p = polyfit(temp,carbonSteel,3);
y = polyval(p,carbonSteel);
What did you build? A model, that predicts carbonsteel, as a function of temp. So temp is the INDEPENDENT variable.
Now, you want to predict the dependent variable, as as a function of temp. So you need to do this:
y = polyval(p,temp);
plot(temp,carbonsteel,'bo',temp,y,'r-')
What you did, as you did it, makes no sense in context of the model you built..
