Difference between controlled rolling and normalized rolling

[metgist]

I came across the terms "controlled rolling" (CR) and "normalized rolling" (NR) in the Indian Standard IS 2062 for hot rolled (HR) medium and high tensile structural steel specification while looking specially at a Fe-.22C-1.5Mn-.4Si steel (E250-GrBr50).

According to the standard, CR is a HR process in which the temperature and reduction ratio are controlled, particularly in the finishing stages, to achieve fine microstructure and optimum properties. I guess the temeprature is kept somewhere between the Ac1 and Ac3 during the last stages.

In NR, according to the standard,final rolling passes are carried out "at a suitable temperature equivalent to normalizing temperature" (which I presume to be above Ac3) followed by cooling in air below the transformation temperature "to produce a structure, analogous to that obtained by a separate normalizing treatment of hot rolled product".

My reading of these is that CR will give better properties (toughness?) than NR.

Now our client specification requies CR plates for section thickness 20-40 mm and NR plates for greater than 40 mm sections. But if I understand correctly, greater section thickness demands higher toughness. So shouldn't this requirement go the other way, that is, NR for less than 40 mm, and CR for greater than 40 mm?

Actually, our problem is that the vendor for the plates can supply only NR and our section thickness is always less than 40 mm. If my first assumption is correct, that is, less thick sections can do with lower toughness, then probably NR could meet the requirement?

Thank you in anticipation.

 

[kumkumvijay]

Controlled Rolling steels are micro alloyed to get better mechanical properties compare to the same grade of steel with normalizing rolling with out micro alloying elements. So you will get better mechanical properties with 20-40mm CR steel (micro alloyed) and need more than 40mm for same mechanical properties with the same grade NR steel ( no micro alloying).

http://bis.org.in/sf/mtd/MTD4(5005).pdf

http://www.bis.org.in/sf/mtd/MTD4(4919).pdf

The link above will give you some details.

 

[weldstan]

There are limits to the effectiveness of CR to prtovide the desired microstructure throughout the cross section as T increases.

 

[redpicker]

But if I understand correctly, greater section thickness demands higher toughness. So shouldn't this requirement go the other way, that is, NR for less than 40 mm, and CR for greater than 40 mm?

Perhaps this is the problem, your understanding.

You are correct a greater section thickness demands a higher toughness when the stress and environmental conditions are the same. The section thickness does not define the stress and environmental conditions, however.

The CR processing can improve toughness, but as already mentioned, there are thickness limits at which the CR can be effective. What this means is if a design calls for a thicker section, it must be able to tolerate a lower toughness or a different material must be used.

You see the same situation with quenched and tempered steels. As the hardness goes up, the actual toughness generally goes down. The toughness demands of a particular design, however, would go up with increasing hardness.

rp

 

[metgist]

First, thank you all for your help and pointing out things I did not consider. Much appreciate!

Apologies for my muddled understanding too - please bear with me as I still cannot understand a copuple of things:

Agreed that CR cannot achieve the same high toughness if section thickness is increased, but if a plate of a given thickness were to be made independently by CR and NR, wouldn't the lower finishing temperature of CR refine the grains more and hence give higher toughness compared to the one produced by NR?

redpicker: It is not necessary that the maximum stress within a component will be higher when it has to withstand higher loads - the designer may choose to keep the maximum stress the same as in case of lower applied loads by the very act of increasing the section thickness. A thicker section also does not necessarily call for higher hardness.

Is it obvious that "if a design calls for a thicker section, it must be able to tolerate a lower toughness"? I am sorry that I probably do not fully understand what you mean by the statement - could you please explain it a bit more?

 

[kumkumvijay]

***** if a plate of a given thickness were to be made independently by CR and NR, wouldn't the lower finishing temperature of CR refine the grains more and hence give higher toughness compared to the one produced by NR? ****

My opinion is as below
I would suggest to know the strengthening mechanism to understand the CR process.

Dynamic re-Crystallization:
In dynamic recrystallization, as opposed to static recrystallization, the nucleation and growth of new grains occurs during deformation rather than afterwards as part of a separate heat treatment. This is influenced by temperature and amount of reduction. At hot working temperature, the grain growth will happen and this growth is hindered by micro alloying elements like Nb, V,Ti & Al. these alloying elements form stable carbide and nitrides which will not dissolve on the austunite phase and remain in the solution at hot working temperature. these particles will stop the grain growth and resulting in the limitation of subsequent hot working and control grain size.

The finishing temperature also play a role on the microstructure like ferrite precipitate formation in sub grain to add value on mechanical properties for CR. That alone done determine the complete mechanical properties.

Three mechanism control the mechanical properties
1. solid solution strengthening
2. Grain size control
3. Dispersion strengthening.

these three determined by chemistry & thermo-mechanical treatment cycle.

 

[metengr]

The problem with thicker is better in pressure vessel design (because of lower membrane stress for increased thickness) is that toughness will be adversely affected because of constraint or plane strain condition through-thickness. This is a fundamental concept especially with materials which have a ductile to brittle transition temperature.

If you look at design requirements for a vessel in low temperature service (which drives the need for normalized plates or quenched and tempered plates), the essential factor that controls the use of plate material at a service temperature is the component thickness. As the component thickness increases minimum design metal temperature increases in similar fashion. So, you have to select plate materials wisely either based on minimizing thickness to have better low temperature toughness (or lower minimum design metal temperature) or use materials which have high toughness at lower temperature.

 

[weldstan]

metgist,
The capability of the plate mill rolls to provide the required reduction at the reduced temps is also at play.

 

[metgist]

kumkumvijay and metengr: thanks for your detailed explanations!

weldstan: Yes, that could be the reason why our vendor cannot supply CR plates below 40 mm. However, since the customer wants CR plates for thicknesses below 40 mm (and accepts NR for > 40 mm), to convince him otherwise (that is, to have him accept NR plates for < 40 mm too), I was thinking along the line that if NR fits the requirement for thicker sections, why should it also not be adequate for thinner sections as well? Am I missing something here?

Again, profound apologies for my evident lack of knowledge!

 

[redpicker]

redpicker: It is not necessary that the maximum stress within a component will be higher when it has to withstand higher loads - the designer may choose to keep the maximum stress the same as in case of lower applied loads by the very act of increasing the section thickness. A thicker section also does not necessarily call for higher hardness.

You are aboslutely correct. But, as metengr has pointed out, while a thicker section will lower the stresses with the same loads, the constraint of the thicker section can create a higher "stress intensity" around unavoidable imperfections in the material, even though the overall stress level is lower.

The hardness has noththing to do with it. I was justing using an analogy between section thickness (and the resulting minimum toughness required) and hardness, or material strength (and the corresponding minimum toughness required). As both increase, the minimum toughness required increases, but in reality, for most materials, as both increase, the actual toughness decreases. Perhaps a poor attempt, but that was what I was trying to do.

Is it obvious that "if a design calls for a thicker section, it must be able to tolerate a lower toughness"? I am sorry that I probably do not fully understand what you mean by the statement - could you please explain it a bit more?

This is just coming to terms with the reality of the situation. As you have noted, with the thicker sections, the toughness is lower (because of limitations in the processing; the CR is no longer possible or, perhaps, effective). If the design is going to require the thicker section (say, to meet a lower general stress level), then it (the design) will have to accomodate a lower toughness, since that is what the material will possess. If the design cannot accomodate a lower toughness, then either a different material will be needed or the design will fail.

rp

 

[metgist]

Thanks rp, I see your point now!

BTW, impact test is optional for this particular grade, but when performed, it has to be at least 27 J (Charpy value at RT). I do not have much feel for these numbers, is it too high or just typical?

After all these discussions, I now have another question:

Consider two materials A and B of the same grade but differing slightly in the treatment (as in the present case). If both satisfy minimum strength requirements (UTS > 410 MPa and YS > 240 MPa) and result in similar section thickness (obviously one will be thicker than the other, but not that much to run into the practical limitations on toughness, as just discussed), then shouldn't either be acceptable? Will it be correct to assume that in this case, differences in other variables, say, corrosion resistance, cost, availability, etc. should guide the choice of material in a particular application?

 

[metengr]

Consider two materials A and B of the same grade but differing slightly in the treatment (as in the present case). If both satisfy minimum strength requirements (UTS > 410 MPa and YS > 240 MPa) and result in similar section thickness (obviously one will be thicker than the other, but not that much to run into the practical limitations on toughness, as just discussed), then shouldn't either be acceptable?

Yes.