How does hot-rolling and thickness affect the resulting microstructure in steel?

[RPI_Quantum]

I am trying to answer a question using the TTT diagram of steel that is given to me in my text. I know how to use the diagram, but the book assumes some prior technical knowledge of steel. It gives the information that the steel samples are 0.3 mm thick hot-rolled strips of 1080 steel, and then it gives me a heat-treating schedule and asks me to determine the final microstructure. I know that 1080 means 0.8% C, but I do not know if the thickness or hot-rolling affect the resulting microstructure. Initially, the sample is held at 860 C for an hour.

Any help would be appreciated. I am not looking for an answer or tutorial on the TTT, but if someone can explain to me if and how hot-rolling and thickness affect microstructure.

Thanks!

 

[RPI_Quantum]

No one can give me any help at all?

 

[Astronuc]

Hot rolling causes large grains to elongate, and then the elongated grains recrystallize to form smaller grains.

860°C takes the 1080 up to the austenitic region of the phase diagram. Cooling produces pearlite and some bainite - but that depends on cooling rate and to what temperature the steel is cooled slowly before quenching or more rapid cooling.

Slow cooling would give coarse pearlite.

 

[RPI_Quantum]

After the austenitizing at 860 C, it is water quenched- it does not give a specific cooling rate. I am guessing that in this case, the quench is assumed to be extremely fast resulting in martensite?

 

[Astronuc]

There is a good discussion of 1080 in ASM's Specialty Hanbook "Carbon and Alloy Steels". To what temperature is the 1080 quenched - below the Martensitic temperature?

 

[Astronuc]

I read some more material on heat treatments of simple carbon steels. Although I am not familiar with quench rates, I imagine that a water quench is perhaps sufficiently 'rapid' to cause martensite to develop.

Slower quenching would produce a ferrite-cementite combination, with either a pearlite structure or a bainite structure.

According to the handbook on Carbon and Alloy Steels, "hot rolling is done in the austenite region, and because of large sections and equipment design, the austenite transforms to microstructures of ferrite and pearlite. In controlled rolling, that is a controlled thermo-mechanical process, the key is the formation of fine austenite grains which transform to fine ferrite grains upon cooling. The fine grains provide for increased strength.

With thin sections, rapid quenching would presumably martensite.

 

[RPI_Quantum]

Thanks, that was very helpful. That hot-rolling causes a transformation to a two-phase structure was the key. I am not sure I understand the exact mechanism behind this transformation, but I do know that once pearlite forms it cannot be transformed to martensite without being re-austenized.

Though I am not perfectly clear on this, I now know where to look (the handbook you mentioned). Thanks a lot!

 

[Astronuc]

Rolling, hot or cold, produces dislocations in the metal crystal structure and grain deformation. The hot working should have a lower dislocation density since the dislocations can glide more easily. Hot worked material usually has finer grain structure than cold-worked material, although cold-worked material with elongated grains can be annealed, i.e. recrystallized, in which dislocation networks form new grain boundaries. I am more familiar with HCP materials than FCC materials.

Anyway, the issue of pearlite/bainite vs martensite has to do with temperature and quench rate. The particular microstructure has a threshold for stability. The pearlite/bainite temperature range sits between the austenite and martensite regions, so that if one quenches (cools) slowly one will form pearlite or bainite, depending on the hold temperature, or if one cools rapidly below the martensitic temperature, then martensite develops.

The time at temperature also has an effect because it the temperature dependent diffusion of Carbon that affects the microstructure.