Effect of B/N ratio on plastic anisotropy behaviour in low carbon aluminium killed steel

Abstract

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 properties 3,4 of low carbon steel. Whenever excess boron is present in solution in austenite, it segregates to the c grain boundary, thus inhibiting the transformation of austenite to ferrite, and resulting in increase in hardenability of steel. 5 Although plenty of works 6,7 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 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¡10uC and coiled at 620¡10uC. As lower coiling temperature (,600uC) results in higher r m values in batch annealed aluminium killed steel, 8 some coils were coiled at 540uC also. Hot rolled coils were cold reduced to 1 mm thickness. The cold rolled coils were annealed with shorter and longer annealing cycles as schematically shown in Fig. 1. Conventionally shorter annealing cycle is practiced for normal cold rolled steel whereas longer annealing cycle is used for extra deep drawing grade. 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 m is defined as r m 5(r 0 z2r 45 zr 90 )/4, where subscripts refer to the angles of tensile tests to the rolling direction. Figure 2 shows the effect o...

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