Grain growth in low carbon steel after heating to 930¦ 3

[mai39]

Hello everybody,

I´ve been experincing a problema with a low carbon Steel. Two materials with similar chemical composition and subjected to the same process show very different behaviour.

Chemical composition: Material 1: C=0.018; Mn=0.17 residual:content low. Al-killed=0.040
Material 2: C=0.020; Mn=0.18 residual:content low. Al-killed=0.040


Both materials have been hot rolled, subjected to a 700ºC annealing, stamped and heated to 930º.
The first one presents very coarse grain size after heating to 930ºC, and the second one doesn´t.
If chemical composition is very similar and the process they have been subjected to is identical,which factors could be influencing this grain growth? Is more difficult this grain growth to happen if carbon content is higher?

Thank you very much,

 

[metengr]

Usually, the following parameters affect grain growth in steels - prior forming, heating rates associated with heat treatment, heat treatment itself and chemical composition. If you are sure about the forming and chemical composition, I would suspect Material 1 was heated at slower rate or the temperature exceeded your 930 deg C target value (because of placement in the furnace) or the hold time was not the same between the two materials.

 

[weldstan]

Did you check the grain size after the 700 C heat treatmenmt? Assuming that the materail was produced onm a hot strip mill, were there differences in finishing and coiling temperatures? Were the reductions the same?

 

[mai39]

Fist of all thank you very much.

Yes, we checked grain size after 700ºC heat treatment and in both cases grain size was 7-8. The hot rolling supplier and final thickness were the same in both cases, so I suppose the reductions and finishing and coiling temperatures were similar in both cases.
Could be that 930ºC heating temperature is too high, or maybe is in the limit to start grain growth and this is why sometimes grain growth happens and other times do not? I have read the normalizing temperature must be Ac3+30ºC for hypoeutectoid steels, so 930ºC should`t be too high.

And, do you think that a higher carbon content could help avoiding it????

Apologies for my english.

 

[EdStainless]

Has this happened to many coils of steel, or material off of a single coil?
It takes very little cold work to trigger grain growth.
I would also look at residual chemistry, some very minor elements can help inhibit growth.

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[TVP]

Further to what EdStainless mentioned, what are the values for Si, V, Nb, and Ti? It is possible that the "good" steel contained one or more of the these elements in sufficient quantity to inhibit grain growth.

 

[MagBen]

<.02 C is super low carbon! we may call it electrical iron, rather than steel. Besides the residual chemistry (which have stronger pinning effect than c), the cooling rate after hold at normalizing tem is key to control the structure and properties.

Identical grain szie after 700C anneal has somewhat indicated the hot rolling process is identical for material 1 and 2. By the way, what is your purpose to anneal at 700C before stamping? For stamping, people normally prefer harder material. Was that for uniformity?

 

[MagBen]

P.P.S. at that low Carbon level, .02 and .018 carbon could make a big difference.
For one of my alloy, I have diffent heat treatment systems according to carbon ranges: .005-.006, .007-.010, .011-.016, the higher the range, the higher the annelaing temperature.

 

[OGMetEngr]

I agree with previous comments. It takes very little V, Ti, Nb to inhibit grain growth (e.g. - microalloyed steels). Also any "cold" work will accelerate. Those are the two most likely explanations. Another possibility would be temperature variation at the higher temperature heat treatment. Were the two materials heat treated together? What was the temperature tolerance, +-25F?

 

[racookpe1978]

"the same process" ?????

Where they rolled and welded ?? the same way?
The same direction w/r grain and width?
Both "thick plates" or very thin sheetmetal-type rolls of material?
Same mill and heat/pour?

 

[mai39]

Effectively the annealing was made to uniformize the material and have lower differences in mechanical properties along the coil before stamping.
The residual content is the one below, it is higher in the case of material 1, so the pinning effect should be higher, isn´t it?
Material 1: Ni=0.018 Cr=0.018 Mo=0.002 Cu=0.010
Material 2: Ni=0.016 Cr=0.013 Mo=0.001 Cu=0.018
On the other hand, analizing carbón content, it´s true it should be considered as a pure iron (<0.02%C) more than a hypoeutectod Steel. Is it a pure iron more prone to grain growth than a hypoeutectoid Steel? In order to avoid this to hapen again, do you think that raising carbon content to values higher than 0.03 could help?

 

[MagBen]

Ni, Cr, Mo, Cu can form solid solution, without much of pinning effect. you can look at Nb, Al, Ti, etc., maybe also N.
my suggestions are 1. decrease normalizing temperature to 910-920C, 2. control your cooling rate. the cooling rate maybe key, during cooling, there will be new nucleation and growth of α-phase from ϒ-phase. Both the original grain size of ϒ and cooling can affect final α grain size.

 

[EdStainless]

Ben is spot on.
Mb, Al, Ti, and Si are key to pinning.
Yes, irons are much more sensitive to grain growth than steels, there are fewer things in them to inhibit it.
With material this clean you should be able to lower the temp.
I would do some lab work and find out where the lower limit really is.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube