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Effect of B/N Ratio on Plastic Anisotropy Behaviour in Low Carbon Aluminium Killed Steel

February 29, 2008

By Deva, A De, S K; Jha, B K

It is well known that dissolved nitrogen in ferrite seriously impairs the formability of hot rolled unalloyed steel.1-3 Boron being a strong nitride former, combines aggressively with dissolved nitrogen in steel, and thereby improves the forming properties. Further, atomic ratio of boron to nitrogen (B/N) plays an important role in influencing the microstructure and properties3,4 of low carbon steel. Whenever excess boron is present in solution in austenite, it segregates to the gamma grain boundary, thus inhibiting the transformation of austenite to ferrite, and resulting in increase in hardenability of steel.5 Although plenty of works6,7 have been carried out on the effect of boron on properties of hot rolled steels, limited literature is available on its effect in cold rolled formable grades particularly when carbon is in the range 0.03- 0.06 wt-%. The present paper discusses the effect of B/N atomic ratio on the forming properties in general and plastic anisotropy ratio r^sub m^ in particular, in low carbon aluminium killed batch annealed steel. The present study has been carried out on the industrially produced low carbon (0.04-0.06 wt-%) steel with varying B/N atomic ratio. The chemical composition of steels used for the present study is shown in Table 1. Steel A is the typical chemistry used for producing extra deep drawing steel.

All the steels were continuously cast to 210 mm thick slabs and were hot rolled to 2.8 mm thickness. The hot rolled bands were finish rolled at 880+-10[degrees]C and coiled at 620+-10[degrees]C. As lower coiling temperature (

Table 2 shows the mechanical properties of steels with varying B/ N ratio processed under different annealing cycles. Properties of boron added steel has shown a significant improvement compared to steel without boron in terms of lower yield strength and higher elongation. In spite of being subjected to similar hot rolling conditions and annealing cycle parameters, lower YS (242 MPa), lower UTS (360 MPa) and higher elongation has been obtained in boron added steel B1 as compared to boron free steel A. It can be attributed to the reduced solute nitrogen and carbon contents in Steel. As expected, increasing the annealing time has led to lowering the strength values and increasing elongation further.

As the tensile properties alone does not depict the forming behaviour of cold rolled steel completely, plastic anisotropy ratio r, which is a good measure of deep drawability of steel, has been assessed. A mean value r^sub m^ is defined as r^sub m^=(r^sub 0^ + 2r^sub 45^ + r^sub 90^)/4, where subscripts refer to the angles of tensile tests to the rolling direction. Figure 2 shows the effect of B/N ratio on r^sub m^ for the steels (with and without boron) annealed with longer cycle. Steel with B/N ratio of 0.8 (steel C) shows lower value of r^sub m^ (1.12) as compared to r^sub m^ value of 1.66 in steel with B/N ratio of 0.3 (steel B2) processed under identical conditions of hot/cold rolling and annealing.

The r^sub m^ value of steel A, subjected to coiling temperature of 540[degrees]C and longer annealing cycle, has also been compared to steel B3 to assess the effect of boron. r^sub m^ for both the steel were found to be nearly same (Fig. 2) with value of 1.76 for steel A and value of 1.74 for steel B3. The results show that lower value of B/N ratio does not affect r^sub m^ adversely. It can be explained in terms of availability of nitrogen for AlN precipitation in steel during batch annealing.

Depending on the Al, B and N concentration in steel, the range of temperature at which AlN and BN precipitate coincide in general. However, Ohmari and Yamanaka9 have reported that BN will form first compared to AlN due to higher diffusivity of boron. In the present study also, it appears that most of nitrogen has been combined by boron before precipitation of AlN in the hot rolled stage, which in turn has resulted in lower availability of nitrogen in solution depending on B/N atomic ratio for combining with aluminium during batch annealing. It is well known that there is strong influence of aluminium nitride during batch annealing of aluminium killed steel. High r^sub m^ values are produced by textures containing a high proportion of grains with (111) planes and low proportion of (100) planes parallel to the sheet surface. The aluminium nitrides lead to enhancement of the (111) texture components and a concurrent reduction of the (100) components. While developing the desirable texture, aluminium nitrides also help at the same time in formation of a pancake grain structure which results in better r^sub m^ value in steel. This emphasises the importance of availability of N and Al in solution before batch annealing. Masatoshi and Ichiro4 have reported in their work carried out on continuous annealed low carbon boron added aluminium killed (~0.04 wt-%Al) sheet steel that r^sub m^ value does not change with B/N atomic ratio upto one. Results obtained in our study is however not in line with their finding and the reason for the same can be attributed to the texture development in steel processed through continuous annealing and batch annealed route. In continuous annealing, free nitrogen does not contribute towards development of (111) texture, whereas it is a must for batch annealing steel.

1 Schematic representations of two batch annealing cycles

2 Effect of B/N atomic ratio on plastic anisotropic ratio

The findings can further be explained in term of microstructural evolution taking place in steels with varying B/N atomic ratio. In spite of similar hot rolling condition and longer annealing cycle, steel C (B/N: 0.8) has equiaxed microstructure (Fig. 3a), whereas steel B3 (B/N: 0-3) exhibited pancake structure as shown typically in Fig. 3b. The decrease in r^sub m^ value from a level of 1.66 to that of 1.12 is thus associated with a characteristics change in grain shape from elongated to equiaxed. Further XRD results conforms a lower (111)/ (100) ratio of 0.45 in steel C compared to that in steel B3 with (111)/(100) ratio of 0.86. The favourable (111) texture in steel B3 can also be attributed to availability of higher nitrogen in solution for AlN precipitation during batch annealing. Humphreys et al.10 in their recent study have shown that addition of boron promote the formation of shear bands under warm rolling condition thus resulting in a stronger {111} recrystallisation texture.

Table 1 Chemical composition of steels, wt-%

Table 2 Mechanical properties with different annealing cycle

The present study has clearly demonstrated the effect of B/N atomic ratio on the forming behaviour of low carbon batch annealed aluminium killed steel. B/N ratio can be optimised in such a way that sufficient nitrogen is available in solution to combine with aluminium during batch annealing. It will result in lower YS, higher elongation and almost similar r^sub m^ value in steel with boron as compared to steels without boron. Advantage of this study can be exploited in development of cold rolled batch annealed formable steel even with higher nitrogen content.

3 a equiaxed ferrite grains in steel C and b pancake ferrite grains in steel B3

Acknowledgement

Authors are grateful to the management of RDCIS, SAIL for their support and encouragement during this work.

References

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9. Y. Ohmari and K. Yamanaka: Proc. Conf. on ‘Boron in steel’, (eds. S. K. Banerji and J. E. Morral), 44-60; 1980, The Metallurgical Society of AIME.

10. A. O. Humphreys, D. Liu, M. R. Toroghinejad, E. Essadiqi and J. J. Jonas: Mater. Sci. Technol., 2003, 19, 709-714.

A. Deva*, S. K. De and B. K. Jha

Research and Development Centre for Iron and Steel, Steel authority of India Limited, Ranchi, 834002, India

* Corresponding author, email anjana@sail-rdcis.com

(c) 2008 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute

Received 21 August 2007; accepted 19 September 2007

DOI 10.1179/174367507X247520

Copyright Institute of Materials Jan 2008

(c) 2008 Materials Science and Technology; MST. Provided by ProQuest Information and Learning. All rights Reserved.




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