1 INTERNATIONAL CONFERENCE ON HEAT TREATMENT AND SURFACE ENGINEERING OF TOOLS AND DIES IFHTSE 2005 Pula, Croatia, June, 2005

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1 st 1 INTERNATIONAL CONFERENCE ON HEAT TREATMENT AND SURFACE ENGINEERING OF TOOLS AND DIES IFHTSE 25 Pula, Croatia, 8-11 June, 25 THE ROLE OF INTERCRITICAL JOMINY TEST IN THE DEVELOPMENT OF DP AND TRIP STEELS M. Réger 1,B. Verő 2, Zs. Csepeli 3, C. H. Gür 4 1 Budapest Polytechnic Dep. of Materials and Technology 2 BAY ZOLTÁN Foundation Institute for Materials and Technology 3 DUNAFERR Co. Innovation Management 4 Middle East Technical University, Ankara, Turkey ABSTRACT The final microstructure of DP and TRIP assisted steels can evolve after hot working (hot rolling) or during post heat treatment process. In the formation of the final structure a number of different technological parameters have important role, e.g. finishing temperature of rolling, cooling rates, temperature of intercritical annealing, etc. As a result of the individual factors and their combinations a lot of product technology routes are feasible. The effect of the different combinations of these technological parameters on the microstructure can be mapped by a special Jominy end-quench test (so called intercritical Jominy end-quench test) described in this paper. Unlike the traditional Jominy test, in this case there is a partial austenizing between A 1 and A 3 temperatures which results in a given amount of ferrite in the microstructure before quenching. The amount of ferrite depends on the temperature. In some cases the quenching process was interrupted for a given period of time in order to model the cooling process on the run-out table. During cooling each point of the Jominy specimen has a different cooling rate, so the effect of cooling rate on the microstructure can be evaluated along the length of the specimen. Keywords: intercritical annealing, Jominy End Quench Method, modeling, DP, TRIP 1. INTRODUCTION The characteristic multiphase structure of DP and TRIP steels can be evolved by hot rolling or by post heat treatment. Only this latter case is discussed in this paper. The transformation processes which result in the expected microstructure and ratio of constituents are controlled by the technological parameters of the treatment. Although the number of affecting parameters is not so large, the determination of important parameters values (intercritical temperature, cooling rate, temperature and duration of quasi isothermal stage) needs numerous pre-experiments before industrial production. The paper describes a new simple method which can be applied for mapping the effect of technological parameters on the 411

2 microstructural properties. This technique was elaborated in the frame of a Hungarian national industrial project which aims at the introduction of DP and TRIP steels production into the Hungarian industrial practice. The project is still under way, so the first results of application of this technique is discussed here. The thermal and physical metallurgical simulation of intercritical annealing is aimed at the complete scanning of possible routes producing multiphase structure (DP and TRIP) and at the determination of heat cycle parameters necessary for optimal microstructure. In addition to the increase of technological reliability the model can help in the prediction of structural and mechanical properties. 2. EXPERIMENTAL MATERIAL Some DP and TRIP steel compositions produced in the frame of the research project can be seen in Table I. The first four grades were smelted in an electric induction furnace and were cast into blocks of 15 kg of weight. After electro slag remelting they were forged and normalized. The 5th grade was produced in the conventional industrial steelmaking route (LD converter). Table I.: Chemical composition of experimental heats Nb. Grade C Mn Si P S Al Cr Nb Mo Ni V 1 DP7,11 1,17,18,45,6,1,5,38,5,75 2 DP6,17 1,18,45,94,6,9,12,12,8 3 TRIP1,22 1,74 1,51,27,5,24,13,1 4 TRIP3,18 1,83,55,124,4,78,8,7 5 DP8,1 1,39,374,14,3,37,28,42,241,29 3. EXPERIMENTAL AND SIMULATION PLAN In the intercritical annealing process two parameters, the intercritical temperature and the cooling rate play the main role from the microstructural point of view. The amount and carbon content of austenite are governed by the chemical composition and the intercritical temperature. The expected amount of austenite at the intercritical temperature is 1-2 % in case of DP, and % in case of TRIP grades. The schemata both of the DP and TRIP steels production are given in Fig. 1. Temperature [ C] Temperature [ C] Ferrite Pearlite Bainite Ferrite Pearlite Bainite Time [s] Intercritical Isothermal bainitic annealing transformation Time [s] Figure 1: Typical time-temperature diagrams of DP (left) and TRIP steel (right) production by intercritical annealing 412

3 Taking into account the wide variety of chemical compositions of these types of steels and the wide range of affecting parameters, the developed method had to be as simple as possible. In practice, the cooling rate range after the intercritical annealing is similar to the cooling rate range inside of end quench specimen used conventionally for hardenability tests of steels. In the conventional Jominy test method the austenizing temperature is over A 3, in our case the temperature is between A 1 and A 3,so this modified test can be called as intercritical Jominy test. The point of this new approach is the partial austenizing at a temperature depending on the chemical composition and on the expected austenite/ferrite ratio. The cooling after intercritical holding in case of DP steel was in accordance with standard. The cooling rate in the Jominy specimen is decreasing as a function of distance from the quenched end, so the connection between the cooling rate and microstructure can be mapped using only one specimen. By hardness and metallographic investigation of these intercritical Jominy probes the distribution of microstructural constituents and hardness can be determined and, knowing the exact cooling rate at a given position inside the specimen, the dependence of microstructure and hardness on the actual cooling rate can be cleared up. That s why in our experimental work, we focused on the precise determination of both the intercritical temperature and cooling rates. In case of TRIP steels the water cooling of the Jominy specimen was interrupted for a given period of time in order to model the step-like cooling schema of TRIP steel production given in Fig 1. During this off-cooling period the temperature differences inside the Jominy specimen are decreasing because of the reheating of the quenched end region resulting in a nearly isothermal period inside the specimen. The microstructure and hardness can be determined after the treatment by the method mentioned earlier EXPERIMENTAL WORK The exact determination of the position dependent time-temperature curves in the Jominy specimen is indispensable in the interpretation of results, so the precise temperature measurement was of great importance. Into the standard Jominy specimens two holes were drilled with a diameter of 1,3 mm squarely to the surface at 1 and 5 mm from the quenched end. Into these holes Ni-CrNi thermocouples were inserted just to the centerline of the specimen. The whole thermal cycle (heating, holding, quenching) was measured and recorded by a data logger. According to the preliminary calculations five intercritical temperatures were chosen (73, 76, 79, 82 and 85 C), these results can be found in [1]. In the present study we are focusing on the reliable description of the temperature distribution inside the Jominy specimen and the possible experimental approach of step-like heat treatment. 4. MATHEMATICAL MODEL OF JOMINY SPECIMEN S COOLING In order to calculate the temperature distribution and cooling in the centerline of Jominy probes a thermal mathematical model was developed and applied. The validation of the model based on temperature measurements discussed above as it follows. The cooling of Jominy specimen was described and published earlier by the adaptation of a FD model [1]. In the present work a FEM model was used in which the temperature dependence of the temperature dependent parameters (i.e. specific heat and the thermal conductivity) were taken into account properly (Fig 2.) on the basis of the calculation results performed by IDS software [2]. The heat transfers for all surfaces are controlled by using proper heat transfer coefficients. 413

4 7,x ,5x Specific heat, J/(m 3 K) 6,x1 6 5,5x1 6 5,x1 6 4,5x1 6 4,x1 6 3,5x1 6 Specific heat (unit volume) Thermal conductivity, W/(mK) Thermal conductivity 3,x Figure 2: The temperature dependencies of specific heat and thermal conductivity By a systematic modification of the coefficients the calculated cooling curves for the position of 1 and 5 mm from the quenched end can be fitted to the measured ones as can be seen in Fig 3. The difference between the measured and calculated results was checked in every case. The values of heat transfer coefficients determined by optimization are in good agreement with literature data quenched end: 5 mm Measured Calculated Interrupted cooling at 36 sec for 25 secs quenched end: 5 mm Measured Calculated 1 quenched end: 1 mm quenched end: 1 mm Figure 3: Measured and calculated time-temperature diagrams for normal (non-interrupted, left) and interrupted (right) cooling conditions It was supposed that in case of optimal fitting the model describes the centerline temperature distribution of the whole specimen properly. The time-temperature functions calculated for the predefined positions of the Jominy specimens in case of a non-interrupted and an interrupted Jominy test are given in Fig. 4. In this latter case the water cooling was interrupted 36 secs after the start of quenching for a holding time of 25 secs with only air cooling. The diagrams contain the calculated temperature just at the quenched surface and below it at a distance of 1,2...1, 15, 2, 25 5, 6, 7, 8 mm, respectively. For the whole cooling process, the cooling rates from the temperature distribution in the characteristic positions of the centerline can be easily calculated. Table II shows the average 414

5 cooling rates between 7 and C along the centerline quenched from different intercritical temperatures. 8 8 Interrupted cooling at 36 sec for 25 secs quenched end (mm): quenched end (mm): Surface Surface Figure 4: Calculated temperature distributions inside the Jominy specimen for normal (non-interrupted, left) and interrupted (right) cooling conditions In order to model the industrial cooling circumstances given in Fig 1. (left) the noninterrupted intercritical Jominy test can be used as it was published earlier [1]. The situation in case of TRIP steel treatment a little bit more complicated because of the presence of quasi isothermal holding stage. By tuning the model parameters the industrial cases can be approached as it follows. 5. APPLICATION FOR MODELING OF INTERCRITICAL HEAT TREATMENT IN HOT DIP CONTINIUOUS GALVANIZING LINE The intercritical heat treatment of strip steels can be combined with hot dip continuous galvanizing process. In this case the cooling rate after the intercritical annealing is about 6-1 K/sec, the residence time in the galvanizing bath about 5-1 secs and the temperature of the molten zinc is about -45 o C. The whole process is very sensitive for the values and stability of these parameters that is why the intercritical interrupted Jominy test was used for modeling the thermal process. By analyzing the cooling curves given in Fig. 4. the following cooling rates can be derived valid between 7- o C. Table II. Average cooling rates between 7- o C at given positions from the quenched end after intercritical annealing of 76 o C quenched end, mm Cooling rate, K/s 92,1 74,9 61,7 5,6 39,9 33,3 28,3 24,1 21,3 19,3 17,4 quenched end, mm Cooling rate, K/s 11,4 8,1 6, 4,7 3,8 3,2 2,8 2,4 1,9 1,6 1,4 415

6 The cooling rate of the required 8 K/s was calculated at around 2 mm from the quenched end, so this region of the Jominy specimen represents the cooling of the strip after the intercritical annealing. By varying the starting time and the duration of water cooling interruption in the Jominy test the temperature and holding time of the isothermal stage can be modified. Fig. 5. shows some preliminary calculation results for determination of appropriate cooling and interruption strategy for approaching the real industrial cases. Start of water cooling interruption 36 secs secs 55 secs Duration of interruption = 25 secs s Start of water cooling interruption Duration of interruption = 15 secs s 45 secs 65 secs Figure 5: Calculated quasi isotherms at 2 mm from the quenched end of the Jominy probe 6. CONCLUSIONS The intercritical annealing of multiphase steels can be simulated in an easy way experimentally by the so-called intercritical and interrupted Jominy test in a wide range of technological parameters. The microstructure developed during the treatment is very sensitive to the intercritical temperature, because it affects both the austenite/ferrite ratio and the carbon content of phases. Thereby the precise knowledge of temperature distribution inside the specimen after temperature holding is essential from the viewpoint of interpretation of the results. By analyzing the microstructural and hardness profile of intercritical Jominy specimens the optimized technological variation can be chosen. Acknowledgement The authors would like to acknowledge the help of all industrial and academic partners involved in the accomplishment of this research project. The financial support of Hungarian Ministry of Education and TÉT Foundation (MISAG-HUN-2, TR-1/23 Turkey-Hungary) is greatly acknowledged. References 1. M. Reger, B. Vero, Zs. Csepeli, J. Pan: Modeling of Intercritical Heat Treatment of DP and TRIP Steels, Transactions on Materials and Heat Treatment, 24, Vol.25, pp Miettinen, J.: Calculation of Solidification-Related Thermophysical Properties for Steels, Metallurgical and Materials Transactions B, 28B (1997), pp