very slowly^*^* ^39) _{amount of} such precipitation is very small. The arnoimt precipitated depends on

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the cooling rate' ' , and fast cooling lowers the amount of Nb(C,N) precipitated in austenite. However, deformation of the austenite accele­ rates the precipitation of N b ( c , N ) ^ ^ * w h i c h is attributed to the

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deformation,nucleation of ITb(C,N) occurs primarily on the sub-grains^^’^2'^. The precipitation kinetics of Nb(C,N) in undefozmed and defozmed austenite

(178 181)

have shown' * 7 that the progress of precipitation is characterized by C-curve behaviour with a nose temperature around about 950°C. This is due to the both changes in the degree of supersaturation and diffusivity of niobium in austenite, associated with temperature. Deformation of

austenite reduces the incubation time for precipitation and therefore moves , ' ' , . A .. (75,110,126,181) T _ . ,(126) , ,

the C-curve to shorter times' 1 1 1 7. LeBon et al' 7 showed that.strain induced precipitates have mean sizes in the range of 30-50 & diameter,in contrast with 1000-3000 S*diameter particles obtained after isothezmal holding of undeformed austenite, .

Watanabe^^^ studied the effect of molybdenum, nitrogen and aluminium additions on the kinetics of precipitation of 2Bd(C,N) in austenite, and

found that an addition of molybdenum (0.29%) moved the C-curve to lower temperatures and shorter times. However, in deformed austenite, 30% reduct-, ion, molybdenum decreased the precipitation rate at 925°C. An increase in nitrogen from 0.006% to 0.010% increased the precipitation rate in the temperature range 870°~925°C. On the other hand an increase in aluminium from 0.025% to 0.047%.did not affect the precipitation kinetics of Nb(C,N) significantly.

The precipitation strengthening contribution due to Nb(C,N) therefore varies in controlled rolled steels, depending on the rolling schedule used. Morrison et al^®2^ have pointed out that only about half the maxim­ um hardening effect of Nb(C,N) precipitation is obtained in controlled

steels.

10.1.2 Precipitation during the austenite-^- fei'rite transformation

Precipitation of 2Tb(C,N) also occurs during the austenite ferrite trans- formation^^^5^ ^ ’^4)^ characterized by interphase precipitation. Precipitates formed during transformation are in the size range 50-

100 and do not impart the maximum strengthening which can be obtained from precipitation of Nb(C,N). The intensity of precipitation

strengthening depends on the size, interparticle spacing and volume fraction of precipitates, and these tend to vary with transformation temperature^

and cooling rate^*^' A fast cooling rate decrease the amount of niobium precipitated^^*^ and lowers the transformation temperature, resulting^ ^ in a closer spacing of precipitates and a decrease in precipitate size.

10.1.3 Precipitation in ferrite

1

The remnant niobium in solution after transformation will precipitate in the ferrite. The preferential nucleation sites for such precipitation are dislocations in the ferrite. There is. an orientation relationship’ between Kb(C,N) and ferrite matrix^

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It is generally accepted that a significant strengthening effect is obtained when Nb(c,W) precipitates semi-coherently in the ferrite. Also the size of the TTb(C,N) precipitates are extremely fine 50 2), and thus are very effective strengtheners.

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Hoogendoom et al studied the effect of the addition of vanadium to 0.03)o Ito steel on the precipitation of Kb(CjN)'in ferrite during cooling, and found that vanadium increases the amount of precipitation, possibly due to a complete series of solid solutions existing between Nb(C,N) and V(C,N) (201).

10.1.4 Precipitation strengthening

Because of the higher precipitation temperature of Nb(C,N), much of it tends to precipitate in austenite, particularly during controlled

rolling, and therefore very small amounts of niobium remain in solid solut­ ion to precipitate in ferrite. The amount of niobium precipitated in

ferrite, and hence the precipitation strengthening, is greatly influenced by solution temperature, theimo-mechanical treatment and transformation temperature. In order to achieve optimum precipitation strengthening from Nb(C,N) it is desirable:- .

(a) to soak the steel at temperature'where complete solution of Nb(C,N) can occur^^* f

(b) to suppress the precipitation of Hb(C,N) in austenite so

that more niobium is available to precipitate in ferrite^ ^75) (c) to control the transformation temperature so as to control

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