What are the materials of screw & bolt? (Part 1)

What are the materials of screw and bolt?

About the daily screw and bolt we use on our DIY crafts or constructional works,
Have you ever wondered how the hack it made of?
You may know it made of steel,
But, what kind of them?
How do they come from?

Here is the brief information for you,
First qusetion to ask will be, 
How does the steel been made?

Arvola, Dustin Alan, "Grain refinement of high alloy stainless steels in sand and directionally solidified castings" (2018).Masters
Theses. 7793.

The pictures demonstrate clear idea of the process of the steel manufacturing,
(Pictures always better than words...)

And now, we move to the main topic of this article,
What are the materials of screw and bolt?

We are not taking "international standard" perspective for this article here,
You could check out ISO 898-1 for that,
We might have another article to discuss about ISO 898-1 if the writer was not that lazy,

Like I am at the lowest class in my family.frown
In the world of screw/bolt material, they do have hierarchy,

R material / K material / A material

R material, Rimmed quality material,
Basically it is recycled steel and put them back to furnace,
There is no refining process for this type of material,
It is kind of middle class material and it can be used for cold forming screw production,
But because it is contained recycled material,
There may be some impurities in the material,
So it is not the best material for screw/bolt production,

K material, Killed quality material,
To be honest, i am also confused about this,
It sounds like Silicon-Killed steel and Aluminum-Killed steel, which are relatively high quality steel,
But the information I could find are quite unclear on this part,
What I conclude for this is that K material is the worst class material,
It is recycled materials as well,
However, the quality of the recylced steel is too bad to refined to be screw/bolt producable material,
Normally, we will only use it for hot forming bolt,

A material, i don't really know the "A" represents for,
This is more like a praticial saying in Taiwan fastener industry,
I can't really find the information about,
Based on what I know and I interview with experience workers,
They define A material is the best level of screw/bolt producable material,
Basically it is newly made steel from iron ore,
Therefore, it contains less of impurities,
Aluminum-Killed steel and Silicon-Killed steel are A material,