Influence of Boron on the Hot Ductility of TWIP steels

1.0 Introduction

The demand for crude steel has increased worldwide according to World Steel Association the steel production in 2010 was 1414 million tones. This shows an increase 15% when compared to the total in 2009 where production was 1229 tonnes. What is interesting is that China accounted for 44% of this total, if China was excluded the annual total increased by 20% when we compare this to 2009. The steel production in December for 66 countries increased by 7.8% to 116 million tonnes.

To meet these production demands, casting is engaged as the chosen fabrication process as it is more economically viable for high production rates than forming operations. In particular, the technique of continuous casting is favored as a primary hot-rolling operation that produces slabs or thin sheets of steel. These are a convenient entity that can then undergo secondary metal-forming operations.

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The conventional way to improve the strength and impact behavior of steel is to add small amounts of micro-alloying elements such as Al or in our case B to benefit grain.Micro-alloying additions enable tensile strengths of over 500 MPa to be achieved as the micro-alloys formed fine precipitates that would attach the austenite grain boundaries from moving as well as to provide strengthening via precipitation hardening. It is important to note that these beneficial mechanical properties engineered into steels are only applicable to the finished product, where the steel is employed at room temperature as automobiles. This point is applicable before the finished steel is even utilized, as at the high temperatures of the continuous casting operation, the steel has undesirable properties.

Over the past forty years, the difficulty in casting some steels has led to great efforts in understanding the science behind the cracking processes. The continuous casting process can to some degree be simulated using a simple laboratory hot tensile test. Investigating the hot ductility of the steel under these laboratory conditions does give great insight into the viability of producing continuous cast steel free from transverse cracking. It has been quantified that after straining a steel sample to failure, a ductility displayed as a reduction of area (R of A) value of greater than 40 % will allow for successful casting in industry. These findings are further enforced with visual probing techniques such as microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). These allow insights into the influence of grain size, precipitation hardening, cooling rates, strain rate and composition on crack propagation which are all important in explaining failure modes and preventing failure. New steel types need to be tested under laboratory conditions before steel companies have the confidence for mass production in industry.

2.0 Literature Review

3.0 Aim & Method of Contribution

The major problem with TWIP steels is the difficult in casting it; this is because during the continuous casting operation, the cast has to be straightened as it is being cast in a curved mould. The straightening process puts the surface and the edges of the slabs into tension and this causes cracks and this can be propagated easily into fragile steel grades this is what TWIP falls into.

The aim of the project is to do a hot ductility test in which a tensile specimen is melted and then cooled to the straightening temperature which ranges from 1000-700°C as used in the commercial casting operation. Following this the specimen is strained to failure by using the strain rate used in the unbending operation as this is a good indicator of a steels likely hood to crack.

There have been indications that that by adding a small amount of boron in our case (0.001%) that this will improve the hot ductility of the new TWIP steels and this would make them easier to cast. As mentioned the experiment involves the ductility test in which a tensile specimen is melted and then cooled to the straightening temperature which ranges from 1000-700°C from this we will get a hot ductility curve. Following this I will perform microscopically and scanning electron microscope examination of the fractured surfaces to find out the mechanism of this improvement. I have 12 specimens which have been provided by POSCO. The machine used for the tensile test is the Hounsfield Tensometer which is situated at City University.

3.1 Gannt Chart Project

4.0 References

1. ISSB: World Steel Review (Iron & Steel Bureau, February 2011)
2. Abu Shousha, R., I: Hot Ductility of Steels (PhD thesis, City University)
3. Discussions with Prof B.Mintz

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