1. Multiphase Structures in Case Hardening Steels following Continuous Cooling
H. Roelofs, S.Hasler, L. Chabbi, U. Urlau, Swiss Steel AG
J. Chen, H.K.D.H. Bhadeshia, University of Cambridge
Buenas dias, senoras y senores
Dear Mr. chairman
In the former presentation we have heart about a very promising simulation tool allowing to predict the steel structure after isothermal heat treatment.
As a steel making company however we are very much interested in predicting steel structures after continuous cooling as it typically occurs under hot rolling conditions. Together with Prof. Bhadeshia and Mr. Chen of the University of Cambridge we therefore worked on first applications considering hot rolling.

2. Typical steel structure of a MnCr-case hardening steel
How to master structure and hardness by rolling conditions and steel composition?
ferrite
pearlite
To explain why we are doing this I show you a metallographic picture from a 41 mm bar of a conventional case hardening steel of the MnCr-family. The position of the picture is half radius. And here all kind of phases can be found: allotriomorphic ferrite, pearlite and bainite. Closer to the center also the occurence of martensite is possible.
For steelmakers it is a challenge to master all these phases from the center to the surface of the bar and throughout the whole range of produced diameters.
Sometimes a ferrite-pearlite structure is demanded if cold headed operations are subsequently applied. If instead high strength is within the focus of our customer complete bainitic-martensite structures can also be interesting.
bainite

3. Development of new products
measuring CCT diagrams
hot rolling trials
simulating the steel‘s microstructure on a computer
„Complete Calculation of Steel Microstructure of Strong Alloys“
Excellent agreement for isothermal treatments
To develop accurate rolling strategies or steel compositions we can do small laboratory steel heats and determine CCT diagrams by dilatometry or do small scale laboratory rolling trials to get first ideas about the steel structures after hot rolling.
Typically next step is a large scale industrial trial. In our case this means a 80 tons heat. If the probable result could be predicted by computer simulation than steel and process development could be accelerated and the financial risk of large scale trials could be reduced.

4. The model Chemical composition (C, Mn, Si, Ni, Mo, Cr, V)
Cooling rate
Austenitic grain size (AGS)
Modified Avrami model including simultanuous nucleation and growth rates of
allotriomorphic ferrite,
pearlite,
widmanstätten ferrite,
bainite and
martensite
Steel Structure
Considering Prof. Bhadeshias model we need the chemical composition of the steel, the cooling curve from the austenite state to room temperature and the austenite grain size at the moment of phase transformation.
The elements which are included in the present model are C, Mn, Si, Ni, Mo, Cr and V. As Mr. Chen explained, the program works with a modified Avrami model considering independent nucleation and growth rates of each phase. Allotriomorphic ferrite, pearlite, Widmanstätten ferrite and bainite are then simultanuously growing and the model makes corrections for overlapping phases.

5. Dilatometry: experimental conditions
Steel composition (wt.-%):
Cooling rates (K/s): 0.2, 0.3, 0.6, 0.8, 1.0, 1.6, 3.0, 6.0
Austenitic grain size (mm): 13, 23
C Si Mn Ni Cr Mo
To get a first impression whether this model can be applied if continuous cooling is considered the phase transformation behaviour of a typical case hardening steel was investigated by dilatometric experiments and by computer simulation.
The cooling rate was varied from 0.2 to 6.0 K/s and two austenitic grain sizes were prepared by heat treatment.

6. Comparison for AGS = 13 mm as measured as calculated
Here you see the comparison between the experimental investigation and the computer model for the smaller, 13 microns, grain size. Before I comment this please let me show the second result with the larger grain grain size.

7. Comparison for AGS = 23 mm as measured as calculated
The overall phase transformation behaviour is qualitatively well predicted. There is a reasonable comparison for allotriomorphic ferrite and for pearlite at least at low cooling rates. And here it is good to remember that the model works perfectly well in case of isothermal heat treatments.
The model prefers to predict Widmannstätten ferrite instead of bainite. However these two phases are also difficult to distinguish in metallographic investigations. The reasons for the found discrepancy is still under investigation.

8. Coilers Garrrett (16 – 38 mm)
Industrial trials: layout of hot rolling mill
Furnace
Precision Sizing Mill
Cooling Bed (16 – 64 mm)
Descaler
Shears
Coilers Garrrett (16 – 38 mm)
Finishing Mill Stelmor (5.5 – 16 mm)
Water Boxes
At the end of the day we want to be able to predict steel structures after industrial hot rolling. This picture shows the Swiss Steel rolling mill. 150x150 mm2 billets pass a walking beam furnace, a descaling unit, a 6 stand roughing mill followed by a 10 stand intermediate mill. The last deformation step for bar and Garett wire production happens in a 5 stand Kocks precision sizing mill. The switch after the precision sizing mill decides whether thick wires, thin wires or bars are produced. The thick wires from 16 to 38 mm are directly coiled on a Garrett coiler. The thinner wires from 5.5 to 16 mm pass a 10 stand finishing mill and are thereafter controlled cooled on a Stelmor conveyor line. The bars from 16 to 64 mm are passing the 78 m long cooling bed.

9. Challenges under real industrial conditions
sampling for AGS determination is sometimes difficult
AGS and phase contributions vary over the cross- section of the wire/bar
the cooling curve is not linear
While laboratory investigations on small samples give quite well defined testing conditions, this is not always the case under an industrial environment. This affects the input data for the simulation model as well as the finale results from industrial trials.
Considering the model input data following remarks can be made: the austenitic grain size is difficult to determine properly. In case of thin wire production sampling cannot be performed at the needed position. Particularly in thick products like bars the AGS and the finale steel structure are not homogenous over the cross section. This has to be taken into account if model calculations and measurements are compared.
The simulation model accepts all kind of cooling curves. However in this work the cooling curve was idealized to be linear. The cooling rates are deduced from temperature measurement during production.

10. Stelmor line Positions of sampling: 6.5 mm wire
Furnace
Precision Sizing Mill
Finishing Mill Stelmor
Water Boxes
phase transformation
First we look at the results from the thin wire production. The sketch shows the Stelmor line. The arrow marks the position of the sampling for the AGS analysis. This position is about 4 seconds after the precision sizing mill.
The correct position of sampling would have been at the position indicated by the second arrow. It is assumed that the grain sizes distribution after the finishing mill and after short time relaxation would be similar to the grain size distribution of the sampling position.

11. Input data for modeling
C Si Mn Ni Cr Mo
Steel composition (wt.-%):
Cooling rate (K/s): K/s ( °C)
mean AGS as calculated by hot rolling simulation model: 33.8 mm
Within this work the hot rolling of a 6.5 mm wire was considered. The steel under investigation was a European standard case hardening steel grade called 16MnCr5. Its composition is shown.
The measured cooling rate at the Stelmor conveyor line was 0.34 K/s. With this very low cooling we try to reduce the amount of bainite as good as possible.

12. Comparison calculation versus experiment 6.5 mm wire
Experminentally it is difficult to distinguish between Widmanstätten ferrite, bainite and martensite. For this reason all these phases are summed up as remaining phases.
It is nearly impossible to give reasonable error bars for the experimental data. So we didn‘t. However keeping in mind all the mentionned experimental uncertianties the found agreement must be considered as very good.

13. Garrett line Position of sampling: 23.5 mm wire
Furnace
Precision Sizing Mill
Coilers Garrrett
Water Boxes
In case of thick wire production we can sample at the coiling station. The AGS at this position are expected to be close to the needed AGS before phase transformation. The steel structure of course is determined on the final product.

14. Input data for modeling
C Si Mn Ni Cr Mo
Steel composition (wt.-%):
Cooling rate (K/s): K/s ( °C)
large scattering AGS due to the hot rolling deformation
A 23.5 mm wire was chosen for investigation. The measured cooling rate was 0.16 K/s. However within the coil the cooling rate is expected to be even lower.
The measured grain sizes show little scatter at the surface and mid-radius position. The hot rolling deformation in the center of the bar is less leading to the occurence of larger grains. Now the scattering of measured data is much more pronounced.
Again the encircled mid-radius value was taken for calculation.

15. Comparison calculation versus experiment 23.5 mm wire
Whereas allotriomorphic ferrite and pearlite are well predicted the calculation overestimates the remaining phases. The mentionned uncertainties in the cooling rate and in the AGS determination however could give an explaination for these discrepancies.

16. Bar production line Position of sampling: 36 mm
C Si Mn Ni Cr Mo
Steel composition (wt.-%):
Cooling rate (K/s): K/s ( °C)
sampling
Furnace
Precision Sizing Mill
Cooling Bed
Water Boxes
Now we switch from wire to bar production. In this case the sampling at the cooling bed is close to the position of phase transformation. The measured grain sizes therefore should be really representative.
The considered bar size was 36 mm. The cooling rate as measured by pyrometers was 0.8K/s. This value is expected to be fairly constant over the cross section of the bar.

17. Comparison calculation versus experiment 36 mm bar
The ticker the as rooled product the larger the scattering in austenitic grain sizes. In the surface area the mean measured AGS was 19.3 microns with a standard deviation of only At the mid-radius position the AGS already goes from 27.8 to 71.4 resulting in the mean value of 47.8 and a standard deviation of 13.2 microns. At the center position the scattering of data is already difficult to handle.
For this reason the results from modelling are shown for each position surface, mid-radius and center separately. For the surface area there is excellent agreement between calculation and measurement. This agreement is getting poorer moving towards the center of the bar.
excellent good modest

18. Conclusions
the predictions of the model agree qualitatively well with the measured data from industrial hot rolling distinguishing ferrite, pearlite and „remaining phases“
there is a first indication that the model underestimates bainite and overestimates Widmannstätten ferrite (this is still under investigation)
Considering allotriomorphic ferrite and pearlite the agreement between modeling and measurement is fairly good. There are still some questions concerning the so called remaining phases. There are some indications that the model favours Widmanstätten ferrite instead of bainite. This is still under investigation. And as soon as the problems are solved we will apply the model to work on high strength bainitic steels.

19. Outlook
The quantitative analysis of phase contributions in modern high-strength bainitic-martensitic steels is a very challenging task.
Together with advanced EBSD techniques this model will in future be applied to bring more light into the microstructure of such steels.
The quantitative analysis of phase contributions in modern high-strength bainitic-martensitic steels is a very challenging task.
Together with advanced EBSD techniques this model will in future be applied to bring more light into the microstructure of such steels.

20. High strength bainitic steel Stelmor line, 13 mm wire rod
cooling rate ~ 3.0 K/s
As an example I present here the structure of a bainitic-martensitic steel of the C-Si-Mn-Cr family. It was hot rolled on the Stelmor conveyor line with an elevated cooling rate of 3 K/s. The microstructure of the steel, whatever it is, looks very fine. This fine fine structure leads to an excellent combination of strength and toughness.
very fine steel structure

21. High strength bainitic steel Stelmor line, 13 mm wire rod
Estimated cooling rate ~ 3.0 K/s
Rp0.2 = MPa
Rm = MPa
A5 = 15 %
Z = 68 %
This is the heart beat for our steel scientists
The Charpy impact level at ambient temperature almost reaches 200 Joules. The tensile strength is above 1000 MPa and the reduction of area has the high value of 68%

22. Thank you !
Muchas gracias !