Texture in an intercritically annealed dual-phase steel

Scripta METALLURGICA

TEXTURE

Vol. 18, pp. 1211-1214, 1984 Printed in the U.S.A.

IN AN INTERCRITICALLY

Pergamon Press Ltd. All rights reserved

ANNEALED DUAL-PHASE STEEL

R . ~ Ray Department of Metallurglcal Engineering Indian Institute of Technology, (Received April (Revised August

Kanpur, India

25, 1984) 27, 1984)

Intro~qn

In the p a s t few years great attention has been paid to the development of the "Dual-Phase" steels. The structures of these steels consist basically of ferrite grains with up to about 2 0 % martenslte islands d/strlbuted within the matrix. The simplest way of producing these steels is by annealing low-carbon steels in the intercrltlcal ( ~ + 7 ) temperature range to produce ferrlte-austenIre mixtures, followed by accelerated coollng which transforms the austenltic phase into ~ r t e n s l t e o Alternatively, dual-phase steels can be produced directly from the rollIng heat in hot strip mills. It has been shown (1) that for the latter type of processing a wide variety of textures can b e developed due to metallurgical and t h e r u o - ~ c h a n i c a l pxDcesses acting in combination. The hot-rolling textures have been studied by analysing the orientation distribution functions (O.D.F.' s) of the crystallltes. In the p r e s e n t w o r k texture produced in a dual-phase steel composition after Intercritical annealing will be reported. The texture has been studied b y the conventional pole-flgure method as well as by determining the crystallite orientation distribution functions. ExDeri~tal The chemical in Table 1.

composition of the steel used in the present work is given TABLE Chemical

1

C~mpQ~iti~n 9f the Steel

Weight percentage

of elements

C

Mn

Si

S

P

V

0.12

1o51

1.47

0.021

0.016

0°09

The alloy was melted in the f o r m of a 30 kg ingot in an induction furnace and cast into a pre-heated cast-lron mould. The heat was deoxidlsed by the addition of alumlnlum shots and mlsch-n~tal. The cross-sectional area of the ingot was 12 cmo x 12 cn~ (approx.)° A part of the ingot was Inltlally forged down into a rod of square cross-section (25 ram. each side). This was cold-rolled 50% in a laboratory rolling mill, annealed at 940°C for 30 minutes and then aircooled. The process was repeated once more. The steel plate was then held at the intercritlcal annealing t e ~ e r a t u r e of 750°C for 30 minutes followed by water-quenching. Optical m i c r o s t r u c t u r e o f the inter-critlcally annealed material was obtained in the usual manner. Crystallographic texture of the midsection of the plate material was determ/ned by measuring the [ 110 } pole-figure using the

1211 0036-9748/84 $3.00 + .00 Copyright (c) 1984 Pergamon Press

Ltd.

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TEXTURE

IN DUAL-PHASE

STEEL

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Schultz reflection method (2) . Later on the { 110}, { 200} and { 112} pole-figures were measured in an automatic texture goniometer (3) and 3-dimensional orlentation distribution functions (ODF's) calculated out of these pole-figure data (4)In this method the crystallographic orientation density is described in a 9-dimensional orientation space formed by the 3 Euler angles ~i" ~" ~2" Results and Discussion The optical microstructure (Fig. I) shows islands of dark-etching martensltic areas distributed in a ferritic matrix. This is a typical dual phase struc ture. The {ii0) pole-figure for the material is shown in Fig. 2. As can clearly be seen, the Dole-fi~ure is Quite flat and smeared-out. In fact, on the basls of this p o l e - f i g u r e a l o n e the material could pass as one having a 'random" texture or no texture at all. However, in contrast to this pole-figure, the O.D.F. plot for the alloy (Fig. 3) shows some definite features which are practically impossible to visualise from the { l l O } p o l e - f i g u r e . The O.D.F. p l o t ahows a d i s t i n c t pattern with rather pronounced maxima at specific regions of the orientation space, Initial analysis has indicated that two intensity maxima are obtained at near (337)I l~Ol and ~III) [II0] with the second peak being the stronger of the two_in intensity. Again, two intensity maxima are found to be present at near (-~37) [ q ~ 6 ] a n d (Iii) [ ~ 2 ~ the second one being the stronger. These results indicate the presence of both {III} and {937} fibre textures in these materials, An examination of the 0,D.~. plot has further shown that in addition to the two fibre-components men~i0nea adore ~wo peaK-type texture components are also present, namely, {310}<001> znd { II0} . It m a y b e men_tloned here that orientation density maxima were found near (~[I) [OllJ and (111) [112 ] for the hot-rolled strips of a few dual-phase steels (I)In many deep-drawing quality steels the ND I| [ III ] fibre has been found to be present as a major component of the recrystallisation texture (5)- The i n c o ~ l e t e ND [I [ 113 ]fibre component reported by Bunge et al. (6) for lowcarbon steel can be taken as very similar to the ND 11 [337J fibre observed for t h e present steel. The other texture components found for the present alloy, although of rather low intensity, are k n o ~ to occur in the annealing textures of a few annealed low-carbon steels. Thus, qualitatively, t h e texture of the present inter-critlcally annealed dual-phase steel is essentially similar to that of a good deep-drawing-quallty low-carbon steel. Conclusion The present results seem to emphasize a very interesting fact t h e 0 .D~F. method is a ~ c h more powerful tool compared to the conventional polefigure technique for the determination of texture of a material. In fact a much higher resolution is attainable with the ODF technique. Thus although the polefigure method suggested a random texture for the experimental alloy, the different texture components could be resolved easily from the correspondino, 0.D.F. plot. References 1• 2. 3. 456 •

C.M. Vlad and H .J. Bunge, Textures of Materials, Proc. of Sixth International Conference on Textures of Materials, Tokyo, p° 649 (1981)• L.G. Schultz, J. Appl. Phys., 20, 1030 (1949). R. Alam, H.D. Mengelberg, K. L ~ k e , Z. Metallko 58¢ 8 6 7 (1967). J. Pospiech and J. Jura, Z. Metallk, 65, 324 (1974) H. Hu, Texture of Crystalline Solids, Vol. 4, p. 13 (1980). H.J. Bunge, D. Schleusener and D. Schlafer, Met. Sci, 8, 413 (1974) •

Vol.

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FIG. 1

TEXTURE IN DUAL-PHASE STEEL

1213

Optical microstructure of the alloy showing the dual phase structure

RD

TO FIG. 2

The { 110} pole-figure for the intercrltically annealed alloy

1214

TEXTURE

, O-

)

IN DUAL-PHASE

STEEL

Vol.

18, No. ii

-.

7o //<>~a.,

<'. ~<,'v,~,,,4~°Q

'

~°

(:>

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">

'

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),<

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.~~., - <~~ , ?

FMAX=

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,~,.,~,E~ 50 70 80%

FIG. 3

The O.D.F. plot for the intercritically

annealed alloy