Question

QUESTION 6 (5+5+5+5 = 20 marks) Microalloyed steels are an important class of steels used extensively by the automotive industry. The two most important contributions to the yield strength of these alloys are the strengthening by precipitates in the form of alloy carbides and the strengthening by ferrite grain size refinement. Consider a typical Nb microalloyed alloy composition: Fe-0.1C-1Mn-0.1Nb (wt. %). These alloys are hot rolled in the austenitic condition and then air cooled from elevated temperature. Figure 1 below shows the yield strength of this microalloyed steel as a function of ferrite grainsize for cooling from three different temperatures and the comparisons with the plain carbon steel. The cooling rates from the different temperatures are identical. 500 0-09% Niobium stool, air cooled from 1250 °C 0-09% Niobium steel, air cooled from 1100 °C 400 Yield stress (Mim-2) 0-09% Niobium stool, air cooled from 950 °C 8 300 Plain carbon steel, air cooled 200 5 6 7 8 11 12 13 9 10 Ferrite grain size, d-1/21mm-12) Figure 1. Yield strength as a function of ferrite grainsize for a Nb microalloyed steel cooled from three different temperatures in the austenitic regime. Comparisons with a plain C steel are also shown. (a) Explain why the ferrite grainsizes achievable in the microalloyed composition cooled from 950°C are much smaller than those achievable in the plain carbon steel. (b) What is the dominant contribution to the strength of the microalloyed steel cooled from 950°C and explain your reasoning?

Answer 1

​​​​​​a) In micro alloyed steels there are NbC precipitates which act as grain refiners and they produce fine grain sized structure. Whereas in plain carbon steels no such grain refiners are present.

b) Dominant contribution to the strength would be grain size refinement. At 950°C precipitation hardly occurs because if low degree of super saturation. Therefore the main contributing factor would be grain size refinement.

c) As you increase the temperature more is the solubility of the Nb and more is the degree of super saturation. This will enhance the precipitate amount as more number of precipitates are obtained for more degree of super saturation.

But at the same time Nb is also a grain refiner and this will dissolve in the matrix as you increase the temperature to a high value and therefore fine grain size cannot be obtained due to the expense of grain refiners.

Therefore yield strength is a result of precipitation hardening as well as grain refining.

d) it is around 160 Mpa.

Given grain size = is um -3 15710 mm 42 8.16.my ㅓ년 15x10 3 for this value of d², yield strength of 0.09%. No steel cooled at han 1100c is around 320 MPa. 5 → For 12508c, gield strength iş around usom Pa. Contribution is 480 - 320 The quantitative 2 160 MPa CS Scanned with CamScanner