ACCELERATED CARBIDE SPHEROIDISATION AND GRAIN REFINEMENT (ASR) IN RST37-2 STEEL

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1 , Brno, Czech Republic, EU ACCELERATED CARBIDE SPHEROIDISATION AND GRAIN REFINEMENT (ASR) IN RST7 STEEL Jaromír DLOUHÝ a, Daniela HAUSEROVÁ b, Zbyšek NOVÝ c, Jozef ZRNÍK d a,b,c,d COMTES FHT a.s., Průmyslová 995, 4 4 Dobřany, Czech Republic, comtesfht@comtesfht.cz Abstract One of current efforts aimed at ordinary structural steels is to achieve high strength combined with sufficient ductility with minimum production cost. These properties should result from a new thermomechanical treatment process leading to ferrite grain refinement, carbide spheroidisation and changes in the carbide distribution in the microstructure. This newlydeveloped ASR process (Accelerated Spheroidisation and Refinement) has the following stages: heating a steel workpiece through thickness to no higher than the A c transformation temperature and subsequent mechanical working. The mechanical energy introduced by forming leads to rapid heating of the entire workpiece above A c. The workpiece is then aircooled in a conventional fashion. The combination of a suitable preheating temperature and an appropriate amount of strain lead to transformation of initial ferritepearlite microstructure with lamellar pearlite into a desired ferrite matrix with spheroidised carbides throughout the workpiece volume. An impact of plastic deformation on the resulting microstructure and mechanical properties of bulk specimens from lowcarbon St7 steel has been explored here. This thermomechanical process has a potential to be applied to controlled rolling or hot drawing. Keywords: thermomechanical treatment, accelerated carbide spheroidisation, grain refinement, steel RSt7.. INTRODUCTION These days, an ever greater emphasis is laid on enhancing mechanical properties of carbon and lowalloyed steels. By using an appropriate type of processing, one can significantly increase their yield strength, ultimate tensile strength and toughness and achieve an excellent ratio between mechanical properties and price of such materials. This can be accomplished only by refining the microstructure and by obtaining carbides with suitable morphology and distribution []. Recently proposed thermomechanical treatment procedure allows the formation of microstructure with fine ferrite grain and globular cementite to be obtained. As a result of structure modification higher yield strength and ultimate tensile strength can be achieved. In addition, cementite in the globular form contributes also to higher toughness. The present paper explores the influence of plastic deformation on ASR process. Significant acceleration of the spheroidisation process is based on the stock annealing near the A c transformation temperature [] and on introducing strain into the material [].. EXPERIMENTAL PROGRAM The experiment was conducted on specimens of lowcarbon structural steel RSt7 with carbon content below 0.7 weight per cent. Its initial microstructure consisted of ferrite and lamellar pearlite. Hardness of the material in its initial state was 0 HV, its 0. proof stress (PS) was R p0, = 75 MPa, ultimate tensile strength R m = 46 MPa and its elongation was A = 9%. The purpose of the thermomechanical treatment procedure was to explore the influence of deformation intensity on carbide spheroidisation, grain refinement and resulting mechanical properties: The following thermomechanical schedules were carried out: Thermomechanical schedules,, graded deformation in one direction (upsetting),

2 , Brno, Czech Republic, EU thermomechanical schedules 4, 5, 6 graded deformations in two perpendicular directions. These schedules are described in details in section.. The specimens were heated in an air furnace. Plastic deformation was accomplished by flat swages using hydraulic press. Metallographic observation was performed on longitudinal sections of all specimens in order to examine and compare microstructures across the entire specimen crosssection. The microstructure was observed using light microscope Nikon Eclipse MA 00 and in JEOL JSM680 scanning electron microscope. Vickers HV0 hardness values were measured. Specimens with the 0 mm gauge length and a diameter of 4 mm were used for tensile testing... Experimental Schedules Thermomechanical schedules,, Input stocks with the diameter of 45 mm and length of 6 mm were heat treated in an air furnace and then formed between flat swages of a hydraulic press with the ram speed of 5 mm per second. The schedule included heating of the workpiece to a temperature just below A c, onehour hold on temperature and subsequent pressing in a direction parallel to the bar axis (upsetting). Different level of upsetting was performed by individual samples. After forming, the workpiece was cooled in air. The soaking temperature was 70 C and the total effective strain ef was calculated for individual schedules by numerical simulation using DEFORM software (Table ). Effective strain in the specimen centre for schedule was 0.8. In specimen no., the calculated effective strain level was.4 and in specimen no. the effective strain was.. Thermomechanical schedules 4, 5, 6 Specimens with identical dimensions as in previous schedules, and were heat treated in an air furnace and then deformed between flat swages of a hydraulic press with the ram speed of 5 mm per second. The schedule included heating of the workpiece to a temperature just below A c, onehour hold and subsequent plastic deformation in a press. In all these schedules, deformation was introduced in two perpendicular directions at two immediately following steps. After forming, the specimens cooled in still air. The soaking temperature was 70 C and the total effective strain ef was calculated for individual schedules by numerical simulation (Table ). Effective strain in the specimen centre for schedule 4 was.. In specimen no. 5, the calculated strain level was.5 and in specimen no. 6 the strain was.... Numerical Simulation Numerical simulation of thermomechanical treatment was carried out using the software DEFORM to monitor the distribution of strain and temperature throughout the specimen. Effective strain magnitude and a temperature increase due to plastic deformation primarily in the centre of the specimen (point P) but also halfway between the centre and the surface (point P) were monitored (Fig., ). Effective strain was calculated by numerical simulation according to the equation (). ef (), where, and are principal strains and is the effective strain.

3 , Brno, Czech Republic, EU Fig. Effective strain ef distribution for schedule no. 6 immediately after deformation. Fig. Temperature distribution for schedule no. 6 immediately after deformation. ef Table Values calculated by numerical simulation run in DEFORM software Schedule Values at P upon first deformation Values at P upon second deformation Values at P upon first deformation Values at P upon second deformation ef [] T [ C] ef [] T [ C] ef [] T [ C] ef [] T [ C] C/ hour, deformation, air cooling RESULTS AND DISCUSSION Plastic deformation introduced at a temperature around A c modified the microstructure, both in terms of ferrite grain and cementite morphology. Changes in the microstructure were reflected in proof stress values, ultimate strength and, to certain extent, in hardness of specimens as well. (Table ).

4 , Brno, Czech Republic, EU Table Mechanical properties Schedule 0.PS Values at point P UTS A 5 RA HV0 0.PS UTS Values at point P A 5 RA HV0 Initial state In specimens no., and (unidirectional deformation) appreciable grain refinement was detected, being caused by either ferrite grain recrystallization or by formation of subgrains in prior deformed ferrite grains (Fig., Fig. 4, Fig. 5). The strain field is very inhomogeneous. This is why significant changes in microstructure only took place in the specimen centre. In specimens and, it is also the point P (halfway between the specimen centre and surface Fig., Fig. ) where cementite lamellae are partially fragmented. Ferrite grain in this location outside the centre, however, has not been significantly changed from its initial state. Fig. Specimen, centre. Fig. 4 Specimen, centre. Fig. 5 Specimen, centre. In centres of specimens (, and ) the proportion of recrystallized ferrite (or ferrite with mosaic subgrain structure) increased with the amount of plastic strain introduced and the degree of cementite lamellae fragmentation was increasing. Specimen showed only partially broken down cementite lamellae and its pearlite regions largely retained their original shape. One can probably find signs of developing substructure in the form of to µm grains or subgrains in original coarse ferrite grains. The size of subgrains and the

5 , Brno, Czech Republic, EU intensity of their development in various prior ferrite grains are very nonhomogeneous. The centre of specimen already exhibits fully converted microstructure (Fig. 5). Cementite lamellae are completely fragmented and cementite is present in the form of globular particles. The pearlite areas are elongated due to intensive plastic deformation, forming bands with dense globular carbides. Ferrite matrix is either fully recrystallized or the initial ferrite grains are divided into subgrains with the size of µm. Mechanical properties of the specimens reflect their microstructure development. Proof stress and tensile strength increased in all specimens above the strength level of their initial state and showed increases with the amount of plastic strain. In specimen no. the proof stress increased from 75 MPa to 98 MPa and tensile strength changed from 46 MPa to 47 MPa (Table ). Work hardening and strengthening by grain boundaries or lowangle subgrain boundaries caused probably higher hardness. Hardness in the centre of specimens increases gradually between specimens and. Increases in mechanical properties, including hardness, were greatest in the centre of specimens. At point P, microstructure strengthening in comparison with the initial state is not large but the decline in elongation is more appreciable. This is probably due to untransformed lamellar pearlite. Fig. 6 Specimen 4, centre. Fig. 7 Specimen 5, centre. Fig. 8 Specimen 6, centre. Specimens 4, 5 and 6 (deformation applied in two perpendicular directions) exhibit fully spheroidised pearlite in their central region. Centres of specimens 5 and 6 contain, in addition to fine ferrite grains and subgrains, new equiaxed recrystallized grains with notably greater size of 0 to 40 µm (Fig. 6, Fig. 7, Fig. 8). These grains lie outside the bands with globular carbides. In specimen 6, complete carbide spheroidisation took place and finegrained ferrite matrix formed even at point P, i.e. halfway between the centre and surface of the specimen. Microstructure at point P in specimens 4 and 5 is much less elongated than that in the specimen s centre. The prior pearlite lamellae are only partially broken down. However, recrystallized ferrite grains and subgrains have evolved fully even at point P (Fig. 9, Fig. 0). Mechanical properties of specimens 4, 5 and 6 do not show great variations. The values characterizing the specimen centre and the point P are equal as well.

6 , Brno, Czech Republic, EU Fig. 9 Specimen 4, point P (centre). Fig. 0 Specimen 4, point P (halfway between the specimen centre and the surface). 4. CONCLUSION Thermomechanical treatment of steel near the critical temperature A c was used to considerably refine the microstructure and produce spheroidised carbide particles (Accelerated Spheroidisation and Refinement). Increases in proof stress from 75 MPa to around 400 MPa and in tensile strength from 46 MPa to about 470 MPa were accompanied by decline in elongation from 9% to about 8%. In locations where only ferrite was strengthened by formation of fine grains or subgrains but pearlite spheroidisation did not take place, elongation dropped to 5% or lower. Fraction of recrystallized grains enhances with increasing degree of deformation. Recrystallized grains are very fine up to effective strain ca.5. As effective strain increases over the value.5, coarse equiaxial recrystallized grains occur in the structure. Further research will have two key objectives. First one is optimizing the ASR process to obtain finest possible ferrite grain and globular carbides in the microstructure. The second is improving plasticity of the material while retaining the strengthening effect of the ASR process. This can be accomplished through recovery of the ferrite lattice without ferrite grain coarsening. ACKNOWLEDGEMENTS This paper includes results achieved within the project GACR P07/0/7: Accelerated Carbide Spheroidisation and Grain Refinement in Steels. REFERENCES [] Storojeva L., Ponge D., Kašpar R., Raabe D. (004) Development of microstructure and texture of medium karbon steel dutiny heavy warm deformation Acta Materialia, vol. 5, 090, ISSN [] Ghosh S. (00), Ratecontrolling parameters in the coarsening kinetics of cementite in Fe 0.6C steels during tempering. Scripta Materialia, Vol. 6, No., 776, ISSN [] Zhang S. L., Sun. X. J., & Dong H. (006), Effect of deformation on the evolution of spheroidization for the ultra high carbon steel. Materials Science and Engineering, Vol. 4, No., 4, ISSN