Microstructural Evolution During Hot Rolling

A. S. K o r h o n e n ( l ) , A R a n t a n e n , J . Larkiola . Hels inki Univers i ty of T e c h n o l o g y / F i n l a n d R e c e i v e d on J a n u a r y 15, 1991

SUMbIXRY

Finite element modeling of the hot rolling of steel plates was studied. The finite element code ABAQUS was complemented by a recrystallization model called MICROPLA. It appeared that the rolling force as well as the final grain size and the yield strength of the plate could be predicted with a fairly good accuracy. To study the accelerated cooling after rolling. the phase transformations were taken into account approximately by considering the different cooling n tes in the center and at the edges of the plate. The predicted distortion of the plate was reduced about tenfold after the phase transformations were taken into account.

Key words: Farming, Hot rolling, Finite element method.

1. INTRODUCTION

The continuous development of computers and numerical techniques have made it possible to model efficiently many metal forming processes. Various finite element codes are commercially available today to study e.g. hot forming. .Applications include e.g. the study of metal flow, die design and defect formation in forming / I ,2/. However. althouzh the modeling of the overall geometrical changes have been accomplished successfully in many cases. relarively little attention has been paid to the associated changes in the microstructure and the resulting mechanical properties which finally determine the usefulness of the product itself.

There are several important microstructural phenomena during the hot forming of metals, and these must be closely controlled to obtain the desired properties of the final product. They include e.g. the heating to the hot forming temperature, recrystallization durins the hot forming, cooling. precipitation and phase transfonnations. For steels i t has become quite common to decrerrse the hot forming temperature below the recrystallization temperature during the final stages of the hot forming period to obtain a fine-grained microstructure. Especially in rolling, this kind of technique is nowadays applied on a large scale. It is known as controlled rolling since close control of both temperarure and time is required. Modem so-called microalloyed steels contain in addition to carbon and magnese also small amounts of various alloying elements like Ti, Zr, Nb, V, B and Al. These elements combine readily with the carbon and/or nitrogen in solution and form carbonitride precipitates when the temperature falls. The precipitates in tum retard the movement of the grain boundaries during recrystalliza- tion and hence prevent the grain growth during the high- temperature deformation.

Further precipitation during the cooling period may affect the me- chanical properties considerably. Precipitation generai!y causes additional strengthening of the material. However, the phase transformations of the metallic lattice finally determine the niechanical properties and control the magnitude of the dimensional changes. It has become con~mon to control the cooling rate from the hot working temperature to obtain the desired properties without any additional heat treatment. Since the early 1980s several plate cooling systems have been installed for controlled cooling of the steel plate. Careful control of the cooling process is required to ensure good mechanical properties and dimensional accuracy of the plate.

The aim of this work is to study the aforementioned phenomena in connection with the hot rolling of steel. Similar types of considerations also apply to other forming processes, like hot forging and even other materials. Thermally coupled elastic-plastic finite element code is used to solve the deformation. It is complemented by microstructural models which take recrystallization. precipitation and phase transformations into account. As a result, predictions for the rolling force and the resulting grain size, yield strength and the dimensional changes in the final product are obtained. These are then compared with the measured values.

2 . MODELING OF ROLLING

2.1 Hot rolling

The modeling of the plastic deformation in this work was accomplished with commercially available finite element code ABAQUS version 4.8 i3/. ABAQUS is a thermally coupled elastoviscoplastic code which is commonly applied to study large plastic deformations.

The finite element program was combined with a microstructural recrystallization model MICROPLA developed at Czntro Sviluppo .Ilateriali ( C S M ) , Rome. Italy /4/. Although in the past only a few microstructural models existed for rolling, a number of new ones have appeared during the past couple of years. Some of the reasons for this are: .

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the rolling force is used to control the rolling process and more accurate models are constantly required controlled rolling leads to lower temperatures and higher rolling forces precise rolling force calculation is therefore required to avoid mill overloading and to control f lames the effects of process variables on the properties of the final product can be studied if microstructural changes are taken into account.

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.

Fig. 1 illustrates schematically how the grain size and the yield strength may vary during the successive hot rolling passes.

Variation of grain size and yield stress during successive hot rolling passes (schematic).

The calculation of the grain size proceeds as shown in Fig. 2 . Since the recrystallization after the pass may not be complete, there may be grains of different size after each pass. These are then divided into different classes based on their size. Finally, the transformation from austenite to ferrite during cooling is taken into account and the average grain size of the ferrite is obtained as ;1 result.

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The results of the calculations have been discussed in an earlier work by Rantanen, Malinen and Korhonen /5 / . The material properties and the boundary condition used in the calculations are shown in Table I. For simplicity. spreading was neglected in the calculations. The heat transfer coefficients were obtained from the mill data provided by Rauraruukki Oy Raahe Steel Works.

I INPLTDATA 1 *I

7 IRECRYSTALLIZATION OF THE AUSTENITE~ I I

CLASSIFICATION OF THE GRAINS INTO GRAIN SIZE CLASSES

v GRAIN SIZES AFER THE LAST PASS

I 4

COOLING AND THE PHASE TRANSFORMATION y->a

CLASSIFICATION OF TH€ FERRITE GRAINS INTO GRAIN SIZE CLASSES

Fia. 2. Principle of the calculation of the grain size in the hot roiling of steel.

To compare the calculated results with the experiments. the hot rolling of a low carbon (0.13 4 C) steel plate was considered and a typical pass schedule at the plate mill at Rautaruukki Oy Raahe Steel works was chosen. The starting slab thickness was 174 mm and rhe discharge temperature 1533 K. The pass schedule is shown in Table 11.

Table I. Material properties and boundary conditions.

U4TERML PROPERTIES Young's modulus Poisson's ntio Density Specific heat Tnermal conduciiviry Coefficient of thermal expansion

BOUNDARY COhVlTIONS Heat wnsfer coefficients: From slab to air from slab to roll coefficient of friction

Table 11. Pass schedule parameters

Pass Thickness Force (mm) (kW

1 164 262 2 142 2 1 6 0 3 121 2297 4 100 21.52 5 81 2936 6 6 1 2359 7 44 2238 8 30 2186

145 Wlm2 K 70 k W h 2 K 0.3

Width Time (mm) 6) 186 35

3350 12 3360 5 3360 7 3360 5 .. .. 3360 8 1960 15 1961 12

2.2 Cooling after rolling

As was stated above there will bz a phase transformation from austenite to ferrite when the steel plate cools after rolling. There is considerable interest in accelerating the cooling rate by water spraying. Consequently, thermal stresses of considerable magnitude may arise and the effect of these will be combined with the subsequent phase transformations. If the cooling rate is high enough. even direct quenching to manensite may take place. It would be of considerable interest to be able to predict the possible dimensional changes which would result from the accelerated water cooling.

In this preliminary study we assumed that the existing plate has a perfect rectangular cross-section, i.e. we neglected the effects of the non-zero crown and possible shape distortion due to rolling. The calculation method itself closely followed the technique which has been used earlier by Toshioka /6/ and Thurander et al. f l / .

The steel plate studied in this work was 60 mm thick and 2000 mm wide. It was assumed that the plate initially had a non-uniform temperature distribution after rolling. The comers of the plate were assumed to be cooler than the center. The temperature distribution corresponded approximately to that which was obtained in the previous section after the last pass. Fig. 3 shows the calculated temperature distribution at the end of the plate. Although the temperature distribution was non-uniform, the material properties of the plate were assumed to be initially uniform. The calculation of the cooling of the plate was accomplished using the ABAQUS finite element code and Vaxstation 3100 work station computer.

Fig. 3. The temperature distribution in the end of the plate after rolling. The calculation was carried out in two steps. First, the cooling of the plate was calculated using the values for material parameters which did not take account of the phase transformation. For simplicity, only the cooling of the center and edges of the plate were considered. The cooling curves corresponding to the center and edge were then superimposed on the continuous cooling transformation diagram for the steel studied. The diagram is shown in Fig. 4. It should be noted that it is only approximate. The location of the phase boundaries depend e.g. on the austenitizing temperature and the initial grain size. When they increase, the transformations become generally slower and the phase boundaries move to the right. In the present preliminary calculation it was thought that these effects could as a first approximation be disregarded.

1200 I I I I I

c 1000 t A

2 - 800

2 600

W LL

!- Q

z w I- 200

E LOO

, 0 ' I I I

( I I 10 l o 2 lo3 10' lo5 0 1

TIME [sl

Fiz. 1. The continuous cooling transformation diagram for the 0.23 Ti C 1.32 % Mn steel /8 / .

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When comparing the calculated cooling curves with the CCT- diagram, the phase transformations within each element could be considered 3s a function of time.

Consequently, corrected values were obtained for the material parameters in the center and at the edge of the plate. The parameters included the yield strength, Young’s modulus, density, thermal conductivity and specific heat.

After repeating the calculations. the dimensional changes of the plate were obtained as a result.

3.0

- 2 2 5 - ,“ 2 0

~ 1.5

i 1.0

a 0 LL

f 0 [L

0.5

3. RESULTS AND DtSCL ’SSION

A

-

-

-

-

-

-

3.1 Hot rolling

The calculated and measured rolling forces are compared in Fig. 5 .

It can be seen from Fig. 5 that the ABAQUS program complemented with the MICROPLA microstructure model predicts the rolling force with good accuracy. The difference between the measured and calculated values is less than 4 %. The predicted temperature values after each pass during the pass schedule do not vary by much /5/. However, there IS a difference between the temperatures corresponding to the surface and center immediately after each pass. The difference is between 100 and 150 K and does not vary much in the successive passes. After the final pass the difference between the predicted and measured temperatures is about 30 K.

The calculated average grain size of the fenite was 26 pm. which corresponds to the measured value a1 the center of the plate. In the surface the calculated value was 23 p. The corresponding calculated yield strength values for the center and surface were 260 and 270 m a , respectively, while the measured average value was 270 MPa. The agreement between the predicted and measured values can be considered good.

i d

- C A L C U L A T E D ii--o- M E A S U R E D

00 I I I I I I 1

1 2 3 L 5 6 7 6 PASS N U M B E R

Fig. 5 , Comparison of the measured and calculated rolling forces 151.

3.2 Cooling after rolling

The calculations with the ABAQUS program made it possible to predict dimensional changes of the plate aftcr subsequent accelerated water cooling. Considerable dimensional changes resulted when the phase transfomiations were ignored. The plate curved strongly in the transverse direction and the deflection was about 20 mm. After taking the phase transformations into account, the resulting deflections were reduced to about 2 mm, which corresponds to the results obtained by other authors nl. It may be noted that the present results are rather preliminary. In the future more realistic cross-sectional profiles and possible inhomogeneous distribution of the material parameters should be considered. Combining the cooling model with a realistic 3D

rolling model would be interesting. In addition, one should improve the phase transformation model and make i t an integral par! of the FE-calculation to be able to predict the changes in material properties within each element continuously as function of time.

1. SUMMARY AND COKCLUSIONS

In this work finite element calculation was combined with various microstructural models in order to study hot rolling and subsequent accelerated cooling of steel piate. The finite element code used was ABAQUS version 4.8 and the microstructural models consisted a recrystallization model called MICROPLA and consideration of phase transformations during the subsequent accelerated cooling.

It was shown that by combining ABAQUS with a recrystallization model the rolling force and the final grain size and yield strength of the plate could be predicted with a good accuracy. One limitation of the present approach was that we considered only C- hin steels without microalloying additions. Although the present and other available models have also been applied to microalloyed steels, they do not consider the possible effects of precipitation in detail. It was usually just assumed that recrystallization proceeds slower in microalloyed steels. Work is, however, underway to exparid the present microstructural model to include a precipitation model.

The modeling of accelerated cooling was Pairly simple. It basically consisted of complementing ABAQUS with the information obtained from the continuous cooling transformation diagram, which took into account the phase transformations approximately by considering the differing cooling rates of the edges and the center of the plate. It appeared that by taking the phase transformation into account the resulting dimensional changes were reduced considerably. Further work. however, should be done in order to make the phase transformation model an integral part of the finite element calculation as well as to consider more realistic modeling of the plate so that cooling so that a meaningful comparison with experiments can be made.

REFERENCES

(1) Altan, T., 1987. “Process simulation of hot die forging processes” in Advanced Techno IOPV of Plasticitv (Ed. by K. Langej, Springer-Verlag, Vol. 11: 1021-1034.

(2) Altan,, T. and Oh, S . 4 , 1990, “Application of FEM to 2-D metal flow simulation: practical examples”, in Advanced Technoloev of Plasticitv. f i e Japan Socieiy for Technology of Plasticity, Vol. IV: 1779-1787.

(3) ABAOUS. Theorv and User’s Manuals. Version 4.8.. 1989, Hibbit, Karlsson s( Sorensen Inc. 1989.

(4) Anelli. E., Ghersi, M., Mascanzoni, A,, Paolichhi, M., Aprile, A., Granato, F., Liguori, G. and Rizzo, G. , 1986. “Plate rolling of HSLA steels with control of microstructure”, in HSLA Steels:

-andAoolications. ASM International: 693-695.

(5) Rantanen, A,, Malinen, M. and Korhonen, A.S.. 1990. “Prediction of the microstructure of steel during hot rolling”, in Advanced Technoloev of Plasticitv, The Japan Society for Technology of Plasticity Vol. 11: 659-664.

(6) Toshioka, Y., 1985, Materials Science and Technology 1: 883-892.

(7) Thurander, A.. Melander, A. and Jansson, R . , 19S8, Calculation of distortion during quenching of steel. Swedish Institute for Metals Research, Report No. Ihl-2361.

(8) Schrader, A. and Rose, A.. 1966, De ferri metallogranhia, Vol. IT. Verlag Stahleisen, Diisseldorf: 81.

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